Thermal Analysis of Rubbers and Rubbery Materials

Thermal Analysis of Rubbers and Rubbery Materials

Editors: P.P. De, N. Roy Choudhury, and N.K. Dutta

Author

Thermal Analysis of Rubbers and Rubbery Materials

Editors: Namita Roy Choudhury Prajna P De Naba K Dutta

iSmithers - a Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.iSmithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra Technology Ltd

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library. Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 978-1-84735-103-6 (hardback) ISBN: 978-1-84735-102-9 (softback) ISBN: 978-1-84735-104-2 (ebook) Typeset by SA Hall Typesetting, Brixham Printed and bound by Lightning Source UK

Intelligent Tyres

This book is dedicated to the curious minds:

The pioneering researcher in the field of thermal analysis of rubbery materials; Professor Anil K. Sircar, formerly of the University of Dayton, Ohio, USA

Budding scholar Ankit K. Dutta, currently at the University of Adelaide, Australia

Thermal Analysis of Rubbers and Rubbery Materials

Contents

Contents

Preface .............................................................................................................xiii 1

Introduction ................................................................................................1 References ....................................................................................................7

2

Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials ...............................................................................11 2.1

Introduction ......................................................................................11

2.2

Differential Thermal Analysis (DTA) .................................................12 2.2.1

DTA Instrument ....................................................................13 2.2.1(a) Sample Holders ....................................................... 14 2.2.1(b) Furnace and Furnace Temperature Programmers .... 15 2.2.1(c) Differential Temperature Detection System.............. 16 2.2.1(d) Low Level DC Voltage Amplifier ............................ 16 2.2.1(e) Recorder.................................................................. 16 2.2.1(f) Atmosphere Control ............................................... 17

2.3

2.4

2.5

Differential Scanning Calorimetry (DSC)...........................................17 2.3.1

Heat-flux DSC .......................................................................17

2.3.2

Power Compensated DSC......................................................18

2.3.3

Temperature Modulated DSC (TMDSC) ...............................20

Thermogravimetry (TG) ....................................................................22 2.4.1

Thermobalance......................................................................24

2.4.2

Temperature Detection in TG or TGA ...................................24

2.4.3

Furnace and Furnace Temperature Programmers ...................24

2.4.4

Controlled Atmosphere .........................................................25

2.4.5

Sample Containers.................................................................25

2.4.6

Recorders ..............................................................................25

2.4.7

Software ................................................................................26

Derivative Thermogravimetry (DTG) ................................................27 i

Thermal Analysis of Rubbers and Rubbery Materials

2.6

Evolved Gas Analysis (EGA) or Evolved Gas Detection (EGD) .........28

2.7

Thermomechanical Analysis (TMA) and Thermodilatometry (TD) or Thermodilatometric Analysis (TDA) ....................................31

2.8

2.7.1

Parallel Plate Rheometry (PPR) .............................................34

2.7.2

Fibre Tension Spectrometry ...................................................35

2.7.3

Stress Relaxation Spectrometry .............................................36

Dynamic Mechanical Analysis (DMA) ..............................................36 2.8.1

2.9

Torsional Braid Analysis (TBA) .............................................39

Thermally Stimulated Current (TSC) .................................................41 2.9.1

Principle ................................................................................41

2.10 Relaxation Map Analysis (RMA) ......................................................44 2.11 Differential Photo Calorimetery (DPC)..............................................46 2.11.1 DPC Instrument ....................................................................47 2.11.2 Principle of Operation ...........................................................48 2.11.3 Uses of DPC ..........................................................................49 2.12 Dielectric Analysis (DEA) or Dielectric Thermal Analysis (DETA).....49 2.12.1 Technique ..............................................................................50 2.13 Newly Developed Thermal Analysis ..................................................52 2.14 New Combined Methods of Thermal Analysis ..................................53 2.14.1 Coupled Thermogravimetry – Infra red Spectroscopy (TG-IR) .............................................53 2.14.2 Coupled Thermogravimetry – Fourier Transform Infra red Spectroscopy (TG-FT-IR) ..........54 2.14.3 Coupled Thermogravimetry – Mass Spectrometry (TG-MS) .................................................55 2.14.4. Coupled Thermogravimetry – Gas Chromatography (TG-GC) .............................................56 2.14.5 Coupled TG-GC-IR ................................................................58 2.14.6 Coupled TG-GC-MS ..............................................................59 2.15 Conclusion ........................................................................................60 Acknowledgements .....................................................................................60 ii

Contents

References ..................................................................................................60 3

Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials ...............................................................................65 3.1

Introduction .....................................................................................65

3.2

Differential Scanning Calorimetry of Rubbery Materials ...................65 3.2.1

Measurement of Specific Heat and Glass Transition Temperature .........................................................66

3.2.2

Significance of Tg ...................................................................67

3.2.3

Factors Affecting Tg ..............................................................69

3.2.4

Effect of Molecular Weight on Tg ..........................................69

3.2.5

Effect of Polymer Architecture on Tg .....................................71

3.2.6

Effect of Composition, Morphology and Thermal History on the Tg of Polymers ...............................................76

3.2.7

Effect of Crosslinking of Rubbers on Tg ................................85

3.2.8

Monitoring Vulcanisation of Rubber Using DSC ...................86

3.2.9

Characterisation of Melting and Crystallisation of Polymer ..88

3.2.10 Decomposition of Polymer ....................................................96 3.2.11 Oxidation Induction Time .....................................................99 3.2.12 Other Miscellaneous Applications of DSC ............................99 3.3

3.4

Thermogravimetric Analysis of Rubbery Materials .........................100 3.3.1

Thermal Degradation and Stability of Rubbers by TGA ......102

3.3.2

Compositional Characterisation of Rubbers by TGA ..........113

3.3.3

Study of Rubber Blend Compatibility Using TGA ...............117

3.3.4

Study of Rubber Degradation Kinetics Using TGA ..............118

3.3.5

Miscellaneous Applications of TGA ....................................124

Conclusion ......................................................................................124

References ................................................................................................124 4

Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites ...............................................149 4.1

Introduction ....................................................................................149

iii

Thermal Analysis of Rubbers and Rubbery Materials

4.2

4.3

4.1.1

Mechanical Models Describing Viscoelasticity.....................149

4.1.2

Linear Viscoelastic Behaviour of Amorphous Polymers .......149

4.1.3

Zones of Viscoelastic Behaviour ..........................................152

4.1.4

Time-Temperature Superposition Principle ..........................154

Instrumentation ...............................................................................156 4.2.1

Working Principle of a Dynamic Mechanical Analyser ........156

4.2.2

Selecting a Clamp for a DMA Experiment...........................157

4.2.3

Running a DMA Experiment...............................................158

Interpretation of Dynamic Mechanical Spectra of Polymers: Case Studies ....................................................................................159 4.3.1

Glassy Polymers ..................................................................159

4.3.2

Crystalline Polymers ............................................................160

4.3.3

Elastomers ...........................................................................162

4.4

Dependence of Storage Modulus on Frequency and Strain ..............180

4.5

Various Other Applications .............................................................182

4.6

Conclusion ......................................................................................182

References ................................................................................................182 5

iv

Characterisation of Rubbers, Polymers and Their Composites Using TMA ..............................................................................................187 5.1

Introduction ....................................................................................187

5.2

Instrumentation ...............................................................................188

5.3

Applications ....................................................................................189 5.3.1

Determination of Tg.................................................................................................189

5.3.2

Effect of Plasticiser on Tg.....................................................................................195

5.3.3

Creep and Stress Relaxation ................................................196

5.3.4

Use of TMA - Parallel Plate Rheometer (PPR) for Curing of Thermoset Polymers ......................................197

5.3.5

Evaluation of Crosslink Density by TMA ............................199

5.3.6

TMA for Fibre Analysis.......................................................203

5.3.7

Why Fibre Properties are Important ....................................206

Contents

5.3.8 5.4

TMA for the Analysis of Composites ..................................208

Use of TMA in Industry ..................................................................210 5.4.1

TMA in the Electronics Industry .........................................210

5.4.2

TMA in the Automotive Industry ........................................212

5.5

Conclusion ......................................................................................213

5.6

Acknowledgments ...........................................................................214

References ................................................................................................214 6

Micro-thermal Analysis of Rubbery Materials ..........................................217 6.1

Introduction ....................................................................................217

6.2

Basic Principles of μTA ....................................................................218

6.3

Modes of Micro-thermal Analysis ...................................................219

6.4

Micro-thermal Analysis of Rubbery Material ..................................220

6.5

Morphological Investigation in Polymer Blends...............................221

6.6

Thin Films/Coating on the Substrate ...............................................226

6.7

Multilayer Material Characterisation ..............................................233 6.7.1

Thermal Properties of Rubbery Micro-Particles...................235

6.8

Thermal Characterisation of Micro-spheres ....................................238

6.9

Powder Particle Characterisation.....................................................239

6.10 Characterisation of Micropores .......................................................240 6.11 Characterisation of Nanostructured Material ..................................240 6.11.1 Micro-thermal Analysis Combined with Chemical Characterisation Techniques ................................................244 6.12 Future Outlook ...............................................................................248 References ................................................................................................249 7

Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends .........................................................................................253 7.1

Introduction ....................................................................................253 v

Thermal Analysis of Rubbers and Rubbery Materials

7.2

Miscibility and Crystallisation of Biodegradable Polymer/Rubber Polymer Blends .....................................................254

7.3

Morphology and Crystallisation of Polyamide/Rubber Polymer Blends ................................................................................260

Acknowledgement ....................................................................................273 References ................................................................................................273 8

Thermal Characterisation of Polymer Nanocomposites ............................277 8.1

Introduction ....................................................................................277

8.2

Thermo-Gravimetric Analysis (TGA) ..............................................277

8.3

8.4

8.2.1

Introduction ........................................................................277

8.2.2

The Apparatus.....................................................................278

8.2.3

Methodology .......................................................................279

8.2.4

Typical TGA Curves ............................................................280

8.2.5

Applications of TGA in Nanocomposites Characterisation ..................................................................282

Differential Scanning Calorimetry (DSC).........................................290 8.3.1

Conventional DSC...............................................................290

8.3.2

The Apparatus.....................................................................291

8.3.3

Procedure ............................................................................293

8.3.4

Typical Data ........................................................................296

8.3.5

Temperature-Modulated DSC (TMDSC) .............................299

8.3.6

Applications of DSC for Thermal Characterisation of Polymer Nanocomposites ................................................301

8.3.7

Applications of TMDSC for Thermal Characterisation of Polymer Nanocomposites ................................................306

Other Characterisation Techniques..................................................309 8.4.1

Thermal Conductivity .........................................................309

8.4.2

Micro-Thermal Analysis (μTA)............................................311

References ................................................................................................313 9

vi

Thermal Analysis in Understanding Rubbery Matrix and Rubber-Filler Interactions .........................................................................321

Contents

9.1

Introduction ....................................................................................321

9.2

Thermal Analysis and Investigation of Heterogeneous Materials .....321

9.3

Structure-Properties Relationships in Particulate Filler/Rubbery Matrix Systems ........................................................322

9.4

Description of the Shape and Space Distribution of Filler Particles ..................................................................................325

9.5

Filler-to-Matrix and Filler-to-Filler Interactions as Investigated by Thermal Analysis ........................................................................325

9.6

9.5.1

Thermogravimetry...............................................................326

9.5.2 etry

Dielectric Thermal Analysis (DETA) and Thermoconductom326

9.5.3

Magnetic Thermal Analysis .................................................327

9.5.4

Dynamic Mechanical Analysis .............................................328

9.5.5

DTA and DSC .....................................................................329

Concluding Remarks .......................................................................329

References ................................................................................................329 10 Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry ..............................................................................................335 10.1 Introduction ....................................................................................335 10.2 Crystallisation .................................................................................335 10.2.1 Overall Crystallisation.........................................................335 10.2.2 Solution-grown Crystallisation ............................................338 10.3 Stem Length and Stem Length Distribution .....................................342 10.4 Effect of Fatty Acids ........................................................................344 10.5 Summary .........................................................................................351 Acknowledgements ...................................................................................351 References ................................................................................................351 11 Thermal Properties of Chemically Modified Elastomers ...........................353

vii

Thermal Analysis of Rubbers and Rubbery Materials

11.1 Introduction ....................................................................................353 11.2 Hydrogenation ................................................................................353 11.3 Epoxidation.....................................................................................356 11.4 Halogenation, Hydrohalogenation ..................................................357 11.5 Chemical Modification by Grafting .................................................360 11.6 Chemical Modification by Introducing Ionic Groups.......................367 11.7 Miscellaneous ..................................................................................372 Summary ..................................................................................................374 Acknowledgements ..................................................................................375 References ................................................................................................375 12 Thermal Analysis of Rubber Products ......................................................381 12.1 Introduction ....................................................................................381 12.2 Thermal Analysis of Rubber Based Vibration Control Devices .......383 12.2.1 Introduction ........................................................................383 12.2.2 Vibration Damping .............................................................383 12.2.3 Vibration Isolation ..............................................................384 12.2.4 Selection of Rubbers for Vibration Damping Application and the Role of Thermal Analysis ....................386 12.2.5 DMA for the Comparison of Different Rubber Based Shock Mounts ...........................................................390 12.2.6 Interpenetrating Polymer Networks (IPN) as Vibration Dampers ..............................................................391 12.2.7 Air Springs .........................................................................393 12.3 Thermal Analysis of Rubber Seals ...................................................395 12.3.1 Introduction ........................................................................395 12.3.2 Major Rubbers used for Seal Manufacturing .......................396 12.3.3 Role of Thermal Analysis in the Formula Reconstruction of Rubber Seals ...........................................396 12.3.4 Other Thermal Studies on Rubber Seals .............................402 12.3.5 Automotive Window Seal ....................................................403 viii

Contents

12.4 Thermal Analysis of Rubber-Based Cable Sheathing Compounds ....406 12.5 Thermal Analysis of Rubber Based Adhesives .................................409 12.5.1 Introduction ........................................................................409 12.5.2 Testing of Adhesives ...........................................................409 12.6 Thermal Analysis of Rubber Based Insulators .................................412 12.7 Thermal Analysis of Thermal Interface Materials (TIM) .................415 12.8 Thermal Analysis of Automobile Tyres ............................................417 12.8.1 Introduction ........................................................................417 12.8.2 Identification of Polymer in an Automobile Tyre Using Thermal Analysis ...............................................417 12.8.3 Isothermal TGA of Tyre Tread Compound ..........................419 12.8.4 Thermal Analysis for the Development of a Tyre Tread Compound ........................................................422 12.9 Concluding Remarks .......................................................................423 Acknowledgements ..................................................................................424 References ................................................................................................424 13 Thermal Analysis in Recycling of Waste Rubbery Materials .....................429 13.1 Introduction ....................................................................................429 13.2 Utilisation of Scrap Elastomers for Material Recovery ...................430 13.2.1 Characterisation of Recycled Rubber .................................430 13.2.2 Polymer Blends Containing Recycled Rubber ......................440 13.2.3 Recycled Rubber Modified Bitumen, Concrete and Composites ...................................................................451 13.3 Pyrolytic Utilisation of Waste Rubber .............................................453 13.3.1 Degradation and Recovery of Monomer, Gas and Carbon .........................................................................453 13.3.2 Energy Recovery Through Incineration ...............................455 13.4 Concluding Remarks .......................................................................456 Acknowledgement ....................................................................................456

ix

Thermal Analysis of Rubbers and Rubbery Materials

References ................................................................................................456 14 Thermal Analysis of Biological Molecules and Biomedical Polymers ........463 14.1 Introduction ....................................................................................463 4.2

Structure and Phase Behaviour of Cells, Membranes and Lipid Bilayers Using TA ...................................................................464 14.2.1 Lipid Blends and Alloys .......................................................468 14.2.2 Liposomes ...........................................................................470 4.2.3

Phospholipid-Additive Interactions .....................................472

14.3 Molecular Dynamics, Conformational Change and Swelling Behaviour of Biopolymers Using TA ..................................475 14.3.1 Level of Hydration on Thermal Characteristics of Proteins ...........................................................................478 14.3.2 State of Water and Molecular Dynamics in Biomaterials by DSC ...........................................................481 14.4 Collagen and Collagen Based Biomaterials ......................................488 14.4.1 Denaturation of Collagen from Different Origins ................491 14.4.2 Collagen Based Composite Biomaterial ...............................491 14.5 Thermal Stability of Silk, and Other Elastic Biomaterials ...............494 14.6 Thermal Characteristics of Biopolymers for Drug Delivery and Drug-Polymer Interaction ........................................................496 14.6.1 Protein-Protein, Protein-DNA and Protein-Ligand Interactions .........................................................................499 14.7 Thermal Characteristics of Synthetic Hydrogels and Scaffolds for Tissue Engineering .....................................................................500 14.8 Thermal Characteristics of Biomimetic Protein Based Hydrogels .....505 Acknowledgement ....................................................................................508 References ................................................................................................509 Abbreviations ..................................................................................................523 Index ...............................................................................................................531

x

Contributors

Contributors

Anil K. Bhowmick Rubber Technology Centre Indian Institute of Technology Kharagpur – 721302 India Namita Roy Choudhury Ian Wark Research Institute University of South Australia Mawson Lakes Campus Mawson Lakes Boulevard Mawson Lakes South Australia 5095 Australia Prajna P. De Retired Professor Rubber Technology Centre Indian Institute of Technology Kharagpur-721 302 India Naba K. Dutta Ian Wark Research Institute University of South Australia Mawson Lakes Campus Mawson Lakes Boulevard Mawson Lakes South Australia 5095 Australia

Yuko Ikeda Kyoto Institute of Technology Faculty of Engineering and Design Matsugasaki Kyoto 606-8585 Japan Takayuki Ikehara Department of Applied Chemistry Faculty of Engineering Kanagawa University 3-27-1, Rokkakubashi Kanagawa-ku Yokohama 221-8686 Japan Luminita L. Ionescu-Vasii Department of Chemical Engineering McGill University Montreal Canada H3A 2B2 Musa R. Kamal Department of Chemical Engineering McGill University Montreal Canada H3A 2B2

xi

Thermal Analysis of Rubbers and Rubbery Materials Seiichi Kawahara Department of Chemistry Faculty of Engineering Nagaoka University of Technology Nagaoka Niigata 940-2188 Japan Shinzo Kohjiya Kyoto University Institute for Chemical Research Uji Kyoto 611-0011 Japan Ivan Krakovsky Charles University Faculty of Mathematics and Physics Department of Polymer Physics V Holesovickach 2 180 00 Prague 8 Czech Republic Suman Mitra Rubber Technology Centre Indian Institute of Technology Kharagpur – 721302 India Amit K. Naskar Department of Chemical Engineering and Center for Advanced Engineering Fibers and Films Clemson University Clemson SC 29634 USA Kinsuk Naskar Rubber Technology Centre Indian Institute of Technology Kharagpur – 721302 India xii

Toshio Nishi Department of Organic and Polymeric Materials Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1 Ohokayama Meguro-ku Tokyo 152-8552 Japan Zhaobin Qiu College of Materials Science and Engineering Beijing University of Chemical Technology Beijing 100029 China R.S. Rajeev Institute of Space Technology and Aeronautics Japan Aerospace Exploration Agency 6-13-1 Osawa Mitaka-shi Tokyo-181 0015 Japan Kinnari Shelat Ian Wark Research Institute University of South Australia Mawson Lakes Campus Mawson Lakes Boulevard Mawson Lakes South Australia 5095 Australia Nikhil K. Singha Rubber Technology Centre Indian Institute of Technology Kharagpur – 721 302 India

Contributors N.D. Tran Ian Wark Research Institute University of South Australia Mawson Lakes Campus Mawson Lakes Boulevard Mawson Lakes South Australia 5095 Australia

xiii

Thermal Analysis of Rubbers and Rubbery Materials

xiv

Introduction

1

Introduction Prajna P. De, Namita Roy Choudhury and Naba K. Dutta

Since the early days of smelting of copper and iron, application of heat to manipulate the properties was known to man. Development of thermometer in eighteenth century gave an impetus to thermal studies of materials. In 1877, Hannay [1] was the first to find out that an examination of the rate at which the volatile constituent of a compound is driven off at a constant temperature may provide valuable information as to the constitution of the body and found out a relation between the vapour tension of a decomposing body and its chemical constitution. Ramsay [2] suggested that the composition and constitution of many of the amorphous hydrates such as aluminum oxide and iron oxide hydrate whose compositions are somewhat indefinite might be accurately determined by this method. In 1886 Le Chatelier [3, 4] obtained heating curves for various minerals in which endothermic (absorption of heat) and exothermic (evolution of heat) effects were distinguished. Gradually the science of thermometry for measurement of temperature of materials and their change of state with temperature changes was developed. In 1891 Roberts-Austen [5] introduced a differential thermocouple, which measured the differences in voltage between thermocouples placed in experimental sample and a control (inert sample). Principles of thermal analysis are based on thermometry [6] and thermodynamics [7]. Thermometry consists of plain measurement of temperature and is used in the study of phase transition or in recording temperatures as a function of time in the form of heating and cooling curves. But in thermodynamics, kinetic parameters of a reaction can be studied [8-11] and the most popularly used method is Arrhenius equation [12], correlating the rate of the reaction and activation energy. Similar kinetic equation in differential scanning calorimetry (DSC) has been used by Borchardt and Daniels [13] for thermoset cure, polymerisation process and chemical decomposition with single heating rate method. The ASTM E698 method is the only means to analyze the reactions with irregular baselines [14] and reactions with multiple exotherms [15] as well as a precursor to isothermal studies [16, 17]. As in the case of DSC and differential thermal analysis (DTA), kinetic equations have also been used in thermogravimetry and the most popular method is the single heating rate method by Freeman and Carrol [18, 19] for determining the order of the reaction and activation energy. Anderson [20] used the multiple heating rates and the kinetic parameters such as order of reaction and activation energy can be deduced from the different thermogravimetric analysis (TGA) curves. Reich [21, 22], 1

Thermal Analysis of Rubbers and Rubbery Materials Doyale [23, 24] and Ozawa [25] also introduced several methods to follow the kinetic parameters for decomposition reactions. Modern development of thermal analysis started in 1940s by the use of recorders, sample holders, weighing balance, thermocouples and improved instrumental techniques. Description of DTA, TGA and dilatometry has been reported by Smothers and Chiang [26], Garn [27], Wendlandt [28] and Mackenzie [29]. Other methods of thermal analysis include DSC, derivative thermogravimetrc analysis (DTG), dynamic mechanical thermal analysis (DMTA) [30, 31], thermomechanical analysis (TMA) [32]. Combination of two or more of these methods along with non-thermal techniques such as spectroscopy [33, 34] and microscopy form the basis of versatile tools for studying macromolecules, nano particles [35, 36], ceramics [37], medicines and biopolymers [38, 39]. Although there has been a large number of publications on thermal analyses, it is only in 1970 that a concerted effort was made by International Confederation of Thermal Analysis (ICTA) to standardise the nomenclature and experimental procedures to allow direct interlaboratory comparison of data. In 1970 ICTA merged with North American Thermal Analysis Society. Simultaneously publication of two journals namely Journal of Thermal Analysis (Wiley) and Thermochimica Acta (Elsevier) were started in 1969-1970. Though there have been dozens of books published on thermal analyses, books on thermal analysis of polymers, in particular books on thermal analysis on rubbers and rubbery materials have been very few. Rubber is different from other polymeric materials in the sense that it needs to be crosslinked or vulcanised and mixed with several additives like fillers to achieve its shape and strength. Mauer [40] was the first to publish a review on application of thermal analysis techniques in the study of the elastomer system and subsequently contributed a book chapter in ‘Thermal Characterisation of Polymeric Materials’ [30]. A few researchers have contributed chapters on thermal analysis of rubber in books [30, 31]. Several scientists have made contribution in this field and the pioneering work of Sircar and Lamond [41-47] and Brazier and Nickel [48-51] has opened new vistas of research. In view of the surge of research activities in this field, we felt the need of compilation of research work on thermal analysis of rubbers and rubbery materials in the form of a book. The present volume is the outcome of this thinking. The book consists of fourteen chapters, including the Introduction. The second chapter entitled ‘Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials’ by Prajna P. De deals with the instrumentation components in the techniques of thermal analysis used for characterisation of rubbers, rubbery materials, and the different constituents in their products. The techniques discussed are DTA, DSC, thermogravimetry (TG), DTG, evolved gas analysis, TMA, dynamic mechanical analysis (DMA), thermally stimulated current spectroscopy, 2

Introduction relaxation map analysis, differential photo calorimetry and dielectric thermal analysis (DETA). Combined methods of thermal analysis provide complete analyses of the constituents of rubber products. The techniques include coupled thermogravimetryinfrared spectroscopy, coupled thermogravimetry-mass spectrometry (MS), coupled thermogravimetry-gas chromatography (TG-GC), coupled TG-GC-infrared (IR), coupled TG-GC-MS. Amit K. Naskar and Prajna P. De contributed the third chapter dealing with the ‘applications of DSC and TGA for the characterisation of rubbers and rubbery materials’. DSC finds applications in locating phase transitions (such as glass-rubber transition, crystalline melting), chemical reactions (such as crosslinking, oxidation and degradation) and micro and macro morphological changes. Glass-rubber transition temperature can be used to study composition and morphology of block copolymers, rubber blends, interpenetrating polymer networks, filled polymers and crosslinking. DSC scans are useful in characterising melting of crystalline polymers and crystallisation of molten polymers. DSC scans along with TG thermograms provide useful information on the pyrolytic decomposition and oxidative degradation of polymers at elevated temperatures. Compositional characterisation of rubber products can be made by TGA, when performed under nitrogen and air environments. TGA can be used to study blend compatibility and rubber degradation kinetics. Combination of other analytical tools such as Fourier transform infrared (FT-IR), gas chromatography and mass spectrometry is extensively used for the analysis of evolved gases in TGA. Recent developments in instrumentation and of software based data analysis made these tools easy and less time consuming for faster qualitative and quantitative evaluation of the components in rubber products. The fourth chapter entitled ‘Dynamic Mechanical Analysis (DMA) for characterisation of polymers, polymer blends and composites’ by Suman Mitra, Kinsuk Naskar and Anil K. Bhowmick deals with theoretical considerations on viscoelasticity of polymeric materials, instrumentation, and application areas. The authors discuss case studies on interpretation of dynamic mechanical spectra of glassy polymers, crystalline polymers, and elastomers. DMA provides useful information on modified rubbers, crosslinking of rubbers, influence of fillers on rubber properties, composition and compatibility of rubber blends, block copolymers and rubber-based nano-composites. Dependence of dynamic properties on frequency and strain is useful in understanding the processing characteristics, tyre performance, vibration isolation and fatigue behaviour. R.S. Rajeev and P.P. De contributed the fifth chapter on ‘Characterisation of rubbers, polymers and their composites using TMA’. TMA conducted with zero load is known as thermodialometric analysis (TDA). An interchangeable sample probe permits the determination of penetration, expansion, tension and dilatometry of samples. The dilatometer mode is used to measure the coefficient of thermal expansion of polymers. TMA of elastomers via a penetrometer probe produces thermograms that closely resemble the master curves obtained by conventional time-temperature superposition 3

Thermal Analysis of Rubbers and Rubbery Materials of modulus data. The most commonly used probe is expansion probe. TMA is believed to be more sensitive than DSC for the measurement of glass transition temperature (Tg) of crosslinked materials, filled materials and composites. Thermally stimulated creep, thermally stimulated recovery and thermally simulated stress relaxation are different ways of performing thermo-mechanical experiments. TMA appears to have high potential in polymer rheology, in which it uses its accessory parallel plate rheometer for measuring the ‘gel time.’ TMA can be used to measure the crosslink density. TMA is widely used in the analysis of fibres. It measures coefficient of thermal expansion, thermal shrinkage, shrinkage force, Tg and Tm and kinetics of shrinkage and shrinkage force phenomena. The sixth chapter of the book is entitled, ‘Micro-Thermal Analysis of Rubbery Materials’ contributed by Kinnari Shelat, Namita Roy Choudhury and Naba K. Dutta. Integration of thermal analysis and microscopic imaging at the micron level resulted in micro-thermal analysis (μTA). The authors discuss the basic principles of μTA and its modes. Imaging of the sample and obtaining thermal conductivity and diffusivity images provide visualisation of sample morphology and spatial arrangement in complex systems. Characterisation of scanned surface involves different forms of localised thermal analysis. This feature provides the facility to perform localised thermo-mechanical analysis, localised dynamic mechanical analysis and localised rheometry experiments. Furthermore, the mode can be used in combination with other techniques such as FT-IR and GC-MS. The authors review the application of μTA in morphological investigation of polymer blends, thin films or coatings for specialised applications such as paints, protective layers, electronic devices and automotive products. μTA can be used to estimate the thickness and homogeneity of the coated film. ‘Miscibility, morphology and crystallisation behaviour of rubber based polymer blends’ by Z. Qui, T. Ikehara and T. Nishi is the subject matter of seventh chapter. In the first section the authors review the miscibility and crystallisation behaviour of polymer blends based on biodegradable polymers and rubber. The second section deals with the morphology and crystallisation behaviour of thermoplastic elastomers (TPE) based on polyamide (PA). While the miscibility studies in the first section are based on changes in Tg and melting temperature (Tm), the spherulite morphology and growth were studied by using polarising optical microscope. Role of compatibilisers on the morphology of thermoplastic vulcanisates based on dynamically vulcanised ethylene-propylene diene terpolymer (EPDM)/PA blends have been studied by measuring crystallisation kinetics and atomic force microscopy images in the second section. An immiscible or partially miscible blend does not typically show depression in melting point, in contrary to what is observed in the case of a miscible blend wherein there occurs depression in melting point with increasing content of the amorphous phase. The melting point is affected not only by thermodynamic factors, but also by the morphological factors such as the crystalline lamellar thickness.

4

Introduction Chapter 8 entitled ‘Thermal characterisation of polymer nano-composites’ is a contribution made by Musa R. Kamal and L.L. Ionescu-Vasii. In the beginning the authors have dealt with principles, instrumentation and methodology of TGA and DSC. They then discussed thermal stability of nano-fillers and nano-filler –polymer composites by using TGA. Final section deals with applications of DSC and temperature modulated DSC (TMDSC) for thermal characterisation of polymer nano-composites. For example, DSC can be used to study the isothermal and non-isothermal crystallisation kinetics providing insight into the nucleating agents for crystallisation, while the heat capacity and the glass transition behaviour of polymer nano-composites can be determined by using TMDSC. The authors conclude by providing a brief description of new thermal characterisation techniques and their application to nano-composites such as thermal conductivity and micro-thermal analysis. Ivan Krakovsky, Yuko Ikeda and Shinzo Kohjiya deal with ‘Thermal analysis in understanding rubbery matrix and rubber-filler interactions’ in Chapter 9. The authors begin their chapter with classification of the methods of thermal analysis on the basis of thermodynamics. Next the authors review the structure-property relationships in particulate filler/rubbery matrix systems and description of the shape and space distribution of filler particles. Finally, the authors describe applications of the following thermal analysis techniques to understand filler-to-matrix and filler-to-filler interactions: TG, DETA and thermo-conductometry, magnetic thermal analysis, DMA, DTA and DSC. For example, DSC is used mainly for the investigation of the presence of filler on the mobility of polymer chains, which is reflected in the change of Tg of the elastomer. The change is most pronounced in the case of nano-fillers. Filler reinforcement of rubber can be understood from DMA studies at small deformations and at large strains. Dependence of magnetic susceptibility on temperature depends strongly on geometrical form of carbon blacks. Curie’s paramagnetism originating from localised spins at structural defects of carbon black crystalline structure can be followed by magnetic thermal analyses. Chapter 10 entitled ‘Study of crystallisation of natural rubber with differential scanning calorimetry’ is contributed by Seiichi Kawahara. The overall rate of crystallisation of natural rubber (NR) may be estimated from the half-life of the crystallisation as an inflection point determined in a plot of degree of crystallinity versus crystallisation time. The fatty acids and branching points play important roles in the crystallisation. Removing free fatty acids that are present as a mixture may suppress the crystallisation, whereas it is recovered to the original level by mixing with stearic acid 1 wt%. The saturated fatty acids may play a role of the nucleating agent for crystallisation of NR. Chapter 11 contributed by Nikhil K. Singha deals with ‘Thermal properties of chemically modified elastomers’. Polymers can be chemically modified to induce changes in important properties such as weatherability, oxidation resistance, adhesion properties, biodegradability, fire resistance, thermal resistance, polarity and reactivity towards specific groups or ions and so on. Modifications include hydrogenation, epoxidation, halogenation, hydrohalogenation, and the thermal techniques include DSC, TGA and 5

Thermal Analysis of Rubbers and Rubbery Materials DMTA. Chemical modification by the grafting of maleic anhydride (MA) is a very useful way to induce polarity in conventional elastomers. Mechanism of maleation can be understood from the changes in Tg. For example, MA-grafting in the case of NR causes an in increase in Tg due to possible interchain interaction between the polar groups. However, there is a decrease in Tg in the case of MA-grafted EPDM due to maleation occurring in the pendant ethylidene norbornene site, thereby increasing the bulkiness of the pendant group and subsequent plasticisation in intermolecular chains. Changes in thermal stability can be followed by TG. Elastomers can also be modified by introducing ionic groups in the form of sulfonate, phosphonate and carboxylate groups. Ionomers are interesting polymeric materials which have a small amount (less than 10%) of ionic groups. The presence of a low amount of ionic groups has a dramatic effect on the physical and mechanical properties of polymers. The elastomeric ionomers can be identified by DMTA. ‘Thermal analysis of rubber products’ by R.S. Rajev and P.P. De is the subject matter of Chapter 12. In the case of product analyses or reverse engineering with the aim for formula reconstruction, single technique will not serve the purpose. Combination of thermal techniques, jointly with spectroscopic, chemical and microscopic techniques is required for qualitative and quantitative analyses of the components of the product. The authors have chosen representative rubber products such as rubber based vibration control devices, rubber seals, rubber-based cable sheathing compounds, rubber-based adhesives, rocket motor insulator, thermal interface materials, and automobile tyres. The authors have provided lists of rubbers and other additives normally used in such applications. Thermal analyses techniques used include DSC, TG, DTG and DMTA in combination with other methods of analyses. In Chapter 13, Amit Naskar deals with ‘Thermal analysis in recycling of waste rubbery materials’. Disposal of solid wastes is a serious challenge to the society and scrap polymeric materials make a major contribution to the solid wastes. Recycling of waste rubbery materials is thus an important area of research from environmental and resource constraints point of view. The authors discuss thermal techniques for characterising waste rubbers and for evaluation of blends and composites based on recycled or waste rubber. DSC, TG and DTG in combination with DMTA and spectroscopic techniques are useful in characterising the rubbers and the additives present in the waste. Chemical modification of ground waste rubber has been studied with a view to enhance its compatibility with other polymers. In the case of TPE based on rubber-plastic blends, considerable proportion of the rubber phase can be replaced by waste finely ground rubber with little deterioration in final properties. The blend morphology can be studied by measuring the Tg, tan  and microscopic studies. DMTA is also useful in evaluating recycled rubber modified bitumen, concrete and other composites. A pyrolytic TG study of waste rubber helps in estimating the operating conditions for converting waste rubbery materials into activated carbon.

6

Introduction ‘Thermal analysis of biological molecules and biomedical polymers’ by N.D. Tran, N.K. Dutta, N. Roy Choudhury forms the contents of the last chapter. DSC and isothermal titration calorimetry are emerging as the important tools in the functional analysis of proteins, lipids and nucleic acid molecules, ligand binding and fundamental understanding of DNA-drug, phospholipid-ligand and protein-protein interactions. Thermal behaviour of biopolymers and the effect of water on the molecular dynamics can be studied by DSC. It is emphasised that most hydrated biopolymers have glass transitions is affected due to the freezing of the cooperative motions of biopolymers and bound waters. The authors have discussed important roles played by thermal techniques in elucidating the morphological structure of collagen, collagen-based biomaterials, bio-artificial polymeric materials and biopolymers for drug delivery.

References 1.

J.B. Hannay, Journal of the Chemical Society, 1877, 32, 399.

2.

W.M. Ramsay, Journal of the Chemical Society, 1877, 32, 395.

3.

H. Le Chatelier, Comptes Rendus, 1886, 102, 1243.

4.

H. Le Chatelier, Comptes Rendus, 1887, 104, 1517.

5.

N.C. Roberts-Austen, Proceedings of Royal Society, 1891, 49, 347.

6.

W.E. Knowles Middleton, A History of the Thermometer and its use in Metrology, John Hopkins Press, Baltimore, MD, USA, 1966.

7.

W.J. Moore, Physical Chemistry, 4th Edition, Prentice-Hall Englewood Cliffs, NJ, USA, 1972.

8.

K.E.J. Barrett, Journal of Applied Polymer Science, 1967, 11, 1617.

9.

R.N. Rogers and E.D. Morris, Jr., Analytical Chemistry, 1966, 38, 412.

10. S. Arrhenius, Journal of the American Chemical Society, 1927, 49, 3033. 11. G.O. Piloyan, I.D. Ryabchikov and O.S. Novikova, Nature, 1966, 212, 1229. 12. S. Glasstone, Textbook of Physical Chemistry, 2nd Edition, Macmillan, London, UK, 1962, p.1098. 13. H.J. Borchardt and F.J. Daniels, Journal of American Chemical Society, 1956, 79, 41.

7

Thermal Analysis of Rubbers and Rubbery Materials 14. N.S. Schneider, J.F. Sprouse, G.L. Hagnauer and J.K. Gillham, Polymer Engineering and Science,1979, 19, 304. 15. T.A.M.M. Mass, Polymer Engineering and Science, 1978, 18, 29. 16. R.B. Prime in Thermal Characterisation of Polymeric Materials, Ed., E.A. Turi, Academic Press, New York, NY, USA, 1981, Volume 1, Chapter 5. 17. A.A. Duswalz, Thermochimica Acta, 1974, 8, 57. 18. E.S. Freeman and B. Carroll, Journal of Physical Chemistry, 1958, 62, 394. 19. E.S. Freeman and D.A. Anderson, Journal of Polymer Science, 1961, 54, 253. 20. H.C. Anderson, Journal of Polymer Science: Part B, 1964, 2, 115. 21. L. Reich, H.T. Lee, D.W. Levi, Journal of Applied Polymer Science, 1965, 9, 351. 22. L. Reich, Journal of Polymer Science: Part B, 1964, 2, 621. 23. C.D. Doyle, Journal of Applied Polymer Science, 1961, 5, 285. 24. C.D. Doyle, Journal of Applied Polymer Science, 1962, 6, 639. 25. T.J. Ozawa, Journal of Thermal Analysis, 1970, 2, 301. 26. W.J. Smothers and M.S. Yao Chiang, Handbook of Differential Thermal Analysis, Chemical Publishing Co., New York, NY, USA, 1966. 27. P.D. Garn, Thermoanalytical Methods of Investigation, Academic Press, New York, NY, USA, 1965. 28. Thermal Analysis, Eds., W.W. Wendlandt and L.W. Collins, Dowden Hutchinson and Ross Publishing Company, Stroudsburg, PA, USA, 1976. 29. Differential Thermal Analysis, Volumes 1 and 2, Ed., R.C. Mackenzie, Academic Press, London, UK, 1970. 30. J.J. Maurer in Thermal Characterisation of Polymeric Materials, Ed., E.A. Turi, Academic Press, New York, NY, USA, 1981, Volume 1, Chapter 6. 31. A.K. Sircar in Thermal Characterisation of Polymeric Materials, Ed., E.A. Turi, Academic Press, New York, NY, USA, 1997, Volume 2, Chapter 5. 32. J.L. Leblanc, Journal of Applied Polymer Science, 1977, 21, 2419. 8

Introduction 33. D.E. Smith, Thermochimica Acta, 1976, 14, 370. 34. G.J. Mol, Thermochimica Acta, 1974, 10, 259. 35. R. Sengupta, S. Sabharwal, A.K. Bhowmick and T.K. Chaki, Polymer Degradation and Stability, 2006, 91, 131. 36. S.K. Srivastava, M. Pramanik and H. Acharya, Journal of Polymer Science: Part B - Polymer Physics, 2006, 44, 471. 37. A.C. Momin, E.B. Mirza and M.D. Mathews, Thermochimica Acta, 1991, 180, 191. 38. V. Samouillan, C. Ande, J. Dandurand and C. Lacccabanne, Biomacromolecules, 2004, 5, 958. 39. N.A. Grunina, T.V.Belopolskaya and G.I. Tsereteli in Statistical Physics of Ageing Phenomena and the Glass Transition, Eds., M. Henkel, M. Pleimling and R. Sanctuary, Journal of Physics: Conference Series, Volume 40, 2006, p.105. 40. J.J. Mauer, Rubber Chemistry and Technology, 1969, 42, 110. 41. A.K. Sircar and T.G. Lamond, Rubber Chemistry and Technology, 1972, 45, 329. 42. A.K. Sircar and T.G. Lammond, Rubber Chemistry and Technology, 1973, 46, 178. 43. A.K. Sircar and T.G. Lammond, Thermochimica Acta, 1973, 7, 287. 44. A.K. Sircar and T.G. Lammond, Rubber Chemistry and Technology, 1975, 48, 301. 45. A.K. Sircar and T.G. Lammond, Rubber Chemistry and Technology, 1975, 48, 631, 640, 653. 46. A.K. Sircar and T.G. Lammond, Journal of Applied Polymer Science, 1973, 17, 2549. 47. A.K. Sircar and T.G. Lammond, Rubber Chemistry and Technology, 1978, 51, 647. 48. D.W. Brazier and G.H. Nickel, Rubber Chemistry and Technology, 1975, 48, 26. 49. D.W. Brazier and G.H. Nickel, Rubber Chemistry and Technology, 1975, 48, 661. 9

Thermal Analysis of Rubbers and Rubbery Materials 50. D.W. Brazier and G.H. Nickel, Thermochimica Acta, 1978, 26, 399. 51. D.W. Brazier, G.H. Nickel and Z. Szentgyorgyi, Rubber Chemistry and Technology, 1980, 53, 160.

10

Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials

2

Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials Prajna Paramita De

2.1 Introduction A single property by which a material can be indentified is its thermal behaviour. Characterisation of materials by thermal analysis was introduced by Le Chatelier [1] in 1886, who used differential thermal analysis (DTA) for the first time to study the thermal characteristics of clay materials. The method consisted of direct determination of the rate of change in temperature of the sample during regular heating. The reactions followed in this way gave a series of plateaux on temperature versus time plots, but their determination was rather inaccurate. In 1891 Roberts-Austen [2] introduced a differential thermocouple which measured the difference in voltage between thermocouples placed in the sample and in an inert standard. After this, continuous developments were made, which included high pressure DTA and differential scanning calorimetry (DSC), thermogravimetry (TG), derivative thermogravimetry (DTG) and simultaneous thermal analysis like TG-DTA, TG-DTG-DTA. A detailed account of the developments of thermal analysis can be found in Smothers and Chiang [3], Garn [4], Wendlandt [5] and Blazek [6]. The term ‘thermal analysis’ now applies to a series of techniques, all of which subject a sample to a programmed temperature treatment, and use a variety of transducers to sense property changes continuously and automatically. Thermoanalytical techniques are presented in Figure 2.1, in which thermal changes related to mass, temperature, energy, dimension and mechanical properties of materials are exhibited as TG, DTG, evolved gas analysis (EGA), evolved gas detection (EGD), (DTA), (DSC), thermo dilatometry (TD), thermo mechanical analysis (TMA), dynamic mechanical analysis (DMA). There are also many other thermoanalytical techniques related to acoustic, optical, electrical and magnetic properties of the materials as shown in Figure 2.1. In general, thermal analysis is important for the study of thermal decomposition, solid state reaction, determination of moisture and volatile matter, pyrolysis of coal, petroleum, wood, decomposition of explosive materials, adsorption, desorption, rate of evaporation and sublimation but for polymers and rubber, thermal analysis is used for determination 11

Thermal Analysis of Rubbers and Rubbery Materials

Figure 2.1 Classification of thermoanalytical techniques

of glass transition temperature (Tg), melting of polymers, crystalline transition, thermal stability, assessment of life of finished products, development of a new compound, miscibility of polymers and rubbers, shrinkage, rigidity and so on. Popular techniques of thermal analysis of rubbers and polymers, are given in Table 2.1. The details of thermal analysis instruments, principles, and techniques will be discussed next.

2.2 Differential Thermal Analysis (DTA) It is a technique in which the difference of temperature ( T) between a substance and a reference material ( - Al2O3) against either time (t) or temperature (T) is recorded as the two specimens are subjected to an identical temperature regime in an environment 12

Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials

Table 2.1 Common techniques of thermal analysis used for characterisation of polymers and rubbers 1 2

3 4 5

6 7 8 9 10 11

DTA DSC: a. Heat flux DSC b. Power compensated DSC c. Modulated DSC (MDSC) or temperature modulated DSC (TMDSC) a. Thermogravimetric analysis (TGA) or TG b. DTG a. EGA b. EGD TMA: a. Parallel plate rheometry (PPR) b. Fibre tension spectrometry. c. Stress relaxation spectrometry. d. TD or thermo dilatometric analysis (TDA) DMA Torsional braid analysis (TBA) Thermally stimulated depolarisation current (TSDC) Relaxation map analysis (RMA) Differential photo calorimetry (DPC) Dielectric thermal analysis (DETA) or dielectric analysis (DEA)

heated or cooled at a controlled rate. In a DTA curve the temperature difference ( T) is usually plotted on the ordinate and T or t on the abcissa increasing from left to right. In a DTA curve an endothermic peak is a peak where the temperature of the sample falls below that of reference material i.e., T is negative. An exothermic peak is a peak, where the temperature of the sample rises above that of reference material, i.e., T is positive. In general, in an endothermic reaction heat is absorbed and in exothermic reaction heat is evolved. With respect to the fixed position of reference and sample, the endothermic peak is always downwards, while the exothermic peak is upwards in T versus T(t) plots. Generally, the phase transition like melting, solvent evaporation, dehydration and decomposition reactions are endothermic in nature, whereas crystallisation, oxidation, adsorption and certain solid state reactions are exothermic.

2.2.1 DTA Instrument A typical DTA apparatus [7] is illustrated schematically in Figure 2.2. The apparatus consists of: 13

Thermal Analysis of Rubbers and Rubbery Materials a. sample holders, b. furnace and furnace temperature programmers, c. differential temperature detection system i.e., a temperature sensor, d. a low level dc voltage amplifier, e. recorder, and f.

atmosphere control.

There are numerous DTA systems described in literature, many of which use novel designs of sample holders, furnace, heating device, differential temperature (Ts-Tr) detectors such as thermocouples, platinum resistance thermometers (PRT) and thermopiles and recorders. Details of each part are given in the next sections:

Figure 2.2 Schematic diagram of a typical DTA apparatus [8]

2.2.1(a) Sample Holders Sample holders are of various shapes, sizes, depending on the nature of the reaction to be studied. Hence, common containers have been constructed from aluminium (crimples used by TA Instruments), stainless steel, nickel, platinum or platinum alloys, 14

Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials fused quartz, boron nitride and graphite according to the temperature requirement for the experiment. Wendlandt and co-workers [8] described these sample containers in ‘Thermal Characterisation of Polymeric Materials’.

2.2.1(b) Furnace and Furnace Temperature Programmers A wide variety of DTA/DSC furnaces are available each designed for a specific temperature range from –150 to 2800 ºC. Most of the furnaces have resistance heater elements, but some use infra red heating for extremely rapid heating and cooling rates. Maximum temperature limits for the various resistance heater elements for the furnaces are shown in Table 2.2. Requirements for a good DTA/DSC furnace include symmetry in heating and the ability of the heater elements to heat uniformly. The furnace temperature distribution must be uniform in the area of the sample container for good results. For operation at low temperature, the furnace may be surrounded by a Dewar flask and precooled with liquid nitrogen. Most temperature programmers do not function efficiently unless a thermal reservoir at least 30 ºC below the temperature is available. The type of temperature controller varies from the simple variable voltage transformer, coupled to a synchronous motor to the more sophisticated feedback, proportional type controller. On the other hand, controllers of the on-off type cannot be used, because the fluctuating power outputs give rise to severe thermal gradients in the furnace and sample holder system. Most commercially available thermal analysis equipment, however, comes

Table 2.2 Approximate maximum temperature limits for furnace resistance elements Serial Element Number 1 Nichrome 2 Chromel A 3 Kanthal 4 Platinum 5 Platinum - 10% rhodium 6 Kanthal Super 7 Rhodium 8 Molybdenum 9 Tungsten Ox = Oxidising Nox = Non oxidising (inert, vacuum)

Approximate Temperature, °C 1000 1100 1350 1400 1500 1600 1800 2200 2800

Required Atmosphere Ox Ox Ox Nox, Ox Nox, Ox Ox Nox, Ox Nox, H2 Nox, H2

15

Thermal Analysis of Rubbers and Rubbery Materials with a specially matched and prepackaged controller as for example, the Model QC25 controller, provided by the Omnitherm corporation, TECO Model TP-2000 Thermocouple Temperature programmer, or TA Instruments furnace temperature programmer.

2.2.1(c) Differential Temperature Detection System The choice of the temperature detection device depends on the nature of the instrument, maximum temperature desired, chemical reactivity of the sample. The most common means of differential temperature detection is with thermocouples, thermistors thermopiles, or platinum-resistance thermometers. Commonly used thermocouples are given in Table 2.3.

2.2.1(d) Low Level DC Voltage Amplifier The output voltage from a differential thermocouple is in the order of 0.1-100/V, depending on the type of thermocouples used (Table 2.3) and the temperature difference between them. Hence, unless a very sensitive recording system is used ( SBR ~ NBR (34% ACN) > NBR (27% ACN) > NR> EPDM ~EPM. Jana and co-workers used the same method for characterisation of DCP curing of low-density polyethylene (LDPE) - silicone rubber blends [147]. Incorporation of LDPE in silicone rubber increased the activation energy of vulcanisation. Peroxide curing of PE-co-methyl acrylate-silicone, and EVA-EPDM blends was also investigated using similar methodology [148, 149]. DCP curing of rubber was found to be mostly first order reaction [147-149]. Brazier and co-workers reported a drawback of DSC measurements of curing kinetics of rubbers indicating several parallel exothermic reactions such as formation of noncrosslinking sulfidic products, and maturation or 86

Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials subsequent polysulfide reactions were taking place [150]. Rheometric measurement of kinetics appeared to be more accurate as the rheometer responds only to crosslinking and chain scission reactions [151]. An advantage of DSC cure monitoring of rubber is the unique exotherm patterns (‘fingerprint’) for different cure systems [152]. Manna and co-workers investigated exothermic crosslinking of ENR by ISAF carbon black in the presence of silane coupling agent using DSC [153]. Usually the Borchardt and Daniel method is not applicable in any of the following: (i) overlapping reaction peaks, (ii) decomposition occurring during curing, and (iii) autocatalytic reactions. To overcome limitations of Borchardt and Daniel method ASTM E698 method [142, A] was developed based on the Ozawa method of studying kinetics using DSC [143]. Again the Ozawa method is a modified form of Kissinger’s method. According to Kissinger’s method [154], it is assumed that the reaction rate varies with temperature, and at the temperature corresponding to the DSC exotherm maximum (Tm) the rate of reaction is the maximum. Lee and co-workers studied the curing kinetics of rubber modified epoxy resin using both the Kissinger and the Ozawa (isoconversional) methods [155]. Chough and Chang measured the kinetics of sulfur vulcanisation of NR, BR and SBR using the Ozawa method [156]. It was observed that the overall rate of vulcanisation followed the order: SBR>BR>NR. Vulcanisation kinetics of octadecyl amine modified clay-NR nanocomposite indicated influence of modified clay to lower the activation energy of vulcanisation [157]. Ginic-Markovic and co-workers utilised the modulated DSC kinetics results to match the curing characteristics of automotive seals based on EPDM rubber and PU coating for enhanced adhesion between rubber and coatings [158]. Crane and co-workers used both differential-integral analysis of a single dynamic DSC data and multiple scan at different scan rate method to obtain E, n and heat of crosslinking of adhesives [159]. Hayes and Seferis studied curing and high temperature degradation kinetics of epoxy using the Ozawa method [160]. Isothermal curing can be modeled as nth order kinetics as expressed in Equation 3.5. Another model includes autocatalytic curing reaction and the rate equation is expressed as: d
k(T) m (1
)n dt

(3.6)

Isothermal curing of silicone elastomers applicable in electronics was found to be Arrhenius type with strong temperature dependence [161]. The kinetic data was useful to solve surface contamination of the product due to deposition of the nonvolatile fraction of uncrosslinked material. Deng and Isayev applied isothermal rubber curing kinetic data at different temperatures to estimate state of curing of rubbers in an injection mould and a compressive press using a nonisothermal model [162, 163]. A modified rate equation, as expressed next, for the isothermal curing of thermoset is also useful for the estimation of kinetic parameters [164, 165]: 87

Thermal Analysis of Rubbers and Rubbery Materials d  (k1
k2 m )(1
)n dt

(3.7)

Keenan validated Equation 3.7 for autocatalytic curing of epoxy adhesives by DSC measurement [166]. The autocatalytic model for cure characterisation of fluororubberclay [167] and NBR-clay [168] nanocomposite was found to be in good agreement with experimental data. Cure kinetic data differed for the composites based on chemically modified and unmodified clay.

3.2.9 Characterisation of Melting and Crystallisation of Polymer Semi-crystalline polymers have a distribution of crystal size in their morphology. Hence, during DSC heating scans, such polymers display a melting endotherm over a broad range of temperatures. The temperature corresponding to the peak of the melting endotherm is termed as the melting point. Characterisation of melting behaviour of polymers is important for predicting the structure and its correlation to the properties. After melting transition in a dynamic DSC, if molten polymer is cooled back at a programmed cooling rate, the melt crystallises and a crystalline exotherm is observed. From a DSC scan, melting point, crystallisation temperature, heat of fusion and heat of crystallisation of the polymers can be estimated. In the cases of binary mixtures, usually the melting point of the polymer matrix is lowered by the diluent. The melting point depression can be expressed by the Flory equation as shown next [169]: 1 1  0 Tm Tm BV  R V2
[1 1 1 ]
Hu V1 1 R Tm

(3.8)

where, Tm0 and Tm are the melting point of pure and diluted polymers, 1 is the volume V2 fraction of diluent, V is the ratio of molar volume of polymer repeat unit and diluent, 1 Hu is the heat of fusion per repeat unit of polymer, B is the interaction energy density for the system, and R the gas constant. A plot of (1/Tm -1/Tm0)/ 1 versus 1/Tm gives the true heat of fusion and the interaction energy density for the system. Heat of fusion is calculated from a melting endotherm by integrating the area under the curve using a proper baseline. The DSC unit should be calibrated with indium, tin or zinc, whose melting point and heat of fusion are known. From the measured heat of fusion ( Hf), % crystallinity in the polymer can be estimated using following relationship: 
100.

88


H f
H 0

(3.9)

Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials where, H0 indicates the theoretical heat of fusion for a 100% crystalline polymer. The reference H0 values are established and can be obtained from literature or polymer handbook [170]. Molecular structure, MW, thermal history and composition are key factors that affect the melting characteristics of polymers. Other than the thermoplastic semi-crystalline materials a few classes of rubbery materials such as thermoplastic elastomers (rubber/plastic blend, block copolymers), and linear rubber molecules (BR, IR, NR, PDMS, CR) display crystallinity.

3.2.9.1 Melting and Crystallisation of Flexible Chain Rubbers Molecular architecture, morphology, composition, and presence of foreign matter are the different factors that affect crystallisation and melting behaviour of elastomeric materials. It has been shown that the green strength of NR is higher than that of the other synthetic rubbers at room temperature. The reason for that is the strain-induced crystallisation of NR at the ambient testing condition [171-173]. NR being of 100% cis-polyisoprene undergoes crystallisation due to orientation of polymer chains during stretching. Generally the crystallisability of the synthetic cis-polyisoprene rubbers correlates with the average cis sequence lengths of the polymer main chain [174]. Decrease in average cis sequence length decreased the heat of fusion of the crystallised samples. Strain induced crystallisation of cis1,4-polybutadiene [175-177], polyisobutylene [178, 179], EPDM [180], and polychloroprene [181, 182] are also well known. Degree of crystallisation obtained by stretching follows the order BR highly aromatic oil (295 C ) > naphthenic oil (253 C) [265]. Swarin and Wims established decomposition temperatures for various plasticisers used in NBR vulcanisates [357]. The type of carbon black present in a butyl elastomer influences the decomposition step involving oxidation of black. High surface area carbon black decomposed in air at a lower temperature than that, that occurred with the low surface area carbon black [303]. Structure of the carbon black (high versus low structure) affects decomposition temperature/time of the black [358]. For a type of carbon black, cure system also 113

Thermal Analysis of Rubbers and Rubbery Materials

Figure 3.15 Schematic of a TGA thermogram of a rubber vulcanisate for compositional analysis

Table 3.11 Decomposition temperature various rubber compounding ingredients Material Plasticiser, Naphthenic oil Highly aromatic oil Paraffinic oil Dibutyl phthalate Dioctyl adipate Dioctyl phthalate Dioctyl sebacate Carbon black: HAF FEF GPF SRF Graphite Calcium carbonate

Temperature (°C)

Environment

253 [265] 295 [265] 381 [265] 220 [357] 255 [357] 264 [357] 282 [357]

Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen

545 [265]; 542-607 [368]

Oxygen

567-621 [368] 550 [265] 585-632 [368] 760 [357], 800 [265] 825 [265]

Oxygen Oxygen Oxygen Oxygen Nitrogen/oxygen

influences the black decomposition temperature in air [358, 303]. The oxidation temperature of carbon black residue from a pyrolysed vulcanisate depends on mixing procedure and loading of carbon black in the elastomer [358]. Therefore, unless otherwise the black surface area or particle sizes are distinctly different, in a vulcanisate containing different types of black, it is very difficult to identify the types of blacks by 114

Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials dynamic TGA [359]. Sometimes isothermal scans at ~540 C under a mild oxidising atmosphere are useful in identifying different blacks in a vulcanisate of char forming Neoprene [358, 360].

Blend Vulcanisates

Elastomer blends containing two or three rubber components can be analysed by TGA, easily, if the components have significantly different thermal stabilities. In this case the DTG thermogram indicates distinct peaks for different components. If the thermal stability of the individual components is close, the decomposition of the rubbers in the blend is usually overlapped. DTG plots for both the types of blends can be used to identify the elastomer composition. Many DTG data (at a specified heating rate) for known blend compositions available in literature can be used as a fingerprint to detect the unknown blend composition [238, 268]. DTG peak temperature of a rubber usually depends on the presence of other rubbers (if any), type of filler and cure system [303]. For qualitative and quantitative prediction of rubber formulation other complementary techniques such as pyrolysis-FTIR, pyrolysis GC-mass spectroscopy, sulfur analysis, and DSC are also used [77]. Sircar and co-workers in a series of works characterised various blend composition of rubber vulcanisates using TGA [76, 101, 237-239, 262]. Maurer [360], Brazier and Nickel [268], Amraee and co-workers [361], Shield and co-workers [367] and Swarin and Wims [357] also employed TGA as a primary tool for compositional characterisation of various elastomer blends. DTG curves of the vulcanisates displayed characteristic peaks of the rubber components. The specific peak intensity depends on the content of the corresponding rubber component in the vulcanisate. For example, in a blend of NR/SBR the peak due to the decomposition of NR in blends would increase with increase in NR content in the blend composition [76]. For quantitative analysis of the DTG data a calibration plot of peak height ratio against composition is usually used to predict relative proportion of the rubbers in the blend [237, 268, 360, 361]. Shield and co-workers [367] found that for the blend of SBR and NBR, temperature corresponding to 70% weight loss linearly decreased with increase in SBR content in the blend. Such statistics are useful to predict composition of an unknown SBR/NBR blend. Comparative TGA data of known compositions of NR/BR, NR/SBR and NR/EPDM blends are displayed in Figure 3.16. As discussed earlier the decomposition temperatures of the rubber components follow the trend EPDM > BR ~ SBR > NR. Therefore, from Figure 3.16 individual blends can be clearly identified. It is noteworthy that for the distinction of BR and SBR other characterisation tools such as DSC, FTIR and GC/mass spectrometer is necessary. TGA data of an unknown tyre rubber vulcanisate is shown in Figure 3.17. It can be found that the vulcanisate is a multi-component rubber blend. It was estimated from the peak positions that the vulcanisate consists of BR, SBR and NR. The FTIR spectroscopy results of the pyrolysates qualitatively confirmed the predicted composition [77]. Maurer 115

Thermal Analysis of Rubbers and Rubbery Materials

Figure 3.16 DTG thermogram of elastomer blends under nitrogen at 10 C/min: peroxide cured 40/60 NR/SBR ( ); peroxide cured 50/50 NR/BR ( ) [76], and tyre sidewall 40/60 NR/EPDM elastomer ( ) [238]

Figure 3.17 DTG thermogram of an unknown tyre rubber under nitrogen at 10 C/min [77]

in an early work discussed the utilisation of TGA in analysing the vulcanisate composition based on NR/EPDM and NR/SBR/EPDM blends [360]. Brazier and Nickel in their early work successfully demonstrated utilisation of DTG data to predict composition of NR/ BR, NR/SBR and NR/SBR/EPDM blend vulcanisates [268]. 116

Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials Results on TG and DTG analysis of different elastomer blends has been reviewed by various authors [362-364]. A list of prior works on quantitative TG and DTG analysis of various binary/ternary/quaternary rubber blends is displayed in Table 3.12 for further reference. Requirement of an extensive database/library of TGA/DTG plots for various blend compositions with known cure systems is a severe drawback of compositional characterisation of blend vulcanisates using TGA only.

Table 3.12 A list of prior works on compositional characterisation of rubber blend vulcanisates using TGA Blend System NR/BR NR/SBR NR/SBR/BR NR/CR NR/CR/Hypalon NR/CIIR NR/EPDM NR/EPDM/SBR NR/EPDM/CIIR NR/SBR/CIIR NR/SBR/EPDM/CIIR NR/IR SBR/BR SBR/NBR SBR/EPDM SBR/CIIR SBR/EPDM/CIIR BR/CIIR CR/Oil extended BR CR/Oil Extended SBR NBR/EPDM IIR/EPDM

Standard Compound Tyre sidewall (black) Tyre sidewall Truck tyre tread Tyre sidewall (white) Tyre sidewall (white) Tyre innerliner Tyre sidewall (white) Tyre innerliner Tyre sidewall (white) Passenger tyre tread Oil seal Tyre innerliner Tyre sidewall (white) Tyre innerliner Tyre sidewall (white) Tyre sidewall (white) Automotive products -

Reference [76], [268], [365] [76], [268], [366] [76] [101] [101] [262] [360], [238] [360], [268] [238] [262] [238] [237] [361] [367] [360] [262] [238] [262] [101] [101] [357] [368]

3.3.3 Study of Rubber Blend Compatibility Using TGA Thermal analysis is a very common tool to detect polymer compatibility in blends. For these studies, however, DMA, DSC, and dielectric thermal analysis (DETA) are used extensively. TG and DTG data are sometimes useful to detect physico-chemical interactions between blend components. Specific interaction usually leads to either more overlapped degradation region of the polymers or, enhanced thermal stability indicated 117

Thermal Analysis of Rubbers and Rubbery Materials by a higher decomposition temperature of the compatibilised phase or, earlier degradation of the compatibilised phase by mutual catalysed degradation. Co-vulcanisation of individual rubber components sometimes causes compatibility by exhibiting higher mechanical properties and thermal stability [369]. Kader and Bhowmick reported that ACM/FKM miscible vulcanisates containing polyfunctional acrylate exhibited improved thermal stability compared to the individual component vulcanisates containing similar dosage of polyfunctional acrylate [334]. Compatibilised blend vulcanisate of CR and IIR exhibited improved thermal stability as shown by a high temperature degradation of the compatibilised interphase [300]. Reactive compatibilised ethylene methylacrylate (EMA) and PDMS blends with EMA/PDMS ratio of 50/50 and 30/70 exhibited an increased decomposition temperature of EMA phase and decreased decomposition temperature of siloxane phase due to free radical assisted crosslinking of the two phases [324]. Similar results were obtained with NR/ EPDM gum unvulcanised blend compatibilised by mercapto-modified EPDM. The compatible blend formed a crosslinked gel due to the reaction of the mercapto group and NR [370]. The EMA/ENR compatible blends also exhibited similar results [371]. PVC/NBR blends are miscible. Recycled PVC/NBR blends show lower properties compared to that of the virgin PVC/NBR blend. Use of compatibiliser such as acrylic acid enhanced the properties of the blends, but lowered the thermal stability possibly due to mutual catalysed degradation [372]. In a ternary blend system of PVC/EVA/PS-co-acrylonitrile (SAN) blend, PVC acts as compatibiliser for the immiscible SAN/EVA system and the overall thermal stability of the ternary blend appears superior to the homopolymers and PVC/SAN and SAN/ EVA blends [373]. A conjugated structure formed by degradation of a polymer (like dehydrohalogenation of PVC) acts as free radical scavenger to enhance the thermal stability of the other component in the blend [374]. Both NBR and EVA exhibit twostep degradation. With increase in NBR content in the NBR/EVA blend, an increase in the initial and final decomposition temperatures were observed due to interfacial interaction [375]. The peroxide cured blend composition exhibited higher decomposition temperature than the sulfur vulcanisation system due the stable C-C bonding in the former. Decomposition temperature of ENR/PMMA partially miscible blend increased with increase in ENR content and the degree of epoxidation in ENR [376]. TGA studies on various other compatibilised blend compositions such as PP/EPDM [377], PE/EPDM [314], LDPE/PDMS [378], polyaniline/EPDM [379], SBR/NBR [367], silicone/EPDM [380], NBR/CSM and CR/CSM [381] have been reported in literature.

3.3.4 Study of Rubber Degradation Kinetics Using TGA In earlier sections (Section 3.2.8), the role of DSC in monitoring kinetics of rubber vulcanisation has been summarised. Similar methods are applicable for thermal degradation of rubber or melt crystallisation of semi-crystalline thermoplastic elastomers. 118

Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials In those methods rate of heat flow associated with the reaction or phase changes was monitored. Kinetics of thermal and oxidative degradation of elastomers and raw rubbers can also be monitored by recording rate of weight loss during thermal treatment under a specific environment. Here, again, two basic approaches in measuring kinetic parameters are isothermal methods and non-isothermal (dynamic) methods. The dynamic analysis is the most commonly followed method [311]. In TG analysis the fraction of materials reacted at time t is estimated as , where: 


ms
m ms
mf

and ms, m, mf are initial mass, mass at time t and the final mass at the end of reaction. d dW where W is the mass reading (in TGA) data which corresponds Therefore,
 dt dt to (1- ). The Freeman and Carroll method is used extensively for the analysis of TG degradation of rubbers and other polymers [382]. According to this method: d dt
n
E d(1 / T) d ln(1
) R d ln(1
) d ln

(3.11)

where, n is the order of degradation reaction, E is the activation energy of the reaction, and T is the temperature that is function of time and . d d(1 / T) dt Data of d ln(1
) against d ln(1
) would result in a linear plot with a negative slope of E/R and intercept n. For accurate estimation of n, usually the data in the vicinity of DTG maxima is plotted. However, the kinetic parameters determined in this method were found to be dependent on sample mass and heating rate [382]. d ln

Another general method, commonly known as multiple heating rate method, proposed by Friedman [383] deals with comparison of rates of conversion (d /dt) at different levels of conversions measured at various heating rates q. In this method: ln q

d E  n ln(1
)
ln A
dT RT

(3.12)

d
For various constant isoconversion method, ln q versus 1/T data would give different linear plots for the estimation of n and E. dT

Other than the previous differential approaches, various integrated methods were proposed for evaluation of degradation kinetic parameters. In these methods: 119

Thermal Analysis of Rubbers and Rubbery Materials q

d d A (
E/RT)
A.e(
E/RT) f() or,
e dT dT f() q

(3.13)

The integral function of conversion of above is usually expressed as g( ). Based on various approximations to find g( ) several methods were proposed to find the activation energy (E) and frequency factor (A). Commonly used methods were proposed by Doyle [384], Flynn and Wall [385], Ozawa [386], Kissinger [154] and Coats and Redfern [387]. Later Flynn and Dickens proposed a method for estimation of kinetic parameters where effect of sample history was minimised [388]. Various approximations, advantages and limitations of all these methods have been extensively reviewed by Hatakeyama and Quinn [389], Wendlandt [390], and Flynn [391]. Table 3.13 summarises the published kinetic parameters for a few selected raw rubbers and the compounded vulcanisates. Activation energy (E) of NR varied with degree of conversion indicating complex decomposition reactions of NR involving multiple steps and alteration of overall rate of reaction with extent of conversion. However, two major decomposition peaks were observed in the DTG curve of NR [392]. Similar results were also obtained with car tyre rubber [393]. Based on an assumption that degradation of rubber is a single order reaction, Williams and Beslar found that with increase in heating rate the E and frequency factors for NR and SBR degradation decreased significantly [394]. The value of E for thermal degradation of IR was reported to be 157 kJ/mol [298]. Addition of fillers increased the activation energy [298]. The greater the filler-rubber interaction, the greater the activation energy associated with degradation. Chlorinated NR (CNR) under oxidative environment decomposes in two steps but leaves no residue. However, single step degradation of CNR under nitrogen leaves 32% carbonaceous residue. The activation energies and frequency factors of the steps increased with increasing heating rate without significantly affecting the order of reaction [271]. The values of true E and frequency factor (i.e., values corresponding to 0 C/min heating rate) were extrapolated from the plots of those at various heating rates. Conesa and Marcilla investigated kinetic parameters of different SBR rubber [395]. Presence of volatiles in oil extended SBR, changes the kinetic parameters significantly. For SBR, the activation energy of decomposition under nitrogen is increased with addition of carbon black, however, the same under oxygen atmosphere is decreased [261]. Filled SBR followed fractional order degradation kinetics under nitrogen, but a first order degradation under oxygen. Under nitrogen the value of E measured by isoconversion method (Flynn-Wall-Ozawa) increased with increase in degree of conversion at a lower black loading, but at higher black loading E remained more or less constant. Under oxygen, E at lower conversion levels was slightly higher. Lin and co-workers attributed BR thermal degradation to two reactions involving two distinct mass losses [396]. The first step involves only 20% weight loss with an activation 120

SBR vulcanisate (30 phr carbon black)

SBR (30% St) SBR-OE (44% St) SBR gum

BR

Medium Number Fractions Activation of Steps Contributed Energy by Each Step (kJ/mol) N 2 0.80 80-210 0.20 125-150 Air 2 239 N 3 0.30 68.4 0.38 219.7 0.32 225.7 O 2 0.60 101.7 0.40 125.0 N 1 1.00 98.6 N 2 0.20 59.8 0.80 197.0 N 3 0.23 45.1 0.64 211.8 0.13 290.1 N 2 0.15 232.2 0.85 289.0 N 2 0.36 72.9 0.64 462.5 N 3 0.35 52.2 0.50 150.6 0.15 169.4 N 1 1.00 251.0 256.4 240.7 O 1* 1.00 169.1 173.0 175.2 -

2.3 x 103 1.5 x 1010 3.5 x 1010 -

-

-

4.7 x 108 2.38 x 108 2.32 x109 2.8 x 103 1.9 x 1013 -

-

-

-

1 4.4 1.2 1.6 1.1 1.1 1.1 1.3 1.5 1.8 2.1 1.4 3.0 1.0 3.7 1.6 2.1 1.3 1.9 -

Frequency Order of Factor Reaction (min-1) -

Flynn-Wall-Ozawa, Friedman, Kissinger [261] Flynn-Wall-Ozawa, Friedman, Kissinger [261]

Friedman [397]

Statistical numerical analysis [395]

Statistical numerical analysis [395]

Statistical numerical analysis [395]

Friedman [396]

Coats–Redfern [271]

Statistical numerical analysis [395]

Freeman and Carroll [263]

Friedman [392]

Method and Reference

Table 3.13 Summary of kinetic parameters for degradation of various rubbers and vulcanisates

CNR (65% Cl)

IR-OE

NR

Rubber

Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials

121

122 2 3

N

N

N

Hypalon 20

CR

2

0.38 0.62 1.00

-

0-0.15 1-0.85 0.04 0.34 0.62 -

1.00

0.13 0.87 0.19 0.81 1.00 7.5 x 1010

147.6 § 140-200 330-365 92.0 112.9 246.6 202.6 94.5 152 ± 25 147 ± 24 281 ± 28 127.0 215 111.8

5.9 x 1013

1.1 1.1 0

-

-

1.5 x 1012 6.5 x 1017 -

-

1 1 -

1.9

2.9 0.6 2.9 1.6 -

-

-

-

-

155.7 260.1 155.7 260.1 140.0

Kissinger [401]

Ozawa [299]

Friedman [298]

Coats–Redfern [311]

Friedman [315]

Friedman [393]

Kissinger [398]

Numerical analysis [399]

Numerical analysis [399]

Kissinger [398]

Method and Reference

Silicone N 1 Freeman-Carroll [343] *Decomposition of carbon black is ignored **Oxidative degradation of EVA at relatively lower temperature than the major degradation step is ignored §Average value of activation energies at different conversion levels §§Polyether polyol and aromatic diisocyanate based PU

N

2

N 3

1

N

Passenger Car Tyre EPDM

2

O 1

2

N

O

Polyurethane §§

Table 3.13 Cont’d ...

Medium Number Fractions Activation Frequency Order of of Steps Contributed Energy Factor Reaction by Each Step (kJ/mol) (min-1) O 1** 122 5.5 x 1012 -

NBR vulcanisate

EVA (28% VA) EVA (18% VA)

Rubber

Thermal Analysis of Rubbers and Rubbery Materials

Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials energy value of 60 kJ/mol, whereas, that of the second step is 197 kJ/mol. Kleps and co-workers reported that E for the thermal degradation of BR at Tmax is 263 kJ/mol [298]. Incorporation of inorganic fillers (silica, calcium carbonate, clay) increased the activation energy of thermal degradation, but addition of carbon black lowered the same [298]. Gamlin and co-workers analysed EPDM rubbers of different PE/PP compositions [315]. It was observed that for a pseudo-first order thermal degradation at lower temperature (0-15% conversion) the activation energy was low (140-200 kJ/mol); however, the same at higher temperature range was very high (330-365 kJ/mol). Smith also reported similar range of E values for different grades of EPDM [311]. The values of E did not follow any trend with the composition of EPDM [315]. It was likely that morphology or microstructure of the rubber controlled the complex degradation reaction. Kleps and co-worker reported that average activation energy of EPDM rubber is 214 kJ/mol [298]. Addition of filler to the vulcanisate increased the value of E. Hypalon (chlorosulfonated PE) exhibited a three-step thermal decomposition. The major degradation occurred at the high temperature region (400-500 C) with the highest activation energy being 246.6 kJ/mol [311]. CR undergoes two-step degradation under nitrogen and leaves carbon residue. The activation energies of the two steps are 203 kJ/mol (Tmax 376 C) and 95 kJ/mol (Tmax 452 C) [298]. Denardin and co-workers employed multi-Gaussian curve fitting of DTG data to obtain multi-step CR degradation (in air or nitrogen) kinetic parameters [299]. Addition of mineral fillers lowers the thermal stability and activation energy for the first step. However, addition of carbon black improves thermal stability of the rubber. Thermal decomposition of EVA is also two-step as found in high resolution TGA studies [400]. The activation energy for the second step is higher than that of the first step (Table 2.13). Oxidative degradation of EVA also followed a similar trend. Under oxygen the fractional conversion involved in first step was higher [399]. PU too, exhibit a two-step thermal degradation. Applying Kissinger’s method Agi and co-workers determined the values of E for PU of different compositions [401]. The E varied from 115 kJ/mol to142 kJ/mol and 197 kJ/mol to 233 kJ/mol for the first and second steps, respectively. The activation energy for thermal degradation of silicone rubber is temperature sensitive [343]. With increase in temperature, the E decreases initially then increases at the very high temperature region. Addition of conductive black lowers the activation energy for thermal degradation of silicone at certain temperature regions, however, addition of silica as filler increases the activation energies. Tyre derived rubber usually consists of various rubber components and the activation energy of degradation lies in the range of 140-200 kJ/mol [402]. Isothermal TGA analysis of rubbery materials is also used to study the kinetics of thermal or oxidative degradation [403-405]. According to the isothermal method: ln


dW  n ln W
ln k dt

(3.14) 123

Thermal Analysis of Rubbers and Rubbery Materials where, k(T) = Ae(–E/RT). Therefore, the Arrhenius plot of ln k against 1/T gives the activation energy from the slope and ln A from the intercept. Isothermal TGA data is also effective to predict the lifetime of the product [406-407].

3.3.5 Miscellaneous Applications of TGA TGA is widely used to analyse raw materials of the rubber product manufacturing industry and the processed materials at various stages. The ability to measure volatile content by TGA is exploited to control the quality of the products. Other than the quantitative compositional analysis of rubber products, TGA is extensively used in (a) determining the effectiveness of antioxidants in compounds [408] or ageing characteristics of raw rubber and finished rubber products [254, 299, 409], (b) evaluation of (composition/ homogeneity) in rubber mixes [355-356, 410], (c) qualitative identification of carbon black or other fillers [358-359, 411-412], and (d) finding optimum processing/storage/ degradation parameters for various rubber ingredients such as plasticisers [357], blowing agents [413], curatives [414-415] and so on. Continued efforts in the development of instrumentation and software-based data analysis further increased the scope of TGA applications in research/quality control activities of rubber products. For example, modulated thermogravimetry, a recently developed tool, is very useful for obtaining continuous kinetic parameters for rubber degradation in a simple dynamic TGA run [416-417].

3.4 Conclusion Different applications of DSC and TGA for the analysis of rubbery materials has been discussed. A combination of other analytical tools such as FTIR, GC, and MS, are also extensively used for the analysis of evolved gases in TGA. Recent development of user friendly equipment and software-based analysis made these tools easy to use and less time consuming for faster qualitative and quantitative evaluation of rubber products.

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Thermal Analysis of Rubbers and Rubbery Materials 402. D.Y.C. Leung and C.L. Wang, Journal of Analytical and Applied Pyrolysis, 1998, 45, 2, 153. 403. H.A. Papazian, Journal of Applied Polymer Science, 1972, 16, 10, 2503. 404. G. Liptay, J. Nagy, J.C. Weis and A. Borbély-Kuszmann, Thermochimica Acta, 1985, 85, 403. 405. W.H. Dickstein, R.L. Siemens and E. Hadziioannou, Thermochimica Acta, 1990, 166, 137. 406. A.S. Deuri and A.K. Bhowmick, Journal of Thermal Analysis, 1987, 32, 3, 755. 407. S. Kole, R. Santra and A.K. Bhowmick, Rubber Chemistry and Technology, 1994, 67, 1, 119. 408. D.J. Burlett in Proceedings of the 17th North American Thermal Analysis Society Conference, 1988, Orlando, FL, USA, p.654. 409. S. Roy, B.R. Gupta and T.K. Chaki, Kautschuk und Gummi Kunststoffe, 1993, 46, 4, 293. 410. A. Adhikary, S. Das Gupta, A.S. Deuri and R. Mukhopadhyay in Proceedings of the Ninth National Symposium on Thermal Analysis, Goa, India, 1993, p.258. 411. Y. Bereznitski and M. Jaroniec, Journal of Porous Materials, 1996, 3, 3, 181. 412. J. Harris, Elastomerics, 1978, 110, 4, 48. 413. S. Morisaki, M. Naito and T. Yoshida, Journal of Hazardous Materials, 1981, 5, 1-2, 49. 414. B. Banerjee, S.N. Chakravarty, B.V. Kamath and A.B. Biswas, Journal of Applied Polymer Science, 1979, 24, 3, 683. 415. R.B. Prime, Thermochimica Acta, 1978, 26, 1-3, 165 416. R.L. Blaine and B.K. Hahn, Journal of Thermal Analysis, 1998, 54, 2, 695. 417. C. Gamlin, M.G. Markovic, N.K. Dutta, N.R. Choudhury and J.G. Matisons, Journal of Thermal Analysis and Calorimetry, 2000, 59, 1-2, 319.

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4

Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites Suman Mitra, Kinsuk Naskar and Anil K. Bhowmick

4.1 Introduction The mechanical properties of elastic solids can generally be conveniently described by Hooke’s law, which states that an applied stress is directly proportional to the resultant strain, but is independent of the rate of strain. On the other hand, for liquids, Newton’s law mainly governs the properties. For liquids, the stress is independent of the strain, but proportional to the rate of strain. In many cases, a material may exhibit the characteristics of both a solid and a liquid and neither Hooke’s law nor Newton’s law can properly explain its behaviour. This typical situation gives rise to the viscoelastic state. In other words, a viscoelastic material is characterised by possessing both viscous and elastic behaviour.

4.1.1 Mechanical Models Describing Viscoelasticity A weightless spring generally represents a system storing energy, which is recoverable; on the other hand a dashpot (loose fitting piston in a cylinder containing a liquid) represents the dissipation of energy in the form of heat.The first viscoelastic model was proposed by James Clark Maxwell, [1], who showed that the model could be constructed by putting a spring and dashpot in series. The second simple model was proposed by Voigt and Kelvin [1], which can be constructed by placing a spring and dashpot in parallel. However, these models (Maxwell and Voigt-Kelvin) are too simple and explain neither the complex viscoelastic behaviour of a polymeric material, nor provide any clear picture to help understand the molecular mechanism of the process.

4.1.2 Linear Viscoelastic Behaviour of Amorphous Polymers The three most common examples of linear viscoelastic behaviour of amorphous polymers are: a.

Creep – where there is a delayed strain response after the rapid application of a stress, 149

Thermal Analysis of Rubbers and Rubbery Materials b.

Stress relaxation – in which the material is quickly subjected to a strain and a subsequent decay of stress is observed, and

c.

Dynamic response – here a steady sinusoidal stress or strain is imposed on a body. This produces a strain or stress oscillating with the same frequency as, but out of phase with the stress or strain. Thus, dynamic mechanical properties generally refer to the response of a material as it is subjected to periodically varying stresses or strains. When equilibrium conditions have been established in a linear viscoelastic material subjected to a sinusoidally varying shear strain, the stress also varies sinusoidally but out of phase with the strain. Viscoelastic behaviour is most commonly characterised in a so-called oscillatory dynamic mechanical test [2-4]. Bhowmick has recently reviewed the various theories of dynamic properties and their applications [5].

The application of an oscillatory strain  of angular frequency  is given by:  (t) = 0 sint

(4.1)

where  (t) is the strain at any time t and 0 is the strain at the maximum stress. For a linear viscoelastic material, sinusoidal stress , which is out of phase with strain can be expressed as: (t) = 0sin(t + )

(4.2)

where 0 is the maximum stress at the peak of sine wave. The strain lags behind the stress by a phase angle . A simplified representation of the dependence of ,  and  is shown in Figure 4.1.

Figure 4.1 Dependence of strain (), stress () with phase angle ()

150

Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites Equation (4.2) can be written as follows: (t)=(0cos) sint+(0sin)cost

(4.3)

This equation shows that the stress consists of two components: one in phase with the strain (0 cos), the other 90º out of phase (0 sin). Therefore, the relationship between stress and strain in a dynamic experiment can be redefined by: (t) = 0[E sin t+ Ecos t]

(4.4)

in which: E


0 cos
0

(4.5)

and E


0 sin
0

(4.6)

Thus, the component of stress E´ is in phase with the oscillatory strain whereas the component E´´ is 90º out of phase. E´ is termed as storage modulus and E´´ the loss modulus. The tangent of the phase angle, which corresponds to the damping property of the material, also called loss tangent, is: tan



E E

(4.7)

In shear mode, this equation can be also written as: tan



G G

(4.8)

where G´ is the shear storage modulus and G´´ is the shear loss modulus. Tan  is a basic parameter for expressing the energy losses relative to the energy stored. Losses in various dynamic test methods, such as, rebound experiments or decay of free vibrations, can all be expressed conveniently in terms of tan . Typical values of G´, G´´ and tan  for a solid polymeric material are 109 N/m2, 107 N/m2, and 0.20, respectively. A typical DMA plot is shown in Figure 4.2. The strain can also be expressed in terms of stresses in phase and 90º out of phase with the stress. The storage compliance [J´ ()], the ratio of strain to stress, may be defined by the following equation:  = 0 [J´ () sint – J´´() cost]

(4.9) 151

Thermal Analysis of Rubbers and Rubbery Materials

Figure 4.2 A typical DMA plot showing Storage modulus (G), Loss modulus (G) and Tan delta () for a typical polymer

Although J´ and G´ are both measure of stored energy, they differ in that G´ compares at corresponding strain, while J´ compares at corresponding stresses. However, it is noteworthy to mention that G´ and G´´ are not the reciprocals of J´ and J´´, respectively.

4.1.3 Zones of Viscoelastic Behaviour Zones of viscoelastic behaviour of a typical polymer are discussed next with the help of Figure 4.3 [6]. The general features of the pattern are more or less similar for all polymers.

(a) The Plateau Zone In the plateau zone, G´ changes slightly with frequency, and G´´ goes through a minimum. This typical behaviour is commonly expressed in terms of ‘entanglement coupling’. 152

Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites

Figure 4.3 Zones of Viscoelastic behavior illustrated by logarithmic plots of G´ and G´´ against frequency for uncrosslinked poly (n-octylethacrylate) at 100 °C, molecular weight 3.6 x 106 From O. Kramer and J.D. Ferry, Science and Technology of Rubber, ed. F.R. Eirich, published by Academic Press, 1978

(b) The Transition Zone At relatively higher frequencies (higher than the plateau zone), the strain corresponding to a given stress is less and the modulus increases with increasing frequency. At this stage, G´´ increases faster than G´.

(c) The Glassy Zone In the glassy zone, the strain in response to a given stress is small, leading to a high value of the storage modulus such as that of a hard glass-like solid. The polymer becomes less viscous than in the transition zone.

(d) The Terminal Zone In this case, the frequency is very small and the period of oscillation is long enough so that the molecules can completely rearrange their configurations. Processing characteristics of polymers at elevated temperature is largely governed by the viscoelastic behaviour at the terminal zone. 153

Thermal Analysis of Rubbers and Rubbery Materials

4.1.4 Time-Temperature Superposition Principle Because of the viscoelastic nature of the polymeric materials, the study of their longterm behaviour is necessary. For a viscoelastic polymer, the modulus is a function of time at a constant temperature. The modulus is in turn a function of temperature at a constant time. According to this time-temperature correspondence, there are two ways to analyse the long-term behaviour of a polymer. First, experiments for extended periods of time can be pursued at a constant temperature and then the response can be measured directly. This technique is however an extremely time consuming one, due to the long response time of most of the polymers. The second method is related to the experiments, which are performed at a constant temperature over a short time frame and then repeated over the same time interval at different temperatures. These two methods are equivalent to each other according to the principle of time-temperature superposition. Thus, a creep curve observed for short times at a given temperature is identical with one observed for longer times at a low temperature, except that the curves are shifted on the logarithmic time axis. They can be superimposed by proper scale changes on this axis. Similarly, portions of a creep curve or stress relaxation curve can be observed at different temperatures and these curve segments can then be shifted along the log-time axis to construct a composite curve or master curve, applicable for a given temperature, extending over many decades of time. Figure 4.4 illustrates this procedure for a plot of relaxation modulus against time.

Figure 4.4 Illustration of time-temperature superposition principle using stress relaxation data for polyisobutylene. The curves are shifted along the axis by an amount represented by aT as shown in the inset. The reference temperature is at 25 °C From J.D. Ferry, Viscoelastic Properties of Polymers, published by John Wiley and Sons, 1980

154

Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites The shift factor for a curve segment is designated by aT, log aT being the horizontal displacement necessary to allow it to join smoothly into the master curve. This is the factor by which the time scale is altered due to the difference in temperature, and is, of course, a function of temperature. For accurate measurement, there is also small vertical shift, modulus values multiplied by T00 T11 (or compliance values by the reciprocal ratio) to take account of the entropy effect of temperature on stress. To and 0 are absolute temperature and density, respectively, for standard conditions or for master curve, and T1 and 1 apply for the curve segment, which is to be shifted. It has been found that for all linear viscoelastic materials over a limited temperature range horizontal shift factors are given by the empirical Williams-Landel-Ferry (WLF) equation [2, 3]: log aT 


C1 (T
Tg ) C2
(T
Tg )

(4.10)

where, C1 and C2 are constants and Tg is the glass transition temperature of the material. The WLF equation provides quite satisfactory shift factors in the range Tg