(Plastics Design Library handbook series) Sina Ebnesajjad - Expanded PTFE Applications Handbook_ Technology, Manufacturing and Applications-William Andrew (2017) - Flipbook by Kishan Patel

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EXPANDED PTFE APPLICATIONS HANDBOOK

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PLASTICS DESIGN LIBRARY (PDL) PDL HANDBOOK SERIES Series Editor: Sina Ebnesajjad, PhD ([email protected]) President, FluoroConsultants Group, LLC Chadds Ford, PA, USA www.FluoroConsultants.com The PDL Handbook Series is aimed at a wide range of engineers and other professionals working in the plastics industry, and related sectors using plastics and adhesives. PDL is a series of data books, reference works and practical guides covering plastics engineering, applications, processing, and manufacturing, and applied aspects of polymer science, elastomers and adhesives. Recent titles in the series Biopolymers: Processing and Products, Michael Niaounakis (ISBN: 9780323266987) Biopolymers: Reuse, Recycling, and Disposal, Michael Niaounakis (ISBN: 9781455731459) Carbon Nanotube Reinforced Composites, Marcio Loos (ISBN: 9781455731954) Extrusion, 2e, John Wagner & Eldridge Mount (ISBN: 9781437734812) Fluoroplastics, Volume 1, 2e, Sina Ebnesajjad (ISBN: 9781455731992) Handbook of Biopolymers and Biodegradable Plastics, Sina Ebnesajjad (ISBN: 9781455728343) Handbook of Molded Part Shrinkage and Warpage, Jerry Fischer (ISBN: 9781455725977) Handbook of Polymer Applications in Medicine and Medical Devices, Kayvon Modjarrad & Sina Ebnesajjad (ISBN: 9780323228053) Handbook of Thermoplastic Elastomers, Jiri G Drobny (ISBN: 9780323221368) Handbook of Thermoset Plastics, 2e, Hanna Dodiuk & Sidney Goodman (ISBN: 9781455731077) High Performance Polymers, 2e, Johannes Karl Fink (ISBN: 9780323312226) Introduction to Fluoropolymers, Sina Ebnesajjad (ISBN: 9781455774425) Ionizing Radiation and Polymers, Jiri G Drobny (ISBN: 9781455778812) Manufacturing Flexible Packaging, Thomas Dunn (ISBN: 9780323264365) Plastic Films in Food Packaging, Sina Ebnesajjad (ISBN: 9781455731121) Plastics in Medical Devices, 2e, Vinny Sastri (ISBN: 9781455732012) Polylactic Acid, Rahmat et. al. (ISBN: 9781437744590) Polyvinyl Fluoride, Sina Ebnesajjad (ISBN: 9781455778850) Reactive Polymers, 2e, Johannes Karl Fink (ISBN: 9781455731497) The Effect of Creep and Other Time Related Factors on Plastics and Elastomers, 3e, Laurence McKeen (ISBN: 9780323353137) The Effect of Long Term Thermal Exposure on Plastics and Elastomers, Laurence McKeen (ISBN: 9780323221085) The Effect of Sterilization on Plastics and Elastomers, 3e, Laurence McKeen (ISBN: 9781455725984) The Effect of Temperature and Other Factors on Plastics and Elastomers, 3e, Laurence McKeen (ISBN: 9780323310161) The Effect of UV Light and Weather on Plastics and Elastomers, 3e, Laurence McKeen (ISBN: 9781455728510) Thermoforming of Single and Multilayer Laminates, Ali Ashter (ISBN: 9781455731725) Thermoplastics and Thermoplastic Composites, 2e, Michel Biron (ISBN: 9781455778980) Thermosets and Composites, 2e, Michel Biron (ISBN: 9781455731244) To submit a new book proposal for the series, or place an order, please contact David Jackson, Acquisitions Editor [email protected]

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EXPANDED PTFE APPLICATIONS HANDBOOK TECHNOLOGY, MANUFACTURING AND APPLICATIONS Sina Ebnesajjad President, FluoroConsultants Group, LLC Amsterdam • Boston • Heidelberg • London • New York • Oxford Paris • San Diego • San Francisco • Singapore • Sydney • Tokyo William Andrew is an imprint of Elsevier

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William Andrew is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright Ó 2017 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means,electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this ﬁeld are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-1-4377-7855-7 For information on all William Andrew publications visit our website at https://www.elsevier.com/ Publisher: Matthew Deans Acquisition Editor: David Jackson Editorial Project Manager: Edward Payne Production Project Manager: Nicky Carter Designer: Victoria Pearson Typeset by TNQ Books and Journals

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Contents Preface ................................................................................................................................................................... xi Acknowledgment ................................................................................................................................................. xiii 1 History of Polytetraﬂuoroethylene and Expanded PTFE Membrane ..................................................... 1 Part I: Discovery of Polytetraﬂuoroethylene ..................................................................................................1 1.1 Discovery of Polytetraﬂuoroethylene .......................................................................................................1 1.2 Roy Plunkett’s Story .................................................................................................................................1 1.3 Commercialization of Polytetraﬂuoroethylene.........................................................................................3 Part II: Invention of Expanded Polytetraﬂuoroethylene ..................................................................................4 1.4 A New Type of Polytetraﬂuoroethylene...................................................................................................4 1.5 Early History of W.L. Gore and Associates.............................................................................................4 1.6 Discovery of Expanded Polytetraﬂuoroethylene......................................................................................5 References.........................................................................................................................................................7 2 Polytetraﬂuoroethylene: Properties and Structure .................................................................................... 9 2.1 Introduction...............................................................................................................................................9 2.2 Impact of F and CeF Bonds on the Properties of Polytetraﬂuoroethylene ............................................9 2.3 Crystalline Structure of Polytetraﬂuoroethylene....................................................................................12 2.4 Branched Tetraﬂuoroethylene Chains: Perﬂuorinated EthyleneePropylene Copolymer .....................12 2.4.1 Perﬂuorinated EthyleneePropylene Copolymer ......................................................................... 13 2.5 Reaction Mechanisms .............................................................................................................................14 2.6 Impact of Solvents on Fluoropolymers ..................................................................................................15 2.7 Molecular Interaction of Polytetraﬂuoroethylene: Low Friction and Low Surface Energy .................16 2.8 Conformations and Transitions of Polytetraﬂuoroethylene ...................................................................18 2.8.1 Images of the Polytetraﬂuoroethylene Molecule ........................................................................ 19 2.9 Microstructure and Fracture of Polytetraﬂuoroethylene ........................................................................20 References.......................................................................................................................................................22 3 Manufacturing Polytetraﬂuoroethylene by Emulsion Polymerization .................................................. 25 3.1 Introduction...........................................................................................................................................25 3.2 Tetraﬂuoroethylene Preparation............................................................................................................26 3.3 Polymerization of Tetraﬂuoroethylene .................................................................................................29 3.4 Tetraﬂuoroethylene Polymers ...............................................................................................................31 3.4.1 Ammonium Perﬂuorooctanoate (Also C8)................................................................................ 32 3.4.2 Alternatives to Ammonium Perﬂuorooctanoate........................................................................ 33 3.5 Preparation of Polytetraﬂuoroethylene by Emulsion Polymerization .................................................35 3.6 Emulsion Polymerization of Tetraﬂuoroethylene With Ammonium Perﬂuorooctanoate Replacements ........................................................................................................................................42 v

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vi CONTENTS 3.7 Mechanism of Emulsion Polymerization of Tetraﬂuoroethylene ........................................................44 3.8 Development of Polytetraﬂuoroethylene for Expanded Polytetraﬂuoroethylene Applications ..........45 Stretch Ratio and Ultimate Stretch Ratio Test ....................................................................... 47 Preparation of Test Specimen ................................................................................................. 47 Stretch Procedure.....................................................................................................................48 Tensile Break Strength Test .................................................................................................... 48 Stretching Rate ........................................................................................................................48 Stress Relaxation Time............................................................................................................ 48 Stretch Test ..............................................................................................................................51 Measurement of Stress Relaxation Time ................................................................................ 52 Stretch Procedure.....................................................................................................................55 Stress Relaxation Time............................................................................................................ 56 Break Strength ......................................................................................................................... 56 Creep Rate ...............................................................................................................................56 Evaluation of Extrusion Pressure and Stretchability ..............................................................57 Measurement of Tensile Break Strength................................................................................. 57 Measurement of the Endothermic Ratio ................................................................................. 57 Measurement of the Stress Relaxation Time .......................................................................... 58 3.9 Fine Powder (Coagulated Dispersion) Products ..................................................................................59 3.10 Characterization of Polytetraﬂuoroethylene.........................................................................................60 Fine Powder Polytetraﬂuoroethylene Resins .......................................................................... 60 Dispersions of Polytetraﬂuoroethylene ...................................................................................62 References.......................................................................................................................................................62 4 Fabrication and Processing of Fine Powder Polytetraﬂuoroethylene.................................................... 65 4.1 Introduction...........................................................................................................................................65 4.2 Background ...........................................................................................................................................65 4.3 Paste Extrusion Fundamentals..............................................................................................................66 4.4 Resin Handling and Storage .................................................................................................................68 4.5 Extrusion Aid or Lubricant...................................................................................................................71 4.6 Blending the Resin With Lubricant......................................................................................................74 4.6.1 Pigment Addition....................................................................................................................... 77 4.7 Preforming ............................................................................................................................................77 4.8 Extrusion Equipment and Process ........................................................................................................79 4.8.1 Extruder...................................................................................................................................... 80 4.8.2 Die .............................................................................................................................................. 81 4.8.3 Drying ........................................................................................................................................83 4.8.4 Sintering and Cooling ................................................................................................................ 83 4.8.5 Reduction Ratio ......................................................................................................................... 84 4.9 Extrusion of Tubing ..............................................................................................................................85 4.9.1 Blending Lubricant and Pigment and Preforming ....................................................................87 4.9.2 Extrusion of Spaghetti Tubing...................................................................................................87 4.10 Unsintered Tape ....................................................................................................................................90 4.10.1 Blending Lubricant and Pigment and Preforming .................................................................. 90

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CONTENTS vii 4.10.2 Extrusion of Round and Rectangular Bead............................................................................. 91 4.10.3 Calendaring ..............................................................................................................................92 4.10.4 Stretching the Polytetraﬂuoroethylene Tape ........................................................................... 94 4.10.5 Final Tape Product................................................................................................................... 95 References.......................................................................................................................................................96 5 Expansion of Polytetraﬂuoroethylene Resins............................................................................................ 99 5.1 Introduction.............................................................................................................................................99 5.2 Manufacturing Expanded Polytetraﬂuoroethylene Articles...................................................................99 5.2.1 Basic Polytetraﬂuoroethylene Expansion Processes ................................................................. 100 5.2.2 Uniaxial Expansion.................................................................................................................... 101 5.2.3 Biaxial Expansion ......................................................................................................................107 5.3 Microstructure of Polytetraﬂuoroethylene............................................................................................111 5.4 Microstructure of Expanded Polytetraﬂuoroethylene ..........................................................................116 5.5 Formation of Expanded Polytetraﬂuoroethylene .................................................................................118 5.6 Amorphous Locking .............................................................................................................................122 5.7 Characterization of Membrane Pores ...................................................................................................122 5.7.1 Bubble Point .............................................................................................................................. 123 5.7.2 Derivation YoungeLaplace Equation........................................................................................124 5.7.3 Mercury Porosimetry ................................................................................................................. 124 5.8 Summary ...............................................................................................................................................125 References.....................................................................................................................................................125 6 Manufacturing of Various Shapes of Expanded Polytetraﬂuoroethylene (ePTFE) ........................... 129 6.1 Planar Expanded Polytetraﬂuoroethylene Membranes ........................................................................129 6.1.1 Uniaxial Orientation ..................................................................................................................130 6.1.2 Biaxial Expansion (Orientation)................................................................................................ 131 6.2 Tubular Expanded Polytetraﬂuoroethylene Shapes..............................................................................134 6.2.1 Complex Shape Tubular Expanded Polytetraﬂuoroethylene ....................................................140 6.3 Expanded Polytetraﬂuoroethylene Fiber ..............................................................................................142 6.3.1 High Tensile Strength Polytetraﬂuoroethylene Fiber................................................................ 142 6.3.2 Production of Expanded Polytetraﬂuoroethylene Fiber............................................................143 6.4 Densiﬁed Porous Polytetraﬂuoroethylene Membranes ........................................................................149 6.5 Expanded Polytetraﬂuoroethylene Sheets ............................................................................................153 6.6 Expanded Polytetraﬂuoroethylene Tapes and Rods .............................................................................159 References.....................................................................................................................................................159 7 Properties, Characteristics, and Applications of Expanded PTFE (ePTFE) Products...................... 163 7.1 Introduction...........................................................................................................................................163 7.2 Properties and Characteristics ..............................................................................................................163 7.3 Applications ..........................................................................................................................................166 7.3.1 Industrial and Process Filtration................................................................................................ 167 7.3.2 Microﬁltration Applications ...................................................................................................... 168 7.3.3 Vent Filters and Breathers ......................................................................................................... 168 7.3.4 Medical and Biological Uses..................................................................................................... 168

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viii CONTENTS 7.3.5 Cables and Cable Assemblies....................................................................................................168 7.3.6 Electronic and Electrochemical Materials ................................................................................168 7.3.7 Sealants ......................................................................................................................................169 7.3.8 Fibers and Fabrics......................................................................................................................169 References.....................................................................................................................................................169 8 Expanded PTFE Use in Fabrics and Apparel ........................................................................................ 171 8.1 Introduction...........................................................................................................................................171 8.2 Breathable Expanded Polytetraﬂuoroethylene Fabric Structure..........................................................172 8.3 Development History ............................................................................................................................178 8.4 Outdoor Apparel ...................................................................................................................................182 8.4.1 Testing Apparel..........................................................................................................................184 8.4.2 Outdoor Footwear ......................................................................................................................186 8.4.3 Testing Footwear........................................................................................................................186 8.4.4 Outdoor Gloves ..........................................................................................................................188 8.5 Protective Apparel ................................................................................................................................188 8.6 Summary ...............................................................................................................................................190 References.....................................................................................................................................................190 9 Medical and Surgical Applications of Expanded PTFE........................................................................ 193 9.1 Introduction...........................................................................................................................................193 9.2 Deﬁnition of Medical Devices..............................................................................................................193 9.3 Classiﬁcation of Devices ......................................................................................................................195 9.4 Designing Medical Devices..................................................................................................................196 9.5 Biomaterials ..........................................................................................................................................196 9.6 Expanded Polytetraﬂuoroethylene........................................................................................................196 9.7 Examples of Applications.....................................................................................................................198 9.7.1 Vascular Grafts...........................................................................................................................198 9.7.2 Patches........................................................................................................................................202 9.7.3 Expanded Polytetraﬂuoroethylene Lipoatrophy Implants.........................................................202 9.7.4 Expanded Polytetraﬂuoroethylene Sutures................................................................................204 9.7.5 Lead Assembly of Implanted Devices ......................................................................................205 9.7.6 Stents .......................................................................................................................................... 207 References.....................................................................................................................................................209 10 Filtration ..................................................................................................................................................... 213 10.1 Introduction.........................................................................................................................................213 10.2 Classiﬁcation of Filtration Processes .................................................................................................213 10.3 Surface Filtration Processes................................................................................................................214 10.4 Types of Filtration...............................................................................................................................215 10.4.1 GaseSolid Filtration .............................................................................................................. 215 10.4.2 SolideLiquid Filtration ......................................................................................................... 223 10.5 Examples of Filtration Applications...................................................................................................224 References.....................................................................................................................................................230

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CONTENTS ix 11 Industrial and Other Applications of Expanded PTFE ........................................................................ 233 11.1 Expanded Polytetraﬂuoroethylene Fiber ............................................................................................233 11.1.1 Oral Care................................................................................................................................233 11.1.2 Sutures.................................................................................................................................... 235 11.1.3 Sewing Threads......................................................................................................................236 11.1.4 Fishing Line ...........................................................................................................................238 11.1.5 Weaving and Knitting Fiber .................................................................................................. 240 11.1.6 Ropes......................................................................................................................................242 11.2 Gaskets and Seals ...............................................................................................................................244 11.2.1 Testing Gaskets ......................................................................................................................247 11.3 Expanded Polytetraﬂuoroethylene Vents............................................................................................248 References.....................................................................................................................................................250 12 Electrical and Electronic Applications of Expanded PTFE.................................................................. 253 12.1 Coaxial Cables ....................................................................................................................................254 12.2 Hook-Up Wire.....................................................................................................................................255 12.3 Electromagnetic Interference Shielding Gasket.................................................................................256 12.4 Disk Drive Filters................................................................................................................................257 References.....................................................................................................................................................258 13 Surface Modiﬁcation of Expanded Polytetraﬂuoroethylene ................................................................. 259 13.1 Introduction.........................................................................................................................................259 13.2 Surface Treatment of Polytetraﬂuoroethylene ...................................................................................260 13.3 Surface Treatment of Expanded Polytetraﬂuoroethylene Membrane................................................263 13.3.1 Surface Modiﬁcation for Hydrophilicity and Adhesion .......................................................263 13.3.2 Surface Modiﬁcation to Reduce Thrombogenicity ...............................................................265 13.3.3 Mechanical Alteration of Expanded Polytetraﬂuoroethylene Surface ................................. 268 References.....................................................................................................................................................271 14 The Competitive Scene .............................................................................................................................. 275 14.1 Introduction.........................................................................................................................................275 14.2 Other Expanded Polytetraﬂuoroethylene Players...............................................................................275 14.2.1 General Electric .....................................................................................................................275 14.2.2 Donaldson Corporation.......................................................................................................... 276 14.2.3 DeWal Industries.................................................................................................................... 276 14.2.4 Zeus Industrial Products ........................................................................................................ 276 14.2.5 C. R. Bard Corporation.......................................................................................................... 276 14.2.6 Maquet Cardiovascular .......................................................................................................... 276 14.2.7 Porex Corporation..................................................................................................................276 14.2.8 Phillips Scientiﬁc ...................................................................................................................276 14.2.9 Asian Manufacturers.............................................................................................................. 277 Index ................................................................................................................................................................... 279

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Preface I have marveled at the elegant and intricate developed for membrane manufacturing. Chapter 4 structure of expanded microporous membranes of describes processing of ﬁne powder PTFE into polytetraﬂuorethylene (PTFE) for decades. Thanks to precursor ﬁlms and other shapes for the production of scanning electron microscopy, one can see the microporous membranes. Chapter 5 focuses on how minutia of the membrane scaffolding. The beauty of expanded PTFE membranes and other shapes are the chaotic yet ordered microporous structure comes formed followed by discussion of techniques to make in tandem with its remarkable utility. The words different shapes of ePTFE in Chapter 6. “elegant” and “beauty” may raise eyebrows as they do in mathematics. David H. Bailey (retired Senior Chapter 7 describes the properties and character- Scientist, Lawrence Berkeley National Laboratory, istics of ePTFE membranes along with a brief University of California, Davis) and Jonathan M. discussion of its important applications. Chapters 8 Borwein (Professor of Mathematics, University of through 12 discuss key applications of ePTFE. Newcastle, Australia) wrote in a 2014 blog post Chapter 13 discusses a number of methods for (Why Mathematics Is Beautiful and Why It Matters, treatment and modiﬁcation of surfaces of micropo- February 14, 2018, Hufﬁngton Post): all (mathe- rous membranes. Those superﬁcial alterations allow matical) esthetic responses seem in part to come from impartation of hydrophilicity or special functions to identifying simplicity in complexity, pattern in chaos, the membrane surface. Chapter 14 discusses the structure in stasis. I have found this description true manufacturers of expanded microporous PTFE of microporous membranes of PTFE. membrane. Naturally, I set out to learn all I could about this This has been a difﬁcult book to write and has beautiful membrane. These days there are few taken almost 6 years to complete. Any publisher commercial products about which one does not ﬁnd other than Elsevier would have long canceled my scores of books and in-depth articles. Consequently, I contract. Now that the book is published I have two expected to ﬁnd books, at least one, and papers. To hopes. First, the book, ﬂawed as it may be, would be my surprise, there was little published about the beneﬁcial to those who seek information about fundamentals of expanded PTFE membranes and expanded PTFE membranes. Second, I hope what is ﬁlms, how they are formed and made, how they work, correct and what is incorrect motivates brighter and a host of other questions. Certainly, there are minds to write their own books about this subject. innumerable articles about the membrane’s varied The colossal impact of these membranes on human applications, plenty of commercial literature and life certainly warrants such efforts. I hope to receive a plethora of patents. as many critiques of this book as possible, with a promise to make corrections in the next edition. So I set out on a decade and a half long journey to explore and learn, on my own, about this micropo- The readers who wish to dispense with discussions rous membrane (ePTFE). This book presents the of polymer and precursor ﬁlm preparation can begin results of my ePTFE educational journey. I hope and reading the book with Chapter 5. A modest knowl- pray it offers a convenient starting point for those edge of PTFE and its properties is required for who wish to learn about the ePTFE membranes. a deeper understanding of the discussions. The applications chapters can be approached without The book begins with two short chapters reading the other chapters. describing a short history of ePTFE and the proper- ties of PTFE. Chapter 3 discusses polymerization and Sina Ebnesajjad ﬁnishing of PTFE by emulsion method (ﬁne powder PTFE) placing emphasis on grades specially Chadds Ford, Pennsylvania August 2016 xi

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Acknowledgment Many people have contributed to this book Parkinson Freudenberg Sealing including everyone whose work has been mentioned Technologies Technologies in this book in some shape and form. I have tried to acknowledge all who have helped me with this book. STM, Inc. Donaldson Corp. To my profound regret I may have missed some names. Please accept my sincere apologies for the International Polymer Poly Fluoro Ltd. oversight. A short note to the publisher or to me Engineering, Co. directly will allow correction in the electronic online copy and future editions. I am most grateful to many Adtech Polymer Diener Electronic companies and individuals who have contributed Engineering data, photographs, and illustrations to this book. Authors and companies have been cited in the book Outdoor Sports Center Henniker Plasma or in the reference section of each chapter. www.eventfabrics.com PVA TePla Co. I have listed the names and website of companies wherever their material has been used. I offer you my Madison Chemical Acton Technologies heartfelt thanks for your enriching contributions to this Industries book. I hope the following list (in the order of appearance in the book) is not missing any companies. REI Co-Op Enercon Ind. If there is an error, a short note to the publisher or me will be appreciated so that the error could be corrected: Atrium Medical Corp. Tri-Star Technologies GE Life Sciences Ingenta Mayo Foundation for DeWal Industries Education and Res W. L. Gore & Pall Corporation Clarcor Industrial Air, Zeus Industrial Products Associates BHA Industrial Filtration C. R. Bard Davol Inc. Coreﬂon Corp. Willy A. Bachofen Amann & Soehne G Eaton Corp. Maquet Cardiovascular DuPont/Chemours Sunteca Corp. Buﬂovak, LLC WLT Dichtungstechnik, PORTEF ePTFE Filters Porex Corporation e.K. Jennings International Virginia Sealing Advantec MFS, Inc. Phillips Scientiﬁc Products QPD Inc. Teadit North America I have used numerous illustrations and much Bruckner USA All State Gasket data from W. L. Gore & Associates in this book. Summit Filter Leader GT Corporation That is simply because of the paramount role Corporation the company has played in the development Marshall and Williams, RAM Gasket Solutions of expanded polytetraﬂuoroethylene (ePTFE) Div. Parkinson membranes and products based on those Technology membranes. Rarely, if ever, a single company has played such a substantial role in the development of a product that has cascaded into billions of dollars throughout the market value chains. To be sure a number of manufacturers of ePTFE have also made technological and applications contributions. My deepest thanks go to W. L. Gore & Associates xiii

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xiv ACKNOWLEDGMENT and other ePTFE manufacturers for the generosity the breadth of subject matters of PDL. Ms. Nicky they have extended to me. Carter was the production manager of this book. Nicky’s good nature and patience, in addition to her I would like to thank the individuals who played support, were vital during the production of this book. a key role in attainment of permission for the use of W. L. Gore illustrations and data in this book. Ms. I would like to thank my friend and soul mate Amy E. Calhoun, the leader of Enterprise Commu- Ghazale Dastghaib for her inﬁnite support and nications at W. L. Gore & Associates devoted patience over the decades while I have been writing signiﬁcant time to this matter. She consulted and editing books. Without her love and comradery I a number of people at W. L. Gore & Associates and could have never had the wonderful career I have had. provided me with guidance and the needed permis- sion to use the illustrations and artwork in this book. None of the views or information presented in this Thank you Amy. Ms. Jenny E. Maher was instru- book reﬂects the opinion of any of the companies mental in facilitating the process of obtaining (especially W. L. Gore & Associates) or individuals permission. Thank you Jenny. that have contributed to the book. If there are errors, I own them. A note indicating the speciﬁc error to the I would like to express my appreciation to publisher, for the purpose of correction, would be Matthew Deans the senior publisher of William much appreciated. Contact information can be found Andrew imprint for his support. David Jackson, in the front matter of this book. acquisitions editor of Plastics Design Library (PDL) has supported me throughout this project and others Sina Ebnesajjad in every possible way. He has been my energetic Chadds Ford, Pennsylvania partner in the efforts to grow the number of titles and August 2016

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1 History of Polytetraﬂuoroethylene and Expanded PTFE Membrane OUTLINE Part I: Discovery of Part II: Invention of Expanded 4 Polytetraﬂuoroethylene 1 Polytetraﬂuoroethylene 1.1 Discovery of Polytetraﬂuoroethylene 1 1.4 A New Type of Polytetraﬂuoroethylene 4 1.2 Roy Plunkett’s Story 1 1.5 Early History of W.L. Gore and Associates 4 1.3 Commercialization of 1.6 Discovery of Expanded Polytetraﬂuoroethylene 5 Polytetraﬂuoroethylene 3 References 7 Part I: Discovery of new ﬂuorinated refrigerants that were safer than old Polytetraﬂuoroethylene [1a] gases because of being nonﬂammable, nontoxic, colorless, and odorless. He reacted tetraﬂuoro- Many people around the world, regardless of ethylene (TFE) with hydrochloric acid (HCl) for whether or not they have been directly involved in the synthesis of a refrigerant, CClF2eCHF2 [2]. As he creation and production of polymer, are familiar with had done on many other occasions, on the morning the origins of ﬂuoropolymers. Teﬂon®, DuPont’s of April 6, 1938, Plunkett checked the pressure on a trademark name for polytetraﬂuoroethylene (PTFE), full cylinder of TFE. He was surprised to ﬁnd no is world renowned. The classic story of the discovery pressure, and yet the weight of the cylinder was the of ﬂuoropolymers is replete with the magical com- same as it had been the previous day. Plunkett and bination of curiosity, perseverance, and serendipity. It his technician removed the valve and shook the is helpful to note the environment and context in cylinder upside down. When they cut open the gas which polymer science began. This section places the cylinder, they recovered a small amount of a slip- invaluable discovery made by Roy Plunkett [1b] in pery white substance (Fig. 1.1). They analyzed the the context of the times and of the events and per- waxy powder and named this new substance poly- sonalities that shaped science, industry, and the world tetraﬂuoroethylene, later trademarked as Teﬂon® by in the 1930s and 1940s. Plunkett’s ﬁnding is even the DuPont Company. The rest, as they say, is his- more impressive when viewed through the prism of tory (Fig. 1.2). this context. 1.2 Roy Plunkett’s Story 1.1 Discovery of Polytetraﬂuoroethylene Roy was born into a poor farm family in New Carlisle, Ohio. When the Great Depression began he For those who do not already know the story, let was a student at Manchester College in North Man- us begin with the ending. By 1938, Dr. Roy Plunkett chester, Indiana, where he shared a room with an had been working at DuPont for 2 years, developing older student named Paul Flory. Roy graduated with a bachelor of arts in chemistry in 1932 and followed Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00001-8 1 Copyright © 2017 Elsevier Inc. All rights reserved.

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2 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 1.1 Depiction of the discovery of polytetra- Paul to graduate school at Ohio State University. ﬂuoroethylene by Roy Plunkett and his assistant, Within 2 years of one another, Roy and Paul both Jack Rebok [3]. earned Masters and PhD degrees from Ohio State Courtesy: The DuPont Co. University. In 1936, Roy joined DuPont Central Research, where Paul had been working since 1934. Roy quickly advanced to Kinetic Chemical Co., a joint venture that DuPont and General Motors (GM) had set up to produce safe refrigerants to replace ammonia and sulfur dioxide. Roy was given a laboratory in DuPont’s Jackson Laboratory on the shore of the Delaware River in Deep Water, New Jersey. Roy’s laboratory was across the hall from a laboratory run by a colleague named Charlie, whose research focused on synthesizing new organic compounds. Roy was trying to expand the line of ﬂuorocarbons, known as Freon, to meet the needs brought on by the explosive growth of automobile production at GM. Excitement erupted in Roy’s lab on April 6, 1938, when he found no pressure in the TFE cylinder and discovered the strange new substance inside. What was this slippery white powder? Because he had time, knowledge, and curiosity, he paused to ask Figure 1.2 Photograph of the notebook page in which Plunkett recorded the discovery of polytetraﬂuoroethylene [3]. Courtesy: The DuPont Co.

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1: HISTORY OF POLYTETRAFLUOROETHYLENE AND EXPANDED PTFE MEMBRANE 3 questions. He was not working under relentless Nazi Germany. Lieutenant General Leslie Richard pressure to meet next month’s deadline because Groves, who led the project, made critical decisions companies like DuPont, that funded research like to prioritize the various methods of isotope separa- Roy’s, understood that success in research required a tion; and he acquired the raw materials needed by the reasonably low-stress work environment. One won- scientists and engineers working on the project. ders how history might have been different had Roy had been given an inﬂexible objective. What would In the course of his search for new materials to have happened if Roy had, upon determining that the meet the novel needs of the Manhattan Project, foreign substance had no properties that would General Groves came across PTFE. After hearing further his ﬂuorocarbon research, wiped up the about the properties of PTFE and its resistance to powder and carried on with his daily tasks? But Roy different chemicals, General Grove is purported to was a well-trained scientist with the freedom and have said that the cost, even at $100 a pound, was a curiosity to investigate this unexpected ﬁnding. bargain! Scientists working on the project badly needed corrosion-resistant materials for the uranium When Charlie heard the racket across the hall, he enrichment process. U-235 had to be separated from walked over to Roy’s lab to investigate. He later said, U-238 using differential diffusion of UF6. UF6 is “I noticed commotion in the laboratory of Roy highly corrosive to most metals, but PTFE stands up Plunkett, which was across the hall from my own. I to it. Once the scientists involved in the Manhattan investigated and witnessed the sawing open of a Project veriﬁed its properties, the US Patent Ofﬁce cylinder from which was obtained the ﬁrst sample of placed PTFE under a national “secrecy order” and Teﬂon® ﬂuoropolymer.” This is the description of from then on it was referred to as “K-416.” Only one that day at Jackson Laboratory that Charlie Pedersen patent, with minimal content, was issued to DuPont shared in his 1987 Nobel Lecture. Pedersen (Fig. 1.3) in 1941 to recognize its rights to the invention [4b]. went on to invent new crown ether compounds, for which he was awarded the Nobel Prize in Chemistry. The next time anyone outside of DuPont heard of PTFE was after World War II, in 1946, under the After logging the results of his discovery that day, now-famous trademark of Teﬂon®. DuPont learned a Roy Plunkett continued with his research. Several great deal about PTFE during its intense efforts to years later, wartime needs rescued his discovery from produce it for the Manhattan Project. When resources oblivion. The Manhattan Project was a covert pro- formerly reserved for the war effort became available gram whose aim was to develop an atom bomb before again to scientists and manufacturers in the US and around the world, it was time to move the production of PTFE from pilot plant to a commercial manufacturing operationdand DuPont was ready. Dr. Plunkett’s own words describe the impact of his discovery: “The discovery of polytetraﬂuoro- ethylene (PTFE) has been variously described as (1) an example of serendipity, (2) a lucky accident and (3) a ﬂash of genius. Perhaps all three were involved. There is complete agreement, however, on the results of that discovery. It revolutionized the plastics in- dustry and led to vigorous applications not otherwise possible” [4a]. Figure 1.3 Dr. Charles J. Pedersen, 1987 Nobel 1.3 Commercialization of Laureate in Chemistry (retired from DuPont). Polytetraﬂuoroethylene Efﬁcient monomer synthesis methods, polymeri- zation technologies, and various forms of PTFE had to be developed. The fact that large-scale monomer synthesis and controlled polymerization had not been fully developed was a technical impediment to

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4 EXPANDED PTFE APPLICATIONS HANDBOOK commercialization of the new polymer. Intensive half a century. PTFE and ePTFE have also generated studies resolved these problems, and small-scale tens of billions of dollars’ worth of business. production of Teﬂon® began in Arlington, New Jer- sey, in 1947. In 1950, DuPont scaled up the com- 1.4 A New Type of mercial production of Teﬂon® in the US with the Polytetraﬂuoroethylene construction of a new plant in Parkersburg, West Virginia. In 1947, Imperial Chemical Industries built W.L. Gore was a successful small company, barely the ﬁrst PTFE plant outside the US, in the United a decade old, when Bob Gore discovered ePTFE, Kingdom. which the company named Gore-Tex. ePTFE trans- formed W.L. Gore into a multibillion-dollar giant of PTFE cannot be dissolved in any solvent, acid, or creativity that has continued to try to ﬁnd new base, and when melted it forms a stiff clear gel with product development opportunities in which ePTFE no ﬂow. Special processing techniques normally used plays a key role. Without the discovery of PTFE and for molding metal powders were modiﬁed to fabri- ﬂuoropolymers, DuPont would have still been a large cate parts from PTFE. Another process, called paste corporation, albeit somewhat diminished. In the extrusion, was borrowed from ceramic processing. absence of ePTFE, W.L. Gore and Associates, whatever its fate, would not have been the company Roy Plunkett’s discovery of PTFE was just the that it became, thanks to Gore-Tex. beginning. Throughout this embryonic stage of polymer science there was much excitement and The discovery and evolution of ePTFE is inextri- curiosity and debate in scientiﬁc circles, and many cably linked to the history of W.L. Gore as a com- scientists around the world built upon this discovery. pany, and so this chapter tells that story. W.L. Gore’s Scientists devoted a great deal of effort, from the unique management style and structure have been 1940s through the 1960s, to developing technologies credited for its sustained growth through innovation to fabricate useful objects from the three forms of and creativity. Over the past ﬁve decades, the com- PTFE: granular, ﬁne powder, and dispersion. Over pany has brought the technology of Gore-Tex into a time, through the 1980s, a variety of TFE copolymers myriad of applications. The end-use products utiliz- were developed that could be processed by melt ing ePTFE have enhanced people’s lives beyond extrusion techniques and solution processing [5]. imagination. There is some controversy over which company or individual ﬁrst invented the concept of Part II: Invention of Expanded PTFE expansion. Regardless of the answer to this Polytetraﬂuoroethylene question, however, it is W.L. Gore that continues to propel ePTFE to new frontiers. In 2008, W.L. Gore and Associates celebrated the 50th anniversary of the founding of their company. Over time, competitors began to produce ePTFE Ironically, 2008 also marked the 70th anniversary of membranes and products containing them. This book Roy Plunkett’s discovery of PTFE at DuPont. W.L. presents the contributions of many of these com- Gore is where expanded polytetraﬂuoroethylene panies and explores the valuable role that W.L. Gore (ePTFE) was discovered and perfected over the and its competitors played in the evolution of ePTFE years. The discovery stories of PTFE and ePTFE are technology. separate yet intertwined, similar though unique. 1.5 Early History of W.L. Gore and Both discoveries were the result of the technical Associates brilliance, creativity, curiosity, perseverance, and business savvy of their respective masterminds. Wilbert (“Bill”) L. Gore was born in 1912 in Foremost among the common qualities is curiosity, Meridian, Idaho. He spent most of his formative because these scientists were curious enough to years in Salt Lake City, Utah. Bill studied chemistry recognize an anomaly and pursue with vigor what and engineering and received both a bachelor of many might have considered a setback or an odd science degree in chemical engineering, in 1933, and effect. Both PTFE and ePTFE are still used in the a Master of Science in chemistry, in 1935, from the development of innumerable new products that have University of Utah in Salt Lake City. Bill was a quiet made vital contributions to humankind for more than

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1: HISTORY OF POLYTETRAFLUOROETHYLENE AND EXPANDED PTFE MEMBRANE 5 and modest man with a passion for innovation and Figure 1.4 First W.L. Gore product [7]. tinkering. In 1935, he married Genevieve Walton, Courtesy: Motion Design System. Bill and Bob Gore, article who also became Bill’s lifelong business partner. on the 50th anniversary of W.L. Gore, Penton Media, Inc., Both Bill and Vieve developed a great love for the 2008. outdoors that they bequeathed to their ﬁve children. That so many of the Gore-Tex apparel fabrics she supported the ﬂedgling company in many ways. enhance the outdoor experience for sportsmen, Vieve maintained her active role in the company hunters, and others is not surprising. until the end of her life. When she died in 2005, the annual sales of W.L. Gore and Associates In 1941, Bill Gore was employed by DuPont, approached 2 billion dollars. where he was assigned to working on advancing the company’s research into polymers, resins, and plas- W.L. Gore’s ﬁrst commercially viable were wire tics. During World War II, when PTFE was placed and cable insulated with PTFE. Bill Gore’s eldest under a secrecy order, DuPont was prevented from son, Bob, played an important role in these in- developing commercial PTFE products. When the novations. Bob, who was at the time a chemical secrecy order was lifted in 1946, opening the way for engineering student at the University of Delaware, is commercialization of PTFE, Bill Gore worked for the credited with coming up with the concept that next 12 years on the development of new applications resulted in Gore’s ﬁrst patent [6a] for PTFE-insulated for PTFE. A new plant was built in Parkersburg, wire and cable [6b]. Virginia, to produce the new polymer, and a ﬂurry of research and development work got underway at W.L. Gore’s ﬁrst order was from the city of Den- DuPont to ﬁnd applications for Teﬂon® PTFE. The ver, Colorado, for 7.5 miles of insulated ribbon cable focus of this work ranged from solving fundamental (Fig. 1.4). For the company’s ﬁrst 10 years, these problems with polymerization and ﬁnishing of tet- products comprised the core of the W.L. Gore’s sales. raﬂuoroethylene polymers to ﬁnding end uses and Multi-Tet cables, as they were called, were recog- markets for the product. nized for high performance in the defense industry and in the nascent ﬁeld of computers. The cables Bill Gore’s interests were focused primarily on were even used in the Apollo space program for the ﬁnding new uses for Teﬂon®. The fundamental ﬁrst moon landing. properties of PTFE rendered the material useful for many applications. No other material possesses all 1.6 Discovery of Expanded the properties of PTFE, which include a low dielec- Polytetraﬂuoroethylene tric constant (it is a good electrical insulator); high thermal resistance; a low coefﬁcient of friction; low By the late 1960s, W.L. Gore was a successful ﬂammability; resistance to UV light, hydrophobicity, wire and cable supplier. Bob Gore, who had earned a and oleophobicity; and chemical inertness. Only bachelor’s degree from the University of Delaware in imagination could expand the breadth of new appli- 1959 and a master’s and a PhD from the University of cations for this special plastic. Bill Gore was a man Minnesota (all in chemical engineering), joined the who possessed such imagination. company’s board of directors in 1961 and began DuPont in the 1950s was a basic materials sup- plier and did not produce many fabricated products; it did not go down the value chain, as they say. The company, which was over 150 years old at this point, had a well-entrenched culture. Corporate environ- ments were (are) hardly fertile ground for non- conformers, mavericks, or those who, like Bill Gore, were possessed of an entrepreneurial spirit and driven to innovate. Bill Gore felt the need to leave the large corporate environment to pursue his in- terests, and so in 1958 he leave DuPont to establish his own business. The suggestion that he leave DuPont has been attributed to his wife Vieve. In addition to being the mother of their ﬁve children,

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6 EXPANDED PTFE APPLICATIONS HANDBOOK working at the company full time in 1963. As Gore ascertained that the ePTFE (trademarked competition grew and other companies began to Gore-Tex) was both “very porous and very strong.” produce similar cables, Bill Gore looked for ways to This discovery, of the conditions under which it reduce costs and to develop new products. would stretch to this degree this quickly, set the stage for the creation of hundreds of products and funda- Bill thought they might cut costs, and perhaps mentally altered the trajectory of the manufacturing create a new form of PTFE, if they could ﬁnd a way efforts of W.L. Gore and Associates. “I guess that to stretch the PTFE insulation [8]. His idea was to would be my biggest discovery, the basic Gore-Tex® introduce air into the polymer structure and basically material,” Bob Gore once noted. create a foam form of PTFE. The cost of the cables would be reduced because they would use less The expanded form possesses the basic properties polymer for insulation. Because PTFE is a thermo- of PTFEdincluding chemical inertness, low friction plastic but not melt processible, stretching it is constant, wide-use temperature range, hydrophobicity, difﬁcult. Bob placed rods of PTFE in an oven and outdoor durability, and biocompatibilitydin addition attempted to stretch the heated rods by hand. But the to porosity, air permeability, and extreme strength. rods broke regardless of the temperature Bob used or Given this range of properties, the potential applica- the rate at which he stretched them. While they knew tions of the expanded form were limitless. ePTFE is that PTFE stretches when it is elongated at very slow found in thousands of medical, industrial, and fabric rates (<5 cm/min), these rates are not commercially products, as well as in electronic products [9]. practical; so Bob needed to ﬁnd a method for stretching it more quickly. Bob Gore and his family originally lived in the Rockies, where they used to “hike and go back- The story goes that, late one night in 1969, Bob packing for several weeks, carrying everything on became frustrated because of his inability to stretch [their] backs.” Given his family’s love for the the PTFE rods. As he later explained, “We were outdoors, Bob Gore ﬁnds the use of ePTFE in out- having really bad luck with that so I started to doors garments and adventure gear personally experiment with it at high temperatures. The more rewarding. carefully I tried to stretch the material, the more easily it broke. That seemed counterintuitive to me. Moreover, ePTFE has facilitated new technologies One evening, I took a piece that had been treated at and treatments in the ﬁeld of medicine because of its high temperature and gave it a fast yank (Fig. 1.5), biocompatibility. Millions of people have received and was surprised to ﬁnd that it stretched 1000 ePTFE medical implants, which are conﬁgured to percent, rather than the 10 to 20 percent we had been exclude or accept tissue in-growth depending on the seeing” [9]. needs of the speciﬁc application. Biocompatible ePTFE is used in vascular grafts, cardiovascular and Figure 1.5 Bob Gore’s depiction of Gore-Tex discov- soft tissue patches, facial implants, surgical sutures, ery [7]. and endovascular prostheses. Courtesy Motion System Design. ePTFE was ﬁrst used as a joint sealant, and since then the number of its applications in the industrial arena has grown rapidly. W.L. Gore still produces sealants and the world’s tightest, most chemically resistant gaskets. The ePTFE membrane is the key to ﬁltration products for a range of particle sizes, from pollutants found in the energy, mineral, metal, and chemical industries to clean room and computer disk drive micro contaminants [9]. The original W.L. Gore product line, which constituted insulated wires and cables, beneﬁted from the discovery of ePTFE. ePTFE combines the chemical, thermal, and ﬂammability characteristics of PTFE with the electrical properties of air. It has greater thermal stability, lower loss tangent, higher velocity of propagation, more ﬂexibility, and a lower dielectric constant than solid PTFE.

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1: HISTORY OF POLYTETRAFLUOROETHYLENE AND EXPANDED PTFE MEMBRANE 7 ePTFE has been also used in printed circuit (b) R.J. Plunkett, US Patent 2,230,654, Assigned boards, electromagnetic interference shielding ma- to DuPont Co, Februaury 4, 1941. terial, and ﬁber optic assemblies. It has diverse ap- [2] R.J. Plunkett, The history of polytetraﬂuoro- plications in the defense industry, industrial ethylene: discovery and development, in: automation, computers, telecommunications, and R.B. Seymour, G.S. Kirshenbaum (Eds.), High medical technologies. Performance Polymers: Their Origin and Development, Proceedings of the Symposium on Today, with more than 2 billion dollars in sales, the History of High Performance Polymers at the the company operates over 45 facilities throughout ACS Meeting Held in New York, April 1986, the world and employs thousands of associates. Elsevier, New York, 1987. [3] A. Kinnane (Ed.), DuPont: From the Banks of the Bob Gore has stated that two fundamental core Brandywine to Miracles of Science, Johns principles have underpinned the company’s growth. Hopkins, Baltimore, MD, 2002. The ﬁrst principle is the pursuit of product develop- [4] (a) R.J. Plunkett, in: Speech at the American ment through leadership in ﬂuoropolymers, and Chemical Society Meeting, New York, April particularly ePTFE. The second principle is a 15e18, 1986. commitment to creating a unique, fulﬁlling work (b) S. Ebnesajjad, Fluoroplastics, Volume 1: environment. Bob’s parents initiated and articulated Non-melt Processible Fluoropolymers, second this commitment in the early days of the company. ed., Elsevier, New York, 2014. [5] S. Ebnesajjad, Fluoroplastics, Volume 2: Melt Numerous organizations have recognized Bob Processible Fluoropolymers, second ed., Gore’s accomplishments. He was awarded the 2005 Elsevier, New York, 2016. Perkin Medal by the Society of Chemical Industry, [6] (a) US Patent 3,082,292, Assigned to Robert W. was elected to the National Academy of Engineers, Gore, September 22, 1964. and received the Society of Plastics Engineers award (b) www.fundinguniverse.com/company- for beneﬁts to society through the use of plastics as histories/WL-Gore-amp;-Associates-Inc- well as an award for lifetime achievement in ﬂuo- Company-History.html. ropolymers from DuPont and the WinthropeSears [7] Bill, Bob Gore, Motion System Design Award from the Chemical Heritage Foundation. In Magazine, Penton Media, Inc, 2008. 2006, he was inducted into the National Inventors [8] C.C. Manz, H.P. Sims, Business Without Bosses: Hall of Fame. How Self-managing Teams Are Building High, John Wiley & Sons, New York, 1993. W.L. Gore’s serious commitment to research and [9] University of Delaware, Alumni News, development has resulted in a continually broadening 2008e2009, May 2, 2015. www.che.udel.edu/ range of products. The company’s unique corporate downloads/2009ChEgNewsletter.pdf. culture, which they refer to as a “ﬂat lattice” struc- [10] http://dedo.delaware.gov/information/ ture, stresses freedom, fairness, commitment, and databook/technology.pdf. good judgment in an open and creative work envi- [11] The Culture of W.L. Gore & Associates. www. ronment. Associates have no titles, communicate gore.com/en_xx/aboutus/culture/index.html. directly with one another, and work closely together [12] J.P. Riederer, M. Baier, G. Graefe, Innovation in teams and task forces. Gore associates believe this managementdan overview and some best unique culture enables the company to respond practices, C-LAB Rep. 4 (3) (2005) 9. quickly to changing market developments and that it [13] G. Hamel, B. Breen, The Future of Management, has been a key element in the company’s success and Harvard Business School Press, Boston, MA, growth [11e13]. 2007. References [1] (a) S. Ebnesejjad, Introduction to Fluoropol- ymers: Materials, Technology, and Applications, Elsevier, New York, 2013 ch. 3.

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2 Polytetraﬂuoroethylene: Properties and Structure OUTLINE 2.1 Introduction 9 2.7 Molecular Interaction of Polytetraﬂuoroethylene: 2.2 Impact of F and CeF Bonds on the 9 Low Friction and Low Surface Energy 16 Properties of Polytetraﬂuoroethylene 2.8 Conformations and Transitions of 18 2.3 Crystalline Structure of Polytetraﬂuoroethylene 12 Polytetraﬂuoroethylene 19 2.8.1 Images of the Polytetraﬂuoroethylene 2.4 Branched Tetraﬂuoroethylene Chains: 12 Molecule 20 Perﬂuorinated EthyleneePropylene 13 22 Copolymer 2.9 Microstructure and Fracture 2.4.1 Perﬂuorinated EthyleneePropylene of Polytetraﬂuoroethylene Copolymer References 2.5 Reaction Mechanisms 14 2.6 Impact of Solvents on Fluoropolymers 15 2.1 Introduction Understanding the role ﬂuorine plays in altering the properties of a polymer will result in a more in- The main ingredient of an overwhelming majority depth appreciation of, and deeper insight into, the of expanded polytetraﬂuoroethylene (ePTFE) pro- characteristics of ﬂuorinated polymers. duced in the world, as the name indicates, is poly- tetraﬂuoroethylene (PTFE) resin. This chapter 2.2 Impact of F and CeF Bonds examines the important properties of PTFE, on the Properties of including the extreme properties exhibited by PTFE, Polytetraﬂuoroethylene and focuses on the signiﬁcant impact of replacing hydrogen with ﬂuorine in hydrocarbon macromole- Fluorine is a highly reactive element with the cules. This substitution enhances a number of PTFE’s highest electronegativity of all the elements (4 properties, including thermal stability, chemical Pauling on a relative scale of 0.7e4) [1]. The change resistance, electrical characteristics, and the coefﬁ- in the properties of compounds where ﬂuorine has cient of friction. replaced hydrogen can be attributed to the differ- ences between CeF and CeH bonds. Another critical area this chapter considers is the mechanical behavior of PTFE under various A simple way to frame the issue is to explore the stress/strain conditions. This is important because differences between linear polyethylene (PE) and the unique mechanical response of PTFE at high PTFE. The two chemical structures appear to be strain rates is foundational for ePTFE products. Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00002-X 9 Copyright © 2017 Elsevier Inc. All rights reserved.

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10 EXPANDED PTFE APPLICATIONS HANDBOOK similar on paper, yet in PTFE replacing H with F the CeH bond, and the CeF bond is more highly results in the distortion of the geometry of PE: polarized (see Fig. 2.1). In other words, ﬂuorine has Polyethylene a higher electron density it pulls the shared pair of HHHH CCCC electrons closer to itself relative to the center point HHHH of the CeF bond. Conversely, in the CeH bond the Polytetrafluoroethylene electron pair is closer to carbon, which has a higher FFFF electron density. CC C C FFFF The difference in polarity of CeH and CeF bonds affects the relative stability of the its conformations Let us compare the CeF and CeH bonds. Table 2.1 of the two polymer (PTFE and PE) chains. Crystal- [1,5] summarizes the key differences in the electronic properties and sizes of F and H. In comparing ﬂuorine lization of PE takes place in a planar and trans and hydrogen, several relevant differences are noted: conformation. 1. Fluorine is the most electronegative of all The crystal structure of PTFE, e(CF2)ne, is un- elements usual because it has a number of crystal forms 2. Fluorine has unshared electron pairs (Fig. 2.2) and because there is substantial molecular 3. F is more easily converted to its ionic FÀ motion within the crystal well below its melting 4. The CeF bond is stronger than the CeH bond 5. Fluorine is larger than hydrogen point. PTFE can only be forced into a planar conformation (form or phase III) at extremely high The electronegativity of carbon (2.5 Paulings) is pressures [6]. In contrast, at below 19C, PTFE somewhat higher than that of hydrogen (2.1 Paul- crystallizes as an incommensurate helix with ings) and signiﬁcantly lower than that of ﬂuorine (4 approximately 0.169 nm per repeat distance [30], Paulings). These electronegativity values imply that the polarity of the CeF bond is opposite to that of thus requiring 13 carbon atoms for a 180-degree turn to be completed. At above 19C, the repeat distance increases to 0.195 nm, which means that 15 carbon atoms will be required for a 180-degree turn to be completed [7]. At temperatures above 19C the chains are capable of angular displacement, and this angular displacement increases at temperatures above 30C until the melting point is reached (342C). Substitution of F for H in the CeH bond increases the bond strength from 99.5 kcal/mol for the CeH bond to 116 kcal/mol for the CeF bond, which is substantial. Consequently, PTFE’s thermal stability and chemical resistance are much higher than those Table 2.1 Electronic Properties of Hydrogen and Halogens [1,5] Element Electronic Electronegativity Ionization Electron CeX Bond CeX Bond (Preferred Conﬁguration (Pauling) Energy in Length in Ionic Energy Afﬁnity Form) 1s1 CX4 CX4 (A˚ ) (kcal/g atom) (kcal/g atom) (kcal/mol) 1.091 H 1s1 XD D ee > X X D ee > Xe (Hþ) 2s22p5 1.317 2.1 315.0 17.8 99.5 F 1s1 1.766 (F) 2s22p5 4.0 403.3 83.5 116 3s23p53d0 Cl 3.0 300.3 87.3 78 (Cl) X ¼ H, F or Cl.

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2: POLYTETRAFLUOROETHYLENE: PROPERTIES AND STRUCTURE 11 Center Point of the C—H Bond Most of the properties of PE and PTFE differ signiﬁcantly. The following four properties in H+α ↓ H+α particular are vastly altered in PTFE: Shared Pair C-2α 1. PTFE has one of the lowest surface energies of Electrons Shared Pair among the organic polymers of Electrons 2. PTFE is the most chemically resistant organic Center Point of the C—F Bond polymer ↓ 3. PTFE is one of the most thermally stable among the organic polymers F-δ C+2δ F-δ 4. PTFE’s melting point and speciﬁc gravity are Shared Pair Shared Pair more than double those of PE of Electrons of Electrons Table 2.2 lists the properties of PTFE and PE. Commercial PE melts at 100e140C, depending δα on the extent of branching, as compared to PTFE, which melts at 327C (ﬁrst melting point 342C). Figure 2.1 The comparative polarization of CeH and One could expect that weak intermolecular forces CeF bonds. in PTFE should result in a lower melting point, or at most in a somewhat higher melting point because Pressure (GPa) 0.6 III of the extremely high molecular weight of PTFE. 0.4 II On the contrary, however, PTFE’s melting point 0.2 IV is signiﬁcantly higher than that of PE. Why? I The nature of the intermolecular forces in PTFE, which are responsible for its high melting point, is not 280 300 320 340 360 380 fully understood. The answer may lie in the differ- Temperature (K) ences between the molecular structure conformation and the crystalline structure of PE and PTFE. Because Figure 2.2 Phase diagram of polytetraﬂuoroethylene ﬂuorine atoms are much larger than hydrogen atoms, [31]. there is less chain mobility in PTFE than in PE. Steric repulsion, due to the size of the ﬂuorine atoms, pre- of PE because more energy is required to break the vents the PTFE from forming a PE-like planar zigzag CeF bond. Additionally, the size of the F atom and conformation. Instead, its conformation is helical the length of the CeF bond (Table 2.1) are such that and steric repulsion is minimized. the carbon backbone of PTFE is blanketed with PTFE is insoluble in common solvents. The ﬂuorine atoms, thus rendering the CeF bond replacement of H with the highly electronegative F impervious to solvent attack. The polarity and renders PTFE immiscible with protonated material. strength of the CeF bond rule out an F atom Conversely, PE can be plasticized and dissolved abstraction mechanism for formation of chain above its melting point much more easily than branches in PTFE. Instead, fully and partially ﬂuo- PTFE. PTFE absorbs only small amounts of per- rinated comonomers with pendent groups are poly- halogenated solvents such as perchloroethylene and merized with tetraﬂuoroethylene (TFE) to produce carbon tetrachloride. The insolubility of PTFE in copolymers. solvents is one of its most important characteristics in many applications, such as in lined pipe and In contrast, highly branched PE (>8 branches per other lined equipment for processing corrosive 100 carbon atoms) can be synthesized with relative chemicals. ease [8]. The branching mechanism is a tool used to In summary, the characteristics of F and CeF reduce the crystallinity of PE to produce polymers bonds give rise to the high melting point, low solu- with differing properties. bility, high thermal stability, low friction, and low surface energy of PTFE.

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12 EXPANDED PTFE APPLICATIONS HANDBOOK Table 2.2 A Comparison of Polytetraﬂuoroethylene transition at 19C between phases II and IV is (PTFE) and Polyethylene Properties [2e4] unraveling in the helical conformation from a well-ordered triclinic structure with 13 atoms/180 Property PTFE Polyethylene degrees turn to a partially ordered hexagonal phase with 15 atoms/180 degrees turn [1,8,12,30,34e36]. Density 2.2e2.3 0.92e1 Further rotational disordering and untwisting Melting 342 (ﬁrst) 327 105e140 of the helices occur above 30C, giving way to phase I to form a pseudohexagonal structure with temperature (C) (second) dynamic conformational disorder and long-range positional and orientational order [1,2]. There also Dielectric 2.0 2.3 exists a fourth phase (III) at high pressure, as seen constant (1 kHz) in Fig. 2.2 [37]. Dynamic 0.04 0.33 Fig. 2.3 shows PTFE crystallites, which appear as coefﬁcient of striations throughout the micrograph. The extent of friction crystallization, the size of the crystallites, and the packing order increase with the decrease in the Surface energy 18 33 cooling rate from the molten state. PTFE molecules (dynes/g) crystallize in an accordion style in which the chain folds back and forth on itself. The uniformity of the Resistance to Excellent, no Susceptible to width of the crystals indicates the regularity of the solvents and structure of PTFE molecules. chemicals known solvent hot The crystal model, in which the chain folding is hydrocarbons regular and sharp with a uniform fold period is called adjacent reentry model (Fig. 2.4). The chains Thermal Stabilitya 505 404 reenter through the adjacent neighbor, with only a T1/2 (C) 0.000002 0.008 few exceptions due to multiple nucleation and 264 chain-end defects. This is a very idealized visuali- K350 (%/min) 339 zation of the chain-folding process and not appli- 1010e1012 e cable to the majority of polymers. There are sharp Eact (kJ/mol) boundaries between the crystal and the amorphous phases. Melt creep viscosityb (Poise) 2.4 Branched Tetraﬂuoroethylene Chains: Perﬂuorinated Refractive index 1.35 1.51 EthyleneePropylene Copolymer Chain branching No Yes TFE polymerization allows an overwhelming propensity majority of the chains to crystallize, despite their very large molecular weight. This high degree of a T1/2 is the temperature at which 50% of the polymer is lost after crystallization is important to the development heating in a vacuum for 30 min; K350 is the rate of volatilization, ie, of properties such as high modulus, low coefﬁ- weight loss, at 350C; Eact is the activation energy of thermal cient of friction, and high heat-deﬂection tem- degradation. perature. Crystallinity of virgin PTFE (never b Melt creep viscosity for PTFE at 380C. Please see S. Ebnesajjad, melted) is in the range of 92e98% [9], which is consistent with an unbranched chain structure. Fluoroplastics, Vol. 1: Non-Melt Processible Fluoroplastics, second Properties of PTFE are altered by the inducement of branching or substitution of a different atom ed., Plastics Design Library, Elsevier, Oxford, UK, 2014, for the for ﬂuorine. An example is described in this section. deﬁnition and procedure to measure melt creep viscosity, which is speciﬁc to PTFE. 2.3 Crystalline Structure of Polytetraﬂuoroethylene Bunn and Howells ﬁrst reported the crystalline structure of PTFE in 1954 [32]. As Fig. 2.2 illus- trates, PTFE has two atmospheric pressure crystal- line transitions, at 19C [32] and at 30C [33]. Substantial molecular motion within the crystal is observed well below the melting point of 327C in once-melted PTFE and of 342C in the as- polymerized PTFE. PTFE has a and g glass-like transitions at À80C and 126C [34]. The ﬁrst-order

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2: POLYTETRAFLUOROETHYLENE: PROPERTIES AND STRUCTURE 13 Figure 2.3 Crystalline structure of polytetraﬂuoroethylene cooled down from 355C to 200C at 4.6C per hour (image formed by scanning electron microscopy) [38]. Figure 2.4 Schematic diagram of three chain-folding model in polymer crystals: (A) adjacent reentry with sharp folds; (B) adjacent reentry with loose folds; and (C) random reentry or switchboard model [39]. 2.4.1 Perﬂuorinated contains a tertiary carbon at the branch point bonded EthyleneePropylene Copolymer to a pendent CF3. This carbon should have less thermal stability than primary carbons and, to a lesser Perﬂuorinated ethyleneepropylene copolymer extent, than secondary carbons that constitute the rest (FEP), a copolymer of TEF and hexaﬂuoropropylene, of the backbone of the polymer chain. This decreased

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14 EXPANDED PTFE APPLICATIONS HANDBOOK Table 2.3 A Comparison of the Properties of FEP and PTFE [10] Figure 2.5 Comparison of thermal degradation of Property FEP PTFE perﬂuorinated ethyleneepropylene copolymer (FEP) 265 327 and polytetraﬂuoroethylene (PTFE) by thermogravi- Melting point (C) 360 400 metric analysis [10]. 380 465 Processing temperature (C) 200 260 104e105 1011e1012 Thermogravimetric 40e50 analysis loss 92e98 temperature of 1%/h (C) Upper continuous use temperature (C) MV (380C) (Poise) Crystallinity of virgin polymer (% wt) stability is due to a steric effect in which the chain FEP, perﬂuorinated ethyleneepropylene copolymer; PTFE, polytetraﬂuoroethylene. departs from a helix at the branch point. Fig. 2.5 2.5 Reaction Mechanisms shows the results of thermogravimetric analysis of Perﬂuorooleﬁns such as PTFE are generally, in PTFE and FEP after 1 h of heating in the air. The spite of broad chemical resistance, more vulnerable to attack by nucleophiles than electrophiles, which is lines in Fig. 2.5 start at a degradation rate of 0.02% the opposite of the case of hydrocarbon oleﬁns. weight loss/hour at 300C for FEP and 0.03% weight Nucleophilic attacks occur on the ﬂuorooleﬁns by the loss/hour at 425C for PTFE. scheme proposed in Fig. 2.6. The nucleophile (Nuc) approaches the carbon side of the double bond (I) FEP FFFFFF searching for a positive charge, which leads to the CC C C C C formation of a carbon ion (II). For example, if the nucleophilic compound was methoxy sodium, the FFF FF CH3eOe side of the molecule would be approaching TFE. The carbon ion (II) is unstable and will give off FCF a FÀ ion and generate reaction products. The nature of the reaction medium determines which product is F generated. In the example of methoxy sodium, in the absence of a proton donor such as water, FÀ would Table 2.3 provides a comparison of the properties combine with Naþ to produce NaF and per- of FEP and PTFE. Melting point, processing tem- ﬂuorovinyl methyl ether (III). perature, degradation temperature, and upper continuous use temperature are all signiﬁcantly lower Reactions of TFE oligomers and nucleophiles for FEP. The most important of these properties is the have been reported, such as the pentamer (1) of TFE use temperature. The reason for lower thermal sta- with alkoxide nucleophiles (see Fig. 2.7), sulfur- bility in FEP lies in the greater susceptibility of the containing nucleophiles and amines. The presence tertiary carbon bonded to the pendent per- of a mobile double bond in the pentamer molecule ﬂuoromethyl group to oxidation. FEP has about half renders it susceptible to attack by nucleophiles. It the crystallinity of PTFE, even though its molecular can either replace a ﬂuorine atom at a vinyl position weight is an order of magnitude lower. CF3 side or attack the double bond, causing rearrangement chains disrupt the crystallization sufﬁciently to towards a terminal position. When the pentamer reduce the crystalline content. The melt viscosity of was reacted with alkoxide nucleophiles such as FEP is almost 100 million times lower than that of PTFE, which places it among the melt-processible thermoplastic polymers.

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2: POLYTETRAFLUOROETHYLENE: PROPERTIES AND STRUCTURE 15 Nucleophile + CF2—CF2 → [ Nuc ]δ+ [CF2—CF2]δ- TFE I [ Nuc]δ+ [CF2—CF2]δ- → Nuc+—CF2—CF2- → F- → Reaction Products II EXAMPLE: Nuc = CH3—O—Na [No proton donor like water is present] CH3—O—Na + CF2—CF2 → NaF + CH3—O—CF—CF2 III Figure 2.6 Proposed reaction scheme for nucleophilic attack on ﬂuorooleﬁns [2]. CF3 CF3 r.t. CF3 CF3 PTFE changes from white to brown and then to RF C C OR + RFCH2COOR black. The black layer is normally comprised of RF C C F NaOR carbon, some oxygen, and small amounts of other 1 F113 34 elements. NaOR RF = C(C2F5)2CF3 -30 - -40 °C F113 Et3r.tN. 2.6 Impact of Solvents on Fluoropolymers CF2 CF3 R = (a) CH2CH CH2 = (b) C2H5 Earlier in this chapter, the structure of PTFE was RF C C F = (c) CH3 likened to a carbon rod completely blanketed with ﬂuorine atoms, which render the CeF bond imper- OR N CH3 vious to solvent attack. Testing the effects of nearly 2 all solvents on this polymer has proved this postulate. r. t. = Room temperature There are no known solvents for PTFE below its Et3N: melting point. PTFE is attacked only by molten alkali metals, chlorine triﬂuoride, and gaseous ﬂuorine. H3C Attacks by alkali metals result in deﬂuorination and surface oxidation of PTFE parts, which is a conve- H3C nient route to render them adherable. Figure 2.7 Reaction of the pentamer of tetraﬂuoro- Small molecules can penetrate the structure of ethylene with alkoxide nucleophiles [41]. ﬂuoropolymers. Tables 2.4 and 2.5 provide a sum- mary of room-temperature sorption of hydrogen- allylic alcohol, methanol, and ethanol at low tem- containing and nonhydrogenated solvents into ﬁlms peratures (À30C to À40C), kinetically controlled of PTFE and FEP. Table 2.4 describes the charac- products (2) were obtained as the main products. At teristics of the ﬁlms used in these experiments. Most room temperature, however, the main products were thermodynamically controlled (3) and accompanied Table 2.4 Characteristics of Films in Sorption by small amounts of degradation products (4) Studies [11] [40,41]. Thickness (mm) PTFE FEP Generally, PTFE is not susceptible to nucleophilic Preparation 50 attack because of the absence of double bonds. It is 50 Melt extruded still susceptible to loss of ﬂuorine by electrophilic Crystallinity (%) attack, particularly under heat and over long periods Cast from 42 of exposure. Alkali metals, which are highly reactive aqueous elements such as cesium, potassium, sodium, and dispersion lithium, are among the most likely candidates for abstraction of ﬂuorine from PTFE by an electrophilic 41 mechanism. Certain other metals, such as magne- sium, can attack PTFE if they are highly activated by FEP, perﬂuorinated ethyleneepropylene copolymer; PTFE, etching or other means. polytetraﬂuoroethylene. Loss of ﬂuorine destabilizes PTFE’s structure. As the ﬂuorine-to-carbon ratio decreases, the color of

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16 EXPANDED PTFE APPLICATIONS HANDBOOK hydrogen-containing solvents are absorbed into Table 2.5 Sorption of Various Compounds by PTFE and FEP at less than 1%. In their case, the Perﬂuorocarbon Polymers at Room Temperature [11] extent of swelling does not depend on the solubility (Continued ) parameter. In contrast, halogenated nonhydrogenated solvents penetrate these polymers as a strong func- Compound Solubility Wt Gain% tion of the solubility parameter. Maximum swelling Parameter FEP (11%) takes place at a solubility parameter of 6, and CCl2]CCl2 (cal/cm3)1/2 it drops to less than 1% swelling at a solubility CS2 PTFE Resin parameter of 10. Br2 9.0 1.9 1.4 Table 2.5 Sorption of Various Compounds by 9.3 0.4 0.2 Perﬂuorocarbon Polymers at Room Temperature [11] 0.7 0.7 10.0 11.5 Solubility Wt Gain% FEP, perﬂuorinated ethyleneepropylene copolymer; PTFE, FEP polytetraﬂuoroethylene; TFE, tetraﬂuoroethylene.a Structure: Parameter (cal/cm3)1/2 PTFE Resin CF2 CF2 Compound 0.8 0.4 CF2 CF CF2 CF2CF2 CF3 0.7 0.5 O Compounds Containing Hydrogen 0.8 0.6 1.2 0.5 Isooctane 6.85 1.1 0.4 b Cyclic dimer of hexaﬂuoropropylene: 0.4 0.3 n-Hexane 7.3 1.5 0.6 F2 F2 F2 0.4 0.3 F2 F2 F2 Diethyl ether 7.4 1.4 1.4 0.5 0.6 n-Octane 7.55 0.8 0.4 0.5 0.2 Cyclohexane 8.2 0.8 0.5 0.4 0.3 Toluene 8.9 10.6 11.0 1,1-Dichloroethane 9.1 11.2 6.1 A useful rule of thumb is that little hydrogen- 10.1 10.4 containing solvent is taken up by perﬂuoropolymers, Benzene 9.15 irrespective of the solubility parameter. The amount 9.1 8.4 will increase as temperatures increase. One way to CHCl3 9.3 6.5 7.2 envision this process is to imagine that the solvent 5.2 3.6 molecules are increasingly energized at higher tem- CH2Cl2 9.7 2.4 1.8 peratures and the polymer structure becomes more 3.4 2.0 open. Both effects lead to more swelling. With 1,2-Dichloroethane 9.8 2.2 1.3 nonhydrogen-containing solvents, swelling decreases when the solubility parameter of the solvent increases. CHBr3 10.5 More swelling occurs at higher temperatures, as with the hydrogen-containing solvents. “The more the Average solvent chemical structure resembles the ﬂuoropol- ymer structure, the greater the swelling,” is the rule of Standard thumb for this group. deviation 2.7 Molecular Interaction of Compounds Without Hydrogen Polytetraﬂuoroethylene: Low FC-75a Friction and Low Surface Energy Perﬂuorokerosene 6.2 Coefﬁcient of friction and surface energy (critical 6.1 surface tension) are very low for ﬂuoropolymers (see Perﬂuorodimethyl- Table 2.6). Both characteristics are essential for cyclohexane 7.6 many applications of these plastics, such as bridge C6F12b 8.6 1,2-Br2 TFE 8.7 SiCl4 CCl4 SnCl4 TiCl4

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2: POLYTETRAFLUOROETHYLENE: PROPERTIES AND STRUCTURE 17 Table 2.6 Coefﬁcient of Friction and Surface Energy of Unﬁlled Fluoropolymers Fluoropolymers Formula Coefﬁcient of Critical Surface Surface Tension Polyethylene eCH2eCH2e Friction Tension [12] [21] (Harmonic- Polyvinylﬂuoride eCHFeCH2 (dyne/cm) Mean Method) Polyvinylideneﬂuoride eCF2eCH2e (Dynamic) 31 Polytriﬂuoroethylene eCF2eCHFe 0.33 28 (dyne/cm) Polytetraﬂuoroethylene eCF2eCF2e 0.3 25 Polyvinylchloride eCHCleCH2e 0.3 22 36.1 Polyvinylidenechloride eCCl2eCH2e 0.3 18 0.04 39 38.4 0.5 40 0.9 33.2 e 22.5 41.9 45.4 expansion bearings (low friction) and nonstick attractive repulsive energies, shown in Eq. (2.3), cookware (low surface energy). This section relates which is known as Lennard-Jones potential [21]. these properties to the intermolecular forces of PTFE and other materials. To help the reader, deﬁnitions of Ea ¼ Ar6 (2.1) the forces are brieﬂy discussed. Er ¼ Br12 (2.2) Over a century ago (in 1879), Johannes Diderik Van der Waals postulated the existence of attractive Et ¼ Ar6 þ Br12 (2.3) intermolecular forces. His framework for the dis- cussion of these forces was a modiﬁed form of the where r is the distance between two molecules, A, B ideal gas law. Other researchers after Van der Waals are constants. have classiﬁed the intermolecular forces into four components: PTFE molecules have little propensity for polari- zation or ionization, which minimizes the nonpolar 1. Dispersion (or nonpolar) force energy, or force, between PTFE molecules and between PTFE and other molecules. Neither are there 2. Dipoleedipole force any permanent dipoles in its structure, which is not the case for polymers such as polychlorotriﬂuoroethylene 3. Dipole-induced-dipole (induction) force and polyvinylﬂuoride, minimizing dipoleedipole energy and force in PTFE. A low polarizability coef- 4. Hydrogen bonding ﬁcient minimizes dipole-induced-dipole energy. The neutral electronic state of PTFE and its symmetric These forces are referred to as Van der Waals geometry rule out hydrogen bonding. Consequently, forces [13e20]. The focus in this section is on short- PTFE is very soft and easily abraded. The molecules range forces between two molecules which are fairly slip by and slide against each other [22]. The absence close to each other. Van der Waals forces can exist of any branches or side chains eliminates any steric between any pair of molecules. A second class of hindrance, which could constrain the slipping of repulsive forces acts in opposition to the Van der PTFE molecules past each other. In PTFE (and ﬂuo- Waals forces. The net result of two forces is the ropolymers in general) relative to engineering poly- actual repulsive force present between two mers, this characteristic gives rise to properties like: molecules. Low coefﬁcient of friction All four forms of attractive energy are propor- tional to 1/r6, therefore allowing the Van der Waals forces to be expressed by Eq. (2.1). Repulsive energy for two neutral molecules that get close to each other is conventionally expressed by Eq. (2.2). The total energy between the two molecules is the sum of the

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18 EXPANDED PTFE APPLICATIONS HANDBOOK Low surface energy assumes a helical conformation to accommodate the High elongation Low tensile strength large atoms of ﬂuorine. High cold ﬂow In 1956, E.S. Clark et al. presented an unusual The electronic balance and neutrality of the PTFE molecule result in: room temperature transition for PTFE that occurs at 19C between forms II and IV, as seen in Fig. 2.2 High chemical resistance Low polarizability [42]. It was interpreted as an untwisting in the helical Low dielectric constant Low dissipation factor conformation of the molecule from a 13/6 confor- High volume and surface resistance mation to a 15/7 conformation. These properties serve as the foundation of the Below 19C, a helix forms with a 13.8-degree applications of this plastic. angle of rotation around each carbonecarbon bond. 2.8 Conformations and Transitions of Polytetraﬂuoroethylene At this angle, repeat units of 13 CF2 are required to complete a 180-degree twist of the helix. At above The special size and electronic relationship of 19C, the number of CF2 groups needed to complete ﬂuorine and carbon atoms set the conformational and a 180-degree twist increases to 15. The crystalline transitional arrangement of PTFE apart from seem- structure of PTFE changes at 19C, which is signif- ingly similar molecules such as PE. Polymerization of TFE produces a linear molecule without branches icant due to its proximity to the ambient temperature: or side groups. Branching would require removal of ﬂuorine from CeF bonds, which does not occur the repeat distance is 1.69 nm and the separation of during the polymerization. The linear chain of PTFE chain axes is 0.562 nm [26]. Above 19C, the repeat does not have a planar zigzag conformation, as is the case with PE. Only under extreme pressure distance increases to 1.95 nm and the separation of (5000 atm) does the chain adopt a zigzag confor- mation [23e25]. Under this pressure the chain chain axes decreases to 0.555 nm. In the phase III (zigzag) crystal state, at a pressure of 12 kbar, density increases to 2.74 g/cm3 and crystal dimensions are a ¼ 0.959 nm, b ¼ 0.505 nm, c ¼ 0.262 nm, and g ¼ 105.5 degrees [26]. The helical conformation of the linear PTFE molecules causes the chains to resemble rod-like cylinders [22] which are rigid and fully extended. The crystallization of PTFE molecules occurs in a banded structure depicted in Fig. 2.8. The length of the bands is in the range of 10e100 mm, while the range of the bandwidth is 0.2e1 mm, depending on the rate of the cooling of the molten polymer [27]. Slowing the cooling rates generates larger crystal bandwidths. There are striations on the bandwidths that correspond to crystalline c.4–9 Å 100 µm Crystalline ‘slice’ 1 µm Crystalline c.5–7 Å 200 Å Disordered region Figure 2.8 Crystalline structure of polytetraﬂuoroethylene [28].

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2: POLYTETRAFLUOROETHYLENE: PROPERTIES AND STRUCTURE 19 Table 2.7 Transitions of Polytetraﬂuoroethylene [9] Temperature (8C) Order Region Affected Type of Disorder 19 First order Crystalline Angular displacement 30 First order Crystalline 90 First order Crystalline Crystal disorder À90 Second order Amorphous Onset of rotational motion Amorphous around CeC bond Amorphous À30 Second order 130 Second order slices, which are produced by the folding over, or not require special interactions between the probe tip stacking, of the crystalline segments. These segments and the surface being analyzed such as conducting are separated by amorphous polymer at the bending current, tunneling current, or magnetic forces. point. The thickness of a crystalline slice is Therefore, AFM investigations of thin ﬁlms and 20e30 nm [28]. crystals of polymers and polymer-related compounds have been conducted successfully [43e45]. PTFE has several ﬁrst- and second-order transition temperatures (Table 2.7) [9]. The actual quantity of AFM studies of PTFE ﬁlm thickness and minor transitions is somewhat dependent on the molecular structure [46e48] have yielded experimental method used. From a practical stand- important results. The image resolution from point, the two ﬁrst-order transitions that occur at these studies, however, was insufﬁcient to clearly 19C and 30C are most important. Fig. 2.2 shows distinguish the individual ﬂuorine atoms from the the phase diagram of PTFE. It can be seen from this PTFE macromolecular chains. A study by the ﬁgure that the only phase that cannot be present at National Aeronautics and Space Administration atmospheric pressure is phase III. Phase III requires (NASA) in 2000 provided the ﬁrst direct obser- elevated pressure under which the polymer molecule vations of individual ﬂuorine atoms, and the ﬁrst assumes a zigzag conformation. measurements of the ﬂuorine-helix and carbon- Below 19C, the crystalline system of PTFE is a Figure 2.9 Atomic resolution image, taken with a nearly perfect triclinic. Above 19C, the unit cell 50-A˚ ﬁeld of view, shows the chain-like structure of changes to a hexagonal conformation. In the range of the polytetraﬂuoroethylene macromolecules with 19e30C, the chain segments become increasing intermolecular spacing of 5.72 A˚ [54]. disorderly; and the preferred crystallographic direc- tion disappears. Between 19C and 30C, there is a large expansion in the speciﬁc volume of PTFE, approaching 1.8% [29], which must be considered in measuring the dimensions of articles made from this plastic. 2.8.1 Images of the Polytetraﬂuoroethylene Molecule There has been an interest in studying the char- acteristics of the unidirectionally oriented PTFE chain. Samples of PTFE transferred to glass surfaces have been studied by atomic force microscopy (AFM). AFM is a powerful scanning probe technique for surface analysis of a variety of materials with nanometer-scale and is a very effective tool for analyzing nonmetallic materials. The technique does

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20 EXPANDED PTFE APPLICATIONS HANDBOOK structures of the PTFE macromolecules are aligned parallel to each other with an intermolecular spacing of 5.72 A˚ (Figs. 2.9 and 2.10), and individual ﬂuorine atoms are clearly observed along these twisted mo- lecular chains with an interatomic spacing of 2.75 A˚ . Furthermore, the ﬁrst direct AFM measurements for the radius of the ﬂuorine helix and of the carbon helix in sub-Angstrom scale are reported as 1.7 and 0.54 A˚ , respectively (Table 2.8). Figure 2.10 Atomic resolution image, taken with a 2.9 Microstructure and Fracture 30-A˚ ﬁeld of view, showing the unique twisted of Polytetraﬂuoroethylene character of the polytetraﬂuoroethylene macromole- cules [54]. PTFE is a semicrystalline polymer used in a large number of challenging mechanical applications helix radii from highly oriented PTFE ﬁlms using where its chemical resistance and broad temperature AFM [49]. resistance are often required. Voids in PTFE structure interact with crystallinity in the microstructure A thin PTFE ﬁlm was mechanically deposited development and failure (fracture) of parts. Whether onto a smooth glass substrate at speciﬁc temperatures it is used in aerospace or in an implanted medical using a friction-transfer technique. Atomic resolution device, understanding the mechanism of PTFE’s images of these ﬁlms show that the chain-like helical fracture failure is quite important. Researchers from Los Alamos National Labora- tory and the US Naval Academy conducted an extensive study of the mechanical properties of PTFE and began publishing the results in 2004. A comprehensive review of past studies and new works Table 2.8 X-ray Diffraction and Atomic Force Microscopy (AFM) Measurements Comparison for Polytetraﬂuoroethylene (PTFE) Molecules [54] PTFE Molecular X-ray [67] AFM Data NASA AFM Data [68] (A˚ ) AFM Data [47] (A˚ ) Conﬁguration Diffraction (A˚ ) Study (A˚ ) 5.80 5.30 5.72 PTFE 5.54 intermolecular 1.43 e e spacing 1.29 2.75 e e 2.60 16.9 11.4 e Bragg spacing 16.8 2.36 e e along chain axis 2.0e2.4 1.70 e e 1.64 0.54 e e Fluorine atomic 0.42 spacing Period length (13- atom chain) CF2 group helix spacing Fluorine-helix radius Carbon-helix radius

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2: POLYTETRAFLUOROETHYLENE: PROPERTIES AND STRUCTURE 21 by Rae, Dattelbaum, Brown, Joyce, and their asso- Brown and Dattelbaum used molded/sintered bil- ciates has shed new light on the behavior and failure lets of Teﬂon® PTFE 7C for machining fracture modes of PTFE under compressive and tensile stress specimens as deﬁned in ASTM E1820. Two sets of [37,50e57]. fracture specimens were machined such that the crack propagation would occur either parallel to, or Brown and Dattelbaum [40] studied the effects of perpendicular to, the compaction direction of PTFE, the crystalline phase on the fracture and micro- as illustrated in Fig. 2.11. Tests were performed structure evolution of PTFE, which is unique because at À75C, À50C, À15C, 15C, 25C, 50C, and of its three ambient pressure crystalline phases near 100C. These test temperatures encompass the three room temperature. The aim of their study was to ambient pressure crystalline phases of PTFE. uncover the effects of temperature-induced phase At 25C the crystalline structure of PTFE consists of transitions on the fracture mechanisms of PTFE. phase IV that converts to phase I at higher tempera- tures (50C and 100C). Brown and Dattelbaum’s study is superior to pre- vious research for a number of reasons. There are a Crack propagation in PTFE was found to be signiﬁcant number of investigations of the chemical strongly phase dependent, with a brittle-to-ductile structure of PTFE, of crystalline phase transitions, transition associated with the room temperature phase and of the percent of crystallinity. Most studies of the transitions. Above 19C, extensive crack tip blunting mechanical behavior of PTFE have either focused on and plastic deformation were observed and crack tip a single temperature [58,59] or overlooked the tran- positions were measured optically. Increases in frac- sitions of the crystalline phase over the temperature ture toughness resulted from the onset of stable ﬁbril range investigated [55,60,61,65,66]. Studies by formation bridging the crack plane and the increased McCrum [62], Vincent [63], and Kisbenyi et al. [64] plastic deformation. The stability of drawing ﬁbrils take phase transitions into account by correlating was primarily determined by temperature and crys- changes of the modulus and loss factor with phase talline phase with additional dependence on loading transitions. They do not, however, consider and rate and microstructure anisotropy. report the characteristics of PTFE. Because PTFE crystals completely melt during sintering and While fracture toughness values associated with recrystallization occurs during cooling, crystallinity the initiation of crack growth have nominal depen- is an important component of fabrication process dence on orientation, crack propagation perpendic- variables. ular to the pressing direction is far less stable than when it is parallel to the pressing direction. This work Due to the nonlinear mechanical behavior of demonstrated that although PTFE has been consid- PTFE, studied by Rae and Brown [51] extensively, ered highly resistant to crack propagation due to its the fracture behavior cannot be captured by linear behavior at room temperature, the onset of brittle elastic fracture mechanics. Hence, a J-integral fracture below room temperature caused by the analysis [65] was performed to measure the temperature-induced phase transition necessitates nonlinear elasticeplastic strain energy fracture consideration of brittle fracture during service at toughness using the single compact tension normal- lower temperatures. ization technique outlined in ASTM E1820. PTFE is heterogeneous because of the mingling of Pressing its crystalline domains in an amorphous matrix. It direction provides a mechanism for the formation of micro- voids in the high-stress region near a crack tip, as PTFE billet Perpendicular (⊥) illustrated in Fig. 2.12. The mechanisms by which Parallel (II) crystalline domains in PTFE orient themselves under uniaxial loading are dependent on the phase. PTFE in Figure 2.11 Compact tension orientation relative to phase II has limited material mobility, and the crys- billet pressing direction [40]. PTFE, talline domains deform and orient out of the principle polytetraﬂuoroethylene. stress direction. Here, fracture either occurs as cleavage (Fig. 2.12A) or microvoid coalescence (Fig. 2.12B) which results in brittle crack growth with a low resistance to fracture. PTFE crystalline domains in phase IV initially deform and orient out of

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22 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 2.12 Schematic of the primary fracture mechanisms observed in polytetraﬂuoroethylene: (A) cleavage, (B) microvoid coalescence, and (C) ductile ﬁbril formation [40]. the principle stress direction but rotate into the or pop-in behaviors, to the smooth ductile-like J-R principle stress direction with additional extension, curve behavior observed here only at the higher and crystalline domains in phase I preferentially loading rates and/or higher test temperatures.” orient into the primary stress direction [66]. The key variable in determining fracture tough- Therefore, PTFE in phases IVor I is able to deform ness and mode is temperature. Other variables, such locally in the vicinity of microvoids to initiate the as orientation, rate, and even adding ﬁllers to PTFE, stable formation of ﬁbrils. Once initiated, the for- have less pronounced impact on the fracture tough- mation of ﬁbrils is an efﬁcient mechanism to dissi- ness of PTFE. The crystalline structure of PTFE pate energy through localized plastic deformation undergoes two transitions in a narrow temperature (Fig. 2.12C). Moreover, as the ﬁbrils are drawn they range at atmospheric pressure, which is the root become oriented and thus stronger and stiffer. As cause of a wide variation of fracture toughness in a the ﬁbrils bridge the crack plane, they slow down the narrow temperature band. crack growth and shield the material ahead of the crack. The irreversible formation of ﬁbrils pro- References vides a signiﬁcant mechanism for plastic deformation of PTFE in phase IV and phase I. Additionally, ﬁbril [1] L. Pauling, The Nature of the Chemical Bond formation is an orientation process and signiﬁcantly and the Structure of Molecules and Crystals: An increases the elastic strength of PTFE. The ability of Introduction to Modern Structural Chemistry, ﬁbrils to bridge the fracture plane retards the rapid third ed., Cornell University Press, Ithaca, NY, crack propagation. 1960. Joyce and Joyce [55e57] reached more or less [2] R.D. Chambers, Fluorine in Organic Chemis- similar conclusions: “Testing this polymer using try, ﬁrst ed., John Wiley and Sons, New York, multi-specimen procedures at standard laboratory 1973. testing rates and ambient temperatures would result in missing most interesting features. Use of the [3] M. Hudlicky, Chemistry of Fluorine Com- normalization procedure allows observation of the pounds, ﬁrst ed., Macmillan, New York, complex transition from creep-crack-growth 1962. behavior, to viscous blunting, through the run/arrest [4] D.W. Van Krevelen, Properties of Polymers: Their Estimation and Correlation with Chemical

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2: POLYTETRAFLUOROETHYLENE: PROPERTIES AND STRUCTURE 23 Structure, second ed., Elsevier, Amsterdam, [24] R.I. Beecroft, C.A. Swenson, Behavior of poly- 1976. tetraﬂuoroethylene (Teﬂon®) under high pres- [5] C.R. Patrick, Advan. Fluorine Chem. 3 (1) sures, J. Appl. Phys. 30 (1959) 1793. (1961). [6] D.C. England, et al., in: Proceedings of Robert A. [25] T.W. Bates, W.H. Stockmayer, Conformational Welch Conference on Chemical Research energies of perﬂuoalkanes. III. Properties of XXVI, R.A. Welch Foundation, 1982, pp. polytetraﬂuoroethylene, Macromolecules 1 193e243. (1968) 17. [7] Clark, L.T. Muus, Z. Krist (1962) 117e119. [8] R.A. Raff, K.W. Doak, Crystalline Oleﬁn Poly- [26] C.A. Sperati, H.W. Starkweather Jr., Adv. mers, Interscience, 1965, pp. 678e680. Polym. Sci. 2 (1961) 465. [9] S.V. Gangal, Polytetraﬂuoroethylene, homo- polymers of tetraﬂuoroethylene, in: Encyclo- [27] C.J. Speerschneider, C.H. Li, Some observations pedia of Polymer Science and Engineering, on the structure of polytetraﬂuoroethylene, second ed. vol. 17, John Wiley, New York, 1989, J. Appl. Phys. 33 (1871) 1962. pp. 577e600. [10] B.B. Baker, D.J. Kasprzak, Thermal degradation [28] K.R. Makinson, D. Tabor, The friction and wear of commercial ﬂuoropolymer in air, Polym. of polytetraﬂuoroethylene, Proc. R. Soc. 281 Degrad. Stab. 42 (1994) 181e188. (1964) 49. [11] H.W. Starkweather Jr., The sorption of chemicals by perﬂuorocarbon polymers, Macromolecules [29] N.G. McCrum, An internal friction study of pol- 10 (5) (1977) 1161e1163. ytetraﬂuoroethylene, J. Polym. Sci. 34 (1959) 355. [12] W.A. Zissman, Inﬂuence of construction on adhesion, Ind. Eng. Chem. (1963) 18e38. [30] E.S. Clark, The molecular conformations of [13] H. Margenau, N. Kestner, Theory of Intermo- polytetraﬂuoroethylene: forms II and IV, Poly- lecular Forces, third ed., Pergamon Press, mer 40 (1999) 4659e4665. London, 1971. [14] J.O. Hirschfelder (Ed.), Intermolecular Forces, [31] R.K. Eby, E.S. Clark, B.L. Farmer, G.J. Piermarini, Interscience, New York, 1967. S. Block, Polymer 31 (1990) 2227. [15] Intermolecular Forces. Discussion, Faraday Society, 1965, p. 40. [32] C.W. Bunn, E.R. Howells, Structures of mole- [16] J.N. Israelachvilli, D. Tabor, Prog. Surf. Mem- cules and crystals of ﬂuorocarbons, Nature 174 ber. Sci. 7 (1) (1973). (4429) (1954) 549e551. [17] J.N. Israelachvilli, Quart. Rev. Biophys. 6 (4) (1974) 341. [33] C.A. Sperati, H.W. Starkweather Jr., Fluorine- [18] J.N. Israelachvilli, in: Yearbook of Science and containing polymers. II. Polytetraﬂuoro- Technology, McGraw-Hill, New York, 1976, ethylene, Adv. Polym. Sci. 2 (1961) 465e495. pp. 23e31. Springer, Berlin. [19] H. Krupp, Adv. Colloid Interface Sci. 1 (1967) 111. [20] J.O. Hirschfelder, C.F. Curtiss, R.B. Bird, Mo- [34] C.A. Sperati, Physical constants of polytetra- lecular Theory of Gases and Liquids, Wiley, New ﬂuoroethylene, in: J. Brandrup, G.H. Immergut York, 1954. (Eds.), Polymer Handbook, second ed., John [21] S. Wu, Polymer Interface and Adhesion, ﬁrst ed., Wiley, New York, 1975, pp. V29eV36. Marcel Dekker, Inc., New York, 1982. [22] T.A. Blanchet, Polytetraﬂuoroethyelne, in: [35] C.W. Bunn, A.J. Cobbold, R.P. Palmer, The ﬁne Handbook of Thermoplastics, ﬁrst ed., Marcel structure of polytetraﬂuoroethylene, J. Polym. Dekker, Inc., New York, 1997. Sci. 28 (117) (1958) 365e376. [23] R.G. Brown, Vibrational spectra of polytetra- ﬂuoroethylene: effects of temperature and pres- [36] J.J. Weeks, I.C. Sanchez, R.K. Eby, C.J. Poser, sure, J. Chem. Phys. 40 (1964) 2900. Order-disorder transitions in polytetraﬂuoro- ethylene, Polymer 21 (3) (1980) 325e331. [37] E.N. Brown, D.M. Dattelbaum, The role of crystalline phase on fracture and microstructure evolution of polytetraﬂuoroethylene (PTFE), Polymer 46 (2005) 3056e3068. [38] A.E. Woodward, Atlas of Polymer Morphology, Hanser Publishers, New York, 1989. [39] R.J. Young, P.A. Lovell, Introduction to Polymers, third ed., CRC Press, Boca Raton, 2011, p. 411. [40] L.F. Chen, J.H. Wang, Acta Chim. Sinica 41 (1983) 375.

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24 EXPANDED PTFE APPLICATIONS HANDBOOK [41] L.F. Chen, Nucleophilic substitution reactions of [53] J.J. Jordan, C.R. Siviour, J.R. Foley, tetraﬂuoroethylene pentamer and tetramer and E.N. Brown, Compressive properties of transformations of the reaction products, extruded polytetraﬂuoroethylene, Polymer 48 J. Fluorine Chem. 67 (1994) 95e101. (2007) 4184e4195. [42] R.H.H. Pierce Jr., E.S. Clark, J.F. Whitney, [54] E.N. Brown, P.J. Rae, C. Liu, Mixed-mode-I/II W.M.D. Bryant, in: Abstract of Papers, 130th fracture of polytetraﬂuoroethylene, Mater. Sci. Meeting of the American Chemical Society, Eng. A 468e470 (2007) 253e258. September 1956, p. 9S. [55] J.A. Joyce, Polym. Eng. Sci. 43 (10) (2003) [43] S.N. Magonov, K. Qvarnstrom, V. Elings, 1702. H. Cantow, Atomic force microscopy on poly- mers and polymer related compounds, Polym. [56] P.J. Joyce, J.A. Joyce, Int. J. Fract. 127 (4) (2004) Bull. 25 (1991) 689. 361. [44] W. Stocker, G. Bar, M. Kunz, M. Moller, [57] J.A. Joyce, P.J. Joyce, Eng. Fract. Mech. 71 Atomic force microscopy on polymers and (16e17) (2004) 2513. polymer related compounds, Polym. Bull. 26 (1991) 215. [58] J.G. Williams, J.M. Hodgkinson, Proc. R. Soc. Lond. A 375 (1761) (1981) 231. [45] G. Meyer, N.M. Amer, Optical-beam-deﬂection atomic force microscopy: the NaCl (001) [59] A. Khan, H.Y. Zhang, Int. J. Plast. 17 (9) (2001) surface, Appl. Phys. Lett. 56 (1990) 2100. 1167. [46] H. Hansma, F. Motamedi, P. Hansma, [60] S. Fischer, N. Brown, J. Appl. Phys. 44 (10) C. Wittmann, Molecular resolution of thin, (1973) 4322. highly oriented poly(tetraﬂuoroethylene) ﬁlms with the atomic force microscopy, J. Polym. 33 [61] F.J. Zerilli, R.W. Armstrong, AIP Conf. Proc. (1992) 647. 620 (1) (2002) 657. [47] S.N. Magonov, S. Kempf, M. Kimmig, [62] N.G. McCrum, J. Polym. Sci. 34 (127) (1959) H. Cantow, Atomic force microscopy on poly- 355. mers and polymer related compounds, Polym. Bull. 26 (1991) 715. [63] P.I. Vincent, Impact strength and mechanical losses in thermoplastics, Polymer 15 (2) (1974) [48] S.N. Magonov, Characterization of polymer 111e116. surfaces with atomic force microscopy, Ann. Rev. Mater. Sci. 27 (1997) 175e222. [64] M. Kisbenyi, M.W. Birch, J.M. Hodgkinson, J.G. Williams, Correlation of impact fracture [49] J.A. Lee, Observation of Individual Fluorine toughness with loss peaks in PTFE, Polymer 20 Atoms from Highly Oriented Poly(tetraﬂuoro- (10) (1979) 1289e1297. ethylene) Films by Atomic Force Microscopy, 2000. NASA Report NASA/TMd2000e [65] J.R. Rice, A path independent integral and the 209962, http://ntrs.nasa.gov/archive/nasa/casi. approximate analysis of strain concentration by ntrs.nasa.gov/20000032164_2000025512.pdf. notches and cracks, J. Appl. Mech. 35 (1968) 379e386. [50] P.J. Rae, D.M. Dattelbaum, The properties of poly(tetraﬂuoroethylene) (PTFE) in compres- [66] S.M. Wecker, T. Davidson, D.W. Baker, sion, Polymer 45 (2004) 7615e7625. Preferred orientation of crystallites in uniaxially deformed polytetraﬂuoroethylene, J. Appl. Phys. [51] P.J. Rae, E.N. Brown, The properties of poly(- 43 (11) (1972) 4344e4348. tetraﬂuoroethylene) (PTFE) in tension, Polymer 46 (2005) 8128e8140. [67] C.W. Bunn, E.R. Howells, Structures of mole- cules and crystals of ﬂuorocarbon, Nature 18 [52] E.N. Brown, P.J. Rae, E.B. Orler, G.T. Gray III, (1954) 549. D.M. Dattelbaum, The effect of crystallinity on the fracture of polytetraﬂuoroethylene (PTFE), [68] P. Dietz, P.K. Hansma, K.J. Ihn, F. Motamedi, Mater. Sci. Eng. C 26 (2006) 1338e1343. Molecular structure and thickness of high ori- ented poly(tetraﬂuoroethylene) ﬁlms measured by atomic force microscopy, J. Mater. Sci. 28 (1992) 1372.

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3 Manufacturing Polytetraﬂuoroethylene by Emulsion Polymerization OUTLINE 3.1 Introduction 25 Tensile Break Strength Test 48 Stretching Rate 48 3.2 Tetraﬂuoroethylene Preparation 26 Stress Relaxation Time 48 Stretch Test 51 3.3 Polymerization of Tetraﬂuoroethylene 29 51 Preparation of Test Specimen 51 3.4 Tetraﬂuoroethylene Polymers 31 Stretch Test 52 3.4.1 Ammonium Perﬂuorooctanoate (Also C8) 32 Measurement of Stress Relaxation Time 55 3.4.2 Alternatives to Ammonium Stretch Procedure 56 Perﬂuorooctanoate 33 Stress Relaxation Time 56 Break Strength 56 3.5 Preparation of Polytetraﬂuoroethylene by 35 Creep Rate Emulsion Polymerization Evaluation of Extrusion Pressure and 57 Stretchability 57 3.6 Emulsion Polymerization of 42 Measurement of Tensile Break Strength 57 Tetraﬂuoroethylene With Ammonium Measurement of the Endothermic Ratio 58 Perﬂuorooctanoate Replacements Measurement of the Stress Relaxation Time 59 3.7 Mechanism of Emulsion Polymerization of 3.9 Fine Powder (Coagulated Dispersion) Products 60 Tetraﬂuoroethylene 44 60 3.10 Characterization of Polytetraﬂuoroethylene 62 3.8 Development of Polytetraﬂuoroethylene for 45 Fine Powder Polytetraﬂuoroethylene Resins Expanded Polytetraﬂuoroethylene Dispersions of Polytetraﬂuoroethylene 62 Applications 47 Stretch Ratio and Ultimate Stretch 47 References Ratio Test 48 Preparation of Test Specimen Stretch Procedure 3.1 Introduction To be suitable for expansion, the polytetraﬂuoro- ethylene (PTFE) must have linear unbranched chains Expanded polytetraﬂuoroethylene (ePTFE) appli- and a high molecular weight, which confers strength cations utilize polymers with high molecular weights to the polymer chains. A phenomenon called ﬁbril- that are made by emulsion polymerization of tetra- lation occurs when particles rub against a surface, ﬂuoroethylene (TFE). In general, the key character- including the surfaces of other particles. Fibrils are istics of the emulsion polymerization regime include groups of molecular chains that have been pulled out presence of ample surfactant and mild agitation at of the surfaces of PTFE particles. These ﬁbrils con- elevated temperatures and pressures. The dispersion nect various PTFE particles, thus creating a structure recovered from the reactor is ﬁnished by two that can be oriented. Orienting the substrate stretches different series of processes, depending on whether the ﬁbrils, the strength of which has a clear impact on an aqueous dispersion or a dry powder (ﬁne powder) PTFE expansion. is the desired ﬁnal product. Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00003-1 25 Copyright © 2017 Elsevier Inc. All rights reserved.

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26 EXPANDED PTFE APPLICATIONS HANDBOOK This chapter reviews methods for manufacturing plasma ﬂame to form a gaseous reaction mixture, high molecular weight PTFE from TFE by emulsion which he then quenched to form TFE. He used the polymerization. TFE manufacturing is discussed turbulent plasma for dissociating a noncarbonaceous brieﬂy, followed by a more in-depth consideration of metal ﬂuoride into a gaseous mixture of metal and polymerization. reactive ﬂuorine in the presence of carbon. This forms a precursor to TFE, which can then be 3.2 Tetraﬂuoroethylene quenched and converted to TFE. Preparation The most widely used method for TFE preparation TFE (CF2]CF2) is the basic building block of is pyrolysis of chlorodiﬂuoromethane (CHClF2), also most commercial perﬂuorinated ethylenic ﬂuoropol- known as HCFC-22. In this procedure, a molecule of ymers, especially PTFE. Publications in the 1890s HCl is removed and the degradation products are report a variety of attempts to synthesize TFE by reacted. Pairs of CF2 free radicals generated by direct reaction of ﬂuorine with carbon, ﬂuorine with dehydrochlorination are combined to yield C2F4 chloromethanes, and tetrachloroethylene with silver molecules. Fully integrated commercial TFE- ﬂuoride [1e4]. Humiston [5] reported the ﬁrst manufacturing operations use ﬂuorspar (CaF2), sulfu- documented preparation of TFE in 1919, which has ric acid, and chloroform as the base ingredients been disputed due to erroneous property data. [13e20]. Fluorspar is the starting point for introducing ﬂuorine into the organic reaction sequence. The con- The ﬁrst reliable and complete description of ventional reaction scheme for synthesizing TFE from TFE synthesis was published in 1933 by Ruff and ﬂuorspar is as follows: Bretachneider [6]. They synthesized TFE by decomposing tetraﬂuoromethane in an electric arc. HF preparation: They separated the TFE by bromination, followed by dehalogenation with zinc, to purify the TFE from the CaF2 þ H2SO4 / 2HF þ CaSO4 (3.1) other products of pyrolysis. Chloroform preparation: (3.2) Researchers have experimented with many CH4 þ 3Cl2 / CHCl3 þ 3HCl different procedures for synthesizing TFE since it Chlorodiﬂuoromethane preparation: was ﬁrst prepared. Farlow [7] obtained TFE by reacting carbon with carbon tetraﬂuoride or hexa- CHCl3 þ 2HF/CHClF2 þ 2HCl (3.3) ﬂuoroethane at 1700e2500C. The resulting product ðSbF3 catalystÞ was quickly quenched and the impurities were recycled. Farlow and Muetterties [8] employed TFE synthesis: another method, reacting elemental ﬂuorine with carbon using a carbon arc. They reported positive 2CHClF242CF2 þ 2HCl (3.4) results in the range of 2500e3500C. Yet another method involves brominating hydrocarbon gases, ðpyrolysisÞ such as methane, then replacing the bromine by hydroﬂuorination, using HF [9]. Vecchio et al. [10] 2CF2 / CF2eCF2 (3.5) used a process for TFE synthesis in which dichloro- (3.6) diﬂuoromethane (also known as R22 or F22) was The overall pyrolysis reaction is: dechlorinated and dimerized. The reaction was car- ried out with an amalgam of alkali or alkaline earth 2CHClF2 4 CF2eCF2 þ 2HCl metal in a reaction medium of one or more organic (an equilibrium reaction) solvents. Another preparation technique that pro- duces high yields of TFE involves contacting penta- A few other side compounds are also produced ﬂuoroethane or triﬂuoromethane with hot gases (at a during pyrolysis including hexaﬂuoropropylene temperature >1730C), followed by rapid quenching (HFP), perﬂuorocyclobutane, octaﬂuoroisobutylene, to a temperature cooler than 540C [11]. 1-chloro-1,1,2,2-tetraﬂuoroethane, 2-chloro-1,1,1,2, 3,3-hexaﬂuoropropane, and a small amount of highly Webster [12] has disclosed that, in the late 1990s, toxic perﬂuoroisobutylene. The type and the amount he prepared TFE by feeding a metal and carbon to a of by-products, also called high boilers, depend on the reaction conditions because both Eqs. (3.4) and (3.5) are equilibrium reactions.

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 27 Downing, Benning, and McHarness [21] disclosed pyrolysis were achieved when an inert gas was pre- the ﬁrst preparation of TFE by pyrolysis of chlor- sent as a diluent during the pyrolysis [18,23]. Typi- odiﬂuoromethane and cooling of the reaction prod- cally, reducing the pyrolysis pressure by 0.5 atm by ucts in 1945. Temperature proved to be a signiﬁcant introducing an inert gas produces a maximum con- variable in the synthesis because of the consecutive version of 50% and yield of 90%. nature of degradation and integration that yields TFE. They reported that a temperature range of Scherer et al. [18] attempted to use Le Chatelier’s 600e1000C was most suitable. Downing et al. principle to decrease the partial pressure of chlor- studied the effect of pressure because of the gaseous odiﬂuoromethane in the pyrolysis reaction pressure nature of the reaction products. Table 3.1 presents the by introducing water vapor into the reaction medium. effect of pyrolysis temperature and pressure on the Signiﬁcant improvement in the conversion of chlor- conversion of chlorodiﬂuoromethane and TFE yield. odiﬂuoromethane and TFE yield was reported. When The data indicate that TFE conversion decreases with 15e70% by mole of water was present in the gaseous an increase in pressure. mixture, Scherer et al. obtained 60e70% conversion and 90e94% TFE yield at 750e900C, which is, The reaction shown in Eq. (3.6) has been found to considering the high cost of chlorodiﬂuoromethane, be a reversible, or equilibrium, reaction [22]. Le signiﬁcant. Chaˆtelier’s (or Le Chaˆtelier-Braun) principle states: if a chemical system at equilibrium experiences a Edwards et al. [22] demonstrated the impact of change in concentration, temperature, volume, or steam on reaction Eq. (3.6) on the conversion of partial pressure, then the equilibrium shifts to coun- chlorodiﬂuoromethane and the yield of TFE at teract the imposed change and a new equilibrium is different residence times. Table 3.2 shows the effect established. This principle predicts the effect of an of residence time on a pyrolysis reaction in which increase in pressure on the overall disintegration/ 3 mol of steam were present for each mole of integration reaction Eq. (3.6). Two moles of chlor- chlorodiﬂuoromethane. The mixture was preheated odiﬂuoromethane are converted into 1 mol of TFE to 400C and then held in a tubular reactor for a brief and 2 mol of HCl. An increase in pressure causes the period of time at 700C. Table 3.3 presents the results reaction to shift to the side with fewer moles of gas. of a control pyrolysis reaction in which no steam was In short, the overall pyrolysis reaction favors for- present. It is clear from the data that far higher yields mation of TFE with a decrease in reaction pressure. of TFE can be achieved in the presence of steam, while conversion of CHClF2 remains near Reducing the partial pressure of chlorodiﬂuoro- constant. methane by adding diluent gas (an inert gas) such as nitrogen or helium has also been attempted to in- The pyrolysis can also be performed in such a way to crease TFE yield. Results similar to subatmospheric produce both TFE and HFP. Typically, low conversions of chlorodiﬂuoromethane at 600e1000C and low Table 3.1 Effect of Pressure and Temperature on the Conversion of Chlorodiﬂuoromethane and Tetraﬂuoroethylene (TFE) Yield [21] Pyrolysis Pressure (kPa) Conversion of TFE Yield (%) Temperature (8C) 103 Chlorodiﬂuoromethane (%) 90 700 379 78 600 379 25 57 740e745 655 37 73 810e820 655 79 68 740 655 100 65 720 655 77 63 205e620 2654 82 60 660 39 60

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28 EXPANDED PTFE APPLICATIONS HANDBOOK Table 3.2 Effect of Steam on Conversion of residence times yield high levels of TFE and a small quantity of HFP. Increasing the chlorodiﬂuoromethane Chlorodiﬂuoromethane (Steam Molar Ratio 3:1) and conversion results in an increase in the high boiler Tetraﬂuoroethylene (TFE) Yield at 700C [22] compounds, which is an undesirable result. Halliwell [24] discovered that, in the range of 86e94% conver- Contact Conversion of TFE Yield (%) sion, the amount of HFP produced approached, and Time (s) CHClF2 (%) 93.1 even exceeded, the amount of TFE. In this range per- 0.17 64.3 93.5 ﬂuorocyclobutane is also formed, along with TFE and 0.17 67.0 90.8 HFP. At 94% chlorodiﬂuoromethane conversion 0.27 69.3 89.4 and above, the perﬂuorocyclobutane level drops to zero 0.28 67.9 90.0 and carbon deposits are formed on the surface of the 0.31 74.0 88.6 pyrolysis tubes. Optimal process conditions include a 0.38 75.0 88.0 temperature range of 700e900C, a pressure range of 0.40 77.5 84.5 0.5e1.2 atm, and under 2 s of contact time while 0.64 80.2 keeping the conversion in the precise range of 86e94%. Tables 3.4 and 3.5 show the effects of these Table 3.3 Conversion of Chlorodiﬂuoromethane and variables on the TFE, HFP, and perﬂuorocyclobutane Tetraﬂuoroethylene (TFE) Yield at 700C [22] yield. Contact Conversion of TFE Yield (%) TFE yields approaching 95% can be achieved at Time (s) CHClF2 (%) 67.5 80% chlorodiﬂuoromethane conversion if the molar 0.25 67.5 65.9 ratio of steam to CHClF2 is in the range of 7:1 to 0.36 61.5 10:1. Fig. 3.1 shows the results of computer-aided 0.48 68.0 47.7 simulation [25]. 0.78 73.9 Chinoy and Sunavala have studied and simulated thermodynamic and kinetics for the manufacture of 78.5 TFE by the pyrolysis of chlorodiﬂuoromethane [25]. They were able to obtain good agreement between kinetic and thermodynamic results by a slight correction of one of the reported expressions for rate constants. The study concluded that operating the Table 3.4 Effects of Temperature, Conversion, and Contact Time on Conversion of Chlorodiﬂuoromethane to Perﬂuorocarbon Yield [24] Pyrolysis Residence Conversion of TFE Yield (%) HFP Yield (%) Perﬂuorocyclo- Temperature Time (s) Chlorodiﬂuoro- 93.1 1.1 butane (%) (8C) 1.8 78.9 3.7 3.2 687 2 methane (%) 63.8 7.5 9.7 797 2 38.4 31.2 31.9 13.8 841 2 69.1 49.7 14.6 6.5 907 2 81.9 46.3 17.5 14.4 806 2 89.0 31.2 36.8 14.3 866 1.8 86.2 12.8 49.4 4.8 809 1.8 93.6 48.5 29.2 3.1 881 0.19 91.9 36.4 36.5 2.7 923 0.19 94.9 3.2 931 92.0 93.0 HFP, hexaﬂuoropropylene; TFE, tetraﬂuoroethylene.

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 29 Table 3.5 Effect of Pressure on Conversion of Chlorodiﬂuoromethane to Perﬂuorocarbon Yield [24] Total Partial Contact Conversion TFE Yield HFP Yield Perﬂuorocyclo- Pressure Pressure, of Time (s) of CHClF2 (%) (%) butane (%) (atm) 28.9 34.7 6.5 CHClF2 3.24 (%) 35.8 38.2 1.5 1 (atm) 3.24 39.5 36.6 2.35 3.24 91.0 41.1 33.5 3.3 1 1 3.24 91.5 0.5 0.66 92.9 1 0.5 91.6 0.5 HFP, hexaﬂuoropropylene; TFE, tetraﬂuoroethylene. 100 R 7.5 Moles steamR = 8.0 = R = Diluent Ratio =1.0 90 Moles CHCI F2 = 80 R = 2.0 R =0 70 R 60 % Conversion R = 0.5 50 40 30 20 10 0 500 550 600 650 700 750 800 850 900 950 1000 Temp (°C) Figure 3.1 Effect of temperature on conversion (R ¼ 0, 0.5, 1.0, 2.0, 7.5, and 8.0) [25]. pyrolysis reaction at above 850e900C is helpful for 3.3 Polymerization three reasons. First, chlorodiﬂuoromethane conver- of Tetraﬂuoroethylene sion is maximized (Fig. 3.1). Second, the HFP con- tent of the product is minimized both in the absence TFE is polymerized in water in the presence of and the presence of a diluent (Figs. 3.2 and 3.3). an initiator and other additives, with or without a Finally, TFE yield is maximized at temperatures surfactant. There are two different methods for above 900C, as Fig. 3.4 shows. These results have commercial TFE polymerization. Suspension poly- been proven in the actual manufacturing of TFE merization produces granular polymers. In this when plants run at the optimal conditions. process, TFE is polymerized aqueously, usually without a surfactant or with a very small amount of The effect of steam on the pyrolysis of chlorodi- surfactant accompanied by vigorous agitation. The ﬂuoromethane continues to be studied because of its positive impact of TFE selectivity [26].

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30 EXPANDED PTFE APPLICATIONS HANDBOOK 100% (Motor) Composition of product gasesCHCI F 2 Moles steam R = Diluent Ratio = 90 Moles CHCI F2 80 70 HCI 60 50 40 C2F4 30 20 10 C3F6 0 500 550 600 650 700 750 800 850 900 950 1000 Temp (°C) Figure 3.2 Effect of temperature on composition of product gases (R ¼ 0) [25]. 100 Moles steam R = Diluent Ratio = 90 Moles CHCI F2 80 % (Motor) Composition of product gases 70 60 HCI CHCI F 2 50 40 30 C2F4 20 10 C3F6 0 500 550 600 650 700 750 800 850 900 950 1000 Temp (°C) Figure 3.3 Effect of temperature on composition of product gases (R ¼ 1) [25].

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 31 100% YieldR = 8.0 Moles steamR = 7.5 R = Diluent Ratio =1=.02.0R=0 90 Moles CHCI F2 R 80= 70R 600.5 50= 40R 30 20 10 0 500 550 600 650 700 750 800 850 900 950 1000 Temp (°C) Figure 3.4 Effect of temperature on yield (R ¼ 0, 0.5, 1.0, 2.0, 7.5, and 8.0) [25]. surfactant is rapidly consumed, which leads to The only means of controlling the extent of the precipitation of the polymer, and thus the recrystallization after TFE homopolymers melt is to term slurry. Emulsion polymerization, on the other drive up the molecular weight of the polymer. There hand, is the method by which dispersion and ﬁne is a much higher probability of chain entanglement powder PTFE (also called coagulated dispersion) among the extremely long chains of PTFE in the are manufactured. Mild agitation, ample surfactant, molten phase and they have little chance to crystal- and a waxy substance are features of the emulsion lize back to extents in the premelt state (>90e95%). polymerization method. Once the TFE is polymer- This is why it is essential to polymerize TFE to ized, different ﬁnishing techniques are used to extremely high molecular weights. The molecular convert the product to dispersion and ﬁne powder weight of PTFE is speculated to approach 50 million products. [27], and it can be controlled by means of certain polymerization parameters such as initiator content, 3.4 Tetraﬂuoroethylene Polymers telogens, and chain transfer agents. TFE polymerizes linearly, without branching, One consequence of the very high molecular which results in a virtually perfect chain structure up weight of PTFE is its immense melt viscosity. The to rather high molecular weights. The chains crys- melt creep viscosity of PTFE is 10 GPa (1011 P) at tallize to form a nearly 100% crystalline structure. 380C [5]. This viscosity is more than a million times Thermoplastics develop good mechanical properties too high for melt processing by extrusion or injection because of the Van der Waals forces arising from molding. Unique processing techniques resembling interchain attractive forces. How can PTFE polymers those used for metal powders are therefore required. with useful properties be produced? The answer lies PTFE may be a thermoplastic, but it develops no ﬂow in controlling the recrystallization of the polymer upon melting. While it is relatively easy to eliminate after melting. voids completely with common thermoplastics, such as polyoleﬁns, this is not the case with articles made from PTFE. A small fraction of void volume remains

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32 EXPANDED PTFE APPLICATIONS HANDBOOK in parts made from homopolymers of PTFE due to 3.4.1 Ammonium the polymer’s slow rate of void closure. Voids affect permeation and mechanical properties such as ﬂex Perﬂuorooctanoate (Also C8) life and stress-crack resistance. Throughout the history of ﬂuoropolymers there Solving the void problem requires a reduction in has been a great deal of controversy about the use of the viscosity of PTFE without extensive recrystalli- ammonium perﬂuorooctanoate (APFO) as a poly- zation. The remedy has been to polymerize a small merization aid or surfactant. C8 was selected early on amount of a comonomer, such as perﬂuoro- because of its special characteristics that aid TFE propylvinyl ether [28] and HFP, with TFE to disrupt polymerization, and it became an essential ingredient the crystalline structure of PTFE. in the manufacture of PTFE and other ﬂuoropol- ymers. As a polymerization aid, it plays a critical role TFE can be polymerized by means of suspension in the polymerization of TFE and ﬂuorinated co- and emulsion techniques to produce PTFE resins monomers for the production of PTFE, PFA, and (Table 3.6). The suspension method yields granular perﬂuorinated ethyleneepropylene copolymer. It has polymers, which are processed by compression always been an intermediate component of the ma- molding methods. Homopolymers and modiﬁed jority of ﬂuoropolymers and is eliminated during the polymers of PTFE are produced by this technique. ﬁnishing steps. Solid-phase ﬂuoropolymers produced Emulsion polymerization is the process by which ﬁne in this way contain extremely small amounts of C8, powder and dispersion PTFE products are manufac- in the order of a few parts per million. A fraction of tured. Fine powder resins are fabricated by paste 1% of APFO remains in aqueous dispersion products. extrusion, in which a hydrocarbon is added to the powder as an extrusion aid and then removed prior to Over the years, C8’s impact on human health and sintering. Dispersion products are primarily applied the environment has been studied. APFO has been by coating methods and ﬁlled co-coagulation found to be bio-persistent, meaning that it accu- methods. mulates in the environment and in living organisms. The discovery of C8’s widespread presence in the All three forms of PTFE are produced by batch blood of humans and wildlife led to regulatory polymerization under elevated pressure in specially activities aimed at limiting its use. Fluoropolymer designed reactors. The polymerization media is highly manufacturers committed to the US Environmental puriﬁed water, which is virtually devoid of inorganic Protection Agency (www.EPA.gov) to work toward and organic impurities that might poison the reaction. the elimination of APFO emissions and its use in The choice of surfactant in these reactions is an anionic products by 2015. Consequently, between 2000 and surfactant and, prior to 2015, a perﬂuorinated car- 2010, ﬂuoropolymer manufacturers took interim boxylic ammonium salt was often used. steps to decrease environmental emissions and to Table 3.6 A Comparison of Polytetraﬂuoroethylene Products and Processes [27] Granular Fine Powder Dispersion Monomers TFE, PPVE TFE (HFP, PPVE, PFBE) TFE Media Regime H2O H2O H2O Reactor Suspension Dispersion Dispersion Agitation Mode Vertical Horizontal Horizontal Initiator Vigorous Mild Mild Surfactant Solids, % Batch Batch Batch (NH4)2S2O8 (APS) Disuccinic peroxide DSAP/ASP (DSP)/APS Fluorinated carboxylic salt Fluorinated carboxylic salt Fluorinated carboxylic salt 25 45 45 HFP, hexaﬂuoropropylene; PFBE, perﬂuorobutyl ethylene; PPVE, perﬂuoropropyl vinyl ether; TFE, tetraﬂuoroethylene.

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 33 reduce and eliminate C8 from dispersion products. Most ﬂuoropolymer manufacturers developed alternative polymerization aids and completely discontinued the use of C8 by the end of 2015. 3.4.2 Alternatives to Ammonium Figure 3.5 Structure of anionic surfactants with per- ﬂuoromethoxy or di perﬂuorometyl amine end groups Perﬂuorooctanoate (inside broken lines) [28,30]. This section discusses the alternative polymeri- Cesium Fluoride ( ) CI F3 zation aids. Because of the extensive and ongoing (n+1) F2C\\—/ CF—CF3 in Tetraglyme F---CI F–CF2– O---CF search for better alternatives to APFO, this review is O nI in no way exhaustive. For the most part, ﬂuoropol- CF3 CF ymer manufacturers have held propriety rights to the II speciﬁc surfactants used in commercial polymeri- O zation. Because of the ubiquitous presence of per- ﬂuorinated compounds in the environment and all Figure 3.6 Anionic ring-opening oligomerization of living organisms, a convergence around the safest hexaﬂuoropropylene oxide [28,31e36]. surfactants would be highly desirable. Such a convergence is practical because the majority of RF –CH2–CF2–CH2–CF2–X these substances are consumed in the process of emulsion polymerization. In spite of differences, the Biodegradable Points overwhelming majority of manufacturers share common practices for emulsion polymerization, RF–CH2–CI H–CH2–CI H–Y which should allow for signiﬁcant surfactant CF3 CF3 convergence. Airframe manufacturers have modeled this sort of convergence with respect to de- X, Y = hydrophilic parts velopments related to the safety and airworthiness of aircrafts. Figure 3.7 Structure of surfactants based on oligomers of vinylidene ﬂuoride or 3,3,3- Ameduri, Zaggia, Kostov, and Boschet have triﬂuoropropene [29]. reviewed the developments of alternative surfactants and have published the results [28,29]. Five groups of perﬂuorooctanoic acid (PFOA) at the same structures have drawn interest for the synthesis of concentration. nonbioaccumulative ﬂuorinated surfactants. They include: (1) compounds containing CF3O or (CF3)2N An example of a ﬂuorinated polyether surfactant is as end groups; (2) compounds based on oligomers of oligomers of HFP oxide formed by anionic ring- HFP oxide; (3) compounds produced from the opening oligomerization of HFP oxide (Fig. 3.6). telomerization of vinylidene ﬂuoride with short per- ﬂuoroalkyliodide; (4) 3,3,3-triﬂuoropropene telom- The possibility of biodegradation is enhanced ers synthesized from either perﬂuoroalkyliodides or when methylene or methyne groups in surfactants other chain transfer agents; and (5) surfactants ob- containing oligo(vinylidene ﬂuoride) or oligo(3,3,3- tained by cotelomerization or by controlled radical triﬂuoropropene) [29] form chains, as shown in copolymerization of vinylidene ﬂuoride and 3,3,3- Fig. 3.7. triﬂuoropropene. Figs. 3.5e3.7 show examples of the ﬁrst three structures. Some of the research has led to interesting results [37]. For instance, C2F5(VDF)2I (vinylidene ﬂuoride The CF3O(CH2)10SO3Na surfactant contains three (CF2eCH2)) is a novel telomer of vinylidene ﬂuoride ﬂuorine atoms but can reduce the surface tension of that can be ethylenated into C2F5(VDF)2CH2CH2I. the water rather substantially. For example, at 0.01 wt Then the ethylenated telomer can be converted into % concentration in water, surface tension is an alcohol by one of the many techniques, such as 25 mNmÀ1 as opposed to 19 dynes/cm for hydrolysis using oleum H2SO4/SO3 in dime- thylformamide. In the ﬁnal step, the alcohol is

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34 EXPANDED PTFE APPLICATIONS HANDBOOK Telomerization: CnF2n+1I + x H2C CF2 CnF2n+1(–H2C–CF2–)xI Ethylenation CnF2n+1(–H2C–CF2–)xI + CH2 CH2 CnF2n+1(–H2C–CF2–)x–CH2–CH2I Oxidation to alcohol CnF2n+1(–H2C–CF2–)x–CH2–CH2I CnF2n+1(–H2C–CF2–)x–CH2–CH2OH +HI H2SO4/SO3 in DMF/H2O Oxidation to acid CnF2n+1(–H2C–CF2–)x–CH2–COOH H2SO4/CrO3 Figure 3.8 Ideal reaction scheme for preparation of 3,3,5,5,7,7,8,8,8-nonaﬂuorooctanoic acid. 50 45 Surface tension, mN/m 40 35 C2F5(VDF)2CH2COOH PFOA 30 25 20 15 1 3579 11 13 –1 Surfactant concentration in water, g/l Figure 3.9 Surface tension of 3,3,5,5,7,7,8,8,8-nonaﬂuorooctanoic acid and perﬂuorooctanoic acid as a function of concentration in water [37]. oxidized using chromic oxide and sulfuric acid into addition to being the site at which chain transfer reactions commence. C2F5(VDF)2CH2COOH (3,3,5,5,7,7,8,8,8- nonaﬂuorooctanoic acid). This carboxylic acid is a CF3CF2CF2CF2I þ CH2] CF2 / CF3CF2CF2CF2CH2CF2I water-soluble surfactant. Fig. 3.8 summarizes these (3.7) reactions. Fig. 3.9 shows that the surface tension of such a CF3CF2CF2CF2CH2CF2I þ TFE / CF3CF2 VDF-containing surfactant is similar to that of CF2CF2CH2CF2CF2CF2I (3.8) PFOA. It has low surface tension (about 19.8 mN/m CF3CF2CF2CF2CH2CF2CF2CF2I / CF3CF2CF2CF2CH2CF2CF2COOH for a surfactant concentration of 5 g/L) and low (3.9) critical micelle concentration (CMC) (1.4 g/L), and therefore it is a replacement candidate for PFOA. In anticipation of completely abandoning APFO as a polymerization aid, a number of companies Murai, Enokida, and Murata have provided developed replacement surfactants. In 2008, Dyneon announced an early example. The company intro- another example of a surfactant that should degrade duced its new surfactant, Dyneon ADONA (CF3OCF2CF2CF2OCHFCF2COONH4), as a poly- in the environment [38], as seen in Eq. (3.7) merization aid to completely replace APFO in its production of ﬂuoropolymers. Fig. 3.10 shows the through Eq. (3.9). The starting compound is CF3CF2CF2CF2I, which can be prepared from telomerizing C2F5 and TFE. In this case the CH2 group is the susceptible point in the molecule, in

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 35 Figure 3.10 Chemical structure of ADONATM, product, as illustrated in Fig. 3.11. This section re- ammonium 4,8-dioxa-3H-perﬂuorononanoate, is views polymerization and these ﬁnishing techniques. 3M’s PFOA replacement in the emulsion polymeriza- tion of ﬂuoropolymers [39] Emulsion polymerization of TFE and other monomers takes place in an aqueous emulsion me- reported structure of the new surfactant. The toxicity dium. The resulting colloidal polymer (Fig. 3.12) proﬁle of ADONA has been studied and reported to remains in a stable emulsion. An early report of this be superior to that of APFO. Dyneon reported 2009 process was made by Renfrew in 1950 [42], who used as its ﬁrst full year of APFO-free operation [40]. disuccinic or diglutaric acid peroxide (0.1e0.4% by Other companies have selected different versions of weight of water) as the polymerization initiator. short perﬂuoroalkyl chains, such as C4, for ﬂuori- Gentle agitation was applied to the reactor while it nated surfactant development. was held under elevated pressure (0.3e2.4 MPa) at a temperature of 0e95C. The polymerization product An extensive discussion of APFO replacements was a stable dispersion of small polymer particles can be found elsewhere [27,41]. containing 4e6.5% PTFE. This dispersion coagu- lated easily after being subjected to agitation. The 3.5 Preparation of resulting dispersion required the addition of a second Polytetraﬂuoroethylene by surfactant to raise the stability further while it was Emulsion Polymerization being concentrated to a level that would allow for transportation and handling. TFE is polymerized commercially in an aqueous dispersion (emulsion) medium to produce dispersion The following is an example of Renfrew’s [42] and ﬁne powder PTFE products. In general, the key work. A stainless steel autoclave with a capacity of characteristics of this polymerization regime include 3400 mL was charged with 1500 mL of distilled water ample surfactant and mild agitation at elevated tem- containing 0.1% by weight disuccinic acid peroxide perature and pressure. The dispersion recovered from and 0.125% by weight chlorendic acid (Fig. 3.13). the reactor is ﬁnished by two different series of pro- Five parts per million by weight of ﬁne iron powder cesses, depending on whether an aqueous dispersion and 100 mL of a mineral oil were added to this so- or a dry powder (ﬁne powder) is the desired ﬁnal lution. The vessel was ﬂushed with nitrogen, evacu- ated, and pressured to 2.8 MPa with TFE. This level of Emulsion pressure was maintained for 1 h as the reaction Polymerization mixture was agitated at a temperature of 85C. At the end of the reaction, monomer ﬂow was stopped, the Cooling excess monomer was vented, and the reaction mixture was removed from the autoclave. The colloidal Wax dispersion of PTFE was separated from the mineral oil Decantation by decantation and then ﬁltered to remove coagulated polymer (coagulum) and the iron powder. The Coagulation and Concentration resulting aqueous PTFE dispersion was found to have Drying of Polymer and Formulation a solid concentration of 20%. The isolated coagulum was less than 2% of the polymer in the dispersion. The Dispersion Fine Powder polymer obtained after coagulating the dispersion and Products Products drying the resulting powder was compression molded into chips and sintered at 380C. The speciﬁc gravity Figure 3.11 Emulsion polymerization of tetraﬂuoro- of the chips was 2.22 Æ 0.01. ethylene and product ﬁnishing processes. Brinker and Bro [43] reported signiﬁcant im- provements to this process by adding a small amount of methane, ethane, hydrogen, or hydroﬂuoroethanes to the reactor prior to the onset of polymerization. The reactor ingredients included a surfactant such as ﬂuoroalkyl carboxylate [44] and an insoluble satu- rated hydrocarbon as an anticoagulant [45]. A typical reaction contained 0.1e3% of a dispersing agent like

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36 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 3.12 Scanning electron micrograph of two emulsion polymerized tetraﬂuoroethylene polymers at 20,000Â and 30,000Â magniﬁcation. (Courtesy: DuPont and Chemours companies) Figure 3.13 Chemical structure of chlorendic acid. was 36%, well in excess of Renfrew’s process. Dispersion stability was deﬁned by the amount of ammonium perﬂuorocaprylate. The initiator of time required for coagulation of the PTFE particles choice was a water-soluble compound such as when the mixture was agitated at 500 rpm. The ammonium persulfate (APS) and disuccinic acid Brinker and Bro [43] process nearly tripled the peroxide. Redox initiators, such as sodium bisulﬁte coagulation time to 6e8 min. with ferric triphosphate, could also be used. Initiator concentration, anywhere from 0.01% to 0.5% of the Following is an example of Brinker and Bro’s [43] weight of the water, depended on the rate and the polymerization method. A 330 mL platinum-lined degree of polymerization. The anticoagulant was a pressure vessel was charged with 200 g of deoxy- saturated hydrocarbon with more than 12 carbons, genated water containing 0.0125% by weight of water also known as wax, which is a liquid at the poly- of APS. The pressure vessel was evacuated and merization temperature. varying amounts of a chain-terminating agent (Table 3.7) were added. The reaction vessel was Brinker and Bro refer to the gases that they added heated to 85C and pressurized with TFE to 2.8 MPa. (eg, ethane) as “stabilizers” because of the surprising Both pressure and temperature were maintained while improvement in thermal stability that their presence the reaction mixture was agitated for a period of provided. They believed that these stabilizing agents 30 min. The reaction vessel was cooled to room aided in the formation of smaller colloidal particles temperature, excess monomer was removed, and the and thus enhanced the stability of the dispersion resulting polymer was ﬁltered and dried. The polymer against coagulation. They also noted that the added was molded into sheets and sintered, and its speciﬁc gases might be chain-transfer or chain-terminating gravity was measured (Table 3.7). The stability of the agents. In other words, these gases might not sup- PTFE was deﬁned by the amount of ﬂuorides evolved press the growth of the polymer chain but might when samples were exposed to moist air at 350C. instead help to form a bond between the stabilizer and the TFE polymer chain. Brinker and Bro, how- The addition of the stabilizing agents signiﬁcantly ever, considered the added gases stabilizing agents reduced the ﬂuoride evolution, which demonstrates rather than chain-transfer agents. the improved thermal stability of the polymers pro- duced. The small variations in the speciﬁc gravity of The addition of 0.008% methane (based on TFE) the PTFE made with the stabilizing agent as to the reactor at 86C and 2.8 MPa signiﬁcantly compared to the speciﬁc gravity of the standard enhanced stability. The colloidal solids concentration polymer containing no additive show that the poly- mers produced with the additives were high molec- ular weight polymers. Brinker and Bro’s work was a signiﬁcant advancement in the emulsion polymeri- zation of TFE. Up to this point, ﬁne powder PTFE with a fairly high molecular weight was available but could not be

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 37 Table 3.7 Emulsion Polymerization of Tetraﬂuoroethylene in the Presence of Stabilizers [43] Run # Stabilizer Stabilizer Polymer Yield Rate of Speciﬁc Fluoride Quantity in (g) Polymerization Gravity Evolution Mole Percent 2.2760 of Monomer (g/L/h)a 2.2710 (mg/h) 2.2773 1 None 123.1 1500 2.2738 2.2 2.2264 2 H2 0.5 71.9 400 2.2756 0.37 87.6 2.2754 3 CH4 0.5 67.7 800 0.4 94 4 H2CF2 0.5 67 400 0.38 60 5 CHF3 0.5 1000 6 CH3eCHF2 0.5 300 7 C2H6 0.05 335 a g/L/h, grams of polymer formed in 1 L of medium in 1 h. used to create thin parts such as wire coating and properties of the ﬁnal product. So, for example, if the tubing. Excessive paste extrusion pressure and ﬂaws modiﬁer is introduced after 70% of the monomer to developed when high molecular weight resins were be polymerized has been consumed, each PTFE converted to parts less than 500 mm in thickness. This particle will contain a core of high molecular weight was primarily due to the high molecular weight of PTFE and a shell of low molecular weight modiﬁed PTFE. See Chapter 4 for a complete description of PTFE. In this example, 30% of the outer shell of the the processing of ﬁne powder PTFE. In 1964, particle, by weight, was modiﬁed. The total modiﬁer Cardinal et al. [46] reported the development of ﬁne content of the polymer was extremely small powder resins with lower molecular weight, as evi- (Table 3.8), yet the impact on the properties was denced by their lower melt creep viscosity. These profound. Melt creep viscosity was 3e6 Â 1010 P, as resins were produced by introducing a modiﬁer to the compared to the polymer made under identical con- polymerization kettle. The modiﬁer consisted of a ditions without the modiﬁer, which had a melt creep nonpolymerizable chain-transfer agent such as viscosity of 10 Â 1010 P. Paste extrusion pressure hydrogen, methane, propane, carbon tetrachloride, was decreased by 20e50%, which led to fewer ﬂaws perﬂuoroalky triﬂuoroethylene, or oxyperﬂuoroalky in the tubing and wire insulation made from these triﬂuoroethylene. These last two gases contain resins. between 3 and 10 carbon atoms. The developments of Cardinal et al. [46] marked In the aqueous phase, Cardinal’s mixture contained another signiﬁcant advance in knowledge of altering water, a dispersing agent, an initiator, and wax. Typical and controling polymer properties. Holmes et al. [47] polymerization was conducted at 85e110C at a have explored the importance of perﬂuoroalkyl vinyl pressure of 2.9 MPa. The reactor, also called an ether comonomers (modiﬁers) such as per- autoclave, consisted of a horizontal cylinder with a ﬂuoromethyl vinyl ether, perﬂuoroethyl vinyl ether, length-to-diameter ratio of 10:1 equipped with a PPVE, and perﬂuorobutyl vinyl ether in dispersion steam/water jacket and a paddlewheel agitator running polymerization. They developed polymers composed the length of the reactor [47]. The agitator speed was entirely of copolymers of TFE and modiﬁers such as fairly slow compared to the speed of the agitator in PPVE with excellent mechanical properties. For suspension polymerization process. The motion of the example, a ﬂex life of 18 million cycles was obtained paddlewheel kept the aqueous phase saturated with after aging samples of PPVE-modiﬁed polymer at TFE. Table 3.8 shows a typical recipe and some 322C for 31 days. Standard speciﬁc gravity (SSG) properties of HFP and methanol modiﬁers. Others of the PPVE-modiﬁed polymers was below 2.175 and have reported using different modiﬁers, such as per- melt creep viscosity remained below 4 Â 1010 P at ﬂuoropropyl vinyl ether (PPVE). 380C. Polymerization rates to produce these modi- ﬁed polymers were increased to commercially The modiﬁer may be introduced at any time during acceptable rates by incorporating puriﬁed modiﬁers the polymerization, depending on the desired

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38 EXPANDED PTFE APPLICATIONS HANDBOOK Table 3.8 An Example of a PTFE Emulsion Polymerization Recipe and Some Properties [46] Reaction Component or Polymer 1 Polymer 2 Property Hexaﬂuoropropylene Methanol Modiﬁer type 0.15 0.009 Modiﬁer content, % by weight of 1500 parts 1500 parts TFE 3000 parts 3000 parts 0.005 Potassium persulfate 0.006 Ammonium persulfate Deionized water 0.15 0.15 TFE 6.3 6.3 Initiator, % by weight of water 85 70 2.9 2.9 Ammonium perﬂuorononaoate, % 125 125 by weight of water 35 40.5 Wax, % by weight of water 0.17 0.17 Temperature, C 2.211 2.211 3e6 Â 109 Pressure, MPa e Agitator speed, rpm Solids content of dispersion, %% by weight of water PTFE particle size, mm Standard speciﬁc gravitya Melt creep viscosity at 380C, poise PTFE, polytetraﬂuoroethylene; TFE, tetraﬂuoroethylene. a Determined according to ASTM Method D 4895. and by replacing disuccinic acid peroxide with a ﬂow was terminated. After a pressure drop of about persulfate-type initiator such as APS. The latter did 60%, agitation was stopped and the autoclave was not slow down the polymerization reaction. vented and its contents discharged. The supernatant solids, which consisted primarily of parafﬁn wax, Holmes and Fasig [47] polymerized TFE in an were removed. The treatment of the dispersion from autoclave similar to the one used by Cardinal et al. this point on depended on whether the desired ﬁnal [46]. The length to diameter ratio of their reactor was product was ﬁne powder or dispersion. Table 3.9 about 1.5:1. The autoclave was equipped with a four- presents a typical recipe and polymerization data. bladed cage-type agitator, rotated at 46 rpm. Typi- cally, the autoclave was ﬁrst evacuated, then charged Poirier [48] reported preparation of dispersion with wax, water, and ammonium perﬂuorocaprylate polymerized PTFE with a composite particle struc- (surfactant). The autoclave was heated to 65C and ture. The inner portion (core) of the particle con- then APS (initiator) was added while stirring. After tained a higher concentration of the comonomer than the autoclave was heated to 72C, PPVE was added its outer portion (shell). The comonomers were from and the vessel was pressurized with TFE at a constant the general family of vinyl ethers such as per- temperature and stirring rate. The temperature was ﬂuoroalkyl vinyl ethers. The advantage of using increased to 75C after a drop in the autoclave these polymers is the possibility of paste extrusion pressure indicated that the reaction had started. Early of their ﬁne powders at a high reduction ratio in the polymerization process, after consumption of without the complications of high extrusion pressure about 10% of the total TFE, additional surfactant was and potential ﬂaws in the extruded parts, which added to stabilize the dispersion. After the desired might include tubing or wire insulation. The reduc- amount of TFE had been fed to obtain a 35% polymer tion ratio of these polymers can exceed 10,000:1, concentration in the aqueous phase, the monomer

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 39 Table 3.9 An Example of a PTFE Emulsion Polymerization Recipe and Properties [47] Reaction Component or Polymer 1 Polymer 2 Property Perﬂuoropropyl vinyl ether Perﬂuoroethyl vinyl ether Modiﬁer type Modiﬁer amount 20.5 mL 3g Deionized water, g 21,800 3600 TFE, g 10,050 1830 Ammonium persulfate initiator, g 0.065 0.33 4.92 Ammonium perﬂuorocaprylate surfactant, g 2 Initial 26.7 141 855 75 Final 65e75 2.8 2.8 105 Wax, g 46 33.7 Temperature, C 35 0.10 Pressure, MPa 0.188 2.160 2.149 0.09 Agitator speed, rpm 0.102 2 Â 109 Solids content of dispersion, % by 0.9 Â 109 weight of water PTFE particle size, mm Standard speciﬁc gravitya Modiﬁer content of the polymer, % by weight Melt creep viscosity at 380C, poise PTFE, polytetraﬂuoroethylene; TFE, tetraﬂuoroethylene. a Determined according to ASTM Method D 4895. which is well above the conventional commercial ﬁnal product. Copolymers tend to improve the range. transparency of the sintered part as, for example, in tubing. Sometimes the modiﬁer improves the The coreeshell polymer [48] was made by evacu- extrudability but diminishes properties of the sintered ating the reactor partway through the polymerization part. For example, a copolymer of TFE with chlor- and repressurizing the autoclave with TFE. This otriﬂuoroethylene (CTFE) could be paste extruded at resulted in a reduction in the concentration of the high reduction ratios and low extrusion pressure [49]. comonomer. The core constituted 65e75% of the total The thermal stability of the CTFE copolymer, how- weight of the particle. The remaining 25e35% of the ever, was substantially reduced. To overcome this polymer formed the shell at a lower comonomer problem, a coreeshell architecture was designed. content than the core. Wire insulation was made from The core was comprised of a copolymer of a ﬂuo- these polymers, and the number of ﬂaws in the insu- roalkyl vinyl ether and TFE, and the shell was lation was detected by subjecting the wire to a high comprised of a CTFE and TFE copolymer. The voltage (2000e8000 V). The number of ﬂaws in the thickness of the shell could be fairly low (5% of the coreeshell polymer made with a lower concentration total weight of the particle) without a loss of good of the comonomer in the shell was minimal compared extrusion properties. Table 3.10 shows examples of to the number of ﬂaws in a polymer made with TFE ﬁve polymers with different compositions that alone, without the comonomer. demonstrate the beneﬁcial effect of coreeshell structure. Introduction of the comonomer improved the paste extrudability of the resin and the properties of the

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40 EXPANDED PTFE APPLICATIONS HANDBOOK Table 3.10 An Example of PTFE Emulsion Polymerization and Polymer Properties [49] Reaction Polymer 1 Polymer 2 Polymer 3 Polymer 4 Polymer 5 Component or 120 120 60 120 e Property 3.75 3.75 4.1 3.75 10 Disuccinic acid 70 70 85 70 70 peroxide, ppm 31.9 31.4 32 31.5 31.8 Ammonium 0.20 0.20 0.24 0.26 0.18 persulfate PPVE PPVE CTFE PPVE PPVE initiator, ppm CTFE CTFE CTFE 0.035 0.280 0.250 e e Temperature, C e e 0.02 0.02 e Solids content of 0.02 0.10 dispersion, % by 2.185 2.184 2.183 weight of water 10 33 40 2.186 2.173 100 64 52 1 1 Particle size, mm 108 118 Core modiﬁer Shell modiﬁer CTFE content of the polymer, % by weight PPVE content of the polymer, % by weight Standard speciﬁc gravitya Thermal instability indexa Extrusion pressure at a reduction ratio of 1500:1 MPa CTFE, chlorotriﬂuoroethylene; PPVE, perﬂuoropropyl vinyl ether; PTFE, polytetraﬂuoroethylene; TFE, tetraﬂuoroethylene. a Determined according to ASTM Method D 4895. Enhancing the properties of elastomers or plastics D4895, 19.2% naphtha, and a reduction ratio of by using PTFE as an additive has been a long-sought goal because such blends have properties superior to 1600:1) was in the range of 6.9 and 17.2 MPa. those of the base resin, such as tear, ﬂame, or abra- sion resistance. Morgan and Stewart [50,51] reported Other monomers, such as CTFE, have been used to on the development of a modiﬁed PTFE, which could be used as an additive. Examples of modifying modify PTFE for use as an additive [52]. About 3 L monomers included HFP and perﬂuoroalkyl vinyl ethers with ether linkage length of 1e4 carbon atoms. of deionized water, 120 g of a parafﬁn wax (with a The modiﬁed PTFE had sufﬁcient molecular weight melting point of 62C), and 4.4 g of APFO were and enough comonomer in the shell so that the tensile elongation at break was greater than 60%, and the charged to an autoclave with a 6-L capacity. The ratio of yield strength to break strength was greater than 0.60. The rheometer pressure (per American vessel was equipped with anchor-type mixing blades Society for Testing Materials (ASTM) Method and a jacket for temperature control. While heating to 85C, the autoclave was purged with N2 gas three times and twice with TFE to remove oxygen. The autoclave was pressurized with TFE to 0.64 MPa. At the same time, 0.26 g of CTFE was introduced. During agitation, an aqueous solution of 12.3 mg of APS in 20 mL of water, as well as an aqueous

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 41 solution of 180 mg of disuccinic acid peroxide small minority of the particles are spherical. The core (DSAP) in 20 mL of water, were fed into the system. in this case comprised 88.3% by weight of the par- ticles, and the shell comprised 11.7% by weight. The The pressure of the autoclave was raised to PTFE resin had a SSG of 2.1917. The concentrated 0.78 MPa. TFE gas was fed continuously to maintain dispersion of 6 wt% Triton X-100 had a critical the pressure at 0.78 MPa while the temperature was cracking thickness (CCT) of 29.1 mm and a gel time controlled at 85C. When TFE consumption during the of 991 s. At 8 wt% Triton, the CCT was increased to process reached a total of 1300 g, 3.5 g of CTFE ﬂuid 42.5 mm. was injected into the autoclave using TFE from a small tank. The reaction continued until a total of 1430 g had 3M Corporation [56e59] has described a process been consumed and then the TFE ﬂow was stopped. for manufacturing a polyperﬂuorovinyl ether homo- The autoclave was cooled and vented, and the solids polymer dispersion in the following manner: concentration of the reaction was measured to be 32.2 wt%. The mean particle diameter of latex parti- 1. An aqueous mixture of a perﬂuorovinyl ether is cles was 0.24 mm, and the SSG was 2.177. CTFE emulsiﬁed in the presence of a ﬂuorochemical content of the polymer was determined to be 0.23 wt%. emulsiﬁer to an average emulsion droplet size of 1 mm or less and Another invention illustrates [53] the polymeri- zation of TFE to make ﬂuoropolymer particles with a 2. Then the perﬂuorovinyl ether is polymerized in high molecular weight core of PTFE and a low mo- the presence of a free-radical initiator at a tem- lecular weight shell of PTFE. A polykettle with a perature and for a time sufﬁcient to produce horizontal agitator and a water capacity of 240 parts particles of polyperﬂuorovinyl ether. by weight was charged with 123.5 parts of demin- eralized water and 5.82 parts of a parafﬁn wax. The The perﬂuorovinyl ether used in this process had evacuated polykettle was loaded with 3.24 parts of a the following formula: CF2]CFeRf wherein Rf solution containing 0.0616 parts of APFO. The represents a perﬂuorinated organic group with a contents of the polykettle were agitated at 50 rpm and chain length of at least two atoms that contains at the temperature was raised to 90C. TFE was then least one carbon atom and at least one oxygen atom. added until the pressure reached 2.72 MPa. Next, The Rf group may be a perﬂuoroalkoxy group, a 1.29 parts of a fresh initiator solution containing 0.01 perﬂuoroether group, or a perﬂuoropolyether group. parts of DSAP and 0.00005 parts of APS per part of The dispersions made by this process are useful for water were added at the rate of 0.129 parts per rendering ﬁbrous substrates such as textiles that are minute. oil repellent, water repellent, and/or stain resistant. After the pressure had declined by 0.1 MPa, the A 2013 patent issued to Asahi Glass Company batch was considered to have kicked off. TFE was describes another method for emulsion polymeriza- added at a rate sufﬁcient to maintain the pressure at tion of TFE [60]. A 100-L stainless steel autoclave 2.72 MPa. After 8.81 parts of TFE had reacted after equipped with a bafﬂe plate and a stirrer was charged the kickoff, 6.47 parts of a 2.46 wt% of ammonium with 35 g of APFO, 872 g of parafﬁn wax, and 59 L perﬂuorooctonate solution were added at a rate of of deionized water. The air in the autoclave was 0.324 parts per minute. After 88.1 parts of TFE had purged with nitrogen, followed by evacuation, before been consumed, an additional 3.24 parts of a solution feeding in TFE. of 0.005 parts of APS and 0.06 parts methanol per part of water were added at the rate of 0.647 parts per The temperature was raised to 70C while stir- minute. After 96.9 parts of TFE had been fed, the ring, and the pressure was raised to 1.765 MPa by TFE feed was shut off and the polykettle pressure adding TFE. Then 5.0 g of disuccinic acid peroxide was allowed to decrease to 0.79 MPa before the (80 wt% solution), dissolved in 1 L of warm water at agitation was stopped. The time from the kickoff to about 70C, was injected into the autoclave. The the second initiator addition was 68 min, and the time autoclave pressure decreased to 1.746 MPa in about to the cessation of agitation was 87 min. 3 min. Polymerization continued as TFE was fed to maintain the pressure of the autoclave at 1.765 MPa. Solids content of the raw dispersion was 45.8 wt The APFO was dissolved in warm water, and the %, and the average raw dispersion particle size was total of 63 g of APFO was added during the 263 nm [53e55]. Typically, these raw dispersion polymerization. particles are cylindrical with rounded ends. Only a

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42 EXPANDED PTFE APPLICATIONS HANDBOOK Ammonium sulﬁte (AMS) was dissolved in water, ultrapure water was added. When TFE consumption and a total of 4 g as AMS was added during the reached 13.7 kg, the stirring and TFE feeding were polymerization [60]. The temperature was lowered to discontinued. The TFE in the polymerization vessel 64C at the halfway point and was raised to 80C at was vented, followed by a nitrogen purge. An the end of the polymerization. The reaction was aqueous PTFE dispersion (solids content 31.4% wt) terminated after 173 min, when a total of 23 kg of was obtained. The aqueous PTFE dispersion was TFE had been added. The residual TFE in the auto- allowed to coagulate without using any coagulant. clave was vented, the PTFE emulsion was cooled, The wet PTFE was separated and dried at 160C for and the supernatant parafﬁn wax was decanted. The 18 h to obtain a ﬁne powder of PTFE. solids concentration of the PTFE emulsion was about 26 wt%. The APFO used was 4122 ppm based on the 3.6 Emulsion Polymerization of PTFE. The average primary particle size was Tetraﬂuoroethylene With 0.25 mm. Only a trace of coagulum was left in the Ammonium Perﬂuorooctanoate reactor. Replacements The PTFE emulsion was then adjusted and diluted This section describes some examples of TFE with pure water to a concentration of 10 wt%, and polymerization using APFO replacements. 7.3 kg of the diluted aqueous emulsion were charged into an 8-L coagulation vessel equipped with a stirring A team at 3M in 2010 prepared a PTFE dispersion blade. The temperature of the vessel was dropped to using the following polymerization process. A 40-L 20C, after which they introduced 110 g of a 20 wt% kettle equipped with an impeller agitator and a ammonium carbonate aqueous solution. Stirring, at bafﬂe was charged with 30 L of deionized water, set 427 rpm, followed in order for coagulation to take to 35C, and evacuated to remove oxygen. Agitation place. The wet PTFE ﬁne powder contained about speed was set to 165 rpm. The oxygen-free kettle was 0.03% ammonium carbonate based on the PTFE. charged with 70 mmol of ammonium 2,4,6 trioxa- perﬂuoro-octanoate [CF3-(OCF2)3-COONH4], and The residual solid content of PTFE in the coagu- the following materials were added: 0.5 mL of a so- lation liquid was less than 0.1 wt% [60]. The wet lution containing 40 mg of copper sulfate pentahy- PTFE ﬁne powder was dried at 180C for 5 h to drate and 1 mg of concentrated sulfuric acid; 15 g of produce the PTFE ﬁne powder. The average particle a 25 wt% of aqueous ammonia solution and 5.6 g of size, the bulk density, and the SSG of the PTFE ﬁne CF3CF2CF2OCF(CF3)-CF2-O-CF]CF2 (PPVE-2) powder were found to be 410 mm, 460 g/L, and [62,63]. 2.150, respectively. A paste extruded beading was obtained at an extrusion pressure of 16.3 MPa. The The reactor was pressurized with TFE to 0.2 MPa tensile strength was 43 MPa and the elongation was and 47 g of HFP were added. The kettle was then 430%. pressurized to 1.5 MPa using TFE. Next, 100 mL of an aqueous initiator solution containing 140 mg of The following is an example of redox polymeri- sodium disulﬁte was pumped into the reactor, fol- zation of TFE [61]. A 50-L polymerization vessel lowed by 100 mL of a solution containing 340 mg of was charged with 30 kg of ultrapure water, 1.2 kg of ammonium peroxodisulfate. A pressure drop indi- parafﬁn wax, and 45 g of APFO, together with 3 g of cated the beginning of the polymerization. succinic acid and 210 mg of oxalic acid. The vessel Throughout polymerization, the pressure was main- was deaerated by purging with nitrogen and heated to tained at 1.5 MPa by feeding in TFE continuously. 55C. When the polymerization reaction temperature After 3.2 kg of TFE had been consumed, the mono- was stabilized, TFE gas was introduced into the mer valve was closed and the reactor was vented. The vessel to a pressure of 2.7 MPa. resulting PTFE dispersion had a solids content of 10.1 wt% and a pH of 9.6. The latex particle diameter While the contents were being stirred, a solution was 115 nm, as determined by dynamic light of 39 mg of potassium permanganate in ultrapure scattering. water was added continuously at a constant rate. Then TFE was fed continuously to maintain the In a series of other experiments, TFE and other pressure at a constant level of 2.7 MPa. The internal monomers were polymerized in the presence of temperature of the vessel was controlled at 55C. After 5.3 kg of TFE had been consumed, the entire solution of 39 mg of potassium permanganate in

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 43 different ﬂuorinated polymerization aids [64]. APFO (1000 mL) of the polymer dispersion was coagulated was used as a control in one of these experiments. All by adding 20 mL of hydrochloric acid under agitation of these polymerization procedures were conducted [64]. When coagulation was conducted, 100 mL of in a 40-L autoclave equipped with an impeller gasoline was added and the dispersion was stirred agitator and a bafﬂe. The autoclave was evacuated again. After dewatering, the coagulated product was and then charged with 33 L of deionized water and rinsed several times with deionized water. The gas- heated to 35C. Agitation started at 160 rpm and the oline was removed by heating the wet PTFE in a vessel was evacuated and purged with nitrogen to vacuum to 40C. The polymer was dried overnight at remove oxygen. A ﬁnal purge was made by pres- 100C in a vacuum oven. Table 3.11 shows the PTFE surizing the reactor to 0.2 MPa with TFE. The gas polymer dispersion for each of the polymerization was then released, followed by a ﬁnal evacuation. aids used. Next the ﬂuorinated emulsiﬁer, as speciﬁed in Table 3.11, was added to the water. The following is an example of redox polymeri- zation of TFE using an alternative surfactant. A 100- The following materials were also charged to the L stainless steel autoclave, equipped with a bafﬂe aqueous solution: 24 mg of copper sulfate pentahy- plate and a stirrer, was charged with 59 L of deion- drate, 0.6 mg of sulfuric acid, and 8 g of a 25 wt% ized water [65]. Then 70 g of a polymerization aid aqueous ammonia solution and 5.6 g of PPVE-2. with the chemical formula C2F5OC2F4OCF2- Finally the reactor was pressurized to 0.2 MPa us- COONH4 (ammonium perﬂuoro-3,6-dioxaoctanoate, ing TFE, and 50 g of HFP was also added. The abbreviated as APFDO), 872 g of parafﬁn wax, and reactor pressure was raised to 1.5 MPa using TFE, 59 L of deionized water were charged. The air in the and 100 mL of an aqueous initiator solution con- autoclave was purged with nitrogen and then evacu- taining 187 mg of sodium sulfate as well as a further ated. The TFE feed was started to raise the autoclave 100 mL of an aqueous initiator solution containing pressure while heating and stirring. The temperature 429 mg of ammonium peroxodisulfate were pumped was increased to 70C and the pressure was elevated into the vessel. to 1.765 MPa by feeding TFE. Five grams of an 80% wt solution of disuccinic acid peroxide in water was A pressure drop indicated the start of the poly- injected into the autoclave. merization. During polymerization the pressure was kept at 1.5 MPa by feeding TFE into the gas phase. In 3 min, pressure decreased to 1.746 MPa. Poly- After feeding the amount of TFE indicated in merization was continued by ﬂowing TFE to keep the Table 3.11 for the particular compound being used in autoclave pressure at 1.765 MPa. APFDO was dis- each procedure, the TFE valve was shut down. Some solved in warm water and pumped into the reactor Table 3.11 Polymerization Conditions and Polymer Properties With Ammonium Perﬂuorooctanoate (APFO) Replacements [64] Polymerization Aid Polymerization APFO Compound Compound Compound Compound Compound Variable and Product 140 11234 Properties Amount polymerization 3.64 210 70 140 70 70 aid, mmol 75 2.55 3.2 3.2 3.2 3.2 Tetraﬂuoroethylene 120 consumption, kg 10.2 84 84 73 79 72 2.171 122 122 129 115 113 Polymerization time, min 7.1 7.1 10.1 10.0 10.2 2.166 2.175 2.159 2.167 2.165 Particle size, nm Solids content, wt% Standard speciﬁc gravity APFO, CF3-(CF2)6-COONH4; Compound 1, CF3-O-CF2-CF2-COONH4; Compound 2, C2F5-O-CF2-CF2-COONH4; Compound 3, C3F7-O-CF2- CF2-COONH4; Compound 4, C4F9-O-CF2-CF2-COONH4.

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44 EXPANDED PTFE APPLICATIONS HANDBOOK during the polymerization for a total of 125 g of Rahl et al. [69] examined emulsion-grade PTFE APFDO. Then 4 g of AMS were dissolved in water particles at various stages of polymerization. In the and a total of 4 g of (NH4)2SO3 was added while early stages of polymerization the nonspherical, polymerization was taking place. The temperature ribbon-shaped objects are formed. These ribbons are was lowered to 64C halfway through the reaction chain-extended single crystals with the chain axis and it was raised to 80C at the end of the parallel to the long axis of ribbons. Rahl et al. sug- polymerization. gested that the folding of these thin ribbons (approximately 6 nm in thickness) occurs as a The reaction was terminated after 23 kg of TFE consequence of hydrodynamic forces acting on was consumed. The residual TFE was released to the extremely thin chain-extended crystals once they atmosphere. The total polymerization time was exceed a certain aspect ratio and presumably 176 min. The recovered aqueous PTFE emulsion was compete with further reaction and crystallization on cooled, and the supernatant parafﬁn wax was removed the surface of the growing particles. At polymeriza- by decantation. The solids concentration of the tion with low-yields, rodlike particles are also formed aqueous emulsion was 26% by wt. The APFDO used as apparently single crystals with the chain axis was 8333 ppm, based on the total PTFE recovered. parallel to the long axis of rod. The average primary particle size was 0.28 mm. Little or no coagulum was found in the autoclave. Seguchi et al. [70] also suggested that the polymer morphology (shape and molecular weight) strongly The aqueous PTFE emulsion was diluted with depends on the polymerization conditions, and pure water down to a concentration of 10% wt, and especially on the concentration of surfactant. When 7.3 kg of the diluted aqueous PTFE emulsion were polymerization is carried out without an emulsiﬁer, charged into an 8-L coagulation vessel equipped with only nearly spherical to elliptical particles are a stirring blade running at 427 rpm [66]. The tem- formed, with an average diameter of 100 nm and a perature was controlled at 20C during the coagula- molecular weight above 106 Da (daltons). When the tion. Then 73 g of a 20% ammonium carbonate concentration of emulsiﬁer is increased, the type of aqueous solution were introduced into a tray polymer in the latex changes from rods, 30e60 nm in (30 cm Â 40 cm). The wet PTFE was loaded into the diameter with molecular weights from 2 Â 105 to tray in an even layer 2e3 cm high and placed in a 5 Â 105 Da, to ﬁbrils 20 nm in diameter with mo- forced convection oven at 180C for 5 h. The average lecular weights below 2 Â 104 Da. particle size and the bulk density of the PTFE ﬁne powder were 590 mm and 480 g/L, respectively, with Luhmann and Feiring reexamined the inﬂuence of a SSG of 2.150. Samples of the paste extrudate of the emulsiﬁer concentration and polymerization time on resin had a tensile strength of 35 MPa and a break the structure of PTFE [71]. They observed three types elongation of 36%. of particles with different morphologies: rodlike, roughly spherical, and, in the case of those with very Additional examples of emulsion polymerization low molecular weight, small hexagonal particles. of TFE and its copolymers using alternative poly- What follows is an abbreviated version of the merization aids can be found in the patent mechanism of formation of emulsion-polymerized art [66e68]. PTFE particles. 3.7 Mechanism of Emulsion Initiation occurs in the aqueous phase in the Polymerization of presence of an ionic surfactant, such as ionic ﬂuori- Tetraﬂuoroethylene nated surfactant (C7F15COONH4). During the initi- ation, a monomer molecule reacts with a free radical Emulsion polymerization of TFE polymerization group generated by the decomposition of the APS follows a typical free radical mechanism similar to initiator, forming R-CF2-CF2 fragments. A very the polymerization of styrene, for example, into large number of these fragments, consisting of one or polystyrene. Jurczuk has summarized the ﬁndings two monomer units, is produced. Many of these about the polymerization mechanism of TFE and the fragments are eliminated by reaction with other free formation of PTFE microstructure. Morphology of radical groups. The fragments that survive become virgin emulsion-polymerized PTFE has been exten- the nuclei of particles in which straight chains of TFE sively studied by a number of researchers. grow until they develop ﬂexibility. After reaching about 100 carbons (surface energy driven), the chains

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 45 Chain folding Fig. 3.17 shows the effect of the surface tension of the emulsion polymerization medium on the shape A flexible PTFE chain (morphology) of PTFE particles. At lower surface tensions (<18 dyn/cm) particles are rod shaped, Figure 3.14 A polytetraﬂuoroethylene (PTFE) chain while at higher surface tensions (>25 dyn/cm) the folding after growing to about 100 carbon atoms. particles are spherical. In the transition range of 20e25 dyn/cm the particles are initially rod shaped Hair pin but fold when they reach a certain size, as dictated by the type and concentration of the polymerization Figure 3.15 Accordion-shaped crystal formed by the surfactant. Thermodynamic laws requiring the par- folding of a growing polytetraﬂuoroethylene chain. ticle to have the geometry for which the Gibbs free energy of the particle surface is minimized govern (A) this phenomenon. The geometry for which the Gibbs free energy is minimized is the one that has the (B) smallest surface area for a given volume. That ge- ometry is spherical, which is why water droplets are Figure 3.16 Schematic of the structures of a single spherical and not rectangular. polytetraﬂuoroethylene particle: (A) rod shape; (B) spherical shape. Fig. 3.18 shows scanning electron micrographs of PTFE particles produced by emulsion polymeriza- fold in a hairpin as depicted in Figs. 3.14 and 3.15. tion at below and above CMC. The surfactant used The folding of the chains continues and results in a was APFO, which has a CMC of 33.0 Â 10À3 mol/L. crystalline geometry resembling an accordion. When the polymerization of TFE was carried out without the surfactant, or the surfactant concentration The accordion-shaped crystallites pack in a was very high (over CMC), the particles aggregated spherical conﬁguration and form entire particles in easily as the reaction proceeded. In particular, most spherical (round) shapes or rod shapes depicted in of the particles of the PTFE dispersion near or above Fig. 3.16. CMC were rod-shaped, as seen in Fig. 3.18C and D. From a processing standpoint, the most desirable It is important, therefore, that a sufﬁciently high shape for emulsion-grade PTFE particles is round or concentration of the surfactant be present while spherical. The PTFE particles, however, do not start polymerization is taking place. This will allow uni- as round. Initially, during the polymerization, the form production of round PTFE particles whether the particles are in the shape of a rod as seen in Fig. 3.17. monomer consists of TFE or a mixture of TFE and a Although the surfactant acts primarily as a stabilizing modifying comonomer. agent to prevent coagulation, it can also inﬂuence the PTFE particle morphology. Indeed, the surfactant 3.8 Development of and its concentration have to be selected carefully to Polytetraﬂuoroethylene for ensure that the emulsion polymerization results in Expanded Polytetraﬂuoroethylene round particles. Applications In the very early days of ePTFE development the importance of maximizing the crystallinity of the resin was discovered. The premium PTFE resin at the time was Teﬂon® 6A, which contained 0.2% HFP and TFE, as polymerized Teﬂon® 6A had a crystal- linity of 95%. Increasing the crystallinity by annealing led to more efﬁcient PTFE (lower stretch rate and temperature) expansion processes. An early patent, US 3,953,566 [73], states: “It is found that some resins are much more suitable for the expansion process than others, since they can be

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46 EXPANDED PTFE APPLICATIONS HANDBOOK Water plus high Polymerization Medium Water concentration of fluorinated surfactants or Water plus 0.12% by water plus perfluorinated weight APFO oils (e.g., Krytox®) <18 dynes/cm 20-25 dynes/cm 72 dynes/cm Surface tension of Polymerization Medium Figure 3.17 Effect of surface tension [ammonium perﬂuorooctanoate (APFO)] of medium in emulsion polymer- ization on the shape of polytetraﬂuoroethylene particles. Figure 3.18 Scanning electron micrographs of polytetraﬂuoroethylene dispersion polymerized for various surfactant concentrations: (A) 3.48 Â 10À3 mol/L; (B) 9.28 Â 10À3 mol/L; (C) 20.0 Â 10À3 mol/L; (D) 46.4 Â 10À3 mol/L [72]. processed over a wide range of rate and temperature and correspondingly low amorphous content. It has and still produce useful products. The primary been found that techniques for increasing the crys- requisite of a suitable resin is a very high degree of tallinity, such as annealing at high temperatures just crystallinity, preferably in the range of 98% or above, below the melt point, improve the performance of the

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 47 resin in the expansion process.” Similar statements 2.6 MPa absolute with TFE [76]. Stirring rate and were also made in later patents [74,75]. temperature were maintained until a 0.07 MPa drop in pressure indicated that kickoff had occurred, and After the successful growth of ePTFE products in the time from the start of the pressurization to the various end uses, DuPont and other ﬂuoropolymer kickoff was recorded. Additional TFE was then manufacturers began to develop resins for the ePTFE added to raise the reaction pressure to 2.8 MPa and to process. The new resins were designed to ﬁbrillate maintain that pressure until a dispersion of approxi- easily, produce large numbers of ﬁbrils, and easily mately 35% solids content by weight (total polymer endure high stretch ratios (SRs). These resins had plus aqueous medium basis) was obtained. The high molecular weights and were at least 98% crys- temperature was maintained, at 98e101C, talline as-polymerized. throughout the polymerization. In one of the early developments in 1977, a PTFE After 10.05 kg of TFE had been fed after kickoff, resin was prepared expressly for porous ePTFE the monomer feed to the autoclave was terminated application [76]. The patent claims a process using and the pressure was allowed to decrease to 1.2 MPa inorganic persulfate initiators at a temperature of before agitation was stopped and the vapor space of 95e125C. This polymerization process resulted in the reactor was vented. The total polymerization time PTFE resins with a higher degree of stretching than was measured from kickoff to when the agitator was those previously available. The stretchability of PTFE turned off. The resulting dispersion was discharged resins produced by the aqueous dispersion polymeri- and cooled. The PTFE was recovered using a similar zation method was demonstrated at temperatures procedure to those described above for conversion of below the sintering temperature of the resin. the dispersion to dry powder. The increased stretchability is characterized in Several tests were devised to evaluate the useful- terms of ultimate SR and SR. SR is the weight per unit ness of PTFE for expansion processes. Some of these of length of the test specimen prior to stretching, as tests are run on production batches of PTFE resins for compared to the weight per the same unit of length quality assurance speciﬁc to ePTFE use. Over time, a after stretching. Ultimate SR refers to the SR value at few of the tests have evolved beyond the initial breakpoint. Holmes set out to improve on these ratios procedures disclosed by Holmes. The actual tests are, as reported in Belgian Patent No. 767,423, which however, proprietary to PTFE manufacturers. The showed that extremely high strain rates were required tests devised by Holmes are described ﬁrst. to expand Teﬂon® 6C ﬁne powder (which also con- tains a small amount of hexaﬂuoropropylene). At a Stretch Ratio and Ultimate Stretch degree of stretch of only 550%, stretch rates of 5000% Ratio Test and 10,000% per second caused the test specimen to break, and the stretch rate of 40,000% per second was The stretchability property of PTFE resins pro- required to avoid breaking at the stretch temperature duced by the aqueous dispersion polymerization of 205 and 315C. method is manifested at temperatures below the sintering temperature of PTFE. The increased A horizontal, water/steam jacketed, cylindrical stretchability is characterized in terms of ultimate SR stainless steel autoclave with a capacity of 36.25 L and SR. Ultimate SR is the ratio of stretched length at and a length-to-diameter ratio of about 1.5:1 was used breaking of the test specimen of the ﬁne powder to for polymerization. The vessel was equipped with a unstretched length. SR is the weight per unit of length four-bladed cage-type agitator capable of being of the test specimen prior to stretching, as compared rotated at 46 rpm that ran the length of the autoclave. to the weight per the same unit of length after The vessel was evacuated and then charged with 855 g stretching. of parafﬁn wax, 21.8 kg of demineralized water, and 30 g of APFC dispersing agent. The autoclave was Preparation of Test Specimen subsequently heated to the designated temperature with stirring. After pressurization, the initiator solu- The sample of resin to be stretched is paste tion was added at a continuous uniform rate and was extruded by a procedure disclosed into beading stopped within the period in which 50e80% by through an oriﬁce of 0.318 cm in diameter. This weight of the total PTFE was formed. beading is tested for uniformity by tensile testing three representative samples (gauge length At the beginning of the polymerization, the auto- clave was pressured over a period of about 2 min to

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48 EXPANDED PTFE APPLICATIONS HANDBOOK 11.43 cm), stretching them along the length of the The tensile break strength of the stretched test beading at 25C and a strain rate of 30.5 cm/s. The specimen is taken as the minimum tensile break load beading is considered uniform if the range of tensile of three stretched test specimens, one from each break loads obtained is within Æ5% of the average stretched end of the specimen (excluding neckdown, tensile break load. If the beading satisﬁes this crite- if there is any, in the region of clamp) and one from rion of uniformity it is subjected to further testing to the center. The measurement is made at 25C at a determine stretchability. The average tensile break strain rate of 30.5 cm/s and gauge length of value obtained while determining the beading’s uni- 11.43 cm, gripped by 5.08-cm standard ﬁlament formity is also taken as the tensile break strength of jaws, with a distance of 1.27 cm between the jaws. the unstretched beading. The beading is then cut into 8.9-cm lengths for further testing. Stretching Rate Stretch Procedure The stretchability of the resins produced by this process occurs at the relatively low rate of stretching The test specimen is clamped at each end, leaving of 5.33 cm/s. Other PTFE resins produced by the a 5.08-cm space between clamps (gauge length). aqueous dispersion method may attain higher SRs, The clamps are part of the apparatus, which moves but only at much faster stretching rates. These faster the clamps apart at a constant speed of 5.33 cm/s. The stretching rates have several disadvantages. These clamped beading is placed in a circulating air oven rates are difﬁcult to attain mechanically and operating at 300C and the specimen is stretched at economically, especially when it comes to stretch- this temperature to the distance corresponding to the ing sheeting, and there is also a production yield SR desired (ratio of weight per unit length to weight penalty caused by extrudate breakage during after stretching for the same unit of length) at the equipment start-up. In addition, even when a faster constant rate indicated. stretching rate is used the extrudate must ﬁrst pass through a slow rate region, which also leads to After stretching, the specimen is examined while extrudate breakage. still clamped to determine if the stretch is uniform along the length of the specimen, excluding some In contrast, the 5.33 cm/s stretch rate used for these possible neckdown in the region of the clamps. Ink resins corresponds to a stretch rate of only about markings made on the specimen in the area under the 100% per second. At this low rate, an ultimate SR of clamps prior to stretching should not be visible, as at least 30:1, and preferably at least 50:1, is attainable. this would indicate slippage rather than stretching. A center marking made on the specimen prior to Stress Relaxation Time stretching should be within 12.5% of the center after stretching. Another unique property of the PTFE aqueous dispersion resins produced by this process is a stress To determine ultimate SR, a recording load cell relaxation time of at least 4 min. Stress relaxation is connected to a high-speed chart that records the time is deﬁned as the length of time it takes for the stressetime curve during stretching at the 5.33 cm/ test specimen to break when fully extended and s rate at 300C. When the load reaches zero (either heated at 395C after stretching to a ratio of 24:1 at by direct measurement or by extrapolation of the 300C and at a strain rate of 38.1 cm/s with an initial predominant negative slope of the stressetime length of 5.08 cm between the clamps (a 12.7 cm curve), the corresponding SR is taken as the ulti- length of stretched beading in this test). These resins mate SR. resist breaking in the molten state in which this test is conducted for at least 4 min, and many times for at Tensile Break Strength Test least 5.5 min, as compared to a maximum of 3 min for the prior art PTFE resin. In addition to testing the ultimate SR and stress relaxation time (described in detail below), the TFE Fluon CD-023 (SSG of 2.175) is the prior art resins produced by this process are also tested for resin that comes closest to the resins produced by strength. These resins have a SR of at least 24:1 and this process in terms of stress relaxation time. their tensile break strength is at least 80% of the Many other prior art resins tested had a consider- tensile break strength of the original unstretched ably lower stress relaxation time [76]. Even Fluon beading, which is preferably at least 2.27 kgf. CD-023 has a tensile break strength for beading

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 49 stretched to a ratio of 24:1 that is less than 40% of A2 m cal/sec the tensile break strength of the unstretched beading. The heating at 395C for this test is done EF G in a circulating air oven operating at that temper- ature, but for a short period of time after the C stretched test specimen is placed in the oven the temperature drops somewhat, to about 365C, and L DB it takes 1e2 min for the oven temperature to return to 395C. The stress relaxation time is measured 560 570 580 590 600 610 620 630 640 starting at the time the test specimen is placed in the oven. Temperature (°K) One signiﬁcant advantage of the improved stress Figure 3.19 Differential scanning calorimetry ther- relaxation time exhibited by these resins is that mogram of an expanded polytetraﬂuoroethylene stretched articles made from them can be sintered resin [77]. (heated to a temperature greater than 340C to allow partial coalescence of the PTFE particles in the Resins produced under this patent were stretchable at stretched article while retaining most of the article’s 100e10,000% per second expansion rates. porosity) in a continuous inline operation with greater assurance of freedom from breaking, which Koizumi et al. [77] discovered the characteristics would cause the operation to shut down. that PTFE must have to be highly stretchable. In Fig. 3.19, a vertical broken line (B) was drawn The ultimate SRs for the competitive PTFE1 and downward from the peak (A) and the cross-point with PTFE2 and Teﬂon® resins made by this process can the baseline (L). A parallel line was drawn at a be increased by raising the stretch rate from 100% temperature 10C lower than the temperature of the per second to 800% per second. At this increased top of the peak [ie, 346C (619K)], and the cross- rate, these resins exhibit ultimate SRs of 113:1 (for points with the endothermic curve and with the PTFE1), 137:1 (PTFE2), and 177:1 (Teﬂon®) [76]. baseline (L) were named (C) and (D), respectively. The value of CD/AB was deﬁned as the endothermic Koizumi et al. [77] disclosed a stretchable ratio. Moreover, at the middle point (F) of the vertical PTFE ﬁne powder with a molecular weight of line AB, a line parallel with the abscissa axis was ! 5,000,000. They followed a process using a drawn, and the cross-points with the endothermic minimal amount of a persulfate initiator and curve were labeled (E) and (G). The distance EG, the changed the polymerization conditions after the width of the peak at half of the height from the initiation of polymerization. They polymerized the baseline of the endothermic curve, was deﬁned as TFE in an aqueous medium in the presence of an “half-value width of the endothermic peak,” and this anionic surface active agent, dispersion stabilizer, unit was shown in degrees. and polymerization initiator at a temperature of 55e85C. After the initiation had taken place and The PTFE ﬁne powders created through this pro- 30e80% by weight of the total TFE was consumed, cess had an average molecular weight of not less than the polymerization conditions were altered. They 5,000,000, preferably not less than 5.5 million, and changed the polymerization reaction by adding an were usually in the range of 5e20 million. When the alkali or a radical scavenger, or both, to the reaction system. They also tried lowering the polymerization temperature by at least 10C. The PTFE ﬁne powders manufactured by this procedure had high molecular weight and high crystallinity. The PTFE appeared to have a narrow distribution of molecular weight because using minimal amounts of initiator limited the number the radicals successively produced in the course of polymerization. In fact, the melting diagram of resin using differential scanning calorimetry (DSC) indi- cated the narrow distribution of molecular weight.

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50 EXPANDED PTFE APPLICATIONS HANDBOOK molecular weight is lower than 5 million, the but <85% by weight of the TFE to be polymerized so stretchability of the ﬁne powders was not sufﬁcient for that the polymerization time was extended by at least most applications, even though other requirements 130% (in comparison with the time of the polymeri- might be met. The value of the amorphous index (AI, zation under the same conditions without the retarder). the fraction of the polymer that is amorphous) should The SSG of colloidal PTFE particles, after 50% by not be more than 0.1, preferably not more than 0.09, weight TFE consumption, had to be <2.210 and SSG and is usually in the range of 0.02e0.1. Higher AI of the ﬁnally produced PTFE particles had to values indicate the presence of more incomplete PTFE be <2.210. The average particle size was in the range crystals. When the value is higher than 0.1, the ﬁne of 0.1e0.5 mm. powders have less stretchability. PTFE ﬁne powders that have a primary particle size of less than 0.1 mm The molecular weight of PTFE ﬁne powder was are not suitable for processing because of excessive calculated from the following relationship: paste extrusion pressure. On the other hand, PTFE ﬁne powders that have a primary particle size of more than Mns=Mnc ¼ 2Mn=Mnc À 1 0.4 mm are also undesirable because the extrudate will have low strength. Mn is number the average molecular weight of the polymer in its ﬁnal state, Mnc is the molecular The crystals of the PTFE ﬁne powders created weight of the polymer after 50% by weight TFE under this patent have an endothermic peak of consumption, and Mns can be derived from Mn and melting, which is measured by DSC at a temperature Mnc. The speciﬁc gravity of this powder, however, between 343 and 350C. Some ﬁne powders occa- was inversely proportional to the SSG of the poly- sionally show two peaks, and when this is the case the mer. Therefore the characteristic feature of the higher peak is deﬁned as the endothermic peak. The PTFE ﬁne powder made under this patent may be higher the polymer’s molecular weight, the higher represented by the following formula: Mns/ the temperature of the endothermic peak will be. Mnc > 1.0. As a result of the experimental study, Taking this fact into consideration, these PTFE ﬁne however, the powder obtained by the process powders had some of the highest molecular weights described in the patent indicates Mns/Mnc > 2.0. of all PTFE polymers. Rather, the polymer satisfying this relationship exhibited excellent stretchability. Typically, the best PTFE ﬁne powders have sharp endothermic peaks, when measured by DSC [77]. A In addition to the above characteristics, the PTFE sharp peak has an endothermic ratio of not more than ﬁne powder made using this patented formula usually 0.3 and a half-value width of not more than 8C. has a heat absorption ratio of not more than 0.3, and When a PTFE ﬁne powder does not have such a sharp preferably not more than 0.27, as seen in the differ- peak its tape does not demonstrate good stretch- ential thermal analysis chart showing measurements ability, even though the powder might have an taken by a differential scanning calorimeter as endothermic peak at 343e350C and other properties described in Japanese Patent Publication (unexam- within the required ranges. ined) No. 60979/1978, a half-value width of heat absorption peak of not more than 6C, and preferably In 1982, Shimizu and Koizumi developed another not more than 5.5C, an average molecular weight of process [78] for preparing PTFE ﬁne powder by not more than 5 million, and an AI of <0.1. These polymerization of TFE in an aqueous medium. The properties were required for carrying out the paste reaction mixture was comprised of a water-soluble extrusion of PTFE ﬁne powder under commercially polymerization initiator and a nontelogenic surfac- acceptable conditions, including relatively low tem- tant capable of keeping colloidal PTFE particles in a perature and low extrusion rate [77]. The SSG of the sufﬁciently stable state. The polymerization was car- PTFE ﬁne powder made under this patent had the ried out at a temperature of 55e120C. A polymeri- following relationship with the number average mo- zation retarder was chosen from aromatic hydroxy lecular weight (Mn): compounds, aromatic amino compounds, and quinone compounds. This solution had to have a water solu- log10Mn ¼ 28:04 À 9:790 Â ðSSGÞ bility of !2.5 Â 10À6 mol/L at 25C without reiniti- ating the reaction or permitting chain transfer. They The samples obtained by cutting a strand of this added 0.7e20 ppm of the retarder to the polymeriza- PTFE ﬁne powder were stretched at a draw ratio of tion reaction mixture after consumption of >10%

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 51 30 at 310C with a distance between chucks of reacted (5.5% of the total TFE polymerized), a so- 50 mm under a pulling rate of 100% per second or lution of 45 g C-8 in 1000 mL water was pumped in 1000% per second. The sample that could be at 50 mL/min. After 2.27 kg of TFE had reacted stretched 30 times at the rate of 100% per second (14% of the total TFE reacted), the polykettle tem- showed no failure (break) on stretching under 1000% perature was increased to 113C. After 6.8 kg of per second. The sample that failed when stretched at TFE had reacted (41.5% of the total TFE reacted), 1000% per second, however, also failed when the polykettle temperature was slowly lowered to stretched at 100% per second. The stretched product 73C by adding cold water at the rate of 100 mL/min that did not fail was divided between that with a after 10 kg TFE had reacted. The ﬁnal temperature uniform appearance and that with a nonuniform of 73C was reached when 95% of the TFE had appearance. The sample with the best stretchability reacted. was the one with a uniform appearance that did not fail on stretching under the rate of 100% per second. After 16.4 kg of TFE had reacted, the feed was The next best sample was the one with a nonuniform stopped and the polykettle was vented, evacuated, appearance that did not break when stretched at and purged with N2. The contents were cooled and 100% per second [77]. The polymers prepared by this discharged from the polykettle. The supernatant wax process do not fail, even if a nonuniform appearance was removed. The dispersion was diluted to 15% is the result of stretching at 100% per second and a solids and coagulated in the presence of ammonium uniform appearance results from stretching at 1000% carbonate under high agitation conditions. The per second. coagulated ﬁne powder was separated and dried at 150e160C for 3 days [79,80]. In lab testing the Malhotra also described a process for preparing polymer stretched very well and produced a highly unsintered TFE ﬁne powder resins for ePTFE uniform ePTFE beading. Following are the test pro- manufacturing [79]. TFE (to which a comonomer cedures Malhotra used. could be added if desired) was polymerized in an aqueous medium in the presence of an initiator and a Stretch Test nontelogenic anionic surfactant to maintain colloidal particles of PTFE product in dispersed Preparation of Test Specimen form. Malhotra varied the temperature during polymerization. At the beginning of the polymeri- A sample of the resin was screened through a zation (1), the temperature was between 70 and 2000-mm sieve. One hundred grams of this resin was 90C. When between 5% and 30% of the total mixed with the desired amount of lubricant at room amount of TFE had been polymerized (2), the tem- temperature by shaking it in a glass jar with a 6-cm perature was raised to 100e125C. After between inside diameter and rolling it for 4 min at 64 rpm. 20% and 80% of the total TFE had been polymer- It was then preformed at room temperature in a tube ized and after step (2) had been carried out, the 26 mm in diameter and 23 cm long at 2.76 MPa. The temperature was lowered by at least 30C from the preform was paste extruded at room temperature temperature used in step (2). through an oriﬁce with 2.4 mm in diameter into a uniform beading. Land length of the oriﬁce was A 36-L polykettle was charged with 17.7 kg of 5 mm. The extrusion speed was 84 cm/min and the demineralized water, 600 g of parafﬁn wax, 10 g of angle of the die was 30 degrees. The beading was succinic acid, 13 g of APFO (C-8) as a dispersing dried at 190C for 20 min. agent, and 0.15 g ZnCl2. The contents of the poly- kettle were heated to 65C, evacuated, and purged Stretch Test with N2. The contents of the polykettle were then agitated and the temperature was increased to 75C. A beading of resin was cut and clamped at each TFE was then added to the polykettle after evacua- end, leaving a space of 50 mm (ink-marked at the tion until the pressure reached 2.75 Â 106 Pa. Then center) between the clamps, and heated to 300C in a 120 mL of APS (1 g/L) was added at the rate of circulating air oven. The clamps were then moved 100 mL/min. apart at the desired rate to the desired length. The stretched specimen was examined for uniformity of After the polymerization began, as evidenced by stretch, even appearance, and surface roughness. The a drop in pressure, TFE was added to maintain the percentage of uniformity was calculated as follows. pressure at 2.75 Â 106 Pa. After 0.9 kg of TFE had

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52 EXPANDED PTFE APPLICATIONS HANDBOOK % uniformity of stretch ¼ 100 Â smaller distance from ink mark to beading edge after stretch 1 total length after stretch 2 Measurement of Stress Relaxation Malhotra’s work [81e83] also provided a process Time for preparing PTFE resins by polymerizing TFE, and optionally a small amount of a comonomer, in an To measure stress relaxation time, a specimen was aqueous medium in the presence of an initiator and a made, using the procedure outlined above for the nontelogenic anionic surfactant. This process consti- stretch test, by stretching a beading at 1000% per tuted a signiﬁcant departure from previous methods second to achieve a 2400% total stretch. Stress because the polymerization initiator was a redox relaxation time is the time it takes for a specimen to catalyst represented by the formula XMnO4. The break when heated at 393C in the extended condition. cation Xþ forms a water-soluble salt with the MnO4À For a short period of time after the specimen is placed anion. The cation X was hydrogen, ammonium, alkali in the oven the temperature drops somewhat, to metal, or alkaline earth metal. The XMnO4 was added 375C, and it takes about 1 min for the oven to return optionally as a precharge, either intermittently or to 393C. Stress relaxation time is measured from the continuously. As soon as the XMnO4 addition was time the test specimen is placed in the oven [80]. shut down, the reaction slowed. The reaction lasted 5e20% longer than a reaction in which an initiator Malhotra [81] set out to reduce the sensitivity of addition is continued to the end of the polymerization. PTFE ﬁne powder resin to lubricant content during paste extrusion. The characteristics of the resin he The most desirable initiator for this process was produced are as follows: potassium permanganate at a concentration of 1e25 ppm based on the aqueous phase. A reducing 1. The primary particle size is between 0.1 and agent, such as oxalic acid, was present to form a 0.5 mm and preferably between 0.15 and 0.3 mm. redox couple with the HMnO4. Oxalic acid can be added to the solution, but it is also formed in situ as a 2. The speciﬁc surface area is greater than 5 m2/g product of TFE oxidation. The reaction is generally and preferably greater than 10 m2/g. carried out in an acidic medium. Succinic acid is a common acid and is preferred because it also pre- 3. The SSG is less than 2.190 and preferably less vents coagulation. When the medium is acidic, the than 2.160. KMnO4 generally forms the acid HMnO4 in situ. Buffers may be used to control the pH. A complexing 4. The rheometric pressure (sometimes referred to agent for manganese, such as a phosphate, may be as extrusion pressure) is at least 24.5 MPa and added to prevent MnO2 from forming. preferably at least 34.3 MPa. In Malhotra’s process, a TFE monomer (and 5. The uniformity of stretch is at least throughout optionally a comonomer) is mixed with an aqueous a 4% band of variability of lubricant loading medium containing a dispersing agent and polymer- level within the loading level of 10e25% wt ization initiator. The polymerization temperature and at a stretch rate of 100% per second. pressure are not critical, provided the procedure described is followed. Temperatures high enough to 6. The uniformity of stretch is at least 75% decompose XMnO4 are desirable to obtain high throughout, at a stretch rate of between 10% molecular weight near the surface of the resin parti- and 100% per second at a lubricant loading cles formed. Ideally, the temperature is in the range level of 17%. of 65e100C, and pressure is between 2.4 and 3.9 MPa. Polymerization is carried out in a gently 7. The stress relaxation time is at least 400 s, stirred autoclave. measured at 393C. Some studies have reported using a temperature of 395C. After polymerization is complete, the batch is cooled and the wax is decanted off from the disper- These resins have an unusual lack of sensitivity to sion. Water is added to reduce the concentration of lubricant loading levels and high stress relaxation the dispersed polymer particles. The diluted mixture times (dwell times), and they can be stretched at low stretch rates even at high lubricant levels. Other PTFE resins do not possess this latter feature.

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 53 can be coagulated by high-speed agitation. Usually a acid redox couple. In some batches the ceric salt and compound such as ammonium carbonate is added to oxalic acid were added together as a precharge, and increase the ionic strength of the dispersion, thus in others one was added as a precharge and the other facilitating coagulation. When coagulation takes was added intermittently or continuously during the place more easily there is no need for harsh agitation polymerization. In the latter cases, the last addition that could possibly deform the PTFE particles. After was made before the end point so that the reaction coagulation the resin particles can be collected and slowed down and the end point occurred 20% later dried. As we have seen, these resins do not have than it would have in a reaction that did not slow excessive sensitivity to lubricant level and produce down. highly stretchable and uniform beads [81]. Yet another process proposed by Malhotra also In an example a 36-L polykettle was charged with produced highly stretchable PTFE resins [85]. In this 19.1 kg of demineralized water, 600 g of parafﬁn process the TFE polymerization took place at wax, 16 g of C-8, 0.15 g of ZnCl2, 1.0 g of oxalic 65e100C in an aqueous medium. A nontelogenic acid, and 3 g of succinic acid. The contents of the anionic surfactant was present in just enough quan- polykettle were heated to 70C, evacuated, and TFE tity to keep the PTFE particles in dispersed form. At purged. The contents of the polykettle were agitated least one polymerization initiator was required that at 46 rpm and temperature was increased to 80C. consisted of a redox couple of xBrO3/yHSO3 in TFE was then added until the pressure reached which x and y can be hydrogen, ammonium, alkali, or 2.75 MPa. A fresh solution of 3.2 g of Mn(C2H3-O- alkaline earth metal ions, and in which either the O)3 (manganese triacetate) dissolved in 1000 mL of bromate or the bisulﬁte is added as a precharge and a watereacetic acid mixture (1:1 by volume) was the other is added intermittently or continuously injected at the rate of 6.5 mL/min until 8.2 kg of during the polymerization. The last addition was TFE had been consumed in the polymerization re- made before the end point so that the reaction slowed action. Polymerization began 4 min after the start of down and the total reaction time was at least 40% the initiator injection, as evidenced by a drop in longer than a reaction that did not slow down. pressure. TFE was added to maintain the pressure at 2.75 MPa [84]. In yet another set of experiments, Attwood and Bridges polymerized ﬁne powder PTFE [86,87] in an After 0.9 kg of TFE had reacted, a solution of 42 g of aqueous medium in the presence of a surfactant as an C-8 in 1000 mL of water was pumped in at 50 mL/min. emulsifying agent and a water-soluble initiator sys- A total of 0.768 g of manganese salt was added. No tem comprised of disuccinic acid peroxide (DSAP) manganese triacetate was added after 67% of the TFE and ammonium sulphite (AMS). The amount of had been polymerized. The reaction lasted 58% longer DSAP added was in the range 0.005e0.02% wt based than if the manganese salt addition had continued until on the weight of water charged before the start of the the end. After 12.3 kg of TFE had reacted, the feed was polymerization reaction. The AMS was added to the stopped and the polykettle was vented, evacuated, and aqueous reaction medium after the start of polymer- purged with N2. The contents were cooled and dis- ization. The amount of AMS was in the range charged from the polykettle and the supernatant wax 25e300% wt, based on the weight of the disuccinic was removed. The dispersion was diluted to 15% solids acid peroxide used. In separate experiments AMS, and coagulated in the presence of ammonium carbon- hydroquinone, ferrous sulfate, and sodium meta- ate under high agitation conditions. The coagulated bisulﬁte were added at various conversions during the ﬁne powder was separated and dried at 150e160C polymerization. Each polymerization was terminated for 3 days. at approximately 30% solids contents. The resins produced by Malhotra’s procedure had In a differential scanning calorimeter, a 10 mg surprisingly high extrusion pressures and molecular sample of the resulting polymer was heated from 280 weights. The high molecular weight made these to 380C at 10C/min. The sample was cooled to resins useful in post-paste extrusion stretching oper- 280C at 10C/min and then reheated to 380C at ations. The stretch performance of these resins was 10C/min. The ratio of reheating and cooling peak excellent when attempts at manufacturing ePTFE heights R/C was measured to give an indication of were made [84]. molecular weight (the larger the number, the higher the molecular weight), as shown in Table 3.12. Similar results were obtained in another process that yielded highly stretchable PTFE resins using an The stretch test was run using a 200-g sample of initiator other than potassium permanganate [80]. polymer. The resin was sieved through an 8-mesh The polymerization initiator was a Ce(IV) salt/oxalic screen to which 43 g of Isopar H lubricant (inert

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54 EXPANDED PTFE APPLICATIONS HANDBOOK Table 3.12 Characteristics of Resins in Expanded Polytetraﬂuoroethylene Testing [86] Initiator Type Stretch Test Results Differential Scanning Standard Speciﬁc Disuccinic acid peroxide Calorimetry Test (R/C) Gravity (DSAP) Very roughdmany 2.157 DSAP þ ammonium translucent areas 1.00 2.149 sulphite (AMS) 1.25 Very smooth, entirely 2.166 DSAP þ sodium homogenous, did not 1.14 2.152 metabisulﬁte 0.97 DSAP þ ferrous sulfate break 2.148 1.05 DSAP þ hydroquinone Fibrillateddbroke into strands Produced discolored polymer when sintered; broke at draw ratio of 4:1 Somewhat lumpy hydrocarbon) was added. The mix was rolled for contents of the polykettle were heated to 55C then 25 min, then conditioned for 24 h at 25C. After cooled to 50C while agitating at 25 rpm. The poly- conditioning the mixture was sieved, extruded at kettle was then evacuated and purged with TFE. 20 mm/min (at 30C), with a reduction ratio of 100:1, through a 2.54-mm die. The extrudate was dried in a With the temperature at 50C, the agitator speed vacuum oven for 2 h at 100C, then baked for 3 min was increased to 46 rpm. TFE was then added until at 190C. The extrudate was then heated to 300C and drawn (6:1) in a tensiometer at 17% per second to the pressure reached 2.75 MPa. Then 0.65 part of a generate a porous product. A 2-cm sample length was fresh initiator solution of 0.00015 part of KMnO4 and used. The results, summarized in Table 3.12, show 0.00007 part of ammonium phosphate per part of that the best results were obtained by using a com- water were added at the rate of 0.22 part/minute (for a bination of disuccinic acid peroxide and AMS. total of 3 min addition time). When this addition was In 2000, Jones devised a process for making complete, a 3.2% wt solution of C-8 was added at the ePTFE-grade resin with improved productivity over rate of 0.022 part/min to the end of the batch, and earlier processes [88]. This process improved the emulsion polymerization of TFE to make PTFE and additional initiator solution was simultaneously modiﬁed PTFE. Jones initiated polymerization at a added at the rate of 0.014 part/min. TFE was added at lower temperature, such as a temperature not higher than 60C, and completed polymerization at a higher a rate sufﬁcient to maintain the pressure at 2.75 MPa. temperature in the presence of liquid saturated hy- With cooling applied to the reactor, the heat of the drocarbon (parafﬁn wax). It is preferable to initiate reaction increased the temperature to 55C about the polymerization at the lower temperature in the 10 min after the initiator began to be added. When absence of the parafﬁn wax. This process was particularly useful because it allowed production, 10 min had elapsed after the end of the ﬁrst initiator with reduced polymerization time, of PTFE that was addition, the polykettle was heated as rapidly as suitable for stretching operations. The PTFE resin possible so that the temperature reached 80C. At this produced by this process can also yield stretched product with superior mechanical properties. point approximately 18 min had elapsed after the start of initiator addition. In one example, a polykettle with a horizontal agitator and a water capacity of 100 parts by weight The temperature was then maintained at 80C for was charged with 51.6 parts of demineralized water, the remainder of the batch. After 12.3 parts of TFE 1.62 parts of parafﬁn wax, 0.24 part of a 20% wt solution of C-8, 0.027 part of a 2% wt oxalic acid had been added, following initial pressurizing with solution, and 0.0027 part of succinic acid. The TFE, the initiator solution addition was stopped. After 36.8 parts of TFE had been added, the TFE feed, the C-8 solution feed, and the agitator were stopped, and the polykettle was vented. The length of the reaction was 152 min. The contents were dis- charged from the polykettle and the supernatant wax was removed. Solids content of the raw dispersion was 44.1% wt and raw-dispersion particle size was

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 55 270 nm. The dispersion was diluted to 11% solids The sample jar is then placed in a 22C water bath for and coagulated in the presence of ammonium car- at least 2 h before extruding. bonate under high agitation conditions. The coagu- lated ﬁne powder was separated and dried at 150C The lubricated resin is paste extruded at a reduc- for three days. tion ratio of 100:1, at room temperature, through an oriﬁce (2.4 mm diameter, 5 mm land length, 30- Jones has described the procedures for the tests he degree entrance angle) into a uniform beading. The used to evaluate these PTFE resin samples [88] and extrusion, or ram, speed is 51 cm/min. The lubricant his original source [89], which are detailed below. is removed from the beading by heating at 230C for Table 3.13 summarizes the polymer properties and 30 min. shows higher break strength and lower creep rate than obtained in other processes, as seen in Conditions 1, A length of dry beading is cut and clamped at each 4, 5, and 6 versus 2. end, leaving a space of either 38 mm or 51 mm be- tween clamps depending on the purpose of the pro- Stretch Procedure cedure, and heated to 300C in a circulating air oven. The clamps are then moved apart at the desired rate A sample of the ﬁne powder resin is screened to a length corresponding to the desired total stretch, through a 2000-mm sieve. This resin (113.4 g) is and the specimen is examined for integrity. This mixed with 17.7% wt, based on combined weight of stretch procedure essentially follows a method dis- resin and lubricant at room temperature, in a glass jar closed in US Pat. No. 4,576,869 [81], although here with an inside diameter of 8.25 cm. The jar is then the extrusion speed is 51 cm/min instead of 84 cm/ closed and turned for 3 min on a vertically disposed min. “Stretch” is the increase in length, normally mixing wheel (horizontal axis) revolving at 14 rpm. expressed relative to original length. Table 3.13 Summary of Polymerization Conditions and Properties of Polytetraﬂuoroethylene Resins [88] Conditions and 1 2 3 4 5 6 Results 50 40 40 40 40 33 Polymerization 80 80 80 80 60 80 Starting T (C) 10 13 13 20 20 12 Completing T (C) 18 18 18 28 23 28 Time to 55C (min) 12.3 12.3 12.3 11.0 12.3 11.90 Time to comp. T (min) 33 36 33 34 45 26 Core TFE (part) 152 253 138 326 323 95 Core time (min) 36.8 36.8 36.8 36.8 30.7 34.0 Batch time (min) Total TFE (part) 270 252 246 249 236 250 3.6 e e e e 7.2 Properties 2.154 2.154 2.153 2.155 2.153 2.153 RDPS (nm) 54.3 59.3 53.2 65.3 62.3 35.6 MV (1010 Pa s) 3.25 2.79 2.70 3.23 3.18 3.18 SSG 723 644 640 702 716 718 Rheometer p (MPa) 0.078 0.224 e 0.076 0.072 0.048 Break strength (kgf) Stress relaxation time (s) Creep rate (minÀ1) SSG, standard speciﬁc gravity; TFE, tetraﬂuoroethylene.

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56 EXPANDED PTFE APPLICATIONS HANDBOOK Stress Relaxation Time The specimen for creep rate measurement was made by stretching a beading, as in the stretch pro- The specimen for the stress relaxation time test cedure above with 3.7 cm between clamps, at a measurement is made by stretching a beading, as in stretch rate of 1000% per second to a total stretch of the stretch procedure above, with 3.7 cm distance 2400%. A sample of stretched beading is tied be- between clamps, at a stretch rate of 1000% per sec- tween two glass rods with hooks at each end such that ond to a total stretch of 2400%. Both ends of this the length of beading that can be stretched under beading sample are tied to a ﬁxture so that there is a tension is 2.5 cm. The upper rod is 2.5 cm long and taut 20-cm span of beading. Stress relaxation time is about 0.8 mm in diameter. The lower rod is 15.2 cm the time it takes for this specimen to break after it is long, 4.76 mm in diameter, and weighs 6 Æ 0.5 g. placed in an oven at 390C, a temperature above the 380C melting point of the extended chain conﬁgu- The rod and beading assembly is inserted into the ration as outlined in US Pat. No. 5470655 [89]. The cavity of the melt indexer such that the top of the specimen, in its ﬁxture, is inserted into the oven beading is approximately 3.2 cm below the top of through a (covered) slot in the side of the oven so that the cavity and the longer rod extends out of the the temperature does not drop during placement of bottom of the apparatus. The mass of the lower rod the specimen and therefore does not require a minute keeps the sample from retracting quickly while the to recover as seen in US Pat. No. 4576869 [81]. specimen heats to the test temperature and before a heavier weight (below) is attached. The upper rod is Break Strength hung from a small PTFE ring that also serves to impede airﬂow up through the heated chamber. Then, The specimen for break strength test measurement as quickly as possible, a weight of 52 g is hung from is made by stretching a beading, as in the stretch the hook on the lower rod. The weight is made from procedure above, using 5.1 cm between clamps, at a 9.5-mm square steel rod stock 7.9 cm long, and on its stretch rate of 100% per second to a total stretch of top end the weight has an eyelet attached to a swivel 2400%. The break strength is the minimum tensile for hanging it from the lower glass rod. On one side break load (force) of three specimens taken from the are 10 machined transverse square notches 3.18 mm stretched beading, one from each stretched end of the wide and deep, separated by lands 3.2 mm wide. The beading (excluding neck-down if there is any in tops of the lands (the surfaces that are 3.2 mm by the region of the clamp), and one from the center. The 78.3 mm) are polished. measurement is made at room temperature using a tensile tester clamping the specimen in its jaws with An SSG specimen (above) is mounted with its ﬂat a 5.1-cm gauge length and driving the moving jaw at surfaces in a vertical plane and the cylindrical surface a speed of 30.5 cm/s. just barely touching the weight, thereby serving as a PTFE bearing to guide the weight and keep it in a Creep Rate desired orientation. A photoelectric sensor with an integral light source (eg, model PS2-61/PS-47, This test was designed to measure the tensile creep Keyence Corporation of America) is mounted fac- at 365C, a temperature above the melting point of ing the surface of the weight containing the polished the as-polymerized resin but below the 380C tops of the lands. The test begins with the light beam melting point of the extended chain conﬁguration directed at the lower edge of the lowest notch, and the according to US Pat. No. 5470655 [89]. The chamber signal from the sensor is recorded on a GC (gas used to provide the constant 365C temperature is a chromatograph) integrator as a function of time as the melt index apparatus without oriﬁce or retaining nut weight drops past the sensor. that conforms to ASTM D1238 standard dimensions. The temperature is measured by a thermometer (to A signiﬁcant signal is obtained when the beam of Æ1C) that is inserted into the top of the indexer light is incident on a polished land, and there is no cavity such that the bulb end is 8.9 cm from the top of signal (or low signal) when incident on a notch. The the cavity, and a collar is placed around the ther- light beam is focused to a small point so that the GC mometer to serve as a barrier to air ﬂow through integrator trace is very nearly a square wave. The test the cavity. signal is usually recorded beginning at a strain (DL/ Lo) of 0.125 and continuing to a strain of 2.5, where DL is the change of length and Lo is the initial length of the sample (2.5 cm). The slope of strain versus

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 57 time over the strain range from 0.625 to 1.5 is The pressure required for extrusion at that time calculated by a least squares method to obtain the was measured and taken as the extrusion pressure. strain rate in units of minÀ1, which is reported as The beading was dried at 230C for 30 min, tensile creep rate or merely creep rate. thereby removing the lubricant. The beading was then cut into a suitable length and each end was A later Jones patent describes a still further clamped so that the distance between the clamps improved PTFE resin that, as a ﬁne powder, was was either 3.7 cm or 5.1 cm. After heating at 300C suitable for use in stretching operations [90]. The resin in an air-circulating oven, the stretching was car- had SSG of 2.160, which indicates high average ried out at a predetermined rate until the distance molecular weight, as well as high rheometer pressure between the clamps reached a predetermined level. of at least 25 MPa at a reduction ration of 400:1, and This stretching method was essentially the same as stress relaxation time of at least 650 s. The PTFE resin the method disclosed in US Pat. No. 4,576,869, created under this patent was further characterized by except that the extrusion speed (51 cm/min) was superior mechanical properties, with a break strength different. “Stretching” is an increase in length and of at least 3.0 kgf at room temperature and/or a creep is usually represented in relation to the initial rate of no more than 0.1 minÀ1 at 365C. length. Break strength test measurement was made by Measurement of Tensile Break stretching a beading, as in the stretch procedure above, with 5.1 cm between clamps, at a stretch rate Strength of 100% per second to a total stretch of 2400%. The break strength was taken as the minimum tensile A sample for the tensile break strength test was break load (force) of three specimens taken from the prepared by stretching the beading as above, using a stretched beading, one from each stretched end of the clamp distance of 5.1 cm, a stretching rate of 100% beading (excluding neck-down if there was any in per second, and a total stretching of 2400%. The the region of the clamp) and one from the center. The tensile break strength was measured as the minimum measurement was made at room temperature using tensile break load (force) of three samples obtained an Instron tensile tester, clamping the specimen in its from a stretched beading, one from each end of the jaws with a 5.1-cm gauge length and driving the stretched beading (excluding any neckdown within moving jaw at a speed of 30.5 cm/s. the clamped range), and one sample from its center. The tensile break strength was measured at room In 2003, Asahi Glass Company reported [91] on temperature with a tensile tester (manufactured by A PTFE resins for expanded applications that had high & D Company) by clamping the sample in a jaw with stretchability with SSG 2.160. The procedures to a gauge length of 5.0 cm and driving a movable jaw evaluate the suitability of these PTFE resins are at a speed of 30 cm/min. described below. Measurement of the Endothermic Evaluation of Extrusion Pressure Ratio and Stretchability A differential scanning calorimeter was used to After 100 g of PTFE ﬁne powder were left to measure the thermal properties of the PTFE. A 10- stand at room temperature for at least 2 h, the ﬁne mg sample was heated to an initial temperature of powder was put into a glass bottle with an internal 200C for 1 min and then heated to 380C, at a rate of capacity of 900 cc. Then 21.7 g of a lubricant were 10C/min, to obtain a differential thermal curve. A added and these ingredients were mixed for 3 min. baseline was obtained by connecting the point of The resulting PTFE mixture was left to stand for 310C and the point of 350C on the differential 2 h in a constant-temperature oven at 25C and was thermal curve. The length of the line from the highest then paste extruded through an oriﬁce with a peak on the differential thermal curve to the baseline diameter of 2.5 cm, a land length of 1.1 cm, and an was designated as A. The length from the point on the entrance angle of 30 degrees. Under such condi- baseline 10C lower than the intersection point of tions at 25C, the reduction ratio (the ratio of the the line from the highest peak to the baseline to the inlet cross section to the outlet cross section of the differential thermal curve was designated as B. A die) to obtain a beading was 100, and the extrusion value of B/A was taken as the endothermic ratio. rate was 51 cm/min.

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58 EXPANDED PTFE APPLICATIONS HANDBOOK Measurement of the Stress highly water-soluble ﬂuorinated compounds with high volatility. The strength of the PTFE produced was Relaxation Time 32.0e49.0 N. SSG of the resin was 2.160. The average primary particle size of the PTFE A sample to measure the stress relaxation time was !150 nm. Stress relaxation time was 500 s, and it was also prepared by stretching a beading as above, had a break strength of 29.7 N or more. The PTFE was using a clamp distance of 3.7 cm, a stretching rate obtained by emulsion polymerization of TFE in the of 1000% per second, and a total stretching of presence of a ﬂuorinated surfactant with log(POW) of 2400%. This sample of stretched beading had a 3.4 or less. The log(POW) is the distribution coefﬁ- total length of 20 cm and was tightly tensioned by cient between 1-octanol and water. The log(POW) is ﬁxing both ends to ﬁxtures. The stress relaxation represented by log(P), in which P is the ratio of a time is the time it takes for breaking of an extended concentration of a ﬂuorinated surfactant in an octanol chain shape after being left to stand in an oven at phase to a concentration of a ﬂuorinated surfactant in a 390C, a temperature higher than the melting point water phase when an octanol/water (1:1) mixture of 380C, as described in US Pat. No. 5,470,655. containing a ﬂuorinated surfactant is phase separated. The sample attached to the ﬁxtures was inserted into an oven through a covered slot located on the Daikin’s method for producing PTFE com- side of the oven. Because the opening was covered prises the following steps: (1) feeding a ﬂuori- the temperature did not decrease during the nated surfactant with log(POW) of 3.4 or less, arrangement of the sample, and accordingly no water, and TFE in a polymerization vessel; (2) time was required to recover the temperature as feeding a redox initiator to the polymerization described in US Pat. No. 4,576,869. vessel to initiate emulsion polymerization of the TFE; and (3) recovering PTFE, wherein a total Tensile break strength of this PTFE was in the feed amount of the ﬂuorinated surfactant is range of 34.3e49.0 N. The extrusion pressure was in 1000e6000 ppm to ﬁnal PTFE yield. the range of 9.8e15.2 MPa. The resin also had an endothermic ratio of, at most, 0.10 as calculated by The ﬂuorinated surfactant is preferably one DSC analysis. SSG tends to decrease as the average selected from the group consisting of a com- molecular weight increases. This PTFE had a rela- pound represented by the general formula tively small SSG value and, accordingly, its average CF3OCF(CF3)CF2OCF(CF3)COOX (in this for- molecular weight was quite high. The SR of the mula X is a hydrogen atom, NH4, or an alkali extruded product exceeded 3000% and the stretched metal atom) and a compound represented by the product had excellent uniformity. The stress relaxa- general formula CF3CF2OCF2CF2OCF2COOX (X tion time was 730 s. being a hydrogen atom, NH4, or an alkali metal atom). In a 2004 patent, Asahi Glass Company described a process for manufacturing PTFE that Table 3.14 Standard Speciﬁcation Methods for was stretchable and had higher strength than any Different Polytetraﬂuoroethylene (PTFE) Product PTFE previously produced [92]. Their process Types consisted of polymerizing the TFE in an aqueous medium in the presence of a dispersant, a stabilizer, PTFE Product and a polymerization initiator. The polymerization initiator was a redox polymerization initiator Type ASTM Method ISO Method comprising a halogen acid salt YXO3 and a sulﬁte Z2SO3 wherein X is a chlorine atom, a bromine Granular D4894 12086-1 and atom, or an iodine atom; Y is a hydrogen atom, 12086-2 ammonium, an alkali metal, or an alkaline earth Fine powder D4895 metal; and Z is ammonium, an alkali metal, or an 12086-1 and alkaline earth metal. Dispersion D4441 12086-2 Daikin Corporation ﬁled a patent application in 12086-1 and 2012 that disclosed the use of alternative surfactants to 12086-2 APFO for manufacturing PTFE suitable for forming expanded ﬁlms. The alternative surfactants were

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 59 Table 3.15 Deﬁnition of Basic Properties of Fine Powder Polytetraﬂuoroethylene (PTFE) According to ASTM D4895 Property Deﬁnition Reference Bulk densitya ASTM Method Particle sizea Mass of 1 L of resin measured under the test conditions Melting characteristicsa D1895 Average particle size and distribution by sieving E11 Water contenta Standard speciﬁc gravity Heat of fusion and melting peak temperature of resin as D4591 (SSG)b determined by differential scanning calorimetry D792, D1505 Thermal instability index (TII)a,b Water present in the PTFE resin D638 Tensile propertiesa,b Speciﬁc gravity of a sample of molded and sintered D792, D1505 PTFE according to this method Extrusion pressurea A measure of decrease in molecular weight of PTFE Stretch void indexa,b (SVI) material determined by the difference between ESG and Strained speciﬁc gravity SSG: TII ¼ (ESG À SSG) Â 1000 Untrained speciﬁc gravity Elongation and strength at break of a sample made according to the speciﬁed method Shrinkage and growth The pressure measured while extruding a paste of ﬁne Extended speciﬁc gravity powder PTFE made with an iso-parafﬁn under speciﬁed (ESG) conditions A measure of change in speciﬁc gravity of a PTFE specimen as a result of being subjected to tensile strain Speciﬁc gravity of a PTFE specimen after being subjected to tensile strain Speciﬁc gravity of a PTFE specimen before being subjected to tensile strain The change in the diameter of SSG preform due to sintering The speciﬁc gravity of a PTFE specimen molded for SSG after sintering for an extended period of time, compared to the sintering time of SSG SVI ¼ (Unstrained speciﬁc gravity À Strained speciﬁc gravity) Â 1000. Unstrained speciﬁc gravity is measured on a tensile specimen prior to straining it. The strained speciﬁc gravity is measured on a sample of PTFE after it has been strained to break at a strain rate of 5.0 mm/min. The break elongation of the PTFE specimen must be greater than 200% or the experiment is repeated. The two speciﬁc gravity values are used to calculate SVI. a Properties required for resin speciﬁcations. b Properties required for molded speciﬁcations. 3.9 Fine Powder (Coagulated Coagulation requires diluting the raw dispersion to Dispersion) Products a polymer concentration of 10e20% wt and possibly adjusting the pH to neutral or basic. A coagulating Three processing steps are necessary to produce agent, such as a water-soluble organic compound or ﬁne powder from the polymerization dispersion. inorganic salt or acid, can be added to the dispersion. Examples of water-soluble organic compounds 1. Coagulation of the colloidal particles include methanol and acetone. Inorganic salts, such 2. Separation of the agglomerates from the as potassium nitrate and ammonium carbonate, and inorganic acids like nitric acid and hydrochloric acid aqueous phase can all aid coagulation. The diluted dispersion is then 3. Drying of the agglomerates agitated vigorously. Primary PTFE particles form

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60 EXPANDED PTFE APPLICATIONS HANDBOOK Table 3.16 Fine Powder Polytetraﬂuoroethylene Resin Speciﬁcations According to ASTM D4895 Type Bulk Density (g/L) Average Particle Tensile Strength Break Elongation I 550 Æ 150 Size Diameter (mm) (MPa) (%) II 550 Æ 150 500 Æ 200 19 200 1050 Æ 350 19 200 Table 3.17 Molded Fine Powder Polytetraﬂuoroethylene Speciﬁcations According to ASTM D4895 Type Grade Class Standard Speciﬁc Extrusion Pressure Maximum Stretch Void I 1 Aa Gravity Range (MPa) Index 2.14e2.18 5e15 e Ba 2.14e2.18 15e55 e Ca 2.14e2.18 15e75 e I 2 Aa 2.17e2.25 5e15 e Ba 2.17e2.25 15e55 e Ca 2.17e2.25 15e75 e Ia 3 Cb 2.15e2.19 15e75 200 Db 2.15e2.19 15e65 100 Ea 2.15e2.19 15e65 200 Ia 4 Bb 2 14e2.16 15e55 50 IIa e Aa 2.14e2.25 5e15 NA a Thermal instability index less than 50. b Thermal instability index less than 15. agglomerates, which are isolated by skimming or 3.10 Characterization of ﬁltration. Polytetraﬂuoroethylene The PTFE agglomerates are dried by vacuum, This section discusses the characterization of ﬁne high frequency, or by heated air such that the wet powder (coagulated) and dispersion PTFE products. powder is not excessively ﬂuidized. Friction or con- A number of properties have to be measured to tact between the particles, especially at a high tem- characterize and identify each PTFE resin. The basic perature, adversely affects the ﬁne powder because it properties of PTFE are established by standard test so easily ﬁbrillates and loses its particulate structure, methods (Table 3.14) published by ASTM. Three leading to poor properties in parts made from the major methods specify types and deﬁne properties resin. Drying temperatures range from 100 to 180C for granular, ﬁne powder, and dispersion products. and have great inﬂuence on the paste extrusion of the The International Standards Organization (ISO) resin. High drying temperatures result in high publishes another, similar set of standards covering extrusion pressures. ﬂuoropolymers. Fine powder resins must be protected from ﬁbril- Fine Powder lation after drying. PTFE does not ﬁbrillate below its Polytetraﬂuoroethylene Resins transition point (19C for TFE homopolymers) dur- ing normal handling and transportation. Storage and ASTM Method D4895 covers speciﬁcations for transportation of the resin after refrigeration below ﬁne powder PTFE resins and test methods for the its transition point are normal commercial practices for handling ﬁne powder PTFE resins.

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 61 Table 3.18 Deﬁnition of Basic Properties of Dispersions of Polytetraﬂuoroethylene (PTFE) According to ASTM D4441 Property Deﬁnition Reference ASTM Method Solids contenta The amount of PTFE in the dispersion as weight% Surfactant contenta D4441 Surfactant added to the dispersion plus the remaining D4441 polymerization surfactant D4441 Dispersion particle size Particle size measured in the presence of added surfactant D4441 D4441 Raw dispersion particle size Particle size measured in the absence of added surfactant E70 Coagulated polymer PTFE that has coagulated as a result of handling and D4441, D792 processing of dispersion D4441, D4591 pH Acidity/alkalinity of the dispersion Standard speciﬁc gravity Speciﬁc gravity of a sample of molded and sintered PTFE isolated from the dispersion according to this method Melting characteristics Heat of fusion and melting peak temperature of resin as determined by differential scanning calorimetry a Properties required for dispersion speciﬁcation. Table 3.19 Dispersion Polytetraﬂuoroethylene Classiﬁcation According to ASTM D4441 ASTM Type Solids Content (wt%) Surfactant Content (wt%) 0.5e1.5 I 23e27 1e5 1e5 II 25e35 6e10 2e4 III 53e57 4e8 6e10 IV 58e62 5e9 e V 57e63 VI 58e62 VII 54e58 VIII 56e60 XIa 20e45 a No surfactant is added to stabilize. Addition of a hydrocarbon oil is optional. as-produced polymer. PTFE does not dissolve in any extrude for 5 min. The extruder is equipped with a solvents. Consequently, direct measurement of PTFE hydraulic system and a ram with an inside diameter of molecular weight is not possible. An indirect prop- 32 mm, capable of pushing the lubricated PTFE paste erty, standard speciﬁc gravity (SSG), is a proxy for out of a small die. There should also be an appropriate molecular weight. SSG is the relationship between pressure-sensing device. The die oriﬁce size is molecular weight and SSG. Properties used in the selected to obtain barrel-to-oriﬁce cross-sectional ra- characterization of ﬁne powder PTFE are deﬁned in tios of 100:1, 400:1, and 1600:1, called the reduction Table 3.15. ratio. The choice of reduction ratio depends on the resin type. The equipment temperature is maintained Extrusion pressure is determined in a paste at 30C during the extrusion. The agreed upon rate of extruder, also called a rheometer. It consists of a extrusion is 19 g/min on a dry resin basis. vertically positioned breech-loaded tubular barrel with an inside diameter of 32 mm. The barrel is approxi- The lubricant is an isoparafﬁn, also called an mately 305 mm long, though the length is not critical extrusion aid, and should be blended with the resin at as long as the barrel can hold enough resin preform to a prescribed ratio. The mixture is placed in a jar and

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62 EXPANDED PTFE APPLICATIONS HANDBOOK blended by rolling. There are alternative techniques [4] S.V. Gangal, P.D. Brothers, Perﬂuorinated to bottle rolling. After blending, the jar and its con- polymers, perﬂuorinated ethylene-propylene tent are stored at 30C for 2 h or longer to allow the copolymers, in: Encyclopedia of Polymer Sci- lubricant to diffuse to the inside surface of the ence and Technology, online ed., John Wiley & polymer particles. The preform is made by molding Sons, New York, 2010. the blend in a tube with an inside diameter of 32 mm and a length of 610 mm. The lubricated blend is [5] B.J. Humiston, Phys. Chem. 23 (1919) 572e577. poured into the tube and is pushed down using a plug- [6] O. Ruff, O. Bretschneider, Z. Anorg. Chem. 210 in press at a pressure of 0.07 MPa. The preform is loaded in the extruder barrel prior to extrusion, dur- (1933) 73. ing which pressure is recorded. [7] M.W. Farlow, US Patent 2,709,192, Assigned to Table 3.16 summarizes the speciﬁcations that DuPont, May 24, 1955. differentiate ﬁne powder PTFE powders. Water [8] M.W. Farlow, E.L. Muetterties, US Patent content and melting peak temperatures are the other speciﬁcations of these resins. Table 3.17 lists speci- 2,732,411, Assigned to DuPont, January 24, ﬁcations for molded parts. 1956. [9] A.C. Knight, US Patent 3,210,430, Assigned to Dispersions of DuPont, October 5, 1965. Polytetraﬂuoroethylene [10] M.M. Vecchio, S.G. Carraro, I. Cammarata, Assigned to Montecatini Edison, November 21, ASTM Method D4441 covers speciﬁcations for 1972. dispersions of PTFE and test methods for the PTFE [11] B.S. Malone, US Patent 4,973,773, Assigned to dispersion. These dispersions are converted into DuPont, November 27, 1990. coatings or used as precursors for compounding by a [12] J.L. Webster, US Patent 5,633,414, Assigned to co-coagulation method. PTFE resins are thermo- DuPont, May 27, 1997. plastics in that they can be remelted, but they cannot [13] J.D. Park, et al., Ind. Eng. Chem. 39 (1947) 354. be processed by the normal melt processing tech- [14] J.M. Hamilton, in: M. Stacey, J.C. Tatlow, nologies due to their extremely high rheology. This A.G. Sharpe (Eds.), Advances in Fluorine polymer does not dissolve in any solvents. These two Chemistry, vol. 3, Butterworth, Kent, 1963, p. facts render direct measurement of PTFE molecular 117. weight virtually impossible and, as seen above, an [15] J.W. Edwards, P.A. Small, Nature 202 (1964) indirect property called SSG is substituted for mo- 1329. lecular weight. Section 4.9.1 discusses the relation- [16] F. Gozzo, C.R. Patrick, Nature 202 (1964) 80. ship between molecular weight and SSG. [17] M. Hisazumi, H. Shingu, Japanese Patent 60 15,353. Table 3.18 lists properties that characterize dis- [18] O. Scherer, et al., US Patent 2,994,723, Assigned persions of PTFE. ASTM Method D4441 classiﬁes to Farbwerke Hoechst Aktiengellscaft, August 1, the different types of dispersions according to the 1961. system summarized in Table 3.19. SSG and melting [19] J.W. Edwards, S. Sherratt, P.A. Small, Brit. characteristics of PTFE in dispersions can both be Patent 960,309, Assigned to ICI, June 10, 1964. measured by ASTM D4895. The polymer has to be [20] H. Ukahashi, M. Hisasne, US Patent isolated from the dispersion by coagulation, ﬁltra- 3,459,818, Assigned to Asahi Glass Co., tion, and drying. Consult ASTM Method D4441 for a August 5, 1969. complete description of these procedures. [21] F.B. Downing, A.F. Benning, R.C. McHarness, US Patent 2,384,821, Assigned to DuPont, References September 18, 1945. [22] J.W. Edwards, A.F. Benning, S. Sheratt, [1] C. Chabrie, Compt. Rend. 110 (1890) 279. P.A. Small, US Patent 3,308,174, Assigned to [2] H. Moissan, Compt. Rend. 110 (1890) Imperial Chemical Industries, March 7, 1967. [23] J.W. Edwards, et al., British Patent 960,309, 276e279. Assigned to ICI, June 1964. [3] H. Moissan, Compt. Rend. 110 (1890) 951e954. [24] R.H. Halliwell, US Patent 3,306,940, Assigned to DuPont, February 28, 1967.

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3: MANUFACTURING POLYTETRAFLUOROETHYLENE BY EMULSION POLYMERIZATION 63 [25] P.B. Chinoy, P.D. Sunavala, Thermodynamics [40] 3M/DYNEON Company Progress Reports for and kinetics for the manufacture of tetraﬂuoro- 2009 Submission Under the EPA 201012015 ethylene by the pyrolysis of chlorodiﬂuoro- PFOA Stewardship Program, October 30, 2010. methane, Ind. Eng. Chem. Res. 26 (1987) www.epa.gov/opptintr/pfoa/pubs/stewardship/ 1340e1344. preports4.html#2010. [26] Z.Y. Li, Chinese Patent, Assigned to Tianjin Tai [41] S. Ebnesajjad, Fluoroplastics vol. 2, Melt Pro- Xu Logistics, Ltd, Publication Number cessible Fluoropolymers, second ed., Elsevier, CN101973843A, Application Number CN 2015. 201010508707, February 16, 2011. [42] M.M. Renfrew, US Patent 2,534,058, Assigned [27] S. Ebnesajjad, Fluoroplastics vol. 1, Non-melt to DuPont, December 12, 1950. Processible Fluoropolymers, second ed., Elsevier, 2014. [43] K.C. Brinker, M.I. Bro, US Patent 2,965,595, Assigned to DuPont, December 20, 1960. [28] A. Zaggia, B. Ameduri, Recent advances on synthesis of potentially non-bioaccumulable [44] K.L. Berry, US Patent 2,559,752, Assigned to ﬂuorinated surfactants, Curr. Opin. Colloid DuPont, July 10, 1951. Interface Sci. 17 (2012) 188e195. [45] S.G. Bankoff, US Patent 2,612,484, Assigned to [29] G. Kostov, F. Boschet, B. Ameduri, Original DuPont, September 30, 1952. ﬂuorinated surfactants potentially non- bioaccumulable, J. Fluorine Chem. 130 (2009) [46] A.J.Cardinal,W.L.Edens,J.W.VanDyk,USPatent 1192e1199. 3,142,665, Assigned to DuPont, July 28, 1964. [30] M. Peschka, N. Fichtner, W. Hierse, P. Kirsch, [47] D.A. Holmes, E.W. Fasig, US Patent 3,819,594, E. Montenegro, M. Seidel, et al., Chemosphere Assigned to DuPont, June 25, 1974. 72 (2008) 1534e1540. [48] R.V. Poirier, US Patent 4,036,802, Assigned to [31] J.L. Howell, E.W. Perez, US2003/0073588 A1, DuPont, July 19, 1977. Assigned to Du Pont de Nemours, 2003. [49] T. Shimizu, K. Hosokawa, US Patent 4,840,998, [32] L. Han, Y. Zhang, H. Li, L. Li, Colloids Surf. A: Assigned to Daikin Industries Ltd., June 20, Physicochem. Eng. Aspects 34 (2009) 176e180. 1989. [33] S.V. Kostjuk, E. Ortega, F. Ganachaud, [50] R.A. Morgan, C.W. Stewart, US Patent B. Ameduri, B. Boutevin, Macromolecules 42 4,952,630, Assigned to DuPont Co., August 28, (2009) 612e619. 1990. [34] N. Durand, D. Mariot, B. Ameduri, B. Boutevin, [51] R.A. Morgan, C.W. Stewart, US Patent F. Ganachaud, Langmuir 27 (2009) 4057e4067. 4,952,636, Assigned to DuPont Co., August 28, 1990. [35] W. Schwertfeger, K. Hintzer, E. Obermaier, Eur Patent 2006/093885 A1, Assigned to 3M, 2006. [52] T. Kitahara, K. Hosokawa, T. Shimizu, US Patent 6,503,988, Assigned to Daikin Industries, [36] K. Hintzer, G. Moore, T. Zipples, H. Kaspar, January 7, 2003. WO2007/140112, Assigned to 3M/Dyneon, 2007. [53] C.W. Jones, W.T. Krakowiak, US Patent 6,841,594, Assigned to DuPont Co, January 11, [37] G. Boutevin, D. Tiffes, C. Loubat, B. Boutevin, 2005. B. Ameduri, New ﬂuorinated surfactants based on vinylidene ﬂuoride telomers, J. Fluorine [54] R.J. Cavanaugh, C.W. Jones, K. Konabe, D.N. Chem. 134 (2012) 77e84. Levy, P.A.F. Thomas, T.A. Treat, US Patent 6,956,078, Assigned to DuPont Co., October 18, [38] D. Murai, T. Enokida, S. Murata, US Patent 2005. 8,148,573, Assigned Unimatec Co., April 3, 2012. [55] C.W. Jones, W.T. Krakowiak, US Patent 7,612,139, Assigned to DuPont Co., November [39] S.C. Gordon, Toxicological evaluation of 3, 2009. ammonium 4,8-dioxa-3H-perﬂuorononanoate, a new emulsiﬁer to replace ammonium per- [56] R.S. Buckanin, L.S. Tan, E.S. McAlister, US ﬂuorooctanoate in ﬂuoropolymer manufacturing, Patent 6,822,059, Assigned to 3M Innovative Regul. Toxicol. Pharm. (September 2010). Sci- Properties Company, November 23, 2004. enceDirect, Elsevier. [57] R.S. Buckanin, L.S. Tan, E.S. McAlister, US Patent 6,833,418, Assigned to 3M Innovative Properties Company, December 21, 2004.

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64 EXPANDED PTFE APPLICATIONS HANDBOOK [58] R.S. Buckanin, L.S. Tan, US 7,045,571, Assigned [72] C.U. Kim, J.M. Lee, S.K. Ihm, Emulsion poly- to 3M Innovative Properties Company, May 16, merization of tetraﬂuoroethylene: effects of re- 2006. action conditions on particle formation, J. Fluorine Chem. 96 (1999) 11e21. [59] R.S. Buckanin, L.S. Tan, E.S. McAlister, US Patent 7,199,196, Assigned to 3M Innovative [73] R.W. Gore, US Patent 3,953,566, Assigned to Properties Company, April 3, 2007. W.L. Gore, April 27, 1976. [60] S. Higuchi, Y. Matsuoka, S. Kobayashi, US [74] US Patent RW Gore, 3,962,153, Assigned to Patent 8,575,287, Assigned to Asahi Glass Co., W.L. Gore, June 8, 1976. November 5, 2013. [75] US Patent RW Gore, 4,187,390, Assigned to [61] S. Kasai, M. Ono, T. Yamanaka, Y. Sawada, US W.L. Gore, February 5, 1980. Patent 7,820,775, Assigned to Daikin Industries, October 26, 2010. [76] D.A. Holmes, US Patent 4,016,345, Assigned to DuPont Co., April 5, 1977. [62] K. Hintzer, M. Yurgens, G.G.I. Moore, T. Zipples, H. Kaspar, H. Koenigsmann, K.H. [77] S. Koizumi, S. Ichiba, T. Simizu, C. Okuno, T. Lochhaas, A.R. Maurer, W. Schwertfeger, L. Kadowaki, K. Yamamoto, US Patent 4,159,370, Mayer, M.C. Dadalas, J.F. Schulz, R.M. Flynn, Assigned to Daikin Kogyo Co., November 11, US Patent 7,671,112, Assigned to 3M Innovative 1977. Properties Company, March 2, 2010. [78] T. Shimizu, S. Koizumi, US Patent 4,363,900, [63] K. Hintzer, M. Yurgens, G.G.I. Moore, Assigned to Daikin Kogyo Co., December 14, 1982. T. Zipples, H. Kaspar, H. Koenigsmann, K.H. Lochhaas, A.R. Maurer, W. Schwertfeger, L. [79] S.C. Malhotra, US Patent 4,530,981, Assigned to Mayer, M.C. Dadalas, J.F. Schulz, R.M. Flynn, DuPont Co., July 23, 1985. US Patent 8222322, Assigned to 3M Innovative Properties Company, March 2, 2010. [80] S.C. Malhotra, US Patent 4,654,406, Assigned to DuPont Co., March 31, 1987. [64] K. Hintzer, H. Kaspar, A.R. Maurer, W. Schwertfeger, T. Zipples, US Patent 7776946, [81] S.C. Malhotra, US Patent 4,576,869, Assigned to Assigned to 3M Innovative Properties Company, DuPont Co., March 18, 1986. March 2, 2010 Assigned to 3M Innovative Properties Company, August 17, 2010. [82] S.C. Malhotra, US Patent 4,640,955, Assigned to DuPont Co., February 3, 1987. [65] US Patent 7,973,127, S. Higuchi, Y. Matsuoka, S. Kobayashi, assigned to Asahi Glass Company, [83] S.C. Malhotra, US Patent 4,725,644, Assigned to July 5, 2011. DuPont Co., February 16, 1988. [66] P.D.Brothers,S.V.Gangal,USPatentUS7932333, [84] S.C. Malhotra, US Patent 4,639,497, Assigned to Assigned to DuPont Co, April 26, 2011. DuPont Co., January 27, 1987. [67] P.D.Brothers,S.V.Gangal,USPatentUS7705074, [85] S.C. Malhotra, US Patent 4,748,217, Assigned to Assigned to DuPont Co., April 27, 2010. DuPont Co., May 31, 1988. [68] P.D. Brothers, S.V. Gangal, US Patent US [86] T.E. Attwood, R.F. Bridges, US Patent 8519072, Assigned to DuPont Co, August 27, 4,766,188, Assigned to Imperial Chemical In- 2013. dustries, August 23, 1988. [69] F.J. Rahl, M.A. Evanco, R.J. Fredericks, [87] T.E. Attwood, R.F. Bridges, US Patent A.C. Reimschuessel, Studies of the morphology of 4,921,922, Assigned to Imperial Chemical In- emulsion-grade polytetraﬂuoroethylene, J. Polym. dustries, May 1, 1990. Sci. Part A-2: Polym. Phys. 10 (1972) 1337e1350. [88] C.W. Jones, US Patent 6,136,933, Assigned to [70] T. Seguchi, T. Suwa, N. Tamura, M. Takehisa, DuPont Co., October 24, 2000. Morphology of polytetraﬂuoroethylene pre- pared by radiation-induced emulsion polymeri- [89] K. Hirai, US Patent 5,470,655, Assigned to Japan zation, J. Polym. Sci. Polym. Phys. Ed. 12 (1974) Gore-Tex, November 28, 1995. 2567e2576. [90] C.W. Jones, US Patent 6,177,533, Assigned to [71] B. Luhmann, A.E. Feiring, Surfactant effects in DuPont Co., January 23, 2001. polytetraﬂuoroethylene dispersion polymeriza- tion, Polymer 30 (1989) 1723e1732. [91] S. Kobayashi, J. Hoshikawa, K. Kato, H. Kamiya, H. Hirai, US Patent 6,518,381, Assigned to Asahi Glass Co., February 11, 2003. [92] S. Kobayashi, J. Hoshikawa, K. Kato, H. Kamiya, H. Hirai, US Patent 6,822,060, Assigned to Asahi Glass Co., November 23, 2004.

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4 Fabrication and Processing of Fine Powder Polytetraﬂuoroethylene OUTLINE 4.1 Introduction 65 4.9 Extrusion of Tubing 85 4.2 Background 65 4.9.1 Blending Lubricant 87 and Pigment and Preforming 87 4.3 Paste Extrusion Fundamentals 90 66 4.9.2 Extrusion of Spaghetti Tubing 4.4 Resin Handling and Storage 90 68 4.10 Unsintered Tape 4.5 Extrusion Aid or Lubricant 91 71 4.10.1 Blending Lubricant 92 4.6 Blending the Resin With Lubricant and Pigment and Preforming 92 4.6.1 Pigment Addition 94 74 4.10.2 Extrusion of Round and 4.7 Preforming 94 77 Rectangular Bead 95 4.8 Extrusion Equipment and Process 96 4.8.1 Extruder 77 4.10.3 Calendaring 4.8.2 Die 4.10.3.1 Calendaring Equipment 4.8.3 Drying 4.8.4 Sintering and Cooling 79 4.10.3.2 Calendaring Operation 4.8.5 Reduction Ratio 80 4.10.4 Stretching the Polytetraﬂuoroethylene 81 Tape 83 4.10.5 Final Tape Product 83 References 84 4.1 Introduction fraction of a millimeter to almost a meter, with wall thicknesses from 100 mm to a few millimeters. Rods Anyone working in expanded polytetraﬂuoro- up to 5 cm in diameter can be produced, calendared, ethylene (PTFE) technologies must have an in-depth and expanded to produce tapes. Unsintered low- understanding of paste extrusion. All parts are pro- density tapes such as thread sealant tapes are manu- duced by paste extrusion of PTFE followed by factured by stretching a paste-extruded rod or thick expansion. The process of coating wire and cable PTFE sheet. conductors with PTFE to insulate them has been described at some length to illustrate the intricacies 4.2 Background of paste extrusion. In all paste extrusion and expan- sion, PTFE ﬁbrils play a critical role in holding the Fine powder PTFE is unique; it is highly crystal- shape of parts and imparting strength to them. line (96e98%) and has a high molecular weight. The crystalline form of PTFE changes from a triclinic to a This chapter discusses the use of coagulated hexagonal lattice at 19C. Above this temperature, dispersion polymerized tetraﬂuoroethylene, known ﬁne powder PTFE becomes softer and more as ﬁne powder or coagulated dispersion powder malleable, which is important to its processing. (Fig. 4.1), to fabricate different shapes and articles. Modiﬁed resinsdthat contain a small amount of The most common commercial forms made from ﬁne comonomerdhave lower transition temperatures. powder PTFE include rods, tapes, wire insulation, tubes, and sheets. Tube diameters range from a Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00004-3 65 Copyright © 2017 Elsevier Inc. All rights reserved.

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66 EXPANDED PTFE APPLICATIONS HANDBOOK operation. For a given extruder barrel, the smaller the cross section of the ﬁnal product, the higher the RR will be. PTFE resins must be able to undergo the reduction during the extrusion. Resin suppliers have developed different ﬁne powder grades to accom- modate the wide range of RRs necessary for com- mercial processes. Figure 4.1 Close-up view of individual polytetra- 4.3 Paste Extrusion Fundamentals ﬂuoroethylene particles and a few ﬁbrils extending The structure of the individual particles shown in Fig. 4.2 is critical to paste extrusion of ﬁne powder among multiple particles. PTFE. In each single polymer particle, chains are packed in a crystalline phase. The orderly packing of Courtesy: DuPont Company. completely linear polymer chains [5] takes place during the polymerization, monomer by monomer or Transition to phase I takes place at 30C. In phase I, it brick by brick. This is why nearly perfect crystal- has the same 15 carbon helical conformation as it has linity is achieved despite very high molecular at 19C, but molecules increase their rotational weights. The chains fold after reaching a certain orientation about their long axis [1]. Because this length, which creates areas with less order at the change further softens the PTFE, ﬁne powder PTFE point where the molecule bends [6]. Fig. 4.3 shows is processed above 30C [14]. the “accordion” structure of PTFE which has been melted and then cooled very slowly. Reducing the Because it does not ﬂow after melting, ﬁne powder cooling rate increases not only the extent of crystal- PTFE is fabricated using a technology adopted from lization but also the size of the crystals (Fig. 4.3). ceramic processing called paste extrusion. In paste extrusion, PTFE powder is ﬁrst blended with a hy- One of the characteristics of PTFE crystals is that drocarbon lubricant (hence the term paste) which they are loosely packed because of the relatively low acts as an extrusion aid. This mixture is then formed Van der Waals attraction forces between the chains into a cylindrical preform at a fairly low pressure (in contrast to other polymers such as polyethylene). (1e3 MPa) and placed inside the barrel of a ram The transition from a triclinic to a hexagonal cell unit extruder, where it is forced through a die at a constant corresponds to 1.3% increase in volume [7]. Chains ram rate. The extrudate is passed through multiple can thus be removed from the surface of particles by ovens where it is dried, then sintered, and ﬁnally the application, above the transition temperature, of a cooled. The lubricant was originally removed by extraction in a hot solvent bath [2]. Structure of one particle In paste extrusion it is essential that, up to the 96-98% crystalline point of sintering and coalescence, the extrudate possess sufﬁcient strength to withstand the extensive Figure 4.2 Structure of one polytetraﬂuoroethylene handling that takes place during the process. The ﬁne powder particle. tendency of PTFE ﬁne powders to ﬁbrillate (form a web of strong ﬁlaments between particles) when extruded provides the necessary strength and the unique characteristics of ﬁne powder articles. Reduction ratio (RR) is deﬁned as the ratio of the cross-sectional surface areas of the preform to the extrudate. RR is an important variable in paste extrusion as it impacts the pressure during the

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 67 Figure 4.3 Crystalline structure of polytetraﬂuoroethylene cooled from 380C; (A) at 0.12C/min, and (B) at 0.02C/min, bar ¼ 1 mm [15]. fairly small force. Decreasing the temperature in- direction(s). In the industry parlance, these particles creases the shear force required for chain pullout. are referred to as abused. They will not be able to The chains abstracted from the particle are called ﬁbrillate properly during the processing of the resin ﬁbrils. The ease with which chains can be pulled out and will appear as defects in the ﬁnal product. Hose at higher temperatures is the main reason that ﬁne leakage and wire spark out are typical examples of powder PTFE is handled, stored, and transported abused particles. They occur at the point where below its transition point. abused particles are located. PTFE ﬁbrils are relatively wide and thin in cross Figure 4.4 Fibrilation of ﬁne powder polytetraﬂuoro- section, with a maximum width around ethylene in an extruded and calendared tape (freeze 100 nmdwhich is about the same as the diameter of fractured). the crystalline particles. The minimum width may be Courtesy: DuPont Company. one or two molecular diameters or in the range of 0.5 or 1 nm. Fibril lengths as high as 23 mm have been reported. Fig. 4.4 shows four SEM micrographs taken from a cross section of a paste-extruded and calen- dared unsintered tape. The particles more or less preserve their round shape and do not appear signiﬁ- cantly deformed. The micrographs taken at 1000 to 30,000 times magniﬁcation exhibit extensive ﬁbrilla- tion in the machine directiondthat is, in the direction of extrusiondconnecting the particles extensively and conferring strength to the tape in this direction. Some deformation of the round particles can be observed, due to the calendaring that the tape has undergone. The changed melting behavior of the paste mate- rial after extrusion is a proof that a reversible defor- mation has taken place, as illustrated in Fig. 4.5. The differential scanning calorimetry diagram shows a uniform melting peak for the original and a “bimodal” melting peak for the extruded paste ma- terial [17]. The crystalline structure has obviously been changed through the kneading. The original orderly alignment of chains has been partly destroyed. A resin that has been subjected to shear stress prematurely contains ﬁbrillated particles in arbitrary

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68 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 4.5 DSC diagram: on the left, unkneaded (original from the drum) and on the right, extruded polytetra- ﬂuoroethylene paste; both samples are unsintered [17]. The principles for ﬁne powder processing can be the lubricant content, the lower the extrusion summarized as follows: pressure will be (Fig. 4.7). Extrusion pressure is a function of several vari- Fine powder PTFE is sensitive to mechanical ables, as follows: shear, especially above its 19C transition point. Resin type (Fig. 4.8) Shear stress causes ﬁbrillation of ﬁne powder RR (Fig. 4.7) particles, which is the removal of a group of Lubricant content (Fig. 4.7) chains of PTFE from the crystalline phase Lubricant type (Fig. 4.6). The pressure that the extruder applies Die cone angle length to the PTFE paste generates the shear force. Die land length Resin ﬁbrillation increases with extrusion Extrusion speed pressure. Temperature All transportation, storage, and handling of the 4.4 Resin Handling and Storage powder must take place below its 19C transi- tion temperature. Fine powder PTFE is susceptible to shear damage, particularly above its transition point (19C). Paste extrusion should take place above the Handling and transportation of the containers could 30C transition temperature of the polymer. easily subject the powder to sufﬁcient shear rate to spoil it if the resin temperature is above transition A hydrocarbon lubricant is added to PTFE to aid point. A phenomenon called ﬁbrillation occurs when in processing and is removed prior to sintering particles rub against a surface including other parti- the article. cles surfaces. Fibrils are pulled out of the surface of PTFE particles. Uncontrolled ﬁbrillation must be The extrudate develops strength in the direction prevented to insure good quality production from the of extrusion as a result of ﬁbrillation, permitting its handling during processing. Extrusion pressure is a function of the molecu- lar weight of the polymer and lubricant content of the preform under the same processing condi- tions. The higher the molecular weight, the higher the extrusion pressure will be. The higher

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 69 Figure 4.6 A depiction of ﬁbrillation of polytetraﬂuoroethylene particles in the paste extrusion die. Courtesy: Prof. Savvas Hatzikirikos, Dept Chemical and Biological Engineering, University of British Columbia, 2014. Figure 4.7 Inﬂuence of extrusion pressure and lubri- powder. Premature ﬁbrillation leads to the formation cant content in tube extrusion at different reduction ratios [17]. of lumps, which cannot be completely broken up. To ensure that the resin does not ﬁbrillate, it should be cooled below its transition temperature prior to handling and transportation. A typical com- mercial container (20e30 kg) should be cooled (for 24e48 h) to <15C [3] to insure temperature uni- formity throughout the container. In practice, drums of resin are stored and transported at <5C. Specially designed shallow cylindrical drums are used to minimize lump formation, compaction, and shearing of the resin. Fig. 4.9 shows the cooling curve as determined experimentally for a plastic drum ﬁlled with 25 kg of ﬁne powder at a temperature of 30C [17]. When the ambient temperature is 5C and the temperature sensor is in the middle of the powder in the drum, the

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70 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 4.8 Dependency of extrusion pressure on reduction ratio (RR) for different types of ﬁne powder [17]. Figure 4.9 Cooling time for a 25 kg powder drum at ambient temperatures of 5 and 15C (curves determined experimentally) [17]. ﬁne powder material is not ready for further pro- (<0.25 mm diameter) primary round particles, which should have the same shape as when they were cessing for more than 24 h. It takes approximately polymerized. This means that the post- three days to cool the material down to 5C. A more polymerization isolation and drying processes should not affect the appearance of the particle. Any practical solution would be a cold room temperature deformation of the resin particles or ﬁbrillation in- of 15C, where the cooling of the PTFE down to dicates the potential for defects in the fabricated part. 15C extends over several days. Fine powder can be compacted to some extent Individual particles of PTFE form agglomerates, during transportation and storage, even when refrig- erated and handled gently, thereby creating lumps. which are roundish and average several hundred Sifting the resin through coarse wire mesh will help break up the majority of the lumps. The size of the microns in size. Fig. 4.10 shows agglomerates and primary particles of a typical PTFE ﬁne powder. Closer examination at higher magniﬁcations reveals that agglomerates are comprised of many small

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 71 Figure 4.10 Agglomerates of a typical ﬁne powder at 100Â and 30,000Â magniﬁcation. Courtesy Dupont Company. sieve should not be smaller than 10-mesh; 4-mesh is (available from Exxon Corp.), mineral spirits, and preferable. The resin should never be scooped out of VM & P Naphtha (available from Shell Corp.). the container but should be poured over the sieve to avoid shearing. The wire mesh should be vibrated Tables 4.1 and 4.2 list the characteristics of a gently up and down, as opposed to movement from number of solvents. They have a range of boiling side to side, to avoid shearing. The remaining lumps points, which inversely correlate with vapor pressure. of powder should be poured into a plastic jar with a The higher the boiling point, the more slowly the wide mouth. When the bottle is one-third full, it solvent will leave the extrudate. Ideally, the lubricant should be shaken gently to break down the lumps [4]. should have a lower surface tension than the critical It is wise to process this portion of the powder surface tension of PTFE, which is very low 18 dyn/ separately by making a different preform to minimize cm. While this is not practical for PTFE systems, the risk of adding damaged powder to the rest of the most solvents have fairly low surface tension, which resin. helps their spreadability on PTFE. Surface tension of Isopar series solvents rises with increasing boiling Treating ﬁne powder PTFE with utmost care is point, which adversely affects their spreadability. For crucial for fabricating high-quality parts at high example, Isopar G has a surface tension of 23.5 dyn/ production yield and it is best to minimize handling cm and spreads more easily than Isopar V, which has of the resin, such as avoiding screening when not a surface tension of 30.8 dyn/cm. required. The amount of lubricant in the compound depends 4.5 Extrusion Aid or Lubricant on the type of product, equipment design, and the desired extrusion pressure. Its lubricant content An extrusion aid is added to ﬁne powder PTFE as a should be as low as possible, but not so low that the lubricant to enable smooth uniform extrusion. The extrusion pressure would be excessively high. A less extrusion aid must coat the resin easily yet also be volatile extrusion aid is often recommended for the readily removable from the extrudate. Neither should manufacture of an unsintered tape [10]. Lubricant the extrusion aid leave a residue, which could alter content can range between 15% and 25% of the total the color of the product. The volatilization tempera- weight of the compound. ture of the lubricant should be lower than the sin- tering temperature of the polymer. The other Petroleum solvents are volatile and their vapors requirements of lubricants include high purity, low can be ignited, causing ﬂash ﬁres or explosions. They odor, low polar components, high autoignition tem- must be kept away from heat, sparks, and open ﬂame. perature, low surface tension, and low skin irritation. Table 4.3 presents ﬂammability data for Isopar sol- Common lubricants are synthetic isoparafﬁnic hy- vents. Fire can be avoided by controlling vapor- drocarbons available in a wide boiling range. Some ization and airevapor concentrations and eliminating of the commercial lubricants include Isopar solvents sources of ignition and spills. To reduce the risk of ﬁre, the solvent with the highest ﬂash point should be selected for paste extrusion. For safety and to

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Table 4.1 Properties of Isopar Solvents [8] Grade Isopar C Isopar E Isopar G Isopar H I 27 29 28 27 Solvency 8 Kauri-butanol 78 (172) 75 (167) 83 (181) 84 (184) value 7.2 7.3 7.3 7.3 5 Aniline point, C ( F) À7 (19) 7 (45) 41 (105) 54 (129) 1 1 Solubility 98 (208) 118 (244) 160 (320) 178 (352) 1 parameter 99 (211) 121 (250) 166 (331) 182 (360) 104 (219) 137 (279) 174 (345) 188 (370) Volatility Flash point, C 98 52 14 6.2 ( F) Distillation, C 0.699 0.722 0.747 0.758 ( F) 5.82 6.01 6.22 6.31 IBP þ30 þ30 þ30 þ30 50% Dry point Vapor pressure, mm Hg at 38 C (100 F) General Speciﬁc gravity at 15 C/15 C (60 / 60 F) Density, lb/gal Color, Saybolt

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Isopar K Isopar L Isopar M Isopar V Test Method 72 EXPANDED PTFE APPLICATIONS HANDBOOK 27 27 27 25 ASTM D 1133 84 (184) 85 (185) 91 (196) 93 (199) ASTM D 611 7.3 7.3 7.2 7.2 Calculated 57 (135) 64 (147) 91 (196)a 129 (264)a ASTM D 56 ASTM D 86 177 (350) 191 (376) 223 (433) 273 (523) 185 (365) 195 (383) 238 (460) 288 (550) ASTM D 2879 197 (386) 207 (405) 252 (487) 311 (592) 5.7 5.2 3.1 0.3 0.760 0.767 0.788 0.817 ASTM D 1250 6.33 6.39 6.56 6.80 Calculated þ30 þ30 þ30 þ30 ASTM D 156

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Viscosity, cP 0.48 0.62 1.00 1.29 3 at 25 C (77 F) 399 (750) 382 (720) 293 (560) 349 (660) E Auto-ignition <5 <5 <10 <10 temperature, C ( F) 100 100 0.01 <0.01 Bromine index None None Composition, mass % 100 100 <1 <3 Saturates 0.01 0.01 <1 <1 Aromatics <1 Trace <2 Purity, ppm None None 1 Acids <3 <2 24.9 Chlorides e <2 23.5 Nitrogen 0 0 51.4 Peroxides <2 <2 51.6 Sulfur Excellent Excellent Surface Properties Surface tension, 21.2 22.5 dyn/cm at 25 C (77 F) Interfacial tension 48.9 48.9 with water, dyn/ cm at 25 C (77 F) Demulsibility Excellent Excellent a ASTM D 93 was used to determine ﬂash points.

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1.39 1.61 2.70 7.50 ASTM D 445 4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 349 (660) 338 (640) 338 (640) 210 (410) ASTM D 2155 10 <10 5 500 ASTM D 2710 100 100 99.9 99.5 Mass spectrometer 0.01 <0.01 <0.05 <0.5 UV absorbance None None None None Exxon method 2 <1 e 7 Exxon method <1 e e Exxon method <1 <1 0 Exxon method <1 <2 <1 1 Exxon method <2 <2 25.9 30.8 duNuoy 25.9 26.6 50.1 49.8 52.2 44.9 ASTM D 371 Excellent Excellent Excellent Excellent Exxon method 73

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74 EXPANDED PTFE APPLICATIONS HANDBOOK Table 4.2 Properties of Lubricants Supplied by Shell Corporation [9] VM & P Shell Sol Shell Sol Mineral Spirit Odorless Naphtha HT 340 HT 142 HT Property 200 HT Mineral Spirits 34.6 32.0 30.0 Solvency 61.1 67.7 70.6 32.1 26 Kauri-butanol value 67.8 84.4 Aniline point (C) 12.8 39.4 61.7 119.4 159 187 43.9 51.7 Volatility 138.9 176 206 Flash point, C 1.4 0.4 162 179 distillation (C) 9.8 206 204 0.773 0.788 1.1 0.5 Initial BP 0.753 0.77 0.79 Dry point 0.75 30 30 0.777 0.759 Vapor pressure 30 0.77 (mmHg at 20C) <0.1 <0.1 30 0.76 <0.01 <0.1 0.25 General 0.01 <1 <1 <0.1 30 Speciﬁc gravity <30 <0.1 (15C/15C) <1 96.5 Density (kg/L at 0 15C) 3.5 Color (Saybolt) <0.1 Composition <1 Parafﬁns (vol%) Naphthenes (vol%) Oleﬁns (vol%) Aromatics (vol%) Benzene (ppm) minimize human exposure, the vapors should be controlled at a relative humidity of 50%. Safety thoroughly ventilated from the workspace. Static features, which should be part of the design of the electricity may be a source of spark and can be blending room, include antistatic ﬂooring and created when blending powder and solvent. Measures clothing, explosion-proof lighting, and grounding for should be taken to reduce chances of spark from all equipment. All clothing and other fabric should be static charge buildup. These include grounding the lint-free to avoid contamination of the extrusion equipment, avoiding splashing, preventing vapor compound. buildup in the workplace, and transferring solvent through conductive hoses and nozzles. The two most common methods for blending are bottle (or jar) blending or by motorized blender. 4.6 Blending the Resin With Neither technique has a clear advantage over the Lubricant other. The bottle process is suitable for manufacturing on a modest scale. Large-scale The lubricant, PTFE, and pigment must be blending is usually done in a V-cone blender such blended in a clean enclosed area where the temper- as the units offered by Buﬂovak LLC in East ature is below the resin’s transition temperature Stroudsburg, Pennsylvania, United States. (19C). The humidity of this area should also be The bottle or jar method (see Fig. 4.11) requires a wide-mouth polyethylene or polypropylene bottle for easy (low shear) powder loading. The jars must be

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Table 4.3 Solvents Flammability Data [8] Flash Point, Autoignition Flammable Limit Vapor Volum TCCb ASTM Temperature vol% in Air at 258C Unit Volume ASTM D 2155 D 56 (778F) 258C 1 8C (8F) 8C (8F) Lower Upper (778F) (2 Iospar C À7 (19) 399 (750) 0.9 7.0 150 Iospar E <7 (<45) 382 (720) 0.9 7.0 143 Iospar G 41 (106) 293 (560) 0.8 7.0 123 Iospar H 54 (129) 349 (660) 0.7 7.0 115 Iospar K 57 (135) 338 (640) 0.7 7.0 113 Iospar L 64 (147) 338 (640) 0.6 7.0 110 Isopar M 91 (196)c 338 (640) 0.6 7.0 100 Isopar V 129 (264) 210 (410)d 0.9 7.0 101 a TLV is a registered trademark of the American Conference of Governmental Industrial Hygienists. It is normal 8-h work day. 40-h work week, to which nearly all workers may be exposed repeatedly without exposure limit for each solvent. b A TLV has not been established for this product. The value shown has been recommended by Exxon c Pensky-Martens method, ASTM D 93. d ASTM method E 659. According to ASTM, autoignition, by its very nature, is dependent on the chemi

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me per Liquid 1008C Vapor Recommended Density Approximate Density, Air TLVa Average mol. 2128F) 400b kg/m3 lb/gal 188 [1 400b wt. 179 3.9 300b 700 5.83 114 153 4.2 300b 722 6.01 123 145 5.1 300b 747 6.22 149 142 5.5 300b 758 6.32 160 137 5.7 300b 761 6.33 164 125 5.9 200b 768 6.39 171 126 6.6 788 6.56 191 6.8 818 6. 81 197 s the threshold limit value or occupational exposure limitdthe time-weighed average concentration for a adverse effect, refer to the most recent Material Safety Data Sheet for latest recommended maximum n Corporation medical research based on consideration of available toxicological data. ical and physical properties of the material and the method.

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76 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 4.11 Blending ﬁne powder polytetraﬂuoro- Figure 4.12 An example of a PK twin-shell liquid ethylene by bottle rolling [10]. solids blender. Courtesy: Buﬂovak, LLC, www.pkblenders.com, 2014. sealed tightly to prevent loss of the lubricant by evaporation. The following steps should be taken to 5. Empty the blender and store the lubricated resin prepare the paste extrusion compound: in a jar or in the original drum and make sure the lid is sealed tightly. Allow the blend to 1. Weigh the powder after screening, and care- age similarly to step 5, above, for jar blending. fully load into the bottle. Other blenders, such as the Turbula Shaker Mixer 2. Create a cavity by giving the bottle a rapid (Turbula is a registered trademark of Willy A. twist. Bachofen AGdMaschinenfabrik, Switzerland, www.wab.ch), can be used for large quantities of 3. Pour the lubricant into the cavity in the middle polymer (Fig. 4.13). This blender consists of a cy- of the powder. lindrical container held in mechanical arms and uses a complex motion pattern. This mixer reportedly 4. Close the lid and place the bottle on rollers minimizes PTFE shearing because it avoids slam- (15 rpm) for 20e30 min. ming the powder against its walls. It does consume signiﬁcantly more energy than V-blenders. 5. Allow the blend to age for at least 12 h at 35C to allow complete diffusion of the lubricant into the polymer particles. 6. Any small lumps should be broken by sieving and the lubricated powder rerolled for 3e5 min. For large quantities of resin (25e70 kg), a twin- shell V-blender (see Fig. 4.12) may be used to blend the lubricant. The following steps should be taken: 1. Load the powder carefully into the V-blender to avoid shearing the resin. 2. Add the lubricant evenly to the resin. 3. Set the blender to tumble at 24 rpm for 13 min for a 25 kg batch of resin. Longer rotation times may be required for larger batches of resin. 4. Screen the compound to break up loose lumps and remove those that do not break easily by placing them in a separate container.

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 77 Figure 4.13 An example of a Turbula shaker mixerd 2. Sieve the pigment, which should be dry, mixing basket has a maximum volume of 55 L. through a 60-mesh (BS 410e1:2000, ISO Courtesy: Willy A. Bachofen AGdMaschinenfabrik, 3310e1:2000) sieve (nominal aperture Switzerland, www.wab.ch. 250 mm) into the polymer. 4.6.1 Pigment Addition 3. Shake the container brieﬂy to disperse the pigment among the polymer particles. Pigments, in dry form or as dispersion, can be added to PTFE. Coloring is done primarily to aid in differ- 4. Tumble the container end-over-end at 50e60 entiating wires. Pigment dispersions (liquid pigments) rev/min for 10 min. are preferable for critical applications such as thin-wall wire insulation and spaghetti tubing. Dispersions The lubricant should be added afterward. It may reduce the formation of ﬂaws due to undispersed be necessary to dry the pigment by heating it in a pigment. Such ﬂaws weaken the extrudate and vacuum oven to remove moisture and other volatile frequently lead to the dielectric breakdown of the wire substances prior to use. insulation. Pigments can be dispersed in hydrocarbons using dispersants. Most pigments are commercially Liquid pigments should be shaken or rolled for available in dispersion form. Pigment of any type in several hours before being added to the resin to PTFE should not exceed 1% because of its detrimental ensure even dispersion in the polymer. The lubricant effect on the dielectric properties of PTFE. and pigment dispersion should be mixed together and quickly added to the PTFE powder because pigment Inorganic pigments should be selected for coloring particles settle rapidly after the addition of the PTFE because organic pigments will degrade at the lubricant. The amount of additional lubricant should sintering temperatures of the polymer. Pigment may be adjusted for the hydrocarbon content of the be incorporated by tumbling the required amount pigment dispersion. with the polymer before lubrication. The steps of the process are [18] the following: 4.7 Preforming 1. Transfer the required weight of sieved polymer Preforming is done after lubricant and pigment to a clean, dry, wide-necked container with an have been added to the PTFE powder and the airtight closure of ample volume for the quan- compound has aged. Preforming, which usually tity of extrusion composition to be mixed. For takes place at room temperature, shapes the com- optimum tumbling this vessel should be be- pound into a billet with the same shape as the barrel tween one-third and two-thirds full. of the ram extruder. The rule of thumb is to compact the resin to one-third of its initial height [3,11]. Preforming removes the air from the PTFE powder and, by compaction, maximizes the quantity of material available for extrusion. The objective is to extrude the longest possible ﬂawless length of the wire. Preforms are made in a cylinder equipped with a mandrel and a pusher similar to the schematic in Fig. 4.14. The mandrel is positioned in the center of the cylinder and the resin is charged in the annular space. The diameter of the preform and the center hole are designed so that: 1. The outer diameter is 0.2e1.3 mm less than the inside diameter of the barrel. 2. The core diameter is 0.25 mm larger than the extruder wire guide (mandrel).

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78 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 4.14 Preforming equipment for billet molding for paste extrusion [11]. The surfaces of the barrel wall and mandrel should Table 4.4 Preforming Compaction Rate (cm/min) [3] be smooth and free of scratches and nicks. A surface ﬁnish of ﬁner than 63 RMS is recommended [3]. Compression Preform Diameter (cm) Stage The aged lubricated resin without lumps is loaded Initial <8 cm >8 cm into the preform cylinder and is evenly distributed 25 25 around the core mandrel to ensure uniform compac- Middle 5 5 tion throughout the preform. The pusher is placed on 5 1 top of the cylinder and compaction started. Resin Final compression may begin at a fairly rapid rate but has to be reduced at the later stages of compaction. This rate reduction, which is size dependent (Table 4.4), is

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 79 to prevent air entrapment which could cause the Figure 4.16 An operator places a preform in the bar- preform to crack. rel of a horizontal paste extruder. Low pressure should be used to compact the powder in the cylinder. At the initial stage, Courtesy: Markel Corp. 0.5e1 MPa pressure is applied. This should be increased to 2 MPa by the end of the compression while moving through the die. It then enters a cycle. The pressure needs to be sufﬁciently high to vaporizing oven, where it is stripped of the lubricant compress the resin and push the air out (since trapped by evaporation. Vaporization temperatures depend on air could cause the preform to crack). Yet pressure the type of lubricant. Heavier hydrocarbons require a should not be so high that it squeezes a high per- higher drying temperature. The dried and coated wire centage of the lubricant out of the resin. A small then goes into the sintering zone, which usually amount of lubricant tends to be pushed out of the consists of a series of individual ovens. The tem- preform even when pressure is adequate, and this peratures of the ovens are set to heat the polymer should not raise concern. Fig. 4.15 shows a scanning above its melting point quickly. After leaving the electron micrograph of a sample of a commercial ovens, the wire is cooled and passed through a spark preform. The particles have been deformed as a result tester, where the wire is subjected to high voltage to of compaction without evidence of ﬁbrillation. “spark out” any ﬂaws, which are counted by the de- Overly ﬁbrillated preforms do not extrude smoothly vice. The ﬁnal step is winding the wire on a spool, and may give rise to defects in the wire insulation. which is done by a motorized take-up system. The preform is quite weak and can easily break or The coating that forms the insulation consists of a deform and should therefore be removed from the thin-walled tube, which is paste extruded onto mov- cylinder with care. The preform can be loaded in the ing conductors, followed by lubricant removal and extruder immediately after removal from the cylinder sintering. The extrusion of the small insulation tube (Fig. 4.16). It can also be stored at ambient temper- around the wire requires the preform to be reduced by ature in a plastic tube prior to the extrusion to avoid forcing the paste through a small die. This reduction contamination, damage, and lubricant loss. gives rise to an important parameter called RR, which is a characteristic of ﬁne powder PTFE. RR 4.8 Extrusion Equipment refers to the ratio of the paste cross-section area in the and Process extruder barrel to the cross-section area of the extrudate. Wire coating requires resins capable of Fig. 4.17 shows a schematic of a paste extrusion moderate to high RR, depending on the thickness of line for wire insulation. The wire is passed through the wall of the insulation. The preform is extruded the paste extruder, where it is coated with PTFE above room temperature (30e100C) to take advantage of the deformability of PTFE at higher Figure 4.15 Scanning electron micrograph of a pre- temperatures. Electric heating bands equipped with form at 20,000Â magniﬁcation. independent controls heat the extruder barrel and the die. Fig. 4.18 shows the changes in PTFE paste as a result of preforming and extrusion at two different RRs, prior to sintering.

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80 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 4.17 Schematic of a polytetraﬂuoroethylene wire extruder, consisting of an unwind roll and dancer roll, extruder, a drying and sintering oven, deﬂector roll, a wire puller, electrical breakdown test device, and wire take-up roll [17]. 4.8.1 Extruder depends on the assembly condition of extrusion equipment. Prepressing could be helpful in driving The ram extruder for this process is a special unit out air if the entrapped air cannot escape through the that can be either horizontally or vertically oriented. back plate seal and the die seal during the extrusion The orientation refers to the direction of the ram because the seals are fairly airtight. Otherwise, it is movement. The extruder consists of a heated barrel, likely that prepressing would increase the number of where the preform is loaded, and a hydraulic or faults. The escape of high-pressure air through the screw-driven ram. A power system draws the insulation during prepressing could leave holes and conductor through a hollow mandrel located at the voids that would be too large to close during center of the barrel. The mandrel terminates in a wire sintering. guide tube that can be adjusted to alter the position of the tip of the guide tube relative to the die. See The wire payoff system is usually motorized and Figs. 4.19 and 4.20 for examples of two commercial equipped with an adjustable tensioning device to paste extrusion units. Fig. 4.21 shows the position of keep the wire from becoming slack or too tight. The the guide tube in the master die. speed of the wire and the ram must be coordinated to produce insulated wire. In commercial extruders, a One option is to prepress the preform prior to start control system synchronizes the changes in the wire of the extrusion. The advisability of prepressing and ram speeds.

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 81 Figure 4.18 Changes in polytetraﬂuoroethylene paste as a result of preforming and paste extrusion [16]. Figure 4.19 A Davis electric vertical extruder [10]. In the extrusion process the force of the ram pushes the preform through the die. It is important to be able to control the speed of the ram throughout the process and keep it at a constant set speed. The uniform thickness of the coating depends on the constancy of the ram speed. The hydraulic or me- chanical drive system must be capable of supplying the force necessary to extrude the preform. Ram pressure capability up to 150 MPa may be required for extrusion at high RRs. 4.8.2 Die The preform ﬁbrillates in the die under ram pressure and forms an extrudate that should have the correct thickness and smoothness. The design of the die plays a key role not only in the properties and quality of the coating but also in the magnitude of the extrusion pressure. The preform is pressed through the extruder cylinder (barrel) with little pressure development until it reaches the die, where the cross-section area for the passage of the preform decreases by the angular design of the wall. At this point, because the polymer particles are forced to compete for ﬂow through this increas- ingly smaller cross-sectional area, they rub past each other and form ﬁbrils.

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82 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 4.20 A Jennings International horizontal extruder. Courtesy: Jennings International, www.jenningsinternational.com, 2014. DE = Diameter of the Extrudate Die AL = Cross -Sectional Insert Area of the Die Land Die Block DL = Land Diameter Guide Guide tube Tube Tip Clearance Tip AR = Cross - Sectional Wire Area of the Guide Guide Tube Tube AC = Cross -Sectional Extruder Area of the Cylinder Cylinder REDUCTION RATIO = AC – AR AL – AWire Figure 4.21 Details of master die [3].

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 83 The angle of the die wall (cone) affects the surface damage. The lubricant content of the insulation, smoothness of the extrudate. The range of the angle is nearly 40% by volume, must be removed prior to 15e60 degrees, but an angle of 20 degrees for thin sintering. The coating may crack if it still contains a coatings on ﬁne conductors and an angle of 30 de- large amount of the lubricant when it reaches the grees for thicker coatings has been recommended [3]. sintering zone. Any remaining hydrocarbons will The surfaces of the cone and the die land areas should degrade at the sintering temperatures and leave a be polished to a mirror ﬁnish of 0.1 mm Ra [11]. The colored residue. higher the extrusion speed and pressure, the more important the surface ﬁnish becomes. There are two conﬁgurations of drying ovens. The ﬁrst is internally heated with a tubular design, The conductor (wire) emerges from the guide tube and the second is a horizontal heated console at the tip, which directs it through the segment called design. Both types of oven are vented to remove the die land. The location of the tip is critical to the the vapors. A typical tubular oven is about 3 m long quality of the insulated wire because it affects how with a diameter of 150e200 mm. One or two 3-m the wire and PTFE are brought together. Adjusting ovens are required for complete drying. The tem- the position of the tip can affect coating thickness, perature at the oven entrance is 150 and 300C at stripability of the PTFE (tightness), and the number the exit. In the heated console-type oven, several of ﬂaws. If the clearance between the tip and the die 10-m lengths of the wire are wound around multi- is too small, the tip constrains the movement of the ple sheaves, which allows longer residence time in paste and extrusion pressure rises. Small clearances this oven than in the tubular kind. The temperatures also reduce the tightness of the insulation around the are lower in console ovens and range between 90 conductor and lead to low strip force. If the clearance and 200C. The exact temperature depends on the is too large, the wire tension increases because of the speed of the wire and lubricant type. A capstan drag of the paste on the conductor. A large clearance drive pulls the wire forward synchronously with the also increases the tightness of the insulation and may ram speed. lead to cracking. A further problem with a clearance that is too large is that it will allow the paste to The drying process removes large volumes of extrude backwards into the tube. The guide tube ﬂammable hydrocarbons and must reduce the con- should be snug around the wire with a maximum centration of these vapors below the ﬂammability clearance of 25e50 mm [11]. It is important to range. Most Isopar solvents have a room temperature experiment with the tip clearance to obtain the best ﬂammability range of <1e7% (Table 4.3). To reduce extrusion condition for a given conﬁguration. solvent concentration so that it is below those limits, a large volume of air must be blown through the The diameter, length, and temperature of the die exhaust system. land area all inﬂuence the coated wire properties. Allowance should be made for swelling (“blow up”) 4.8.4 Sintering and Cooling of the coating when it leaves the die. Relaxation of built-in strains leads to die swelling of the polymer, The wire enters the sintering zone immediately which is a transient effect. The diameter of the die after it leaves the drying oven. At this point it still should accommodate 3e10% die swell. A die land lacks strength because of its porous unsintered length of 6e13 mm and a die temperature >35C structure. The PTFE is heated to temperatures above have been found to produce smooth extrudates [3]. its melting point and undergoes coalescence and void elimination during sintering. After sintering and Die design is a fairly complex process and requires cooling, PTFE insulation assumes its permanent an in-depth understanding of the effects of its pa- dimension and ends at 50e60% crystallinity. rameters on the extrusion process and product qual- ity. An iterative process by trial and error can be The polymer must be heated to at least its melting costly. Benbow and Bridgwater [12] offer an excel- point of 342C for a brief period of time before lent source for the design of paste extrusion dies. melting occurs. In practice, higher temperatures well above the PTFE melting point are used to reduce the 4.8.3 Drying melt creep viscosity of the polymer for rapid void closure. The oven temperatures are typically set at While it still contains the lubricant, the polymer 400e600C, based on a number of variables coating is fairly fragile and susceptible to mechanical including the speed of the wire and thickness of the

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84 EXPANDED PTFE APPLICATIONS HANDBOOK coating. Heat transfer to the polymer accelerates as 4.8.5 Reduction Ratio temperature increases. Care must be taken to prevent the exposure of PTFE to temperatures above 380C, Section 4.2 provided a qualitative description of at which point degradation begins to accelerate. The RR. In this section, we will deepen the discussion of sintering ovens should be equipped with an exhaust this important aspect of the polymer and paste system to remove the toxic by-products of PTFE extrusion process. degradation. Fig. 4.21 shows the basic design of a die for Multiple ovens, sometimes as many as eight, are wire extrusion. RR is deﬁned as the ratio of the used for sintering. The most common sintering ovens cross section of the polymer before extrusion to the are tubular, about 1 m long, with a typical diameter of ratio after extrusion. This ratio can be written as 250 mm. While older ovens have radiant electrical Eq. (4.1). heating elements, the newer ovens are electrically heated with quartz lining, which emits infrared RR ¼ AC À AG ¼ pD2C À pD2G (4.1) radiation. AL À AW pDL2 À pDW2 Cooling of the coated wire is relatively easy AC, cross-sectional area of the extruder cylinder because it is thin compared to granular PTFE parts. (barrel), mm2; AG, cross-sectional area of the guide The wire insulation exiting the sintering ovens is in tube (mandrel), mm2; AL, cross-sectional area of die the molten state and solidiﬁes upon contact with the land, mm2; AW, cross-sectional area of the wire ambient air. The wire is usually allowed to cool by (conductor), mm2. natural convection in the ambient air. Blowers can be installed to move the warm air away from the area. Eq. (4.1) can be simpliﬁed, as shown in Eq. (4.2). The crystallinity of the PTFE is about 50%. It is possible to quench the coating by blowing cold air or RR ¼ DC2 À DG2 (4.2) by passing it through a cold water bath. Such prac- DL2 À D2W tices can drive the crystallinity below 50% and have a measurable impact on the properties of thicker DC, diameter of the extruder cylinder (barrel), mm; coatings. DG, diameter of the guide tube (mandrel), mm; DL, diameter of die land, mm; DW, diameter of the wire After cooling the wire enters a spark tester, which (conductor), mm. is a dielectric breakdown tester. It operates in a similar manner to ASTM Method D149 by subjecting A smaller cylinder, larger mandrel, larger die the wire insulation continuously to a known voltage. land, and smaller wire diameter can each decrease The objective is to measure the number of spots in a the RR. Since in practice the size of the wire is length of wire that are too weak to stand up to the test ﬁxed, the preform is the only variable. A reason- voltage. The failure makes a dielectric arc accom- ably sized preform is required to make a long panied with a buzzing sound, and thus they are called length of wire, which is why high RRs similar to sparks. The number of sparks can be automatically those in Tables 4.5 and 4.6 are encountered. It is measured and recorded. Sparks are a measure of the therefore important to use smaller barrels where quality of the wire and determine its functional value. possible. Resin manufacturers have developed The voltage that should be applied depends on the polymers with RRs that range from very low to thickness of the coating. For example, if a coating has very high, and some of these are capable of un- a thickness of 0.25 mm, and short-term dielectric dergoing RRs as high as 4000:1. The most impor- breakdown resistance of PTFE is around 24 kV/mm, tant polymer property affecting the operating range the voltage should be 6 kV/mm. In practice, half this of the RR is molecular weight, which can be easily voltage (3 kV/mm) may be used for testing, since manipulated during polymerization. 6 kV/mm would represent the maximum value for the polymer. A higher RR increases the extrusion pressure, but the type and amount of lubricant used can reduce this Tables 4.5 and 4.6 show examples of manufacturing pressure. Fig. 4.22 illustrates the effect of RR on process variables for wire coatings that comply with extrusion pressure for two commercial resins. The four US Military (MIL) standards, using two different relationship is close to lineardthat is, a doubling of commercial resins. the RR nearly doubles the extrusion pressure.

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 85 Table 4.5 Recommended Tooling and Processing Conditions for Extruding E223 Constructions of Teﬂon CFP 6000 Fluoropolymer Resin [3] Variable Unit 1.75 (44.5) Barrel Size (mm) 2.5 (63.5) Mandrel, O.D. in. 0.625 0.75 (mm) (16.9) 2.0 (50.8) (19) Conductor, O.D. in. 0.0296 0.625 (mm) (<0.75) (16.9) 0.0296 Isopar G wt% 16.5 0.0296 (0.75) concentration (0.75) 18.5 Die size in. 0.056 18.0 (mm) (1.42) 0.056 Tip, I.D. Â O.D. 0.032 Â 0.042 0.056 (1.42) in. (0.81 Â 1.06) (1.42) 0.032 Â 0.042 Tip clearance (mm) 0.08e0.10 0.032 Â 0.042 (0.81 Â 1.06) (2.03e2.53) (0.81 Â 1.06) 0.08e0.10 Blow up, O.D. in. 0.057e0.058 0.08e0.10 (2.03e2.53) (mm) (1.45e1.47) (2.03e2.53) 0.058e0.059 Finished O.D. (hot) 0.050 0.058e0.059 (1.47e1.49) in. (1.26) (1.47e1.49) 0.050 Wire speed (mm) 260 0.050 (1.26) (79) (1.26) 260 Pressure in. 8000 260 (79) (mm) (57.6) (79) 11,000 Reduction ratio ft/min 1250:1 9000 (79.3) Vaporizer (m/min) 177 (350) (64.8) 2500:1 Sintering oven zone psig 1700:1 177 (350) 1 (MPa) 482 (900) 177 (350) 2 538 (1000) 3 e 566 (1050) 4 C (F) 593 (1100) 5 C (F) 482 (900) 482 (900) 482 (900) 538 (1000) 538 (1000) 566 (1050) 566 (1050) 593 (1100) 593 (1100) 482 (900) 482 (900) 4.9 Extrusion of Tubing from ﬂuid transfer in healthcare to fuel and hydraulic transfer in jet engines. Tubing is divided into three The majority of tubes made from PTFE by paste different categories according to size and wall extrusion have fairly thin walls (<8 mm) and are thickness. Table 4.7 summarizes the sizes and ap- produced in a wide range of sizes from a fraction of a plications of each typedspaghetti tubing, pressure millimeter to several centimeters in diameter. These hose, and pipe-liner. Pressure hoses are composite tubes are used in a variety of applications ranging devices made of one or two layers of PTFE lining

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86 EXPANDED PTFE APPLICATIONS HANDBOOK Table 4.6 Examples of Electric Wire Insulation Molded From Daikin Polytetraﬂuoroethylene [4] Item I F-201 III Core wire structure (no. of 7/0.320 II 7/0.127 strands/diameter mm) Wire plating Silver 19/0.127 Silver Wire outside diameter (mm) 0.96 0.38 Insulation thickness (mm) 0.25 Silver 0.15 MIL standard E-20 0.64 ET-28 0.25 Extruder 38 E-24 38 Cylinder diameter (mm) 16 16 Mandrel diameter (mm) 20 38 20 Die angle (degrees) 1.60 16 0.762 Die tip diameter (mm) 1.067 20 0.406 Guide tube diameter (mm) 732 1.321 2751 Reduction ratio (RR) 19.0 0.686 22 Amount of extrusion aid blended 899 (weight part) 15.9 21 18.0 VM & P Naphtha (wt%) 25 Â 5 25 Â 5 Preforming pressure and time 17.3 (kgf/cm2 Â min) 0.8 25 Â 5 0.3 Guide tube/guide tip clearance (0.78) (0.62) Calculated value (mm) 0.6 Die temperature (C) 50 (0.76) 50 Extruder 18.3 50 11.0 ram speed (mm/min) 8.2 18.2 Haul-off speed (m/min) 615 19.0 1015 Extrusion pressure (kgf/cm2) 14.0 95 500 95 Oven Temperature 205 205 #1 (Drying) (C) 400 95 400 #2 (Drying) (C) 205 #3 (Sintering) (C) 1.52 400 0.68 0.28 0.15 Molded Product Dimensions None 1.10 None Outside diameter (mm) (3.4) 0.23 (1.5) Insulation thickness (mm) None Number of sparks (3.4) (Test voltage) (kV)

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 87 1000 Figure 4.22 Relation of reduction ratio and extrusion pressure [4]. Note: Extrusion F-201 aid used: Super VM & P Naphtha (Shell). 500 (18 % by wt.) Extruder used: Jennings extruder. F-104 20 % by wt. F-201 Extrusion pressure (kgf/cm2) (18 % by wt.) 19 % by wt. 100 50 19 % by wt. 10 50 100 500 1000 2000 Reduction ratio (R.R.) reinforced with overbraiding, usually metal wire, to preforms may be different from those used for wire increase its pressure rating. Each of these types of insulation. tubing requires a somewhat different processing method because of the signiﬁcant differences in their 4.9.2 Extrusion of Spaghetti sizes. Tubing 4.9.1 Blending Lubricant and A small vertical paste extruder can be used to Pigment and Preforming manufacture spaghetti tubing (Fig. 4.23). The small size of this kind of tubing eliminates most of the Resin lubrication, pigmenting, and preforming for handling problems associated with larger products. A tubing should all be carried out similarly to the vertical machine can extrude upwards or downwards. preparations described above for wire coating. The Frequently the extruder is placed at a height of amount and type of the lubricant and pigment may 10e15 m above the shop ﬂoor, which allows the change, but the basic methods of blending, mixing, drying, sintering, and cooling to be accomplished and handling remain the same. Tubing preforming is during the downward fall of the tubing. A motorized also done the same way, though the sizes of the wind-up unit on the shop ﬂoor then collects the tubing. Table 4.7 Types of Tubing and Applications Made From Fine Powder Polytetraﬂuoroethylene Type of Tubing Diameter (mm) Wall Thickness Applications Spaghetti tubing 0.2e8 (mm) Pressure hose 6e50 0.1e0.5 Electrical insulation, ﬂuid handling in medical equipment, and chemical applications Pipe-liner 12e500 1e2 Fuel and hydraulic transfer in aerospace, chemical and gas transfer in chemical processing 2e8 Lining metal pipes and ﬁtting for chemical processing

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88 EXPANDED PTFE APPLICATIONS HANDBOOK Dc Dm Mandrel Ram Seal ring Cylinder (θ = 30 ~ 60°) Core pin Ram head θ Die: Temp. (50 ~ 60°C) Band heater (50 ~ 60°C) Die orifice Centering bushing Figure 4.24 Schematic of tube extrusion die [4]. Figure 4.23 Hydraulic paste extruder for tube fabri- area. Wire coating paste extruders can be set up in cation [17]. PTFE, polytetraﬂuoroethylene. a modiﬁed arrangement to make tubing. In this case the guide tube should extend beyond the die Fig. 4.24 shows a die design for tubing extru- land exit to prevent blockage by the polymer. The sion. It resembles the design for wire coating core pin or the guide tube diameter should be sized (Fig. 4.21) except that instead of the guide tube to give the correct internal tubing diameter. The there is a core pin that extends into the die land ovens should be long enough to allow straight tube production. A horizontal or upward machine will require motorized take-up to move the tubing through the ovens. Extrusion conditions are quite similar to those required for wire coating. Extrusion pressure can reach very large values (100e150 MPa) because high RRs are necessary to obtain small diameters and wall thicknesses. The extruder barrel and the die must be structurally sound to withstand the high extrusion pressure. The lubricant needs to be removed prior to sintering, as it does in the wire coating process. Tables 4.8 and 4.9 provide examples of extrusion conditions for three commercial resins. Start-up threading problems are frequently encountered in the fabrication of spaghetti tubing. Since the tubing is small and is mechanically weak in the gel state, it tends to break under the fairly moderate tension required to pull the tube through the process. With larger tubing, a commonly used trick is to con- nect a leader of wire or chain to the tubing to guide it through the line. But the weight of a leader can be too much for ﬁne tubing and can end up breaking the tube. A method that is usually successful with smaller

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 89 Table 4.8 Typical Extrusion Conditions for Table 4.9 Examples of “Spaghetti” Tubes Extruded “Spaghetti” Tubing [11] From Daikin-Polyﬂon Tetraﬂuoroethylene [4] (Continued ) Polymer Fluon CD509 Fluon CD509 VM & P Type of lubricant Isopar H Naphtha F-201 19.0 Lubricant 19.0 Item IP m content (% of 2.75 total mix) 1.75 Extruder 38 38 38 0.650 16 Extrusion 0.625 0.694 Cylinder diameter 20 cylinder 0.040 0.759 (mm) 50 diameter (in.) 0.055 20 Mandrel diameter 16 16 21 Mandrel 20 75:1 (mm) 17.4 diameter (in.) 1900:1 100 25 Die angle 20 20 540 Core pin tip (in.) 150 1 (degrees) 16.0 30 0.683 Die land 0.054 0.625 Die temperature 50 50 100 diameter (in.) 0.040 0.029 (C) 250 0.007 Die included Amount of Extrusion Aid Blendeda 400 angle (degrees) (Weight part) 23 22 Reduction ratio (RR) (wt%) 18.7 18.0 Die temperature Preforming 25 25 (F) pressure (kgf/cm2) Extrudate speed Extrusion pressure 880 600 (ft/min) (kgf/cm2) Sintered outside Extrusion speed 22.0 18.0 diameter (in.) (m/min) Sintered inside Drying Zone Temperatureb (8C) diameter (in.) #1 100 100 Sintered wall #2 250 250 thickness (in.) Sintering Zone Temperatureb (8C) #3 400 400 Table 4.9 Examples of “Spaghetti” Tubes Extruded Product Dimensions From Daikin-Polyﬂon Tetraﬂuoroethylene [4] Outside diameter 1.08 1.27 1.54 (mm) 1.07 1.24 F-201 0.10 0.15 Inside diameter 0.94 P (mm) Item I 1.60 m 1.32 1.90 Thickness (mm) 0.07 Die diameter 1.08 (outside, mm) 0.12 Tensile Strength 2112 Core pin diameter 1.27 1.50 Longitudinal 720 720 (inside, mm) direction (kgf/cm2) Clearance 0.165 0.20 Elongation (thickness, mm) 1266 870 Longitudinal 350 350 Reduction ratio direction (%) (RR) a Extrusion aid used; Super VM & P Naphtha (Shell). (Continued ) b Corresponding to Jennings extruder (30 ton).

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90 EXPANDED PTFE APPLICATIONS HANDBOOK tubing is to thread the tubing before sintering ovens important properties of electrical wrapping tape have reached the melting temperature of PTFE. The include adequate physical properties for handling, unsintered extrudate, before melting, has sufﬁcient appropriate thickness, and excellent layer-to-layer strength to withstand the tension of threading. The adhesion (which is a property of the resin). The plies oven temperatures can be raised after threading with of the tape, which have been wrapped around the minimal material loss because the small size of the cable, melt during the sintering cycle and must bond tubing will allow it to heat up rapidly to the sintering together ﬁrmly for satisfactory performance as temperature. insulation. The choice of a lower molecular weight PTFE, such as a modiﬁed grade, because of lower The die angle is about 20 degrees in the majority melt creep viscosity, would improve interlayer of spaghetti tubing dies. The core pin is made of a adhesion. hard plastic material such as polyacetal (eg, Delrin by DuPont). Plastic is used instead of metal because the Thread sealant tape, on the other hand, must ﬂexibility of the plastic allows the tube to have better perform after it has been applied to pipe thread. concentricity. Lower density enhances the drapability (ability to deform) of the tape around the threads. A balance 4.10 Unsintered Tape between the tensile strength in the machine and transverse directions in the range of 15:1 [13] is Major applications of unsintered PTFE include required to achieve the desirable deformability. thread sealing, wrapping electrical cables, and rod Elongations of 100e200% in the transverse direction and tape in packings. Important properties of PTFE and better than 800% in the machine direction are like chemical resistance, broad service temperature, desirable. The amount of ﬁbrillation determines the low friction, ﬂexibility, high machine direction tensile properties and deformability of the tape. Too strength, and deformability in the cross direction little ﬁbrillation will yield a tape with insufﬁcient make unsintered ﬁne powder PTFE ideal for these tensile strength while too much ﬁbrillation will create applications. a hard tape lacking sufﬁcient deformability. Thread sealant tape is used in pipes and ﬁttings in a 4.10.1 Blending Lubricant and variety of industries including water pipe, chemical, Pigment and Preforming pharmaceutical, semiconductor manufacturing, and food processing, among others. In insulation Resin mixing with lubricant and preforming applications, electrical grade tape is wrapped around should be done according to the procedures described cable or wire and then the construction is sintered. earlier. The choice of the lubricant should be made Some of the PTFE tape is sintered, treated to impart based on the further processes that the extrudate will adherability, and then coated with pressure-sensitive undergo. For example, if the rod or the ribbon is adhesive for wrapping objects to reduce friction or intended for an application that does not require provide quick-release properties. further change in its characteristics, such as packing, the lubricant should be fairly volatile to be easily Another important area of application is oriented removable. Isopar C or E or other isoparafﬁns with tapes and webs, which are used in the fabrication of equivalent volatility (for distillation temperature porous ﬁber, fabric, tubing, and sheeting. These range, see Table 4.1) are suitable lubricant selections. porous articles are used for protective clothing, Both can be removed in a convection oven at fairly waterproof and weatherproof fabrics, gaskets, ﬁlter low temperatures. bags, and many other items. When a rod or thick ribbon is intended for calen- The unsintered tape is not produced directly in the daring to produce thin unsintered tape, a less volatile ﬁnal thickness and width. Rather, it is made by lubricant is needed to ensure that it will remain in the extruding a round or rectangular bead (thick ribbon), extrudate and on its surface without signiﬁcant followed by calendaring and stretching, which con- evaporation prior to, and during, the calendaring verts it to a thin or low-density tape. Calendaring is operation. In this case a heavier lubricant like Isopar necessary to obtain tapes that are thin enough to be M, with a concentration of 18e20% in the extrudate, conformable to the substrate. Normally, the lower is critical for proper calendaring. limit of thickness is 50e75 mm, but tape that is 25 mm thick can be made by calendaring. The

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 91 4.10.2 Extrusion of Round and Extrusion conditions affect the quality of the rod Rectangular Bead products. It is important to operate both the barrel The general principles for extruding a bead from a and the die above the ﬁrst room temperature transi- PTFE paste are similar to those for wire and tubing tion of the resin (19C) to obtain a smooth and fabrication. Fig. 4.25 illustrates a typical die and extrusion cylinder for manufacturing a round bead coherent extrudate. The temperature of both units (rod). The ram forces the polymer through the die, should be in the range of 25e35C; best results are which is a simple oriﬁce. There are no guide tubes or obtained at >30C. The amount of work (extrusion mandrels because the bead is solid. The die land length to diameter ratio inﬂuences the quality of the pressure) put into the extrudate determines the extrudate. Its value is not critical and is typically in the range of 5e10. Die cone angles in the range of hardness or deformability of the bead. The more the 30e60 degrees are common. Rectangular beads follow the same principles except that the shape of polymer is worked, the harder it will turn. A bead the die land is rectangular. intended for calendaring into a tape should be signiﬁcantly softer than a rod intended for packing applications. A bead made at an excessively high extrusion pressure will be stiff and will not calendar well. A bead that is too soft (because the extrusion pressure is too low) will yield narrow tape that is Figure 4.25 Extrusion cylinder and die for rod [10].

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92 EXPANDED PTFE APPLICATIONS HANDBOOK Table 4.10 Typical Extrusion Conditions for Extrusion of a Roda [11] Type of Lubricant Mineral Oil (Boiling Low-Odor Parafﬁn VM & P Naphtha Range 260e3858C) (Boiling Range: (Boiling Range: 191e2468C) Petroleum Ether 118e1398C) (Boiling Range Packing 95e1168C) 16 3.8 Rod application Calendaring into tape Calendaring into tape or 0.127 packing 900:1 20 Lubricant content, wt% 20 Æ 2 20 29 1982 Extruder barrel diameter (cm) 6.2 11.3 75.9 Die land diameter (cm) 1.27 1.27 Reduction ratio 25:1 80:1 Die cone angle (degrees) 30 30 Die temperature (C) 35 35 Extrudate speed (cm/min) 91.5 183 Extrusion pressure (MPa) 6.2 6.9 a Imperial Chemical Industry Resin Grade Fluon CD-1 was extruded in these trials. weak in the transverse direction. Typical conditions density. Reducing the cost of manufacturing the tape for the extrusion of a bead are given in Table 4.10. is the primary purpose of density reduction. The RR for extruding rods is usually fairly low. Calendaring a large bead into a much wider tape For example, a RR of 100 is required to make a 1 cm and then slitting the tape allows higher productivity diameter bead in an extruder with a barrel diameter of than calendaring the bead into the ﬁnished width. 10 cm, implying a fairly low extrusion pressure Slitting also reduces the amount of scrap produced in (10e20 MPa). Less lubricant may be needed to in- the process. Single- or multiple-stage calendaring is crease the pressure if the bead is too weak for possible, though the latter has little advantage over handling. Another useful change is to increase the die the former. Two rolls with a deﬁned gap are used to cone angle to 60 degrees, which will increase the “squeeze” the thicker bead into a thin tape. Heating extrusion pressure. An added beneﬁt of increasing the rolls assists in the calendaring process. this angle is that it will reduce the effective length of the die cone, which decreases the quantity of heel 4.10.3.1 Calendaring Equipment material in the cone. Theoretically, calendaring can be done by any 4.10.3 Calendaring twin rolls with an adjustable gap in the thickness range. The width of the tape to be made determines Calendaring is the process by which the thickness the width of the rolls. More than one set of calen- of a bead or sheet is reduced. Manufacturing unsin- daring rolls can be used to achieve the desired tered tape from a round or rectangular bead requires thickness. In the case of multiple sets of calendars, calendaring, followed by removal of the extrusion only the gap in the last calendar would have to be aid, slitting, and winding into rolls (Fig. 4.26). The precisely controlled. Calendar rolls are made from a calendaring process reduces the speciﬁc gravity of hard rigid material, such as chilled cast iron, to pre- the tape by about one-third to 1.4e1.5 g/cm3 because vent/reduce deﬂection under the substantial pressures of the lubricant and voids. Removing the lubricant that can develop during the calendaring [11]. While leaves the volume it occupied behind in the form of the initial rolls can be rough, the ﬁnish of the ﬁnal voids. In a sintered part, melting and coalescence of calendar should be smooth (0.25 mm). A surface the polymer eliminate the voids. Stretching after ﬁnish that is too ﬁne will prevent sufﬁcient grip calendaring introduces additional voids, called the porous structure, into the tape and thus reduces its

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 93 Figure 4.26 Outline of process for making thread seal tape [13]. between the bead and the rolls, thereby reducing the 300 mm. The rolls should be equipped with internal effectiveness of calendaring. water, electric, or oil heating capable of reaching a The diameter of the rolls depends on the desired temperature of 50e80C to facilitate the calendaring width of the tape. Rolls with a larger diameter should be selected for wider tapes. For example, to calendar of the bead. A continuous drive should afford roll a bead into a 100e200 mm wide tape, the calendar speeds in the range of 1.5e40 m/min. diameter should be 150 mm and its width about Guides must be installed on the calendar so the bead can be safely fed into the nip point of the rolls.

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94 EXPANDED PTFE APPLICATIONS HANDBOOK positive tension to keep the tape moving. Excess tension will stretch the tape and reduce its width. Figure 4.27 Diagram of a ﬁsh-tail guide and its use 4.10.3.2 Calendaring Operation in a calendar [11]. It is important to retain sufﬁcient lubricant in the The bead-containing lubricant is limp and slippery, bead prior to calendaring, particularly if the extrudate with a tendency to move sideways on the rolls. The is not calendared immediately. The bead can be kept guide should constrain the erratic movement of the in a sealed container until it is calendared. Preheating bead and the tape to maintain a smooth process. One the bead helps produce a wider and more uniform type of guide is a tubing between 8 and 15 cm long tape. The recommended method for warming the with a slightly larger diameter than the bead [13]. extrudate is to place it in an airtight plastic bag and Another type of guide is called “ﬁsh-tail” and is quite immerse it in 45e60C water [13]. effective for feeding rod-shaped beads. Fig. 4.27 shows a schematic of a ﬁsh-tail guide in which a hole Starting the calendaring process can be difﬁcult bored through the guide accommodates the bead. The because of the slippery nature of the bead, particularly guides can be made from a metal like aluminum or with round cross-section beads. The ﬁrst 20e30 cm of from a hard plastic such as polyacetal (Delrin by the bead can be ﬂattened prior to feeding to make it DuPont). The guide should be machined to match the easier to feed into the gap between the rolls. Another curvature of the rolls at the feed point and should be method is to wrap the end of the bead in a rough paper narrower than the width of the tape product without to allow friction to develop to carry the extrudate restriction to control the movement of the tape. A through the nip. For a rectangular bead, the gap can be dancer roll (Fig. 4.26) should be installed to supply increased at the start and then reduced. The variables of calendaring are roll speed and the surface temperature of the rolls. In general, it is difﬁcult to predict the impact of these variables on the tape properties because they depend on the characteristics of the bead itself. Sometimes an in- crease in the roll speed leads to a wider tape. Tem- perature increase, up to a point, is beneﬁcial to soften the bead and reduce the calendaring pressure. Pre- mature lubricant loss occurs if the temperature is raised excessively. Tape splitting and rapid lubricant loss result from high roll temperature. 4.10.4 Stretching the Polytetraﬂuoroethylene Tape Lubricant must be removed from the tape before stretching and before winding up the ﬁnal product. The lubricant can be removed by direct drying in an oven or by extracting the lubricant with a more vol- atile solvent and a lower oven temperature. The extraction technique [2] is less common because of safety, health, and cost issues, but when this method is used the evaporated solvent is usually removed from the oven efﬂuents and recycled. It is sometimes processed in a pollution abatement device to recover the heat for use in the process. A variety of oven designs have been used to remove lubricant with satisfactory results. They include inter- nally heated ﬂat plates, heated drums, resistance

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 95 heaters, and infrared lamps. The ovens must be thor- performance. Typical uniaxial stretch is less than 150% and is carried out at less than 10% per second [13]. oughly exhausted to assure that hydrocarbon concen- tration is well below its lower explosive limit. Fig. 4.28 The difference in linear speed of the two rolls determines the amount of stretch achieved. Eqs. (4.3) depicts a typical convection drying oven in conjunction and (4.4) offer simple formulas to calculate the total with a stretching step. Temperature settings depend on stretch and stretch rate. the volatility of the solvent, with a typical range of 150e300C air. It should be noted that the tape itself Speed 2 Roll should remain well (by 20e30C) below the melting Total Stretch ¼ Speed 1 Roll À 1 Â 100 (4.3) point of PTFE (342C). Even when only some of the polymer has reached its melting point, the tape loses its Stretch Rate ¼ Total Stretch (4.4) usefulness for wire wrapping. Distance Roll 1 to Roll 2 Tapes are basically stretched uniaxially, which Roll 2 SpeedÀRoll 1 Speed means that they are drawn in one direction. Two rolls No stretch takes place if the rolls move at the same with controlled variable speed and heat are the basic speed. An example where the tape is drawn is illus- trated below, where the two rolls are 14 m apart. requirements of stretching. The usual arrangement is illustrated in Fig. 4.28 where a section of the oven separates rolls 1 and 2. In addition to increased pro- duction yield, stretching is reported to improve the tape Roll 1 (speed m/min) Roll 2 (speed m/min) Total stretch (%) Stretch rate (%/min) 50 120 140 (From Eq. (4.3)) 700 (From Eq. (4.4)) Quality control of the tapes is accomplished by measuring the following properties of the ﬁnal product: Width and thickness Density (ASTM D4895-97) Tensile strength and elongation at break (ASTM D882) A record of these properties can often provide clues for troubleshooting the process. Typical prop- erties of calendared tape made from a commercial resin are given in Table 4.11. The ﬁlm or sheet can be stretched up to a ratio of between 1:10 and 1:15 without the formation of ir- regularities and with minimal reduction of thickness and width. Tape material becomes highly porous and stretching results in a substantial reduction of the speciﬁc gravity of the sheet/ﬁlm. Fig. 4.29 shows the development of the ﬁlm or sheet’s density in relation to the stretch ratio; tape thickness is expressed by density as proxy. Figure 4.28 Drying oven for removing lubricant and 4.10.5 Final Tape Product stretching the polytetraﬂuoroethylene tape [13]. Whether stretched or not, the ﬁnal tape product is slit to remove irregular edges and obtain the desired width. Slitting is sometimes done after drying and

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96 EXPANDED PTFE APPLICATIONS HANDBOOK Table 4.11 Typical Properties of Tape Produced by Calendaring [11] Calendar Roll Temperature (8C) Tape Properties 50 80 Width (mm) 95e100 115e120 Thickness (mm) Tensile strength (MPa) 75 75 Machine direction Transverse direction 11 16 Break elongation (%) 0.8 0.7 Machine direction Transverse direction 185 260 255 700 Figure 4.29 Dependence of ﬁlm density on the stretching ratio at a stretching temperature of 300C and a stretching speed of 1.7 m/min [17]. stretching (if any) has been completed. A separate machine is often used to slit the tape into the variety of widths required by the end use. A slitting machine (Fig. 4.30) is equipped with a payoff and a wind-up station and two parallel and adjustable knives. The speed and tension of the tape are also adjusted to obtain the best conditions for slitting. Figure 4.30 Top view of slitting zone in a tape ma- References chine with a width tolerance of 0.25 mm. Courtesy: QPD Inc., www.qdpusa.com, 2014. [1] F.A. Quinn, D.E. Roberts, R.N. Work, Volume- temperature for the room temperature transition in Teﬂon®, J. Appl. Phys. 22 (1951) 1085. [2] J.F. Luntz, J.A. Jaffe, L.E. Robb, Extrusion prop- erties of lubricated resin from coagulated disper- sion, Ind. Eng. Chem. 44 (8) (1952) 1805e1810.

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4: FABRICATION AND PROCESSING OF FINE POWDER POLYTETRAFLUOROETHYLENE 97 [3] Teﬂon® PTFE Fluoropolymer Resin, Processing Molding Powders, Asahi Glass Corp., Guide for Fine Powder Resins, DuPont Materials September 2002. for Wire and Cable, DuPont, Wilmington, DE, [11] The Processing of PTFE Coagulated Dispersion 1994. Powder, Fluon® PTFE Resins, Imperial Chemi- cal Industries, Ltd., 1986. [4] Fluorocarbon Polymers of Daikin Industries, [12] Benbow and Bridgwater. Paste Flow and Extru- Daikin-Polyﬂon® TFE Fine Powder, Daikin In- sion, Clarendon Press, Oxford, 1993. dustries, Ltd., Osaka, Japan, 1986. [13] pub no. RWJ220, Teﬂon® PTFE Fluoropolymer Resin, Thread Sealant Tape Processing Guide, [5] S.V. Gangal, Polytetraﬂuoroethylene, homopol- DuPont, March 2003. ymers of tetraﬂuoroethylene, in: second ed.En- [14] S. Ebnesajjad, Fluoroplastics, in: Non-melt cyclopedia of Polymer Science and Engineering, Processible Fluoropolymers, second ed., vol. 1, vol. 16, John Wiley & Sons, New York, 1989, pp. Plastics Design Library, Elsevier, 2014. 577e600. [15] G. Hougham, P.E. Cassidy, K. Johns, T. Davidson (Eds.), Fluoropolymers 2 e Prop- [6] K.R. Makinson, D. Tabor, The friction and wear erties, Kluwer Academic, Dordrecht, 1999. of polytetraﬂuoroethylene, Proc. R. Soc. 281 [16] P.D. Patil, J.J. Feng, S.G. Hatzikiriakos, (1964) 49. Constitutive modeling and ﬂow simulation of polytetraﬂuoroethylene (PTFE) paste extrusion, [7] N.G. McCrum, An internal friction study of J. Non-Newtonian Fluid Mech. 139 (2006) 44e53. PTFE, J. Polym. Sci. 34 (1959) 355. [17] Processing of Dyneon PTFE Fine Powder, Pub No. PTFEFP201309EN, Dyneon GmbH, 3M [8] Isopar® Solvents, Publication DG-1P from Advanced Materials Division, September 2013. Exxon Corp., 1994. [18] Technical Service Note F11 Coloring Polytetra- ﬂuoroethylene, Asahi Glass Corp., September [9] Solvents, Shell Mineral Spirits. Shell Corp. 2002. Pub. SC 1996;1237e96, and Shell Chemical Co. Tech. Bulletin SC 1998:1219e98R, Shell Hydrated Hydrocarbon Solvents Typical Prop. [10] The Processing of PTFE Coagulated Dispersion Powder. Technical Service Note F3/4/5,

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5 Expansion of Polytetraﬂuoroethylene Resins OUTLINE 5.1 Introduction 99 5.5 Formation of Expanded Polytetraﬂuoroethylene 5.2 Manufacturing Expanded 99 118 Polytetraﬂuoroethylene Articles 5.6 Amorphous Locking 5.2.1 Basic Polytetraﬂuoroethylene Expansion 100 122 Processes 101 5.7 Characterization of Membrane Pores 5.2.2 Uniaxial Expansion 107 5.7.1 Bubble Point 122 5.2.3 Biaxial Expansion 5.7.2 Derivation YoungeLaplace Equation 123 5.7.3 Mercury Porosimetry 124 5.3 Microstructure of Polytetraﬂuoroethylene 111 124 5.8 Summary 5.4 Microstructure of Expanded 116 125 Polytetraﬂuoroethylene References 125 5.1 Introduction manufacture ePTFE parts using published examples. A discussion of molecular changes follows. Addi- Polytetraﬂuoroethylene (PTFE) is a simple poly- tional topics have been included throughout to assist mer made of two elements of carbon and ﬂuorine. in clarifying the subject. PTFE has a long linear chain consisting of carbon atoms to which the ﬂuorine atoms are bonded. A 5.2 Manufacturing Expanded unique characteristic of PTFE is its response to tensile Polytetraﬂuoroethylene Articles high strain rates. When a PTFE ﬁlm is elongated at a sufﬁciently high rate, at a temperature close to but This section discusses ePTFE, beginning with below its melting point, it does not break but instead methods for manufacturing simple shapes such as expands and produces a strong porous structure. planar membranes. Methods for producing speciﬁc shapes, including three-dimensional, tubular ePTFE PTFE resins for manufacturing expanded mem- shapes, are described in Chapter 6 in this book. branes are produced by the process known as emul- sion (or dispersion) polymerization [1]. This process To begin, when working with ePTFE it is crucial to yields an aqueous dispersion of small particles that are establish and understand strain and strain rate, which coagulated and dried to obtain coagulated dispersion are deﬁned according to the following relationships: resin, or ﬁne powder. Successful expansion of PTFE requires highly crystalline and extremely high mo- True strain ¼ et ¼ lnðL=LoÞ (5.1) lecular weights and MWs resins. Standard speciﬁc gravity (SSG) of PTFE may approach 2.14, which is Engineering strain ¼ e ¼ ðL À LoÞ=Lo (5.2) roughly equivalent to a MW of 30 million, although resins with SSG up to 2.17 can be expanded. where Lo is the original length and L is the ﬁnal length This chapter describes the basic processes for producing expanded PTFE (ePTFE) articles and ex- plains changes in PTFE structure during expansion. It begins with a discussion of the required steps to Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00005-5 99 Copyright © 2017 Elsevier Inc. All rights reserved.

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100 EXPANDED PTFE APPLICATIONS HANDBOOK Stretch ratio ¼ L=Lo ¼ l (5.3) Stretch or strain rate ¼ l; (5.4) 1=sðmultiply by 100 to convert to %=sÞ Engineering strain rate ¼ e ¼ ½ðL À LoÞ=Lo; 1=s (5.5) e¼lÀ1 Figure 5.1 Major steps in the process of manufacturing a planar expanded polytetraﬂuoro- Expansion rate, strain rate and stretch rate have ethylene membrane. PTFE, polytetraﬂuoroethylene. been used interchangeably in this chapter. preform (“preformed paste”), neither of which 5.2.1 Basic Polytetraﬂuoroethylene exhibit ﬁbrillation. The micrograph in the lower left Expansion Processes (“extruded paste”) shows the microstructure of the paste at the early stages of extrusion in a funnel- Fig. 5.1 describes, step by step, the process for shaped die. The paste is characterized by ﬁbrilla- manufacturing a tubular ePTFE shape. See Chapter 4 tion primarily in the direction of extrusion. for details on paste extrusion, including lubricant The micrograph on the lower right (“extruded mixing, preforming, calendaring, and sintering. paste”) shows the microstructure of the paste after Because of the importance of the structure of the it has exited the sizing die. The key characteristic paste-extruded ﬁlm to the expansion, this section of the extrudate is that it contains ﬁbrils in brieﬂy reviews the paste extrusion process. different directions. Using a coat hangereshaped die would create ﬁbrils in both extrusion and The ﬁrst step is the selection of an appropriate cross directions. Proper ﬁbrillation in both di- resin. Table 5.1 shows examples of commercial rections is important to biaxial expansion of the PTFE resins that could be used in expansion process. PTFE ﬁlm. These resins have high MW and are highly crystal- line. First the PTFE resin is blended with a lubricant (extrusion aid), usually a naphtha solvent. The con- centration of the lubricant in the mixture is in the range of 15e22% by weight. The extrusion aid must be able to saturate the powder even though it may not completely wet the particles of the resin. It must also be easily removable from the extrudate after extru- sion and/or calendaring. Mixing the powder and the lubricant is critical to assure the paste’s uniform ﬂow. To prevent premature ﬁbrillation or shear damage to the resin, the blending operation must be performed while the resin is kept below the transition temperature. After aging, the paste is formed into a cylindrical preform that is loaded into the barrel of a paste extruder to produce the ﬁlm (tape) that will then be ready for calendaring and expansion. Fig. 5.2 shows scanning electron microscopy (SEM micrographs) of the microstructures of PTFE mixed with a lubricant (“virgin paste”) and the

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 101 Table 5.1 Examples of Commercial Polytetraﬂuoroethylene Resins for Expanded Poly- tetraﬂuoroethylene Production Resin Designation Standard Speciﬁc Reduction Ratio Particle Teﬂon® 601X Gravity <100:1 Size (mm) Teﬂon® 602A 2.16 <150:1 Fluon PTFE CD123 2.17 500 Hostaﬂon TF 2029Z 2.16 25e300:1 550 2.15 5e100:1 475 500 Figure 5.2 Changes in polytetraﬂuoroethylene paste as a result of preforming and paste extrusion [2]. The extruded ﬁlm, whether calendared or not, is 5.2.2 Uniaxial Expansion then dried to evaporate a signiﬁcant portion of the lubricant. In the following step the extrudate is Following are some examples, beginning with expanded longitudinally. Longitudinal expansion of uniaxial expansion capable of producing PTFE tapes the dried extrudate results in an ePTFE structure with speciﬁc gravity <1.4, to illustrate the expan- characterized by node and ﬁbril microstructure sion process [7]. Fig. 5.3 shows the process for [3e5]. The dried and longitudinally expanded expanding the structure in the machine direction extrudate is then put into a sintering oven and the (MD). A roll of unsintered paste-extruded PTFE ends are restrained to prevent longitudinal shrinkage tape (15 cm wide and 75 mm thick) was strung while it is partially sintered [6]. The product is a through the rolls in the equipment arrangement uniaxially expanded membrane. shown in Fig. 5.3. The PTFE ﬁlm (1) is pulled from the feed roll (2) by two rubber-coated rolls (3 oven

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102 EXPANDED PTFE APPLICATIONS HANDBOOK 3 Oven Inlet Roll 4 Oven Outlet Roll Table 5.2 Properties of Expanded Polytetraﬂuoro- ethylene Tape [7] Oven Machine Direction 1 Paste Extruded Stress Relaxation Rolls Expansion Thickness Speciﬁc Tensile PTFE Film 7 Strength 6 Level (%) (mm) Gravity (MPa) Unstretched 75 1.49 10.5 18 75 1.29 9.4 35 75 1.17 9.2 65 75 0.93 9.2 2 Feed Roll 8 Wind-up Roll Although the stretching between oven inlet and oven outlet rolls could be carried out at room tem- Figure 5.3 Schematic diagram of polytetraﬂuoro- perature, heating the tape in the oven during the ethylene (PTFE) tape expansion process [7]. expansion resulted in both improved uniformity and greater degrees of expansion. If the PTFE tape was inlet rolls). These rolls must be adjusted so that the heated in the oven to temperatures >200C, and tape does not stretch in between the feed and oven optimally to 300C, the stress-relaxation roll (6) did inlet rolls. The tape then passes though an oven (or not need to be heated as long as the tape speed two heated platens) so that the tape temperature remained fairly slow. After the expanded PTFE tape reaches 300C. had cooled, it could be slit and wound onto cores for use. There was very little, if any, retraction, and the The oven outlet rolls (4) are coated with silicone resulting wound cores did not collapse, nor did the rubber so that they can withstand contact with the tape self-adhere (also called blocking). heated PTFE tape. They rotate at a higher speed than the oven inlet rolls, which results in the Table 5.2 shows that the thickness of the drawn stretching of the PTFE tape. In this example, ﬁlm remained unchanged at about 75 mm, regardless the inlet rolls (3) moved at 9 m/min and the outlet of the percentage of expansion. Speciﬁc gravity, rolls (4) speed was increased to 10.7, 11.9, and however, decreased with increased stretch. Once- 15 m/min. The stress-relaxation roll (6) was heated expanded tape can be expanded a second time, tak- to 310C. Although it can be set at a somewhat ing care to insure uniform thickness. Multiple ex- higher or lower temperature, it must be below pansions can yield tape with low speciﬁc gravity (less 342C, which is the melting temperature of unsin- than 0.5). tered PTFE. The hot tape was then cooled down by contact with the cooling roll (7). Finally, the tape Another early example of PTFE expansion [3] was collected on a wind-up roll driven at low tor- provides additional insights into the expansion pro- que by a slip clutch (not shown). Rolls 6 and 7 are cess. In this case it was reported that uniaxial or rotated at the same peripheral speed as rolls 4 and biaxial stretching, under appropriate process condi- 5. They are not pinch rolls, though they may be in tions, could expand unsintered parts. The ﬁnal close proximity to each other. The stretching products were porous and mechanically strong. The between pinch rolls 3 (oven inlet rolls) and draw key variables include PTFE resin type, the state of the rolls 4 (oven outlet rolls) is controlled by gear paste extrusion, calendaring conditions, the rate of adjustments in their drive mechanisms (not shown). stretching, and the optimal temperature (between 300 A common driving device is used for each pair of and 325C). Though temperatures must always rolls, ie, 3, 4, and 6/7 [7]. remain below the melting point, higher temperatures may be required depending on the chemical structure of the resin. Heat treatment of the expanded products at a temperature in the range of 350e370C enhanced their mechanical strength.

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 103 At temperatures below the range of 300e325C, Figure 5.4 Alternative process for uniaxial expan- an upper limit to the rate of expansion existed beyond which the part fractured. The products expanded at sion of polytetraﬂuoroethylene (PTFE) [4]. lower temperatures also exhibited signiﬁcantly lower tensile properties. The lower temperature limit is was preheated to the expansion temperature. Rolls 2 quite important in the commercial expansion of and 3 had the same diameter and were connected PTFE products. At higher temperatures within this such that their relative rates of rotation could be range, only a lower limit for expansion rate was changed. Roll 3 could be driven faster than roll 2 so found. The lower limit of the expansion rate inter- that the ﬁlm was stretched in the gap (“A”) between acted with temperature in a roughly logarithmic the two rolls. The difference in speeds determined the manner. In other words, the lower limit of expansion amount of stretch and thus the amount of expansion. was much higher as the temperature increased. Most When roll 3 was driven three times faster than roll 2, of the commercially desirable products were ob- the ﬁlm was expanded approximately 200%. Unlike tained when expansion was carried out at tempera- nearly every other material, the thickness, width, and tures within the range of 300e325C. The balance of length of the PTFE ﬁlm all increased by 200%. This the orientation of the extruded shape also affected the is because the increase in porosity increased the relationship between the proper range of rates and volume and also reduced the speciﬁc gravity. temperature. The relative positions of rolls 2 and 3 were It was reported [3] that some resins were much adjustable so that the gap between them could be more useful for the expansion process than others as varied and the expansion rate could be controlled. If illustrated by the wider processing ranges of expan- the gap distance was halved, the rate of expansion sion rate and temperature. The primary requirement was doubled. The rate of expansion also, of course, of suitable resins was a very high degree of crystal- depended on the speed of feeding the ﬁlm into rolls 2 linity (!98%). The best choice was to process a and 3. The expanded ﬁlm left roll 3 and went onto PTFE resin that was polymerized with extremely roll 4, which was running at the same linear speed. high crystallinity. Alternatively, annealing at high Here the ﬁlm was heated to 370C so that amorphous temperatures just below the PTFE melting point locking could take place. The time the ﬁlm spent on could raise the crystallinity of a PTFE resin. The this roll was controlled by the position of roll 5, resulting beneﬁt was a signiﬁcant improvement in the which can be moved around the periphery of roll 4. performance of the resin in the expansion process, Roll 6 is water cooled to reduce the temperature of such as increase in the product yield. the ﬁlm before it is wound up on roll 7. Throughout this process the temperature, expansion rate, and Copolymers of tetraﬂuoroethylene (TFE) with extent of expansion can be easily controlled. other monomers do not work well, contrary to its homopolymers, in the expansion process. This is Multilayer ﬁlms can be made by laminating two or because the comonomers appear as defects in the more layers of expanded ﬁlm [4]. A quantity of polymer chain [8], which reduces the efﬁciency of expanded tape was produced by setting the heated the expansion process. Resins with very low mono- roll 4 in Fig. 5.4 at 300C, which is less than the mer content were found to be expandable with amorphous-locking temperature. This ﬁlm had a reasonable efﬁciency. For example, standard per- speciﬁc gravity of 0.60 in MD, tensile strength of ﬂuorinated ethyleneepropylene copolymer (FEP) is 13 MPa (a matrix tensile strength of 50 MPa), a copolymer of TFE and hexaﬂuoropropylene (HFP) containing 13% of the latter by weight. Standard FEP cannot be expanded, while a copolymer of 0.2% weight HFP and TFE could be expanded at high temperatures just below the upper end of the tem- perature range, which is still below its melting point. Fig. 5.4 presents an alternative process for uniaxial expansion that is useful for manufacturing long lengths of expanded ﬁlm [4]. Unsintered ﬁlm from the paste extrusion process was fed to the machine from roll 1 onto heated rolls 2 and 3, where the ﬁlm

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104 EXPANDED PTFE APPLICATIONS HANDBOOK transverse direction (TD) tensile strength of 0.7 MPa, An even more important observation was made and a thickness of 88 mm. when studying the interaction of water and ePTFE membranes. When an attempt was made to ﬂow water Two sections of this ﬁlm were stacked at right through the (air-saturated) ePTFE at a pressure of angles to each other. They were placed in a clamp 34 kPa, no ﬂow occurred. When the pressure excee- with all four edges of the sandwich secured while ded 68 kPa, however, water began to ﬂow through the the two layers pressed against one other. This as- air-saturated ePTFE membrane. Water ﬂow continued sembly was given an amorphous-locking treatment similarly to the ﬂow of wetting organic liquids [4]. by heating it to about 370C for 7 min. Then the whole assembly was rapidly cooled with a stream of The next ﬁnding was the beginning of revolu- cold air and the clamps were released, yielding the tionizing apparel and led to a number of other ap- desired one-piece laminated ﬁlm. The tensile plications as well [9]. ePTFE was used in waterproof strength of the expanded, amorphous-locked lami- garments and tents because it was resistant to surface nate was 30 MPa in both MD and TD and its active agents in perspiration but still permitted the thickness was 160 mm. evaporation of the perspiration and the transfer of moisture vapor through the layered article. At least A ﬁltration membrane could be produced by two layers were combined for this application: (1) an expanding a paste-extruded PTFE ﬁlm according to interior, continuous hydrophilic layer (eg, poly- the process outlined in Fig. 5.3 at a temperature of urethane) that allowed water to diffuse through, 300C followed by amorphous locking at 370C (on prevented the transport of surface active agents and heated roll 4). The properties of the original ﬁlm and contaminating substances such as those found in of the expanded and amorphous-locked ﬁlm are perspiration, and was substantially resistant to the summarized in Table 5.3, which reveals the superi- pressure-induced ﬂow of liquid water; and (2) an ority of the expanded ﬁlm. ePTFE layer that permitted the transmission of water vapor and provided thermal insulating properties Signiﬁcant useful ﬁltration characteristics were even when exposed to rain. discovered for the ePTFE membranes. For instance, smoke could be separated from air using ePTFE Garments made of these materials were perma- membranes. In another experiment, samples of the nently waterproof. They repelled all exterior water membranes were used to ﬁlter solids from suspen- yet allowed the evaporation of perspiration whenever sions of the solids in various organic liquids. Again, the partial pressure of water vapor inside the garment good separations were obtained and ﬁltering exceeded that outside. In practice, these garments rates were reasonably high. Yet similar attempts withstood nearly all climate conditions. The hydro- using samples of the dry paste-extruded ﬁlms philic ﬁlm had a moisture vapor transmission rate were unsuccessful because of their extremely low >1000 g/m2 day, and preferably above >2000 g/ ﬁltering rates. m2 day that permitted no detectable transmission of surface active agents and permitted no detectable Table 5.3 Properties of Expanded Polytetraﬂuoro- ﬂow of liquid water at hydrostatic pressures up to ethylene as a Filtration Membrane [4] 172 kN/m2. The hydrophobic layer had a moisture vapor transmission rate >1000 g/m2 day, and pref- Film Property Paste Expanded erably >2000 g/m2 day, and an advancing water Extruded Amorphously contact angle >90 degrees [9]. Thickness (mm) Surface areaa 100 Locked Another type of laminate consisted of a ﬂexible (relative units) 1 88 inner layer of ePTFE (hydrophobic) with a moisture 2.8 vapor transmission rate >1000 g/m2 day and a con- Permeability to air 1 tact angle >90 degrees, combined with a continuous (relative units) 320 outer hydrophilic layer attached to the outer face of 7 the inner hydrophobic layer. This hydrophilic layer Permeability to 2300 had a moisture vapor transmission rate >1000 g/ kerosene (relative m2 day. It contained a particulate solid or a liquid units) additive such as color pigments and antistatic agents. An additional textile layer could be attached to the a Width Â length. inner surface of the hydrophobic layer for strength or

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 105 aesthetic reasons if desired. This laminate’s other 3. Gauge blocks were placed on the perimeter of signiﬁcant properties included bacterial penetration the sheet. (Gauge blocks were not used for in excess of 1000 min, water entry pressure above densiﬁcation to maximum density.) 138 kPa, and a moisture vapor transmission rate >2000 g/m2 day. It could also be made antistatic. 4. The plates, with ﬁlm between, were placed in- This type of laminate is particularly useful in both side the press. biological and healthcare applications [10]. 5. The platens were closed until contact was Coarse and highly porous ePTFE parts that are made. strong and have microstructures of relatively large nodes interconnected by relatively long ﬁbrils as 6. The steel plates were heated to the desired compared to those described so far have also been temperature for densiﬁcation. manufactured. Such microstructures are desirable in many applications, and particularly in the biological 7. Pressure was applied and both steel plates ﬁeld, where the microstructure must be large enough were slowly brought into contact with the to allow cellular ingrowth and incorporation of body thickness gauge blocks (or specimen, where tissue. ePTFE parts with these qualities are used, for gauge blocks were not used). example, as a mesh to repair hernias. 8. The pressure was held for a sufﬁcient time to In a trial to manufacture coarse ePTFE, the prop- obtain the desired density. erties of the dry and calendared ﬁlm were as follows: thickness about 406 mm, density about 1.6 gm/cc, 9. The pressure was released. matrix tensile strength in the direction of extrusion 11 MPa, and matrix tensile strength in the transverse 10. Materials that were densiﬁed at higher than (width) direction 4 MPa [5]. The dry, calendared ambient temperatures were cooled in water extrudate was cut into specimens approximately upon removal from the press. 11.4 cm by 11.4 cm. Some of the specimens were then densiﬁed by compression in a Carver press that The 11.4 cm by 11.4 cm specimens were weighed could be heated, and the remaining specimens were prior to the densiﬁcation step. Thickness measure- left undensiﬁed at the 1.6 g/mL density level to serve ments were taken at the four corners, about 2.5 cm as test controls. from each edge, and these four readings were aver- aged. The density was calculated by dividing the A range of densities was examined, from 1.6 g/mL weight of the specimen by the area times the average (the undensiﬁed control) to values approaching the thickness. This procedure yields a specimen’s nom- maximum achievable density of 2.3 g/mL. Densiﬁ- inal density, since the thickness of the specimen cations were carried out at temperatures from varied due to local inconsistencies. Materials ambient (22C) to slightly above 300C. The times to (densiﬁed and undensiﬁed) were then stretched on a reach the desired densiﬁcation temperature and the pantograph in the MD (ie, the primary direction of times to reach the desired densiﬁcation at these both extrusion and calendaring) to expand them. The temperatures were noted. The control pieces were ﬁlm was heated to the desired temperature for subjected to the same temperatures and time condi- stretching by heater plates directly above and below tions. For convenience, two ﬁlm samples were the specimens. stacked together with a sheet of polyimide ﬁlm be- tween them so that two 11.4 cm by 11.4 cm samples The stretch conditions were: of ﬁlm could be densiﬁed simultaneously [5]. Temperature: approximately 300C. The following steps were used to densify the dry ﬁlm: Stretch ratio: 4:1 (300% increase in length). 1. Carver press platens were heated to speciﬁed Stretching rate: approximately 400%/s (deter- temperature. mined by dividing the percent change in length by the duration of the stretching 2. Film was inserted between two ﬂat steel operation). plates, along with polyimide ﬁlm, to serve as a release agent. The stretched specimens were then placed on a pin frame to restrain them from shrinking and immersed in a 370C salt bath for about 20 s to sinter them.

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106 EXPANDED PTFE APPLICATIONS HANDBOOK Table 5.4 Properties of Coarse and Highly Porous Uniaxially Expanded Polytetraﬂuoroethylene Samples [5] Prestretch Density (g/cc) Property 1.63a 1.83 2.01 2.27 Thickness (mm) 302 300 290 284 Density (g/mL) 0.56 0.57 0.59 0.58 Matrix tensile strength (MPa) 107 108 110 114 Fibril length (mm) 23 Node width (mm) 4 5 5 15 EBP (kPa) 3 3 4 17 Gurely no. (s) 54 47 44 6.5 Crushability (%) 27.5 23.8 19.4 Coarseness index (g/mL/kPa) 15 14 14 9 0.010 0.012 0.013 0.033 EBP, ethanol bubble point (isopropanol is commonly used because of lower volatility and better wetting PTFE) is a measure of the maximum pore size in the test specimen (see ASTM F316). Speciﬁcally, the EBP is the minimum pressure required to force air through an ethanol- saturated article. Gurley no., indicator of coarse structure is relatively low resistance to the passage of air (Gurley number). Gurley number is deﬁned as the time in seconds for 100 cm3 of air to ﬂow through one square inch of material for a pressure of 12.5 cm of water across the material. See ASTM D-726 for a method of measuring the Gurley number. Coarseness index, deﬁned here as the density of the stretched porous article divided by the EBP of that article. a Control, no densiﬁcation step. Temperature did not appear to signiﬁcantly affect measure crushability, the specimen was placed under the densiﬁcation process. The data reported in a tensile load by applying a 2.2 N force to the ma- Table 5.4, therefore, are averages of the measure- terial in the direction of stretching. Thickness was ments obtained for given densities irrespective of the measured and was deﬁned as the original thickness. densiﬁcation temperature. All data for matrix tensile Next, an 680 g weight covering a 7.5 mm2 area was strength, ﬁbril length, and node width were reported applied to the specimen for 0.5 min and the resulting for measurements made in the direction of stretch thickness was recorded with the weight still in place. (which was also the primary direction of extrusion Percent crush, or crushability, is deﬁned as (tÀC)/t and calendaring). Break forces were measured using (100%), in which t is the original thickness and C is specimens that were 25 mm long; the tensile tester the thickness under load. Lower values of crush- crosshead speeds were 25 cm per minute [5]. ability, therefore, indicate a higher resistance to being crushed (ie, a higher crush resistance). The density prior to stretching is listed as a single number and is the aforementioned nominal value. In spite of the relative simplicity of the expansion The actual densities after densiﬁcation varied due to process, the characteristics of the resulting ePTFE experimental variability and inevitable small mea- can be altered by changes in the process. For surement error. Thus the individual measurements for example, it was possible to obtain different micro- the 1.63 g/mL materials ranged from 1.60 to 1.64 g/ structures on two side of the same membrane mL. The individual measurements for the 1.83 g/mL (Fig. 5.5) by manipulating the temperatures of the materials ranged from 1.75 to 1.85 g/mL. The indi- fast and slow rolls used to stretch the ﬁlm in MD. The vidual measurements for the 2.01 g/mL materials ﬁbril lengths and thickness and the homogeneity of ranged from 1.97 to 2.04 g/mL. The individual the nodes could be altered on the two sides of the measurements for the 2.27 g/mL materials ranged membrane [11]. from 2.19 to 2.35 g/mL. Therefore, the nominal range of 2.27 g/mL includes the maximum obtain- In one attempt, a calendared PTFE ﬁlm was able densities. stretched using a pair of 120 mm rolls capable of being heated to 330C. The distance of stretching the Crushability is the extent to which the ePTFE ﬁlm was 8.5 mm, the rotation ratio of the rolls thickness is decreased under mild pressure. To was 1:9, the surface speed of the low-speed roll was

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 107 Figure 5.5 Microstructures on two sides of expanded polytetraﬂuoroethylene (white matter is polytetraﬂuoroethy- lene ﬁbrils and nodes) [11]. Table 5.5 Properties of Asymmetric Expanded Polytetraﬂuoroethylene Membrane [11] Temperature of Porosity (%) Machine Direction Tensile Thickness (mm) High-Speed Strength (kPa) Rolla (8C) 280 Before After Before After Before After Sintering Sintering Sintering Sintering Sintering Sintering 230 79 74 24 42 120 90 180 82 75 18 42 120 80 82 77 14 47 110 80 130 84 78 9.5 40 100 90 a Slow roll is at 130C. 2 m/min, and its temperature was 130C. The tem- laboratory stretching equipment, also called panto- perature of the high-speed roll was varied as shown in graph. Two pieces of equipment have been used in Table 5.5. Finally, the ﬁlm was sintered at a tem- the laboratory for ﬁlm stretching. The T.M. Long perature of !327C. Characteristics of the product stretcher, supplied by the AccuPull division of ﬁlm are shown in Table 5.5. Inventure Laboratories, was one of the ﬁrst labora- tory ﬁlm-stretching devices. The second machine is The asymmetric porous ePTFE ﬁlm in Fig. 5.5 the Karo IV unit from Bruckner [12]. consisted of nodes linked by very ﬁne ﬁbrils. The length and thickness of the ﬁbrils or state of the nodes The T.M. Long stretcher operates by the move- on one surface are different from those on the other ment of two bars that are perpendicular to each other surface. The degree of asymmetry depended on the and correspond to the MD and TD of the ﬁlm. The process condition during manufacturing of the stretching heads are hydraulically driven and the expanded ﬁlm. In ﬁltration applications the ﬁlm was grips that hold the sample during the stretching installed such that the surface with the smaller pore size process are also driven by hydraulics. Figs. 5.6 and was on top, in contact with process ﬂuids (solution). 5.7 show two different styles of frames and grips. The device can orient ﬁlms in both uniaxial and biaxial 5.2.3 Biaxial Expansion mode at stretching ratios up to seven times. Sample size is 50 mm Â 50 mm. In biaxial stretching, the Before outlining the process for the biaxial device stretches the ﬁlm in the MD and TD expansion of PTFE, it is necessary to describe the

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108 EXPANDED PTFE APPLICATIONS HANDBOOK Machine Cross Direction Direction Figure 5.6 Photograph of the stretch frame (head) of a biaxial laboratory stretcher. Courtesy: AccuPull, Division of Inventure Laboratories, http://accupull.com/gallery.php, 2012. Figure 5.8 Photograph of a biaxial laboratory stretcher [13]. Courtesy: Inventure Laboratories, Inc. Figure 5.7 Photograph of a biaxial laboratory second stretching oven in the Karo IV stretcher stretcher during operation. (Fig. 5.9) that can be operated completely indepen- Courtesy: AccuPull, Division of Inventure Laboratories, dent of the ﬁrst oven. With this second oven, opera- http://accupull.com/gallery.php, April 2015. tors can study the effects on the material of stretching in one direction before stretching it in another di- simultaneously. The device can also be set to stretch rection. This is signiﬁcant because the deformation of in these different directions sequentially. The ﬁlm a ﬁlm in one direction can signiﬁcantly affect its deformation takes place in a temperature-controlled subsequent crystallization and orientation in addi- environment. AccuPull has made signiﬁcant up- tional steps. grades to the T. M. Long stretcher, including addi- tional automation and computerization (Fig. 5.8). Some examples show the process and the effect of variables on the ﬁnal product [3]. A biaxial stretcher The Karo IV unit (from Bru¨ckner, www.brueckner- used in these experiments was capable of stretching usa.com) operates on similar principles [12]. Its 10 cm Â 10 cm samples of ﬁlm to 40 cm Â 40 cm. stretch ratios extend up to 10 times in both the MD Actuated clamps gripped the 10-cm square ﬁlm on and TD. A maximum stretching temperature of each side and moved apart uniformly. The ﬁlm was 400C is attainable, which means that practically any heated by hot air ﬂowing from all directions. polymer can be evaluated with this equipment. An advantage of the Karo IV unit is its ability to more In another experiment, a sample of ﬁlm that was directly simulate the tenter frame process. This is 15 cm wide, 914 mm thick, and of continuous length made possible through the optional addition of a was produced by paste extrusion. After the lubricant was evaporated, the unsintered ﬁlm was tested. It had the following properties: speciﬁc gravity of 1.65, MD tensile strength of 0.2 MPa, and TD tensile strength of 1.7 MPa. The following paragraphs describe a number of stretching trials that were run on samples of this ﬁlm. A sample of paste-extruded ﬁlm that had not been dried and that contained lubricant was calendared down to a thickness of 109 mm. The physical prop- erties of the ﬁlm were measured and recorded as follows: speciﬁc gravity of 1.60, MD tensile strength

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 109 Figure 5.9 Multiple oven arrangements in a Karo IV biaxial stretcher. Courtesy: Bru¨ ckner, www.brueckner-usa.com. of 15.5 MPa, and TD tensile strength of 1.9 MPa. Another 10 cm Â 10 cm sample of the paste- Samples of this ﬁlm were stretched at different extruded ﬁlm was stretched in the pantograph ma- temperatures and expansion rates. Table 5.6 sum- chine [4]. In this case, the ﬁlm was heated to 300C marizes the results. The data indicate that the ﬁlm and stretched simultaneously in two directions, 100% responded differently depending on which axis was stretched but that, at low expansion rates, rupture in each direction. The surface area of the stretched occurred irrespective of the direction of expansion. ﬁlm was thus four times the surface area of the original ﬁlm. Linear stretching rates of about 400% Table 5.6 Effects of Expansion Rate and Low to Moderate Temperature on the Process and Product [3] Temperature (8C) Expansion Rate Expansion Rate Result 225 in Longitudinal in Transverse 225 Direction (%/s) Direction (%/s) Ruptured 500 500 0 Longitudinal tensile ¼ 27 MPa (a matrix tensile strength of 88 MPa) 500 500 500 Transverse tensile ¼ 8 MPa 225 0 Speciﬁc gravity ¼ 0.70 50 500 0 50 500 500 Ruptured 50 0 5 Ruptured 225 5 0 225 5 5 Ruptured 225 0 5 50 5 0 Longitudinal tensile ¼ 16.5 MPa (a matrix tensile 50 5 5 strength of 51 MPa psi) 50 0 Transverse tensile ¼ 19 MPa Speciﬁc gravity ¼ 0.75 Ruptured Ruptured Ruptured Ruptured Ruptured Ruptured

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110 EXPANDED PTFE APPLICATIONS HANDBOOK per second in each dimension were used. With the the remaining specimens were left undensiﬁed at the expanded ﬁlm still in tension (as stretcher clamps 1.6 g/mL density level to serve as test controls. held the stretched ﬁlm), hot air was circulated over the ﬁlm for 5 min to raise the air temperature to All four samples were stretched simultaneously in 360C. This rise in temperature caused amorphous two directions at right angles to each other in the locking within the ﬁlm. In the ﬁnal step, with the biaxial stretching machine, 100% in each direction. stretcher clamps still holding it in tension, the ﬁlm The surface area of the stretched ﬁlm was thus four was cooled rapidly to room temperature by blowing times greater than the surface area of the original cold air over it. The cooled ﬁlm was then removed ﬁlm. The ﬁlm temperature was about 30C at the start from the clamps for testing. Table 5.7, which com- of the stretching operation. Stretching rates of about pares the properties of the original unexpanded ﬁlm 130% per second in each direction were used. The with those of the ﬁnal expanded, amorphous-locked stretching rate was determined by dividing ﬁlm, shows the advantage of this process. the percent change in length by the duration of the stretching operation. (The clamps of the pantograph In another biaxial expansion trial to manufacture moved apart at constant velocity.) The stretched coarse ePTFE, properties of the dry and calendared specimens were then placed on a pin frame to restrain ﬁlm were as follows: thickness of about 406 mm, them from shrinking, removed from the pantograph density of about 1.6 g/cc, matrix tensile strength in machine clamps, and immersed in a 370C salt bath the direction of extrusion of 11 MPa, and matrix for about 20 s, thereby sintering them. Finally, the tensile strength in the transverse (width) direction of specimens were cooled in water. The data in 4 MPa [5]. The dry, calendared extrudate was cut Table 5.8 show the properties of these samples. into specimens 11.4 cm by 11.4 cm approximately. Some of the specimens were then densiﬁed by The data in Table 5.8 show that the material that compression in a Carver press that could be heated; was densiﬁed the most prior to stretching was far more crush resistant than the materials that were Table 5.7 Properties of a Polytetraﬂuoroethylene densiﬁed less or not at all. Four additional samples Film Biaxially Expanded by 100% [4] were produced from the same raw materials, using the same processes, to further examine the beneﬁts of Property Original Expanded densiﬁcation with respect to crush resistance. The Unexpanded Amorphous- same range of densities prior to stretching was examined. These samples, unlike those whose data Film* Locked appear in Table 5.8, were not sintered subsequent to 1.9 stretching. The data for these unsintered specimens Film length 1 2.0 appear in Table 5.9. The crushability for the stretched (relative unit) 800 materials with prestretching densities of 1.63, 1.89, 0.45 2.06, and 2.29 g/mL were 30.1, 19.7, 10.2, and 3.6%, Film width 1 respectively. Therefore those materials that were (relative unit) 13.1 (Matrix 64) densiﬁed the most produced the most crush-resistant ﬁnal products. A comparison of the data for the sin- Film thickness 914 12.1 tered and unsintered materials that were not densiﬁed (mm) indicates that sintering serves to decrease crush- 6 ability for undensiﬁed materials (from 30.1% to Speciﬁc gravity 1.65 14.6%, in this case). The material that was densiﬁed the most but not sintered was still far more crush Machine 2.1 resistant (with a crushability of 3.6%) than the direction undensiﬁed material that was sintered (which had a tensile crushability of 14.6%). strength (MPa) It is also possible to expand ﬁlled ﬁlms of PTFE. Cross- 1.7 In one of the original experiments [14], PTFE was direction blended with an asbestos powder in a weight ratio of tensile four to one, resin to asbestos. The mixture was strength (MPa) lubricated and paste extruded into a ﬁlm 15 cm wide with a thickness of 900 mm. The ﬁlm was then Permeability to 400 air (relative units) * As paste-extruded, dried.

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 111 Table 5.8 Properties of Coarse and Highly Porous Biaxially Expanded Polytetraﬂuoroethylene Samples [5] Prestretch Density (g/cc) Property 1.61a 1.83 2.02 2.25 Thickness (mm) 282 277 267 310 Density (g/mL) 0.57 0.54 0.65 0.54 Ethanol bubble point (kPa) 67 48 114 Matrix MD tensile strength (MPa) 70 57 8 45 Matrix TD tensile strength (MPa) 70 77 48 44 Crushability (%) 15 17 59 Coarseness index (g/mL/kPa) 0.0014 0.0017 17 4 0.0019 0.0048 MD, machine direction; TD, transverse direction. a Control, no densiﬁcation step. Table 5.9 Properties of Coarse and Highly Porous Biaxially Expanded Polytetraﬂuoroethylene Samples [5] Prestretch Density (g/cc) Property 1.63a 1.89 2.06 2.25 Thickness (mm) 371 330 305 295 Density (g/mL) 0.58 0.63 0.61 0.72 Ethanol bubble point (kPa) 97 56 33 21 Matrix MD tensile strength (MPa) 30 30 33 26 Matrix TD tensile strength (MPa) 16 15 19 16 Crushability (%) 30.1 19.7 10.2 3.6 Coarseness index (g/mL/kPa) 0.0057 0.0114 0.0180 0.0328 MD, machine direction; TD, transverse direction. a Control, no densiﬁcation step. calendared to 200 mm thickness and the extrusion aid gravity of 0.95, longitudinal tensile strength of was removed by drying. The properties were 20 MPa, and transverse tensile strength of 5.2 MPa. measured and found to be as follows: speciﬁc gravity The heating of the ﬁlm therefore substantially of 1.44, MD tensile strength of 6.9 MPa, and trans- increased its tensile strength but had little effect on its verse tensile strength of 1.4 MPa. speciﬁc gravity. A 10 cm Â 10 cm sample was stretched in a 5.3 Microstructure of pantograph at a rate of 500% per second at a tem- Polytetraﬂuoroethylene perature of 225C. It was stretched to three times its original length in the MD while no stretch was One of the questions about PTFE that has puzzled applied in the TD. A sample of the ﬁlm was tested researchers is “why can it be expanded at high stretch and found to have the following properties: speciﬁc rates?” An entirely different product is obtained gravity of 0.82, MD tensile strength of 10.4 MPa psi, when PTFE is stretched at lower strain rates. What is and transverse tensile strength of 1 MPa. The even more unusual is that, in spite of its high work- remainder of the sample was placed in clamps to hardening rate (it becomes mechanically stronger restrain it from shrinking, heated to 370C for 5 min, when stretched), PTFE is expandable. PTFE also has and then cooled to room temperature. This sample was found to have the following properties: speciﬁc

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112 EXPANDED PTFE APPLICATIONS HANDBOOK a very stiff chain. Other polymers break when of some limitations and the age of the study, Rae and stretched beyond a certain strain rate at a given Brown deemed it the most extensive and reliable data temperature. available. In this section we explore PTFE molecules using a Previous studies looked at electron micrographs of granular (suspension polymerized) version of the multiple samples of PTFE structures exposed by resin (fundamental some PTFE molecule as emulsion cryofracture before and after deformation at various polymerized polymer). It is important to remember temperatures. Crystalline regions were observed and that emulsion-polymerized PTFE, not granular grade, their size was determined by crystallinity concentra- is used to produce expanded products. As- tions. These crystalline regions are long narrow bands polymerized emulsion PTFE possesses a high level with striations parallel to the long axis. The amor- of crystallinity [15], and its crystalline morphology is phous regions reside between the crystalline regions. quite different from that of the polymer recrystallized from a melt. The high degree of crystallinity in virgin In these studies, two deformation mechanisms PTFE (between 93 and 98) can be obtained only were identiﬁed in tension. It was assumed that at because the chain structure is unbranched [16,17]. temperatures higher than À196C, the amorphous regions became oriented while slip occurred in the Several in-depth studies were conducted in the crystalline regions along the parallel striations. As ﬁrst decade of the 21st century at Los Alamos Na- slip occurs, the crystalline regions tend to orient by tional Laboratory and the United States Naval rotation so that the long axis is along the pulling Research Academy. They focused on the tensile direction. This orientation reduces the deformation properties of granular PTFE, the effect of polymer that is possible. At higher strains, the crystals bow or crystallinity on those properties, and the failure kink around the striations (ie, they are no longer modes of parts. At Los Alamos National Laboratory, straight and parallel to the long axis of the crystal). Rae and Brown [18] evaluated a large number of past O’Leary and Geil [41] presented a menu of various studies on the tensile (and compressive) properties of types of deformation that PTFE crystals can undergo PTFE in addition to contributing data generated in under stress (Fig. 5.10). Koo accepted this defor- their own research [19e39]. A discussion of a mation mechanism and used it to explain his number of Rae and Brown’s published works and observation that, at low strains, samples with higher citations can be found elsewhere [40]. Rae and crystallinity were stiffer than samples with lower Brown found that Koo had undertaken by far the crystallinity. At higher strains, however, higher most thorough examination of the tensile response of crystallinity samples were less stiff than those with PTFE [27,29] even though he conducted his work in lower crystallinity. See Fig. 5.11 for SEM micro- 1969. Koo’s doctoral thesis presented a vast amount graphs of PTFE crystals before and after of research into a commercial grade of PTFE. In spite deformation. SHEAR SHEAR BOWING ROTATION KINKING Figure 5.10 Schematic of the types of deformation of polytetraﬂuoroethylene crystalsdarrows indicate direction of stress [41].

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 113 Figure 5.11 Scanning electron micrographs of polytetraﬂuoroethylene (A) before and (BeD) after deformation at different temperatures: (B) À40C; (C) 21C; (D) 70C [42]. Rae and Brown [18] conducted most of their own these different temperatures, with the exception of a studies of the tensile properties of PTFE at signiﬁ- deviation at 100C at high strains. At 100C, strain- cantly lower strain rates than those required for the ing the PTFE with higher crystallinity requires lower preparation of ePTFE. They report samples with stress than the less crystalline sample. The data in 28e38% crystallinity. These percentages are signif- Fig. 5.12 reveal the unusual nature of PTFE, even icantly below the minimum level (45%) believed to though the data were obtained at strain rates (stretch be attainable after melting PTFE, even by quenching ratios) several orders of magnitudes smaller than in ice water [1,43]. Regardless of the absolute values those used to expand PTFE. of the crystallinity in Rae and Brown’s samples, they were much lower than that of PTFE (c. 98%) used in Implicit in Koo’s model is the assumption that the expansion process. The evidence in Rae and increasing the crystallinity of PTFE results in larger, Brown’s data shows that increased crystallinity has not more crystalline, domains. This assumption has little impact on reducing the tensile stress of PTFE. been proven in recrystallization experiments in which PTFE was heated to a temperature well above Fig. 5.12 (stressestrain graphs) shows the effect of its melting point. The molten resin was then cooled at crystallinity on the tensile response of PTFE to strain different controlled rates. Davidson, Gounder, at 23 and 100C. PTFE appears to behave similarly at Weber, and Wecler examined samples of solidiﬁed

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114 EXPANDED PTFE APPLICATIONS HANDBOOK 200True Stress / MPa PTFE under different cooling rates using SEM Low crystallinity PTFE 23°C (Fig. 5.13) [43]. Several observations are made from High crystallinity PTFE 23°C the micrographs in Fig. 5.13. At the fastest cooling Low crystallinity PTFE 100°C rate (2C/min), crystal bands are narrow and scat- tered in different directions and there are signiﬁcant 150 High crystallinity PTFE 100°C amorphous areas in between the crystalline regions. At lower cooling rates, an increase in the size and 100 order of the crystals is accompanied by a reduction in the intracrystalline space. Finally, at 0.02C/min, the 50 crystal bands are quite wide and approximately par- allel to one another. 0 0 0.5 1 1.5 2 The crystals of high-molecular-weight PTFE are lamellar and large and lack the complexity of True Strain spherulitic or fringed superstructures (Figs. 5.14 and 5.15). Clearly, both crystalline and amorphous phases Figure 5.12 Effect of crystallinity on the tensile have an impact on the mechanical behavior of PTFE. response of polytetraﬂuoroethylene (PTFE) at 23 Table 5.10 summarizes the properties of the PTFE and 100C. Low crystallinity PTFE was 28%, samples in Fig. 5.13. The determination of the pro- high crystallinity PTFE was 38% (strain rate was portions of the crystalline and amorphous phases in 10À3 sÀ1) [18]. PTFE has been somewhat controversial. At extremely slow cooling rates, such as those used by Davidson et al., crystalline segments could reach a maximum of 80e85wt%, which is close to the Figure 5.13 Crystalline structure of polytetraﬂuoroethylene cooled from 380C at controlled cooling rates: (A) at 2C/min; (B) at 0.48C/min; (C) at 0.12C/min; and (D) at 0.02C/min (Bar ¼ 1 mm) [43].

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 115 Figure 5.14 Optical micrograph showing spherulite as-polymerized PTFE resin. Bunn et al. published of linear polyethylene crystallized from a molten thin similar micrographs of PTFE crystallization as those ﬁlm [44]. reported by Davidson et al., in 1958, nearly three decades earlier [46]. Figure 5.15 Schematic of fringed-micelle model for semicrystalline thermoplastics polymers [45]. The assumption that Koo made in his 1969 research [27,29] that increasing the crystallinity of PTFE results in larger, as opposed to more, crystal- line domains, was proven to be correct by Davidson et al. The fully extended chain length of PTFE with a MW of 10 million (typical of PTFE used to make ePTFE) would be around 26 mm. The relatively low thickness of PTFE lamella suggested that PTFE might be composed of extended-chain crystals similar to polyethylene. A polymer crystal in which the chains are in an essentially semicrystalline state with a MW over 20,000 is considered a high polymer, though these are many orders of magnitude smaller than the MW of PTFE. According to Wunderlich and Davidson, MW of 20,000 corresponds to a fully extended chain of 200 nm [47]. Incidentally, there are two basic routes to grow crystals of macromolecules. Either an already poly- merized ﬂexible macromolecule is crystallized from molten or dissolved state, or the crystallization oc- curs during polymerization. Extended chain crystals of PTFE are formed during polymerization and may be replicated from polymer melt, though to a lower extent of crystallinity than in polymerization. In terms of its structure, commercial PTFE has an extremely high MW, is completely linear, and re- crystallizes in a lamellar manner, especially at low cooling rates. For the most part PTFE does not form spherulites during recrystallization from the melt phase. The spherulite structure of emulsion poly- merized PTFE particle is nearly lost during paste extrusion. The emulsion PTFE molecule is capable of producing long ﬁbrils. Extruded, unsintered PTFE can be stretched even at the room temperature Table 5.10 Results of Cooling Rate Experiments With Polytetraﬂuoroethylene [43] Cooling Photo Mean Standard Standard Density Percent Rate (A) Lamellar Deviation (A˚ ) Error of the (g/cc) Crystallinity (8C/min) (B) Thickness 2.138 (C) 329 Mean (A˚ ) 2.146 45 Quench (D) (A˚ ) 493 52 2.156 53 501 78 2.192 58 2 1110 839 79 2.205 65 2037 68 0.48 1600 132 322 0.12 1440 0.02 1850 2590

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116 EXPANDED PTFE APPLICATIONS HANDBOOK because of its very high MW. Additional information about PTFE crystallinity and molecular shape has been covered earlier in this book. 5.4 Microstructure of Expanded Figure 5.17 Scanning electron micrograph (1000Â) Polytetraﬂuoroethylene of a uniaxially expanded polytetraﬂuoroethylene membranedthe arrows indicate the direction of When ePTFE was developed in the 1960s it was expansion [1]. unclear why emulsion-polymerized PTFE powder, Courtesy: Summit Filter Corporation, www.summitﬁlter. when converted to ﬁlm by paste extrusion, could com/images/unipore-surface-1000x-magni.jpg, 2014. withstand high stretch (strain) rates to create porous membranes and other shapes. Many studies have Figure 5.18 Schematic structure of a balanced biax- been conducted on polymers, including PTFE, in the ially expanded polytetraﬂuoroethylene membrane [1]. decades since. The ﬁndings of this research explain the unusual behavior of PTFE in response to rapid strain rates. The processing conditions, especially temperature and expansion rate determine the porous micro- structure of the expanded material. The structure has two components: nodes and small ﬁbrils. These ﬁ- brils interconnect the nodes. Uniaxial expansion elongates the nodes such that the longer axis of each node is perpendicular to the expansion direction (Figs. 5.16 and 5.17). The nodes are consequently at a 45-degrees angle to MD in an expanded part that has been equally oriented in MD and TD, as seen in Figs. 5.18 and 5.19. Unlike other plastic ﬁlms, unsintered PTFE changes very little in thickness or width, even though the length may increase by 100% or more. The volume increases because of an increase in the porosity and a corresponding decrease in speciﬁc gravity [4]. These ﬁbrils appear to be characteristically wide and thin in cross section, the maximum width being Direction of Uniaxial Expansion Figure 5.19 Scanning electron micrograph (1000Â) of a biaxially balanced expanded polytetraﬂuoroethy- Figure 5.16 Schematic structure of a uniaxially lene membranedthe perpendicular lines indicate the expanded polytetraﬂuoroethylene membrane [1]. direction of expansion [1].

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 117 equal to about 0.1 mm (100 nm), which is the diam- ﬁlms were analyzed by the DSC at the heating/ eter of the crystalline particles. The minimum width cooling rate of 10C/min. Sample weights were in the may be one or two molecular diameters, or in the range of at 4e5 mg. range of 0.5 or 1 nm. The nodes vary in size from about 400 mm to less than a micron, depending on the Fig. 5.20 shows DSC scans of the raw tape (as- conditions used in the expansion. extruded) and of ﬁlms that have been uniaxially and biaxially stretched. Table 5.12 summarizes the Products that have been expanded at high tem- values of melting peak temperatures. There are two peratures and high strain rates have a more homo- melting peaks, labeled 1 and 2. The high melting geneous structure. That is, they have smaller, more point of ePTFE crystal, around 380C, seems to be closely spaced nodes that are interconnected by a related to the level of molecular chain orientation. greater number of ﬁbrils. These products are also The powder, raw tape (calendared sample), and uni- much stronger. Table 5.11 shows the strength and axial 5X drawn sample do not share this high melting porosity of various sintered ﬁlms, including point. The heat of fusion of the melting point around expanded ones. The strength of expanded PTFE far 340e345C tends to decrease with increasing stretch exceeds the strength of PTFE that has been subjected ratios, while the heat of fusion associated with the to molding, extrusion, or calendaring. melting point around 380C tends to increase with increasing stretch ratios for both uniaxial and biaxial ePTFE has other unique characteristics, in addi- stretching (Fig. 5.21). Other researchers have pre- tion to great strength and tunable porosity that can be sented similar data, as found in Figs. 5.21 and 5.22. examined through differential scanning calorimetry (DSC). We will explore those properties through re- Fig. 5.23 shows the DSC thermogram of a fully view of past research. Spruiel and Choi [48] reviewed sintered densiﬁed ePTFE ﬁlm sample. Densiﬁcation the studies of other researchers and published them process took place after the expansion by placing the along with the results of their own research. Spruiel expanded ﬁlm under pressurized compression fol- and Choi experimented with the expansion of a ﬁne lowed by the sintering. All pores of this ﬁlm sample powder PTFE homopolymer with a MW of 107. The were eliminated during the sintering. The appearance ﬁne powder they used has an SSG of about 2.149 to of the ﬁlm was similar to standard PTFE ﬁlm. The about 2.165. They expanded the PTFE by adding a DSC thermogram peaks at 327.1 and 381.75C lubricant to the powder, molding the paste into a indicate preservation of nodes and ﬁbrils post preform, and extruding it under high pressure. The densiﬁcation and sintering. The peaks are useful for extrudate was calendared at 60C to make PTFE tape veriﬁcation that a ﬁlm sample has been expanded and then dried to remove the lubricant. prior to densiﬁcation [51]. They ﬁrst expanded the PTFE tape in the MD at a Another unique characteristic of ePTFE is that its temperature of 200e300C. Some of the tape was microstructure can be manipulated using resin and only stretched uniaxially. Other samples were sub- process variables. The even distribution of desirable sequently stretched in the TD in a tenter frame at pore size and the manipulation of average size can be 275e325C. After stretching, the samples were achieved in PTFE membranes, which is quite useful quickly heated to just below 342C to heat-set them. in the ﬁltration industry and in other applications. They then rapidly cooled the samples to room tem- The temperature and the rate of expansion specif- perature and held them overnight at 20C to allow the ically affect pore size. As we have seen, products that formation of phase IV PTFE crystals. Samples of the have been expanded at high temperatures and high Table 5.11 Tensile Strength of Various Polytetraﬂuoroethylene (PTFE) Films [3] PTFE Film Type Tensile Porosity Strength Sintered, extruded, or molded Strength (vol%) Based on Sintered calendared 100% Solids Expanded PTFE (MPa) <0.5% w0 (MPa) 21 90 21 35 35 69 690

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118 EXPANDED PTFE APPLICATIONS HANDBOOK 0 Powder –0.5 Uniaxial 5X –1 –1.5 Uniaxial 36X W/g –2 Uniaxial 48X –2.5 Biaxial 36X by 36X –3 Biaxial 48X by 36X –3.5 –4 260 280 300 320 340 360 380 400 Temperature (Celsius) Figure 5.20 Differential scanning calorimetry scans of polytetraﬂuoroethylene powder and uniaxial and biaxial stretched ﬁlmdthe numbers in the graph labels (eg, “Biaxial 36X by 36X”) show the stretch ratio in each biaxial direction [48]. Table 5.12 Peak Melting Temperatures of Polytetraﬂuoroethylene (PTFE) Powder (1, Unsintered PTFE) and Expanded PTFE (2, ePTFE) From Differential Scanning Calorimetry Scans [48] Sample ID Sample Description 1 (Melting Point 2 (ePTFE Peak Relative Heat of Powder of Virgin PTFE) Temperature) Fusion at PTFE u5 PTFE resin Melting Point (1) 345 Absent u36 Calendared ﬁlmd5 times 345 Absent 1 the original length 1 u48 346 378 Uniaxial expansion at 0.76 bi36 Â 36 stretch ratio 36 345 379 0.71 bi48 Â 36 Uniaxial expansion at 341 381 stretch ratio 48 0.62 341 381 Biaxial expansion at 0.46 stretch ratios 36 Â 36 Biaxial expansion at stretch ratios 48 Â 36 strain rates have a more homogeneous structure with one takes all of the unique characteristics of ePTFE smaller, more closely spaced nodes that are inter- into consideration, it is possible to view it (Fig. 5.24) connected with a greater number of ﬁbrils. as nearly a new material, invented by Bob Gore. Extreme tensile strength is yet another unique 5.5 Formation of Expanded property of ePTFE products. The orientation of ﬁbrils Polytetraﬂuoroethylene imparts ultimate tensile strength to ePTFE that is orders of magnitude greater than the tensile strength The key question to address at this point is how of molded parts. For example, a 90% microporous ePTFE is formed. Let us begin with the resin. It was PTFE membrane can easily have tensile break discovered early on that the crystallinity of even the strength of 69 MPa. If the porosity is taken account, the matrix strength is an astounding 690 MPa. When

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 119 S(A) S(B) Copolymers of TFE, which have defects in the crystalline structure and hence a higher amorphous Heat absorption PTFE = A content, do not work as well as PTFE homopolymers in the preparation of ePTFE. In the early days of B = ePTFE ePTFE development, special PTFE resins with maximized crystallinity did not exist. The early 250 300 350 400 ePTFE studies were conducted using Teﬂon® 6A, Temperature (°C) which was an earlier version of today’s Teﬂon® 6C that contained less than 0.2% HFP as a comonomer. Figure 5.21 Differential scanning calorimetry ther- Teﬂon® 6A worked for ePTFE manufacturing at very mogram of expanded polytetraﬂuoroethylene high rates of expansion at temperatures just below the (ePTFE) exhibiting two temperature peaks for polyte- melting point of PTFE. Today, there are special PTFE traﬂuoroethylene (PTFE) (A) and ePTFE (B)dS(A) resins with high crystallinity that have been devel- and S(B) represent the area under the curve for A oped especially for ePTFE manufacturing (see and B peaks [49]. Table 5.1). Figure 5.22 Comparison of differential scanning As we have seen, PTFE is completely linear and calorimetry thermograms of polytetraﬂuoroethylene folds neatly into accordion- or lamellar-shaped (PTFE) powder (sintered PTFE) and expanded crystals. The PTFE molecule is also symmetrical, PTFE (ePTFE) [50]. inert, and weakly polarizable and thus is minimally interactive. Because PTFE molecules slip past one best available PTFE resin had to be increased [3] to another easily and are also inert, they have one of the produce ePTFE. The primary requisite of a suitable lowest coefﬁcients of friction of all materials. resin was a very high degree of crystallinity, prefer- ably in the range of 98% or higher, with corre- The length and/or width of paste-extruded ﬁlm spondingly low amorphous content. Techniques for increases many times during expansion. How is this increasing the crystallinity of PTFE resin, such as increase in length accommodated? Part of the answer annealing at high temperatures just below the melting lies in the formation of pores, which result from point, have been developed and have been shown to expansion in both machine and cross directions. improve the performance of the resin in the expan- Tables 5.13e5.15 summarize the impact of stretch sion process. ratio in the two directions. The increase in length has to stem from another source as well, and it is most likely that it comes from the unwinding of PTFE molecules. By looking at Fig. 5.15 we can infer that it would be far easier for the molecules to “unwind” in the lamellar crystalline phase than in the amorphous phase. PTFE molecules become entangled in the amorphous phase, which is precisely why the crys- tallinity of the ePTFE resin must be maximized. Expansion (orientation) of a paste-extruded PTFE sample in one direction (or axis) affects the degree of orientation of ﬁbrils present in the other directions (or axes). To keep the discussion from becoming overly complicated, for our purposes here in this section let us overlook the secondary impact of expansion, in a single direction, on the ﬁbrils in other directions. Fig. 5.25 depicts a section of a paste-extruded ﬁlm in which the circles represent the PTFE particles and the line segments represent the ﬁbrils. If expansion takes place in Direction 1, the ﬁlm is rapidly strained is that direction and thus experiences stress. The ﬁ- brils that are positioned in the general perpendicular directions of 1 and 2 are affected when expansion

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120 EXPANDED PTFE APPLICATIONS HANDBOOK 2.00 WT = 19.17 mg MAX = 327.15 SCAN RATE = 10.00 deg/min MAX = 381.75 ENDO> PEAK FROM = 268.79 1.00 TO = 338.09 MCAL / SEC ONSET = 319.27 CAL / GRAM = 4.96 PEAK FROM = 345.87 TO = 390.33 ONSET = 385.81 CAL / GRAM = 2.46 0.00 210.00 230.00 250.00 270.00 290.00 310.00 330.00 350.00 370.00 390.00 Temperature (°C) DSC Figure 5.23 Differential scanning calorimetry thermogram of fully sintered densiﬁed expanded polytetraﬂuoro- ethylene [51]. Table 5.15 Effect of Stretch Speed in the Transverse Direction on Porosity and Pore Diameter [53] Stretching rate, m/min 4.8 6 8 Porosity, % 60.4 64.2 70.8 Mean pore diameter, mm 0.12 0.11 0.09 Figure 5.24 Scanning electron micrograph of a typical expanded polytetraﬂuoroethylene membrane [52]. Table 5.13 Effect of Stretch Ratio in the Machine Direction on Porosity and Pore Diameter [53] Stretching ratio 5 6 8 Porosity, % 76.4 87.6 89.1 Mean pure 0.35 0.47 0.51 Figure 5.25 Microstructure of a section of paste- extruded polytetraﬂuoroethylene (PTFE) ﬁlm. diameter, mm takes place in these directions. These ﬁbrils are ori- Table 5.14 Effect of Stretch Ratio in the Transverse ented by the stress, and in addition they consume a Direction on Porosity and Pore Diameter [53] portion of the stress by pulling additional length, groups of chains of PTFE, out of the crystalline Stretching ratio 2.1 2.9 4.7 8.5 phase. Ideally, the PTFE tape should be extruded 56.5 58.4 60.4 78.0 such that ample ﬁbrils are formed in the direction Porosity, % 0.08 0.09 0.12 0.24 cross to the extrusion direction. Mean pore diameter, mm

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 121 The ﬁbrils that interconnect the nodes are oriented signiﬁcantly in the expansion phenomenon. In Sec- parallel to the direction of expansion [3]. These ﬁ- tion 5.2.2 of this chapter the positive effect of brils appear to be characteristically wide and thin in increasing the crystallinity of PTFE by annealing was cross-section. The maximum ﬁbril width is about discussed. The data in Table 5.12 clearly exhibit the 0.1 mm (100 nm), which is equal to the diameter of “consumption” of PTFE crystalline phase as a result the crystalline particles. The minimum width may be of expansion, at high strain rates at elevated one or two molecular diameters, or in the range of 0.5 temperatures. or 1 nm. Amorphous phase is known to govern the elastic The data in Table 5.12 support the proposition that deformation of polymers. At low strain rates the the PTFE chains unwind out of the crystalline phase amorphous phase undergoes stretching by untangling during expansion. Note that the relative heat of fusion PTFE chains up to a strain of <1e2%. It takes time (PTFE powder ¼ 1) of PTFE decreases as the extent for the amorphous phase chains to be untangled, thus of expansion increases. The heat of fusion of sample a low strain rate is required. At higher strains the u5, drawn to ﬁve times the length of the paste- crystalline phase undergoes a variety of deformations extruded tape, is the same as that of the PTFE such as slippage described in the Section 5.3. powder. The heat of fusion of PTFE is reduced to less than half of that of the powder for the most The behavior of silly putty, an entirely amorphous extensively biaxially expanded sample: bi48 Â 36. material, is instructive. Its behavior can be modeled Fig. 5.26 shows an example of a highly ﬁbrillated by a Maxwell model (Fig. 5.27). That model deﬁnes ePTFE membrane that has been expanded at high the relationship between viscosity and modulus of stretch ratios. The round contour of the PTFE elasticity of the polymer. Hook’s law is the basic emulsion particles has been entirely lost due to relationship between instantaneous stress (s), and extensive ﬁbrillation and expansion. strain (ε) is deﬁned by Eq. (5.6). Maxwell’s linear viscoelasticity model relates the viscosity (h) and Operationally one can envision elevating the Young’s modulus of elasticity (E) by Eq. (5.7). The temperature to below melting point, say around factor s is called relaxation time. 300C, during expansion weakens the already weak intermolecular PTFE interactions. High strain rates If one pulls gently (low strain rate) on a piece of during expansion rapidly unwind PTFE chains out of silly putty, it ﬂows easily as soon it is stretched the crystalline phase. Macroscopically viewed, PTFE (Hookean spring). A rapid pull (high strain rate) behaves similar to an elastomer because it responds causes the piece of silly putty to break in two pieces to stress by assuming a higher length. It is all but (Newtonian dashpot) a behavior expected from a certain the crystalline phase of PTFE participates rigid solid with little elasticity. This is an interesting example of relaxation time, which is a measure of the time taken for the material to ﬂow. The relaxation time of silly putty is about 0.1 s. When a large pull force (stress) is applied the time available for relax- ation could be much lower than 0.1 s, which does not Figure 5.27 The Maxwell model including Hookean spring (E) and Newtonian dashpot (h). Hooks Law s ¼ ε$E (5.6) Figure 5.26 A highly ﬁbrillated expanded polytetra- Maxwells model h ¼ s$E (5.7) ﬂuoroethylene membrane expanded at high stretch ratios. Courtesy: DuPont Co.

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122 EXPANDED PTFE APPLICATIONS HANDBOOK give the molecule chain sufﬁcient time for relaxation. amorphous locking process. Regardless of the That causes the silly putty to break like a solid ma- explanation, heat treatment at above 327C results in terial. (www.physics.usyd.edu.au/wcross/SillyPutty. a signiﬁcant increase in strength, and the heat-treated htm) material often has double the strength of the material that has not been heat treated [3]. Imagine a piece of silly putty is attached to a rubber band. A rapid pull on the silly putty is unlikely Because the upper end of the melting range of as- to break it because the stress is transferred to the polymerized PTFE is about 345C, the heat treatment rubber band that can dissipate the incoming energy appears to be more effective if it is carried out above by stretching. A far more rapid pull is required to this temperature. Treatment at lower temperatures break the silly putty attached to a rubber band than can have the same effect, however, if the exposure when it is pulled without the rubber band. time is increased. The optimum temperature for heat treatment is in the range of 350e370C, and the At low strains, PTFE samples with lower amor- heating periods required may range from about 5 s to phous content are stiffer than samples with higher about 1 h. The microstructure of the expanded amorphous content. At higher strains, however, lower product is not substantially changed by the amor- amorphous content PTFE samples are less stiff than phous locking step. Table 5.16 shows the impact of those with higher amorphous content. At high strain the heat-setting temperature on the pore size in the rates and elevated temperatures the amorphous phase ePTFE membrane, made from Asahi Glass CD123 of PTFE behaves like a rigid solid. It does so by PTFE, in a study reported in 2005. Similar results are transferring the stress to the crystalline phase leading seen at temperatures above 300C. to the unwinding of crystallized chains. Although most materials fracture when subjected to a high rate If the heat treatment, or amorphous locking, is of strain, highly crystalline PTFE ﬁlm withstands this carried out at a high temperature for too long, the operation without breaking. Actually, stretching the microstructure may become coarse as the nodes PTFE ﬁlm at very high rates increases its mechanical increase in size and the ﬁbrils rupture. When this strength. occurs there is a noticeable deterioration in strength, but this should not be a problem since one can very 5.6 Amorphous Locking readily determine the optimum time and temperature for heat treatment of any TFE polymer being pro- When ePTFE products are heated to above the cessed. Temperatures above about 390C may cause lowest crystalline melting point of PTFE, disorder disintegration and loss of strength in less than 1 min. begins to occur in the geometric order of the crys- Whenever ﬁlms are subjected to heat treatment it is tallites and the crystallinity decreases. At the same essential that they be held in place so they cannot time, the amorphous content of the polymer retract during the amorphous locking process. increases, to as much as 10%. These amorphous regions within the crystalline structure appear to 5.7 Characterization of greatly inhibit slippage along the crystalline axis of Membrane Pores the crystallite and appear to lock ﬁbrils and crystallites so that they resist slippage under stress. The most unique characteristic of ePTFE mem- Therefore, the heat treatment can be considered an branes is their porosity. The open pores come in a Table 5.16 Effect of Heat-Setting Temperature on Pore Size and Distribution of Expanded Polytetraﬂuoroethylene Membrane [53] Heat Setting Mean Pore Minimum Pore Maximum Pore Temperature Diameter (mm) Diameter (mm) Diameter (mm) 245 0.382 0.319 0.424 280 0.589 0.469 0.646 300 0.685 0.618 0.835

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 123 variety of sizes. Pore size distribution, average and curve is drawn. The wet curve yields the ﬁrst maximum pore size of ePTFE in combination with bubble point as seen in Fig. 5.28. Mean ﬂow pore surface characteristics of PTFE form the properties size corresponds to the pore size calculated at the of those membranes in applications. Characterization pressure where the wet curve and the half-dry of pore structure of ePTFE is, therefore, important to curve meet. The smallest pore size is deter- quantifying the membranes. A widely accepted mined from the pressure at which wet and dry standard apparatus and method for assessing pore curves intersect. size is ASTM F 316, which measures Bubble Point and Mean Flow Pore Test. The relationship between the operating pressure and pore size of the membrane is described by the 5.7.1 Bubble Point YoungeLaplace equation, Eq. (5.8). The size of the largest pores in the membrane is calculated from An indication of the maximum pore size of ePTFE the ﬁrst bubble point. Mean ﬂow pore size is calcu- membranes disregarding of their shapes is the Bubble lated from the pressure at the intersection of the wet Point. ASTM F-316 deﬁnes the ﬁrst or initial Bubble and half-dry curves. Similarly, smallest pores are Point as the pressure at which the ﬁrst continuous gas determined from the pressure value at the intersection bubbles is detected. The method prescribes the material of wet and dry curves. While smallest and largest to get saturated with a test liquid such as isopropanol, pore are present in a membrane the mean ﬂow pore is held in a holder and subjected to progressively a calculated value without physical signiﬁcance. increasing pressure of a test gas like air. The relation- ship between pressure applied and ﬂow through the rp ¼ 2gLA b cosðqÞ (5.8) material is observed using a pressure gauge and a ﬂow DP meter downstream of the material. A bubble point de- tector notes the initial breakthrough of gas [54]. rp, pore diameter; gLA, surface tension of liquid (liquid-air), dyn/cm (mN/m) [56]; DP, pressure dif- A dry curve is established by plotting airﬂow ference applied across the membrane; ß, capillary (l/min) through the membrane against the pres- constant; q, contact angle. sure. Another curve is drawn assuming airﬂow of one-half that of the dry curve, called half dry Water entry pressure is the minimum pressure curve. Next the membrane is saturated with a required to force water through the largest opening of liquid, usually isopropanol for ePTFE. Airﬂow a dry hydrophobic membrane. The pressure required through the membrane is established. Airﬂow to force water through the structure is inversely values are plotted against pressure and a wet proportional to opening size and dependent on the polymeric properties of the membrane. Water entry Figure 5.28 A generic wet, dry curve and half-drive curve for a typical membrane [55].

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124 EXPANDED PTFE APPLICATIONS HANDBOOK pressure represents the ﬁlter’s ability to serve as an After simpliﬁcation Eq. (5.10) is obtained: aqueous barrier. Factors that affect water entry pressure include pore size, surface tension of the gSL À gSA ¼ DP$ðDÞ=4 (5.10) liquid, surface free energy of the membrane, and size of the largest opening [57]. Water breakthrough or Substituting in Eq. (5.10) from YoungeDupre penetration pressure testing of hydrophobic mem- equation Eq. (5.11) and rearrangement yields Eq. branes such as ePTFE gives a relative measure of the (5.12). D can be calculated from Eq. (5.12) when the pore size of a membrane. Water breakthrough pres- values of gLA, q, and DP are known. Eq. (5.13) is sure is the minimum pressure required to force water obtained after ﬁnal rearrangement. through the largest pore of a dry hydrophobic mem- brane [54]. gSL À gSA ¼ gLA$cosðqÞ (5.11) 5.7.2 Derivation YoungeLaplace gLA$cosðqÞ ¼ DP$ðDÞ=4 (5.12) Equation D ¼ 2r In this section the YoungeLaplace equation is derived for a single cylindrical pore in a PTFE r¼ 2gLA cosðqÞ (5.13) membrane. Assume a case where liquid such as water DP or isopropanol works under pressure against the hy- drophobic resistance force of the capillaries arising The difference between Eqs. (5.8) and (5.13) is the from surface energy of the liquid. When liquid coefﬁcient ß aimed at correction for noncylindrical pressure is sufﬁciently large to overcome the coun- pore shape. Below examples of maximum pore size teracting force it would push the liquid through an calculation, using Eq. (5.13), is presented for water ePTFE membrane. An axial force balance is con- entry into a pore of ePTFE. ducted on a single capillary interfacing with a water droplet (Fig. 5.29). DP ¼ 1.65 bar Conducting a balance on the forces working on q ¼ 108e112 degrees (assume 110 degrees the wall surface of a single pore capillary yields for water) Eq. (5.9): gLA ¼ 72 dyn/cm (at 25C) ðp$DÞ$gSL À ðp$DÞ$gSA ¼ DP$Àp$D2Á4 (5.9) DP ¼ 1.65 bar ¼ 1.65 Â 105 Pa capillary wall single capillary gLA ¼ 72 dyn/cm ¼ 72 Â 0.001 N/m cos(q) ¼ cos(110) ¼ À0.342 r ¼ 2(72 Â 0.001)(0.342)/(1.65 Â 105) ¼ 2.98 Â 10À7 m ¼ 2.98 Â 10À7 Â 106 mm θ r ¼ 0.3 mm (at 1.65 bar) γSA 5.7.3 Mercury Porosimetry DγLA γSL This method is used to evaluate the pore-volume ΔP = force across distribution of the membranes [58]. The method is the membrane based on the fact that mercury is a strongly nonwetting liquid on most materials. When mercury ΔP = force across is forced into a dry membrane with the volume the membrane of mercury being determined at each pressure, a cu- mulative volume of mercury as a function of Figure 5.29 Depiction of a liquid droplet at the entry the applied pressure is established, from which the point of a cylindrical pore. pore-size distribution is deduced. Again the

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 125 relationship between the operating pressure and pore [6] T.J. Edwin, S. Randall, E.P. 1741544, Assigned size of the membrane can be described by the Lap- to Bard Peripheral Vascular, October 1, 2007. lace equation in Eq. (5.6). [7] W.L. Gore, U.S. Patent 3,664,915, Assigned to This equation assumes cylindrical pores, which is W. L. Gore Associates, May 23, 1972. generally not the case for most membranes; there- fore, a morphological constant must be introduced to [8] S. Ebnesajjad, Fluoroplastics vol. 2, Melt Pro- correct the results. During the experiment, the largest cessible Fluoropolymers, second ed., Elsevier, pores are ﬁlled with mercury at a certain minimum 2015. pressure. As the pressure increases further, smaller pores are ﬁlled until a maximum intrusion value is [9] R.W. Gore, U.S. Patent 4,194,041, Assigned to reached, that is all pores are ﬁlled. Thus, the pore- W. L. Gore Associates, March 18, 1980. size distribution of the membrane can be deter- mined because every pressure valve is related to one [10] D.J. Gohlke, U.S. Patent 4,344,999, Assigned to speciﬁc pore size. W. L. Gore Associates, August 17, 1982. 5.8 Summary [11] K. Okita, U.S. Patent 4,277,429, Assigned to Sumitomo Electric Ind., July 7, 1981. This chapter is devoted to establishing an under- standing of the ePTFE product and manufacturing [12] M.T. DeMeuse, Biaxial Stretching of Film: process at a molecular level. The intent is to shed Principles and Applications, Woodhead, Elsev- light on the fundamental reasons for the unusual ier, Cambridge, 2011. characteristics of ePTFE. The unique properties of ePTFE membranes and other shapes make them [13] AccuPull Biaxial Film Stretcher, Inventure useful for a broad number of applications across most Laboratories, Inc., 2012. http://accupull.com. industries. Microporous PTFE membrane is used to construct a variety of vascular grafts, drug delivery [14] J.B. Bowman, D.E. Hubis, J.D. Lewis, S.C. devices, ﬁltration products, apparel, hunting gear, Newman, R.A. Staley, U.S. Patent 4,096,227, wire and cable insulation, electronics, sealants, ﬁ- Assigned to W. L. Gore Associates, November bers, geochemical, and others. Next chapter con- 13, 1984. centrates on the description of basic shapes of ePTFE for different applications. [15] H.W. Starkweather Jr., P. Zoller, G.A. Jones, A.J. Vega, The heat of fusion of polytetra- References ﬂuoroethylene, J. Polym. Sci. Polym. Phys. Ed. 20 (4) (1982) 751e761. [1] S. Ebnesajjad, Fluoroplastics, in: Non-melt Processible Fluoropolymers, second ed., vol. 1, [16] F.J. Rahl, M.A. Evanco, R.J. Fredericks, Elsevier, 2014. A.C. Reimschuessel, Studies of the morphology of emulsion-grade polytetraﬂuoroethylene, [2] P.D. Patil, J.J. Feng, S.G. Hatzikiriakos, Consti- J. Polym. Sci. Part A-2 Polym. Phys. 10 (1972) tutive modeling and ﬂow simulation of poly- 1337e1350. tetraﬂuoroethylene (PTFE) paste extrusion, J. Non-Newtonian Fluid Mech. 139 (2006) [17] H.W. Starkweather Jr., A comparison of the 44e53. rheological properties of polytetraﬂuoro- ethylene below its melting point with certain [3] R.W. Gore, U.S. Patent 3,953,566, Assigned to low-molecular smectic states, J. Polym. Sci. W. L. Gore Associates, May 21, 1970. Polym. Phys. Ed. 17 (1979) 73e79. [4] R.W. Gore, U.S. Patent 4,187,390, Assigned to [18] P.J. Rae, E.N. Brown, The properties of poly- W. L. Gore Associates, February 5, 1980. tetraﬂuoroethylene (PTFE) in tension, Polymer 46 (2005) 8128e8140. [5] J.B. Bowman, D.E. Hubis, J.D. Lewis, S.C. Newman, R.A. Staley, U.S. Patent 4,482,516, [19] N. Brown, M. Parrish, Effect of liquid nitrogen Assigned to W. L. Gore Associates, November on the tensile strength of polyethylene and 13, 1984. polytetraﬂuoroethylene, J. Polym. Sci. Polym. Lett. Ed. 10 (1972) 777e779. [20] R.C. Doban, C.A. Sperati, B.W. Sandt, The physical properties of Teﬂon®, polytetraﬂuoro- ethylene, Soc. Plast. J. 11 (1955) 17e21 (see also pp. 24 and 30). [21] DuPont-Fluoroproducts, Teﬂon®, PTFE, Prop- erties Handbook, DuPont, 1996. Tech Rep H-37051e3.

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126 EXPANDED PTFE APPLICATIONS HANDBOOK [22] J. Dyment, H. Ziebland, The tensile properties of [36] T. Tervoort, J. Visjager, B. Graf, P. Smith, Melt- some plastics at low temperatures, J. Appl. processable poly(tetraﬂuoroethylene), Macro- Chem. 8 (1958) 203e206. molecules 33 (2000) 6460e6465. [23] B. Fayolle, L. Audouin, J. Verdu, Radiation [37] P.E. Thomas, J.F. Londz, C.A. Sperati, induced embrittlement of PTFE, Polymer 44 J.L. McPherson, Effects on fabrication on the (2003) 2773e2780. properties of Teﬂon® resins, Soc. Plast. J. 12 (1956) 89e95. [24] S. Fischer, N. Brown, Deformation of poly- tetraﬂuoroethylene from 78 to 298 K and the [38] L. Wall, R. Florin, Polytetraﬂuoroethyleneda effect of environmental crazing, J. Appl. Phys. radiation-resistant polymer, J. Appl. Polym. Sci. 44 (10) (1973) 4322e4327. II (5) (1959) 251. [25] J. Joyce, Fracture toughness evaluation of poly- [39] F.J. Zerilli, R.W. Armstrong, Thermal activa- tetraﬂuoroethylene, Polym. Eng. Sci. 43 (2003) tion constitutive model for polymers applied to 1702e1714. polytetraﬂuoroethylene, in: M.D. Furnish, N.N. Thadhani, Y. Horie (Eds.), AIP Confer- [26] T. Kletschkowski, U. Schomburg, Y. Katsumura, ence Proceedings: Shock Compression of Endochronic viscoplastic material models for Condensed Matter-2001, vol. 620, American ﬁlled PTFE, Mech. Mater. 34 (2002) 795e808. Institute of Physics, Melville, NY, 2001, pp. 657e660. [27] G.P. Koo, R.D. Andrews, Mechanical behavior of polytetraﬂuoroethylene around the room- [40] S. Ebnesajjad, Fluoroplastics, in: Non-melt temperature ﬁrst-order transition, Polym. Eng. Processible Fluoropolymers, second ed., vol. 1, Sci. 9 (4) (1969) 268e276. Elsevier, 2014 (Chapter 18). [28] G.P. Koo, E.D. Jones, M.N. Riddell, [41] K. O’Leary, P.H. Geil, Polytetraﬂuoroethylene J.L. O’Toole, Engineering properties of a new ﬁbril structure, J. Appl. Phys. 38 (1967) polytetraﬂuoroethylene, Soc. Plast. J. 21 (9) 4169e4181. (1965) 1100e1105. [42] R.J. Young, Deformation mechanism in poly- [29] G.P. Koo, Cold Drawing Behavior of Polytetra- tetraﬂuoroethylene, Polymer 16 (1975) ﬂuoroethylene (Ph.D. dissertation, reprinted by 450e458. University Microﬁlms Inc.), Stevens Institute for Technology, Ann Arbor, Michigan, USA, Sc.D, [43] T. Davidson, R.N. Gounder, D.K. Weber, 1969. S.M. Wecler, A perspective on solid-state microstructure in polytetraﬂuoroethylene, in: [30] H. Kudoh, T. Sasuga, T. Seguchi, Y. Katsumura, G.G. Hougham, P.E. Cassidy, K. Johns, High energy ion irradiation effects on polymer T. Davidson (Eds.), Fluoropolymers 2d material: 4 heavier ion irradiation effects on Properties, Springer, 1999. mechanical properties of PE and PTFE, Polym. Commun. 37 (1996) 3737e3739. [44] P.H. Geil, Polymer Single Crystals, ﬁrst ed., Krieger, 1973. [31] A. Nishioka, M. Watanabe, Viscosity and elas- ticity of polytetraﬂuoroethylene resin above the [45] Z. Huang, Crystallization and melting behavior melting point, J. Polym. Sci. 24 (106) (1957) of linear polyethylene, in: Crystallization and 298e300. Melting Behavior of Linear Polyethylene and Ethylene/Styrene Copolymers and Chain Length [32] G. Peng, H. Geng, D. Yang, S. He, An analysis on Dependence of Spherulitic Growth Rate for changes in structure, tensile properties of poly- Poly(Ethylene Oxide) Fractions, Virginia Tech, tetraﬂuoroethylene ﬁlm by protons, Radiat. 2004 (Ph.D. dissertation), scholar.lib.vt.edu/ Phys. Chem. 69 (2004) 163e169. theses/available/etde055504/unrestricted/ Chapter2.pdf. [33] P. Rae, D. Dattelbaum, The properties of poly(- tetraﬂuoroethylene) (PTFE) in compression, [46] C.W. Bunn, A.J. Cobbold, R.P. Palmer, J. Polym. Polymer 45 (2004) 7615e7625. Sci. 28 (1958) 365e376. [34] M.M. Renfrew, E.E. Lewis, Polytetraﬂuoro- [47] B. Wunderlich, T. Davidson, Extended-chain ethylene. Heat resistant, chemically inert plastic, crystals. I. General crystallization conditions Ind. Eng. Chem. 38 (9) (1946) 870e877. and review of pressure crystallization of poly- ethylene, J. Polym. Sci. Part A-2 Polym. Phys. 7 [35] J.A. Sauer, K. Pae, Flow of solid polymers under (12) (1969) 2043e2050. high pressure, Colloid Polym. Sci. 252 (1974) 680e695.

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5: EXPANSION OF POLYTETRAFLUOROETHYLENE RESINS 127 [48] K.J. Choi, J.E. Spruiel, Structure development [53] X. Ha, J. Zhang, Y. Guo, H. Zhang, Studies on in multistage stretching of PTFE ﬁlms, porous and morphological structures of J. Polym. Sci. Part B: Polym. Phys. 48 (2010) expanded PTFE membrane through biaxial 2248e2256. stretching technique, INJ (2005). www. jeffjournal.org/INJ/inj05_2/p31-38.pdf. [49] K. Hirai, U.S. Patent 5,470,655, Assigned to Japan Gore-Tex Inc., November 28, 1995. [54] ASTM Method F316 for Pore Size Characteris- tics of Membrane Filters by Bubble Point and [50] D.I. Lutz, N.E. Clough, U.S. Patent 7,740,020, Mean Flow Pore Test, October 2015. www.astm. Assigned to Gore Enterprise Holdings, org/Standards/F316.htm. June 22, 2010. [55] N.M. Amsler, U.S. Patent 6,267,252, Kimberly- [51] J.B. Knox, W.E. Delaney, J.M. Connelly Jr., U.S. Clark Worldwide, July 31, 2001. Patent 5,374,473, Assigned to W. L. Gore and Associates, December 20, 1994. [56] S. Ebnesajjad, C.F. Ebnesajjad, Surface Treat- ment of Material for Adhesive Bonding, second [52] M. Wikol, B. Hartmann, J. Brendle, M. Crane, ed., Elsevier, Oxford, UK, 2014. U. Beuscher, J. Brake, T. Shickel, Expanded PTFE membranes and their applications, in: [57] Materials Technology, Water Entry Pressure M.W. Jornitz, T.H. Meltzer (Eds.), Filtration Testing, W. L. Gore & Associates, 2011. www. and Puriﬁcation in the Biopharmaceutical gore.com. Industry, second ed., CRC Press, Boca Raton, November 2007. Informa Healthcare. [58] C. Charcosset, Membrane Processes in (Chapter 23). Biotechnology and Pharmaceutics, Elsevier, Oxford, UK, 2012.

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6 Manufacturing of Various Shapes of Expanded Polytetraﬂuoroethylene (ePTFE) OUTLINE 6.1 Planar Expanded Polytetraﬂuoroethylene 6.3.2 Production of Expanded Membranes 6.1.1 Uniaxial Orientation 129 Polytetraﬂuoroethylene Fiber 143 6.1.2 Biaxial Expansion (Orientation) 149 130 6.4 Densiﬁed Porous Polytetraﬂuoroethylene 6.2 Tubular Expanded Polytetraﬂuoroethylene 131 Membranes Shapes 6.2.1 Complex Shape Tubular Expanded 134 6.5 Expanded Polytetraﬂuoroethylene Sheets 153 Polytetraﬂuoroethylene 6.6 Expanded Polytetraﬂuoroethylene Tapes and 6.3 Expanded Polytetraﬂuoroethylene Fiber 6.3.1 High Tensile Strength 140 Rods 159 Polytetraﬂuoroethylene Fiber 142 References 159 142 This chapter is in a way the continuation of the bubbles appears in this test is the bubble point pres- Chapter 5 that focused on manufacturing of planar sure. The bubble point test is signiﬁcant not only for ﬂat membrane of expanded polytetraﬂuoroethylene indicating maximum pore size, but may also indicate (ePTFE). The original shape of ePTFE was a thin a damaged membrane, ineffective seals, or a system membrane that still forms the majority of the con- leak. sumption volume. In time the beneﬁts of ePTFE membrane resulted in demand for this material in The words graft and stent are used frequently in other parts and applications though different shapes this chapter. A stent is a tiny tube placed into an ar- were required. An important feature of the PTFE tery, blood vessel, or other hollow structure in your expansion technology turned out to be its versatility body (such as the tube that carries urine) to hold it in manufacture of various shapes. Table 6.1 shows open. The word graft sometimes refers to an entire the signiﬁcant shapes of ePTFE and examples of device and sometimes to a speciﬁc component. A components in which they are used. graft is used to replace, repair, or bypass a damaged or blocked part of an artery. The graft may be a PTFE molecular weight and structure along with plastic tube (eg, ePTFE), or it may be a blood vessel the process conditions affect the extent of porosity, taken from the body during the same surgery [1]. pore size distribution, and average pore size of ePTFE products. Membranes of PTFE have discrete 6.1 Planar Expanded pores from one side to the other of the membrane, Polytetraﬂuoroethylene similar to capillary tubes. The bubble point test is Membranes based on the principle that a wetting liquid is held in these capillary pores by capillary attraction and sur- There are a number of patents that describe a face tension, and the minimum pressure required to number of methods of manufacturing basic porous force liquid from these pores is a function of pore ePTFE membranes. Technology has also been diameter. The pressure at which a steady stream of Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00006-7 129 Copyright © 2017 Elsevier Inc. All rights reserved.

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130 EXPANDED PTFE APPLICATIONS HANDBOOK Table 6.1 Shapes and Applications of Expanded Paste Extruded Film Polytetraﬂuoroethylene (ePTFE) Calendaring ePTFE Shape Examples of Component Membrane/ﬁlms Containing ePTFE Drying Tubes Laminates, ﬁltration laminates, Longitudinal stent grafts, vents, fuel cell Expansion Fibers membrane electrode (at elevated assemblies, dielectric temperature) Densiﬁed materials, battery/capacitor ePTFE separators Sintering Sheets Peristaltic pump tubes, vascular Expanded PTFE Rods/tapes grafts, environmental screening Membrane modules Figure 6.1 Schematic diagram of uniaxial expanded polytetraﬂuoroethylene membrane manufacturing. Weaving/sewing threads, dental ﬂoss, packings, ﬁltration feedstock of the expansion process as shown in Fig. 6.2. felts In the subsequent step the extrudate is longitudi- Barrier materials, gaskets nally [in machine direction (MD)] expanded using slow and fast rolls. Longitudinal expansion of the Sealing gaskets, medical dried extrudate under heat (<320C) results in an patches, electromagnetic ePTFE structure represented by node and ﬁbril microstructure [3,4,6]. Fig. 6.2 shows a uniaxial interference gaskets expansion process. The key components of the pro- cess are a feed roll, driven slow rolls, multiple zone Gasket strips (three in Fig. 6.2), driven fast rolls, and a chill roll and ﬁnally a product take-up roll. developed for manipulation of the properties of the membranes such as average pore size (0.02e40 mm) The feed roll delivers the dry extruded PTFE feed and the distribution of pore size. Unique products to the expansion process beginning with the driven have also been developed by uniaxial and biaxial slow rolls. These rolls feed the ﬁlm into the heating/ expansion. The properties of membrane surfaces can sintering oven. At the end of oven the ﬁlm is fed into be further modiﬁed by plasma treatment, for driven fast rolls. The feed PTFE ﬁlm heats up and example, to render the surface hydrophilic. Another stretches (expands) according to the ratio of the linear approach has been the application of a coating or (surface) speeds of the fast and slow rolls. The even bonding drugs to ePTFE surface to impart expansion of the ﬁlm occurs in zone 1 or 2 depending properties tailored to speciﬁc applications [2e6]. on the process conditions. The ﬁlm is sintered fully or Membranes of ePTFE can be slit to a wide range of partially in zone 3 of the oven before exiting [8]. The width depending on requirements of the end use chill roll cools the ePTFE membrane to room tem- application. perature prior to it is wound up on the product take- up roll. 6.1.1 Uniaxial Orientation Sintering occurs while PTFE ﬁlm is under tension Fig. 6.1 shows the major steps of expansion pro- thus preventing longitudinal shrinkage while it is cess for manufacturing of an ePTFE membrane. The sintered [9]. A sintered ePTFE ﬁlm tends to shrink preliminary step is the selection of a resin suitable for very little after the tension on the web is released. the end use and its extrusion into a ﬁlm [7]. The The expanded ﬁlm can be quite thin and sometimes it extruded ﬁlm may or may not be calendered is placed on a carrier web to make later handling depending on the requirements of the end use. The easier (Fig. 6.3). hydrocarbon lubricant accompanying the ﬁlm is substantially removed by drying. This ﬁlm is the

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 131 Figure 6.2 Schematic diagram of a polytetraﬂuoroethylene expansion process. There is commercial uniaxial orientation equip- 6.1.2 Biaxial Expansion ment for ﬁlm processing (Fig. 6.4). One such unit (Orientation) is offered by Marshall and Williams Company called machine direction orientation (MDO). In ePTFE membrane is biaxially oriented by this equipment, MDO of plastic ﬁlms is accom- stretching a web oriented in MD in transverse or plished by heating the web and stretching it in the cross direction (CD) as depicted in Fig. 6.5. In other MD over a single or multistage draw sections. The words an extruded (and calendared) PTFE ﬁlm is ﬁrst draw may take place over a series of rollers in oriented in the MD, uniaxially without being sin- horizontal of vertical conﬁguration. Heat transfer tered. It is then oriented in the CD either sequentially simulation program can be used to predict the (continuously) with MD orientation, or wound up behavior of the material such as PTFE ﬁlm as it and oriented in CD in a separate process step, in a passes through the process. That type of informa- tenter frame under heat. tion can be used to select the appropriate style machine and the correct number and size of rollers The origin of the name tenter frame is from the needed to manufacture the best possible ePTFE device originally used for stretching cloth between membrane. Figure 6.3 A scanning electron micrograph of ﬁbrils Figure 6.4 Commercial uniaxial orientation (ma- and nodes in a uniaxially expanded polytetraﬂuoro- chine direction) equipment for expanded polytetra- ethylene ﬁlm. ﬂuoroethylene manufacturing. Courtesy: Marshall and Williams, Div. Parkinson Technology, www.parkinsontechnologies.com/marshall-and-williams/ machine-direction-orientation-mdo-systems, October 2015.

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132 EXPANDED PTFE APPLICATIONS HANDBOOK Paste Extruded Film Figure 6.7 Photograph of inside of a tenter frame with the clips in the rest position. Calendaring Courtesy: Parkinson Technologies, Woonsocket, Rhode Island, USA, www.parkinsontechnologies.com. Drying be often more highly oriented in the MD. Fig. 6.6 Longitudinal illustrates a typical proﬁle of sequential stretching. Expansion (at elevated After MD stretching is completed the ﬁlm enters temperature) the tenter frame. At the entrance to the tenter frame the ﬁlm is gripped by a clip system (Fig. 6.7) and via Transverse a track rail is stretched in the direction of the width. Expansion Fig. 6.7 shows a photograph of the clips system and (at elevated the two rails in rest position. Fig. 6.8 shows two temperature) examples of available clip type of tenter frames. Sintering Tentering consists essentially of a heating tunnel in which the ﬁlm is heated to below the melting point Expanded PTFE of PTFE. As the ﬁlm passes through the tenter frame, Membrane it is progressively stretched in the transverse Figure 6.5 Schematic diagram of biaxially oriented expanded polytetraﬂuoroethylene membrane manufacturing. grips known as tenterhooks. Film stretching is car- ried out at a temperature below the melting point of the PTFE and results in a partial orientation of polymer molecules in the direction of stretch. Biaxial stretching of MD-expanded ﬁlm improves mechanical strength and void content of the ﬁlm. Biaxially oriented ﬁlm may be isotropicdthat is its properties are the same in both MD and CD. In practice, ﬁlm produced by the tenter process tends to Figure 6.6 Typical width proﬁle of sequential biaxial orientation [10]. MD, machine direction.

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 133 Figure 6.8 Examples of two styles of tenter frame clips. Courtesy: STM, Inc., Danville, Virginia, USA, www.stm-inc.net/tenter/parts.html. direction (TD) as the clips move apart at high speed. 4. Rail design to process a wide range of stretch The edge grip mechanism must withstand large cross conditions, widths, and thicknesses loads and be capable of operating at high line speeds because of the unusually high stretch rates that 5. Temperature capability for ePTFE processing expansion of PTFE requires. (!340C). The tenter frame consists of two horizontal chain Fig. 6.9 depicts an isometric view of a commercial tracks, on which clip and chain assemblies ride, tenter frame. MD-oriented ﬁlm enters the tenter enclosed in an oven. Some of the other characteristics frame at one end and moves through a heating sec- of a tenter frame suitable for ePTFE processing are: tion. It next enters the stretch zone in which the rails move apart at a speed to match the speciﬁed stretch 1. Typically three heated zones: preheat, stretch, rate. The rails reach their maximum distance in the and anneal sintering zone (Fig. 6.6). In the cooling zone the rails are brought in slightly to allow for shrinkage 2. Adjustable rail widths and stabilization of the width of the ePTFE web. Table 6.2 shows some of the typical characteristics of 3. A clip design that accommodates a wide range commercial MD and TD stretching equipment. of operations Figure 6.9 Commercial transverse orientation equipment for expanded polytetraﬂuoroethylene manufacturing. MD, machine direction. Courtesy: Marshall and Williams, Div. Parkinson Technology, www.parkinsontechnologies.com/marshall-and-williams/ transverse-direction-orientation-tdo-systems, October 2015.

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134 EXPANDED PTFE APPLICATIONS HANDBOOK Table 6.2 Characteristics of a Machine Direction Table 6.3 Comparison of Original (Unexpanded) and (MD) and Transverse Direction (TD) Stretching Expanded Polytetraﬂuoroethylene Tubing [11] Equipment Expanded, Speciﬁcation Value Original Amorphous- Unexpanded MD Property Locked Stretch ratio Tubing Tubing Line speed, m/min Width, m 20:1 Length, relative 2.5 2.0 Temperature capability, C 450 units 4 Outside diameter, 0.5 1.4 400 cm TD Wall thickness, mm 750 600 Stretch ratio 30:1 Speciﬁc gravity of 1.50 0.75 tubing walls 2 Â 10À5 1 Â 10À3 Line speed, m/min 450 Air permeability of Inlet width, m 0.1 tubing walls, metric units Exit width, m 10 Courtesy: Parkinson Technologies, Woonsocket, Rhode Island, USA, www.parkinsontechnologies.com. 6.2 Tubular Expanded porous moldings. The porosity can be calculated by Polytetraﬂuoroethylene Shapes measurement of speciﬁc gravity in air and in water. Pore size is measured according to different methods. Tubular shape is an important conﬁguration of In this case the pore size speciﬁcation was deﬁned as ePTFE membrane in medical devices designed for the maximum pore size [12]. vascular, endovascular, and other prosthetic grafts. One of the earliest references to an ePTFE tube goes The maximum pore size can be calculated by back to the 1970s [11]. The starting material in this measurement of the bubble point, namely, pressure at method was extruded, unsintered PTFE tubing with which the ﬁrst bubble generates when the molding is an outside diameter of 5.1 cm and a wall thickness of wetted by a liquid having a low surface tension such 0.76 mm (US). A section of 38 cm long section of as propyl alcohol, etc. and air pressure is applied to this tubing was plugged off at one end, and the other one face of the wetted tubing while gradually end was clamped to a steel tube that was connected to increasing the air pressure, for example, ASTM a source of compressed gas. Method F316. The bubble point is in inverse pro- portion to the maximum pore size. The larger the The tubing was placed in an air oven, and the as- bubble point, the smaller the maximum pore size. sembly was heated to about 300C. Compressed gas was admitted to the tubing in such a way that the Fig. 6.10 illustrates the stretching process ac- diameter of the tube was increased in about 2 s from cording to the invention. The plug functions as a the original 5.1 mm to about 15.2 mm. Then, with mandrel and is composed of two column parts, one pressure maintained in the tubing so that no collapse having a size smaller than the initial inside diameter took place, the temperature of the assembly was of the tube and a tapered part. On the other hand, the raised to about 360C and held there for about 5 min. die is composed of an oriﬁce part having a size While still maintaining pressure to prevent tubing smaller than the initial outside diameter of the tube collapse, the assembly was cooled rapidly using a and a tapered part. It is preferred that the angle of the stream of cold air, yielding the desired expanded, tapered part in the die be larger than that in the plug. amorphously locked tubing (Table 6.3). In an example a PTFE ﬁne powder was mixed with Characteristics of ePTFE tubing are demonstrated trichloroethylene (TCE) and extruded at a reduction by porosity, pore size, thickness, inside diameter, ratio (RR) of 580:1 to form a tube with a 5.5 mm strength, etc. Among these, the porosity and the pore outside diameter and a 4.0 mm inside diameter. The size are the most important characteristics of the lubricant TCE was removed before the tubing was stretched by fourfold at 300C at a speed to 20 cm/min

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 135 Plug Die PTFE Tubing Figure 6.11 Porosity versus bubble point of expanded polytetraﬂuoroethylene tubing [12]. Draw The longitudinal axes of the nodes were oriented at an Direction angle between about 85 degrees and about 15 degrees relative to the longitudinal axis of the tube. Figure 6.10 Die and plug arrangement for drawing polytetraﬂuoroethylene (PTFE) tubing [12]. The equipment for extruding the PTFE tubing consisted of a hollow, cylindrical barrel that con- tube supplying rate (Fig. 6.10). It was then stretched tained a centrally positioned mandrel (Fig. 6.12). two- to threefold by drawing using a die with a 4.2 mm The barrel had a resin supply region consisting of a oriﬁce diameter and a 30 degrees taper angle and a tapered region leading from the supply region to an plug having 4.0 and 3.0 mm outside diameters of the annular outﬂow region. The outﬂow section con- column parts and a 20 degrees taper angle at a 300C sisted of an external, hollow, circular cylindrical die tube temperature (below the crystalline melting point) that was positioned central to the circularly cylin- in the lengthwise direction at a 20 cm/min tube sup- drical tip of the mandrel. The tip and die formed an plying rate. The tube stretched by drawing was sin- tered by passing through an electric furnace having Figure 6.12 Equipment for extrusion of polytetra- 510C at 60 cm/min. ﬂuoroethylene (PTFE) tubing. ePTFE tubing with bubble points of 0.26e0.8 kg/ cm2 was obtained while keeping the porosity !80% or more. Even if the bubble point was 1.0 kg/cm2 the tubing had porosity of 75%. These relations are shown in detail in Fig. 6.11 indicating advantage over the art prior to this invention. As can be understood from this ﬁgure, it is possible to obtain moldings having porosities between 20% and 60% and bubble points between 1.5 and 2.0 kg/cm2. This range is shown as an area surrounded by straight lines linking each point A, B, C, and D. The porosity and the bubble point at each point are as follows: Point A (96%, 0.2 kg/cm2), Point B (54%, 2.0 kg/cm2), Point C (20%, 2.0 kg/cm2), and Point D (30%, 1.0 kg/cm2). Campbell et al. [13] reported a process to produce an ePTFE tube along both longitudinal and transverse axes. Examination of the ePTFE tube indicated that the nodes were interconnected by ﬁne ﬁbrils.

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136 EXPANDED PTFE APPLICATIONS HANDBOOK Grooves Grooves direction of expansion was !four times its original in the die in the tip length. The ﬁnal length could reach seven times its original length. Optionally the stretched PTFE tube The Die was heated to a temperature above its crystalline melting temperature of PTFE. One option was to slit The tip the stretched tubing along its length to produce a sheet of ePTFE. The sheet was further rolled down. Figure 6.13 The cross-sectional view of the die and elevational view of the tip [13]. Before heating the tubing to a temperature above its crystalline melt point it had a crystallinity greater annulus with either one of the tip and die containing than about 95% and one which has been heated to a one or more groove in its surface. The groove in the temperature above the crystalline melt point of PTFE die was oriented in the opposite direction as the has a crystallinity below about 95%. The longitudinal groove in the tip. axes of the nodes that were located adjacent to the inside radius of the tube were oriented at an angle of The grooves were oriented helically with respect about 30 degrees to about 60 degrees with respect to to the longitudinal axis of the tip and die (Fig. 6.13). the longitudinal axis of the tube (Fig. 6.14). The The equipment included a ram capable of recipro- longitudinal axes of the nodes that were located cation to push a preformed tubular billet of lubri- adjacent to the outside radius of the tube were ori- cated PTFE through the annulus. The tubing had at ented at an angle of about 30 degrees to about least one helically oriented ridge on one of its sur- 150 degrees with respect to the longitudinal axis of faces. Both the tip and the die had at least one the tube. The tube could be slit longitudinally to form groove in the surface; the grooves were oriented a sheet, which could be further rolled down. helically with respect to the longitudinal axis of the tip and die. Fig. 6.15A shows a 1000 times magniﬁcation photomicrograph of the inside wall (lumen) of a PTFE was extruded using the equipment shown in PTFE tube extruded and expanded [13]. The longi- Fig. 6.12 to produce unsintered tubing with at least tudinal axes of most nodes are oriented at an angle of one helically oriented ridge on at least one of its approximately 60 degrees with respect to the longi- surfaces. The process also allowed applying pressure tudinal axis of the tube. The angle of pitch of the to the external surface of the extruded tubing thus grooves in the tip used in extrusion is indicated to be smoothed its surface and eliminated the ridge on its 45 degrees. surfaces. Pressure was applied using a roller over the external surface of the extruded tubing while the Fig. 6.15B shows a 1000 times magniﬁcation tubing was held on a mandrel that extended through photomicrograph of the outside wall of the extruded the bore of the tubing. and expanded tube [13]. The longitudinal axes of most nodes are oriented at an angle of approximately The next process step was to remove the liquid 60 degrees with respect to the longitudinal axis of the lubricant by drying and then stretching the unsintered tube. The angle of pitch of the grooves in the die used tubing at a rate !100%/s while the tube was at a in extrusion was opposite to that of the grooves in the temperature below the crystalline melt point of the tip, ie, the angle of pitch of the die grooves was PTFE. After the expansion the ﬁnal length in the 135 degrees with respect to the longitudinal axis of the tube. The relationship between the orientation of the nodes at the inside wall of the tubes and at the outside wall after expansion is not completely un- derstood. What is believed is that, if the longitudinal axis of the nodes at the inside wall are oriented at an angle between about 85 degrees and about 15 de- grees with respect to the longitudinal axis of the tube, the longitudinal axes of substantially all nodes near the external surface of the tube may vary between about 15 degrees and about 165 degrees, depending upon many variables other than but including the helical angles of the grooves in the tip and die.

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 137 (A) (B) Nodes Θ Fibrils Direction of uniaxial expansion Θ = angle of nodes’ longitudinal axes with the direction of expansion Figure 6.14 Schematic of orientation of nodes and ﬁbrils relative to the direction of expansion: (A) expanded pol- ytetraﬂuoroethylene (ePTFE) using smooth surface extrusion tip and die and (B) ePTFE using helically grooved extrusion tip and die [13]. (A) A technology reported in 1980 describes biaxial expansion of tubes in radial and axial directions (B) 45º [14]. A tubular composite structure was produced with a pore size of 1e5 mm at the inside surface 135º and !3 mm at its outside surface. The average ﬁbril diameter was 0.1e2 mm at the inside surface and Figure 6.15 Scanning electron micrographs showing twice that value for the outside surface. The porosity the node-ﬁbril orientation of an expanded poly- of expanded tube was 70e95% and its ﬁbril tetraﬂuoroethylene tubing using helically grooved length 40 mm. Expansion was achieved by longi- extrusion tip and die at (A) 45 degrees and (B) tudinal stretch rate of 100e500%/s and radial 135 degrees [13]. stretch rate of 20e200%/s. The expanded tube exhibited enhanced tear strength in the implantation surgery. Porosity was determined by measurement of speciﬁc gravity using ASTM Method D276 and the pore size distribution and bubble point using ASTM Method F316. Okita [14] offered a relationship between the mean pore size and the length and diameter of ﬁbrils among the nodes in a microstructure consisting of very ﬁne ﬁbrils of PTFE and nodes connected to one another by the ﬁbrils (eg, Fig. 6.14). If the length of each ﬁbrils connecting nodes is l and the distance between two ﬁbrils is d, then the sectional surface of a rectangle surrounded by the two ﬁbrils and the nodes has the following relation with regard to the ﬂuid dynamical equivalent pore size b as deﬁned by Eq. (6.1). 2=b ¼ ð1=lÞ þ ð1=dÞ (6.1) Since l is usually far larger than d, b becomes approximately equal to 2d. Ultimately, the structure

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138 EXPANDED PTFE APPLICATIONS HANDBOOK can be described as a porous structure having a ﬂuid was further developed. Porosity was deﬁned by intermodal distance as measured using a scanning dynamical equivalent pore size twice the inteﬁriluar electron micrograph of a sample taken from the surface, as seen in Fig. 6.15. The average pore size in distance. Okita suggested that the number of ﬁbrils the two tube surfaces ranged from 10e40 mm [15e17]. The procedure to make a dual porosity occurring between two nodes is approximately the PTFE tube begins with forming an inner preformed tubular billet and an outer preformed tubular billet; same for both the outside surface and inside surface the outer billet is adapted to closely ﬁt concentrically within the inner billet. The inner and outer billets are of the tubing. concentrically joined thus merging the two billets into a composite whole. Next the composite billet is In an example, PTFE tubing was extruded by the coextruded into a composite tubular extrudate. paste method, dried, and a length of 20 cm was Altering the lubricant concentration and PTFE resin particle size in the billets can change porosity of expanded. Longitudinal expansion of the tubing was the inner and outer surfaces. Extrusion parameters also affect the properties of the ﬁnal graft. For done by stretching it to a length of 100 cm at a high example, a forward-positioned mandrel and a slower rate in an oven at 200C. The two ends of the speed will result in a decrease in the porosity of outer surface. Heating the mandrel during extrusion also stretched tubing were ﬁxed to prevent shrinkage. A decreases porosity of outer surface. If the mandrel is moved forward and heated and if the extrusion speed pipe for introducing a cooling air was connected to is reduced, the decrease of porosity in the outer sur- face is compounded. one end of the tubing, and the other end was sealed. Fig. 6.16 shows the schematic of a longitudinal The tubing was placed in a furnace, and the tem- expansion device. A pair of crimping tools secures two ends of the composite extruded tube. This device perature of the oven was gradually increased until the allows stretching of the tube longitudinally at temperature reached 320C. At that point air (at controlled rates and temperatures. The crimping tool 200C) at a pressure of 39 kPa was abruptly intro- consists of metal band or clamp adapted to closely circumscribe the extrudate and plug. The expanded duced and pressure was held constant. The temper- length was varied to determine the impact on the ature of the furnace was raised to 440C. At that point porosities. the tubing was rapidly cooled to room temperature Crimped extrudate is then dried in an oven by (20e30C). evaporating the lubricant. After drying, crimped extrudate is heated to a temperature in the range of The inside and outside surfaces of the resulting 225e300C and then longitudinally expanded by stretching to a desired dimension. The extrudate is tubing were analyzed using a scanning electron mi- expanded at a rate of 5e10%/s by pulling the hooks of the crimp device in opposite directions. The croscopy and micrographs were obtained. The ﬁbril expansion ratio (ﬁnal length divided by initial length) was predetermined with the desirable ratio ranging diameter was measured to be 0.5e1.0 mm at the in- side surface and 1.0e3.0 mm at the outside surface. The ﬁbril length was 15e30 mm both at the inside and outside surfaces. The tubing as a whole had a porosity of 81%. In another example, air pressure, at 200C, was increased to 147 kPa. The tubing stretched ﬁve times when the oven temperature reached 330C. This resulted in the expansion of the outside diam- eter of the tubing to 16 mm. The air pressure was reduced to 39 kPa, and the furnace temperature was increased to 465C before the tubing was rapidly cooled. The ﬁbril diameter of the inside surface was 0.1e0.2 mm, and the tubing as a whole had a porosity of 93% [14]. In another study, dual porosity ePTFE tube with different porosities in its inner and outer surfaces Extrudate Metal band Plug Crimping end Crimping end Direction of expansion Figure 6.16 Schematic diagram of tube expansion apparatus.

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 139 from about 6.3 to about 2.5. The expanded extrudate at a rate of 10%/s in an oven at air temperature of was sintered and cooled and cut away from crimping 275C. The expanded extrudate was then sintered in tools for use as graft. an oven at 380C. Following sintering, the tubes were cooled and then cut from the crimping appa- An inner PTFE billet was formed using PTFE with ratus. The porosities of the inner and outer surfaces average particle size less than 355 mm with mineral of the grafts were measured and are summarized in spirits at a lubricant level of 16% by weight. The inner Table 6.4. billet had an outer diameter (OD) of 3.2 cm and inner diameter (ID) was 1.3 cm. An outer PTFE billet was Two groups of inner and outer PTFE billets were formed using PTFE with average particle size less produced. In Group 1, the inner billet had a low than of 450 mm 24% by weight. The outer billet OD lubricant level while the outer billet had a high was 5.1 cm and the ID was 3.2 cm. The inner billet lubricant level. In Group 2, the inner billet had a was then slid through the outer billet so that the outer high lubricant level while the outer billet had a low billet ﬁt snugly concentric to the inner billet. The lubricant level. In other words, Group 2 represents concentrically aligned inner and outer billets were the reverse of Group 1. Both sets of billets were then pulled over a cylindrical supporting shaft that had prepared under conditions as those in Table 6.4. The an OD of 1.27 cm. The shaft-supported billets were extruded tube has a length of 17.8 cm and was then loaded into a paste extruder attaching the shaft to stretched to 96.5 cm thus had an expansion ratio of a mandrel, and several lengths of tube (graft) with ID 5.1:1 (Table 6.5). The porosity data indicate com- of 6 mm was extruded at extrusion pressure of plete reversal between the two groups. 7.9 MPa and speed of 14.4 cm/s. The extrusion mandrel was in the normal position and unheated. The inner surface of the resulting PTFE tube ex- hibits porosity different from that of the outer surface The extrudate was used to prepare four sections of of the tube. Lowering the porosity is aimed to reduce tube at lengths of 20.3, 25.4, 30.5, and 35.6 cm and blood leakage of a vascular graft, while the outer were placed in the apparatus shown in Fig. 6.16. The surface is made more porous to enhance tissue tubes were dried at 40C for 1 h, incubated for ingrowth. By reversing the porosity the inner surface 5 min, and then longitudinally expanded to 96.5 cm is made more porous while the outer surface is made Table 6.4 Dual Porosity of Expanded Tubing at Different Expansion Ratios [17] Tube Expansion Inner Surface Outer Surface Length (cm) Ratio Porosity (mm) Porosity (mm) 20.3 4.8:1 25.4 3.8:1 24 68 30.5 3.2:1 20 60 35.6 2.7:1 12 52 12 40 Table 6.5 Reversal of Porosity of Expanded Tubing With Dual Porosity [15] Group 1 Group 2 Average Inner Billet Outer Billet Inner Billet Outer Billet polytetraﬂuoroethylene 450 450 450 450 particle size, mm 14 26 26 14 Lubricant level, wt% 20 60 60 20 Porosity, mm

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140 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 6.18 Scanning electron micrographs showing the node-ﬁbril orientation of expanded polytetra- ﬂuoroethylene tubing made using a rotating extrusion die [18,19]. Figure 6.17 Schematic diagram of the rotating Figure 6.19 Examples of commercial expanded mandrel inside a polytetraﬂuoroethylene paste extru- polytetraﬂuoroethylene tubing. sion die [18]. Courtesy: International Polymer Engineering, Co., Tempe, Arizona, USA, www.ipeweb.com. less porous, to accelerate healing of an implanted vascular graft [15e17]. from ePTFE. Surgeons in need of grafts suitable for use as vein or arterial grafts, shunts, or the like, have Another technology described a method to pro- had to adapt the tubing shape structures to the duce tubular ePTFE that had high radial tear strength. particular applications. Peripheral artery disease re- The microporous structure of the tubular product fers to narrowing and hardening of the arteries that contained mostly tilted PTFE ﬁbrils and nodes. The supply blood to the legs and feet. In such a case end- extrusion of the PTFE green tube took place while the to-side attachment (anastomosis is the medical term) die was rotating (Fig. 6.17). The extrusion apparatus of a distal bypass ePTFE graft is accomplished by was kept at a constant temperature to avoid a tem- techniques such as Miller Cuff or a Taylor Patch as perature gradient during extrusion. At least one of the depicted in Fig. 6.20 [20,21]. The basic aim is to die surfaces was rotated during extrusion to enhance avoid compliance mismatch between the ePTFE and ﬁbrillation of the PTFE in perpendicular direction to the receiving native artery. Consequently, the cuff the direction of the tube extrusion. The green tube and patch are made using venous tissue [22]. was expanded in secondary operations to form an ePTFE tube suitable for medical use. The ePTFE Edwin and Randall [9,23] developed a method tube exhibits a microporous structure deﬁned by (Fig. 6.21) for fabricating ePTFE into a ﬂange graft nodes interconnected by ﬁbrils. The nodes in that with a tubular body and a ﬂanged end section that is microporous structure were not oriented perpendic- ular to the longitudinal axis of the tubular body (Fig. 6.18) [18]. Fig. 6.19 shows an example of different diameter ePTFE tubes. 6.2.1 Complex Shape Tubular Expanded Polytetraﬂuoroethylene Conventional processes for making ePTFE prod- ucts are typically limited to fabricating sheets, tubes, rods, or ﬁlaments. In actual applications often three dimensional, irregular-shaped, complex medical grafts are required which are difﬁcult to fabricate

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 141 Figure 6.20 Schematic of Miller Cuff and Taylor Figure 6.22 Schematic diagram of an integral Patch attaching expanded polytetraﬂuoroethylene expanded polytetraﬂuoroethylene (ePTFE) graft (ePTFE) graft to a vein [20,21]. with ﬂange for direct attachment to a native vein [24]. Wire scaffolding ePTFE cover Figure 6.23 Schematic view of an expanded polytetraﬂuoroethylene (ePTFE) endoluminal stent graft [23]. Figure 6.21 Major process steps for manufacturing constrained regions, or highly tortuous regions may of tubular expanded polytetraﬂuoroethylene shapes require external wrapping to ensure close confor- [9,23]. PTFE, polytetraﬂuoroethylene. mation with the shaping mold [23]. Fig. 6.22 shows the schematic of an integral ePTFE graft with ﬂange angularly displaced from the longitudinal axis of the for direct attachment to a native vein. graft. That requires molding ePTFE tube by radially expanding a portion of ePTFE tube into a shaping More complex shape grafts are manufactured by mold such that the exterior surface of the ePTFE tube shaping tubes of ePTFE around molds and mandrels is in contact with the mold surface. The method by thermoforming. Fig. 6.23 illustrates an endolu- described consists of essentially thermoforming minal stent-graft device that was manufactured a by a complex conformations to produce grafts from thermoforming technique. The endoluminal stent is ePTFE. metallic, made of nitinol or stainless steel, and re- places the forming mold described previously. It is The ePTFE tube is radially expanded to a diameter placed inside a radially expanded ePTFE tubular that is relatively greater than the unexpanded diam- shape. Next it is heated resulting in radially eter of the ePTFE tube. The radially expanded contraction about the stent. Fig. 6.24 shows an ePTFE tube is then engaged about a shaping mold ePTFE aortic arch graft. It consists of a central lumen and heated. The radially expanded ePTFE tube con- and branch members that project outwardly from the tracts about the shaping mold thus forming to the crest of the aortic arch. It is made by heat contraction external conﬁguration of the shaping mold. Highly of a radially expanded ePTFE tube around a shaping mandrel [23].

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142 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 6.24 Schematic view of a complex shape Figure 6.25 Side and cross-sectional views of expanded polytetraﬂuoroethylene prosthetic aortic the double stents [27]. ePTFE, expanded arch graft [23]. polytetraﬂuoroethylene. Kink and crush resistance of ePTFE grafts is not described in this book. The reader may consult enhanced by wrapping it with an external textile tape other sources for further information [7,30]. or ﬁtting a cylindrical structure to the exterior of the graft. The reinforcement materials must be remov- In the paste extrusion process, PTFE resin is pro- able, without damaging the ePTFE layer, to allow the cessed into membrane, tape, and ﬁbers and expanded. surgeon to tailor the graft to the patient’s vein. A To produce ﬁber from tape, membranes are slit into number of techniques have been reported [25,26]. ﬁber after expansion. Another approach is paste extrusion of a monoﬁlament followed by expansion. Stents have becoming smaller resulting in minia- ePTFE ﬁbers are produced using different turization of ePTFE tubes. The latter are susceptible manufacturing processes from those used to make to tearing during implantation. To solve this problem matrix-spun ﬁber. The extrusion process typically novel designs have been proposed such as use of delivers a much higher tensile strength ( 4 g/denier) double layers of ePTFE in between two stents. Side with a lower shrinkage (3e5%) compared to the and cross-sectional views of the subject stents are matrix-spun PTFE ﬁber [31]. shown in Fig. 6.25. The side and the cross-sectional views show the stent graft after expansion. Here the There is a third method for manufacturing “high device works as both a stent and a graft. After tensile strength PTFE ﬁber” that is usually comprised placement in the patient and expansion of the stents of direct past extrusion of a monoﬁlament followed any hole in the inner and outer ePTFE layers are by heat treatment and stretch of the ﬁber. The ﬁber unlikely to be aligned. The design thus provides made by this technique is not porous. No commercial protection against leakage. manufacturer using this process has been identiﬁed. 6.3 Expanded 6.3.1 High Tensile Strength Polytetraﬂuoroethylene Fiber Polytetraﬂuoroethylene Fiber Commercial PTFE ﬁber is offered in two main A reported method for the production of a strong forms: a matrix-spun ﬁber and a paste extrusione PTFE ﬁber begins with paste extrusion of mono- based ﬁber. In a matrix-spun ﬁber, PTFE is processed ﬁlaments [32e34]. The extrudate monoﬁlament is using a cellulose binder that is subsequently volatil- ized, resulting in a characteristic brown PTFE ﬁber. PTFE ﬁber’s tensile strength is in the range of 1e2 g/denier at the room temperature though ten- sile strength decreases with increasing tempera- tures. Shrinkage of matrix-spun ﬁbers is relatively high (10e20%). More thermally stable ﬁbers at the lower end of shrinkage range have been developed by Toray Fluoroﬁbers America [28,29]. This PTFE ﬁber, in spite of stretch during spinning, is not considered expanded. Matrix spinning of PTFE is

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 143 subjected to heat treatment and stretching/expan- Fusion enthalpy of 100% crystalline measured by sion, which results in very high tensile strength ﬁ- DSC was assumed to be 93 J/g [36]. bers. Fibers manufactured by slitting ePTFE sheet/ tape also develop high tensile strength when they A ﬁber with uniform tensile strength with a undergo heat treatment and stretching. In the ﬁrst diameter of 50 mm was produced at RR ! 800 from process, heat treatment and sintering are used a paste extruded monoﬁlament (extrudate diameter of interchangeably. 0.4 mm). The monoﬁlament had to be drawn by 25,000% or 250 times its original length at a rate of In a process paste extrusion of monoﬁlament took 1000%/s. The axial strength obtained for a ﬁber place at a die temperature in the range of 40e60C diameter of 70 mm was !500 MPa. A ﬁber with axial and a die taper angle of 30e60 degrees [34]. The strength !1000 MPa had a diameter of 50 mm. The impact of RR on ﬁber strength was found to be sig- optimal drawing temperature had to be precise in the niﬁcant. The RR is a ratio of the cross-sectional area range of 387e388C although an acceptable product of the cylinder of the extruder to the cross-sectional could be manufactured at !360C. area of the die. Generally, RR is an important fac- tor in paste extrusion process, but especially impor- Shimizu emphasizes the importance of unre- tant to manufacturing super high strength ﬁber from strained heat treatment and extremely low cooling the PTFE polymer. rate to drive up the crystallinity. Successful stretching required maximum crystallinity in the ﬁber thus the High strength PTFE ﬁber was produced by altering required cooling rate of 0.5C/min. Chapter 5 of the helical structure of the polymer molecules by this book should be consulted for an in-depth un- stretching them [7]. The molecules extend such that derstanding of PTFE behavior at high stretch rates the ultimately extended chains are oriented in the and the role of crystallinity in the polymer expansion direction of the ﬁber axis. There were three prepa- process. ratory steps prior to stretching the PTFE mono- ﬁlament. Those steps were selection of an appropriate 6.3.2 Production of Expanded RR, heat treatment (annealing), and cooling. Polytetraﬂuoroethylene Fiber Shimizu considered the heat treatment condition the most important factor in manufacturing high The most common way to produce ePTFE ﬁbers is strength ﬁbers of PTFE [34]. In other words, PTFE to slit webs or tapes of ePTFE. These ﬁbers are often can be super drawn easily, but, if the heat treatment further processed to improve one or more of their condition is not adequate, its axial strength may not properties. Stretching, twisting, and applications of be uniform. Ideal heat treatment required a temper- other materials to the ﬁber surface are among the post ature of 350C for 1.5 h. The two ends of the slitting processes. Their applications include sewing monoﬁlament ﬁber were unrestrained during heat threads, dental ﬂoss, and weaving yarns. The dis- treatment. Alternatively a slack (sag) of 10e20% cussion begins with a short description of slitting could be allowed, if the ﬁlament ends were technology. restrained, as a prerequisite for super drawing to obtain axially strong ﬁber [35]. Relaxation occurring Most web goods like paper, fabric, and plastic as a result of complete or partial restrain prevented ﬁlms are manufactured much wider than the majority the ﬁber from latent shrinkage. of the applications for the material. The webs are slit or cut into narrower width either on the production Cooling rate was used to tune the degree of crys- line or in a separate stage (Fig. 6.26) followed by tallinity of the heat-treated PTFE monoﬁlaments. rewinding on smaller core tubes. For example, plastic Increasing the crystallinity of the ﬁber remarkably ﬁlm is produced at 1.5e2 m width, coated with ad- increased its post-stretch axial strength, reduced the hesive, and slit into narrow-width individual rolls number of defects and enhanced the longitudinal sold in retail. uniformity. The best results were obtained when cooling rate was 0.5C/min although acceptable There are a wide variety of slitting machines and ﬁber properties were achieved when the cooling rate cutting elements designed to maximize precision was <5C/min. Inﬂuence of cooling speed on the and productivity of the operations. There are three degree of crystallinity of PTFE monoﬁlament was basic types of machine including razor blade slit- determined using differential scanning calorimetry ting, shear slitting, and score cut slitting. The ma- (DSC) on ﬁber that was annealed for 1.5 h at 350C. chines differ based on the actual mechanism of web cutting. The type and thickness of material

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144 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 6.26 Example of a plastic ﬁlm slitting machine. Courtesy: Parkinson Technologies, www.parkinsontechnologies.com. (A) Razor (B) Razor Web Smooth roll Web feed feed Grooved roll Figure 6.27 (A) Razor blade slitting in air and (B) razor blade slitting on grooved roll [37]. determines whether shear cut, razor blade, or score honed area of the blades keeping abrasion of the ﬁlm cut slitting should be employed. to a minimum. When hollow ground blades are used there is less interference between the blade and the Soft thin ﬁlms like ePTFE are slit using razor ﬁlm being cut [37]. blade slitting. Fig. 6.27 shows two designs of razor blade slitting. Fig. 6.27A shows air cutting (without a Slitting can produce narrow tapes down backing roll), which is not precise and cannot hold to <1 mm width. A problem with ePTFE materials tight tolerances. Fig. 6.27B shows grooved roll slit- is their tendency to be difﬁcult to process because ting, in which as the name indicates cutting takes of structural problems. Other ﬁbers used for place on the web wrapped around the grooved back- weaving consist of multiple ﬁlaments twisted into a up roll. The grooved roll with the web wrapped single ﬁber with uniform dimensions. The ePTFE around it allows a cleaner, smoother, and more ac- ﬁbers generally consist of thin ﬂat ﬁlament strands. curate cut because, of the very close support the roll Leaving the thin edges of ePTFE ﬁbers exposed gives to the web as it is being cut. Fig. 6.28 shows a during processing may cause the ﬁber to fray or magniﬁed view of a section of the grooved roll and a ﬁbrillate during handling. Traditionally, folding comparison of hollow and tapered razor blades. In process has been difﬁcult to control during pro- this arrangement the ﬁlm touches only the ﬁnely cessing thus resulting in a ﬁber with inconsistent

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 145 Razor step the ﬁber was amorphously locked by exposure to blade 400C for 1 s using a heated plate. Web A number of reports describe development of ePTFE ﬁber for speciﬁc applications such as sewing Groove thread or oral care ﬂoss [42,43]. Sewing ﬁber must have low friction and high toughness for high-speed Magnified section sewing and withstand application requirements. The of grooved roll ﬁber made by this process could also be used as a ﬁlament for dental ﬂoss and bearing material. An Hollow Taper additive such as titanium dioxide had to be applied to ground blade ground blade the ﬁber to improve the “grippability” of a dental ﬂoss ﬁber [44]. Figure 6.28 Magniﬁed razor blade slitting in grooved roll [37]. The important property for many ePTFE ﬁber applications is high toughness deﬁned as resistance of materials to fracture. It is determined by the amount of mechanical energy materials can absorb and plastically deform without fracture. Toughness of a material is a function of its hardness and ductility. Mathematically, Eq. (6.2) deﬁnes toughness in en- ergy per volume units. width and thickness along its length. More recently Zε (6.2) technology has been developed that allows effective folding of the edges of ﬂat ﬁber one or more times T ¼ s$dε [38]. Twisting the ﬁbers can also solve the prob- lems plus twisting increases the strength of the 0 ﬁber. T is the toughness, ε the engineering strain, and s Attempts have been made to solve the problems is the engineering stress. For a given material and a associated with slit ePTFE ﬁber. Abrams et al. have set of conditions s ¼ f(ε). It is difﬁcult to directly developed an ePTFE ﬁber with useful properties calculate T because f(ε) is not known for different folding or twisting prior to or during the weaving materials. Practically proxy properties such as impact [39e41]. A thick ePTFE sheet was made and slit strength are used to assess the toughness of materials. into narrow-width strips, for example, by using razor T may be determined by graphical measurement of blade slitting on grooved roll. The larger aspect ratio the area under the stressestrain curve of a material. of the ﬁber allowed spooling the slit ﬁber without folding or creasing. The ﬁber was expanded in the Fig. 6.29 shows typical shapes of stressestrain curve longitudinal direction at an expansion ratio of 15:1 for various materials. Kelmartin et al. proposed using to 35:1. Finally, this ﬁber was heat treated at a temperature above 342C to effect amorphously an approximation of Eq. (6.2) expressed by Eq. (6.3) locking. [42,43]. For example, a mono or biaxially oriented ePTFE sheet was manufactured such that its ﬁnal length was 1.5e2.5 times its original length. The sheet had a Toughness ¼ Xn εðiþ1Þ À εi si þ sðiþ1Þ (6.3) thickness of 0.5e1.0 mm and a density range of 1e1.9 g/cm3. The sheet was slit into narrow strips i¼1 2 with the width ranging from 5 to 7 mm. Next the ePTFE strips were expanded longitudinally into a In Eq. (6.3), i varied from 1 to n where n was the 0.5e3 mm ﬁnal width and thickness of 50e250 mm. total number of data points. The engineering stress is The width of the ﬁber was controlled by expansion expressed in g/denier. process variables such as the slit width, expansion temperatures, and the expansion ratio. In the ﬁnal Fig. 6.30 shows the stressestrain curve of an un- waxed dental ﬂoss ePTFE with width of 1.1 mm, thickness of 76 mm, and 1170 denier [44]. Five samples of this ﬁber were tested to obtain the stressestrain curves. A special high-speed tensile test device was used for testing at ambient temperature (20C).

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146 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 6.29 Stressestain curve for material with different toughness. Engineering Stress (g/denier) 4 3.5 3 2.5 2 1.5 1 0.5 0 –0.5 0% 5% 10% 15% 20% 25% 30% 35% 40% –1 Engineering Strain Figure 6.30 Stressestrain of an expanded polytetraﬂuoroethylene ﬁber before heat treatment [42]. This tensile test machine was equipped with gives summary of peak engineering stress and its pneumatic ﬁber grips set to a gauge length of corresponding strain value, break strain, and tough- 270 mm. The sample was loaded into the grips and ness for each ﬁber sample. clamped. The grips were moved 50 mm closer together to slacken the ﬁber. Care was taken to make The ﬁber of Fig. 6.30 [44] was heat treated at a sure that the ﬁber did not get tangled. The test began temperature above the melting point of PTFE. Using as the grips moved apart at a crosshead speed of nip rolls, the ﬁber was overfed at a rate of about 13%, 2000 mm/s. Care was taken to ensure constant ve- to allow slack, over a plate heated to 400C. The locity was reached at the point where the grips were residence time of the ﬁber on the heated plate was 270 mm apart. In other words, in 1 s the length of 5.5 s. The ﬁber was subsequently wound onto a core. ﬁber was increased nearly eight times. The end of the After heat treatment, the denier of this ﬁber was test was indicated when the ﬁber sample broke. measured to be 1430. Five random samples of this ﬁber were tested in the high-speed tensile test device The engineering strain was calculated as a percent to obtain the stressestrain curves according to the of the original ﬁber length (270 mm) to plot the data high-speed test. The data were plotted in Fig. 6.31 in graph form up to the point after break where the and summarized in Table 6.7. engineering stress returns to zero. The data was examined to identify the peak engineering stress, the Heat treatment had a signiﬁcant impact on the engineering strain at peak engineering stress, and toughness of the ﬁber by nearly quadrupling its the break strain. The value of toughness for each toughness as seen in Tables 6.6 and 6.7. Peak engi- sample was calculated from Eq. (6.3). Table 6.6 neering stress is reached at about 3.5 g/denier for the ﬁber before and after treatment. The engineering

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 147 Table 6.6 Summary of Peak Engineering Stress and Its Corresponding Strain Value, Break Strain, and Toughness for Each Fiber Sample [42] Sample Peak Strain at Peak Break Strain Toughness Number Engineering Engineering (%) (g/denier) 1 Stress (%) 12.4 2 Stress 12.4 0.275 3 (g/denier) 7.52 12.8 0.165 4 12.1 0.141 5 4.69 7.76 12.5 0.210 Mean 12.4 0.207 2.44 8.93 0.200 1.87 7.89 3.65 7.44 3.37 7.91 3.20 Engineering Stress (g/denier) 4 3.5 3 2.5 2 1.5 1 0.5 0 -0.50% 5% 10% 15% 20% 25% 30% 35% 40% Engineering Strain -1 Figure 6.31 Stressestrain curve of expanded polytetraﬂuoroethylene ﬁber in Fig. 6.30 before heat treatment [42]. Table 6.7 StresseStrain Curves for Fibers According to the High-Speed Test [42] Sample Peak Strain at Peak Break Strain Toughness Number Engineering Engineering (%) (g/denier) 1 Stress (%) 29.0 2 Stress 50.1 0.654 3 (g/denier) 18.9 37.7 1.226 4 23.8 0.918 5 3.67 20.0 25.6 0.449 Mean 33.3 0.536 3.73 19.6 0.757 3.65 17.8 3.46 19.3 3.56 19.1 3.61

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148 EXPANDED PTFE APPLICATIONS HANDBOOK Table 6.8 Comparison of Two Heat Treatment Methods for Teadit Corp Yarn [50] Monofilament Multifilament Ply yarn Twisted Property US Patent Neto yarn yarn yarn Fiber count, denier 5,989,709 Process Elongation at max Figure 6.32 Examples of ﬁber (yarn) construction stress, % 617 394 including monoﬁlament and twisted yarn [46]. Shrinkage, % 11.20 3.50 Tenacity, g/denier 4.00 1.20 1.47 2.84 strain values at peak engineering stress and break developed with near circular cross sections. Exam- strain value were both signiﬁcantly higher for the ples of technologies for preparation of twisted ePTFE heat-treated ﬁber. Untreated ﬁber broke relatively ﬁbers with near round cross sections include methods shortly after reaching maximum engineering stress in published by Neto [48,49]. contrast to the tougher heat-treated ﬁber that deformed and continued to bear lower levels of en- Neto’s technique described stretching the twisted gineering stress. To put it simply the tougher ﬁber has ePTFE ﬁber during heat treatment as opposed to more “give” to it than the untreated ﬁber thus making allowing a slack by overfeeding the ﬁber as pre- it more suitable as dental ﬂoss than the untreated ﬁber scribed by Kelmartin et al. [42]. Table 6.8 shows a described in Fig. 6.30 and Table 6.6. comparison of the properties of ﬁbers prepared by the two techniques. Neto applied his technique to ePTFE Fiber with best properties was heat treated at a ﬁbers obtained from Teadit Corp (Brazil). The ﬁla- temperature for a few seconds at a temperature in the ments were twisted at 400 turns per meter in the Z range of 350e450C while it was overfed 10e20% direction and then subjected to heat treatment over to retain a slack during the heat treatment. Optimal hot plates. The ﬁber was fed at a stretching rate of range of the ﬁber toughness was from 0.50 to about 70% at 400C for 5.5 s. After treatment the 0.80 g/denier and a peak engineering stress 3e5 g/ ﬁber count was measured to be 394. denier and a break strain !15.5%. A desirable ﬁber would have a peak engineering stress of 4.4 g/denier Versatility and utility of the unique properties of and break strain of 24% [42]. ePTFE ﬁbers has spurred its use in innumerable applications. Description of some innovative ex- The improvements described by Kelmartin et al. amples illustrates the point. Clough, Lutz, and Harp [42] could also be applied to twisted multiﬁlament at W. L. Gore [51] have described a composite ﬁbers using the same procedures [43,45]. Fig. 6.32 bundle for repeated stress applications that con- shows a comparison of the shape and conﬁgurations sisted of one or more high strength ﬁbers (core ﬁ- of twisted and untwisted ﬁbers (yarns). Twisting ber) and 10% by weight of a ﬂuoropolymer ﬁber. multiple ﬁlament ﬁbers helps hold the ﬁlaments The high strength ﬁber was a liquid crystal polymer together and strengthen the strand plus allows or ultrahigh molecular weight polyethylene increasing the denier of the strand [47]. Optimal peak (UHMWPE) or their combinations. engineering stress for the heat-treated twisted ePTFE ﬁber was between 3.0 and 5.0 g/denier, break strain The ratio of break strengths after abrasion test was of 20e50%, and toughness of 0.50 to about most preferably of at least 4.0. The ﬂuoropolymer 0.80 g/denier. ﬁber is an ePTFE ﬁber, which may be a mono- ﬁlament or multiﬁlament, either of which can be low One of the drivers of ePTFE ﬁber development has or high density. The schematic of one style of the been thrust at obtaining controlled appearance and ﬁber is shown in Fig. 6.33 in which one strong core uniform structure fabrics. Strategies in achieving ﬁber (eg, UHMWPE) was wrapped with ePTFE ﬁber. these goals have included altering the rectangular cross section of individual slit ﬁbers closer to circu- A single UHMWPE ﬁber was tested for abrasion lar. More signiﬁcantly twisted ﬁbers have been resistance using a modiﬁed ASTM Method D-6611. Three complete wraps were applied to the ﬁber. The test was conducted at 65 cycles per minute, under

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 149 a = yarn bundle consisted a a of core fiber wrapped with a a ePTFE fibers a b b = ePTFE fiber, single or multi filament a b b b Figure 6.33 Schematic diagram of structure of one strong core ﬁber (eg, ultrahigh molecular weight polyeth- ylene) wrapped with expanded polytetraﬂuoroethylene (ePTFE) ﬁber [51]. 762 g tension (which corresponded to 1.5% of the Clough and Sassa reported on the development of break force of the UHMWPE ﬁber). a wire rope including at least one metal wire and an ePTFE ﬁber [52]. The ﬁber was present in an amount An ePTFE monoﬁlament ﬁber HT400 d Rastex less 25% by weight of the rope. Alternative ﬁbers to ﬁber was sourced from W. L. Gore and Associates. ePTFE included PTFE and melt processable ﬂuo- This ﬁber possessed the following properties: 425 d ropolymer ﬁbers though ePTFE ﬁbers were prefer- weight per unit length, 2.29 kg break force, able. The wire rope is useful in tensioned and 5.38 g/denier tenacity, and 1.78 g/cc density. bending applications. The UHMWPE ﬁber was wrapped with the ePTFE There have been reports about use of ePTFE in monoﬁlament ﬁber. The combination of the two ﬁ- ﬁshing lines [53]. UHMWPE and ePTFE ﬁbers were bers was tested for abrasion resistance in the same combined and stretched to increase the line tenacity. manner. Three complete wraps were applied to the The stretching conditions could be chosen to signif- combination of the ﬁbers. The test was conducted at icantly reduce the denier of the line. 65 cycles per minute and 775 g tension. The abra- sion tests were run for 500 cycles, after which point 6.4 Densiﬁed Porous the test samples were tensile tested to determine their Polytetraﬂuoroethylene break force. Membranes The composite bundle and the UHMWPE ﬁber Expansion of PTFE enhances its mechanical exhibited 42.29 and 10.90 kg break forces after properties. The improvements in ePTFE and PTFE abrasion, respectively. Adding the single ePTFE ﬁber itself are still short of the requirements of a number increased the break force by 2% prior to abrasion of applications. For instance, insulated wires require testing and resulted in a 288% higher break force cut-through resistance beyond the capability of after the abrasion test, a signiﬁcant improvement. ePTFE. Cooper and Lane offered one of the early The abrasion rates for the UHMWPE ﬁber alone and solutions to the problem in 1988 [54]. One approach the composite of the UHMWPE ﬁber and the ePTFE monoﬁlament ﬁber were 79.8 and 18.8 g/cycle, respectively [51].

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150 EXPANDED PTFE APPLICATIONS HANDBOOK was to insulate the conductor with a cut-through each successive pass was applied with opposite tape lay direction. resistant material and then cover with standard The tape-wrapped conductor was then passed PTFE. A more elegant method was to use a single through a salt bath heated at a temperature of about 390C for a period of 5e7 s. A standard PTFE insulation layer of “densiﬁed ePTFE” that provided insulated conductor was prepared as follows: three layers of an unsintered PTFE tape with a thickness of sufﬁcient cut-through resistance. 0.076 mm and a density of about 1.54 g/cm3 were tape wrapped on a 30(1) AWG conductor in one pass. A dry tape with a thickness of about 0.38 mm, The insulated conductor was then heated at about 390C in a salt bath for a period of 5e7 s. a width of about 124.5 mm, and a density of about 0.97 g/cm3 was expanded using a three-stage plate Cut-through resistance of densiﬁed ePTFE, stan- dard PTFE, and Tefzel insulations are summarized in expander. The slow/fast roll distance in both ﬁrst and Table 6.9. A comparative sample of a 30-gauge conductor insulated with Tefzel ETFE (a copolymer second stage was 61 cm. The ﬁrst stage expansion of ethylene and tetraﬂuoroethylene) insulation with a ratio was set at 20:1 at 325C. The second expansion 0.13 mm wall was also tested. Tefzel is known to plate was done 325C at a ratio of 2:1. In the third have excellent cut-through resistance for most wire and cable applications. Densiﬁed ePTFE insulation is stage the distance between the two rolls was 122 cm clearly superior to the other two materials [54]. with a 1:1 expansion at 330C. Thus, the expansion Fillers can be added to PTFE dispersions to create ratio on the plate machine was 40:1 and the total intimate mixtures of the two components. These can then be co-coagulated to partition the mixture, ﬁlter expansion ratio was 80:1. The properties of this out the liquid, and dry the solids to recover a ﬁlled PTFE composition that can then be processed into expanded tape were as follows: thickness of about ﬁlms and other parts. The most common method for making ﬁlms and tapes from ﬁlled PTFE compounds 0.05 mm, width of about 43.2 mm, and density of is paste extrusion. about 0.56 g/cm3. A DSC test showed two crystalline melt points, one at about 344C and a second at about In a number of examples, various ﬁllers were 379C. added to PTFE dispersion, co-coagulated, and even- tually processed into tape. The extrudate was then The ePTFE tape was then compressed (densiﬁed) calendered using heated rolls to produce thin tapes, which were then dried. The tape was stretched uni- between two polished steel rolls (calendar), heated to axially in MD or biaxially in MD and TD. The a temperature of about 90C, so that the ﬁnal density expanded ﬁlled PTFE tape was then compressed by of the tape was about 1.96 g/cm3. The ﬁnal thickness running it through heated rolls [55]. was about 0.015 mm and the ﬁnal width was about 43.2 mm. Testing by DSC showed two crystalline melt points, one at about 345C and another at about 383C [54]. This compressed, ePTFE tape was then slit and helically wrapped according to conventional tape wrapping techniques onto a 30(1) AWG (American Wire Gauge) conductor. Eleven layers were applied resulting in a ﬁnal wall thickness of about 0.15 mm. These layers were applied in three passes of three layers and one pass of two layers; Table 6.9 Comparison of Cut-Through Resistance of Densiﬁed Expanded Polytetraﬂuoroethylene (ePTFE) With Polytetraﬂuoroethylene (PTFE), and Tefzel [54] High Strength Densiﬁed Standard PTFE Film Tefzel Insulation PTFE Conductor, Final OD Cut-Through Final OD Cut-Through Final OD Cut-Through AWG (mm) Resistance (mm) Resistance (mm) Resistance (Stranding) 0.58 (kg) 0.61 (kg) 0.51 (kg) 30(1) 0.46 1.40 0.46 0.85 1.08 38(1) 1.35 2.08 1.35 0.99 1.01 18(1) 3.21 1.57 OD, outer diameter.

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 151 Table 6.10 Comparison of the Properties of Densiﬁed Filled Expanded Polytetraﬂuoroethylene Membranes [55] Variable Compound 1 Compound 2 Compound 4 Compound 5 Uniaxial Biaxial Uniaxial Uniaxial Expansion type 6:1 3.5:1 2:1 6:1 Machine direction (MD) NA 5:1 NA NA expansion ratio Carbon black Titanium oxide Tungsten powder Poly ethereether Transverse direction (TD) ketone powder expansion ratio 56 50 0.07 0.09 41.5 Filler type 230 659 0.08 54 30 134 Filler volume, % 28 47 Film thickness, mm 0.03 Weight/area, g/m2 36.9 Matrix tensile strength in 112 MDa, MPa Matrix tensile strength in 3.8 55.9 3.6 6.2 TD, MPa a Matrix tensile strength (MTS) is calculated from equation: MTS ¼ gm=foot of 423 one inch wide Â weight force at break ðpsiÞ sample: (6.4) sample fraction of PTFE in The ﬁlled membranes in Table 6.10 were inspected The procedure consisted of placing two or more visually in a light box. The appearance was uniform layers of expanded porous PTFE inside a thermally with no evident holes indicating ﬂaws. The MTS of stable and pressure resistant ﬂexible (eg, rubber) the membrane of Compound 2 was virtually balance container or mold. Air was evacuated from the inside because of biaxial expansion. Conversely, uniaxial of the mold until the pressure in the ﬂexible mold was expansion imparted unbalance MTS to Compounds less than 67.7 kPa. Next, the ﬂexible mold was sub- 1, 4, and 5 in which MD was overwhelming favored jected to a pressure between 1 and 2.5 MPa and a over TD. temperature in the range of 368 and 400C in an autoclave. Then the container was cooled while the A process was disclosed for making formed ar- pressure on the container was reduced and the ticles of densiﬁed ePTFE. The process produced densiﬁed ePTFE part was removed. The densiﬁed densiﬁed ePTFE ﬁlms, sheets, or formed parts. At PTFE layers may contain one or more reinforcing sufﬁcient thickness and reduced porosity the parts layers of a fabric material [56]. were useful as barrier layers either alone or in constructions combined with other materials. Spe- Densiﬁcation of ePTFE membranes has been used ciﬁc gravity of useful parts was !2.14. Samples of to manufacture creep resistant parts. This is a parts made by this process exhibited remnants of a particularly important point because standard PTFE ﬁbril and node structure as characterized by peaks is soft and tends to creep [7]. In a process one or more at about 327C and about 380C in a DSC ther- layers of ePTFE were densiﬁed [58]. The densiﬁed mogram (heating and cooling rate 10C/min) as ePTFE material exhibited remnants of a ﬁbril and seen in Fig. 6.34. An important application of node structure. The part was also resistant to creep at densiﬁed parts made by this process was pump high temperatures and under high loads. Compared to diaphragm (Fig. 6.35) made of a layer of the PTFE and ﬁlled PTFE parts the densiﬁed parts made densiﬁed ePTFE laminated to a ﬂexible elastomeric by this process were exceptionally creep resistant at polymer. The diaphragm was capable of with- temperatures up to 327C under a load. The creep standing ﬂexural fatigue arising from the pump resistant parts of the present invention had to have the operation [56]. appropriate density, >2.1 g/cm3.

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152 EXPANDED PTFE APPLICATIONS HANDBOOK MCAL/SEC 2.00 WT= 19.17mg MAX= 327.15 ENDO> SCAN RATE= 10.00deg/min MAX= 381.75 1.00 PEAK FROM= 268.79 TO= 338.09 ONSET= 319.27 CAL/GRAM= 4.96 PEAK FROM= 345.87 TO= 390.33 ONSET= 385.81 CAL/GRAM= 2.46 0.00 210.00 230.00 250.00 270.00 290.00 310.00 330.00 350.00 370.00 390.00 TEMPERATURE (C) DSC Figure 6.34 Differential scanning calorimetry thermogram of fully sintered densiﬁed expanded polytetraﬂuoro- ethylene [56]. Figure 6.35 An example of Garlock’s one-up pump of 2.190 g/cm3. Plot C represents graphite-ﬁlled diaphragm [57]. PTFE material having a graphite ﬁller content of 19% and a density of 2.164. Plot D represents the Graph of strain (%) versus time (hour), at a pres- creep-resistant densiﬁed ePTFE material with a sure of 10.5 MPa and a temperature of 176C, for density of 2.168 g/cm3. four tested materials, are illustrated as plots A, B, C, and D (Fig. 6.36). Plot A represents skived PTFE Plots AeD were obtained by testing samples having 0% ﬁller material and a density of 2.194 g/ with a diameter around 3.8 cm and a thickness of cm3. Plot B represents glass-ﬁlled PTFE material about 3.2 mm. The testing was accomplished using having a glass ﬁller content of 27% and a density an extensometer test machine. An environmental chamber that completely enclosed the samples and the testing equipment was used for heating. All tests were done under a constant load with thick- ness change (measured in %) as the dependent variable. Fig. 6.36 indicates densiﬁed ePTFE has signiﬁcantly lower creep than all the other three types of PTFE. Densiﬁed ePTFE technology arose because of a problem. ePTFE is porous, not useful as a barrier layer to low surface tension ﬂuids since such ﬂuids with surface tensions below 50 dyn-cm pass through the pores of the membrane. Knox et al. [56] described a method for producing densiﬁed ePTFE parts. These parts had excellent barrier properties rendering them useful in applications like pump diaphragms. Water vapor permeation coefﬁcients in the order of 0.10 g-mm/m2/day were

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 153 % STRAIN 100 A B C 10 D 1 0.1 1 10 100 1000 0.01 Time (hour) Figure 6.36 Plot of % strain versus time (h), at a pressure of 10.5 MPa and a temperature of 176C, illustrating plots of a skived polytetraﬂuoroethylene (PTFE) (A) material, a glass-ﬁlled PTFE material (B), a graphite-ﬁlled PTFE material (C), and the densiﬁed expanded polytetraﬂuoroethylene material (D) [58]. obtained. These materials have been successfully multiplying its water vapor transmission rate by the implemented in a number of applications. A need thickness of the test sample and was reported as g- still exists for materials with further improved mm/m2/day. performance for even more demanding barrier applications. Kennedy and Hollenbaugh [59] made a rough assessment of the relative water vapor permeation Kennedy and Hollenbaugh [59] developed dense coefﬁcient of the densiﬁed versus commercially PTFE sheets or ﬁlms with water vapor permeation available ﬂuoropolymer materials. A series of coefﬁcients as low as 0.003 g-mm/m2/day. Desir- commercially available ﬂuoropolymer ﬁlms was able thickness of the barrier materials is less than evaluated. Four samples of each of the following 0.5 mm and as low as 2 mm. Tensile strengths of the ﬁlms were sent to a third party laboratory for deter- materials of the barrier are in the order of !103 MPa mination of water vapor permeation coefﬁcient. The in one of the MD or CD and !172 MPa in the other barrier material performs even better than Aclar direction. known for their extraordinary resistance to water vapor permeation (Fig. 6.37). Fabrication of the material consisted of three steps. First, one layer or multiple layers of sintered Densiﬁed ePTFE has continued to ﬁnd use in or unsintered ePTFE were densiﬁed using one of many applications where good barrier properties are the techniques described in this section. Second, required. New developments continue to be reported the densiﬁed sheet was preheated at a temperature [60e62]. above the melting point of PTFE. Third, the heated sheet was stretched at !5%/s in MD and CD at a 6.5 Expanded stretch ratio of !4:1. Stretching could be per- Polytetraﬂuoroethylene Sheets formed in multiple steps or simultaneously in a single step. Interactions of the mechanical proper- PTFE was used to manufacture sealing gaskets at ties of densiﬁed ePTFE membranes and the stretch its inception in the Manhattan Project [7]. Extraor- rate and ratio can impact the performance of the dinary chemical resistance, broad operating temper- barrier material. Water vapor permeability of the ature range, and compressibility made PTFE an ideal densiﬁed barrier materials was measured according material as sealing gasket in direct contact with to ASTM Method F-1249. Water vapor permeation uranium hexaﬂuoride. After the end of World War II, coefﬁcient of each sample was calculated by

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154 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 6.37 Comparison of densiﬁed expanded polytetraﬂuoroethylene barrier material with commercial material [59]. PTFE gaskets began to enter into chemical process- resulted in a signiﬁcant increase in the resin cost. ing applications. Considering the high creep of PTFE, even a whop- ping 60e70% decrease in creep was insufﬁcient to In the beginning, gaskets and seals were fabricated allow its use in many applications. from skived PTFE sheets made by molding suspension-polymerized resin. It could not, however, Development of ePTFE gasket material brought be used as universal gasket material. PTFE ﬂows about a signiﬁcant performance change without under load (creeps) resulting in the loss of bolt load at detrimental trade-offs. It offered a gasket made from a fairly high rate; temperature elevation makes mat- 100% PTFE material with dramatic improvements in ters only worse. Another disadvantage is the high creep and cold ﬂow characteristics. The gasket was sealing stress (torque) required by PTFE gaskets. produced by stacking and laminating layers of Broadening the use of PTFE as gasket material ePTFE sheets. Both batch and continuous process required a step change in its thermomechanical may be used to produce laminates of ePTFE. These behavior. continuous, form-in-place, conformable ePTFE gas- kets have their strength due to the expansion of the In an effort to enhance the mechanical properties PTFE in only the longitudinal direction. Thus, the of PTFE, technology was subsequently developed to resistance to creep relaxation is primarily in just the incorporate ﬁllers into PTFE to mitigate the creep longitudinal direction. and torque loss behavior. Filled compounds of PTFE exhibited incremental improvements because PTFE’s Uniaxially expanded porous ePTFE joint seal- nonstick property precluded intimate contact be- ants and gasket tapes provide excellent seals in tween the polymer and ﬁllers. The applications where applications with relatively wide sealing surfaces the improvements were sufﬁcient from the standpoint and sufﬁcient clamping loads. The sealant com- of creep faced new limitations. Chemical resistance presses between the sealing surfaces to form a thin decreased and permeation of gaskets increased and and wide gasket. The level of compressive stress purity was lost because of the incorporation of ﬁllers. must be sufﬁciently high to densify the gasket thus Applications in pharmaceutical, bio-pharmaceutical, provide the desired sealability. These gaskets are and semiconductor manufacturing are among the not always suitable for applications where the industries that require high purity of process surfaces. sealing surfaces are narrow or require thick gaskets. For optimum sealing performance in a gasket, A slight modiﬁcation in the molecular structure of resistance to creep relaxation is desired in both the PTFE using another, per-, ﬂuorinated comonomer longitudinal and transverse directions. Biaxially resulted in some improvements. Modiﬁed PTFE oriented gaskets are made by stacking individual improved creep resistance of standard PTFE by as sheets of ePTFE and subjecting them to heat and much as 60e70%. The new type of PTFE had other pressure in batch or continuous manner. Processes beneﬁts such as weldability, lower permeation rate, to produce continuous long rolls of such gaskets and higher dielectric breakdown strength. On the have been developed [63]. downside the higher cost of the second monomer

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 155 Figure 6.38 Stacking of expanded polytetraﬂuoroethylene (ePTFE) ﬁlm layers by batch process [63]. Fig. 6.38 shows an example of a batch process that required dimensions. A beneﬁt of amorphous locking consists of a pay-off station, an idler drum, a take-up of the ePTFE before gasket fabrication is less fraying station, and a mandrel. The idler roller controlled the of its edges as a result of pin penetration (Fig. 6.38). tension and position of the ePTFE ﬁlm. The mandrel on the take-up roll has pins installed on its two ends. Fig. 6.39 shows an example of a continuous pro- The pins poke through the outer edge of the ePTFE cess for fabricating the ePTFE gasket. The process ﬁlm and restrain it from slippage. ePTFE is coiled consisted of two heated metal drums, a system with when it is fed from the pay-off over the idler drum multiple payoff rolls, and a take-up roll to collect the onto the mandrel. The ePTFE was layered upon itself laminate. The individual sheets of ePTFE were fed as the mandrel is rotated by the take-up. When the between the two drums and nipped under pressure. required number of PTFE layers had been delivered, The heated roll surfaces were nonstick and could the ePTFE ﬁlm is cut and the mandrel is removed reach a temperature in the range of 300e450C. The from the take-up. pressure, temperature, and speed applied by the heated rolls should be such that the layers of ePTFE If the ePTFE ﬁlm had not been amorphously adhere to one another during the nipping by the locked the mandrel containing the layers was heated heated rolls. Temperatures of the rolls should at a temperature !342C and for a period of time be !342C. The ePTFE ﬁlms and their laminate long enough to allow the layers of ePTFE to self- webs must not droop and be taut. adhere to each other. The mandrel and PTFE were cooled and the gasket tape was cut spirally at the While gaskets formed from pure ePTFE perform very well in many applications, they have a number Figure 6.39 Schematic of a continuous process for fabricating the expanded polytetraﬂuoroethylene (ePTFE) gasket [63].

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156 EXPANDED PTFE APPLICATIONS HANDBOOK of deﬁciencies. One problem with this material is that reduces the ability of the users, easily modify these it is extremely ﬂexible. This ﬂexibility makes the gaskets. Unless mounted under high stress, the gasket difﬁcult to handle and/or install in many in- selectively densiﬁed gaskets do not supply a wide stances, especially where sealing surfaces are in sealing area over the entire gasket face. Conventional awkward locations or where the gasket may be prone ePTFE sheet gaskets may be trimmed and modiﬁed to bending or folding during installation. Different to address particular sealing needs. Another advan- approaches have been proposed to solve these prob- tage of these gaskets is that the entire gasket material lems [64]. placed between sealing surfaces serves as a seal [65]. Consequently, a better solution was needed. Some manufacturers have attempted to stiffen the material by attaching the ePTFE to a stiff substrate of In a proposed solution the sealing material con- metal or similar material. While a metal substrate sisted of a composite sheet of ﬂexible conformable improves handling characteristics, it tends to ePTFE layers bonded to at least one embedded layer constrain the gasket use, leaving the metal subject to of rigid ﬂuoropolymer, such as densiﬁed ePTFE attack by harsh chemicals or other environmental material [64]. The composite material is quite rigid factors. Another approach attempted has been to load while retaining the advantages of conventional the ePTFE material with ﬁller that supplies some ePTFE material, such as strength, ease of sealability limited rigidity. Examples of ﬁllers include glass and and customization, and wide effective sealing areas silica microspheres. These ﬁllers tend to diminish the (Fig. 6.41). The rigidity of the sealing material of the overall performance of the gasket materialdlimiting present invention allows the material to be easily chemical or temperature resistance or other qualities. handled and installed without the difﬁculty associ- ated with excessively ﬂexible gasket products. Yet another solution to the problem of insufﬁcient rigidity has been developed by W. L. Gore is its The use of conformable outer layers provides good insertable Gore-Tex gasket product. This gasket sealing properties, permitting the sealing material to consists of a ring gasket constructed entirely from ﬁll gaps and imperfections on or between sealing ePTFE that had a sealing surface of low-density surfaces. The embedded rigid material assures that ePTFE and a densiﬁed area next to it that supplied the sealing material will retain its position during rigidity to the gasket. Examples of the structure of handling, cutting, and mounting without the prob- two O-ring gaskets with variable density ePTFE are lems a “ﬂoppy” gasket material may encounter. The shown in Fig. 6.40. Densities of the two sections of material of the proposed structure is also consistent the gasket were measured. The densities of the con- across its entire sealing surface, allowing the material ventional and densiﬁed ePTFE areas were 0.5e0.7 to be cut or modiﬁed into a wide variety of shapes and and 1.2e1.8 g/cm3. assuring maximum effective sealing area between sealing surfaces. The structure that provides the improved handling characteristics of the gasket also restricts its use in Bowen et al. [67] improved the multilayer other sealing applications. Selective densiﬁcation compressible ePTFE gaskets developed by Dolan Figure 6.40 Examples of mixed expanded polytetraﬂuoroethylene (ePTFE) and densiﬁed ePTFE gasket [65].

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 157 Figure 6.41 Structure of multilayer compressible expanded polytetraﬂuoroethylene (ePTFE) gasket and typical thickness of each layer [64,66]. PTFE, polytetraﬂuoroethylene. et al. [64,66,68]. The improved ePTFE gasket could this full density sheet were wrapped around a 584- form a seal with greater bolt load retention than was mm diameter mandrel. One hundred layers of a possible with the previous ePTFE gaskets. The second biaxially expanded ePTFE sheet (1600 mm ePTFE membranes were prepared with an MTS wide) were wrapped over the full density ﬁlm. The of !309 MPa in one or both directions. And the MTS thickness of 100 layers was 0.038 mm. Next ﬁve ratio in MD and TD was between 0.25 and 4, more layers of the full density ePTFE sheet were orientation index was 20 degrees or less, and a again wrapped onto the mandrel covering the density of 2.0 g/cm3. The improved gaskets undensiﬁed ePTFE sheet. The ends of the sheets exhibited improved mechanical properties such as on the mandrel were secured to prevent the high bolt load retention, low creep, high tensile shrinking at elevated temperatures. All the layers strength, low stress to seal, and crystallinity were then sintered while secured to the mandrel in index !70% (determined by wide angle X-ray an oven at 370C for 45 min to bond all the layers scattering). together [69]. “Low stress to seal” meant a gasket that provides After cooling, the PTFE material was longitudi- an air impermeable, seal after application of a rela- nally cut from the mandrel in the form of a sheet. An tively low stress. That is a stress below the amount annular ring shape having an ID of 89 mm and OD of required to fully densify a porous ePTFE gasket, less 135 mm was cut from the sheet. It was selectively than about 20.7 MPa and as low as 2.1 MPa [69]. compressed to form the air impermeable area be- These gaskets are especially useful in applications tween the full density ePTFE layers. For example, an such as of glass-lined steel ﬂanges. In spite of a air impermeable segment was formed in gasket of relatively smooth surface ﬁnish, these ﬂanges are this example by compressing the gasket between often uneven and not entirely ﬂat. The unevenness annular dies with an ID of 104.8 mm and OD of requires the gasket to conform to fairly sizable vari- 108.0 mm. The dies were heated to 200C, and ations around the perimeter as well as between the applied a pressure around 51.7 MPa for 15 s. The air internal and external diameter of the ﬂange to create impermeable section had a thickness of 0.025 mm an effective seal. Gaskets have developed that in and width of 1.6 mm (Fig. 6.42). Overall gasket addition to sealing at a comparably low stress, they thickness was 3.0 mm. also contain a section that is air impermeable. The gasket thus does not allow permeation of ﬂuids Development of special performance gaskets uti- through its porous structure. lizing ePTFE in their structure has continued over the years. Hisano and Urakami [71], for example, have The air impermeable gaskets are extensively reported gaskets with even higher creep resistance engineered, an example of which is given in and acceptable interlayer adhesion than previously Fig. 6.42. A biaxially expanded ePTFE sheet with a available. Fibrils correspond to PTFE molecules with thickness of 0.015 mm was rolled in calendar to high degree of crystallization in contrast to nodes that densify it fully. This sheet had a thickness of about are in amorphous state. These nodes are easily plas- 0.005 mm and width of 1270 mm. Five layers of tically deformed by compression stress, which causes

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158 EXPANDED PTFE APPLICATIONS HANDBOOK First (A) section Air impermeable Second region section (B) Air impermeable region First Second section section Figure 6.42 Example of a low sealing stress gasket with an air impermeable segment [67e70]. creep. In Japanese Patent Laid-Open Publication No. obtain an ePTFE ﬁlm with a thickness of 0.041 mm 11-80705 [72], creep resistance was improved by and a density of 0.36 g/cm3. increasing the expansion ratios to reduce the size of the nodes as much as possible. The ePTFE ﬁlm prepared was wound 125 times around a stainless steel hollow mandrel with a Hisano and Urakami, however, believed nodes diameter of 1000 mm and a length of 1550 mm. This play an important role in adhesion between ePTFE cylindrical layered product was placed in an oven, ﬁlms. When nodes are in amorphous state, they and sintered at a temperature of 365C for 60 min. are softened (deformed) at lower temperatures After the baking, the cylindrical layered product was compared to ﬁbrils. When ePTFE ﬁlms are laminated taken out from the oven, and cooled to room tem- by thermal compression bonding, bonding can take perature. The laminated product was cut open in the place between the nodes under the inﬂuence of heat. axial direction, and the edge portions on four sides The adhesion between ePTFE ﬁlms is decreased were removed by cutting, thereby obtaining two when node sizes were reduced to improve creep ePTFE ﬁlm-laminated sheets each having a size of resistance. A new approach was proposed to improve 1524 mm Â 1524 mm, a thickness of 2.8 mm, a both creep resistance and interlayer adhesion be- density of 0.67 g/cm3, and a mass per unit area of tween the ePTFE plies illustrated by an example. 0.19 g/cm2 [71]. An extruded ﬁlm was calendared to obtain a The sheet had a speciﬁc surface area of 9.0 m2/g or tape with a thickness of 0.6 mm. This tape was higher and a density of from 0.4 to <0.75 g/cm3, expanded in the transverse direction at a tempera- wherein the laminated sheet has a 180-degree peeling ture of 80C using an expansion ratio 6 at a rate of strength of 0.20 N/mm or higher between the ePTFE 180%/s. After drying, the tape was expanded in the layers of the laminated sheet. The gasket had a stress MD at a temperature of 300C at a rate of 400%/s relaxation rate of 45% or lower after 1 h when and an expansion ratio of 5. The tape was next compressed at a surface pressure of 50 MPa. The expanded in the transverse direction at a tempera- sheet had an MTS of !120 MPa or higher in at least ture of 330C at an expansion speed of 200%/s and one direction among in-plane directions of the sheet the expansion ratio of 16 times. After the expan- and has a ratio of matrix strengths from 0.5 to 2 in the sion, the tape was sintered by heating at 360C to MD and TD of the sheet.

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 159 Figure 6.43 An example of an expanded polytetraﬂuoroethylene joint sealant gasket and its applications to a ﬂange. 6.6 Expanded Early patents reporting on PTFE expansion have Polytetraﬂuoroethylene Tapes and described methods of manufacturing rods and joint Rods seals [4,73,74]. ePTFE joint sealant is a conformable product that A cylindrical rod, with a diameter of 4 mm, was can provide a tight seal under extreme conditions extruded by PTFE paste extrusion. The lubricant was (Fig. 6.43). ePTFE is softer and far more ﬂexible than removed by drying. The dry extruded rod had a regular PTFE. A topical adhesive on a carrier paper is speciﬁc gravity of 1.63, a tensile strength of 3.7 MPa, usually applied to ePTFE joint sealant to facilitate its and an elongation of 183%. A device was fabricated application on the sealing surface. The paper is to allow samples of the rod to be stretched at various peeled at the time of applying the joint sealant to a rates and temperatures. It consisted of two clamps for ﬂange (Fig. 6.44). Overlapping the joint sealant at a holding the rod, one clamp being held ﬁxed within an point completes the seal. Joint sealant works well oven while the other clamp was attached to a wire with complex ﬂanges and equipment. leading outside the oven to a rack-and-pinion pulling device driven by a variable speed motor. Most joint seals have ﬂat rod shapes that can be produced by paste extrusion of PTFE ﬁne powder. After the sample had been expanded by stretching Manufacturing of ePTFE joint seal is relatively at the given controlled temperature, the oven tem- simple compared to other shapes. Expansion takes perature was raised to 370C for 10 min while the place only in one direction and that is longitudinal. samples were held clamped in their extended condi- tion. Samples were stretched 1500% successfully at a rate of 1000e40,000%/s at 315C. Porosity of expanded rods ranged from 68% to 96% [4,73,74]. Figure 6.44 An example of expanded polytetra- References ﬂuoroethylene joint sealant with adhesive on carrier paper. [1] Medical Dictionary, US National Library of Courtesy: Adtech Polymer Engineering, www.adtech.co.uk. Medicine, NIH, November 2015. https://www. nlm.nih.gov/medlineplus. [2] W.L. Gore, U.S. Patent 3,664,915, Assigned to W. L. Gore Associates, May 23, 1972. [3] R.W. Gore, U.S. Patent 3,953,566, Assigned to W. L. Gore Associates, May 21, 1970. [4] R.W. Gore, U.S. Patent 4,187,390, Assigned to W. L. Gore Associates, February 5, 1980. [5] K. Okita, U.S. Patent 4,277,429, Assigned to Sumitomo Electric Ind., July 7, 1981. [6] J.B. Bowman, D.E. Hubis, J.D. Lewis, S.C. Newman, R.A. Staley, U.S. Patent 4,482,516, Assigned to W. L. Gore Associates, November 13, 1984.

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160 EXPANDED PTFE APPLICATIONS HANDBOOK [7] S. Ebnesajjad, Fluoroplastics, in: Non-melt [23] T.J. Edwin, S. Randall, U.S. Patent 6,203,735, Processible Fluoropolymers, second ed., vol. 1, Assigned to Impra, Inc., March 20, 2001. Plastics Design Library, Elsevier, 2014. [24] H. Scholz, U. KrUˆ ger, U. Settmacher, U.S. Pat- [8] Harada, H. Mano, U.S. Patent 5,234,751, ent Application 20060030935, Assigned to Bard Assigned to Sumitomo Electric Ind., August 10, Peripheral Vascular, Inc., February 9, 2006. 1993. [25] D.L. Bogert, J. Abbott, U.S. Patent 8,066,758, [9] T.J. Edwin, S. Randall, EP 1741544, Assigned to Assigned to C.R. Bard, Inc., November 11, 2011. Bard Peripheral Vascular, October 1, 2007. [26] D.L. Bogert, J. Abbott, U.S. Patent 8,652,284, [10] M. Wolf, J. Breil, R. Lund, Develop of New Assigned to C.R. Bard, Inc., February 18, 2014. BOPP Barrier Films by Coextrusion and Simultaneous Biaxial Orientation, Brueckner [27] R. Bregulla, G. Stockert, U.S. Patent Application Maschinenbau GmbH, Siegsdorf, Germany, 20120172977, Assigned to Abbott Cardiovas- October 2015. www.brueckner-maschinenbau. cular Systems, July 5, 2012. com. [28] M. Donckers, Patent W.O. 2008157307, [11] R.W. Gore, U.S. Patent 3,953,566, Assigned to Assigned to Toray Fluoroﬁbers America, W. L. Gore, April 1976. December 24, 2008. [12] K. Okita, U.S. Patent 4,177,334, Assigned to [29] N.L. Blankenbeckler, M. Donckers, et al., U.S. Sumitomo Electric Ind., December 1979. Patent 5,820,984, Assigned to DuPont Co., October 13, 1998. [13] M.L. Campbell, B.G. Williams, R.G. Rifﬂe, C.E. Biggerstaff, U.S. Patent 4,876,051, Assigned to [30] A.R. Nelson, C.D. Moon, P. WO2013159020, W. L. Gore, October 24, 1989. Assigned to Toray Fluoroﬁbers America, October 24, 2013. [14] K. Okita, U.S. Patent 4,208,745, Assigned to Sumitomo Electric Industries, June 24, 1980. [31] N. Clough, Introducing a New Family of Gore PTFE Fibers, White Paper, W. L. Gore & Asso- [15] R. Calcote, R.R. Kowligi, S. Wollner, USP ciates, 2007. 5,453,235, Assigned to Impra Inc., September 26, 1995. [32] S. Katayama, E.P. Application 0 352749, Assigned to Asahi Kasei Kogyo Kabushiki, [16] R. Calcote, R.R. Kowligi, S. Wollner, USP January 31, 1990. 5,641,443, Assigned to Impra Inc., June 24, 1997. [33] S. Katayama, U.S. Patent 5,061,561, Assigned to Asahi Kasei Kogyo Kabushiki, October 29, [17] R. Calcote, R.R. Kowligi, S. Wollner, USP 1991. 5,935,667, Assigned to Impra Inc., August 10, 1999. [34] M. Shimizu, U.S. Patent 5,686,033, Assigned to Hitachi Cable Ltd., November 11, 1997. [18] R.J. Zdrahala, N. Popadiuk, D.J. Lentz, E.J. Dormier, U.S. Patent 5,874,032, Assigned to [35] T.P. Kelmartin Jr., et al., U.S. Patent 6,071,452, Meadox Medicals, February 23, 1999. Assigned to W. L. Gore, September 12, 2000. [19] R.J. Zdrahala, N. Popadiuk, D.J. Lentz, E.J. [36] H.W. Starkweather, et al., J. Polym. Sci. Polym. Dormier, U.S. Patent 6,530,765, Assigned to Phys. Ed. 20 (1982) 751e761. Meadox Medicals, March 11, 2003. [37] Guide to Winding and Slitting, Ashe Converting [20] R.S. Taylor, et al., Improved technique for PTFE Equipment, November 2015. www.ashe.co.uk. bypass grafting: long-term results using anasto- motic vein patches, Br. J. Surg. 79 (April 1992) [38] K. Yoshida, U.S. Patent 7,892,468, Assigned to 348e354. Japan Gore-Tex, Inc., February 22, 2011. [21] S. Raptis, J.H. Miller, Inﬂuence of a vein cuff on [39] B.F. Abrams, R.B. Minor, G.L. McGregor, J.W. polytetraﬂuoroethylene grafts for primary fem- Dolan, U.S. Patent 5,571,605, Assigned to W. L. oropopliteal bypass, Br. J. Surg. Soc. 82 (4) Gore & Associates, November 5, 1996. (April 1995) 487e491. [40] B.F. Abrams, R.B. Minor, G.L. McGregor, J.W. [22] U. KrUˆ ger, H. Scholz, U. Settmacher, E.P. Dolan, U.S. Patent 5,591,526, Assigned to W. L. EP1011521, Assigned to Bard Peripheral Gore & Associates, January 7, 1997. Vascular, Inc., January 26, 2005. [41] B.F. Abrams, R.B. Minor, G.L. McGregor, J.W. Dolan, U.S. Patent 5,635,124, Assigned to W. L. Gore & Associates, June 3, 1997.

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6: MANUFACTURING OF VARIOUS SHAPES OF EXPANDED POLYTETRAFLUOROETHYLENE (ePTFE) 161 [42] T.J. Kelmartin Jr., G.M. Roberts, J.D. Dolan, [59] M.E. Kennedy, D.L. Hollenbaugh Jr., U.S. Patent R.B. Minor, U.S. Patent 5,989,709, Assigned to 7,521,010, Gore Enterprise Holdings, April 21, Gore Enterprise Holding, November 23, 1999. 2009. [43] T.J. Kelmartin Jr., G.M. Roberts, J.D. Dolan, [60] E.G. Ashmead, E.C. Gunzel, M.P. Moritz, U.S. R.B. Minor, U.S. Patent 6,071,452, Assigned to Patent 8,722,178, Assigned to W. L. Gore & Gore Enterprise Holding, June 6, 2000. Associates, May 13, 2014. [44] J.W. Dolan, R.B. Minor, J.W. Spencer Jr., U.S. [61] E.G. Ashmead, E.C. Gunzel, M.P. Moritz, U.S. Patent 5,518,012, Assigned to W. L. Gore, May Patent Application US 2012/0251748, Assigned 21, 1996. to W. L. Gore & Associates, October 4, 2012. [45] T.J. Kelmartin Jr., G.M. Roberts, J.D. Dolan, [62] E.G. Ashmead, E.C. Gunzel, M.P. Moritz, W.O. R.B. Minor, U.S. Patent 6,117,547, Assigned to 2011059823, Assigned to W. L. Gore & Asso- Gore Enterprise Holding, September 12, 2000. ciates, May 19, 2011. [46] T. Gries, D. Veit, B. Wulfhorst, Textile Tech- [63] D. Mills, R. Nelson, W. Nibler, H. Gutsmiedh, nology, Carl Hanser Verlag, 2015. U.S. Patent 5,964,465, Assigned to W. L. Gore and Associates, October 12, 1999. [47] V.B. Gupta, V.K. Kothari, Manufactured Fiber Technology, Springer, 1997. [64] J.W. Dolan, D.J. Mills, U.S. Patent 5,879,789, Assigned to W. L. Gore and Associates, March 9, [48] J.A. Neto, U.S. Patent Application 2003033784 1999. A1, Assigned to Almeida Neto Jose Antonio, March 10, 2005. [65] R.A. Snyder, E. Patent EP0416031 B1, Assigned to W. L. Gore and Associates, March 29, 1995. [49] J.A. Neto, U.S. Patent Application 2003074770 A1, Assigned to Almeida Neto Jose Antonio, [66] J.W. Dolan, D.J. Mills, U.S. Patent 6,030,694, March 10, 2005. Assigned to W. L. Gore and Associates, February 29, 2000. [50] J.A. Neto, U.S. Patent Application 20050053783 A1, Assigned to Almeida Neto Jose Antonio, [67] C. Bowen, K.E. Dove, C. Jones, R.B. Minor, U.S. March 10, 2005. Patent 7,829,170, Assigned to W. L. Gore and Associates, November 9, 2010. [51] N.R. Clough, D.I. Lutz, G. Harp, U.S. Patent 7,296,394, Assigned to Gore Enterprise Hold- [68] C. Bowen, K.E. Dove, C. Jones, R.B. Minor, U.S. ings, Inc., November 20, 2007. Patent 8,231,957, Assigned to W. L. Gore and Associates, July 31, 2012. [52] N.R. Clough, R. Sassa, U.S. Patent 7,409,815, Assigned to Gore Enterprise Holdings, Inc., [69] R.B. Minor, K.E. Dove, R.G. Egres Jr., A. Riedl, August 12, 2008. H. Hisano, D.J. Mills, U.S. 6,485,809, Assigned to W. L. Gore and Associates, November 26, [53] R. Cook, J. Thelen, J. Meyer, U.S. Patent 2002. 8,181,438, Assigned to Pure Fishing, Inc., May 22, 2012. [70] C. Bowen, K.E. Dove, C. Jones, R.B. Minor, U.S. Patent 7,829,171, Assigned to W. L. Gore and [54] P.B. Cooper, S.J. Lane, U.S. Patent 4,732,629, Associates, November 9, 2010. Assigned to W. L. Gore and Associates, March 22, 1988. [71] H. Hisano, S. Urakami, U.S. Patent 8,784,983, Assigned to W. L. Gore and Associates, July 22, [55] W.P. Mortimer, U.S. Patent 4,985,296, Assigned 2014. to W. L. Gore and Associates, January 15, 1991. [72] K. Hiroichi, I. Junji, M. Katsuhiko, S. Toru, S. [56] J.B. Knox, W.E. Delaney, J.M. Connelly Jr., U.S. Fumihiro, Y. Masanobu, Japanese Patent Laid- Patent 5,374,473, Assigned to W. L. Gore and Open Publication No. 11e80705, March 1999. Associates, December 20, 1994. [73] R.W. Gore, U.S. Patent 3,953,566, Assigned to [57] Gorlock Corp., www.garlockdiaphragmshop. W. L. Gore Associates, April 27, 1976. com, November 2015. [74] R.W. Gore, U.S. Patent 3,962,153, Assigned to [58] J.P.L. Fuhr, M.M. Gentile, R.K. Hutter, M.E. W. L. Gore Associates, June 8, 1976. Kennedy, U.S. Patent 5,792,525, Assigned to W. L. Gore and Associates, August 11, 1998.

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7 Properties, Characteristics, and Applications of Expanded PTFE (ePTFE) Products OUTLINE 7.1 Introduction 163 7.3.4 Medical and Biological Uses 168 7.2 Properties and Characteristics 163 7.3.5 Cables and Cable Assemblies 168 7.3 Applications 7.3.6 Electronic and Electrochemical Materials 168 7.3.1 Industrial and Process Filtration 7.3.2 Microﬁltration Applications 166 7.3.7 Sealants 169 7.3.3 Vent Filters and Breathers 167 7.3.8 Fibers and Fabrics 169 168 References 169 168 7.1 Introduction Figure 7.1 Scanning electron micrograph of an expanded polytetraﬂuoroethylene membrane. Polytetraﬂuoroethylene (PTFE) is the base ther- Courtesy: Outdoor Sports, www.outdoorsports.com/ moplastic material for manufacturing expanded pol- outdoor-apparel.aspx?page¼4, 2015. ytetraﬂuoroethylene (ePTFE) membranes and other products. PTFE’s remarkable extreme properties those applications in further detail. Table 7.1 lists the include resistance to nearly all commercial chemicals key attributes of ePTFE. and steady mechanical endurance in the temperature range of À260 to 260C. The major drawback to this 7.2 Properties and Characteristics plastic is its relatively low strength as compared to other engineering plastics. Consequently, it has a A key characteristic of ePTFE is that it allows tendency to ﬂow under modest tensile or compressive the passage of water vapor and other gases but does loads, and this tendency is aggravated as the tem- not permit most liquids, including liquid water, to perature increases. The traditional remedy has been penetrate through the membrane. In gas ﬁltration to incorporate ﬁllers into PTFE. But these ﬁllers alter and venting applications that use materials other a number of PTFE’s properties, thereby limiting its than ePTFE, liquid eventually ﬁlls the porous applications and usefulness [4]. ePTFE (Fig. 7.1) is a porous membrane that shares many of PTFE’s unique properties and has also overcome many of PTFE’s shortcomings, including its lack of strength under pressure. As a result, ePTFE has made possible literally thousands of new appli- cations. These applications impact nearly every area of human life, from life-saving endovascular grafts to industrial ﬁlters to comfortable apparel and footwear. This chapter explores the important applications of ePTFE and the rest of the book describes some of Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00007-9 163 Copyright © 2017 Elsevier Inc. All rights reserved.

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164 EXPANDED PTFE APPLICATIONS HANDBOOK Table 7.1 Key Attributes of Expanded Polytetraﬂuoroethylene Porosity (low and high) High speciﬁc surface area High chemical resistance in harsh environments Chemical inertness of polytetraﬂuoroethylene surface High thermal stability Resistance to ultraviolet rays Excellent outdoor weatherability Low coefﬁcient of friction Low water adsorption (0.04% at room temperature) Low ﬂammability (limiting oxygen index is 95%) High performance at extreme low and high temperatures Strength (high strength-to-weight ratio) Low dielectric constant (2.0) Low loss coefﬁcient Biocompatibility media, followed by stoppage of the gas ﬂow. This Figure 7.2 Typical porosity/pore-size relationships phenomenon is called “wetting out.” Non-ePTFE for expanded polytetraﬂuoroethylene ﬁlm [2]. membranes usually permit initial airﬂow but clog quickly, thus preventing consistent airﬂow. In water vapor. This pore size, therefore, is what makes contrast, ePTFE membranes ensure a steady ﬂow of ePTFE membranes functionally waterproof while at gases. the same time they allow perspiration vapors to escape from inside a piece of clothing [5]. An ePTFE web (membrane), tube, or rod can be described as a porous structure with signiﬁcantly Due to its porous structure, heat-treated ePTFE lower density than a similar unexpanded PTFE web has a higher permeability to gases and liquids structure. Paste extrudates have a density of 1.5 g/ than normally sintered PTFE. It can also act as a cm3 before sintering. A solid sintered PTFE part has semipermeable membrane by allowing wetting liq- a density of about 2.15 g/cm3. The density of an uids through while being impermeable to nonwetting expanded part can be as low as <0.1 g/cm3, with a ﬂuids. For example, a gas-saturated membrane in porosity of 96%. Fig. 7.2 shows the relationship of contact with a mixture of gas and water will allow the porosity to pore size and density. Density and gas through while holding back the water. The porosity have a linear relationship. Small pore size, membrane will continue to hold the water as long as less than 1 mm, results in up to 90% porosity; larger the water pressure does not exceed the water entry size pores (1e6 mm) contribute to driving the porosity to !95%. ePTFE membranes are usually quite thin (down to <1 mm). One square centimeter of an ePTFE membrane has over 1.4 billion pores. While those pores are actually 20,000 times smaller than a water droplet, they are 700 times larger than a molecule of

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7: PROPERTIES, CHARACTERISTICS, AND APPLICATIONS OF EXPANDED PTFE (EPTFE) PRODUCTS 165 Figure 7.3 Typical air-permeability/water-entry pressure relationships [2]. (A) Relationship of porosity (%) to density (g/cm2). (B) Relationship of porosity (%) to largest pore size (m). pressure. Fig. 7.3 indicates that, at above 90% web for this increase we have to consider two parame- porosity, air permeability increases drastically while ters: Poisson’s ratio (vxy) and Young’s modulus the water entry pressure of the web decreases to a of elasticity (Ex). Poisson’s ratio (Eq. (7.1)) is fairly small value. A comparison of Fig. 7.3A, B deﬁned as the ratio of the lateral strain (ex) to the reveals that there is a range in which porosity can be longitudinal extension caused by uniaxial tensile selected to balance air permeability and water extension. impermeability, which is particularly useful in ap- plications such as clothing. In Fig. 7.4A, assuming extension occurs in the x direction, Poisson’s ratio is deﬁned as the ratio of Table 7.2 presents a comparison of the properties dimension change in the y direction to the extension of expanded and full-density (unexpanded) PTFE. in the x direction. Most materials would experience The crystallinity of the amorphously locked PTFE is necking, or a reduction in the y direction, thus pre- about 95%, which is signiﬁcantly above the highest serving their volume. For example, Poisson’s ratio commercially attainable value with unexpanded parts. of rubber is in the range of 0.5e1 and for engi- The most striking advantage of ePTFE is its tensile neering polymers it is vxy > 1. An increase in the y strength, which is orders of magnitude above that of direction is rare and would be indicated by a nega- the full density material and has therefore opened new tive sign [3]. Spectacular changes of n also occur in applications for PTFE. The tensile strength of ePTFE anisotropic auxetic materials outside the isotropic is calculated for the matrix by multiplying the range of À1 n 1/2 in response to small strains. measured value of tensile strength by the ratio of the For instance, n was found to decrease from 0 to À14 densities of the full-density PTFE to ePTFE. The ﬂex for an anisotropic ePTFE in a true strain range of life and maximum service temperature of ePTFE are 0.03 [1]. both higher than those of the full-density material. Young’s modulus of elasticity is the relationship How does expansion build strength in the between stress and strain (Eq. (7.2)) in a uniaxial structure of PTFE? To understand the mechanism extension.

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166 EXPANDED PTFE APPLICATIONS HANDBOOK Table 7.2 Comparison of the Typical Properties of Expanded Versus Full-Density Polytetraﬂuoroethylene [2] Property Units Full-Density Expanded Speciﬁc gravity G/cm3 2.1 0.1e1.0 Crystallinity 95 Porosity % 50e70 25e96 Pore size range % <1.0 0.02e15 Matrix tensile strength m N/A 50e800 Flexural fatigue MPa 20e30 3 Â 107 resistance Cycles to failure 1 Â 106 Service temperature 280 (maximum) C 260 Thermal conductivity <0.1 Thermal expansion kCal/mhC 0.2 1 Â 10À4 coefﬁcient Per C 3 Â 10À4 Resistance to cold ﬂow Excellent Abrasion resistance As creep Poor Excellent Chemical resistance e Moderate Excellent e Excellent Figure 7.4 Schematic diagram of structural changes strain of 2000dhas been reached. Evans [3] has observed in microporous polytetraﬂuoroethylene un- proposed a model to explain this phenomenon. dergoing tensile loading in the x direction [2]. Fig. 7.4A shows the structure of unexpanded paste- extruded PTFE in which the nodes lie ﬂat and are vxy ¼ εy (7.1) connected by ﬁbrils. As the stretching begins, the εx nodes start to move. At some point as they are pulled they tilt up, thus increasing the width, or bulk, of Ex ¼ sx (7.2) the material Fig. 7.4C. The tilting movement leads to εx the perpendicular orientation of the long axis of the nodes to the direction of draw Fig. 7.4D. ePTFE has a negative Poisson’s ratio (Fig. 7.5) until signiﬁcant extension or straindthat is, about the Zone I of Fig. 7.5 shows a linear Young’s modulus, indicating an elastic behavior by the expanded material in which the ﬁbrils possibly act as elastic bands, as proposed by Evans [3], storing energy. At the end of Zone I, Poisson’s ratio reaches zero, and it ﬁnally levels at 1e2 in Zone III. The material be- haves as a plastic in Zone III, where the original anisotropic structure breaks up. In the expanded structure, the tensile load is borne by highly oriented ﬁbrils with a virtually 100% crystalline structure, which explains their much higher tensile strength. 7.3 Applications ePTFE products have penetrated all major indus- trial sectors, and this section explains some examples of these products. The availability of ePTFE in

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7: PROPERTIES, CHARACTERISTICS, AND APPLICATIONS OF EXPANDED PTFE (EPTFE) PRODUCTS 167 4I II III 1.6 × 10–1 Table 7.4 Key Industries in Which Expanded 2 1.4 Polytetraﬂuoroethylene Products Play an Important Role 0 1.2 Aerospace –2 1.0 Vxy Automotive Ex (GPa) –4 0.8 Chemical processing Ex –6 Vxy Computers, telecommunications, and electronics –8 0.6 Energy 0.4 Environment –10 0.2 Industrial and manufacturing –12 0 0 5 10 15 20 25 30 35 × 10–2 Medical ε xv Military Figure 7.5 Poisson’s ratio (vxy) and Young’s modulus of elasticity (Ex) plotted against engineering Pharmaceutical and biotechnology strain (εx) and showing the three regions of behavior: I, II, III [2]. Semiconductor and microelectronics multiple forms, including tape, membranes/ﬁlms, Textiles tubes, ﬁbers, sheets, and rods, has been critical to the growth of its use. Typically one component in a de- Table 7.4 gives examples of a number of important vice or system, usually a signiﬁcant or even critical industries in which ePTFE products play an impor- one, is made of ePTFE. ePTFE is frequently used in tant role [6]. the fabrication of high-performance fabrics, medical implants, ﬁbers, gaskets and sealants, membranes, Speciﬁc applications of ePTFE products depend and dielectric materials for electronics. Table 7.3 lists on one or more of the properties of ePTFE. Because some examples of parts containing ePTFE as a key ePTFE is so versatile, a large number of performance component. characteristics and properties can be engineered for speciﬁc products and functions. Those characteristics Table 7.3 Examples of Components and Parts Made are listed in Table 7.5. From Expanded Polytetraﬂuoroethylene (ePTFE) [6] The following sections brieﬂy describe examples ePTFE Components Containing ePTFE of applications and end uses of ePTFE in only a few Form key industries. Some of the material in these sections Tape Electronic wire and cable is based on the website publications of WL Gore and Membrane/ Associates (www.Gore.com). ﬁlms Laminates, ﬁltration laminates, stent grafts, vents, fuel cell membranes, 7.3.1 Industrial and Process Sheets Filtration electrode assemblies, dielectric Fibers materials, battery/capacitor ePTFE membranes are important components in Tubes separators the construction of industrial, automotive, and pro- cess ﬁltration systems. Emissions of polluting dust Electromagnetic interference and fumes into the environment and workplace are gaskets, sealing gaskets, medical increasingly subject to more stringent legislative control and scrutiny. These legal mandates have patches driven the demand for more efﬁcient, effective, and economical ﬁlter media. In cement manufacturing, Weaving/sewing threads, dental for example, the key challenges include reduction in ﬂoss, packaging, ﬁltration felts energy consumption and carbon dioxide. In the production of ﬁne chemicals, emissions during the Peristaltic pump tubes, vascular manufacturing process must approach zero, primar- grafts, environmental screening ily because of the high cost of these products. Another sector that strives for near-zero emissions is modules

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168 EXPANDED PTFE APPLICATIONS HANDBOOK Table 7.5 Performance Characteristics of Expanded microﬁlters such as chemical inertness and cleanli- Polytetraﬂuoroethylene Products That can be ness. In addition, they do not shed particles and yield Engineered minimal extractables. Electrical conductivity 7.3.3 Vent Filters and Breathers Thermal conductivity Catalytic activity ePTFE membranes repel water and can be used as Chemical activity permeable water barriers for venting in gas sensors, Absorption acoustical applications, pressure venting, industrial Magnetic activity battery venting, electronics, and other areas. Gas- Antibacterial functionality permeable venting and oil-repelling membranes can Odor absorbency be made using ePTFE for applications that require oil Radiation resistance resistance but also call for gases to be able to vent Reﬂectivity through the membrane. Selective permeability Transparency/opacity 7.3.4 Medical and Biological Uses Dimensional stability High dielectric constant ePTFE membrane ﬁlms are hydrophobic and Low dielectric constant oleophobic, making them a good ﬁt for medical Varied hardness and stiffness applications. When formed into membranes, ﬁlms, Low/high surface energy laminates, and tubular shapes, ePTFE can be used to Low/high abrasion resistance manufacture a wide variety of medical accessories Nonpermeable barrier properties including wound-care materials, face masks, trans- Controlled ﬂuid delivery ducer protectors, ostomy bags, urine bags, drainage bags, medical device enclosures, vent caps, IV waste disposal by incineration, as legislative con- administration sets, spike vents, surgical smoke ﬁl- straints have continually pushed plants to improve ters, and suction ﬁlters. their performance standards. ePTFE also has a nanoﬁbrillar structure that is Other important industries in which ePTFE-based suitable for use as a cell culture surface. ePTFE ﬁlters are used include food/confectionary, plastic surfaces provide a biomimetic environment for more processing, metallurgical plants, nonmetallic min- consistent and reproducible in vivo-like cell erals mining and recovery, and pharmaceutical phenotypes. manufacturing. The technological requirements of various industries differ widely based on materials, 7.3.5 Cables and Cable processes, and the performance and quality expec- Assemblies tations of each sector. PTFE has electrical properties that render it 7.3.2 Microﬁltration Applications suitable, especially in its expanded form, for elec- tronic and electrochemical applications. Its volume ePTFE membrane ﬁlms for microﬁltration appli- resistivity is 1018 U cm, its dielectric constant 2.0, cations have high ﬂow rates and outstanding reten- and its loss factor <0.0004 (at <108 Hz). In addition, tion characteristics. They resist bacterial challenge its dielectric breakdown increases with decreasing and toxicity deﬁned by USP 23, Class VI (www.USP. thickness. These electrical properties of PTFE are org). ePTFE ﬁlter membranes possess important crucial for cable applications that must meet rigorous characteristics that are necessary for effective electrical, mechanical, and environmental re- quirements. Examples of these parts are listed in Table 7.6. 7.3.6 Electronic and Electrochemical Materials PTFE’s electrical properties also render it suitable, especially in expanded form, for electronic and

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7: PROPERTIES, CHARACTERISTICS, AND APPLICATIONS OF EXPANDED PTFE (EPTFE) PRODUCTS 169 Table 7.6 Examples of Cables Made Using Table 7.8 Examples of Expanded Expanded Polytetraﬂuoroethylene Products Polytetraﬂuoroethylene Use in Sealing and Gasket Applications Coaxial cable Fiber-optic cable Gaskets Flat cable Hook-up wire Packing ﬁber Round cable High data rate copper cabling Pump diaphragms Microwave/RF cable assemblies Pump tubes Table 7.7 Some Examples of Expanded Polytetraﬂuoroethylene Use in Electronic/ Aircraft sealant Electrochemical Materials Fibers may be knitted, woven, braided, or sewn into Dielectric materials myriad applications. Examples include: Electromagnetic interference shielding Fuel cell components Compression packing Batteries and capacitors Specialty electronic products ePTFE ﬁbers electrochemical applications. See Table 7.7 for ex- Sewing thread and weaving ﬁbers amples of applications that take advantage of PTFE and other polymeric dielectric materials to meet a Rope ﬁber broad array of high-performance signal transmission requirements. ePTFE membranes have been used extensively in a variety of fabrics for apparel as well as in in- 7.3.7 Sealants dustrial and architectural applications. Fabrics for civilian, sporting/hunting, military, and safety gar- In both its expanded and unexpanded forms, PTFE ments have been developed that include ePTFE has a vast number of applications in the production of membranes as a functional component. High- gaskets, sealants, packing, and other components. Its performance outerwear, footwear, gloves, and ac- extreme ﬂex fatigue makes it an ideal material for cessories depend on the properties of ePTFE diaphragms and other applications in which the parts membranes. Industrial fabrics made with ePTFE are required to move regularly. The fact that PTFE have a large number of applications as media in and ePTFE can be combined with other materials liquid and gas ﬁltration, electrical insulation, and also contributes to its usefulness in these applica- vent windows. tions. A few examples of its uses in the transport industry and in sealing industrial ﬂuids are summa- Numerous industries, including geochemical im- rized in Table 7.8. aging services in environmental and exploration ap- plications, petroleum exploration, and minerals 7.3.8 Fibers and Fabrics exploration, all have found end uses for ePTFE. Pharmaceutical and biotechnology industries use ePTFE ﬁbers possess high strength, exhibit low ePTFE products to ensure the purity and security of shrinkage, and resist abrasion. They also offer process streams. Signiﬁcant applications of ePTFE excellent resistance to degradation from ultraviolet membranes and other shapes are described in detail rays. This combination of characteristics ensures in the rest of this book. stability and integrity in extreme environments. References [1] US Patent 3,962,153, R.W. Gore, assigned to W.L. Gore & associates, Inc., June 8, 1976. [2] E.G. Norman, Gore W.L., Associates, UMIST expanded PTFE properties & applications, in: Fluoropolymers Conference, RAPRA Technol- ogy, UK, 1992. Paper 9.

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170 EXPANDED PTFE APPLICATIONS HANDBOOK [3] B.D. Craddock, K.E. Evans, University of [5] Outdoor Sports Center, 2015. www.outdoorsports. Liverpool, J. Phys. D Appl. Phys. 22 (1989) com/outdoor-apparel.aspx?page¼4. 1877e1882. [6] WL Gore and Associates, June 2015. www.gore. [4] S. Ebnesajjad, Fluoroplastics Vol. 1, Non-melt com/en_xx/technology/technicalcapabilities.html. Processible Fluoropolymers, second ed., Elsevier, 2015.

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8 Expanded PTFE Use in Fabrics and Apparel OUTLINE 8.1 Introduction 171 8.4.2 Outdoor Footwear 186 8.4.3 Testing Footwear 186 8.2 Breathable Expanded Polytetraﬂuoroethylene 8.4.4 Outdoor Gloves 188 Fabric Structure 172 8.5 Protective Apparel 188 8.3 Development History 178 8.6 Summary 190 8.4 Outdoor Apparel 182 References 190 8.4.1 Testing Apparel 184 8.1 Introduction Fabrics treated with silicone, ﬂuorocarbon, and other water repellants usually allow evaporation of The use of expanded polytetraﬂuoroethylene perspiration but are only marginally waterproof. (ePTFE) began to revolutionize the apparel industry, They allow water to leak through them under very and generally fabric design. The discomfort of low pressures, and usually leak spontaneously when clothing and footwear from overheating, such as rubbed or mechanically ﬂexed. Rain garments must when wearing winter coats and boots, has been an old withstand the impingement pressure of falling and issue that has been resolved by the use of ePTFE. We wind blown rain and the pressures that are generated begin with a few words about the issue of clothing in folds and creases in the garment. “comfort” with respect to body temperature. Comfort is a complicated concept and has many components Garments must be “breathable” to be comfortable. including weight, drapeability, and feel of a garment. Passage of some air through the garment makes it Numerous applications beyond coats have been more comfortable in addition to the transmitted water developed for breathable and waterproof fabric ex- vapor from inside to outside. In the absence of water amples of which include tents, gloves, and uniforms accumulation the undergarments do not become as well. wet allowing the natural evaporative cooling effect to take place. Breathability and ability to transport Protective garments for wearing in rain and other interior moisture vapor to the external environment wet conditions must keep the person wearing the are used interchangeably in this discussion. clothes dry by preventing the leakage of water into the garment and by allowing perspiration to evapo- The transport of water through a layer can be rate from the wearer to the atmosphere [1]. In the achieved in a number of ways. Wicking is the most past, and through a long history of rainwear devel- common way when large quantities of moisture are to opment, truly waterproof materials have not allowed be transferred. Wicking materials are hydrophilic in the evaporation of perspiration. Consequently, an that a drop of water placed on the surface of these active person wearing the clothes becomes soaked materials forms an advancing water contact angle of with perspiration. “Breathable” materials that do less than 90 degree so that they wet spontaneously. permit evaporation of perspiration have tended to wet They are also porous with pores that interconnect to through from the rain, and they are not truly water- make complete pathways through the wicking proof. Waterproof materials like vinyl do not allow material. water in but do not allow evaporation of perspiration. Liquid water moves by capillary action from inte- rior surface to exterior surface where it evaporates. Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00008-0 171 Copyright © 2017 Elsevier Inc. All rights reserved.

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172 EXPANDED PTFE APPLICATIONS HANDBOOK Although some wicking materials may resist pressure- Figure 8.2 Basic model of moisture vapor transmis- induced ﬂow of liquid water through them due to the tortuosity and length of ﬂow path, they readily trans- sion and liquid and particulate repellence of a fabric [3]. port water by capillary action from the exterior surface to the interior surface and so are unsuitable as rain side. This model was implemented using multiple material. The comfort attributed to cotton garments in layers of especially designed materials including warm climates results from its ability to transport microporous membrane of ePTFE. water to the exterior surface where it can readily evaporate and provide cooling. Another natural Fig. 8.3 shows the implementation of the model wicking material is leather that owes its great comfort shown in Fig. 8.2 in actual apparel like a winter to breathability via wicking. jacket. Those coats are a popular example for the application of ePTFE membrane. The outer shell is 8.2 Breathable Expanded usually made from a strong durable fabric such as Polytetraﬂuoroethylene Fabric nylon, which is made water repellent by topical Structure treatment with a ﬂuorocarbon or silicone compound. The water that is not repelled travels through the Expanded PTFE membranes have more than outer shell but is stopped by the ePTFE membrane 1.4 billion pores per square centimeter (Fig. 8.1). The that is quite hydrophobic. Water drops, even the pores are 20,000 times smaller than a drop of water, smallest ones, are simply too large to enter the ePTFE but 700 times larger than a molecule of water. So the membrane pores. It holds back water penetration pores in the membrane are too small to allow water in because of its very low surface energy and small its liquid form to penetrate the membrane, but mois- pores. Indeed a signiﬁcant amount of pressure is ture vapor (a gas) can easily escape [2]. required to force water into the pores of ePTFE membranes. That pressure is far larger than the hy- Fig. 8.2 shows a basic model of a breathable fabric drodynamic pressure generated by even the most laminate and how it works. Ideally, water, liquids, torrential downpours. and particulate matter are repelled by the external (exposed) surface of the fabric/garment. The fabric The water vapor generated by bodily perspiration thus may not absorb (like cotton) or wick (like goes through the polyurethane (PU)/ePTFE layer. PU leather) the liquid water. Simultaneously, the fabric is permeable to water vapor. In the vapor phase, water allows uninhibited transmission of moisture vapor molecules are sufﬁciently small to diffuse through from the inside surface of the fabric to the ambient the pores of ePTFE membrane. The body thus re- environment. Moisture transmission takes place as mains cool because vapor removal allows continued long as the partial pressure of the water on the interior evaporation of perspiration, which is the body’s main side of the fabric is higher than it is on its external cooling mechanism. Figure 8.1 An example of expanded polytetraﬂuoro- Fig. 8.4 shows the components of the fabric ethylene membrane at 40,000Â magniﬁcation [2]. including the ePTFE membrane. The outer layer of the fabric is usually only resistant to abrasion and tear. It can be made permanently water repellent by applying a silicone or ﬂuorocarbon chemical to its surface; also called durably water repellent. Those

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 173 Figure 8.3 A practical structure of water repellent and breathable fabric containing expanded polytetraﬂuoroethylene. chemicals usually endure many wash cycles but Investigation by W. L. Gore & Associates revealed would need to be reapplied after a period of time. The PTFE is not oleophobic, thus allows entry of oily next layer is usually a mesh intended to protect the materials into the pores. Gore chose to attach (coat) layers beneath it. the ePTFE membrane to a thin layer of PU to render it oleophobic. An alternative and newer method to Early in the history of ePTFE development the achieve oleophobicity of ePTFE involves covering membranes were used in outdoor coats. After a the walls of the pores within the membrane with an period of time the ﬁrst GORE-TEX coats began to oleophobic coating without blocking the pores. This leak water in and the breathability lessened! In- technology has been in use by eVent Corporation, vestigations showed contamination (caused by dirt, which is related to BHA Corp, in fabrics for outdoor body oils, sweat, sunscreen, insect repellent, or coats. similar foreign matter) was the root cause of the leak. Contamination had become the unexpected enemy of The ePTFE membrane (Fig. 8.4) is coated with a the early ePTFE laminates that were designed with PU resin to keep out the bodily oils, surfactants, and plain ePTFE membranes. The membranes worked other oily compounds from blocking its pores. PU ﬁneduntil they collected dirt and oils, which possess partially penetrates the near-surface pores of the higher surface energies than PTFE. Gradually, water ePTFE, thus keeps the two layers together. One or makes contact with dirty or oily ePTFE membranes, more fabric layers are placed under the PU layer as which eventually allow water through and thus inner shell (or backer). Aside from the ePTFE/PU leakage occurs. layer being ubiquitously present in breathable Figure 8.4 Construction of breathable and moisture repellent layers in an expanded polytetraﬂuoroethylene fabric.

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174 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 8.5 Schematic of water vapor transmission through expanded polytetraﬂuoroethylene membrane. waterproof fabrics, there are many fabric designs pores. The answer is no because the pores are orders depending on the intended end use and its of magnitude larger than the water molecules; thus do requirements. not hinder their movement. Fig. 8.5 is not drawn to scale; in reality water molecules are much smaller There is, however, a difference between mois- than they are depicted. ture vapor transmission of ePTFE alone and the PU/ePTFE combination membrane. In the case of An example of the chemical structure of typical the expanded microporous PTFE membrane a PU can be seen in Fig. 8.6. In this case PU is the simple driving force removes water vapor from the product of the reaction of a polyol with diisocyanate interior of the apparel (like a coat) to the outside that produces a fairly polar polymeric structure environment as seen in Fig. 8.5. That driving force capable of absorbing water. Examples of polar is the partial pressure of the water vapor that must functional groups include the following. be higher in the fabric interior, next to the body, than the exterior environment. The larger the dif- II ference in partial pressures the more breathable – O –, – C – and – N –. the membrane. The moisture vapor transmission rate (MVTR) is thus proportional to the difference I between the partial pressure of water on the two H sides of the membrane. For example, on a cold dry winter day a breathable coat works very well The PU used for coating ePTFE membranes is because of the relatively large magnitude of the hydrophilic and thus absorbs some water (Fig. 8.7). It driving force. Table 8.1 lists the most common then acts as a reservoir or “jumping off platform” for methods for measurement of breathability of ﬁlms water molecules to enter the pores of the membrane. and coatings. The singular beneﬁt of the PU coatings is it pro- One reasonable question is whether the hydro- tects the membrane pores from oils and oily com- phobicity of the microporous PTFE membrane af- pounds. The earliest ePTFE fabric designs did not fects the diffusion of water molecules through the include a PU coating. Oils and other oily compounds produced by the body or applied to the body clogged the pores of ePTFE membrane relatively quickly

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 175 Table 8.1 Common methods for measurement of breathability of ﬁlms and coatings Method Name Standard Code Explanation of Purpose Sweating guarded hot ISO 11092 plate test ISO 1999 Measurement of thermal and water vapor resistance under steady state conditions Upright cup method ASTM F1868 ASTM E96 Standard test methods for water vapor transmission Inverted cup method of materials ASTM E96 Desiccant inverted cup Standard test methods for water vapor transmission method ASTM E96 Method M-05 of materials Dynamic moisture permeation test ASTM F2298 Standard test methods for water vapor transmission of materials Moisture vapor ASTM D1653 transmission cell Standard test methods for water Vapor diffusion Dynamic moisture ASTM F2298 resistance and air ﬂow resistance of clothing permeable cell materials using the dynamic moisture permeation cell Standard test methods for water vapor transmission of organic coating ﬁlms Standard test methods for water vapor diffusion resistance and air ﬂow resistance of clothing materials using the dynamic moisture permeation cell Figure 8.6 Example of the polymerization reaction of a two-part polyurethane [4]. causing the fabric to stop being breathable. PU air through the fabric [6]. So, can ePTFE membrane coatings were selected because of their high water be used without a PU coating (Figs. 8.4 and 8.7) permeation rate and oleophobicity that prevents successfully? The answer is yes and accomplished by clogging of the membrane. applying an oleophobic ﬂuoropolymer coating to the ePTFE membrane. An example of the technique is Silicone resin has also been reported as a substi- described here. tute material for PU as a protective layer for ePTFE. US Patent 5,362,553 [5] reported on the discovery It is coalesced on the surfaces of the nodes and that ePTFE coated with a silicone resin exhibited ﬁbrils to provide resistance to oil and contaminating improved resistance to surfactant activity. The in- agents without completely blocking the pores in the ventors reported no loss in MVTR of silicone ePTFE membrane. An example of the oleophobic resinecoated ePTFE compared to the membrane by ﬂuoropolymer is a perﬂuoroalkyl acrylic copolymer itself. In contrast, they contended ePTFE/PU ﬁlm had with ﬂuorocarbon side chains. The ﬂuorocarbon side a lower MVTR than that of the ePTFE alone. chains extend in a direction away from the surface of Commercially either PU or no coating on the back- the nodes and ﬁbrils that the coalesced oleophobic side of ePTFE remains the prevailing system for ﬂuoropolymer coats. The oleophobic ﬂuoropolymer manufacturing breathable waterproof fabrics. coating is coalesced on surfaces of the nodes and ﬁbrils to provide resistance to oil and contaminating It is known that some degree of air permeability is agents without completely blocking the pores in the desirable to increase user comfort. A drawback cited membrane. for PU is its impact on reducing permeation (ﬂow) of

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176 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 8.7 Schematic of water vapor transmission through polyurethane-coated expanded polytetraﬂuoroethy- lene membrane. A dispersion of perﬂuoroalkyl acrylic copolymer the molecular attraction between the solid and the with ﬂuorocarbon side chains was diluted with a water- miscible wetting agent (Fig. 8.8). The dispersion was liquid. The free energy of the solid relative to a diluted at a ratio of water-miscible wetting agent to dispersion in a range of about 1:5 to 20:1. The diluted liquid is often referred to as the surface energy gSL dispersion had surface tension and relative contact of the solid relative to the liquid. The free energy of angle properties that enable the diluted dispersion to wet the membrane and coat surfaces of the membrane. liquid relative to air is normally called the surface That included diluting the dispersion in a material selected from the group including ethanol, isopropyl tension of the liquid gLA. The free energy of the alcohol (IPA), methanol, n-propanol, n-butanol, N-N- solid relative to air is normally referred to as the dimethylformamide, methyl ethyl ketone, and water- soluble e- and p-series glycol ethers. surface energy of the solid gSA. The YoungeDupre equation relates all the free energies to the contact Fig. 8.9 shows scanning electron micrographs of ePTFE membranes coated with a PU ﬁlm and an angle as q [8]: oleophobic coating like the one described in Fig. 8.8. The interior side of the ePTFE membrane is contin- gSA À gSL ¼ gLA$cosðqÞ (8.1) uously covered with the PU ﬁlm while its exterior side retains the basic node and ﬁbril structure. In case of The degree to which a challenge liquid may wet a the oleophobic coating the pore wall surfaces and the challenged solid depends on the contact angle q. At a node and ﬁbril structure of the membrane is contact angle q of 0 degree, the liquid wets the solid preserved. so completely that a thin liquid ﬁlm is formed on the solid. When the contact angle q is between 0 degree Wetting mechanism of PTFE surface is described and 90 degree the liquid wets the solid. When the further because of the importance of the subject to contact angle q is more than 90 degree the liquid does the coating of the membrane pores. The free energy not wet the solid. between a solid and a liquid is inversely related to For example, consider two different liquids on a polytetraﬂuoroethylene (PTFE) solid surface that has a surface energy gSA of 19 dyn/cm. One liquid, such as IPA has a surface tension gLA of 22 dyn/cm (which is a higher value than the surface energy gSA value of

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 177 Figure 8.8 Schematic of coating perﬂuoroalkyl acrylic copolymer on the walls of expanded polytetraﬂuoroethy- lene membrane pores. Figure 8.9 Scanning electron micrographs of expanded polytetraﬂuoroethylene (ePTFE) membranes coated with a polyurethane (PU) ﬁlm and an oleophobic coating [7]. Images courtesy of Dr Philip Gibson.

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178 EXPANDED PTFE APPLICATIONS HANDBOOK the PTFE material and in theory cannot wet the PTFE of SCCO2 and coating materials also have material) and a relative contact angle q of about viscosity <0.5 cP. The viscosity and surface tension 43 degree relative to PTFE. Therefore, IPA “wets” of the resultant solution are low compared to tradi- PTFE very well. The gSL of IPA relative to PTFE can tional solvents so resistance to ﬂow is reduced; thus now be calculated by rearranging Eq. (8.1) to lending itself to entering even the smallest pores in Eq. (8.2) to: the membrane. Most solvents’ viscosity is >0.5 cP and surface tension >15 dyn/cm which make it gSL À gSA ¼ gLA$cosðqÞ (8.2) difﬁcult for them to enter small pores of ePTFE. Consequently, it is difﬁcult to coat all the surfaces of gSL ¼ 19 À 22 Â cosÀ43Á ¼ 3 dyn=cm ePTFE membrane with those liquids. Another liquid such as deionized water has a The polymer coatings in the described method surface tension of about 72 dyn/cm and a contact form very small “particle-like” precipitates in the angle q of 112 degree relative to PTFE and, therefore, CO2 ﬂuid. These particles are very small as compared does not wet PTFE or is held out. The calculated to conventional dispersed particles. As the polymer value for the surface energy gSL of water relative to particles precipitate from the low surface tension PTFE would be 38.5 dyn/cm [6,9]. ﬂuid the polymer stays highly swollen and the ePTFE material of base membrane remains completely Another aspect of contact angle q is important. If wetted with the ﬂuid and the CO2-plasticized poly- the contact angle q that a given liquid makes relative mer [10]. to a solid is less than 90 degree, the liquid can be drawn into capillaries existing in even an apparently 8.3 Development History solid material. The amount of capillary force drawing the liquid into the capillary will depend on the size of The development of breathable fabrics took place the capillary. A relatively smaller capillary exerts a at W. L. Gore & Associates in the 1970s. One of the relatively greater force on the liquid to draw the early patents is US 4,194,041 that describes the liquid into the capillary. If the contact angle q is construction of breathable and waterproof fabrics greater than 90 degree, there will be a force to drive using ePTFE membranes. There have been a large the liquid out of the capillaries. number of patents since the Gore disclosure many of which have innovated over the initial invention. The capillary force relates to the surface energy Today, a number of fabric designs with water repel- gSA of the solid material and to the surface tension lence and breathability characteristics are available gLA of the liquid. The capillary force drawing from various companies. the liquid into the capillaries increases with the increasing surface energy gSA of the solid. ePTFE was used in waterproof garments and tents The capillary force drawing the liquid into the cap- because it kept liquid water out while permitting the illaries also increases with decreasing surface tension evaporation of the perspiration and the transfer of gLA of the liquid. YoungeLaplace Eq. (8.3) governs moisture vapor through the layered fabric [1]. At the equilibrium state of liquid entry into a capillary. least two layers had to be combined for this appli- cation: (1) an interior, continuous hydrophilic layer rp ¼ 2gLA b cosðqÞ (8.3) (eg, PU) that allowed water to diffuse through, pre- DP vented the transport of surface active agents and contaminating substances such as those found in rp ¼ pore diameter; gLA ¼ surface tension of solide perspiration, and was substantially resistant to the liquid, dyn/cm (mN/m) (calculated from Young’s pressure-induced ﬂow of liquid water and (2) an equation); DP ¼ pressure difference applied across ePTFE layer that permitted the transmission of water the membrane; ß ¼ capillary constant; q ¼ contact vapor and provided thermal insulating properties angle. even when exposed to rain. In another method the solvent used for coating Fabrics of these materials were permanently material is carbon dioxide in supercritical phase. The waterproof. They repelled all exterior water surface tension of the supercritical carbon dioxide yet allowed the evaporation of perspiration whenever (SCCO2) solution is less than 0.1 dyn/cm so it can the partial pressure of water vapor inside the garment enter very small pores of base membrane. Mixtures

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 179 exceeded that outside. In practice, these garments The fabric exhibited elastomeric properties of withstood nearly all climate conditions. The hydro- stretch to break of 275% in the machine direction, philic ﬁlm had an MVTR >2000 g/m2 day, permitted and 145% in the transverse direction, and a total no detectable transmission of surface-active agents stretch recovery of at least 39% after being stretched and no detectable ﬂow of liquid water at hydrostatic to 75% extension for 100 cycles. The waterproof and pressures up to 172 kN/m2. The hydrophobic layer breathable elastomeric ePTFE laminate bonded to a had an MVTR >2000 g/(m2 day), and an advancing stretch fabric was proven durable and possessed water contact angle >90 degree [1]. MVTR of over 2000 g/m2 day. Donovan [11] reported development of a breath- A water vaporepermeable, waterproof, and highly able fabric using ePTFE that was quite strong and elastic ﬁlm of ePTFE was developed by impregnating/ ﬂame resistant. The exterior layer was Nomex poly- coating it on both sides with a water vaporepermeable aramide, followed by the ePTFE membrane and the PU elastomer. The membrane had elongation of interior layer was Kevlar polyaramide. This fabric >40% in at least one direction. The membrane was was waterproof, windproof, and permeable to water durable in repeated stretching to 80% of its ultimate vapor, properties that are desirable in tents for severe elongation for over 200,000 cycles. The membrane is service applications, such as continued rough usage useful for construction of clothing, tents, and other and usage under severe weather conditions. Tensile end uses in which water vapor transmission and strength of the fabric was 2.3 and 2 MPa in the ma- waterproofness are required [14]. chine (warp) and ﬁll (cross) directions. US Patent 4,961,985 [15] described innovations A laminate developed for medical and biological over previously reported developments in surgical applications exhibited strong resistance to bacterial gowns, drapes, and similar fabrics that protect sur- penetration. It consisted of a ﬂexible inner layer of gically prepared areas of the skin from contamina- ePTFE with an MVTR >1000 g/(m2 day) and a tion. Similarly, they also protect surgeons and nurses contact angle >90 degree, combined with a against contamination through contact with unpre- continuous outer hydrophilic layer such as PU pared or contaminated areas of patient’s skin. In attached to the inner surface of ePTFE. This PU summary, surgical gowns must provide a sterile layer had a minimum MVTR of 1000 g/(m2 day). It barrier to protect patients from contamination contained a solid powder or a liquid additive such through contact with the surgeons and operating as color pigments and antistatic agents. An addi- room staff, and vice versa. tional textile layer was attached to the inner surface of the ePTFE layer for strength and aesthetic rea- An ePTFE membrane with a minimum porosity of sons. This laminate had, in addition to being 65% was used that had a per unit area weight of breathable and waterproof, strong resistance to 1e10 g/m2. A layer of hydrophilic PU resin was bacterial penetration in excess of 5000 min, water applied to one side of ePTFE. The PU resin diffused entry pressure above 138 kPa, and an MVTR into the pores near the surface of the ePTFE (see >2000 g/(m2 day). This type of laminate is partic- Figs. 8.4 and 8.7). The PU layer had a per unit area ularly useful in both biological and health-care weight of 10e20 g/m2. The laminate of ePTFE/PU applications [12]. had a minimum MVTR of 15,000 g/m2/24 and had a signiﬁcant resistance to the passage of microor- In addition to being waterproof and breathable, it ganism barrier. For instance, when challenged by a is desirable for fabrics to have the distinguishing Virus Barrier Efﬁciency Test at 27.6 kPa no virus characteristic of stretch [13]. Stretchability offers passed through the membrane. The laminate exhibi- many advantages including comfort, ﬁt, reduction in ted air permeability <6 cm3/min by the Gurley air pucker, improved wrinkle resistance, required fewer permeability test (ASTM D726) [15]. sizes, alterations, and greater design ﬂexibility. In its broad concept, “stretch” might be deﬁned as an A 2005 patent application [16] provides another important comfort factor in textile products. To type of breathable fabric with a capability to control accomplish the goal of stretchability, ePTFE was microorganisms like bacteria. In addition to infection coated with an elastomeric hydrophilic polymer control in medical applications, growth of microor- with an MVTR exceeding 1000 g/m2 day. The ganisms such as molds and bacteria damages fabrics stretchability of the elastomer coating had to be at or produces an unpleasant odor. Examples include least 5% higher than its yield point. garments worn for an extended period of time without being changed, or used under circumstances

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180 EXPANDED PTFE APPLICATIONS HANDBOOK where it will not dry for extended periods. Military resistance to water. Manufacturers typically describe uniforms are sometimes worn for long periods of the waterproof breathability of fabrics using two time in extreme environments. Control of microor- numbers. The ﬁrst is in millimeters and is a measure ganisms in such instances is important. of how waterproof a fabric is. In the case of a 10,000 mm fabric, if you put a square tube with inner Silver compounds such as silver acetate, silver dimensions of 2.54 cm Â 2.54 cm (100 Â 100) over a nitrate, silver protein, and silver sulfadiazine piece of said fabric, you could ﬁll it with water to a are known for antimicrobial effects. They generate height of 10,000 mm (32.8 feet) before water would silver ion treatment purported to place some bacteria begin to leak through. The higher the number, the in an active but nonculturable state and eventual more waterproof the fabric. death [17]. Silver is formulated into ﬁbers such as X- Static available from Noble Biomaterials. The silver- The second number is a measure of how breath- laden ﬁbers are incorporated in various medical able the fabric is, and is normally expressed in terms devices such as advanced wound care treatments, of how many grams of water vapor can pass through a dressings, medical socks, and orthopedic soft square meter of the fabric from the inside to the goods. They are also incorporated into soft surfaces, outside in a 24-h period. In the case of a 20k such as privacy curtains, scrubs, lab coats, patient (20,000 g) fabric, this would be 20,000 g. The larger apparel, and bedding to prevent the growth and the number the more breathable the fabric would be. cross-contamination of bacteria on the surface of fabrics [18]. US Patents 5,026,591 and 4,532,316 described the formulation and preparation technique for PU coat- Silver-containing ﬁbers are included in multilayer ings [19,20]. The PU described is a 100% solids breathable fabric structure of ePTFE membrane thus mixture of polyol and an isocyanate such as diiso- providing protection against microbial growth, along cyanate. A coating lamination process was reported with anti-odor, anti-static, heat and moisture transfer that produced a laminate of PU, ePTFE membrane, attributes. The breathable fabric included a mem- and a fabric substrate. The coated ePTFE may be brane containing a porous ePTFE scaffold material used in waterproof-breathable products, such as with a void volume (>60%). A resin such as PU can garments, shoes, or gloves. The fabric acted as a be applied to at least one surface of the scaffold protective layer in the construction of the apparel. material. The fabric also included silver-containing substrate placed in contact with the ePTFE mem- One of the beneﬁts of the laminate is the wide brane. One method of securing the silver-containing variety of fabric substrates that can be processed into substrate to the ePTFE membrane was by an adhe- a laminated product. This is true because the sub- sive system. strate does not control the ﬁlm-forming process, nor does the substrate’s geometry, properties, or charac- Table 8.2 lists waterproof rating of fabrics in teristics control the penetration of coating into the which pairs of numbers are used to describe the substrate. The ePTFE membrane controls the amount Table 8.2 Waterproof rating of fabrics using pairs of numbers Waterproof Rating (mm) Resistance Provided What it can Withstand 0e5000 mm No resistance to some resistance to Light rain, dry snow, no pressure moisture 6000e10,000 mm Rainproof and waterproof under light Light rain, average snow, light pressure pressure 11,000e15,000 mm Rainproof and waterproof except under Moderate rain, average snow, light high pressure pressure 16,000e20,000 mm Rainproof and waterproof under high Heavy rain, wet snow, some pressure pressure 20,000 mmþ Rainproof and waterproof under very Heavy rain, wet snow, high pressure high pressure

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 181 of PU entering and adhering to the fabric substrate, adhering adjacent layers of seam tape together and further controls in a unique way the geometry strongly enough that upon unrolling the spool, the and the continuity of the coating. The fabric may expanded porous PTFE layer is damaged. The have any geometry such as thickness, texture, open- densiﬁed surface of the ePTFE layer prevents the ness, etc. The substrate, therefore, is selected pre- entry of the thermoplastic adhesive through cold dominantly as the needs of the end use dictate. creep into the ePTFE layer while the same tape is stored in roll form (Fig. 8.11). The coated products of this technique have unique characteristics. It was discovered that the combina- The ePTFE ﬁlms prior to densiﬁcation have den- tion of PU and ePTFE membrane was attached to the sities between 0.3 and 0.5 g/cm3, thickness fabric substrate in a unique way. The PU/ePTFE was 25e50 mm, and porosity !40%. The PU (25e200 mm attached only at select points. This was in contrast to thick) must have a sufﬁciently low viscosity as a what was normally seen in previous techniques (see liquid to ﬂow into the pores of the ePTFE and when Fig. 8.4) where the PU/ePTFE seemed to follow the cured or partially cured must melt well above the contour of the fabric substrate. It did not thus have an melting point of the thermoplastic hot melt adhesive overall regular thickness. The US Patents 5,026,591 layer to prevent delamination. Materials with melt and 4,532,316 produced laminates with regular viscosities between 10 and 200 P, at 100C, are thickness (Fig. 8.10). required. The melting point of the cured or partially cured thermosetting adhesive is in !200C and The problem of blocking of seam tape contain- preferably does not melt, but decomposes. In the solid ing an ePTFE layer during storage, especially in form, it must be insoluble in water and unaffected by warm months, used to be a big impediment for the dry cleaning solvents. breathable waterproof garment manufacturing. US Patent 5,162,149 reported on a non-blocking The preferred thermoplastic hot melt adhesive is waterproof seam tape for covering sewn seams PU. Its melting point should be >100e180C and in 1992. have a melt ﬂow rate (as determined by ASTM 1238 under conditions KISS/15) of >20 g/min and <150 g/ The seam tape consisted of an ePTFE layer in min to ensure adequate ﬂow of the thermoplastic hot which one surface had been densiﬁed, a cured, or melt layer when applied to a seam. Fig. 8.12 illus- attached to a partially cured PU adhesive layer and a trates how the sealant tape is applied to a sewn seam thermoplastic hot melt adhesive layer. Compressive of ePTFE fabric. forces especially in warm environment induce the thermoplastic hot melt adhesive to creep into the More recently, copolymers of tetraﬂuoroethylene pores of the adjacent layer of ePTFE layer, thus, (TFE) of the ﬁne powder type have been developed. Figure 8.10 Construction of a breathable and moisture-repellent fabric using an expanded polytetraﬂuoroethy- lene laminate with uniform thickness [19,20]. Figure 8.11 Schematic design of seam sealing tape for apparel [21].

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182 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 8.12 Schematic design of seam sealing tape for apparel [21]. Comonomer concentrations of over 5% by weight 8.4 Outdoor Apparel have been demonstrated yielding copolymers that are expandable. Examples of comonomers included A few examples of commercial product are chlorotriﬂuoroethylene, hexaﬂuoropropylene, vinyl described in this section. Even though there are many ﬂuoride, vinylidene diﬂuoride, hexaﬂuoroisobutylene, suppliers the selected examples are limited to a few and triﬂuoroethylene. The TFE copolymers produced major suppliers that provide descriptions of those strong, useful, expanded membranes with a micro- products. structure of nodes interconnected by ﬁbrils. Up to and beyond a 25:1 stretch ratio allowed formation of a Fig. 8.13 shows a basic GORE-TEX coat (GORE- uniform viable membrane. Expandability of TFE co- TEX is a trademark of W. L. Gore & Associates, polymers is entirely unexpected, and contrary to past Inc.). The two-layer construction, designed for a reports that a TFE copolymer can not be expanded wide range of outdoor activities, uses a speciﬁc [22e24]. GORE-TEX membrane bonded to the outer material and protected on the inside by a separate lining. The Figure 8.13 Example of basic GORE-TEX coat. Courtesy: W. L. Gore & Associates, www.GORE-TEX.com, January 2015.

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 183 separate lining ensures better wearing comfort and Figs. 8.14 and 8.15 show examples of two fabric versatility. designs that eliminate the added oleophobic (pro- tective) layer of traditional products thus allowing for The three-layer construction, designed for more increased breathability while preserving its protec- demanding activities, uses a speciﬁc GORE-TEX tion against contamination. The design in Fig. 8.14 is membrane sandwiched between the outer material likely based on the technology discussed in the and backer material. Three-layer provides added Section 8.2 (Fig. 8.8) in which the pores are coated durability without additional weight or bulk. Figure 8.14 Design of the eVent fabric without polyurethane oleophobic layer. Courtesy: www.eventfabrics.com/ products/#protective. Figure 8.15 Example of GORE- TEX Pro Coat. Courtesy: W. L. Gore & Associates, www.GORE-TEX.com, January 2015.

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184 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 8.16 Example of GORE-TEX Paclite Coat. Courtesy: W. L. Gore & Associates, www. GORE-TEX.com, January 2015. using a supercritical carbon dioxide solution of an both the outer material and a specially developed oleophobic polymer. The construction in Fig. 8.15 robust inner lining. Outer materials meet demanding uses a 100% ePTFE based multilayer membrane performance criteria (denier !40) and the thin, low construction. It has up to 28% increase in breath- denier Gore Micro Grid Backer technology which ability over current ePTFE laminates. The interior enhances breathability, reduces weight, internal comfort of the garment is increased, thus enabling the abrasion, and snag resistance allow high performance wearer to feel less clammy and more comfortable of the three-layer products. over a wide range of temperatures and conditions. The new membrane technology performs better in 8.4.1 Testing Apparel lower humidity conditions, is more comfortable in warmer conditions caused by solar loading, and has Fabrics containing ePTFE membrane must meet improved dry out times in conditions with frequent the requirements of the intended outdoor wear just workerest cycles. like any other materials would. The essential tests include mechanical durability, resistance to cold ﬂex Fig. 8.16 is an example of GORE-TEX Paclite fatigue, waterproofness, and comfort. These tests are apparel advertised to have the lightest, most packable described brieﬂy. fabrics. The garments are durably waterproof, windproof, and breathable and are built for activities The Wyzenbeek abrasion test (ASTM D4157) is when weight and space are critical, but protection is used primarily in North America. Although designers still important. The outside fabric is constructed of in North America are less familiar with the Martin- high-performance polyester or nylon, and the inside dale test (ASTM D4966), it is gaining recognition as uses a speciﬁc ePTFE membrane with a protective a reliable test. The Martindale is considered by many layer made from an oil repellent substance and car- to be a more accurate measurement of “real life” use. bon. So no separate lining is required which makes The fabric is mounted ﬂat and rubbed in a modiﬁed the shells lighter and smaller to pack away. Special ﬁgure-eight motion with a piece of worsted wool as tape technology is said to ensure the seams and are the abradant. The number of cycles that the fabric can completely sealed waterproof. withstand before showing an objectionable change in appearance is counted. The inspection interval is GORE-TEX Pro products for mountain sport ac- dependent on the end point of the fabric and is usu- tivities (Fig. 8.17) use a revolutionary 100% ePTFE- ally every 1000 up to 5000 rubs, every 2000 between based multilayer membrane system with a unique microstructure. The membrane is strongly bonded to

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 185 Figure 8.17 Example of GORE-TEX Pro products for mountain sport activities. Courtesy: W. L. Gore & Associates, www.GORE-TEX.com, January 2015. 5000 and 20,000, every 5000 between 20,000 and management inside a garment. Other factors include 40,000, and every 10,000 above 40,000 [25]. lightweight, garment ﬁt, and softness. In the cold ﬂex text the goal is to determine the The body regulates heat in four primary ways: resistance of the fabric to failure under stress at radiation, convection, conduction, and evaporation. reduced temperatures. The fabrics are squashed and In hot and humid environments or during physical stretched repeatedly in extreme low temperatures for activity, evaporative cooling (wet heat transfer) is the many hours. The fabrics must survive this punishing primary heat loss method. test and emerge still durably waterproof. Figure 8.18 Factors contributing to heat stress [27]. W. L. Gore tests every garment style for water- proofness before production. The testing facility is designed to simulate a variety of rain conditions. Specially engineered rain nozzles are strategically positioned in the chamber to subject the garment to conditions that range from light drizzle to wind- driven rain [26]. While comfort is important in everyday garments, it is critical during physical activity because of the possibility of heat stress. Clothing is one of the four key factors contributing to heat stress (Fig. 8.18). Comfort is the combination of the garment properties and each individual’s perception or preference, with the region and work environment having signiﬁcant inﬂuence on this comfort perception. However, the issue of comfort can seem very confusing because there are many variables involved. Two important factors of comfort are heat and humidity

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186 EXPANDED PTFE APPLICATIONS HANDBOOK Managing sweat/moisture is one of the most GORE-TEX Performance Comfort footwear important aspects of heat management. Moisture design is suitable for outdoor use in moderate travels in both vapor and liquid forms away from the weather conditions. They are waterproof and body. Poor moisture management can make a garment breathable and may be worn in a wide range of feel clammy, clingy, sticky, and heavy. Fabrics that use outdoor activities and changing weather conditions. ePTFE membranes solve this problem. When moisture The GORE-TEX membrane in Fig. 8.20 is a com- is removed rapidly through a breathable fabric/ bination of ePTFE and other materials to render it garment, less liquid perspiration is formed because of waterproof. its evaporation away from the body. GORE-TEX Insulated Comfort footwear 8.4.2 Outdoor Footwear (Fig. 8.21) are designed for outdoor use in rain, snow, and cold conditions. They combine waterproofness Breathable waterproof footwear has been devel- and effective breathability with insulation for use in oped by a number of companies, using ePTFE tech- cold weather conditions. Water and snow remain on nology. W. L. Gore has developed a variety of designs the outside while moisture generated by perspiration for a broad range of wear such as everyday life and escapes from the inside. The footware’s insulated strenuous sports such as skiing, climbing, and inner lining allows reliable protection from the cold hunting. A few examples of GORE footwear are thus making them suitable for a wide range of out- described. doors activities (Fig. 8.21). Shoes constructed using GORE-TEX Extended A number of patents provide detailed information Comfort technology may be worn indoor and outdoor about the development of breathable and waterproof in moderate and warmer conditions or during higher footwear material and the construction of shoes and activity levels. The footwear is (Fig. 8.19) waterproof boots [29e32]. combined with effective breathability, offering enduring weather protection. Water stays on the 8.4.3 Testing Footwear outside while perspiration can easily escape from the inside. Their noninsulated construction allows There are a number of footwear tests for each type outstanding climate comfort and heat release [28]. of shoe and boot. Breathable and waterproof foot- The GORE-TEX membrane in Fig. 8.19 is a com- wear are subjected to speciﬁc tests: walking simu- bination of ePTFE with other materials to ensure the lator, wicking test, leak test, and breathability. footwear is waterproof and does not leak. The walking simulator tests the waterproof per- formance of the footwear (Fig. 8.22). Test shoes are Figure 8.19 Shoes constructed using GORE-TEX Extended Comfort technology [28].

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 187 Figure 8.20 Shoes constructed using GORE-TEX Performance Comfort technology [28]. Figure 8.21 Shoes constructed using GORE-TEX Insulated Comfort technology [28]. placed on ﬂexible foot forms equipped with moisture and laces to ensure that the whole shoe or boot meets sensors that are subjected to 200,000 steps in a water the waterproof performance standards. Fig. 8.23 bath. If moisture enters the shoe, the testing stops and shows the setup for the wicking test. the sensor indicates the source of the leak. Leak test is run in a centrifuge using boots ﬁlled In addition to the ePTFE membrane, there are other with water spun at high speeds. The pressure gener- components in footwear that ensure durable water- ated by centrifugal force enables water to go through proofness. All materials must also be nonwicking to even the smallest of holes. prevent water from being transported into the shoe or boot. Those include the shoe’s leather, foam, stitching, Breathability is tested to ensure all the shoe components work together properly.

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188 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 8.22 Example of a walking simulator test as- 8.4.4 Outdoor Gloves sembly [28]. Gloves have been made with ePTFE membranes to Figure 8.23 The setup for footwear wicking test [28]. protect hands from wind and keep them cool and dry during activity (Fig. 8.24). Less moisture is trapped in the insulation, so it remains drier thus keeping hands warmer. According to W. L. Gore the gloves provide enduring weather protection and personal comfort, balanced heat transfer, and optimum mois- ture managementdeven in harsh conditions. Hands stay warmer when it is cold, and drier when as the wearer perspires. Gloves designed for sports such as skiing and hunting have specialized grips to enhance dexterity. 8.5 Protective Apparel Attempts have been made to utilize the properties of microporous PTFE membranes in developing protective apparel. An important consideration is the breathability characteristic of ePTFE membranes. This is a useful property for a broad range of uniforms for ﬁreﬁghters, policemen, military personnel, and other personnel. Yet the ability of water vapor to move through breathable fabrics im- plies other vapors and gases also could. This implies ePTFE fabrics are useful when protecting against liquid challenge as opposed to vapor challenge. Figure 8.24 Example of a breathable water repellent glove. Courtesy: W. L. Gore & Associates, www.GORE-TEX.com, January 2015.

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 189 First responders perform physically demanding (ASTM F903) with liquid splash protection. activities that increase the risk of heat stress. Multi- Clothing of this type is designed to protect the Threat suits have been developed that are light- wearer from liquid contact, but allows exposure to weight, ﬂexible, and give the wearer freedom of vapors. Permeation data is not appropriate for movement, increased range of motion, improved deciding material performance for the level of pro- peripheral visibility, and excellent dexterity. Wetting tection it provides [34]. the outer layer of this fabric reduces heat stress on the wearer by promoting evaporative cooling, allowing Penetration test procedures are speciﬁed in the wearer to remain engaged longer. An example of National Fire Protection Association (NFPA) this type of fabric, that is not breathable, is GORE 1992dStandard on Liquid Splash-Protective En- Chempak Ultra Barrier Fabric and suits [33]. These sembles and Clothing for Hazardous Materials suits are certiﬁed to NFPA 1994, Class 2 and NFPA Emergencies. These procedures are identical to 1992 (Table 8.3). those in ASTM F903, Procedure C. The penetration test measures the resistance of protective clothing Liquid splash protection is needed, but not vapor materials to penetration by liquids using a 1-h, one- protection, a certiﬁed liquid splash protective sided liquid exposure to the normal outside material ensemble that meets NFPA 1992 must be selected. surface. The test is conducted at atmospheric pres- These protective uniforms are selected for their sure and room temperature. During the sixth minute, capability to protect against a speciﬁc chemical the test is conducted at 13.8 kPa to simulate the based on penetration data (ASTM F903). Penetra- pressure from a burst pipe. Liquid penetration is tion is the bulk ﬂow of a liquid chemical through the detected visually at the end of the test. Penetration material, seams, or suit closures. NFPA 1992 asso- results are recorded as either “Pass” or “Fail” ciates liquid-tight integrity and penetration data (Table 8.4). Table 8.3 Certiﬁed Protection in CB Hot Zone Environments [33]. Requirement Multi-Threat Typical Results NFPA 1994, Class 2 ensemble overall function and integrity !2100 PPDFsys Man in simulant test Systemic physiological !361 PPDFsys (MIST) protective dosage factor !310 lbf !190 lbf/2 in (PPDFsys) >720 min Material performance >720 min Burst strength !35 lbf >480 min >480 min Seam break strength !15 lbf/2 in >480 min >480 min Chemical permeation Max level >480 min Chemical warfare agents <4.0 mg/cm2 60 min Mustard (HD) <1.25 mg/cm2 60 min Soman (GD) Toxic industrial chemicals <6 mg/cm2 60 min Dimethyl sulfate (DMS) <6 mg/cm2 60 min Acrolein <6 mg/cm2 60 min Ammonia (NH3) <6 mg/cm2 60 min Chlorine (Cl2) <6 mg/cm2 60 min Acrylonitrile

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190 EXPANDED PTFE APPLICATIONS HANDBOOK Table 8.4 Examples of Chemical Penetration Data [34]. Green (gray in print version)dThese chemicals represent liquid splash hazards as deﬁned by NFPA 1992 Standards. GORE Chemical Splash Fabric passes the penetration test for chemicals printed in green. Yellow (light gray in print version)dThese chemicals represent both potential vapor and liquid splash hazards3. GORE Chemical Splash Fabric passes the penetration test for chemicals printed in yellow. Signiﬁcant amounts of chemical vapor permeate this material. Red (dark gray in print version)dDo not usedGORE Chemical Splash Fabric fails the penetration test for chemicals printed in red. 8.6 Summary [3] www.gore.com, December 2015. [4] Introduction to Polyurethane Technology, Mad- This chapter describes applications of expanded membrane of PTFE in fabrics aimed at apparel. The ison Chemical Industries, December 2015. great advantage of ePTFE as a porous material is its www.madisonchemical.com. ability to block liquid water but allow water vapor [5] J.A. Dillon, M.E. Dillon, U.S. Patent 5,362,553, through. The membrane is mechanically strong and Assigned to Tetratec Corp, November 8, 1994. possesses the basic properties of PTFE resin. ePTFE [6] R.J. Klare, D.E. Chubin, U.S. Patent 6,228,477, has found use in nearly every type of apparel ranging Assigned to BHA Technologies, Inc., May 8, from outdoor garments to protective gear to medical 2001. uniforms. Technology development for modifying [7] REI Co-Op, http://www.rei.com/learn/expert- the basic ePTFE membrane for manufacture of new advice/rainwear-how-it-works.html, Image and improved apparel products has continued nearly courtesy of Dr. Philip Gibson, December 2015. half a century after its discovery. [8] S. Ebnesajjad, C.F. Ebnesajjad, Surface Treat- ment of Material for Adhesive Bonding, second References ed., Elsevier, Oxford, UK, 2014. [9] R.J. Klare, U.S. Patent 6,854,603, Assigned [1] R.W. Gore, U.S. Patent 4,194,041, Assigned to to BHA Technologies, Inc., February 15, W.L. Gore Associates, March 18, 1980. 2005. [10] J. DeYoung, R.J. Klare, U.S. 7,407,703, [2] W.L. Gore & Associates, www.GOREprotective Assigned to BHA Technologies, Inc., August 5, fabrics.com, January 2016. 2008.

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8: EXPANDED PTFE USE IN FABRICS AND APPAREL 191 [11] J.G. Donovan, U.S. Patent 4,302,496, Assigned [22] L.A. Ford, U.S. Patent 8,637,144, Assigned to to Albany International Corp, November 24, W.L. Gore & Associates, January 28, 2014. 1981. [23] L.A. Ford, U.S. Patent 9,040,646, Assigned to [12] D.J. Gohlke, U.S. Patent 4,344,999, Assigned to W.L. Gore & Associates, May 26, 2015. W. L. Gore Associates, August 17, 1982. [24] L.A. Ford, U.S. Patent 9,193,811, Assigned to [13] D. Worden, F.T. Wilson, L.J. Grubb, U.S. Patent W.L. Gore & Associates, November 24, 2015. 4,443,511, Assigned to W.L. Gore & Associates, April 17, 1984. [25] Place Textiles, January 2016. http://placetextiles. com/abrasion-testing. [14] H. Nomi, U.S. Patent 4,692,369, Assigned to Japan GORE-TEX®, September 8, 1987. [26] W.L. Gore & Associates, www.GORE-TEX. com/en-us/experience/quality/testing- [15] R.L. Henn, D.J. Sakhpara, C.E. Bailey, J.J. outerwear, January 2016. Bowser, P.L. Brown, U.S. Patent 4,961,985, Assigned to W.L. Gore & Associates, October 9, [27] J. DeNardis, FR Garment Comfort e Explaining 1990. the Mystery, DuPont Protection Technologies, 2014. [16] M.E. Carr, B. Parker, W.F. McNally, S. Chandra, V. Naik, J.M. Furey, U.S. 2005/0196603, Mer- [28] W.L. Gore & Associates, www.GORE-TEX. chant & Gold PC, September 8, 2005. com/products/footwear/, January 2016. [17] W.K. Jung, H.C. Koo, K.W. Kim, S. Shin, [29] G. Sacre, U.S. Patent 4,599,810, Assigned to S.H. Kim, Y.H. Park, Antibacterial activity and W.L. Gore & Associates, July 15, 1986. mechanism of action of the silver ion in Staph- ylococcus aureus and Escherichia coli, App [30] R.J. Wiener, U.S. Patent 6,935,053, Assigned to Environ. Microbiol. 74 (April 2008) 7. W.L. Gore & Associates, August 30, 2005. [18] Noble Biomaterials, January 2016. http:// [31] A.W. Jessiman R.J. Wiener, U.S. Patent noblebiomaterials.com. 8,296,970, Assigned to W.L. Gore & Associates, October 30, 2012. [19] R.L. Henn, U.S. Patent 4,532,316, Assigned to W.L. Gore & Associates, July 30, 1985. [32] A.W. Jessiman, R.J. Wiener, U.S. Patent 8,607,476, Assigned to W.L. Gore & Associates, [20] R.L. Henn, C.H. Morell, E.J. Daniel, U.S. Patent December 17, 2013. 5,026,591, Assigned to W.L. Gore & Associates, June 25, 1991. [33] Chempak® Fabric, Multi-threat Suit, Published W. L. Gore & Associates, www.GORE.com, [21] J. Reaney, U.S. Patent 5,162,149, Assigned 2009. to W.L. Gore & Associates, November 10, 1992. [34] Chemical Splash Fabric-Technical Data and Application Guide, Published by W. L. Gore & Associates, www.GORE.com, 2009.

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9 Medical and Surgical Applications of Expanded PTFE OUTLINE 9.1 Introduction 193 9.7.1 Vascular Grafts 198 9.2 Deﬁnition of Medical Devices 9.3 Classiﬁcation of Devices 193 9.7.2 Patches 202 9.4 Designing Medical Devices 9.7.3 Expanded Polytetraﬂuoroethylene 9.5 Biomaterials 9.6 Expanded Polytetraﬂuoroethylene 195 Lipoatrophy Implants 202 9.7 Examples of Applications 196 9.7.4 Expanded Polytetraﬂuoroethylene Sutures 204 9.7.5 Lead Assembly of Implanted Devices 205 196 9.7.6 Stents 207 196 References 209 198 9.1 Introduction 9.2 Deﬁnition of Medical Devices Most people have beneﬁted from a medical device What is the deﬁnition of a modern medical device? while receiving health-care services. Common in- The most authoritative source for the deﬁnition of struments including thermometers, blood pressure medical devices is the United States Food and Drug gauges, syringes, blood bags, forceps, scalpels, Administration (FDA) and the World Health Orga- catheters, stents, and titanium rod implants to repair nization (WHO). In Europe the situation is more bone fractures are all considered medical devices. In complex because different countries have their own spite of their ostensible simplicity, many of these regulations. The European Commission has been devices are highly regulated by government and moving towards a uniﬁed set of regulations. The FDA industry agencies. provides the following comprehensive deﬁnition of medical devices: The history of the use of medical devices dates back to ancient Egypt. The goddess Sakhmet (also Medical devices range from simple tongue depres- spelled Sekhmet or Sachmis) was endowed with sors and bedpans to complex programmable pace- medicinal healing powers. She appeared in the form makers with microchip technology and laser of a lioness, with a sun disk adorned with a cobra on surgical devices. In addition, medical devices her head. Her cult was strong between 1800 and 2000 include in vitro diagnostic products, such as BC in Upper Egypt (in the south). Inscriptions on the general-purpose lab equipment, reagents, and test wall of the Temple of Kom Ombo (Fig. 9.1) depict kits, which may include monoclonal antibody ancient Egyptian medical instruments. The devices in technology. Certain electronic radiation emitting these inscriptions include scalpels, curettes, forceps, products with medical application and claims a dilator, scissors, medicine bottles, hooks, drills, meet the deﬁnition of medical device. Examples forceps, pincers, spoons, scales, saws, and a vase with include diagnostic ultrasound products, X-ray burning incense. Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00009-2 193 Copyright © 2017 Elsevier Inc. All rights reserved.

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194 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 9.1 Ancient Egyptian medical instruments depicted in inscriptions on the wall of the Temple of Kom Ombo. Photo: courtesy Mrs. Ghazale Dastghaib. machines, and medical lasers. If a product is This deﬁnition provides a clear distinction be- labeled, promoted, or used in a manner that meets tween a medical device and other FDA- the following deﬁnition in Section 201(h) of the regulated products such as drugs. If the primary Federal Food Drug and Cosmetic Act it will be intended use of the product is achieved through regulated by the Food and Drug Administration chemical action or by being metabolized by the as a medical device and is subject to premarketing body, the product is usually a drug. Human and postmarketing regulatory controls. A device is: drugs are regulated by FDA’s Center for Drug Evaluation and Research (CDER). Biological an instrument, apparatus, implement, machine, products that include blood and blood products, contrivance, implant, in vitro reagent, or other and blood banking equipment are regulated by similar or related article, including a component FDA’s Center for Biologics Evaluation and part, or accessory which is: Research (CBER). FDA’s Center for Veterinary Medicine (CVM) regulates products used with recognized in the ofﬁcial National For- animals. If a product is not a medical device mulary, or the United States Pharmaco- but regulated by another Center in the FDA, poeia, or any supplement to them [1], each component of the FDA has an ofﬁce to assist with questions about the products they intended for use in the diagnosis of dis- regulate [2]. ease or other conditions, or in the cure, mitigation, treatment, or prevention of It is evident that the term “medical device” disease, in man or other animals, or covers a wide range of tools and instruments used for healthcare. According to the WHO, there are intended to affect the structure or any roughly 10,000 medical devices available for use function of the body of man or other an- in healthcare today. The complexity of each imals, and which does not achieve its device depends on its functional use. Devices do primary intended purposes through not all come in direct contact with the human body, chemical action within or on the body and many serve important purposes indirectly. of man or other animals and which is The selection of appropriate medical equipment not dependent upon being metabolized for the achievement of any of its pri- mary intended purposes.

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 195 always depends on local, regional, and national Table 9.1 Sixteen Medical Specialties (Panels) and requirements. Factors to consider include the Regulatory Codes [4] type of health facility where the devices are to be used, the health workforce available, and the Regulation Panel burden of disease experienced in that Citation Anesthesiology speciﬁc geographic area. It is therefore impossible Code Cardiovascular to draw a comprehensive list of core medical 868 Clinical chemistry and clinical equipment [3]. 870 862 toxicology Because most medical devices that contain Dental ePTFE come into physical contact with patients, this 872 chapter will focus on those devices. 874 Ear, nose, and throat 876 Gastroenterology and urology 9.3 Classiﬁcation of Devices 878 General and plastic surgery 880 General hospital and personal use There are a variety of ways to classify medical 864 devices. One way to classify them is according to the 866 Hematology and pathology regulatory status of the devices. The FDA has 882 Immunology and microbiology established classiﬁcations for approximately 1700 884 different generic types of devices and grouped them 886 Neurology into 16 medical specialties referred to as panels. Each 888 Obstetrical and gynecological of these generic types has been assigned to one of 890 three regulatory classes based on the level of control 892 Ophthalmic necessary to ensure the safety and effectiveness of the Orthopedic device. The three classes and the requirements that Physical medicine apply to them are as follows: Radiology 1. Class I: General controls The in vivo (or living) environment of the human body exposes devices to a variety of relatively harsh 2. Class II: General controls and special controls factors that could result in a device’s deterioration. These factors include blood coagulation, corrosion, 3. Class III: General controls and premarket acid ambience (in the stomach), protein coating, approval vibration, and environmental stress cracking. Vibra- tion is not intuitive, but because of heartbeats the lead This classiﬁcation is risk-based, in that the risk a assembly of a pacemaker is subjected to approxi- device poses to the patient and/or the user is a major mately 400 million ﬂex cycles over a period of factor in determining to which class it is assigned. 10 years [5]. Class I includes devices with the lowest risk and those with the greatest risk are assigned to Class III. A device’s degree of invasiveness is another Table 9.1 shows the 16 medical specialties, or panels, measure of classiﬁcation. A device that penetrates the and the applicable codes for FDA regulatory body in whole or in part, either through a body purposes. oriﬁce, or through the body surface, is called invasive. Because of the impact of time on device perfor- mance, the duration of patient exposure to the device There is a separate category for surgically invasive is an important consideration in classiﬁcation: devices, which are invasive devices that penetrate the body in the context of a surgical operation. A surgi- Transient device: Normally intended for contin- cally invasive device always enters the body through uous use for less than 60 min. an artiﬁcially created opening. Short-term device: Normally intended for contin- An implantable device is totally introduced into uous use for not more than 30 days. the human body or replaces an epithelial surface, Long-term device: Normally intended for contin- uous use for more than 30 days.

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196 EXPANDED PTFE APPLICATIONS HANDBOOK or the surface of the eye, by surgical intervention. adversely impacting the adjacent tissues and will not Any implantable device remains in place after the interfere with the systemic functions of the human or procedure. Any device that is introduced into the animal body. human body through surgical intervention and is intended to remain in place after the procedure for Parts of medical implants, extracorporeal de- at least 30 days is also considered an implantable vices, and disposables utilized in every aspect of device [5]. patient healthcare, from medicine, surgery, and dentistry to veterinary medicine, are all composed 9.4 Designing Medical Devices of biomaterials [11]. Biomaterials can be con- structed from metals, ceramics, polymers, or natu- When specialists are designing and developing ral matter. In general, biomaterials must not react medical devices, they need to take several factors with bodily tissues, must be nontoxic, and in the into consideration when selecting a potential case of permanent implants they must not be material. First, it is important to understand the biodegradable. component or device’s intended use. Clearly, the requirements for nondisposable devices such as Biomaterials include not only substances implan- diagnostic equipment or imaging parts will be quite ted inside the human body but also any nondrug different from those for a disposable blood bag, material that comes in contact with living tissues. A which will be very different again from those for a simple example of such a biomaterial is latex, which permanent spinal implant. Physical and mechanical is used to make gloves worn by medical personnel. properties, thermal and electrical properties, Latex is known to cause allergic reactions and is chemical and sterilization resistance, biocompati- therefore not an ideal biomaterial. bility, and joining and welding capabilities are just some of the criteria that must be evaluated in the Although biomaterials are primarily used for selection of the appropriate plastic material [6]. medical applications (the focus of this chapter), they There are a number of excellent sources for the are also used to grow cells in culture, to assay for study of medical device design [6e9]. blood proteins in the clinical laboratory, for implants to regulate fertility in cattle, and for investigational 9.5 Biomaterials cell-silicon “biochips” as well as in equipment for processing biomolecules for biotechnological appli- The consensus deﬁnition of a biomaterial in the cations, in diagnostic gene arrays, and in the aqua- professional literature is as follows: “A biomaterial is culture of oysters. The common thread is the a nonviable material used in a medical device, interaction between biological systems and synthetic intended to interact with biological systems.” How- or modiﬁed natural materials [12]. ever, more expansive deﬁnitions and descriptions of biomaterials that provide additional information are 9.6 Expanded more useful to plastics practitioners. Polytetraﬂuoroethylene According to Merriam-Webster (www.merriam- Expanded polytetraﬂuoroethylene (PTFE) is an webster.com), a biomaterial is a natural or synthetic exemplary biomaterial. In addition to possessing the material (as a metal or polymer) that is suitable for basic properties of PTFE, ePTFE products offer a introduction into living tissue especially as part of a number of beneﬁcial properties. The combination of medical device (as an artiﬁcial joint). The medical these two sets of properties makes possible a vast dictionary [10] deﬁnes a biomaterial as any substance number of applications for medical devices contain- (other than a drug), synthetic or natural, that can be ing ePTFE. Table 9.2 (which is the same as Table 7.1) used as a system or part of a system that treats, lists the basic properties of ePTFE, a signiﬁcant augments, or replaces any tissue, organ, or function number of which are required in medical devices. of the body, especially material suitable for use in prostheses that will be in contact with living tissue. Shaped ePTFE membranes are useful in the con- Clearly, an ideal biomaterial will function without struction of a wide variety of medical accessories including wound-care materials, face masks, trans- ducer protectors, ostomy bags, urine bags, drainage bags, medical device enclosures, vent caps, IV

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 197 Table 9.2 Important Properties of Expanded administration sets, spike vents, surgical smoke ﬁl- Polytetraﬂuoroethylene ters, and suction ﬁlters. Porosity (low and high) ePTFE grafts exhibit certain in vivo behaviors that High speciﬁc surface area are important for their use as graft materials. For High chemical resistance in harsh environments example, an abdominal wall hernia is a protrusion of Chemical inertness of polytetraﬂuoroethylene the intestine through an opening or area of weakness surface in the abdominal wall. Treatment involves surgery to High thermal stability repair the hernia using a patch [13]. One of the sur- Resistance to ultraviolet rays gical patches used in surgery is a composite of Excellent weatherability outdoors ePTFE and polypropylene ﬁber. The side facing the Low coefﬁcient of friction wall of the abdominal cavity is constructed of two Low water adsorption (0.04% at room temperature) layers of monoﬁlament polypropylene mesh, inten- Low ﬂammability (limiting oxygen index is 95%) ded to provide rapid tissue in-growth and strong Performance at extreme low and high incorporation into the abdominal wall. The visceral temperatures side is made of submicron ePTFE that provides a Strength (high strength-to-weight ratio) permanent barrier, minimizing tissue attachment (or Low dielectric constant (2.0) adhesion) (Fig. 9.2). Low loss coefﬁcient Biocompatibility Conversely, because ePTFE is so versatile its surface can be made favorable to tissue in-growth or endothelialization. One of the bigger challenges to using biomaterial implants effectively is inducing a natural regenerative and healing response within the human body. This is a challenge for prosthetic replacements of small diameter blood vessels in humans. In the majority of cases, ePTFE is a Figure 9.2 Surgical implantation of Ventrio Hernia Patchdtop photo shows two different sizes of patch. Courtesy: C. R. BARD Davol Inc., www.davol.com.

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198 EXPANDED PTFE APPLICATIONS HANDBOOK desirable substitute for blood vessels of 6 mm in several advantages over polyester. PTFE’s surface is diameter. The voids in the ePTFE membrane provide less thrombogenic (causing coagulation of the blood) the ideal space for tissue growth. One strategy for than that of woven polyester. With regard to tissue inducing more natural tissue and capillary in-growth reaction, PTFE is preferable to polyester because is to embed within the implant pro-angiogenic mol- while PTFE can cause a low-grade tissue reaction, ecules (encouraging blood vessel growth) that will polyester more readily induces an inﬂammatory mimic the body’s natural healing response. Signiﬁ- tissue reaction and granulation. As far as dimen- cant research has been conducted on identifying and sional stability is concerned, PTFE is better than reﬁning key biomolecules that can be embedded into polyester because the latter tends to dilate more than ePTFE implants to encourage capillary and tissue in- PTFE does. Despite several advantages claimed by growth [14e16]. ePTFE, studies regarding abrasion resistance show a much lower resistance of these ePTFE fabrics ePTFE is used in vascular, cardiovascular, cardiac, when compared with polyethylene terephthalate orthopedic, neurological, and general surgical pro- grafts [19]. cedures. Medical products made of ePTFE have been implanted in millions of people. Because of its 9.7 Examples of Applications physical attributes, porousness, and strength, spe- cialists have developed a signiﬁcant number of ap- The use of ePTFE in medical applications owes a plications for ePTFE. ePTFE can be formed into great deal to W.L. Gore and Associates. While there many shapes including ribbon, tape, multilumen are many other companies that produce innovative tubing, monoﬁlament, grafts, sheets, patches, sutures, products based on ePTFE, W.L. Gore has been the and customized forms such as implants. ePTFE is pioneer in the ﬁeld. The company began introducing soft and ﬂexible, microporous, air permeable, and medical products made with ePTFE in the 1970s, and ﬂuid impermeable, and it has a low dielectric con- over the decades W.L. Gore has played a vital role in stant [17]. developing and increasing the usage of vascular grafts in medical procedure [20]. Today Gore Medi- Table 9.3 lists some of the venting and ﬁltration cal offers the broadest range of medical ePTFE-based applications of ePTFE membranes. products available. Examples of the variety of W.L. Gore’s medical products can be seen at www.gore. Although polyester is another biomaterial com/en_xx/products/medical/index.html. commonly used in medical applications, PTFE has A large number of medical conditions are treated Table 9.3 Venting and Filtration Applications of with devices containing ePTFE. These treatment Expanded Polytetraﬂuoroethylene Membranes [18] areas span a variety of medical specialties. Tables 9.4 and 9.5 summarize these medical conditions treated IV ﬁlter and venting components with ePTFE devices. Medical pumps and sensor vents 9.7.1 Vascular Grafts Drainage bag vents ePTFE has been in use longer than any other material in a variety of grafts for vascular repair Surgical suction ﬁlters (Fig. 9.3). Vascular grafts are intended either to replace or bypass the body’s own diseased or Surgical smoke ﬁltration damaged vessels. The applications range from the aorta through the peripheral vascular system and into Laboratory ﬁlters the carotids. Vascular grafts made with ePTFE are manufactured in a wide range of stretch and non- Renal dialysis transducers stretch conﬁgurations, including straight, tapered, and bifurcated, and many have external customizable Microﬁltration cartridges (liquid and sterile air) ring reinforcement. FEP (ﬂuorinated ethylene pro- pylene polymer) rings are used to prevent kinking in Syringe ﬁlters IV ﬁlter vents Transducer protectors Medical pumps and sensor vents Fluid reservoir vents Protective gaskets

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 199 Table 9.4 Examples of Medical Conditions Treated Figure 9.3 Stretch vascular grafts made from Gore- With Devices Containing Expanded Polytetraﬂuoro- Tex expanded polytetraﬂuoroethylene. ethylene in Their Structures Courtesy: Gore Medical, W.L. Gore and Associates, www.GoreMedical.com. Abdominal aortic aneurysm Abdominal wall reconstruction Figure 9.4 Kink-resistant expanded polytetraﬂuoro- Atrial septal defects ethylene vascular grafts using removable ﬂuorinated AV access and dialysis ethylene propylene polymer rings. Biliary disease Cardiac ePTFE vascular grafts (Fig. 9.4). To protect vascular Cardiovascular disease grafts against dilatation and aneurysm formation, a Carotid artery stenosis two-layer wall structure has been devised (as seen in Colon disease Fig. 9.5). Embolic protection Heart defect Blood is naturally compatible with vascular Hernia endothelium (which lines the interior surface of Lung disease blood vessels), but not with artiﬁcial surfaces. Part Obesity of the body’s defensive response is to coat a foreign Peripheral disease (PAD) (PVD) object introduced into the body with blood clots. Portal hypertension and liver disease ePTFE vascular grafts are no exception, thus a so- Spinal disc disease lution is required to increase the useful life of the Stroke Thoracic aortic aneurysm Trauma Table 9.5 Some of the Medical Specialty Areas That Use Devices With Expanded Polytetraﬂuoroethylene in Their Structures Bariatric surgery Cardiovascular Colorectal surgery Endovascular and interventional Gastroenterology General surgery Hernia Interventional cardiology Interventional radiology Pediatric cardiovascular Peripheral disease (PAD) (PVD) Spine surgery Thoracic surgery Vascular surgery

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200 EXPANDED PTFE APPLICATIONS HANDBOOK 12 34 Heparin Antithrombin Thrombin Thrombin-antithrombin complex Figure 9.7 Working mechanism of CARMEDA BioActive Surface heparin chemistry [23]. Figure 9.5 Two-layer structure of vascular grafts to treatment of thrombosis. Pharmaceutical grade hep- prevent formation of dilatation and aneurysm [21]. arin is derived from the mucosal tissues of animals slaughtered for meat, such as pig intestines or cow Figure 9.6 Scanning electron micrograph of an lungs. expanded polytetraﬂuoroethylene graft with heparin immobilized on its inside surface (it is in contact Fig. 9.6 shows a scanning electron micrograph of blood) [22]. the surface of an ePTFE vascular graft that has been treated with immobilized heparin. The typical node- grafts. Grafts made with ePTFE are treated with ﬁbril microstructure of ePTFE is evident, indicating heparin, which is bonded to the surfaces of ﬁbrils that the heparin immobilization does not alter the and nodes to solve the problem. The surface of ePTFE microstructure. nearly every ePTFE graft is rendered non- thrombogenic using a bioactive coating such as Procedures have been developed to bond heparin heparin. to ePTFE nodes and ﬁbrils. For example, CAR- MEDA BioActive Surface (CBAS) is a durable, Heparin is a natural compound found in the liver nonleaching heparin-coating technology with end- and other tissues that inhibits blood coagulation. It is point attachment that enhances blood compati- a sulfur-containing polysaccharide (sulfated glycos- bility and provides thromboresistant blood- aminoglycan) that works as an anticoagulant in the contacting surfaces for cardiopulmonary bypass circuit devices. CBAS is offered by CARMEDA, which is a subsidiary of W.L. Gore [23]. From pediatric patients to adults, CBAS is an important component of routine as well as complex circula- tion procedures that take place outside the human body. The ePTFE vascular grafts offered by Gore Medical and other companies require no preclotting treatment because they are coated with surface- immobilized heparin (CBAS) and resist dilatation and the spread of infection as well as thrombosis (blood clotting). CARMEDA’s proprietary CBAS chemistry mimics the endothelial function, as seen in Fig. 9.7. Antithrombin binds to a speciﬁc structure in the heparin molecule (1). The binding involves a minor conformational change (2) that speeds up the inactivation of thrombin and other coagulation agents (3). Finally, the complex formed between the

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 201 antithrombin and thrombin is released, re-exposing severe kidney dysfunction. This operation cleans the the heparin molecule and thereby sustaining the blood by removing it from the body and passing it anticoagulant activity (4). It is important that CBAS through a dialyzer, or artiﬁcial kidney. The blood is heparin coating is not consumed, and thus it lasts for accessed through a large aorta using a catheter, an many years [23]. arteriovenous graft (AV), or an arteriovenous ﬁstula (AVF) graft. The preferred device is the AVF graft Procedures have been developed for bonding because of its overall effectiveness, durability, and bioactive agents directly to the polymer backbone of low rate of complication (clotting, infection, etc.). A an implantable device. For example, heparin has been ﬁstula directly connects an artery to a vein. Once the covalently linked to the amino groups of a ﬂuorinated ﬁstula has been created, it is a natural part of the body polyurethane coating [24]. These coating reactions (Fig. 9.8). were carried out directly on the device’s surface. The disadvantage was decreased bioactivity and, conse- The schematic in Fig. 9.8 indicates the entry of quently, increased thrombogenicity. The solution has needles in the AVF graft, thus requiring it to be self- been to position heparin at a distance from the sub- sealing. This is achieved by placing an elastomeric strate surface so that it will interact optimally with layer over the ePTFE layer, followed by an overlay of blood. This solution has been applied to ePTFE grafts. another ePTFE layer as shown in Fig. 9.9. The surface of ePTFE is extremely hydrophobic Stretch vascular grafts have some advantages over and therefore requires modiﬁcation before heparin standard grafts [31]. Stretch grafts have better can be bonded to its surface. In spite of the resistance handling characteristics, kink resistance, and con- of polytetraﬂuoroethylene to most common surface formability. In addition, the “stretch” feature is modiﬁcation techniques, effective plasma treatment thought to confer ease of sizing and anastomotic techniques have been developed for PTFE [25e28]. (connections between blood vessels) accuracy. Plasma treatment does not affect the node and ﬁbril Fig. 9.10 shows some of the features of stretch structures of ePTFE grafts but imparts hydrophilic vascular grafts. groups to the surfaces of the grafts to allow for binding of the heparin coating. Vein Artery Graft Patnaik et al. [26] developed a technique in which Graft a bioactive coating (like heparin) containing a connection polymer backbone bonds, via an amide or amine chemical bond, to one end of a hydrophilic, amine- Venous Arterial terminated spacer that has at least one amine group needle needle at its ﬁrst and second ends. The heparin molecule is covalently bonded to the unreacted end of the hy- Figure 9.8 Schematic diagram for placement of an drophilic spacer. The hydrophilic spacer is repelled arteriovenous ﬁstula graft in a human arm [29]. by the hydrophobic surface of the medical device in such a way that the heparin molecule is extended away from the hydrophobic surface (Fig. 9.7). This technique may also be used to bond antibiotic agents, antibacterial agents, and antiviral agents to various surfaces. Once the plasma-primed substrate surfaces of ePTFE grafts are contacted (or coated) with a bioactive coating, pressure is applied to the surface to force the coating into the interstices of the graft. Pressure or force can be applied by different me- chanical means. The grafts are allowed to dry prior to sterilization and eventual introduction into the body. Naturally, the bioactive coatings must be able to withstand these various treatments. Another example of the application of ePTFE vascular grafts is in hemodialysis for patients with

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202 EXPANDED PTFE APPLICATIONS HANDBOOK ePTFE with congenital heart defects, with coronary Self-sealing elastomer artery bypass grafts, in young patients requiring ePTFE valve replacement, and in ventricular assist device Figure 9.9 Generic depiction of the construction of implantation. an arteriovenous ﬁstula graft [30]. There is a high probability that patients who un- Figure 9.10 Features of stretch vascular graft [32]. dergo repair of congenital heart defects will eventu- ally need a repeat operation. Using a pericardial 9.7.2 Patches membrane substitute can preclude adhesion forma- tion, thereby helping to avert the complications of a Patches are used in reconstructive and cardiac repeat sternotomy [33]. Pericardial membrane sub- procedures, for example, in pericardial membranes, stitutes made from ePTFE, such as W.L. Gore’s to minimize tissue adhesions. Surgeons might Preclude, are effective solutions for minimizing tis- perform pericardial membrane repair in patients sue attachment to the material (Fig. 9.11). They are used for cardiac reconstructions or repair within the pericardial space in procedures with a likelihood of reoperation [34]. Other examples of patch use include cardiovas- cular patches that, thanks to their strength, resist aneurysmal dilatation. Cardiovascular patches are also implanted to “seal” around the penetrating suture to decrease suture line bleeding, impres- sively, by almost 90% (Fig. 9.12). This improve- ment is accompanied by enhanced suture retention, multidirectional strength, and low thrombogenicity. Traditional treatments of diseases of the thoracic aorta have involved high-risk surgeries and large incisions in the chest to place synthetic grafts to repair diseased arteries. These surgeries result in long hospital stays and painful recoveries [36]. Patches now enable surgeons to treat these diseases with minimally invasive procedures. Patches are used in thoracic aortic aneurysm repair with endografts. The W.L. Gore TAG endoprosthesis, a nitinol-supported ePTFE tube graft, has proven to be the best device for this application since it gained the approval of the US FDA for general use in March 2005 (Fig. 9.13) [37,38]. 9.7.3 Expanded Polytetraﬂuoroethylene Lipoatrophy Implants Facial fat is distributed naturally throughout the face as subcutaneous fat, in fat pockets around the orbits, and in buccal (inside lining of the cheeks) and temporal fat pads. Facial lipoatrophy is char- acterized by fat loss throughout the face, most commonly in the cheeks, temples, and nasolabial regions. A number of HIV-infected patients experi- ence facial lipoatrophy. While it is not a typically

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 203 Figure 9.11 An example of Gore’s Preclude, made from expanded polytetraﬂuoroethylene, used for pericardial membrane repair [34]. Figure 9.12 Gore ACUSEAL cardiovascular ﬁlling agents to treat HIV facial lipoatrophy. Im- patch [35]. plants composed of low-porosity ePTFE materials may be preferable to solid silicone or high-density life-threatening syndrome, it can lead to comorbid- porous polyethylene for correction of HIV lipoa- ities and is one of the most stigmatizing complica- trophy since capsule formation and ﬁbrous tissue in- tions of HIV. Facial lipoatrophy is believed to be an growth are limited with ePTFE, making the implants adverse effect of antiretroviral therapy [39]. easier to remove should facial fat recover over time [40e42]. There are a variety of invasive and noninvasive procedures to treat lipoatrophy. Implantation pro- ePTFE implants have also been used successfully cedures, such as those using ePTFE, require surgery for augmentation of thinning lips; deep nasolabial under general anesthesia. Subcutaneous augmenta- folds; and marionette lines without the risk, recovery tion material implants manufactured from ePTFE time, and expense of major surgery. Fig. 9.14 shows have been used alone and in concert with soft-tissue Advanta ePTFE implants produced by Atrium Med- ical Corp (www.atriummed.com). Advanta features a dual-porosity conﬁguration that differentiates it from the uniform porosity that characterizes other ePTFE implants. Advanta ePTFE implants undergo a sintering process that heats the material and stabilizes the dimensions and porosity. This heating process en- hances to the physical properties and decreases implant shrinkage and stretching. While other available ePTFE implants consist of a uniform structure of 20 mm, the Advanta ePTFE implant has a dual porosity conﬁguration (Fig. 9.15). Cross- sectionally, the Advanta implant has a smooth, medium-porosity outer core of 50 mm and a soft, high-porosity inner core of 100 mm. The softer, more porous construction improves cellular inte- gration with less inﬂammatory response and less rigid encapsulation.

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204 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 9.13 The Gore TAG expanded polytetraﬂuoroethylene thoracic endoprosthesis [36]. Figure 9.14 Advanta expanded polytetraﬂuoroethy- Figure 9.15 Advanta ePTFE dual-porosity conﬁgu- lene implants are available in round (top four ration for implantation [44]. strips) or oval (bottom three strips) shapes in various diameters [43]. The porous microstructure of the ePTFE suture allows tissue attachment while ePTFE’s inertness and 9.7.4 Expanded biocompatibility result in benign tissue response to Polytetraﬂuoroethylene Sutures the suture. Consequently, the usage of ePTFE sutures results in less capsule formation and less inﬂamma- The choice of suture has a signiﬁcant impact on tion as compared to polypropylene and polyester achieving an optimal soft tissue approximation, sutures [46,47]. Fig. 9.17 compares a fresh medical particularly where reduced capsule formation, cord with a 6-month-old suture in the mitral valve of inﬂammation, and suture hole leakage are key con- a sheep. The latter exhibits complete endotheliali- siderations [45]. ePTFE sutures such as GORE-TEX zation, or tissue encapsulation of the suture by are microporous and nonabsorbable monoﬁlaments. in-growth into the structure of the ePTFE. Epi- The structure of the ePTFE suture is similar to the cardiography indicated complete mitral valve repair. structure of the materials used in patches and vascular grafts (Fig. 9.16).

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 205 Figure 9.16 The GORE-TEX suture is a nonabsorb- 9.7.5 Lead Assembly of Implanted able expanded polytetraﬂuoroethylene suture [45]. Devices Different types of medical electrical leads for use in cardiac rhythm management systems have been developed. Those leads typically extend intravascu- larly to an implantation location within or on a pa- tient’s heart, and coupled to a pulse generator or another implantable device for sensing cardiac elec- trical activity or delivering therapeutic stimuli. The leads must be highly ﬂexible to accommodate the patient movement, yet have minimized size. At the same time, the leads are exposed to various external forces such as the human muscular/skeletal system, the pulse generator, other leads, and surgical in- struments used during implantation and explanation procedures. This is the reason development to improve lead design has continued. An important application of ePTFE is as a part of the insulation coating structure on the lead assembly of some cardiac rhythm management (CRM) devices. Examples of CRM devices include implantable car- dioverter deﬁbrillator (ICD) and cardiac resynchro- nization therapy (CRT) device (seen in Fig. 9.18). ICD devices handle high voltage thus making use of ePTFE in their structures. (A) (B) Figure 9.17 Expanded polytetraﬂuoroethylene sutures in a sheep heart: a fresh medical cord (A), and after 6 months (B) [48].

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206 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 9.18 Example of cardiac resynchronization are inserted into the heart through a vein in the upper therapy device is St. Jude Medical’s Unify Quadra chest. The ICD will monitor the heart rhythm. If the equipped with quadripolar lead [49]. device detects an irregular rhythm in the ventricles, it will use low-energy electrical pulses to restore a An ICD is a small device that is placed in the chest normal rhythm. If the low-energy pulses do not or abdomen. Human heart has its own internal elec- restore the normal heart rhythm, the ICD will switch trical system that controls the rate and rhythm of the to high-energy pulses for deﬁbrillation. The device heartbeat. Doctors, thus, use the device to help treat also will switch to high-energy pulses if the ventricles irregular heartbeats called arrhythmias using electric start to quiver rather than contract strongly. The high- pulses. The wires with electrodes on the ends are energy pulses last only a fraction of a second, but inserted into the heart through a vein in the upper they can be painful [52]. chest. Fig. 9.19 shows the location and general size of an ICD in the upper chest. A number of CRM devices use ePTFE as a covering or as a part of the insulation layer of the lead An ICD has wires with electrodes on the ends that assembly. Sometimes other materials are combined connect to the heart chambers. Fig. 9.20 shows the with ePTFE to produce a better lead performance. A actual location and general size of an ICD in the common insulation structure consists of a laminate of upper chest. The wires with electrodes on the ends silicone rubber and ePTFE. Sufﬁcient thickness and dielectric strength can provide adequate electrical Figure 9.19 Location and general size of an implant- insulation from the local potential gradients [53]. able cardioverter deﬁbrillator in the upper chest [50]. Another ﬂuoropolymer ethylene tetraﬂuoroethylene copolymer (ETFE) has excellent insulation and physical properties. ETFE is used as a thin primary insulation in different implantable CRM devices. Typically, deﬁbrillating coils are covered with ePTFE to prevent tissue in-growth as shown in Fig. 9.21 and Table 9.6. The shocking coils allow for the internal deﬁbrillation of the heart with a high- energy current delivery from the generator if ventricular tachycardia (heart beat is too fast) is detected. ePTFE acts only as a partial insulation because of its porosity. It does prevent inﬂamma- bility and ﬁbrotic changes following the implanta- tion of devices [56]. Application of ePTFE in the construction of lead insulation has grown over the years. Direct contact of ePTFE with blood requires wet-out of its hydropho- bic surface to be made hydrophilic. The open-cell porous structure of ePTFE, when the pores are ﬁl- led with conductive ﬂuid, allows the lead to deliver deﬁbrillation energy through the pores. The ePTFE sheath is treated to accelerate the normally slow occurring wet-out process. An effective agent is gelatin, another protein (eg, collagen, albumin) or another similar material (eg, agar). Impregnation of the ePTFE can be accomplished by immersion/vac- uum, perfusion, or pressure processes. The gelatin does not require cross-linking and can dissolve away immediately. After the gelatin accomplishes its function to wet out the ePTFE it may be allowed to wash away from the surface as it dissolves into the blood [53].

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 207 Figure 9.20 Example of actual implantable cardioverter deﬁbrillator placed inside a patient (X-ray) [51]. Figure 9.21 Photograph of the distal portions of the There are two different types of stents namely self- Endotak Reliance 0157 and the Endotak Reliance G expandable and balloon-expandable. Self-expanding 0185 ICD leads (manufactured by Guidant, Inc., St stents have much lower radial strength in spite of Paul, MN, United States) [54]. their ﬂexibility and conformability. The balloon- expandable stents are highly precise and have 9.7.6 Stents excellent radial strength. These stents are suitable for calciﬁed vessels. For example, they are used to treat A basic stent is a tiny wire mesh tube (Fig. 9.22) that aortoiliac occlusive disease in which the iliac nar- can be expanded at the target spot. It has a ﬁne wire rows and stiffens because of aging. Iliac arteries may mesh structure called scaffolding that can pop open be narrowed or even blocked (Fig. 9.23). somewhat similar to a geodesic dome. It is placed in artery at the target point, propped open, and is left there Both types of stents may be made of nitinol, stain- permanently. When a coronary artery (an artery less steel, or ePTFE-covered metal. Covered stents feeding the heart muscle) is narrowed by a buildup of have somewhat stiffer scaffolding than uncovered de- fatty deposits called plaque, it can reduce blood ﬂow. If vices. Early on covered stents were used for the treat- blood ﬂow is reduced to the heart muscle, chest pain ment of a limited number of special vascular can result. If a clot forms and completely blocks the conditions. Today, covered stents (Fig. 9.24) are blood ﬂow to part of the heart muscle, heart attack broadly applied in the treatment of various stenotic results. Stents help keep coronary arteries open and lesions that is deﬁned as abnormal narrowing in a blood reduce the chance of a heart attack [57]. vessel. One common application is the treatment of cardiac diseases. Fig. 9.25 displays a depiction of an ePTFE-covered stent during placement in a vein. Gore Viabahn endoprosthesis is an ePTFE- covered stent graft. It is constructed with a durable, reinforced, biocompatible, ePTFE liner attached to an external nitinol scaffolding. The ePTFE surfaces are bonded to heparin covalently. The end-point co- valent bonding keeps the heparin anchored to the endoprosthesis surface in contrast to a physical coating that depletes in time. The bioactive site re- mains free to interact with the blood [61].

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208 EXPANDED PTFE APPLICATIONS HANDBOOK Table 9.6 Materials Structure of Two Styles of Implantable Cardioverter Deﬁbrillator (ICD) Leads [55] Material Endotak Reliance SG Endotak Reliance G External insulation DF-1 terminal pin Silicone rubber IS-1 terminal pin Titanium Pace/sense conductor Stainless steel Shocking conductor MP35N nickelecobalt alloy, Tip electrode (helix) polytetraﬂuoroethylene (PTFE) sleeve Coil electrode covering Drawn brazed strand cable, PTFE coated Steroid Compatibility Platinum iridium Expanded polytetraﬂuoroethylene Approximately 1.0 mg dexamethasone acetate Boston Scientiﬁc ICD pulse generators Figure 9.22 A stent is inserted into the clogged ar- Figure 9.23 Schematic of occlusion in iliac aorta [58]. tery to hold the artery open and allow blood to ﬂow more freely [57].

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 209 Figure 9.24 Examples of ePTFE-covered stents illustrating design ﬂexibility [59]. ePTFE, expanded polytetraﬂuoroethylene. Figure 9.25 Depiction of placement of a Gore Via- Unit B2 “Cosmetics and Medical Devices”, bahn endoprosthesis stent (ePTFE-covered) in a MEDDEV 2.4/1 rev 89, June 2010. partially occluded vein [60]. ePTFE, expanded [6] V. Sastri, Plastics in Medical Devices e Prop- polytetraﬂuoroethylene. erties, Requirements and Applications, in: Plas- tics Design Library, Elsevier, 2010. References [7] M.B. Privitera, Contextual Inquiry for Medical Device Design, ﬁrst ed., Academic Press, June [1] www.usp.org, August 2015. 2015. [2] www.fda.gov, http://www.fda.gov/Medical [8] P.J. Ogrodnik, Medical Device Design: Innova- tion from Concept to Market, ﬁrst ed., Academic Devices/DeviceRegulationandGuidance/ Press, December 2012. Overview/ClassifyYourDevice/ucm051512.htm. [9] R.C. Fries, Reliable Design of Medical Devices, [3] www.who.int, August 2015. third ed., CRC Press, September 2012. [4] http://www.fda.gov/MedicalDevices/Device [10] Medical Dictionary, MedlinePlus, US National RegulationandGuidance/Overview/Classify Library of Medicine, October 2015. www.nlm. YourDevice/ucm051530.htm, August 2015. nih.gov/medlineplus/mplusdictionary.html. [5] Medical Devices Guidance Document, Classiﬁ- [11] An Introduction to Tissue-Biomaterial In- cation of Medical Device, European Commis- teractions, John Wiley & Sons, 2002, pp. sion DG, Health and Consumer Directorate B, 165e214. [12] B.D. Ratner, A.S. Hoffman, F.J. Schoen, J. Lemons, Biomaterials Science, second ed., Elsevier, 2004. [13] D.A. Iannitti, W.W. Hope, V. Tsikitis, Strength of tissue attachment to composite and ePTFE grafts after ventral hernia repair, J. Soc. Laparoendo- scopic Surg. 11 (4) (OctobereDecember, 2007) 415e421, 2007. [14] E.S. Wijelath, J. Murray, S. Rahman, Y. Patel, A. Ishida, K. Strand, S. Aziz, C. Cardona, W.P. Hammond, G.F. Savidge, S. Raﬁi, M. Sobel, Novel vascular endothelial growth factor binding domains of ﬁbronectin enhance vascular endothelial growth factor biological activity, Circ. Res. 91 (2002) 25e31.

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210 EXPANDED PTFE APPLICATIONS HANDBOOK [15] E.S. Wijelath, S. Rahman, J. Murray, Y. Patel, [29] Mayo Foundation for Education and Research, G. Savidge, M. Sobel, Fibronectin promotes www.mayoclinic.org, September 2015. VEGF-induced CD34 cell differentiation into endothelial cells, J. Vasc. Surg. 39 (2004) [30] R. Bond, Advances in Access for Renal Patients, 655e660. Renal Society of Australia, September 2015. http://rsaannualconference.org.au. [16] E.S. Wijelath, S. Rahman, M. Namekata, J. Murray, T. Nishimura, Z. Mostafavi-Pour, [31] R. Chiesa, G. Melissano, R. Castellano, Y. Patel, Y. Suda, M.J. Humphries, M. Sobel, S. Frigerio, Extensible expanded PTFE vascular Heparin-II domain of ﬁbronectin is a vascular grafts for aortoiliac and aortofemoral recon- endothelial growth factor-binding domain: struction, Cardiovasc. Surg. 8 (7) (2000) enhancement of VEGF biological activity by a 538e544. singular growth factor/matrix protein synergism, Circ. Res. 99 (2006) 853e860. [32] Gore-Tex® Stretch Vascular Grafts, for Modiﬁed Blalock-Taussig Shunts, W. L. Gore & Associ- [17] Gore Medical, W. L. Gore and Associates, www. ates, July 2006. AF1569eEN4. GoreMedical.com, September 2015. [33] M. Loebe, V. Alexi-Meskhishvili, Y. Weng, [18] GE Life Sciences, www.gelifesciences.com, G. Hausdorf, R. Hetzer, Use of polytetraﬂuoro- October 2015. ethylene surgical membrane as a pericardial substitute in the correction of congenital heart [19] I.C.T. Santos, et al., Mechanical Properties of defects, Tex. Heart Inst. J. 20 (3) (1993). Stent-Graft Materials, October 2015. https:// web.fe.up.pt/wtavares/downloads/publications/ [34] Cardiac Surgical Products, Pub No. AK0363e artigos/JMDA226(4).pdf?. EN1, W. L. Gore and Associates, October 2006. [20] S. Ebnesajjad, Introduction to Fluoropolymers e [35] Gore Medical, Gore® Acuseal Patch, www. Materials, Technology and Applications, ﬁrst goremedical.com/acuseal/, October 2015. ed., Elsevier, Oxford, UK, 2013. [36] Gore Medical, Gore® TAG® Thoracic Endo- [21] Surgical Vascular Grafts, Gore Medical, www. prosthesis, www.goremedical.com/me/tagap/, goremedical.com, September 2015. October 2015. [22] P.C. Begovac, R.C. Thomson, J.L. Fisher, [37] J.-S. Cho, S.E. Haider, M.S. Makaroun, Endo- A. Hughson, A. Ga¨llhagen, Improvements in vascular therapy of thoracic aneurysms: GORE GORE-TEX vascular graft performance by TAG trial results, Semin. Vasc. Surg. 19 (1) Carmeda BioActive surface heparin immobili- (March 2006) 18e24. zation, Eur. J. Vasc. Endovasc. Surg. 25 (5) (May 2003) 432e437. [38] M.S. Makaroun, E.D. Dillavou, et al., Endovas- cular treatment of thoracic aortic aneurysms: [23] Technology Overview e Carmeda® BioActive results of the phase II multicenter trial of the Surface (CBAS®), www.carmeda.se/upl/ﬁles/ GORE TAG thoracic endoprosthesis, J. Vasc. 663.pdf, November 2006. Surg. 41 (1) (January 2005) 1e9. [24] C.B. Hu, D.D. Solomon, U.S. Patent 5,077,372, [39] J.S. Roth, Restorative Treatment Approaches for Assigned to Becton, Dickinson and Co., HIV-Associated Lipoatrophy, vol. 10, The PRN December 1991. Notebook®, September 2005. No. 3, www.prn. org. [25] S. Ebnesajjad, C.F. Ebnesajjad, Surface Treat- ment of Materials for Adhesive Bonding, second [40] D. Carey, S. Liew, S. Emery, Restorative in- ed., Elsevier, Oxford, 2014. terventions for HIV facial lipoatrophy, AIDS Rev. 10 (2008) 116e124. [26] K. Patnaik, H.B. Lin, D.J. Lentz, R.J. Zdrahala, U.S. Patent 6,306,165, Assigned to Meadox [41] B. Mole, Lasting treatment of facial HIVand non Medicals, October 23, 2001. HIV lipoatrophies through the use of SAM GoreTex malar implants and polyacrylamide [27] A. Kondyurin, M.F. Maitz, U.S. Patent hydrogel ﬁller Eutrophill, Ann. Chir. Plast. 7,597,924, Assigned to Boston Scientiﬁc Esthet. 51 (2006) 129e141. Scimed, October 2009. [42] T. Romo, J. Baskin, A. Sclafani, Augmentation of [28] G. Garlough, U.S. Patent 8,524,097, Assigned to the cheeks, chin and pre-jowl sulcus, and naso- Medtronic Corp, September 2013. labial folds, Facial Plast. Surg. 17 (2001) 67e78.

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9: MEDICAL AND SURGICAL APPLICATIONS OF EXPANDED PTFE 211 [43] K.P. Redbord, C.W. Hanke, Expanded PTFE [52] What Is an Implantable Cardioverter Deﬁbril- implants for soft-tissue augmentation: 5-year lator, National Institutes of Health, November follow-up and literature review, Dermatol. 2015. www.nhlbi.nih.gov/health/health-topics/ Surg. 34 (2008) 735e744. topics/icd. [44] J. Niamtu, Advanta® implants in cosmetic facial [53] D.F. Carson, U.S. Patent 5,931,862, Assigned to surgery, J. Oral Maxillofac. Surg. (2006). Pacesetter Co., August 3, 1999. [45] Gore Medical, Gore-Tex® Suture, www. [54] J.M. Cooper, et al., Covering sleeves can shield goremedical.com/la/suture, October 2015. the high-voltage coils from lead chatter in an integrated bipolar ICD lead, Europace (February [46] G. Setzen, E.F. Williams III, Tissue response to 1, 2007) 137e142. First published online, http:// suture materials implanted subcutaneously in a dx.doi.org/10.1093/europace/eul180. rabbit model, Plast. Reconstr. Surg. 100 (7) (1997) 1788e1795. [55] Physician’s Manual, Endotak Reliance® G and SG, Steroid Eluting Extendable/Retractable [47] The Perfect Close to Your Surgical Procedure, Helix Deﬁbrillation Leads, Guidant, Inc., Boston W. L. Gore & Associates, June 2015. Pub No. Scientiﬁc, August 2011. Pub No. 356920e004. AB0101eEN6, www.GoreMedical.com. [56] F.M. Kusumoto, N.F. Goldschlager, Cardiac [48] J.S. Sauer, H.R. Gorea, et al., Evaluation of a Pacing for the Clinician, Springer Science & novel automated ePTFE suturing and coaxial Business Media, September 21, 2007. fastener system for mitral chordae tendineae replacement: strength, feasibility and healing, [57] What Is a Stent?, American Heart Association, poster presentation, in: 2014 Meeting, The In- November 2015. www.Heart.org. ternational Society for Minimally Invasive Cardiothoracic Surgery, October 2015. http:// [58] Vascular Web, Aortoiliac Occlusive Disease, meetings.ismics.org/abstracts/2014/P97.cgi. The Society for Vascular Surgery, November 2015, www.vascularweb.org. [49] https://professional-intl.sjm.com/products/crm/ crt/crt-d/unify-quadra, November 2015. [59] M.H.H. Tenholt, Theresien Hospital, Man- nheim, Germany. [50] D. Fornell, Advances in Implantable Car- dioverter Deﬁbrillators (ICD) Technology, [60] GORE® VIABAHN® Endoprosthesis Videos, Diagnostic and Interventional Cardiology, http://www.goremedical.com, November 2015. October 23, 2015. www.dicardiology.com. [61] VIABAHN® Endoprosthesis, Gore Medical, [51] www.drugs.com, November 2015. www.goremedical.com/viabahn, November 2015.

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10 Filtration OUTLINE 10.1 Introduction 213 10.4.1 GaseSolid Filtration 215 10.2 Classiﬁcation of Filtration Processes 213 10.4.2 SolideLiquid Filtration 223 10.3 Surface Filtration Processes 214 10.5 Examples of Filtration Applications 10.4 Types of Filtration 215 References 224 230 10.1 Introduction of Industrial Chemistry has offered a classiﬁcation system based on four parameters. The short section Filtration is a process in which solid particles in a from Ullmann is quoted here. mixture are separated from a liquid or gas using some type of a porous medium. Generally, solids are “Filtration processes can be classiﬁed in accor- retained on the ﬁlter while allowing the liquid or gas dance with different criteria: through. Filtration process is used in nearly every segment of industry. The suspension to be ﬁltered is 1. Location of particle retention: The particles called slurry when the ﬂuid is a liquid. The porous can be separated on the outer surface of the ﬁl- medium used to separate the solids is known as ﬁlter ter medium (surface ﬁltration, cake ﬁltration) medium. The solids on the ﬁlter are referred as ﬁlter or inside of the ﬁlter medium (depth ﬁltration, cake and the clear liquid passing through the ﬁlter is deep bed ﬁltration) ﬁltrate. The pores of the ﬁlter medium are smaller than the size of particles to be separated. Because of 2. Generation of the pressure difference: Pressure their unique properties expanded polytetraﬂuoro- ﬁltration, vacuum ﬁltration, gravity ﬁltration, ethylene (ePTFE) microporous membranes have centrifugal ﬁltration found a vast number of applications in various market segments. 3. Operation mode: discontinuous, continuous, quasi-continuous. Dynamic ﬁltration and static 10.2 Classiﬁcation of Filtration (normal) ﬁltration. In case of dynamic ﬁltration Processes there are mechanisms active, while ﬁltration process is running, that help reduce the buildup Filtration is an extensive area of technology of a ﬁlter cake. The most common dynamic because of its application to natural and artiﬁcial ﬁltration process is cross-ﬂow-ﬁltration processes. Global demand for ﬁlters is projected to increase a healthy 6.2% annually to $80.0 billion in 4. Application: For example, water ﬁltration, beer 2018 [1]. Filtration has been divided according to ﬁltration” [2]. various aspects of ﬁltration processes. There are, however, a number of different classiﬁcations Several useful sources are provided for further depending on the authors. Ullmann’s Encyclopedia reading about ﬁltration processes [3e7]. Surface ﬁltration is deﬁned as a process that traps contaminants larger than the pore size on the top surface of the ﬁlter, usually a membrane, wire cloth, or monoﬁlament fabric. Contaminants smaller than Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00010-9 213 Copyright © 2017 Elsevier Inc. All rights reserved.

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214 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 10.1 Comparison of (A) depth ﬁltration showing material preparation and cake formation on the surface and (B) surface ﬁltration showing material accumulation on the surface [10,11]. the speciﬁed pore size may pass through the medium and packaging, pharmaceutical, minerals, power or may be captured within the medium by some other generation, metals, chemicals, engineering, automo- mechanism, such as surface afﬁnity, turboelectric tive, and aerospace. The membrane is laminated to a potential, or other means, which prevent particle wide variety of substrates (Fig. 10.2) such as poly- penetration [8]. Mechanisms at work include strain- ester needle felts and woven glass ﬁber to be made ing and impingement. ePTFE microporous mem- into ﬁlter bags, and pleatable materials such as branes work by surface ﬁltration. The membrane polyester and cellulose for ﬁlter cartridges and ele- works by a screening action in which the pores ments. The substrate acts as a stable supporting base formed by nodes and ﬁbrils prevent the passage of for the membrane. The type of substrate is deter- solids. This is why ePTFE with back support of mined based on the speciﬁc application requirements fabrics and plates with holes is preferred for a wide to which the ﬁlters will be subjected [13]. Fig. 10.3 variety of processes [9]. Of the total ﬁlter market the shows a scanning electron micrograph of ePTFE size of membrane ﬁltration is estimated at membrane laminated to a nonwoven backing mate- $2.6 billion in 2018. rial (scrim). Surface and depth ﬁltration have differences to be considered. With conventional ﬁlter fabrics, such as standard needle felts, the spaces between the ﬁbers within the structure of the media are usually considerably larger than the size of particles to be separated. The solid (dust) particles penetrate the surface of the media to close off the open pores, thus forming a ﬁlter cake on the surface of the media called depth ﬁltration. In depth ﬁltration (Fig. 10.1), particles penetrate the structure of the media and form a ﬁlter cake on the surface. In surface ﬁltration (Fig. 10.1), particles are collected on the surface of the membrane. 10.3 Surface Filtration Processes Figure 10.2 Lamination of expanded polytetraﬂuoro- ethylene membrane (GORE-TEX) to backing mate- ePTFE is used as a ﬁlm or membrane on a growing rial [12]. number of ﬁlters across every industry including food

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10: FILTRATION 215 Figure 10.3 Scanning electron micrograph of Figure 10.4 Gas ﬁltration mechanisms [15]. expanded polytetraﬂuoroethylene membrane (GORE- TEX) laminated to a nonwoven backing material [12]. electrostatic attraction (Fig. 10.4). The ﬁrst three of these mechanisms apply mainly to mechanical ﬁlters 10.4 Types of Filtration and are inﬂuenced by particle size. Filters for both gas and liquid media are con- Impaction and interception are the dominant structed using ePTFE membranes. Most of the gas collection mechanisms for particles greater than ﬁltration serves the removal of dust particulates from 0.2 mm, and diffusion is dominant for particles less ambient air, various types of air intakes, vehicle than 0.2 mm. The combined effect of these three exhaust, ﬂy ash from power plant ﬂue gas, and collection mechanisms results in the classic collec- incinerator ﬂue gas. Liquid ﬁltration involves an tion efﬁciency curve, displayed in Fig. 10.5. extremely broad range of solids and liquids. Gas ﬁltration constitutes 16% of total annual ﬁltration PTFE yarns, felts, and microporous expanded investment while 84% is devoted to liquid ﬁlters and membranes give outstanding temperature resistance, other separation means [14]. PTFE as a woven fabric good chemical resistance, low differential pressure, and as a needle-felt material has been used in various and high removal efﬁciency. PTFE ﬁlter media are ﬁltration applications, particularly those involving used where extremes of chemical and thermal con- aggressive liquid chemical environments. ditions are encountered, such as waste burning, heavy fuel oil-ﬁred systems, nonferrous metal melting 10.4.1 GaseSolid Filtration plant, slurry burners, carbon black producers, chlo- rine gas cleaners for PVC production, and so on. Gas ﬁltration is a common operation with Mineral and glass ﬁber needle-felt ﬁlter media are numerous applications: Figure 10.5 Efﬁciency of gas ﬁltration as function of Heavy-duty engine collection mechanism and particle size [15]. Gas turbine air inlet Natural gas transportation Heating, ventilating, and air conditioning (HVAC) Air pollution control Industrial air Gas ﬁltration removes particle matter by four different collection mechanisms. Those are (1) iner- tial impaction, (2) interception, (3) Diffusion, and (4)

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216 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 10.6 Nonmembrane ﬁlter lets dust penetrate collecting ﬁne particulate efﬁciently. Eventually, inside ﬁlter media and increasing pressure drop [17]. over time media blinding and atmospheric emissions take place as individual dust particles penetrate into used where temperatures range from 180 to 300C. and beyond the ﬁlter media (Fig. 10.6). ePTFE They are found in dedusting of gases from electric laminated ﬁlters use the “surface ﬁltration” method- melting furnaces and replacing precipitators in boiler ology. The ePTFE surface acts as the contact area and ﬁring and power stations [16]. because of its microporous structure with millions of pores per square centimeter even submicron particles Without a cake on the surface of the ﬁlter are captured on its surface. Fig. 10.7 shows a number conventional ﬁlter media are seldom capable of of ﬁlter materials including a hybrid material with a range of pore sizes deﬁned by the number of ﬁbers in 1 cm2. The ePTFE membrane has the smallest pores (Fig. 10.7). The backing substrate is merely a support and plays no part in the ﬁltration process. The build-up of a dust cake is not an operational requirement thus the ﬁlter can be cleaned more effectively. A stable dif- ferential pressure is maintained as shown in Fig. 10.8. Surface ﬁltration can, therefore, prolong the service life of the ﬁlter. In addition, it provides substantial savings of compressed air and pressure and fan power. ePTFE manufacturing process allows production of membranes with structures tailored to the re- quirements of applications. Manufacturers such as W. L. Gore have developed a variety of ePTFE membranes for different applications as seen in Fig. 10.9. These choices along with rigorous ﬁlter construction process accommodate high ﬂow, dura- bility, efﬁciency, versatility, and ruggedness re- quirements of various. Fig. 10.10 (also see Fig. 10.3) depicts the way an ePTFE gas ﬁlter works. Dust is ﬁltered at the membrane surface though no dust cake buildup required for ﬁltration to take place. No particle enters Figure 10.7 Comparison of pores sizes of various ﬁlter media materials [18].

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10: FILTRATION 217 400.00 Pressure difference across filter 350.00 300.00 Conventional 250.00 filter 200.00 150.00 ePTFE filter 100.00 50.00 0.00 Time Figure 10.8 Pressure difference across the ﬁlter for expanded polytetraﬂuoroethylene (Tetratex by Donaldson) and conventional ﬁlter [10]. Figure 10.9 Scanning electron micrographs of expanded polytetraﬂuoroethylene membranes for manufacturing different ﬁlters [17]. the backer. The ePTFE membrane capable of sub- The example in Table 10.1 shows a comparison of micron ﬁltration can be selected. This type of ﬁlter a conventional ﬁlter and an ePTFE Tetratex ﬁlter has exhibited both high capture efﬁciency over wide media (manufactured by Donaldson) in a cement- range of system conditions and long-term durability. ﬁnish milling process. The ePTFE ﬁlter improves Reversing the gas ﬂow using clean air cleans a dirty airﬂow by an estimated 10%. Where other system ﬁlter. Nearly all dust falls off the membrane surface. parameters are acceptable, the use of ePTFE ﬁlter The hydrophobic nature of ePTFE allows even wet signiﬁcantly reduces the cost/ton of product and in- dust cakes to be cleared from the ﬁlter surface. creases mill capacity.

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218 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 10.10 Schematic of how an expanded polyte- There are numerous types of contaminants in the traﬂuoroethylene works during normal operation and air. Fig. 10.11 lists the types and sizes of common cleaning [17]. contaminants. Fig. 10.12 shows the choice of ﬁltra- tion method as a function of solid particle size. One of the major application areas of ePTFE ﬁlters is ﬂue gas and combustion system baghouse (Figs. 10.11 and 10.12) throughout the world. ePTFE ﬁltration products can improve performance in boiler and incineration systems (Fig. 10.13). Filter bags manufactured with ePTFE membrane (by BHA-TEX) are available in a variety of base fabrics to meet your system’s speciﬁc requirements (Fig. 10.14 and Table 10.2). The available variety allows the selection of a base fabric best suited for the process environment. PulsePleat ﬁlter elements for baghouses by Clar- cor consist of a combination of pleated high- efﬁciency ﬁltration media and an inner support core (Fig. 10.15). It forms a one-piece element that ﬁts directly into existing baghouse tube sheets. BHA PulsePleat replaces the traditional ﬁlter bags and cages [22]. Some of the key beneﬁts of PulsePleat ﬁlters include requiring less compressed air pressure to pulse clean, operating across a wide range of temperatures and applications, increasing ﬁltration area 2e3 times, dramatically reduces air-to-cloth ratios, and reducing the operating pressure differential. Table 10.1 Filter Media Comparison in a Cement Finish Mill Process [10,11] Example Conventional Filter TetratexdPower Saving Production MediadPower Drive Consumption 24/7 Â 8000 h Air volume Dust loading 24/7 Â 8000 h 2.4 MW Current media Filter DP 2.4 MW 165,000 AM/h Result 150,000 AM3/h 750 gr/Nm3(85.9T/h) 750 r/Nm3 (78.1 T/h) Tetratex felt Conventional felt 150 mmWG 200 mmWG Above equates to 27.94 Kw Above equates to h/Tonne of cement @ V0. 10/ 30.73 Kw h/Tonne of Kw h ¼ V2794/Tonne of cement cement @ V0. 10/Kw h producing 687,200 T/annum V3.073/Tonne of cement producing 624,800 T/ annum

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10: FILTRATION 219 Relative sizes of common air contaminants Particle diameter, microns·logarithmic scale 0.0001 0.001 0.01 0.1 0.3 1 10 100 1000 10,000 Gelatin Fog Mists Rain Tobacco smoke Moulds Carbon black Beach sand Oil smokes Diameter of human hair Gas molecules Bacteria Pollen Virus Milled flour Plant spores Unsettling atmospheric impurities Settling atmospheric impurities Heavy industry dust Flumes Dusts Fty·ash Colloidal silica Visible by human eye 0.3 Mioron Fouling zone Erosion zone Figure 10.11 Common air contaminants [19]. >40 Bar 10 Bar < 1 Bar High Pressure Low Pressure Membranes Filters Membranes Ultrafiltration Reverse Microfiltration Osmosis Nano- filtration 0,0001 0,001 0,01 0,1 1 10 100 µm Organic Collolds Algae Compound Salts Organic Yeasts Pollens Macromolecules Virus Bacterla Smallest Red Cell Protozoa Polio Virus Bacteria Hair Figure 10.12 Choice of ﬁltration method as a function of the solid particle size [20]. Fig. 10.16 shows an ePTFE membrane ﬁlter to efﬁciency particulate arresting (HEPA) and ultralow remove dust and dirt from the ambient environment. penetration air (ULPA). An example of HEPA ﬁlter Higher performance types of ﬁlters are called high can be seen in Fig. 10.17. The membrane is usually

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220 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 10.13 Examples of expanded polytetraﬂuoro- commonly those developed by the US Department of ethylene ﬁlters for baghousesdGORE-TEX [17]. Energy (DOE)dto qualify as a HEPA ﬁlter. The US standard (DOE-STD-3020-2005) requires that a Figure 10.14 Examples of expanded polytetraﬂuoro- HEPA ﬁlter be capable of removing 99.97% of ethylene ﬁlters for baghousesdBHA-TEX [21]. contaminant particles 0.3 mm in diameter. Most backed by one of spun-bonded polyester, synthetic standards also specify that HEPA ﬁlters must feature cellulose, or other nonwoven media. HEPA ﬁlters minimal pressure drop and maximum airﬂow when in are, however, available with a broad range of backing operation. material based on the end-use application. Ultralow particulate ﬁlters (sometimes called HEPA and special tests have been developed to penetration) ﬁlter more efﬁciently than HEPA ﬁl- test the efﬁciency of HEPA ﬁlters [24,25]. Air ﬁlters ters. ULPA ﬁlters are required to remove 99.999% must satisfy certain standards of efﬁciencydmost or more of particles !0.12 mm in diameter. There is an overlap in the capabilities of ULPA and HEPA ﬁlters. Airline cabin air puriﬁers, biomedical air ﬁltration, electronics manufacturing, pharmaceutical manufacturing, and vacuum cleaner ﬁlters are among the important applications of ULPA and HEPA ﬁlters. A particularly important application of HEPA/ ULPA ﬁlters is to purify the air in certain medical environments. Specialized medical procedures impose various demands on health-care facility HVAC air ﬁltration and air puriﬁcation systems, which must continuously operate at the highest efﬁ- ciency. This usually includes operating rooms, outpatient surgery suites, bone marrow transplant, isolation rooms, critical care, intensive care, and other areas [26]. Fig. 10.18 shows a ﬁlter for operating room air. It is a combination of depth and surface ﬁlter. The ﬁlter, for surgical smoke ﬁltration, combines a microﬁber glass preﬁlter with an ePTFE membrane. The mi- croﬁber glass preﬁlter captures the smoke particles generated during surgical cauterizing. Simulta- neously, the ePTFE membrane provides a liquid and microbial barrier [27]. There has been large growth in gas ﬁltration over the recent decades because of the focus on reduction of pollution and enhancement of sustainability. Ac- cording to Ken Sutherland [28,29], there are three broad classes of ﬁlters for gas/solid separations: 1. Gas ﬂow treatment within processes and in transit from one process to another; 2. Cleaning of input/intake gases, usually air, to remove minor contamination ahead of such systems as engines or building ventilation; and 3. Cleaning of exhaust gases, mostly air, that are usually heavily contaminated with dust and other gases.

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Table 10.2 A comparison of Backing Fabrics for Expanded Polytetraﬂuoroeth Fabrics Polypropylene Acrylic Polyester PPS (To 275 F Proco Maximum 170 F (77 C) 265 F 375 continuous (130 C) (135 C) (190 operating temperature Excellent Goo Excellent Goo Abrasion Excellent Good Poor Goo Energy Good Good Fair Excell absorption Fair Excell Excellent Moist heat Excellent Excellent Poo Alkalines Excellent Fair Mineral acids Excellent Good Oxygen Excellent Excellent (15%þ) a Sensitive bag-to-cage ﬁt. b Fair with chemical acid resistant ﬁnishes. c Must oversize bag for shrinkage above 232 C.

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10: FILTRATION hylene Filters [21] orcon/ Aramid P84c Fiberglassa PTFEc on) (Nomex) (Teﬂon®) 356e500 F 500 F F 400 F (180e260 C) (260 C) 500 F C) (204 C) (260 C) od Excellent Fair Fair Good od Good Gooda Faira Good od Good Good Excellent Excellent lent Good Fair Fair Excellent lent Excellent Fair Good Poorb Excellent or Excellent Excellent Excellent 221

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222 EXPANDED PTFE APPLICATIONS HANDBOOK (A) (B) Figure 10.17 Example of expanded polytetraﬂuoro- ethylene high efﬁciency particulate arresting ﬁlter. (C) this is a very inexact classiﬁcation, since most appli- cations mentioned vary widely in size [29]. This Figure 10.15 (A)e(C) Examples of PulsePleat ﬁlter expansion is covered by: elements for baghouses by Clarcor [22]. Cleaning of large volumes of ambient air, eg, for Figure 10.16 Example of a ﬁlter for removal of dirt gas turbine intakes or the input of air as feed to particles [23]. air separation plants. Process gas ﬂow is treated in bulk and ﬁne Cleaning of moderate volumes of ambient air, chemical industries to recover airborne products. The eg, for general ventilation and cleanroom other two classes pertain to cleaning of air to prevent systems. pollution as well as avoid exposure of animals and plants to harmful materials. Cleaning of small volumes of ambient air, eg, for engine intakes, for vehicle cabin, and air Cleaning of natural gas takes place ahead of conditioning unit ﬁlters, for equipment breather liquefaction for long-distance transport purposes. The purposes, for personal respiration, and for suc- other one is ﬁltration of carbon dioxide prior to rein- tion cleaner usedplus for free-standing air jection underground to capture carbon and seques- cleaners. tration. Both applications are expected to grow. Processing of small volumes of compressed The two gas cleaning classes (input and exhaust) gases, eg, to supply as compressed air pneu- can be further classiﬁed by size of gas ﬂowdalthough matic systems, or underwater diving sys- tems, or medical air supply, including recycle. Processing of large volumes of dirty exhaust gases usually at elevated temperatures and heavily contaminated with dust and acid gases, eg, steam-raising boiler furnace ex- hausts, “clean” solid fuel production, metal- lurgical process furnaces, cement works, or oil reﬁneries, some of which may involve the injection of reactive dusts upstream of the ﬁlter. Processing of moderate volumes of dirty exhaust gases, usually hot, eg, a variety of en- gine exhausts, but especially from diesel en- gines, also drying, curing, degreasing and

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10: FILTRATION 223 Anatomy of a Smoke Filter Microfiberglass prefilter: Large capacity for Liquid, smoke, and aerosolized bacteria trapping smoke particles and viruses remain on one side. Hydrophobic ePTFE membrane: Robust contamination barrier Airflow through filter is Polymeric support: maintained as smoke Adds strength, particles collect. facilitates device integration Detail of middle layer showing node and fibril structure of expanded PTFE membrane Figure 10.18 Surgical smoke ﬁlter from W. L. Gore & Associates (www.Gore.com) combines depth and surface ﬁltration [27]. baking ovens, spray booths, and general factory intermittently? Is worker exposure to the process ventilation system exhausts, especially with a liquid during ﬁlter cleaning or replacement a prob- volatile organic compound contaminant. lem? These and other factors must be weighed when choosing the right ﬁltration method for a particular Processing of small volumes of possibly application [30]. contaminated exhaust gases, for example, controlled process vessel vents, and gases From a mechanistic point of view, there are two recycled through gas blanketing systems. These regimes of surface ﬁltration of liquids (Fig. 10.19): broad application types occur throughout indus- direct ﬂow ﬁltration (DFF) or dead-end and try, commerce and, to a smaller extent (in size, tangential ﬂow ﬁltration (TFF) or cross-ﬂow [2]. but not in number), in domestic and institutional Each method has advantages depending on the use. The corresponding ﬁlter needs are largely characteristics of the solideliquid mixture to be satisﬁed by simple pieces of ﬁlter medium, or ﬁltered. In DFF all the solids are deposited on the by the bag, pocket, cartridge, and panel types ﬁlter media. The driving force is usually gravity. of ﬁlter [29]. Additional pressure applied to the suspension above the ﬁlter only compresses the ﬁlter cake without 10.4.2 SolideLiquid Filtration increasing the ﬁltration. TFF, also known as cross ﬂow ﬁltration, where the feed stream passes par- Solideliquid ﬁltration includes any process in allel to the membrane face as one portion passes which solids are separated from a liquid suspension through the membrane (permeate) while the by running the mixture through a porous medium remainder (retentate) is recirculated back to the [28]. In addition to removing undesirable/desirable feed reservoir. material from a liquid stream, the ﬁltration method selected must also satisfy other requirements. Solideliquid techniques have found use in a great Installed costs must be weighed against operating many industries including those listed below: costs. Waste disposal costs must be considered. Is continuous ﬂow a requirement of the application, or Water (municipal, industrial, waste water) can the ﬁltration equipment be operated Industrial processes (fuels, chemicals, inks)

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224 EXPANDED PTFE APPLICATIONS HANDBOOK (A) Dirty Fluid (B) Dirty Fluid Clean, Filtered Fluid MEMBRANE Flowrate Clean, Filtered Fluid Flowrate Time Time Dead End Filtration Crossflow Filtration Figure 10.19 Two modes of liquid ﬁltration (A) direct (or cake) ﬂow ﬁltration and (B) tangential (or surface) ﬂow ﬁltration [31]. Microelectronics (ultrapure water, cleaning Other properties of ePTFE membranes, beneﬁcial to chemicals) ﬁlters, include its availability with very small pores size (0.05e0.1 mm) such as bacteria, availability in a Food and beverages (dairy products, wine, broad range of pore sizes (!0.1e0.2 mm) and beer) extremely low extractible content. Biopharmaceuticals (antibodies, proteins, 10.5 Examples of Filtration vaccines) Applications Biomedical (blood, plasma ﬁltration, hospital Fig. 10.20 provides data for a ﬁlter cartridge with a water) diameter of 25 cm with an effective ﬁltration area of 0.65 m2. Flow rate of iso-propanol through the ﬁlter Most applications allow the use of common ther- for different thicknesses and pore sizes is plotted moplastics. A variety of polymers are used to against the pressure drop across the ePTFE ﬁlter. manufacture ﬁlter media. Each type has speciﬁc at- Table 10.4 shows the characteristics of ePTFE tributes and could be best for certain applications but membranes (without backing) in the pore size range could be a complete failure for other applications. of 0.1e3 mm. The same ﬁlter material from different manufacturers can differ in physical properties and in ﬁltration Fig. 10.21 shows a high-ﬂow ﬁlter (Ultipleat high- characteristics. However, the chemical compatibility ﬂow ﬁlter by Pall Corp.) for wet etching and cleaning of the material is almost the same irrespective of the solutions. Two alternative ﬁltration media options manufacturer as seen in Table 10.3. Among all have been exhibited: PTFE and ECTFE (ethylene ﬁltration media materials, PTFE stands out for its chlorotriﬂuoroethylene) membranes. Table 10.5 outstanding chemical resistance. provides the speciﬁcations of the two types of the ﬁlter shown in Fig. 10.21. Basic PTFE properties including chemical inert- ness and resistance, surface hydrophobicity, broad An example of the “ultimate” ﬁlter cartridge is operating temperature range, thermal stability, purity, entirely made of ﬂuorinated polymers. The ﬁlter and others render ePTFE liquid ﬁlters unique and medium is microporous ePTFE placed over a layer highly useful under extreme conditions. Speciﬁc of ECTFE support membrane. The core, cage, and applications include ﬁltration of concentrated acids end caps are all made of solid ECTFE. The O-rings and bases, solvents, de-ionized water, etchants, photoresist solutions, and high purity bulk chemicals.

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Table 10.3 Chemical Compatibility of Common Filter Membranes With Widely U Chemical Nylon PTFE PVDF PS P Acids LC C C C Glacial acetic acid NC C C C Hydrochloric acid NC C NC NC Sulfuric acid NC C C NC Nitric acid NC C Phosphoric acid LC C (25%) C C C C Bases C C NC NC Ammonium hydroxide C NC (25%) C C C ND Sodium hydroxide C C C 3 mol LÀ1 C C C C C C C NC Common Solvents C C NC Acetone C LC Benzene Benzyl alcohol Butanol Carbon tetrachloride Chloroform

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Used Solvents [32] Regenerated Cellulose Cellulose Cellulose Cellulose Nitrate Acetate Triacetate Polypropylene C NC NC NC C C NC NC NC C NC NC NC NC C NC NC NC NC C LC LC C C C C LC C C C C LC NC NC NC LC C NC NC NC C C C C C C C C C LC LC LC C C C C LC C C LC C LC LC NC NC (Continued )

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Table 10.3 Chemical Compatibility of Common Filter Membranes With Widely U Chemical Nylon PTFE PVDF PS P Dichloromethane C C C NC Dimethylformamide C NC DMSO LC C NC NC Ethanol methanol C C NC Ethyl acetate C C C Ethyl ether C C C NC Glycerol C C C NC Hexane C C LC Isopropanol C C C C Methyl ethyl ketone C C C NC Tetrahydrofuran C C C Application C LC C MF LC NC MF MF, UF NC MF, UF, RO C, compatible; DMSO, dimethyl sulfoxide; LC, limited compatibility; MF, microﬁltration; NC, noncomp RO, reversed osmosis; UF, ultraﬁltration.

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Used Solvents [32] (Continued ) Polypropylene Regenerated Cellulose Cellulose Cellulose LC Cellulose Nitrate Acetate Triacetate C C C NC C LC NC NC NC C C NC NC NC LC C LC C NC C C NC NC C C NC NC C C C C C NC C C C C NC ND C LC C C C LC LC C MF C NC NC C MF, UF MF MF, UF, C RO LC NC MF, UF, RO patible; ND, not done; PTFE, polytetraﬂuoroethylene; PVDF, polyvinylidene ﬂuoride; PS, polysulfone;

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10: FILTRATION 227 Figure 10.20 Water ﬂow rate versus pressure through expanded polytetraﬂuoroethylene ﬁlter for different pore sizes [33]. Table 10.4 Properties of Hydrophobic Expanded Polytetraﬂuoroethylene Membrane [34] Bubble Point Flow Rates Pore MPa psi Acetone Air Porosity (%) Maximum Thickness Size !0.12 !17.4 (mL/min/cm2) (L/min/cm2) 68 Operating (mm) (mm) !0.091 !13.2 74 Temperature (8C) 70 0.10 !0.063 !9.1 27.0 e 78 80 !0.039 !5.7 55.0 e 76 260 75 0.20 !0.031 !4.5 100 e 79 75 !0.013 !1.9 200 e 83 260 75 0.50 300 e 75 750 e 260 0.80 260 1.00 260 3.00 260 Figure 10.21 Ultipleat high ﬂow ﬁlter containing are made of Viton ﬂuoroelastomer encapsulated with expanded polytetraﬂuoroethylene membrane (by a copolymer of tetraﬂuoroethylene and per- Pall Corp.) [35]. ﬂuoropropyl vinyl ether. This ﬁlter cartridge is suit- able for the most extreme corrosive chemicals. Fig. 10.22 shows cartridges of ﬁlter with ePTFE membrane media. It has a retention rating in the range of 0.03e0.1 mm using hydrophilic micropo- rous polytetraﬂuoroethylene membrane. The support and drainage layers and core, cage, and end caps are all made of polypropylene. Hydrophilic surface of the ePTFE membrane facilitates ﬁltration of aqueous chemicals and solutions. Table 10.6 gives properties of a ﬁlter with hydro- philic ePTFE membrane (unrelated to Fig. 10.22) in the pore size range of 0.1e3 mm. Note the difference between the maximum operating temperatures of hydrophobic and hydrophilic membranes by

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228 EXPANDED PTFE APPLICATIONS HANDBOOK Table 10.5 Speciﬁcations of Ultipleat High Flow Filter Containing Expanded Polytetraﬂuoroethylene (ePTFE) and Ethylene Chlorotriﬂuoroethylene (ECTFE) Membranes [35] Components Materials Filter medium Removal rating PTFE ECTFE Support, drainage 3 mm 10 mm Support core Polypropylene End caps Polypropylene Q-ring Polypropylene (glass ﬁber ﬁlled) Maximum operating Fluorinated ethyleneepropylene encapsulated ﬂuoroelastomer temperature 82C/180F Maximum forward differential pressure 0.34 MPa at 50C 50 psi at 122F Housings Ultipleat high ﬂow plastic housing P/N: 1HFW-09GF25H, 1HFW-2GF41H (For availability of speciﬁc options and housing details, please contact your Pall Corporation representative) Figure 10.22 Example of hydrophilic expanded poly- comparing the data in Tables 10.4 and 10.6. The tetraﬂuoroethylene membrane ﬁlter cartridge [36]. temperature drops from 260C for hydrophobic to 100C for the hydrophilic membrane due to the hy- drophilic treatment of the ePTFE surface. Surface modiﬁcation of hydrophobic of ePTFE to make it hydrophilic is discussed separately in Chapter 14. Compared to other porous membranes, ePTFE displays superior performance as a barrier to liquids, particles, and aerosols even containing viable bacte- ria and viruses. PTFE and ePTFE are both inherently inert, biocompatible, and biostable. In addition to these advantages ePTFE deliver it must possess the Table 10.6 Properties of Hydrophilic Expanded Polytetraﬂuoroethylene Membrane [34] Bubble Pointa Flow Ratesb MPa psi Pore Water Air Porosityc Thickness Maximum Size (%) (mm) Operating (mm) (mL/min/cm2) (L/min/cm2) Temperature (8C) 0.10 !0.38 !55.1 14 1.6 71 35 100 0.20 !0.24 !34.8 21 2.1 71 35 100 0.50 !0.14 !20.3 39 2.9 79 35 100 1.00 !0.083 !12.0 73 5.7 83 35 100 a Bubble point is the minimum pressure required to force air through a membrane which has been prewet with water. b Flow rate indicates initial ﬂow rate at 10 psi using a KGS 47 ﬁlter holderWater: using water preﬁltered to 0.1 pm pore sizeAir: using preﬁltered nitrogen at 10 psi. c Porosity refers to the percent open area.

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10: FILTRATION 229 attributes of a ﬁlter including such as pressure Figure 10.24 A urine collection device [37]. equalization and air/gas elimination, and air/gas delivery [37,38]. Fig. 10.23 demonstrates the venting performance of ePTFE membrane in terms of airﬂow and resistance to liquid breakthrough. Microporous membranes made of different polymers and pro- cesses (eg, phase separation) are also included for reference. Airﬂow and the resistance to liquid breakthrough are opposing material properties. In urine collection devices, oleophobic ePTFE membranes are used as vents, equalizing pressure as liquid ﬁlls the device (Fig. 10.24). Air residing in the device is displaced, and the membrane blocks liquid and airborne contaminants [37]. During the drug delivery process, it is common to use in-line intravenous ﬁlters to remove fungi, bacteria, and particulate from parenteral solutions (Fig. 10.25). Air bubbles can collect within these ﬁlter devices, which can negatively affect the performance of the medical device, drug delivery regimen, and ultimately patient safety. The ePTFE membranes provide an effective and reliable solution Figure 10.25 A drug delivery ﬁlter to remove fungi, bacteria, and particulate from parenteral solutions [37]. Figure 10.23 Airﬂow (bottom) and water (top) break- by safely eliminating entrained air bubbles while through performance of microporous membranes [37]. preventing leakage of parenteral solution from the ﬁlter device. Insufﬂation is used in laparoscopic (or “key hole”) surgery to inﬂate the abdomen with CO2 gas, allowing space for surgical and visualization in- struments (Fig. 10.26). Insufﬂation ﬁlters are used to protect the patient from bacteria and particulate that may be present in the CO2 tanks and delivered to the patient via the insufﬂator device. The ﬁlter also protects the insufﬂator from potential bio-hazardous contamination from the patient. The ePTFE mem- brane is commonly used in insufﬂation ﬁlters due to its desirable combinations of membrane gas ﬂow, particle retention, and hydrophobicity [37]. To summarize, this chapter illustrates that ePTFE membranes have a large number of desirable prop- erties and characteristics as ﬁlter medium. These

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230 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 10.26 An insufﬂation ﬁlter to remove bacteria [10] P. Bickers, Donaldson Corp, Membranes: and particulate from the CO2 for laparoscopic surgery expanded PTFE ﬁnds new markets, Fil- in the abdominal cavity [37]. trationþSeparation (November/December, 2012). membranes are an important building block for [11] Surface Filtration versus Depth Filtration, designing safe and effective solutions for the chal- Donaldson Corp, 2015. Pub code: lenging industry and medical ﬁltration and separation DFvSF005.EN.01.15. problems. [12] G.G. Pranhofer, PM2.5 emissions from indus- References trial sources, in: Presentation by W. L. GORE & Associates, DustConf2007, The Netherlands, [1] Freedonia Group, www.freedoniagroup.com/ April, 2007. World-Filters.html, January 2016. [13] D. Corp, www2.donaldson.com/tetratex/en-sg/ [2] S. Ripperger, W. Go¨sele, C. Alt, T. Loewe, pages/news-item.aspx?NewsID¼23, January Filtration, 1. Fundamentals, in: Ullmann’s 2016. Encyclopedia of Industrial Chemistry, on-line ed., Wiley, Sep 2013. [14] K.S. Sutherland, G. Chase, Chap 6 gas ﬁltration, in: Filter and Filtration Handbook, ﬁfth ed., BH, [3] A.Y. Tamime, Membrane Processing: Dairy and Elsevier, 2008. Beverage Applications, ﬁrst ed., Wiley- Blackwell, 2013. [15] Guidance for Filtration and Air-cleaning Sys- tems to Protect Building Environments from [4] C. Tien, Principles of Filtration, ﬁrst ed., Elsev- Airborne Chemical, Biological, or Radiological ier, 2012. Attacks, National Institute for Occupational Safety and Health, April 2003. [5] S. Tarleton, R. Wakeman, Solid/Liquid Separa- tion: Principles of Industrial Filtration, ﬁrst ed., [16] K.S. Sutherland, G. Chase, Filter and Filtration Elsevier, 2005. Handbook, ﬁfth ed., BH, Elsevier, 2008. [6] T. Chase, G. Chase, Filters and Filtration Hand- [17] Gore™ Filtration Products, W.L. Gore & Asso- book, sixth ed., Butterworth-Heinemann, ciates GmbH, www.Gore.com, 2006. Elsevier, 2015. [18] T. Norton, Hybrid membrane technology: a [7] U.S. Environmental Protection Agency, Mem- new nanoﬁbre media platform, Filtrationþ brane Filtration Guidance Manual, CreateSpace Separation (March 2007). Independent Publishing Platform, 2015. [19] K. Sutherland, G. Chase, Section Gas [8] B. Perlmutter, Solid-Liquid Filtration e Prac- Filtration in Filters and Filtration Handbook, tical Guides in Chemical Engineering, Elsevier, ﬁfth ed., Butterworth-Heinemann, Elsevier, 2015. 2008. [9] R. Patel, D. Shah, B.G. Prajapti, M. Patel, [20] A. Doucoure, Overview of Concepts, Practices Overview of industrial ﬁltration technology and & Applications in Liquid Filtration, Tutorial, its applications, Indian J. Sci. Tech. 3 (10) FILTRATION2011, Chicago, IL, USA, (October 2010). November 15, 2011. [21] ePTFE Membrane Filters BHA-TEX, High Ef- ﬁciency Fine Filtration, General Electric Co (now Clarcor), 2006. [22] Clarcor Industrial Air e BHA Industrial Filtra- tion, www.clarcorindustrialair.com/Products/ Industrial-Filtration/Replacement-Filters/BHA- PulsePleat-Filter-Elements, February 2016. [23] www.efcﬁltration.com, February 2016. [24] E. Bryan, B. Kitch, J. Meek, D. Milholland, N. Nance, Alternative Test Methodology for In-Situ Testing of EPTFE HEPA Filters for Pharmaceutical Applications, Pharmaceu- tical Engineering, November/December 2011, pp. 1e6.

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10: FILTRATION 231 [25] E. White, Hepa and ULPA ﬁlters, J. Validation [32] R. Sahai, Membrane separations, ﬁltration, in: Technol. (Summer 2009). Encyclopedia of Separation Science, Elsevier, 2000, pp. 1717e1724. [26] Guidelines for Environmental Infection Control in Health-care Facilities, Recommendations of [33] PORTEF ePTFE ﬁlters, ABSFIL Absolute CDC and the Healthcare Infection Control Filtration, www.absﬁl.com, February 2016. Practices Advisory Committee (HICPAC), February 2016. www.cdc.gov/mmwr/preview/ [34] Membrane Filters, Advantec MFS, Inc, www. mmwrhtml/rr5210a1.htm. advantecmfs.com/catalog/ﬁlt/membrane.pdf, February 2016. [27] V. Burt, A Few Guidelines for Selecting Filters, Medical Design, July 1, 2005. http://medicaldesign. [35] Pall Corporation Data Sheet, Ultipleat® High com/components/few-guidelines-selecting-ﬁlters. Flow Filter, www.pall.com/pdfs/Microelectronics/ MEHFUEFEN.pdf, August 2012. [28] K. Sutherland, G. Chase, Filters and Filtration Handbook, ﬁfth ed., Butterworth-Heinemann, [36] GORE® Filters for high purity chemical pro- Elsevier, 2015. cessors, PP Cartridge, Hydrophilic PTFE Membrane, W.L. Gore & Associates, 2012. [29] K. Sutherland, Market Driving Forces in http://gore.com/ﬁlters. Gaseous Filtration, FiltrationþSeparation (July/ August, 2011). [37] E.C. Wigner, K. Fritsky, Closing Remarks e Use of Expanded PTFE Membranes in Medical [30] Choosing the Best Filtration Method for Your Filtration, W.L. GORE & Associates, December Liquid Processing Application, Eaton Corp, 2009 pub No. AN1014eEN1. February 2016. www.eaton.com/Eaton/Products Services/Filtration/index.htm. [38] Intervene™ Gas Filter with Gamma Sterilizable ePTFE Membrane, product data, Pall Life [31] Highland Fluid Technology, http://highlandﬂuid. Sciences Corp, pub No., GN06.1415, 2006. com/cross-ﬂow-ﬁltration, February 2016.

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11 Industrial and Other Applications of Expanded PTFE OUTLINE 11.1 Expanded Polytetraﬂuoroethylene Fiber 233 11.2 Gaskets and Seals 244 11.1.1 Oral Care 247 11.1.2 Sutures 233 11.2.1 Testing Gaskets 11.1.3 Sewing Threads 248 11.1.4 Fishing Line 235 11.3 Expanded Polytetraﬂuoroethylene Vents 11.1.5 Weaving and Knitting Fiber 236 250 11.1.6 Ropes 238 References 240 242 Expanded polytetraﬂuoroethylene (ePTFE) prod- Dental ﬂoss is a large market served with ﬁbers made ucts have a wide range of applications indoor and from a number of materials such as nylon. ePTFE ﬁber outdoor (Fig. 11.1) across various market segments. has captured a signiﬁcant share of the ﬂoss market. The main forms of ePTFE in industrial applications This section describes preparation of dental ﬂoss from include ﬁber, ﬁlm, and sheet. Fiber is a highly ver- ePTFE. satile material that can be converted into different parts as listed in Table 11.1. Vents and gaskets are One problem with ePTFE ﬁber is its extremely low planar products both which may be produced from friction coefﬁcient. It makes it difﬁcult to handle by membranes or woven ﬁbers of ePTFE. Chapter 6 the user. To increase the ease of handling, a coating of describes the procedures for manufacturing of a composition capable of increasing the coefﬁcient of various ePTFE shapes. friction was applied to the ﬂoss. That resulted in an increase in the friction coefﬁcient of the ﬂoss. One of 11.1 Expanded the preferred coatings was wax and particularly a Polytetraﬂuoroethylene Fiber coating of a microcrystalline wax with a low to me- dium molecular weight [3]. ePTFE ﬁbers are most commonly produced by slitting webs or tapes of ePTFE. These ﬁbers are Table 11.1 Applications of Expanded Polytetra- often further processed to improve one or more of its ﬂuoroethylene Products properties. Stretching, twisting, and applications of other materials to the ﬁber surface are among the Oral care post-slitting processes. Examples of available types and properties of ePTFE ﬁber are shown in Sutures Tables 11.2e11.4. Outdoor sewing ﬁbers 11.1.1 Oral Care Kite lines Flossing teeth does about 40% of the work required to remove sticky bacteria, or plaque, from Fishing lines the teeth. Plaque generates acid, which can cause cavities, irritate the gums, and lead to gum disease [2]. Weaving and knitting yarn Packings Gaskets and seals Vents Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00011-0 233 Copyright © 2017 Elsevier Inc. All rights reserved.

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234 EXPANDED PTFE APPLICATIONS HANDBOOK 120 TENARA® THREAD POLYESTER COTTON/POLYESTER Table 11.4 Flammability Characteristics of 100 Expanded Polytetraﬂuoroethylene Fibers [1] Break strength 80 Limiting oxygen index >95% Vertical ﬂame test (ASTM D6413) PASS 60 Continuous operating temperature 260C Water regain 40 0% 20 0 Initial 1st Year 2nd Year 3rd Year The ﬂoss was produced from PTFE that had been stretched at elevated temperatures. The resulting EXPOSURE: ARIZONA DESERT, USA ePTFE had a highly porous structure consisting of nodes interconnected by very small ﬁbrils. The Figure 11.1 Effects of UV sunlight on various ﬁbers preferred size of the ﬂoss ranged from 600 to as a result of exposure in Arizona Desert (Tenara is a 1200 denier. The ﬂoss had a tensile strength greater trademark of W.L. Gore & Associates) [1]. than 69 MPa and polymeric matrix strength around 690 MPa. Table 11.2 Example of Available Types of Expanded Polytetraﬂuoroethylene Fibers [1] The individual ePTFE strands could have a denier of 100e1500 denier. When the strand denier Flat and round 30 denier/33 dtex and was <600 denier, it had 2e12 strands usually in a monoﬁlament higher lightly twisted condition to form the strands into a Fibrillated cohesive single thread. In multistranded ePTFE 200 denier/222 dtex and ﬂosses the wax served two purposes. It maintained “Staple” ﬁber (dpf) higher the individual strands in the twisted single thread shape. Second, it increased the coefﬁcient of friction Sewing thread 5 denier/5.6 dtex and from 0.08 to 0.25. Round ﬁber diameter higher The coating process consisted of a strand of the >500 denier/>555 dtex ePTFE immersed in a bath of a microcrystalline wax either in a molten condition or dispersed in an 45 mm and higher organic solvent or carrier. The strand was removed from the bath, and if solvent had been used, allowed Table 11.3 Properties of Expanded Polytetraﬂuoro- to dry to remove the solvent. When the coated ﬂoss ethylene Fibers [1] was inserted between the teeth the wax coating ten- ded to be removed as the ﬂoss was pulled between the Tenacity Up to 7 g/denier contact points of adjacent teeth. That resulted in (62cN/tex) exposing the PTFE thus allowed ease of insertion Tenacity at 200 C between the teeth. Thinner ﬂoss in the range of Up to 2.5 g/denier 600e800 denier was particularly easy to use Modulus (22cN/tex) and provided sufﬁcient tensile strength to resist Density breakage [3]. Melting point 50e8000 ksi Coefﬁcient of friction 0.2e2.2 g/cc An alternative ﬂoss achieved the necessary di- Flex endurance (MIT) mensions for ﬂoss while maintaining an unfolded Thermal conductivity 340 C orientation along its entire length. This was accom- Thermal conductivity 0.01e0.04 plished by employing a relatively thick ePTFE sheet (ﬁlled) >10E6 cycles that is slit from an ePTFE tape/web into the ﬁnal Volume resistivity 0.1e0.3 W/mK dimensions of the ﬂoss and carefully wound on (ﬁlled) 1.5 W/mK spools to avoid rolling, folding, or bending. The ﬂoss comprised a minimum, unfolded thickness of 75 mm >1 Ohm-cm and a minimum width of 0.7 mm [4].

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 235 Figure 11.2 Schematic representation of a ﬂoss ﬁ- elliptical or rectangular. The ﬂoss had an average abrasion break strength 2.8 Â 10À3 kg per denier. ber compressing while passing through a tight con- The ﬂoss had an average surface roughness >0.3 mm, a root mean square roughness >0.35 mm, and a peak to tact between two teeth [4]. valley distance >1.7 mm. Other ﬁbers besides ePTFE could be included in the ﬂoss, each with the same or This ﬂoss had a number of advantages over other different composition. ePTFE ﬂosses. Some of the improved properties were more uniform dimensions (width and thickness) along Several beneﬁts were stated for the low-density its entire length, signiﬁcantly improved compress- ePTFE ﬂoss including having a good feel for the ibility, improved grip ability and handling, and comfort user. The feel included the overall handling charac- during ﬂossing. The inventors discovered that the ﬂoss teristics of the ﬁber as well as the perceived effect of densiﬁed when passed between teeth during ﬂossing the ﬁber in a user’s mouth as it cleans the teeth. (Fig. 11.2). While the unused ﬂoss has an opaque white Baillie et al. further stated that an ePTFE dental ﬂoss color, following ﬂossing the length of ﬂoss used will should be soft and conformable to slide comfortably densify to a remarkably different transparent or trans- between a user’s teeth. At the same time, they said, it lucent color. This property provided an automatic should provide a scrubbing or cleaning sensation for indication of those areas of ﬂoss that had been used. the user when manipulated under the gums. Finally, Another improved property of the invention was its the ﬂoss should have a soft feel to the hands and a signiﬁcantly improved ﬁbrillation resistance over rough feel in the mouth [5]. conventional ePTFE ﬂosses. A 2014 disclosure offers an example of an inno- A dental ﬂoss consisted of ePTFE and non-PTFE vative device thus a change in the ﬂossing method ﬁlaments. The ﬂoss contained from 30% to 70% [7]. The device is a band of ePTFE that could further PTFE ﬁlaments and the rest were non-PTFE ﬁla- stretch from a ﬁrst state to a second stretched state. ments. The ﬁlaments ranged from 100 to 800 denier. The circumference of the band of expanded ﬂuo- The use of ePTFE ﬁlaments was found to be optional ropolymer could be increased by a factor of at least because of cost. The non-PTFE ﬁber was selected two after it was stretched. The cross-sectional area of from a wide range of ﬁbers such as nylon, poly- the band in the stretched state could be reduced to ethylene terephthalate, cellulose, cotton, thermo- one-fourth or less of the cross-sectional area of the plastic polyurethanes, and others. The ﬁbers could be band in the ﬁrst state. The ePTFE ﬂoss device had wax-coated or non-wax coated with the coating choice wax or other materials on its surface to increase the including ﬂavorants and medications including ﬂuo- friction of the band and increase dental cleaning rides, antibacterial agents, cooling agents, coagulants, performance. It could also be placed on a support antibiotics, antiplaque agents, antitartar agents, and device to provide the user with a means to stretch the polishing agents. The ﬂoss as it is formed underwent a band (Fig. 11.3). twisting to form the ﬁlaments into a more cohesive form by 0.4e2 twists per centimeter of ﬁlament [4]. 11.1.2 Sutures Baillie et al. reported on a low-density ePTFE dental Surgical sutures made from ePTFE are used ﬂoss with a denier between 100 and 3500, density as mostly in vascular and cardiac surgery. Pores of the low as 0.2 g/cm3 and as high as 0.8 g/cm3 [5,6]. The ePTFE sutures are interlaced by soft tissues and ﬂoss had a strength in the range of 1.5e10 lbs. The ﬂoss vessels, thus the sutures are permanently incorpo- may be a hollow ﬁber with any cross-section such as rated in the body. As far as wound closure in the oral cavity is concerned, this property is unnecessary dsurgical sutures in the mouth are removed 7e10 days after surgery [8]. Threads of ePTFE possess unique softness and smoothness that enable it to go through soft tissues. That minimizes microdamage reaction around tissue duct, which prevents bacterial colonization in deeper layers of the wound. The thread does not retain coiled shape after unpacking, which makes it easier to work

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236 EXPANDED PTFE APPLICATIONS HANDBOOK First State: Finger Unstretched Stretch Direction Floss Band Second State: Stretched Floss Band Figure 11.4 Use of expanded polytetraﬂuoroethy- lene suture in the surgical repair of Mitral valve chor- Figure 11.3 Isometric view of a dental ﬂoss band of expanded polytetraﬂuoroethylene in an unstretched dae tendineae [12]. (ﬁrst state) and stretched (second state) [7]. 11.1.3 Sewing Threads with. The surgical knot made with a PTFE suture is In many nondemanding applications, conven- durable and does not loosen. The ends of the suture tional polyester sewing threads are used. When do not cause irritation, for example, of the cheek, exposed outdoors polyester threads lose stability lips, and tongue [8]. and become weak within a short period of time depending on the intensity of sunrays, duration of One advantage of ePTFE sutures is that, as exposure, and other environmental factors. Poly- compared to braided absorbable materials, they do ester sewing threads are not suitable for extreme not absorb blood, saliva, bacteria, and food residues. conditions such as intense sunlight, wind, and That promotes wound healing. In spite of its softness moisture. Even water repellent polyester threads are and delicate nature, ePTFE ﬁrmly supports tissues only incrementally more resistant to stress and during the whole healing process. In contrast, moisture than other grades [13]. absorbable materials maintain tissues only in the early stages of healing due to the presence of en- ePTFE threads have a number of properties in both zymes in the mouth. The ePTFE sutures can be indoor and outdoor applications especially where mounted with a smooth transition between the needle long-term durability in challenging environments is and the thread contributing to minimizing trauma of required (Fig. 11.5). Examples include general out- the sewed tissues [8]. door sewing, ﬁshing lines, kite lines and marine awnings, ropes, and sails. PTFE is resistant to sun- The impairment of the components of the mitral light and UV ray resistant, weatherproof, resistant to valve prevents the valve from working properly aggressive cleaning ﬂuids, water and saltwater causing a condition called mitral regurgitation. ePTFE resistant, and extremely colorfast. Plus it does not rot sutures for the surgical repair of ruptured mitral and resists decay. The ePTFE ﬁbers perform well chordae tendineae are one of the most popular treat- when subjected to abrasion and tensile stress. Even ment techniques. Experimental animal studies have long-term sunlight and mechanical stressdfor demonstrated that ePTFE sutures can effectively instance the tensile stress awnings are put under reduce regurgitation. Clinical studies of the long-term (when the fabric is mechanically opened and closed) effect of the ePTFE sutures used for MV repair have or maritime applications (Table 11.5)ddo not reduce reported that these sutures are safe, effective, and seam quality. Tensile strength and the visual quality reproducible. However, the ePTFE sutures occasion- of the seam remain constant under the harshest ally fail due to degeneration, calciﬁcation, and rupture environmental conditions. likely due to different material behaviors between the ePTFE sutures and the native chordae [9e11] (Fig. 11.4).

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 237 Figure 11.5 Example of outdoor awnings [14]. Table 11.5 Outdoor Applications of Expanded Polytetraﬂuoroethylene Sewing Threads [15] Outdoor Marine Awnings Bimini tops Garden structures Boat covers Outdoor upholstery Boat Patio furniture Sun protection and shade enclosures Dodgers sails Sail covers Swimming pool covers Sails Umbrellas Seat cushions Side curtains Upholstery Figure 11.6 An example of ﬁlter bag sewn using expanded polytetraﬂuoroethylene thread [16]. Filter bags (Fig. 11.6), cartridges, and other sewn ﬁlter media are exposed to extreme temperatures Development work to increase the toughness of (230e260C), chemicals, abrasives, and, occasion- ePTFE ﬁbers has lead to improvement of their ally, moist environments, for extended periods. These sewability. For instance a ﬁber with a size in the range conditions degrade the ﬁlter media and thread, and of 700e2200 denier possessed toughness !0.36 g/ the thread often gives out ﬁrst. In some cases the denier had widths of about 0.5 mm to about 5.0 mm, thread is not appropriate for the application, or it and thicknesses of about 30 mm to about 300 mm. simply wears out from chemical, temperature, or Filaments could be optionally twisted together using abrasive attack before the ﬁlter media. This is a two or more monoﬁlaments. As a sewing thread the demanding application ﬁt for ePTFE sewing threads improved properties allowed the ﬁber to be sewn at because of its ability to withstand exposure to harsh high speeds, !1500 stitches/min with fewer thread elements [16]. breaks than conventional PTFE ﬁber (emulsion spun) [17,18]. Clear ePTFE sewing threads are available that virtually disappear in lightly shaded fabrics.

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238 EXPANDED PTFE APPLICATIONS HANDBOOK 11.1.4 Fishing Line These raised regions, or islands, were connected at their bases to the ePTFE structure. The islands were The majority of ﬂuoropolymer ﬁshing lines are distinguishable from the nodes and ﬁbrils because of made of polyvinylidene ﬂuoride. Efforts have been their much larger sizes. The largest length dimension made to develop ePTFE ﬁshing lines. In one inven- of the islands was at least twice that of the dimension tion, “islands” were attached to the ePTFE ﬁber of the nodes. This length difference could even structure that increased its drag coefﬁcient while also exceed 100 times that of the nodes. This island improving its abrasion resistance [19,20]. These structure is unique to the surface of the article and is structures exhibit islands of PTFE attached to and not present below the surface (Fig. 11.8). raised above the ePTFE structures (Fig. 11.7). By “raised” it was meant that when the article was Treating the ePTFE surface in argon plasma, or viewed in cross section the islands were seen to rise using an alternative gas, generated the islands. Heat above the outer surface of the underlying node-ﬁbril treatment following the plasma etch step was structure by a length “h.” required. Neither ePTFE alone nor plasma treating alone without subsequent heat-treatment generated Island the islands. The plasma-treated ﬁber was next sub- jected to a heat-treating step by passing it over a Fiber heated plate set to 390C at fairly high speed [19,20]. Surface Morphology of the islands tended to be different h from the ePTFE structure as determined by differ- ential scanning calorimetry. The thermograms indi- ePTFE Fiber h = height of island cated the ePTFE ﬁber exhibited melt temperatures at above surface or about 327C and 380C as exhibited in graph one in Fig. 11.9. The transition temperature at 380C is Figure 11.7 Schematic view of a cross section of speciﬁc to ePTFE parts. The raised islands (graph expanded polytetraﬂuoroethylene ﬁber showing two) do not exhibit the melt temperature at 380C islands of polytetraﬂuoroethylene above the surface thus do not have the structure, ie, ﬁbril and node, of of the ﬁber [19]. the ePTFE ﬁber. One example of a plasma-/heat-treated ePTFE ﬁber, containing islands, had the following proper- ties: bulk density of 1.52 g/cm3, longitudinal matrix tensile strength of 428 MPa, width of 1.1 mm, and thickness of 0.05 mm. The ﬁber had a drag resistance of 0.196 as deﬁned in US Patents 7,615,282 and Figure 11.8 Scanning electron micrograph of ﬁber surface (A) before, precursor and (B) after plasma and heat treatment of expanded polytetraﬂuoroethylene Finer Showing the Islands on its Surface [20].

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 239 Figure 11.9 Differential scanning calorimetry Ther- line was stretched by the under tension and heat such mograms of (1) raised islands and (2) expanded pol- that tenacity of the resulting ﬁshing line increased by ytetraﬂuoroethylene Fiber [19,20]. at least 10% relative to its tenacity before the stretching. The composite ﬁshing line counteracted 7,736,739. In comparison the untreated material had the buoyancy of high tenacity polyoleﬁn lines that bulk density of 1.52 g/cm3, thickness of 0.05 mm, prevent the sinking of the lines in water. It actually width of 1.2 mm, and matrix tensile strength of had negative buoyancy in water thus allowing it to 561 MPa in the length direction, drag resistance of sink because the speciﬁc gravity of the composite 0.148 [19,20]. line was >1. Other than re-orienting the ePTFE and UHMWPE stretching reduced the diameter of the A 2012 patent describes a composite ﬁshing line ﬁbers but with a tenacity gain at least 10% over the consisted ultrahigh molecular weight polyethylene original composite ﬁber [21]. (UHMWPE) and ePTFE ﬁbers. The composite was either twisted or braided. The bi-component ﬁshing The optimal denier ratio of UHMWPE to ePTFE ﬁber was 1:3 to 3:1. For example, a construction of four yarns of UHMWPE and four yarns of ePTFE were combined, abbreviated to PE 4x100 þ ePTFE 4x200. A braided ﬁshing line was made with those four yarns to determine whether ePTFE could be drawn under two sets of conditions that would be sufﬁcient to redraw the UHMWPE. The latter had a tenacity of 40 g/denier before it was braided and af- terward a tenacity of 13.3 g/denier. The composite line was redrawn in a three-stage process in which the line was heated prior to being drawn using slow-fast roll combinations. Table 11.6 shows the conditions and the results. Table 11.6 Two Examples of Redrawing Composite Fishing Lines of Ultrahigh Molecular Weight Polyethylene and Expanded Polytetraﬂuoroethylene Fibers [21] Example 1 2 Construction 4x100 PE þ 4x200 ePTFE 4x100 PE þ 4x200 ePTFE Total draw ratio Draw roller 1 draw ratio 1.25 1.5 Oven 1 temp (C) 1.083 1.167 Chen 1 res. Time (s) 150 152 Draw roller 2 draw ratio 32.0 30.7 Oven 2 temp (C) 1.077 1.143 Oven 2 res. Time (s) 145 151 Draw roller 3 draw ratio 29.6 26.6 Oven 3 temp (C) 1.071 1.125 Oven 3 res. Time (s) 145 151 Avg. Diameter (in.) 27.6 23.5 Avg. Break strength (kgforce) 0.0133 0.0119 Final denier 14.7 12.8 Final tenacity (g/denier) 1054.0 883.5 14.2 14.5

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240 EXPANDED PTFE APPLICATIONS HANDBOOK Ten samples of the resulting ﬁshing line were then spooling process. This folding process is difﬁcult to measured for diameter to determine whether either control and to maintain in the ﬁnal product. It thus component was relaxing once tension had been results in a ﬁber with inconsistent width and thick- removed or otherwise not irreversibly extended as a ness along its length. Another weakness is the result of the drawing process. The results in exposed thin edges of ePTFE ﬁbers during process- Table 11.6 show that braids made with gel-spun ing fray or ﬁbrillate as a result of mechanical polyethylene and ePTFE can, in fact, be redrawn in handling. postformation processing. The fairly narrow range of diameters around the average showed that the braid is To address some of these concerns a number of not exhibiting post-relaxation puckering or loosening alternative ePTFE ﬁber constructions have been of the braid. attempted. Folding and/or twisting the ePTFE ﬁber can signiﬁcantly reduce its tendency to fray or 11.1.5 Weaving and Knitting Fiber ﬁbrillate. These processing steps are often difﬁcult to perform while maintaining uniform width and From the early days of ePTFE, it has been used as thickness dimensions. Moreover, for certain appli- a thread and as a component in woven fabrics along cations where a very ﬂat weave is desired, these with other ﬁbers. The fabrics containing ePTFE yarns alternative processing steps have been relatively un- have a number of advantages over others materials. successful in delivering a suitable product. For example, ePTFE ﬁbers are chemically inert, are resistant to high temperatures, possess high tensile Abrams et al. developed an ePTFE ﬁber with strength, have a high dielectric constant and are uniform width dimensions that retained width uni- highly lubricious. Additionally, these materials can formity when it was woven into a fabric [22,23]. The be modiﬁed to impart other desirable properties by ﬁber was not folded or twisted prior to or during incorporating ﬁllers to enhance the thermal and/or weaving plus it was resistant to fraying, ﬁbrillation, electrical conductivity of ePTFE ﬁbers [22,23]. and shredding. An example of a procedure to produce such a ﬁber is described. Fig. 11.10 shows examples One of the drawbacks of ePTFE materials is that of woven and knit patterns of ePTFE ﬁbers. they tend to be difﬁcult to process as well as having a number of structural problems. Common weaving A dry paste extruded PTFE tape was expanded yarns and ﬁbers such as nylon or polyester consist of uniaxially in the longitudinal direction 1.9 times its multiple ﬁlaments that are either twisted or interlaced original length by passing the dry coherent extrudate into a thread with uniform dimensions. In contrast, over a series of rotating heated rollers at a tempera- ePTFE ﬁbers are formed from thin ﬂat tapes slit into ture of 275C. The expanded extrudate was slit to single ﬁlament strands and then folded prior to the 5.1 mm widths by passing it between a set of gapped blades. The slit extrudate was further expanded uni- axially in the longitudinal direction over hot plates at Figure 11.10 Examples of woven and knit patterns of expanded polytetraﬂuoroethylene ﬁbers [24].

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 241 a temperature of 335C at a total ratio of 26 to 1 to commercial ﬁber. The ﬁgure indicates a signiﬁcant reduction of the thickness variability of the new ﬁber form a ﬁber. This ﬁber was subsequently subjected to versus the commercial ﬁber. an amorphous locking step by passing the ﬁber over a A 2015 patent described woven and knit fabrics of heated plate set at a temperature of 400C for about ePTFE and one or more other ﬁber(s). The ePTFE ﬁbers had a rectangular conﬁguration. The ePTFE 1 s. The following procedure was used to determine ﬁber could be a single, non-twisted ﬁber, or twisted or braided with another ﬁber in either of the warp thickness variability: and/or the weft directions. The woven and knit fab- rics were breathable, lightweight, durable, drapable, 1. Start at a random place on the ﬁber’s length by and fast drying. The woven and knit fabrics were selecting a point on the ﬁber by unwinding the quiet, soft, and drapable. The surface of the ePTFE ﬁber off its spool or core. ﬁber or its fabrics could be treated to impart desired functionalities such as oleophobicity, antimicrobial, 2. After selecting a starting point at random ﬁnd contamination resistance, or UV stability. Fabrics the largest and smallest thickness within a made of ePTFE ﬁbers alone were ﬂame resistant [25]. 50 cm section (at least 10 measurements must be taken) starting from the random starting (1) One example was combination of ePTFE ﬁbers point. Measure the thickness using a snap with rectangular cross-sectional conﬁguration and (2) gauge having a precision of 2.54 mm. at least one non-ePTFE ﬁber. Density of the ePTFE ﬁbers before weaving ranged from 0.1 to 2.2 g/cm3; 3. Continue by selecting another random starting its aspect ratio was >1. The ePTFE ﬁber could be point and repeat the Step 2. partially ﬁlled with oil and/or a polymer. The fabric had a dry time <30 min, a vertical wicking of 4. Repeat Step 3 until 10 random lengths have >90 mm/10 min, and weight per unit area been sampled. of <1000 g/m2. The woven fabric was also ﬂexible and had an average stiffness of <1000 g. A polymer 5. Compute the delta thickness percent by the membrane could be attached to one or both sides of following formula: the ePTFE fabric to form a laminated structure. The woven fabric had high moisture vapor transmission Dt ¼ 2*ðtmax À tminÞ * 100 (11.1) (ie, highly breathable). ðtmax þ tminÞ Another example related to a knit fabric that Dt ¼ thickness variability; tmax ¼ maximum thick- included (1) ePTFE ﬁbers having a substantially ness, mm; tmin ¼ minimum thickness, mm. rectangular conﬁguration and a density greater than about 1.2 g/cm3 and (2) knitting ﬁbers. The ePTFE Fig. 11.11 shows a plot of the results of running ﬁbers may have a preknitting density from 1.0 to 2.2 g/cm3. The knit fabric had a vertical wicking of the above procedure on an ePTFE ﬁber produced >10 mm/10 min and a weight per unit area of <1000 g/m2. The ePTFE ﬁbers may be at least according to the Abrams et al.’s procedure and a partially ﬁlled with oil and/or a polymer. By choosing a ﬁre-resistant ﬁber to knit with ePTFE the fabric was Figure 11.11 Graph of the thickness uniformity of ﬁre resistant [25]. the Abrams et al. expanded polytetraﬂuoroethylene ﬁber compared with a commercial polytetraﬂuoro- A unique application of woven ePTFE ﬁber is in ethylene ﬁber [22]. the construction of divider membrane of chloralkali cells. The anode and cathode cell used to be sepa- rated mostly using mercury and some asbestos. Per- ﬂuorinated ionomer membranes such as Naﬁon by DuPont and Flemion by Asahi have replaced the mercury and asbestos. Fig. 11.12 shows the sche- matic of a cell membrane containing sulfonic and carboxylic perﬂuorinated ionomers. Either ePTFE ﬁbers or woven cloth of ePTFE ﬁbers are used to

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242 EXPANDED PTFE APPLICATIONS HANDBOOK Reinforcement ‘Shadow’ Effect because of the low friction coefﬁcient of PTFE, thus electrically invisible reducing the elevated temperatures normally brought Woven ePTFE Fiber Improved current about by friction. uniform distribution United States Patent No. 7,296,394 [28] described Sacrificial Carboxylic Polymer a composite bundle for repeated stress applications. It fiber/channel contained one or more high strength ﬁbers and one or Improved Gas more ePTFE ﬁbers. Improvements in reducing fric- Sulfonic Polymer Release Coating tion and preserving the ﬁber bundle strength were observed even at a weight content of 5% ePTFE. Improved Gas Abrasion testing was done according to ASTM Release Coating Method D6611, yarn-on-yarn abrasion. Examples of high strength ﬁbers included one or a combination of Figure 11.12 Schematic of a cell separation mem- a liquid crystal polymer (LCP) or an UHMWPE. A brane using expanded polytetraﬂuoroethylene composite bundle without and with ePTFE ﬁber had (ePTFE) ﬁber/woven cloth reinforcement [26]. a post abrasion test ratio of break strengths >1.8 and as high as >4.0 (see Fig. 11.14). reinforce the separation membrane of high perfor- mance chloralkali cells. The narrow ePTFE ribbons A single ePTFE ﬁber was combined with another are twisted (4e10 twists/cm) into a ﬁlament. These ﬁber such as a LCP ﬁber (Vectran, Celanese Acetate) twisted ribbons can be made to have a very high and subjected to the abrasion test (ASTM Method tensile modulus. Monoﬁlaments with a circular cross D6611). The results from this test were compared section have been introduced in this application to against the results from the test of a single LCP ﬁber resolve problems with ﬂat ePTFE ﬁbers [26]. without (ePTFE). The ePTFE monoﬁlament ﬁber of the tests was obtained from W.L. Gore and Associate 11.1.6 Ropes (HT400 d Rastex). This ﬁber had the following properties: 425 d weight per unit length, 2.29 kg Ropes are used in numerous applications break force, 5.38 g/denier tenacity, and 1.78 g/cm3 including in deep-water subsea operations, elevators, density. The results of abrasion testing according to hoists, cranes, and in mining. In high-stress situations ASTM Method D6611 are summarized in Table 11.7. synthetic ropes take a severe beating. These appli- It can be seen the post abrasion test break strength cations encounter high-tension, high-stress bending ratios of combination ﬁbers with and without ePTFE and lifting where rope reliability and performance are ranged from 1.8 to 4.1. critical (Fig. 11.13). Adding ePTFE ﬁber to the rope construction enhances its performance reliability and Fig. 11.15 illustrates the extended service life of prevents premature failure. The ePTFE ﬁber acts as a ropes using ePTFE combined with various high durable dry barrier to abrasion, reduces friction, strength rope materials. The addition of ePTFE ﬁber dramatically increased the number of cycles that each Figure 11.13 Example of rope containing expanded rope could endure. The ﬁbers tested were LCP and high polytetraﬂuoroethylene ﬁber in high-tension, high- molecular weight polyethylene (HMPE). Bend-over- stress bending and lifting application [27]. sheave tests were run for 12-strand LCP and HMPE ﬁber ropes constructed with and without the addition of ePTFE ﬁber. Test conditions were load 20% break strength, D/d ¼ 20:1, and (a) rate of 500 cycles/h for LCP and (b) rate 360 cycles/h for HMPE [30]. D/d is the ratio of pulley disk diameter and the rope. The reasons for improvement reside in the char- acteristic behavior of PTFE when subjected to abra- sion. The subject has been researched thoroughly since 1960’s. In contrast to other high performance polymers, PTFE is quite soft and when rubbed against a surface leaves a thin ﬁlm behind. The transferred material, in a low friction regime, is in the form of thin ﬁlm with a thickness range of

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 243 Crank and Yarn Attachment Pulley Pulley 70 mm 70 mm Gear Motor 254 mm Tension Weight Interwrapped Yarn Region Pulley Figure 11.14 Yarn-on-yarn test setup according to ASTM Method D6611 [29]. Table 11.7 Abrasion Test Results of Combining a Single ePTFE Fiber With Other Fibers [28] Ratio of Abrasion Rate Ratio of Break (g/cycle) Abrasion Strengths After Rates Break Strength after Abrasion Abrasion Test (kg) Test Composition (weight%, Fiber Type) With Without With With Without With 21% Monoﬁlament ePTFE, 79% LCP ePTFE ePTFE ePTFE/ ePTFE ePTFE ePTFE/ 26.38 Without Without 21% Monoﬁlament ePTFE, 79% 42.29 13.21 ePTFE 5 14.2 ePTFE UHMWPE 18.8 41.37 10.9 2 79.8 2.84 20% Multiﬁlament ePTFE, 80% 18 UHMWPE 17.4 3.88 4.24 57.5 17% Monoﬁlament ePTFE, 83% 36.73 10.9 3.8 79.8 4.43 para-aramide 25.2 99.07 9.29 1.87 77.7 1.35 23% C-ﬁlled monoﬁlament ePTFE, 35.9 77% UHMWPE 100.49 10.9 3.37 79.8 3.17 36.7 8% Monoﬁlament ePTFE, 92% 44.26 23.9 4.14 167.6 4.67 UHMWPE 10 39.64 23.9 4.2 167.6 4.57 31% Monoﬁlament ePTFE, 69% 19.74 UHMWPE 27.87 10.9 4.06 79.8 7.98 45.14 38% Monoﬁlament ePTFE, 62% 10.9 3.64 79.8 4.04 UHMWPE 10.9 2.56 79.8 1.77 20% Matrix-spun PTFE, 80% UHMWPE 21% ETFE, 79% UHMWPE ETFE, ethylene-tetraﬂuoroethylene copolymer; LCP, liquid crystal polymer; PTFE, polytetraﬂuoroethylene; UHMWPE, ultrahigh molecular weight polyethylene.

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244 EXPANDED PTFE APPLICATIONS HANDBOOK (A) Bending Fatigue Life 12 LCP Strands 12 LCP Strands + one ePTFE Strand 2000% 1500% 1000% 500% 0% (B) 500% Bending Fatigue Life 400% 300% 200% 100% 0% HMWPE Strands 12 HMWPE Strands + one ePTFE Strand Figure 11.15 Bend over sheave tests results for 12- Figure 11.16 Example of a rope, which consisted of strand liquid crystal polymer (LCP) and high molecular bundles of ﬁbers including an expanded polytetra- weight polyethylene (HMPE) ﬁber ropes constructed ﬂuoroethylene (ePTFE) ﬁber [28]. with and without adding expanded polytetraﬂuoroethy- lene (ePTFE) ﬁber [30]. 10e40 nm. Contact surfaces are covered with PTFE, weight though the effectiveness was detectable at thus have low coefﬁcient of friction. The thin PTFE 5% ﬁber content. The metal wire was preferably ﬁlm is removed and regenerated as long PTFE is steel or copper (>2000 denier) and possibly lubri- rubbed against the surface. PTFE has found a number cated using oil. A monoﬁlament of ePTFE ﬁber of applications as a lubricant in powder and oil worked best [33]. dispersion. This phenomenon occurs even if the bulk of PTFE is highly oriented and strong [31,32]. 11.2 Gaskets and Seals Based on the contribution of PTFE to reducing The earliest PTFE gaskets were made from sheets yarn-on-yarn friction ropes have been design. manufactured by peeling or skiving a compression- Although blending of ﬂuoropolymer ﬁbers within molded cylinder of PTFE. Gaskets were punched ropes without particular attention to speciﬁc posi- out of the PTFE sheet and at a fairly low cost. Creep tioning within the rope signiﬁcantly enhanced fatigue or cold ﬂow of PTFE led to the loss of bolt load thus life, speciﬁc positioning of the ﬂuoropolymers within leakage, aggravated by elevated temperatures. In an the rope structure offered the ability to even further effort to enhance the mechanical properties of PTFE enhance life. Fig. 11.16 shows an example of a rope ﬁllers were incorporated into the PTFE to reduce that consisted of bundle groups. Each group was creep, cold ﬂow, and bold load loss behavior. Im- comprised of bundles of ﬁbers. Each bundle group provements in those properties of PTFE were incre- was wrapped with an ePTFE ﬁber. Alternatively, mental accompanied with loss of purity and reduced each bundle of ﬁber could be wrapped with an ePTFE chemical resistance due to the ﬁllers [34]. ePTFE ﬁber. gaskets exhibited signiﬁcant creep reduction and bolt-load retention, maintained purity, and preserved In another example a low-friction wire rope was all chemical resistance characteristics of PTFE developed that included one or more metal wires (Fig. 11.17). and at least one ePTFE ﬁber. The wire rope is useful in tensioned and bending applications. The ﬁber was present in an amount less than 25% by

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 245 Figure 11.17 Comparison of bold load retention for various 6.3-mm thick gaskets [35]. ePTFE, expanded poly- tetraﬂuoroethylene; PTFE, polytetraﬂuoroethylene. Figure 11.18 A perspective view of stacked plies of expanded polytetraﬂuoroethylene membrane lami- nated into a gasket [36]. Chapter 6 describes ePTFE gasket manufacturing Figure 11.19 Remaining thickness of expanded pol- methods, all of which aim to stack layers of biaxially ytetraﬂuoroethylene gaskets at different tempera- oriented membranes. Pressure and heat are used to tures and surface pressures [37]. induce self-adherence of the membrane plies (Fig. 11.18). A description of a process for lamina- Figure 11.20 An example of expanded polytetra- tion of ePTFE membranes is described in Ref. [36]. ﬂuoroethylene gasket sheet [39]. The high-compressibility ePTFE gaskets are ﬂoppy and can be difﬁcult to install. In operation, ePTFE gaskets compress to be thin (Fig. 11.19), thus pre- vent any appreciable gasket recovery or rebound which may affect reliability in process cycling ap- plications [38]. The biaxial stretching process by which ePTFE sheets (Fig. 11.20) are manufactured imparts a multidirectional strength that gives them advantage over the other PTFE gaskets. Even at elevated tem- peratures and higher surface pressures ePTFE gasket showed little or no increase in width or length because of movement restraints caused by the node

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246 EXPANDED PTFE APPLICATIONS HANDBOOK range of sealing applications including those listed here [40]: Steel pipe ﬂanges Ventilation ducts Heat exchangers Plastic pipe ﬂanges Manhole and hand-hole Compressor housing covers ﬂanges Glass-lined pipe ﬂanges Hydraulic and pneumatic systems Fiberglass-reinforced Fume ducts vessels Figure 11.21 A variety of complex and simple Ceramic joints Turbine cases gasket shapes punched out of expanded polytetra- ﬂuoroethylene sheets [39]. Fiberglass pipe ﬂanges Water supply systems Pump housing ﬂanges Concrete lids Fig. 11.23 shows examples of ﬂat face and raised ﬂanges. Both surfaces can be sealed using an ePTFE joint sealant beads. To facilitate seating the joint sealant on the ﬂange surface accurately sometimes a contact adhesive is applied to one surface of the bead. Typically, the adhesive was applied along with a protective release paper attached to the bead. At the time of application of the joint sealant gasket the release paper was peeled off (Fig. 11.24). Figure 11.22 Example of commercial packages of expanded polytetraﬂuoroethylene joint sealant gasket [41]. and ﬁbril structure. While compressibility of these Figure 11.23 Two styles of expanded polytetra- gaskets could cause problem in some cases their ﬂuoroethylene joint sealants: (A) ﬂat faced and adaptability to unevenness and damages on the ﬂange (B) raised faced [42]. surfaces is a major advantage. Another feature of ePTFE gaskets is the ability to cut or punch out Figure 11.24 Correct removal of the protective strip complex and precise gasket shapes (Fig. 11.21) for from an expanded polytetraﬂuoroethylene joint applications including heat exchangers, agitators, sealant bead [43]. and pipeline ﬂanges; and pressure vessels. Joint sealants beads made of ePTFE are soft and conformable, resembling the texture of marsh- mallow. They conform to seal ﬂanges with irregular and damaged surfaces quite readily (Fig. 11.22). These beads are expanded in one axis and tend to be narrow and thick. Joint sealants made of ePTFE are versatile thanks to chemical and thermal prop- erties of PTFE. These gaskets have found a wide

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 247 Table 11.8 Deﬁnition of Gasket Constants Using Room Temperature Operational Tightness Test Technique [44] a The slope obtained by linear regression. It indicates the capacity of the gasket to ensure tightness. Gs The gasket stress at Tp ¼ 1 when unloading the gasket. It indicates the capacity of the gasket to maintain tightness when pressure is applied, as well as the gasket’s sensitivity to unloading. Tp The tightness parameter is dimensionless. A value of 1 corresponds to a helium leak rate of 1 mg/s under atmospheric pressure for a gasket with an outside diameter of 150 mm. Note: The greater the Tp, the greater the gasket tightness. Tpmax The maximum tightness obtained when loading the gasket. Tpmin The minimum tightness obtained when unloading the gasket. 11.2.1 Testing Gaskets are taken with at least two different helium pressure levels. Some 75% of gaskets are installed to seal pipes. Pressure Vessel Research Council (PVRC) has put Part B represents the changes on gasket stresses in forth a protocol called Room Temperature Opera- service (unloading from hydraulic ﬂuid pressure, tional Tightness Test (ROTT), in the United States, to external loads, etc.) by applying successive loading characterize pipe gaskets (Table 11.8) [45]. There are and unloading cycles and measuring the changes in several standards for pipe gaskets in the UK, Ger- leak rates at a constant inner pressure. Representing, many, European Union, and Japan [46]. ePTFE gas- in ordinates, the logarithm of the loading and kets have been qualiﬁed for food and drug contact unloading gasket stress and in abscissae, the loga- rules and standards such as FDA 21 CFR 177.1550, rithm of Tp, a plot (Fig. 11.25) is obtained with three USP Class VI, TA-Luft (German), and 3-A sanitary new gasket factors, Gb, a, Gs. Gb, is the loading stress standards. corresponding to Tp ¼ 1 (log Tp ¼ 0), a is the slope of the gasket loading line. ROTT provides constants by monitoring leakage over a series of applied stresses to a gasket sealing a Combined values of Gb describe the seating small vessel ﬁlled with helium. He was selected characteristics of a gasket and its capacity to develop because it has a small molecule thus hard to seal and tightness upon initial sealing. Gs, is a theoretical is safe to handle. In the ﬁrst part of the test (Part A) point, representing the intercept of the set of possible a sequence of increasing compressive stress levels is unloading lines with the Tp ¼ 1 line. It represents the applied to the gasket while leakage is measured. gasket potential to maintain tightness after pressuri- Minimum seating stress is measured at gas pressures zation and during operation [47]. of 2.8 and 5.5 MPa internal pressure. The second part of the test (Part B) is a series of unload, reload Figure 11.25 Logelog gasket stress versus tight- sequences at higher gasket stress levels while ness parameter for Room Temperature Operational 5.5 MPa internal vessel pressure is maintained (see Tightness Test procedure [47]. both Table 11.8 and Fig. 11.24 for an ePTFE pipe gasket). This test provides the gasket constants Gb, a, and Gs. The dimensionless number, Tp (Tightness param- eter), was introduced to link the tightness level of a connection with the internal pressure. For each gasket stress there is a Tp value. The higher the Tp value the higher the tightness of the connection. The ROTT test is a combined compression, relaxation, and helium leakage test at room temperature using a 10 cm Nominal Bore ﬂanges. Part A refers to the assembly conditions during which growing stresses are applied on the gasket, and leakage measurements

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248 EXPANDED PTFE APPLICATIONS HANDBOOK 0.100000 0.010000 He Leak Rate (mg/s) 0.001000 0.000100 0.000010 0.000001 6.9 69 690 0.69 Gasket Stress (MPa) Figure 11.26 Room temperature operational tightness test resultsdHe leak rate versus gasket stress for Gore Universal Pipe Gasket-Style 800. The horizontal red (dark gray in print versions) and blue (gray in print versions) lines separate the leak rate into low, medium, and high [48]. HP sequence He pressure ¼ 6 MPa (blue (gray in print versions)); LP sequence He pressure ¼ 2 MPa (red (black in print versions)). The ROTT test results characterizing the tightness Table 11.10 Speciﬁcations of a Flat Expanded Pol- behavior of an ePTFE-based gasket (Gore Universal ytetraﬂuoroethylene (ePTFE) Gasket for Sealing Ap- Pipe Gasket-Style 800) are summarized in Fig. 11.26. plications in the Chemical Processing Industry [50] The gasket constants Gb, a, and Gs related to a tightness parameter Tp can be determined from this 1.5 mm 3.0 mm ﬁgure. Table 11.9 lists the gasket constants of a biaxially oriented ePTFE gasket (Gore GR sheet TF-0-0 gasketing). DIN 28091-3 ePTFE gaskets provide fast, easy installation and tight, reliable sealing for glass-lined steel, plastic, ePTFE and steel ﬂanges under demanding conditions. They are usually made from 100% PTFE and are available White in tape, cord, and sheet gasketing forms. Table 11.10 shows speciﬁcations of ePTFE gaskets at two FDA 21 CFR 177.1550 thicknesses. USP Class VI (þ121C/þ250F) TA Luft Table 11.9 Room Temperature Operational Tight- 11 N/mm2 0.85 g/cm3 ness Test Constants for an expanded Polytetra- (transverse) 14.5 N/mm2 (transverse) ﬂuoroethylene Gasket [49] 69% 66.2% Gasket Gasket Value 7.6% 11.2% Constant Thickness, mm 4.7 Gb, MPa 5.3 À268C to þ315C Gb, MPa 1.6 a 3.2 0.271 11.3 Expanded 1.6 0.274 Polytetraﬂuoroethylene Vents a 3.2 4.3Â10À2 Gs, MPa 1.6 6.5Â10À9 There are many systems that either require or Gs, MPa 3.2 beneﬁt from venting. For example, automobile gas

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 249 Oil, Liquid water, Ostomy and urine bags Powder Solar panels Display (OLED, LCD, LED, etc.) Gas Caps and closures Medical device vents ePTFE Bottles, vials, boxes, and containers Vent Household and consumer chemical packaging Industrial chemicals Figure 11.27 Schematic depiction of the working Agricultural products mechanism of an expanded polytetraﬂuoroethylene Life science and biotechnological substances membrane. In most applications the ePTFE membrane is tanks have a vent to prevent over-pressurization at thermally bonded to polypropylene, polyethylene, high air temperatures or vacuum in low air temper- and polyester substrates for reinforcement. The vents atures. Membrane vents are designed to enhance the are available in ﬁlm thicknesses from 5.1 to 254 mm ingress protection of gasketed enclosures. The with pore sizes ranging from 0.05 mm up to 7.0 mm microporous ePTFE membrane lets continuous free [52]. Fig. 11.28 shows a few different types of plug passage of gases and vapors, equalizing the pressure vents. The membrane (Fig. 11.29) is installed on the difference between the enclosure and ambient pre- topside of the plug. venting pressure build-up that may compromise a seal. A number of hearing instruments contain micro- phones that convert sound waves into electrical sig- Water, dust, dirt, cleaning agents, and most oils are nals that are processed and retransmitted. These repelled by the ePTFE membrane thus protecting microphones must be protected from liquids and expensive and sensitive electronics from those haz- particulates without blocking the sound waves. ards (Fig. 11.27). The free ﬂow of gases through Acoustic ePTFE membrane vents protect hearing membrane vents is vital to a waterproof battery- devices by blocking the entry of contaminants while powered device. The ePTFE membrane allows minimizing transmission loss and attenuation. The diffusion of hydrogen gas out of the device enclosure vent is integrated into Siemens’ new Aquaris water- thus keeping the hydrogen concentration below proof hearing instrument series to preserve the mi- explosive levels. Membrane vents are installed in crophone’s sound quality while protecting it from automotive, electronic, medical, electrochemical water, dirt, and dust particles (Fig. 11.30). A micro- sensors, liquid packaging, and others [51]. Some phone cover protects the GORE acoustic vent from speciﬁc examples include [52,53]: Figure 11.28 Examples of expanded polytetraﬂuoro- Mobile phones ethylene membrane plug vents [51]. Electronic enclosures MEMS/microsystems Hearing aids Sensors Headlamps Transducer protectors

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250 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 11.29 Rolls of ready-to-use expanded polyte- membrane, dust particles, or water cannot stick or traﬂuoroethylene membranes [54]. penetrate the membrane surface [55]. Figure 11.30 Sound wave transfer in an expanded In summary, ePTFE membranes, as compared to polytetraﬂuoroethylene acoustic membrane vent [55]. other porous materials, show superior performance as physical damage and reduces unpleasant noises a barrier to liquids, particles, and aerosols that may caused by sources such as wind, raindrops, and contain viable bacteria and viruses. While protecting rustling hair [55]. against contaminants is critical, the membrane must also provide other required functionalities such as ePTFE membranes for acoustic immersion pro- pressure equalization, air/gas elimination, and air/gas tected applications have engineered mechanical delivery. The sum of PTFE and ePTFE properties properties that reduce acoustic losses and distortions. have resulted in a vast array of applications that Consequently, the air sound vibrations are converted continue to be further expanded as time passes. into membrane mechanical vibrations, which are in turn reproduced on the other side of the membrane References (Fig. 11.29). Low surface energy of the ePTFE [1] ePTFE Fiber Solutions, Product Information, W.L. Gore & Associates, 2009. www.gore.com/ ﬁbers, 001.03. [2] www.webmd.com, March 2016. [3] J.P. Curtis, J.H. Kemp, US Patent 5,209,251, Assigned to Colgate-Palmolive Co, May 11, 1994. [4] J.W. Dolan. R.B. Minor, J.W. Spencer, Jr., US Patent 5,518,012, Assigned to W.L. Gore & Associates, May 1996. [5] R.L. Baillie, J.H. Chastain, J.W. Dolan, W.H. Wiley, US Patent 6,539,951, Assigned to Gore Enterprise Holdings, April 2004. [6] R.L. Baillie, J.H. Chastain, J.W. Dolan, W.H. Wiley, US Patent 7,060,354, Assigned to Gore Enterprise Holdings, June 2006. [7] J.W. Dolan, M.F. Altman, D.L. Hollenbaugh, Jr., R. Radspinner, A.R. Hobson, US Patent 8,726,917, Assigned to W.L. Gore & Associates, May 2014. [8] Coreﬂon® PTFE Surgical Suture, March 2016. www.coreﬂon.pl/ptfe.php. [9] R.G. Gayle, J.R. Wheeler, R.T. Gregory, S.O. Snyder Jr., Evaluation of the expanded polytetraﬂuoroethylene (ePTFE) suture in pe- ripheral vascular surgery using ePTFE prosthetic vascular grafts, J. Cardiovasc. Surg. (Torino) 29 (5) (SeptembereOctober 1988) 556e559. [10] T.E. David, A. Omran, S. Armstrong, Z. Sun, J. Ivanov, Long-term results of mitral valve repair for myxomatous disease with and without chordal replacement with expanded polytetra- ﬂuoroethylene sutures, J. Thorac. Cardiovasc. Surg. 115 (6) (1998) 1279e1285 discussion pp.1285e1276.

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11: INDUSTRIAL AND OTHER APPLICATIONS OF EXPANDED PTFE 251 [11] Y. Rim, S.T. Laing, D.D. McPherson, H. Kim, [28] N.E. Clough, D.I. Lutz, G. Harp, US Patent Mitral valve repair using ePTFE sutures for 7,296,394, Assigned to Gore Enterprise Hold- ruptured mitral chordae tendineae: a computa- ings, November 2007. tional simulation study, Ann. Biomed. Eng. 42 (1) (January 2014). [29] Yarn-on-Yarn Abrasion Test, Quantitative Mea- sure of Yarn Durability, Tension Technology Int, [12] GORE-TEX® Suture for Chordae Tendineae, by 18, January 2005. Technical Notes. W.L. Gore & Associates, www.goremedical. com/suturechordae, March 2016. [30] GORE™OMNIBEND® Fiber e Unprece- dented Bending Life for the most Demanding [13] Service & Technology Information for the Synthetic Rope Applications, W.L. Gore & As- Sewing Industry, Amann & Soehne GmbH, sociates, December 2008. www.gore.com/ www.amann.com, December 2009. omnibend. [14] MakMax Umbrellas, Sunteca Corp, Australia, [31] S.K. Biswas, K. Vijayan, Friction and wear of www.awnings.com.au, March 2015. PTFE e a review, Wear 158 (1992) 193e211. [15] Tenara® Sewing Thread, W.L. Gore & Associ- [32] X. Lu, K.C. Wong, P.C. Wong, K.A.R. Mitchell, ates, 2015. J. Cotter, D.T. Eadie, Surface characterization of PTFE transfer ﬁlms during rollingesliding [16] GORE® Rasted® Fibers Sewing Thread e tribology tests using X-ray photoelectron spec- Proven Reliability, W.L. Gore & Associates, troscopy, Wear 261 (2006) 1155e1162. 2011-2013. [33] N.E. Clough, R. Sassa, US Patent 7,409,815, [17] T.J. Kelmartin, Jr, G.M. Roberts, J.D. Dolan, Assigned to Gore Enterprise Holdings, August R.B. Minor, US Patent 5,989,709, Assigned to 2008. Gore Enterprise Holding, November 23, 1999. [34] S. Ebnesajjad, Fluoroplastics: Non-melt Pro- [18] T.J. Kelmartin, Jr, G.M. Roberts, J.D. Dolan, cessible Fluoropolymers, in: Plastics Design R.B. Minor, US Patent 6,071,452, Assigned to Library, second ed., vol. 1, Elsevier, 2014. Gore Enterprise Holding, June 6, 2000. [35] C.P. Ganatra, C. Jones, Gasket Standardization: [19] D.I. Lutz, N.E. Clough, US Patent 7,615,282, It’s Now Becoming a Reality in the Chemical Assigned to Gore Enterprise Holding, November Processing Industry, White Paper, W.L. Gore & 2009. Associates, April 2016. www.Gore.com. [20] D.I. Lutz, N.E. Clough, US Patent 7,736,739, [36] H. Hisano, S. Urakami, US Patent Assigned to Gore Enterprise Holding, June 2010. 8,784,983, Assigned to W.L. Gore & Associ- ates, July 2014. [21] R. Cook, J. Thelen, J. Meyer, US 8,181,438, Assigned to pure ﬁshing, May 2012. [37] WT-A ePTFE Gasket Sheet, Product Informa- tion, WLT Dichtungstechnik, e.K., www.wlt- [22] B.F. Abrams, R.B. Minor, G.L. McGregor, J.W. dichtungstechnik.de, April 2016. Dolan, US Patent 5,571,605, Assigned to W.L. Gore & Associates, November 5, 1996. [38] PTFE Gasket Technology, Virginia Sealing Products, www.vsptechnologies.com, April [23] B.F. Abrams, R.B. Minor, G.L. McGregor, J.W. 2016. Dolan, US Patent 5,591,526, assigned to W.L. Gore & Associates, January 7, 1997. [39] WT-A ePTFE Gasket Sheet, WLT Dichtung- stechnik, e.K., www.wlt-dichtungstechnik.de, [24] Gore PTFE Fiber Solutions, W.L Gore & Asso- April 2016. ciates, March 2016. www.gore.com/en_xx/ products/ﬁbers/architectural/gore_eptfe_ﬁbers. [40] Expanded PTFE Joint Sealant, Product Infor- html. mation No. R.050415, Teadit North America, www.teadit-na.com, (undated). [25] D.J. Minor, US Patent Application 20150361599, Assigned to W.L. Gore & Asso- [41] ePTFE Flange Joint Sealant, April 2016. www. ciates, December 17, 2015. gasket-tools.com/sealants-ptfe-joint-seal.asp. [26] W. Grot, Fluorinated Ionomers, second ed., [42] Gore Joint Sealants, Installation Instructions No. Plastics Design Library, Elsevier, 2011. SEAL-82-TEC-EN-AUG13, W.L. Gore & As- sociates, 2010e2013. [27] GORE® OMNIBEND® Fiber for High Perfor- mance Ropes, W.L. Gore & Associates, March [43] Adtex Expanded Sealing Tape, ADTech Corp., 2016. www.gore.com/en_xx/products/ﬁbers/ April 2016. www.adtech.co.uk. ropeﬁber/rope_ﬁber.html.

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252 EXPANDED PTFE APPLICATIONS HANDBOOK [44] GORE® Universal Pipe Gasket-Style 800, [50] Flat Gaskets for the Process Industry, Freudenberg Room Temperature Tightness with Crush Sealing Technologies, April 2016. www.fst.com. (ROTT) Test Method Overview, April 2016. www.gore.com/en_xx/products/sealants/ [51] Gore™ Membrane Vents, Product Info No. IP- gaskets/upg-gaskets-performance-tests.html. MFT/333E 06.05/GB, W.L. Gore & Associ- ates, www.gore.com/ventsolutions, 2005. [45] Testing Gasket Leakage Performance, Leader GT Corporation, www.leadergt.com, 2008. [52] Tetratex® ePTFE Venting Solutions, Product Info No. TEVS001/EN/04/08, www.donaldson. [46] RAM Gasket Solutions www.ramgaskets.com/ com, 2008. gaskets/pipe-gaskets, April 2016. [53] Venting Products, CLARCOR Industrial Air, [47] F.J. Montero, Tightness in Gasketed Flanged www.clarcorindustrialair.com, April 2016. Unions e Part II: New Standards for Calculation and Testing, November 2012, pp. 163e164. [54] GE Micro-venting ePTFE Membrane Technol- www.valve-world.net. ogies, General Electric Co, January 2005 now CLARCOR Industrial Air, www. [48] C.P. Ganatra, C. Jones, Gasket Standardization, clarcorindustrialair.com. GEA-13962. White Paper, W.L. Gore & Associates, April 2016. www.Gore.com. [55] GoreTM Acoustic Vents for Hearing Instruments, Membrane Properties and Acoustic Functionality, [49] Gore® GR Sheet Gasketing for Steel Pipes & W.L. Gore & Associates, 2013. No. PB1750- Equipment, W.L. Gore & Associates, 2011. REV1-TEC-US-APR13, www.Gore.com.

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12 Electrical and Electronic Applications of Expanded PTFE 12.1 Coaxial Cables OUTLINE 257 254 12.4 Disk Drive Filters 258 12.2 Hook-Up Wire 255 References 12.3 Electromagnetic Interference 256 Shielding Gasket Aside from excellent resistant to chemicals, the electrical properties of PTFE in addition to its environmental stress cracking, low friction, weath- own unique characteristics. erability, and low and high temperature operability, polytetraﬂuoroethylene (PTFE) has outstanding Dissipation factor and dielectric constant values electrical properties. The PTFE molecule is sym- of PTFE vary little up to and beyond 10 MHz metric, free of electrical charges, and resistant to frequency (Table 12.1). The value of dielectric polarization by magnetic or electric inducement. In constant is 2.1 essentially over the entire spectrum contrast, polyvinylidene ﬂuoride (eCF2eCH2e) of frequency. PTFE dielectric constant and contains permanent dipoles and can be further dissipation factors remain constant over a broad polarized. temperature range (À40 to 240C). Dissipation factor of PTFE remains <0.0004 up to 100 MHz. Excellent electrical stability of PTFE lasts over a It approaches a peak value at around 1 GHz [2]. wide range of frequency and environmental condi- tions. PTFE is a strong electrical insulator at normal Applications of ePTFE include signal trans- operating temperatures. Its dielectric strength drops mission (wire and cable), shielding in computers, off more slowly than most other materials when telecommunications, and measurement cables. The frequency is increased. The void content of expanded low dissipation factor and dielectric constant of PTFE (ePTFE) considered, this membrane possesses ePTFE make it an ideal insulation for many elec- trical/electronic products such as wire and cable Table 12.1 Electrical Properties of Polytetraﬂuoroethylene [1] Property Method Value Dielectric strength (2.03 mm ASTM D149 24 thickness), kV/mm Surface arc resistance, s ASTM D459 >300 Volume resistivity, U cm ASTM D257 1018 Volume resistivity, ohm-square ASTM D257 1018 Dielectric constant (<2 GHz) ASTM D150 Dissipation factor (<2 GHz) ASTM D150 2.1 <0.0001e0.0003 Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00012-2 253 Copyright © 2017 Elsevier Inc. All rights reserved.

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254 EXPANDED PTFE APPLICATIONS HANDBOOK Table 12.2 Advantages of Dielectric Expanded Polytetraﬂuoroethylene (ePTFE) Material for Fast Signal Speeds (!85% Speed of Light) [3] Insulation Dielectric Velocity of Temperature Flammability (L01) Gore ePTFE Constant Propagation (%) Limits (C) >95 PTFE >95 FEP 1.3 >85 À70 to þ260 >95 Irradiated polyoleﬁn 2.1 70 À70 to þ200 29 Polyethylene 2.1 70 À70 to þ260 2.5 65 À50 toþ125 2.5 65 À60 to þ80 PTFE, polytetraﬂuoroethylene. Figure 12.1 Cable design with low dielectric con- Because of elimination of the separation of the stant and low-loss tangent, an expanded polytetra- ePTFE insulation layer the “noise” in the form of ﬂuoroethylene (ePTFE) dielectric layer [6]. triboelectric currents were signiﬁcantly reduced in the cable while the beneﬁcial characteristics of the (Table 12.2). The porous structure allows signals to ePTFE were preserved. Triboelectric effect is a type travel nearly at the speed of light with minimum loss of contact electriﬁcation in which certain materials or distortion plus thermal stability and mechanical become electrically charged after they come ﬂexibility. ePTFE also allows reduction of the overall into frictional contact with a different material. The interconnect size and weight [4,5]. efﬁciency of the cable to keep the noise done was sufﬁciently useful to eliminate the need for a low 12.1 Coaxial Cables noise semiconductive layer to dissipate triboelectric currents in low noise cables. In addition to smaller Development of low noise cables (Fig. 12.1) for size and lighter weight the cable has lower capaci- use in sensitive electrical signal transmission has tance, improved ﬂexibility, and reduced susceptibil- been reported in a number of patents. The cable ity to damage during use. Manufacturing time and contained an insulation layer of ePTFE that was cost are reduced thanks to ease of assembly and bonded using an adhesive such as tetraﬂuoroethylene reduced material costs [8,9]. Typical applications of copolymers like perﬂuoro ethylene hexaﬂuoro- ePTFE-insulated cables include microwave coaxial ethylene copolymer (FEP) and perﬂuoroalkoxy assemblies which include test, aerospace, defense, polymer. The adhesive “ﬁlls” the membrane voids. telecommunication, and general purpose. An The ePTFE layer was placed either directly or indi- example of the structure of a commercial cable is rectly over a surrounding shield layer to maintain a given in Fig. 12.2. ﬁxed relative position between the insulation layer and the shield layer. The bonding process produced a Figure 12.2 Structure of a coaxial cable insulated tightly coherent interface between the insulation with expanded polytetraﬂuoroethylene (ePTFE) [10]. layer and the shield which is resistant to separation and movement in ﬁeld application [7e9].

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12: ELECTRICAL AND ELECTRONIC APPLICATIONS OF EXPANDED PTFE 255 Figure 12.3 Expanded polytetraﬂuoroethylene A hybrid ﬂexible round cable (Fig. 12.4) with (PTFE) cables with 49% lower attenuation or outer optimized electrical and mechanical characteristics diameter reductions 47% [11]. for high performance in difﬁcult applications has been commercialized. Rugged features, robust envi- ronmental protection, and high shielding capabilities have been developed for computer, industrial, and military applications with stringent performance circumstances. The beneﬁts of such a cable include precise electrical properties, signiﬁcantly lower capacitance, velocity propagation !85%, size reduction and weight for high-density applications, and high ﬂex life in multiple axes. 12.2 Hook-Up Wire Hook-up wire is deﬁned as a single insulated conductor wire in the family of lead wires usually used for low voltage and low current applications (Fig. 12.5). The lead wires are used in control panels, automotive, meters, ovens, internal wiring of com- puters, electronic equipment, business machines, and appliances. The wire rating depends on the operating and environmental conditions to which it is exposed. Simple appliances such as microwaves, toasters, and washer/dryers are subject to thousands of thermal cycles during their uses. PTFE-insulated hook-up wires can withstand demanding conditions (Table 12.3). PTFE insulation is applied to conductors by paste extrusion [2] or tape wrapping [15]. Even though PTFE has the most outstanding electrical properties of all ﬂuoropolymers, it is disadvantaged because of poor mechanical properties such as low cut-through resistance, low cold ﬂow resistance, and low tensile Figure 12.4 Example of a rugged hybrid round cable Figure 12.5 High strength toughened expanded with robust environmental protection [12]. polytetraﬂuoroethylene insulation with improved mechanical performance [13]. The performance of ePTFE-insulated cables is signiﬁcantly better than those of similar RG con- structions. In an example (Fig. 12.3) an ePTFE cable had 49% lower attenuation or outer diameter reduction of 47% compared to equivalent full density PTFE cable. The ePTFE cables transmit over longer distances, have lower density and allowed higher bandwidth than their RG counterparts [11]. Cables containing ePTFE insulations are applied in coaxial and microwave and radiofrequency cable assemblies, high-speed data cable and power and signal cable.

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256 EXPANDED PTFE APPLICATIONS HANDBOOK Table 12.3 Advantages and Disadvantages of Expanded Polytetraﬂuoroethylene Insulation Materials [14] Electrical Pros Cons Mechanical Environmental Dielectric withstanding voltage Abrasion and cut- Dielectric constant through resistance Application constraints Radiation resistance Flexibility Tensile strength Additional processing required Liquid and gas resistance Temperature and UV resistance No outgassing Coefﬁcient of friction Used as insulation, dielectric, and jackets Flame resistance Performance standards Table 12.4 Cut-Through Resistance of ePTFE, PTFE, and ETFE Insulated Wires [15] Densiﬁed ePTFE Paste-Extruded PTFE ETFE Insulation Conductor Final Cut-Through Final Cut-Through Final Cut-Through Size 30(1) Insulated OD Resistance Insulated OD Resistance Insulated OD Resistance 38(1) 0.58 1.40 0.61 0.85 0.51 1.08 30(1) 0.46 2.08 0.46 0.99 e e 1.35 3.21 1.35 1.57 e e ePTFE, expanded polytetraﬂuoroethylene; ETFE, ethylene and tetraﬂuoroethylene; PTFE, polytetraﬂuoroethylene. strength. Cut-through resistance is deﬁned as the wires have higher cut-through resistance than those amount of force needed for a sharp edge to penetrate insulated by PTFE paste extrusion (Table 12.4). through the insulation wall and make contact with the conductor. Typically, PTFE insulated wires have 12.3 Electromagnetic Interference signiﬁcantly lower cut-through resistance than such Shielding Gasket insulating materials as polyimide and polyester ﬁlms and a melt-extruded copolymer of ethylene and An electromagnetic interference shielding (EMI) tetraﬂuoroethylene. gasket is a conductive interface material that is used to connect an electrically conductive shield The issue of poor cut-through has been addressed with a corresponding section of an electrical by different approaches. One method is insulation of ground, such as a ground trace of a circuit board. wires with densiﬁed ePTFE tape [16]. The expanded EMI gaskets are required to shield circuit boards tape was densiﬁed between two polished steel rolls, and other devices against interference. They are heated to a temperature of about 90C, so that the always, with minor exceptions, installed directly ﬁnal density of the tape was about 1.96 g/cc. This onto a conductive surface. Optionally, the gaskets compressed, ePTFE tape was then slit and helically could be formed in place or precut to ﬁt the wrapped the conductor. The tape-wrapped conductor circuit board. was then passed through a salt bath heated at a temperature of 390C for a period of 5e7 s [15]. The Manufacturing techniques for installing these ePTFE tapeeinsulated wires meet military speciﬁ- gaskets include: (1) Dispensing a conductive paste cations (eg, MIL-W-16878) because they offer better or a conductive liquid material directly onto a cut-through resistance, absence of pinholes, and conductive surface and curing the dispensed reduced weight and diameter. Tape-wrapped PTFE

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12: ELECTRICAL AND ELECTRONIC APPLICATIONS OF EXPANDED PTFE 257 material in situ; (2) Die-cutting a conductive sheet D4935. The shielding effectiveness (SE) was material having an adhesive backer and then trans- measured to be À26 to À30 dB. ferring, positioning, and adhering the dimensioned material directly to a conductive surface; (3) Me- In another experiment the EMI shielding material chanically fastening a conductive material to a was patterned, cut, and fashioned into a close-ﬁtting conductive surface [17]. cover for a shipboard radar housing. The cover was installed on the radar housing and ﬁeld tested at sea There are various methods of fabricating EMI over a period of about one week. The cover was materials; an example is described. Woven fabric of repeatedly removed and reinstalled during the test ePTFE ﬁbers was coated on one surface with a period and electromagnetic radiation measurements dispersion of a thermoplastic ﬂuoropolymer like were made inside the housing with and without the FEP. The FEP dispersion was applied by roll cover in place. Durability and EMI shielding per- coating, followed by oven drying to remove the formance of the structure during the test period were water yielding a 4% dry weight ﬁlm [18]. excellent and appeared to be unaffected by handling or weather conditions of the test. Average SE was The coated fabric was laminated to a porous determined to be À30 dB [18]. ePTFE membrane by heating the porous ePTFE membrane on the surface of a metallic hot-roll to a 12.4 Disk Drive Filters temperature high enough to melt the FEP disper- sion particles. The FEP-coated side of the fabric Hard disk drive (HDD) ﬁlters are designed to was forced against the heated surface of the porous protect the HDD from particles, hydrocarbons, acid ePTFE membrane with a silicone rubber coated gases, and the effect of water vapor such as pinch roll. The porous ePTFE membrane had a condensation and corrosion (Fig. 12.6). The ﬁltration thickness of approximately 25 mm, 95% porosity, membrane removes particles from incoming air. The and typical nodes and ﬁbrils structure. adsorbent element is specially formulated to control humidity and to adsorb acid gases and hydrocarbons The fabric constructed was laminated to an from internal and external sources. The ﬁlter system electrically conductive ePTFE containing electri- is called adsorbent breather assembly (ABA) [19]. cally conductive carbon black particles using a layer of FEP ﬁlm as adhesive assisted by heat and A large variety of adsorbent ﬁlters are available for pressure using a hot roll and a pinch roll. The as- different models of HDDs. Fig. 12.7 shows an sembly obtained so far was laminated to an ePTFE example of a simpler adsorbent breather ﬁlter [20]. membrane (76.2 mm, 90% porosity) using a lower melting thermoplastic polymer as adhesive. The integral diffusion channel reduces the amount of hydrocarbons, water vapor, and acid gases A sample of the composite EMI shielding material entering the drive from the surrounding environment was prepared and tested according to ASTM Method Figure 12.6 Adsorbent breather assembly design and installation relative to hard disk drive [19].

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258 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 12.7 Adsorbent breather ﬁlter design and installation relative to hard disk drive ﬁlter [20]. and thereby extending the life of the adsorbent. [6] Gore Microwave Coaxial Assemblies, W. L. The most common adsorbents and their associated Gore & Associates, May 2001. No. 1159. functions are as follows: [7] F.A. Kennedy, W.G. Hardie, J.J. Hegenbarth, US Activated carbon: Relative humidity control Patent 5,210,377, Assigned to W. L. Gore & and adsorption of hydrocarbons Associates, May 1993. Chemically treated activated carbon: Relative [8] D.T. Singles, G. Walter, W.P. Mortimer Jr., US humidity control and adsorption of acid gases Patent 5,477,011, Assigned to W. L. Gore & and hydrocarbons Associates, December 1995. Silica gel: Relative humidity control [9] D.T. Singles, G. Walter, W.P. Mortimer Jr., US Patent 5,554,236, Assigned to W. L. Gore & The ABA is installed on the inside of the drive Associates, September 1996. with the ﬁlter located in a low-pressure area of the drive so that air can enter through the breather. [10] Poly Fluoro Ltd., Performance Plastics, April Generally, the best location is on the cover or the base 2016. www.polyﬂuoroltd.com. casting as close as possible to the spindle [19]. [11] Surpassing RG Cable, W. L. Gore & Associates, This chapter has described a few of the more No. JK060510e03, April 2006. signiﬁcant applications of ePTFE porous membranes in electrical and electronic devices. Additional in- [12] Hybrid Round Cable, W. L. Gore & Associates, formation can be obtained from the manufacturers April 2016. www.gore.com/en_xx/products/ including W. L. Gore & Associates (www.Gore. cables/round/harsh/hybrid_round_cable.html. com), Donaldson Corp (www.Donaldson.com), and Clarcor Corp (www.Clarcor.com). [13] pub No. 11/03 AGS by, Gore™ High Strength Toughened Fluoropolymer (HSTF) Wire, W. L. References Gore & Associates, 2003. [1] No. H-37051-3, Teﬂon® PTFE Properties [14] Improving Cable Performance in Harsh Handbook, DuPont Co., July 1996. Environments, White Paper, W. L. Gore & Associates, 2013. [2] S. Ebnesajjad, Fluoroplastics: Non-melt Pro- cessible Fluoropolymers, in: Plastics Design [15] P.B. Cooper, S.J. Lane, US Patent 4,732,629, Library, second ed., vol. 1, Elsevier, 2014. Assigned to the Inventors, March 1988. [3] Signal Parameters of EPTFE vs Other Materials, [16] J.B. Knox, W.E. Delaney III, J.M. Connelly Jr., W. L. Gore & Associates, April 2016. www.gore. US Patent 5,374,473, Assigned to W. L. Gore & com/en_xx/products/cables/round/harsh/ Associates, December 1994. hybrid_round_cable.html. [17] B.E. Reis, D.R. King, US Patent 6,255,581, Gore [4] W. L. Gore & Associates, April 2016. www.gore. Enterprise Holdings, July 2001. com/en_xx/technology/applications.html. [18] M. Gerry, D.Z. Kelly, M.G. Ryan, T.E. Dykes, R. [5] W. L. Gore & Associates, April 2016. www.gore. Sassa, US Patent 5,401,901, W. L. Gore & com/en_xx/industries/computer/computer_ Associates, March 1995. telecom_electronics.html. [19] Adsorbent Breather Assembly, Donaldson Company, July 2010. diskdrive@donaldson. com. [20] Adsorbent Breather Filter, Donaldson Company, July 2010. [email protected].

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13 Surface Modiﬁcation of Expanded Polytetraﬂuoroethylene OUTLINE 13.1 Introduction 259 13.3.2 Surface Modiﬁcation to Reduce Thrombogenicity 13.2 Surface Treatment of 260 265 Polytetraﬂuoroethylene 13.3.3 Mechanical Alteration of Expanded Polytetraﬂuoroethylene Surface 268 13.3 Surface Treatment of Expanded 271 References Polytetraﬂuoroethylene Membrane 263 13.3.1 Surface Modiﬁcation for Hydrophilicity and Adhesion 263 13.1 Introduction ePTFE has a node and ﬁbril porous structure in which the ﬁbrils have a cross-sectional diameter of Surface preparation, modiﬁcation, or treatment is less than about 1 mm. Fig. 13.1 is a scanning electron usually done to make that surface adherable. In gen- micrograph (SEM) of an extruded PTFE ﬁlm. This eral, surface modiﬁcation could include one or more ﬁgure shows the PTFE particles still maintain their of a number of operations such as cleaning, removal of roundish appearance in spite of moderate ﬁbrillation. loose material, physical and/or chemical modiﬁcation The ﬁbrils connect the PTFE particles while being of a surface, and application of a coating to the sur- primarily oriented in the direction of extrusion. A face. In plastic bonding, surface preparation is aimed comparison of Figs. 13.1 and 13.2 indicates the at increasing the surface polarity (energy), improving changes that PTFE particles have undergone up to the surface wettability, and creating chemical sites end of extrusion. See Chapter 5 of this book for amenable to adhesion. Methods to modify plastic additional information. surfaces include chemical, plasma, corona, and ﬂame treatment methods. These methods act similarly in Figure 13.1 Scanning electron micrograph that they raise the surface energy, although each (20,000X) of changes in polytetraﬂuoroethylene parti- technique has advantages and disadvantages [1]. cles as a result of preforming and paste extrusion [2]. The main reason for modifying the surface of expanded polytetraﬂuoroethylene (ePTFE) mem- brane is to create a speciﬁc chemical composition on the substrate. Usually the surface is oxidized in which F atom is replaced by oxygen or nitrogen. In a subsequent step the surface is functionalized by either wet or dry chemistry techniques. A special example is the modiﬁcation of the hydrophobic sur- face of polytetraﬂuoroethylene (PTFE) to make it hydrophilic. Chemical composition and morphology of the PTFE surface is altered so that the surface energy of the substrate would be elevated. Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00013-4 259 Copyright © 2017 Elsevier Inc. All rights reserved.

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260 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 13.2 Scanning electron micrograph (20,000X) of polytetraﬂuoroethylene (PTFE) particle in paste (ie, after addition of lubricant and blending) and in preform states in PTFE [2]. Fig. 13.3 shows an SEM of a biaxial ePTFE incompatibility with blood. Lack of good tissue membrane. The PTFE particles have lost their round ingrowth was another problem. appearance due to extensive ﬁbrillation. The red ar- rows indicate examples of locations on the membrane 13.2 Surface Treatment of ﬁbrils and nodes where surface modiﬁcation must Polytetraﬂuoroethylene take place. This is an important consideration in the selection of surface modiﬁcation method further The most common surface treatment method for discussed in Section 13.2. unexpanded basic solid PTFE for adhesive bonding is etching by a sodium solution in anhydrous liquid An example of adverse effect of surface hydro- ammonia. A similar method is dissolution of sodium phobicity of ePTFE is in vascular grafts. Long-term in naphthalene, which is considered safer. Both application of an ePTFE graft as an arterial substi- techniques are practiced commercially. Other ﬂuo- tute used to be limited because of the high incidence ropolymers also respond positively to this etching of occlusion by thrombus as a result of technique. An alternative method for modiﬁcation of PTFE surface is plasma treatment. Figure 13.3 Scanning electron micrograph (50,000X) of a highly ﬁbrillated expanded polytetraﬂuoroethylene Etching has a profound effect on the surface membrane expanded at high stretch ratios. chemistry of the ﬂuoroplastics. The consistent Courtesy: DuPont Co. changes in surface composition of ﬂuoroplastics brought about by surface modiﬁcation are reduction in ﬂuorine and/or chlorine content, and increase in carbon and oxygen contents (Table 13.1). Sodium- etched PTFE surface is comprised virtually of car- bon and oxygen, with a small amount of ﬂuorine. Theoretical atomic ratio of ﬂuorine to carbon for PTFE is 2; the lower ratio (1.6) in Table 13.1 indicates surface contamination. Disregarding of the F/C before treatment after sodium etching the ratio is lower by orders of magnitude. The color of sodium- etched PTFE surface ranges from light brown to black. Historically, PTFE surface has proven as an exceptionally difﬁcult ﬂuoropolymer for modiﬁca- tion by plasma treatment. The early work showed

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13: SURFACE MODIFICATION OF EXPANDED POLYTETRAFLUOROETHYLENE 261 Table 13.1 Effect of Sodium Etching on the Surface Composition [3] Surface Chemical Analysis (%) by ESCAa Polymer Treatment F/C Ratio Cl/C Ratio O/C Ratio CI CF O PTFE e e e 38.4 61.6 e PTFE None 1.60 e e 82.2 0.9 16.9 0.20 PTFE Tetra-Etchb 0.011 e e 87.2 0.4 12.4 (1 min) 0.14 N/1 minc 0.005 PTFE, polytetraﬂuoroethylene. a Electron spectroscopy for chemical analysis. b Supplied by W.L. Gore Corporation. c Treatment with a 1 mol solution of sodium naphthalenide in tetrahydrofuran at room temperature. PTFE is the only commercial ﬂuoropolymer that did catheters were heparinized according to the not respond well to plasma treatment. Old data following sequential three-step procedure: indicated PTFE was the only commercial ﬂuoropol- ymer that did not respond well to low-pressure 1. Steeping in a 15% aqueous solution of dodeca- plasma treatment (LPT) as indicated by the adhe- methylmethyl ammonium chloride, pH 7.5, sion bond strength in Table 13.2. A signiﬁcant 16 h, 65C, followed by thorough rinsing in amount of research has been conducted to improve distilled water and drying with nitrogen. PTFE response to plasma treatment at both low and at atmospheric pressures [1]. 2. Digestion in a 9% aqueous solution of sodium heparin for 16 h at 65C, followed by thorough A study of plasma treatment followed by cova- rinsing with distilled water and drying with lent bonding of heparin to the ePTFE surface done nitrogen. in the 1980s is described in US Patent number 4,613,517 [4]. One hundred 3.1 cm radiopaque 3. Submersion in a 1% aqueous solution of glutar- PTFE (solid not porous) catheters weighing 20 g aldehyde for 2 h at 60C, followed by thorough each were placed in a plasma generator. The system rinsing with distilled water, 5% aqueous Triton was evacuated for 6 min to a pressure of X405, and distilled water. In the ﬁnal operation 120 mm Hg, then an oxygen bleed was started and the catheters were dried with nitrogen and maintained for 1 min at a pressure of 180 mm of stored in a vacuum oven at 25C. Hg. A plasma stream was initiated and maintained at 13.56 MHz and 50 W power for 10 min. The Another study exhibited the effectiveness of LPT chamber was air quenched, opened, and the [5]. Fig. 13.4 shows bond strength of PTFE treated Table 13.2 Comparative Values of Adhesion Bond Strength for Fluoropolymers Treated by Na-Etching and Low Pressure Plasma Methods Material Treatment PTFE FEP ETFE PFA Untreated Negligible 0.1 Negligible 0.04 Sodium etched 8.2 6.4 Plasma treated 5 e 8.3 2.2 10.4 15.8 ETFE, ethylene tetraﬂuoroethylene copolymer; FEP, ﬂuorinated ethyleneepropylene copolymer; PTFE, polytetraﬂuoroethylene; PFA, peril- uoroalkoxy copolymer. Courtesy: Gasonics/IPC applications notes. San Jose, California: Gasonics International.

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262 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 13.4 Bond strength versus plasma exposure with nitrogen plasma at a 2 cm distance to plasma time for pretreated polytetraﬂuoroethylene-treated N2 source in a Planartrons source. It increases from plasma at a distance of 2 cm to plasma source [5]. 0.25 N/mm2 for the untreated material to 5.9 N/mm2 after 50 seconds of plasma treatment. At longer Figure 13.5 Bond strength versus plasma exposure treatment times the bond strength actually decreases time for pretreated polytetraﬂuoroethylene-treated N2 to 2.6 N/mm2 (after 200 seconds). The maximum plasma at a distance of 0 cm to plasma source [5]. bond strength of PTFE ﬁlm, treated with N2 at a distance of 40 cm to plasma source (Fig. 13.5) was reached in 5 seconds at 5 N/mm2. This represents a factor of 20 times increase compared to the untreated material. It is remarkable that after a treatment of 5 seconds the same bond strength is achieved as treating the PTFE for 20 seconds at a distance of 2 cm from the source. Fig. 13.6 shows the topical view of PTFE surface as is and after low pressure plasma treatment. The beneﬁt of etching is increased surface roughness leading to increase in available adhesive bonding surface area. Adhesion bond strength always increases as a result of an increase in the bonding surface area. Many LPT research projects on PTFE surface modiﬁcation have focused [6e13] on the effect of LPT using different gases and compounds, grafting, plasma process variables, surface wettability, and analysis of surface composition and topography. These studies are useful in shedding light on the state of the modiﬁed PTFE surfaces versus the as is sample. A major disadvantage of low-pressure plasma method is the requirement to apply the treatment inside a vacuum chamber. The requirement of a vacuum chamber adds to the complexity of the pro- cess and limits shape and type of material to be Figure 13.6 Scanning electron micrograph of polytetraﬂuoroethylene ﬁlm: (A) before plasma treating and (B) after plasma treating. Courtesy: Diener Electronic, www.plasma.de/en/index.html.

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13: SURFACE MODIFICATION OF EXPANDED POLYTETRAFLUOROETHYLENE 263 Peel strength (g/20 mm)treated. Consequently, LPT continues to be a moreas Diener Electronic (www.plasma.de/en/index. costly process than other methods because of vacuum html), Henniker Plasma (www.plasmatreatment.co. pumps and operational limitations. To overcome the uk), PVA TePla (www.pvatepla.com), Enercon Ind vacuum issue a great deal of research and develop- (www.enerconind.com), and Acton Technologies ment has been concentrated on atmospheric plasma (www.actontech.com) provide equipment and/or treatment by the industry and academic institutions contract services for low and atmospheric pressure [14e27]. An example of the PTFE adhesion peel treatment of PTFE products. strength obtained by atmospheric plasma treatment is shown in Fig. 13.7. The mixture of oxygen and he- 13.3 Surface Treatment of lium proved to be the most effective gas for the Expanded Polytetraﬂuoroethylene modiﬁcation of PTFE and other ﬂuoropolymer sur- Membrane faces. A somewhat longer treatment time, compared to other polymers, was required to enhance the PTFE and ePTFE have hydrophobic surfaces, bondability of PTFE sufﬁciently. which is an advantage for many applications. Yet a large number of applications such as vascular grafts The interest in the development of a more tunable require a hydrophilic and/or biocompatible surface. treatment method than sodium etching and to avoid Technologies have been developed to modify the the process hazards and disposal of hazardous ma- ePTFE surface by functionalization. In some cases terial produced by sodium etching has driven in- the ePTFE surface is mechanically altered to impart vestigations on plasma techniques. Both low pressure other desirable characteristics to the membrane such (vacuum) and atmospheric effective techniques have as suturability strength. Occasionally, surface modi- been developed for PTFE, although the atmospheric ﬁcation is aimed at making the ePTFE surface more method has been lagging behind the LPT. Effective hydrophobic, for example, to discourage ingrowth of PTFE surface treatment by LPT has been achieved by tissues. the combined academic and industrial efforts. The new processes offer lower cost though they are still The majority of surface modiﬁcations are more expensive than other methods. Companies such intended: 500 1. To render the surface hydrophilic O2/He a. To enhance biocompatibility 400 b. To reduce clotting of blood (thromboge- N2/He nicity) He only c. To make it more adherable 300 2. To mechanically alter the membrane surface Ar/He 3. To make the ePTFE surface more hydrophobic 200 13.3.1 Surface Modiﬁcation for Hydrophilicity and Adhesion 100 A number of approaches and methods have been Untreated developed to render the surface of PTFE hydrophilic or nonthrombotic [28e30]. One popular approach for 0 modiﬁcation was to alter the surface chemistry of 0 60 120 180 240 300 PTFE by the introduction of oxygenated moieties Treatment time (s) such as OH, CO, COOH, and others. Another approach was to modify the surface of ePTFE Figure 13.7 Peel strength of polytetraﬂuoroethylene as membrane to mimic the design of the biocompatible a function of atmospheric plasma treatment time [22]. surface of the endothelial cell wall. Costello and McCarthy presented an early process that created hydroxyl groups bonded to the PTFE

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264 EXPANDED PTFE APPLICATIONS HANDBOOK surface. The reaction sequence consisted of a liquids having surface tensions >39.3 dyn/cm indi- reduction step followed by hydroboration and an cating a retention of the original materials’ non- oxidation step. A drawback, however, was the depth wetting characteristics for these liquids. The of the reduction step into the bulk of the PTFE, measured angles for these liquids also indicate the ranging from 15 to 2000 nm. The Costello and retention of surface ﬂuorine functionality and espe- McCarthy process altered the node and ﬁbril micro- cially a large degree of hydrophobicity as indicated structure of expanded because of the penetration from the angles measured for water and glycerol. depth of the treatment [28]. Below 39.3 dyn/cm, the contact angles of the uti- An oxy-ﬂuoropolymer comprising of a ﬂuoropol- lized liquids showed a larger degree of wettability ymer in which up to about 98% of the surface ﬂuorine indicating an increase in surface energy that was atoms to depths of 1e10 nm were permanently ascribed to the presence of small amounts of surface substituted with H2 and O2 or O2-containing groups oxygen functionality. A 0 measurement (ie, yC) was [30]. Of those from 3% to 30% of the substituted F is observed at 27.6 dyn/cm for modiﬁed ePTFE replaced with O2 or O2-containing groups. From compared to 23.8 dyn/cm measured on the unmodi- about 70% to 97% was substituted with H atoms, the ﬁed ePTFE membrane. This indicates an increase in morphological and hydrophobic properties of the surface energy again, attributed to the creation of oxy-ﬂuoropolymer remaining substantially un- surface oxygen functionality in close proximity to the changed from those of the ﬂuoropolymer. ﬂuorine functionality [30]. The wettability with respect to low surface tension Yamada et al. [31e33] devised a unique approach liquids and surface free energy (yS) as determined by overheating one surface of the ePTFE membrane. through critical surface tension (yC) was increased A porous PTFE tube (OD 0.9 mm, ID 0.2 mm, (see Table 13.3) [1]. This method allowed wetting of porosity 33%) with an isopropanol bubble point of the modiﬁed ePTFE surface with “low surface ten- 1.54 kg/cm2 was treated using a ﬂame. The tube was sion” liquids <50 dyn/cm but would not be wetted by moved through the tip of a gas burner ﬂame at a water with a surface tension of 72 dyn/cm. It also linear speed of 5 m/min. The gas was a mixture of suffered from imparting large changes to the bulk 32% C3, 54% O2, and 14% air by volume. All tube ePTFE ﬁbrils because of the depth of modiﬁcation parameters were unchanged except the bubble point ranging from 1 to 10 nm. was reduced to 0.65 kg/cm2. Flame treatment increased the adhesion bond strength of the tube to Table 13.3 shows that a decrease in contact angles silicone rubber to 5.4 kg/cm2 versus 1.5 kg/cm2 for as measured on the modiﬁed ePTFE was small for the Table 13.3 Contact Angles of Various Liquids Measured by a Rame Hart Goniometer Model 100 [30] Water Liquid / Vapor Surface Unmodiﬁed Modiﬁed ePTFE Glycerol Tension yLV (dyn/cm) ePTFE (degree) 20 min H2 (H2O) (degree) Formamide Thiodiglycol 72.4 w140 110 Methylene iodide 64.8 130 115 1-Bromo-naphthalene 58.9 130 112 1-Methyl-naphtbalene 53.5 125 120 Dicyclohexyl 49.0 120 115 n-Hexadecanc 45.0 100 110 n-Tridecane 39.3 100 90 n-Decane 32.7 93 60 27.6 20 (Spread) 0 26.0 10 23.8 0 (Spread) 0 0 ePTFE, expanded polytetraﬂuoroethylene.

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13: SURFACE MODIFICATION OF EXPANDED POLYTETRAFLUOROETHYLENE 265 Table 13.4 Results of Treatment of Expanded Polytetraﬂuoroethylene (0.03 mm thickness) by Atmo- spheric Pressure Microwave Plasma Porous Membrane [35] Line Wettabilityb Break Speed Strength Gas Flow Power Voltage (m/ Day N% 34 Example Gas Rate, SLMa (W) (kV) min) Day 0 71 28 81 Control 97% Test He 3 2400 6.5 3 96% 96% a SLM ¼ standard liter per minute (at 0C and 1 bar). b The percentages indicate the minimum amount of methanol in water that would wet the surface. the untreated tube. Raising the moving speed of the During the ﬁltration of outgassing liquids with a tube to 10 m/min reduced the bubble point to hydrophobic porous membrane, the porous mem- 0.92 kg/cm2 and the adhesion bond strength to sili- brane can provide nucleating sites for dissolved gases cone rubber increased to 2.8 kg/cm2 versus 1.5 kg/ to come out of solution under the driving force of the cm2 for the untreated tube. pressure differential during the ﬁltration process. Gases that come out of solution at these nucleating Gardella and Vargo used radio frequency glow sites on the hydrophobic membrane surfaces, discharge plasmas consisting of water or methanol including the interior pore surfaces and the exterior vapor mixed with hydrogen gas to hydroxylate or geometric surfaces, can form gas pockets, which ePTFE. They also described the preparation of adhere to the membrane. As these gas pockets grow silanized ePTFE by treatment with (3-aminopropyl) in size due to continued outgassing, they may begin triethoxysilane and subsequent derivation with to displace liquid from the pores of the membrane ﬂuorescein isothiocyanate. The method did not pro- that can reduce the effective ﬁltration area of the vide the capability to control the degree of hydrox- membrane. This phenomenon is usually referred to ylation [34]. as dewetting of the porous membrane since the liquid-wetted, or liquid-ﬁlled portions of the porous Surfaces of ePTFE membrane have been modiﬁed membrane are gradually converted into gas-ﬁlled by atmospheric pressure microwave (APMW) portions [35]. plasma [35]. The plasma-modiﬁed membrane is non- dewetting and can be wetted on contact with a 13.3.2 Surface Modiﬁcation to solution of CH3OH in water. The minimum amount Reduce Thrombogenicity of CH3OH in water in the solution to wet the APMW plasmaemodiﬁed porous membrane is less than the Prosthetic graft materials prove to be inferior to minimum amount of CH3OH in water in a reference autologous conduits, especially when the vessel or control solution that wets an untreated sample of diameter is less than 5 mm. Problems include the porous membrane. increased risk of thrombosis and infection, limited durability, lack of compliance both of the graft and The plasma-modiﬁed membrane is non-dewetting around the anastomosis, and failure due to restenosis, and wettable with a solution of 96% wt or less of thus necessitating further surgery. This section de- CH3OH in water while an untreated sample is wetted scribes strategies to overcome thrombogenicity and with a solution of 97% wt of CH3OH in water. The propensity for infection interventions [36e38]. surface of the membrane can be modiﬁed in APMW plasma using an oxygen gas. The non-dewetting Thrombin catalyzes a complicated biochemical modiﬁed membrane has higher oxygen to carbon cascade, which ultimately leads to the formation of a ratio (O/C) than the untreated membrane. The O/C thrombus. Thrombus is a blood clot formed within ratio of the modiﬁed membrane surfaces is between the vascular system of the body and impeding blood 0.04 and about 0.08. The modiﬁed porous mem- ﬂow. Thrombus is composed of elements of the branes can have strength in the range of 70e90% or blood, eg, platelets, ﬁbrin, red blood cells, and leu- more of the strength of the untreated ePTFE mem- kocytes. Thrombus formation is caused by blood brane without the APMW plasmaemodiﬁed surface (Table 13.4).

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266 EXPANDED PTFE APPLICATIONS HANDBOOK coagulation and platelet adhesion to, and platelet spectroscopy, it showed 4% hydroxyl groups on the activation on, foreign substances. When this occurs, a membrane surface. The repeat of the procedure graft is occluded by thrombogenic material, which in including metallization/removal on the same sample turn, results in decreased patency because of raised the hydroxyl concentration to 10%. A third obstruction of the graft. Various anti-thrombogenic treatment on the same sample raised the hydroxyl agents, such as heparin, have been developed and level to 16%. The ePTFE tube had 18% hydroxyl incorporated into biocompatible articles to combat after running the metallization/removal procedure thrombus formation. In a living system, heparin in- multiple times. hibits the conversion of a proenzyme (prothrombin) to its active form (thrombin) [39]. The tubular ePTFE was inverted before the removal of the aluminum layer for the last time. This ePTFE is hydrophobic thus requires surface inversion allowed the modiﬁed surface to be the modiﬁcation before any functional groups such as luminal surface (exposed to blood) of the tube. After heparin can be grafted to its surface. Plasma treat- removal of the aluminum the luminal surface is ment is the method of choice for imparting hydro- silanized using (3-aminopropyl) triethoxysilane. The philic functionality to ePTFE surface after which a silanized surface was next treated with monomethyl bioactive coating covalently bonded to it. Hydro- ester of azelaic acid. It was then treated with aqueous philic groups allow the application of aqueous NaOH to hydrolyze the methyl ester. In the ﬁnal step chemistry in bonding nonthrombogenic group to the the surface of ePTFE tube was treated with lyso- ePTFE surface. caprylyllecithin. The tissue surface of the tube can be treated similarly thus creating a vascular bioma- Thompson et al. used metallization as an terial [40]. intermediate step to impart changes to the ePTFE membrane surface [40]. The outer surface of a well- In a 2001 study, a bioactive coating was bound to cleaned ePTFE tube was metalized with a 40-nm the hydrophobic surface of an implantable medical thick ﬁlm of aluminum using conventional vacuum device. The bioactive coating contained a polymer metal deposition equipment. The resultant metal- backbone bound via an amide or amine chemical coated ePTFE was wetted prior to metal layer- bond to one end of a hydrophilic, amine-terminated removal step, by immersion into ethanol (100 mL) spacer that had at least one amine group at its ﬁrst for 1 min. It was washed with ultra-pure water and second ends. A bioactive molecule was cova- (100 mL, twice) for 10 min under ultrasonic lently bonded to the unreacted end of the hydrophilic agitation. spacer. The hydrophilic spacer was repelled by the hydrophobic surface of the medical device in such a The Al layer was removed from the ePTFE surface way that the bioactive molecule was extended away by immersion of the metalized ePTFE tube into an from the hydrophobic surface. The bioactive agent ultrasonically agitated, aqueous solution of NaOH included a polymer structure, which was deﬁned by a (0.1 M) for 15 min. Afterward, ultrasonic agitation biocompatible polymeric backbone and one or more for 20 min with ultrapure water (100 mL, three pendant groups with the general formula exhibited in times) was used to wash the ePTFE tube thoroughly. Fig. 13.8 [39]. When this procedure was run with an ePTFE ﬁlm sample analysis of its surface by X-ray photoelectron R1 is a spacer group selected from the group consisting of oxygenated polyoleﬁn, aliphatic poly- Figure 13.8 General chemical structure of polymeric esters, polyaminoacids, polyamines, hydrophilic bioactive agent [39]. polysiloxanes, hydrophilic polysilazanes, hydrophil- ic acrylates, hydrophilic methacrylates, linear and lightly branched polysaccharides. R2 is a spacer group with a chain length from 60 to about 250 atoms. The spacer group may include oxygenated polyoleﬁn, aliphatic polyesters, polyaminoacids, polyamines, hydrophilic polysiloxanes, hydrophilic polysilazanes, hydrophilic acrylates, hydrophilic methacrylates, linear and lightly branched poly- saccharides. R3 is a bioactive agent selected from the group consisting of antithrombogenic agents,

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13: SURFACE MODIFICATION OF EXPANDED POLYTETRAFLUOROETHYLENE 267 Figure 13.9 Position of the bioactive group bonded low-energy plasma discharge followed by application of negative high voltage pulses for short durations. to the expanded polytetraﬂuoroethylene polymer This resulted in the formation of high-energy ion ﬂux from the plasma discharge. They generate ions, chains (P) [39]. which formed free radials on the surface of the ePTFE. The molecular and physical structure of the antibiotic agents, antibacterial agents, antiviral bulk of ePTFE did not change. The high-voltage agents, or their pharmaceutical salts. Fig. 13.9 shows pulses modiﬁed the surface of the ePTFE without position of the bioactive group bonded to the ePTFE destroying the node and ﬁbril structure of the ePTFE. polymer chains (P) [39]. That was true even when applying the high-voltage pulses etched or carburized the surface of the In some cases [41,42] it may be desirable to pro- ePTFE. The modiﬁed surface had a depth of up to vide either dual antiinfective or antithrombogenic 500 nm. The ions were dosed onto the ePTFE sample action with two or more agents. Both antithrombo- at concentrations from 1013 ions/cm2 to about genic and antiinfective agents were grafted to the 1016 ions/cm2. polymer backbone. A spacer group with functional group like carboxylic acid or reactive nitrogen groups Most of the modiﬁcations were done with the was placed at both ends. The best spacer was a hy- equipment of the Forschungszentrum Rossendorf drophilic amino end-blocked polyethylene oxide (FZR), Dresden, Germany. The pressure of residual (PEO) because of its low interfacial free energy, lack air was 10À3 Pa, working pressure at discharge was binding sites, and exhibit highly dynamic motion. 10À1 Pa. N2, O2, and Ar gases were used for plasma These characteristics were important because they discharge generated by radio frequency generator of increase the activity of a PEO-linked bioactive agent, 13.56 MHz. Plasma power was regulated in the range eg, heparin [43]. of 50e400 W. High-voltage pulses were applied to the sample holder at the stable plasma discharge after The length of the spacer group may be used to 0.5e1 min after plasma start. The high-voltage pul- control the bioactive agent’s activity. It is known that ses had 5 ms duration; 20, 10, 1, and 0.5 kV values of the antithrombogenic activity of heparin increases peak voltage were used. A pulse repetition frequency when it is positioned a certain distance from the from 0.2 to 200 Hz was used. The regulation of pulse substrate to which it is bonded. For example, in a frequency was used to control the temperature during study of ePTFE-spacers heparin coatings with a C6 the PIII treatment. The PIII treatment induced a alkyl spacer, PEO200, PEO1000, and PEO4000, the change of color of the samples at high doses; for ePTFE-PEO4000-heparin surface maintained the example, ePTFE samples became gray. A homoge- highest bioactivity. neous distribution of the color is an indicator of an inhomogeneous dose distribution on the sample sur- An alternative method to modify the surface of face [44]. ePTFE was by spraying one of the previously dis- cussed compositions as a coating on the surface of the The modiﬁed samples were treated by a 10% so- ePTFE. Before applying the coating, the ePTFE sur- lution of acrylamide in water and a 10% solution of face was subjected to treatment using hydrogen-rich the modiﬁed polysaccharide hydroxyl ethyl starch in plasma followed by application of the coating [41,42]. water. The posttreatment was done immediately after PIII, as well as after 20 days and after different time Another method for modifying the ePTFE surface of an accelerating aging procedure. The samples was by plasma immersion ion implantation (PIII) were immersed in the solutions for 2 h at the room [44]. An ePTFE sample was placed in a plasma temperature. After the posttreatment the samples treatment chamber and was subjected to continuous were washed with deionized water and dried on air. The wetting angle measurement of these samples was determined at the following day to exclude any in- ﬂuence of residual water on surface of the polymers. The etching process increased the roughness of the ePTFE surface as determined by atomic force mi- croscopy. The node and ﬁbril structure of the modi- ﬁed ePTFE was not destroyed or substantially

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268 EXPANDED PTFE APPLICATIONS HANDBOOK altered. The contact angles of the water drops Figure 13.11 Molecular structure of ﬂuorocarbon sur- changed dramatically after PIII treatment. The wet- factant with pendent dextran and perﬂuoroundecanoyl ting measurement of ePTFE directly by the wetting chains along the polyvinyl amine backbone [45]. angle technique is inadequate, because of the Courtesy: www.ingenta.com. abnormal form of the drop. But the chemical struc- ture of ePTFE and PTFE is quite similar and allows The surfactant polymers underwent spontaneous the transfer of the wetting measurement results of surface-induced adsorption and assembly on ePTFE. PTFE to ePTFE samples. The adhesion stability of the surfactant polymer Fig. 13.10 depicts contact angles of water on on PTFE was examined under dynamic shear con- ePTFE (top) and PTFE (middle) with PIII treatment ditions in phosphate buffer saline and human whole with Nþ ions at 20 keV and posttreatment as indi- blood with a rotating disk system. Fluorocarbon cated. At posttreatment by acrylamide and modiﬁed surfactant polymer coatings at three different dextran polysaccharide hydroxyethyl starch the wetting an- to perﬂuoroundecanoyl ratios (1:0.5, 1:1, and 1:2) gles of PTFE decreased to very low values of 20e were compared in the context of platelet adhesion on 30 degree. The treatment dose of 1014 and 1015 ions/ ePTFE surface under dynamic ﬂow conditions. cm2 is enough for a good wettability. At high dose the effect of etching for PTFE surface also was observed Suppression of platelet adhesion was achieved for for posttreated surfaces [44]. all three coated surfaces over the shear-stress range of 0e75 dyn/cm2 in platelet-rich plasma or human A 2009 study reported about ﬂuorocarbon sur- whole blood. The effectiveness depended on the factant polymers acting as surface-modifying agents surfactant polymer composition such that platelet for lowering the thrombogenicity of ePTFE vascular adhesion on coated surfaces decreased signiﬁcantly graft by the reduction of platelet adhesion. The sur- with increasing ﬂuorocarbon branch density at 0 dyn/ factant polymers consisted of a polyvinyl amine cm2. The results (Fig. 13.12) suggested that the backbone with pendent dextran (Dex) and per- ﬂuorocarbon surfactant polymers could effectively ﬂuoroundecanoyl (FC11) branches (Fig. 13.11). suppress platelet adhesion and demonstrate the po- Dextran is a complex branched polysaccharide made tential application of the ﬂuorocarbon surfactant of many glucose molecules. Surface modiﬁcation polymers as nonthrombogenic coatings for ePTFE was accomplished by dip-coating the vascular graft. vascular grafts [45]. Figure 13.10 Contact angles of expanded polytetra- 13.3.3 Mechanical Alteration of ﬂuoroethylene (ePTFE) and polytetraﬂuoroethylene Expanded Polytetraﬂuoroethylene (PTFE) as a function of plasma dosage [44]. PIII, Surface plasma immersion ion implantation. A high degree of porosity may enhance lamination bond strength or improve certain ﬁlter performance properties. In a number of medical applications, more open structure may be beneﬁcial since the structure

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13: SURFACE MODIFICATION OF EXPANDED POLYTETRAFLUOROETHYLENE 269 Figure 13.12 Epiﬂuorescence images of platelet adhesion and activation on expanded polytetraﬂuoroethylene (ePTFE) surfaces in human whole blood: (A) uncoated ePTFE and (B) PVAm(Dex:FC11) 1:2 coated ePTFE under static conditions; (C) uncoated ePTFE and (D) PVAm(Dex:FC11) 1:2 coated ePTFE under dynamic con- ditions. Images show both platelet adhesion (green (White in print versions), FITC antihuman CD 41a) and platelet activation (red (Light gray in print versions), PE antihuman CD62p) [45]. Courtesy: www.ingenta.com. can encourage tissue ingrowth and attachment. There desirable to increase surface friction, ﬂow turbu- is, however, a limit to the amount of ingrowth lence, sound abatement, or exposed surface area. structure that can be achieved through only manip- ulation of nodes and ﬁbrils during expansion. To Seiler, Jr., et al. [47,48] described a PTFE surface further enhance tissue ingrowth beyond that of a treatment method that creates full density PTFE ribs highly porous surface, a roughened or textured sur- on the outer surface of an ePTFE tube. Although this face is believed to be required. In medical applica- process increased macro-roughness by the producing tions requiring rapid tissue ingrowth, an ideal ePTFE stiff ridges, the ridges were unexpanded and not surface may have both a high degree of porosity and porous. The ribs also conferred a good suture holding further surface modiﬁcation to provide some degree property to the tube. Close spacing of the ribs ensured of macroscopic texturing [46]. that any suture would always be adjacent to a rib. A rib would present an increased thickness to a suture and, In addition to enhancing the rate of tissue attach- being less porous than the tube wall and thus denser ment, a textured surface may be desirable in other and tougher, add resistance against the suture being applications, for instance to enhance bond strength, torn through the porous wall by excess tension. These abrasion, resistance, heat transfer, optical, or other ridges were believed to be undesirable because of properties. Increased roughness may also be achieving minimal tissue attachment and ingrowth.

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270 EXPANDED PTFE APPLICATIONS HANDBOOK Figure 13.13 Schematic diagram of expanded polytetraﬂuoroethylene membrane (A) before and (B) after removal of surface ﬁbrils by plasma treatment [50]. A somewhat unique surface treatment process was interconnecting ﬁbrils. This structure imparted both disclosed in the US Patent numbers 5,462,781 and porosity and stiffness to the ridges. The valleys were 5,437,900 [49,50]. The patents described a plasma formed by the substantial removal of nodes along treatment process that removes ﬁbrils to a selected with their interconnecting ﬁbrils, resulting in a depth to leave freestanding nodal ridges (Fig. 13.13). porous valley ﬂoor [46,51]. These freestanding nodes are easily bent or deﬂected due to the lack of supporting ﬁbrils. Such a treated The process also created a series of distorted or surface affects the hydrophobicity, bondability, and gnarled nodes along the valley ﬂoor. These gnarled appearance. When used as a graft it may not neces- node structures, remaining in and projecting from the sarily elicit an optimum tissue response due to an valleys, contributed to the surface “roughness” and excessively “soft” exposed surface. texture of the valleys, but did not signiﬁcantly compromise the porosity. The modiﬁed surface thus Another invention applied an unfocused laser had a texture that could be simultaneously macro- beam to both alter and remove selected ePTFE ﬁbrils rough and micro-porous along both the ridges and and nodes, resulting in a ridge and valley texture valleys. (Fig. 13.14). The ridges constituted clustered nodes that are internally supported by shortened The process generated repeatable, consistent tex- tures onto an ePTFE surface after the base ePTFE Figure 13.14 Enlarged partial cross-section views of an expanded polytetraﬂuoroethylene surface at sequential stages that occur during laser exposure [46,51].

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13: SURFACE MODIFICATION OF EXPANDED POLYTETRAFLUOROETHYLENE 271 material had been created. Thus common ePTFE [7] C. Wang, J.-R. Chen, Studies on surface graft materials could be subsequently processed into a polymerization of acrylic acid onto PTFE ﬁlm by variety of textures, each texture being optimized for a remote argon plasma initiation, Appl. Surf. Sci. speciﬁc application. 253 (2007) 4599e4606. There are several approaches to improve the sur- [8] C.-Y. Tu, Y.-C. Wang, C.-L. Li, K.-R. Lee, face hydrophilic properties of PTFE and ePTFE J. Huang, J.-Y. Lai, Expanded poly(tetraﬂuoro- including chemical reduction with sodium naphtha- ethylene) membrane surface modiﬁcation using lene, ion beam bombardment, ﬂame treatment, UV or acetylene/nitrogen plasma treatment, Eur. e-beam irradiation, and plasma modiﬁcation Polym. J. 41 (2005) 2343e2353. [1,5,7e11,14,15,28,43,52]. The plasma modiﬁcation technique has advantages over other methods mainly [9] C.-Y. Tua, Y.-L. Liua, K.-R. Lee, J.-Y. Lai, because it avoids environmental contamination Surface grafting polymerization and modiﬁcation problems and highly efﬁcient plasma-based treat- on poly(tetraﬂuoroethylene) ﬁlms by means of ments are also well suited to production-scale pro- ozone treatment, Polymer 46 (2005) 6976e6985. cesses. They provide good uniformity even for complex geometries. Plasma modiﬁcation allows [10] H.-X. Sun, L. Zhang, H. Chai, H.-L. Chen, Sur- cross-linking, functional group attachment, or abla- face modiﬁcation of poly(tetraﬂuoroethylene) tion. It can be surface-speciﬁc, leaving the bulk ﬁlms via plasma treatment and graft copoly- polymer and hence the mechanical properties unaf- merization of acrylic acid, Desalination 192 fected. A wide variety of gas precursors make avail- (2006) 271e279. able numerous surface treatments to alter the chemical and physical surface properties [52]. [11] H. Xu, Z. Hu, S. Wu, Y. Chen, Surface modiﬁca- tion of polytetraﬂuoroethylene by microwave References plasma treatment of H2O/Ar mixture at low pres- sure, Mater. Chem. Phys. 80 (2003) 278e282. [1] S. Ebnesajjad, C.F. Ebnesajjad, Surface Treat- ment of Materials for Adhesive Bonding, second [12] Q. Chen, Investigation of corona charge stability ed., Elsevier, Oxford, UK, 2013. mechanisms in polytetraﬂuoroethylene (PTFE) Teﬂon® ﬁlms after plasma treatment, [2] P.D. Patil, J.J. Feng, S.G. Hatzikiriakos, Constitu- J. Electrost. 59 (2003) 3e13. tive modeling and ﬂow simulation of polytetra- ﬂuoroethylene (PTFE) paste extrusion, [13] Z. Kolska´, A. Rezn´ıckova´, V. Hnatowicz, J. Non-Newtonian Fluid Mech. 139 (2006) 44e53. V. Svorc´ık, PTFE surface modiﬁcation by Ar plasma and its characterization, Vacuum 86 [3] J.A. Marchesi, K. Ha, A. Garton, G.S. Swei, (2012) 643e647. K.W. Kristal, Adhesion to sodium naphthalenide treated ﬂuoropolymers. Part II. Effects of treat- [14] A.A. Bujanda, V. Rodriguez-Santiago, C.C. Ho, ment conditions and ﬂuoropolymer structure, B.E. Stein, R.E. Jensen, D.D. Pappas, Atmo- J. Adhes. 36 (1991) 55e69. spheric Plasma Treatment of Polymer Films and Alumina Ceramics for Enhanced Adhesive, [4] J.L. Williams, T.S. Dunn, J.P. O’Connell, D. United States Army Research Laboratory, 2008. Montgomery, U.S. Patent 4,613,517, Assigned to Becton Dickinson and Co, September 1986. [15] S. Jinka, R. Behrens, C. Korzeniewski, V. Singh, A.I. Arunachalam, S. Parameswaran, et al., [5] K.-M. Baumgartner, J. Schneider, A. Schulz, Atmospheric pressure plasma treatment and J. Feichtinger, M. Walker, Short-time plasma breathability of polypropylene nonwoven fabric, pre-treatment of polytetraﬂuoroethylene for J. Ind. Text. 42 (4) (2013) 501e514. improved adhesion, Surf. Coat Technol. 142e144 (2001) 501e506. [16] K.H. Kale, A.N. Desai, Atmospheric pressure plasma surface treatment of textiles using non- [6] T. Shi, M. Shao, H. Zhang, Q. Yang, X. Shen, polymerizing gases, Indian J. Fibre Text. Res. Surface modiﬁcation of porous poly(tetraﬂuoro- 36 (2011) 289e299. ethylene) ﬁlm via cold plasma treatment, Appl. Surf. Sci. 258 (2011) 1474e1479. [17] M. Kogoma, H. Nakamura, H. Jinno, S. Okazakim, Effect of Atmospheric Pressure Glow Plasma on Adhesive Bond Strength of Plastics, Faculty of Science and Technology, Sophia University, 1992.

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272 EXPANDED PTFE APPLICATIONS HANDBOOK [18] I. Murokh, Atmospheric Plasma Surface [31] K. Yamada, K. Okita, A. Toyo-oka, S. Asako, Treatment Technique, Tri-Star Technologies, El U.S. Patent 5,296,510, Assigned to Sumitomo Segundo, CA, 2005. www.tri-star-technologies. Elec Ind, March 1994. com. [32] K. Yamada, K. Okita, A. Toyo-oka, S. Asako, [19] I.Y. Murokh, A.A. Kerner, Us Patent U.S. Patent 5219,894, Assigned to Sumitomo No.5,798,146, Assigned to Tri-Star Technolo- Elec Ind, June 1993. gies Corp., August 25, 1998. [33] K. Yamada, K. Okita, A. Toyo-oka, S. Asako, [20] M. Okubo, M. Tahara, Y. Abratani, T. Kurki, U.S. Patent 5252,626, Assigned to Sumitomo T. Hibino, Preparation of PTFE ﬁlm with adhe- Elec Ind, June 1993. sive surface treated by atmospheric-pressure nonthermal plasma graft polymerization, in: [34] J.A. Gardella Jr, T.G. Vargo, U.S. Patent Proceedings Electrostatics Joint Conference e 5,627,079, Assigned to Research Foundation of Session 11: Non-Thermal Plasma, Boston State Univ. of New York, May 1997. University, June 16e18, 2009. [35] J. Ge, A. Gjoka, J.H. Shyu, U.S. Patent [21] A.C. Sparavigna, Plasma Treatment Advantages 8,668,093, Entegris, Inc., March 2014. for Textiles, Cornell University Library, arXiv, 2008, pp. 1e16, 0801.3727v1 physics. [36] K. Singha, M. Singha, Cardio vascular grafts: pop-ph, http://arxiv.org/ftp/arxiv/papers/0801/ existing problems and proposed solutions, Int. J. 0801.3727.pdf. Biol. Eng. 2 (2) (2012) 1e8. [22] K. Tanaka, T. Inomata, M. Kogoma, Improve- [37] R.Y. Kannan, H.J. Salacinski, K. Sales, P. Butler, ment in adhesive strength of ﬂuorinated polymer A.M. Seifalian, The roles of tissue engineering ﬁlms by atmospheric pressure glow plasma, Thin and vascularisation in the development of micro- Solid Films 386 (2001) 217e221. vascular networks: a review, Biomaterials 26 (1) (2005) 1857e1875. [23] C. Tendero, C. Tixier, P. Tristant, J. Desmaison, P. Leprince, Atmospheric pressure plasmas: a [38] S.T. Rashid, H.J. Salacinski, G. Hamilton, review, Spectrochim. Acta B 61 (2006) 2e30. A.M. Seifalian, The use of animal models in developing the discipline of cardiovascular tis- [24] H. Uchiyama, S. Okazaki, M. Kogoma, U.S. sue engineering: a review, Biomaterials 25 Patent No. 5,124,173, Assigned to E. C. Chem- (2004) 1627e1637. ical Co. et al., June 23, 1992. [39] B.K. Patnaik, H.B. Lin, D.J. Lentz, R.J. Zdra- [25] A. Villermeta, P. Cocoliosa, G. Rames- hala, U.S. Patent 6,306,165, Assigned to Meadox Langlade, F. Coeuret, J.L. Gelot, E. Prinz, et Medicals, October 2001. al., ALDYNEs: surface treatment by atmo- spheric plasma for plastic ﬁlms converting in- [40] M. Thompson, N.B. McKeown, P.G. Kalman, dustry, Surf. Coat. Technol. 174e175 (2003) U.S. Patent 5,118,524, Assigned to the Toronto 899e901. Hospital, June 1992. [26] R. Wolf, Atmospheric plasma: the new func- [41] B.K. Patnaik, R.J. Zdrahala, U.S. Patent tional treatment for nonwovens, in: Proceedings 6,713,568, Assigned to SciMed Life Systems, of TAPPI PLACE, Conference, Boston, 2002. March 2004. [27] R. Wolf, Atmospheric Surface Modiﬁcation of [42] B.K. Patnaik, H.B. Lin, D.J. Lentz, R.J. Zdra- Polymers for Biomedical Device Adhesion, hala, U.S. Patent 6,803,069, Assigned to SciMed Enercon Industries, 2003. www.EnerconInd.com. Life Systems, October 2004. [28] C.A. Costello, T.J. McCarthy, Surface-selective [43] K.D. Park, T. Okano, C. Nojiri, S.W. Kim, introduction of speciﬁc functionalities onto Heparin immobilization onto segmented poly- PTFE, Macromolecules 20 (11) (1987) urethane urea surfacesdeffect of hydrophilic 2819e2828. spacers, J. Biomed. Mat. Res. 22 (1988) 977e992. [29] M. Morra, E. Occhiello, F. Garbassi, Langmuir 5 (1989) 872. [44] A. Kondyurin, M.F. Maitz, U.S. Patent 7,597,924, Assigned to Boston Scientiﬁc [30] J. A. Gardella Jr., T. G. Vargo, US Patent Scimed, October 2009. 4,946,903, Assigned to Research Foundation of State Univ. of New York, August 1990. [45] S. Wang, A.S. Gupta, S. Sagnella, P.M. Barendt, K. Kottke-Marchantc, R.E. Marchant, Bio- mimetic ﬂuorocarbon surfactant polymers

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13: SURFACE MODIFICATION OF EXPANDED POLYTETRAFLUOROETHYLENE 273 reduce platelet adhesion on PTFE/ePTFE sur- [49] S. L. Zukowski, U.S. Patent 5,437,900, Assigned faces, J. Biomater. Sci. Polym. Ed. 20 (5e6) to W. L. Gore & Associates, August 1995. (2009) 619e635. [46] J.T. Walter, U.S. Patent 6,780,497, Assigned to [50] S.L. Zukowski, U.S. Patent 5,462,781, Assigned Gore Enterprise Holdings, August 2004. to W. L. Gore & Associates, October 1995. [47] L. Seiler Jr., R. F. Rosenbluth, U.S. Patent 4,550,447, Assigned to Shiley Inc, November [51] J.T. Walter, U.S. Patent 7,736,576, Assigned to 1985. Gore Enterprise Holdings, June 2010. [48] L. Seiler Jr., R. F. Rosenbluth, U.S. Patent 4,647,416, Assigned to Shiley Inc, [52] H.H. Chien, K.J. Ma, C.H. Kuo, S.W. Huang, March 1987. Effects of plasma power and reaction gases on the surface properties of ePTFE materials during a plasma modiﬁcation process, Surf. Coat. Technol. 228 (2013) S477eS481.

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14 The Competitive Scene OUTLINE 14.1 Introduction 275 14.2.4 Zeus Industrial Products 276 14.2.5 C. R. Bard Corporation 276 14.2 Other Expanded Polytetraﬂuoroethylene 275 14.2.6 Maquet Cardiovascular 276 Players 275 14.2.7 Porex Corporation 276 14.2.1 General Electric 276 14.2.8 Phillips Scientiﬁc 276 14.2.2 Donaldson Corporation 276 14.2.9 Asian Manufacturers 277 14.2.3 DeWal Industries 14.1 Introduction Gore Scotland produces electronic interconnects in Dundee. Two plants in Livingston are dedicated to Naturally, the lucrative business that W.L. Gore fabrics and ﬁltration products and medical sales. created attracted competitors. The expiration of Gore’s basic patents accelerated the competition’s Gore Shenzhen manufacturing facility produces growth in all economic regions of the world. The fabrics. The Shenzhen facility is also licensed to supplier’s size, technological capabilities, and prod- make certain electronic products. A Gore joint ven- uct offerings fall within a broad range anywhere from ture company in Shanghai, Shanghai Bag Filtration small independent companies to global powers such Equipment Co., manufactures ﬁltration products. as GE and Donaldson Corporations. Nearly every company has adopted Gore’s business model of 14.2 Other Expanded selling products primarily fabricated with expanded Polytetraﬂuoroethylene Players polytetraﬂuoroethylene (ePTFE). This approach, generally, allows capture of a larger value for the There are a number of producers of ePTFE for ePTFE by the manufacturers. internal consumption of the companies and external supply as membranes. Gore EU and Japan Japan Gore-Tex Inc. (JGI), a wholly owned Gore 14.2.1 General Electric subsidiary, functions as the operational base of the Gore group in Japan. JGI is engaged in the devel- General Electric acquired BHA Corporation in opment, manufacturing, and sales of diversiﬁed June 2004, which is currently a division of GE Po- GORE-TEX products in the industrial, fabrics, and wer (www.gepower.com). BHA Group is a wholly medical ﬁelds. owned subsidiary of GE Environmental Service Gore Germany plants are clustered in Putzbrunn team targeted at reducing particulate emissions. and Feldkirchen-Westerham, near Munich, and Prior to acquisition by GE, BHA Group Holdings Pleinfeld, which is close to Nuernberg. The Pleinfeld was a global ﬁltration company. Its principal busi- team focuses on electronic interconnect products. ness was the design, manufacture, and sale of Associates in Putzbrunn and nearby Feldkirchen- replacement parts and the performance of rehabili- Westerham produce fabrics, membranes, industrial tation conversion services for the types of industrial sealants, ﬁltration products, and vents. Medical air pollution control equipment known as baghouses, product sales are centered in Putzbrunn. cartridge collectors, and electrostatic precipitators. Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00014-6 275 Copyright © 2017 Elsevier Inc. All rights reserved.

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276 EXPANDED PTFE APPLICATIONS HANDBOOK The story of BHA did not end here. In November 14.2.5 C. R. Bard Corporation 2013, Clarcor Inc. announced the acquisition of the air ﬁltration business of GE’s Power and Water di- C. R. Bard Corporation is involved with the vision for approximately $265 million. The business design, manufacture, packaging, distribution, and was a leading supplier of air ﬁltration systems and sale of medical, surgical, diagnostic, and patient care ﬁlters used in gas turbine applications, as well as devices worldwide. Bard manufactures its own industrial air ﬁltration products and membranes. The ePTFE membranes, in contrast to the majority of company has manufacturing operations in Missouri, medical device manufacturers that buy membranes. the UK, and China. It has continued to supply It offers vascular, urology, oncology, and surgical gas turbine air inlet ﬁltration systems and ﬁlters to specialty products. The company’s vascular products GE. include percutaneous transluminal angioplasty cath- eters, guide-wires, introducers and accessories, pe- 14.2.2 Donaldson Corporation ripheral stents, stent grafts, vena cava ﬁlters, and biopsy devices; electrophysiology products, such as Donaldson Corporation (www.Donaldson.com) is electrophysiology laboratory systems and diagnostic, the largest ﬁltration company in the world. It ac- therapeutic, and temporary pacing electrode cathe- quired Tetratec Company in 1994. Acquisition of ters; and fabrics, meshes, and implantable vascular Tetratec has allowed Donaldson a signiﬁcant degree grafts. of vertical integration in products that contain ePTFE membranes. The company purchases PTFE resin 14.2.6 Maquet Cardiovascular and sells ﬁltration products. Prior to acquisition by Donaldson, Tetratec produced and supplied ePTFE Maquet Cardiovascular (part of Getinge Group of membrane. It did not participate in the manufacturing Sweden) acquired Atrium Medical Corporation, of fabricated products from its membranes. The engaged in medical device technologies for inter- company’s membrane business was focused on ventional cardiology and radiology, chest trauma multiple markets of ﬁltration, apparel, vents, and care and thoracic drainage, vascular surgery, and other markets. general surgery. The acquisition further enhanced the extensive experience of Maquet with ePTFE extru- 14.2.3 DeWal Industries sion and processing. DeWal Industries (www.DeWal.com) produces 14.2.7 Porex Corporation ePTFE for ﬁltration and venting applications. DeWal produces ePTFE for specialized uses such Porex Corporation has several global sites. Its as blood ﬁlters, gas sensor membranes, and parts and products are used in a wide range of other applications that require submicron pore products ranging from consumer products such as sizes. children’s markers and home scent diffusers to medical and bioscience devices such as dialysis 14.2.4 Zeus Industrial Products systems and DNA research equipment. In 1999, Porex acquired Mupor Ltd., a manufacturer of porous Zeus Industrial Products (www.ZeusInc.com) PTFE membrane and solid PTFE labware, to expand began producing ePTFE products in 2008. Zeus was its range of polymer capabilities into the growing founded in 1966 and has developed a broad array of PTFE membrane market. multilumen tubing for medical applications. They have been producing ePTFE tubing for a number of 14.2.8 Phillips Scientiﬁc years and have now extended their ePTFE offering to variety of shapes, including ultrathin membranes Phillips Scientiﬁc is a small manufacturer of (<<10 mm). Since the 1990s, Zeus has established ePTFE products and production systems equipment. an extensive research and development sector that The company’s advertised products include tubing, has resulted in products made from high performance ﬁlters, valve stem packing, protective pads, hex rod thermoplastics. proﬁling, joint sealant, thread, tubing, tapes, mem- branes, and sheeting. It also provides laminating such

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14: THE COMPETITIVE SCENE 277 as ePTFE membrane to felts, spunbonds, mesh, and Other manufacturers of ePTFE membranes and ﬁlm. products from ePTFE include: 14.2.9 Asian Manufacturers Nitto Denko/Ambic/Kyowa (Toyobo), Japan Shanghai Lingqiao Environmental Protecting Sumitomo Electric Industries, Japan, produces ePTFE membranes and products from ePTFE mem- Works, China branes. They offer porous separation membranes Shanghai Da Gong New Materials Co., China exclusively, and as membrane modules. Applications Anhui Lite Environment, China include wastewater treatment, clariﬁcation, and Shanghai Linﬂon Film Technology, China sterile ﬁltration. The modules are used for gas Yeu Ming Tai Chemical Ind., Taiwan dissolution and deaeration.

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Index ‘Note: Page numbers followed by “f ” indicate ﬁgures and “t” indicate tables.’ A Anionic ring-opening oligomerization of coating, 266 hexaﬂuoropropylene oxide, 33f molecule, 266 ABA. See Adsorbent breather assembly Biocompatible ePTFE, 6 (ABA) Anionic surfactants, 33f Biological products, 194 Aortoiliac occlusive disease, 207 Biomaterials, 196 Accordion-shaped crystallites, 45 APFDO. See Ammonium perﬂuoro-3, Blending lubricant and pigment and Activated carbon, 258 Adhesion 6-dioxaoctanoate (APFDO) performing, 87, 90 APFO. See Ammonium Blocking process, 102 adhesive backer, 256e257 Bolt-load retention, 244, 245f adhesive bonding, 260 perﬂuorooctanoate (APFO) Bond strength, 261e262, 262f bond strength, 260e262, 261t, 264e265 APMW. See Atmospheric pressure Bottle blending, 74e76 surface modiﬁcation, 263e265 Branched TFE chains, 12e14 Adjacent reentry model, 12 microwave (APMW) Branching mechanism, 11 Adsorbent breather assembly (ABA), APS. See Ammonium persulfate (APS) Break strength, 56 Arrhythmias, 206 Breathable and moisture-repellent fabric 257e258, 257f Arteriovenous ﬁstula graft (AVF graft), AFM. See Atomic force microscopy construction, 181, 181f 201, 201fe202f Breathable ePTFE fabric structure, (AFM) Arteriovenous graft (AV), 201 AI. See Amorphous index (AI) Artery disease, 140 172e178, 172f Alkali metals, 15 Asahi Glass Company, 58 American Society for Testing Materials Asian Manufacturers, 277 coating perﬂuoroalkyl acrylic ASTM. See American Society for Testing copolymer, 177f (ASTM), 40 Materials (ASTM) construction of breathable and moisture ASTM F 316 standard, 122e123 Atmospheric plasma treatment, repellent layers, 173f ASTM Method D4441, 62 ASTM Method D4895, 60e61 262e263, 263f methods for measurement of ASTM method D4935, 257 Atmospheric pressure microwave breathability of ﬁlms and coatings, ASTM Method D6611, 242, 243f, 243t 175t 3-Aminopropyl, 265 (APMW), 265, 265t Ammonium carbonate, 52e53 Atomic force microscopy (AFM), 19 model of moisture vapor transmission, Ammonium perﬂuoro-3, Autoclave, 37e43 172f 6-dioxaoctanoate (APFDO), 43 stainless steel, 35, 47 polymerization reaction, 175f Ammonium perﬂuorooctanoate (APFO), AV. See Arteriovenous graft (AV) scanning electron micrographs, 177f AVF graft. See Arteriovenous ﬁstula graft structure of water repellent, 173f 32e33, 41 water vapor transmission, 174f, 176f (AVF graft) Bubble Point, 227te228t alternatives to, 33e35 Azelaic acid, 266 anionic surfactants, 33f test, 123e124, 129 effect of surface tension, 46f B polymerization conditions and polymer C Balloon-expandable stents, 207 properties with, 43t BHA Group, 275e276 C.R. Bard Corporation, 276 TFE emulsion polymerization with Biaxial expansion process, 107e111 C2F5(VDF)2I, 33e34 C8. See Ammonium perﬂuorooctanoate replacements, 42e44 biaxial laboratory stretcher, 108f Ammonium persulfate (APS), 35e36 coarse and highly porous uniaxially (APFO) Ammonium sulﬁte (AMS), 42, 53 Cables Amorphous index (AI), 49e50 ePTFE, 111t Amorphous locking process, 122 multiple oven arrangements in Karo IV and cable assemblies, 168 Amorphous phase, 121 coaxial, 254e255, 254f Amorphously locked PTFE, 165 biaxial stretcher, 109f Calendared extrudate, 105 AMS. See Ammonium sulﬁte (AMS) photograph of stretch frame, 108f Calendaring, 92e94, 93f Anastomosis, 140 properties of PTFE ﬁlm, 110t Anhydrous liquid ammonia, 260 Biaxial orientation, 131e133, 132f equipment, 92e94 Biaxial stretching process, 245e246 ﬁsh-tail guide, 94f Bioactive operation, 94 agent, 266e267 279

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280 INDEX Capillary force, 178 Crushability, 106 EMI gasket, 256e257 Cardiac resynchronization therapy device Crystalline structure of PTFE, 12, 13f, hook-up wire, 255e256 Electromagnetic interference shielding (CRT device), 205, 206f 18f Cardiac rhythm management devices CTFE. See Chlorotriﬂuoroethylene gasket (EMI gasket), 256e257 Electronic and electrochemical (CRM devices), 205 (CTFE) CARMEDA BioActive Surface (CBAS), Cut-through resistance, 255e256, 256t application, 168e169 CVM. See Center for veterinary medicine Electrophiles, 14 200, 200f Electrostatic attraction, 215, 215f Cartridges, 237 (CVM) EMI gasket. See Electromagnetic CBAS. See CARMEDA BioActive D interference shielding gasket Surface (CBAS) (EMI gasket) CBER. See Center for biologics Daikin Corporation, 58 Emulsion polymerization, 25, 32, 35f, 99 Degree of invasiveness, 195 evaluation and research (CBER) Deionized water, 178 with APFO replacements, 42e44 CCT. See Critical cracking thickness Densiﬁed porous PTFE membranes, development for ePTFE applications, (CCT) 149e153, 154f 45e58 CD. See Cross direction (CD) Garlock’s one-up pump diaphragm, 152f PTFE preparation, 35e42 CDER. See Center for drug evaluation scanning calorimetry thermogram, 152f TFE, 44e45 strain vs. time, 153f and research (CDER) Dental ﬂoss, 233, 236f polymerization, 29e31 Cell membrane, 241e242, 242f Depth ﬁltration, 214, 214f polymers, 31e35 Center for biologics evaluation and DeWal Industries, 276 preparation, 26e29 Dextran (Dex), 268, 268f Emulsion techniques, 32 research (CBER), 194 DFF. See Direct ﬂow ﬁltration (DFF) Endothermic ratio, 49 Center for drug evaluation and research Dichlorodiﬂuoromethane, 26 Die, 81e83 measurement, 57 (CDER), 194 land, 83 ePTFE. See Expanded Center for veterinary medicine (CVM), Dielectric constant values, 253, 254t Differential scanning calorimetry (DSC), polytetraﬂuoroethylene (ePTFE) 194 Etching technique, 260, 267e268 CeF bonds on properties of PTFE, 9e11 49, 49f, 117, 143 Ethylene tetraﬂuoroethylene copolymer CHClF2. See Chlorodiﬂuoromethane Diffusion, 215, 215f Direct ﬂow ﬁltration (DFF), 223 (ETFE), 206 (CHClF2) Disk drive ﬁlters, 257e258 Expanded polytetraﬂuoroethylene Chemically treated activated carbon, 258 Dispersion Chlorendic acid, 36f (ePTFE), 4, 9, 25, 99, 129, 171, Chlorodiﬂuoromethane (CHClF2), 26 polymerization, 99 196e198, 197f, 197te198t, 214, of PTFE, 62 253, 259, 270f, 275. See also conversion, 28t Dissipation factor, 253 Medical and surgical preparation, 26 Disuccinic acid peroxide (DSP), 40e41, applications; Chloroform preparation, 26 Polytetraﬂuoroethylene (PTFE); Chlorotriﬂuoroethylene (CTFE), 39 53 Tetraﬂuoroethylene (TFE) CMC. See Critical micelle concentration DOE. See US Department of Energy advantages and disadvantages of (CMC) (DOE) insulation materials, 256t Coagulated dispersion, 29e31, 59e60 Donaldson Corporation, 276 Drug delivery ﬁlter, 229, 229f amorphous locking process, 122 polymerized tetraﬂuoroethylene, 65 Dry curve, 123, 123f aortic arch graft, 141, 142f powder, 65 Drying, 83 cables, 255f Coaxial cables, 254e255, 254f DSAP. See Disuccinic acid peroxide characteristic of membranes pores, Common air contaminants, 218, 219f Cooling rate experiments, 115t (DSP) 122e125 Coreeshell polymer, 39 DSC. See Differential scanning development Costello process, 263e264 Crack propagation in PTFE, 21 calorimetry (DSC) opportunities, 4 Creep rate, 56e57 DSP. See Disuccinic acid peroxide (DSP) of PTFE for applications, 45e58 Critical cracking thickness (CCT), 41 DuPont, 1e5, 47 discovery, 5e7 Critical micelle concentration (CMC), 34 Durably water repellent, 172e173 ePTFE-based ﬁlters, 168 CRM devices. See Cardiac rhythm ﬁber, 140e149, 233e244, 233te234t E and fabrics, 169 management devices (CRM ﬁshing line, 238e240, 238f devices) Electrical and electronic applications high tensile strength PTFE ﬁber, Cross direction (CD), 131 coaxial cables, 254e255 Cross ﬂow ﬁltration, 223 disk drive ﬁlters, 257e258 142e143 CRT device. See Cardiac oral care, 233e235, 235f resynchronization therapy device plastic ﬁlm slitting machine, (CRT device) 144f

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INDEX 281 razor blade slitting, 144f Extruder, 80e81 gasesolid ﬁltration, 215e223 ropes, 242e244, 242f, 244f solideliquid ﬁltration, 223e224 sewing threads, 236e238, 237f, 237t Davis electric vertical extruder, 81f surface ﬁltration processes, 214 stressestrain, 146fe147f, 147t Jennings International horizontal Fine powder, 65, 70e71 strong core ﬁber, 149f sutures, 204, 205f, 235e236, 236f extruder, 82f polytetraﬂuoroethylene resins, 60e62 weaving and knitting ﬁber, 240e242, master die, 82f products, 59e60 Extrusion. See also Paste extrusion PTFE fabrication and processing 240f ﬁlters, 217e218 aid, 61e62, 71e74 background, 65e66 isopar solvents properties, 72te73t blending resin with lubricant, 74e77 for baghouses, 220f lubricant properties, 74t extrusion aid, 71e74 comparison of backing fabrics, 221t solvents ﬂammability data, 75t extrusion equipment and process, high efﬁciency particulate arresting equipment and process, 79e84 79e84 ﬁlter, 222f changes in PTFE paste, 81f extrusion of tubing, 85e90 manufacturing process, 216 die, 81e83 individual PTFE particles, 66f microporous membranes, 213 drying, 83 paste extrusion fundamentals, 66e68 Tetratex ﬁlter media, 217 extruder, 80e81 preforming, 77e79 formation, 118e122 PTFE wire extruder, 80f resin handling and storage, 68e71 DSC thermogram, 119fe120f reduction ratio, 84, 85te86t, 87f unsintered tape, 90e96 Maxwell model, 121, 121f sintering and cooling, 83e84 Fish-tail guide, 94f microstructure of section of Fishing line, 238e240, 238f pressure, 52, 61, 68, 69fe70f pasteeePTFE ﬁlm, 120f evaluation, 57 differential scanning calorimetry SEM of, 120f thermograms, 239f joint sealant, 159 of tubing, 85e90, 87t lipoatrophy implants, 202e203 blending lubricant and pigment and scanning electron micrograph of ﬁber advanta ePTFE dual-porosity performing, 87 surface, 238f extrusion of spaghetti tubing, 87e90, conﬁguration, 204f 88f, 89t Flame treatment, 264e265 advanta ePTFE implants, 204f “Flat lattice” structure, 7 manufacturing, 129 F “Floppy” gasket material, 156 densiﬁed porous PTFE membranes, Fluon CD-023, 48e49 F22. See Dichlorodiﬂuoromethane Fluorescein isothiocyanate, 265 149e153 Fabrics, 184 Fluorinated ethylene propylene polymer ePTFE articles, 99e111 Fabrics and apparel, ePTFE use in planar ePTFE membranes, 129e133 (FEP polymer), 12e14, 103, shapes and applications, 130t breathable ePTFE fabric structure, 198e199 tubular ePTFE shapes, 134e142 172e178, 172f Fluorinated surfactant, 58 membrane, 173e174, 180e181 Fluorine (F), 9 ﬁlms, 168 development history, 178e182, 180t microstructure, 116e118 outdoor apparel, 182e188 bonds on properties of PTFE, 9e11 products, 163, 164t protective apparel, 188e189, 189te190t CeF bonds on properties of PTFE, 9e11 applications, 166e169, 167te168t FDA. See Food and Drug Administration Fluoropolymers, 1, 4, 7, 260, 264 properties and characteristics, (FDA) solvents, 15e16 163e166, 166f, 166t FEP. See Hexaﬂuoroethylene copolymer Fluorspar (CaF2), 26 scanning electron micrograph of, 163f Food and Drug Administration (FDA), sheets, 153e158 (FEP) fabricating ePTFE gasket, 155f FEP polymer. See Fluorinated ethylene 193e194 low sealing stress gasket, 158f Footwear, testing, 186e187, 188f stacking of ePTFE ﬁlm layers, propylene polymer (FEP Forschungszentrum Rossendorf (FZR), polymer) 155f Fiber, 233 267 structure of multilayer compressible Fibril(s), 25, 66e69 Fracture of PTFE, 20e22 Friction, 16e18 ePTFE gasket, 157f formation, 22 tapes and rods, 159 porous structure, 259 unﬁlled ﬂuoropolymers, 17t Fibrillation, 25, 68e69 FZR. See Forschungszentrum Rossendorf ePTFE joint sealant gasket, 159f Film stretching, 131e132 vents, 248e250, 249f Filter(s), 215 (FZR) rolls of ready-to-use, 250f bags, 218, 237, 237f G sound wave transfer, 250f cake, 213 Extendedechain crystals, 115 medium, 213 Garments, 171 “Extruded paste”, 100, 101f Filtrate, 213 Gas ﬁltration, 215, 215f Filtration, 213, 215 Gaskets, 244e248, 245f, 248t applications, 224e230 complex and simple gasket shapes, 246f classiﬁcation, 213e214 testing, 247e248

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282 INDEX Gasesolid ﬁltration, 215e223. See also High molecular weight polyethylene L Solideliquid ﬁltration (HMPE), 242, 244f LCP. See Liquid crystal polymer (LCP) choice of ﬁltration method, 219f High tensile strength PTFE ﬁber, 142e143 Le Chaˆtelier-Braun principle. See Le common air contaminants, 218f HMPE. See High molecular weight comparison of pores sizes of various Chaˆtelier’s principle polyethylene (HMPE) Le Chaˆtelier’s principle, 27 ﬁlter media materials, 216f Hollow ground blades, 144 Lead assembly of implanted devices, ePTFE ﬁlters for baghouses, 220f Hook-up wire, 255e256 ﬁlter media comparison in cement ﬁnish 205e206 high strength toughened, 255f mill process, 218t Horizontal heated console design, 83 actual implantable cardioverter nonmembrane ﬁlter, 216f Hybrid ﬂexible round cable, 255, 255f deﬁbrillator, 207f pressure difference across ﬁlter for Hydrochloric acid (HCl), 1 Hydrophilic spacer, 266 distal portions of Endotak Reliance ePTFE, 217f Hydrophilicity, surface modiﬁcation for, 0157, 207f SEM of ePTFE membranes, 217f Gauge blocks, 105 263e265 Lennard-Jones potential, 17 General Electric (GE), 275e276 Hydrophobic membrane surfaces, 265 Liquid General Motors (GM), 2 Gore, W.L., 216, 275 I ﬁltration, 215 Gore EU and Japan, 275 modes, 223, 224f Gore Germany, 275 ICD devices. See Implantable Gore Scotland, 275 cardioverter deﬁbrillator devices liquid-Filled Portions, 265 Gore Shenzhen, 275 (ICD devices) liquid-wetted portions, 265 Gore Viabahn endoprosthesis, 207 pigments, 77 GORE-TEX, 6 ID. See Inner diameter (ID) splash protection, 189 Impaction, 215, 215f Liquid crystal polymer (LCP), 242, 244f coats, 173, 182e183, 182f Implantable cardioverter deﬁbrillator Log(POW), 58 gasket product, 156 Long-term device, 195 insulated comfort footwear, 186, 187f devices (ICD devices), 205e206, Low surface tension liquids, 264, 264t membrane, 186 206f, 208t Low-pressure plasma treatment (LPT), Paclite apparel, 184 Implantable device, 195e196 performance comfort footwear, 186, In vivo environment, 195 260e263 Industrial and process ﬁltration Lubricant, 61e62, 71e74 187f application of ePTFE, 167e168 pro products, 184 Inner diameter (ID), 139 blending resin with, 74e77 trademark, 182e183 Inorganic pigments, 77 isopar solvents properties, 72te73t Gore-Tex. See GORE-TEX Inorganic salts, 59e60 properties, 74t Insufﬂation, 229 solvents ﬂammability data, 75t H ﬁlters, 229, 230f Interception, 215, 215f M Half dry curve, 123, 123f International Standards Organization Hard disk drive ﬁlters (HDD ﬁlters), 257, (ISO), 60 Machine direction (MD), 101e102, 130, Invasive, 195 134t 258f IPA. See Isopropyl alcohol (IPA) HCFC-22. See Chlorodiﬂuoromethane ISO. See International Standards Machine direction orientation (MDO), Organization (ISO) 131 (CHClF2) Isopar solvents properties, 72te73t HCl. See Hydrochloric acid (HCl) Isoparafﬁn, 61e62 Manhattan Project, 3 HDD ﬁlters. See Hard disk drive ﬁlters Isopropyl alcohol (IPA), 176 Maquet Cardiovascular, 276 Martindale test, 184e185 (HDD ﬁlters) J Matrix tensile strength (MTS), 151 Heat treatment, 142, 146e148 Matrix-spun ﬁber, 142 HEPA ﬁlter. See High efﬁciency J-integral analysis, 21 Maxwell model, 121, 121f Japan Gore-Tex Inc. (JGI), 275 particulate arresting ﬁlter (HEPA Jar blending, 74e76 linear viscoelasticity model, 121 ﬁlter) JGI. See Japan Gore-Tex Inc. (JGI) McCarthy process, 263e264 Heparin, 199e200 Joint sealants beads, 246, 246f MD. See Machine direction (MD) Hexaﬂuoroethylene copolymer (FEP), MDO. See Machine direction orientation 254, 257 K Hexaﬂuoropropylene (HFP), 26, 103 (MDO) Knitting ﬁber, 240e242, 240f Mean ﬂow pore size test, 123 HFP oxide, anionic ring-opening graph of thickness uniformity, 241f Medical and biological uses of ePTFE, oligomerization, 33f Koo’s model, 113e114 168 High boilers, 26 Medical and surgical applications. High efﬁciency particulate arresting ﬁlter See also Expanded (HEPA ﬁlter), 219e220 polytetraﬂuoroethylene (ePTFE) ancient Egyptian medical instruments, 194f biomaterials, 196

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INDEX 283 classiﬁcation of devices, 195e196, 195t National Fire Protection Association Perﬂuoro ethylene, 254 designing medical devices, 196 (NFPA), 189 Perﬂuoroalkoxy polymer, 254 ePTFE lipoatrophy implants, 202e203 Perﬂuoroalkyl acrylic copolymer, 175 ePTFE sutures, 204 Neto’s technique, 148 Perﬂuorocyclobutane, 27e28 lead assembly of implanted devices, NFPA. See National Fire Protection Perﬂuorooleﬁns, 14 Perﬂuoropropyl vinyl ether (PPVE), 37 205e206 Association (NFPA) Perﬂuorovinyl ether, 41 medical applications, 198, 199t Non-dewetting, 265 Pericardial membrane, 202 medical devices, 193e195 3,3,5,5,7,7,8,8,8-Nonaﬂuorooctanoic Petroleum solvents, 71e74 patches, 202 Phillips Scientiﬁc, 276e277 stents, 207 acid, 34f Pigment vascular grafts, 198e201, 199f Nonmembrane ﬁlter, 216f Medical devices, 193e195 Nontelogenic anionic surfactant, 53 addition, 77 Nucleophiles (Nuc), 14 dispersions, 77 classiﬁcation, 195e196, 195t PIII. See Plasma immersion ion designing, 196 reaction scheme for nucleophilic attack, Melt creep viscosity, 37 15f implantation (PIII) Membranes pores, 122e125 Planar ePTFE membranes, 129e133 Nylon, 240 Bubble Point test, 123e124 biaxial orientation, 131e133, 132f YoungeLaplace equation, 124 O uniaxial orientation, 130e131, 131f Mercury, 124e125 Plasma O/C ratio. See Oxygen to carbon ratio porosimetry method, 124e125 (O/C ratio) plasma-modiﬁed membrane, 265 Microﬁltration applications of ePTFE, plasma-treated ﬁber, 238 OD. See Outer diameter (OD) treatment, 266, 270f 168 Oleophobic ﬂuoropolymer, 175 Plasma immersion ion implantation Microporous membranes, 229, 229f Microstructure coating, 175 (PIII), 267, 268f Oral care, 233e235, 235f Platelet adhesion, 268, 269f of ePTFE, 116e118 Oral cavity, 235 Poisson’s ratio, 165, 167f DSC scans of PTFE powder, 118f Outdoor apparel, 182e188 Polyester, 198, 240 SEM of biaxially ePTFE, 116f Polyethylene (PE), 9e10, 10f SEM of uniaxially ePTFE, 116f design of eVent fabric, 183f Polyethylene oxide (PEO), 267 structure of uniaxially ePTFE outdoor footwear, 186, 186f Polykettle, 51, 53 membra, 116f outdoor gloves, 188, 188f Polymer testing apparel, 184e186, 185f of PTFE, 20e22, 111e116 testing footwear, 186e187, 188f chains, 266e267 crystalline structure of PTFE, 114f Outer diameter (OD), 139 coatings, 178 deformation of PTFE Oxy-ﬂuoropolymer, 264 structure, 266, 266f crystalsearrows, 112f Oxygen to carbon ratio (O/C ratio), 265 Polytetraﬂuoroethylene (PTFE), 1, 9, 25, fringed-micelle model, 115f Oxygen-free kettle, 42 SEM of PTFE, 113f 65, 99, 129, 163, 176e178, 196, P 215, 224, 253, 259. See also Miller Cuff techniques, 140 Expanded polytetraﬂuoroethylene Mitral regurgitation, 236 Pantograph, 107 (ePTFE); Tetraﬂuoroethylene Modiﬁed polytetraﬂuoroethylene, 154 Parafﬁn wax, 54 (TFE) Moisture vapor transmission rate Paste extrusion, 66e68, 142e143 amorphous locking process, 122 (MVTR), 174 crystalline structure of PTFE, 67f biaxial expansion process, 107e111 Molecular interaction of PTFE, 16e18 DSC diagram, 68f billet, 139 Molecular weights (MWs), 99 ﬁbrilation of ﬁne powder PTFE, 67f branched tetraﬂuoroethylene chains, Monoﬁlaments, 241e242 ﬁbrillation of PTFE, 69f Motorized blender, 74 one PTFE ﬁne powder particle, 66f 12e14 MTS. See Matrix tensile strength (MTS) pressure, 37, 69fe70f characterization, 60e62 MVTR. See Moisture vapor transmission Patches, 202 commercialization, 3e4 Gore ACUSEAL cardiovascular patch, comparison of products and processes, rate (MVTR) MWs. See Molecular weights (MWs) 203f 32t Gore TAG ePTFE thoracic conformations, 18e20 N crystalline structure, 12, 13f, 18f endoprosthesis, 204f development for ePTFE applications, Naphthalene, 260 Gore’s preclude, 203f NASA. See National Aeronautics and PE. See Polyethylene (PE) 45e58 Pedersen, Dr. Charles J., 3f break strength, 56 Space Administration (NASA) Peeling, 244 creep rate, 56e57 National Aeronautics and Space Penetration, 189, 220 endothermic ratio measurement, 57 Pentamer, 14e15, 15f Administration (NASA), 19e20 PEO. See Polyethylene oxide (PEO)

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284 INDEX Polytetraﬂuoroethylene (PTFE) (Continued ) PPVE. See Perﬂuoropropyl vinyl ether Round and rectangular bead extrusion, evaluation of extrusion pressure and (PPVE) 91e92, 91f, 92t stretchability, 57 preparation of test specimen, 47e48 “Preformed paste”, 100, 101f RR. See Reduction ratio (RR) stress relaxation time, 48e51, Preforming, 77e79, 78fe79f Rugged hybrid round cable, 255f 56, 58 stretch procedure, 48, 55 compaction rate, 78t S stretch test, 51 scanning electron micrograph, 79f stretching rate, 48 Pressure vessel, 36 Scaffolding, 207 tensile break strength test, 48, 57 Pressure Vessel Research Council Scanning electron micrograph (SEM), ultimate SR test, 47 (PVRC), 247 100, 259, 259fe260f electrical properties, 253t Prosthetic graft, 265 electronic properties of hydrogen and Protective apparel, 188e189, 189te190t high stretch ratios, 260f PTFE. See Polytetraﬂuoroethylene PTFE ﬁlm, 262f halogens, 10t SCCO2. See Supercritical carbon dioxide expansion processes, 100e101, 100f (PTFE) ﬁber, 142 PU. See Polyurethane (PU) (SCCO2) ﬁbrils, 67 PulsePleat ﬁlter elements, 218, 222f SE. See Shielding effectiveness (SE) microstructure, 20e22, 111e116 PVRC. See Pressure Vessel Research Sealants, 169 molecular interaction, 16e18 Seals, 244e248 PE vs., 12t Council (PVRC) Seam sealing tape for apparel, 181fe182f phase diagram, 11f Pyrolysis, 27e28 Self-expandable stents, 207 preparation by emulsion polymerization, SEM. See Scanning electron micrograph R 35e42 (SEM) AMS, 42 R22. See Dichlorodiﬂuoromethane Sewing threads, 236e238, 237f, 237t autoclave, 37e38 Ram extruder, 80 Sewn ﬁlter media, 237 coreeshell polymer, 39 Reaction Shielding effectiveness (SE), 257 polymer properties, 40t Short-term device, 195 pressure vessel, 36 mechanisms, 14e15 Silica gel, 258 recipe and properties, 38te39t vessel, 36 Silicone resin, 175 stabilizers, 36 Reduction ratio (RR), 61, 66, 84, Silver compounds, 180 TFE consumption, 42 Sintering 3M Corporation, 41 85te86t, 134e135 primary fracture mechanisms, 22f and cooling, 83e84 properties of ﬁne powder, 59t for extruding rods, 92 sintered ePTFE, 130 reaction mechanisms, 14e15 parameter, 79 Slurry, 213 Roy Plunkett’s story, 1e3 Relaxation time, 121e122 Sodium solvents on ﬂuoropolymers, 15e16 Repulsive forces, 17 three chain-folding model in polymer Resin etching, 260, 261t, 263 heparin, 261 crystals, 13f blending with lubricant, 74e77 sodium-etched PTFE surface, 260 transitions, 18e20, 19t blending ﬁne powder PTFE, 76f Solideliquid ﬁltration, 223e224. tubing, 135e136, 135f pigment addition, 77 uniaxial expansion process, 101e107 PK twin-shell liquid solids blender, See also Gasesolid ﬁltration X-ray diffraction and AFM 76f Turbula Shaker Mixer blender, 77f chemical compatibility of common ﬁlter measurements comparison, 20t membranes, 225te226t yarns, felts, and microporous expanded handling and storage, 68e71 agglomerates of ﬁne powder, 71f hydrophobic ePTFE membrane membranes, 215e216 cooling time, 70f ﬁlter cartridge, 228f Polyurethane (PU), 172, 180 properties, 227te228t Polyvinylidene ﬂuoride, 238, 253 Rheometric pressure. See Extrusion Porex Corporation, 276 pressure water ﬂow rate vs. pressure, 227f Porosity, 122e123, 129, 135f, 137e138 Solvents on ﬂuoropolymers, 15e16 Porous medium, 213 Room Temperature Operational Spaghetti tubing extrusion, 87e90, 88f, Tightness Test (ROTT), 247e248, 247f, 247t, 248f, 89t 248t Sparks, 84 Spherical geometry, 45 Ropes, 242e244, 242f, 244f SPM. See Scanning probe microscopy ROTT. See Room Temperature (SPM) Operational Tightness Test Spooling process, 240 (ROTT)

P:306

INDEX 285 SR. See Stretch ratio (SR) TCE. See Trichloroethylene (TCE) tube expansion apparatus, 138f Standard speciﬁc gravity (SSG), 37e38, TD. See Transverse direction (TD) Turbula Shaker Mixer blender, 76, 77f Teﬂon 6A, 119 62, 99 TeﬂonÒ ﬂuoropolymer, 3 U Tefzel ETFE, 150 specimen, 56 Tensile break strength UHMWPE. See Ultrahigh molecular Stents, 129, 207 weight polyethylene (UHMWPE) measurement, 57 into clogged artery, 208f test, 48 ULPA ﬁlter. See Ultralow penetration air ePTFE-covered stents, 209f Tenter frame, 133, 133f ﬁlter (ULPA ﬁlter) Gore Viabahn endoprosthesis stent, 209f Tenterhooks, 131e132 ICD leads, 208t Tentering, 132e133 Ultimate SR test, 47 occlusion in iliac aorta, 208f Testing apparel, 184e186, 185f “Ultimate” ﬁlter cartridge, 224e227 self-expandable and balloon- Tetraﬂuoroethylene (TFE), 1, 10e11, 25, Ultipleat high ﬂow ﬁlter, 224, 227f Ultrahigh molecular weight polyethylene expandable, 207 103, 181e182. See also Stress relaxation time, 48e51, 56, 58 Expanded polytetraﬂuoroethylene (UHMWPE), 148, 239, 239t Stretch ratio (SR), 47 (ePTFE); Polytetraﬂuoroethylene Ultralow particulate ﬁlters, 220 Stretch(ing), 57 (PTFE) Ultralow penetration air ﬁlter (ULPA procedure, 48, 55 emulsion polymerization, 35f, 37t, ﬁlter), 219e220 PTFE tape, 94e95, 96f, 96t 44e45 Uniaxial expansion process, 101e107, drying oven for removing lubricant, polymerization, 29e31 103f 95f polymers, 31e35 asymmetric ePTFE membrane, 107t preparation, 26e29 coarse and highly porous uniaxially rate, 48 test, 51 pressure effect, 27t, 29t ePTFE, 106t vascular grafts, 199f, 201, 202f temperature effect, 27t, 29f ePTFE as ﬁltration membrane, 104t Stretchability, 179 synthesis, 26 PTFE tape expansion process, 102f TFE. See Tetraﬂuoroethylene (TFE) Uniaxial orientation, 130e131, 131f evaluation, 57 TFF. See Tangential ﬂow ﬁltration (TFF) Unsintered tape, 90e96 Succinic acid, 52 Thermodynamic laws, 45 blending lubricant and pigment and Sumitomo Electric Industries, 277 Thread sealant tape, 90 Supercritical carbon dioxide (SCCO2), 178 Three chain-folding model in polymer preforming, 90 Surface calendaring, 92e94 crystals, 13f extrusion of round and rectangular bead, energy, 16e18, 17t, 176 3M Corporation, 41 ﬁltration, 213e214, 214f Thrombin catalyzes, 265e266 91e92, 91f, 92t Thrombogenicity, surface modiﬁcation to ﬁnal tape product, 95e96, 96f lamination of ePTFE membrane, 215f stretching PTFE tape, 94e95 SEM of ePTFE membrane, 215f reducing, 265e268 Urine collection device, 229, 229f free energy, 264 Thrombus, 265e266 US Department of Energy (DOE), 220 hydrophobicity, 260 Tissue surface, 266 modiﬁcation of ePTFEs Toughness, 145 V for hydrophilicity and adhesion, Transient device, 195 Transverse direction (TD), 103e104, Van der Waals forces, 17, 31 263e265 Vascular grafts, 198e201, 263 mechanical alteration of surface, 132e133, 134t Trauma, 236 arteriovenous ﬁstula graft, 201fe202f 268e271 Triboelectric effect, 254 CARMEDA bioactive surface heparin membrane treatment, 263e271 Trichloroethylene (TCE), to reduce thrombogenicity, 265e268 chemistry, 200f treatment of PTFE, 260e263 134e135 Kink-resistant ePTFE vascular grafts, treatment of PTFE, 260e263 Triethoxysilan, 265 Surgical smoke ﬁlter, 220, 223f Tube extrusion die, 88f 199f Surgical sutures, 235 Tubing, 134 scanning electron micrograph, 200f Surgically invasive devices, 195 Tubular ePTFE shapes, 134e142, 135f stretch vascular grafts, 199f, 202f Suspension techniques, 32 two-layer structure, 200f Sutures, 235e236, 236f complex shape, 140e142 Vent ﬁlters and breathers, 168 Swelling, 16 dual porosity of expanded tubing, W T 139t orientation of nodes and ﬁbrils, 137f W.L. Gore & Associates, 173, 178, 198 Tangential ﬂow ﬁltration (TFF), 223 PTFE tubing, 135e136, 135f W.L. Gore TAG endoprosthesis, 202 Tape wrapping, 255e256 Wax, 35e36 Taylor Patch techniques, 140 Weaving ﬁber, 240e242, 240f graph of thickness uniformity, 241f

P:307

286 INDEX Wet curve, 123, 123f Wire payoff system, 80 Y Wettability, 264 Word graft, 129 Wetting mechanism of PTFE surface, World Health Organization (WHO), Yarn-on-yarn abrasion, 242 Young’s modulus of elasticity, 165e166, 176 193e194 “Wetting out” phenomenon, Wyzenbeek abrasion test, 184e185 167f YoungeDupre equation, 176 163e164 X YoungeLaplace equation, 123e124, 178 WHO. See World Health Organization XMnO4, 52 Z (WHO) Wicking, 171 Zeus Industrial Products, 276