Particulate-Filled Polymer Composites,

Particulate-Filled Polymer Composites Second Edition

Editor

R.N. Rothon

Rapra Technology Limited

Particulate-Filled Polymer Composites 2nd Edition

Editor: Roger N. Rothon

rapra TECHNOLOGY

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2003 by

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2003, Rapra Technology Limited

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 apologize if any have been overlooked. Cover micrograph of aramid fibre-reinforced polyamide 6,6, reproduced with permission from P.R. Hornsby, Brunel University, UK.

ISBN: 1-85957-382-7

Typeset by Rapra Technology Limited Cover printed by The Printing House, Crewe, Cheshire, UK Printed and bound by Rapra Technology Limited, Shrewsbury, UK

Contents

Preface ................................................................................................................... 1 Contributors .......................................................................................................... 3 1

General Principles Guiding Selection and Use of Particulate Materials ........... 5 1.1

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

1.2

Basic Characteristics of Particulate Fillers .............................................. 5 1.2.1

Cost ........................................................................................... 6

1.2.2

Chemistry, Composition and Impurities..................................... 7

1.2.3

Density or Specific Gravity ........................................................ 9

1.2.4

Hardness .................................................................................. 10

1.2.5

Abrasiveness ............................................................................ 11

1.2.6

Optical Properties .................................................................... 11

1.2.7

Thermal Properties .................................................................. 14

1.2.8

Particle Shape and Size............................................................. 16

1.2.9

Shape ....................................................................................... 17

1.2.10 Particle Size .............................................................................. 20 1.3 Surface Modification ............................................................................... 26

1.4

1.3.1

Stearic Acid and Stearates ........................................................ 26

1.3.2

Coupling Agents ...................................................................... 27

1.3.3

Polymer Modifications............................................................. 27

1.3.4

Direct Bonding ......................................................................... 28

Particle Packing and the Maximum Packing Fraction .......................... 29 1.4.1

Introduction ............................................................................. 29

1.4.2

Determination of Maximum Packing Fraction (Pf) by Oil Absorption Procedures............................................................. 29

1.4.3

Particle Packing Theory ........................................................... 30 i

Particulate-Filled Polymer Composites

1.4.4

Applications of Packing Principles to Particulate Filled Composites .............................................................................. 33

1.5

Interparticle Spacing ............................................................................ 34

1.6

Particle Effects on the Structure of Polymers ........................................ 36 1.6.1

Introduction ............................................................................. 36

1.6.2

Molecular Weight Reduction During Processing ...................... 36

1.6.3

Molecular Weight and Crosslinking Changes due to Cure Modifications .................................................................. 37

1.6.4

Preferential Adsorption of Polar Species .................................. 37

1.6.5

Formation of an Interphase of Immobilised Polymer ............... 38

1.6.6

Effects on Polymer Conformation due to the Presence of Particle Surfaces and Interparticle Spacing ............................... 42

1.6.7

Effects on Crystallinity............................................................. 42

References ..................................................................................................... 45 2

Principal Types of Particulate Fillers ............................................................. 53 2.1

Introduction ......................................................................................... 53

2.2

Particulate Fillers from Natural Origins (Mineral Fillers) .................... 53 2.2.1

Introduction ............................................................................. 53

2.2.2

Minerals and Rocks ................................................................. 54

2.2.3

Rocks ....................................................................................... 55

2.2.4

Calcium Carbonate Minerals ................................................... 57

2.2.5

Dolomite .................................................................................. 61

2.2.6

China Clay or Kaolin ............................................................... 61

2.2.7

Calcined Clay .......................................................................... 66

2.2.8

Mica ........................................................................................ 69

2.2.9

Talc .......................................................................................... 70

2.2.10 Montmorillonite (AlMg)8(Si4O10)3-(OH)10.12H2O ................... 72 2.2.11 Barite (BaSO4).......................................................................... 73 2.2.12 Calcium Sulfate Products ......................................................... 74 2.2.13 Wollastonite (CaSiO3) .............................................................. 74 2.2.14 Crystalline Silicas ..................................................................... 76

ii

Contents

2.3

Synthetic Particulate Fillers .................................................................. 78 2.3.1

Carbon Black ........................................................................... 78

2.3.2

Synthetic Silicas ....................................................................... 81

2.3.3

Hydroxides and Basic Carbonates ........................................... 84

2.3.4

Precipitated Calcium Carbonate (PCC) .................................... 96

Acknowledgements ....................................................................................... 96 References ..................................................................................................... 97 3

Analytical Techniques for Characterising Filler Surfaces ............................. 101 3.1

Introduction ....................................................................................... 101

3.2

Acid-Base Theory ............................................................................... 104 3.2.1

Introduction ........................................................................... 104

3.2.2

Gutmann Approach ............................................................... 105

3.2.3

Drago Approach .................................................................... 106

3.2.4

Use in Characterising Fillers .................................................. 106

3.3

Analytical Techniques ........................................................................ 108

3.4

Reactive Techniques ........................................................................... 109

3.5

3.6

3.7

3.4.1

Flow Microcalorimetry .......................................................... 109

3.4.2

Inverse Gas Chromatography ................................................ 119

Spectroscopic Techniques ................................................................... 124 3.5.1

Introduction ........................................................................... 124

3.5.2

X-Ray Photoelectron Spectroscopy ........................................ 124

3.5.3

Secondary Ion Mass Spectrometry ......................................... 130

3.5.4

Diffuse Reflectance Fourier Transform Infrared Spectroscopy . 134

Methods for Examining Structural Order in Filler Coatings .............. 145 3.6.1

Wide Angle X-Ray Diffraction ............................................... 145

3.6.2

Differential Scanning Calorimetry ......................................... 146

Summary ............................................................................................ 147

References ................................................................................................... 148

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Particulate-Filled Polymer Composites

4

Surface Modification and Surface Modifiers ............................................... 153 4.1

Introduction ....................................................................................... 153

4.2

Reasons for Using Surface Modifiers ................................................. 153

4.3

General Principles of Surface Modification ........................................ 154

4.4

Methods of Using Surface Modifiers .................................................. 155

4.5

Choice of Coating Level ..................................................................... 156

4.6

Techniques for Determining the Amount of Coating Present, and Assessing the Amount Needed for Mono-layer Coverage ........... 158

4.7

4.6.1

Determination of Amount of Additive and it’s Distribution ... 158

4.6.2

The Monolayer and it’s Determination .................................. 160

4.6.3

Effects of Processing on the Coating Structure ....................... 163

Surface Modifier Types ...................................................................... 163 4.7.1

Monomeric Organic Acids and their Salts ............................. 163

4.7.2

Stearic Acid (CH3(CH2)16COOH ........................................... 166

4.7.3

Other Saturated Fatty Acids and Related Substances ............. 170

4.7.4

Effects of Stearic Acid Coating in Composites ....................... 171

4.7.5

Fatty Acid Salts ...................................................................... 173

4.7.6

Unsaturated and other Functional Organic Acids in Composites ........................................................................ 173

4.7.7

Polymeric Acids and Anhydrides ........................................... 175

4.7.8

Organo-silicon Compounds ................................................... 177

4.7.9

Examples of Silane Coupling Agent Effects in Filled Polymers ...................................................................... 190

4.7.10 Organo-Titanates and Zirconates .......................................... 191 4.7.11 Aluminates and Zircoaluminates ........................................... 198 4.7.12 Phosphates and Borates ......................................................... 199 4.7.13 Organic Amines and Amino-acids ......................................... 200 4.8

Conclusions ....................................................................................... 200

References ................................................................................................... 201

iv

Contents

5

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds ............................. 207 5.1

Introduction ....................................................................................... 207

5.2

Functional Characteristics of Compounding Machinery .................... 209

5.3

5.4

5.5

5.2.1

Transport of Feedstock .......................................................... 210

5.2.2

Melting and Shear Heating .................................................... 213

5.2.3

Mixing ................................................................................... 214

5.2.4

Melt Devolatilisation ............................................................. 217

5.2.5

Melt Pumping and Pressurisation .......................................... 218

Constructional Design of Compounding Plant ................................... 219 5.3.1

Low and Medium Intensity Premixing Procedures ................. 220

5.3.2

High-Intensity Compounding Machinery .............................. 221

Characterisation of Filled Compounds .............................................. 228 5.4.1

Introduction ........................................................................... 228

5.4.2

Residence Time Distribution .................................................. 229

5.4.3

Specific Energy Input ............................................................. 231

5.4.4

Screen Pack Analysis .............................................................. 231

5.4.5

Rheological Analysis .............................................................. 232

5.4.6

Ultrasonic Measurement ........................................................ 233

5.4.7

Microstructural Analysis........................................................ 235

5.4.8

Miscellaneous Methods of Analysis ....................................... 239

Process Enhancement of Particulate Polymer Composites .................. 240 5.5.1

Addition of Rigid Particulate Fillers ....................................... 241

5.5.2

Effects on Polymer Molecular Weight .................................... 243

5.5.3

Short Fibre-Reinforced Thermoplastics Composites .............. 244

5.6

Woodflour and Natural Fibre-Filled Thermoplastics ......................... 246

5.7

Supercritical Fluid Assisted Processing of Filled Compounds ............. 249

5.8

Processing of Thermoset Recyclate Waste Materials .......................... 250

5.9

Preparation of Silicate Layer Polymer Nanocomposites ..................... 251

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Particulate-Filled Polymer Composites

5.10 Conclusions ....................................................................................... 254 References ................................................................................................... 254 6

Effects of Particulate Fillers on Flame Retardant Properties of Composites ... 263 6.1

Introduction ....................................................................................... 263

6.2

General Effects of Fillers on Polymer Flammability ........................... 263

6.3

Fire Retardant Testing........................................................................ 264

6.4

6.3.1

Oxygen Index Test (ASTM D2863-87) .................................. 266

6.3.2

Underwriters Laboratory Vertical Burn Test (UL94 -1980) .... 266

6.3.3

Horizontal Burn Test (ASTM D635-88) ................................ 267

6.3.4

Ignitability Test (ISO5657 1986) ........................................... 267

6.3.5

Cone Calorimeter (ASTM E1354, ISO 5660) ........................ 267

6.3.6

Smoke and Corrosive Gas Tests ............................................. 268

Fire Retardant Fillers that Rely on Endothermic Decomposition ....... 269 6.4.1

Historical Background ........................................................... 269

6.4.2

Potential Endothermic Flame Retardant Fillers ...................... 270

6.4.3

Performance of Endothermic Flame Retardant Fillers ............ 273

6.4.4

Smoke and Corrosive and Toxic Gases .................................. 290

6.5

Nano-Clays ........................................................................................ 296

6.6

Ammonium Polyphosphate (APP) ...................................................... 297

6.7

Fillers for Use in Conjunction with Halogens .................................... 297

References ................................................................................................... 298 7

Particulate Fillers in Elastomers .................................................................. 303 7.1

Introduction ....................................................................................... 303

7.2

Uses of Elastomers ............................................................................. 303

7.3

Elasticity of Rubber ........................................................................... 303

7.4

Formulation of Elastomers ................................................................. 306 7.4.1

vi

General .................................................................................. 306

Contents

7.5

7.6

7.7

7.4.2

Selection of Polymer .............................................................. 306

7.4.3

The Curing System ................................................................. 308

7.4.4

Antioxidants and Antiozonants ............................................. 310

7.4.5

Coupling Agents .................................................................... 311

7.4.6

Process Oils and Plasticisers ................................................... 311

7.4.7

Fillers ..................................................................................... 313

7.4.8

Specialty Additives ................................................................. 314

The Performance of the Polymer ........................................................ 317 7.5.1

Specification of the Polymer .................................................. 317

7.5.2

Processing Considerations ..................................................... 321

7.5.3

Strength Characteristics of Polymers ...................................... 321

7.5.4

Compounding Considerations ............................................... 322

The Performance of Fillers ................................................................. 326 7.6.1

Reinforcement of Rubber by Fillers ....................................... 326

7.6.2

Processing Considerations ..................................................... 330

7.6.3

Compounding Considerations ............................................... 333

Filler Types ........................................................................................ 338 7.7.1

Specification of Fillers for Elastomers .................................... 338

7.7.2

Carbon Black ......................................................................... 340

7.7.3

Synthetic Silicas and Silicates ................................................. 343

7.7.4

Clay Minerals ........................................................................ 344

7.7.5

Calcium Carbonates .............................................................. 346

7.7.6

Alumina Trihydrate ............................................................... 348

7.7.7

Talcs ...................................................................................... 349

7.7.8

Natural Silicas ....................................................................... 349

7.7.9

Barytes and Blanc Fixe ........................................................... 350

7.7.10 Miscellaneous ........................................................................ 350 Acknowledgements ..................................................................................... 350 References ................................................................................................... 351

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Particulate-Filled Polymer Composites

8

Filled Thermoplastics .................................................................................. 357 8.1

8.2

8.3

8.4

8.5

viii

Introduction ....................................................................................... 357 8.1.1

Thermoplastics and Typical Applications .............................. 357

8.1.2

Thermoplastic Composites..................................................... 359

Bulk and Process Related Properties .................................................. 361 8.2.1

Specific Gravity or Relative Density ....................................... 361

8.2.2

Acoustic Properties ................................................................ 362

8.2.3

Melt Viscosity (MFI) .............................................................. 363

8.2.4

Compounding and Extrusion ................................................. 364

8.2.5

Thermal Conductivity and Specific Heat Capacity ................ 367

8.2.6

Thermal Expansion ................................................................ 368

8.2.7

Electrical Properties ............................................................... 369

8.2.8

Barrier Properties ................................................................... 370

Mechanical Properties ........................................................................ 371 8.3.1

Introduction ........................................................................... 371

8.3.2

Modulus – Tensile and Flexural ............................................. 372

8.3.3

Heat Deflection Temperature (HDT) ..................................... 374

8.3.4

Yield Strength ........................................................................ 375

8.3.5

Impact Strength (Toughness) .................................................. 376

Effects of Filler on the Polymer Phase ................................................ 380 8.4.1

Introduction ........................................................................... 380

8.4.2

Nucleation ............................................................................. 380

8.4.3

Transcrystallinity ................................................................... 381

8.4.4

Interphase .............................................................................. 382

Surface Science Aspects ...................................................................... 383 8.5.1

Introduction ........................................................................... 383

8.5.2

Surface Energy and Surface Tension....................................... 383

8.5.3

Wetting and Spreading ........................................................... 384

8.5.4

Adhesion ................................................................................ 384

8.5.5

Dispersion and Agglomeration .............................................. 387

8.5.6

Surface Treatments – Dispersants and Coupling Agents ........ 388

Contents

8.6

8.7

8.8

8.9

Aesthetics ........................................................................................... 390 8.6.1

Introduction ........................................................................... 390

8.6.2

Colour/Pigmentation .............................................................. 390

8.6.3

Surface Finish and Gloss ........................................................ 390

8.6.4

Scratch and Abrasion Resistance ........................................... 391

Stabilisation and Recycleability ......................................................... 392 8.7.1

Introduction ........................................................................... 392

8.7.2

The Effect of Filler Chemistry and Impurities on Stability ..... 393

8.7.3

The Effect of Antioxidant Adsorption on Stability ................ 394

8.7.4

Recycleability ......................................................................... 395

Uses of Filled Thermoplastics............................................................. 396 8.8.1

Uses of Fillers ......................................................................... 396

8.8.2

Fillers in PVC ......................................................................... 397

8.8.2

Uses of Fillers in Unplasticised PVC ....................................... 401

8.8.3

Uses of Fillers in Polypropylene ............................................. 403

8.8.4

Uses of Fillers in Polyethylene ................................................ 406

8.8.5

The Use of Fillers in Polyamides ............................................ 408

8.8.6

Polybutylene Terephthalate .................................................... 409

8.8.7

Polyethylene Terephthalate .................................................... 410

Conclusions ....................................................................................... 411

Acknowledgements ..................................................................................... 413 References ................................................................................................... 413 9

Filled Thermosets ........................................................................................ 425 9.1

Introduction ....................................................................................... 425

9.2

Brief Chemistry of Thermoset Polymers ............................................. 427

9.3

9.2.1

Free-Radical Chain-Growth Curing Resins ............................ 427

9.2.2

Step Addition Curing Resins .................................................. 435

9.2.3

Condensation Resins .............................................................. 440

Mechanical Properties ........................................................................ 444

ix

Particulate-Filled Polymer Composites

9.4

9.3.1

Modulus ................................................................................ 444

9.3.2

Fracture Toughness and Fracture Energy ............................... 450

9.3.3

Failure Stress .......................................................................... 463

9.3.4

Fatigue ................................................................................... 469

Applications ....................................................................................... 477 9.4.1

Cost Reduction ...................................................................... 477

9.4.2

Modified Mechanical Properties ............................................ 477

9.4.3

Exotherm Control .................................................................. 478

9.4.4

Shrinkage Control .................................................................. 480

9.4.5

Processing Aids ...................................................................... 481

9.4.6

Flame Retardants ................................................................... 482

9.4.7

Metal Fillers ........................................................................... 482

9.4.8

Structural Adhesives .............................................................. 483

Acknowledgements ..................................................................................... 483 References ................................................................................................... 484 10 Composites Using Nano-Fillers ................................................................... 489 10.1 Introduction ....................................................................................... 489 10.2 Scope ................................................................................................. 489 10.3 General Comments ............................................................................ 490 10.4 Nano-Filler Forms ............................................................................. 490 10.4.1 Regular Shapes ...................................................................... 491 10.4.2 Rods, Fibres, etc. ................................................................... 492 10.4.3 Platy Nano-Fillers (Nano-Clays and Related Materials) ........ 493 10.5 Summary and Future Perspectives ...................................................... 510 Acknowledgements ..................................................................................... 511 References ................................................................................................... 511 Abbreviations and Acronyms ...................................................................... 515 Author Index ............................................................................................... 521 Index ........................................................................................................... 527 x

Preface

I was delighted to accept the chance offered by Rapra Technology to produce this updated second edition of Particulate Filled Polymer Composites, a book first published in 1995. The first edition had been very well received but has been out of print for some time. Despite the relatively short time since the publication of the first edition, much has changed and hence considerable new material has been introduced, including a completely new chapter covering the latest developments in nano-filler technology. The use of particulate fillers in polymers has a long history, and they continue to play a very important role today. Despite the apparent commodity status, the area is still very dynamic and considerable changes have occurred in applications over the last few years. The most significant of these have been the dramatic growth in the use of precipitated silica in energy efficient tyre applications and the emergence of ‘nano-filler’ technologies. The multidisciplinary nature of this topic is well illustrated by the scope of the references in the book. These are drawn from many fields, including: mineralogy, crystallography, precipitation and crystal growth, powder technology, surface and colloid science, organic and organo-metallic chemistry, optics, materials science, and polymer science and technology. It also brings together people from many backgrounds, such as filler producers, machinery manufacturers, polymer compounders and suppliers of processing aids and surface modifiers. Together with my co-authors, I have set out to describe the fundamentals involved in producing, characterising and using particulate fillers in as clear and concise a way as possible. The authors have been encouraged to pay particular attention to those areas of their subjects which their experience shows cause the most confusion, or which are poorly covered elsewhere. Most importantly, where a topic is controversial, or poorly understood, they have been encouraged to interpret the existing literature, not merely quote opposing views, as is often the case. Where possible, use has been made of previously unpublished work to illustrate key points and extensive literature references are provided so that any subject can be followed up in depth, if needed. Chapter 4, on surface modification, is a good example of the approach taken. Most works concentrate on the organo-silanes. While they are still treated in detail here, significant space is also given to fatty acids and organo-titanates. Fatty acids are of considerable commercial importance and exhibit complexities that can cause problems

1

Particulate-Filled Polymer Composites for the unwary. Considerable controversy exists over the mode of action of the organotitanates and the available scientific literature is critically reviewed here. As with any editor, I am very much indebted to my co-authors for their hard work. I am also indebted to the staff at Rapra Technology, especially Frances Powers, for their help and support in turning the various contributions into a very well presented composite. Finally, I would like to express my particular thanks to the late Professor Derek Birchall, FRS, who first aroused my interest in this area and to the late Hugh Olmstead founder of Intertech Corporation and to my many friends and colleagues, especially those at Manchester Metropolitan University, Avecia Limited, Colin Stewart Minchem, CSIRO Division of Minerals, Electrolux and the Intertech Corporation, who have helped me to continue this interest in recent years. Roger N. Rothon July 2003

2

Contributors

David Ashton ICI Acrylics PO Box 34 Darwen Lancashire BB3 1QB David Briggs Siacon Consultants Ltd. 21 Woodfarm Road Malvern Worcestershire WR14 4RL Chris DeArmitt BASF Aktiengesellschaft KSP/S-E100 67056 Ludwigshafen Germany Michael Hancock 6 Poltair Avenue St Austell Cornwall PL25 4LY Graham Jackson LGC Ltd. The Heath Runcorn Cheshire WA7 4QD Peter Hornsby The Wolfson Centre Brunel University Uxbridge Middlesex UB8 3BH

Christopher Liauw Manchester Metropolitan University Centre For Materials Science Research John Dalton Building Chester Street Manchester M1 5GD Michael Orton 24, School Lane Hartford Cheshire CW8 1PE Roger Rothon Rothon Consultants and Manchester Metropolitan University 3 Orchard Croft Guilden Sutton Chester CH3 7SL David Skelhorn Performance Materials Division Imerys 100 Mansell Court East Suite 300 Roswell Georgia 30076 USA Howard Taylor Manchester Metropolitan University Department of Engineering and Technology John Dalton Building Chester Street Manchester M1 5GD

3

Particulate-Filled Polymer Composites

4

1

General Principles Guiding Selection and Use of Particulate Materials Roger N. Rothon and Michael Hancock

1.1 Introduction Particulate-filled polymer composites have a long history and consequently newcomers to the field usually expect to find an area of well-understood science with few intellectual challenges remaining. However, they are usually amazed to find that this is far from the truth in many areas, with few reliable generalisations (but several unreliable ones) available and much basic information yet to be established. This is largely due to the way in which the technology has developed, with different filler and polymer combinations tending to be developed largely piecemeal to meet the specific demands of various industries. The initial, and to some extent continuing, emphasis on cost reduction has also meant that many fillers have been poorly characterised. The purpose of this chapter is to give readers sufficient background so that they may approach the subject with an open mind and see the whole area as one with stimulating intellectual challenges, not as an area of mystery and witchcraft. This chapter discusses how fillers can be characterised and defined with the least ambiguity, and hence how their behaviour in composites can be understood and hopefully predicted. We also highlight the principal sources of misunderstandings and the limitations imposed by commonly used methods of measurement, and explain why and where deviations from general rules are likely to occur.

1.2 Basic Characteristics of Particulate Fillers In developing a particulate-filled composite, the formulator needs to be able to answer the following questions: 1. What property benefits are being sought? 2. What deleterious changes may also occur and can they be tolerated? 3. How easy is the filler to handle and how might it affect processing?

5

Particulate-Filled Polymer Composites 4. Are special additives needed? 5. What is the true cost of using the filler, is it justifiable, and are there more cost effective alternatives? Important information for answering the above questions includes, cost, purity, particle size and shape, density, hardness, optical and thermal properties and chemistry. The primary source of this information should be the filler supplier, although frequently data may be sparse. A brief description of each factor, its measurement and significance follows.

1.2.1 Cost In theory this is a fairly simple topic, but one that causes considerable problems for the unwary. It is widely assumed that polymers are expensive and fillers are cheap, and many articles on filled polymers start with the statement that fillers are used primarily to reduce costs. While this can often be the case, these savings are frequently not as great as anticipated and, in quite a few instances, compound costs, even with the lowest cost filler, can be higher than the unmodified polymer. There are two principal reasons for this. Firstly, the process of compounding filler into polymer costs money in the form of capital investment, manpower and energy [1]. In cases where compounding is essential, because other additives such as stabilisers or curatives have to be added to the polymer, then the cost of incorporating a filler is markedly reduced. In these cases, exemplified by elastomers, polyvinyl chloride (PVC) and thermosets, the use of fillers is the rule rather than the exception, unlike the case with, say, polyethylene. Secondly, prices of fillers and polymers are usually quoted in weight terms, while the majority of their composites are used on a volume basis. The specific gravity of most fillers is two to three times that of common polymers and, while raw material savings may accrue on a weight basis, more mass of the final compound will have to be used to achieve the same volume than would be the case with the unfilled polymer. When looking at potential cost savings, comparison of polymer and filler costs must therefore be made on a volume basis. On this basis one should generally regard the effective raw material costs of mineral fillers as two to three times their price (by weight). As a consequence many fillers will not give significant cost savings in the commodity plastics, such as polypropylene (PP) and this explains why the emphasis today is on achieving specific effects. However, while fillers may not ‘cheapen’ a filled composite they may well produce a cost-property performance that allows the composite to compete against and often replace more expensive systems.

6

General Principles Guiding Selection and Use of Particulate Materials

1.2.2 Chemistry, Composition and Impurities While the chemical nature of the filler is frequently of little direct importance to its use in composites, it plays two important roles, in that it determines the structure of the mineral, and also the nature of the interaction between polymer and filler. However, away from these a priori considerations, only in a few cases does the chemical reactivity of the filler play a significant role in the properties of a composite.

1.2.2.1 Bulk Chemistry Intrinsically fillers can be divided into two types, reactive and inert. Reactive fillers will react with their environment. A good example of this is gibbsite (aluminium hydroxide), which will react with both acidic and basic substances. Aluminium hydroxide also loses its water of crystallisation at around 200 °C and this enables it to provide fire retardancy in polymer formulations. The silicate minerals: (kaolin, mica, talc, quartz, etc.), are, in classical chemical terms, virtually inert, only being attacked by very strong acids and alkalis. The carbonate minerals and the hydroxide minerals are very reactive to acids. The interactions between the constituent elements in a filler determine its molecular structure and hence crystallinity, which then dictates all the intrinsic properties of the filler.

1.2.2.2 Surface Chemistry Filler surface chemistries are of more significance than the bulk ones, as they determine both the rate of wetting and the strength of interaction with polymers. They are invariably different from bulk chemistry but, unfortunately, they are poorly characterised for many fillers. Because of the interest in this very important topic, techniques for surface characterisation are covered in detail in Chapter 3.

1.2.2.3 Surface Interactions Polymers have a very much higher (20-30 times) thermal expansion coefficient than mineral fillers. Thus, in many well-dispersed, hot processed composites, a radial compressive stress develops as the polymer cools leading to an intimate interaction between matrix and filler. The value of this stress can be calculated, and depends on both polymer and filler [2]. Because of this, the interaction will at the very least be mechanical, but other types will exist depending on the surface chemistry of the filler and also the chemistry of the polymer.

7

Particulate-Filled Polymer Composites Wettability of filler by polymer is an indication of compatibility between the two. Wettability by polymer is not readily measured directly, but the effect that the material has on surface tension of a liquid is a measure of its surface energy. Usually water is chosen as the medium. Schlumf [3] has reported surface energies for a variety of fillers (Table 1.1). There have been a wide number of claims that surface energies determine the forces between polymer chains and different phases, determining mechanical properties of the composite. Adhesion at a polymer-filler interface has been shown to exert a considerable influence on mechanical responses, and a correlation between acid-base characteristics of filler and polymer (as determined by inverse gas chromatography) and properties has been established [4]. Lewis acid-base interactions between filler and polymer are claimed to be the most important component in the adhesion of a filler to polymers and thus in determining properties of composites [5]. For some filler-polymer systems, the strength of interfacial acid-base bonding may be appreciably enhanced by surface modification of the filler, or by modification of the polymer, giving large increases in properties.

Table 1.1 Surface energies of fillers and plastics Material

Surface energy (m Jm-2)

Diamond

10,000

Mica

240-500

Glass

1,200

Titanium dioxide Kaolin

650 500-600

Calcium carbonate

65-70

Stearate coated calcium carbonate

25-30

Talc

65-70

Polymers

15-60

Polypropylene

31

1.2.2.4 Chemical Analysis and Impurities The purity of a filler is of importance both commercially and technically, i.e., the users need to know what they are buying, whether it contains components that may be detrimental to the properties of their product and whether it will cause environmental problems. Impurities include trace elements that may be on the filler surface or in the structure of the filler and ancillary minerals, which will have been formed at the same

8

General Principles Guiding Selection and Use of Particulate Materials time as the main mineral (see Chapter 2). Sometimes additives from the filler manufacture may be present, (e.g., surfactants). The form of the impurity can be very important in determining it’s importance. Sometimes a potentially detrimental impurity can be tolerated in relatively high levels, if present in an inert form, but small traces of the same impurity, if present on the surface in an active form, can be very deleterious. Examples are certain transition metals, (e.g., iron, manganese, copper), which can seriously affect colour and thermal stability if present in an active form, even at levels of a few parts per million. Thus some micas and talcs can contain high levels of iron in an inert form, which is not detrimental to polymer ageing, while others can have lower levels of more active material. Some impurities may also pose health hazards, examples being crystalline silica and asbestos. Fillers are not always analysed for such low-level impurities and it is a mistake to believe that, if a possible deleterious impurity is not quoted in a data sheet, it is not present in the product. Conversely, it must be stressed that, if an element or substance is present in a non-active form, it may have little effect on polymer properties [6]. It is also necessary to point out that the way chemical composition is presented, can, and does, cause misunderstandings. Because of the methods of analysis used, many mineral compositions are quoted as a list of oxides rather than the actual constituent chemical. Thus a calcium carbonate may be specified as so many percent CaO or a clay may be reported in terms of SiO2 and Al2O3. This does not imply that any of the oxides are present in the free state. It should also be recognised that some standard methods of analysis do not detect very light elements.

1.2.3 Density or Specific Gravity This is one of the simplest of properties to define, understand and use. Density is mass per unit volume with SI (Système Internationale) units of kilograms per cubic meter. Specific gravity and relative density, the weight of the substance in relation to the weight of an equivalent volume of water, are often used synonymously. They are dimensionless but have the same numerical values as density. Density is determined by the size of the atoms forming the mineral, the closeness with which they are packed together and by impurities present in the crystal lattice: the heavier the atoms and the more tightly they are packed together, the higher the density. Thus, smithsonite, ZnCO3, has a higher specific gravity (4.3) than calcite, CaCO3 (2.7) because zinc has a much higher atomic mass than calcium, but magnesite, MgCO3, has a higher specific gravity (3.0-3.2) than calcite despite the fact that magnesium is lighter than calcium. This is because its ions pack much more closely together. In some fillers air is deliberately introduced into a matrix to reduce its density.

9

Particulate-Filled Polymer Composites True specific gravities of particulate additives can range from less than 1.0 for hollow glass beads to over 6.0 for some metallic fillers, but most lie in the range 1.6-2.8, while common polymers range from 0.9 to 1.4. Powder densities are of commercial importance, indicating ease of handling, and polymer compounding, and even affecting plant design. Values for bulk and packing densities are usually supplied by filler producers and are a measure of how the powder packs under various conditions. They are determined to some extent by the true density, particle shape and size of the filler, but the processing involved in its production also plays an important role. Surface treatments, and the methods used to apply them, can also be important. A powder with a low bulk density relative to its true density will contain a great deal of air. It will flow easily but will be difficult to incorporate into polymers, especially in equipment such as internal mixers and screw compounders.

1.2.4 Hardness Hardness is the resistance of a mineral to scratching. It is related to the structure of the mineral, the strength of the chemical bonds and the density of packing of its constituent atoms. F. Mohs, a mineralogist drew up a table of hardness in 1812 called the Mohs Hardness Scale (Table 1.2), which is still used to rank minerals by their resistance or susceptibility to scratching by other minerals. This scale is widely used for mineral fillers, but care must be taken in using the data because the scale is not linear, with the differences becoming greater at the high end. The hardness of most common particulate fillers range from 1 to 4. Talc is the softest, with a value of one.

Table 1.2 Mohs scale of mineral hardness 1

Talc (softest)

5

Apatite

8

Topaz

2

Gypsum

6

Orthoclase

9

Corundum

3

Calcite

7

Quartz

10

Diamond

4

Fluorite

Hardness must not be confused with toughness, which relates to the ability of the mineral to resist fracture. Hard minerals can often fracture very easily, for example, by cleavage along crystal planes.

10

General Principles Guiding Selection and Use of Particulate Materials Where mixed fillers are used, if one is much harder than the other, then abrasion of the softer one may occur. This is of particular importance when hard particulates are used with easily damaged glass fibres. Minerals such as talc are soft because they readily delaminate due to weak structural features (see Chapter 2). Such soft particles can fracture or cleave when polymer composites are deformed, thus limiting their reinforcing properties [7].

1.2.5 Abrasiveness Abrasion due to the filler has quite an important role in the processing of filled polymers. It has several origins. Mohs hardness is a major factor with the harder minerals, such as quartz and wollastonite (CaSiO3), approaching, or even exceeding the hardness of the metals used in processing. Large particles and angular particles with sharp edges are particularly detrimental, probably due to a scratching mechanism, followed by attrition of the exposed edges. Sometimes the coarse component of a filler may be predominantly a harder impurity, which leads unfortunately to a combination of the two mechanisms. Chemical attack may also be caused by some reactive fillers, leading to etching and progressive erosion.

1.2.6 Optical Properties The colour, opacity and gloss of a composite are very important, both aesthetically and functionally, and will be strongly affected by the incorporation of fillers. When light strikes any surface, it is subjected to several effects: absorption, scattering, reflection, polarisation and interference. The extent of each effect is dependent on the nature of both exit and entrance media, but for all practical purposes only three media, air, filler and polymer, will be considered here in a superficial treatment. The reader is referred to works such as by Bohren and Huffman for a detailed explanation [8]. A light ray incident upon an air-solid interface at an angle, i, will be bent or refracted somewhat normal to the interface, at an angle of refraction, r. A relationship of the incident angle, i, and the refracted angle, r, sin i/sin r, is constant for a given isotropic solid. The constant always denoted as n is called the refractive index. The refractive index of a filler is one of the most important parameters affecting the optical properties of a filled polymer system and is always quoted relative to air. It is, in fact, a manifestation of the ratio of the speed of light in air to its speed in the solid.

11

Particulate-Filled Polymer Composites The refractive index of a mineral is the sum of the specific refractive indices of all individual components normalised for concentrations. Simplifying, it can be said that the refractive index of a solid is directly related to its density. When light strikes a surface, some will be reflected with the amount reflected being determined by the density of the constituent atoms and is thus affected by the refractive index, n. Reflection (r) and transmission (t) coefficients are given by: r=

1− n 1+ n

t=

2 1+ n

(1.1)

To a first approximation this leads to the fraction of incident light being reflected as simply (n – 1)2/(n + 1)2 and approaches zero as the refractive index approaches unity. The response of a plane surface to an incident beam of light is complicated, as briefly described previously. Mineral fillers, however, complicate the subject even more deeply in that they are a priori made up of particles that usually have sizes in the order of the wavelengths of visible light. Thus large surfaces will reflect and transmit the light waves but particles will scatter them. Therefore, light entering a typical powdered filler, comprising many small particles, will experience reflection, refraction, diffraction and interference at each air - particle interface. Refraction into the filler particles can lead to absorption of the light (the radiation absorbed will be converted into another form such as heat) and a measure of this is the absorption coefficient. The remainder of the light is diffused by scattering and the measure of this is its scattering coefficient. The colour (sometimes referred to as brightness, reflectance or whiteness) is related to absorption and scattering coefficients by Kubelka-Munk equations [9]. The reader is referred to the original paper and to some of the speciality books on the subject [10] for more details. The level of scattering is very dependent on particle size, so that finer fillers appear whiter than coarser ones: as an approximation, scattering in air is at a maximum when the particle size of powder is one-third to one-half the wavelength of the radiation. Levels of reflectance and refraction, and hence scattering, are directly related to the refractive index of the mineral. When incorporated into a polymer matrix, the ratio of its refractive index to that of the polymer will strongly affect the optical properties of the composite. As the ratio approaches unity (which is the case for most common polymers and filler): reflection at the interfaces between filler and polymer approaches zero. This means that light scattering is reduced dramatically and light absorption becomes obvious. Mie’s theory [11] now widely used for particle size determinations from light-scattering data, predicts that as differences in refractive indices become small, not only does scattering become less but particle size effects become less and maximum scattering moves to larger sizes, often several times greater than the wavelength of the light.

12

General Principles Guiding Selection and Use of Particulate Materials A filler such as titanium dioxide, having a refractive index much higher than that of the polymer, will therefore exhibit considerable scattering and will give white products, unless its absorption coefficient is high in which case dark, opaque composites will be produced (carbon black or black iron oxide are the ultimate in this instance). Most fillers are white or off-white powders due to the reflection of visible light. This reflection consists of two components, absorption and scattering, and, as discussed previously, most fillers have low absorption but high scattering coefficients. However, the amount of light scattered depends on the refractive index of the mineral relative to the medium in which it is measured. In air the differences in refractive indices mean that most minerals scatter light and appear white. Most plastics, on the other hand, have refractive indices close to those for minerals (1.5-1.6) and much of the scattering is lost (the light, in fact cannot ‘see’ the boundary between filler and polymer). There are a few cases in which mineral and polymer have almost identical refractive indices. In these instances, scattering virtually vanishes and the composite becomes almost transparent until loadings are increased to a level at which particle-particle interaction occurs [12]. This transparency is of commercial importance in several applications such as silica-filled styrene-butadiene rubber (SBR) shoe soles, and calcined clay- and glass fibre-filled unsaturated polyester roof panels. On the other hand, for example, calcite has a refractive index of 1.6, which is sufficiently different from most of the common plastics for some scattering and hence pigmentation to occur, but whiteness and opacity will only become very noticeable at high loadings and with thick products. With the loss of light scattering, light absorption becomes the dominant optical property of a mineral-filled plastic. Light is absorbed by transition metal ions, with different wavelengths being absorbed by different species of ion, and by naturally occurring organics, which are poly-aromatic species with strong absorption across the whole spectrum (that is they appear brown or black). The impurities in mineral fillers are mainly transition metal ions, particularly iron, and humates or lignates. Colours due to these become frequently more marked in a plastic. Some minerals exhibit birefringence with refractive indices different in differing directions, due to an asymmetric crystal structure. Plastics filled with these are dichroic and exhibit unusual visual appearances. As well as being a direct cause of the colour in a filled plastic, a filler may indirectly generate colour by either degrading the polymer (forming radicals and unsaturated species) or by deactivating antioxidants/stabilisers, etc., used in the polymer. In some cases, breakdown of filler surface treatments can also generate colour problems. Filler effects can be observed in most thermoplastics but are particularly relevant in PVC where dehydrochlorination, giving a polyene conjugated structure, is readily promoted by Lewis acid sites on a mineral surface.

13

Particulate-Filled Polymer Composites During the processing of filled thermoplastics, but particularly during their injection moulding, white markings are observed at points where high melt strain rates are encountered. These are especially apparent around the sprue and are very much more noticeable than for unfilled plastics. Some may be due to water or gas bubbles (splash markings), some to poor dispersion or filler separation, but most seem to result from flow patterns. These have been frozen in a strained state before they have had time to relax due to the high thermal conductivity of filled polymers [13]. When filled polymers are strained, they whiten to some extent. This is partly due to orientation of polymer chains around filler particles but also to light scattering from vacuoles formed as the polymer is pulled away from the particle surfaces. Optical properties can also be affected by the fact that filler particles can affect the morphology of the plastic by nucleating its crystallisation at the interface. Thus a composite with different light scattering properties may be obtained.

1.2.6.1 Spectral Absorption The ultraviolet and visible spectra of the common fillers are largely featureless, unless there are significant levels of transition metal ions in the mineral, when strong absorptions will occur. However, the most important practical aspect is the infra-red (IR) spectrum of a filler. This arises from the nature of the chemical bonds present and will be unique for each filler type. A common use of IR spectroscopy is as an analytical tool to identify a filler. An important practical consequence of a filler’s IR absorption is the use of certain fillers in horticultural and agricultural films (see Chapter 8). Spectra of some of the more common minerals incorporated in a lowdensity polyethylene (PE) film are shown in Figure 1.1. Calcium carbonate shows only two strong absorption bands because the only energy absorptions occur in the CO3; ion (bending, stretching and deformation modes). Kaolin, a complex aluminosilicate, shows a much more complicated IR spectrum: after calcining, the —Al—O—Si— bonds rearrange and the resultant product has a very complex IR pattern with several broad, overlapping bands.

1.2.7 Thermal Properties Leaving aside the special cases of phase change and decomposition, the principal thermal properties of interest are specific heat, thermal conductivity and coefficient of expansion.

14

General Principles Guiding Selection and Use of Particulate Materials

Figure 1.1 Infra-red spectra of mineral filled polyethylene films

1.2.7.1 Specific Heat This is usually defined as the energy required to raise 1 gram of material by 1 K with SI units of J.kg-1.K-1. Specific heats are easily measurable and available for most particulates [14]. Where they are not, rules exist for estimating the value [7]. The volume specific heats of most inorganic fillers are similar to the common polymers and the rule of mixtures gives a good approximation to the values found in composites.

1.2.7.2 Thermal Conductivity This is the rate at which heat energy is transmitted through a substance, SI units are W m-1 K-1 but it is usually quoted as cal cm-1K-1. Mineral fillers are about an order of magnitude more conducting than most polymers, while metallic fillers are even more so. Filled composites are therefore more conductive in general than the base polymer. The simple law of mixtures is only applicable at low concentrations because, as with electrical conductivity, particle-particle contact will occur at high loadings leading to a sudden

15

Particulate-Filled Polymer Composites sharp increase in conductivity, the so-called percolation threshold [15], which is dependent on particle shape and packing. This increased conductivity is of considerable importance in some processing operations, especially injection moulding.

1.2.7.3 Coefficient of Thermal Expansion As the name implies, this is the rate at which a material changes volume on heating or cooling. SI units are K-1. Most polymers have coefficients of expansion at least an order of magnitude greater than mineral fillers: thus mineral-filled composites have lower coefficients of expansion than unfilled polymers and, in well-annealed systems, the rule of mixtures applies unless strong bonding occurs between filler and polymer. Some fillers have a negative coefficient of expansion and are used to achieve composites with zero expansion. Other fillers show anisotropy, that is they have coefficients that are direction dependent. The differences in expansion between particle and matrix can result in intimate contact between them in a composite and consequently considerable stresses occur. These can have the same properties as chemical bonding at low strains and can mask any effects of changes in polymer-filler interactions in some tests.

1.2.8 Particle Shape and Size The importance of these factors is felt at all stages of composite production and use. They affect powder flow, compounding behaviour, composite viscosity, and mechanical, thermal and optical properties. Indeed, most of the current predictive equations for the properties of filled composites use shape and size factors, often determined using model particles such as glass spheres and flakes. Unfortunately such particles are rare in the real world and it is important in applying equations to appreciate the limitations imposed by problems of adequately measuring and describing the morphology of fine particles. One of the principal difficulties is due to the ability of many fillers to exhibit a variety of particle shapes and sizes depending on the work done in dispersing them. Their ‘effective’ shape and size can therefore vary at any stage of composite formation and use. In principle one would like to characterise them in situ. This is, however, far less easy than characterising the initial particulate material itself and one is usually reduced to trying to carry out measurements under conditions that will represent as near as possible those encountered in use. In this context, the concept of ‘effective’ particle, which is the size and shape achieved in the actual application, is a very useful one to keep in mind and is returned to later in this chapter.

16

General Principles Guiding Selection and Use of Particulate Materials

1.2.9 Shape 1.2.9.1 Introduction Particle shape (as will be discussed in following chapters) is very important in determining the stiffness, or rigidity, of a composite, the flow and rheology of a melt or liquid, tensile and impact strength, and the surface smoothness of a component, i.e., many of the important properties of a composite. Shape is determined by the genesis of the filler, by its chemistry, its crystal structure and by the processing it has undergone. Unfortunately, it is usually poorly defined, the literature abounding with vague terms such as roughly spherical, blocky, irregular, platy, acicular, etc. Some typical particles likely to be found in fillers are shown in Figure 1.2. All the fillers commonly used are microscopic in size imposing major difficulties both in how to measure, and then how to describe and quantify shape in any simple yet meaningful way. In addition, there are problems of distinguishing between primary particles, agglomerates and aggregates that occur in a filler, especially in synthetic materials, in which a ‘structure’ is sometimes deliberately designed. The subject is complicated further because aggregates and even particles may break down during processing (see Section 1.2.10.3 for a definition of these various terms).

Figure 1.2 Some particle types likely to be found in common fillers

17

Particulate-Filled Polymer Composites Despite the complex problems outlined previously, fillers are used because of their shape in a very wide range of polymers to give specific properties. For example, conventional clays are used in hose and chemical lining because their shape reduces permeability to fluids; platy talcs give rigidity to PP; the complex aggregate structure of precipitated calcium carbonates contributes to the structure of liquid polysulfides; and the special structured shape of many carbon blacks and synthetic silicas is important to their performance in elastomers. A further example is the emerging use of very high aspect ratio, nano-clays as reinforcing, fire retarding and gas and fluid barrier fillers.

1.2.9.2 Origins of Particle Shape Synthetic products will have their shape determined by their chemical composition and by the production conditions. For example, precipitated calcium carbonate (CaCO3) can be produced with different shapes by changing precipitation conditions to produce aragonite, calcite or vaterite. These conditions can be chosen to produce single crystals or complicated aggregates, which can be modified during drying and milling. Another example is the very complex shape of most carbon black particles arising from the partial fusing and solidification of pyrolysing droplets into three-dimensional chain-like aggregates during manufacture. The external shape of a natural mineral is a manifestation of its crystal structure, which will be briefly discussed in Chapter 2. It is also dependent on the environmental conditions in which the mineral was formed. If allowed to grow without constraint, then the particles are bounded by crystal faces, which are disposed in a regular way such that there is a particular relationship between them in any one mineral species, which is derived from regular atomic arrangement. However, under pressure, temperature or the effects of impurities, the crystal may adopt different shapes or habits. These include cubic, fibrous (fine, long, needles), acicular (needle-like), lamellar (plate-like) and prismatic. It is very unusual for perfect crystals to be found, but even poorly formed ones will always show evidence of their intrinsic symmetry. Many fillers (as will be discussed later) being extracted from the earth or rocks and processed by fairly simple methods will exhibit the same basic original shapes. Others, however, may be extracted by complicated routes or involve intensive comminution, thus altering particle shape. This is determined by the chemical nature and strength of the bonds between the atoms and groups in the mineral. These depend on electronegative differences between atoms and the bonding can range from covalent through to ionic. Two other types of interaction or bonding are important. Hydrogen forms co-ordinate bonds with atoms with very high electro-negativities, such as oxygen. This is very important in hydrated minerals such as kaolinite and gibbsite,

18

General Principles Guiding Selection and Use of Particulate Materials with layers being bonded together by these relatively weak hydrogen bonds. In ‘neutral units’ another weak force, known as Van der Waals bonding, due to non-uniform charge distributions, bonds group together. The types and strengths of bonds present in the crystal determine the methods by which the minerals break down on processing. Thus in the calcite crystal, the weakest bonds exist between the calcium ion and the carbonate group and these cleave invariably, giving rise to a regular rhombohedral shape. Kaolin platelets, held by hydrogen bonds, separate more readily from each other rather than fracture. In talc, magnesium silicate layers are held together by weak Van der Waals forces and very readily undergo cleavage into progressively thinner plates.

1.2.9.3 Assessment and Measurement of Shape The shapes outlined previously are very difficult to specify precisely using Euclidean geometry, but various approaches have been attempted to describe shape, anisotropy and ‘structure’ as numbers. Structure is particularly interesting and, while only really recognised currently in the elastomer field, it probably has significance, as yet overlooked, in other areas. In synthetic fillers it is sometimes difficult to separate fundamental from aggregate shape but, where there is sufficient incentive, then ways will usually be found to overcome such difficulties. Such incentives arose in carbon blacks and more recently in precipitated silica, where shapes are very complex, but an understanding is critical to their high value usage in the tyre industry. Much work has been done, especially by Medalia and Heckman [16], and by Hess and co-workers [17], to develop automatic image analysis procedures. Using such procedures, all the aspects described previously have been investigated. This work has much to teach us about other filler particles. Fractal geometry is now showing great promise for describing the shape of various complex, irregular particles. As usual, the carbon black industry is leading the way in exploring the potential of fractals for describing fillers and promising results are already emerging. A good introduction to fractal geometry and its applications to particles can be found in the excellent work by Kaye [18], and a number of recent papers have explored the measurement and significance of fractal dimensions of filler particles including carbon black [19-21]. The importance of measuring ‘structure’ results from its role in describing the ability of a particle to trap and partly shield a portion of the polymer matrix from deformation. This is generally known as an occluded polymer and its presence is important, especially in many of the unique properties of carbon and silica-filled elastomers. A simplified picture is given in Figure 1.3 and structure can be thought of as increasing the effective filler volume. While the carbon black industry is leading the way in using sophisticated techniques to measure shape and assess structure, they are also prepared to use fairly simple measurements, and in particular the oil absorption procedure with certain refinements, to give a quantification of this parameter [22].

19

Particulate-Filled Polymer Composites

Figure 1.3 Schematic illustration of the effect of particle structure and occluded polymer

The shape of simple particulate fillers is usually expressed as its aspect ratio (AR), which is the ratio of the particle’s diameter to its thickness. Measurement of aspect ratio is relatively simple for large particles, although time consuming. But, for the micrometer-sized particles that are normally encountered in fillers used in polymers, it is difficult, needs expensive equipment (such as scanning electron microscopes), and is very time consuming, depending on the spread of aspect ratios normally found. Other techniques are being sought to give an easier, more rapid measurement. Fourty reports that a comparison of particle-size distribution curves measured by sedimentation and laser diffraction techniques can give a shape factor [23]. Electrical conductivity of an aqueous suspension of a mineral under flow compared with it at rest (after the particles have randomised) gives a mean value for AR or a shape factor. Reasonable agreement with electron microscopy has been found [24]. This complex situation is further complicated by the fact that not only is shape often dependent on size, but measurement of size is affected by shape. This interdependence is discussed next and also in the descriptions of individual minerals given in Chapter 2.

1.2.10 Particle Size 1.2.10.1 Introduction Particle size is a very important, property of a filler, but is a particularly complex area where great confusion still occurs. For an individual, naturally occurring, filler it will have been determined by the origin and mineralogy of the deposit from which it has been extracted, by the method used in mining, and by separation procedures used during processing. For synthetic fillers, size will be determined by the conditions used

20

General Principles Guiding Selection and Use of Particulate Materials in its synthesis such as precipitation and possibly by the drying and any coating procedures. Size is one variable that can be controlled (in theory at least), and its importance is felt at all stages of composite production and use. Hence, there is considerable interest in its measurement. Particle size distributions are more useful than single average values, although the latter have the merit of simplicity. Size is an easy property to measure reproducibly using a variety of techniques including sieving, sedimentation, optical scattering and diffraction from particulate suspensions. Each, however, has a different dependence upon particle dimension and varies with particle shape. Thus correlation between different minerals measured by different techniques, in different laboratories is difficult. Again the reader is referred to one of the specialised books on the subject [25]. The problems arise from the fact that most filler particles are irregular in shape and contain a wide range of sizes, some of which will be individual particles and others agglomerates (also with a different shape from the individual particles). In such instances it is not possible to describe fully the particle size by a few numbers, as is often attempted. Commercially, most producers will quote two or three values to indicate the size of their fillers (see Section 1.2.10.2 and Chapter 2). Thus the data used in published studies are often a poor approximation of the real situation and can be very misleading. Wherever possible, the reader should try to determine and to use the full particle size distribution curve and be very cautious in interpreting literature studies in which only limited data such as average particle sizes or surface areas are given. In particular very small amounts of particles above or below a critical size can sometimes dominate certain properties, (e.g., viscosity, fatigue, tensile and impact strength) but will not be apparent in many particle size determinations. An interesting example of the effect of small amounts of oversize particles on the properties of carbon black-filled elastomers has been given by Gent [26].

1.2.10.2 Particle Size Measurement As already stated, filler suppliers usually quote three or four properties as a measure of the particle size: 1. Top cut. This is the size below which the majority (usually 99% or 99.9% by weight) of the particles are finer. 2. Coarse particles may also be reported as that percentage above a certain size. 3. Average particle size is obtained from particle size distribution curves and is the size that 50% of the particles are below. This can be defined in a variety of ways, e.g., by weight, volume or number, and care must be taken to understand the definition being used.

21

Particulate-Filled Polymer Composites 4. Specific surface area. This is determined by a number of adsorption techniques and is affected by size, shape and structure of the filler particles. Coarse particles, i.e., particles above 40 μm in size are usually quoted separately and are measured by screening the filler through standard mesh sieves. This may be carried out in aqueous suspension or as a dry powder. If the filler is heavily agglomerated then erroneous results may be obtained especially in dry screening. For accuracy, aqueous measurements are preferred with the filler being completely dispersed both by mechanical means and also using a dispersing agent. An idea of the structure and ease of dispersion of a filler may be obtained by comparing screen residues obtained by both wet and dry screening. If the filler is hydrophobic, then difficulties will be encountered in wet screening. These may be overcome by dispersing the filler in an organic liquid such as butanol, but a wetting agent and a dispersing agent are often both required. For some applications it is important to know the amount of particles that exceed 10 μm. This may be obtained by sieving but practical difficulties due to sieve screen blinding can occur. The most widely used techniques in this case involve sedimentation in a fluid, usually water and applying Stokes’ law. This states that, under standard equilibrium conditions, the time, t, taken for a particle to settle to a fixed depth is inversely proportional to the square root of its spherical diameter, d. This is given in Equation 1.2: ⎡

1/ 2

⎤ ⎥ d = 10 ⎢ ρ s − ρ f gt ⎥ ⎣ ⎦ 4⎢

(

18ηl

)

(1.2)

where d is the particle diameter, η is the viscosity of the fluid, ρs and ρf are the densities of the solid and the fluid, respectively, g is the gravitational constant and t is the time for the particle to settle a distance, l. Diameters obtained in this manner are precise descriptions of the particles but will compare with the results of other techniques only when the particles are perfectly spherical. In practice, most fillers have irregularly shaped particles and it is common to interpret experimental data in terms of theories applicable to spherical particles. Dimensions obtained are then for ‘equivalent spherical diameters’ (esd), which are the diameters of spheres that would give the same behaviour as that obtained from the sample by the method in question. Specific surface area is usually measured by the quantitative adsorption of nitrogen following the procedure originally described by Brunauer, Emmett and Teller, and known as the BET method (Equation 1.3) [27]: 22

General Principles Guiding Selection and Use of Particulate Materials P V (Po − P )

=

C −1 P 1 + VmC VmC Po

(1.3)

where V is the volume of gas, under standard conditions, adsorbed at equilibrium pressure, P; Vm is the volume of gas necessary for monolayer coverage; Po is the saturation pressure; and C is the BET constant. Often only a single point measurement is made to obtain surface area, but it is much more reliable to measure adsorptions at several pressures and then plot P/ V(Po – P) against P/Po. Permeability of a bed of packed particles to a fluid can also be used to give a surface area [28]. However, there are major sources of difficulty and confusion in describing particle size and shape. These include adequately defining what subdivision of material is to be described as the particle, in relying on particle size measurements carried out on the powdered particulate as an adequate description of it in a composite, and how shape and size interact.

1.2.10.3 Primary Particles, Agglomerates and Aggregates It surprises many people that a variety of ‘particles’ can exist in a powder, each of which can be determined as the particle size under certain conditions of dispersion and measurement. Moreover, the appropriate particle size for consideration can itself vary according to whether one is dealing with powder flow, behaviour during compounding and dispersion, or the properties of the final composite. These different types of particles are generally described as primary or ultimate particles, agglomerates and aggregates. In some instances these particle types are readily distinguishable but in others there can be appreciable overlap. Before discussing the types of particle in detail it is necessary to clarify the terminology as two contradicting conventions are widely used, and this in itself is a cause of considerable confusion. The need is to distinguish between collections of particles that are weakly and strongly bonded together. In this book we shall use the term agglomerate for weakly bonded particle collections and aggregate for strongly bonded ones. This is opposed to the views of Kaye [18] based on the derivations of the two words. The chosen terminology is, however, at least as widely used and is especially prevalent in the carbon black industry. The reader should always ascertain which terminology is in use when reading articles in the literature. An idealised view of particle type and breakdown with work during composite formation is presented in Figure 1.4. This goes beyond primary particles and considers the case where fragmentation of these may occur, e.g., hollow glass beads during thermoplastic compounding. With such simple systems it is usually fairly easy to match the particle sizing method to the degree of dispersion expected in the composite and obtain realistic answers. 23

Particulate-Filled Polymer Composites

Figure 1.4 Idealised view of the way filler particles disperse and of the different forms of particle types that might be encountered

Unfortunately, many filler systems do not exhibit such simple profiles, and the steps shown are often less sharp and overlap. Some possibilities are shown in Figure 1.5. A further complication arises when agglomerates form from initially well-dispersed systems. These agglomerates are sometimes referred to as flocs and can arise due to loss of colloidal stability in polymerising systems, or to reticulation (filler network formation) above the glass transition, especially in cured elastomers, an effect often observed with carbon blacks. The most difficult situation to deal with is quite frequently met with synthetic products, especially those formed by precipitation. This is where quite strong, complex aggregates are present, in addition to agglomerates. These aggregates often break down slowly, leading to a drawn-out step in the effective size profile. Frequently, they do not fully break down to primary particles under normal processing conditions. The effective particle size will then be critically dependent on the exact processing conditions and will be very difficult to predict in advance. Precipitated calcium carbonates are a good example where this type of situation is encountered. Most particle sizing techniques attempt to break down powders to their primary sizes by use of intensive mixing, ultrasonics and dispersants, but even these can be insufficient. Curves of the type shown in Figures 1.4 and 1.5 would be very useful for predicting filler behaviour, especially if the work input could be related to different compounding procedures. In principle, this should be possible, at least in a semi-quantitative way, by varying the energy used in dispersing the powder. Such procedures have been found

24

General Principles Guiding Selection and Use of Particulate Materials

Figure 1.5 Complex particle dispersion behaviour, as often encountered with fine, precipitated fillers

valuable by one of the authors and some very useful preliminary work of this type has been reported by Thoma and co-workers [29]. Some typical results obtained by the author (RR) with a coated precipitated calcium carbonate are given in Table 1.3 (Note only average particle size values are given for illustrative purposes, the earlier comments about

Table 1.3 Effect of measuring conditions on the apparent particle size of a coated precipitated calcium carbonate Measuring Condition

Average particle size (μm)

Comments

Laser diffraction of organic dispersion using weak ultrasonics

20

Detecting agglomerates

Laser diffraction of organic dispersion using medium ultrasonics

4

Detecting basic aggregate size

Laser diffraction of organic dispersion using strong ultrasonics

0.2

Detecting some form of sub-aggregate structure

Electron microscopy

0.07

Detecting primary crystallite size

X-ray line broadening

0.07

Detecting primary crystallite size

25

Particulate-Filled Polymer Composites the dangers of using a single value for a distribution still apply). Furnace-type carbon blacks provide a good example of the problems involved with concepts and terminology in this area. Such carbon black particles are formed by partial fusion and solidification of very small spherical units. These spherical units are generally referred to as the primary particles. Under no compounding or particle-sizing conditions, however, is it possible to break down the actual particles completely into these ultimate units. Unfortunately, some breakdown of the fused structures does occur, depending on the processing conditions, so that it is not appropriate to consider these fused structures as the ultimate particle either. In view of these complexities, the best rule is to keep the basic principles as outlined previously firmly in mind when trying to relate particle size as determined on a powder to what happens in an actual composite. As mentioned before, many of the previous problems would be removed if there were simple and reliable methods for assessing the ‘effective’ particle size in the polymer matrix. Various methods can be used including microscopy and contact radiology, both coupled with image analysis, and recovery from the polymer followed by conventional sizing techniques, although none are completely satisfactory. A more detailed discussion of some of these aspects will be found in Chapter 5. In the authors’ opinions the understanding of filled composites would be greatly improved by advances in this area.

1.3 Surface Modification Surface modification of fillers to give improved properties to a polymer composite is a topic that has received enormous attention over the last 30 years. Improvements in mechanical properties, dispersion of the filler (which leads to improved properties), improved rheology and higher filler loading have all been reported to accrue from rendering the surface more hydrophobic and hence compatible with the polymer or by enabling the filler to bond covalently, through hydrogen or ionic bonds to the polymer; or by changing the physical nature of the interface so that energy absorption can occur. This section will deal mainly with mineral surface interactions. The use of surface modifiers is dealt with at length in Chapter 4 and specific examples are described in Chapters 2, 7, 8 and 9.

1.3.1 Stearic Acid and Stearates The most widely used surface modification is treatment with stearic acid. This is believed to result in a stearate salt coating on most fillers and metal stearates are also used. Stearic acid will react with basic minerals to give a surface that is covered with strongly bonded long organic ions (this is discussed in more detail in Chapter 4). Stearic acidmodified silicates are commercially available but in these cases the stearic acid is almost

26

General Principles Guiding Selection and Use of Particulate Materials certainly weakly adsorbed and probably desorbs during melt compounding. Similarly, metal stearates will form weak bonds with mineral surfaces and desorb from them. Organo-amines have been used to render silicate surfaces hydrophobic, bonding probably through strong coordinate bonds. These relatively labile coatings will help improve dispersion in the initial stages of a compounding operation, before they desorb. They also give a protective coating to the filler, minimising any polymer degradation that may occur before stabilisers, antioxidants, etc., are fully dispersed.

1.3.2 Coupling Agents Surface treatments with bi-functional additives, which form very strong covalent bonds to the filler and then bond to a polymer by a variety of mechanisms, are widely available. They are based on organo-metallic compounds with the general formula: (RI)a M(RII)b

(1.4)

where M is a metal ion with valency a + b; RI is an organic group, the choice of which depends on the polymer in which it is to be used and RII is a group designed to react with a mineral surface. The most commonly encountered compounds are organo-silanes [30] and organotitanates [31], but others, including organo-zirconates [32], organo-borates [33] and complexes of chromium [34] have been proposed. R II in most of the coupling agents is an alkoxy group (methoxy, ethoxy or 2-methoxyethoxy are common), which is reactive to hydroxyl groups and to water. The preferred reaction mechanism is for hydrolysis to occur, initially by reaction with environmental or adsorbed water, with the kinetics being affected by the nature of both the alkoxy and other reactive groups on the molecule. The organo-metallic hydroxide then condenses with hydroxyls on the mineral surface or it can form oligomers followed by polymers by self-condensation reactions [35]. Multiple layers of many coupling agents can be adsorbed onto a mineral surface, with the packing of the coupling reagent on the surface being determined by the size of the RI organic group [36]. The structure of these layers can be very complex, with both strongly and weakly adsorbed species being present. This is discussed in detail in Chapter 4.

1.3.3 Polymer Modifications To try to improve energy absorption at the interface, and hence improve composite toughness, modification of the filler with a polymeric, low modulus, interlayer has

27

Particulate-Filled Polymer Composites been reported [37]. There have been many papers published on this technique. Aivazyan and co-workers [38] review the surface modification of mineral fillers by polymer deposition, using polymer adsorption from solution, mixing of polymer with dispersed filler and hetero-coagulation of latex polymer particles on the filler. Good bonds between coating and filler are often achieved by grafting a reactive group on to the interfacial polymer before or during filler treatment. Thus, kaolin can be coated with a styrenediacetone acrylamide copolymer by dispersing the two monomers with the clay in an aqueous medium and polymerising with a persulfate catalyst [39]. Unsaturated monomers have been adsorbed on to fillers and then polymerised to give encapsulated products. The modulus of the polymer could be modified by selecting the monomer [40]. Acrylic acid-vinyl chloride (1:99) has been polymerised on to calcium carbonate [41]; reports of the use of 3,5-triacryloxyhexahydro-S-triazine [42], bis-phenol A and epichlorohydrin [43], methyl methacrylate [44], and acrylic acid [45] have also been published. Graft copolymers between unsaturated acids, especially acrylic acid and maleic anhydride (MA), and polyolefins (PE and PP) are widely used as surface modifiers and compatibilisers, sometimes in combination with bi-functional coupling agents [46], for talc, calcium carbonate and calcined clays. Such polymer coatings include polypropylene-maleic anhydride [47], polypropylene cis-4-cyclohexene-1,2 dicarboxylic acid [48], polystearyl or polylauryl acrylate [49], polypropylene-acrylic acid, partially oxidised poly(butane diol) [50] and ethylene-vinyl acetate copolymers [51]. Acidcontaining products can react with basic fillers. With most other types, they will simply adsorb on to the mineral surface, but they can form esters with some non-basic metal hydroxyls, notably silanols.

1.3.4 Direct Bonding Polymers themselves can be grafted on to a mineral surface and the resulting composites have been reported to be significantly better than simple mixes because of strong covalent polymer-filler bonds. Composites of clay with ethylene-MA graft copolymer, in which the anhydride groups interact with the hydroxyl groups on the surface of the clay, give increased impact strength and flexural modulus compared with a physical blend of clay and ethylene-MA random copolymers [52]. Polymerisation of monomers can be induced on the mineral by natural catalytic sites on its surface [53] or by adsorbing polymerisation catalysts on to the mineral prior to treatment with monomers [54, 55].

28

General Principles Guiding Selection and Use of Particulate Materials

1.4 Particle Packing and the Maximum Packing Fraction 1.4.1 Introduction The packing behaviour of particles in a polymer matrix determines at what loading particle/particle effects become important and is a critical factor in the understanding and design of polymer composites, especially when highly filled systems are involved. The maximum packing fraction, Pf, is a particularly useful concept. This is the maximum volume fraction of particulate that can be incorporated before a continuous network is developed and voids begin to appear in the composite. In addition to indicating the maximum practical loadings obtainable, Pf is also useful in understanding and describing the effect of filler loading on composite properties. Many properties change rapidly as one approaches Pf, almost in a percolation way, and this parameter is now incorporated into many mathematical treatments of property-loading dependence [56]. Indeed, it often goes a long way to explaining why different physical forms of the same filler material can give markedly differing results at the same loading.

1.4.2 Determination of Maximum Packing Fraction (Pf) by Oil Absorption Procedures Despite its utility, Pf is difficult to measure or predict with great precision. This is due to complex interactions between the particles and the polymer matrix, and the influence of the fabrication method used. Both of these factors affect the way the particles pack and make conventional measurements of packing, such as the tap density of the powder, of limited quantitative value. This is especially true for many typical particulate fillers, which, due to their fine size and irregular shapes, exhibit particle agglomeration and do not pack well in the dry state. Such powders often pack considerably better when wetted by suitable liquids, especially if dispersants are present. These wetting and dispersing effects lie behind the pragmatic approach of using oil absorption procedures to determine an effective Pf. Originally introduced for application in putty and paint technology, oil absorption methods rely on titrating a sample of powder with an oil or other liquid, while continually rubbing and mixing the mass. End-points are readily detected in these procedures, often defined as the point at which a putty of a certain consistency is obtained. It is generally believed that at this point all the particles are dispersed and wetted, and all the gaps between the particles are just filled with liquid. In practice, only two liquids are used to any extent, linseed oil and phthalate esters. Both simple manual and instrumental methods are available for determining oil absorption values. It is important that standard procedures are closely followed if reproducible results are to be obtained and a number of standards have been issued. The simplest procedure is the spatula rub-out method, which is embodied in ASTM D281 [57]. A useful description of its application to particulate fillers has been given by Ferrigno [58].

29

Particulate-Filled Polymer Composites Oil-absorption results are usually quoted as cubic centimetres of oil per 100 grams of particulate at the end point. Simple mathematical treatment using oil and particle densities can then convert this to a volume fraction. As a guide, linseed oil absorptions for calcium carbonate of 20 cm3 100 g-1 and 50 cm3 100 g-1 correspond to filler volume fractions of 0.64 and 0.41, respectively. The attraction of the oil absorption methods lies in their simplicity and their crude approximation to polymer processing. Different liquids are also observed to give markedly different values and this is believed to be due to their respective abilities to wet out and disperse the particles. Thus in theory, by choosing a liquid of similar properties to the polymer in question, one should be able to improve the usefulness of the data, although this does not seem to have been greatly used to date. As mentioned previously, only two liquids are commonly used and, of these, linseed oil generally gives lower values suggesting that it wets and disperses particles better than phthalates. The phthalates are popular because they are widely used as plasticisers in PVC compounds. Liquid paraffins have been used as model systems for olefinic polymers, and squalane could be used as a model system for PP. A detailed study of the oil absorption test and its meaning in terms of particle packing has been reported by Huisman [59]. He found that the agglomerates present in the original powder are frequently not fully dispersed during the test, but may be compressed and reduced in porosity. The degree to which this occurs critically affects the packing and hence the oil-absorption value. Differences in the degree of compaction obtained go a long way to explaining variation in oil-absorption values between different operators, and between manual and instrumental methods, as well as between different liquids. While oil absorption serves a useful purpose as a guide to the effective value of Pf, great care must be taken in applying it too literally. In some polymer systems there may be a good match with the polymer wetting properties and the dispersion procedures. In others this may be very poor. Oil absorption values are widely used for assessing carbon black properties in the rubber industry. In this field, special procedures have been developed to attempt to reproduce the effect of rubber compounding conditions on particle morphology [22].

1.4.3 Particle Packing Theory Much effort has been devoted to studying the effect of particle size and shape on the packing properties of powders. Special attention has been given to identifying those factors that allow high packing fractions to be obtained. This subject is very relevant to the design of filler systems and the general principles, which often cause considerable confusion, are outlined here. For greater detail specialist publications such as that by German should be consulted [60].

30

General Principles Guiding Selection and Use of Particulate Materials The packing behaviour of particulate materials depends largely on their particle size, shape and surface characteristics. The behaviour of model systems with closely defined size and shape distributions is now well understood. Real particulate materials are much harder to treat, largely due to the difficulty in determining and describing their size and shape distributions accurately. Nevertheless, the principles derived for the model systems can be applied in a semi-quantitative way and appear to work reasonably well. The aims of particle packing theory are to predict the maximum volume fraction that can be obtained under a given set of circumstances and the structure of the particle assembly at this point. The first point to appreciate is that there are several ways of packing the same collection of particles, each one resulting in a different maximum volume fraction. The circumstances under which the particles are assembled will determine which of these structures is obtained in practice. In the extreme case consider a three-dimensional jigsaw puzzle. With care this can be assembled to give perfect packing with no free space. This is known as ordered packing. However, if the pieces are put in a container and shaken, it is extremely unlikely that they will ever discover this perfect packed structure. Nevertheless, it will be found that, providing there are enough particles, a certain packed density can be reproducibly obtained implying that some statistically balanced structure is obtainable. This is known as a random packing situation. More than one ordered and random packing fraction may be observed with many systems. Appreciation of the nature of ordered and random packing is important in understanding the application of packing theory. Ordered packing results in long-range structure as in a crystal lattice and, although it can sometimes be observed in suspensions of monodisperse spherical particles, is not of great importance in our context. As its name implies, random packing is a statistical process with no long-range structure and it is more relevant to particulate-filled composites. While some ordered packing configurations can be of very low density, ordered packing will always be capable of giving higher packing densities than random packing. Two classes of random packing are recognised, loose and dense random packing. Loose packing refers to the sort of packing obtained when particles are randomly assembled under conditions where they cannot easily move past each other, while in dense random packing, conditions are such that movement is possible. In powder technology terms, loose random packing corresponds with a pour density and dense random packing to a tapped density. The simplest case to consider first is the packing of smooth, regular, mono-sized particles. This has been well studied for smooth spherical particles, which can readily move past one another. With these particles a maximum ordered packing fraction of 0.74 has been established, although other, less dense packings are feasible. Random loose packing fraction is difficult to predict accurately but is about 0.60. Random dense packing is more readily predicted and is about 0.64.

31

Particulate-Filled Polymer Composites The theory has been extended quantitatively to a number of regular, non-spherical shapes. Many of these such as cubes, rectangles and plates can have ordered packing fractions of unity but most have less dense random packing than spheres, largely due to the difficulty of the particles moving with respect to one another to optimise packing. Cubic particles do, however, seem to have a slightly higher random packing density than spheres (about 0.75). Quantification of the packing behaviour of irregular particles is poorly developed at present but the enhanced interparticle friction soon causes the packing fraction to decrease as the particles depart from sphericity. Values of 0.50 are not uncommon for three-dimensional irregular particles. Very low packing fractions (<0.1) are obtained with high aspect ratio fibres. The general approach to obtaining high packing fraction powders is to use small particles to fill in the pores in the packed structure obtained from large particles and then to use even smaller particles to fill in the remaining pores and so on. A simple view of this is presented in Figure 1.6. The approach is known as multimodal packing, and much effort has been devoted to defining the optimum particle size ratios and quantities of the various modes and the resulting packing fraction obtained. The case of spherical particles provides the simplest example. As we have already seen, the maximum ordered packing fraction is 0.74. From geometric principles it can be shown that this structure has two sizes of pores present, which can be filled by spheres with 22.5% and 41.4% of the diameter of the largest spheres. This results in an increase in packing fraction to 0.81. Unfortunately, such simple geometric calculations are not possible for the more relevant random packing situation, even when spherical particles are involved. This is because there will be a random distribution of pore sizes and the small particles will not spread out evenly among them. In particular, the small particles show a tendency to cluster around the

Figure 1.6 Classical bimodal packing effect

32

General Principles Guiding Selection and Use of Particulate Materials surface of the larger ones, increasing their spacing [61]. It can be shown, however, that the limiting case for random packing of a bimodal distribution of spherical particles of a single material is achieved at a packing fraction of 0.86. This is obtained with 73% large and 27% small particles and a size ratio of at least 7:1 and preferably up to 20:1 [62]. As with ordered packing, increasing the number of modes improves the packing, but similar ratios are needed for each mode. This soon results in an excessively wide particle size distribution accompanied by ever decreasing benefits. Three modes give a limiting random packed fraction of 0.91 and the ultimate limiting value appears to be 0.96. The packing behaviour of multimodal mixtures of irregular particles is very poorly understood at present, although some empirical studies have been made on simple systems. Where different sizes of similar shapes are mixed, then improvements in packing similar to those observed with spheres are usually obtained but starting from a lower level. It also appears that a wide difference in sizes and a large proportion (by weight) of the bigger particles is advantageous. Where different shapes are involved then the picture is very unclear and the packing density may even decrease. While the multimodal approach is theoretically attractive and is useful for understanding the general principles, it has some practical limitations. Continuous particle size distributions are often more readily achieved and less costly and so more relevant. Much work has gone into trying to predict the optimum shape of distribution for maximum packing. This is a very complex subject but it has been shown that packing fractions as good as those obtained from the multimodal approach can be obtained from carefully controlled continuous particle size distributions. These distributions are hard to define and are usually developed empirically. They can, however, be looked upon as being derived from multimodal principles. In general, such distributions are skewed having an abundance by weight of large particles and a long tail to the fine end.

1.4.4 Applications of Packing Principles to Particulate Filled Composites The packing principles outlined in the previous section are of great value in preparing filler size distributions capable of being processed at high loadings in polymeric systems. In the extreme case monomodal spherical particles can be specially synthesised and mixed together but this is only worthwhile in a few high value applications, such as light curable dental resins. More often adjustments are made to the natural particle size distribution of standard fillers composed of irregular particles to improve the packing. These adjustments may be by classification to remove certain fractions or by blending in extra material in specific size ranges. In some instances narrow sieve fractions are prepared and recombined in set proportions. Where comminution is used to produce fillers then the processing can be controlled to some extent to produce favourable size distributions. Practical procedures for producing maximum packing systems were originally worked

33

Particulate-Filled Polymer Composites out by Furnas [63] and have recently been described in some detail by Ferrigno [64]. It must be borne in mind, however, that other considerations may restrict the particle size range that can be tolerated in a composite. While particle distributions prepared according to packing principles generally give the highest processable loadings in polymers, the prediction of these loadings and the measurement of the effective maximum packing fraction, is not very easy. As mentioned earlier a number of different random packing fractions can exist for the same collection of particles depending on the assembly conditions. Particle packing is usually determined on a powder, by procedures such as pour or tap density, or by some compression process. These are not very relevant as the particles used as fillers tend to be relatively fine and irregular and do not pack well under these conditions. When processed into polymers, the matrix itself, or additives, together with the compounding procedures, can drastically alter particle packing behaviour. Different types of packing are also relevant to different processing conditions. Where flow is required then some sort of random loose packing is probably the maximum achievable, whereas if the dispersion is subsequently compression moulded with squeezing out of excess matrix, then a denser random packing becomes more relevant. Where particles can readily move with respect to one another and a dense packed arrangement is readily obtained, then the phenomenon of dilatancy, or shear thickening, may be observed. This is caused by the need for the particles to move to a more open structure before flow can occur. In such cases processing difficulties may rule out use of compositions based on the dense packing configuration. As has been discussed earlier, procedures such as oil absorption, although imperfect, probably remain the best way of determining a relevant Pf.

1.5 Interparticle Spacing The distance between filler particles in a polymer matrix is an important consideration, which often does not receive the attention it deserves. It can be appreciated that composite properties may be markedly affected when this distance becomes very small. This is partly because the polymer chains may not now have the full freedom of movement they would have in the bulk polymer. Equally importantly, if there is a shell of modified polymer adsorbed on the particles, then at some limiting spacing this shell will overlap with that on adjacent particles leading to a percolation effect and to this phase dominating some properties (Figure 1.7). It can also be shown that the critical distance of separation is likely to be a small number of radii of gyration of polymer chains (20-200 nm). Interparticle spacing is directly determined by the number of particles present and this changes markedly with particle size. This is dramatically illustrated in Figure 1.8 by the micrographs of two poly(methylmethacrylate) composites containing equal volume fractions of aluminium hydroxide of differing particle size.

34

General Principles Guiding Selection and Use of Particulate Materials

Figure 1.7 Idealised view of the effect of interparticle distance and of absorbed polymer on composite structure and properties

Figure 1.8 Scanning electron microscope images of cross-sections of poly(methylmethacrylate) filled with equal volume fractions of two different particle size aluminium hydroxide fillers: a) 55% coarse particles, b) 55% fine particles

35

Particulate-Filled Polymer Composites

1.6 Particle Effects on the Structure of Polymers 1.6.1 Introduction There is now considerable evidence that particulate fillers, especially inorganics, can significantly affect the structure of the matrix polymer itself, and hence the properties of the final composite. Such effects can vary enormously from system to system and have largely been overlooked in many studies of filled composites. The existence of these effects may explain many of the anomalies reported in trying to develop and apply generalised predictive equations [65]. Matrix modifications can occur in a number of ways and the effect on properties can sometimes be at least as important as that arising from the much more widely recognised interfacial bonding effects. Unfortunately, no systematic study of this area has yet been made. The principal ways in which a particulate phase may affect polymer structure are: 1. Molecular weight reduction during processing. 2. Molecular weight and crosslink modifications due to interference with curing processes. 3. Adsorption of polar, low molecular weight species such as surfactants, plasticisers, stabilisers and antioxidants and oxidation products. 4. Formation of an immobilised shell of polymer around the particles. Often this shell is rich in a certain molecular weight fraction, which is then depleted in the matrix. 5. Modification of crystallinity in semi-crystalline polymers. 6. Effects on polymer conformation due to particle surfaces and inter-particle spacing. A brief description of the various effects together with examples of their importance is given in the next section.

1.6.2 Molecular Weight Reduction During Processing It is well known that polymers may undergo mechanochemical degradation during compounding and moulding operations [66]. Indeed this is an important feature of natural rubber processing. As most properties are dependent on molecular weight, this can play an important role in determining final properties and conditions (including the use of stabilisers) are usually chosen to limit any degradation.

36

General Principles Guiding Selection and Use of Particulate Materials As has already been discussed, and will be returned to later, the introduction of a particulate phase may markedly affect the amount of degradation. The usual effect is for degradation to be increased due to increased mechanical work, to chemical interactions at the interface and possibly to deactivation of stabilisers and antioxidants. The effect is most pronounced under conditions (especially high loadings) where viscosities are high, leading to excessive shear forces being developed. Such effects have been reported in filled elastomers [67] and polypropylene [68].

1.6.3 Molecular Weight and Crosslinking Changes due to Cure Modifications The detailed microstructure of free-radical cured polymers determines their overall properties, and is governed by the rate of cure and relative rates of various competitive polymerisation processes such as: initiation, propagation, combination and termination. Inorganic particles can modify the cure process in several ways. In particular, by lowering the overall exotherm, they can significantly reduce the cure temperature and kinetics, thus altering the relative rates of the various processes described above. The surface of the particles can also play a role by absorbing and altering the stability of the growing polymers, or by absorbing and altering the activity of the initiator and curative. Most of these effects have been reported but, unfortunately, structure determination is very difficult with crosslinked systems and little or no direct evidence for resulting structure changes has been reported. An example of the effect of different filler particles on the curing of unsaturated polyesters (and by inference on their structure), as measured by the cure exotherm, has been given by Plueddemann and Stark [69].

1.6.4 Preferential Adsorption of Polar Species Polar materials are usually more strongly adsorbed on particulates than is the matrix polymer and, where present, can displace the polymer altogether from the surface. This can modify the composite properties in two ways. Firstly, it may lead to a weak interfacial bond. Secondly, where the polar species was providing a significant matrix function such as plasticisation, cure modification or flow enhancement, this can be lost. Good examples of the preferential adsorption of polar materials have been given by Barnett and Jones [70], Evans and co-workers [71] and by Karlivan and co-workers [72]. Barnett and Jones carried out some elegant and painstaking work on the effect of mineral fillers on the reinforcement of emulsion-polymerised SBR. This type of elastomer contains high levels of organic surfactants remaining from the polymerisation process. Certain mineral fillers gave exceptional and unexpected reinforcement properties, which were

37

Particulate-Filled Polymer Composites demonstrated to be due to the preferential adsorption of these surfactants, resulting in a significant improvement in the strength of the matrix. This strength improvement was, at least in part, due to modification of the curing process showing how complex and inter-related the effects of adding particulates can sometimes be. More recently, Evans and Rothon have shown how fatty acids prevent the formation of bound rubber when precipitated calcium carbonate is used as a filler in SBR elastomers. When they are absent, high levels of bound rubber are formed, accompanied by a marked increase in reinforcement. Karlivan demonstrated how certain inorganic fillers could be used to preferentially adsorb low molecular weight polar species produced by thermo-oxidation of polyolefins. These species tend to form during heat welding of polyolefins to metals and, by accumulating at the interface, limit the adhesive strength. Their removal markedly improved adhesion.

1.6.5 Formation of an Interphase of Immobilised Polymer It is now well established that, in the absence of competing polar species, molecules of the matrix polymer can be effectively adsorbed on to the surface of a particulate phase, even when this is an inorganic mineral. The strength of this adsorption can play a key role in determining composite properties as will be discussed in later chapters. It is also generally recognised that this adsorbed polymer may have different properties from the bulk, but the consequences of this for composite properties are often overlooked. This is partly because, as we shall see, the effects only become significant at high loadings and with small particles, but also because the adsorbed layers are hard to detect and characterise. Where they are observed, the effects can be at least as great as those associated with particle matrix bonding. The principal effect of this adsorbed layer is to increase the effective size and hence volume fraction of the particulate phase. A complication that has caused much confusion in the past, and is only just becoming recognised and understood, is the apparent variability in the thickness of the immobilised layer according to the method of measurement and the composite property being studied [73]. Once this difficulty is appreciated, its origins can be clearly understood. Thus, in the absence of other adsorbing species, there will be a primary layer of strongly adsorbed polymer at the filler surface. The conformation and properties of this are likely to be severely modified and probably in all instances it can be regarded as immobilised and contributing to particle volume. As one moves beyond this layer, the effects due to the particle do not suddenly disappear but will gradually tail away. This outer region will be less strongly associated with the particle and may only appear to be immobilised under certain test conditions.

38

General Principles Guiding Selection and Use of Particulate Materials As we shall see, while effects of particles can sometimes extend for large distances, the immobilised layer thickness generally appears to range from 1 to 50 nm and in many instances is much less than 10 nm. The effect of various layer thicknesses on the effective volume of different sized cubic particles is presented in Table 1.4. Only when the layer is of the order of 50 nm does it have much effect on particles over 1 μm in size, but even thin layers of 10 nm or less have a significant effect on particles about 0.5 μm, and this becomes dramatic as very small particles (0.1 μm) are reached. Because of their increased surface to volume ratio, anisotropic particles such as plates will show greater effects than those recorded in Table 1.4. Determination of the thickness of immobilised layers is far from straightforward and can give a variety of answers, as mentioned earlier.

Table 1.4 Calculated effect of surface layers on the apparent volume of various sized cubic particles Particle size (μm)

% volume increase for a layer thickness of: 1 nm

10 nm

100 nm

1000 nm

0.1

6

73

2600

-

0.5

1.2

12

174

-

1.0

0.6

6

73

2600

5.0

< 0.1

1.2

12

174

10.0

< 0.1

0.6

6

73

The adsorption and conformation of polymer molecules on particle surfaces is of great interest to colloid scientists, especially with regard to steric stabilisation of dispersions, and general guidelines can be obtained from their studies. A good description of the subject can be found in treatise such as that by Napper [74]. It is found that single point adsorption results in the molecule extending preferentially in a direction perpendicular to the surface and a layer thickness that approaches a value of twice the radius of gyration of the free polymer. The radius of gyration is dependent on molecular weight but for our purposes the effective layer thickness due to this type of conformation will be in the range of 20-40 nm. With most polymers of interest to us, multipoint attachment is more likely. This will significantly reduce the layer thickness until, in the limit of flat adsorption, this can be as low as 1 nm or less. The most direct method of determining the thickness of adsorbed polymer is by carrying out adsorption studies, either from a suitable solution of the polymer, or by extracting an actual composite and determining the residual or ‘bound’ polymer. Both approaches have

39

Particulate-Filled Polymer Composites been used and the layer thickness is calculated from the volume of adsorbed polymer by dividing by the surface area of the particulate phase. The bound polymer approach has been frequently used in studies on carbon black-filled elastomers and several useful reviews have been published [75]. Unfortunately, the origin of bound polymer is complex and may be of limited use in characterising effects in the composite itself. Bound layer thicknesses determined in this way for carbon black-elastomer systems are usually of the order of 1-5 nm. There has been only limited work on determining bound polymer with mineral fillers and in polymers other than elastomers. Several studies have shown that bound rubber readily forms with high surface area silicas in elastomers [76], where calculated layer thicknesses are similar to those reported for carbon blacks. Evans and Rothon have reported on bound rubber formation with precipitated calcium carbonate in an elastomer system [71] and again the calculated layer thickness based on their data is less than 5 nm. Similar low values have been reported by Kosfeld and co-workers for kaolin and zinc sulfide [77] in a rubber-modified polypropylene and by Lipatov [78] and Boonstra [79] for silica-filled polyethylene. It is probable that these determinations only detect the polymer molecules that are strongly attached to the particles, probably in multipoint fashion. This explains the low values of thickness observed, but it does not necessarily follow that the immobilised, or modified thickness in composites is not significantly greater than this. In addition to bound polymer of the type described previously, polymer may also be shielded from some deformation processes by being occluded within the particle shape (See Section 2.8.2) and this is especially important with some carbon blacks. Viscosity measurements provide another fairly simple method for determining a layer thickness. For non-agglomerated particles, a fairly simple equation (Equation 1.5) exists relating relative viscosity of a suspension to the effective volume fraction of the solid phase: ηs – η = 1 + KΦ + K´Φ + …

(1.5)

Where ηs and η are the viscosities of the suspension and the solvent, respectively, K is a coefficient depending on particle size, shape and concentration, and Φ is the effective volume concentration. K can be determined in pure solvent without polymer being added and then used to determine the change in Φ resulting from addition of polymer. Viscometric determinations of this type tend to give quite high layer thicknesses of the order of several tens of nanometres. An example of this approach can be found in the work of Chibowski who studied the adsorption of polyacrylamide of different molecular weights on to the surface of titanium dioxide from aqueous solution [80]. He found effective layer thicknesses of 30-70 nm increasing with polymer molecular weight. The results obtained are, of course, influenced by the solvent. If possible, the measurements

40

General Principles Guiding Selection and Use of Particulate Materials should be carried out in a theta solvent, (i.e., one in which the dimensions of the polymer coil are the same as in the melt). Various procedures have been applied to composite materials to attempt to determine the amount and properties of the immobilised layer in situ. Much of the interest in this has been in the former eastern block countries of Europe and has been reviewed by Enikolopyan [81]. Among the measurements used have been density, specific heat, dielectric constant and dynamic mechanical analysis. The general conclusions are that, as one might expect from reduced configurational entropy, the surface layers are denser and more rigid than the bulk polymer. Again most work has been carried out on carbon black-filled elastomers. This has shown that a slight broadening can be observed in the glass transition of the polymer, which is interpreted as being that of the surface layers. Deconvolution indicates that the glass transition of these layers is about 10 °C higher than the matrix and the calculated volume of modified material approximates to a 3 nm layer [82]. Only isolated studies have been reported in other systems. Galeski has carried out thermal analysis of chalk-filled isotactic polypropylene [83]. Uncoated chalk was found to modify all the transitions present in the unfilled polymer and to lead to two new transitions assigned to restricted motion in the interface layers. These effects were largely eliminated when the chalk was surface treated, confirming their surface origin. No attempt was made to calculate layer thickness. Gerard and co-workers have reported on the effect of glass beads in a crosslinked epoxy matrix [84]. They used dynamic mechanical analysis and found the uncoated beads to reduce the mobility of the polymer, an effect that could be largely overcome by coating the beads. Analysis of this sort has much to offer and hopefully will be more widely applied in future work. Workers such as Pukansky and Tudos have developed procedures for calculating effective layer thicknesses from the mechanical properties of composites [85]. They have applied this to calcium carbonate-filled polypropylene and found the effective layer thickness to vary according to the property being measured. A value of 5 nm was obtained from Young’s moduli in agreement with the thicknesses expected for strongly bound polymer. A value of 150 nm was, however, calculated from tensile yield and strength calculations. It was postulated that the size of the effective interlayer depends on the elongation at which it is being determined. The aspect of bound or modified polymer still has many secrets to reveal and its existence and importance should always be assessed when new polymer/filler combinations are being developed, especially with high filler loadings and high surface area particles.

41

Particulate-Filled Polymer Composites

1.6.6 Effects on Polymer Conformation due to the Presence of Particle Surfaces and Interparticle Spacing The properties of polymers depend to a large degree on the conformations available to their chains. The presence of solid, particulate material can significantly modify these conformations, even in the absence of any particle-polymer interaction or other effects such as already discussed. Firstly, the presence of a solid surface will restrict the conformation of polymer molecules in its vicinity. This type of effect is widely recognised in colloid science where, as already mentioned, it has been extensively studied with regard to steric stabilisation of suspensions. Detailed treatment can be found in works such as those of Napper [74]. The basic findings are that, in the absence of multipoint attachment, chain molecules respond to this restriction by expanding in a direction parallel to the surface. This expansion can double the effective molecular size in that direction. Secondly, polymer molecules occupy quite considerable volumes, depending on their molecular weight and temperature. At high filler loadings, especially with ultrafine particles, the interparticle spacing may be of similar size or smaller. This must mean that the polymer molecules will be forced to take up a strained conformation and thereby be considerably modified in their properties. While these effects are undoubtedly present in particulate composites they have not, to the authors’ knowledge, been addressed in any detail so that their significance is still largely unknown.

1.6.7 Effects on Crystallinity There is sufficient evidence available to show that particulate fillers can affect the crystallinity of some polymers and that this may in turn affect mechanical properties. Despite this, the area has received surprisingly little attention and many studies of filler/ property effects in semi-crystalline polymers do not consider this possibility. Many thermoplastic materials are semi-crystalline and the precise nature of this crystallinity plays a major role in determining their properties. In addition to the level of crystallinity, factors such as crystal phase, crystallite numbers, orientation, size and shape are all important, but the exact way in which they affect properties is far from understood. In unfilled polymers the crystallites generally seem to be present in spherulitic aggregates [86]. Because of hindered motion, thermoplastics have a marked tendency to super cool before crystallising. The degree of super cooling will itself determine the number of

42

General Principles Guiding Selection and Use of Particulate Materials crystallites and will vary according to forming conditions. As a consequence of this super cooling, thermoplastics are very prone to nucleating effects and specially chosen nucleating agents are frequently added to ensure rapid crystallisation during forming. Among other things, this reduces cycle times. In some cases, nucleants may be used to favour certain crystal phases. Such nucleating agents are often salts of organic acids and sometimes fine fillers. Their use does not generally alter the spherulitic nature of the crystallinity, only the size and number of crystallites. Even so, there can be marked changes in properties accompanying the use of such nucleants, examples of which are given by Jansen [87]. In particular their use generally seems to increase impact strength due to the reduction in crystallite size. Given the importance of crystallinity in determining mechanical and other properties and the susceptibility of polymers to nucleation, it is pertinent to enquire whether normal filler particles have any significant effects in this context. This area is surprisingly little studied, but there is sufficient evidence to show that marked nucleation can occur, although this may vary from filler to filler, with surface treatment also playing a role. There are also indications that the type of crystallinity may be affected and thtat there is some circumstantial evidence for a link between nucleation effects and mechanical properties. This area is, however, not yet well understood and we are a long way from having a good generalised theory. The subject is probably best advanced for polymers in contact with glass fibres and certain platy particles such as sheet silicates. As well as acting as nucleants, this type of material can alter the type of crystal structure present. Thus, it is now well established that glass-fibre surfaces can lead to trans-crystallinity. This is the growth of radially oriented lamellae extending for a few hundred micrometres from the fibre surface. This topic has been discussed in detail by Thomason [88]. Transcrystallinity is also discussed in Chapter 8 of this work. Outside of these areas the picture is much less clear. Straight nucleation affects are easily detected by thermal analysis techniques. These generally manifest themselves in an earlier onset temperature and peak temperature, but not necessarily in any increase in total crystallinity. There are many reports of significant effects for various polymer/filler systems. Thus, talc has been reported as nucleating PE [89], PP [90] and Nylon 6 [89]. Calcium carbonate has been reported as nucleating PP [91-93] and PE [94], while sepiolite (Mg4 Si6 O15 (OH)2 . 6H2O) has been reported as nucleating PP [95]. Surface treatment of the filler particles has also often been found to reduce or eliminate these nucleating effects [89, 96, 97]. One of the authors (RR), has observed similar nucleating effects for aluminium and magnesium hydroxide in polypropylene and also confirmed the reduction of this effect by surface treatment. Typical results illustrating these effects are presented in Figure 1.9. What is far less clear is whether and how these nucleation effects alter the crystal structures present and whether, like organic nucleants, they alter mechanical properties.

43

Particulate-Filled Polymer Composites

Figure 1.9 Effect of magnesium hydroxide fillers on the crystallisation of a polypropylene copolymer (DSC experiment, cooling at 10 °C/min)

There is some evidence for a link with mechanical properties in the work of Hutley and Darlington [96, 97]. They found a good inverse correlation between the nucleating ability of fillers such as calcium carbonates and falling weight impact strength in PP. Surprisingly, the organic nucleants were also found to decrease the impact strength in line with their nucleating ability, although the opposite effect is usually observed, as discussed earlier. According to these workers, other mechanical properties were little affected. Mitsuishi and co-workers have also studied the effect of calcium carbonate on the nucleation and mechanical properties of PP [91]. They reported significant correlation with the loss modulus, tensile modulus and tensile impact strength. They confirmed the decrease of impact strength with increasing nucleating effect reported by Hutley and Darlington. Quantifying the effects of the fillers on crystal structure and hence explaining the mechanical property effects described above is very difficult. This is largely because of the problems of carrying out direct observations using traditional microscopy procedures with such highly filled systems. Among the effects that could be looked for are: changes in total level of crystallinity, changes in crystal phase and its orientation, changes in the number and size of crystalline particles, and changes in the type of crystalline particle. In none of these areas is the picture completely clear and the effects obtained probably vary from system to system. One of the more detailed studies in this field has been by McGenity and co-workers on calcium carbonate and talc-filled polypropylene [98].

44

General Principles Guiding Selection and Use of Particulate Materials While there are some reports of significant effects on the overall level of crystallinity, most studies show little or no effect [92, 98]. Chacko and co-workers reported that the effect of calcium carbonate on the crystallinity of polyethylene was dependent on molecular weight with reduced crystallinity being observed at low molecular weight and no effect at higher molecular weight [94]. Multiple peaks are often observed in melt cooling studies, (e.g., Figure 1.9) and have been claimed as evidence for the nucleation of new crystal phases [92]. When X-ray studies have been carried out, they have usually failed to confirm this [99], although Murthy did report finding that talc altered the crystal phase present in Nylon 6 [89]. The present author (RR), also found that reheating of magnesium hydroxide-filled polypropylene displaying the twin peaks in Figure 1.9 only gave a single melting peak, suggesting that only one phase was present. On the other hand, fillers have been reported to increase the amounts of beta-phase in polypropylene [98] and to affect the crystal phase present in Nylon 6 [89]. The few studies that have been carried out to date strongly suggest that particulate fillers at normal usage levels markedly disrupt the normal spherulitic morphology. This is not surprising as the interparticle spacing would probably be insufficient to allow these to develop properly. There is also some evidence that new forms of crystallinity can be present associated with the particle surfaces. Thus Chacko and co-workers [94], report lamellar growths originating from selected spots on calcite surfaces in polyethylenes and radiating outwards for up to 400 μm. Rybnikar reported similar effects for calcite in polypropylene [93]. In their work, Kowalewski and Galeski carried out a detailed study of calcium carbonate in polypropylene including work with large single crystals [92]. They reported that the effects observed depended on the crystal faces present. A form of trans-crystallinity was observed on some crystal faces and epitaxial growth was postulated for others. In summary, it seems clear that particulate fillers can have significant nucleating effects in semi-crystalline polymers and that this may lead to effects on mechanical properties. Much work remains to be done to clarify this and provide a clear, coherent description of the effects involved, however. This would be greatly helped if simple techniques for determining polymer crystal structures in filled systems were available. It would also be highly desirable to have a better understanding of how the structure of polymer crystallinity affects composite properties.

References 1.

P. Challinger, Fillers in Plastics Conference, Filplas 81, British Plastics Federation, London, UK, 1981.

45

Particulate-Filled Polymer Composites 2.

M.E.J. Dekkers and D.J. Heikens, Journal of Materials Science, 1983, 18, 3281.

3.

H.P. Schlumf, Polypropylene Compounds — Sustaining the Market Growth, Corporate Developments Consultants Conference, Brussels, Belgium, 1991.

4.

H.P. Schreiber and F. St. Germain, Journal of Adhesion Science and Technology, 1990, 4, 4, 319.

5.

F.M. Folkes, Proceedings of the SPE Fillers and Additives for Plastics Conference, Gothenberg, Sweden, 1988.

6.

K.F. Heinisch, Dictionary of Rubber, Applied Science Publishers Ltd., London, UK, 1974.

7.

R.P. Sheldon, Composite Polymeric Materials, Applied Science Publishers, London, UK, 1982.

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C.F. Bohren and D.R. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley and Sons, New York, NY, USA, 1983.

9.

P. Kubelka and F. Munk, Zhurnal Tekhnicheskoi Fiziki, 1931, 12, 593.

10. M. Kerker, The Scattering of Light and other Electromagnetic Radiation, Academic Press, New York, NY, USA, 1969. 11. G. Mie, Annalen der Physik, 1908, 25, 325. 12. P.N. Dunlap and S.E. Howe, Polymer Composites, 1991, 12, 1, 39. 13. M. Hancock, Developments in PVC Symposium, Loughborough, UK, 1986. 14. T.H. Ferrigno in Handbook of Fillers for Plastics, 2nd Edition, Eds., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987. 15. A.A. Berlin, S.A. Volfson, N.S. Enikolopian and S.S. Negmatov, Principles of Polymer Composites, Springer-Verlag, Berlin, Germany, 1991. 16. A.I. Medalia and F.A. Heckman, Carbon, 1969, 7, 567. 17. W.M. Hess, G.C. McDonald and E.M. Urban, Rubber Chemistry and Technology, 1973, 46, 204. 18. B.H. Kaye, A Random Walk through Fractal Dimensions, VCH Publishers, New York, NY, USA, 1989.

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General Principles Guiding Selection and Use of Particulate Materials 19. T.W. Zerda, H. Yang and M. Gerspacher, Rubber Chemistry and Technology, 1992, 65, 130. 20. C.R. Herd, G.C. McDonald and W.M. Hess, Rubber Chemistry and Technology, 1992, 65, 107. 21. H. Barthel, F. Achenbach and H. Maginet, Proceedings of Moffis-93, Namur, Belgium, 1993, p.301. 22. R.E. Dollinger, R.H. Kallenberger and M.L. Studebaker, Proceedings of the 90th ACS Rubber Division Meeting, New York, NY, USA, Fall, 1966, Paper No.19. 23. G. Fourty, Proceedings of Moffis-91, Conference on Mineral and Organic Functional Fillers in Plastics, Le Mans, France, 1991, p.59. 24. L.F. Gate and T.W. Webb, inventors; ECC international Ltd., assignee; GB 2,240,398, 1994. 25. T. Allen, Particle Size Measurement, 3rd Edition, Chapman and Hall, London, UK, 1981. 26. A.N. Gent, Journal of Materials Science, 1980, 15, 2884. 27. S. Brunauer, P.H. Emmett and E. Teller, Journal of the American Chemical Society, 1938, 60, 309. 28. Characterisation of Powder Surfaces: with Special Reference to Pigments and Fillers, Eds., G.D. Parfitt and K.S.W. Sing, Academic Press, London, UK, 1976. 29. S.G. Thoma, M.C. Ciftioglu and D.M. Smith, Powder Technology, 1991, 68, 53. 30. R.E. Godlewski, Proceedings of the 40th Annual Conference of the Reinforced Plastics/Composites Institute, Atlanta, GA, USA, 1985, Paper 4-F. 31. S.J. Monte, Modern Plastics Encyclopedia, 1976, 53,10A, 161. 32. L.B. Cohen, Plastics Engineering, 1983, 39, 11, 29. 33. M.M. Fein, B.K. Patnaik and F.K.Y. Chu, inventors; Dart Industries Inc., assignee; US Patent 4,073,766,1978. 34. J.A. Robertson, R.L. Adelman and H.E. Bergna inventors; E I DuPont de Nemours, assignee; US Patent 3,951,680, 1976.

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Particulate-Filled Polymer Composites 35. J.D. Miller, K-P. Hoh and H. Ishida, Polymer Composites, 1984, 5, 1, 18. 36. B.D. Favis, L.P. Blanchard, J. Leonard and R.E. Prud’homme, Polymer Composites, 1984, 5, 1, 11. 37. W. Pompe, S. Völlmar and H.J. Weiss, Plaste und Kautschuk, 1974, 21, 9, 664. 38. G.B. Aivazyan and co-workers, Armianskii Khimicheskii Zhurnal, 1990, 43, 3, 186. 39. S.E. Gebura, inventor; Interpace Corporation, assignees; US Patent 3,557,038, 1971. 40. R.L. Kaas and Z.G. Gardlund, Proceedings of the 35th SPE Technical Conference, Montreal, Canada, 1977, p.333. 41. V.A. Popov and co-workers, Doklady Akademii Nauk SSSR, 1984, 275, 5, 1109. 42. F. Nitto, inventor; Japan Kokai Tokyo Koho, 80,110,138, 1980. 43. Shirashi Kogyo Co Ltd, assignee; Japan Kokai Tokyo Koho 81,100,860, 1981. 44. K. Iwaku and J. Fujimura, Journal of Applied Polymer Science, 1979, 24, 4, 975. 45. K. Choi and C.K. Kim, Pollimo, 1989, 13, 5, 426. 46. S. Cartasegna, SPE Symposium, Polymer Grafting in Twin Screw Extrusion, Antwerp, Belgium, 1990. 47. A. Ohhira and M. Tuzawa, inventors; Japan Kokai Tokyo Koho, 77,109,546, 1977. 48. S. Fujii, N. Miyazaki and I. Unno, inventors; Japan Kokai Tokyo Koho, 70,54,034, 1974. 49. K. Senda and co-workers, Japan Kokai Tokyo Koho, 74,07,336, 1974. 50. Chisso Corporation, assignee; Japan Kokai Tokyo Koho, 81,149,452, 1981. 51. N. Iguchi and T. Ono, inventors; Japan Kokai, 75,107,046, 1975. 52. N.G. Gaylord and A. Takahashi, Polymer Science and Technology, Volume 21: Modification of Polymers, Plenum Press, New York, NY, 1983, 183. 53. M. Litvinenko and co-workers, Zhurnal Fizicheskoi Khimii, 1989, 63, 12, 284. 54. M. Bokhitski and co-workers, Plasticheskie Massy, 1988, 8, 34.

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General Principles Guiding Selection and Use of Particulate Materials 55. E.G. Howard, B.L. Glazar and J.W. Collette, Proceedings of the SPE High Performance Plastics, National Technical Conference, 1976, p.36. 56. T.H. Ferrigno in Handbook of Fillers for Plastics, 2nd Edition, Eds., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987, 33. 57. ASTM D281-95, Standard Test Method for Oil Absorption of Pigments by Spatula Rub-out, 2002. 58. T.H. Ferrigno in Handbook of Fillers for Plastics, 2nd Edition, Eds., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987, 17. 59. H.F. Huisman, Journal of Coatings Technology, 1984, 56, 712, 65. 60. R.M. German, Particle Packing Characteristics, Metal Powder Industries Federation, Princeton, NJ, USA, 1989. 61. R.M. German, Particle Packing Characteristics, Metal Powder Industries Federation, Princeton, NJ, USA, 1989, p.148. 62. R.M. German, Particle Packing Characteristics, Metal Powder Industries Federation, Princeton, NJ, USA, 1989. Chapter on bimodal mixtures of spherical particles. 63. C.C. Furnas, Industrial and Engineering Chemistry, 1931, 23, 9, 1052. 64. T.H. Ferrigno in Handbook of Fillers for Plastics, 2nd Edition, Eds., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987, 15. 65. B. Pukanszky, F. Tudos and I. Kelen in Polymer Composites, Ed., B. Sedlacek, Walter de Gruyter, New York, NY, USA, 1987. 66. O.S. Kauder in Thermoplastic Polymer Additives: Theory and Practice, Ed., L.J. Lutz Jr., Marcel Dekker, New York, NY, USA, 1989. 67. B.B. Boonstra and G.L. Taylor, Rubber Chemistry and Technology, 1965, 38, 4, 943. 68. W.V. Titow and B.I. Lanham, Reinforced Plastics, Applied Science Publishers, London, UK, 1975. 69. E.P. Plueddemann and G.L. Stark, Proceedings of the 31st SPI Annual Technical Conference, Washington, DC, USA, 1976, Section 6-D. 70. C.E. Barnett and H.C. Jones, Industrial and Engineering Chemistry, 1949, 41, 7, 1518.

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Particulate-Filled Polymer Composites 71. M.B. Evans, R.N. Rothon and T.A. Ryan, Proceedings of Fillers-86, BPF/PRI, London, UK, 1986, Paper No.12. 72. V.P. Karlivan, M.M. Kalnin’, L.Y. Malers, Y.Y. Avolin’sh, Y.Y. Malers and A.V. Viksne, Plasticheskie Massy, 1976, 11, 46. 73. Filled Polymers I, Ed., N.S. Enikolopyan, Advances in Polymer Science No.96, Springer-Verlag, Berlin, Germany, 1990. 74. D.H. Napper, Polymeric Stabilisation of Colloidal Dispersions, Academic Press, London, UK, 1983. 75. B. Pukansky and F. Tudos in Proceedings of the 3rd International Conference on Composite Interfaces (ICCI-III), Cleveland, OH, USA, p.691. 76. M. Ashida, K. Abe and T. Watanabe, Nippon Gomu Kyokaishi, 1976, 49, 11, 821. 77. R. Kosfeld, K. Schaefer, E.A. Hemmer, M. Hess, A Theisen and T.H. Uhlenbrich, Proceedings of the 3rd International Conference on Composite Interfaces (ICCIIII), Cleveland, OH, USA, 1990, p.385. 78. Y.S. Lipatov and F.G. Fabuliak, Vysokomolekulyarne Soedineniya, 1968, 10, 1605. 79. B.B. Boonstra, Polymer, 1979, 20, 691. 80. S. Chibowski, Powder Technology, 1990, 63, 75. 81. N.S. Enikolopyan, Filled Polymers, Advances in Polymer Science, 96, SpringerVerlag, Berlin, Germany, 1990. 82. G. Kraus, Advances in Polymer Science, 1971, 8, 155. 83. A. Galeski in Proceedings of the 3rd International Conference on Composite Interfaces (ICCI-III), Cleveland, Ohio, USA, May 1990, p.623. 84. I.F. Gerard in Proceedings of the 3rd International Conference on Composite Interfaces (ICCI-III), Cleveland, OH, USA, 1990, p.441. 85. B. Pukansky and F. Tudos in Proceedings of the 3rd International Conference on Composite Interfaces (ICCI-III), Cleveland, OH, USA, 1990, p.691. 86. A. Galeski, J. Grebowicz and M. Kryszewski, Die Makromoleculare Chemie, 1983, 184, 1323.

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General Principles Guiding Selection and Use of Particulate Materials 87. I.J. Jansen, in Plastics Additives, 2nd Edition, Ed., R. Gachterand and H. Muller, Hanser Publishers, Munich, Germany, 1987. 88. J.L. Thomason and A.A. van Rooyen in Proceedings of the 3rd International Conference on Composite Interfaces (ICCI-III), Cleveland, Ohio, USA, May 1990, p. 423. 89. N.S. Murthy, A.M. Kotliar, J.P. Sibilia and W. Sacks, Journal of Applied Polymer Science, 1986, 31, 2569. 90. I. Menczel and I. Varga, Journal of Thermal Analysis, 1983, 28, 161. 91. K. Mitsuishi, S. Ueno, S. Kodama and H. Kawasaki, Journal of Applied Polymer Science, 1991, 43, 2043. 92. T. Kowalewski and A. Galeski, Journal of Applied Polymer Science, 1986, 32, 2919. 93. F. Rybnikar, Journal of Applied Polymer Science, 1991,42, 2727. 94. V.P. Chacko, F.E. Karasz, R.J. Farris and E.L. Thomas, Journal of Polymer Science: Polymer Physics Edition, 1982, 20, 2177. 95. J.L. Acosta, A. Linares, M.C. Ojeda and E. Morales, Revista de Plasticos Modernos, 1985, 49, 346, 441. 96. T.J. Hutley and M.W. Darlington, Polymer Communications, 25, 226. 97. T.J. Hutley and M.W. Darlington, Polymer Communications, 1985, 26, 264. 98. P.M.McGenity, J.J. Hooper, C.D. Paynter, A.M. Riley, C. Nutbeem, N.J. Elton and J.M. Adams, Polymer, 1992, 33, 5215. 99. L. Jingjiang, W. Xiufen and G. Qipeng, Journal of Applied Polymer Science, 1990, 41, 2829.

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52

2

Principal Types of Particulate Fillers Michael Hancock and Roger N. Rothon

2.1 Introduction One of the fascinations of working in the field of particulate-filled polymer composites is the wide variety of materials used as fillers, from relatively simple chalks and limestones to complex rare-earth magnetic powders. Even apparently, mundane, relatively inexpensive, fillers can vary in subtle, but important, ways according to their precise origin and method of manufacture, thus adding their interest to the scientific and technological mind. In the space available here it is only possible to cover the major fillers and a few of the less common filler materials, the latter having been selected on the basis of lack of adequate treatment elsewhere and for illustration of important principles.

2.2 Particulate Fillers from Natural Origins (Mineral Fillers) 2.2.1 Introduction Mineral fillers are a vital and significant part of the world’s polymer industry. Consumption in rubber and plastics is currently estimated to be over 2.5 million tonnes per year in Western Europe. In the first edition of this book growth projections were very bullish and use in plastics especially was projected to grow at 8-12% per year. On a worldwide basis, it was predicted that 20 Mt per year would be used in polymers by the year 2000 [1]. However, local and world economies have suffered several blows since 1997 and this volume has not been reached. As a best ‘guestimate’ the authors believe that filler use in polymers currently is about 15 Mt per year. The majority of use consists of low-cost products in which price is the predominant specification property. Although this will continue to be an important area for mineral fillers, increasingly sophisticated applications for plastics, rubbers and composites mean that the requirements placed on fillers and the specific properties imparted by them will become more demanding; that is, they will be required to perform as functional additives conferring specific benefits in applications, which will be narrowly defined. It is therefore

53

Particulate-Filled Polymer Composites important to understand the intrinsic properties of the minerals, how these affect bulk properties and how these then influence the final filled polymer. The most important mineral fillers used are carbonates, clays and talcs, while other silicates are also of interest. Several carbonate minerals are known with some having potential for use as mineral fillers, although only a few are of industrial importance in plastic and rubber applications. Calcite (calcium) and dolomite (calcium-magnesium) are the main carbonate fillers and are very widespread, being exploited in many countries. The other carbonate mineral of any importance is, in fact, a mixture of two carbonates: hydromagnesite (a hydrated basic magnesium carbonate: Mg5(CO3)4(OH)2.4H2O) and Huntite (CaCO3.3MgCO3). A large, relatively pure deposit is found in Greece and a smaller one in Texas, USA, with the former being exploited as a filler for paint and paper applications, and as a flameretardant filler in plastics and rubber. Clay minerals are aluminium silicates of either the two-layered kaolinite type or threelayered montmorillonite type. Only three clay minerals are commonly used in the polymer industries, kaolinite, montmorillonite and chlorite, and these will be discussed below. Chlorite, because it usually occurs with talc and has essentially the same properties, is discussed with that mineral. Talc (magnesium silicate) is widespread but is commonly found with other magnesium minerals such as magnesite. For a complete analysis of the world’s industrial minerals, the reader is referred to publications such as the Minerals Yearbook published by the US Department of the Interior [2] and The Industrial Minerals Handbook [3].

2.2.2 Minerals and Rocks Strictly speaking, a mineral is a substance of inorganic origin with definite chemical composition, which is found in the earth’s crust. Rocks are naturally occuring mixtures of minerals. With few exceptions, minerals possess an internal ordered arrangement of their constituent atoms or groups of atoms, that is, they are crystalline. Some lack any ordered internal structure and are said to be amorphous. The arrangements (or motifs) of the atoms are determined by the forces between them; distances and directions from each other are also determined by these forces. These arrangements are linked together in a three-dimensional repeating network known as a lattice. From a consideration of the physics of particles packing around each other, it can be shown that there are only 14 space lattices possible. These are known as Bravais lattices. There are also limitations on how the motifs are arranged in the lattice such that they fill

54

Principal Types of Particulate Fillers all space. There are only 32 possible ways of arranging atoms around a point in a lattice leading to 230 possible arrangements of objects in space, the so-called space groups. The forces between the atoms result in specific three-dimensional repeatable orders. The limiting factors depend on the shape of the unit cell and the environment in which it grows. The external faces of a mineral are therefore partly an accident of growth: changes imposed by environmental factors do not alter the fundamental properties of the crystal, although other effects may be noticed. The regular ordered structure of the crystal means that it may have different properties in different planes and directions. The nature of the atoms forming the mineral, the forces between them and their order determine its fundamental properties such as chemistry, density, optical properties, hardness and shape. They will also determine the mechanisms by which minerals fracture and thus play a strong role in determining shape, size and size distribution. Impurities, which are invariably present in the mineral from its original formation, may also be present in the crystal and will play a significant role in all the above. Impurities will also play a major role in determining the surface chemistry of the minerals due to adsorption of organics or inorganics on to faces. The chemistry of the faces may also be altered, compared with the bulk, by weathering, and ‘fresh’ faces that may be formed during processing may have different adsorption behaviour and different reactivities than weathered ones.

2.2.3 Rocks Approximately 2000 minerals are known but relatively few are common, and these are based mainly on the elements aluminium, calcium, carbon, iron, magnesium, oxygen, potassium, silicon and sodium. Some elements, such as, sulfur and phosphorus, occur in localised high concentrations. All other elements are found in trace amounts. Minerals based on silicates and carbonates dominate and these are the ones mostly used in polymers. This is illustrated in Table 2.1, which shows the principal mineral fillers used in plastics in Western Europe. They originate from one of the three main divisions of rocks: igneous, sedimentary or metamorphic. Igneous rocks are formed by the cooling and solidification of a molten rock mass. During cooling, ordered crystallisation occurs. However, minerals may be formed not only by crystallisation, but may also result from reactions between already formed minerals and molten ingredients. Minerals may be formed throughout a rock, concentrated in certain areas, or in very localised veins or strata. The bulk of the silicates such as kaolin, mica, talc, feldspar, wollastonite and quartz are all found in igneous rocks.

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Particulate-Filled Polymer Composites

Table 2.1 Some types of mineral fillers used in plastics in Europe Main mineral (crystalline phase)

Chemical composition (simplistic formulae used for the silicates, see individual entries for details)

Shape

Calcite

CaCO3

Aspect ratio about 3:1, blocky

Talc

MgSiO3

Platy

Kaolinite

Al2O3.2SiO2.2H2O

Platy

Calcined clay

Metakaolinite

Al2Si2O7

Low aspect ratio, platy

Calcined clay

Amorphous

Al2Si2O7

Low aspect ratio, platy

Alumina trihydrate

Gibbsite

Al(OH)3

Low aspect ratio, platy

Magnesium hydroxide

Brucite

Mg(OH)2

Variable

Wollastonite

Wollastonite

CaSiO3

Acicular

Mica

Muscovite

KFeMgAlSiOn

Platy

Mica

Phlogopite

KFeMgAlSiOn

Platy

Silica

Quartz

Si O 2

Cubic

Silica

Amorphous

SiO2

Aggregated

Filler

Natural calcium carbonate Talc China clay

Sedimentary rocks are derived from weathered rock masses and deposited by the action of water or other means, or by the sedimentation of bioliths (mineral skeletons, shells, etc., of plants and animals) into layers, or, rarely, by chemical precipitation. Massive deposits of china clay or kaolin are found as a result of weathering, followed by movement and deposition. Limestone, the general term for natural calcium carbonate rocks, is the most abundant of the sedimentary rocks and is formed by the deposition of countless skeletons and shells. Gypsum and diatomaceous earths occur widely in sedimentary rocks. Sedimentary rocks are almost universally found in layered beds, which may have folded or otherwise been altered by subsequent geological events. Layers will differ from each other in texture, mineralogy and particle size. Metamorphic rocks are formed mostly from sedimentary deposits, which have been subjected to stresses occurring within the earth’s crust. These forces will cause bending, breaking, generation of considerable heat and inclusion of magma. The temperatures

56

Principal Types of Particulate Fillers and pressures involved deform rocks, melt and recrystallise them. By far the most important metamorphic rock is marble, produced from limestone. Impurities in the limestone can lead to formation of dolomite, tremolite, wollastonite and phlogopite in the marble. Brucite (magnesium hydroxide) sometimes is found in metamorphosed limestone. Other minerals of interest to the polymer industries formed by metamorphic action are biotite, feldspar and chlorite.

2.2.4 Calcium Carbonate Minerals 2.2.4.1 Properties Calcium carbonate exists in three crystal modifications, aragonite, calcite and vaterite, but only the calcite form is of real importance. Because of calcite’s perfect rhombohedral cleavage, it is a soft mineral with a Mohs hardness of 3.0. It has a specific gravity of 2.7 and is birefringent having refractive indices of 1.65 and 1.48.

2.2.4.2 Occurrence and Processing Commercially viable deposits of calcite occur throughout the world. These deposits differ considerably not only in purity, but in size and genesis, and the variations affect the nature of a filler produced from them. Over the years various terms have been used to describe these different materials but the ones in general use today are chalk, limestone and marble. Chalk formations are soft-textured limestones, which were laid down in the cretaceous period between 70 and 130 million years ago. Nanofossils are still very commonly found in these deposits and a typical coccolith (with a diameter of about 8 μm) found in a chalk from Wiltshire in the south of the United Kingdom is shown in Figure 2.1. Limestone is a consolidated sedimentary calcium carbonate rock. Consolidation may be due to packing by the overlaying rocks or soil or by recrystallisation of calcite through the leaching effects of water. The deposits are usually organic in origin being formed by the deposition of shells and skeletons of marine organisms – predominantly coccoliths. Consolidation pressures may have substantially altered these nanofossils. Marble is metamorphic limestone. Under high pressures and temperatures in, for example, volcanic activity, the original calcite (or aragonite) has been recrystallised giving rise to a coarse-grained rock, which is more dense and harder than chalk. Both limestone and marble deposits can be heavily contaminated with dolomite, calcium magnesium carbonate, and magnesite, magnesium carbonate, from the interaction of calcite and magnesium ions in the sea water in which deposition occurred.

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Particulate-Filled Polymer Composites

Figure 2.1 Coccosphere from a UK chalk deposit

During the formation of the original calcium carbonate deposits, other minerals may also have been deposited, or organic matter and salts may have percolated through the rocks under action of ground water. During metamorphic transformation, this will have been incorporated into the marble structure. Thus, a large proportion of the calcium carbonate throughout the world is contaminated resulting in dark-coloured marbles and limestones, of little use to the polymer industry. However, there are many deposits around the world of sufficient purity and close enough to the surface that products suitable for use in a wide range of applications can be obtained by a relatively simple dry-mining operation followed by grinding. Grinding can be carried out dry or in aqueous suspension depending on the exact nature of the impurities present. Chalk deposits, as mentioned previously, consist of the skeletal remains of marine animals. These skeletons consist of rounded calcite crystals around 3 μm in size, loosely bonded together. Grinding operations break these agglomerates into their constituent crystals to produce fillers with average particle size around 3 μm (40 wt% of particles finer than 2 μm) and a surface area of around 1 m2 g-1 (Figure 2.2). The top cut will depend on the energy put into the grinding but, for the plastics industry, is usually around BS 300 mesh. Coarse particles (above 10 μm in diameter) can also be removed by classification. Fine white powders produced from chalk deposits are very widely known as chalk whitings or simply whitings.

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Principal Types of Particulate Fillers

Figure 2.2 Typical particles of a ground UK chalk whiting

Marble deposits can be processed in the same way as chalk but, because marble contains different sized calcite crystals, or grains, a wider range of particle sized products, with wider size distributions are obtained, especially as there may be a need to fragment large crystals. Actual particle sizes will depend on the extent of processing, but typical products will have a mean value of 3-5 μm, with 20-30 wt% finer than 2 μm and a specific surface area around 1 m2g-1. The colour of products, from whichever rock they are produced depends on the purity of the deposit and whether selection is used prior to grinding and whether purification procedures, such as magnetic separation are used during processing. In general, ground marble powders aimed at the polymer industry are coarser, but whiter than ground whitings. There are also a large number of products produced from heavily compacted limestone, which are known as ground limestones. Their average particle-size distributions will be similar to ground-marble products, while colours can range from fairly pure white to grey. All of these products, because of the rhombohedral cleavage pattern of calcite crystals, consist of (mostly) uniform calcite crystals with a fairly symmetrical aspect ratio of 3:1. Producing even finer products than the above requires very large energy inputs because of the need to cleave these ‘primary’ calcite crystals, and energy required is approximately proportional to the new surface produced by grinding [4]. For example,

59

Particulate-Filled Polymer Composites grinding calcite to obtain particles with 0.25 μm equivalent spherical diameter (esd) will use more than 20 times the energy of reducing it to 5 μm. Wet grinding is much more efficient in dissipating such energies and also in transmitting applied stresses. The calcite crystals formed by grinding very finely in either wet or dry processes will have the same aspect ratio, approximately 3:1 as the coarse powders discussed earlier. Although it is found with a large variety of habits, calcite crystals, in fact, have a rhombohedral shape, which is independent of its particle size [5]. These calcite crystals are usually fairly pure calcium carbonate with only traces of transition-metal ions (iron in particular) occurring in the lattice. Surface layers may be calcium carbonate, hydroxide or, in freshly broken crystals, possibly calcium oxide. It is now well established that coccoliths in chalk are coated with very thin aluminosilicates (smectites and other clay minerals) and organic matter (humates, etc) [6]. During comminution, compaction or metamorphosis, these coatings may be disrupted or may still adhere to the calcite surface. Aluminium silicates have been identified in pure limestones and pure marbles, and have been shown to be unattached to calcite fragments.

2.2.4.3 Surface Modification Although it seems that the calcite surface in these fillers is often covered by either organic materials or silicate minerals, their chemistry is determined by the basic nature of calcium carbonate and its reactivity towards acids. This is of considerable importance. In particular, their reaction with fatty or other organic acids, but especially stearic acid, has been used for many years [7, 8] to improve compatibility with, and dispersion in, polymers. The coated fillers are much more hydrophobic than uncoated ones, reducing water pick-up, and they have also been shown to have an effect on polymer morphology, and hence modifying properties (see Chapter 1). It is postulated that the stearic acid molecules interact with the calcium carbonate, with the carboxylate ion reacting with the surface and the organic chains sticking out normal from the surface. It has been reported that oleic acid, or any other acid, in which the presence of double bonds alters chain configuration, adsorbs up to a monolayer with the molecules lying flat on the mineral surface [9]. Much work has been carried out on other surface modifications of calcium carbonate [10] and a more detailed discussion can be found in Chapter 4. From the authors’ experience, many beneficial claims have been made (mostly in patents) for surface treatments and coupling reactions, but the evidence when examined is flawed. Certainly, the market place is sceptical of the supposed benefits because very few coupling agents/surface modifications are used commercially.

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Principal Types of Particulate Fillers

2.2.4.4 Uses Calcium carbonate is used in many rubber and plastic applications, mainly to reduce cost, but ultrafine grades, especially when stearate coated, can give technical advantages, for example, in polypropylene (PP) mouldings, unplasticised polyvinyl chloride (PVC) extrusions and polysulfide sealants.

2.2.5 Dolomite 2.2.5.1 Properties, Occurrence and Extraction The other carbonate mineral in common use as a filler is dolomite: calcium magnesium carbonate (CaCO3. MgCO3). It is in the trigonal crystal system and is usually found as rhombohedral crystals with curved composite faces. Dolomite is harder (3.5), denser (2.85), and slightly more acid resistant than calcite, but in general, the mineral properties and properties in a polymer are similar. Dolomite is very widespread with at least 60 producers in 27 countries, but the bulk of the production goes into building and chemical industries; there are only a handful of suppliers of fine powders to the polymer industries. Production is by dry mining and dry grinding, with air classification to remove coarse particles (down to 10 μm).

2.2.5.2 Uses The uses of dolomite follow the same pattern as calcite, although, because its occurrence is much less widespread, tonnage is much lower. Ultrafine products (i.e., with an average particle size of 1 μm), are not available. Because dolomite is more difficult to grind than calcite due to its higher hardness, comminution is more costly.

2.2.6 China Clay or Kaolin 2.2.6.1 Occurrence, Extraction and Properties Kaolin (Al2O3.2SiO2.2H2O), is probably more widely known as the clay mineral, china clay [11]. It is found in hydrothermal, residual and sedimentary deposits (probably about 1000 are still commercially worked) around the world with the most important resources being in Cornwall in SW England and in South Carolina and Georgia in the USA. Large

61

Particulate-Filled Polymer Composites deposits are found, and are being progressively exploited in, for example, Russia, the Ukraine, the Amazon basin in Brazil, Spain, Australia, Bavaria and Bohemia. There has been a significant consolidation of the industry in the last few years. Hydrothermal and residual deposits are classified as primary occurrences, although some authorities call them secondary as they are found in the rocks from which they have been formed. Sedimentary deposits are always secondary. The kaolins are formed by the hydrothermal alteration and weathering of feldspathic igneous and metamorphic rocks (especially granite) under relatively low temperatures and pressures. The most common parent minerals are feldspars and muscovite micas. The primary deposits will usually be mixed with unaltered granite, mica, feldspar and quartz. Secondary deposits are usually found in ‘layered qualities’ with very variable purity. The main primary deposits in SW England and Brazil are exploited by hydraulic mining, while other small commercial deposits are utilised by a variety of techniques, with varying levels of sophistication. The most widespread involves dry mining of selected areas followed by a batch wet-washing and sometimes refining in terms of particle size. Products are then dried. In hydraulic mining the clay is washed out of the granite matrix using high-pressure jets of water. Refining into different particle size fractions is carried out by sedimentation of this aqueous slurry, using the principle of Stoke’s law for particles falling through a viscous medium to select the required particle size. To achieve good separation, the particles must be deflocculated (separated from each other); this is usually achieved at a neutral pH and by treatment with a polyanion (see below). Mineralogical separation is also achieved in the refining step with ancillary minerals (mostly feldspar, quartz and mica) remaining in the coarse fractions. During aqueous processing, products may be reductively or oxidatively bleached to reduce or remove coloured inorganic (usually hydrated iron oxides), and organic (humus type materials) coatings on the particle surface. The clays are then filtered, dried and, for the polymer industries, pulverised to break down agglomerates, which form during drying. Sedimentary deposits are mined by a variety of techniques depending on the nature and extent of the impurities. The simplest and cheapest production route involves dry mining, crushing and milling. More sophistication is used for air-float products, where the clay after crushing and grinding passes into an air stream of constant velocity and grit and coarse particles remain behind. More controlled, purer, products are produced by wetrefining with the dry-mined clay being dispersed in water, degritted and refined using hydrocyclones or centrifuges. Micromineral separation is being used increasingly in both types of production routes with flotation, selective flocculation and magnetic separation being used to remove impurities from the clay.

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Principal Types of Particulate Fillers

Figure 2.3 Stack of kaolinite crystals found in granite

Particle-size distributions (expressed as equivalent spherical diameter, esd) of clays depend on the inherent particle size of the deposit and the amount of refining that has been carried out during production. In primary deposits the kaolin plates are usually bound together in a book-type structure (Figure 2.3) and refining will separate them to a certain extent. Most commercial products for the polymer industries will be degritted at 300 mesh, so the normal top cut is at least 75 μm. More commonly, clays will have been refined to 20 or 10 μm for a range of applications, and speciality products will have a top cut of 5 μm or finer. Secondary clays are usually very much finer than primary clays and products that are approximately 100% finer than 5 μm can be obtained by a fairly simple air-float or degritting procedure. Although kaolin, china clay (and even clay) are often used interchangeably as names, the main mineral present is usually (but not invariably) kaolinite. In fact some ‘china clays’ are sold that contain only 25% kaolinite, but normally kaolins will contain 50-99% kaolinite with the main impurities being mica, quartz and feldspar. Other silicates, metal oxides and organic matter are usually found in trace amounts. Pure kaolinite has the idealised chemical composition Al2 O3.2SiO2.2H2O. It is a crystalline material with a triclinic form found in microscopic pseudo-hexagonal plates (Figure 2.4),

63

Particulate-Filled Polymer Composites

Figure 2.4 Typical kaolinite plates

which readily undergo cleavage. As a consequence it has a low Mohs hardness, between 2.5 and 3, depending on the ancillary minerals present. Its specific gravity is 2.6 and refractive index is 1.56. Its structure can be regarded as a gibbsite (Al(OH)3), layer bonded to a siloxane (Si2O5) layer. The silica atom in the siloxane layer is tetrahedrally bound to four oxygen atoms and the aluminium atom in the gibbsite grouping completes its favoured octahedral environment by coordinating to three oxygen atoms in the siloxane group. Thus, one face of this structure comprises oxygen atoms and the other of hydroxyl groups. These will interact through hydrogen bonding giving a layered kaolinite crystal, which has a pseudohexagonal platy shape. The idealised structure described above is invariably spoilt by nature. Isomorphous substitution of both silica and aluminium ions by transition metals, and particularly by iron, occurs. This leads to electrical charges occurring on the plates with edges being positively charged and faces negatively charged; the charges are countered by ions, which surround the particles in a double layer. (The reader is referred to Jepson [11] and references therein for a more complete discussion of the properties of clay and kaolinite). Because of the isomorphous substitution and also because of the occurrence of broken bonds at the edges, there are both Lewis and Bronsted acid sites on the surface of a kaolinite particle [12]. These can be very reactive. There is also evidence that some kaolinite particles are covered with a monolayer of amorphous silicic acid [13], with organic matter [14] and with hydrated iron oxides [15]. These thin, hexagonal plates have aspect ratios in

64

Principal Types of Particulate Fillers the range 5:1 to 50:1 (but very fine clays can have even higher aspect ratios), which are dependent on particle size [16]. Aspect ratios of the plates depend on the nature of the clay deposit with, for example, US clays being chunkier than clays from SW England, but they are also dependent on the processing used because kaolinite stacks or ‘booklets’ readily cleave during processing. Kaolinite reacts only with very strong acids and bases, is not affected by organic solvents, and undergoes ion exchange reactions, but from most points of view, it is an inert mineral. It does undergo a complex series of reactions when heated, which are of commercial importance and will be discussed fully next.

2.2.6.2 Surface Chemistry and Modification Although kaolin can be regarded as chemically inert it does have a complicated surface chemistry. Hydroxyl ions either from the gibbsite layer or from adsorbed gels readily react with commercial bifunctional silanes [17], titanates or other coupling agents [18]. Kaolinite plates have negatively charged faces and positively charged edges and adsorption of both positive and negative ions are of great commercial importance. For example, small amounts of polyanions adsorb on to edges, deflocculating or flocculating the clay depending on the molecular weight of the polymer, and are an essential feature of its use in aqueous media (the paper and paint industries especially). The presence of Lewis and Bronsted acid sites gives rise to a variety of chemical reactions. Amines, or other Lewis bases, readily adsorb and the use of fatty amines to render the clay organophilic has been applied for many years to modify properties in a number of applications [19, 20]. Because of these reactive sites, kaolin will enter into organic reactions and, of particular interest for plastics, will catalyse the polymerisation of certain monomers. Sometimes depolymerisation can occur and it can promote the dehydrochlorination reaction of PVC [21].

2.2.6.3 Uses China clay is a widely used white filler in the rubber industry. Depending on particle size, it can be used as a semi-reinforcing filler (hard clay) or a non-reinforcing filler (soft clay) in such applications as chemical liners, bicycle tyres, conveyor belts, shoe soles, gaskets and flooring. Its use in plastics is much more limited. In thermoplastics it is used for speciality antiblocking, in thermosets it is used in urea-, phenol- and melamine formaldehyde, in unsaturated polyesters, and in epoxy resins.

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Particulate-Filled Polymer Composites

Figure 2.5 Differential thermal analysis trace for a typical china clay

2.2.7 Calcined Clay When kaolinite is heated (Figure 2.5) to above 500 °C it dehydroxylates endothermically, that is, it loses its water of crystallisation forming metakaolinite. This is then stable up to 980 °C, when a defect spinel structure, which is virtually amorphous, forms exothermically. Above 1100 °C there is a slow transformation of the defect spinel with mullite forming in an amorphous silica matrix. If the heating is carried out very rapidly, then the outside of the clay particles can fuse, forming a glass before dehydroxylation occurs. The water formed cannot escape and forms closed pores, giving a lighter weight aluminosilicate, which still has a defect spinel structure. Each separate step produces aluminosilicates with unique properties having considerable commercial importance in the polymer industries.

2.2.7.1 Metakaolin Above 500 °C kaolinite starts to lose its water of crystallisation and, by 650 °C, approximately 90% of this dehydroxylation is complete, leaving residual OH groups randomly distributed but isolated so that condensation will not occur readily. The product formed is known as metakaolin. Some crystalline structure is retained [22] but X-ray diffraction patterns are so diffuse and weak that no better, more recent, identification has been made. After these structural changes the aluminium, which was originally in six-fold octahedral sites, occupies four- and five-fold sites almost equally [23].

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Principal Types of Particulate Fillers Metakaolin is much more chemically reactive than the kaolinite from which it was formed [24] and also, deduced from the properties of the polymer composites in which it is used; it has a very reactive surface. This is probably a consequence of an increase in the number of Lewis sites due to the reduction in coordination numbers of the aluminium ions and to the presence of reactive hydroxyls.

2.2.7.2 Calcined Clay Metakaolin is stable up to 980 °C, although between 900 °C and 950 °C the remaining OH groups become mobile and condense. This causes the rearrangement of the poorly crystalline metakaolinite structure to an amorphous defect spinel, which is commonly called calcined clay. This may also be regarded as formation of poorly crystalline mullite and a spinel phase with the separation of amorphous silica [25]. However, experimentally, the material produced up to 1100 °C is X-ray amorphous and the various phases suggested above are deduced from other techniques, such as nuclear magnetic resonance (NMR). Some isolated hydroxyl ions still remain on the surface of the calcined clay [26] and these enable it to participate extensively in coupling reactions, which are of considerable importance (see Section 2.2.7.4).

2.2.7.3 Refractory Material Above 1100 °C the rearrangement of the defect spinel into amorphous silica and mullite accelerates with the mullite becoming progressively richer in aluminium up to 1500 °C. On cooling the mullite crystallises into needles, which are embedded in an amorphous aluminosilicate glass. It is a very hard, abrasive mineral, which is virtually inert to all chemicals and environments. During calcination, kaolinite plates tend to fuse face-to-face with the consequences that calcined clays are coarser than the feed clays from which they are produced, but also have significantly lower aspect ratios, approximately 5:1 (Figure 2.6). Up to a calcining temperature of 1100 °C, cleavage is the preferred mechanism of comminution, but energy requirements are high. Above 1100 °C the rearrangement into defect spinel, amorphous silica and mullite, and subsequent recrystallisation means that particles become spherical and undergo fracture on grinding. Metakaolin and the amorphous aluminosilicate produced at 980-1100 °C have approximately the same Mohs hardness of 4.0, specific gravity 2.6 and refractive index 1.6. Above 980 °C, calcined clays are almost completely inert reacting only with very strong acids and alkalis under normal conditions. Their surfaces are likewise inert except for

67

Particulate-Filled Polymer Composites

Figure 2.6 Particales of clay after calcination at 1100 °C

the fact that between 980 and approximately 1100 °C there are still some reactive hydroxyls present. These react readily with hydrolysable groups such as found in the standard ranges of coupling agents, giving rise to a number of commercial fillers; these are mostly based on bifunctional silanes.

2.2.7.4 Uses There is some overlap in the performance of metakaolinite and calcined clay in certain polymers and hence some contiguous applications such as low-voltage rubber cable insulation, but there are significant differences, which leads to separate use patterns. Thus, metakaolinite is mainly used in PVC cable insulation because it improves (uniquely) the electrical resistivity of plasticised PVC, while calcined clay is used in polyethylene (PE) film, rubber cables, rubber pharmaceutical applications and rubber extrusions for a variety of reasons linked to its shape, size and chemical inertness. The refractory product finds limited use in epoxy and unsaturated polyester mouldings, which have to resist abrasion and chemical attack. When silane-treated, calcined clay can be used in polyamide and polybutylterephthalate (PBT) moulding compounds and in very high-voltage rubber cable insulation. Each application needs a silane with a different functionality.

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Principal Types of Particulate Fillers

2.2.8 Mica 2.2.8.1 Properties Mica is the name of a group of phyllosilicate minerals with the generalised chemical formulation KM(AlSi3O10)(OH)2, where M may be Al, Fe, Mg or combinations of these metals. All are characterised by a plate morphology and perfect basal cleavage, a consequence of their layered atomic structure. They are all monoclinic. Crystals are rare but are usually tabular in form. Their hardness is between 2.0 and 2.5, and specific gravity between 2.76 and 2.88, both depending on impurity levels. Colour is also variable. Refractive indices are directionally dependent between 1.56-1.57 (α direction) and 1.601.615 (β direction), so they show strong birefringence.

2.2.8.2 Occurrence and Extraction There are three commercially important micas: muscovite, which is aluminium rich, with colours varying from white to silver to pale grey or green; phlogopite, which is rich in magnesium, and biotite, which is rich in magnesium and iron. Both phlogopite and biotite are usually darker in colour than muscovite micas. There are also zinnwaldite [a lithium rich mica: KLiFe2+Al(AlSi2)O10(F, OH)2], and lepidolite K(Li, Al)3(Si, Al)4O10(F, OH)2, which are widespread but of no commercial importance in the polymer industries. Muscovite is a widespread and common rock-forming mineral, which is characteristic of granite. It is therefore commonly found in conjunction with other silicates and silicas such as kaolinite, feldspar and quartz, sometimes as the major, sometimes as the minor constituent. Production in these cases is by wet refining followed by flotation. Muscovite is also found in isolated massive deposits and production follows a dry mining (for the purest products these may involve hand-selection), grinding route. It is a very platy mineral that undergoes perfect planar cleavage (see later in this section) but plates can also undergo fracture. Therefore, both dry grinding and wet grinding are employed using a variety of grinding media to achieve different particle-size products with different aspect ratios. Usually careful wet milling, using say a ball mill, is used to give products with the highest aspect ratios. Commercial deposits occur in Argentina, Brazil, Canada, France, India, Republic of South Africa, Russia, UK, USA and Zambia. Phlogopite is also widespread being formed by the metamorphosis of magnesium limestones, dolomites and tetrabasic rocks. It is found usually in isolated massive deposits and processing follows a selection, dry mining, grinding procedure. Deposits occur in Canada, India and Madagascar.

69

Particulate-Filled Polymer Composites Biotite is an important and widely distributed rock-bearing mineral occurring in igneous rocks and thus is a major impurity in many other silicate minerals. Vermiculite, an alteration product of biotite and phlogopite, has its mica sheets interlayered with water. During calcination, exfoliation occurs due to water loss, and this expanded product has considerable use in heat and sound insulation because it has a very low specific gravity. Muscovite, ideally KAl2(AlSi3O10)(OH)2, has a dioctahedral structure but with some of the tetravalent silicon atoms being substituted by aluminium. This leads to negative charges on the aluminosilicate sheets, which are balanced by cations. Phlogopite and biotite have trioctahedral structures with, again, aluminium substituting partly for silicon and subsequent charges balanced by cations. In all three potassium is the common cation. These layered structures are only bonded weakly through large monovalent cations and thus micas cleave very readily to, in theory at least, monolayer thickness. Some rarer micas have more than one silicon atom substituted by aluminium; the extra negative charge produced by this is balanced by bivalent cations such as calcium. Stronger bonding occurs and these micas do not cleave so readily, are harder and much more brittle.

2.2.8.3 Uses Currently mica is used in phenolic moulding compounds aimed at the electrical industry and in some polypropylene mouldings. Because of the birefringence exhibited, some mica is used in decorative articles, sometimes with additional coatings.

2.2.9 Talc 2.2.9.1 Properties of Talc Pure talc has the chemical composition Mg3(Si4O10)(OH)2. It has a trioctahedral structure in which an octahedral brucite, Mg(OH)2, sheet is sandwiched between two tetrahedral siloxane (S2O5) sheets. This structure is electrically neutral and will bond to an adjacent layer only through Van der Waal’s forces. Thus talc undergoes cleavage very readily, is very soft and has a soapy feel. Faces of the layers consist of oxygen atoms, hydroxyl groups are only found at broken edges so that it is fairly hydrophobic. Pure talc is the softest known mineral with a hardness of 1 on the Mohs hardness scale, specific gravity is 2.8, and it has three indices of refraction, 1.539, 1.589 and 1.589, according to crystal direction. Typical particles are shown in Figure 2.7.

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Figure 2.7 Particles of a pure, fine talc

2.2.9.2 Occurrence and Processing Talc is a secondary mineral, which is found characteristically in metamorphic rocks from the degradation of minerals such as olivine, pyroxene and amphibole and also along faults in magnesium rich rock. The designation talc covers a wide range of natural products because the metamorphic changes that gave rise to talc also produced other magnesium minerals. Impurities commonly encountered include magnesite (magnesium carbonate), chlorite (a mixed magnesium silicate-magnesium hydroxide in which Fe2+, Fe3+ and Al3+ may substitute for Mg2+ and Al3+ for Si4+) and tremolite (calcium magnesium silicate). In fact, commercial talcs may contain the mineral talc only as a minor constituent or, in some (all chlorite) not at all. The only two products of significance to the plastics industry are talc and chlorite. Processable talc deposits are widespread, although not as extensive as those of calcium carbonate or clay, with main production coming from north-east, south and west USA, Norway, France, Italy, India, China and Australia. Some massive forms, known as steatite or soapstone (after the characteristic soapy feel of talc) occur, but most of the talc is found in veins. It is produced by conventional selective opencast mining or by underground mining followed by crushing, grinding, beneficiation and classification. After the crude rock is crushed it is often subjected to a manual selection process, although automated techniques are now being introduced (based on colour selection). It is then ground in a variety of ball mills, hammer mills and fluid mills with air-classification systems included in a closed-

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Particulate-Filled Polymer Composites loop system to produce products with a range of particle-size distributions but with the majority having top cuts of 100, 200, and 300 mesh, 20 and 10 μm. Some talcs may be subjected to beneficiation by froth flotation. The talc is ground, dispersed in water, subjected to froth flotation in which the talc may be floated from the impurities or vice versa using a surfactant, selective for the appropriate surface, to transfer it to a foam and effect separation. The talc may then be subjected to grinding, classification or both to achieve the desired particle size distribution.

2.2.9.3 Other Talcs Although described as talcs, many commercial products can contain large quantities of other minerals (as described above). These can markedly affect overall aspect ratio and hardness, and care should be taken in choosing talc for a specific application. Some talcs may contain fibrous minerals which pose health problems and the use of such talcs, especially where the fibrous mineral is tremolite, is strictly controlled in many countries. While normally lamellar or platy the aspect ratio of talc can vary considerably and is not usually specified, although high aspect ratio is the most important property in its use.

2.2.9.4 Surface Modification Conventional surface treatments do not appear to be very effective on talc. This may be due to the low concentration of reactive surface hydroxyls or to the weak nature of the particles themselves.

2.2.9.5 Uses Talc, because of its high aspect ratio, is used in a variety of applications. In rubber, it is used as a partitioning agent and as a semi-reinforcing filler; in membranes, to reduce gas and fluid permeability; and in electrical insulation and sheathing. In plastics it is used to give rigidity to thermoplastics, mostly PP but also some PE and Nylon mouldings.

2.2.10 Montmorillonite (AlMg)8(Si4O10)3-(OH)10.12H2O 2.2.10.1 Properties and Occurence The montmorillonite group, also known as bentonite, comprises a number of clay minerals with dioctahedral (gibbsite) and trioctahedral (Si2O5) groups in three layer structures. They 72

Principal Types of Particulate Fillers have charge deficiencies partly due to isomorphous substitution of Al3+ and Si4+ in each layer, which are balanced by cations (principally sodium and calcium). Water, other hydroxylcontaining groups and amines readily break these ionic bonds. Once the sheets have been parted, many layers can be adsorbed producing a marked expansion of the structure. Bentonites treated with up to 40 wt% long chain fatty amines (such as octadecylamine), often called organoclays, give structure or thixotropy to organic liquids and, for example are widely used in solvent-based paints. Under certain conditions, separation of the platelets into primary layers can occur giving very high aspect-ratio products. Montmorillonite is monoclinic occurring in very thin, small platelets with specific gravity 2.5, Mohs hardness of 1-1.5 and refractive index of 1.50-1.64. It is the predominant clay mineral in bentonite and is found with other clay minerals, such as kaolin, and with silica. Commercial deposits are found in the Morocco, Spain, Turkey and USA.

2.2.10.2 Uses The main uses of montmorillonite stem from its characteristic expansion, and it is used to control viscosity or impart thixotropy to a variety of liquid polymers based on unsaturated polyesters, PVC plastisols, polysulfides, alkyds, etc. It has also been reported to control the melt rheology of thermoplastics and to reinforce polyamides. There has, over the last few years, been enormous industrial and research interest, with many papers and patents, published on montmorillonite, especially as an organoclay as the basis of polymer nanocomposites. Because of the delamination process described above plastic-organoclay nanocomposites have been reported to have very high rigidity, low permeability to fluids, and fire resistance. This subject is covered in more detail in Chapter 10.

2.2.11 Barites (BaSO4) 2.2.11.1 Occurence and Properties

This is widespread usually as a gangue (impurity) mineral in hydrothermal veins associated with metallic ores; some also occurs in limestone or clay. Its crystals are usually tabular and often diamond shaped, but fairly symmetrical. It has two features of commercial interest. Its specific gravity, 4.5, is the highest of the non-metallic minerals and it is chemically inert. It has a Mohs hardness of 3.5, and a refractive index 1.64-1.65.

2.2.11.2 Uses It is used in a variety of polymers to give a very high density, for example, for sound deadening. It is also used in applications that require very high levels of chemical resistance.

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2.2.12 Calcium Sulfate Products 2.2.12.1 Occurence and Properties Gypsum (CaSO22H2O), is a common mineral very widely distributed in sedimentary beds throughout the world. It comprises monoclinic crystals with perfect cleavage yielding thin folia. It is a soft mineral with a Mohs hardness of 2, specific gravity 2.32 and refractive index 1.52-1.53. It is slightly soluble in water and hydrochloric acid, but its principal chemical reaction is that it starts to dehydrate between 65 and 140 °C giving a hemi-hydrate known as plaster of Paris, which further dehydrates up to 200 °C giving anhydrite, which is also found commercially. Gypsum can recrystallise from aqueous media to give a fibrous form, which can be dehydrated to give plaster of Paris or anhydrite in fibrous form. The hemi-hydrate will slowly rehydrate ‘hardening and setting’. Many millions of tonnes of gypsum are used worldwide as a consequence of this dehydration - rehydration reaction and are used in building boards, moulds, casts, plasters, etc. There have been significant developments in recent years in the production of calcium sulfate dihydrate, anhydrous and fibrous calcium sulfate, and their applications as flameretardants and as fillers for plastics. These are described fully by Socha and the reader is referred to that paper [27].

2.2.13 Wollastonite (CaSiO3) 2.2.13.1 Properties Wollastonite is a white, needle-like mineral with a specific gravity of 2.9, Mohs hardness of 4.5 and refractive index between 1.63 and 1.67. It is triclinic and undergoes cleavage along all three faces so it can be obtained in acicular forms with high aspect ratios. However, these needles also readily fracture to give low aspect-ratio fragments. Their aspect ratio is usually 1:8 but higher values can be obtained depending on production route. Typical wollastonite particles are shown in Figure 2.8.

2.2.13.2 Occurrence and Processing Although wollastonite is a constituent of many types of rock, it is rarely found in an economically processable form. The majority of deposits are formed by contact metamorphism of limestone and quartz. Extraction is concentrated on distinct, fairly pure, layers of wollastonite-rich rock. Worldwide production is currently about 250,000 tonnes per year, with China, Finland, India, Mexico and USA being the main producing countries.

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Figure 2.8 Typical wollastonite particles

It is obtained by both opencast and underground mining. After extraction and crushing, the material is beneficiated either magnetically to remove garnet and diopside, or by flotation to remove quartz and calcite. The various wollastonite fractions are then blended, dried and ground, either in a pebble or attrition mill. The latter mill produces the high aspect ratio particles of most use in polymers. Finally, the products are classified and bagged; surface treatments are also often added to grades intended for polymer uses.

2.2.13.3 Surface Modification Numerous surface treatments are used to improve dispersion, particle alignment and particle to polymer adhesion. They can also aid in reducing the abrasiveness of the hard particles to processing equipment, especially dies. Silane coupling agents are very effective and are widely used. Stearate coatings have also been reported to have some applications, but have not been specified by the wollastonite producers.

2.2.13.4 Applications The polymer applications of wollastonite are mainly based on its combination of acicularity, chemical inertness, whiteness and low water adsorption. Primary consuming polymers include thermoplastics, such as polypropylenes and polyamides, and thermosets, such as polyester dough and sheet-moulding compounds, epoxies and phenolics. Manufacturers are predicting considerable growth in reinforced reaction injection moulding (RRIM) systems.

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2.2.14 Crystalline Silicas 2.2.14.1 Properties, Occurrence and Extraction Crystalline silica is the most ubiquitous mineral on earth, being found as a component in almost all other mineral deposits. There are over 20 crystalline phases of silica, differing only slightly in physical properties. Most of these occur naturally, but only a few find any significant use in polymers, notably quartz, novacite and crystobalite. Quartz is the most widely used and is usually found as prismatic crystals. Mohs hardness is 7, specific gravity is 2.6 and refractive index 1.54-1.55. Despite the widespread nature of crystalline silicas, high-purity deposits that are close enough to the surface for economically feasible recovery are not numerous. For polymer applications the purity normally has to be 99% or greater. Good-quality silica deposits have been commercially developed in the US, Europe, India and Australia, although processing methods are most developed and the widest range of grades are available in the US and Europe. Japan has begun processing imported Australian and Indian silica, and now has some grades available. Naturally occurring silica is categorised as crystalline or microcrystalline according to the size of the primary particles in the deposit. Some deposits contain mostly large primary particles, well over 600 μm in size, while others may contain primary particles of under 100 μm. All deposits will contain mixtures of many primary particle sizes, either tightly or loosely bonded together. High concentrations of small loosely bonded primary particles usually characterise microcrystalline deposits and the matrix is thus usually quite friable. Larger, more tightly bonded primary particles are usually referred to as crystalline silica. Primary particle shape can range from round to angular, depending on the natural deposit, but once the primary particle is broken by grinding, all crystalline silica particles become angular in shape. Once ground, the performance of micrometre and submicrometre crystalline silica particles in a polymer composite is influenced by both particle size distribution and an acceptable topsize. Extraction is predominantly by crushing, removal of major impurities, drying, grinding and classification. Typical silica particles are shown in Figure 2.9.

2.2.14.2 Particle Size Crystalline silicas as used for filler applications are generally low surface area, low-oiladsorption materials with angular particles graded by topsize. Identification is on the basis of 98% passing a given spherical micrometre size. A 44 μm product would therefore have 98% by weight of particles below 44 μm. (NB: This odd-looking 76

Principal Types of Particulate Fillers

Figure 2.9 Typical particles of a crystalline silica used in polymers

definition is due to the inability of 100% passing to be precisely defined by practical particle-size equipment. Passing 98% is more readily characterised and is still near enough to 100% to be meaningful.) Particle size distribution is as important as top size for many applications and can vary due to the nature of the deposit or the processing conditions used. A major issue is consistency of both top size and size distribution. In coatings and liquid composites with relatively low loadings, a narrow particle size distribution is generally preferred. At high loadings, a controlled, broad distribution is preferred for more efficient particle packing.

2.2.14.3 Uses in Polymer Composites The uses of crystalline silica are a result of its high hardness, chemical inertness, heat resistance, low coefficient of thermal expansion and good electrical insulation properties. Crystalline silicas also respond readily to silane coupling agents and have useful optical effects due to their lack of birefringence and low refractive index. Their major drawbacks are their abrasivity (causing wear of machinery) and possible health hazards (see Section 2.2.14.4). The properties mentioned previously make silica a very good functional extender for thermoset plastics, such as epoxy moulding and potting compounds for electrical and electronic parts, and unsaturated polyester compounds for microwave dishware. Crystalline silica has a natural affinity for silicone elastomers making it semi-reinforcing, especially when a silane coupling agent is used.

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2.2.14.4 Health Issues Crystalline silica differs from the other minerals discussed previously in that there are considerable concerns about its safety; while all the other minerals are currently regarded as nuisance dusts, users and potential users are strongly advised to obtain specific advice and information from individual suppliers. The primary hazard with crystalline silica is the development of silicosis due to the inhalation of fine particles (<10 μm). There are reports that the exposure to silica (or the existence of silicosis) is associated with an increase in the incidence of lung cancer; the International Agency for Research on Cancer classifies silica in Group 2A ‘probably carcinogenic to humans’. The applicable governmental regulations in various countries should be consulted and followed. Also, various codes of practice, such as ASTME 1132 [28], Standard Practice for Health Requirements Relating to Occupational Exposure to Silica, and ANSI Z 88.2 [29], Practices for Respiratory Protection, may be consulted. The EU Scientific Committee on Occupational Exposure Limits (OEL) for Silica (Crystalline) issued in July 2002, recommendations that its OEL should lie below 0.05 mg/m3 of respirable dust.

2.3 Synthetic Particulate Fillers 2.3.1 Carbon Black 2.3.1.1 Introduction Although synthetic and of relatively high cost, carbon blacks are produced in vast quantities for use in polymers. This is mainly due to their widespread use as a reinforcing agent for elastomers, especially in tyre applications. Worldwide production was estimated to be 6.2 Mt in 1999, of which about 90% was used in rubber applications [30]. In addition to their economic importance, carbon blacks exhibit extreme forms of some of the most difficult characterisation issues in the particulate fillers area, especially regarding size and shape determinations, and surface chemistry. However, largely because of their commercial value, more has been done to make advances in these fields than with most other fillers and this pioneering work has much to teach us in a general sense. In elastomers, carbon blacks are used at relatively high, ‘filler’, loadings, but function as reinforcing agents, improving many properties of the composite, especially tensile strength, tear strength, stiffness, abrasion resistance and dynamic properties. In thermoplastics, they are used at lower loadings to improve weathering performance. A specialised

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Principal Types of Particulate Fillers application is as an electrically conductive filler in the production of antistatic compounds from both thermoplastics and thermosets. They are also widely used as pigments. Because of their importance, carbon blacks have been studied extensively and have a voluminous literature. Only a brief outline of their manufacture, properties and uses will be given here. The reader is referred to specialist publications, such as that by Donnet and coworkers, for more details [31].

2.3.1.2 Production Carbon blacks are, in effect, soots produced by incomplete combustion of volatile organic materials, principally oil and gas. As such they have been made and used as pigments for well over a thousand years. Their production as fillers for polymers has only been carried out since the early part of this century. During this time, there have been four main processes resulting in products of different characteristics: furnace, channel, thermal and lamp blacks. Today, the furnace process dominates, accounting for well over 95% of all black production, the rest being largely thermal black for specialised applications. The furnace process, as its name implies, is based on combustion of gas or oil in a specially constructed furnace. A carefully controlled air to fuel ratio is used. This, together with the turbulence in the furnace, largely controls the particle shapes and sizes produced. The nature of the feedstock also affects size, with the finest particles being produced from natural gas rather than oil. Residence times in the furnace are very short and, on leaving, the hot gases are quenched by a water spray. The carbon black is collected by a combination of electrostatic precipitation, cyclones and bag filters. As discussed in Chapter 1, the morphology or structure of carbon black is a very important property. In addition to turbulence in the furnace, this is controlled by introducing additives into the flame. Because blacks produced by this route are of very low bulk density, they are difficult to transport and handle. They are therefore usually formed into free-flowing pellets by a wet-pelletising process. This has to be controlled carefully so that the pellets are strong enough to pass through solids handling equipment without breaking down, but are readily dispersed to their primary particles when used in polymers. In the channel black process, diffusion flames burning natural-gas impinge on reciprocating metal channels where carbon is deposited. Rotating drums may also be used. The carbon is scraped off, collected, micro-pulverised and then usually pelletised. These blacks have a much higher combined oxygen content than furnace blacks. This process is little used now largely due to unfavourable economics and environmental problems.

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Particulate-Filled Polymer Composites The thermal process produces low-structure blacks of fairly large particle size. These have certain niche applications and, hence, while not as economically attractive as the furnace route, some thermal black continues to be made. The process is carried out batch-wise by decomposing methane (from natural gas) into carbon and hydrogen in the absence of air, in a furnace at about 1300 °C. The furnace is preheated by burning an airfuel mixture, the fuel often being the hydrogen from the process itself. A related process is used to produce acetylene blacks. Acetylene is decomposed to carbon and hydrogen in a controlled manner. Being exothermic, this can be carried out in a continuous fashion. The black produced has laminar or platy particles. Its primary use in polymers is to impart electrical conductivity.

2.3.1.3 Properties As the name implies, carbon black is largely composed of carbon, although considerable quantities of hydrogen and oxygen can also be present. The particles are submicrometer in size, with surface areas in the range of 5-150 m2 g-1. The thermal blacks are at the lower end of the surface area range, and are approximately spherical; but the others have complex three-dimensional shapes, which can be thought of as the partial fusing together of a number of nanometre-sized spherical primary particles. This complex shape is known as ‘structure’ and is usually measured by a variant of the oil absorption procedure. Structure plays an important role in determining many of the key properties of elastomer composites [32]. Because of this, much effort has been, and still is being, devoted to describing the shape of carbon black particles and understanding how this affects composite properties. A further discussion of shape measurement can be found in Chapter 1. Carbon blacks have very chemically active surfaces resulting from their production process, with hydrogen, oxygen and sulfur being the principal surface atoms combined with the carbon. The species of most importance are acidic and basic oxides, active hydrogen groups and highly reactive sulfur moieties. These lead to strong, covalent bonds with many polymers, especially unsaturated elastomers. It is this strong bonding that helps make carbon blacks such good reinforcing agents for elastomeric compounds. One example of their high reactivity is the formation of bound (i.e., non-extractable), rubber on simple compounding. This has also been discussed in Chapter 1. Careful heat treatment can remove the reactive surface species. This process is known as graphitisation as some limited crystalline growth is also observed, although, strictly speaking, no three-dimensional graphitic order is produced. Such ‘graphitised’ blacks show little, or no, bound rubber formation and their elastomer reinforcing properties are severely reduced [33]. Electron microscopy also reveals much earlier void formation on stretching than is

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Principal Types of Particulate Fillers observed with normal blacks, confirming reduced polymer-particle interaction [34]. Because of their large-scale use in very demanding applications, the measurement and effect of particle characteristics has received much more study with carbon blacks than with any other filler. As detailed previously, the principal factors are particle size, shape and surface reactivity. Dispersion also plays a critical and often overlooked role, and excellent work on this topic has been reported by Boonstra and Medalia [35, 36].

2.3.1.4 Uses Carbon blacks are widely used in polymers. Principal applications are as reinforcing agents for elastomers, especially in automotive tyres. Other applications include their use as conductive fillers, pigments, and as stabilisers, especially in outdoor exposure.

2.3.2 Synthetic Silicas 2.3.2.1 Introduction Two forms of synthetic silica find significant use in filled polymers. These are both very small particle, high specific surface area forms, and are known as precipitated and fumed silicas. Long known as the ‘white carbon black’ because of it’s reinforcing ability in elastomers, the use of precipitated silica has increased markedly recently, due to it’s growing use in low rolling resistance (energy or green) automotive tyres [37].

2.3.2.2 Precipitated Silica a) Production Precipitated silicas are normally made from solutions of sodium silicate (water glass). The sodium silicate solution is usually made by digesting a massive form of silica, such as sand, with sodium hydroxide solution. Precipitation is carried out by adjusting the pH with acid (sulfuric is usually used). Like all precipitations, the process is complex and governed by many variables, including: temperature, pH profile, electrolyte concentration, agitation and time. Useful information can be found in Iler [38], in Watson [39] and in manufacturer’s literature [40]. The conditions can be adjusted to give products with specific surface areas in the range 25-700 m2/g-1. For filler applications, the range 25-250 m2/g-1 is the most favoured. The products are usually isolated by filtering and washing on filter presses,

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Particulate-Filled Polymer Composites followed by drying, sieving and grinding. The products are often granulated to reduce dusting and improve handling and to speed incorporation into polymers. Precipitated silica technology has a long history and, until recently, it had all the appearance of a mature technology. This changed with the advent of the large, energy tyre application, which generated the resources for a fresh look at the product and manufacturing processes. The result has been products with significant improvements in dispersability, leading to similar improvements in vulcanisate properties such as abrasion resistance [41]. The surface structure is also claimed to have been improved, leading to products that need less silane coupling agent, a significant cost factor [42]. Although frequently used with organo-silane treatments, these fillers have not traditionally been pre-coated, the in situ method of addition being preferred (see Chapter 4). Largely due to developments in tyre technology, some pre-coated products are now becoming available. b) Properties Precipitated silicas are amorphous, and this has several advantages. Firstly, there is no problem with varying refractive indices, which helps in formulating colourless and transparent products. Most importantly, the health problems associated with crystalline silicas should be absent. In simplistic terms, the particles can be thought of as being similar to carbon blacks, and to consist of very small primary particles that are aggregated into larger structures and which are the effective particles. The primary particle size is in the range 5-100 nm and the aggregates can contain several hundred primary particles. The way in which the primary particles are combined in the aggregates can vary from open, chain-like structures, to denser, spherical structures and this will very much affect both their physical properties and performance. As with the carbon blacks, ‘structure’ is a very important parameter and is usually measured by an oil absorption method. One feature that surprises many people is the high water content of precipitated silica. The actual silica content is only about 90%. The rest of the mass is made up of three types of water: that combined as surface silanols, strongly hydrogen bonded water and loosely held water. The latter is about 5% by mass of the filler and can be removed by drying at 105 °C. The effects of this water are not often considered, but it can obviously be important, especially in reactions with surface modifiers, such as organo-silanes. Normal products also contain significant amounts of sodium and sulfate from the precipitation process.

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Principal Types of Particulate Fillers c) Surface Modification The true particle surface is highly hydroxylated in the form of silanols, and hence organo-silanes are very effective surface modifiers and widely used. As mentioned previously, the real surface also has significant amounts of water present, which can influence both the efficiency of the organo-silanes and the types of surface structure formed. Three types of surface silanols can be distinguished, isolated, vicinal and geminal and they have different reactivity towards silane coupling agents. More on this topic can be found in Chapter 4. For tyre applications, sulfur-containing silanes are used. Traditionally these have been added during compounding, but pre-coated forms are now becoming available. d) Polymer Applications The main applications, by far, are as reinforcing fillers in hydrocarbon elastomers. Organo-silanes, such as the mercapto, polysulfides, amino and vinyl are used to improve filler to polymer interactions and reinforcement. Particularly important uses are in tyres and in footwear. There is also significant use as a reinforcing filler in silicone elastomers.

2.3.2.3 Fumed Silicas a) Production These silicas are produced by a gas phase, as opposed to a solution process. There are a number of possible gas phase routes, but the predominant one is by hydration of silicon tetra-chloride. This is carried out in a flame of hydrogen and oxygen at a temperature of 1000 °C or above, and is similar to the chloride route for the manufacture of titanium dioxide pigments. Details of the process can be found in the work by Watson [39] (see also Figure 2.10).

2H2 + O2  2H2O SiCl4 + 2H2O  SiO2 + 4HCl 2H2 + O2 + SiCl4  SiO2 + 4HCl Figure 2.10 Process for manufacture of fumed silica fillers

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Particulate-Filled Polymer Composites The solid product is separated by cyclones or gas filters and contains some residual absorbed hydrogen chloride. This is removed in a hot moist air stream, rather than by washing. For the best economics, the hydrogen chloride should be recovered and used to generate more silicon tetrachloride. Particle formation is very similar to that for carbon black, with very small, primary particles forming first and then fusing, while still mobile, into chain-like aggregates. Properties like particle size, surface area and shape are controlled by factors such as reactant ratio, temperature and turbulence. Because of the high temperature formation process, the surface is less hydroxylated than that of the precipitated silicas, and as a result adsorbs less water. The moisture loss at 105 °C is usually less than 1.5%. b) Surface Modification The presence of surface silanol groups makes these fillers very responsive to organosilanes and these are the main modifiers used. Many of the applications of fumed silica require some degree of hydrophobicity. This is achieved by reacting the surface with non-functional organo-silanes, such as dimethyldichlorosilane. Such hydrophobic silicas contain less than 0.1% moisture. c) Applications The main polymer applications for fumed silicas are as reinforcing fillers for silicone elastomers and as rheology control agents in coatings, sealants and adhesives.

2.3.3 Hydroxides and Basic Carbonates 2.3.3.1 Introduction Hydroxides and basic carbonates are classed together here, as their distinguishing feature is a marked flame retarding effect when used in polymers at high loadings. This is because they decompose endothermically, with evolution of inert gases, at temperatures at which polymers pyrolyse. As a result, large quantities of these, predominantly synthetic, fillers are used, with one in particular, alumina trihydrate (more properly aluminium hydroxide), currently dominating the market. The mechanisms by which such fillers reduce polymer flammability is discussed in depth in Chapter 6. In this chapter, only the production of the fillers, and the properties relevant to their use as fillers and flame retardants are discussed.

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2.3.3.2 Aluminium Hydroxide This is by far the most widely used flame-retardant filler, being available at relatively modest cost and with a wide range of particle sizes, shapes and surface treatments to suit various applications. Although its chemical structure is that of the hydroxide, it is often referred to as alumina trihydrate (Al2O3.3H2O) or simply ATH. There is more than one crystal form of aluminium hydroxide, but that used as a flame retardant is gibbsite. For convenience the common acronym, ATH, will be used throughout this book. a) General Properties ATH is a white, non-toxic, material, which is soluble in strong acids and alkalies. It has a specific gravity of 2.4, is relatively soft (Mohs hardness about 3) and nonabrasive. It starts to decompose at about 200 °C with the loss of 34.6% by weight of water when fully decomposed. b) Production Gibbsite occurs naturally in many parts of the world but in an impure form in the rock bauxite. This is the starting point for the manufacture of filler-grade ATH. The principal source of ATH is associated with the conversion of bauxite into alumina and aluminium metal by the Bayer process. In this process, the bauxite is extracted with hot sodium hydroxide, resulting in a solution of sodium aluminate. After filtration, the solution is seeded with fine gibbsite crystals and allowed to cool. This results in crystal agglomerates of 60-80 μm in diameter, which are separated, washed and dried. They are then ground and classified to produce ATH fillers with a range of particle sizes. Considerable purification takes place during the process, but some organic matter (largely humate residues) from the bauxite remains, resulting in a reduction in whiteness. These products also contain considerable quantities of sodium ions trapped in the crystal lattice. These adversely affect water pick-up and electrical performance. They are difficult to remove completely and appear to diffuse slowly to the surface of the particles. Products obtained from the Bayer process are relatively inexpensive and hence widely used, despite their colour limitations. The Bayer process results in a waste stream known as ‘red mud’, which contains considerable amounts of sodium aluminium silicates. These can be converted into ATH if the ‘red mud’ is mixed with limestone and sodium carbonate and calcined, to form sodium aluminate. This is extracted into water and gibbsite then precipitated as in the Bayer process. These products are known as sinter hydrates. They are whiter than the Bayer products as the calcination destroys the organics, but they still contain similar levels of sodium.

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Particulate-Filled Polymer Composites The main problem of the Bayer and sinter processes from a filler point of view, is the relatively large size of the product. Although this can be ground to produce finer grades, there is an economic limit to this. Also the grinding tends to produce platy particles, due to preferential cleavage of the ATH along certain crystal planes. It is becoming more common for ATH to be specially precipitated from purer feed materials than the Bayer process, with the precipitation conditions (temperature, concentration, seed contents) also being modified. This gives purer, finer and more spherical products. c) Thermal Decomposition Starting at about 200 °C, ATH dissociates into Al2O3 and H2O, but this apparently simple reaction is actually quite complex. The first issue is stability. The onset of decomposition is critical in determining the safe processing temperature in composites, but is difficult to specify. Quoted values vary by as much as 50 °C depending on the method of measurement. The processing temperatures of some of the major polymers, (e.g., PP) lie close to the decomposition temperature and hence a realistic assessment is essential. Isothermal tests are probably the best guide and these suggest that 200 °C is probably the upper limit for safe processing unless this is carried out under high pressures. This decomposition makes ATH difficult to use with polymers such as polypropylene and considerable effort has been expended in trying to overcome this limitation. The main approach has been to use surface treatments that minimise shear heating effects. Unlike magnesium hydroxide (see Section 2.3.3.3), there is no evidence that surface treatments have any direct effect on stability. The rate and extent of decomposition is complicated by the possibility of two pathways being followed. Thus, in addition to direct decomposition to oxide, a hydroxy-oxide intermediate (boehmite, AlO(OH)) can also be formed. This only decomposes to oxide at relatively high temperatures (approximately 500 °C). Decomposition by this second route probably reduces flame-retardant effectiveness. Boehmite formation is favoured by high partial pressures of water and the extent to which it occurs will depend on heating conditions and the local environment of the ATH. It is well known, for example, that the tendency to boehmite formation increases with ATH particle size [43]. This is illustrated by the thermal analysis data for a coarse and a fine ATH presented in Figure 2.11. It is also thought that polymer matrices may impede water escape during pyrolysis and so favour boehmite formation. In support of this, surprising levels of boehmite have been reported in the residue from pyrolised polymethyl methacrylate/ATH composites [44]. This may be one of the reasons why marked ATH particle size effects are observed for such composites in some flammability tests [45].

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Figure 2.11 The effect of particle size on the decomposition pathway of aluminium hydroxide (ATH)

Even in the absence of boehmite formation, ATH decomposition requires high temperatures to go to completion. This is probably due to the formation of some isolated hydroxyls, which need to diffuse for relatively long distances before they can react. The ATH flame retardant producers have long sought to develop products with sufficiently increased thermal stability to enable wider use in polyolefins and similar polymers. Some success in this area has recently been reported by Weber of Martinswerk [46]. The more stable mono-hydrate, boehmite has also recently been commercialised as a flame retardant filler [47]. d) Surface Modification ATH responds well to organo-silanes and these appear to be the materials of choice for modifying the surface, with the reactive organo-group being chosen to suit the polymer matrix. Carboxylated polymers are also effective [48] and various organic acids, both saturated and unsaturated are also utilised. Rothon and co-workers have recently reported that some unsaturated acids can be give similar coupling efficiency to the silanes in polymers such as ethylene vinylacetate [49]. e) Uses ATH is widely used as a flame-retardant filler in elastomers, thermosets and some thermoplastics. It is also used in significant quantities as a filler in other applications,

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Particulate-Filled Polymer Composites where flame retardancy is not the prime consideration. This is especially true in thermosets; where good aesthetic qualities, (e.g., translucency), are often required and the low hardness enables polishing out of scratches, etc., to be carried out.

2.3.3.3 Magnesium Hydroxide a) Introduction Magnesium hydroxide is a white crystalline material, with similar flame-retardant properties to ATH but with superior thermal stability. Until recently, commercial interest in its use as a flame-retardant filler was minimal outside Japan. This is because the only products generally available were either of poor quality or expensive. This picture has changed rapidly, as environmental concerns over halogen-containing flame-retardant systems have developed, leading to attempts to replace them in thermoplastic formulations, which process at too high a temperature for ATH to be safely used. Progress has been made on many fronts. More economic routes to highquality product forms have been developed. Significant applications have also emerged for the lower quality synthetic products, notably in the United States, where roofing and decking applications are of growing importance. Success has also been achieved in utilising natural sources for some applications. There is now every possibility that magnesium hydroxide will become a significant flame retardant, complementing ATH. b) General Properties Magnesium hydroxide is a white crystalline material. Its specific gravity is 2.4. Like ATH, it is soft and non-abrasive with a Mohs hardness about 3. It starts to decompose at about 300 °C. The naturally occurring mineral is known as brucite. The natural crystal form is as a flat hexagonal shaped plate. c) Production and Forms Available Magnesium hydroxide is potentially available in a number of product forms, which are all being developed and optimised for flame-retardant uses. Three distinct types of magnesium hydroxide can be recognised: natural, synthetic seawater or brine, and synthetic large crystal. Although uncommon, workable deposits of magnesium hydroxide (known mineralogically, as brucite) occur in several parts of the world, especially the United States and China. Some of these are relatively pure, and after milling and beneficiation processes, appear capable of producing material of filler-grade purity and size. Despite

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Principal Types of Particulate Fillers the promise of the natural source, development of suitable products has taken a very long time. Most of the problems now seem to have been resolved and significant quantities of natural product are now being used in cable applications in Europe [50]. The commonest form of magnesium hydroxide in commercial production is generally referred to as sea-water or brine type. This is composed of roughly spherical, somewhat porous, aggregates of very small, platy magnesium hydroxide particles. The aggregates are usually 3-10 μm in diameter, are strong enough to resist breakdown during most polymer processes, and are thus the effective particle form. This type of product is produced by precipitation of the magnesium salts present in sea water or brine, by adding lime or dolomitic lime. This type of precipitation produces very fine crystals, and the conditions are deliberately chosen to lead to aggregate growth in order to facilitate product isolation and handling. Many of the impurities in the lime can be transferred to the magnesium hydroxide, and hence this has to be carefully selected for best results. Products of this type are produced on a large scale as an intermediate in the manufacture of magnesium oxide refractories and thus, in principle, are available fairly cheaply. As a result, considerable effort has been expended in trying to develop filler-grade materials from them. The particle size, porosity, surface area and purity can vary considerably according to the exact conditions used in the precipitation. Even in the most favourable circumstances these products are far from ideal for polymer applications and considerable further processing is usually required, especially if low levels of soluble impurities are to be obtained. Most flame retardant applications require very high loadings, and hence place severe demands on filler morphology and have not been accessible to these products. Some, potentially large applications, now appear to be emerging, where lower levels of flame retardancy are needed and these products are beginning to take a significant part of the magnesium hydroxide flame retardant market. Leaving aside cost, the large-crystal synthetic products have the most desirable physical form for filler applications. These are typified by crystals of the order of 0.5-2 μm in size and particle sizes not very different (i.e., little aggregation). Surface area is generally in the range of 1-10 m2 g -1. The aspect ratio of the crystals can vary considerably but is often about 3:1. They can be produced by a number of methods, including hydration of specially formed magnesium oxide [51], precipitation from a magnesium salt solution by addition of ammonium, calcium or sodium hydroxide [52] and by hydrothermal conversion of basic magnesium chloride, itself produced by precipitation from a magnesium salt by addition of a base [53]. In all cases, the conditions have to be controlled if the desired morphologies are to be obtained. The hydration route has the advantage that no

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Particulate-Filled Polymer Composites soluble by-products have to be removed by washing, as is the case with the precipitation routes, but morphology control tends to be more difficult. Among recent developments, has been the commercial launch of nickel doped large crystal magnesium hydroxide. The nickel is claimed to significantly improve the fire retardancy, and allow lower filler loadings to be used [54]. d) Thermal Decomposition The key property of magnesium hydroxide compared to ATH is its superior thermal stability. This is illustrated by the isothermal data presented in Figure 2.12, which suggests that decomposition only becomes significant at about 300 °C. Unlike ATH there is no evidence for two decomposition pathways and for formation of a stable basic oxide intermediate. Even so, decomposition is slow to go to completion and thermal gravimetric analysis (TGA) traces usually show two steps, the first making up only about 85% of the theoretical weight loss. It is believed that the first step is due to removal of bulk hydroxyls while the second slow step, is due to loss of surface hydroxyls accompanied by oxide sintering [55]. Hornsby has reported that the thermal decomposition varies with the form of the magnesium hydroxide [56]. Surprisingly some coatings do appear to modify the decomposition, as shown for a stearate-coated material included in Figure 2.12. The reasons for this are not clear. Metal impurity doping has also been claimed to alter the thermal stability, presumably by introducing some strain into the crystal structure [57].

Figure 2.12 Decomposition of magnesium hydroxide under isothermal conditions

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Principal Types of Particulate Fillers e) Surface Chemistry and Modification Because these fillers are relatively new, this is an area where much still needs to be done. There is little information concerning the surface chemistry of filler-type magnesium hydroxide, but preliminary work by one of the present authors (RR) suggests that this can be quite complicated and that surfaces of some forms may well be highly carbonated. As would be expected, organic acids react readily with the surface and both fatty acids and the unsaturated polymeric acids described in Chapter 4 bring about useful modifications. The latter have been shown to be effective coupling agents in crosslinked elastomers and thermosets [58]. Silane coupling agents also appear to be effective treatments. The situation is constantly evolving and other coatings, especially phosphate esters, are beginning to appear [59]. The potential effect of coatings on the thermal stability of the filler must always be bourne in mind, as described earlier for fatty-acid coatings. Surprisingly, the carboxylated unsaturated polymers appear to have no effect on stability. Surface modification is likely to be a crucial area of development over the next few years as producers attempt to differentiate their products and minimise the detrimental effects on some composite properties. There is also an interest in use of surface treatments to reduce the susceptibility of the filler to carbonation, which can even lead to leaching from polymers by aqueous carbon dioxide [59]. f) Applications Currently magnesium hydroxide is predominantly used in polyethylene and ethylene vinyl acetate copolymers for electrical wire covering. The areas of greatest potential are, however, in PP and polyamides for a wide variety of uses, and it is in these areas where most development work is currently directed.

2.3.3.4 Basic Magnesium Carbonates There are a number of basic magnesium carbonates, but the ones used for filler applications approximate to the composition of the mineral hydromagnesite [4MgCO3.Mg(OH)2.3H2O]. This material is insoluble, white and dissociates over a temperature range suitable for observing flame retardant effects. It occurs naturally as the mineral hydromagnesite and can be made synthetically, e.g., by the decomposition of magnesium bicarbonate solutions. There is little exploitation of the natural material, except where mixed with huntite as discussed in the next section. Synthetic hydromagnesite normally has a very platy shape, with high oil absorption. The platy nature gives useful properties in elastomers where it finds some use, but

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Particulate-Filled Polymer Composites limits the loadings obtainable, and also the flame-retardant properties. Thus, interest in it as a flame retardant is minimal, although it is being promoted as a smoke suppressant in PVC. a) Thermal Stability This is discussed here, despite the lack of interest in hydromagnesite itself, because of the interest in mixtures with huntite. Thermal decomposition is quite complex with release of water and carbon dioxide, following more than one pathway. Thermal stability is not well established but is probably about 20 °C higher than ATH, indicating a safe processing temperature of about 220 °C. This would seem to make it suitable for PP but, in practice, the platy nature tends to cause greater viscous heating and reduce this apparent advantage. The decomposition might be expected to proceed in three distinct stages: loss of water of hydration, loss of hydroxyls, and finally loss of carbonate. In practice, these overlap considerably and one observes a broad decomposition over the range 200-500 °C. Under ideal conditions, with rapid removal of gaseous products, loss of CO2 occurs well before the temperature one would expect for magnesium carbonate itself, but, in practice, considerable magnesium carbonate can be formed on heating, delaying complete decomposition. b) Huntite/Basic Magnesium Carbonate Mixtures These mixtures occur naturally in Greece, USA and possibly elsewhere, in the form of a white, fairly pure, easily milled form suitable for filler use [60]. Although of natural origin, the main polymer interest is in their flame-retardant nature and hence they are discussed here. c) General Properties Hydromagnesite has already been discussed. Huntite is a rare, mixed magnesium calcium carbonate with the formula 3MgCO3.CaCO3. The properties of huntite have been reported in detail by Faust [61]. Specific gravity is 2.70. It is strongly bi-refringent. It appears to decompose in two steps on heating. The first, at about 645 °C, is believed to be due to decomposition of the carbonate associated with the magnesium, while the second, at about 900 °C, is believed to be that associated with the calcium. In the principal deposit in Greece, the ratio of huntite to hydromagnesite is about 1:1 and there are only traces of other minerals after milling and beneficiation. Other deposits exist with higher hydromagnesite content, but have not been exploited to any extent to date. The following information is based on material from the main deposit.

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Principal Types of Particulate Fillers d) Filler Properties and Surface Treatments The basic magnesium carbonate appears to occur in a more favourable morphology in the natural ‘mixture’ than when made synthetically. Nevertheless, it is still quite platy, existing in the milled product as plates of about 4 μm diameter and a thickness of about 0.6 μm. The huntite, unfortunately, is even more platy, having a mean diameter of about 0.5 μm and a thickness of only about 0.04 μm. Because of the presence of these two different particles, the mixture has a distinctly bimodal particle size distribution. (NB: The mean sizes determined by standard particle-sizing techniques tend to be distorted by the platy nature of the particles.) The surface area is quite high (approximately 18 m2 g-1)) and so is its oil absorption. This gives relatively high-viscosity compounds and dispersions making the achievement of high loadings difficult. Little is known about the surface chemistry and the effect of surface modification. Organic acids would be expected to react with both components and a fatty-acid coated product is commercially available. Other coatings are also offered, including silanes, but the nature of the surface reaction with these is unclear. e) Fire Retardancy When the mixture is examined by differential scanning calorimetry (DSC), four main endothermic peaks are observed at 275, 440, 550 and 690 °C. From information on each separate component it seems reasonable to assume that the first two are due to the basic magnesium carbonate and the last two are mainly due to the huntite, but are lower than reported by Faust [61] for huntite itself (644, 901 °C). The reasons for this are not clear. Faust used a similar heating rate but used a fairly primitive differential thermal analysis (DTA) technique and this may have affected the results. Thus it would appear that the stability in use will be determined by the basic magnesium carbonate and is likely to be around 220 °C. Indeed, this is what the producer claims. This is about 20 °C higher than ATH, making it a more attractive filler for PP, if flame retardancy and other properties are satisfactory. The effect of the fine platy particles on viscosity may cause a greater heat rise during processing than is observed with ATH and reduce the stability advantage somewhat. Incorporation of the mixture into a polymer can also significantly modify its decomposition behaviour. This is illustrated by the DSC data presented in Figure 2.13 for the filler itself and a PP compound containing 60 wt% filler. It is seen that the first two peaks are considerably raised in temperature by incorporation into the polymer, while the second two are relatively unaffected. Presumably, most of the polymer will have disappeared by the time the huntite starts decomposing and thus the latter will have no effect on the burning of the polymer. The weak shoulder on the 550 °C peak in the neat filler is more pronounced in the presence of polymer. It is

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Particulate-Filled Polymer Composites

Figure 2.13 The effect of the prescence of a polypropylene matrix on the decomposition of a huntite - basic magnesium carbonate filler

postulated that this is magnesium carbonate resulting from alteration of the basic magnesium carbonate decomposition as discussed in subsection 2.3.3.4. The fire-retardant effectiveness is a key question, which is particularly difficult to answer in the light of present information. The difficulty lies in the presence of the huntite fraction, which constitutes about half the material. As we have seen, this only appears to decompose at about 500 °C when most of the polymer has gone and its fire-retardant effect must be in doubt, and this must apply in the mixed filler. However, the mixture performs well in tests such as oxygen index. One of the present authors (RR) has himself obtained excellent results with the mixed filler in oxygen-index testing in a number of polymers and has shown by X-ray analysis that the huntite decomposes under the test conditions. There are significant problems in oxygen-index testing as discussed in Chapter 7. One of these is that the flame travels down through the filler residue during the test, raising it to a high temperature. This would certainly be sufficient to decompose huntite and significantly affect heat feedback to the underlying polymer. Such conditions may not be observed in all flame tests, however, and oxygen index may show the filler up in a particularly favourable manner. A detailed study of the effect of the two filler phases and of their mixture on the flame retardancy of PP has recently been reported by Doyle and co-workers [62].

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Principal Types of Particulate Fillers f) Applications Huntite/hydromagnesite mixtures are used as flame-retardant and smoke-suppressant fillers in a variety of polymer types. Their utilisation very much depends on price relative to ATH, prices of which have been quite low in recent years and this has limited commercial utilisation.

2.3.3.5 Dawsonite Dawsonite is the mineral name for sodium aluminium hydroxycarbonate, NaAl(OH)2CO3. It can be produced synthetically in microfibre form, and decomposes at about 300 °C with release of water and carbon dioxide. There has been significant interest in its use as a reinforcing flame-retardant filler but this seems to have waned recently, possibly due to possible toxicity concerns. More information on the material and its performance can be found in an article by Milewski [63].

2.3.3.6 Antimony Oxides Both antimony trioxide (Sb2O3) and pentoxide (Sb2O5) are used in polymers because of their high flame-retardant efficiency when used in conjunction with halogen compounds. The trioxide is the preferred form for most applications and is used widely in the plastics industry. The trioxide is white but the pentoxide is yellow and both have pigmenting properties due to their high refractive indices. They have been used as pigments but have now been largely replaced in this by titanium dioxide. Their pigmenting effect must be allowed for in formulating compounds, but its strength can be controlled to a certain extent by controlling particle size. Antimony trioxide (Sb2O3) is a white crystalline powder. It occurs in nature in two crystalline forms but deposits are impure and virtually all antimony trioxide used in polymers is produced synthetically. Production is mainly by blowing air through molten sulfide ore or metal, when the metal oxide sublimes, and is collected in relatively pure form simply by cooling. Particle size is controlled by the temperature of volatilisation and cooling rate. Nearly all the synthetic antimony oxide is in the senarmontite crystal form. This is of cubic crystal habit with a specific gravity of 5.2 and refractive index of 2.087. While antimony oxide itself has few flame-retardant effects it does, as mentioned above, markedly improve the effectiveness of halogen compounds. This allows overall additive levels to be reduced and lessens the loss in favourable polymer properties that generally accompanies their use. The mechanisms by which the antimony-halogen combinations

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Particulate-Filled Polymer Composites produce enhanced flame retardancy are still the subject of debate. It seems clear that much of the effect is due to the formation of volatile antimony halides, which operate in the gas phase to interrupt flame propagation. Solid-state, char-forming processes may also be significant. As one might expect, the ratio of antimony to halogen in the compound is a critical factor as well as the overall level of additives. Further information on antimony oxides can be found in the article by Touval [64].

2.3.4 Precipitated Calcium Carbonate (PCC) Very small crystalline (<0.1 μm) calcium carbonate can be produced by precipitation. All three crystal forms, aragonite, calcite and vaterite, can, be produced this way, but calcite is by far the most widely used in polymer applications. The production of PCC has been described by Rothon [65]. A variety of precipitation processes may be used, but today the most common involves blowing carbon dioxide through a slurry of calcium hydroxide (milk of lime). The calcium hydroxide is itself produced by calcination of limestone, followed by hydration. The ultimate crystal size, the degree of aggregation and the aggregate strength are important factors in controlling performance in polymer applications. These are, in turn, determined by many factors in the precipitation process, many details of which are proprietary. For polymer application, PCC is usually coated with a fatty acid during manufacture. This not only improves polymer compatibility, but also aids filtration during production and reduces aggregation effects during drying. Conventional coupling agents such as silanes do not perform well with PCC but coupling may be brought about in many instances by carboxylated unsaturated polymers [48]. These may be added during manufacture, like fatty acids, or added during polymer compounding. The traditional areas of use have been as fillers in elastomers and PVC. In elastomer applications they give a unique combination of low modulus, high strength and low set. Today they are widely used as rheological control agents in sealants, especially PVC plastisols.

Acknowledgements The authors gratefully acknowledge the valuable contributions of Dr D R Brown (Croxton & Garry Ltd, Curtis Road, Dorking, Surrey, RH4 1XA, UK) to the sections on calcium sulfate and wollastonite, of Joseph L Scaries (US Silica Company, PO Box 187, Berkeley Springs, WV 25411, USA) to the section on crystalline silica, and of Mrs Caryl Gould (ECC International, Europe) for the SEM micrographs of mineral fillers.

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References 1.

J.B. Griffiths, Proceedings of Filplas ’89, Plastics and Rubber Institute/British Plastics Federation Conference, Manchester, UK, 1989, Paper No.1.

2.

Minerals Year Book, US Geological Survey, US Department of the Interior, Reston, VA, USA, 2002.

3.

P.W. Harben, The Industrial Minerals Handbook: A Guide to Markets, Specifications, & Prices, 3rd Edition, Metal Bulletin plc, London, UK, 1999.

4.

Proceedings of the European Symposium of Particle Technology, Description of Comminution, Amsterdam, The Netherlands, 1980.

5.

F.R. Noble, unpublished results.

6.

L.S. Dent Glasser and D.N. Smith in The Scientific Study of Flint and Chert, Eds., G. de C. Sievaking, and M.B. Hart, Cambridge University Press, Cambridge, UK, 1986, p.105-109.

7.

P.R.S. Gibson and C.P. Prat, Rubber Journal, 1969, 151, 9, 33.

8.

A.H. Riley, C.D. Paynter, P. McGenity and J.M. Adams, Plastics and Rubber Processing and Applications, 1990, 14, 2, 85.

9.

R.H. Ottewill and J.M. Tiffany, Journal of the Oil Colour Chemists Association, 1967, 50, 844.

10. K. Mitsuishi, Hyomen, 1990, 28, 1, 49. 11. W.B. Jepson, Philosophical Transactions Royal Society London, 1984, A311, 411. (A comprehensive, brief review of kaolin, its occurrence, properties and production). 12. W.M. Bundy, Proceedings of Keller ‘90 Symposium, Columbus, OH, USA, 1990. 13. A.P. Ferris and W.B. Jepson, Journal of Colloid and Interface Science, 1975, 51, 2, 245. 14. A.P. Ferris and W.B. Jepson, Analyst, London, 1972, 97, 940. 15. B.R. Angel and W.E.J. Vincent, Clays and Clay Minerals, 1978, 26, 263. 16. F.R. Noble and C. Golley, unpublished results.

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Particulate-Filled Polymer Composites 17. A.L. Barbour and A. Rice, Proceedings of the 131st ACS Rubber Division Meeting, Montreal, Canada, Spring 1987, Paper No.76. 18. S.J. Monte and G. Sugerman, Modern Plastics Encyclopaedia, 1976, 53, 12A, 161. 19. N.O. Clarke and T.D. Parker, inventors; ECLP Co. Ltd., assignee; GB 630,418, 1949. 20. L.W. Carter, J.G. Hendricks and D.S. Bolley, inventors; National Lead Company, assignee; US 2,531,396, 1950. 21. D.H. Solomon in Physical Chemistry of Pigments in Paper Coating, Ed., C.L. Garey, Tappi Press, Atlanta, GA, USA, 1977. 22. L. Tscheischwili, W. Bussem and W. Weyl, Berichte Deutsche Keramische Gesellschaft, 1939, 20, 249. 23. J. Rocha and J. Klinowski, Physics and Chemistry of Minerals, 1990, 17, 179. 24. Chemistry of Clays and Clay Minerals, Ed., A.C.D. Newman, Longman Scientific and Technical, Harlow, UK, 1987. 25. K.J.D.MacKenzie and co-workers, Journal of the American Ceramic Society, 1985, 68, 293. 26. A.P. Ferris and W.B. Jepson, Proceedings of the 3rd European Clay Conference, Stockholm, Sweden, 1977, 54. 27. D.A. Socha, Proceedings of the Filplas ‘89 Conference on Filled Plastics, BPF/ PRI, Manchester, UK, 1989, Paper No.2. 28. ASTM E1132, Standard Practice for Health Requirements Relating to Occupational Exposure to Respirable Crystalline Silica, 1999. 29. ANSI 288.2, Respiratory Protection, 1992. 30. L. White, European Rubber Journal, 1999, 181, 6, 28. 31. J-B. Donnet, R.C. Bansal and M.J.Wang, Carbon Black, Science and Technology, Marcel Dekker, New York, NY, USA, 1993. 32. W.M. Hess, K.A. Burgess, F. Lyon and V. Chirico, Kautschuk und Gummi, 1968, 21, 12, 689. 33. G. Kraus, Advances in Polymer Science, 1971, 8, 178.

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Principal Types of Particulate Fillers 34. W.M. Hess, Reinforcement of Elastomers, Ed., G. Kraus, Interscience Publishers, New York, NY, USA, 1965, Chapter 6. 35. B.B. Boonstra and A.I. Medalia, Rubber Age, 1963, 92, 6, 892. 36. B.B. Boonstra and A.I. Medalia, Rubber Age, 1963, 93, 1, 82. 37. L. White, European Rubber Journal, 1998, 180, 1, 21. 38. R.K. Iler, The Chemistry of Silica, Solubility, Polymerization, Colloid and Surface Properties and Biochemistry, Wiley Interscience, New York, NY, USA, 1979. 39. S.K. Watson in Handbook of Fillers for Plastics, Ed., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987, Chapter 9. 40. Precipitated Silicas and Silicates for the Rubber Industry, Degussa AG, Frankfurt, Germany, 1997. 41. P. Cochet, Tire Technology International, 2000, June, 43. 42. P. Cochet, Proceedings of the Functional Tire Fillers 2001 Conference, Intertech, Fort Lauderdale, FL, USA, 2001. 43. I. Sobolev and E.A. Woycheshin in Handbook of Fillers for Plastics, Ed., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987, Chapter 16. 44. G.V. Jackson, R.N. Rothon and G.A. Moorman, Proceedings of Filplas ‘92, Manchester, UK, 1992, Paper No.10. 45. P. Hughes, G.V. Jackson and R.N. Rothon, Die Makromolekulare Chemie Makromolecular Symposia, 1993, 74, 179. 46. M.Weber, Proceedings of the Functional Effect Fillers 2000 Conference, Intertech, Berlin, Germany, 2000, Session 2, Paper No.4. 47. R. Sauerwein, Proceedings of the Fire and Materials 2001 Conference, Interscience, San Francisco, CA, USA, 2001, 395. 48. M.B. Evans, R.N. Rothon and T.A. Ryan, Plastics and Rubber Processing and Applications, 1988, 9, 4, 215. 49. R.N. Rothon, J. Schofield, D. Thetford, P. Sunderland, G.C. Lees, C.M. Liauw and F. Wild, Proceedings of the Functional Fillers for Plastics 2002 Conference, Intertech, Toronto, Canada, 2002, Paper No.5.

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Particulate-Filled Polymer Composites 50. E. Redondo Grizante, F. Peruzotti, D. Tirelli, A. Zaopo and E. Albizzati, inventors; Pirelli Cavi E Sistemi SPA, assignee; WO 9905688A1, 1999. 51. R.N. Rothon, A.M. Ryder and A.G. Bourke, inventors; DEFPEO, assignee; EP 0,568,488A2, 1993. 52. A. Packter, Crystal Research and Technology, 1985, 20, 3, 329. 53. No inventors; Kyowa Chemical Industry, assignee; GB 1,514,081, 1978. 54. H. Kurisu, T. Kodani, A. Kawase and T. Oki, inventors; Tateho Chemical Industries Co. Ltd., assignees; US 5,766,568, 1998. 55. J. Green, Journal of Materials Science, 1983, 18, 3, 637. 56. P.R. Hornsby and C.L. Watson, Proceedings of the Third Meeting on Fire Retardant Polymers, Turin, Italy, 1989, p.66. 57. S. Miyata, inventor; Kabunshiki Kaisha Kaisui Kaguku Kenkyujo, assignee; EP 0498566A1, 1992. 58. R.N. Rothon, Proceedings of Moffis ‘91, Le Mans, France, 1991, p.37. 59. S. Miyata and M. Yoshii, inventors; Kyowa Chemical Industry Co. Ltd., assignee; EP 0408285A1, 1990. 60. C.C. Briggs, Proceedings of Fillers and Additives in Plastics ‘91 Technical Conference, Lund, Sweden, 1991. 61. G.T. Faust, American Mineralogy, 1953, 38, 4, 24. 62. M. Clemens, M. Doyle, G.C. Lees, C. Briggs and R. Day, Proceedings of Flame Retardants ‘94, London, UK, 1994, p.193. 63. J.V. Milewski in Handbook of Fillers and Reinforcements for Plastics, Eds., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1978, Chapter 24. 64. I. Touval in Handbook of Fillers for Plastics, Eds., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987, Chapter 15. 65. R.N. Rothon, Advances in Polymer Science, Ed., J. Jancar, Springer Verlag, Heidelberg, Germany, 1999, 139, 103.

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3

Analytical Techniques for Characterising Filler Surfaces David P. Ashton, David Briggs and Christopher M. Liauw

3.1 Introduction It is the constant theme throughout this volume that introducing fillers into polymers can substantially modify the properties and cost of the material. However, in most systems to achieve the full benefits of the effects offered by the filler, the interaction between the filler and the polymer matrix must be optimised. Although, it should be noted that optimising does not necessarily mean maximising the strength of adhesion between filler and matrix, in some instances this causes embrittlement, and then a lesser level of interaction is needed. To achieve the required effects a large range of surface reactive chemicals have been developed. These may have a single functionality which is reactive towards the filler surface, and used to improve the wetting by the polymer, and hence the dispersion within the polymer. Alternatively, the surface-active component may have dual functionality being capable of reaction with the filler surface, and also crosslinking into the polymer matrix to form continuous chemical bonding between the filler particles and the polymer matrix. Examples containing a single and a dual functional reagent are described later in this section to illustrate these functions, although a more comprehensive account of current surface coatings science and technology is provided in Chapter 4. The first example is of the single functionality reagent stearic acid that is commonly used to surface modify calcium carbonate for improving the rheology when filling thermoplastics such as polypropylene. In this system, the inorganic surface of the carbonate particles is incompatible with the polymer matrix, i.e., it is not solvated, and without surface modification would lead to very high viscosity melts. This in turn would require higher mould filling pressures or lower filler concentrations to be used, and the final component may, due to poor wetting, contain voids at the filler-matrix interface with consequences for its mechanical integrity. Whereas, in surface modification of the calcium carbonate with stearic acid, the acid groups react with the surface carbonates to form the stearate salt leaving each filler particle with an outer surface comprised essentially of hydrocarbon chains. This is solvatable by the molten polypropylene and hence is fully wetted, leading to low melt viscosities, low mould filling pressures and maximum filler loadings. In the

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Particulate-Filled Polymer Composites second example, the surface reactive chemical has dual functionality being capable of interacting with the filler surface and crosslinking into the matrix to form continuous chemical bonding from the filler particles through to the polymer matrix. The compound chosen for this illustration is 3-(trimethoxysilyl)propyl methacrylate (MPS), which is widely used as the coupling agent in filled thermoset systems, an example being silica dispersed in methyl methacrylate which is commonly used for moulding into articles such as kitchen sinks, baths and bathroom fitments. In this system the purpose of the silane compound is to obtain strong chemical bonding between the filler and the matrix, and to achieve the improved mechanical properties of the final component and not necessarily to improve the rheology of the system. The methoxy groups attached to the silicon atom in the silane molecule can be hydrolysed to silanols which can then form hydrogen bonds with silanols on the silica surface, or more preferably undergo a condensation reaction with the silanols to produce covalent siloxane bonds between silane and the filler particle. The methacrylate group at the other end of the molecule having a similar reactivity to methyl methacrylate reacts into the matrix during the free radical polymerisation. However, in liquid dispersions such as these there is an additional requirement: that of avoiding, during the pre-cured liquid state, flocculation of the filler particles due to the Van der Waals forces acting between them. Therefore, to achieve the necessary spacing between the silica particles, the coating has a thickness requirement in addition to the solubility requirement. The thickness of this ‘steric barrier’ is dependent upon filler density and particle size, and for practical systems the MPS molecule is too short to achieve a fully deflocculated dispersion of silica in the uncured state. Therefore, to obtain the deflocculated state an additional polymeric or oligomeric dispersant is required, although a similar short-term effect can be obtained using thixotropes. To illustrate the importance of the bonding between filler and matrix on the mechanical performance of the final composite, measurements were carried out by one the authors (DPA) on silica filled polymethylmethacrylate containing silica (50% by volume), and a mono functional polymeric dispersant, with and without MPS. The composite containing the MPS had a flexural failure stress of 115 MPa, whilst the composite with no coupling agent had a flexural failure stress of 65 MPa. The two examples described previously illustrate the importance of a number of factors concerning the surface of the filler and the interface between the filler particles and the matrix. These are summarised next: The surface of the filler must be compatible with the molten polymer or, if a thermoset system, with the uncured system, (i.e., the resin or elastomer), to achieve the optimum rheology and filler dispersion.

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The interface is critical to achieving the required mechanical properties from the composite.

•

The thickness, which is usually related to the molecular mass, of the surface coating is important for obtaining the required rheological properties. It may be necessary to use more than one surface modifier to achieve all the effects required of the system.

An extremely important factor not implicitly obvious in the two examples described previously is the specificity of the surface chemistry to each composite system. Of course many surface reactive agents can be used in more than one system, and MPS and stearic acid fall into this category. However, the MPS and stearic acid could not be interchanged between the two examples, not only due to the mono functionality of the stearic acid, but due to the relative lack of reactivity between silanes and calcium carbonate, and stearic acid and silica. In the latter case, the hydroxyl groups, which are formed at the silica crystal surfaces to satisfy valency demands, are acidic in nature and hence do not form strong bonds with the carboxylic acid group. If the silica was substituted, for example, with alumina then the stearic acid would interact with the surface of the filler, as the hydroxyls formed at crystal discontinuities in alumina are basic in character. Interactions between polymer additives such as stabilisers and filler surfaces are also an important consideration as excessive interaction can lead to a loss of stabilisation. Whilst an understanding of these interactions can lead to methods of preventing additive adsorption, it is also important to appreciate that such interactions can be used to advantage in controlled release of stabilisers from filler surfaces. Such an effect can result in improved retention of the stabiliser and hence significantly enhanced stabilisation. This concept can be extended to other functional additives such as biocides and perfumes, etc. Any study of interfaces in filled polymer systems will show that this acid-base interactivity is the constantly recurring driving force for the surface chemistry and its understanding is crucial to predicting melt and composite properties. A theoretical approach to the subject is discussed in some detail in Section 3.2 of this chapter. It can be concluded from this introduction that to obtain the surface modification required to achieve the optimum or desired composite properties an intimate understanding of the chemical nature of the filler surface is required, and of the chemical reactivity of the surface modifier. The ideal situation, of course, would be to have a comprehensive analysis of all the chemical moieties at the filler surface and their precise spatial arrangement, and then to design the surface modifier with functional groups that interact with the surface moieties and which are correspondingly spaced. This ideal is a long way from current scientific capabilities and may never be fully realisable, however, the target is there and huge strides have been made on the road towards this ultimate goal.

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Particulate-Filled Polymer Composites Sophisticated techniques have now been developed for characterising and quantifying the chemical nature of material surfaces, and the more important ones, listed below, are discussed in some detail in this chapter: • • • • •

Flow Microcalorimetry Inverse Gas Chromatography X-Ray Photoelectron Spectroscopy Secondary Ion Mass Spectroscopy Fourier Transform Infrared Spectroscopy

FMC IGC XPS SIMS FTIR

In addition, with the use of fast powerful computers, there is significant activity in the modelling of material surfaces. Much can be expected from this work over the coming years, although the spatial characterisation of typical commercial fillers with the added complications of surface contamination and inhomogeneity will remain a challenge for many years to come. However, despite the large chemical differences between even the most commonly used fillers, such as alumina silicates, aluminium hydroxide, calcium carbonate, silica, titanium dioxide, the most active groups at the surface and which dominate the surface chemistry are more often than not hydroxyl groups. These form, as discussed previously, to satisfy valence and electrostatic requirements at the fractured surfaces and discontinuities of the crystal lattices, or amorphous particles. Furthermore, it is often the acidic or basic nature of these hydroxyl groups that determine the chemical properties of the surface, and therefore, which surface modifier is most suitable for the filler. Consequently, much of the characterisation of filler surfaces is confined to measuring the relative acidity or basicity of the active surface sites. It is worth commenting at this point that all superficially chemically similar groups across any filler particle surface will not necessarily have the same acidic or basic character due to variations in the immediate electronic environment of each moiety. It will be more common for a spectrum of sites from acidic through to basic to be present; therefore a simple pH measurement of an aqueous dispersion of the filler will only give a mean value. It requires more sophisticated techniques to determine the spectrum.

3.2 Acid-Base Theory 3.2.1 Introduction In the main introduction to this chapter the importance of acid-base interactions in determining the interfacial chemistry between fillers and matrices in particulate composites was emphasised. In this section, a summary of the more commonly used acid-base theories will be presented.

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Analytical Techniques for Characterising Filler Surfaces There are two main types of molecular interaction: firstly, the non-polar dispersion forces or London-Van der Waals forces caused by the fluctuations in the charge symmetry of the molecules produced by their thermal energy. Few compounds exhibit only dispersion forces, perhaps the most common examples are saturated hydrocarbons. Secondly, there are polar interactions that are due to asymmetric charge distributions across individual molecules, caused by the different electron affinities of the elements within the compound. However, it has been shown that the strengths of these polar interactions are not simply dependent upon the polarity of the molecules, but on their electron accepting or electron donating, e.g., acidic or basic, character. In fact, Fowkes, [1-4] has shown that the strength is independent of the dipole moments of the interacting molecules except when an acid site is interacting with a basic site. Lewis [5] was the first to describe acids and bases in terms of their electron accepting and electron donating properties. Mulliken [6] further refined the understanding of the acid base interactions for which he was awarded the Nobel Prize for Chemistry. His quantum mechanical approach introduced the concept of two contributions, an electrostatic and a covalent, to the total acid-base interaction. Pearson [7] introduced the concept of hard and soft acids and bases, the HSAB principle, based on the relative contributions from the covalent (soft) interaction and the electrostatic (hard) interaction. In his mathematical treatment he defined the absolute hardness of any acid or base in terms of its ionisation potential and electron affinity. Pearson’s is probably the most robust approach, but the approaches in most common use are those developed by Gutmann [8] and Drago [9], who separately developed equations and methods to quantify the acid or basic strength of compounds, from which their heats of interaction could be calculated.

3.2.2 Gutmann Approach The Gutmann approach was to allocate a donor number (DN) for bases, and an acceptor number (AN) for acids, corresponding to the definitions first proposed by Lewis. The DN was determined by measuring in a calorimeter the heat of reaction between the base under investigation and antimony pentachloride in a dilute 1,2 dichloroethane solution. The AN was determined from the 31P NMR shift of triethylphosphine oxide (TEPO) when dissolved in the acid under investigation. The number allocated was determined from a linear scale in which n-hexane was designated as zero and antimony pentachloride was designated as 100. Gutmann then proposed that the heat of reaction between acids and bases (ΔHab) could be calculated using Equation (3.1): ΔHab =

AN × DN 100

(3.1)

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Particulate-Filled Polymer Composites There are a number of weaknesses in the Gutmann donor-acceptor number approach. Firstly, Riddle and Fowkes [10] showed that AN numbers determined from the 31P NMR shift of TEPO using the technique disclosed by Gutmann contained a contribution due to Van der Waals or dispersion interactions. They showed that even n-hexane, a compound with no dipole, caused a shift in the 31P NMR spectra of the TEPO. An adjustment to the AN numbers can be made to account for the contribution from the dispersion forces. Fowkes has described [11] how the choice of a hard base, TEPO, and a soft acid, antimony pentachloride, for the Gutmann test methods give higher values to hard acids and soft bases than if the reverse was chosen; a soft base and a hard acid. This problem of the order of basicity or acidity being dependent upon the test acid is a consequence of trying to reconcile a single acid or base parameter with the two contributions, the ionic and covalent, to acid-base interactions so elegantly presented by Mulliken.

3.2.3 Drago Approach Drago [9] developed his theoretical approach for relating the enthalpy of interaction with the acidity and basicity of the interacting species, by allocating two constants to define the strength of acids and bases. These are the E and C constants which represent the electrostatic and covalent, or hard and soft, contributions to the acidity or basicity of the compound which loosely correspond to those determined by the quantum mechanics treatment of Mulliken. The constants are determined from calorimetric measurements, and are fitted empirically to the acids and bases and do not necessarily relate precisely to the electrostatic and covalent contributions of the acid base interactions. However, this has not prevented the Drago approach from being used with significant success. The relationship is expressed in Equation (3.2): –ΔHab = CACB + EAEB

(3.2)

ΔHab is the enthalpy of reaction between an acidic species A, and a basic species B. CA and CB are the covalent constants for the acid and base, respectively, and EA and EB are the electrostatic constants for the acid and base. This approach is also not without limitation as it assumes that the two interacting species are purely acidic or basic, and does not take into account any interactions due to acid sites on the basic species, or basic sites on the acid.

3.2.4 Use in Characterising Fillers The most significant work to use acid-base theory to understand the surface chemistry of inorganic fillers has been carried out by Fowkes and his co-workers [1-4, 10-12].

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Analytical Techniques for Characterising Filler Surfaces They made use of the Drago Equation (3.2) to characterise a series of particulate inorganic materials, e.g., ferric oxide, silica and titania, and ascribed E and C constants to their surfaces. The method adopted by Fowkes and co-workers was to determine the heat of interaction between the fillers and probes with known Drago constants using calorimetric techniques. Two techniques were adopted for measuring the heats of adsorption. The first used a static Tronac calorimeter in which the filler surface was titrated with an excess of the chosen probe, the integral heat of adsorption was determined from the calorimeter response. Whilst the molar heat of adsorption was calculated by back titrating to quantify the unreacted probe. The second was to use a flow microcalorimeter in conjunction with a high pressure liquid chromatography (HPLC) detector, a technique described in detail in Section 3.4.1.3, to determine the molar heat of adsorption (ΔHads) of the probes onto the filler surfaces. These data were then used to produce plots derived from the rearranged Drago Equation (3.3): EA =

⎛C ⎞ –ΔHads − CA ⎜ B ⎟ EB ⎝ EB ⎠

(3.3)

In this example of the rearranged equation the probe is a base with known constants CB and EB and is being used to determine the EA and CA constants of the acid sites on the filler surface. But clearly, the A and B subscripts could be interchanged and an acid probe used to characterise the basic sites on the filler surface. The plots of EA versus CA, of which an example from reference [12] is shown in Figure 3.1, are constructed for each probe base by calculating the intercept on the EA-axis, i.e., –ΔHads/EB when CA is zero, then drawing a line from it with gradient CB/EB. For any given surface only one point on each plot is valid and that is at the EA and CA values corresponding to those of the surface. This point is therefore where the plots produced from the different probes intersect. The example given in Figure 3.1 is produced from data collected from the adsorptions of ethylacetate, pyridine and triethylamine onto silica, the EA and CA constants for the acid silanol sites were determined as 4.4 and 1.1; the source of the silica was not disclosed. This is an extremely powerful method for characterising the acidic and basic sites on filler surfaces and produces data that corresponds, at least qualitatively, with the most sophisticated quantum mechanics analyses of Mulliken. However, it is important to appreciate that the method described [12] only gives an average value for the EA and CA constants, as mentioned earlier different active sites may have quite different characteristics. However, an adaptation to the FMC technique offers an opportunity to obtain a semiquantitative understanding of this surface heterogeneity, this is described in Section 3.4.1.3.

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Figure 3.1 Determination of the Drago E and C parameters for the surface SiOH sites of silica, using heats of adsorption of test bases determined by isothermal microcalorimetry. (Reproduced with permission from F.M. Fowkes, Journal of Adhesion Science and Technology, 1987, 1, 1, 17. Copyright 1987, VSP.)

3.3 Analytical Techniques The analytical techniques have been divided into three groups; the first group contains those techniques that are used to characterise the surface of fillers by understanding their reactivity with probe molecules. These are FMC and inverse gas chromatography (IGC) and are discussed in Section 3.4. The second group of techniques are the spectroscopic techniques that provide elemental and chemical analysis. These are diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), XPS, and SIMS. XPS and SIMS probe the first few nanometres of the surface whereas DRIFTS probes an order of magnitude deeper. These techniques are discussed in Section 3.5. The third group of techniques [wide angle X-ray diffraction (WAXS) and differential scanning calorimetry (DSC)] are those that are able to examine formation of structural order within adsorbed layers of surface treatments on filler surfaces. These are discussed in Section 3.6.

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3.4 Reactive Techniques 3.4.1 Flow Microcalorimetry Probably the most notable feature associated with the adsorption (or desorption) of any compound onto a solid surface is the change in enthalpy that occurs during the process; and is captured in Drago’s equation (see Equation 3.2). For an adsorption process the enthalpy change is invariably an exotherm, and is a function of the acidic and basic characters of the adsorbate and the adsorbent surface. With modern calorimetric techniques this enthalpy change is relatively easily measured. Furthermore, for adsorptions onto materials with low specific surface areas it can be more accurately measured than the quantities of adsorbate. However, as will be described in more detail, FMC in conjunction with frontal analysis using HPLC detectors offers the ability to measure both the enthalpic changes and the quantities of adsorbate. Many workers have used both static and flow calorimetry for measurement of the enthalpic changes associated with adsorption and desorption processes. Two basic techniques are used in static calorimetry. In the first, the adsorbent and adsorbate are placed in the calorimeter, but in isolation from each other, the two are then mixed, for example by breaking a membrane, and the heat of reaction is measured. In the second, the adsorbent is charged to the calorimeter cell fully wetted with the solvent for the adsorbate, then a solution of the adsorbate is injected into the calorimeter. Static calorimetry would appear to have some advantages for studying batch systems, as the measured enthalpy change would be the integral of all the surface and solution chemistry taking place within the system. However, within the system there may be many interacting equilibria that a single enthalpy change measurement can do little to elucidate. In flow calorimetry the enthalpy change that is measured is that which occurs as the adsorbant changes from being in equilibrium with a known fluid, e.g., pure solvent, to that of a second known fluid, e.g., a solution of an adsorbate in solvent. Therefore, the initial and final states are relatively well defined and the enthalpy can be more easily attributed to known adsorption-desorption processes taking place at the surface of the adsorbent. Further advantages of FMC over alternative techniques include the ability to measure heats of interactions of polymers in solution with solids, i.e., filler surfaces: characterisation of filler surfaces by IGC is limited to reversibly adsorbed volatile molecules. Also, HPLC detectors can be connected in series with the calorimeter and used to determine quantities adsorbed, and hence provide a measure of surface coverage. The technique has been found by the authors to be most useful for gaining insight into adsorption interactions between a range of probe molecules and filler surfaces. Probes have included: polymers, filler surface treatments and polymer additives. In general FMC is an underused technique in this field.

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3.4.1.2 Flow Microcalorimeter (a) Description. Almost any calorimeter can be adapted to operate in flow mode by making a flow cell which fits into the standard calorimeter cell. However, the calorimeter which will be described here is an instrument designed and produced by Microscal Ltd. [(79 Southern Row, London, W10 5AL; (www.microscal.com)] as a dedicated flow microcalorimeter, although the method of operation, data collection and interpretation described in later sections would be valid for any flow calorimetry system. A line diagram of the Microscal flow calorimeter and the flow system is presented in Figure 3.2. The instrument contains four highly sensitive thermistors. In the more recent models from Microscal, the two working thermistors are placed behind thin polytetrafluoroethylene (PTFE) membranes that form the walls of the cell. These are referenced, using a Wheatstone bridge arrangement to two thermistors, positioned about

Figure 3.2 Schematic diagram of a Microscal Flow microcalorimeter. (Reproduced with kind permission from D.P. Ashton and R.N. Rothon in Controlled Interfaces in Composite Materials, Ed., H. Ishida, Kluwer Academic Publishers, Dordrect, The Netherlands, 1990, p.297. Copyright 1990, Kluwer Academic Publishers.)

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Analytical Techniques for Characterising Filler Surfaces half way into the thick PTFE walls of the cell. The circuit can respond to differences in temperature of 1 x 10-5 °C, the response is amplified for measurement with a simple chart recorder, or can be fed into a chromatography data station (as used by the authors). Microscal have produced their own PC based FMC control and data analysis system called Calorimeter Digital Output and Sequencer (CALDOS). This removes the reliance on chromatography data stations and software and hence provides the user with a system designed especially for FMC. (b) Method of Operation. The filler is charged to the calorimeter cell, where it can be pretreated by heating under vacuum, or more usually by equilibrating in a flow of the pure solvent chosen for the experiment. The cell of the Microscal flow microcalorimeter has a volume of 0.15 cm2. As described previously the flow microcalorimeter is used to measure the enthalpy change for the transition between the filler being in equilibrium with fluid A and the filler being in equilibrium with fluid B. Fluid A is normally a pure solvent, and fluid B is normally a dilute solution of the probe compound in the solvent. Syringe No.1, see Figure 3.2, is filled with fluid A, and syringe No.2 is filled with fluid B containing the probe. After completing any pretreatment of the filler, fluid A is continuously passed through the filler in the calorimeter cell at a constant and preset rate until equilibrium is achieved; this is indicated by a constant signal from the bridge amplifier. The four-way valve is then switched to direct the flow of fluid B from syringe No.2 to the calorimeter cell. The flow of fluid B is also constant and at the same preset rate as fluid A during the initial equilibration. Flow is continued until the output signal indicates equilibrium with the solution has been established. When this is achieved, the flow may be switched back to that of the solvent, fluid A, to test for the reversibility of the process. More recently electrically actuated valves have become available and can be integrated into the Microscal CALDOS PC based FMC control and analysis system. This allows the user to leave the FMC unattended for the duration of the entire experiment, as there is no need to return to the instrument and switch the valves over manually (c) Output from Flow Microcalorimeter. As the solution of the probe, fluid B, passes through the filler, the probe adsorbs onto the surface of the filler, the heat of adsorption causes an increase in temperature which is detected by the thermistors and registered by the recorder or data station. Conversely, after switching the flow from solution to solvent, desorption of the probe species will cause a decrease in temperature. Therefore, the output is the millivolt output from the amplifier, which is a linear function of the temperature increase in the calorimeter cell, and this is plotted against time. Examples of the outputs from the adsorption and desorption of methyl methacrylate onto alumina trihydrate are presented in Figure 3.3.

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Figure 3.3 Flow microcalorimeter response during adsorption and desorption of methyl methacrylate onto ATH

The peak areas in the thermal data are proportional to the energy changes that have occurred. The constant of proportionality is determined from calibration peaks that are formed by passage of a known electric current for pre-selected time interval through a coil of resistance wire that is integrated into some models of the FMC cell outlet connector. Further insight into the nature of the interaction between irreversibly adsorbed species and the filler surface can be gained from DRIFTS analysis of the filler sample taken from the FMC cell after completion of the adsorption – desorption cycle. DRIFTS is described in more detail in Section 3.5.4, but in summary, it is an infrared spectroscopic technique, that by virtue of a significant proportion of glancing angle reflections, affords enhanced resolution of filler surface functional groups. The authors have found this technique particularly useful when studying competitive adsorption of polymer stabilisers and carboxylic acids onto silica and metal hydroxides, respectively. (d) Choice of Reagents. In any single experiment there are essentially three reagents to be considered, the filler charged to the calorimeter cell, the solvent, and the probe compound. The filler under investigation will usually be determined by the composite system under study. Sometimes, however, it is equally valid to use model fillers with properties chosen to maximise the accuracy and reproducibility of the technique. The preferred filler properties are good powder flow characteristics to avoid bridging and hence voiding within cell, and a high surface area to maximise the signal and hence signal to noise ratio. Certain very fine fillers such as fumed silica can create other practical problems such as escape of filler though the outlet connector filter. This can be minimised by use of a special purpose fine particle outlet connector featuring a polyvinylidene fluoride (PVDF)

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Analytical Techniques for Characterising Filler Surfaces membrane filter. In extreme cases very fine (colloidal) filler particles can pass though or block the PVDF membrane, in such cases a static adsorption experiment can be carried out with only minor modifications to the FMC. The choice of solvent is also important, as the enthalpy measured is essentially that of the displacement of the solvent from the filler surface by the probe molecule. For acidbase characterisation a non-polar solvent such as n-heptane is ideal. However, poor solubility of the desired probe molecule may demand the use of a polar solvent, when this is the case the enthalpy of interaction of the solvent with the filler must be considered when interpreting the results. If the probes in question are only capable of weak physical adsorption, (i.e., not via hydrogen bonding), and are only soluble in highly polar solvents such as tetrahydrofuran and dimethyl formamide, situations can arise where apparently no adsorption occurs. This is due to the solvent having equal or stronger interaction with the substrate than the probe. In such cases the value of performing FMC experiments should be questioned and the use of more soluble model compounds considered. If chemical adsorption of the probe occurs the polarity of the solvent is somewhat less critical, provided the solubility of reaction products is not enhanced. Of further importance is the need to use a consistently pure solvent, with special attention paid to the water content. The authors have found drying of n-heptane over sodium metal or freshly activated molecular sieves (3A or 4A) achieves low and consistent water levels in the solvent. Another factor influencing solvent choice is the solubility of any products formed as a result of chemical adsorption; for example, water and metal carboxylates in the case of studies of adsorption of fatty acids onto metal hydroxides. Toluene is a much better solvent for such salts, and the resulting water, than heptane. Generally it is the salts of branched chain fatty acids that are more soluble due to their reduced ability to pack into an ordered array. This is exemplified in Figure 3.4 [13] where the heat of adsorption of isostearic acid (16-methyl heptadecanoic acid) onto magnesium hydroxide is reduced to a far greater extent, when adsorbed from toluene, than stearic acid (octadecanoic acid). Equivalent adsorption onto aluminium hydroxide shows that this is not merely a solvent displacement effect. Formation of a fully neutralised, and hence potentially soluble carboxylate, is far easier in the case of magnesium hydroxide than aluminium hydroxide. This is due to the greater basicity of the former, and the 2+ charge on the magnesium ion: this enables it to become more easily detached from the ionic lattice. The probe compounds may be chosen as model acids (for example, carboxylic acids or chloroform) and bases (for example, pyridine or ether), with which to measure the acidbase character of the filler surface. However, the probes may be actual interfacial modifiers used in the composite system under investigation, for example an organosilane coupling agent or fatty acid dispersant. Furthermore other interesting interactions have been studied by the authors and include stabiliser – filler interactions during evaluation of controlled release/displacement effects.

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Figure 3.4 FMC data for adsorption of stearic acid and isostearic acid onto Mg(OH)2 and Al(OH)3 from heptane and toluene; ■ adsorption data, ■ desorption data. The dashed lines represent vertically adsorbed theoretical monolayer levels. M: magnesium hydroxide, A: aluminium hydroxide, hept: heptane, tol: toluene. (Reproduced with permission from C.M. Liauw, R.N. Rothon, G.C Lees and Z. Iqbal, Journal of Adhesion Science and Technology, 2001, 15, 8, 899. Copyright 2001, VSP.)

It is this ability to use these ‘real’ components that is the main advantage of this technique. The technique was used by Ashton and Rothon [14] to characterise the interaction between 3-trimethoxysilylpropyl methacrylate and aluminium hydroxide from n-heptane solution.

3.4.1.3 Use of HPLC Detectors (a) Description. Whilst use of the flow microcalorimeter can generate important enthalpic data, the power of the technique is substantially increased when it is used in conjunction with one or more HPLC detectors in series with the calorimeter. These can be used according to the method, described in (b) below, to determine the quantity of the probe adsorbed, and therefore the molar heat of adsorption. In the study of the adsorption of 3-trimethoxysilyl-propyl methacrylate on to aluminium hydroxide from n-heptane solution [14], refractive index (RI) and UV detectors were used, and found to be

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Analytical Techniques for Characterising Filler Surfaces

Figure 3.5 Line diagram of flow microcalorimeter in series with HPLC detectors. (Reproduced with kind permission from D.P. Ashton, R.N. Rothon in Controlled Interfaces in Composite Materials, Ed., H. Ishida, Kluwer Academic Publishers, Dordrect, The Netherlands, 1990, p.296. Copyright 1990, Kluwer Academic Publishers.)

complementary allowing different components in the eluent from the FMC to be quantified. A line diagram showing the system containing these detectors is shown in Figure 3.5. (b) Output from HPLC Detectors. The eluant from the FMC is passed through the HPLC detectors, the output from each detector correlates with the RI or UV absorbance at the preset wavelength of the detector. Hence as the flow changes from solvent to solution, a plot of response versus time gives a sigmoidal response as shown by line A in Figure 3.6. However, if the probe is adsorbed onto the filler surface in the FMC, the transition will be shifted to a longer time, as indicated by line B in Figure 3.6. The area marked as C in Figure 3.6 is proportional to the mass of the adsorbed probe. To measure this area requires subtracting the transition that occurs during adsorption of the probe, from a transition when no adsorption takes place, but using the same flow and FMC packing conditions. This subtraction can be handled by most standard chromatography software packages and by the Microscal CALDOS system. The proportionality factor can be determined by injecting pulses of solution, via the calibration loop, shown in Figure 3.5, and measuring the area of the peaks produced in the output signal.

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Figure 3.6 Typical response curves from the HPLC detectors during flow microcalorimetry

In reactive adsorption studies such as that described previously, i.e., adsorption of 3trimethoxysilyl-propylmethacrylate on to aluminium hydroxide from n-heptane [14], or reactions between fatty acids and basic filler surfaces [13], the small molecule split off as a product of the chemisorption reaction, (i.e., methanol and water for the first and second examples, respectively), can affect the differential refractometer response. If the small molecule has a lower refractive index than the carrier solvent and the probe a higher refractive index than the carrier solvent, the calculated level of amount of probe adsorbed will be greater than it really is. If however, both the probe and the small molecule have lower refractive index than the carrier solvent, the amount of probe adsorbed will be smaller than it really is. The extent to which this problem occurs also depends upon the solubility of the small molecule in the carrier solvent. This effect has caused problems in a recent study [13] of adsorption of various carboxylic acids on to metal hydroxides. Figure 3.4 shows both the heat of adsorption and amount of carboxylic acid adsorbed. It is evident that adsorption of stearic acid (octadecanoic acid) and isostearic acid (16-methyl heptadecanoic acid) from toluene affords apparently reduced levels of adsorption, relative to when adsorbed from heptane. This is due to both the probe and the water having lower refractive indices than toluene. DRIFTS analysis of the respective filler samples retrieved from the FMC cell, however, indicates similar levels of adsorption. Only in the case of adsorption from heptane is the level of adsorption in concordance with theoretical values. This is due to reduced solubility of water in heptane, relative to toluene. A further treatment of the output data was used by Joslin and Fowkes [2], in which they time sliced the outputs from the FMC detector and HPLC detector. This technique enables

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Analytical Techniques for Characterising Filler Surfaces the molar heat of adsorption to be calculated as a function of time through the adsorption process, and gives an indication of the heterogeneity of strength of the acid or base adsorption sites on the filler surface. Microscal Limited have developed a variation on Joslin and Fowke’s approach, using the latest Microscal system, controlled additions of the probe solutions are injected into the cell and the reduction in the molar heat of adsorption recorded as a function of amount adsorbed. The injections are carried out under computer control via the Microscal CALDOS software.

3.4.1.4 Use of FMC in Competitive Adsorption Studies Competitive adsorption of components of filler surface modifier systems, stabilisers and lubricants, etc., is an inevitable event that occurs in the vast majority of filled polymer formulations. Such phenomena can lead to inferior product performance in terms of reduced coupling agent efficiency and poor stabilisation. Therefore an understanding of such events is very beneficial if performance is to be optimised. Studies have been carried out using combinations of acrylic acid and isostearic acid in an attempt to produce surface modifier systems that combine effective coupling with good dispersant characterisics [13]. In this study sequential adsorption of isostearic acid and acrylic acid on to magnesium and aluminium hydroxides in the FMC cell indicated that acrylic acid will displace isostearic acid from both fillers, however, isostearic acid will adsorb onto a layer of adsorbed acrylic acid producing a hybrid layer. Combinations of stabilisers, (e.g., a hindered phenolic primary antioxidant, a phosphite secondary antioxidant and a hindered amine light stabiliser) are frequently used. Knowledge of the adsorption activity of a filler towards such additives can indicate what additives can be avoided or how the filler surface may be modified, for example with an epoxy resin or fatty acid, so that it no longer adsorbs the additive [15]. Conversely, it can be argued that the filler surface can be also be used as a reservoir of stabiliser, provided it can be released from the surface in a controlled manner. Liauw and co-workers have examined the effect of incorporation of gel silica (323 m2 g-1) at 0.1% w/w, on the thermal stabilisation (melt and solid state) and UV stabilisation of linear low density polyethylene [16, 17]. The adsorption characteristics of all the stabilisers investigated were examined by FMC [17] and heat stabiliser/UV stabiliser combinations were devised on the basis of relative strength of adsorption. The stabilisation performance was found to be enhanced in the presence of silica only when the more active stabiliser was weakly adsorbed relative to the less active one; this indicated a controlled release mechanism that was associated with displacement of the weakly adsorbing stabiliser

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Particulate-Filled Polymer Composites [16]. This mechanism was confirmed by performing competitive adsorption experiments using the FMC [17]. Figure 3.7 shows FMC data investigating the competitive adsorption of the hindered phenolic antioxidant Irganox 1010 (I1010) and the polymeric hindered amine light stabiliser Chimassorb 944 (C944). These probes are adsorbed from heptane on to silica. Data are shown for adsorption of I1010 followed by C944 and for adsorption of C944 followed by I1010. It is evident that C944 is able to initially adsorb at sites not occupied by I1010 as there is solvent released from the silica. DRIFTS studies on the silica isolated from the cell clearly indicated that C944 totally displaced the I1010. It is evident from Figure 3.7 that I1010 will not displace adsorbed C944 and that the latter will adsorb on to all the sites receptive to I1010.

Figure 3.7 Raw FMC data showing (a) adsorption of Irganox 1010 followed by switch over to Chimassorb 944, and (b) adsorption of Chimassorb 944 followed by switch over to Irganox 1010. (Replotted from C.M. Liauw, A. Childs N.S. Allen, M. Edge, K.R. Franklin and D.G. Callopy, Polymer Degradation and Stability, 1999, 63, 3, 391, p.396. Copyright 1999, Elsevier)

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3.4.2 Inverse Gas Chromatography 3.4.2.1 Introduction This technique [18] has now been in use for a number of years. In inverse gas chromatography (IGC), a pulse of a small quantity of a probe is injected into a stream of gas which passes over the filler which is packed into the separation column of the chromatograph. The time taken for the probe to elute from the column, compared to that of a non adsorbing compound, is a measure of the interaction between the probe and the surface of the filler. A mathematical treatment can then relate this difference in elution time to the free energy change in the adsorption process. The technique can be considered as complementary to FMC, although it is constrained by the need to use volatile probes, a number of experimental factors usually result in more accurate thermodynamic data being obtained. The most important difference between IGC and FMC is that in the former technique, the probe is at infinite dilution, therefore, only the most active sites on the filler surfaces are likely to be probed. As adsorption from the gas phase occurs during IGC, only weakly bound molecules of carrier gas need to be displaced during the adsorption process, it is this aspect that may lead to production of more accurate thermodynamic data. In FMC the probe is at finite dilution and it is likely, in the case of some filler/solvent combinations, that the most active sites will always be occupied by the solvent molecules, therefore these sites may never be probed during the FMC experiment. This will almost certainly be true in cases where the solvent and filler have strong hydrogen bonding activity, i.e., alcohols with silica. However, it can be argued that in a polymer composite, particularly when additive or filler surface modifier adsorption from the matrix melt (or a liquid resin) is considered, the conditions of the FMC experiment are closer to reality. IGC may on the other hand be more useful for analysis of filler-matrix interaction where the matrix chains are essentially displacing nitrogen and oxygen molecules during incorporation. Even in this situation, a question mark still hangs over IGC; in real systems, adsorbed water will certainly be present on a filler surface before it comes into contact with the matrix melt. A significant amount of this water will desorb due to the high temperatures experienced in melt processing, though it is most unlikely that all of the strongly adsorbed water will be totally removed. Under IGC conditions water is usually considered to be completely absent; furthermore, the adsorption of the model compounds may take place at temperatures that are far higher than normally encountered processing temperatures. It is important to appreciate that probes used for IGC must not adsorb on to the filler too strongly as they may take too long to elute from the column, or in extreme cases, may not elute at all. Despite the reservations associated with FMC and IGC these techniques are still valuable for gaining insight in to filler surface chemistry and how this may influence product performance.

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3.4.2.2 Thermodynamic Considerations As described previously the important measurement that is made in this technique is the elution time between injection and elution of the probe. Although more precisely, the value used in the mathematical treatment is the ‘net retention volume’ VN. VN is the additional volume of carrier gas eluted from the chromatograph before elution of the probe, compared to the volume eluted before elution of a non-adsorbing gas, for example methane. VN is calculated using Equation (3.4): VN = kF(tp-tn)

(3.4)

Where, F is the carrier gas flow rate, tp is the retention time of the probe, tn is the retention time of the non-adsorbing gas, and k is a factor which corrects for the drop in pressure of the carrier gas as it passes through the chromatographic column. Equation (3.5) has been used [19] for calculating this pressure change factor: 2 Pi / Po ) − 1 ( k= (Pi / Po )3 − 1

(3.5)

where Pi is the gas pressure at injection, and Po is the gas pressure at the detector. The net retention volume VN is related to the free energy of the acid-base interaction G°ab between the probe and the surface of the filler by Equation (3.6). This relationship is valid only at infinite dilution when interactions between the probe molecules are zero, it is therefore essential that very small volumes of the probe are injected into the carrier stream to get as close to the ideal state as possible. Of course the detector sensitivity dictates the minimum volume, it is usual to operate close to this limit of sensitivity: o ΔGab = RT ln VN + K

(3.6)

where K is a constant which depends [20, 21] upon the chosen reference state. The enthalpy change ΔHab for the adsorption process can be calculated using the Gibbs free energy equation (3.7). o ΔGab = ΔHab − TΔSab

(3.7)

Although the entropy change (ΔSab) is an unknown quantity this can be determined by conducting a series of adsorptions at different temperatures. The entropy change is then calculated from the slope of the plot of ΔG°ab versus absolute temperature.

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Analytical Techniques for Characterising Filler Surfaces Therefore, by measuring the net retention volumes, over a range of temperatures, of a series of acid and base probes with known Drago constants, then fitting the calculated enthalpy data to the Drago Equation (3.2), the acid and base constants for the filler surface are able to be determined. The method is described in Section 3.3 of this chapter and uses plots (see Figure 3.1), derived from the rearranged Drago Equation (3.3). A similar approach was adopted by Schultz and co-workers [22] but using Equation (3.8) derived from the Gutmann Equation (3.1) to characterise the acid base characteristics of carbon fibres using IGC: − ΔHab = AN f DN p + DN f AN p

(3.8)

This method, based on the Gutmann equation, is also preferred by Panzer and Schreiber [23] who believe that the dominant weakness of the Drago approach is its failure to account for the amphoteric nature of the probe.

3.4.2.3 Inverse Gas Chromatography (a) Description. IGC can be carried out on any modern gas chromatograph. The type of detector used is not critical to the application, flame ionisation detectors (FID) and thermal conductivity detectors (TCD) are the more commonly used. The FID is generally a more sensitive detector and therefore permits experiments to be carried out at a greater degree of dilution. However, on modern gas chromatographs the TCD also have high sensitivity and are equally appropriate for IGC. The output can be measured on a traditional chart recorder and retention volumes measured with a rule, or more conveniently using PC based chromatography software that electronically measures the retention time, and can be further programmed to calculate VN, see Equation (3.4). The technique differs from the standard gas chromatography only in the material used for the column packing and the processing of the data. The only additional hardware required are pressure gauges at the injection point and detector, which enable the k factor in Equation (3.4) to be calculated. A schematic diagram of an IGC is shown in Figure 3.8. The critical parameters are a constant and accurately measured gas flow rate, temperature, and injection and detector pressures. A small injection volume of the probe molecule is required, to ensure that the conditions of the thermodynamic treatment are met. (b) Method of Operation. The filler under examination is packed into a standard gas chromatograph column, typically a stainless steel tube 0.5 or 1 m in length, and ~5 mm

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Figure 3.8 Schematic diagram of an inverse gas chromatograph system

internal diameter. The column may be packed with undiluted filler, or the filler may be mixed with a non-interacting low surface area diluent. This would be used where a very high surface area filler caused an excessively long retention time, or where filler packing caused an excessively high pressure drop across the length of the column. The next stage is to find the minimum detection limit of the detector and determine the minimum injection volumes of the probes which still produce well defined peaks, with good signal to noise ratio. Then an injection of a non interacting gas (for example, methane) is made, the retention time of this gas gives the value tn for use in Equation (3.4). The experiments can then be carried out by injecting the predetermined volumes of the probes and measuring their retention times tp. If the enthalpies of the interactions are required, then the injections need to be repeated at different column temperatures to enable the entropy change to be calculated. (c) Example of output. Figure 3.9 is a compilation of typical output plots obtained during an IGC experiment. Time zero corresponds to the time of injection of the sample, a nonadsorbing probe takes time tn to elute from the column, whilst the experimental probe takes time tp to elute. The time difference between tp and tn relates to the strength of adsorption of the probe onto the filler surface. By adsorbing a series of probes with different acidic and basic nature a qualitative characterisation of the filler surface is achieved. Although as described previously by inserting the experimental data into Equations (3.4) to (3.7) it is possible to determine enthalpies of adsorption.

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Figure 3.9 Typical response curves from an IGC experiment, with responses from nonadsorbed control and adsorbed probe superimposed

3.4.2.4 Multiple Probe Temperature-Programmed IGC This variation on traditional IGC has been developed from the McReynolds approach [24], for characterisation of stationary phases, by McMahon and co-workers [25]. It is particularly suited to rapid comparative screening of families of solid surfaces, for example a range of carbon blacks with differing surface chemistry. However, it does not provide quantitative measures of thermodynamic parameters. The multi-probe set includes those specified by McReynolds together with several n-alkanes. Classified groups of these probes can be injected into the column as mixtures. The temperature of the column is increased at a constant rate and the temperature at which the probe elutes is recorded as the retention temperature (TR). The TR values of the probes can be plotted against probe boiling point. For n-alkane probes this relationship is linear but data for other probes will not necessarily coincide with this linear relationship. If the concentration of polar sites on the filler is higher than that of non-polar sites then data for the other probes will fall above the n-alkane line. In the reverse case, i.e., if the concentration of non-polar sites is higher than the concentration of polar sites, the data for the other probes will fall below the n-alkane line. The retention characteristics of the probes can be expressed as Kovats indices (KI), i.e., the carbon number of the n-alkane that will co-elute with a given probe multiplied by 100. The higher the KI, relative to the carbon number of the probe, the greater the strength of interaction between the probe and the filler.

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Particulate-Filled Polymer Composites One of the filler samples can be selected as a reference column and the probe TR values in the sample column can plotted as a function of respective TR values in the reference column. The surface area of filler in each column must be identical. If the surface chemistry of the samples is exactly equivalent a linear plot will result. The direction and magnitude of deviation from linearity provides a measure of the relative strength of interaction. The multiple probe temperature programmed approach to IGC certainly shows potential for rapid screening of fillers and yields data that can be treated using chemometric classification tools such as principle component analysis.

3.5 Spectroscopic Techniques 3.5.1 Introduction Surface modification of filler particles is common and frequently mandatory in order to achieve desirable properties such as dispersability, filler-matrix bonding and even stability. Such modifications can range from the adsorption of less than one monomolecular layer of a surfactant to the precipitation of a coating many nm thick. Surface composition analysis therefore requires methods which posses sampling depths of order 1-10 nm to several μm, and that can characterise both inorganic and organic species, ideally in a fully quantitative manner. Of the variety of ultra-high vacuum (UHV) based surface spectroscopic techniques which have emerged since the late 1960s only two: X-ray photoelectron spectroscopy (XPS) (also frequently referred to as ESCA), and secondary ion mass spectrometry (SIMS), come close to meeting the nanometer sampling depth requirements and are discussed briefly here. It is worth noting that Auger electron spectroscopy (AES), which has many of the necessary attributes, fails in this context because of charging problems with highly insulating powders and beam damage to organic materials. For much more detailed descriptions of the techniques the interested reader should consult references [26-28].

3.5.2 X-Ray Photoelectron Spectroscopy 3.5.2.1 Physical Basis In XPS the sample, inside a high vacuum system (pressure <10-5 Pa), is irradiated with soft X-rays, usually Mg Kα (1253.6 eV) or Al Kα (1486.6 eV). The primary event is photoemission of a core (atomic) electron, but relaxation processes also lead to emission of Auger electrons, as shown in Figure 3.10. The emitted electrons are collected by an electrostatic energy analyser and detected as a function of kinetic energy (Ek), producing a spectrum such as that shown in Figure 3.11. The intense, narrow peaks are due to

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Analytical Techniques for Characterising Filler Surfaces

Figure 3.10 X-ray photoemission from a 1s core level and subsequent relaxation processes leading to X-ray fluorescence and Auger electron emission

photoelectrons emitted from core orbitals, (e.g., carbon 1s). The binding energies (EB) of these electrons, are obtained from the Einstein relationship (Equation 3.9): EB = hν – Ek – ø

(3.9)

(where hν is the X-ray photon energy and ø the sample work function) are highly characteristic and allow the identification of all elements except hydrogen (and helium). The peak intensities are proportional to the number of atoms sampled, and with the aid of appropriate sensitivity factors, atomic compositions can be calculated, with typical detection limits of 0.1 atom%. The much weaker series of peaks at very low binding energy is due to photoelectron emission from the valence band (molecular orbitals). The other, broader peaks are due to Auger electrons, for which the kinetic energy (Ek) is given by Equation (3.10): Ek(ABC) = EA – EB – EC – ø

(3.10)

where A, B, and C are the photoionised level, the level of the electron that fills this vacancy, and the level of the emitted Auger electron, respectively, (Figure 3.10). These kinetic energy values are also element characteristic, although not all elements produce strong Auger signals under typical XPS conditions, and are obviously independent of hν. The information from XPS is surface specific because the electrons that give rise to the useful peaks in the spectrum have emerged from the material elastically, i.e., without loss of energy. The inelastic mean free path (λm) of electrons of energy E in an inorganic solid is given, approximately, by Equation (3.11): λm =

2170 E

2

+ ( aE) 2

1

(3.11)

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Particulate-Filled Polymer Composites

Figure 3.11(a) XPS survey spectra of uncoated commercial TiO2 pigment

where λm is in monolayers and the monolayer thickness, a (nm), is given by Equation (3.12): a3 =

A × 1024 pnN

(3.12)

where A is the atomic or molecular weight, n is the number of atoms in the molecule,

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Analytical Techniques for Characterising Filler Surfaces

Figure 3.11(b) XPS survey spectra of coated commercial TiO2 pigment. Note the marked attenuation of Ti 2p signal relative to Figure 3.11(a) following coating.

N is the Avogadro number and p is the bulk density in kg m-3. From this ‘universal curve’ [29] typical values of λm range from ~2 (100 eV) to ~11 (1500 eV) monolayers. Expressed in nm (λn = aλm) this range is from ~6 to ~40 nm. The vertical sampling depth, corresponding to 95% of the measured electron intensity is given by 3λ sin θ where θ is the electron detection angle with respect to the surface (the ‘take-off angle’).

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3.5.2.2 Instrumental Aspects The photon sources commonly used are Mg Kα1,2 and Al Kα1,2; the doublet nature of the X-ray line is not resolved and the line widths are 0.7 and 0.85 eV, respectively. Mg Kα therefore has the advantage for spectral resolution but Al Kα covers a greater energy range (see section 3.5.2.1). Al Kα can also be monochromated using Bragg diffraction from quartz crystals, reducing the line width to ~0.3 eV. In the past this was at the expense of intensity/sensitivity but modern instruments have overcome this problem through more efficient collection of the photoelectrons. Electron energy analysis is achieved by means of an electrostatic concentric hemispherical analyser operating in the fixed analyser transmission mode. All the photoelectrons are retarded to a constant ‘pass energy’ through the analyser; the higher this energy the greater the sensitivity but the lower the energy resolution. Insulating samples charge up under X-ray bombardment due to secondary election emission. With conventional sources, this is counterbalanced by the flood of low-energy electrons emanating from the front face of the window separating the X-ray source from the analysis chamber. With monochromated sources this mechanism is not available and a discrete source of electrons needs to be provided from a low-energy ‘flood gun’.

3.5.2.3 Information Levels The main source of information is the core levels. With Mg or Al Kα sources at least one core level electron is excited from all atoms in the periodic table except hydrogen. The peak positions are highly element specific with relatively few overlap problems allowing straightforward elemental analysis. As noted previously the peak intensity data is inherently quantitative. The appropriate ‘sensitivity factors‘ can be calculated from theoretical cross-sections and known instrumental factors, but are often best determined empirically using pure stoichiometric standards. Core level binding energies (BE) vary slightly (typically over a range of few eV) depending on oxidation state, electronegativity of attached ligands/chemical environment, etc., (chemical shift effect). This allows more detailed structural interpretation. Thus in the example shown in Figure 3.11, from the BE of the appropriate core levels the oxidation states of the elements aluminium, phosphorus, titanium and zirconium could be established. For organic systems the C ls, etc., chemical shifts reveal information about functionality as illustrated in Figure 3.12. Often this information can be augmented via use of the Auger peaks (from the same element) to give a number, the Auger Parameter, independent of charging effects, that is characteristic of that chemical state (the Auger peaks also suffer chemical shifts as a consequence of Equation 3.10). These data can be very material

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Analytical Techniques for Characterising Filler Surfaces specific and large tabulations of chemical shift and Auger Parameter data are available [30]. By using higher energy X-ray sources, (e.g., Ag Lα at ~3 keV) to excite deeper core levels more Auger Parameters can be measured.

Figure 3.12 C 1s spectra from filler surfaces following interaction with MPS. (a) Alumina after treatment (solid line), alumina before treatment multiplied by (1–x), where x is the calculated coverage by MPS, subtraction (broken line); (b) Subtraction spectrum obtained as in (a) for MPS on quartz and fitted with three components representing C-C/C-H, C-O and C(-O)= functionalities (increasing order of binding energy). These components have relative intensities of ~5:1:1 in agreement with the structure of MPS. (Reproduced with permission from F. Garbassi, E. Occhiello, C. Bastioli, G. Romano, Journal of Colloid and Interface Science, 1987, 117, 1, 266. Copyright 1997, Elsevier.)

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Particulate-Filled Polymer Composites Bonding electrons are also photoemitted and these appear in the ‘valence band’ between, say 0-30 eV BE. Emission from many closely spaced levels with different cross-sections gives rise to a complex spectrum, often rich in structure, which in principle contains more direct structural information than the core level peaks. The spectrum is rather low in intensity (typically only a few percent that of major core lines) but with higher power instruments it is routinely accessible. The ‘fingerprint’ utility of the valence band is increasingly being augmented by full interpretations based on theoretical calculations. A range of carbon black samples of varying degree of oxidation and sulfur content were examined using XPS by Pena and co-workers [31]. In this study the O 1s spectra (543523 eV) and S 2p spectra (177-157 eV) were used to determine the relative amounts and the nature of oxygen and sulfur containing functional groups, respectively. The surface chemistry of the carbon black samples was very well resolved by XPS and was related to FMC data for adsorption of a wide variety of polymer stabilisers and relevant model compounds [31-34]. The multi-probe temperature programmed IGC study by McMahon [25] used two of the same carbon black samples as in [31-34]. The latter IGC data fitted in well with the XPS and FMC data obtained (note CB-1 and CB-2 in [25] are CB-B and CB-A, respectively in [31]).

3.5.3 Secondary Ion Mass Spectrometry 3.5.3.1 Physical Basis In SIMS the sample, inside an ultra-high vacuum system (pressure <10-7 Pa) is bombarded with a primary ion beam, (e.g., Ar+, Cs+, Ga+, O2+) of energy 1-30 keV. Of the fragments of material ejected (sputtered) from the surface a few percent are charged either positively or negatively. These are extracted into a mass analyser to yield, separately, positive or negative secondary ion mass spectra. The fragments can be single elemental ions or clusters of atoms with a collective, usually single, charge. An immediate distinction needs to be made between dynamic and static SIMS experiments. In the former, a primary ion current density of typically > 10 μA cm-2 is used and this leads to rapid sputtering of material. The surface is eroded at a rate of order nm s-1 and by following the intensity of chosen peaks in the mass spectrum as a function of time, a concentration depth profile can be constructed. In this mode SIMS can be very sensitive, with trace element detection limits in the ppm-ppb range. However, quantification is not straightforward. Secondary ion intensities are strongly matrix-dependent and extensive calibration procedures involving closely related standards of known composition and under identical experimental conditions must be used to extract quantitative concentrations. In static SIMS, on the other hand, the aim is to reduce sputtering to the absolute minimum whilst obtaining a spectrum of adequate intensity. For inorganic systems a primary ion

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Analytical Techniques for Characterising Filler Surfaces dose of ~1014 ions cm-2 may be tolerated, resulting in the sputtering of ~10% of the uppermost monolayer. For organic systems, ion-induced damage leads to loss of spectral integrity more quickly and so a maximum dose of 1012-1013 ions cm-2 is permissible. The spectra obtained under these ‘static’ conditions provide a molecular fragmentation pattern representative of the surface molecular structure. Figure 3.13 shows the positive and negative ion mass spectra from the same pigment examined by XPS in Figure 3.11(b). The negative ion spectrum is dominated by fragments from one or more organic species on the surface - either contamination from sample handling/storage or preparation for analysis (powder deposited onto glass from methanol slurry). The pattern of intensities represents a fingerprint as in conventional gas phase mass spectrometry (but the ionisation/fragmentation processes are different). The sampling depth in SIMS is only 1-2 monolayers (~10 Å).

Figure 3.13(a) ToF SIMS spectra from coated pigment giving XPS spectrum of Figure 3.9(b): positive ion spectrum; annotated peaks are due to Na+ (m/z 23), Al+ (27), Ti+ (48), TiO+ (64), TiO(OH)+ (81), ZrO+ (106) and ZrO(OH)+ (123). See next page for Figure 3.13(b)

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Particulate-Filled Polymer Composites

Figure 3.13(b) ToF SIMS spectra from coated pigment giving XPS spectrum of Figure 3.9(b): negative ion spectrum; annotated peaks are due to PO2- (63), PO3- (79), H2PO4- (97) all other peaks are molecular fragments from organic species.

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Analytical Techniques for Characterising Filler Surfaces

3.5.3.2 Instrumental Aspects Since dynamic SIMS is principally a tool for trace element analysis and depth profiling it will not be considered further here. In static SIMS the major instrumental differences are related to ion sources and mass analysis. Early experiments involved the combination of a noble gas (electron impact) ion source with a quadrupole mass spectrometer. Typically 4 keV Ar+ or Xe+ ions in either a raster-scanned focussed spot or a broad stationary spot would be used. Either way a current density in the 1 nA cm-2 regime is the prerequisite for static SIMS analysis. The quadrupole mass spectrometer (QMS) is a compact device with the advantage of a large collection area, suited to the above ion bombardment conditions. A good QMS will have a mass range of 0-1000 atomic mass units (amu) and a transmission of ~0.1% (ions detected:ions ejected). However, transmission decreases with mass (Tα M-1 to M-2) and mass resolution is poor (typically, close to 1 amu over the mass range covered). During the last decade time-of-flight (ToF) mass analysers have completely eclipsed the QMS to the extent that the terms static SIMS and ToF - SIMS have become synonymous [28]. ToF analysers give much greater transmission (>10%, mass independent) and extended mass range (up to 10,000 amu). In ToF - SIMS the primary ion beam is pulsed and all the secondary ions are detected in parallel between each pulse. In contrast the QMS is a serial device, the mass range being scanned. Thus ToF - SIMS instruments are ~104 times more sensitive than QMS instruments and this allows them to perform static SIMS analysis of much smaller areas. Liquid metal ion sources (Ga+) giving small probe diameters (~400 Å) are ideal for imaging at high spatial resolution. ToF instruments also give high mass resolution (of the order of 10,000) which greatly enhances the analytical power of the technique through the resolution of ions with the same nominal mass (a trivial case is a metal ion having the same nominal mass as an organic fragment). Intense Cs+ sources, but with spot sizes of order 30 μm, are used to increase data acquisition rates when high spatial resolution is not required. Insulating samples charge up under ion bombardment and this effect must be prevented during SIMS analysis. With QMS systems a stationary ‘flood’ of relatively low energy electrons is used to ‘charge neutralise’. In ToF systems where the primary ion beam is pulsed at high frequency the electron flood takes place between individual ion pulses when the secondary ion flight times are being measured. Because the pulse averaged current density is also much lower than with a QMS system, this is a much more effective solution to the problem.

3.5.3.3 Comparison of XPS and SIMS The fingerprint quality of static SIMS spectra has already been noted. Identification of surface species is aided by the existence of an expanding library of static SIMS spectra

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Particulate-Filled Polymer Composites with a search engine [35] and the use of accurate mass measurement to assign empirical formulae to chosen peaks. Because of the high surface sensitivity, surface coverage of the substrate by organic molecules is somewhat easier to judge than by XPS. The cluster ion spectra of inorganics are similarly reflective of structure but much less work has been done in this area. SIMS can be much more sensitive to elemental species than XPS, particularly to highly electro-positive or -negative elements. The two techniques are very complementary in respect of quantification, structural insight and sampling depth. In situations such as filler surface analysis, where layered systems involving both organic and inorganic components are involved, SIMS is a valuable adjunct to XPS.

3.5.3.4 Examples There are relatively few detailed studies of particulate fillers by surface analysis and space limitations preclude a review. The interested reader is encouraged to study a series of papers by Johansson and co-workers [36-39] that address the characterisation of coated TiO2 pigments. These discuss in considerable detail the relationship between data from XPS, SIMS and X-ray fluorescence as well as relating surface analytical information to other methods of pigment surface characterisation and to pigment behaviour.

3.5.4 Diffuse Reflectance Fourier Transform Infrared Spectroscopy 3.5.4.1 Background Infrared spectroscopy is a technique that gives intimate structural detail of the molecules under examination. The infrared radiation is absorbed by changes in the vibrational state of the molecule, that is the bending or stretching of the bonds within the molecule. Consequently, the interpretation of the spectra not only gives the chemical groupings present in the molecule but information about the environment of each of the groupings and hence the structure of the molecule. The reader should refer to [40] and [41] to gain a more fundamental understanding of the science. The basic transmission infrared spectroscopy technique has been a workhorse for chemical analysis for many years. As computers and computing techniques became more advanced (FTIR) spectrometers were developed. The introduction of FTIR revolutionised infrared (IR) spectroscopy because for the first time weak signals could be cleared of noise thanks to co-addition and Fourier analysis of many scans. The ability to produce high quality spectra from weak signals led to new sampling techniques, such as diffuse reflectance and microscopy, that simply could not be used with the older dispersive IR spectrometers. The advent of FTIR therefore brought about

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Analytical Techniques for Characterising Filler Surfaces a step change in the resolution and ease of use of IR spectroscopy. FTIR is now no longer the preserve of prestigious university and industrial research laboratories, now quality instruments can be purchased for as little as £12,000. This enhanced sensitivity and ability to subtract spectra with precision led to many adaptations of the technique. One of these adaptations was DRIFTS. This technique was designed to enable spectra of powders to be analysed without the need for grinding and elaborate preparation that can of course modify the chemical nature of the material. The added benefit of DRIFTS is that it is also far more surface specific than transmission IR due to a significant proportion of low and glancing angle reflections though filler surface layers. This effect was effectively demonstrated by Gilbert and co-workers [42] who showed that stearic acid in physical mixtures with various fillers gave very weak IR absorbance in DRIFTS spectra, relative to equivalent amounts of stearic acid that had been adsorbed on the filler surface. FTIR data analysis software enables spectral data for the uncoated filler to be subtracted from that of the coated specimen, thus creating a difference spectrum that enhances absorption bands associated only with the surface treatment. It is important to point out that subtraction does not always yield additional information, especially when a small absorption associated with a surface treatment is obscured by a large absorption associated with the filler. Spectral changes relative to the unbound surface treatment/additive provide insight into the nature of interaction with the surface. Examples of use of DRIFTS are described in Section 3.5.4.4. DRIFTS spectra can be quantified provided sample preparation/presentation and representation of the data is correct.

3.5.4.2 Comparison of DRIFTS and XPS It is important to appreciate that whilst DRIFTS is an IR spectroscopy method that has significant surface specificity, it is by no means an exclusively surface analytical technique as is the case with XPS and SIMS, which have penetration depths in the order of nanometres. The penetration depth in DRIFTS is an order of magnitude higher, i.e., micrometres. However, the high proportion of low and glancing angles of incidence afford the high degree of surface specificity that is the key advantage of the technique. Sutherland [43] has found that with a 20 m2 g-1 calcium carbonate filler, detection limits of less than 0.05 monolayers can be achieved. Rather surprisingly this level of sensitivity was higher than that achievable using XPS on the same series of samples. This can be explained, however, by the greater sampling depth of DRIFTS that enables resolution of surface modifier within pores in the filler surface. The different sampling depths available using these techniques have been exploited by Gilbert and co-workers in more recent studies of fatty acid treated fillers [44].

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3.5.4.3 Sample Preparation It is possible to analyse the filler in an undiluted state using DRIFTS, however, specular reflection and excessively strong absorption can lead to poor quality spectra featuring negative peaks. It is now widely accepted that the filler sample must be diluted with an IR transparent powder such as KBr or KF. The latter must be finely ground and ideally the same batch of ground diluent must be used for a whole sample series. The level of dilution should be such that there is no compression or saturation of the spectrum. This can be observed by comparison of the DRIFTS spectrum with a transmission spectrum of the untreated filler in a KBr disk. Usually 1-10% w/w filler in the diluent will give uncompressed spectra. It is also very important that the ground KBr and filler are not mixed roughly, i.e., by grinding, as the surface layers will be disrupted and non-representative data will be generated; the two components should be mixed by a gentle folding motion with a micro-spatula.

3.5.4.4 Description of DRIFTS Cell A simplified line diagram of a cell is shown in Figure 3.14. The essential features of the cell are mirrors and optics to focus the incident infra-red beam onto the surface of the powdered filler which is held in the sample holder. The reflected radiation that is now scattered across a wide angle, due to the random orientation of the surfaces of the filler

Figure 3.14 Schematic diagram of a typical DRIFTS cell

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Figure 3.15 Schematic diagram of DRIFTS sample holder with specular reflection blocker

particles from which it is reflecting, is captured by the elliptical output mirror. The mirror refocuses the radiation back into a beam that is further corrected by the output optics before passing through to the detector. A further refinement of the sample holder has been the introduction of a specular reflection blocker. Specular reflection is the radiation that is reflected from the front surface of the sample and has not penetrated the powder. This can occur if the filler sample is not sufficiently diluted with KBr or KF. Specularly reflected radiation can render some data transformations invalid and can therefore potentially reduce the accuracy of any quantitative measurements. To overcome this problem blocker devices have been developed that are adaptations of the sample holder. The most basic type is illustrated in Figure 3.15, this simply introduces a physical barrier to the specularly reflected radiation. Correct dilution of the filler powder with finely ground KBr or KF can, however, eliminate the vast majority of specular reflection components without recourse to the blocker. The authors have found that the blocker also removes a significant fraction of the diffuse reflection components and can therefore needlessly weaken the signal.

3.5.4.5 Treatment of Data for Quantitative Analysis Treatment of diffuse reflectance IR data normally involves the Kubelka-Munk (KM) Equation (3.13) that takes account of scattering and absorption characteristics [45]. The constants s and k are used to describe the scattering and absorption properties, respectively, and R∞ is the reflectivity of an infinitely thick sample:

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Particulate-Filled Polymer Composites k (1 − R ∞ ) = 2R ∞ s

2

(3.13)

This relationship is only valid if there are no specular reflection components. Sutherland [43], however, has compared plotting of ratioed data both in absorbance and in KM units as a function of added surface treatment level. As linear relationships were obtained in both cases it was therefore considered that there was no advantage to be gained in using the KM equation. In transmission IR spectroscopy, using a constant path length, absorbance is proportional to concentration; this relationship (the Beer-Lambert law) will only be true for DRIFTS if the nature of dispersion of filler particles in the diluent does not change with the level of surface treatment. Such an ideal condition will rarely be realised. Therefore it is essential to use internal standards within spectra and ratio variable absorptions to these internal standards. Use of absorbance peak area is preferable to the absolute absorbance value (peak height) as area takes the overall energy of the absorbance into account and thus accounts for changes in peak width. The internal standard should be selected carefully as it should not be affected by treatment level. For example with metal hydroxide fillers the OH stretching bands at 3750-3070 cm-1, (depending on actual hydroxide) are a convenient internal standard. This may at first seem surprising considering the fact that many surface modifiers react with the OH groups/ions of the filler, but the reasoning behind use of these bands is clear when the micrometre penetration depth of DRIFTS sampling is considered. The surface area of most mineral fillers for polymers is relatively low (< 30 m2 g-1) therefore the reactive groups on the filler surface are insignificant in number relative to those in the bulk of the filler particles. Hence under such circumstances it can be considered valid to use the IR absorbance of surface treatment reactive groups as internal standards. However, in the case of filler with high surface area, i.e., some grades of silica, the latter argument is not valid. This therefore requires the use of a truly non-reactive IR absorbing group as the internal standard, for example the siloxane related adsorption of silica, centred at 800 cm-1.

3.5.4.6 Examples The following examples illustrate the power of the DRIFTS technique for analysis of interaction between fillers and surface treatments or polymer additives. The first example examines the adsorption of maleanised polybutadiene (MPBD) onto aluminium hydroxide (ATH) and magnesium hydroxide. These are two very effective flame retardant fillers, ATH is the most commonly used, but magnesium hydroxide with

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Analytical Techniques for Characterising Filler Surfaces its higher decomposition temperature has become established in some sectors of the market. MPBD is an effective dispersant and coupling agent, both in thermoset composites or rubber compounding. The spectra (Figures 3.16, 3.17 and 3.18) are presented in transmittance mode and not Kubelka-Munk units as discussed previously. However, the purpose of this investigation was to understand the chemical reactions taking place at the filler surface and not to make any quantitative measurements. Figure 3.16 shows the spectrum of MPBD. The sample was prepared by coating a film onto a sodium chloride disc and the spectrum was measured in transmission mode. The key features are the peaks at 1860 cm-1 and 1781 cm-1 corresponding to the symmetric and asymmetric carbonyl stretching frequencies. Figure 3.17 contains the difference spectrum between ATH and ATH which had been coated, from solution, with the same grade of MPBD. In this spectrum relatively weak adsorptions at 1860 cm-1 and 1781 cm-1 are present indicating some unreacted MPBD, but in addition a strong peak at 1700 cm-1 exists which corresponds to the absorption frequency of the carbonyl stretch in the carboxylic acid. Clear evidence that the anhydride has hydrolysed, but the lack of a strong peak at 1580 cm-1, the carbonyl stretching frequency of the salt, shows that the acid had not reacted with the surface of the ATH to form salt bridging between the polybutadiene backbone and the filler surface.

Figure 3.16 Transmission infra-red spectrum of MPBD

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Figure 3.17 DRIFTS difference spectrum of ATH subtracted from ATH coated with MPBD

Figure 3.18 DRIFTS infra-red difference spectrum of magnesium hydroxide subtracted from magnesium hydroxide coated with MPBD

Figure 3.18 is also a difference plot, but between magnesium hydroxide and magnesium hydroxide coated in MPBD by the same solution coating procedure used for the ATH. In this spectrum there are again relatively weak absorptions at 1860 cm-1

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Analytical Techniques for Characterising Filler Surfaces and 1781 cm -1 from unreacted MPBD, but now only a very weak absorption at 1700 cm-1 from carboxylic acid. However, a strong absorption at 1580 cm-1 shows that the MPBD had reacted with the magnesium hydroxide to form the salt, to fully couple the butadiene backbone with the filler surface. The second example shows how DRIFTS can give insight into the environment in which adsorbed species reside and how this environment changes with addition level [46]. Figure 3.19 shows the carbonyl region of substrate subtracted DRIFTS spectra of magnesium hydroxide that has been treated with varying levels of stearate (from ammonium stearate).

Figure 3.19 Substrate subtracted DRIFTS spectra for the carbonyl region of magnesium hydroxide treated with increasing amounts of ammonium stearate. (a) 10 mg g-1, (b) 20 mg g-1, (c) 30 mg g-1, (d) 45 mg g-1, (e) 60 mg g-1, (f) 90 mg g-1, (g) magnesium stearate and (h) stearic acid. (Reproduced with permission from C.M. Liauw, R.N. Rothon, G.C Lees and W.C.E. Schofield, Journal of Adhesion, 2002, 78, 7, 603. Copyright 2002, Taylor & Francis).

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Particulate-Filled Polymer Composites Interesting changes are evident in both the asymmetric and symmetric carboxylate carbonyl stretching frequencies (1580-1550 cm-1 and 1420-1360 cm-1, respectively). It is evident that the asymmetric stretching frequency increases to a limiting value somewhere between 30 mg g-1 (the monolayer coverage) and 45 mg g-1. Changes in the symmetric stretching vibration frequency relative to the asymmetric vibration can provide insight in to the degree of coordination of the magnesium ion [47]. At a stearate level of 10 mg g-1 adsorption is via the monocarboxylate (C17H35COOMgOH) (here the wavenumber difference between the symmetric and asymmetric stretching frequencies are greater than in pure magnesium stearate). Increasing the addition level to 20 mg g-1 led to to mixed adsorption via the monocarboxylate and dicarboxylate (the symmetric stretching vibration is now split in two). Increasing the level to 30 mg g-1 and beyond indicated exclusive production of the dicarboxylate. Formation of the dicarboxylate on top of the first monolayer is manifested as attainment of a limiting value of the asymmetric stretching frequency at an addition level greater than the monolayer level, i.e., between 30 and 45 mg g-1. The fact that the limiting value of the asymmetric stretching vibration frequency is identical to that in pure magnesium stearate indicates that the adsorbed layers are only weakly associated with the filler surface.

Figure 3.20 Infrared absorption peak area ratio (C-H stretch [from AS] (A(C-H)s)/O-H stretch [from Mg(OH)2] (A(O-H)s), obtained from DRIFTS spectra of ammonium stearate treated magnesium hydroxide. (Reproduced with permission from C.M. Liauw, R.N. Rothon, G.C Lees and W.C.E. Schofield, Journal of Adhesion, 2002, 78, 7, 603, Figure 3. Copyright 2002, Taylor & Francis)

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Analytical Techniques for Characterising Filler Surfaces Continual adsorption beyond the monolayer level is confirmed by plotting the C-H deformation band to OH deformation band area ratio versus the addition level. Whilst there is a linear increase in the peak area ratio up to the monolayer level, there is still a progressive increase beyond it. The ability of IR spectroscopy to probe the environment in which bonds vibrate allows it to be used to examine structural ordering of surface modifier molecules. If alkyl chains for example are in a liquid-like state of disorder the dispersion forces of attraction between chains are not maximised and vibration of the C-H bonds is influenced almost solely by the bond stiffness. If on the other hand the chains are in an ordered crystalline array, dispersion forces are maximised. This causes the C-H bonds to become very slightly longer (and therefore weaker) than when in the liquid state. This reduction in stiffness causes the asymmetric C-H stretching vibration frequency of a C18 alkyl chain to reduce by 3-5 cm-1. Such frequency shifts have been used by Kellar and co-workers [48] to investigate ordering of oleic acid monolayers and by Liauw and co-workers [13, 49]. Vaia and co-workers [50] have also used this approach to examine the order of alkyl chains of quaternary alkyl ammonium intercalants in organo-clays for nanocomposite applications. The third example shows how DRIFTS can be used to gain further insight into what interactions occur in the FMC cell. A study of the adsorption of isostearic and acrylic acids on to aluminium and magnesium hydroxide, was carried out with the intention of evaluating mixed surface modifier systems that possess dispersant and coupling capabilities. Figure 3.21(a) shows a DRIFTS spectrum of a sample of magnesium hydroxide recovered from the FMC cell (and air-dried) after adsorption and desorption of isostearic acid followed by adsorption and desorption of acrylic acid. Figure 3.21(c) shows a DRIFTS spectrum of filler obtained after the reverse experiment, i.e., adsorption and desorption of acrylic acid followed by adsorption and desorption of isostearic acid. Figures 3.21(b) and 3.21(d) shows DRIFTS spectra after adsorption and desorption of acrylic and isostearic acids, respectively. It is evident that acrylic acid will completely displace isostearic acid, however, isostearic acid will adsorb onto a layer of adsorbed acrylic acid. It is envisaged that mixed isostearate and acrylate binary salts may form during the latter adsorption process but not when adsorption of isostearic acid is followed by adsorption of acrylic acid. This is likely to be due to the higher acidity of acrylic acid and magnesium isostearate being more soluble in the toluene carrier solvent than magnesium acrylate. Further examples of application of DRIFTS, and other surface analytical techniques, to organosilane treated fillers and interphase characterisation in organosilane modified composite, have been reviewed by Suzuki and Ishida [51].

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Figure 3.21 DRIFTS spectra of magnesium hydroxide recovered from the FMC cell after adsorption from toluene of: (a) isostearic acid followed by acrylic acid, (b) acrylic acid, (c) acrylic acid followed by isostearic acid and (d) isostearic acid. (Reproduced with permission from C.M. Liauw, R.N. Rothon, G.C Lees and Z. Iqbal, Journal of Adhesion Science and Technology, 2001, 15, 8, 908. Copyright 2001, VSP)

3.6 Methods for Examining Structural Order in Filler Coatings Whilst structural order within adsorbed layers of surface modifier molecules is usually considered to be confined to linear chain fatty acids, it is important to appreciate that silane-based treatments can also undergo a degree of structural ordering under certain conditions. Gähde [52] may have indirectly observed such structural ordering with MPS adsorbed on glass, as the latter gave rise to preferred orientation of polyethylene chains. In a study of adsorption of MPS on to lead oxide, Miller and Ishida [53] attributed a shift in the Si—O—Si asymmetric stretching band (in DRIFTS spectra (see Section 3.5.4))

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Analytical Techniques for Characterising Filler Surfaces to structural ordering as polysilsesquioxane ladder polymers absorb in the same region. In this last section, brief details of other suitable analysis methods that may confirm the presence of structural order are given.

3.6.1 Wide Angle X-Ray Diffraction By virtue of the conveniently sized wavelength diffraction/scattering of X-rays is a long established tool for probing the structure of materials. X-ray diffraction (or WAXS) can be used to measure distances between planes in crystalline structures. For a review of the concepts and methods of WAXS the reader is referred to Cullity’s book [54]. The authors [13, 49] have found WAXS useful for establishing if any structural order is present in fatty acid coatings on filler surfaces. Gilbert and co-workers [55] have also found the technique useful for analysis of fillers treated with fatty acids and fatty acid salts. Stearic acid adsorbed from heptane solution on to magnesium hydroxide (as magnesium stearate) was shown to exhibit structural order [49]. This was manifested as a reflection that became apparent at coverage levels at and beyond monolayer. The d-spacing of the reflection corresponded to the short spacing of magnesium stearate. Isostearic acid and oleic acid adsorbed under the same or similar conditions as the stearic acid showed no evidence of structural order due to the inability of the alkyl/alkenyl chains to pack into an ordered structure [13]. WAXS has become the one of the main techniques for analysis of organoclays and polymer layered silicate nanocomposites. These aspects have been thoroughly reviewed by Vaia [56].

3.6.2 Differential Scanning Calorimetry DSC can be used to observe phase transitions in materials such as melting behaviour (first order transitions) and second order transitions that are associated with the glass transitions of polymers [57]. In essence DSC measures the energy required to maintain a constant temperature in the sample relative to an inert reference. Therefore processes that lead to a change in heat capacity, i.e., melting and glass transitions can be observed, as can endothermic or exothermic chemical reactions. DSC instruments are now extremely sensitive and are quite capable of resolving phase transitions of adsorbed surface modifiers. For analysis of fillers dynamic DSC is most useful. In this mode, the difference in electric power input required to keep the reference pan at the same temperature as the sample pan (i.e., the heat flow), is recorded as a function of steadily increasing (or decreasing) temperature. A typical dynamic DSC analysis may consist of a heat-cool-heat cycle.

145

Particulate-Filled Polymer Composites The same stearic acid treated magnesium hydroxide samples, discussed in Section 3.6.1 showed evidence of melting of stearic acid and/or magnesium stearate even at the monolayer level. Use of heat-cool-heat cycles can indicate structural changes that occur as a result of further reaction of a surface modifier with the substrate. This effect occurred with magnesium hydroxide that had been treated with 2.5 monolayers of stearic acid [13]. DSC data from the first heat segment of the cycle showed evidence of free stearic acid but data from the second heat segment showed that this may have become magnesium stearate (Figure 3.22) [58]. Coatings derived from oleic and isostearic acids showed no melting endotherm peaks and were thus considered amorphous, in concordance with the WAXS data.

Figure 3.22 First heat (upper) and second heat (lower) DSC data (from a heat–coolheat cycle) showing the change in structure within adsorbed layers of (a) stearic acid and (b) isostearic acid on magnesium hydroxide, together with untreated magnesium hydroxide (c). Note change in heat flow scale from first to second heat. (Reproduced with data from C.M. Liauw, R.N. Rothon, G.C Lees and Z. Iqbal, Journal of Adhesion Science and Technology, VSP, 2001, 15, 8, 896. Copyright 2001, VSP.)

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3.7 Summary This chapter has described how the most important property of a particulate filler, with regard to its reactivity, is the acidic or basic character of its surface sites. Furthermore, that the surface of a single filler particle can have sites ranging from strongly acidic to strongly basic. It has been shown that techniques for characterising filler surfaces can be broadly divided into three groups. In the first, the reactivity of the filler surface to well defined probe molecules is observed. Of these, FMC is perhaps the more powerful, giving the ability to measure heat of adsorption directly and using the polymers, coupling agents or additives of interest. IGC is more instrumentally developed, but generally requires the use of model probes because of the need for them to be volatile and reversibly adsorbed. The second group, XPS and SIMS, are the analytical techniques that are capable of determining the chemical composition of filler surfaces, as opposed to the bulk. DRIFTS has a greater penetration depth than XPS or SIMS but in many cases, such as when the filler has a porous surface, this is advantageous. The DRIFTS examples clearly demonstrated how insight into the nature of interactions between surface modifiers and filler surfaces could be acquired. The third group of techniques (WAXS and DSC) can be used to establish the existence of structural order in adsorbed layers of surface modifiers. DSC in particular can be linked to FTIR if a suitable DSC hot stage is used on an infrared microscope, this combination has exciting potential for following the reactions between fillers and surface modifiers. The fact that certain fillers are now valuable functional additives for polymers is a testament to the ever-increasing importance of surface characterisation. Insight into the surface chemistry of fillers opens the door to surface modification; a technology that in-turn leads to high added value products. The emergence of nanocomposite technology places even more emphasis on the filler surface by default due to the fact that the interfacial region in a fully exfoliated nanocomposite forms a very significant fraction of the composite volume. In such systems there is huge potential for modification of the interfacial region by modification of the platelet surfaces. Therefore improvements on the techniques described here will be inevitable and will be driven by an ever-increasing quest to use filler surfaces to promote unique composite properties. It is highly likely that data from these improved techniques may help in creation of improved computer models of surfaces, leading to advanced modelling of adsorption interactions.

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10. F.L. Riddle and F.M. Fowkes, Journal of the American Chemical Society, 1990, 112, 9, 3259. 11. F.M. Fowkes, Acid-Base Interactions: Relevance to Adhesion Science and Technology, Eds., K.L. Mittal and H.R. Anderson, Jnr., VSP Publications, Utrecht, The Netherlands, 1991, 93. 12. F.M. Fowkes, Journal of Adhesion Science and Technology, 1987, 1, 1, 7. 13. C.M. Liauw, R.N. Rothon, G.C. Lees and Z. Iqbal, Journal of Adhesion Science and Technology, 2001, 15, 8, 889. 14. D.P. Ashton and R.N. Rothon in Controlled Interphases in Composite Materials, Ed., H. Ishida, Elsevier, New York, NY, USA, 1990, 295. 15. J. Wolfschwenger, A. Hauer, M. Gahleitner and W. Neissl, Proceedings of Eurofillers 97, Manchester, UK, 1997, p.375. 16. C.M. Liauw, A. Childs, N.S. Allen, M. Edge, K.R. Franklin and D.G. Collopy, Polymer Degradation and Stability, 1999, 65, 2, 207.

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Analytical Techniques for Characterising Filler Surfaces 17. C.M. Liauw, A. Childs, N.S. Allen, M. Edge, K.R. Franklin and D.G. Collopy, Polymer Degradation and Stability, 1999, 63, 3, 391. 18. A.V. Kiselev and Y.I. Yashin, Gas-Adsorption Chromatography, Plenum Press, New York, NY, USA, 1969. 19. D.R. Williams in Controlled Interphases in Composite Materials, Ed., H. Ishida, Elsevier, New York, NY, USA, 1990, 219. 20. C. Kembell and E. K. Rideal, Proceedings of the Royal Society, London, 1946, A187, 53. 21. L. Lavielle and J. Schultz, Langmuir, 1991, 7, 5, 978. 22. J. Schultz, L. Lavielle and C. Martin, Journal of Adhesion, 1987, 23, 1, 45. 23. U. Panzer and H.P. Schreiber, Macromolecules, 1992, 25, 14, 3633. 24. W.O. McReynolds, Journal of Chromatographic Science, 1970, 8, 685. 25. A.W. McMahon, D.G. Kelly and P.J. McLaughlin, The Analyst, 2002, 127, 1, 17. 26(a) Practical Surface Analysis, Volume 1: Auger and X-ray Photoelectron Spectroscopy, Second Edition, Eds., D. Briggs and M.P. Seah, Wiley, Chichester, UK, 1990. 26(b).Practical Surface Analysis, Volume 2: Ion and Neutral Spectroscopy, Second Edition, Eds., D. Briggs and M.P. Seah, Wiley, Chichester, UK, 1992. 27. Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Eds., D. Briggs and J.T Grant, SurfaceSpectra/IM Publications, Manchester, UK, 2003. 28. ToF-SIMS: Surface Analysis by Mass Spectrometry, Eds., J.C. Vickerman and D. Briggs, SurfaceSpectra/IM Publications, Manchester, UK, 2001. 29. M.P. Seah and W.A. Dench, Surface and Interface Analysis, 1979, 1, 2. 30. The NIST X-Ray Photoelectron Spectroscopy Database, http://srdata.nist.gov/xps. 31. J.M. Pena, N.S. Allen, M. Edge, C.M. Liauw, F. Santamaria, O. Noiset and B. Valange, Journal of Materials Science, 2001, 36, 12, 2885. 32. J.M. Pena, N.S. Allen, M. Edge, C.M. Liauw, O. Noiset and B. Valange, Journal of Materials Science, 2001, 36, 18, 4419. 33. J.M. Pena, N.S. Allen, C.M. Liauw, M. Edge, B. Valange and F. Santamaria, Journal of Materials Science, 2001, 36, 18, 4443.

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Particulate-Filled Polymer Composites 34. J.M. Pena, N.S. Allen, M. Edge, C.M. Liauw and B. Valange, Polymer Degradation and Stability, 2001, 72, 1, 31 35. The Static SIMS Library (version 3), Eds., J. C. Vickerman, D. Briggs and A. Henderson, SurfaceSpectra, Manchester, UK, 2002. 36. L-S. Johansson and T. Losoi, Surface and Interface Analysis, 1991, 17, 5, 230. 37. L-S. Johansson, Surface and Interface Analysis, 1991, 17, 9, 663. 38. L-S. Johansson, T. Losoi and J. Jukanoja, Surface and Interface Analysis, 1993, 20, 2, 155. 39. L-S. Johansson, Surface and Interface Analysis, 1993, 20, 4, 304. 40. G.M. Barrow, Introduction to Molecular Spectroscopy, McGraw-Hill, New York, NY, USA, 1962. 41. C.N. Banwell, Fundamentals of Molecular Spectroscopy, 2nd Edition, McGrawHill, New York, NY, USA, 1972. 42. M. Gilbert, I. Sutherland and A. Guest, Proceedings of Filplas 92, Manchester, UK, 1992, Session 1, Paper No.3 43. I. Sutherland, Proceedings of Eurofillers 97, Manchester, UK, 1997, p.93. 44. M. Gilbert, I. Sutherland and A. Guest, Journal of Materials Science, 2000, 35, 2, 391. 45. P. Kubelka and F. Munk, Zeitschrift Für Technische Physik, 1931, 12, 593. 46. R.N. Rothon, C.M. Liauw, G.C. Lees and W.C.E. Schofield, Journal of Adhesion, 2002, 78, 603 47. P.J. Thistlethwaite and M.S. Hook, Langmuir, 2000, 16, 11, 4993. 48. J.J. Kellar, C.A. Young, K. Knutston and J.D. Miller, Journal of Colloid and Interface Science, 1991, 144, 2, 381. 49. C.M. Liauw, R.N. Rothon, S.J. Hurst and G.C. Lees, Composite Interfaces, 1998, 5, 6, 503. 50. R.A. Vaia, R.K. Tuekolsky and E.P. Giannelis, Chemistry of Materials, 1994, 6, 7, 1017.

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Analytical Techniques for Characterising Filler Surfaces 51. N. Suzuki and H. Ishida, Macromolecular Symposia, 1996, 108, 19. 52. J. Gähde, Plaste und Kautschuk, 1975, 22, 8, 626. 53. J.D. Miller and H. Ishida, Proceedings of the SPE 40th Annual Conference of the Reinforced Plastics and Composites Institute, Atlanta, GA, USA, 1985, Session 17-B, p.1. 54. B.D. Cullity and S.R. Stock, Elements of X-ray Diffraction, 3rd Edition, Prentice Hall, London, UK, 2001. 55. M. Gilbert, P. Petiraksakul and I. Mathieson, Materials Science and Technology, 2001, 17, 11, 1472. 56. R.A Vaia in Polymer-Clay Nanocomposites, Eds., T.J. Pinnavaia and G.W. Beall, John Wiley and Sons, New York, NY, USA, 2000. 57. Calorimetry and Thermal Analysis of Polymers, Ed., V.B.F. Mathot, Hanser Publishers, Munich, Germany, 1993. 58. R.N. Rothon, C.M. Liauw and C.G. Lees, Proceedings of Eurofillers 99, British Plastics Federation, Lyon, France, 1999, Session 4, Paper No. 44.

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4

Surface Modification and Surface Modifiers Roger N. Rothon

4.1 Introduction As described in the earlier chapters, the filler surface plays a vital role in determining the processing behaviour and properties of composites. The main methods for characterising filler surfaces were discussed in the previous chapter. This chapter covers the use of additives to beneficially modify the surface of fillers, and thus optimise composite processing and properties. A wide range of such surface modifiers are offered and used commercially, ranging from the inexpensive fatty acids, to silanes, titanates and zirconates and all are discussed. Reference is also made to non-commercial treatments, such as borates and phosphates.

4.2 Reasons for Using Surface Modifiers With the notable exception of carbon blacks, the natural surfaces of most particulate fillers are less than optimum for dispersion into, and interaction with, polymers. Surface modifiers can frequently improve this, and other filler properties. Sometimes materials which are effective surface modifiers will be found to have been added for filler manufacturing reasons. The main reasons for finding surface modifiers present are: •

Improved filler production. Thus, they can be used as milling aids and to improve filtration and reduce hardness development during drying.

•

Protection of the filler in storage and in the end application. Thus, they can reduce water pick up and protect fillers such as magnesium hydroxide from attack by atmospheric carbon dioxide.

•

Improved processing. Surface modification often improves the rate of incorporation into the polymer and reduces the viscosity of the mix. This can lead to significant savings in energy, or to faster throughput. Indeed, some form of surface modification is often essential when high filler loadings are used.

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Reduced adsorption of expensive additives, notably curatives and anti-oxidants. Resulting improvements in curing can also lead to improved properties.

•

Improved composite properties. These can include: increased strength and stiffness, better impact strength, increased abrasion resistance, reduced adsorption and less swelling in the presence of water and other fluids.

It should also be noted that the presence of surface modifiers can also influence other properties. This includes properties such as bulk density, powder flow and dustiness. They can also have unexpected effects on properties such as thermal stability, fire retardancy and ageing. These effects may be positive or negative.

4.3 General Principles of Surface Modification Virtually all treatments in commercial use are chosen to chemically bond an organic species to the filler surface, thus improving compatibility with organic polymers. The most common functionalities used for this are acids or acid precursors, such as anhydrides (for basic or amphoteric fillers) and alkoxy-silanes (for fillers with metal hydroxyls present, especially siliceous fillers). Organo-titanates and related compounds are also proposed for use with a wide variety of fillers. Two distinct types of surface modifier can be recognised: non-coupling and coupling. The non-coupling types have a chemical bond to the filler surface, but no strong bonding to the polymer matrix. The widely used saturated fatty acids are typical of this class. The coupling types, usually described as coupling agents, have both a chemical bond with the filler surface, and some means of strong interaction with the polymer matrix. This interaction with the polymer is usually due to some chemical functionality, which can react with it. In some instances, it is due to the anchored species having long enough chains to entangle, or co-crystallise, with the matrix. Where the matrix polymer undergoes a polymerisation, or crosslinking process, during composite formation; then it is fairly simple to choose a suitable polymer reactive group (e.g., a carbon=carbon double bond). However, with polymers such as the polyolefins, this can be difficult. Highly reactive functionalities, such as azides, are sometimes used in this case. It is also possible to pre-functionalise a part of the polymer matrix, by grafting on a species such as an acid, anhydride or alkoxy-silane, in a separate process.

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4.4 Methods of Using Surface Modifiers Modification of the surface of fillers may be brought about in a variety of ways. The most appropriate will depend on many factors, including: the nature of the filler and the process used in producing it, the nature of the additive, and of the polymer and the composite processing conditions. The first differentiation is into pre-coating and ‘in situ’ methods. In the latter, the coating agent is added during compounding, making its way to, and reacting with, the filler surface during this process. The main advantages and disadvantages of the two procedures are summarised in Table 4.1. The relative importance of the various factors will vary depending on the nature of the coating and filler and on the composite processing conditions. In some cases, especially where some new surface may be generated during processing, a combination of the two approaches may give optimum results.

Table 4.1 A comparison of the pre-coating and in situ methods of filler treatment Pre-coating

In situ

Usually requires an expensive additional step, but is claimed to give more efficient use of the additive.

Usually less expensive, but may require higher additive levels.

Volatile by-products such as alcohols are easily dealt with.

Volatile by-products can be a problem.

Essential where improved filler processing and stability are sought. Surface reaction and coverage can be strictly controlled. Interference from other compounding additives can be minimised.

Surface reaction is very much at the mercy of processing conditions, including competition from other additives. Residual, unreacted, additive more likely to be present and may cause problems.

Unable to treat fresh surface generated during compounding.

Opportunity to treat fresh surface generated during compounding.

Interpenetration of coating and matrix may be limited.

Greater opportunity for coating/matrix interpenetration.

Limited flexibility for adjusting the formulation.

Greater formulation freedom, although the additive must have some degree of ‘solubility’ in the polymer.

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Particulate-Filled Polymer Composites Where the filler is pre-coated, various methods may be used, depending on the nature of the filler, the coating, and the filler preparation procedure. When the filler is produced from aqueous solution, such as by precipitation, it is often advantageous to add a water dispersible or soluble, form of the coating, prior to filtration and drying. If the coating is carefully chosen, it can actually aid filtration and reduce caking during drying. Fatty acid treatments fulfil such a role on precipitated calcium carbonates. In most cases however, the cost of solvent removal makes solution coating uneconomic, and dry coating is favoured wherever possible. Dry coating is often achieved during a milling process, or by use of a suitable mixer. In the dry milling procedures, the conditions must be carefully chosen to ensure complete coverage and surface reaction. The best form of mixer will depend on the filler. In most instances, a high shear mixer is used to ensure rapid de-agglomeration and coating. Such mixers usually generate a significant amount of heat, which can be useful, especially where the additive has to be melted, or a reaction product like water or alcohol has to be removed. With shear sensitive fillers, such as acicular grades of wollastonite, low shear mixers are necessary to avoid excessive degradation of the filler. In such cases, coating times are longer and external heating may be needed with some types of additive. Coating methods are returned to in the sections dealing with specific additive types.

4.5 Choice of Coating Level After deciding on the types of modifier to use, one has to decide on the amount that is needed. Many modifiers are relatively expensive, so it is important not to waste any. Moreover, excess amounts can sometimes lead to deleterious effects. The optimum level will depend on the effects that are being sought, as well as factors such as: how the modifier reacts with the surface, how well it covers that surface, what the thickness and structure of the coating layer are, and how it interacts with the matrix. Some useful guidelines exist to enable an approximate level to be decided on, but final optimisation usually requires trial and error. The most important theoretical concept is probably that of the mono-layer. By this is meant the amount of additive required to just produce a surface layer, one coating molecule in thickness. This appears to be a straightforward concept, but there are quite a few complications in practice. Theoretical mono-layer amounts can be calculated using factors such as the density of reactive sites and the area occupied by a molecule of coating. The smoothness and accessibility of the surface also has to be taken into account. Some of the main issues that can arise are summarised in Figure 4.1.

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Figure 4.1 Different idealised structures for surface mono-layers on a filler surface

Figure 4.1c shows the ideal situation: where the surface is smooth and its reactive sites are spaced at just the right distance for a tightly packed surface layer to be formed. While this situation may be thought unlikely, it seems that stearic acid coatings on calcium carbonate come very close to it, which may explain their great success with this filler. Figure 4.1a shows a situation where the surface sites are widely spaced. Only part of the surface layer can now be strongly bound and the layer is unable to pack so tightly. Small molecules such as water could easily penetrate to the surface. Figure 4.1b shows the reverse case: the reactive sites are too closely spaced for complete reaction. Again, small molecules could reach the surface and possibly destabilise the bonding. A further area of confusion is in assessing molecular orientation at the surface. Most molecules have a number of possible orientations, which makes calculation of geometric coverage difficult. This is especially true of some organo-silane coupling agents, where the functionality intended to react with the matrix may in some circumstances be strongly adsorbed onto the filler surface, resulting in flat, or bridged, rather than vertical orientation (see Section 4.7). Even long chain carboxylic acids may adsorb flat rather than vertically at low coverages. This situation is illustrated in Figure 4.1d. Loopy adsorption such as is illustrated in Figure 4.1e is possible when multifunctional adsorbates are used.

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Particulate-Filled Polymer Composites A further factor not usually taken into account is the surface topography. Thus, while good packing of adsorbed species may well be achieved on a molecularly flat surface such as glass or mica, most particulate mineral fillers will have a rough surface which may prevent such densely packed layers forming. Ishida and Koenig have reported on the effects of surface topography on the structure of some silane coupling agents [1]. Finally, it must be realised that adsorption onto a surface may not be uniform. There may be patches, several molecules thick, in some places and bare surface in others. While the issues discussed previously may seem academic, they are of great practical importance. This is particularly so with regard to the durability of coating effects under humid conditions, where the ability of the surface to adsorb water, which may hydrolyse surface bonds, is a key issue. In general, properties start to change well before complete surface coverage is reached and in some applications, full coverage may not be essential. In many cases, the maximum effect is often found at about the mono-layer level, but multi-layers are found to be beneficial in some instances, particularly where they are able to interpenetrate with the matrix polymer. A good example of the effect of additive level on a wide number of properties of a magnesium hydroxide filled ethylene vinyl acetate (EVA) co-polymer can be found in work by Rothon and co-workers [2]. This includes effects on flame retardant properties and on ageing. The latter is particularly interesting, as it was found that excess of the fatty acid led to a significant decrease in ageing resistance. This was most marked when a commercial blend of fatty acid was used, but was still very significant with pure stearic acid.

4.6 Techniques for Determining the Amount of Coating Present, and Assessing the Amount Needed for Mono-layer Coverage The main techniques for characterising the structure of filler surfaces have been covered in detail in the previous chapter. This section deals with methods for determining the amount present on a filler surface and for obtaining an indication of the amount of additive needed for mono-layer coverage.

4.6.1 Determination of Amount of Additive and it’s Distribution A number of analytical methods are possible, depending on the chemical nature of the filler and additive, and the techniques available. Some of the main ones are described

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Surface Modification and Surface Modifiers next. One of the problems with many techniques is in differentiating between additive that is actually on the filler surface, and that which is merely present as a physical mixture with the filler.

4.6.1.1 Chemical Analysis Methods These can be used where the additive has an element that is clearly different from the filler, and where the amount of additive is sufficient for the sensitivity of the technique being employed. Thus, carbon analysis can be used for many additives on silica surfaces, and titanium analysis is possible for organo-titanates on many fillers. This approach does not distinguish between surface and admixed additive though.

4.6.1.2 Pyrolysis This can also be used with success in many cases providing any weight loss from the filler itself is corrected for. Again, it does not distinguish between surface and admixed amounts.

4.6.1.3 Infra-red Methods These are particularly useful where, as often happens, the additive has at least one characteristic spectral band that is clearly different from the filler. The hydrocarbon bands present in many additives usually provide a good marker. Pre-calibration is necessary, using fillers with known amounts of additive, and it helps if it is possible to use a strong filler peak as an internal standard. Transmission methods will detect both surface and admixed additive, while reflectance methods such as diffuse reflectance Fourier transform infra-red spectroscopy (DRIFTS) [3] are surface sensitive, and thus provide a method for assessing how much additive is actually on the filler surface. As described in the previous chapter, these techniques can also often give information concerning the way in which the additive has reacted with the filler surface. High quality equipment is relatively inexpensive and infra-red methods are well worth considering for quality control.

4.6.1.4 Adsorption Methods Various adsorption methods, including dye adsorption (see Section 4.6.2.3), can be used to determine the amount of uncoated surface remaining. This provides an indirect estimate of the amount of coated surface present.

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4.6.2 The Monolayer and it’s Determination A number of relatively simple methods are used to find out what the apparent monolayer level is. The most common ones are described in the rest of this section.

4.6.2.1 Theoretical Calculation The amount of additive needed to cover unit area of surface has been determined experimentally, or calculated from molecular models, for many common additives. This enables the theoretical mono-layer amount to be calculated, if the specific surface area of the filler is known. As described previously, factors such as surface irregularity and additive orientation have to be taken into account. The reported coverage for a number of common surface modifiers is given in Table 4.2. As demonstrated by the data for the two vinyl silanes, the nature of the alcohol has a major effect on the covering power of these additives.

Table 4.2 Approximate surface coverage of some modifiers Modifier

Coverage (m2 of available surface, per gram of additive)

γ-Aminopropyltriethoxysilane

360

γ-Mercaptopropyl trimethoxysilane

350

Vinyltrimethoxysilane

525

Vinyltrimethoxyethoxy silane

280

γ-Methacryloxypropyltrimethoxysilane

315

Lauric acid (C12)

690 vertical

Stearic acid (C18)

420 vertical, 1,800 horizontal

Oleic acid (unsaturated C18)

940 vertical, 1,800 horizontal

4.6.2.2 Viscosity Reduction or Settling Volume of a Suspension of the Filler in a Simple Organic Liquid It is frequently observed that a minimum viscosity or settling volume is reached at about the mono-layer level.

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4.6.2.3 Dye Adsorption This is an old fashioned technique, cheap and easy to perform, and not given the attention it warrants in determining levels of geometric coverage. The principle is simple, and adapted from that used to determine surface area [4, 5]. A dye is used which strongly adsorbs on the uncoated filler surface, but not onto coated areas, and the level of adsorption relative to uncoated filler is then used to assess the amount of untreated surface. An example of the use of this method follows, and also shows the potential for combining ultrasonics with adsorption techniques in studies related to the use of coated fillers in composites. Dye adsorption curves for congo red (CI: 22120) on various precipitated calcium carbonates are presented in Figure 4.2. The carboxylated polybutadiene coated sample is seen to give a much lower dye adsorption than the uncoated filler, as would be expected. When subjected to an ultrasonic probe treatment, slurries of both the coated and uncoated fillers show a marked rise in dye adsorption. This is assumed to be due to break up of filler aggregates exposing fresh uncoated surface. Similar results were obtained with a stearate coated filler. Such generation of uncoated surface might also be expected when fillers of this type are subjected to high shear mixing processes.

Figure 4.2 Use of the dye adsorption technique to follow the effect of ultrasonic treatment on the amount of bare surface in both uncoated and coated precipitated calcium carbonates (congo red dye adsorbed from aqueous solution)

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Particulate-Filled Polymer Composites

4.6.2.4 Adsorption Isotherms Simple adsorption isotherm studies are widely used to assess the amount of coating that is needed to fully treat a filler surface. The isotherms may be determined by a number of techniques; for example following either the reduction in solution concentration (gravimetrically, spectroscopically, titrimetrically, etc.), or the build up on the filler after isolation and washing. Reversibility of adsorption may also be studied. The main use of the technique has been to assess the amount of coating needed for full coverage without necessarily understanding how the coating is adsorbed, whether it reacts with the surface, and what full coverage means. In some instances, the shape of the adsorption isotherm and reversibility studies can provide information concerning the degree of surface reaction as opposed to physical adsorption. Combination of adsorption studies with calorimetry can be more informative, as in the flow microcalorimeter methods discussed in the previous chapter. Typical isotherms for the adsorption of a carboxylated polybutadiene onto an aluminium and a magnesium hydroxide filler of similar surface areas are presented in Figure 4.3. The isotherms were determined by using infra-red to follow the decrease in solution

Figure 4.3 Adsorption isotherms for a carboxylated unsaturated polymer on to aluminium and magnesium hydroxides. Determined by the change in toluene solution concentration method and normalised to the specific surface area of the aluminium hydroxide (7 m2g-1)

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Surface Modification and Surface Modifiers concentration of the additive in the presence of filler. The isotherm for magnesium hydroxide is typical of very strong filler polymer interaction, with the filler initially removing virtually all of the polymer from solution. With aluminium hydroxide, the interaction appears less strong, but the equilibrium adsorption level is about twice that for magnesium hydroxide. When adsorption levels were determined gravimetrically, by recovering and washing the filler, then the value for the magnesium hydroxide was unchanged but that for aluminium hydroxide was reduced to a similar level, confirming that considerable reversible adsorption appeared to occur with this filler.

4.6.3 Effects of Processing on the Coating Structure Before leaving this section it is essential to remind the reader that, while studies on coated fillers give useful information, great caution must be taken when extrapolating to real coating layers in composites. Marked changes in these layers may well occur due to the processing conditions and the presence of polymer and other additives. Unfortunately ‘in situ’ characterisation of polymer/filler interfaces is still in its infancy. Some progress is currently being made in instrumental techniques and considerable advances can be expected in this area.

4.7 Surface Modifier Types 4.7.1 Monomeric Organic Acids and their Salts Acidic groups provide a convenient method for anchoring organic species to basic and amphoteric fillers, and have been widely studied for filler surface modification. Both non-coupling and coupling structures can be envisaged, depending on the organic group used. Saturated fatty acids are examples of non-coupling types, while unsaturated and amino-acids can be thought of as possible coupling types. Some comment on terminology is needed here, as it is particularly difficult and potentially misleading in this area. The treating agent is usually the free acid, but its bonding to the filler surface is thought to be through the formation of a partial salt, which is bound to the filler. The literature often refers to coatings as, for example, stearic acid or stearate, when it more properly means the product of the reaction of the acid with the filler surface. It would, on the other hand, be inappropriate, to simply refer to salt coatings, as this implies that the salt has been used to carry out the coating. This may be possible, but may also lead to weakly bound material with no partial salt link to the filler surface. To further complicate matters, the actual structure of the coating on the surface has frequently not been elucidated and could be a mixture of free acid, part salt and free salt. In the following discussions,

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Particulate-Filled Polymer Composites acid refers to the species used to produce the filler treatment, with the coating being the product of the reaction of this with the surface, whatever form this may take.

4.7.1.1 Saturated Fatty Acids Saturated fatty acids are the classic non-coupling additive, and are the workhorse surface treatment used on calcium carbonate and other basic fillers, with which the acid group can give strong surface interaction. This type of coating is easily applied, inexpensive and can give a range of useful effects, including easier processing and reduced water adsorption. Under favourable circumstances, dispersion can also be improved, but under other conditions, this may actually be made worse due to the decreased shear during compounding. The fatty acids most used consist mainly of straight, saturated, hydrocarbon chains 1618 carbon atoms long, and with even numbers of carbon atoms. (For reasons associated with their natural product origin, fatty acids with odd numbers of carbon atoms are rare, and there has been little or no study of their effects). The chain lengths used are too short to entangle with a polymer matrix. They also have no means of chemically reacting, and so coatings are generally weakly interacting, and produce low bond strengths between filler and matrix. This results in filler de-bonding at relatively low stress levels. The consequences of this are usually marked stress whitening and a decrease in ultimate strength, although there can sometimes be significant improvements in properties such as elongation and toughness. Indeed fatty acid coated calcium carbonate is widely used in polypropylene because of the excellent impact strength obtainable. While there has doubtless been a great deal of industrial study of the effect of hydrocarbon chain length and structure, there is very little useful work reported in the scientific literature. The work of Ivanishchenko and Gladkikh, referred to again later, indicated that maximum hydrophobicity is obtained at about 14 carbon atoms [6]. Similar results were obtained by Cave and Kinloch for straight chain hydrocarbon derivatives of alkoxysilanes [7]. There is good evidence that, at least on smooth surfaces, chain lengths of about 14 carbon atoms are needed if a tightly packed, crystalline, layer is to be obtained and this is probably important for hydrophobic effects. Liauw and co-workers have reported evidence for crystalline order in stearic acid coatings on magnesium hydroxide [8]. Haworth and Raymond [9] reported no differences in the processing behaviour and property improvements obtained in polypropylene homo-polymer filled with calcium carbonate, whether they used C10 or C22 fatty acid treatment at the mono-layer level. On the other hand, Haworth and Birchenough [10] reported significant differences related to chain length in the same range, when working with magnesium hydroxide in polypropylene. Strength and stiffness were found to increase, but toughness to decrease, with increasing chain length. The work of Tabtaing and Venables [11] discussed in the

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Surface Modification and Surface Modifiers section on unsaturated acids, indicates that a chain length of C10 is sufficient to lead to significant improvements in ductility of a calcium carbonate/polypropylene composite, as determined by elongation and impact properties. They found no marked additional effect up to chain lengths of C20. Fatty acids are most economically derived from natural sources, where they occur as complex mixtures in both animal and vegetable matter. In practice, economics rules, and fatty acids with about 18 carbons are used, as they are the most readily available and of lowest cost. Some of the shorter chain acids also have extremely unpleasant odours, which is not desirable. Fortunately, it does seem that the C18 chain length is somewhere near the optimum in terms of building a stable, hydrophobic layer. Economics also dictates that blends of acids are normally used industrially, as separating out pure components can be quite costly. These blends can be very complex, containing a number of different chain lengths, and even small amounts of unsaturated chains and non-acid material. The composition of the fatty acids used in pre-coating fillers is rarely specified and can vary markedly, making accurate scientific comparisons very difficult. Fatty acids are usually pre-coated onto fillers and two methods of coating are generally used: wet coating and dry blending. In the wet coating process, a hot concentrated aqueous solution of a salt of the fatty acid is added to an aqueous slurry (usually also hot) of the filler. Under these conditions, rapid reaction with the surface of fillers such as calcium carbonate occurs. (Note, there is some evidence that precipitation may occur first). As mentioned earlier, this procedure can aid product isolation and handling. While sodium salts are often used for convenience, this can lead to undesirable levels of sodium in the final product, unless extensive washing is carried out. Ammonium salts are used to overcome this problem. In the dry coating procedure, filler and fatty acid are usually reacted together in a high shear mixer. The conditions have to be carefully selected and controlled if complete surface coverage and reaction is to be achieved. Most fatty acids are solids, and unless a solvent is used, they have to be melted before coating can take place. In some cases the mixer generates enough heat, but it is often necessary to supply external heating. It is also possible to coat from organic solutions of the fatty acid but, although sometimes used in laboratory work, this is usually too expensive for commercial use. It can also give false results, especially where the salt of the acid is soluble in the organic solvent. The chemistry of fatty acids is far more complex than might be imagined from this brief description, with features such as micellisation in aqueous solution and dimerisation in organic solvent being important factors likely to affect coating behaviour. Unfortunately these aspects have largely been overlooked in this context. For a

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Particulate-Filled Polymer Composites comprehensive treatment of the properties of fatty acids, the reader is referred to textbooks such as that edited by Markley [12]. It should also be noted that, while the hydrocarbon chain has been thought of as inert, it is now beginning to be realised that this can have a significant hydro-peroxide content, depending on the exact nature of the blend and the way in which it has been handled [13]. This could have significant consequences in some polymer applications. Careful thought has to be given to the way in which the fatty acid reacts, if strong attachment to the filler surface is to be obtained. Thus, it is necessary to only achieve the formation of a partial salt with the metal cation of the filler. If a full salt is produced, then the metal atom will no longer be a part of the filler structure, and may be readily removed from the surface. While it has received little recognition until recently, this aspect is most important with fillers such as calcium carbonate and magnesium hydroxides and may explain a lack of consistency in fatty acid treatment on these fillers.

4.7.2 Stearic Acid (CH3(CH2)16COOH As mentioned earlier, this is the most widely used fatty acid for filler treatment. However, the reader must bear in mind that most commercial grades of stearic acid, as used in pre-coating fillers, are actually complex blends containing both saturated and unsaturated acids, with the stearic content often under 60%. Pure stearic acid is at least twice the price of common blends, with other acids generally being more expensive still. The compositions of some typical fatty acid blends used for filler treatment are given in Table 4.3. The iodine value is a guide to the total amount of unsaturation

Table 4.3 Composition of some fatty acid grades used for filler surface treatment Component

% w/w in A

B

C

C12/14 saturated

2

2

3

C14 monounsaturated

-

-

-

48

48

45

-

-

-

C18 saturated

34

48

52

C18 unsaturated

13

2

-

C20/22 saturated

3

-

-

Iodine Value g/100 g

13

4

0.5

C16 saturated C16 monounsaturated

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Surface Modification and Surface Modifiers present. The minor components can have an importance influence in some applications and hence the blend has to be carefully chosen. Some of the unwanted effects of minor components include poor colour, odour and reduced composite stability. This section will deal with studies where pure stearic acid has been used. The adsorption and conformation of stearic acid at surfaces has been widely studied, especially using the Langmuir Trough procedure [14]. This has clearly shown that stearic acid adsorbs onto polar surfaces through the carboxylate group and with the hydrocarbon chain perpendicular to the surface. Under these conditions, the surface area occupied per molecule is approximately 0.21 nm2 [15] and this is often used for calculating theoretical surface coverages.

4.7.2.1 Stearic Acid and Calcium Carbonate As mentioned earlier, stearic acid is most widely used on calcium carbonate fillers. This section therefore concentrates on the interaction between these two materials. The adsorption isotherm of organic solutions of stearic acid onto the calcite form of calcium carbonate has been determined by workers such as Suess [16]. This work has shown that mono-layer coverage is obtained when the area occupied per molecule is around 0.21 nm2, which implies that orientation is in this case again perpendicular to the surface. It has also been calculated by Suess that this coverage level corresponds well with the density of calcium atoms in the surface of the inorganic substrate. Thus it is seen that fatty acids, like stearic acid, provide a very good match for the surface of calcite, thereby probably explaining the widespread use of such treatments with calcium carbonate fillers. Ivanishchenko and Gladkikh have studied the adsorption of stearic and other fatty acids onto chalk, using isopropyl alcohol solutions [6]. This work demonstrated that multilayer adsorption is possible. On measuring the water adsorption and ohmic resistance of the coated chalk they found this to vary in a cyclic manner with the level of coating as shown in Figure 4.4. This behaviour was interpreted in terms of the layers orienting oppositely, thus alternately exposing hydrocarbon and carboxylated surfaces. As we shall see later (see Section 4.7.3), by comparing the behaviour of acids of different chain lengths they were also able to draw other interesting conclusions. This work has not perhaps received the attention it deserves. Papirer and co-workers used tritium labelled stearic acid to study adsorption, from a solvent onto a high surface area precipitated calcium carbonate [17]. They found that adsorption (presumably at room temperature) was very slow with up to 24 hours being

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Figure 4.4 Effect of lauric acid coating (from iso-propyl alcohol) on the water adsorption and resistivity of a chalk powder. (G.P. Ivanishchenko and Y.P. Gladkikh. Reproduced with permission from Colloid Journal USSR, 1979, 41, 3, 661. Copyright 1979, Kluwer/Plenum.)

required to reach equilibrium. (This could be due to the stearic acid being present as a dimer in the solvent). They found that a monolayer coverage was obtained at 8% w/w for the 32 m2g-1 surface area filler, a value which again is in excellent agreement with the 0.21 nm2 value for perpendicularly oriented molecules. The previous adsorption studies have been carried out at room temperature, where the adsorption may be physical, rather than due to salt formation. This must be borne in mind when relating this work to commercial coating processes, where conditions probably ensure chemical reaction with the surface. There has been very little work on adsorption of stearic acid from aqueous solution, probably due to the low solubility of the acid. Suess has however made some measurements using carbon-14 labelled acid [16]. As with adsorption from organic solvent, a Langmuir type isotherm showing mono-layer adsorption was observed, however this was at a much lower level than for solvent adsorption. The reasons for this are not clear but were postulated to be due to adsorption as a hydrated complex salt. Presumably, this is converted to the stable coating layer during drying. Commercially, aqueous coating is carried out using hot dispersions of fatty acid salts. Very little information has been published about these processes where precipitation, rather than surface adsorption and reaction, is probably the controlling mechanism.

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Surface Modification and Surface Modifiers Dry coating techniques have also received little scientific study. There has however been some good work by Fekete and co-workers [18]. They showed that in high shear coating, the adsorption is very dependent on the type of mixer and the exact conditions used. Under optimised conditions, they found they were able to define an addition level up to which complete reaction with the filler was obtained (as shown by no solvent extractable material). Above this level, some further reacted material was generated, but accompanied by residual extractable acid and finally a plateau was reached above which all further stearic acid was extractable. They define a value C100 where extractable material is first observed. This value was found, as expected, to be proportional to surface area for a range of calcium carbonates. More interestingly, calculations based on their data show an area per molecule at the C100 point of about 0.21 nm2. This again suggests vertical alignment and provides some evidence for the dry-coating process giving layers of a similar nature to solvent coating. It does not, however, explain how further reaction seems possible in the dry coating process. There has been a great deal of work on the characterisation of stearic acid coated calcium carbonate using a variety of techniques. The work of Ivanishchenko using electrical conductivity and water adsorption has been referred to earlier [6]. Fekete and co-workers [18] examined fractional coating levels up to their C100 value on a high shear coated calcium carbonate. Using inverse gas chromatography (IGC) they found both the dispersive and polar components of the surface energy to fall with stearic acid addition. Surprisingly, perhaps, the majority of the effect on both components was observed when only a quarter of the C100 monolayer value was obtained. This may be interpreted to mean that while perpendicular orientation is observed at a true mono-layer, the stearic acid lays flat at lower coverages. It could also be interpreted as resulting from a non-uniform distribution of energetic sites. Papirer and co-workers have also used IGC to study stearic acid coated calcium carbonates [17]. In their work, a high surface area precipitated filler was used, and coating was from toluene solution. They also prepared fractional coating levels based on solution adsorption isotherms. Stearic acid treatment was again found to decrease both the dispersive and polar contributions of surface energy to values typical of a hydrocarbon. Both acidic and basic probes were used in this work and interestingly, the uncoated filler was found to contain sites capable of interaction with both. In further work, Schmitt and co-workers studied the adsorption of hydrocarbons with varying chain length on the stearate coated filler [19]. Short probes (< 8 carbon atoms) appeared to completely insert into the coating layer, but only partial insertion occurred with higher chain lengths. Ahsan and co-workers have used gas chromatography to study the effect of coating an especially pure precipitated calcium carbonate with stearic acid [20]. The stearic acid was deposited from aqueous solution of the ammonium salt and at significantly less than a mono-layer coverage. They found the calcium carbonate to have

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Particulate-Filled Polymer Composites a heterogeneous surface, with exposed polar cationic sites capable of strong interaction with unsaturated hydrocarbons. The stearic acid coating appeared to preferentially block these sites producing a lower energy surface.

4.7.2.2 Stearic Acid and other Fillers Among the other fillers where stearic acid could be used are aluminium and magnesium hydroxide, basic magnesium carbonates, dolomite (CaCO3.MgCO3) and magnesite (MgCO3). There is little published information concerning the adsorption of stearic acid on the carbonates, although in several cases commercial products exist. Suess studied the adsorption of stearic acid onto dolomite from organic and aqueous solutions at room temperature [16]. In both cases, monolayer coverage was observed at almost exactly half the quantity of stearic acid required by calcite. It was concluded that the magnesium sites were inactive for adsorption under the conditions of the experiment. They may however react at higher temperatures. Rothon and co-workers have reported on the coating of a magnesium hydroxide filler with stearic acid deposited from aqueous solution of the ammonium salt. Mono-layer coverage was determined by a number of techniques and found to correlate well with the amount to be expected for a vertically adsorbed, close packed mono-layer [2]. Liauw and co-workers have studied the adsorption of stearic acid onto magnesium hydroxide, using a variety of techniques, including DRIFTS and flow micro-calorimetry [8, 21]. The mono-layer level was again found to correspond well with a close packed vertical mono-layer. Evidence for order in the adsorbed layer was obtained by a number of techniques, including X-ray and differential scanning calorimetry.

4.7.3 Other Saturated Fatty Acids and Related Substances There has inevitably been a great deal of industrial interest in examining the effect of fatty acids other than stearic acid. However little of this work has been published. Pure fatty acids are all considerably more expensive than the blends generally described as ‘stearic acid’ and dramatic effects would be needed to make their use worthwhile. This probably explains why there are very few commercial examples of other acids being used. Iso-stearic acid is sometimes used, probably because it is a liquid and easier to use in dry coating processes. The branched structure may also improve the dispersion effects obtainable, but would be expected to reduce the mono-layer amount and prevent a tightly

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Surface Modification and Surface Modifiers packed surface layer being formed. Liauw and co-workers have reported that the monolayer coverage on magnesium hydroxide is less than for stearic acid and that no ordered structure is formed [8]. The study of the adsorption of stearic acid onto calcium carbonate from alcohol solution by Ivanishchenko and Gladkikh [6] has already been mentioned. They also determined adsorption isotherms and the variation of ohmic resistance and water adsorption with coating level for shorter chain acids. Acids with ten and twelve carbon atoms appeared to form mono-layers on chalk with 1.5 x 10-5 moles g-1 of filler while longer chain acids only required 0.9 x 10-5 moles g-1 (unfortunately the surface area of the filler was not specified). With the shortest chain acid (C10), about three molecular layers were required to maximise hydrophobicity, but when more than 14 carbon atoms were present then only one mono-layer was needed. Their results also indicated that, for the shorter acids at least, the first molecular layer was packed differently from subsequent layers.

4.7.4 Effects of Stearic Acid Coating in Composites As already alluded to, stearic acid is often added to aid filler handling during production. It is especially useful with very fine precipitated materials, such as calcium carbonate, where it reduces shrinkage on drying and thus improves dispersability. It also makes the filler hydrophobic, and reduces water adsorption, but can make some fillers considerably dustier. Most interest revolves around the effects that the stearic acid generates when the coated fillers are used in composites. Much is written in general terms claiming improved incorporation, better dispersion and improved physical properties. There have however been few comprehensive studies and several conflicting reports in the literature, which suggests that the effects may be system specific, even possibly depending on filler particle size and loading. As more comprehensive details will be found in later chapters, only a brief account will be given here. In thermoplastics it is generally held that fatty acid treatments reduce melt viscosity, improve filler dispersion, decrease modulus, reduce tensile strength but improve elongation and impact resistance. While generally true, there are a significant number of exceptions reported. Fulmer and co-workers [22] have shown clear evidence of faster incorporation and lower melt viscosity in calcium carbonate filled polypropylene homo-polymer. However, despite the widespread use of fatty acids for improving processability, melt viscosity reductions are not always obtained. Bohlin and co-workers have published an interesting paper on the effect of stearic acid treatment of a dolomite filler in polypropylene [23]. They found

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Particulate-Filled Polymer Composites no effect on viscosity as measured by a capillary rheometer over a wide range of shear rates. A small strain rheological technique showed much lower storage modulus for the coated filler however, which was mainly attributed to improved dispersion. In a fairly comprehensive study of the effect of stearic acid treatment of calcium carbonate in polypropylene, Hancock and co-workers confirmed most of the mechanical property effects mentioned previously, but again observed no improvement in melt viscosity [24]. On the other hand Miyata and co-workers found a marked reduction in melt viscosity when stearic acid was used on a magnesium hydroxide filler in polypropylene [25]. Rothon and co-workers found only small improvements in melt flow index of EVA filled with magnesium hydroxide treated with varying amounts of stearic acid [2]. As mentioned earlier, Rothon and co-workers have also reported on the effect of stearic acid on many properties of the magnesium hydroxide filled EVA [2]. They confirmed the general picture of decreasing tensile strength, and increasing elongation. They also found a small benefit in flame retardant performance. Levels of stearic acid above a monolayer were found to be quite detrimental to ageing. The ageing effect was significant with a pure stearic acid, but even more so with a commercial blend, even though it was a high quality type, often used for treating fillers. Fulmer and co-workers have also reported ageing effects in a calcium carbonate filled polypropylene [22]. Pukansky and co-workers have also carried out a comprehensive study of stearic acid treatment of calcium carbonate in polypropylene. The coating was found to decrease tensile strength and modulus and to increase elongation at break [26]. In the author’s experience, correctly chosen fatty acid treatments can improve the colour of some filled polymers. Fulmer and co-workers have also reported this effect [22]. The role of treatments such as fatty acids in altering thermoplastic composite properties may be quite complex. Thus Hutley and Darlington found that high loadings of most fillers could affect the crystallisation behaviour of polypropylene, leading to reduced impact strength [27]. Treatments such as stearic acid could to some extent overcome this (this is also referred to in Chapter 8). Raymond and Gilbert also found that stearic acid treatment of a platy magnesium hydroxide led to increased modulus in polypropylene, which was found to be due to increased particle alignment [28]. Among other factors of possible significance is the presence of variable amounts of hydroperoxide in many fatty acids, as mentioned earlier. In elastomers the effects of stearic acid coating of fillers appear much more dramatic in solution polymerised polymers than in emulsion ones. As shown by Rothon, this is due to the presence of large amounts of residual surfactants in the emulsion polymerised polymer [29]. These probably adsorb on the filler surface, producing effects similar to stearic acid. In the absence of these competing effects, stearic acid reduces viscosity but also reduces the filler polymer interaction, resulting in decreased bound rubber and

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Surface Modification and Surface Modifiers reinforcement. Even in emulsion polymers stearic acid coating can be detrimental to some properties however, especially tensile strength. Fatty acid treatments do not seem to have much use in thermoset applications.

4.7.5 Fatty Acid Salts It has to be admitted that the situation regarding fatty acid salts is not well established. Indeed there is much ambiguity in the literature with the terms stearic acid and stearates often being used interchangeably. As discussed earlier, some form of salt is the effective coating material after a fatty acid has reacted with the surface of fillers, but a part salt has to be involved if the acid is to be thought of as reacted onto the filler surface. A full salt will only be weakly attached. Full fatty acid salts themselves, are frequently used as polymer additives, usually as processing aids and acid scavengers. Both Gilbert and co-workers [30] and Hornsby and Watson [31] have indicated that such fatty acid salts can be used to beneficially pre-treat fillers, with the effectiveness depending on the metal present in the salt and the filler. Fatty acids and their salts when used as additives may adsorb onto the surface of untreated fillers and this has to be taken into account when developing formulations [32]. Fulmer and co-workers, also claim that using fatty acid salts as additives can be as effective as pre-coating fillers [22]. More work is needed to clarify this area.

4.7.6 Unsaturated and other Functional Organic Acids in Composites By analogy with the vinyl and methacryl silane discussed later, unsaturated organic acids such as acrylic, fumaric, maleic, methacrylic and oleic might be expected to give a coupling agent effect in appropriate systems. Amino-acids might also be considered as potentially having similar effects to amino-silanes. Results have, however, been quite mixed, and there isn’t much commercial use of such additives. There have been few academic studies of the adsorption of unsaturated acids onto fillers. Ottewill and Tiffany studied the adsorption from heptane solution of acids with 18 carbon atoms and various levels of unsaturation, onto uncoated rutile titanium dioxide [33]. As with calcium carbonate, stearic acid was found to give monolayer coverage at a concentration suggesting vertical orientation. With all the unsaturated acids, stepped isotherms were obtained, which were interpreted as multipoint attachment through both acid and unsaturated groups at low concentrations, but with re-orientation to a conventional vertical orientation at higher concentrations.

173

Particulate-Filled Polymer Composites In the work mentioned previously, Liauw and co-workers also examined the adsorption of acrylic, linoleic, linolenic and oleic, acids from organic solution onto a magnesium hydroxide filler [8]. Salt formation was observed for all the acids. All the unsaturated long chain acids gave lower apparent mono-layer levels than stearic acid, which was thought to be due to steric effects and even multi-point adsorption through the unsaturation. Miyata and co-workers have compared stearic and oleic acid derived coatings on magnesium hydroxide fillers in polypropylene and found better impact strength for the oleic acid treatment [25]. The reason for this is unclear and the results do not appear to have been confirmed by others. Nevertheless, some commercial magnesium hydroxides are coated with unsaturated acids similar to oleic, suggesting that there may be a real benefit from its use in some circumstances. In the past, there has been significant interest in acids with especially reactive unsaturation, such as acrylic and maleic acid (or its anhydride) [34]. More recently, pre-grafting of these species onto a polymer backbone has become the preferred method of use. This is discussed in Section 4.7.7. Another interesting material, which is used commercially to some extent is rosin. This contains unsaturated structures and is claimed to give better performance than stearic acid in some elastomers. This is demonstrated by the data for precipitated calcium carbonate given in Table 4.4.

Table 4.4 A comparison of a rosin and a fatty acid coated precipitated calcium carbonate in a sulfur cured styrene-butadiene rubber (SBR) elastomer (100 phr filler) Fatty acid treatment (2.6% w/w)

Rosin acid treatment (2.6% w/w)

Tensile Strength, MNm-2

5.5

6.7

Elongation to break, %

560

600

1.6

1.9

1.6

2.4

66

67

Elastomer property

300% modulus, MNm Tear strength, Ncm Hardness, IRHD

-1

-2

IRHD: International Rubber Hardness Degrees

Furukawa and Yamashita have reported on the effect of various functional organic acids as coatings on precipitated calcium carbonate. Unfortunately the acids were added prior

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Surface Modification and Surface Modifiers to the precipitation, and most of the effects observed seem to be due to modification of the crystallisation reaction rather than any effect on filler polymer interaction [35]. Related to the functional acids, dimaleimides have also been attracting attention, mainly for use in filled polypropylene [36]. There is currently considerable interest in nano-clays as reinforcing fillers for polymers. One of the most successful combinations is in polyamides, where amino-acid treatments can be thought of as effective coupling agents. This is discussed in Chapter 10.

4.7.7 Polymeric Acids and Anhydrides These are an interesting class of surface modifiers, now finding significant commercial uses and capable of being used as dispersants and coupling agents with a wide range of filler types. With these products, the unsaturated acid, or pre-cursor such as anhydride, is pre-reacted onto a suitable polymer backbone. Two main classes of product are available, one with a saturated hydrocarbon backbone, the other with an unsaturated one.

4.7.7.1 Acid Functional Saturated Polymers These are based on polyolefin homo- and co-polymers. Acid functionality is usually introduced by use of acrylic acid or maleic anhydride. Peroxide induced grafting is usually used, but co-polymerisation can also be used. One of the problems with peroxide grafting is that extensive chain scission and loss in molecular weight can occur with some polymers, notably polypropylene. When acrylic acid grafting is used there can be significant homopolymerisation, leading to quite long chains between the acid group and the polymer backbone. This doesn’t occur with the maleic anhydride, which is highly resistant to homo-polymerisation. These additives are intended for use in filled polyolefins, where coupling to the polymer matrix is thought to be due to processes such as chain entanglement and co-crystallisation [37]. The acid levels are usually not high enough to provide solubility and enable fillers to be pre-treated and they are usually used as formulation additives. In some instances, emulsions can be prepared and these may be used for pre-treatment. Where the filler is responsive to silanes, but not acid functionality, then coupling can be achieved by using an amino-silane on the filler and this is now one of the main ways of coupling glass fibre into polypropylene. There has been little scientific study of the effect of molecular weight and acid content, but Pukansky has reported that a molecular weight of about 10,000 was necessary in a wood fibre/polypropylene system [38].

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Particulate-Filled Polymer Composites

4.7.7.2 Acid Functional Unsaturated Polymers The use of such materials was prompted by the need to find coupling agents for calcium carbonate, as silanes were largely ineffective with this filler. Simple unsaturated acids also appeared to have little effect, but Hutchinson and Birchall found unsaturated polymeric acids to be excellent treatments for precipitated calcium carbonate [39]. The concept was developed and extended by Evans and co-workers [40, 41]. The structure of a typical unsaturated polymeric acid anhydride surface modifier based on maleanised polybutadiene (MPBD) is shown in Figure 4.5 together with its postulated mode of action. The anhydride groups are believed to lead to reaction with the filler surface, probably by salt formation, while the residual unsaturation is available for participation in various curing or crosslinking processes.

Figure 4.5 Typical structural features of a maleanised polybutadiene surface modifier

Sufficiently high levels of acid can be introduced to allow water soluble products to be produced, and so this type of additive can be used either pre-coated onto a filler, or added separately during compounding. The relative merits of the two processes have been discussed earlier. The effectiveness of an MPBD type additive when pre-coated onto precipitated calcium carbonate is demonstrated in Table 4.5. This compares uncoated, fatty acid coated, and MPBD coated fillers in a crosslinked ethylene-propylene-diene terpolymer (EPDM) elastomer and clearly shows the benefits arising from the use of the latter coating.

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Surface Modification and Surface Modifiers

Table 4.5 A comparison of fatty acid and MPBD coating of precipitated calcium carbonate on performance in a sulfur cured EPDM elastomer (100 phr filler, both coatings at 2.6% w/w on filler) Property Tensile strength, MNm

Untreated filler

Fatty acid coated

MPBD coated

-2

10.0

4.5

13.3

-2

2.3

1.4

3.7

1.6

1.0

2.4

300% modulus, MN m -1

Tear strength, Ncm

While originally developed for calcium carbonate, these additives have since been found to be effective on a wide range of fillers including aluminium and magnesium hydroxides, clays and talcs. Rothon has reported work by Carey, which investigated the effect of molecular weight and anhydride content of MPBD additives on the properties of a precipitated calcium carbonate filled emulsion SBR elastomer [42]. This work showed that properties in this system generally improved with increasing anhydride content and molecular weight, but began to plateau at a molecular weight of about 10,000 and 20-25% by weight of anhydride. Unpublished work by the present author has since found that the molecular weight effect is not as marked in solution polymerised elastomers. It is postulated that the difference is due to the need for the MPBD to compete for the filler surface with residual surfactants in the emulsion polymerised polymer. Ashton and co-workers have studied MPBD adsorption onto magnesium hydroxide using Fourier Transform Infrared (FTIR) spectroscopy, flow microcalorimetry and classical adsorption techniques [43]. They confirmed the formation of a carboxylate salt at the filler surface and found the chemisorbed layer to be only about 15 Å thick. This implies loopy adsorption and multiple point attachment. The MPBD type additives have also been shown to be effective dispersants and coupling agents for fillers in peroxide cured methacrylate resins [44].

4.7.8 Organo-silicon Compounds 4.7.8.1 General Organosilicon compounds are in widespread use for modifying the surface of mineral fillers. The compounds used have the general formula:

177

Particulate-Filled Polymer Composites (R)4-n — Si — Xn where n = 1 to 3 The group X is an hydrolysable group chosen to react with surface hydroxyls of the filler to produce a stable bond, and is usually halogen or alkoxy. The group R is attached through a hydrolytically stable carbon silicon bond and may be inert or contain a reactive organic functionality such as vinyl, amino, etc. In the latter case strong bonds can be produced with the polymer matrix as well as with the filler surface and these materials are known as silane coupling agents or often simply as silanes. The silane coupling agents in commercial use are generally alkoxy based and only contain one organic group (OR) attached to silicon, the general formula being: R — Si — (OR)3 Both non-reactive and reactive organo-silanes are important filler treatments and will be discussed here.

4.7.8.2 Filler Types Susceptible to Silane Treatment As mentioned in the previous section, organo-silanes rely on reaction with surface hydroxyls to produce a stable layer. They are thus most effective on fillers with high concentrations of reactive hydroxyls. Materials such as silica, silicates (including glass), oxides and hydroxides are most receptive. Even with these surfaces, problems can arise if the surface has a high pH, as under exposure to moisture the attachment of the silane to the surface may slowly degrade. Silanes have little effect on carbon blacks. They are also not generally effective on surfaces such as sulfates and carbonates, although attempts have been made to improve silane effectiveness on such surfaces by pre-coating them with silica [45] or by phosphate layers [46]. It has been reported that some organo-silanes, notably, amino-silanes, can produce an effect on calcium carbonate by some form of encapsulation [47].

4.7.8.3 Coating Techniques Organosilanes are complex species, which often have the potential of reacting with themselves in the presence of moisture as well as with filler surfaces. The nature of the surface layer and its reaction with the filler will thus vary markedly according to the method of coating. Some early discussion of the coating techniques in use is thus necessary as there is a great deal of confusion in this area.

178

Surface Modification and Surface Modifiers The non-coupling silanes in use are generally quite volatile, and filler coating is often carried out in the gas phase by exposing the filler to their vapours. This ensures maximum reaction with the surface and minimum self-condensation. The silane coupling agents, on the other hand, are less volatile and are usually coated in liquid form, either as neat additives, or from a suitable solvent. Silane coupling agents have been most used for coating glass fibres. In this application pre-hydrolysis and partial self-condensation have been found to aid the coating process and this is the usual procedure used. Similar solution coating procedures have been widely used in much of the scientific work on silane treatment of particulate fillers. Unfortunately such procedures do not lend themselves readily to most commercial filler production processes, where some form of direct reaction between the silane and filler powder is frequently used. In many instances the filler coating is actually carried out ‘in situ’ during the compounding process, essentially utilising the polymer matrix as the solvent. These distinctions must be borne in mind when trying to relate laboratory studies to results achieved with commercial products.

4.7.8.4 Reaction of Organo-silanes with Filler Surfaces Two distinct processes can be recognised for the reaction of the Si—X group with surface hydroxyls. Thus there can be direct reaction with the surface, producing a siloxane bond and eliminating HX (Equation 4.1) or there can be prehydrolysis with surface moisture followed by silanol condensation and release of water (Equations 4.2, 4.3). Si—X + HO-M or

Si—X + H2O

→ →

Si—OH + HO—M →

Si—O—M + HX Si—OH + HX Si—O—M + H2O

Equation (4.1) Equation (4.2) Equation (4.3)

In addition, the organo-silanes can react with themselves to produce complex threedimensional networks. For this part of the discussion we will however concentrate on the surface reactions. The simplest case to consider is the reaction of molecules such as (CH3)3SiX with an amorphous silica surface under dry conditions where direct reaction with the surface is experienced.

179

Particulate-Filled Polymer Composites The structure of such a silica surface has been extensively studied and the work has been reviewed by Iler [48]. It is generally accepted that, in the absence of surface moisture, a fully annealed surface has about 4.6 surface silanol groups (Si—OH) per nm2. It might then be thought that surface coverage would require an equivalent number of organosilane molecules. This proves not to be the case, however, as not all the silanols are reactive, the bulk of the organo-silane is too great to allow such coverage and the hydrolysis product of the group X can itself react with and block some of the surface. Under anhydrous conditions, the surface silanols are of two types: isolated and freely vibrating, and hydrogen bonded, with the former being about 30% of the total. Workers such as Armistead and Hockey [49], Hertl [50], and Hair and Hertl [51] have found, using spectroscopic methods, that reaction only seems to occur with the isolated silanols even at high temperature and where a very reactive group X such as halogen is used. Eakins has also reported that because of packing considerations only about 2.45 (CH3)3SiO groups per nm2 would be obtainable [52]. Information such as this is very important in understanding hydrophobic effects. Thus Zettlemoyer and Hsing found that water was still able to penetrate through a fully trimethylsilylated layer and form a hydrogen bonded monolayer with unreacted surface silanols [53]. The trimethylsilylation was, however, effective in preventing subsequent multi-layer development. The situation becomes much more complex when one moves to compounds such as (CH3)2SiX2 and CH3SiX3. They can now react with more than one surface site and with each other. Various surface structures can then be produced. As an example the compound (CH3)2SiX2 can lead to the surface structures shown in Figure 4.6. Under essentially anhydrous conditions, the self-condensation will be very limited and surface reactions will predominate. Such reactions have been studied by Hertl [50, 54] who found that surface reaction is still predominately with the isolated silanols, although some reaction with the hydrogen bonded ones can occur, especially where X is an halogen. A particularly interesting part of Hertl’s work is concerned with the molecule methyltrimethoxy silane CH3Si(OCH3)3, which can be taken as a model for the silane coupling agents. Again it was found that reaction was only through the free surface silanols and, from an analysis of the kinetic data and the amount of methanol released, he deduced that about 40% of the silane molecules react mono-functionally with the surface and 60% di-functionally. (On steric grounds it is very unlikely that tri-functional reaction will occur). A considerable proportion of the methanol by-product was found to form surface ester groups reducing silanol availability. He also showed that partial dehydration of the silica surface by heating increased the number of available silanols

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Surface Modification and Surface Modifiers

Figure 4.6 Some structures possible after the reaction of dimethyldihalogenosilane with a silica surface

and allowed more silane to react. This principle is reported as having been applied to enhance silane coupling agent treatment of clays [55]. When moisture is present, then the picture becomes far more complex with hydrolysis and condensation reactions between both silane and the surface and with itself the main processes. The most studied case is where a tri-alkoxy silane is deliberately pre-hydrolysed before coating the filler, as is common practice in glass-fibre treatment. Plueddemann has given a particularly good description of the processes involved [56] and only a brief outline will be given here. It is believed that best results are obtained if treatment is carried out with a dilute aqueous solution in which the additive is predominately present in silanol form and the pH is such as to minimise self condensation. With cationic silanes pH control is also necessary to produce good surface alignment. Coating solutions are prepared by dissolving the tri-alkoxysilane in an acidic water alcohol mixture. The alcohol is used to enhance solubility of the starting material and the acidic pH is used to produce rapid hydrolysis and relatively stable silanols. Coating then relies on adsorption of the hydrolysed species onto the filler surface followed by silanol condensation both with other silanols in the coating and with surface hydroxyls.

181

Particulate-Filled Polymer Composites The trick is to produce conditions at the surface that favour these reactions even though the solution pH is chosen to minimise it. In the case of glass fibre and some fillers, the surface may be sufficiently alkaline to accomplish this. In other cases a two-step process has to be resorted to in which a filler slurry is first produced in the acidic solution which is then made alkaline before the filler is filtered and dried. Thermal treatment is also used to drive the condensation reaction. As might be envisaged, coatings produced by these processes are extremely complex and poorly characterised in terms of structure and surface bonding. (As one is working in an aqueous environment the earlier discussions regarding preferential reaction with nonhydrogen bonded silanols are now irrelevant). Current views regarding the structure of these coatings will be discussed in Section 4.7.8.5. Finally some mention must be made of organo-silane reaction when applied without pre-hydrolysis from organic solvent. This could be thought of as a model for ‘in situ’ coating processes where the solvent is a polymer or polymer precursor. In this type of process, hydrolysis is assumed to take place with surface moisture followed by condensation reactions. Bascom and Timmons have studied the hydrolysis of triethylethoxysilane with an amorphous silica surface using carbon tetrachloride as the solvent [57]. They found that no hydrolysis occurred in wet solvent or in the presence of dry filler, but proceeded rapidly at room temperature when water was present on the filler surface. The reaction rate increased abruptly at a water level corresponding to a 1:1 ratio of water to surface silanols, implying that a certain specific arrangement of water molecules is necessary. They believed this to be due to the establishment of a hydrogen bonded network allowing the acidic surface silanol species to catalyse the hydrolysis reaction. A surprising feature of their results was the discovery of a maximum hydrolysis rate at about 25 °C and a sharp decrease above this. This was put down to disruption of the hydrogen bonded network at the higher temperatures. Unfortunately, silanol condensation reactions were not studied in Bascom and Timmons work. It is widely reported however that the reaction can be slow on many mineral surfaces unless a catalyst or heat is used. Amines are effective catalysts and the amino-silanes are self-catalytic in this respect. Alkoxy-titanium compounds are also useful condensation catalysts and this may explain synergistic effects claimed for silane/titanate mixtures [58]. Overall, the ‘in situ’ method remains very poorly characterised. Relying as it does on diffusion through polymer to the filler surface, where conditions must be right for hydrolysis and condensation. It sounds very haphazard, but seems to work remarkably well in practice.

182

Surface Modification and Surface Modifiers While the surface of silica fillers is generally well understood and models for their reaction with organosilanes can be developed as described previously, this is not true for most other filler types. Thus while the general principles will still apply, great care must be taken in attempting to extrapolate to the surface of fillers such as clays and hydroxides which are less well characterised.

4.7.8.5 Structure of Coatings Produced with Reactive Organo-silanes The ability of tri-alkoxy silanes to condense with themselves to produce various threedimensional networks makes the concept of monolayer coverage based on simple surface reaction of restricted value when considering this type of molecule. This is especially true of the silane coupling agents, where the picture is often further complicated by the tendency of some of the functional groups present to also interact with the surface. As will be discussed later, it has also been demonstrated that optimum results are sometimes obtained with coating levels well in excess of a notional monolayer. Elucidation of the exact nature of the surface layers, and their relationship to the coating conditions has proved difficult. This has been made more so by marked differences in behaviour imposed by the different functional groups present in the reactive silanes, which have severely inhibited the development of a generalised picture. More recently, the advent and application of modern analytical techniques [especially FTIR, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS)] have allowed considerable advances to be made, especially in relation to coatings produced from aqueous solution. A brief outline of the current views follows. The reader is referred to some excellent reviews by Ishida [59] and Plueddemann [60, 61] for more details. Much of the work has been on silane treatments deposited from dilute aqueous solution onto glass fibres. Here, the current view is that coupling agent layers need to be much thicker than a nominal monolayer for optimum reinforcement to be obtained. It is also believed that patchy adsorption often occurs, especially at low treatment levels. These thick layers are very complex and depend sensitively on the nature of the coating conditions, the surface being treated and the chemistry of the reactive functionalities present. In general terms it is believed that coatings as normally produced (and before use of the coated material in a composite) consist of a mixture of chemisorbed and physisorbed material. The physisorbed material is readily removable by solvent washing and consists of silane oligomers. The chemisorbed material can be further subdivided into a surface monolayer, the structure of which depends critically on the surface of the substrate and the nature of the organo-functional group, (i.e., its ability to interact with the surface). Above this there is a layer of tightly bound material and further out more loosely bound structures.

183

Particulate-Filled Polymer Composites The picture is less clear for particulate mineral fillers. The same general concepts appear to apply, although there seems to be less benefit from using multi-layers. Ishida and Miller have carried out a detailed study of the structure of coatings produced from a water/butanol solution of [γ-(methacryloxy)—propyl] trimethoxy silane (γ -MPS) on a variety of fillers [62]. This work confirmed the existence of physisorbed and chemisorbed layers, the relative proportions of which depended on the acid-base balance of the surface, as well as surface chemical nature and silane loading. Fillers with a near neutral surface (assessed by slurry pH) gave the greatest proportion of chemisorbed silane. The physisorbed silane was found to be composed of unhydrolysed and partly hydrolysed monomers and polysiloxane oligomers and polymers. The molecular weight of this fraction was again greatest on a neutral surface. Interestingly, similar products could be produced in the absence of filler by adjusting the solution pH to that of the filler surface. While the previous picture has largely been worked out for silane coatings produced from aqueous solution, it seems that it probably applies, in very general terms at least, to other coating methods. The relative proportion of physisorbed and chemisorbed material and their detailed structure will however probably vary markedly. Among the important factors will be the concentration of suitable surface hydroxyls, the surface pH and the amount of water present. In one of the few relevant studies, Nakatsuka and co-workers [46] examined calcium carbonate and clay fillers coated with γ -MPS by dry blending (from water/alcohol solution, however). They found that a considerable proportion of silane was actually lost from the calcium carbonate system, due to evaporation during drying. Physisorbed material was found on both fillers, but while that on calcium carbonate was of very low molecular weight, that on clay was of much higher molecular weight. They also found that pre-treating the calcium carbonate with phosphoric acid resulted in the formation of higher molecular weight physisorbed material and enhanced the coupling agent effect, as assessed in a vulcanised elastomer. Ishida has reported that dry coating, as often used in particulate filler treatment, results in an approximate monolayer of chemi-sorbed silane, giving imperfect coverage with the rest of the silane being weakly physisorbed [59]. Unlike the situation with solution coating of glass fibre, it is unusual to use more than a monolayer. Hence, the interphase concepts are less important in this case. It is now generally recognised that the exact nature of the chemisorbed and physisorbed layers plays a significant part in the effectiveness of silane coated fillers in polymers. This aspect is also very complex but beginning to be elucidated by the use of modern techniques. The relevance of these structures to polymer properties will be dealt with later (Section 4.7.8.8).

184

Surface Modification and Surface Modifiers

4.7.8.6 Applications of Non-Coupling Compounds These additives are used for making filler surfaces hydrophobic and oleophilic. The trimethyl compounds are most widely used for this purpose, with both the chloro- and methoxyderivatives being used. The most useful compound for introducing the trimethylsilyl group is however, hexamethyldisilazane [HMDS; (CH3)3SiNHSi(CH3)3]. This reacts with surface moisture to form only volatile by-products, and with surface silanols to release only ammonia, so the reaction is particularly clean. More recently, long chain hydrocarbon trialkoxy silanes, such as CH3(CH2)n,Si(OCH3)3 have been made available. Fillers, especially high surface area silicas, are commercially available with such hydrophobic surface coatings. They are expensive and seem to be mainly used in sealant and adhesive applications rather than in polymer composites, where they produce low particle matrix adhesion similar to fatty acid and other cheaper additives. Typical effects obtained by use of these treatments on fillers in elastomer systems can be found in the work of Dannenberg and Cotten [63]. They examined trimethylsilane treatment of a fumed silica and found effects consistent with reduced filler rubber interaction. Thus rebound resilience, modulus, tear strength and bound rubber were all reduced. Surprisingly, the treatment gave a considerable improvement in abrasion resistance, which it was believed resulted from the increased hysteresis.

4.7.8.7 Application of Reactive Compounds (Silane Coupling Agents) Silane coupling agents are much more widely used in particulate filled composites than the non-coupling analogues. This is because of their ability to improve filler polymer adhesion, as well as improving dispersion and reducing hydrophilicity. In the simple model, a monolayer is envisaged in which one end of the silane coupling agent bonds to the filler, and the other end reacts with the polymer matrix. Unfortunately the real situation is much more complex. The complexity of the reaction at the filler surface and of the coating structure has already been described. The complexity of the coupling agent layer interaction with the polymer must now be considered.

4.7.8.8 Current Views on Silane Coupling Agent-Polymer Interaction With the advent of modern analytical techniques, especially FTIR, XPS, SIMS and solid state nuclear magnetic resonance (NMR), the true nature of the interaction between silane coupling agent layers and polymers is beginning to be unravelled. We have already seen (Section 4.7.8.5) how silane coupling agent layers on fillers are believed to consist

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Particulate-Filled Polymer Composites of a mixture of chemisorbed and physisorbed material. This idea has been extended into the filled polymer itself, leading to the concept of the existence of an interphase of graded structure between the filler and polymer matrix, the exact properties of which control polymer reinforcement. The nature of this interphase will depend on the structure of the silane, the coating conditions, the nature of the filler surface, the nature of the matrix polymer and the compounding conditions. Hence one has a very complex picture, which can only be covered in outline here. Again the reader is referred to the excellent reviews by Ishida [59] and Plueddemann [60, 61] for more details. The first issue to be addressed is the importance of the physisorbed silane. There are mixed views on this and very little data. Two different aspects have to be considered, processing and reinforcement. Of these, processing is easiest to understand. The physisorbed material will be similar to a silicone oil, and hence should be a good lubricant and aid processing of mineral filled polymer melts. This probably explains the beneficial effects of silanes on rheology of such systems. Han and co-workers have reported on the effect of reactive and non-reactive silanes (amino-, propyl- and octyl-) on the viscosity of calcium carbonate and glass bead filled polypropylene [64]. Both silanes markedly reduced the viscosity of the calcium carbonate system, but not of the glass bead one. As proposed by Ishida this can be explained by the existence of considerable low molecular weight oligomers able to act as a lubricant on the calcium carbonate, but not present on the more reactive glass beads [60]. The effect of the physisorbed material on reinforcement is much more difficult to predict and will probably depend very much on the system. In some cases this material will dissolve in the matrix and play little role in the interphase. In other cases it may penetrate a little way only and during processing and cure may react with the chemisorbed silane (through silane condensation or its reactive functionality), with itself, and with the matrix polymer. Under favourable circumstances then it could lead to an extensive interphase of an interpenetrating network type, with bonding to both filler and matrix. More often, however, reaction may only occur with the matrix and not with the chemisorbed layers. This will give poor results. In the few studies where the physisorbed material has been removed before use, properties have indeed been improved [65, 66]. We now come to the nature of the chemisorbed layers. It is well known (in glass fibre systems at least) that considerably more than a notional monolayer of silane is required for optimum composite mechanical strength. This is in conflict with the simple chemical bonding theory and has usually been explained by surface contamination, incomplete coverage, etc. As reported by Ishida, several observations suggest that there is a more fundamental cause [59]. Firstly, the optimum thickness is found to be very reproducible for a given system. More importantly the vinyl functional silane is frequently found to be less effective then the methacryl version, even where both should be able to copolymerise with the matrix.

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Surface Modification and Surface Modifiers In order to explain these observations, Plueddemann suggested that interpenetration of matrix and coating may be an essential requirement [67]. This is believed to be partly due to the layer of silane at the mineral surface being relatively unreactive, which itself is probably due to deactivation of the organo-functional group by adsorption onto the surface. Such adsorption has been observed with methacryl and amino functional silanes [60]. The outer layers do not suffer from this problem and hence are free to react with the matrix. A certain looseness of the structure of these layers will allow very good mixing between polymer matrix and coating and promote bonding (especially where the polymer is initially present in low molecular weight, liquid form, as in many thermoset systems). This is proposed as the reason for the better performance of methacryl- as opposed to vinyl-silane [59]. The latter is believed to be very prone to molecular ordering, leading to a tight packed network, while the former gives a very loose, open structure. Interestingly, Ishida [59] also reported that the effectiveness of both silanes was similar on powdered, as opposed to fibre glass, indicating that the ability to form tightly packed films, also depends on the smoothness of the surface. Despite the recent advances in analytical methods we do not as yet know how well the first silane layer is bonded to the remainder of the chemisorbed material or, more importantly, the detailed structure of the interphase region once polymer processing has been carried out. Hopefully, this will become clearer as techniques are further developed.

4.7.8.9 Principle Silane Coupling Agent Types and their Use A considerable range of silane coupling agent types are available commercially, or have been reported in the literature. A vast literature detailing their characteristics and performance also exists. A number of excellent reviews of these subjects are available [61, 68] and hence only the silane types most commonly met will be covered here together with limited examples of the effects they can produce. Three parts of the silane need to be considered, the alkoxy end, the spacer group and the polymer reactive end. The silanes are almost always tri-alkoxy derivatives. While the alkoxy group is lost during the filler treating process, it’s nature plays an important role and needs to be carefully selected. It controls volatility and flash point, affects toxicity, and plays an important role in determining the rate of hydrolysis. The most common groups used are, methoxy-, ethoxy-, and methoxy-ethoxy. Economics dictate that there is little scope for varying the length of the hydrocarbon spacer group between the silicon and reactive functionality. Jang and co-workers have examined the effect of the length of this group in the methacrylic functional silanes, and found only minor differences in performance [69].

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Particulate-Filled Polymer Composites The functional groups most used commercially in particulate filled composites are: amino, epoxy, vinyl, mercapto, methacryl and polysulfide. The principle features of each of these and their utility will now be discussed briefly. a) Vinyl This is the workhorse silane coupling agent, and commercial products generally have the vinyl directly attached to the silicon atom. The hydrolysable groups are usually methoxy, ethoxy, methoxy-ethoxy or acetoxy. This type of functionality is mainly used in polymers that cure or are crosslinked by a free radical process such as peroxide. Gent and Hsu have shown, using infra-red spectroscopy, that the vinyl group does react during peroxide curing [70]. The vinyl group is however not sufficiently reactive for all systems, and methacryl functionality is sometimes preferred. The tendency for this silane to form close packed structures due to molecular orientation has been referred to previously. b) Methacryl This is a more reactive form of unsaturation than vinyl, and is used extensively in free radical curing formulations, where the extra reactivity is of benefit, e.g., acrylics, some polyesters. Commercial products usually contain γ-methacryloxy propyl groups, and hence the double bond is further away from the silicon than is the case with the vinyl silane. As mentioned previously, Jang and co-workers have studied the effect of the length of the spacer group [69]. The presence of a carbonyl group in the molecule leads to a tendency for it to lie flat on filler surfaces under some conditions [59, 60]. This is thought to lead to a reduction in its ability to copolymerise with matrix polymer. Garbassi and co-workers have also reported that some filler surfaces, (e.g., quartz), appear to cause selfpolymerisation of the methacryl groups [71]. c) Epoxy Commercial products are based on 3-glycidoxy propyl trimethoxy silane. While they can perform well in a number of systems, they are relatively expensive and are mainly used in epoxies for which they are generally superior to other silane types. The epoxy products have received less scientific study than some of the other silanes but as with methacryl and amino, there is a strong possibility for deactivation of the epoxy group on a filler surface. d) Amino Amino functional silanes are fairly widely used. The commercial ones are usually based on the γ-aminopropyl functionality. They have a wide versatility, being used in epoxies, phenolics, urethanes, polyamides, some thermoplastic polyesters and elastomers.

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Surface Modification and Surface Modifiers Unlike most silanes they give aqueous solutions of great stability. This is believed to be due to hydrogen bonding between the silanol groups resulting from hydrolysis, and the primary amine. It is most marked when the amine is in the 3 position, as it is in the standard commercial products, and can form an internal 5 or 6 membered ring [56]. The reactivity of the amine group has made structural characterisation of the nature of surface layers resulting from its adsorption on fillers very difficult. The amine itself may absorb strongly on a variety of surfaces and has also been shown to be very prone to bicarbonate salt formation with atmospheric carbon dioxide. Modern analytical procedures are beginning to elucidate some of the important features of these coatings and Ishida has given a very good account of the current state of knowledge [59]. e) Sulfur Functional These are specialist coupling agents, specifically designed for sulfur curing elastomer systems, but nevertheless are used in considerable amounts. Two principal forms are available, γ-mercapto propyl silane and various polysulfidic silanes (especially tetrasulfide). Both types are very effective, the mercaptosilane is probably the most efficient, but tends to be more scorchy than the polysulfides. There has been a remarkable growth in the use of the tetrasulfide over the last decade, due to it’s key role with precipitated silica in the development of low rolling resistance, or ‘green’ tyres [72]. Largely as a result of this success, other polysulfides, especially the disulfide, are becoming of commercial interest [73]. f) Mixed Silane Systems While not often described in the scientific literature, the use of mixed silane layers is becoming of significant industrial importance. As described by Pape and Plueddemann [74] mixtures of reactive and non-coupling silanes can still give high property levels with improved hydrolytic stability. Mixtures of two different reactive silanes are also reported as being beneficial. g) Oligomeric Silanes Another recent development is the use of oligomerised silanes. These are partly self-condensed products and have the advantage of reduced volatility and decreased alcohol release [75].

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4.7.9 Examples of Silane Coupling Agent Effects in Filled Polymers 4.7.9.1 Introduction A brief account of the use of silanes in various polymer systems follows, more details will be found in the appropriate chapters later in the book.

4.7.9.2 Elastomers Silane coupling agents are widely used to improve the performance of mineral fillers in elastomer systems. The silanes most commonly used are the various sulfur-based ones and the vinyl and amino functional. The sulfides are generally used both pre-coated and ‘in situ’. The fillers most often used in conjunction with silane coupling agents are precipitated silicas and clays. The primary objective of the treatment is to improve polymer to filler bonding, although other benefits such as lower viscosity, better dispersion and reduced water adsorption are also observed. The main benefits to be observed from the improved adhesion are higher modulus, higher tear strength, better fatigue resistance and higher abrasion resistance. More complete details can be found in the chapter on elastomers. It should also be recorded here that Schwaber and Rodriguez used dilatometry to provide direct evidence for improved adhesion resulting from the use of silane coupling agents in silica and clay filled elastomers [76].

4.7.9.3 Thermosets Silane coupling agents are widely used in thermoset systems, especially unsaturated polyesters, acrylics and epoxies. The silanes most commonly used are vinyl, methacryl, epoxy and amino. Among the fillers commonly treated are various silicas and silicates and aluminium hydroxide. The latter is particularly widely used for its flame retardancy. The in situ treatment method is frequently used with thermosets. The coupling agents are again predominantly used to produce stronger filler to matrix adhesion, especially after exposure to moisture, though other benefits, especially better dispersion, may also be observed. In addition to leading to better strength retention in wet conditions, the improved adhesion results in improved impact strength. Specialised dispersants are often also used in thermoset systems and have to be carefully chosen to minimise competition with the silane for the filler surface. More details will be found in the chapter on thermosets.

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4.7.9.4 Thermoplastics In discussing thermoplastics we have to distinguish between condensation polymers (like polyamides and polyesters), and polyolefins (such as polyethylene and polypropylene). The condensation polymers are fairly reactive under melt processing conditions and can react with silane functionalities such as primary amine. Such silanes are thus widely used with this class of polymers. Standard silane coupling agents are generally less effective in polyolefins, due to their chemical inertness. A very reactive azido silane was developed specifically for use in such polymers [67], but has now been withdrawn, as other technologies appear to have replaced it. One of the alternative approaches, originally developed by Union Carbide and now marketed by Crompton OSi Specialties is based on the use of organo-silicon chemicals which are of a proprietary nature. Details of the performance of this approach have been given by Godlewski [77, 78]. The use of silane coupling agents as additives for thermoplastics has also been reviewed by Godlewski and Heggs [79]. More recently, the use of acid functional polyolefins appears to have become the dominant technology. These can be used directly with some fillers (such as calcium carbonate and aluminium hydroxide), but in other cases, the filler has to be treated with an amino-silane, which then forms an amide linkage with the acid functional polymer. The fillers most commonly treated are silicas, clays and other silicates and flame retardants such as aluminium and magnesium hydroxides. While both ‘in situ’ and pre-coating methods are utilised, pre-coating is most popular. This is in part at least due to the problems that can be encountered due to alcohol release in compounding machinery when the ‘in situ’ process is used.

4.7.10 Organo-Titanates and Zirconates 4.7.10.1 Introduction Organo-titanates are a very interesting class of surface modifier, which have aroused great interest in recent years. They can all be regarded as derivatives of ortho-titanic acid, Ti(OH)4, and hence are commonly known as organo-titanates rather than by their systematic names. This convention will be adopted here, in order to be consistent with existing literature. While several companies produce organo-titanates for a variety of uses, one group of workers (Monte and Sugerman) have specialised in producing compounds for filler modification, which they refer to as coupling agents. As well as producing an enormous

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Particulate-Filled Polymer Composites range of potential modifiers they have devised a methodology for describing the action of the organo-titanates, and a system for classifying the various types available in the light of this methodology. Most of the scientific and trade literature follows this approach. As we shall see later, some scientific studies have thrown doubt on the validity of this approach. Nevertheless it will be followed in the early part of this article, for simplicity and compatibility with the existing literature. As well as arousing interest, the organo-titanates have also aroused a great deal of controversy. This is mainly related to their mode of action, and in particular, whether they act as true coupling agents, or merely as very effective dispersants. Unfortunately while there is an extensive literature, most of this is of a commercial or purely effect describing nature and there have been virtually no useful basic scientific studies. This has combined with the bewildering range of available products to create confusion and some distrust of the products. The natural chemistry of titanium causes problems with organo-titanates in some applications. To overcome this, organo-zirconate products have been recently introduced. The structure and general principles of the zirconates are similar to the titanates and no separate discussion of them will be made, except to point out reasons why zirconates are preferred in some applications.

4.7.10.2 General Principles Conceptually, the organo-titanates are similar to silanes. Thus, they are designed to have groups (generally alkoxy) that readily hydrolyse to titanium hydroxy groups which can condense with surface hydroxyls and to also carry other organic groups, which are more hydrolytically stable, and may also provide some reactive functionality. Depending on their chemical nature, organo-titanates can be pre-coated from organic or aqueous solution, dry blended or used in situ. As discussed next, the chemistry of titanium causes some difficulties in producing suitable structures, and partly accounts for the complex nature of the products that are available. The most important factor is that, unlike the silicon to carbon bond, the titanium to carbon bond is very unstable and cannot be used to permanently attach organic groups for surface treatment applications. The organo-titanates are thus generally based on tetra-substituted titanium, where all the substituents are linked by titanium-oxygen-carbon bonds, and their chemistry is dominated by the hydrolytic sensitivity of these. Alkoxy radicals are rapidly hydrolysed, with the rate in water decreasing as the chain length increases, due to reduced solubility. Acyloxy derivatives are also fairly readily hydrolysed,

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Surface Modification and Surface Modifiers again with a chain length effect. They are generally thought to be somewhat more stable than the alkoxides. Fairly good hydrolysis resistance can be obtained with chelate groups attached to titanium, particularly where a five or six membered ring is formed. Alkanolamines, diketones, keto esters, hydroxy acids and certain glycols are useful chelating agents. In some instances inorganic acids such as phosphates are used to attach organic groups to titanium, the titanium-oxygen-phosphorus bond being fairly hydrolytically stable. It must also be borne in mind that titanates are very active chemicals and can exhibit a variety of crosslinking and catalytic effects in polymeric systems, which may in part contribute to their effectiveness as filler treatments. Crosslinking occurs through alcoholysis reactions with polymers containing active hydrogen atoms. A wide variety of catalytic effects are also observed. The most important ones in the present context are esterification, trans-esterification, polyamide formation and olefin polymerisation. Finally the sensitivity of titanium IV compounds to photo-reduction must be mentioned as, under certain circumstances this can lead to undesirable colour effects, which limit the usefulness of organo-titanates in some applications. Very useful articles on the chemistry and application of organo-titanates are available from some of the manufacturers [80] and are worth consulting by anyone interested in more detail.

4.7.10.3 Organo-Titanate Types Proposed for Surface Modification Monte and Sugerman’s basic approach is to use alkoxy groups as the hydrolysable group on the titanium and acid groups (carboxylate, phosphate, sulfonate) to give stable attachment of other organic groups. Initially single isopropoxy groups were used as the hydrolysable group and these products are described as monoalkoxytitanates. More recently neopentyl (diallyl) oxy groups have also been used. These are claimed to have better thermal stability and be more suited to ‘in situ’ use in thermoplastics compounding. Derivatives of this type are known as neoalkoxytitanates. Chelate, co-ordinate and quaternary types based on pyrophosphates are also available. The reader is referred to the manufacturers literature for more details [81] and to the extensive publications by Monte and Sugerman [82].

4.7.10.4 Types of Filler Susceptible to Organo-Titanate Treatment While no comprehensive study has been made, it is generally claimed that organo-titanates are effective on a wide variety of particulate mineral surfaces, including calcium carbonates and carbon black. This claim is however often based on dispersant action in simple tests, where adsorption rather than strong surface bonding is often sufficient to produce an

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Particulate-Filled Polymer Composites effect. Based on the proposed mode of action, one would expect best performance on heavily hydroxylated surfaces such as silica, silicates and hydroxides. Organo-titanates seem especially effective on calcium carbonate, however, even though this surface has few hydroxyls and ortho-titanic acid is too weak to displace carbonate. Some other bonding mechanism must thus be involved. It must be borne in mind that considerable free carboxylic acid is potentially present and may play a significant role on this particular surface (see Section 4.7.10.5).

4.7.10.5 Monoalkoxy Types The simplest organo-titanates in general use as filler modifiers are nominally triacyloxy isopropoxy derivatives. As mentioned earlier, the supposed mode of action is hydrolysis and surface condensation through the isopropoxy group and stable bonding of the acyloxy groups to the titanium. Where these acyloxy groups are long chain fatty acids, then simple hydrophobing and dispersion effects should be observed. If however they contain some reactive functionality, then chemical coupling to the matrix polymer is theoretically possible. Typical compounds of these types are: Non-reactive: [CH3 (CH2)16COO] 3Ti—OR Reactive: CH3(CH2)16COO Ti—OR [CH2=C(CH3)COO]2 The sort of structure claimed to be formed on filler surfaces is shown in Figure 4.7a. Several points need to be made about this structure. Firstly, the molecule is very bulky and in this scheme only has one attachment point to the surface. Thus it is unlikely to be able to react with more than a small fraction of the surface hydroxyls present on many fillers. Secondly these idealised structures differ from the silane coupling agents in having no bonding between adjacent coating molecules. One would expect that such a coating may exhibit poor hydrolytic stability. Finally the acylate groups themselves are also of doubtful hydrolytic stability. It could well be then that some of these hydrolyse leading to further surface reaction and condensation similar to that observed with the silanes.

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Figure 4.7 Some structures proposed for the reaction of monoalkoxytitanates with filler surfaces. a) Monofunctional reaction; b) bifunctional reaction; c) complex, partly hydrolysed and condensed layer

While basic scientific work is relatively scarce, there have been some useful studies which tend to confirm that the simple idealised picture is indeed far from correct. Perhaps the most important of these has been by Cans and co-workers [83]. They worked with a commercial organo-titanate (KR TTS from Kenrich) claimed to be isopropyltriisostearoyltitanate, containing a small amount of free alcohol. This is probably the most widely used titanate for filler modification and features in much of the scientific and commercial literature. The usual method of coating is from an isopropanol solution. Cans and co-workers used proton and 13C NMR to establish the structure of this material and were surprised to find that it appeared to be a mixture of diisopropyldiisostearoyl titanate and free isostearic acid. It would thus appear that the excess alcohol present has solvolysed one of the acyloxy groups. They also found that further solvolysis readily

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Particulate-Filled Polymer Composites occurred if the titanate was dissolved in alcohol (as is often the case when coating fillers) and that both solvolysis and hydrolysis could be catalysed by filler surfaces. Similar results were also obtained with another commercial titanate (said to be isopropyldimethacryl isostearoyl titanate with free isopropanol but analysed as diisopropyl methacryl isostearoyl titanate and free methacrylic acid). It would thus seem that, in practice, the titanates most commonly used are probably diacyloxydialkoxy compounds with free acid present. This means that, even if the residual acyloxy groups are stable, multiple surface attachment and self condensation more analogous to silane chemisty can take place. If, as seems probable, some of the remaining acyloxy groups are also hydrolysed, then these effects will be further enhanced. The nature of the surface layer will then depend markedly on the method of surface treatment used. If coating is carried out from an aprotic solvent and the filler is relatively dry, then one may approach simple difunctional reaction with two remaining acyloxy groups still attached (Figure 4.7b). However where further solvolysis and hydrolysis is possible then in the extreme case one can envisage a partially formed titanium dioxide network with some random surface attachment and some remaining chemically bound organic groups (Figure 4.7c). In addition, the role of the free organic acid must be considered, both that apparently present in some commercial titanates and that formed by further hydrolysis. These complications have not been recognised in the vast majority of the available literature, especially that concerned with relating structure to performance effects. One group has however recognised that in some applications the organic acid alone may achieve the same results as the titanate [84]. A few papers relating to adsorption onto particle surfaces and the structure of coating layers are worth a brief mention at this point. Landham and co-workers synthesised a disopropoxy diacyl titanate using 12-stearoyloxystearic acid to give a branched, long chain hydrophobic group and studied its adsorption on to alumina and barium titanate [85]. The solvent was hexane and the particles well dried, so this should have approached the simple difunctional situation outlined previously. This additive was found to be an effective dispersant for both particles but to require considerably in excess of the calculated monolayer coverage for maximum effect. The stability of the adsorbed coating on alumina was assessed by soxhlet extraction of the filler with ethyl acetate. Very little was removed while under the same conditions up to 90% of a carboxylic acid (unspecified) was removed. Golander and Sultan used infra-red and XPS techniques to study titanate coatings (applied from isopropanol!) on an aluminium surface [86]. They found a coating thicker than the XPS sampling depth but with a higher titanium to carbon ratio than expected, indicating hydrolysis had occurred. Tsuchiya and Iwatsuki have examined the reaction between

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Surface Modification and Surface Modifiers titanates and a silica powder using FTIR spectroscopy, thermal analysis and gas chromatography, and claimed to find evidence for reaction with the surface silanols [87]. Finally Abu-Zeid and co-workers have used photoacoustic spectroscopy to examine the interaction between KR-TTS and calcium carbonate [88]. Coating was from mineral oil. The results were interpreted as showing the existence of chemical bonding beneath rather than at the surface of the particles. These conclusions need further investigation.

4.7.10.6 Other Organo-Titanates Despite the wide range of organo-titanate types available there have been virtually no scientific studies with types other than the monoalkoxytitanates.

4.7.10.7 Effects produced by Organo-Titanates Irrespective of the controversy over the structure of the organo-titanates and their mode of action, they do appear to give useful effects and this is after all what matters. Their main use appears to be in giving excellent dispersion of particles and hence low viscosity, easily processable, formulations with good end properties. In this respect they must then be judged against other materials used to achieve the same effects, notably surfactants and fatty acids. The latter are especially relevant as the organotitanates in use are largely based on these and, as we have seen, may indeed contain or form considerable quantities of the free acids. Unfortunately very little data exists on which one can properly judge the relative merits of the organo-titanates and other treatments. As mentioned earlier, Kenrich KR-TTS is probably the most widely studied organo-titanate at present and serves as a good example of the utility of organo-titanates. Several examples of its use to produce low viscosity dispersions of fillers such as calcium carbonate and aluminium hydroxide in organic liquids can be found in the manufacturers literature [81] and the paper by Monte and Sugerman [82]. It is claimed that a significant part of their effectiveness is due to chemical removal of the surface water layer on the particles, which significantly improves dispersion. Such an effect is not present with other common types of dispersant. Unfortunately no good comparison between organo-titanate and other dispersants has been made, although Landham and co-workers did report that organo-titanates formed more stable layers on alumina particles than fatty acids [85]. A great deal of interest has been focussed on calcium carbonate filled polypropylene where materials such as KR-TTS are claimed to give excellent results. The work of Sharma

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Particulate-Filled Polymer Composites and co-workers is typical [89]. They coated ground calcium carbonate with KR-TTS from toluene solution in a high speed mixer. The filler was dry blended with polypropylene powder and compounded in a Buss Ko-Kneader. Test specimens were produced by both compression and injection moulding. The coating was found to improve melt flow index, tensile elongation and notched Izod impact strength relative to the uncoated filler. Scanning electron microscopy indicated better dispersion of the filler due to the presence of the titanate, but gave no evidence of a chemical bond between filler and matrix. Unfortunately, no comparison with a simple fatty acid treatment, which would be expected to give some, at least, of these benefits was carried out. Unfortunately there have been no useful scientific studies with organo-titanates containing unsaturated acyloxy groups to show whether they provide true coupling ability.

4.7.10.8 Limitations of Organo-Titanates and use of Organo-Zirconates In addition to the tendency to colour formation due to photo-reduction already mentioned, organo-titanates can also give undesirable colour effects in the presence of phenolic functionality such as often encountered in antioxidants and light stabilisers. The ready reduction of titanium IV also causes problems in peroxide cures where deactivation of the free radicals and reduction in cure efficiency can occur. These problems are largely eliminated if titanium is replaced by zirconium and hence organo-zirconates analogous to the organo-titanates have been developed. They are considerably more expensive to produce than the titanates and hence are unlikely to completely replace them. The same questions over hydrolytic stability of the attachment of the organic functionalities apply as for the titanates.

4.7.11 Aluminates and Zircoaluminates Use of aluminates is in its infancy and there is very little information available concerning the nature of the products and their effects other than that they are chemically modified alkoxy aluminium chelates. Their use is being pioneered by Kenrich Petrochemicals [81]. Zirco-aluminates have been available for several years, but do not seem to have made much headway in the particulate fillers area. Again structural details and useful scientific data is scarce. They are said to be based on a low molecular weight zirco-aluminate backbone carrying two ligand types. One type confers hydrolytic stability to the backbone, while the other contributes organo-functionality. A number of functionalities are possible including methacryloxy, mercapto, and amino.

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Surface Modification and Surface Modifiers While described as coupling agents, the main benefits illustrated appear to be in dispersion and especially in producing low viscosity filler dispersions of excellent stability in liquid systems such as unsaturated polyester resin. Among the advantages claimed are the ability to produce hydrolytically stable coatings easily and rapidly with aqueous slurries of particulate fillers including calcium carbonate. The reader is referred to publications by Cohen [90-92] for more details.

4.7.12 Phosphates and Borates Organic derivatives of both boric acid and phosphoric acid have been proposed as filler treatments especially for use on calcium carbonate. Borates have been proposed in a patent assigned to Dart Industries [93]. The standard principle of having a readily hydrolysable group for surface attachment and more stably attached groups for retained organo-functionality is followed. In the patent, the hydrolysable group is isopropoxy and the other groups are long chain alkoxy groups such as stearoyl. Calcium carbonate was coated with these additives by dry blending in a high speed mixer and found to give improved properties (melt flow, impact strength, elongation) in polypropylene homo and copolymer compared to untreated filler. As in so much of this work, one has to ask whether these effects are better than could be achieved with fatty acid alone and thus justify the higher cost of such treatment. This information is not available but as this method of treatment has not become generally adopted it would appear that it offers little real benefit. A more interesting case would be where the retained groups had polymer reactive functionality and were able to give a true coupling effect. This is hinted at, but not exemplified in the patent. The hydrolytic stability of the attachment points would then become critical and might be found wanting. Organophosphates are more interesting, and their use as surface modifiers for calcium carbonate has been the subject of a number of papers by Nakatsuka and co-workers (46, 94, 95]. They have concentrated on organofunctional dihydrogen phosphates (RO-PO3H2) obtained by phosphorylation of organic hydroxy compounds using phosphorus trichloride. By this method they obtained compounds where the R group was a simple hydrocarbon (ethyl, butyl, hexyl, octyl) or contained additional functionality such as olefinic double bonds, methacryloxy, chloro or mercapto groups. In addition to simple functional alcohols they also used a low molecular weight hydroxyterminated polybutadiene. These organophosphates were used to coat both precipitated and ground calcium carbonates, and the coated fillers were assessed in SBR and EPDM elastomers. Coating was by adding an aqueous or organic solution of the phosphate modifier to an aqueous

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Particulate-Filled Polymer Composites slurry of the calcium carbonate, filtering and drying. Microscopy showed evidence for surface reaction and precipitation of calcium salts of the additives. The non-functional alcohols were found to improve wetting and dispersion in the polymers but not to increase reinforcement. The unsaturated additives on the other hand gave marked improvements in peroxide cures typical of increased filler to elastomer adhesion. The unsaturated additives were found to be less effective in sulfur cures where as might be expected, a mercapto derivative gave the best performance. Despite the apparent promise of this approach, there seems to be little or no commercial use of this type of coating. The additives are probably expensive to prepare and the benefits obtainable may not then warrant their use.

4.7.13 Organic Amines and Amino-acids Until recently this has been a minor topic, with a small use of organic amines to treat clays and related materials. The growing interest in the use of organo-clays as nanofillers is creating what may become a significant market opportunity for this type of modifier. More details will be found in Chapter 10.

4.8 Conclusions It can be seen that modification of filler surfaces both to aid processing and improve composite properties is an important and active area of research. While there are a considerable number of treatments proposed, they all follow the principle of a filler surface reactive group linked to an organic backbone, which may carry further functionality. The main variation is in the group used to achieve surface reaction. As we have seen this may be an acid or acid precursor, an aluminate, borate, phosphate, silane, titanate or zirconate. There is little difference in the organic groupings available in the different coating types; with long chain alkanes, vinyl and methacryl unsaturation and amines predominating. By and large, these groupings produce similar effects irrespective of the method of filler attachment and hence the cheapest, easiest to use forms, with best hydrolytic stability are used. More recently, greater sophistication is being brought to bear with ever increasing performance of the surface treatments being required. The concept of a thick interphase region is being developed with control of its microstructure seen as crucial to maximising performance. Newer coating types are being developed to achieve this control, especially polymeric modifiers and the organosilicon chemical mixtures. We can expect to see this trend continue.

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H. Ishida and J.L. Koenig, Journal of Polymer Science: Polymer Physics Edition, 1979, 17, 10, 1807.

2.

R.N. Rothon, C.M. Liauw, G.C. Lees and W.C.E. Schofield, Journal of Adhesion, 2002, 78, 603.

3.

M. Gilbert, I. Sutherland and A. Guest, Proceedings of Filplas 92, Manchester, UK, 1992, Paper No.4.

4.

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20. T. Ahsan, B.A. Colenutt and K.S.W. Sing, Journal of Chromatography, 1989, 479, 17. 21. C.M. Liauw, R.N. Rothon, S.J. Hurst and G.C. Lees, Composite Interfaces, 1998, 5, 503. 22. M. Fulmer, J. van der Kooi and K.E. Koss, Proceedings of Antec 2000, Orlando, FL, USA, 2000, p.552. 23. L. Bohlin, G. Malhammar and H.E. Stromvall, Plastics Compounding, 1990, 13, 1, 32. 24. M. Hancock, P. Tremayne and J. Rosevear, Journal of Polymer Science: Polymer Chemistry Edition, 1980, 18, 11, 3211. 25. S. Miyata, T. Imahashi and H. Anabuki, Journal of Applied Polymer Science, 1980, 25, 3, 415. 26. B. Pukansky, E. Fekete and F. Tudos, Die Makromolekulare Chemie Macromolecular Symposia, 1989, 28, 165. 27. T.J. Hutley and M.W. Darlington, Polymer Communciations, 1985, 26, 264. 28. C. Raymond and M. Gilbert, Proceedings of Eurofillers ’99, Villeurbane, France, 1999, Communication No.16. 29. R.N. Rothon, European Rubber Journal, 1984, 166, 11, 39. 30. M. Gilbert, P. Petriraksakul and I. Mathesion, Proceedings of Eurofillers ‘99, Villeurbane, France, 1999, Communication No.5. 31. P.R. Hornsby and C.L. Watson, Journal of Materials Science, 1995, 30, 5347. 32. R.P. Petrich and J.T. Lutz Jr., in Thermoplastic Polymer Additives: Theory and Practice, Ed., J.T. Lutz, Jr., Marcel Dekker, New York, NY, USA, 1989, Chapter 10. 33. R.H. Ottewill and J.M. Tiffany, Journal of the Oil and Colour Chemists Association, 1967, 50, 844.

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Surface Modification and Surface Modifiers 34. I.A. Fujisawa, I. Aishima, J. Seki, K. Matsumoto, Y. Furusawa, R. TSukiska and Y. Takahashi, inventors; Asahi Kasei Kogyo Kabushiki Kaisha, and Shiraishi Central Laboratories Co., Ltd., assignees; US 4,242,251, 1980. 35. J. Furukawa and S. Yamashita, Nippon Gomu Kyokaishi, 1963, 36, 3, 295. 36. C.M. Liauw, V. Khunova, G.C. Lees and R.N. Rothon, Macromolecular Materials and Engineering, 2000, 279, 34. 37. R.E. Godlewski and R.P. Heggs in Thermoplastic Polymer Additives: Theory and Practice, Ed., J.T. Lutz, Jr., Marcel Dekker, New York, NY, USA, 1989, 67. 38. B. Pukansky and E. Fekete in Mineral Fillers in Thermoplastics I, Ed., J. Jancar, Advances in Polymer Science, No.139, 1999, 109. 39. J. Hutchinson and J.D. Birchall, Elastomerics, 1980, 112, 7, 17. 40. M.B. Evans, R.N. Rothon and T.A. Ryan, Plastics and Rubber Processing and Applications, 1988, 9, 215. 41. R.N. Rothon, Proceedings of the International Rubber Conference, Moscow, Russia, 1984. 42. R.N. Rothon in Controlled Interphases in Composite Materials, Ed., H. Ishida, Elsevier, New York, NY, USA, 1990, 401 43. D.P. Ashton, A.G. Glynn and R.N Rothon in Interfacial Phenomena in Composite Materials ‘91, Eds., I. Verpoest and F. Jones, Butterworth-Heinemann, Oxford, UK, 1991, 149. 44. R.N. Rothon, T.A. Ryan and P.J. Tavener, inventors; Imperial Chemical Industries, assignee; EP 0295005A2, 1988. 45. R.D. Kulkarni and E.D. Goodard, inventors; Union Carbide Corporation, assignee; US 4,374,178, 1983. 46. T. Nakatsuka, H. Kawasaki, K. Itadani and S. Yamashita, Journal of Applied Polymer Science, 1979, 24, 1985. 47. Z. Demjen, Modification of Interfacial Interaction with Trialkoxyfunctional Silane Compounds in Polypropylene – CaCO3 Composites, Technical University of Budapest, Hungary, 1997. [Ph.D. Thesis].

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Particulate-Filled Polymer Composites 81. Ken React Reference Manual, Kenrich Petrochemicals, Inc., 140 East 22nd Street, PO Box 32, Bayonne, NJ, USA. 82. S. J. Monte and G. Sugerman in Developments in Plastics Technology-2, Eds., A. Whelan and J.L. Craft, 1985, Elsevier Applied Science Publishers, Barking, UK, 87. 83. C.H.K. Cans and co-workers, Proceedings of the Fatipec Congress, Aachen, Germany, 1988, 341. 84. Technical Committee of the Cleveland Society for Coatings Technology, Journal of Coatings Technology, 1979, 51, 655, 38. 85. R.R. Landham, M.V. Parish, H.K. Bowen and P.D. Calvert, Journal of Materials Science, 1987, 22, 5, 1677. 86. C-G. Golander and B-A. Sultan, Journal of Adhesion Science and Technology, 1988, 2, 2, 125. 87. E. Tsuchiya and M. Iwatsuki, Journal of the Japanese Society for Colour Materials, 1986, 59, 11, 657. 88. H.E. Abu-Zeid, L.A. Tahseen and A.A. Anani, Colloids & Surfaces, 1985, 16, 34, 301. 89. Y.N. Sharma, R.D. Patel, I.H. Dhimmar and I.S. Bhardwaj, Journal of Applied Polymer Science, 1982, 27, 1, 97. 90. L.B. Cohen, Plastics Engineering, 1983, 39, 11, 29. 91. L.B. Cohen, Proceedings of the 39th Annual SPI Reinforced Plastics/Composites Institute Conference, New York, NY, USA, 1984, Section 19E. 92. L.B. Cohen, Proceedings of the 41st Annual SPI Reinforced Plastics/Composites Institute Conference, Atlanta, GA, USA, 1986, Section 26A. 93. M.M. Fein, B.K. Patnaik and F.K.Y. Chu, inventors; Dart Industries, assignee; US4,073,766, 1978. 94. T. Nakatsuka, H. Kawasaki, K. Itadani and S. Yamashita, Journal of Applied Polymer Science, 1982, 27, 259. 95. T. Nakatsuka, H. Kawasaki and S. Yamashita, Rubber Chemistry and Technology, 1985, 58, 107.

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5

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds Peter R. Hornsby

5.1 Introduction The term polymer compounding, as it applies to plastics and rubbers, normally describes operations undertaken once the polymer leaves the polymerisation reactor and up to the point where conversion to a finished product takes place, for example, using moulding techniques or die-forming operations based on extrusion. Frequently, compounding is associated with methods for combining polymer with additives, such as fillers, pigments or reinforcements, which may aim to influence its end-performance, appearance, processibility, or simply to reduce overall cost. However, many variations exist to this generalised description of polymer compounding technology, which may also include procedures for alloying and blending together of different polymer types, or reactive modification of polymers, to induce chain grafting, or adjust the polymer molecular weight. Materials separation processes, concerned, for example, with removal of moisture or other volatiles and monomer polymerisation, can also be considered as forms of compounding technology. Whilst many of these areas fall outside the scope of this chapter, particulate polymer composites are becoming increasingly complex and commonly require more than just inclusion of a filler or particle additive in order to achieve optimum properties. For example, rubber modification of mineral-filled thermoplastics to yield a balance of enhanced toughness and stiffness, is an area of commercial importance. In these ternaryphase systems, there is not only a requirement to attain good dispersion of the filler component, but also a need for breakdown of the rubbery inclusion to yield the most effective size and spatial location within the composition. Whilst this may depend to a large extent on characteristics of the material’s formulation, it can also be influenced by the material’s compounding route. Reactive modification of polymers is also highly relevant to the preparation of particulate-polymer composites, since the incorporation of functional groups onto an otherwise non-reactive polymer chain, can promote adhesion to the particulate phase, with attendant improvements to physical properties. Adjustments to polymer

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Particulate-Filled Polymer Composites reactivity or polarity in this way, has been exploited in filled polymer composite technology, for example, using carboxylated polybutadiene, and polypropylene functionalised with maleic anhydride (MA) or acrylic acid. This system is also used for the preparation of silicate layer nanocomposites in polypropylene and as a means for enhancing interfacial bonding in natural fibre-filled thermoplastics composites. Historically there has been a clear distinction between the objectives of compounding to achieve well-mixed polymer formulations, usually in pellet form, and conversion processes, where the compound pellets are reformed into finished products. More recently however, through developments in machinery design, combined compounding and end-forming is possible on one machine, with resulting economic and quality benefits, since the material experiences reduced overall shear and heat history. This may take the form of direct compounding extrusion into a profile such as sheet, or combined compounding and injection moulding on the same machine. Further factors, which can have an important bearing on the approach adopted to compounding, are the nature of the particulate additive and its level of addition. Thermally sensitive fillers, such as starch, natural fibres, or organic pigments, have limited heat stability, beyond which they may undergo decomposition, depending on the temperature and time of exposure experienced. Some additives are highly shear sensitive and may readily fracture in the severe environment of a high-intensity compounding machine. Such attrition is commonly encountered in the preparation of fibre-reinforced polymeric composites, but can also be evident with brittle particulate additives like hollow glass microspheres, which may collapse under high pressure, or from application of a critical shear stress. Conversely, finely divided particles, such as carbon black, tend to aggregate into larger agglomerates, sometimes with significant inter-particle attraction, requiring generation of a high shear stress during compounding to ensure effective dispersion in the polymeric phase. An increasing number of polymer formulations contain high levels of filler to meet certain property or processing requirements. Mineral fillers, for example, calcium carbonate and talc, may be added at loadings of 40% by weight to enhance stiffness and heat distortion resistance of thermoplastics. Pigment masterbatches may contain well-dispersed additives of around 60% and simulated wood products up to 80% by weight of woodflour filler. Even higher filler loadings are encountered in ceramics and metal injection moulding compositions, where additions of 80-90% by weight of the particulate species is bound within an organic binder, typically comprising a high polymer and other components, such as waxes and process aids. In these systems, after compounding and subsequent moulding of the composition, the minor organic

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Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds phase is removed in a controlled burn-off procedure, and the ceramic or metallic residue densified by sintering at high temperature. Additional demands may be placed on compounding methods when using high filler loadings, such as the need to convey large amounts of powdered species through the machinery, exacerbated if additives are heat or shear sensitive, or are especially difficult to disperse. It will be evident from the previous discussion that the design and effective operation of compounding plant to achieve optimum end-performance of a filled polymeric composition, is a matter requiring close attention. Criteria used to assess the quality of the compound produced, poses a further complication, since frequently this is non-quantitative and often highly subjective. This chapter will review current practice in the preparation of particulate polymer compositions, with emphasis on the basic processing steps demanded from polymer compounding operations in relation to feedstock characteristics, the application of these principles in the design of both batch, and continuous compounding machinery and quality assessment of the compound produced. The discussion will focus extensively on the preparation and analysis of high-viscosity filled polymer compositions, although it should be noted that particulate fillers are also widely used in low-viscosity systems, such as polyvinyl chloride (PVC) pastes and thermoset moulding compounds. The interrelationship between compounding route and material properties will also be considered using examples of recent developments, which have resulted in refinements or variations to established compounding methods and means for enhancing end product performance through modified processing technology.

5.2 Functional Characteristics of Compounding Machinery In order to prepare polymeric composites containing particulate additives to meet acceptable quality criteria, it is necessary to perform a series of well-defined and sometimes overlapping steps during the compounding operation. The precise requirements will depend on the form of polymer being processed, for example, whether it is thermoplastic, thermosetting, or elastomeric in character, together with the nature of the additive present. Further distinction can be made between batch and continuous processes and the form of ancillary equipment needed. The characteristics of these stages will be reviewed from a fundamental viewpoint and then in a subsequent section, their practical application in the design of polymercompounding machinery considered.

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Particulate-Filled Polymer Composites

5.2.1 Transport of Feedstock Feedstock characteristics can have a profound effect on the ease of handling prior to introduction into the compounder, on conveying behaviour during processing and on their tendency to agglomerate. Polymers are used in many forms, including granules of different size and shape, powders and flake, liquid resins and with many rubbery materials, crumb and bales. An equally wide variety of forms are available with particulate additives, differing greatly in shape (spherical, plates, fibres and related variants), and size (submicrometre to tens or even hundreds of micrometres in diameter). Further characteristics relevant to handling of both polymeric and particulate feedstocks are their ability to deform under pressure, their tendency to absorb water and surfaceinteractive forces promoting attraction. Particle size and shape influence packing characteristics and hence material bulk density, an important factor determining throughput, particularly in continuous compounding operations. The ability of polymeric or additive particles to compact and occupy less volume under load is also relevant to the early stages of compounding, since material may be under pressure inside a hopper and to a much greater extent during processing prior to and during melting. Compaction may occur in two broadly different ways, through a particle rearrangement mechanism and from compressibility effects, if the solid is deformable. With powder systems, including polymer additives, compaction behaviour is determined by particle size and shape, surface characteristics and the presence of adsorbed water. On compression, applied forces are transmitted through the material at the points of contact, generating an internal stress field. During the early stages, consolidation and particle rearrangement occurs, which is greatly influenced by inter-particle friction [1]. As pressures are raised and further rearrangement becomes more difficult, deformation of the particle will occur by elastic, plastic or even destructive mechanisms, depending on the magnitude and rate of application of the applied stress. For example, the compaction behaviour of a range of pigment powders has been studied using an instrumented tableting machine [2, 3]. A wide variation in elasticity was found, with the pigments tested all showing evidence of yielding and deformation beyond their elastic limit. In general, organic pigments tested were more elastic than inorganic ones. This observation has important implications on the mechanism of agglomerate formation, since, during compaction, points of surface contact will increase together with interparticle attractive forces, and supports practical experience that more elastic agglomerates, such as those formed from phthalocyanines, are usually more difficult to disperse [3]. Powder compaction is relevant to many processing industries and has been considered with other material forms such as coal [4].

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Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds As mentioned earlier, surface interaction and the extent of particle combination is closely related to compaction behaviour, together with the physical and chemical nature of the material. Pigment powders, for example, may contain three basic types of particle differing in strength and size [3]. Primary particles or crystallites (usually between 0.01 and 5 μm) can unite to form larger strongly bound aggregates. These may then cluster to produce more loosely associated agglomerates, which may be several hundreds of micrometres in size. When added to polymers, the last mentioned structural form is generally of most concern, necessitating effective dispersion procedures, which will be discussed in Section 5.2.3. Agglomeration will occur naturally in fine powders due to attractive forces existing at the surface, which vary enormously in magnitude, creating particle associations of differing strength. The principal bonding mechanisms in operation increase agglomerate strength in the following order [5]: electrostatic forces < van der Waals forces << liquid bridge forces << solid bridges. Solid bridges are formed by crystallising salts or by sintering and are generally too strong to overcome through generation of shear stresses prevalent during polymer compounding processes. The presence of even small amounts of moisture can result in the formation of strong liquid-bridge forces between fine particles, being about four times greater than van der Waals interactive forces, which in turn are an order of magnitude greater than electrostatic adhesion forces arising from contact potential. The magnitude of adhesion forces may also depend on the surface roughness of particles and, as mentioned earlier, the extent of elastic or plastic deformation induced during compaction. Estimation of the strength of agglomerates has been determined both theoretically, through consideration of the adhesion forces prevailing [5] and also by direct measurement of the mechanical strength of compacted powders [3, 6, 7]. Regular flow of feedstock into compounding machinery is an important consideration, if fluctuations of feed rate are to be avoided, and with multi-component compositions prepared from separate feed sources, if uniform concentration of the components is to be achieved. Such considerations have a bearing on the design of hoppers, for example, to avoid bridging and ensure uniform mass transport, output consistency of metering feeders, and also the early stages of movement in single-screw extruders, where Archimedean screw conveying predominates. In response to external forces, particulate solids have characteristics of solid- and liquidlike behaviour, capable of resisting flow until a limiting shear stress is exceeded, after which infinite flow will occur. Unlike liquids, however, shear stress is proportional to the normal load applied rather than the rate of deformation and, unlike solids, the magnitude of shear stress is generally indeterminate, depending on the inter-particle static coefficient

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Particulate-Filled Polymer Composites of friction. In this context, the internal cohesiveness of the powder (its tendency to agglomerate) and its coefficient of friction against a containing external surface, are important considerations, particularly in the optimum design of containers and hoppers, where gravitational flow predominates and flow disturbances, (e.g., powder bridging) must be minimised [8]. Conveying in polymer-processing equipment, including most continuous forms of compounding plant, generally results from frictional effects between polymer and metal surfaces, resulting in so-called drag flow, or through positive displacement created, typically, by intermeshing screws. The former mechanism is evident in most forms of single-screw extruders, although some important differences are apparent in extruders modified to achieve enhanced mixing capability, such as ko-kneaders. Polymer conveying in the feed zone of a single-screw extruder has been analysed with varying degrees of rigour. Full details are beyond the scope of this review, but can be found in the literature [9]. It is generally assumed, however, that material, present in the feed zone before melting occurs, exists as a solid plug capable of behaving elastically and generating pressure internally. For effective transport, a higher coefficient of friction should exist between the material and barrel than between the material and screw. Complications arise in predicting conveying performance for a given polymer type due to effects from the many variables that may influence values of frictional coefficient, including material temperature and pressure, the geometry of the screw (in particular helix angle) and the surface finish of both screw and barrel. Reduced sensitivity to frictional coefficient and increased material throughout is possible using longitudinal tapered grooves in the feed section of the barrel, which increase the relative friction of material to the barrel [9]. Further uncertainties are apparent when processing multiphase polymeric compositions, due to the possible effect of the minor phase(s) on frictional behaviour. Indeed, with many filled polymer formulations containing appreciable additive levels, material transport through a drag-flow mechanism may be seriously hindered, resulting in limited, or no material movement through the extruder. Under these circumstances the positive displacement action, which arises in many forms of intermeshing twin-screw extruder can be particularly effective in overcoming this difficulty. In such designs, material contained in the screw channels is propelled forward axially due to penetration by the adjacent screw flight. Displacement efficiency in twin-screw extruders depends on the flight geometry, the extent of conjugation (intermesh) between the screw flights (generally greatest for trapezoidal screws and channels) and the number of thread starts. Screw design and positive-displacement action will also be influenced by the direction of screw rotation,

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Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds i.e., in the same or opposite directions [10]. So-called self-wiping screw geometries have limited flight penetration into adjacent screw channels and therefore exert reduced positive transport, showing greater dependency on frictional effects and hence drag flow. Non-intermeshing twin-screw extruders generally behave more like two singlescrew extruders arranged side by side depending almost entirely on drag flow for material conveyance.

5.2.2 Melting and Shear Heating During melt compounding, uniform and rapid melting of polymer is necessary in order to maximise production rates, and when fillers are present, to ensure that the particles are effectively coated by polymer. The heat required to melt the polymer may be derived from a number of sources, in particular thermal conduction from surrounding hot metal surfaces and from viscous dissipation of mechanical energy through repeated shear deformation of the material. Reliance solely on the former, however, would result in unacceptably low rates of heating due to the low thermal conductivity of the melt. Shearing of polymers provides a much more efficient and uniform method of heating, but can sometimes result in excessive temperature generation necessitating application of cooling measures. Since dissipation of mechanical energy into heat is strongly dependent on the rheology of the polymer medium, the presence of fillers will increase viscosity and exacerbate shear heating. Generally, in continuous compounding operations based on extrusion, the early stage of melting occurs through the formation of a melt film at the heated wall of the barrel or process chamber. This heat is derived from external sources such as electrical resistance heater bands or circulating oil. As mentioned earlier, melting by conduction only, is a relatively inefficient means for heating polymers, which would result in prohibitively long production times. In extrusion-based compounding processes, the so-called mechanism of conductive melting with forced (drag-induced) melt removal has been well studied, particularly with respect to single-screw extruders and variations may be found in other compounding procedures [9]. In a common version of this mechanism, the melt film produced at the barrel wall is wiped by the traversing screw flight, causing polymer to collect and circulate at the rear of the channel. The solid bed of polymer granules or powder further forward in the channel is pressurised against the barrel surface. Extensive shear deformation of polymer occurs at the solid-melt interface generating considerable heat and making a substantial contribution to the overall melting process.

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Particulate-Filled Polymer Composites In other forms of compounding plant, such as internal mixers, dissipation of mechanical energy is normally the dominating factor determining the rate of heating, both for material in solid form, from inter-particle frictional effects and subsequently as the polymer melts [8]. The influence of fillers on melting mechanism has not been considered in detail. However, at low or modest filler levels it is unlikely to be changed much. Increased rates of heating are likely from particle attrition and increases in polymer melt viscosity. Set against this will be the additional heat required to compensate for heat absorption by the filler component depending on its specific heat capacity.

5.2.3 Mixing A requirement central to most compounding situations is intimate blending of the components, both at the micro and macro level. This is strongly influenced by machinery design and to a large degree by material formulation. Dispersive mixing involves the breakdown of agglomerates or clumps of solid particles, generally in a deforming viscous polymer melt, by forcing the mixture to pass through high shear zones generated between narrow clearances evident in polymer compounding equipment. Agglomerate rupture occurs when hydrodynamic forces exerted on the particles exceed the cohesive surface attraction within the agglomerate. As mentioned earlier, particles may be bound together by forces of physical or physiochemical origin enhanced by the presence of moisture or by applied pressure. The overall process of dispersion can be considered as a number of stages: (i) encapsulation and wetting of solids by the melt; (ii) repeated rupture of agglomerates when internal forces generated by viscous drag on the agglomerate exceed a critical threshold value; and (iii) separation and distribution of the resulting fragments in the polymer matrix, so that re-agglomeration will not occur. Various models have been proposed describing the dispersion process, which can be categorised into two general types, depending on the physical characteristics of the agglomerate and the dynamics of the flow field [11]. Models for the dispersion of cohesive agglomerates, in which the nature of the flow field is important, involve abruptly yielding rupture, where fragmentation into relatively large pieces occurs at discrete time intervals.

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Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds In slowly yielding agglomerate breakdown models, break-up of cohesionless agglomerates occurs over finite intervals, more gradual erosion taking place through detachment of small fragments from the outer surface of the agglomerate over an extended period of time. At present it is not possible to generalise on the breakdown mechanism for all particulate agglomerates. Carbon black, for example, appears to disperse predominantly by rupture, whereas titanium dioxide agglomerates are thought to gradually erode. It is useful to analyse the forces developed during the dispersion process leading to agglomerate rupture. These can be estimated by considering a single agglomerate in the form of a rigid dumb-bell consisting of two unequal beads of radii, r1 and r2, connected together and placed in a homogeneous velocity field of an incompressible Newtonian fluid [12]. A force is developed in the connector, tending to separate the beads, which depends on the dumb-bell orientation and the degree of viscous drag from the surrounding fluid. The maximum separating force (Fmax) acting between the beads in simple shear, occurs at a dumb-bell orientation of 45° to the direction of shear and is given by: ⎛ rr ⎞ Fmax = 3πηγ˙ ⎜ 1 2 ⎟ L ⎝ r1 + r2 ⎠

(5.1)

where η is the viscosity of the surrounding fluid, γ˙ the applied shear rate and L, the length of the connector joining the beads. For elongational flow, the maximum separating force occurs when the dumb-bell is aligned in the direction of flow as follows: ⎛ rr ⎞ Fmax = 6πηε˙ ⎜ 1 2 ⎟ L ⎝ r1 + r2 ⎠

(5.2)

where ε˙ is the rate of elongation. It is apparent from this analysis that it is easier to rupture combinations of larger beads than smaller ones and that dispersive mixing is favoured by conditions of high shear stress or elongational flow. In practice, however, most forms of dispersive mixer operate under conditions of high shear obtained by flow through narrow clearances. Models have also been developed to analyse the dispersibility of ellipsoidal clusters [13] and the relative effects of different flow geometries, from which biaxial extension was found to be most efficient in influencing particle rupture [14, 15]. Furthermore, although it is generally assumed that in the agglomerate rupture process the parent agglomerate produces two identical fragments, controlled experimental studies have also been

215

Particulate-Filled Polymer Composites undertaken using carbon black, which cast doubt on this assumption [11]. It has also been shown, again using carbon black and a specially constructed test rig that the number of passes through the high shear zone has a dominant role in influencing dispersive mixing [16]. Surface treatment of fillers is common to minimise particle interaction and to facilitate dispersion. Additionally, appropriate treatment will lower the filler surface energy, thereby increasing compatibility with the polymer phase. This will aid wet-out by the melt and may result in enhanced physical properties depending on the relative interaction between surface modifier, filler and polymer. It will be evident from the previous discussion that dispersion will be determined by the level of shear stress experienced by the filler particles, which in turn is dependent on process-operating conditions and above all, by compounding machinery design (Figure 5.1). In addition to breaking down particle structures into their smallest physical components, a further goal during mixing is to randomise their spatial location within the polymer. Convective mixing is a general term often used to describe this process of rearrangement from a less to a more probable order. Redistribution of solid components in a mixture

Figure 5.1 Materials and processing parameters influencing dispersion of particulate fillers in polymers

216

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds may be undertaken in low intensity compounding operations, for example, by tumble mixing, or in solid/melt combinations, where the convective mixing results from laminar shear flow. Some additives may deform, however, leading to a reduction in distance between rows of particles (or striations) and an associated increase in interfacial surface area, as mixing progresses. The total shear strain experienced by the mixture during compounding is therefore indicative of the degree of mixing. This quantity is difficult to calculate, however, except in processes where the rheology of melt flow is well understood, for example, in two-roll mills, or in flow through a single-screw melt pump. In the conventional design of a single-screw extruder, where melt mixing is relatively poor, the weighted average total strain (WATS) can be defined as the product of shear-rate and residence time for fluid elements at different channel depths, then integrated across the entire channel, namely: WATS = ( γ ) =

∫

∞ t0

γ˙tf ( t ) dt

(5.3)

where γ˙ is the shear rate, t0 is the minimum residence time in the extruder, and f(t) is the residence time distribution function. Using this analytical approach, mixing effectiveness can be determined in an extruder using different screw geometries or operating conditions [17]. It should be noted, however, that in plasticating extruders WATS defines mixing only in the melt-conveying zone of the extruder and neglects the very substantial contribution to overall mixing evident during polymer melting [18]. Distribution of components in a mixture through laminar shear mixing frequently occurs in a disordered manner, although motionless mixers are designed to blend in a more regular way through a process of repeated stream splitting and material combination.

5.2.4 Melt Devolatilisation There are several reasons why it may be important during compounding to devolatilise polymer melts containing particulate additives. Firstly, there may simply be a need to remove entrapped air introduced into the compounder together with the feedstock. In single-screw extruders this should pass back through a solid bed of partially or unfused plastic as melting progresses and escape through the hopper. However, in some singlescrew variants of more complex design and in closely intermeshing twin-screw extruders, this process may be hindered by the closed nature of the screw channels preventing escape of the air. In such cases, downstream melt venting is very common through the combination of a low-pressure decompression screw profile and an open port on the

217

Particulate-Filled Polymer Composites barrel, often aided by vacuum extraction. Under different circumstances, however, this form of screw and barrel design may serve as a convenient point for downstream filler addition into the compounder. Volatile removal may also be necessary in polymer compositions containing moisture, for example, with hygroscopic polymers and additives, such as wood flour, or where residual quantities of monomers or reactive by-products must be removed. Generally, in these situations, amounts of volatiles extracted are small, typically less than 3% by weight. Polymer melt devolatilisation has been the subject of extensive theoretical analysis [19], resulting in important practical implications for the preparation of filled polymer composites, with optimum particle wet-out and freedom from volatile matter. Hence, in order to enhance efficiency of volatile removal during compounding, it is advantageous to generate large surface areas per unit mass of material, with frequent renewal of the mass interface. In this way diffusion distances through the polymer can be minimised and steep concentration gradients achieved within the melt. Diffusion and volatile extraction is improved with increasing melt temperature, a decrease in equilibrium volatile concentration at the interface between polymer and volatile using reduced environmental pressure, and long material residence times in the devolatilisation zone. With polymer melts, devolatilisation is normally diffusion controlled. Within the bulk of the polymer melt, diffusion rate depends on a volatile concentration gradient existing towards the polymer-free surface. Through diffusional mass transport, volatiles are moved to the interface between the melt and surrounding environment. Differences in volatile concentration at this surface and in the surrounding medium, determine the rate of volatile removal by convective mass transport. Normally, however, mass transport through the melt is the ratedetermining step. Under some circumstances, creation of bubbles in the melt phase can help convect volatiles at increased rates, with subsequent release into the environment. This may be achieved by injection of a carrier substance, such as water, into polymer causing foaming and diffusion of volatiles to the melt-cell interface [20].

5.2.5 Melt Pumping and Pressurisation In most compounding procedures there is a requirement to generate sufficient melt pressure in order to convey material through defined geometrical configurations, such as extrusion dies, clearances between roll mills or specially designed mixing devices. Pressures generated

218

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds tend to be large due to the high viscosity of polymer melts, which is further increased by the presence of particulate additives, and the need to pump molten polymer through small flow channels at high production rates, thereby generating sufficiently large shear stresses to effect filler dispersion. Of particular relevance to the design of polymer-compounding plant are drag-induced flow fields caused by one or more moving external surfaces and positive displacement flow, resulting from external mechanical pressurisation [8]. The former mechanism, also termed viscous dynamic pressurisation, is applicable to the design of single-screw extruders and roll mills, for example, whereas twin-screw compounding extruders generally behave as positive melt-displacement pumps, depending on the direction of screw rotation, the clearances between the screws and the extent to which they intermesh. Detailed analysis of these pressurisation methods has been reported for various flow situations, forming a basis for design of processing machinery [21].

5.3 Constructional Design of Compounding Plant Machinery used to prepare particulate-filled polymer composites may be classified in terms of the applied shear intensity and whether the operation is of a batch or continuous nature. Furthermore, most compounding processes are critically dependent on the efficiency of ancillary equipment to undertake various functions, including additive and polymer feeding, melt filtration and pelletising procedures. The form of the product may also differ, depending on the type of polymer and components used in the formulation. Most particulate-filled polymer compounds based on thermoplastics matrices are made in pellet form, although it should be noted that granule shape may differ from a cubical to lenticular or rod-like geometry, depending on the method of pelletisation used. Some thermoplastics formulations, notably PVC-containing particulate additives, are compounded to give a so-called dry-blend powder. Compounds made from thermosetting polymers often require rather different preparation methods. For example, polyester-based dough-moulding compounds (DMC) require appropriate measures to combine solid inclusions uniformly into the viscous resin phase. Rubber mixes may leave the compounding stage as a strip or sheet, to be subsequently shaped and vulcanised into the final product form. With this wide range of material types and approaches to compounding, incorporation of particulate additives into polymers can vary greatly from one compound to another. Attention will therefore be directed only on the main types of compounding plant used industrially, highlighting their principles of design and operation specifically for the preparation of particulate-filled polymeric compositions.

219

Particulate-Filled Polymer Composites

5.3.1 Low and Medium Intensity Premixing Procedures Frequently the compounding process entails preliminary combination of polymer and additive(s) through an initial blending stage, to pre-distribute the formulation, prior to high shear mixing with polymer in a molten state. During premixing, the components are randomly interspersed with each other and levels of developed shear are generally low. Formulations include solid/solid mixtures of polymer and additive particles, where temperatures are maintained below the polymer melting point, or low-viscosity liquid/solid formulations, as is found with PVC pastes and DMC premixes. For this requirement, various forms of machinery are available, including tumble mixers, Vblenders and double-cone blenders, where powders and particulate components of a mixture are gently moved from end to end through a tumbling action as the mixer revolves (Figure 5.2). Differences in mixing efficiency may be apparent depending on the ability of the blender to divide repeatedly and combine the contents during operation. Some pre-mixers such as ribbon blenders, incorporate impellers, often with complex geometry, which rotate at speeds up to 60 rpm, transporting the components back and forward along a trough-shaped horizontal mixing chamber, as well as creating movement from top to bottom. Differences in ribbon design are possible, which can vary the intensity of end to end flow, in addition to double trough arrangements with improved cross-flow characteristics.

Figure 5.2 Low-intensity mixer designs (a) double-cone blender; (b) twin-shell (‘V’) blender; (c) conical screw mixer; (d) conical screw mixer

220

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds Conical screw mixers also contain a screw for conveying and mixing, but this is located inside an inverted conical-shaped mixing chamber. The screw is angled from the base of the mixer to the outside of the top. It may revolve on its own axis at about 60 rpm, whilst slowly orbiting the cone at around 3 rpm. Pelletised or powdered polymer, and additives are transferred from bottom to top by the rotating screw, from the walls to the centre of the chamber as the screw circulates, to fall on to material being carried upwards.

5.3.2 High-Intensity Compounding Machinery Mixers that impart high shear to the components may differ according to whether the polymer melts or not, and if the process is of a batch or continuous nature.

5.3.2.1 Non-Fluxing Mixers High-intensity non-fluxing batch mixers are frequently used in the preparation of PVC dry-blends, which may contain appreciable quantities of particulate additives and liquid plasticisers [22]. The equipment comprises a revolving impeller, capable of high rotational speeds (up to about 4000 rpm located in the bottom of a jacketed mixing chamber. During blending the components are thrown out radially by the rotor, travel up the walls of the mixer and fall down through the central zone forming a vortex. A baffle may also be introduced to create additional turbulence to this fluid-like flow. The process of producing a PVC dryblend involves the generation of heat, which can be applied externally, but arises principally from inter-particle frictional and shear effects. Mixing times and procedures, for example, the sequence of addition of the various components, depend on the formulation, but may vary from 20 s to 20 min, resulting in material temperatures up to 140 °C. Hence, to ensure rapid and uniform cooling, after blending, the batch of powdered compound is fed from a high-speed mixer to a lowintensity cooling unit, where it is gently agitated until cool. It is important to note that the particle morphology and resulting processing characteristics of the powder blend produced depend strongly on the mixing regime adopted and may have a substantial effect on material flow and gelation during subsequent melt conversion by extrusion or injection moulding [23]. High-speed mixers can also be an extremely effective means of pre-blending powder forms of polymer (other than PVC), with particulate additives, such as pigments or mineral fillers. The action is essentially one of inter-particle randomisation. It is questionable

221

Particulate-Filled Polymer Composites whether high-speed mixing in this manner aids the dispersion of all but loosely aggregated particle structures, and may even contribute to agglomerate formation by compacting particles together, particularly near the chamber walls and mixing impeller [24]. Mixers of this type provide a convenient method for surface application of treatments to fillers, in order to modify their interfacial properties. Since filler is in a fluidised state and may be cold or heated, surface treatment can be introduced through an opening on the mixer lid to achieve rapid and uniform coating. Special forms of high speed mixer have been developed which produce agglomerated mixtures of thermoplastics and fillers in compacted form, thereby enhancing feeding capability into extruders, reducing dust and moisture content associated with hygroscopic materials, such as organic fillers [25].

5.3.2.2 Two-Roll Mills Two-roll milling is a well-established and widely practised melt-mixing process, which can be used for blending particulate additives into rubbers, thermoplastics and thermosetting polymer compositions. Although most frequently encountered in laboratories, due to the highly effective mixing action created, it is still used in some large-scale industrial plants. The machine comprises two contra-rotating rolls, which may be heated or cooled, and can often run at differential speeds. Softened polymer adheres to one of the rolls, forming a continuous band passing through an adjustable gap between the rolls. In this region it experiences high levels of localised shear, which, together with manual (or sometimes automated) lateral mixing across the rolls, provides a highly effective means for both dispersion and distribution of additives into the polymer. Additional, but relatively low levels of laminar shear occurs within a rolling bank of excess polymer situated on the upstream side of the rolls. The estimation of shear imposed on fluid elements passing through the nip of a two-roll mixer can be made for power-law fluids, such as polymer melts, which have highly shear-dependent viscosities. The reader is referred elsewhere for details [21].

5.3.2.3 Internal Mixers High-intensity internal mixers are used for incorporating particulate additives into most polymer types, including thermoplastics, thermosets and rubbers. Limitations, such as the manual intervention generally encountered with two-roll mills, can be overcome, whilst maintaining a significant dispersive and distributive mixing capability.

222

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds Batch designs of internal mixer comprise a temperature-controlled mixing chamber containing two fluted mixing rotors, which commonly revolve in opposite directions and sometimes at different speeds. Materials introduced into the chamber are pressurised using a floating ram, which is lowered onto the feedstock, causing intensive mixing within the chamber. This action is strongly dependent on features of the rotor design, such as clearances between the chamber wall and helical projections on the rotors, together with prevailing operating conditions, notably temperature, rotor speed and ram pressure. The mechanism of mixing is complex, since material is sheared both at the walls of the mixing chamber and between the rotors, in addition to being repeatedly transferred from one side of the chamber to the other. Considerable levels of frictional heat may be generated during the mixing cycle, arising from viscous dissipation of mechanical energy from the rotor drive, as the material is sheared. The widespread use of internal mixers for polymer compounding over many decades, has resulted in many refinements to mixer, and particularly rotor design, together with optimisation of mixing procedures. Despite the wealth of practical experience gained from the application of internal mixers to a wide range of polymer compounding tasks, it is only in recent years that more fundamental studies have been undertaken to analyse and model their mixing action. These include flow visualisation experiments to monitor different flow fields and levels of mixing, created by various rotor designs [26], and process parameters [27], such as mixing times and thermal boundary conditions. Several workers have applied finite element analysis techniques to predict flow behaviour of polymers and in particular their mixing efficiency [28-30]. For example, using a fluid-dynamics package, a threedimensional analysis has been carried out for the whole mixing chamber, providing information on the effects of processing conditions and design parameters on velocity profiles, pressure contours, the development of shear stress and elongational flow behaviour [31]. On completion of the mixing cycle, compound is discharged from the underside of the mixing chamber, often on to a two-roll mill to undergo further homogenisation, or into a pump-extruder equipped with a pelletising facility. Continuous forms of internal mixer are also available, in which polymer and additive feedstock are continuously starve-fed on to twin counter-rotating screws, before progressing into a mixing zone, housing two rotors, similar in design to those used in batch-internal mixers described previously [32]. Compound moves forward and discharges continuously through an adjustable orifice gate, or into an extruder pelletiser. Mixing intensity can be varied through control of temperature, rotor speed, feed rate and discharge orifice opening. Of particular relevance to the preparation of polymer compounds

223

Particulate-Filled Polymer Composites containing heat- or shear-sensitive particulate fillers, is the availability of special continuous mixer designs with provision for the introduction of the additive component after the melt has been formed.

5.3.2.4 Extrusion Compounding Continuous forms of compounding plant engineered for the preparation of particulatefilled polymer composites are frequently based on variants of the extrusion machine, modified to impart increased mixing capability. The application of single-screw extruders for conversion of plastics and rubbers into finished products by die forming is widespread, and the reader is referred to specialist texts, which consider their design, operation and fundamental analysis [9, 21]. As mentioned earlier, their use as mixing devices is less successful, due to an inability of the basic helical screw form to generate sufficiently high levels of shear strain or localised shear stress within the polymer melt. Hence, single-screw extruders (and single-screw reciprocating injection moulding machines) are frequently fed with pre-compounded material, or pre-dispersed additive masterbatch. Common means of enhancing the mixing efficiency of single-screw extruders include use of modified screw forms containing special mixing devices, or static mixers, which are located between the extruder and die. The various forms of mixing screw design available may function in either a dispersive and/or distributive manner. The latter type frequently operate in the melt phase to randomise the position of the minor component, by interrupting the laminar flow path of the polymer melt, causing repeated stream splitting and recombination of fluid elements, and generating substantially increased shear strain in the melt. Static mixers are normally introduced between the extruder and die, containing helical or other forms of element design, which divide and reunite flow streams in a controlled manner. A further alternative is the cavity transfer distributive mixer, which comprises a rotor with hemispherical depressions located within a closefitting stator, also configured with hemispheres. Polymer melt continuously transfers between rotor and stator cavities, and experiences substantially increased shear strain, resulting in increased mixing. Screw designs created to improve dispersive mixing impose the polymer melt and additive components, to high levels of shear stress by intensive flow through narrow clearances, often located between the screw tips and barrel wall. Variants using this principle include Union Carbide and Egan mixers. These, and other forms of mixer design for singlescrew extruders, have been reviewed [33].

224

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds The inadequacies of single-screw extruders, such as melt compounders, are overcome in the ko-kneader [33, 34]. This incorporates a single screw with gaps between the screw flights contained within a barrel, having rows of teeth projecting from and running axially along its length. The screw can both rotate and reciprocate, such that the teeth on the barrel pass forwards and backwards between the gaps on the screw. The rotation and axial oscillation of the kneading screw produces a relative motion between kneading teeth and screw flights providing combined axial and radial mixing. Such movement also creates a self-wiping action against the barrel and screw surfaces, and generates a positive pumping movement on the material, reducing its reliance on frictional and drag flow as the primary means of material conveyance. This unique operating concept has established ko-kneader mixers as a widely accepted means of preparing polymer composites containing many different forms of particulate additive, including masterbatch formulations with high levels of pigments or mineral fillers. Many other specialised forms of single-screw compounding extruder have been commercialised, such as Transfermix, EVK and pin-barrel extruders [9]. The design of the latter has some similarities to the ko-kneader, in that adjustable pins protrude from the barrel into the screw channel, which contains slots at various pin locations. During rotation of the screw (which does not reciprocate), the pins pass through the gaps on the screw flight enhancing mixing capability and providing good melt temperature uniformity. This is found to be particularly useful for rubber applications. An increasing number of compounding tasks are being undertaken on twin-screw extruders, due to their continuous operation at high throughput rates, their highly effective mixing action and provision for interchangeable screw and barrel geometries. This enables machine configuration and process optimisation for a wide variety of compounding tasks, such as preparation of thermoset powder-coating compounds, heat-sensitive flame retardant formulations, or very heavily loaded ceramic- or metal-injection moulding compositions. Versatility in screw and barrel assembly also facilitates application in multifunctional compounding situations, for example, requiring downstream or split feeding of particulate additives, introduction of liquid reactants into the melt and melt devolatilisation. Extremes in mixing intensity are readily achievable, through the use of various forms of mixing elements and helical screw types. In view of the complexity of these systems and the numerous design permutations available, it is not easy to generalise on their mode of operation. However, several indepth accounts have been published, which detail their design principles and theoretical treatment [35, 36]. An overview of pertinent features is presented below. In classifying twin-screw extruders, distinction must first be made between the direction of screw rotation (co- or counter-) within a figure-of-eight screw barrel, whether or not the

225

Particulate-Filled Polymer Composites

Figure 5.3 Screw configurations used in twin-screw extruders

barrel is conical (as can be found in some counter-rotating designs) and the extent to which the screws intermesh, if at all. Further differences can be found in the form of screw flight. Co-rotating designs, for example, may have rounded or so-called self-wiping screw flights, which have large channel clearances across the intermeshing zone, or trapezoidal-shaped channels and flights with more restricted interchannel flow (Figure 5.3). This can have important implications with regard to their positive conveying efficiency and the nature of mixing between the screws. Trapezoidal-shaped screws are generally more effective in both these respects. Furthermore, with these designs, controlled material stream splitting and combination can occur across the intermeshing zone [37]. Industrially, co-rotating, rather than counter-rotating intermeshing twin-screw extruders are widely, but not exclusively, used for compounding a wide variety of polymer-based formulations to specified quality standards. Counter-rotating twin-screw extruders with tightly intermeshing screws have an established role in the extrusion of unplasticised PVC dry-blends into pipe or other profile, on account of their exceptional conveying efficiency using powdered feedstock and close temperature control. In conical form these designs are ideally suited to the preparation of highly loaded woodflour-filled thermoplastics composites, as discussed in Section 5.4. Non-intermeshing counter-rotating designs have limited positive displacement action but, due to effective surface renewal of melt, are especially helpful for efficient polymer devolatilisation.

226

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds An essential feature in most co-rotating twin-screw extruders used for preparing particulate-filled polymer compounds, is the incorporation of special mixing elements and screw designs, which enhance mixing or melting capability, depending on their location in the screw profile. These frequently take the form of reverse-flighted screws, segmented discs and, very frequently, bilobal or trilobal kneading elements. An alternative approach to the same end is to use so-called barrel valves, which regulate material flow through clearances of variable size. With kneading elements, shear and mixing intensity is determined by the frequency of lobes (two or three), the width and number of elements, their relative configuration, (i.e., degree of stagger in a forward or reverse helical pattern), and the clearances existing between the element tips and barrel wall. The action of kneading discs has been analysed theoretically demonstrating the influence of element design and configuration on mixture quality [35]. This treatment is particularly relevant to the blending of particulate fillers into polymers and can provide useful pointers to the optimisation of kneading-element assembly. Under some circumstances, for instance during melt compounding of PVC formulations, it can be beneficial to separate mixing and pumping functions of the extruder to minimise shear heat input. This can be achieved by mixing material on a co-rotating twin-screw extruder, before delivering the compound under low pressure into a cross-head singlescrew extruder running at low-speed, to pressurise material gently through a strand die for pelletisation.

5.3.2.5 Ancillary Equipment Ancillary equipment has an essential role to play in the preparation of filled polymer compounds with reproducible quality. Firstly, delivery of the polymer plus additives into the compounder must be accurately controlled. Often this involves weighing and premixing the ingredients, or sequential addition in batch processes, such as internal mixing. Most twin-screw compounding extruders are starve-fed, where individual components of the compound may be metered independently using volumetric or weigh-feeding devices. The former rely on the use of dosing screws, hence output is highly sensitive to feedstock bulk density. Weigh belt feeders deliver material on a mass basis and are therefore capable of a much higher degree of accuracy. Loss-in-weight gravimetric feeders also use a weighbridge principle but control the rate of weight change of the feeder hopper plus its contents, relative to previously set parameters. Melt filtration is commonly used during continuous melt compounding operations to enhance product quality, by reducing adverse effects from contaminants or unwanted

227

Particulate-Filled Polymer Composites material residues, which may be detrimental to visual appearance or adversely influence material end-performance. This includes, for example, screening of polymer gel particles, degraded polymer and poorly dispersed additive particles, such as pigment agglomerates. This aspect will be discussed again (see Section 5.5.1) in relation to the effects of compound quality on physical properties. Many different melt filtration systems are available commercially ranging from simple screen pack and breaker plate assemblies, which require the process line to be halted every time the screen is replaced, to more sophisticated continuous screen changers, where contaminated filter mesh is automatically moved away from the melt flow path when the pressure drop across the filter exceeds a set level. Particulate-filled polymer compounds generally leave the compounding stage in granular or pellet form. Several routes are available for the conversion of polymer compound from melt to solidified granule, for example, using a dicer for material produced as sheet or strip, a pelletiser for compound made into continuous strands then cooled, or a dieface cutter, where melt extrudate is cut by rapidly rotating knives at the die exit, then cooled in circulating air or water. Die-face cutting is a particularly effective means of compound pelletisation, when processing polymer compositions of limited melt strength, (e.g., at significant filler loadings), and when throughput rates are very high, causing possible difficulties in strand extrusion stability, as may be the case with large twinscrew extrusion compounding lines.

5.4 Characterisation of Filled Compounds 5.4.1 Introduction To obtain optimum additive efficiency in polymers, it is necessary to compound the materials in the most appropriate manner to achieve defined property requirements, depending on the end-application for the material. Thus for example, a pigmented compound may only need to satisfy arbitrary and often non-quantifiable visual criteria, perhaps in terms of colour uniformity and the absence of poorly dispersed additive, evident as streaks or regions of high pigment concentration. However, electrically conducting compounds made from polymers filled with conductive additives, such as forms of carbon black, must achieve a much more stringent level of quality performance, defined in terms of the required electrical properties. Polymers containing flame-retardant fillers are formulated to pass specified testing procedures, which rate the material in terms of its ability to inhibit combustion. Frequently, fillers such as hard minerals or soft rubbers are combined with polymers to modify mechanical properties, such as elastic modulus or resistance to crack propagation

228

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds under impact loads. In these circumstances, the quality of the compound is assessed by determination of specific mechanical properties. It is evident, therefore, that necessary property measurements depend on the functional role of the additive, being strongly influenced by the method of combination in the polymer and, in particular, by the form of microstructure produced. Of great relevance to filled polymer formulations, is the uniformity of particle distribution, the degree of dispersion achieved and the interaction between the filler and matrix phases. Other influencing factors include the presence of micro-porosity, reductions in the molecular mass of the polymer, and changes to the form and level of crystallinity in semi-crystalline thermoplastics. It can be beneficial to consider the measurement of quality in filled polymers in terms of characterising these structural parameters, since, as will be considered later, often an identifiable relationship exists with the compounding route used. Measurement of mixture uniformity and, in particular, the level of filler dispersion is generally considered of utmost importance. To this end, information can be obtained from analysing the operating performance of compounding machinery through measurement of residence time distribution, machine power input and pressure development. Additionally, rheological analysis may be undertaken on filled polymer melts, or solid specimens may be characterised using a variety of microstructural identification techniques. There is an increasing interest in the development of quality assessment techniques at the compounding stage, either in-line with the compound produced, or on-line, through discontinuous measurement of diverted process flow stream. Direct and indirect approaches more commonly used for determination of filler compound quality are summarised in Table 5.1, together with pertinent measured parameters and influencing factors. Further discussion of characterisation procedures follows in Section 5.4.2.

5.4.2 Residence Time Distribution Polymer feedstock and modifying additives entering continuous compounding machinery at the same time will generally exit it at different times, depending on the extent of longitudinal mixing experienced. This can be quantified using an appropriate tracer, which is added to the feed stream (generally as a pulse) then measuring its concentration in the output as a function of time [38, 39]. Subsequently, a residence time distribution curve can be constructed, providing a record of the heat and shear history of material passing through the compounder. This will be a function of the material composition, the operating conditions used and, most importantly, the design of the compounder, in particular clearances between screw flights in intermeshing twin-screw extruders, or the

229

Particulate-Filled Polymer Composites

Table 5.1 Methods for the characterisation of filled polymer compounds Characterisation method

Measured parameter

Ashing Dissolution

Filler concentration

Ultrasonics Screen pack analysis (pressure filter test)

Filler dispersion

Light/electron microscopy

Filler dispersion/distribution

X-ray radiography

Wet-out

Image analysis

Interfacial bonding (qualitative) Filler dispersion

Rheometry (capillary/dynamic/meltflow index) Gel-permeation chromatography Solution viscometry Infrared spectroscopy

Polymer molecular mass changes Effects of surface treatment on melt-flow behaviour Molecular mass changes Chemical effects from polymer degradation

Physical property measurements Filler dispersion/distribution (mechanical, electrical, optical, colour, etc).

configuration of mixing elements present. Optimisation of residence time is most important when processing heat-sensitive filler compositions or in reactive formulations, where chemical activity can be influenced by the uniformity of heat exposure. The residence time distribution of polypropylene-containing barium sulfate has been determined through a co-rotating twin-screw extruder [40]. Filler was added as a pulse into the feed stream, and its concentration in the extrudate determined by ashing and weighing material collected at different time intervals. Results were expressed as an internal age distribution (or 1-F(θ)) function representing the quantity of filler remaining in the compounder at increasing times, actual time being normalised against the mean residence time (θ). More accurate determination of residence time was achieved by neutron activation analysis using a radioactive tracer. Residence time distribution measurements not only generate information about the processing history of a polymer, but have also been used to characterise the conveying and self-cleaning efficiency of compounding machinery.

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Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds

Table 5.2 Specific energy consumption values for pigment concentrates produced on co-rotating intermeshing twin-screw extruders (adapted from [42]) Specific energy consumption (kWh kg-1)

Polymer

Pigment*

LD polyethylene

75% TiO2 + 12.5% wax

0.11

LD polyethylene

60% TiO2

0.17

LD polyethylene

40% carbon black

0.31

Polypropylene

30% organic blue pigment + 10% TiO2

0.70

Polypropylene

20% inorganic yellow pigment

0.80

Polystyrene

50% TiO2 + lubricant

0.10

Polyester

20% organic blue pigment

0.18

Polyamide 6

35% TiO2

0.22

*Additive additions in % by weight.

5.4.3 Specific Energy Input Specific energy consumption may be expressed as the ratio of the required power input to the effective material throughput and is expressed in kWh.kg-1 [41]. Since mixture uniformity is sensitive to specific energy input, measurement of this parameter can provide an indirect indication of overall product quality for a particular compounding process. With filled polymer formulations, specific energy input depends on the melt rheology during compounding, in addition to machinery design and operating conditions, such as temperature, screw, rotor speed, and degree of fill. Table 5.2 lists specific energy input values for various pigment masterbatch formulations prepared on co-rotating intermeshing twin-screw extruders.

5.4.4 Screen Pack Analysis A direct approach for the assessment of additive dispersion, is to analyse pressure development in polymer melt flowing through a screen pack of defined restriction. This so-called pressure filter test has been specifically applied to pigment-containing compositions, where a premix of pigment masterbatch and virgin polymer (with total pigment content of 4% by weight) is melted in an extruder then metered through a screen pack using a gear pump. Pressure developed in front of the screen is monitored

231

Particulate-Filled Polymer Composites and plotted as a function of time. The test is completed either when a pressure of 12 MPa is reached or after 90 minutes [42]. A pressure filter value (DF) can be calculated from: DF =

(Pmax − P0 )F × 100 tKG

(Pa cm

2

g −1

)

(5.4)

where Pmax is the final pressure (Pa), P0 the pressure using unpigmented polymer (Pa), F the screen area (cm2), t the measuring time (min), K the pigment concentration (%) and G the mass throughput (g. mm-1). A low value of DF is indicative of good dispersion. A similar approach has also been considered for on-line measurement of dispersion in a 40% by weight-pigment masterbatch, by analysing the side-stream output from a corotating twin-screw extruder [43]. Although emphasis has been given to the use of this technique in analysing pigment dispersion, it may also be suitable for use with other filled polymer compositions.

5.4.5 Rheological Analysis Rheological data can provide useful information about the influence of filler type, content and surface treatment on the overall melt viscosity of polymer containing particulate additive. Many reports exist describing these effects [44-46]. At high shear rates, changes in shear viscosity may correlate with variations in filler content. However, often these become most evident at very low shear rates, as do apparent differences in the level of structure formation within the filled composition. Low shear viscosity is conveniently undertaken using a parallel plate rheometer oscillating at low angular frequencies. When applied to a study of polypropylene containing calcium carbonate fillers, using particles of 4 μm mean diameter, dynamic storage modulus (G´) was found to increase with filler loading up to 30% by weight, although little difference was seen in the value of G´ at low frequencies [47]. However, with finer (0.25 μm mean diameter) calcium carbonate particles at a filler loading of 30% by weight, a noticeable increase in G´ was observed, which was attributed to increased filler agglomeration in this composition. Similarly, with polypropylene containing different forms of magnesium hydroxide filler, but at the same level of addition, their behaviour at low shear amplitudes can be distinguished, resulting from the relative interaction between the particles (Figure 5.4). Beyond a critical level of shear, dependent on the filler type and whether or not the filler is surface treated, structural changes occur, which are attributed to reduced particle agglomeration. On-line rheometers are also available, suitable for use with continuous compounding lines, such as co-rotating twin-screw extrusion processes. Exploratory work has been

232

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds

Figure 5.4 The relationship between dynamic storage modulus and angular frequency for polypropylene containing uncoated magnesium hydroxide fillers (60% by weight filler)

reported using this approach with thermoplastics pigment concentrates where on-line rheological measurements have demonstrated that changes in additive concentration can be accurately determined [43].

5.4.6 Ultrasonic Measurement Ultrasonic methods have been considered for non-destructive evaluation of compositional changes and mixture uniformity in filled polymers. In principle, it is feasible to determine the size, shape and distribution of filler particles, to detect agglomeration and to assess the extent of filler-matrix interaction, through appropriate application of ultrasonic procedures [48]. The method involves determination of the elastic behaviour of solids by measurement of ultrasonic wave velocity. Propagation of a plane wave in a linear elastic material can be related to its elastic modulus (E) and density (ρ) according to: ⎛E⎞ V=⎜ ⎟ ⎝ ρ⎠

12

(5.5)

where V is the ultrasonic wave velocity. It also follows that the wave transit time through the material (ts) can be determined from:

233

Particulate-Filled Polymer Composites

ts =

ls V

(5.6)

with ls , being the transit distance. Off-line ultrasonic studies have been undertaken on calcium-carbonate-filled polypropylene injection mouldings at a frequency of 5 MHz [49]. Over filler levels between 0% and 40% by volume, correlation was observed between the compression velocity of the ultrasound and filler loading (Figure 5.5). The non-linear behaviour shown was attributed to the relative effects of calcium carbonate filler on the density and elastic properties of the composite. Compressional wave velocity appears to be influenced more by density changes at low concentrations and by changes to elastic properties above a threshold concentration value [50]. From this study it was also concluded, using ultrasound and microfocus radiography, that the filler particles were uniformly dispersed, mostly with dimensions much less than λ/4 (λ being the wavelength of the compressional waves), i.e., agglomerates or voids above 100 μm were largely absent. Earlier attempts to apply ultrasonic wave-velocity measurements to in-line determination of filler concentration during compounding met with mixed success, however. In one study [51], measurements were undertaken by measuring ultrasonic transit times across

Figure 5.5 The influence of calcium carbonate filler loading on ultrasonic compressional velocity through polypropylene injection mouldings

234

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds polymer melt passing through a rod-die. In contrast to off-line measurements on solid materials, fluctuations in ultrasonic velocity increased substantially with increasing filler concentration. Inaccuracies in the in-line results were attributed to two causes, instrumentand process-related errors. These included limitations due to triggering of the electronic timing circuit of the particular instrument used and a lack of resolution of real transit time measurements. Further contributing factors to the lack of reproducibility were the sensitivity of ultrasonic transit time measurements to changes in melt temperature and pressure (no attempt was made to compensate for such fluctuations) and real changes in filler concentration observed, particularly at higher filler loadings. In another study, in-line measurements of polyethylene melt containing 20% by weight of glass beads of different sizes, were undertaken. Focused ultrasound was used to detect the presence of particles with diameter greater than 50 μm [52]. More recently, using magnesium hydroxide filled low density polyethylene (LDPE) and high density polyethylene (HDPE) polymer melts, in-line ultrasonic measurements have been made to determine the effect of melt temperature, pressure and filler concentration on ultrasonic velocity through the melt [53]. Tests were also carried out on static samples of melt under conditions of no flow and used in combination with extrusion processing data to predict filler concentration. It was found that ultrasonic velocity was most sensitive to changes in temperature, least affected by pressure and decreased with increasing filler concentration.

5.4.7 Microstructural Analysis The most common approach to the assessment of quality in particulate-filled polymer composites is by direct observation of microstructure, using a variety of analytical techniques, including light, electron-optic and X-ray methods. For example, BS2782 (Methods 823A and 823B) [54] describes the determination of carbon-black dispersion in polyethylene using transmitted light microscopy, however, the procedure is highly subjective. The choice of techniques and their application is complicated by the aims of the analysis and whether or not this is quantitative. Scanning electron microscopy (SEM), for example, is widely applied to assess the extent of interfacial adhesion in filled polymer composites, usually through examination of fracture surfaces. However, the information obtained is generally non-quantitative, although estimates of interfacial shear strength are possible from fibre pull-out measurements in short-fibre reinforced polymer composites [55]. In semi-crystalline thermoplastics, it is well known that particulate additives can strongly influence polymer morphology and hence physical properties [56]. Indeed, some particulate additives, such as sodium benzoate, are deliberately added to polymers to improve mechanical properties or clarity, through controlled nucleation of the melt during cooling.

235

Particulate-Filled Polymer Composites Morphology of the polymer at and away from the surface of the additive particles, is conveniently determined by polarised light microscopy and, where higher resolution is required, by electron microscopy. More detailed consideration of the effects of particulate additives on polymer crystallinity is presented elsewhere in this book. Since the state of additive distribution and dispersion is generally of prime concern in the preparation of filled polymer compositions, attention will be focused on this aspect of characterisation. An initial requirement is to select a portion of the material that is representative of the sample as a whole. Macroscopic examination, perhaps at low magnification, can establish whether unusual features, such as voids or regions depleted in additive, are present. Of crucial importance then is the method of specimen preparation in relation to the chosen characterisation technique, in order to highlight contrast between the phases present. This requires specialised expertise, for example using staining or selective etching procedures to achieve satisfactory results, depending on the components present in the system. Examples are presented below, to illustrate some possibilities available for the characterisation of dispersion in filled polymer composites, with specific reference to polypropylene containing 40% by weight of calcium carbonate filler [57]. Specimen preparation techniques include isolation of the filler through removal of the polymer by ashing or dissolution. However, this introduces the probability of changes in particle structure. Microtomy (at room or preferably at sub-ambient temperatures), pressing at elevated temperature, or controlled deformation of thin sheet using compressed air (bubble blowing), are all possible methods for producing thin specimens (5-10 μm thick), suitable for examination by transmission techniques. Frequently the former approach is the preferred option, although with materials containing high levels of poorly bonded filler, particle pull-out is a common problem. Specimen polishing, for example, using successively finer grades of diamond paste, provides an alternative preparation method for surface examination. Filler particles may be subsequently etched, however, this can complicate interpretation of the image produced, since air voids contained in the structure may be indistinguishable from extracted filler. From the array of microstructural analytical methods available, light microscopy may be conveniently applied in transmission or reflection modes to identify the presence of particle agglomerates. In transmission, errors due to particle overlap through the specimen thickness must also be considered. SEM and transmission electron microscopy (TEM) give a much greater degree of resolution, which is useful when working with very finely divided fillers, or when details of agglomerate structure are required, e.g.,

236

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds with pigments. When combined with elemental analysis, the compositional identity of unusual features or structures can be confirmed. X-ray microradiography has been widely applied to assess orientation distributions in short glass fibre-reinforced thermoplastic mouldings [58, 59], titanium dioxide filler dispersion in polymers [60] and is equally applicable to the study of mixture quality in other particulate-filled thermoplastics compositions, including the presence of calcium carbonate. It should be appreciated from this overview of microstructural characterisation methods, that many variations of these techniques exist, which can be applied to enhance additive contrast or yield greater information on structural hierarchy. Whilst the qualitative analysis of filler dispersion in polymer composites poses its own difficulties, quantitative evaluation of mixing in these systems creates further challenges. Firstly, to establish the spatial location or size distribution of the additive, a statistically representative number of particles must be examined, preferably from various fields of view within the specimen. Providing there is sufficient contrast between the phases, as is discussed later, automatic image analysis techniques can be applied to rapidly assimilate and process data. Secondly, additive particles frequently have an irregular geometry and may also be exposed in a two-dimensional array at sections other than their mid-point, (i.e., only the tips of the particles may be on view). Thirdly, there is the question of how to define mixing and express this numerically. With reference to the last mentioned point, extensive literature exists on the theoretical representation of mixture quality [38]. With polymer-based systems containing particulate fillers, diffusive mixing is generally absent and it is necessary to consider the relative positioning of the components, together with their clumpiness in the mixture. The intensity of segregation defines the extent to which the composition at each point differs from the average concentration in the mixture and may be determined through measurement of the variance:

S2 =

1 N −1

N

∑ (C i =1

i

−C

)

2

(5.7)

where

C=

1 N

N

∑C i =1

i

(5.8)

i.e., from a randomly sampled mixture, N measurements of concentration Ci are made, giving C as the average concentration.

237

Particulate-Filled Polymer Composites Intensity of mixing (I) is then represented as: I=

S2

(5.9)

S20

where S02 is the variance for total segregation of the components, given by:

(

S20 = C 1 − C

)

(5.10)

Scale of segregation also has statistical origins and relates to the size of clumps in a mixture. It is based on the idea of correlation of concentrations between nearby points in a mixture. The correlation coefficient R(r) can be determined as a function of r (a chosen distance between pairs of points in the mixture), namely: 1

R( r ) =

S2 N

N

∑ ( C ( x ) − C )( C ( x + r ) − C ) i

i

(5.11)

i= 1

The graphical representation of the relationship between R(r) and r is termed a correlogram and the linear scale of segregation (SL) representing the average size of the clumps, defined as the area under the correlogram: SL =

∞

∫ R(r ) dr 0

(5.12)

A related volume scale of segregation can also be written. Polymer mixtures containing deformable components tend to contain streaky or layered structures resulting from laminar shear flow. In such cases, the striation thickness (the distance between the streaks) can be measured to provide an indication of degree of mixing. As mixing progresses, a decrease in striation thickness is accompanied by an increase in interfacial area between the components. Reductions in striation thickness will depend on the level of imposed shear strain and the orientation of the components relative to the direction of shear [38]. Returning to the quantitative analysis of mixture quality using image analysis, a digital representation of the mixture can be obtained by direct attachment of a video camera to a microscope containing the sample under investigation or, if necessary, by viewing previously prepared micrographs of the specimen. The camera signal is transferred to a video digitiser, which divides the picture into many thousands of small rectangular units

238

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds (called pixels) and numerically measures the light intensity in each area, according to a defined scale of grey levels. The image can then be processed by relating the light intensity at each pixel to the mixture composition at that position. The digital image obtained is stored in a computer and may be conveniently analysed using appropriate software to generate numerical quantities, which describe the quality of the mixture. The technique can be applied to the study of mixing (and specifically the degree of dispersion) in polymer compositions containing particulate fillers. For example, reflected light images of polypropylene containing 40% by weight of calcium carbonate filler have been examined to assess the level of additive dispersion [57]. To overcome the difficulties mentioned earlier of irregular particle shapes, recorded particle areas obtained from the raw count data were converted to equivalent spherical areas (expressed as the number of particles per unit area) and plotted as a function of particle diameter. These data were transformed into the number of particles per unit volume and accounting for sectioning errors by applying the Schwartz-Saltykov diameter analysis, which assumes that the view in two dimensions is statistically repeated in the third dimension [61]. Since it was the small number of larger particles and agglomerated structures, which were of interest in this study, results were then expressed as a distribution of the volume of particles per unit volume. A mean volume diameter from this distribution can provide a single number to describe average level of dispersion. Analysis of the data can also provide information about concentration fluctuations of the minor component within and between different fields of view, and hence the variance of the mixture. Other approaches have been reported for mixture analysis of digital images including the use of fast Fourier transform algorithms as a means of determining correlation coefficients [62].

5.4.8 Miscellaneous Methods of Analysis A variety of other analytical techniques have been considered for the assessment of mixture quality in filled polymer compositions following compounding, in both off-line and inline situations. It has been suggested, for example, that nuclear magnetic resonance (NMR) imaging could provide a non-destructive three-dimensional technique, which might be used to determine the spatial distribution, chemical nature and physical characteristics of components in highly filled polymer mixtures [63]. Using high loadings (60-70% by volume) of ammonium sulfate powder in an acrylonitrile-terminated polybutadiene matrix, proton NMR images were obtained, which together with measurements of the relative incidence of grey levels within the image, provided a means for assessing the spatial location and degree of dispersion of the additive phase.

239

Particulate-Filled Polymer Composites Radiometric density measurements using specially developed sensors have been proposed for on-line monitoring of pigment or filler additive levels, which together with feedback to additive and polymer dosing units, could enable accurate control of additive concentrations [64]. The technique does not provide spatially resolved information, however, and may be less applicable when blending together several components of similar densities. In-line Fourier transform infrared spectrophotometers can provide both qualitative and quantitative information about additives during continuous processing, in addition to determining possible structural transformations occurring. This may be particularly useful during reactive compounding operations, for example, to assess degree of grafting of vinyl methoxysilane to polyethylene chains [64]. Details of an on-line procedure for colour control during compounding have been given [65]. Pigmented pellets from a twin-screw extrusion compounding line are diverted to an adjacent injection moulding machine to prepare test plaques. These are transported automatically by robot to a spectral photometer, where colour intensity is measured and, if necessary, to maintain consistent quality, subsequent automatic adjustments are made to the rate of colourant addition at the compounder. The whole system is centrally computer controlled. Instrumentation for direct on-line analysis of particulates in polymers is in commercial use, for detection and sizing of additive agglomerates and impurities, such as polymer gels [66]. Intense light is transmitted through a stream of polymer melt and, using a shadowgraph principle, particle images detected on the other side. These are then analysed by a real-time high-speed computer, enabling the particles to be counted and classified according to their size range. Particles as low as 5 μm in size can be resolved by this method.

5.5 Process Enhancement of Particulate Polymer Composites The interrelationship between compounding route, compound micro-structure and properties is less well documented, due in part to a limited understanding of what is happening inside the compounder and, in many instances, to incomplete characterisation of the processed materials. With particulate-filled polymer compositions, however, the effects of poor compounding practice can be profound. For example, mechanical properties, such as fracture toughness and strain to failure under tensile deformation, are very sensitive to the state of filler dispersion and the size of additive inclusions. The aesthetic qualities of pigmented polymers are also dependent on the level of mixing experienced, which may result in specks or streaks in unfavourable cases. Electrical conduction pathways in polymer compounds containing conductive fillers, such as carbon

240

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds black or metal powders, critically depend on the relative association of the additive particles and their level of dispersion. The effectiveness of functional fillers, such as flame retardants, heat and light stabilisers, can be markedly enhanced through optimisation of mixing procedures during compounding. This may even allow additive content to be reduced, whilst maintaining an acceptable level of performance. In most of the situations outlined previously, there is a need to ensure high levels of additive dispersion together with uniform spatial distribution within the polymeric phase. The conditions needed to achieve this state may result in excessive degradation of the polymer, however, resulting in a loss in properties of the composite as a whole. Similarly, hollow glass microspheres used in the preparation of syntactic foams and brittle reinforcing additives of high aspect ratio, such as glass fibres and platey minerals, may experience severe damage during compounding, again leading to inferior product quality. Examples of filled polymer compositions are discussed, where the role of the preparation method has been clearly identified in relation to compound structure and end properties. Particular attention is also given to recent developments in the field of natural fibre-filled composites, supercritical fluid assisted processing of filled polymers, re-use of thermoset recyclate fillers and silicate layer polymer nanocomposites.

5.5.1 Addition of Rigid Particulate Fillers Mineral fillers can change the physical properties of polymers in several ways. Firstly, the particle characteristics (size, shape and modulus) can have a significant effect on mechanical properties. Whilst most minerals have a large modulus relative to the polymer, a wide range of sizes and aspect ratios may be encountered. Secondly, the filler may cause a change in the micromorphology of the polymer by acting as a nucleant, thereby altering the amount or type of crystallinity. Both of these effects can be influenced by compounding route. It is well known that to achieve good impact strength in mineral-filled thermoplastics composites, effective dispersion of the filler is of overriding importance. For a linearly elastic material the fracture stress (σF) for a plate containing a sharp crack of length a is given by [67]: ⎛ 2Eγ ⎞ σF = ⎜ ⎟ ⎝ πa ⎠

12

(5.13)

241

Particulate-Filled Polymer Composites where E is the Young’s modulus, γ is the surface energy and a the crack length. A large aggregate will constitute a flaw, which will reduce the stress needed to cause the composite to fracture and fail. For a material subjected to a uniform tensile stress, the calculated values of stress introduced in the region of an elliptical flaw in an otherwise homogeneous matrix, can be described as a function of the major:minor axis ratio of the ellipse [68]. From this analysis, it is clear that high aspect ratio filler particles, which are needed to achieve stiffness in a composite will inevitably cause increased stresses in the polymer matrix near the particle edges, which will facilitate failure under impact loads. A systematic investigation has been given detailing the effects of filler dispersion on the mechanical properties of calcium-carbonate-filled polypropylene composites prepared using different mixing conditions [69]. Dispersion index, expressed in terms of the size and number of agglomerates, was found to have no effect on tensile modulus, tensile yield strength and notched Izod impact strength over the specified levels of dispersion achieved. However, ultimate tensile strength and, most dramatically, falling weight impact strength increased with improved filler dispersion. With the latter mentioned property, extensive toughening of the composite was found when the particles were well dispersed but, significantly, a transition to brittle failure occurred as dispersion deteriorated. Filler dispersion is influenced by many factors, including polarity of the matrix, surface treatment applied and the magnitude of shear stress experienced during compounding. For example, differences have been reported in impact strength of polypropylene filled with calcium carbonate subjected to low-intensity single-screw compounder and a highintensity twin-screw extruder. In this work there was clear evidence of filler agglomeration present in the former sample [67]. The change in filler dispersion at different positions along the screws of a co-rotating twin-screw extruder, has been determined by image analysis on polished specimens. A marked increase in dispersion was observed in the region of polymer melting, where intense shear is generated at the solid-melt interface [70]. Factors found to be detrimental to filler dispersion include the presence of moisture and applied pressure, causing an increase in inter-particle adhesion. In cases where elimination of agglomerates is essential to ensure that optimum compound properties are achieved, melt filtration can be used. This is important in pigment masterbatch production or in coloured compounds to be spun into fibres, where agglomerates can lead to fibre breakage. Removal of impurities or large defect particles by melt filtration can also increase the fatigue life of HDPE pipe and the resistance to breakdown of polymer subjected to an electrical stress [71, 72].

242

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds More subtle differences in the properties of filled polymer compounds result from changes to the micromorphology of polymer surrounding the filler particles. For calcium carbonatefilled polypropylene a correlation has been observed between falling weight impact strength and onset temperature of crystallisation when cooled from the melt. High onset crystallisation temperatures were associated with composites having poor impact strength [73]. The origin of this effect is not fully understood but may be related to the capacity of the filler to nucleate crystals, and has been shown to be sensitive to the method of compounding adopted. Blending together thermoplastics and rubber modifiers is an established means for enhancing toughness. Whilst the properties of the blend depend on the nature, amount and distribution of each phase present, the compounding procedure used can also have an important influencing role. In the following example, polypropylene modified with 30% by weight of ethylene-propylene diene terpolymer rubber was prepared on singleand twin-screw extruders under conditions of differing mixing intensity [74]. Compounds made from this formulation exhibit phase separation, with rubber particles dispersed in the polypropylene matrix. However, the extent of this dispersion and the effect of the rubber on properties was found to be sensitive to the compounding conditions adopted. Notched impact strength of the polypropylene increases significantly when rubber particles are present, however, dramatic differences were apparent between materials made on the different types of extruder, results being superior using the twin-screw compounder. In the case of the single-screw extruder, perhaps surprisingly, filler dispersion decreased with increasing screw speed, however, this was attributed to the much greater viscosity of the rubber relative to the polypropylene (it was noted that improved rubber dispersion was possible using an optimum viscosity ratio, i.e., close to 1). Properties of a compound, made on the twin-screw extruder showed some sensitivity to screw speed, screw profile (shear intensity) and mass output rate, with the best results obtained using a high screw speed and low material throughput. Blend homogeneity and impact resistance were closely related to the energy input through the compounder drive. It was also recorded, however, that examination of impact strength after injection moulding revealed much less difference between the various forms of compounding procedure used.

5.5.2 Effects on Polymer Molecular Weight Frequently, during compounding, the attention given to achieving good dispersion or distribution of a filler phase within a polymer matrix can result in generation of high melt temperatures and excessive shear. Whilst these conditions may be beneficial in obtaining compositional uniformity, they may also be detrimental to the polymer structure,

243

Particulate-Filled Polymer Composites resulting in chain scission and a consequent reduction in mechanical properties. This may be particularly evident if the material is processed more than once, as is the case with recycled compounds. The sensitivity of molecular weight changes in polystyrene and polypropylene to compounding procedure using a co-rotating twin-screw extruder have been reported [75]. Variables considered were melt temperature, screw speed (hence shear rate) and screw profile. Polystyrene was considered to degrade more by mechanical degradation at lower melt temperatures and by thermal means at high values, whereas polypropylene showed a much greater tendency to degrade during compounding, with thermo-oxidative degradation being the dominant mechanism.

5.5.3 Short Fibre-Reinforced Thermoplastics Composites To maximise the mechanical properties of short fibre-reinforced thermoplastic composites, it is necessary to achieve effective stress transfer between fibre and polymer matrix and appropriate fibre alignment in the finished component relative to the direction of applied stress. The former requirement is governed primarily by the method of compounding, whilst fibre orientation is very dependent on moulding conditions [76]. Hence, during compounding, it is generally desirable to obtain the longest possible wellwetted fibres in the polymer. However, common fibre reinforcements (glass and carbon) are very brittle, showing a marked tendency to break during melt blending. This problem is exacerbated as the fibre loading is increased and under intense mixing conditions needed to achieve good fibre wet-out. Thus, fibres entering the machine with 6 mm lengths will generally end up no longer than about 500 μm in the final compound. Steps can be taken to minimise this problem through attention to screw profile (in extrusion compounders), the location of addition (preferably by feeding through a downstream port into the melt in extruders and continuous internal mixers), and the melt temperature and shear rate experienced. With aramid fibres (Kevlar), similar considerations are necessary. Although these fibres are not brittle in the same way as glass or carbon, they can suffer serious damage during over-intensive compounding, manifested by fibrillation (or fibre splitting) and kink (or shear) bands formed along the fibre length (Figure 5.6). Whereas qualitative information about the level of interaction between fibre and matrix can be obtained by electron microscopy, various methods are available for quantitative evaluation of fibre-matrix interfacial shear strength in thermoplastics and thermoset polymers [55]. Table 5.3 shows interfacial shear strength and critical fibre length values

244

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds

Figure 5.6 Scanning electron micrograph of aramid fibre-reinforced polyamide 6,6 fracture surface. Compound was prepared in a co-rotating twin-screw extruder.

Table 5.3 Critical fibre length and interfacial shear strength values for glass-fibre reinforced polypropylene compositions determined by a single-fibre fragmentation technique (adapted from [78]) Fibre diameter (μm)

Silane surface treatment

Critical fibre length (μm)

Interfacial shear strength (MPa)

A

13

Type 1

650

34.0

B

18

Type 2 (1%)

950

31.9

C

18

Type 2 (1.75%)

860

35.6

D

18

Type 2 (2.5%)

660

46.4

E

9

Type 2 (2.5%)

760

20.1

F

10

No treatment*

750

22.7

10

**

650

26.2

Fibre

G

No treatment

*

Transcrystallinity present at fibre-polymer interface. No transcrystallinity. Ultimate tensile strength of fibres assumed to be 3.4 GPa.

**

245

Particulate-Filled Polymer Composites

Table 5.4 The effect of compounding route in the ‘average’ fibre-matrix interfacial shear strength of glass-fibre-reinforced polyamide 6,6 mouldings Compounding route

Interfacial shear strength (MPa)

Co-rotating twin-screw extruder (high-intensity profile)

33

Co-rotating twin-screw extruder (low-intensity profile)

26

Pultruded compound (‘long fibre material’)

22

Notes: 1. 2.

Direct measurement of interfacial shear strength by single fibre fragmentation technique 46 MPa. Shear yield strength of matrix 48 MPa.

for different fibre forms, surface treatments and microstructural effects in a polypropylene matrix, when determined by a single fibre fragmentation technique [77]. Average values for interfacial shear strength can also be obtained by analysis of stressstrain curves determined from compounded materials [78]. This approach can distinguish between the effects of compound preparation method on interfacial shear strength and hence degree of fibre-matrix interaction (Table 5.4).

5.6 Woodflour and Natural Fibre-Filled Thermoplastics There is currently widespread commercial interest in the use of natural fibres as reinforcing additives for thermoplastics. Within Europe, flax and hemp reinforced polypropylene are being increasingly used for automotive applications, normally as mats interwoven with polypropylene fibres, then compression moulded into products such as door liners and parcel shelves [79]. Shorter forms of these fibres have also been developed as reinforcements in injection moulding compositions [80, 81]. The driver for these applications stems from the high specific mechanical properties of the natural fibres, yielding weight savings in parts, and the fact that they are derived from renewable resources. In the USA, and to an increasing extent in Europe, woodflour filled thermoplastics are being used as wood substitutes, in areas such as decking and window frames [82]. Although still fibrous in character, their aspect ratio is generally no greater than 4:1, hence their reinforcing potential is limited. In these compositions, filler loadings may be as high as 80% by weight, necessitating special processing measures and inclusion of processing aids to facilitate flow.

246

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds Common to the preparation of natural fibre-reinforced thermoplastics made by melt processing, is the need to maximise fibre-matrix interfacial bonding, minimise the moisture content and heat history of the organic filler and reduce odour arising from exposure to processing temperatures of around 200 °C. Frequently, MA modified polypropylene is introduced into polypropylene compositions to aid fibre bonding to the matrix [83], which also gives these composites greater resistance to moisture pick-up in processed composites. For outdoor applications, polyethylene is the preferred matrix due to its greater ultraviolet stability than polypropylene. Other methods reported for enhancing fibre-matrix bonding include the use of silane treatments, fatty acids (which also assist processibility) and modification of the fibres by corona discharge, or more speculatively, by acetylation [84, 85]. Pre-treatment of natural fibre-reinforcements can be difficult and often inappropriate, due to their variable physical nature and the fact that new fibre surfaces are exposed during transport through high intensity polymer processing equipment. In situ treatment of fibres can therefore be more effective, either using maleinised polypropylene or polyethylene as mentioned above, or with liquid surface treatments, by drip feeding these into the melt during processing or by using porous carriers added in granular form containing absorbed surface modifying additive. Since the presence of moisture can result in porosity in processed material, this can be removed by pre-drying the additive before processing, downstream melt devolatilisation, for example during extrusion of profile, or by use of low moisture content filler, available from some manufacturers. There have also been technological developments, which facilitate processing of natural fibre-filled thermoplastics. Figure 5.7 demonstrates an integrated extrusion compounding procedure, which enables pulping of natural fibres during the first part of the process, where moisture can also be extracted and surface treatment added. Polymer melt is then introduced through a downstream feed port, using a secondary extruder, for combination with the fibres before die-forming into profile or pellets [86]. Conical twin-screw extruders are also being used to perform a similar function, with the advantage that the high volumetric capacity of their feedthroat assists in the introduction of low bulk density organic filler. Direct compounding injection moulding technology (Figure 5.8), which combines the stages of twin-screw extrusion compounding and injection moulding into a unified process unit, offers the advantage of reduced heat input to thermally sensitive fillers. Although proposed originally in the 1960s, then developed commercially for injection moulding of ceramics and highly filled polymer formulations [87, 88], this concept has recently been proposed for moulding natural fibre-filled thermoplastics [89].

247

Particulate-Filled Polymer Composites

Figure 5.7 Integrated polymer compounding procedure

Figure 5.8 Direct compounding injection moulding machine

For automotive use, natural fibre-filled thermoplastics must meet odour and emission standards specified by car manufacturers [90]. These requirements can be achieved

248

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds using proprietary odour suppressants, use of purer natural fibres (with reduced lignin and pectin content) and most importantly, by minimising thermal exposure of the fibres during processing.

5.7 Supercritical Fluid Assisted Processing of Filled Compounds Supercritical fluids are neither in the gaseous or liquid state and can be formed above a critical temperature and pressure. For example CO2 becomes supercritical above a temperature of 31 °C and a pressure of 7.3 MPa. Under these conditions supercritical fluids can provide a more environmentally acceptable alternative to organic solvents and have consequently found widespread application in a range of process industries, such as extraction aids in both pharmaceutical and food preparation [91-93]. There is also much potential for the application of supercritical fluids in polymer processing due to the unique ability of supercritical CO2 (scCO2) to impregnate, extract from, and modify the behaviour of polymers [94]. The use of scCO2 is particularly well established for the preparation of microcellular foams from polyolefins and other commodity plastics [95, 96]. Additionally, dissolution of scCO2 in polymer melts can strongly plasticise the material, yielding significantly reduced viscosities [97-99] The likely explanation for this effect is a shift of the glass transition temperature to lower temperatures.. It has also been reported that addition of supercritical fluids during melt processing can improve the miscibility of polymer blends. For example, the viscosity reduction of polystyrene containing scCO2 was found to be greater than that measured for polyethylene, enabling greater similarity in the viscosity of these polymers when combined as a mixture [100]. The implications of these effects to the processing of filled thermoplastics is profound, since by reducing the melt viscosity, processing temperatures and/or pressures can be lowered, providing faster throughput rates, less thermal damage to heat sensitive fillers, or allowing increased filler levels without compromising processibility. Although at a preliminary stage, these benefits have been demonstrated using in-line extrusion rheometry using polymers containing glass beads and ceramics injection moulding compositions, containing high levels of filler dispersed in a sacrificial polymer binder [101]. Figure 5.9 demonstrates this effect using silicon carbide/polyethylene mixtures. It was also found that under certain process conditions and using low scCO2 addition levels, foaming could be prevented as the mixture emerged into a lower pressure environment at the exit of the die. Furthermore, this approach has also been adapted to the injection moulding process with, for example, highly filled particulate biomedical composites and melt processible ceramics formulations [101, 102]. Products were moulded in either a foamed or unexpanded state as required, by judicious control of processing parameters, in particular the mould cavity pressure.

249

Particulate-Filled Polymer Composites

Figure 5.9 Supercritical fluid assisted processing of filled polymer compositions. Blend of 50% by volume silicon nitride/polyethylene extruded at 180 °C

5.8 Processing of Thermoset Recyclate Waste Materials Whereas thermoplastics can be mechanically recycled at end of product life, thermosetting plastics are infusible and cannot be reprocessed by this means. Apart from landfill, which is becoming increasingly expensive and environmentally unacceptable, strategies for disposing or reusing these materials are mostly limited to incineration or comminution, for subsequent application as a low cost filler. Polyester, epoxy or phenolic resin based compositions generally contain high levels of non-combustible fillers, reinforcing fibres and often, fire retardants, hence the calorific value of these materials is generally low. Nevertheless, their incorporation in cement kilns is one option, which has been considered, where fuel costs are reduced by combustion of the polymer phase and inorganic components are incorporated into the cement mixture. Using specially designed fluidised bed reactors combustion of the polymer and recovery of reinforcing glass fibres is also feasible [103]. The reduction in glass fibre strength during combustion limits their reuse as a reinforcement for polymers however. Preliminary work on the combustion of carbon fibre containing thermosets shows greater potential, since these fibres are inherently stronger, stiffer and more valuable. Oxidation and loss of strength is small at temperatures and low exposure times around 500 °C, resulting in fibre properties similar to virgin glass [104, 105].

250

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds Controlled comminution of thermoset waste, for example using a hammer mill, can yield so-called ‘microcomposite’ particles, containing a mixture of glass fibres, mineral fillers and thermoset resin. With appropriate surface treatment technology, they can be applied as low cost reinforcing additives for thermoplastics [106-108]. Table 5.5 compares mechanical properties of polypropylene, with various filled compositions, in each case containing 30% by weight of filler. Polyester and phenolic recylates used in this study included 5% and 24% of glass fibres, respectively, which is reflected in the different reinforcing efficiencies of these systems.

Table 5.5 Mechanical properties of PP-thermoset recyclate composites at 23 °C Glass in composite, wt%

Tensile strength, MPa

Tensile modulus, GPa

Notched Charpy impact strength, J mm-2

PP

-

26.5 (0.3)**

1.8 (0.1)

7.7 (0.7)

PP-CaCO3

-

20.4 (0.1)

2.1 (0.2)

3.2 (0.5)

PP-Glass fibre

30

72.3 (0.6)

7.7 (0.4)

5.4 (0.20)

PP-DMC untreated

5

18.4 (0.9)

2.7 (0.3)

2.4 (0.2)

PP-DMC treated

5

29.1 (0.1)

3.1 (0.1)

2.4 (0.2)

PP-GWP untreated

24

28.9 (0.2)

5.2 (0.6)

4.7 (0.2)

PP-GWP treated

24

62.1 (0.4)

5.1 (0.2)

7.4 (0.2)

Composition*

*

30 wt-% filler; GWP = glass (woven fabric) phenolic recyclate. Figures in parentheses show standard deviations.

**

5.9 Preparation of Silicate Layer Polymer Nanocomposites Silicate layer polymer nanocomposites consist of delaminated clay platelets with ultimate dimensions around 1 nm x 500 nm, that are well dispersed in a continuous polymer matrix. It is the small size and uniquely high aspect ratio of these inorganic particles that distinguishes them from conventional mineral fillers and reinforcing glass fibres, conferring a significant enhancement in properties to the host polymer even at very low filler loadings, typically between 2% and 5% by weight. In this regard, it is significant that a single 8 μm montmorillonite particle contains over 3000 platelets [109].

251

Particulate-Filled Polymer Composites Even at these small addition levels, marked increases in mechanical properties, heat distortion resistance, gas barrier properties and flame retardancy are apparent, stimulating extensive academic research and commercial interest in these materials [110]. Of central importance to achieving this enhanced performance, however, is the extent of polymer penetration between the silicate layers and the resulting inter-layer spacing achieved. When this distance is relatively small the polymer is said to be intercalcated within the silicate galleries, producing a well ordered multilayer structure with alternating polymer/ inorganic layers and a repeat distance of only a few nanometers. However, delamination and dispersion of these particles creates a so-called exfoliated structure [111]. It is often assumed that the most desirable properties are achieved in the exfoliated condition, although there are notable exceptions to this generalisation. For example, it has been shown the reduced toughness of some exfoliated systems can be alleviated in an intercalcated morphology, although at the expense of modulus [112]. The ultimate microstructure formed, critically depends on a number of factors, including: (i)

the nature of clay pre-treatment used to make it organophilic, for example using alkyl ammonium cationic surfactants,

(ii) whether or not the polymer is in a melt state, in solution, or made by in situ polymerisation of monomer in the presence of the filler; (iii) the polarity of the polymer; (iv) the inclusion of functionalised polymer, such as MA modified polypropylene; and (v) the processing conditions and technology used to effect filler dispersion. A more extensive account of the formation, structural characterisation and physical properties of silicate layer nanocompsites can be found in Chapter 10. The discussion below focuses on nanocomposite preparation by melt processing, since this provides the most promising route for more widespread acceptance and subsequent commercial exploitation of these materials. To achieve delamination and exfoliation of organically modified clays (organoclays) during processing, high shear intensity melt mixing technology is usually recommended, as considered earlier in this chapter. However, a number of issues are known to affect the degree of delamination and dispersion, in particular the compatibilty of the chemical treatment on the clay with the resin matrix and the specific compounding conditions used. The following mechanism has been proposed for this process [109]: (i)

252

Particles of montmorillonite (MMT) clay shear apart giving tactoids, or polymer intercalated MMT, in stacks around 100-150 nm high.

Compound Preparation, Mixture Characterisation and Process Enhancement of Particulate-Filled Polymer Compounds (ii) Polymer entering the galleries of the MMT clay pushes the platelets further apart eventually causing them to peel off the intercalated stacks. This peeling process is time-dependent and does not require intense shear. Hence increased residence time increases delamination and dispersion, whereas this may be limited under high shear conditions. (iii) The more compatible the organoclay and polymer the faster this process occurs. With polar polymers, in particular polyamides, the task of achieving exfoliation is relatively easy and can even be achieved using a single-screw extruder configured with an appropriate mixing head or screw profile [113]. For example, exfoliated polyamide-12 nanocomposites have been successfully prepared by melt compounding in a conventional single screw extruder fitted with a barrier screw design. In this work, a surface modified fluoromica was used at 4% by weight loading. However, this task becomes more challenging with polyolefins, and it is generally necessary to have some polar functionality present, most commonly by inclusion of MA grafted polymer in the composition (typically at a 5% polymer addition level) [114]. A co-rotating twin-screw extruder is frequently recommended for this purpose, however the form of screw profile used and feed location of the components, can influence the structure and properties of compounds produced [115]. For example, it was reported that combined dosing of polymer and filler at the feed end of the extruder gave better intercalation than downstream addition of the clay. It has also been shown that a pre-dispersed organoclay/polypropylene masterbatch concentrate also containing MA-grafted polypropylene compatibiliser, can be successfully let-down to 6% by weight using a single screw extruder [116]. In this work, the following screw designs were compared: standard flights, a Union Carbide dispersive mixer, a distributive mixer and a newly developed ‘dispersionary’ mixing section. The last mentioned variant gave comparable composite properties to material let down within a twin-screw extruder. Reactive extrusion provides an alternative approach to melt compounding, for the preparation of polyamide-6 nanocomposites [117]. In this study, a co-rotating twinscrew extruder was used to anionically polymerise ε-caprolactam in the presence of organically modified montmorillonite clay. Hence, intercalcation and delamination occur concurrently with polymer formation. To assist in the production of silicate layer nanocomposites scCO2 has also been used. Intercalcated poly(methyl methacrylate) clay nanocomposites, containing 25-50% by weight of clay, have been made by polymerisation of methyl methacrylate in a scCO2 pressure vessel [112]. This overcame previous melt processing difficulties caused by the

253

Particulate-Filled Polymer Composites increased viscosity from inclusion of large amounts of filler. Polystyrene nanocomposites containing 10% of organoclay filler have also been prepared in a twin-screw extruder in the presence of scCO2. The principal objective here was to minimise surfactant degradation on the clay by processing at lower temperatures. Using this approach, it was also demonstrated, using TEM, that improved clay dispersion could be obtained [118].

5.10 Conclusions The principal objective of this chapter has been to demonstrate that the preparation of polymer compounds containing particulate additives to meet specific performance criteria requires attention to not only the design of the compounding machinery, but also an understanding of how the process influences components within the composition and ultimately its structure. Hence, there is a need for a basic knowledge of the functional stages of the compounding process to aid optimum machine design and the development of appropriate (and preferably quantitative) characterisation procedures to identify relevant structural parameters in the composition under examination. It has also been shown that the performance of the composite can critically depend on the material and process technology applied during compound or end-product manufacture.

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6

Effects of Particulate Fillers on Flame Retardant Properties of Composites Roger N. Rothon

6.1 Introduction Particulate fillers play a very significant role in flame retardant technologies for polymers. The main focus of this chapter is on those particulate fillers which function by endothermic decomposition and act as primary flame-retardants in their own right. They have the advantage of a low environmental impact, being of low toxicity themselves, and with much lower smoke and toxic fume generation than some of the more traditional approaches. As a result, the use of this type of filler is now well established, and is continuing to expand as environmental awareness grows. Nano-clays are also discussed, as they are starting to show commercial promise. For completeness, fillers like antimony oxide, which work in combination with other additives, such as halogens, are also briefly discussed. This chapter concentrates on fire retardant effects, the synthesis and general properties of the various fire retardant fillers having already been described in Chapter 2.

6.2 General Effects of Fillers on Polymer Flammability Before discussing the special effects of fillers that are active fire retardants, it is useful to recognise that the addition of any particulate, non-combustible, filler to polymers can considerably affect their thermal stability, resistance to ignition and combustion, and the amount and nature of the combustion gases in terms of smoke, corrosion and toxicity. The main general effects are: i)

Simple dilution, reducing the amount of fuel available for combustion.

ii) Changing the heat capacity and thermal conductivity, thus altering the rate of heating and heat distribution. iii) Various thermal effects, such as reflectivity and emissivity. iv) Formation of a solid residue (ash). This may possibly strengthen any polymer char.

263

Particulate-Filled Polymer Composites v) Slowing down the rate of diffusion of oxygen and pyrolysis products. vi) Effects on polymer melt rheology (especially where dripping is a significant factor). The relative importance of these different effects is likely to vary with the polymer type, and its mode of degradation, and also with the type of flammability test. On their own, the above effects are not usually sufficient to pass fire resistance tests and additional features are required. The main effects of this type are: i) Various solid state effects due to the chemistry of the additive, or it’s surface, or shape, which promote polymer charring and the formation of a strong char. ii) Heat adsorption due to endothermic decompositions. iii) Release of gases such as water, which can affect solid state processes, provide a significant dilution and cooling of the pyrolysis products, and possibly insulate the substrate from radiative energy transfer. Phosphorus and phosphates, notably ammonium polyphosphate (APP), are an example of the first effect. Nano-clays also appear to enhance char properties, although the mechanisms are not fully understood at present. The main focus of this chapter is on fillers, generally known as fire retardant fillers, which have an endothermic decomposition, coupled with the release of inert gases. These two processes are always found together, principally in some carbonates, hydrates and hydroxides. For these effects to be important, they must occur above the polymer processing temperature, and near to the polymer pyrolysis temperature. Thus, calcium carbonate is not effective, although it decomposes, with a large endotherm and release of carbon dioxide, as this occurs well above the pyrolysis temperatures of common polymers. As described in Chapter 2, there is some commercial interest in doping the surface of endothermic fillers with metals such as nickel, to add char promotion to their other effects. Doping has also been claimed to be able to alter the decomposition temperature to some extent [1].

6.3 Fire Retardant Testing Tests play a vital and somewhat controversial role in determining the effect of fillers on polymer flammability and some discussion is essential before examining the fire retardant performance of fillers. Large scale tests are used for assessing the real fire resistance of articles containing polymers. The construction of the article and even it’s method of mounting can be important, and

264

Effects of Particulate Fillers on Flame Retardant Properties of Composites these tests are usually carried out on complete items. They are also usually designed to replicate the fire hazard situation the article is likely to experience. Thus, the appropriate test for a cable jacketing will be different to that for chair upholstery or a wall covering. Various smaller scale tests are used for product development and quality control purposes. These are usually carried out on flat specimens, and are more related to material properties. The main characteristics one is trying to assess in small scale tests are: •

Ignitability: How readily a material will catch fire under certain conditions.

•

Propagation: How rapidly fire will travel once ignition is achieved.

•

Heat Release: How much the fire will raise the temperature of the surroundings, thus causing the problem to spread.

•

Afterglow and smouldering: This is an important, but often ignored factor, as it can cause re-ignition after a fire has apparently been extinguished.

•

Dripping: Dripping can reduce the apparent flammability by removing heat from the specimen. On the other hand, it can also lead to propagation, if the drops themselves are burning.

•

Smoke and toxic and corrosive gases.

•

Char integrity: This is becoming of some importance for building products.

Unfortunately, all of these parameters are very dependent on the test conditions, especially the amount of radiant heat from an external source. Indeed, the rating of materials can be reversed in some tests merely by changing the radiant heat conditions. Virtually all the basic work has been carried out using these small scale tests. The main ones being the oxygen index test, the Underwriters Laboratory UL94 test [2] and the ASTM D635-88 [3] horizontal burn test. More recently great interest has been expressed in the cone calorimeter test [4], which appears to have significant potential for basic studies under more realistic conditions. The equipment is expensive, but it is becoming more widely used. A brief description of the main small scale tests follows. A useful description of some of the larger scale tests, and attempts to correlate them with the oxygen index can be found in the paper by Wharton [5]. It must be borne in mind that Wharton’s work did not include filled polymers, and hence any correlations found may not be applicable to such composite products.

265

Particulate-Filled Polymer Composites

6.3.1 Oxygen Index Test (ASTM D2863-87) [6] This measures the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a material initially at room temperature. The sample is ignited and burns from the top downwards. The rating or oxygen index is expressed in terms of this volume percent oxygen concentration. There is no external heat provided once the sample has ignited and the heat required to keep the sample burning comes from the flame itself. A useful description of the test and factors affecting results has been published by Wharton [7]. As discussed later, this test has been successfully modelled. The oxygen index itself does not measure ignitability, afterglow, smoke or dripping. It is possible to observe whether afterglow occurs and how much smoke is formed, but it must be remembered that this is under unrealistically high oxygen concentrations. There is an upwards burning variant of the test, which is sometimes used. There is also a temperature index variant, in which radiant heat is used, and the temperature variation of the oxygen index is determined. The temperature index is quoted as the temperature at which the oxygen index is 20.8 (corresponding to the oxygen content of air). The temperature index is regarded by some as more relevant, especially as the oxygen concentration is more realistic, but it is more time consuming to determine and has been little used to date.

6.3.2 Underwriters Laboratory Vertical Burn Test (UL94 -1980) [2] A strip of the test material is held vertically in air and ignited from the bottom with a Tirrell burner using a set procedure. The material receives a rating according to its burning characteristics. The top rating in the test is Vo. To achieve this, the material must extinguish very rapidly after removal of the flame, show very little afterglow, and not have burning drips. The test is very dependent on sample thickness and this must always be taken into account when comparing results from different workers. While more comprehensive than the oxygen index test, it does not measure smoke. There is some component of ignitability in this test, and Ashley and Rothon have adapted it to give a direct measure of ignitability [8]. The same burner, sample size and configuration are used, but instead of a fixed application time, the burner is applied for varying times until the time to achieve lasting ignition is found. While crude, this can give good reproducibility and is a very cheap way of assessing ignitability.

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Effects of Particulate Fillers on Flame Retardant Properties of Composites

6.3.3 Horizontal Burn Test (ASTM D635-98) [3] The sample material is tested in air in the form of strips held with their longitudinal axis horizontal and their transverse axis at 45° to the horizontal. The sample is ignited at one end with a Tirrell burner and the rate and extent of burning is reported.

6.3.4 Ignitability Test (ISO 5657 1997) [9] This measures the susceptibility of a material to be ignited by an external flame and when exposed to a radiant heat source. The radiant energy is applied from a conical heater, thus giving a uniform heat flux over the sample. The severity of the test can be varied by varying the energy output from the radiant heater. The pyrolysis gases are allowed to build up in the cone heater and a small pilot flame is intermittently applied. The time to ignition is determined visually and quoted together with the heat output of the radiant heater.

6.3.5 Cone Calorimeter (ASTM E1354 [10], ISO 5660 [11]) This is a much more complex test than those described previously, and is thought to come closer to real fire conditions. The method is based on what is known as oxygen depletion (or consumption) calorimetry. The development of the method has been described by Babrauskas [4]. It has been found empirically that the amount of heat produced during combustion of most types of material is equal to 13.1 MJ for each kg of oxygen consumed from air. The rate of heat release can thus be determined from the rate at which oxygen is consumed. This allows heat release to be followed under typical combustion conditions, rather than in some artificial set-up. In the commercial equipment, the sample is heated by radiant heat from an electrically heated truncated cone. This ensures an even heat flux, which can be controlled in the range 0 – 100 kW/m2. Ignition is by a spark igniter. The sample is mounted on a load cell, which allows the rate of mass loss to be followed. The combustion gases pass though an exhaust hood into a pipe-work system, where measurements are made that allow the oxygen depletion and hence heat release, to be automatically computed. Obscuration (smoke) is measured in the exhaust gases using a laser method. Carbon monoxide and dioxide are also determined in the exhaust gases, and corrosivity and toxicity tests may also be made. A large number of parameters may be derived from the data. The most important are:

267

Particulate-Filled Polymer Composites •

Time to ignition

•

Peak heat release rate

•

Time to peak heat release

•

Total heat release

•

Peak smoke production rate

•

Time to peak smoke production

•

Total smoke production

•

Fire growth rate index (FIGRA)

•

Smoke growth rate index (SMOGRA)

FIGRA is now believed to be one of the most relevant parameters and is related to the size and growth rate of a fire. FIGRA is calculated from the peak heat release rate divided by the time to peak. Similarly, SMOGRA is the peak smoke production rate divided by the time to peak.

6.3.6 Smoke and Corrosive Gas Tests As with flammability, tests play a key role in studies of smoke formation from burning polymers. A wide variety of tests have been used, often in conjunction with flammability tests. Hirschler has given a brief description of most of the main tests [12]. Until recently, one of the main tests was the National Bureau of Standards (NBS) smoke chamber (ASTM E662) [13]. In this test, a vertical sample is decomposed under radiant heat in a sealed cabinet and the build up of optical density is monitored. The test can be run under both smouldering and flaming conditions, depending on whether a pilot ignition flame is present. Various parameters can be obtained from the NBS test. Those usually quoted are: maximum optical density, time to maximum density, the time to reach an optical density of 16 (the critical observation time) and the optical density after 4 minutes. The critical observation time is an indication of how long before visibility would become too bad to allow one to find a way out of a room, while the density at four minutes is relevant to whether smoke would affect escape from, say an aircraft, before other life threatening aspects of a fire became important. Today, smoke measurements are more and more being obtained by cone calorimetry as described in Section 6.3.5.

268

Effects of Particulate Fillers on Flame Retardant Properties of Composites Various corrosion tests are in use, often based on absorbing combustion or pyrolysis gases in water and determining the pH. In one test the effect of combustion gases on the electrical resistance of a printed circuit board is determined. Again, corrosion data can be obtained from the cone calorimeter.

6.4 Fire Retardant Fillers that Rely on Endothermic Decomposition These fillers are of great industrial importance, and are the main subject of this chapter. They owe their fire retardant effectiveness to their ability to decompose endothermically at polymer pyrolysis temperatures, with the release of inert gases such as water. Thus, unlike some other flame retardants, they are able to combine a high level of flame retardancy, with low smoke and low toxic and corrosive gas emissions, and are thus becoming of increasing importance. One of the simplest such materials is aluminium hydroxide (also known as alumina trihydrate, ATH).

6.4.1 Historical Background The ability of ATH to flame-retard cellulose was recognised almost a century ago. A patent on it’s use as a flame retardant for elastomers appeared in 1921 [14] and a further patent on it’s use to improve arc resistance of various polymers in 1956 [15]. There appears to have been little or no commercial exploitation of these effects at that time, however. Significant commercial use of ATH as a flame retardant seems to have started in unsaturated polyesters in the mid-1960s, following work by Connolly and Thornton [16]. Interest in elastomer systems followed in the early 1970s when workers such as Lawson and co-workers [17] and Nelson [18] recognised the low smoke benefits associated with the use of ATH as a flame retardant. The effectiveness of other fillers, especially magnesium hydroxide, was also recognised by them. The commercial use of hydrated fillers was given a great boost in the mid-1970s, by legislation in the USA requiring carpet backing to be flame retarded, an application for which aluminium hydroxide was ideally suited. Today ATH is a well established flame-retardant additive, especially for unsaturated polyesters, elastomers and some thermoplastics. Growing concerns over the problems (real or imagined) associated with organo-halogen flame retardants is now creating further opportunities, especially in other polymers, notably polyolefins and polyamides, where ATH is not ideally suited because of its relatively low decomposition temperature. This has led to a surge of interest in other

269

Particulate-Filled Polymer Composites more thermally stable materials, notably magnesium hydroxide. Concurrently with this, the demands on the performance of ATH have become more sophisticated requiring the development of new grades to maximise performance. Thus, the area is one of considerable activity and development at the present time.

6.4.2 Potential Endothermic Flame Retardant Fillers At first glance, there seem to be many materials that decompose endothermically and might be thought of as potentially useful, but there are other important criteria to consider. In particular, the material must be inexpensive, freely available in quantity, non-toxic, noncoloured, of low solubility and with a morphology suitable for filler use. A significant decomposition (by experience at least 25% weight loss) is needed, if the endothermic process is to have a useful effect. Furthermore, the decomposition temperature must be high enough to survive polymer processing, but low enough to exhibit a fire retarding effect. Only a few materials meet some, if not all of these criteria. The main candidates are summarised in Table 6.1. This list excludes other, well known materials, which have significant endotherms, but where they are not sufficient to make the additive a primary flame retardant. These include, some clays and talc, and some stannates and borates. The materials in Table 6.1 are seen to cover a wide range of decomposition temperatures, and to include release of carbon dioxide as well as water. With such a range of materials available, it would be thought that a clear ranking of their performance would allow conclusions to be drawn, regarding the relative importance of the different parameters. Unfortunately, this is not the case at present. This is partly because, as we shall see in Section 6.4.3, particle morphology can play a significant role, leading to marked variations between different forms of the same material. A proper comparison would thus have to use forms of the different materials with the same morphology and this has not been possible. More importantly, most comparative work has been carried out using the simplistic oxygen index test to assess flame retardant performance. As we shall also see in Section 6.4.3.2, this test has its limitations and may not accurately predict the relative performance of fillers in other tests, or indeed in real fire situations. Clearly more could be done in this area. Despite these reservations, the presently available data suggests that, with the possible exception of nesquehonite, none of the candidate materials is a significantly more effective flame retardant than aluminium hydroxide (also known as alumina trihydrate or ATH), although as will be shown later, magnesium hydroxide may have some advantages in terms of smoke production. ATH also comes closest to meeting all of the criteria mentioned previously, and this explains its current pre-eminence. It owes its relatively low cost and wide availability to its production on a vast scale as an intermediate in alumina manufacture. The main limitation of ATH is

270

Effects of Particulate Fillers on Flame Retardant Properties of Composites

Table 6.1 Principle candidate flame-retardant fillers Approximate onset of decomposition (°C)*

Approximate enthalpy of decomposition (kJ kg-1)

H2O

CO2

Total

Nesquehonite MgCO3.3H2O

70-00

1750

39

32

71

Calcium sulfate dihydrate, Gypsum CaSO4.2H2O

60-130

Not available

21

0

21

Magnesium phosphate octahydrate, Mg3(PO4)2.8H2O

140-150

Not available

35.5

0

35.5

Alumina trihydrate, Aluminium hydroxide Al(OH)3

180 -200

1,300

34.5

0

34.5

Basic magnesium carbonate, Hydromagnesite 4.MgCO3.Mg(OH)2.4H2O

220-240

1,300

19

38

57

Dawsonite (sodium form) NaAl(OH)2CO3

240-260

Not available

12.5

30.5

43

Magnesium hydroxide Mg(OH)2

300-320

1,450

31

0

31

Magnesium carbonate sub-hydrate (MCS) MgO.CO2(0.96)H2O(0.30)

340-350

Not available

9

47

56

Boehemite AlO(OH)

340-350

560

15

0

15

Calcium hydroxide Ca(OH)2

430-450

1,150

24

0

24

Candidate material (common names and formula)

Volatile content % w/w

* The decomposition temperatures are only approximate, as they are usually determined under dynamic conditions and depend on heating rate and sample conditions.

it’s relatively low decomposition temperature (about 180 ºC) which has restricted it’s use in applications where processing temperatures are above this, notably many thermoplastics. ATH also loses some of its cost advantages when the morphology or purity available from the alumina processes is not sufficient and special grades are required.

271

Particulate-Filled Polymer Composites A brief discussion of the other materials follows. More complete details, including aspects other than flame retardancy, will be found for most of these and for ATH in Chapter 2. Nesquehonite (MgCO3.3H2O) is an effective flame retardant, as is to be expected from its large volatile content and endotherm, but suffers from a very low decomposition temperature, and is not produced commercially with attractive morphology or price. It’s effect in unsaturated polyester is illustrated later in Figure 6.3 (Section 6.3.4.2). Calcium sulfate dihydrate is a low cost material with significant fire retardant effectiveness. It is restricted by its low thermal stability, but is reported to have some applications in thermosets, such as unsaturated polyesters [19]. Magnesium phosphate octahydrate appears to have interesting properties, although of limited thermal stability, and the present author has obtained excellent results with it in some thermosets. It has received little scientific or commercial interest to date. It has a relatively low specific gravity, which could be attractive in some applications. There are other magnesium phosphates with similar properties. Basic magnesium carbonate (hydromagnesite, 4MgCO3.Mg(OH)2.4H2O) is also an effective flame retardant, and can be formed from nesquehonite, although it is usually synthesised directly. It is more stable than nesquehonite and used on a small scale as a flame retardant and smoke suppressant, principally in elastomers. It is generally more expensive than ATH, and its readily available forms tend to consist of high surface area, platy particles, not ideally suited for highly filled polymer applications. This has probably limited its use. Significant workable deposits containing natural basic magnesium carbonate exist, notably in Greece. These are usually associated with the mineral huntite (3MgCO3.CaCO3). Flame retardant fillers based on these deposits have been commercially developed. Unfortunately, it is not economic to remove the huntite. The present author has found (by x-ray analysis of the ash) clear evidence that the huntite decomposes under oxygen index test conditions (see Chapter 2 for further details). Even so, it does not appear to be such a good flame retardant as the basic salt and this reduces the effectiveness of the mixture [20]. Various forms of synthetic hydrated alkali alumino-carbonates such as dawsonite (the sodium form) have been proposed and indeed marketed as flame retardant fillers [21]. Such materials are of proven effectiveness, but have made little commercial progress. They are relatively expensive, have some water solubility problems and there have also been toxicity concerns over the acicular nature of some of the products. Magnesium carbonate sub-hydrate is included here to illustrate the problems that can be encountered in this area. Like basic magnesium carbonate, it can be produced from

272

Effects of Particulate Fillers on Flame Retardant Properties of Composites nesquehonite, but it is more stable and can be used at processing temperatures up to at least 300 °C. Initial work by the present author showed oxygen index results equivalent to ATH, suggesting a good flame retardant performance, but other tests subsequently showed virtually no flame retardant effect (see Section 6.4.3.2 for more detail). Currently great interest is being shown in magnesium hydroxide. On the basis of present information, its flame retarding effects are similar to ATH: it appears to have better smoke suppression effects under some conditions. On the other hand it may be more prone to afterglow problems (see Section 6.4.4.3). Like ATH it is the intermediate in large scale refractory manufacture (MgO) and should in principle be relatively inexpensive. However, the morphology and purity available from this process is not usually ideal for filler use and more expensive, specially prepared, forms are generally utilised. The attraction of magnesium hydroxide compared to ATH lies in the much higher decomposition temperature (~300 ºC). This allows use in applications such as thermoplastics, which are processed at too high a temperature for ATH. Workable deposits of natural magnesium hydroxide (brucite) exist, principally in the United States and China and after much unsuccessful work, these are now beginning to make a significant commercial impact [229]. Boehmite is a partly dehydrated form of ATH. It is appreciably more stable, but would be expected to have less fire retarding effect, due to the lower endotherm and water content. Nevertheless, it appears to be of some commercial interest [23]. Calcium hydroxide is an interesting material as, although appearing to have suitable properties, both Nishimoto and co-workers [24] and Ashley and Rothon [8] have found it to exhibit relatively poor flame retardant properties in the oxygen index and other tests. Ashley and Rothon showed that, in some polymers at least, the final product from the oxygen index test was calcium carbonate rather than the oxide. As carbonate formation is exothermic, they associated this with the poor performance. More recently, Miyata has claimed that doping with certain metals significantly improves the performance of calcium hydroxide [25], but this has not been verified.

6.4.3 Performance of Endothermic Flame Retardant Fillers 6.4.3.1 Introduction While descriptive papers on the performance in various tests abound, scientific understanding is only partly developed at present. This is due to a number of factors, especially the notorious difficulty is defining and assessing the fire performance of materials using laboratory tests. A great deal of the available literature is also in a patent or product promotional form, and of limited scientific use. The inability to find fillers where the

273

Particulate-Filled Polymer Composites endothermic and gas release effects are separated also makes mechanistic studies difficult. Nevertheless, some progress is being made and details of the most useful work in the area follow. For clarity the issues of flammability and smoke generation, while closely related, are dealt with separately.

6 4.3.2 Effects of Fillers in Various Tests a) Oxygen Index As mentioned earlier, this test has been the most widely used, despite its limitations. The popularity of the test is due to its simplicity, use of relatively inexpensive equipment and small samples and good reproducibility. Nevertheless there is considerable controversy over it’s relevance. Ashley and Rothon [8] and Hornsby and Watson [26] have shown how it fails to correlate with other tests when applied to filled systems. As described later in this Section, this is probably due to the dominant role filler ash structures can play in the heat balance so important in this test. The general effects of fillers on the oxygen index of a polymer are illustrated in Figure 6.1 taken from work by Case and Jackson [27]. An inert filler such as calcium carbonate is seen to have given a small but significant increase in oxygen index, becoming more pronounced at high loadings. An endothermic flame retardant filler such as ATH is seen to have much greater effect. The oxygen index values increase rapidly and non-linearly with filler loading, but high levels of ATH are needed before the oxygen index reaches values, which by experience would be regarded as giving good flame retardancy (about 30%). This need to use high filler levels is the main drawback of endothermic fillers, and makes control of their effects on other factors, such as processability and mechanical properties, a vital concern. A further feature of flame retardant fillers in this test is that different grades of the same filler can give widely differing results, despite no change in their endotherm or inert gas release. Such effects usually become noticeable as enhanced performance of very fine fillers and are generally described as a particle size effect, although other factors such as shape may also be important. Typical results of this type for ATH in polymethylmethacrylate (PMMA) are presented in Figure 6.2, taken from work by Hughes, Jackson and Rothon [28]. The enhanced performance of fine fillers in this test has created considerable interest as, if reflected in larger scale tests, it may point the way to allowing reduced filler loadings to be employed. Unfortunately, fine fillers also generally have a more detrimental effect on processing and physical properties so there may be no overall gain.

274

Effects of Particulate Fillers on Flame Retardant Properties of Composites

Figure 6.1 A comparison of the effect of calcium carbonate and aluminium hydroxide fillers on the oxygen index of a filled polyethylene

Figure 6.2 The effect of filler particle size on the oxygen index of PMMA filled with aluminium hydroxide

275

Particulate-Filled Polymer Composites Some of the problems of trying to relate the oxygen index test on filled polymers to other flammability tests have been described by Ashley and Rothon [8] They found that materials such as calcium carbonate were able to give oxygen indices in crosslinked ethylene vinyl acetate (EVA), far higher than glass beads and approaching those of ATH. Furthermore, dilution of magnesium hydroxide with up to 75% w/w of calcium carbonate had little effect on it’s oxygen index in the same polymer. Nevertheless, both calcium carbonate and diluted magnesium hydroxide hardly improved the ignitability characteristics under UL94 conditions, relative to the unfilled polymer. The same workers also found that additives such as coupling agents could dramatically reduce the oxygen index of certain filled systems, again without any apparent effect in other tests. This effect was put down to increased spalling (thermal cracking), resulting in weakening of the ash structure. The interpretation problems outlined previously and the so far unexplained effects of particle size, make comparisons of the performance of various potential flame retardant fillers reported in the literature very questionable. This is graphically illustrated by the case of MCS referred to earlier (Section 6.4.2). MCS is a synthetic material that showed great promise when assessed by the oxygen index test in a number of polymers, giving values as good as, if not better than ATH. However, when other tests, such as ignition time or the UL94 test, were carried out the performance was little or no better than the base polymer. Typical results illustrating this point are presented in Table 6.2.

Table 6.2 A comparison of the flame retardant performance of aluminium hydroxide (ATH) and MCS [125 phr in crosslinked EVA] Result with Test Oxygen index (%) Ignition time (seconds) (UL 94 method)

No Filler

Glass Beads

ATH

MCS

18.5

21.5

30.5

33.0

<2

<2

20

5

Ashley and Rothon used X-ray methods to analyse the ash from the oxygen index tests performed on MCS filled polymers, and found virtually complete decomposition to magnesium oxide [8]. However, when the ash was continuously removed during the test they found that the oxygen index fell markedly. Thus it would seem that the relatively stable MCS is given time to decompose under oxygen index conditions,

276

Effects of Particulate Fillers on Flame Retardant Properties of Composites probably as the flame travels down through the ash layer, but decomposes too slowly to be effective in ignitability tests. Just as the oxygen index test has been the one most widely used in research, so it has been the one test that people have attempted to model. Bolodyan and co-workers have modelled the test in terms of a heat balance equation [29]. They have used this to account for the effect of inert fillers in terms of their heat capacity and found reasonable agreement with experiment. More recently, the heat balance approach has been extended by Case and Jackson [27] and by Khalturinskii and Berlin [30] to include the thermal properties of flame retardant fillers. The Case and Jackson work has taken the analysis further than Khalturinskii and Berlin, and is reproduced in detail here. Case and Jackson consider the heat balance over the whole combustion sequence. This heat balance is applied to combustion of unit weight of polymer having a gasification endotherm Q, forming fuel gases with heat of combustion H and oxygen requirement, r parts by weight of oxygen. The polymer is loaded with l parts by weight of filler, with a decomposition endotherm, E, evolving a fraction ω of its weight as non-combustible gas, (i.e., CO2 and/or H2O in the present work). The weight fraction oxygen indices for the filled and unfilled polymers are Y and YO, respectively, and it is assumed that the oxygen containing gas stream and the unburnt polymer system have the same initial temperature, TO . The temperature at which the gaseous products and solid filler residues exit from the combustion cycle are TE and TS, respectively. The constant pressure heat capacity terms, c, are identified by subscripts referring to the filler residue (S), filler generated inert gas (I), diluent nitrogen (N) and combustion products (p). The heat balance leads to the Equation 6.1, in which an energy term R has been included to allow for all heat losses: cp ⎞ cN (TE − TO ) (H − Q) R ⎛ = − ⎜ cp − cN + ⎟ (TE − TO ) r r⎝ r⎠ YO

(6.1)

To take account of the enthalpy absorbed by endothermic breakdown of the filler and raising the temperature of its decomposition products, an extra term directly proportional to the loading must be subtracted from the right hand side of this equation. Assuming that all the terms in Equation 6.1 are not significantly affected by filler incorporation, the reciprocal of the oxygen index can be directly related to filler loading l as shown in Equation 6.2: 1 1 − = μl YO Y

(6.2)

277

Particulate-Filled Polymer Composites

Where: μ =

E + (1 − ω)cS (TS − TO ) + ω cI (TE − TO ) rc N (TE − TO )

Equations 6.1 and 6.2 thus provide a basis for analysing the way in which the oxygen index of a filled polymer is determined by (a) the known thermochemical properties of the components and (b) the unknown parameters TE, and TS, and the heat loss term R. Case and Jackson’s analysis predicts that a plot of the reciprocal of the weight fraction oxygen index against filler loading (on a parts by weight per hundred of polymer basis) should be a straight line of slope μ. They examined the effect on the oxygen index of various loadings of aluminium hydroxide and nesquehonite (MgCO3.3H2O) in PMMA, polystyrene, polyethylene, unsaturated polyester resin and phenol formaldehyde and of calcium carbonate in polyethylene only. Providing the fillers were not too fine, good agreement between experimental and theoretical predictions were obtained. Typical results are presented in Figure 6.3 for nesquehonite and alumina hydroxide in unsaturated polyester. On the basis of this work they concluded that the thermal effects of the fillers were sufficient to account for their effects on the oxygen index. They were also able to

Figure 6.3 Linearity of the reciprocal of oxygen index versus filler loading plots, as predicted by the model of Case and Jackson for two fillers (aluminium hydroxide and nesquehonite) in PMMA

278

Effects of Particulate Fillers on Flame Retardant Properties of Composites conclude that for a given filler/polymer system, TE, TS and R do not change significantly over the range of filler loadings studied. Case and Jackson also showed by analysis of the combustion gases produced just above the oxygen index value, that the fillers had little effect on their composition and thus on the heat release per unit of polymer consumed. They were then able to derive values for TE and TS of about 900 and 600 °C, respectively. These were largely independent of filler and polymer type. Finally, by using these derived values and the known filler decompositon endotherms, they were able to calculate the contribution to the overall heat balance of the different filler effects. This is summarised for two of the fillers in Table 6.3.

Table 6.3 Contribution of various factors to the oxygen index effect (using the analysis of Case and Jackson) % Contribution to heat balance effects Enthalpy of decomposition

Heat capacity of residue

Heat capacity of the evolved gases

Al(OH)3

51.4

18.5

30.1

MgCO3.3H2O

56.9

7.6

35.5

Filler

While clearly demonstrating the importance of the endotherm in the heat balance, it is interesting to note that the heat capacity effects, especially of the evolved gases, are also significant contributors. The success of Case and Jackson’s analysis is demonstrated by the fact that they were able to show that the previously published endotherm for ATH decomposition could not be correct. This led to a redetermination of the value by Jackson and Jones who obtained a value in keeping with the predictions of the model and with theoretical consideration [31]. Case and Jackson also carried out studies on upward burning oxygen index values and again found good agreement with their model, the lower oxygen index values in this configuration being due to reduced heat losses. The one condition where the model does break down is with very fine fillers. These do not give straight line plots when the data is treated as described, the oxygen index

279

Particulate-Filled Polymer Composites value increasing faster with loading than predicted. This is in keeping with the particle size effect noted earlier. As the decomposition endotherm is not affected by particle size and complete decomposition is already assumed in the analysis, then the effect must be due to a change in one or more of TE, TS and R. Case and Jackson were able to show that anomalously high values were always associated with a fine grained ash structure and that continuous removal of this ash residue gave values in keeping with prediction. These observations suggest that the main effect may be on TS and R rather than on T E. Indeed the present author’s own observations are that the flame actually enters this fine ash structure, and raises it to incandescence. Thus TS may approach 900 °C, increasing the contribution from the heat capacity of the residue and possibly also increasing radiative heat losses. Unfortunately, while the oxygen index test appears fairly well understood, its relevance to other tests is not well founded, and these other tests are themselves poorly understood in terms of filler effects. As seen previously, the endotherm and heat adsorbing effects can contribute markedly to the heat balance in the conventional oxygen index test, where only a small fraction of the heat of combustion is fed back. In other tests such as UL94 and particularly radiant heat tests, these effects probably become less significant and other factors may become dominant. Thus, Hornsby and Watson have highlighted insulation by a robust ash layer and solid state stabilisation effects as possible important factors [32]. They have also drawn attention to the high surface area of the oxides produced from decomposition of fire retardant fillers as possibly playing an important role. These may well undergo sintering reactions leading to strong ash structures being developed. This is an aspect not often recognised and might explain some of the differences observed between different grades of the same material. b) Other Tests Unfortunately, there has been little scientific work using tests other than oxygen index. Probably the most comprehensive is that by Hughes, Jackson and Rothon on the effect of ATH on PMMA. They have investigated the effect of filler loading and particle size using the oxygen index, ignitability (ISO 5657) [9], UL94 vertical burn [2] and ASTM horizontal burn tests [3, 28].

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Effects of Particulate Fillers on Flame Retardant Properties of Composites The oxygen index results have already been presented in Figure 6.2. Results for the same fillers in the ISO ignitability test are presented in Figure 6.4. This shows the coarsest fillers to have very little effect in this test, while the finest has a very dramatic effect. In the UL94 vertical burn test, the finest fillers achieved a Vo rating at 50% by weight, while 60% was required for the intermediate sizes, and the coarsest was still unclassifiable at 60%. The effect of filler loading and size in this test configuration is illustrated by time to ignition data determined by the procedure developed by Ashley and Rothon [8]. This is presented in Figure 6.5. Data for the ASTM horizontal burn test are presented in Table 6.4 and again show a particle size effect.

Figure 6.4 The effect of filler particle size on the ignitability (ISO 5657 [9]) of PMMA filled with aluminium hydroxide. Test carried out at 20 kWm-2

Table 6.4 The effect of ATH particle size on the horizontal burn performance (ASTM D635) [3] of 50% w/w filled PMMA composites Filler specific surface area (m2 g-1)

Burn time (s)

Extent burnt (mm)

~0.1

654

72

0.5

20

9.1

2.2

6.5

6.7

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Particulate-Filled Polymer Composites

Figure 6.5 The effect of filler particle size on the ignition times (UL 94 configuration) of PMMA filled with alumina hydroxide

Thus, the beneficial effect seen in oxygen index has been shown, in this system at least, to be mirrored in a range of other tests. Furthermore, the particle size effect is also observed in all of the tests, although it appears more marked in some than in others. In an attempt to explain the particle size effect, the fillers were all examined by differential scanning calorimetry (DSC) but only minor differences, insufficient to account for the observed variation in performance, were found (see the section on ATH in Chapter 2 for a detailed discussion of particle size effect on kinetics of decomposition). Miyata and co-workers have examined the effect of magnesium hydroxide on the flammability of polypropylene, using the UL94 vertical burn test [33]. They found that the performance again increased with decreasing effective particle size and that levels of about 58% w/w were needed to achieve Vo rating at 0.3 cm thickness. The rate of change was very steep below this filler level. Hornsby and Watson have also investigated the effect of various magnesium hydroxides on polypropylene. They found some effect of filler type on UL94 vertical burn rating, but were unable to correlate this with particle morphology [26]. Herbert has carried out a comprehensive study of the effect of ATH on the fire resistance of a peroxide crosslinked EVA (18% vinyl acetate), using the cone calorimeter [34]. This work showed that ATH was an effective fire retardant in this test, with significant reductions in most parameters, notably reduced peak heat release rate and smoke level.

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Effects of Particulate Fillers on Flame Retardant Properties of Composites

Figure 6.6 A comparison of the effect of aluminium hydroxide and calcium carbonate fillers on the heat release rate from an EVA copolymer using cone calorimetry. Filler loading 60% w/w, irradiance 35 kW m2. (After Herbert [34])

Typical results are shown in Figure 6.6. ATH is seen to be much more effective than an inert filler, such as calcium carbonate, thus confirming that any effects are more than simple dilution. Somewhat surprisingly, given the results of some of the other studies referred to previously, no significant particle size effect was observed. Although a considerable size range was used in the study, they were all fairly fine and it may be that an effect would still be seen with coarser material. In work not yet published, the present author and co-workers used cone calorimetry to examine the effects of various magnesium hydroxides on the fire resistance of a polypropylene copolymer. The fillers were all fatty acid coated, and compounds were prepared using a Buss ko-kneader, followed by injection moulding. Magnesium hydroxide was found to increase the time to ignition, and to reduce the rate of heat release, peak heat release rate and smoke formation. They found significant differences in performance among magnesium hydroxides from different suppliers, but no significant effect of particle size (over a limited range) or of coating level, for a given magnesium hydroxide type. Typical results demonstrating the differences in performance are presented in Figures 6.7 and 6.8. The heat release rate curves are

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Particulate-Filled Polymer Composites

Figure 6.7 A comparison of the effect of two similar magnesium hydroxide fillers on the heat release rate from polypropylene using cone calorimetry. Filler loading 65% w/w, irradiance 50 kWm-2

Figure 6.8 A comparison of the effect of two similar magnesium hydroxide fillers on the smoke level from polypropylene using cone calorimetry. Filler loading 65% w/w, irradiance 50 kWm-2

284

Effects of Particulate Fillers on Flame Retardant Properties of Composites particularly interesting as, while one shows twin peaks, the other does not. This finding was confirmed by repeat compounding and testing. In the work referred to previously, Herbert often found similar twin peaks for ATH filled composites and speculated that the first peak was the point at which the fire retardant filler first became effective, while the second occurred once it had all been used up. The results of Hughes and co-workers, referred to in the preceding section, are worthy of further comment here. Thus, while a particle size effect is apparent in all the tests investigated, its magnitude appears to vary considerably from one test to another. A poor correlation is also often observed if one attempts to plot results from one test against another, even for the coarser fillers. This implies that the various factors that govern filler performance are varying in relative importance according to the nature of the test. By analogy with halogenated flame retardants, most workers put great emphasis on the importance of the filler decomposition closely matching that of the polymer. While this is undoubtedly true for the halogens, which by their mechanism of action must accompany the volatile pyrolysis products, it is unproved for flame retardant fillers, which function by different mechanisms. While too high a decomposition temperature is almost certainly undesirable, more rapid decomposition than the polymer may well give excellent results in some tests (try lighting wet wood!). One can imagine that a combination of fillers, to achieve a broad decomposition range could be beneficial in some tests, (e.g., cone calorimetry), and this is sometimes used in practice.

6.4.3.3 Application of Thermal Analysis Various thermo-analytical procedures are frequently used for assessing the temperature and rate of decomposition of fillers, polymers and composites. While these undoubtedly have an important part to play, great care must be taken in carrying out and interpreting such analysis. Thus, as found by Jackson, the results can vary markedly according to the exact nature of the sample (powder, chip, etc.) [35]. Also as reported by Rychly and co-workers, the decomposition of both the filler and polymer may be different in the composite to those of the separate components [36]. Jackson and Rothon have observed that elevated pressure can delay the decomposition of hydroxides and hydrates [37] and such an effect may well be observed in a polymer matrix. Of particular interest is the tendency of ATH to form boehmite, rather than undergoing complete dehydration. This may well be favoured by the presence of polymer and has been discussed in Chapter 2.

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Particulate-Filled Polymer Composites Other factors that have to be considered are: i) the faster rate of heating in flammability tests than is possible in most thermal analysis. ii) the concentration on solid state effects in thermal analysis, while gas phase processes are also important in flammability. iii) both surface and bulk effects have to be taken into account. iv) the nature of the heating source: conductive, radiative, etc., can be important. The application of thermal analysis is illustrated in Figures 6.9-6.12 based on unpublished work by Ashley and Rothon. Figure 6.9 shows DSC traces for the effect of magnesium hydroxide on Nylon 6 under nitrogen. The endotherm for the filler is clearly seen and occurs well before that for polymer pyrolysis, which in this case is apparently unaffected in position by the filler. Figure 6.10 shows similar DSC traces in air. These are much more complex. The early decomposition of the filler is still apparent as an endotherm, but now may be overlapping the start of the polymer exotherm. The shape of the exotherm also seems to have been much affected by the presence of the filler.

Figure 6.9 DSC traces (in nitrogen) showing the effect of magnesium hydroxide in Nylon 6

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Effects of Particulate Fillers on Flame Retardant Properties of Composites

Figure 6.10 DSC traces (in air) showing the effect of magnesium hydroxide in Nylon 6

The dramatic effect of magnesium hydroxide on the solid state processes involved in polypropylene air pyrolysis is illustrated by the DSC data in Figure 6.11. The presence of the filler is seen to cause both a delay in the exotherm, and a marked sharpening when it does occur. This is in keeping with increased char formation, followed by sudden oxidation of the char, as mentioned elsewhere. Surprisingly a very similar trace is observed with calcium carbonate as the filler. While not as dramatic, this effect is also demonstrated by the thermo-gravimetric analysis (TGA) data presented in Figure 6.12. This shows the weight loss to be delayed but to go very rapidly once it does occur. Rychly and Pavlinec have developed a method for analysing thermo-gravimetric data for multi-step processes obtained under non-isothermal conditions to provide kinetic data for each step [38]. Rychly and co-workers have applied this and other methods to a detailed study of the flame retardant effects of fillers [36]. In addition to the oxygen index, they used an ignition test based on an adapted thermal analysis method to assess flammability. Rychly and Pavlinec [38] put particular emphasis on the effect of fillers on the activation energy for polymer pyrolysis determined in air. Movement of this value

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Particulate-Filled Polymer Composites

Figure 6.11 DSC traces (in air) showing the effect of magnesium hydroxide in polypropylene

from that determined on the neat polymer in air towards that for the same polymer in nitrogen is interpreted as indicating efficient surface blanketing by filler decomposition gases. Their work is particularly interesting as it attempts to understand the effects of fillers on ignition. Increased polymer thermal stability in air as shown by a move of the associated weight loss in TGA or by higher activation energy gave good correlation with longer ignition times. The most pronounced effect on ignition time they found was for magnesium hydroxide in polypropylene and this correlated well with increased polymer stability. In polyethylene, the magnesium hydroxide they used had a much more marked effect on activation energy than did the aluminium hydroxide and was also found to produce a higher ignition time. These workers also observed that in EVA copolymer they only obtained two steps in the TGA trace, while summation of independent filler and polymer traces would indicate

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Effects of Particulate Fillers on Flame Retardant Properties of Composites

Figure 6.12 Thermogravimetric traces (in air) showing the effect of magnesium hydroxide in polypropylene

that three should be obtained. It appears that in this polymer water release is delayed from aluminium hydroxide but accelerated from magnesium hydroxide. This is probably due, at least in part, to the effects of acetic acid released from the polymer. They also observed that carbon retention and subsequent oxidation could be very important and was particularly noticeable with magnesium hydroxide in polypropylene leading to a sharp exotherm of the type already illustrated in Figure 6.11. They also claimed that this could lead to greater heat feedback in the oxygen index test, thus reducing fire retardant filler effects. In separate work aimed at elucidating filler effects on surface, oxidation Vesely and coworkers [39] have examined the oxidation of polyethylene and polypropylene containing high levels of calcium carbonate. The grade of filler used was found to decrease polymer thermal stability, an effect they associated with transition metal impurities. They also believe that, in the presence of fillers, heteregeneous oxidation reactions become more important. They observed black polyenes to be formed above about 320 ºC and postulated that these could be acting as photo-sensitisers.

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Particulate-Filled Polymer Composites Finally Baillet and Delfosse have developed an apparatus for determining the temperatures of ignition and incandescence of filled polymers [40]. They have applied this to an EVA copolymer filled with either aluminium or magnesium hydroxide. Both fillers were found to increase the self-ignition temperature with the particular magnesium hydroxide used being the more efficient at low loadings [41]. In the same work they used TGA to examine filler effects on polymer decomposition, and concluded that the main solid state effect in EVA is the oxidation of carbonaceous residues. Their work is discussed in more detail in Section 6.4.4.3.

6.4.4 Smoke and Corrosive and Toxic Gases 6.4.4.1 Introduction Conventional methods for flame retarding polymers are usually halogen-based and rely on interruption of gas phase combustion processes for their efficiency. Consequently, these additives generally give rise to relatively high smoke levels compared to non-flame retarded polymers. The combustion gases are also usually rich in hydrogen halides, which can pose hazards to both humans and equipment in a fire scenario. Furthermore, although not proven, there is a concern that toxic organohalogen compounds may be evolved during the processing and use of such flame retardant polymers as well as in a fire. As mentioned earlier in Section 6.1, one of the driving forces behind the growth in the use of hydrated fillers is their low level of smoke generation, coupled with the lack of evolution of corrosive and toxic off-gases. This is because they do not function by interrupting the combustion process and only add water and in some cases carbon dioxide to the combustion products. In addition to producing less smoke than halogenated flame retardants, there is also some evidence that hydrated fillers can reduce smoke levels compared to the unfilled, non-flame retarded, host polymer, and that some fillers are more effective than others in this respect. Because of the importance of smoke, this topic is worthy of further examination. Unfortunately, the present state of knowledge in this area is even less developed than with flame retardancy. This is due in part at least to the complexity of the smoke issue in flammability studies and the difficulties in carrying out meaningful measurements under laboratory conditions. Smoke generation is known to vary according to whether the sample is smouldering, or burning, and to depend on the oxygen level which can fall considerably in some types of fire situation. Furthermore, the effects of hydrated fillers on smoke generation will probably vary considerably according to the nature of the host polymer, thus making generalisations very difficult.

290

Effects of Particulate Fillers on Flame Retardant Properties of Composites The whole issue of smoke formation and measurement in burning polymer systems has been extensively reviewed by Hirschler [12, 42, 43] and this provides a very useful background.

6.4.4.2 Endothermic Fire Retardant Filler Effects An important early study on the effect of fillers on smoke formation was carried out by Lawson and co-workers [17]. They studied the effects of aluminium hydroxide, calcium carbonate and magnesium hydroxide fillers on soot and smoke formation from styrenebutadiene rubber (SBR) foams. While their results for smoke were obtained under unrealistically high oxygen concentrations and must be treated with caution, soot was measured under ambient air combustion and some useful observations were made. i)

All the fillers reduced soot formation compared to unfilled foam. The order of effectiveness was CaCO3 < Al(OH)3 < Mg(OH)2. The calcium carbonate effect was attributed to dilution, that of the other fillers to a combination of dilution and promotion of solid state crosslinking and charring.

ii) Soot particle size distribution and CO:CO2 ratio in the combustion gases was very similar for both calcium carbonate and aluminium hydroxide fillers. This was taken as evidence for the relative unimportance of the water released from the hydrated filler in reducing smoke levels (by carbon oxidation via the water gas reaction for instance). iii) Although obscured by the unrealistic experimental conditions, both hydrated fillers gave less smoke under flaming conditions than calcium carbonate, with magnesium hydroxide being the more effective. iv) The increased char formation observed with the hydrated fillers was attributed to their endothermic cooling effect, causing slower heating and favouring crosslinking as opposed to pyrolytic degradation processes. The difference in effectiveness between aluminium and magnesium hydroxides was further attributed to the higher decomposition temperature of the latter better favouring the crosslinking processes in the SBR foam. v) The possible synergy of aluminium/magnesium hydroxide mixtures was briefly explored. vii) The occurrence of afterglow, possibly due to slow burn-off of carbon residues, was also noted with magnesium hydroxide.

291

Particulate-Filled Polymer Composites Mosesman and Ingham examined the effect of various fillers on smoke generation from ethylene-propylene-diene monomer (EPDM) elastomers under both flaming and nonflaming conditions, using the NBS smoke chamber [44]. The fillers used were calcium carbonate, soft clay, ATH and magnesium hydroxide. Very little difference was noted between the fillers under non-flaming conditions. Under flaming conditions, both hydrated fillers performed better than the soft clay with magnesium hydroxide being far superior to ATH. Surprisingly, calcium carbonate gave results almost as good as the magnesium hydroxide. Unfortunately, no mechanistic studies were carried out. Keating, Petrie and Beekman compared magnesium hydroxide and calcium carbonate in polypropylene using the NBS smoke chamber [45]. Magnesium hydroxide gave some smoke reduction compared to the inert filler under flaming conditions and also under non-flaming conditions at high heat fluxes. They also compared magnesium and ATH in polyethylene, where the magnesium compound again gave better results under flaming and higher heat flux, non-flaming conditions. In both polymers, magnesium hydroxide was observed to promote char formation compared to the other fillers; as determined by the carbon content of the residue from pyrolysis experiments. This was thought to be the origin of the reduced smoke levels observed. Hornsby and Watson have examined the effect of fillers on smoke generation from a number of polymers [26, 32, 46]. Magnesium hydroxide was found to give marked smoke reductions (more than expected from dilution effects) under both flaming and non-flaming NBS smoke chamber conditions when added to acrylonitrile-butadienestyrene (ABS), polypropylene oxide (PPO) and polybutylene terephthalate polymers. All these polymers give very high levels of smoke when unfilled. Polypropylene was examined using the UITPE4 test (International Union of Public Transport Method E4, also known as the 3 metre cube test from the size of the test chamber). The expected dilution effect on smoke was observed with calcium carbonate, with significantly lower smoke levels being observed with both aluminium and magnesium hydroxides. In this case, the ATH gave slightly better results. Magnesium oxide was found to give similar smoke reduction levels to the hydroxide, indicating again that the water release does not play a significant role in smoke reduction. The levels of carbon monoxide evolution were also found to be reduced from ABS and PPO containing both magnesium hydroxide and oxide, again indicating that the water gas reaction has little effect. They also noted a reduced effect from magnesium hydroxide under non-flaming conditions, which they associated with incomplete filler decomposition.

292

Effects of Particulate Fillers on Flame Retardant Properties of Composites Overall, their conclusions were that the smoke reducing effect is predominately a consequence of the high surface area oxides resulting from filler decomposition. They adsorb soot precursors leading to solid state coking processes and also promote subsequent oxidation of the carbon to non-obscuring oxides. The reduced combustion rates may also play a part in improving oxygen to fuel ratios, thus further reducing smoke. These workers also draw attention to the afterglow phenomenon associated with slow carbon burn-off. Hirschler and Thevaranjan investigated the effect of magnesium oxide and hydroxide on smoke formation from polystyrene [47]. Both additives were found to produce marked reductions in smoke, again showing that the endotherm and water release are unimportant in this effect. In other work, Hirschler found that very small quantities of high surface area silica were an effective smoke suppressant for polystyrene [48]. This was thought to function by sintering to form a glassy surface layer. Herbert has reported on the effect of ATH in a peroxide crosslinked EVA [34]. Using the cone calorimeter with an irradiation of 35 kW/m2, the ATH was found to give much lower smoke levels than calcium carbonate and aluminium oxide fillers. In addition to their use in non-halogenated systems, fillers, especially of the hydrate type, have been extensively studied for use in halogenated polymers such as polyvinyl chloride, or in conjunction with halogenated flame retardants. The aim is to combine their effects and produce a composite with high flame retardancy, low smoke and low corrosive gas emission. One of the problems of using fillers to adsorb hydrogen halides is that this prevents them functioning as gas-phase flame retardants and hence can reduce the flame retarding effect, although this can to some extent be offset if hydrated fillers are used. This approach appears to have had reasonable success but has received little fundamental study. One of the earliest studies was by Stewart and co-workers who demonstrated the benefit of ATH in reducing smoke from polychloroprene foam [49].

6.4.4.3 Afterglow The afterglow phenomenon is a recurring theme in the work described previously. This effect can seriously reduce the effectiveness of flame-retardants in certain applications and hence must be taken seriously. A detailed study of this effect in filled ethylene-vinyl acetate copolymer has been made by Delfosse and co-workers [41]. Using a heated quartz reactor purged with air, they were

293

Particulate-Filled Polymer Composites able to follow self-ignition and incandescence effects both visually as well as by monitoring the temperature of the sample and of the gas phase just above it. Depending on the reactor temperature a polymer sample will slowly oxidise, incandesce or burst into flame. The unfilled polymer was found to have a self-ignition temperature of about 456 °C. Below this, it slowly oxidised with weak exothermicity and no incandescence. Fillers such as aluminium and magnesium hydroxide were found to progressively raise the ignition temperature, with magnesium hydroxide being the more effective at low loadings. At the same time, however, incandescence appeared at lower temperatures. The onset temperature of incandescence decreased with filler loading and the magnesium hydroxide used was found to have a greater effect than the aluminium hydroxide at all loadings. These effects are summarised in Figure 6.13. It was also observed that the residue from the experiments under incandescent conditions was a white ash with no carbonaceous matter. By contrast, a kaolin filled sample showed no incandescence and a black residue below the self ignition temperature. Thermal analysis

Figure 6.13 The effect of aluminium and magnesium hydroxide fillers on the selfignition and incandescence temperatures of an EVA coplymer. (Reproduced with permission from L. Delfosse, C. Baillet, A. Brault and D. Brault, Polymer Degradation and Stability, 1989, 23, 4, 337, Figure 1 [41]. Copyright, 1989, Elsevier Science)

294

Effects of Particulate Fillers on Flame Retardant Properties of Composites studies showed that both flame retardant fillers affected the pyrolysis of the polymer with more complete oxidation observed in the final stages. Overall they concluded that the afterglow or incandescence was, like smoke reduction, due to catalytic oxidation of carbonaceous residues by the oxides resulting from filler decomposition. The greater effect of magnesium hydroxide was then assigned to the known greater catalytic effects of its oxide. In later work Baillet and Delfosse examined the effect of the flame retardant fillers on the formation of carbon oxides during pyrolysis of an EVA polymer and tested various additives for incandescence suppression effects [50]. Using the same quartz reactor described previously, they demonstrated that below the self-ignition temperature, the filled systems gave much more complete oxidation (higher CO2:CO levels) than the unfilled polymer. This supports the greater degree of char combustion referred to previously. In their search for incandescence inhibitors, they screened a number of known organic catalyst poisons but without success, probably due to insufficient thermal stability. Inorganic additives proved more promising. Ammonium polyphosphate (APP) was found to be very effective but suffers from water sensitivity problems and was rejected on this basis. Surprisingly antimony trioxide, which is often regarded as an incandescence promoter itself, was found to be very effective and was their preferred material. Unfortunately, the effectiveness of antimony trioxide on flame retardancy and smoke formation was not reported on.

6.4.4.4 Summary While the results are far from conclusive, and the danger of generalising must be borne in mind, the following picture is emerging: i)

Fire retardant fillers such as aluminium and magnesium hydroxide generally reduce the smoke from burning polymers, especially under flaming as opposed to nonflaming conditions.

ii) The mechanism of their action seems independent from their endothermic and water releasing characteristics. It seems to be composed of a simple dilution effect exhibited by all fillers, plus solid state carbon formation and oxidation capabilities associated with the catalytic nature of the oxide formed and the high surface area resulting from the filler decomposition process. Improved oxygen to fuel ratios due to slower burning may also play a part. iii) There is some evidence that magnesium hydroxide is a more effective smoke suppressant, probably due to greater catalytic activity of the oxide formed. However, this also manifests itself in an afterglow phenomenon, which may be a problem in some applications.

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Particulate-Filled Polymer Composites

6.5 Nano-Clays Currently there is much interest in the use of nano-clays as flame retardant additives for polymers. The topic has recently been reviewed by Porter and co-workers [51]. Present understanding is not well developed, but Gilman and co-workers at NIST are carrying out an extensive study of the area [52, 53]. Commercial applications are also starting to appear [54]. The production and characteristics of nano-clays are covered in Chapter 2 of the present work. Their uses in polymers, including further discussion of fire retardant effects, are the subject of Chapter 10. Some aspects of their compounding are dealt with in Chapter 5. Nano-clay composites contain very thin, high aspect ratio, alumino-silicate platelets, derived from the stacks present in the parent clay. Two types of structure can be recognised, intercalated and delaminated. In the intercalated form, the stacks of platelets are still present, but with polymer chains present between clay platelets, pushing them apart. In the delaminated form, the original stacks are no longer present, and the platelets are fully dispersed. Due to their nature, nano-clay loadings are usually restricted to about 10% w/w because of processing difficulties. At these levels, they show little benefit in the traditional small scale tests, such as oxygen index and UL94 and this probably led to a lack of interest in them until recently. It has now been found that very good results can be obtained with filler loadings of 2-10% in some fire retardant test parameters, particularly using the cone calorimeter, and they are now receiving serious study. The principal effects observed in cone calorimeter tests are a marked reduction in peak and average rate of mass loss and in heat release. There appears to be little reduction in total heat of combustion or in smoke levels. The workers at NIST report that there seems to be little difference between intercalated and delaminated forms of nano-clay, despite evidence that polymer thermal stability can be more improved by the intercalated structure. As an example, Gilman and co-workers [52] reported a reduction of over 60% in peak heat release rate for a 5% w/w loading of a nanoclay in polystyrene. There was little benefit from increasing the loading beyond 5%. They also found that the processing conditions were critical to obtaining good fire retardant effects. Similar reductions in peak heat release rate have been reported for other polymers, including polyamides and polypropylene [55]. Alexandre and co-workers [56], reported similar results for an EVA composite. They found a 47% reduction in the peak heat release rate at 5% w/w of a nano-clay and no further benefit by increasing the loading. The effect on ignition time is not clear, but the workers at NIST have reported that the ignition time can sometimes be reduced, an effect that they think may be due to instability of the surface treatment present on the clay that was used.

296

Effects of Particulate Fillers on Flame Retardant Properties of Composites The primary effect of the nano-clays seems to be related to char formation. The workers at NIST have found that a reduction in mass loss and heat release rate only starts once the surface of the polymer is at least partly covered by char. Beyer reported that, while no char was produced by burning unfilled EVA, the filled composite formed a strong char early in the process [54]. Once the amount of clay is taken into account, final char levels are often similar to unfilled polymer, indicating that while a stronger, more insulating, char may form and retard combustion, it is eventually consumed in this test. Currently, nano-clays are of most interest as admixtures with flame retardant fillers such as ATH, rather than as primary flame-retardants. Beyer [57] has reported that replacement of a small amount of ATH can significantly improve fire retardancy and increase char integrity at the normal filler loadings used. Alternatively, the overall filler loading can be significantly reduced, while maintaining the existing level of fire retardancy. This work is still in it’s infancy, and further developments in understanding, products and applications can be expected.

6.6 Ammonium Polyphosphate (APP) Although not strictly a filler, this additive deserves a brief mention. Its mode of action is predominately by the promotion of char formation, resulting from high acidity phosphoric acids produced on pyrolysis. The polyphosphate is used, as it is less water soluble and hygroscopic than the orthophosphate. APP is effective on its own in polymers with suitable chemistry for char formation to occur by this mechanism. In other polymers, it is used in conjunction with a charring agent, such as a melamine derivative [58].

6.7 Fillers for Use in Conjunction with Halogens Organo-halides, especially bromides, are very effective fire retardants for most polymers. They work mainly by interruption of the gas phase combustion processes. This leads to incomplete combustion and high levels of smoke formation. It has been known for a long time that the efficiency of the halogenated flame retardants can be much increased by the use of certain additives, usually known as synergists. Most of these synergists are particulate inorganic fillers. Their main mode of action is believed to be the result of the formation of volatile metal halides. The main synergist in commercial use is antimony trioxide, which is believed to function by forming volatile antimony halides, which interfere with the gas phase

297

Particulate-Filled Polymer Composites combustion processes. Antimony oxide appears to have little solid phase activity, with most of the antimony being lost to the gas phase during combustion. High levels of smoke result from the gas phase action. Maximum efficiency is generally found at an antimony to available halogen molar ratio of 3:1. Touval has provided a good review of this additive [59]. Alternative synergists include borates, molybdates and stannates. The borates and stannates are often in the form of zinc salts. Cook and Musselman have recently reviewed the performance of these materials [60]. Cusack and co-workers have also carried out extensive studies of the zinc stannates [61, 62]. Briefly, it appears that, in addition to gas phase effects similar to antimony trioxide, these additives show significant solid state activity, increasing char formation, with considerable amounts of the metal remaining in the solid phase. It appears that most of the solid state effects are due to halide salts and so require the presence of halogen. The borates may also give a stronger, glassy char. There may also be some endotherm and inert gas contribution from some of the materials. Higher addition levels tend to be needed than with antimony oxide, but smoke levels are lower. These additives are usually used in combination with antimony oxide for best overall effect. Cusack and co-workers have reported that the efficiency of the zinc stannates can be markedly improved if they are finely divided and carried on a filler such as ATH to improve dispersion.

References 1.

S. Miyata, inventor; Kabushiki Kaisha Kaisui Kagaku Kenkyujo, assignee; EP 0 498 566A1, 1992.

2.

UL94, Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, 1996.

3.

ASTM D635-98, Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position, 1998.

4.

V. Babrauskas in New Technology to Reduce Fire Losses and Costs, Eds., S.J. Grayson and D.A. Smith, Elsevier Applied Science Publishers, London, UK, 1986, 78.

5.

R.K. Wharton, Fire and Materials, 1981, 5, 3, 93.

6.

ASTM D2863-00, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), 2000.

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Effects of Particulate Fillers on Flame Retardant Properties of Composites 7.

R.K. Wharton, Fire and Materials, 1979, 3, 1, 39.

8.

R.J. Ashley and R.N. Rothon, Plastics & Rubber & Composites Processing & Applications, 1991, 15, 19.

9.

ISO 5657-97, Reaction to Fire Tests — Ignitability of Building Products using a Radiant Heat Source, 1997.

10. ASTM E1354-02de1, Standard Test Method for Heat and Visible Smoke Release Rates for Materials and Products Using an Oxygen Consumption Calorimeter, 2002. 11. ISO 5660, Reaction-to-fire Tests – Heat Release, Smoke Production and Mass Loss Rate, 2002. 12. M.M. Hirschler, Journal of Fire Sciences, 1985, 3, 5, 343. 13. ASTM E662-01, Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials, 2001 14. H. Frood and H.P. Alger, inventors; no assignee; GB 183, 922, 1921. 15. No inventors; General Electric, assignee; GB 831, 490, 1960. 16. W.J. Connolly and A.M. Thornton, Proceedings of the SPI 20th Reinforced Plastics Technology and Management Conference, Chicago, IL, USA, 1965. 17. D.F. Lawson, E.L. Kay and D.T. Roberts, Jr., Rubber Chemistry and Technology, 1975, 48, 1, 124. 18. G.L. Nelson, Journal of Fire and Flammability, 1974, 5, 2, 125. 19. D.A. Rust, Proceedings of Functional Fillers and Reinforcements ’99, Intertech Conference, Atlanta, GA, USA, 1999, Paper No.22. 20. M.L. Clemens, M.D. Doyle, G.C. Lees, C.C. Briggs and R.C. Day, Proceedings of Flame Retardants ’94, Interscience, London, UK, 1994, 193. 21. J.F. Kalina, Modern Plastics, 1975, 5, 5, 42. 22. E. Redondo Grizante, F. Peruzzotti, D. Tirelli, A. Zaopo and E. Albizzati, inventors; Pirelli Cavi e Sistemi SpA, assignee; WO 9905688A1, 1999. 23. R. Sauerwein, Proceedings of Fire and Materials 2001, Interscience, San Francisco, CA, USA, 2001, 395.

299

Particulate-Filled Polymer Composites 24. K. Nishimoto, T. Muraida and T. Murata, Japanese National Technical Report, 1975, 21, 3, 367. 25. S. Miyata, inventor; Kabushiki Kaisha Kaisui Kagaku Kenkyujo assignee; US 5,480,929, 1996. 26. P. R. Hornsby and C. L. Watson, Polymer Degradation and Stability, 1990, 30, 73. 27. J.R. Case and G.V. Jackson, ICI, Runcorn, Unpublished work. 28. P. Hughes, G.V. Jackson and R.N. Rothon, Die Makromolekulare Chemie Macromolecular Symposia, 1993, 74, 179. 29. L.A. Bolodyan, A.F. Zherlakov and co-workers, Khimicheskie Volokna, 1976, 5, 28. 30. N.A. Khalturinskii and A.A. Berlin, International Journal of Polymeric Materials, 1990, 14, 1-2, 109 31. G.V. Jackson and P. Jones, Fire and Materials, 1978, 2, 1, 37. 32. P.R. Hornsby and C. L. Watson, Proceedings of the Institute of Physics Short Meetings Series No.4 on Fundamental Aspects of Polymer Flammability, London, UK, 1987, p.17. 33. S. Miyata, T. Imahashi and H. Anabuki, Journal of Applied Polymer Science, 1980, 25, 3, 415. 34. M.J. Herbert, Proceedings of Flame Retardants ‘94, Interscience, London, UK, 1994, 59. 35. G.V. Jackson, ICI Runcorn, Private Communication. 36. J. Rychly, K. Vesely, E. Gal, M. Kummer, J. Jancar and L. Rychla, Polymer Degradation and Stability, 1990, 30, 1, 57. 37. G.V. Jackson and R.N. Rothon, ICI Runcorn, unpublished work. 38. J. Rychly and J. Pavlinec, Polymer Degradation and Stability, 1990, 28, 1, 1. 39. K. Vesely, J. Rychly, M. Kummer and J. Jancar, Polymer Degradation and Stability, 1990, 30, 1, 101. 40. C. Baillet, L. Delfosse, S. Antonik and M. Lucquin, Comptes Rendus de l’Academie des Sciences, Serie C, 1972, 274, 146.

300

Effects of Particulate Fillers on Flame Retardant Properties of Composites 41. L. Delfosse, C. Baillet, A. Brault and D. Brault, Polymer Degradation and Stability, 1989, 23, 4, 337. 42. M.M. Hirschler, Journal of Fire Sciences, 1985, 3, 6, 380. 43. M.M. Hirschler, Journal of Fire Sciences, 1986, 4, 1, 42. 44. H. Mosesman and J.D. Ingham, Rubber Chemistry and Technology, 1978, 51, 970. 45. L. Keating, S. Petrie and G. Beekman, Morton Thiokol Technical Report, Ventron Division of Morton Thiokol, Danvers, MA, USA, 1985. 46. P.R. Hornsby and C.L. Watson, Proceedings of Flame Retardants 87, (PRI/BPF), London, UK, 1987, Paper No.18. 47. M.M. Hirschler and T.R. Thevaranjan, European Polymer Journal, 1985, 21, 4, 371. 48. M.M. Hirschler, Journal of Fire Sciences, 1986, 4, 1, 42. 49. C.W. Stewart, Sr., R.L. Dawson and P.R. Johnson, Rubber Chemistry and Technology, 1975, 48, 132. 50. C. Baillet and L. Delfosse, Polymer Degradation and Stability, 1990, 30, 1, 89. 51. D. Porter, E. Metcalfe and M.J.K. Thomas, Fire & Materials, 2000, 24, 1, 45. 52. J.W. Gilman, T. Kashiwagi, A. Morgan, R. Harris, L. Brasell, W. Awad, R. Davis, L. Chyall, T. Sulto, P. Trulove and H. DeLong, Proceedings of Fire and Materials 2001, Interscience, San Francisco, CA, USA, 2001, 273. 53. J.W. Gilman, R. Davis, W.H. Awad, A.B. Morgan, P.C. Trulove, H.C. DeLong, T.E. Sutto, L. Mathias, C. Davis and D. Schiraldi, Proceedings of Flame Retardants 2002, Interscience, London, UK, 2002, 139. 54. G. Beyer, Proceedings of Nanocomposites 2002, E-Map Conference, Amsterdam, The Netherlands, 2002. 55. J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, Jr., E. Maniac, E.P. Giannelis, M. Wuthenow, D. Hilton and S. Phillips, Proceedings of Flame Retardants 2000, Interscience, London, UK, 2000, 49. 56. M. Alexandre, G. Beyer, C. Henrist, R. Cloots, A. Rulmont, R. Jerome and P. Dubois, Macromolecular Rapid Communications, 2001, 22, 8, 643.

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Particulate-Filled Polymer Composites 57. G. Beyer, Proceedings of Flame Retardants 2002, Interscience, London, UK, 2002, 209. 58. B. Nass, O. Schacker, E. Schlosser and W. Wanzke, Proceedings of Flame Retardants 2002, Interscience, London, UK, 2002, 63. 59. I. Touval, Handbook of Fillers for Plastics, 2nd Edition, Eds., H.S. Katz and J.V. Milewski, Van Nostrand Reinhold, New York, NY, USA, 1987, Chapter 15, 279. 60. P.M. Cook and L.L. Musselman, Proceedings of Flame Retardants 2000, Interscience, London, UK, 2000, 69. 61. R.G. Baggaley, P.R. Hornsby, R. Yahya, P.A. Cusack and A.W. Monk, Fire and Materials, 1997, 21, 4, 179. 62. P. Cusack, M. Cross and P. Hornsby, Proceedings of Flame Retardants 2002, Interscience, London, UK, 2002, 83.

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7

Particulate Fillers in Elastomers David Skelhorn

7.1 Introduction This chapter is devoted to the use of particulate fillers in elastomers, which make greater use of fillers than do most other polymers, largely because fillers are able to improve greatly many of their properties to an extent not found in other composites. This is particularly true of their use in automobile tyres where the high performance levels taken for granted today would not be possible without very sophisticated carbon blacks and silicas. Because of the value of fillers to the elastomer industry, it is not surprising that the most advanced work on filler characterisation and links with composite properties have been carried out in this area. Many of the filler characterisation aspects have been covered elsewhere. This chapter concentrates on the basic principles of elastomer formulation and the role of fillers in this.

7.2 Uses of Elastomers Elastomers are used in a wide range of industrial, transportation, domestic, construction and aerospace end-applications. The major consumer of both natural and synthetic rubber is the tyre industry. Five types: natural rubber (NR), styrene butadiene rubber (SBR), polybutadiene, polyisoprene and the butyl (and halobutyl) rubbers are used in massive quantities around the world. Smaller applications for rubbers include: conveyor belting, footwear, pharmaceutical closures, plant lining, hoses, extruded goods, flooring, power cables, gaskets and seals, and many, many other applications.

7.3 Elasticity of Rubber Rubber as an engineering material is unique in its physical behaviour. It exhibits physical properties that lie mid-way between a solid and liquid, giving the appearance of solidity, while possessing the ability to deform substantially. Most solid materials have an extensibility of only a few percent strain and only a portion of that is elastic, being typically Hookean in character, exhibiting a linear stress-strain relationship. Rubbers, however, may be extensible up to over 1000% strain, most of which is 303

Particulate-Filled Polymer Composites elastic and non-Hookean. This is true of rubbery materials that have been vulcanised. Rubber in the unvulcanised state exhibits substantial flow in addition to elasticity and should be regarded as a high-viscosity fluid, a factor that is vital to the manufacturing operations. Like all polymers, rubber is a viscoelastic material with complex flow behaviour [1]. Rubbers exhibit these properties because unlike conventional solids, which are comprised of atoms that occupy fixed positions relative to each other, they are formed from molecules that are arranged to form a flexible, long-chain macromolecule or polymer. While not all polymers are rubber-like, all rubbers are polymeric. The characteristic that makes a polymer rubber-like is its ability to undergo rapid molecular movement, allowing it to deform readily, and the ability of the molecule to return to its original configuration after the deforming forces have been removed. A number of qualities are necessary for attainment of rubber-like properties. Polymers may exhibit a number of different properties, which are dependent on temperature. Heat is a major source of the energy required for molecular mobility (Figure 7.1). Under low-energy conditions (cold), molecular rotation is slow so the polymer behaves as a rigid solid. As energy availability increases, the polymer progressively becomes leathery, then rubbery, followed by a transition to a rubbery fluid and ultimately total fluidity is achieved. A rubber, therefore, is a polymer whose rubbery region coincides with ambient temperature. It also follows that, on cooling, the rubbery behaviour will be lost (below the glass transition temperature) and, on heating, the rubber will ultimately become more fluid and flexible.

Figure 7.1 Variation of stiffness with temperature for polymers.

304

Particulate Fillers in Elastomers It is also essential that the polymer be of sufficient length, otherwise the molecule will behave essentially as a fluid and will possess little elasticity. Relative molecular masses in the range 100,000-2,000,000 are typical for rubbers, which ensures that a significant amount of chain entanglement occurs. Only a relatively small number of polymers have sufficient mobility to be rubbery at room temperature. The molecular mobility depends heavily on the composition of the polymer backbone, which often contains a significant proportion of simple hydrocarbon species, such as those derived from ethylene, butadiene or isoprene. These species are small and are able to undergo bond rotation with relative ease, since they do not suffer problems due to steric hindrance [2] or the presence of strong dipoles. The rubber molecule is also able to undergo extension easily because the forces acting on the material are relatively weak secondary intermolecular forces, i.e., those acting between molecules, and not the primary inter-atomic forces, i.e., those existing within a molecule [3]. A further requirement, that of recovery of shape after deformation, is also provided by high molecular mobility and the fact that long-chain polymers have a preference to exist as a randomly coiled chain, which represents a minimum energy-state condition for the polymer. A move away from this state, such as occurs during mechanical deformation of the polymer, is only achieved by the input of energy, which creates a state of higher order in the thermodynamic sense, i.e., a reduction in the entropy (disorder) of the system. The return to maximum entropy is one of the key elements of a high elastic recovery [4, 5]. Another important element for rubbery behaviour is the fixation of the polymer molecules relative to each other, since flow would occur as a result of a deforming force, if this were not achieved. This state is conventionally achieved in rubber by a crosslinking or curing process, where chemical reactions are initiated with the specific intention of joining adjacent polymer molecules via a small number of chemical bonds or bridges. The frequency of bond formation must be carefully controlled, since too few would result in excessive fluid behaviour, while too many results in loss of flexibility. This is an area of great complexity for the rubber chemist. Other materials that achieve rubbery behaviour may achieve this state by physical processes, such as when a copolymer contains one element that has elastic properties and another that is glassy or crystalline. In this case, the glassy or crystalline portion may exist as a separate rigid phase, binding the polymer spatially. Such materials are generally described as thermoplastic elastomers, since on heating to a temperature above the melting point of the glassy or crystalline element, the polymer attains fluidity and may be processed as a thermoplastic. This class of material is not considered further in this chapter.

305

Particulate-Filled Polymer Composites

7.4 Formulation of Elastomers 7.4.1 General Elastomers are complex mixtures of a large number of different materials. Essential additives for elastic behaviour would include crosslinking agents and anti-degradants. These are often complex blends of a number of chemicals designed to achieve a balance of process and end-use performance at minimum cost. Many other specialised additives are used according to the needs of the product being considered. A rubber produced using only the additives described thus far would have physical properties which are totally unsuitable for most engineering applications. The modulus (resistance to deformation) is very low and the product highly extensible. For many polymers physical properties, such as tensile or tear strength, may also be very poor. One of the main ways of modifying elastomers, so that acceptable engineering properties are obtained, is by the incorporation of fillers. The filler system may be composed of one or more of a wide variety of materials, but the main classifications used are carbon blacks, silicas and mineral products such as kaolin (china clay) or calcium carbonate. Each filler imparts a particular balance of properties, and selection, therefore, is made based on the property requirements of the end product. Typically, carbon blacks would be used to enhance strength characteristics, while minerals provide a more modest strength enhancement at significantly reduced cost. All modify processing performance and allow the production of articles that could not otherwise be fabricated. A rubber formulation may be broken down into a number of component groups, which has a specific function. These are listed in Table 7.1 and discussed by class below.

7.4.2 Selection of Polymer The first decision in the design of a rubber formulation is the selection of the base polymer to be used. This choice will be determined principally by the end-product specification and/or service conditions. This will stipulate such conditions as upper and lower operating temperatures, type and level of chemical or fluid resistance, mechanical or electrical properties, etc. This task requires a comparison of the basic properties of the polymer types and grades available with a view to making a selection on a cost-effective basis. There are few areas of ambiguity about the choice of polymer to be used as most polymers have a specific profile of chemical and physical properties. Where a single polymer cannot provide the requisite properties, then a blend of two or more may often be used. The performance aspects of the polymer are discussed further in Section 7.5.

306

Particulate Fillers in Elastomers

Table 7.1 The rubber formulation Ingredient

Addition range (phr)

Function

Polymer

100

Determines the base properties of the end product, (e.g., elasticity, resistance to oxidation, fluids and chemicals; lowtemperature, dynamic and electrical properties)

Curing system

5-15

Determines the rate of cure to an elastic material. Influences the mechanical properties and stability of the rubber

Antioxidants/ antiozonants

0-7

Inhibit attack by oxygen and ozone. Improve flex fatigue and inhibit the degradation effects of metal impurities

Coupling agents

0-2

Modify the polymer-filler interface

Oils/plasticisers

0-250

Modify mechanical and processing properties and low-temperature flexibility

Fillers

20-400

Modify mechanical properties, processing characteristics and cost

Examples of specialised additives Process aids

0-20

Influence the processing behaviour of the rubber compound

Peptisers

0-3

Reduce compound viscosity

Mill release

0-10

Prevent sticking to processing equipment

Fire retardants

0-300

Inhibit the rubber's ability to burn

Smoke suppressants

0-20

Inhibit smoke evolution on burning

Tackifiers

0-30

Promote building tack

Pigments and dyes

0-20

Used to colour the compound

Bonding agents

0-6

Allow direct bonding of the rubber to a variety of substrates, (e.g. metals, textiles)

Antistatic agents

0-4

Prevent build-up of surface charges

Odourants

0-2

Mask or modify the odour of rubber

Blowing agents, etc.

0-20

Generate gas during cure to form expanded open- or closed-cell foamed products

307

Particulate-Filled Polymer Composites

7.4.3 The Curing System The selection of chemicals used for crosslinking the rubber has to be made based on the chemical reactivity of the polymer, and the stability and type of crosslink that will perform best in the end product. A number of basic types are used but within these categories the complexity of chemicals in use may be high.

7.4.3.1 Sulfur-based Systems This type of curing system is used for those rubbers containing unsaturation, which is either along the polymer backbone or, as in the case of ethylene propylene diene elastomer (EPDM), it is pendant to the backbone. Sulfur-based systems are the most frequently used system for several reasons: 1. The polymers using this system form the bulk of rubbers in use. 2. The systems are relatively inexpensive. 3. The system is extremely flexible due to the diversity of materials available for use. This allows considerable scope for engineering the system to the needs of both production and performance requirements. Sulfur-curing systems are comprised of several components, which are listed next [6]. (a) Activators. The materials normally used are zinc oxide and stearic acid. Other oxides of cadmium or lead may be used for specific properties, and other fatty acids may be used. Zinc carbonate or stearate may be used where translucency is required. (b) Elemental sulfur or a sulfur donor. Several grades of sulfur are available and a number of organic accelerators are capable of generating free sulfur during the curing reaction. Sulfur forms the interchain crosslinks. (c) Organic accelerator(s). These materials are often used in combination to gain improved efficiency or to ensure solubility of the system in the polymer. They control the rate of crosslink formation and influence the type and stability of the crosslink. (d) Retarders and prevulcanisation inhibitors. Retarders may be added to delay and slow down the rate of crosslink formation. Prevulcanisation inhibitors delay the onset of cure but do not reduce rate of cure. These are not essential components of the sulfur-curing system but provide additional control of the cure process.

308

Particulate Fillers in Elastomers

Figure 7.2 Sulfur vulcanisation of diene rubbers

The chemistry of the sulfur system is extremely complex, but the generally accepted mechanism involves the formation of zinc-accelerator complexes, which interact with sulfur to form a zinc perthio salt [7]. This salt reacts with a rubber hydrocarbon to form a rubber-bound intermediate. This intermediate extracts hydrogen from another rubber hydrocarbon (which is most frequently in the alpha position relative to the double bond) to form a crosslink. Accelerator is regenerated during the process to continue the reaction with sulfur and zinc oxide (Figure 7.2).

7.4.3.2 Metal Oxides Several rubbers may be crosslinked using divalent metal oxides, usually zinc oxide. There are a limited number of polymers that utilise this method, which is used with halogenated polymers such as polychloroprene [8], chloro- and bromobutyl, and chlorosulfonated polyethylene and carboxylated nitrile rubbers. The system may utilise the metal oxide alone or in combination with the organic accelerators used with sulfur-curing systems. In the case of halogenated polymers, magnesium oxide may be added to act as an acid scavenger.

7.4.3.3 Peroxides Peroxide-curing agents are effective at crosslinking most rubbers. Exceptions to this are butyl and chlorobutyl rubber, some polyacrylates and some fluoropolymers (these being grade dependent). Peroxides form direct carbon-carbon crosslinks, which are

309

Particulate-Filled Polymer Composites inherently, more stable than many other crosslink types. The short length of the crosslink, however, results in reduced extensibility and mechanical strength. The use of co-agents, such as acrylates or cyanurates, enhances the efficacy of the peroxide crosslinking reaction by reducing the losses resulting from chain scission and results in a modified crosslink structure in which the co-agent is substituted for the direct carbon to carbon bond.

7.4.3.4 Other Curing Systems Many other chemicals are available for crosslinking rubbers. Some of these materials have broad applicability while others are specific to certain polymer grades. Examples are: Resin-curing systems

Silane crosslinking

Soap/sulfur systems

Isocyanates

Urethane crosslinking agents

Poly-functional amines

Bisphenol curing agents

High-energy radiation

7.4.4 Antioxidants and Antiozonants Most of the rubbers used commercially require addition of an antioxidant whose function is to protect against high temperature or long-term oxidative degradation. Only the more thermally stable polymers may have sufficient stability not to require antioxidant protection. There are many classes of antioxidant but for simplification these can be categorised as staining types (amine based) or non-staining (phenol based). Antioxidants often have secondary protective properties such as protection against light, flex fatigue and inhibition of heavy-metal catalysis. Antiozonant protection is required for most rubber products, which are based on diene rubbers. Ozone attack is different to oxidative ageing in that specific conditions are required for it to occur. These conditions are: (1) presence of ozone; (2) unsaturation in the polymer backbone; and (3) a specific strain level must be exceeded (critical strain). Many of the chemical antiozonants also provide powerful antioxidant protection. The most important of these are the paraphenylene diamines but other products also have commercial importance.

310

Particulate Fillers in Elastomers Wax blends are also widely utilised for static ozone protection. The wax migrates to the rubber surface where it forms a protective film. This film cracks, however, when the product is flexed. The most comprehensive system uses a combination of both materials. The wax enhances the chemical system by carrying it to the surface more efficiently where it is required.

7.4.5 Coupling Agents Coupling agents are chemicals, which are able to form a strong, physical or chemical bond between a filler and the polymer in which it is used. By so doing, the properties of the composite can be significantly modified. Coupling agents are important technically, as they allow composite materials to be produced with enhanced performance characteristics. Coupling agents are discussed in detail in Chapter 4. In rubber systems, coupling agents can improve a wide range of properties, so much so that they are indispensible for high performance products such as power cable [9] and tyres [10, 11]. One point of fundamental importance when selecting a coupling agent is to ensure that it is able to react with both the filler and the polymer, which is being used. The level of addition must also be determined experimentally, if the desired results are to be achieved. With most coupling agents it is possible to either pre-treat the filler in advance of mixing the compound, or to add the coupling agent directly to the mixer where reaction takes place in situ. In many applications, either method can be used to achieve similar results. In some cases, both methods of treatment must be used together to achieve optimum performance [12]. A number of coupling agent types are available commercially and include organo-silanes [13], organo-titanates and carboxylated polybutadienes [14].

7.4.6 Process Oils and Plasticisers Plasticisers used in rubbers are materials, which have the effect of reducing compound viscosity, thus making the material more plastic during processing. These materials also function to control physical properties such as hardness and low-temperature flexibility. Plasticisers have an influence on many other properties and thus must be selected in order to provide the overall property profile for the end product. Basic types available are as described in the following sections.

311

Particulate-Filled Polymer Composites

7.4.6.1 Mineral Oils (a) Paraffinic oils. Composed of mixed hydrocarbons but containing, predominantly, non-cyclic linear or branched hydrocarbon species. These are used in non-polar rubbers such as EPDM and butyl rubber. (b) Naphthenic oils. Composed of mixed hydrocarbons but containing, predominantly, unsaturated cyclic hydrocarbon species. These are used as general-purpose plasticisers in many rubbers because of their relatively low cost and broad compatibility with non-polar and moderately polar rubbers. (c) Aromatic oils. Composed of mixed hydrocarbons but containing, predominantly, unsaturated cyclic aromatic species. These are used in general-purpose rubbers such as NR and SBR because of their low cost and efficacy. These materials are dark coloured and cause staining, which limits their use. The composition of the oils may be inferred from several of the key properties measured [15] which includes the viscosity-gravity constant, refractive index, density and aniline point. Other properties measured include viscosity, flash point, volatility and colour.

7.4.6.2 Ester Plasticisers A wide range of products fall into this category based on monomeric and polymeric types. Used with polar polymers for compatibility reasons. Selection of type used must be made according to properties required [16]. These may be classified as generalpurpose, low-temperature, high temperature, flame-retardant or permanent plasticisers.

7.4.6.3 Other Types Many other classes of material are used as plasticisers for rubber for specific property enhancement. Examples are: Chlorinated paraffins

Low-cost flame retardant

Reactive monomers

Polymerise during cure

Thermoplastic polymers

High-temperature plasticisers

Resinous materials

High-temperature plasticisers

Liquid polymers

Co-vulcanise with the rubber

312

Particulate Fillers in Elastomers

7.4.7 Fillers Fillers of many types are used in rubber formulations. A ‘filler’ may be considered to be any particulate material, which is added for one or more of the following reasons: 1. Modification of physical properties. 2. Modification of processing performance. 3. Reduction in cost.

7.4.7.1 Modification of Physical Properties Fillers influence the whole spectrum of physical characteristics of a rubber, including hardness, strength, extensibility, heat build-up, vibration damping, electrical insulation or conductivity, colour, long-term ageing performance, permeability to fluids, swelling in and absorption of fluids, and adhesion to substrates.

7.4.7.2 Modification of Processing Performance It is true to say that, of the many materials used in the rubber formulation, it is the polymer that is the cause of poor processing behaviour. This is to be expected, because it is the very quality of possessing elastic behaviour that confers poor processing performance. For ease of processing the compound must often be formulated to behave as a non-elastic (plastic) material for the purposes of shaping and shape retention. This must be followed by conversion to an elastic material for the end product (vulcanisation). These conflicting requirements must be rationalised in the formulation of the compound. The main method of achieving a formulation with good processing performance is to replace a portion of the polymer with materials, which do not exhibit elastic behaviour. Most compounding ingredients are non-elastic and addition of fillers provides one important means of influencing this property by direct replacement of polymer, by physical interaction with the polymer and by permitting increased levels of liquid plasticisers to be used without sacrificing other valuable properties.

7.4.7.3 Reduced Cost The volume cost of fillers used in the rubber industry is usually below that of the polymers in which they are used. The extremes of cost vary from approximately onetwentieth of the polymer cost to costs that are similar in magnitude to the lower cost, general-purpose rubber grades.

313

Particulate-Filled Polymer Composites Selection of filler type must be made with careful consideration of materials available and their cost-performance characteristics. It is quite common for a multiplicity of products to be used to gain specific benefits in one area or another. The use of fillers is discussed in detail in Section 7.6.

7.4.8 Specialty Additives A wide variety of chemicals may be added to impart specific properties to a rubber. These include the following types:

7.4.8.1 Process Aids The term process aid is a general description for any material that will improve one or more aspects of the processing behaviour of rubber. These materials generally improve the flow behaviour, providing smooth surface finish to calendared or extruded products. Typical process aids include the following material types: Paraffin wax

Fatty acids

Soaps

Emulsified esters

Polyethylene waxes

Polyethylenes

Polyoctenamer

Liquid polymers

7.4.8.2 Peptisers Peptisers are materials that reduce the viscosity of rubber. They fall into two classes, chemical peptisers and physical peptisers. Chemical peptisers are materials that are added to a rubber compound and cause a reduction in the molecular weight of the polymer, reducing viscosity. Physical peptisers reduce viscosity by functioning as internal lubricants for the polymer, allowing polymer chains or localised polymer domains to slip easily past each other during processing. The molecular weight of the polymer remains relatively little changed [17].

7.4.8.3 Mill and Mould-Release Agents Many additives aid the release of rubber compound from the steel processing equipment used for processing. The materials used are often chosen for their effectiveness in specific polymer types and include the following:

314

Particulate Fillers in Elastomers Cis-polybutadiene

Fatty acids and soaps

Waxes (paraffin and polyethylene)

Emulsified fatty-acid esters

Factices

7.4.8.4 Fire Retardants A wide range of materials are available that will inhibit combustion. Many may be classified in other ways, e.g., fillers or plasticisers. Examples are: Aluminium trihydrate (ATH)

Fine calcium carbonates

Antimony trioxide

Chloroparaffin plasticisers

Magnesium hydroxide

Zinc borate

Phosphate ester plasticisers

Halogenated hydrocarbons

7.4.8.5 Smoke Suppressants A number of materials may inhibit smoke generation during burning such as ATH and magnesium hydroxide.

7.4.8.6 Tackifiers Many resinous products increase building tack, which is important for the construction of many rubber products. Examples of these are: Coumarone-indene resins

Terpene resins

Petroleum hydrocarbon resins

Phenol-formaldehyde resins

7.4.8.7 Pigments and Dyes For coloured products, it is necessary to use a colouring agent (pigment) in conjunction with a white base. This is normally achieved by addition of titanium dioxide (or alternatively lithopone or zinc oxide) plus a pigment, which may be either organic or inorganic in nature. Soluble dyes may be used in some systems to allow compound identification to be made easily by rubbing of the final product with solvent wipe to reveal the dye, giving positive identification of the composition.

315

Particulate-Filled Polymer Composites

7.4.8.8 Bonding Agents Many rubber composites require that the rubber adheres permanently to a substrate such as steel, brass, textiles or other media. There are a number of established proprietary additives, which may be added to the rubber compound, which function in this capacity. The resorcinol-formaldehyde-silica (RFS) system is commonly used to achieve adhesion to metals. This system relies on the interaction of resorcinol (or a resorcinol donor), a methylene donor such as hexamethylene tetramine and precipitated silica. Other systems that may rely on cobalt complexes or blocked isocyanates may also be used.

7.4.8.9 Antistatic Agents These are usually highly polar liquids, which form a surface film that is antistatic in character. This prevents the build-up of electrostatic charges on the surface of the product. This type of additive is normally added to light-coloured compounds, as most carbonblack-filled composites are inherently antistatic. Many of these proprietary additives are quaternary amines.

7.4.8.10 Odourants A number of products are available to mask the characteristic odours of rubber products. This may be desired for domestic products.

7.4.8.11 Blowing Agents Foamed products may be produced by adding a chemical, which decomposes, usually during curing, to generate gas (usually nitrogen or carbon dioxide). This gas remains dissolved in the rubber under high pressure during cure but expands immediately on release of the moulding forces applied during cure, forming a micro-cellular or open cell foam.

7.4.8.12 Other Additives This list is not by any means exhaustive but does give some idea of the complexities involved in formulating rubber products. Many other specialty additives may be added according to the requirements of the application.

316

Particulate Fillers in Elastomers

7.5 The Performance of the Polymer

7.5.1 Specification of the Polymer The properties used to describe a polymer are many and varied, but depend to a degree on the special characteristics of the individual products. The basic properties that describe the polymer, such as composition or rheology, are required in order to select the appropriate polymer for any application. These properties, which indicate such performance characteristics as upper and lower temperature resistance or oil resistance, are usually described in general terms, partly because the values obtained depend on the method of measurement and partly because the exact properties are formulation dependent. These do allow the chemist to select from a narrow range of polymers those, which are likely to conform to performance criteria. Data of this type are often presented in graphical or tabular form (Figure 7.3 and Table 7.2).

Figure 7.3 Performance characteristics of various polymers.

317

318 -50 -80 -50 -50 -40 -60 -50 -50 -50 -50 -50 -40 -50 -50 -40 -10 -40

Butadiene rubber

Styrene butadiene rubber

Polynorbornene

Ethylene propylene rubbers

Polyolefin elastomers

Butyl rubber

Chloroprene rubber

Polysulfide rubber

Chlorosulfonated polyethylene

Nitrile rubber

PVC-nitrile

Poly(ether)urethane

Poly(ester)urethane

Epichlorhydrin copolymer

Epichlorhydrin homopolymer

Chlorinated polyethylene

Minimum

150

150

130

100

100

100

125

140

70

100

130

175

150

70

120

100

100

Maximum

Working temperature (°C)

Natural rubber

Rubber type

E

E

E

E

E

E

P

E

VG

VG

VG

E

E

P

P

P

P

Ozone

G

E

G

G

G

G

P

E

VG

M

M

P

P

P

P

P

P

Light

Resistance to:

Table 7.2 Polymer characteristics

VG

L

G

E

E

E

VG

G

E

G

M

P

M

P

P

P

P

Oil

H

H

M

H

H

L

M-L

H

VL

H

VL

M

H

H

H

H

H

M

M

P

P

G

P

P

M

P

P

VG

E

E

P

G

G

VG

Gas Electrical permeability insulation

Particulate-Filled Polymer Composites

-50 -50 -40 -100 -40 -50 -40 -30

Ethylene vinyl acetate

Hydrogenated nitrile rubber

Acrylic rubbers

Silicone rubber

Fluoroelastomers

Perfluoroelastomer

Polyfluorophosphazene

Ethylene acrylic elastomer

170

170

300

250

250

180

175

180

135

Maximum

E

E

E

E

E

E

E

E

E

Ozone

VG

E

E

G

VG

E

E

P

E E

E

VG

VG

G

Oil

G

G

G

E

Light

Resistance to:

H

-

L

L

H

M

M- L

H

H

P

-

E

E

E

P

P

P

M

Gas Electrical permeability insulation

P = poor; M = moderate; G = good; VG = very good; E = excellent; VL = very low; L = low; H = high

-50

Minimum

Working temperature (°C)

Polypropylene oxide

Rubber type

Table 7.2 Continued

Particulate Fillers in Elastomers

319

Particulate-Filled Polymer Composites Cost considerations also have a strong influence on selection. When a specific polymer type has been selected for use, it then becomes necessary to look in detail at the differences between grades. Firstly, if the polymer is a copolymer, the level and regularity of the two monomers in the polymer must be examined. In the case of nitrile rubber, for instance, the level of acrylonitrile influences most of the properties of the polymer. High acrylonitrile materials are tough, highly oil-resistant, relatively thermoplastic elastomers with high brittle point and low gas permeability. Lowacrylonitrile-content polymers, however, are elastic, moderately oil resistant polymers with low brittle point and high gas permeability. Intermediate substitution levels of acrylonitrile provide intermediate properties. Similarly, the styrene level in SBR, or chlorination or chlorosulfonation level in chlorinated or chlorosulfonated polyethylenes provide significant property variation [18, 19]. The next basic property that must be fixed is the viscosity of the grade to be used. This will require some knowledge of the viscosity limitations of the processes to be used. Viscosity is normally given in Mooney degrees determined on a Mooney viscometer (ASTM D1646-03) [20] using one of several test temperatures (usually l00 °C or 120 °C) and measured after a set period of time. Mooney viscosity influences such properties as processability, and the ability for extension with fillers and plasticisers. NR, as a specific example is often selected according to its oxidation resistance. This may be assessed by measurement of its plasticity retention index (where Wallace rapid plasticity is measured before and after heat ageing at 140 °C, e.g., ASTM D319499 [21]). The drop in plasticity found after ageing is an indicator of this polymer’s oxidation resistance. For general purpose rubbers, the quantity and type of monomer incorporated for cure may be of vital importance. This will determine the method and rate of cure that is possible. Several of the more exotic polymers have recently become available with termonomers incorporated to allow crosslinking by peroxides rather than by the use of more exotic chemicals. This has provided a greater degree of latitude to the chemist in polymer choice. Stereoregularity is of importance in some cases. Polyisoprenes are available with cisisoprene contents between 92% and 98%. This influences the processing and mechanical properties of the compound substantially. Similarly, SBR are available as random or block copolymers with different configurations. These influence strength, rolling resistance and wet skid properties of tyres. In emulsion polymers, the type of coagulation system and stabiliser added influences the end-application. Similarly, the type and level of oil and/or carbon black extension of specific grades of polymer must be controlled.

320

Particulate Fillers in Elastomers

7.5.2 Processing Considerations The term processability describes the ease with which a rubber may be processed. There are a multitude of processing operations in use within the industry so the term has a very broad applicability. In general terms, it describes the ease with which the rubber is able to flow during mixing and shaping processes such as extrusion, calendaring or moulding. The processability of a compound is influenced by many of the ingredients used in its manufacture. Indeed, many are added specifically to modify process behaviour. The effect of fillers is discussed in Section 7.6.2. The polymer influences processing performance through a number of parameters. Viscosity is dictated by molecular weight and weight distribution of the polymer. The rate of change of viscosity with processing temperature varies considerably from polymer to polymer. A high molecular weight polymer, indicated by a high Mooney viscosity, will be more difficult to process than its low molecular weight equivalent. However, it may be possible to incorporate higher loadings of filler and plasticisers to obtain a lower cost than would be possible with the lower viscosity grade. High-viscosity grades will usually disperse fillers more readily through the higher shear forces applied during mixing. They will be less prone to flow at high temperatures, e.g., exhibit good collapse resistance on extrusion, and cause fewer problems with porosity during low-pressure cure. Heat buildup during processing, however, will be greater. Many polymers have their own process characteristics. NR and SBR process very easily on a two-roll mill. Other polymers process differently. Nitrile rubbers are well known for ‘bagging’ (non-adherence to the rolls), while other polymers, especially those that are chlorinated, tend to stick firmly to the mill roll. Consequently, specific solutions have been found for each polymer’s processing idiosyncrasies. The requirement for ease of processing demands that the rubber behave as a plastic material. While the effect of heating will aid in converting the tough elastic polymer into a plastic, the process is far from complete. Many grades of polymer are available in a pre-crosslinked form to assist in this respect. These polymers have usually been crosslinked prior to coagulation of the emulsion from which they were derived. The three-dimensional network helps to reduce the elastic component of the compound but at the expense, usually, of mechanical strength. Reclaimed rubbers form a supply of polymer with similar properties because of the residual crosslink structure from the parent material.

7.5.3 Strength Characteristics of Polymers The strength of rubbers is influenced strongly by the effect of filler type and loading. This is discussed further in Section 7.6.1. The addition of plasticisers is known to reduce

321

Particulate-Filled Polymer Composites strength substantially. The elastomers may be classified into three distinct types according to their strength properties: 1. Those elastomers that exhibit low modulus plus high strength in the unfilled gum state because they undergo stress-induced crystallisation. 2. Those with a high modulus and high strength. 3. Amorphous polymers, which have low modulus plus low strength. Other factors are known to influence strength. If the molecular weight of the polymer can be maintained at a high level, then strength characteristics will be maintained. This may be achieved by selection of high-molecular-weight (high Mooney) polymer initially and/or by using processing procedures that do not cause molecular weight reduction, (e.g., short mixing cycles and physical rather than chemical peptising).

7.5.4 Compounding Considerations The principal properties that the formulator attempts to optimise are discussed next.

7.5.4.1 Strength Characteristics and Hardness The strength characteristics of a rubber, such as tensile stress-strain properties, tear strength, abrasion resistance and hardness are influenced by the interacting effects of polymer, additives, vulcanisation conditions and reinforcing characteristics of the filler(s) used in the formulation. The strength aspects of rubbery polymers were discussed briefly in Section 7.5.3. The influence of additives may be significant but is generally deleterious to strength enhancement, especially if the additives themselves are inherently weak, e.g., if they are liquids or waxes. Vulcanisation conditions vary enormously according to the process used and chemistry selected, but may be optimised to provide limited control over product strength. A full discussion of these parameters is outside the scope of this chapter. The influence of fillers is generally described under the topic of reinforcement, since this term describes the influence of particulate materials over strength of the resultant composite.

322

Particulate Fillers in Elastomers

7.5.4.2 Permanent Set Permanent set measurements provide a means of assessing the longer-term stability of the crosslink structure under the influence of deforming forces, heat, oxidation, etc. This type of test may be carried out either in tension (tension set) or, more commonly, under compression (compression set) by application of fixed stress or fixed strain conditions applied for controlled time and temperature. Permanent deformation occurs as a result of a variety of processes (see below), which result in a net permanent deformation of the article. This is obviously undesirable as it leads to both changes in dimensions and, where sealing properties are required, a reduction or loss of the sealing forces. Processes influencing permanent set are: 1. Slippage and flow of polymer chains. 2. Chain disentanglement. 3. Chain scission/chain extension. 4. Crosslink modification. 5. Crosslink formation. 6. Oxidation of the polymer. 7. Effects from the use of fillers. These are discussed further in Section 7.6.3.1. No doubt, there are other agents that may also lead to permanent set in rubber products but, whatever the cause, it is an aspect of product design to which much attention is devoted.

7.5.4.3 Gas Permeability The permeation rate of fluids (gases and liquids) through rubber is of particular importance in applications where the rubber’s function is to contain a fluid, such as a tyre inner liner, fuel and liquid petroleum gas hoses, diaphragms, etc. Permeability is the product of a pressure gradient across a membrane and the diffusivity (the rate at which fluids will enter and leave a polymer) [22]. Solubility determines the amount of fluid held in the polymer. These are all influenced by temperature, which results in increased permeability with increasing temperature through increased polymer mobility. Permeability is determined partly by polymer structure and by compounding considerations. If the structure contains polar groups, then permeability is reduced, e.g., nitrile rubbers [23]. Likewise, the presence of methyl groups reduces permeability (butyl rubber). Permeability is also influenced strongly by the amount and type of filler used in the rubber (see Section 7.6.3.2).

323

Particulate-Filled Polymer Composites

7.5.4.4 Electrical Properties The electrical properties of rubbers may be varied enormously but can be considered to fall into one of three classes, dependent on the resistivity of the end product. Testing for electrical performance of rubber is not entirely consistent and may involve testing of an article or composite, or determination of the fundamental performance characteristics. Three types of product may be identified based on the resistivity requirements of the end product. These are conductive, antistatic and insulating. The three material types fall approximately into the resistivity ranges shown in Table 7.3.

Table 7.3 Classification of elastomers by electrical properties Classification Conductive

Volume resistivity < 104 ohm cm

Antistatic

104 - 109 ohm cm

Insulating

> 109 ohm cm

Most commercial polymers may be classified as insulating, having volume resistivities of 109 ohm cm or above. However, their properties are extensively modified by compounding, with fillers having the largest effect on properties. These are discussed in Section 7.6.3.3.

7.5.4.5 Dynamic Properties One of the main reasons for manufacturing a product from rubber is the need for the product to accommodate movement of some description. This requirement may be minimal as in the case of static sealing where the movement may be single cycle compression or it may be more extreme as in the case of a tyre, which undergoes repeated high-frequency cycling under load. Cyclic movement requires that a number of properties are considered [24]: Fatigue resistance – crack initiation and growth. Dynamic mechanical properties. Vibration damping. Repeated stressing of any product will eventually lead to failure through crack formation. The action of oxygen, ozone, heat, light and other agencies result in crack formation and

324

Particulate Fillers in Elastomers crack growth on flexing. Formulation for fatigue resistance normally involves attainment of stability towards the agents responsible for degradation. This normally requires that the polymer be stabilised towards oxidation and ozone attack especially. Low modulus is also desirable for best fatigue resistance in many applications. This may be achieved by control of the crosslink type, density and stability. It is known that short mono- and disulfidic crosslinks perform poorly on repeated flexing. Polysulfidic crosslinks perform much better, presumably because of the increased polymer mobility. These crosslinks are thermally less stable and on ageing may give poor long-term performance. A low crosslink density is also known to give good fatigue resistance. The effects of filler are discussed in Section 7.6.3.4. During repeated flexing of a viscoelastic material, such as rubber, heat is produced internally. This results from the fact that the rubber is not perfectly elastic but is a viscoelastic material. The viscous element of the rubber leads to internal frictional heating on repeated cycling. This is influenced strongly by the nature of the polymer and its crosslink structure. It is also affected by the filler system in use and the polymer-filler interface. Many rubber applications involve use of the rubber for vibration damping purposes. For this, the response of the filled polymer to cyclic deformation through a frequency spectrum must be determined as must the way in which this changes with temperature. The damping characteristics are primarily determined by the viscoelastic behaviour of the polymer, but is influenced also by the filler and filler loading used [25, 26].

7.5.4.6 Resistance to Liquids Many rubber products must operate in contact with fluids of various types. An important part of compound design is formulation for resistance to fluids. Selection of suitable polymer is important, (e.g., use of a polar rubber for oil resistance), while for minimal swelling it is desirable to have a high crosslink density. Other design considerations are also important, such as the effect of the liquid on the filler and plasticiser systems. Plasticisers will often be extracted by fluids and occasionally replaced by the fluid within the compound. Filler effects are discussed in Section 7.6.3.5.

7.5.4.7 Burning Behaviour The requirement for fire retardance in many applications has never been greater. Not only is the requirement for flame retardance increasing, but the control of airborne byproducts has become more important. In small-scale fires, it may be sufficient for a product to be self-extinguishing. There are, however, in the modern building, ship, train

325

Particulate-Filled Polymer Composites or factory an increasing number of computers or microprocessors. It has been found that conventional flame-retardant rubbers, which are halogen containing, may do far more damage from acid gas evolution than the damage caused by the primary fire. There is also an increasing requirement for low-smoke or smoke-free rubber products (for cables, flooring, etc.), which will permit easier evacuation in a fire situation than older systems, which generate smoke on combustion. As a result, recent attention has centred on those polymers that are both halogen free and burn cleanly. There are, of course, applications where the more conventional systems may be used. The polymer may be modified significantly by the use of fire-retardant additives, which may act as fillers, plasticisers, and so on [27]. The combustion behaviour of fillers is discussed in Section 7.6.3.6

7.6 The Performance of Fillers 7.6.1 Reinforcement of Rubber by Fillers The phenomenon of reinforcement by fillers is unique to rubbery materials. The relative order of reinforcement by fillers appears to be the same for all dry rubber polymers but the relative magnitude of the effect is influenced strongly by the inherent strength characteristics of the unfilled elastomer. Those elastomers that exhibit high strength in the gum state, (i.e., those which may undergo stress-induced crystallisation and those with a high modulus), are influenced to a lesser degree than those which are amorphous and exhibit low modulus. Several definitions of reinforcement have been proposed over the years [28, 29]. These include measurement of the tensile product (the product of tensile strength and elongation at break) or energy of rupture; but the definition preferred by the author is based upon the influence of the filler on a high-quality natural rubber gumstock. This convention classifies the spectrum of fillers into three types: reinforcing, semi-reinforcing and non-reinforcing. As with most systems of classification, there are grey areas, especially in the regions between types. A reinforcing filler is a particulate material that is able to increase: (1) the tensile strength; (2) the tear strength; and (3) the abrasion resistance of NR. A semi-reinforcing filler is a particulate material that is able to: (1) increase tensile strength; and (2) tear strength, but does not improve abrasion resistance.

326

Particulate Fillers in Elastomers A non-reinforcing filler is unable to provide any increase in these properties and functions only as a diluent.

7.6.1.1 Factors that Influence Reinforcement of Rubber by Fillers The reinforcing ability of fillers is influenced by three primary characteristics of the filler: particle size, polymer-filler bonding, and particle shape complexity [30]. Particle size. The primary particle size of the filler has the most significant influence on reinforcement. This assumes, of course, that the filler in question is adequately dispersed. Particle size has a direct influence on the specific surface area of the filler and it is the increase in surface area that is in contact with the rubber phase that probably leads to the increase in reinforcement. It can be argued that reducing particle size simply leads to a greater influence of polymer-filler interactions. One area of great importance to reinforcement is the presence (or absence) of large particles or agglomerates in the rubber [31, 32]. These detract from strength, not only because of the reduced surface contact, but through localisation of stresses which would thus lead to premature failure by functioning as failure initiation sites. Polymer-filler bonding. The ability of the filler to react with the polymer resulting in adhesion, increases strength significantly. This process may be physical or chemical, although in the case of carbon blacks it is considered to be physical adsorption by the filler that is primarily responsible for reinforcement [33]. Polymer-filler bonding, particularly in the case of carbon black, develops naturally between filler and rubber through active sites on the filler surface resulting in ‘bound rubber’ attached to the filler surface. The effect of these surface interactions has been clearly demonstrated by comparing the effect of using carbon black before and after graphitisation [34, 35]. Adhesion may also be induced by addition of a coupling agent, which participates in the vulcanisation reaction to form polymer-filler crosslinks. Inorganic fillers in particular often respond to use of coupling agents to create polymer-filler crosslinks, which would not be formed in their absence. On the other hand, fillers with active surfaces, such as carbon black, develop a high level of polymer-filler bonding in the absence of coupling agents through formation of physical and chemical bonding. Both mechanisms lead to the formation of high modulus compounds, which is a very clear indicator that polymerfiller bonding has taken place. The increased modulus occurs as a direct result of attachment of the rubber to the filler, which has the effect of reducing polymer mobility. The particle complexity. Particle shape may provide further changes in reinforcement. This is especially true for carbon blacks where products with differing ‘structures’ are

327

Particulate-Filled Polymer Composites produced. Increasing carbon black structure has the effect of reducing tensile strength and increasing abrasion resistance, but has little effect on tear strength [35]. Overall, the effect on reinforcement is relatively small. Particle complexity has a more pronounced effect on processing behaviour than on reinforcement and provides important benefits in this area. It can also significantly increase modulus due to occlusion or shielding of some of the rubber phase [34]. Table 7.4 provides a list of commonly used fillers, categorised according to their ability to reinforce elastomers.

7.6.1.2 General Properties of Fillers The reinforcement of rubber may be illustrated by examining the response of two dissimilar general-purpose polymers to the increase in filler loading (Figure 7.4). NR, which can stress crystallise, exhibits very high mechanical strength in the absence of filler and is used in a number of applications (rubber bands, baby feeders, etc.), because of this fact. SBR offers no such properties, and must be filled in order to achieve even modest strength. The bulky benzene rings along the rubber molecule prevent approach of the polymer chains on extension, which is necessary for crystal growth to occur. As a result, its gum strength is low. When compounded with fillers, however, the two materials may exhibit similar mechanical strengths.

Figure 7.4 Reinforcement of rubber by fillers. Filler type versus strength.

328

Particulate Fillers in Elastomers

Table 7.4 Classification of fillers by reinforcement Primary particle size (μm)

Polymer-filler adhesion

Particle shape

Fumed silicas

0.005-0.025

Very high

Structured

Precipitated silicas

0.015-0.060

Very high

Structured

Precipitated silicates

0.015-0.060

Very high

Structured

N100 carbon blacks

0.011-0.019

Very high

Structured

N200 carbon blacks

0.020-0.025

Very high

Structured

N300 carbon blacks

0.026-0.030

Very high

Structured

S300 carbon blacks

0.026-0.030

Very high

Structured

N500 carbon blacks

0.040-0.048

Very high

Structured

N600 carbon blacks

0.049-0.060

Very high

Structured

N700 carbon blacks

0.061-0.100

Very high

Structured

Precipitated calcium carbonate

0.050-0.100

Low

Spherical

N900 carbon blacks

0.201-0.500

Very high

Spherical

Silane treated fine kaolin

0.200-1.000

Very high

Platy

Fine kaolins

0.200-1.000

Low

Platy

Zinc oxide

0.090-0.150

Low

Spherical

Synthetic aluminium trihydrate

0.500-1.000

Low

Particulate

Calcined clays (fine)

0.600-1.000

Low

Complex

Micronised talcs

1.000-2.000

Very low

Platy

Calcined clays

1.5-2.5

Low

Complex

Coarse kaolins

1.0-5.0

Low

Platy

Neuburger chalk

1.0-5.0

Low

Semi-platy

Coarse talcs

2.0-50.0

Very low

Platy

Ultrafine chalks and marbles

0.7-2.0

Very low

Particulate

Natural aluminium trihydrate

5.0-1000

Very low

Particulate

Barytes

20.0-1000

Very low

Particulate

Chalk/limestone/marble

2.0-1000

Very low

Particulate

Filler Reinforcing fillers

Semi-reinforcing fillers

Non-reinforcing fillers

329

Particulate-Filled Polymer Composites

7.6.2 Processing Considerations Fillers have a major influence on most aspects of the processing behaviour of rubbers. It is true to say that many of the products manufactured from rubber could not be produced effectively without the addition of fillers to control processing behaviour. It may be considered that the component of the formulation that provides poor processing is the polymer. By virtue of being an elastic material its flow (plastic component) performance is poor and it will always exhibit some tendency to recover after deformation (a property often described as ‘nerve’ or extrusion shrinkage in the industry). Obviously, if the rubber component is replaced by other additives, which result in dilution, the elasticity of the component will be reduced. This is only part of the story, however, as many fillers have an extraordinary effect on this aspect of processability. There is a useful (but not total) relationship between control of ‘nerve’ and reinforcement, in that the fillers that exhibit the greatest influence are those that are the most reinforcing because the parameters that influence reinforcement also influence processability. Fine carbon blacks, and synthetic silicas and silicates exhibit an extraordinary influence on nerve and extrusion shrinkage, while natural platy minerals have a limited effect, and the particulate fillers have a minor effect. The anomaly in this model is precipitated calcium carbonate, which is a semireinforcing filler, and yet exhibits very poor control of nerve. The phenomenon is probably influenced by polymer-filler bonding brought about during mixing and is influenced by the surface area of the filler. Processing aspects where this property is important are die swell during extrusion and dimensional stability during calendaring. Compound viscosity, green strength and heat build-up follows the same pattern with the fine surface-active fillers having highest viscosity and generating greatest heat build-up during processing. This is primarily due to the increased viscosity, which correlates with the formation of bound rubber. Bound rubber content is influenced by surface area, surface activity and particle-shape factors in much the same way as reinforcement. Platy minerals such as clays and talcs, are useful for providing stiffness to hot, semiprocessed rubber products. This is particularly useful for thin-walled extrusions, which emerge from the die at high temperatures. The stiffening effect is usually sufficient to prevent collapse under the product’s own weight immediately prior to and during cure. Low moisture contents are required for many processes as temperatures often exceed the boiling point of water, resulting in the potential for porosity. For most rubber processes, fillers with less than 2% moisture are satisfactory, although it should be

330

Particulate Fillers in Elastomers noted that many of the fine, high surface-area fillers, (e.g., precipitated silicas), may have a moisture content above 5%. This is often unavoidable and therefore accommodated by compound adjustment with moisture scavengers. For particularly sensitive applications, (e.g., low-pressure curing media), low moisture content fillers such as calcined clays or calcium carbonates may be preferred. Cure rate is strongly influenced by the nature of the filler surface and the amount of filler used. Generally speaking, fillers are preferred with a high surface pH, as this will usually provide positive benefits for the cure reaction. This rule of thumb, however, may not always be satisfactory, as fast-curing formulations may be prepared from acidic china clays without the need for specific compound adjustments. The complexity of interactions available within commercial rubber formulations is such that prediction may not be possible with any certainty. Mill sticking and bagging are problems that most rubber processors encounter with annoying frequency when processing on a two-roll mill. High green-strength and stiff compounds tend to release from the roll easily and may fail to adhere to the roll surface (bagging). Low-stiffness compounds with poor green strength or adhesive qualities may stick strongly to the rolls (mill sticking). Both conditions make it difficult to process rubber, but mill sticking is the more serious of the two conditions. Extremes are relatively easy to predict but subtle changes to compound design may result in significant effects on these properties. Specific additives are usually added to control these phenomena. The effects of mill sticking are often most serious with halogenated polymers, where mild corrosion of process equipment may provide a clean steel surface, thus promoting high bond strengths. Heat build-up exacerbates the problem severely. Within the clay minerals it is known that the total surface hydroxyl content (which is the product of surface area and hydroxyl concentration) influences mill sticking and other properties markedly [36]. Other aspects of processability are also known to be influenced by this (Table 7.5). Although the effect of filler on some properties has been discussed, their use in rubber has an effect on almost every property of the end product. The final choice of filler as with all ingredients of the compound, will be made based on an overall assessment of the performance of the end product.

331

332 Difficult

Ease of dispersion

Chlorinated polymers

General purpose rubbers Very high

High

Very high

Accelerator and peroxide adsorption

Mill-sticking

Very high

2.0

Equilibrium moisture content (%)

High

Moderate

1.2

114

8

13

Very high

High

Moderate Moderate

Moderate Moderate

High

High

1.4

128

8

16

Low

0.7

60

8

7.5

High

Low

Easy

Zero

Very easy

Very low

Very low

0.3

12

1

12

Very low

Zero

Very easy

Zero

Very low

02

85

1

8. 5

Metakaolin Amorphous

Calcined clays

Moderate Moderate

Low

Easy

Moderate Moderate

Low

1.0

80

8

10

Finer Kaolins …………… Coarser Kaolins

Water absorption

240

8

Surface hydroxyl concentration (nm-2)

Total surface hydroxyls (1018 g-l)

30

Surface area BET (m2g-1)

Clay type

Table 7.5 Typical properties of clays

Particulate-Filled Polymer Composites

Particulate Fillers in Elastomers

7.6.3 Compounding Considerations 7.6.3.1 Permanent Set Permanent set is a complicated phenomenon discussed further in Section 7.5.4.2. The choice and nature of any filler used, however, may play a significant role in attaining low values. Several general statements may be made relating to filler effects: 1. As filler becomes finer, permanent set increases. With most types of filler it is known that the permanent set performance is poorer for finer particle sizes. It may be necessary to sacrifice reinforcement in order to achieve lowest permanent set. 2. High polymer-filler adhesion reduces compression set. The ability for the polymer to be immobilised at the filler surface limits the ability for the polymer to flow to a different configuration when stressed. This effect may be demonstrated readily by examining the effect of silane coupling agents on the set performance of silicatemineral-filled rubber. The resultant compression set may be as low as 50% of the value without coupling agent. 3. High aspect ratio fillers give high permanent set, (e.g., talcs). It is thought that increases in aspect ratio have the effect of increasing the stresses generated internally between filler particles when a compound is stressed. This in turn leads to increased flow rates between the filler particles, which results in increased permanent set 4. Fillers with polar surfaces may give poor permanent set as a result of curative adsorption, (e.g., silicas and fine clays). Silicas and clays have surfaces that contain polar hydroxyl groups. These are able to attract polar materials, (e.g., curing agents and accelerators), by hydrogen bonding and thus remove them from solution within the polymer phase. The effect of this is similar to addition of a lower level of curing agent, i.e., reduced crosslink density. This is obviously a costly waste of curing agent. The mechanism proposed for this cure retardation is through reaction with fatty-acid-solubilised zinc formed during sulfur vulcanisation. The zinc reacts by replacement of the surface hydroxyl group on the silica surface. The effect is proportional to the total surface area of silica available, hence it is surface area and loading dependent [37]. This problem is relatively easy to deal with as the curing agents adsorbed may be displaced by addition of a polar additive, such as diethylene or polyethylene glycol, triethanolamine, etc. These materials are preferentially adsorbed on to the filler surfaces and thus inhibit accelerator adsorption [38].

333

Particulate-Filled Polymer Composites

7.6.3.2 Gas Permeability Fillers reduce permeability as a consequence of their own impermeability. Replacement of the polymer by filler naturally results in a reduced permeation rate. Some fillers, notably those with a platy shape (fine clays and talcs) have a considerable influence on permeability [39]. These fillers work by forcing the fluid to take a longer path around the plates to escape from the opposite side of the membrane. In addition, the plates reduce the effective area through which the fluid may pass. Thus, the net result is the same as if a thicker membrane of smaller area were used (Figure 7.5).

Figure 7.5 Permeation paths through unfilled and filled rubber

7.6.3.3 Electrical Properties The effect of fillers on the electrical properties of rubber is substantial. Varying the filler type and loading can result in compounds that fall anywhere into the range of volume resistivities from <10 to >1016 ohm cm, i.e., from conductive to insulating. Electrical behaviour of networked and filled composites may be described by percolation theory [40]. Both the polymer, and filled polymer composites may be considered to be networks of connecting material. In such networks, the ability for electrons to flow requires the formation of a conductive pathway through the material. The pathway requires a critical density of connections for effective passage of current. Highly structured carbon blacks, in particular, are effective in forming interconnecting, percolation, networks which permit passage of current. Because the system is elastomeric, and subject to deformation, these networks often weaken during periods of deformation but usually re-build after a period of rest. Simple concept models of low and high density percolation systems are shown in Figure 7.6. Conductive rubbers are normally produced by incorporation of specific grades of conductive carbon black having a high surface area and a high structure. These systems

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Particulate Fillers in Elastomers

Figure 7.6 Examples of weak and strong percolation network structures

may be considered to be efficient, disordered electrical percolation networks. Values as low as single figures (< 9 ohm.cm) are obtainable with these fillers. Semi-conductive materials may be produced by addition of conventional carbon blacks. Resistivity is lowest for those grades with the highest structure and highest surface areas. Non-black products generally rely on the use of antistatic additives, which reduce the surface resistivity of the product, since most non-black fillers are insulating in nature. Insulating properties require careful selection of polymer and additives in order to achieve the specific properties sought. Many of the more polar polymers have volume resistivities, which restrict their application in this area. The major application requiring insulating properties is, of course, in power cable insulations. The requirements are such that the insulation properties must be maintained under operating conditions, which may involve high working temperatures and exposure to water, and yet the product is expected to perform satisfactorily for many years. Achieving this level of performance requires careful selection of polymer and raw materials used. Other aspects of dielectric performance, such as dissipation factor, relative permittivity or breakdown strength, may become important in power cable technology. The intricacies of this sector of the rubber industry are beyond the scope of this chapter.

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Particulate-Filled Polymer Composites

7.6.3.4 Dynamic Properties There are many aspects to consider when formulating for dynamic performance, where the elastomer experiences repeated cyclic deformation during service. The key properties include fatigue resistance, dynamic heat build up and adhesion to other components of the composite product. Selection of filler can play an important role in achieving good fatigue resistance. Of paramount importance is the selection of grades that contain minimal levels of coarse particles, which may initiate cracking [30, 31]. In addition, finer particle sized fillers are known to improve flex cracking resistance, presumably due to the improvements in tear strength that results. In fixed-strain deformations, low-modulus compounds are known to perform better than those with a high modulus. This favours the use of finer, non-black, fillers. High levels of filler are not favoured because of the increase in modulus that results. The increased modulus increases stress on the material when it is deformed, reducing the critical strain at which ozone attack may take place. Increased filler loadings also reduce the volume fraction of polymer in the composite. This often increases the rate of property degradation from oxidation and/or ozone attack of the reduced polymer phase as a result of the increased gas:polymer ratio at the surface, particularly in general purpose elastomers which are more easily degraded. One of the major recent changes in filled elastomer technology has been the introduction of ‘green’, or energy saving tyres. These first came into use to provide lower rolling resistance, and so better fuel economy. This was achieved by utilisation of precipitated silicas in combination with high levels of silane coupling agent in order to achieve reduced heat build-up during service of the tyre tread. This was achieved somewhat at the expense of abrasion resistance and ease of processing of these tyres and considerable research continues in order to optimise the contradictory properties required of this application, not just with silicas but also with new generation carbon blacks. It is desirable to formulate, not just for mechanical and construction parameters, but also around minimum cyclical energy losses at different temperatures or frequencies [41-43].

7.6.3.5 Resistance to Liquids The effect of fillers on the resistance to change or deterioration in the presence of liquids is quite marked. Fillers influence the degree of swelling by replacement of polymers that swells by filler that does not, resulting in reduced swelling through reduction of the volume fraction of polymer in the compound. Some fillers are obviously not suited to specific applications, e.g., carbonate fillers decompose in the presence of acids.

336

Particulate Fillers in Elastomers Filler surface effects may be important. Silicas and silicates that contain hydroxyl groups on their surfaces may result in increased water attraction. Attainment of high filler-polymer bonding may reduce or eliminate this type of effect by eliminating the interface. Hydrophobic treatments may have a similar effect, but increased crosslink density, resulting from strong filler-polymer interactions, can be an additional advantage of coupling agents.

7.6.3.6 Burning Behaviour A number of fillers are produced specifically to reduce the rubber’s ability to burn, generate smoke or liberate corrosive gases. These are often used in combination with each other and with careful selection of other compounding ingredients. Examples of these fillers are shown in Table 7.6.

Table 7.6 Fillers which influence burning behaviour Filler

Function

Antimony trioxide (+ halogen donor)

Flame retardant

Precipitated calcium carbonate

Acid absorber

Magnesium hydroxide

Acid absorber, flame retardant and smoke suppressant

ATH

Flame retardant and smoke suppressant

Zinc borate

Flame retardant and intumescent

These materials have different effects on the way in which the composite burns. Precipitated calcium carbonates reduce acid gas emissions by reaction and neutralisation during combustion. This may in turn increase the ease of combustion, as the halogen has a gas-phase quenching effect. Magnesium hydroxide and ATH decompose endothermically, thus removing energy from the combustion source, and generate quenching gases, which dilute the oxygen availability. When added in sufficient quantities they can be highly effective at inhibiting combustion. Zinc borate decomposes endothermically to generate a porous glassy material that acts as a physical insulating barrier to inhibit combustion. Antimony trioxide requires the presence of a small quantity of a halogen containing material. During combustion antimony pentahalides are formed, which are extremely powerful freeradical scavengers. They remove the active species from the combustion source, and so inhibit combustion effectively. 337

Particulate-Filled Polymer Composites Many of these materials may be considered to be specialty additives but the volume of material required for efficacy requires that their properties as fillers (especially the effect on reinforcement) be given significant attention.

7.6.3.7 Colour Most rubber products are black because of the supremacy of carbon black as a reinforcing agent, so colour does not play an important role in most industrial applications. For those applications where colour is important, the selection of filler is restricted to those non-black fillers, which are able to provide the properties required. The effect of these materials on colour is readily predictable from the whiteness of the filler. It is important, however, not to rely too heavily on the optical qualities of fillers as they are not produced for primary pigmentation purposes.

7.7 Filler Types 7.7.1 Specification of Fillers for Elastomers The specification for fillers for rubber varies considerably according to the nature of the product. Users of fillers must also be quite clear of the fact that no two fillers are exactly the same. Conventions that categorise fillers such as the American Society for Testing and Materials (ASTM) system for blacks provide only a system for grouping products with similar characteristics. If two fillers with identical specification values are compared in critical products, significant differences in performance will still often be found. Filler interchangeability must be approached with a certain degree of caution. Fillers vary in particle size by many orders of magnitude, their production and processing methods vary enormously, and the chemical nature of the types available varies considerably. Probably the most important single requirement of any product is batch to batch uniformity. Properties most usually described are: Moisture content

Sieve residue

Mean particle size

Particle size distribution

Colour (for non-blacks)

Impurity levels

Surface area

Iodine number (blacks)

Oil absorption

Chemical composition

pH

Density

338

Particulate Fillers in Elastomers Moisture content should be controlled to be as low as possible, bearing in mind the surface area and chemistry of the filler. Low surface area fillers with hydrophobic surfaces will naturally maintain a low moisture content once dried. High surface area fillers with hydrophilic surfaces, such as precipitated silica, will regain moisture from the atmosphere rapidly and relatively high levels of moisture (5-7%) are unavoidable. Other minerals such as kaolin are also sensitive to moisture regain, but to more acceptable levels of moisture (0.5-2.0%). Total absence of moisture, however, should be avoided as water plays a subtle role in many crosslinking reactions. Low sieve residues are important for all but the most undemanding of rubber applications. Coarse impurities may result in die blockage on extrusion, pinholes in calendared sheet and abrasive wear to processing equipment generally. In application they may act as failure sites in tension or on flexing, may cause failure electrically in cables, and cause leakage of gas or fluid through membranes. Mean particle size, particle size distribution, surface area and oil absorption are all indicators, directly or indirectly, of particle size. For coarse fillers, particle size measurements are easily performed, while for fillers with mean particle sizes below 0.1 μm, size may be measured by microscopy (very time consuming) or by indirect measurement techniques such as surface area (BET), iodine absorption, oil absorption, etc. While most fillers are not produced for their primary pigmenting properties, consistency of colour may be of concern for non-black applications. Most non-black fillers are produced to a basic colour or whiteness specification. Carbon blacks, too, may be categorised by their colour or tinting strength. While this is useful to know, it is often of little value to the end-user who rarely possesses the equipment necessary for colour determination. There are also a large number of measurement methods in use, which leads to some confusion in this area. Most processors must content themselves with making visual comparison with an established reference sample. A number of impurities found in compounding ingredients are known to influence the ageing behaviour of rubbers, especially NR. High iron oxide impurity levels, and some forms of copper and manganese impurities are known to be the cause of rapid oxidative degradation. There is little literature available that relates either the active form of impurities or that identifies the level of impurities found in fillers to specific effects on polymer degradation. Nonetheless, it is occasionally an area of concern. It should be noted that every source of filler will be different in its level and type of impurities, so each material must be considered in isolation. Specification values may not be an indicator of performance in polymer. The chemical composition of fillers is often provided with the specification. This will often provide information about chemical purity, which may be important. With naturally derived fillers, however, this is likely to provide typical information only,

339

Particulate-Filled Polymer Composites since the composition is substantially fixed by the nature of the material’s source. The chemical composition of many minerals expresses the main components, by convention, as their oxides. Most fillers, when produced, have a pH that arises as a consequence of their origins. The pH is rarely controlled by the production process unless a surface treatment of some sort has been applied to the filler. Nonetheless, it is useful to have information of the filler’s pH as this is one of the simplest methods of characterising the filler surface. Likewise, the density of a filler is a property that is rarely influenced by the filler production process and therefore should not form part of a production specification. It is of great importance, however, to know the density, if for no other reason than for costing purposes. This description of specification parameters is not meant to be at all exhaustive. Many other properties may be provided by individual filler producers on the composition and characterisation of their fillers. These are often directed specifically at one application where this type of filler is commonly used. These should be considered on their own merits and discussed with the supplier. The importance of the relevance of specification parameters to applicability should be borne in mind at all times. As we have seen, fillers play a vital role in the formulation of most rubber products. They are produced using a wide variety of processes and may have either natural or synthetic origins. Hence, fillers vary enormously in their chemical characteristics and in particle size, which, in turn, influences the filler’s overall behaviour in rubber. Fillers provide the formulator with a range of materials that can modify processing behaviour, and physical and chemical properties of the polymer. Details of the production routes used and the characteristics of the individual filler types are shown in greater detail in Chapter 2. This chapter will concentrate on those aspects of particular importance to elastomers.

7.7.2 Carbon Black Carbon blacks [44] are a form of carbon produced by controlled pyrolysis of hydrocarbon oil or gas. They are the most important filler type for use in rubber as they are the main agents for providing high-strength compounds. These materials also have a pronounced effect on the processing behaviour of rubbers. Two main production routes are used for the manufacture of carbon black, the thermal process and furnace process. The furnace process is used for the bulk (approximately 98%) of carbon black production today.

340

Particulate Fillers in Elastomers The most important properties used to characterise carbon blacks are: (1) Particle size or surface area (2) Structure (3) Surface chemistry [45] As a material, carbon blacks are available with primary particle sizes between 15 nm and 450 nm. Surface area is usually used for characterisation. Surface area measurements may be made using nitrogen BET [46] (Brunauer, Emmett and Teller) surface area or by iodine adsorption. A technique is also used based on cetyl trimethyl ammonium bromide (CTAB), which more directly relates to rubber reinforcement. The primary particles exist in an aggregated form where they are fused together in a randomly branched, chain-like form described as ‘structure’. The structure is formed during the production process and is not broken down by subsequent processing operations. A secondary ‘structure’ also exists, which is formed by loose attraction of black particles. This structure is easily destroyed during processing and is not described by the term ‘structure’. Structure is normally measured using dibutyl phthalate (DBP) absorption, which fills the voids between the particles. This method is also used in a modified form where the black is initially crushed to breakdown any secondary structure prior to DBP absorption. The crushed DBP is claimed to relate more closely to the way in which carbon black is found in a rubber mix [47]. The chemical and physical nature of the carbon-black surface is known to influence reinforcement strongly. There is no doubt that polymer interacts very strongly with the surface of carbon black to form a layer of bound rubber that cannot easily be removed. The attraction forces are considered to be primarily physical in character but many reactive chemical sites [33] are also present on the surface. These may play an important role on vulcanisation behaviour. The surface of carbon blacks are also known to contain varying degrees of porosity (from surface area measurements) [48]. The most universal system for categorising carbon blacks is ASTM 1765 [49]. This system classifies blacks according to particle size and cure rate.

7.7.2.1 Furnace Blacks The furnace process is based on incomplete combustion of oil fractions in purpose-designed furnaces. The product is removed from the air stream by cyclone and bag filters. It then passes to a pelletising unit (either wet or dry). Dry-pelletising is carried out by tumbling the black in large rotating drums. Wet-pelletisation is carried out by mixing with water

341

Particulate-Filled Polymer Composites followed by drying in heated rotating horizontal drums. The particle sizes produced are between 14 and 90 nm. The range of products available is diverse in both particle size and structure available, allowing this class of blacks to be tailored to a broad range of end-uses. For this reason, furnace blacks have become the dominant filler for highly reinforced systems, and particularly in tyres. However, with the recent introduction of precipitated silica in tyres for low rolling resistance, the furnace blacks have come under threat. As a result, new grades of furnace blacks are under development. New products based on modified reactor conditions are described as ‘nano-structure’ blacks [50], having increased surface roughness, providing enhanced filler-polymer interaction. Other approaches include physical modification of the blacks and development of hybrid black/silica systems [41]. The key strength of carbon black is in its processability and ability to confer high abrasion resistance to the tyre tread.

7.7.2.2 Thermal Blacks The thermal process involves thermal decomposition of natural gas at 1300 °C in the absence of air. The process involves heating one of two kilns with a mixture of hydrogen and air. At 1300 °C, gas is introduced. This breaks down to form hydrogen and carbon black. The hydrogen is used to heat the second kiln. When the second kiln reaches temperature, production is switched to this kiln where the cycle recommences. A second variant (the Jones process) exists where oil is used to heat the kiln. A mixture of steam and oil is then introduced. The oil cracks to form a medium thermal black plus a mixture of gases.

7.7.2.3 Special Grades A number of specialised carbon blacks are used in rubbers. Acetylene black is produced using a variation of the thermal process but, with acetylene decomposing exothermically, heat is only required to start the process. This black is primarily used for its electrical conductivity. Lampblack is produced by burning oil residues in shallow pans. The smoke is directed to chambers where the black flocculates. Lampblack replacements are now manufactured by the furnace process. Channel blacks are made by the impingement process where natural gas (or natural gas enriched with oil) flames impinge on either steel channel irons or a rotating drum. The black is collected by scraping. Very fine products may be made by this process with

342

Particulate Fillers in Elastomers particle size ranging from 10 to 30 nm. Only one plant remains in production (in Germany) manufacturing this type of product.

7.7.3 Synthetic Silicas and Silicates Synthetic silicas and silicates are powerful reinforcing agents for rubber. These materials may be classified according to their method of production, thermally produced (pyrogenic or fumed) silicas and wet-produced (precipitated) silicas [51].

7.7.3.1 Fumed Silica Fumed silicas used in rubber are made by flame hydrolysis of silicon tetrachloride. The products that result have a primary particle size in the range of 5-50 nm. The product is found in the form of agglomerates. The surface is highly active and contains silanol groups, which make the unmodified product hydrophilic. Modified hydrophobic forms are available. As a product, fumed silicas have limited use in rubbers but they are used as a reinforcing filler for silicone rubbers.

7.7.3.2 Precipitated Silica and Silicates Precipitated silicas and silicates have widespread applications in rubber. They have two distinct functions in rubber: (1) as a reinforcing filler; and (2) as an integral part of proprietary bonding systems. The products are produced by precipitation from sodium silicate (water glass) by reaction with acids. Replacement of the acid, in part or in total, by metal salts (of calcium or aluminium) results in the production of metal silicates. Because of the purity of the water glass used, the products are free from contaminants. The precipitate is filtered and dried using a variety of techniques. The products available have primary particle sizes in the range of 5-100 nm. They have a structured or agglomerated form similar to that of carbon black. Precipitated silicas contain approximately 12% water, 6% as chemically bound silanol groups plus 6% as free water, physically adsorbed on the surface [30]. The high level of silanol groups at the surface provides this filler with an extremely polar surface, which will readily attract polar additives. The problem of accelerator adsorption can be eliminated through addition of polar activators such as polyethylene glycol, ethylene glycol or triethanolamine, which are attracted in preference to organic accelerators. This high surface activity is also responsible, with the fine particle size, for the high level of reinforcement achieved. One of the outstanding qualities of precipitated silica is the very high tear strength. Silica is

343

Particulate-Filled Polymer Composites the only significant competitor to carbon black for reinforcement in tyres and recently become a significant filler in tread reinforcement because of its inherent low rolling resistance [50, 52, 53]. The silica manufacturers are improving the processability performance of traditional precipitated silica by developing modified products to meet the demands of the tyre community. Three new generation sub-groups have become available, these being classified as (a) Easy Dispersing Silicas (EDS), (b) Highly Dispersible Silicas (HDS) [53] and, (c) Highly Dispersible and Reactive Silicas [54] (HDRS). These grades are produced under modified conditions to improve dispersion characteristics in an attempt to improve abrasion resistance. Surface treatment with coupling agents, usually of the organo-silane class, is also offered to further improve dispersability and processability. Precipitated silica has wide applications in integral bonding systems based on resorcinol and methylene donor (the RFS system) or for systems based on cobalt complexes [55]. High levels of adhesion to steel, zinc plate, brass and a variety of textile substrates can be achieved with these systems.

7.7.4 Clay Minerals Clay minerals are widely used in rubbers because of their cost effectiveness in terms of providing beneficial reinforcing and processing properties at a modest cost. The main clay mineral of importance is kaolin (china clay) and the derivatives produced by chemical treatment and/or heating (calcining). These clays may be classified in many ways. The method most commonly used by the rubber industry is to classify clays into one of two types, hard clays and soft clays. This terminology dates back to the early part of the last century when only two basic grades of clay were produced, these being very fine (secondary clays) with mean particle sizes between 0.2 and 1 μm and coarser grades (primary clays) having a mean diameter of 1.5-5 μm. The two types clearly provided very different performance characteristics being semi-reinforcing and non-reinforcing, respectively. This classification system is totally unrealistic today but nonetheless persists, even though the types available and the processes used have changed substantially. The following classifications are more useful [56].

7.7.4.1 Primary Clays The term primary clay refers to those clays that are found in the 1ocation in which they were geologically formed. They are found in deposits that contain substantial quantities of other minerals. The kaolin is processed in an aqueous slurry and must be separated

344

Particulate Fillers in Elastomers from the impurities using a number of processes, which are followed by de-watering, drying and pulverising. It is possible to apply a number of techniques to the slurry to produce many different particle-size fractions ranging in size from 5 to 0.4 μm. These products may be either semi-reinforcing or non-reinforcing in rubber.

7.7.4.2 Secondary Clays Secondary clays are those clays, which have been transported away from their primary source to another location, where they have been deposited as a sediment. The transportation process was usually effective at both mineral separation and at particle size selection, in that only the finer particles remained in suspension long enough to reach the final deposit. Important secondary deposits are found in the kaolin belt of south-eastern United States (Alabama, through Georgia into South Carolina) and southwestern England. These clays are very fine with mean particle sizes from 0.1 to 2.0 μm. These are all semi-reinforcing fillers. Secondary clays may be processed hydraulically (water washed) or by dry processing (air classification or air flotation). Wet-processed products are recognised as having lower levels of coarse contaminants, and are often whiter and purer because of the opportunity of applying purification processes such as flotation or magnetic separation. A number of grades are also available from a fraction of the deposit in which the clay exists in stacks (rather like a pack of cards). These clay platelets may be separated for use in applications where the higher aspect ratio of the delaminated clay can be utilised. The costs of wet processing are, however, substantially higher than dry processing. Dry processing involves very simple blending of feedstocks, dry milling, and passing through an air classifier or separator to remove oversize particles. The product is then packaged for use in rubber. The dry route provides very low-cost semi-reinforcing fillers, which are extremely cost effective in rubber.

7.7.4.3 Calcined Clays When kaolin is heated, it undergoes chemical and physical changes. Between approximately 450 °C and 700 °C, dehydroxylation occurs (often described as loss of water of crystallisation) to form a product called metakaolin. This is accompanied by approximately 13% loss of mass as water. At higher temperatures, between 950 °C and 1030 °C, metakaolin undergoes an internal rearrangement to form an amorphous product described as a defect spinel. For rubber applications, products are manufactured at temperatures just above these transitions. Metakaolin is produced primarily for use in flexible polyvinyl chloride wire and cable insulations, but is also used in EPDM cable insulations.

345

Particulate-Filled Polymer Composites The defect spinel form is used extensively in rubber applications. The primary application is in low- and medium-voltage power-cable insulations where it provides both excellent insulation stability under wet conditions and useful extrusion performance. Pharmaceutical closures form another important application due to the need for a chemically inert, white, filler. It is also used in a variety of extruded products, especially those vulcanised under lowpressure conditions because of it’s very low moisture content.

7.7.4.4 Treated Clays Both kaolin and calcined clays are available with surface treatments. Many treatments may be applied but few have any commercial importance. Kaolins are available treated with surfactants and pH adjusters, which produce kaolins that may be dispersed directly into water, (e.g., rubber latices), or with amines to enhance cure performance. Stearic acid treated products may provide ease of dispersion. The most important treatments, technically, are the organo-silanes. (a) Silane-treated kaolins. Fine kaolins are produced in the USA for use in rubber applications such as white sidewalls, tyre inner-liner, footwear, flooring, and so on [57, 58]. Materials available range in particle size from 0.2 to 1.0 μm and are usually treated with either a mercapto-silane, amino-silane or other coatings. (b) Silane-treated calcined clays. Silane-treated calcined clays are produced specifically for use in high-voltage power-cable insulations where they provide the processing performance necessary for production and provide the low water absorption characteristics required of the insulation in service. These grades are usually produced to minimise dissipation factor which imparts energy losses to the cable, which is critical in high performance applications [9]. No other fillers offer such a high level of performance for this type of product [59].

7.7.5 Calcium Carbonates Calcium carbonates have a number of characteristics of particular importance to rubber. Products used in rubber are all based on the crystalline form of calcium carbonate, calcite (although some precipitated grades of aragonite are available, they are not generally used in rubber). As a group of products, they provide low hardness at a given loading, and have the advantage of low compression set and low inherent moisture content. Disadvantages associated with calcium carbonates are mainly concerned with limited reinforcement and poor processing performance and sensitivity towards acids. They must be subdivided into naturally derived and precipitated products to describe their applicability to rubbers.

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Particulate Fillers in Elastomers

7.7.5.1 Precipitated Calcium Carbonate Precipitated calcium carbonates are produced by controlled precipitation from calcium hydroxide solution by carbonation. The products of these processes are fine particles (0.05-0.2 μm) and are often coated with a fatty acid (stearic) or a reactive resin (carboxylated polybutadiene). Uncoated grades are also available. Some precipitated calcium carbonate is also available as a by-product of water-softening plants. These grades are generally coarser with poorer control of properties and, naturally, less costly. Other precipitated calcium carbonate grades are available from 0.2 to 5 μm but do not find general applicability in rubber. The ultrafine materials are all semi-reinforcing in character, as a consequence of their fine particle size. The uncoated and fatty-acid-coated grades exhibit very poor interaction with polymer, so the modulus (a function of polymer-filler interaction) is very low, thus soft compounds are readily obtained. The high purity ensures that these fillers are very white and heat-ageing performance is superior to naturally derived calcium carbonates. One of the outstanding features of these fillers is their good high-temperature tear resistance [60]. Precipitated calcium carbonate with a reactive coating is available and exhibits increased modulus and fatigue properties compared with stearate coated grades. This product is competitive with coarse carbon blacks [61, 62].

7.7.5.2 Natural Calcium Carbonates Natural calcium carbonates are produced from three types of rock: chalk, limestone and marble [63]. All are forms of the mineral calcite. Chalk and limestone are very similar and differ only in the degree of compaction (initial hardness of the source mineral) and are most usually formed from the skeletal remains of micro-organisms such as coccoliths. Chalk is less compacted than limestone but in practice there is a gradual transition from one form to the other, the principle difference being degree of compaction and cementation, which reflects the geologic history of the source. Marble, the metamorphic form, has been re-crystallised under pressure and is usually produced commercially from sources with high whiteness and purity. Calcium carbonate is abundant throughout the world, thus the cost of the product is very low and this forms the basis for much of its use. Stearic acid coated grades are also readily available. The products obtained may be divided into six types according to particle size and coating (Table 7.7). Those products with a mean diameter of 1 μm or finer do provide superior properties than those that are coarser [64, 65]. This is especially important where hot tear strength is a requirement and the ultrafine products may compete with precipitated grades in less critical applications. 347

Calcium carbonate types Particulate-Filled Polymer Composites

Table 7.7 Ground calcium carbonate types Mean diameter

Uncoated

Coated

Production route and sources

Below 1 μm

Yes

Yes

Wet ground. Chalk, limestone and marbles.

1-4 μm

Yes

Yes

Wet or dry ground available. Chalk, limestone and marble.

5-12 μm

Yes

-

Wet or dry ground available. Generally limestone and marble.

Above 12 μm

Yes

-

Dry ground. Generally marble.

Stearate coating provides products with superior water resistance and a significant improvement in dispersability, providing shorter mixing cycles and improvements in consistency. The water resistance is of particular value in cable applications, where wet electrical properties are important and in extruded products, which are cured under low-pressure conditions (salt bath, fluidised bed, microwave or hot air) or in steam (cables or hoses).

7.7.6 Aluminium Trihydrate The most important feature of ATH is its function as a flame retardant and smoke suppressant. Its flame retardancy is due to the fact that the product decomposes endothermically at temperatures at or below the temperatures required for combustion of most polymers. This strong endotherm (Equation 7.1) removes heat from the ignition source. On decomposition, inert gases are produced (water), which reduces oxygen availability. Smoke suppression [66] is thought to be due to heat dissipation effects that occur during burning, favouring the formation of polymeric char rather than soot particles. To be effective, high loadings are necessary, thus in rubber the filler qualities must be considered as well as combustion behaviour. These fillers are available in a wide range of sizes with mean particle diameters from 100 μm (non-reinforcing) to 0.5 μm (semi-reinforcing) and with a variety of surface treatments available on demand.

Equation 7.1 Endothermic decomposition of aluminium trihydrate

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Particulate Fillers in Elastomers

7.7.7 Talcs Talc is a hydrated magnesium silicate, which has a platy shape, similar to clays. Unlike clays, the mineral does not possess a significant level of surface hydroxyl groups, which is the primary reason for the application differences between the two minerals. Talcs have a surface that interacts poorly with rubber (it has a characteristic slippery feel), is basic in character, and is relatively hydrophobic. Good sources of talc are relatively scarce. This has ensured that the cost of talc is relatively high. Talcs are used in rubber applications for one of two reasons: (1) as a dusting agent to prevent rubber from sticking to itself, and (2) as a compounding ingredient in specific applications. The former category utilises large quantities of coarse, often relatively impure talcs. As a compounding ingredient, however, the finer, micronised products are used because of their specific qualities. The micronised talcs are useful semi-reinforcing minerals. The platy shape is useful in extruded or calendared applications. The lubricating effect of the surface provides a lower processing viscosity than clays with similar aspect ratio and size, plus better mill-release behaviour. The basic nature of the surface is often advantageous in acid-sensitive polymers such as polychloroprene and will often provide improved heat stability to filled products. The finer grades are used to provide low gas permeability in many membrane applications. Electrical insulation properties are good, especially under wet conditions in low-voltage applications. Specific drawbacks of talcs include poor wetting by the polymer, resulting in longer mixing cycles. The platy shape and poor polymer-filler interaction leads to very poor compression-set performance in most applications. This can be reduced by using a silanetreated grade. Airborne cross-contamination is another hazard associated with talc, which can cause problems, especially of poor part-to-part adhesion, with manufacturing operations that are remote from the location in which it is being used.

7.7.8 Natural Silicas Natural silicas have some application in rubbers. One cause for concern with these products is the presence of high levels of free crystalline silica. The most frequently used types are based on deposits found in Bavaria [67], known as Neuburger silicas. These products are mixed minerals being typically 70% silica and 30% kaolin in the form of an intimate blend. The major application is in extruded products where they are able to combine excellent surface finish with reasonable mechanical properties and low compression set. They are also used in a wide range of products, including low-voltage cable insulations. A wide range of surface-treated products is also available. Other grades of silicas are available, particularly in North America, which are of a high purity. These are often used in silicone elastomers.

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7.7.9 Barites and Blanc Fixe Barites and blanc fixe are both forms of barium sulfate. Barites is found as a naturally occurring product and is processed to provide a range of relatively coarse products for use in rubber. Blanc fixe is a precipitated form of the mineral and is used in more technically demanding applications. These materials have several useful qualities: 1. They have a high specific gravity, SG = 4.3. 2. They are chemically inert. 3. They are thermally stable to very high temperatures. The high SG provides a useful filler for sound-deadening applications. Chemical inertness allows the filler to be used in chemical plant lining and industrial gaskets of different types, which also require high thermal stability. Blanc fixe has some application in pharmaceutical compounds for its chemical inertness and purity.

7.7.10 Miscellaneous The fillers described previously form the bulk of materials used in rubber applications. The list is by no means exhaustive but those that are not covered are more specialised in use. Other fillers known to be used in rubber include: calcium sulphate, cork, dolomites, feldspar, graphite, lead, lithopone, magnesium carbonate, molybdenum disulphide, rubber crumbs, textile fibres (flocks), wollastonite, wood flour, zinc oxide and many others.

Acknowledgement The author would like to thank Imerys for encouragement and permission to publish.

References 1.

R.G.C. Arridge, Mechanics of Polymers, Clarendon Press, Oxford, UK, 1975.

2.

Science and Technology of Rubber, Ed., F.R. Eirich, Academic Press, London, UK, 1978.

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Particulate Fillers in Elastomers 3.

Rubber Technology, 3rd Edition, Ed., M. Morton, Van Nostrand Reinhold, New York, NY, USA, 1987.

4.

L.R.G. Treloar, The Physics of Rubber Elasticity, 3rd Edition, Clarendon Press, Oxford, UK, 1975.

5.

J.M.G. Cowie, Polymers: Chemistry and Physics of Modern Materials, International Textbook Company, Aylesbury, UK, 1973.

6.

E.R. Rodger in Developments in Rubber Technology-1, Eds., A. Whelan and K.S. Lee, Applied Science Publishers, London, UK, 1979.

7.

Rubber Technology and Manufacture, 2nd Edition, Eds., C.M. Blow and C. Hepburn, Butterworth Scientific, London, UK, 1982.

8.

R.M. Murray and D.C. Thompson, The Neoprenes, EI DuPont de Nemours Inc., Wilmington, DE, USA, 1963.

9.

D.A. Skelhorn, Proceedings of Developments In The Use Of Silane Treated Clay In Cable Insulations, International Rubber Conference, Prague, Czechoslovakia, 1989.

10. E.R. Pohl, Proceedings of Reactivity of Silanes with Silica: Silane carriers for Rubber: Functional Tire Fillers 2001, Intertech, Fort Lauderdale, FL, USA, 2001, Paper No.7. 11. N. Hewitt, Proceedings of Silane Coupling of Precipitated Silica: Functional Tire Fillers 2001, Intertech, Fort Lauderdale, FL, USA, 2001, Paper No.6. 12. D.A. Skelhorn, Proceedings of Fillers ‘86, PRI/BPF Conference, London, UK, 1986, Paper No.19. 13. E.P. Pleuddemann and B. Thomas in Developments in Rubber Technology -1, Eds., A. Whelan and K.S. Lee, Applied Science Publishers, London, UK, 1979. 14. R.N. Rothon, Proceedings of Second European Conference on High Performance Additives, BPF/PRI Conference, London, UK, 1991, p.12/1. 15. BP Process Oils: Selection and Applications, British Petroleum, London, UK. 16. P. Clutterbuck, Proceedings of Rubber Compounding, Winter Symposium PRI Leicester Section, Loughborough, UK, 1991, Paper No.2. 17. B.G. Crowther, Proceedings of Rubbercon 89, Prague, Czechoslovakia, 1989, Paper No.B29. 351

Particulate-Filled Polymer Composites 18. Developments in Rubber Technology-2, Eds., A. Whelan and K.S. Lee, Applied Science Publishers, London, UK, 1981. 19. R.O. Babbit, The Vanderbilt Rubber Handbook, RT Vanderbilt, Norwalk, CT, USA, 1978. 20. ASTM D1646-03, Standard Test Methods for Rubber—Viscosity, Stress Relaxation, and Pre-Vulcanization Characteristics (Mooney Viscometer), 2003. 21. ASTM D3194-99, Standard Test Method for Rubber From Natural SourcesPlasticity Retention Index (PRI), 1999. 22. Rubber Technology and Manufacture, 2nd Edition, Eds., C.M. Blow and C. Hepburn, PRI, Butterworths, London, UK, 1982. 23. F. Leibbrandt, Diffusion of Gases Through Nitrile-Butadiene Rubber Vulcanisates, Bayer Technical Notes for the Rubber Industry No.50, Bayer, Leverkusen, Germany, 1978. 24. E. Engelman, Proceedings of Rubbercon 1977, PRI, London, UK, 1977, Volume 1, p.9/1. 25. R.F. Ohm, Rubber & Plastics News, 1991, 20, 22, 43. 26. R.F. Ohm, Rubber & Plastics News, 1991, 20, 23,17. check 27. G. Matenar and E. Rhode, Kautschuk und Gummi Kunststoffe, 1977, 30, 5, 289. 28. Rubber Technology, 2nd Edition, Ed., M. Morton, Robert E. Krieger Publishing Co, Malabar, FL, USA, 1981, p.51. 29. B.B. Boonstra, Polymer, 1979, 20, 691. 30. Rubber Technology, 2nd Edition, Ed., M. Morton, Robert E. Krieger Publishing Co, Malabar, FL, USA, 1981, p.69. 31. W.A. Brown and A.C. Patel, Proceedings of Rubbercon 81, PRI, Harrogate, UK, 1981, Paper No.G5. 32. Rubber Technology, 2nd Edition, Ed., M. Morton, Robert E. Krieger Publishing Co, Malabar, FL, USA, 1981, p.78. 33. C. Stevens, Proceedings of Rubber Compounding, Winter Symposium, PRI Leicester Section, 1991, Paper No.6.

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Particulate Fillers in Elastomers 34. Science and Technology of Rubber, Ed., F.R. Eirich, Academic Press, New York, NY, USA, 1978, Chapter 8. 35. Science and Technology of Rubber, Ed., F.R. Eirich, Academic Press, New York, NY, USA, 1978, Chapter 10. 36. D.A. Skelhorn, Clays in Rubber – A Review, ECC International, St Austell, UK, 1990. 37. M.Q. Fetterman, Precipitated Silica – Coming of Age, PPG Industries, reference 1833 1M 988, Pittsburgh, PA, USA, 1986. 38. Manual for the Rubber Industry, Bayer, Leverkusen, Germany, 1970, p.490. 39. Barrier Polymers and Structures, Ed., W.J. Koros, ACS Symposium Series No.423, Dallas, TX, USA, 1989, Chapter 11. 40. G. Grimmett, Percolation, 2nd Edition, Springer, Berlin, Germany, 1999, Chapter 1. 41. M-J. Wang, Proceedings of Functional Tire Fillers 2001, Intertech, Fort Lauderdale, FL, USA, 2001, Paper No.10. 42. A. Blume, H-D. Luginsland, S. Uhrlandt and A. Wehmeier, Proceedings of Silica 2001, 2nd International Conference on Silica Science, Mulhouse, France, 2001. 43. H. Deckmann, Proceedings of the 160th ACS Rubber Division Meeting, Cleveland, OH, USA, Fall 2001, Paper No.22. 44. F. Lyon and K.A. Burgess in Encyclopedia of Polymer Science and Engineering, Volume 2, 2nd Edition, Eds., H.F. Mark and J.I. Kroschuritz, Wiley, New York, NY, USA, 1990, p.623. 45. J-B. Donnet and A. Voit, Carbon Black: Physics, Chemistry and Elastomer Reinforcement, Marcel Dekker, New York, NY, USA, 1976. 46. S. Brunauer, P.H. Emmett and E.J. Teller, Journal of the American Chemical Society, 1938, 60, 309. 47. A.I. Medalia and R.L. Sawyer, Proceedings of the 5th Conference on Carbon, Volume 2, Pergamon Press, New York, NY, USA, 1961, p.563.

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Particulate-Filled Polymer Composites 48. J-B. Donnet and A. Voit, Carbon Black: Physics, Chemistry and Elastomer Reinforcement, Marcel Dekker, New York, NY, USA, 1976, p.64. 49. ASTM DI765-03, Standard Classification System for Carbon Blacks used in Rubber Products, 2003. 50. A. McNeish, Proceedings of Functional Tire Fillers 2001, Intertech, Fort Lauderdale, FL, USA, 2001, Paper No.1. 51. Amorphous Synthetic Silica Products in Powder Form. Production and Characterisation, Degussa Technical Bulletin Pigments No.32, Degussa AG, Frankfurt, Germany, 1980. 52. T. Harris T, Proceedings of Functional Tire Fillers 2001, Intertech, Fort Lauderdale, Florida, USA, 2001, Paper No.2. 53. S. Uhrlandt and A. Blume, Silicas for ‘Green Tires’ – Processes, Products and Performance, Degussa AG, D-50389, Wesseling, Germany, 2000. 54. P. Cochet, Proceedings of Functional Tire Fillers 2001, Intertech, Fort Lauderdale, FL, USA, 2001, Paper No.4. 55. Bayer Manual for the Rubber Industry, 2nd Edition, Bayer AG, Leverkusen, Germany, 1993, p.521-535. 56. D.A. Skelhorn, Proceedings of Rubbercon ‘88, Sydney, Australia, 1988, Paper No.25. 57. G.W. MacDonald, Rubber Age, 1970, 102, 4, 66. 58. A.L. Barbour and A. Rice, Rubber and Plastics News, 1987, 17, 2, 48. 59. Polarite 503A in Power Cable, Technical Data Sheet, Imerys, Imerys Pigments and Additives Division, St. Austell, UK, 2000. 60. Winnofil S, ICI Datasheet, ICI Chemicals and Polymers, Runcorn, UK. 61. J. Hutchinson and J.D. Birchall, Elastomerics, 1980, 112, 7, 17. 62. D.L. Harrison and R.N. Rothon, Proceedings of Fillers ’86, BPF/PRI Conference, London, UK, 1986, Paper No.2. 63. D.A. Skelhorn, Proceedings of the 149th ACS Rubber Division Meeting, Spring 1996, Montreal, Canada, Paper No.A.

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Particulate Fillers in Elastomers 64. Polcarb & Polcarb S, Reference 2M/8174, English China Clays Paint and Polymer Division, St. Austell, UK, 1974. 65. Non-black Fillers for Rubber from ECC International, Fillers in Compound Design, Reference R87, ECC International (now Imerys), St. Austell, UK, 1981, p.5. 66. BACO Alumina Trihydrate Flame Retardants, Publication number M300 3K 8190, BA Chemicals Ltd., Gerrards Cross, UK, 1990. 67. Neuburg Siliceous Earth, Hoffmann Mineral GmbH & Co. KG., Neuburg (Donau), Germany.

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8

Filled Thermoplastics Chris DeArmitt and Michael Hancock

8.1 Introduction 8.1.1 Thermoplastics and Typical Applications Thermoplastics have become an essential part of our everyday lives. Our cars and appliances contain more and more plastics every year. Even our clothes are often made from synthetic thermoplastics. They are a very important class of material for many reasons. They combine good mechanical and electrical properties with low density and high formability. Clearly, the driving force for their success has been that they can often provide an overall solution that is less expensive than that achievable with other materials such as glass, wood, metal, thermosetting polymers or ceramics. Thermoplastics, as implied by their name, are materials that flow upon heating, and harden when cooled. They can be formed using a wide variety of techniques, such as injection moulding, thermoforming, blow moulding and rotational moulding. Injection moulding in particular allows complex shapes, so it is possible to integrate several smaller parts into one larger part, thus saving on assembly costs. As well as being easily processed when molten, they also have the potential to be recycled by remelting them to form new articles, or burnt and used to generate electrical energy. New legislation is being introduced to encourage recycling of used products; this is expected to favour thermoplastics over other materials, which are not as easy to reprocess. Thermoplastic demand in Western Europe is 37 x 106 tonnes compared to 10 x 106 tonnes for thermosetting polymers. A breakdown of thermoplastics by application area is given in Figure 8.1. Some of the main properties and applications are given next for the five main thermoplastics, which together account for 75% of the total thermoplastics market.

• Polyethylene (low density) LDPE, (linear low density) LLDPE: 7.6 x 106 tonnes Properties: Flexible, translucent, very tough, weatherproof, good chemical resistance, low water absorption, easily processed by most methods, low cost.

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Figure 8.1 Use of thermoplastics in Western Europe

Applications: Squeeze bottles, toys, carrier bags, high frequency insulation, chemical tank linings, heavy-duty sacks, general packaging, gas and water pipes.

• Polyethylene (high density) HDPE: 5.0 x 106 tonnes Properties: Semi-rigid, translucent, weatherproof, good low temperature toughness (to –60 °C), easy to process by most methods, low cost, good chemical resistance. Applications: Chemical drums, jerricans, carboys, toys, picnic ware, household and kitchenware, cable insulation, carrier bags, food wrapping material.

• Polypropylene PP: 7.0 x 106 tonnes Properties: Semi-rigid, translucent, good chemical resistance, tough, good fatigue resistance, integral hinge property, steam sterilisable, good heat resistance. Applications: Sterilisable hospital ware, ropes, car battery cases, chair shells, integral moulded hinges, packaging films, electrical kettles, car bumpers and interior trim components, video cassette cases.

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• Polyvinyl chloride PVC: 5.8 x 106 tonnes Properties: Rigid or flexible, clear or opaque, durable, weatherproof, flame resistant, good impact strength, good electrical insulation properties, limited low temperature performance. Applications: Window frames, drain pipes, sewage and soil pipes, roofing sheets, cable and wire insulation, floor tiles, hose and pipes, stationary covers, fashion footwear, leathercloth.

• Polystyrene (general purpose) GPPS: 3.1 x 106 tonnes combined with high impact polystyrene (HIPS) Properties: Brittle, rigid, transparent, low shrinkage, low cost, excellent X-ray resistance, free from odour and taste, easy to process. Applications: Toys and novelties, rigid packaging, refrigerator trays and boxes, cosmetic packs and costume jewellery, lighting diffusers, audio cassette and CD cases.

• Polystyrene (high impact) HIPS Properties: Hard, rigid, translucent, impact strength up to seven times that of GPPS, other properties similar. Applications: Yoghurt pots, refrigerator linings, vending cups, bathroom cabinets, toilet seats and tanks, closures, instrument control knobs.

• Polyesters (thermoplastic) PET: 3.1 x 106 tonnes Properties: Rigid, clear, extremely tough, good creep and fatigue resistance, wide range of temperature resistance (–40 °C to 200 °C). Applications: Carbonated drink bottles, synthetic fibres, video and audio tape, microwave utensils.

8.1.2 Thermoplastic Composites One disadvantage of thermoplastics is that they soften appreciably as they are heated. As this happens, their modulus decreases and they begin to creep (slowly deform over time)

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Particulate-Filled Polymer Composites and at higher temperatures they progressively lose their shape, and then melt. There has been a great deal of effort spent in trying to overcome this limitation. Filling PP with talc is an early example of this. In fact, addition of mineral fillers increases the modulus of all thermoplastics, and increases their heat distortion temperature (HDT) [1]. It is commonly thought that the primary goal of adding fillers is to lower the overall materials cost of the composite compared to the unfilled polymer. This is, however, rarely the case. Polyethylene (PE) and PP are the world’s number one and two highest volume polymers, respectively, and they are also the least expensive per unit volume. Addition of any common filler, with the exception of calcium carbonate, increases the material cost of these polymers. Even in cases where the main goal is to decrease cost, the addition of filler changes nearly every property of the polymer. It is therefore now common to use the term ‘functional fillers’ to emphasise that the fillers change the polymer, giving many advantages, and naturally, some disadvantages. Making good composites is all about knowing how to find a good balance of properties at the lowest cost. In order to make the right decision, one needs to know about polymers, engineering, fillers and surface science. That is what makes the study and application of composites so challenging, fascinating and rewarding. In this chapter, we will discuss the main polymer properties and how they are influenced by the addition of various common fillers. The global fillers market is estimated as between 5-10 million tons per year, with over 90% of the filler going into rubbers, PVC and polyolefins, e.g., PE and PP. PP is the one of the most commercially important filled polymers [2] and it will therefore be used to illustrate some of the main points. Similar trends are seen when fillers are compounded into other semi-crystalline thermoplastics such as PE¸ PVC [3] and the polyamides. The amorphous polymers such as polystyrene and polycarbonate also respond similarly to filler addition. Incorporation of fillers into thermoplastics alters all the properties of the material. Some of the changes will be beneficial and some will be detrimental. It should be noted that these are not absolutes and the determination of pros and cons is only meaningful to the proposed end-use of the material. Burditt listed 21 reasons why filler may be added to a polymer [4]. In addition to those intentional changes, there are a multitude of unintentional effects that must also be considered. In this chapter the main properties of composites and how they vary with filler type and level of addition will be discussed. An essential point to note, is that the properties of the composite depend upon the volume percentage of filler added [5, 6]. Often in the literature, one sees properties plotted versus the weight percentage of filler, which is not particularly useful and may even be misleading. It is more meaningful to plot properties versus the volume percentage of filler [7]. In many cases, this latter approach gives straight lines, allowing simple, accurate extrapolation and prediction of properties [8, 9]. The properties of a composite are usually in between those of the component materials. Several of the properties such as

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Filled Thermoplastics density, modulus and yield strength can be predicted by the rule of mixtures, according to the volume fraction of each component. In other cases, there are more complex mathematical models that can be used to describe and predict properties as the volume fraction of the ingredients is varied. Sometimes, the inclusion of filler can chemically or physically modify the polymer phase to such as extent that it becomes more difficult to apply simple models. It is important to realise that these complications may occur, and to look for them when characterising composites. This will help in the understanding and design of new composite materials. One example of this behaviour is changes in crystallisation, such as nucleation of crystal growth [10-14] and changing of the crystal phase of the polymer [15, 16]. Chemical degradation of the polymer may be catalysed by the filler, or impurities in the filler, especially transition metals [17, 18]. Another common example is where the filler surface adsorbs stabilisers and antioxidants, which are then unable to protect the polymer during processing and during its service life [19-21]. Alternatively, mechanical degradation may occur when high levels of filler cause unduly high viscosity, thereby inducing chain scission due to the excessive shear needed to process the material. In this chapter, an attempt has been made to mention each of the factors that influence composite design and performance. For a given application, one must identify the key properties that are important and concentrate on those. It will be seen that there are many different parameters to consider and that in some cases, optimisation of one precipitates an inevitable worsening of some other property. There is no one optimal composite; rather the goal is to seek the best balance of properties through compromise and an awareness of the entire picture in terms of economics and performance. In cases where the filler is less expensive than the polymer, then the goal is to increase the filler loading as much as possible, while still retaining sufficient processability and properties. Conversely, when the filler is more expensive than the polymer on a volume basis, then one seeks to identify the minimum filler loading that gives sufficient properties.

8.2 Bulk and Process Related Properties 8.2.1 Specific Gravity or Relative Density The common fillers used in plastics are minerals (densities from 2.4-2.8 g/cm3), which give a composite of higher density than that of the unfilled polymer (densities of 0.8-1.9 g/cm3). The density of a composite of known composition can be calculated according to the linear rule of mixtures (Equation 8.1), where ρc, ρf and ρp are the densities of the composite, filler and matrix, respectively, and mf is the mass fraction of filler.

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ρc =

ρf ρ p ρ pm f + ρf (1 − m f )

Equation 8.1 Composite density Such calculations usually agree well with measured values. A lower than expected density may be due to inclusions of air resulting from poor mixing or poor wetting of the filler surface by the polymer. Higher than expected density occurs when the filler nucleates crystal growth in the polymer. The increased crystallinity increases the density of the composite because crystals are of higher density than amorphous regions in the polymer. The expected density of the composite can be calculated using Equation 8.1, assuming no air inclusions and that the filler does not influence density of the polymer phase by nucleation of crystal growth for example. The filler loading mf is usually determined by ashing, namely burning away the polymer from a known mass of composite and weighing the amount of residual filler. This method is simplest if the filler has enough thermal stability so that it does not lose mass at the high temperatures needed to burn off common polymers (≥ 300 °C). For fillers that are somewhat thermally unstable, such as aluminium trihydrate, it is necessary to correct the ashing result using the mass loss for the filler alone under the same ashing conditions. Recently ‘ovens’ based on microwave ashing have been introduced. These operate at low temperatures, removing the polymer while leaving the filler unaffected. Density can also be determined by measuring the volume of liquid displaced by a known mass of composite or using a density gradient column. Increased density is usually undesirable because products must inevitably be transported to be sold, or installed. This may result in increased transportation costs. Weight increases are undesirable when the material is to be used to make cars, trucks, trains, aeroplanes or spacecraft. Recent European legislation on packaging will also penalise by weight. Decreased density is possible through use of fillers such as wood flour (or fibre), hollow glass microspheres, hollow polymer microspheres [22] (e.g., Expancel®) or hollow spheres from fly ash. Low-density thermoplastic composites are useful for products that must float.

8.2.2 Acoustic Properties It is appropriate to mention acoustic properties here, as they are affected by density. Adding filler usually increases the density compared to the host polymer, and this is usually an unwanted side effect. However, it is common to make sound deadening composites by using high-density fillers such as barium sulfate (BaSO4, 4.5 g/cm3) or magnetite (Fe3O4, 5.1 g/cm3) [23].

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Filled Thermoplastics In order to dampen sound, a material must be able to absorb vibrational energy (sound) and transform the energy into thermal energy (heat). The loss factor, tan δ, is the parameter that best describes the sound damping ability of materials [5]. It is the inelastic component of the material’s response to deformation. The loss factor can be measured by dynamic mechanical thermal analysis (DMTA), which shows its behaviour with changes in frequency and temperature. It is common to select a matrix with a high tan δ in the frequency range to be damped. Then, high-density filler can be added in order to further improve performance. Mica is also used, as the platy particles cause multiple reflection of the sound waves so they may be absorbed in the composite instead of passing straight through [5]. Adding too much filler should be avoided however, because this leads to particle-particle interactions, and eventually percolation, where a continuous path exists from particle to particle. Under those conditions, the acoustic energy can pass through the interparticle contacts, largely avoiding interaction with the matrix.

8.2.3 Melt Viscosity (MFI) The melt viscosity of polymers is usually measured as the melt flow index (MFI), also known as melt viscosity index (MVI) and melt flow rate (MFR). A pressure is applied to force the molten polymer through a hole at a controlled temperature [24]. All three parameters are set out in standards appropriate to the polymer being measured [24]. The measured value is in grams of polymer extruded through the hole in a set period of time, e.g., ten minutes. This of course, is actually the reciprocal of the viscosity, so a high MFI means low viscosity and vice versa. There are some caveats when using MFI as a measure of viscosity. Firstly, MFI measurements are performed in the medium shear rate range [24], whereas polymer processing is often performed at higher shear rates. Therefore, the MFI may not correspond well to the flowability of the polymer melt during compounding and processing. A further point is that raw MFI data should not be used to compare viscosities of polymer melts containing different filler loadings. This is because adding filler increases the density of the melt, and therefore the MFI will increase because more mass of polymer melt flows in a given time, even if the volume of material flowing remains constant. Therefore, the MFI data must be adjusted by dividing by the density. This gives the volume of material flowing in unit time and allows fair comparison of samples with differing filler amounts [8]. A detailed description of filled polymer melt rheology can be found in a book by Shenoy [24]. Of particular interest is Shenoy’s method for extrapolating MFI data to give an idea of the expected rheology of the polymer melt at the higher shear rates encountered during polymer processing. Another good review has been made by Hornsby [25]. This latter work includes an investigation of the degree of filler dispersion at various points as it passes through a twin-screw extruder.

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Particulate-Filled Polymer Composites An inevitable consequence of adding filler is that the viscosity of the polymer melt increases [8, 24-26]. This is exacerbated at high volume fractions of filler, and when the filler particles are smaller, especially for nano-sized fillers such as carbon black [27]. In fact, the increase in viscosity often limits the amount of filler in the compound. At low filler addition levels, this effect is very low and is masked by the high inherent viscosity of the polymer melt. As the filler addition increases, the viscosity begins to rise more sharply, until eventually it approaches infinity at some critical filler level, which depends upon particle size, shape and amount of agglomeration. High melt viscosity is to be avoided, as it lessens the throughput during extrusion, increasing production costs. It may also prevent complete filling of the mould, leading to high reject levels. The viscosity of a very dilute dispersion of rigid spherical particles in a Newtonian fluid is described by the Einstein equation (Equation 8.2) [26]. Where η is the viscosity of the dispersion, ηl is the viscosity of the fluid alone, φ is the volume fraction of particles and kE is the Einstein coefficient, which is 2.5 for spherical particles. kE depends upon both particle shape and orientation. η = ηl (1 + kE φ)

Equation 8.2 Einstein equation of dispersion viscosity Although it is a starting point for understanding the effect of filler on viscosity, the Einstein equation is not applicable to filled polymer melts. Polymer melts are nonNewtonian, and the filler concentrations are often too high to ignore particle-particle interactions. A plethora of equations exist for modelling dispersions of particles. However, the best approach is to make the desired formulation and test it under real conditions, such as measuring the torque and volume throughput during extrusion, plus mould filling when injection moulding.

8.2.4 Compounding and Extrusion 8.2.4.1 Introduction Extruders are used to mix ingredients into thermoplastics [28]. The polymer is fed into a hopper and is then forced into the barrel, either by gravity, or by mechanical means, such as a feeder screw. The barrel is heated to melt the polymer and a rotating single or twinscrew arrangement transports the polymer melt down the barrel and out of the die (hole) at the end. There are usually ports at various points along the barrel to allow for the introduction of additives such as lubricants, antioxidants, pigments and fillers. These additives may be added individually, but more commonly they are fed in together as a

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Filled Thermoplastics concentrate known as a masterbatch. After it emerges from the die, the molten polymer string is usually cooled rapidly by running it through a water bath. The polymer is then dried and lastly, the cooled polymer strand is chopped into small granules using a pelletising machine. If the material is to be stored before use, then it is common to seal the pellets to protect them from contamination and water pick-up. The extrusion step is not particularly costly in comparison with the price of the raw materials, but the cost is still significant and impacts on the overall economics of the final material. It is therefore worthwhile to devote effort to optimisation of the extrusion process in terms of increased throughput (productivity) and decreasing energy consumption and machine wear. In the author’s opinion, the subject of throughput does not receive the attention it deserves. There are countless reports of the mechanical properties of thermoplastic composites but no mention of the extrusion characteristics of the materials. For a meaningful comparison of different composites, one must consider not only their mechanical and aesthetic properties, but also the relative economics of extrusion.

8.2.4.2 Volume Throughput As with all of the other properties of a composite, when considering extruder output one must think in volume terms, not in terms of weight. If one is making a certain amount of composite, that material must be of sufficient volume to create a certain number of parts, each of which has a fixed volume determined by the size of the mould to be filled. Therefore, the mass of material produced is, in itself, of no interest. There have been various reports that filler increases extruder throughput. These claims are often erroneous, or at least greatly exaggerated, because the throughput is given in units of mass per unit time. That is not a valid way to compare throughput, for the same reasons mentioned previously for MFI. Namely, that the addition of mineral fillers increases the density of the filled polymer compared to its unfilled counterpart. Naturally, this elevates the mass throughput of the extruder, giving a false impression of improved productivity. One possible cause for the confusion over throughput, may be the way in which extruders are rated by the manufacturer. Usually, the capacity of the extruder is given in terms of kilograms of polymer extruded per hour. This value conjures up an image that the maximum throughput of the extruder is limited to a given mass of polymer, as shown on the side of the machine. In reality, that value is valid only in the case of a standardised grade of unfilled polymer, and is merely a convenient means for the extruder manufacturer to show the relative capacities of different machines. Perhaps in the future, the extruder manufacturers will consider expressing the maximum throughput in volume terms to avoid confusion. There are two classes of extruder, single-screw and twin-screw. The primary drawback of single-screw extruders is that they give poor dispersion of fillers compared to the

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Particulate-Filled Polymer Composites more expensive twin-screw variants. Thus, twin-screw extruders are used almost exclusively when good filler or pigment dispersion is required. Recently, it has been suggested that the lack of dispersion associated with single-screw extruders is not an intrinsic limitation [29]. It is argued that single-screw extruders had not been optimised, as all of the attention and R&D has been on the more lucrative twin-screw machines. Furthermore, it is claimed that the newly designed single-screw extruders can now give sufficient dispersion for filled systems along with advantages such as being simpler, less expensive, easier to maintain and giving higher volume throughput compared to twinscrew machines. It will be interesting to see whether single-screw extruders do indeed gain acceptance for the manufacture of thermoplastic composites. The volume throughput and the quality of filler dispersion are the main parameters to consider. One might assume that it would be a simple matter to measure the volume MFI, as described previously, and then correlate that to extruder throughput. However, it has been shown that the transport of the polymer melt in an extruder is more complicated than the simple flow used in measuring MFI. In fact, the mechanism is different for melt transportation through a single-screw compared to a twin-screw machine [28]. The best approach is to extrude the proposed formulations in the production extruder that will be used. The volume throughput can be measured as well as the torque on the screw and the energy input to the motor. However, production extruders can have outputs of well over one thousand kilos (litres) per hour, which means a lot of filler and polymer is needed to run a test. Even more problematic is the loss of productivity when a production extruder is used for testing purposes. Usually, the test material made will not conform to existing specifications and may have to be scrapped/discarded. More commonly an instrumented laboratory extruder is used for initial testing. The feeding and screws of the laboratory machine should be set up to mimic the configuration of the production extruder. When this is done, one can obtain good correlations between the properties of compounds made in the laboratory and production material. A more detailed treatment of compounding is given in Chapter 5.

8.2.4.3 Dispersion Good dispersion is nearly always beneficial for the properties of a composite so one tries to optimise the dispersion of filler. The level of dispersion can be measured directly or indirectly. The most common direct measurement is to perform scanning electron microscopy (SEM) on a cross-section. It is advisable to use two different magnifications to examine the filler distribution on a macroscopic and dispersion on a microscopic scale. An indirect measurement is to measure the unnotched impact strength of the composites, as that property is sensitive to agglomerates. In a rather insightful study, Hornsby showed the degree of dispersion of filler as it passed through a twin-screw

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Filled Thermoplastics extruder [25]. This was achieved by using a clamshell extruder that could be stopped and opened so that samples of compound could be removed for analysis. It was found that the greater part of the dispersion was imparted in the melt zone where the pellets of polymer are just melting. The high viscosity in that region requires a high energy input and encourages deagglomeration. Feeding the filler into the unmelted polymer may give good dispersion, but it also results in higher wear of the extruder so that this approach is only advisable for soft, surface treated fillers. It is more usual to add the filler when the polymer is already molten although a multitude of feeding possibilities exist [25].

8.2.4.4 Machine Wear Machine wear is a concern, partly because it costs money to replace a worn barrel or screw. The main problem though, is the loss of productivity when the machines must be shut down for maintenance. Potentially, wear may lead to significant metal contamination levels with accompanying polymer stability problems. Although machine wear is an issue, it is not a subject that has received much attention. There are a few studies that have investigated the effect of filler properties on wear [17, 30]. One simple method is to extrude through a plate of soft metal and measure weight loss from the perforated plate at regular intervals. It was concluded that hard, large irregular particles cause most wear. Surface treatment with a lubricating additive such as stearic acid helps alleviate wear because the additive forms a protective layer around the particles.

8.2.5 Thermal Conductivity and Specific Heat Capacity It is not only the final properties of the composite that matter, processability and economy of manufacture are also important. If the part can be cooled more quickly, then money can be saved through improved productivity. Mineral fillers typically have thermal conductivities in the range 0.02–3 WK-1m-1 [5, 22, 31] that are an order of magnitude higher than those of polymers [2, 32-34]. The volume specific heat capacity of mineral fillers (~1900-2000 J litre-1K-1) [5, 22, 31] are very similar to those of polymer melts (~1500-3000 J litre-1K-1) [2, 33-35]. An equation has been proposed for calculating the heat capacity of composites based on the composition and the heat capacities of the components [36]. The result is that the filler speeds heating and cooling of the filled polymer melt through improved conduction. Therefore, filling a polymer often allows for reduced cycle times in injection moulding [30] and thermoforming [34] because the part cools and hardens more quickly, allowing it to be removed from the mould earlier. In the literature, there have been some contradictory statements about •

•

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Particulate-Filled Polymer Composites the relative specific heat capacities of mineral fillers and polymers. This confusion may have arisen because the specific heat capacity is usually given in terms of mass with units of J·kg-1K-1, whereas to compare fairly, these values must be converted into volume terms, i.e., J·L-1K-1. There is a niche market for thermoplastics with very high thermal conductivity. These are marketed for CPU cooling in laptop computers and other high performance applications. An interesting point to note is that thermal and electrical conductivity actually benefit from poor filler dispersion. Agglomeration and network formation (percolation) allows better heat conduction due to the network of particle – particle contacts.

8.2.6 Thermal Expansion The thermal expansion coefficients (CTE) for polymers (~10 x 10-5 mm mm-1 °C-1) [2, 36, 34] are approximately an order of magnitude higher than those for mineral fillers (~10 x 10-6 mm mm-1 °C-1) [31] or for metals (~20 x 10-5 mm mm-1 °C-1) [31]. This may lead to problems in applications where plastics and metals are in contact, as differential expansion and contraction can cause such parts to warp. It is possible to estimate the CTE of a composite based on the composition and knowledge of the CTE for each component [37]. The polymer chains can become oriented during flow of the polymer melt and this can give rise to a difference in the amount of shrinkage parallel and perpendicular to the flow direction. Addition of particulate fillers such as calcium carbonate, silica and talc tend to lessen the amount of polymer chain orientation, and thereby reduce not only shrinkage, but also shrinkage differentials, with a corresponding decrease in warpage [22, 34]. In contrast, fibrous fillers and other highly anisotropic fillers, tend to partially align in the flow direction, leading to an increased shrinkage differential and a tendency for the part to warp during cooling. Warpage is not easy to predict, and so it is common to use low aspect ratio fillers for parts where warpage must be avoided. Another approach is to use a mixture of low aspect ratio filler and fibres [38], which can ameliorate the high warpage observed when fibres alone are used, whilst maintaining sufficiently good mechanical properties. •

•

•

It is often not possible to use the same mould for filled and unfilled polymer because the change in shrinkage gives parts that are out of specification. On the other hand, adding filler to a polymer allows its shrinkage to be systematically tuned. This tuning method is useful for example, if one is attempting to use an existing mould with a different polymer. Filler may be added to the newly chosen polymer to achieve similar shrinkage to that of the previously used polymer. Injection moulding tools (moulds) are very expensive and it is preferable to keep using the same mould rather than purchasing a new one specifically made to accommodate the new polymer.

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8.2.7 Electrical Properties The high tonnage, commercially important polymers are, in general, excellent insulators with resistivities in the range 1012 – 1018 Ω cm [22]. The top three polymers in volume terms (in descending order) PE, PP and PVC are all used extensively as cable insulation. There are some intrinsically conductive polymers [39-41] such as polyaniline [42, 43] polythiophene and polypyrrole [43], but these are relatively expensive, intractable, niche materials, that must be modified to impart processability [44-46]. •

Most fillers, while having good dielectric properties and resistivities, are in general worse than a plastic. Also, incorporation of a filler will introduce flaws and interfaces, charged ions and traces of water (all fine particles adsorb some water from their environment) and hence reduce electrical properties. Usually, however, because of the good electrical properties of the plastic, fillers may be used simply as an extender, the composite still giving good properties. Calcined clay (produced at just above 1000 °C) and other calcined silicates, do not degrade electrical performance as severely as other fillers, because of low water pickup, and immobilisation of matrix ions. Conversely, metakaolin, because of its highly reactive surface absorbs ions and thus improves the electrical performance of polymers such as plasticised PVC and ethylene vinyl acetate (EVA) copolymers [47]. Due to their good insulation characteristics thermoplastics can suffer from tracking, i.e., the build-up of surface charge which then discharges across the surface, because that path has less resistance than passage through the plastic. Filler particles can reduce this problem by acting as a physical barrier and by distributing the charge before critical build-up occurs [48, 49]. In many cases, it is beneficial to introduce some level of electrical conductivity into a polymeric material [5, 22]. The highly insulating polymers are susceptible to static build up. This may be a nuisance, attracting dust to the surface, or it may lead to damage of sensitive electronic parts, when manufacturing integrated circuits, for example. Even very low levels of surface conductivity will resolve this problem [6]. Organic based antistatic additives are available or alternatively conductive fillers such as carbon black, graphite, or metals may be added [6]. Sometimes, a material of much higher conductivity is required. One example is for electromagnetic interference (EMI) shielding [22]. This is achieved by adding a sufficient level of conductive filler. Initially, as one adds more conductive filler, the conductivity does not rise appreciably, because the conductive particles remain isolated from one another. As the filler loading is gradually increased, the conductivity suddenly rises sharply, by many orders of magnitude, approaching the conductivity of the conductive filler itself, and then levels off [50]. This discontinuity is known as the percolation threshold and it signifies the volume percentage of filler required to attain a continuous network of interconnected particles throughout the matrix [22]. Usually, 10-30 volume percent of filler is needed to

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Particulate-Filled Polymer Composites achieve percolation, depending upon particle size, shape and the level of dispersion. Smaller, more anisotropic particles show lower percolation thresholds [39]. As with thermal conductivity, good dispersion is detrimental to electrical conductivity [22, 51]. On the contrary, the aim is to achieve percolation by inducing an agglomerated network of particles.

8.2.8 Barrier Properties Thermoplastics are widely used as packaging materials due to their low cost, excellent chemical resistance, good barrier properties and the potential for recycling. In fact, over 35% of all thermoplastic is used in packaging. They are also used in a variety of other applications where barrier properties are required. These include water and gas pipes, as well as car petrol tanks. Permeability of a material to small molecule penetrants, such as oxygen and water, increases with the solubility of the small molecule in the matrix [52] and with the diffusion coefficient in that material [53]. Polymers are very sensitive to plasticisation by small molecules. Thus, the presence of small molecules may greatly increase the diffusion coefficient. Based on these observations, one can envision ways to decrease permeability by reducing the solubility and/or the diffusion coefficient. Molecules can neither dissolve in, nor diffuse through, mineral fillers to any appreciable extent. Therefore the presence of filler reduces the solubility of the diffusant in the composite material, and thereby the permeability, in proportion to the volume fraction of filler. In addition, the presence of impermeable filler in a polymer forces the diffusant molecule to travel further around the filler particles. This physical blocking effect is known as tortuosity, because the filler forces the diffusant to take a more indirect, or tortuous, path through the material. The degree of tortuosity imposed is dependent upon the anisotropy and orientation of the filler particles with respect to the direction of diffusion. For example, platy particles oriented perpendicularly to the diffusion vector will be particularly effective in retarding diffusion. The permeability of a composite can be calculated using an equation that allows for the reduction in permeant solubility and for the tortuosity (Equation 8.3). Where Pc and Pp are the permeability of the composite and the unfilled polymer, respectively. The terms w and t refer to the width and thickness of the filler and φp and φf represent the volume fraction of polymer and filler. φp Pc = Pp 1 + (w / 2t )φ f

Equation 8.3 Composite permeability

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Filled Thermoplastics As mentioned previously, the addition of filler may also change the amount of crystallinity in the polymer. As polymer crystals are impermeable even to low molecular weight species, an increase in crystallinity also results in improved barrier properties, through increased tortuosity [54]. This effect is expected to be especially prevalent for fillers that induce a high degree of transcrystallinity. Dispersion and wetting of the filler can also affect the permeability of the composite. It has been shown that PE filled with 25 volume percent calcium carbonate was actually four times more permeable to oxygen compared to the unfilled reference PE. This was attributed to poor wetting of the filler, so that the diffusant was able to travel unimpeded along the polymer/filler interface. In contrast, stearic acid coated calcium carbonate at the same loading resulted in three times lower oxygen permeability than the unfilled PE [55]. Similarly, Tiburcio and Manson showed that the water vapour permeability of glass-bead filled phenoxy films decreased sharply as the degree of adhesion between the filler and the matrix was increased [56]. In some cases, it is desirable to increase the permeability of a polymeric material. One example is breathable films. For example, calcium carbonate filled PP films are first made by solvent casting, or extrusion casting or as blown film and subsequently stretched to delaminate the filler – polymer interface [57]. High filler loadings are used to ensure interconnecting voids, giving unimpeded diffusion [58].

8.3 Mechanical Properties 8.3.1 Introduction For any given application, certain mechanical properties will be of more importance than others. It is therefore, essential to identify and rank the most relevant properties and formulate or purchase the least expensive composite material that satisfies the requirements. The key mechanical properties for most applications are modulus (tensile or flexural), yield strength, impact strength and possibly HDT. A distinction is often made between reinforcing and nonreinforcing fillers, but unfortunately, the term reinforcement is rarely defined explicitly. Fibres are usually considered to reinforce and isotropic fillers are not, with platy fillers somewhere in between. As shown later, it is not appropriate to define reinforcement in terms of particle shape, because that definition breaks down with variations in anisotropy and particle size. In agreement with Ram [59], the definition of reinforcement as the simultaneous improvement of both modulus and yield strength will be used in this chapter. Polymer mechanics is a broad subject and the interested reader is directed to specialised texts [60]. Each of the main properties is described here, along with a consideration of

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Particulate-Filled Polymer Composites how it changes with filler type and level of addition. The effect of surface treatments such as dispersants and coupling agents are also mentioned, where applicable. A more detailed description of surface treatments is given in Chapter 4.

8.3.2 Modulus – Tensile and Flexural One of the main reasons for adding mineral fillers to thermoplastics is to increase the modulus (stiffness). Tensile (under tension) modulus is the ratio of stress to strain, at some low amount of strain, below the elastic limit. Flexural (bending) modulus is also often measured. The most relevant modulus to measure depends upon the expected deformation mode of the part that will be made from the material. Often, the flexural modulus and tensile modulus are rather similar. The exception is the case of anisotropic fillers, which can become aligned in the flow direction when the test specimens are moulded. There is a good understanding of how addition of filler affects the modulus of a polymer. Chow has done an extensive review of the area, and the interested reader can consult that work for a more detailed description [61]. In fact, the simple rule of mixtures is fairly accurate at low strain levels (Equation 8.4). Ec, Ep and Ef are the moduli of the composite, polymer and filler, respectively, and φf is the volume fraction of filler. The moduli of thermoplastics are in the range 1-3 GPa [1, 32, 35, 34] whereas common fillers have much higher moduli [5, 22, 31] (calcium carbonate and dolomite ~ 35 GPa, mica ~ 17.2 GPa and wood flour ~10 GPa). Ec = (1 − φ f )E p + φE f

Equation 8.4 The dependence of composite modulus on volume fraction of filler The modulus increases with increasing volume percentage of filler [62]. In most cases, this relationship is linear for filler concentrations up to approximately 20 volume percent. Filler orientation strongly affects modulus [63]. This is important because injection moulded ASTM standard ‘dog-bone’ shaped test specimens give high filler orientation. The moduli measured on such specimens may be far higher than those attained in a real injection moulded part where the orientation is usually not optimal. Increasing crystallinity in the polymer phase can also lead to a higher modulus because the crystal phase is stiffer than amorphous regions [64, 65]. Various attempts have been made to account for other factors such as polymer-filler interaction and interparticle interactions. One such semi-empirical model is known as the Nielsen equation, also known as the LewisNielsen or modified Kerner equation [66, 67]. In these theories, modulus is independent of particle size. However, Heikens [68] has found that when the polymer is strongly bonded to the plastic composite, modulus is

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Filled Thermoplastics affected by the filler particle size. Particle size effects are allowed for to some extent in the modified Kerner equation by the introduction of an effective filler volume fraction. The most important filler parameter affecting modulus is its shape. Unfortunately, when the filler is non-spherical theories become much more complicated and the reader is advised to refer to Chow’s review [61]. Shape factors can be incorporated in the models mentioned previously but are only useful when applied to very high aspect ratio materials, e.g., fibres. There is also an almost insurmountable problem with particulate fillers: the difficulty and effort to measure aspect ratios of micrometre sized particles. Pukansky examined the effects of 11 different fillers in polypropylene [69] and concluded that Young’s modulus is affected by the amount of bonded polymer, which is in turn related to surface area, and therefore to both particle size and shape. That observation helps to explain the strong effect that nano-fillers have on the modulus of a composite. Schreiber and Germain showed that modulus depends on the strength of interaction between the polymer and the filler surface [62]. To exemplify the effect of fillers on a thermoplastic, PP homopolymer filled with differing filler types and consequently very different shapes is shown in Figure 8.2. It can be seen that a linear fit can be used successfully for most of the fillers. The exception is mica, which deviates from linearity at high filler levels where interparticle interactions become important. The modulus values also reflect the expected shapes of each of the fillers. Similar trends are reported for other common polymers [22].

Figure 8.2 The effect of common fillers on the tensile modulus of PP homopolymer

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Particulate-Filled Polymer Composites In conclusion, it can be stated that the effect of fillers on modulus is relatively well understood and may be predicted. The other properties are less easy to predict as they are measured under conditions where the composite is deformed to a greater extent. This means that other factors such as particle debonding, particle re-orientation and polymer orientation, must be considered.

8.3.3 Heat Deflection Temperature (HDT) The HDT is the temperature at which a beam of polymer deflects by a given amount under a specified load. The HDT is a complex function of the composite’s modulus and polymer properties such as glass transition temperature (Tg), melting temperature (Tm), degree of crystallinity and amount of bonded polymer in the filler-polymer interphase. Examining the effect of fillers on the HDT of PP homopolymer (Figure 8.3) shows that the trends are similar to those for modulus. There are two common standard conditions for testing the HDT of PP; one uses a force of 0.46 MPa, whereas the other uses 1.8 MPa. Care must be taken when comparing results for different composites to ensure that the same test conditions were used. The greatest enhancement of HDT is seen with semi-crystalline thermoplastics such as PE, PP, polyamides and PET, with only minor enhancements achieved for filled amorphous

Figure 8.3 The effect of common fillers on the HDT of PP homopolymer

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Filled Thermoplastics polymers like polystyrene, acrylonitrile-butadiene-styrene (ABS), polycarbonate and polysulfone [1]. Addition of glass fibre to semi-crystalline polymers gives an HDT approaching the melting point of the matrix. In the case of amorphous polymers, incorporation of glass fibre gives an HDT (at 1.8 MPa stress), which is close to the Tg of the matrix. For this reason, filler is most commonly used in semi-crystalline polymers where the HDT is improved most because the crystalline regions help transfer stress to the filler under load [70].

8.3.4 Yield Strength Yield strength is a measure of the force that a material can withstand before it suffers macroscopic plastic deformation. For most materials, e.g., metal, it is taken as the point on the stress-strain curve when the line becomes non-linear (the elastic limit). However, for plastics, it is taken as the peak of the stress-strain curve, as that is simpler to measure. In practice, most parts are designed so that they never experience a force approaching the yield stress because yielding represents failure of the material. Yield strength is a key property when designing parts. Fillers are often added because they increase the yield strength of the polymer, this effect is known as reinforcement if the modulus is also improved [59]. The explanation for reinforcement lies in the fact that adding filler actually changes the polymer phase. It has been shown that polymers interact with the filler surface, forming an interphase of adsorbed polymer [71-74]. The thickness of the interphase can vary widely from system to system. That is to be expected; for example polar polymers such as polyamides are capable of strong, specific interactions with groups on the filler surface. In contrast, non-polar polymers such as PE and PP have weaker interactions with fillers. The apparent thickness of the interphase also depends strongly upon the measurement method. Lower values of around 0.004 μm are reported from extraction experiments, whereby all non-adsorbed polymer is solvent extracted [75, 76]. Values deduced from mechanical data such as by dynamic mechanical analysis or modulus tend to be much larger, in the range 0.012 to 1.4 μm [77]. This interphase has mechanical properties intermediate between those of the polymer and the filler [78, 79], thereby allowing an increased yield strength for the composite. Several factors determine the level of reinforcement attained by adding filler. These include, the volume fraction of filler added, the surface area of the filler (related to particle size), particle shape, the level of adhesion between the filler and polymer [80], as well as the thickness and nature of the interphase between the two phases. A linear correlation between yield strength and heat of crystallisation has also been reported in the case of PP filled with calcium carbonate [81]. It is well known that spherical fillers give least

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Figure 8.4 The effect of common fillers on the yield strength of PP homopolymer

reinforcement, platy fillers are better and fibrous fillers are best of all [5, 30]. Usually, it is considered that spherical fillers such as calcium carbonate and dolomite do not reinforce at all, and in fact they usually reduce the yield strength of the material (Figure 8.4). However, that is not necessarily true, as it has been shown that it is possible to increase the yield strength of PP by using very fine spherical filler with a mean diameter of 0.01 μm [78, 79]. This improvement must be due to the high surface area of the filler as the filler is isotropic. The high surface area increases overall polymer-filler adhesion and thereby improves yield strength. It is observed that spherical fillers do not reinforce whereas platy fillers like mica may do, and glass fibres are most effective (Figure 8.4). In this particular example, talc does not reinforce, probably because the talc grade used did not have sufficient anisotropy. As with yield strength, the data shown is for PP homopolymer, but similar trends are seen for a wide range of other thermoplastics [3, 22]. The filler creates an additional complication especially for injection moulded parts. Namely, during mould filling, the filler distribution becomes non-homogeneous due to the flow. One consequence is flow lines and weld lines. These are created when two fronts of molten polymer meet. For an unfilled polymer the melt can easily mix when two melt fronts meet and so the mechanical properties are normally unchanged (except for the special case of liquid crystalline polymers). The uneven distribution of filler at the weld lines creates a weak point, so for example, the measured yield strength and elongation to break are reduced. This effect is not as great for isotropic fillers but for more anisotropic fillers the yield strength may be reduced by more than fifty percent. It is

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Filled Thermoplastics therefore essential to design with this in mind. This can be done by designing parts to avoid weld lines and by judicious placement of injection points. Additionally, it should be remembered that the reported mechanical properties for composites are for ideal specimens with no weld lines, whereas the actual yield strength in the part may be far lower.

8.3.5 Impact Strength (Toughness) The impact strength of polymers and composites is another key property. In contrast to the other mechanical properties, it is not possible to predict the impact strength of a thermoplastic composite. The reason is that there are too many factors to be considered. One of the major complications is that adding hard filler can change the mode of failure from ductile to brittle [2, 7], or vice versa for rubbery fillers [7]. The filler may act as a flaw, if there are large particles or agglomerates [82, 83]. Alternatively, well dispersed, small particles can improve the impact strength by a crack-pinning mechanism. Another problem in predicting impact strength of composites is that mechanical properties of polymers are very dependent upon the rate at which the testing is performed. Most mechanical data are acquired at low speeds; for example, tensile testing is often performed at a strain rate of 0.1–10 mm·mm-1min-1. In contrast, impact testing is a very rapid event (>10 000 mm·mm-1min-1) and the polymer often responds very differently. For example, a polymer that fails in a ductile way during tensile testing may become brittle under impact test conditions because when the deformation is fast, the polymer chains have insufficient time to move and accommodate the deformation. It is well known that the response of polymers is dependent upon testing rate [60, 84-86]. The WLF (Williams, Landel and Ferry) equation [87] can be used to account for this effect at temperatures near to the Tg. The WLF equation predicts that the effective Tg of a polymer changes by about 6.9 °C for every decade of change in the rate of testing [84]. That means that a polymer which has a Tg below room temperature under tensile testing conditions, may have a Tg well above room temperature under impact test conditions. Therefore, the polymer may behave in a ductile manner, with high strength, under tensile testing but it may be brittle under impact, giving low impact strength. In cases where the mode of fracture does not change with testing speed, then it is expected that the energy to break determined by tensile testing (the area under the stress-strain curve) will correlate with the impact strength. Similar complications exist when it comes to measuring impact strength. Many methods are available, the most common of which involve either a pendulum striking the sample (Izod and Charpy), tensile impact testing, or a falling dart. In general, impact tests do not correlate well with each other, although it has been shown that there is a correlation between Izod and Charpy values [88]. Impact tests may be performed on unnotched (as moulded) samples or pre-notched samples, whereby a well-defined flaw is introduced to ensure that the sample fails at the desired point. Adding a notch improves the reproducibly noticeably,

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Particulate-Filled Polymer Composites but it may not be realistic as parts in actual use are not notched, at least not intentionally. Unnotched testing is best for detecting agglomerates and flaws because the crack initiates at such imperfections. Notched impact testing is comparatively insensitive to agglomerates and large particles because the sample fails preferentially at the large, introduced flaw. Reverse notched testing such as Izod E is especially suitable for composites. This sample is struck on the opposite side to the notch. In this way the stress is concentrated opposite the notch but the introduced notch is not the site where crack growth occurs. The notch encourages reproducibility by localising the stress on the opposite side of the sample, but the crack initiation occurs at some imperfection in the composite and so the method is still sensitive to agglomerates. In the final analysis though, the lab scale impact tests are of limited value. They may be used to compare materials qualitatively, but the ultimate test is to make the part or prototype and perform impact tests on that, in a manner that closely simulates the way in which the part is to be handled during manufacture, or used. There are many reports describing the effect of fillers on the impact strength of polymers. These are however, hard to compare as they often use different polymers, test methods and filler particle size distributions. It is therefore difficult to make any general comments. One study showed that increased filler anisotropy (aspect ratio) resulted in reduced falling weight impact strength [89]. That is particularly interesting because increased filler anisotropy is known to improve modulus. This is therefore an excellent illustration that one must be prepared to sacrifice some property for the sake of improving another, more important one. There has been some success in creating high modulus composites with good impact strength. This was done using ternary composites where a dispersed, stiff filler is encapsulated in a rubbery layer dispersed in the matrix polymer [90]. It can be stated that fillers can either decrease or increase the impact strength compared to the unfilled polymer. A common technique for increasing impact strength is to disperse a soft, or rubbery, filler in a harder polymer to increase its impact resistance. This method is used to make HIPS and to improve the impact resistance of PP, especially at low temperatures [91]. Hard fillers on the other hand may either increase or decrease impact strength [92]. For example, fine calcium carbonate or talc can increase the impact strength of PP homopolymer by more than a factor of two [93, 94]. This only holds if there is good dispersion to avoid agglomerates, if there are no large particles, above about 10-20 μm in diameter, the calcium carbonate is stearate coated and the particle size distribution has been optimised [94]. There seems to be a definite interaction or synergy between particle size and coating level, and the highest impact strengths are found with calcium carbonates with 1% stearate coating and a mean particle size 1-2 μm, depending on the type of polymer and the nature of the impact test (se Figure 8.5).

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(a)

(b) Figure 8.5 (a) Effect of stearate coating level on a ultrafine calcium carbonate (d50: 0.8 μm) on the properties of polypropylene (b) Effect of particle size of stearate coated calcium carbonate on the properties of polypropylene

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Particulate-Filled Polymer Composites In the case of PP copolymer and impact modified PP, the effect of filler is very different; impact strength is lowered significantly by hard particulate fillers. This is because the filler interacts with the soft or rubbery component and nullifies its ability to adsorb energy during impact. In conclusion, it seems that soft rubbery fillers improve impact strength by helping to dissipate the energy of impact. Hard fillers decrease the impact strength of ductile materials, which already have high impact strength. On the other hand, well dispersed, hard particles of the correct particle size may improve the impact strength of brittle materials like PP homopolymer or polystyrene [22] by promoting crazing.

8.4 Effects of Filler on the Polymer Phase 8.4.1 Introduction To understand and predict the properties of composites, it is necessary to realise that adding filler may affect the polymer phase, both chemically and physically. Chemical changes may occur if the filler, or impurities on the filler surface, catalyse degradation of the polymer. Alternatively, various physical changes may result from the incorporation of filler. Some fillers nucleate crystal growth in certain polymers, which in turn influences manufacturing and mechanical properties. In the literature, the possibility that the filler may have altered the polymer phase is rarely considered. It is important to realise that the polymer may be altered by the filler, especially in the case of semi-crystalline polymers [95] like PE, PP, PVC, PET and the polyamides. Therefore, these effects are mentioned here to allow a better understanding of the factors affecting the performance of a composite.

8.4.2 Nucleation It is widely recognised that fillers may affect the crystallisation of polymers [96]. The filler may increase the rate of cooling, as mentioned previously, and may thereby affect crystallisation. Some polymers have more that one type of crystal phase, which occur preferentially when cooling within a certain temperature range [97, 98]. These crystal types have different mechanical properties [99, 100] and therefore, the relative amount of each phase will influence to the properties of the thermoplastic composite. In other instances, the filler may nucleate crystal growth. This is often beneficial, as it causes the material to harden more rapidly on cooling, giving the possibility of faster production. In fact, it is common practice to add a small amount of fine talc to nucleate

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Figure 8.6 The effect of some common fillers on the impact strength of PP homopolymer

crystallisation of PP, especially in thin-walled parts. Interestingly, dolomite is also rather effective, and calcium carbonate and mica have a slight effect on crystallisation onset temperature (Figure 8.6). In some cases surface treatment of the filler affected nucleation and in other cases no effect was seen. As well as the crystallisation onset temperature, the peak of the crystallisation endotherm may also be used as a measure of nucleating effectiveness. The two methods show the same qualitative trends. Another important parameter is the proportion of crystallinity in the composite. The crystalline phase has a higher modulus than the amorphous phase, and it has been reported that the yield strength is linearly proportional to the heat of crystallisation [81]. Clearly, the mechanical properties of the composite are influenced by the degree of crystallinity. For a given PP copolymer grade, the degree of crystallinity was 43 weight%, whereas this was 45-48 weight% when 60 weight% of magnesium hydroxide, dolomite or talc was added. When measuring the degree of crystallinity one must correct for the amount of filler added. For example adding 50 volume % filler will reduce the apparent crystallinity [as measured by differential scanning calorimetry (DSC)] by 50%. The explanation is quite simply that 50% of the polymer has been removed. It is quite common that this dilution effect is not accounted for, or it is not stated whether it has been corrected for. This leads to some confusion in the literature.

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Particulate-Filled Polymer Composites Unfortunately, it is not possible to predict whether a particular filler type will nucleate crystal growth. For some time it was thought that high energy surfaces nucleated, but this has since been proven incorrect [95]. Instead, Hobbs has shown that surface microtopology influences nucleating ability [101]. In that work, Hobbs made a replica of a nucleating surface using a non-nucleating material, to produce a surface that was strongly nucleating. It can be concluded that any thorough study of filled polymers must also consider the possibility of nucleation and changes in the degree of crystallinity.

8.4.3 Transcrystallinity Transcrystallinity may occur when a polymer is cooled in contact with a highly nucleating filler or surface [95, 102]. Usually, polymer crystals are spherulitic, growing out radially from the nucleation site [2, 85, 86]. In contrast, when crystals are nucleated very close together, they impinge on each other almost immediately, and are forced to grow in one direction, away from the nucleating surface [95]. For a polymer where only one crystal form occurs, it has been shown that the microstructure of the transcrystalline phase is the same as that for the spherulites [103]. The transcrystalline layer is typically 10-30 μm thick, which means that for higher filler loadings, the whole matrix may be composed of transcrystalline material [95]. Transcrystallinity has been reported in several systems including glass [104], PET [105, 106] and some types of carbon fibre in PP [107], as well as Kevlar fibres in Nylon 6,6 and even in Nylon-PP blends. Sheets of transcrystalline PP have been prepared by cooling PP from the melt in between PET (Melinex®) sheets and the properties studied [108]. This rather elegant work by Fowkes and Hardwick showed that the transcrystalline sheet had a much higher Young’s modulus (1.09 GPa) compared to the control PP sample that was quenched to give a fine spherulitic sheet (0.67 GPa). The tensile yield strength of the transcrystalline material was also higher at 25.0 MPa, compared to 18.6 MPa for the spherulitic sheet. Some of the properties were much worse compared to normal, spherulitic PP. For example, the transcrystalline sheet showed just 4% elongation to break and an energy to failure of just 28.0 kJm-2, compared to >300% and 48.5 kJm-2, respectively, for the fine spherulitic analogue. Clearly then, the filler can greatly influence the type and level of crystallinity, leading to profound changes in the properties of the resultant composite material.

8.4.4 Interphase Aside from changes in crystallinity, there is another way in which the presence of filler may alter the host polymer. It has been shown that polymer adsorbs onto the filler and that this

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Filled Thermoplastics adsorbed material has different properties compared to those of the bulk matrix [68, 71, 109, 110]. Pukansky and Fekete have written a review of the importance of the interphase in thermoplastics [9]. The relevance of the interphase to adhesion at planar interfaces, and in composites, has been discussed by Berg [109]. As expected, the thickness of the interphase varies depending upon the extent of interaction between the polymer and filler. It has been shown that the thickness of the interphase is proportional to the reversible work of adhesion [8]. Reported thicknesses are usually in the range 0.004–0.15 μm, depending upon the polymer filler combination and the method used to estimate the thickness [9, 22]. It is anticipated that the interphase thickness should be influenced by the ubiquitous van der Waals forces plus any specific chemical interactions such as Lewis acid-base or hydrogen bonding or covalent bonding [22]. Therefore it should be affected by surface treatment of the filler. The degree to which the interphase affects the properties of the composite should also therefore depend on the total surface area of the filler [22] and is therefore especially important for nano-composites.

8.5 Surface Science Aspects 8.5.1 Introduction The importance of surface interactions in composites is widely recognised, and yet, it is probably the one that leads to most confusion. This is partly because it is difficult to characterise the interphase, but also because the composites field is interdisciplinary, requiring not only surface science and adhesion science but also an understanding of polymer science. This section will briefly mention some of the relevant points, but surface and colloid science is a wide area and specialist texts should be consulted if more detail is required [111-114]. Pugh has reviewed the wetting and dispersion of ceramic powders in liquids, which is highly informative and relevant to the case of mineral particles dispersed in a polymer [115]. The interfacial interactions in particulate filled composites have been reviewed by Pukánsky and Fekete [9]. Most recently, Berg has written an excellent review on the subject of wetting and adhesion [109].

8.5.2 Surface Energy and Surface Tension Surface energy must be introduced in order to be able to understand the forces that drive wetting and adhesion. If one imagines an atom or molecule located in the bulk of a material, the forces acting upon that entity are symmetrical. Each unit is attracted to its neighbours equally. If one then cleaves the material, then the forces acting on the units (atoms or molecules) at the newly formed surface are no longer symmetrical. Those surface units are still attracted towards their neighbours, but they lack attractive

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Particulate-Filled Polymer Composites interaction in the direction of the new interface. Because these units at the interface are in a new environment, energetically speaking, there must be a change in the total free energy. This change in energy relative to the bulk is termed the surface free energy, with units of mJm-2. For liquids, the net force towards the bulk manifests itself as a tension at the interface. This gives rise to the concept of surface tension in liquids, with units of mNm-1. There are various old and new units of surface energy and surface tension, but fortuitously, it turns out that they are numerically equivalent, i.e., 1 mNm-1 = 1 mJm-2 = 1 dyne cm-1 = 1 erg cm-2. So, in summary, the surface energy is the energy required per unit area to form a new surface of that material in a vacuum. The symbol for surface tension and surface energy is γ, where γ S, γ L and γ SL refer to the surface energies of the solid, the liquid and the interfacial surface energy between the solid and the liquid, respectively. •

•

The surface tension of liquids is easily measured by a wide variety of methods, whereas it is much more difficult to measure the surface energy of solids. A comprehensive overview covering methods applicable for liquids and solids is presented by Adamson [111].

8.5.3 Wetting and Spreading When the filler is mixed with the polymer melt, it is important that the polymer wets the filler. It is desirable for the polymer to penetrate in between the particles, displacing the air and giving intimate contact between the polymer and filler surface. This will prevent trapped air bubbles and aid adhesion between the filler and polymer. Wetting, and more specifically, immersional wetting, is the term used to describe this process [112, 115]. The driving force for wetting to occur is related to the surface energies of the wetting fluid and the solid [8, 109, 111-115]. The surface energy of solids is usually determined using static contact angle measurements. This is a useful method, although the results are affected by surface roughness, drop size and contaminants [116]. Furthermore, contact angle measurements are only valid when the liquid does not penetrate into the solid under analysis [116], which is of particular importance for polymers, as they are often swollen by liquids. Theoretically, under equilibrium conditions, a low surface energy liquid, such as a polymer melt, should completely wet the high energy mineral filler surface [112, 115]. However, this view is overly simplistic, and does not apply in reality for several reasons. One reason is that high energy surfaces attract contaminants and are usually covered in a layer of hydrocarbon material spontaneously adsorbed from the atmosphere [112]. This means that the ‘high energy’ surface no longer behaves as such. Another, more fundamental problem is that polymer processing is a dynamic process and so kinetic aspects of wetting are more important than thermodynamics. Dynamic wetting has been studied as it is of great industrial importance. It can be shown that the low surface energy and the high

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Filled Thermoplastics viscosity of the polymer melt both disfavour wetting of the filler [111, 112]. Dynamic wetting in composites is an area that deserves further study in order to understand and optimise wetting under real processing conditions. Some studies have been performed, in particular for fibre reinforced composites [117]. The spreading coefficient represents the thermodynamic driving force for the liquid to spread onto and wet the solid (Equation 8.5). For spontaneous spreading to occur, the spreading coefficient must be positive, implying a contact angle of zero. S L / S = γ S − (γ L + γ SL )

Equation 8.5 The spreading coefficient

8.5.4 Adhesion The degree of adhesion between the filler and polymer is expected to influence the mechanical properties of the composite. This can be predicted theoretically, although it is much more difficult to prove experimentally. It is a major challenge to change the adhesion between the polymer and the filler without invalidating the experiment by unintentionally altering other parameters [109] such as filler dispersion level or polymer crystallinity. Even so, many workers have tried to correlate the calculated reversible work of adhesion with the mechanical properties of composites. Studies on planar surfaces have shown that measured adhesive bond strengths are, at best, only one tenth of the calculated value based on van der Waals interactions alone. It might therefore be assumed that the reversible work of adhesion would have rather limited utility as an indicator of adhesion at the interface. Despite that, it has been shown numerous times that the reversible work of adhesion often does correlate rather well with the mechanical properties of composites, in particular yield strength [8, 109]. Berg addressed the question of whether it was worthwhile to use concepts such as reversible work of adhesion to predict and tune adhesion [109]. The conclusion was that the approach is reasonably effective already and will improve in the future. The first criterion for good adhesion is intimate contact, that is good wetting of the surface. Wetting is a necessary, but not sufficient, condition for good adhesion [109]. In addition, one should seek to maximise the work of adhesion. The simplest case is to consider the work of adhesion to be attributable solely to non polar (London) forces between the materials. This is indeed the case when at least the adhesive (polymer melt) or adherand (filler surface) is non-polar. So, for thermoplastics such as PE, PP and polytetrafluoroethylene (PTFE) this simple case should apply.

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Particulate-Filled Polymer Composites The work of adhesion is given by Equation 8.6. This implies that the work of adhesion can be maximised for a given adherand by increasing the surface energy of the adhesive. However, we also know that wetting is required and so the surface tension of the adhesive must not exceed that of the adherand. So, the ideal case is when the surface tension of the polymer melt is as high as possible, but without exceeding that of the filler surface, so that spontaneous spreading still occurs. This has been verified experimentally for a range of polymeric adhesives on a set of surfaces with controlled surface energy [109]. WA = 2 γ Sγ L

Equation 8.6 Work of adhesion (WA) for the non-polar case Later, it was realised that polar interactions could also play a role in wetting and adhesion. The surface energy was divided into non-polar and polar components and new equations were then proposed to allow for the possibility of polar interactions across the interface (Equation 8.7). From that equation it can be expected that maximal adhesion will occur when the ratio of polar to non-polar surface energy is the same for the adhesive and the adherand. WA = WAd + WAp = 2 γ dS γ Ld + 2 γ Spγ Lp

Equation 8.7 Work of adhesion for the polar case Wu proposed another equation using an harmonic mean approach to estimate the nonpolar and polar interactions across the interface [8, 109, 116]. This approach gives the same criterion for maximising adhesion. Folkes went a step further by arguing the importance of Lewis acid and base interactions. This leads to a new elaboration of the equation for calculating the work of adhesion (Equation 8.8). WA = WAd + WAp + WAAB

Equation 8.8 Work of adhesion in Lewis acid-base terms The premise is that each component can only interact with its corresponding component in the other material. Intuitively, it seems reasonable that this should be the case, although again, it is not clear whether the geometric or harmonic mean, or some other method, is best for estimating the magnitude of the Lewis acid-base interactions across the interface. Fowkes’ proposal has been widely accepted and verified experimentally. For example

386

Filled Thermoplastics Sinicki and Berg varied the Lewis acid-base interactions systematically and found a correlation between adhesion measured by peel testing and the calculated thermodynamic work of adhesion [118]. Schreiber and Germain used plasma treatment to alter the Lewis acid-base balance of three fillers [62]. These were then compounded into two different matrix polymers. Firstly, polyethylene (LLDPE), which is completely apolar and cannot form Lewis acid-base interactions. The other polymer was poly(ethylene-vinyl acetate) [EVA], which is based on polyethylene, but contains some (28 mole%) acetate groups, which they found to be Lewis acidic (although acetate groups are also reported to be Lewis basic [109]). They then measured tensile modulus and elongation at break to assess the affect of adhesion on mechanical performance of the composites. In LLDPE, a fluoropolymer (PTFE-like) coating on the filler gave lowest modulus and highest elongation to break, attributed to poor adhesion. An ammonia plasma was used to give a polar, Lewis basic surface. For this combination, the modulus was lower and the elongation to break higher than for the untreated filler. This was consistent with reduced adhesion compared to the untreated filler, but better than for the fluoropolymer coated filler. The modulus and adhesion were maximised when the filler was plasma treated with methane to make the surface non-polar like the polymer. When the matrix polymer was changed to EVA, the results were quite different. In that case, the highest modulus and least elongation to break were recorded for the ammonia plasma treated filler. This was interpreted as being due to enhanced adhesion from the bonding between the Lewis acidic groups in the polymer and the Lewis basic sites introduced on the filler surface through ammonia treatment.

8.5.5 Dispersion and Agglomeration Good dispersion of fillers and pigments is a prerequisite for good mechanical and aesthetic properties. For example, multiple studies have shown that filler agglomerates act as stress concentrators, reducing tensile and impact strength [82]. Similarly, good dispersion results in a smoother surface, with a concomitant increase in specular gloss [119]. Shenoy [24] and Hornsby [25] have each given detailed descriptions of the factors influencing dispersion of particulates in polymers. In the case of pigments, good dispersion results in a higher tinting strength so that less pigment is needed to attain a given level of pigmentation. The principal exceptions are thermal and electrical conductivity, which are improved by particle agglomeration to form a percolated (continuous) network [22]. The ubiquitous van der Waals forces ensure that particles attract each other, thus favouring agglomeration. The magnitude of this attractive force can be calculated if the Hamaker

387

Particulate-Filled Polymer Composites constants for the disperse phase (filler particles in this instance) and the dispersion medium, i.e., the polymer melt are known. The Hamaker constants for several inorganic materials, including fillers such as calcium carbonate, muscovite mica, silica and titania have been calculated by Bergström and co-workers using Lifshitz theory [120, 121]. In a subsequent study, it was possible to directly measure the van der Waals forces between materials using atomic force microscopy (AFM). The results were found to be in good agreement with the magnitude of the attractive force predicted from the calculated Hamaker constants [122]. Once the particles have agglomerated, the surfaces come into intimate contact and other, short-range forces can become important. Thus, the force required to separate the particles will depend on the van der Waals force plus any specific surface interactions that have occurred such as water bridging, hydrogen bonding or Lewis acid-base interactions. The polymer melt competes to interact with the surface so that the overall energetics depend on whether it is more favourable for the filler surface to interact with another particle, or with the polymer phase. In order to separate the particles, mechanical energy must be added [25]. Then, once separated, the particles must be kept from reagglomerating. This can be achieved by reducing the attractive forces between the particles (by adding a dispersant), or by increasing the affinity of the polymer phase for the filler surface (by adding some polymer modified to interact with the filler). These two approaches are well understood and are described in texts on surface and colloid science [111-115]. A brief description of dispersants and coupling agents is given below, but is dealt with more thoroughly in Chapter 4. A theoretical model has been developed to help in optimising dispersion in twin-screw extruders [123].

8.5.6 Surface Treatments – Dispersants and Coupling Agents Surface treatments for fillers have been extensively reviewed [124]. Ernstsson and Larsson studied a range of mineral fillers in terms of the Lewis acid-base character [125, 126]. It was found that each one had a different surface chemistry and they were all amphoteric (possessing both Lewis acid and basic sites). Another point was that trace impurities of iron on the filler surface, presumably picked up during processing, significantly changed the surface chemistry of silica. In unpublished work, DeArmitt and Breese used a novel rheological method to characterise a wide variety of mineral fillers. The method was especially attractive in that it allowed investigation of the surface chemistry of the fillers, while at the same time showing which types of organic probe molecules were most effective as dispersants. This work confirmed the findings of Ernstsson and Larsson [125, 126]. Each mineral displayed a unique affinity for the probe molecules and each was appreciably amphoteric, adsorbing Lewis acid and

388

Filled Thermoplastics Lewis basic probes. This shows that the dispersant should be chosen to optimise its affinity for the filler or pigment. It is important to ensure that the chosen surface treatment agent does in fact bind to the filler. If the additive does not bond to the surface then it cannot fulfil its function. Excess additive is, at best, a waste of money, but in some cases it may have worse consequences such as destabilising the polymer. For example, it has been shown that calcium stearate, a common dispersant, can destabilise polyolefins and cause yellowing, by interacting with the antioxidant [127].

8.5.6.1 Dispersants As mentioned, dispersant design and mode of action are well understood by surface and colloid scientists [43, 112-115]. In that field, the term dispersant refers to any additive that reduces the interparticle interactions, thereby encouraging dispersion of the particles. This is achievable via a number of mechanisms using low molecular weight, oligomeric, or polymeric additives [128]. Steric stabilisation is most relevant to mineral fillers in polymers because it is the main way to achieve colloidal stability in low polarity solvents. This stabilisation mechanism operates by strong adsorption of a layer of organic additive that physically prevents close interparticle approach. Stearic acid and metal stearates are widely used as dispersants, especially in cases where high filler loadings are required. Examples are polyolefins filled with aluminium hydroxide or magnesium hydroxide where 60 weight percent of filler or more may be needed to achieve sufficient flame retardancy [129, 130]. Of course the correct level of addition depends upon the amount of filler surface to be covered, and therefore upon the amount of filler, and its specific surface area. Excess additive is to be avoided as it can seriously destabilise some polymers and give yellowing problems [127].

8.5.6.2 Coupling Agents Dispersants need only adhere to the filler, and help reduce particle – particle interactions. Whereas for coupling agents, as implied by the name, the additive must be bi-functional, adhering both to the filler and to the polymer. It is therefore found that the best choice of coupling agent varies depending on the filler and polymer. In some instances, the socalled ‘coupling agent’ may not couple to either phase, or may only adhere to the filler. In the latter case it may act solely as a dispersant, rather than as a coupling agent. It is therefore important to note that an additive marketed as a coupling agent will only provide coupling for certain combinations of filler and polymer.

389

Particulate-Filled Polymer Composites

Filled Thermoplastics

It is often assumed that coupling (adhesion) is good, but that is not always the case. For example, good coupling is undesirable during extrusion, because it would dramatically increase the torque (and energy) needed for extrusion. This would reduce extruder throughput and that would result in a significantly more expensive material. Coupling is advantageous when high yield strength is required, but it is often detrimental to the elongation at break. Impact strength is more difficult to predict, it may increase or decrease depending on the filler – polymer combination. The effect of coupling agents on modulus is less clear. The equation for describing modulus has no term for adhesion between filler and polymer, so coupling agents should not affect modulus (Equation 8.4). However, there are reports that they do affect modulus. This may be due to increased orientation of the filler or due to the way that the modulus was determined. The modulus should be determined at low stress, in the linear part of the stress-strain curve, where filler debonding has not yet occurred, and should therefore be insensitive to adhesion.

8.6 Aesthetics 8.6.1 Introduction Often thermoplastic composites are used in applications where the consumer will see the part [131]. In those instances, it is vital to consider the aesthetic aspects of the material as well as the mechanical, electrical and other properties. Fillers affect the surface finish, colour and scratch-resistance of the composite and these factors should be optimised to give a marketable product.

8.6.2 Colour/Pigmentation Pigments may be either organic or inorganic particles that are added to give colour to a plastic, as opposed to dyes, which dissolve in the plastic. Pigments can be considered as a special class of filler, and they influence the polymer in much the same way as any other filler would. So, for example, they can lower the thermal stability of the polymer matrix [18], adsorb antioxidant and nucleate crystallisation of the polymer. They also affect the mechanical properties in the same way as any other filler, but usually pigments are used at concentrations that are too low to significantly affect modulus, yield strength or HDT. It is essential to disperse pigments thoroughly in order to achieve the maximum tinting strength, and to avoid agglomerates, which would lower the impact strength, as discussed previously. As dry powders, several common fillers appear white, because they scatter light strongly, and it might therefore be supposed that they would make good pigments for polymers. This is usually not the case however, because the amount of scattering is determined by the difference in refractive index between the particulate and continuous phases. This

390

Filled Thermoplastics refractive index difference is small for common fillers and thermoplastics and so scattering is limited. In fact, in some cases the filler is invisible in the polymer because the refractive indices match very closely. An example is glass beads in PVC. Calcium carbonate is available in high whiteness and gives a mild pigmentary effect, but if high whiteness is required, then a pigment of higher refractive index, such as titanium dioxide, must be used. Titanium dioxide and carbon black are used to protect against UV radiation as they scatter and adsorb it, respectively. It should be noted that commercial titanium dioxide is always has an inorganic coating, e.g., aluminosilicate, plus an organic additive such as a polyol or silicone [132]. This is necessary because naked titania can oxidatively degrade polymers when exposed to UV light.

8.6.3 Surface Finish and Gloss Surface finish is important to the end-user. It can be altered to be glossy or matte, as fashion dictates, and similarly, textures can be used to convey the right feel when handling the product. The surface may also be tuned for more functional reasons. For example, smoother surfaces are generally more hygienic and easier to clean, whereas a rough surface can prevent blocking (self adhesion) of films or increase the coefficient of friction. It is well known that fillers can affect the surface finish of the host polymer [133]. Coarse particles tend to give a rough surface finish, whereas fine particles decrease roughness with a concomitant increase in gloss [119]. Surface treated filler can give better gloss by aiding dispersion. For a given filler type the gloss is steadily reduced as more filler is added. However factors such as the mould surface and processing parameters can have a very large effect on the final gloss. For low shear situations like gas-assisted injection moulding even 10 weight% of filler may decrease the gloss to unacceptable levels. For a normal injection moulded part the gloss may still be high at filler levels as high as 40 weight%. At low filler loadings it is possible to retain much of the gloss of the unfilled polymer, but above a certain filler level (depending on filler type, size and treatment), there is a sharp reduction in gloss. Special surface treatment of the filler has been shown to decrease scratch visibility.

8.6.4 Scratch and Abrasion Resistance Scratch and abrasion resistance is very important, especially in the home appliance and automotive industries [134]. The manufacturer may be forced to use a more expensive polymer or an unfilled polymer in order to maximise scratch resistance. There are two different aspects of scratch resistance. One is the actual size of the physical scratch, but the more important aspect is usually the visibility of the scratch, because that is what the user sees. Often filler increases the visibility of scratches. A

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Particulate-Filled Polymer Composites commercially important example is talc-filled polypropylene where the talc particles become exposed by scratching, giving an undesirable white mark. Other fillers suffer the same problem and a great deal of work has been devoted to lessening the scratch depth and visibility of thermoplastic composites. Attempts to correlate the mechanical properties of polymers to their scratch-resistance have been largely unsuccessful. This is partly because the response of polymers to deformation is very dependent on the speed of testing. The mechanical properties of polymers and composites are usually determined at much lower rates than those encountered during scratching. Evans and Fogel used the WLF equation to correct tensile testing results and were then able to correlate the energy to break to the scratch resistance [135]. Another complication is that there is a wide variety of scratch-testing methods. These vary from scratching the surface using pencils of varying hardness (B, HB, H, 2H, etc.), through scratching with hard styli whilst incrementally increasing the load, (e.g., Erichsen pen), all the way to fully instrumented methods that examine the scratch profile and its dependence on load [136, 137]. Many polymeric materials display some recovery over time and this too must be allowed for if the true scratch performance is to be established [138]. Krupicka has reviewed methods and factors affecting scratch performance of organic coatings [138]. One can say that scratch resistance may be improved by one, or some combination of three methods. Lowering the coefficient of friction may make it more difficult to scratch the surface. This can be achieved for example by adding a lubricant that migrates to the surface [136, 137]. Another method is to increase the hardness of the surface, although this is only effective if the scratching medium is not too hard. For example, this method might protect against scratching by another polymer, but not against sand, grit or metal objects. By far the best method is to make the surface elastic using a soft, rubbery coating for example. One prime example is in the plastic flooring industry, where the flooring is usually coated in a polyurethane layer optimised to give scratch-resistance, the desired gloss level and the right level of friction.

8.7 Stabilisation and Recycleability 8.7.1 Introduction Contrary to popular public perception, polymers are not very stable and are readily attacked by atmospheric oxygen, heat and UV light [132, 139, 140]. Most of the high volume polymers require additives that stabilise them during processing at high temperatures, during their service life, and during any subsequent recycling operations. In fact, polypropylene is so unstable that all commercial grades must be stabilised [18] and the same is true of PVC. Once properly stabilised, these polymers can have a useful service life of tens or even hundreds of years.

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Filled Thermoplastics Fillers often affect the stability of polymers via a variety of mechanisms. Although this is recognised, at least to some extent, it has not been studied as thoroughly as the stability of unfilled polymers. As the stability and recycleability can be critical issues, hopefully this subject will receive more attention in the future. Thermoplastics are usually processed in the molten state, at temperatures in the range 150-350 °C depending on the melting point and viscosity of the polymer. There are many standard stabilisation packages on the market, often containing a process stabiliser and a long-term stabiliser. Most of the stabilisers are synthetic, although recently, a natural hindered phenol, α-tocopherol (vitamin E) was found to be effective in polyolefins [141143] and has been commercialised. For further information, the reader can consult books explicitly dedicated to stabilisation of polymers [132, 144]. During service, most polymers experience mean temperatures in the range 20-40 °C, whereas peak temperatures may be much higher for short periods. The mechanical properties of polymers depend upon molecular weight, and it takes relatively little degradation to seriously impair mechanical performance. The degradation may result in crosslinking or chain scission depending on the chemistry of the polymer and the conditions the polymer is exposed to. By far the most important stabilisers are the hindered phenols, which are used in a wide range of polymers including the polyolefins, (e.g., PE and PP), polyamides, polycarbonate and PET. These stabilisers are effective both during processing at high temperature and for long-term use under ambient conditions. For increased effectiveness, they are usually combined with other stabilisers to attain an optimised combination of stabilisation and other properties such as discoloration. Often, the antioxidant is physically lost, primarily by extraction or volatilisation, rather than by chemical consumption [145]. The trend is therefore to use higher molecular mass antioxidants [132, 139, 146]. Fillers may affect the stability of polymers via a number of mechanisms. The two most important ones are discussed here. Those are the catalysis of degradation by the filler’s surface and the indirect lowering of stability that occurs when the filler surface adsorbs, and thereby deactivates, the antioxidants.

8.7.2 The Effect of Filler Chemistry and Impurities on Stability It is well documented that transition metals such as chromium, copper, iron and vanadium can catalyse the degradation of polymers [132, 139, 144]. These metals promote the decomposition of hydroperoxides, which are important in the degradation mechanism of most polymers.

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Particulate-Filled Polymer Composites Most common fillers are not based on these elements, but they may be present as impurities in the filler, or they may be picked up by the filler during processing operations such as milling. Talc and mica commonly contain traces of iron. In fact, talc grades with low iron content command a price premium over less pure grades because it is assumed that iron content correlates to polymer stability. In fact, that assumption is completely erroneous. For example, iron oxide (Fe2O3) is used as a pigment, but it does not cause polymer degradation because it is not the total iron concentration that is important, but more the chemical form of the iron, or other metal [5]. It would be more accurate to measure the actual destabilisation caused by each talc grade and then adjust the price of the talc accordingly. However, this has not been done, probably because it would be rather labour intensive and therefore expensive. There is a fast, inexpensive alternative and that to measure the effect of filler on the stability (oxidation induction time) of a low molecular weight model liquid. For example, instead of compounding the filler into PE or PP, one can mix in a small amount of filler into squalane and measure the effect on stability [147]. It has been shown that squalane degrades by the same mechanism as PP [148]. This approach requires only very small samples and the procedure can be automated so it is very rapid to perform. Other model liquids may be used to simulate other polymers.

8.7.3 The Effect of Antioxidant Adsorption on Stability Antioxidants often contain functional groups that are capable of interaction with the filler surface. This can result in antioxidant adsorption depending upon the surface chemistry of the filler and the type of antioxidant. Once adsorbed, the antioxidant becomes ineffective because it is unable to diffuse to, and react with, the radicals that cause polymer degradation. The amount of deactivated antioxidant can be significant, and the usual response in industry is to add more antioxidant to attain the required level of stability. However, that approach raises the cost of the compound significantly. Another commercial approach is to use an epoxy additive that preferentially adsorbs onto the filler surface, physically blocking antioxidant adsorption. That helps to reduce cost, but the epoxy additive is itself still a relatively expensive chemical. DeArmitt, Breese and Lamèthe [149] studied the propensity of calcium carbonate to adsorb Irganox 1010 using squalane as a model liquid to simulate PP (Figure 8.7). Oxidation induction time (OIT) is a well-accepted method for measuring antioxidant concentration [145]. First a calibration curve of Irganox 1010 in squalane was made. Then increasing amounts of Irganox 1010 were added to a 20 weight percent dispersion of calcium carbonate in squalane. This was mixed and left for some time to allow the antioxidant to adsorb. The dispersion was then centrifuged to give a clear supernatant solution, which was analysed by OIT to determine the residual

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Filled Thermoplastics

Figure 8.7 Effect of fillers on the nucleation of PP

Figure 8.8 The effect of calcium carbonate on the stability of squalane

antioxidant concentration in the squalane. The results showed that the Irganox 1010 was completely ineffective until enough had been added to saturate the filler surface. The 20 weight percent dispersion of calcium carbonate (specific surface area 5 m2g-1)

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Particulate-Filled Polymer Composites adsorbed and deactivated 300 ppm of Irganox 1010. The same results were found when the calcium carbonate was not removed by centrifugation. The OIT measurements were performed at 190 °C, so the antioxidant must have been strongly bound to the filler, otherwise it would have desorbed and raised the OIT of the squalane. Interestingly, calcium carbonate surface treated with stearic acid did not adsorb any antioxidant. Stearic acid treated calcium carbonate is more expensive than untreated grades, but the cost differential is largely compensated for by the reduction in antioxidant required. If this were more widely recognised, it might promote the use of surface treated calcium carbonate.

8.7.4 Recycleability Thermoplastics may be recycled in a variety of ways such as mechanical recycling (collection, sorting, and reprocessing), burning to give energy, or biological recycling [150, 151]. There is a public perception that synthetic polymers are less friendly to the environment than natural polymers such as cellulose, poly(lactic acid) and poly(hydroxyalkanoates). That view is not supported by the facts. Life cycle analysis reveals a very different picture, favouring the synthetic polymers, especially polyolefins [150]. Although thermoplastics and thermoplastic composites are potentially easy and economical to recycle, in practice there are some impediments to the implementation of widespread recycling. The main one is that the used materials must be collected, separated and cleaned economically. This is feasible in some instances but often it is not. In general, polymers are immiscible with one another, and, if melt processed as a mixture, the result is phase separation to give domains of one polymer in the other. This morphology leads to rather poor mechanical properties. Therefore, there are efforts to find better separation techniques in order to avoid the problem or to use compatibilisers [152] that lower the interfacial tension, improve the adhesion of the two phases, and encourage smaller domains of the disperse phase.

8.8 Uses of Filled Thermoplastics 8.8.1 Uses of Fillers As shown in Table 8.1 more than 80% of the filler used in thermoplastic is based on calcium carbonate minerals. Most is used in PVC, with major sectors being cables, flooring, hose, plastisols, pipe, profiles and fittings. The main reason for this is firstly due to the fact that PVC has to be compounded in order for it to be used. The incorporation of stabilisers is an essential prerequisite for its successful use and therefore fillers can be

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Fillers European consumption Filled Thermoplastics

Table 8.1 Estimated European consumption (in kilotonnes) of fillers in plastics, 1998 Calcium carbonate

PVC

PP

PE

Thermoset

ETP

Total

850

75

50

100

1

1076

10

1

131

6

15

Talc Calcined kaolin

120 7

Mica Wollastonite Kaolin

5

Total

862

small

2

small

Small

4

4 1

199

52

111

8 6

12

1236

ETP: engineering thermoplastics

included without increasing processing costs substantially. Secondly, PVC is compatible with both organic and inorganic substances so that changes in properties due to the filler can be more readily controlled than the other thermoplastics. Not withstanding the previous statement, the use of fillers in polypropylene, polyethylene and polyamide is of considerable commercial importance and seeing greatest growth and most research and development. The main fillers used are talc, ground calcium carbonate and calcined kaolin. The bulk of the filler comprises low-cost products for which price is the main specification requirement but in all applications, certain criteria are important. The filler should not greatly worsen the colour either directly by increasing light absorption (K) or indirectly by reducing heat stability or by degrading other additives in the plastic. However, in several applications the functionalities – shape, size, size distribution and coating, are very important. In unplasticised PVC (uPVC), extrusions and mouldings, PP mouldings, PP film and PE film, the particle size, size distribution and surface coating play an important role in determining processing, mechanical and aesthetic properties. Talc is used principally in PP, and to a lesser extent polyamide and PE, to give rigidity, a consequence of its (usually) high aspect ratio. In PP it is also used as a nucleating agent. Calcined kaolin is used in film as an anti-blocking agent, in thermal barrier agricultural film, and to allow for carbon dioxide laser printing. Aminosilane treated calcined clay is used principally in polyamide to give rigidity, toughness with low anisotropy. Wollastonite is also used in polyamide to give rigidity.

397

Particulate-Filled Polymer Composites

8.8.2 Fillers in PVC 8.8.2.1 Introduction PVC is produced by the free-radical polymerisation of vinyl chloride in suspension, emulsion, solution and mass, with the first two being the most important. Its fundamental unit is:

CH2

CH Cl

and commercial polymers have molecular weights between 50,000 and 120,000. They have approximately 5% crystallinity. All PVC polymers are unstable to heat and light with hydrogen chloride being evolved in an ‘unzipping’ mechanism. The resulting polymer chains are highly coloured, rigid and infusible. Stabilisers must be added to the PVC for it to be processed and used satisfactorily. These are added in a compounding operation at which time other additives can be added with little cost penalty. Density, refractive index, Tg, melt viscosity and other properties are dependent on the additives used. PVC is produced as a powder containing irregular grains with diameters between 65 and 170 μm (for suspension grades, other types are different). These grains are quite complex structures with a ‘strawberry’ looking surface, because they are composites with very small domains of 10-30 nm diameter which have agglomerated to form ‘spherical’ primary particles of 0.2–1.5 μm in diameter. The surface of the PVC grain (except for the mass polymer) is a skin of surfactants, polymerisation aids and other additives. During processing lubricants, processing aids and plasticisers penetrate this structure aiding melting and homogenisation. This process is known as gelation or fusion and fillers affect it significantly. The level of fusion determines many of the properties of the final PVC article. As a consequence of its complex chemistry and formulations, PVC is used in a very wide range of applications, making it the plastic with the third largest tonnage. However, the last few years have seen a very strong movement against PVC as a material because of its chlorine content and the possibility that, during its production, converting and disposal, chlorinated organic compounds known as dioxins may be formed. Possible formation of dioxins is also of concern during burning and disposal. As a consequence of this converters of PVC have made determined efforts to replace it with non-halogenated polymers. This has led to some changes in the uses of fillers. There are also moves occurring to replace the most commonly used lead-based stabilisers with organic and heavy metal free stabilisers because of concerns over its toxicity. These changes impose more stringent requirements on the purity of the filler. One of the most obvious changes is in colour because lead stabilisers give much greater opacity than the alternatives.

398

Filled Thermoplastics

Table 8.2 Estimated use of calcium carbonate in thermoplastics in Europe Application

Grade of calcium carbonate

kilo tonnes

PVC Cables

Medium and fine coated and uncoated

250

uPVC Extrusions

Ultrafine and fine coated

110

uPVC Windows and Profiles

Ultrafine coated

80

PVC Plastisols

Precipitated, ultrafine coated, fine and coarse

25

PVC Flooring

Ultrafine coated, fine and very fine

177

PVC Flexibles (general)

Fine, coated and uncoated

133

PE Compound and Masterbatch

100

PP Compounds

70

8.8.1.1 Fillers in Plasticised PVC Calcium carbonate is used in virtually all plasticised (flexible) PVC applications as an extender as seen in Table 8.2. The effects that the filler has on mechanical properties, colour and stability, are similar to those reported in Section 8.8.1.2 for cables, and will not be discussed in detail, although the formulations are far more variable.

8.8.1.2 Cable Coverings Most of the calcium carbonate used in plasticised PVC cable (insulation, sheathing and filling) in Europe is fine, good-quality chalk whiting often with a stearate coating, although where non-lead stabilisers are being used white marble based products are being used because of higher colour needs. In North America, Italy, Spain and other countries where relatively pure ground limestone or marble is abundant, then these are already being used. The use of stearate is not necessary as it can be added as extra lubricant in the formulation. In this case allowance must be made for the reaction that will occur between stearic acid and calcium carbonate. Even so, many users prefer a stearate coated filler as the coating improves the powder flow and general handling of the filler. The main type has a high purity usually 94-98% CaCO3, good whiteness 80-95 ISO, a mean particle size of 2-3 μm and low levels of coarse particles 1-10% above 10 μm. The

399

Particulate-Filled Polymer Composites

Figure 8.9 The effects of calcium carbonate fillers in plasticised PVC

Table 8.3 PVC Cable Sheathing Formulation Compound

ph r

PVC (suspension grade K67)

100

Di-2-ethylhexylphthalate

50

Tribasic lead sulfate

5

Coated calcium carbonate

10 0

properties of the calcium carbonate have little effect on the mechanical and electrical properties of a PVC compound. This is shown in Figure 8.9 for the cable-sheathing compound given in Table 8.3, in which the calcium carbonate filler is based on chalk whiting that has been ground to different particle size distributions, and the compounds were extruded as a flat strip. Gloss values show a significant particle size effect increasing markedly with finer fillers. Many high-gloss cable covers are produced using ultrafine fillers. Particle size also effects stress whitening and scratch marking. As a generalisation, it may be said that insulation compounds will be filled with 40-70 parts per hundred resin (phr) and sheathing with 20-100 phr; filler loading, plasticiser level and lubricants are used to control properties of the cable covering. In some applications, such as high temperature resistant or high voltage compounds the electrical properties obtained using calcium carbonate as filler are not good enough. In these cases 400

Filled Thermoplastics metakaolinite (calcined clay produced at around 700 °C) is used at between 5 and 15 phr, although loadings as high as 20 phr are sometimes encountered.

8.8.1.3 Floor Tiles and Homogeneous Flooring Coarse calcium carbonate with a broad particle size distribution, based on chalk, limestone or marble, or dolomite with an average particle size of approximately 15 μm, is the main filler used in PVC floor tiles. Price is the main specification but control of colour and of levels of coarse particles is needed. It is used as an extender at 200-450 phr. To give extra dimensional stability, reduced water pick-up, and green or hot strength during calendering and extrusion of the carpet high aspect ratio platy particles are used with the calcium carbonate. Stabilisation systems have to be modified to allow for the extra reactivity of the silicate surface. In homogeneous flooring all types of calcium carbonate are used but finer products are more common at loadings between 25 and 250 phr. Sometimes stearate coated grades are used. The biggest developments in PVC flooring are, unfortunately, its replacement with other types of floor coverings such as wood and carpet due to fashion, or the replacement of PVC with other polymers such as polypropylene and polyethylene. These can be heavily filled but because the developments are new and ongoing little can be said concerning the filler requirements.

8.8.1.4 Wall Coverings/Leather Cloth/Spread Coatings/Calendered Sheet Fine calcium carbonates (average particle size 2-5 μm) is used as an extender at loadings between 25 and 50 phr; precipitated calcium carbonate is sometimes used as a rheological control, although whatever filler is used there will be a significant effect on rheology, which is of great importance in these applications. Filler dispersion is also important.

8.8.1.5 Hose and Profiles Most grades of calcium carbonate are used, except very coarse, (i.e., above 10 μm average particle size), as extenders at levels of 20-50 phr.

8.8.1.6 Footwear Precipitated and 1-3 μm grades are used for rheological control in rotational moulded products and as extenders in injection moulded products.

401

Particulate-Filled Polymer Composites

8.8.1.7 Plastisol/Sealants Mostly precipitated grades are used as rheological control additives in combination with medium (2-5 μm) or fine (1 μm) grades as extenders, the latter at levels of 40-200 phr.

8.8.2 Uses of Fillers in Unplasticised PVC 8.8.2.1 Introduction As with plasticised PVC applications, most uPVC uses natural calcium carbonate as a filler. Loadings, however, are usually lower, in the range 3-30 phr. Low levels of impact modifier and processing aids are frequently used with the higher loadings to achieve desired mechanical and processing properties. Often non-specification products will use higher loadings with levels up to 100 phr being encountered, especially in pipe in China, Indonesia and the Indian sub-continent. Another quite significant difference with plasticised products is that in uPVC the filler affects processing and end-properties significantly. Usually fine and ultrafine products with top cuts of 10 μm and average particle sizes of 1-2 μm are used, but in lower specification products 3 μm grades are being used. The filler affects processing by affecting fusion of the PVC particles. This is a consequence of the interaction and reaction of the lubricants used in the compound with the surface of the calcium carbonate changing the balance of internal and external lubrication. This, of course, depends on the surface coating used on the filler and the filler’s surface area [153]. The speed and level of fusion or gelation play significant roles on efficiency of processing, and in determining mechanical properties, especially impact strength [154]. The particle size of the filler, especially levels of coarse particles, also affects tensile and impact strengths either positively by the fine particles acting as stress diffusers or crack stoppers or adversely with the coarse particles acting as ‘flaws’ or stress concentrators. Filler loading has a significant effect on processing and on properties, especially on impact strength, which can reach a maximum often higher than the unfilled. The optimum loading is determined by particle size, coating levels and by other additives in the formulation. For coated ground calcium carbonates with a top cut of 10 μm and a d50 of about 0.8 μm, maxima have been found at between 15 and 20 phr for lead stabilised compositions without any impact modifier [153], with acrylic modifier [153] chlorinated PE [155], and with tin stabiliser and styrenics [156].

402

Filled Thermoplastics

8.8.2.2 Pipes, Conduit and Fittings The fillers used are mostly 1-3 μm grades based on chalk in Europe, limestone and marble in the rest of the world principally to reduce costs. The finer grades also act as impact modifiers and processing aids and are preferred to coarser grades in more demanding applications, and in decorative areas where gloss becomes important. In pressure pipes and fittings levels are usually between 1 and 5 phr; in rainwater goods, sewage, soil and agricultural drainage levels are 8-20 phr; and in conduit and ducting levels are 3-40 phr. In non-specification pipes, loadings can be as high as 100 phr but most national and international specifications are now including limit values for fillers, either directly or indirectly by specifying the maximum specific gravity permissible. This differs from application to application but typically will be around 1.46 g/cm3. This equates to a stabiliser plus filler content of around 20 phr.

8.8.2.3 Window and Other Profiles Technical properties supplied by the filler are as described previously for pipes, although in most 4-5 phr TiO2 is added to give whiteness, light and some heat stability. Stearate-coated ultrafine (0.7-0.8 μm) produced from chalk, white limestone and white marbles are most widely used. Growth in the whiter fillers is being spurred by changes in aesthetic requirements; by the move from lead stabilisers to calcium-zinc or tin-based which do not give the same levels of opacity as lead composites; and by developments in coloured profiles. Filler levels for window and other building profiles have been creeping higher in the last few years from 3-5 phr to 5-15 phr with the limiting factors being cold impact strength, extrudate gloss and corner weld-strength. In various non-critical applications such as blinds and roller blinds coarser grades (1-3 μm) are used with levels up to 75 phr being encountered. In these cases, small amounts of plasticisers (7.5 phr) are added to aid processing and properties.

8.8.2.4 Film The market for uPVC film is diminishing under environmental pressure and consequently the use of particulate fillers is also diminishing. Most of the film is transparent for food packaging, display, blister packs and so on, and kaolins and other silicate minerals are used as antiblocking additives, without detriment to colour and transparency of the film.

403

Particulate-Filled Polymer Composites

8.8.3 Uses of Fillers in Polypropylene 8.8.3.1 Introduction Polypropylene has the general formula:

CH2

CH

n

CH3 with n being about 2000. It is a linear polymer, essentially a hydrocarbon with many similarities to PE, being chemically inert, flexible, tough and having fairly low softening and melting points. The presence of the methyl groups, however, introduces several significant differences. Tacticity is introduced due to the various spatial arrangements of the methyl groups that are possible. Commercial polymers are 90-95% isotactic; that is the methyl groups occur on one side of the polymer chains. This introduces some crystallinity (approximately 50%), and higher softening points. However, the methyl groups also induce greater susceptibility to oxidation and chemical attack (usually at the hydrogen atom β to the methyl group). It is the lightest common plastic with a specific gravity of about 0.9. Ethylene can be polymerised at levels of 4-15% with propylene, either randomly or as blocks to give copolymers that are more flexible and tougher than the homopolymer. Alternatively the polypropylene may be compounded with ethylenepropylene rubber to give copolymers as a physical mixture or rubber-modified-grades (depending on the level of the rubber). All suffer from oxidative instability and are always stabilised in service. Particulate fillers and coupled glass fibres are used in all these polymers for many applications to increase rigidity and heat distortion.

8.8.3.2 The Uses of Filled Polypropylene As mentioned previously, the principle reason why fillers are used in PP is to increase rigidity, especially at higher than ambient operating temperatures. The rigidity or modulus of a composite is affected by the modulus of the inorganic component, its loading and by its aspect ratio, that is the ratio of its length (or largest dimension of a particle) to its thickness (or smallest dimension). The modulus of any inorganic filler is very much higher than that of a plastic, and thus differences between types of fillers are not so important in determining modulus as the other two factors. As volume loading of filler increases, the modulus of a composite increases almost linearly at low to moderate filler levels. The higher the aspect ratio the higher the modulus. Thus, where rigidity of the final product is the most important single parameter, high-aspect-ratio fillers such as talc

404

Filled Thermoplastics or glass fibre are preferred. Mica and wollastonite are available with high aspect ratios but their main use is in North America where their cost-performance ratio is advantageous; in Europe, talc is most widely used. Original developments in filled PP were compounds with much higher stiffness than the unfilled, especially at higher than ambient temperatures, but which would be low cost to match the styrenics, especially ABS. Talc, being widely available, low cost and usually having a high aspect ratio is the preferred filler type, and all polypropylene compounders have several talc-filled grades available. Typical uses are in: automotive components such as air-filter covers, timing chain covers, heater boxes, and battery box tops, domestic appliances, such as washing machine soap dispensers, and in some disposable food packaging such as skeletal fruit packages. New initiatives on recycling, particularly in the automotive industry, are tending to limit polymer types used. Polypropylene is strongly favoured and this is helping drive the market for filled grades. Crudely, the talcs that are used can be divided into five types: three based on ‘pure’ talc (less than 10% impurities) with top cuts of 300 BS mesh, 20 and 10 μm; and two based on less pure minerals with top cuts of 300 BS mesh and 20 μm. Particle size has no effect per se on rigidity but, depending on the method of processing, finer types may have higher aspect ratios and therefore will give higher rigidities. Particle size does affect composite impact and tensile strength with smaller particle size products giving higher strengths [157], although the results are not unambiguous because methods of producing fine talcs also produce higher aspect ratio particles [158]. Virtually all applications for talc-filled PP are those that do not require toughness or high strains because the rigidity imparted by talc is accompanied by brittleness. The toughness can be improved by changing the base polymer to a copolymer with ethylene as comonomer, by incorporating ethylene-propylene rubber, or by changing the mineral to stearate coated calcium carbonate. All methods, however, reduce the rigidity of the composite compared with the talc filled equivalent. As discussed in [94], particle size and coating of the calcium carbonate affect impact strength and toughness, while other properties are not affected greatly. Loading of the coated calcium carbonate also affects properties. Rigidity increases, and tensile strength decreases virtually linearly with loading but impact strength can go through a maximum at between 20 and 40 wt%. This high level of toughness coupled with a rigidity which is higher than the unfilled, good flow in mouldings, good light and temperature stability means that calcium carbonate filled PP is regarded as a separate material, with major uses in injection moulded garden furniture, automotive components, food packaging, fibres, tapes and blown oriented PP (BOPP) packaging film. A very rapidly growing market for fine coated calcium carbonate is in breathable films, in which micropores form around the calcium carbonate particles during film orientation (mostly oriented PP). In fibres and tapes, the stearate-coated

405

Particulate-Filled Polymer Composites calcium carbonate is used at around 8 wt% and acts as a delustering agent and also reduces fibrillation. Another large and growing market is in BOPP film used in packaging. Loadings up to 70 wt% are used in the central layer of a three-layer film, produced by co-extrusion, with the outer layers unfilled. Mineral filled polypropylene often replaces ABS, polyamide and other engineering plastics, although their mechanical, processing and optical properties are different. However, by experimenting with the large number of permutations and combinations of type of filler, type of polypropylene and rubber toughening agent, satisfactory matching of cost-property performance is achievable. Reductions in cycle time, shrinkage, sink marking and improved noise reduction are among the benefits resulting from mineral filling, but detrimental effects have usually been observed in the aesthetic properties. Gloss is normally reduced but the surface can be tailored by choice of filler. For example, a high gloss moulding can be obtained by using a blend of talc and calcium carbonate [159]. Colour is also changed by the mineral filler: pure talc gives translucent, almost colourless compounds while high quality calcium carbonate gives a white colour with some opacity. Increasing levels of impurities in both minerals causes colour degradation. Colour will also be caused by any instability introduced into the polypropylene by the mineral. One problem from which all particulate filled plastics, and in particular filled PP, suffer is that of scratch marking and marring; as the plastic is scratched, light scattering occurs at exposed filler particles. This has been overcome to some extent by choosing the correct filler and by modifying the filler plastic interface. Calcined talc is being used in automotive compounds for its improved resistance to scratching. Filled PP sheet is also being used extensively as thermoformed packaging materials because the filler confers good sheet extrudability and thermoforming characteristics [160].

8.8.4 Uses of Fillers in Polyethylene 8.8.4.1 Introduction Polyethylene –(–CH2–CH2–)–n is essentially a high-molecular-weight paraffin, which as a consequence, is inert to most chemicals, flexible with low softening and melting points. Three types are now commercialised: LDPE, produced by polymerisation over a freeradical source at high pressures; HDPE, produced by polymerisation over Ziegler catalysts (complex catalysts based on metallic co-ordination compounds); and LLDPE, produced by a variety of techniques designed to give chains with limited short-chain branching and incorporating low levels of other olefins – butene, hexene and octene. All grades are semi-crystalline with levels of about 60% for low density, and up to 90% for high density,

406

Filled Thermoplastics but these can be lowered by branching. There is much argument about the Tg with values from –20 °C to –130 °C being reported. They have softening points from about 77 °C to 124 °C. The main applications for filled polyethylene, film and bags, blow moulding and electrical insulation dictate the required properties and limit the potential for fillers.

8.8.4.2 The Uses of Filled Polyethylene A considerable amount of calcium carbonate is used in LDPE, LLDPE and HDPE. Medium particle-size grades (with d50 of between 2 and 3 μm) are used in masterbatch either alone or to extend pigments, principally white TiO2 and carbon black. These in fact dominate the masterbatch market, with film and bags being the final destination for most. In natural masterbatch, the filler level will usually be 70-75 wt%; in pigmented products it is used to dilute the prime pigment by amounts dictated by the requirements (opacity, colour and gloss) of the end application. Mostly dilution at 10-20 wt% is used but some masterbatches can be formulated with 10% prime pigment and 60 wt% calcium carbonate. Levels of course are also dictated by any other additives (stabilisers, lubricants, etc.), in the masterbatch. Cost is naturally very important, but the filler does play an important role in several film properties [161]. For example, in LLDPE extruded film, ground calcium carbonate improves efficiency by both increasing the cooling rate of the bubble and the level of fusion; it improves printability; primary pigment dispersion can be improved; it reduces the coefficient of friction by increasing the surface hardness of the film; and it acts as an anti-blocking agent. Other minerals are used to give specific properties. Medium and coarse china clays (average particle size 2 and 5 μm) are used at 5-10 wt% to reduce stretch, to reduce slip, act as anti-blocking additives and to give thermal barrier properties (see below). A variety of minerals are used to anti-block all types of film (although HDPE film is not so prone to blocking because it is much stiffer, so anti-blocking needs are less). Talc, silica (natural and synthetic), aluminosilicates, zeolites and calcined kaolin are all used at levels of 0.11.0 wt%, the loading depending on the ‘stickiness’ of the film and the temperatures at which it may be used. The surface chemistry, shape, purity and refractive index of the mineral determine the blocking, friction, clarity, haze and colour of the film. A major use for calcined clay is in agricultural film. Because of its very strong absorption bands in the far infrared (IR), it renders the plastic opaque to heat (and consequently more like a flexible glass) [162]. Other minerals absorb IR radiation in lesser amounts but are still used by some film producers. All of course are added via masterbatch, which will also include stabilisers and lubricants the levels of which being dictated by the intended service length of the agricultural film.

407

Particulate-Filled Polymer Composites Linear low-density polyethylene is principally used in film and bags, with similar criteria applying as in LDPE. There is some interest in using it (because of its toughness) in engineering and automotive applications, but its stiffness has to be increased by using fillers. In recent years there has been a rapidly growing and very large market for ground calcium carbonates in LLDPE in the production of microporous films, where holes have been incorporated into a plastic sheet or film through de-bonding of the polymer from filler particles dispersed in the matrix. The most common plastic used is LLDPE (often in blends) and the most common filler is coated ground calcium carbonate with average particle size 1-3 μm. The de-bonding is achieved by stretching and orienting the film. Loading levels of the calcium carbonate are around 55-60 wt%, and the particle size and efficiency of coating are key to producing film with controlled pore size. The principal uses are in the production of controlled atmosphere or breathable films and in white opaque films. The former has various uses – hygiene, medical and industrial garments, building membranes, house wrap but the biggest single use is in the production of babies’ nappies. The latter includes food packaging, labels and paper replacement. High-density polyethylene finds its major outlets in blow-moulded bottles and containers, film and carrier bags and pipes (the last often used with LDPE and the blends are known as medium density polyethylene (MDPE)). Although fillers have been and are still being looked at periodically in blow mouldings, processing imposes severe restrictions before costs and product properties are considered. Film and bags will have some ground calcium carbonate included via masterbatch as an extender for the prime pigment. The main pipe sectors are for gas and water transportation, and end-users have imposed very strict regulations on additives and properties; potential use of fillers is very low. There has been some interest in using HDPE in ducting, competing against uPVC, and a filler, at high loadings, will be essential to achieve the required stiffness and cost balance, if this replacement is to be successful.

8.8.5 The Use of Fillers in Polyamides 8.8.5.1 Introduction Nylon is the generic name for the family of polyamides (PA) with PA6 and PA6, 6 being the most common. They are named after the chemical group:

O

H

C

N

formed during the condensation polymerisation which occurs when an organic acid is heated with an organic amine. The numbers which always occur with the name Nylon or 408

Filled Thermoplastics polyamide refer to the types of acid or amine used in the production. The regular spacing of the amide groups means that the polymers crystallise with a high intermolecular attraction leading to high-strength polymers with high melting points. Levels of crystallinity depend on thermal history and can vary from 15% to 50%. Nylon is fairly hygroscopic and its Tg (temperature below which the polymer become brittle) and mechanical properties depend on the amount of absorbed water.

8.8.5.2 Properties of Filled Nylon All polyamides are tough, rigid plastics with high heat distortion temperatures but they are thermoplastic, that is, they exhibit plastic flow (high creep) and soften at elevated temperatures. Mostly they are used unfilled but there is a significant application sector in which higher rigidities and higher heat distortion temperatures are required than can be achieved from unfilled polyamide. Glass fibre (with a size or a silane coating) fills most requirements giving very high rigidity, etc. Other mechanical properties of a properly coupled glass-fibre filled Nylon are also good. Some 200,000 tonnes per year of glass-filled Nylon is now used, and it is regarded as a separate engineering material. However these fibre filled composites suffer from anisotropy due to fibre orientation during processing; that is, the properties of the composite vary depending on the direction in which they are measured. Fibre orientation also causes uneven shrinkage that can lead to unpredictable warpage (bending and distorting), which is not acceptable in products that require good dimensional stability. Anisotropy and warpage are proportional to the aspect ratio of the filler used and can be reduced to zero by using glass spheres [163]. Calcined kaolins give good, low, anisotropy and when treated with a bifunctional aminosilane, give very good mechanical properties and tend to dominate the European and Pacific filled Nylon market. Both particle size and coating level affect impact strengths. Pre-treatment gives much better properties than when calcined clay and silane are added separately to the compounding operation [164]. Some talc and wollastonite is also used giving high rigidity (although this depends on the grade and its aspect ratio), although impact properties are worse. In the case of talc, this has been related to its inability to couple through silanes with the polyamide. Calcined talc is being developed to improve the interaction [154].

8.8.5.3 Uses of Filled Nylon Mineral-filled Nylon is widely used: in automotive applications, such as wheel discs, headlamps, water pumps, air inlets and grills; in electrical engineering; in electronics; appliances and consumer goods [165]. Significant recent developments have seen its use in the production of automotive engine covers. The mineral, as mentioned previously, is

409

Particulate-Filled Polymer Composites frequently used in combination with glass fibre. Loading levels have, in the automotive industry, dropped to around 20 wt% under pressures to reduce vehicle weight but in some cases are still as high as 40 wt%.

8.8.6 Polybutylene Terephthalate Polybutylene terephthalate (PBT) is a fairly tough, rigid thermoplastic which has a very good gloss in mouldings. It is less susceptible than Nylon to moisture when moulded but otherwise most of its mechanical properties are not so good. Many glass-fibrefilled compounds are available from specialty compounders and polymer producers and some mineral filled grades are also widely used. Talc, aminosilane-treated calcined clay, ultrafine calcium carbonate, ultrafine china clays, glass beads and glass flakes have all been encountered. Benefits from the particulate fillers have been reported to be rigidity with good dimensional stability, high impact strength and exceptional surface finish [166].

8.8.7 Polyethylene Terephthalate One of the most important uses of PET is in producing many types of film and tapes. In these a variety of speciality fine grades of china clays, calcined clays, calcium carbonates (natural and precipitated), synthetic silicas and silicates are used at levels of about 0.1 wt% as anti-blocking agents; they may also nucleate crystallite formation. Fillers seem to be precluded from the largest single market for PET – bottles, not only because of the difficulties in successfully blow moulding filled products in general, but also their application properties such as optical clarity and burst strength will be adversely affected by fillers. Many bottles are now being recycled into a variety of plastic and fibre applications; fibres and particulate fillers are being looked at to extend the range of applications. The high HDT and softening temperature of PET have recently led to developments in its use in food ‘cook-in’ applications. However, it’s HDT is not quite high enough for conventional oven use and fillers are being looked at to improve this.

8.8.7.1 Polystyrene, High-Impact Polystyrene and ABS The styrenics are a family of low to medium price, rigid, easily processed plastics with good gloss and optical properties. ABS and HIPS are more tough, due to the acrylonitrile rubber content, which has either been incorporated into the plastic or copolymerised with the styrene monomer, respectively. Fillers have been looked at by several producers

410

Filled Thermoplastics and compounders but do not give good enough properties to allow them to be used in any significant amounts, although products containing kaolin, calcium carbonate and talc have been reported. These may have the filler incorporated as a diluent in colour masterbatches used in the plastic. Some very old patents cover the incorporation of china clays and ultrafine calcium carbonates into the rubber before compounding this into the polystyrene but again no noticeable commercial success has been achieved. Talc is used in expanded polystyrene as a nucleating agent.

8.8.7.2 Polyphenylene Oxide (or Ether) Polyphenylene oxide is an engineering plastic that is commonly blended with polystyrene, with the blend having significantly better properties than the separate plastics. Modified polyphenylene oxide has been available now for approximately 30 years and it’s uses are widespread including automotive components, business machines, domestic appliances, wire covers and water treatment. Other blends with PBT and PA are in the market place offering different property balances. These composites (and other polymer blends) are being trialed for automotive body parts, and particulate fillers are needed to give the correct coefficient of thermal expansion. This type of filler is also needed to give rigidity without affecting impact properties. Very fine kaolins (with mean equivalent spherical diameter (esd) of about 0.3 μm) with and without silane coupling agent give good impact strength [167].

8.8.7.3 Polyphenylene Sulfide (PPS) Polyphenylene sulfide is a high temperature, stiff, fire-resistant, chemically resistant plastic, which has a very low melt viscosity and accepts fillers and reinforcing agents very well. Talc, china clay, dolomite, quartz and glass fibre filled grades are all available but volumes are very small and the situation changes very rapidly.

8.8.7.4 Polyformaldehyde or Polyoxymethylene Polymers These plastics are characterised by high stiffness, good impact resistance, high HDT, good chemical resistance and have the best fatigue resistance of any plastic. Glass fibreand mineral-filled grades are used to increase stiffness, hardness and resistance to distortion. The type of filler that can be used is important as the polymers are badly depolymerised by acids, the unzipping of the polymer chain giving gaseous formaldehyde. Minerals with acid surfaces, such as china and calcined clays, can cause this depolymerisation. Calcium carbonate reduces some mechanical properties but one calcium carbonate filled grade is claimed to have better abrasion resistance. 411

Particulate-Filled Polymer Composites

8.8.7.5 Others The properties of particulate fillers in a number of other thermoplastics are being investigated by plastics producers and academic institutions but usually with a low priority rating. This activity has been growing less and less in recent years as companies, in particular, have been reducing the staffing levels in the research and development departments.

8.9 Conclusions Thermoplastic composites are all around us and their use is increasing every year. The reason for this is that thermoplastics have an excellent combination of cost and performance. The performance can often be further enhanced by addition of fillers while maintaining a favourable cost. The recycleability of thermoplastic composites is an advantage compared to rubbers and thermosetting polymers because the latter two types cannot be melted and reshaped. This favours the continued growth of the thermoplastics and their composites at the expense of other polymeric materials. To understand and optimise composites, one must have an overview of all the different economic, chemical, surface and physical aspects. Furthermore, one must have a clear goal, and be able to correctly prioritise the properties of most import for the intended application. The best composite is the one that makes the best compromise between the multitude of properties, at the lowest cost. The use of fillers has been increasing incrementally for many years and that trend is expected to continue as the use of traditional fillers is optimised and as new nano-fillers eventually become economically attractive. What can we expect in the future? In the near future, composites must be designed with re-use in mind. That means proper stabilisation so that the polymer can be recycled with sufficient retention of mechanical and aesthetic properties. There will be an increased tendency to use fewer, standard materials to reduce cost and to reduce the need for extensive separation of materials for recycling. It can also be anticipated that products will be designed for easy disassembly. It seems probable that surface treated filler will become more popular. Although the treatment adds cost it gives many advantages and when these are summed, the overall cost and performance of the material may be better for the surface treated type. For example, stearic acid coated calcium carbonate in PP homopolymer gives higher extruder throughput, better gloss, better impact strength and improved stability (because it prevents antioxidant from adsorbing onto the filler and becoming inactive). Any one of these

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Filled Thermoplastics benefits may not justify the extra cost of pre-treated filler, but taken together they give a very attractive combination of price and performance. Another trend for the future may be the increased use of single-screw extruders to make composites. At the moment twin-screw extruders are used almost exclusively, because they are able to achieve better dispersion. However, recent developments have improved mixing in single-screw extruders. Therefore, it may become common to use single-screw extruders because they are cheaper, easier to maintain and give higher throughput. Again, surface treatment of the filler also helps here to give good filler dispersion even for a single-screw extruder. Progress in polymer composites has been held back because it is expensive and timeconsuming to prepare multiple formulations and then perform thorough mechanical testing. It is possible to save time and money by screening new fillers, antioxidants, dispersants and coupling agents in a model liquid instead of the polymer. The screening can then be followed up by full testing, using the polymeric matrix. Hopefully, this method will be used to help develop new filler grades and surface treatments, more quickly and cheaply.

Acknowledgements We would like to thank several people for their assistance in writing this chapter. Firstly, the editor Professor Rothon who has been a great help in making suggestions, proofreading and for general discussions. Professor John Berg (who deserves a special thanks for being such a help with the section on adhesion), Professor Ulf Gedde, Professor Aubrey Jenkins, Kevin Breese, Massimo Sanità, Carlo Tomaselli, Roy Goodman, Chris Paynter, Richard Day, Anna Kron and Werner Posch are all warmly thanked for making significant contributions by reading draft versions and for making valuable comments that improved the quality of the chapter.

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Filled Thermosets Graham V. Jackson, Michael L. Orton and Howard Taylor

9.1 Introduction Thermosets is the term used to describe those polymers produced by polymerisation of relatively low-molecular-weight precursors either within a mould cavity or in some other in situ situation. Thus, the part shape is set by chemical reaction within the mould cavity and this distinguishes thermosets from thermoplastics for which shape is set by the cooling of a melt. Most polymers processed as thermosets are crosslinked and therefore undergo an irreversible liquid to solid transition unlike thermoplastics, which show a reversible liquid to solid transition. Consequently, thermoplastics may, in principle, be directly reprocessed, while thermosets cannot, though they can be reused by granulating and using as a filler. In this chapter the term ‘resins’ is used to describe the relatively lowmolecular-weight precursors of thermosets and ‘polymer’ is used for the high-molecularweight cured product. During the curing or polymerisation of a crosslinked polymer, two phenomenological steps occur - gelation and vitrification. Gelation occurs when conversion of the reactive groups on resin/monomer has proceeded to such an extent that the amount of branching ensuing is sufficient to generate a ‘global’ network of essentially infinite molecular weight [1-4]. A physical picture of gelation is that of a giant molecule swollen by unreacted resin/monomer. The conversion at which gelation occurs depends upon the mechanistic type of the polymerisation, but for free-radical addition polymerisations, can be as low as a few per cent (see Sections 9.2.1-9.2.3). Clearly, mould filling has to be complete before the gel point. As conversion proceeds, the glass transition temperature (Tg) of the resin swollen gel increases and, when it corresponds to the temperature at which the resin is polymerising, vitrification occurs, i.e., the swollen gel is hardened to a glass. The significance of the point of vitrification is that, after vitrification, the mobility of the reactive groups within the glass is severely inhibited and the rate of further reaction falls sharply, eventually becoming zero. Thus, in moulding thermosets, it is important that the mould temperature or the maximum exotherm temperature be close to the Tg of the fully cured polymer, i.e., vitrification occurs as late as possible in the conversion process, otherwise undercure will result [5]. If a resin is initially undercured, it can be postcured to achieve full cure, though if too much time elapses before postcure, full cure and mechanical strength cannot be achieved due to loss of mobility of reactive groups within the resin.

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Particulate-Filled Polymer Composites Crosslinked polymers can be characterised conveniently by defining their crosslink density as branch points per unit volume or average molecular weight between crosslinks. This parameter in conjunction with the molecular nature of the polymer defines whether the material will behave as an elastomer or as a rigid material, which shows either ductile or brittle failure behaviour. Fillers can be used to modify properties further across the whole range of polymer behaviour. Because inorganic fillers are, compared to most polymers, much stiffer and less extensible materials, their incorporation into a polymer will usually produce a composite material of reduced strain to failure and increased stiffness relative to the polymer, i.e., the composite will be less elastomeric or less ductile. Nevertheless, large quantities of fillers are used in polymers that already have low strains to failure and show brittle failure behaviour. This chapter will confine itself to a discussion of the use of fillers in ductile and brittle crosslinked polymers. The function of the filler in a thermoset can be to offer material cost reduction, since most fillers are of low cost relative to polymers or to confer property modification. Highly crosslinked polymers are generated in their final shape by in situ processing of lower molecular weight monomeric or oligomeric precursors. Since the lowmolecular-weight precursors are usually low-viscosity liquids at their moulding temperatures, an important use for fillers is for rheology control both before and during the polymerisation or curing step. Other features to be considered in the processing of crosslinked polymers are: (1) the polymerisations are exothermic and, if rapid cure is required, exotherm temperatures can be high (as high as 200 °C); and (2) the polymerisations often occur with a significant reduction in volume since the polymer, with extremely few exceptions, has higher density than the precursors from which it has been derived. Fillers, through their essentially volumetric and thermal inertness at typical thermoset resin-processing temperatures, offer a valuable means of exotherm and shrinkage control. Fillers can modify the properties of the cured polymer in a wide variety of ways. Some examples of properties readily modified by incorporation of fillers are mechanical properties such as stiffness, hardness, strength and toughness, and thermal properties such as expansion, conductivity or thermal decomposition. The latter can have a considerable effect on flammability and smoke generation. Fillers are also an important way of varying and enhancing aesthetics.

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Filled Thermosets

9.2 Brief Chemistry of Thermoset Polymers Many highly crosslinked polymers have been synthesised and reported since the emergence of ‘Bakelite’ as a synthetic moulding material [6, 7]. However, to keep the subject matter of this chapter within reasonable bounds, we have restricted our discussions of filled polymers to examples of the more common crosslinked matrices, unsaturated polyesters, epoxies, methacrylics and phenolics. The resins are cured by different mechanisms: •

free-radical chain growth (unsaturated polyesters and acrylics);

•

step addition (epoxies, although occasionally epoxies are cured by ionic mechanisms); and

•

condensation (phenolics).

9.2.1 Free-Radical Chain-Growth Curing Resins 9.2.1.1 Unsaturated Polyester Resins An unsaturated polyester resin has two primary components, a polyester containing polymerisable double bonds and a copolymerisable solvent monomer, of which the most commonly used is styrene. Unsaturated polyesters are made by esterification of glycols with mixtures of maleic anhydride and saturated diacids (Structure 9.1). The term ‘alkyd’ is used to describe low-molecular-weight polyesters, where molecular weight is broadly

Structure 9.1 Preparation of unsaturated polyester resins

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Particulate-Filled Polymer Composites controlled by reacting polyhydroxy and polycarboxylic compounds in non-stoichiometric amounts, or by modification with monocarboxylic acids. Thus unsaturated polyester resins are often referred to as comprising unsaturated alkyds in styrene. The other resins, which cure by a free-radical chain-growth mechanism, also have a lowmolecular-weight double-bond functional species dissolved in a solvent comonomer but, since these species are not polyesters, they cannot be termed alkyds. The term oligomers is used to describe them instead, although it is not a strictly correct use of this term, since the species are not always a multiplicity of repeat units. The properties of the polyester resin and the final polymer after curing can be varied by choice of diacid and glycol, and by the molar ratio of maleic anhydride to saturated diacid. This ratio is typically from 1:1 to 3:1 m/m and is particularly important in determining reactivity during cure and crosslink density after cure. High maleic anhydride to diacid ratios give high-reactivity resins with fast gelation and cure characteristics, and final polymers having high heat distortion temperatures (HDT) but which show the most brittle failure behaviour due to their high crosslink density. The higher double bond content also leads to high-volume shrinkage on curing. Conversely, reducing maleate ratios reduces reactivity and crosslink density with the consequences of long gel times and low heat distortion temperatures, but lower volume contraction on curing and a less brittle polymer. It is very rare to find an unsaturated polyester with a maleate to phthalate ratio less than 0.5:1 m/m. The most common diacids are the phthalic acids, orthophthalic acid (phthalic anhydride is actually used) giving general purpose resins and isophthalic acid giving resins of improved corrosion resistance. Adipic acid is also used, but on a smaller scale and where chain flexibility is particularly required. The most commonly used glycol is propylene glycol because it does not lead to branching in the polyester chain and has no tendency to crystallise. Other diols are used to achieve special properties, e.g., propoxylated bis-phenols to confer corrosion resistance. Typically, the resin would incorporate the three components in the molar ratio, maleic anhydride:phthalic anhydride:propylene glycol (5:5:11). A typical unsaturated polyester is shown in Structure 9.1. Esterification of the three components would produce a distribution of species [1, 8, 9] and the previous ratio would produce a polyester alkyd with a number average molecular weight (Mn) of about 2000 and a weight average molecular weight (Mw) of about 4500, but with a spread of molecular species ranging from, say 500 to 10,000 molecular weight. The bulk of the material has from 3 to 15 unsaturated groups per polyester chain and the unsaturation is predominantly in-chain with only a statistical terminal unsaturation. Although maleic anhydride is the starting material, esterification isomerises much of this to fumarate and the resulting polyesters probably contain some 70-90% of unsaturation as fumarate.

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Filled Thermosets Properties of the unsaturated polyester resin and cured polymer are also influenced strongly by the nature and amount of solvent comonomer used. The solvent comonomer polymerises with the polyester unsaturation and with itself to form bridges or crosslinks between the polyester chains. The following discussion is restricted to the use of styrene as comonomer, it being by far the commonest, with some reference to methyl methacrylate for speciality uses. Full discussion of other comonomers can be found in [10]. High styrene contents lead to low-viscosity resins and this is desirable if high filler loadings are required, but there are tight limits on the polyester/styrene ratio, if an acceptable cure is to be achieved. If the styrene:maleate/fumarate ratio is too low (< 1:1 m/m), inadequate cure results because there is insufficient styrene to bridge the maleate/fumarate units in an alternating manner, or the resin vitrifies at too low a conversion under the moulding conditions used. The result is that, although all the styrene would be polymerised, only about 75% of the maleate/fumarate would be reacted. Conversely, if the styrene:maleate/fumarate ratio is too high (> 3:1 m/m), cure is slow and inadequate due to the relatively slow rate of the styrene homopropagation step. Under the typical initiation conditions used, the rate of decomposition of the initiator becomes fast relative to the styrene homopropagation step leading to an increased tendency for primary radical termination, and consequent undercure and high residual styrene. Thus, the preferred styrene:maleate/fumarate ratio is usually around 2:1 m/m, i.e., the average length of crosslink between polyester chains is 1-2 styrene units, and this ratio under ‘good-practice’ cure conditions would be expected to lead to > 95% of both styrene and maleate/fumarate unsaturation being reacted. These boundary conditions imposed by the reactivities in polymerisation lead to styrene contents typically limited to the range 30-40 wt% which puts a lower viscosity limit of around 0.2 Pa-s on unsaturated polyester resins. The 2:1 m/m ratio produces resins with a double bond concentration of about 6 mol litre-1 and a volume contraction on curing of about 8.5% (see Section 9.4.4). With compositions close to the preferred styrene:polyester ratio, the overall rate of conversion depends on the reactivity of styrene with maleate and fumarate, the ratio of maleate to fumarate in the polyester, and the reactivity of styrene with itself. Data from the literature on relevant reactivity ratios are conflicting, but the consensus of evidence is that styrene is more reactive with fumarate than with maleate and is more reactive with either than with itself. Similarly, maleate/fumarate is more reactive with styrene than with itself, hence the tendency to form alternating copolymers from styrene and maleate/fumarate unsaturation. Methyl methacrylate is sometimes used as a component in unsaturated polyesters to reduce refractive index and thereby derive aesthetic variations when the polymers are filled with particulates or glass. However, methyl methacrylate could not wholly replace styrene in polyester resins because, unlike styrene, methyl methacrylate is much more reactive with itself than

429

Particulate-Filled Polymer Composites with maleate/fumarate and would, therefore, tend to homopolymerise in unsaturated polyester/methyl methacrylate compositions. Nevertheless, styrene and methyl methacrylate do copolymerise readily and the upper limit of methacrylate incorporation is that sufficient styrene be present to act as the necessary link between maleate/fumarate and methacrylate.

9.2.1.2 Vinyl Ester and Vinyl Urethane Resins A second group of thermosets curing by free-radical chain addition are those that use methacrylate, or acrylate-derived double bonds, in the oligomer component of the resin. It is a feature of these resins that the unsaturation is thus always terminal, although some members of this class do contain in-chain maleate/fumarate unsaturation as well. The group comprises vinyl esters, vinyl urethanes and urethane acrylics/methacrylics, the resins being differentiated by the specific molecular nature of the oligomer backbone and the choice of solvent comonomer. Those resins that have entirely acrylic functional oligomers are reactive with styrene or methyl methacrylate, and either monomer can be used alone or in a mixture of any proportion. Those that do contain some in-chain maleate/fumarate unsaturation must have an appropriate amount of styrene if full cure is to be achieved. When styrene is used as the solvent comonomer, then, because of the relatively slow homopropagation of styrene, composition limits still have to be applied if undercure is to be avoided (see discussion of unsaturated polyester alkyds in Section 9.1.2.3). Again, the ratio of styrene:oligomer unsaturation is typically < 3:1 m/m, the average crosslink between oligomers being 2-3 styrene units. The vinyl esters are derived from epoxy resins, adducts of bis-phenol A and epichlorhydrin, by reaction of the terminal epoxide groups with methacrylic acid (Structure 9.2) [11, 12]. Thus, these resins have only terminal unsaturation, but do have hydroxy functionality on the oligomer. The vinyl urethanes are normally derived from hydroxyl-terminated unsaturated polyester alkyds, e.g., propoxylated bisphenol A fumarate, which have been end-capped with a polyisocyanate and then subsequently end-capped with an hydroxy alkyl methacrylate. Thus, these resins have both terminal acrylic and in-chain maleate/fumarate unsaturation, the ratio depending on the oligomer molecular weight and the functionality of the polyisocyanate. High molecular weight results in a lower terminal: in-chain unsaturation ratio, while a polyisocyanate functionality > 2 increases the ratio. A typical oligomer structure is shown in Structure 9.3 [13, 14].

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Structure 9.3 Preparation of vinyl urethane resins

Structure 9.2 Preparation of vinyl ester resins

Filled Thermosets

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Particulate-Filled Polymer Composites

9.2.1.3 Urethane Methacrylate Resins Urethane methacrylates are made by end-capping isocyanate-terminated, low molecular weight urethanes with hydroxyalkyl methacrylates [15]. The synthesis is illustrated in Structure 9.4. Although Structure 9.4 implies that a single molecular species will result from reaction of the diisocyanate, diol and hydroxyalkyl methacrylate, in practice the oligomers will consist of a mixture of species of type ABA, ABCBA, ABCBCBA and higher, where A is hydroxyalkyl methacrylate, B is diisocyanate and C is diol. In commercial resins, the structures may be further complicated by the use of isocyanates and polyols of functionality > 2, which introduce branching into the oligomer. In the production of these urethane oligomers, it is common practice to use mixtures of polyols of low molecular weight, e.g., propylene glycol, and of molecular weights > 2000, e.g., polypropylene ether diol. In such cases, the oligomer molecular weight distribution becomes bimodal (or multimodal) in nature because some oligomers contain high molecular weight diol and some contain only low molecular weight diol or no diol. In such distributions, Mn and Mw have little meaning. Unsaturated polyester alkyds and vinyl urethanes rarely contain high molecular weight diols, as incorporation of these would lead to lower reactivity and even higher viscosity. Therefore, polyesters generally have a single peak distribution of molecular weights. Vinyl esters also have a single distribution of molecular weights. The incorporation of unsaturated diols in urethane methacrylate resin oligomers is reported but is very rare, and it can be reasonably assumed that commercial resins contain urethane oligomers having unsaturation only in terminal positions. The actual average functionality of the oligomer depends on the average functionalities of the polyol and isocyanates as indicated in Structure 9.4. As with other acrylic functional oligomers, the resin can be formulated in either styrene or methyl methacrylate, but the same arguments still hold requiring the ratio of styrene: oligomer unsaturation to be < 3:1 m/m. In contrast, oligomers with acrylic unsaturation can be copolymerised with, and therefore formulated with, methyl methacrylate in any proportion and high conversion is still achieved in reasonably short times. This ‘high-dilution’ feature of urethane methacrylate oligomers in methyl methacrylate renders such resins particularly suitable for filling to high volume fraction, because the resin viscosity can be tailored to meet specific volume fraction and rheological requirements without the cure being impaired. Dilution does increase gel times but gel to peak times are less sensitive to dilution, and ‘snappy’ cures still result, even at methyl methacrylate:oligomer unsaturation ratios of 10:1 m/m [16]. The reason for the excellent cure, even at high methyl methacrylate contents, lies in the auto-acceleration phenomenon that occurs in the homopolymerisation of methyl methacrylate. This does not occur in styrene homopolymerisation [17-19].

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Filled Thermosets

Structure 9.4 Preparation of urethane methacrylate resins

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Particulate-Filled Polymer Composites

Figure 9.1 Cure exotherms for urethane methacrylate/methyl methacrylate resins (a), compared with unsaturated polyester/styrene resins (b)

The practical consequences of an auto-acceleration effect on exotherm and conversion are illustrated by the exotherm curves in Figure 9.1. The urethane methacrylate/methyl methacrylate resin that shows auto-acceleration at all oligomer/monomer ratios is compared with an unsaturated polyester/styrene resin, which only shows an autoacceleration (or ‘gel effect’) at high alkyd/styrene ratios. The urethane methacrylate oligomer copolymerised with styrene shows virtually the same exotherm behaviour as the unsaturated polyester for equivalent styrene/oligomer unsaturation ratios. The freedom in monomer/oligomer unsaturation ratio in the ‘all-acrylic’ functional resins allows crosslink density and ‘hardness/softness’ properties of the cured polymer to be varied

434

Filled Thermosets over a wide range by changing the nature and amount of the urethane segment. The hard properties derive from the methacrylate and associated isocyanate and the soft properties from the polyol segments. Thus, the urethane methacrylates can range from ‘all hard’, i.e., no polyol used, to increasingly soft as the polyol content increases. Indeed, the properties of the increasingly soft urethane methacrylates polymerised by free-radical chain growth overlap with those of high-modulus polyurethanes made by step addition [20].

9.2.2 Step Addition Curing Resins 9.2.2.1 Epoxy Resins ‘Epoxy resin’ is a term that covers a diverse range of molecular types, the common feature being the presence of epoxide groups through which curing the crosslinking occurs. Also included within the general descriptive term are the hardeners or co-reactants, which are used in conjunction with the actual epoxy resin and which again are of diverse molecular type related only through possession of functional groups reactive with epoxide. Only the more common types of epoxy resin are discussed here. The common epoxide resins fall into four molecular types: (1) (2) (3) (4)

bisphenol A-derived resins; epoxy novolacs; glycidyl amines; and cycloaliphatic epoxides.

In (1) and (2), epichlorhydrin is reacted with a di- or polyphenol, in (3) with a di- or polyamine, and (4) is made by peroxidation of a bis cyclic olefin. A characteristic parameter, which depends on functionality and molecular weight, is the ‘epoxide equivalent weight’; the weight of resin that contains one epoxide group (also sometimes expressed as number of epoxide groups per kilogram). This parameter has to be known when considering the curing reaction. The majority of epoxy resins are in categories (1)(3), derived from epichlorhydrin, and thus have epoxide groups present as the glycidyl entity. Epichlorhydrin reacts, in the presence of alkali, with compounds containing active hydrogen atoms by a two-step reaction, initial formation of a chlorhydrin and generation of a new epoxide by ring closure with sodium hydroxide.

9.2.2.2 Bisphenol A Diepoxides The most common epoxy resins are the reaction products of bisphenol A with excess epichlorhydrin and have the structure shown in Structure 9.5. Because the simple diepoxide

435

Particulate-Filled Polymer Composites

Structure 9.5 Preparation of bisphenol epoxy resins

(n = 0) is reactive with the phenolic groups of bisphenol A (see later), chain extension or ‘advancement’ occurs and higher molecular weight species are generated (n = 1, 2, 3, …), each chain extension reaction generating a hydroxyl group. For n < 1, resins are supercooled liquids at ambient temperature; as molecular weight increases n > 2, resins are solids with softening points in excess of about 70 °C. The control of reactant ratios and the addition rate of NaOH can be used to keep molecular weights low if required, but typically n = 0.1-0.2 even for the lowest molecular weight resins. A gel-permeation chromatograph of a typical low molecular weight resin could show n = 0 (84%), n = 1 (11%), n = 2 (4%) and n = 3 (1%) (parts by weight), giving an average of n = 0.12. Because of side reactions in the preparation, epoxide functionalities are slightly less than 2, being typically 1.95. However, as n increases, the hydroxyl to epoxide ratio increases and for very high molecular weight resins, reactions through the hydroxyl groups may be more dominant in the curing process than ring opening of the epoxides. Chain extension of the notional diepoxide with tetrabromo bisphenol A gives fireretardant resins.

9.2.2.3 Epoxy Novolacs Epoxy novolacs are made by glycidylation of phenol formaldehyde novolacs, the latter being the low molecular weight thermoplastic condensation products from phenol and formaldehyde that are formed under conditions of excess phenol and acidic catalysis (see Section 9.2.3.1). The resins are of the structural type shown in Structure 9.6, the methylene

Structure 9.6 Epoxy novolac resins

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Filled Thermosets bridges being a mixture of ortho and para. The molecular weight of the epoxy novolac clearly depends on the molecular weight of the starting novolac and, similar to the bisphenol diepoxides, the low members are liquids at room temperature (n < 2) but become solid as molecular weight increases (n > 2). The simplest species present in the novolac (n = 0.15) is the diepoxide of bisphenol F. In contrast to the bisphenol A diepoxides, the epoxide functionality of the novolacs increases with increasing molecular weight, and also novolacs do not possess aliphatic hydroxyl groups and have a mixture of o-o, o-p and p-p linkages.

9.2.2.4 Glycidyl Amines Glycidyl amine resins are prepared by reaction of epichlorhydrin with either primary diamines, monoamines or aminophenols. Each primary amine gives rise to two epoxide substituents. The reaction again involves two steps: reaction of the epichlorhydrin with an active hydrogen to form the chlorohydrin, followed by ring closure with sodium hydroxide to generate the new epoxide. The common glycidyl amines are the reaction products of epichlorhydrin with diamino diphenyl methane and with p-aminophenol. As with bisphenol A diepoxides, reaction of the diepoxide with the amine still present generates higher molecular weight species but, in the case of the glycidyl amines, chain extension leads to an increase in epoxide functionality and branching, as well as generation of hydroxyl groups. Furthermore, the combination of high functionality and the presence of tertiary amine and hydroxyl groups, both of which catalyse epoxide homopolymerisation, can lead to problems of short pot lives with glycidyl amine resins. Thus, particular commercial activity is directed to keeping molecular weights low in these resins. Typical epoxide equivalent weights for diaminodiphenyl-methane based resins are 115-130 compared with a theoretical weight of 105 for the simple non-extended tetraepoxide and, for p-aminophenol-based resins, 105-115 compared with 92 for the simple triepoxide.

9.2.2.5 Reactive Diluents Reactive diluents are often added to epoxy resins to lower viscosity and ease processing. They are normally low molecular weight glycidyl compounds and either monofunctional, e.g., n-butyl glycidyl ether and C12-C14 aliphatic glycidyl ethers, or difunctional, e.g., butane diol diglycidyl ether, neopentyl glycol diglycidyl ether. When amine curing agents are used, multifunctional acrylates are sometimes used as diluents and curing of the diluent is achieved by Michael addition to the primary or secondary amine.

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Particulate-Filled Polymer Composites

9.2.2.6 Curing of Epoxy Resins Epoxide groups may homopolymerise or react with active hydrogen atoms in other molecules, usually termed hardeners or co-reactants, to produce copolymers. Cure of an epoxy resin may involve either or both of these reactions, and can be very complex, since reaction of an epoxide with one functional group can produce new functional groups for additional reaction with epoxides, and so on. Homopolymerisation is promoted by tertiary amines and certain Lewis acids, which do not contain acidic hydrogen. The amines promote a mechanism in which an anion is the propagating species; the Lewis acids promote a cationic propagation mechanism. In polyepoxide resins, both mechanisms lead to polymers that are essentially highly crosslinked polyvinylene ethers in which each vinylene unit carries a crosslink. Copolymerisation is typically with polyfunctional primary or secondary amines, or with polycarboxylic acids or anhydrides. Diepoxides plus primary diamines give the stepaddition polymer (Structure 9.7), a linear polymer containing both hydroxyl and secondary amine groups. These both provide sites where further reaction with epoxide groups can occur to generate crosslinked polymers. The reaction with the secondary amine generates further hydroxyl groups and also a tertiary amine, which can promote the etherification reaction of epoxides discussed previously. For the special case of a primary monoamine, a linear polymer is still generated if reactions of the hydroxyl groups are ignored, because both hydrogens are active. Secondary amines have to be at least difunctional to continue polymerisation. Typical amine curing agents are linear polyamines, cyclo-aliphatic amines and polyamide polyamines. Examples of the linear amines are diethylene triamine, triethylene tetramine and higher homologues, these being liquids that contain both primary and secondary amine groups. For high temperature use, diaminodiphenyl sulfone is commonly used and for latent cure, dicyandiamide. The cycloaliphatics are typically isophorone diamine, 1, 2-diamino cyclohexane and N-methyl piperazine. To reduce vapour pressure and odour, the low boiling polyamines are often adducted with diepoxides,

Structure 9.7 Curing of bis-epoxides with primary diamines

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Filled Thermosets which increases molecular weight. The polyamide curing agents are widely used and are typically the reaction products of stoichiometric excess of diethylene triamine with dimer acid, the latter being the Diels-Alder adduct of 9,11- and 9,12-linoleic acids. Because of the diversity of amines and polyepoxide resins, it is necessary to determine an appropriate stoichiometry for each resin/co-reactant pair if good cure is to be obtained. Provided etherification side reactions are not considered, the stoichiometry can be derived by equating the weight of polyamine containing one active hydrogen to the weight of resin containing one epoxide group. Where commercial mixtures are supplied, these values are normally given by the manufacturer. Species containing acidic hydrogen, polyfunctional carboxylic acids and phenols are commonly used, and these will react with epoxide groups. A difunctional carboxylic acid reacting with a diepoxide produces a linear hydroxy functional polyester. Crosslinking is through the independent etherification reaction, itself catalysed by acid. In the absence of etherification, monofunctional acids would only produce trimers with diepoxide. In acid conditions, protonation of the epoxide facilitates etherification through reaction, either with hydroxyl groups or a second epoxide (Structure 9.8). If the desire is to suppress etherification, basic catalysts can be used, which promote only the esterification reaction. With bisphenol A diepoxides containing hydroxyl groups, cyclic anhydrides from difunctional acids can be used as co-reactants. These produce crosslinked polymers directly without the need for independent etherification reactions, because the initial reaction is the opening of the anhydride group by the pendant hydroxyl group, which produces a carboxylic functional ester. This then reacts with an epoxide group to generate another hydroxyl group and so on (Structure 9.9). Curing by the reaction of epoxide with phenolic OH is usually done using a phenolic novolac. These are high-temperature cures and produce very highly crosslinked polymers due to the frequency of OH groups on the novolac.

Structure 9.8 Protonation of epoxide leading to etherification

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Particulate-Filled Polymer Composites

Structure 9.9 Curing of bis-epoxides with anhydrides

Thus, the family of epoxy resins, depending on how and by what they are cured, can generate polymers containing a variety of linking groups, ether, ester and secondary and tertiary amine, in addition to any functional linkages present in the initial resin and co-reactant. The reader will therefore appreciate that the cured polymers are certainly not polyepoxides.

9.2.3 Condensation Resins 9.2.3.1 Phenolic Resins Phenolic resins are the reaction products of formaldehyde (usually in the form of aqueous formaldehyde), with phenol. Sometimes alkyl phenols, such as cresol and xylenol, are incorporated to reduce crosslink density and brittleness, although reactivity is also reduced. Phenolic resins are offered in two forms, novolacs and resols, characterised by their different methods of manufacture and curing.

440

Filled Thermosets (a) Novolacs These are prepared using excess phenol and strong acid catalysts, and are essentially linear or lightly branched, low molecular weight thermoplastic resins. Formaldehyde:phenol molar ratios are typically 0.6-0.85:1. Aqueous formaldehyde can be considered to exist as methylene glycol and, in the presence of strong acid, the phenol nucleus undergoes electrophilic substitution at the 2 or 4 position by the protonated form of this glycol according to Structure 9.10. Other phenol nuclei can then undergo electrophilic substitution by the protonated methylol phenol and so on, the average molecular weight being determined by the starting formaldehyde to phenol ratio. A broad range of molecular weight distributions are obtained in this way. The low formaldehyde ratios give average molecular weights of a few hundred and ‘n’ about 1 in Structure 9.10. The higher formaldehyde ratios give average molecular weights of around 1000, with an average ‘n’ about 6-9 in Structure 9.10. At low molecular weights, branching is small because of the lower reactivity of the doubly substituted phenol units but, as molecular weight increases, branching does likewise. Strong acid catalysis gives a predominance of 2,4 linkages, and weaker acids, such as divalent metal acetates, give a preponderance of 2,2 coupling.

Structure 9.10 Preparation of novolac resins

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Particulate-Filled Polymer Composites (b) Resols Resols are produced using alkaline catalysts (typically sodium hydroxide) and an excess of formaldehyde, typical formaldehyde:phenol molar ratios being 1.2 to 3:1. The first step in the reaction sequence is the production of a mixture of methylolated phenols. These then undergo partial condensation to higher molecular weight structures, which possess either dimethylene ether links, via elimination of water, or methylene links, via further elimination of formaldehyde at higher temperatures. Liquid resols have a low level of condensation and average less than two benzene rings per molecule; solid resols are more condensed and average 3-4 benzene rings. (c) Curing of phenolic resins Resols possess a plurality of methylol groups through which crosslinking during curing primarily occurs. In contrast, novolacs are cured through reaction of the vacant ortho and para positions on the phenolic nuclei and require a co-reactant. Resols are the true thermosets and cure is effected by heat, which causes a continuation of the manufacturing process. Heating at about 180 °C at alkaline pH condenses the remaining methylol groups to form initially ether links, followed by elimination of formaldehyde and generation of methylene links (Structure 9.11a). Room-temperature curing of liquid resols is carried out with strong acid catalysts, e.g., p-toluene sulfonic acid. Cure then effectively mimics novolac formation, i.e., protonated methylols substitute at free ortho and para positions on phenol nuclei (Structure 9.11b). Thus, either cure mechanism involves elimination of water but further methylol condensation can also involve formaldehyde elimination, although the liberated formaldehyde can itself then enter into crosslinking reactions. This elimination of volatiles can be a problem in closed mould processing, as it could be as high as 14 wt% and formulations have to be chosen to minimise it. So the final crosslinked polymer could look something like Structure 9.12, a mixture of methylene links, ether links and some unreacted methylol groups.

Structure 9.11(a) Heat cure of resol

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Filled Thermosets

Structure 9.12 Cured crosslinked resol

Structure 9.11(b) Acid cure of resol

Novolacs are cured by reaction with a co-reactant to supply latent methylene links. Hexamethylene tetramine, the reaction product of formaldehyde with ammonia, (CH2)6 . N4, at about 5-15 wt% is normally used. Curing occurs via a reaction at free

443

Particulate-Filled Polymer Composites ortho and para positions on phenolic nuclei with elimination of ammonia and the cured polymer is a mixture of methylene and secondary amine links. Because of their high crosslink density and very brittle behaviour, phenolic moulding compounds are invariably filled with reinforcing fillers such as wood flour, wollastonite, mica, mineral wool flour and glass fibres. Solid moulding compounds are normally novolac based; resins for transfer moulding (RTM) and other liquid laminating processes are resols.

9.3 Mechanical Properties While many different mechanical properties may be measured for polymer composites, the description of the mechanical behaviour is afforded by the critical stress intensity factor, Kc, the fracture energy, Gc, Young’s modulus, E, yield stress, σy , together with a knowledge of failure stress and fatigue behaviour. These properties are explained further in this section.

9.3.1 Modulus It was observed empirically by Hooke that, for many materials under low strain, stress is proportional to strain. Young’s modulus may then be defined as the ratio of stress to strain for a material under uniaxial tension or compression, but it should be noted that not all materials (and this includes polymers) obey Hooke’s law rigorously. This is particularly so at high values of strain but this section only considers the linear portion of the stress-strain curve. Clearly, reality is more complicated than described previously because the application of stress in one direction on a body results in a strain, not only in that direction, but in the two orthogonal directions also. Thus, a sample subjected to uniaxial tension increases in length, but it also becomes narrower and thinner. This quickly leads the student into tensors and is beyond the scope of this chapter. The subject is discussed elsewhere [21-23]. There are four elastic constants usually used to describe a macroscopically isotropic material. These are Young’s modulus, E, shear modulus, G, bulk modulus, K, and Poisson’s ratio, v. They are defined in Figure 9.2 and they are related by Equations 9.1-9.3. E = 2G(1 + ν)

(9.1)

E = 3K(1 − 2ν)

(9.2)

(3K − 2G) 2(3K + G)

(9.3)

ν=

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Filled Thermosets

Figure 9.2 Definition of E, G, K and v

It must also be remembered that, because polymers are viscoelastic materials, the measured value of modulus is dependent on strain rate. The following discussion concentrates on Young’s modulus (often measured under three-point bend), but is equally applicable to any of the other elastic constants. The introduction of a rigid particulate filler into a polymer matrix results in an increase in Young’s modulus. This is shown in Figure 9.3 for (i) a urethane methacrylate polymer containing no polyol soft-block and (ii) a copolymer of methyl methacrylate (60 wt%) and an ethoxylated bisphenol A dimethacrylate (40 wt%). Both polymers are filled with silica sand. Note that the gradient of the curve increases as filler volume fraction increases. This behaviour is typical of that seen for many other systems, for example various epoxies filled with silica sand [24], glass spheres [25-28] and polyester resins filled with glass spheres [25]. For unidirectional, continuous fibre composites, relationships are well developed which predict modulus from a knowledge of the fibre modulus and volume fraction and matrix modulus. However, the situation for particulate composites is more complex, because variations in the stress and strain fields are introduced by the discontinuous nature of the

445

Particulate-Filled Polymer Composites

Figure 9.3 Graph of Young’s modulus against filler volume fraction for two acrylic polymers

reinforcement. Several models have been reviewed in detail by McGee and McCullough [29]. The modulus of a composite can, in principle, be determined from the corresponding volume fraction and modulus of the components. In practice, either assumption must be made to simplify the analysis, or else the analysis must carried out for a special case. The authors categorise the various approaches by the nature of the assumptions made: 1. Mechanics of materials approach 2. Embedding or self-consistent field approach 3. Bounding approach All the approaches assume that the phases are in direct contact (but not necessarily chemically bonded) to allow stress transfer and attention is directed towards overall response of the material to loads or deformation. The ‘mechanics of materials’ approach is the simplest and least useful. It assumes that each phase is subjected either to the same strain (the Voigt model; Equation 9.4) or the same stress (the Reuss model; Equation 9.5). This yields the relationships: Pc = VmPm + Vf Pf

446

Voigt

(9.4)

Filled Thermosets where P = K or G (E may then be calculated from Equations 9.1 or 9.2) 1 ⎛ Vm ⎞ ⎛ Vf ⎞ =⎜ ⎟ +⎜ ⎟ E c ⎝ E m ⎠ ⎝ Ef ⎠

Reuss

(9.5)

The Voigt model results in a linear relationship between Ec and Vf and gives a gross over-estimate of Ec while the Reuss model gives a reasonably accurate estimate of Ec for Vf < 0.3. The Voigt and Reuss models are given in Figure 9.4 with data for a urethane methacrylate polymer (containing no soft-block) filled with silica sand.

Figure 9.4 Comparison of Voigt and Reuss models with experimental data for a urethane methacrylate polymer (no polyol soft-block) filled with silica sand

The ‘self-consistent field’ approach models the microstructure of the composite in order to deal more realistically with the internal distribution of stress and strain. The various treatments in this category have been brought together by Wu and McCullough [30]. The treatment relates to an arbitrary reference material and encompasses a bounding approach. The lower bound of calculated modulus refers to a dispersion of filler particles in a continuous matrix of polymer and the upper bound refers to inclusions of polymer in a continuous phase that has the properties of the filler. The approach is further modified in reference [29] and the derived equations which relate modulus to Vf are shown next: Pc = Pm

(1 + ξ pl χ plVf ) (1 − ψχ plVf )

where P = K or G and

447

Particulate-Filled Polymer Composites (Pf − Pm ) where χ pl = (P + ξ P ) f pl m ⎛ VfVm (1 − γVm )(Pf − Pm )(ξ pu − ξ pl ) ⎞ ψ = 1+ ⎜ ⎟ Pf + ξ pu (Vf Pf + VmPm ) ⎠ ⎝

γ=

2v c − 1 vc

vc is a critical volume fraction at which there is a sharp change in phase continuity between continuous matrix phase and continuous filler phase. ξpu and ξpl are given by: property

ξpu

ξpl

K

2(1 − 2νf )Kf (1 + νf )Km

2(1 − 2ν m ) (1 + ν m )

and G

(7 − 5νf )Gf (8 − 10νf )Gm

and (7 − 5ν m ) (8 − 10ν m )

where vf and vm are the Poisson’s ratios for the filler and matrix, respectively. E may then be calculated from the calculated values of K and G for the composite. Despite the complexity of this model and the difficulty in readily assigning physical significance to all the parameters, it does nevertheless model well the relationship between E and Vf, as is shown in Figure 9.5, where our data for the urethane methacrylate polymer (containing no soft-block) filled with silica sand are plotted together with the predicted curve. Guild and Young [31] have developed a finite element model to predict the modulus of an epoxy resin filled with glass spheres. Monodisperse spherical particles are assumed to be dispersed randomly in an infinite matrix and the authors consider a cylinder of resin containing a single particle. The cylinder is represented by a plane with one corner at the centre of the spherical particle and of sides equal to the half height of the cylinder and to its radius. Finite element analysis is carried out on a one-radian segment which is considered to be deformed along two edges so that the deformed segment retains parallel sides. The analysis was repeated for various ratios of cylinder radius to particle radius. In order to relate the result to a specific filler volume fraction, the analysis must be carried out on a grid, the dimensions of which relate to that volume fraction. This

448

Filled Thermosets

Figure 9.5 Comparison of data for a urethane methacrylate polymer (no polyol softblock) filled with silica sand, with the model of McGee and McCullough

takes into account the fact that interparticle distances are variable in a real material. The model gives rise to two values of modulus, an upper bound that assumes equal strain in the cylinders and a lower bound that assumes equal stress. These bounds are much closer together than those in the previously described models. Modulus was calculated by the authors for a range of Vf from zero to 50% and the theoretical predictions compared very favourably with the experimental data. In order to determine the modulus of composites with different values of Em and εf, the whole analysis has to be repeated and therefore, regrettably, the predictive value of this approach is limited.

9.3.1.1 The Effect of Strain Rate This was studied by Spanoudakis and Young [28] for an epoxy resin (Epikote 828) hardened with tetraethylene pentamine and filled with glass ballotini. These authors found that for the unfilled polymer, Young’s modulus (measured in flexure) increased by 9% from 3.5 GPa to 3.8 GPa over two decades of strain rate. As Vf was increased then the strain rate dependence increased so that for Vf = 0.46, Young’s modulus increased from 10.8 GPa to 12.4 GPa over the same two decades of strain rate. This increase is 15%. The authors found the same trend for all the filler particle sizes studied. No explanation of this behaviour was offered, but it was noted that this was an area warranting further investigation. It is not expected that the presence of a large area of bonded filler surface should reduce segmental mobility within the polymer matrix,

449

Particulate-Filled Polymer Composites

9.3.1.2 The Effect of Particle Size The same authors studied the effect of filler particle size on Young’s modulus (again measured in flexure) for the same epoxy resin filled with monodispersed glass ballotini. It was found that, particularly at high Vf (> 0.2), E decreased linearly with increasing filler particle size, d. Thus at Vf = 0.46, E was 11.3 GPa for a particle size of 4.5 μm and E fell to 10.1 GPa for a particle size of 62 μm. The authors attribute this fall to a ‘skin’ effect, which was first described by Lewis and Nielsen [32]. In flexural measurements, the properties of the surface dominate the magnitude of the property that is measured. For large particles it is argued that the skin is more depleted of filler than is the case for small particles. If this is the case, then composites containing the smaller particles will have a higher modulus than those containing larger particles, at the same volume fraction. If this explanation is correct then the particle size effect is, in truth, merely an artefact of the flexural experiment and would not be observed either for specimens where the surface layer had been abraded away prior to measurement or for modulus measured in a tensile experiment.

9.3.2 Fracture Toughness and Fracture Energy The failure stress of a material depends not only upon its structure, but also on the size of the largest flaw it contains. Failure stress is, thus, not a material property and it is not valid therefore to compare the inherent integrity of materials on this basis. Such a comparison is provided by fracture mechanics with the parameters Kc and Gc. The stress intensity factor, K, was introduced by Irwin [33] who derived a model for the stress field round a crack tip. In this model, K relates the magnitude of the stress at a point near the crack tip to the applied stress and the geometry of the sample. K thus characterises the intensity of the stress field around the crack tip. As applied stress is increased, so K increases until it reaches a critical value, Kc, at which crack propagation begins. Kc is the critical stress intensity factor or fracture toughness and is a measure of a material’s ability to resist crack propagation. Clearly, the stress field depends on the direction of the applied stress relative to the crack. For the case where the stress is pulling the crack open (that is in a tensile experiment), the loading is termed mode I, the critical stress intensity factor is given this subscript and is written KIC. Kc is a material property and the units of Kc are MPa.m1/2. The bigger its value then the more resistant is the material to crack propagation. The value of KIC for poly(methyl methacrylate) (PMMA) is about 1 MPa.m1/2 while brittle thermosets, for example some epoxies, may have values of KIC below 0.5 MPa.m1/2. Polycarbonate has a value of KIC of about 2 MPa.m1/2. Gc is defined as the energy required to form 1 m2 of new surface. It is related approximately to Kc through the equation:

450

Filled Thermosets

Gc =

K2c E

where E is Young’s modulus. The value of GIC for PMMA is 500 J/m2, that of a brittle thermoset is typically 100 J/m2 and GIC for polycarbonate is 1100 J/m2. Note that, just because a material has a high value of KIC, it does not follow that GIC is also high. Certain ceramics have high values of KIC but low values of GIC. For example, partially stabilised zirconia has a KIC of 6 MPa.m1/ 2 but a GIC of only 180 J/m2 [34]. In order to derive a meaningful comparison between materials it is necessary to consider both KIC and GIC (in addition to σy and E). There are many methods available for measuring KIC and GIC of which the more popular are three point bend [35], compact tension [36] and double torsion [37-39]. The threepoint bend method is a European standard. It is relatively straightforward to carry out and requires only small samples. This geometry does not, however, allow crack propagation to be studied, because once the crack starts to propagate, it accelerates to the end of the sample. Compact tension and double torsion geometries allow crack propagation and the effect of crack propagation rate to be investigated. In order to measure a meaningful value of KIC it is essential to have a sharp crack and the dimensions of the specimen are also important. A more detailed discussion of the principles of fracture mechanics is beyond the scope of this chapter, but the reader is directed to [21, 22, 40].

9.3.2.1 Observations The addition of a rigid particulate filler to a brittle thermoset matrix significantly affects KIC and GIC. While general statements may be made regarding these effects exceptions will be found and the theories to explain the effects are still being developed. (1) As filler volume fraction, Vf , increases, then KIC increases, see Figure 9.6. Thus, perhaps contrary to expectation, the addition of a rigid filler to a thermoset makes it more resistant to crack growth. (2) As Vf increases, then GIC passes through a maximum, see Figure 9.7. Note that this statement is not in contradiction to (1) because GIC is related to KIC by E, which also increases with filler volume fraction, (G = K2Ic /E). Once again, contrary to expectation, the addition of a rigid filler results in a composite that is usually tougher than the thermoset matrix. (3) The maximum in the plot of GIC against Vf moves to higher Vf as mean filler particle size decreases.

451

Particulate-Filled Polymer Composites

Figure 9.6 Effect of filler volume fraction on KIc

Figure 9.7 Effect of filler volume fraction on GIc

(4) For Vf in a broad range around the GIC maximum, crack growth is not continuous in samples that are subjected to a continuous cross-head displacement test, but instead propagation is observed, see Figure 9.8. This behaviour is usually observed in unfilled polymers of low yield stress and therefore in materials where plastic flow may account for crack-tip blunting. While the mechanism may be different, crack-tip blunting is also believed to be responsible for stick-slip propagation in particulate composites, see Section 9.3.2.5. Briefly, in the arrest condition, the crack tip is blunt relative to that during propagation and so the crack overloads until the stress is high enough to initiate propagation. Once this occurs the crack tip becomes sharper and a rapid release in strain energy occurs as the crack propagates rapidly. The applied stress soon falls below the value critical for crack propagation however, at which point the crack arrests and the tip then blunts by whatever blunting mechanism is operating. This condition remains until the stress once more builds up to a value at which the

452

Filled Thermosets

Figure 9.8 Slip-stick propagation

blunted crack will propagate and the cycle is then repeated. Consequently, a sawtooth curve of load against crosshead displacement is observed. This behaviour is known as stick-slip propagation, see Figure 9.8. (5) The degree of adhesion between filler and matrix affects GIC and KIC, but improving adhesion may increase these parameters or decrease them depending on the matrix and the rigid filler. (6) The incorporation of a rigid filler into a tougher matrix leads to a smaller percentage increase in KIC than is observed when a less tough matrix is filled.

9.3.2.2 Effect of Vf on KIC and GIC Figure 9.9 is a plot of KIC against Vf for two composites: •

a copolymer of methyl methacrylate (60 wt%) and an ethoxylated bisphenol A dimethacrylate (40 wt%) filled with silica.

•

a urethane methacrylate polymer, which contains no soft-block, filled with silica.

The silica is well bonded to the matrix using 0.3 wt% (on filler) of a silane coupling agent (γ-methacryloxypropyl (trimethoxy) silane) for both sets of composites. Figure 9.10 is a plot of GIC against Vf for the same composites. In both cases there is a maximum in GIC at about a Vf of 0.1, but for all filled samples GIC is as high, or higher, than the value for the unfilled matrix. It was also found that for samples with Vf of 0.1,

453

Particulate-Filled Polymer Composites

Figure 9.9 Plot of KIc against Vf for two acrylic composites

Figure 9.10 Plot of GIc against Vf for two acrylic composites

stick-slip crack propagation occurred, whereas for all other volume fractions of filler and for the unfilled polymers, crack propagation was continuous. These observations have been observed for composites of several other thermosets and fillers. Lange and Radford [41] studied an epoxy (ERL 2774; Union Carbide Co.) filled with aluminium hydroxide and, indeed, were the first to report the toughening effect of a rigid particulate filler in a polymer matrix. Brown [42] investigated an unsaturated polyester

454

Filled Thermosets (Crystic 191MV; Monsanto) and a range of fillers. Mallick and Broutman [26] studied an epoxy resin (Epon 828; Shell Chemicals) cured with 14 phr m-phenylene diamine) and Broutman and Sahu [43] studied this system and an unsaturated polyester (Co Rezyn1808-2; Interplastic Corp.); both thermosets were filled with 45 μm diameter glass beads. All the composites in these studies exhibited the type of behaviour shown for the acrylic composites in Figures 9.9 and 9.10. Perhaps the most comprehensive study to date is that of Spanoudakis and Young [28, 44]. These authors also used an epoxy resin (Epikote 828 cured with tetraethylene pentamine) filled with glass beads and they too observed a maximum in the plot of GIC against Vf. More recent work on both thermoplastic and epoxy matrices by Pukansky and Maurer [45] and epoxy materials by Kim and Khamis [46], Hussain and co-workers [47] and Lee and Yee [48] all show the same trend. Thus a wide variety of thermosets filled with rigid particulates of different morphology, size and chemistry all show the same general behaviour.

9.3.2.3 The Crack Pinning Model The increase in fracture energy of a brittle matrix by a rigid particulate filler was first discussed by Lange [49] and his theory was later developed by Evans [50] and by Green [51, 52]. The theory and its developments have been reviewed [22, 28]. From a study of the fracture surfaces of magnesium oxide crystals containing small voids, Lange observed that the crack front became pinned when it met these voids. In order to propagate further, he proposed that the crack must bow out between the voids, while remaining pinned, until eventually it is able to break away and continue. This bowing increases the length of the crack (Figure 9.11) and, Lange proposed that there was an extra energy term associated with this increase in crack length. He derived the expression: Gc (c ) = Gc (m) + 2

T D

where: Gc(c) is the fracture energy of the composite Gc(m) is the fracture energy of the matrix T is the line energy per unit length of crack D is the interparticle spacing Lange considered several other possible mechanisms to account for the increase in GIC: (1) The fracture surfaces are not smooth and so an increase in fracture energy associated with increased roughness should be taken into account.

455

Particulate-Filled Polymer Composites

Figure 9.11 Crack pinning in a particulate composite

(2) Energy may be absorbed by the particles through plastic deformation. (3) Friction between the parting fracture surfaces can absorb energy. The contributions from (1) and (3) were calculated to be extremely small, while (2) is extremely unlikely for inorganic particles. Spanoudakis and Young [28] and Green and co-workers [52] have also considered crack-tip blunting as a mechanism of increasing fracture energy and, as will be shown later, this does occur. Nevertheless, it is generally agreed that the major contribution to the increase in fracture energy, in particulate composites, is from crack pinning. Lange’s model correctly predicts that Gc(c) increases with filler volume fraction (that is as D decreases), but it does not explain why Gc(c) is often observed to pass through a maximum at higher values of Vf.

9.3.2.4 The Effect of Filler Particle Size Further limitations to Lange’s model are revealed when the effect of filler particle size on Gc(c) is considered. Lange himself [53] measured Gc(c) against Vf for three series of sodium borosilicate glass/alumina composites in which the average alumina particle size was 3.5 μm, 11 μm and 44 μm, respectively. For any given value of Vf, Gc(c) increased

456

Filled Thermosets with particle size, as predicted by the model (D decreases as particle size decreases). However, Lange then plotted Gc(c) against 1/D. Each series gave the expected linear relationship, but the slopes were different (the larger the particle size then the larger the gradient), implying that each particle had its own associated line energy. Mallick and Broutman [26] observed similar behaviour in their epoxy/glass bead composites as have Spanoudakis and Young [28] for low values of 1/D. Similarly, Lange and Radford [41] reported this behaviour for epoxy/alumina trihydrate composites. Lange introduced a dimensionless parameter F(D) to account for these experimental results: Gc (c ) = Gc (m) + 2

T D

but this parameter has dubious physical significance. Building on this work Evans [50] and Green and co-workers [51, 52] carried out extensive theoretical analyses of crack pinning, taking into account particle shape, particle penetration by the crack front, secondary crack shape and secondary crack interactions. These studies have proven that the contribution of the line energy to Gc(c) is indeed dependent, not only on interparticle spacing, but also on particle size. Using Lange’s results [53], Evans showed that data for the different composites all fell on a theoretically derived curve of Gc(c) against r/D, where r is the particle radius. Furthermore, a similar treatment of Lange and Radford’s data for alumina trihydrate/epoxy composites [41] also showed that the data fitted the theoretically derived curve at low values of r/D. However, Gc(c) fell below the theoretical line at higher r/D values. Penetration of filler particles by the crack front was the explanation of this phenomenon and alumina trihydrate is susceptible to cleavage (see later discussion in Section 9.3.3). Thus, toughening by crack pinning is much less effective when the filler particles can be penetrated or cleaved by the advancing crack front. What can be concluded is that the major factors in determining the fracture toughness and fracture energy are the properties of the matrix material, the decrease in volume of matrix material due to the introduction of particulates and the interaction between the matrix and particulate. In the case of rigid particulates, the stress field ahead of a crack generally results in debonding, which leads to significant plastic deformation of the matrix material. A model proposed by Pukansky and Maurer [45] assumes that property changes due to interaction are proportional to the actual value of the given property and arrived at a relationship in the form: Gc =

1− ϕ Gc0 . exp(Bf ϕ) E / E0 1 + 2.5ϕ

They got very good correlations with experimental data from a range of composites, including some reanalysis of earlier data from Spanoudakis and Young [28, 44].

457

Particulate-Filled Polymer Composites In summary, the crack pinning model, as described so far, explains why fracture energy increases with filler volume fraction and it accounts for the particle size effect. In the next section a rationalisation of the maxima in the plots of GIC (c) against Vf is presented.

9.3.2.5 Extensions to the Crack Pinning Model and the Effect of Particle Matrix Adhesion To date, the most comprehensive and systematic experimental study of rigid particulate composites is that of Spanoudakis and Young [28, 44]. These authors have studied the effect of crack propagation rate, particle volume fraction, particle size and particle matrix adhesion on KIC, Young’s modulus and fracture energy. They point out that previous studies have concentrated on the dependence of GIC (c) rather than KIC (c) on Vf and the maxima observed have been ascribed to changes in failure mechanism. This can be misleading, however, because Gc(c) is a function of both KIC(c) and composite modulus, Ec. Using values of KIC (c)/KIC (m) predicted from Green’s analysis [52] and values of Ec from Ishai and Cohen [24], Spanoudakis and Young were able to show that maxima in GIC (c) versus Vf could be demonstrated solely from the crack pinning model, without invoking a change in crack propagation mechanism. The crack pinning model would now appear to explain all of the phenomena observed for the behaviour of particulate filled composites. Not surprisingly, reality is a little more complicated than this. For many samples Spanoudakis and Young observed stick-slip propagation, as was described for the acrylic composites previously and where this occurred the KIC of initiation was greater than the arrest value, or the continuous propagation value usually seen at the highest measured crack propagation rates. The authors point out that stick-slip propagation is associated with crack-tip blunting in certain polymers and their analysis concluded that it was probably occurring in this case. Thus, a secondary toughening mechanism may also operate in certain ranges of Vf. Particle-matrix adhesion further complicated the situation. While KIC (c) was relatively unaffected by the degree of adhesion, Ec was smaller in composites where adhesion was poor compared with composites where adhesion was good. This then generally resulted in GIC (c) being higher in composites where poor particle-matrix bonding occurred. Nevertheless, scrutiny of the authors’ data does reveal compositions where the reverse is true and, once again, the secondary toughening mechanism of crack-tip blunting is superimposed on all of these data. Lastly, Spanoudakis and Young also observed that, as adhesion improved between filler particle and matrix, then the position of maximum stress moves from the particle’s equator (no bonding) towards its poles. The result is that fracture surfaces of well-bonded composites show less evidence of filler particles because the crack front is deflected towards the poles and often remains within the polymer. This too may reduce the effectiveness of the crack pinning mechanism.

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Filled Thermosets

Figure 9.12 Graph of KIC against coupling agent concentration for an acrylic composite

Figure 9.13 Graph of GIC against coupling agent concentration for an acrylic composite

Thus, while the basic mechanism of toughening of thermosets by rigid particles is well established, it is clear that further work is required before a comprehensive understanding of the subject is gained. As further illustration of this consider Figures 9.12 and 9.13, which are plots of KIC (c) and GIC (c) against coupling agent concentration for composites of silica filled (60 wt%) PMMA made in our laboratory. Good adhesion results in a 50% increase in KIC and a 100% increase in GIC. These results are different from those reported by Spanoudakis and Young and suggest that good adhesion is extremely beneficial for these composites. All of these samples exhibited continuous crack propagation. It is not clear from the foregoing arguments if good adhesion merely promotes debonding at a higher stress thus resulting in massive shear yielding. The same behaviour has been observed in rubber-toughened epoxies, where the common argument centres on the role of cavitation as detailed in a review by Bagheri and Pearson [54]. Some argue that cavitation within rubber particles precedes localised

459

Particulate-Filled Polymer Composites shear banding and is the cause of the localised deformation. Huang and Kinloch [55, 56] have found from finite element analysis that cavitation can occur before or after shear banding particularly if particles have a high modulus and low Poisson’s ratio. It seems reasonable to suppose that once a critical strain is reached in the matrix then localised shear banding will occur regardless of cavitation in the rubber particles. Bagheri and Pearson [57] in a study comparing rubber modifiers and hollow particles, showed however that cavitation resistance of the rubbery phase does not increase toughness in the epoxy matrix.

9.3.2.6 The Effect of Polymer Fracture Toughness We have found that the ratio KIC(c)/KIC(m), for a given value of Vf, is dependent on the inherent matrix brittleness. For example consider the data for three sets of acrylic composites each containing 40 volume per cent silica sand (mean particle size 10 μm). These data are summarised in Table 9.1.

Table 9.1 Effect of polymer fracture toughness on composite fracture toughness Polymer matrix

KIC(m) (MPa.m1/2)

KIC(c) KIC(c)/KIc(m) (MPa.m1/2)

Urethane methacrylate (no soft-block)

0.55

1.15

2.10

Ethoxylated bisphenol A/MMA copolymer

0.65

1.30

2.00

Urethane methacrylate (soft-block)

1.05

1.50

1.30

The urethane methacrylate polymer which contains no soft-block and the ethoxylated bisphenol A/MMA copolymer both have relatively low values of KIC and their composites have fracture toughnesses approximately double that of the respective polymer. The urethane methacrylate polymer which contains a polyol soft-block, on the other hand, has a fracture toughness which is approximately twice that of the other polymers, but for the composite KIC is increased only modestly. Thus, the higher the fracture toughness of the polymer then the less is the fracture toughness of the particulate composite enhanced relative to it. Recently we have begun a programme of work on similar urethane methacrylate polymers, but have used glass ballotini coupled into the matrix with γ-methacryloxypropyl (trimethoxy) silane. Values of KIC were measured by double torsion for composites of Vf = 0.4. The results are shown in Figure 9.14. For the urethane methacrylate polymer (containing no soft-block) filled with ballotini of mean particle size 10 μm, KIC(c)/KIC(m) is 2.1, the same

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Filled Thermosets

Figure 9.14 Plot of KIC against crosshead speed for various acrylic composites

value as that obtained for composites of the same polymer and silica sand of mean particle size 10 μm. For the less notch sensitive urethane methacrylate polymer which contains a polyol soft block and is filled with the same ballotini, KIC(c)/KIC(m) is only 1.1. When a grade of ballotini with a larger particle size range (50-100 μm) was used then stick-slip propagation occurred and high values of KIC at the initiation of crack growth (KICi) were 461

Particulate-Filled Polymer Composites observed. However, if the methodology of Spanoudakis and Young is followed and if the extrapolated continuous crack propagation value of KIC(c) (1.15 MPa.m1/2) is taken, then once again KIC(c)/KIC(m) is 1.1, but clearly the larger filler particles are hindering crack growth. Our preliminary observation is that when the filler particles are smaller than the calculated critical crack opening displacement of the polymer, then KIC(c)/KIC (m) is unity or slightly greater. When the filler particles are larger than the critical crack opening displacement, however, KIC(c)/KIC(m) may be significantly increased (> 2), if KICi is used. One of the major problems associated with much of the work carried out to show the effect of polymer matrix toughness on the composite toughness, is that the polymers used often have very different chemical structures, which may lead to differences in fracture behaviour. One study by Lee and Yee [48] used a series of glass bead filled epoxies to investigate the effect of inherent matrix toughness on fracture behaviour. The matrix toughness was varied by increasing the molecular weight of epoxides, which in turn decreased the crosslink density, increasing the inherent matrix toughness. As expected the fracture toughness of the filled epoxies increased as the inherent matrix toughness increased. However, the toughening effect was also seen to increase and the effect of glass beads was therefore more effective in tougher epoxide matrices. From microscopy studies of the fractures they found that a number of micro-deformation mechanisms occurred during the fracture of the filled epoxies. These were classified as step formation, debonding, diffuse matrix shear yielding and micro shear banding. At the higher inherent matrix toughness, diffuse matrix shear yielding and micro shear banding were found to be the most prevalent toughening mechanism. It was also concluded that localised shear yielding is the major energy absorption mechanism for both filled and unfilled epoxies.

9.3.2.7 Ductile to Brittle Transition It is generally accepted that as the thickness of a fracture specimen is increased, the measured toughness will reduce. This effect can be seen both in toughened and untoughened blends and is generally considered to be due to a change in dominant stress state from plane stress at small thickness to plane strain at larger thickness. A similar transition has been observed by Bagheri and Pearson in rubber-modified and micro-void toughened epoxies where the toughness reduces with increasing inter-particle distance [57]. They studied the effect of a wide range of particle size and spacings and concluded that the occurrence of particle cavitation relieves plane strain constraint from the matrix allowing plastic deformation to occur locally. Reducing the interparticle distance on a range of particle sizes from 0.2 to 40 μm increased the measured toughness in the epoxy, the effect being consistent for both rubber and micro-void toughening spacings of 0.2 to 50 μm. These spacings are all comfortably less than the plane strain plastic zone sizes at these toughnesses (1 to 2 MPa.m1/2) which are in the range 20 to 80 μm. The effect of

462

Filled Thermosets particle size was found to induce the same amount of deformation in the matrix at larger interparticle distances. The work showed clearly that the transition in toughened epoxies does not occur at a critical interparticle spacing and that increasing the particle size shifts the transition to larger interparticle spacings.

9.3.3 Failure Stress Failure in polymers may occur in a brittle or a ductile manner. Let us illustrate this using PMMA as an example, for which the tensile yield stress is about 90 MPa and KIC is about 1.0 MPa.m1/2. For a specimen containing a through crack, failure stress, σum, is related to KIC by the equation based on Griffith’s theory: 1

σ um

⎛ K2 ⎞ 2 = ⎜ IC ⎟ ⎝ πa ⎠

(strictly this is only true for an infinite plate)

If σum is calculated for different values of a, we obtain the plot shown in Figure 9.15.

Figure 9.15 Plot illustrating the concept of inherent flaw size

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Particulate-Filled Polymer Composites Note that this intersects the yield stress at a value of 3 x 10-5 m or 30 μm and this point is termed the inherent flaw size. For cracks above 30 μm in size, PMMA breaks in a brittle manner, because the overall stress in the polymer is too low to cause significant bulk yielding during the timescale of the experiment. However, if the flaws present in the sample (naturally occurring or deliberately introduced) are below 30 μm, then a sufficiently high stress may be applied to the sample that yielding will occur and any further applied strain will result in plastic flow and not in an increase in stress. In thermosets, brittle failure is usually observed because yield stresses are usually high (> 120 MPa). There are exceptions though, an example being the urethane methacrylate polymer which contains a polyol soft-block, for which the tensile yield stress is 56 MPa and the inherent flaw size is about 1 mm. In particulate composites the situation is considerably more complicated because there are two phases present, each with very different mechanical properties and, in addition, the interface between them may also be critical in determining the magnitude of the failure stress. Failure occurs almost exclusively in a brittle manner. It is usually observed that: (1) For composites with poorly bonded filler particles, where either no coupling agent has been employed, or indeed the filler has been pre-treated with a mould release agent, then failure stress decreases very significantly with filler volume fraction. (2) For composites with well-bonded filler particles the variation of failure stress on filler volume fraction depends greatly on the system concerned. For example stress may be independent of filler volume fraction, increase slightly with filler volume fraction or, more usually, exhibit a slight or moderate decrease followed by a recovery. These statements are illustrated in Figures 9.16 and 9.17 for two acrylic matrices one filled with silica and one with aluminium hydroxide. It has also been shown that epoxy composites [25, 58] and polyester composites [59] exhibit similar behaviour. These are the observations that require explanation. As a starting point, let it be assumed that failure is not initiated within a filler particle. This should usually be true because the failure stress of most mineral fillers is considerably larger than the polymer matrix. Exceptions may occur, however. For example, many layer minerals, including the fire retardant filler aluminium hydroxide, may be mechanically weak in the [001] direction and strongly aggregated particles may also be relatively mechanically weak. Figure 9.18 is a scanning electron micrograph of the two fracture surfaces of a mica-containing acrylic composite which has undergone tensile failure. The locus of failure is seen to be a mica particle and, since the particle appears on both failure surfaces, it may be inferred that its fracture initiated the failure of the composite. More usually, though, it is either the matrix or, in particular, the filler-matrix interface, from which failure initiates. The most comprehensive description of the failure of particulate composites is that of Leidner and Woodhams [59]. These authors consider a specimen of a particulate composite containing

464

Filled Thermosets

Figure 9.16 Graph of tensile failure stress against filler volume fraction for an acrylic composite

Figure 9.17 Graph of tensile failure stress against filler volume fraction for an acrylic composite

465

Particulate-Filled Polymer Composites

Figure 9.18 Scanning electron micrograph of the two fracture surfaces of a micacontaining acrylic thermoset that has undergone tensile failure. The mica particle appears on both fracture surfaces and is at the locus of failure

spherical filler particles under load. They base their approach on the theory for fibre-reinforced composites developed by Kelly [60] and Outwater [61] by treating the spherical filler particle as a series of cylinders. Where there is no adhesion between filler particles and the matrix it is argued that stress is transferred from the matrix to a filler particle by frictional forces only. The authors thus derive an expression for the average stress (σav) in a particle: σav = 0.83pα where: p is the pressure exerted by the matrix on the particle and α is the coefficient of friction. In our experience however, polymerisation shrinkage at high filler volume fractions will occur away from filler particles to leave voids around them and in this case no stress transfer from matrix to filler can occur.

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Filled Thermosets The maximum load carried by the filler phase at composite failure is then: σav = 0.83pαVf where: Vf is the volume fraction of filler. Since the filler particles are themselves stress concentrators, the load carried by the matrix (σm) at the failure point of the specimen is not simply the product of the failure stress of the unfilled polymer (σum) and its volume fraction (1 - Vf). Instead, the authors use the expression: σm = Kσum(1 - Vf) where K is the relative change in strength of the matrix due to the presence of the filler. Thus, their final expression for the ultimate strength of the composite (σuc), for the case of no filler-matrix adhesion, is: σuc = σb + σm = 0.83pαVf + Kσum(1 - Vf) This equation predicts: (1) σuc is a linear function of Vf. (2) σuc (as Vf approaches unity) is 0.83 pα and is independent of d. If we also consider Griffith’s theory then: (3)

for constant Vf, σuc is a linear function of d-1/2 where d is the filler particle diameter.

(4)

σuc (for Vf = 0) is equal to Kσum and depends on d.

In this case as Vf approaches zero, the failure of the polymer is governed, not by the inherent flaws it contains, but by the size of an isolated filler particle, if this acts as a larger flaw. As filler size decreases then σf increases according to the proportionality: σ f ∝ 1 d1 2

However, once d is smaller than the size of the inherent flaws in the polymer, σf becomes independent of filler particle size (Figure 9.15), which is not made clear in reference [59]. Over the range of Vf studied, Leidner and Woodhams confirmed predictions (1) and (2), above, for a polyester resin filled with spherical glass beads, although there

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Particulate-Filled Polymer Composites was considerable scatter in their data. It is found that for acrylic composites (Figures 9.16 and 9.17) σuc is not necessarily a linear function of Vf and this is also the case for epoxies [25, 58]. Leidner and Woodhams treatment assumes that σum is constant, independent of Vf and related, through Griffith’s equation, to the diameter of the largest filler particle. However, the flaw size, even in a well dispersed composite, may be much larger than the size of the individual filler particles, because of the finite probability that two or more particles will find themselves in close proximity to one another. On this basis, the size of the largest flaw increases with increasing Vf and this yields the concave plot shown in Figure 9.17. This behaviour was fitted mathematically by Nicolais and Nicodemo [58]. Leidner and Woodhams also considered the case of good adhesion between filler and matrix. Stress is transferred from matrix to filler in two ways: (1)

through shear stresses at the matrix-filler interface (σs).

(2)

through the tensile stress at the matrix-filler interface (σa).

When the filler loading is small they argue that the extra load placed on the matrix, when the matrix-filler bond fails, is small and so σb approximates to the case for no filler-matrix adhesion. At high filler loadings the authors derive the following expression for the load carried by the filler at composite failure: σb = σa + σs = (σa + 0.83τm)Vf where: τm is the shear stress of the matrix. The load carried by the matrix at failure is given by: σ m = σ aS(1 − Vf )

where: S is a stress concentration factor. The stress concentration around the filler particle means that the average tensile stress in the matrix is lower than the stress at the filler/matrix interface. Thus, the ultimate strength of the composite is given by: σuc = (σa + 0.83τm)Vf + σaS(1 - Vf)

high Vf

σuc = 0.83pαVb + Kσm(1 - Vf)

low Vf

This treatment predicts behaviour as shown in Figure 9.19 and this is indeed the behaviour shown in Figures 9.16 and 9.17 for acrylic composites and in [25, 58, 59]. 468

Filled Thermosets

Figure 9.19 Prediction of tensile strength against filler volume fraction (after Leidner and Woodhams)

9.3.4 Fatigue A material subjected to an applied stress, well below its failure stress under monotonic loading at conventional strain rates, is nevertheless often observed to fail after a certain period of time. This process is known as fatigue; if the loading has been constant, it is termed static fatigue or creep rupture and if the loading has been cyclical, it is termed dynamic fatigue. Note that there is no demarcation between monotonic loading and static fatigue experiments, hence the use of the term ‘conventional strain rates’ in the first sentence. There are many variables to be considered in the design of fatigue experiments and particularly so for dynamic fatigue experiments. The principal ones are: •

Specimen geometry

•

Specimen preparation

•

Wave-form, e.g., sine, square, saw-tooth

•

Maximum applied stress or strain

•

Minimum applied stress or strain (it does not have to be zero)

•

Frequency

•

Temperature

•

Environment

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Particulate-Filled Polymer Composites For brittle thermosets, parallel-sided dumb-bell or waisted dumb-bell specimens are preferred; simple parallel-sided specimens often fail in the jaws of the testing instrument. Moreover, in our experience, sample preparation is absolutely critical. Even minor imperfections and scratches on the surface of the specimen can lead to failure occurring in times several orders of magnitude earlier than expected. The result of a typical dynamic fatigue experiment is a plot of maximum applied stress or stress amplitude against the number of cycles to failure (an S-N curve) and these have been generated for many polymers for a wide range of the variables listed previously. For static fatigue experiments results are usually plotted in the form of applied stress against the logarithm of time to failure. Such plots are often, but not always, linear for unfilled polymers and once again a large body of data exists from work done on them. In all fatigue experiments, the relative importance of crack initiation and crack propagation to fatigue life is often unknown and indeed the former is often ignored. Crack growth is usually found to obey the Paris equation [62, 63]: da = A.ΔK n dN

where:da/dN is the crack growth per cycle ΔK is the applied stress intensity factor A and n are constants Experiments are usually conducted on specimens in a double-cantilever beam geometry. ΔK is calculated from a knowledge of specimen dimensions, maximum and minimum applied load and crack length. Crack growth rate may be measured with a travelling microscope or from the change in resistance of a thin metal film attached to one face of the specimen. A typical plot is shown in Figure 9.20 where it may be seen that crack growth rate is linear for a wide range of ΔK. Once again such curves have been generated for many unfilled polymers and for a wide range of experimental conditions. In static fatigue experiments the time to failure is also governed by the time required for a crack to initiate from a flaw, ti, and the time for the crack to propagate through the specimen tp. Once again, if initiation is ignored a specimen failure time may be derived from the Paris equation and this has been applied successfully by several authors. We may speculate on the physical transition from a flaw into a crack. For example a urethane methacrylate polymer containing sufficient polyol soft-block undergoes yield prior to failure in a tensile experiment. The fracture surface of these tensile specimens usually show no evidence that particulate inclusions are the flaws responsible for failure. Polymerisation of thermosets rarely occurs uniformly and we hypothesise that even in post-cured specimens microscopic regions will have different crosslink densities and therefore have different yield stresses. The regions of low yield stress, it is argued, will undergo yield first and thus lead to the

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Filled Thermosets

Figure 9.20 Typical Paris plot

formation of a crack. In particulate composites, even well bonded ones, the filler-matrix interface is usually the weakest feature and it may be envisaged that a poorly bonded area on a filler particle is the flaw which, under stress, leads to the generation of a crack. An extensive review of this broad area is beyond the scope of this chapter and the reader is referred to Kinloch and Young [22] and McCrum [40] for a fuller review. In the case of filled thermosets, relatively little work has been reported in the literature and of that which has, none has been carried out on identical compositions or under identical conditions. Examples are given in references [64-72] (dynamic fatigue) and references [27, 73, 74] (static fatigue). In the authors’ view it is too early for a robust model which describes fatigue in particulate filled thermosets to be presented from the existing literature. Individual studies give insight into specific aspects of one specific type of composite, but the area lacks a comprehensive and systematic study that will enable the individual pieces of the jigsaw to be assembled into a coherent whole and we are no exception! However, two aspects of fatigue in particulate composites from our work, which illustrate that this is indeed a challenging and rewarding area for study will be discussed next.

9.3.4.1 The Effect of Filler Particle Size on Dynamic Fatigue From the foregoing discussions it would be expected that any changes to the composite structure, apart from having an effect on static properties would also have some effect on

471

static properties Silica filled PMMA Particulate-Filled Polymer Composites

Table 9.2 Static properties of silica filled PMMA Volume Fraction

Tensile σ (MPa)

Modulus, E (MPa)

KIC (MPa.m1/2)

10

0.44

65

8875

1.5

350

0.48

38

8056

2.4

590

0.64

22

13449

1.9

SiO2 Particle Size (μm)

the fatigue life of a particulate composite. A recent study by Antunes and co-workers [75] has looked at both fatigue crack growth rates and total fatigue life in silica filled PMMA. Table 9.2 shows some static properties which were measured. The fatigue tests resulted in much lower fatigue crack growth rates in the composite materials compared to the bulk PMMA. The bigger particles gave a bigger improvement (reduction) in fatigue crack growth rate. The same was not true for the total fatigue life, with the bigger filler particles reducing the total fatigue life. This can be explained when considering that the total life is made up of both the crack growth and the number of cycles to initiate a crack. When compared with the static results a trend may be observed. The larger particles result in larger potential initial defect sizes, from flaws in the silica particles, which then grow to form cracks. In a tensile test these initiate failure at a lower applied stress. The fracture toughness is measured from a pre-notched specimen where the notch is much larger than the particle size and this shows a rising toughness with increasing particle size.

9.3.4.2 The Effect of Filler-Matrix Adhesion on Dynamic Fatigue It might also be expected that an improvement in bonding between filler and matrix would lead to an increase in the fatigue life of a particulate composite. Indeed this may be true as shown in Figure 9.21, where S-N curves obtained in flexural geometry are plotted for a methyl methacrylate/ethylene glycol dimethacrylate copolymer filled with 60 wt%. The data for curve (a) are derived from samples in which no coupling agent has been added to the formulation and those for curve (b) are for specimens prepared from a formulation containing 0.4 wt% (on filler) γ-methacryloxypropyl (trimethoxy) silane. Clearly, the addition of coupling agent has increased the failure stress of the composite and therefore at any specific value of stress, the fatigue life has also been increased. However, now consider the data plotted as applied stress as a fraction of failure stress against log cycles to failure (Figure 9.22). This plot contains data for four different coupling agent concentrations (including zero) and it may be seen that they all fall on the same master curve. Thus, in absolute terms, fatigue life is improved, but this improvement is only the result of an increase in failure stress. Better coupling for these composites does not affect the S-N curve in any fundamental way.

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Filled Thermosets

Figure 9.21 S-N curves for two acrylic composites, one containing coupling agent and one without coupling agent

Figure 9.22 Dry flexural fatigue of silica-filled methacrylate composites containing different concentrations of coupling agent

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Particulate-Filled Polymer Composites

9.3.4.3 The Effect of Water on Dynamic Fatigue Water and organic liquids can have a catastrophic effect on the fatigue life of polymers and particulate composites, with specimens failing several orders of magnitude earlier than for equivalent specimens tested dry. For example, consider the silica filled methacrylate composite containing no coupling agent, which was described in the previous section. Figure 9.23 shows two sets of data obtained from a flexural dynamic fatigue experiment, one set for the composite tested dry and the other for the composite tested under water at 20 °C. Clearly the effect of water is profound, for example at 50 MPa, fatigue life is reduced by three orders of magnitude. Now let us consider the effect of filler-matrix adhesion on dynamic fatigue life in an aqueous environment. Figure 9.24 is a plot of applied stress as a fraction of failure stress against number of cycles to failure for the four composites described above that contain 0-0.4 wt% γmethacryloxypropyl (trimethoxy) silane. The upper curve is the master curve for the dry specimens already shown in Figure 9.23. It may be seen that the fatigue life is moderately improved by the presence of > 0.1 wt% coupling agent and this improvement is genuine and not just related to an increase in failure stress over the uncoupled case. It is, however, only a modest one and still does not compare with the fatigue life of the dry specimens. Filler-matrix bonding with silane coupling agents is susceptible to hydrolysis [76] and therefore it is perhaps not surprising that such coupling chemistry is only so modestly effective in improving fatigue life. In the case of methacrylic thermosets, the filler-matrix

Figure 9.23 Wet and dry flexural fatigue for an acrylic composite

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Filled Thermosets

Figure 9.24 Wet flexural fatigue for silica-filled methacrylate composites containing different amounts of coupling agent

interface is but one factor, the matrix itself is also sensitive to water. This is illustrated in Figure 9.25, which shows the S-N curve obtained from a flexural geometry for an ethoxylated bisphenol A/MMA copolymer. It may be seen that even the unfilled polymer shows a catastrophic response to water. It should be emphasised that this discussion is specific to methacrylic thermosets and need not apply generally. As an example consider the S-N curves in Figure 9.26 for an epoxy matrix containing 50 wt% silica. These two sets of data were obtained for specimens in flexural fatigue, one set under dry conditions and the other set under water. It may be seen that for this system water has no measurable effect on fatigue life. This phenomenon may be explained by considering the action of a fluid on the surface of the specimen under water. If the wetting angle of the composite is large then the water will be able to enter the growing fatigue crack. The action of the water in the growing crack will cause hydraulic ‘jacking’ of the crack flanks as the flanks try to close during the unloading part of the fatigue cycle. This action will result in an increased fatigue crack growth rate. If however the wetting angle of the composite is small then water will not penetrate down the fatigue crack to the same degree and the additional hydraulic jacking will not occur.

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Particulate-Filled Polymer Composites

Figure 9.25 S-N curve for MMA/ethoxylated bisphenol A dimethacrylate co-polymer

Figure 9.26 Dynamic fatigue of an epoxy polymer filled with 50 wt% silica

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Filled Thermosets

9.4 Applications 9.4.1 Cost Reduction Reduced cost is a reason often quoted for adding filler to thermoset resins. However, the shape and size of a moulding, together with requirements for strength and stiffness dictate that the moulding should have a certain volume rather than a certain weight. So, although fillers are typically much cheaper by weight than resins, their higher density means that volume costs are not reduced by as great an extent. For example, let resin cost = x kg -1, density = 1.1 g ml-1 then volume cost = 1.1x litre-1. Let filler cost = 0.1x kg-1, density = 2.6 g ml-1, volume cost = 0.26x litre-1. Thus, a 50/50 w/w mixture would cost 0.55x kg-1 or 0.86x litre-1, i.e., a volume-cost reduction over the resin of only 22% compared with a weight-cost reduction of 45%.

9.4.2 Modified Mechanical Properties The modification of mechanical properties by incorporation of fillers has been extensively discussed in Section 9.3 and, even where this is not the primary aim, the filler being incorporated for some other purpose, the effect on mechanical performance is always of importance. Probably the two physical property changes for which fillers are commonly added are to increase modulus and reduce coefficient of thermal expansion. The increased modulus allows a thinner part, for example, for equivalent centre span deflection of a rectangular beam: 3

⎛ d′ ⎞ E 1 = ⎜ ⎟ = ⎝ d⎠ E′ 2

where d´ and E´ are the thickness and modulus of the filled polymer, d and E those of the unfilled polymer. If at a filler loading of 50/50 w/w an approximate doubling of modulus is assumed (from Section 9.3), then d´ = 0.79d, i.e., the volume of material falls to about 80%. So, if a criterion were equivalent deflection and other aspects of the design of the moulding permitted it, use of filler could permit a volume reduction with an attendant further cost reduction. Thus, cost in unfilled resin = 1.1x, cost in filled resin = 0.79 x 0.86x = 0.68x, i.e., a 38% cost reduction over unfilled resin. The potential for cost reduction therefore exists, but it is by no means universal and each resin/filler combination and each application has to be worked through separately. Coefficients of linear thermal expansion ‘a’, for typical crosslinked thermoset polymers over the temperature range 10-70 °C, (i.e., below the Tg), are in the range 60-100 ppm K-1. (Coefficients of volume expansion approximate to ‘3a’.) This range of values is high

477

Particulate-Filled Polymer Composites relative to metals, 12-28 ppm K-1 and to ceramics, 0.3-5 ppm K-1. If used to make large parts, the high ‘a’ of polymers can lead to excessive dimensional changes in normal use and, if polymers are used in conjunction with other materials in fabricating assemblies, differential stresses can be set up as a result of temperature changes. Fillers, having low expansion coefficients, typically < 6 ppm K-1, can be used to offset the high polymer values but, because their effect is dependent on volume rather than weight fraction, the magnitude of the decrease is modest. For example, on a ‘rule of mixtures’ basis, 33 vol% of filler (50 wt%) would bring the linear expansion coefficient down to 40-68 ppm K-1, still considerably higher than that of steel (15 ppm K-1) or aluminium (25 ppm K-1). The incorporation of filler does reduce the expansion anisotropy in glass-fibre laminates. The latter use either continuous or very high-aspect-ratio fibre, l/d > 1000 and the simple ‘rule of mixtures’ for thermal expansion does not apply, as the expansion is governed principally by the fibre in the plane of the laminate (x-y direction). The consequence is that with an increase in temperature, the matrix, because it is coupled to the fibre, is constrained from expanding in the plane of the laminate and is thus put under compression. In this state, some stress relaxation by creep can occur. In the direction perpendicular to the plane (the z direction), the aspect ratio of the fibre is clearly one and the rule of mixtures can be applied. Thus, for a random chopped-strand laminate of fibre volume fraction 0.2, coefficient of thermal expansion in the plane of the laminate might be 25-35 ppm K-1, whereas in the perpendicular direction it would be 50-80 ppm K-1. Incorporation of 50 wt% filler in the matrix polymer could reduce this to 35-55 ppm K-1.

9.4.3 Exotherm Control Fillers are also useful for moderating exotherms during the cure of thermoset resins. High exotherms can occur in thick parts or in mouldings which are polymerised rapidly. The actual temperature reached depends on the balance between the rate of heat release (heat of polymerisation, part thickness and rate of polymerisation) and the rate of heat transfer, the latter depending on the thermal conductivity and thermal capacity of the mould and to some extent of the curing resin. Lowest exotherms are for slow-cured thin parts in metal tooling, where the high thermal conductivity and usually high mass of the latter can lead to approximately isothermal conditions, and the part temperature then remains essentially that of the tool. Conversely, for rapidly cured thick parts in fibrereinforced plastic tooling, where the thermal conductivity is very much lower, near adiabatic conditions might pertain in the centre of the part. For example, the upper limit of adiabatic temperature rise for unsaturated polyester or urethane methacrylate resins, assuming 100% conversion, could be as high as 200-220 °C. A special and extreme case is the foaming of thermosets, where the very low thermal conductivity of the foamed curing resin itself promotes the development of an adiabatic condition.

478

Filled Thermosets Thus, in particular circumstances, high exotherms can be generated and these can be damaging in a number of ways. They are most likely to occur when plastic or ‘soft’ tooling is being used. Subjecting the tool to temperatures, which might be in the region of the glass-transition temperature of the tooling gel coat, can lead to creep in the tooling polymer with print through of the backing fibre pattern on to the gel coat. It may also lead to delamination of the tooling gel coat due to differential thermal expansion. Reduced tool life is the consequence in both cases. If the resin is being used for encapsulation, a high exotherm can cause damage to the encapsulated component. High exotherms can also cause distortion in the moulded part, particularly if the part is of non-uniform thickness. Local ‘hotspots’ during curing can lead to stress gradients in the final part. In extreme cases and where volatile comonomers such as methyl methacrylate are used in free radically cured resins, monomer evaporation and even boiling can occur, leading to very poor surface finish on the moulding. ‘Exotherm cracking’ is another consequence of high exotherms and occurs in highly crosslinked polymers and particularly in thick sections. If the crosslinked networks are developed to high conversion at high temperature, then, on cooling, the thermal contraction is sufficient to generate stresses, which in the presence of stress-concentrating flaws, cause rupture of the polymer. Fillers can be used to reduce exotherms and moderate some of these extremes of behaviour. They work by reducing the amount of polymerisable material in the mould and by providing a heat sink. However, because of their low thermal capacity relative to the resin/polymer, the effect is moderated, although still very significant. For example, let resin exotherm = 90 cal g-1 (typical of unsaturated polyester), resin specific heat 0.45 cal g-1 °C-1 and filler specific heat = 0.2 cal g-1 °C-1, for a 50/50 w/w resin/filler mix; exotherm of mix = 45 cal g-1 and specific heat of mix = (0.5 x 0.45 + 0.5 x 0.2) = 0.325 cal g-1 °C-1. Thus, the adiabatic temperature rise of mix = 139 °C compared with 200 °C for resin alone. Whilst fillers are beneficial in moderating exotherms and taking out ‘hotspots’, it is important that exotherms are not over-suppressed, otherwise the extent of cure will be inadequate and will result in the part having poor green strength out of the mould. This in turn will necessitate post-curing at elevated temperature. It is necessary for the exotherm to attain the region of the polymer Tg to allow sufficient molecular mobility for the crosslinking reactions to proceed to upwards of 95% conversion. Undercuring leads to low polymer modulus and strength, depression of the Tg/HDT and a tendency subsequently to warp. Although the moderating effect of fillers can reduce the extent of cure at the highconversion final stage, their presence can enhance gelation in the early stages of cure. The filler surface, through adsorption and reactive coupling of resin components, effectively participates in the branching and network generation such that the gel point

479

Particulate-Filled Polymer Composites (where an infinite network exists) occurs at a lower conversion and in a shorter time. For free radically cured resins exhibiting auto-acceleration or the ‘gel effect’ (unsaturated polyesters, methacrylates), the fast conversion stage of the cure is then triggered.

9.4.4 Shrinkage Control Fillers are also used to counteract excessive shrinkage in some thermoset resins. Shrinkage occurs on curing thermoset resins because the polymers have higher densities than the low-molecular-weight monomers or precursors. This is a consequence of the Van der Waal’s distances separating the reactive groups of the low molecular weight species being replaced by chemical bonds of shorter length as curing proceeds. Thus, the overall shrinkage is dependent on the molecular interactions in the liquid uncured state and on the number of functional groups that react to form new chemical bonds in the cured polymer. Shrinkage can be expressed in absolute terms as the percentage change in volume going from uncured resin to cured polymer at the same temperature, thus: Shrinkage = –100(Vp – Vr)/Vr = 100(1–(Pr/Pp)) where: V is the volume P the density r the uncured resin and p the cured polymer. Polymerisation shrinkages range from values of 3-6.5% for epoxies, 6-9% for unsaturated polyesters to 10-12.5% for urethane methacrylates. The actual volume-change profile shown by a curing thermoset is quite complex. The absolute polymerisation shrinkage as a function of conversion is offset in the early stages by the thermal expansion of the resin due to any elevated initial mould temperature and expansion of the resin/curing polymer mixture due to the polymerisation exotherm itself. After the exotherm maximum has passed, the polymer then undergoes a cooling contraction. This behaviour is quite distinct from that of thermoplastics, where mould shrinkage is due to the melt being cooled by contact with a cooler mould. The consequences of the polymerisation shrinkage of thermosets depends on the moulding technology being employed but are most severe in closed-mould technologies such as casting, compression moulding (sheet moulding compound, dough moulding compound), RTM and structural-reaction injection moulding, and less so in open-mould processes such as hand-lay and filament winding. In developing formulations for closed-mould processing, shrinkage is often measured as a linear shrinkage, ‘cold mould to cold part’, in which the distance between two etched lines on the mould is compared with that

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Filled Thermosets transferred to the moulding. This is convenient for thin essentially planar mouldings but it does not take into account any shrinkage in the perpendicular ‘z’ direction, and it is the latter that has a marked effect on the surface finish. A small amount (1-2%) of shrinkage in a moulding can be desirable as it aids demoulding, but excessive amounts have increasingly deleterious effects. For example, loss of dimensional accuracy, sink marks over ribs or in thick sections, cracking in thick parts, fibre print-through in laminates and undesirable surface finish effects due to separation of the moulded part from the mould surface before full conversion is reached are all potential consequences of high polymerisation shrinkage. The shrinkage can be counteracted by incorporation of thermoplastic or inorganic fillers. Thermoplastic additives are widely used in the free radically cured styrene or methacrylate-based resins, and offset the shrinkage by the generation of voids in the thermoplastic phase during polymerisation [77-81]. Fillers work by either simply providing inert bulk but again their relatively poor volumetric efficiency moderates the benefit achievable, or by expanding during the exotherm through liberation of water, for example. In the latter case ‘zero shrink’ can be attained as with the thermoplastic additives [82]. Used for their inertness, fillers at 50/50 w/w will only reduce the shrinkage of the mix to about 70-75% of that of the unfilled resin and, even increasing filler to 60/40 w/w, which is about the limit for viscosity reasons, will only drop it to 60-65% of the neat resin. Thus, 50/50 w/w filled compositions of urethane methacrylates will still show shrinkages of about 8.5% and of unsaturated polyesters about 5%. At these loadings of filler, the interparticle spacing is such that, if the interface is perfectly bonded, the interface and polymer are in a state of hydrostatic tension. If the interfacial adhesion is poor, void formation can occur due to polymer shrinkage away from the filler-particle surface. Only if filler volume fractions are low is the interface in a state of compression due to polymerisation shrinkage.

9.4.5 Processing Aids Fillers can help to reduce air entrainment when processing fibre composites by reaction injection moulding (RIM) or pultrusion. Low-viscosity resins (< 0.05 Pa-s) have the advantage of rapid penetration of fibre rovings and wetting of individual filaments. However, the attendant disadvantage is that, in RIM, as resin is injected into the preplaced fibre mat, resin wicks along the rovings ahead of the main resin front, and can result in air bubbles being bypassed and thus entrained at the interstices where rovings cross one another. Low resin viscosity also means that the resin pressure in the tool is essentially atmospheric and there is no driving force for displacement of the bubbles. Incorporation of filler provides competing surface, reduces wicking and gives a more stable flow front.

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Particulate-Filled Polymer Composites The increased viscosity from filler addition also raises resin pressure within the tool cavity further aiding elimination of air. The same effect is seen in pultrusion using the wet-bath process. Resin-impregnated fibre rovings are pulled through a heated shaping die where the curing takes place. If resin viscosity or fibre volume fraction is too low, resin can run back down the fibre rovings and the entrance to the die does not then act as a tight restrictor to squeeze out air fully. Increased viscosity through addition of filler to the resin, reduces resin backflow and increases the packing in the front end of the die. Air is then effectively squeezed out as the rovings are pulled into the die and a high-quality profile is produced. If high filler loadings are used to ‘fill’ the front end of the die in pultrusion, a good profile can still be made out at lower glass loadings. If the reduced stiffness and strength resulting from lower glass is acceptable, this provides a means of lowering costs.

9.4.6 Flame Retardants Lastly, a major use of fillers is for flame-retardant applications, with aluminium hydroxide being extensively used, especially in unsaturated polyesters. This is dealt with in depth in Chapter 6.

9.4.7 Metal Fillers The use of semi-metallic soft tooling has increased in recent years due to an increased demand for more complex dies, reduced costs and increased use of rapid prototyping. Chung and co-workers [83] have looked at the effect of adding aluminium short fibres and cast iron chips to a heat-cured epoxy casting resin. The short aluminium fibres were around 5 mm in length and the cast iron flakes of more random size up to around 150 μm and they looked at the effect on the tensile strength, wear resistance, shrinkage and thermal conductivity. They found that the tensile strength was generally lower with aluminium flakes than that of the epoxy due to voids or bubbles thought to have been introduced during the mixing process. The tensile strength increased quite markedly with the cast iron flakes however and this was thought to be a result of the high density of the mixture eliminating such bubbles. It is also possible that the high reactivity of aluminium could react with components in the epoxy to cause foaming during cure at high temperature. The wear resistance showed a clear increase as more aluminium flakes were added; the maximum wear resistance though was found in the cast iron flake filled material. Shrinkage was also found to decrease as the filler content was increased though the metal fillers could not be added easily at more than 50% by volume as the high viscosity made mixing very difficult. Not unexpectedly, the thermal

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Filled Thermosets conductivity increased as more aluminium fibres were added with the epoxy having a conductivity of around 0.23 W/mK and the filled composite around 1.82 W/mK at 50% aluminium fibre content. Tools were successfully tested using the aluminium fibre composite to mould wax patterns for investment casting. This was done with a metal mould tool having an aluminium fibre/epoxy soft tool insert. More work is being done on metallic fillers to investigate the filler/matrix adhesion, as other workers have found that additions of copper, zinc and aluminium produce quite varied results when tested in tension. Indeed the authors have also found that zinc will react readily with dicyandiamide, which is commonly used as a hardener in epoxies. The reaction can cause severe foaming and a change in the structure of the cured epoxy at the interface with the zinc.

9.4.8 Structural Adhesives The use of structural adhesives in the automotive sector for bonding bodyshell structures has often been highlighted as a major growth area [84-86]. These high strength adhesives are a group of adhesives that play a truly structural role in the construction of the bodyshell. The adhesive is the major connector between panels and has to be capable of supporting high loads throughout the life of the vehicle. The most common structural adhesive in this type of application is high toughness mineral filled epoxy. They have the ability to bond directly to oil contaminated steel and the toughened variants can provide the necessary impact performance for many applications. To date however, their use has been very limited due to the lack of the information about their long-term durability. While the strength of bonded joints, as measured in a simple lap shear test, is equal to or greater than that of their spotwelded counterparts, when loaded in warm, humid environment the performance can be significantly poorer. The improvements in performance have been largely due to the addition of fillers, often wollastonite and talc in combination with rubber toughening additions. This has produced a family of adhesives which have excellent toughness up to 50 °C and down to –40 °C.

Acknowledgements The authors would like to thank ICI for permission to publish and W.S. Balch and S.H. Rogers for allowing us to present some of their work and for many useful discussions.

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P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, USA, 1953.

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W.H. Stockmayer, Journal of Chemical Physics, 1944, 12, 125.

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H. Jacobsen and W.H. Stockmayer, Journal of Chemical Physics, 1950, 18,1600.

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K. Dusek, H. Galina and J. Mikes, Polymer Bulletin, 1980, 3, 19.

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J.K. Gillham, Polymer Engineering and Science, 1979, 19, 10, 676.

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L.H. Baekeland, inventor; no assignee; US 939,966, 1909.

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L.H. Baekeland, inventor; no assignee; US 942,852, 1909.

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W.H. Carothers, American Chemistry Society, 1929, 51, 2548.

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Collected Papers by Wallace Hume Carothers on High Polymeric Substances, High Polymers, Volume 1, Eds., H. Mark and G.S. Whitby, Interscience, New York, NY, USA, 1940.

10. H.V. Boenig, Unsaturated Polyesters: Structure and Properties, Elsevier, Amsterdam, The Netherlands, 1964. 11. No inventor; H.H. Robertson Co., assignee; GB 1,006,587, 1965. 12. F. Fekete, P.J. Keenan and W.J. Plant, inventors; H.H. Robertson Co., assignee; US 3,256,226, 1966. 13. E.C. Ford and A.J. Restaino, inventors; ICI United States, assignee; US 3,876,726, 1975. 14. E.C. Ford, inventor; ICI Americas, assignee; US 4,182,830, 1980. 15. F. Fekete, W.J. Plant and P.J. Keenan, inventors; H.H. Robertson Co., assignee; US 3,297,745, 1967. 16. M.L. Orton, R.D. Howard and W.I. Spurr, Proceedings of the PRI Conference on Fibre Reinforced Composites 84, Liverpool, UK, 1984, Paper No.2. 17. R.G.W. Norrish and R.R. Smith, Nature, 1942, 150, 336. 18. V.E. Trommsdorf, H. Kohle and P. Lagally, Die Makromoleculare Chemie, 1947, 1, 169.

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Filled Thermosets 19. A.M. North and G.A. Reed, Transactions of the Faraday Society, 1961, 57, 859. 20. M.L. Orton and D.J. Sparrow, Cellular Polymers, 1988, 7, 309. 21. R.J. Young, Introduction to Polymers, Chapman and Hall, London, UK, 1981 22. A.J. Kinloch and R.J. Young, Fracture Behaviour of Polymers, Elsevier, Amsterdam, The Netherlands, 1983. 23. I.M. Ward, Mechanical Properties of Solid Polymers, John Wiley & Sons, New York, NY, USA, 1971. 24. D. Ishai and L.J. Cohen, International Journal of Mechanical Science, 1967, 9, 539. 25. S. Sahu and L.J. Broutman, Polymer Engineering and Science, 1972, 12, 91. 26. P.K. Mallick and L.J. Broutman, Materials Science and Engineering, 1975, 18, 63. 27. R.J. Young and P.W.R. Beaumont, Journal of Materials Science, 1977, 12, 684. 28. J. Spanoudakis and R.J. Young, Journal of Materials Science, 1984, 19, 473. 29. S. McGee and R.L. McCullough, Polymer Composites, 1981, 2, 149. 30. C.D. Wu and R.L. McCullough in Developments in Composite Materials - 1, Ed., G.S. Holister, Applied Science Publishers, London, UK, 1977, p.119. 31. F.J. Guild and R.J. Young, Journal of Materials Science, 1989, 24, 298. 32. T.B. Lewis and L.E. Nielsen, Journal of Applied Polymer Science, 1970, 14, 6, 1449. 33. G.R. Irwin, Applied Materials Research, 1964, 3, 65. 34. W.J. Clegg, K. Kendall, N.McN. Alford, T.W. Button and J.D. Birchall, Nature, 1990, 347, 6292, 455. 35. G. Spetz, Polymer Testing, 1990, 9, 1, 27. 36. ASTM E399-90, Standard Test Method for Plane Strain Fracture Toughness of Metallic Materials, 1997. 37. A.G. Evans, Journal of Materials Science, 1972, 7, 1137. 38. G.P. Marshall, L.H. Coutts and J.G. Williams, Journal of Materials Science, 1974, 9, 1409.

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Particulate-Filled Polymer Composites 39. R. Frassine, T. Ricco, M. Rink and A. Pavan, Journal of Materials Science, 1988, 23, 4027. 40. N.G. McCrum, C.P. Buckley and C.B. Bucknall, Principles of Polymer Engineering, Oxford University Press, Oxford, UK, 1988. 41. F.F. Lange and K.C. Radford, Journal of Materials Science, 1971, 6, 1197. 42. S.K. Brown, British Polymer Journal, 1980, 12, 1, 24. 43. L.J. Broutman and S. Sahu, Materials Science and Engineering, 1971, 8, 98. 44. J. Spanoudakis and R.J. Young, Journal of Materials Science, 1984, 19, 487. 45. B. Pukansky and F.H.J. Maurer, Polymer, 1995, 36, 1617. 46. H.S. Kim and M.A. Khamis, Composites: Applied Science and Manufacturing, 2001, 32A, 1311. 47. M. Hussain, A. Nakahira, S. Nishijima and K. Nihara, Materials Letters, 1996, 27, 1-2, 21. 48. J. Lee and A.F. Yee, Polymer, 2000, 41, 8375. 49. F.F. Lange, Philosophical Magazine, 1970, 22, 983. 50. A.G. Evans, Philosophical Magazine, 1972, 26, 1327. 51. D.J. Green, P.S. Nicholson and D.J. Embury, Journal of Materials Science, 1979, 14, 1413. 52. D.J. Green, P.S. Nicholson and D.J. Embury, Journal of Materials Science, 1979, 14, 1657. 53. F.F. Lange, Journal of American Ceramics Society, 1971, 54, 614. 54. R. Bagheri and R.A. Pearson, Polymer, 1996, 37, 20, 4529. 55. Y. Huang and A.J. Kinloch, Journal of Material Science Letters, 1992, 11, 8, 484. 56. Y. Huang and A.J. Kinloch, Journal of Material Science, 1992, 27, 10, 2763. 57. R. Bagheri and R.A. Pearson, Polymer, 2000, 41, 1, 269. 58. L. Nicolais and L. Nicodemo, Polymer Engineering and Science, 1973, 13, 6, 469.

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Filled Thermosets 59. J. Leidner and R.T. Woodhams, Journal of Applied Polymer Science, 1974, 18, 6, 1639. 60. A. Kelly, Proceedings of the Royal Society, 1964, 63, 282. 61. J.O. Outwater, Modern Plastics, 1956, 33, 7, 156. 62. P.C. Paris, M.P. Gomez and W.E. Andrews, Trends in Engineering, 1961, 13, 9. 63. P.C. Paris and F. Erdogan, Journal of Basic Engineering, 1963, 85, 528. 64. L.E. Nielson, Journal of Composite Materials, 1975, 9, 149. 65. T. Fugii and Z. Maekawa, Zairyo, 1978, 27, 301, 984. 66. T. Yamazaki and K. Chiba, Zairyo, 1984, 33, 373, 1304. 67. T. Yamazaki and K. Chiba, Zairyo, 1986, 35, 393, 629. 68. R. Spaude and T. Kaiser, Kunststoffe, 1986, 76, 2, 141. 69. B.W. Staynes and V.C. Kearley, Proceedings of the International Conference of Fatigue in Polymers, 1983, London, UK, Paper No.14. 70. T.A. Freitag and S.L. Cannon, Journal of Biomedical Materials Science, 1977, 11, 4, 609. 71. H. Hojo, W. Toyoshima, M. Tamura and N. Kawamura, Journal of Materials Science, 1974, 11, 604. 72. J. Lankford, W.J. Astleford and M.A. Asher, Journal of Materials Science, 1976, 11, 9, 1624. 73. P.W.R. Beaumont and R.J. Young, Journal of Materials Science, 1975, 10, 8, 1334. 74. R.J. Young and P.W.R. Beaumont, Journal of Materials Science, 1975, 10, 8, 1343. 75. F.V. Antunes, J.M. Ferreira, J.D. Costa and C. Capela, International Journal of Fatigue, 2002, 24, 10, 1095. 76. E.P. Plueddeman, Silane Coupling Agents, Plenum Press, New York, NY, USA, 1982. 77. E.J. Bartkus and C.H. Kroekel in Polyblends and Composites, Ed., P.F. Bruins, Applied Polymer Symposium No.15, 1970, Wiley Interscience, New York, NY, USA, 113.

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Particulate-Filled Polymer Composites 78. V.A. Pattison, R.R. Hindersinn and W.T. Schwartz, Journal of Applied Polymer Science, 1974, 18, 9, 2763. 79. V.A. Pattison, R.R. Hindersinn and W.T. Schwartz, Journal of Applied Polymer Science, 1975, 19, 11, 3045. 80. C.B. Bucknall, P. Davies and I.K. Partridge, Polymer, 1985, 26, 1, 109. 81. C.B. Bucknall, I.K. Partridge and M.J. Phillips, Polymer, 1991, 32, 4, 636. 82. C.D. Armeniades and E. Haque, inventors; C.D. Armeniades and P.E. Krieger, assignees; EP 136,920, 1989. 83. S.I. Chung, Y.G. Im, H.D. Jeong and T. Nakagawa, Journal of Materials Processing Technology, 2003, 134, 1, 26. 84. P. Fay, Materials World, 1994, 2, 8, 418. 85. G.S. Cole and A.M. Sherman, Lightweight Materials for Automotive Applications. Materials Characterization, 1995, 35, 1, 3. 86. K. Lowe, Materials World, 1998, 6, 5, 281.

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10

Composites Using Nano-Fillers Roger N. Rothon and Chris DeArmitt

10.1 Introduction Although they have been known for a long time, composites using extremely small (nanoparticle) fillers have until recently been of minor commercial interest. This is now changing, and nano-filler technology is currently experiencing a period of intense activity, mainly focussed on plate shaped particles derived from layer silicates. Although present commercial use is small, it is forecast to grow rapidly. One estimate puts the demands for layer mineral based nano-composites as several million tons by 2010 [1]. Even with the low filler levels involved, this would mean tens of thousands of tons of nano-filler. There are, however, many challenges to be overcome if this is to be achieved. Some aspects of the technology are covered in other chapters of this work, but it was considered that the topic was of sufficient current interest to warrant it’s own chapter. However, it must be stressed that what is presented here is only a ‘snap-shot’ in what is a rapidly changing and developing technology.

10.2 Scope The term nano-fillers is vague and often abused in the literature. For the purposes of this chapter, it is restricted to particles which can reasonably be described by the adjective nanoand which are also of low enough cost to be considered for conventional polymer applications. While there is no precise definition, nano-fillers can be considered as particles which, when dispersed in polymers, are very small in at least one dimension. This concept is pushed quite far in some of the literature, with particles of up to at least a hundred nanometers being described as nano-particles. A reasonable working definition would seem to be that at least one dimension of the effective particle, when dispersed in a polymer matrix, should be no more than 20 nm, or 200 Å. As a result, the specific surface area, which plays a significant role in the effects observed, will be at least 150 m2/g. The term effective particle is used to eliminate fillers, such as carbon blacks, where the primary particle could be in the specified range, but are strongly aggregated into larger structures, that become the effective particles.

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10.3 General Comments Extremely small particles are not that difficult to prepare, and have been known for a very long time, usually prepared in the forms of sols or smokes (fumes). The challenge is to isolate them in a form that is easily handled and readily dispersible. Some type of agglomeration/aggregation is generally used to isolate the particles, but this is usually partly irreversible, resulting in products that cannot be redispersed into a polymer in their nanoparticle form. Carbon blacks and most fumed and precipitated silicas are examples of such products. One way around this is to form the particles directly in the polymer, or in a liquid monomer, but this has restricted applicability, partly because there are usually coproducts from the particle formation process to deal with. It is only recently that sufficient commercial incentive has been identified to focus real attention on this problem. Because of their nano-dimensions, the surface is particularly important for nano-fillers. Their specific surface area is one to two orders of magnitude higher than for conventional fillers. For example, while most minerals fillers would have specific surface areas in the range 1-10 m2g-1, a spherical silica nano-filler, with a diameter of 10 nm, would have a specific surface area of about 250 m2g-1, and a nano-clay can have a specific surface area of 1,000 m2g-1 or more. Although nano-fillers are used at significantly lower volume fractions, the total surface present in a composite is still much higher than for most conventional fillers. Thus, the silica nano-filler at 5% filler level would contribute about 1,250 m2 of surface to 100 g of a composite, compared to about 200 m2 for a fine calcium carbonate used at 40% w/w. The amount of surface modifier present is also a very significant factor. It can easily be 30-40% by weight of the nano-particle, which even at 5% filler level, can be over 1% of the total composite. This high surface area can have profound effects on the composite, as the presence of a mineral surface can significantly modify the properties of the polymer in its near vicinity. The amount of modified polymer is small with most conventional fillers, but can be quite significant with nano-particles (up to about 20% of the polymer can be affected) and its presence has to be considered in calculating composite properties. Finally, the high surface area can have deleterious effects on polymer stabilisers and composite stability. This is often overlooked, but it is becoming apparent that new stabiliser systems may be needed if optimum composite performance is to be achieved [2]. The influence of the filler surface on thermoplastic composites is covered in some detail in Chapter 8.

10.4 Nano-Filler Forms Three potential forms of nano-particle can be distinguished, depending on how many dimensions are in the nano-range. These are regular, rod-like and platy and are illustrated in Figure 10.1.

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Figure 10.1 Three basic types of nano-particle

All three types have some current interest as fillers. The platy ones (mainly in the form of layered silicates) are receiving by far the most attention, and will be focussed on in this work. The various forms are briefly discussed next.

10.4.1 Regular Shapes The most commonly met regular nano-fillers are silicas and titanias. These are generally made by controlled precipitation in the presence of surfactants, to prevent agglomeration and fusion of particles. Sol-gel processes starting from metal alkoxides are often utilised. Some very fine particles produced by more conventional gas phase processes are just within the definition used for this discussion. For example, fumed silicas, with diameters of under 10 nm are available. With the precipitation methods, organosilanes can be added during the precipitation, so that they become built into the surface layers. In order to obtain good dispersion, the precipitation is sometimes carried out in the host polymer, or its pre-cursor, such as a monomer. Another method of achieving the same result is to directly transfer (flush) a stabilised suspension, resulting from precipitation, into a liquid monomer. By the use of such methods, high loadings can be achieved in some polymers, such as poly(methyl methacrylate (PMMA). More details can be found in various publications [3-6]. In principle, similar things can be achieved with other materials that can be precipitated in fine form, such as calcium carbonate (PCC), although little seems to have been done

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Particulate-Filled Polymer Composites with them so far. In the case of PCC, acid-based surface modifiers would probably need to be used in place of the organosilanes. One of the notable features of composites produced with such fillers is high transparency, as the particles are too small to scatter visible light. On present information, the applications for this type of nano-composite seem to be specialised, and they have not, so far, become of widespread interest. Part of the reason for their relative unimportance is their high cost. Their main potential applications to date seem to be at conventional (relatively high) loadings, where the amount of expensive surface modifier needed is very significant. Systems, such as the PCC-based ones, have the potential for significant cost reductions and could repay further investigation. In a particularly interesting recent study, Nelson and co-workers examined the use of surface modified spherical silica particles of various sizes in PMMA and in polystyrene (PS) [7]. They found significant strength and modulus increases at low loadings of the smallest sizes (under 10 nm). This suggests that there may be more scope for these materials than realised to date. Low levels of PCC have also been reported to give significant improvements in modulus and impact strength of polypropylene (PP) [8] There are some other products, which fall into the size definition, but are currently too expensive to be considered as fillers for other than very specialised applications. The ultimate regular nano-particles are probably the carbon fullerenes [9] and the polyhedral oligomeric silsesquioxanes, or POSS [10-12]. The fullerenes, usually known as buckyballs, are soccer-ball shaped 60 or 70 carbon atom clusters, under 1 nm in diameter. The POSS materials are similar, extremely small, silica cages, carrying reactive and/or non-reactive organic groups. The cost of the POSS products are predicted to fall to levels where they can be considered for large-scale polymer applications as production is scaled up and this could be a very interesting development.

10.4.2 Rods, Fibres, etc. There are a number of products which are described as nano-fibres or nano-tubes. These are mainly carbon or metallic in nature and principally intended for electrical applications, where they produce conductivity at significantly lower volume fractions than more conventional conductive fillers. Most of these products are several tens of nanometers in diameter and so are outside of the definition of nano-particles adopted for this discussion. Pyro-graphitic fibres are a typical example. Their diameter is from 60-200 nm, with specific surface areas of 100-20 m2/g, respectively [13].

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Composites Using Nano-Fillers The finer products are usually too expensive to be considered as commercially viable fillers and are generally produced in a tangled, or birds nest, form making dispersion difficult. The nearest products to true nano-fibres are probably fullerene derived single and multiwall carbon tubes, which can have diameters as small as 10 nm and aspect ratios of several hundred. They have excellent theoretical reinforcement potential, but problems of dispersion, maintenance of aspect ratio and control of surface chemistry have to be overcome for this to be realised [14]. One important feature of this type of filler is its electrical conductivity and exceptionally high thermal conductivity. The high aspect ratio gives percolation at much lower volume fractions than observed for conventional fillers. Currently, these are niche products produced in small quantities at high price.

10.4.3 Platy Nano-Fillers (Nano-Clays and Related Materials) The main commercial interest is currently in composites based on platy nano-particles [15]. This is due to a combination of several factors. The first is that they can be obtained from relatively inexpensive raw materials, especially certain clays. The second is that the combination of shape and high specific surface area gives rise to some useful effects at relatively low loadings (below 10% w/w). This not only reduces the overall cost, it allows products to be made with lower density and better surface finish. The useful effects that can be obtained include: gas barrier properties, fire retardancy, increased stiffness and increased heat distortion temperature (HDT). Finally, it seems to be easier to overcome the isolation, handling and redispersion issues with platy particles than with the other types. Conventional platy fillers, such as mica and talc, are well known, but the plates are quite thick and the aspect ratios accessible are limited, if the other dimensions are to be kept to reasonable levels. The plate thickness also means that quite high filler loadings are still required. Certain layer silicates, both natural and synthetic, allow very thin plates, with a high aspect ratio to be produced, while keeping the other dimensions to a reasonable level. The materials of most interest are made up of alumino-silicate layers which are imperfect and so carry a negative charge, neutralised by various cations, usually sodium and calcium, intercalated between them. The layers themselves are nano-sized in thickness, with the other two dimensions being such that plates of aspect ratio in the range 50-1,000 can be obtained, without the other dimensions being so large as to pose problems. Montmorillonite clay, with a plate thickness of about 1 nm, is particularly useful. The challenge is to both separate these plates and make them compatible with (wetted by) a polymeric matrix. Various swelling agents can enter into the space (gallery) between the clay layers, pushing them apart. These agents include water and polar organic species such as alcohols and glycols. The swelling agents can also carry soluble species, which can react with the clay surface, or monomers that can be subsequently polymerised. The spaces between the plates are known as galleries and materials that penetrate them as intercalants.

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Particulate-Filled Polymer Composites There are a number of potential ways of incorporating and dispersing swollen layer silicates into polymer matrices. Some of these are outlined in the scheme in Figure 10.2. Some make direct use of the dispersion in which the swelling took place, but these are of restricted applicability. In many practical situations, it is desirable to isolate a dry solid for use in subsequent compounding procedures and this type of processing will be concentrated upon here.

Figure 10.2 Some processing strategies for making nano-clay polymer composites

There are several important factors in producing a suitable dry clay. The most important are that it should not collapse back into its original state on drying, and that the plate surfaces are converted into an organophilic state to make them compatible with the intended polymers. Fortunately, an established technology can be used for this purpose. This is organo-clay thickener technology, in which suitable clays are swollen in aqueous solutions of organic amines and their derivatives. These additives ion-exchange with, and displace, the gallery cations. The basics of this procedure are illustrated in Figure 10.3.

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Figure 10.3 Much simplified view of the expansion of the layer silicate gallery resulting from ion exchange with a long chain organic cation

Two other factors have to be considered. Firstly, the clay needs to have a suitable purity and plate diameter. This is achieved by careful selection and processing of the raw material but can add considerably to the costs. Secondly, any salts produced from the cations by the ion-exchange process usually have to be removed from the product by extensive washing. The key to the performance of nano-clays is how well they can be dispersed into a polymer matrix. Before discussing this, the types of dispersion that can be achieved need to be considered. Three extreme types of structure are usually recognised. These are: normal clay stacks (sometimes known as micro-composites), intercalates, where there is some polymer between the plates, but the stack morphology is preserved, and exfoliates with completely delaminated and dispersed nano-particles. (In practice, dispersed structures known as tactoids are also frequently observed. These are small stacks of intercalated plates). The three main types of structure are illustrated in Figure 10.4. The fully dispersed form is most useful for the majority of commercial applications, and is the one that is normally aimed for, although conventional processing methods often give mixed structures. A further sub-division of the fully dispersed state should also be made, this is into oriented and randomly aligned states, as also shown in Figure 10.4. For many applications, it is the oriented form that gives the best results.

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Figure 10.4 Different states of dispersion for a layered silicate in a polymer composite

Some brief description of the methods used to characterise the type of dispersion is also needed. Two approaches are commonly used, X-ray diffraction and electron microscopy. The X-ray method relies on the fact that the layer silicate diffraction pattern contains a readily identified peak, the position of which is directly related to the inter-plate spacing. When intercalation takes place (as with the organic amines), this peak moves in response to the increasing spacing. When complete delamination and dispersion (exfoliation) take place, this peak is lost. These concepts are illustrated in Figure 10.5. While apparently very simple, this method is far from perfect and must be used with caution. Electron microscopy data can be more informative, but also needs to be treated with care. Some structures seen by this method are shown in Figure 10.6. It is worth noting that while most of the literature, especially theoretical treatments, implies flat plates, curved structures are often seen in real composites. Intercalation of polymer and subsequent delamination is a difficult process, and the best results obtained to date have been where the organo-clay has been dispersed into a

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Figure 10.5 Schematic illustration of the use of wide angle x-ray diffraction for studying the dispersion of montmorillonite clay plates. (a) sodium form, (b) quaternary ammonium form and (c) quaternary ammonium form dispersed in polymer

monomer prior to polymerisation. This was the basis of the first commercial applications by Toyota [16], who developed nano-clay/polyamide composites. A very neat approach is possible here, with the introduced organic moiety becoming an integral part of the polymer matrix. In the first step, an amino-acid is ion-exchanged into the clay galleries. The treated clay is then dispersed into monomer prior to polymerisation. The chemistry of this process is illustrated in Figure 10.7, where it can be seen that the organo-treatment is acting as a coupling agent. (Coupling agents are discussed in detail in Chapter 4). The polymer is sometimes said to be tethered to the clay plate in this type of structure and the process is sometimes called polymerisation filling. Unfortunately, the polymerisation filling processes are not so easy with many polymer types, although they are apparently being used for some polyolefins, with polymerisation catalysts being carried on the surface of the clay plates [17]. They also tend to put the technology into the hands of the polymer producers and other, more complex, processing methods are needed for general application. Most interest has been with what is known as melt intercalation. In this process, an organo-clay is added to a polymer melt during

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Particulate-Filled Polymer Composites

Figure 10.6 High resolution electron micrograph of a cross-section of a nano-clay/ polypropylene composite illustrating some intercalated, partly dispersed clay stacks (tactoids) (Micrograph kindly supplied by Monica Celotto)

Figure 10.7 Illustration of how in situ polymerisation can lead to grafted polyamide chains (one plate surface only shown for clarity)

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Composites Using Nano-Fillers the compounding process. If the correct organic treatment is present and the compounding conditions are carefully controlled, a significant amount of intercalation and delamination can be achieved. It is rare for complete delamination to result from this process however. As a result, somewhat more organo-clay is required to achieve the same level of performance as compared to the polymerisation method. This melt processing area is one where a considerable amount of effort is currently being expended and where the situation could be significantly different in a few years time. Among the avenues being explored are: •

Optimisation of the compounding equipment and procedure [18]

•

Use of ultrasonics to enhance dispersion [19]

•

Use of a supercritical fluid, such as carbon dioxide, to aid processing [20]

•

Use of co-agents to improve incorporation and dispersion [21]

•

Freeze drying [1]

There is also some interest in what is known as one-pot compounding, in which the clay is ion-exchanged, delaminated and dispersed during the compounding process itself [22]. One of the problems with this approach is the formation of metal salts arising from the ion-exchange. Alternative intercalants are being examined and may offer a possible way of overcoming this [23]. A further refinement is the addition of water, or solvent, swollen clay direct into the polymer matrix [24].

10.4.3.1 Particle surfaces and the organic intercalant. As discussed in detail in Chapter 4, the level of interaction between a particle and polymer matrix has a big impact on the processing, dispersion and final properties of filled composites and various organic treatments are used to obtain optimum results. These treatments have to be carefully designed to suit the filler surface and application. The same situation exists for nano-clays, but is complicated by the presence of surfaceactive species used in their preparation. These organic treatments comprise a significant part of most nano-clays, and fulfil a number of very important functions. Thus, they increase the gallery spacing, making it easier for other species, such as monomers or polymers, to intercalate. The increased spacing also makes it easier to disperse, or delaminate the structure. In the simplest cases, these intercalants would also provide the compatibility and interaction with the polymer matrix described previously. It is thus obvious that the nature and amount of intercalant have to be chosen carefully and controlled. For ease of treatment, it is also helpful for the intercalant to be water soluble.

499

Particulate-Filled Polymer Composites These competing demands are difficult to reconcile and it is by no means clear that optimum surface treatments have yet been developed. Indeed there are reports that some conventional intercalants can degrade during processing and even initiate degradation of the matrix polymer [25]. Virtually all of the reported work to date has been with organic species such as amines and amino-acids, which can be dissolved in the form of salts and then form onium compounds by ion-exchange with cations on the clay surfaces. The main variables are: the number of chains attached to the nitrogen of the amine, (i.e., is it a primary, secondary, tertiary or quaternary amine?), the length distribution and branching of these chains, whether they are simple hydrocarbons or carry some other functionality such as carboxylic acid or unsaturation and finally, their concentration on the filler surface. Some general comments can be made. First is the level of treatment. The current approach relies on ion-exchange with the clay surface to anchor the additives. The amount of exchangeable cation varies with the clay source, but is usually 100 milli-equivalents per gram (meq/g) or less. This will result in a much lower density of surface attachment and less tightly packed chains than is normal with filler treatments. Thus a fully delaminated and dispersed clay will have a specific surface area of over 1,000 m2/g and would need an ion exchange capacity of up to 750 meq/g to form a fully packed surface layer (depending on the bulkiness of the organic intercalant). At a level of 100 meq/g, there is likely to be a considerable amount of free surface. On the other hand, the levels of additive needed for such a packed layer would be very high (up to about 200% w/w on clay). It seems that patchy coverage, with some available plate surface is helpful in getting intercalation and dispersion. The intercalation of polymeric species between the silicate plates seems at first sight to be unlikely, due to extra constraints imposed on the polymer chains. Close analysis suggests that intercalation can become thermodynamically favourable under certain conditions. In melt processing the main driving force appears to be interaction between the polymer and the filler surface. It also seems that having too strongly packed an interlayer of surfactant can actually impede polymer penetration and be detrimental. Secondly, we need to consider the position of the modifier. The clay chemistry dictates that conventional onium ions will only become firmly attached to the basal plate surfaces, leaving the edges and other faces untreated. These untreated surfaces will probably have sites amenable to treatment with more conventional additives, such as organo-silanes. Thus, mixed surface treatments may be most effective in covering all active surface sites. Indeed, it has been reported that all common mineral fillers are amphoteric to some extent and therefore have sites for adsorbing both acidic and basic surface treatment additives [26].

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Composites Using Nano-Fillers There is also the question of the method of attachment. Currently this is by onium ion, resulting from the ion-exchange process. As mentioned previously, there is some concern over the thermal and hydrolytic stability of this type of attachment and alternatives are being sought. Finally, the role of added ‘coupling agents’ needs to be considered. For nano-clays in polypropylene, it is common practice to add maleated PP to aid delamination and enhance adhesion and thereby improve some properties [27]. The full consequences of adding materials such as maleated PP are almost always overlooked. For example, the amount of maleated PP must be adjusted depending upon the surface area of the filler. Even more importantly, it is not generally recognised that the maleated PP reduces the thermooxidative stability of nano-composites. This is probably due to a combination of two factors. Firstly, carboxylic acids such as stearic acid destabilise PP, as seen by a reduction in oven ageing time for PP containing a standard hindered phenolic antioxidant package. Secondly, maleated PP is made by grafting maleic anhydride onto PP using peroxide initiators. These peroxides may not be fully consumed in the grafting process and residual peroxides are expected to have a very deleterious effect on the oxidative stability of the material. One recent unpublished study by Sanità in the Electolux laboratories in Italy showed that unfilled PP with normal stabilisation levels had an oven ageing time of 820 hours at 150 °C, whereas the same PP had an oven ageing time of only 250 hours once 22 weight% of maleated PP had been added. One can expect that clay-based nano-composites may show three modes of destabilisation. That due to transition metal impurities in the filler, that due to the organic intercalant and that due to maleated PP (if present). This may necessitate the addition of significant extra antioxidant, adding to the overall composite cost.

10.4.3.2 Alternatives to Natural Clay-Based Particles While attractive on cost grounds, products from natural clay sources have some limitations. First, extensive purification is needed, removing some of the cost advantage. Secondly, the clay will usually have some colour, which will be apparent at high loadings, or in thick articles. Thirdly, the surface chemistry may be sub-optimum for some purposes. This is especially true of the use of ionic bonding of the intercalant. Finally, the plate aspect ratios and concentration of ion-exchangeable sites are quite limited. Synthetic layer silicates, particularly fluoro-micas, are starting to be exploited where these issues are a real concern. There is also interest in other layered inorganics: especially magnesium salts like hydrotalcite, which can be synthesised as very thin plates and for which other forms of modifier attachment are possible.

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Particulate-Filled Polymer Composites

10.4.3.3 Composite Effects and Applications The principal exploitable properties that nano-plates can bring to a polymer composite are shape related, with the main ones being: •

Increased stiffness

•

Increased HDT

•

Reduced gas and fluid permeability

•

Reduced flammability.

The main attraction is that, because of the high surface area and aspect ratio, these benefits are potentially obtainable at much lower volume fractions than with most other established fillers. This has several potential advantages, notably: lower cost, lower density, less opacity, and potentially less reduction in other properties such as impact strength. Because of viscosity effects associated with the plate shape and potential particle damage, and difficulties in obtaining and maintaining exfoliation, the practical loading level is, however, limited to about 10% which severely restricts the absolute property values achievable. These various aspects are discussed further next, with the emphasis on thermoplastic applications, which are currently of most interest.

10.4.3.4. Modulus, heat distortion temperature and strength Low levels of nano-plates have a big effect on these properties in thermoplastic composites, and this was the first area to arouse interest [16]. Typical results are presented in Table 10.1 and show how much benefit can be achieved at very low loadings, and when conditions favour exfoliation, dispersion and strong interaction with the polymer.

Table 10.1 The effect of an amino acid based nano-clay in a Nylon 6 polymer Wt% of nano-clay

Flexural Modulus MPa

Tensile Modulus MPa

Heat Distortion Temperature °C

0

2,800

2,900

55

2

4,320

4,400

125

However, the real test is how the nano-plates compare with other reinforcing fillers like talc, mica and glass fibre. Currently there is much controversy over the ability of nanoplates to provide real benefits in properties such as stiffness, compared to conventional reinforcing fillers, especially glass fibres. Present results are very mixed and complicated 502

Composites Using Nano-Fillers by factors such as the amount of dispersion and orientation achieved and by nucleation effects. As an example, Motha and co-workers have found low levels of talc to be able to give similar modulus increases in polypropylene to similar levels of a nano-clay [28]. Most interest is in thermoplastic applications, but these are probably the most complex systems to model. The effect of a filler can arise from a number of means: 1. The simple volume fraction of strong, non-extensible, material present. 2. The shape of the particles. 3. Changes in any polymer crystallinity arising from the presence of the particles. 4. Changes in the polymer morphology near to the filler surface, creating a third phase (interphase). 5. The strength of interaction between the particles and the polymer. 6. Secondary effects due to adsorption of stabilisers, promotion of degradation, etc. The relative importance of these factors will vary markedly from system to system and even from one property to another. As an example, not all thermoplastics contain crystallinity, and filler polymer interaction will be more important in high strain situations. They also make the interpretation of much of the present experimental data relating to nano-clay filled composites difficult. It is thus appropriate to look at theoretical predictions first. Starting with the simplest case, where there are no changes in crystallinity or polymer morphology and no secondary effects to consider, the effect of anisotropic fillers on the modulus and strength of a polymer composite (but not impact strength) can be described by a number of equations, the most successful approach being that of Halpin [29] and Tsai. The reader is referred to works, such as those by Chow [30] for a detailed discussion of what is a very complex topic. The author’s much simplified view is presented next. The main features for the limiting case where the plates are fully dispersed plates are: •

The stiffness and strength should be predictable by the rule of mixtures, i.e. the volume fraction of filler will be the ultimate determining factor. In practice this ideal state is very difficult to obtain, due to various processing problems, especially affecting dispersion and particle alignment. (Note, the linear rule of mixtures also only strictly applies for two components that have the same Poisson’s ratios and have interfacial coherence [31]).

503

Particulate-Filled Polymer Composites •

With plates or fibres, the main effects will be in the plane of the particles. (Flexural testing is a special case, where the best effects can be obtained with the particles oriented normal to the applied force).

•

Modulus will increase with aspect ratio, up to a certain limiting value. This aspect ratio limit will depend on the modulus difference between the plate and the polymer, but is about 100:1 for the types of material with which we are concerned.

•

The actual amount of modulus increase will depend on the plate modulus. Data for the relevant moduli of filler particles are scarce, and somewhat inconsistent, but all mineral fillers are much stiffer than common thermoplastics.

•

Yield strength will also increase with aspect ratio. Again, it is expected that a limiting value will be reached at similar aspect ratios (about 100:1) as for the modulus.

•

Modulus is little affected by filler/polymer interaction, while it does play a major role in composite strength. Strong interaction is needed to maximise strength, and would normally be achieved by the use of a suitable coupling agent. As discussed in Section 10.4.3.1, the surface of nano-clays is complex, due to the presence of intercalation agents, and in many cases may not give optimum results.

In real systems, the other factors described previously may come into play. Thus: •

Polymer close to a filler surface can become modified by that surface. In elastomers and thermoplastics, this usually means an increase in rigidity. With conventional fillers, the amount of modified polymer is usually small enough to be ignored, but calculations show that up to about 20% of the polymer could be affected by the surface of nano-particles. One could thus expect an additional increase in modulus and yield strength from this source.

•

It is well known that fillers can affect the level and type of crystallinity in polymers such as polypropylene and polyamides. Nano-clays are known to promote significant changes in crystallinity in polyamides [32]. Mixed results have been reported for polypropylene, with most studies reporting no changes in the level of crystallinity, but some orientation of the polymer crystallites. Where these effects are present, they could contribute to the modulus and yield strength.

•

One of the biggest problems with plates and fibres is in achieving good dispersion and alignment while avoiding degradation in aspect ratio during processing. Alignment efficiency varies very significantly with the type of processing, being especially high in extrusion processing. In injection moulding and some other

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Composites Using Nano-Fillers processing methods, the orientation varies throughout the thickness of the sample, being good near the surface and poor in the centre. In this case, there can be marked effects of sample thickness. •

The flexural modulus is of most interest in many applications. This is much more complex than tensile, compressive or shear moduli. It not only has elements of both tensile and compression, but is very much influenced by particle orientation and by the surface layers. These are often polymer rich, an effect that becomes greater as the filler size increases.

To summarise: 1. From the rule of mixtures predictions, nano-plates should not give higher modulus and tensile strength than is obtainable from the same volume fraction of thicker plates and fibres of the same modulus, strength and aspect ratio. 2. In practice, results should be better than for equivalent amounts of mica and talc, because of higher effective aspect ratios, nearer to the optimum level. The higher aspect ratio should also lead to better orientation. Aspect ratio retention could also be better than for larger platy or fibrous particles. 3. For the same reasons, results are unlikely to be superior to good, long glass fibre composites. In reality, this is bourne out in practice. 4. Polymer modification through interface and crystallisation effects will be superimposed on the classical effects discussed. These may be most marked for nano-particles, due the much greater surface area, and this could give significant deviations from theoretical predictions. Despite the apparent limitations of nano-clays compared to more conventional anisotropic fillers, the nano-clays appear to combine their effects with other benefits such as: good surface finish and better scratch and mar resistance than can be achieved with talc, mica or glass fibres and thus, if the costs are not too high, they appear to have a good future in applications where these are important considerations.

10.4.3.5. Toughness and impact strength These are important properties, and usually decrease as the stiffness is increased. One of the great expectations for nano-plates in thermoplastics was that they would give products with better toughness for a given stiffness.

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Particulate-Filled Polymer Composites As a general rule, large particles give rise to a loss of impact strength. So for example classical mica composites show a marked decrease in impact strength because the particles act as stress concentrators. In contrast, nano-particles are able to provide the benefits of high anisotropy (increased modulus and yield strength) while keeping a low particle size so that impact strength is retained. Unfortunately, this is not an area that is readily treated by theoretical analysis, and present experimental results are mixed. Many factors contribute to toughness, including: particle size and shape, polymer degradation and crystallinity and interface properties. Some of the current problems with nano-plates in this area probably stem from formulation issues and from the present sub-optimum nature of surface treatments and a clearer picture should emerge as these are eliminated.

10.4.3.6 Gas and Fluid Barrier Properties This is an area where the nanoclays appear to offer very distinct advantages and is already undergoing commercialisation. Thus, as little as 2 wt% of a nano-clay is sufficient to decrease the oxygen transmission rate of a polyamide film by over 40%. Their performance is believed to be due to the creation of a tortuous diffusion path, as shown in Figure 10.8. (Note alignment is necessary for this. Truly random dispersion would not give the same effect). According to Lopez, the theoretical permeability coefficient of a composite containing aligned and overlapping plates can be calculated using Equation 10.1 [33]: Pc =

Pp τ

(10.1)

where Pc and Pp are the permeability coefficients for the composite and polymer, respectively, and τ is the tortuosity. This equation only accounts for the tortuosity and not for the ‘dilution’ of polymer. See also Equation 8.3 which allows for both effects to give more accurate predictions. Vf .AR ; where Vf is the volume fraction of filler and AR is the 2 aspect ratio. (Note: this assumes good wetting between the filler and polymer).

τ is defined as τ = 1 +

Because of their high aspect ratios, nano-plates have the potential to give very good results at low volume fractions, accompanied by good clarity and low density. However, as with other properties, their absolute performance is restricted by the volume fractions

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Figure 10.8 Tortuous diffusion path created by aligned plate particles in a polymer film

that are obtainable and with the difficulties of dispersion, alignment and plate damage. The film formation processes usually used, do appear to favour optimum alignment however. The main attraction of the nano-plate approach is the ability to use thinner films (down-gauging) to achieve the same barrier properties as for a thicker unfilled film. There is also growing pressure on some of the conventional barrier film materials containing chlorine and nano-plate prospects in this area look bright.

10.4.3.7 Fire Retardancy Platy nano-particles, such as nano-clays and micas, have some potentially useful flame retardant effects, and are currently receiving a lot of attention for this application. This topic has recently been reviewed [34, 35]. The other forms of nano-particle do not seem to have the same effectiveness. This subject, which was briefly treated in Chapter 6, is discussed in more detail here, but the earlier chapter should be referred to for details of fire retardant tests, especially the cone calorimeter which is widely used in studies involving nano-plate fillers. The flame retardant potential of nano-clays was recognised as early as 1976 [36], but serious work has only begun recently. A comprehensive study of the subject is under way, led by workers at the National Institute of Standards and Technology (NIST) in the United States, and commercial products and applications are starting to appear, notably in cable applications. Scientific understanding is still rudimentary, but is evolving fast. Only a limited number of nano-clays and polymers have so far been studied, so generalisations are risky. It has already been shown that delaminated clays have excellent gas barrier properties. Both delaminated and intercalated clays have also been found to improve the thermal

507

Particulate-Filled Polymer Composites stability of some polymers, especially under oxidative conditions. There is thus reason to expect that nano-clays might posses some fire retardant properties. There is a paucity of data for the standard small scale oxygen index and UL 94 tests [37], but what little there is strongly suggests that nano-clays are not very good in these tests at the normal loadings (up to 10%) that they are used at. Thus, 7% of a nano-clay was found to have little effect on the oxygen index of an ethylene methylacrylate copolymer [38]. This may be the reason why interest in this area was slow to develop. Nearly all of the available data is with the more recently developed cone calorimeter test, where excellent results are achievable. Nano-clays have been tested in a number of polymers, including; PP, PS, epoxy, polyamide 6 and ethylene vinyl acetate (EVA). Similar results have been reported in all cases and are: •

Effects on time to ignition vary considerably, with a decrease in this parameter sometimes being reported.

•

A marked decrease in the very important, peak rate of heat release, accompanied by a similar decrease in the rate of mass loss.

•

Some increase in the time to peak heat release and a lengthening of the time taken for complete combustion.

•

No significant decrease in total heat of combustion, smoke generation or carbon monoxide formation. Indeed, increases in smoke and carbon dioxide levels have sometimes been reported [28].

Typical results for a cone calorimeter experiment are shown in Figure 10.9. These effects were obtainable with only 3-5% by weight of additive, but did not change much when higher loadings were used. While not firmly established, present results indicate that both intercalated and delaminated forms of the same nano-clay have similar effects. This is perhaps surprising, given that the delaminated form is by far the best gas barrier. The lack of effect on total heat release, smoke and carbon monoxide, is different from halogenated flame retardants, and is taken to show that the effects are all in the condensed phase. Using gasification equipment, which duplicates the pyrolysis conditions in the cone calorimeter, without flaming taking place, Gilman and co-workers found that the melt that formed on the surface of unfilled polymer was quickly converted to a solid, black char when nano-clays were present [39]. It is postulated that this char is reinforced with nano-particles and slows down the combustion processes. It is,

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Composites Using Nano-Fillers

Figure 10.9 Schematic of the sort of cone calorimeter results obtainable from low levels of nano-clays. Note the much lower peak heat and slower combustion. The decrease in ignition time and double peak are often observed, but are not always seen.

however, eventually consumed under the radiant heat flux of the cone test and so the final char content is not greatly increased. Recent work has indicated that the acidic sites on the clay surface become exposed by decomposition of the organic treatment and play a significant role in promoting char formation [40]. The workers at NIST have published quite widely [41-44]. They have reported that there seems to be little difference between intercalated and delaminated forms of nano-clay, despite evidence that polymer thermal stability can be more improved by the intercalated structure. They have also reported that the reduction in ignition time may be due to instability of the quaternary ammonium salt and could be overcome by use of an alternative intercalant. The primary effect of the nano-clays seems to be related to char formation. The workers at NIST have found that a reduction in mass loss and heat release rate only starts once the surface of the polymer is at least partly covered by char. Le Bras has also reported that, while no char was produced by burning unfilled EVA, the nano-clay filled composite formed a strong char early in the process [45]. Once the amount of clay is taken into account, final char levels are often similar to unfilled polymer, indicating that, while a stronger, more insulating, char may form and retard combustion, it is eventually consumed. Given this char effect, the lack of performance in the oxygen index test is surprising. It may be because the sample is vertical and the melt flows away, preventing a char layer from forming, while the sample is horizontal in the cone experiment.

509

Particulate-Filled Polymer Composites The workers at NIST found that the processing conditions had to be carefully controlled if good results were to be obtained. In particular, when working with PS, they found that all fire retardant effects could be lost if the processing temperature was too high, or if air was not rigorously excluded. They put this down to degradation of the quaternary ammonium species on the clay. It has also been claimed that this degradation is the cause of the reduction in ignition time sometimes seen. As discussed earlier, more stable treatments are now being developed. While the reduction in peak rate of heat release, the increase in time to peak, and the general slowing down of combustion are all very valuable effects, they are probably not enough, on their own, to meet final application tests. Considerable effort is thus being spent on examining nano-clays in conjunction with other flame retardants. Beyer has reported how replacement of a small amount of aluminium trihydrate (ATH) in an EVA compound can significantly improve performance in the cone calorimeter test. Alternatively, a larger amount of ATH can be removed allowing the same performance at a lower overall filler level and with improved physical properties [46]. Burbigot and co-workers have shown improved performance when replacing part of the ammonium polyphosphate in an intumescent formulation for PP [47]. Inoue and Hosakawa have shown good effects in a number of polymers using melamine intercalated into nano-clay structures [48]. Finally, nano-clay combinations with halogen/antimony oxide systems are showing some promise [49].

10.5 Summary and Future Perspectives The topic of nano-fillers and nano-composites is in vogue at present. This is predominately due to the surprisingly good properties that are attainable at low loading levels of nanoplates, and the ability to produce such plates from certain clays. As we have seen in this chapter, absolute property levels in many polymer types, are currently much restricted by the combined problems of dispersion, alignment, polymer and intercalant degradation, plate damage and obtainable volume fraction. These problems limit nano-plate filler levels in composites to 10% or less. Glass fibre filled composites, on the other hand, are commonly made with 40% or more of fibre, where the properties far exceed those attainable from present technology nano-clay filled composites. Doubtless progress will be made with the absolute performance levels of nano-plates and they do have some advantages over glass fibres in terms of

510

Composites Using Nano-Fillers surface finish and scratch resistance which are important in some major applications. However, if they are to achieve widespread commercial importance they must become more cost-competitive with conventionally filled polymers. Much progress still needs to be made on the theoretical understanding of these materials. This is particularly important with the plate like particles derived from natural sources, which are currently of most commercial interest. These are complicated by the competing contributions from the shape, surface area, delamination and alignment, polymer degradation and surface chemistry issues. This area is still largely at the research stage and even so the materials produced are rather close to being commercially attractive on a wide-scale. With some further work it is certain that improvements in performance will be seen and that the cost will decline once large volumes are sold. At some point it is expected that the performance:cost ratio will become attractive compared to the traditionally filled composites and a breakthrough will occur, leading to wide-scale commercialisation of nano-filler composites.

Acknowledgements The authors would like to thank Dr Chris Liauw of Manchester Metropolitan University and Dr Kevin Breese of Electrolux for useful discussions regarding the contents of this chapter. They would also like to thank Massimo Sanità of Electrolux for the information regarding the effects of maleinised polypropylene on composite stability and Monica Celotto, also of Electrolux for the electron micrograph. We also thank Professor Ulf Gedde of KTH Stockholm for discussions on the permeability of nano-composites.

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16. A. Okada, Y. Fukushima, M. Kawasumi, S. Inagaki, A. Usuki, S. Sugimyama, T. Kurauchi and O. Kamigaito inventors; KK Toyota Chou Kankyusho, assignee; US 4,739,007, 1988. 17. M. Alexandre, P. Dubois, R.J.E.G. Jerome, M. Garcia-Marti, T. Sun, J.M. Garces, D. Millar and A. Kuperman, inventors; Dow Chemical Company, assignee; WO 9947598, 1999. 18. H.R. Dennis, D.L. Hunter, D. Chang, S. Kim, J.L. White, J.W. Cho and D.R. Paul, Proceedings of Antec 2000, Orlando, FL, USA, 2000, Paper No.75. 19. I. Rhoney and R.A. Pethrick, Proceedings of E-MAP Conference on NanoComposites 2002, Amsterdam, The Netherlands. 20. C. Zeng and L.J. Lee, Proceedings of Antec 2002, San Francisco, CA, USA, 2002, Paper No.470.

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Composites Using Nano-Fillers 21. H. Ishida, S. Campbell and J. Blackwell, Chemistry of Materials, 2000, 12, 5, 1260. 22. M. Alexandre, G. Beyer, C. Henrist, R. Cloots, A. Rulmont, R. Jerome and P. Dubois, Chemistry of Materials, 2001, 13, 11, 3830. 23. J.W. Gilman, R. Davis, W.H. Awad, A.B. Morgan, P.C. Trulove, H.C. DeLong, T.E. Sutto, L. Mathias, C. Davis and D. Schiraldi, Proceedings of Flame Retardants 2002, London, UK, p.139. 24. U. Wagenknecht, P. Potschke, B. Kretzschmar and D. Wolf, Proceedings of Eurofillers ’99, Lyon-Villeurbanne, France, Paper No.C30. 25. J.W. Gilman, A.B. Morgan, R.H. Harris, P.C. Truelove, H.C. De Long and T.E. Sutto, ACS Polymeric Materials: Science and Engineering, Fall Meeting 2000, Washington, DC, USA, 2000, p.59. 26. M. Ernstsson, Surface Characterisation of Minerals Used in Asphalt Systems and Filled Plastics: A Multianalytical Approach, Royal Institute of Technology, Stockholm, Sweden 1999. [Licentiate Thesis] 27. C. Varela, C. Rosales, R. Perara, M. Matos, T. Poirier and J. Blunda, Proceedings of Nanocomposites 2002, San Diego, CA, USA, Session 1, Paper No.4 28. K. Motha, J. Huhtala, T. Ikonen and E. Hasari, Proceedings of Nanocomposites 2002, San Diego, CA, USA, Session 3, Paper No.4 29. J.C. Halpin and J.L. Kardos, Journal of Applied Physics, 1972, 43, 2235. 30. T.S. Chow, Journal of Materials Science,1980, 15, 8, 1873. 31. Physical Ceramics for Engineers, L.H. Van Vlack, Addison – Wesley Publishing Company, Reading, MA, USA, 1964. 32. L.M. Liu, Z.N.Qi and X.G. Zhu, Journal of Applied Polymer Science, 1999, 71, 7, 1133. 33. G.A. Lopez, Proceedings of TAPPI Polymers, Laminations and Coatings Conference, Chicago, IL, USA, 2000, Volume 3, p.1063. 34. D. Porter, E. Metcalfe and M.J.K. Thomas, Fire & Materials, 2000, 24, 1, 45. 35. J.H. Koo, S. Bourbigot and W.K. Chow, Proceedings of Functional Fillers for Plastics 2002, Toronto, Canada, Paper No.16.

513

Particulate-Filled Polymer Composites 36. S. Fujiwara, and T. Sakamoto, inventors; Kokai Patent Application no.SHO 51(1976)-109998, 1976. 37. UL-94, Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, 1996. 38. K. Motha, J. Huhtala, T. Ikonen and E. Hasari, Proceedings of Nanocomposites 2002, San Diego, CA, USA. 39. J. Gilman, T. Kashiwagi, A. Morgan, R. Harris, L. Brassell, W. Awad, R. Davis, L. Chyall, T. Sutto, P. Trulove and H. DeLong, Proceedings of Fire and Materials 2001, San Francisco, CA, USA, p.273. 40. M. Zanetti, G. Camino, P. Reichert and R. Mulhaupt, Macromolecular Rapid Communications, 2001, 22, 3, 176. 41. J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, E. Manias, E.P. Giannelis, D. Hilton and S. Phillips, Proceedings of Flame Retardants 2000, London, UK, p.49. 42. J.W. Gilman, T. Kashiwagi, A.B. Morgan, R.H. Harris, Jr., L.D. Brassell, M.R. Van Landingham and C.L. Jackson, Flammability of Polymer Clay Nanocomposites Consortium: Year One Annual Report, 2000, NISTIR 6531, NTIS, Springfield, VA, USA. 43. J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, E. Manias, E.P. Giannelis, M. Wuthenow, P. Hilton and S. Phillips, Chemistry of Materials, 2000, 12, 7, 1866. 44. D.L. VanderHart, A. Asano and J.W. Gilman, Chemistry of Materials, 2001, 13, 10, 3796. 45. S. Bourbigot, M. Le Bras, R. Leeuwendal, K.K. Shen and D. Schubert, Polymer Degradation and Stability, 1999, 64, 419. 46. G. Beyer, Proceedings of Flame Retardants 2002, London, UK, p.209. 47. S. Bourbigot, M. Le Bras, F. Dabrowski, J. Gilman and T. Kashiwagi, Fire and Materials, 2000, 24, 4, 201. 48. H. Inoue and T. Hosokawa, inventors; Showo Denka K K, assignee, JP 10 81,510 (98 81 510), 1998. 49. K. Okada, inventor; Sekisui Chemical Co. Ltd., assignee; JP 11228748 A2, 1999.

514

Abbreviations and Acronyms

ABS

Acrylonitrile-butadiene-styrene

AES

Auger electron spectroscopy

AFM

Atomic force microscopy

amu

Atomic mass units

AN

Acceptor number

APP

Ammonium polyphosphate

AR

Aspect ratio

ASTM

American Society for Testing and Materials

ATH

Aluminium hydroxide

BE

Binding energy(s)

BET

Brunauer, Emmett and Teller surface area

BOPP

Blown oriented PP

C944

Chimassorb 944

CALDOS

Calorimeter Digital Output and Sequencer

CD

Compact disk

CPU

Central processing unit

CTAB

Cetyl trimethyl ammonium bromide

CTE

Thermal expansion coefficients

DBP

Dibutyl phthalate absorption

DMC

Dough-moulding compound

DMTA

Dynamic mechanical thermal analysis

DN

Donor number

DPA

David P Ashton

DRIFTS

Diffuse reflectance Fourier transform infrared spectroscopy

DSC

Differential scanning calorimetry

515

Particulate-Filled Polymer Composites EDS

Easy dispersing silicas

EMI

Electromagnetic interference

EPDM

Ethylene-propylene-diene elastomer

ESCA

Photoelectron spectroscopy

esd

Equivalent spherical diameter(s)

ETP

Engineering thermoplastics

EVA

Ethylene vinyl acetate

FID

Flame ionisation detectors

FIGRA

Fire growth rate index

FMC

Flow microcalorimetry

FTIR

Fourier transform infrared spectroscopy

GPPS

General purpose polystyrene

HDPE

High density polyethylene

HDRS

Highly dispersible and reactive silicas

HDS

Highly dispersible silicas

HDT

Heat distortion temperature

HIPS

High impact polystyrene

HMDS

Hexamethyldisilazane

HPLC

High pressure liquid chromatography

HSAB

Hard and soft acids and bases

I1010

Irganox 1010

IGC

Inverse gas chromatography

IR

Infra-red

IRHD

International rubber hardness degrees

ISO

International Standards Organisation

KI

Kovats indices

KM

Kubelka-Munk

l/d

Length to diameter

LDPE

Low density polyethylene

LLDPE

Linear low density polyethylene

516

Abbreviations and Acronyms m/m

Mol/mol

MA

Maleic anhydride

MCS

Magnesium carbonate subhydrate

MDPE

Medium density polyethylene

MFI

Melt flow index

MFR

Melt flow rate

MMA

Methyl methacrylate

MMT

Montmorillonite

Mn

Number average molecular weight

MPBD

Maleanised polybutadiene

MPS

3-(Trimethoxysilyl)propyl methacrylate

MVI

Melt viscosity index

Mw

Weight average molecular weight

NBS

National Bureau of Standards

NIST

National Institute of Science and Technology

NMR

Nuclear magnetic resonance spectroscopy

NR

Natural rubber

OEL

Occupational exposure limits

OIT

Oxidation induction time

PA

Polyamide

PBT

Polybutylene terephthalate

PC

Personal computer

PCC

Precipitated calcium carbonate

PE

Polyethylene

PET

Polyethylene terephthalate

phr

Parts per hundred resin

PMMA

Poly(methyl methacrylate)

POSS

Polyhedral oligomeric silsesquioxanes

PP

Polypropylene

ppb

Parts per billion

517

Particulate-Filled Polymer Composites ppm

Parts per million

PPO

Polypropylene oxide

PPS

Polyphenylene sulfide

PS

Polystyrene

PTFE

Polytetrafluoroethylene

PVC

Polyvinyl chloride

PVDF

Polyvinylidene fluoride

QMS

Quadrupole mass spectrometer

R&D

Research and Development

RFS

Resorcinol-formaldehyde-silica

RI

Refractive index

RIM

Reaction injection moulding

rpm

Revolutions per minute

RR

Roger N. Rothon

RRIM

Reinforced reaction injection moulding

RTM

Resin transfer moulding

SBR

Styrene-butadiene rubber

scCO2

Supercritical CO2

SEM

Scanning electron microscopy

SG

Specific gravity

SI

Système Internationale

SIMS

Secondary ion mass spectroscopy

SL

Linear scale of segregation

SMOGRA

Smoke growth rate index

TCD

Thermal conductivity detectors

TEM

Transmission electron microscopy

TEPO

Triethylphosphine oxide

Tg

Glass transition temperature

TGA

Thermal gravimetric analysis

Tm

Melting temperature

518

Abbreviations and Acronyms ToF

Time-of-flight

ToF-SIMS

Time-of-flight secondary ion mass spectroscopy

UHV

Ultra-high vacuum

UL

Underwriter’s Laboratory

uPVC

Unplasticised PVC

UV

Ultraviolet

WATS

Weighted average total strain

WAXS

Wide angle x-ray diffraction

WLF

Williams Landel Ferry

XPS

X-Ray photoelectron spectroscopy

γ-MPS

[γ-(Methacryloxy)-propyl] trimethoxy silane

519

Particulate-Filled Polymer Composites

520

Author Index A Adamson, A.W. 384 Ahsan, T. 169 Aivazyan, G.B. 28 Alexandre, M. 296 Allen, N.S. 118 Antunes, F.V. 472 Armistead, C.G. 180 Ashley, R.J. 266, 273, 274, 276, 281, 286 Ashton, D.P. 101, 110, 114, 115, 177

B Babrauskas, V. 267 Bagheri, R. 459, 460, 462 Baillet, C. 290, 294, 295 Barnett, C.E. 37 Bascom, W.D. 182 Bastioli, C. 129 Beekman, G. 292 Berg, J.C. 383, 385, 387 Bergström, L. 388 Berlin, A.A. 277 Beyer, G. 297, 510 Birchall, J.D. 176 Birchenough, C.L. 164 Bohlin, L. 171 Bohren, C.F. 11 Bolodyan, L.A. 277 Boonstra, B.B. 40, 81 Brault, A. 294 Brault, D. 294

Breese, K.D. 388, 394 Briggs, D. 101 Broutman, L.J. 455, 457 Brown, S.K. 454 Burbigot, S. 510 Burditt, N. 360

C Callopy, D.G. 118 Cans, C.H.F. 195 Case, J.R. 274, 277, 278, 279, 280 Cave, N.G. 164 Celotto, M. 498 Chacko, V.P. 45 Chibowski, S. 40 Chow, T.S. 372, 373, 503 Chung, S.I. 482 Cohen, L.B. 199 Cohen, L.J. 458 Connolly, W.J. 269 Cook, P.M. 298 Cotten, G.R. 185 Cusack, P. 298

D Dannenberg, E.K. 185 Darlington, M.W. 44, 172 DeArmitt, C. 357, 388, 394, 489 Delfosse, L. 290, 293, 294, 295 Donnet, J-B. 79 Doyle, M. 94 Drago, R.S. 105, 106

521

Particulate-Filled Polymer Composites

E Eakins, W.J. 180 Edge, M. 118 Enikolopyan, N.S. 41 Ernstsson, M. 388 Evans, A.G. 455, 457 Evans, M.B. 37, 38, 40, 176 Evans, R.M. 392

F Faust, G.T. 92 Fekete, E. 169, 383 Ferrigno, T.H. 29, 34 Fogel, V. 392 Folkes, M.J. 386 Fourty, G. 20 Fowkes, F.M. 106, 107, 109, 116 Fowkes, M.J. 382 Franklin, K.R. 118 Fulmer, M. 171, 172 Furnas, C.C. 34 Furukawa, J. 174

G Gähde, J. 144 Galeski, A. 41, 45 Garbassi, F. 129, 188 Gent, A.N. 21, 188 Gerard, I.F. 41 German, R.M. 30 Gilbert, M. 135, 145, 172, 173 Gilman, J. 508 Gilman, J.W. 296 Gladkikh, Y.P. 164, 167, 169, 171 Godlewski, R.E. 191 Golander, C.G. 196 Green, D.G. 455 Green, D.J. 456, 457, 458

522

Guild, F.J. 448 Gutmann, V. 105

H Hair, K.L. 180 Halpin, J.C. 503 Han, C.D. 186 Hancock, M. 5, 53, 172, 357 Hardwick, S.T. 382 Haworth, B. 164 Heckman, F.A. 19 Heggs, R.P. 191 Heikens, D. 372 Herbert, M.J. 282, 283, 285 Hertl, W. 180 Hess, G.C. 19 Hirschler, M.M. 268, 291, 293 Hobbs, 382 Hockey, J.A. 180 Hornsby, P.R. 173, 207, 274, 280, 282, 292, 363, 366, 387 Hosakawa, T. 510 Hsing, H.H. 180 Hsu, E.C. 188 Huang, Y. 460 Huffman, D.R. 11 Hughes, P.J. 274, 280, 285 Huisman, H.F. 30 Hussain, M. 455 Hutchinson, J. 176 Hutley, T.J. 44, 172

I Iler, R.K. 81, 180 Ingham, J.D. 292 Inoue, H. 510 Iqbal, Z. 115, 145, 146 Irwin, G.R. 450 Ishai, D. 458

Author Index Ishida, H. 110, 115, 143, 144, 158, 183, 184, 186, 187 Ivanishchenko, G.P. 169 Ivanishchenko, O.I. 164, 167, 171 Iwatsuki, M. 196

J Jackson, G.V. 274, 277, 278, 279, 280, 285, 425 Jang, J. 187, 188 Jansen, I.J. 43 Jepson, W.B. 64 Johansson, L.S. 134 Jones, H.C. 37 Jones, P. 279 Joslin, S.T. 116

K Karlivan, V.P. 37, 38 Kaye, B.H. 19, 23 Keating, L. 292 Kellar, J.J. 143 Kelly, A. 466 Khalturinskii, N.A. 277 Khamis, M.A. 455 Kim, H.S. 455 Kinloch, A.J. 164, 460, 471 Koenig, J.L. 158 Kosfeld, R. 40 Kowalewski, I. 45 Krupicka, A. 392

L Lamèthe, 394 Landham, R.R. 196, 197 Lange, F.F. 454, 455, 456, 457 Larsson, A. 388 Lawson, D.F. 269, 291

Le Bras, M. 509 Lee, J. 455, 462 Lees, G.C. 115, 140, 143, 145, 146 Leidner, J. 464, 468 Lewis, A.G. 450 Lewis, G.N. 105 Liauw, C.M. 101, 115, 117, 118, 140, 143, 145, 146, 164, 170, 171, 174 Lipatov, Y.S. 40 Lopez, G.A. 506

M Mallick, P.K. 455, 457 Manson, J.A. 371 Markley, K.S. 166 Maurer, F.H.J. 455, 457 McCrum, N.G. 471 McCullough, R.L. 446, 447, 449 McGee, S. 446, 449 McGenity, P.M. 44 McMahon, A.W. 123, 130 McReynolds, W.O. 123 Medalia, A.I. 19, 81 Milewski, J.V. 95 Miller, J.D. 144, 184 Mitsuishi, K. 44 Miyata, S. 172, 174, 273, 282 Mohs, F. 10 Monte, S.J. 191, 193, 197 Mosesman, H. 292 Motha, K. 503 Mulliken, R.S. 105, 106, 107 Murthy, N.S. 45 Musselman, L.L. 298

N Nakatsuka, T. 184, 199 Napper, D.H. 39, 42 Nelson, G.L. 269, 492

523

Particulate-Filled Polymer Composites Nicodemo, L. 468 Nicolais, L. 468 Nielsen, L.E. 450 Nishimoto, K. 273

280, 281, 285, 286, 489 Rybnikar, F. 45 Rychly, J. 285, 287

S O Occhiello, E. 129 Orton, M.L. 425 Ottewill, R.H. 173 Outwater, J.O. 466

P Panzer, U. 121 Pape, P.G. 189 Papirer, E. 167, 169 Pavlinec, J. 287 Pearson, R.A. 459, 460, 462 Pearson, R.G. 105 Pena, J.M. 130 Petrie, S. 292 Plueddemann, E.P. 37, 181, 183, 186, 187, 189 Porter, D. 296 Pugh, R.J. 383 Pukansky, B. 41, 172, 175, 373, 383, 455, 457

St. Germain, F. 373, 387 Sahu, S. 455 Sanità, M. 501 Schlumf, H.P. 8 Schmitt, P. 169 Schofield, W.C.E. 140, 143 Schreiber, H.P. 121, 373, 387 Schultz, J. 121 Schwaber, D.K. 190 Sharma, Y.N. 197 Shenoy, A.V. 363, 387 Sinicki, R.A. 387 Skelhorn, D. 303 Socha, D.A. 74 Spanoudakis, J. 449, 455, 456, 457, 458, 459, 462 Stark, G.L. 37 Stewart, C.W. 293 Suess, E. 167, 168, 170 Sugerman, G. 191, 193, 197 Sultan, B.A. 196 Sutherland, I. 136, 138 Suzuki, N. 143

R Radford, K.C. 454, 457 Ram, A. 371 Raymond, C. 172 Raymond, C.L. 164 Riddle, F.L. 106 Rodriguez, F. 190 Romano, G. 129 Rothon, R.N. 5, 38, 40, 43, 53, 94, 96, 110, 115, 140, 143, 145, 146, 153, 158, 170, 172, 177, 263, 266, 273, 274, 276,

524

T Tabtaing, A. 164 Taylor, H. 425 Thevaranjan, T.R. 293 Thoma, S.G. 25 Thomason, J.L. 43 Thornton, A.M. 269 Tiburcio, A.C. 371 Tiffany, J.M. 173

Author Index Timmons, R.B. 182 Touval, I. 96, 298 Tsai, S.W. 503 Tsuchiya, E. 196 Tudos, F. 41

V Vaia, R.A. 143, 145 Venables, R. 164 Vesely, K. 289

Y Yamashita, S. 174 Yee, A.F. 455 Yee, R.J. 462 Young, R.J. 448, 449, 455, 456, 457, 458, 459, 462, 471

Z Zettlemoyer, A.C. 180

W Watson, C.L. 173, 274, 280, 282, 292 Watson, S.K. 81, 84 Weber, M. 87 Wharton, R.K. 265, 266 Woodhams, R.T. 464, 468 Wu, C.D. 447 Wu, S. 386

525

Particulate-Filled Polymer Composites

526

Index A Abrasiveness 11 Accelerator adsorption 343 Acetylene black 80, 342 Acid-base interactions 103 free energy of 120 Acid-base theory 104 Acoustic properties 362, 363 Acrylic acid 117, 143 Acrylic composites 455, 458, 461, 473, 474 tensile failure stress 465 Acrylic thermoset mica-containing 467 Acrylonitrile-butadiene-styrene 410 Additive-filler interactions 103, 113, 117 Additives distribution 158 reduced adsorption 154 Adsorption isotherms 162 Adsorption methods 159 Aesthetics 390 abrasion resistance 391 colour 390 gloss 391 pigmentation 390 scratch 391 surface finish 391 Agglomerates 23 rupture 214 Agglomeration 210, 368 Aggregates 23

Aluminates 198 Aluminium hydroxide 85, 113, 138, 162, 270, 348 cone calorimeter 282 decomposition pathway 86 DRIFTS difference spectrum 140 exothermic decomposition 348 in PMMA 278 production 85 properties 85 surface modification 87 thermal decomposition 86 uses 87 uses in polymers 464 Aluminium trihydrate see aluminium hydroxide Ammonium polyphosphate 295, 297 Ammonium stearate treated magnesium hydroxide DRIFTS spectra 143 Analytical techniques Auger electron spectroscopy 124 FTIR 183 SIMS 183 surface analysis methods 108 XPS 183 Antimony oxides 95 Antimony trioxide 95, 295, 297 Antioxidants 117, 310 Antiozonants 310 Aspect ratio 20 Auger electrons 124, 125 Auger parameter 128

527

Particulate-Filled Polymer Composites

B Ballotini 461 Barites 73, 350 occurence 73 properties 73 uses 73 Barrier properties 370 Basic carbonates 84 Basic magnesium carbonates 91, 272 Bayer process 85 Biotite 70 Birefringence 13 Bis-epoxides curing of with anyhrides 440 curing of with primary diamines 439 Bisphenol epoxy resins preparation of 437 Bisphenol A diepoxides 435 Bisphenol A-derived resins 435 Bisphenol curing agents 310 Blanc fixe 350 Boehmite 86, 273 Borates 298 Bravais lattices 54 Brucite 88 Burning behaviour aluminium hydroxide 337 antimony trioxide 337 magnesium hydroxide 337 precipitated calcium carbonate 337 zinc borate 337

C Calcined clays 66, 67, 331, 369 uses 68 Calcium carbonate 167 chalk 347 fatty acid coating 174, 177 MPBD coating 177

528

occurrence 57 particle size 25 precipitated 347 processing 57 properties 57 rosin 174 stearic acid modification of 101 surface modification 60 types of 348 uses of 61 Calcium carbonates 331, 346 calcite 347 limestone 347 marble 347 natural 347 stearic acid coated 169 Calcium hydroxide 273 Calcium sulfate 74 dihydrate 272 occurence 74 properties 74 Carbon black 78, 340, 369 active surfaces 80 channel black process 79 furnace process 79 graphitisation 80 interaction with stabilisers 130 particle size 341 production 79 properties 80 special grades 342 structure 80, 341 surface chemistry 341 thermal process 80 uses 81 XPS analysis of 130 Carboxylated polybutadiene 162 Chalk 57 deposits 58 whiting 58

Index Channel blacks 342 Characterisation electron microscopy 496 X-ray diffraction 496 Chemical analysis 159 Chemical composition 7, 8 bulk chemistry 7 Chemical impurities 7, 8 Chimassorb 944 118 China clay. See Kaolin Clay 68 calcined clay 345 secondary clay 345 treated clay 346 Clay minerals 344 china clay 344 hard clay 344 kaolin 344 primary clay 344 soft clay 344 Clays properties of 332 silane-treated calcined clay 346 silane-treated kaolins 346 Coating lauric acid 169 Coating level mono-layer 158 Coating structure processing 163 Coccosphere 59 Colour 13, 59, 338 Composite Composite density 362 Composite modulus 372 Composite permeability 370 Composite properties 154 Composites flame retardant properties 263 fracture energy 450

fracture toughness 450, 460 properties of 360 Compound characterisation FTIR spectroscopy 240 image analysis 237, 238 intensity of mixing 238 light microscopy 236 methods of analysis 239 microstructural analysis 235 nuclear magnetic resonance 239 on-line flow visualisation 240 residence time distribution 229, 230 rheological analysis 232 scale of segregation 238 screen pack analysis 231 specific energy input 231 specimen preparation 236 spectral photometry 240 striation thickness 238 ultrasonic measurement 233 Compound preparation 207 Compounding 207, 364 abrasion resistance 322 accelerator adsorption 333 additive feeding 227 ancillary equipment 227 breakdown strength 335 burning behaviour 325 compression set 323 conductive rubbers 334 dispersion 208, 366 dissipation factor 335 dynamic properties dynamical mechanical properties 324 fatigue resistance 324 vibration damping 324, 325 electrical properties 324, 334 gas permeability 323, 334 hardness 322 high filler levels 208

529

Particulate-Filled Polymer Composites insulating properties 335 internal mixers 222 ko-kneader 225 machine wear 367 melt filtration 227 mixer designs 220 non-fluxing mixers 221 pelletising 228 permanent set 323, 333 permeability 323 premixing procedures 220 reactive modification 207 relative permittivity 335 resistance to liquids 325 semi-conductive materials 335 shear sensitive fillers 208 single screw mixer design 224 single-screw extrusion 224 strength characteristics 322 tension set 323 ternary-phase 207 thermally sensitive fillers 208 twin-screw extruders 225 two-roll mills 222 volume throughput 365 Compounding machinery functional characteristics 209 high-intensity 221 internal mixers 222 non-fluxing mixers 221 two-roll mills 222 Compounding of particulate fillers effects of dispersion and impact strength 241 factors affecting dispersion 242 inclusion of rubber modifiers 243 Compounding plant design of 219 Compounds characterisation 228

530

Cone calorimeter 508, 509 Coupling agents 27, 311, 344, 459, 501 complexes of chromium 27 organo-borates 27 organosilanes 27, 113 organo-titanates 27 organo-zirconates 27 silane 453 Crack pinning 456 model 455, 458 Crack-tip blunting 458 Crosslinking 305 density 426 Crystalline silicas 76 extraction 76 health issues 78 occurrence 76 particle size 76 properties 76 uses 77 Crystallinity 372, 381 Crystallisation 44 Cure exotherms 435 Cure modification 37 Curing system 305, 308 activators 308 bisphenol curing agents 310 high-energy radiation 310 isocyanates 310 magnesium oxide 309 metal oxides 309 organic accelerator 308 organic accelerators 309 peroxides 309 poly-functional amines 310 prevulcanisation inhibitors 308 resin 310 retarders 308 silane crosslinking 310 Soap/sulfur 310

Index sulfur-based 308 urethane crosslinking agents 310 zinc oxide 309 Cycloaliphatic epoxides 435

D Dawsonite 95, 272 Density. See Specific gravity Devolatilisation melt 217 Diene rubbers sulfur vulcanisation 309 Differential scanning calorimetry (see also DSC) 145 Diffuse reflectance Fourier Transform Infrared...(see also DRIFTS) 134 Diffusion path 507 Dilatancy 34 Dimethyldihalogenosilane 181ß∑ Dispersant fatty acid 113 Dispersion 366, 371 characterise 496 Dispersion viscosity Einstein equation 364 Dolomite 61 extraction 61 occurrence 61 properties 61 uses 61 Drago equation 107 heat of interaction 106, 107, 109 parameters 109 use in FMC 109 use in IGC 121 DRIFTS 108, 147 analysis of cell content 143 comparison with XPS 135 DRIFTS cell 136

internal standards 138 Kubelka-Munk equation 137 post FMC analysis 112, 143 quantitative analysis 137 sample preparation 136 specular reflection 136 spectra 140 surface specificity 135 Dry coating 156, 169 DSC 108, 147 data 146 Dye adsorption 161 Dynamic fatigue effect of filler-matrix adhesion 472 effect of filler particle size 471 effect of water 474 Dynamic properties 336 Dynamic storage modulus 233

E Effective particle concept 16 Einstein equation 125 Elastomers electrical properties antistatic 324 conductive 324 insulating 324 formulation of 306 particulate fillers in 303 uses of 303 Electrical conductivity 369 Electrical properties 369 Electromagnetic interference shielding 369 Endothermic fillers. See Fire retardant fillers Epoxide protonation 439 Epoxy novolac resins 435, 436437 glycidylation 436

531

Particulate-Filled Polymer Composites Epoxy polymer dynamic fatigue 476 Epoxy resins copolymerisation 438 curing of 438 homopolymerisation 438 Equivalent spherical diameters 22 ESCA (see also XPS) 124 Ester plasticisers 312 Etherification 439 EVA coplymer self ignition and incandescence temperatures 294 EVA copolymer cone calorimetry 283 heat release rate 283 Extruder clamshell 367 single-screw 365, 366 twin-screw 365, 366 Extrusion 364 machine wear 367 volume throughput 365

F Fatigue effect of fillers 469 Fatty acid treatments 171, 173 Fatty acids 145, 165 composition 166 saturated 170 Filled polymer processing supercritical fluid 250 Filled thermoplastics uses 396 Filled thermosets 425 Filler particle size 338 specification 338

532

Filler dispersion 366 Filler loading calcium carbonate 235 Filler orientation 372 Filler particle dispersion 25 Filler particle size effect 456 Filler particles 457 Filler production 153 Filler protection 153 Filler surface functional groups hydroxyl groups 104 Filler surface modifier methods of examination 144 structural ordering of 143 Filler surface treatments interaction with fillers 109 Filler surfaces 101 characterising 101 reaction of monoalkoxytitanates 195 Filler treatment in situ 155 pre-coating 155 Filler volume fraction 452 Filler-additive interactions 130 Filler-matrix adhesion 101 Filler-surface modifier 101 coupling agent 101 dispersant 101 Fillers 313 abrasion resistance 326 ageing behaviour 339 chemical composition 339 colour 339 cost reduction 313 density 340 European consumption 397 general properties 328 impurities 339

Index inherent strength 326 market 360 modification of physical properties 313 modification of processing performance 313 moisture content 339 natural rubber 326 non-reinforcing 327 oil absorption 339 organosilane treated 143 particle size 339 performance 326 pH 340 reinforcement 329 reinforcement of rubber 326, 328 particle complexity 327 particle size 327 polymer-filler bonding 327 reinforcing 326 semi-reinforcing 326 sieve residues 339 specification parameters 340 stability 394 stress-induced crystallisation 326 uses of 396 wettability 8 Fire retardancy 507 Fire retardant fillers affect on afterglow 293 historical background 269 Flame retardant. See Fire retardancy Flame retardant fillers (see also flame retardant fillers) 270, 272 aluminium hydroxide 271 ASTM horizontal burn test 280 basic magnesium carbonate 271 boehemite 271 calcium hydroxide 271 calcium sulfate dihydrate 271 dawsonite 271

endothermic 273 ignitability 280 magnesium carbonate sub-hydrate 271 magnesium hydroxide 271 magnesium phosphate octahydrate 271 nesquehonite 271 oxygen index 277, 279, 280 particle size effect 280, 282 smoke formation 291 UL94 vertical burn test 280 Flame retardant performance aluminium hydroxide 276 MCS 276 Flammability testing 264 afterglow and smouldering 265 char integrity 265 cone calorimeter 267 corrosive gas tests 268 corrosive gases 265 dripping 265 FIGRA 268 heat release 265 horizontal burn test 267 ignitability test 265, 267 oxygen index test 266 propagation 265 SMOGRA 268 smoke 265, 268 toxic gases 265 Underwriters Laboratory vertical burn test 266 Flexural fatigue wet and dry 474 Flexural modulus 372 Flow microcalorimetry (FMC) 107, 108, 109, 110, 115, 147 choice of reagents 112 comparison with IGC 119 competitive adsorption studies 117

533

Particulate-Filled Polymer Composites data 115, 118 HPLC detectors 114, 115 method of operation 111 output from 111 Fluid barrier properties 506 Fractal geometry 19 Fracture energy 457 Fracture toughness 457 Fullerenes 492 Fumed silicas 83 applications 84 manufacture of 83 production 83 surface modification 84 Functional fillers 360 Furnace blacks 341

G Gas barrier properties 506 Gelation 425 Gibbs free energy equation 120 Glass ballotini 460 Glass spheres 445, 448 Glass transition 41 Glass-fibre laminates 478 Glycidyl amines 435, 437 Griffith’s theory 463, 467 Gutmann equation 105 IGC 121

H Hamaker constants 388 Hardness 10 HDPE 358 Heat deflection temperature 374, 375 Heat of reaction Gutmann equation 105 High-energy radiation 310 High-impact polystyrene 359, 411

534

HIPS (see high-impact polystyrene) Huntite 92 applications 95 decomposition of 95 general properties 92 Hydrogen bonding 113 Hydroxides 84

I IGC (see inverse gas chromatography) Igneous rocks 55 Impact strength 380, 505 effect of fillers 378 Izod and Charpy 377 toughness 377 Impact testing notched 378 Impurities 55 Infra-red methods 159 Inherent flaw size 462, 464 Injection moulding machine direct compounding 249 Injection moulding technology direct compounding 247 Intercalants 500 organic 499 Intercalation 496 Interparticle distance 34 Interparticle spacing 42 Interphase concept 186 chemisorbed layers 186 effect on processing 186 physisorbed layers 186 Inverse gas chromatography (IGC) 108, 119, 123, 147 comparison with FMC 119 Kovats indices 123 method of operation 121 multiple probe temperatureprogrammed 123

Index output 122 response curves 123 thermodynamics 120 Irganox 1010 118 Isocyanates 310 Isostearic acid 113, 116, 117, 143, 145, 146

K Kaolin 61, 63 aspect ratios 65 differential thermal analysis 67 extraction 61 occurrence 61 properties 61 surface chemistry 65 surface modification 65 uses 65 Kaolinite 63 crystals 62 plates 65 Kerner equation 372, 373 Kovats indices 123 Kubelka-Munk equation 12, 137

L Lampblack 342 Layered silicate states of dispersion 497 Lewis acid-base theory 105 Lewis and Bronsted acid sites 64, 65 Light stabilisation 117 Limestone 57 Linear low density polyethylene 117, 357 Low density polyethylene 357

M Magnesium carbonate sub-hydrate 272 Magnesium hydroxide 88, 113, 138, 146, 162, 273 applications 91 brine type 89 cone calorimetry 283 DRIFTS spectra 145 forms available 88 isothermal decomposition 91 large-crystal type 89 natural form 89 production 88 properties 88 sea-water 89 surface chemistry 91 surface modification 91 thermal decomposition 90 thermal stability 92 Magnesium phosphate octahydrate 272 Maleanised polybutadiene (MPBD) 138 DRIFTS infra-red difference spectrum 140 infra-red spectrum of 139 surface modifier 177 Marble 57 Maximum packing fraction 29 oil absorption procedures 29 Mechanical properties 371 modification 477 Melt devolatilisation mechanism of 218 Melt pressurisation 218 Melt pumping 218 Melt viscosity 363 Metakaolin 66, 369 uses 68 Metamorphic rocks 56 Methyltrimethoxy silane 180 Mica 69, 464

535

Particulate-Filled Polymer Composites extraction 69 occurrence 69 properties 69 uses 70 Microcalorimeter schematic diagram 110 Microcalorimetry HPLC detectors 116 Mie’s theory 12 Mineral fillers 53, 56 thermal conductivities 367 Mineral oils aromatic 312 naphthenic 312 paraffinic 312 Minerals 54 Mixing 214 convective 216 dispersive 214 effect of surface treatment on 216 laminar shear 217 randomisation 220 weighted average total strain 217 Mixture characterisation 207 Modulus 322 Mohs Hardness Scale 10, 11 Molecular interaction 105 Molecular weight effects of compounding on 243 Molecular weight reduction 36 Molybdates 298 Monolayer concept 156 coverage 142 determination 160 theoretical calculation 160 level by DRIFTS analysis 143 Montmorillonite 72 occurence 72 properties 72

536

uses 73 Montmorillonite clay. See Nano-clay Montmorillonite clay plates dispersion 497 Mooney viscometer 320 Morphology effects 274 Muscovite 69, 70

N Nano-clays 296 co-agents 499 composite effects and applications 502 dispersion 495 exfoliates 495 freeze drying 499 heat distortion temperature 502 intercalates 495 maleated PP 501 melt intercalation 497 modulus 502 platy 493 polymer composites processing strategies 494 polymerisation filling 497 processing 493 stability of intercalant 500 strength 502 supercritical fluid 499 ultrasonics 499 Nano-fillers 489 particle forms 490 regular shapes 491 rods, fibres 492 Nano-particle basic types 491 Nanocomposites

Index exfoliation 252 intercalated 253 melt compounding 253 microstructure of 252 reactive extrusion 253 silicate layer polymer 251 Natural clay-based particles alternatives 501 Natural fibre filled composites 247 emission standards 248 fibre-matrix bonding 247 integrated extrusion compounding 247 melt processing 247 moisture removal 247 odour standards 248 Natural fibre-filled thermoplastics woodflour 246 Natural silicas 349 Nesquehonite 272, 273 Nesquehonite in PMMA oxygen index 278 Network structures percolation 335 Neuburger silica 349 Nielsen equation 372 Novolac resins preparation of 441 Novolacs 443 Natural rubber 320 Nuclear magnetic resonance 105 Nucleation of polypropylene effect of fillers 395 Nylon 6 effect of magnesium hydroxide 286, 287 filled properties 409 uses 409

O Oil-absorption 30 Oleic acid 145, 146 Optical properties 11, 14 Organo-silanes 344, 346 Organo-silicon compounds. See also Silane coupling agents applications of non-coupling compounds 185 coating techniques 178 fillers 178 polymer interaction 185 reaction with filler surfaces 179 silane coupling agents 185 structure of coatings 183 Organo-titanates 191 effects produced 197 general principles 192 limitations of 198 monoalkoxy types 194 types for surface modification 193 types of filler 193 Organo-zirconates 191 use of 198 Ozone attack 310

P Packing multimodal 32 ordered 31 random 31, 32 Paris equation 470 Particle effects 36 dispersion 25 matrix adhesion 458 packing 33 packing fraction 29 shape 16, 17

537

Particulate-Filled Polymer Composites assessment of 19 measurement of 19 origins of 18 size 16, 20 distributions 21 measurement 21 spacing 34 structure 19 occluded polymer 20 types 16 volume 39 Particulate fillers 53 characteristics of 5 chemistry 7 composition 7 cost 6, 477 dispersion of 217 impurities 7 natural origins 53 types of 53 use in thermosets 445 Particulate materials 5 selection and use 5 Particulate polymer composites process enhancement 240 Percolation 368, 369, 370 effect 34 Permeability 370 Permeation paths 334 Phenolic resins 440 curing of 442 novolacs 441 resols 442 Phlogopite 69 Pigment twin-screw extruder 231 Pipes 359 Plastic deformation 375 Plasticised PVC cable coverings 399

538

calcium carbonate fillers 400 calendered sheet 401 fillers 399 floor tiles 401 footwear 401 homogeneous flooring 401 hose and profiles 401 leather cloth 401 plastisol 402 sealants 402 spread coatings 401 wall coverings 401 Plasticisers 311, 312 Platy fillers mica 376 Platy minerals clays 330 talcs 330 PMMA 459, 472 PMMA filled with aluminium hydroxide horizontal burn performance 281 ignitability 281 ignition times 282 oxygen index 275 Polar species absorption of 37 Poly-functional amines 310 Polyamide 6,6 aramid fibre-reinforced 245 glass-fibre-reinforced 246 Polyamide chains grafted 499 Polyamides use of fillers 408 Polybutylene terephthalate 410 Polyester unsaturated 428, 429 Polyester resins preparation of unsaturated 426 properties 428

Index unsaturated 427 Polyesters 359 Polyethylene 357 filled polyethylene oxygen index 275 uses of fillers 406, 407 Polyethylene films infra-red spectra 15 Polyethylene terephthalate 359, 410 Polyformaldehyde 411 Polyhedral oligomeric silsesquioxanes 492 Polymer characteristics 318, 319 direct bonding 28 fracture toughness 460 immobilised 38 modifications 27 performance characteristics 317 performance of 317 selection of 306 specification 317 Polymer additives interaction with fillers 109 Polymer compounding 248 Polymer compounds characterisation of filled 230 Polymer conformation 42 Polymer crystallinity 42 Polymer flammability corrosive gases 290 effects of fillers 263 smoke 290 thermal analysis 285 toxic gases 290 Polymer fracture toughness 460 Polymer melting influence of fillers 214 mechanism 213 shear heating 213

Polymer phase effects of filler 380 interphase 382 nucleation 380, 381, 382 transcrystallinity 382 Polymers modulus 322 stiffness 304 strength characteristics 321 stress-induced crystallisation 322 Polyoxymethylene polymers 411 Polyphenylene oxide 411 Polyphenylene sulfide 411 Polypropylene 101, 358 cone calorimetry 284 effect of magnesium hydroxide 288, 289 filled uses 404 heat release rate 284 properties 379 smoke level 284 uses of fillers 404 Polypropylene compositions glass-fibre reinforced 245 Polypropylene homopolymer HDT 374 impact strength 381 tensile modulus 372 yield strength 377 Polystyrene 410 general purpose 359 Polyvinyl chloride 359 cable sheathing formulation 400 fillers 398 Powder densities 10 Precipitated silica 81 polymer applications 83 production 81 properties 82

539

Particulate-Filled Polymer Composites silica structure 82 surface modification 83 Primary particles 23 Process enhancement 207 Process oils 311 Processing 153, 330 bagging 331 cure rate 331 green strength 330 mill sticking 331 nerve 330 viscosity 321, 330 Properties bulk and process 361 Pyrolysis 159

Q Quadrupole mass spectrometer 133 Quaternary ammonium salts 143

R Reactive diluents 437 Reactive techniques 109 Recycleability 396 Refractive index 12 Refractory material 67 Reinforcement level of 375 Relative density 362 Resins 310 condensation 440 epoxy 435 free-radical chain-growth curing 427 low-viscosity 429 phenolic 440 preparation of urethane methacrylate 432 preparation of vinyl ester 431 preparation of vinyl urethane 431

540

step addition curing 435 urethane methacrylate 432 vinyl ester 430 vinyl urethane 430 Resistance to liquids 336 Resols 442 acid cure of 442 cured crosslinked 442 heat cure of 443 Reuss model 446, 447 RFS system 344 Rolling resistance 344 Rubber elasticity of 303 formulation 307 specialised additives 307

S Saturated fatty acids 164 Sedimentary rocks 56 Sedimentation 22 Settling volume method 160 Shear heating 213 Shrinkage 368, 480, 481 Silane coupling agents amino functional 188 effects in elastomers 190 effects in filled polymers 190 effects in thermoplastics 191 effects in thermosets 190 epoxy functional 188 interpenetration of matrix 187 methacryl functional 188 mixed silane systems 189 oligomeric silanes 189 polymer interaction 185 sulfur functional 189 types 187 uses 187 vinyl functional 188

Index Silane crosslinking 310 Silanes 102 Silica fumed 112 natural crystalline 453, 464, 472 synthetic 81 Silica filled PMMA static properties 472 Silica sand natural crystalline 445, 448, 460 Silica-filled methacrylate composites 475 Silica-filled methacrylate composites 472 Silicas and silicates 343 fumed silica 343 precipitated silica 343 silicates 343 synthetic 343 Silicate gallery 495 SIMS 108, 124, 147 comparison with XPS 133 dynamic 130 instrumental aspects 133 static 130 time-of-flight 133 Sinter hydrates 85 Slip-stick propagation 452 Smoke suppressant 348 Soap/sulfur systems 310 Specialty additives 314 antistatic agents 316 blowing agents 316 bonding agents 316 dyes 315 emulsified esters 314 fatty acids 314 fire retardants 315 liquid polymers 314 mill 314 mould-release agents 314

odourants 316 paraffinic wax 314 peptisers 314 pigments 315 polyethylene waxes 314 polyethylenes 314 polyoctenamer 314 precipitated silica 316 process aids 314 resorcinol-formaldehyde-silica 316 smoke suppressants 315 soaps 314 tackifiers 315 Specific gravity 9. See also Relative density Specific heat 15 capacity 367 Specific surface area 22 Spectral absorption 14 Spectroscopic techniques 124 DRIFTS 108, 134, 135, 136, 140, 141, 143, 145, 147 ESCA 124 FTIR 104, 183 SIMS 104, 108, 124, 130, 133, 147, 183 ToFSIMS 130, 133 UHV 124 WAXS 108, 147 XPS 104, 124, 133, 135, 183 Spherical fillers 376 Squalane 394 stability 394 Stabilisation 393 Stabilisation and recycleability 392 Stabilisers 117 hindered phenols 393 Stability antioxidant adsorption 394 filler chemistry 393 impurities 393

541

Particulate-Filled Polymer Composites Stannates 298 Static calorimeter 107 Static calorimetry 109 Static fatigue 471 Static fatigue experiments 470 Stearate coating. See also Surface treatment Stearates 26 Stearic acid 26, 101, 103, 116, 141, 145, 146, 167, 170, 347 properties 166 Stearic acid coating effects in composites 171 Steric barrier 102 Stokes’ law 22 Strain rate 449 Stress whitening 14 Stress-induced crystallisation 322 Supercritical fluids viscosity reduction effects 249 Surface analysis methods flow microcalorimetry 104 Fourier transform infrared spectroscopy 104 inverse gas chromatography 104 secondary ion mass spectroscopy 104 X-ray photoelectron spectroscopy 104 Surface chemistry 7 Surface energies 8 Surface interactions 7 Surface modification 26, 153 general principles 154 Surface modification of particles 453 Surface modifier anhydrides 175 coating level 156 coupling 154 effect of surface topography 157 effect on polymer crystallinity 172 monomeric organic acids 163 non-coupling 154

542

surface coverage 160 types 163 Surface modifiers 153 acid functional saturated polymers 175 acid functional unsaturated polymers 176 amino-acids 200 borates 199 dimaleimides 175 dry coating 165 fatty acid salts 173 functional organic acids 173 methods of use 155 organic amines 200 phosphates 199 polymeric acids 175 reasons for use 153 rosin 174 wet coating process 165 Surface mono-layers structures for 157 Surface science adhesion 385 agglomeration 387 dispersion 387 spreading 384 spreading coefficient 385 surface energy 383, 384 surface tension 383, 384 wetting 384 Surface treatments 344 coupling agents 388, 389 dispersants 388, 389 Surface treatments. See also Coupling agents Synthetic particulate fillers 78

T Talc 70, 349 occurrence 71

Index processing 71 properties 70 surface modification 72 uses 72 Tensile modulus 372 Tensile yield stress effect of fillers 463 Thermal blacks 342 Thermal conductivity 15, 367 Thermal expansion 368 coefficient of 16 Thermal properties 14 Thermoplastic composites 359, 360 Thermoplastics applications 357 calcium carbonate 399 fillers 360 particulate fillers 412 use in Western Europe 359 Thermoplastics composites interfacial shear strength 244 short fibre-reinforced 244 Thermoset polymers chemistry 427 Thermoset recyclate composites mechanical properties 251 Thermoset recyclate fillers 250 Thermosets applications 477 cost reduction 477 exotherm control 478 failure stress 463 fatigue 469 flame retardants 482 fracture energy 450 fracture toughness 450 mechanical properties 444 metal fillers 482 modulus 444 processing aids 481

shrinkage control 480 structural adhesives 483 TiO pigments 2 surface analysis of 134 ToF SIMS spectra 130, 133 Toughness 505 ductile to brittle transition 462 Toughness of composites effect of particle size 450 effect of strain rate 449 Transcrystallinity 43, 371 Transport of feedstock compaction 210 flow of particulate solids 211 frictional effects 212 hopper design 211 particle characteristics 210 (Trimethoxysilyl)propyl methacrylate 129. See also Silanes silanes 102, 103 Twin-screw extruders positive-displacement 212 screw configuration 226

U Ultra high vacuum spectroscopy 124 Unplasticised PVC conduit 403 fillers 402 film 403 fittings 403 pipes 403 profiles 403 window 403 Urethane crosslinking agents 310 Urethane methacrylate polymer 449

543

Particulate-Filled Polymer Composites

V

X

Viscosity measurements 40 Viscosity reduction 160 Vitrification 425 Voigt model 446, 447

X-ray photoelectron spectroscopy (see also XPS) physical basis 124 X-ray photoemission 124 XPS 108, 124, 147 Auger electrons 125 comparison with DRIFTS 135 comparison with SIMS 133 Einstein equation 125 information levels 128 instrumental aspects 128 penetration depth 125 TiO pigment 126, 127

W Wallace rapid plasticity 320 Warpage 368 WAXS 108, 147 Wet flexural fatigue 475 Wetting 371 Wide angle X-ray diffraction 145 WLF equation 377 Wollastonite 74 applications 75 occurrence 74 processing 74 properties 74 surface modification 75 Work of adhesion Lewis acid-base terms 386 non-polar case 386 polar case 386

544

2

Y Yield strength 375, 376 Young’s modulus 444, 445, 446, 451

Z Zircoaluminates 198

ISBN: 1-85957-382-7

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