Adhesion to Fluoropolymers: Rapra Review Report 183 (Rapra Review Reports)

Report 183

ISSN: 0889-3144

Adhesion to Fluoropolymers

D.M. Brewis and R.H. Dahm

Volume 16, Number 3, 2005

Expert overviews covering the science and technology of rubber and plastics

RAPRA REVIEW REPORTS A Rapra Review Report comprises three sections, as follows: 1. A commissioned expert review, discussing a key topic of current interest, and referring to the References and Abstracts section. Reference numbers in brackets refer to item numbers from the References and Abstracts section. Where it has been necessary for completeness to cite sources outside the scope of the Rapra Abstracts database, these are listed at the end of the review, and cited in the text as a.1, a.2, etc. 2. A comprehensive References and Abstracts section, resulting from a search of the Rapra Polymer Library database. The format of the abstracts is outlined in the sample record below. 3. An index to the References and Abstracts section, derived from the indexing terms which are added to the abstracts records on the database to aid retrieval.

Source of original article Title

Item 1 Macromolecules

33, No.6, 21st March 2000, p.2171-83 EFFECT OF THERMAL HISTORY ON THE RHEOLOGICAL BEHAVIOR OF THERMOPLASTIC POLYURETHANES Pil Joong Yoon; Chang Dae Han Akron,University The effect of thermal history on the rheological behaviour of ester- and etherbased commercial thermoplastic PUs (Estane 5701, 5707 and 5714 from B.F.Goodrich) was investigated. It was found that the injection moulding temp. used for specimen preparation had a marked effect on the variations of dynamic storage and loss moduli of specimens with time observed during isothermal annealing. Analysis of FTIR spectra indicated that variations in hydrogen bonding with time during isothermal annealing very much resembled variations of dynamic storage modulus with time during isothermal annealing. Isochronal dynamic temp. sweep experiments indicated that the thermoplastic PUs exhibited a hysteresis effect in the heating and cooling processes. It was concluded that the microphase separation transition or order-disorder transition in thermoplastic PUs could not be determined from the isochronal dynamic temp. sweep experiment. The plots of log dynamic storage modulus versus log loss modulus varied with temp. over the entire range of temps. (110-190C) investigated. 57 refs. GOODRICH B.F.

Location

USA

Authors and affiliation

Abstract

Companies or organisations mentioned

Accession no.771897

DOCUMENT DELIVERY SERVICE Almost all of the documents which are listed in the References and Abstracts section are available in full text form, as photocopies or pdf files from Rapra Technology’s Document Delivery Service. Documents can be delivered by a variety of methods, including email, post or fax. Customers may pay for individual copies at the time of ordering by credit card or alternatively open up a deposit account. See the back of this report for further information. Please contact the Document Delivery Department for availability, current prices and delivery methods. Document Delivery Department Rapra Technology, Shawbury, Shrewsbury, Shropshire SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 Email: [email protected]

RAPRA REVIEW REPORTS VOLUME 16 Series Editor Dr. S. Humphreys, Rapra Technology Rapra Review Reports comprise a unique source of polymer-related information with useful overviews accompanied by abstracts from hundreds of relevant documents. A Rapra Review Report is an excellent starting point to improve subject knowledge in key areas. Subscribers to this series build up a bank of information over each year, forming a small library at a very reasonable price. This series would be an asset to corporate libraries, academic institutions and research associations with an interest in polymer science. Twelve reports are published in each volume and these can be purchased individually or on a subscription basis. Format: Soft-backed, 297 x 210 mm, ISSN: 0889-3144 Order individual published Rapra Review Reports (see the following pages for a list of available titles), or purchase a subscription to Volume 16 (12 issues).

ORDER FORM Title of Publication

Price £/$/€

I would like to order the following Rapra Review Report(s) at £85 / US$136 / €136 each Report Number(s) ....................................................................................................(please state quantity if more than one) Please add postage at the following rates: UK £5 total, Overseas £7 / US$11 / €10 per item Subtotal: I would like to order ................. subscription(s) to Volume 16 of the Rapra Review Report Series at £650 / US$975 / €1105 each Please add postage at the following rates: UK £35 total, Overseas £65 / US$110 / €110 per subscription All prices are subject to change and orders will be charged at the price indicated on www.polymer-books.com on the date of processing

enclosed (Please make cheques payable to Rapra Technology in ❑ Remittance £ Sterling drawn on a UK bank or in US$ / Euros - Unesco coupons are also accepted.)

❑ Please invoice my company ❑ Please charge my credit card American Express/Visa/Mastercard (delete as appropriate) For credit card orders we require all of the following details to be completed prior to processing your order.

Total Order Value:

IMPORTANT - Value Added Tax (VAT) The above prices do not include VAT. Customers in EU member countries may be liable to pay VAT if their Registration Number is not supplied. Please enter your EU Registration Number (VAT - BTW - IVA - TVA - MWST - MOMS - FPA) below: VAT Number: ________________________________________

Card Number:

Full Name: ________________________________________________ Please enter the cards security code below, or provide us with your telephone number or email address. (Visa/Mastercard - the last 3 digits from the number on the signature strip on the back of the card, Amex - 4 digit code from the front of the card.) 3 or 4 Digit Security Code: ____________

Signature: ______________________ Exp. date:__________________ Issuing Bank: ______________________________________________ Cardholder's Name (as on card): ________________________________

Company: _________________________________________________ Job Function:_______________________________________________ Delivery Address (if different from Cardholder's Address): ___________ __________________________________________________________ __________________________________________________________

Cardholder's Address: ________________________________________ __________________________________________________________ __________________________________________________________ Postcode: _______________________ Country:___________________ Telephone: ______________________ Fax: ______________________ Company PO#: _____________________________________________

Please Return to:

Publications Sales, Rapra Technology Shawbury, Shrewsbury, Shropshire SY4 4NR, United Kingdom

Postcode: _______________________ Country:___________________ Telephone: ______________________ Fax: ______________________ If you would like to receive regular electronic updates informing you of new titles and offers please enter your E-mail address below. E-mail:____________________________________________________ Tel. +44 (0)1939 250383 Fax: +44 (0)1939 251118 E-mail: [email protected]

www.rapra.net

Previous Titles Still Available Petrochemicals Inc.

Volume 1 Report 35

Polymers in Household Electrical Goods, D.Alvey, Hotpoint Ltd.

Report 36

Developments in Additives to Meet Health and Environmental Concerns, M.J. Forrest, Rapra Technology Ltd.

Report 1

Conductive Polymers, W.J. Feast

Report 2

Medical, Surgical and Pharmaceutical Applications of Polymers, D.F. Williams

Report 3

Advanced Composites, D.K. Thomas, RAE, Farnborough.

Report 4

Liquid Crystal Polymers, M.K. Cox, ICI, Wilton.

Volume 4

Report 5

CAD/CAM in the Polymer Industry, N.W. Sandland and M.J. Sebborn, Cambridge Applied Technology.

Report 37

Report 8

Engineering Thermoplastics, I.T. Barrie, Consultant.

Polymers in Aerospace Applications, W.W. Wright, University of Surrey.

Report 10

Reinforced Reaction Injection Moulding, P.D. Armitage, P.D. Coates and A.F. Johnson

Report 38

Epoxy Resins, K.A. Hodd

Report 39

Polymers in Chemically Resistant Applications, D. Cattell, Cattell Consultancy Services.

Report 40

Internal Mixing of Rubber, J.C. Lupton

Report 41

Failure of Plastics, S. Turner, Queen Mary College.

Report 42

Polycarbonates, R. Pakull, U. Grigo, D. Freitag, Bayer AG.

Report 43

Polymeric Materials from Renewable Resources, J.M. Methven, UMIST.

Report 11

Communications Applications of Polymers, R. Spratling, British Telecom.

Report 12

Process Control in the Plastics Industry, R.F. Evans, Engelmann & Buckham Ancillaries.

Volume 2 Report 13

Injection Moulding of Engineering Thermoplastics, A.F. Whelan, London School of Polymer Technology.

Report 44

Report 14

Polymers and Their Uses in the Sports and Leisure Industries, A.L. Cox and R.P. Brown, Rapra Technology Ltd.

Flammability and Flame Retardants in Plastics, J. Green, FMC Corp.

Report 45

Composites - Tooling and Component Processing, N.G. Brain, Tooltex.

Report 15

Polyurethane, Materials, Processing and Applications, G. Woods, Consultant.

Report 46

Quality Today in Polymer Processing, S.H. Coulson, J.A. Cousans, Exxon Chemical International Marketing.

Report 16

Polyetheretherketone, D.J. Kemmish, ICI, Wilton.

Report 47

Report 17

Extrusion, G.M. Gale, Rapra Technology Ltd.

Chemical Analysis of Polymers, G. Lawson, Leicester Polytechnic.

Report 18

Agricultural and Horticultural Applications of Polymers, J.C. Garnaud, International Committee for Plastics in Agriculture.

Report 48

Plastics in Building, C.M.A. Johansson

Report 19

Recycling and Disposal of Plastics Packaging, R.C. Fox, Plas/Tech Ltd.

Report 20

Pultrusion, L. Hollaway, University of Surrey.

Report 21

Volume 5 Report 49

Blends and Alloys of Engineering Thermoplastics, H.T. van de Grampel, General Electric Plastics BV.

Materials Handling in the Polymer Industry, H. Hardy, Chronos Richardson Ltd.

Report 50

Automotive Applications of Polymers II, A.N.A. Elliott, Consultant.

Report 22

Electronics Applications of Polymers, M.T.Goosey, Plessey Research (Caswell) Ltd.

Report 51

Biomedical Applications of Polymers, C.G. Gebelein, Youngstown State University / Florida Atlantic University.

Report 23

Offshore Applications of Polymers, J.W.Brockbank, Avon Industrial Polymers Ltd.

Report 52

Polymer Supported Chemical Reactions, P. Hodge, University of Manchester.

Report 24

Recent Developments in Materials for Food Packaging, R.A. Roberts, Pira Packaging Division.

Report 53

Weathering of Polymers, S.M. Halliwell, Building Research Establishment.

Report 54

Health and Safety in the Rubber Industry, A.R. Nutt, Arnold Nutt & Co. and J. Wade.

Report 55

Computer Modelling of Polymer Processing, E. Andreassen, Å. Larsen and E.L. Hinrichsen, Senter for Industriforskning, Norway.

Volume 3 Report 25

Foams and Blowing Agents, J.M. Methven, Cellcom Technology Associates.

Report 26

Polymers and Structural Composites in Civil Engineering, L. Hollaway, University of Surrey.

Report 56

Injection Moulding of Rubber, M.A. Wheelans, Consultant.

Plastics in High Temperature Applications, J. Maxwell, Consultant.

Report 57

Joining of Plastics, K.W. Allen, City University. Physical Testing of Rubber, R.P. Brown, Rapra Technology Ltd.

Report 27 Report 28

Adhesives for Structural and Engineering Applications, C. O’Reilly, Loctite (Ireland) Ltd.

Report 58

Report 29

Polymers in Marine Applications, C.F.Britton, Corrosion Monitoring Consultancy.

Report 59

Polyimides - Materials, Processing and Applications, A.J. Kirby, Du Pont (U.K.) Ltd.

Report 30

Non-destructive Testing of Polymers, W.N. Reynolds, National NDT Centre, Harwell.

Report 60

Physical Testing of Thermoplastics, S.W. Hawley, Rapra Technology Ltd.

Report 31

Silicone Rubbers, B.R. Trego and H.W.Winnan, Dow Corning Ltd.

Report 32

Fluoroelastomers - Properties and Applications, D. Cook and M. Lynn, 3M United Kingdom Plc and 3M Belgium SA.

Report 33

Polyamides, R.S. Williams and T. Daniels, T & N Technology Ltd. and BIP Chemicals Ltd.

Report 34

Extrusion of Rubber, J.G.A. Lovegrove, Nova

Volume 6 Report 61

Food Contact Polymeric Materials, J.A. Sidwell, Rapra Technology Ltd.

Report 62

Coextrusion, D. Djordjevic, Klöckner ER-WE-PA GmbH.

Report 63

Conductive Polymers II, R.H. Friend, University of Cambridge, Cavendish Laboratory.

Report 64

Designing with Plastics, P.R. Lewis, The Open University.

Report 65

Decorating and Coating of Plastics, P.J. Robinson, International Automotive Design.

Report 66

Reinforced Thermoplastics - Composition, Processing and Applications, P.G. Kelleher, New Jersey Polymer Extension Center at Stevens Institute of Technology.

Report 67

Plastics in Thermal and Acoustic Building Insulation, V.L. Kefford, MRM Engineering Consultancy.

Report 68

Cure Assessment by Physical and Chemical Techniques, B.G. Willoughby, Rapra Technology Ltd.

Report 69

Toxicity of Plastics and Rubber in Fire, P.J. Fardell, Building Research Establishment, Fire Research Station.

Report 70

Acrylonitrile-Butadiene-Styrene Polymers, M.E. Adams, D.J. Buckley, R.E. Colborn, W.P. England and D.N. Schissel, General Electric Corporate Research and Development Center.

Report 71

Rotational Moulding, R.J. Crawford, The Queen’s University of Belfast.

Report 72

Advances in Injection Moulding, C.A. Maier, Econology Ltd.

Volume 7

Separation Performance, T. deV. Naylor, The Smart Chemical Company. Report 90

Rubber Mixing, P.R. Wood.

Report 91

Recent Developments in Epoxy Resins, I. Hamerton, University of Surrey.

Report 92

Continuous Vulcanisation of Elastomer Profiles, A. Hill, Meteor Gummiwerke.

Report 93

Advances in Thermoforming, J.L. Throne, Sherwood Technologies Inc.

Report 94

Compressive Behaviour of Composites, C. Soutis, Imperial College of Science, Technology and Medicine.

Report 95

Thermal Analysis of Polymers, M. P. Sepe, Dickten & Masch Manufacturing Co.

Report 96

Polymeric Seals and Sealing Technology, J.A. Hickman, St Clair (Polymers) Ltd.

Volume 9 Report 97

Rubber Compounding Ingredients - Need, Theory and Innovation, Part II: Processing, Bonding, Fire Retardants, C. Hepburn, University of Ulster.

Report 98

Advances in Biodegradable Polymers, G.F. Moore & S.M. Saunders, Rapra Technology Ltd.

Report 99

Recycling of Rubber, H.J. Manuel and W. Dierkes, Vredestein Rubber Recycling B.V.

Report 100

Photoinitiated Polymerisation - Theory and Applications, J.P. Fouassier, Ecole Nationale Supérieure de Chimie, Mulhouse.

Report 73

Reactive Processing of Polymers, M.W.R. Brown, P.D. Coates and A.F. Johnson, IRC in Polymer Science and Technology, University of Bradford.

Report 74

Speciality Rubbers, J.A. Brydson.

Report 75

Plastics and the Environment, I. Boustead, Boustead Consulting Ltd.

Report 101

Report 76

Polymeric Precursors for Ceramic Materials, R.C.P. Cubbon.

Solvent-Free Adhesives, T.E. Rolando, H.B. Fuller Company.

Report 102

Advances in Tyre Mechanics, R.A. Ridha, M. Theves, Goodyear Technical Center.

Plastics in Pressure Pipes, T. Stafford, Rapra Technology Ltd.

Report 103

Gas Assisted Moulding, T.C. Pearson, Gas Injection Ltd.

Report 104

Plastics Profile Extrusion, R.J. Kent, Tangram Technology Ltd.

Report 105

Rubber Extrusion Theory and Development, B.G. Crowther.

Report 106

Properties and Applications of Elastomeric Polysulfides, T.C.P. Lee, Oxford Brookes University.

Report 77 Report 78

PVC - Compounds, Processing and Applications, J.Leadbitter, J.A. Day, J.L. Ryan, Hydro Polymers Ltd.

Report 79

Rubber Compounding Ingredients - Need, Theory and Innovation, Part I: Vulcanising Systems, Antidegradants and Particulate Fillers for General Purpose Rubbers, C. Hepburn, University of Ulster.

Report 80

Anti-Corrosion Polymers: PEEK, PEKK and Other Polyaryls, G. Pritchard, Kingston University.

Report 107

High Performance Polymer Fibres, P.R. Lewis, The Open University.

Report 81

Thermoplastic Elastomers - Properties and Applications, J.A. Brydson.

Report 108

Chemical Characterisation of Polyurethanes, M.J. Forrest, Rapra Technology Ltd.

Report 82

Advances in Blow Moulding Process Optimization, Andres Garcia-Rejon,Industrial Materials Institute, National Research Council Canada.

Report 83

Report 84

Molecular Weight Characterisation of Synthetic Polymers, S.R. Holding and E. Meehan, Rapra Technology Ltd. and Polymer Laboratories Ltd.

Volume 10 Report 109

Rubber Injection Moulding - A Practical Guide, J.A. Lindsay.

Report 110

Long-Term and Accelerated Ageing Tests on Rubbers, R.P. Brown, M.J. Forrest and G. Soulagnet, Rapra Technology Ltd.

Report 111

Polymer Product Failure, P.R. Lewis, The Open University.

Report 112

Polystyrene - Synthesis, Production and Applications, J.R. Wünsch, BASF AG.

Rheology and its Role in Plastics Processing, P. Prentice, The Nottingham Trent University.

Volume 8 Report 85

Ring Opening Polymerisation, N. Spassky, Université Pierre et Marie Curie.

Report 113

Report 86

High Performance Engineering Plastics, D.J. Kemmish, Victrex Ltd.

Rubber-Modified Thermoplastics, H. Keskkula, University of Texas at Austin.

Report 114

Developments in Polyacetylene - Nanopolyacetylene, V.M. Kobryanskii, Russian Academy of Sciences.

Report 87

Rubber to Metal Bonding, B.G. Crowther, Rapra Technology Ltd.

Report 115

Metallocene-Catalysed Polymerisation, W. Kaminsky, University of Hamburg.

Report 88

Plasticisers - Selection, Applications and Implications, A.S. Wilson.

Report 116

Compounding in Co-rotating Twin-Screw Extruders, Y. Wang, Tunghai University.

Report 89

Polymer Membranes - Materials, Structures and

Report 117

Rapid Prototyping, Tooling and Manufacturing, R.J.M.

Report 118

Liquid Crystal Polymers - Synthesis, Properties and Applications, D. Coates, CRL Ltd.

Volume 13

Report 119

Rubbers in Contact with Food, M.J. Forrest and J.A. Sidwell, Rapra Technology Ltd.

Report 145

Multi-Material Injection Moulding, V. Goodship and J.C. Love, The University of Warwick.

Report 120

Electronics Applications of Polymers II, M.T. Goosey, Shipley Ronal.

Report 146

In-Mould Decoration of Plastics, J.C. Love and V. Goodship, The University of Warwick.

Report 147

Rubber Product Failure, Roger P. Brown.

Volume 11

Report 148

Plastics Waste – Feedstock Recycling, Chemical Recycling and Incineration, A. Tukker, TNO.

Report 121

Polyamides as Engineering Thermoplastic Materials, I.B. Page, BIP Ltd.

Report 149

Analysis of Plastics, Martin J. Forrest, Rapra Technology Ltd.

Report 122

Flexible Packaging - Adhesives, Coatings and Processes, T.E. Rolando, H.B. Fuller Company.

Report 150

Mould Sticking, Fouling and Cleaning, D.E. Packham, Materials Research Centre, University of Bath.

Report 123

Polymer Blends, L.A. Utracki, National Research Council Canada.

Report 151

Rigid Plastics Packaging - Materials, Processes and Applications, F. Hannay, Nampak Group Research & Development.

Report 124

Sorting of Waste Plastics for Recycling, R.D. Pascoe, University of Exeter.

Report 152

Report 125

Structural Studies of Polymers by Solution NMR, H.N. Cheng, Hercules Incorporated.

Natural and Wood Fibre Reinforcement in Polymers, A.K. Bledzki, V.E. Sperber and O. Faruk, University of Kassel.

Report 153

Report 126

Composites for Automotive Applications, C.D. Rudd, University of Nottingham.

Polymers in Telecommunication Devices, G.H. Cross, University of Durham.

Report 154

Polymers in Building and Construction, S.M. Halliwell, BRE.

Report 127

Polymers in Medical Applications, B.J. Lambert and F.-W. Tang, Guidant Corp., and W.J. Rogers, Consultant.

Report 155

Styrenic Copolymers, Andreas Chrisochoou and Daniel Dufour, Bayer AG.

Report 128

Solid State NMR of Polymers, P.A. Mirau, Lucent Technologies.

Report 156

Life Cycle Assessment and Environmental Impact of Polymeric Products, T.J. O’Neill, Polymeron Consultancy Network.

Report 129

Failure of Polymer Products Due to Photo-oxidation, D.C. Wright.

Report 130

Failure of Polymer Products Due to Chemical Attack, D.C. Wright.

Report 131

Failure of Polymer Products Due to Thermo-oxidation, D.C. Wright.

Report 132

Stabilisers for Polyolefins, C. Kröhnke and F. Werner, Clariant Huningue SA.

Volume 12 Report 133 Report 134

Volume 14 Report 157

Developments in Colorants for Plastics, Ian N. Christensen.

Report 158

Geosynthetics, David I. Cook.

Report 159

Biopolymers, R.M. Johnson, L.Y. Mwaikambo and N. Tucker, Warwick Manufacturing Group.

Report 160

Emulsion Polymerisation and Applications of Latex, Christopher D. Anderson and Eric S. Daniels, Emulsion Polymers Institute.

Report 161

Advances in Automation for Plastics Injection Moulding, J. Mallon, Yushin Inc.

Emissions from Plastics, C. Henneuse-Boxus and T. Pacary, Certech.

Report 162

Infrared and Raman Spectroscopy of Polymers, J.L. Koenig, Case Western Reserve University.

Analysis of Thermoset Materials, Precursors and Products, Martin J. Forrest, Rapra Technology Ltd.

Report 163

Polymer/Layered Silicate Nanocomposites, Masami Okamoto, Toyota Technological Institute.

Report 164

Cure Monitoring for Composites and Adhesives, David R. Mulligan, NPL.

Report 165

Polymer Enhancement of Technical Textiles, Roy W. Buckley.

Report 135

Polymers in Sport and Leisure, R.P. Brown.

Report 136

Radiation Curing, R.S. Davidson, DavRad Services.

Report 137

Silicone Elastomers, P. Jerschow, Wacker-Chemie GmbH.

Report 138

Health and Safety in the Rubber Industry, N. Chaiear, Khon Kaen University.

Report 166

Developments in Thermoplastic Elastomers, K.E. Kear

Report 139

Rubber Analysis - Polymers, Compounds and Products, M.J. Forrest, Rapra Technology Ltd.

Report 167

Polyolefin Foams, N.J. Mills, Metallurgy and Materials, University of Birmingham.

Report 140

Tyre Compounding for Improved Performance, M.S. Evans, Kumho European Technical Centre.

Report 168

Plastic Flame Retardants: Technology and Current Developments, J. Innes and A. Innes, Flame Retardants Associates Inc.

Report 141

Particulate Fillers for Polymers, Professor R.N. Rothon, Rothon Consultants and Manchester Metropolitan University.

Volume 15

Report 142

Blowing Agents for Polyurethane Foams, S.N. Singh, Huntsman Polyurethanes.

Report 169

Engineering and Structural Adhesives, David J. Dunn, FLD Enterprises Inc.

Report 143

Adhesion and Bonding to Polyolefins, D.M. Brewis and I. Mathieson, Institute of Surface Science & Technology, Loughborough University.

Report 170

Polymers in Agriculture and Horticulture, Roger P. Brown.

Report 171

PVC Compounds and Processing, Stuart Patrick.

Rubber Curing Systems, R.N. Datta, Flexsys BV.

Report 172

Troubleshooting Injection Moulding, Vanessa Goodship, Warwick Manufacturing Group.

Report 144

Report 173

Regulation of Food Packaging in Europe and the USA, Derek J. Knight and Lesley A. Creighton, Safepharm Laboratories Ltd.

Report 174

Pharmaceutical Applications of Polymers for Drug Delivery, David Jones, Queen's University, Belfast.

Report 175

Tyre Recycling, Valerie L. Shulman, European Tyre Recycling Association (ETRA).

Report 176

Polymer Processing with Supercritical Fluids, V. Goodship and E.O. Ogur.

Report 177

Bonding Elastomers: A Review of Adhesives & Processes, G. Polaski, J. Means, B. Stull, P. Warren, K. Allen, D. Mowrey and B. Carney.

Report 178

Mixing of Vulcanisable Rubbers and Thermoplastic Elastomers, P.R. Wood.

Report 179

Polymers in Asphalt, H.L. Robinson, Tarmac Ltd, UK.

Report 180

Biocides in Plastics, D. Nichols, Thor Overseas Limited.

Volume 16 Report 181

New EU Regulation of Chemicals: REACH, D.J. Knight, SafePharm Laboratories Ltd.

Report 182

Food Contact Rubbers 2 - Products, Migration and Regulation, M.J. Forrest.

Adhesion to Fluoropolymers

D.M. Brewis and R.H. Dahm (IPTME, Loughborough University)

ISBN-10: 1-85957-524-6 ISBN-13: 978-1-85957-524-6

Adhesion to Fluoropolymers

Contents Abstract ...............................................................................................................................................................3 1

Introduction .................................................................................................................................................3

2

Principles .....................................................................................................................................................3

3

2.1

Theories of Adhesion........................................................................................................................3

2.2

Wettability ........................................................................................................................................4

2.3

Diffusion ...........................................................................................................................................5

Methods Used to Study Surfaces ................................................................................................................5 3.1

Introduction ......................................................................................................................................5

3.2

X-Ray Photoelectron Spectroscopy (XPS).......................................................................................5

4

Adhesion Without Pretreatment ..................................................................................................................6

5

Pretreatments ...............................................................................................................................................7 5.1

Summary ..........................................................................................................................................7

5.2

5.5

Wet Chemical Treatments ................................................................................................................8 5.2.1 Treatments with Solvated Electrons and Radical Anion Salts .............................................8 5.2.2 Treatment with Strong Aqueous Bases ..............................................................................10 5.2.3 Other Reductive Pretreatments ..........................................................................................13 Electrochemical and Related Methods ...........................................................................................13 5.3.1 The Indirect Electrochemical Pretreatment of PTFE .........................................................14 5.3.2 Treatment of PTFE with Metal Amalgams ........................................................................15 5.3.3 The Direct Electrochemical Pretreatment of PTFE ...........................................................16 Plasma Treatments..........................................................................................................................17 5.4.1 Principles and Equipment ..................................................................................................17 5.4.2 Studies of Plasma Treatments ............................................................................................20 5.4.3 Flame Treatment ................................................................................................................24 Photochemical Pretreatments .........................................................................................................25

5.6

Miscellaneous Pretreatments ..........................................................................................................26

5.3

5.4

General Discussion ...........................................................................................................................................26 Conclusions .......................................................................................................................................................27 References .........................................................................................................................................................27 Abbreviations and Acronyms............................................................................................................................28 Subject Index ..................................................................................................................................................103 Company Index ...............................................................................................................................................121

1

Adhesion to Fluoropolymers

The views and opinions expressed by authors in Rapra Review Reports do not necessarily reflect those of Rapra Technology or the editor. The series is published on the basis that no responsibility or liability of any nature shall attach to Rapra Technology arising out of or in connection with any utilisation in any form of any material contained therein.

2

Adhesion to Fluoropolymers

Abstract Fluorinated polymers have a number of very useful properties such as excellent chemical resistance. However, they are usually difficult to bond without a pretreatment. There are exceptions and untreated poly(vinylidene fluoride), for example, may be successfully bonded with an amine-cured epoxide. The most effective methods to pretreat fully fluorinated polymers were developed in the 1950s. Much work, especially on electrochemical and plasma treatments, has since been carried out but to date these have not proved as effective as the early methods. Partially fluorinated polymers may be treated like polyolefins, for example, flame, and plasma treatments are very effective. Aqueous and alcoholic solutions of Group I hydroxides are also very effective.

to adhere to, while partially fluorinated polymers such as PVDF and PVF may cause problems depending on the circumstances. To achieve a satisfactory level of adhesion, it is often necessary to carry out a pretreatment with both fully and partially fluorinated polymers. In this review the principles of adhesion are considered first (Section 2). Techniques that have greatly increased our understanding of adhesion to fluoropolymers are described in Section 3. Cases where good adhesion is achieved without a pretreatment are examined in Section 4. However, as pretreatments are usually needed to get the required level of adhesion, this review is mainly concerned with the wide variety of methods available to pretreat fluoropolymers (Section 5). A general discussion and conclusions follow.

2 Principles 1 Introduction

2.1 Theories of Adhesion

Fluoropolymers can provide a combination of properties including high temperature resistance, excellent resistance to many chemicals and ultraviolet (UV) radiation, fire resistance and low friction. Fluoropolymers are relatively expensive and they are generally used in specialised applications such as linings for chemical plant, spacecraft coatings that are resistant to atomic oxygen, fire-resistant coatings for cables, and of course, non-stick functions. There are more than twenty different fluoropolymers that are commercially available. Important examples of fluorinated plastics include:

It is first necessary to consider why materials adhere to each other. There are four main theories of adhesion, namely adsorption, electrostatic, diffusion and mechanical keying. According to the adsorption theory, macromolecules of the mobile phase (adhesive, printing ink, etc.) are adsorbed onto the substrate and held there by forces ranging from weak dispersion forces to chemical bonds so that an interface exists. In the electrostatic theory there is a transfer of charge between the mobile phase and the substrate such that they are held together by electrostatic forces. The third theory requires the diffusion of macromolecules, or monomers, of the mobile phase into the substrate, thereby eliminating an interface. With the mechanical keying theory, the mobile phase flows into the irregularities (pits and troughs) of the substrate surface and after hardening, a keying action occurs.

Fluorinated ethylene-propylene copolymer Poly(tetrafluoroethylene) Poly(vinyl fluoride) Poly(vinylidene fluoride) Copolymer of ethylene and tetrafluoroethylene Perfluoroalkoxy copolymer

FEP PTFE PVF PVDF ETFE PFA

An example of a fluorinated elastomer is a terpolymer of vinylidene fluoride (VDF), hydropentafluoropropylene (HPFP) and tetrafluoroethylene (TFE). Good adhesion is required in a number of technologies involving fluoropolymers including adhesive bonding, painting, printing, metallisation (via vacuum or solution) and composite production. However, fully fluorinated polymers such as PTFE and FEP are notoriously difficult

These theories will each be important with particular systems, but the adsorption theory is likely to be the most generally applicable. Aspects of diffusion are briefly discussed in Section 2.3. To these four theories should be added a theory of non-adhesion, due to the existence of regions of low cohesive strength in the interfacial region. Bikerman (a.1) first suggested that adhesion problems may be due to weak boundary layers. He suggested that molecules of low molecular weight create a region of low strength on the surface.

3

Adhesion to Fluoropolymers

The weak boundary theory received considerable support from other research work and some of this is outlined next. It is certainly easy to envisage various possible sources of weak boundary layers on polymer surfaces, namely: •

impurities arising from the polymerisation process;

•

the low molecular weight tail of a polymer;

•

additives, e.g., antioxidants and slip agents;

•

external processing aids, e.g., mould release agents;

•

contamination after the polymerisation process.

Schonhorn and co-workers have put forward evidence in favour of the weak boundary concept using polyethylene as the substrate. In 1966, Hansen and Schonhorn (a.2) reported work in which they bombarded polyethylene and certain other polymers with activated inert gases and found that the adhesion of an epoxide adhesive to the polymers greatly increased, although the critical surface tensions of the polymers were unchanged. Also, using reflection infrared (IR) analysis they were unable to detect any chemical changes in the surface. They proposed therefore, that regions of low molecular weight on the surface had been crosslinked to the long polymer chains thereby eliminating weak boundary layers. In fact, Hansen and Schonhorn suggested that surface treatments in general act primarily by eliminating weak boundary layers. In some later work Schonhorn and Ryan (350) exposed polyethylene to UV radiation. They found joint strengths with an epoxide adhesive much increased, but there was no evidence of oxidation using reflection IR and contact angle measurements. They concluded that crosslinking at the surface had occurred thereby eliminating a potential weak boundary layer. However, at the time of this research, reflection IR analysis was not sufficiently surface sensitive to detect the chemical changes that were later shown, by X-ray photoelectron spectroscopy (XPS), to have occurred. Before any adhesion mechanism can operate, good contact between the two materials is necessary. The question of wettability is therefore of crucial importance and this topic will now be considered briefly.

2.2 Wettability A satisfactory level of contact between the mobile phase, for example an adhesive, and the substrate is essential for good adhesion. A direct measure of wettability may be obtained via a contact angle measurement. This is the angle (θ) between the tangent and the substrate surface when a drop of liquid is placed upon it.

When there is a strong attraction between the liquid and the solid, θ will be small (or zero for perfect wetting). Conversely when the attraction between a liquid and solid is poor a large contact angle is obtained, possibly greater than 90° as illustrated next:

Such is the case with fluorinated polymers; the water contact angle with PTFE is about 110°. The poor wetting is due to relatively low attractive forces between water, which is a very polar molecule, and PTFE, which is non-polar. Contact angle values from various pure liquids may be used to estimate the surface free energy of a substrate via various thermodynamic theories. The Owens, Wendt and Kaelble approach enables the polar and dispersion components of the surface free energy (SFE) to be evaluated from a knowledge of the contact angle of various liquids of known polar and dispersion values. The equation employed is: (1 + cos θ )γ l / 2(γ ld )1/2 = (γ sp )1/2 (γ lp / γ ld )1/2 + (γ sd )1/2

4

(1)

Adhesion to Fluoropolymers

where: γ1 is the SFE of the liquid γ1p is the polar component of the liquid SFE γ1d is the dispersion component of the liquid SFE γs is the SFE of the solid γsp is the polar component of the solid SFE γsd is the dispersion component of the solid SFE This is the equation of a straight line where (1 + p d 1/ 2 cosθ)γι/2(γιd)1/2 is plotted against (γ l / γ l ) . Hence the square of the gradient is γsp, i.e., the polar component of the surface free energy of the solid and the square of the γ intercept is γsd, i.e., the dispersion component of the surface free energy of the solid. Although the greatest contribution to the surface energy of a polymer comes from the dispersion component, the polarity of the surface is more easily altered with a pretreatment. For example, an untreated low density polyethylene (LDPE) sample with a zero polar component and a 31.9 m/Nm dispersion component, showed an increase in the polar component to 8.0 m/Nm after a flame treatment. The dispersion component stayed fairly constant at 30.7 m/Nm (a.3). Typically a ten-fold increase in adhesion may be observed from such a treatment. Adhesion improvement comes from better wetting and stronger interfacial attraction due to the new functionality. Hence surface energy estimation can be a useful tool to assess adhesion performance.

2.3 Diffusion If two pieces of the same polymer are heated to a sufficiently high temperature and brought together under pressure, then chain segments from the two pieces will interpenetrate and the interface will be eliminated. This process is often termed autohesion. There is no dispute that this process will occur when two pieces of the same polymer are involved as in heat sealing, although ethylene-propylene copolymers are known to exhibit relatively poor autohesion. Whether diffusion between two different polymers occurs will depend on a number of factors including temperature, time available and chemical compatibility. It is often desirable to combine the properties of two or more polymers and this can be achieved by coextrusion. If two polymers A and B are insufficiently compatible to achieve good adhesion, then a tie-layer, often a copolymer, may be used. This is a polymer that is compatible with both A and B. Raghava and Smith (277) showed that

interdiffusion had occurred in compression-moulded laminates formed between PVDF and poly(methyl methacrylate) (PMMA). Using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray microanalysis they demonstrated the existence of interphases several microns thick. Yang and Garton (214) examined the use of primers as an alternative to pretreatments for PTFE. The authors used triphenylphosphine (TPP) as a primer for a cyanacrylate adhesive and diaminodiphenylamine (DDM) with an epoxide adhesive. The authors proposed that the primers mix with the PTFE in the surface regions and are then able to form interphases involving the adhesives and PTFE. With the TPP/cyanoacrylate system shear bond strengths in excess of 10 MPa were obtained compared to a negligible strength in the absence of a primer. With the DDM/epoxide system a maximum strength of 2.2 MPa was obtained. It is therefore possible under some circumstances to avoid pretreatments. However, in general to achieve good adhesion to fluoropolymers, especially if fully fluorinated, a pretreatment is necessary. It is also possible for a primer or adhesive to diffuse into a substrate.

3 Methods Used to Study Surfaces 3.1 Introduction A knowledge of the surface chemistry of substrates is important in our understanding of adhesion. There are many techniques which provide information on the surface chemistry of plastics. Of particular importance is XPS, also known as electron spectroscopy for chemical analysis (ESCA). This technique is outlined in Section 3.2. Static secondary ion mass spectrometry (SSIMS) and reflection infrared analysis are also widely used. Topography, which can have an important effect on adhesion, may be examined using electron microscopy or atomic force microscopy.

3.2 X-Ray Photoelectron Spectroscopy (XPS) In this technique a solid, e.g., a plastic film, is bombarded with X-rays of known energy under high vacuum. Photoelectrons from different core levels are ejected. Photoelectrons from the first few atomic layers

5

Adhesion to Fluoropolymers

have a characteristic kinetic energy depending on the elements present in the surface regions. The binding energy of a photoelectron from a particular core level is given by the equation: Eκ = hν - EΒ – Φ

(2)

where: Eκ is the kinetic energy of the photoelectron hν is the X-ray energy EΒ is the binding energy of the photoelectron Φ is the constant for a given instrument A schematic diagram of the equipment used is shown in Figure 1. By scanning different energies, a spectrum is obtained. The elements present in the first few atomic layers can be readily identified from their characteristic binding energies. The percentage concentration of each element can be calculated from the following equation: Ix Cx =

∑ (I

Sx n

Sn

4 Adhesion Without Pretreatment Good adhesion can be achieved by any of the following mechanisms namely: •

Strong interaction across an interface

•

Diffusion

•

Mechanical keying

The magnitude of the interaction across an interface will be determined by (a) the degree of contact between the mobile phase, e.g., an adhesive and the substrate, and (b) the type of interaction between the two materials. A direct measure of wettability is given by surface energy (see Section 2). Some values of surface energies for fluoropolymers are given in Table 1.

× 100 )

(3)

where: Cx = percentage concentration of element X Ix = quantity of photoelectrons from element X Sx = sensitivity factor for element X ∑ = summation of I/S for all elements It is thus a routine matter to obtain a quantitative elemental analysis of the surface regions of a solid.

The ‘non-stick’ nature of PTFE can be explained by the lack of chemical functionality resulting in poor wetting and weak interfacial interactions. It will be noted later that weak boundary layers can also play an important role in the adhesion to fluoropolymers. Partially fluorinated polymers like PVF and PVDF have higher surface energies than PTFE and in particular possess relatively high polar contributions to surface energy. It might be expected that partially fluorinated materials would have

Figure 1 Schematic of XPS instrument

6

Adhesion to Fluoropolymers

Table 1 Surface energies and polar and dispersion components of surface energy of some polymers Polymer

Chemical structure of monomer compared with ethylene

Polytrifluoroethylene

4H replaced by F

γsd (mJ/m2)

γsp (mJ/m2)

γs (mJ/m2)

18.6

0.5

19.1

Poly(vinylidene fluoride)

3H replaced by F

19.9

4.0

23.9

Poly(vinyl fluoride)

2H replaced by F

23.2

7.1

30.3

31.3

5.4

36.7

33.2

-

33.2

Low density polyethylene

1H replaced by F -

d

Key: γs Dispersion component of surface energy γsp Polar component of surface energy γs Total surface energy

good adhesion with adhesives where there are polar interactions across the interface. In fact, Schonhorn and Luongo (278) provided infrared evidence to show that certain amines could chemically modify the surface of PVDF while simultaneously ‘curing’ an epoxide resin: a) Reaction of PVDF with amines

b) If R contains more than one amino group as in many epoxy resin hardeners further reaction with the epoxy group of a resin results in covalent bonding between adhesive and substrate, for example:

Thus chemical bonding across the interface could occur. Schonhorn and Luongo found, for example, that if diethylaminopropylamine (DEAPA) was used in conjunction with an epoxide, strong adhesive joints were formed with PVDF. Krueger and co-workers (30) have discussed the use of a two-component adhesive for bonding of filled and unfilled PTFE in a process that does not require a surface pre-treatment. A proprietary twopart adhesive made up by adding one part of A containing a multifunctional aziridine (a saturated heterocyclic compound which behaves cationically towards a polyethylene amine), a polyester adipate, a dimethylsiloxane and hexamethylenediamine amongst others. The composition of component B is not disclosed. The cure time is of the order of seconds at room temperature. In lap shear tests bond strength is up to 14.5 MPa, which exceeds the cohesive strength of PTFE and results in a plastic-elastic deformation. The adhesion is not affected by immersion in water at 60 °C.

5 Pretreatments 5.1 Summary Effective pretreatments for PTFE were developed in the 1950s. These were sodium in liquid ammonia (a.4) and sodium naphthalenide in tetrahydrofuran (THF) (a.5). Many other treatments for PTFE have since been investigated including plasma treatment, direct electrochemical reduction, treatment with an alkali

7

Adhesion to Fluoropolymers

metal amalgam and reduction with benzoin dianion. In recent years the vast majority of research studies have centred on plasma treatment. This is reflected in the account of pretreatments given next.

5.2 Wet Chemical Treatments

5.2.1 Treatments with Solvated Electrons and Radical Anion Salts Sodium in liquid ammonia and sodium naphthalenide in THF are both powerful reducing agents, i.e., they are capable of acting as powerful electron donors as shown in Equations (4) and (9): Na + nNH 3 → Na + + e n– ( NH 3 )

(4)

(5) A possible mechanism of reduction of the carbon halogen bond in fluoropolymers is outlined next:

(6)

(7)

(8) (9) The first step, Equation (6), involves electron transfer from a solution of solvated electrons, or a naphthalenide radical anion or a cathode to the fluorinated polymer with elimination of a fluoride ion to produce a neutral radical. This then reacts further to produce new carboncarbon bonds resulting in crosslinking as shown in Equation (7) or the radical may accept a second electron to form a carbanion which in turn reacts with a protic

8

solvent (HS) to yield carbon-hydrogen bonds as shown in Equation (8), or undergoes elimination of a fluoride ion with formation of carbon carbon double bonds as shown in Equation (9). The treatment of fully fluorinated polymers with sodium in liquid ammonia or sodium naphthalenide in THF is very rapid. In the 1970s the changes in surface chemistry caused by these treatments were studied using XPS shortly after the technique became commercially available. Brecht, Mayer and Binder (353) showed that treatment of PTFE with sodium naphthalenide in THF for 30 seconds reduced the F:C ratio in the surface region from 2 to 0.17 and introduced substantial quantities of oxygen-containing groups (O:C ratio = 0.2). Dwight and Riggs (347) treated FEP with sodium in liquid ammonia. Complete defluorination took place and a large quantity of oxygen was introduced into the surface in the form of carbonyl and carboxylic acid groups. Some more recent studies are now described. Ha and co-workers (262) using reflection infrared analysis (IR-IRS) and XPS concluded that the sodium naphthalenide treatment of PFA introduced unsaturation fairly evenly to a depth of 112 nm while oxygencontaining functional groups such as —OH, C=O and —CO2H were concentrated in the first few nm. They calculated that in the top 5 nm of treated surface, 1 in 20 carbon atoms was involved in a hydroxyl group and 1 in 60 in a carbonyl group. The same group studied the effect of sodium naphthalenide on PTPE, FEP and PFA (a.6). They found that the washing procedure had a marked effect on the surface chemistry of treated PFA. Washing by THF alone left a surface with much sodium carbonate but few hydroxyl groups. The authors concluded that hydroxyl groups were produced during the normal washing procedure, i.e., with water. They found that the surface compositions of PFA and PTFE as determined by XPS were broadly similar after a 1 hour treatment (Table 2). The SEM study indicated that the etching treatment did not cause any changes in topography to PFA or PEP but cracks were apparent with skived PTFE. Brewis and co-workers (229) compared the effectiveness of sodium naphthalenide towards PTFE and PVF. They found that the increase in lap shear joint strength with a two-part epoxy was much more rapid with PTFE, as can be seen from Table 3. In a later study (215), the same authors studied the treatment of PTFE, copolymer of ethylene and

Adhesion to Fluoropolymers

Table 2 A comparison of surface functionality as measured by XPS of PFA and PTFE after a one-hour treatment with sodium complex (a.9) Group

PFA

PTFE

1 per 2.9 carbons

1 per 3.7 carbons

—OH

1 per 20 carbons

1 per 18 carbons

—CO

1 per 60 carbons

1 per 100 carbons

—F

1 per 80 carbons

1 per 40 carbons

C=C, C≡C

Table 3 Joint strengths for ‘Tetra Etch’ – treated PTFE and PVF (229)

PTFE

PVF

Treatment time (s)

Joint strength (MPa)

0 2 10 30 60 600 1800 3600

2.1 18.3 21.4 20.5 21.3 22.2 20.8 22.3

0 2 10 30 60 600 1800 3600

1.8 3.6 4.0 5.0 10.4 15.9 13.4 14.1

chlorotrifluoroethylene (ECTFE), PVDF and PVF with sodium naphthalenide (Table 4). As can be seen, the rate of chemical modification is in the order PTFE > PVDF > ECTFE > PVF. With PTFE, almost complete defluorination occurred in 10 seconds whereas with PVF defluorination was still far from complete after 1 hour. Rye (201) compared the effects of a sodium naphthalenide etch on PTFE and FEP. Although the chemical modification, as shown by XPS, was similar, there were marked differences in the topographical changes that occurred in the two polymers. With FEP there was little change in the topography, whereas with PTFE extensive pitting occurred. This difference was attributed to differences in the processing of the two polymers. FEP can be melt processed, e.g., by extrusion, whereas PTFE is intractable, and film and thin sheet must be obtained by skiving (essentially planing with a sharp blade) cylinders of the polymer. The skiving process results in long unidirectional scratches. Upon etching, cracks parallel

Table 4 Effect of ‘Tetra Etch’ treatment on PTFE, PVF, PVDF and ECTFE Polymer

Treatment

Colour

XPS (at.%) C

Cl

F

O

Failure load (N)*

PTFE

None 10 s 1 min

White Brown Black

38.4 87.6 82.2

— — —

61.6 0.8 0.9

— 11.6 16.9

420 4280 4260

PVF

None 10 s 1 min 60 min

Colourless Colourless Colourless Colourless

70.4 72.4 75.4 87.3

— — — —

28.8 26.7 23.0 11.4

0.8 0.9 1.6 1.3

360 800 2080 3020

PVDF

None 1 min 60 min

Colourless Faint brown Faint brown

51.4 77.4 79.5

— — —

47.9 12.9 9.2

0.7 9.7 11.3

— — —

ECTFE

None 1 min

Cream Cream

53.2 72.5

14.3 3.7

32.5 17.7

— 6.0

240 3300

*For a bonded area of 20 mm wide x 10 mm long; a two-part epoxide adhesive was used

9

Adhesion to Fluoropolymers

to these scratches are produced. Rye found that adhesion of copper to etched FEP was poor, whereas with etched PTFE it was good. Rye attributed the good adhesion to mechanical keying with the porous PTFE. Whereas XPS can be used to study chemical changes up to about 10 nm, Rutherford back scattering (RBS) can be used to study chemical changes, although in less detail, to much greater depths. Rye, using RBS, found the depth of etching was about 300 nm with PTFE but less than 50 nm with FEP. Lin and co-workers (107) studied the treatment of PTFE with sodium naphthalenide in THF and its effect on copper adhesion to the polymer. They found that the adhesion of electroless copper improved as defluorination increased. They concluded that the adhesion was also enhanced by the ‘groove-like’ topography caused by the chemical treatment. Bening and McCarthy (254) reduced FEP with sodium naphthalenide in THF at –78 °C and 0 °C. They then determined the thickness of the treated layers using a combination of gravimetric measurements and UV-vis spectroscopy. After 8 hours, at –78 °C the thickness was about 9 nm and at 0 °C was about 70 nm. Marchesi and co-workers (251) treated various fluorinated polymers with sodium naphthalenide in THF at ambient temperature. Using IR-IRS, they determined the treatment depth after 1 hour. For smooth films the treated depth was within the range 112 – 150 nm for PFA, FEP and PTFE. A number of reagents can attack PTFE and other perfluoropolymers resulting in substantial surface modification but without complete carbonisation of the surface. For example Chakrabati and Jacobus (281) found that the product obtained from the reduction of PTFE by a solution of lithium in liquid ammonia depended on the lithium to fluorine ratio in the mixture. Of particular interest is the observation that for a lithium to fluorine ratio of four a white, oxygen-free material of composition C90 H179 F was obtained. Infrared and 13CNMR spectroscopy showed that this consists of a layer of high molecular weight polyethylene formed by the following reaction (see also Equation 6): -(CF2)- + 4Li + 2NH3 → (-CH2-) + 2LiF + 2LiNH2 (10)

5.2.2 Treatment with Strong Aqueous Bases Crowe and Badyal (a.7) treated PVDF overnight at room temperature with a saturated aqueous solution of

10

lithium hydroxide. Very different surface chemistries were obtained depending on the washing procedure. If the treated PVDF was washed in water, two new peaks were evident in the XPS spectrum, the dominant one being at 285.2 eV (most likely –C x H y -) and 287.4 eV (probably >C=O or >C PFA > FEP > PTFE Surprisingly Teflon AF (copolymer of TFE and perfluoro-2,2-dimethyl-1,3-dioxole) is totally inert except when the TFE content is very high in which case bond strengths similar to those of PTFE were observed. The maximum bond strengths for various polymers after the benzoin dianion treatment are presented in Table 9.

a b

Polychlorotrifluoroethylene % tetrafluoroethylene in Teflon-AF

5.3 Electrochemical and Related Methods The majority of wet pretreatments of polymers involve surface functionalisation by oxidation or reduction of the polymer. The oxidising or reducing agent may be replaced by an anodically or cathodically polarised electrode. We can make use of this in two ways. The electrode may be brought into direct contact with the polymer surface, the so-called direct electrochemical process or, alternatively, an indirect approach may be used where a soluble redox reagent is generated at an electrode which then transfers electrons to or from the polymer, regenerating the original species which merely acts as an electron carrier or mediator transferring electrons between the electrode and the polymer. Both approaches have been used to treat the surface of polymers with the aim of improving their bondability. PTFE may be regarded as a highly oxidised polymer which is thermodynamically unstable relative to its reduction products as shown by the following data (a.8): 1/n (-CF2-)n + H2 → 1/n (-C-)n + 2 HF

(10)

ΔG0 = -193.18 kJ/mol With the exception of Teflon-AF (40) these peel strengths would be satisfactory for most purposes but clearly the treatment times are excessive. It was also shown that exposure of the reduced films to the strongly nucleophilic sodium methylthiolate anion (CH3SNa) resulted in the incorporation of sulfur into the surface which in turn greatly enhanced the adhesion of sputtered gold and electroless copper.

E0 = 1.00 V This clearly shows that PTFE although generally regarded as a highly inert material is thermodynamically unstable in contact with most metals and even with such mild reducing agents as iodide ions or stannous ions. The apparent stability of this polymer is due to kinetic inhibition which may be overcome by contacting the polymer with powerful one electron reducing agents such as alkali metal amalgams, solvated electrons or

13

Adhesion to Fluoropolymers

solutions of radical ion salts in aprotic solvents. All of these species behave as powerful electron donors capable of transferring an electron to a halogenated species including PTFE with simultaneous expulsion of a halide ion as shown in Equations (6-9), Section 5.2.1. For PTFE, reduction under aprotic conditions can lead to complete removal of halogen to yield a product consisting mainly of carbon and some surface oxygen. The precise mechanism for the formation of carbon is not known but could involve elimination of a fluoride ion from the intermediate carbanionic species R- as in Equations (8) and (9) or successive reductive eliminations as in Equation (6).

5.3.1 The Indirect Electrochemical Pretreatment of PTFE The black carbonaceous layer formed on PTFE adheres well to the substrate and can be readily bonded using conventional epoxy adhesives. Brewis and Dahm showed that tetrabutylamonium naphthalenide produced electrochemically by reducing a solution of naphthalene in DMF containing tetrabutylammonium tetrafluoroborate (TBAT) at a platinum cathode was just as effective as commercially available ‘Tetra-Etch’ (H L Gore and Associates, Newark, NJ or Flagstaff, AZ) or sodium naphthalenide in reducing the surface of PTFE. This was the first recorded example of an electron carrier mediated electrochemical reduction of PTFE (344). Figure 3 illustrates in simplified form the principle of this electrocatalytic treatment. Amatore and co-workers (a.9) have recently carried out a detailed study of this process using a gold band microelectrode separated from the PTFE by a thin layer of inert Mylar polyester film taking into consideration the intercalation (doping) process omitted from the

previous, simplified scheme and first shown to play an important role in the electrochemical pretreatment of PTFE by Dahm and co-workers (a.10), see below. Amatore and co-workers found that reduction takes place only if mediators with reduction potentials in excess of –2.15 V saturated calomel electrode (SCE) such as benzonitrile or naphthalene are used to drive the carbonisation which takes place via two pathways. One of these involves direct electron transfer to the PTFE by diffusion of radical anions directly to the site of reduction as shown in Figure 3 and is essentially a surface reaction resulting only in lateral growth of the carbonaceous layer whereas the second process involves initial electron transfer to the carbonaceous (electronically conducting) film followed by reduction of PTFE at the carbon PTFE interface resulting in both widening and thickening of the carbonaceous film as proposed earlier by Dahm and co-workers (a.10). For the direct electrochemical pretreatment, see below. Amatore’s elegant quantitative analysis clearly shows that the ‘chemical’ amalgam and ‘wet’ electrochemical reduction of PTFE proceed by essentially the same mechanism. Similar schemes can be written for the reduction of the PTFE surface by solvated electrons generated for example, by the dissolution of sodium in liquid ammonia. Brace and co-workers (166) have used electrochemical techniques to generate solutions of solvated electrons with magnesium counter ions by electrolysing a solution of ammonium tetrafluoroborate in liquid ammonia using a magnesium anode and an inert stainless steel cathode. This treatment appears to be surprisingly mild compared with that observed for the sodium in liquid ammonia in that even after prolonged treatment times the appearance of the PTFE surface remains largely unchanged unlike the intensely black coloured surfaces produced by the sodium in liquid ammonia treatments. Combellas and co-workers in a later publication (75) report on the effect

Figure 3 Naphthalene mediated electrochemical pretreatment of PTFE Reproduced with permission from D.M. Brewis and R.H. Dahm, International Journal of Adhesion and Adhesives, 2001, 21, 401. Copyright Elsevier 2001.

14

Adhesion to Fluoropolymers

of the magnesium treatment on the adhesion to PTFE. Peel tests on PTFE/rubbery epoxy/PET film assemblies resulted in modest improvements in adhesion which could however be improved greatly by the addition of sodium or potassium salts to the liquid ammonia solution which then, not unsurprisingly, produced the deeply coloured surfaces characteristic of the sodium in liquid ammonia treatment.

5.3.2 Treatment of PTFE with Metal Amalgams Carbonisation with alkali metal amalgams was investigated extensively by Jansta and co-workers who were the first workers to demonstrate the electrochemical nature of the carbonisation process (341). They showed that only the early stages of the process involved purely chemical interaction between the alkali metal and the polymer resulting in the formation of an intimate mixture of more or less finely divided electronically conducting carbonaceous material and the pore filling by the solid metal fluoride. The overall cell reaction is described by Equation (13): (-CF2-) + 2M(Hg)x → (-C-) + 2MF + 2 x Hg (13) Defluorination then continues as a series of processes taking place at the anode and cathode of a short circuited galvanic cell which may be represented by the following half cell reactions: At the anode, i.e., the amalgam metal fluoride interface: M(Hg) → M+ + e + Hg

(14)

The electrons travel along the electronically conducting ‘carbon wire’ to the carbon PTFE interface which now acts as a cathode at which PTFE is electronated according to: (-CF2-) + 2e → (-C-)+2F-

(15)

Simultaneously metal cations migrate from the vicinity of the anode to the cathode region in order to maintain charge neutrality. At that point the cations combine with the fluoride ions to form the metal fluoride: M+ + F- → MF

(16)

Because the newly defluorinated PTFE is also electronically conducting, the process continues until either the metal amalgam or the PTFE has been consumed. The system is shown schematically in Figure 4. Jansta and co-workers (341) showed that the thickness (l) of the carbonised layer increases with time according to: l = kt1/2

(17)

Where k is a rate constant whose magnitude is determined mainly by the electronic and ionic conductivity of the composite layer and by the thermodynamic cell voltage. More recently Kavan and co-workers (71) found that the reduction of PTFE with magnesium amalgam differs significantly from that with alkali metal amalgams in that this treatment does not result in blackening of PTFE surface even after prolonged contact with the amalgam at 150 °C thus strongly resembling the behaviour

Figure 4 Schematic of the metal amalgam - PTFE corrosion cell Reproduced with permission from D.M. Brewis and R.H. Dahm, International Journal of Adhesion and Adhesives, 2001, 21, 401. Copyright Elsevier 2001.

15

Adhesion to Fluoropolymers

of PTFE towards magnesium in liquid ammonia as described earlier (a.9, 75, 166). Kavan showed that the magnesium amalgam reaction is confined to the top 100 nm surface layer producing carbon and magnesium fluoride but does not, unlike the sodium amalgam treatment, propagate into the bulk polymer. This is somewhat surprising because the magnesium amalgam has a higher standard potential (–2.09 V) than sodium (–1.959 V) or potassium (–1.975 V) amalgam so that lack of sufficient thermodynamic driving force cannot be the cause of the lower reactivity of the magnesium amalgam. For most treatments leading to intensive carbonisation a dense impervious carbonaceous film is formed. The only way in which such a film can grow in thickness (other than through crack formation) is by the mechanism outlined in Figure 4. This necessitates that the carbonaceous film is capable of acting as an electronic conductor and at the same time allows the movement of metal cations from the amalgam through the film to the PTFE-carbon interface. If either of these processes is blocked film thickening cannot take place. Kavan and co-workers speculate that ion transport is inhibited in the case of magnesium ions both here and in the case of magnesium in liquid ammonia. The precise mechanism by which the cations move through the film has not been established. It is unlikely that the solid metal fluorides, e.g., NaF, LiF or MgF2 are capable of acting as solid state ionic conductors at the low temperatures involved in these carbonisation reactions. Dahm and co-workers (a.10) have shown below, that the carbonaceous film can take up an excess negative charge (doping) and suggest that this is due to intercalation of (or doping by) cations into the film.

It is suggested here that it is these mobile cations that sustain the film thickening process.

5.3.3 The Direct Electrochemical Pretreatment of PTFE Brewis and Dahm reasoned that because the carbonaceous reduction product of PTFE was known to be an electronic conductor, direct electron transfer from a suitably polarised cathode in contact with a PTFE surface should also result in the formation of essentially the same carbonaceous reduction product as that observed for the chemical reductive treatments. Using a simple electrochemical cell they showed that the surface of PTFE could be reduced electrochemically to the extent of about 1 cm2 by contacting the surface of PTFE skived tape or ram-extruded rod with a metal cathode held at a potential in excess of –1.5 V versus the SCE under the surface of a 0.1 M tetrabutylammonium tetrafluoroborate solution in dry DMF (344, a.10). The treated, blackened area grows radially outwards from the point of contact to cover an area of about 1 cm2. Prolonged contact caused the film to thicken at a rate which also obeys Equation (17). The magnitude of k appears to be a function of the size of the cation used as the support electrolyte. Thus the rate of increase in the thickness of the carbonaceous layer is greatest for the relatively small tetraethylammonium cations and very much smaller for the much larger tetraoctylammonium ions (a.10). The processes of lateral and in depth growth of the film is shown schematically in Figure 5.

Figure 5 Schematic of the metal amalgam - PTFE corrosion cell Reproduced with permission from D.M. Brewis and R.H. Dahm, International Journal of Adhesion and Adhesives, 2001, 21, 401. Copyright Elsevier 2001.

16

Adhesion to Fluoropolymers

The electrochemical surface carbonisation does not proceed at a uniform rate in all directions from the point of contact. For example, in the case of skived tape the carbonisation proceeds at a faster rate in the direction of skiving resulting in marked asymmetry of the treated film (a.10). This may be due to preferential reduction along polymer chains which have been aligned to some extent by the high shearing stresses during processing. Similar anisotropic carbonisation is observed for ram extruded rod and for tapes which have been elongated by stretching in a tensometer (327). On the other hand, specimens which have not been subjected to severe processing such as cast film, pressed sheet or skived tapes from which the altered surface layer has been removed by first reducing the surface layer and then removing the carbonaceous film by oxidation with, for example, a nitric/sulfuric acid mixture, do not exhibit anisotropic surface carbonisation (a.11). The reduction of PTFE requires the stoichiometry of four electrons per C2F4 unit. However, Brewis and Dahm using coulometric measurements showed that the number of electrons required for the electrochemical reduction is always slightly but consistently greater than four (a.10). The authors interpret this in terms of a two step mechanism involving reduction of PTFE to carbon according to: (C2F4)n + 4ne → (C2)n + 4nF-

(18)

followed by a process which may involve intercalation or simply doping of the previous carbonaceous product according to: (C2)n + pe + pR4N+ → (C2)np- p(R4N+)

(19)

The latter process was found to be reversible by cyclic voltammetry (a.10) and was accompanied by a huge change in the electronic conductivity of the film and by a change in its appearance from a metallic bronze colour to black. Similar but completely irreversible changes are observed when the film is exposed to air or protic solvents. The XPS spectra of such films are very

similar to those obtained using sodium napthalenide as the reducing agent and contain relatively large amounts of oxygen as well as hydrogen and carbon. The effectiveness of the electrochemical treatment was compared directly with films treated with sodium naphthalenide in THF using lap shear joints (a.11) and a commercially available epoxide adhesive cured for 16 hours at 60 °C. A brief summary of the results obtained is given in Table 10. The failure loads obtained for the electrochemical treatment are at least as good as those obtained using sodium naphthalenide which appears to produce a maximum in the failure load after about 30 seconds immersion. The carbonaceous film appears to adhere strongly to the polymer substrate and the locus of failure is mostly within the polymer particularly in the case of the electrochemical treatment. It should of course be pointed out that the electrochemical treatments have not been developed for large-scale operations.

5.4 Plasma Treatments

5.4.1 Principles and Equipment Although flames and corona discharges consist of plasmas, i.e., activated gases, the term plasma treatment usually refers to a process carried out at reduced pressure. Much research has been carried out since the 1960s on the use of plasmas to pretreat fluoropolymers and other polymers. The improvements in adhesion obtained with PTFE have been much less than those achieved with partially fluorinated and other polymers, and much less than can be achieved with wet chemical methods (see Section 5.2). Plasma treatment is an increasingly common method of modifying the surfaces of many natural and synthetic polymers in order to improve adhesion, wettability, printability, dye uptake, etc. This is in part due to

Table 10 Failure loads obtained for lap shear joints of PTFE treated with various reducing agents Sample

Treatment

Time

Failure Load (N)*

Skived tape

None

400

Skived tape

Na Naphthalenide/THF

10 s

2420

Skived tape

Na Naphthalenide/THF

30 s

2730

Skived tape

Na Naphthalenide/THF

4h

2760

Skived tape

Direct electrochemical

3240

* Single lap shear tests with a 12 x 12 mm overlap using an epoxy adhesive

17

Adhesion to Fluoropolymers

ever stricter environmental requirements which have rendered many previously used wet chemical methods uneconomic or even led to their complete ban. Corona discharge treatment of polymer films has been used commercially for many years. It makes use of an atmospheric pressure air plasma and is therefore only effective for the treatment of a limited number of materials generally of simple shape such as flexible webs and objects of simple geometrical shape. This discussion will be limited to the application of low pressure glow discharge plasmas which by making use of a variety of working gases can be used for the treatment of a variety of polymers including objects of complex shapes and varying in size from car bumpers to fine powders as well as simple webs, yarns and fabrics. Plasmas are gaseous mixtures of partially ionised gases produced by subjecting a gas at low pressure (0.1-103 Pa) to an intense electric field. Electrons are removed from the neutral gas molecules to yield positive ions and electrons. The kinetic energy of these particles increases as they accelerate in the electric field resulting in collision with the remaining gas molecules to form more charged particles (as well as other activated but neutral species produced by excitation of atoms formed by cleavage of some of the covalent bonds of the working gas). The presence of the charged species increases the conductivity of the gas and sustains the plasma. The glow discharge plasma may be regarded as consisting of a mixture of electrons with relatively high temperature (103 K) and much heavier positive

ions and other activated species which are at a much lower temperature (300 K). The fact that the temperature of the plasma is relatively low renders it particularly useful for the treatment of heat sensitive materials such as polymers. Figure 6 gives a schematic indication of some of the processes taking place in a glow discharge (cold) plasma and of the interactions of the plasma with the polymer surface. The dissociation of the processing gas produces a variety of reactive species which can interact with the polymer surface. Neutral radicals can abstract surface atoms leaving trapped surface radicals which can either react with other gas phase fragments resulting in surface functionalisation or react with other surface radicals to form crosslinks or simply remain trapped and react later when the polymer is exposed to air or to some other reactive species, usually a monomer. It is also possible for the substrate to undergo fragmentation up to a depth of about 1 nm upon bombardment by reactive ions particularly if the substrate is placed in close proximity to a radio frequency (RF) driven electrode. Positive ions are accelerated towards this electrode gaining in kinetic energy compared with ions in other regions of the plasma. This process commonly referred to as ion sputtering can result in the cleaning of the surface by removing low molecular weight material or, under more vigorous conditions, result in the roughening of the surface due to the fact that ablation of amorphous regions takes place much more readily than that of crystalline regions. The surface is also subject to attack by highly energetic and abundant vacuum ultraviolet

Figure 6 Schematic of possible reactions of species produced in a glow discharge plasma with the surface of a polymer

18

Adhesion to Fluoropolymers

(λ ≤ 175 nm) photons emitted by excited state gas phase species. Such photons are sufficiently energetic to break covalent bonds within the polymer surface even very strong bonds such as C-C, C-H and C-F resulting in further crosslinking, photoablative roughening or desaturation by abstraction of, for example, fluorine atoms from the surface of fluoropolymers. The major components of a plasma processing system are a vacuum chamber, a vacuum pumping system, a power supply, a power transfer matching system and some kind of process control system to ensure stability of the plasma. The polymer may be processed either directly in the plasma or remote from the plasma as shown in Figures 7a and 7b. In the former process the sample is placed directly in the plasma chamber Figure 7a usually but not exclusively powered by a RF source. In the remote configuration the sample is placed in a separate chamber some distance from the plasma chamber and is therefore not subjected to the energetic

ion bombardment described earlier and is only subject to attack by relatively stable long-lived species capable of surviving the journey from the plasma chamber to the reaction chamber. The parameters that exert the greatest influence on the concentrations and reactivities of the species attacking the polymer surface include the feed gas composition and flow rate, the chamber pressure, the frequency and power of the excitation source and in the case of an RF process the configuration, size and shape of the electrodes and their position with respect to the sample. Most commercial plasma systems are designed for batch operation but demand for the treatment of flexible webs, yarns and fabrics has led to the development of continuous processes based on either air vacuum air or cassette to cassette configurations with web speeds in excess of 80 m/min. In addition batch reactors with

Figures 7a and 7b Schematic of ‘in plasma’ and ‘remote from plasma’ treatment systems

19

Adhesion to Fluoropolymers

capacities in excess of 9 m3 capable of treating large automotive parts such as car bumpers are available using both in plasma and remote plasma processes. On the other hand reactors for the treatment of small objects and even powders are also available. The excitation frequencies vary from DC to a number of allocated frequencies including low frequencies (LF) 40-450 kHz, RF 13.56 and 27.12 MHz and microwave (MW) at 915 MHz and 2.45 GHz with excitation power in the range 10 W to > 30 kW. In summary, plasma treatments of polymer surfaces result in improved adhesion due to one or more of the following effects: •

Removal of low molecular mass material

•

Stabilisation of polymer surfaces by crosslinking

•

Surface roughening by ion sputtering and VUV ablation

•

Functionalisation of the surface to improve wetting and interaction with the adhesive.

The effects of oxygen and argon plasmas on the surface energy and bondability of PTFE were studied by Kinbara and co-workers (234). The surfaces became considerably more water-wettable after a 20 second treatment in either an oxygen or argon plasma. The adhesion of a vacuum deposited gold film, as measured by a scratch test, showed a considerable increase after these treatments. The authors concluded from XPS data that an oxygen plasma introduced -C-O-, -CH2 - and -CHF groups into the PTFE surface. Klemberg-Sapieha and co-workers (195) treated PFA with a low pressure plasma at 2.45 GH3 using a variety of gases and assessed the adhesion of thermally evaporated copper to treated and untreated PFA. The effectiveness of the gases was found to be in the order: N2 > O2 > (N2 + H2) > (O2 + H2) > H2

5.4.2 Studies of Plasma Treatments Much research was carried out in the 1960s and 1970s on the use of plasmas to treat fluorinated and other polymers; see for example reference (a.2). This work has continued, in increasing volume, to the present day and a selection of more recent publications is given next. Some of these publications give adhesion data while others limit themselves to information on wettability, surface chemistry and topography. The publications which include adhesion data may be divided into those involving metallisation and those involving bonding with an adhesive.

The adhesion of evaporated copper to PFA, as assessed by both a peel test and a scratch test, was much enhanced by various plasma treatments (213). The effectiveness of the plasma treatments towards metal adhesion as assessed using the scratch test decreased in the order: N2 > O2> (N2 + H2) > (O2 + H2) > H2 No topographical changes were found by SEM after the plasma treatments and the improved adhesion was attributed to cleaning, improved wetting and formation of Cu—O and Cu—N bonds. Some chemical changes caused by the different plasmas under a particular set of conditions are given in Table 11. Kasemura and co-workers (266) used a domestic microwave oven with an output of 560 W at 2450 MHz operating at a pressure of 26 Pa to pretreat PTFE and FEP. As the pressure of the air was decreased from 133 to 26 Pa, the efficiency of the treatment increased as measured by lower contact angles with various liquids. XPS indicated that a ‘significant’ amount of oxygen had

Table 11 Atomic ratios, after different plasma treatments of PFA, as determined by XPS (treatment conditions: 100 W and 60 s) Treatment

F/C

O/C

N/C

Untreated

1.86

0.004

—

O2

1.78

0.014

—

N2

1.58

0.031

0.032

O2+H2

1.51

0.069

—

N 2 + H2

0.50

0.13

0.19

H2

0.35

0.064

—

20

Adhesion to Fluoropolymers

a 45 second treatment with argon. A 12-fold increase in peel strength was achieved with an acid etch consisting of acetic, phosphoric and nitric acids. However, the peel strength after the acid etch was 48 N/m which is surprisingly low.

been introduced into the surfaces after a 10 s treatment in air at 26 Pa. Peel strengths using a pressure sensitive adhesive tape increased by about 50 and 200% for PTFE and FEP, respectively. However, it should be pointed out that even the highest peel strengths obtained (2 kg per 25 mm) are only moderate.

In a study comparing ammonia plasma treatment with a sodium naphthalenide etch, the latter was found to be much more effective at increasing the bondability of PTFE (a.12). A joint strength of 12.9 MPa was obtained with the sodium naphthalenide treatment compared with 2.8 MPa for the best plasma treatment. The authors used XPS to study the pretreated surfaces and also the failure surfaces after the adhesive joints had been destroyed. Some of these results are given in Table 12.

The bondability of an RF plasma-treated ethylenetetrafluoroethylene (E-TFE) copolymer was investigated by Hansen and co-workers (252). They found that O2 + SF6, O2 and NH3 plasmas increased the strength of double lap joints bonded with various epoxide adhesives by a factor of 20-30; this compared with an 11-fold increase with the commercial etchant, Tetra-Etch. However, the highest value obtained with a plasma treatment (2.3 MPa) was surprisingly low. The present authors have obtained lap shear strengths in excess of 20 MPa with various treatments of PVF, PVDF and PTFE and in excess of 16 MPa for an ethylenechlorotrifluoroethylene copolymer (215).

The bondability test involved bonding a bolt to the PTFE and then pulling off the bolt. It is apparent that, with both treatments, failure is largely cohesive within the PTFE. Kaplan and coworkers (a.12) concluded that the much lower joint strength obtained with the plasma treatment was due to a cohesively weak layer between the bulk PTFE and the chemically modified layer. On the other hand, they believe the naphthalenide treatment may give higher joint strengths due to considerable crosslinking.

Hansen and co-workers (252) used XPS to examine the E-TFE copolymer after it had been treated for 15 minutes in an oxygen plasma. They found 7-8 at.% oxygen had been introduced into the polymer surface and attributed this mainly to ester groups. Anderson and co-workers (247) studied the use of RF plasmas and an acid etch to improve bonding to PVDF. All four plasmas examined (O2, N2, Ar and NH3) caused substantial reductions in water contact angle after 10 second treatment. The best improvements in bondability with a plasma was a 7-fold increase in peel strength after

Kang and co-workers (128) treated PTFE with an argon plasma at 40 kHz. The treated PTFE was exposed to the atmosphere for about 30 minutes and then laminated to copper foil in the presence of a monomer at 120 °C. Three monomers were used, namely 1-vinyl imidazole,

Table 12 XPS elemental analysis (at.%) of pretreated PTFE and failure surfaces of PTFE bonded with an epoxide (a.12) Sample description

C

S

Mg

Ca

N

0

F

Na

PTFE virgin

33

—

—

—

—

—

67

—

Treated surface

71

0.4

1.2

0.9

1.9

17.0

7.9

—

PTFE side

37

—

—

—

1.2

1.0

61

0.2

Bolt side

40

—

—

—

2.2

2.8

53

2.1

Tetra-Etch

Plasma Activation Treated surface

46

—

—

—

6.4

6.3

39

—

PTFE side

33

—

—

—

—

—

67

—

Bolt side

35

—

—

—

0.7

0.4

64

—

The bondability test involved bonding a bolt to the PTFE and then pulling off the bolt

21

Adhesion to Fluoropolymers

4-vinyl pyridine and 2-vinyl pyridine. Peel and lap shear strengths of the laminates were determined. The peel strengths of up to 7.5 N/cm were satisfactory but the lap shear strengths were surprisingly low. The maximum values of about 120 N/cm2 are very low. Full details of the bonding procedure are not given, but it appears that the flexible laminate was not reinforced and therefore peeling, rather than shear, forces would dominate, leading to low adhesion values. Inagaki and co-workers (273) studied the effect of NH3 plasma treatment at 20 kHz using contact angle measurements, XPS and ATR-FTIR. The hydrophilicity of the surface was measured, water contact angles as low as 16° being observed. A combination of XPS and ATR showed that extensive defluorination had occurred and this was accompanied by the formation of carbonyl and amide groups. The treated PTFE was bonded to nitrile rubber with a phenolic adhesive. Peel strengths as high as 8 kN/m were obtained but only after a treatment of 10 minutes at 200 °C. Zou and co-workers (67) examined the pretreatment of PTFE with a hydrogen plasma and/or plasma polymerised glycidyl methacrylate (ppGMA) and the effects of these treatments on the adhesion of evaporated copper to PTFE. The adhesion results are summarised in Table 13. Using XPS, Zou and co-workers found that after washing with acetone, the surface of GMA plasma polymerised PTFE was almost identical to untreated PTFE. However, the coating on PTFE that had been

treated with a hydrogen plasma followed by a GMA plasma polymerisation was resistant to acetone washing. The authors concluded that covalent bonding existed between PTFE treated with a hydrogen plasma and the GMA layer. However, the present authors note that even the highest peel strengths shown in Table 13 are only moderate. Koh and co-workers (169) irradiated PTFE with argon ions at l keV. A fibrous texture was demonstrated by SEM. Except at low ion doses, the water contact angle increased after treatment despite the introduction of oxygen-containing groups as shown by XPS. This was attributed to the increased surface roughness. The adhesion levels of untreated and treated PTFE were measured using a tensile test in which the polymer was bonded to ‘sample holders’. The treatment led to a 3.75-fold increase in adhesion in the best case. It should be pointed out that the value for untreated PTFE was 200 kg/cm2, which is an order of magnitude higher than previously reported results. Badey and co-workers (186) treated PTFE with hydrogen and ammonia microwave plasmas (2.45 GHz) and also with sodium naphthalenide in dimethyl ether of ethylene glycol. XPS showed that all treatments brought about extensive defluorination and the introduction of oxygen, and in the cases of treatment with sodium naphthalenide or an ammonia plasma, also nitrogen. The authors compared the bondabilities of untreated PTFE in a pull-off test in which an aluminium stud was bonded to sheets of polymer using an epoxide adhesive. The results are summarised in Table 14.

Table 13 Treatment of PTFE with a hydrogen plasma and/or plasma polymerised GMA (67) Treatment

Maximum 180° peel strength N/m

Failure mode

20

-

H plasma only (a)

180

-

pp GMA (b)

50

at ppGMA-PTFE interface

(a) followed by (b)

480

cohesive within unmodified PTFE

None

Table 14 Surface compositions (atom %) and failure loads (Lf) of PTFE (214) C (%)

F (%)

O (%)

N (%)

Lf (N)

Untreated

27.7

70.5

1.8

-

31 + 17

SN treateda

68.0

14.7

13.2

2.7

800 + 100

NH3 plasmab

49.4

36.6

4.9

9.1

195 + 42

41.4

5.2

-

189 + 21

H2 a

22

plasmac b

60 s, 500 W, 30

53.4 cm3/min,

c

120 s, 300 W, 100

cm3/min,

120 s: bonded area 3.14

cm2

Adhesion to Fluoropolymers

It can be seen that the most extensive chemical modification occurred with the sodium naphthalenide treatment. It is interesting to note that failure occurred in a chemically unmodified zone in all cases as indicated by the F:C ratios. The authors commented that the joint strengths were limited by weak boundary layers. However, the failure loads are much lower than observed in other studies. Kunz and Bauer (62) have developed a process for the pretreatment of difficult to coat polymers which involves three steps. A plasma treatment followed by the grafting on of a photoinitiator which results in a stable but light sensitive surface. The surface is then coated with a UV hardenable formulation such as an acrylate lacquer and exposed through a mask, if desired, to UV light. Unexposed lacquer is removed with ethanol after which other coatings or adhesives may be applied to the hardened lacquer which is covalently bonded to the initiator which is in turn covalently bonded to the plasma treated surface. The authors claim ‘astonishingly’ good results for the coating of PTFE using this so called ‘Smart Priming Process’. Morra and co-workers (a.13) showed that treatment of PTFE for 30 seconds in an oxygen plasma did not cause any significant topographical changes according to SEM, but treatment for 15 minutes produced a spongelike surface. However, with argon no topographical changes were evident, even after 15 minutes treatment. In contrast to the topographical changes, chemical changes were more pronounced with an argon plasma as can be seen in Table 15. It is interesting to note that the amount of defluorination and the amount of oxygen introduced into the surface decrease with increasing treatment time. With an oxygen plasma treatment for 15 minutes, the amount of surface oxidation is less than that for the untreated PTFE. Griesser and co-workers (a.14) studied the treatment of FEP using argon, air, oxygen or water vapour plasmas most of the work utilised argon. A custom-built oscillator operating at 700 kHz and 10 W was used at a pressure of 80 Pa. XPS data showed that treatment for a few seconds in an argon plasma introduced about 4 at.% O, significantly reduced the F content and increased the C content. They found that water contact angles decreased markedly after treatment for a few seconds. This effect was partially reversed over a period of three weeks. The authors attributed this to a partial reorientation of chain segments such that oxygen-containing functional groups moved towards the bulk of the polymer. From the decreased F:C ratio, the authors concluded that some crosslinking had occurred and that this might account for the incomplete recovery in contact angles.

Table 15 XPS composition (atomic percentages) of oxygen and argon plasma-treated PTFE as a function of treatment time Treatment time (min)

C

F

O

Untreated

39.8

60.4

0.8

Oxygen plasma 0.5 1.0 2.0 5.0 10.0 15.0

44.6 42.7 42.6 40.9 38.3 38.3

48.9 51.1 50.9 57.0 60.5 61.4

6.4 7.1 6.5 2.1 1.2 0.3

Argon plasma 0.5 1.0 2.0 5.0 10.0 15.0

50.2 50.8 46.9 45.6 38.5 37.6

27.0 25.0 37.2 44.9 52.5 53.9

22.8 24.2 15.9 9.5 9.0 8.5

In contrast to the contact angle data, Griesser and coworkers found that the oxygen content within the XPS sampling depth increased on storage over a few weeks and suggested that a possible mechanism could involve the relatively slow breakdown of peroxy radicals. These radicals could decompose into stable oxygen-containing groups plus carbon-centred radicals which, in turn, could react with oxygen. The treatment of PTFE using air, oxygen, argon and water plasmas was studied by Youxian and co-workers (255). After treatment for a few seconds, substantial increases in surface energy occurred mainly due to changes in the polar component. However, XPS showed that little oxygen was introduced by the treatment. A reduced F:C ratio on treatment indicated crosslinking. By using mixtures of hydrogen and methanol, high levels of oxygen (approximately 17 at.%) were introduced into PTFE (255). It was found that H2/O2 mixtures caused major changes in topography to porous PTFE whereas no topographical changes were observed for H2/H2O or H2/methanol mixtures when SEM was used at l0000x magnification. Egitto (272) studied the mechanism of plasma treatment using a variety of polymers including PTFE. He examined the effect of treatment, downstream from a He microwave plasma, on the water contact angle with PTFE. By excluding He metastables with a lithium fluoride crystal filter, he demonstrated that photons

23

Adhesion to Fluoropolymers

alone resulted in a reduction in the receding angle of up to 40°. Busscher and co-workers (248) showed that the hydrophobicity of FEP can be increased by ion etching; the advancing water contact angle increased from 10 to about 120°. However, if ion etching was followed by exposure to an oxygen glow discharge (GLD) advancing water contact angles in excess of 140 ° were observed in some cases. The increased hydrophobicity was attributed to changes in topography. Ion etching caused extensive roughening but the resulting surfaces contained high levels of oxygen (Table 16). The GLD treatment removed much of this O-functionality, while retaining much of the topographical change, and hence the hydrophobicity was greater than with ion etching alone. Badey and co-workers (217) studied the surface modification of PTFE following downstream microwave plasma treatment (2.45 GHz). They found no modification with O2 or O2/N2 mixtures whatever the conditions. However, using NH3, under suitable conditions, major changes were noted using contact angle measurements and XPS, under suitable conditions. Large increases in the polar component of surface energy were observed. XPS demonstrated that under suitable conditions a NH3 plasma caused extensive defluorination accompanied by the introduction of substantial quantities of N and O-containing groups. The use of an atmospheric plasma to treat polymeric films is described by Yializis and Markgraf (43). As far as PTFE was concerned, they assessed the effectiveness of the treatment using dyne levels according to ASTM D2578 (a.15). With most gases the effect of the treatment lasted only a few hours, but if acetylene was used the dyne level remained constant at about 60 dynes/cm for 350 hours. No adhesion measurements were carried out.

Gengenbach and co-workers (219) made a detailed study of the ammonia plasma treatment of PTFE and PEP. For P, the N:C ratio was determined as a function of ageing time at take-off angles (TOA) of 15 and 90° with respect to the sample surface. Initially the ratio was higher at 15°, indicating a shallow treatment. At both angles, the N:C ratio decreased substantially over a period of 325 days, the decrease being much more rapid in the first few days. In marked contrast, the O:C ratio increased from about 0.05 to about 0.28 (at 90° TOA) over 325 days, with most of the change taking place in the first four days. The rapid uptake of oxygen is attributed to reaction between atmospheric oxygen and radicals produced by the plasma treatment whereas the slow uptake is thought to be due to secondary reactions. The corresponding water contact angles for the treated PEP increased markedly over the first few days of ageing, despite the increased uptake of oxygen, although not to the level of untreated polymer. The reduced wettability is attributed to both reorientation of polar groups towards the bulk of the polymer and chemical changes in the first few atomic layers. Changes in topography and surface chemistry for a range of plasmas (O2, N2, H2, He, Ne, Ar and CF4) were studied by Badyal and Ryan (208). The changes in chemistry are summarised in Table 17. It can be seen the smallest chemical changes occur with the O2 and CF4 plasmas. On the other hand, an oxygen plasma gave the greatest topographical change.

5.4.3 Flame Treatment As noted earlier, flame treatment is an example of plasma treatment. Mathieson and co-workers (215) showed that large increases in adhesion could be achieved by flame treating PVF and ECTFE (Table 18). With PTFE there was no surface modification and an

Table 16 Elemental atomic composition by XPS of ion etching-treated (8 mA, 6 kV and 4 x 10-4 Torr argon pressure) and oxygen GLD treated (1500 Pa oxygen pressure at 50 W for 5 min) FEP (248) Ion Etching Treatment

GLD Treatment

O/C

F/C

Al/C

No

No

0.02

1.93