Thermal Analysis of Rubbers and Rubbery Materials
Editors: P.P. De, N. Roy Choudhury, and N.K. Dutta
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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.
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W.M. Ramsay, Journal of the Chemical Society, 1877, 32, 395.
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H. Le Chatelier, Comptes Rendus, 1886, 102, 1243.
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H. Le Chatelier, Comptes Rendus, 1887, 104, 1517.
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N.C. Roberts-Austen, Proceedings of Royal Society, 1891, 49, 347.
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W.E. Knowles Middleton, A History of the Thermometer and its use in Metrology, John Hopkins Press, Baltimore, MD, USA, 1966.
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W.J. Moore, Physical Chemistry, 4th Edition, Prentice-Hall Englewood Cliffs, NJ, USA, 1972.
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K.E.J. Barrett, Journal of Applied Polymer Science, 1967, 11, 1617.
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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.
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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.
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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 (<100/V), the Ts-Tr signal must be amplified. The amplifiers must have low noise, low drift and high stability to be used in DTA instrument. The Spectrum Scientific Company manufactures a line of active electronic filters that provide a wide range of adjustable time constants and amplification.
2.2.1(e) Recorder Generally, three types of analog recorders are used in modern thermal analysis instruments: the time-base potentiometric strip chart and multipoint recorders and the temperature-based X-Y or X-Y1Y2 function plotters. One advantage of time-base
Table 2.3 Thermocouples commonly used for DTA, DSC, TGA Serial Name of Thermocouple Number 1 Copper – Constantan 2 Iron – Constantan 3 Chromel – Constantan 4 Chromel – Alumel 5 Platinum – Platinum - 10% rhodium 6 Tungsten – Tungsten - 26% rhenium 7 Chromel – Constantan differential thermocouple
16
Approximate Maximum Temperature, oC 250 450 1000 1350 1600 2400 -150 to 600
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials strip chart recorders is that the furnace heating rate can be observed and checked for variation. However, modern thermal analysis instruments are computer-controlled and capable of collecting data digitally and simultaneously comparing them with results from previous experiments.
2.2.1(f) Atmosphere Control In DTA and DSC instruments, during experimental work, control of the atmosphere is important, an inert gas like nitrogen, argon, helium is passed through the instrument to drive the effluent gases generated during heating out. The purging of gases takes place with extremes of pressure from 1.33 x 10-4 Pa to hundreds of pascals. Depending on the nature of the experiment, air or oxygen is also purged. Generally, the flow rate is maintained at 20-30 ml/min, so that the efficiency of temperature programming is not affected.
2.3 Differential Scanning Calorimetry (DSC) Watson and co-workers [9] developed a DSC instrument, in which H the difference of heat flow is plotted against temperature, T. Unlike DTA, the apparatus maintains a sample temperature equal to a reference substance by supplying heat to the sample or reference material. Basically three types of DSC instruments are used, the heat-flux DSC (e.g., TA Instruments Q2000, Q200, Q20 DSC, Mettler Toledo DSC 1, Mettler DSC 30), power compensated DSC (e.g., Perkin Elmer DSC, Setaram 101 DSC and temperature modulated DSC (TMDSC) or modulated DSC of TA Instruments (MDSC).
2.3.1 Heat-flux DSC Figure 2.3 shows the schematic diagram of Dupont DSC cell [10], which uses a constantan disk as its primary means of heat transfer to the sample as well as reference positions. The sample and reference are placed in pans that sit on raised platforms on the constantan disk. The constantan disk serves as the major path of heat transfer to and from the sample and also as one-half of the measuring thermocouple. The differential heat flow to the sample and reference is monitored by a chromel-constantan differential thermocouple formed by the function of the constantan disk and a chromal wafer that covers the underside of each platform. The chromel and alumel wires are connected to the underside of the chromel wafers and the resultant chromel-alumel thermocouple is used to monitor the sample temperature directly. Constant calorimetric sensitivity is maintained throughout the usable range of cells via electronic linearisation of the cell calibration co-efficient. This system is usable in the temperature range, -150 to 600 °C in an inert atmosphere. 17
Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.3 Disk type thermocouple used for Heatflux DSC [10]
2.3.2 Power Compensated DSC The Perkin-Elmer DSC system is different from Heatflux DSC and was introduced by Watson and co-workers [9]. A schematic diagram [7] of power compensated DSC is presented in Figure 2.4. The Pyris 1 DSC is one such differential scanning calorimeter. It maintains sample temperature equal to the reference compound by supplying heat to the sample or reference material. The amount of heat required to maintain the isothermal condition is recorded as a function of time or temperature. The instrument contains two control loops, one for the average- temperature control and the other for differential-temperature control. In the former, a programmer provides an electrical output signal that is proportional to the desired temperature of the sample and reference holders. The programmer signal that reaches the average-temperature amplifier is compared with signals received from platinum resistance thermometers permanently embedded in the sample and reference holders and then the necessary power supplied equally to both heaters. Actually, when an exothermic (heat yielded) or endothermic (heat absorbed) change occurs in the sample, power or energy is applied or removed from one or both the calorimeters to compensate for the energy change occurring in the sample. The power compensated DSC system is maintained in a ‘thermal null’ state at all times. The amount of power required to maintain the system in an equilibrium condition is directly proportional to the energy changes occurring in the sample. The power compensated DSC therefore provides a true measure of the calorimetric properties of the sample since the fundamental measurement with the power compensated DSC is energy flow. 18
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.4 Schematic diagram for power compensated Perkin-Elmer DSC [7]
In contrast, the more common heat-flux DSC instruments have the sample and reference in a single furnace. Thermocouples measure the temperature differential (not energy differential) between the sample and reference platforms. With the heat-flux DSC units, the temperature differential is converted to energy flow via a mathematical equation and is a more indirect approach as compared to the pure energy flow measurements obtained via the Pyris1 power compensated DSC. Besides measuring the enthalpy, the DSC unit is useful to measure the specific heat, particularly with the software package available from the manufacturers. One major advantage of the power compensated design over the heat-flux DSC cell is that the masses of the individual furnaces of the power compensated system are much lower than that of the heat-flux DSC. The faster response time of the power compensated DSC also provides a much higher degree of resolution compared to heat-flux DSC this may be due to the use of a PRT, used for the measurement of temperature. Like the DTA apparatus, the DSC instruments also consist of specimen holder, furnace having 19
Thermal Analysis of Rubbers and Rubbery Materials low thermal inertia, temperature programmer-controller system for atmosphere control and an X-Y multipen recorder. Details of the previously mentioned accessories have already been described.
2.3.3 Temperature Modulated DSC (TMDSC) TMDSC was recently introduced by some TA instrument manufacturers for their temperature modulated differential scanning calorimeter [11]. In a heat-flux DSC, the difference in heat flow between a sample and inert reference material is measured as a function of time, as both the sample and reference are subjected to a controlled temperature profile. The temperature profile is generally linear (heating or cooling) varying in the range from 0 K/min (isothermal) to 60 K/min. Thus the programmed sample temperature T(t) is given by: T(t) = To + t
(2.1)
Where To (K), (K/min) and t (min) denote the starting temperature, linear constant (heating or cooling) rate and time, respectively. TMDSC uses the heat-flux DSC instrument design and configuration to measure the differential heat flow between a sample and an inert reference material as a function of time. However, in TMDSC, a sinusoidal temperature modulation is superposed on the linear (constant) heating profile to yield a temperature programme in which the average sample temperature varies continuously in a sinusoidal manner: T (t) = To + t +AT Sin t
(2.2)
Where AT ( K) denotes the amplitude of the temperature modulation, (s-1) is the modulation frequency and = 2/p, where p (s) is the modulation period. Figure 2.5 illustrates a modulated temperature profile for a TMDSC heating experiment, which is equivalent by decomposition to applying two profiles simultaneously to the sample: a linear (constant) heating profile and a sinusoidal heating profile. The temperature profiles of these two simultaneous experiments (constant heating profile and sinusoidal heating profile) are governed by the following experimental parameters: • constant heating rate ( = 0-60 K/min) • modulation period (p = 10-100 s) • temperature modulation amplitude (AT = 0.01-10 K) The total heat flow at any point in a DSC or TMDSC experiment is given by: dQ C p f (T ,t ) dt
20
(2.3)
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.5 Typical TMDSC heating profile - a sinusoidal temperature modulation [11]
Where Q (J) denotes heat, t (s) time, Cp (J/K) sample heat capacity and f (T, t), the heat flow from kinetic processes, which are absolute temperature and time dependent. Like conventional DSC, TMDSC also measures the total heat flow, but by effectively applying two simultaneous temperature profiles to the sample, this can estimate the individual contribution to Equation 2.3. The constant heating profile (dashed line in Figure 2.5) provides total heat flow information, while the sinusoidal heating profile (solid line in Figure 2.5) gives heat capacity information corresponding to the rate of temperature change. The heat capacity component of the total heat flow, Cp is referred to as the reversing heat flow and the kinetic component, f (T, t) is referred to as the non-reversing heat flow. TMDSC data are calculated from three signals: time, modulated heat flow, modulated heating rate. But raw data for TMDSC is very complicated, it requires deconvolution to obtain standard data. TMDSC measures the heat capacity, Cp, using the equation: Q p C p K amp Tamp 2
(2.4)
where K denotes heat capacity calibration constant, Qamp, the heat flow amplitude, Tamp, the temperature amplitude. Heat capacity calibration is performed using a sapphire standard reference material. For data analysis, TA Instrument’s Thermal Solutions Software is used. 21
Thermal Analysis of Rubbers and Rubbery Materials The following trademarks are used by different TA instrument manufacturers for their temperature modulated differential scanning calorimeters: Modulated DSC (MDSC) of TA Instruments Inc, Oscillating DSC (ODSC) of Seiko Instruments Inc., alternating DSC (ADSC) of Mettler – Toledo Inc, and dynamic DSC (DDSC) of Perkin – Elmer Corporation. DTA/DSC provide important information that can be used to characterise materials especially polymers and rubbers, design rubber products, predict product performance, optimise processing conditions, improve quality, and select the best materials for specific applications. Thus, specific measurements made by DSC include: 1. Tg, 2. Melting temperature (Tm), 3. Purity, 4. Crystallisation time and temperature, 5. Percentage crystallinity, 6. Polymorphism, 7. Heat of fusion and heat of reaction, 8. Specific heat and heat capacity, 9. Oxidative stability, 10. Rate of cure, degree of cure, completeness of cure 11. Reaction kinetics, and 12. Thermal stability.
2.4 Thermogravimetry (TG) Thermogravimetric analysis (TGA) is the most important tool for quantitative analysis of rubber vulcanisates and their additives. The standard test method for compositional analysis by TGA describes a general technique to determine the quality of four arbitrarily defined compounds such as: a. highly volatile matter, b. matter of medium volatility, 22
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials c. combustible material, and d. ash left after decomposition (mainly inorganic components in rubber vulcanisates. The principle of thermogravimetry is based on a ‘thermobalance’ which describes an instrument, that continuously measures the weight changes of a substance at gradually varying temperatures [12]. The modern thermobalance [13] is illustrated schematically in Figure 2.6. It consists of the following components: a. recording balance, b. furnace, c. furnace temperature programmer or controller, and d. recorder. The specific details of each component depend on the particular application that is required of the instrument, for example, furnaces can be obtained that operate up to 2800 °C (Table 2.2).
Figure 2.6 Schematic diagram of TG apparatus [13]
23
Thermal Analysis of Rubbers and Rubbery Materials
2.4.1 Thermobalance The most important component of TGA is the recording thermobalance. Recording balances can be divided into three types, based on their mode of operation: a) deflection type of instruments, b) null type instruments, and c) those based on changes in a resonance frequency – which give tremendous sensitivity. The null-type balance principle is now used in almost all commercially available thermo balances, e.g., TGA 2050 of TA Instruments. The system operates on a null-balance principle, using a highly sensitive transducer coupled to a taut-band suspension system to detect minute changes in the mass of sample. Actually an optical servo loop maintains the balance arm in the horizontal reference (null) position by regulating the amount of current flowing through the transducer coil. An infra red light source and a pair of photosensitive diodes detect movement of the beam. A flag at the top of the balance arm controls the amount of light reaching each photosensor. As sample weight is lost or gained, the beam becomes unbalanced, causing the light to strike the photodiodes unequally. The unbalanced signal is fed into the control programme, where it is zeroed. This changes the amount of current supplied to the meter movement, causing the balance to rotate back to its null (zero) position. The amount of current required is directly proportional to the change in mass of the sample.
2.4.2 Temperature Detection in TG or TGA In TG or TGA, the mass change of the sample is continuously recorded as a function of temperature. The temperature in this definition, may be that of the furnace chamber, or the temperature near the sample. A thermocouple is used for temperature detection, it should be in contact either with the sample or with the sample container. For maximum temperature accuracy, temperature calibration is done with either well characterised curie temperature materials or high purity metals with well documented melting points. Various types of thermocouples used are shown in Table 2.3.
2.4.3 Furnace and Furnace Temperature Programmers A wide variety of furnaces are available for thermobalances, each designed for a specific temperature range from –150 to 2800 oC. Maximum temperature limits for various resistance heater elements are shown in Table 2.2. Various types of furnaces are used according to the needs of the experiment. The furnace may be positioned above, below or parallel to the balance. Each configuration has its own advantages and disadvantages. Placing the furnace above the balance appears to be the preferred configuration for high temperature ranges, where as for lower temperatures the furnace below the balance is more convenient. 24
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials In the DuPont thermobalance, a furnace parallel to the balance is used, whereas in the Perkin-Elmer Model, TGS-2, a novel microfurnace is placed inside the sample space on a ceramic tube. A thermocouple in close proximity to the sample pan is provided as an alternative measure of sample temperature. The type of temperature controller varies from the simple variable voltage transformer to a synchronous motor to the more sophisticated feed back, proportional type controller. Furnace programmers and controllers are of course, available from a wide variety of manufacturers like the DuPont furnace temperature programmer, TECO model TP-200 programmer [8]. The calibration of the temperature of the furnace and sample chamber has been discussed by various workers such as Stewart [14] and Norem and co-workers [15]. The criteria that are considered characteristic of an ideal standard are the following: 1. The transition must be sharp, 2. Should be unaffected by the chemical nature of the atmosphere, 3. Should be readily observable using a standard sample in the milligram range.
2.4.4 Controlled Atmosphere Purging of gas during the TG experiment is very important. Purge gases can be inert or reactive. A positive flow of inert gas (N2, He) from the balance chamber into the furnace protects delicate components against back diffusion of furnace purge gases or sample effluents.
2.4.5 Sample Containers Numerous sample containers are available for containment of the sample in a thermobalance. The type employed usually depends on the nature, amount and reactivity of the sample and the maximum temperature desired. Common materials of construction include alumina, platinum, platinum - 10% rhodium, aluminum, quartz, nickel, tungsten and graphite. A detailed description of sample containers is given by Wendlandt and co-workers [8].
2.4.6 Recorders Like DSC, three types of analog recorders are currently used in modern thermobalances – the timebase potentiometric strip chart and multipoint recorders and temperature based X-Y and X-Y1Y2 function plotters. However, all modern TGA instruments are 25
Thermal Analysis of Rubbers and Rubbery Materials computer-controlled and capable of collecting data digitally and then simultaneously comparing it with results from previous experiments.
2.4.7 Software Special software libraries provide complete analysis capabilities for all thermal techniques including microthermal analysis (TA). With the help of software for thermal analysis today’s material characterisation problems are solved. Hence, for data analysis, special software is used. The DuPont software library provides a wide range of data analysis software, which enables the user to choose from standard routines, calibration and advanced methods through to writing their own software. All DSC, TGA, TMA modules require a moduleinterface for running the software, for example, IBM personal systems are packaged with TA Instruments. DSC software can determine: 1. Tg 2. crystallisation time and temperature, 3. heat of fusion, melting point, 4. oxidative stability, 5. temperature of transition, 6. kinetic parameters, 7. generic equation, 8. curve overlay. Similarly modern TGA loaded with software can determine: a. the composition of multicomponent systems of material, b. thermal stability of the materials, c. oxidative stability of the materials, d. the lifetime of a product through reaction kinetics, e. decomposition kinetics, f.
the effect of reactive atmosphere on materials, and
g. moisture and volatile content of the materials. 26
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials In addition to universal analysis, an extensive library of optional specialty data analyses programs are available for interpretation, evaluation and optimisation of DSC/TGA experiments. These programmes include software for: 1. Borchardt and Daniel’s kinetics [16], 2. Method of Freeman and Carroll [17, 18], 3. Thermal stability by ASTM E698 [19, 20], 4. Isothermal Method for analysis of cure exotherm [21], 5. Heat capacity.
2.5 Derivative Thermogravimetry (DTG) DTG describes the rate of weight change with respect to time or temperature and is d d or represented as . In order to study the multicomponent systems, the role of dt dT DTG is very important, as it easily distinguishes two or more components, when the rate of weight change is reflected in the form of peaks. This is not surprising as the thermal stabilities of many materials in the component are very similar. Although it is apparent that the rate of weight loss changes with temperature, it is difficult to determine the exact temperature range associated with a loss of a specific component from the mixture. The DTG thermogram could be constructed from the TG curve by manual methods or by data processing and this would present little difficulty where the weight loss events are well separated on the temperature scale. For overlapping events DTG peaks are important as shown in Figure 2.7, where TG/DTG curves of a tyre compound based on NR/BR blend are recorded [22]. The primary weight loss in TG curve shows a continual weight loss over the temperature range 150 to 400 °C, then the rate of weight loss becomes slow for a short time, then again the rate of weight loss increases until the temperature reaches 510 ºC, after which the weight loss becomes constant. It is difficult to determine the exact temperature range associated with a loss of a specific component from the mixture; to measure this DTG is required. The presence of oil is not observed in the TG curve (Figure 2.7) but a small peak in the DTG curve, shows the existence of oil. In the TG curve, the line of demarcation of natural rubber (NR) and polybutadiene rubber (BR) is not clear, but the two peaks (maximum at 365 and 450 °C) clearly indicate that the first peak is due to NR and the second peak is due to BR. Thus, the DTG curve is used for quantitative analysis. The experimental details under which TG/DTG curves are obtained should be given with each analysis. The experimental factors to be considered in obtaining TG/DTG data have been reviewed by several authors [22, 23].
27
Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.7 TG/DTG Curve of a tyre compound based on NR/BR blend [22]
Commercial thermal analysis (DTA, DSC, TGA) instruments are listed in Table 2.4 and 2.5, so that choices can be made according to the requirements of the polymer analyst.
2.6 Evolved Gas Analysis (EGA) or Evolved Gas Detection (EGD) EGA is the general term for any technique, which determines the nature and amount of volatile products evolved by a sample as it is subjected to a controlled temperature programme. EGA is preceded by EGD, which merely detects the presence of evolved gases. When used in tandem with TG or DTA, EGA is primarily employed to determine the composition and concentration of evolved gases from mass loss reactions. Wendlandt [7, 13], Lodding [24] and Langer [25] have discussed the details of the instrumentation for EGD and EGA. In many cases, EGD/EGA is determined simultaneously with other thermal analysis techniques such as DTA and TG. It is a simple matter to add a thermal conductivity detector to a TG/DTA apparatus and determine the EGD curve of the sample. The schematic diagram of EGA is shown in Figure 2.8, where a thermal conductivity detector is connected to a Netzsch DTA system [8]. Here the evolved gases from the samples are swept from the furnace by means of a carrier gas and detected by the thermal conductivity detector (Figure 2.8). A flow meter permits adjustment of carrier gas (He, N2) flow rate. The output voltage of the detector is proportional to the concentration of the reaction products (evolved gases or carrier 28
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
Table 2.4 Name of common commercial thermal analysers (DTA, DSC) Serial Name of Instrument No.
Temperature Range, in an Inert Atmosphere, °C
1
DuPont DSC Cell, Model 910*
2
DuPont DTA 990*
60 to 1600
3
Perkin-Elmer DTA 1700
20 to 1500
4
Perkin-Elmer DSC-2 Calorimeter
5
MettlerToledo TGA/DSC1 (Simultaneous TG – DSC) Room temperature to 1200
6
Mettler DSC 20
- 20 to 600
7
Mettler DS, Mettler Toledo DSC 1
- 170 to 600
8
Stanton Redcroft DTA Systems: a. DTA 6713 b. DTA 672 c. DTA 673 d. DTA 674
-150 to 500 25 to 500 25 to 1000 25 to 1500
9
a. Netzsch TG-DTA b. Model DSC M444
-180 to 1600 -180 to 500
10
Setaram a. DTA 2000K b. DTA 1500K c. Micro DTA M5 d. DSC
25 to 1750 25 to 1250 -170 to 1500 -123 to 827
Theta DTA System a. TG – DTA
25 to 1600
Hungarian Optical Works, MOM Derivatograph (TG-DTA-DTG)
25 to 1100
ULVAC (Sinku-Riko) DTA, HPTGD-3000-M, High pressure TG-DTA
25 to 800
11 12 13
- 150 to 725
- 175 to 725
14
Perkin-Elmer Pyris1 DSC (Power compensated)
-150 to 1000
15
TA Instrument DSC 2010, Q20, Q200, Q2000
-180 to 725
16
a. DuPont 920 Auto DSC or 2920 Auto DSC* b. DuPont DSC 10 cell base*
17
a. b. c. d.
MDSC of TA Instruments ODSC (Seiko Instrument, Inc. ADSC of Mettler – Toledo Inc. DDSC, Perkin Elmer Corporation.
-180 to 700, 62 Samples at a time -170 to 725 Ambient to 1000 Ambient to 1000 Room temperature to 1000 Ambient to 1000
*: Currently TA Instruments
29
Thermal Analysis of Rubbers and Rubbery Materials
Table 2.5 List of common commercial thermogravimetric analysers (TGA, DTG) Serial Number 1 2 3
4
5
6 7 8 9 10
Name DuPont Model 951* Perkin Elmer Model TGS – 2 a. Mettler TA-1, Thermoanalyzer (Simultaneous TG-DTG-DTA) b. Mettler TA-2 (TG-DTG-DTA) c. Mettler 2000C (TG-DTG-DSC) Netzsch Model STA 429 (TG-DTA)
Theta Instrument a. Cahn Model 1000 (TG-DTA) b. Cahn Model 2000 Setaram Model G 70 Shimadzu Thermo balances a. TGA-20B Micro Thermo Balance Rigaku TG – DTA or TG-DSC
Temperature, °C RT to 1000 RT to 1000 RT to 2400 25 to 1000 20 to 1200 a. –150 to 420 b. 25 to 1000 c. 25 to 1350 d. 25 to 2400 (furnaces are different) RT to 1400 RT to 1400 or 1600 -196 to 2400 Option for 5 furnaces RT to 1000 RT to 1000 RT to 1500 RT to 1000 RT to 1100
Stanton Redcroft TG-750 (Thermobalance) Hungarian Optical Works (MOM Derivatograph) TG-DTA RT to 1000 11 a. TGA 2950a TA Instrument b. TGA 2050a TA Instrument RT to 1000 c. SDT, 2960 (DSC – TGA) RT to 1500 a : An optional quartz-lined evolved gas analysis (EGA) furnace can replace the normal TGA furnace RT: Room temperature *: Currently TA Instruments
gas absorbed). Langer [25] has discussed the various types and applications of EGA, involving gas chromatography (GC) and mass spectrometry (MS). Pyroprobe Model 100 are pyrolysis probes available for coupling with gas chromatography to provide the temperature capabilities. The use of mass spectrometers for EGA has been particularly rewarding because of their high sensitivity, which gives them an ability to directly identify the vapour species. The Netzsch simultaneous TG-DTA-EGA apparatus provides a line of sight-path so that readily condensable species can be measured. 30
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.8 Schematic diagram of a typical EGA apparatus [8]
Fourier transform infra red (FT-IR) spectroscopy, continuously monitors the purge gas as a function of time or temperature, when attached to EGA. Some problems due to condensation of gases, time lag, reaction between gases, sensor selectivity, differences in diffusion rate, should be avoided, in order to achieve good results. In order to compile a complete EGA report with the results from a TG-EGA experiment, the following points should be considered: •
Record of the evolved gas spectrum,
•
Description of gas components,
•
Flow rate, total volume, design type and temperature of the interface between the TG and EGA instruments,
•
Delay between evolution and analysis of gas,
•
Relationship between the signal amplitude and concentration of evolved gases.
2.7 Thermomechanical Analysis (TMA) and Thermodilatometry (TD) or Thermodilatometric Analysis (TDA) Thermomechanical analysis (TMA) is a technique in which the deformation of a substance is measured under non-oscillatory load as a function of temperature programme. 31
Thermal Analysis of Rubbers and Rubbery Materials Thermodilatometry (TD) or thermo dilatometricanalysis (TDA) on the other hand is a technique in which the dimension of a substance is subjected to a controlled temperature programme [26]. Both of these techniques may be obtained using the same apparatus; only the sample probe or probe loadings are different. With TMA, the dimensional properties of a sample are measured as the sample is heated, cooled or held under isothermal conditions. The loading or force applied to the sample can be varied with TMA. The technique is used to assess the following important properties of polymers: 1. Softening temperature, 2. Melting temperature, 3. Co-efficient of thermal expansion, 4. Dimensional compatibilities of two or more different materials, 5. Relative degree of cure of thermosets, 6. Composite delamination temperature, 7. Percentage shrinkage of films and fibres, 8. Shrinkage forces, 9. Testing of coatings on metal films. A typical thermomechanical analyser [7], such as is incorporated in Perkin-Elmer TMS-2, is shown in Figure 2.9(a). In the penetration and expansion modes, the sample is placed on the platform of a quartz sample tube. The quartz probe is connected to the armature of linear variable differential transformer (LVDT) and any change in the position of the armature results in an output voltage from the transformer, which is then recorded. The probe assembly includes a weight tray, which permits a choice of loadings on the sample surface. The weight tray on the probe assembly is supported by a plastic float rigidly fixed to the shaft and totally immersed in a high density fluid such as mercury, glycerol, paraffinic oil, depending on the plastic used in the float. When the expansion of the sample takes place, it will disturb the float, then by adding weight on to the tray, it will be adjusted. The sensitivity of the apparatus provides an amplification in displacement of 4 x 10-5 on a 10 mV recorder. Two furnaces are used to cover the ranges, -150 to 325 °C and 25 to 725 °C. A first derivative computer accessory can be fixed to Perkin-Elmer TMS-2 for simultaneous recording of TMA and DTMA. The TMA probes offered by PerkinElmer includes, expansion, penetration, compression, flexure, extension, dilatometry, a few are shown in Figure 2.9(b). 32
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.9(a) TMA apparatus, Perkin-Elmer TMS–2 [7]
Figure 2.9(b) Probes of TMA apparatus [7]
33
Thermal Analysis of Rubbers and Rubbery Materials TMA is used to determine the linear thermal expansion co-efficient ( ) of polymers and also co-efficient of thermal expansion (CTE), which is a quantitative assessment of the expansion of material over a temperature interval. But in order to measure the volume expansion co-efficient, dilatometry (TDA) is used, as this cannot be measured from TMA data, because Poisson’s constant is not 1.0 for many polymers. Thus, in TDA, glass capillary dilatometers were designed and built by individual researchers using mercury as a filling medium. The full description of TDA is given by Hatakeyama and co-workers [27]. The sample (1-2 g) is inserted into the glass tube of the dilatometer followed by a glass rod, which fits the inner diameter of the glass tube and acts as a spacer. The inner volume of the dilatometer is measured with mercury. Now the dilatometer containing the sample is placed in an oven and heated at a programmed rate. The height of the mercury in the glass capillary of the dilatometer is measured as a function of temperature. By this method the volume expansion co-efficient of the sample can be calculated, if the sample mass and its density at room temperature are known, since the mass expansion co-efficient of mercury and the diameter of the dilatometer capillary are known. The DuPont Model 943 Thermomechanical Analyser module TA Instruments’ DMA Q400 is capable of handling samples in the form of plugs, films, powders or fibres in the temperature range −180 to 800 °C. Interchangeable sample probes permit the determination of penetration, expansion, tension and dilatometry of the samples. 943 TMA has wide selections of optional accessories like: i)
Parallel Plate Rheometry,
ii) Fibre Tension Spectrometry, iii) Stress Relaxation Spectrometry.
2.7.1 Parallel Plate Rheometry (PPR) Time-temperature viscosity data on thermosets can be generated during the early stages of cure, when the TMA is employed as a parallel-plate rheometer. Bartlett [28] has applied the PPR technique for characterisation of fibre–reinforced resins. a. Two types of experiments are possible when a sample fills the space between the plates and exudes from between them after the load is applied, e.g., Wallace plastimeter. b. When the radius of the test plates is larger than that of the sample throughout the test and the sample volume is constant, e.g., Williams plastimeter.
34
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials Both methods require a flat ended cylindrical sample as shown in Figure 2.10. Viscosities from 10-107 Pa-s can be measured with low shear rates of 10-3 to 10°/s. The viscosity is calculated as: 1 [ 4F / 3R 2 ] / [ d / dt ] h2
(2.5)
Where, F = applied force, R = sample radius, h = distance between the plates, and t = time. For method (b) experiments, the relationship is slightly different: 1 8F 3V 2 / d / dt h4
(2.6)
Where, V = sample volume. The parallel plate method actually measures the creep compliance, which has elastic, time-dependent elastic (viscoelastic) and viscous components. Cessna and Jabloner [29] used TMA-PPR for thermoset cure studies.
2.7.2 Fibre Tension Spectrometry It is used to measure shrinkage tension of fibres as a function of temperature held under constant elongation. In this case the primary transducer is a load cell with a low spring constant in series with the LVDT sensor of TMA.
Figure 2.10 Schematic diagram of parallel plate rheometry Dupont Company Instrument System, Concord Plaza, Wilmington, Delaware, 1989, USA
35
Thermal Analysis of Rubbers and Rubbery Materials
2.7.3 Stress Relaxation Spectrometry It is used for measurement of relaxation modulus versus time on viscoelastic materials. A similar transducer to the fibre tension spectrometer is used, with a high force load cell and using steel probes for up to 1 kg load.
2.8 Dynamic Mechanical Analysis (DMA) It is a technique in which dynamic modulus or damping of a substance is measured under oscillatory load as a function of temperature as the substance is subjected to a controlled temperature programme. So in DMA, the sample is clamped into a frame and the applied stress (which varies sinusoidally) of frequency can be presented as : (t) = o sin (t + )
(2.7)
where o is the maximum stress amplitude and the stress proceeds the strain by a phase angle . The strain is given by: (t) = o sin (t)
(2.8)
where o is the maximum strain amplitude. These quantities are related by: ( t ) * ( ).(t )
(2.9)
where E*() is the dynamic modulus, and: *( ) i
(2.10)
where E () and E () are the dynamic storage modulus and the dynamic loss modulus, respectively. For a viscoelastic polymer, E characterises the ability of the polymer to store energy (elastic behaviour) while Ereveals the tendency of the material to dissipate energy (viscous behaviour). The phase angle is calculated from: tan
(2.11)
Normally E, E and tan are plotted against time or temperature. DuPont Model 981 DMA module [8] is illustrated schematically in Figure 2.11 where the sample is oscillated at its resonant frequency and an amount of energy, equal to that lost by the sample, is added on each cycle to keep the sample in oscillation at constant amplitude. The logarithm of this extra energy (called damping) and the frequency are displayed digitally as well as recorded as a function of sample temperature [26].
36
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.11 Dupont Model 981 DMA module [8]
In Figure 2.11, the sample is clamped between two arms, one of which is a passive support, while the other is driven by an electromechanical transducer. Rotation of the driven arm a few tenths of a millimetre, puts the sample in flexure stress, so that, when the displacing force is released, the deflection energy started in the sample, causes it to go into the resonant oscillator. The frequency and amplitude of this oscillator are deflected by the LVDT positioned at the opposite end of the active arm. The LVDT signal is fed to the driver circuit, which then feeds back enough energy to the electromechanical transducer to keep the sample in oscillation at a constant amplitude. The temperature range is –150 to 500 ºC (with an optional accessory). Software is fixed with DMA instruments, so that values of E, E and tan are directly calculated. Not only this, computer controlled DMA instruments allow the deforming force and oscillating frequency to be selected and to be scanned automatically through a range of values, in the course of the experiment. Hence, DMA can be applied to a wide range of materials using the different clamping configurations and deformation modes as shown in Table 2.6. 37
Thermal Analysis of Rubbers and Rubbery Materials
Table 2.6 DMA probes and deformation modes for different applications Sample Solid polymers
Film, fibre, coatings
Viscous fluids, gel
Parameter Dynamic modulus Glass transition temperature Melting temperature Crosslink density Relaxation behaviour Crystallinity Cure behaviour Dynamic modulus Glass transition temperature Creep Cure Relaxation behaviour Viscosity Gelation Gel-sol transition Cure behaviour
Clamp/mode Clamp with sharp teeth Deformation mode
Clamps which are flat type Deformation mode
Clamps with a small nipple to retain material Shear mode
Hard samples or samples with a glazed surface use clamps with sharp teeth to hold the sample firmly in place during deformation. Soft materials and films use clamps which are flat to avoid penetration or tearing. When operating in shear mode flat-faced clamps, or clamps with a small nipple to retain the material, can be used. From the variation in the temperature of the tan peak of a DMA curve as a function of frequency, a transition map can be compiled from this map, and an activation energy for the phenomenon, can be calculated using the Arrhenius relationship or William–Landel– Ferry (WLF) equation [30]. While reporting TMA or DMA results, the following things should be included: •
The type of TMA or DMA instrument used,
•
Method of sample preparation, including dimensions and orientation,
•
Deformation mode,
•
Shape and dimensions of probe (TMA),
•
Size and type of clamps and frame (DMA),
•
Temperature range, heating/cooling rate, isothermal conditions,
•
Atmosphere,
38
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials •
Flow rate,
•
Exact location and type of sample thermocouple, and
•
Description of temperature.
Popular instruments for TMA and DMA are shown in Table 2.7.
Table 2.7 Manufacturers of TMA and DMA systems Manufacturer
Model Number
Temperature, °C
TMA 7/DMA 8000
RT to 800
2. Netzsch
TMA 402
-160 to 1700
3. Stanton-Redcroft
TMA 691
-180 to 500
Numerous*
-150 to 3000
5. DuPont*
DMA 981
-150 to 500
6. DuPont*
DMA 2980
-150 to 600
7. DuPont*
TMA 2940
-150 to 1000
8. TA Instruments
DMA 800
-150 to 500
9. DuPont**
TMA 943
-180 to 800
Rheovibron (DVP-II-EP)
-150 to 400
1. Perkin Elmer
4. Theta
10. Orientec Corporation
* Furnaces are different ** TA Instruments was divested from the DuPont Company in 1990
2.8.1 Torsional Braid Analysis (TBA) The technique of torsional braid analysis (TBA) was first introduced by Gillham [31] for the investigation of the mechanical properties of polymeric substances. It permits thermomechanical ‘fingerprints’ of polymer transitions in the temperature range, -190 to 500 °C in controlled atmospheres. The apparatus used in TBA determines the frequency (less than 1 Hz) and decay of a freely oscillating pendulum, which provides information on the modulus and mechanical damping of the polymer under examination [32]. TBA measures the rigidity of the material and is used for the curing of thermosets. Here the sample is prepared by impregnating a glass braid or thread substrate with a solution of the polymeric material to be tested, followed by evaporation of the solvent [33]. During the heating of the sample impregnated braid, it is subjected to free torsional oscillations. An electrical analog of the decaying pendulum oscillation is obtained by attenuating light with a circular transmission disk, which features a linear relationship between light transmission and displacement angle (Figure 2.12). 39
Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.12 Sketch of a TBA pendulum [33]
From these oscillations, the relative rigidity parameter: 1 p2 1 Where, p = the period of oscillation in seconds and the mechanical damping index n (n = number of cycles for the damping peak amplitude to decrease by a fixed amount) are obtained intermittently through the isothermal cure of a thermoset. The relative rigidity is proportional to the shear modulus (G) and the mechanical damping index is a measure of the logarithmic decrement, which is proportional to the ratio of the outG of-phase shear modulus(G´´) to the storage shear modulus (G). Hence, tan G can easily be found. Sykes and co-workers [34] describe a novel use of TBA, in which a 200 x 2 mm composite specimen replaces the braid. Using this technique, they show that absorption of moisture causes a decrease in rigidity, a broadening and a decrease in
-transition and increase in intensity of -transition. Their results lead them to conclude that polymer is plasticised by absorbed water, which is in part reversible. Hence, TBA is used to find out the gel point of a thermosets. Schneider and co-workers [35] determined the gelation and vitrification times of epoxy-dicyandiamide during isothermal curing from TBA experiment.
40
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
2.9 Thermally Stimulated Current (TSC) Thermally stimulated depolarisation current (TSDC) or thermally stimulated current (TSC) spectroscopy is used to characterise the relaxation processes and structural transitions occurring in samples that have been polarised at a temperature greater than the temperature where molecular motion in the sample is enhanced and subsequently quenched so that the high mobility state is frozen. TSC was first discovered by Lacabanne and her coworkers [36]. TSC is ideal for the investigation of the fine structure of polymers, semicrystalline polymers, co-polymers and blends, polymer complexes and resins and is uniquely suitable to study the influence of additives, and plasticisers [37].
2.9.1 Principle The principle of TSC is to orient polar molecules (pendant polar groups) of macromolecules, by applying a high voltage field at a temperature, then quenching the material to a much lower temperature, where molecular motion ceases. After this polarisation, the material is heated at constant rate causing it to depolarise and in so doing – it creates a depolarising current. This thermally stimulated depolarisation current relates directly to molecular mobility, giving the research analyst a true opportunity to study the physical and morphological structure of material. Here in Figure 2.13, the mobile units of the samples are oriented by a static electric field (E) at a given polarisation temperature Tp. When the polarisation (P) has reached its equilibrium value, the temperature is decreased to ‘freeze’ this configuration, then the field is to cut off. The method is to polarise the sample at temperature Tp for the time tp. Now the sample is quenched to temperature Td (10 ºC below Tp), then the polarisation voltage is cut off and is kept at Td for a time td, which allows depolarisation of another fragment to oriented dipoles. Next the sample is quenched to T0 << Td. The sample is reheated at a constant rate and the current is measured. The spectrum, described by single relaxation time, is a function of temperature. The depolarisation current (J) flowing through the external circuit is measured by an electrometer indicating the dipolar conductivity (Figure 2.13). A TSC apparatus is schematically illustrated in Figure 2.14. The sample is mounted between parallel condenser-type electrodes and heated to the depolarisation temperature, Tp, under a controlled atmosphere. A DC voltage up to 500 V is applied across the electrodes for a time, tp, producing an electric field which polarises the sample. A TSC curve plots the thermally stimulated current versus temperature or depolarisation current J ersus time as shown in Figure 2.13. If isothermal polarisation varies exponentially with time, then: (relaxation time) =
P
(2.12) 41
Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.13 Windowing polarisation of TSC, Solomat Instrumentation, Glenbrook Industrial Park, Stanford, CT, USA, 1989
42
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.14 Schematic diagram of TSC Instrument, Solomat Instrumentation, Glenbrook Industrial Park, Stanford, CT, USA, 1989
Where P = polarisation = dipolar conductivity E = electrical field. Assuming a Debye-type relaxation process, where all dipoles have a single relaxation time and there are no interactions between dipoles, the instantaneous thermally stimulated current J(t) or depolarisation current J is given by: J(t) = (t) P (t)
(2.13)
1 Where P(t) denotes the polarisation decay and (t) the relaxation frequency t . Heating at a constant rate () the temperature dependent relaxation time is described by Arrhenius relationship:
T o exp H
KT
(2.14)
Where 0, H and K denote the pre-exponential factor, activation enthalpy and Boltzman constant, respectively. The relaxation time is: 43
Thermal Analysis of Rubbers and Rubbery Materials P(T ) ) (T ) J(T ) P(T J(T )
(2.15)
This is the ratio of the area under the peak of depolarisation at temperature (T) and of the value of depolarisation current density J at that temperature (Figure 2.13). Equation 2.15 is known as Kelvin – Voigt model. The following items should be included when presenting the results of TSC measurement: •
Type of TSC apparatus,
•
Method of sample preparation including dimensions,
•
Type and configuration of electrodes,
•
Electric field, polarisation temperature, polarisation time,
•
Quenching rate, and
•
Heating rate and final temperature.
2.10 Relaxation Map Analysis (RMA) It is a unique method that reveals a material’s physical properties and discovers more about the solid state matter than ever before. It has the sensitivity to monitor the influence of external stress, processing conditions, degree of cooling or annealing, chemical composition and percentage of crosslinking on molecular mobility. By varying the value of Tp (as shown in Figure 2.13) and repeating the process, the elementary modes can be isolated or ‘windowed’ and used to construct the material’s ‘relaxation map’. Hence, TSC/RMA is the only quasi-equilibrium technique that operates from liquid nitrogen to about 100 degrees above the Tg, showing all transitions along the way, which is free of the annoying problems that plague calorimetry. From low temperatures up to the molten state, for example, it can indicate why an adhesive or paint has formed a good bond, or show how to identify a good matrix/fibre composite and specify its conditions of use. Research and development conducted with a Solomat spectrometer (Model 41000 or 41000 plus) fitted with a powerful software (Multitasking) so that it can analyse data while the determinations are in progress. TSC/RMA is ideally suited to the investigation of the effects of molecular weight, chemical structure, internal stress, orientation, curing, crystallisation and thermodynamic 44
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials history or individual relaxation modes and thus it supercedes other techniques like DSC, DTA, DMA TMA. Hence, the Solomat TSC/RMA Spectrometer performs an entire series of complex experiments, without operator attention, as the software remembers the complete experimental set up. For semicrystalline materials, the power resolution with RMA makes it possible to specify the difference between macromolecules trapped in the interlamellar regions and those which belong to the true amorphous region. In TSC, amorphous polymers display a very strong relaxation mode at the Tg, which is attributed to microbrownian motions of the amorphous chains (e.g., Figure 2.15(a), drawn and undrawn PP). In Figure 2.15(b), the window polarisation analysis reveals two distinct relaxation modes at the lower temperatures, clearly indicating the existence of a fine structure within the amorphous region. The relaxation component observed at the lower temperature is attributed to those regions free from constraint – the inter-spherulite regions. The component at the higher temperature corresponds to the amorphous chains under constraints from crystallites i.e., inter-crystalline regions where both ends of the chains may be included in crystallites. Lacabanne and co-workers [37] studied the structure of polyether block amide (PEBA) adhesives, and characterisation of latex co-polymers with Solomat 41000. Similarly the thermal transitions of toluene diisocyanate based polyurethane elastomers with polytetramethylene oxide were characterised by TSC/RMA [38]. Using TSC/RMA Shin and co-workers [39] characterised the low temperature relaxation of epoxy-resin modified with amine-terminated butadiene acrylonitrile copolymers (ATBN). TSC and
Figure 2.15(a) TSC spectrum for (……….) undrawn and _________ drawn polypropylene Solomat Instrumentation, Glenbrook Industrial Park, Stanford, CT, USA, 1989
45
Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.15(b) Relaxation map analysis for undrawn polypropylene Solomat Instrumentation, Glenbrook Industrial Park, Stanford, CT, USA, 1989
RMA were used to characterise the molecular behaviour of several coating systems [40], thermal transitions and relaxations in styrene-butadiene block and random co-polymer samples, both unfilled and compounded with SBR tyre reclaim.
2.11 Differential Photo Calorimetery (DPC) DPC is a new tool for understanding photosensitive materials. It is the key to the development of new materials. DPC applies the well-known principle of DSC for the measurement of chemical reactions, which are initiated by UV or visible light. DPC measures the heat absorbed or released by a sample as it and an inert reference, are exposed simultaneously to radiation of known wavelength and intensity in a temperaturecontrolled environment. It also measures Tg, curie temperature, and degree of cure of a photopolymer. As DPC is quantitative and objective it can be used to characterise a wide range of light sensitive materials used in many end-use applications, such as: 1. Coatings for paper and wood, plastics, metals, fibre optics. 2. Films for imaging, protection (solder mask), electrical insulation. 46
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials 3. Adhesives. 4. Dental fillings. 5. UV stabilisers. 6. Printing inks. 7. Photo initiators.
2.11.1 DPC Instrument The DuPont 930 DPC is an instrument with the dual capability of a DPC and a DSC system, which is a versatile, cost effective instrument, having automated analysis, and producing very fast and accurate results. It is a good marketing aid with advanced software. It can calculate the heat flow of a curing polymer. Figure 2.16(a) shows the instrument, which consists of 1) lamp compartment, 2) Optical system, 3) DSC cell, 4) DPC power supply, 5) DSC cell base.
Figure 2.16(a) DPC spectrophotometer DuPont Company Instrument System, Concord Plaza, Wilmington, Delaware, 1989, USA
47
Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.16(b) Optical system of DPC spectrophotometer DuPont Company Instrument System, Concord Plaza, Wilmington, Delaware, 1989, USA
Figure 2.16(b) shows the optical system in detail which contains: 1. Pressurised xenon lamp, 2. Pressurised mercury - xenon arc lamp or high pressure mercury arc lamp, 3. Spherical collection mirror, 4. Photo feedback sensor, 5. Collimating optics, 6. Heat sink. It also has a shutter, filter holder, and a DSC cell with radiometer probe.
2.11.2 Principle of Operation DPC exposes a sample to a precisely controlled beam of high intensity UV or visible light. The sample as either a liquid or a solid is placed in an open pan in a DSC cell. The DSC 48
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials cell provides precise temperature control, high sensitivity for quantitative measurement of heat of reaction associated with thermal and photo initiated reactions. It needs a light source, i.e., a lamp and the type used depends on wavelengths required: i)
High pressure mercury arc lamp for UV light, while pressurised mercury-xenon lamp is used for visible light.
ii) The light beam passes through a series of focusing lenses, which provide constant beam focus for all wavelengths. iii) The focus beam then strikes an infra red (IR) absorbing mirror, which reflects UV and visible wavelength absorbing all IR radiation. Heat buildup in the mirror, is dissipated by a metallic heat sink (Figure 2.16(b). A computer controlled photo feedback censor (No. 4 in Figure 2.16(b)) mounted near the heat-sink, monitors and controls light intensity. Light reflected from the IR absorbing mirror, passes through specially designed optics, which assure uniform intensity across the entire beam. The computer controlled shutter assures precise sample exposure. While performing DPC experiments, a precisely machined silver lid with openings above the sample positions and quartz window are used to cover the sample compartment of the cell. These assure superior baseline performance and permit absolute control of the atmosphere.
2.11.3 Uses of DPC The oxidation induction time (OIT) is measured by DPC, which was used as a tool for evaluating the UV stability of isotactic polypropylene [41]. DPC appears to be an efficient tool in the evaluation of photo-initiated, free radical polymerised systems like a newly synthesised tertiary aromatic amine methacryloxy substituted 4-N,N-(dimethylamino) phenylacetamide [42]. DPC was used for estimating the degree of cure of photo curable coatings composed of Ebecryl 270 resin, 1,6 hexanediol diacrylate and Darocur 1173 photo initiator [43].
2.12 Dielectric Analysis (DEA) or Dielectric Thermal Analysis (DETA) DEA or DETA measures the changes in the properties of a polymer, when it is subjected to a periodic electric field (sinusoidal electric field). This produces quantitative data from which the researcher can determine the capacitative and conductive nature of the materials characterising different molecular motions present in the polymeric system. In the dielectric experiment, the molecules are required to interact with an applied electric field and thus forces are only imposed on dipolar or charged species. Polarisation 49
Thermal Analysis of Rubbers and Rubbery Materials is the basic property of any dielectric material which measures the extent of restricted displacement of charged particles present in any dielectric material under the influence of electric field. Polarisation is a vector quantity, which depends on the density of charge particles in a particular direction depending on the impressed electric field [44, 45]. In the dielectric technique a small sinusoidal electric field is applied to the sample and dQ
. Extent the electric displacement (charge Q) is usually followed via current ‘i’, as i dt of polarisation which also measures the capacitative nature of the material is measured by the dielectric constant (permittivity). If the material is subjected to an alternating electric field, the polarisation lags behind the field by phase angles [44].
The complex dielectric permittivity (*) is obtained from the amplitude and phase measurement which allows resolution into the storage part (dielectric constant: ‘) and the loss part (dielectric loss: ‘’). Then the loss tangent is: tan e
dissipation factor
(2.16)
Dielectric analysis measures the two fundamental electrical characteristics of a material: capacitance and conductance as a function of time, temperature and frequency. The capacitive nature of a material is its ability to store electric charge and the conductive nature is its ability to transfer electric charge. Hence, DEA or DETA has four major properties: 1. = permittivity also called the dielectric constant 2. = loss factor 3. Tan = dissipation factor
4. = dielectric specific conductivity = mhos/cm i.e., (ohm-cm)-1
is proportional to capacitance, and
is proportional to conductance
, conductivity is derived from measurement of
2.12.1 Technique Dupont 2970 Dielectric analyser (DEA), is used for measuring the capacitative and conductive nature of the material. The instrument (Figure 2.17) consists of: 1) two parallel electrodes, 2) frequency generator, 3) module microprocessor, 4) response interface, 5) A/D converter, 6) digital signal processor (DSP), and 7) controller/analyser.
50
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials The technique involves placing a sample between two electrodes and exposing it to an alternating electric field. The field is created by applying a sinusoidal voltage from a frequency generator to the input electrode (Figure 2.17). The output electrode receives the response current from the sample and it passes to the response – interface which amplifies the signal and passes it to A/D Converter, which transforms the signal to a digital format passing it to DSP. The processed phase and gain signals are sent to the module microprocessor, where they are combined with sample thickness measurement signals from LVDT to calculate (permittivity) and (loss factor). The polarisation produced within the sample causes an oscillation, which is at the same frequency as the field but with a phase angle () shift. The phase angle shift is measured by comparing the applied voltage to the measured current. Values for capacitance and conductance are measured as follows: Capacitance (C) faraday = I measured Sin . V applied 2f Figure 2.17 Schematic diagram of dielectric thermal analyser Dupont 2970 Dielectric Analyzer, (2000) Dupont Company, Instrument system, Wilmington, Delaware, USA, 1989
= (2f ).0
Conductance 1 I measured .Cos mho V applied R
(2.17)
(2.18)
Where, f = applied frequency (Hz) R = resistivity (ohms) (2.19)
Where, 0 absolute permittivity of free space. , both provide valuable information about molecular motion, measures the alignment of dipoles, while represents the energy required to align dipoles and move ions. 51
Thermal Analysis of Rubbers and Rubbery Materials The technique of operation is similar to DSC, only the sample holder is simply a changeable disposable sensor, like a ceramic parallel plate, ceramic single surface, or a remote single surface. The temperature range is –150 to 500 ºC and the frequency range is 0.001 kHz to 100 kHz. The atmosphere is inert (He, N2). DETA measures several relaxations of polymeric materials like , , , where the relaxation corresponds to microbrownian motion of the whole chain and is operative only above the Tg, the process corresponds to dipolar group orientation occurred through limited mobility of small section of polymers and occurs at lower temperature. Like DMA, DETA can measure the compatibility and incompatibility of polymers/ rubbers. For example, SBR-nitrile-butadiene rubber (NBR) blend, is dominated by NBR due to strong cyano dipoles, and as a result it is an incompatible blend [46]. Radhakrishnan and Saini [46] studied the dielectric relaxation properties of polyvinyl chloride/thermoplastic elastomer (Hytrel grade 5526) blends of different composition and found a single -transition, which indicated the compatibility of the blend. The applications of DETA for polymers are as follows: 1. Molecular relaxations, 2. Oxidation, 3. Ageing, 4. Change in crystallinity, 5. Curing and rheological studies, and 6. Filler analysis.
2.13 Newly Developed Thermal Analysis There are many newly developed thermal analysis methods [47], which are also used nowadays for characterisation of polymers, these are: 1) Thermo microscopy, 2) Thermoluminescence 3) Alternating current calorimetry, 4) Thermal diffusivity (TDF), 5) Microthermal Analysis (TA), 6) Optothermal Transient Emission Radiometre (OTTER), 7) Specific Heat Spectroscopy, and 8) Thermal conductivity measurements. But these methods are very expensive, so they are not used very frequently for the characterisation of rubbers. 52
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
2.14 New Combined Methods of Thermal Analysis The capability of thermal techniques for material characterisation can be greatly increased if techniques other than thermal are connected to the thermal analyser to identify further either the residue or the effluence during a certain thermal event. Such techniques include, infra red spectroscopy (IR/FT-IR), MS, nuclear magnetic resonance spectroscopy, X-ray methods, ultraviolet and visible light spectrophotometer, electrochemical methods, emission and atomic absorption spectroscopy. For complex mixtures, a separation step may be desired before identification. Gas or liquid chromatography (GC, LC) is most convenient for this purpose. Analytical techniques [48] commonly considered for coupling with TG are IR, MS, GC. But many factors are to be considered for such combination. The heating rate in TG is relatively slow under typical operating conditions and some adjustments may have to be made to make it compatible with the evolved gas technique. A TG analysis is normally performed in a flowing gas atmosphere, thus, the gas flow rate has to be carefully chosen to allow proper evolved gas analysis. For some analytical techniques such as MS, only vacuum or very low pressures are permitted. Sample size is also an important parameter. If the sample is too large, it cannot be tolerated, whereas if it is too small, then it cannot be detected by others. For optimum operations of a typical TG analyser, sample weights should be in the range 1-100 mg, which may be beyond the limit of some gas analysis techniques. Good judgment should always be exercised when the analysis of off-gases from the TG analyser is performed. Thermal decomposition, recombinations, thermal reactions, condensations on cold spots may all occur. Hence, a combination of two or more major analytical techniques require multiple skills of the user. Of course the increased cost of investment in either capital or maintenance should be considered. A few popular processes of combined techniques will be discussed.
2.14.1 Coupled Thermogravimetry – Infra red Spectroscopy (TG-IR) The simplest and most inexpensive approach was described by Smith [49] as shown in Figure 2.18. The volatile effluents were drawn by slight vacuum via a 0.6 m long and 1.25 mm id (internal diameter) stainless steel hypodermic tubing through a Grubb Parson (1 m, 45 ml volume) multireflection IR cell mounted in the infra red beam of a Pye Unicom Sp1000 spectrophotometer to collect total volatiles - an evacuated PerkinElmer (1 m, 1 litre gas) cell was used. The gas transfer tubing was connected through silicone rubber septa to an IR cell and to a glass ball joint at the end of furnace tube. A DuPont 950 TG analyser was used because of its horizontal balance configurations. This technique has been used to distinguish many homopolymer blends [49] from copolymers, e.g., polyethylene (PE)-polymethylmethacrylate (PMMA) blends. Evidently, this technique is only good for analysis of major components in the sample gas stream and the precision is inadequate for critical work. 53
Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.18 Coupled thermogravimetry infrared spectroscopy (TG-IR) [49]
2.14.2 Coupled Thermogravimetry – Fourier Transform Infra red Spectroscopy (TG-FT-IR) With the advent of sophisticated commercial instrumentation of FT-IR in recent years, the potential of using IR for analysis of evolved gases from the TG analyser is greatly increased. One typical setup was described by Cody and co-workers [50, 51]. The interfacing system [48] is shown schematically in Figure 2.19. A Dupont 951 TG Analyzer controlled by a Dupont 1090 programmer/plotter was used in conjunction with a Nicolet 7199 FT-IR. A Nicolet GC/FT-IR sampling attachment served as the interface [50, 51]. Volatiles from TG were carried by dry nitrogen to a gold coated light – pipe via a heated 1.6 mm stainless steel transfer line. A broad band mercury-cadmium-telluride detector measured the signal transversing the light pipe. The
Figure 2.19 Coupled TG-FT-IR [50]
54
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials computer system associated with the FT-IR, stored the data in interferogram, performed phase calculation and Fourier transform and displayed the resulting spectrum in less than 2 seconds. The computer programme also allowed addition and subtraction of spectra. FT-IR is widely used in EGA owing to its relatively high sensitivity and short spectrum acquisition time. Corrosive and reactive decomposition products are more easily handled by TG-FT-IR coupling mechanism than by TG-MS.
2.14.3 Coupled Thermogravimetry – Mass Spectrometry (TG-MS) MS is a high-sensitivity, non specific technique used to identify unknown compounds. Infra red spectra identify the functional groups of polymer, but sometimes it is difficult to analyse the mixtures of compounds with similar functional groups. In MS, when bombarded by electrons all substances ionise and fragment in a unique manner. The mass spectrum, which records the mass and relative abundance of the ion fragments gives a fingerprint for each compound. Hence, MS using quadrupole mass spectrometers, is most commonly used with TG/EGA. MS have the capability of analysing simultaneously and independently a number of volatile components from a weight loss step. Direct connection of two instruments (TG and MS) is only possible, when the TG experiment can be performed only under high vacuum [52, 53] or under certain types of reagent gas to allow chemical ionisation in the MS [54, 55]. For flexible control of atmosphere and easy adaptation to various types of instrumentation, an interface is used to reduce the atmospheric pressure normally used in TG to below 0.01 Pa encountered in most MS operations. The general principle of the interface is to allow a small portion of effluent gas to leak into the MS so as not to reduce its vacuum below its tolerance limit. By doing so, the sampled gas should also be reasonably, reproducible and representative of the original gas stream. Different types of interfaces [56-58] were used with different types of thermal analyser. Yuen and co-workers [57] interfaced a ‘Mettler’ thermoanalyser with a Hewlett–Packard quadrupole mass spectrometer. A schematic diagram of TG-MS [57] is shown in Figure 2.20, where (A) is a HP-5992 quadrupole mass spectrometer, (B) is a Varian leak valve, (C) is a GC oven (D) is a fore pump (a mechanical pump which exhausts to the atmosphere and maintains a specific pressure in the inlet)), (E) is a leak valve from control shaft for fine adjustment, (F) is a TA/MS transferline, and (G) is a 9.5 mm to 6.35 mm stainless steel swagelock union. Two HP-9825 calculators were used for data acquisition and retrieval. Both thermal and mass spectral data were collected once every 10 seconds. All DTA, TG, DTG and selected mass abundance scans were plotted simultaneously. Another versatile and inexpensive interface based on the capillary principle has been reported by Chiu [58]. It is simple to construct, easy to maintain and readily adaptable to most TG and MS instruments, without modification. Some of the biggest problems for the TG-MS interface for routine use are contamination and clogging, a main advantage of this interface is its easy removal for cleaning by a suitable solvent (hydroflouric acid). 55
Thermal Analysis of Rubbers and Rubbery Materials
Figure 2.20 Coupled TG – MS using a leak valve as an interface [57]. A - HP 5992 quadrupole mass spectrometer; B - Varian leak valve; C - GC oven; D - Fore pump; E Leak valve from control shaft; F - TA-mass transfer line; G - Swage lock union
The interface samples the TG effluence for MS analysis directly and continuously as the TG experiment progresses, thus providing selective monitoring of individual components independent of each other. Chiu and co-workers [58] used this method to distinguish polymer blends of polystyrene (PS)-PMMA and a copolymer of PS-PMMA. Slusarski and co-workers [59] calculated the activation energy of destruction of cis-1,4 polybutadiene using TG-FT-IR and TG-MS methods and also determined the various products of thermal destruction of the elastomer cis-1,4 polybutadiene.
2.14.4 Coupled Thermogravimetry – Gas Chromatography (TG-GC) Pyrolysis gas chromatography has long been established as a powerful technique for polymer characterisation. The use of TG preceding GC is essentially equivalent to oven-pyrolysis GC. It thus possesses both the advantages and drawbacks of the latter except that TG-GC has the additional benefit of gaining weight loss information, which is important for quantitative examination of the original sample and guidance to the pyrolysis process. For TG, the addition of GC provides a means for separation and identification of volatile products at various weight loss steps, which is vital for the correct interpretation of a TG curve. One immediate benefit is the prior separation of 56
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials solvents, monomers and additives from the polymer by TG, thus avoiding the polymer degradation process. The use of TG-GC for polymer characterisation was first reported by Chiu [60, 61]. In contrast to TG-MS which requires a reduction in pressure, in most cases, TG-GC demands pressurisation of the TG effluent for efficient introduction of the sample into the GC. Also GC is a batch analysis and sampling from the continuous flow of TG effluence has to be intermittent. A sampling loop is typically used to accomplish this purpose by applying a pressurised carrier gas to sweep periodically the TG effluent into the GC column. A typical flow diagram of TG-GC is presented in Figure 2.21. A series of four switching stop cocks is used to direct the gas flow. After TG starts, the effluent gas flows through stopcocks A and B and the two traps to the atmosphere. If the concentration of the sample of interest is too small, a proper refrigerant may be added to the trap to collect a large segment of the effluent along the weight loss curve. After trapping, both stopcocks A and B are closed to the outside and connected to stopcocks C and D. Trap 1 is then heated to volatilise the condensed products. Now stopcock D is switched toward stopcock A to pressurise the system before stopcock C is turned to inject the vapourised sample as a plug to the GC column. If the concentration of sample gas is high no trapping is necessary. By rotating the stopcock, the trap becomes full of gas which can be used for weight loss measurements. A similar interface was built with larger capacity allowing eight samples to be collected for each weight loss measurement.
Figure 2.21 Schematic diagram of coupled TG-GC system [60]
57
Thermal Analysis of Rubbers and Rubbery Materials
2.14.5 Coupled TG-GC-IR Although GC can identify materials on the basis of their retention time in the column, its analytical capability is greatly enhanced if GC is further combined with IR or MS for positive identification of the components already separated by GC. Once TG has been coupled with GC as described previously, combinations of TG-GC-IR or TG-GCMS are straight forward and constitute extremely powerful tools for rubber product characterisation. Figure 2.22 shows the schematic diagram of TG-GC-IR [63]. A DuPont 950 thermogravimetric analyser, a Perkin Elmer 900 gas chromatograph, and a PerkinElmer 700 Infra red Spectrophotomer, equipped with a Wilks 41C vapour phase cell are combined [63]. The interface between TG and GC is a condensation trap connecting to a sampling valve. The effluent gas from the GC column is split so that 0.0625 of the gas goes to a flame ionisation detector and the rest to a hot wire thermal conductivity detector. The exhaust of the hot wire detector is connected to a series of microtraps each having a volume of 0.7 ml and equipped with inlet and outlet valves made of Teflon. The vapour fraction of each microtap is fed to the vapour phase infra red cell positioned in the spectrophotometer through a diverter valve as shown in Figure 2.22. Cukor [63] uses this technique to analyse a coating used to protect photographic emulsion masks from scratching. In TG, it shows weight losses in three steps (95%, 4%, 1%) in temperature ranges 30-150 ºC, 275-375 ºC and 430-500 ºC. GC scan from retention time and IR spectra identify the fractions as acetaldehyde, water, acetic acid, a mixture of butyric acid and butyraldehyde. Since these products have resulted from decomposition of polyvinyl alcohol, then the coating is nothing but a solution of polyvinyl alcohol. Of course, both sensitivity and speed will be much improved, if FT-IR is used in place of IR. Hence, use of TG-GC FT-IR will be more fruitful than TG-GC-IR.
Figure 2.22 Schematic diagram of coupled TG-GC-IR [63]
58
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials
2.14.6 Coupled TG-GC-MS Like TG-GC-IR, the principle of combining TG-GC with MS is the same. MS has greater sensitivity than IR, so TG-GC-MS is more frequently used than TG-GC-IR. Chang and Mead [64] first reported TG-GC-MS, in which interface was constructed from an 8-way valve, a 6-way valve and two collecting traps, while the TG effluent was collected in one trap, the contents of other trap were analysed by GC-MS. The interface for GC-MS was a standard Biemann-Watson molecular separator used in conjunction with a GC stream splitter. The pyrolysis of PS foam and ethylene vinyl acetate co-polymer was studied by this apparatus. However, most recent devices of TG-GC-MS have capability for TG-MS alone and a combination of TG-GC-MS. A typical setup [58] is shown in Figure 2.23. Here a Dupont 951 TGA is connected to HP 5710A GC through a TG-GC interface, which is a six port microvalve connected with a condensation trap, which can introduce the condensed volatiles into either GC or MS. For selective ion monitoring, a TG-MS capillary interface shown in Figure 2.23 has been used. The GC-MS interface is a jet separator provided by Hewlett-Packard for its GC-MS instrument. All the transfer lines are heated to prevent pre-condensation of volatile products. For easy connection transfer lines are made of 1.6 mm stainless steel tubing. Here analysis of sample less than 1 mg is possible. A HP 21 Max computer and TEKTRONIX 4012 CRT are used for data analysis. Few examples of TG-GC, TG-MS and TG-GC-MS are reported here for characterisation of rubber/polymer analysis. Bart and Raemaekers [65] have studied several ethylenepropylene-diene rubber (EPDM) products and EPDM-SBR blends by TG-MS method.
Figure 2.23 Schematic diagram of TG-GC-MS [58]
59
Thermal Analysis of Rubbers and Rubbery Materials Möhler and co-workers [66] have reported TG-DSC-MS of the thermal decomposition of the vulcanisation accelerator tetramethyl thiuram disulfide in rubber. Meuzelaar and coworkers [67] have used high pressure TG-GC-MS to simulate a solvent-free thermal and catalytic liquefaction reaction for co-processing of waste polymers (non vulcanised SBR or a mixture of waste plastics composed of PE, PS and waste rubber tyres) with coal. Gorman [68] has proposed a controlled thermal desorption (TDS) and concentration method for separating volatile additives from vulcanisable rubber in a TD-GC-MS configuration without the need for prior sample preparation. A major challenge in TG/DTG based analysis of elastomer vulcanisates is to demarcate oil/plasticiser and elastomer regions, which often show overlapping TG events. Deconvolution of the overlapping oil/plasticiser and oil/elastomer TG curves is expected to be feasible with high resolution thermogravimetry (HRTG), MS and principal component analysis (PCA) i.e., HRTG-MS-PCA, which would substitute the dated methods for graphical resolution of oil and polymer weight loss [69]. There are many other coupled TG methods like coupled thermogravimetry – photometry [70], coupled thermogravimetry – electrochemical analysis [71], which are not so popular for characterisation of polymers and rubber.
2.15 Conclusion It is concluded that, the previously mentioned thermal analysis procedures (DSC, DTA, TGA) give information on qualitative and quantitative analyses of rubbers, rubbery materials, rubber-blends, rubber-products. Mechanical properties like modulus, creep, stress relaxations of polymers and rubbers are obtained by TMA and DMA. Optical and electrical properties of rubbers and rubbery materials are determined by DPC and DETA. Modern coupled TG-GC, TG-IR, TG-MS, TG-GC-IR, TG-GC-MS give the complete analyses of the constituents of rubber compounds and rubber products.
Acknowledgements Thanks are due to Mr. Anjan Bhattacharjee, Mr. Ajoy Gayen, Ms. Subhra Dutta for assistance in preparing the manuscript.
References 1. H. Le Chatelier, Comptes Rendus des Seances de l’Academie des Sciences, 1886, 102, 1243. 60
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials 2. N.C. Roberts-Austen, Proceedings of the Royal Society of London, 1891, 49, 347. 3. W.J. Smothers and Y. Chiang, Handbook of Differential Thermal Analysis, Revised Edition, Chemical Publishing Company, New York, NY, USA, 1966. 4. P.D. Garn, Thermoanalytical Methods of Investigation, Academic Press, New York, NY, USA, 1965. 5. W.W. Wendlandt and L.W. Collins in Benchmark Papers in Analytical Chemistry, Volume 2, Dowden, Hutchinson and Ross, Inc., Stroudsburg, PA, USA, 1976. 6. A. Blazek, Thermal Analysis, Van Nostrand Reinhold, London, UK, 1973. 7. W.W. Wendlandt, Thermal Methods of Analysis, Volume 19, 2nd Edition, Wiley Interscience, New York, NY, USA, 1974,. 8. W.W. Wendlandt and P.K. Gallagher in Thermal Characterisation of Polymeric Materials, Ed., E.A. Turi, Academic Press, New York, NY, USA, 1981, Chapter 1. 9. E.S. Watson, M.J. O’Neill, J. Justin and N. Brenner, Analytical Chemistry, 1964, 36, 7, 1233. 10. R.A. Baxter in Thermal Analysis, Eds., R.F. Schwenker, Jr., and P.D. Garn, Academic Press, New York, NY, USA, 1969, Volume 1, p.65. 11. T. Hatakeyama and F.X. Quinn, Thermal Analysis: Fundamentals and Applications to Polymer Science, 2nd Edition, John Wiley and Sons, New York, NY, USA, 2000, p.13. 12. C. Duval, Inorganic Thermogravimetric Analysis, 1st Edition, Elsevier Publishing Company, Amsterdam, The Netherlands, 1953. 13. W.W. Wendlandt, Handbook of Commercial Scientific Instruments, Volume 2, Marcel Dekker, New York, NY, USA, 1974, p.144. 14. L.N. Stewart in Proceedings of the 3rd Toronto Symposium on Thermal Analysis, Toronto, Canada, 1969, p.205. 15. S.D. Norem, M.J. O’Neill and A.P. Gray, Thermochimica Acta, 1970, 1, 1, 29. 16. J.H. Borchardt and F. Daniels, Journal of the American Chemical Society, 1956, 79, 1, 41. 17. E.S. Freeman and B. Carroll, Journal of Physical Chemistry, 1958, 62, 4, 394. 18. D.A. Anderson and E.S. Freeman, Journal of Polymer Science, 1961, 54, 159, 253. 61
Thermal Analysis of Rubbers and Rubbery Materials 19. T.J. Ozawa, Thermal Analysis, 1970, 2, 3, 301. 20. ASTM D698-07e1, Standard Test methods for the Laboratory Compaction Characteristics of Soil using Standard Effort [12,400 ft-lbf/ft3 (600 kN-m/m3)], 2007. 21. D.N. Waters and J.L. Paddy, Analytical Chemistry, 1988, 60, 1, 53. 22. D.W. Brazier and G.H. Nickel, Rubber Chemistry and Technology, 1975, 48, 4, 661. 23. J.J. Maurer, Rubber Chemistry and Technology, 1969, 42, 1, 110. 24. W. Lodding, Gas Effluent Analysis, Marcel Dekker, New York, NY, USA, 1967. 25. H.J. Langer in Treatise on Analytical Chemistry, Part 1, 2nd Edition, Eds., I.M. Kolthoff, P.J. Elving and E.B. Sandell, Wiley, New York, NY, USA, 1980, Chapter 15. 26. G. Lombardi, For Better Thermal Analysis, 2nd Edition, Institute of Minerology, Rome, Italy, 1980, p.18. 27. T. Hatakeyama and F.X. Quinn, Thermal Analysis: Fundamentals and Applications to Polymer Science, 2nd Edition, John Wiley and Sons, New York, NY, USA, 2000, p.133. 28. C.J. Bartlett, Journal of Elastomers and Plastics, 1978, 10, 4, 369. 29. D.P. Bloechle, Journal of Elastomers and Plastics, 1978, 10, 4, 377. 30. M.L. Williams, R.F. Landel and J.D Ferry, Journal of the American Chemical Society, 1955, 77, 3701. 31. J.K. Gillham in Thermoanlysis of Fibers and Fiber-Forming Polymers, Ed., RF Schwenker, Jr., Applied Polymer Symposium, Interscience, New York, NY, USA, 1966, 2, 45. 32. J.K. Gillham, Polymer Engineering Science, 1979, 19, 10, 676. 33. R. Bruce Prime in Thermal Characterisation of Polymeric Materials, Ed., E.A. Turi, Academic Press, New York, NY, USA, 1981, Chapter 5. 34. G.F. Sykes, H.D. Burks and J.B. Nelson in Proceedings of the Natural SAMPE Symposium Exhibition, 1977, 22, 350. 35. N.S. Schneider, J.F. Sprouse, G.L. Hagmauer and J.K. Gillham, Polymer Engineering Science, 1979, 19, 4, 304. 62
Instrumental Techniques used for Thermal Analysis of Rubbers and Rubbery Materials 36. C. Lacabanne, Contribution à l’étude des Propriétés Diélectriques des Polysulfonamides,1974, University of Toulouse, France. [PhD Thesis] 37. C. Lavergne and C. Lacabanne, IEEE Electrical Insulation Magazine, 1993, 9, 2, 5. 38. J-M. Hsu, D-L. Yong and S.K. Huang, Journal of Polymer Research, 1999, 6, 2, 67. 39. S.M. Shin, D.K. Shin and D.C. Lee, Polymer Bulletin, 1998, 40, 4–5, 599. 40. C.M. Neag, J.P. Ibar, J.R. Saffell and P. Denning, Journal of Coatings Technology, 1993, 65, 826, 37. 41. J.E. Volponi, L.H.I. Mei and D.S. Rosa, Polymer Testing, 2004, 23, 4, 461. 42. R.E. Kerby, A. Tiba, B.M. Culbertson, S. Schricker and L. Knobloch, Journal of Macromolecular Science A, 1999, A36, 9, 1227. 43. C.S.B. Ruiz, L.D.B. Machado, J.A. Vanin and J.E. Volponi, Journal of Thermal Analysis and Calorimetry, 2002, 67, 2, 335. 44. B.M. Tareev, Physics of Dielectric Materials, Mir Publishers, Moscow, Russia, 1975. 45. N.G. McCrum, B.E. Reed and G. Williams, Anelastic and Dielectric Effects in Polymeric Solids, Wiley, London, UK, 1967. 46. S.R. Radhakrishnan and D.R. Saini, Journal of Applied Polymer Science, 1994, 52, 11, 1577. 47. T. Hatakeyama and F.X. Quinn, Thermal Analysis: Fundamentals and Applications to Polymer Science, John Wiley and Sons, New York, NY, USA, 2000, Chapter 6. 48. Jen Chiu in Applied Polymer Analysis and Characterisation: Recent Developments in Techniques, Instrumentation and Problem Solving, Ed., J. Mitchell, Jr., Hanser Publishers, Munich, Germany, 1987, Chapter IIG. 49. D.E. Smith, Thermochimica Acta, 1976, 14, 3, 370. 50. C.A. Cody, L. Dicarlo and B.K. Faulseit, American Laboratory, 1981, 13, 1, 93. 51. C.A. Cody, L. Dicarlo and B.K. Faulseit in Proceedings of the 10th North American Thermal Analysis Society Conference, 1980, Boston, MA, USA, p.137. 52. E.K. Gibson, Jr., and S.M. Johnson, Thermochimica Acta, 1992, 4, 1, 49. 63
Thermal Analysis of Rubbers and Rubbery Materials 53. G.J. Mol, Thermochimica Acta, 1974, 10, 3, 259. 54. S.M. Dyszel in Analytical Calorimetry, Volume 5, Eds., J.F. Johnson and P.S. Gill, Plenum Press, New York, NY, USA, 1984, p.277. 55. S.M. Dyszel, Thermochimica Acta, 1983, 61, 1-2, 169. 56. F. Zitomer, Analytical Chemistry, 1968, 40, 7, 1091. 57. H.K. Yuen, G.W. Mappes and W.A. Grote, Thermochimica Acta, 1982, 52, 1-3, 143. 58. J. Chiu in Analytical Calorimetry, Volume 5, Eds., J.F. Johnson and P.S. Gill, Plenum Press, New York, NY, USA, 1984, p.197. 59. G. Janowska and L. Slusarski, Journal of Thermal Analysis and Calorimetry, 2001, 65, 1, 205. 60. J. Chiu, Analytical Chemistry, 1968, 40, 10, 1516. 61. J. Chiu, Thermochimica Acta, 1970, 1, 3, 231. 62. D.W. Brazier and N.V. Schwartz, Rubber Chemistry and Technology, 1978, 51, 5, 1060. 63. P. Cukor and E.W. Lanning, Journal of Chromatographic Science, 1971, 9, 487. 64. T-L. Chang and T.E. Mead, Analytical Chemistry, 1971, 43, 4, 534. 65. K.G.H. Raemakers and J.C.J. Bart, Thermochima Acta, 1997, 295, 1-2, 1. 66. H. Möhler, A. Stegmayer and E. Kaisersberger, Kautschuk und Gummi Kunststoff, 1991, 44, 4, 369. 67. K. Liu, E. Jakab, W. Zmierczak, J.S. Shabtai and H.L.C. Meuzelaar, ACS Preprints, Division of Fuel Chemistry, 1994, 39, 2, 576. 68. W.B. Gorman, Jr., inventor; Bridgestone/Firestone Inc., assignee; US 5,191,211, 1993. 69. S.J. Swarin and A.M. Wims, Rubber Chemistry and Technology, 1974, 47, 5, 1193. 70. B.B. Johnson and J. Chiu, Thermochimica Acta, 1981, 50, 1-3, 57. 71. S.G. Fischer and J. Chiu, Thermochimica Acta, 1983, 65, 1, 9. 64
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
3
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials Amit K. Naskar and Prajna P. De
3.1 Introduction Thermal analysis is an essential technique to measure the temperature or time dependent response of physical and chemical changes that occur in materials. In these techniques the sample is placed in a small chamber whose temperature can be controlled by a multi-step program. During each test scan, the temperature is usually set constant or altered periodically. Any change that occurs in sample against time or temperature is detected by a specific electronic device and recorded. In differential scanning calorimetry (DSC) mode, difference in heat flux between sample and an inert reference placed in the chamber is measured. Sometimes difference in temperatures between sample and reference is recorded and the technique is called differential thermal analysis (DTA). In a thermogravimetric analyser (TGA), change in mass of the sample is measured with variation in temperature or time using a force transducer. These techniques are extensively used to characterise polymeric materials [1]. Details of instrumentation for these techniques have been discussed in an earlier chapter (Chapter 2). This chapter summarises various applications of DSC and TGA for the characterisation of rubbers and rubbery materials.
3.2 Differential Scanning Calorimetry of Rubbery Materials In general, DSC is used for many applications including characterisation of materials by measuring parameters associated with phase transition (such as temperature and activation energy of phase transition), chemical reaction (such as crosslinking, oxidation and degradation) and micro- or macro- morphological alteration. For polymeric materials, morphological characteristics and thermo-chemical reactivity are important to control the physico-mechanical properties, processability and stability. A schematic of a generalised DSC thermogram of a polymer is displayed in Figure 3.1. Endothermic baseline shift at the lower temperature region indicates glassy to rubbery transition (Tg) of the polymer. For rubbery materials this temperature is below the ambient temperature. Like semi-crystalline thermoplastics a few rubbery materials often show cold crystallisation (exotherm) and a subsequent melting (endotherm) during a heating scan. Cis-1,4-polybutadiene, for 65
Thermal Analysis of Rubbers and Rubbery Materials
Figure 3.1 Schematic DSC thermogram of an imaginary rubbery material: (a) baseline shift indicating glass-rubber transition, (b) cold crystallisation exotherm, (c) melting endotherm or endotherm for volatilisation of plasticiser, (d) curing and/or oxidative degradation and (e) pyrolytic thermal degradation
example, displays a Tg at –100 °C, a cold crystallisation at –56 °C and a melting transition at –6 °C, when scanned at 20 °C/min [2]. Polyisoprene and natural rubber exhibit a cold crystallisation temperature at –35 to –23 °C [3]. Rubbery materials containing volatile plasticiser display endothermic broad peak due to loss of plasticisers [4]. Curative compounded rubbers display a crosslinking exotherm at a temperature ranging from 100-200 °C. Oxidative degradation of rubbers is also exothermic in nature. At very high temperature (250-500 °C) rubbery materials usually pyrolyse and display an endotherm. It can be reasonably stated that a rubbery material does not necessarily exhibit all these transitions shown in Figure 3.1 in a single DSC run. For polymeric materials, in general, DSC is used for the determination of following: i)
Specific heat and Tg,
ii) Melting point (Tm) and heat of fusion, iii) Curing characteristics, iv) Decomposition characteristics.
3.2.1 Measurement of Specific Heat and Glass Transition Temperature Specific heat (CP) of any material can be measured easily in a DSC apparatus using a constant rate of heating scan, a calibration run of empty cell and a controlled scan on a calibration specimen (sapphire, in general) following standard ASTM E1269 method [5]. Modulated temperature DSC is used to measure CP from a single experimental run 66
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials of the material using a calibrated instrument (ASTM E 1952). Temperature modulated DSC analyses of polymers have been broadly reviewed by various authors [1, 6]. Amorphous polymeric molecules, at low temperature, exist in a glassy state where molecular motions are frozen (such a state is called super-cooled liquid). With rise in temperature a subnano-scale process involving segmental molecular motion starts in the glassy polymer. The temperature (actually a range of temperatures), where segmental mobility in the polymer chain starts is called the Tg. With increase in degrees of freedom at a temperature beyond Tg, the heat capacity of the polymer increases. Therefore, in a DSC scan of a polymer, usually a shift in baseline is observed at the Tg. The most precise determination of Tg is done by cooling an equilibrium melt (rubbery) at a specified cooling rate and finding the temperature at half freezing [7]. Tg of a polymer depends on temperature scan rate, thermal history, molecular structure and composition of the material. It also depends on the experimental methods, e.g., DSC, dynamic mechanical analysis (DMA), thermomechnical analysis (TMA), dilatometry, etc., used to measure the Tg. However, for the DSC measurements, different methods of locating Tg (such as, extrapolated on-set, half height (ASTM E1356), derivative plot), are reported by Sircar and co-workers [8]. Commonly the point on thermal curve corresponding to half of the heat flow difference between the shifted base line is called the Tg. More precisely, Tg represents the temperature corresponding to the point on the curve at half heat capacity difference between the extrapolated baselines.
3.2.2 Significance of Tg Glass to rubber transition occurs during heating of the polymer from very low temperature, where amorphous molecular chains cannot exhibit segmental mobility. With increase in temperature, at a certain point, the segmental mobility starts and the polymer chains behave as flexible, rubbery material. The most successful theory behind Tg is the free volume theory of Flory [9]. It states that the glass transition (from rubbery state) occurs in a polymer when the fraction of free or unoccupied volume in a material reaches a constant value, which cannot be lowered further by cooling the material. For practical applications a Tg of a polymer represents the lower end use temperature. A polymer cannot be processed at a temperature below its Tg. For elastomeric products a rubbery polymer with very low Tg is desired. For example natural rubber (cispolyisoprene) has a Tg of –70 °C and is extensively used in rubber products. However, the trans-polyisoprene (Gutta percha) is a semi-crystalline plastic material and has a Tg of 20 °C to 35 °C depending on the molecular weight [10]. It has very limited applications (used in golf ball covers and adhesives only). Polybutadiene and polyisobutylene both are rubbery but the former having a lower Tg (–80 °C) exhibits a higher degree of rebound resilience than the latter (Tg = –60 °C). Silicone has the lowest Tg (–120 °C) and its oligomer acts as a lubricant. 67
Thermal Analysis of Rubbers and Rubbery Materials Glass transition temperature of a polymeric material is used to characterise polymers, polymer blends and their compatibility, block copolymers, degree of plasticisation, and extent of crosslinking in elastomer systems. Table 3.1 summarises the Tg of different rubbery materials.
Table 3.1 Glass transition temperatures (Tg) of some common rubbers and polymers* Name of Polymers Cis-Polyisoprene or natural rubber (NR) Polybutadiene (syndiotactic) (BR) Styrene butadiene rubber (SBR) Neoprene or chloroprene (CR) Ethylene propylene diene rubber (EPDM) Polyurethane rubber Butyl rubber (IIR) Epoxidised natural rubber (ENR) Silicone rubber Polyethylene Polypropylene (PP) Polycaprolactam (Nylon 6) Polystyrene (PS) Polyvinyl chloride (PVC) Polyacrylonitrile (PAN) Polymethyl acrylate Polyethyl acrylate Polybutyl acrylate Fluoro rubber (FKM) Fluoro silicone rubber (FVMQ) Acrylonitrile butadiene rubber (NBR)
Tg (°C) -75 to -70 -100 to -85 -65 to -50 -45 to -20 -60 to -55 -45 to -20 -70 to -65 -40 to -20 -125 to -120 -120 (linear) to -80 (branched) -19 to -8 50 to 57 100 83 (neat), -8 (40% plasticised) 105 (when moisture free) 9 to 0 -20 to -25 -63 to -55 -50 to -18 -70 -45 (low nitrile content) -34 (medium nitrile content) -20 (high nitrile content) Halogenated butyl rubber (BIIR or CIIR) -66 to -60 Chlorinated polyethylene (CM) -40 to -25 (depends on content of Cl) Chlorosulfonated polyethylene (CSM) -25 (depends on level of Cl and SO2) Polyepichlorohydrin (CO) -26 Polyacrylate rubber (ACM) -44 to -22 Ethylene acrylate rubber (EAM) -40 (depends on monomer ratio) Polyfluorophosphazene (PNF) -66 Polyethylene glycol -50 Ethylene vinyl acetate (EVA) copolymer -30 (depends on monomer ratio) *Tg depends on experimental condition e.g., rate of heating, reference used, atmosphere and thermal history, and morphology/microstructure of the polymer
68
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
3.2.3 Factors Affecting Tg Various factors that affect polymer chain mobility control the Tg. Those factors can be classified as follows: i)
Molecular weight,
ii) Molecular architecture, iii) Morphology, composition and thermal history, and iv) Degree of crosslinking. The following sections illustrate the effect of these factors on Tg for different elastomer systems.
3.2.4 Effect of Molecular Weight on Tg The physical characteristics of a polymer depend on its molecular weight. Monomers are generally gas or liquids. Oligomers are mostly liquid. In general, high molecular weight polymers are solid (often termed as super cooled liquid, if totally amorphous). Fox and Flory established a mathematical relationship between Tg and molecular weight (MW) as given in Equation 3.1 [11]: Tg Tg () (MW)1
(3.1)
()
where, Tg is the limiting Tg at infinite molecular weight and Kg is a constant, characteristic of the polymer.
3.2.4.1 Chain Length Effect Cowie and McEwen used DSC to measure the Tg of silicone oligomers and polymers [12]. The Tg values for silicones of different molecular weight are presented in Table 3.2. The Tg() and Kg values, obtained from Tg – MW-1 plot, correspond to 148 K and 5.9 x 103 K.g/mol, respectively. A similar trend in Tg and MW has also been found for glassy polymers such as PS [13], and a polysulfide [14]. Kow and coworkers investigated dependence of Tg of polyisoprene (IR) with MW [15]. Sometimes Tg – MW-1 plot for IR does not follow the Fox-Flory equation. This is due to the variation in proportion of 3,4-, cis-1,4 and trans-1,4 microstructures in IR.
69
Thermal Analysis of Rubbers and Rubbery Materials
Table 3.2 Tg of polydimethylsiloxanes (PDMS) of varying chain length [12] Mn Tg ( °C) (± 1 °C)* 236 -150 540 -137 2400 -127 7700 -125 19000 -125 57000 -125 * Onset Tg data obtained using DuPont DSC 900 module at 10 °C /min heating rate Mn: number average molecular weight Reproduced with permission from J.M.G. Cowie and I.J. McEwen, Polymer, 1973, 14, 9, 423. ©1972, Elsevier
Table 3.3 Effect of end groups on the Tg of perfluoro oligomers E-CF2O[(CF2CF2O)p(CF2O)q]CF2-E [16] Degree of polymerisation = [p + q + 3], where p/q ~1
Tg ( °C) * of the polymers having end group -CH2OCH3 –CH2OSi(CH3)3 -CH2OH 9 -97.5 -127.8 -120.0 14 -107.0 -126.6 -121.6 24 -113.9 -124.2 -120.7 42 -117.9 -122.5 -121.3 * Measured by DSC (Mettler TA 3000) cooling scans from 35 to –175 °C at 10 °C/min Reproduced with permission from F. Danusso, M. Levi, G. Gianotti and S. Turri, Polymer, 1993, 34, 17, 3687. ©1993, Elsevier
3.2.4.2 End Group Effect Low MW polymers have a large number of end groups that have greater mobility compared to that of the molecular body. Thus, cohesive state of the system progressively loosens with decrease in chain length. This is true when body and end units are similar. For telechelic oligomers or telomers, however, the chemical dissimilarity of body and end units affects the trend. For example, oligomers of perfluorinated chain E-CF2O[(CF2CF2O)p(CF2O)q]CF2-E with two equal foreign end groups (E) show both increase and decrease in Tg with increase in MW depending on how dissimilar the end groups and body chain are [16]. The representative data for such cases are shown in Table 3.3. When the end groups are similar to the backbone chain, (i.e., when E is –OCF3 or –CH2OCH3), body and end groups have improved cohesion due to compatibility and hence display an increase in Tg with a rise in MW [17]. If the body and end groups significantly differ chemically, incompatibility causes sub-phase or phase separation. 70
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
3.2.4.3 Cyclic Polymers For cyclic polymers the dependence of Tg on MW is different from that of the linear polymers. Clarson and co-workers measured the Tg of cyclic and linear counterparts of polydimethylsiloxane PDMS and polyphenylmethylsixonaes (PPMS) [18-19]. Non-linear Tg – MW-1 dependence in cyclic PPMS suggested that ring structure, size and strain affects glass formation during cooling. The disparity in Tg – MW-1 plots of cyclic PDMS and cyclic PPMS was attributed to the manifestation of difference in molecular topology, i.e., replacement of the methyl group by a bulkier phenyl ring in the cyclic siloxanes.
3.2.4.4 Block-copolymers For elastomeric block copolymers of the A-B-A type, generally, a soft rubbery segment is connected to the hard segments in both ends. The segmental MW affects the Tg of the material. However, the segmental MW are less influential than the morphology of the phases and processing conditions to control the Tg. Ikeda and coworkers found that Tg of the hard phase in styrene-butadiene-styrene (SBS) block copolymer randomly varied between 95 °C to 105 °C with increase in MW [20].
3.2.5 Effect of Polymer Architecture on Tg 3.2.5.1 Linear Homopolymer As discussed in the previous section about the difference in Tg – MW-1 plots of cyclic PDMS and PPMS, it is clear that polymer chain architecture is a key factor to control the Tg. At a similar MW range linear PPMS displays higher Tg than that of the linear PDMS. The limiting Tg for PDMS and PPMS are –123 °C and –33 °C, respectively [18-19].
Presence of Polar Group
Incorporation of a polar group in a polymer chain, usually, increases Tg. Polyethylene (PE) displays a Tg at –120 °C. Replacement of hydrogen in the PE repeat unit by a methyl group (polypropylene) or a chlorine group (PVC) increases Tg to -20 °C or 83 °C, respectively. Due to the strong dipolar interaction, the Tg is very high in PVC. Acrylic rubber obtained from acrylic esters exhibits different a Tg range depending on the alkyl group present on the ester moiety. For example, if the alkyl groups are methyl, ethyl, propyl, and butyl, the Tg of the polymers are 0 °C, -23 °C, -51 °C and –63 °C, respectively [21].
71
Thermal Analysis of Rubbers and Rubbery Materials Cis-trans Isomerism
Microstructure of the diene rubbers is very critical to control the physical properties. Cis-1,4-poybutadiene is amorphous and displays a Tg of –100 °C. However, the trans1,4-BR is crystalline and it melts in the temperature range of 40-60 °C [22]. The Tg of the trans-isomer is very indistinct by thermal analysis. Tg values depend on the degree of crystallinity present in the trans-1,4-BR. Cis-1,4-BR crystallises during stretching, like NR [23]. In the actual BR composition 1,2-BR segment also exists along with cisand trans-1,4 microstructure. With increase in 1,2 microstructure content in BR the Tg of the polymer increases and the polymer becomes semi-crystalline [24-26]. Table 3.4 displays the Tg of BR having different microstructure content. Like butadiene rubber, Tg of isoprene rubber also depends on the polymer microstructure. Onset of Tg for cis-1,4polyisoprene is -65 C. However, trans-1,4-polyisoprene exhibits onset of Tg at a higher temperature (-60 C) [27]. Presence of the 3,4-isomer increases the Tg of polyisoprene [27-28]. Polyisoprene containing 76% cis-1,4-; 16% trans-1,4-; and 8% 3,4- shows the lowest Tg (-63 C), whereas that containing 28% cis-1,4-; 12% trans-1,4-; 56% 3,4- and 4% 1,2- shows the highest Tg (-20 C). For a tyre tread formulation having excellent skid and rolling resistance, Tg of the base rubber is a critical factor. Thus, physical characteristics of the tailored base polymer (elastomer) with varied proportion of microstructure can be predicted from thermal analysis data.
Table 3.4 Tg of BR with different microstructures* Microstructurea 1,2-BR 1,4-cis 1,4-trans 6 41 53 8 36 56 17 35 48 35 27 38 52 21 27 56 15 29 65 12 23 78 9 13 90 3 7 * molecular weights (Mn) of the polymers are high 10,000-100,000 a 13 C-NMR data b DSC cooling @10 °C/min data [26] c DSC heating @10 °C/min data [25]
72
Tg (°C) -95.0 b -95.2 c -87.5 c -72.3 c -59.6 c -51.8 b -45.0 b -29.6 c -16.2 c
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials Tacticity
Tacticity of a polymer is also a critical factor that controls Tg. Hydrogenated 1,2-polybutadiene of atactic microstructure displayed a Tg onset at –28.8 °C. However, the syndiotactic counterpart at similar MW range exhibited slightly higher Tg (–24.9 °C) [29]. Hydrogenated 1,4-poly(2,3-dimethyl-1,3-butadiene) (H-PDMB) displayed lower Tg than that of its non-hydrogenated counterpart (PDMB: 74% trans, 23% cis and 3% 1,2 units) [30]. H-PDMB is completely amorphous elastomeric material that has a Tg 17 °C lower than that of the atactic head-to-tail polypropylene.
3.2.5.2 Random Copolymer The Tg of a random copolymer is an intermediate temperature between two Tg temperatures of the corresponding homopolymers. The following mathematical equations correlate Tg of a co-polymer with that of the homopolymers: (1) Gordon and Taylor Equation [31]: Tg
Tg A .w A K.Tg B .w B w A K.w B
(3.2)
where wA and wB are the weight fractions of the homopolymers, TgA and TgB are the Tg temperatures of the respective homopolymers, and TgB > TgA. K is the volume additivity parameter and defined as K = (l/2)( 2/ 1), where 1 and are the density and 1 and 2 are the change in coefficient of thermal expansion of two constituent polymers. (2) Couchman-Karasz Equation [32]: This equation considers that the copolymer is a statistical mixture of individual components. The equation is given as: TgB Tg w B C PB .ln Tg A ln
w A CPA w B C PB Tg A
(3.3)
(3) Fox Equation [33]: w w 1 A B Tg Tg A TgB
(3.4)
The Fox equation can be derived from Gordon-Taylor equation, considering the fact that Tg is constant for linear polymers and l/2 1 [34]. 73
Thermal Analysis of Rubbers and Rubbery Materials Wood experimentally verified the Gordon-Taylor equation with different copolymer systems [35]. Penzel and co-workers also utilised DSC data of various copolymers to verify the Gordon-Taylor Equation [36]. De Sarkar and coworkers utilised DSC to measure the Tg of SBR rubbers having different styrene contents and found that the Tg of copolymers follows the GordonTaylor Equation [37]. The relationship between percentage styrene content and Tg of SBR is shown in Figure 3.2. Increase in styrene content linearly increases the Tg of the copolymer. However, refractometric Tg data of such compositions, published in the earlier original work, displayed deviation from linearity [38]. Figure 3.2 also shows the Tg of polyethylene-co-vinyl acetate EVA at different vinyl acetate content (wt%) and the Tg of polyacrylonitrile-butadiene (NBR) at different acrylonitrile contents. It is apparent that the data for EVA does not follow a linear equation, but both SBR and NBR follow a linear trendline [37, 39-40]. For epoxidised SBR, increase in epoxy content increases Tg of the SBR rubber. An SBR with 43% epoxy content displayed a Tg of -10 °C, when epoxy content was 68% the Tg was 2 °C [41]. Bhattacharjee and co-workers analysed EPDM produced by a slurry process at different ranges of ethylene content [42]. The Tg of the statistically random terpolymers at constant diene (ethylidene norbornene) level of ~5% increased with increase in ethylene content. At a very high level of ethylene content, the material turned semi-crystalline and displayed a melting endotherm. The physical properties and the
Figure 3.2 Relationship between percentage monomer content, (- -) styrene in SBR; (- -) vinyl acetate in EVA; (- -) acrylonitrile in NBR and the Tg of different copolymers. Tg values were measured using DSC at 10 °C/min for SBR [37], 20 °C/min for EVA [39] and 3.5 °C/min in modulated DSC mode for NBR [40]
74
-38 46 980
-57 48 702
-63 50 1000
-84
Hard segment wt(%) Soft segment Mn 1000
a
Polyurethane a
Polyimide b
Polyester a
PTMO
20 °C/min heating rate [48]
b
20 oC/min heating rate [49]
-
Hard segment
additive mixing characteristics of the polymer depend on the Tg of the terpolymer. It is desired that the Tg of the EPDM be low and the material be amorphous. Thus, DSC analysis provides an excellent tool for the characterisation of EPDM elastomers that are applicable at low temperature.
3.2.5.3 Block Copolymers
Soft segment Polymer type
Table 3.5 DSC Tg of PTMO segment in homopolymer and different block copolymers
Tg ( °C) of soft segment
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
Diblock or triblock copolymers display a broader Tg of the segments than that of homopolymer segments. This is due to mixing of microphases [43] and also the rigidity imparted by the adjacent hard phases to the soft segments [44]. For a pom-pom shaped star polystyrene-block-polyisoprenestar polystyrene polymer [S4-I-S4] the Tg of isoprene (-40 C) is higher than that of S-I-S triblock copolymer (-49 C) or linear polyisoprene (-73 C). For uncoupled S4-I block copolymer the T g of isoprene segments is -57 C, which is simply due to more mobility of the openended isoprene chain [44]. Thermoplastic polyurethane elastomeric block copolymers of (A-B)n type consisting of siloxanes and urethane phases having 60/40soft/hard phase ratio displayed a Tg of siloxane phase at -97 °C. However, if the siloxane soft segment is block copolymerised with polytetramethylene oxide (PTMO) of MW 2000, the Tg is further lowered to –102 °C [45]. 75
Thermal Analysis of Rubbers and Rubbery Materials For block copolymers, molecular structure of the adjacent hard segment affects the Tg of the specific soft segment. For example, let us consider thermoplastic elastomeric block copolymers based on common PTMO soft segment but different chemical structure of hard segments as shown in Table 3.5. All these samples had approximately similar soft segment molecular weight and hard phase content (wt%). Variation in soft segment structure keeping MW the same, also affects the Tg of the elastomers having similar hard phase structure [46-47].
3.2.6 Effect of Composition, Morphology and Thermal History on the Tg of Polymers
3.2.6.1 Composition and Morphology of Block Copolymers A block copolymer having two or more dissimilar homopolymer segments along the chain backbone segregate and form micro-phases. These polymers are mostly used as thermoplastic elastomers. A particular block copolymer elastomer can have different morphology depending on prior thermal history or annealing conditions. DSC offers a striking tool to characterise the morphology of such block components.
Figure 3.3 DSC scans of elastomeric PU block-copolymer taken after annealing at 170 °C for 5 minutes, followed by post annealing at room temperature for different period: (1) non-annealed a control; (2) 5 minutes after annealing; (3) 20 minutes after annealing; (4) 2 hour after annealing; (5) 7 days after annealing [50] Reproduced with permission from Journal of Applied Physics, 1975, 46, 4148. ©1975, American Institute of Physics
76
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials Block copolymers of hard and soft segments at room temperature display a Tg of the soft segments at equilibrium phase morphology. At this condition the soft segments exists in extended form. With increase in temperature, during annealing, relaxation of soft segment (extended to coiled form) pulls out hard phase segments and causes phase mixing. As a result Tg of the soft phase increases. After annealing, however, quenched annealed samples show time dependent phase separation and display gradual decrease in Tg [47, 50]. Figure 3.3 shows a DSC scans of elastomeric polyurethane before and after annealing at 170 °C. The Tg of the block copolymer also depends on the composition (ratio of hard and soft segment masses). Increase in the hard phase content usually increases the Tg of the soft phases. Compression moulded polyether esters (similar structure to that shown in Table 3.5) having hard phase content (wt%) 33, 57, and 84 display Tg of –68 °C, -55 °C, and –9 °C, respectively [51]. A similar trend was observed with multiblock poly(ether-ester-amides).
3.2.6.2 Rubbery Blends Plasticised thermoplastics can act as melt-processible rubber. For this purpose a plasticiser or a rubber is usually blended with thermoplastics. In these cases, phase morphology affects the physical characteristics of the material. DSC is used to measure the Tg of a blend’s components. If Tg of the components in the blend does not alter, then the blend is incompatible. However, if the Tg of the polymer having a lower Tg is increased and that of the high Tg polymer is lowered, the blend is compatible. The extent of shortening the Tg interval is a measure of the degree of compatibility. If both the Tg merge, the blend is miscible. Characterisation of different miscible and immiscible rubbery blends using various thermal analysis tools are reviewed elsewhere in this book.
Miscible and Partially Miscible Blends
A simple example of a blend could be plasticised thermoplastics. Plasticisers enhance segmental mobility of the plastics and lower the Tg of the plastics. For example, PVC is not processable without plasticisers. Depending on the loading of plasticiser the material behaves as plastic, leathery or rubbery. Brennan studied the effect of plasticisation of PVC using DSC [52]. The Tg of the PVC phases at different dosage of di(2-ethylhexyl) phthalate (DOP) decreased from 95 C (for PVC without DOP). For example, PVC containing 50% loading of DOP exhibits a Tg of -51 C [52]. Tg of hydrocarbon blends used in pressure sensitive adhesives is very critical for the performance of products. The Gordon-Taylor equation (Equation 3.2) was found to be useful to predict the desired composition of the pressure sensitive adhesive for practical usage [53]. Ethylene-propylene rubber mixed with N-hexane displayed reduced Tg of the extended rubber [54]. 77
Thermal Analysis of Rubbers and Rubbery Materials Wang and Cooper reported the DSC analysis of the blends of PVC and acrylonitrilebutadiene rubber (NBR). It was observed that the miscibility or compatibility criteria depends on the acrylonitrile content in NBR [55]. An NBR with 31% acrylonitrile (NBR31) was miscible with rigid PVC. DSC scans of such blends are displayed in Figure 3.4a. Blends displayed a single Tg between –27 °C (Tg of NBR31) and 84 °C (Tg of PVC). However, blends of NBR with 44% acrylonitrile content (NBR44) exhibit partial miscibility with PVC. DSC traces of these blends are shown in Figure 3.4b. The reason for the immiscibility of the NBR44 and PVC is likely to be the strong dipolar interaction between acrylonitrile and the PVC segments, which excludes the mixed phase from the butadiene rubbery phases. Manoj and co-workers investigated similar miscible blends of PVC and NBR and observed that the properties of such miscible blends improve if sufficiently melt-mixed over longer period [56]. Increase in rheological torque with time at high temperature and insolubility (swelling) in solvents of melt-mixed blends indicated crosslinking, which was further supported by spectroscopic measurements. Authors termed such polyblends as self-crosslinkable rubber blends. Pruneda and coworkers recently reported the thermal characterisation of such blends [57]. Addition of tribasic lead sulfate, as stabiliser, in PVC based rubbery blends plays a crucial role in the morphology. Stabilised PVC is miscible with epoxidised NR containing
Figure 3.4 DSC scans of various NBR-PVC blend compositions for NBR with (a) 31% acrylonitrile content and (b) 44% acrylonitrile content: (1) neat NBR; (2) NBR/PVC, 75/25; (3) NBR/PVC 50/50; (4) NBR/PVC, 25/75; (5) neat PVC [55] Reproduced with permission from C.B. Wang and S.L. Cooper, Journal of Polymer Science: Polymer Physics Edition, 1983, 21, 1, 11. ©1983, John Wiley and Sons
78
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials 50 mol% epoxy group (ENR-50) [58, 59]. However, the non-stabilised blend appears immiscible [60]. Immiscible blends exhibit interfacial crosslinking during thermal annealing. Miscible blends based on stabilised PVC cannot crosslink and such blends are soluble in tetragydrofuran (THF). Addition of carboxylated nitrile rubber (XNBR) in the immiscible ENR-50/PVC blend composition, at a critical loading, makes the blend miscible, which is self-crosslinkable too. A blend consisting of 1:2 ENR-50 and XNBR is miscible and self-crosslinkable [61]. Addition of intermediate super abrasive carbon black enhances self-crosslinking behaviour without affecting the miscibility criterion [62]. Examples of other miscible and self-crosslinkable blends are Hypalon/XNBR [63], ENR/Hypalon [64], and CR/ XNBR/ENR [65]. The blend of CR and ENR-50 was partially miscible and addition of XNBR made the blend miscible [66]. Roland and co-workers have reported that CR epoxidised polyisoprene with 25 mol% epoxy group blends (EPI-25) were miscible and displayed a single Tg depending on the composition, within a temperature ranging from –44 °C to –42 °C [67] and the relaxation time of EPI segments became shorter in the blends [68]. Hypalon/ENR blends were miscible at lower epoxy content (25 mol%) in ENR rubber at all blend ratios. However, the blends of Hypalon with ENR having higher epoxy level (50 mol%) appeared immiscible when ENR content was low [69]. Hydrogenated NBR/PVC blends were miscible [70-71]. Blends of cis-1,4-IR and atactic 1,2-BR are always miscible [72]. Interaction parameters for miscible blends of cis-1,4-IR and 1,2-BR is more when the latter is syndiotactic [73]. DSC scans of miscible blends of IR and 1,2-BR are shown in Figure 3.5. At higher IR contents the Tg shifts to a higher temperature and the crystallinity in the 1,2-BR segments diminishes. Yamada and Funayama investigated the miscibility of IR and a copolymer of cis-1,4-BR and 1,2-BR [74]. Blends of BR and SBR are partially miscible [75-76]. Several authors reported single broad Tg of the SBR/BR blend vulcanisates [76-77]. A broad Tg that was observed in a 60/40 SBR/BR blend actually consisted of two Tg, as indicated in a differential heat flow curve [78]. This indicates incomplete homogenisation of the blend. Hourston and Song measured the interface content in an SBR/BR partially miscible blend using modulated DSC data from blend and unmixed samples in the same proportion in the DSC pan [79]. Figure 3.6 shows the glass transition behaviour of the polyblend and a non-blended physical mixture of 50/50 SBR/BR. Overlapped transition in the blend indicates partial miscibility of SBR and BR. Analysis showed that with increasing styrene content in SBR of SBR-BR blends, the amount of interfacial mix is decreased [79]. Poly(ethylene oxide) and EVA are miscible at all compositions in the amorphous phase [80]. Similarly FKM and ACM also form a miscible elastomeric blend [81]. Addition of a polyacrylate plastic phase to the miscible binary FKM/ACM blend produced thermoplastic 79
Thermal Analysis of Rubbers and Rubbery Materials
Figure 3.5 Representative DSC thermograms of blends of 1,2-BR and IR at different volume fractions of IR: (1) 0%; (2) 10%; (3) 25%; and (4) 40% [72] Reproduced with permission from C.M. Roland, Macromolecules, 1987, 20, 10, 2557. 1987, American Chemical Society
Figure 3.6 dCP/dT versus temperature plots for the SBR–BR (50:50 by weight) processed blend (- - -) and the granular mix ( ) placed in a DSC sample holder [79] Reproduced with permission from D.J. Hourston and M. Song, Journal of Applied Polymer Science, 2000, 76, 12, 1791. ©2000, John Wiley and Sons
80
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials elastomeric (TPE) blend, which is (i) processable like thermoplastics and (ii) elastic like crosslinked rubber. In such blends, however, immiscibility of the rubber and plastic phases are desired. For improved properties of such blends the interaction at the interphase should be high. For a miscible blend, however, a single Tg is observed. Thus, DSC offers a measurement of compatibility of such blend components by the measurement of Tg. Coran and co-workers studied extensively and developed TPE formulations based on different rubber plastic blends using dynamic vulcanisation technique [82-83]. Choudhury and co-workers studied the compatibility of NR/PP blends using DSC. Increase in Tg of NR phase after dynamic vulcanisation of the blend in comparison to that of the neat NR indicated compatibilisation [84]. DSC was used to characterise dynamically vulcanised thermoplastic elastomeric blends of PP/oil extended EPDM [85] and SAN/NBR [86]. Tg of semi-crystalline plastic phases such as PP, PE is very difficult to detect in DSC but here DSC scans are used to detect the melting transition of such blends. Phase morphology of thermoplastic elastomeric rubber-plastic blends based on recycled rubbers are also characterised by DSC and other thermal analysis tools [86]. DSC was used to study the compatibility or partial miscibility of the following rubberplastic blends: Santoprene/Nylon 66 [87], epoxy resin/styrene-butadiene block copolymer [88], polymethylmethacrylate (PMMA)/polyvinyl acetate (PVAc) [89], S-I-S/PPO [89], PMMA/CO blends [90], ENR-50/PE-co-acrylic acid) [91], ENR/ethylene-methyl acrylate copolymer [92].
Immiscible Blends
Different rubber blends are used in many applications because of technological requirements. For example, in tyre formulations, blends of NR, SBR, BR, IIR, and EPDM are used because of specific properties of each component. BR has excellent wear resistance but poor tear resistance. NR has excellent tear resistance but moderate wear resistance. SBR exhibits very good road holding characteristics in comparison to NR and BR. IIR and EPDM show excellent ageing resistance [93]. Blends of all the combinations are not always compatible, although filler loaded co-vulcanised blends are used in actual applications. Thermal analysis provides an excellent tool to determine the optimum level of homogeneity that can be reached in such blends by appropriate mixing conditions. NBR/EPDM blends are incompatible blends. However, addition of chlorinated PE or Hypalon at the 5 wt% level was found to be effective for compatibilisation of the system [94]. NBR/linear low-density PE incompatible blends were also compatibilised by gamma irradiation [95]. Bair and coworkers studied the blend of PVC and acrylonitrilebutadiene-styrene terpolymer (ABS) and found that the blends are incompatible [96]. Yehia and co-workers suggested that measurement of Cp for Tg of each phase at different blend ratios could indicate incompatibility of the blends [97]. This technique showed incompatibility of NR-BR, NR-NBR, and CR-NBR blends. Typical DSC scans showing 81
Thermal Analysis of Rubbers and Rubbery Materials variation of Cp with temperature for NR/BR blends are shown in Figure 3.7. Several authors described incompatibility of different blend systems, earlier, using DSC; such blends are NR-BR [75-76], SBR-NR [75-76], SBR-IR [75, 98], NR-ENR [99], BR-SBR with high styrene content [79], NBR-NR [75], NR-acrylic rubber [100], NR-CR [101], and ENR50-cis-1,4-BR [102], to name a few. PS-BR blends are immiscible but BR-SBS or PS-SBS blends are compatible. The Tg of compatible blends follows the Couchman equation (Equation 2.3) [103]. Chlorobutyl rubber and carboxylated NBR are immiscible but self-vulcanisable as detected by DSC and DMA [104]. CR and EPDM produce incompatible blends. However, replacement of neat EPDM by dibutyl maleate grafted EPDM (DBM-g-EPDM) or CM narrowed down the breadth of Tg indicating compatibilisation of the blend [105]. The shifting in Tg towards each other as observed from the DSC traces, shown in Figure 3.8, proves that addition of 5-10% of DBM-g-EPDM or CM are effective in making the CR/EPDM blends technologically compatible. The DSC scans of neat components, incompatible and compatible blends are also displayed in Figure 3.8. Poly(3-hydroxybutyrate), a biopolymer, is incompatible with ENR-50. However, they react during high temperature annealing (190 °C) and form a mixed phase [106]. PU elastomer is immiscible with silicone rubber. However, grafting of vinyl triacetoxy silane onto urethane rubber made a compatible blend with silicones [107]. Many technologically compatibilised blends such as, PVC/polyether-ester elastomer blends [108], EPM/Hypalon blends [109], CR/polyacryamide blends [110] and SBS/asphalt mixes [111] were reported by various authors.
Interpenetrating Polymer Network
Another way of developing technologically important polyalloys is the formation of interpenetrating polymer network (IPN). In such cases, components mostly form separated phases with little compatibility. In the immiscible IPN two distinct Tg could be identified. A very broad Tg indicating partial miscibility was reported by Hermant and co-workers in a system based on PU-PMMA [112]. Various other authors also characterised semi-compatible IPN based on PU/methacrylates using DSC [113-114]. PU-based semi-IPN with polyvinylpyrrolidone (PVP) displayed the Tg for PU phase and two Tg for PVP phase in the DSC scans [115]. Incomplete phase separation was believed to be the cause for two Tg for the PVP phase. Incompatible IPN based on thermoplastic PU (TPU) and cis-polyisoprene was developed for improved mechanical properties [116]. DSC analysis was shown to be useful for the characterisation of morphology of IPN based on polysiloxane-polytetrafluoroethylene [117], PDMS/PS [118], PDMS/polyacrylate (PAC) matrix with polymethacrylate (PMAC) [119], NBR/EVA [120], and PU/PAN [121] systems. 82
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
Figure 3.7 Temperature dependence of the heat capacity, Cp of (1) pure BR; (5) pure NR; and their blends (2) NR/BR, 25/75; (3) NR/BR, 50/50; (4) NR/BR, 75/25 [97] Reproduced with permission from A.A. Yehia, A.A. Monsour and B. Stoll, Journal of Thermal Analysis, 1997, 48, 1299. ©1997, Springer Publishers
Figure 3.8 DSC curves of control CR, EPDM and their blends, showing shift of Tg towards each other for compatibilised systems: (1), CR; (2), EPDM; (3), CR/EPDM or CR/DBM-g-EPDM blends; (4), CR/EPDM/DBM-g-EPDM compatibilised blends; (5), CR/ EPDM/CM compatible blends [105] Reproduced with permission from A.K. Kalidaha, P.P. De, A.S. Bhattacharyya and A.K. Sen, Angewandte Makromolekulare Chemie, 1993, 204, 1, 19. ©1993, Wiley-VCH
83
Thermal Analysis of Rubbers and Rubbery Materials Ionomeric Polyblends
A polymer containing ionic group is called an ionomer. Ionically terminated ends of two immiscible polymers when mixed form a technologically compatible polyblend [122]. Grafting potential ionomeric short chains onto two incompatible polymers also makes compatible ionomers. The physical properties of such blends are superior to those of the non-ionomeric blends because of the ion-ion interaction in the blend. Ionomeric polyblends exhibit various morphologies and hence properties depending on the processing and/or thermodynamic conditions. Thermal analysis is one of the tools that characterise the morphology of such materials. In an ionomer, non-polar polymeric chains exhibit their own Tg along with new Tg of restricted polymeric segments bound with micro-phase separated ionic domains [123-124]. As shown in Figure 3.9, Mandal and co-workers found that ionomeric polyblends based on XNBR/zinc oxide/stearic acid/zinc-stearate exhibited two T g, one at –19 °C due to XNBR and the other at +34 °C due to the restricted segment of XNBR associated with an ionic cluster. At low temperatures zinc-stearate strengthens the ionomer formation in the XNBR-ZnO system and at high temperature it functions as an ionic domain plasticiser [125]. Several other ionomeric blends were characterised using thermal analysis tools. These are zinc salts of sulfonated EPDM [126-127], blends
Figure 3.9 DSC thermograms of XNBR/ZnO/stearic acid/Zn-stearate system. 100/12/2/0 ( ); 100/12/2/15 (----); 100/12/2/30 ( - ), 100/12/2/60 ( ~ ); zinc stearate ( - - ) [125] Reproduced with permission from U.K. Mondal, D.K. Tripathy and S.K. De, Polymer Engineering Science, 1996, 36, 2, 285. ©1996, John Wiley and Sons
84
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials of EPDM/Na-salt of poly(ethyl-co-acrylic acid) (Surlyn) [128], SBS block copolymer/ polyacrylamide blend compatibilised with sodium salt of maleic anhydride grafted SBS [129], ionomeric PS-PIB-PS triblock copolymer-silicate nanocomposite containing sulfonated PS segments based on benzyltrimethylammonium [130] or sodium salts [131] prepared by the sol-gel method, and statistical polymethacrylate comb copolymers with tail-end 4-(dimethylamino) pyridinium mesogen ionic groups synthesised by partial quaternisation of high molar mass poly(12-bromododecyl methacrylate) [132].
3.2.6.3 Filled Elastomers For the reinforcement of elastomers or elastomer blends carbon black is used extensively. Kenny and co-workers observed that the Tg of a neat NR and carbon black filled NR did not differ significantly [133], indicating loose binding of rubber on the filler particles. Mansencal (and co-workers) characterised different polybutadiene rubbers having different microstructure (1,2-PB content) but having constant (33.3%) loading of super abrasion furnace (SAF) black [134]. It was observed that Tg of the gum, filled and bound rubber (residue after solvent extraction of filled rubber) remained almost same. However, DSC data revealed that normalised Cp values associated with Tg for each bound rubber sample was always less than that of the gum and filled samples. This indicated that part of the bound rubber was not detected by the DSC for the response of the Tg, i.e., part of the bound rubber displayed reduced mobility. The observation was very significant when 1,2-PB content was very high or the original chain flexibility was a little reduced. Essawy and El-Nashar observed that montmorillonite filler acts as compatibilising agent for the vulcanisate blends of SBR and NBR [135]. Tg of both the components increased slightly after addition of montmorillonite clay, indicating a specific interaction between the clay and rubber components. DSC analysis of TPU filled with mica, revealed that with increase in mica loading the Tg of the TPU soft segment significantly increased, but that of the hard segment remained almost unchanged [136].
3.2.7 Effect of Crosslinking of Rubbers on Tg It is well known that crosslinking of polymer chains increases the Tg [137-138]. Crosslinking of cis-1,4-BR to a higher degree increased the Tg of the rubber from –103 °C to –96 °C [139]. Cook and co-workers investigated variation in Tg of various rubber vulcanisates at different crosslinking densities using DSC and NMR spectroscopy [140]. The Tg versus crosslinking density data followed a linear regression. The NR and BR vulcanisates display similar Tg versus crosslinking density plot, but the increase in Tg of SBR with crosslink density was much greater than that of the NR or BR.
85
Thermal Analysis of Rubbers and Rubbery Materials
3.2.8 Monitoring Vulcanisation of Rubber Using DSC Kinetic study of the vulcanisation process for compounded rubbers provide an elastomer technologist with valuable information such as optimum temperature and time for curing, activation energy, kinetic order and frequency factor of the vulcanisation reaction. Usually, cure monitoring of rubber is performed using a rheometer to measure the torque required to shear a specimen undergoing curing process at elevated temperature. Since crosslinking reactions are exothermic, i.e., the reactions are accompanied by a measurable heat effect, DSC can be used to monitor the curing reaction of thermosets. Different methods that are used for the cure characterisation of rubber using DSC are i)
Borchardt and Daniels method [141],
ii) ASTM E698 method [142, 143A] and iii) Isothermal method [144, 145]. Borchardt and Daniel method was developed to calculate kinetic parameters from the area of a DTA or DSC curve. The method mainly assumes that the curing reaction essentially completes before the highest temperature is reached in a dynamic DSC scan and the rate of reaction is very small at the lowest temperature [141]. The method also assumes that there is no mass loss during the reaction and it follows nth order kinetics. Therefore, the rate equation can be expressed as: d k(T)(1 )n dt
(3.5)
where, = the fractional conversion usually obtained from fractional area under the DSC curve; k(T) = the specific rate constant at temperature T and that presumably follows the Arrhenius equation [i.e., k(T) = A.e-E/RT , where, A is frequency factor, E is the activation energy and R is the universal gas constant] and n = order of the reaction. Brazier and Schwartz used this method to calculate apparent activation energies for dicumyl peroxide (DCP) curing of elastomers [146]. The enthalpy of curing followed the order: BR> 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
Thermal Analysis of Rubbers and Rubbery Materials
Table 3.6 Melting transition temperatures of different rubbers Rubber Cis-1,4-IR (natural rubber) Cis-1,4-IR (synthetic rubber) Trans-1,4-IR (natural) (Gutta percha) Trans-1,4-IR (synthetic) Cis-1,4-BR Trans-1,4-BR IIR CR EPR PDMS
Tm (°C) * (-12, 2) a (-20, 10) b 1 (-16, 13) b 2 30 10 c (-10, 1) a (-7, 2) a (58, 67) b 3 52 a (60-67, 50-55) a 60-65 a 3 a , 13 d 150 a 129-142 e 15 f 42-47 a 47 -45 g -40 a
Reference [186] [191] [23] [192-193] [4] [191] [194] [195] [196] [23] [194] [197] [179] [4] [42, 198] [199] [18]
* Two melting peaks observed in IR does not represent metamorphic crystals rather similar crystals of different lamellar sizes [3] a DSC, 10 °C/min b DSC, 0.3 °C/min: (b 1, stored at –25 °C for 6 months, b 2, crystallised at –25 °C for 16 h, b 3, crystallised at 54 °C) c Partially crosslinked samples stretched up to 200% after crystallising at -25 °C, DSC, 10 °C/min d Crosslinked samples stretched up to 200% after crystallising at -6 °C, DSC, 10 °C/min e Dilute solution grown crystals, DSC 20 °C/min f Stress-strain-temperature measurement (non DSC data) g DSC, 6 °C/min
copolymer [200]. High ethylene content EPDM rubber exhibits more strain-induced crystallisation [180]. It can be noted that the melting transition of EPR occurs at a temperature far lower than that of the PE. The morphology of the EPR is also influenced by the catalyst system used to produce EPR. Silicone (PDMS) elastomers show a melting transition at a temperature ranging from –40 °C to –45 °C [18, 199]. Maxwell and coworkers reported cold crystallisation of PDMS around –60 °C [201].
3.2.9.2 Transitions in Liquid Crystalline Elastomers Liquid crystalline elastomers that are lightly crosslinked molecules containing rigidrod, mesogenic moieties either in main backbone or side chains, display special optical, mechanical and piezoelectric properties. Other than polarised microscopy and x-ray scattering, DSC thermal analysis also provides a tool for characterisation 90
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials of liquid crystalline behaviour of the elastomers. Smectic liquid crystalline elastomers generally display four transition zones in their DSC thermograms. The first zone is the glassy smectic region, which changes to soft smectic phase during heating through a baseline shift in the thermogram. Further heating registers a tiny endotherm indicating transformation of smectic to nematic form. Continued heating makes the material isotropic through an endothermic transition [202]. Sometimes the smectic to nematic and nematic to isotropic transitions are indistinguishable. Transition temperatures depend on the structure of the polymers. Figure 3.10 shows the DSC traces of side chain polyisoprene liquid crystalline block copolymers [203]. PI blocks containing 34% 1,2 addition, 59% 3,4- addition and 7% 1,4-addition microstructures were first hydroxylated via hydroboration/oxidation reaction. A liquid crystalline monomer 6-[(4-cyano-4´-biphenyl)oxy]hexanoic acid (5CNCOOH) was grafted onto the hydroxylated PI producing side chain liquid crystalline PI. Incorporation of rigid mesogens increases the Tg of elastomers. The endotherm in each trace indicates clearing temperature (Ti, anisotropic to isotropic transition). The higher the content of grafted liquid crystalline monomers, the higher is the clearing temperature. Saenger and Gronski modified polybutadiene and SBS block copolymers by attaching similar side chain liquid crystalline mesogens (5CN-COOH) having an aliphatic chain
Figure 3.10 DSC traces of (1) polyisoprene (Mn = 3600 g/mol) and liquid crystalline derivatives of the same polyisoprene containing different levels of grafted 6-[(4-cyano-4´biphenyl)oxy]hexanoic acid: (2) 5%; (3) 50%; (4) 80%; (5) 100% during second heating scan at 20 °C/min [203] Reproduced with permission from K.M. Lee and C.D. Han, Macromolecules, 2002, 35, 8, 3145. ©2002, American Chemical Society
91
Thermal Analysis of Rubbers and Rubbery Materials as soft spacer to the hydroxylated butadiene segments [204]. Other than the clearing transition (Ti) it was also observed that an endothermic transition, which is heating rate (annealing) dependent, appeared at an intermediate temperature between Tg and Ti. During the cooling cycle, however, a corresponding exotherm to the previous enthalpy relaxation could not be detected due to supercooling. Such a monotropic behaviour is common to many other liquid crystalline polymers [205]. Zhang and co-workers synthesised liquid crystalline elastomers based on silicones containing rigid mesogens as crosslinker and side chains [206]. The polymers exhibited a melting transition due to aliphatic long chains in the crosslinkers and a tiny liquid crystalline-isotropic transition. The transitions overlap and form a broad endotherm at a high level of crosslinking mesogen content. Further increase in crosslinking mesogen content causes disappearance of mesophase behaviour due to the ‘inhibiting effect of high crosslinking on the liquid crystalline order’ [207]. Main chain liquid crystalline, segmented PU elastomers are high performance materials. Morphological characterisation of such materials has been extensively studied using DSC [208]. Liquid crystalline phases of different thermodynamic stability in those PU materials have been detected. The heat of isotropisation depends on the molecular architecture of the liquid crystalline elastomers. The presence of higher mole fraction of soft segment spacer decreases the heat of isotropisation. It has been established that the mesophase formation through microphase separation is both thermodynamically and kinetically controlled phenomenon. At higher annealing temperature (below the Ti) it takes longer time to develop mesophase, conversely, at a lower annealing temperature it takes a shorter time to develop the mesophases. Thermotropic liquid crystalline elastomers of polyester [209] and polyester-amide [210] types have also been characterised by thermal analysis. Mesophase formation in such materials depends on the structure of the hard phase and hard to soft segment ratio.
3.2.9.3 Melting and Crystallisation in Filled and Blended Elastomers Presence of impurity or foreign matter influences crystallisation of rubbery materials. Addition of trans-PI in cis-PI enhances strain-induced crystallisation of the latter [211]. Trans-PI acts as a nucleating agent for cis-PI. Addition of carbon black initially lowers the rate of crystallisation of NR but at high filler loading the rate of crystallisation increases [212]. Crystallisation temperatures of rubbers usually increase with increase in carbon black loading. Addition of black increases the extent of shear and hence molecular alignment during the mixing process. This causes ready nucleation during cooling scans of the samples. In a study by Lee and Singleton, SBR/BR 80/20 blend was mixed with carbon black (20 phr) for different durations and the compounds were then analysed thermally [213]. As shown in Figure 3.11, it was observed that with increase in mixing time the crystallisation exotherm and the melting endotherm 92
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
Figure 3.11 DSC scans of a filled rubber blend containing 80 parts SBR, 20 parts BR, and 20 phr carbon black blends as a function of mixing time: (a) 0.5 min, (b) 1.1 min, (c) 3.2 min and (d) 8.7 min. These blends were prepared by a free black mixing process [213] Reproduced with permission from B-L. Lee and C. Singleton, Journal of Applied Polymer Science, 1979, 24, 10, 2169. ©1979, John Wiley and Sons
of BR phase was significantly reduced. Increase in mixing time caused enhanced immobilisation of BR molecules on black surface (formation of bound rubber) and improved interfacial mixing of SBR and BR phases. Therefore, less BR remained available for crystallisation and subsequent melting. SBR/BR blends at certain black loading and different mixing times could have differential filler distribution in two phases. The DSC scans displaying decrease in heat of fusion of BR with increase in mixing time also indicated migration of filler from SBR to BR, and the results were verified by electron microscopy [213]. Measurement of crystallisation temperatures of IR/BR blends was shown to be a useful technique to quantify dispersion of carbon black in the polymeric phases [214]. DSC analysis could detect the difference in the polymer dispersion in NR/IR blends, by measuring the crystallisation behaviour of the compounds [215]. Crystallisation of NR was more adversely affected by addition of IR than the addition of carbon black. Addition of 20 phr IR caused 80% reduction in the heat of fusion of the polymer. 93
Thermal Analysis of Rubbers and Rubbery Materials In situ formation of nano-silica in semicrystalline trans-polybutadiene reduced the rate of hexagonal crystallisation of the matrix [216]. Crosslinking of rubber lowers cold crystallisation of polybutadiene [190]. In an attempt to develop thermo-reversible crosslinking in polybutadiene, the rubber was first hydrochlorinated via epoxydation and then a sulfonyl isocyanate derivative was added to incorporate hydrogen bonding in the system [217]. The modified rubber displayed excellent improvement in mechanical properties. The cold crystallisability of the rubber gradually diminished with increase in degree of such modification. PP based thermoplastic elastomeric blends are commercially important. Addition of rubber in to the PP matrix lowered the crystallisation rate and heat of fusion of PP in PP/ NR [84], PP/IIR and PP/EPR blends [218]. For all the blends the rubbery phase acts as a nucleating agent for PP. However, net crystal growth of PP remains low for blends of PP/ NR and PP/IIR. It was claimed that the NR and IIR phases act as diluents in the PP melt. For PP/EPM blends, the EPM phase accelerates the crystal growth at a very high ethylene content in EPM (>60 wt%) [218]. Consequently, the Tm and the crystallinity of the PP phase in blends of EPM with high ethylene content exceed the values of the neat PP. Wenig and Asresahegn extensively studied the crystallisation kinetics of PP in PP/EPDM blends using DSC and a polarising microscope [219]. Crystallisation kinetics is influenced by the interface layer formed between two components through diffusion of PP molecules in EPDM domains. Dynamically vulcanised PP/EPDM blends displayed a narrower melting endotherm than the non-vulcanised blends [220]. Zerjal and co-workers investigated crystallisation kinetics of PP/CaCO3/EPDM blend system using DSC [221]. Addition of CaCO3 or CaCO3 filled EPDM also lowered the crystallisation rate of PP. Heat of fusion and hence, crystallinity of PE/NR thermoplastic elastomeric blends decreased with increase in loading of NR [222]. The Tm of the PE phase was lower for blends than that for pure PE. Kalfoglou also made a similar observation in blends of high-density polyethylene (HDPE) and EPM or EPDM, however, the % crystallinity of the HDPE phase remained almost constant [223]. Rubber molecules nucleate small crystals of PE are shown during heating scans of the blends, particularly at high PE loadings. These tiny crystals melt at a lower temperature. Crystallisation rate is also decreased by addition of rubbers in the PE phase. Silane and peroxide induced crosslinked blends of PE and EPM exhibit reduced Tm, and low crystallinity in comparison to those of the neat PE [224]. The reduction in Tm and crystallinity is prominent in the case of peroxide crosslinked materials. The crystallisation and melting behaviour is a function of the sol content and network density of gel present in PE.
3.2.9.4 Melting and Crystallisation in Elastomeric Block Copolymers Elastomeric block copolymers having glassy or semicrystalline hard phase segments and soft segments are commercially important materials. Morphological characterisation of such materials is necessary to quantify interaction between two segments that leads to 94
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials microphase-separated structure. In an earlier section the role of DSC to detect the Tg of block copolymers has been summarised. Here, in this section, we review the melting or crystallisation transition in semi-crystalline elastomeric block copolymers. Block copolymers of polyisobutylene (PIB) and polybutylene terephthalate (PBT), synthesised by condensation of telechelic PIB, butane diol, and di-methyl terephthalate, displayed melting transition of PBT in the range of 200 to 220 °C depending on the block composition [225]. Ionic polyurethane elastomers containing bulky side groups in the hard phase segments sometimes display a small endotherm (at 80 °C) in the initial DSC scans due to the liberation of water from the coordination environment of anions and vaporisation [226]. Thermal analysis data of segmented polyether ester elastomers are presented in Table 3.7. DSC analysis of such elastomers based on polytetramethylene terephthalate (PTMT) and PTMO as hard and soft segments, respectively, revealed that with increase in hard segment weight fraction the Tm and heat of fusion increased. Polymers containing 45, 54 and 72 wt% hard segments displayed a Tm of 173, 197, and 216 °C, respectively, [51, 227-228]. A similar trend was observed in polymers based on PTMT and polyethylene oxide (PEO) [229-230]. Heat of fusion and percentage crystallinity of these polymers depend on the annealing temperature and time. During annealing of the polymers below the Tm the crystals becomes more perfect and the Tm increases [231]. Segmented polyether urethanes based on PTMO of different molecular weights and 4,4 diphenylmethane diisocyanate (MDI) having almost 50 wt% hard segments displayed differential melting transition and morphological characteristics under similar annealing conditions [48]. When the PTMO MW was 980, no soft segment melting transition
Table 3.7 Thermal analysis data of polyether ester Soft Segment (MW)
Hard Segment
Wt% Tm (soft Tm (Hard Wt% of Reference of Hard Segment) Segments) Crystallised Segment (°C) (°C) Hard Segment a 162 30 [231] PTMO (1000) PTMT 36 173 -b [228] PTMO (1000) PTMT 45 -a a 197 42 [228] PTMO (1000) PTMT 54 a 216 54 [228] PTMO (1000) PTMT 72 [229] PEO (6000) PTMT 20 52 162 -b b [229] PEO (6000) PTMT 40 44 212 b [229] PEO (6000) PTMT 80 20 223 PEO (1000) PTMT 79 220 40 [230] PEO (1000) PTMT 50 -12 185 32 [229-230] PEO (1000) PTMT 24 11 [230] a PTMO soft segments below 2000 MW do not exhibit crystallinity b Data not reported
95
Thermal Analysis of Rubbers and Rubbery Materials was observed in the PU copolymer, but the soft segment melting appeared prominent (at 14 °C) when the PTMO MW was 2000 in the PU. The former displayed a melting transition at a temperature ranging from 150 to 200 °C depending on the annealing conditions, however, the latter displayed melting transition at a higher temperature range (210 to 235 °C). Larger soft segment facilitates phase separation and causes more perfect crystal formation during annealing. Samuels and Wilkes have shown that the multiple melting behaviours of thermoplastic urethanes depend on the thermal history not on the lack or availability of hydrogen bonding sites in the hard segment [232]. Polyether-ester-amide block copolymers based on carboxyl terminated polyamide 6 (PA6) (obtained by polymerising -caprolactam in presence of adipic acid) and polyethylene glycol (PEG) of different PA6/PEG ratio displayed different morphology and melting behaviour, as shown in Table 3.8 [233]. Increase in PEG content lowered the Tm and heat of fusion and increase in PEG molecular weight increased the Tm and heat of fusion of the polymers.
Table 3.8 Thermal analysis data of poly(ether-ester-amide) block copolymer and the homopolymer segments [233] Soft Segment (MW)
Hard Segment
Wt % Tg ( °C) Tm (°C) Tm ( °C) H of Fusion of Hard Soft (Hard for (J/g) Hard Segment Segment Segment) Segment PEG (1000) PA6 80 -45 214 45.5 PEG (1000) PA6 70 -47 0 207 41.0 PEG (1000) PA6 52 -42 0 195 32.0 PEG (400) PA6 74 -42 189 23.0 PEG (400) PA6 66 -44 178 19.5 PEG (400) PA6 51 -46 175 17.0 PA6 100 28 220 68.0 PEG (1000) -67 38 158 PEG (400) -67 -6 Reproduced with permission frm S. Fakirov, K. Goranov, e. Bosvelieva and a. Du Chesne, Makromoleculare Chemie, 1992, 193, 9, 2391. ©1992, Wiley-VCH
3.2.10 Decomposition of Polymer Usually pyrolytic decomposition and oxidative degradation of polymers at elevated temperature are associated with absorption and liberation of heat, respectively. Hence, measurement of heat exchange occurring during thermal degradation of polymers allows one to determine oxidation induction time and kinetic parameters associated with the overall decomposition reactions. DSC has been widely used to study rubber degradation [234-235]. Details of different methods that can be used for the kinetic evaluation of a 96
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials decomposition reaction using DSC is similar to that have been summarised in the earlier section on monitoring vulcanisation (Section 3.2.8). Rubbers can be classified into different categories based on the nature of pyrolytic decomposition under an inert atmosphere. Sircar and Lamond have used DSC as a ‘fingerprint’ tool in the identification of elastomeric systems [76]. Diene rubbers (BR, SBR, NR, IR, NBR, and CR) mostly display an exotherm under nitrogen due to cyclisation prior to an endotherm for the decomposition reaction. Polyolefinic (EPM, EPDM) rubbers exhibit an endotherm for the decomposition reaction. Halogenated polyolefinic rubbers (halobutyl, Hypalon), depending on the halogen content, could display an endotherm due to elimination of halogen, then an exotherm for the cyclisation followed by endothermic decomposition reaction(s). In oxygen or an air atmosphere all rubbers display exothermic transitions. Oxidative decomposition temperature is diffusion controlled, and hence depends on sample size (less reproducible). Vulcanisation of rubber, usually, causes an increase in the thermo-oxidative decomposition temperatures. DSC results for degradation of different rubbers are summarised in Table 3.9. As shown in Table 3.9, microstructure of BR influences its decomposition temperature. Therefore, it is expected for all other polymers that the decomposition temperature of rubber may alter depending on the variation in grade and microstructure content. With increase in vinyl content, oxidative decomposition temperature of BR increases [236]. Decomposition behaviours of elastomer systems containing carbon black and other additives follow a more or less similar trend as that of the neat rubbers under a nitrogen atmosphere. Commercial elastomer products based on NR or IR exhibited similar degradation behaviour even with different formulation recipes. A tyre carcass compound of NR had a decomposition temperature of 369 °C while that of an IR based compound was 349 °C. Single elastomeric vulcanisate of NR and IR showed decomposition temperature of 372 °C and 340 °C, respectively [237]. For NR/IR blends, a single peak at the intermediate temperature of individual rubbers is observed depending on the ratio of the blends. Curatives used in vulcanisation of CR influence the decomposition of the vulcanisate both in inert and oxidative atmosphere [101]. Presence of sulfur accelerates dehydrochlorination and subsequent crosslinking exotherms in CR. Based on these decomposition temperatures not only single elastomers but also some blends of elastomers can be identified [101, 238-240]. Fluoroelastomers are thermally more stable than the general-purpose elastomers. They usually display higher exothermic decomposition temperatures (a large peak at 460510 °C and a small peak at 500-585 °C) at static air atmosphere [241]. The homogeneity of litharge (PbO) dispersion in a fluoroelastomer and other rubbers can be detected by exothermic decomposition of PbO and conversion to elemental lead [242].
97
Thermal Analysis of Rubbers and Rubbery Materials
Table 3.9 Decomposition temperatures of different rubbers by DSC analysis Polymer
Transition Temperatures (ºC) Under O2 Atmosphere N2 Atmosphere Exotherm Endotherm Exotherm c c 470 [4] 466 b [245] BR 382 [4] 466 c [246], Cis-1,4 BR 370 c [246-247] 460 c [247] 462 c [247] Trans-1,4 BR 387 c [247] c 1,2 BR 355 [247] 470 c [247] 460 c [4] 188, 378 c [4] SBR 381 c [4] SBR (vulcanised) 240, 430 c [4] c c 460 [4] NBR 372 [4]; 330 d [248] ABS 380 c [239] 412, 440 c [239] 225, 370, 390 c [239] c c 460 [4] CR 382 [4] CR (S Vulcanised) 316 c [4] 293 c [4] c 252, 318 c [4] EPDM 392, 474 [4] 432 c [4] 397 c [4] IIR 315 (small) c [4] 228, 403 c [4] 205, 271, 277 [4] CIIR 285 c [4] c c 330, 390 [4] 170 b [240] NR 377 [4] NR (vulcanised) 378 c [4] Balata 388 c [287] c Balata (Cured) 351 [237] IR 363 c [4] 337, 387 c [4] 155 b [240] 410 c [237] 242, 453, 475 c [4] IR (vulcanised) 350 c [4] Hypalon [chlorosulfonated PE] 188, 308, 475 c [4] 234, 293, 367 c [4] d Viton [poly(hexafluoro508, 585 [241] propene-vinylidene fluoride)] PU (Conathane 3010/AH 31) 360, 400 a [243] 290, 385 a [243] Scan rates : a 2 °C/min; b 8 °C/min ; c 10 °C/min; d 20 °C/min
PU usually display endothermic decomposition under nitrogen and exothermic oxidation in air [243]. Endothermic decomposition is mainly the depolymerisation of an elastomer to produce isocyanates. Toluene diisocyanate based PU decomposes in a single step at a temperature less than 400 °C, whereas, 4,4 diphenyl methane diisocyanate based PU decomposes at a temperature above 400 °C [244]. 98
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
3.2.11 Oxidation Induction Time Oxidation induction time (OIT) of polymers is determined by using DSC. Samples are isothermally scanned at a temperature between 180-220 °C under an oxygen atmosphere and the time required for the onset of exotherm due to oxidative degradation is used as a measure of OIT [236]. The OIT can also be used to determine the apparent activation energy for the oxidative degradation (using Equation 3.10, given next). Montanari and co-workers utilised DSC OIT data to compare the performance of high voltage cable insulating compounds [249]. Mason and co-workers found that the OIT of EPM and crosslinked polyethylene (XLPE) insulating compounds decrease exponentially with increasing radiation dose and with decreasing antioxidant concentration [250]. Stenberg and Bjoerk measured OIT by DSC to compare the effectiveness and minimum dosage requirement of different types of antioxidants in NR vulcanisates [251]. Lye and Toh [252] studied oxidation of NR vulcanisates using DSC analysis methods developed by Kissinger [154], Doyle [253] and Ozawa [143]. The major advantage of such analysis is that the estimation of isothermal OIT of several hours for a particular sample in a relatively short time. Activation energy for the oxidation calculated by Kissinger [154] or Ozawa-Doyle method [143] can be used to calculate the OIT for a low temperature oxidation (T0) following Doyle’s approximation: log t 0 log
E E E 2.315 (log e) 0.4567 R RT0 RTP
(3.10)
where TP = exotherm peak temperature, T0 = reference temperature, = scan rate, and t0 = isothermal induction time or isothermal equivalent for the reaction. This equation E can only be used if RT > 20, below which the approximation differ significantly from the exact value [253]. Nichols and Pett correlated OIT and tearing energy of chlorinated PE and chlorosulfonated PE vulcanisates after thermoxidative ageing for different periods [254]. Considering the reciprocal of OIT as the kinetic rate constant, for the degradation reaction, an Arrhenius plot can be obtained. Based on the Arrhenius plot and tearing energy data correlation curve, one can fairly easily predict the life of products or OIT at ambient temperature. Goh observed that the OIT at 127 °C and reciprocal of dynamic DSC oxidation peak temperature (1/TP) follows a linear relationship for most of the rubbery materials [255]. The curve fits better if the dynamic scan rate is low (2 °C/min).
3.2.12 Other Miscellaneous Applications of DSC DSC analysis has been shown to be effective in detecting the ageing effects of thermoplastic elastomeric materials. An aged material would display enthalpy relaxation at the Tg of the 99
Thermal Analysis of Rubbers and Rubbery Materials material [26]. Sircar and Wells modified a DSC unit and used that to measure the thermal conductivity of rubbery materials [256]. A DSC unit equipped with a photo-cell can be used to study photon-initiated reaction(s) of the materials [257]. It is also called differential photo-calorimetry. Modulated DSC units provide increased transition sensitivity and these are used extensively nowadays [258]. DSC units with a pressure controllable cell can be used to study pressure-sensitive transitions. Since vaporisation is a pressure-sensitive transition, such units are useful for studying loss of plasticiser in a rubbery formulation at different pressure and temperature [259]. For medical or pharmaceutical grade rubbery products, calorimetric purity of the material gives useful data for quality control purposes. High sensitivity micro DSC is useful for these applications [260].
3.3 Thermogravimetric Analysis of Rubbery Materials TGA of rubbery materials is a useful tool for the investigation of various products in terms of thermal degradation, compositional characterisation, and life of finished materials. The technique is very useful for the analysis of processes such as oxidative degradation, pyrolytic decomposition, and volatilisation and for the evaluation of kinetic parameters involved in these processes. The following sections of this chapter review the utility of TGA in those areas. Usually a plot of weight loss data against temperature or time is obtained in this study. In derivative TG (DTG) analysis rate of weight loss data is plotted against temperature or time. In a dynamic TGA study mass loss is monitored as a function of temperature; in isothermal TGA mass loss is monitored as a function of time at constant temperature. The dynamic TGA or DTG is used extensively for rubbery materials. Isothermal scans are usually done for quality control purpose when volatilisation or degradation temperature of a compound is well known. In dynamic TGA, the temperature corresponding to the derivative weight loss peak of a rubber is dependent on temperature scan rate, medium of environment, composition and thermal history. An increase in heating rate shifts the DTG peak to a higher temperature. Under nitrogen the rubber thermally degrades, but in air or oxygen degradation is thermo-oxidative in nature. Presence of filler, vulcanising agent, and antioxidant also influence the thermal degradation of rubber. Representative TGA thermograms of a gum and carbon black filled SBR vulcanisates are shown in Figures 3.12a and 3.12b, respectively. The weight losses (and derivative weight losses) of the rubber in air and nitrogen atmospheres are shown at temperatures ranging from 50 to 800 °C. Under nitrogen the DTG peak appears at a higher temperature than that, that occurs in oxygen (Figure 3.12a). Decomposition in oxygen is diffusion controlled and hence depends on sample size and nature of the rubber. The gum vulcanisate leaves no carbon residue in either air or nitrogen atmosphere. Carbon black filled rubber vulcanisate leaves the filler residue beyond 500 °C, under nitrogen (Figure 3.12b). In an oxygen atmosphere, however, carbon black filled vulcanisate leaves 100
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
Figure 3.12 TGA and DTG thermogram of (a) gum SBR vulcanisate and (b) black filled SBR vulcanisates under nitrogen (- - -) and oxygen ( ) atmosphere [261] Adapted with permission from G.T. Mohanraj, T. Vikram, A.M. Shanmugharaj, D. Khastgir and T.K. Chaki, Journal of Materials Science, 2006, 41, 15, 4777. ©2006, Springer
no residue. Carbon black oxidises, in presence of oxygen, at a temperature above 500 °C and the filled vulcanisates show mainly a two-step decomposition involving oxidative degradation of rubber in the first step and oxidation of carbon black in the second step. Thus, by altering the gaseous environment one can identify the relative ratio of polymer and fillers in a rubber compound. The details of compositional characterisation will be discussed later. The following section illustrates thermal stability of various rubbers and their vulcanisates. 101
Thermal Analysis of Rubbers and Rubbery Materials
3.3.1 Thermal Degradation and Stability of Rubbers by TGA Thermal degradation of rubbers is evaluated by measuring the weight loss with constant rise in temperature in a thermogravimetric analyser. The temperature corresponding to maximum weight loss (determined from the maxima in derivative weight loss plots) under inert (nitrogen) atmosphere and is usually termed as the decomposition temperature. Under an oxygen atmosphere the weight loss occurs due to oxidative degradation through chain scission. The higher is the degradation temperature the greater is the thermal stability of the material. The following sections describe the thermal degradation behaviour of various general purpose and specialty rubbers, their derivatives and vulcanisates. Table 3.10 summarises different decomposition temperatures of rubbers under nitrogen and oxygen atmospheres. Representative DTG thermograms of general purpose elastomers and polyolefin elastomers under a nitrogen environment are displayed in Figure 3.13a and Figure 3.13b, respectively.
Table 3.10 Decomposition temperature of rubbers by TGA Rubber NR
Temperature ( °C) * 365-370, 430 [76, 237]; 387 [264]; 408 [265] 360, 490 [263] 375 [262] Hydrogenated NR ** 400-449 [264] CNR (Chlorinated NR) 150, 300, 350-450 [274]; 300 [271] 290, 510 [271] BNR (Brominated NR) 160, 410-450 [274] ENR (ENR-50) 428 [273] Hydrogenated ENR 455 [273] Chlorinated ENR (ENR-25) 110, 270, 405 [274] Brominated ENR (ENR-25) 150, 375 [274] IR 360, 420 [237] 394, 534 [270] BR 355, 465 [76]; 373, 470 [275] Cis-1,4 BR 447 [277]; 486 [278] 430, 500 [281] Trans-1,4 BR 330, 475 [216] 1,2 BR 461 [277] SBR 420-430, 447 [76, 284]; 473 [265] 380, 420, 445 [284] High vinyl SBR 340-440 (80-10 % styrene content) [287] Hydrogenated SBR ** 425[284]; 462 [285]; 473 [286] 380, 400[284]; 407, 424, 448, 488 [285]
102
Conditions N (10 °C/min) Air (10 °C/min) O (10 °C/min) N (20 °C/min) N (10 °C/min) O (10 °C/min) N (10 °C/min) N (20 °C/min) N (20 °C/min) N (10 °C/min) N (10 °C/min) N (10 °C/min) Air (20 °C/min) N (10 °C/min) N (10 °C/min) Air (10 °C/min) N (20 °C/min) N (10 °C/min) N (10 °C/min) O (10 °C/min) Air (5 °C/min) N (10 °C/min) Air (10 °C/min)
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
Table 3.10 Cont’d... Rubber Epoxidised SBR NBR Hydrogenated NBR XNBR oligomer Hydrogenated XNBR oligomer CR IIR BIIR CIIR EPDM EVA CM CSM PU Polyether-urethane Polyester-urethane siloxane chain extended FKM ACM Silicone (PDMS) Trimethylsilyl-terminated Dimethylvinylsilylterminated
Temperature ( °C) * 450 [41] 400, 430, 496 [292]; 433, 476 [294] 300, 395, 460, 570 [57] 495 [294] 380, 460 [297] 465 [297]
Conditions N (20 °C/min) N (20 °C/min) Air N (10 °C/min) N (10 °C/min) N (10 °C/min)
346, 439 [101]; 360, 450 [301] 428 [262] 301, 420 [262] 300-310, 430-440 [262] 399 [262] 484 [262]; 469 [313]; 490 [265] 340, 470 [318] 328, 465 [307] 321, 456 [309] 285, 475 [310] 323, 390 [325] 341, 539 [325] 335, 410 [325]
N (10 °C/min) N (10 °C/min) N (10 °C/min) N (10 °C/min) O (10 °C/min) N (10 °C/min) N (20 °C/min) N (2.5 °C/min) Air (10 °C/min) N (20 °C/min) N (10 °C/min) Air (10 °C/min) N (10 °C/min)
493 [334]; 500 [333]; 535 [336] 411 [325, 352] 575 [340] 418, 512 [340] 390, 485 [340]
N (20 °C/min) N (20 °C/min) N (10 °C/min) Air (10 °C/min) N (10 °C/min)
408, 442 [340] Air (10 °C/min) * X1, Y1, ...Z1; X2, Y2, ...Z2 temperature data sets indicate multi-step degradation, where first maximum weight loss occurs at Xi °C and the second one at Yi °C and the last weight loss maximum appears at Zi °C ** depends on degree of hydrogenation
3.3.1.1 Natural Rubber and Polyisoprene NR exhibits two-step degradation under nitrogen. In the first step (DTG maximum around 370 °C) major depolymerisation along with cyclisation occurs and the cyclised residue is eliminated by a second step at higher temperature (400-430 °C) [237]. Synthetic polyisoprene (IR) also decomposes in two steps under nitrogen (350 and 414 °C) [237]. For IR, the weight loss in the first step is usually a little less than that occurs in the first 103
Thermal Analysis of Rubbers and Rubbery Materials
Figure 3.13a DTG thermogram of peroxide cured general purpose elastomers under nitrogen ( , NR); ( SBR); ( BR) [76] and ( , CR) [101] at 10 C/min
Figure 3.13b DTG thermogram of olefinic elastomers under nitrogen at 10 C/min: , EPDM) [262] and tyre liner compounds, ( + , IIR); ( O , BIIR); and ( uncrosslinked fluororubber ( , Aflas) [336]
104
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials thermal decomposition step of NR. The presence of natural fatty acids in NR enhances the depolymerisation reaction in the first step [263]. Singha and co-workers [264] and Zeyen [265] reported single step decomposition of NR under nitrogen with a maximum at 387 °C and 408 °C, respectively. Possibly, the decomposition steps (depolymerisation and decomposition of cyclised NR) were overlapped. Schneider showed that cis-1,4-polyisoprene is less stable than the PI containing 50% 1,2 microstructure [266]. The activation energy for the depolymerisation of 1,4IR is lower than that of the 1,2 product. Vinyl side groups cyclise during thermal decomposition and form a stable network, which enhances thermal stability. Like NR and synthetic cis-1,4-IR, distinction between natural trans-1,4-polyisoprene (balata) and it’s synthetic counterpart could be made by DTG data [237]. Trans-IR has a two-step decomposition temperature (350-360 °C and 400-410 °C) that is a little lower than the cis-isomer (370 °C, 420-430 °C). Balata (unless purified) usually contains other resin impurities and hence starts decomposing at 270 °C [237]. Vulcanisation ingredients sometimes affect the decomposition characteristics of the rubber. The gum NR, peroxide and sulfur vulcanised NR exhibit more or less similar TGA thermograms [237]. However, for the IR vulcanisates, with increase in sulfur content, the first peak in the DTG thermogram becomes obtuse and short, whereas the second DTG peak becomes sharp and intense [237, 267]. An increase in carbon black loading at relatively high sulfur dosage (2-2.5 phr) also shows a similar effect in DTG thermograms of IR vulcanisates [237, 267]. However, in these cases particle size of carbon black is an important factor. Larger size carbon black particles (medium thermal (MT) black or fast extrusion furnace (FEF) black) does not cause an increase in intensity of the second degradation peak of IR [268]. Addition of silica and or feldspar mineral powder as filler improved the thermal stability of NR vulcanisates [269]. Oxidative decomposition usually starts at a temperature lower than that observed under nitrogen. However, the process is diffusion controlled. For thick specimens incomplete combustion leaves a pyrolysed carbonaceous residue which oxidises at elevated temperatures. IR shows DTG maxima at 394 °C and 534 °C under an air atmosphere at 20 °C/min heating rate [270]. The first peak represents the oxidative decomposition temperature of IR, whereas the second peak is related to the combustion of char formed during the first step. NR vulcanisates usually oxidatively decompose at 375 °C [262]. Hydrogenation of NR enhances thermal stability. Fully hydrogenated NR displayed a decomposition temperature of 449 °C [264]. Modified NR such as halogenated NR derivatives display two or multiple step degradation where the first step occurs around 100 °C due to elimination of volatile matter (water, residual solvents). The next step is dehydrohalogenation around 150 °C and final decomposition is at temperatures ranging from 300 to 450 °C [271]. Epoxidation of natural rubber enhances thermal stability. Epoxy modification of NR up to 50 mole% epoxidation increased the DTG maximum from 388 °C to 419 °C [272]. ENR with 100 mole% epoxidation, however, 105
Thermal Analysis of Rubbers and Rubbery Materials shows a TGA curve like NR with a DTG maximum at 397 °C [272]. Hydrogenation of a 50 mole% epoxidised NR (ENR-50) further enhanced the DTG maximum [273]. Halogenated ENR decomposes in multiple steps involving dehydrohalogenation and final decomposition of cyclised hydrocarbon [274].
3.3.1.2 Polybutadiene and Butadiene-Based Rubbers BR, in general, decomposes in two steps under nitrogen atmosphere, a little weight loss occurs at 355-375 °C and major degradation occurs at 460-470 °C [76, 268, 275]. The first step is due to the volatile depolymerisation and the second step is attributed to the decomposition of cyclised and crosslinked BR residue resulting after the first step [275]. Unlike NR or 1,4-IR, 1,4-BR exhibits cis-trans transformation and cyclisation at a very low temperature (~200 °C), even before detectable weight loss in a TGA thermogram [276]. Heating rate affects the butadiene decomposition significantly. With increase in heating rate the peaks shift to a higher temperature and the extent of first weight loss stage increases [268, 275]. Microstructure of the BR is a key factor on the decomposition behaviour of the rubber. Usually decomposition temperature follows the order: trans-1,4-BR < cis-1,4-BR < 1,2-BR [277]. Yildirim and co-workers reported that under a nitrogen environment cis-1,4 BR decomposes at 486 °C at 10 °C/min heating rate [278]. Zhou and Mark found that trans-1,4 BR decomposes at 475 °C with little devolatilisation at 330 °C under nitrogen at 20 °C/min scan rate [216]. The 1,2 BR cyclises under nitrogen [277, 279] and produces more carbonaceous residue than that formed by BR with other microstructures [276]. Grassie and Heaney showed that in a butadiene rubber containing both vinyl (1,2) and trans (1,4) unsaturation, the vinyl group thermally cyclised at a faster rate than with the trans moiety [280]. Compounded BR vulcanisates decompose similarly as gum BR under nitrogen, but those decompose at a broad temperature ranging from 400-450 °C in oxygen [76]. Irradiation (both in oxic and inert media) of 1,4-BR decreased the onset degradation temperature of rubber [281]. Oxic irradiated BR produced more non-volatile residue in the TGA scans under air. During oxidative degradation of BR both chain scission and crosslinking reaction occurs [282]. SBR under a nitrogen atmosphere degrades in two steps: the first step involves cyclisation and isomerisation of BR units at 380-420 °C and the second broad step deals with the scission of cyclised residue with a DTG maximum at 450-460 °C [76, 283-284]. The major broad peak involves a doublet and with increase in styrene content, the doublet nature of the DTG peak becomes more prominent with shift of the peak towards a lower temperature [268]. In an air atmosphere SBR oxidatively decomposes around 390460 °C with relatively higher weight loss in the first step of degradation in comparison to that under nitrogen [284-287]. With increase in styrene content, the decomposition temperature of SBR in air decreases [287]. Both hydrogenation and epoxidation of SBR enhance the decomposition temperature. Hydrogenated SBR (HSBR) [284-286] and 106
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials epoxidised-SBR (ESBR) [41] decompose in a single step under nitrogen atmosphere due to lack of unsaturation. HSBR decomposes in multiple steps under an oxygen atmosphere [284-285]. In air, hydrogenated SBR of high vinyl content degrades at a faster rate than the SBR due to pendant groups [284]. Degradation behaviour of block copolymer (di, tri and star blocks) of styrene and butadiene are similar to that of the random copolymer SBR [284, 288]. Vulcanisation of SBR by either peroxide or sulfur does not significantly affect the decomposition behaviour under a nitrogen atmosphere [289]. Ahmed and co-workers reported that peroxide-cured SBR has slightly higher thermal stability than the sulfurcured SBR vulcanisate, although the former has a lower temperature for onset of degradation than the latter [290]. Under oxygen the sulfur vulcanisates of SBR decompose at a very broad temperature ranging from 360 to 450 °C [76]. Loading of conducting carbon black in the SBR vulcanisates does not significantly affect degradation behaviour of the polymer under nitrogen or oxygen [261]. NBR exhibits thermal decomposition at a temperature higher than that of the butadiene rubber. Acrylonitrile units undergo cyclisation and form carbonaceous residue which enhances thermal stability of the overall polymer. Polyacrylonitrile homopolymer thermally cyclises at 250-350 °C and eliminates hydrogen cyanide and ammonia gases [291]. NBR containing 30 mol% acrylonitrile displays a DTG maximum at 496 °C [292]. With increase in acrylonitrile content in NBR the initial decomposition temperature decreases and the carbonaceous residue content increases [293]. NBR under oxidative atmosphere degrades in multiple steps with a major degradation at 460 °C [57]. Hydrogenated NBR displayed a higher decomposition temperature than that of the pure NBR [294]. However, hydrogenation of NBR significantly lowers the carbonaceous residue content after TG decomposition under nitrogen at a temperature above 550 °C [295]. The amount of carbonaceous residues formed by thermal cyclisation reaction is much more for NBR (than the HNBR) because of the presence of an unsaturated butadiene segment (in NBR). Electron beam cured HNBR exhibits slightly higher carbonaceous residue than the uncured HNBR [295]. The vulcanisation system plays a key role in the thermal stability of an NBR compound. The onset temperature for degradation and the temperature at DTG maxima for NBR vulcanisate follow the trend: sulfur-cured elastomer < peroxide-cured elastomer < E-beam cured elastomer [290]. Carbon black filled NBR vulcanisates show improved thermal stability compared to the unfilled NBR vulcanisates [296]. Addition of phenolic resin in black filled vulcanisate further improves thermal stability of the elastomer compound. Liquid XNBR decomposes in two steps under nitrogen. The first step at 380 °C involves elimination of compounds during cyclisation of acrylonitrile and butadiene units; while the second step at 460 °C involves volatilisation of depolymerised material [297]. Hydrogenation of the liquid XNBR (with 26 mol% acrylonitrile content) makes the polymer thermally stable and almost eliminates the first step of decomposition.
107
Thermal Analysis of Rubbers and Rubbery Materials
3.3.1.3 Chloroprene Rubber and its Derivatives CR undergoes multi-step decomposition under nitrogen. The first step at 320-380 °C involves dehydrohalogenation and cyclisation, [101, 298-300]. The fi nal pyrolytic depolymerisation occurs at 440-455 °C. CR usually forms 20-23% carbonaceous residue (char) under a nitrogen atmosphere. GC/Mass spectra analysis of CR pyrolysis indicates CR monomer and dimer in the pyrolysis off-gas [301]. CR vulcanisates often show multiple DTG peaks due to loss of volatile ingredients and plasticisers [299]. The presence of fillers also influences the degradation behaviour of CR vulcanisates. Addition of silica, clay or carbonates lowered the first decomposition temperature of CR vulcanisates, however, presence of carbon black did not affect the same [298]. Ageing of CR vulcanisates in air caused lowering in the decomposition temperatures [299]. Hydrogenated CR displays improved thermal stability [302].
3.3.1.4 Isobutylene-isoprene Rubber or Butyl Rubber IIR vulcanisates decompose at 420-428 °C [262]. Halogenated derivatives of IIR display two-step decomposition, where the first step involves dehydrohalogenation (around 300 °C) [238, 262]. The second decomposition temperature (405-420 °C) of the halobutyl rubber is slightly lower than that of the IIR [262, 303]. Gum uncrosslinked IIR exhibits a lower decomposition temperature (370-400 °C) than the vulcanisates [268, 300, 304]. Cure system and type of carbon black has little or no effect on decomposition temperature of vulcanised IIR [303]. Isobutylene and styrene block copolymer usually decompose in a single step at 417 °C under nitrogen. Sulfonated ionomers of such copolymer exhibit improvement in thermal stability [305]. Acrylate grafted IIR prepared by electron beam modification showed two-stage decomposition-where the first step involves decomposition of acrylates [306].
3.3.1.5 Thermal Degradation of Specialty Rubbers
Ethylene-based Rubbers
CM decomposes in two steps: the first step of dehydrochlorination (328 °C) is followed by the degradation of dehydrochlorinated mass (450 °C) that leaves little carbon residue [307]. Increase in the degree of chlorination lowers the temperature associated with the first step [308]. In air, CM decomposes in multi-steps, the first step is the liberation of hydrogen chloride and the next steps involve thermo-oxidative decomposition of the unsaturated residue. The main chain degradation occurs, in air, at a lower temperature than that, that occurs in nitrogen [309]. CSM decomposes in two major steps under 108
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials nitrogen. The first step involves elimination of hydrogen chloride and sulfur dioxide at temperatures ranging from 260 °C to 340 °C, whereas, in the second step the residual polymer chain decomposes at 475-480 °C through formation of 2-3% char [310]. Smith reported that during the initial decomposition of CSM, loss of a –SO2Cl group occurs first, and then dedydrochlorination occurs [311]. Very high chlorine contents (~50%) and very high sulfur contents (>2%) significantly lowered the thermal stabilities of the CSM polymers. Varma and co-workers reported that among a series of synthesised CSM those having chlorine and sulfur contents in the range of 27-37% and 0.6-1.6%, respectively, were thermally the most stable [312]. EPDM, which exhibits excellent ozone resistance, degrades at 470-490 °C under nitrogen [262, 265, 313]. Under air, EPDM decomposes at a temperature lower than that, that occurs in nitrogen [314]. As shown in Figure 3.14, an increase in ethylene to propylene ratio in EPDM rubber increases the degradation onset and DTG maximum to higher temperatures [315]. Incorporation of antioxidant (0.1 to 1.5 phr level) in the radiation cured EPDM vulcanisate improves the thermal stability of the compound [316]. Vulcanisation of EPDM by conventional sulfur curatives and silanes improves thermal stability of EPDM and its thermoplastic blends [314]. Addition of reinforcing fillers such as silica, or carbon black improves thermal stability of EPDM elastomers [298, 317].
(
Figure 3.14 TGA thermograms of the EPDM rubbers with of different PP/PE ratio: ) PP/PE, 60/40; ( ) PP/PE, 41/59; ( ) PP/PE, 22/78; ( O ) PP/PE, 0/100 at 2 °C/min [315]
Reproduced with permission from C. Gamlin, N. Dutta, N. Roy-Choudhury, D. Kehoe and J. Matisons, Thermochimica Acta, 2001, 267-368, 185. ©2001, Elsevier
109
Thermal Analysis of Rubbers and Rubbery Materials EVA decomposes in two steps and the first step involves the elimination of acetic acid from the vinyl acetate segment through the formation of an unsaturation in the main chain (340 °C) and the second step involves decomposition of a PE rich residue (470 °C) [318-319]. The first step weight loss follows a linear relationship with the vinyl acetate content. Thus, TGA offers an excellent tool for the quality control and compositional identification of EVA copolymers [320]. Under an air atmosphere the extent of the first step decomposition is increased (presumably due to elimination of vinyl acetate along with adjacent CH2 segments under an oxidative environment) and the second-step decomposition maximum is slightly lowered relative to that, that occurs in inert atmosphere [321]. In air, EVA decomposes through hydroperoxidation, which can be hindered by addition of antioxidant [322]. Incorporation of mineral nanoclay particles in the EVA matrix does not significantly alter decomposition temperature under nitrogen, however, it causes an increase in degradation temperature under air due to excellent impermeability of nano-reinforced matrix [321]. In some cases the presence of an organic moiety in the organically modified clay-EVA nanocomposites accelerates the first decomposition step [323]. Thermal stability of polyethylene-co-methacrylate (EMA) rubber follows similar trends as EVA [321]. An EMA containing 21% methyl acrylate showed a DTG maximum at 450 °C under nitrogen (at 10 °C/min scan rate) with a low temperature (350-400 °C) shoulder [324].
Polyurethanes
Polyether-urethanes depolymerise at higher temperatures under inert atmospheres to produce diisocyanate and diol. During decomposition of PU, diisocyanates react and form carbodiimide which subsequently decompose at elevated temperatures [325]. Under nitrogen and at 10 °C/min scan rate, about 40-50% weight loss occurs in the first step (323 °C) and the rest is eliminated in the second step (390 °C). However, the same polymer oxidatively decomposes under air and displays 90% weight loss in the first step with a maximum at 341 °C. The carbonaceous residue burns at 540 °C. Polyester-polyurethane shows better thermal stability than the polyether-polyurethane [325-326]. Under nitrogen, the former shows a two-step degradation with the first step (335 °C) of 20% mass loss, and the second step decomposition at 410 °C. Chuang and co-workers observed two-step decomposition in different PU samples under a nitrogen atmosphere [327]. Incorporation of siloxane chain extender in the PU improved the thermal stability [327]. Addition of layered silicate clay in the PU matrix slightly improves thermal stability under nitrogen but it causes a significant improvement in thermal stability under air [328]. Layered silicate improves the barrier property and causes formation of thermally decomposed products (in the absence of air) such as crosslinked substituted urea from carbodiimide and diol and an isocyanurate product from trimerised isocyanate ending chains. Polyhedral oligomeric silsesquioxane (POSS) when used as a nanofiller and crosslinking agent in the PU networks, also improves the thermal stability of the PU elastomers [329]. 110
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials Koscielecka and Dzierza reported that thermal stability of PU is independent of the crosslinking density and the structural characteristics of crosslinks and hard segments [330]. However, Stanciu and co-workers reported that poly(ester-siloxane)urethanes based on different diisocyanate (hard segments) exhibited different thermal stability in the following order: 2,4-toluylene diisocyanate (TDI) < 4,4-methylene diphenyl diisocyanate (MDI) < hexamethylene diisocyanate (HDI) [331]. Structure of soft segments always appeared to be a controllable factor for the thermal stability of the PU [332]. PU based on polytetramethylene glycol (PTMG) is more stable than PU based on polyethylene glycol (PEG) and polypropylene glycol (PPG). Increase in soft-segment molecular weight of PTMG and PEG improved the thermal stability of the PU [326; 331-332]. PPG chains, because of the steric effect, reduce the inter-chain hydrogen bonding and hence the thermal stability. UV irradiation of PU block copolymers improved thermal stability of the polymers due to UV assisted crosslinking or structural modification of the polymer [326].
Fluorinated Rubbers
Flouroelastomers (such as copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), Viton A, and terpolymer of VDF, HFP and tetrafluoroethylene (TFE), Viton B-50), commonly termed FKM, are thermally stable. They decompose under nitrogen, in a single step, with a maximum rate at a temperature ranging from 490-500 °C [311, 333-334]. Copolymer of tetrafluoroethylene and propylene (Aflas) is the first fluoroelastomer containing non-fluorinated alkene comonomer. Increase in propylene content lowers the degradation temperature [335]. These copolymeric fluororubbers decompose at an intermediate temperature between decomposition temperatures of polytetrafluoroethylene and PP [336]. A -radiation crosslinked polytetrafluroethyleneco-perfluoromethyl vinyl ether shows inferior thermo-oxidative stability to that of the uncrosslinked rubber [337]. Fluorinated polysiloxanes are excellent materials that provide both high thermal stability and low temperature resistance. Incorporation of long fluorinated chains onto siloxane improves low temperature behaviour but causes low temperature elimination of the chain during TGA scans. However, beyond the first elimination temperature modified polymer shows better thermal stability than the polymethyltrifluoropropyl siloxane and comparable thermal stability as that of the PDMS [338]. Incorporation of as low as 20 wt% perfluorocyclobutane-containing silphenylene repeat unit in a poly(3,3,3trifluoropropylmethyl siloxane) fluorosilicone backbone minimised isothermal weight loss at a temperature above 250 °C [339].
Silicone Rubbers
PDMS rubber exhibits high thermal resistance. Under nitrogen neat trimethylsilylterminated PDMS displays a DTG maximum at 575 °C at 10 °C/min heating rate [340]. 111
Thermal Analysis of Rubbers and Rubbery Materials Dimethylvinylsilyl-terminated PDMS exhibits a two-step thermal degradation (390 °C and 485 °C) under nitrogen and the decomposition temperatures are much lower than that of the trimethylsilyl-terminated PDMS. Obviously, the decrease in thermal stability is due to the presence of vinyl group at the end of PDMS chain. Under air both the siloxanes exhibit two-step decomposition but the vinyl terminated PDMS again exhibits reduced thermo-oxidative stability [340]. Incorporation of cation exchanged montmorillonite clay [341], precipitated silica [342-343], and conductive carbon black [343] into PDMS vulcanisate improved thermal stability of the rubber. Conductive carbon black enhanced thermal stability of the PDMS more than that caused by silica [343]. A crosslinked product of liquid PDMS rubber exhibits slight hygrothermal degradation after prolonged treatment (2 y) under boiling water leading to ~20% decrease in volume. The treated vulcanisate however has an identical TGA thermogram to that of the untreated control vulcanisate [344]. During the life time of silicone rubbers as electrical insulator, surface degradation occurs and it produces volatile products that are eliminated at the temperatures ranging from 150-250 °C during TGA scans of used insulators [345]. Insulating silicone compounds are usually filled with alumina trihydrate (ATH). Degradation of ATH produces Al2O3 and water at a temperature ranging from 250-350 °C. The %weight of water released from ATH can be used to determine ATH content in such silicone formulation [346]. Silphenylene siloxane elastomer shows very high thermal stability (TGA: 50% weight loss temperature is above 600 °C) under nitrogen and oxygen and excellent low temperature flexibility [347-348]. Incorporation of a vinyl group in p-silphenylene siloxane segment improves thermal stability significantly and causes more formation of nonvolatile residues due to thermal polymerisation and cyclisation of vinyl segments [349-350]. Polyester-siloxane block copolymer of polybutylene terephthalate (PBT) and PDMS shows slightly improved thermal stability compared to the neat PBT under nitrogen. However, under an oxygen atmosphere copolymers exhibit multi-step decomposition. The first step involves decomposition of siloxane segments to form gaseous silicon compounds, CO, CO2, HCOOH and so on, whereas the second and third step involves the elimination of cyclised siloxane oligomers and degradation of PBT [351].
Miscellaneous Rubbers
Polyacrylic rubber (ACM) decomposes under nitrogen at around 410 °C [334, 352]. Poly[oxy(chloromethyl)ethylene] (or polyepichlorohydrin), under nitrogen, mostly degrades in a single step with a decomposition temperature of 344 °C, whereas poly[oxy(ethylthiomethyl)ethylene] decomposes at 354 °C. Although the former decomposes at lower temperature it leaves more nonvolatile residue (char) than the latter [353]. Polynorbornene (PNBE) prepared by ring opening metathesis showed a two-step decomposition with a major peak at 455 °C and a minor degradation peak at 520 °C [354]. 112
Applications of DSC and TGA for the Characterisation of Rubbers and Rubbery Materials
3.3.2 Compositional Characterisation of Rubbers by TGA TGA or DTG data provides useful information for the quantitative analysis of the rubber vulcanisates. A vulcanisate usually consists of several chemical ingredients (other than raw rubbers) that are used as filler, emulsifier (in emulsion polymerised raw rubber), processing oil (for filler dispersion and mixing), vulcanising agents and accelerators, antioxidants, and activator to accelerator. Broadly, a rubber vulcanisate consists of polymer rubber, fillers (inorganic, organic and carbon black) and acetone extractable volatiles (plasticising oil, antioxidant, unutilised vulcanising agents, and so on). In a TGA scan volatiles leave the samples in the low temperature region followed by decomposition of polymers at higher temperature. The weight loss due to plasticisers and polymers are usually overlapped, therefore, it is very difficult to distinguish volatile content in a rubber vulcanisate from a single TGA run [303, 355]. Both carbon black and inorganic fillers are left as residue after TGA runs under nitrogen. Generally, the polymer is completely decomposed under nitrogen at 600 °C. At this temperature the TGA gas environment is usually changed to air/oxygen. Carbon black oxidises at 600-800 °C and inorganic fillers are left as an ash residue during this TGA scan in air/oxygen. Thus inorganic ash content (say, A) and carbon black content (say, B) in wt% can be estimated from TGA scans in air and nitrogen. The amount of acetone extractable volatiles is removed by an acetone extraction (using Soxhlet extractor) of the vulcanisate at reflux temperature (60 °C). Ratio of polymer to carbon black content (say, R) of acetone extracted rubber can be used to back calculate the percentage of polymer (say, P) in the unextracted rubber, P = B/R. Then volatiles (V) in the original rubber can be estimated from V + P + B + A = 100. A schematic of a TGA thermogram of a vulcanisate is shown in Figure 3.15. A detailed methodology for the compositional estimation (formulation reconstruction) of elastomer compounds was reviewed by Zeyen [265]. Macaione and co-workers found that TGA data for percentage inorganic varied in different runs, mostly due to heterogeneity in sample. Ash content obtained on a large mass of sample by a simple gravimetry after combustion in a muffle furnace appeared close to the known value for a vulcanisate [356]. DTG data of various rubber compounding ingredients are summarised in Table 3.11. Under a nitrogen atmosphere, thermal stability of the plasticising oils follows the order: paraffinic oil (DTGmax, 381 C) > 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 ()
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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
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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
Figure 4.5 Tearing energy Gc against effective rate RaT of tear propagation at Tg for PNF samples crosslinked to various extents: (▲) 0.5% DCP (E = 50 kPa); (●) 1% DCP (E = 97 kPa); (■) 3% DCP (E = 121 kPa) From A.K. Bhowmick, Journal of Materials Science, published by Springer, 1986, 21, 3929
155
Thermal Analysis of Rubbers and Rubbery Materials Dynamic properties are dependent on both frequency and temperature and it is possible to approximately relate the two effects quantitatively. Preferably, results would be obtained over the range of frequencies and temperatures of interest but if it is required to transform modulus results to other temperatures or frequencies, use may be made of the so-called WLF equation. The effect of increasing or decreasing frequency is to shift the curves to the right or left, respectively, along the temperature axis. At room temperature, the order of magnitude of the effect of temperature on modulus for a typical rubber is 1% per ºC and the effect of frequency of the order of 10% per decade [2].
4.2 Instrumentation
4.2.1 Working Principle of a Dynamic Mechanical Analyser Dynamic mechanical analysers can be divided into two main categories: stress (force) controlled and strain (amplitude) controlled, based on their ability to apply a constant force or a constant displacement to the sample, respectively. DMA Q800 (TA Instruments) is a typical example of a stress-controlled instrument. As shown in Figure 4.6, in a stress controlled DMA, a motor applies a force (stress) to the sample and a displacement sensor measures the strain (amplitude). While running an experiment, the instrument such as DMA Q800 simply applies force (control parameter) until it achieves the pre-set amplitude. As the sample stiffness varies considerably during the test, applied control force also varies to maintain the measured pre-set amplitude. Typical output signals from DMA Q800 include storage modulus, loss modulus, loss tangent, sample stiffness as a function of temperature and frequency [8].
Figure 4.6 A schematic diagram of a stress controlled DMA
156
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites A brief specification of DMA Q800 is as follows: Force range:
0.0001 N to 18 N
Amplitude range:
0.50 μm to 10,000 μm
Modulus range:
103 Pa to 1012 Pa
Frequency range:
0.01 Hz to 200 Hz
Temperature range:
-150 ºC to 600 ºC
4.2.2 Selecting a Clamp for a DMA Experiment Selection of proper clamp while running a DMA experiment is of great importance. The sample geometry and the stiffness of the material determine the choice of clamp. Improper or wrong selection of clamp may even lead to erroneous results. Table 4.1 summarises the general guidelines for selecting a proper clamp while running a DMA experiment.
Table 4.1 Guidelines for selecting a DMA clamp Clamps 3-Point bending
Type of Sample Stiff, low damping
Cantilever (single/ Weak to moderately stiff dual) Shear
Unsupported viscous liquids to elastomers above glass transition
Examples Metals, ceramics, highly filled thermosetting polymers, highly filled crystalline, thermoplastic polymers Thermosetting resins, elastomers, amorphous or lightly filled thermoplastic materials Uncured resins, b-staged material, tyre rubber
Compression
Gels and weak elastomers Personal care products, toothpaste, hydro-gels Film tension Thin films and fibres Various types of films Fibre tension Single/bundied fibres Various types of fibres Penetration Any material Various samples for DMA penetration, Tg, or melting analysis (not used for quantitative DMA experiments.) Reproduced with permission from Reference [8]
157
Thermal Analysis of Rubbers and Rubbery Materials
4.2.3 Running a DMA Experiment In the DMA Q800, before starting an experiment, one has to choose, install and calibrate a suitable clamp guided by the sample shape and nature (stiffness) of the material. After installing the clamp, the appropriate mode of operations such as temperature scan, multifrequency scan, multistrain scan, stress relaxation etc. need to be selected. Once the mode selection is over, pre-programmed templates or standard procedures are available for that particular mode with all modern DMA instruments, which guide the user from start to end of an experiment. Sample dimensions, frequency, amplitude, heating rate and temperature ranges are commonly used as input parameters. The user has the option to modify or create a new programme based on specific requirements. Then sample is mounted on the chosen clamp, followed by the closure of the isolated thermal chamber or furnace. The run can now be started by pressing the start button housed on the instrument panel or from the software control. Although the previous description is applied to the DMA Q800, there are many other machines available in the market (such as the Diamond DMA from Perkin Elmer, the Viscoanalyser from 01db-Metravib, StressTech DMA from Rheologica Instruments AB), which can perform very similar operations. Various modes of tests that are possible nowadays can be broadly classified into two groups: single sweep and combined sweep. • Single sweep: - Frequency sweep, - Dynamic displacement sweep, - Dynamic strain sweep, - Dynamic force sweep, - Static displacement sweep, - Temperature sweep. • Combined sweep: - Frequency/temperature sweep (stages), - Frequency/temperature sweep (ramps), - Dynamic displacement/temperature sweep, - Dynamic strain/temperature sweep, - Dynamic force/temperature sweep, - Static displacement/temperature sweep, - Time/temperature sweep (ramps and stages). 158
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites
4.3 Interpretation of Dynamic Mechanical Spectra of Polymers: Case Studies
4.3.1 Glassy Polymers Glassy polymers show several different relaxation transitions, which are known as alpha ( ), beta (), gamma () and delta (), in the order of their appearance with decreasing temperature. At first, at relatively high temperatures, -relaxation or transition, which is better known as Tg, occurs accompanied by a large-scale change in storage and loss modulus. For polymethylmethacrylate the -transition at 125 ºC is shown in Figure 4.7. Side chain motions of the functional groups such as ester groups are responsible for -relaxation. The and relaxations are due to the localised measurements of methyl group present in the main and side chains, respectively, [4, 5]. The transition temperatures are greatly influenced by the change in the molecular flexibility or free volume, which are governed by molecular weight, plasticisers, crosslinking etc. Below the Tg, the molecular weight has no effect on the transition temperature in the case of high molecular weight glassy polymer. However, in the case of low molecular weight polymer, the presence of a large number of chain ends affects the Tg considerably [4]. Molecular weight has a profound effect in the transition zone.
Figure 4.7 DMA plot of PMMA showing transition
159
Thermal Analysis of Rubbers and Rubbery Materials Increase in chain length or chain entanglements leads to the disruption of irreversible flow. As the number of crosslinks is increased, the Tg shifts towards higher temperature and the transition is also broadened. Tg is not generally found in highly crosslinked polymers. This can be exemplified with the help of phenol-formaldehyde (PF) crosslinking reaction [9]. With the addition of 2% hardener (hexamethylene tetramine, which is commonly known as ‘hexa’), PF resin shows a Tg at 120 ºC. However, at 10% hardener concentration, it does not yield any Tg.
4.3.2 Crystalline Polymers The relaxation transitions in crystalline polymers can be represented with the help of low-density polyethylene as shown in Figure 4.8 [10]. A low temperature relaxation transition appears at –120 ºC, which is due to the restricted motion of several successive CH2 groups of the main chain (crank shaft mechanism). Between –13 ºC to +1 ºC at 3 Hz, another peak is observed, because of the relaxation of the side groups or branching. The -peak (105 ºC–109 ºC) is a manifestation of a crystalline relaxation. Similar behaviour is shown by high-density polyethylene (HDPE), although it does not give any -relaxation and the -relaxation seems to consist of two transitions, and ´
Figure 4.8 DMA spectra of pure low-density polyethylene (LDPE) at different frequencies From Chaki et al., Journal of Polymer Engineering, published by Freund Publishing House, 1994, 13, 20
160
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites [4]. For HDPE, increase in lamellar thickness and annealing reduces the -relaxation transition, which is associated with chain folding. Figure 4.9 shows the dynamic mechanical plot of Nylon-6, which reveals mainly two transitions, and , which appears at 98 ºC and –50 ºC, respectively [11]. The -peak is designated as its Tg, which is due to the motion within the amorphous phase, whereas the damping peak is because of the H-bonds formed between the carboxyl group of the polyamide and absorbed water. At –100 ºC or below, another transition, -relaxation is also observed. For isotactic polypropylene, the -transition or the Tg appears at about 0 ºC as shown in Figure 4.10 [12]. Another broad peak can be seen at ~ 70 ºC, which is believed to be due to the amorphous isotactic portion of the polymer. Similarly, polyethylene terephthalate (PET) shows two relaxations: and , where the
-relaxation is primarily due to the presence of the amorphous region [13]. Neat PET has a Tg of around 93 ºC. This relaxation is solely dependent on the crystallinity and the relaxation is observed at –60 ºC. For polytetrafluoroethylene, three main transitions can be identified [9].
Figure 4.9 Dynamic mechanical plot of nylon-6
161
Thermal Analysis of Rubbers and Rubbery Materials
Figure 4.10 Different -transitions in isotactic polypropylene
4.3.3 Elastomers
4.3.3.1 Glass Transition of Raw Rubbers The Tg can be determined more accurately from the plot of dynamic mechanical properties of raw rubber. A number of factors such as chain flexibility, molecular weight and so on influence the Tg. Figure 4.11 shows the dynamic mechanical spectra, i.e., plot of tan and log (storage modulus) as a function of temperature for raw natural rubber (NR) obtained from a NR latex having 60% dry rubber content [14]. It can be clearly seen that the tan passes through a maxima whereas the storage modulus falls rapidly at the transition zone. The tan peak maxima or Tg appears at about –50 ºC with a very high value of 2.4, indicating the predominant damping or viscous nature of the raw rubber. Beyond the Tg, tan decreases steadily with the increase in temperature and tends to level off at a comparatively higher temperature. The storage modulus undergoes a characteristic sharp decrease at the Tg, where it reduces from ~108 Pa to about ~106 Pa in magnitude. Thereafter, it decreases continuously with increase in temperature, resulting in a very low plateau modulus values. 162
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites
Figure 4.11 DMA plot of raw natural rubber (NR)
The DMA plot of control and hydrogenated styrene butadiene rubber (HSBR) with 17% styrene content is shown in Figure 4.12 [15]. The glass to rubber transition temperature of the amorphous phase increases with increasing saturation level. At a hydrogenation level of the 94%, Tg increases by 19 ºC, because of the gradual replacement of the amorphous segments by crystalline units of HSBR. The value of storage modulus (E´) increases accordingly over the whole temperature range. With the increase in styrene content, the Tg of styrene butadiene rubber (SBR) changes linearly, which is not observed in the case of HSBR.
4.3.3.2 Glass Transition of Modified Rubber On modification of polymers especially rubbers, the Tg changes drastically. This can be detected by the DMA very easily. For example, polyethylene octene elastomer (POE) (commercially known as Engage), having 27% crystallinity (POE-27), has been modified with 10 wt% acrylic acid by a solution grafting method, using 0.3 wt% benzoyl peroxide (BPO) as initiator and toluene as medium at 70 ºC for 6 hours [16]. DMA studies have revealed that after the modification, as shown in Figure 4.13, there is a substantial shift in the Tg of the polyethylene octane elastomer from –31 ºC (POE-27) to –20 ºC (POE27-g-AA), indicating the delay in the segmental movement, which is probably due to the relatively strong interaction between the polar clusters formed because of the acid 163
Thermal Analysis of Rubbers and Rubbery Materials
Figure 4.12 Dynamic mechanical spectra of hydrogenated styrene butadiene rubber (17% styrene content) De Sarkar et al., Journal of Materials Science, published by Springer 1999, 34, 1742
Figure 4.13 DMA plot of ethylene octene copolymer modified with 10 wt.% acrylic acid
164
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites grafting. However, the storage modulus value of the acid modified sample (POE27-g-AA) is considerably lower than that of the virgin elastomer, particularly in the low temperature regions due to the drop in the crystallinity of the former after modification. In an another interesting example, hydrogenated nitrile rubber (HNBR) has been synthesised from nitrile rubber (NBR) having varying levels of acrylonitrile (ACN) content, by homogeneous catalytic hydrogenation process and the effect of hydrogenation on the dynamic mechanical properties has been studied extensively by Bhattacharjee and co-workers [17]. It has been found that, the Tg of the hydrogenated NBR is lower than that of the pure polymer. The higher the degree of hydrogenation, the lower is the Tg. This is attributed to increased chain flexibility and absence of crystallinity upon hydrogenation.
4.3.3.3 Influence of Crosslinking Crosslinking means the anchorage of different parts of large polymer chains by chemical agents. Generally rubber has poor mechanical properties without crosslinking. There are other polymers, which also need crosslinking for better performance such as epoxies and polyesters. Crosslinking of rubber is thus a common practice to impart better elastomerlike properties. In Figure 4.14, the effect of sulfur crosslinking on the dynamic mechanical
Figure 4.14 Tan as a function of temperature for crosslinked and uncrosslinked NR compounds
165
Thermal Analysis of Rubbers and Rubbery Materials properties of an unfilled NR is demonstrated. NR latex has been prevulcanised with 2.4 phr of sulfur and 1.2 phr of accelerator zinc diethyl dithio carbamate (ZDC), to form crosslinked NR and compared with an uncrosslinked NR [18]. Introduction of sulfide linkages between the NR chains due to the crosslinking restricts the segmental motion, leading to a positive shift in tan peak (Tg) with an accompanying decrease in tan max value. Due to similar reasons, the storage modulus value of the crosslinked NR is much higher than that of the uncrosslinked one. The influence of curing systems on tan for HNBR is compared in Figure 4.15 [19]. Here the sulfur-cured system is having a similar degree of crosslinking as the peroxide cured one. The tan peak height is reduced for the sulfur-cured system in the transition zone, due to the formation of sulfur crosslinks in the polymer, which in turn considerably increases the monomeric frictional coefficient. Similar results have also been found for NR [2]. For the sulfur-cured system, the tan decreases first in the temperature range of 90-180 ºC and then increases steadily with temperature because of the post-vulcanisation reaction and cleavage of sulfide linkage, respectively. It has been generally observed that with the increase in crosslink density, tan shifts towards a higher temperature. In the case of electron beam crosslinked fluorocarbon rubber system, for example, with an increase in radiation dose, the Tg shifts by about 4–6 ºC,
Figure 4.15 Tan as function of temperature for the compounds cured with different curing systems (––– peroxide cured) and (----- sulphur cured) From P. Thavamani and A.K. Bhowmick, Journal of Materials Science, published by Springer, 1992, 27, 3246
166
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites with a corresponding decrease in tan peak value (tan max) [20]. Segmental motions are hindered by the crosslinking, which require higher temperature for the inception of rotation. At a high radiation dose, the peak temperature does not show any significant shift, because of the balancing effect of crosslinking and chain scission reactions. Similar results have also been found for ethylene-propylene-diene terpolymer (EPDM) rubber [21]. The unirradiated control EPDM rubber shows a Tg of –33 ºC, which shifts slightly towards higher temperatures as the radiation dose increases due to the increase in crosslink density. This also brings about a marginal decrease in the tan max values. Use of radiation sensitiser such as trimethylol propane triacrylate (TMPTA) enhances the crosslinking for a given radiation dose. With the increase in TMPTA concentration level, log E´ is increased along with a decrease in tan peak height [21].
4.3.3.4 Influence of Filler Reinforcing fillers are generally incorporated into rubber to achieve better mechanical and failure properties. In Figure 4.16, the effect of carbon black on tan for HNBR filled with varying levels of super abrasion furnace carbon black (10-50 phr) and crosslinked with
Figure 4.16 Variation of tan and storage modulus as a function of temperature for SAF filled HNBR (––– gum; ----10 phr; – - –20 phr; – – – 30 phr; – - - – 40 phr; – ---- –50 phr) From P. Thavamani and A.K. Bhowmick, Journal of Materials Science, published by Springer, 1992, 27, 3250
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Thermal Analysis of Rubbers and Rubbery Materials 3 phr of dicumyl peroxide, is shown [19]. The peak tan height reduces progressively with the increasing filler loading. The trend gets reversed in the plateau zone where the 50 phr carbon black containing system produces the highest value of tan . The value of G´ increases steadily with the filler loading. Payne proposed following contributing factors [22], for the considerable increase in the modulus of the filled elastomers (a) contribution from pure gum rubber, which depends on the crosslink density and entanglement; (b) the strain amplification effect or hydrodynamic effect. Here the fillers increase the amount of local strain; (c) due to the elastomer-carbon black bonds, the number of crosslinks per unit volume increases which lead to the increase in modulus; (d) carbon black structure is believed to be responsible for increase in the dynamic and static modulii at low (<1%) strain. The limiting value of storage modulus can be achieved by swelling the elastomer in appropriate solvents in absence of the energy dissipation [23]. A very similar effect has been reported for modified carbon black filled rubber vulcanisates by Medallia [24] and Bandyopadhyay and co-workers [25]. As shown in Figure 4.17a-b, Bandyopadhyay and co-workers [26] have studied the effect of surface oxidised carbon black on the dynamic mechanical properties of carboxylated nitrile rubber. At room temperature, with the increase in the oxygen content on the carbon black filler surface, both storage and loss modulus increase, with the storage modulus having the higher rate of increase (Figure 4.17a). This is ascribed to the physical and chemical bonding between the rubber and filler leading to the reinforcement. Due to similar reasons, the loss factor (tan ) gradually decreases with increasing oxygen content and tends to level off at higher oxygen content (Figure 4.17b). Ray and Bhowmick have studied extensively the effect of electron beam modified surface coated silica [27] and clay fillers [28] on the dynamic mechanical properties of ethylene octene copolymer. Surface modified fillers shifted the Tg of the base polymer towards a higher temperature with an accompanying increase in the storage modulus value compared to the unmodified fillers, due to the better polymer to filler interaction. Shanmugharaj and Bhowmick have also reported the dynamic mechanical behaviour of surface treated dual phase fillers in SBR [29]. Using the DMA, the distribution of filler has been calculated for a series of silica filled 50-50 binary blends of epoxidised natural rubber with 25 mol% epoxy content (NRENR 25) by Maiti and co-workers [30]. For example, as the silica filler is distributed more in the ENR-25 phase, the extent of decrease in tan peak value is more prominent. This is due to the differences in polymer to filler ratio, polymer to filler interaction and the immobility of the dispersed phase (NR in this case). Normally addition of fillers to rubber, lowers the peak tan value (tan max). However, in the case of the binary blends, due to preferential distribution of the fillers in different phases, this lowering is different for both the phases. This has been assumed to be proportional to the quantity of filler remaining or migrated to the individual rubber phases. A simple mathematical 168
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites
Figure 4.17 Variation of (a) Storage (E´) modulus and Loss (E´´) modulus, respectively and (b) tan max with percent oxygen content of surface oxidized carbon blacks From Bandyopadhyay et al., Journal of Applied of Science, published by John Wiley & Sons Ltd., 1995, 58, 723
169
Thermal Analysis of Rubbers and Rubbery Materials calculation based on the reduction of tan peak value on incorporation of filler gives us an estimate of the amount of the filler in each phase. This has been given by the following expression: R = [(tan g) max - (tan f) max]/tan (4.11) Where g and f are the gum and filled systems, respectively. The following relationship relates the term R to the weight fraction of the filler, w: R= w (4.12) Where is the polymer-filler interaction parameter. Equation 4.12 applies to the component single elastomers and their blend systems (i.e., R1 = 1w1 and R´1 = ´1w´1 for blend systems). The total weight fraction of filler, w, in a binary blend is the sum of weight fractions, w1 and w2, in individual polymers. Hence: w = w´1 + w´2 (4.13) The subscripts 1 and 2 denote two different single-phase elastomer systems. Assuming that the ratio of interaction parameters in the single phases is equal to those of the same elastomers in the blend i.e. [ 1/ 2 = ´1/ ´2], the following final equation has been proposed by Maiti and co-workers [30]: W´1 = (R´1R2w)/(R´1R2 + R´2R1) (4.14) The amount of filler (%) in each system can be obtained by multiplying w´1 and w´2, by 100.
4.3.3.5 Elastomer Blends In order to understand the dynamic mechanical properties over a range of temperatures and frequencies, and also to predict the compatibility of various components in polymer and elastomer blends, dynamic mechanical measurements are frequently used. An example of compatibility between acrylate rubber (ACM) and fluorocarbon rubber (FKM) is shown in Figure 4.18 [31]. The tan max (peak) for ACM and FKM, corresponding to their Tg, appear at 4 ºC and –4 ºC, respectively. Other than the Tg, ACM shows two major peaks at –65 ºC and –100 ºC, due to the side chain acrylate moiety and the –CH2 sequence, respectively. Miscible blends with homogeneous phases produce a characteristic sharp single Tg peak irrespective of their blend composition. However, the tan max of the blend is shifted towards higher temperature. With the 170
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites
Figure 4.18 Compatibility between acrylate rubber and fluorocarbon rubber through dynamic mechanical spectrum From A. Kader and A.K. Bhowmick, Rubber Chemistry and Technology, published by Rubber Division, ACS, 2000, 73, 896
increase in ACM content in the blend, tan and Tg are gradually increased because of the higher tan value of ACM compared to FKM. The presence of strong interaction is reflected from the highest shift in the absorbance peak of the carbonyl band in IR spectra, with a corresponding higher enthalpy of interaction and a finely dispersed microstructure (~2 μm) as evident from the transmission electron micrograph, which indicates miscibility of the blends. For immiscible blends, for example, the blend of silicone rubber and EPDM rubber, the individual peaks due to the components are clearly identifiable (Figure 4.19). The loss tangent peak at –109 ºC appears due to the glass transition of silicone rubber, whereas the tan peak at –23 ºC is because of the dual effect of the Tg of EPDM and the melting of silicone crystallites [32]. At –55 ºC, a very small hump appears due to the amorphous to crystalline transition in silicone rubber and the magnitude of this hump indicates a low level of crystallisation. The same authors have studied the effect of physical compatibilisers and chemical interaction between the components on the dynamic properties [33]. The influence of chemical interaction on the dynamic mechanical properties is exemplified from the following interesting example of a thermoplastic elastomer (TPE) produced from 171
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Figure 4.19 Loss tangent behavior for the A- AM-g-Si + MA-g-EP blended and moulded at 35 °C; B- AM-g-Si + MA-g-EP blended and moulded at 150 °C; C- AM-g-Si + MA-gEP blended and cured at 150 °C; D-control silicone + EPDM blend cured at 150 °C. The components were blended in a 50-50 ratio From Kole et al., Polymer, published by Elsevier, 1995, 36, 3276
reactive processing of a rubber-plastic blend. Figure 4.20 shows the effect of interaction or reaction on the dynamic mechanical properties of a 40-60 (w/w) Nylon-6/ACM blend. The reaction has been carried out in a mixer and the reaction time includes specified time of mixing in the mixer at 220 ºC and an additional two minutes moulding at 230 ºC in press. The magnitude of tan max decreases progressively up to 9 minutes of reaction. For ACM, the Tg decreases with time up to 11 minutes. The most interesting feature is the appearance of a secondary tan peak at a high temperature region with increasing level of interaction. In the DMA curve of the blends, two shoulder peaks appear at 13 ºC and 17 ºC corresponding to 9 and 11minutes of reaction time, respectively, whereas a distinct peak is observed at 22.5 ºC after 13 minutes of reaction time (Figure 4.20). A similar trend is observed in the modulus versus temperature plot. The storage modulus also increases with reaction time, showing a progressive increase of the interfacial reaction between Nylon-6 and ACM phases, which leads to the improved adhesion between the components. The authors have further confirmed these facts with the help of IR, nuclear magnetic resonance spectroscopy, ellipsometry and adhesion studies [11]. 172
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Figure 4.20 Temperature dependence of tan and E´ of nylon-6/ACM (40/60) blends interacted at different times From A. Jha and A.K. Bhowmick, Rubber Chemistry and Technology, published by Rubber Division, ACS, 1997, 70, 808
The reduction in tan max of ACM in the blend is attributed to a decrease in the relative amount of bulk rubber active in the dynamic transitions. As the weight fraction of ACM in the blend is held constant, such a reduction may be interpreted as the progressive immobilisation of the ACM chains residing close to the boundary between the two phases where they are increasingly grafted to the Nylon matrix. The mobility of the rubber chains when grafted to the plastic matrix is expected to be greatly reduced compared to a chain in the bulk polymer, with the mobility increasing gradually as chains go away from the boundaries. Thus, the formation of a layer of restricted chain mobility near the phase boundary is responsible for the appearance of a secondary tan peak. In the case of the reactive blend, the decrease of Tg of ACM indicates that the segmental motion in the bulk occurs at lower temperatures than it appears in homopolymers. When the samples are cooled from the melt at lower temperatures, a state of tri-axial tension is formed on the outer shells of the rubber phases due to the greater thermal contraction of rubber compared with the glass matrix. This thermal stress can only be developed when there is sufficient adhesion between the phases, which generally results from the grafting reaction. As a consequence of this, the free volume of the rubber increases 173
Thermal Analysis of Rubbers and Rubbery Materials and leads to a decrease in the Tg. The decrease of Tg of nylon-6 is due to the increased flexibility of Nylon chains, as Nylon-6 is grafted to ACM chains, which are rubbery in nature [11]. The self-vulcanisable elastomer and self-crosslinkable rubber plastic blends through the DMA have been extensively studied. A number of such blends based on epoxidised natural rubber (ENR) and chlorosulfonated polyethylene (CSM) [34], Neoprene (CR) and carboxylated nitrile rubber (XNBR) [35] and a ternary blend based on ENR, CR and XNBR [36] have been developed. In the plots of tan versus temperature of such a self-vulcanisable blend of CR and XNBR [37], as shown in Figure 4.21, two distinct Tg for the blends indicate the presence of two phases in the blend. The higher Tg is due to the XNBR phase while the lower one is due to the CR phase. For the blends, the Tg of the individual phases approach each other indicating a very strong interaction between the two phases. It can be also seen that the tan peak height for XNBR decreases and that of CR increases as the proportion of CR in the blend increases. The crosslinking mechanism in such cases is due to the chemical reaction occurring between the different
Figure 4.21 Variation of tan with temperature for different neoprene and XNBR blends having 100%(N100), 75%(N75), 50%(N50), 25%(N25) and 0%(N0), Neoprene, respectively From S. Mukhopadhyay and S.K. De, Journal of Applied of Science, published by John Wiley & Sons Ltd., 1991, 43, 2290
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Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites functional groups presents in the different components. Several researchers have reported the dynamic properties of self crosslinkable rubber plastics blends [38, 39]. Anandhan and co-workers [40] have studied the effects of dynamic vulcanisation and blend ratios on mechanical properties and morphology of TPE compositions, based on blends of NBR and polystyrene-co-acrylonitrile (SAN) and DMA has been used as one of the tools to investigate blend miscibility. The storage modulus of the blends, which represents the relative stiffness of the materials, increases with an increase in the plastic content of the blends (Figure 4.22). The E´ values decrease with an increase in the NBR content of the blends because of the higher modulus value of SAN than that of NBR. The E´ values of the dynamically vulcanised blends are higher than those of the unvulcanised blends as the modulus of the NBR phase is highly increased as a result of the introduction of crosslinks. Also due to the crosslinking, the loss tangent peak of the rubber phase is considerably broadened with a reduction in the peak height. This can be explained by the fact that, for unvulcanised blends, the spectra arise because of the multiple relaxations of the loosely connected or unconnected polymeric chains. Dynamic vulcanisation reduces the number of relaxations.
Figure 4.22 A dynamic mechanical spectrum of NBR/SAN blends. Log E´ versus temperature plot From Anandhan et al., Journal of Applied Polymer Science, published by John Wiley & Sons Ltd., 2003, 88, 1985
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4.3.3.6 Block Copolymers Thermoplastic elastomers from poly [styrene-b-{ethylene-co-butylene)-b-styrene] triblock copolymer (SEBS) has two Tg due to its micro-phase separated self-assembling structure. Figure 4.23 shows two distinct Tg, one at –58 °C for the rubbery polyethylene-co-butylene phase and another at 64 °C for the plastic polystyrene phase, which are characteristics of this TPE [41].
Figure 4.23 Tan versus temperature plot for SEBS
4.3.3.7 Rubber based nanocomposites Nanotechnology is recognised as one of the most promising fields of research of 21st century. The term, ‘nanocomposite’ refers to every type of composite materials having one of the components in the nanometer size range at least in one dimension. More specifically, polymer nanocomposites are polymer matrix composites that are reinforced with rigid inorganic or organic particulate having nano-metric size-range. Polymerclay nanocomposites possess unique properties that are not shared by conventional composites, such as excellent mechanical properties, high thermal stability, improved barrier properties and flame retardance [42]. Better mechanical properties originate from stronger polymer-filler interaction and a large interfacial area per unit volume of the filler particles. Addition of nanofillers considerably modifies the low strain dynamic mechanical properties of the rubber vulcanisates depending on the nature of the fillers used. Rubber-based nanocomposites have been prepared from SBR and NBR with varying 176
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites acrylonitrile content (19%, 34% and 50%) using unmodified (N) and octadecyl amine modified (OC) Na-Montmorillonite clays [42]. Figure 4.24 illustrates the temperature dependencies of storage modulus for SBR and its nanocomposites. With the addition of 4 phr of nanoclays, the clay filled nanocomposites give higher storage modulus values compared to unfilled SBR in the glassy as well as in the rubbery regions. The peak tan value, i.e., the Tg shifts from –57 ºC in unfilled SBR to –50 ºC for a modified clay filled sample. The magnitude of the tan peak also reduces drastically from the control SBR (1.74) to OC-filled SBR (1.10). Better polymer-filler interaction is responsible for the lowering in tan peak height and shifting of the Tg towards higher temperatures. It has been found that the dynamic mechanical properties are greatly influenced by the nature and polarity of the base rubber due to the change in degrees of intercalation and interactions. For example, the storage modulus of OC-filled NBR systems increases steadily with nano filler loading. However, NBR with 50% ACN content shows the maximum increment in storage modulus with clay loading. Figure 4.25 shows the plot of loss tangent as a function temperature of different fluorocarbon rubber based nanocomposites having different loadings of unmodified (NA) and modified nano clays (20A)[43]. The Tg of these nanocomposites has been calculated from the peak maximum in the curve. With the addition of the unmodified clay, the Tg shifts towards a higher temperature by 5 C (FNA4-V), whereas a 2 C rise
Figure 4.24 log E´ versus temperature plot for SBR and its nanocomposites From S. Sadhu and A.K. Bhowmick, Rubber Chemistry and Technology, published by Rubber Division, ACS, 2005, 78, 327
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Figure 4.25 Variation of tan with temperature for different fluorocarbon rubber nanocomposites From M. Maiti and A.K. Bhowmick, Polymer Engineering Science, published by John Wiley & Sons Ltd., 2007, 47, 1779
is observed in the modified clay filled system at 4 phr loading (F20A4-V). The change in Tg is attributed to the increase in volume fraction of the rubber, arising from the increased rubber-filler interaction. This results in restricted segmental mobility of the polymer chains. Among the entire filler loaded samples, the maximum shift in Tg can be observed at 4 phr loading. The magnitude of tan max is also lowest for this nanocomposite compared to the gum vulcanisate. At still higher loading the peak height increases. As the rubber-filler interaction increases, the available free-chains decrease, resulting in the decrease in tan max. The lowest value of tan max in FNA4-V is due to better filler dispersion and higher interaction between fluoroelastomer and polar unmodified clay compared to the organically modified clay. This observation has been found to be very unique for the fluoroelastomer nanocomposite system. Over a long range of temperatures, both the unmodified and modified clay filled systems show increased storage modulus compared to the gum vulcanisate. For example, at 25 C, 10% improvement in log E´ can be observed with 4 phr of unmodified clay compared to the control. The improvement in storage modulus is higher for the unmodified clay filled system than the modified one. The storage modulus increases marginally on changing the filler loading from 4 to 16 phr in the transition region while in rubbery region it does not change appreciably [43]. 178
Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites Figure 4.26a-b compares the storage modulus as well as tan for various nanocomposites prepared from ethylene vinyl acetate (EVA) rubber, having 50% vinyl acetate content, and multi-wall carbon nanotubes (MWNT), as a function of temperature [44]. Throughout
(a)
(b)
Figure 4.26 DMA plots of EVA with MWCNT, showing temperature dependence of (a) Storage modulus and (b) Tan , respectively
179
Thermal Analysis of Rubbers and Rubbery Materials the temperature range of -35 to 20 °C, the moduli show a steady increase from the virgin rubber with the concentration of CNT. For example, there is a 10% increase of storage modulus with 4% CNT. The increase in modulus on addition of CNT is due to rubber-filler interactions. For tan , it shows a significant decrease in the peak height with CNT concentration in the EVA from that of the virgin rubber. A steady shift in the tan max, which indicates the Tg of the system, to higher temperature regions as a result of addition of CNT is also noted.
4.4 Dependence of Storage Modulus on Frequency and Strain Bandyopadhyay and co-workers [45] have studied the effect of nanosilica concentration on the frequency dependence of dynamic storage modulus of the nanocomposites prepared from acrylic rubber (ACM D) and nanosilica (tetraethoxysilane, TEOS) by sol-gel technique at 50 ºC. The frequency sweep plot of ACM nanocomposites having 10% (ACMD10), 30% (ACMD30) and 50% (ACMD50) nanosilica, with control ACM is shown in Figure 4.27. The nanocomposites containing a higher silica concentration have higher modulus, which increases with the increase in frequency. With the increasing frequency, in case of the neat polymer, maximum increase in the storage modulus is observed from the initial value, compared to their nanocomposites counterparts, up to 8 Hz frequency. The low frequency region corresponds to the behaviour at higher temperature, whereas the high frequency region is equivalent to low temperature characteristics. A consistent increase in dynamic modulus with frequency, in ACM/silica nanocomposites, indicates that more time is required for the relaxation of the polymer chains. This is due to the relatively weak polymer-filler interaction which gives polymer chains higher mobility at the polymer-silica interface to change their conformation. It can be seen from Figure 4.27 that the slopes of the modulus–frequency curves are slightly higher at higher frequency (beyond 8 Hz) in the ACM/silica nanocomposites compared with those in ACM, because of the easier detachment of the polymer chains in nanocomposites under high frequency or higher deformation conditions. Similar results are obtained for other nanocomposites systems [42]. The same authors have also studied the effect of dynamic deforming strain on the storage modulus of ACM/silica nanocomposites [45]. Figure 4.28 illustrates the dynamic strain dependence of storage modulus for the above system at 50 ºC. It can be clearly seen, that the storage modulus of the neat polymer (ACM D) does not change appreciably with the increase in applied strain amplitude over the experimental range. At very low strain (up to 0.2%), modulus is independent of the applied strain showing linear viscoelastic behaviour. After that, it reduces slightly with the increasing strain indicating the onset of non-linear viscoelastic regime. However, in the case of ACM/silica nanocomposites, the storage modulus decreases drastically with the increasing strain in the non-linear viscoelastic portion. This type of behaviour is commonly known as the Payne effect, which has also been observed extensively in the case of carbon black filled rubber composites [46]. 180
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Figure 4.27 Plots of storage modulus (log scale) against frequency at 50 °C for the hybrid ACM/silica nanocomposites From Bandyopadhyay et al., Journal of Polymer Science: Part B: Polymer Physics, published by John Wiley & Sons Ltd., 2005, 43, 2410
Figure 4.28 Plots of dynamic storage modulus (log scale) against strain amplitude for ACM/silica hybrid nanocomposites at different silica concentrations at 50 ºC From Bandyopadhyay et al., Journal of Polymer Science: Part B: Polymer Physics, published by John Wiley & Sons Ltd., 2005, 43, 2406
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4.5 Various Other Applications The other applications of dynamic mechanical analysis are as follows: 1. Processing characteristics: such as viscosity and extrusion shrinkage, and so on. 2. Tyre performance: such as rolling resistance, traction, tread wear and heat build up, and so on. 3. Mechanical goods: for vibration isolation, dynamic stiffness and so on. 4. Others: such as fatigue behaviour.
4.6 Conclusion DMA is a very accurate and useful technique to elucidate the dynamic properties of polymeric materials and composites. Information obtained from DMA can be used to evaluate the material properties of polymeric substances and to design and develop newer grades of materials. The dynamic mechanical thermal properties are dependent on many factors such as molecular weight, crystallinity, crosslinking, amount and nature of filler and even on the experimental conditions like applied frequency, amplitude, rate of heating and so forth. In this chapter, the dynamic mechanical properties of different rubbers, plastics and their blend and composites were discussed in detail with respect to various influencing factors.
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Dynamic Mechanical Analysis (DMA) for Characterisation of Polymers, Polymer Blends and Composites 6. O. Kramer and J.D. Ferry in Science and Technology of Rubber, Ed., F.R. Eirich, Academic Press, New York, NY, USA, 1978, p.180. 7. A.K. Bhowmick, Journal of Materials Science, 1986, 21, 11, 3927. 8. DMA Q-Series getting started guide, Chapter 3, p.50, TA Instruments Inc., New Castle, DE, USA, 2000. 9. P. Gradin, P.G. Howgate, R. Selden and R.A. Brown in Comprehensive Polymer Science: The Synthesis, Characterisation, Reactions and Applications of Polymers, Volume 2, Eds., G. Allen and M. Bevington, Pergamon Press Oxford, UK, 1989. 10. T.K. Chaki, D. Roy, A.B. Majali, V.K. Tikku and A.K. Bhowmick, Journal of Polymer Engineering, 1994, 13, 1, 17. 11. A. Jha and A.K. Bhowmick, Rubber Chemistry and Technology, 1997, 70, 5, 798. 12. P. Chakraborty, A. Ganguly, S. Mitra and A.K. Bhowmick, Polymer Engineering and Science, 2008,48, 960. 13. A. Jha and A.K. Bhowmick, Polymer, 1997, 38, 17, 4337. 14. S. Mitra, S. Chattopadhyay, Y.K. Bharadwaj, S. Sabharwal and A.K. Bhowmick, Radiation Physics and Chemistry, 2008, 77, 5, 630. 15. M. De Sarkar, P.P. De and A.K. Bhowmick, Journal of Materials Science, 1999, 34, 8, 1741. 16. A. Biswas, A. Bandyopadhyay, N.K. Singha and A.K. Bhowmick, Journal of Polymer Science Part A: Polymer Chemistry, 2007, 45, 23, 5529. 17. S. Bhattacharjee, A.K. Bhowmick and B.N. Avasthi, Industrial and Engineering Chemistry Research, 1991, 30, 6, 1086. 18. S. Mitra, S. Chattopadhyay and A.K. Bhowmick, Journal of Applied Polymer Science, 2008, 107, 5, 2755. 19. P. Thavamani and A.K. Bhowmick, Journal of Materials Science, 1992, 27, 12, 3243. 20. I. Banik and A.K. Bhowmick, Journal of Applied Polymer Science, 1998, 69, 10, 2079. 21. P.S. Majumder and A.K. Bhowmick, Journal of Applied Polymer Science, 2000, 77, 2, 323. 183
Thermal Analysis of Rubbers and Rubbery Materials 22. A. Payne, Rubber Chemistry and Technology, 1963, 36, 2, 444. 23. C. Neogi, S.P. Basu and A.K. Bhowmick, Plastics and Rubber Processing and Applications, 1989, 12, 3, 147. 24. A.I. Medalia, Rubber Chemistry and Technology, 1973, 46, 4, 877. 25. S. Bandyopadhyay, P.P. De, D.K. Tripathy and S.K. De, Polymer, 1995, 36, 10, 1979. 26. S. Bandyopadhyay, P.P. De, D.K. Tripathy and S.K. De, Journal of Applied of Science, 1995, 58, 4, 719. 27. S. Ray and A.K. Bhowmick, Polymer Engineering and Science, 2004, 44, 1, 163. 28. S. Ray, and A.K. Bhowmick, Radiation Physics and Chemistry, 2002, 65, 3, 259. 29. A.M. Shanmugharaj and A.K. Bhowmick, Journal of Applied Polymer Science, 2003, 88, 13, 2992. 30. S. Maiti, S.K. De and A.K Bhowmick, Rubber Chemistry and Technology, 1992, 65, 2, 293. 31. M.A. Kader and A.K. Bhowmick, Rubber Chemistry and Technology, 2000, 73, 5, 889. 32. S. Kole, S. Roy and A.K. Bhowmick, Polymer, 1995, 36, 17, 3273. 33. S. Kole, R. Santra and A.K. Bhowmick, Rubber Chemistry and Technology, 1994, 67, 1, 119. 34. S. Mukhopadhyay, T.K. Chaki and S.K. De, Journal of Polymer Science, Polymer Letters Edition, 1990, 28, 25, 1. 35. S. Mukhopadhyay and S.K. De, Journal of Applied Polymer Science, 1992, 45, 1, 181. 36. R. Alex, P.P. De and S.K. De, Polymer, 1991, 32, 13, 2345. 37. S. Mukhopadhyay and S.K. De, Journal of Applied Polymer Science, 1991, 43, 12, 2283. 38. P. Ramesh and S.K. De, Polymer Networks and Blends, 1992, 2, 209. 39. P. Ramesh and S.K. De, Journal of Materials Science, 1991, 26, 11, 2846. 184
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5
Characterisation of Rubbers, Polymers and Their Composites Using TMA R.S. Rajeev and P.P. De
5.1 Introduction Thermomechanical analysis (TMA) is one of the most important characterisation techniques in the field of thermal analysis. TMA measures the deformation of a material under non-oscillatory load either as a function of a controlled temperature programme or as a function of time at a constant temperature. With TMA, the dimensional properties of a sample are measured as the sample is heated, cooled or held under isothermal conditions. The loading or force applied to the sample can be varied in TMA. TMA conducted with zero applied load is known as thermodilatometry (TD) or thermodialometric analysis (TDA). It is a technique in which the dimension of a sample is measured as a function of temperature, as the sample is subjected to a controlled temperature programme [1]. TMA experiments are generally conducted under static loading with a variety of probe configurations. The mode of operation is by tension, compression, expansion, penetration, flexure and dilatometery. A few of the TMA probes are shown in Figure 5.1. Typical probe configurations for TMA as used in elastomer applications have been described by Maurer [2, 3]. The TMA technique is used to assess the following important properties of polymers and rubbers: 1. Expansion and contraction. 2. Modulus. 3. Glass to rubber transition temperature (Tg). 4. Softening point. 5. Gel point. 187
Thermal Analysis of Rubbers and Rubbery Materials 6. Viscosity at low temperature. 7. Crosslink density. 8. Shrinkage forces and percent shrinkages of films and fibres. 9. Coefficient of thermal expansion (CTE). 10. Testing of coatings on metals, films, optical fibres and electrical wires. 11. Composite delamination temperature. 12. Melting temperature. 13. Dimensional compatibilities of two or more different materials. The dilatometer mode in TMA is used to measure the coefficient of thermal expansion of polymers (for example, polyvinylchloride (PVC), polyethylene (PE) and polypropylene (PP)). TMA is used to measure the behaviour of fibres, which are mainly used to make rubber composites.
5.2 Instrumentation Details of the TMA instrument and various manufacturers are discussed in Chapter 2. The DuPont model 943 TMA/TA Instruments’ Q400 is capable of handling powders and fibres in the temperature range of -180 to 800 °C. An interchangeable sample probe permits the determination of penetration, expansion, tension and dilatometry of samples (Figure 5.1). The DuPont 943 TMA has three accessories: 1. Parallel plate rheometry (PPR) is used to measure the low shear viscosity of viscous materials. 2. Fibre tension spectrometry measures the shrinkage tension of fibre as a function of temperature. 3. Stress relaxation spectrometry measures the relaxation modulus versus time of viscoelastic materials.
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Characterisation of Rubbers, Polymers and their Composites Using TMA
Figure 5.1 Different probes (penetration and expansion) used for TMA
5.3 Applications
5.3.1 Determination of Tg TMA of elastomers via a penetrometer probe produces thermograms that closely resemble the master curves [2, 3] obtained by conventional time-temperature superposition of modulus data. Figure 5.2 shows the thermogram of isobutylene-isoprene rubber (IIR). Several types of information can be obtained from such data including the Tg and flow behaviour; –59 °C corresponds to the Tg of IIR. Noshay and McGrath [4] have observed that the random versus block copolymers can be distinguished by the type of variation in the master curve (Figure 5.2) that accompanied variations in polymer composition. For random polymers, the Tg changes in a regular fashion with composition but for block polymers, the Tg values of the two constituents remain constant and the height of the modulus plateau varies with composition. The most commonly used TMA probe is the expansion probe. This probe rests on the surface of two test specimens under low loading conditions. As the sample expands during heating, the probe is pushed up and the resulting expansion of the sample is measured. Schwartz [5] used TMA to study the influence of heating and cooling rate on expansion coefficients and Tg of a carboxy-terminated polybutadiene (CTPB). The detection of Tg by TMA in expansion and penetration modes are reported in Figures 5.3 and 5.4. The results on the effect of heating and cooling rates are shown in Figure 5.5. From this figure, it is evident that the value of Tg is dependent on both the heating 189
Thermal Analysis of Rubbers and Rubbery Materials
Figure 5.2 TMA (penetrometer) data resemble polymer master curves [3]. 1: glassy region; 2: leathery region; 3: rubbery plateau and 4: flow
Figure 5.3 Linear expansion of polybutadiene rubber [5]
and cooling rates. Extrapolation to zero rates gives the same Tg for both cooling and heating (Figure 5.5). Schwartz [5] has demonstrated that the expansion coefficient is not influenced by the rate of heating or cooling or by sample thickness. TMA is significantly more sensitive than differential scanning calorimetry (DSC) for the measurement of the Tg of crosslinked materials, filled materials and composites. 190
Characterisation of Rubbers, Polymers and their Composites Using TMA
Figure 5.4 Penetration test of polybutadiene rubber [5]
Figure 5.5 Tg of polybutadiene rubber as a function of heating and cooling rate [5]. A represents heating curve and B, cooling curve
TMA is useful for the measurement of thermal expansion or contraction of cured printed circuit boards (PCB), which are composite materials based on epoxy resin and glass fibre [6]. A typical TMA plot of a PCB is shown in Figure 5.6. The break in the expansion profile is the Tg of the board. TMA facilitates measurement of Tg [6] since 191
Thermal Analysis of Rubbers and Rubbery Materials the expansion of the board above its Tg is greater than that below Tg. If the starting thickness of the board is known, an exact value of thermal expansion (and contraction, if the board is cooled in a controlled manner) can be calculated. Determination of this expansion coefficient should be made for the full range of processing and in service working temperatures, including local fluctuations induced by soldering. The expansion measurement shown in Figure 5.6 is z-axis (out of plane) expansion. In multilayer boards, in plane x-axis and y-axis expansions are also important, because of the tight tolerances required, particularly between board and conductive layers. Gill and coworkers [7] have made these measurements using another TMA probe, which permits evaluation of the board while standing vertically. The CTE is a quantitative assessment of the expansion of a material over a temperature interval. In order to manufacture products with multiple components, it is essential to take care of the CTE, because if the CTE values are not identical, the build up of thermal stresses will cause malfunctioning of the board and there may be leak between the board and the electrical components. For example, better product lifetimes of electronic flip-chip packaging can be obtained on solder joints by ensuring that the CTE values of solder and the epoxy composite of PCB are identical. The high sensitivity of the TMA technique allows it to detect weak transitions that may not be observed by DSC. An example is the characterisation of brake linings, which are highly filled and crosslinked. The TMA expansion [8] results shown in Figure 5.7 demonstrate that TMA in expansion mode can detect the Tg of brake linings.
Figure 5.6 TMA measurement of out of plane expansion of printed circuit board [6]
192
Characterisation of Rubbers, Polymers and their Composites Using TMA The TMA penetration probe [8] provides another means of assessing T g. When performing measurements with a penetration probe, loading is added to the probe so that it moves down through the material as it softens. The penetration probe is useful for measuring Tg of coatings on a substrate. Figure 5.8 shows the results of TMA penetration generated on a wire sample with two coatings. The wire is used to produce electrical motor coils. The inner coating prevents electrical contact between adjacent wires and the outer coating is used to bond the coil. The TMA penetration results show that the outer coating has its Tg at 128 °C and for the inner coating, the Tg occurs at 176 °C. The decomposition of the resin is observed at 260 °C.
Figure 5.7 TMA expansion results of brake linings [8]. Curve A shows Tg at 85 °C and curve B shows Tg at 93 °C
Figure 5.8 TMA penetration probe results on electrical coil wire [8]
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Thermal Analysis of Rubbers and Rubbery Materials TMA is useful in the identification of Tg of rubbers like polychloroprene (Figure 5.9), where it is evaluated over a temperature range from –100 °C to +20 °C, at a heating rate of 5 °C/min. For both indentation and tension, a 50 g load is used. These expansion and penetration data enable CTE and Tg to be evaluated. The derivative plot [that is, derivative thermomechanical analysis (DTMA)] is a sensitive means to aid in detecting minor transitions and relaxations. Below the Tg, coefficients of expansion agree for both the loaded and the unloaded probes. Above Tg, indentation occurs, the extent of which is varying with hardness of the compound. DTMA and tension measurements appear to be sensitive techniques for evaluating polymer blends since several ‘transitions’ are detected reproducibly for a large number of rubber vulcanisates [9] as shown in Table 5.1.
Figure 5.9 TMA free expansion, indentation and tension thermograms of polychloroprene vulcanisate; 50 °C/min and 50 g load in indentation and tension mode. l versus temperature and ( l)/ t versus temperature [9]
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Characterisation of Rubbers, Polymers and their Composites Using TMA
Table 5.1 TMA of rubber vulcanisates [9] -1 4
(°C ) x 10
Vulcanisate Polyacrylate NBR (medium acrylonitrile content)
Neoprene WRT NBR (low acrylonitrile content) BR/SBR/NR
±0.03 below Tg ±0.1 above Tg 1) 0.54 (-120 to -40) 2.30 (+80) 1. 0.45 (-80) 1.80 (+40) 2. 0.51 (-80) 1.80 (+40) 0.49 (-60) 1.69 (+40) 0.45 (-120 to -60) 1.47 (+40) 0.46 (-110) 1.57 (+20)
Expansion (°C) c Tg Tgb
Tension (°C) Tgd
Tgo
-8
-16
0
-11
-18 -22
-24 -30
-15 -18
-21 -26
-33
-40
-27.5
-36
-32
-40
-28
-36
-74
-89
-58 -70 -38 -5 BR/SBR 0.60 (-110) -78 -70±5 -57 -70 1.53 (+40) -27 -6 c Tg , temperature of DTMA peak at maximum rate of indentation or tensional strain Tgb, glass to rubber transition temperature calculated via conventional extrapolation procedures Tgd, glass to rubber transition temperature in tension probe Tgo, glass to rubber transition in indentation probe
The possibility of using TMA for evaluating the low temperature properties of vulcanisates has been explored by Brazier and Nickel [9]. They have found a linear relationship between Gehmann rigidity modulus and Tg0 and Tgd. Several Tgd values are observed in blends (Table 5.1). Tg0 and Tgd are explained as footnotes in Table 5.1. Dallas [10] found that TMA is more sensitive than DSC for detecting the Tg of the cured laminates. He observes that in TMA curve, Tg is seen as a sharp increase in the expansion coefficient at 162.5 °C whereas delamination is clearly seen at 356.6 °C.
5.3.2 Effect of Plasticiser on Tg Analysis of Tg of a compound is sometimes a useful means of establishing plasticiser effectiveness and concentration. In such cases, TMA offers a rapid means for such evaluations. It has been noted by various researchers [9, 11] that in some cases, the 195
Thermal Analysis of Rubbers and Rubbery Materials addition of plasticisers and process oils may have no effect on the Tg of the compound. The influence of plasticising resins on acrylonitrile-butadiene rubber (NBR) vulcanisates has been evaluated with TMA by Brazier and Nickel [9]. Plasticiser reduces the hardness of the vulcanisate. By indentation technique, the hardness can be measured indirectly. Effect of plasticisation by using different resins like 10 phr coumarone indene resin, 10 phr hydrocarbon resin, 10 phr hydroxyl resin on NBR vulcanisate (formulation: Krynac 363 40 SP, 100; MT black, 85; FEF black, 15, Betanox Special, 2; antioxidant MB, 2; Dicup 40C, 3.5) is shown in Figure 5.10. The indentation by TMA is found to be the maximum for hydroxyl resin, where the Shore hardness is found to be the minimum. It is evident from the thermogram that the Tg of the rubber is not affected by the presence of plasticisers (-22 °C). However, there is a change in hardness with the incorporation of plasticisers. Different plasticisers give different hardness values for the vulcanisate.
Figure 5.10 Effect of plasticiser on nitrile rubber [9]. 1: NBR vulcanisate without plasticiser; 2: vulcanisate with coumarone indene resin; 3: vulcanisate with hydrocarbon resin and 4: vulcanisate with hydroxyl resin. Numbers 56, 59, 60 and 68 represent Shore A hardness of the vulcanisates
5.3.3 Creep and Stress Relaxation Creep and stress relaxation are two important properties of polymers. Creep is a measurement of the increase of strain with time under constant force, whereas stress relaxation is the measurement of change of stress with time under constant strain. Set is the measurement of recovery after the removal of applied stress or strain. TMA of polymers involves measurement of the response of the material to a constant load or 196
Characterisation of Rubbers, Polymers and their Composites Using TMA deformation while the temperature is changed. The simplest technique is the thermally stimulated creep (TSC), which means measurement of extensional, compressional or torsional deformation under constant load by linearly increasing the temperature. Besides TSC and thermally stimulated recovery (TSR), there is another way of performing thermomechanical experiments in polymers [13]. This involves measurement of the force (stress) at constant deformation (strain) as a function of the temperature and this technique is referred to as thermally stimulated stress relaxation (TSSR). Maurer [2] has applied classical creep experiment at constant stress to a sample and subsequently monitored the time-dependent strain over long periods. On application of the load, the resultant creep curve exhibits instantaneous elastic response, delayed elastic response and finally viscous flow. An illustration of TMA creep analysis of an IIR compound is presented in Figure 5.11, which shows the elastic recovery of a compound based on IIR as determined in the TMA creep recovery analysis. Thus it appears that TMA offers a rapid means for qualitative comparison of viscoelastic characteristics of polymers.
Figure 5.11 TMA creep analysis of IIR compound [2]
5.3.4 Use of TMA - Parallel Plate Rheometer (PPR) for Curing of Thermoset Polymers TMA appears to have high potential in polymer rheology, in which it uses its accessory parallel plate rheometer (PPR) for measuring ‘gel time’. Gelation is a characteristic of thermosets. From a processing standpoint, gelation is critical since the polymer does not flow and is no longer processable beyond this point. Gelation and vitrification are the two common terms in the curing of thermosets. The formation of a gel is attributed to a mesh-like structure of dispersed phase or collolid with dispersion medium, whereas in vitrification, solution turns into glass by fusion and heat. Gelation does not inhibit the curing process i.e., the reaction rate remains unchanged whereas vitrification brings an abrupt halt to curing. Gelation depends on functionality, reactivity and stoichiometry of the reactants. Beyond the gel point, the reaction proceeds to form infinite network with substantial increase in crosslink density and 197
Thermal Analysis of Rubbers and Rubbery Materials Tg, whereas vitrification is a reversible transition and can be resumed by heating (in which devitrification takes place). Vitrification causes a shift from chemical control to diffusional control. In the generalised cure programme of a thermoset, gelation precedes vitrification and forms a crosslinked rubbery network. Gelation and vitrification are very important for adhesive preparation and gel point determination is only possible with torsional braid analysis (TBA). Both gelation and vitrification are manifested as mechanical damping peaks accompanying large changes in rigidity or modulus [13]. TMA-PPR is complementary to TBA and dynamic mechanical analysis (DMA) as it is the most sensitive to rheological changes occurring at and below the gel point. Thus TMA, when modified to a microparallel plate rheometer can be used to generate viscosity data during early stages of curing. Blaine and Lofthouse [14] have used this method to investigate curing of several polyaromatic thermosetting resins as shown in Figure 5.12. Figure 5.12a shows the change in viscosity with temperature obtained from TMA data. The increase in viscosity around 80 °C may be due to volume increase on melting, which can be confirmed by DSC measurement. It is evident from Figure 5.12b that curing takes place faster as the temperature changes from 100 °C to 115 °C. Another potential application of TMA to the cure of thermosets involves the use of a volume dilatometer to follow volume changes during cure. A volume measurement is important when characterising thermosets, as reported by Bailey [15]. TBA, DMA and TMA are complementary to DSC and TGA in the study of thermosets. DSC and TGA directly measure the extent and rate of chemical conversion, are insensitive
Figure 5.12 (a) and (b) TMA-PPR used for measuring viscosity of polyaromatic thermosetting resin [14]. In (a), A indicates melting and B, initiation of cure.
198
Characterisation of Rubbers, Polymers and their Composites Using TMA to gelation and become less sensitive beyond the gel point as the reaction rate diminishes. Mechanical spectroscopy (DMA and TBA) is little affected prior to the gel point, but at the gel point, the resin starts to become rigid because of the formation of networks and subsequent changes in rigidity as a consequence of curing are detected. These methods are not able to measure the degree of conversion directly, but when coupled with a method (such as DSC), they are able to monitor the chemical conversion reactions in the rubbery state between the gel point and either the vitrification or the end of reaction. TMA-PPR is complementary to TBA and DMA, as it is the most sensitive technique to rheological changes occurring at and below the gel point. Dienes and Klemm [16] have established a basis for evaluating the melt viscosities of various polymeric systems by the analysis of the flow characteristics of polymer samples, contained between parallel plates, under the application of an applied load. Figure 5.13 shows the relationship between the weight average molecular weight (Mw) of a series of IIR fractions and the flow characteristics of small samples of these materials in a simple ‘microscale’ TMA-PPR [2]. Applications of TMA for the analysis of elastomer systems are discussed by many authors [17-20] for the evaluation of the state of cure or crosslinking in a vulcanisate. The detection and monitoring of cure state of finished rubber parts by TMA (penetrometry and expansion) have also been reported in the previous references. TMA penetration testing and flexure probe are very often used to find out the relative state of cure of a polyester/polyamide coating of motor winding wire.
5.3.5 Evaluation of Crosslink Density by TMA Evaluation of crosslink density by equilibrium solvent swelling is an established technique based on the well known Flory-Rehner [21] equation. The conventional solvent swelling procedure is lengthy and tedious. A swollen sample is removed from
Figure 5.13 Flow characteristics of IIR fractions [2]. Flow region; A 30 to 60 min and B, 10 to 40 min
199
Thermal Analysis of Rubbers and Rubbery Materials the solvent, placed between pieces of filter paper and weighed in a weighing bottle. It is then removed from the bottle, which is weighed again. Swollen sample weight is obtained by difference. However, the swollen sample weight may change over long periods (days, weeks, months), necessitating extrapolation procedures in order to determine ‘zero time’ equilibrium swell value. Prime [20] has developed a micro-technique for rapid measurement of solvent swell of crosslinked polymer. The key feature of this technique is the elimination of solvent evaporation from the sample, thus enabling the use of thin samples which equilibrate rapidly. Thin samples (0.1-0.4 mm thick) of silica filled dimethyl siloxane elastomers were swollen in hexane and toluene. The swollen sample suspended from a Cahn balance was weighed in an atmosphere saturated with the swelling solvent. These conditions enable both rapid equilibrium (Figure 5.14) and precise measurement of swell ratio. For the samples studied, precision was ±1-2% for swell ratio and ±10-12% for sol fraction. Accuracy of the technique was established by comparison with a conventional method for samples ranging in crosslink density from 7 to 35 x 10-5 moles/cm3 and having 1 to 4% sol fraction. The correlation between Young’s modulus and swell ratio thus established by Prime [20] demonstrates an unambiguous analysis for monitoring crosslink density (shown in Figure 5.15). Based on the speed and precision of the technique, Prime foresees wide applicability in research, quality control, ageing and degradation of crosslinked polymers. The crosslinking reaction in a two component methyl silicone rubber (Sylgard-186) has
Figure 5.14 Equilibrium swelling of poly dimethyl siloxane versus soak time [20]. Sample thickness and solvent type are as follows: A = 0.19 mm in hexane; B = 0.19 mm in toluene and C = 0.39 mm in hexane
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Characterisation of Rubbers, Polymers and their Composites Using TMA
Figure 5.15 Correlation of swell ratio by TMA with Young’s Modulus [20]. A: in hexane and B: in toluene
been studied by using DSC and TMA. TMA is run at room temperature, on the same samples used for finding heat of reaction using DSC. Elastic modulus (Em) is calculated from the penetrometer measurements via an equation developed by Gent [22]: F 9 E m 1/2 1/2 p 16r
(5.1)
where Em is the elastic modulus, F the load, p, the penetration and r, the probe radius. The crosslink density (e) measurements are evaluated by conventional solvent swelling procedures and also from TMA elastic modulus Em using the equation: e
Em 3RT
(5.2)
where T = 239 K, R (universal gas constant) = 8.314 x 107 ergs/mol/K. Heat of reaction is calculated from the total area under the DSC curve and the energy of activation of the curing process is determined by the Arrhenius equation. Heat of reaction, energy of activation and crosslink density are evaluated in terms of prepolymer concentration, dilution and swelling. Barrall and Flandera [17] have observed that the crosslink density of silicone rubber obtained by using TMA does not changing linearly with the weight fraction of rubber 201
Thermal Analysis of Rubbers and Rubbery Materials as shown in Figure 5.16. It has been assumed to this point that effective crosslinks are due only to chemical bonds, whose formation is detectable thermally. Assuming simple volume additivity, a linear relationship between crosslink density and dilution ratio would be expected but practically it is not true, as some of the crosslinks in the undiluted sample are due to entangled but not overlapping chains. Upon dilution with solvent, these chains disentangle, thereby reducing the number of effective crosslinks. This may be the reason for the non-linear relationship in the plot.
Figure 5.16 Variation of crosslink density of silicone rubber with weight fraction of rubber during reaction [17]. A represents extrapolated crosslink density due only to network bond
202
Characterisation of Rubbers, Polymers and their Composites Using TMA
5.3.6 TMA for Fibre Analysis TMA is widely used in the analysis of fibres. It measures: a. coefficient of linear thermal expansion (CTE), b. thermal shrinkage (ST), c. shrinkage force (SF - usually reported in units of stress, i.e., cN/dtex), d. Tg and melting temperature (Tm) of fibre, e. kinetics of shrinkage and shrinkage force phenomena. The key variables in TMA experimentation are time/temperature, atmosphere and applied load. Addyman and Ogilvie [23] have discussed the use of TMA in the solution of complex problems in fibre processing. Fibre tension spectrometry, an attachment of TMA, is used to measure shrinkage tension of fibre as a function of temperature. Experimentally, the largest problem encountered in TMA is to obtain accurately and reproducibly the desired level of fibre loading, especially zero load. Due to shrinkage, decitex value will change, so a standard 100-250 decitex is taken as a safe range. Figure 5.17 shows a typical shrinkage thermogram at zero load and first derivative of the shrinkage ds/dT for a drawn partially crystalline fibre. Starting at room temperature, four regions of length change behaviour are observed (Figure 5.17). The mechanistic interpretation of the fibre shrinkage is shown in Figure 5.18. The four zones of fibre process history areas are explained next: 1. From room temperature up to Tg, the sample undergoes reversible thermal expansion; a small amount of irreversible shrinkage may occur due to elimination of solvent or moisture. The value of ds/dT is constant (Figure 5.17). 2. At Tg, the fibre undergoes a rapid irreversible shrinkage process due to the relaxation of oriented amorphous chains not bound in crystalline regions. The corresponding peak observed in ds/dT curve is due to the temperature of the maximum rate of this relaxation (Figure 5.17). 3. In the intermediate temperature region between Tg and Tm, shrinkage takes place due to reorganisation, chain folding, recrystallisation and general perfecting of the fibre structure (Figure 5.18). Shrinkage is dependent on process history and molecular chain relaxation process. Therefore, ds/dT curves change accordingly. 4. The rapid shrinkage prior to sample failure is a consequence of melting (Figure 5.18), i.e., the molecules are pulling out of crystalline units and disorienting, as are the crystalline units themselves. The start of this rapid shrinkage is about equivalent to the start of melting in DTA/DSC. The ds/dT peak reflects that temperature at 203
Thermal Analysis of Rubbers and Rubbery Materials which the sample is sufficiently molten that it is unable to support its own weight. The high temperature ds/dT peak is used to measure Tm (Figure 5.17). Hence each region reflects an aspect of the fibre process history and structure, which can differ widely from sample to sample.
Figure 5.17 Shrinkage and differential shrinkage of a partially crystalline fibre [24]
Figure 5.18 Structural changes occurring within a fibre in four length change regions [24]
204
Characterisation of Rubbers, Polymers and their Composites Using TMA Berndt and Heidemann [25] have studied the reversibility of Nylon 66 and polyethylene terephthalate (PET) yarn shrinkage as shown in Figure 5.19. After heating to a temperature T>Tg, the shrinkage up to that temperature is eliminated on reheating the cooled sample, the CTE of the relaxed polymer is observed up to the temperature T, above which the shrinkage recommences. Hence only length changes associated with inherent thermal expansion are reversible; all shrinkage processes are irreversible. It is possible, however, to perform heating and cooling experiments at rates faster than the disorientation rate of fibre structure, resulting in some shrinkage (or shrinkage force) still being observed during a second heating. Kimmel [26] has extensively studied the shrinkage behaviour of acrylic yarns. Finding the shape and magnitude of the shrinkage thermograms strongly reflects processing conditions. Jaffe [27] has interpreted the parameters for the first shrinkage temperature to imply melting in other semicrystalline fibres that undergo chemical transformations at elevated temperature such as poly(p-phenylene terephthalamide), i.e., Kevlar. It is possible that for the chemical transformation to proceed in these types of semicrystalline polymers, the molecular mobility introduced by melting is a prerequisite. In fibre identification, shrinkage or shrinkage force measurement is very important. This shrinkage force depends upon the rate of spinning. In Figure 5.20, polyester fibre is spun at three different rates (2700, 3500 and 3700 m/min) and the samples are heated
Figure 5.19 Length changes in Nylon 66 and PET yarn shrinkage during cycled heating and cooling [24]. A: Nylon 66 multifilament yarn and B: PET multifilament yarn
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Thermal Analysis of Rubbers and Rubbery Materials
Figure 5.20 Shrinkage force of polyester (spun) as a function of take up speed [24]. A: 2700 m/min; B: 3500 m/min and C: 3700 m/min
by a Kanebo thermal tester at a heating rate of 3 °C/min [28, 29]. The low temperature peak is due to the molecular orientation and yarn age and high temperature peak is due to the increasing yarn crystallinity. In commercial fibre characterisation, processing behaviour is very important. The exact position of the peak depends on annealing time, temperature, stress, and media as well as spinning history. It has been shown that the temperature rate and extent of deformation experienced by a yarn will define the dimensional and structural stability of the resistant yarn as monitored by TMA directly.
5.3.7 Why Fibre Properties are Important Properties of fibre in a composite are as important as its thermal shrinkage determines the end use potential of a given composite. For example, for the performance of reinforcing cords in rolling tyres, V-belts and so on, where a few properties like CTE, process history, structural state of the fibre as well as that of the elastomer should match each other. TMA is largely used to determine the coefficient of linear expansion ( ), which holds a universal relationship between cubical () expansion of all materials according to the following equation: x y z
206
(5.3)
Characterisation of Rubbers, Polymers and their Composites Using TMA This equation is valid for a fibre-reinforced composite i.e., a fibre and particle filled thermoset [30]. In Equation 5.3: - coefficient of cubic thermal expansion
- coefficient of linear thermal expansion x and y - thermal expansion parallel to the fibre direction z - thermal expansion perpendicular to the fibre direction. Linear thermal expansion of thermosets filled with a uniform distribution of spherical particles is isotropic. x y z
3
(5.4)
For a unidirectional fibre-reinforced composite, thermal expansion parallel to the fibres is given by 11 and that perpendicular to the fibres is given by 22. For this case, the relationship between volumetric and linear expansions is: 11 222
(5.5)
For a two-dimensional fibre reinforced isotropic composites, the in plane linear expansion is given by:
E E22 11 22 1 ISO X Y 11 22 11 E11 1 v12 E22 2 5v E f m m m v f Ef
(5.6)
where: f - filler m - matrix
- coefficient of linear expansion E - Young’s modulus (tensile modulus) v - volume fraction 11 - parallel to fibre direction in unidirectional layer 22 - normal to fibre direction in unidirectional layer The exact equation (Equation 5.6) is from Ashton and co-workers [31]. For a pseudoisotropic composite containing 50 volume % E glass in epoxy, Equation 5.6 yields an exact value of iso = 16.4 x 10-6 °C-1 which is almost identical to copper i.e., 20 x 10-6 °C-1 207
Thermal Analysis of Rubbers and Rubbery Materials
5.3.8 TMA for the Analysis of Composites Short fibre reinforced polymer composites having a fibre aspect ratio less than 250, iso will increase in a predictable manner [32]. Linear thermal expansion perpendicular to the fibre plane is simple. Prime and co-workers [33] have studied the thermal expansion of composites below the Tg. They found that for accurate measurement of thermal expansion below the Tg, the specimen should be dry as moisture in the specimen may affect CTE measurements. The CTE of the wet specimen was lower than that of the dry specimen. Reed [34] has observed, by using TMA and DMA, a significant influence of silane-finished glass fibre reinforcement on properties of unidirectional composites. Expansion and shrinkage are the two properties, which should be considered for the studies of rubber-fibre composites. Fogiel and co-workers [35] have developed a fundamental definition of the factors that control the three dimensional shrinkage of fluoroelastomers (FKM) with a DuPont 941 TMA. They made a special calibration so that reliable expansion and shrinkage measurements could be made. Vulcanised elastomer standards of known expansion coefficients are required for such studies because metal standards, due to low expansion coefficient, give TMA displacements too small for reliable calibration over the desired temperature range of 25-200 °C. The volumetric expansion coefficients of these standards obtained by dilatometry, were used to develop an equation: a b(t 25)
(5.7)
that relates the coefficient of linear expansion ( ) to temperature (t); a and b are calibration constants. This equation is used to calibrate the TMA with vulcanisate standards described previously. Three expansion coefficients are considered to be equilibrium values based on the following criteria: a. the same is obtained for repeat runs on a given sample including the use of probe load and heating rate variations, b. is not affected by repeated heating over a two week period, and c. both stepwise and constant rate heating give the same results. Hence in tyres, vulcanised elastomer standards of known expansion coefficients are required because metal standards due to low expansion coefficients will mismatch. Thus TMA is used to evaluate average shrinkage and anisotropy in three dimensions. Based on theoretical consideration, the shrinkage S of press-cured materials averaged over three dimensions, is given by: S c m t 25
208
1 t 25 c
(5.8)
Characterisation of Rubbers, Polymers and their Composites Using TMA where c and m refer to similarly averaged for the cured compound and mould, respectively, at the press cure temperature, t. In general good agreement with this relationship was observed, results being better for the sample cured at 177 °C than that cured at 150 °C. Fogiel and co-workers [35] predicted the shrinkage of fluoroelastomer (FKM) vulcanisate filled with medium thermal carbon black using equations (5.8) and (5.9) as given next:
c v r r 1 v r f
(5.9)
where r and f are linear isotropic for the rubber and the filler, respectively. vr is the volume fraction of rubber. Calculated values for c are found to be in good agreement with experimental values as shown in Figure 5.21, in which it is observed that only 1% deviation takes place between the expected and the experimental values.
Figure 5.21 Predictability of shrinkage from of unfilled FKM vulcanisate and volume fraction of filler [35]. ( ______ ) calculated from equations; (oooooo) experimental
209
Thermal Analysis of Rubbers and Rubbery Materials Hence it is concluded that: a. TMA properly calibrated is eminently suitable for the measurement of the thermal coefficient of rubbers, and, b. the relative simplicity and precision of TMA method for the determination of the coefficient of isotropic expansion and hence of the volumetric expansion of rubbers make this important thermodynamic parameter easily accessible experimentally.
5.4 Use of TMA in Industry
5.4.1 TMA in the Electronics Industry TMA is largely used in the electronics industry. Electronic materials are composed of circuit boards, low density packaging foams, encapsulants and wire and cables used for telephone. Thermal analysis, through its diversified techniques, can provide useful information on all these materials. Thus TMA measures: 1. softening points, 2. curing, 3. Tg, 4. thermal expansion, 5. moisture content, 6. viscosity at low shear rate. These are applied in the following fields: a. optical fibre testing, b. printed circuit board (PCB), c. thermal fatigue of soldered joints, d. effect of temperature on adhesion, e. design flow and bleed.
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Characterisation of Rubbers, Polymers and their Composites Using TMA PCB consist of paper web or glass fibre matting saturated with thermosetting resin (usually phenolic or epoxy resin). TMA is used to measure the thermal expansion and contraction of cured boards (Figure 5.6). Board dimensional changes are important because they can cause stresses in the board, which in turn can lead to failure of solder joints and components. In multilayer boards, expansions in the X and Y axes are important because of tight connections; enough gap is required particularly between the board and the conductive layers. Figure 5.22 illustrates the use of a TMA-PPR technique in the analysis of prepreg gel time and flow behaviour [36]. TMA results depict dimensional change as a function of temperature as the system is heated through the processing temperature regime. Flow commences at the softening point of the prepreg (Tg) and continues until gelation or crosslinking occurs. Data analysis permits direct determination of onset, inflection and completion temperatures and total displacement. From these tangential extrapolations, gel time is determined by the time difference between the onset of flow and gel and resin flow is determined by comparing the total displacement with the initial sample thickness. The gel time observed by this method is 102 seconds. The instrument used is a DuPont 943 TMA and the rate of heating is 10 °C/min. Usamani and Slayer [37] and Ritchie and co-workers [38] have used TMA for the Tg measurement of electronic encapsulants. The encapsulants are generally made with
Figure 5.22 Gelation and cure of epoxy-glass prepeg by TMA-PPR [36]. A represents softening process and B represents gelation process
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Thermal Analysis of Rubbers and Rubbery Materials epoxys, polyurethanes, polyesters, PVC, phenolics, polysulfones or silicones. Semicured epoxy powders and phenolic resins are specially suitable for transfer moulding process. Polyurethanes and modified polyester rigid foams, in addition to solid polymer encapsulants, are suitable for embedding electronic components. In general, epoxy powders are mixed with inorganic powders to reduce the coefficient of thermal expansion, in addition to reducing the cost and increasing the stiffness. The coefficient of thermal expansion must be known in order to match the encapsulants’ expansion/contraction with that of the electronic component and its leads. Otherwise, stress might create an intermargin gap between the encapsulant and the lead, which would be an easy pathway for moisture penetration. TMA is an ideal instrument for monitoring thermal expansion and contraction (as shown in Figures 5.6 and 5.7). The value of these coefficients is dependent on the polymer, the amount of preparation for processing, the filler content, and the method of fabrication, hence TMA is used for both research and quality control.
5.4.2 TMA in the Automotive Industry Like the electronic industry, TMA is used in the automotive industry as well. Tyres are a composite material consisting of rubber, fibre and metal. Hence, along with other thermal analysis methods such as DSC, TGA and DMA, TMA used for the measurement of: i)
expansion of rubber (i.e., coefficient of thermal expansion), metal and composite,
ii) hardness by indentation method, iii) shrinkage of fibre used in rubber, iv) mould shrinkage, v) modulus of rubber blends used in tread and sidewall. Besides tyres, TMA is also important for other automotive parts, which are made from polymers, thermoplastic elastomers or thermosets. As TMA measures changes in dimension of materials (e.g., fibres, films, polymers, metals) as a function of temperature, it is possible to obtain expansion coefficient values, Tg and softening temperatures from which correlations can be made to obtain several parameters such as: a. degree of cure of thermosetting resins [39], b. ASTM modulus measurements, c. brake lining evaluations [40], d. gasket quality [41].
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Characterisation of Rubbers, Polymers and their Composites Using TMA Tg determined by TMA can relate directly to the degree of cure of resin as described earlier in this chapter. In the white sidewall of a tyre a blend of natural rubber (NR) and polychloroprene rubber is used. TMA is used to measure the CTE and Tg of rubbers so that the values can be compared with that of the mould (as calculated earlier in Equation 5.8). In the cable industry TMA is used to measure CTE, viscosity, material softening, material flow and Tg. Dimension changes, even in the order of micrometers, are often critical to an elastomer’s ability to perform in close tolerance situations. In a TMA experiment, depending on the stiffness of the material and the load used, positive dimensional change (expansion) or negative dimensional change (penetration) occurs as the material is heated. Laird and Liolios [42] have used TMA to differentiate between the dimension changes of uncured and fully cured samples of chlorosulfonated polyethylene. They have also used TMA curves as an indicator of degree of cure of such rubbers. Their TMA profiles show the ability of the technique to provide data on CTE. This is valuable information for use in product design and applications development and can provide quality control standards. TMA can also evaluate sponge materials for blowing agent activation and sponge formulation, as suggested by Laird and Liolios [42]. In a typical experiment, they have compared three different formulations based on ethylene-propylene diene terpolymer rubber: i)
the masterbatch alone,
ii) the masterbatch with curatives, iii) the complete formulation with blowing agents. In such TMA curves, the initial activation temperature and the rate of expansion provide information that can be used to screen blowing agents and cure systems. Through experimentation, the optimum TMA heating rate for studying sponge formulation is found to be 50 °C/min, which gives the most accurate correlation with total expansion for the compound when cured in liquid curing medium (oil).
5.5 Conclusion The application of TMA techniques with various attachments is used by different industries and laboratories. The wide range of applications, as well as the ease of operation and quick turn around have resulted in an increase in the number of TA analysis including TMA in rubber, polymer and fibre industries. Though the equipment is accurate, careful attention is still required for test procedures and calibration should be done two or three times to get accurate and reproducible results. 213
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5.6 Acknowledgments One of the authors, R.S. Rajeev, would like to thank Dr. Yutaka Sato of Japan Aerospace Exploration Agency (JAXA), Tokyo, for permitting him to prepare the manuscript using the facilities at JAXA along with the assigned research activities.
References 1. G. Lombardi, For Better Thermal Analysis, ICTA, Rome, Italy, 1980, p.18. 2. J.J. Maurer in Thermal Methods in Polymer Analysis, Ed., S.W. Shalaby, Franklin Institute Press, Philadelphia, PA, USA, 1978, p.129. 3. J.J. Maurer in Thermal Characterisation of Polymeric Materials, 1st Edition, Ed., E.A. Turi, Academic Press, New York, NY, USA, 1981, p.571. 4. A. Noshay and J.E. McGrath, Block Copolymers: Overview and Critical Survey, Academic Press, New York, NY, USA, 1977. 5. A. Schwartz, Journal of Thermal Analysis, 1978, 13, 3, 489. 6. J.V. Wood, Thermal Analysis of Electronic Materials, DuPont (UK) Ltd., Stevenage, UK. 7. P.S. Gill, P.G. Fair and J.N. Leckenby, Multilayer Characterisation by Thermal Analysis, DuPont Instruments Publication E-60856, 1983. 8. W.J. Sichina, Characterisation of Polymers by TMA, Perkin-Elmer Instruments, 2000, Perkin Elmer Inc., Norwalk, CT, USA. 9. D.W. Brazier and G.H. Nickel, Thermochimica Acta, 1978, 26, 1-3, 399. 10. G. Dallas, DuPont Company, Instrument Systems, Wilmington, DE, USA. 11. J.J. Maurer, Rubber Chemistry and Technology, 1967, 40, 5, 1592. 12. P. Hedvig in Applied Polymer Analysis and Characterisation - Recent Developments in Techniques, Instrumentation and Problem Solving, Ed., J. Mitchell, Jr., Hanser Publishers, Munich, Germany, 1987, Chapter II-L. 13. R.B. Prime in Thermal Characterisation of Polymeric Materials, 1st Edition, Ed., E.A. Turi, Academic Press, New York, NY, USA, 1981, p.435. 14. R.L. Blaine and M.G. Lofthouse, Dupont Application Brief, TA-65. 214
Characterisation of Rubbers, Polymers and their Composites Using TMA 15. W.J. Bailey, Journal of Macromolecular Science, Part A: Chemistry, 1975, 9, 5, 849. 16. G.J. Dienes and H.F. Klemm, Journal of Applied Physics, 1946, 17, 6, 458. 17. E.M. Barrall, II, M.A. Flandera and J.A. Logan, Thermochimica Acta, 1973, 5, 4, 415. 18. P.M. James, E.M. Barrall, II, B. Dawson and J.A. Logan, Journal of Macromolecular Science, Chemistry, 1974, A8, 1, 135. 19. E.M. Barrall, II and M.A. Flandera, Journal of Elastomers and Plastics, 1974, 6, 1, 16. 20. R.B. Prime, Thermochimica Acta, 1978, 26, 1-3, 165. 21. P.J. Flory and J. Rehner, Jr., Journal of Chemical Physics, 1943, 11, 11, 521. 22. A.N. Gent, Transactions of the Institution of the Rubber Industry, 1958, 34, 1, 46. 23. L. Addyman and G.D. Ogilvie, British Polymer Journal, 1979, 11, 3, 151. 24. M. Jaffe in Thermal Characterisation of Polymeric Materials, Ed., E.A. Turi, Academic Press, New York, NY, USA, 1981, p.709. 25. H.J. Berndt and G. Heidemann, Melliand Textilberichte, 1977, 1, 83. 26. R.M. Kimmel, Fibre Society Lecture, 1971. 27. M. Jaffe, Thermal Analysis Symposium, American Physical Society, APS Preprint, 1977, p.1. 28. D.R. Buchanan and G.I. Hardegree, Journal of Textile Research, 1977, 47, 732. 29. S.C. Aleksandriiskii, E.M. Aisenshtein and B.V. Petukhov, Mekhanika Polimerov 1968, 4, 2, 369. 30. L. Holliday and J. Robinson, Journal of Materials Science, 1973, 8, 301. 31. J.E. Ashton, J.C. Halpin and R.H. Petit, Primer on Composite Materials: Analysis, Technomic, Stamford, CT, USA, 1969. 32. J.C. Halpin and N.J. Pagano, Journal of Composite Materials, 1969, 3, 720. 33. R.P. Prime, E.M. Barrall II, J.A. Logah and P.J. Duke in AIP Conference Proceedings, 1974, 17, 72. 215
Thermal Analysis of Rubbers and Rubbery Materials 34. K.E. Reed, Polymer Composites, 1980, 1, 1, 44. 35. A.W. Fogiel, H.K. Frensdorff and J.D. MacLachlan, Rubber Chemistry and Technology, 1976, 49, 1, 34. 36. J.N. Leckenby, P.S. Gill and P.G. Fair, Multilayer Characterisation by Thermal Analysis, DuPont Instrument Publication, E-60856, 1983. 37. A.M. Usmani and I.O. Slayer, Journal of Materials Science, 1981, 16, 1402. 38. K. Ritchie, W. Hunter, C. Malkiewicz and C. Maze, Society of Plastics Engineers, Technical Papers, 1972, 18, 1, 114. 39. DuPont Applications Brief TA-27 (A-64904), DuPont Company, Analytical Instrument Division, Wilmington, DE, USA, 1974. 40. DuPont Applications Brief TA-22 (A-61908), DuPont Company, Analytical Instrument Division, Wilmington, DE, USA, 1974. 41. DuPont Applications Brief TA-13 (A-58776), DuPont Company, Analytical Instrument Division, Wilmington, DE, USA, 1974. 42. J.L. Laird and G. Liolios, American Laboratory, 1990, 47.
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6
Micro-thermal Analysis of Rubbery Materials Kinnari Shelat, Namita Roy Choudhury, and Naba K Dutta
6.1 Introduction Rubbers or rubber-like materials are unique polymers that can withstand large deformation and recover on relaxation. In many applications, they are used as composites, blends and coatings with different plastics or elastomers so as to provide excellent strength, flexibility and also widen the areas of application. In recent years, research and development of nano/ micro-structured rubber composites, micro-particles, nano-films or protective coating on substrates has emerged as important areas of research in polymer chemistry. In case of such multi-component systems, distribution of filler micro or nanoparticles within the rubber matrix is a prerequisite to achieve high performance of the composite. Thus, their characterisation at the micro/nano level becomes critical to understand the morphology and performance of such nano/micro-structured materials Modern thermo-analytical techniques such as temperature modulated differential scanning calorimetry are powerful tools to identify existence and quantity of the phases in the multi-component system [1]. Using this technique one can identify phases present at a level of 7% in a mixture of four polymers [2]. The information obtained from these techniques is from the bulk of the material giving overall information. To obtain information about spatial distribution the other alternative is to use microscopic techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both these techniques require sample preparation either by coating or cutting the sample into thin film, which is difficult in the case of elastomeric samples. Also conventional spectroscopic techniques such as infra red spectroscopy (IR), Raman spectroscopy and secondary ion mass spectroscopy (SIMS) are some of the other techniques operating under high vacuum, which causes problems to many specific samples containing oil and plasticisers. Therefore, there is a strong need to combine thermal analysis with microscopy. Integration of thermal analysis and microscopic imaging at the micron scale resulted in micro-thermal analysis (μTA). Introduction of micro-thermal analysis instrument by TA Instruments in 1998 has given opportunities to study different types of materials such as polymers, pharmaceuticals, thin films, coatings and metals. μTA opens new avenues to obtaining this type of information using different combinations of methods. In order to highlight state-of-the-art use of this technique and to demonstrate different types of applications of μTA, this chapter primarily focuses on rubbery material 217
Thermal Analysis of Rubbers and Rubbery Materials and is subdivided into different types of elastomeric systems ranging from composites, blends, micro-particles and thin films.
6.2 Basic Principles of μTA The origin of Micro-thermal analysis technique is the result of advancement in scanning probe microscopy (SPM) as shown in Figure 6.1. Scanning tunnelling microscopy (STM) was initially used to obtain a sub-micrometer image, followed by introduction of AFM and then μTA. μTA combines the capabilities of AFM and thermal analysis, and gives useful information about the morphology of the material. This technique combines both imaging and thermal analysis of the samples. In all these SPM techniques, an area on the sample as small as a few microns is scanned providing information of spatial distribution in the sample. The technique of μTA involves analysis using a locally (near-field) heated thermal probe and microscopic imaging. In this technique, firstly, an image of a small area of the sample is obtained by scanning and the scanned area is characterised by a thermal probe combined with different techniques of analysis. The innovation in μTA is associated with the tip design [3]. The AFM component of μTA consists of a sharp silicon nitride tip resting over the cantilever, a photodetector and a mirror. On the other hand, the set-up of μTA consists of a thermal probe, a mirror and a photodetector as shown in the Figure 6.2. Two types of images of the samples are obtained, thermal conductivity and diffusivity, by scanning the tip on the surface of the samples and monitoring deflection of the tip via a feedback loop. The thermal probe used in μTA consists of 5 μm diameter Wollaston wire, containing a platinum filament enclosed in a silver sheath. The wire is bent to certain degree and the platinum is etched to reveal the silver sheath. This probe plays a dual role, as a local heater and sensor for measuring the resistance. The mirror on the cantilever acts as a laser reflector spot forming part of the feedback loop of the microscope. The probe tip rasters over the sample and images can be obtained in terms of its topography
Figure 6.1 Evolution of SPM
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Figure 6.2 Thermal probe schematic Reproduced with permission from TA Instruments
and response to an AC and DC temperature program. The response to the DC signal gives rise to a thermal conductivity image and response to AC signal leads to thermal conductivity and thermal diffusivity of the sample. Thermal conductivity images are obtained by measuring power required to maintain the tip at a constant temperature when it rasters over the sample surface. Thermal diffusivity images are obtained by applying modulation at the isothermal temperature.
6.3 Modes of Micro-thermal Analysis The micro-thermal analysis technique has two different modes, visualisation and characterisation of the visualised sample area. Imaging of the sample and obtaining thermal conductivity and diffusivity images provide visualisation of sample morphology and spatial arrangement in complex system. Characterisation of scanned surface involves different forms of localised thermal analysis (LTA). LTA is an important and very useful feature of μTA. This feature provides the facility to perform localised calorimetric, localised thermomechanical analysis (L-TMA), localised dynamic mechanical analysis (L-DMA) and localised rheometry experiments. Lately, this function of μTA has proven to be one of the most versatile, as this can also be used in combination with other techniques such as FTIR, gas chromatography – mass spectrometry (GC–MS). One of the major advantages is that only part of the sample is destroyed preserving other parts, and the chemical and mechanical property identification is at nano/micro scale. Different researchers have shown the use of this technique combined with other techniques and presented important research in the areas of polymer films, pharmaceuticals, rubber materials and complex system containing more than one component. A detailed flow chart as shown in Figure 6.3 shows different functionalities of the microthermal analysis technique. 219
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Figure 6.3 Modes of μTA
Micro-thermomechanical analysis using μTA involves analysis of local elastic and viscoelastic properties of the samples at high resolution of few micrometers. Different modes can be used to get different information based on the type of tip used, data requirements and type of sample to be analysed. Modulation of temperature can be used in pulsed force mode, L-TMA, and L-DMA. The basic working principle in this technique is a temperature modulation applied on the probe which transfers heat to the sample on contact initiating thermal expansion producing mechanical strain in the sample [4]. Another approach used is application of force in different frequencies on the probe coming in contact with the sample and recording elastic response of the sample. Also, it is possible to obtain mechanical properties of the sample at several micrometer size with a special arrangement using AFM [5].
6.4 Micro-thermal Analysis of Rubbery Material The technique of μTA and its different modes have been used extensively by researchers to analyse a variety of systems. Price and co-workers [3, 6, 7] demonstrated use of μTA by analysing a range of materials that included pharmaceutical drugs, polymer brush layers, nano-films, polymer blends, multilayer films and coated substrates. Also, these authors [8] showed different combinations of μTA with FTIR, GC and microthermomechanical analysis. Recently, Ye and co-workers [9] used μTA to establish corelation of changes in microstructure and melting behaviour of original and zone drawn polypropylene. However, limited work is done so far in the area of rubbery materials. Utilisation of μTA to analyse complex elastomeric systems has several advantages over using conventional techniques. As rubbers are non-conductive, the most suitable 220
Micro-thermal Analysis of Rubbery Materials technique to study the morphology of a system containing rubbers is μTA. Using thermal conductivity and thermal diffusivity images, one can differentiate between the conductive and non-conductive phase in the sample which is otherwise difficult using TEM or SEM techniques. Tsukruk and co-workers [10] have used scanning thermal microscopy and scanning force microscopy to understand micro-thermomechanical and micro-mechanical properties of poorly conducting polymers to highly conducting metals and established the use of μTA for such different materials. These techniques have proven to be the most useful techniques for understanding surface/interfacial morphology, distribution of elastomeric phases and physical properties in nanocomposites. In order to demonstrate and highlight the use of this technique this chapter mainly discusses different applications, in the current context, for systems containing elastomers or rubbers. The following part of the chapter provides information about μTA technique used to analyse elastomers, thermoplastic elastomers and other polymeric systems in the form of polymer blends, micro-spheres, microparticles, nanofilm and nanocomposites including a few state-of-the-art uses of μTA to establish new directions. A systematic collection of a range of materials studied using μTA and LTA by different researchers and the usefulness of the experimental results are discussed.
6.5 Morphological Investigation in Polymer Blends Preparation of polymer blends to achieve distinct and favourable characteristics is one of the extensively used approaches in the polymer industry [11]. Addition of a small amount of a property enhancing polymer into the matrix of a commonly available polymer such as a polyolefin may result in improved mechanical, chemical and physical properties and has been reported extensively [12-15]. While the majority of the work reported on the use of μTA so far focuses on thermoplastics, herein we give an overview of its use in the field of elastic materials. Blends of elastic materials ranges from blends containing two elastomers or a combination of thermoplastics and elastic materials based on application requirements. Performance of the blends is mainly governed by their thermodynamics of mixing and their resulting morphology. Detailed investigation of morphology is one of the key factors in order to optimise processing parameters and achieve desired properties. Spectroscopic techniques such as SEM and TEM are long established techniques giving information about bulk morphology of the blends. (Etching or solvent extraction methods are used to obtain phase contrast and observed in SEM and TEM.) Use of μTA to analyse morphology of a chemically complex system is a direct means and can be very informative for example studies of two blends having very close melting temperatures or blends having components with high conductivity differences. Moreover, localised thermal analysis can provide useful information about spatial distribution and homogeneity of the blends. There are number of examples to demonstrate these capabilities of μTA, a few of them are discussed later in this section. 221
Thermal Analysis of Rubbers and Rubbery Materials In heterogeneous polymer blends such as rubber modified plastics, micro-scale morphology is a major determinant of bulk and surface properties. μTA is an excellent technique for non-destructive visualisation/characterisation of surface and sub-surface morphology in rubber modified thermoplastics. Rubber toughened plastics such as polycarbonate (PC) and ASA have been studied using μTA to investigate the variation in core morphologies that exists between the gloss and dull region of injection moulded parts [1]. The blends were prepared to impart or enhance toughness of the glassy polymer. Topography and conductivity images of such samples are shown in the Figure 6.4. It is clear from the images
Figure 6.4 Topography and conductivity images of rubber toughened blends of polycarbonate and ASA Reproduced with permission from S.A. Edwards, M. Provatas, M.Ginic-Markovic and N. Roy Choudhury, Polymer, 2003, 44, 3661. ©2003, Elsevier
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Micro-thermal Analysis of Rubbery Materials that μTA can extensively differentiate circular shaped less conductive rubber particles dispersed in a polymer matrix of high conductivity. The LTA experiments performed on the image by selecting nine different locations show the presence of SAN and PC and their glass transition (Tg) between 100 -124 °C and 139 -159 °C, respectively. The studies of the same blends at the interfaces between the two components show the coexistence of the phase which explains the ranges of Tg as measured by LTA (Figure 6.5). The authors also demonstrated the use of μTA in relation to the processing parameter and structural development effects with the surface finish of the PC-ASA blends [1, 16, 17]. Effects of processing parameters such as temperature and pressure on the morphology can be studied using this technique in conjunction with other techniques such as rheometry. As shown in Figures 6.6 and 6.7, it is observed that increase in injection pack pressure changes the orientation of the molecules in the blend system. Price and co-workers [7] observed miscible and immiscible phases in blends of PC with polymethyl methacrylate (PMMA). As shown in Figure 6.8a, there are two separate phases while 6.8b shows the existence of one phase. Also, LTA performed on these images as shown in Figure 6.9, detects three distinct softening points, two for the single phase materials and one for the miscible phase, and as expected the softening point of miscible phase lies between two distinct phases. Song and co-workers [18] investigated a unique elastomer blend system using μTA. The topographic and thermal conductivity images of the blends of natural rubber (NR)
Figure 6.5 LTA of rubber toughened blends of polycarbonate and ASA Reproduced with permission from S.A. Edwards, M. Provatas, M.Ginic-Markovic and N. Roy Choudhury, Polymer, 2003, 44, 3661. ©2003, Elsevier
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Figure 6.6 Thermal diffusivity image of the dull area of injection moulded blends of PC/ ASA showing that a change in pack pressure changes the morphology Reproduced with permission from M. Provatas, S.A. Edwards and N. Roy Choudhury, Thermochimica Acta, 2002, 392-393, 339. ©2002, Elsevier Science
Figure 6.7 Thermal diffusivity image of the dull area of injection moulded blends of PC/ ASA showing that a change in pack pressure changes the morphology
and nitrile rubber (NBR) were mapped. Figure 6.10 shows the thermal conductivity image with excellent contrast between two phases, while SEM fails to provide such information even after staining. This simplicity makes μTA very powerful for studies of different rubber blends. The phase size and distribution can be calculated precisely from such direct measurement. 224
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Figure 6.8 Thermal conductivity images of two samples of polyethylmethacrylate/PC blends Reproduced with permission from M. Provatas, S.A. Edwards and N. Roy Choudhury, Thermochimica Acta, 2002, 392-393, 339. ©2002, Elsevier Science
Figure 6.9 Thermomechanical analysis (TMA) data for the PMMA/PC blend showing the existence of three phases Reproduced with permission from S.A. Edwards, M. Provatas, M.Ginic-Markovic and N. Roy Choudhury, Polymer, 2003, 44, 3661. ©2003, Elsevier
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Figure 6.10 Thermal conductivity image of NR-NBR blends showing morphology of the blend Reprinted with permission from M. Song, D.J. Houston, D.B. Grandy and M. Reading, Journal of Applied Polymer Science, 2000, 81, 9, 2136. ©2000, Wiley Interscience
6.6 Thin Films/Coating on the Substrate μTA has also been applied extensively to investigate thin films or coatings for specialised applications such as paints, protective layers, electronic devices, and automotive products. To attain a homogenous, stable, scratch resistant thin film or coating is a critical challenge for the scientists. The factors affecting performance of the films/coating are morphology, thickness, adhesion of the film and substrate and stability. Conventional techniques such as SEM, time-of-flight secondary ion mass spectrometry and x-ray photoelectron spectrometry (XPS) are useful to understand and analyse characteristics of the films. However, understanding of spatial resolution in the film/coating, measurement of thickness and roughness of the film are few parameters unidentified. The technique of μTA and its different functionalities such as LTA and μ-TMA can be used to understand the thermal and mechanical properties at submicron/nanoscale. This can reveal interesting calorimetric and mechanical properties along with thickness and roughness [17]. Edwards and co-workers [17a] have characterised a plasma polymer coating of fluoropolymers on silicone, glass and elastomeric surfaces using μTA. Also, the authors showed the use of this technique to measure the thickness of the coating on the substrate using μTA. Figure 6.11 shows the topography and conductivity images of the plasma coated perfluoromethlycyclohexane on ethylene-propylene-diene terpolymer (EPDM) surface. The detailed morphology is clear from the topographic image, showing a smooth edged, granular morphology. The tetramethyldisiloxane plasma coated glass substrate shows sharp and rough morphology as shown in the Figure 6.12. These results show that the type and film quality is directly related to the monomer interaction and the type of the substrate used. The LTA thermograms, obtained by 226
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Figure 6.11 3D representation of the μTA topography (a) of the perfluoromethylcyclohexane plasma polymer onto the EPDM surface at power 60 W for 6 min (a) and its conductivity image (b) Adapted with permission from S.A. Edwards, N.D. Tran, M. Provatas, N. Roy Choudhury and N. Datta in Mechanical Tribology: Materials Characteristics and Applications, Eds., G.E. Totten and H. Liang, Marcel Dekker, New York, NY, USA, 2004. ©2004, Marcel Dekker
Figure 6.12 3D representation of the μTA topography of (a) the perfluoromethylcyclohexane plasma polymer onto the glass surface, and (b) the topography of the tetramethyldisiloxane onto the glass surface Adapted with permission from S.A. Edwards, N.D. Tran, M. Provatas, N. Roy Choudhury and N. Datta in Mechanical Tribology: Materials Characteristics and Applications, Eds., G.E. Totten and H. Liang, Marcel Dekker, New York, NY, USA, 2004. ©2004, Marcel Dekker
heating the tip from room temperature to 450 °C at 10 °C heating rate, are shown in Figure 6.13. The Tg of perfluorinated plasma film is 140 °C and melting occurs at 407 °C. Also, the thermograms superimpose on each other with minimal deviation, which can be due to a slight inconsistency in the coating. 227
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Figure 6.13 Micothermomechanical results for the plasma polymer film Adapted with permission from S.A. Edwards, N.D. Tran, M. Provatas, N. Roy Choudhury and N. Datta in Mechanical Tribology: Materials Characteristics and Applications, Eds., G.E. Totten and H. Liang, Marcel Dekker, New York, NY, USA, 2004. ©2004, Marcel Dekker
The roughness of such film, can be measured in tapping mode AFM of μTA. The roughness was measured for the coating of perfluoromethylcyclohexane and tetramethyldisiloxane on EPDM. The root mean square rms values are indicators of roughness of approximately 3.5 nm and 4.7 nm (root mean square rms) respectively. The 3D images are shown in Figures 6.14 and 6.15 for perfluoromethylcyclohexane and tetramethyldisiloxane, respectively. The technique of μTA can also be used to estimate the thickness and homogeneity of the coated film. The principle of thickness measurement using μTA is the thermal tip is heated at comparatively faster heating rate of 5-25 °C/s. When temperature increases, the sample softens and the tip displaces. Displacement of the tip can be estimated by a position-sensitive detector, estimating thickness of the coated film. Another confirmation would be change in temperature of the tip when it comes in contact with the substrate. Dutta and co-workers [19] showed clearly for the first time the measurement of the thickness of a nanoscale film on the silicon substrate by burning holes on the surface of the film and then taking line measurement, as shown in Figures 6.16a and 6.16b. Reading and co-workers performed measurement of polyethylene film on styrene-butadienerubber (SBR) by cross-sectioning and taking topographic and DC thermal images. The thickness was estimated to be 80-90 μm. μTA has been [20] specially used to analyse homogeneity of a polystyrene coating on a steel substrate. The thickness estimated was 228
Micro-thermal Analysis of Rubbery Materials
Figure 6.14 AFM topographic image acquired in tapping mode of (a) the fluoro plasma polymer deposited onto the EPDM surface at power 60 W for 6 min, and (b) surface roughness analysis of the coating Adapted with permission from S.A. Edwards, N.D. Tran, M. Provatas, N. Roy Choudhury and N. Datta in Mechanical Tribology: Materials Characteristics and Applications, Eds., G.E. Totten and H. Liang, Marcel Dekker, New York, NY, USA, 2004. ©2004, Marcel Dekker
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Figure 6.15 AFM topographic image acquired in (a) tapping mode of the disiloxane plasma polymer coated on the EPDM surface at power 40 W for 4 min, and (b) surface roughness analysis Adapted with permission from S.A. Edwards, N.D. Tran, M. Provatas, N. Roy Choudhury and N. Datta in Mechanical Tribology: Materials Characteristics and Applications, Eds., G.E. Totten and H. Liang, Marcel Dekker, New York, NY, USA, 2004. ©2004, Marcel Dekker
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Micro-thermal Analysis of Rubbery Materials (a)
Figure 6.16 (a) Pyrolysis crater and (b) film thickness as determined by μTA Reproduced with permission from N.K. Dutta, N.D. Tran and N. Roy Choudhury, Journal of Polymer Science Part B - Polymer Physics, 2005, 43, 11, 1392. ©2005, Wiley Interscience
80-90 nm. This value is lower than that already documented, which was explained to be due to polystyrene chains which were stretched more. Direct measurement of thermoelastic properties of polymer brush layer using μTA and AFM was firstly demonstrated by Lemieux and co-workers [21]. The authors measured the nanomechanical properties of a polystyrene-co-2,3,4,5,6-pentafluorostyrene (PSF) and polymethylacrylate polymer brush layer of 50-60 nm thick using μTA and AFM. μTA mode of SThM was used to perform an independent measurement of heat dissipation. As shown in Figure 6.17 typical Tg was observed at 109 °C in line with that observed from DSC. This indicates that PSF retains the thermal properties of the bulk polymer with high molecular weight. Also, on measurement of thermoelastic properties of the polymer brush layer by force-volume testing, indentation depth suddenly increases at 90 °C so as sudden change in elastic moduli at 90 °C. Nanomechanical properties can also be determined from SPM as shown in studies of ultrathin films of polyisoprene and styrene-butadiene-styrene (SEBS) block copolymer by Chizhik and co-workers [22]. The elastic modulus of these rubbery ultrathin films was measured by force-distance and penetration-load data. In a similar work Luzinov and co-workers [23] analysed, bilayer nanocomposite coatings using SPM to obtain images of the surface of the thin film and study micromechanical properties. A hard 231
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Figure 6.17 Direct measurement of Tg of brush layers M. Lemieux, S. Minko, D. USov, M. Stamm and V.V. Tsukruk, Langmuir, 2003, 19, 15, 6126. ©2003, ACS
monolayer of EP/PPP was deposited on an elastomeric SEBS layer. The substrate used was functionalised silicon wafer with epoxy-terminated SAM and the force-distance curve was measured The results confirm that SEBS film between the substrate and the film acts as a compliant reducing stresses and deformations developed in the thin polymer film due to non-similarities in the physical properties of substrate and the film. The phase miscibility of thin films coated on a substrate can be obtained from pulloff-force images, when a pulsed force module is combined with μTA. Oaten and coworkers [24] used this mode of μTA to obtain topography and pull-off force images to understand stability and phase miscibility of hybrid coated steel substrate. Thin films of hybrid were prepared from hexamethylene diisocyanate and trisilanol isobutyl polyhydral oligomeric silsesquioxane (T-POSS) on a steel substrate. Understanding of local adhesion was done using μTA attached with PFM. Topography and pull-off force images as shown in Figure 6.18, show homogeneity with a single phase indicating the constituent polymers in coated hybrid are covalently bonded. Recently, Risio and coworkers [25] used μTA and PFM to study spatial distribution of a kaolin coating on the paper and the thickness of the coating. Use of μTA to deposit a micro-film on a substrate in order to obtain controlled film characteristics is demonstrated by Krupers and co-workers [26]. 232
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Figure 6.18 Topography and pull-off force image of hybrid on steel substrate Reproduced with permission from M. Oaten and N. Roy Choudhury, Macromolecules, 2005, 38, 15, 6392. ©2005, ACS
6.7 Multilayer Material Characterisation Film and packaging material often consist of either blends of polymers or multilayers of different polymers and may comprise different polymers in the form of laminate layers, multilayer or single layer, with or without a metallic layer. Preparation of the most suitable material requires detailed characterisation of constituent polymers, their melting temperatures, barrier properties for oxygen and moisture, mechanical strength, chemical resistance and compatibility for printing the outer layer. Especially in laminated or multilayered packaging film it is important to identify the layered structure, and the melting temperatures of different layers independently and adhesion between these layers. Multilayered material is also used in semiconductor industry. In order to identify the presence of different layers and their phase contrast, AFM can be used to obtain topographic and phase difference images. However, to obtain melting temperatures and spatial distribution in multilayered or laminated packaging film, μTA would be the only tool, which can give us melting and Tg temperatures along with topography, conductivity and diffusivity images. Unique information can be obtained and performance can be judged from this information obtained using μTA. 233
Thermal Analysis of Rubbers and Rubbery Materials For example, Price and co-workers [6], examined commercially available packaging laminated film of seven layers containing a barrier polymer laminated by polyolefin: low-density polyethylene (LDPE), high density polyethylene (HDPE) using μTA. As it can be promptly identified using plot of sensor response versus temperatures, as shown in Figure 6.19, 6 layers exist in the film, and from the softening temperatures one can identify the polymers present. The curve showing lower softening point is HDPE and middle layer with higher softening point is EVOH, also presence of tie layer and possibly medium density polyethylene (MDPE) can be identified by such measurement. Woodward and co-workers [26a] analysed seven layered packaging film using a combination of three different techniques giving complementary information using AFM, photo-acoustic FTIR spectroscopy and μTA. Firstly, AFM height and phase images showed presence of seven layers. The thermal conductivity image obtained using a thermal probe of the same cross-sectional area is shown in Figure 6.20.
Figure 6.19 LTA of packaging film Reproduced with permission from D.M. Price, M. Reading, A. Hammiche, H.M. Pollock and M.G. Branch, Thermochimica Acta, 1999, 332, 2143. ©1999, Elsevier
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Figure 6.20 Thermal conductivity images of a seven layered packaging film Reproduced with permission from I. Woodward, S. Ebbens, J. Zhang, S. Luk, N. Patel and C.J. Roberts, Packaging Technology and Science, 2004, 17, 129. ©2004, Wiley Interscience
As shown in Figure 6.20, a resin layer appears to be the darkest due to its lowest conductivity, also it shows the existence of a central highly conductive layer in the middle. This layer is not seen in AFM images. Initial LTA performed on the film showed two melting temperatures, 135 °C and 194 °C. Later, repetitive measurements gave one melting temperature 166 °C. The first observed two melting temperatures were explained by authors to be due to layers present in the samples. As the thermal tip comes in contact with the first layer, the heat is being lost in the first layer and the probe overestimates the results as it consumes more power to melt the second layer of polypropylene. When a series of the same measurements are performed, the first layer has been removed and the exposed second layer of polypropylene gives only one melting temperature as shown by schematic in Figure 6.21.
6.7.1 Thermal Properties of Rubbery Micro-Particles Conventional thermal analysis techniques are widely used to obtain thermal properties of different types of micro-particles. However, due to critical size and difficulty of sample preparation, techniques like μTA are more suitable and informative. There has been interesting research carried out by different researchers to characterise and measure the size and stability and micro-thermomechanical properties of micro-particle systems. This includes core-shell micro-particles [27] micropores found in snakeskin [28], microspheres [29] and pharmaceutical powder particles. Core-shell micro-particles in the range of 4-5 μm with a rubbery core were investigated by Kuo and co-workers [27] Such core-shell rubbery microparticle systems consist of 235
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Figure 6.21 Schematic of contact between thermal tip and the sample Reproduced with permission from I. Woodward, S. Ebbens, J. Zhang, S. Luk, N. Patel and C.J. Roberts, Packaging Technology and Science, 2004, 17, 129. ©2004, Wiley Interscience
polybutadiene-polysulfonated styrene-polyaniline (PSSB-PANi) microparticles covalently bonded to different amount of polysulfonated styrene- polyaniline (PSS-PANi) outer layer. It exhibits contrast in thermal conductivity and stiffness of the constituent material. To understand morphology, stability, formation and size of these core-shell particles, topography and conductivity images were mapped using μTA and confirm the formation and stability of this system. Spatial distribution and morphology reveals characteristic features of the system. Use of μTA in this system is an ideal technique because the system contains contrast in size, conductivity and stiffness. The micron size probe and LTA functionality of μTA were the two major advantages of μTA in this system. The LTA experiment carried out on the surface of the solid film and on individual microparticles reveals important information on the thermal properties of the particles. LTA thermal profile of the solid film of the micro-particles shows that the rubbery core melts with 16% PANi loading while the core is stable with 58% of PANi loading as shown in Figure 6.22 and Figure 6.23, respectively. The results of LTA, SThM and topographic analysis showed that with higher loadings of PSS-PANi one can obtain a rubber-polyaniline homogenous system and with low loadings rubber particles aggregate and form larger particles of about 20 μ. Also, the promising results from the first set of experiments and the capability of μTA to form the basis to carry out LTA on the single micro-particle. This was done by isolating single micro-particle on silicon wafers. This experiment confirmed the size, stability and elastic performance of rubber micro-particles. 236
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Figure 6.22 LTA of 16% of loading of PANi showing instability of microparticles Reproduced with permission from C. Kuo, C-C. Chen and W. Bannister, Thermochimica Acta, 2003, 403, 1, 115. ©2003, Elsevier
Figure 6.23 LTA of 58% loading of PANi showing instability of microparticles before and after thermal analysis Reproduced with permission from C. Kuo, C-C. Chen and W. Bannister, Thermochimica Acta, 2003, 403, 1, 115. ©2003, Elsevier
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Figure 6.24 LTA of core of single isolated microparticle Reproduced with permission from C. Kuo, C-C. Chen and W. Bannister, Thermochimica Acta, 2003, 403, 1, 115. ©2003, Elsevier
LTA performed in triplicate at the core of the particle shows softening between 350 ºC and 420 °C as shown in the Figure 6.24. The stability of the particle can be observed from the morphology before and after the LTA experiments.
6.8 Thermal Characterisation of Micro-spheres One of the important characteristics of the μTA is to perform thermal analysis on highly specific region with high resolution and localised thermal analysis. This functionality is well utilised by Royall and co-workers [29] in their work to identify the presence of progesterone on the surface of polylactic acid (PLA) microspheres. The authors performed DSC, MDSC and TEM analyses and confirmed the presence of the progesterone in PLA microspheres. However, these techniques were not capable of finding exact location of the drug in the system. Therefore, the authors obtained topography, conductivity and LTA data using μTA on the PLA microspheres (Figure 6.25). The LTA of 0%, 30% and 50% loading shows the prominent change and melting by penetration of the probe with increasing temperature. These LTA results confirm the presence of progesterone in the PLA microspheres. 238
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Figure 6.25 LTA of PLA microspheres containing 0 w/w%, 30 w/w% and 50 w/w% progesterone Reproduced with permission from P.G. Royall, V.L. Hill, D.Q.M. Craig, D.M. Price and M. Reading, Pharmaceutical Research, 2001, 18, 3, 294. ©2001, Plenum Press
6.9 Powder Particle Characterisation There has been a significant increase in the use of μTA in the pharmaceutical industries [4, 29, 30]. Researchers have been using μTA to analyse complex multi-component pharmaceutical systems. This area of application of μTA has got its own strengths and weaknesses. It has been proven by some researchers that it is one of the most suitable techniques to differentiate complex systems and also to identify each of the phases. Royall and co-workers [29] used LTA and μTA in support of DSC to identify the presence of polymorphs and assess the interparticulate composition in some of the pharmaceutical powders. This was not possible using conventional thermal analysis techniques such as DSC. The authors carried out experiments on different pharmaceutical powders such as ibuprofen, salbutamol sulfate, trehalose dehydrate and indomethacin. The sample preparation includes the use of double sided tape and epoxy resin. Utmost care was taken to prepare the samples in order to utilise μTA and analyse discrete micro powder particles. The particle size on the glass plate after spray-drying was approximately 1020 μm. The sensor response as a function of temperature for ibuprofen samples shows results in conjunction with DSC data confirming consistency in the results. In a similar work by Craig and co-workers [30, 31] observed the necessity to consider scanning rate used in μTA. The LTA performed on two different forms of trehalose shows inequality in DSC and μTA results. On the other hand, LTA performed on polymorphs 239
Thermal Analysis of Rubbers and Rubbery Materials of indomethacin shows two distinct melting supporting results of DSC. The authors have shown capability of μTA to analyse individual micro powder particles outlining a few limitations of the same.
6.10 Characterisation of Micropores To imitate an IR receptor from micropores present in snake skin, SPM can be used. Although, TEM was used initially and the size was measured but it was underestimated due to dried sample and high vacuum. Also, shrinkage was observed in the sample [28]. With specialised attachments, it was possible to obtain topography and conductivity images in wet conditions using SPM [28, 31a]. The results of SPM as shown in Figure 6.26 revealed interesting structural information showing terraces like microstructure present in all the nonspecific skin areas and presence of nanopit array structure in pit organ receptor areas including average distance between pit and non-pit areas important for IR transmission. It is predicted that thermal properties may contribute to the specialised characteristic of this pit areas. Significant differences can be observed from conductivity images in pit receptor areas and non-receptor as shown in Figure 6.27. Also, thermal analyses performed on these areas show rapid increase in surface temperature of nanopit areas compared with non-receptor areas as shown in Figures 6.28a and 6.28b. Measurement of surface elastic modulus highlighted specific mechanical properties of these pit areas showing elastic moduli comparable with crosslinked rubber. These measurements performed spotting on receptor areas and compared with non-receptor areas reveal important information about basic difference between them and reason of receptor areas having IR receptor properties.
6.11 Characterisation of Nanostructured Material Nanostructured comb like polymer of stearyl methacrylate was studied by Thompson and co-workers using pulsed force mode (PFM) and μTA. The unique comb like structure of this homopolymer contains an amorphous and crystalline region, which has been demonstrated by μTA [32]. They employed thermal AFM (-TA) and pulsedforce mode (PFM) scanning force microscopy for visualisation and characterisation of nanostructured side-chain crystalline polymeric materials. Pulsed force microscopy (PFM) coupled with local thermal analysis for poly stearyl methacrylate (PSMA) confirms the growth of the crystals and formation of self-organised nanodomains below melting with a typical size of 20-30 nm (Figure 6.29a). The local thermal analysis performed on nine different lateral positions on the topographic image (Figure 6.29b). The melting point (Tm) was determined from the onset of the slope change of the sensor signal with 240
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Figure 6.26 Topography and thermal conductivity images obtained on snakeskin nonreceptor and receptor areas Reproduced with permission from N. Fuchigami, J. Hazel, V.V. Gorbunov, M. Stone, M. Grace and V.V. Tsukruk, Biomacromolecules, 2001, 2, 3, 757. ©2001, ACS
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Figure 6.27 Topography and conductivity of snakeskin pit organ surface only at different magnification Reproduced with permission from V.V. Gorbunov, N. Fuchigama, M. Stone, M. Graca and V.V. Tsukruk, Biomacromolecules, 2002, 3, 106©
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Figure 6.28a and b showing thermal properties/conductivity of receptor (a) and nonreceptor areas (b) Reproduced with permission from V.V. Gorbunov, N. Fuchigama, M. Stone, M. Graca and V.V. Tsukruk, Biomacromolecules, 2002, 3, 106©
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Figure 6.29 Pulsed-force-mode (PFM) scanning force microscopic image (a) and local thermal analysis (LTA) of the stearyl methacrylate homopolymer. (b) The unique phase separation nature of the side chain crystallising comb like polymers are clearly identified Reproduced with permission from S. Thomspon, N.K. Dutta and N. Roy Choudury, International Journal of Nanoscience, 2004, 3, 6, 839. ©2004, World Scientific Publishing
temperature. The Tm observed in the range of 20-25 °C. Most of the curves retrace each other indicating the uniform distribution of the nanocrystals in the amorphous matrix. The MDSC data on (PSMA) reveals the complex melting behaviour of the polymer and indicates that alkyl groups of monomeric units aggregate in the melt. In comb-like polymer the regular spacing imposed by the main chain sequence contributes to retain some ordering in the molten state and the fusion process is complex, and close to above melting directs the side chain to organise in a quasihexagonal arrangement and acts as a nucleating agent for rapid crystallisation upon cooling. At higher temperatures, above the melting point, Tm such comb like chains form random coil, however, if the branches are long and regular enough, the branches can crystallise below Tm in lamellae.
6.11.1 Micro-thermal Analysis Combined with Chemical Characterisation Techniques Calorimetric, thermometry and viscoelastic information can be obtained using different functionalities of μTA. However, to obtain chemical information of the samples one needs to rely on the techniques such as XPS or SIMS. These techniques, which give chemical information analysing surface properties of the sample requires high vacuum conditions. Also, the presence of multiple components in the system reaches limitations of the techniques. Therefore, the approach to combine chemical characterisation 244
Micro-thermal Analysis of Rubbery Materials techniques with micro-thermal analysis technique may result in the best combination to perform chemical characterisation at the μm scale. In order to expand the use of μTA researchers have further enhanced the capability of the technique of μTA by combining it with chemical characterisation techniques such as evolved gas analysis with mass spectroscopy (MS), GC-MS and near-field IR spectroscopy. Combination of μTA with different chemical characterisation techniques has been demonstrated by several researchers [8, 33, 34]. In case of combining μTA with GCMS or MS for chemical characterisation, the innovative part is, when localised thermal analysis is done the probe is kept very close to the sample and the sample is rapidly heated to a very high temperature. A specially designed collector tube with a special absorber is kept close to the probe to entrap gases evolved combined with a syringe to draw the gas through the tube. This is then placed in the GC-MS to obtain the chemical composition of the material. Using a similar set up Reading and co-workers [33] obtained chromatograms of polyethylene coated SBR film as shown in Figure 6.30a and 6.30b. However, use of MS significantly reduces analysis time compared with GC-MS. Reading and co-workers [33] also obtained the mass spectra of polyethylene coated SBR film as shown in the Figure 6.31. A similar setup was used by Hammiche and co-workers
Figure 6.30a Localised pyrolysis-GC-MS data for the top layer of film showing single ion chromatograms for m/e 57(C4H9+) and m/e 104 (styrene) Reproduced with permission from M. Reading, D.M. Price, D.B. Grandy, R.M. Smith, L. Bozec, M. Conroy, A. Hammiche, H.M. Pollock, Macromolecualr Symposia, 2001, 167, 1, 45. ©2001, Wiley Interscience
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Figure 6.30b Localised pyrolysis-GC-MS data for film substrate following initial pyrolysis experiments shown in Figure 6.30a Reproduced with permission from M. Reading, D.M. Price, D.B. Grandy, R.M. Smith, L. Bozec, M. Conroy, A. Hammiche, H.M. Pollock, Macromolecular Symposia, 2001, 167, 1, 45. ©2001, Wiley Interscience
Figure 6.31 Single ion intensities for m/e 57 and m/e 104 (styrene-broken line) versus time as a heated tip is brought into contact with a polyethylene-coated SBR film Reproduced with permission from M. Reading, D.M. Price, D.B. Grandy, R.M. Smith, L. Bozec, M. Conroy, A. Hammiche, H.M. Pollock, Macromolecular Symposia, 2001, 167, 1, 45. ©2001, Wiley Interscience
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Micro-thermal Analysis of Rubbery Materials [34] to analyse polystyrene film sandwiched between two PMMA films and an image is constructed by performing MS as shown in Figure 6.31. To obtain spectroscopic information based on microscopic spatial resolution, Hammiche and co-workers [35], for the first time, combined FTIR spectroscopy with μTA. The authors used Wollaston wire thermal probe as a probe to measure temperature fluctuations induced by irradiation of IR radiation on the sample. Using this technique one can obtain IR spectrum independent of diffraction limit, but based on size of area covered by the thermal probe. Using a similar setup Bozec and co-workers [36] performed ‘real world’ samples such as a fungicide powder and concentrated surfactant solution. The results obtained on these two samples were comparable with those from attenuated total reflection-FTIR. Also, the authors demonstrated the use of this technique by testing a typical sample of polyisobutylene on top of polystyrene confirming feasibility of subsurface detection. Reading and co-workers [33] gave a schematic of a similar setup as shown in Figure 6.32 which is combination of FTIR and μTA. Using this set-up, the authors successfully identified surface of a polyethylene coated SBR sample. Recently, Slough and co-workers [37] performed micro-spectroscopy using the similar setup to a forensic type of analysis. Interestingly, a drop of polystyrene LDPE, previously (90/10) blend, was absorbed onto a tissue and the technique of micro-spectroscopy as described above was used to perform FTIR on this droplet. Successful identification of both the components was obtained showing the versatility of the technique.
Figure 6.32 Schematic diagram of interface that allows the microscope to be mounted in the compartment of a standard infra red spectrometer Reproduced with permission from M. Reading, D.M. Price, D.B. Grandy, R.M. Smith, L. Bozec, M. Conroy, A. Hammiche, H.M. Pollock, Macromolecular Symposia, 2001, 167, 1, 45. ©2001, Wiley Interscience
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6.12 Future Outlook The technique of micro-TA and its usage in various areas ranging from elastomeric systems, thermoplastic elastomers, polymer blends to nanocomposites, biopolymers, nanofilms and pharmaceuticals has confirmed that it can be used both for visualisation and simultaneous thermal characterisation of materials in sub micron scale. Hyphenated μTA with GC-MS, MS and FTIR surely opens new directions for the researchers to obtain further information about chemical composition; and open up the possibilities of total characterisation of materials in submicron scale resolution. Further research in this area has recently developed a local thermal analysis technique which combines the high spatial resolution imaging capabilities of atomic force microscopy with the ability to obtain understanding of the thermal behaviour of materials with a spatial resolution of nano-level (sub-100 nm) and has been introduced commercially by Anasys Instruments (AI), Santa Barbara. This heated tip AFM (HTAFM) and nano-TA the conventional AFM tip is replaced by a special nano-TA probe that has an embedded miniature heater and is controlled by the specially designed nano-TA hardware and software. This is fundamentally an AFM equipped with an AI nano-thermal analysis (nano-TA) accessory and AI micro-machined thermal probe. The nano-TA system is compatible with a number of commercially available Scanning Probe Microscopes. Heated tip AFM (HT-AFM) refers to any AFM operation where a heated tip is used instead of a normal tip. Nearly any AFM imaging mode (tapping/contact/ Force-Volume, Pulsed Force Mode etc) can accommodate a heated tip to yield new information tied to the thermal properties of the sample [38-43]. Figure 6.33 shows a surface plot of the thermomechanical analysis hole pattern of poly trimethyl silyl propyne (PTMSP) thin film from the thermal scan using nano-TA. From the image the tip radius was revealed to be approximately ~50 nm and local heating area prior to tip penetration is less than 100 nm. Due to the relatively fast heating rate, tip penetration occurs quickly after Tg. The lack of thermal drift in the technique is evident by the symmetry of the hole. AI has also introduced a new transition temperature microscopy (TTM). In principle a TTM image is obtained by plotting the localised transition temperatures at different points on the sample. This powerful new form of microscopy enables one to identify localised thermal inhomogeneities on the sample surfaces including: (i) map thermal inhomogeneities on surfaces at the microscale, (ii) measure thermal gradients in thin films and multi-layers, (iii) identify defects / contaminants with in situ failure analysis, (iv) visualise the variation of crystallinity across your sample, (v) characterise interface properties between two solid phases that cannot be identified by conventional forms of microscopy and bulk thermal analysis [44]. The possibilities of the quantity of information and quality of visualisation; significant scientific excitement and continuous commercial development in the area of micro/nano thermal analyses confirm a very bright future prospect of this technique.
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Figure 6.33 Surface plot of the thermomechanical analysis hole pattern of poly trimethyl silyl propyne (PTMSP) thin film from the scan Adopted from J. Killgrove and R.Overney, Heated Tip AFM of Nanocomposite Polymer Membranes, Nano Thermal Application Note No.5, Anasys Instruments. ©Anasys Instruments
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Micro-thermal Analysis of Rubbery Materials 22. S.A. Chizhik, V.V. Gorbunov, I. Luzinov, N. Fuchigami and V.V. Tsukruk, Macromolecular Symposia, 2001, 167, 1, 167. 23. I. Luzinov, D. Julthongpiput, P.D. Bloom, V.V. Sheares and V.V. Tsukruk, Macromolecular Symposia, 2001, 167, 1, 227. 24. M. Oaten and N. Roy Choudhury, Macromolecules, 2005, 38, 15, 6392. 25. S. Di Risio and N. Yan in Proceedings of the 91st Annual Meeting of the Pulp and Paper Technical Association of Canada, Montreal, Quebec, Canada, 2005. 26. M.J. Krupers, H.R. Fischer, A.G.A. Schuurman and F.F. Vercauteren, Polymers for Advanced Technologies, 2001, 12, 9, 561. 26a. I. Woodward, S. Ebbens, J. Zhang, S.Luk, N. Patel and C.J. Roberts, Packaging Technology and Science, 2004, 17, 129. 27. C. Kuo, C-C. Chen and W. Bannister, Thermochimica Acta, 2003, 403, 1, 115. 28. N. Fuchigami, J. Hazel, V.V. Gorbunov, M. Stone, M. Grace and V.V. Tsukruk, Biomacromolecules, 2001, 2, 3, 757. 29. V.L. Hill, D.Q.M. Craig and M. Reading, Pharmaceutical Research, 2001, 18, 3. 30. J.R. Murphy, C.S. Andrews and D.Q. Craig, Pharmaceutical Research, 2003, 20, 3, 500. 31. K. Six, J. Murphy, I. Weuts, D.Q.M. Craig, G. Verreck, J. Peeters, M. Brewster and G. Van den Mooter, Pharmaceutical Research, 2003, 20, 1, 135. 31a. V.V. Gorbunov, N. Fuchigami, M. Stone, M. Grace and V.V. Tsuknik, Biomacromolecules, 2002, 3, 106. 32. S. Thompson, N.K. Dutta and N. Roy Choudhury, International Journal of Nanoscience, 2004, 3, 6, 839. 33. M. Reading, D.M. Price, D.B. Grandy, R.M. Smith, L. Bozec, M. Conroy, A. Hammiche and H.M. Pollock, Macromolecular Symposia, 2001, 167, 1, 45. 34. A. Hammiche, M. Reading, D. Grandy, D. Price, M. German, L. Bozec, J.M.R. Weaver, P. Stopford, G. Mills, and H.M. Pollock in Proceedings of the 12th International Conference on Scanning Tunnelling Microscopy/Spectroscopy and Related Techniques, Eds., P.M. Koenraad and M. Kemerink, American Institute of Physics Conference Proceedings No.369, Melville, USA, 2003, p.369.
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Thermal Analysis of Rubbers and Rubbery Materials 35. A. Hammiche, H.M. Pollock, M. Reading, M. Claybourn, P.H. Turner and K. Jewkes, Applied Spectroscopy, 1999, 53, 7, 810. 36. L. Bozec, A. Hammiche, H.M. Pollock, M. Conroy, J.M. Chalmers and N.J. Everall, Journal of Applied Physics, 2001, 90, 10, 5159. 37. C.G. Slough, A. Hammiche, M. Reading and H.M. Pollock, Journal of ASTM International, 2005, 2, 10. 38. J. Killgore and R. Overney, Heated tip-AFM of Nanocomposite Polymer Membranes, Nano Thermal Analysis, Application Note #5, http://www.anasysinstruments.com 39. L. Harding, W.P. King, X. Dai, D.Q. Craig and M. Reading, Pharmaceutical Research, 2007, 24, 11, 2048. 40. K. Park, J. Lee, Z.M. Zhang and W.P. King, Review of Scientific Instruments, 2007, 78, 4, 043709. 41. Z. Dai, W.P. King and K. Park, Nanotechnology, 2009, 20, 9, 95301. 42. B.A Nelson and W.P. King, Review of Scientific Instruments, 2007, 78, 2, 023702. 43. W.P. King, S. Saxena, B.A. Nelson, B.L.Weeks and R. Pitchimani, Nanoletters, 2006, 6, 9, 2145. 44. Anasys Instruments, Santa Barbara, CA, USA, http://www.anasysinstruments.com
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Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends
7
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends Zhaobin Qiu, Takayuki Ikehara and Toshio Nishi
7.1 Introduction Rubber is one of the most widely used polymeric materials. Without rubber products, today’s transportation and engineering industries all over the world could not survive at all. The people of today’s modern society could not enjoy their lives as a result. Rubber can be mainly classified into two categories, namely natural rubber (NR) and synthetic rubber (SR). Natural rubber (or cis-1,4-polyisoprene) is one of the most well known natural polymers. Ancient Mayans and Aztecs harvested it from the Hevea tree and used it to make waterproof boots and also the balls for them to play a game similar to basketball. The NR is normally treated to give it crosslinks, which makes it an even better elastomer. Like most diene polymers, polyisoprene has a carbon-carbon double bond in its backbone chain. Polyisoprene can be harvested from the sap of the Hevea brasiliensis, but it can also be made by Ziegler-Natta polymerisation. This is a rare example of a natural polymer that we can make almost as well as nature does. Natural rubber products have very useful technical characteristics of good tensile strength, high resilience and excellent flexibility, and resistance to impact and tear. However, on the other hand, NR is less resistant to oxidation, ozone, weathering and a wide range of chemicals and organic solvents due mainly to its unsaturated chain structure and nonpolarity. These drawbacks must cause limitations in the usage of NR. There are mainly two methods to modify the properties of NR. One is the chemical modification. Chemical modification can be classified into three main categories: modification by bond rearrangement without introducing new atoms, modification by attachment of new chemical groups (like chlorine and epoxy) through addition or substitution reactions at the olefinic double bonds, and grafting a second polymer onto the NR backbone. The examples of the first category are carbon-carbon crosslinking, cyclisation, cis-transisomerisation, and depolymerisation. The typical examples for the second category are chlorinated NR, hydrochlorinated NR and epoxidised NR. For the third category, grafting is mostly carried out using vinyl monomers such as styrene and methacrylate. The other method to modify the properties of NR is to blend it with SR. Compared with the chemical modification method, blending NR with SR is an easy and economical way to produce rubbery materials with suitable properties [1, 2]. Much attention has been paid to this method from the viewpoints of practical application and cost. 253
Thermal Analysis of Rubbers and Rubbery Materials Synthetic rubbers have also been developed to replace NR since World War II. There are three main types of synthetic rubbers. The first type is rubbers with an unsaturated carbon backbone, including polybutadiene (BR), polystyrene-co-butadiene (SBR), polybutadiene-co-acylonitrile (NBR) and so on. The second type is rubbers with a saturated –C–C– main chain, including copolymers of ethylene and propylene (EPR), ethylene-propylene-diene monomer (EPDM) and so on. The third is rubbers with carbon and oxygen in the main chain, such as polyepichlorohydrin (PECH) and polyepichlorohydrin-co-ethylene oxide (PECH-co-EO). Sometimes one synthetic rubber is also blended with the other synthetic rubber in order to modify the properties and extend the application field. Rubber - rubber blends are used widely in industry, for example, in tyre manufacturing. Rubber is often blended with thermoplastics to prepare new polymeric materials. Rubber is usually an impact modifier to thermoplastic. These blends, consisting of thermoplastics and rubber, are called as thermoplastic elastomers (TPE), which combine the processing characteristic of plastics at elevated temperature with the physical properties of conventional elastomers at service temperatures. One of the most important advantages of TPE is that the products can be reprocessed and remoulded. Therefore, TPE have started to be used instead of thermoset crosslinked rubber in many applications. TPE can play an increasingly important role in the polymer material industry. So TPE are of significant commercial interest. Several typical TPE are thermoplastic polypropylene (PP), thermoplastic polyurethanes (PU), thermoplastic copolyesters, and thermoplastic polyamides (PA). Most of the thermoplastics are semicrystalline polymers, such as PP, PA and biodegradable polymers. It is well known that the physical properties of crystalline polymeric materials depend strongly on their crystallinity and their microstructure. The studies of morphology and crystallisation of polymers are very important to get a better understanding of the relationship between structure, properties and processing. It is apparent that the crystallisation of the thermoplastics must be affected by the addition of rubber in the TPE. In this chapter, we focus on the study of the miscibility, morphology and crystallisation behaviour of plastic/rubber polymer blends. The miscibility and crystallisation behaviour of polymer blends based on biodegradable polymers and rubber will be reviewed first. Second, the morphology and crystallisation behaviour of TPE based on PA will be briefly introduced based mainly on the authors’ research.
7.2 Miscibility and Crystallisation of Biodegradable Polymer/Rubber Polymer Blends Miscibility plays a significant role in the structure and properties of polymer blends. Miscibility describes the capability of a mixture to form a single phase over certain ranges 254
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends of temperature, pressure and composition. Whether a single phase exists or not depends on many factors, such as the chemical structure, molar mass distribution, and molecular architecture of the components and so on. The single phase in a polymer blend may be confirmed by many experimental methods, including light scattering, X-ray scattering and neutron scattering. However, the most convenient one is to study the glass transition temperature (Tg) of the polymer blend using thermal analysis including differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) method. It is well known that a single Tg is the most widely and conventionally used criterion for determining the miscibility of a polymer blend. A single composition-dependent Tg indicates full miscibility with a dimension in the order of 20-40 nm. Conversely, an immiscible polymer blend exhibits more than one Tg. Some equations, such as the Fox equation – Equation 7.1 [3], Gordon-Taylor equation – Equation 7.2 [4] and Kwei equation – Equation 7.3 [5], have been proposed to predict the variation of the glass transition temperature of a miscible polymer blend as a function of composition. 1 w1 w 2 Tg Tg1 Tg2 Tg Tg
(7.1)
w1Tg1 k(1 w1)Tg2 w1 k(1 w1) w1Tg1 kw 2 Tg2 w1 kw 2
(7.2)
qw1w 2
(7.3)
Where Tg, Tg1, Tg2 are the glass transition temperatures of the blend, polymer 1 and polymer 2, respectively, w1 and w2 are the weight fraction of polymer 1 and 2, respectively, and k and q are fitting parameters. In a crystalline/amorphous polymer blend, the miscibility can also be judged from the depression of the melting point of the crystalline component with the increase of the amorphous component. It is well known that the depression of the melting point of a crystalline polymer blended with an amorphous polymer provides important information about its miscibility and its associated polymer-polymer interaction parameter. An immiscible or partially miscible blend does not typically show the depression of the melting point, which is depressed significantly with increasing the content of the amorphous polymer for a miscible blend, especially one containing specific interaction between the components. However, the melting point of a polymer is affected not only by the thermodynamic factors but also by the morphological factors such as crystalline lamellar thickness. Therefore, the equilibrium melting point should be used to separate the morphological effect from the thermodynamic effect in discussing the melting point depression as described by the Flory-Huggins theory [6, 7]. Hoffman and Weeks [8] have shown a relationship between the apparent melting point Tm and the isothermal crystallisation temperature T c: 255
Thermal Analysis of Rubbers and Rubbery Materials Tm = T c + (1-T c) Tmo
(7.4)
where Tmo is the equilibrium melting point, and may be regarded as a measure of the stability, i.e., the lamellar thickness, of the crystals undergoing the melting process. The equilibrium melting point can be obtained from the intersection of this line with the Tm = T c equation [8]. The equilibrium melting point data obtained can be analysed by the Nishi-Wang equation [7] based on the Flory-Huggins theory [6]. The melting point depression is given by Equation 7.5: RV2 ln 2 1 1 1 1 2
1 12 1 Tmo blend Tmo pure H o V1 m 2 m 2 m1 o
(7.5)
o
where Tm (pure) and Tm (blend) are the equilibrium melting point of the pure crystallisable component and of the blend, respectively. Ho is the molar heat of fusion of the repeat unit for a perfectly crystallisable polymer, V is the molar volume of the repeating units of the polymers, m and are the degree of polymerisation and the volume fraction of the component in the blend, respectively. Subscripts 1 and 2 refer to the amorphous and crystalline polymer, respectively. R is the universal gas constant, and 12 is the polymer-polymer interaction parameter. When both m1 and m2 are large, as for high molecular weight polymers, these related terms in Equation 7.5 can be neglected. The interaction parameter 12 can be written as:
Ho V1 1
12 12 o 1 RV2 Tm blend Tmo pure
(7.6)
For crystalline/amorphous polymer blends, a negative value of 12 indicates that the two components are thermodynamically miscible in the melt. Furthermore, the polymerpolymer interaction becomes stronger with the decrease of the value of 12. The development and application of biodegradable polymers has recently received more and more attention because of environment protection and the recycling of resources. Based on the difference in the preparation method, biodegradable polymers can usually be classified into two types. One is the biosynthetic polymers, such as bacterial poly(3hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-hydroxyvalerate) (PHBV). The other is the chemosynthetic polymers, such as the aliphatic polyesters. Polymer blending is often used to improve the properties and extend the application field of biodegradable polymers [9-11]. Through the blending, the properties of biodegradable polymers can be modified significantly, which influences not only the performance of biodegradable polymers but also affects their subsequent biodegradation. Although miscibility and crystallisation of biodegradable polymer blends have been investigated extensively, much less attention has been paid to the blending of biodegradable polymer with rubber. PHB is reported to be immiscible with rubbers, 256
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends such as EPR, ethylene-vinyl acetate (EVA), and poly(cis-1,4-isoprene) (PIP) [12-14]. In PHB/EPR blends, the Tg values for both components do not change with composition at all, which is indicative of their complete immiscibility [12]. Furthermore, although the crystallinity of the PHB phase is only slightly influenced by blend composition, no change in the radial growth rate (G) of spherulites occurs with increasing EPR content. It is found that the spherulites of PHB grow in the presence of a two-phase melt consisting of molten PHB containing EPR domains as a dispersed phase in the blend. The EPR particles are first rejected and then occluded in the intraspherulitic region during growth. The resulting morphology consists of PHB spherulites occluding particles of EPR in intraspherulitic regions. The miscibility of PHB with EVA copolymers containing 70 wt% vinyl acetate was investigated. The Tg, the melting temperature and the spherulite growth rate under the isothermal crystallisation conditions are independent of the blend composition, indicating that the two components are immiscible [13]. PIP was blended with PHB to improve the mechanical properties of PHB. With the change in the blend composition there is virtually no shift in the Tg of either PIP or PHB, indicating that PHB and PIP are immiscible [14]. Gassner and Owen studied the thermal and mechanical properties of PHBV/EVA blends [15], which are immiscible. The blends of PHBV and EVA show two Tg, corresponding to the two components. The mechanical properties of PHBV/EVA blends are strongly affected by blend composition. It is reported that only the PHBV component showed degradation in soil degradation tests. However, miscible, biodegradable polymer/rubber blends are essential to modify the properties of biodegradable polymers. Until now, only a few miscible bacterial polyester/ rubber blends are found, including PHB/EVA (85 wt% vinyl acetate) [13], PHB/PECH [16-19], PHB/PECH-co-EO and PHBV/PECH-co-EO blends [20]. PECH is a linear and amorphous elastomer with a low Tg of around −23 °C. PECH-coEO is an epichlorohydrin copolymer rubber. Both PECH and PECH-co-EO are reported to be miscible with crystalline polyethylene oxide (PEO) [21]. The two polymers have also been blended with PHB and PHBV to form miscible crystalline/amorphous polymer blends. PECH is also reported to be completely miscible with PHB and PEO in the ternary blends of PHB/PECH/PEO. [22] Goh and co-workers reported that PHB and PHBV were miscible with PECH-co-EO using DSC and polarising optical microscopy (POM) [20]. Both PHB/PECH-co-EO and PHBV(14 mol% hydroxyl valerate (HV) content)/PECH-co-EO blends were prepared by casting them from chloroform solutions. The miscibility of the blends was investigated by DSC. The Tg of the blends are composition dependent and intermediate between those of the component polymers, indicating that both PHB/PECH-co-EO and PHBV/ PECH-co-EO blends are miscible. Furthermore, the Tg composition curves of both blends can be fitted by the Kwei equation [5]. The composition dependent Tg fits the Kwei equation well, using k = 1.0 and q = -16.3 in PHB/PECH-co-EO blends. Similarly, 257
Thermal Analysis of Rubbers and Rubbery Materials the composition dependent Tg of PHBV/PECH-co-EO blends can also be described well by the same equation using k = 1.05 and q = 15.1. It can also be found that the melting point temperature of PHB or PHBV decreases slightly with the increase of the PECHco-EO content. Furthermore, the cold crystallisation temperature (T cc) of PHB is lower than that of pure PHB at lower PECH-co-EO content, and is higher than that of pure PHB at higher PECH-co-EO content in the PHB/PECH-co-EO blends. This indicates that the addition of PECH-co-EO may have some positive effects on the crystallisation of PHB when the PECH-co-EO content is lower. On the other hand, for the PHBV/ PECH-co-EO blends, the T cc of PHBV increases with the increase of the PECH-co-EO content, indicating that the cold crystallisation of PHBV is suppressed by the addition of the amorphous diluent PECH-co-EO in the blends. For PHB/PECH-co-EO and PHBV/PECH-co-EO blends, both PHB and PHBV are biodegradable crystalline polymers, while PECH-co-EO is an amorphous component. It is well known that the depression of the melting point of a crystalline polymer blended with an amorphous polymer provides important information about its miscibility and its associated polymer-polymer interaction parameter. The melting behaviour of PHB/ PECH-co-EO and PHBV/PECH-co-EO blends were studied by DSC. The equilibrium melting point is obtained for both polymer blends using the Hoffman-Weeks method. It was found that the Tmo of both PHB and PHBV decreases with the increase of the amorphous diluent PECH-co-EO in the blends. The equilibrium melting point data obtained were analysed by the Nishi-Wang equation [7] based on the Flory-Huggins theory [6]. For PHB/PECH-co-EO blends, a negative value 12 = −0.089 was obtained using V1 = 54.2 cm3/mol, V2 = 75 cm3/mol, Ho = 1.25 × 104 J/mol in Equation 7.6. As for the PHBV/PECH-co-EO blends, a negative value 12 = −0.075 was obtained using V1 = 54.2 cm3/mol, V2 = 75 cm3/mol, Ho = 109 J/g in Equation 7.6. The negative polymerpolymer interaction parameters for both polymer blends, indicate that both polymer blends are thermodynamically miscible in the melt. On the other hand, as compared to a slightly more negative interaction parameter for PHB/PECH-co-EO blends, it can be concluded that the interaction between PHB and PECH-co-EO is suppressed by the presence of HV units in the case of PHBV/PECH-co-EO blends. Therefore, the miscibility of PHB/PECH-co-EO and PHBV/PECH-co-EO blends has been confirmed not only from the single composition dependent Tg but also from the negative polymer-polymer interaction parameters. The spherulitc morphology and growth for both PHB/PECH-co-EO and PHBV/PECHco-EO blends were studied using POM. Typical spherulitic textures for various samples are observed when they crystallise at various crystallisation temperatures isothermally. Volume-filling spherulites for PHB or PHBV were obtained in the blends, indicating that the uncrystallisable component was rejected into the interlamellar or interfibrillar regions. Furthermore, the spherulitic growth rate (G) of PHB or PHBV showed a bell curve as a function of crystallisation temperature. The value of G decreased with the increase of PECH-co-EO content for a given crystallisation temperature in the blends. Meanwhile, the temperature showing the maximum values of G of PHB and PHBV 258
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends spherulites also shifted to the low temperature range with the addition of PECH-co-EO in the blends. Spherulitic growth kinetics can usually be analysed using the Lauritzen-Hoffman equation [23]: logG
Kg U* log Go 2.3R(Tc T ) 2.3Tc (T)f
(7.7)
where Go is a pre-exponential factor, U* is the transport activation energy, T is a hypothetical temperature below which all viscous flow ceases, Kg is a nucleation parameter, T is the degree of supercooling, and f is a correction factor to account for the variation in the bulk enthalpy of fusion per unit volume with temperature, f = 2T c / (Tmo+ T c). It was found that the value of Kg decreases with the increase of PECH-co-EO content for both polymer blends. PHB/PECH blends were prepared by solution casting from dichloromethane. PHB/ PECH blends showed a single Tg which fits the Fox equation well, indicating that both components are miscible. The polymer-polymer interaction parameter 12 = −0.054 was obtained from the Nishi-Wang equation, indicating that both components are thermodynamically miscible [16]. Therefore, the miscibility of PHB/PECH blends was confirmed both from the single composition dependent Tg and from the negative polymerpolymer interaction parameter. Similar results were also found in PHB/PECH-co-EO and PHBV/PECH-co-EO blends [20]. As mentioned previously, the polymer-polymer interaction parameter of PHB/PECH-co-EO blends was –0.089, indicating that the introduction of the EO segment into the backbone of PECH increases the polymerpolymer interaction with PHB. The spherulitic growth rates of PHB/PECH blends were also studied using POM, which decreased with the increase of the PECH component at a given crystallisation temperature. It is also concluded that the PECH component was rejected into the interlamellar or interfibrillar regions of the PHB spherulites since the volume was space filling. Furthermore, the small angle X-ray scattering (SAXS) studies on the PHB/PECH blends provided more information about the localisation of the amorphous component in the spherulitic structure of the crystalline polymer [17]. The results showed that the PECH component was dispersed at the molecular level in the interfibrillar region. The addition of the PECH component to PHB also causes a reduction of the overall crystallisation rate. It is expected that the addition of PECH to PHB must also have a remarkable influence on the degradation of PHB in the blends. Doi and co-workers found that the biodegradability and the tensile properties of PHB were improved markedly by blending with PECH [24].
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7.3 Morphology and Crystallisation of Polyamide/Rubber Polymer Blends Thermoplastic elastomers, which are prepared by mixing elastomers with thermoplastics, are of significant interest. The essential feature of all TPE is two polymeric phases, where one is rubbery and the other is either glassy or crystalline. These materials combine the processability of a thermoplastic with the functional performance of a rubber. Such materials are tending to replace traditional rubber in a host of applications offering an easier or cheaper route for producing products which can also be easily recycled. As a result, TPE have started to be used instead of thermoset crosslinked rubber in many applications. There are seven main TPE groups found commercially – styrene block copolymers, impact modified and super soft PP, polyolefinic blends of PP and crosslinked EPDM, thermoplastic PU, melt processible rubber, thermoplastic copolyesters, and thermoplastic PA. Among the previously mentioned TPE, elastomeric PA offer the best heat resistance in TPE and also have good chemical resistance so that its applications are cable jacketing and aeronautics. Many researchers have reported the preparation, characterisation, mechanical properties, thermal properties, morphology and crystallisation behaviour of PA (or Nylon) and rubber (elastomer) blends [25-33]. Paul and co-workers prepared blends of PA 6 and EPR grafted with maleic anhydride (EPR-g-MA) using a melt blending process [25, 26]. Two different rubbers were used. One was EPR-g-MA, which was nearly free of crystallinity. The other had a higher level of ethylene crystallinity, and was called H-EPR-g-MA. It was found that the reaction of the PA amine end groups with the grafted MA had the potential to form TPE with controlled morphology and chemical bonding between the phases. They further studied the effects of PA 6 content and crystallinity of the maleated rubber on morphological, thermal and mechanical properties of these blends. The morphology of blends of both EPR-g-MA and H-EPR-g-MA with PA 6 was studied by transmission electron microscopy (TEM) over the whole composition (100/0 to 0/100). Both blends showed similar morphology variation trends. Generally, discrete particles of the minor phase in a matrix of the major phase were observed at 20 and 80% of PA 6. Meanwhile, a tendency for co-continuity was observed for the intermediate compositions containing 40-60% of PA 6. It was found that an elongated PA 6 phase was observed at a level of 40% of PA 6, and this was more obvious at 50% PA 6. Furthermore, complete phase inversion was found at 60% of PA 6. The rubber existed as a dispersed phase within the PA 6 matrix. The morphology studies showed that the phase inversion composition was about 50% of PA 6 for both PA 6 and rubber blends. But there were some morphological differences between EPR-g-MA and H-EPR-g-MA in the blends. The PA particles were found to be smaller in the matrix of H-EPR-g-MA than those of EPR-g-MA at 20% of PA 6 because the melt viscosity of H-EPR-g-MA was higher than that of EPR-g-MA. Furthermore, the EPR-g-MA phase showed a more elongated structure than H-EPR-g-MA for blends 260
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends with intermediate compositions. Thermal and mechanical properties of PA 6 and two different rubber blends were also investigated using DSC and DMTA. Both polymer blends showed a melting peak at around 217 °C, corresponding to the melting peak of PA 6. However, they showed very different melting behaviour at a low temperature range. A melting peak at 45 °C was found for H-EPR-g-MA phase with a high heat of fusion, while a melting peak at 125 °C was found for EPR-g-MA with a low heat of fusion. The previously described, different melting behaviour was believed to result from the difference in the crystallinity formed from the sequences of ethylene units in the rubber. The melting peaks for PA 6 and the rubber did not change significantly in the blends. The mechanical properties of both blends were studied by DMTA. The Tg of both the rubber and the PA 6 phase were also obtained from the locations of the tan peaks with DMTA. Both blends showed two Tg corresponding to the Tg of rubber and that of PA 6. Both polymer blends showed similar trends. The Tg of the EPR-g-MA rubber phase increased from −38.5 to −34.3 °C when the PA content increased from 0 to 40%. For H-EPR-g-MA, the Tg of the rubber phase increased from −23 to −18.1 °C when the PA content increased from 0 to 40%. However, for both blends, the Tg of the dispersed rubber phase decreases below that of neat rubber when the PA content increases further from 50 to 80% and becomes the major matrix phase. It should also be noted that the values of Tg for the rubber phase of H-EPR-g-MA blends were higher than those of EPR-g-MA blends due to the higher crystallinity of H-EPR-g-MA. On the other hand, the Tg of PA 6 decreased from 65.8 to 52.3 °C in the blends with EPR-g-MA when the rubber phase increased from 0 to 60%, while for PA 6 and H-EPR-g-MA blends, the Tg of PA 6 decreased from 65.8 to only 61.6 °C when the rubber content increased from 0 to 60%. In the blends with 80% rubber, the Tg of PA 6 could not be observed. Moreover, strain-hardening and cold-drawing were observed for both blend systems in the intermediate and PA-rich composition range. Modulus values from stressstrain measurements and dynamic, mechanical, thermal measurements were compared with predictions using a model by Hill for composite materials. Blends based on rubber with high ethylene crystallinity gave better agreement with the model than those based on amorphous rubber. Groeninckx and co-workers studied the melt rheology and morphology of PA-6/EPR blends as a function of composition, temperature, and compatibiliser loading [27]. It was found that incompatibilised blends with a higher PA 6 content (or rubber content) had viscosities approximately intermediate between those of the component polymers. EPR was found to be dispersed as spherical inclusions in the PA matrix up to 30 wt% of its concentration. A kind of co-continuous morphology was observed between 30 and 50 wt% PA and a phase inversion beyond 70 wt% PA. Experiments were also carried out on in situ compatibilisation using MA-modified EPR (EPR-g-MA), which reacted with the amino end groups of PA. This reaction produced a graft copolymer at the blend interface as a compatibiliser. When a few percent of modified EPR were added, the viscosity of the blend was found to increase, however, the viscosity levelled off at higher concentrations, indicating a high level of interaction at the interface. 261
Thermal Analysis of Rubbers and Rubbery Materials Further morphological investigations showed that the size of the dispersed phase initially decreased when a few percent of the graft copolymer was added followed by a clear levelling off at higher concentration. Zhang and co-workers studied the effect of several compatibilisers on mechanical properties and morphology of EPDM/PA copolymer (PA) blends [28]. A significant reduction of the dispersed phase dimension was observed when chlorinated polyethylene (CPE) was added to an EPDM/PA blend, due to the interaction that exists between CPE and PA. A speculative description of configuration was proposed to interpret the morphological investigation made on these blends based on DSC, DMTA, scanning electron microscopy and TEM characterisation. The studies of mechanical properties showed that the materials obtained possess useful strength and excellent heat resistance. It should be noted that the previously mentioned PA/rubber blends are also called thermoplastic vulcanisates (TPV), which combine the excellent processing characteristics of thermoplastics with the elastic properties of elastomers. However, the elastomer is prepared by static vulcanisation. Recently, dynamic vulcanisation, the process of vulcanising elastomer during melt-mixing with molten thermoplastic, has become the best way to prepare TPV combining the excellent processing characteristics of thermoplastics with the elastic properties of elastomers. The elastic properties of TPV are similar to the more conventional class of thermoplastic elastomers based on hard segment and soft segment block copolymers. This technology has led to a significant number of new thermoplastic elastomeric products commercialised during the midto-late 1980s. Some thermoplastic elastomers, through dynamic vulcanization, have been commercialised. A well-known commercial example of dynamically vulcanised thermoplastic elastomer compositions are blends of PP and EPDM. These TPV are prepared by first melt-mixing PP with EPDM where a kind of co-continuous phase morphology is formed. Subsequently, a vulcanising agent is added to crosslink the EPDM rubber phase, and the crosslinked rubber phase will be sheared into small particles and finally finely dispersed in the thermoplastic matrix. Researches on the dynamic vulcanisation of blends of nitrile elastomer with PA and blends of CPE rubber with PA have also been reported [29, 30]. However, there are few publications available on dynamic vulcanisation of EPDM/PA blends. The difficulty in preparing the EPDM/PA TPV is due to the high interfacial energy between the two components. For PA 6/rubber blends with 50 or 60 wt% of an EPDM rubber, Groeninckx and co-workers found that the blends exhibited co-continuous morphologies and thereby relatively poor mechanical properties [31]. Therefore, in order to improve the mechanical properties, they used a suitable compatibiliser and slightly crosslinked the rubber phase during melt-mixing. Through this process, it was possible to disperse up to 60 wt% rubber in the PA matrix and to improve the mechanical properties markedly. Such materials exhibited good elastic properties with 262
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends a thermoplastic processability. They investigated the influence of the compatibiliser, the crosslinking agent and the viscosity ratio of rubber/thermoplastic on the blend phase morphology using TEM. The viscosity of the PA phase should be low enough to shift the phase inversion towards higher rubber content. On the other hand, if the viscosity of the PA was too low, a kind of coarse blend morphology was achieved resulting in poor mechanical properties. Therefore, the viscosity ratio of rubber/PA played a crucial role in order to achieve a PA6/rubber TPV with fine rubber dispersion. Huang and co-workers prepared EPDM/PA copolymer TPV using suitable compatibilisers and dynamic vulcanisation [32, 33]. In EPDM/PA copolymer TPV, the rubber/ thermoplastics ratio was chosen to be 65/35. Therefore, PA was the minor component and formed the dispersed phase in an EPDM matrix for a 65/35 EPDM/PA blend. During dynamic vulcanisation of such a blend, EPDM and PA had to undergo a phase inversion to maintain the thermoplasticity of the blend. The general way for preparing TPV was to blend rubber with a thermoplastic through melt-mixing. Thus, a co-continuous phase was generated. A vulcanisation agent was then added to crosslink the rubber phase, which was no longer able to coalesce into a continuous phase. As the degree of crosslinking advanced during mixing, the continuous rubber phase became elongated further and then broke up into polymer droplets. Therefore, the thermoplastic became the continuous phase, while the rubber became the dispersed phase. Thus TPV were obtained with finely dispersed rubber particles in the thermoplastics matrix. However, Groeninckx and co-workers reported in their studies of the dynamic vulcanisation of PA 6/EPDM blends, that the phase inversion region could not be shifted due to the compatibilisation reaction between EPDM-g-MA and PA 6 [31]. In their study, peroxide was premixed with the rubber before the PA 6/EPDM blend was compounded. Huang and co-workers [32] prepared the EPDM/PA copolymers TPV in a different way. EPDM/PA copolymers were prepared as follows: a PA copolymer (a copolymer of 70% PA 1010, 20% PA 66 and 10% PA 6) with a compatibiliser (EPDM-g-MA or EPR-g-MA) was first melt-mixed, and then EPDM was melt-mixed. Later, a sulfur vulcanizing agent and a coagent were added to crosslink the rubber phase. There were different variables that may affect the properties of EPDM/PA blends: the ratio of EPDM to PA, the volume fraction of compatibiliser, the molecular weight and the viscosity of EPDM and PA, composition and functionality of the compatibiliser and the crystalline structure of these systems. Huang and co-workers [33] studied the effect of dynamic vulcanisation on the morphology and crystallisation behaviour of EPDM/PA copolymer TPV in more details. It should be noted that the morphology study for TPV is very important to confirm the phase inversion. For TPV, the phase morphology of the blends is usually observed with TEM. Recently, atomic force microscopy (AFM) has been recognised as a powerful surface characterisation technique and has been widely used to study surface morphology of homopolymers, block copolymers, and polymer blends. AFM does not require the staining of the sample as TEM does. Therefore, it has become a useful and convenient tool to study the phase morphology for TPV. 263
Thermal Analysis of Rubbers and Rubbery Materials Figure 7.1 shows the phase morphology for the dynamically vulcanised EPDM/PA copolymers sample, which was obtained by tapping mode AFM. The morphology of large crosslinked white EPDM particles was observed to be dispersed in the dark PA matrix. The large rubber particles had a tendency to be co-continuous since no compatibiliser was used. EPDM-g-MA and EPR-g-MA were used as compatibilisers in EPDM/PA copolymer TPV. The effects of the variety and the content of the compatibiliser on the phase morphology of the EPDM/PA copolymer TPV had also been investigated by AFM. Figure 7.2 shows the phase morphology of EPDM/EPDM-g-MA/PA copolymer TPV with different amounts of EPDM-g-MA as a compatibiliser. The bright area represented the crosslinked rubber phase, and the dark area was the PA phase. It could be observed from the AFM images shown in Figure 7.2 that the rubber phase could be finely dispersed in the PA phase by increasing the content of the EPDM-g-MA, indicating that phase inversion existed due to the introduction of EPDM-g-MA as a compatibiliser. These results indicated that the phase inversion region could be shifted although there was a compatibilisation reaction between EPDM-g-MA and PA. It can be explained as follows: PA kept its flow mobility in the molten state. Some PA droplets may be encapsulated by the rubber phase during the dynamic vulcanisation process because of the steric stabilisation effect of the EPDM-g-PA formed. The sub-inclusions of PA inside the rubber particles could be observed in the AFM images. However, the PA phase tended to coalesce and could form the continuous phase because the degree of crosslinking of the rubber phase was not high enough to form a network and the PA phase still kept the flow mobility in the molten state. Furthermore, the size of the crosslinked rubber particles decreased with increasing amount of EPDM-g-MA content due to the compatibilisation reaction. When EPDM was totally replaced by EPDM-g-MA, the size of the rubber particles were at their smallest.
Figure 7.1 AFM image of dynamically vulcanised EPDM/PA (65/35)
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Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends
(a)
(b)
(c)
(d)
(e)
Figure 7.2 AFM images of dynamically vulcanised EPDM/EPDM-g-MA/PA TPV. (a) 58.5/6.5/35; (b) 52/13/35; (c) 39/26/35; (d) 13/52/35; (e) 0/65/35
265
Thermal Analysis of Rubbers and Rubbery Materials The effect of EPR-g-MA as a compatibiliser on the phase morphology of EPDM/ PA copolymer TPV has also been studied using AFM. Figure 7.3 shows the phase morphology of EPDM/EPR-g-MA/PA copolymer TPV with different contents of EPRg-MA as a compatibiliser. Since EPR-g-MA could not be crosslinked by sulfur, three phases existed in these TPV. It is observed in Figure 7.3a and 7.3b that the bright crosslinked EPDM particles were dispersed in the dark matrix of PA and EPR-g-MA. It should be noted that the morphology of TPV using EPR-g-MA as the compatibiliser showed much smaller dispersed rubber particles compared with morphology of TPV using the same content of EPDM-g-MA as the compatibiliser. The small dispersed rubber particles were expected to have a positive effect on the mechanical properties of TPV. When the EPR-g-MA content was increased to 26 wt%, it could be seen from Figure 7.3c that the size of the crosslinked EPDM particles decreased and the larger bright regions seemed to be composed of small particles, which was more obvious in the enlarged image inside Figure 7.3c. The uncrosslinked EPR-g-MA at the interface between the crosslinked EPDM and PA tended to coalesce with increasing compatibiliser content of EPR-g-MA. When the EPR-g-MA content was 52 wt%, the uncrosslinked EPR-g-MA became the dark continuous phase, and PA and crosslinked EPDM became the bright dispersed phase as shown in Figure 7.3d. When EPDM was totally replaced by EPR-g-MA, the blend was composed of PA and EPR-g-MA only. Since EPR-g-MA could not be vulcanised by sulfur, the bright PA particles were found to be dispersed into the dark matrix of EPR-g-MA. The crystallisation behaviour of EPDM/PA copolymer TPV had also been studied using DSC. Five different samples were used including neat PA copolymer (PA), unvulcanised EPDM/PA (65/35), vulcanised EPDM/PA (65/35), vulcanised EPDM/EPDM-g-MA/PA (52/13/35), and vulcanised EPDM/EPR-g-MA/PA (52/13/35). There were two reasons for selecting the five different samples. One was to study the effect of vulcanisation on the crystallisation behaviour of PA of binary EPDM/PA blends. The other was to investigate the effect of different compatibilisers (EPDM-g-MA and EPR-g-MA) on the crystallisation behaviour of PA in the ternary TPV. Both nonisothermal crystallisation and isothermal crystallisation processes were used to fulfill these two purposes. For the nonisothermal crystallisation process, all the samples were cooled from the crystal-free melt at a cooling rate of 5 °C/minute. Figure 7.4 shows the DSC traces for the five different samples. Neat PA copolymer exhibited a broad crystallisation exotherm with the peak at around 100 °C. The crystallisation peak temperature of PA copolymer almost did not change in the binary blends of EPDM/PA, irrespective of the vulcanisation. But it was found that the crystallisation enthalpy of the PA phase decreased in the case of vulcanised EPDM/PA compared with that of unvulcanised EPDM/PA blend. However, the crystallisation peak temperature shifted to 110.5 and 107.5 °C, for EPDM/EPR-g-MA/PA and EPDM/EPDM-g-MA/PA TPV, respectively, indicating that the introduction of the compatibilisers EPR-g-MA and EPDM-g-MA had had a positive effect on the crystallisation of the PA phase. This phenomenon could be ascribed to the 266
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends
(a)
(b)
(c)
(d)
(e)
Figure 7.3 AFM images of dynamically vulcanised EPDM/EPR-g-MA/PA TPV. (a) 58.5/6.5/35; (b) 52/13/35; (c) 39/26/35; (d) 13/52/35; (e) 0/65/35
267
Thermal Analysis of Rubbers and Rubbery Materials fact that addition of compatibiliser would help to form fine dispersion of crosslinked rubber particles, which may subsequently act as heterogeneous nucleating centres for the crystallisation of the PA phase. The T c and the crystallisation enthalpy ( H) of the PA phase for the five different samples are summarised in Table 7.1. Furthermore, the effect of the content of the compatibilisers EPR-g-MA and EPDM-gMA on the crystallisation peak temperature of the PA phase in the TPV has also been studied. For both TPV with different compatibiliser, the T c increased and finally arrived at a constant value with increasing compatibiliser content as shown in Figure 7.5. The values of H of the PA phase in the TPV were less than that of neat PA. The decrease of crystallinity could be ascribed to the crystallisation in confined spaces between rubber
Figure 7.4 DSC cooling traces at a cooling rate of −5 oC/minutes: (a) PA; (b) EPDM/PA (unvulcanised) (65/35); (c) EPDM/PA (65/35); (d) EPDM/EPR-g-MAH/PA (52/13/35); (e) EPDM/EPDM-g-MAH/PA (52/13/35)
Table 7.1 Crystallisation peak temperature and enthalpy of crystallisation of PA phase for the five different samples during nonisothermal crystallisation at a cooling rate of 5 oC/min PA EPDM/PA (unvulcanised) (65/35) EPDM/PA (65/35) EPDM/EPR-g-MAH/PA (52/13/35) EPDM/EPDM-g-MAH/PA (52/13/35)
268
Tc (°C) 41.2 39.4 31.1 39.1 39.1
H (J/g) 41.2 39.4 31.1 39.1 39.1
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends particles. Figure 7.5 shows that the value of H of the PA phase went through a maximum value at a compatibiliser content of 20% in both TPV. The addition of compatibiliser may reduce the particle size of the dispersed rubber phase which was beneficial for nucleation of PA, but at the same time introduced the compatibilisation reaction between compatibiliser and PA which would restrict the growth of PA crystallinity. Therefore, the value of H of the PA phase went through a maximum with increasing compatibiliser content.
Figure 7.5 Effect of compatibiliser content on T c and H in EPDM/PA TPV (EPDM+compatibiliser/PA: 65/35)
269
Thermal Analysis of Rubbers and Rubbery Materials Apart from the nonisothermal crystallisation behaviour, the isothermal crystallisation of the five different samples was also been investigated by DSC. The isothermal crystallisation temperature was chosen in the range of 100-108 °C. The well-known Avrami equation is often used to analyse the isothermal crystallisation kinetics – it assumes that the relative degree of crystallinity develops with crystallisation time t as: 1 Xt = exp ( k t n) (7.8) where n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals, and k is a composite rate constant involving both nucleation and growth rate parameters [34]. The Avrami parameters n and k can be obtained from the plots of log(-ln(1-Xt)) versus log t. The Avrami exponents n and crystallisation rate constants k of the five different samples at 100-108 °C are listed in Table 7.2. It was found
Table 7.2 Avrami parameters for the five samples at various crystallisation temperature Sample Neat PA
EPDM/PA (unvulcanised) (65/35)
EPDM/PA (65/35)
EPDM/EPDM-g-MAH/PA (52/13/35)
EPDM/EPR-g-MAH/PA (52/13/35)
270
Tc (oC) 100 102 104 106 108 100 102 104 106 108 100 102 104 106 108 100 102 104 106 108 100 102 104 106 108
n 2.2
2.3
2.5
2.5
2.4
k (s-n) 7.58 10 6.76 10 5.88 10 5.24 10 4.36 10 1.73 10 1.31 10 1.09 10 8.51 10 6.76 10 3.71 10 2.63 10 2.18 10 1.44 10 1.07 10 6.02 10 5.24 10 4.46 10 3.46 10 2.57 10 3.16 10-5 1.94 10-5 1.51 10-5 8.91 10-6 6.16 10-6
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends that the value of the average n did not change significantly despite the sample and the crystallisation temperature. For the neat PA copolymer, the value was around 2.2, which increased a little to 2.3-2.5 in the blends with EPDM despite the vulcanisation and the compatibiliser, indicating that the crystallisation mechanism of the PA phase remained the same. On the other hand, the overall crystallisation rate k was found to be dependent on the crystallisation temperature. The value of k decreased with increasing the crystallisation temperature for the same sample. Furthermore, it was also found that the value of k changed with the different samples used in this study, which is shown in Figure 7.6.
Figure 7.6 Plots of half crystallisation time versus T c (a) and plots of crystallisation rate t0.5-1 versus T c (b) for various PA samples
271
Thermal Analysis of Rubbers and Rubbery Materials The half-life crystallisation time t0.5, the time required to achieve 50% of the final crystallinity of the samples, is an important parameter for the discussion of crystallisation kinetics. The value of t0.5 can be calculated by the following equation based on the Avrami equation: t0.5 = (ln2/k) 1/n
(7.9)
where k and n are the same as in the Avrami equation. Figure 7.6a shows the plots of the values of t0.5 for the five different samples as a function of crystallisation temperatures. It could be concluded from Figure 7.6a that the crystallisation time increased with increasing the crystallisation temperature. Meanwhile, the variety of the samples also had a significant influence on the crystallisation time. For a given crystallisation temperature, neat PA copolymer needed the longest crystallisation time, and EPDM/EPR-g-MA/PA required the shortest crystallisation time. Usually, the crystallisation rate was described as the reciprocal of t0.5. Therefore, the values of t0.5-1 were plotted as a function of crystallisation temperature in Figure 7.6b for all the samples to compare the effect of the variety of the sample on the crystallisation rate. From Figure 7.6b, the crystallisation rate was found to decrease with increasing the crystallisation temperature for each sample. Meanwhile, the crystallisation rate showed the following order among the five samples at a given crystallisation temperature, PA < unvulcanised EPDM/PA < vulcanised EPDM/PA < EPDM/EPDM-g-MA/PA < EPDM/EPR-g-MA/PA. The crystallisation rate was highest in compatibilised TPV and lowest in neat PA while intermediate in uncompatibilised TPV and unvulcanised EPDM/PA blends. Comparing with unvulcanised EPDM/PA blends with the vulcanised EPDM/PA blends, it seemed that the dynamic vulcanisation process could increase the crystallisation rate for the PA copolymer obviously, especially when a suitable compatibiliser is used. The dynamic vulcanisation introduced fine crosslinked rubber particles which could act as heterogeneous nucleating centres, which would enter the PA matrix and cause an increase in nucleating rate. For unvulcanised EPDM/ PA blends, PA copolymer was the island phase dispersed in the unvulcanised rubber matrix. In this case, the crystallisation of PA would be restricted. While for TPV, the immiscibility was improved and the crystallisation of PA was increased accordingly. Without any compatibiliser, it would be difficult to disperse a rubber phase in a PA matrix during dynamic vulcanisation and the crosslinked rubber particles were large (as shown in Figure 7.1). Using EPDM-g-maleic anhydride (MAH) or EPR-g-MAH as compatibiliser, the corresponding succinic anhydride groups could react readily with the terminal amine group of the PA copolymer leading to a graft copolymer formed in situ. The copolymer acted as a macromolecular surfactant and its presence, during mixing, permits the formation of very small droplets of the elastomer which later, during dynamic vulcanisation, became very small particles of vulcanised rubber. These finely dispersed vulcanised rubber particles increased the density of nucleating centres and thus increased the nucleating rate. On the other hand, the compatibilisation reaction restricted the mobility of PA copolymer chains and decreased the crystal growing rate. However, Figure 7.6 indicates that the nucleating rate was the main influencing factor and the crystallisation rate increased by adding compatibiliser. Compared 272
Miscibility, Morphology and Crystallisation Behaviour of Rubber Based Polymer Blends with the morphology of TPV with the same dosage of EPDM-g-MAH compatibiliser (Figure 7.2b), the morphology of TPV using EPR-g-MAH as a compatibiliser showed much smaller dispersed rubber particles (Figure 7.3b), which may contribute to the higher crystallisation rate. Therefore, the preparation of TPV through the dynamic vulcanisation was an efficient route to produce plastic and rubber blends with suitable properties. The development of plastic/rubber blends will find more and more application in the near future in industry and improve the quality of modern lives.
Acknowledgement The authors thank Dr. Hua Huang for her kind contributions to this chapter.
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Thermal Characterisation Of Polymer Nanocomposites
8
Thermal Characterisation of Polymer Nanocomposites Musa R. Kamal and Luminita L. Ionescu-Vasii
8.1 Introduction Thermal Analysis refers to the group of techniques that follow changes in sample properties with temperature. In this chapter, we discuss mainly the two most commonly used techniques for the thermal characterisation of nanocomposites: thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC), which also includes modulated differential scanning calorimetry (TMDSC). Some reference is also made to thermal conductivity (TC) and micro-thermal analysis (TA). The presentation of the different techniques starts with the description of the principles involved and the commonly used apparatus and experimental procedures. Subsequently, the application of the technique to nanocomposites is illustrated with typical examples and results reported in the literature.
8.2 Thermo-Gravimetric Analysis (TGA)
8.2.1 Introduction Thermogravimetric analysis is a technique that measures the mass and the change of the mass (usually, but not always, loss of mass) of a sample during heating as a function of time and/or temperature. The change in the mass of a sample could be the result of chemical reactions, particularly decomposition that might occur during heating, or physical changes, such as evaporation or sublimation. It should be pointed out that not all thermal events are accompanied by a change in sample mass. The main applications of TGA are to evaluate the thermal stability of materials under various conditions and to study the kinetics of the physico-chemical processes that cause change of mass of the sample. TGA is particularly useful for the following measurements: thermal stability, decomposition kinetics, composition, estimated lifetime, oxidative stability, moisture and volatile contents.
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Thermal Analysis of Rubbers and Rubbery Materials
8.2.2 The Apparatus The changes in the mass of the sample are measured using a thermobalance. As shown in Figure 8.1, the thermobalance consists of an electronic microbalance enclosed in a furnace with controlled temperature and atmosphere. A temperature sensor measures the specimen/ furnace temperature and a purge gas flows through the chamber [1]. The experiment may be performed under an inert atmosphere by using nitrogen, helium or argon, in a reactive atmosphere, e.g., oxygen or air, or in vacuum. Heat transfer to the sample depends on the gas flow rate. The furnace temperature is controlled by computer software. Generally, thermobalances are designed for maximum loads between 1 mg and 500 g [2]. New thermogravimetric instruments are equipped with microbalances with sensitivities down to the nanogram range. Photoelectric or electromagnetic sensors are used to follow the balance deflection. The sensors for temperature are either platinum resistance thermometers or thermocouples. The heating rates that can be used are from less than 1 °C/min to ~1000 °C min, with the option of running isothermal experiments. Both, ultrahigh vacuum and high pressure (up to 50 MPa) balances are commercially available [3]. Sometimes, for polymer analysis, TGA is used in combination with Fourier transform infra red spectroscopy (FTIR) or mass spectrometry (MS). The gases evolved during heating in the TGA furnace are transferred into another chamber for sampling and analysis by these techniques. When mass spectrometry is used, TGA measurements are performed in vacuum. In some commercial systems, the TGA apparatus is part of a network that includes some or all of the following devices: differential thermal analysis (DTA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA).
Figure 8.1 Schematic of a thermobalance [3] Reproduced with permission from Introduction to Thermal Analysis, Techniques and Applications, Ed., M.E. Brown, Chapman and Hall, New York, NY, USA, 1988, p.7. ©1988, Chapman & Hall
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8.2.3 Methodology The following American Society for Testing and Materials (ASTM) standards cover the application of TGA to study various aspects of the thermal behaviour of polymer systems: •
polymer content, plasticiser fraction, filler content, weight of volatile components or adsorbed liquids, ASTM E 1131-98 [4], ASTM D 6370-99 [5],
•
decomposition kinetics, ASTM E 1641-98 [1],
•
the temperature and time of drying, ASTM E 1868-97 [6].
Various factors influence TGA measurements, including sample weight and characteristics, the pan, the purge gas flow rate, the heating rate, and the position of the thermocouple. Therefore, the following steps are required before starting TGA measurements: instrument calibration, sample preparation and the selection of the purge gas and its flow rate.
8.2.3.1 Instrument Calibration The calibration of TGA must be performed under the same conditions as those employed during the planned measurement: the same purge gas and purge gas flow rate, the same heating rate, and the same thermocouple position. According to ASTM E1582 [7], either magnetic transition standards or melting point standards may be used for temperature calibration. Magnetic transition standards calibration is based on the use of a property of ferromagnetic substances called Curie temperature (Curie point), which is the temperature at which a ferromagnetic material loses its ferromagnetic properties. Mass calibration is performed with calibrated weights (10-100 mg) in the absence of the purge gas [8]. During TGA experiments, the symmetry of the balance can be disturbed by several factors, such as: change of the purge gas density with temperature, change of pressure in the furnace, hang-wire – purge gas friction, and sample – purge gas friction. Thus, it is necessary to make a correction for buoyancy arising from the lack of symmetry in the weighing system [8, 9]. The TGA pan must have a low mass. It is also important that the pan is made of an inert material, in order to avoid a chemical reaction between the sample and the pan. It is possible to make the buoyancy correction by using the equation of state for the purge gas. Another possibility is to perform a TGA experiment without a sample in the pan, under the same conditions as in the planned experiment (temperature, type of purge gas, and purge gas flow rate). The recorded TGA curve for the experiment, in the absence of the sample, should be then subtracted from the TGA experimental curve. 279
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8.2.3.2 Preparation of the Sample As a general rule, small amounts of the sample (1-100 mg) and, if possible, thinly spread powdered samples in the pan are preferred [8]. The thermal properties of bulk materials differ significantly from those of powders. Solid samples with the same chemical composition, may behave differently, due to differences such as the content of defects, the porosity and the surface properties. The mass of the sample is very important. If the mass of the sample is too large, the temperature of the sample could become nonuniform. The rate of gas diffusion out of the sample is also reduced. Such effects cause poor reproducibility of the results [9].
8.2.3.3 Flow Rate of the Purge Gas Variations in the flow rate of the purge gas do not necessarily affect the noise level of the recorded signal, but they may shift the weighing zero. The noise increases, as the temperature increases, especially for dense gases at pressure higher than ~20 kPa, due to thermal gradients that cause convection. Therefore, a low density gas, e.g., helium, should be introduced above the hot zone.
8.2.4 Typical TGA Curves TGA curves record mass as a function of temperature. Different types of TGA curves are obtained, depending on the transformations that occur in the sample during heating. If the sample does not exhibit a change of mass, this does not imply that no changes have occurred in the sample. Phase changes (e.g., melting) or reactions such as polymerisation might occur without a change of mass. If no such changes occur, then the sample is stable over the temperature range used. The TGA curve will be a horizontal line. A rapid loss of mass is recorded when desorption or sample drying occurs. The mass declines for a while, and finally becomes constant. The TGA curve exhibits an inflection point, if decomposition occurs (Figure 8.2). The final percentage loss of mass, mL is: mL
( mi mf )x100% mi
(8.1)
where the initial mass of the sample is mi and the final mass is mf The mass loss at any time may be estimated similarly. The rate of mass loss and its variation with time may be calculated from the derivative of the mass loss curve. This allows the estimation of the temperature at which the maximum rate of mass loss takes place. Most commercial TGA instruments are equipped with software to supply both the mass loss and the rate 280
Thermal Characterisation Of Polymer Nanocomposites of mass loss data. If the decomposition of the sample occurs in several stages, the TGA curve shows several inflection points, with relatively stable intermediates. Figure 8.3 illustrates a typical TGA curve for a two step-decomposition [5]. If the intermediates are not stable, the corresponding TGA curve is illustrated in Figure 8.4 [9, 10]. Similar curves may be obtained when the heating rate is very high. The derivative of the TGA curve may also be used in order to evaluate the rate of loss of mass in each stage. In some
Figure 8.2 Typical TGA curve
Figure 8.3 TGA curve for a two steps loss of mass process
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Figure 8.4 TGA curve for a multi-step loss of mass process with non-stable intermediates
cases, the TGA curve may show an increase of mass, as a result of a chemical reaction between the sample and the surrounding atmosphere, e.g., oxidation of the sample. In such cases, the mass gain and rate of mass gain are analysed in the same fashion as for the cases of mass loss.
8.2.5 Applications of TGA in Nanocomposites Characterisation
8.2.5.1 Thermal Stability of Nanofillers The most common nanocomposite systems are based on a polymer matrix and inorganic clay minerals composed of layered silicates, particularly the smectite mineral montmorillonite (MMT) which belongs to the general family of 2:1 layered silicates. Its structure consists of two fused silica tetrahedral sheets sandwiching an edge-shared octahedral sheet of either aluminium or magnesium hydroxide. Chemically, MMT is a hydrated sodium calcium aluminium magnesium silicate hydroxide (Na,Ca)x(Al,Mg)2(Si4O10)(OH)2·nH2O. Potassium, iron, and other cations are common substitutes. The exact ratio of cations varies with the source. The silicate layers are coupled through relatively weak dipolar interactions and Van der Waals forces [11, 12]. Layered silicates are not easily dispersed in most polymers because of their hydrophilic nature. In order to improve their dispersion, the ions of sodium or calcium residing in the interlayers are replaced via ion-exchange reaction by organic cations, such as alkylamonium. This replacement makes the initial hydrophilic layered silicates organophilic, thus increasing 282
Thermal Characterisation Of Polymer Nanocomposites their compatibility with polymer matrices. The second advantage of the replacement is that it enlarges the spaces between the silicate layers (called the galleries) and facilitates polymer penetration into the galleries. Mineral clays obtained by the above process are called organoclays [13-15]. The thermal stability of organoclays is of particular importance for the processing of high melting temperature polymers. TGA was used to evaluate the thermal stability of MMT, derivative organoclays, and polystyrene nanocomposites incorporating these clays [16, 17]. The study revealed the influence of the modifiers on the thermal stability of MMT and the influences of the different clays and processing conditions on the thermal stability of the polymer matrix. Four clays were used as reinforcement for polystyrene: Cloisite Na+ (neat Montmorillonite) and Cloisite-10A (from Southern Clay Products, Gonzales USA), and Cloisite Na+ treated with two phosphonium organic modifiers (Organoclay A and Organoclay B). Cloisite-10A is a Na+-Montmorillonite treated with dimethyl-benzyl, hydrogenated tallow ammonium. Figures 8.5 and 8.6 show, the TGA and the derivative TGA traces, respectively, of MMT and the four organoclays used [16, 17]. A stable region is observed until above 110 °C. A mass loss of 2.5% occurs in the TGA trace of the neat MMT between 124 °C and 132 °C. The loss of mass between 124 °C and 132 °C was attributed to the loss of crystallisation water in the silicate stacks. The neat MMT is thermally stable between 150 °C and 550 °C. At 650 °C, a total mass loss of 4.4% was observed. Similar work by Chen [18] and coworkers showed a 2% mass loss below 250 °C and a total mass loss of 8% at 800 °C. They attributed the mass loss below 250 °C to the evaporation of the superficial water and of the water located between interlayers, while the mass loss at temperatures higher than 400 °C was attributed to the water formed from the structural –OH groups of the clay. The TGA trace of the neat MMT treated with dimethyl-benzyl, hydrogenated tallow ammonium (Cloisite-10A) indicates that this clay is stable until above 200 °C. It starts to degrade at 220 °C (onset of decomposition 239 ± 2.5 °C). It has been suggested that, upon heating, the ammonium cation in the clay is substituted by a proton, via the Hoffmann elimination reaction [19, 20]. The total organic content of Cloisite-10A is 34.4%. The TGA trace of Cloisite-10A does not show any mass loss around 100 °C, suggesting that no free moisture was present in this sample. The low thermal stability of this organoclay suggests that it could be lost from the galleries, if the material is processed at high temperature. In such an event, the d-spacing will decrease and the galleries could collapse, thus reducing the degree of intercalation. Figure 8.6 for Cloisite-10A shows a peak between 240 °C and 300 °C, corresponding to 29% modifier decomposition and a second peak between 300 °C and 390 °C, corresponding to 30% modifier decomposition. The total decomposition at 390 °C is 59%. Similar results were reported by Doh and co-workers [21]. They studied the kinetics of thermal decomposition of MMT treated with dimethyl-benzyl, hydrogenated tallow ammonium, and they found in the TGA derivative trace a small peak around 270 °C and a larger peak around 320 °C, indicating that the maximum thermal degradation occurs near the latter temperature. 283
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Figure 8.5 TGA traces of montmorillonite, Cloisite10-A and organoclays A and B [16, 17] Reproduced with permission from M.R. Kamal in Proceedings of the Polymer Nanocomposites Workshop, 3rd International Symposium on Polymer Nanocomposites Science and Technology, Boucherville, Quebec, Canada, 2005, p.69. ©2005, National Research Council of Canada
Figure 8.6 Derivative TGA traces of montmorillonite, Cloisite10-A and organoclays A and B [16, 17] Reproduced with permission from M.R. Kamal in Proceedings of the Polymer Nanocomposites Workshop, 3rd International Symposium on Polymer Nanocomposites Science and Technology, Boucherville, Quebec, Canada, 2005, p.69. ©2005, National Research Council of Canada
284
Thermal Characterisation Of Polymer Nanocomposites The TGA traces of the phosphonium-based organoclays A and B are shifted to higher temperatures, suggesting that they are more thermally stable than the ammonium treated MMT (Cloisite A-10). The total organic content is 25.0% for A and 34.4% for B. The onset decomposition temperatures for these clays are 340 ± 1.9 °C and 342 ± 2.6 °C, respectively. The TGA curves show fast decomposition of organoclay B (decrease of mass between 320 °C and 440 °C) and a slower decomposition of organoclay A (between 330 °C and 580 °C). Xiao and co-workers [22] compared a cetyl pyridinium chloride modified montmorillonite (CPC-OMT) with a hexadecyl trimethyl ammonium chloride modified montmorillonite (C16-OMT) and a neat montmorillonite. Figure 8.7 shows the TGA traces of the three fillers. Neat MMT showed a stable region up to 100 °C, a mass loss between 120 °C and 150 °C, and a thermally stable region between 150 °C and 550 °C. The onset decomposition temperature of C16-OMT was 218 °C, while it was 242.7 °C for CPCOMT. These results indicate that CPC-OMT clay has better thermal stability than C16OMT, probably due to the higher thermal stability of the pyridinium cations, which have an aromatic ring structure.
Figure 8.7 TGA curves of pure MMT and of C16-OMT and CPC-OMT [22] Reproduced with permission from J. Xiao, Y. Hu, Z. Wang, Y. Tang, Z. Chen and W. Fan, European Polymer Journal, 2005, 41, 5, 1030. ©2005, Elsevier
8.2.5.2 Thermal Stability of Nanocomposites [16, 17, 19, 21-31] Figure 8.8 shows the TGA traces for a polystyrene resin and for nanocomposites incorporating the same resin with unmodified MMT, for clay concentrations in the range of 0 to 10 %wt [16, 17]. The unfilled PS starts to decompose around 400 °C, while the decomposition of the nanocomposites starts at higher temperatures. These 285
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Figure 8.8 TGA traces of a pure PS and its nanocomposites with Montmorillonite [16, 17] Reproduced with permission from M.R. Kamal in Proceedings of the Polymer Nanocomposites Workshop, 3rd International Symposium on Polymer Manaocomposites Science and Technology, Boucherville, Quebec, Canada, 2005, p.69. ©2005, National Research Council of Canada
results suggest that the thermal stability of PS is improved by the addition of the neat MMT. Higher concentrations of the neat MMT in the nanocomposites produce higher thermal stability. Figure 8.9 shows the TGA thermograms for nanocomposites based on the same PS resin with Cloisite-10A. The same processing conditions and the same filler concentrations were used as for the neat MMT. As in the case of MMT-nanocomposites, the TGA traces of the nanocomposites were shifted toward higher temperatures, as the concentration of the organoclay was increased. However, it seems that the improvement in thermal stability becomes smaller for organoclay concentrations above 5%wt. Comparison between Figures 8.8 and 8.9 shows that the nanocomposites with the neat MMT exhibit better thermal stability than the nanocomposites based on the organoclay. For the latter, the decomposition starts at lower temperatures and the rates of decomposition are higher than those for MMT nanocomposites. The thermal degradation of dimethyl-benzyl hydrogenated tallow ammonium modifier leads to the diffusion of the decomposition products from the galleries, which might cause the lowering of the onset decomposition temperature of the polymer matrix. This has been attributed to the Hoffmann elimination reaction of the polystyrene with the degradation products of the organic modifier at elevated temperature [19, 20]. 286
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Figure 8.9 TGA traces of PS/Cloisite10-A nanocomposites [16, 17] Reproduced with permission from M.R. Kamal in Proceedings of the Polymer Nanocomposites Workshop, 3rd International Symposium on Polymer Manaocomposites Science and Technology, Boucherville, Quebec, Canada, 2005, p.69. ©2005, National Research Council of Canada
Xiao and co-workers [22] prepared composites based on polybutylene terephthalate (PBT) and the three fillers: MMT, C16-OMT (hexadecyl trimethyl ammonium chloride modified montmorillonite) and CPC-OMT (pyridium chloride modified montmorillonite). The concentration of clay in the composites was 3%wt. Table 8.1 shows the 3%wt loss temperature (T3%), the maximum decomposition temperature (Tmax), and the burn residue at 600 °C, for the pure polymer and the nanocomposites. The addition of the two organoclays causes a lowering of the thermal stability (T3% and Tmax) of the pure polymer. This decrease is larger for the composite with C16-MMT and is very small for the composite with CPC-OMT. The burn residue at 600 °C is lower for the composite with C16-MMT than for the pure polymer, but is higher for the composite with cetyl pyridium chloride modified montmorillonite than for the pure polymer. The above results indicate that the thermal stability of PBT was not improved by the addition of the organoclays. The authors suggest that the absorbed water in clay layers and
Table 8.1 Thermal properties of pure PBT and of the PBT nanocomposites [22] Sample Pure PBT PBT + C16-OMT PBT + CPC-OMT
T3% (°C) 371.0 366.5 370.0
Tmax (°C) 407.9 404.2 407.0
Residue at 600 °C 5.27 2.03 7.05
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Thermal Analysis of Rubbers and Rubbery Materials the hydroxyl groups of the montmorillonite silicates may accelerate the decomposition of the polyester. Also, the Hoffmann elimination reaction of the alkyl ammonium cations accelerates the degradation of PBT at elevated temperature. Thus, the burn residue at 600 °C in C16-MMT is smaller than in pure PBT [22]. The composite based on the cetyl pyridium chloride modified clay has higher thermal stability than the composite based on the C16-MMT modified clay, due to the higher thermal stability of CPC-MMT [22]. XRD results showed that PBT cannot diffuse into the galleries of neat MMT, but it penetrates the galleries of both C16-OMT and CPC-OMT, leading to intercalated and exfoliated/intercalated structures of the nanocomposites, respectively. Studies of degradation kinetics may be facilitated by conducting scanning TGA experiments at different scanning rates. As mentioned earlier, non-isothermal scanning TGA results depend on the heating rate. Generally, faster heating rates are associated with a shift of the mass loss to higher temperatures, as shown in Figure 8.10 [31]. Thus, if different samples are compared, they must be scanned at the same rate. Otherwise misleading conclusions might be reached regarding the comparative thermal stability of the materials.
Figure 8.10 Thermal stability of a PS resin as a function of the heating rate [31] Reproduced with permission from M.R. Kamal and J. Uribe. ©M.R. Kamal and J. Uribe
8.2.5.3 Reproducibility and Sample Uniformity An important application of TGA measurements is for the evaluation of the uniformity of nanocomposite samples [16, 17, 32]. Figure 8.11 shows the results of five separate TGA experiments on different specimens taken from a polystyrene sample and similarly 288
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Figure 8.11 Reproducibility of TGA experiments and sample uniformity [16, 17] Reproduced with permission from M.R. Kamal in Proceedings of the Polymer Nanocomposites Workshop, 3rd International Symposium on Polymer Nanocomposites Science and Technology, Boucherville, Quebec, Canada, 2005, p.69. ©2005, National Research Council of Canada
eight experiments on different specimens taken from a nanocomposite based on the same polystyrene resin. Such experiments are used to check the reproducibility of the experiments and the uniformity of the materials. In both cases, the results show good reproducibility and sample uniformity.
8.2.5.4 Environmental Reactions of Nanocomposites During processing in a number of applications, nanocomposites are exposed to gases and chemicals at high temperature. Under these conditions, reactions might occur that would influence the performance characteristics of the material. Thus, it is important in such situations, to conduct TGA experiments under environmental conditions comparable to those encountered during processing or in the application of interest [14, 32, 33]. Bertini and co-workers [33] prepared nanocomposites based on a commercial montmorillonite modified with octadecyl-ammonium ions (Nanofil 848 from SudChemie, Moosburg Germany). They used two isotactic PP homopolymers (iPP) with different rheological properties, and a maleic anhydride grafted PP. As expected, the material behaves differently under the two environmental conditions. Such studies provide useful information regarding the reactions that might occur at high temperature and the kinetics of these reactions. 289
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8.3 Differential Scanning Calorimetry (DSC) DSC measures temperatures and heat flows associated with thermal transitions in a material. The DSC is used in the investigation, selection, comparison and end-use performance evaluation of materials. Properties measured by DSC techniques include glass transitions, ‘cold’ crystallisation, phase changes, melting, crystallisation, product stability, cure/ cure kinetics, and oxidative stability.
8.3.1 Conventional DSC Calorimetry is a technique which measures the amount of heat evolved or absorbed by a substance (sample) undergoing a physical or chemical change. DSC is a technique which measures the difference in heat flow, as a function of temperature or time, between a sample which undergoes a physical or chemical transition and an inert reference which does not experience any transition over the temperature range of the experiment. The possible transitions induced by heat in an organic polymer are: glass transition, melting, crystallisation, volatilisation, and thermally-induced reactions, such as crosslinking, oxidation, decomposition, etc. During the experiment under controlled heating or cooling, a polymer sample can experience one or more of the above transitions, depending on chemical structure, thermal history, the temperature range of the experiment, the heating/cooling rate of the experiment, etc. Two kinds of experiments may be conducted using the DSC: isothermal experiments which yield results in the form of heat flow rate as a function of time, and non-isothermal experiments, with output in the form of heat flow rate as a function of temperature or time. The latter are referred to as scanning experiments. Commonly for polymers, the temperature range varies from -150 ºC to 500 ºC and the heating/cooling rates vary from 0.25 ºC/min to 100 ºC/min. However, these ranges may be extended, depending on the material and application. Unless the experiment requires a reactive medium, DSC measurements are usually performed under an inert atmosphere (a purge inert gas), in order to avoid chemical reactions or sample degradation. In a typical DSC experiment, the specimen and a reference, in two identical separate pans, are kept at the same temperature, either during an isothermal experiment or a temperature scanning experiment (heating or cooling). The difference in the amounts of heat received by the reference and the specimen is recorded continuously as a function of time, while maintaining the temperatures of the specimen and the reference identical. If the thermal properties (specific heat) of the reference are known, then the thermal properties (specific heat, glass transition temperature, melting point, onset of reaction, etc.) and/or the amount of heat release during thermal events (heats of melting, solidification, reaction, etc.) may be identified and quantified. If during the heating, chemical or phase changes occur, the change in enthalpy measured with the calorimeter, shows a displacement of the heat flow, Q* (W/g), from the baseline, which 290
Thermal Characterisation Of Polymer Nanocomposites appears on the thermogram as an exothermic or an endothermic peak, proportional to the amount of heat evolved or absorbed by the sample. As a result, it is possible to estimate the rates of reaction or phase change and the rate constants, in both isothermal and non-isothermal experiments. According to ASTM E794-06 [34] and D 3417-97 [35], by convention, endothermic events are considered as being negative (below the base line) and the exothermic events are represented as being positive (above the base line). A change of slope is observed at the glass transition temperature. In DSC experiments, the specific heat is determined from the slope of the heat flow rate versus temperature scanning rate curve, during a scanning experiment. Cp
Q* = (dQ/dt) /m (dT/dt) mv
(8.2)
*
Q v Cp m
(8.3)
where Q* = (dQ/dt) is the rate of differential heat input (output), v is the rate of temperature scanning (heating/cooling), m is the mass of the specimen, and Cp is the heat capacity (specific heat) at constant pressure. The amount of heat associated with a given transition is measured by calculating the area under the corresponding peak from the onset to the end of the transition (melting or crystallisation). The glass transition temperature is associated with a change in specific heat, as indicated by an inflection point in the heat flow curve. It should be noted that the results of DSC experiments are affected significantly by the scanning rate and the sample mass.
8.3.2 The Apparatus
8.3.2.1 Heat Flux DSC In a heat flux DSC, only one furnace is used to heat both the sample and the reference. The sample, enclosed in a pan and an empty reference pan are placed on a thermoelectric disk. The furnace is heated at a linear heating rate and the heat is transferred to the sample and reference pans through the thermoelectric disk. If the sample and the reference respond to the temperature program in the same way, their heat flows remain constant, and the difference between the two points of measurement is also constant. If at a certain temperature, the sample responds differently, e.g., it melts, the heat provided by the instrument is used for this phase transformation and even if the instrument continually provides the same amount of heat, the sample temperature remains constant during the melting. The DSC instrument will add enough heat in order to completely melt the sample. During the melting, the temperature of the sample is lower than the temperature 291
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Figure 8.12 Schematic representation of a heat-flux DSC
of the reference, which continues to heat up uniformly. The difference between the two temperatures, T, generates the change in the heat flow recorded, Q*. Figure 8.12 [36] shows schematically a heat-flux DSC instrument. The advantages of the heat flux DSC are: robustness, ease of handling, and straightforward measurement. The heating curves have a stable baseline, and they allow easy measurement of the glass transition temperature (Tg). The Tzero cell is a modified heat flux DSC. This instrument incorporates a third thermocouple located on the heat flow plate, so as to account for temperature gradients there. In this case, the heat flow equation includes several terms, which incorporate individual time constants and thermal resistances of the reference and sample sensors.
8.3.2.2 Power-Compensation DSC In power compensated calorimeters, Figure 8.13 [36], the sample and the reference are located in separate furnaces, which are heated by separate heaters. Both, the sample and the reference, are maintained at the same temperature and the difference in thermal power required to maintain them at the same temperature is measured and plotted as a function of temperature or time. If an endothermic (or exothermic) event occurs in the sample, the instrument will supply to or remove heat from the sample furnace, in order to maintain both the sample and the reference at the same temperature. The energy (power) compensation is recorded by the instrument. Because the furnaces are small, they need 292
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Figure 8.13 Schematic representation of a power-compensation DSC
little time to compensate for the small energy difference. Thus, power-compensation DSC is used for measuring very rapid events, such as fast reactions. The quality of different DSC instruments is evaluated according to the following two criteria: ‘sensitivity’ and ‘time constant’. (i) Sensitivity, governed by noise, is represented by the smallest detectable signal recorded. It is determined by recording a baseline with or without a specimen. Sensitivity is usually expressed as effective noise in W. (ii) Time constant is the time required by the instrument to compensate for the difference in temperature between the sample and the reference. It is given in seconds. The time constant is a function of the thermal conductivity of the heating system. The time constant of the power-compensation DSC is smaller, because of the smaller size of the furnaces. The following factors influence DSC results: sample mass, heating rate, the choice and the preparation of the reference, specimen preparation, the starting and the end temperatures of the DSC experiment and the type of the purge flow.
8.3.3 Procedure
8.3.3.1 Instrument Calibration Standard reference materials are used for both the temperature and the heat flow calibration of the DSC instrument. The calibration should be carried out under the 293
Thermal Analysis of Rubbers and Rubbery Materials same experimental conditions as for the proposed experiment, i.e., the same heating rate and the same atmosphere. ASTM E967-97 [37] lists a number of materials, which may be employed for temperature calibration. Generally, metals with purity 99.99% and known melting temperature are used for calibration. The calibration process involves heating of the standard material at a controlled rate, in a controlled atmosphere, over a region of its thermal transition (melting). The recorded melting temperature is compared with the accepted value. If they are different, the instrument reading is adjusted to the accepted value. At least two standard reference materials, whose melting temperatures span the sample transition interval, should be used to calibrate the instrument. The calibration is generally performed in the heating mode, in order to avoid complications due to supercooling. High purity standard reference materials, with known enthalpy of melting, such as indium, tin or zinc, may be used for heat calibration. The melting endotherm of the standard material is recorded as a function of time. The peak is integrated over time to yield the area measurement proportional to the enthalpy of melting of the standard material. The instrument calibration coefficient is calculated with the equation: E
Hm ABS
(8.4)
E = calibration coefficient at the temperature of the melting endotherm H = enthalpy of fusion of the standard material m = mass of the standard A = melting endotherm peak area B = recorder time scale sensitivity, s/cm S = recorder heat flow scale sensitivity The proportionality factor (the instrument calibration coefficient) is determined as a function of temperature. The temperature and enthalpy of melting of the reference materials should be known. Another method for heat flow calibration employs the specific heat of sapphire ( -alumina), which is known as a function of temperature. Two experiments are performed: one with two empty pans and another with sapphire as a reference material. The difference between the two results is multiplied by the calibration factor, which, in this case, is the Cp of sapphire, to obtain the true heat flow difference. The Cp of sapphire changes with temperature. For this reason, the measurements must be performed over narrow temperature ranges. At least two measurements have to be performed for the heat flow calibration by this method. 294
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8.3.3.2 Sample Preparation During sample preparation, special attention must be paid to avoid specimen damage, deformation, or heating. The optimum sample mass should take into account the desired objective of the measurement: glass transition temperature, melting and/or crystallisation, or specific heat. Generally, for DSC measurements, the mass of the sample is between 10 and 40 mg. Larger samples risk the possibility of significant gradients of temperature and composition. Aluminium pans are the most commonly used DSC pans. They should be covered with lids, which are usually sealed with a special press. It is important to ensure that the contact between the sample and the pan is very good. Large contact area is desirable, in order to have uniform heat transfer. Accordingly, different pan shapes are employed for liquid or solid samples. Usually, the reference is an empty pan covered with a lid and sealed with a press.
8.3.3.3 Methodology In order to avoid reactions with the environment, the DSC chamber is purged with an inert gas during the experiment. Typical gases include nitrogen, argon, or helium. Gas flow must be not too high, in order to avoid the modification of the temperature in the furnace. Yet, it must be not too low, in order to ensure an inert atmosphere in the furnace. The flow rate is usually around 50 ml/min. The starting temperature should be at least 50 °C below the first expected transition. The specimen should be kept at the starting temperature long enough to establish that equilibrium is reached. Ideally, scanning measurements should be made at different heating (cooling) rates in order to select a practical rate that provides results that are independent of the heating (cooling) rate. The heating rate used for determining melting and crystallisation parameters is usually 10 °C/min, while a heating rate of 20 °C/min is employed for determining the glass transition temperature. If all these transitions are determined during the same experiment, a heating/cooling rate of 10 °C/min is used. Sometimes, for semi-crystalline thermoplastics, especially those with higher crystallinity, the Tg is less evident, because of the presence of an unmelted crystalline phase, characterised by low mobility of the chains. In this case, a heating rate higher than 20 °C/min-1 should be employed to determine Tg. However, the heating rate should not surpass 40 °C/min-1. In isothermal measurements, the specimen must be rapidly heated or cooled to the desired temperature, but the rate must not be higher than 50 °C/min. For accurate determinations, the end temperature should be ~50 °C higher than the transition temperature. Usually, the cooling and heating rates used in the analysis of a given sample in the same experiment are the same. The first heating scan (the first run) reveals information about the current condition of the sample (processing influences, crystallinity and degree of cure, service temperature, etc.). At the end of the first run, the chemical and/or physical structure of the specimen 295
Thermal Analysis of Rubbers and Rubbery Materials would be modified, as a result of the heating and cooling cycles. Sometimes, at the end of the first heating run, the melt is quenched to temperatures below Tg. It freezes in a vitreous state, and subsequently a second heating scan is employed. The second heating scan provides information regarding the characteristic properties of the specimen in the new state.
8.3.4 Typical Data
8.3.4.1 Glass Transition Temperature, Tg The glass transition temperature is the temperature at which an amorphous polymer or the amorphous domain of a semicrystalline thermoplastic passes from a glassy state to a rubbery state, during heating. At Tg, the mobility of the chain segments is modified, and a step-like change occurs in the magnitude of specific heat (Cp) and other properties, such as thermal expansion coefficient, refractive index, and mechanical properties. Tg depends on the morphology of the polymer, which is affected by the processing conditions and the thermal history of the polymer. For this reason, Tg is determined in the second heating. The transition at Tg is a relaxation and not a phase transition, and it occurs over a certain interval, called the glass transition range. Figure 8.14 shows a typical glass transition, and the temperatures related to it. The temperature labels are according to ASTM E1356-98 [38]. According to some conventions, Tg is defined as the temperature
Figure 8.14 Typical glass transition range [38] Reproduced with permission from ASTM E1356-03. ©2003, ASTM
296
Thermal Characterisation Of Polymer Nanocomposites at which half of the change in Cp has occurred. However, various researchers have used one or another of the transition points indicated in Figure 8.14. The actual value of Tg is somewhere between. T0, the temperature of the 1st deviation from the baseline, and Tr, the temperature of return to the baseline. Various researchers use as Tg one or the other among the following points: Tf, the extrapolated onset temperature; Tm, the midpoint temperature; Ti, the inflection temperature; or Te, the extrapolated end temperature. Commercial DSC instruments are equipped with software which allows the estimation of Tg. The determination of Tg is considered correct if the difference between the values obtained in 2 or 3 experiments are within 1 °C.
8.3.4.2 Melting and Crystallisation Melting is an endothermic physical process, which marks the change from a solid crystalline state into an amorphous liquid state. This change does not involve a loss of mass. Semi-crystalline polymers melt over a temperature range, which is mainly governed by the polymer structure. Melting depends on the thermal and mechanical history of the specimen. The appropriate choice of the heating rate is essential in DSC experiments. Very low heating rates could promote crystal reorganisation or recrystallisation of the polymer. A typical melting curve for a semi-crystalline polymer is shown in Figure 8.15. The melting range is between T´im, the onset temperature, and T´em, the end temperature The melting temperature is usually considered to be either the melting peak temperature (Tpm) or the extrapolated onset melting temperature (Tim).
Figure 8.15 A typical melting curve of a semicrystalline polymer
297
Thermal Analysis of Rubbers and Rubbery Materials The equilibrium melting temperature (T0m) is the melting point of a perfect, infinitely large crystal, for which the melting temperature and the crystallisation temperature are equal. T0m is also known as the thermodynamic equilibrium melting point. In order to determine this temperature by DSC, the sample is melted and cooled very fast from the melt to a certain isothermal crystallisation temperature, T c, where the sample is held until crystallisation is complete. The melting point, Tm, of the resulting material is determined by heating the sample. The measurements are repeated for several isothermal crystallisation temperatures. Finally, the Tm values evaluated in each experiment are plotted against the crystallisation temperature T c. A linear relationship is obtained. The line giving Tm = f(T c) is extrapolated until it intersects a line drawn through the origin corresponding to Tm = T c. The intersection point is T0m. The heat of fusion is the energy needed to melt the crystalline fraction of the sample. The enthalpy change H corresponding to melting is equal to the heat of fusion, Hm. It is calculated from the area bounded by the curve and the line connecting T´im and T´em, in Figure 8.15. The crystalline fraction of the polymer is calculated by dividing the experimental value of the heat of fusion of the polymer, Hm, by the heat of fusion of a 100% crystalline material, H0m. H0m values for different polymers can be found in the literature: wc %
Hm 100 H 0m
(8.5)
In DSC, the crystallisation curve (exothermic curve) shows the change in enthalpy which occurs, when a melt (liquid) material is transformed into a crystalline solid, during cooling. Similar treatment of DSC crystallisation curves to the treatment of melting curves yields data on the heat of crystallisation and the fraction crystallinity obtained during cooling.
8.3.4.3 Specific Heat Specific heat (Cp) represents the quantity of energy needed to raise the temperature of a unit of mass of the sample by 1 °C. The first step, the calibration, consists of measuring the difference in heat flow between an empty pan and the reference material, which is usually sapphire (Q*empty pan). The second step is to measure the difference in the heat flow between an empty pan and a pan containing the specimen (Q*sample) [39]. The specific heat is calculated by: Q* sample Q* Cp
empty pan
mv
m = specimen mass (g) v = heating rate (°C/min) 298
(8.6)
Thermal Characterisation Of Polymer Nanocomposites The above assumes that no chemical reaction or physical change occurs in the sample.
8.3.4.4 Chemical Reactions Exothermic or endothermic curves, similar to the curves recorded for melting and crystallisation, are obtained in the case of chemical reactions. The heat of reaction, r is given by the area bounded by the curve and the line which connects the onset temperature with the end temperature. Care must be taken in the analysis of data, if mass changes occur during the chemical reaction.
8.3.5 Temperature-Modulated DSC (TMDSC) TMDSC is a variant of DSC, which overlays the isothermal temperature or the linear heating rate of the conventional DSC with a sinusoidal temperature modulation of one frequency [40-52]. The modulated temperature superimposed on the linear heating rate gives a modulated heating rate (as indicated in Figure 8.16). The average heating rate is a straight line similar to that obtained in the conventional DSC. The heating rate amplitude may be represented by the equation: A t v Amod
2 2 cos P P
(8.7)
Figure 8.16 Modulated temperature
299
Thermal Analysis of Rubbers and Rubbery Materials A(t) is the heating rate amplitude; P is the period of the signal; v is the linear heating rate; t is time; and Amod is the modulation amplitude. A variant of TMDSC technique is based on a multi-frequency (broad band of frequencies) temperature modulation [53, 54]. TMDSC methods allow the separation of temperatureand time-dependent processes. The modulation of temperature is a relatively fast changing signal, compared to the heating ramp which is slow. The output of TMDSC consists of three plots of heat flow versus temperature: (i) overall heat flow, (ii) reversing flow and (iii) non-reversing flow, as depicted in Figure 8.17 [54]. The reversing flow signal corresponds to the process that can be reproduced by repeated heating. It is a function of the heating rate and heat capacity. The irreversible flow signal corresponds to the process that cannot be reproduced by repeated heating. The kinetics of thermal processes, which depend on time and absolute temperature, often appear in non-reversing heat flow, while the specific heat appears in reversing heat flow. The specific heat may be calculated using the equation: Cp K
Amod H Amod v
(8.8)
Figure 8.17 TMDSC experimental curves of an SBR elastomer with oil [54] Reproduced with permission from N.K. Borse, M.R. Kamal and V. Mollett in Proceedings of the 61st SPE Annual Conference – ANTEC 2003, Nashville, TN, USA, 2003, p.2260. ©2003, SPE
300
Thermal Characterisation Of Polymer Nanocomposites where K is the calibration factor; Amod H is the amplitude of the modulated heat flow; and Amod v is the amplitude of the modulated heating rate (C/min). The specific heat multiplied by the mean heating rate gives the reversible heat flux. The irreversible heat flow may be obtained by subtracting the reversible heat flow from the total heat flow. TMDSC allows the separation of processes that overlap or occur in rapid succession, to distinguish between reversible and irreversible transitions. Thus, it measures heat capacity effects simultaneously with the kinetic effects. The sensitivity and the resolution of TMDSC is higher than that of conventional DSC. The high temporal resolution of TMDSC is given by the slow linear increase of temperature. Its high sensitivity is due to the rapid periodic change in the heating rate. The typical parameters of TMDSC experiments are: heating rate from isothermal to 5 °C/min, modulation amplitude from 0.01 to 10 °C and a modulation period that can vary from 10 to 100 seconds. TMDSC has found applications in the separation between glass transition, melting and thermally induced cross-linking in one experiment. TMDSC also allows the easy determination of Cp from the reversing signal and the relaxation enthalpy from the non-reversing signal.
8.3.6 Applications of DSC for Thermal Characterisation of Polymer Nanocomposites The use of DSC yields information about the thermal history of polymers, annealing, polymer crystallisation behaviour and kinetics, nucleation, polymorphism, crosslinking, curing of thermosets, ageing, heat of vapourisation, polymer blends, etc. Xiao and co-workers [22] used the DSC to follow the phase changes (melting and crystallisation) of pure polybutylene terephthalate (PBT) and of its composites with montmorillonite (MMT) (PBT1), hexadecyl trimethyl ammonium chloride modified montmorillonite (C 16-OMT) (PBT2) and with cetyl pyridium chloride modified montmorillonite (CPC-OMT) (PBT3). Both DSC heating and cooling measurements were performed at of 10 °C/min. Figure 8.18 and Figure 8.19 show the melting and crystallisation curves, respectively, for the pure polymer and the two nanocomposites in the range 100 °C to 250 °C for heating and 150 °C to 250 °C for cooling. During heating, the DSC curve of the pure PBT shows multiple melting peaks. These peaks are likely due to the melting-recrystallisation process during heating. Small melting peaks were also observed for the composites. The cooling results show that the crystallisation temperatures and the crystallisation peak areas are larger for the two nanocomposites than for the pure polymer. The increase of the crystallisation rate and the crystallinity of the polymer matrix, when the organoclays are used, suggest that the clays act as nucleating agents for crystallisation. Mehrabzadeh and Kamal [55] have shown, using DSC measurements that the presence of clay in polyamide 6 (PA6) promotes the formation of the -phase during the crystallisation 301
Thermal Analysis of Rubbers and Rubbery Materials
Figure 8.18 DSC thermograms recorded during heating with a rate of 10 °C/min;
(a) PBT; (b) PBT2; (c) PBT3 [22] Reproduced with permission from J. Xiao, Y. Hu, Z. Wang, Y. Tang, Z. Chen and W. Fan, European Polymer Journal, 2005, 41, 5, 1030. ©2005, Elsevier
Figure 8.19 DSC curve during cooling; 10 °C/min cooling rate; (a) PBT; (b) PBT2;
(c) PBT3 [22] Reproduced with permission from J. Xiao, Y. Hu, Z. Wang, Y. Tang, Z. Chen and W. Fan, European Polymer Journal, 2005, 41, 5, 1030. ©2005, Elsevier
302
Thermal Characterisation Of Polymer Nanocomposites of these systems. The results, shown in Table 8.2 and Figure 8.20 were supported by FTIR and wide-angle X-ray diffraction (WAXD) and dilatometry measurements [56]. This effect was not observed in high-density polyethylene (HDPE) composites. It is interesting to note that PA6,6 did not seem to show a similar transformation to the -phase in the presence of clay [57]. The nucleating role of the silicate organoclays in the crystallisation of polymer matrices was also reported in a number of studies by Kamal and co-workers [57-59]. The DSC crystallisation kinetics data for the pure polymers and their composites followed the Avrami equation in the early stages of crystallisation:
Figure 8.20 DSC traces of heating scans for PA6, PA6/clay (5 wt%) and PA6/clay (10 wt%) [55] Reproduced with permission from M. Mehrabzadeh and M.R. Kamal, Canadian Journal of Chemical Engineering, 2002, 80, 6, 1083. ©2002, Canadian Society for Chemical Engineering
Table 8.2 Tm, Tc and onset of Tc for PA6 and HDPE and nanocomposites [55] Sample PA6 PA6/clay (5 wt%) PA6/clay (10 wt%) HDPE HDPE/clay (5 wt%) HDPE/clay (10 wt%)
Tm (C)
220 210 219 211 219 211 131.0 130.4 130.4
Tc (C)
Onset (C)
Crystallinity (%)
187.8 185.3 182.8 116.2 116.9 117.2
191.0 189.3 186.0 118.5 119.3 119.3
38.1 35.8 38.1 66.4 66.4 65.7
303
Thermal Analysis of Rubbers and Rubbery Materials 1 X exp( Kt n )
(8.9)
where K, n and X are the Avrami rate parameter, the Avrami exponent, and the relative crystallinity, respectively. The crystallisation kinetics data were plotted as ln [-ln (1X)] against ln t. Figure 8.21 shows such plot for the crystallisation of PA6,6 and its nanocomposite (PA6,6NC) at atmospheric pressure by differential scanning calorimetry for different experimental crystallisation temperatures. The data at different temperatures are fitted to straight lines. The slopes give the values of the Avrami exponent n and the intercepts on the y-axis give the values of the Avrami rate parameter K. The Avrami crystallisation rate constants for PA6,6 and its nanocomposites PA6,6NC with 3.0 wt% Cloisite 30B were 2.38 x 10-5 and 1.32 x 10-3, respectively. This indicates the important role of the nucleating agent. The Avrami rate constants for PA6 and PA6 NC, with the same organoclay, were not very different, implying that the clay did not influence the rate of crystallisation. However, microscopic examination suggested that the clay acted as a nucleating agent for both PA6 and PA6,6. Bertini and co-workers [33] used DSC to study the isothermal and non-isothermal crystallisation kinetics of three PP resins: two isotactic (iPP) homopolymers (Q and R) and a maleic anhydride grafted PP (G), in addition to composites with montmorillonite modified with octadecyl-ammonium ions (N). The half time of crystallisation, t0.5, defined as the time required to reach 50% of complete crystallisation was reported for
Figure 8.21 Crystallisation kinetics of PA66 and PA66NC at atmospheric pressure by DSC: PA66: ( ) 237.5 °C, ( ) 235 °C, PA66NC: ( ) 237.5 °C, ( ) 245 °C [57] Reproduced with permission from N.K. Borse, M.R. Kamal and S. Hasni in Proceedings of the 61st SPE Annual Conference - ANTEC 2003, Nashville, TN, USA, 2003, p.1413. ©2003, SPE
304
Thermal Characterisation Of Polymer Nanocomposites different crystallisation temperatures (T c). For the non-isothermal crystallisation tests, the samples were heated to 200 °C and held for 3 minutes, then cooled at various cooling rates ranging from 5 to 30 °C min. The results are presented in Table 8.3. Non-isothermal crystallisation data were analysed using the method proposed by Khanna [60]. The crystallisation rate constant (CRC) values were similar for the pure polymers R and G and for their nanocomposites RN and GN. The CRC value for the composite QN (155 h) was higher than the CRC value for the pure polymer Q (134 h). Nowacki and co-workers [61] used DSC to study the isothermal and non-isothermal crystallisation of nanocomposites based on a ternary mixture of iPP (PPF), compatibiliser, and montmorillonite modified with octadecyl amine (M). The compatibiliser (PB1) was iPP grafted with 1% maleic anhydride (MA). Two concentrations of PB1 were used: 20 wt% and 33 wt%. Organoclay (M) concentrations in the composites were 3, 6 and 10% (M3, M6, M10). Crystallisation was carried out at 128, 132 and 135 °C. The isothermal crystallisation at each temperature was also monitored, using a video camera. Non-isothermal crystallisation was conducted by cooling from 220 °C to room temperature at 5, 10 and 20 °C/min. All experiments were carried under nitrogen. The addition of the compatibiliser to the iPP resulted in slowing down the crystallisation. Nanocomposites with 3% MMT showed slower crystallisation than pure iPP, but faster than the compatibiliser/iPP blends. Nanocomposites having 6% and 10% MMT crystallised faster, at a rate similar to that of iPP. Similar behaviour was observed for isothermal crystallisation at 128 and 132 °C. Table 8.4 gives the DSC data recorded during the non-isothermal crystallisation of the various compositions. The crystallisation peaks for all the samples were shifted to lower temperatures, as the cooling rates increased. For the same cooling rate, the crystallisation temperatures of the composites
Table 8.3 Crystallisation and melting data of the PP samples and the nanocomposites [33] Tc (°C)
Pure polymers Q
t0.5 (min)
Tm (°C)
16.9 24.3 29.2
163.8 166.5 167.2
128 129 130
R
7.5 9.3 11.5
129 131 135
G
4.6 7.5 13.0
127 128 129
Nanocomposites QN
t0.5 (min)
Tm (°C)
5.9 7.9 9.8
160.5 161.2 161.5
165.5 165.6 166.2
RN
8.9 10.8 12.8
157.1 157.3 158.5
165.0 166.6 167.6
GN
5.2 7.7 14.9
156.8 158.1 158.5
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Thermal Analysis of Rubbers and Rubbery Materials
Table 8.4 DSC crystallisation peaks of polypropylene blends and composites, at different cooling rates [61] Material PPF PPF/PB1/6 PPF/PB1/10 PPF/PB1/M3 PPF/PB1/M6 PPF/PB1/M10
Tc (°C) at 5 K/min 118.1 116.5 117.3 117.7 119.6 120.2
Tc (°C) at 10 K/min 115.4 113.6 114.2 115.0 117.0 118.0
Tc (°C) at 20 K/min 112.4 109.9 110.0 110.8 112.2 114.0
with 6% and 10% organoclay were slightly higher than those of the pure polymer, while the composite with 3% organoclay had a slightly lower crystallisation temperature. At the same rate, both crystallisation temperatures of the blends were lower than that of the pure polymer, and the crystallisation temperature of the blend with 20% modifier was lower than that of the blend with 33% modifier.
8.3.7 Applications of TMDSC for Thermal Characterisation of Polymer Nanocomposites [62-70] Privalko and co-workers [64] studied by TMDSC the melting behaviour of a neat polyamide 6 homopolymer (PA6) and of a series of commercial nanocomposites (PNC) containing up to 7.5 wt.% of exfoliated organoclay nanoparticles at three underlying heating rates and five modulation frequencies. Their results correlated well with wide angle X-ray scattering results. They indicated that both - and -crystal modifications of PA6 were formed during cooling from the melt in all samples. The melting endotherms obtained in the complex specific heat capacity (Cp*) versus temperature (T) plots showed a single maximum at the apparent melting point, Tm, for samples prepared by cooling at 20 K/min, whereas for samples prepared by cooling at 0.5 K/min, a subsidiary maximum at Tm
Thermal Characterisation Of Polymer Nanocomposites Using the same technique, TMDSC, Hu and Zhao [67] investigated the effects of annealing, solid-state annealing (190 °C) and melt-state annealing (230 °C and 250 °C) and of the thermal history on the polymorphic behaviour and thermal property of polyamide 6 (PA6)/layered-silicate nanocomposites (PA6LSN) and of pure PA6. The study indicated a similar polymorphic behaviour of the pristine PA6 and its nanocomposites, when they were annealed in the solid state for different time duration. However, significant differences were recorded for annealing at 230 °C and 250 °C. The -phase became absolutely dominant for PA6LSN, while the crystal was the most predominant phase in neat PA6. The new endothermic peak around 235 °C in the TMDSC scans of all composites was attributed to the melting of PA6 lamellae formed in the confined environment on the surface of the nano-silicate. Miltner and Mele [69] used MTDSC to separate reversible from irreversible processes during the isothermal crystallisation of several pure polymers and their nanocomposites, reinforced with unmodified and modified clay and carbon nanotubes. Their results are illustrated in Figure 8.22. The apparent heat capacity may be described by the equation: Cpapp(T,t) = Cpbase (T) + Cpexcess (T,t)
(8.10)
where Cpexcess is temperature- and time-dependent for polymers and changes with the progress of the transformation. According to the authors, the origin, magnitude and time-dependency of Cpexcess in crystallising nanocomposites are related to melting/ crystallisation processes in the interphase region between the polymer matrix and the inorganic reinforcement. Also, the value of Cpbase at a constant temperature during crystallisation depends on the isothermal crystallisation time t, due to the changing degree of crystallinity. The addition of the filler to the pure polymers caused a decrease of the magnitude of the signal for the polar matrices (PA6, EVA). No change was observed for the non-polar HDPE matrix. The authors suggest that the magnitude of the excess in the heat capacity can be used to quantify the polymer segmental mobility and interaction efficiency in the filler/polymer matrix interphase region. The mobility restriction in the vicinity of the filler was confirmed by quantitative MTDSC analysis of the amount of polymer taking part in the glass transition of PA6 or PVAc. Li and Ishida [70] used the TMDSC to study the heat capacity (Cp) and the glass transition behaviour of monodisperse polystyrene/montmorillonite nanocomposites. Their study indicated that the normalised Cp of polystyrene in the nanocomposites (Equation 8.11), which directly relates to the change of molecular mobility, strongly depends on the polymer concentration. The intercalated polystyrene does not contribute to Cp, indicating that intercalated polystyrene does not exhibit glass transition behaviour at the usual glass transition temperature. The glass transition temperature of the bulk portion of the polystyrene in nanocomposites is similar to that of pure polystyrene. 307
Thermal Analysis of Rubbers and Rubbery Materials
Figure 8.22 TMDSC for the quasi-isothermal crystallisation of PA6 (210 °C ± 0.5 °C/60 s) - Top; Apparent heat capacity measured during quasi-isothermal crystallisation of PA6 and different PA6/clay nanocomposites: (a) 1 vol%; (b) 5 vol%; (c) 10 vol% [69] H.E. Miltner and B.V. Mele in Proceedings of Polymer Nanocomposites 2005, the3rd International Symposium on Polymer Nanocomposites Science and Technology, Boucherville, QC, Canada, Paper 7.02. ©2005, National Research Council of Canada
308
Thermal Characterisation Of Polymer Nanocomposites A normalised Cp may be defined: C p,normalized
C p,bulkWbulk Cp,intercalatedWintercalated WPS
(8.11)
where Cp,normalised, Cp, bulk, and Cp,intercalated refer to the normalised Cp of the nanocomposites and the Cpof bulk and intercalated polystyrene, respectively; WPS, Wbulk, and Wintercalated refer to the weight of the total polystyrene, bulk polystyrene, and intercalated polystyrene in the nanocomposite, respectively. The amount of intercalated polystyrene was calculated for polystyrene concentrations larger than 0.4 with Cp = 1. Cp,normalised was the experimental result from TMDSC, while WPS and Wclay were obtained from TGA measurements. The authors suggested that the differences that are observed for the glass transition behaviour of polymers inside and outside the silicate layers may be due to the small amount of polymer that can intercalate into the silicate layers. In their study, the total amount of intercalated polystyrene was almost constant at 40 wt% of the clay weight, no matter how high the polystyrene concentration was.
8.4 Other Characterisation Techniques In this section, we provide a brief description of a few other important or new thermal characterisation techniques and their application to nanocomposites.
8.4.1 Thermal Conductivity Thermal conductivity, or , is a property of the material that determines the heat flux, that flows through a material of thickness d, when a temperature difference, T, exists across the material:
d T
(8.12)
Another important property of the material is thermal diffusivity , which is defined as:
cp
(8.13)
where is the density of the material and cp is its specific heat. Generally, the techniques used to determine thermal conductivity can be classified into steady-state and non-steady-state techniques. The advantage of steady-state techniques is the ease of measurement of and the analysis of the steady state (relatively constant) output signal. However, the measurements require a long time to reach steady state. 309
Thermal Analysis of Rubbers and Rubbery Materials The unsteady-state techniques can be made relatively quickly, during heating or cooling experiments. In this case, the calculation of thermal conductivity is more difficult. Several methods to determine thermal conductivity are listed in the ASTM procedures. The selection of measurement technique depends upon the type of material, the magnitude of the conductivity and the desired test temperature range. Liu and co-workers [71] measured the thermal conductivity of composites obtained by mixing unpurified carbon nanotubes with a commercial silicon elastomer. The unpurified CNT, (~ 90% purity), contained both single-walled nanotubes and multi-walled nanotubes. Loading was studied up to 4 wt%. For comparison purposes, silicon elastomer loaded with carbon black was also prepared. The thermal conductivities of the composites were measured with an apparatus similar to that described in ASTM D5470 [72]. The thermal conductivities () were found to increase with the carbon amount. A 65% enhancement in was obtained for 3.8 wt% CNT loading. The enhancement by equal loading of carbon black was found to be a little lower than that by the CNT loading. The authors also studied composites with chemically modified CNT (CNT treated in concentrated nitric acid) [73]. For composites with 2 wt% CNT, an enhancement of 70% in thermal conductivity, compared with that of the pristine polymer matrix, was obtained. Two other techniques that have been used to measure the thermal conductivity of polymer nanocomposites are: the 3 method and the microflash method. In the 3 method [74, 75] an ac electric current of the form I0sin t is fed in the sample or the metal strip. It creates a temperature fluctuation on the sample or the metal strip at the frequency 2 , and accordingly a resistance fluctuation at 2 . This further leads to a voltage fluctuation at 3 across the sample which is measured. Microflash instrumentation determines the thermal properties of the material by the laser flash method [76, . A single laser beam pulse uniformly impinges on one side of the sample. An infra red detector measures the temperature rise on the other side of the sample as a function of time. The thermal diffusivity is calculated by using the recorded temperature rise data and sample thickness. The magnitude of the temperature rise for the unknown sample is compared to that of a reference or calibration sample. The temperature of the sample back face is a function of several variables: sample geometry, thermal diffusivity, and heat loss from the sample. These variables are grouped into dimensionless parameters. Thermal diffusivity can be determined by comparing the measured data with the appropriate mathematical model. Putnam and co-workers [78] used the 3 method to study the thermal conductivity of nanocomposites, based on polymethyl methacrylate and 60 nm alumina nanoparticles, in the temperature range 40-280 K. The concentration of the particles was 10%wt, or 3.5% by volume. Below 100 K, the thermal conductivity of the nanocomposite decreased compared to the pure polymer, meanwhile above 100 K it increased slightly. At room temperature, a maximum increase of 4% was reported. The thermal conductivity 310
Thermal Characterisation Of Polymer Nanocomposites values of microcomposites with the same concentration of filler increased in the whole temperature range by 2 to 3% compared to the pure polymer. Kuriger and co-workers [79] reported the study of composites based on polypropylene reinforced with aligned vapour grown carbon nanoscale fibres. They measured the thermal conductivity employing a microfl ash instrument in the longitudinal and transverse directions for 9%, 17% and 23% fibre reinforcement by volume. They found anisotropy in thermal conduction, but the thermal conductivity of the nanocomposite was higher than that of the pure polymer. The values were 2.09, 2.75 and 5.38 W/m/K, respectively, in the longitudinal direction. The corresponding values were 2.42, 2.47 and 2.49 W/m/K/ in the transverse direction. Fukushima and co-workers [80] prepared graphite nanoflakes, composites by combining the exfoliated graphite nanoplatelets with PA6, PA6,6 and HDPE. The thermal conductivity of these nanocomposites was measured by DSC, modifi ed hot-wire technique, and flash method. The modified hot wire method uses a heating element, supported on a backing, placed in contact with the material to be evaluated. The temperature of the heating element is monitored, and the rate of temperature increase at the sensor surface is inversely proportional to the ability of the sample to transfer heat. Using this approach, the thermal conductivity of the specimen is quickly and nondestructively measured. The three methods showed good agreement up to 1.5 W/m/K. Above 1.5 W/m/K the results obtained by DSC diverged from those obtained with the flash method. The halogen lamp flash method revealed that exfoliated graphite nanoplatelet-PA composites, with up to 20 vol.% exfoliated graphite flakes, exhibited thermal conductivities of more than 4 W/m/K, which are significantly higher than those of the polymer matrices. The modified hot wire transient technique was also used by Mojumdar and co-workers [81] to measure the thermal conductivity of a polyvinyl alcohol (PVA) and its composites with calcium silicate hydrate (C–S–HPN; Ca/Si ratio 0.7-0.15) at 25 °C and 50 °C. Their study leads to new routes for developing new cement-based nanocomposite materials. The PVA composite exhibited the higher conductivity at 50 °C, while the C–S–HPN material exhibited the highest thermal conductivity at both 25 °C and both 50 °C.
8.4.2 Micro-Thermal Analysis (μTA) TA refers to a group of techniques which use a combination of localised thermal analysis with a microscope that uses a near-field thermal probe. Several microscopy techniques [82-88] that produce thermal contrast imaging associated with variations in the thermal properties of a sample such as thermal conductivity [87, 89-92], thermal diffusivity [89, 93] and thermal expansion coefficient [81, 94-96] have been developed. These techniques use a scanning thermal microscope equipped with a temperature-sensing 311
Thermal Analysis of Rubbers and Rubbery Materials scanning probe. A typical instrument is an atomic force microscope (AFM) equipped with a near-field thermal probe. The near-field thermal probe is a temperature-sensing near-field optical scanning probe, which achieves a spatial resolution of about 10-100 nm, for property measurement, sub-surface imaging and thermal mapping. The probe scans the sample surface and provides both topographic and thermal image contrast, thus allowing the thermal characterisation of subsurface regions simultaneously while recording the topographic image. Areas of interest may be then selected and localised thermal analysis (modulated temperature calorimetry and thermomechanical analysis) carried out. Localised dynamic mechanical measurements are also possible. Spatially resolved chemical analysis can be performed using the same basic apparatus by means of pyrolysis gas chromatography-mass spectrometry or high-resolution photothermal IR spectrometry. The probe may perform three functions: (i) to exert a force on the sample surface (it generates the AFM topographic image), (ii) to locally heat the sample (it is the heat source), and/or (iii) to measure the heat flow (it is the thermal sensor). The heat flow may be constant, modulated or ramped. High heating and cooling rates may be used because of the small size of the heating element and because of the small amount of the material employed. The highest reported rate is 1400 °C/min. The probe may be used in two operating modes: the constant temperature mode (also called active or self-heating mode) and the constant current mode. In the constant temperature mode, the probe is kept at a fixed predetermined temperature. The resistance and the temperature of the probe are kept constant via a feedback loop of the probe heating circuit. The feedback voltage, required to keep the probe at a predetermined temperature, is monitored and used to generate the contrast in the thermal image. The image contrast represents variation in the amount of heat flowing out of the probe, which is determined by the variation in the thermal conductivity at and near the surface, as illustrated in Figure 8.23. In the constant current mode, a small constant current is passed through the probe, which acts as a thermometer. When the tip contacts successive regions with different thermal conductivities, varying amounts of heat will flow from the tip to the surface and the probe temperature fluctuates. TA has been used to study multilayer systems [84, 97, 98], interfaces and interphases [84, 86, 87, 99-103], polymorphic forms of polymers and their thermal history [82, 89, 93, 96, 100], and to differentiate the constituents of heterogeneous materials [82, 84, 85, 97, 100]. TA is also used in microelectronics [82, 85, 102] and the biological field. [82, 84, 103, 104]. Hammiche [96] and co-workers used TA and TMA to study some composite polymeric samples and established the versatility of cantilever-type resistive thermal probes.
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Figure 8.23 TA images of a polished cross-section of carbon fibre-carbon composite: a) Topography image confirms that the surface is flat to within 200 nm; b) Thermal conductivity image reveals the fibres which have higher thermal conductivity [81] Reproduced with permission from S.C. Mojumdar, L. Raki, N. Mathis, K. Schmidt and S. Lang, Journal of Thermal Analysis and Calorimetry, 2006, 85, 1, 235. ©2006, Springer
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9
Thermal Analysis in Understanding Rubbery Matrix and Rubber-Filler Interactions Ivan Krakovsky, Yuko Ikeda and Shinzo Kohjiya
9.1 Introduction Generally, commercial rubber products are manufactured as composites with inorganic fillers of nanometer size (nano-fillers). For example, pneumatic tyres are made from a rubber or a rubber blend with a suitable reinforcing nano-filler in conjunction with a suitable sulfur/accelerator system for crosslinking [1, 2]. Among lots of reinforcers for rubber, carbon black has been most widely used [2, 3], thus many rubber products are coloured black as is typically seen in automobile tyres. In materials design of rubber nano-composites, dispersion of the nano-filler in the amorphous rubbery matrix has long been known as the determining factor for specifying the mechanical properties. On this line, rubber technologists have paid much attention on the dispersion of nano-fillers, which has been and is the most challenging problem in rubber industry. One of the most important factors influencing their dispersion is the relative magnitude of fill-to-filler and filler-to-rubber interactions. However, the nature of these interactions is difficult to characterize exactly, since they are depending heavily on a particular system. The interactions are elucidated by various means. In this chapter, thermal analyses applied to the elucidation of these interactions involving fillers in rubbery materials are reviewed.
9.2 Thermal Analysis and Investigation of Heterogeneous Materials The International Confederation for Thermal Analysis and Calorimetry (ICTAC) defines thermal analysis as: a group of techniques in which a property of the sample is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed [4]. The programme may involve heating or cooling at a fixed (or variable) rate of temperature change, or holding the temperature constant, or any sequence of these. Since a thermal analysis measurement is a kind of thermodynamic process, classification of the methods of thermal analysis can be made on the basis of the thermodynamics. 321
Thermal Analysis of Rubbers and Rubbery Materials According to the First Thermodynamic Law, internal energy of a system, U, can be changed by supplying a heat Q and/or carrying out a work W on the system: U=Q+W Particularly, for an infinitesimal quasistatic process, the change of internal energy of a system, dU, can be expressed by [5]: dU = Q + W where Q is heat supplied to the system and W work carried out on the system (symbols d and denote exact (total) and inexact differential, respectively, see [5]). Heat and work are related to the physical properties involved in individual methods (see Table 9.1). A quasistatic process is an idealised process which is carried out so slowly that the thermodynamic system under consideration remains arbitrarily close to equilibrium at all stages of the process. From the microphysical point of view, changes of the internal energy of the system during thermal analysis measurement reflect changes of the internal structure of the system investigated. Therefore, methods of thermal analysis can provide useful information about structure-properties relationships in materials [6] including heterogeneous materials such as, for example, polymer blends and composites [7]. The important methods of thermal analysis are summarized in Table 9.1. The top two, thermogravimetric analysis (TGA) and thermodilatometry, are for quantifying the most basic properties: mass and volume. Between the two, TGA has been applied to inorganic filler/rubber composites as described in the Section 9.5.1. However, thermodilatometry has not been much used on rubbery materials. One paper deals with temperature effect on a thin film [8]. The last two, i.e., differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are the most popular techniques in thermal analysis, and those are also of use for elucidating interactions. The others are more or less sophisticated and specialised than popular techniques such as TGA, DTA and DSC, and may afford useful results in a variety of particular purposes.
9.3 Structure-Properties Relationships in Particulate Filler/Rubbery Matrix Systems In most of their industrial applications, elastomers are used as composites, i.e., they constitute a rubbery matrix in which a kind of filler is dispersed. It is a well-known fact that properties of an elastomer are significantly changed by addition of fillers. The most common fillers are carbon black and amorphous silica, both of them in the form of fine particles ranging from tens to thousands of nanometers in size. Other fillers such as clay silicates (montmorillonite, hectorite, bentonite, and so on) are of growing interest in relation to so called nanotechnology. A great improvement of the filler compatibility has been achieved by organic modification of the clay silicates (organoclays) [9-14]. 322
w 1 Tr[( T )1 d( T ) 1 ] 2 = stress tensor = deformation gradient tensor q =Tds T = absolute temperature s = entropy density
w = E.dD or w = d E= electric field D = electric induction = electric potential = charge density w = H.dB H=magnetic field B= magnetic induction
---
Change of internal energy density measured ---
MS: Mass spectrometry FTIR: Fourier transform infra red spectroscopy
Specific heat, latent heats
Magnetic permeability (generally: relation between magnetic induction and magnetic field) Elastic moduli (generally: relation between mechanical stress and strain)
Volume or linear expansion coefficient Dielectric constant or electric conductivity (generally: relationship between electric induction and electric field)
Mass
Physical property of interest
Derived special methods TGA-MS, TGAFTIR, TGA-DSC
Differential thermal analysis (DTA), Differential scanning calorimetry (DSC)
Thermal mechanical analysis (TMA), Dynamic mechanical analysis (DMA)
Magnetic thermal analysis (MTA) or thermomagnetometry
Temperature modulated DSC
Dielectric thermal analysis Thermally stimulated (DETA), depolarisation Thermoconductometry current measurement
Thermodilatometry
TGA
Method
Table 9.1 The most important methods of thermal analysis
Thermal Analysis in Understanding Rubbery Matrix and Rubber-Filler Interactions
323
Thermal Analysis of Rubbers and Rubbery Materials During preparation of a filler such as carbon black or silica, primary filler particles usually fuse together giving rise to 3-D branched clusters or aggregates. High structured particles contain a high number of primary particles per aggregate (strong aggregation due to strong filler-to-filler interaction relative to filler-to-rubber matrix interaction). These aggregates again may associate to form agglomerates linked by van der Waals interactions (agglomeration). These situations are schematically represented in Figure 9.1 [15, 16]. In this figure, (a) shows a hypothetical single nano-particle and bound rubber on its surface. Nano-particles usually exist as an aggregate from the time of their manufacturing due to their high surface energy. Thus, aggregates as shown in (b) are the basic entity of existence of them in rubbery matrix. Quite often, further association of aggregates results in formation of agglomerates. They are often dispersed in a network structure of rubbery matrix [17]. A good way of illustrating the factors controlling the structure-property relationships of such a composite material can be provided by computer modelling of its effective properties [18]. It becomes clear soon that description of the shape and space distribution of filler particles is required, apart from the knowledge of rigorous continuum physics principles of the phenomenon of interest and macroscopic properties of individual phases, i.e., those of matrix and filler. Information about the behaviour of the boundaries between filler particles and matrix (filler-to-matrix interaction) and (at higher filler concentration) mutual influence of the filler particles (filler-to-filler interaction) might complete the input information for a computer programme. Comparison of the numerical results obtained by computer modelling of an effective property of a composite with experimental data available can serve as a good test of inclusion of all-important factors.
Figure 9.1 Morphology of nano-filler in rubbery matrix
324
Thermal Analysis in Understanding Rubbery Matrix and Rubber-Filler Interactions There is also another point of view to structure-properties relationships, which is based on the molecular approach. Composite is treated as a binary mixture of supermacromolecules (filler) and polymer chains (matrix). The equation of state for the mixture and hence effective behaviour of the composite is found as a function of the interplay between the repulsion and attraction of the constituents and molecular characteristics of each constituent. This approach was used for the calculation of effective compressibility and thermal expansivity by Simha and co-workers [19, 20].
9.4 Description of the Shape and Space Distribution of Filler Particles Much of the early work on characterizing the microstructure of heterogeneous materials was done via sectioning. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are now well-established non-invasive techniques but they are limited to projected two-dimensional information [21-24]. However, interpretation of physical properties of composites requires 3-D information about the structure. X-ray microtomography and confocal microscopy are relatively new non-invasive techniques that provide such information, but their resolutions are not enough to elucidate the details of nano-fillers. Therefore, TEM has to be used for the nanometer scale resolution. For 3-D structural observation, a series of TEM images at various angles should be obtained by tilting an ultra-thin sample [22], the thickness of which may be thicker than that for conventional high-resolution TEM. With the recent development of computer tomography combined with TEM, the technique has developed into electron tomography or 3-D-TEM [15, 22], which is becoming established as a very powerful tool for the evaluation of morphology in materials science [15, 23-25]. Information about the shape of fillers can also be obtained by indirect techniques, such as scattering methods (small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) [26, 27], however, only in the case of particular dilute systems. For concentrated systems, interpretation of scattering profiles becomes too complicated due to interference between the particles.
9.5 Filler-to-Matrix and Filler-to-Filler Interactions as Investigated by Thermal Analysis As mentioned previously, information about the matrix-to-filler and filler-to-filler interactions represents a very important part of understanding the properties of composites. These interactions manifest themselves in different ways when investigated by methods of thermal analysis.
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9.5.1 Thermogravimetry The simplest method of thermal analysis is thermogravimetry (TGA). A change of the mass of sample with increasing temperature (non-isothermal TGA) or time (isothermal TGA) under a defined atmosphere of air, oxygen, nitrogen or an inert gas is measured. Of course, the mass of a closed system is constant, however, from an open system volatile (gaseous) components are released and measurement of the change of mass of the system can give useful information about the processes occurring in the system. Since decomposition temperatures of matrix and filler are well separated in inorganic filler/organic rubber composites, the method is useful in determination of the composition of such composites, e.g., rubber/filler weight fractions. Very good examples are quantification of the amount of in situ silica in rubber vulcanisates [28-30]. Because the silica was produced in the rubbery matrix, TGA affords the most suitable method to quantify the amount of silica based on the big difference in thermal stability between organic rubbers and inorganic fillers. Decomposition processes can be studied in much more details if the method is coupled with an analytical method for gases (FTIR or MS). Filler-to-matrix and filler-to-filler interactions influence the values of all three important parameters obtained from TGA [31]: decomposition temperature, rate of mass change and (using a model) activation energy of the decomposition process. The interactions were found to have an opposite effect on the activation energy of the decomposition of the matrix, with increase of the filler-to-matrix interaction activation energy increases due to the physical crosslinking and reduction of the mobility of polymer chains in the interface layer [32]. On the other hand, stronger filler-to-filler interaction accompanied by appearance of the physical network of filler particles at higher filler content increases the amount of unbound rubber and lowers the activation energy of the decomposition. Some authors reported no change of the activation energy in the presence of carbon black filler [33]. TGA of natural rubber and synthetic rubbers loaded with carbon black are not influenced by the presence of carbon particles [34].
9.5.2 Dielectric Thermal Analysis (DETA) and Thermoconductometry In this method a thin slab of material is sandwiched between metal electrodes (or the electrodes are sputtered onto the surface of sample) and an oscillating electric field is applied. Real and imaginary parts of complex dielectric constant (or electric conductivity) are determined from the values of amplitude and phase shift of electric current passing through the sample. Electric properties can be very sensitive to the presence of even very low amount of filler. Electrical properties of polymer composites are very important for industrial applications and have been the subject of intensive research for many years [25, 35-39]. Unfilled rubbers are very good insulators with values of their electrical conductivity lower than 10-13 -1m-1 and low frequency dielectric constants of 2-5. Fillers like silica are also good 326
Thermal Analysis in Understanding Rubbery Matrix and Rubber-Filler Interactions insulators, however, electrical conductivity of carbon black is in the semiconductor range giving values of approximately 10-5 -1m-1. Difference in the electric properties of matrix and filler gave rise to the accumulation of charge carriers at interface boundaries (Maxwell-Wagner-Sillars effect [40]). Influence of filler-to-matrix and filler-to-filler interaction on the dipole relaxation processes in the system were investigated for example on the polyurethane-silica system [41] and on the polydimethylsiloxane-silica system [42]. For conducting fillers, such as carbon black, at low concentrations of filler, the filler particles are diluted and, consequently, electrical conductivity of the composite is close to that of the matrix, i.e., very low. With an increase in concentration a 3-D network of the conducting phase begins to expand (a kind of agglomeration in the rubbery matrix) which is reflected in a big increase of the conductivity. The increase is the most pronounced just after a certain (critical) concentration was achieved and spans a few orders. Finally, the increase becomes gradual until maximum packing of the filler particles is achieved [25, 38]. These behaviours are explained well by the percolation theory [43]. The magnitude of conductivity depends on the number of contacts between aggregates of conducting filler particles in the polymer matrix. The existence of a 3-D network of the aggregates which represents a conduction path across the sample is strong evidence of a percolation phenomenon [25]. However, the processes controlling the conduction onset in the composites with carbon black are still not entirely understood. One recent report is much concerned with this difficult problem [44].
9.5.3 Magnetic Thermal Analysis So far there has been much less investigation of the magnetic properties of carbon black composites. The magnetic response of materials consists of an electron and (a few orders weaker) a nuclear part. The electron magnetic response of carbon black consists of both, diamagnetic and paramagnetic contribution [45]. The dependence of the magnetic susceptibility on temperature is known to depend strongly on the geometrical form of carbon blacks [46]. Paramagnetism of the carbon black has a number of sources [47, 48]. For example, the Pauli’s paramagnetism is caused by the presence of free charge carriers in the carbon black and is almost temperature independent. Another type is the Curie’s paramagnetism originating from localised spins at structural defects of the carbon black crystalline structure. Electron paramagnetic resonance (EPR) in combination with measurement of static magnetic susceptibility proved to be very useful methods in the investigation of carbon black aggregates and networks [49-51]. Nuclear magnetic resonance was used in the investigation of the influence of the presence of filler on the mobility of rubber chains [52-55]. Three kinds of chains were found in 327
Thermal Analysis of Rubbers and Rubbery Materials the composite systems - tightly bonded chains presumably confined in the proximity of filler particles (see Figure 9.1a), loosely bound chains, and chains which are far from filler particles and show mobility close to that in the pure matrix.
9.5.4 Dynamic Mechanical Analysis A strip of material, which is usually stretched by a static force, is subject to an oscillating (dynamic) force of defined amplitude and frequency in DMA. After achieving stationary state, amplitude and phase shift of strain induced in the strip relative to stress is measured. From these parameters, values of storage and loss parts of elastic modulus are determined. Material can be also investigated in shear or torsion mode, and the values of storage and loss parts shear modulus are determined in this case. Loss tangent may be calculated from loss modulus divided by storage modulus. Changes of storage modulus, loss modulus and loss tangent with temperature are usually followed, which can give information on matrix phase transitions such as glass-transition and melting or freezing. Any changes of them by introduction of a nanofiller can be due to filler-tomatrix interaction. Presence of fillers in rubbery composites has a strong impact on their static and dynamic behaviour. Magnitude of static (low frequency) Young’s modulus of pure elastomer in a rubbery state is controlled by its crosslinking density. Since elastic modulus of filler particles is usually many orders of magnitude higher, they can be considered as rigid and the part of the volume of the composite they occupy is almost undeformable. Consequently, the regions in composite consisting of elastomer are deformed more than in a pure elastomeric sample of the same size and at the same loading conditions [56]. The difference depends on the distance from filler particles and filler-to-matrix interaction. A part of the elastomer can also be completely occluded by filler aggregates (the occluded rubber) and thus shielded from external strain. As a result of all these factors, effective modulus of the composite with respect to elastomeric matrix is increased. The filler-topolymer interaction can be attributed to physical (van der Waals) interaction of chemical groups present on the surface of filler and/or to chemical linkages when the filler is used in combination with a suitable silane coupling agent. Filler-to-filler interaction is also reflected in dynamic mechanical properties of rubbery composites. First deformation cycles of virgin sample break the filler network leading to the ‘stress softening’. At small amplitudes, this is attributed to the breakdown of the filler network. This stress softening at small deformations is called the Payne’s effect [57, 58] and plays an important role in the understanding of reinforcement mechanism of filled rubber samples [59]. Fletcher and Gent [60] and Payne [61, 62] carried out a detailed study of the low frequency dynamic properties of filled natural rubber. In the case of carbon black, the filler-to-matrix interactions are mainly physical (physisorption), while in case of chemisorption chemical bonding is involved. The filler-to-matrix interaction is also influenced by heat treatment. 328
Thermal Analysis in Understanding Rubbery Matrix and Rubber-Filler Interactions At large deformations the ‘stress softening’ is called the Mullins effect [63, 64]. In the behaviour of rubbery composites at large strains, the stress is also influenced by finite chain extensibility reached in some regions by strain-amplification effects. Stretching of rubber chains connecting the filler particles depends on the local concentration of filler particles.
9.5.5 DTA and DSC In differential thermal analysis, sample and reference are heated in identical environments with a defined heating rate using a programmed furnace. The differential heat rate is proportional to the temperature difference between the sample and reference, which is measured. In power compensation DSC, sample and reference are heated separately according to a temperature programme. Temperatures of sample and reference are measured and kept equal by supplying additional power into the sample or reference heater. Heat evolved (absorbed) by sample is thus equal to this power. 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 the glass transition-temperature (Tg) of the elastomer. The change is the most pronounced in the case of nano-fillers. Several studies on polymer nano-composites show an increase of Tg, suggesting that the mobility of the entire volume of the polymer is restricted by the presence of the nano-particles [65]. However, a decrease of Tg has also been reported [66] for the weak filler-to-matrix interaction, and in other cases, the addition of nano-particles does not cause any significant change to the Tg of the polymer. This is possible since the effects causing the increase and decrease of polymer mobility act simultaneously and the two may effectively cancel each other out [67].
9.6 Concluding Remarks Elucidation of filler-to-rubber and filler-to-filler interactions has been one of the central subjects for rubber technologists for a long time. Still, their nature is to be clarified more for controlling and designing filler effect onto rubbery matrix. Not only thermal analysis but also many other techniques need to be fully utilised.
References 1. Natural Rubber Science and Technology, Ed., A.D. Roberts, Oxford University Press, Oxford, UK, 1988. 329
Thermal Analysis of Rubbers and Rubbery Materials 2. Reinforcement of Elastomers, Ed., G. Kraus, Interscience, New York, NY, USA, 1965. 3. Carbon Black: Science and Technology, 2nd Edition, Eds., J-B. Donnet, R.C. Bansal and M-J. Wang, Dekker, New York, NY, USA, 1993. 4. B. Wünderlich, Thermal Analysis of Polymeric Materials, Springer, Berlin, Germany, 2005. 5. F. Reif, Fundamentals of Statistical and Thermal Physics, McGraw-Hill, New York, NY, USA, 1965. 6. J. esták, Thermophysical Properties of Solids: Their Measurements and Theoretical Thermal Analysis, Elsevier, Amsterdam, The Netherlands, 1984. 7. Thermal Characterization of Polymeric Materials, 2nd Edition, Ed., E.A. Turi, Academic Press, San Diego, CA, USA, 1997. 8. S. Hirano and A. Kishimoto, Japanese Journal of Applied Physics, 2000, 39, Part 1, 3A, 1193. 9. A. Usuki, Y. Kojima, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi and O. Kamigaito, Journal of Materials Research, 1993, 8, 5, 1179. 10. Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi and O. Kamigaito, Journal of Materials Research, 1993, 8, 5, 1185. 11. P. Maiti, P. H. Nam, M. Okamoto, N. Hasegawa and A. Usuki, Macromolecules, 2002, 35, 6, 2042. 12. M. Kato, H. Okamoto, N. Hasegawa, A. Tsukigase and A. Usuki, Polymer Engineering and Science, 2003, 43, 6, 1312. 13. M. A. López-Manchado, M. Arroyo, B. Herrero and J. Biagiotti, Journal of Applied Polymer Science, 2003, 89, 1, 1. 14. A. Mousa and J. Karger-Kocsis, Macromolecular Materials and Engineering, 2001, 286, 4, 260. 15. S. Kohjiya, A. Katoh, J. Shimanuki, T. Hasegawa and Y. Ikeda, Polymer, 2005, 46, 12, 4440. 16. S. Kohjiya and A. Katoh, Kobunshi Ronbunshu, 2005, 62, 10, 467. 17. S. Kohjiya, A. Katoh, T. Suda, J. Shimanuki and Y. Ikeda, Polymer, 2006, 47, 10, 3298. 330
Thermal Analysis in Understanding Rubbery Matrix and Rubber-Filler Interactions 18. S. Torquato, Random Heterogeneous Materials: Microstructure and Macroscopic Properties, Springer Verlag, New York, NY, USA, 2002 19. R. Simha, E. Papazoglou and R.H.J. Maurer, Polymer Composites, 1989, 10, 6, 409. 20. E. Papazoglou, R. Simha and R.H.J. Maurer, Rheological Acta, 1989, 28, 4, 302. 21. W.M. Hess, Rubber Chemistry and Technology, 1991, 64, 3, 386. 22. Electron Tomography: Three-Dimensional Imaging with the Transmission Electron Microscope, Ed., J. Frank, Plenum Press, New York, NY, USA, 1992. 23. Y. Ikeda, A. Katoh, J. Shimanuki and S. Kohjiya, Macromolecular Rapid Communications, 2004, 25, 12, 1186. 24. M. Weyland and P.A. Midgley, Materials Today, 2004, 7, 12, 32. 25. S. Kohjiya, A. Kato, J. Shimanuki, T. Hasegawa and Y. Ikeda, Journal of Materials Science, 2005, 40, 9-10, 2553. 26. R-J. Roe, Methods of X-Ray and Neutron Scattering in Polymer Science, Oxford University Press, Oxford, UK, 2000. 27. T. Koga, M. Takenaka, K, Aizawa, M. Nakamura and T. Hashimoto, Langmuir, 2005, 21, 11409. 28. Y. Ikeda, A. Tanaka and S. Kohjiya, Journal of Materials Chemistry, 1997, 7, 3, 455. 29. Y. Ikeda and S. Kohjiya, Rubber Chemistry and Technology, 2000, 73, 3, 534. 30. S. Kohjiya, K. Murakami, S. Iio, T. Tanakashi and Y. Ikeda, Rubber Chemistry and Technology, 2001, 74, 1, 16. 31. S. Montserrat, J. Málek and P. Colomer, Thermochimica Acta, 1998, 313, 1, 83. 32. S.A.S. Venter, M.H. Kunita, R. Matos, R.C. Nery, E. Radovanovic, E.C. Muniz, E.M. Girotto and A.F. Rubira, Journal of Applied Polymer Science, 2005, 96, 6, 2273. 33. P. Thavamani, A.K. Sen, D. Khastgir and A.K. Bhowmick, Thermochimica Acta, 1993, 219, 1, 293. 34. M.J. Matheson, T.P. Wampler and W.J. Simonsick, Jr., Journal of Analytical Applied Pyrolysis, 1994, 29, 2, 129. 331
Thermal Analysis of Rubbers and Rubbery Materials 35. L.C. Burton, K. Hwang, T. Zhang, Rubber Chemistry and Technology, 1989, 62, 4, 838. 36. F. Carmona, Physica A, 1989, 157, 1, 461. 37. D. van der Putten, J.T. Moonen, H.T. Brom, J.C.M. Brokken-Zijp and M.A.J. Michels, Physics Review Letters, 1992, 69, 3, 494. 38. L. Karásek and M. Sumita, Journal of Materials Science, 1996, 31, 2, 281. 39. J-C. Huang, Advances in Polymer Technology, 2002, 21, 4, 299. 40. A.R. Blythe, Electrical Properties of Polymers, Cambridge University Press, Cambridge, UK, 1979 41. Z.S. Petrovi , I. Javni, A. Waddon and G. Bánhegyi, Journal of Applied Polymer Science, 2000, 76, 2, 133. 42. K.U. Kirst, F. Kremer and V.M. Litvinov, Macromolecules, 1993, 26, 5, 975. 43. D. Stauffer and A. Aharony, Introduction to Percolation Theory, 2nd Edition, Taylor & Francis, London, UK, 1992. 44. A. Katoh, J. Shimanuki, S. Kohjiya and Y. Ikeda in Proceedings of the 168th ACS Rubber Division Meeting Fall, 2005, Pittsburgh, PA, USA, Paper No. 40. 45. C. Kittel, Introduction to Solid State Physics, 8th Edition, Wiley, Hoboken, NJ, USA, 2005. 46. C. Brosseau, P. Molinie, F. Boulic and F. Carmona, Journal Applied Physics, 2001, 89, 12, 8297. 47. J.F. Baugher and B. Ellis, 48. B. Ellis and J.F. Baugher, Journal of Polymer Science, Part A: Polymer Chemistry, 1973, 11, 7, 1461. 49. C. Brosseau, F. Boulic, P. Queffelec, C. Bourbigot, Y. Le Mest, J. Loaec and A. Beroual, Journal of Applied Physics, 1997, 81, 2, 882. 50. C. &$'')$)! )!'($# &$)! # !!& $!)"% 51. Y. Nakamura, K. Nishizawa, N. Motohira and H. Yanagida, Journal of Materials Science Letters, 1994, 13, 11, 829. 332
Thermal Analysis in Understanding Rubbery Matrix and Rubber-Filler Interactions 52. M.A. Waldrop and G. Kraus, Rubber Chemistry and Technology, 1969, 42, 5, 1155. 53. S. Kaufman, W.P. Slichter and D.D. Davis, Journal of Polymer Science, Part A-2: Polymer Physics, 1971, 9, 5, 829 54. N.K. Dutta, N. Roy Choudhury, B. Haidar, A. Vidal, J-B. Donnet, L. Delmotte and J.M. Chezeau, Polymer, 1994, 35, 20, 4293. 55. R. Mansencal, B. Haidar, A. Vidal, L. Delmotte and J-M. Chezeau, 56. S. Poompradub, M. Tosaka, S. Kohjiya, Y. Ikeda, S. Toki, I. Sics and B. S. Hsiao, Journal of Applied Physics, 2005, 97, 10, 103529. 57. A.R. Payne in Reinforcement of Elastomers, Ed., G. Kraus, Wiley, New York, NY, USA, 1965. 58. A.R. Payne and R.E. Whittaker, Rubber Chemistry and Technology, 1971, 44, 2, 440. 59. A. I. Medalia, Rubber Chemistry and Technology, 1978, 51, 3, 437. 60. W.P. Fletcher and A.N. Gent, Transactions of the IRI, 1953, 29, 166. 61. A.R. Payne, Journal of Applied Polymer Science, 1962, 6, 19, 57. 62. A.R. Payne, Journal of Applied Polymer Science, 1964, 8, 6, 2661. 63. L. Mullins, Rubber Chemistry and Technology, 1948, 21, 2, 281. 64. L. Mullins, Rubber Chemistry and Technology, 1969, 42, 1, 339. 65. D. Fragiadakis, P. Pissis and L. Bokobza, Polymer, 2005, 46, 16, 6001. 66. B.J. Ash, L.S. Schadler and R.W. Siegel, Materials Letters, 2002, 55, 1-2, 83. 67. V.A. Bershtein, L.M. Egorova, P.N. Yakushev, P. Pissis, P. Sysel and L. Brozova, Journal of Polymer Science, Part B: Polymer Physics, 2002, 40, 10, 1056.
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10
Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry Seiichi Kawahara
10.1 Introduction Differential scanning calorimetry (DSC) is a powerful technique because it is nondestructive in nature, which is indispensable for studying the crystallisation of natural rubber (NR) which is slow compared to those of other polymers but rapid compared to other rubbers. This makes it possible to investigate important factors influencing the crystallisation process. Furthermore, after the crystallisation, thermodynamic parameters, i.e., melting temperature, heat of fusion and so forth, are determined in a heating process. Thus, DSC plays an important role in the science and technology of NR. The methods of investigating the crystallisation of NR may be classified into two: 1. detection of calories released exothermically versus crystallisation time, and 2. measurement of heat of fusion versus crystallisation time. The crystallisation of raw NR may be investigated by method (1), because of its comparatively rapid phase transition. However, when NR is purified or modified, the crystallisation may be investigated by method (2). In this chapter, recent research on the crystallisation of NR through DSC are described: particularly, overall crystallisation from melted polymer or polymer solution. Crystallisation may be associated with the branching structure of NR and the effect of fatty acids.
10.2 Crystallisation
10.2.1 Overall Crystallisation Overall crystallisation of NR has been investigated by DSC, as well as dilatometry, since crystallisation is a first order phase transition. Because of its slow phase transition, the overall crystallisation is assessed principally by measurement of the heat of fusion, which is converted to the degree of crystallinity as a function of crystallisation time. A 335
Thermal Analysis of Rubbers and Rubbery Materials typical DSC thermogram for NR is shown in Figure 10.1. Two melting endothermic peaks appear in the thermogram, which are identified to and forms [1-3], respectively. Melting temperature (Tm), of and forms, determined to be the top of the peaks, is dependent on crystallisation temperature (Tc). Thus, in a plot of Tm versus T c, equilibrium melting temperatures (Tm0), are determined to be 39 ºC for and forms, respectively, as reported by Kim and Mandelkern [2]. The endothermic peaks increase with crystallisation time, reflecting an increase in the degree of crystallinity [1]. Figure 10.1 shows a plot of degree of crystallinity of natural rubber versus crystallisation time, in which a sigmoidal curve is drawn [4], similar to that obtained with dilatometry [5, 6]. Thus, overall rate of crystallisation of 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 rate of overall crystallisation is recognised as being of importance to investigate an effect of non-rubber components on the crystallisation. In Figure 10.2 the crystallisation of acetone-extracted NR and transesterified NR also are shown [4]. The overall crystallisation is suppressed by acetone-extraction and it is recovered by transesterification. This may be associated with structure of NR. The plausible structure [7, 8] is shown in Figure 10.3. NR has been proposed to consist of an initiating terminal unit, two trans-1,4-isoprene units, a long sequence of cis-1,4-isoprene units (more than 5,000), and terminal groups, although the structure of both terminal groups has not been identified. Based on the structure, deproteinisation of NR may result in the
Figure 10.1 DSC thermogram for natural rubber crystallised at –25 °C for 600 minutes
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Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry
Figure 10.2 Isothermal crystallisation of natural rubbers at –25 °C observed by DSC: (a): control, (b): extracted with acetone, (c): transesterified
Figure 10.3 Plausible structure of natural rubber
337
Thermal Analysis of Rubbers and Rubbery Materials decomposition of branching points containing proteins as a component and to form linear and branched molecules. In contrast, transesterification may decompose the branching points comprised of long-chain fatty acids. Thus, the fatty acids and the branching points play important roles in the crystallisation. The effect of branching points and fatty acids may be investigated through solution-grown crystallisation and bulk crystallisation, respectively.
10.2.2 Solution-grown Crystallisation When hexane solutions 1% w/v of NR, naturally occurring cis-1,4-polyisoprene from sporophores of Lactarius volemus (L-rubber) and synthetic cis-1,4-polyisoprene (CPI) are cooled at the isothermal setting temperature of –70 ºC, a white precipitate appears [9]. The precipitate disappears at room temperature. This disappearance of the precipitate suggests two possibilities: dissolution and melting. For the former, little heat transport for dissolution is expected to be measured. In contrast, for the latter, the heat of fusion of crystal can be shown in the DSC thermogram as an endothermic peak. In Figure 10.4, an endothermic peak of 24.8 J/g for the hexane solution of CPI cooled at –70 ºC is shown at around –39 ºC. This may be explained because of the melting of the CPI crystal, since the endothermic peak was not observed in the thermograms of aluminium pan and hexane. Assuming that the equilibrium heat of fusion of CPI in hexane solution is the same as that of bulk specimen [2, 10], i.e., 64.8 J/g, the degree of crystallinity of the CPI crystal grown from the solution is estimated to be 38.3% from the ratio of the endothermic peak at 24.8 J/g to the equilibrium heat of fusion. It is noteworthy that the estimated degree of crystallinity of the CPI crystal grown from the solution is identical to the final degree of crystallinity of the crystal grown from melted polymer [11]. The melting temperature of cis-1,4 polyisoprene is dependent on the T c at which the polymer is crystallised. If the temperature necessary for the appearance of the endothermic peak in the DSC thermogram is dependent on the cooling temperature, the white precipitate at low temperature can be proved to be a crystal of the polymer. The temperature, at which the endothermic peak top appears, Tm, is significantly dependent on the isothermal cooling temperature, as shown in Figure 10.5. Plotting Tm versus T c, in Figure 10.6, the Tm is revealed to increase linearly as the isothermal cooling temperature increases. The Tm increase in the Tm is, however, divided into two temperature regions bordering around –65 ºC. Below –65 ºC, the Tm may be regarded as a definite temperature, while it increases linearly by increasing the cooling temperature to higher than –65 ºC. These two variations of Tm on the cooling temperature showed a similar trend to that of the Tm of solution-grown crystal of polymer. Therefore, the white precipitate grown from the dilute hexane solution of CPI may be concluded to be a solution grown crystal. 338
Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry
Figure 10.4 DSC thermograms for (a) aluminum sample holder (pan), (b) hexane, and (c) hexane solution of CPI crystallised at –70 °C
In Figure 10.6, CPI, NR and L-rubber shows a similar T c -dependence of Tm and boundary temperature of about –65 °C which is dependent on the size of vessel used for the crystallisation. However, the Tm values of CPI, NR and L-rubber are distinguished from each other at the same Tm. In contrast, to NR, transesterified natural rubber (TENR), deproteinised natural rubber (DPNR), acetone-extracted DPNR (AE-DPNR) and transesterified DPNR (TE-DPNR), all have a similar Tm versus Tc relationship, despite the fact that they contain different non-rubber components as shown in Table 10.1. From the Tm of these rubbers, equilibrium melting temperature, Tm0, is estimated to be –18 ºC by a Hoffman-Weeks plot, which is significantly lower than the Tm0 of 39 ºC [2] or 36 ºC [3] determined for a NR crystal grown from a polymer melt. The lower Tm0 may be due to the depression of melting temperature attributed to the use of hexane as a solvent, as is evident from the depression of Tm the mixtures of NR with dodecane, tetradecane or methyl oleate [12]. On the other hand, L-rubber and CPI showed distinctly separate lines (Figure 10.6). This demonstrates the negligible effect of non-rubber components, such as protein, mixed fatty acids and linked fatty acids, on the Tm0 of solution-grown crystal of cis-1,4-polyisoprene. 339
Thermal Analysis of Rubbers and Rubbery Materials
Figure 10.5 DSC thermograms for hexane solution of CPI crystallised between –80 and –52 °C
In Table 10.1, despite the significant difference in the average molecular weight, the Tm of low molecular weight DPNR (LF-DPNR) is the same as that of NR, while it was different from the Tm of L-rubber which is a homologue of TE-DPNR. It is well known that the Tm increases with increasing molecular weight in the lower molecular weight (Mw) region, but it does not change in the higher molecular weight region. In order to ascertain the molecular weight dependence of Tm in the lower molecular weight region, the Tm value of fractionated L-rubbers may be compared to each other. The Tm of two fractionated L-rubbers is shown in Figure 10.7. The high molecular weight fraction of L-rubber, whose number-average molecular weight, Mn = 3.50 x 104, Mw = 3.26 x 105 and Mw/Mn = 9.31, showed the Tm to be higher than Tm of the low molecular weight fraction, whose Mn = 1.42 x 104,
Figure 10.6 Dependence of melting temperature on crystallisation temperature; (O) CPI, ( ) natural rubber, ( ) TE-NR, ( ) DPNR, ( ) AE-DPNR, ( ) TE-DPNR, ( ) LFDPNR and ( ) L-rubber, respectively
340
Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry
Table 10.1 Average molecular weight, molecular weight distribution, content of 3,4 units, nitrogen content (N%) and fatty acid ester content of the samples Specimen Mn (104) Mw (104) Mw/Mn 3.4 (%) CPI 20.3 104 5.10 0.3 NR 29.8 195 8.55 0 TE-NR 27.4 169 7.29 0 DPNR 24.6 140 7.80 0 AE-DPNR 31.8 122 5.25 0 TE-DPNR 20.8 119 7.76 0 LF-DPNR 4.4 15 7.35 0 L-rubber 2.8 8.7 2.01 0 LF-DPNR: lowest molecular weight fraction of DPNR
N% 0 0.38 0.38 0.01 0.01 0.01 -
Ester (mmol/kg) 0 14.8 0 14.8 4.5 0 -
Dependence of melting temperature on crystallisation temperature; ( ) high molecular weight fraction and (O) low molecular weight fraction fractionated from L-rubber
Mw = 5.29 x 104 and Mw/Mn = 3.73. This suggests that the difference between the Tm of LF-DPNR and L-rubber is due to the lower molecular weight nature of L-rubber. Thus, at the higher molecular weight fraction of NR, the Tm of a cis-1,4 polyisoprene crystal grown from solution was clearly shown to be independent of the slight decrease in the average molecular weight by deproteinisation and transesterification. On the other hand, the content of 3,4 units in CPI can be related directly to the lowest Tm of the rubber. The heterogeneity in this microstructure may result in the lowest Tm. 341
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10.3 Stem Length and Stem Length Distribution Stem length and stem length distribution may be investigated by ozonolysis followed by size exclusion chromatography (ozonolysis-SEC) [13, 14]. It has provided the distribution of chain length for only the stem part of the folded chain crystal at the temperatures lower than –30 ºC. A typical SEC curve of ozonolysis products from a TE-NR crystal is shown in Figure 10.8. Twelve peaks in the lower molecular weight region and one broad peak in the higher molecular weight region are well separated on the chromatogram [15]. The lower molecular weight peaks are attributed to the single chain traverse through the crystalline core of the thinned crystal layer (single traverse), whereas the higher molecular weight peak is ascribed to a chain length approximately twice that appropriate to a single traverse (double traverse), as shown in Figure 10.9. In the chromatogram, the single traverse peaks show overlapping around the centre of the single traverse. Figure 10.10 shows SEC and supercritical fluid chromatography (SFC) curves for ozonolysis products from the crystals of NR, DPNR and CPI. The single traverse and double traverse are separated at a minimum peak between single and double traverse distributions. The number of isoprene units, n, at the centre and the polydispersity of the single traverse, Mw/Mn, estimated from SEC curves, is given in Table 10.2. The value of n is almost identical in the rubbers, whereas the polydispersity of NR and DPNR
Figure 10.8 SEC curve for ozonolysis product of TE-NR crystal obtained from 0.2 w/v% hexane solution at –70 °C for 240 minutes
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Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry
Figure 10.9 Schematic representation of ozonolysis product crystal monolayer and double layer plus single traverse and double traverse elements
Figure 10.10 SEC and SFC traces of ozonolysis products from NR, CPI, DPNR and NR-TE crystal
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Table 10.2 Number of isoprene units at the distribution centre and Mw/Mn for single traverse element in ozonolysis products of rubbers crystallised at –70 °C Specimen NR DPNR TE-NR CPI
n-mer 10 9 10 11
Mw/Mn 1.10 1.09 1.05 1.06
Table 10.3 Tc dependence of ozonolysis product stem length characteristic of solution-grown TE-DPNR crystal Tc (ºC) -70 -65 -60 -55 -50
O3 uptake (%) 48.6 47.3 48.8 37.4 40.8
n-mer 10 10 11 12 13
Nn 8.95 8.91 8.99 9.63 9.34
Mw/Mn 1.10 1.11 1.11 1.12 1.13
is the largest. Since NR is a branched polymer in which branch points are composed of proteins and a certain group containing long chain fatty acid esters while TE-NR mainly comprises linear polymer, the difference in the polydispersity is attributed only to the presence of branching points in NR and DPNR. Since the branching points hinder the conformational arrangement of the polymer in the course of crystallisation, a more regular crystal structure for TE-NR is expected to grow in dilute solution. This demonstrates that the rapid crystallisation of TE-NR, which is much faster than that of acetone-extracted NR (AE-NR), is related to the crystallisability increase due to regular crystal structure. In Table 10.2, a large polydispersity of CPI than TE-NR may be due to the influence of very small amounts of trans-1,4- and 3,4-isoprene units. In Table 10.3, the n, Mw/Mn and the number average degree of polymerisation (Nn), estimated from Mn are shown for the ozonolysis products of solution grown TE-DPNR crystals at crystallisation temperatures between -70 and –50 ºC. The sequence length at the centre of the single traverse of the crystal obtained at a crystallisation temperature higher than –65 ºC increases as the crystallisation temperature rises, while it is definite when crystallisation is carried out below –65 ºC. This is consistent with the fact that isothermal crystallisation occurs above –65 ºC [9].
10.4 Effect of Fatty Acids The rapid crystallisation of NR has been recognised, so far, to be due to certain nonrubber constituents present in the rubber, i.e., lipids. Removing free fatty acids that are 344
Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry present as a mixture may suppress the crystallisation [5], whereas it is recovered to the original level by mixing with stearic acid 1 wt% [4]. By comparing the crystallisation behaviour of NR to those of DPNR, AE-NR and TE-NR, the rapid crystallisation is attributed to the synergistic effect of mixed fatty acids and linked fatty acid ester groups present at a terminal unit of the rubber molecule [4, 6]. NR may contain various fatty acids as a mixture [16, 17]: saturated fatty acids such as decanoic acid, myristic acid, palmitic acid and stearic acid, and unsaturated fatty acids such as palmitoleic acid, oleic acid, linoleic acid and linolenic acid. These fatty acids are also expected to link to the rubber molecule as phospholipids, because the mixed fatty acids are reported to be derived by hydrolysis of phospholipid during storage. Figure 10.11 shows the aliphatic region of an 1H-NMR spectrum for the toluene– methanol soluble fraction, which is isolated from DPNR after acetone extraction. Signals at 0.88, 1.27 and 2.38 ppm are assigned to the protons in methyl, long sequence methylene, and methylene attached to carboxyl group in fatty acid ester groups, respectively. Here, the residual low molecular weight fraction of DPNR shows signals at 1.6 and 2.0 ppm, corresponding to the methyl and methylene protons of isoprene units, respectively. To isolate fatty acids, the solution was saponified followed by esterification with BF3·2CH3OH in diethyl ether. The resulting fatty acid esters were characterised by gas chromatography. Figure 10.12 shows the gas chromatogram for the toluene–methanol soluble fraction, in which sharp peaks appeared separately. These peaks may be identified by using standard fatty acids as given in Table 10.4, together with mixed fatty acids recovered by acetone-
Figure 10.11 1H NMR spectrum of soluble fraction in toluene–methanol separated from rubber fraction after transesterification of AE-DPNR
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Figure 10.12 Gas chromatogram of fatty acid ester groups linked to AE-DPNR
Table 10.4 Composition of linked fatty acid and mixed fatty acid Fatty acid (carbon : double bond) C10:0 C12:0 C14:0 C14:1 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0 C22:0
Linked fatty acid (mmol/kg) 0.11 0.32 0.05 0.19 0.36 0.54 1.05 0.71 0.39 0.50 0.15 0.56
Mixed fatty acid (mmol/kg) 0.03 0.04 0.01 0.05 0.10 0.72 4.53 1.74 1.37 3.39 0.25 0.08
extraction of DPNR. The composition of linked fatty acids is similar to that of mixed fatty acids [18]; i.e., the major of fatty acids are stearic acid, linoleic and linolenic acids for both, although the ratio is not the same. The saturated fatty acids may play a role of the nucleating agent for crystallisation of NR [19], while unsaturated fatty acids may be a plasticiser [20]. The saturated fatty acid such as stearic acid was miscible with the other saturated fatty acid, but immiscible with unsaturated fatty acids. NR is a multicomponent system consisting of various fatty acids and a rubber hydrocarbon bonding 346
Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry to fatty acid ester groups. This feature makes it difficult to investigate directly the role of each fatty acid on the crystallisation behaviour of the rubber. Thus, model polymers for NR are prepared by hydoroboration of CPI followed by esterification with acyl chloride [21], in which CPI is esterified with various fatty acid ester groups selectively at the 3,4 isomeric units. Figure 10.13 shows a plot of degree of crystallinity versus crystallisation time for CPI and modified CPI, in which the crystallisation is investigated by DSC because of the small amount of the modified CPI. The crystallisation of CPI is significantly suppressed by introducing hydroxyl group at 3,4 units of CPI. However, the crystallisation is recovered to the original level by esterifing OH group with acyl group (CPI-C2) and it is further promoted by esterifing with stearoyl group (CPI-C18). The difference in the crystallisation between CPI-C2 and CPI-C18 may be attributed to a nucleating effect concerned with a carbon sequence of linked fatty acid ester groups. Table 10.5 shows the content of fatty acid ester groups determined by Fourier transform infra red spectroscopy (FT-IR) measurement, where subscript figures represent the number of carbon atoms, i.e., C10 for decanoyl group, C14 for myristoyl group and C18 for stearoyl group. The ester content is approximately 17 mmol/kg rubber for CPI-C10, CPI-C14 and CPI-C18. This implies that the number of fatty acids per rubber chain is about four, based upon the number average molecular weight of 2.2 x 105.
Figure 10.13 Crystallisation behaviour at –25 °C for ( ) decanoyl (C10) esterified CPI mixed with decanoic acid (C10), ( ) myristoyl (C14) esterified CPI mixed with decanoic acid (C10) and ( ) stearoyl (C18) esterified CPI mixed with decanoic acid (C10)
347
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Table 10.5 Content of linked fatty acid ester groups in CPI-C10, CPI-C14 and CPI-C18, determined by FT-IR measurement Specimen CPI-C10 CPI-C14 CPI-C18
Ester content (mmol/kg) 17.1 16.1 17.8
Table 10.6 Rate of crystallisation at –25 oC estimated from t1/2 for CPI linking with fatty acid ester groups Specimen CPI CPI-C2 CPI C10 CPI-C12 CPI-C14 CPI-C16 CPI-C18
Rate of crystallisation (10-6/s) 9.3 8.8 7.5 8.0 8.3 8.0 17.3
Table 10.6 shows the rate of crystallisation for the IR esterified with C10 to C18, abbreviated as CPI-C10 to CPI-C18. No significant difference in the rate of crystallisation is observed for CPI-C10, CPI-C12, CPI-C14 and CPI-C16. Only CPI-C18 showed a dramatic increase in the rate of crystallisation compared to untreated CPI. Figure 10.12 shows the crystallisation behaviour of CPI-C10, CPI-C14 and CPI-C18 mixed with 4.0 x 10-2 mol/kg decanoic acid at isothermal crystallisation temperature of –25 ºC. The overall crystallisation of CPI-C10, CPI-C14 and CPI-C18 mixed with decanoic acid is apparently similar to that of CPI, showing no effect from the mixed fatty acid on the crystallisation. In contrast, when 4.0 x 10-2 mol/kg myristic acid is mixed with the rubbers, the overall crystallisation is significantly promoted. The rate of crystallisation of the mixtures, estimated to be the reciprocal t1/2, is shown in Table 10.7 together with that of neat CPI, CPI-C10, CPI-C14 and CPI-C18. The rate of crystallisation shown in Table 10.7 indicates that the combination of linked fatty acids and mixed fatty acids brings about a significant effect on the rate. For example, the rate of crystallisation of CPI-C10 and CPI-C14 increases when 4.0 x 10-2 mol/kg decanoic acid (C10) is mixed with them, whereas that of CPI-C18 decreases. In contrast, when 4.0 x 10-2 mol/kg myristic acid is mixed with CPI, CPI-C10, CPI-C14 and CPI-C18, the crystallisation of the rubbers is promoted. The effect of fatty acids on the crystallisation of CPI may be related to an onset temperature of crystallisation, T cfat, of the fatty acids dispersed in the rubber during 348
Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry
Table 10.7 Rate of crystallisation at –25 ºC for CPI, CPI-C10, CPI-C14 and CPI-C18 Mixed fatty acid C10 C12 C14 C16 C18 C20 Non
CPI 8.1 14.2 27.7 22.8 19.3 12.8 9.3
Rate of crystallisation (10-6/s) CPI-C14 CPI-C10 11.5 10.5 16.6 14.2 27.5 33.7 27.2 34.7 19.8 21.4 15.0 15.9 7.5 8.3
CPI-C18 9.1 8.4 29.2 29.9 22.2 19.6 17.3
Table 10.8 Tcfat of mixed fatty acid in CPI, CPI-C10, CPI-C14 and CPI-C18 Mixed fatty acid C10 C12 C14 C16 C18 C20 Non
Tcfat (K) CPI -265 -245 -250 -254 -260 -264
CPI-C10 -262 -246 -246 -253 -258 -266
CPI-C14 -263 -239 -238 -252 -257 -265
CPI-C18 -264 -244 -243 -251 -253 -256
cooling after melting at 100 ºC [19]. The crystallisation of CPI is promoted by mixing with the fatty acids when the T cfat is higher than the isothermal crystallisation temperature of CPI, but it does not change when the T cfat is lower than that. Table 10.8 shows T cfat for mixtures of CPI, CPI-C10, CPI-C14 and CPI-C18 with various saturated fatty acids. The T cfat for the mixture with decanoic acid is lower than the isothermal crystallisation temperature of CPI, compared to the other mixtures. The relationship between the rate of crystallisation of the mixtures versus T cfat of fatty acids is shown in Figure 10.14. The rate of crystallisation for CPI, CPI -C10, CPI -C14 and CPI -C18 mixed with fatty acids is significantly dependent upon T cfat. This implies that the crystallisation of CPI is promoted, when the saturated fatty acids form crystalline phase in the CPI matrix. The effect of mixed, unsaturated fatty acids on the crystallisation behaviour of CPI-C10, CPI-C14 and CPI-C18 is shown in Figure 10.15. The overall crystallisation of CPI-C18 is promoted by mixing with 4.0 x 10-2 mol/kg methyl linoleate [20], but the crystallisation of CPI-C10 and CPI-C14 did not change as in the case of CPI. Since methyl linoleate is a plasticiser for CPI [20], the chain mobility of the rubber molecules should be enhanced 349
Thermal Analysis of Rubbers and Rubbery Materials
Figure 10.14 Rate of crystallisation at –25 °C of various esterified CPI mixed with fatty acids, : CPI, : CPI-C10, : CPI-C14, and : CPI-C18
Figure 10.15 Crystallisation behaviour at –25 °C of decanoyl esterified CPI mixed with methyl linoleate ( ), myristoyl (C10) esterified CPI mixed with methyl linoleate ( ), and stearoyl (C18) esterified CPI mixed with methyl linoleate ( )
350
Study of Crystallisation of Natural Rubber with Differential Scanning Calorimetry by mixing them. These two effects, i.e., the nucleating effect of the C18 group and the plasticising effect of methyl linoleate, will result in the rapid crystallisation of CPI. This demonstrates the validity of our assumption about the synergistic effect of linked fatty acid and mixed fatty acid on the crystallisation behaviour of NR. The synergistic effect should be attributed to both the nucleating effect of saturated fatty acid and the activated chain mobility of the rubber.
10.5 Summary The effects of fatty acids and branching points on the crystallisation of natural rubber is investigated with DSC in conjunction with chemical treatments, as described in the present chapter. The slow crystallisation of the AE-NR in the plot of degree of crystallinity versus crystallisation time is associated with the crude folded chain crystal formed for the rubber, as is evident from the results of ozonolysis-SEC measurement. In contrast, the rapid crystallisation of NR is proved by both transesterification of NR and hydroboration-oxidation of CPI followed by esterification with fatty acid. The results may contribute to seek for a long-standing mystery ‘why natural rubber is excellent compared to synthetic rubbers’.
Acknowledgements The author expresses his sincere thanks to Dr. Yasuyuki Tanaka of Mahidol University.
References 1.
B.C. Edwards, Journal of Polymer Science: Polymer Physics Edition, 1975, 13, 7, 1387.
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H-G. Kim and L. Mandelkern Journal of Polymer Science Part A-2: Polymer Physics, 1972, 10, 6, 1125
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E.N. Dalal, K.D. Taylor and P.J. Phillips, Polymer, 1983, 24, 12, 1623.
4.
E-A. Hwee, S. Ejiri, S. Kawahara and Y. Tanaka in Proceedings of an International Seminar on Elastomers, Eds., J.L. White and T. Inoue, Applied Polymer Symposia Series No.53, Wiley-Interscience, New York, NY, USA, 1994, p.5.
5.
A. N. Gent, Transactions of the Institution of the Rubber Industry, 1954, 30, 6, 139. 351
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N. Nishiyama, S. Kawahara, T. Kakubo, E-A. Hwee and Y. Tanaka, Rubber Chemistry and Technology, 1996, 69, 4, 608.
7.
Y. Tanaka, S. Kawahara and J. Tangpakdee, Kautschuk und Gummi Kunststoffe, 1997, 50, 1, 6.
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Y. Tanaka, Rubber Chemistry and Technology, 2001, 74, 3, 355.
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S. Kawahara, Y. Inomata, Y. Tanaka and N. Ohya, Polymer, 1997, 38, 16, 4113.
10. E.H. Andrews, P.J. Owen and A. Singh, Proceedings of the Royal Society of London, 1971, A324, 79. 11. L.E. Alexander, S. Ohlberg and G.R. Taylor, Journal of Applied Physics, 1955, 26, 1068. 12. D.E. Roberts and L. Mandelkern, Journal of the American Chemical Society, 1955, 77, 781. 13. G.N. Patel and A. Keller, Journal of Polymer Science: Polymer Physics Edition, 1975, 13, 12, 2259. 14. Y. Tanaka, P. Boochathum, M. Shimizu and K. Mita, Polymer, 1993, 34, 5, 1098. 15. S. Kawahara, Y. Inomata, T. Kakubo, Y. Tanaka, K. Hatada, K. Ute and N. Miyatake, Rubber Chemistry and Technology, 1998, 71, 2, 277. 16. C.C. Ho, A. Subramaniam and W.M. Young in Proceedings of the International Rubber Conference, Kuala Lumpur, Malaysia, 1975, 2, 441. 17. R. C. Crafts, J. E. Davey, G. P. McSweeney and I. S. Stephens, Journal of Natural Rubber Research, 1990, 5, 4, 275. 18. S. Kawahara, T. Kakubo, J.T. Sakdapipanich, Y. Isono and Y. Tanaka, Polymer, 2000, 41, 20, 7483. 19. S. Kawahara, K. Takano, J. Yunyongwattanakorn, Y. Isono, M. Hikosaka, J.T. Sakdapipanich and Y. Tanaka, Polymer Journal, 2004, 36, 5, 361. 20. S. Kawahara and Y. Tanaka, Journal of Polymer Science: Polymer Physics Edition, 1995, 33, 5, 753. 21. T. Kakubo, A. Matsuura, S. Kawahara and Y. Tanaka, Rubber Chemistry and Technology, 1998, 71, 1, 70.
352
Thermal Properties of Chemically Modified Elastomers
11
Thermal Properties of Chemically Modified Elastomers Nikhil K Singha
11.1 Introduction Chemical modification of existing rubber and rubber like materials contributes even more to the progress of rubber chemistry, and technology than the development of the new ones [1-5]. Chemical modification tailors, and enhances, the important properties of polymers, for example, weatherability, oxidation resistance, adhesion properties, biodegradability, fire resistance, thermal resistance, polarity, reactivity towards specific groups or ions and so on. For these reasons chemical modification has been important field of research as shown by a number of reviews or book chapters [1-6]. The chemical modification influences the physical as well as thermal properties of polymers, especially for rubber, and rubber like materials. These properties include low as well as high temperature properties, crystallinity, thermal ageing characteristics and so on. These properties can be characterised using thermal analysis methods, including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA). Principles of these analyses have been explained in several other chapters in this book. This chapter will delineate how different thermal properties vary with the different types of chemical modification and how they are characterised. It is easy to do different chemical modifications in diene elastomers because of the presence of reactive carbon-carbon unsaturation. The chemical modification can take place directly attacking the carbon – carbon double bond or at the site of reactive allylic hydrogen.
11.2 Hydrogenation Hydrogenation is one of the most important methods of chemical modification of diene elastomers [1, 6]. Hydrogenation improves weatherability, thermal ageing resistance, resistance to oxidative and ozonolytic degradation. Hydrogenation has a great effect on the high temperature as well as low temperature properties of elastomers [6, 7]. Among all the diene elastomers the hydrogenation of nitrile-butadiene rubber (NBR) has been well-studied. The oil resistance property of hydrogenated NBR (HNBR) is much 353
Thermal Analysis of Rubbers and Rubbery Materials better than NBR at high temperature. Hydrogenation influences the glass transition temperature (Tg) of HNBR [6, 8, 9]. The change of Tg depends on the acrylonitrile (ACN) content as well as on the degree of hydrogenation in HNBR. Figure 11.1 shows the variation of Tg of HNBR with its iodine values (degree of hydrogenation), and its ACN content. The Tg of HNBR with a low ACN content (18 to 28 mol%) decreases as the degree of hydrogenation increases. For HNBR with medium ACN content the Tg increases with increase in hydrogenation after passing through a minimum. The Tg of HNBR with a higher ACN content (>41 mol%) increases with increasing degree of hydrogenation. Table 11.1 shows the Tg of other different elastomers, and their hydrogenated products. Hydrogenation of natural rubber (NR) leads to a strictly alternate ethylene propylene copolymer [10]. It was observed there was no significant change in the Tg of hydrogenated NR (HNR). However, the initial decomposition temperature (IDT) as well as Tmax of HNR increases with increase in degree of hydrogenation [6, 7]. The Tg of styrene-butadiene rubber (SBR) decreases on hydrogenation of the butadiene part [11]. Interestingly, hydrogenated SBR (HSBR) having more than 60% hydrogenation shows an endotherm indicating the development of crystallinity in HSBR [7] (Figure 11.2). The 13C-NMR study showed the presence of a large segment of a ~CH2~ group which underwent crystallisation to show well-defined melting peak [12]. In the presence of the rhodium-based Wilkinson’s catalyst [rhodium tris(triphenylphosphine) chloride] the hydrogenation of polychloroprene rubber (CR) leads to dehydrochlorination followed by hydrogenation [13]. The resultant hydrogenated chloroprene rubber (HCR)
Figure 11.1 The effect of hydrogenation on Tg of HNBR
354
Thermal Properties of Chemically Modified Elastomers
Figure 11.2 DSC thermogram of hydrogenated styrene-butadiene rubber (HSBR)
Table 11.1 Tg of different rubbers and their hydrogenated products No. Elastomers Hydrogen (%) 1 NBR 0 2 HNBR 64 3 HNBR 96 4 NR 5 HNR 60 6 HNR 80 7 HNR 100 8 SBR 9 HSBR 40 100 10 HSBRa 11 CR 12 HCR 59 96 13 HCRb a It shows a melting temperature (Tm) at 45 ºC b It shows a melting temperature (Tm) at 100 ºC
Tg ( ºC) -18 -22 -24 -59 -58 -57 -56 -49 -53 -54 -35 -20 -24
355
Thermal Analysis of Rubbers and Rubbery Materials contains a large segment of the -CH2- group and a minor quantity of ethylene-co-vinyl chloride segment which was due to the direct hydrogenation of CR. The HCR has a Tg less than that of CR. Interestingly, it shows a Tm at 100 ºC which is due to a crystallisable very long -CH2- sequence in the hydrogenated elastomer. Table 11.1 shows the change in Tg of different elastomers on hydrogenation. Hydrogenation also has tremendous influences on the high temperature properties of elastomers. Hydrogenation improves the IDT as well as maximum decomposition temperature (Tmax) of different elastomers. Table 11.2 shows the IDT and Tmax of different elastomers and their hydrogenated products. It clearly indicates that hydrogenated elastomers have higher IDT and Tmax and can be used for higher temperature applications relative to unhydrogenated elastomers. For example, HNBR can be used as an oil resistant rubber at a much higher temperature than NBR. The IDT as well as Tmax of HNBR increases with increase in degree of hydrogenation [6, 9]. Dynamic mechanical properties are important in large number of engineering applications of rubber products. Hydrogenation influences the dynamic mechanical properties of the elastomers. Obrecht and co-workers [14] studied the DMA analysis of NBR and HNBR. They observed that HNBR has two secondary relaxations at –75 ºC and –130 ºC, whereas NBR has only one relaxation at –70 ºC. The additional relaxation in HNBR at -130 ºC is attributed to the rearrangements of rotational isomers of ethylene sequences. For other elastomers such as SBR, NR and CR, the high temperature properties improve with the increase in hydrogenation. The absence of carbon-carbon unsaturation in hydrogenated elastomers improves its resistance to thermal ageing and thermal degradation.
Table 11.2 Initial decomposition and maximum decomposition temperature of elastomers and hydrogenated elastomers Elastomers NR HNR SBR HSBR CR HCR NBR HNBR
IDT (0 ºC) 361 420 397 430 226 336 400 445
Tmax (0 ºC) 387 449 445 473 433 and 476 495
11.3 Epoxidation Epoxidation induces the polarity in the diene elastomer which influences the different properties like oil resistance, adhesion to polar substrates, compatability to polar polymers 356
Thermal Properties of Chemically Modified Elastomers in a polymer blend [1]. Epoxidation has been mostly studied on NR [15-18]. On epoxidation, the Tg of the modified elastomer increases. It is not only due to the increased polarity but also due to the decrease in free volume, thus restricting segmental mobility in the epoxidised elastomers. Epoxidation of NR influences resilience, oil resistance, rolling resistance as well as its Tg. As Figure 11.3 Effect of epoxidation on Tg of the degree of epoxidation increases, epoxidised NR (ENR) its T g increases (Figure 11.3). Commercially available epoxidised NR (ENR), for example, ENR-25 (ENR with 25% epoxidation) and ENR-50 (ENR with 50% epoxidation) have Tg of -48 ºC and -23 ºC respectively. Because of the higher Tg, high wet grip and low air permeability, these ENR are attractive for use in tyre tread and liners. Epoxidation of 1,2-polybutadiene improves its adhesive strength, solvent resistance and heat stability [19]. Epoxidation of styrene-butadiene-styrene (SBS) block copolymer improves its stress relaxation behaviour [20].
11.4 Halogenation, Hydrohalogenation Among the chemical modification via the halogenation, chlorination is the most commercially used process. It can be carried out in solution, suspension and also in the solid state of polymer. Chlorination as well as hydrochlorination induces the polarity in the polymers, increases its limiting oxygen index (LOI), and thus improves its resistance to flammability. It also influences its Tg as well as the crystallinity of the polymer. Chlorination converts the semicrystalline polyethylene into elastomers [21]. Chlorination reduces the chain regularity and thus restricts the ability of polyethylene to crystallise. A higher degree of chlorination increases the interchain attractions, and thus raises the Tg. The chlorinated polyethylene (CPE) with a high degree of chlorination loses its rubberiness. Table 11.3 shows the properties of polyethylene with different degrees of chlorination. Thermal analysis especially the thermal degradation of the CPE is well reported [22-25]. The rate of dehydrochlorination depends on the chlorine content as well as the distribution of chlorine groups in the macromolecular chain (i.e., microregularity of the CPE polymer chains). Depending on the chlorine content, the maximum decomposition temperature varies from 310 to 335 ºC. Stoeva and co-workers [24] studied and compared the thermal analysis of high density polyethylene (HDPE), CPE (with different chlorine content) and polyvinyl chloride (PVC). Figure 11.4 shows that HDPE has mainly a single stage of 357
Thermal Analysis of Rubbers and Rubbery Materials
Table 11.3 The properties of polyethylene at different chlorination contents Cl content (%) 8 25 40 45 54 60
Structure at room temperature Crystalline Amorphous Amorphous Amorphous Amorphous Amorphous
Physical form at room temperature More flexible Rubbery Soft, flexible Flexible, leathery Rigid Rigid, brittle
Brittleness temperature (ºC) < -70 < -70 < -70 -20 20 40
Melting point (ºC) 69 20 20 30 52 67
Figure 11.4 Comparison of TGA of HDPE (1), CPE [Chlorinated PE; 6.26% Cl (2) 22.16% Cl (3) 56.09% Cl (4)] and PVC (5)
decomposition whereas CPE have two stages of decomposition one starting from 250 to 375 ºC, another at 400 up to 550 ºC. Interestingly, there is a difference between the thermal stability of CPE with 56.09% Cl, and PVC, although they have the same chlorine content. In PVC there is regular distribution of chlorine groups (Cl) which commonly correspond to the ‘head-to-tail’ pattern. Hence, the dehydrochlorination is more facile because of 358
Thermal Properties of Chemically Modified Elastomers the easier elimination of hydrogen chloride from chloroallylic fragments. It also leads to a stable polyethylene structure. In CPE (with 56.09% Cl), chlorine atoms are randomly distributed ‘block wise’, or ‘head-to-head’ or ‘tail-to-tail’. So PVC decomposes at faster rate than CPE though both of them have the same chlorine content. It can be mentioned here that the hydrogen chloride produced during the decomposition of PVC catalyses the intermolecular and intramolecular dehydrochlorination in PVC [26]. CPE shows mainly two stages of decomposition [22-25]. The first one is characterised by dehydrochlorination (by evolution of hydrogen chloride), both intermolecular and intramolecular at temperatures below 300 °C in an inert atmosphere [22]. The second stage is due to the destruction of the main polymer chain which usually occurs above 400 ºC. Dehydrochlorination is also accompanied by the rearrangement of double bonds in macromolecular chains resulting in the formation of polyacene rings and/or a crosslinked polycyclic structure. The planar crosslinked structure can be converted to a graphite-like structure at a very high temperature of ~800 ºC [24]. In air the dehydrochlorination takes place above 210 ºC. Thermal degradation of CPE in air accompanies the evolution of gases such as HCl, H2O, CO, CO2, CH3OH, and so on. Evolution of these gases indicates some oxidation processes take place. It also releases methane, ethane, propane and to lesser extent chlorinated hydrocarbons [23, 24]. Chlorination of PVC enhances its high temperature properties such as flammability which are determined by LOI. Chandler and co-workers [26] increased the chlorine content in PVC up to 70 wt% by chlorination. Interestingly, on this chlorination of PVC the LOI increased from 45-49 (for pristine PVC) to 70. The amount of smoke evolved decreased drastically with increasing chlorination of PVC. Saturation of cis butadiene rubber (BR) with chlorination yields almost the same Tg as PVC (56.5% Cl). It can be mentioned that trans-CR (39.7 wt% Cl) has a lower Tg (-40 ± 5 ºC) than PVC because of the high unsaturation content of CR. Hydrochlorination of crystalline polydiolefins with trans structure (such as trans 1,4 polybutadiene) (trans-BR) reduces the crystallinity in the polymer chain by introducing the bulky substituents in the chain. Bruzzone and co-workers [27] reported the hydrochlorination of trans-BR using tin(VI)chloride, as the catalyst at a temperature range of -30 to + 300 ºC. Figure 11.5 shows the Tg of transBR at different chlorine content obtained at different reaction conditions. The large scattering of Tg values is due to the different hydrochlorination reaction; the lowest Tg is at the mildest reaction condition. However, the general trend is that the Tg increases with increasing chlorine content in trans-BR. There was decrease in Tm, crystallinity and enthalpy change (Hm) on hydrochlorination of trans-BR. Figure 11.6 shows the effect of Tm and crystallinity (obtained by x-ray diffraction analysis) on the hydrochlorination of BR. The Tg and Tm of the hydrochlorinated BR were calculated from the DSC analysis. Figure 11.7 shows a typical DSC spectrum of a hydrochlorinated trans-BR with a chlorine content of 7.9 wt%. The Hm was calculated to be 8 cal/g. Interestingly, the decrease in Tm in hydrochlorinated trans-BR was almost in conformity with the Flory’s equation [28]: 359
Thermal Analysis of Rubbers and Rubbery Materials 1 1 R 0 ln A Tm Tm H m
(i)
Where Tm = melting point of chlorinated trans-BR T0m = melting point of trans-BR A
= mole fraction of butadiene unit
Hm = Enthalpy change in trans-BR
11.5 Chemical Modification by Grafting Chemical modification via grafting is a useful way to alter the properties of the existing elastomers and to achieve the desirable properties which are difficult to obtain from either of the homopolymers. Grafting and graft copolymerisation can be carried out in both diene as well as in saturated elastomers. The field of grafted copolymers is a very vast topic and is beyond the scope of this chapter. However, in this sub-chapter a few typical examples of the thermal properties of different grafted elastomers will be discussed. It will explain how the different thermal properties change with the extent of grafting. Chemical modification by the grafting of maleic anhydride (MA) is a very useful way to induce polarity in conventional elastomers. There are reports of grafting of MA on
Figure 11.5 Effect of chlorination on Tg of trans-BR
360
Thermal Properties of Chemically Modified Elastomers
Figure 11.6 Effect of hydrochlorination on Tm of trans-BR
Figure 11.7 DSC thermogram of a hydrochlorinated trans-BR
361
Thermal Analysis of Rubbers and Rubbery Materials ethylene-propylene diene terpolymer (EPDM) [29-31] ethylene-propylene rubber [32, 33] and on NR [34, 35]. These maleated graft copolymers are used as blend compatibilisers, in reactive blending, as the main blend components. The Tg of the maleated NR was studied using DSC analysis. Table 11.4 shows the Tg of NR, maleated NR (mNR) at different degrees of grafting of MA. The pure NR has a Tg of –70 ºC. The Tg of mNR increases with increasing MA content in mNR (Table 11.4). It can be attributed due to the restriction in chain mobility and chain flexibility because of the presence of the bulky MA group. There is also increase in interchain interaction between the polar groups on increasing the MA content in NR. Hydrogen bonding is also more pronounced for the graft copolymer when the content of MA is very high. All these factors lead to an increase of Tg in mNR. Ray and co-workers [31] reported the melt grafting of MA on EPDM having 5 mol% ethylidene norbornene (ENB). The thermal properties of the maleated EPDM (m-EPDM) (with 6% degree maleation) were studied using DSC, and DMA analysis. DSC analysis showed there was decrease in Tg from –50 ºC (for EPDM) to –55 ºC (for m-EPDM). DMA analysis (Figure 11.8) also shows the same trend of shift in tan from –31 ºC to –36 ºC. In EPDM the maleation takes place in the pendant ENB site. The increase in bulkiness of the pendant group probably causes the plasticisation in intermolecular chains in the EPDM elastomers, and hence decreases the Tg. Figure 11.8 also shows that there was decrease in storage modulus (E´) and in tan (by 0.23) in the m-EPDM. Chemical modification of elastomers by grafting with hydrophilic moieties makes them suitable for many important applications like hygroscopic coating reagents, waterleak sensors, biomedical application and so on. There are reports of grafting of polydimethylacrylamide on NR [36], polyethylene oxide (PEO) on chlorosulfonated PE [37], PEO on butyl rubber (IIR) [38-39], and carbohydrates on IIR [40]. PEO of different molecular weights (Mn = 750, 2000, 5000) were grafted on chlorobutyl rubber (CIIR). Table 11.5 shows the results of DSC analysis of CIIR, CIIR-g-PEO, and blends of CIIR, PEO (designated as Blend-750, Blend-2000, Blend-5000). In the blends of PEO and CIIR the PEO content of the blend materials is equal to those of the corresponding PEO-g-CIIR. There is no large difference between the Tg of the grafted blends and the corresponding blends. But the Tm of the grafted materials was
Table 11.4 Effect of degree of maleic anhydride grafting on the Tg of NR Sample Name NR mNR3 mNR6 mNR9 mNR10
362
Maleic Anhydride (wt%) 0 3 6 9 10
Tg (ºC) -70.0 -63.0 -60.4 -59.9 -58.8
Thermal Properties of Chemically Modified Elastomers
Figure 11.8 DMA analysis of maleated EPDM
Table 11.5 DSC Analysis of IIR, IIR-g-PEO Sample Code
Mw of PEO
CIIR PEO-750 750 Blend –750 750 IIR-g-PEO-750 750 PEO-2000 2000 Blend-2000 2000 IIR-g-PEO-2000 2000 PEO-5000 5000 Blend-5000 5000 IIR-g-PEO-5000 5000 a : Heat of fusion of PEO domains b : Not measured
PEO Content (wt%)
Tg (ºC)
DSC Tm (ºC)
100 10.2 10.2 100 11.3 11.3 100 11.5 11.5
-63.8 -65.2 -66.3 -65.2 -65.7 -65.4 -65.5
32.0 27.6 4.5 52.0 52.4 33.2 59.0 60.2 46.0
Ha (mJ/mg) -b 158 89 -b 163 138 -b 184 168
363
Thermal Analysis of Rubbers and Rubbery Materials shifted to the lower temperature than that of the blends. There was no significant change in Tm of the blend materials from that of PEO. The IIR-g-PEO film was optically clear though the blends of CIIR and PEO were opaque. Further studies of the film properties indicate that the blend of CIIR and PEO undergoes macrophase separation, whereas IIR-g-PEO undergoes microphase separation. DMA is widely used to study the domain-matrix separation in copolymers or in polymer blends. If the two components are compatible (i.e., there is complete phase mixing) in any block or in any graft copolymer, the respective polymer shows only single transition somewhere in between the transition temperatures of the constituents. If the segments are incompatible, they show two transitions at the transition temperature corresponding to that of two segments. When the system has partial microphase mixing or with different interphase, it shows a broad transition which differs from either homopolymers. The dynamic modulus becomes less steep and it gradually decreases over a wide range of temperature. For all IIR-g-PEO samples the dynamic loss (tan ) is broad. It indicates longer grafted PEO segments aggregate to form larger PEO domains. For example, although the number of PEO segments are more in IIR-g-PEO 750, the larger domains are formed in IIR-g-PEO 5000, with a peak with shoulder (Figure 11.9). The broad peak was shifted to higher temperature with increase in molecular weight of PEO segment. The broad peak is considered to overlap with tan peak for the melting of PEO segments. The E´ of the grafted samples showed distinct decrease due to the melting of PEO segments at 30-50 ºC (Table 11.6). TGA was carried to determine the thermal stability of IIR-g-PEO. Table 11.7 shows the decomposition temperature (T10) of different grafted samples at which there was 10% weight loss. The decrease in T10 of the blend or the grafted IIR with respect to CIIR is due to the introduction of PEO oligomers which had T10 at 235 ºC. However, all grafted IIR have higher T10 than the CIIR-PEO blends. It can be attributed due to the better compatibility in the grafted polymers than in the blends.
Table 11.6 DMA of IIR-g-PEO Sample Code
Number of PEO Transition Transition Side Chains on temperature temperature 100 Butylene (ºC)a (ºC)b Units IIR-g-PEO-750 0.84 -52 -31 IIR-g-PEO-2000 0.36 -52 -27 IIR-g-PEO-5000 0.13 -52 -25 a : Corresponding to the Tg of rubber matrix b : Corresponding to the mechanical relaxation of PIB segment of IIR
364
E´ at 150 ºC (Pa)
0.0115 0.0142 0.0131
Thermal Properties of Chemically Modified Elastomers
Figure 11.9 DMA analysis of IIR and IIR-g-PEO
Table 11.7 Thermal stability of IIR-g-PEO Sample Code T10a (ºC) CIIR 313 Blend- 2000 297 IIR-g-PEO-750 307 IIR-g-PEO-2000 301 IIR-g-PEO-5000 302 a : Decomposition temperature in which 10 wt% of polymer degraded
Nowadays, elastomers are being used in optoelectronics because of its greater flexibility and many other important properties [41]. Silicone elastomers are important components in optical fibres, because of their many unique properties such as excellent thermal stability, chemical resistance, nonflammability and most importantly they are very transparent, which is a determining factor in reducing optical loss [42]. Different silicone rubbers like poly(methyl hydrogen siloxane) (PMHS) and poly(dimethyl siloxane-comethyl hydrogen siloxane) (PDMS-MHS) have been chemically modified by grafting with various materials like styrene, 2-vinyl naphthalene (VN), 9-vinyl anthracene (VA), perfluoro-octyl ethylene (PFOE). Grafting of these materials increases the refractive index of the silicon rubber [43] leading to several applications in optoelectronics. There was change in the Tg of the grafted copolymers (Figure 11.10). There are several equations in the literature which describe the relation between Tg, the composition of the copolymers. 365
Thermal Analysis of Rubbers and Rubbery Materials The Gordon-Taylor-Wood equation [44, 45]: W1 (Tg – Tg1) + KW2 (Tg – Tg2) = 0 The Fox equation [46]: W W 1 1 2 Tg Tg1 Tg2
The Dimarzio, Gibbs’ equation [47]: Tg = m1 Tg1 + m2 Tg2 Where Wi = Weight fraction of monomer unit ‘i’ Tgi= The glass transition of homopolymer ‘i’ Tg = The glass transition of the copolymer k = A constant in the Gordon-Taylor-Wood equation mi = The molar fraction of monomer unit ‘i’ In the graft copolymers of polysiloxanes the Gordon-Taylor-Wood equation gave the best fit. The Tg of the grafted copolymer increased with increasing the degree of grafting as well as with size of the substituents. Polysiloxanes grafted with VN have higher Tg than
Figure 11.10 Tg of silicone rubber and its graft copolymers
366
Thermal Properties of Chemically Modified Elastomers those grafted with styrene. The k value in the Gordon-Taylor-Wood equation increases with increasing size of the substituents.
11.6 Chemical Modification by Introducing Ionic Groups Chemical modification of elastomers can be carried out by introducing the ionic groups in the form of sulfonate, phosphonate or carboxylate groups. These ionic groups are in the salt form of carboxylate, sulfonate or phosphonate groups. Because of the presence of these ionic groups, these polymers show unique properties which lead to different important applications [48-50] such as batteries, sensors, displays, in fuel cells, and in thermoplastic elastomers for high temperature application. Sulfonation can cause considerable change in overall thermal stability of a polymer [51-53]. Sulfonated polymers usually show a three step degradation pattern upon TGA: dehydration, desulfonation and degradation. Suleiman and co-workers [51] sulfonated polystyrene-isobutylene-styrene (SIBS) block copolymers and characterised them by TGA. Figure 11.11 shows the TGA of SIBS (S-SIBS-O), sulfonated SIBS with 42% sulfonation (S-SIBS-42). SIBS shows only a single decomposition at 432 ºC. S-SIBS-42 shows a small mass loss at 150 ºC, another at 290 ºC. The maximum degradation takes place at 448 ºC. Sulfonation also influences the Tg of the polymers. Mokrini and Acosta [54] carried out sulfonation (15%) of hydrogenated polystyrene butadiene (sHSB) and blended it with the unsulfonated polymer (designated with HSB). They determined the Tg of the sHSB, and its blends with HSB, by DMA analysis. Figure 11.12 shows the variation of tan
Figure 11.11 TGA of styrene-isobutylene-styrene block copolymers and its sulfonated derivative
367
Thermal Analysis of Rubbers and Rubbery Materials
Figure 11.12 DMA analysis of styrene butadiene block copolymers and its sulfonated derivative
versus temperature in the sHSB and sHSB/HSB blends. Tg was calculated from maximum signal of tan and results are shown in Table 11.8. It indicates there is slight increase of 5 ºC in the Tg of the hydrogenated polybutadiene (PHB) segment. But the Tg of the polystyrene segment (PS) increases by 30 ºC after sulfonation (15%). This increase in Tg is due to the restrictions of segmental movement in the PS blocks due to the introduction of sulfonate groups. In the blends Tg increases as the sHSB content increases. This increase in Tg is directly associated to the ion content [48]. Ionomers are interesting polymeric materials which have a small amount ( 10%) of ionic groups. The presence of a low amount of ionic groups has a dramatic effect on the
Table 11.8 Tg of hydrogenated poly(styrene-b-butadiene)(HSB), sulfonated HSB (sHSB) and their blends Sample HSB sHSB (15% sulfonation) Blend - 1 Blend - 2
368
Composition (%) sHSB HSB-PS 0 100 100 0 90 10 70 30
Tg (ºC) 90 120 121 109
Tg (ºC) PHB -51 -46 -45 -49
Thermal Properties of Chemically Modified Elastomers physical and mechanical properties of polymers [55-59]. The balance between coulombic and elastic forces produces different microstructures which control the material properties. Several theories have been postulated to define the microstructures in ionomers. In the elastomeric ionomers the ionic groups associate to form ion-rich domains which enable them to be processed like a thermoplastic at elevated temperatures. The ultimate properties of elastomeric ionomers depend on the level and type of ionic functionality (e.g., sulfonate, carboxylate, phosphate and so on), the degree of neutralisation, the types of cation (e.g., ammonium salt, valency of the metal ions), the crystallinity of the backbone and so on. The effect of introducing a sulfonate ionic group into a polymeric material (in this case PS [59]) is shown in Figure 11.13. It shows that the Tg of ionomeric PS occurs at higher temperature than the unmodified PS. After heating the ionomer above its Tg it exhibits an extended plateau in modulus analogous to a rubber. So the ionic species effectively crosslinks the PS matrix. The ionic crosslinks are thermally labile. At low temperatures the ionomers are glassy, at higher temperature they can be elastically deformed like a rubber and at still higher temperatures they can be processed like a viscous fluid. Figure 11.14 shows the typical loss tangent results of PS and a sodium salt of sulfonated PS. These are two relaxations, and named in order of decreasing temperature. The
relaxation is ascribed due to the break-up of the ionic association, as the ionomers flow at a temperature above the peak. The temperatures depend on the nature of metal salts. For example, ionomers prepared with zinc salts have peaks at a lower temperature than those prepared with sodium salts. It also depends on the nature of
Figure 11.13 Modulus of polystyrene and its sulfonated ionomer (7.5% sulfonation) as a function of temperature
369
Thermal Analysis of Rubbers and Rubbery Materials
Figure 11.14 Tan of polystyrene and its ionomers as a function of temperature
ionic groups. The carboxylate ionomers have relaxations at much lower temperatures than the sulfonate ionomers, because the ionic interactions in sulfonate salts are stronger than the carboxylate ones. The preparation and the properties of different types of elastomeric ionomers have been reviewed [58, 60]. The important ionic elastomers are zinc as well as sodium salt of sulfonated EPDM, zinc salt of maleated EPDM, zinc salt of carboxylated nitrile rubber (Zn-XNBR). Uniroyal has commercialised a number of ionic elastomers for a wide variety of applications such as adhesives, impact modifiers, footwear, hoses and so on. Ghosh and co-workers [61, 62] carried out the sulfonation of m-EPDM) and studied the dynamic mechanical thermal analysis (DMTA) of the sulfonated-m-EPDM (s-m-EPDM) based on different salts. Figure 11.15 shows the plots of tan versus temperature for m-EPDM, s-m-EPDM, Na-s-m-EPDM and Zn-s-m-EPDM. Table 11.9 shows the results of tan for different polymers. The peaks at lower temperature (Figure 11.15) is due to the glass-rubber transition. The sulfonated EPDM and its salts have higher Tg than its parent rubber. The peak height of the tan curve decreases in these cases. This is due to the restriction of chain mobility in the ionic crosslinks in sulfonated salts of EPDM. Furthermore, these sulfonated EPDM show a second transition at higher temperatures, known as the ionic transition. There is also a drastic change in the storage modulus for the sulfonated ionomer. The variation of the storage modulus (E´) of the polymer versus temperature is shown in 370
Thermal Properties of Chemically Modified Elastomers
Figure 11.15 Tan versus temperature for m-EPDM, s-m-EPDM, Na-s-m-EPDM and Zn-s-m EPDM
Table 11.9 Dynamic mechanical thermal analysis of m-EPDM and its sulfonated derivatives Polymer
Tg (ºC)
tan at Tg
Ti (ºC)
m-EPDM -35.0 1.27 a s-m-EPDM -30.6 1.24 a Na-s-m-EPDM -29.0 0.85 89.5 Zn-s-m-EPDM -27.4 0.69 119.6 a: no high temperature (ionic) transition
tan at Ti a a 0.16 0.14
Storage modulus (Pa) x 108 25 ºC 140 ºC 2.4 0.6 2.8 1.2 3.3 5.5 8.9 9.6
Figure 11.16. Compared to the base elastomers, the ionomers show a broad rubbery plateau, with a higher, stronger modulus, which is due to the physical crosslinks arising out of the ionic aggregates. When the ionic association is stronger, the storage modulus became higher. The zinc-salt of sulfonated EPDM showed a higher storage modulus because of the stronger ionic associations. Zn-s-EPDM containing a few metal sulfonate groups has unique properties, and can be considered as replacement of vulcanised EPDM rubber, because of the strong ionic linkages [63, 64]. The ionic elastomer based on 371
Thermal Analysis of Rubbers and Rubbery Materials
Figure 11.16 Log E´ versus temperature for m-EPDM and its sulfonated derivatives
Zn-m-EPDM behaves as an ionic thermoplastic elastomer. It shows a Tg at –37 ºC and a high temperature ionic transition above 50 ºC [65]. Weiss and co-workers reported the properties of ionomers based on sulfonated polystyrene/polyethylene-co-butene/ polystyrene block copolymers (SEBS)] [66]. Phosphate containing polymers are frequently used in fire retardant materials. Mequanint and co-workers [67] reported phosphated polyurethane (PU) ionomers by introducing phosphate group into the soft segment in PU. They carried out TGA studies of phosphate containing PU, observed three stages of degradation at 200 ºC, 350 ºC and maximum degradation at 450 ºC. When the phosphate content in the PU increases (from 1% to 2.5% phosphorus content), the decomposition residue (char) were high and the polymer was thermally stable up to 700 ºC. The higher char yield of phosphated PU was related to its fire retardant property and could be used in fire retardant coatings.
11.7 Miscellaneous Thermoplastic elastomers are interesting materials having unique properties [67-69]. The structure property relationship and mechanical properties of polystyrene-polybutadienepolystyrene (SBS) block copolymers are well-reported [68, 69]. These block copolymers lose most of their strength above 70 ºC, even though the Tg of the PS segment is 100 ºC [68-71]. When the mid-segment is hydrogenated, the resultant SEBS (polystyrene-b-poly 372
Thermal Properties of Chemically Modified Elastomers (ethylene-co-butene)-b-polystyrene block copolymers can have higher service temperature than SBS. But mainly the service temperature of SBS or SEBS thermoplastic elastomer is restricted to the Tg of the end segments, i.e., the PS segments. Research is underway to improve the service temperature of these block copolymers by chemical modification of the hard segment. Since this segment has an aromatic ring, there have been a lot of efforts to perform chemical reactions on this ring, so that the Tg of this segment will increase. Nitration [72, 73] of the aromatic groups in SBS block copolymer (Kraton G) increases the Tg of the end block from 100 ºC to 148 ºC. Being a strong polar group, nitration induces the polarity in the macromolecular chain, hence the interchain interaction increases. Thus the mobility of these segments decreases, which eventually increases the Tg of the PS segment. Nitration has an adverse effect on the very high temperature properties of the polymers. On nitration [74] the initial decomposition temperature (Ti) of polyacenaphthylene decreases from 404 ºC to 341 ºC. Higher the degree of nitration, larger is the decrease in Ti. Molnar and co-workers [75] reported the different chemical modification on the aromatic ring in polysulfones. Ti decreases from 442 ºC to 334 ºC on nitration of the aromatic ring. The nitrated polysulfones have several stages of decomposition in which the first stage starts at 334 ºC. It could be due to the conversion of the nitro group to the nitroso group. It is also reported in the decomposition of nitrated PS [76]. Benzoylation increases the modulus and ultimate strength and ultimate elongation. SEBS having 30% styrene unit (Kraton G 1652) was benzoylated using benzoyl chloride as reagent and AlCl3 as catalyst [77]:
The Tg of the benzoylated derivatives were determined by DMTA, as shown in Table 11.10. It indicates that on benzoylation of the aromatic ring in the end segment i.e., the PS unit, the Tg increases. As the mol% of benzoylation increases, the Tg also increases. However, the Tg of the midsegment i.e., the ethylene-co-butylene, segment remains unaffected. Figure 11.17 shows the stress-strain plot of Kraton G 1652 and its benzoylated derivatives. It indicates that the 70 mol% benzoylated SEBS shows elastomeric behaviour at 85 ºC. Kraton G. 1652 shows viscous behaviour at 65% benzoylation. It means on benzoylation there is significant increase in the upper service temperature. Benzoylation increases the apparent volume fraction of the hard phase in 373
Thermal Analysis of Rubbers and Rubbery Materials
Figure 11.17 The stress-strain plot of SEBS and its benzoylated derivatives at 85 °C
Table 11.10 The Tg of Kraton G 1652, and its benzoylated derivative Benzoylation (mol %) 0 35 45 70
Tg of the middle segment (ºC) -34 -38 -34 -34
Tg of the end segment (ºC) 98 114 116 121
these polymers as well as the interchain interaction which lead to higher softening point as well as its high temperature utility. Naphthoylation [78, 79] as well as acetylation [80, 81] also have a similar effect in thermoplastic elastomers.
Summary The different types of chemical modification are used to modify the elastomers to achieve interesting and improved properties especially thermal properties which are difficult to achieve otherwise. This chapter surveys the basics of the change of thermal properties of the elastomers on different chemical modification as well as the typical examples of the chemically modified rubbers. 374
Thermal Properties of Chemically Modified Elastomers
Acknowledgements The author wishes to thank his students for their input into this manuscript. Thanks are also due to Satya Sadhan Dutta for his help in typing and in editing this chapter.
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Thermal Properties of Chemically Modified Elastomers 65. S. Datta, S.K. De, E.G. Kontos, J.M. Wefer, P. Wagner and A. Vidal, Polymer, 1996, 37, 15, 3431. 66. R.A. Weiss, A. Sen, L.A. Pottick and C.L. Wills, Polymer, 1991, 32, 15, 2785. 67. K. Mequanint, R. Sanderson and H. Pasch, Polymer Degradation and Stability, 2002, 77, 1, 121. 68. G. Holden in Applied Polymer Science: 21st Century, Eds., C.E. Carraher and C.D. Craver, American Chemical Society, Washington, DC, USA, 2000. 69. Thermoplastic Elastomers, 2nd Edition, Eds., G. Holden, N.R. Legge, R.P. Quirk and H.E. Schroeder, Hanser Publishers, Munich, Germany, 1996. 70. D.M. Brunwin, E. Fischer and J.F. Henderson, Journal of Polymer Science, Part C: Polymer Symposia, 1969, 26, 1, 135. 71. L.J. Fetters and M. Morton, Macromolecules, 1969, 2, 5, 453. 72. H.J. Harwood and S.W. Jolly, inventors; University of Akron, assignee; WO9701548, 1997. 73. S.W. Jolly, Arylsulfonylation and Nitration of a Poly[styrene-b-(Ethylene-coButylene)-b-Styrene] Thermoplastic Elastomer, The University of Akron, OH, USA, 1993. [Ph D Thesis] 74. A. Flores, M. Rodriguez, M.L. Morras and L.M. Leon, Polymer Degradation and Stability, 1998, 61, 2, 259. 75. G. Molnar, A. Botray, L. Poppl, K. Torkos, J. Borossay, A. Mathe and T. Torok, Polymer Degradation and Stability, 2005, 89, 3, 410. 76. M.J. Fernandez and M.D. Fernandez, Polymer Degradation and Stability, 1998, 60, 2-3, 257. 77. T. Wright, A.S. Jones and H.J. Harwood, Journal of Applied Polymer Science, 2002, 86, 5, 1203. 78. H.J. Harwood, A.S. Jones and M.A. Smook, inventors; The West Company, Inc., assignee; US 5110876, 1992. 79. H.J. Harwood, A.S. Jones and M.A. Smook, inventors; The West Company, Inc., assignee; US 5272215, 1993. 80. L. Liu and M. Jiang, Macromolecules, 1995, 28, 25, 8702.
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Thermal Analysis of Rubbers and Rubbery Materials 81. L.E. Carson, Chemical Modification of Polystyrene and Poly(Styrene-bEthylene-co-Butylene)-b-Styrene, The University of Akron, OH, USA, 1994. [PhD Thesis]
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12
Thermal Analysis of Rubber Products R.S. Rajeev and P.P. De
12.1 Introduction Selecting and compounding the right rubber for product development is a balancing act. A lot of demanding and often contradictory requirements need to be met within a single rubber product. Often a single rubber may not fulfil all the property requirements of the finished product. In many cases, the product will be a combination of different individual rubber compounds having different compositions, assembled together (for example, tyre). The compound should also meet the requirements of the processing equipment. Analysis of such a product, which may often contain more than one rubber and different parts having different rubber and/or rubber-rubber blends, is a challenging task. There is a possibility that the properties of some of the constituent rubbers may be close to each other which makes the analysis complicated. In such cases, analysis using single equipment may not yield conclusive results, especially if the aim is reverse engineering or formula reconstruction. For example, if the rubber blend contains two rubbers in which the glass to rubber transition temperature (Tg) of one rubber is close to that of the other, spectroscopic and chemical analysis techniques should be carried out in conjunction with thermal analysis. Again, determination of Tg using a single thermal analysis technique may not be often sufficient. Thus, differential scanning calorimetry (DSC) results may need to be confirmed by carrying out dynamic mechanical analysis (DMA) or thermo mechanical analysis (TMA). Secondary transitions in a polymer are more evident in DMA results compared to DSC results [1]. The thermal stability of a rubber product can be determined by using thermogravimetric analysis (TGA). However, information regarding the dynamic properties of the product at elevated temperatures needs to be obtained by using DMA. The variety of additives used in the rubber compound pose further challenges. Plasticisers will reduce the Tg of the rubber and reinforcing fillers will lower the tan (also called damping, loss tangent and loss factor) peak height. Thermal analysis is an important analytical technique used in the rubber industry for raw material analysis, product development, quality assurance, failure analysis and reverse engineering of the final product. The common techniques used are DSC, TGA, DMA and TMA. A complete thermal analysis of a rubber product is not possible with a single technique. For example, as pointed out by Sircar [1], it may not be possible to 381
Thermal Analysis of Rubbers and Rubbery Materials detect the minor transitions arising from the side-chain motion or crankshaft motion of a few segments of the main chain of the polymer by DSC. Such transitions, termed as , and , as they occur in the order of decreasing temperature, with melting temperature or Tg termed as transition, can be recorded using DMA. In fact, in many cases, for the successful identification of molecular motions responsible for relaxation, a combination of techniques involving DMA, dielectric relaxation and nuclear magnetic resonance spectroscopy are needed [2]. Rubber products are ubiquitous (from household products to those used in aircraft and space applications). Some of the household rubber products include shoe soles, carpet backing, gloves, toys and bushes apart from sports goods. The automobile and transportation industry use a substantial number of rubber products such as tyres, window channels and seals, vibration dampers, cable sheaths, gaskets and mats. Space applications include thermal insulators, seals, O-rings and gaskets. Thermal analysis of a rubber product is complex because of the complexity of the product itself, which requires specific procedures for characterisation and analysis. However, the procedure for the thermal analysis of many of the rubber products follows a certain pattern. The most popular thermal analysis technique for rubber application is DSC followed by TGA. Derivative thermogravimetry (DTG) is helpful for the accurate determination of maximum temperature of decomposition of the product at various stages of the thermal programme. For product development, failure analysis and product improvement, a combination of techniques including combined TGA and DTG (TG-DTG), DSC, DMA and TMA are used. Dielectric analysis has not yet been widely used for rubber product analysis though reports are available on its use on the analysis of composition of nitrile rubbers [3, 4] and analysis on the compatibility of rubber blends [5]. One of the advantages of dielectric method is the possibility of exploring an extended frequency range and, therefore, avoiding reliance on the time-temperature superposition principle [1]. A comprehensive description of thermal analysis of rubbers has already been given by Sircar [1] and Maurer [6]. The various ASTM methods on the thermal analysis of polymers are also listed out in [1]. A detailed description of thermal analysis of all the rubber products is beyond the scope of this book. In this chapter, application of thermal analysis for the development and analysis of some of the common rubber products are described with special emphasis on DSC, DMA and TGA. TMA is not explained in detail because of the detailed description of TMA of rubber and rubber products in Chapter 5. The users are advised to follow their national and international standards for the accurate thermal analysis procedures. The authors would also like to mention that the instruments and methods described in this chapter are mentioned only as examples. The authors do not endorse the use of any particular instrument or method for any of the analysis purposes. The use of the methods and instruments mentioned in this chapter shall be made purely based on the reader’s discretion. Before attempting any of the procedures described in this chapter, it is advised that the users should refer to the instructions in the operation manuals of their instruments and/or contact the instrument representative. 382
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12.2 Thermal Analysis of Rubber Based Vibration Control Devices
12.2.1 Introduction Control of noise and damping of vibrations are required in many day-to-day applications including automotive engines, machinery parts, railroads, bridges and other structural applications. These noise and vibrations are controlled mainly by: 1. absorption of the vibrations; 2. use of barriers and enclosures; 3. damping of the vibrations, and 4. isolation of the vibrations. Because of their viscoelastic characteristics, rubbers are used as vibration control devices in the latter two areas. That is, they are used as vibration dampers and vibration isolators. The application of a damping material like rubber (having a loss factor in the order of 0.1-0.4) to a vibrating surface like metallic material (with very small damping or internal friction, loss factor of the order of 0.001-0.004) converts the vibration energy, which otherwise would have been radiated as airborne noise, into heat, which is then dissipated within the damping material. High damping or internal friction is essential in decreasing the effect of undesirable vibrations. Thermal analysis techniques like DMA and DSC help in identifying the suitable rubbers for the development of vibration control devices by determining their properties like Tg, modulus and tan . TGA and DMA help in identifying the usable temperature range for the optimum performance of such products.
12.2.2 Vibration Damping Damping is the dissipation of energy, usually by releasing it in the form of heat. In vibration damping, vibration energy in the structure is dissipated mainly as heat before the building up and spreading of the vibration. A damping treatment consists of any material applied to a structural component to increase the latter’s ability to dissipate mechanical energy. By bringing structures, made by using materials having very little internal damping (steel, aluminium and glass), into intimate contact with a highly damped, dynamically stiff material (for example, rubber), it is possible to control the vibrations. Since rubbers are viscoelastic, they are capable of storing strain energy when deformed, while dissipating a portion of this energy as heat unlike steel, which will radiate the vibration. 383
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12.2.2.1 Types of Vibration Damping Two typical ways of attaching a viscoelastic material as vibration damping layer on a structure are: a.
Free-layer or extensional damping. Here the viscoelastic material is attached to the plate to be damped by means of a strong bonding agent, as shown in Figure 12.1a [7]. The energy is dissipated as a result of extension and compression of the damping material under flexural stress from the base structure. Damping increases with damping layer thickness. Changing the composition of a damping material may also alter its effectiveness.
b.
Constrained-layer damping (CLD). This type of vibration damping device is usually used for very stiff structures. A ‘sandwich’ is formed by laminating the base layer to the damping layer and adding a third constraining layer, as shown in Figure 12.1b [7]. When the system flexes during vibration, shear strains develop in the damping layer. Energy is lost through shear deformation, rather than extension of the material. In constrained layer damping, damping is affected by the relative thickness of the various layers, the stiffness of the base and top plates, and the loss tangent and shear storage modulus of the viscoelastic layer.
12.2.3 Vibration Isolation In vibration isolation, the transmission of vibration energy from one system to another is reduced by the isolator. Steel springs and rubber pads are examples of common vibration isolators apart from automobile suspension and shock absorbers. Portable electronics, CD drives and vehicle mount electronics are protected from vibration and shock by vibration isolators. Because of their viscoelastic characteristics, rubbers provide both the spring force and damping and therefore can act as vibration isolators. Incorporating a vibration isolation system reduces the transmitted vibration levels in a structure by allowing the relative motion between the vibration source and the supporting structure. This is typically accomplished by providing a resilient connection between the two. Controlling the natural frequency of a vibrating system is one of the methods to control the vibration; another method being damping, which can be accomplished by using rubbers. Damping reduces the amplitude of resonant vibration by converting a portion of the energy into heat. Thus, rubbers in vibration isolation applications provide both resilience and damping required for the isolation of vibration. Rubbers control the vibration by undergoing hysteretic damping to dissipate energy. The loss factor is used to quantify the level of hysteretic damping of a material. The loss factor is the ratio of energy dissipated from the system to the energy stored in the system for every oscillation. A loss factor of 0.1 is generally considered a minimum value for 384
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Figure 12.1 Two typical ways of attaching a viscoelastic material as vibration damping layer on a structure: (a) extensional damping and (b) constrained layer damping [7] Reproduced with permission from N. Plesner, Technical White Papers, E-A-R Specialty Composites, Indianapolis, USA Copyright ©2002-2004, Aearo Company
significant damping. Compared to this value, materials such as steel and aluminium do not have a high level of damping. Thus it can be seen that rubbers act as both vibration dampers and vibration isolators because of their viscoelastic characteristics. Rubbers for vibration damping applications should have tan as high as practicable over a wide temperature-frequency range whereas 385
Thermal Analysis of Rubbers and Rubbery Materials those for vibration isolation applications should have high damping as well as resilience. Therefore an unfilled and unvulcanised rubber may be the ideal vibration damper (because of the high tan value) whereas a filled vulcanised rubber may be a better vibration isolator. However, for practical purposes, rubbers for both the applications should have the structural stability, shape and adequate physical properties which make vulcanisation and use of fillers necessary. Though the design of vibration isolator primarily requires resilient action as that of a spring and vibration damper requires damping as in the case of a dash pot, damped rubber may be a better vibration isolator because of the minimisation of the amplification of vibration by converting a part of the vibration energy into heat.
12.2.4 Selection of Rubbers for Vibration Damping Application and the Role of Thermal Analysis DMA and DSC are the two major thermal analysis techniques widely used for the design and development of rubber-based vibration control devices. Dynamic modulus and loss factor of rubber, which are important parameters for the design of vibration control devices, are determined by using DMA. DSC is mainly used for the determination of Tg and crystallinity of the rubber, which are important factors in the development of vibration control devices [1]. The other techniques like TGA and TMA aid in the reverse engineering or the formula reconstruction purposes to find out the composition of the final product. They are also used for general quality assurance purpose, for example, to determine the thermal stability or the dimensional changes of the product with temperature. The first requirement in the design of rubber-based vibration dampers is the selection of the rubber itself. The ideal rubber should have high loss tangent (tan ) for vibration energy dissipation as heat. At the same time, it should have a storage modulus comparable to that of the structure to which it is attached. These two requirements are difficult to meet simultaneously [1]. The highest loss tangent occurs at Tg where the highest storage modulus has not yet been attained. Careful selection of carbon black may cause an increase in the storage modulus while causing minimal decrease in tan [3]. Focusing on Tg in the desired range, Ball and Salyer [8] propose different methods, which may help in the selection of a polymer for vibration damping applications. Some of the suggested methods are: 1. Selection or synthesis of a homopolymer. 2. A copolymer with an intermediate Tg compared to that of the corresponding homopolymers (styrene-butadiene rubber (SBR) or acrylonitrile butadiene rubber (nitrile rubber or NBR)). 3. Use of polyblends of limited compatibility, which often give a broad Tg. 386
Thermal Analysis of Rubber Products Though the Tg can be determined both by static methods (for example, DSC, DTA, TMA and dilatometry) and dynamic methods (for example, DETA and DMA), for vibration damping applications, Tg is determined more commonly by using DMA because of the relationship of Tg with frequency -Tg increases with frequency. It should be noted that the Tg values determined by different thermal methods are somewhat different. Even within the same method, Tg can be different depending on the criteria chosen for the determination. Sircar and co-workers [1] have made a detailed analysis of Tg of elastomers determined by using various thermal analysis techniques including DSC, TMA and DMA. As can be seen in Figure 12.2, there are several locations in the DMA curve where Tg can be specified, such as the point of intersection of the two tangents at the inflection of storage modulus curve (onset temperature), the peak of loss modulus curve and the peak of tan curve [9]. Wetton [10] has mentioned the reasons for the preference of tan peak height over loss modulus peak height for denoting Tg though reports are available where the maximum value of loss modulus in the loss modulus-temperature curve is also considered as Tg [11,
Figure 12.2 A typical DMA curve showing the temperature dependence of storage modulus (E), loss modulus (E) and loss tangent (tan ) of a rubber sample. Tg can be specified at least in three different locations in this figure, such as the point of intersection of the two tangents at the inflection of storage modulus curve (onset temperature), the peak of loss modulus curve and the peak of tan curve [9] Reprinted with permission from J. Richard, Polymer, 33, 562. ©1992, Elsevier Ltd
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Thermal Analysis of Rubbers and Rubbery Materials 12]. Unlike loss modulus curve, the tan peak height and shape change in a systematic way with increasing amorphous phase. Hagen and co-workers [12] compared the Tg determined by the dynamic mechanical measurements of peroxide cured natural rubber vulcanisates using two different instruments, a Polymer Laboratories dynamic mechanical thermal analyser (DMTA) and a Perkin Elmer DMA. As mentioned earlier, one of the advantages of DMA over DSC is that minor transitions occurring due to the side-chain motions or crankshaft motion of a few segments of the polymer chain in a rubber can be determined by using DMA [13-15]. Rubbers for vibration damping applications should have tan as high as practicable over a wide temperature-frequency range. Tg can be considered as the lowest end use temperature for damping applications because in many instances, tan will essentially be zero below Tg. Ohm [16] mentions typical tan values for high damping rubbers. Crystallinity may reduce the damping and should be avoided. Crystallinity of rubber is determined by using DSC [1]. Though intensity of the tan peak height reduces with crosslinking, adequate crosslinking is required for suitable physical properties and structural stability. For many applications, the rubber selected for vibration control purposes should have high damping at room temperature along with good low temperature properties in a single formulation. This is particularly true in the case of vibration dampers for aerospace applications. Rubbers having high Tg, for example, fluorocarbon rubbers, exhibit reasonably high damping at room temperature. However, they cannot be used extensively for dynamic applications because of their high Tg (around -30 °C). Flexibility and elasticity at low temperature are essential for better dynamic applications. Among various polymers, butyl rubber (isobutylene-isoprene rubber or IIR) is the most commonly used damping material because of its damping characteristics over a broad temperature range with a peak between 10 and 100 kHz at ambient temperature. However, the problem with the butyl rubber is its low temperature crystallisation which makes the polymer stiff. On the contrary, polybutadiene rubber (BR) has good low temperature flexibility, but its damping characteristics are poor. This may lead to the choice of a 75:25 blend of cis-1,4- polybutadiene and butyl rubber which is expected to have good damping as well as low temperature properties [17]. These types of possibilities can be easily explored using DMA. The effect of incorporation of fillers like carbon black to the compound formulation to adjust the modulus of the vibration dampers can also be assessed using DMA. In general, increase of carbon black loading will decrease the tan peak height, thereby reducing the damping characteristics. However, incorporation of carbon black may diffuse the tan peak thereby damping characteristics can be extended to a wide range of temperatures [3]. Bandyopadhyay and co-workers [18] have used DMA to study the miscibility of polychloroprene rubber (CR)/epoxidised natural rubber with 50 mol% oxirane ring (ENR-50) blends in presence of high abrasion furnace (HAF) carbon black filler with an aim to develop vibration dampers. They report that incorporation 388
Thermal Analysis of Rubber Products of 30 phr HAF carbon black reduces the tan peak height in the 30/70 and 50/50 CR/ ENR-50 blends (Figure 12.3) from 1.732 to 1.023 and 1.171 to 0.522, respectively. It has also been noted that even with 5 phr filler, the tan peak is broadened in the 70/30 CR/ENR-50 blend (Figure 12.4). The broadening increases with increasing filler loading, resulting in the formation of two separate peaks, one occurring at the transition temperature of CR and the other occurring at a temperature higher than the transition temperature of ENR. The two different peaks show immiscibility of the rubbers in presence of carbon black. Such a blend composition can act as a broadband vibration damper. The authors assume formation of furanised ENR and the thermovulcanisation of CR as the reasons for the broadening of the peak with the furanisation catalysed by the presence of carbon black.
Figure 12.3 Effect of incorporation of HAF carbon black filler on the damping characteristics of CR/ENR-50 blends; ( __...__ ) 30/70 CR/ENR-50 unfilled; ( __x__ ) 30/70 CR/ENR with 30 phr filler; ( __.__ ) 50/50 CR/ENR-50 unfilled and ( __..__ ) 50/50 CR/ENR-50 with 30 phr filler [18] Reproduced with permission from S. Bandyopadhyay, P.P. De, D.K. Tripathy and S.K. De, Polymer, 1995, 36, 10, 1979. ©1995, Elsevier Ltd
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Figure12.4 Effect of loading of HAF carbon black filler on the damping characteristics of 70/30 CR/ENR-50 blend; ( __.__ ) 0 phr filler; ( __..__ ) 5 phr filler; ( __...__ ) 10 phr filler; ( __ __ ) 15 phr filler and ( __x__ ) 30 phr filler [18] Reproduced with permission from S. Bandyopadhyay, P.P. De, D.K. Tripathy and S.K. De, Polymer, 1995, 36, 10, 1979. ©1995, Elsevier Ltd
Though high damping (high tan value) is desired for vibration and noise control, for applications involving flexing, high loss factor may increase the heat build up; the same is true with a high filler loading. Very high loading of carbon black may improve damping due to interaction between carbon black particles which may increase hysteresis and damping. The effect of plasticisers on Tg is well documented [19-21]. Though plasticisers may reduce the Tg, they may broaden the glass to rubber transition region.
12.2.5 DMA for the Comparison of Different Rubber Based Shock Mounts The following example shows how DMA can be used to compare different rubber-based shock mounts. Figure 12.5 shows the comparative DMA damping curves (tan curves) 390
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Figure12.5 Comparison of three different elastomeric shock mounts using the tan versus temperature curve of DMA. A is the poor shock mount, B better and C, the best [22] Reproduced with permission from J. Foreman, Application Note TS-16A, TA Instruments Inc., New Castle, DE, USA. ©TA Instruments
of three shock mount rubbers [22]. All the three shock mounts exhibit a large damping peak in the range -50 °C to -30 °C indicative of their Tg. Material A has the highest Tg and the sharpest glass transition damping peak, as well as the lowest damping level above the Tg. All these factors contribute to its poor performance in standard shock mount tests. Materials B and C, on the other hand, have lower transition temperatures, broader glass transition damping peaks, as well as reasonable damping above the Tg. Both perform well as shock mounts – Material C which has the highest damping above Tg performs the best.
12.2.6 Interpenetrating Polymer Networks (IPN) as Vibration Dampers Recently, DMA has been used for the development of vibration dampers based on interpenetrating polymer network (IPN) systems. IPN can be broadly identified as any material containing two polymers, each in network form. According to the IUPAC definition [23], a mixture of two or more preformed polymer networks is not an IPN. IPN are multicomponent crosslinked systems and are distinguished from other classes of polymers in that the crosslinking takes place exclusively within, rather than between, the component polymers. The chains of one network are threaded through the chains of the other network, causing more intimate mixing of the components than in a polyblend. The component polymers selected have widely separated Tg with extensive 391
Thermal Analysis of Rubbers and Rubbery Materials but incomplete miscibility, however, they are held together by interpenetrating crosslinks. IPN are distinguishable from blends, block copolymers, and graft copolymers in two ways: (1) an IPN swells but does not dissolve in solvents, and (2) creep and flow are suppressed. Reviews of IPN are available in the literature [24-27]. Compared to homopolymers, which exhibit damping in a lower range of temperature, IPN may show high damping in a wide temperature range. This observation can be confirmed by using DMA. Recently Manoj and co-workers [28] used DMA for developing vibration damping materials based on IPN of carboxylated nitrile rubber (XNBR) and polymethyl methacrylate (PMMA). The DMA of the IPN has been carried out on a Rheometric Scientific Dynamic Mechanical Thermal Analyzer model PL Mk III. The samples have been scanned from -50 °C to +125 °C at a heating rate of 3 °C/min. The dynamic stress applied on to the sample is such as to produce a dynamic (oscillatory) strain of 32 μm. The loss tangent versus temperature curves of XNBR and the corresponding IPN (Figure 12.6) show that the magnitude of tan peak for the XNBR is the maximum (having value 1.4), but the value decreases on IPN formation (0.68 for XNBR and
Figure 12.6 Plots of loss tangent versus temperature of XNBR and IPN based on XNBR and PMMA; A, pure XNBR; B, XNBR/PMMA 76/24 and C, XNBR/PMMA 51/49 [28] Reproduced with permission from N.R. Manoj, L. Chandrasekhar, M. Patri, B.C. Chakraborty and P.C. Deb, Polymers for Advanced Technologies, 2002, 13, 9, 644. ©2002, John Wiley and Sons Ltd
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Thermal Analysis of Rubber Products PMMA in the ratio 76:24 and 0.34 for XNBR and PMMA in the ratio 51:49). This shows that nitrile groups in the XNBR contributes a maximum towards damping and XNBR is a good narrow band damper in a narrow temperature range. The IPN out of XNBR (XNBR/PMMA 76:24 and XNBR/PMMA 51:49) have lower tan values, but they have broad loss tangent peaks that are shifted to a temperature between the Tg of XNBR and PMMA. Using the phenomenological relaxation theory and timetemperature superposition principle based on the DMA experiments, the authors have determined the dynamic modulus and loss tangent of the XNBR and IPN in the frequency range of 1-105 Hz. Because of the fact that extensional damping systems should have high modulus and constrained layer damping systems should have high tan , the authors suggest XNBR/PMMA 76:24 IPN for a constraint layer damping system because of its high modulus and XNBR/PMMA 51:49 IPN for extensional damping because of its high modulus and reasonably good tan in the entire frequency range.
12.2.7 Air Springs An air spring is defined as a mechanical device which uses confined air to absorb the shock of motion. When vehicles move, the vibrations are transmitted into different body parts and cause flex fatigue to all objects which they disturb. The noise level is also increased. Air springs help in isolating unwanted vibrations. The problem with steel and hard rubber springs is that they deliver a harsh ride when the vehicle is underweighted. Air springs provide a smooth ride loaded or unloaded by adding or releasing air from the air springs. Air springs, in general, consist of four material layers: a synthetic fibre inner liner, two body plies, and an outer cover. They are placed between a suspension member and the vehicle’s body and provide isolation from road noise and vibration, and also serve as pneumatic actuators.
12.2.7.1 Identification of Polymer in an Air Spring Using Thermal Analysis The normal practice of thermal analysis of an air spring is the sampling from different parts of the material as inside rubber may be different from that of the outside rubber. Since the air spring is reinforced with fabric, the fabric coating may be of yet another composition. The fabric is separated from the fibre by swelling in chloroform. Figure 12.7 shows the plot of weight percentage against temperature for the inside rubber sample of an air spring. The experiments are done using a DuPont 9000 TGA in a nitrogen atmosphere [29]. There is a typical pattern of multistage degradation. The derivative graph indicates initial Tmax (temperature corresponding to the maximum rate of decomposition in the DTG curve) around 375 °C, which coincides with that of 393
Thermal Analysis of Rubbers and Rubbery Materials
Figure 12.7 Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves in nitrogen atmosphere of the inside portion of an air spring [29]
peroxide cured polychloroprene rubber [6]. Such observations are further confirmed by carrying out the DSC analysis and comparing the Tg values of the sample and the neat polychloroprene rubber. Both Tg values are found to be -42 °C. Chemical analysis and infrared spectra of air spring compound confirm the presence of chloroprene rubber [29]. The outer rubber of an air spring may be of different composition compared to that of the inner rubber. This compositional difference can also be identified using thermal analysis techniques. Figure 12.8 shows the TGA of the outer rubber layer of the same air spring material. Two prominent peaks of decompositions are displayed here. The peak at 375 °C corresponds possibly to natural rubber (NR) whereas the peak around 500 °C may be due to the presence of SBR. Additional chemical analysis techniques like burning the sample on a copper wire distinguish NR from CR. The latter gives a blue-green flame on copper wire whereas such a flame is not associated with NR. DSC and IR analyses can also be used to confirm the above observations. The amount of polymer, filler and other compounding ingredients in the material can be determined using TG-DTG analysis carrying out both in an inert atmosphere like nitrogen and in an oxidative environment. A more detailed description of such an analysis is given in section 12.3 describing thermal analysis of rubber seals. 394
Thermal Analysis of Rubber Products
Figure 12.8 TG-DTG curves in nitrogen atmosphere of the outer rubber of an air spring [29]
12.3 Thermal Analysis of Rubber Seals
12.3.1 Introduction Seals and O-rings are materials that prevent or control the passage of matter across the separable members of a mechanical assembly. O-rings are moulded seals with a precise circular cross section suitable for use in a machined groove for static or dynamic applications. Rubber-based seals are used in a wide variety of applications from automotive to aircraft engines in lubrication, hydraulic and fuel systems. Window seals are used for protection from weather. Seals may be subjected to severe service conditions while in operation, such as high temperature and pressure, thermal expansion and exposure to various types of fluids which may lead to structural degradation or additional crosslinking. Sometimes the structural degradation or change in material properties that are caused by the high temperature environment may be evident only at low temperatures when during thermal contraction, which will make the seal material insufficiently elastic to maintain the minimum sealing pressure. The seal industry uses thermal analysis techniques to evaluate the composition by TGA, to ascertain the glass to rubber transition temperature and mechanical strength using DSC, TMA and DMA, and to evaluate the oxidative stability using TGA, DSC and 395
Thermal Analysis of Rubbers and Rubbery Materials pressure DSC. Many times a combination of thermal analysis techniques are used for the design, development and formula reconstruction of rubber seals [30]. Specifications for physical properties of seals can be very general or extremely specific in nature. Some of the common standards used for the selection of rubber based seals are ASTM D2000-05, Japanese Industrial Standard (JIS) B2401 for O-rings and Society for Automotive Engineers (SAE) standards SAE J120a for rubber rings in automobile applications.
12.3.2 Major Rubbers used for Seal Manufacturing The majority of the seals were initially made of nitrile and polychloroprene rubbers though various other synthetic rubbers have taken their place in order to meet stringent and sometimes conflicting service requirements such as high operating temperature coupled with stable viscosity for aerospace applications. Window seals are made mainly of ethylene-propylene diene terpolymer (EPDM) rubber because of its outstanding weather resistance. Some common rubbers used in seal industry are given in Table 12.1.
12.3.3 Role of Thermal Analysis in the Formula Reconstruction of Rubber Seals Thermal analysis techniques such as DSC, TGA and DMA coupled with spectroscopic characterisation techniques and elemental analysis are being widely used in the industry for reverse engineering and formula reconstruction purposes. The following example describes the role of thermal analysis techniques in the formula reconstruction of rubber seals, such as the identification of polymer and quantification of amount of filler. It should be noted that reverse engineering and formula reconstruction of rubber products involves a number of characterisation techniques other than thermal analysis. The following example is a generalised procedure followed in the laboratories. The first step in the reverse engineering and formula reconstruction of a rubber product is the identification of the base polymer, mainly by determining the Tg, preferably by using DSC. The Tg of an unfilled and unvulcanised rubber takes place over a narrow temperature range whereas for a vulcanisate containing fillers and other compounding ingredients, Tg can be much wider. Precise determination of Tg of polymers by using different thermal analysis techniques has been explained in detail by various authors [30-32]. Loadman [31] notes that different points in a heat flow versus temperature DSC curve can be used to cite Tg, for example temperature of maximum slope, temperature of departure of baseline from its linear extrapolation and temperature of completion of transition. Since the Tg of the rubber is below ambient temperature, improper temperature calibrations of the instrument at these temperatures may affect the precise determination of Tg, apart from factors like presence of compounding ingredients, 396
-55 - 150 °C
-50 - 125 °C
-40 - 150 °C
-55 - 135 °C
Fluorosilicone rubber
Nitrile rubber (NBR)
Hydrogenated nitrile rubber (HNBR)
Carboxylated nitrile rubber (XNBR)
Polychloroprene -40 -135 °C rubber
-30 – 205 °C (up to 300 °C for shorter periods)
Fluororubber
Resistance to Freon and ammonia. Used as refrigeration seals.
Low temperature resistance; abrasion resistance.
Resistance to petroleum oils and sour gas. Use in seals, O-rings, washers and gaskets requiring additional chemical and temperature resistance.
Resistant to petroleum oils, steam, hydrogen sulfide and amine corrosion inhibitors; Good sealing force and resilience; low compression set; suitable for hard vacuum services. Use in tough sealing applications requiring extreme chemical resistance. Combines good high and low temperature stability of silicone with the fuel, oil and solvent resistance of fluororubber. Most widely used rubber in seal industry. Low cost combined with good mechanical properties. Use in pneumatic seals, O-rings, oil seal lips, gaskets and washer.
Usable temperature Remarks range
Rubber
Table 12.1 Common rubbers used in seal industrya Do not use with
Refrigerants, alcohol, ozone
Petroleum oil, sour gas, CO2, rapeseed oil methyl ester. Petroleum oils, water
Petroleum oils, water, hydraulic oils
Petroleum oils, gasoline
Brake fluids, phosphate esters Petroleum oils, toluene.
Acetone, ethyl acetate Brake fluids, ketones, phosphate estes, H2S Brake fluid
Acetone, Petroleum oils, engine lubricants, lacquers H2S, steam,
Use with
Thermal Analysis of Rubber Products
397
398
Dry heat, alcohol, Petroleum vegetable oil oils and fuels Brake fluids, Petroleum refrigerant, steam, oils water.
Petroleum oils, hydraulic oils, water based hydraulic fluids.
Ethylene propylene rubber a for guideline purpose only
Silicone rubber
Polyurethane rubber
Table 12.1 Continued ...
Rubber
Usable temperature Remarks range -40 - 110 °C Good abrasion resistance and mechanical properties. Used in high pressure hydraulic systems being subjected to high stress and wear. Can be used with pressure up to 41.4 MPa psi. Use in hydraulic cylinders as rod seals and piston seals. -65 – 260 °C Resistant to high and dry heat; low compression set characteristics and wide temperature range. Use in O-rings, special low pressure seals and gaskets. -50 – 150 °C Ozone and chemical resistance; used in automotive brake systems. Use in U-cups, wiper rings, O-rings and gaskets.
Use with
Do not use with Brake fluids
Thermal Analysis of Rubbers and Rubbery Materials different grades of the same rubber, and different microstructure and microphase separation of the same rubber. Thus Sircar, and Chartoff [32] emphasise that the reports of the Tg of a rubber should include not only the experimental method (DSC, DMA) and the method of Tg determination (extrapolated, onset, half-height), but also the exact name and description of the rubber with the number index, the recipe used and the cure conditions. In order to determine the T g of the base polymer as accurately as possible, the samples are normally subjected to acetone extraction using standard procedures and the residue is tested for polymers and filler using DSC and TGA [29]. Acetone extraction removes some of the compounding ingredients from the rubber which may affect the thermal analysis results. The DSC analysis of the residue of the seal sample performed on a DuPont 9000 thermal analyzer by heating from -150 °C to +100 °C, at a heating rate of 20 °C/min, is shown in Figure 12.9. The results are given in Table 12.2 [29]. The DSC analysis indicates that Seal A has T g values at -38 °C and -12 °C, whereas Seal B shows values at temperatures of -37 °C and -20 °C. The presence of a particular rubber in the composition is confirmed by carrying out the chemical analysis (in this case, the Beilstein test), which shows the presence of a fluororubber. This is further confirmed by comparing the
Thermal Analysis of Rubber Products
Figure 12.9 DSC analysis of seal samples, Seal A and Seal B. Seal A shows Tg values of -38 °C and -12 °C whereas Seal B shows Tg values of -37 °C and -20 °C [29]
Table 12.2 Results of DSC analysis of elastomeric seals Sample Seal B Viton B-50 Viton B-201C
1 -37 -40 -40
Transition temperatures (°C) 2 -20 -12 -20
3 -+10 +13
DSC results of the pure fluororubber though the transition temperature can be affected by the presence of compounding ingredients like fillers. Thus Viton B-50 shows transitions at -40 °C, -12 °C and +10 °C whereas Viton B-201C has transitions at -40 °C, -20 °C and +13 °C (Table 12.2). This shows that Seal A may contain Viton B-50 and Seal B, Viton B-201C. The determination of the amount of filler and other compounding ingredients present in the seals and the further confirmation of the type of polymer present are done by carrying out the TG-DTG analysis of the samples. The experimental factors to be considered in obtaining TG data have been reviewed by several authors [33, 34]. The 1988 ASTM symposium on Compositional Analysis by Thermogravimetry [35] and the ASTM standard [ASTM E-1131-03] shows the wide acceptance of TG-DTG method in compositional analysis. It should be noted that because of the complexity in the composition of many rubber products, the analysis may not always be accurate. For example, curatives and antidegradants may not show independent weight losses in the 399
Thermal Analysis of Rubbers and Rubbery Materials TG curve and their weight loss may occur in a wide range of temperatures, overlapped by weight, loss of other compounding ingredients. Low molecular weight volatile products such as oil and plasticiser may overlap with polymer decomposition. Though such overlaps are minimum or negligible for rubbers having high thermal stability (for example EPDM rubber and fluororubber), an accurate demarcation of oil/plasticiser and rubber decomposition regions in a TG-DTG analysis often poses a challenge for compounds based on rubbers having low thermal stability (for example, NR or SBR) [1, 36]. Even curatives and antioxidants may start volatilising at the volatilisation temperature of oils and plasticisers. The degree of overlap also depends on the type of oil and plasticiser. Irrespective of these limitations, TG-DTG is widely used as a routine method for quality control and formula reconstruction in rubber industries. Sircar [1] has given a detailed description of the various methods to be followed for the TG-DTG analysis of compounded rubber and vulcanisates. In general, all the weight loss in an inert atmosphere such as nitrogen is due to the polymer, plasticiser and oil. The general method of determination of carbon black is by oxidation of the residue left after a TG run in nitrogen to a constant high temperature, usually 550-600 °C (most carbon blacks oxidise below 600 °C). Silicas and clays show weight loss after 550 °C. If the polymer is not a char forming one and the product contains no mineral fillers, the final residue after carbon black oxidation may give information regarding the presence of zinc oxide. The results of the TG-DTG analysis are given in Table 12.3. Representative TGA curves of Seal A both in nitrogen and in air are shown in Figure 12.10 [29]. In nitrogen, the sample shows a single stage degradation starting at 400 °C, which is completed at around 520 °C, the final residue being 15 wt% (Table 12.3). In air the decomposition
Table 12.3 Results of thermogravimetric analysis of seal samples [29] Sample
Seal A
Seal B
400
Decomposition temperatures and weight loss Atmosphere Initial Decomposition Temperature decomposition temperature corresponding temperature range (°C) and to the peak (°C) corresponding maximum in weight loss (%) DTG curve (°C) Nitrogen 400 (i) 400-520; 54 (i) 520 (ii) 520-800; gradual Air 400 (i) 400-500; 50 (i) 490 (ii) 590 (ii) 520-600; 5 (iii) 690 (iii) 600-800; 28 Nitrogen 480 (i) 410-520; 55 (i) 510 (ii) 520-700; 10 Air 380 (i) 380-530; 55 (i) 510 (ii) 530-590; 5
Residue (%)
15
12
35 37
Thermal Analysis of Rubber Products
Figure 12.10 Thermgravimetric curves of seal sample designated as Seal A. Curve A (dotted line) shows experiment done in nitrogen atmosphere and curve B (solid line), that in air atmosphere. Analysis in air shows multistage decomposition [29]
takes place in multiple steps. The decomposition of Seal A in air causes 50% weight loss in the temperature of 400-500 °C; 5% in the temperature range of 520-600 °C and 28% in the temperature range of 600-800 °C. The final residue in air is 12 weight%, which is due to the presence of metal oxides present in the sample as compounding ingredients. The major degradation step is due to the degradation of the polymer. Seal B shows a single stage decomposition starting at 400 °C in nitrogen atmosphere and leaves a white residue of 35%. In this temperature range of 410-520 °C, 55% weight loss takes place. The decomposition of Seal B is also taking place in multiple steps in air. The final white residue in this case is 37%. On comparing the TG-DTG results of Seals A and B, it can be seen that both the samples decompose in a similar manner in air as well as in nitrogen. However, the second stage decomposition of Seal B is slow both in air and in nitrogen compared to that of Seal A. For Seal A, there is a sharp decomposition in air. This phenomenon indicates that the types of fillers and curatives used in the seal samples are different. The comparison of TG-DTG analysis results (in a nitrogen atmosphere) of a Viton based seal compound prepared in the laboratory and the seal sample B analysed (Figure 12.11) confirm that the polymer used for the manufacturing of the seal samples is Viton. A similar comparison can be made with DSC analysis results as well [29]. 401
Thermal Analysis of Rubbers and Rubbery Materials
Figure 12.11 TG-DTG curves in nitrogen atmosphere of (A) seal sample supplied for analysis (seal B), represented by dotted lines and (B) Viton® rubber based seal compound prepared in the laboratory, represented by solid lines [29]
As mentioned previously, the findings of the DSC and TG-DTG analyses are often confirmed by following other analytical techniques including chemical analysis and IR spectroscopy studies. For example, the presence of fluororubber is confirmed by the dry test and the Beilstein test. IR studies are often done by comparing the spectra of the product with that of the standard polymer. The presence of some of the compounding ingredients can also be identified using IR spectra. The IR analysis of the acetone extract is also used to identify the compounding ingredients present in the product. The type of carbon black in the compound is determined by carrying out the iodine absorption and dibutyl phthalate absorption studies. Atomic absorption spectroscopy (AAS) is also used to identify the presence of the compounding ingredients.
12.3.4 Other Thermal Studies on Rubber Seals Thermal techniques are used for evaluating new and used rubber seals and parts and for quality assurance purposes and polymer identification [29, 37]. Riga [38] has developed a TGA and TMA quality control and failure analysis protocol for automotive rubber seals made by using nitrile rubber and fluororubber, Viton. The characterisation of several failed seals has revealed the source of some unique field problems. One of the major reasons for the degradation of automotive rubber seals is the one caused by the 402
Thermal Analysis of Rubber Products contact with hot lubricant oils, which may lead to crosslinking or thermal degradation. The chemistry of the oil affects the stability of such seals [39]. A Seiko-Haake RTG 220 TGA coupled with a differential thermal analyser (DTA) has been used for the studies both in nitrogen and in air. The experimental conditions described are: 10 mg sample mass, 250 ml/min air flow rate, 10 °C/min heating rate, and 900 °C maximum temperature. The sample containers were platinum pans. A Seiko-Haake TMA is used to assign the Tg of the samples. The TMA conditions are cool to -80 °C, heat to 50 °C at 10 °C/min under a 5 mg load. A modulated DSC (MDSC) has been used to ascertain Tg in the reversing heat flow mode. The analysis was started using TGA and TMA with limited knowledge of the polymer type and composition. The polymer Tg is determined by evaluating the TMA curve, by using the extrapolated onset temperature in the dimensional change (Y-axis) versus sample temperature (X-axis). The coefficient of thermal expansion (CTE) measured before Tg as well as the Tg values of the different seal materials aided in identifying the various seals and parts. Riga [38] establishes the reason for the failure of a silicone rubber seal as over heating based on the results of the thermal analysis techniques such as MDSC coupled with X-ray methods and structural examination. However, in some cases, the failure may be related to the clutch hardware or seal surface cracking, which are not detected by TMA or TGA. In this study, TGA is widely used for identifying the decomposition temperature and approximate composition of the polymeric seals. One of the most widely-used rubbers in seal industry is NBR. Degrange and co-workers [40] suggest that specific application of NBR in the seal industry, such as lip seals for ball bearings, requires specific thermo-mechanical and tribological properties, especially in presence of lubricants. They have studied the influence of viscoelasticity on the tribological behaviour of carbon black filled NBR compounds for lip seal application. Such studies are important because weak contact friction decreases the heat generation and improves the durability of ball bearings. Though it is not a finished rubber product analysis, such studies using thermal analysis techniques will help the research and development of rubber seals.
12.3.5 Automotive Window Seal EPDM rubber is widely used for the manufacturing of automotive window seals because of its excellent weather resistance. In many cases, these seals are coated with polyurethane (PU) to obtain low friction, high abrasion resistance and release properties. Ginic-Markovic and co-workers [41] have used thermal analysis techniques to study the durability of window seals, which is affected by the complex service environment conditions like heat, light and humidity. Since ultraviolet (UV) radiation is one of the reasons for the degradation of the seals, they have studied the effect of UV radiation on the PU coating and EPDM rubber underneath by using controlled irradiation in a 403
Thermal Analysis of Rubbers and Rubbery Materials weatherometer. When coated window seals are exposed to UV radiation, either the coating or the bulk rubber or both may undergo photodegradation. Using thermal analysis techniques, the actual degradation pattern of such seals can be accurately determined. The EPDM rubber is surface and bulk modified to achieve better adhesion with PU coating. DMA studies have been carried out to investigate the effect of photodegradation on the viscoelastic properties by operating the instrument in tension mode from -100 to 100 °C at 1 Hz frequency and 0.2% strain amplitude, at a heating rate of 2 °C/min. Liquid nitrogen is used to achieve sub-ambient conditions. The compositional analysis has been done using TGA. Samples are subjected to UV exposure for more than 1000 hours. An increase in storage modulus of the UV irradiated samples may indicate hardening due to photodegradation. The hardening could either be due to oil/volatile loss from the composition or due to change in network structure. While TGA can monitor the volatile loss efficiently, the minor change in crosslink density or hardness could be best understood from the elastic modulus, for which DMA is a powerful probe. Comparison of the storage modulus values of the samples at three different temperatures (-25, 0 and +25 °C) reveal that there is an initial steady increase in modulus up to 800 hours of UV exposure, which is followed by a plateau. Thus, the change in modulus in the early part could be accounted for by the change in EPDM network structure in the form of post curing, and consequently an increase in crosslink density. In order to confirm such observations, the changes in crosslink density associated with degradation are detected by analysing the tan versus temperature curves of the seal samples. Figure 12.12 is a representative plot of the temperature dependence of tan of the PU coated window seal compounds without and with UV exposure. Ageing and corresponding crosslink density can cause both the reduction in the tan peak height and shift of tan peak temperature to the higher value [1]. Since only a reduction in tan peak height is observed here at around -36.7 °C, the rubber material in the seal compound might have undergone only a marginal degradation due to UV exposure. The rubber compound is covered and protected by primer/coating and, thereby, less affected. However the tan peak height corresponding to the PU coating (indicated by the broad transition with peak at 5.3 °C in Figure 12.12) also shows a reduction in height suggesting that the coating is degraded due to UV exposure. This observation can be confirmed by carrying out the TGA of the coating film. Figure 12.13 shows the DTG curves of the coating film before and after UV exposure. A shift in the decomposition temperature to a lower value is observed after the UV exposure. The shift is more pronounced with the coating than with the rubber compound indicating that the coating film is more affected by the UV radiation. By carrying out the TGA at different heating rates, activation energy of decomposition is determined using various methods, which will give further insight into the nature of decomposition of such materials. Delor-Jestine and co-workers [42] have observe that an EPDM formulation designed for engine and outdoor applications shows a limited resistance to thermal and photo404
Thermal Analysis of Rubber Products
Figure 12.12 Temperature dependence of tan for an automotive window seal compound; A (solid line), polyurethane coated sample without UV exposure and B (dotted line), polyurethane coated sample subjected to 800 hours of UV exposure [41] Reproduced with permission from M. Ginic-Markovic, N. Roy Choudhury, M. Dimopoulos and J. G. Matisons, Polymer Degradation and Stability, 2000, 69, 157, Copyright ©2000, Elsevier Ltd
Figure 12.13 DTG curves of the coating film on the window seal compound showing the effect of UV exposure on thermal stability. A, before UV exposure and B, after UV exposure [41] Reproduced with permission from M. Ginic-Markovic, N. Roy Choudhury, M. Dimopoulos and J. G. Matisons, Polymer Degradation and Stability, 2000, 69, 157, Copyright ©2000, Elsevier Ltd
405
Thermal Analysis of Rubbers and Rubbery Materials oxidation, respectively. They have carried out the photoageing, natural ageing and thermal ageing of compounds based on EPDM rubber designed for automotive applications using both IR spectroscopy and DMA to describe the two competitive phenomena involved in ageing of rubbers (oxidation and crosslinking). DMA is used here mainly to study the influence of thermo-oxidation on viscoelastic properties. Thermo-oxidation of the compounds for longer duration causes significant increase in the storage modulus with a shift in tan peak height temperature to higher values.
12.4 Thermal Analysis of Rubber-Based Cable Sheathing Compounds Power cables are used for conveying electricity to industrial equipment and household electrical appliances [43]. Generally rubber blends are used for the production of cable sheathing compounds (protective cover or jacketing) rather than a single rubber because of the property requirements like abrasion, flame and oil resistance coupled with heat, ageing and ozone resistance. One such example is the development of heat, oil and fire resistant cable sheathing compounds by combining the flame and oil resistance of polychloroprene rubber with heat and ageing resistance of EPDM rubber. Kalidaha and co-workers [45] observed three types of incompatibility in such a blend leading to poor mechanical properties: i)
Thermodynamic incompatibility arising due to the structural difference of the two polymers which may prevent mixing at the molecular level.
ii) Incompatibility due to viscosity mismatch arising due to the difference in molecular weight, branching and molecular weight distribution. iii) Cure rate incompatibility arising due to the difference in the degree of unsaturation, chemical structure and polarity. To overcome these limitations, they used dibutyl maleate grafted EPDM rubber (EPDM-g-DBM) and chlorinated polyethylene (CPE) as compatibilisers to prepare technologically compatible CR-EPDM blends. The compatibility is assessed by many techniques including DSC where a shift in the Tg of the two rubbers towards each other is observed. DSC and DMA are the common thermal analysis techniques used to determine the compatibility of rubber blends through the determination of Tg [45-47]. Incompatible blends will have two unbroadened glass to rubber transitions with values similar to that of the component rubbers [48]. In the case of partially compatible blends, Tg of the low Tg polymer will be higher and that of the high Tg polymer will be lower, thus shortening the Tg interval between the two rubbers. As pointed out by Sircar [1], the extent of this shortening is a measure of compatibility. The DSC curves of the control CR and EPDM rubbers as well as their blends are given in Figure 12.14. The shift in Tg towards each other for compatibilised systems is evident here. 406
Thermal Analysis of Rubber Products
Figure 12.14 DSC curves showing the shift in Tg with polymer blending in a compatibilised system; (I) control CR; (II) control EPDM; (III) CR/EPDM blend without compatibiliser; (IV) CR/EPDM blend with compatibiliser EPDM-g-DBM and (V) CR/ EPDM blend with compatibiliser chlorinated polyethylene (CPE) [44] Reproduced with permission from A.K. Kalidaha, P.P. De, A.S. Bhattacharyya and A.K. Sen, Die Angewandte Makromolekulare Chemie, 1993, 204, 19. Copyright ©1993, Hüthig & Wepf Verlag
It is always desirable to have a sheathing compound which is fire resistant on the one hand and has minimum hazards from generation of smoke, toxic and or corrosive fumes [49]. Therefore, polyvinyl chloride (PVC) based sheathing compounds are replaced by non-halogen fire retardant thermoplastic elastomeric compounds, although PVC has good flame resistance properties coupled with good mechanical properties and easy processibility. This is because, in the event of fire, PVC based compounds may form large amounts of dense black smoke together with acidic and toxic fumes. Sen and coworkers [49] have developed low halogen and non-halogen fire resistant low smoke (FRLS) cable sheathing compounds from blends of functionalised polyolefin and PVC. For non-halogen compounds, both polyethylene (PE) and EPDM are functionalised by grafting 10% dibutyl maleate (DBM) using dicumyl peroxide (DCP) as initiator (0.2% for EPDM and 0.5% for PE). The compounds are made fire and smoke resistant by mixing with ingredients like anitomony trioxide (Sb2O3), molybdenum trioxide (Mo2O3), pentabromodiphenyl oxide, aluminium trihydrate (ATH) and zinc magnesium sulfate. The decomposition characteristics of the control compounds (thermoplastic elastomers made by simultaneous grafting and dynamic curing using DBM and DCP, designated as TPE-g as well as those made by dynamic curing without grafting, designated as TPE) and the cable sheathing compounds (designated as TPE-FRLS, and TPE-g-FRLS for 407
Thermal Analysis of Rubbers and Rubbery Materials compositions without and with grafting respectively) are studied by TGA under nitrogen atmosphere at a heating rate of 20 °C/min (Figure 12.15). Though both TPE-g and TPE show the same decomposition behaviour, the FRLS compound TPE-g-FRLS possesses higher thermal stability in terms of onset of decomposition and weight loss. The authors assume increased interaction between ATH and the polymer matrix through DBM moiety is the reason for the improved thermal stability. This shows the compatibilisation of ATH with the polymer matrix of TPE-g-FRLS compound which involves the reaction of the pendant ester groups with the hydroxyl groups of ATH.
Figure 12.15 The decomposition characteristics of thermoplastic elastomers and the corresponding cable sheathing compounds. A, Thermoplastic elastomer made by simultaneous grafting and dynamic curing, designated as TPEg as well as those made by dynamic curing without grafting, designated as TPE; B, the cable sheathing compound based on thermoplastic elastomer made by grafting and dynamic curing, designated as TPE-g-FRLS and C, the cable sheathing compound based on thermoplastic elastomer prepared without grafting, designated as TPEFRLS [49] Reproduced with permission from A.K. Sen, B. Mukherjee, A.S. Bhattacharya, L.K. Singhi, P.P. De and A.K. Bhowmick, Journal of Applied Polymer Science, 1991, 43, 9, 1673. ©1991, John Wiley & Sons, Inc
408
Thermal Analysis of Rubber Products
12.5 Thermal Analysis of Rubber Based Adhesives
12.5.1 Introduction Though rubber-based adhesives are primarily used for joining and bonding two substrates, if correctly used, they can perform a variety of other functions. Because of their elasticity, these adhesives can absorb a part of the force when stress is applied to the substrate. The ability of the rubber adhesives to yield slightly under peel or shear stress conditions distributes the force over a larger area. The elastomeric nature of the bond also allows it to resist rupture or delamination due to impact. One of the most interesting properties of such adhesives is their acoustic behaviour. If the bonding dimensions are correct, a part of the kinetic vibration energy is converted to heat-that is removed from the system. In a simplified form, this behaviour is similar to a combination of a spring (stored energy) and a shock absorber (lost energy). Particularly, PU adhesives have a damping factor which is used with advantage in some applications such as bonding the floor to the steel frame of a bus body. The transmission of the engine and wheel noise through the body of the bus is thus considerably reduced. The result is a noticeable reduction in the noise level in the passenger compartment. This same property makes rubber-based adhesives the best solution to assemble parts that are submitted to shocks and impacts.
12.5.2 Testing of Adhesives The most common rubber-based adhesives are made by using polychloroprene rubber. It is compatible with a variety of porous and nonporous substrates, creating bonds that resist degradation by oils, chemicals, water, heat, sunlight and ozone. Polychloroprene delivers a combination of polarity and crystallinity that further improves its strength and versatility in bonding a wide range of substrates. Compositional analysis of an adhesive is challenging as the adhesive may contain several ingredients like rubber, polymer, resin, solvent, filler, plasticiser, flexibiliser, curing agent, hardener, catalyst, accelerator, adhesion promoter, surface active agent, wetting agent, protective colloid, antioxidant, flow control agent, UV stabiliser, tackifier, thixotropic agent, flame retardant, inhibitor and pigment. Adhesives can be in the form of solid, gel, sol, film, powder, paste or tape. Type of adhesive will vary depending on the surfaces to be bonded, environment of the bonded joints, and the atmosphere (air or inert). The viscosity is determined by using a Brookfield Viscometer or parallel plate rheometer. IR spectroscopy and chemical analysis are used to find out the chemical nature of the adhesives. Thermal analysis techniques such as DSC, DMA and TMA are used to determine the Tg and other thermal properties. DSC is used to find out the change of state, melting and solvent type in the adhesive. Compositional analysis is done by TG-DTG 409
Thermal Analysis of Rubbers and Rubbery Materials coupled with IR analysis, chromatography and atomic absorption spectroscopy. Modern techniques like secondary ion-mass spectrometry, X-ray photoelectron spectroscopy, gas chromatography (GC) and coupled multiple thermal analysis techniques like TG-MS, TG-IR, TG-GC are also used for the analysis of adhesives. The low and high temperature DSC curves of commercially available acrylate-based pressure sensitive adhesive, Quickfix are shown in Figure 12.16a and b, respectively [29]. The broad endothermic peak from 80 °C to 150 °C in Figure 12.16b indicates
Figure 12.16 (a) Low temperature DSC scan of the commercial adhesive Quickfix®, showing the Tg; (b) high temperature DSC scan of the same adhesive showing the evaporation of solvent (endothermic peak) and polymer degradation (exothermic peak) [29]
410
Thermal Analysis of Rubber Products evaporation of the solvent. The degradation of the polymer is shown by the sharp exothermic peak at 200 °C in the same figure. Because of its instant fixation, acrylates in the form of aqueous dispersion are used in such adhesives. Analysis of these types of adhesives starts with a knowledge on the type of adherents with which they find application. TGA is done without and with solvent. Persson and co-workers [50] have used DMTA to study the cure characteristics of rubber-to-metal bonding agents involving a commercial primer/adhesive bonding system and sulfur/accelerator cured NR/BR blend containing carbon black. The instrument is operated in an air atmosphere with the samples clamped in the shear mode. For load symmetry, samples have been mounted on both sides of the drive shaft. The average strain is 0.24% at a frequency of 2 Hz (forced vibration) and the shear modulus (G) is measured both under isothermal conditions and at a constant heating rate (2 °C/ min).The dynamic shear modulus response of the primer, adhesive and the rubber at 150 °C is shown in Figure 12.17. The primer is found to be scorchy and fast curing with high crosslink density whereas the adhesive is less scorchy and slower curing than the primer. Figure 12.17 shows that the rubber has a typical sulfur/accelerator delayed action cure, which has been further amplified by the addition of a cure retarder. DMA analysis of adhesives may result in dimensional changes during heating mainly due to swelling, which may affect the results as the specimen dimension is a vital parameter in
Figure 12.17 Dynamic shear modulus response for the primer (P), adhesive (A) and rubber (R) in a rubber-to-metal bonding agent system under isothermal conditions (150 °C) [50] Reproduced with permission from S. Persson, M. Goude and T. Olsson, Polymer Testing, 2003, 22, 671. Copyright ©2003, Elsevier Ltd
411
Thermal Analysis of Rubbers and Rubbery Materials the modulus calculation in DMA. The authors believe that the dimensional changes are related to entrapped gases and/or too high concentrations of the remaining solvents. A more complete solvent evaporation is expected to reduce this artifact. On analysing the DMTA results in Figure 12.17, it is evident that the primer has reached a degree of conversion of more than 90% at the time when the curing reaction of the rubber commences. Simultaneously, the adhesive has started to cure but since the adhesive lacks a definite cure plateau, it is difficult to determine any specific degree of conversion when the rubber has reached the cure onset. It is also evident that if the scorch time of the rubber is reduced to such an extent that the adhesive and the rubber will co-cure; a stronger interfacial bond could be achieved. Conversely, a delayed action adhesive may also improve the interfacial bonding. Such analyses using DMA will help the adhesive industry in research and development of new products with improved performance. Though it is still in the infant stage, DMA can be used for the determination of cure characteristics of rubber in a way similar to that of rubber rheometers like oscillating disc rheometers (ODR) or moving die rheometers (MDR). Wetton [10] shows that DMA plot of shear modulus versus time for an unfilled SBR sample with sulfur cure system which looks very similar to a modulus-time curve by ODR. Monsanto rheometers like MDR 2000 and RPA 2000 have provision for measuring the tan of the samples, apart from the provision for shear modulus determination in the RPA 2000 [51].
12.6 Thermal Analysis of Rubber Based Insulators Rajeev and co-workers [52] have developed solid rocket motor insulator compositions using short melamine fibre filled EPDM rubber, maleated EPDM (mEPDM) rubber and NBR composites. Melamine fibre is one of the recent generation of high performance fibres and is recommended for heat and flame resistant applications [53, 54]. A solid propellant rocket motor consists of a solid propellant placed inside a metallic or composite casing, which is protected from the hot combustion gases by means of insulators, liners and inhibitors. The solid propellant on combustion is decomposed into low molecular weight gaseous products liberating very high temperature, in the order of 2200-3200 °C. The casing is to be protected from these hot combustion gases for its smooth functioning. Organic materials such as rubber, at this high temperature, undergo thermal ablation with the formation of a surface char layer. This surface layer plays a predominant role in the absorption of heat by the endothermic processes involved in its formation and also because of its high heat capacity. The char formed is reinforced by using particulate or fibrous fillers to prevent their erosion due to mechanical shear forces, thermal stresses and internal pressure generated by the hot volatile gases [55, 56]. The use of TGA in the development of rocket motor insulators is very important as TGA gives information regarding the amount of char generated by the insulators at high temperatures. 412
Thermal Analysis of Rubber Products The insulators have been developed by using a dry bonding system consisting of resorcinol, hexamethylene tetramine and silica to improve the adhesion between the fibre and the rubber matrix [57]. The mEPDM rubber-based compositions have been cured by using sulfur/accelerator, ionic (involving zinc oxide and zinc stearate) and mixed (involving both sulfur/accelerator system and ionic crosslinking system) crosslinking systems [58]. The TGA experiments are done in nitrogen atmosphere at a heating rate of 10 °C/min., using TA Instruments’ Q 50 model TGA. Based on the TG-DTG analysis, it is confirmed that the incorporation of melamine fibre to the rubber matrices causes the degradation of the corresponding insulators to occur in two steps. Though the fibre incorporation causes the degradation to start at an early temperature compared to the unfilled vulcanisates, the overall thermal stability of the fibre filled insulators is found to be higher, especially at higher temperatures. The presence of melamine fibre decreases the rate of decomposition in the second degradation step of the insulators compared to that of the unfilled vulcanisates. The authors have also observed that the degradation pattern of EPDM and mEPDM rubber-based insulators are the same, whereas for nitrile rubber based composites, the two degradation steps are overlapping (Figure 12.18). The rate of degradation of nitrile rubber based insulator is lower than that of its EPDM and mEPDM rubber based counterparts. A higher char residue is obtained from the mEPDM rubber based insulators cured by using both mixed and ionic crosslinking systems because of the presence of higher concentration of zinc oxide and zinc stearate. The analyses of the thermograms are given in Table 12.4.
Figure 12.18 TG-DTG curves in nitrogen atmosphere of the solid rocket motor insulators based on (A) EPDM, (B) maleated EPDM sulfur cure, (C) maleated EPDM mixed cure, (D) maleated EPDM ionic cure and (E) nitrile rubber containing 30 phr melamine fibre and dry bonding system. Heating rate, 10 °C/min [52] Reproduced with permission from R.S. Rajeev, S.K. De, Anil K. Bhowmick and B. John, Polymer Degradation and Stability, 79, 449. ©2002, Elsevier Ltd
413
414 4.1 5.5 3.5
453 452 451 450 428
Weight lossII degradation step (%) 59.5 59.6
EPDM 390 341 Maleated EPDM409 344 Sulfur crosslinking Maleated EPDM331 400 9.1 52.4 349 Mixed crosslinking Maleated EPDM340 407 8.0 53.5 354 ionic crosslinking Nitrile 302 384 13.2 52.8 336 a Onset I – onset temperature corresponding to first degradation step. Onset II – onset temperature corresponding to second degradation step. Tmax - temperatures corresponding to the maximum rate of decomposition (peak maximum) in the DTG curve. dw/dt – derivative of the weight loss in the DTG curve.
Weight lossI degradation step (%) 9.4 10.4
dw/dt at Tmax1 (%/min) 3.5 3.6
Onset II (°C)
Tmax2 (°C)
Onset I (°C) 320 320
Tmax1 (°C)
Base polymer
9.0
16.3
15.1
dw/dt at Tmax2 (%/min) 17.3 17.1
22.4
24.3
23.8
Char residue (%) 19.9 18.7
Table 12.4 Analysis of the thermograms of the solid rocket motor insulators in nitrogen atmosphere, at a heating rate of 10 °C/min.a [52]
Thermal Analysis of Rubbers and Rubbery Materials
Thermal Analysis of Rubber Products
Table 12.5 Thermal erosion rate and density of the rocket motor insulators [52] Base polymer
Thermal erosion rate (mm/s) EPDM 0.20 Maleated EPDM-sulfur crosslinking system 0.29 Nitrile rubber 0.17
Density (g/cm3) 1.09 1.08 1.17
Because of the higher concentration of the diene content, nitrile rubber based insulators start degrading at a temperature lower than that of EPDM and mEPDM rubber based composites. However, the overall performance as an insulator composition is better for the nitrile rubber based composites because of the lowest thermal erosion rate among the composites developed (Table 12.5). A combination of low-density and good ablative properties are shown by EPDM rubber based insulators (Table 12.5). Thermal erosion rate of the insulators are determined by using plasma arc jet facility, which would simulate the high temperature environment of the insulators. Polymeric insulators are widely used as outdoor high voltage insulators, due to their superior service properties, high hydrophobicity, low dielectric permittivity, high breakdown voltage and high surface and volume resistance. A wide range of polymers have been used for wire and cable insulation, older materials being NR, IIR and SBR. Newer materials include crosslinked polyethylene, silicone rubber, EPDM rubber and thermoplastic elastomers. According to Mead and co-workers [59], insulation materials should have a low value of tan to avoid dielectric losses. Operating conditions for power cables would require low values of tan at frequencies of 60 Hz over the expected operating temperature of application.
12.7 Thermal Analysis of Thermal Interface Materials (TIM) Thermal interface materials (TIM) are used to effectively dissipate heat in electronic materials like transistors. Miniaturisation of transistors allows integration of more transistors in a single device to improve the performance. However, in such cases, effective heat dissipation is important for reliable performance. Though high thermal conductivity and low CTE heat sinks are used for this purpose, their performance is often limited due to interfacial thermal resistance arising from non-surface flatness and surface roughness of both the devices and heat sink, which may cause interfacial air entrapment and lower heat dissipation because of the low thermal conductivity of air. TIM are a viable alternative in such instances, and are used to reduce the thermal contact resistance between two surfaces. One type of TIM, that is rubber-based thermal pads, typically made up of silicone rubber matrix reinforced with thermally conductive 415
Thermal Analysis of Rubbers and Rubbery Materials but electrically insulating fillers such as aluminium nitride, boron nitride, alumina or silicon carbide [60, 61]. The requirements for a TIM are high thermal conductivity, low CTE and easy deformability to contact all the uneven areas of the surface using a very small contact force. The work done by Sim and co-workers [62] shows the application of different thermal analysis techniques such as TGA and TMA for the development and characterisation of Al2O3 and ZnO reinforced silicone rubber based thermal pads as TIM. TGA (in air atmosphere, from room temperature to 250 °C, at a heating rate of 5 °C/min) has been used to study the increase in thermal stability of silicone rubber with the incorporation of the previously discussed fillers. The temperature limits used for the TG analysis are based on the lower and upper limits of a device operating temperature. CTE measurements have been done using TMA. From the TGA curve (Figure 12.19), it is observed that the addition of fillers into the silicone rubber matrix improves its thermal stability. At 125 °C, a relative weight loss of 1.36 weight% is observed for the unfilled thermal pads compared to 0.77 and 0.78 weight% for Al2O3 and ZnO filled thermal pads, respectively. When further heated up to 250 °C, a relative weight loss of 2.65% is observed for the unfilled thermal pads compared to 1.66% and 1.67% for Al2O3 and ZnO filled counterparts, respectively. They attribute the reasons for the improvement in thermal stability of the filled thermal pads to the increase in physical
Figure 12.19 Comparison of the thermal stabilities of thermal interface materials. A, unfilled silicone rubber; B, silicone rubber with10 vol% Al2O3 and C, silicone rubber with 10 vol% ZnO [62] Reprinted with permission from L.C. Sim, S.R. Ramanan, H. Ismail, K.N. Seetharamu and T.J. Goh, Thermochimica Acta, 430, 155. ©2005, Elsevier Ltd
416
Thermal Analysis of Rubber Products and chemical crosslinking points, and to the interactions between the filler and silicone rubber [63]. Based on the TGA results in Figure 12.19, a suggestion is made that the application of silicone rubber as thermal pads should be limited to low power devices, such as chip sets to avoid significant degradation of the product which would reduce its thermal performance.
12.8 Thermal Analysis of Automobile Tyres
12.8.1 Introduction As pointed out by Maurer [6], analysis of formulations containing rubber blends is a challenging task because of the general complexity of the system. This is further complicated by the difficulty of identifying the types and relative proportions of the polymers in the system. Thus, thermal analysis of tyres should be done based on systematic experimental procedures because the formulations for the tread, sidewall, bead coat, fabric coat and so on are different as each part involves compositions based on different rubbers and/or rubber blends. A detailed description of thermal analysis of different parts of a tyre is given by Maurer [6]. Different thermal analysis techniques are simultaneously used for the total thermal analysis of automobile tyres. These include DSC for the determination of Tg, cure characteristics and level of sulfur and accelerator, TG-DTG for the determination of amount of rubber, amount of filler and nature of filler (silica or carbon black), DMA for the determination of composition of the polymer (blend or homopolymer), modulus of the polymers, damping characteristics and interactions (polymer-filler and filler-filler), and TMA for the expansion of rubber-metal composites, hardness, shrinkage, cure characteristics and modulus of the compound.
12.8.2 Identification of Polymer in an Automobile Tyre Using Thermal Analysis In a typical thermal analysis of a tyre tread compound involving DSC, TGA and DMA for reverse engineering purposes [29], the DMA of the sample is done first to get an idea of the type of polymer(s) present in the compound. From the tan peak heights of the samples shown in Figure 12.20 [29], the tread compound is a blend of two rubbers. The Tg values (corresponding to the tan peak height temperatures) of -80 °C and -50 °C indicate that the rubbers may be BR and SBR, respectively. NR, SBR and BR are the most common polymers used in the manufacturing of tyre tread compounds. Blends of SBR and BR are common in rubber industry as tread rubber compounds for passenger car tyres. The good hysteresis property of BR is combined with the abrasion 417
Thermal Analysis of Rubbers and Rubbery Materials resistance of SBR. The determination of Tg of these blends is complicated because of the crystallisation exotherm of BR [64]. The results obtained by using DMA are generally confirmed by carrying out the DSC analysis of the sample in a nitrogen atmosphere (Figure 12.21) because analysis of Tg via DSC is useful for determining the composition
Figure 12.20 Tan versus temperature curve of a tyre tread compound consisting of a blend of BR and SBR. A represents tan peak height corresponds to BR (Tg = -80 °C) and B that corresponds to SBR (Tg = -50 °C) [29]
Figure 12.21 DSC curve of a tyre tread compound consisting of a blend of BR and SBR. A corresponds to the Tg of BR (-85 °C) and B, that of SBR (-55 °C). C represents the melting of BR [29]
418
Thermal Analysis of Rubber Products of certain rubber blends. DSC analysis has also given similar observations [6], showing Tg at -85 °C for BR and -55 °C for SBR. Since BR-SBR blends are generally regarded as compatible systems, the DSC and DMA of vulcanised blends should give a single Tg, intermediate between those of the individual rubbers. However, in the present study, both DSC and DMA analyses show two distinct Tg for the blend. Callan and co-workers [65] have studied a wide range of both uncured and cured blends of BR and SBR and the effect of filler loading on blending using differential thermal analysis (DTA). They report that the uncured blends, with or without carbon black, show two Tg in the DTA curve whereas the peroxide cured blends show only a single Tg. According to Callan and co-workers, uncured BR-SBR blends are microheterogeneous with very small domain sizes which are capable of covulcanisation and, thus, give a single Tg when cured. This leads to a broadening of thermal response, giving a diffuse Tg interval which is difficult to detect. Though the Tg of a vulcanised BR-SBR blend is intermediate to the homopolymer Tg indicating their miscibility [65, 66], a higher concentration of BR may give two Tg for the cured blend. Sircar and Lamond [67] report that Tg of the cured blend is intermediate between those of the components, except at BR concentrations exceeding 60%. In the present study, the actual formulation of the tread rubber compound is found to be containing 45 phr SBR and 55 phr BR. The higher concentration of BR in the formulation may be the reason for the two distinct Tg in the blend as is shown in the DMA and DSC curves. As already explained, TG-DTG analysis is performed both in nitrogen and in air atmospheres to determine the amount of polymer and filler, respectively. The TG-DTG analysis is performed from room temperature up to 300 °C in nitrogen atmosphere at a heating rate of 40 °C/min so that low molecular weight components volatilise first, then the polymer is heated at a heating rate of 20 °C/min up to 600 °C as polymers burn at this temperature, then oxygen or air is introduced in to the TG instrument, disconnecting the nitrogen gas flow, so that carbon black is burnt off. The TG-DTG curves are shown in Figure 12.22 [29]. From these curves, it is evident that the volatile contents in the compound are approximately 17% and the polymer is approximately 43%. The use of DTG peak heights for quantitative analysis of polymer blend composition is described by Maurer [6] and Brazier and Nickel [68]. Carbon black in the tread compound constitutes 35.4% of the total weight. The residue obtained after doing the experiment in an oxidative environment may be due to the presence of metal oxides (1.6% in this case) like zinc oxide.
12.8.3 Isothermal TGA of Tyre Tread Compound A better separation of oil or plasticiser in a tyre rubber can be done by isothermal TGA analysis at a temperature well below the degradation temperature of the polymer. In a particular method called the controlled rate thermal analysis [69], the sample is heated at a constant rate in the TGA and, when a significant weight loss is detected, the TGA 419
Thermal Analysis of Rubbers and Rubbery Materials
Figure 12.22 TG-DTG curves of a tyre tread compound containing blends of BR and SBR both in nitrogen (N2) and oxygen (O2) atmospheres [29]
instrument automatically holds the sample under isothermal conditions. Once the rate of the mass loss becomes very small, the TGA automatically resumes heating the sample until the next significant weight loss event occurs. Using this approach, the TGA will automatically provide the best possible separation of the various components in a tyre tread compound. On comparing the standard TGA results [70] and the results of the controlled rate thermal analysis (Figure 12.23a and b respectively), it can be seen that there is a severe overlap in the weight losses between 200 °C and 500 °C when the experiment is done using standard TGA. In Figure 12.23a, the sample is heated from room temperature to 700 °C at a heating rate of 20 °C/min in a nitrogen atmosphere followed by heating in an oxygen atmosphere from 700 °C to 900 °C. The evaporation of the oil at about 300 °C and the decomposition of the polymer at around 450 °C are overlapped because of the relative slowness of the evolution of the oil causing its weight loss to run into that of the polymer degradation. In Figure 12.23b, the TGA automatically detects the weight losses of the oil and the polymer by automatically holding the rubber under isothermal conditions while the oil is evaporated and the polymer thermally degrades. This approach to tyre rubber shows that excellent resolution is obtained between all of the weight loss events, which permits better quantitative compositional analysis of the rubber. The results shown in Figure 12.23b give the following compositional information on the tyre rubber, oil - 21.8%, polymer - 43.9%, carbon black - 32.2%, and inert filler/metallic oxide - 2.11%. Perkin Elmer calls this feature the Auto Stepwise TGA approach [70], which is featured with their TGA instrument. Seiko Instruments USA has also marketed similar device [1]. Several reports are available in the literature describing the studies dealing with the overlap between oil volatilisation and polymer decomposition in TGA [33, 36, 71-73] 420
Thermal Analysis of Rubber Products
Figure 12.23 (a) Standard TGA and (b) controlled rate thermal analysis results of a tyre tread compound. A, B and C represents the weight loss regions corresponding to oil, polymer and carbon black respectively. Overlap of weight losses in the oil and polymer decomposition region is evident in the standard TGA result whereas better resolution is obtained between the weight loss events in the controlled rate thermal analysis. The purge gas is changed from nitrogen to air at 700 °C [70] Reproduced with permission from W.J. Sichina, Compositional Analysis of Tyre Elastomers using Auto Stepwise TGA, Thermal Analysis Application Note, Perkin Elmer Instruments, PETech-59, 2005. ©2005, PerkinElmer, Inc
Thus, it can be concluded that a reasonably accurate identification of polymers in a tyre tread compound could be achieved by total thermal analysis. The other ingredients present in the compound are identified by using techniques like chemical analysis, IR, AAS, surface analysis techniques and microscopy coupled with thermal analysis techniques. 421
Thermal Analysis of Rubbers and Rubbery Materials
12.8.4 Thermal Analysis for the Development of a Tyre Tread Compound Thermal analysis techniques are being widely used in the development of tyre compounds. A tyre tread compound is a complex mixture containing a combination of crosslinked polymer network, filler network and filler-polymer network. Carbon black, silica and silica-silane combination, all interact in different ways with the polymer. The interactions and the networks formed by these filler systems respond differently when a tyre tread element deforms at various loads, temperature and frequencies in service. The influence of filler-filler networks and filler-polymer networks on dynamic properties is often characterised by DMA. Compounds containing carbon black or silica fillers show a dramatically higher dynamic shear modulus (G) at low strain compared to the G at higher dynamic strain [74]. Wang and co-workers [75] have used DMA for the development of carbon-silica dual phase fillers (CSDPF) for passenger tyre application. This new filler shows better polymer-filler interaction and lower filler-filler interaction relative to the conventional fillers used in tyre tread compounds. This will result in improved wet skid resistance compared to the earlier products without affecting the low rolling resistance at higher temperatures. The much less developed filler networking in the CSDPF would show a great impact on the hysteresis for filled vulcanisates which can be determined from the loss modulus and tan curves of DMA. A depressed filler network or agglomeration will give a lower viscous modulus at a particular temperature. It is known that at elevated temperatures such as 70 °C, polymers would remain in their rubbery state and the loss modulus is mainly determined by the hydrodynamic effect and energy dissipation caused by the breakdown and reformation of the filler network upon dynamic strain. The tan of the tread compound is also an important parameter for tyre performance. At high temperatures (over the range of 50-80 °C), tan should be low to have a low rolling resistance. At the same time, high hysteresis (high tan ) at lower temperature is important for improved wet skid resistance. CSDPF developed by Wang and co-workers [75] shows significantly low tan at higher temperatures and high hysteresis at lower temperatures, as shown in Figure 12.24. The tan was measured at 2.5% strain amplitude and at 10 Hz frequency. Dynamic mechanical properties have been used to predict tyre winter and wet traction by Waddell and co-workers [76] when they used butyl rubber, bromobutyl rubber, chlorobutyl rubber and brominated isobutylene-co-para methyl styrene (BIMS) rubbers in place of solution polymerised SBR having 20% bound styrene for the development of model winter passenger tyre tread formulation containing BR and NR. They concluded that BIMS is the isobutylene based rubber of choice for a winter passenger tyre tread compound since it has the highest tan values between -40 °C and 20 °C.
422
Thermal Analysis of Rubber Products
Figure 12.24 Temperature dependence of tan of tyre tread compounds (blends of 70/30 SBR/BR) filled with (A) N234 carbon black, (B) carbon-silica dual phase filler and (C) silica. Experimental conditions: heating rate 2 °C/min, strain 2.5% and frequency, 10 Hz [75] Reproduced with permission from M.J. Wang, Y. Kutsovsky, P. Zhang, L.J. Murphy, S. Laube, K. Mahmud, Rubber Chemistry and Technology, 2002, 75, 247. ©2002, Rubber Division, American Chemical Society
12.9 Concluding Remarks Based on the previous discussions, it is evident that thermal analysis techniques are vital in a rubber industry for the design, development, failure analysis, quality control and formula reconstruction of various rubber products. The wide acceptance of thermal analysis in rubber related research is evident here. However, compared to the number of research reports on the thermal analysis of rubber and rubbery materials for the design and development of rubber products, those on the finished rubber products are fewer. Thermal analysis of finished rubber products are mainly done for quality control, failure analysis and formula reconstruction in reverse engineering. Nevertheless, there is a wide acceptance of thermal analysis techniques in all fields of rubber science and technology. More sophisticated and advanced instruments are being developed to accommodate the demands of the academic and industrial community, which include coupled and simultaneous thermal analysis techniques. One of the important requirements for the thermal analysis of rubber products is the prior knowledge of the design concepts (both compound design and product design) of the product. This will not only help in the planning of the experimental procedures in a 423
Thermal Analysis of Rubbers and Rubbery Materials more systematic way but also aid in the more accurate scientific interpretation of the final results. For example, prior knowledge of the compound design and construction details is important for effectively carrying out the thermal analysis of samples from an automobile tyre. As mentioned in various parts of this chapter, thermal analysis results need to be confirmed by using other analytical techniques like chemical analysis, spectroscopy, surface analysis and microscopy. The authors expect that the thermal analysis techniques described in this chapter may help interested readers in developing laboratory procedures based on national and international standards and aid them in the interpretation of the analysis results.
Acknowledgements The authors would like to record their gratitude to Professor S.K. De, formerly with Rubber Technology Centre, Indian Institute of Technology, Kharagpur, for his help and support during the preparation of the manuscript. One of the authors, R.S. Rajeev, would like to thank Dr. Yutaka Sato of Japan Aerospace Exploration Agency (JAXA), Tokyo, for permitting the author to prepare the manuscript using the facilities at JAXA and Dr. V. Vijaybaskar of Leibniz-Institut für Polymer Forschung, Germany, for arranging some of the valuable reference materials.
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Thermal Analysis of Rubber Products 43. J.T. Henderson in Electric Cables Handbook, Ed., D. McAllister, Granada, London, UK, 1982, Chapter 2. 44. A.K. Kalidaha, P.P. De, A.S. Bhattacharyya and A.K. Sen, Die Angewandte Makromolekulare Chemie, 1993, 204, 19. 45. R. Buchdahl and L.E. Nielsen, Journal of Applied Physics, 1960, 21, 482. 46. M.C. Morris, Rubber Chemistry and Technology, 1967, 40, 2, 341. 47. N. Yoshimura and K. Fujimoto, Rubber Chemistry and Technology, 1969, 42, 4, 1009. 48. J.R. Fried, F.E. Karasz and W.J. MacKnight, Macromolecules, 1978, 11, 1, 150. 49. A.K. Sen, B. Mukherjee, A.S. Bhattacharya, L.K. Singhi, P.P. De and A.K. Bhowmick, Journal of Applied Polymer Science, 1991, 43, 9, 1673. 50. S. Persson, M. Goude and T. Olsson, Polymer Testing, 2003, 22, 3, 671. 51. J.L. LeBlanc and A. Staelraeve, Journal of Applied Polymer Science, 1994, 53, 8, 1025. 52. R.S. Rajeev, S.K. De, A. K. Bhowmick and B. John, Polymer Degradation and Stability, 2003, 79, 3, 449. 53. T.J. Hopen, The Microscope Journal, 2000, 48, 107. 54. BASOFIL Fibre, Technical literature, BASF Corporation, Enka, NC, USA, 1999 55. A.P Foldi in Short Fibre-Polymer Composites, Eds., S.K De and J.R White, Woodhead Publishing Limited, Cambridege, UK, 1996, p.242. 56. M. Fabrizi, G. La Motta and H.F.R. Schoeyer in Proceedings of the 32nd AIAA/ ASME/SAE/ASEE Joint Propulsion Conference, Lake Buena Vista, FL, USA, 1996. 57. R.S. Rajeev, A.K. Bhowmick, S.K. De, G.J.P. Kao and S. Bandyopadhyay, Polymer Composites, 2002, 23, 4, 574. 58. R.S Rajeev, A.K. Bhowmick, S.K De and S. Bandyopadhyay, Journal of Applied Polymer Science, 2003, 89, 5, 1211. 59. J.L. Mead, Z. Tao and H.S. Liu, Rubber Chemistry and Technology, 2002, 75, 4, 701.
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Thermal Analysis of Rubbers and Rubbery Materials 60. Y. Xu, D.D.L. Chung and C. Mroz, Composites - Part A, 2001, 32, 12, 1749. 61. S. Rimdusit and H. Ishida, Polymer, 2000, 41, 22, 7941. 62. L.C. Sim, S.R. Ramanan, H. Ismail, K.N. Seetharamu, and T.J. Goh, Thermochimica Acta, 2005, 430, 155. 63. J. Zhang, S. Feng and Q. Ma, Journal of Applied Polymer Science, 2003, 89, 6, 1548. 64. A.K. Sircar and T.G. Lamond, Rubber Chemistry and Technology, 1973, 46, 1, 178. 65. J.E. Callan, W.M. Hess and C.E. Scott, Rubber Chemistry and Technology, 1971, 44, 3, 814. 66. L.Y. Zlatkevich and V.G. Nikolskii, Rubber Chemistry and Technology, 1973, 46, 5, 1210. 67. A.K. Sircar and T.G. Lamond, Rubber Chemistry and Technology, 1975, 48, 2, 301. 68. D.W. Brazier and G.H. Nickel, Rubber Chemistry and Technology, 1975, 48, 4, 661. 69. F. Paulik and J. Paulik, Journal of Analytical Chemistry Acta, 1978, 13, 429. 70. W.J. Sichina, Compositional Analysis of Tyre Elastomers using Auto Stepwise TGA, Thermal Analysis Application Note, Perkin Elmer Instruments, PETech-59, 2005. 71. S.J. Swarin and A.M. Wims, Rubber Chemistry and Technology, 1974, 47, 5, 1193. 72. R.L. Zeyen, Rubber World, 1989, 199, 4, 14. 73. K. Baker and J. Leckenby, Kautschuk Gummi Kunststoffe, 1987, 40, 3, 223. 74. J.T. Byres, Rubber Chemistry and Technology, 2002, 75, 527. 75. M.J. Wang, Y. Kutsovsky, P. Zhang, L.J. Murphy, S. Laube and K. Mahmud, Rubber Chemistry and Technology, 2002, 75, 2, 247. 76. W.H. Waddell, J.H. Kuhr and R.R. Poulter, Rubber Chemistry and Technology, 2003, 76, 2, 348.
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Thermal Analysis in Recycling of Waste Rubbery Materials
13
Thermal Analysis in Recycling of Waste Rubbery Materials Amit K. Naskar
13.1 Introduction Utilisation of scrap polymeric materials is important from both environmental and economic perspectives. Disposal of waste polymers is strict nowadays because of regulations imposed by environmental protection agencies and minimal landfi ll requirements. Recycling is mandatory for all the polymer-processing units including automotive industries. Thermoplastics are easy to reuse by melt processing, however, elastomers or thermosets are difficult to reutilise. Reuse of rubber vulcanisates like scrap tyres and other post-consumer rubbery materials was brought to the attention of scientists in the last few decades. These are generally used as low cost fillers after granulation, and as raw materials for energy and materials recovery. Waste rubbers are also used for impact modification of brittle polymers and composites including high strength concrete and asphalt. To facilitate reuse very often scrap rubbers are modified by chemical and physical methods. Reclamation of rubber is carried out by devulcanisation. Monomers, fuel gases and carbons can be regenerated by pyrolysis of waste rubbery materials. Different rubber recycling techniques have been extensively reviewed in a recent publication [1]. A material engineer or elastomer compounder is required to know the types of waste polymers and their compositions prior to using them, with other raw materials, in a formulation of new product. In a review, Baranwal briefly discussed different ASTM standards for classification of recycled rubber, various techniques to generate recycled rubber, and their storage and characterisation test methods [2]. Evaluation of materials’ quality, thermal stability, and recyclability of waste rubbery materials is indispensable. Thermal analysis is not only the best tool for qualitative and quantitative characterisation of waste polymeric materials but also the most useful technique to evaluate the effects of such materials on processing parameters, morphology, physical properties and stability of the waste derived products. In the previous chapters, the basic instrumentation and the methods of different thermal analysis techniques have been discussed. This chapter highlights the role of thermal analyses on recycling and recyclability of rubbery materials. The first section of this chapter deals with utilisation of rubbery waste for recovery of materials. In 429
Thermal Analysis of Rubbers and Rubbery Materials this section the characterisation of composition of different recycled rubbery materials such as granulated rubber, reclaimed and surface modified rubber powder is discussed. Analyses of elastomer blends containing recycled rubbers, scrap rubber modified toughened plastics, thermoplastic elastomers, bitumen and cementitious composites are also described in this section. The next section includes the role of thermal analysis in pyrolytic recovery of fuel gases, monomer and activated carbon.
13.2 Utilisation of Scrap Elastomers for Material Recovery Reutilisation of primary scrap from the production unit of elastomeric materials is relatively easy as the composition of the material is known. However, regeneration of materials from used and returned waste dump is difficult. Reuse of unsorted waste material certainly results in low quality products. For the specific utilisation of scrap rubber and to avoid incompatibility in blends, understanding of the composition of recycled elastomer is necessary.
13.2.1 Characterisation of Recycled Rubber Scrap rubbers are used in the form of pulverised vulcanisate, devulcanised rubber and surface modified vulcanisate powders as recycled material. In the following sections thermal characterisation of these materials is discussed.
13.2.1.1 Waste Vulcanisate Elastomer formulations are usually complex in nature. Formulations consist of several ingredients such as a base rubber, process oil, reinforcing or diluent fillers, additives and curatives. Sometimes a mixture of base rubbers is incorporated depending on specific end-use [3]. For example, a typical tyre tread formulation comprises a mixture of natural rubber (NR), styrene-butadiene rubber (SBR) and butadiene rubber (BR). ASTM E1131 [4] describes a standard test method for the compositional analysis of rubbers by thermogravimetric analysis (TGA). ASTM E1356 [5] and D3418 [6] describe standard test methods for determining glass transition temperature (Tg) and other transition temperatures of polymers by differential scanning calorimetry (DSC) or differential thermal analysis (DTA). For the qualitative chemical analysis of elastomer composition, ASTM D297 [7] provides a standard method. Another qualitative method deals with pyrolysis of acetone extracted mass and infra red spectroscopy of the pyrolysates for the identification of base rubbers (ASTM D3677 [8]). The thermal analysis technique is relatively simpler and requires a minimum number of experimental steps.
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Thermal Analysis in Recycling of Waste Rubbery Materials The characterisation of vulcanisates by different thermal analysis techniques has been extensively studied. Elastomers from different sections of tyre such as tread, sidewall, inner liner, and so on, containing different rubbers and rubber blends have been analysed by DSC [9, 10], TGA [11-15] and total thermal analysis, i.e., a combination of DSC and TGA [16-22]. Exothermic peaks in DSC thermograms characterise decomposition temperatures of rubbers in an air or nitrogen atmosphere. However, the characteristic decomposition temperature varies not only on the temperature scan rate, but also on degree of crosslinking, filler loading, monomer ratio in the copolymer composition and presence of other rubbers in blends. Therefore, as a supportive technique TGA is conventionally used for compositional characterisation. During TGA analysis volatilisation of oils and plasticisers generally occurs in a broad temperature range of 200-400 C, in a nitrogen atmosphere [23]. This temperature range overlaps with the polymer decomposition region. Naskar and co-workers investigated compositional characterisation of waste rubber vulcanisates [15]. A certain mass of rubber vulcanisate was extracted in acetone to eliminate extractable oil and plasticiser content. Both acetone extracted dry mass and non-extracted rubber were analysed in TGA at constant heating rate in a nitrogen atmosphere up to 600 C, then in an oxygen atmosphere up to 900 C. A typical TGA and corresponding derivative thermogravimetric curve (DTG) of an acetone extracted granular waste rubber tyre is shown in Figure 13.1. In a nitrogen atmosphere the rubber starts to degrade at ~ 300 °C, the mass reduces until all rubber is thermally decomposed. There is no loss of weight in a nitrogen atmosphere with further rise in temperature. At 600 °C the chamber atmosphere is changed to oxygen. Carbon black starts burning at ~ 625 °C, weight loss occurs until all the carbon black is oxidised. At this stage only inorganic residues are left. A further decomposition due
Figure 13.1 Representative TGA and DTG curves for acetone extracted waste rubber tyre [15] Reproduced with permission from Naskar and co-workers, Rubber Chemistry and Technology, 2000, 73, 5, 902. ©2000, Rubber Division, American Chemical Society, Inc
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Thermal Analysis of Rubbers and Rubbery Materials to carbonate materials is observed at the higher temperature range of 750-800 °C. The DTG curve displays two peaks (413 °C and 455 °C) in the rubber decomposition zone (350-530 °C). This indicates the presence of a rubber blend in the vulcanisate. The details of formulation reconstruction have been discussed in relevant references [15, 23] and in an earlier chapter of this book. Typical decomposition temperatures of rubber vulcanisates (reported as DTG peak temperature) are summarised in Table 13.1. NR and isoprene rubber (IR) display a twostage thermal decomposition. For NR one sharp DTG peak with a maximum at 370 °C and an inflection around 430 °C has been reported [11]. IR displays two peaks at 370 and 420 °C. It has also been reported that the intensity of second peak and its location varies with sulfur content in the IR vulcanisate formulation [11]. Different loadings of carbon black in the vulcanisate cause variation in decomposition temperature. Increase in carbon black content reportedly lowers the decomposition temperature of IR. For NR, however, decomposition temperatures of gum and black filled vulcanisates remain same [16]. SBR generally displays an inflection at 420-430 °C due to a decomposition peak at 447 °C. BR shows a flat decomposition peak at 355 °C and a sharp peak at 465 °C [16]. Another investigation also reports two-stage (373 °C and 470 °C) decomposition of BR [24]. Butyl rubber (isobutylene-isoprene rubber, IIR) degrades at 428 °C [14]. Halogenated rubbers such as chloroprene rubber (CR) and halobutyl rubbers (BIIR and CIIR) undergo two-stage decomposition, where first step corresponds
Table 13.1 Decomposition temperatures of elastomers Rubber NR IR IR-Black filled BR SBR CR EPDM IIR BIIR CIIR EVA
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Temperature (°C) 365-370, 430 375 370, 420 336, 379 355, 465 373, 470 420-430, 447 346, 439 484 469 428 301, 420 300-310, 430-440 399 340, 470
Atmosphere Nitrogen Oxygen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Oxygen Nitrogen
Reference [11, 16] [19] [11] [16] [16] [24] [16] [18] [19] [20] [19] [19] [17, 19] [19] [25]
Thermal Analysis in Recycling of Waste Rubbery Materials to dehydrohalogenation. For blends of CR with NR, dehydrohalogenation of halogenated rubber leaves a carbon residue and shifts the decomposition temperature of NR to a higher temperature [18]. For CR the presence of sulfur lowers the first decomposition temperature [18]. Ethylene propylene diene rubber (EPDM) degrades at 470-485 °C [19-20]. Ethylene vinyl acetate rubber (EVA) decomposes in two steps and the first step involves elimination from vinyl acetate component [25]. A study of compositional characterisation of cryogenically shattered waste tyre rubbers of different particle size range, using TGA, revealed that with a decrease in particle size, % rubber content, % carbon black content and % acetone extractable volatiles decreased, but % inorganic filler content increased [15, 26]. Figure 13.2 shows the TGA thermograms of acetone extracted ground rubber tyres (GRT) of different particle sizes. It is evident that smaller particles have a higher amount of ash residue. In another study, composition of solid-state, shear extruded, rubber powder was reported to be independent of particle size [27]. The extent of thermo-oxidative degradation was dependent on the surface area of samples. However, thermal degradation in an inert atmosphere was independent of particle size or surface area. For the identification of rubber type, Tg of the base polymers can be investigated. In general Tg is measured by thermal analysis techniques such as DSC, dynamic mechanical thermal analysis (DMA), thermomechanical analysis (TMA) and dielectric thermal analysis (DETA). Sircar and co-workers reviewed the different techniques and results of Tg measurements [28]. Although the Tg of a rubber vulcanisate depends on type of base rubber, microstructure of synthetic rubber, filler-rubber interaction, degree of crosslinking, and so on, thermal analysis techniques can predict fairly accurately the base
Figure 13.2 TGA thermograms of GRT powders of different particle sizes, ( ), <52 mesh; (----), 52-72 mesh; ( - ), 72-100 mesh; ( - - ), 100-150 mesh [26]
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Thermal Analysis of Rubbers and Rubbery Materials rubber if the source of waste rubber is known. For confirmatory test a supplementary chemical analysis (ASTM D297 [7]) and infra red spectroscopic analysis (ASTM D3677 [8]), as mentioned earlier, are recommended. Details of the chemical and spectroscopic analysis are beyond the scope of this chapter.
13.2.1.2 Devulcanised Rubber Devulcanisation is one of the most useful methods for regeneration of material from waste rubber. In this process the crosslinking sulfidic bonds are cleaved by a combination of chemical, mechanical, ultrasonic and microwave treatments. Several authors have reviewed the devulcanisation process of rubber [29-32]. Bilgili and co-workers reported that devulcanisation occurred during solid-state shear extrusion of rubber vulcanisates [27]. Crosslink density and gel fraction of shear extruded rubber powder was smaller than that of the original granulates. Thermal stability of the reclaimed rubber in nitrogen remained unaffected. In air, the heat of thermo-oxidative degradation of smaller particles increased, although the DTG peak position did not significantly alter. Isayev and co-workers investigated ultrasonically devulcanised rubbers [33, 34]. The Tg of the rubber vulcanisates after treatment with ultrasonic waves of different amplitudes under similar processing conditions was measured using DSC. The Tg of NR and SBR vulcanisates after ultrasonic treatment increased, although the gel fraction and crosslink density of the treated rubber decreased. The increase in Tg was attributed to the cleavage of linear polysulfidic crosslinks and formation of cyclic sulfidic structures chemically linked to the polymer backbone [33]. For the EPDM rubber containing ethylidene norbornene (ENB), vulcanisates after ultrasonic treatment exhibited decrease in Tg. Presence of the bulky ENB group hinders formation of cyclic sulfidic structure in EPDM devulcanisates [34]. Thermal analysis was used to investigate the effect of ultrasonic devulcanisation of BR elastomers and thermal stability of revulcanisates [35]. As estimated by onset of thermal degradation from TGA traces, thermal stability of the rubbers follows the trend: virgin gum > gum vulcanisates > devulcanised rubber > revulcanisates. In comparison to the gum rubber, vulcanisates exhibited an accelerated degradation which was likely to be due to the presence of curatives. Ultrasonic treatment weakens the chemical bonds and hence lowers the decomposition temperature. Non-volatile content remains same in both vulcanisate and devulcanisates. For IIR, however, the trend is reversed [36]. Presence of unsaturation in raw IIR is likely accelerates the process of bond scission by thermal degradation. BR exhibits two-stage degradation in DSC scans, first cyclisation at 390 °C and decomposition at 470 °C. Exothermic heats of cyclisation ( Hcx) gradually decrease on 434
Thermal Analysis in Recycling of Waste Rubbery Materials vulcanisation of gum BR, devulcanisation of vulcanisates and finally revulcanisation of devulcanisates. The H values are summarised in Table 13.2. Endothermic heats of decomposition ( Hd) of vulcanisates are lower than those of the unvulcanised rubbers. The devulcanised sample displays higher Hd than that of the vulcanisates. From the DSC scans of gum and devulcanised butyl rubber and their vulcanisates, Tg was measured. As shown in Table 13.2, Tg of cured IIR is higher than that of the gum and ultrasonically severed vulcanisates.
Table 13.2 DSC data of gum, vulcanised, and devulcanised BR [35] and IIR [36] rubbers Sample
BR
Hcx (J/g) Gum 751 Vulcanisate 621 Devulcanised * 481 Revulcanisate 477 * 10 μm ultrasonic wave treatment at 120 °C
Hd (J/g) 247 243 287 181
IIR Tg (°C) -63.4 -58.2 -60.0 -58.2
Figure 13.3 TG and DTG curves of NR vulcanisate ( ) and its devulcanisates (-----) in nitrogen atmosphere, heating rate 10 °C/min [37] Redrawn from Kleps and co-workers, Journal of Thermal Analysis and Calorimetry, 2000, 60, 1, 271. ©2000 Akadémiai Kiadó, Budapest. Redrawn with kind permission of Springer Science and Business Media
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Thermal Analysis of Rubbers and Rubbery Materials Kleps and co-workers conducted TGA analysis of rubber devulcanisates obtained by microwave treatment [37]. With an increase in microwave dosage, total organic polymer content in the devulcanisate decreased while organic non-polymer content increased. During thermal degradation, organic polymers decompose along with cleavage of S-S and C-S linkages to form soot. TGA and DTG thermograms of NR elastomers before and after microwave devulcanisation are shown in Figure 13.3. Microwave treatment causes decrease in decomposition temperatures (as observed in DTG maximum, TDTG max) of the rubbers. For example, a microwave energy dosage of 612-650 kJ per kg of NR and SBR vulcanisates decreased TDTG max in TGA curves under nitrogen atmosphere by 15 °C from 385 °C and 465 °C, respectively. EPDM elastomer displayed a 10 °C fall in TDTG max from 480 °C under similar conditions. Kojima and co-workers studied devulcanisation of sulfur cured NR in supercritical carbon dioxide using diphenyl disulfide [38]. Although devulcanised rubber contained about 50% sol fraction, and gel of significantly low crosslink density, it displayed an increase in Tg from –59 °C (for cured NR) to –51 °C. The increase in Tg was attributed to hindered mobility of the molecules due to addition of diphenyl disulfide onto polymer chains during the devulcanisation process. Devulcanised product, however, displayed an exotherm at 174 °C due to a residual delinking reaction of S-S bonds. Thus DSC can be used for monitoring the extent of devulcanisation at different processing times. In a recent study, thermal devulcanisation of NBR by nitrobenzene has been reported [39]. Nitrobenzene cleaves the S-S bonds in NBR vulcanisate and forms a reactive adduct. Butadiene segments also reacts with nitrobenzene and forms a degraded fraction of NBR. Devulcanised rubber displays a lowering in decomposition temperature due to formation of low molecular weight fractions and leaves more carbon residue due to formation of aromatised material.
13.2.1.3 Surface Modified Vulcanisate Powders Recycled rubber particles when used in polymeric matrices are often surface modified to overcome most of the deleterious effects. Also, surface modification of ground rubber enables its incorporation at higher loading in a thermoplastic or rubbery matrix without significant decrease in the physical properties. Methods of surface modification of ground rubbers are broadly classified into two major techniques, e.g., chemical and physical techniques. In these methods additional functionality is incorporated onto the particles to improve surface wettabilty and/or chemical bonding with matrix. Thermal analysis tools are extensively useful to characterise modified rubber. Waste rubber particles obtained by solid state, shear extrusion were chemically modified by polymerising acrylic acid in presence of toluene as swelling solvent for the particles to produce particulate phase semi-interpenetrating polymer network (PPSIPN) [40]. Such materials are useful in variety of aqueous media application such as additives to 436
Thermal Analysis in Recycling of Waste Rubbery Materials waterborne emulsions and wastewater treatments. DSC and TGA can be used to evaluate the morphology of the products. Two types of PPSIPN having similar polyacrylic acid content produced DSC and TGA thermograms which behaved differently depending on the particle morphology. Polyacrylic acid (PAA) segments polymerised inside rubber particles exhibited a Tg of 105 °C and 110 °C for samples with high and low interpenetrating nature (IPN), respectively, [40]. A shift in Tg of PAA segments toward that of the rubbery phase in the modified particles indicated partial interpenetration of PAA into rubber network. TGA weight loss versus temperature plot for these samples is shown in Figure 13.4. The sample with less IPN nature (i.e., PAA is not deeply interpenetrated inside the particles, is labelled as modified rubber particle B in Figure 13.4) displays a faster decomposition of PAA segments within the 200-340 °C region. For the other type of sample, modified rubber particle A, major fractions of PAA decompose along with rubber in the high temperature region 300-460 °C. Modified rubber particles with 45-50% PAA content have been reported [41]. Naskar and co-workers successfully grafted maleic anhydride onto GRT particles to enhance compatibility of GRT in acrylated high-density polyethylene (HDPE) matrix [42]. DSC scans of the modified and unmodified GRT are shown in Figure 13.5. It shows
Figure 13.4 TGA thermogram of rubber particles; ( ), unmodified rubber particles; ( ), modified rubber particles-A with high degree of interpenetrated network; ), modified rubber particles-B with low degree of interpenetrated network [40] ( Redrawn with permission from Shahidi and co-workers, Polymer, 2004, 45, 15, 5183. ©2004, Elsevier
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Thermal Analysis of Rubbers and Rubbery Materials
Figure 13.5 DSC thermograms of maleated and non-maleated rubber particles, mGRT and GRT, respectively, (a) glass-rubber transition; (b) high temperature transition [42] Reproduced with permission from Naskar and co-workers, Journal of Applied Polymer Science, 2002, 84, 2, 370. ©2002, John Wiley and Sons, Inc
that GRT exhibits a broad baseline shift (Tg) in the temperature range of –100 °C to –25 °C indicating a mixture of rubbers in the composition of GRT. However, maleated GRT (mGRT) exhibits a high temperature base line shift in the temperature range of 25 to 80 °C along with the broad rubbery transition in the low temperature range. The high temperature transition is believed to be due to the dissociation of the ionic clusters arising out of the salt of mGRT. Salt formation is likely due to interaction of mGRT with unreacted zinc oxide present in the waste vulcanisate [42]. A similar high temperature transition in the compression moulded samples was observed when GRT was chlorinated using a mild chlorinating agent [43]. Chlorination of GRT enhances its compatibility with polyvinyl chloride (PVC). The dynamic mechanical loss tangent spectra of the compression moulded GRT specimens at different degrees of chlorination are presented in Figure 13.6a. GRT shows a broad maximum at –30 oC, which is the Tg of the rubbery phase. The peak appeared broad due to mixture of base rubbers in GRT. With increase in chlorine content (from G1 to G5 in Figure 13.6a), the Tg of the rubbery phase does not shift but the (tan )max value 438
Thermal Analysis in Recycling of Waste Rubbery Materials
Figure 13.6 Dynamic mechanical spectra of moulded GRT samples, where degree of chlorination follow the order G0 < G1 < G3 < G5; (a) loss tangent, (b) storage modulus [43] Reproduced with permission from Naskar and co-workers, Rubber Chemistry and Technology, 2001, 74, 4, 645. ©2001, Rubber Division, American Chemical Society, Inc., Washington, DC, USA
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Thermal Analysis of Rubbers and Rubbery Materials is lowered, indicating that incorporation of –Cl groups lowers the relative proportion of flexible rubbery phase. The higher the chlorine content, the lower is the (tan )max value. Apart from the peak due to the Tg of the rubbery phase, the chlorinated samples show another peak in the high temperature region (60-70 °C). This peak is due to the relaxation of the hard phase formed by C-Cl dipole-dipole interaction. Non-chlorinated GRT, however, does not display this peak. The (tan )max in the high temperature region increases with increase in relative proportion of the hard phase and levels off at high degrees of chlorination. The storage modulus spectra as shown in Figure 13.6b, shows biphasic structure, (i.e., the soft and the hard phases), in chlorinated GRT samples (in contrast to the single-phase behaviour of the unmodified GRT). It is also evident that the storage modulus increases with increase in degree of chlorination. Occurrence of biphasic morphology was also detected in the DSC scans [43]. Ground rubber tyre particles when treated with nitric acid produced partially degraded nitro oligomers [44]. The product was sulfomethylated by sodium hydroxymethane sulfonate to yield a sodium salt of N-sulfonic acid of nitro oligomers [45]. These materials are useful as brighteners in cyanide-less electrolyte. TGA and DTA studies of these waste derived oligomers and a control oligomer obtained from neat SBR provides comparable thermal stability data for the products [45]. Among various rubbery materials, saturated elastomers are very difficult to recycle, because of their inherent resistance to chemicals, gases, radiation and microbes. Among these rubbers IIR (butyl) and EPDM are mostly used as thermoset elastomers. Recycling of IIR by gamma irradiation at a dosage of more than 70 kGy was found to significantly enhance plasticity of butyl elastomers [46, 47]. This kind of reclamation of IIR has been commercially implemented [48]. Although, thermal analysis of radiation degradation of rubber is not extensively followed, a few radiation degradation studies on BR and SBR vulcanisates by gamma rays have been reported [49]. Irradiation of butadienecontaining polymers in oxygen caused a decrease in onset temperature of mass loss in TGA thermograms.
13.2.2 Polymer Blends Containing Recycled Rubber Recycled rubber, either in the form of vulcanisate powder or surface modified vulcanisate powder or rubber devulcanisate, is used along with virgin rubber or plastic or blends of rubbers and plastics for the development of new materials. The following section reviews the application of thermal analysis for the blends containing recycled rubber.
13.2.2.1 Rubbery Blends Thermal analysis of rubbery products containing recycled rubber is a widely accepted tool for characterisation of thermal stability, morphology and dynamic mechanical properties 440
Thermal Analysis in Recycling of Waste Rubbery Materials of the waste derived products. DSC analysis has been shown as a method to study the heterogeneity of waste tyre rubber-polyurethane (PU) composite [50, 51]. In dynamic mechanical loss tangent spectra of the neat PU, two segmental relaxations with rise in temperature were observed at -13 and 10 °C, as a sharp peak and a broad shoulder, respectively. The first peak was due to rubbery transition and the second one was due to relaxation of relatively rigid segments in PU. For the composite containing 20% PU and 80% finely ground tyre rubber, the broad shoulder transition due to PU appeared more intense than it occurred in neat PU. The peak intensity of the rubbery transition of PU in composite was low due to quantity effect. But the stronger shoulder intensity around 10-20 °C indicated interfacial interaction between PU and rubber particles [51]. TGA degradation of the waste tyre rubber revealed a two-step decomposition process. A low temperature decomposition region for the rubber appeared in the region of the single step decomposition of PU. Activation energy for the decomposition reaction at low temperature was measured for both the neat components and the composite. The difference in the observed activation energy for the low temperature degradation of the composite and the calculated value (based on weight fraction of the components) also suggested interaction between PU and the recycled rubber [52]. In an attempt to recycle PVC from scrap electrical wire and cables, acrylic acid was used as compatibiliser for its blends with NBR. Although, mechanical properties of the blends containing 4 phr of acrylic acid were superior to those of the control NBR/recycled PVC blends, the former displayed inferior thermal stability. Acrylic acid was found to have a decreasing influence on the thermal stability of the blends [53]. Silicone and fluoro-elastomer blends have both low and high temperature oil/chemical resistant applications. Ghosh and co-workers utilised recycled fluororubber vulcanisate powders in the blend up to 50% replacement of fluororubber component without significant effect on mechanical properties, Mooney viscosity and scorch time [54]. However, addition of silicone rubber vulcanisate powder in the blend formulations for the substitution of neat silicone rubber showed deleterious effect on the physico-mechanical properties of the blends. Thermodynamic immiscibility of the blends was investigated by dynamic mechanical thermal analysis. Although, incorporation of vulcanisate powders in each component of the blend did not significantly affect the relaxation temperature of the two phases, the storage modulus decreased. The decrease was significant when silicone rubber was replaced by silicone vulcanisate powder. In spite of the adverse effect on dynamic mechanical properties with replacement of neat rubber by recycled rubber powder, the waste derived blend of selected composition displayed technological compatibility based on static mechanical properties. Jacob and co-workers reported on reuse of finely ground waste EPDM vulcanisate powders in virgin EPDM formulations. Carbon black filled waste EPDM vulcanisate powder (denoted as wEPDM) could be successfully loaded up to 100 phr of neat EPDM [55]. The wEPDM was found to act as a reinforcing filler. At very high loading 441
Thermal Analysis of Rubbers and Rubbery Materials of wEPDM, a decrease in crosslinking density was observed in the final vulcanisates. Dynamic storage modulus plots of the selected blend formulations are shown in Figure 13.7. Storage modulus increases with incorporation of wEPDM initially, up to 50 phr loading, due to its reinforcing effect. A marginal shift in Tg towards a higher temperature has also been attributed to the reinforcing filler effect. Further loading of wEPDM in virgin EPDM causes a decrease in the storage modulus. This decrease in dynamic modulus is likely to be due to the decrease in crosslinking density. Plasticised PVC containing chlorinated GRT (Cl-GRT) was investigated as melt processable rubber [56]. DMA results revealed that the storage modulus of the blend compositions containing Cl-GRT in the PVC matrix is higher than that of the control composition containing non-chlorinated GRT. The loss tangent value at the Tg of the PVC phase for the blend containing Cl-GRT was also lower than that of the control blend, indicating enhanced interaction between Cl-GRT and PVC phases. Results of dielectric thermal analysis of the blends, as shown in Figure 13.8, indicate that at high frequencies (104-105 Hz) when interfacial polarisation in plasticised PVC is significantly
Figure 13.7 Effect of waste black-filled EPDM vulcanisate powder (wEPDM) on variation of storage modulus with temperature; legends ( ), (—O—), ( ), and ( ) represent formulation containing 0, 20, 50 and 100 phr wEPDM, respectively [55] Reproduced with permission from Jacob and co-workers, Journal of Applied Polymer Science, 2001, 82, 13, 3293. ©2001, John Wiley and Sons, Inc
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Figure 13.8 DETA Results showing temperature dependence of dielectric loss factor () of neat PVC compound ( ), PVC compounded with GRT ( O ), and PVC compounded with chlorinated GRT ( ● ) at a frequency of (a) 104 Hz and (b) 105 Hz [26]
low, two relaxation transitions ( and ) of PVC phase appears due to non-uniform plasticiser distribution [57]. At high frequency (105 Hz) these transitions overlap and form a broad peak. At low frequencies (<104, not shown in Figure) transition remains masked by strong dielectric polarisation in the plasticised PVC. Upon loading of GRT in the matrix, the peaks due to the relaxation of PVC phases broaden. Loss factors at the high temperature region also increase on GRT loading. The increase in loss factor is significantly higher for blends containing Cl-GRT due to strong interfacial polarisation. Strong interaction between Cl-GRT and PVC also masks the relaxation transitions of PVC phase particularly at low frequencies. The shoulder that appears at 105 Hz, due to the relaxation in Cl-GRT/PVC blend at 105 Hz (Figure 13.8b) almost disappears in the dielectric spectra at 104 Hz (Figure 13.8a). Reclaimed rubbery material was obtained by decomposition of the NR and SBR vulcanisate wastes in a reactor at 310 °C and high pressure (4-9 MPa) of nitrogen for 45-60 minutes [58]. In the DSC scan, the degraded rubber displayed a Tg at –58 °C, which is about 6 °C lower than that of the waste vulcanisates. The degraded product was 443
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Figure 13.9 TGA thermograms of NR vulcanisates compounded with 40 phr oil ( and 40 phr degraded NR ( ) in nitrogen atmosphere [58]
)
Reproduced with permission from Tripathy and co-workers, Polymer Engineering and Science, 2004, 44, 7, 1338. ©2004 John Wiley and Sons
successfully used as plasticiser in NR and SBR formulations. As shown in Figure 13.9, the NR vulcanisate containing 40 phr decomposed NR (DNR) exhibits enhanced thermal stability compared to a control commercial plasticiser (oil) compounded NR vulcanisate. Replacement of oil by DNR also increased crosslink density of the final vulcanisates.
13.2.2.2 Toughened Plastics Brittle plastics filled with waste rubber powders are generally processed to produce toughened plastics. Soft rubbery dispersed domains toughen the brittle matrix. To enhance interfacial adhesion, surface modified ground rubber vulcanisates are used. Karger-Kocsis and Gremmels utilised waste polyester based PU from the footwear industry to toughen epoxy resin [59]. The waste PU was hygrothermally degraded prior to its use in the epoxy matrix. The compounded materials were characterised by DMA. Dynamic mechanical complex modulus at rubbery plateau (ER) was used to calculate the mean molecular weight between crosslinks (Mc) based on the equations of basic rubber elasticity: ER =3n RT =3 (d/Mc) RT 444
(13.1)
Thermal Analysis in Recycling of Waste Rubbery Materials where, d is the resin density; R is the universal gas constant; T is the plateau onset absolute temperature. Incorporation of recycled and degraded PU shifted the Tg of the epoxy matrix towards a lower temperature and increased Mc. The decrease in Tg of the epoxy matrix containing recycled PU was significantly greater than that of the control compounds containing carboxyl-terminated acrylonitrile butadiene, a commercial toughening agent for epoxy. The recycled PU acts not only as phase separating additive but also as an active plasticiser in epoxy matrix. Kim and co-workers reported a toughened HDPE composition based on acrylamide grafted recycled rubber [60]. To understand the effect of surface modification of recycled rubber on the properties of the HDPE, interfacial characteristics were analysed by dynamic mechanical studies. The variation of the storage modulus as a function of the temperature for the neat HDPE and its blends containing 20 wt% rubber powder (both modified and unmodified), and the 10 wt% maleic anhydride grafted polypropylene (PP) (as compatibiliser) is shown in Figure 13.10. With the addition of rubber powders, the storage modulus at low temperature is increased. In both the blends, a two-stage drop in storage modulus was observed due to the rubbery and the plastic components. The blend
Figure 13.10 Variation of storage modulus as a function of temperature for neat HDPE ) and HDPE/20 ( ), HDPE/20 wt% unmodified rubber powder composition ( wt% modified rubber powder composition ( ). For the blends 10 weight% maleic anhydride grafted PP was used as compatibiliser [60] Reproduced with permission from Kim and co-workers, Journal of Applied Polymer Science, 2000, 77, 12, 2595. ©2000, John Wiley and Sons, Inc
445
Thermal Analysis of Rubbers and Rubbery Materials containing surface-modified rubber powder shows higher values of the storage modulus than that of an unmodified rubber powder-filled composition. This increase in the storage modulus of the modified rubber blend is due to the interfacial interaction between the acrylamide of the modified rubber powder and the maleic anhydride of the compatibiliser via interfacial reaction. The surface-modified rubber powder-filled HDPE reportedly displayed lower tan values than that of the unmodified rubber powder-filled blend. Waste crosslinked linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) foam (obtained from scrap packaging materials) were pulverised and used as an elastomeric filler in HDPE for toughening the matrix [61]. Thermal characterisation of the blends revealed that incorporation of foam particles did not affect onset of the melting temperature. However, the melting peak temperature and the heat of fusion of the compositions were reduced by incorporation of foam particles. On the other hand, the onset crystallisation temperature during cooling remained unaffected but the crystallisation peak temperature increased by incorporation of waste polyethylene particles. Tantayanon and Juikham reported toughening of the PP matrix by GRT or reclaimed tyre rubber (RTR) [62]. Impact strength of the matrix is marginally higher in the GRT or RTR based blends. Dynamic vulcanisation of GRT by a sulfur cure system does not help to improve the impact strength. However, the RTR based compositions display significantly higher impact strength when they are dynamically vulcanised by a sulfur or peroxide cure system. Maleic anhydride acts as the compatibiliser for the peroxide cure system and the impact strength improves further. As measured by DSC, 4-6% drop in crystallinity of PP was observed when the blends containing RTR were dynamically vulcanised. Finely dispersed crosslinked domains of RTR do not allow close packing of the PP chains and reduce crystallinity of the matrix. Liu and co-workers also reported a similar decrease in crystallinity in PP by incorporation of recycled EPDM powder [63]. The effect was more pronounced when the blends were dynamically vulcanised. Phinyocheep and co-workers discussed impact toughening of PP by scrap rubber dust (obtained by buffing of sport shoe soles) in the presence of compatibilisers [64]. It was reported that the temperatures corresponding to the DSC crystallisation peak and to the onset of crystallisation of PP/scrap rubber dust blends were increased in comparison to that of the neat PP. In these cases, however, increased crystallinity in PP after incorporation of rubber dust was observed. The rubber dust in PP acts as nucleating agent. Ismail and Suryadiansyah investigated thermal degradation behaviour of PP/NR and PP/reclaimed rubber (RR) blends [65]. From the TGA thermograms of the blends, as shown in Figure 13.11, it is clear that the thermal stability of the blend containing RR is higher than that of the control blend containing neat NR. Additives present in RR enhance the thermal stability of the blends. Grigor’eva and co-workers investigated plasticisation of rigid PVC by using recycled PU foam coated with PVC film (from waste sound proofing material) using DSC and DMA 446
Thermal Analysis in Recycling of Waste Rubbery Materials
Figure 13.11 TGA thermograms of blends of PP/NR ( , 70/30; PP/reclaim rubber (
, 70/30; , 50/50) [65]
, 50/50) and
Reproduced with permission from H. Ismail and Suryadiansyah, Polymer–Plastics Technology and Engineering, 2004, 43, 2, 319. ©2004, Taylor & Francis Group
[66]. PVC coated PU foam was degraded by a thermomechano-chemical process inside an extruder and used as a plasticiser. Combination of plasticiser (such as mixture of phthalate and sebacate) and reclaimed polymer was used for the study. The Tg values of the compositions are summarised in Table 13.3. The Tg of the PVC matrix is low when an equivalent amount of reclaim mix is used instead of commercial ester plasticisers. PVC/PU reclaim mix acts as a better plasticiser than the esters and significantly enhances the impact strength of PVC.
Table 13.3 Tg of the various PVC formulations containing recycled PU foam and ester plasticiser [66] Composition Tg of PU phase (°C) Tg of PVC phase (°C) (rigid PVC/reclaim mix/ester plasticiser mix) 100/0/0 — 78 100/0/40 — 60 100/20/20 -28 57 100/40/0 -33 58 0/100/0 -40 53
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13.2.2.3 Thermoplastic Elastomer In the previous section, analysis of recycled rubber toughened plastics, where the plastic phase dominates, has been discussed. Incorporation of crosslinked rubber domains in the plastic matrix at higher proportion (50-70%) leads to the formation of a thermoplastic elastomeric (TPE) material. These materials have a significantly higher elongation at break (Eb 200%) indicating ductility and a low tensile set % (<35%) that indicates high elasticity. In contrast, toughened plastics display low ultimate elongation (Eb <50%). TPE materials can be processed as thermoplastics but they behave as elastomers. Recycling of waste rubber to produce TPE is beneficial for the environment because unlike the waste rubber vulcanisate, the TPE product based on waste rubber is thermally reprocessible or recyclable. TPE materials possess an inherent biphasic morphology and rubbery domains are dispersed in plastic matrices. Thermal analysis tools are extremely useful to investigate glass-rubber relaxation and melting/crystallisation transition spectra of the phases. Naskar and co-workers investigated TPE composition based on GRT [67]. GRT powder was mixed with EPDM rubber and the rubbery material was blended with an acrylic acid modified HDPE (A-HDPE), which was subsequently dynamically vulcanised using dicumyl peroxide at 1 phr dosage. GRT contained about 44% rubber (rGRT). A 60:40 rubber:plastic blend where the rubbery phase consists of 50:50 ratio of rGRT:EPDM, exhibited a thermoplastic elastomeric nature. Various rubber:plastic 60:40 blend formulations having different rGRT:EPDM ratios, shown in Table 13.4, were investigated by DMA [26]. Loss tangent (tan ) spectra of blends are shown in Figure 13.12a. It is evident that gradual replacement of EPDM by rGRT leads to broadening of the tan peak and a decrease in tan value at –35 °C, the Tg of rubbery phase. Since GRT consists of a
Table 13.4 Formulations showing variation of EPDM/rGRT ratio at rubber/ plastic ratio of 60:40 and DCP dose of 1 phr [26] Ingredients (Parts by weight)
Blend C2 C4 C5 C6 C8 EPDM 60 40 30 20 0 GRT 0 45 68 91 136 a (rGRT) (0) (20) (30) (40) (60) A-HDPE 40 40 40 40 40 b DCP 0.6 0.6 0.6 0.6 0.6 a :Values in the parentheses refer to the parts of the rubber phase in GRT b :Dicumyl peroxide (DCP) concentrations are equivalent to 1 part per 100 parts of rubber component (EPDM + rGRT) in the blend
448
Thermal Analysis in Recycling of Waste Rubbery Materials
Figure 13.12 Plots of (a) tan versus temperature, and (b) storage modulus (E´) versus temperature of 60/40 rubber/plastic blends at different EPDM/GRT ratios; GRT loadings in the compositions follow the order C2 < C4 < C5
449
Thermal Analysis of Rubbers and Rubbery Materials mixture of rubbers in a crosslinked state, its incorporation in place of EPDM leads to broadening of the tan peak. The broad loss tangent shoulder in the 30-120 °C region is due to vibrational and rotational motion in HDPE crystallites [67]. Incorporation of GRT in the HDPE matrix affects semi-crystalline morphology in the matrix and increases the loss tangent value in this region. Two-phase morphology of these blends is clear from the storage modulus (E´) plot displayed in Figure 13.12b. The first drop in E´ at the low temperature region is due to the Tg of the rubbery phase and the second drop in E´ at the high temperature region is due to melting of the plastic phase. With increase in degree of replacement of EPDM by rGRT, E´ in the low temperature region, i.e., below the melting point of A-HDPE increases, but at higher temperature, i.e., beyond the melting transition of A-HDPE, E´ decreases. At very high substitution of EPDM by rGRT, drop in E´ due to glass rubber relaxation transition becomes less sharp. However, discrete rubbery domains reduce the elasticity of the molten matrix at very high level of substitution of EPDM by rGRT, probably due to lack of interfacial adhesion. As for the physical properties, a formulation containing an intermediate loading of GRT in EPDM represents the best TPE material based on waste rubber. Very high GRT loading leads to formation of toughened plastics only. Naskar and co-workers reported a TPE composition based on mGRT in similar rubber/ plastic blend formulation [60/40, (EPDM + m-rGRT)/A-HDPE] where the rubbery phase consists of mixture of mGRT and EPDM [42]. Dynamic mechanical loss tangent of the blend containing mGRT was significantly lower than that of the blend containing GRT, especially at a temperature greater than the Tg of the rubbery phase. However, there was no change in the Tg (−37 °C) and the loss tangent at Tg by the replacement of GRT by mGRT in the TPE composition. A higher degree of interaction between the rubbery phase containing GRT, mGRT and the A-HDPE matrix could be surmised from the DMA results. Grigoryeva and co-workers reported similar dynamic mechanical properties of TPE composition based on blends of recycled HDPE, EPDM of high ethylene content and bitumen modified GRT [68]. Bitumen modification with GRT enhanced the compatibility of the waste rubber with the HDPE matrix. Incorporation of GRT in the EPDM/HDPE blend did not affect its thermal degradation behaviour. However, bitumen treatment of GRT lowered the decomposition temperature of the TPE blend by 30 °C. Thermal stability of the TPE containing modified GRT remained unchanged even after six times extrusion of the product, indicating excellent recyclability. Incorporation of GRT or modified GRT lowered the melting point of HDPE slightly. Bitumen modified GRT which was two-roll milled under hot conditions and then masticated with the blend of EPDM and HDPE formed a TPE composition with improved interfacial adhesion of the matrix and GRT. This was evident from the shift in Tg of rubbery phase to higher temperature and lowering of HDPE melting point in DMA results. Li and co-workers characterised TPE composition based on ternary blends of HDPE, scrap rubber and EPDM that were dynamically vulcanised after plasticisation by silicone 450
Thermal Analysis in Recycling of Waste Rubbery Materials oil [69]. Plasticisation of the rubbery phase led to a lowering of the Tg and the elastic storage modulus. Thus, a high impact resistant TPE was developed. TPE compositions based on rubbery component consisting of GRT or devulcanised GRT mixed with neat rubbers such as NR, SBR and EPDM and a plastic component of LDPE at a rubber to plastic ratio of 50:50 were reported [70, 71]. The compositions were characterised before and after dynamic vulcanisation using dicumyl peroxide or sulfur (efficient vulcanisation system). Addition of devulcanised GRT made by mastication with or without a commercial chemical decreased the Tg of the rubbery phase in the blend composition. But, dynamic curing of the blend increased the Tg of the rubbery phase. The increase in Tg was more with the sulfur cure system than that observed with the peroxide cure system. The properties of the TPE composition based on EPDM were better than the others (NR and SBR) due to the compatibility of EPDM with LDPE and interfacial crosslinkability with GRT or devulcanised GRT. Nevatia and co-workers also characterised similar TPE compositions based on reclaimed rubber and scrap LDPE (at a 50:50 ratio of rubber:plastic) dynamically vulcanised by sulfur system or DCP [72]. Since the reclaimed rubber contained inorganic and organic fillers, the Tg and the storage modulus of the recycled compound were greater than that of the control TPE based on neat NR. Guo and co-workers prepared TPE based on scrap rubber powder and LLDPE by reactive compatibilisation with epoxidised natural rubber (ENR) grafted LLDPE [73]. A DSC scan of the compatibilised TPE displayed a distinct Tg at 74 °C due to the interfacial region of LLDPE bonded ENR. Such a transition was absent in the control LLDPE/ scrap rubber blend that is incompatible. Several authors have conducted research on TPE composition based on PP where PP/ scrap rubber blend is dynamically vulcanised with or without addition of virgin rubber in the blend formulations. Jacob and co-workers applied DMA to investigate biphasic morphology of the PP/EPDM blends containing waste EPDM rubber [74]. The Tg of the rubbery phase remained unchanged with replacement of virgin EPDM by waste EPDM vulcanisate powder in the blend. The PP phase displayed a glass-rubbery relaxation in the temperature range of 15-20 °C. Liu and co-workers also made a similar observation from a DMA study of blends of recycled EPDM or tyre rubber and PP [63].
13.2.3 Recycled Rubber Modified Bitumen, Concrete and Composites Bitumen is generally used to seal cracks and joints in pavements. It is also used as waterproofing material in buildings. Such material is very brittle in a cold environment and soft at high temperatures. Polymer modification is performed to improve viscoelastic properties of bitumen besides maintaining its advantages. The high cost of virgin polymer provides an opportunity to utilise rubbery waste for bitumen modification. Singh and co-workers utilised rubbery scrap PVC particles for modification of bitumen 451
Thermal Analysis of Rubbers and Rubbery Materials and characterised the mix by DMA [75]. A higher storage modulus and less loss tangent values for waste modified bituminous material indicated its superior quality in contrast to its non-modified counterpart. Masson discussed application of thermal analysis for characterisation of bituminous sealant [76]. A loss tangent plot from DMA, modulated DSC scans, and changes in dimension of specimen against temperature in TMA plots were used for estimation of the Tg of the bituminous sealants. At a temperature less than the Tg, the sealant is stiff. At the Tg, the lower amplitude of tan indicates easy dissipation of cyclic stress. This way DMA results relate the viscoelastic properties to modulus and stress relaxation, both of which have great practical importance. Garcia-Morales and co-workers investigated thermal behaviour and microstructure of the recycled EVA modified bitumen [77]. Incorporation of 9 wt% recycled EVA lowered the Tg of neat bitumen from –22 °C to –33 °C. The loss tangent master curve obtained from time temperature superposition of dynamic mechanical frequency sweep data at different temperatures revealed that up to 7 wt% loading of recycled EVA could retain the linear viscoelasticity of the mix. More than 7 wt% loading of EVA led to formation of a semi-continuous polymer phase and complex dynamic mechanical behaviour. The Arrhenius activation energy for thermorheological relaxation of a grade of bitumen decreased with gradual incorporation of EVA. To improve crack resistance and damping behaviour of concrete, crumbled tyre rubber is generally used. Waste rubber modified concrete exhibits reduced plastic shrinkage cracking in comparison to control specimens. Hernandez-Olivares studied dynamic behaviour of recycled rubber filled concrete [78]. It was observed that although incorporation of waste rubber crumb reduced the mechanical strength and dynamic elastic modulus slightly, it increased loss tangent or the damping behaviour of the concrete. The study revealed that the concrete containing 5 wt% recycled rubber is an optimal candidate for absorbing and dissipating energy under dynamic action without damage. Polymeric mortars based on recycled rubber were developed for machine tool structures. Bignozzi and co-workers investigated damping and relaxation phenomena of such materials using DMA and DETA [79]. Incorporation of coarse tyre rubber particles in a mortar based on sand and unsaturated polyester (as binder) increased its damping behaviour. However, incorporation of micronised tyre rubber or milled cable polymer waste did not alter the damping behaviour of the composite. Use of a silane coupling agent significantly improved filler dispersion, phase adhesion, dielectric constant, composite stiffness and activation energy for relaxation of the binder chains. Short natural fibre reinforced tyre rubber, used as mat and industrial flooring, has been investigated to evaluate the effect of fibre length, content and chemical treatment on the dynamic mechanical properties of the composite [80]. Mercerisation of sisal fibre enhances the strength, dynamic loss and storage modulus of the composite, indicating improved fibre matrix adhesion. 452
Thermal Analysis in Recycling of Waste Rubbery Materials
13.3 Pyrolytic Utilisation of Waste Rubber Pyrolysis is one of the destructive utilisations of waste rubbers. Recovery of monomer, fuel gases and carbon or recovery of energy by incineration of pyrolytic products is achieved by this method. Thermal analysis provides a tool for fundamental investigation of stability and decomposition of polymers. Based on the results of decomposition temperature and relative mass loss upon heating the waste mass, operating parameters of a pyrolysis reactor is determined. The kinetics of the decomposition of rubbery waste can be studied considering TGA as a miniature pyrolysis reactor.
13.3.1 Degradation and Recovery of Monomer, Gas and Carbon Several authors investigated pyrolysis kinetics of rubber mixtures by TGA. Mass loss behaviour of rubber mixture could be adequately described by the weighting sum of mass loss of individual components [81-84]. Senneca and co-workers studied the pyrolysis kinetics of solid fuels such as waste plastics, rubbers and wood materials in both inert and oxidative media [85-87]. Heterogeneity of rubbery waste causes complex degradation. With increase in heating rate, the decomposition temperatures due to different rubber components shift to a higher temperature and merge. Shift in decomposition temperature of tyre rubber to higher values with increase in heating rates is shown in Figure 13.13. Char formation on the particles due to early decomposition of a relatively unstable component complicates the degradation kinetics [86, 88]. During oxidative decomposition multiple peaks appear. A synergistic effect between purely thermal degradation and heterogeneous oxidation needs to be accounted for when modelling the oxidative degradation kinetics [86]. Williams and Besler found that the activation energy for decomposition of rubber decrease with increasing heating rate in nitrogen atmosphere [14]. Considering one-step reactions, Chen and co-workers also utilised one reaction model for analysis of pyrolysis kinetics of scrap automotive tyres [89]. It was observed that the reaction temperature range increased when the heating rate was increased. Leung and Wang modeled the thermal degradation behaviour of tyre rubber to derive the kinetic parameters [90]. The main components of tyre rubber are NR, BR and SBR. Tyre samples show two distinct areas of weight loss representing lower and higher temperature decomposition. SBR decomposes mainly at higher temperatures, NR at lower temperatures and BR at both higher and lower temperatures. In the proposed model by Leung and Wang tyre rubber pyrolysis rate was considered to be the sum of two reaction rates. Both the reactions were assumed to follow the Arrhenius law. The first reaction occurs at lower temperature region than the second reaction and both the reactions occur at intermediate temperature regions. The model simulation curves along with experimental normalised weight loss rate against temperature curve are shown in Figure 13.14. The simulation agrees well with the experimental data. It was 453
Thermal Analysis of Rubbers and Rubbery Materials
Figure 13.13 Weight loss profiles of tyre rubber at different heating rates: 5° C/min, ( ); 20 °C/min, ( ); 100 °C/min, ( ) and 900 °C/min, ( ) [87] Redrawn with permission from Senneca and co-workers, Fuel, 1999, 78, 13, 1575. ©1999, Elsevier
observed that during pyrolysis apparent activation energy and frequency factor for low temperature decomposition reaction increased with increase in heating rate indicating a difficult decomposition reaction at higher heating rates. In contrast, the apparent activation energy and frequency factor of pyrolysis reaction at the high temperature region decreased with increase in heating rate. Size of tyre rubber particles did not significantly affect the rate of pyrolysis. Pyrolysis of tyre rubber produces primarily isoprene, dipentene or other dimer and styrene. Further decomposition of them produces smaller molecules such as H2, CH4, C2H6, C3H6, C3H8 and C4H6 along with CO and CO2. At high temperatures, the primary gases aromatise and form hydrocarbon oil containing a mixture of benzene, toluene, xylene, styrene, phenantherene, indene, vinyl alkenes, alkanes and alkenes [14, 82]. Pyrolysis of waste natural rubber products produced 72% oil, 3% solid char and 25% gaseous products [91]. The gross heating value of waste derived oil was comparable to petroleum products. Although the density of the oil is slightly higher than diesel, they have a similar kinematic viscosity. A pyrolytic TGA study of waste rubber materials was also conducted to estimate the operating conditions for processing of waste rubbery materials to obtain activated 454
Thermal Analysis in Recycling of Waste Rubbery Materials
Figure 13.14 Plots of theoretical (low temperature range, ; high temperature range, ; and entire temperature range, ) and experimental ( O ) normalised weight loss rate data against temperature for the pyrolysis of tyre rubber [90] Redrawn with permission from Leung and co-workers, Journal of Analytical and Applied Pyrolysis, 1998, 45, 2, 153. ©1998, Elsevier
carbon [92]. Qiao and co-workers also discussed utilisation of TGA for heat treatment conditioning of waste PVC to obtain chlorine free carbon materials [93]. Pyrolysis of EVA waste from the footwear industry revealed a two-step decomposition in both a nitrogen and an oxygen atmosphere [94]. Activation energy remained similar in both the environments, indicating domination of thermal degradation over combustion during pyrolysis in an oxygen atmosphere.
13.3.2 Energy Recovery Through Incineration Incineration of waste tyres is an alternative recycling method. Excellent heating value of tyre rubbers (28-37 MJ/kg) makes it comparable to coal. Energy recovery is also accompanied by a drastic volume reduction of waste that is ready for disposal. In contrast to coal char, which has significantly high heat value (~25 MJ/kg), char from waste rubber does not have any heat value. It is noteworthy that tyres need to be pulverised to small particles for complete combustion in a coal fired boiler. Though size reduction 455
Thermal Analysis of Rubbers and Rubbery Materials is a cost-intensive process, it is necessary for any reutilisation process of waste rubber tyres including landfilling. Different waste rubber materials decompose at different temperatures. To classify waste fuels based on their decomposition temperature and rate of decomposition, TGA is used extensively. TGA experiments are done at a relatively slower heating rate than the actual combustion process. Atal and Levendis pointed out that although the particle size of waste rubber does not significantly affect TGA degradation behaviour (at slower heating rate), combustion of such fuel, as measured by pyrometry, is slower if the particle size is larger [95]. The real combustion kinetics of waste rubber fuels in an incinerator was determined by gas chromatography of evolved gases [96]. The actual combustion in an incinerator followed multi-stage reactions, which were similar to those of the TGA degradation.
13.4 Concluding Remarks Thermal analysis is recognised as a sensitive technique for qualitative and quantitative characterisation of polymeric materials. This is an essential tool not only to characterise waste rubbery materials for compositional verification but also to evaluate thermal stability, compatibility issues, morphology, damping behaviour and other physical characteristics of waste reutilised products. To estimate or optimise different process parameters pertaining to recycling and recyclability of scrap rubbers various thermal analysis methods are indispensable.
Acknowledgement The Author wishes to thank the many individuals who contributed to this chapter by supplying information, and suggestions. He wishes to thank Professor (Mrs) P.P. De, Moni, Didi and Santanu for stimulation. Support from the Center for Advanced Engineering Fibers and Films, and Robert Muldrow Cooper Library, Clemson Univeristy is gratefully acknowledged. Support was also provided by the ERC Program of the National Science Foundation under Award Number EEC-9731680.
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Thermal Analysis of Rubbers and Rubbery Materials 82. J. A. Conesa, R. Font and A. Marcilla, Journal of Analytical and Applied Pyrolysis, 1997, 43, 1, 83. 83. J.A. Conesa and A. Marcilla, Journal of Analytical and Applied Pyrolysis, 1996, 37, 1, 95. 84. J.M. Heikkinen, J.C. Hordijk, W. de Jong and H. Spliethoff, Journal of Analytical and Applied Pyrolysis, 2004, 71, 2, 883. 85. O. Senneca, R. Chirone, S. Masi and P. Salatino, Energy & Fuels, 2002, 16, 3, 653. 86. O. Senneca, R. Chirone and P. Salatino, Energy & Fuels, 2002, 16, 3, 661. 87. O. Senneca, P. Salatino and R. Chirone, Fuel, 1999, 78, 13, 1575. 88. M.J. Kawser and N.A. Farid, Plastics, Rubber and Composites, 2000, 29, 2, 100. 89. J.H. Chen, K.S. Chen and L.Y. Tong, Journal of Hazardous Materials, 2001, 84, 1, 43. 90. D.Y.C. Leung and C.L. Wang, Journal of Analytical and Applied Pyrolysis, 1998, 45, 2, 153. 91. M.J. Kawser and N.A. Farid, Plastics, Rubber and Composites, 2000, 29, 8, 427. 92. J. Caponero and J.A. Soares Tenorio in Proceedings of the 55th Congresso Anual - Associacao Brasileira de Metalurgia e Materiais, Rio de Janeiro, Brazil, 2000, p.2771. 93. W.M. Qiao, S.H. Yoon, Y. Korai, I. Mochida, S. Inoue, T. Sakurai and T. Shimohara, Carbon, 2004, 42, 7, 1327. 94. A.N. Garcia and R. Font, Fuel, 2004, 83, 9, 1165. 95. A. Atal and Y.A. Levendis, Fuel, 1995, 74, 11, 1570. 96. J.C. Lou, G.F. Lee and K.S. Chen, Journal of Hazardous Materials, 1998, 58, 1-3, 165.
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Thermal Analysis of Biological Molecules and Biomedical Polymers
14
Thermal Analysis of Biological Molecules and Biomedical Polymers N.D. Tran, N.K. Dutta, and N. Roy Choudhury
14.1 Introduction Thermal analyses (TA) of macromolecules of biological origin are very complex, the transition phenomena involved are poorly defined, broad and of low energy; for example, the enthalpy of denaturation of the proteins and enzymes, and order/disorder transition of lipid bilayers are only of the order of 4 to 40 J/g [1-3]. Reversible conformational changes in biopolymers are of even lower enthalpy. Thus, the thermal equipments required for thorough investigation of biological materials need to be highly sensitive with very minimum signal-to-noise ratio, and a very steady baseline. The recent development in new materials of construction, novel heating/cooling system and sophisticated electronic components has made it possible to design and manufacture highly sensitive thermal analyses equipment able to measure heat pulses as small as fraction of micro-calories; and extended the use of the technique to bio-molecules, bio-macromolecules and their conjugates [4]. Differential scanning calorimetry (DSC) and other closely related techniques are the most commonly used in samples of biological interest. As DSC follows heat capacity as a function of time and temperature, most of the thermal events of interest including the events do not involve an enthalpy change, such as Tg can also be detected. Thermogravimetric analysis (TGA) is also used for the analysis of evaporation and stability, however, very few thermal transitions of importance in biological systems are associated with a mass loss (temperature change within the natural environment are usually too small to initiate changes in mass loss). The biological samples are very scarce due to the difficulty of their isolation and purification, thus techniques that need significant amount of sample, such as dynamic mechanical analysis (DMA) are not often attempted. Very often the biological systems operate in an aqueous environment and the presence of water affects the properties and the amount of sample that can be investigated when studying the bulk properties. Consequently, difficulties are experienced in measuring some of the thermodynamic parameters, however, the heat of transition (even minor change in the position and shape of the peak), or change in enthalpy is very useful and reflects the thermal event. The biomolecular interactions are best investigated by thermodynamic methods and isothermal titration calorimetry (ITC) is emerging as the key tool in the analyses of proteins, lipids and nucleic acid molecules, ligand binding, 463
Thermal Analysis of Rubbers and Rubbery Materials and fundamental understanding of DNA-drug, phospholipid-ligand and protein-protein and membrane-protein interactions, which are critical for new drug design and design of efficient biomedical and pharmaceutical treatments. ITC is very sensitive technique and can measure energetics of protein folding, protein-protein interactions in solution at concentrations in nanomole range involving heat effect of as small as ~100 nanojules. Considering the limitations and importance calorimeters, particularly DSC and ITC of the required sensitivity and capacity has now been developed and are available commercially. For example the nano ITC2G has been designed and manufactured by TA instruments to perform highly-sensitive analyses on nanomolar quantities of biomolecules. VP-ITC, iTC200 and Auto-iTC200 are also commercially available ultrasensitive isothermal titration calorimeter from Setaram Instrumentations. It is claimed that with the Nano ITC2G, heat effects as small as ~120 nanojoules are detectable using one nanomole or less of biopolymer [5]. It uses a solid-state thermoelectric heating and cooling system to precisely control the temperature (temperature stability 0.0002 °C). The highsensitivity cells of the Nano ITC2G are made of 99.999% gold or hastelloy to allow for the widest range of reagent chemistry. These unique design parameters results in calorimetric measurement of unparallel sensitivity, accuracy and precision. Classical DSC instruments are designed for measurement on a wide range of samples but often lack the ultrahigh sensitivity essential for biological molecules. Recently, DSC such as Nano DSC from TA Instruments has been specifically designed for bio-molecules. The Nano DSC uses a dual-capillary design, and operates in power compensation mode and has baseline repeatability of 0.028 Watts, scan rate as low as 0.001 °C/min and cell volume of ~0.30 ml. Setaram Instruments’ μDSC3 evo and μDSC7 evo are the most popular high-sensitivity DSC from Setaram. μDSC7 is mainly designed for the study of substances in a confined environment (denaturation, transition, gelification, reaction, etc.) with rising and falling temperatures (no outside cooling system is needed) over a wide temperature range (-45 to 120 °C) [6]. This chapter offers a general treatise on the application of thermal analysis to biomacromolecules, bio-mimetic elastomers with some focus on the synthetic polymers for biomedical applications. We focus our attention particularly to the application of DSC and ITC to materials of interest in the field of biochemistry and molecular biology. However, other thermal techniques will also been discussed when appropriate.
4.2 Structure and Phase Behaviour of Cells, Membranes and Lipid Bilayers Using TA The cell membrane is the biological membrane that separates the interior of a cell from the outside environment. It contains a wide variety of biological molecules, primarily proteins and lipids and is involved in a vast array of cellular processes including cell adhesion, ion channel conductance, cell signalling and selectively permeable to selected 464
Thermal Analysis of Biological Molecules and Biomedical Polymers chemicals that pass in and out of cells. The cell membrane consists primarily of a thin layer of amphipathic phospholipids, which spontaneously arrange to form a continuous, spherical lipid bilayer. The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and steroids. The amount of each lipid depends upon the type of cell, but in the majority of cases phospholipids are the most abundant. Of the ~30% chemicals in a typical cell (rest ~70% water), 24% consists of bio-macromolecules (proteins, 15%, polysaccharides, 2%, RNA, 6%, DNA 1%]), 2% phospholipids and rest 4% small molecules, ions, etc. [7]. Though phospholipids are not macromolecules in strictest sense, however, they self-organize using secondary forces of interactions to make supramolecular structure. Calorimetric studies are highly suitable to gain insight into their structure-function relationship, self-organisation, molecular dynamics, phase changes/transitions, denaturation from the native state, freeze induced dehydration, depression of freezing point, state of water in the system, binding of ligands to the receptors molecules reactions/interactions and many more. Figure 14.1 shows the DSC thermograms for a representative whole cell of the bacterium Acholeplasma laidlawii and its different individual components [8]. The DSC traces of the whole cell (curve (a)) and that for the membrane (curve (b)) (consists of ~50% lipid and ~50% protein) appears to be similar and reflecting two endothermic peaks during
Figure 14.1 DSC thermogram of (a) whole cells, (b) membranes before heat denaturation, (c) membranes after protein heat denatured, (d) extracted membrane lipid Reproduced with permission from D.L. Melchior, F.J. Scavitto, M.T. Walsh and J.M. Steim, Thermochimica Acta, 1977, 18, 1, 43. ©1977, Elsevier
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Thermal Analysis of Rubbers and Rubbery Materials temperature ramp. The observed lower temperature endotherm is reversible and is similar in living organisms, and aqueous dispersions of extracted membrane lipids. It also remains unaffected by thermal protein penetration, enzymic digestion of the membranes, or even by the absence of protein [9-11]. The second endotherm is absent in the denatured protein (curve (c)) [12]. The first endotherm is related to the order/ disorder transition in the lipid bilayer and the second one is associated with the change in organisation of protein on heating. The order/disorder transition of the lipids can be varied as much as 70 °C from -20 to 50 °C, depending on the presence of different fatty acids into the membrane lipids [8]. Most membrane lipids are phospholipids and the structure of the molecules can be simply described as having a polar region, the head; and two hydrophobic hydrocarbon side chains. Table 4.1 shows the most common types
Table 14.1 Generic structure of different series of lipids Structure
Name 1-Palmitoyl-2-Myristoyl-snGlycero-3-Phosphocholine
Group Phosphatidylcholine (PC)
1-Palmitoyl-2-Oleoyl-snPhosphatidylethanol Glycero-3-Phosphoethanolamine amine (PE) (POPE)
466
1-Palmitoyl-2-Oleoyl-snGlycero-3-[Phsopho-L-Serine] (Sodium Salt) (POPS) 1,2-Dioleoyl-sn-Glycero-3Phosphoinositol (Ammonium Salt)
Phosphatidylserine (PS)
1,2-Dipalmitoyl-sn-Glycero-3Phosphate (Monosodium Salt) (DPPA)
Phosphatidic Acid (PA)
1,2-Dialmitoyl-sn-glycero3-[Phospho-rac-(1-glycerol)] (Sodium Salt) (DPPG) 1,1’,2,2’-Tetramyristoyl Cardiolipin (Ammonium Salt)
Phosphatidylglycero 1 (PG)
1,2-Dipalmitoly-sn-Glycerol c
Diacylglycerides (DG)
Posphatidylinositol (PI)
Cardiolipin (CA)
Thermal Analysis of Biological Molecules and Biomedical Polymers and generic structures of different series of lipids. In presence of water the phospholipids act like a surfactant and self-organised themselves; and the conformational organisation depends on the chemical structure of the lipid, its concentration, the environment, temperature, and history. In excess water, lipids generally form a bilayer arrangement with their polar head towards the aqueous environment and the hydrophobic tail forming a hydrophobic region in the centre of the bilayer. Due to the unique supramolecular interaction these bilayers arrange themselves in concentric sheet arrangements separated by water [13]. On heating the hydrophobic core of the phospholipid membranes exhibits a sharp phase transition, similar to the liquid-crystalline phase transition, undergoing a thermally-induced reversible transition to a disordered fluid state at a specific temperature, T c. The value of T c depends on the environment and structure of the phospholipid, including the length and rigidity of the hydrophobic side chains. This transition is accompanied by a loss of order in the two dimensional lattice, with a sudden increase in the chain mobility, a lateral expansion in the plane of the membrane, a decrease in the bilayer thickness and an increase in the net volume per lipid molecules. This order disorder transition (ODT) is reflected as an endothermic peak on a DSC thermogram (Figure 14.1). The morphological change at ODT in two typical phospholipid bilayers is shown schematically in Figure 14.2. Dipalmityl phosphatidylethanolamines (DMPA) undergo a phase change from the lamellar L to the L phase, which may be considered as a melting of the bilayers with conservation of the lamellar conformation [3,14]. On the molecular level this transformation consists of a change in the lipid hydrocarbon chains from a largely all trans conformation in a hexagonally packed lattice structure (lateral movement is greatly inhibited) to a more disordered state, where molecules are free to diffuse laterally. This is accompanied by lateral expansion and a decrease in thickness of the bilayer [1] and the DSC thermogram of DMPA (Figure 14.3d, e and f) is relatively simple. However, dipalmityl Figure 14.2 Schematic drawing of phosphatidylcholines (DMPC), with the phase transitions of dipalmityl identical saturated hydrocarbon side phosphatidylethanolamine (DMPA) and chains, exhibit a more complicated dipalmitoyl phosphatidyleholine (bottom) behaviour (Figure 14.2) due to the presence of two different crystalline Reproduced with permission from A. Blume, phases. The pre-transition in which Thermochimica Acta, 1991, 193, 299, the bilayer is distorted by a periodic p.301. ©1991, Elsevier 467
Thermal Analysis of Rubbers and Rubbery Materials
Figure 14.3 DSC thermograms of Dipalmityl Phosphatidylcholines (PC) (a,b,c) and Phosphatidylethanolamines (PE) (d,e,f) Reproduced with permission from A. Blume, Thermochimica Acta, 1991, 193, 299, p.307. ©1991, Elsevier
ripple, which is characterised by a small endotherm before the pronounced endotherm indicating ODT (Figure 14.3a, b and c) [14].
14.2.1 Lipid Blends and Alloys In the presence of blends of lipids of different chemical structure and architecture, the ODT behaviour of lipid in membranes depends on the nature of the lipids present and the ratio in which they are combined in the membrane. An increase in the number of double bonds in the hydrocarbon tails in the lipid decreases the ODT temperature, which is related to the disruption in close packing due to loss of flexibility in the chain. As shown in the DSC thermogram in Figure 14.4 when lipids with different ODT are combined in a binary system it produces broad ODT transitions of individual component, indicating the lateral diffusion of the lipid molecules [14]. The presence of minor constituents of the living cell, such as cholesterol nestled in between the fatty tails can modify the mechanical properties of the lipid bilayers and hence bio-membrane flexibility. The presence of electrolytes such as sodium chloride has a screening effect on 468
Thermal Analysis of Biological Molecules and Biomedical Polymers
Figure 14.4 DSC thermogram of dipalmityl phosphatidylcholines (PC) and phosphatidylethanolamines blends of different ratios Adapted with permission from A. Blume, Thermochimica Acta, 1991, 193, 299, p.307. ©1991, Elsevier
the electrostatic interactions and may affect the thermal undulations of electrostatically charged bilayers [15]. Demel and co-workers [16], Yeagle and co-workers [17], and Ladbrook and co-workers [18] investigated the lipid-cholesterol mixtures in details. It was clearly demonstrated that the addition of cholesterol to DMPC causes disappearance of the pre-transition at about 5 mol% cholesterol [18] and broadening of the main transition, however, the Tg remains unaffected. The rigid ring structure of cholesterol acts as a spacer between the hydrocarbon side chains, and interferes with the liquid crystalline organisation [19-21] of the lipids. Gardikis and co-workers [22] investigated the interactions of dimethoxycurcumin, a lipophilic bioactive curcumin derivative with DMPC using DSC and Raman spectroscopy to extract information on the membrane integrity and physico-chemical properties that are essential for the rational design lipid based drug delivery systems. They also revealed that dimethoxycurcumin influences the thermotropic properties of DMPC lipid membrane causing abolition of the pretransition and broadening of the phase-transition profile and slightly decreases the Tm at increased concentrations. The results of these studies provide information on the membrane integrity and physicochemical properties that are essential for the rational 469
Thermal Analysis of Rubbers and Rubbery Materials design lipidic drug delivery systems. The relationship between molecular architecture of the nestled molecule and the nature of interactions with lipid bilayers has been investigated systematically using a series of polyethylene oxide-b-polypropylene oxide-b-polyethylene oxide (PEO-PPO-PEO) triblock copolymers using small-angle X-ray scattering (SAXS) and thermal analysis (DSC) by Firestone and co-workers [23]. The number of molecular repeat units in the hydrophobic PPO block has been reported to be a critical determinant of the nature of triblock copolymer-lipid bilayer association. From this investigation it has been demonstrated that in DMPC based biomembrane structures, when the nestled PEO-PPO-PEO copolymer possesses a PPO chain length commensurate with the acyl chain dimensions of the lipid bilayer, it yields highly ordered swollen lamellar structures consistent with well-integrated (into the lipid bilayer) PPO blocks. Increase in the concentration of wellintegrated triblock copolymers from 4 to 12 mol% enhanced the structural ordering of the lamellar phase, while concentrations exceeding 16 mol% resulted in the formation of a hexagonal phase. The examination of the temperature-induced changes in the structure of these mesophases revealed that if the temperature is reduced sufficiently, all compositions exclude polymer and exhibited pattern for hydrated DMPC bilayers. Triblock copolymers of lesser PPO chain length insufficient to permit full incorporation into the lipid bilayer yielded no integration and no temperature-induced structural changes [23]. Schullery and co-workers [24] investigated the mixtures of DMPC with palmitic, stearic, and myristic acids and the sodium salts of these acids using, differential thermal analysis (DTA) over a wide range of lipid compositions, all in excess water. It was confirmed that all three fatty acids raise the liquid-crystal phase transition and form sharp-melting complexes. The effects of anaesthetics cis- and trans-9,10-tetradecenols on the phase transitions of dimyristoyl-, dipalmitoyl-, and distearoyl-phosphatidylcholines were investigated using high sensitivity DSC and Raman spectroscopy by O’Leary and co-workers [25]. The results demonstrate that lipid-anaesthetic interactions obtained from model membranes may be misleading to the problem of clinical anaesthesia, since qualitatively different results may be obtained when lipids of differing acyl chain lengths are employed. Sada and co-workers [26] prepared two types of artificial membranes with a phospholipid containing hydrophobic and hydrophilic solutes and measured their permeabilities around the phase-transition temperature of the phospholipid. The permeability of the membranes to a hydrophobic solute was reported to be higher than to a hydrophilic solute, and exhibited an abrupt change at the phase-transition temperature of the phospholipid, similar to that in biomembranes and liposomes, confirming the fluidity change of the phospholipids at this temperature.
14.2.2 Liposomes Vesicles (= liposomes) consisting of essentially closed spherical lipid bilayers enclosing an aqueous compartment in analogy to cell membranes are formed when amphiphilic 470
Thermal Analysis of Biological Molecules and Biomedical Polymers phospholipid molecules are mixed with water. Depending on the preparation methods, these self-assembled macromolecular aggregates can exist as uni- or multilamellar lipid bilayer systems [27, 28]. The hydrated lipid sheets detach during agitation and self-close to form large, multilamellar vesicles (MLV) which prevent interaction of water with the hydrocarbon core of the bilayer at the edges. Once these particles have formed, reducing the size of the particle requires energy input in the form of sonic energy (sonication) or mechanical energy (extrusion). Unilamellar vesicles (Figure 14.5) consist of only one bilayer and represents the model most similar to natural membranes of cells. Liposome technology particularly liposome-based drugs have become a highly successful and rapidly developing area of preclinical and clinical research. Figure 14.5 depicts different organisations of lipids and its vesicle form. Several new aspects of liposome research have been discovered including methods for drug encapsulation into liposomes, modification of the liposomal surface to control drug behaviour in biological environments, longcirculating liposomes and the use of cationic liposomes as transfection vectors, pH-sensitive liposomes, liposomes for diagnostic imaging, liposomal DNA vaccines, and so on [29-35]. A large number of synthetic and natural lipids of different series including phosphocholine (PC) (e.g. 1,2-didecanoyl-sn-glycero-3-phosphocholine-DDPC; 1,2-dilauroyl-snglycero-3-phosphocholine-DLPC, 1,2-dimyristoyl-sn-glycero-3-phosphocholineDMPC; 1,2-dipalmitoyl-sn-glycero-3-phosphocholine-DPPC, 1,2-distearoyl-snglycero-3-phosphocholine-DSPC; 1,2-dioleoyl-sn-glycero-3-phosphocholine-DOPC,palmitoyl,2-oleoyl-sn-glycero-3-phosphocholine-POPC; 1,2-erucoyl-sn-glycero-3phosphocholine-DEPC; 1-palmitoyl,2-stearoyl-sn-glycero-3–phosphocholine-PSPC; 1-stearoyl,2-myristoyl-sn-glycero-3–phosphocholine-SMPC;1-stearoyl,2-oleoyl-snglycero-3-phosphocholine-SOPC; 1-stearoyl,2-palmitoyl-sn-glycero-3-phosphocholineSPPC); phosphoglycerol (PG) (dimyristoyl phosphatidylglycerol-DMPG; dipalmitoyl phosphatidylglycerol-DPPG; palmitoyl-oleoyl phosphatidylglycerol-POPG; distearoyl
Figure 14.5 Illustration of a typical phospholipid, phospholipid bilayer, unilayer vesicle and a bilayer vesicle
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Thermal Analysis of Rubbers and Rubbery Materials phosphatidylglycerol; DMPG-DSPG); phosphatidic acid (PA) (1,2-dimyristoyl-sn-glycero3-phosphatidic acid-DMPA, 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid-DPPA, 1,2-distearoyl-sn-glycero-3-phosphatidic acid-DSPA); phosphoethanolamin(PE) (e.g. 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-DMPE; 1,2-dipalmitoyl-sn-glycero3-phosphoethanolamine-DPPE; 1,2-diostearpyl-sn-glycero-3-phosphoethanolamineDSPE; 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-DOPE; 1-palmitoyl-2-oleoylsn-glycero-3-phosphoethanolamine-POPE), phoshoserine(PS) (e.g. 1,2-distearoyl-snglycero-3-phosphoserine-DSPS; 1,2-dioleoyl-sn-glycero-3-phosphoserine-DOPS) and their head group modified and fatty acid modified derivatives are available commercially. Calorimetric studies are suitable to gain insights in structural properties of lipidmembrane and vesicle structure, their phase behavior and dynamics, order and disorder, freeze induced dehydration, effects of repeated freezing and thawing and binding of ligands to receptor molecules [36-39].
4.2.3 Phospholipid-Additive Interactions The understanding of phospholipid-ligand and lipid-additive interactions is of fundamental importance in the biochemical process. Together with the structure and mechanism, the specificity and energetics of such interactions in the natural or clinical environment forms a central fundamental knowledge that is required to completely understand biochemical systems. Calorimetry is the only technique that allows the direct measurement of enthalpy and ITC provides the opportunity to obtain thermodynamic information in specific environment. A typical titration curve of a phospholipid (DMPE) with sodium hydroxide at three different temperatures obtained using ITC is shown in Figure 14.6. The characteristic calorimetric heat signals provides information on the binding of monovalent cation Na+ to the negatively charged lipid bilayer [40, 41]. The heat of reaction may depend on the experimental temperature, particularly, whether the reaction is carried out above or below the ODT of the lipid vesicle. Normally, at pH 7 the head group of DMPE has one negative charge and titration with sodium hydroxide induces the dissociation of the second proton and the head group becomes doubly charged. In the temperature range of 24 to 51 °C, in DMPE the dissociation reaction also associated with the change in the bilayer state. Therefore, the total heat of reaction is a combination of heat of dissociation of the second proton, the heat of dissociation of the lipid bilayers and the heat of neutralisation. The slope of the curve at three different temperatures (Figure 14.6b) is also observed to be different, indicating the changes in the exposure of the hydrophobic surfaces. This occurrence of this exposure is superimposed on the release of water of hydration from the charged head group of DMPE caused by an increase in counter ion condensation [40, 41]. The binding of divalent cations such as Ca2+ and Mg2+ on negatively charged lipid bilayers of DMPE and DMPG has also been investigated and reported to be more complicated due to low permeability of the divalent cations through the lipid bilayer (the equilibrium 472
Thermal Analysis of Biological Molecules and Biomedical Polymers
(a)
(b)
Figure 14.6 Calorimetric heat signals (a) and integrated heat of reaction (b) for the titration of 1mM DMPA with 0.1 M NaOH (5 l steps) Reproduced with permission from A. Blume and J. Tuchtenhagen, Biochemistry, 1992, 31, 19, 4636. ©1992, ACS
state is not instantaneously reached). In addition the biding of divalent cations usually induces gross change in phase and morphological state. Investigation using ITC has been proven to be particularly well suited for the incorporation of surfactants into the lipid bilayers and the solubilisation process of the lipid vesicles by the surfactants; and significant research has been dedicated in this area for scientific interest and technological importance [42-44]. Investigations on lipid/surfactant mixtures can provide important information on the molecular interactions within membranes. Addition of lipid vesicles to the surfactant solution below the critical micelle concentration (cms) (partition experiment) provides the partition coefficient of the surfactant between water and bilayers and the transfer enthalpy of the surfactant from water to the bilayer. Figure 14.7 shows a typical experimental data of Q versus surfactant concentration for the titration of octaglucoside (OG) into water and DMPC vesicle solutions. The coexistence of the vesicles and micelles are determined from the extremes of the first derivative curve. In the plot, Dtsat represents the surfactant concentrations, where the lipid bilayer becomes saturated with surfactant and the first mixed micelles appear; and Dtsol is the surfactant concentration where all vesicles have been completely transformed into mixed micelles. The location of the phase boundaries between mixed micelles and mixed vesicles depends on the nature of the head group of the lipid. Figure 14.8 shows the location of the phase boundaries 473
Thermal Analysis of Rubbers and Rubbery Materials
Figure 14.7 Titration of Octylglucoside (OG) micellar solutions into water and into dimyristoyl phosphatidylcholine (DMPC) vesicles Reproduced with permission from J.E. Ladbury and B.Z. Choudhury, Biocalorimetry Application of Calorimetry in the Biological Sciences, Wiley, New York, NY, USA, 1998, p.85. ©1998, Wiley
Figure 14.8 Phase diagram of solubilisation of phospholipids with different head groups and palmitoylchain; DPPG = 1,2 dipalmitoylphosphatidylglycerol; DPPA=1,2dipalmitoylphosphatidic acid, DPPC= dipalmitoyl phosphatidylcholine; DPPE=1,2-dipalmitoylphosphoethanolamine Reproduced with permission from J.E. Ladbury and B.Z. Choudhury, Biocalorimetry Application of Calorimetry in the Biological Sciences, Wiley, New York, NY, USA, 1998, p 87. ©1998, Wiley
474
Thermal Analysis of Biological Molecules and Biomedical Polymers of a variety of phospolipids with different head groups and palmitoyl chains when solubilised by OG.
14.3 Molecular Dynamics, Conformational Change and Swelling Behaviour of Biopolymers Using TA Thermal properties of biopolymers are considered to be one of the most important properties as in the synthetic polymers. However, biopolymers are more complicated
Table 14.2 Standard amino acid abbreviations Amino Acid
Structure
3-Letter Code
Alanine
ala
1-Letter Code A
Arginine
arg
R
Asparagine
asn
N
Aspartic acid
asp
D
Cysteine
cys
C
Glutamic acid
glu
E
Glutamine
gln
Q
Glycine
gly
G
Histidine
his
H
Isoleucine
ile
I
Leucine
leu
L
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Table 14.2 Continued ... Amino Acid
Structure
3-Letter Code
Lysine
lys
1-Letter Code K
Methionine
met
M
Phenylanine
phe
F
Proline
pro
P
Serine
ser
S
Theonine
thr
T
Tryptophan
trp
W
Tryosine
tyr
Y
Valine
val
V
than synthetic ones; not only due the diversity of the biological origin and complexity in their chemical structure, but also due to the presence of precise three dimensional organisations. For example the natural proteins are biopolymers based on unique linear arrangement of just 20 different amino acid monomers (Table 14.2). However, the structural diversity and self-organisation behaviour of protein molecules in different environment attribute a tremendous breadth of physical and chemical activities to folded proteins; ranging from exquisitely specific room temperature catalysis (enzymes) to the formation of unusually strong and tough biomaterials, such as collagen and spider silk [45, 46]. The self-organisation of biopolymers is dramatically influenced by the environment, particularly the presence of water. Biopolymers including proteins organise themselves in a unique native structure in water environment to perform and maintain biological activity. During dehydration- more precisely at the loss of bound water (water that does not freeze at cooling) biopolymer’s steric structure is lost. Normally the process of destruction/restoration of the native structure at dehydration/hydration 476
Thermal Analysis of Biological Molecules and Biomedical Polymers
Figure 14.9 Temperature dependence of the absolute heat capacity for: (a) DNA (CH2O = 16%), (b) elastin (CH2O = 17%),(c) lysozyme(CH2O = 11%), (d) rice starch (CH2O = 15%),potato starch ((CH2O = 15%). Solid lines correspond to the first heating of samples, dash lines are the second heating Reproduced with permission from N.A. Grunina, T.V. Belopolskaya and G.I. Tsereteli, Journal of Physics, Conference Series, 2006, 40, 105. ©2006, Institute of Physics
is completely reversible [47]. Figure 14.9 demonstrates the temperature dependences of the absolute heat capacity for different biopolymer-water systems containing only bound water. DSC thermograms of the proteins in both their native and denatured state are shown in Figure 14.9 [48]. The thermograms (solid lines) clearly demonstrate the T g and different stages of destruction of the native structures and the loss of bound water. On first heating, globular protein lysozyme, potato and rice starches exhibit only one peak of heat of absorption, which represents the cooperative destruction of the native structure at given conditions. This is not observed in the second heating due to the irreversible nature of this transition. The figure demonstrates that the native structure of different biopolymer is destroyed completely at different level of hydration. Note that the corresponding maximum absent in DNA in the first heating indicates that DNA is denatured in its original hydrated state of 18% water. It is known that at hydration level of less than 25% in DNA, the structure is completely destroyed. Also in case of elastin, an elastic protein, no endothermic peak is observed in the first heating and the thermogram of the first 477
Thermal Analysis of Rubbers and Rubbery Materials heating and the second heating coincides. Elastin is an amorphous protein and does not have a regular melting temperature. The granules of starch from rice and potato contain crystallinity of different bound water and the thermal destruction of rice starch ordered structure takes place at much higher temperature compared to that of the potato starch. The characteristic thermograms for all the biopolymer samples in the second heating exhibit only an amorphous polymer like Tg due to the destruction of the unique native structure in the first heating. Figure 14.10 illustrates the effect of water content on the Tg of the humid denatured biopolymers and amorphous elastin. It is clearly observed that the position of the Tg depends on the concentration of solvent and the Tg shifts to a lower temperature with hydration confirming its role as a plasticizer [48].
Figure 14.10 Dependence of the glass transition temperature on water content for: 1-DNA (x), 2-potato starch (), 3-rice starch (), 4-elastin (), 5-lysozyme (o) Reproduced with permission from N.A. Grunina, T.V. Belopolskaya and G.I. Tsereteli, Journal of Physics, Conference Series, 2006, 40, 105. ©2006, Institute of Physics
14.3.1 Level of Hydration on Thermal Characteristics of Proteins Protein is one of the most important biological macromolecules and significant progress has been made to illustrate the structure(X-crystal structures of over 6000 proteins are listed in the protein data bank [49]), however, relatively little information is available on the energetic and consistent interpretation of the existing data. Some heat capacity information on hydrated protein has been determined [50], however, very few papers on thermodynamic information on dehydrated proteins are available in the literature [51-53]. 478
Thermal Analysis of Biological Molecules and Biomedical Polymers
Figure 14.11 MDSC thermogram of freeze-dried elastin over a wide range of temperature Reproduced with permission from V. Samouillan, C. Andé, J. Dandurand, and C. Lacabanne, Biomacromolecules, 2004, 5, 3, 958. ©2004, ACS
Figure 14.11 shows a thermogram of freeze-dried elastin in the modulated differential scanning calorimetry (MDSC) mode for dehydrated elastin over a wide range of temperature [54]. The conventional DSC measures the total heat flow (HF) in a sample, which sums up both the reversing and the non-reversing heat parts. On the other hand, MDSC is a new extension to the conventional DSC, which provides the possibility of deconvoluting the HF into a component that tracks the temperature modulation (reversing heat flow (RHF) rate), leaving the part, which does not track the temperature modulation (non-reversing heat flow (NHF)). The RHF component is ascribed to thermodynamic specific heat changes upon heating, whereas the NHF component is ascribed to kinetic processes and the enthalpic changes that accompany structural reorganisation. This additional information aids interpretation and allows unique insights into the structure and behaviour of materials [55, 56]. In the case of MDSC, experimentally, a sinusoidal component is added to the linear heating profile, and the order of temperature is as follows: 2 T To qt A q sin t z
(4.1)
where Aq is the amplitude of modulation, z is the period of modulation, and t is the time. 479
Thermal Analysis of Rubbers and Rubbery Materials The heat flow signal (Figure 14.11) corresponds to the normal DSC experiment of dehydrated elastin sample and exhibits two principal thermal events: an endothermic peak centred around 80 oC, due to the loss of non-bound water with temperature, and a baseline shift related to increase in specific heat capacity related to glass transition, Tg at 200 °C of dehydrated elastin. The pseudo-second-order transition such as Tg are known to be reflected in the RHF component and clearly observed in the figure. On the RHF signal the jump in the specific heat corresponds to the Tg. Whereas in the NHF signal the extrinsic transition due to water loss at 80 °C and a weak endothermic reflection at ~200 °C related to structural relaxation of elastin (the disruptions of hydrogen bonds between N-H and C=O groups of the polypeptide chain, formed during the physical ageing [57] of elastin at temperature below Tg) is observed. Wunderlich and co-workers [58-60] have demonstrated a new theoretical evaluation technique to calculate specific heat of proteins using ‘The Advanced Thermal Analysis System’ (ATHAS) computations based on the approximate vibrational spectrum and the empirical additional scheme. A typical data point and experimental observation and calculated lines are shown in Figure 14.12 and good agreement between them is observed. In the figure ‘Cp experimental low’ refers to adiabatically measured data starting at low temperature; ‘Cp experimental high’ refers to DSC data, ‘Cp addition’ is a plot of the heat capacities gained addition scheme, ‘Cp calculation’ reports the calculated heat capacity at constant pressure, ‘Cv calculation’ is heat capacity at constant volume; NAA is the number of amino acids in the molecule, Mw is the molar mass in Daltons.
Figure 14.12 Heat capacities of bovine lactoglobulin in the solid state. The origins of the plotted data are indicated in the figure [49] Reproduced with permission from B. Wunderlich, Pure and Applied Chemistry, 1995, 67, 6, 1019. ©1995, IUPAC
480
Thermal Analysis of Biological Molecules and Biomedical Polymers The authors claim that based on the approximate vibrational spectra and using the mathematical scheme proposed it is now possible to calculate the heat capacities and the integral functions enthalpy, entropy, and Gibbs function for all dehydrated proteins of known composition using the ATHAS to a good estimation. The ATHAS computation scheme to derive theoretically from the chemical structure of protein has been discussed in detail elsewhere [58-60].
14.3.2 State of Water and Molecular Dynamics in Biomaterials by DSC Water plays a crucial role in the formation; stabilisation and association of protein structure and function, and the importance of the role of water in biochemical processes have long been recognised. The solubility of protein is also of critical importance in many areas – including biochemical and pharmaceutical industry – and it varies widely, ranging from completely soluble to almost insoluble. Protein structural transformation from native to the -rich and self-assembly state is critically influenced by solvent
Figure 14.13a DSC thermograms of differently hydrated elastins. The hydration level h is given in each thermogram Reproduced with permission from V. Samouillan, C. Andé, J. Dandurand, and C. Lacabanne, Biomacromolecules, 2004, 5, 3, 958. ©2004, ACS
Figure 14.13b DSC thermograms of highly hydrated elastins. The hydration level h is given for each thermogram Reproduced with permission from V. Samouillan, C. Andé, J. Dandurand, and C. Lacabanne, Biomacromolecules, 2004, 5, 3, 958. ©2004, ACS
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Thermal Analysis of Rubbers and Rubbery Materials conditions [61-63] and the importance of the hydration state for protein stability has been investigated in detail [64-71]. Figure 14.13 shows the effect of hydration level on the molecular dynamics and Tg of elastic protein elastin. This was purified from bovine ligament neck and freeze dried. The hydration level h (%) is defined as: h(%)
m water 100 m elastin
where, mwater is the mass of water uptake and melastic is the mass of the elastin. Figure 14.13a illustrates the thermograms of elastin hydrated from 5.5 to 60% and exhibits that Tg is critically dependent on the level of hydration. A hydration of 5.5% causes the Tg to drop from 200 to 100 °C, and a hydration of 30% brings the Tg of elastin down to ambient temperature. This result illustrates the dramatic role of water on molecular dynamics, which undoubtedly facilitates the motion of the polypeptide chains. The hydrated relaxed elastic protein undergoes mainly chaotic, Brownian-like motions and behaves as a fractal system of high entropy [72,73]. Figure 14.13b shows the DSC thermograms of highly hydrated elastin. In this case, an endothermic peak related to the melting of the bulk water observed to be superimposed on the Tg. This observation indicates the presence of bulk water for a hydration level higher than 30%. At weight fraction of water superior to 30%, Tg is independent of water content and remains equal to ~0 °C, the melting point of the crystallisable water [54]. Almost all biological activity occurs in largely aqueous solution well above their freezing temperature; and the process of freezing of proteins in aqueous solution normally disturbs some of their properties [74]. In has been confirmed by many researchers using different experimental techniques that in hydrated protein a fraction of water of up to ~0.30.4 h gwater/g protein remains mobile down to very low temperatures and does not crystallise even after long periods [75, 76]. Hydration of protein above this critical level exhibits crystallisation behaviour of water on slow cooling but can be vitrified with increasing rate of cooling. This water forms a hydration layer around the protein and transforms into a glass-like structure when the temperature drops below the freezing point of the aqueous solution. Water in polymeric/biopolymeric systems is known to be affected by the specific interaction with polymer/biopolymer chains or the internal structure of the polymer/biopolymer matrix. Therefore, studies on the physical state of water absorbed in polymeric/biopolymeric systems may provide useful insight to their internal structure. Specifically, water that does not freeze as a result of the strong interaction with polymer systems has been called ‘non-freezing bound water’ (Figure 14.14) [77]. Water in polymeric/biopolymeric systems can be classified into three types of species: freezing free water, freezing bound water, and non freezing bound water. Non-freezing bound water is affected by the strong interaction with polymer systems and has no detectable phase transition. Freezing bound water has a phase transition at lower than 0 °C. This can be attributed to the weak interaction between water and polymer. Freezing free water has a same transition temperature at 0 °C as bulk water [78]. Among three types 482
Thermal Analysis of Biological Molecules and Biomedical Polymers
Figure 14.14 Classification of water structure in a hydrated polymer/protein based on DSC analysis Reprinted with permission from M. Tanaka and A. Mochizuki, Journal of Biomedical Materials Research, Part A, 2004, 68, 4, 684. ©2004, Wiley Interscience
of water in polymeric systems, non-freezing bound water is considered to be related to the water interacting with polymer chains. Stability of a low-temperature structure in protein could come from more ordered water molecules, which creates additional strong water-protein contacts. Kurinov [79] performed structure and dynamic simulation of protein and water (lysozyme crystals). The ordered water molecules are found to constitute only about one-quarter of all crystalline water. Much of the solvent remains disordered, even at very low temperatures, and has an amorphous structure that can not be modelled by an atomic model and the majority of ordered waters are in the 4 Å shell. For side chains in proteins, the number of ordered molecules near oxygen and nitrogen atoms marginally increased as the temperature decreased largely due to the immobilisation of the longer groups. However, main-chain hydration remains nearly temperature independent. Dynamic disorder of protein groups and water molecules at high temperatures is transformed into static disorder at low temperatures, as indicated by the temperature independence of the overall temperature factors. It has been reported that temperatures in excess of 200 °C is required to remove tightly bound water from proteins. The heating of enzymes to this temperature caused no cleavage of the polypeptide chains [80]. Chymotrypsin has to be heated to approximately 200 °C to achieve stabilisation of the weight of the sample. The temperature at which 483
Thermal Analysis of Rubbers and Rubbery Materials a protein undergoes thermal denaturation is strongly dependent on the amount of water associated with the protein. Protein denaturation could be correlated with thermodynamic water activity, the lower the water activity, the higher the protein thermal denaturation [80]. Figure 14.15 shows the DSC curves of anionic collagen and of the anionic collagen-hydroxyapatite composite film [81]. The main feature in the DSC curve of anionic collagen is the denaturation endotherm at 59.47 °C. Water in the triple helix of collagen (structural protein of connective tissues) and in the double-helical molecules of DNA is more ordered than water in a typical globular protein (RNase A) [82]. It is emphasised that most hydrated biopolymers have Tg due to freezing of the cooperative motions of biopolymers and bound waters. It is well known that in aqueous solution the structure of native collagen triple helix stabilised not only by intermolecular H-bonds but also by the ‘fourth chain’ formed by water molecules and tightly built into three polypeptide chains of the protein. These water molecules inserted in the structure of triple helix were deprived not only of translational degrees of freedom, but of rotational ones, as well [82]. This gives the possibility of building the so-called ‘water-carbonyl helix’ model of collagen molecules. The process of organisation of the macromolecular
Figure 14.15 DSC curve of the samples (A) anionic collagen film, (B) collagen-HA composite film Reproduced with permission from C.C. Silva, D. Thomazini, A.G. Pinheiro, N. Aranha, S.D. Figueiro, J.C. Goesand and A.S.B. Sombra, Materials Science & Engineering, B: Solid-State Materials for Advanced Technology, 2001, B86, 210. ©2001, Elsevier
484
Thermal Analysis of Biological Molecules and Biomedical Polymers structure of DNA is realised due to the immobilisation of water molecules by the structure of the double helix. The heat capacity of bound water in DNA double helix is 3.0–3.5 J/g-K, that is midway between the heat capacity of ice (4.2 J/g-K) and liquid water (2.1 J/g-K). The calorimetric investigations suggest that the binding of more tightly held water to a collagen and DNA can cause a decrease in biopolymer entropy as well as decrease in water entropy. The denaturation induced increment of the entropy change caused by the destruction of ordered water clusters inside the structure of the double helix, and of additional layers of ‘liquid-like’ water, should be accompanied by an increase of heat capacity and entropy. There is evidence that immobilised water in globular protein structure has characteristics similar to those of loosely bound water in the channels of zeolites and related minerals, for which entropy contribution is larger. It is understood that most water molecules occur on the polar surface of proteins and organise the H-bounded water network including the water present in the ‘hydrophobic channels’ inside the globule. The protein surface water behaves more like liquid water, similar to the loosely bound non-structured water, but individual water molecules at the protein surface may significantly contribute to the thermodynamics of folding. The native structure of protein is the result of a symbiosis of two fractal objects – respectively, the folded globule, and the fractals of bound water forming the hydrate layers of protein surface [82].Lee and Ha reported an interesting observation from the DSC curves of the silk fibroin (SF) and SF/S-carboxymethyl kerateine (SCMK) blend films as shown in Figure 14.16 [78]. SF exhibits an endothermic peak at 223 °C and an exothermic
Figure 4.16 DSC curves of (a) SF; (b) SF/SCMK=50/50; and (c) SCMK film Reproduced with permission from K.Y. Lee and W.S. Ha, Polymer, 1999, 40, 14, 4131. ©1999, Elsevier
485
Thermal Analysis of Rubbers and Rubbery Materials peak at 227 °C. The endothermic peak corresponds to the weakening of interhelical interactions in the -helix of SF. The exothermic peak attributed to the transition from random coil to -structure of SF. In case of the blend film, the exothermic peak of SF is weakened and the transition peaks of --structure of keratin still remain at 224 °C and 238 °C, respectively [78]. This observation indicated that the structural change of SF from random coil form to -structure has occurred because of addition of SCMK. DSC measurements of the blend films, fully hydrated in water, were performed and the results are shown in Figure 14.17. During the cooling cycle one sharp exothermic peak is observed except for the SF film. This peak can be attributed to the freezing of free water. But water absorbed in the SF film shows two different exothermic peaks. One is for freezing of free water and the other is for freezing of freezable bound water. During the heating cycle one endothermic peak of a similar shape is observed. Thermodynamic characteristics of hydrated elastin-mimetic synthetic polypentapeptide poly(ValProGlyValGly) (see Table 4.2 for abbreviation used) in water and in a series of ethylene glycol-water mixtures was reported by Luan and co-workers using DSC technique [83]. The endothermic heat of the inverse temperature transition for the polypentapeptide in ethylene glycol-water is progressively decreased as the ethylene glycol is progressively increased to the 30:70 ratio. It follows that G for dissolution in water is small. As G = H - TS and as H is negative, the quantity (-TS), must be positive, which requires that the S for dissolving a hydrophobic moiety in water must
Figure 14.17 DSC cooling (A) and heating (B) curves of fully hydrated SF/SCMK blend films SF (a); 75/25 (b); 50/50 (c); 25/75 (d) and SCMK (e) Reproduced with permission from K.Y. Lee and W.S. Ha, Polymer, 1999, 40, 14, 4131. ©1999, Elsevier
486
Thermal Analysis of Biological Molecules and Biomedical Polymers be negative. Therefore, the negative change in entropy must come from an increase in order of the water surrounding the hydrophobic moiety [83]. The endothermic heat and positive entropy change of the inverse temperature transition of poly(ValProGlyValGly) is also due to the change in hydration. An effect of addition of a single -CH2 moiety for the polypentapeptide, the endothermic heats and the positive entropy change observed for the inverse temperature transition are reported as primarily arising from the melting of the more ordered water of hydrophobic hydration. The entropy change due to hydrophobic side chain is not the primary source of the entropic elastomeric force [83]. Experimental study of the glassy behaviour of the water of hydration is challenging task. Slow dynamics of protein interfacial water has been observed by neutron diffusion [84] studies and dielectric measurements [85-87]. Recently, Sartor and co-workers [76] performed an elegant experiment using DSC to identify the characteristics and the role of water in hydrated protein and opened the way for a better understanding of interfacial water in proteins. Figure 14.18 shows the DSC thermogram of hydrated lysozyme containing 0.25 (g-water/g-lysozyme) annealed at different temperatures. The unannealed sample is also shown for reference. For curve 2-5 the samples were subjected to the following thermal cycle: first cooled from 298 K to 103 K at ~150 K/min, subsequently heated to the annealing temperature at 30 K/min; kept at the annealed temperature for 60 min, cooled further to 103 K at 30 K/min, and finally reheated to 298 K at 30 K/min when the scan was recorded. The authors also examined lysozyme, haemoglobin, and myoglobin globular proteins with various levels of hydration. DSC studies of the glassy states of unannealed proteins with different level of hydration (curve 1 - Figure 14.18) shows that the heat capacity increases progressively with increasing temperature, without any abrupt increase - the characteristics of glass to liquid transitions. This Figure 14.18 DSC thermogram of hydrated lysozyme containing 0.25 (g-water/geffect has been attributed to the lysozyme) annealed at different temperatures. presence of a large number of local The unannealed sample is also shown for structures in the macromolecular reference segment that diffuses at different time scales over a broad temperature Reprinted from G. Sartor, E. Mayer and G.P. range. Below 200 K, the relaxation Johari, Biophysical Journal, 1994, 66, 1, rates of the solvent and the protein 249. ©1994, Elsevier 487
Thermal Analysis of Rubbers and Rubbery Materials motions are decoupled. Thus, the Tg of the hydration water at the surface of a protein should influence protein reactivity; and this influence is very likely to be reflected in kinetic measurements made at the Tg. It is assumed that the ‘unfrozen’ water in the solvation layer of a protein has a lower limit, because the water molecules are restricted to move in the layer, and they interact with polar and charged surface residues of protein. The annealing of hydrated proteins causes their DSC scan to exhibit an endothermic region immediately above the annealing temperature (Figure 14.18). The annealing effect confirms that the individual Tg of the relaxing local regions is spread over. The frustration introduced by the coupling of the water molecules to the protein surface prevents a true crystallisation and induces a smooth transition to glassy state [88].
14.4 Collagen and Collagen Based Biomaterials The collagen is the most abundant protein found in the animal kingdom and is the major protein of the extracellular matrix (ECM); and makes up about 25% of the total protein content [89]. ECM is a complex structural entity surrounding and supporting cells that are found within mammalian tissues. The ECM is often referred to as the connective tissue, which surrounds the fibrous and cellular elements of varying size, shape and number, which make up the body and is composed of three major classes of biomolecules: structural proteins (collagen and elastin), specialised proteins: (e.g., fibrillin, fibronectin, and laminin) and proteoglycans. Proteoglycans are composed of a protein core to which is attached long chains of repeating disaccharide units termed glycosaminoglycans (GAG), which forms extremely complex high Mw components of the ECM. The collagens exist in variety of morphological forms and there are at least 12 types of collagens. Types I, II and III are the most abundant and form fibrils of similar structure. Type IV collagen forms a two-dimensional reticulum and is a major component of the basal lamina. The collagens are predominantly synthesised by fibroblasts, but epithelial cells can also synthesise these proteins. The collagens are the main load-bearing structural element, providing structural integrity and strength to soft and hard tissues including blood vessels, tendons, skin, bone and dentin in teeth. Collagen presents a variety of structural forms: as sheets that support the skin and internal organs, cable like fibres in tissues such as tendons and heart valves. Collagen is an attractive class of biomaterial that has a wide spectrum of clinical applications [90]. In particular, collagen can readily undergo chemical modification at various pendant sites of its associated amino acids. For example, collagen-rich animal tissue used in bio-prostheses are easily and routinely crosslinked with glutaraldehyde to render the tissue mechanically more durable. This versatility for modification enables collagen to be converted into a variety of function-specific biomaterials [90]. Hybrid biomaterials based on collagen and conducting polymers have found biological applications in the separation and recovery of charged biological molecules. [90]. Figure 14.19 shows the schematic of a connective tissue and typical TGA, and DTA data of the connective tissue samples. The degradation characteristics of different regimes are clearly observed in the 488
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Figure 14.19 The structure of connective tissue (a), and typical thermal traces of connective tissue sample (b) Reproduced with permission from M. Bihari-Varga, Journal of Thermal Analysis, 1982, 23, 7. ©1982, Springer
figure [91].Thermal stability of human bone tissue and its bone-extracted type-I collagen was investigated by Lozano and co-workers [81]. From DSC investigation for type-I Collagen, variations of an exothermic maximum peak were observed between 500 and 530 °C depending on the extraction method. High thermal stability of extracted collagen as opposed to thermal stability found in bone tissue (maximum exothermic peak was found at ~350 °C) is reported to be caused by interactions that induce a change in the molecular properties of collagen during mineralisation, (specifically in its crosslinks and other chemical interactions). These interactions have an effect on fibre elasticity and on strength of bone tissue as a whole [92]. Mkukuma and co-workers [93] investigated the thermal stability and structure of calcellous bone mineral from the femoral head of patients with osteoarthritis or osteoporosis. In exaggerated loss of mainly calcellous bone results in bone fragility and increased risk of fracture followed by minimal trauma. Samouillan and co-workers performed detail thermal characterisation of connective tissue related to the aortic tissues for cardiac valve for bioprostheses [94]. Figure 14.20 shows the DSC traces of leaflet and aortic wall from the heart of young pig. The aortic valves consist of three layers of morphologically distinct tissue: the fibrosa consists mainly of collagen fibre bundles, while the ventricularis is a much thinner collagen and elastin layers. The separation between the two layers is a very loose spongiosa zone that mainly consists of water and glycosaminoglycans. This zone enables localised movements 489
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Figure 14.20 DSC thermogram of aortic wall and valvular leaflet; elastin and collagen are indicated for reference Reproduced with permission V. Samouillan, J. Dandurand-Lods, A. Lamure, E. Maurel, C. Lacabane, G. Gerosa, A. Venturini, D. Casarotto, L. Gherardini and M. Spina, Journal of Biomedical Materials Research, 1999, 46, 4, 531. ©1999, Wiley
and shearing between the layers. The aortic wall is also a three layered structure, which must resist repeated flex – during an average lifetime the aortic arch undergoes more than a billion stretch-relaxation cycles [95]. Complementary traces of pure collagen and elastin have been superimposed on the figure for comparison and to help understand the thermal event in the aortic matrices. The four thermograms (Figure 14.20a) for the first heating exhibit a very broad endothermic peak with maxima close to 100 °C. This first order transition is attributed to the release of bound water [96-98]. The broadness of the peak implies complexity of this transition, which includes: disruption of protein water interaction, evaporation, and vaporisation. The samples were hydrated and obtained by sorption of water with 85% relative humidity at 25 °C. The water content in different experimental samples was: collagen - 29.4%, elastin - 19.7%, leaflet - 28% and aortic wall - 22%. Figure 14.20b represent the corresponding second and third DSC heating scans. The transitions recorded on the first scan are associated with protein-water interactions, second scans are specific for dry material. The DSC thermogram from the second heating of collagen is characterised by a first-order transition at around 220 °C, which is attributed to the denaturation of dry collagen and corresponds to the helix-coil transition induced by thermal disruption of hydrogen bonding. In solution this collagen-gelatine transition occurs between 20 and 80 °C and is widely 490
Thermal Analysis of Biological Molecules and Biomedical Polymers used as a measure of thermal stability of collagen, which originates from the collagen shrinkage. In a third DSC scan the collagen-gelatine transition is absent, demonstrating the irreversibility of this transition; only a step in specific heat related to the Tg of gelatine at 180 °C is observed (collagen after denaturation behaves like an amorphous polymer).
14.4.1 Denaturation of Collagen from Different Origins Collagens are observed in a wide range of biological systems, where they have evolved to fulfil precise biological functions. Like other vertebrates, the skin of fish contains a high amount of collagen [99] and fish species with reported collagen denaturation temperatures above 30 °C are very limited (e.g., skipjack and carp). The majority of fish collagens denatures below 30 °C, indicating that fish collagen is generally less stable than its mammalian counterparts. There are very few fish species that contain collagen with high denaturation temperatures such as the tropical and subtropical fish inhabiting the Gulf of Mexico may contain collagen with denaturation temperatures above 30 °C. Collagen represents only 2% of the total protein in muscle, but is responsible for many of the textural changes in meat during heating [100]. Collagen goes through structural denaturation and solubilisation during heating. The rate and extent of these changes depend on the maturity of the collagen as well as exogenous factors such as rate of heating, relative humidity, and restraint during cooking. In solution, isolated collagen molecules are denatured at ~36 °C. However, the structure of collagen molecules in muscle remains relatively stable due to the crystallisation energy of the triple helix until the temperature reaches 64 °C. At this temperature, the triple helix begins to break down, starting at the ends of the molecules [100]. As denaturation continues, the molecule eventually shrinks to approximately one-quarter its original length, resulting in toughening of the muscle. Continued heating above 70 to 75 °C causes partial solubilisation of collagen, resulting in gelatine formation [100].
14.4.2 Collagen Based Composite Biomaterial Bone tissue consists of mainly hydroxyapatite (HAP) (~80 wt%) and collagen (~20 wt%). Therefore, development of HAP/polymer composites is considered to be one of the most prospective biomaterial in the reconstruction surgery for the repair of defects in human hard tissue. Such bio-composites are required to be biocompatible, biodegradable, nontoxic (i.e., sterile), as well as having mechanical properties similar to those of natural bone. Uskokovic and co-workers [101] developed such composite materials from tendon derived collagen and HAP. They identified the characteristic endothermic peaks at temperatures of 70, 122 and 222 °C as well as a complex endothermic peak with two separate maximums at 320 and 325 °C from DSC thermogram of pure collagen. The appearance of three endothermic peaks is a characteristic of the proteins. Heating of 491
Thermal Analysis of Rubbers and Rubbery Materials collagen leads to dehydration and a part of the released water comes from hydroxyl groups of amino acid hydroxyproline and from thermal breakage of H-bonds, which stabilise polypeptide chains. Effects of collagen heating can be reversible or irreversible. Mild heating results in local unfolding of the polypeptide chain, which is a reversible process, and so the polypeptide chain returns to its initial folded structure after restoring the normal (physiological) temperature. It is considered that these later transformations occur due to the break of longer sequences of H-bonds, which stabilise protein helices. Certain sequences along the molecule could be more sensitive to breakage of (sequential) H-bonds. Heat induced breakage of crosslinking bonds between adjacent polypeptide chains can have a significant role in the process of denaturation. DSC traces of HAP/ collagen composites show an endothermic peak resulting from collagen denaturation, which appears at temperatures between 89 and 119 °C; and an exothermic peak resulting from the presence of calcium phosphate with temperature maximum between 353 and 443 °C. Uskokovic and co-workers reported that denaturation of collagen is a reaction of the first order, and the reported activation energy obtained using both BorchardtDaniels, and Piloyan’s has an average value of 2.8 kJ mol-1 [101]. Shapes of the TGA curves of tendon derived collagen composites show that the mass loss of composite during heating results from degradation of collagen. Pure HAP did not show any loss of its mass during heating up to 1000 °C [101]. Major mass loss is observed in the temperature range from 200-600 °C, mostly between 200 and 400 °C. Complete mass loss of collagen in the composite occurs at 800 °C and a pure collagen at 900 °C, with a residue of 5%. Roveri and co-workers [102] used a biomimetic approach to direct nucleation of HAP on self-assembled collagen fibres to set up a collagen-HAP nanocrystal composite as a new particularly attractive material for bone repair and reconstruction. X-ray diffractometric technique, thermogravimetric (TG-DTG), spectroscopic (FT-IR, ICP), microscopic (SEM, TEM) analyses have been used to highlight the likeness of the artificial biomimetic HAP/collagen composite with natural bone tissue. Thermal analysis of polyetheretherketone(PEEK)/HAP biocomposite mixtures for bone tissue engineering was investigated by Meenan and co-workers [103] using a series of PEEK/HAP mixtures. TGA, DSC, and MDSC were employed for the analyses of the composites. Thermal degradation of collagen and collagen containing -carotene was investigated in detail by Kaminska and co-workers [104]. Collagen, extracted from porcine pericardium and copolymer with poly(sodium 4-(3-pyrrolyl) butanesulfonate) (PSPBS) was reported by Li and Khor [90]. Dehydrated collagen did not show any thermal transition peak in the 25–100 °C range. However, when hydrated with a little water, an endothermic peak was observed at 45.6 °C corresponding to collagen melting. When glutaraldehyde solution was used to hydrate the collagen instead of water, the endotherm appears at a higher temperature of 75 °C. Treatment with gluteraldehyde for 24 hours increased this point marginally to 77 °C. It is suggested that the collagen in the hybrid was not crosslinked by gluteraldehyde. This may be due to the short gluteraldehyde molecules (5 atoms long), being unable to find adjacent crosslinking sites to effect crosslinking in a more dispersed collagen network in the hybrid or a lack of crosslinking sites on collagen arising from interaction with PSPBS. The decomposition profiles and associated 492
Thermal Analysis of Biological Molecules and Biomedical Polymers first derivative curves of PSPBS and collagen shows two major decomposition steps at around 340 °C and 535 °C, respectively. Giusti (105) and co-workers reported blends of collagen (water soluble type of collagen (TC)) with either polyvinyl alcohol (PVA) or polyacrylic acid (PAA) by mixing aqueous solutions of the two polymers. DSC and dynamic mechanical thermal analysis (DMTA) have been used to investigate the miscibility and the mechanical behaviour of the blends. Chiellini and co-workers [106] reported thermal characteristics of gelatine-based blends and composites. Bioartificial materials using PVA, PAA and PMAA as synthetic components, and collagen (SC), gelatine, starch, hyaluronic acid and dextran as biological components, were investigated by Cascone [107]. The materials were prepared in the form of films or hydrogels and treated by glutaraldehyde vapor or thermal dehydration in order to reduce their solubility in water. The results indicate that SC/PVA, gelatin/PVA and starch/PVA films behave as biphasic systems, showing good mechanical properties over a wide range of temperature. It was observed that the gluteraldehyde procedure affects only the biological component of the SC/PVA and gelatine/PVA blends, whilst the thermal treatment influences mainly the synthetic polymer. Barbani and co-workers [108] reported the interactions between soluble collagen (SC) from calf skin and PAA and observed that mixing aqueous solutions of collagen and PAA, at various pH values (2.5-4), leads to the formation of complexes that precipitate in the form of insoluble aggregates. The effects of mixture composition, pH, and ionic strength on SC/PAA complex formation were investigated by gravimetric, turbidimetric, and conductometric analysis. Mixture composition and pH appear to influence the thermal properties of both complexes and films. Thermal and rheological behaviour of collagen/chitosan blends were reported by Salomé Machado and co-workers [109]. Sarti and co-workers [110] investigated, blends of PVA with collagen and gelatine, prepared from aqueous solution by solvent casting, and the blends were investigated by DSC and DMTA. It was concluded that, though thermodynamically immiscible with both native and denatured collagen, PVA forms mechanically compatible blends with collagen and gelatine. Tsai and co-workers [111] fabricated porous chitosan/collagen-based composite scaffolds for skin-related tissue engineering applications. To improve the performance of chitosan/collagen composite scaffolds, they added 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and amino acids (including alanine, glycine, and glutamic acid) in the fabrication procedure of the composite scaffolds, in which amino acid molecules was employed as crosslinking bridges to enhance the EDC-mediated crosslinking. The miscibility of the components and thermal properties of the scaffolds were analysed using DSC that demonstrated that collagen and amino acids were well-mixed with the chitosan [111]. Pederson and co-workers [112] reported a novel strategy to exploit temperature driven self-assembly of collagen and thermally triggered liposome mineralization to form a mineralized collagen composite from an injectable precursor fluid. Combining an acid-soluble collagen solution with calcium- and phosphate-loaded liposomes resulted in a liposome/collagen precursor fluid, which formed a mineralized 493
Thermal Analysis of Rubbers and Rubbery Materials collagen gel when heated from room temperature to 37 °C. They employed DSC to demonstrate that mixtures of calcium- and phosphate-loaded liposomes composed of dipalmitoylphosphatidylcholine (90 mol%) and dimyristoylphosphatidylcholine (10 mol%) are stable at room temperature but form calcium phosphate mineral when heated above 35 °C, a consequence of the release of entrapped salts at the lipid chain melting transition. The formation of calcium phosphate mineral induced by triggered release of calcium and phosphate was detected as an endothermic transition (H=6.2±1.1 kcal/mol lipid) near the lipid chain melting transition (Tm=37 °C) [112].
14.5 Thermal Stability of Silk, and Other Elastic Biomaterials Nature produces a wide range of biomaterials including elastomers and fibres of amazing range of properties and thermal analyses techniques enable us to develop in depth understanding of the unique characteristics of these materials. Such understanding has encouraged scientist to mimic biology and develop novel bio-mimetic materials for wide range of applications. For example spiders produce a variety of polymer ‘silks’ with a wide range of mechanical properties (Figure 14.21): reeling out their dragline as they move, spooling it as they fall or descend, and producing it to build the frame and radii of their webs [113, 114]. It is known that the spiders can rapidly adjust the material properties and thread diameter of major ampullate silks to achieve this matching of properties to function. Spider dragline silk fibre has attracted much scientific interest due to its unique and technologically significant combination of high tensile and compressive strength. These silks are unique protein polymers, since they have evolved to function in air, rather than in aqueous tissues. The most well-defined spider silks are (i) viscid silk, which forms the extensible, sticky catching spiral of the web which traps prey, and (ii) dragline silk - the strong, stiff silk used as the frame for the web and as a safety line for the spider. Spider silk opens a window into the structure-function relationship of many biological protein elastomers. The most dominant repeat sequence, in spider silk GlyProGlyGlyX (three letter code for amino acid residues used, Table 4.2, X variable amino acid residue), appears up to 63 times in tandem arrays. The secondary structure of the elastic repeat sequences is not clear, however, the sequence GlyProGlyGlyX, may form random coils or -turns [113]. Viscid silk relies on water as a plasticizer – despite the fact that silk has evolved to function in air. If viscid silk is allowed to dry, the fibres become glassy and rigid. In comparison to viscid silk, dragline silk contains shorter elastic repeat sequences and higher degrees of crosslinking, which result in less extensible but stronger fibre. Dragline silk is uniquely different from viscid in that it functions in a non-plasticised, air-dried state. When dragline silk is immersed in water, it swells and is transformed into rubbery elastic materials [115]. Ortlepp and Gosline [116] employed thermal analyses to study the characteristics of spider silks. TGA demonstrated that the silk is stable to approximately 230 °C after which there is a rapid loss in weight on additional heating. Figure 14.22 shows the 494
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Figure 14.21 Nephila golden silk spider showing its silk glands as well as the use of each silk and its mechanical properties in the form of comparative experimental stress-strain plots, where the highlighted line in each graph is that of the graph label; the symbols D and W in the flagelliform graph are dry and wet irrespectively Reproduced with permission from F. Vollrath and D. Porter, Soft Matter, 2006, 2, 3777. ©2006, RSC Publishing
typical dynamic mechanical properties of major silk of Nephila clavipes over a wide range of temperature. In the DMA, two transitions are noted, the first one at about -70 °C, presumably representing localised transition in the amorphous domain, and the second at around 210 °C, representing a main chain motion associated with partial melt. Young’s modulus (E) at the room temperature region is 12-20 GPa and is maintained until about 170 °C [114, 116]. The marginal increase in modulus after heat treatment is presumably due to an increase in stiffness, resulting from loss of moisture and increased crystallinity. Creep compliance under a constant force 1 N at -40, 25, 100 and 150 °C indicates that the initial creep is strongly dependent on the initial temperature. However, 495
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Figure 14.22 Dynamic mechanical properties of silk:both elastic modulus and loss tangent are shown, Model prediction line is also shown in the graph Reproduced with permission from F. Vollrath and D. Porter, Soft Matter, 2006, 2, 3777. ©2006, RSC Publishing
after the initial elongation, creep behaviour is not significantly influenced by temperature, presumably due to the extensive H-bonding which may tend to limit long-term creep. Magoshi and Magoshi [117] reported a Tg of 175 °C for silkworm silk using DMA and dielectric properties. From dielectric loss tangent measurement, they observed Tg of -40 °C and 175 °C, with the transition at 175 oC reported to be due to crystallisation. Magioshi and Nakamura [118] studied silk fibroin using DSC and reported the endotherm at 175 °C due to Tg and an exotherm at 212 °C related to crystallisation. The exotherm at 280 °C was attributed to degradation. Osaki [119] reported thermal properties of spider silks using TGA and DSC that exhibited an endotherm at 100 °C (loss of water) and exotherms at 300, 340, 500 and 580 °C respectively.
14.6 Thermal Characteristics of Biopolymers for Drug Delivery and Drug-Polymer Interaction Natural polysaccharides are widely used as thickening agents in food and drug products. They are also used for the extended release of drugs after oral administration. It has 496
Thermal Analysis of Biological Molecules and Biomedical Polymers been suggested that biodegradable plant polysaccharides, such as pectin, dextrans and galactomannans, could be used for specific delivery of drugs into human colon if properly modified (e.g., by crosslinking to form hydrogels) to reduce their water solubility [120]. It was reported that Guar gum could be crosslinked with glutaraldehyde to reduce its swelling properties. The reduction in the swelling properties was required to prevent entrapped drug leakage right after its oral ingestion. Figure 14.23 shows the thermal behaviour of Guar gum. Endothermic peaks were detected at 256 and 296 °C, and an exothermic peak was detected at 310 °C. Its weight loss in TGA started at a temperature of 180 °C and maximum weight loss was reached at 300 °C. The DTG measurements of guar gum and its various crosslinked products showed temperature maxima at 300, 290, 280 and 270 °C. The activation energies for decomposition reported to be increased with crosslinking density. By crosslinking guar gum with glutaraldehyde, new covalent bonds were also introduced to the polysaccharide. Glutaraldehyde identified to substitute part of the hydroxyl groups and the crosslinking caused a change in the polymer structure. These physico-chemical changes were also reflected in the thermal behaviour of the crosslinked guar gum [120]. Bioartificial polymeric materials, based on blends of polysaccharides with synthetic polymers such as PVA and PAA have also been reported as hydrogels. The physicochemical, mechanical, and biological properties of these materials were investigated using different thermal techniques such as DSC, DMTA. The results indicate that while dextran is perfectly miscible with PAA, dextran/PVA, chitosan/ PVA, starch/PVA, and gellan/PVA blends behave mainly as two-phase systems, although interactions can occur between the components. It is reported that crosslinked starch/PVA films can be used as dialysis membranes – they exhibit transport properties comparable to, and in some cases better than, those of currently used commercial membranes. Hydrogels based on dextran/PVA and chitosan/PVA blends could be used as delivery systems. They appear to be able to Figure 14.23 DSC curve of GG at a release physiological amounts of temperature range of 50-500 °C and a human growth hormone, offering heating rate of 1 oC/min the possibility of modulating the release of the drug by varying the Gliko-Kabir, A. Penhasi and A. Rubenstein, content of the biological component Carbohydrate Research, 1999, 316, 1-4, 6. [121]. ©1999, Elsevier 497
Thermal Analysis of Rubbers and Rubbery Materials Biopolymer gels are used as structuring additives in a variety of food applications. Mixed protein/polysaccharide gels often exhibit phase separation, which is emulsion-like in nature, with spherical inclusions present within a continuous matrix [122]. Figure 14.24 illustrates a schematic temperature state diagram for an aqueous carbohydrate solution, showing the Tg line, defined by viscosity, the equilibrium freezing line, a ‘non-equilibrium’ freezing path, the theoretical eutectic line, and a description of the reactivity of the various states [123]. Above and to the left of the Tg line, solutions or complex systems of such foods are in the rubbery or liquid state, in which they are unstable and reactive, so that ice crystal growth can occur in time frames significant to food storage. Below and to the right of the Tg curve the system transforms to the glassy (vitreous) state as a result of extremely high viscosity and exists as an unreactive amorphous solid. When a weak interfacial bond occurs between the two phases, debonding can be anticipated if the particles included have significantly higher elastic modulus than the continuous matrix. Conversely, when a relatively strong interfacial bond occurs, fracture through the particles is more likely.
Figure 14.24 A schematic temperature state diagram for an aqueous carbohydrate solution, showing the Tg line, defined by viscosity, the equilibrium freezing line, a ‘non-equilibrium’ freezing path, the theoretical eutectic line, and a description of the reactivity of the various states. Point Tg´ represents the Tg of the maximally freeze-concentrated solution and Wg´ represents the amount of unfrozen water (100 % solute) which becomes trapped in the glass. Points Tg and Wg represent an example of a temperature concentration relationship in a glass formed as a result of less than maximal ice formation Reproduced with permission from D.H. Goff, Pure and Applied Chemistry, 1995, 67, 11, 1801. ©1995, IUPAC
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14.6.1 Protein-Protein, Protein-DNA and Protein-Ligand Interactions Protein-protein, protein-DNA and protein-ligand interactions are central to many aspects of biomolecular processes and ITC is particularly a powerful and straightforward technique to yield fundamental thermodynamic parameters of intermolecular interactions. The type of ITC experiment to be used depends on the specific system to be investigated. For heterogeneous interactions the conventional calorimetric approach of simply injecting one protein (ligand) into the solution of the other (macromolecule) will provide the necessary titration curve (solubility of the protein is a prerequisite for this experiment). However, for investigation of homogeneous interactions such as monomer, or oligomerisation reactions, a heat of dilution method involving injection of protein mixture into a buffer, where heat of dissociation is analysed to extract thermodynamic parameters for the process. Evans and co-workers [124, 125] investigated colicincell membrane receptor interactions using ITC, from which affinity, entropies and stoichiometry of binding were evaluated. Colicins are a family of ionophoric/antibiotic proteins produced by and active against sensitive Escherichia coli and related bacteria; and act by binding to the trimeric outer membrane protein-OmpF (OmpF porin), translocating across the periplasmic space to form a pore in the cytoplasmic membrane. Homogeneous interaction of peptide antibiotics of the vancomycin family, particularly in the presence of specific peptide ligand analogues related to their cell wall target has been successfully investigated by Cooper and co-workers [126]. The thermodynamic parameters of the association-coupled folding, its stability, and association of engineered heterodimeric coiled coil or leucine zipper has also been successfully investigated using ITC [127-130]. The interaction of ribonuclease-S (RNase S) with ligands from random peptide libraries has been investigated extensively using ITC due to its importance as a new approach to drugs for therapeutic targets [131]. Since its discovery in 1959, RNase S has been investigated extensively as a model for protein structure [132-134]. Thermodynamic properties for bonding of S-peptide (and S-peptide analogues) to S-protein can be obtained directly, and illuminate not only the bimolecular interaction itself but also the intermolecular interactions that hold the corresponding intact polypeptide in its native three dimensional conformation. The sequence-specific binding of protein to DNA is an interesting problem in biomolecular recognition. It is now understood from the protein-DNA complexes that the recognition of a specific binding site involves the formation of a highly complementary interface with a large number of non-covalent interactions. This binding process also involves extensive dehydration of the interacting surface, release of counter ions and conformational changes as well as change in dynamics in the macromolecules. Thermodynamic contribution to the specificity of binding and its stability may be evaluated by ITC and has been observed that for such interaction dehydration dominates the thermodynamic of complex formation [135, 136-138]. ITC has been very successful in understanding the thermodynamic and kinetic parameter of this trimolecular complex and demonstrated the versatility of ITC in understanding the interactions in biomolecules. 499
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14.7 Thermal Characteristics of Synthetic Hydrogels and Scaffolds for Tissue Engineering Hydrogels are unique condensed matter that exhibit excellent bio-compatibility and hydrophilicity, and closely resemble natural living tissues because of their high water content and elastomeric consistency. In recent years, tissue engineering has emerged as a highly researched area for developing biological substitutes that restore, maintain, or improve the lost or damaged tissues and organs. Because of their unique properties, hydrogels have been exploited for a wide range of biomedical applications, including drug delivery [139], protein delivery [140], gene delivery [141], wound dressing [142], water elimination from oedema [143], cosmetic reconstruction [144], contact lenses [145], and tissue engineering [146-154]. Synthetic polymeric biomaterials can have the advantages of reproducible mechanical properties, tunable biochemical behaviour, and controllable degradation. The most widely investigated biodegradable polymers for tissue engineering scaffolds are polyurethanes (PU) due to their chemical versatility, excellent biocompatibility and superior mechanical properties. PU composed of hard and soft segments has been extensively used in the manufacturing of biocompatible prosthesis and medical devices. A broad variety of PU can be obtained by modifying the balance between both segments. Extensive research has been carried out for a variety of applications including cardiovascular devices such as vascular prosthesis, intra-aortic balloons, cardiac valves, and insulating sheaths for peacemakers, membranes for dialysis, craniofacial reconstruction, breast implants, and so on. In these applications the PU used are normally non-degradable. Recently, interest has been also focused on degradable PU for tissue engineering [155]. Most degradable PU are developed by introducing biodegradable building blocks in the final polymer; such as caprolactone, lactides, hydroxybutyric acid, saccharide, amino acids, as either soft segment of chain extenders, which are degraded by the hydrolysis of the ester linkage into naturally occurring substances. The susceptibility of the polymer chain to hydrolysis may be tuned by introducing polyethylene glycol (PEG) as the soft segment. PU hydrogels based on PEG and its derivatives or copolymers as soft segment have received much attention in the medical fields [156], due to their chemical functionality, superhydrophilic chain motion, nontoxicity and non-immunogenicity. Polycaprolactone diol (PCL) is considered as most popular building block as the soft segment for PU because its degradation products are non-toxic and have FDA approval. Table 4.3 shows DSC data from typical segmented PU from PCL-diol and PEG prepolymer. Different basically-flexible PU have been prepared using different combinations of aliphatic hexamethylene diisocyanate (HMDI), PEG (Mw 400 Da), poly(-caprolactone) diol (Mw 530 Da), and 1,4-butanediol. Thermal analysis of the synthesised PU demonstrated high thermal stability and the assumption of glassy state well below room temperature, in agreement with their marked flexibility. The observed Tg at about -95 °C (PEG-diol series) and -45 °C (PCL-diol series) can be attributed to soft segments, whereas the higher temperature Tg can be associated with PU hard segments. The endothermic transitions between 9 and 23 °C were associated to the 500
Type BD (mol) Pre-polymer mmol HGBu1 HG 3 3 HGBu1 HG 4 2 HGC1.2 HG 50 HGC1.9 HG 46 HCBu0.7 HC 3 4 HCBu1 HC 4 4 HCBu2 HC 4 2 HG= Tg1=glass transition of soft segment, Tg2=glass transition of the hard segment, Tm1=melting of crystalline zone in soft segment, Tm2=melting and rearrangement of hard segment, Tcc=cold crystallisation temperature, Tc=crystalline temperature BD = Butadiene
Sample designation
NCO/OH (mol/mol) 1.0 2.0 1.2 1.9 0.7 1.0 2.0
PCL550 43 24 -
-93 -93 -93 -95 -47 -48 -38
Tg1 (oC) -32 -34 -37 -39 -40 12
Tg2 (oC) 9 20 23 -
Tm1 (oC)
Table 14.3 DSC data on typical segmented polyurethanes
112 109 66 91 112
Tm2 (oC)
-8 -8 -
Tcc (oC)
92 96 9 -8 13 98
Tc (oC)
Thermal Analysis of Biological Molecules and Biomedical Polymers
501
Thermal Analysis of Rubbers and Rubbery Materials melting of crystalline zones from soft segments, while the transitions between 66 and 112 °C were attributed to the melting and rearrangement of hard domains and to the dissociation of the diverse types of H-bonds [156]. Flexible segmented PU containing soft polyether or polyester segments and hard semiflexible hexamethylene segments can be conveniently prepared by using 1,4-butadiol or di-hydroxyl terminated PCL as chain extruder. By taking into account their solubility, melting temperature, and rather high thermal stability these materials appear promising for processing from melt. The low Tg indicates that the synthesised PU can be conveniently tested as biocompatible materials for the design of devices of pharmaceutical and biomedical interest [156]. Figure 14.25 shows the representative DSC thermograms of PCL 530, 1250 and 2000 with HMDI containing 50% mesogen content [157]. An increase in the molecular weight of the PCL soft segment from 530 to 2,000 results in the lowering of Tg, melting and isotropisation temperatures. Thermoplastic PU elastomers based on PCL [158] showed that the aromatic isocyanate-based PU degraded primarily into amine and carbon dioxide while the aliphatic isocyanate-based PU dissociated primarily into isocyanate and glycol. The kinetic parameters obtained for PU elastomers based on the PCL and aromatic type of diicocyanate before UV irradiation were studied by Bajsic and co-workers [159]. The activation energy (Ea) values of these materials were found to be higher than Ea values reported for PU elastomers based on the polyether soft segment. The PU elastomers based on PCL degraded slowly than those based on polyether soft segment [159]. It was observed that the Ea increases with increasing hard segment content.
Figure 14.25 DSC thermogram of a) PC-5-H-50, b)PC-1-H-50 and c) PC-2-H-50 Reprinted with permission from K.S.V. Srinivasan and T. Padmavathy, Molecular Symposia, 2003, 199, 277. ©2003, Wiley-VCH
502
Thermal Analysis of Biological Molecules and Biomedical Polymers A variety of different PCL blends have been reported in the literature for prospective application in the medical field including drug delivery devices, because of its excellent permeability to drugs [160]. Thermal analyses have been used as a major technique to understand the miscibility and morphological characteristics of such blends. Figure 14.26 shows the DSC traces of PCL/TDP 4,4´-thiodiphenol (TDP) blends with various TDP contents recorded during the first heating scan. The melting point of the PCL component in the blends decreased with the TDP content from 64 °C for pure PCL and only 46 °C for the PCL/TDP30. The presence of the H-bond network in the miscibility and crystallisation characteristics of the blend has been discussed in details by He and coworkers [161]. However, Tg of the blends were higher than Tg of pure PCL. The Tg of a PCL/TDP blend increased with the content of TDP in the blend except for the Tgs of PCL-TDP10 (-44 °C), which was higher than that of PCL-TDP20 (-47 °C). That could be due to: 1.
the intermolecular H-bond, the molecules of TDP might act as a physical bulk side group of the PCL chain in the blend;
2.
the formation of H-bond network, TDP should also act as a physical crosslinking agent in the blend. Both the physical bulk side group and crosslink network should lower the flexibility of the PCL chain and then heighten the Tg of the blend [161]. Blends of PCL with 1,6-hexanediol (HDO), 1,4-di-(2-hydroxyethoxy) benzene (DHEB) and 1,4-dihydroxybenzene (DHB) were also investigated by He and co-workers using DSC and these compounds were observed to be molecularly immiscible by DSC.
The behaviour of polymer blends with various compositions of PCL and poly-4hydroxystyrene was reported by Lezcano and co-workers [162]. It was apparent from the investigation that PCL crystallised from blends having weight fraction of PCL (w2) > 0.7. For w2 < 0.6 no fusion endotherms were detected; there was only one Tg that revealed the presence of a unique amorphous phase, which was confirmed by the transparency of the samples. The system falls within the class of miscible crystalline amorphous polymer blends. The blends with w2 > 0.7 displays both the melting peak of PCL and the Tg of the corresponding amorphous phase and the thermograms exhibit double melting peaks. PCL based polymer blend with L-lactide (50/50) were used for the reconstruction of meniscal lesions (cartilage tear) (menisci act as a structural transition zone between the femoral condyles and tibial plateau) [162]. This material showed a very good adhesion to the meniscal tissue and, therefore, a good healing of the meniscal lesion. However, it was observed to have certain drawbacks such as, the degradation rate was somewhat too high, due to the very high Mw of the polymer maximal concentration of 5% could be reached, and the poly(L-lactide) crystals, which are still present after eight years of in-vitro degradation, may induce an inflammatory reaction since cells cannot digest them unlike PCL and polyglycolide crystals. To avoid lactide crystallinity, an amorphous 50/50 copoly(-caprolactone/L-lactide) was used for the production of nerve guide 503
Thermal Analysis of Rubbers and Rubbery Materials
Figure 14.26 DSC traces during the first heating scan of pure PCL, TDP, and their blends with various TDP contents Reproduced with permission from Y. He, N.Asakawa and Y. Inoue, Journal of Polymer Science, Part B: Polymer Physics, 2000, 38, 14, 1848. ©Wiley Interscience
conduits [163,164]. Polyurethanes based on a 50/50 copoly(-caprolactone/L-lactide) prepolymer and butanediisocynate were attempted by de Groot and co-workers [165]. Compared to the high molecular weight 50/50 copoly(L-lactide/-caprolactone) the polyurethane showed better thermo-mechanical properties, and a slower degradation rate. These factors make them useful for in-vivo tissue engineering in for instance meniscal reconstruction material, nerve guide and artificial skin. Polyester based scaffolds fabricated from elastic polyesters, in particular polyoctanediol citrate (POC) or polyglycerol sebacate (PGS), are compatible for regeneration of different tissues such as blood vessel [166-168], cartilage [169], bone [170], and nerve tissue [171]. In vivo immunological response of polyester elastomer, such as PGS, has shown no gross inflammation or fibrosis [171]. Apart from their easy thermal processabilities, the properties of elastomeric citrates could also be tailored by mixing them with natural bone mineral hydroxy apatite (HA) [171] or with other biocompatible polymer [172] in order to meet specific tissue regeneration requirements. Djordjevic and coworkers [173] recently reported the synthesis and characterisation a new class of these materials, polyoctanediol citrate/sebacate [p(OCS)] by a catalyst-free polyesterification of multifunctional 1,8-octanediol (OD), citric acid (CA), glycerol and sebacic acid (SA), as monomers. DSC was employed as the main tool to develop detail understanding of 504
Thermal Analysis of Biological Molecules and Biomedical Polymers the morphological behaviour of these biocompatible polymers. Besides matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-ToF-MS), nuclear magnetic resonance spectroscopy (NMR), photo-acoustic Fourier transform infrared spectroscopy (PA-FTIR), X-ray photoelectron spectroscopy (XPS), DSC were employed for detail morphological investigation. The morphology of the copolymer so obtained was co-related with the cell survival, growth and proliferation [174].
14.8 Thermal Characteristics of Biomimetic Protein Based Hydrogels Biomimetic (mimicking nature) protein-based polymers, which are composed of natural and non-natural amino acid sequences have emerged as a promising new class of material, for novel applications in biotechnology, tissue engineering and drug delivery [175-187]. Biomimetics is essentially the practice of taking ideas and concepts from Nature and using them to develop novel materials, processes or methods. It is now understood that mimicking Nature’s strategy and borrowing building blocks from biology has enormous potential to synthesise remarkable materials. For example amazingly diversified proteins and nucleic acids of precise functionality are synthesised by Nature, using mild conditions and starting from a limited number of rather simple building blocks. Biomimetic proteins are inherently biocompatible and biodegradable and have the potential to find many in vivo applications as ideal materials for tissue engineering scaffold, control drug delivery and wide range applications as soft condensed material. Thermal, thrmomechanical and rheological investigation has been used extensively to understand and evaluate such bmaterials characteristics. Significant research efforts has been dedicated to understand structural information of flagelliform silks at cDNA at gene level [188] because of their combination of strength, elasticity and exceptional toughness [114, 189, 190]. The diversity of spider silks confirms the wide possible variability for one natural elastomeric biomaterial on both the chemical (genetic) and physical (mechanical) level. The protein coding genes at the nucleotide sequence level have been recently identified [114, 190- 192] and it has been suggested that the key structural feature of the flagelliform silk protein is Gly-Pro-GlyGly-(Xaa)n (Figure 14.27) and a -spiral structure has been proposed to account for the observed unique mechanical properties. The predicted silk protein is organised into a non-repetitive amino-terminal (NR) and carboxy-terminal (CR) regions, shown in grey, that flank a repetitive region made up of several iterations of an ensemble repeat B. Each ensemble repeat is approximately 440 amino acids in length. Three types of sub-repeats are present in an ensemble, shown in blue, red and yellow boxed. The predominant unit is Gly-Pro-Gly-Gly(Xaa)n. The 28 amino acid sub-repeat is referred as ‘spacer’ between glycine-rich regions. Based on this understanding of the architecture and design parameter many biomimetic silk-like proteins have been developed and thermal analysis has been used as a major tool for the characterisation and performance evaluation of such materials [114, 193]. 505
Thermal Analysis of Rubbers and Rubbery Materials
Figure 14.27 Alignment of amino acid repeat sequence in flagelliform silk. One-letter abbreviation of the amino acids are used Reproduced with permission from J. Gosline, P. Guerette, C. Orfepp and K. Savage, Journal of Experimental Biology, 1999, 202, Part 23, 3295. ©1999, The Company of Biologists
Elastin is another unique elastic protein whose repeat motif has been exploited extensively to develop soft biomaterials for numerous biomedical applications [179, 194]. Elastin is one of the major proteins of connective tissue and is responsible for imparting elasticity to organs and tissues, and plays a role in several biological mechanisms. As discussed previously, the biomimetic elastic-like-polymer posed unique elastic behaviour and thermal analysis has been a major tool for the evaluation of it unique characteristics. Recent development in genetic engineering of synthetic polypeptides has enabled synthesis of block co-polypeptide composed of complex sequences in which the individual blocks have different mechanical, chemical or biological properties. Like synthetic block copolymers the peptide based block copolymers exhibit unique and tunable association characteristics. Figure 14.28 presents the DSC thermograms of dilute aqueous solution (approximately 1 mg/ml) of elastin-like recombinant ABA type protein triblock co-polypeptide 1 and 2 (see Figure 14.28d) that exhibits tunable association. Sharp endothermic transitions at 23 °C and 21 °C are clearly observed, which are reversible in nature and are related to the lower critical solution temperature of the block co-peptides. The enthalpy of the process was evaluated to be in the range of 400440 kcal/mol or 500-540 kcal/mol indicating an entropy driven process (hydrophobic or other solvation related interaction). The decrease in apparent heat capacity of the polypeptide above the Tg is characteristics of the cooperative formation of condensed aggregate [195, 196]. Nagapudi and co-workers reported synthesis of high molecular weight elastin-like recombinant ABA type protein triblock copolymes that exhibit tunable properties, which 506
Thermal Analysis of Biological Molecules and Biomedical Polymers
(d)
Figure 14.28 Raw DSC traces for the endothermic thermal transition of 1 and 2 in dilute aqueous solution. a) Protein 1 at high pH (40mM NaOH), with rescan(dotted line) showing reversibility. The instrument base line is shown for comparison. (b) Protein 1 at low pH (40 mM acetic acid). c) Protein 2 at high and low pH. The temperature maximum for each transition is as indicated. d) Amino acid sequence of protein based block-copolymer 1 and 2 derived from elastin-mimetic peptide sequence Reproduced with permission from E.R. Wright, R.A. McMillan, A. Cooper, R.P. Apkarian and V.P. Conticello, Advanced Functional Materials, 2002, 12, 2, 149. ©2002, Wiley-VCH
may find use in novel scaffolds for tissue engineering and new biomaterials for controlled drug delivery, and cell encapsulation [197]. Development of large molecular weight ABA type block protein based biopolymers offers a unique opportunity to systematically modify microstructure on both nano and meso scale. They also demonstrated that through the rational choice of processing conditions including solvent type, temperature, and pH an array of thermoplastic elastomer (TPE) materials could be produced systematically from the ABA block copolymer that display a wide range of physical properties. Petka and co-workers [198] Minich and co-workers [199], Breedveld and co-workers [200], Pochan and co-workers [201], and Nowak and co-workers [202] have recently reported the development of many different protein based block copolymers that exhibit thermoplastic elastomer like behaviour. These functional materials could be used in novel scaffolds for tissue engineering and as new biomaterials for controlled drug delivery and cell encapsulation. 507
Thermal Analysis of Rubbers and Rubbery Materials Recently, Elvin and co-workers [203] reported successful cloning and expression of the exon-1 of the Drosophila melanogaster CG15920 gene-encodes, a resilin-like protein [204] to bio-synthesise a water soluble protein named pro-resilin. This precursor protein, when crosslinked, produces unique stimuli-sensitive elastic hydrogels of unusual resilience and unique tunable characteristics and is identified as rec1-resilin. Resilin is composed of naturally occurring protein polymer in which biological control of the polypeptide sequence allows fabrication of a material with mechanical properties that exceed those of any synthetic and non-peptide natural polymer. Resilin is a member of the family of elastic proteins that include elastin, gluten, gliadin, abductin and spider silks [179,205] and occurs as a highly elastic extracellular skeletal component in insects. It is purported to be the most resilient elastic material known [194,203] . It can be compressed to store energy very efficiently for a quick release, and it remains extremely functional over an insect’s lifetime indicating extraordinarily high fatigue. The elasticity of resilin is best known for its roles in insect flight and the remarkable jumping ability of fleas, and spittle bugs. Froghoppers and fleas achieve a jump acceleration of more than 400 times of gravity in just one millisecond and flies take advantage of resilin’s durability to flap their wings more than 720,000 times an hour. Dutta and co-workers [206-209] explored the unique molecular, physical, thermal and thermo-mechanical and rheological characteristics of the protein including molecular self-organisation and its tuneability to understand the sophisticated chemical structural scheme, and its relation to the functionality that is crucial to develop novel functional smart materials synthetically from this unique protein based materials. Stimuli-sensitive biomimetic polymer hydrogels are extensively investigated due to their potential application as ‘smart’ biomaterials and controlled drug delivery systems and medicine. It is now clearly understood biomimetism and bioinspiration as tools for the design of innovative materials and novel artificial functional molecule will have impressive impact on the development of novel functional material with unique responsiveness and tuneability. Therefore, in conclusion we remark that thermo-analytical techniques have made and continue to make profound contribution to the fundamental understanding of structureproperty-performance relationship and utilization of polymeric materials including bio-macromolecules, elastomers, bio-mimetic polymers and biomedical polymers and other soft condensed matters. In future with the invention of novel thermo-analytical and hyphenated thermo-analytical techniques and associated electronics and software it will continue to generate wealth of knowledge.
Acknowledgement The authors acknowledge the financial support of Australian Research Council, Government of Australia to carry out some of the research works presented in the chapter. Two of the co-authors (N.K. Dutta and N. Roy Choudhury) acknowledge 508
Thermal Analysis of Biological Molecules and Biomedical Polymers the conscientious and patient cooperation of M. Ankit K. Dutta that made this book chapter possible.
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521
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522
Abbreviations
Abbreviations
TA
Micro-thermal analysis
3-D
Three-dimensional
5CN-COOH
6-[(4-Cyano-4´-biphenyl)oxy]hexanoic acid
AAS
Atomic absorption spectroscopy
ABS
Acrylonitrile-butadiene-styrene terpolymer
AC
Alternating current
ACM
Acrylate rubber
ACM
Polyacrylic rubber
ACM D
Acrylic rubber
ACN
Acetonitrile
ADSC
Alternating differential scanning calorimetry
AE-DPNR
Acetone-extracted deproteinised natural rubber
AFM
Atomic force microscopy
A-HDPE
Acrylic acid modified
ASA
Acrylonitrile-styrene-acrylate
ASTM
American Society for Testing and Materials
ATBN
Amine-terminated butadiene acrylonitrile copolymers
ATH
Aluminium trihydrate
ATHAS
Advanced thermal analysis system
BIIR
Bromobutyl rubbers
BIMS
Brominated isobutylene co-para methyl styrene
BNR
Brominated natural rubber 523
Thermal Analysis of Rubbers and Rubbery Materials BPO
Benzoyl peroxide
BR
Butadiene rubber
BR
Cis butadiene rubber
BR
Polybutadiene rubber
C16-OMT
Hexadecyl trimethyl ammonium chloride modified montmorillonite
CaCO3
Calcium carbonate
CD
Compact disk
CDNA
Complementary deoxyribonucleic acid
CIIR
Chlorobutyl rubber(s)
CLD
Constrained-layer damping
Cl-GRT
Chlorinated ground rubber tyres
CM
Chlorinated polyethylene
Cms
Critical micelle concentration
CNR
Chlorinated natural rubber
CNT
Carbon nanotubes
CO
Polyepichlorohydrin
Cp
Sample heat capacity
CPC-OMT
Cetyl pyridinium chloride modified montmorillonite
CPE
Chlorinated polyethylene(s)
CPI
Cis-1,4-polyisoprene(s)
CPI-C18
Esterified CPI with stearoyl group
CPI-C2
Esterified CPI with acyl group
CR
Chloroprene rubber
CRC
Crystallisation rate constant
CSDPF
Carbon-silica dual phase fillers
C-S-HPN
Calcium silicate hydrate
CSM
Chlorosulfonated polyethylene
CTE
Co-efficient of thermal expansion
524
Abbreviations CTPB
Carboxy-terminated polybutadiene
DBM
Dibutyl maleate
DBM-g-EPDM
Dibutyl maleate grafted ethylene-propylene diene terpolymer
DC
Direct current
DCP
Dicumyl peroxide
DDPC
1,2-Didecanoyl-sn-Glycero-3-phosphocholine
DDSC
Dynamic differential scanning calorimetry
DEA
Dielectric analysis
DETA
Dielectric thermal analysis
DHB
1,4-Dihydroxybenzene
DHEB
1,4-Di-(2-hydroxyethoxy) benzene
DLPC
1,2-Dipalmitoyl-sn-Glycero-3-phosphocholine
DMA
Dynamic mechanical analysis
DMPA
Dipalmityl phosphatidylethanolamine
DMPC
1,2-Distearoyl-sn-Glycero-3-phosphocholine (s)
DMPE
Dipalmityl phosphatidylethanolamine(s)
DMPG
Dipalmitoylphosphatidylglycerol
DMTA
Dynamic mechanical thermal analysis
DNA
Deoxyribonucleic acid
DNR
Decomposed natural rubber
DOP
Di(2-ethylhexyl) phthalate
DPC
Differential photocalorimetry
DPNR
Deproteinised natural rubber
DPPA
1,2-Dipalmitoylphosphatidic acid
DPPC
Dipalmitoyl phosphatidylcholine
DPPE
1,2-Dipalmitoylphosphoethanolamine
DPPG
1,2 Dipalmitoyl-sn-glycero-3[phosphor-rac-(1-glycerol)] (Na salt)
DSC
Differential scanning calorimetry
DSP
Digital signal processor 525
Thermal Analysis of Rubbers and Rubbery Materials DSPG
Distearoyl phosphatidylglycerol
DTA
Differential thermal analysis
DTG
Derivative thermogravimetry
DTMA
Derivative thermomechanical analysis
Ea
Activation energy
EAM
Ethylene acrylate rubber
ECM
Extracellular matrix
EGA
Evolved gas analysis
EGD
Evolved gas detection
Em
Elastic modulus
EMA
Polyethylene-co-methacrylate
ENB
Ethylidene norbornene
ENR
Epoxidised natural rubber
ENR25
Epoxidised natural rubber with 25 mol% epoxy content
ENR-50
Epoxidised NR containing 50 mol% epoxy groups
EP/PPP
Epoxy resin/poly(paraphenylene)
EPDM
Ethylene-propylene diene terpolymer
EPDM-g-DBM
Dibutyl maleate grafted ethylene-propylene diene terpolymer rubber
EPM
Ethylene-propylene monomer
EPR
Copolymer of ethylene and propylene
EPR
Electron paramagnetic resonance
EPR-g-MA
Maleic anhydride-modified EPR
ESBR
Epoxidised-styrene-butadiene rubber
EVA
Ethylene vinyl acetate copolymer
EVOH
Ethylene vinyl alcohol
FDA
Food and Drug Administration (USA)
FEF
Fast extrusion furnace
FKM
Fluororubber
526
Abbreviations FKM
Fluorocarbon rubber
FKM
Fluoroelastomers
FNA4-V
Fluorocarbon rubber based nanocomposite with a 4 phr loading of unmodified nanoclay
FRLS
Fire resistant low smoke
FTIR or FT-IR
Fourier-transform infrared spectroscopy
FVMQ
Fluorosilicone rubber
GAG
Glycosaminoglycan(s)
GC
Gas chromatography
GC-MS
Gas chromatography – mass spectrometry
GRT
Ground rubber tyre(s)
HAF
High abrasion furnace
HAP
Hydroxyapatite
HCR
Hydrogenated chloroprene rubber
HDI
Hexamethylene diisocyanate
HDO
1,6-Hexanediol
HDPE
High-density polyethylene
H-EPR-g-MA
Maleated ethylene-propylene rubber with some ethylene crystallinity
HF
Heat flow
HFP
Hexafluoropropylene
HMDI
Hexamethylene diisocyanate
HNBR
Hydrogenated nitrile rubber
HNR
Hydrogenated natural rubber
HP
Hewlett Packard
H-PDMB
Hydrogenated 1,4-poly(2,3-dimethyl-1,3-butadiene)
HRTG
High resolution thermogravimetry
HSBR
Hydrogenated styrene butadiene rubber
HV
Hydroxyl valerate
527
Thermal Analysis of Rubbers and Rubbery Materials ICP
Inductively coupled plasma
ICTA
International Confederation of Thermal Analysis
ICTAC
International Confederation for Thermal Analysis and Calorimetry
id
Internal diameter
IDT
Initial decomposition temperature
IIR
Butyl rubber(s)
IIR
Isobutylene-isoprene rubber
IPN
Interpenetrating polymer network(s)
iPP
Isotactic polypropylene
IR
Infrared
IR
Isoprene rubber
IR
Synthetic polyisoprene
IR-OE
Oil-extended isoprene rubber
ISAF
Intermediate super abrasion furnace carbon black
ITC
Isothermal titration calorimetry
IUPAC
International Union of Pure and Applied Chemistry
LC
Liquid chromatography
L-DMA
Localised dynamic mechanical analysis
LDPE
Low-density polyethylene
LF-DPNR
Low molecular weight deproteinised natural rubber
LLDPE
Linear low-density polyethylene
LMV
Multilamellar vesicles
LOI
Limiting oxygen index
L-rubber
Naturally occurring cis-1,4-polyisoprene from sporophores of Lactarius volemus
LTA
Localised thermal analysis
L-TMA
Localised thermomechanical analysis
LVDT
Linear variable differential transformer
MA
Maleic anhydride
528
Abbreviations MAH
Maleic anhydride
MDI
4,4-Methylene diphenyl diisocyanate
MDPE
Medium-density polyethylene
MDR
Moving die rheometers
MDSC
Modulated differential scanning calorimetry
mEPDM
Maleated ethylene-propylene diene terpolymer
M-EPDM
Maleated ethylene-propylene diene terpolymer
M-GRT
Maleic anhydride grafted ground rubber tyres
MMT
Mineral montmorillonite
Mn
Number average molecular weight
mNR
Maleated natural rubber
MS
Mass spectrometry
MT
Medium thermal black
MTA
Magnetic thermal analysis
MW
Molar mass
MW
Molecular weight
MW
Molecular weight(s)
Mw
Weight average molecular weight
MWNT
Multi-wall carbon nanotubes
NA
Unmodified nanoclays
NAA
Number of amino acids in the molecule
NBR
Acrylonitrile-butadiene rubber
NBR
Nitrile rubber
NBR
Nitrile-butadiene rubber
NBR
Polyacrylonitrile-butadiene
NBR
Polybutadiene-co-acylonitrile
NHF
Non-reversing heat flow
NMR
Nuclear magnetic resonance
529
Thermal Analysis of Rubbers and Rubbery Materials NR
Natural rubber
OC
Octadecyl amine modified
ODR
Oscillating disc rheometers
ODSC
Oscillating differential scanning calorimetry
ODT
Order disorder transition
OG
Octaglucoside
OIT
Oxidation induction time
OTTER
Optothermal transient emission radiometer
PA
Polyamide
PA 66NC
Polyamide 6,6 nanocomposite
PA6
Polyamide 6
PA6,6
Polyamide 6,6
PA6LSN
Polyamide 6 layered-silicate nanocomposites
PAA
Polyacrylic acid
PAC
Polyacrylate
PAN
Polyacrylonitrile
Pani
Polyaniline
PBT
Polybutylene terephthalate
PC
Phosphocholines
PC
Polycarbonate
PCA
Principal component analysis
PCB
Printed circuit board(s)
PCL
Polycaprolactone
PCL-TDP
Polycaprolactone – thiophenol
PDMS
Polydimethylsiloxane(s)
PDMS-MHS
Poly(dimethyl siloxane-co-methyl hydrogen siloxane)
PE
Polyethylene
PEBA
Polyether block amides
530
Abbreviations PECH
Polyepichlorohydrin
PECH-co-EO
Polyepichlorohydrin-co-ethylene oxide
PEG
Polyethylene glycol
PEO
Polyethylene oxide(s)
PET
Polyethylene terephthalate
PF
Phenol-formaldehyde
PFM
Pulsed force mode
PFOE
Perfluoroctyl ethylene
PG
Phosphatidyl glycerols
PHB
Hydrogenated polybutadiene
PHB
Poly(3-hydroxybutyrate)
PHBV
Poly(3-hydroxybutyrate-co-hydroxyvalerate)
phr
Parts per hundred rubber
PI
Polyisoprene
PIB
Polyisobutylene
PIP
Poly(cis-1,4-isoprene)
PLA
Polylactic acid
PMAA
Polymethacrylic acid
PMAC
Polymethacrylate
PMHS
Poly(methyl hydrogen siloxane)
PMMA
Polymethylmethacrylate
PNBE
Polynorbornene
PNC
Commercial nanocomposites
PNF
Polyfluorophosphazene
POE
Polyethylene octene elastomer
POE-27-G-AA
Nitrile rubber
POGP
Palmitoyloleoylphosphatidylglycerol
POM
Polarising optical microscopy
531
Thermal Analysis of Rubbers and Rubbery Materials POSS
Polyhedral oligomeric silsesquioxane
PP
Polypropylene
PPF
Non-isothermal crystallisation of nanocomposites based on a ternary mixture of iPP
PPG
Polypropylene glycol
PPMS
Polyphenylmethylsixonaes
PPO
Polypropylene oxide
PPR
Parallel plate rheometry
PPSIPN
Particulate phase semi-interpenetrating polymer network
PRT
Platinum resistance thermometer
PS
Polystyrene
PSF
Polystyrene-co-2,3,4,5,6-pentafluorostyrene
PSPBS
Poly(sodium 4-(3-pyrrolyl)butanesulfonate)
PSS-pani
Polysulfonated styrene – polyaniline
PTMG
Polytetramethylene glycol
PTMO
Polytetramethylene oxide
PTMT
Polytetramethylene terephthalate
PU
Polyurethane(s)
PVA
Polyvinyl alcohol
PVAc
Polyvinyl acetate
PVC
Polyvinyl chloride
PVP
Polyvinylpyrrolidone
Reca
Recombinant protein A
rGRT
Ground rubber tyres containing 44% rubber
RHF
Reversing heat flow
RMA
Relaxation map analysis
rms
Root mean square
RR
Reclaimed rubber
RT
Room temperature
532
Abbreviations RTR
Reclaimed tyre rubber
SAF
Super abrasion furnace
SAM
Self-assembled monolayer
SAN
Polystyrene-co-acrylonitrile
SANS
Small-angle neutron scattering
SAXS
Small-angle X-ray scattering
SBR
Polystyrene-co-butadiene
SBR
Styrene-butadiene rubber
SBR-OE
Oil-extended styrene-butadiene rubber
SBS
Polystyrene polybutadiene-polystyrene or styrene-butadienestyrene
SBS
Styrene-butadiene-styrene
SC
Collagen
SCMK
S-carboxymethyl kerateine
SEBS
Poly[styrene-b-{ethylene-co-butylene}-b-styrene] triblock copolymer
SEBS
Polystyrene/polyethylene-co-butene/polystyrene block copolymers
SEBS
Styrene-butadiene-styrene
SEC
Size exclusion chromatography
SEM
Scanning electon microscopy
S-EPDM
Sulfonated ethylene-propylene diene terpolymer
SF
Shrinkage force
SF
Silk fibroin
SFC
Supercritical fluid chromatography
Shsb
Hydrogenated poly(styrene butadiene)
SIBS
Poly(styrene-isobutylene styrene)
SIMS
Secondary ion mass spectrometry
S-I-S
Styrene-isoprene-styrene
S-m-EPDM
Sulfonated-m-ethylene-propylene diene terpolymer
533
Thermal Analysis of Rubbers and Rubbery Materials SPM
Scanning probe microscopy
SR
Synthetic rubber
SRF
Semi-reinforcing furnace carbon black
ST
Thermal shrinkage
SThM
Scanning thermal microscopy
STM
Scanning tunneling microscope
SWNT
Single-walled nanotubes
TBA
Torsional braid analysis
Tc
Crystallisation temperature
TC
Soluble type of collagen
TC
Thermal conductivity
T cc
Cold crystallisation temperature
TD
Thermal desorption
TD
Thermodilatometry
TDA
Thermodialometric analysis
TDF
Thermal diffusivity
TDI
2,4-Tolulene diisocyanate
TDP
4,4´-Thiodiphenol
TDS
Thermal desorption
TE-DPNR
Transesterified deproteinised natural rubber
TEM
Transmission electrom microscopy
TE-NR
Transesterified natural rubber
TEOS
Tetraethoxysilane
TFE
Tetrafluoroethylene
Tg
Glass transition temperature(s)
TG
Thermogravimetry
TGA
Thermogravimetric analysis
TG-DTG
Combined thermogravimetry analysis and derivative thermogravimetry
534
Abbreviations TG-GC
Thermogravimetry-gas chromatography
TG-GC-IR
Thermogravimetry-gas chromatography - infrared spectroscopy
TG-GC-MS
Thermogravimetry-gas chromatography - mass spectrometry
TG-IR
Thermogravimetry - infrared spectroscopy
TG-MS
Thermogravimetry - mass spectrometry
THF
Tetrahydrofuran
Ti
Clearing temperature
TI
Initial decomposition temperature
TIM
Thermal interface material(s)
Tm
Melting temperature
Tm0
Equilibrium melting temperatures
TMA
Thermomechanical analysis
Tmax
Maximum decomposition temperature
TMDSC
Temperature-modulated differential scanning calorimetry
TMPTA
Trimethylol propane triacrylate
TPE
Thermoplastic elastomer(s
TPE
Thermoplastic elastomeric blend
TPE-FRLS
Thermoplastic elastomer fire retardant - low smoke
TPE-g
Grafted thermoplastic elastomer
TPE-g-FRLS
Thermoplastic elastomer grafted to a low smoke fire retardant
T-POSS
Trisilanol isobutyl polyhydral oligomeric silsoqoxane
TPU
Thermoplastic polyurethane
TPV
Thermoplastic vulcanisates
Trans BR
Trans 1,4 polybutadiene
TSC
Thermally stimulated current
TSDC
Thermally stimulated depolarised current
TSR
Thermally stimulated recovery
TSSR
Thermally stimulated stress relaxation
535
Thermal Analysis of Rubbers and Rubbery Materials Ts-Tr
Differential temperature detector
UV
Ultraviolet
VA
9-Vinyl anthracene
VA
Vinyl acetate
VDF
Vinylidene fluoride
VN
2-Vinyl naphthalene
VPGVG
Valyl-prolyl-glycyl-valyl-glycine polypeptide
WAXD
Wide angle X-ray diffraction
WEPDM
Waste black-filled ethylene-propylene diene terpolymer vulcanisate powder
WLF
Williams-Landel-Ferry
XLPE
Crosslinked polyethylene
XNBR
Carboxylated nitrile rubber
XPS
X-ray photoelectron spectrometry
ZDC
Zinc diethyl dithiocarbamate
Zn-XNBR
Zinc salt of carboxylated nitrile rubber
536
Index
Index
A Acrylate rubber 170-171, 173 Acrylic rubber 71 Acrylonitrile butadiene rubber 386 Adhesives, rubber-based 409 Air springs 393 Aliphatic polyesters 256 Alloys 468 Alternating current calorimetry 52 Annealing 76-77, 96 Arrhenius equation 1, 86, 201 ASTM E698 1, 87 Aluminium trihydrate 408 Atomic absorption spectroscopy 402, 410, 421 Atomic force microscope 4, 218, 220, 228, 231, 233-234, 248, 263-264, 312 Avrami equation 272, 303
B Benzoylation 373 Biological molecules 463-464 Biomacromolecules 464, 508 Biomaterial 481, 491 Biopolymers 475, 496 Bitumen 451 Blend morphology 6 Blends, immiscible 81 Blends, miscible 79 Block copolymers 71, 76-77, 176 Borchardt and Daniel method 87
Bulk crystallisation 338 Butyl rubber 108, 388, 417-419, 422, 432, 434-435, 453
C Cable sheathing compounds, rubberbased 406 Calorimetry 290 modulated temperature 312 Cells 464-465 Chlorinated polyethylene 357-359 Chlorination 360 Chloroprene rubber 108, 356, 432-433 Chloroprene rubber, hydrogenated 354 Chymotrypsin 483 Coefficient of linear thermal expansion 34, 188, 192, 194, 203, 206, 208, 213, 416 Collagen 490-493 gel 494 Couchman-Karasz equation 73 cis-1,4-polyisoprene 341-344, 347-349 Creep 149, 196 compliance 495 thermally stimulated 197 Crystalline fibre 204 Crystallisation 5 isothermal 307, 337 kinetics 94 rate 94 rate constant 305 time of 22 Crystallisation, solution-grown 338 537
Thermal Analysis of Rubbers and Rubbery Materials Curie temperature 279 Curing 97, 197, 210, 407 isothermal 87
D Debye-type relaxation process 43 Dehydrochlorination 358-359 Deproteinised natural rubber 339-346 acetone-extracted 339-340, 345-346 Devulcanisation 434 Dielectric thermal analysis 3, 5, 13, 4950, 52, 117, 323, 326, 433 analyser 50-51 Differential photo calorimetery 3, 13, 46-49, 100 Differential scanning calorimetry 1, 5, 11, 17-19, 190, 255, 277-278, 290, 322-323, 353, 381, 430, 463, 335 alternating 22 analysis 363, 394, 401, 483 curve 302 dynamic 22 heat flux 13 power compensated 13 modulated 22, 238, 277, 479, 492 oscillating 22 power compensating 17-18, 292 temperature modulated 13, 20-21, 299-301, 306-309 thermal analysis 90 thermal modulated 300, 307 Differential thermal analysis 1-2, 5, 11, 15, 28-29, 45, 65, 203, 278, 322323, 329, 387, 403, 419, 430, 470, 488 apparatus 19 differential scanning calorimetry instruments 17 furnace temperature programmers 15 instrument 13differential temperature detection system 16 low level DC voltage amplifier 16 Netzsch system 28 sample holders 14 Digital signal processor 50 Dilatometry 2, 11, 32, 34, 67, 335, 387 538
Dimyristoyl phosphatidylcholine 469470, 473 Dynamic mechanical thermal analysis 2, 6, 255, 261-263, 370, 373, 411-412, 441, 497 Dynamic mechanical analysis 2, 36, 11, 67, 149, 156, 198, 278, 323, 328, 353, 381, 433, 463 localised 219-220 Dynamic mechanical test, oscillatory 150
E Elastin 478, 480, 482, 506 Elastomer blends 170 Elastomeric block copolymers 94 Elastomers 494 bio-mimetic 464 chemically modified 353 filled 85 liquid crystalline 90, 92 scrap 430 Electron paramagnetic resonance 327 Entanglement coupling 152 Enzymes 476 Epoxidation 5, 105-106, 356-357 Epoxidised natural rubber 174, 357, 451 Epoxy resin 211, 239, 444 Equilibrium melting temperature 298 Equilibrium phase morphology 77 Ethylene vinyl acetate rubber 179, 433, 452 waste 455 Ethylene-based rubbers 108 Ethylene-propylene copolymer rubber 89 Ethylene propylene diene rubber rubber 90, 262-264, 266, 271, 370, 400, 404, 406, 412, 413, 415, 434, 436, 440-442, 446, 448, 450-451 Evolved gas analysis 2, 11, 13, 28, 30-31 Evolved gas detection 11, 13, 28 Extracellular matrix 488
F Fatty acid 344, 346, 348-349 Fillers 167, 324
Index carbon-silica dual phase 422 loading 168, 178 First thermodynamic law 322 Flexible chain rubbers 89 Flory’s equation 359 Flory-Huggins theory 255-256 Flory-Rehner equation 199 Fluorinated rubbers 111 Fluorocarbon rubber 170-171 Fluororubber 397-399, 402 Fluorosilicone rubber 397 Fourier transform - infrared spectroscopy 3, 31, 55, 58, 124, 219, 247-248, 278, 347-348, 492 Fox equation 255, 259, 366 Freeman and Carroll method 119 Furnace temperature programmer 25
G Gas chromatograph 58 Gas chromatography 30, 57, 124, 410, 456 Gas chromatography – mass spectrometry 4, 59, 219, 245 Gehmann rigidity modulus 195 Gel point 187, 199 Gel time 197, 211 Gelation 197, 211 Glass transition range 296 Glass transition temperature 4, 12, 66, 68, 155, 295-296, 354 Glycosaminoglycans 488 Gordon-Taylor equation 73-74, 77, 255 Gordon-Taylor-Wood equation 366-367 Graft copolymerisation 360 Grafting 6, 84, 165, 173, 360, 365, 407 Ground rubber 436, 440, 444 tyres 433, 437-440, 443, 446, 448, 450-451 waste 6
H Halogenation 5, 357 Heat capacity calibration 21 Heterogeneous materials 321
Hevea brasiliensis 253 High-density polyethylene 160, 161, 234, 303, 311, 357, 437, 445-446, 448, 450 Hoffman-Weeks method 258 Hooke’s law 149 Hydrochlorination 361 Hydrogels 500, 505, 508 Hydrogenation 5, 106, 353-354, 356 Hydrohalogenation 5, 357 Hydrophobic hydration 487
I Inductively coupled plasma 492 Indentation 196, 212 Infrared analysis 402, 406, 410 Infrared spectroscopic analysis 217, 434 Insulators, rubber based 412 Interpenetrating polymer network 82, 391-392, 437 Isobutylene-isoprene rubber 388, 415, 434-435, 440 Isoconversion method 120 Isoprene rubber 172, 421, 432 Isothermal titration calorimetry 7, 463464, 472-473, 499
K Kelvin-Voigt model 44 Kissinger’s method 87 Kwei equation 255, 257
L Lauritzen-Hoffman equation 259 Limiting oxygen index 357 Linear low-density polyethylene 446 Linear variable differential transformer 32 Lipid 466, 469 bilayers 464, 468 blends 468 Liposomes 470 Low-density polyethylene 160, 234, 446 539
Thermal Analysis of Rubbers and Rubbery Materials
M Mass spectroscopy 60, 124 Maxwell-Wagner-Sillars Effect 327 Medium density polyethylene 234 Melt grafting 362 Microflash method 310 Micro-scale morphology 222 Microscopy, confocal 325 Microscopy, polarised 90 Microscope, polarising optical 4 Microscopy, pulsed force 240, 244 Microscopy, scanning thermal 218 Micro-thermal analysis 4, 26, 52, 217221, 223-224, 226-228, 231-236, 238-240, 244-245, 247-248, 277, 311, 313 Micro-thermomechanical analysis 220 Mineralisation 489 Miscibility 254-257 Mooney viscosity 441 Moving die rheometers 412 Mullins effect 329 Multilamellar vesicles 471 Multi-wall carbon nanotubes 179
N Nanocomposites 285, 289 Nano-filler 282, 324-325 Nanostructured material 240 Nanotechnology 176, 322 Naphthoylation 374 Natural polysaccharides 496 Natural rubber 5, 28, 81, 85, 87, 94, 97, 99, 105, 120, 166, 335-336, 338339, 342-347, 351, 356-357, 415, 417, 422, 432-433, 435-436, 446, 453 acetone extracted 345 decomposed 444 deproteinised, lowest molecular fraction of 340 deproteinised, transesterified 339-340 transesterified 339-340, 345 Neutron scattering 255 Newton’s law 149 540
Nishi-Wang equation 256 Nitrile rubber 165, 175-176, 196, 224, 353, 356, 386, 397, 402-403, 436, 441 carboxylated 174, 392-393, 397 hydrogenated 165-167, 353-356, 397 Non-freezing bound water 482 Non-reversing heat flow 479-480 Nuclear magnetic resonance spectroscopy 53, 85, 172, 327, 505 Nylon-6 161, 174
O Octylglucoside 475 Oligomers 69 Optothermal transient emission radiometre 52 Order disorder transition 467-468, 472 Oscillating disc rheometers 412 Oxidation induction time 49, 99 Ozawa method 87 Ozonolysis 343
P Parallel plate rheometer 34-35,188, 197 Paramagnetism 327 Phenolic resins 212 Phospholipid-additive interactions 472 Photo-acoustic Fourier transform infrared spectroscopy 234, 505 Photothermal infrared spectrometry, high-resolution 312 Plasticiser 195 Plastics, toughened 444 Poisson’s constant 34 Polyacrylic rubber 112 Polybutadiene rubber 27, 190-191, 254, 388 Polybutadiene-co-acylonitrile 254 Polycaprolactone diol 500 Polychloroprene rubber 397 Polyester 504 Polyether ester 95 Polyethylene-co-methacrylate rubber 110 Polymers
Index architecture 71 biomedical 463, 508 bio-mimetic 508 blending 256 blends 221 chemosynthetic 256 crystalline 160 cyclic 71 glassy 159 liquid crystalline 92 matrix 286, 301, 327 nanocomposites 306 semicrystalline 88-89, 254, 297 synthetic 464, 475, 497 thermoset 197 waste 429 Polymerisation 1 Ziegler-Natta 253 Polymorphism 22 Polystyrene-co-butadiene 254 Polyurethanes 110, 504 Principal component analysis 60 Printed circuit board 191-192, 210-211 Protein, bio-mimetic 505 Proteoglycans 488 Proton nuclear magnetic resonance 345 Pyrolysis 430, 454-455 crater 231 Fourier transform infrared 115 gas chromatography 56 gas chromatography - mass spectrometry 115, 312
Q Quadrupole mass spectrometer 55 Quasi-isothermal crystallisation 308
R Raman spectroscopy 217, 469-470 Raw rubbers 162 Reclaimed tyre rubber 446 Recycled rubber 430, 440-441 Relaxation map analysis 3, 13, 44-46 Resilin 508 Resins, thermosetting 212
Reversing heat flow rate 479-480 Rocket motor insulators 415 Rubber, modified 163 Rubber recycling 429 Rubber, scrap 430 Rubber seals 395-396, 402 Rubber, synthetic 253-254, 396, 433 Rubber, waste 434, 436, 452-454, 456 Rubbery materials, waste 429 Rubbery blends 77-78, 440 ionomeric polyblends 84 miscible blends 77 partially miscible blends 77 Rubbery matrix 321-322, 324
S Scanning electron microscopy 217, 221, 226, 262, 325, 492 Scanning force microscopy 221 Scanning probe microscopy 218, 240 Scanning thermal microscopy 221 Size exclusion chromatography 343 Secondary ion-mass spectrometry 217, 244, 410 Shrinkage force 203 Shock mounts, rubber based 390 Silicone elastomers 365 Silicone rubber 111, 171, 201-202, 366, 417 Size exclusion chromatography 342 Small-angle x-ray scattering 259, 325, 470 Small-angle neutron scattering 325 Softening point 187 Spectrophotomer, infra red 58 Spectrometer, Solomat 44 Spectrometry, fibre tension 34-35, 188 Spectroscopy, near-field infrared 245 Spectroscopy, specific heat 52 Spectroscopy, thermally stimulated current 2 Spider dragline silk 494 Spongiosa zone 489 Stress relaxation 150, 196 Stress relaxation spectrometry 34, 36, 188 541
Thermal Analysis of Rubbers and Rubbery Materials Stress softening 328-329 Styrene-butadiene rubber 163, 228, 354, 386, 430 hydrogenated 163-164, 355 Sulfonation 367, 369 Supercritical fluid chromatography 342343 Surface analysis techniques 115, 421 Surface modification 436 Swelling 201 Synthetic hydrogels 500
T Thermal analysis 381, 464 local 219, 221, 223, 226, 244 magnetic 5, 327 Tacticity 73 Thermal conductivity 309, 313 measurements 52 3 method 310 steady-state 309 non-steady-state 309 Thermal degradation 102 Thermal desorption 60 Thermal desorption-gas chromatographymass spectroscopy 60 Thermal diffusivity 52 Thermal erosion rate 415 Thermal interface materials 415 Thermal mechanical analysis 323 Thermal shrinkage 203 Thermally stimulated current 41, 45 Thermally stimulated depolarisation current 13 Thermally stimulated recovery 197 Thermally stimulated stress relaxation 197 Thermoanalyser 55 Thermoanalytical techniques 12 Thermoconductometry 5, 326 Thermodialometric analysis 3, 13, 31-32, 34, 187 Thermodilatometry 11, 31-32, 187, 322 Thermodynamics 1 Thermogravimetry 1, 11, 22-23, 326, 394 542
controlled atmosphere 25 furnace 24 furnace temperature programmers 24 recorders 25 sample containers 25 software 26 temperature detection 24 thermobalance 24 Thermogravimetric analysis 1, 13, 22, 25, 30, 100, 277, 322, 353, 400, 430, 463 analyser 58 derivative 2, 6, 11, 27, 30, 101, 115117, 120, 382, 394, 404-405, 431432, 434-436 high resolution 60 isothermal 419 Thermogravimetry – differential scanning calorimetry – mass spectroscopy 60 Thermogravimetry – electrochemical analysis 60 Thermogravimetry – Fourier transform infra red spectroscopy 54-56, 58 Thermogravimetry – gas chromatography 3, 56-57, 60, 410 Thermogravimetry – gas chromatograpy – Fourier transform infrared 58 Thermogravimetry – gas chromatography - infrared 3, 58-60 Thermogravimetry – gas chromatography – mass spectroscopy 3, 55-60 Thermogravimetry – infrared spectroscopy 3, 53-54, 60, 410 Thermogravimetry – mass spectroscopy 3, 55-56 Thermoluminescence 52 Thermomechanical analysis 2, 4, 26, 3132, 34, 38-39, 45, 67, 187-201, 203, 206, 208, 210-213, 225, 249, 278, 312, 382, 386-387, 395, 402-403, 409, 416-417, 433, 452 apparatus 33 derivative 32,194-195 fibre tension spectrometry 13 localised 219-220 parallel plate rheometry 13 stress relaxation spectrometry 13
Index Thermomechanical analyser 34 Thermometers, platinum resistance 14 Thermometry 1 Thermoplastic elastomer 4, 6, 254, 260, 372, 430, 448, 450-451, 507 Thermoplastic vulcanisates 262-264, 266, 268-269, 273 Time-of-flight secondary ion mass spectrometry 226 Time-temperature superposition principle 154 Tissue engineering 500, 507 Torsional braid analysis 13, 39, 198-199 pendulum 40 Transmission electron microscopy 217, 221, 238, 262-263, 325, 429 Tyres, automobile 417 tread compound 419, 421-423
X-ray photoelectron spectrometry 226, 244, 410, 505 X-ray scattering 90, 255
Y Young’s modulus 200, 328, 495
Z Viscoelastic behaviour, zones of 152 glassy 153 plateau 152 terminal 153 transition 153
V Van der Waals interactions 324, 328 Vibration dampers 391 Vibration damping 383, 385-386 constrained-layer damping 384 free-layer or extensional damping 384 Vibration isolation 384, 386 Viscoelasticity 149 Viton 402 Vitrification 197, 199 Vulcanisation 86-87, 97, 105, 107, 109
W Wallace plastimeter 34 Water-carbonyl helix 484 Wide-angle x-ray diffraction 303 Williams plastimeter 34 Williams-Landel-Ferry euqation 38, 155156 Window seal, automotive 403
X X-ray diffractometric technique 492 X-ray microtomography 325
543
Thermal Analysis of Rubbers and Rubbery Materials
544
Index
545
Thermal Analysis of Rubbers and Rubbery Materials
546
Published by Smithers Rapra, 2010
The term ‘thermal analysis’ describes a group of techniques in which a physical property of a substance is measured as a function of temperature, while the substance is subjected to a controlled temperature programme. In differential thermal analysis, the temperature difference that develops between a sample and an inert reference material is measured, when both are subjected to identical heat treatments. The related technique of differential scanning calorimetry relies on the differences in energy required to maintain the sample and reference at an identical temperature. This book describes the use of this technique: - For determining additives in rubbery materials - In recycling of rubbers - In understanding the interactions of rubber - fillers and the rubber matrix - For characterisation of rubber nano-composites and other modified rubbers and their blends - For measuring the crystallisation of rubbers, and - The different instrumental techniques that can be used
Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 Web: www.iSmithers.net