Fire Retardancy of Polymers: New Strategies and Mechanisms

Fire Retardancy of Polymers New Strategies and Mechanisms

This book is dedicated to our families, for the support and tolerance they showed during its production, particularly to Helen, Sam, Matthew, Isobel, Baldev, Abhineet, Navrohit and Amitarun.

Fire Retardancy of Polymers New Strategies and Mechanisms

Edited by T Richard Hull Centre for Fire and Hazard Sciences, University of Central Lancashire, Preston, UK

Baljinder K Kandola Centre for Materials Research and Innovation, The University of Bolton, Bolton, UK

ISBN: 978-0-85404-149-7 A catalogue record for this book is available from the British Library r Royal Society of Chemistry 2009 All rights reserved Apart from fair dealing for the purposes of research for non-commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 0WF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org

Preface This volume follows in a tradition of ten previous biennial meetings and publications by those working to reduce the hazards associated with burning polymers. The 11th Meeting on Fire Retardant Polymers (FRPM’07), which took place in the Albert Halls in Bolton in July 2007, brought together over 200 scientists from across the globe with representatives from over 20 countries, to discuss the latest developments in fire retardant technology. There were around 100 presentations at the meeting, and as editors we have extracted the highlights, summarised in 25 chapters, in order to present the state of the art in fire retardant strategies and mechanisms. All of the papers in this volume were subject to peer review, by two independent experts in fire retardancy. We are extremely grateful for the efforts put in, both by our authors and by the reviewers in helping us to maintain the highest standards in this volume. The volume opens with a brief overview on Polymers and Fire setting the context by outlining the stages of a fire and the broad groups of fire retardants and their modes of action. The first section discusses the new Fire Retardant Strategies which are currently emerging. The Synergy between nanometric alumina and organoclay in conventional fire retardant systems for ethylene-vinyl acetate leads in with the novel approach of investigating the effect of conventional fire retardants in nanoscopic form. Strained organophosphorus compounds as reactive flame retardants for polymeric materials continues the quest for halogen free fire retardants by designing phosphorus compounds which will decompose in sequence with the polymer to optimise the flame retardant effect. Amorphous silicon dioxide as additive to improve the fire retardancy of polyamides describes the novel application of smooth silica microspheres for flammability reduction. Use of organosilicone composites as flame retardant additives and coatings for polypropylene demonstrates the dual effects on flame retardancy and reducing surface adhesion of treatments based on polyborosiloxane. Organo-modified Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Preface

ultrafine kaolin for mechanical reinforcement and improved flame retardancy of recycled PET shows how the interaction of ultrafine kaolins with triphenylphosphite leads to the formation of a barrier which enhances charring. Complex micro-analysis assisted design of fire retardant nanocomposites-contribution to the nano-mechanism uses a novel laser-Raman system to simultaneously decompose and probe the decomposition products of fire retardant polymer systems. The second section is focussed on Nanoparticulate Fillers which have combined the potential to improve the physical properties of polymeric materials while also reducing their flammability. The section opens with a discussion on the Impact of nano-particle shape on the flammability of nanocomposites. The majority of fire retardant nanotechnology is currently focussed on the incorporation of nanoscale carbon and clay to form polymer nanocomposites. The Thermal and combustion behaviour of polymer-carbon nanofibre composites describes an approach with one form of these new materials. This is followed by an extensive investigation of the Combination of carbon nanotubes with fire retardants: the thermal and fire properties of polystyrene nanocomposites. Significant assessment of nanocomposites combustion behaviour by the proper use of the cone calorimeter describes the interpretation of the novel burning behaviour of polymer nanocomposites, and its investigation using cone calorimetry. The combination of traditional and nanocomposite approaches to fire retardancy using Phosphorus based epoxy resins nano-clay composites is reported. A novel approach to the somewhat elusive, and variable, mechanisms of nanocomposite fire retardancy is approached using viscosity measurements in the Study of the relationship between flammability and melt rheological properties of flame retarded poly(butylene terephthalate) containing nanoclays. The section concludes with another investigation combining novel and nanocomposite fire retardants outlining the Thermal and fire performance of flame-retarded epoxy resin: Investigating interaction between resorcinol bis(diphenyl phosphate) and epoxy nanocomposites. The third section investigates another important class of fire retardants, Intumescents, whose effect is created by the formation of a barrier layer which is then driven from the surface of the substrate by the release of a volatile or gaseous component. These may take the form of coatings or may be incorporated into the bulk of the material. A major application is in the creation of a polymer-based coating which is applied to structural members such as steelwork for fire protection. The section opens with an analysis of the bubble formation responsible for blocking the transfer of heat using Image analysis of 2D intumescent char sections to estimate porosity. This is followed by a description of Efficient modelling of temperatures in steel plates protected by intumescent coating in fire. The section finishes with a discussion focussed around Fire retardancy and fire protection of materials using intumescent coatings - a versatile solution? The fourth section describes one of the major and most challenging application areas for fre retardant systems, namely Fibres and Textiles. The section opens with a discussion of Trends in textile flame retardants – a market review.

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This is followed by an overview of New and potential textile flammability regulations and test methods within the USA. From this contextual setting of the market and regulatory framework, an investigation of the Flame retardancy of cellulosic fabrics: interactions between nitrogen additives and phosphoruscontaining flame retardants is described. The section then covers two detailed investigations into the fire retardancy of polyacrylonitrile fibres. The Synergistic flame retardant copolymeric polyacryonitrile fibres containing dispersed phyllosilicate clays and ammonium polyphosphate is reported, followed by reactive fire retardant approach to Flame retardance of polyacrylonitriles covalently modified with phosphorus- and nitrogen-containing groups. This is followed by Novel fire retardant back-coatings for textiles. The section concludes with a description of The effect of yarn and fabric construction and colour in respect of red reflectance and pigmentation on the thermal properties and LOI of flame retardant polypropylene fabrics. The final section of the book describes investigations into the effects of fire retardant on the major hazard to life in fire, that resulting from Fire Toxicity. The Influence of fire retardants on toxic and environmental hazards from fires describes a large body of work assessing the hazards associated with burning polymers and their flame retardant counterparts. The volume concludes with a discussion on the specific influence of nanocomposite formation and fire retardants on the toxic product yields of nylon 6 and polypropylene in Assessment of fire toxicity from polymer nanocomposites. We trust that the book will provide fire retardant materials’ developers with latest developments in research or design of new fire retardant materials and understanding their mechanisms of action. Finally, we would like to thank our colleagues, Prof Dennis Price, Prof Richard Horrocks, Dr Shonali Nazare, Dr Everson Kandare and Dr Anna Stec for their support during completion of this book.

Contents Introduction

Polymers and Fire T.R. Hull and A.A. Stec 1 2

Hazards from Fire Fires and Fire Growth 2.1 Conditions of Each Fire Stage 2.2 Chemical and Physical Processes 2.3 Studying Polymer Decomposition 3 Fire Effluent Toxicity 4 Structural Deformation 5 Fire Retardant Strategies 5.1 Physical Action 5.2 Chemical Action 5.2 Polymer Nanocomposites 6 Conclusions References

1 2 5 6 7 8 9 9 10 10 11 12 14

Fire Retardant Strategies Chapter 1

Synergy between Nanometric Alumina and Organoclay in Conventional Fire Retardant Systems for Ethylene–Vinyl Acetate N. Cinausero, J.-M. Lopez-Cuesta, F. Laoutid, A. Piechaczyk and E. Leroy 1.1 1.2

Introduction Experimental 1.2.1 Materials 1.2.2 Processing 1.2.3 Testing

17 18 18 18 18

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Contents

1.3 Results and Discussion 1.4 Conclusion References Chapter 2

Strained Organophosphorus Compounds as Reactive Flame Retardants for Polymeric Materials Bob A. Howell 2.1 2.2

Introduction Experimental 2.2.1 Materials 2.2.2 Initiator, 2,4,4,5,5-Pentaphenyl-1,3,2-Dioxaphospholane 2.2.3 Polymers 2.3 Results and Discussion 2.3.1 Thermal Properties of Styrene Polymers Containing Phosphorus Units 2.3.2 Evaluation of Flammability 2.4 Conclusions References

Chapter 3

28 29 29 29 29 30 33 33 34 34

Amorphous Silicon Dioxide as Additive to Improve the Fire Retardancy of Polyamides G. Schmaucks, B. Friede, H. Schreiner and J.O. Roszinski 3.1 3.2

Introduction Experimental 3.2.1 Materials 3.2.2 Sample Preparation 3.2.3 Test Methods 3.3 Results and Discussion 3.4 Conclusion Acknowledgement References

Chapter 4

19 26 27

35 40 40 40 40 41 47 47 48

Use of Organosilicone Composites as Flame Retardant Additives and Coatings for Polypropylene + A. Szabo´, K. Kiss and G. Marosi B.B. Marosfoi, 4.1 Introduction 4.2. Experimental 4.2.1 Materials 4.2.2 Sample Preparation 4.2.3 Preparation of PP Compounds

49 50 50 51 51

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4.2.4

Preparation of Composites with Multilayer Structures 4.2.5 Characterisation 4.3 Results and Discussion 4.3.1 Thermo-Oxidative Stability 4.3.2 Combustion Characteristics of Polypropylene-Based Composites 4.3.3 Multilayer Structure PP–(pBSil–OSEP–MB) 4.4 Conclusion Acknowledgements References Chapter 5

53 56 57 57 58

Organomodified Ultrafine Kaolin for Mechanical Reinforcement and Improved Flame Retardancy of Recycled Polyethylene Terephthalate B. Swoboda, E. Leroy, J.-M. Lopez Cuesta, C. Artigo, C. Petter and C.H. Sampaio 5.1 5.2

Introduction Experimental 5.2.1 Materials 5.2.2 Processing 5.2.3 Characterization Techniques 5.3 Results and Discussion 5.3.1 Properties of Unmodified Kaolins 5.3.2 Grafting of TPP onto Kaolin Surface 5.3.3 Morphological, Rheological and Mechanical Properties of Polymer Compounds 5.3.4 Thermal Stability and Reaction to Fire of Polymeric Compounds 5.4 Conclusion 5.5 Acknowledgements References

Chapter 6

51 52 52 52

59 61 61 61 62 64 64 64

67 69 73 73 73

Complex Micro-analysis Assisted Design of Fire-Retardant Nanocomposites – Contribution to the Nanomechanism + P. Anna and Gy. Marosi A. Szabo´, B.B. Marosfoi, 6.1 6.2

Introduction Experimental 6.2.1 Materials 6.2.2 Methods

74 78 78 78

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Contents

6.3

Results and Discussion 6.3.1 Nanonetwork Formation 6.3.2 Intumescent Polymeric Particle Formation 6.4 Conclusion Acknowledgements References

80 80 84 88 89 89

Nanoparticulate Fillers Chapter 7

Impact of Nanoparticle Shape on the Flammability of Nanocomposites F. Yang, I. Bogdanova and G. L. Nelson 7.1 7.2

Introduction Experiment 7.2.1 Preparation of Polymer–Inorganic Nanocomposites 7.2.2 Mechanical Testing of Polymer–Inorganic Nanocomposites 7.2.3 Morphology Study for Polymer–Inorganic Nanocomposites 7.2.4 Thermal Degradation of Polymer–Inorganic Nanocomposites 7.2.5 Flammability of Polymer–Inorganic Nanocomposites 7.3 Results and Discussion 7.3.1 Polycarbonate–Inorganic Nanocomposites 7.3.2 PS–Inorganic Nanocomposites 7.4 Conclusion References

Chapter 8

95 96 96 96 96 97 97 97 97 102 107 108

Thermal and Combustion Behaviour of Polymer–Carbon Nanofibre Composites D. Tabuani, S. Pagliari, W. Gianelli and G. Camino 8.1 8.2

Introduction Materials and Methods 8.2.1 Melt Blending 8.2.2 From Solution 8.2.3 Characterization 8.3 Results and Discussion 8.3.1 Morphology 8.3.2 Thermal Behaviour 8.4 Conclusions References

110 111 111 111 112 113 113 115 123 123

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Chapter 9

Combination of Carbon Nanotubes with Fire Retardants: Thermal and Fire Properties of Polystyrene Nanocomposites Florentina Tutunea and Charles A. Wilkie 9.1 9.2

Introduction Experimental 9.2.1 Materials 9.2.2 Preparation of Composites 9.2.3 Instrumentation 9.3 Results and Discussion 9.3.1 Thermogravimetric Analysis 9.3.2 Cone Calorimeter Evaluation 9.4 Conclusions References

Chapter 10

Significant Assessment of Nanocomposites’ Combustion Behaviour by the Appropriate Use of the Cone Calorimeter A. Fina, F. Canta A. Castrovinci and G. Camino 10.1 10.2

Introduction Experimental 10.2.1 Materials 10.2.2 Preparation and Characterization 10.2.3 Combustion Tests 10.3 Results and Discussion 10.3.1 8 mm Specimens Combustion Behaviour 10.3.2 16 mm Specimens Combustion Behaviour 10.4 Conclusions Acknowledgements References

Chapter 11

125 126 126 127 127 127 127 135 145 146

147 148 148 148 150 151 151 153 157 158 158

Phosphorus-Based Epoxy Resin–Nanoclay Composites Jianwei Hao, Yanbing Xiong and Na Wu 11.1 11.2

Introduction Experimental 11.2.1 Materials 11.2.2 Preparation of Phosphorus-Based Epoxide 11.2.3 Preparation of Phosphorus-Based Epoxy–Nanoclay Composites 11.2.4 Characterization 11.3 Results and Discussion

160 161 161 162 162 162 163

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11.3.1

Structure of EPO–P Analysis with FTIR 11.3.2 Structure of EP–P–nano Composites Analysis with XRD and TEM 11.3.3 Combustion Performance and Mechanical Properties 11.4 Conclusion References Chapter 12

163 164 167 167

Study of the Relationship Between Flammability and Melt Rheological Properties of Flame-Retarded Poly(Butylene Terephthalate) Containing Nanoclays S. Nazare, T. R. Hull, B. Biswas, F. Samyn, S. Bourbigot, C. Jama, A. Castrovinci, A. Fina and G. Camino 12.1 12.2

Introduction Experimental 12.2.1 Materials 12.2.2 Sample Preparation 12.2.3 Characterization and Testing 12.3 Results and Discussion 12.3.1 Nanodispersion 12.3.2 Differential Scanning Calorimetry and Thermal Analysis 12.3.3 Melt Viscosity 12.3.4 Flammability 12.4 Conclusions Acknowledgements References Chapter 13

163

168 170 170 170 171 172 172 174 177 178 182 182 183

Thermal and Fire Performance of Flame-Retarded Epoxy Resin: Investigating Interaction Between Resorcinol Bis(Diphenyl Phosphate) and Epoxy Nanocomposites Charalampos Katsoulis, Everson Kandare and Baljinder K. Kandola 13.1 13.2

Introduction Experimental 13.2.1 Materials 13.2.2 Sample Preparation and Characterization

184 185 185 185

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13.2.3 13.2.4 13.3 Results 13.3.1 13.3.2

Thermogravimetric Analysis Flammability Tests and Discussion XRD and TEM Analysis Thermal Degradation Behaviour of Epoxy Resin and Its Composites 13.3.3 Flammability Behaviour 13.4 Conclusions Acknowledgements References

186 186 187 187 188 195 202 203 203

Intumescents Chapter 14

Porosity Estimates of Intumescent Chars by Image Analysis J.E.J. Staggs 14.1 14.2 14.3

Introduction Pore-Finding Algorithm Relationship Between Area Porosity and Volume Porosity 14.4 Relationship Between 2D and 3D Pore Distributions 14.4.1 Test Case 1 (Identical Spheres) 14.4.2 Test Case 2 (Spheres with Uniformly Distributed Radii) 14.5 Construction of 3D Distributions from 2D Distributions 14.6 Analysis of a Real Char Section 14.7 Conclusion Acknowledgements References Chapter 15

209 212 213 215 216 217 217 219 223 224 224

Efficient Modelling of Temperatures in Steel Plates Protected by Intumescent Coating in Fire J.F. Yuan and Y.C. Wang 15.1 Introduction 15.2 Mathematical Modelling 15.3 Validation 15.4 General Analysis of Intumescence Process 15.5 Parametric Studies 15.6 Conclusions Nomenclature References

225 227 230 231 233 238 238 239

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Chapter 16

Contents

Fire Retardancy and Fire Protection of Materials using Intumescent Coatings – A Versatile Solution? S. Duquesne, M. Jimenez and S. Bourbigot 16.1 16.2

Introduction The Use of Intumescent Coatings for The Fire Protection of Steel Structures 16.3 Fire Protection of Polyurethane Foams using Intumescent Systems 16.4 Conclusion References

240 241 245 252 252

Fibres and Textiles Chapter 17

Trends in Textile Flame Retardants – a Market Review R. Hicklin, R. Padda and G. Lenotte 17.1 Introduction 17.2 Burning Behaviour of Cotton Fabrics 17.2.1 Factors that Affect the Burning Behaviour of Cotton Fabrics 17.2.2 Combustion of Cotton 17.3 Mechanism of Phosphorus Flame Retardants 17.4 Classification of Flame Retardants 17.5 Flame Retardant Selection 17.5.1 Non-Durable Flame Retardants 17.5.2 Semi-Durable Flame Retardants 17.5.3 Durable Flame Retardants 17.6 Flammability Standards and Testing 17.7 Health, Safety and Environmental Considerations 17.8 Fibre Blends 17.9 Future Developments 17.9.1 Multifunctional Fabrics 17.9.2 Alternative Chemistries 17.9.3 Application Technologies 17.10 Conclusions References

Chapter 18

255 256 256 256 256 257 257 257 259 259 261 261 262 263 263 263 263 264 264

New and Potential Textile Flammability Regulations and Test Methods within the USA P.J. Wakelyn 18.1 Introduction 18.2 Mattresses and/or Foundation (Box Springs)

266 270

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18.2.1 US CPSC 18.2.2 CA BHFTI 18.3 Bedclothes 18.3.1 US CPSC 18.3.2 CA BHFTI 18.4 Upholstered Furniture 18.4.1 US CPSC 18.4.2 CA BHFTI (TB 117, TB 116) 18.4.3 Upholstered Furniture Action Council Voluntary Furniture Smoulder/ Cigarette Test 18.4.4 ASTM Standard 18.5 Children’s Sleepwear 18.5.1 1996 Amended Standard 18.5.2 New CPSC Data Collection Tool for Clothing-Related Burn Injuries to Children 18.6 Clothing Textiles 18.6.1 US CPSC General Apparel Standard (16 CFR 1610) 18.6.2 US CPSC Updated Standard (2008) 18.7 Carpets and Rugs 18.8 Ignition Sources 18.8.1 Cigarette Lighters 18.8.2 Candles and Candle Accessories 18.8.3 Matches 18.8.4 Cigarettes 18.9 Flame Retardant Chemicals 18.10 Summary and Conclusions References Chapter 19

270 271 272 272 272 275 275 280

282 283 283 283

284 284 285 285 286 287 287 287 287 287 288 289 290

Flame Retardancy of Cellulosic Fabrics: Interactions between Nitrogen Additives and Phosphorus-Containing Flame Retardants Sabyasachi Gaan, Gang Sun, Katherine Hutches and Mark Engelhard 19.1 19.2

Introduction Experimental 19.2.1 Material 19.2.2 Sample Preparation 19.2.3 Measurements 19.3 Results and Discussion 19.3.1. Flammability of Treated Fabrics 19.3.2. Surface Morphology of Char 19.3.3 ATR-FTIR Spectra of Char Surfaces

294 296 296 296 296 297 297 297 300

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19.3.4 19.3.5

XPS Analysis of Char Mechanism of Formation of Surface Coating on the Char 19.4 Conclusion References Chapter 20

Chapter 21

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres Containing Dispersed Phyllosilicate Clays and Ammonium Polyphosphate A.R. Horrocks, J. Hicks, P.J. Davies, A. Alderson and J. Taylor 20.1 Introduction 20.2 Experimental Method and Results 20.2.1 Materials and Characterization 20.2.2 In-Situ Radical Polymerization of Nanocomposite Copolymers 20.2.3 Dope Blending of Clays 20.2.4 Polymer Spinning 20.2.5 Physical Characterization 20.5.6 Preparation and Flammability Testing of Flame Retarded Experimental Tows and Polymer Samples 20.6 Conclusions Acknowledgements References

303 305 305

307 308 308 310 313 313 314

320 328 329 329

Flame Retardance of Polyacrylonitriles Covalently Modified with Phosphorus- and Nitrogen-Containing Groups John R. Ebdon, Barry J. Hunt, Paul Joseph and Tara K. Wilkie 21.1 Introduction 21.2 Experimental 21.3 Characterization 21.4 Results and Discussion 21.5 Conclusions Acknowledgements References

Chapter 22

300

331 332 334 335 338 339 339

Novel Fire Retardant Backcoatings for Textiles M.A. Hassan 22.1 22.2

Introduction Experimental

341 342

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22.2.1 22.2.2 22.2.3 22.2.4 22.3 Result 22.3.1

22.3.2 22.3.3

22.3.4 22.3.5 22.3.6 References

Chapter 23

Materials Preparation of Flame Retardant Compounds Preparation of Coating Paste Characterization and Discussion Thermal Characterization of A1 and A5 Organophosphorus Compounds Pyrolysis Behaviour of Uncoated and Back-coated Cotton Samples Thermal Pyrolysis Process of Uncoated and Back-coated Polyacrylic Samples Flammability Properties Smoke Density Measurements Conclusion

342 342 345 346 346

346 348

351 354 355 357 358

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance and Pigmentation on the Thermal Properties and Limiting Oxygen Index of Flame Retardant Polypropylene Fabrics C. Kindness B.K. Kandola and A.R. Horrocks 23.1 23.2

Introduction Experimental Methods 23.2.1 Materials 23.2.2 Flammability Testing 23.2.3 Thermal Analysis 23.2.4 Air Permeability 23.2.5 Colour Measurement and Pigment Analysis 23.3 Results and Discussions 23.3.1 Commercial Fabrics 23.3.2 Experimental Fabrics 23.3.3 Air Permeability and the Effect on LOI 23.3.4 Effect of Colour on LOI 23.3.5 Thermal Analysis of Fabrics and Colour Pigments 23.4 Conclusions Acknowledgements References

359 362 362 365 366 366 366 367 367 368 369 370 372 373 376 376

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Fire Toxicity Chapter 24

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires David Purser 24.1 24.2 24.3

Introduction Major Determinants of Toxic Product Yields Influence of Different Fire Retardant Systems on Toxic Product Yields and Toxic Hazards 24.3.1 Inert and Active Fillers 24.3.2 Phosphorus-based Systems 24.3.3 Nitrogen, Melamine and Melamine– chlorinated Phosphate Systems 24.3.4 Halogen Acid Vapour-phase Systems 24.3.5 Fluoropolymers and Ultrafine Particles (Nanoparticles) 24.3.6 Environmental Contamination by Dioxins and Furans from Halogenated Materials 24.4 Conclusions References Chapter 25

387 387 390 390 393 394

398 402 402

Assessment of Fire Toxicity from Polymer Nanocomposites Anna A. Stec and T. Richard Hull 25.1

Introduction 25.1.1 Fire Scenarios 25.1.2 The Steady-state Tube Furnace (ISO 19700) 25.1.3 Toxic Potency of Fire Effluent 25.2 Experimental 25.2.1 Materials 25.3 Results 25.3.1 Yields of Toxic Products from PA6 and PP 25.3.2 Fractional Effective Dose 25.3.3 LC50 of Different Polymeric Materials 25.4 Conclusions References

Subject Index

381 383

405 406 407 408 410 410 410 410 413 415 416 416 419

INTRODUCTION

Polymers and Fire T. RICHARD HULL AND ANNA A. STEC Centre for Fire and Hazards Science, School of Forensic and Investigative Sciences, University of Central Lancashire, PR1 2HE, Preston, Lancashire, United Kingdom

Unwanted fires account for significant losses to life and property. In the UK about 600 lives are lost each year,1 and the cost of unwanted fire has been estimated at about 1% of the UK’s gross domestic product.2 The vast majority of unwanted fires are fuelled by organic polymers and, as manufacturing technology has advanced, there has been a rapid shift from natural polymers (contained in wood, cotton, leather and wool) to synthetic polymers. Synthetic polymers are generally more flammable than their natural counterparts – polyethylene, polypropylene (PP) and polystyrene have calorific values comparable to that of petroleum. However, unlike natural polymers, which can only be fire retarded by coatings or other surface treatments, for most plastic materials the manufacturing process is ideal for the incorporation of fire retardants.

1 Hazards from Fire The flammability of a material is not an intrinsic property, like its density or heat capacity, but is dependent on the fire conditions. The apparent order of flammability of two materials may be reversed if tested under different conditions. Similarly, changing the material composition, for example by the addition of a fire retardant, will also change its reaction to fire behaviour. The incorporation of a nanofiller will reduce the dripping tendency. In one fire scenario, dripping away from a flame will reduce the ignitability, while in Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

1

2

Introduction Heat

Fuel

Figure 1

Oxygen

The fire triangle.

another, drips, especially flaming drips, will cause downward flame spread, which significantly increases the fire hazard. The fire triangle (Figure 1) demonstrates the interdependence of the material properties with ventilation and heat. In general, fire growth is more favourable as the heat flux or oxygen supply increases, or if the material is more ‘‘flammable’’. However, excessive ventilation may remove heat from the flame, while additional heat may also result in melting or char formation, each of which could reduce fire growth. This scenario dependence ultimately favours certain materials under certain conditions. This is the heart of the difficulty in defining flammability, and explains why the materials development described in the different sections of this book often give apparently inconsistent results when tested under different conditions. Recent developments in flammability testing have brought us nearer to addressing the ultimate goal of predicting large-scale fire behaviour from smallscale tests, as have measurements of material properties coupled to models of full-scale fire behaviour. However, large-scale fires show considerable variation, so there is no universal benchmark against which to judge a material’s fire performance.

2 Fires and Fire Growth Fire tests that focus on particular fire stages should address the prevailing conditions appropriately. Most fires start from small beginnings. There may be an induction period (involving smouldering) before flaming ignition takes place, then a rise in temperature until ventilation-controlled burning occurs (usually 800–1000 1C), followed by decay as fuel is consumed, shown schematically in Figure 2. During the ignition phase, the impact of heat on a polymeric material causes an increase in temperature. If a sufficiently high temperature is reached then chemical bonds break and volatile fragments are produced. As they are hot, they are buoyant above the surface of the polymer. Once a sufficient concentration is reached, if the products are flammable then a flame may stabilize. Ignition may either be piloted by a flame or spark or be spontaneous; the latter typically occurs when the polymer surface is 200 1C hotter. There will be convected heat above the flame and radiative heat in all directions, including downwards, as shown in Figure 3.

Polymers and Fire

Figure 2

Stages in a fire.

Figure 3

Primary ignition process.

3

These heat-transfer processes are critical to the ignition and fire behaviour. Once ignited, the fire initially grows by a process of flame spread. The surfaces near the pyrolysis and flame zone are heated and decompose to form more flammable products. The flame spreads by piloted (i.e. flame) ignition of these areas beyond the burning zone. Hence flame spread is essentially a process of repeated ignitions, as depicted in Figure 4. Horizontal flame spread is relatively slow because the material ahead of the flame is heated by gas-phase conduction only, enhanced by downward

4

Introduction

Figure 4

Horizontal flame spread (slow).

Figure 5

Vertical flame spread (rapid).

radiation. Upward flame spread (Figure 5) is more rapid, because radiative, convective and some conductive heat transfer occurs. During the early growth phase, flaming is normally confined to the item first ignited. As flames become more than about 1 or 2 m high, radiative heat transfer to adjacent items becomes important, even to objects several metres away from the flame (Figure 6). These items decompose predominantly as a consequence of radiative heat transfer, pyrolyze and may then spontaneously ignite. As the flame reaches the ceiling, it spreads across it, dramatically increasing the radiant flux to other objects in the room. These then pyrolyze, filling the room with a flammable, or even explosive, fuel–air mixture. Once this occurs then the whole room will burn, and flashover is said to occur (Figure 7). At this point, the speed of flame spread will be faster than the running speed, and the fire can no longer be controlled. Burning will continue until the available fuel is consumed.

5

Polymers and Fire

Figure 6

Ignitability at a distance.

Radiation

Pyrolyzing fuel

Figure 7

2.1

Flashover conditions.

Conditions of Each Fire Stage

To simulate the effects of fire for materials development and testing, the test conditions should be related to the appropriate scenario.
Ignition. It is more difficult to obtain repeatable results from spontaneous ignition so, even though unwanted fires may result in this way, piloted ignition is generally the preferred scenario to assess the onset of flaming combustion. This is dependent on the ignition source (flame, cigarette, glow wire, etc.), sample size (1–10 cm) and ambient temperature.
Developing fire. The continuation of flaming combustion during fire growth involves an external heat flux of around 20–60 kW m2, which

6

Introduction

requires larger sample sizes (10 cm to 1 m), ambient temperatures above the ignition temperature (400–600 1C), with adequate ventilation.
Fully developed fire. The major stage of fire growth involves high external heat fluxes (450 kW m2), large sample sizes (1–5 m), ambient temperatures above the spontaneous ignition temperatures (4600 1C) and low ventilation. These conditions are not generally easy to replicate on a small scale, and materials which are required to perform well in developed fires normally need to be tested under these extreme conditions, and may perform differently in bench and large-scale scenarios.

2.2

Chemical and Physical Processes

The chemical composition of a polymeric fuel and the presence of fire retardants, additives, etc., are important in determining the degree to which flammable products will be released as the temperature increases. Untreated natural materials, such as wood, cotton and paper, tend to release flammable products and ignite at relatively low temperatures in comparison with synthetic materials [polyethylene, polyvinyl chloride (PVC), etc.]. However, the physical nature of the material also plays an important part (sometimes more so than the chemistry) in determining whether a material will reach decomposition temperatures. The thermal inertia (krc) is the product of the thermal conductivity, density and specific heat capacity. It dictates the time for the surface temperature to reach ignition temperature, describing the characteristics of materials according to their heat insulation or heat sink properties. A block of wood is more difficult to ignite with a small ignition source than are wood shavings. Cellular polymers of inherently combustible compositions (such as polyurethane foam) burn very rapidly in comparison to their solid counterparts because their heat insulation properties cause heat to be retained at the surface. The thermal inertia is low for insulating materials and high for heat-conducting materials. Ultimately, most fire science and hence most fire testing is focussed on specific protection goals, for good reasons. Common protection goals include preventing sustained ignition, limiting the contribution to fire propagation or acting as a fire barrier. Most of the better-established fire tests try to simulate a specific, realistic fire scenario and monitor a specific fire risk or hazard from a specific specimen within that scenario, rather than to determine the material’s properties. Furthermore, the way a specimen responds in a fire, or in a fire test, may make a significant contribution to the overall fire scenario. Hence, three general remarks can be made:
To compare the fire behaviour in different fire tests is difficult. Exact predictions often fail because different material properties determine the performance in different scenarios. However, rough correlations or correlations limited to specific classes of materials have been successful.
Scaling up and down is a key challenge in fire science, since the sample size plays such a major role. Typically, empirical approaches fail to predict fire

Polymers and Fire

7

behaviour satisfactorily, particularly attempts to span multiple orders of magnitude. Advanced predictive models have been developed which are moving towards reliable predictions of fire behaviour.
The interactions between properties of components and ‘‘intrinsic’’ material properties are complex and variable. Different polymers decompose in different ways and fire retardants act to inhibit the decomposition or flaming combustion processes. When a polymer is heated its chains will start to break down, which eventually results in the formation of volatile fuel molecules. The pyrolysis of a polymer, which turns polymer chains of 10 000–100 000 carbon atoms into species small enough to be volatilized, often involves breaking the polymer chain. In some cases, the chain releases groups from its ends most easily, known as end-chain scission or unzipping. In others, the chain breaks at random points along its length, known as random chain scission. A third process, in which groups that easily release are attached to the backbone as side chains, is known as chain stripping. This is often the preferred mechanism of the fire-retardant chemist, especially if the resulting chain may be prevented from undergoing chain scission to form volatiles or lose further substituents, and instead undergo carbonization that results in char formation. Thus, the conversion of organic polymer into volatile organic molecules may follow four general mechanisms. While some polymers fall exclusively into one category, others exhibit mixed behaviour. This process can be accelerated by chemical attack on the polymer chains, for example by atmospheric oxygen. In the presence of an ignition source, when the concentration of fuel molecules above the surface reaches a critical level, the proportion of their heat of combustion transferred back to the polymer is sufficient to replace the fuel by further pyrolysis. This is essentially the criterion for piloted ignition. It correlates well with the critical surface temperature for ignition. Once ignition has occurred, a proportion of the heat from the flame will be transferred back to an adjacent non-flaming part of the polymer surface, pyrolyzing the polymer and causing a repeat of the ignition process. This results in flame spread across the surface.

2.3

Studying Polymer Decomposition

Thermogravimetric analysis (TGA) provides a valuable insight into the decomposition behaviour of polymers under controlled conditions. The temperature at which significant mass loss occurs during decomposition in air gives an indication of the ignition temperature, as this is the point when a significant amount of fuel is lost from the polymer. This can be affected by gas-phase flame inhibitors and, to some extent, by the production of CO on the surface, which simultaneously reduces the oxygen and fuel concentrations. Once ignition has occurred, the mass loss in nitrogen is more representative of the fuel production rate, since the oxygen concentration under a flame is close to 0%.

8

Introduction

3 Fire Effluent Toxicity Analysis of fire statistics shows that most fire deaths are caused by inhalation of toxic gases.1 While some real-life fires may be represented by a single fire stage, most fires progress through several different stages.3 Burning behaviour and particularly toxic product yields depend most strongly on a few factors. Of these, material composition, temperature and oxygen concentration are normally the most important. The formation of carbon monoxide (CO), often considered the most toxicologically significant fire gas, is favoured by a range of conditions from smouldering to fully developed flaming. CO results from incomplete combustion, which can arise from:
Insufficient heat in the gas phase (e.g. during smouldering).
Quenching of the flame reactions (e.g. when halogens are present in the flame or excessive ventilation cools the flame).
The presence of stable molecules, such as aromatics, which survive longer in the flame zone and so give high CO yields in well-ventilated conditions, but lower than expected yields in underventilated conditions.4
Insufficient oxygen (e.g. in underventilated fires large radiant heat fluxes pyrolyze the fuel even though there is not enough oxygen to complete the reaction). The high yields of the asphyxiant gas CO from underventilated fires are held responsible for most of the deaths through inhalation of smoke and toxic gases,

Polymer Heat Pyrolysis

No

n-F

lam

ing

Ignition Flaming, Combustion (Heat flux 2-10x greater, Rapid increase in pyrolysis)

Op

en

Ve

ntil

atio

n

Restricted Ventilation Products rich in CO, smoke, organics and HCN if nitrogen present

Figure 8

Products rich in organics and partially oxygenated species

Effect of fire stage on toxic gas production.

Products mostly CO2 and H2O, (also SO2, NO2, acrolein and formaldehyde)

Polymers and Fire

9

but this underventilated burning is the most difficult to create on a bench scale. For most materials the yields of toxic species have been shown to depend critically on the fire conditions. Figure 8 illustrates the generalized change in toxic product yields during the growth of a fire from non-flaming through wellventilated flaming to restricted ventilation. Although the toxic product yields are often highest for non-flaming combustion, the rates of burning and the rate of fire growth are much slower, so underventilated flaming is generally considered the most toxic fire stage. Other toxic species include hydrogen cyanide (HCN, the other asphyxiant gas) and incapacitating irritants that cause blinding pain to the eyes and flooding of the lungs and respiratory tracts, which inhibit breathing and prevent escape. The wide variety of these irritants has led to groupings such as acid gases, organo-irritants and particulates, in order to estimate incapacitation.5 The effect of different fire conditions on the yields of these different toxicants is summarized in Figure 8. Data from large-scale fires6,7 show much higher levels of the two asphyxiant gases CO and HCN under conditions of reduced ventilation. It is therefore essential to the assessment of toxic hazard from fire that these different fire stages can be adequately replicated, and preferably the individual fire stages treated separately. Analysis of fire hazard requires data that describe the rate of burning of the material, and data that describe the toxic product yield of the material. This is best achieved using the steady-state tube furnace,8 in which the air supply and rate of burning are fixed as the sample is driven into a furnace, and then subjected to an increasing applied heat flux. Fire toxicity is also scenario dependent, but using this technique a clear relationship has been demonstrated between the yield of toxic products (for example in grams of toxicant per gram of polymer) and the fire condition for a given material composition.9 A more detailed account of current protocols in fire toxicity testing10 has recently been published.

4 Structural Deformation The increased use of polymer materials to replace structural members, such as the carbon fibre composites used in aircraft bodies, increases the importance of maintaining structural integrity during a fire. In many other cases, such as electrotechnical products, the failure of plastic components early on in a fire could radically alter the course of a fire, with potentially devastating consequences. As new materials with greater rigidity and structural integrity are being developed, synthetic polymer composites are increasingly being used to replace metal components. Incorporation, for example, of a 1% loading of a nanofiller can have a very large, beneficial effect on these physical properties.

5 Fire Retardant Strategies These can be broadly separated into those that block the fire physically, and those that use alternative chemical reactions to stop the material from burning.

10

Introduction

They are outlined here to set the context for the specific approaches described in detail in the individual sections.

5.1

Physical Action

There are several ways in which the combustion process can be retarded by physical action:
By cooling. Endothermic reactions cool the material.
By forming a protective layer. Obstructing the flow of heat and oxygen to the polymer, and of fuel to the vapour phase.
By dilution. Release of water vapour or carbon dioxide (CO2) may dilute the radicals in the flame so it goes out. For example, the most widely used fire retardant, aluminium hydroxide [Al(OH)3], breaks down endothermically to form water vapour, which dilutes the radicals in the flame, while the residue of alumina (Al2O3) builds up to form a protective layer. Unfortunately, relatively large amounts may be needed to be effective (up to 70%) and the freshly formed Al2O3 can lead to afterglow.11 180200  C

2AlðOHÞ3 ðsÞ ! Al2 O3 ðsÞ þ 3H2 OðgÞ DH ¼ þ1:3 kJ g1

5.2

Chemical Action


Reaction in the gas phase. The radical reactions of the flame can be interrupted by a flame retardant. The radical concentration falls below a critical value, and the flame goes out. The processes that release heat are thus stopped, and the system cools down. However, interfering with the flame reactions often results in highly toxic and irritant partially burnt products, including CO, which generally increase the toxicity of the fire gases while reducing fire growth.
Reaction in the solid phase. The flame retardants work by breaking down the polymer so it melts like a liquid and flows away from the flame (just like trying to light candle wax without a wick). Although this allows materials to pass certain tests, sometimes fire safety is compromised by the production of flammable drops. Char formation. Better solid-phase flame retardants are those which cause a layer of carbonaceous char to form on the polymer surface. This can occur, for example, by the fire retardant removing the side chains and thus generating double bonds in the polymer. Ultimately, these form a carbonaceous layer by forming aromatic rings.

Polymers and Fire

11

Char formation usually reduces the formation of smoke and other products of incomplete combustion. Intumescence. The incorporation of blowing agents causes swelling behind the surface layer, and provides much better insulation under the protective barrier. The same technology is used for coatings for protecting wooden buildings and steel structures.

5.3

Polymer Nanocomposites

Polymeric materials that contain fillers with at least one dimension of only a few tens of nanometres have opened up an enormous range of possibilities in fire retardant research. Fillers may have dimensions that extend over four orders of magnitude, and their effects include:





reinforcing organic char as a barrier layer; providing a catalytic surface to promote char-forming reactions; enhancing the structural rigidity of the polymer; changing the melt–flow properties of the polymer close to its ignition temperature; and
providing intimate contact between a fire retardant and the host polymer. Initially, investigations involved polymer–clay nanocomposites, but more recently investigations have included the use of single and multi-walled carbon nanotubes,12 and other nanoscopic fillers with potential fire retardant properties, including layered double hydroxides12 (or hydrotalcites), Al(OH)313 and others.

5.3.1 Types of Nanofillers for Fire Retardancy Although the effect of polymer–clay nanocomposites on fire behaviour was first investigated over two decades ago, the wider study of the full range of sizes, morphologies, chemistries and surface treatments of polymer nanocomposites has only just begun.

5.3.2 Filler Morphology Traditionally, particles with a platy morphology, and especially montmorillonite, have been investigated, as it was generally assumed that these would most easily assist in the formation of a barrier layer. The influence of nanocomposite formation and the different mechanisms of breakdown of different polymers make generalizations regarding filler morphology difficult. In some cases fillers with aspect ratios greater than 1000 have been successfully deployed to enhance fire retardancy. For example, a clay filler with a mean diameter of 25 mm has been used commercially to reduce the flammability of cable sheathing materials.14

12

Introduction

5.3.3 Filler Coating and Dispersion To produce appropriately dispersed polymer nanocomposites, it is generally necessary to add a compatibilizing agent, such as a surfactant, to the polar filler surface in order to insert it between the polymer chains. In partially ionic polymers, such as nylon, dispersion is much easier than in hydrophobic, crystalline polymers, such as isotactic PP. In these cases it is generally necessary to attach a grafting agent, such as maleic anhydride, onto the polymer to ensure adequate dispersion. While the mechanical properties of the polymer depend on adequate dispersion at ambient temperatures, the fire behaviour is a function of the dispersion of the nanofiller in the molten bubbling polymer. In many cases, the surfactant decomposes to leave the polar nanofiller. There has been intense speculation as to whether this results in incompatibilization and migration of the filler to the surface, or whether the preferential loss of the first few hundred nanometres of polymer results in accumulation of filler at the surface. In some cases no fire retardant effect is observed without adequate dispersion. In others it is evident, which suggests that dispersion occurs in the molten, decomposing bubbling polymer.

5.3.4 Effects of Nanofiller Composition on Thermal Decomposition Burning Behaviour The evolution products from nanocomposites made from polyethylene, ethylene vinyl acetate (EVA) and polystyrene, with organically modified clays, single and multi-walled carbon nanotubes and layered double hydroxides has been studied.12 It was found that the relative amounts and the identity of the degradation products change when both well-dispersed cationic and anionic clays are used, but there is no difference in the degradation products when carbon nanotubes are utilized. When the nanodimensional material is not well dispersed, the degradation products are not changed. Unlike clays, polymer-layered double hydroxide nanocomposites give reasonably good reductions in peak heat-release rate, even when nanodispersion has not been obtained. These data suggest that the enhancement in the fire behaviour must be, at least in part, due to different mechanisms for montmorillonite, layered double hydroxides and carbon nanotube-based nanocomposites.

6 Conclusions Fire is a complex process and no two real fires are identical. In developing materials with enhanced fire safety, such as lower ignitability, lower heatrelease rates during burning and lower fire toxicity, it is essential to relate the desired properties to the end-use scenario. It is normally the role of regulators to select appropriate test methods to protect people and property from the most likely fire scenarios.

Polymers and Fire

13

The thermal decomposition of polymers is a complex process, which may follow a number of different routes, depending on the material and the conditions. Polymers burn by breakdown of their long-chain structures, which releases fuel into the gas phase, where flaming combustion can occur. The mechanism of breakdown is often unique to a particular polymer and, in general, flame retardant methods cannot be directly exported from one polymer system to another. Fire retardants may be classified by their mode of action (physical or chemical, condensed phase or gas phase, char forming or intumescent, etc.) or by their physical or chemical structure. Polymer nanocomposites are an important new class of materials that offer thermal and mechanical properties not evident in their parent polymers, with great potential for fire retardancy. Some of their physical properties, for example the typically massive increase in viscosity modulus, may cause processing problems that prevent large-scale production using conventional extruders. The incorporation of additives, which have dimensions that range over four orders of magnitude, has a range of physical effects, such as barrier layer formation, loss of compatibilizer, migration to the surface, inhibition of bubble movement and reduction in the flow of the molten polymer. In addition, a number of chemical effects have been observed, including catalyzing decomposition reactions, promoting graphite formation and altering the decomposition pathway, which has been seen to influence the decomposition behaviour. The recent approach of preparing conventional fire retardants in nanoscopic form increases the range of chemical effects. The degree of dispersion has often been cited as a prerequisite for improved fire behaviour (typically a shorter time to ignition, but a lower peak of heat-release rate), but this is certainly not always the case. The controlling parameter is the degree of dispersion at the point of ignition, rather than that in the cold polymer, since either the compatibilizer may decompose, reducing the degree of dispersion, or the nanofiller may disperse under the more extreme agitation within the decomposing polymer. Measurements of rheological properties as a function of temperature have been shown to be an effective tool to demonstrate this.15 The complexities of fire behaviour and the difficulties in quantifying that behaviour in a scenario-independent way compound the problems of understanding the thermal decomposition of polymer nanocomposites. However, the large number of empirical studies that have produced encouraging results provides evidence that the future of fire retardancy will follow the nanocomposite route.9 Even if it is not yet possible to predict which type of nanofiller (in terms of chemistry, morphology, compatibilization and dimensions ranging over four orders of magnitude) and what degree of dispersion and filler loading are required for optimum performance. That nanofillers uniquely improve physical properties, while almost all other fire retardants worsen them, suggests that until such optimization has been reached, and the vast numbers of experiments required to achieve it have been undertaken, fire retardant formulations will be based on a combination of nanofiller and conventional flame retardant.

14

Introduction

References 1. Fire Statistics United Kingdom 2005, Office of the Deputy Prime Minister: London, April 2007. 2. The cost of fires – A review of the information available, Home Office Publications Unit, UK, 1997. 3. ISO TS 19706:2004 Guidelines for assessing the fire threat to people. 4. T.R. Hull, J.M. Carman and D.A. Purser, Prediction of CO evolution from small-scale polymer fires, Polym. Int., 2000, 49, 1259–1265. 5. ISO 13571:2007, Life-threatening components of fire – Guidelines for the estimation of time available for escape using fire data. 6. P. Blomqvist and A. Lonnermark, Characterization of the combustion products in largescale fire tests comparison of three experimental configurations, Fire Mater., 2001, 25, 71–81. 7. B. Andersson, F. Markert and G. Holmstedt, Combustion products generated by hetero-organic fuels on four different fire test scales, Fire Safety Journal, 2005, 40, 439–465. 8. T.R. Hull, J.M. Carman and D.A. Purser, Prediction of CO evolution from small-scale polymer fires, Polym. Int., 2000, 49, 1259. 9. A.A. Stec, T.R. Hull, K. Lebek, J.A. Purser and D.A. Purser, The effect of temperature and ventilation condition on the toxic product yields from burning polymers, Fire and Materials, 2008, 32, 49–60. 10. T.R. Hull and K.T. Paul, Bench-Scale Assessment of Combustion Toxicity – A Critical Analysis of Current Protocols, Fire Safety J., 2007, 42, 340–365. 11. T.R. Hull, R.E. Quinn, I.G. Areri and D.A. Purser, Combustion toxicity of fire retarded EVA, Polym. Degrad. Stab., 2002, 77, 235–242. 12. M.C. Costache, M.J. Heidecker, E. Manias, G. Camino, A. Frache, G. Beyer, R.K. Gupta and C.A. Wilkie, The influence of carbon nanotubes, organically modified montmorillonites and layered double hydroxides on the thermal degradation and fire retardancy of polyethylene ethylene-vinyl acetate copolymer and polystyrene, Polymer, 2007, 48, 6532–6545. 13. K. Daimatsu, H. Sugimoto, Y. Kato, E. Nakanishi, K. Inomata, Y. Amekawa and K. Takemura, Preparation and physical properties of flame retardant acrylic resin containing nano-sized aluminum hydroxide, Poly. Degrad. Stab., 2007, 92, 1433–1438. 14. T.R. Hull, D. Price, Y. Liu, C.L. Wills and J. Brady, An investigation into the decomposition and burning behaviour of ethylene-vinyl acetate copolymer nanocomposite materials, Polym. Degrad. Stab., 2003, 82, 365–371. 15. S. Nazare, T. R. Hull, B. Biswas, F. Samyn, S. Bourbigot, C. Jama, A. Castrovinci, A. Fina and G. Camino, Chapter 12, this book.

Fire Retardant Strategies

CHAPTER 1

Synergy between Nanometric Alumina and Organoclay in Conventional Fire Retardant Systems for Ethylene–Vinyl Acetate N. CINAUSERO,a J.-M. LOPEZ-CUESTA,a F. LAOUTID,a A. PIECHACZYKb AND E. LEROYa,c a

Ecole des Mines d’Ale`s, Centre des Mate´riaux de Grande Diffusion, 6, avenue de Clavie`res, 30319 Ale`s Cedex, France; b Nexans Research Center, 170 Av. Jean Jaure`s, 69633 Lyon Cedex 07, France; c Laboratoire de Ge´nie des Proce´de´s-Environnement-Agroalimentaire (GEPEA), 37, Bd de l’Universite´, 44606 Saint-Nazaire BP 420 Cedex, France

1.1 Introduction Hydrated mineral fillers like aluminium hydroxide (ATH) or magnesium hydroxide (MDH) are used in the cable industry as flame retardants for polyolefins such as ethylene–vinyl acetate (EVA) copolymers. The very high filler loadings usually required to obtain satisfactory fire properties,1,2 mean this results in a decrease in the mechanical performance of the materials. Nevertheless, enhancement of the efficiency of ATH or MDH may be achieved by partially substituting them with synergistic additives, in particular high-aspect ratio inorganic particles such as oMMTs or delaminated talcs.3,4 In addition to Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

17

18

Chapter 1

improvement in the mechanical properties, the presence of such lamellar particles leads to an intumescence phenomenon that occurs before ignition in cone calorimeter tests. A foam-like charred structure is formed as a consequence of heterogeneous bubble nucleation, increased viscosity and the promotion of charring of the host polymer.4 This structure leads to the formation of a porous protective residue (mainly inorganic), which limits both heat transfer and the diffusion of fuel and oxygen.3 In a recent patent5 we showed that the addition of alumina nanoparticles improved the reactions to fire of flame retardant EVA compositions that contain metal hydroxide and oMMTs. In this chapter we present a detailed study of these complex systems and discuss the influence of the size of alumina particles.

1.2 Experimental 1.2.1 Materials EVA [Elvax 260, containing 28 weight percent (wt%) of vinyl acetate] was purchased from DuPont. Magnesium hydroxide (MDH; Magnifin H10, d50 ¼ 0.85 mm, specific surface area ¼ 10 m2 g
1) and oMMT (Nanofil 5: distearyldimethylammonium ion-exchanged bentonite) were supplied by Martinswerk (now Albemarle) and Su¨d Chemie (now Rockwood Holdings), respectively. Alumina particles of different physical properties were obtained from Degussa (ALU nano, Aeroxide Alu C, d50 ¼ 13 nm, SBET ¼ 86 m2g
1) and Alcan (ALU micro, d50 ¼ 0.47 mm, SBET ¼ 6.5 m2g
1), respectively.

1.2.2 Processing Blending of molten EVA copolymer with the different minerals was performed using a Haake internal mixer at 160 1C and 60 revolutions per minute (rpm) for 10 minutes. Thick (4 mm) sheets were then compression moulded at 160 1C at a pressure of 100 bars (1  107 Pa) for five minutes. These sheets were cut to the size required for the experiment to be performed. For all the different compositions studied, the total filler content was 10% or 60% w/w. As an example (EVA 40/ MDH 50/ALU nano 5/oMMT 5) means a formulation that contains 40% w/w of EVA, 50% of MDH, 5% of ALU nano and 5% of oMMT.

1.2.3 Testing E´piradiateur tests (AFNOR NF P 92-505) were carried out on 70  70  4 mm3 samples to determine the flammability and the self-extinguishability of the different compositions. The heat flux of e´piradiateur measured using a flux meter is around 30 kW/m2. This test allows the time to ignition (TTI) for the sample placed under the radiator (500 W) to be determined. After first ignition, the radiator is successively removed and replaced as soon as extinction occurs, the procedure being repeated for a period of 5 minutes. The mean inflammation

Synergy between Nanometric Alumina and Organoclay

19

period (MIP) and the number of ignitions (N) are then calculated from results obtained from four experiments for each formulation. Cone calorimeter tests (ISO 5660) were performed on filled polymer samples (100  100  3 mm3) placed horizontally, using a FTT cone calorimeter. Irradiances of 30, 50 and 70 kW/m2 were used. Ignition is piloted by a spark generator in contrast to the e´piradiateur test, during which ignition is spontaneous. TTI and peak of heat-release rate (PHRR) values are discussed later. The results given correspond to mean values obtained from two experiments for each formulation. Thermogravimetric analysis (TGA – Perkin Elmer PYRIS 1) was used to study the thermo-oxidative degradation of composites. Samples of typically 15 mg were placed in alumina crucibles and subjected to a temperature ramp from 25 1C to 700 1C in air at a heating rate of 5 1C min
1.

1.3 Results and Discussion To study the interactions between the various components of the formulations above, we performed three series of TGA. Figure 1.1 shows the effect of the presence of oMMT, ALU micro and ALU nano on the thermo-oxidative degradation of EVA. Pure EVA shows two main mass losses, the first one corresponding to EVA deacylation and the second one to EVA main-chain degradation. In the presence of oMMT, the first mass loss takes place at lower temperatures, while the second one is shifted towards higher temperatures, in agreement with literature results.6 The main explanation of the acceleration of the acetic acid loss is the catalytic effect of hydroxyl groups on the clay.6,7 In contrast, the presence of ALU nano does not affect the temperature of either of the two mass losses, while the presence of ALU micro leads to a slight change in the temperature of the second mass loss. Figure 1.2 displays the effect of nanofillers on the dehydration of MDH. Pure MDH dehydration takes place between 270 and 390 1C, with a maximum mass loss rate at 370 1C. In comparison, pure oMMT shows a mass loss between 200 and 390 1C, which corresponds to the degradation of its organic part, while ALU nano (and ALU micro) does not show any mass loss in this temperature range. When MDH is mixed with ALU nano (50/50 w/w powder mix), the dehydration of MDH starts at lower temperature, with a maximum mass loss rate at 355 1C. In comparison, when MDH is mixed with oMMT (50/50 w/w powder mix), the opposite occurs, with a maximum mass loss rate at 380 1C. In the meantime, the degradation of the organic part of oMMT does not appear to be influenced by the presence of MDH. Figure 1.3 shows the TGA and differential thermogravimetric (DTG) curves for the flame retardant formulations. The first mass loss, which corresponds to both the deacylation of EVA and the dehydration of MDH, is slightly influenced by the presence of ALU nano (the maximum mass loss rate is shifted towards lower temperatures). This may confirm its influence on the decrease of the MDH dehydration temperature observed in Figure 1.2. When both ALU

20

Chapter 1 100 90

Residual mass (%)

80 70 60 50 EVA 100

40

EVA 90 oMMT 10 30

EVA 90 ALU micro 10

20

EVA 90 ALU nano 10 EVA 90 ALU micro 5 oMMT 5

10

EVA 90 ALU nano 5 oMMT 5 0 250

300

350

400 Temperature (°C)

450

500

550

450

500

550

0

Mass loss rate (%/min)

-2 -4 -6 -8 EVA 100 -10

EVA 90 oMMT 10 EVA 90 ALU micro 10

-12

EVA 90 ALU nano 10 -14

EVA 90 ALU micro 5 oMMT 5 EVA 90 ALU nano 5oMMT 5

-16 250

300

350

400 Temperature (°C)

Figure 1.1

TGA and DTG curves for samples that do not contain MDH.

nano (or micro) and oMMT are present, a shoulder appears on the low temperature side of the mass loss rate peak. In this case, two phenomena can accelerate the mass loss: ALU nano seems to sharpen the reaction of deacylation of EVA catalyzed by oMMT, as observed in Figure 1.1; in addition, when incorporating both nanofillers, water release may be restricted, which contributes to the acceleration of mass loss at lower temperatures.

21

Synergy between Nanometric Alumina and Organoclay 100

Residual mass (%/min)

95

90

85 oMMT 80

ALU nano MDH

75

MDH - ALU nano MDH - oMMT

70 100

150

200

250 300 Temperature (°C)

350

400

450

350

400

450

0

Mass loss rate (%/min)

-0,5 -1 -1,5 -2

oMMT ALU nano

-2,5

MDH MDH - ALU nano

-3

MDH - oMMT -3,5 100

150

200

250

300

Temperature (°C)

Figure 1.2

Thermogravimetric effect of the presence of nanofillers on MDH dehydration.

Furthermore, the presence of oMMT shifts the first maximum mass loss rate towards a higher temperature, in agreement with the results of Figure 1.2 (dehydration of MDH at higher temperatures). The second mass loss peak is clearly shifted towards higher temperatures in the presence of oMMT, a shift due to EVA charring. From these TGA results we can make the assumption that the presence of ALU nano (or micro) in flame retardant formulations does not affect the flame

22

Chapter 1 100

Residual mass (%)

90 80 70 EVA 40 MDH 60 60

EVA 40 MDH 55 oMMT 5 EVA 40 MDH 50 ALU nano 10

50

EVA 40 MDH 50 ALU micro 5 oMMT 5 EVA 40 MDH 50 ALU nano 5 oMMT 5

40 250

300

350

400 Temperature (°C)

450

500

550

450

500

550

0

Mass loss rate (%/min)

-1 -2 -3 EVA 40 MDH 60 -4

EVA 40 MDH 55 oMMT 5 EVA 40 MDH 50 ALU nano 10

-5

EVA 40 MDH 50 ALU micro 5 oMMT 5 EVA 40 MDH 50 ALU nano 5 oMMT 5

-6 250

300

350

400 Temperature (°C)

Figure 1.3

TGA and DTG curves for samples containing MDH.

retardant action of oMMT, but can modify the dehydration of MDH, which starts at a lower temperature. Besides, oMMT used with alumina particles accentuates the deacylation of EVA in the low temperature range (250–340 1C), which could have a favourable effect on further charring due to double bond formation at lower temperature. The data obtained from the e´piradiateur tests are presented in Figure 1.4. The reference flame retardant formulation (EVA 40/MDH 60 w/w%) has the lowest TTI, which means the highest flammability, and the highest MIP, which means a poor auto-extinguishing ability. The introduction of oMMT clearly improves these two characteristics, while in the case of ALU (nano or micro) no significant improvement is observed. When both oMMT and ALU (nano or

23

Synergy between Nanometric Alumina and Organoclay 180

9,5 TTI épiradiateur (s)

170

9

MIP (sv)

160

8,5

TT I ( s)

150 8 140 7,5 130 7

120

6,5

110 100

6 EVA 40 MDH 60

Figure 1.4

EVA 40 MDH 55 oMMT 5

EVA 40 MDH 50 oMMT 10

EVA 40 MDH 50 Alu 10

EVA 40 EVA 40 MDH 50 MDH 50 oMMT 5 oMMT 5 Alu nano 5 Alu micro 5

E´piradiateur test results.

HRR (kW/m²)

500 450

30 kW/m²

400

50 kW/m²

350

70 kW/m²

300 250 200 150 100 50 0 0

Figure 1.5

200

400 Time (s)

600

800

Cone calorimeter HRR curves for the EVA 40/MDH 60 formulation.

micro) are present, the TTI increases significantly, compared to formulations that contain only oMMT. The best increase in TTI is obtained for ALU nano. The size of alumina particles is therefore an important parameter. Figures 1.5 and 1.6 show the cone calorimeter HRR curves obtained at various incident heat fluxes for (EVA 40/MDH 60), and (EVA 40/MDH

24

Chapter 1 500 30 kW/m²

HRR (kW/m²)

450 400

50 kW/m²

350

70 kW/m²

300 250 200 150 100 50 0 0

Figure 1.6

200

400 Time (s)

600

800

Cone calorimeter HRR curves for the EVA 40/MDH 50/oMMT 10 formulation.

50/oMMT 10), formulations, respectively. The behaviour shown in Figure 1.5 is ‘‘classical’’: when the incident heat flux decreases, the TTI increases and the PHRR decreases. In contrast, the presence of oMMT (Figure 1.6) leads to an unusual behaviour at low irradiance (30 kW/m2): the TTI becomes extremely long while the PHRR strongly increases. Similar behaviour was observed for the (EVA 40/MDH 55/oMMT 5) formulation, as shown in Table 1.1. This strong increase of PHRR at low incident-heat flux caused by the presence of oMMT is likely to be a problem for cable applications of the material. Effectively, studies8,9 have shown a good correlation between the PHHR in the cone calorimeter at low incident-heat flux (typically below 50 kW/m2) and the passing of the FIPEC cable test,10 which is a vertical tray test using a 20 kW burner. As Table 1.1 shows, the introduction of ALU nano allows the negative effect of oMMT on the PHRR at 30 kW/m2 to be decreased. In contrast, when ALU micro is used, the PHRR is increased, showing again that the size of alumina is an important parameter. As regards ignition, when incorporating nanoclays in flame retardant EVA, an increase of TTI occurs that we have already observed in a previous study.4 If we now focus on the TTI values obtained in the cone calorimeter test at 30 kW/m2, it is striking that they strongly differ from those obtained in e´piradiateur tests, although the incident heat flux is nearly the same. Contrary to what is observed for e´piradiateur tests (Figure 1.4), the introduction of ALU nano leads to a relative decrease of the TTI in cone calorimeter tests relative to that of the EVA/MDH/oMMT composition. These contradictory evaluations of the TTI are undoubtedly related to different experimental conditions: in the

Cone calorimeter results. 30 kW/m2

EVA EVA EVA EVA EVA EVA

40/MDH 40/MDH 40/MDH 40/MDH 40/MDH 40/MDH

60 55/oMMT 5 50/oMMT 10 50/ALU nano 10 50/oMMT 5/ALU nano 5 50/oMMT 5/ALU micro 5

50 kW/m2

70 kW/m2

TTI (s)

PHRR (kW/m2) TTI (s)

PHRR (kW/m2) TTI (s)

PHRR (kW/m2)

171 314 415 173 279 384

204 388 496 194 306 410

293 255 286 253 249 250

398 297 315 332 286 289

(+84%) (+143%) (+1%) (+63%) (+124%)

(+90%) (+143%) (–5%) (+51%) (+100%)

77 91 75 80 84 92

(+18%) (– 1%) (+4%) (+9%) (+19%)

(– 13%) (– 2%) (– 14%) (– 15%) (– 15%)

45 53 43 48 48 52

(+18%) (– 4%) (+7%) (+7%) (+17%)

(– 25%) (– 21%) (– 17%) (– 28%) (– 27%)

Synergy between Nanometric Alumina and Organoclay

Table 1.1

25

26

Chapter 1

case of e´piradiateur tests, the gases emitted from the sample are not aspirated, as in the cone calorimeter tests. To sum up, both the acetic acid and water are produced more efficiently owing to the effect of ALU on the deacylation of EVA catalyzed by oMMT observed in TGA, as well as the regulated water release from the dehydration of MDH. Therefore, we can assume that acetic acid and water may dilute the combustible gases at the surface of the sample during the pre-ignition period of e´piradiateur tests. This is likely to delay ignition, which in addition is not promoted by a spark in this test. Then the effectiveness with which ALU nano increases the TTI of a cable in a real fire will depend on the fire scenario. Nevertheless, the ‘‘static’’ conditions of the e´piradiateur are more likely than the forced flow of the cone calorimeter. Let us now come back to the PHRR values at 30 kW/m2. Comparison of the shape of the curves in Figure 1.6 suggests that, for this low external heat flux, a more important flux of combustible gas evolves at the time of ignition, and results in a strong PHRR. Such a ‘‘critical phenomenon’’ observed for (EVA 40/ MDH 50/oMMT 10) could be explained by a stronger migration of clay platelets11 towards the surface at low irradiance. This forms a protective layer before ignition that becomes ‘‘unstable’’ after ignition because of the additional external heat flux provided by the flame. Such a ‘‘critical phenomenon’’ is not observed for (EVA 40/MDH 60; Figure 1.5) and (EVA 40/MDH 50/ALU nano 10; Table 1.1) formulations, and is significantly reduced when both ALU and oMMT are present (EVA 40/MDH 50/ALU nano 5/oMMT 5; Table 1.1). Eventually, this last formulation is the best compromise concerning TTI and PHRR. In contrast, when ALU micro is used (Table 1.1), the PHRR is increased compared to that of the (EVA 40/MDH 55/oMMT 5) formulation, which confirms the size dependence of alumina particles on flammability properties.

1.4 Conclusion The effect of alumina particles on the thermo-oxidative degradation and the reaction to fire of conventional flame retardant formulations for EVA that contains MDH and oMMT has been studied. The introduction of alumina particles did not have any direct effect on the thermo-oxidative degradation of the EVA copolymer, but was shown to shift the dehydration of MDH towards lower temperatures. Besides, when mixed with oMMT, alumina particles may accentuate the deacylation of EVA catalyzed by oMMT. It was suggested that the restriction of water release and the acceleration of acetic acid loss could have an effect on the reaction to fire. Effectively, the e´piradiateur test showed a strong increase in the TTI in the presence of both alumina particles and oMMT, the best improvement being obtained for nano alumina particles. In addition, the use of alumina nanoparticles allowed the PHRR of EVA/MDH/ oMMT formulations to decrease at low external heat flux in cone calorimeter, which showed high values. This phenomenon is particularly relevant since the PHRR at low external heat flux is known to correlate with larger scale cable fire tests.

Synergy between Nanometric Alumina and Organoclay

27

References 1. F. Montezin, J.M. Lopez-Cuesta, A. Crespy and P. Georlette, Fire Mater., 1997, 21, 245–252. 2. R.N. Rothon, Particulate-Filled Polymer Composites, Longman Scientific & Technical, Harlow, Essex, England, 1995. 3. L. Ferry, P. Gaudon, E. Leroy, J.M. Lopez-Cuesta in Fire Retardancy of Polymers: New Applications of Mineral Fillers, ed. M. Le Bras, C.A. Wilkie, S. Bourbigot, S. Duquesne and C. Jama, The Royal Society of Chemistry, Cambridge, 2005, ch. 22 pp. 345–358. 4. L. Clerc, L. Ferry, E. Leroy and J.M. Lopez-Cuesta, Polym. Degrad. Stab., 2005, 88, 504–511. 5. A. Piechaczyk, J. Fournier, E. Tavard, J.M. Lopez-Cuesta, F. Laoutid and E. Leroy, European Pat., 2007, 1752490. 6. M. Zanetti, G. Camino, R. Thomann and R. Mu¨lhaupt, Polymer, 2001, 42, 4501–4507. 7. M.C. Costache, D.D. Jiang and C.A. Wilkie, Polymer, 2005, 46, 6947–6958. 8. M.M. Hirschler, Fire Technol., 1997, 33, 291–315. 9. M.M. Hirschler, Proceedings of Interflam’2001, Edinburgh, UK, 2001, Interscience Communications, London, UK, pp. 137–148. 10. S.J. Grayson, P. Van Hees, A.M. Green, H. Breulet and U. Vercellotti, Fire Mater., 2001, 25, 49–60. 11. M. Lewin, Polym. Degrad. Stab., 2006, 17, 758–763.

CHAPTER 2

Strained Organophosphorus Compounds as Reactive Flame Retardants for Polymeric Materials BOB A. HOWELL Center for Applications in Polymer Science, Central Michigan University, Mt. Pleasant, MI, USA

2.1 Introduction The development of new, more effective and environmentally friendly flame retardant strategies for styrenics is of increasing urgency as the criticism of the use of organohalogens, primarily brominated aromatics, becomes more prominent around the world.1–5 Flame retarding species that can be chemically incorporated into the polymer are particularly attractive, since volatility, blooming and loss during processing are not limitation issues for the use of the retardant. A goal of this work was to develop an organophosphorus (phosphate or phosphonate) that contains a thermally labile carbon–carbon bond that might be cleaved homolytically to generate a diradical that could act as an initiator for styrene polymerization.6,7 The use of such a compound as initiator would generate a polymer with a phosphorus-containing unit incorporated into the main chain. The polymer should display desirable flammability properties and be readily processable using standard techniques. It is possible that such a molecule with a strained carbon–carbon bond might also be reactive towards propagating radicals, i.e., it might act as a monomer. In this case, several phosphorus-containing units may Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

28

Strained Organophosphorus Compounds as Reactive Flame Retardants

29

be incorporated into the polymer main chain. In fact, if the compound were to be sufficiently reactive as a monomer, the level of incorporation could be controlled by simply adjusting the concentration of the phosphorus compound in the polymerization mixture.

2.2 Experimental 2.2.1 Materials Benzpinacol was obtained by photoreduction of benzophenone. Benzophenone, dichloro(phenyl)phosphine and triethylamine were obtained from the Aldrich Chemical Company and used as received.

2.2.2 Initiator, 2,4,4,5,5-Pentaphenyl-1,3,2-Dioxaphospholane A solution of 1.00 g (2.71 mmol) of benzpinacol and 0.553 g (5.50 mmol) of triethylamine in 50 ml of anhydrous diethyl ether was placed into a 100 ml, threenecked, round-bottomed flask fitted with a magnetic stir bar, a pressure-equalizing dropping funnel and an Allihn condenser bearing a gas-inlet tube. Dichloro(phenyl)phosphine (0.49 g, 2.71 mmol) was added dropwise over a period of 0.25 hour. The mixture was stirred at solvent reflux for two hours, allowed to cool to room temperature and triethylammonium chloride was removed by filtration. The filtrate was subjected to rotary evaporation at reduced pressure to remove the solvent. The solid residue was crystallized twice from hexane to afford 2,4,4,5-pentaphenyl-1,3,2-dioxaphospholane (0.90 g, 70.5 % yield) as a white crystalline solid, Melting point (m.p.) 72 1C [differential scanning calorimetry (DSC)]; 1H nuclear magnitic resonance (NMR) (d, CDCl3), 7.01–7.42 (m, 25 H, aromatic protons); P-31 NMR (d, CDCl3), 22.1(s); Fourier transform infrared (FTIR) spectroscopy (cm 1, NaCl), 3028(vs) (aromatic C–H stretch), 2925[very strong (vs)] (aliphatic C–H stretch), 1601(vs) (aromatic nucleus), 1059 and 759 (O–P–O stretch and bending); mass spectrum m/z, (% of base), 77 (70), 106 (62), 141 (21), 182 (100), 473 (53).

2.2.3 Polymers 2.2.3.1

Polymerization of Styrene (1% 2,4,4,5,5-Pentaphenyl1,3,2-Dioxaphospholane as Initiator) at 70 1C

A polymerization tube fitted with a two-way stopcock was charged with inhibitor-free dry styrene monomer (10.0 g, 96.0 mmol) and 2,4,4,5,5-pentaphenyl1,3,2-dioxaphospholane (0.10 g, 0.21 mmol). The solution was degassed with pure dry nitrogen for 30 minutes and the tube placed in a constant-temperature

30

Chapter 2

oil bath at 70 1C for 15 hours. Size exclusion chromatographic (SEC) analysis of the polymer after purification by repeated dissolution in toluene followed by precipitation with methanol gave molecular weight (MW) 3.65  105, mean numerical molecular weight (Mn) 1.80  105 with a polydispersity of 2.03 and spectral properties 1H NMR (d, CDCl3) 1.58 (3H, methylene and methine protons), 6.8–7.5 (aromatic protons); P-31 NMR (d, CDCl3) 31.4(s); FTIR (cm 1, NaCl), 3025(vs) (aromatic C–H stretch), 2922(vs) (aliphatic C–H stretch), 1601(vs) (aromatic nucleus), 1492, 1452 (characteristic bands for polystyrene), 1059 and 759 (O–P–O stretch).

2.2.3.2

Preparation of Other Polymers

Polymers that contain various levels of phospholane were prepared in an analogous manner. A reference polymer that contained no phospholane fragments was prepared similarly using azobisisobutyronitrile (AIBN) as an initiator.

2.3 Results and Discussion The synthesis of an appropriate phosphorus compound was based on an earlier observation that when 2,3-diphenyl-2,3-butanediol was treated with phosphoryl chloride at 56 1C, a cyclic phosphate, which could be isolated and characterized, was formed.8,9 This ester contains hydrogen atoms alpha to the ester functionality, so it is unstable at higher temperature in the presence of base and readily undergoes elimination of phosphate to form the corresponding diene. A similar ester in which elimination is prohibited may be prepared from 1,1,2,2-tetraphenylethanediol (benzpinacol). Several suitable phosphorus esters may be prepared, depending on the reagents used. For example, treatment of the diol with dichloro(phenyl)phosphine affords 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane Scheme 2.1. In this case, the phosphine served as both reactant and solvent. The cyclic compound 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane can be obtained in good yield as a white crystalline solid with a melting point of 72 1C (DSC). The proton NMR spectrum contains absorptions for aromatic protons and

Scheme 2.1

Synthesis of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane.

Strained Organophosphorus Compounds as Reactive Flame Retardants

31

the phosphorus spectrum consists of a singlet at d 22.1 parts per million (ppm), relative to the absorption for 85% aqueous phosphoric acid as external reference. The infrared spectrum of this contains an aromatic absorption band at 1601 cm 1, as well as strong bands for O–P–O absorption at 1059 and 759 cm 1. As noted above, this five-membered ring structure contains a sterically strained carbon–carbon bond that might be expected to undergo thermally induced homolysis at modest temperatures. This may be easily demonstrated for 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane using thermogravimetry. The compound readily undergoes thermal decomposition with a maximum rate of degradation at 120 1C and a degradation onset temperature of 70 1C. Isothermal thermogravimetry was used to determine a half-life for decomposition at various temperatures. For example, the half-life for its decomposition at 72 1C is 10 hours. The 10 hour half-life temperature is a convenient way to classify polymerization initiators. The ease with which the central carbon– carbon bond of this bond undergoes homolysis means it should function as a polymerization initiator. This is illustrated in Scheme 2.2 for the polymerization of styrene. If the compound functions as initiator, each polymer chain generated should contain one phosphorus moiety. Alternatively, it is possible that the phosphorus compound could also function as a co-monomer in the polymerization, i.e., the propagating poly (styryl) radical could add to the strained carbon–carbon bond (see Scheme 2.3). A series of mixtures was prepared in standard polymerization tubes. The first tube contained pure styrene monomer and was used as a control. The remaining tubes contained solutions of 1, 5 and 10% by mass of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane in styrene monomer, respectively. Tubes were placed in an oil bath maintained at 70 1C. The progress of polymerization was

P O

P O

P O •

Scheme 2.2

(m + n)

O •

O •

O •

H H2

H2

* C C mCH C

O

P

O

H2 H2 H C CH C C n*

Initiation of styrene polymerization with 2,4,4,5,5-pentaphenyl-1,3,2dioxaphospholane.

32

Chapter 2

.

P

H2C CH

O

O H2C CH

H2C CH

Scheme 2.3

C O

P

O

C O

P

O

..

H2 . C CH

Copolymerization of styrene and 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane.

Table 2.1

SEC Characterization of poly(styrene) produced using 2,4,4,5,5-pentaphenyl-1,3,2dioxaphospholane as initiator at 70 1C.

Initiator present (weight %)

MN

MW

Polydispersity

0 1 5 10

Na 180 000 120 000 90 000

Na 365 400 302 400 273 600

Na 2.03 2.52 3.04

a

N: no polymer formed.

followed by removing aliquots of the mixture as a function of time for viscosity measurement. The aliquots were diluted with benzene (1 g of mixture per 100 ml of benzene). The relative viscosity increased as a function of time, which reflected the extent of polymerization. SEC data for the polymers formed are contained in Table 2.1. From the chromatographic data, it is apparent that thermal decomposition of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane is effective in initiating styrene polymerization. In fact, when no phospholane was present in the monomer, no polymer was produced at 70 1C. In the presence of phospholane, polymer is readily produced under the same conditions. Further, the molecular mass of the polymer formed decreases regularly, as expected, as the concentration of initiator (phospholane) is increased. Analysis of the of polymers by both infrared and phosphorus-31 NMR spectroscopy indicated that phosphorus units were incorporated into the polymer main chain at a much higher level than can be accounted for by initiation, i.e., the phosphorus moiety probably functions as both initiator and co-monomer.

33

Strained Organophosphorus Compounds as Reactive Flame Retardants

2.3.1 Thermal Properties of Styrene Polymers Containing Phosphorus Units The thermal stabilities of the styrene polymers generated using one, five and ten mass percent 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane as initiator were examined using thermogravimetry. The relevant decomposition data are given in Table 2.2. From these results, it would appear that the thermal stability of the polymers that contain a phospholane unit is similar to that of poly(styrene), i.e., incorporation of the phospholane into the polymer main chain does not diminish the thermal stability of poly(styrene).

2.3.2 Evaluation of Flammability The flammability of these polymers was evaluated using the UL 94 vertical burn test. Results are presented in Table 2.3. Table 2.2

Comparison of the extrapolated onset and maximum decomposition temperature of styrene polymers produced in the presence of one, five, and ten percent 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane at 70 1C.

Phospholane in polymerization mixture (weight %)

Extrapolated onset temperature for decomposition (1C)

Maximum decomposition rate temperature (1C)

0 1 5 10

418.1 424.2 429.3 433.2

438.9 443.0 448.3 454.1

Table 2.3

Flammability behaviour of styrene polymers generated by initiation with 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane at 70 1C.

Observation Total flaming combustion for each specimen Total flaming combustion for all five specimens of any set Flaming and glowing combustion for each specimen after second burner Cotton ignited by flaming drips from any specimen Glowing or flaming combustion of any specimen to holding clamp Classification

Level of phospholane in polymerization mixture (Weight %) 0 30s 250s

1 30s 250s

5 30s 250s

10 30s 80s

60s

60s

60s

60s

YES

YES

YES

NO

YES

YES

NO

NO

94V-1

o94V-1

34

Chapter 2

The results in Table 2.3 suggest that, at the level of incorporation, the phospholane imparts a modest flame retardancy to poly(styrene). It may be anticipated that similar results can be obtained from other common evaluation techniques, such as limiting oxygen index (LOI) or cone calorimetry.

2.4 Conclusions 2,4,4,5,5-Pentaphenyl-1,3,2-dioxaphospholane undergoes thermally stimulated homolysis of a carbon–carbon bond at 70 1C to generate a diradical which efficiently initiates styrene polymerization. Polymers produced from initiation in this manner contain phosphorus in the main chain. As assessed by thermogravimetry, these polymers display thermal stability comparable to that of conventional poly(styrene), i.e., incorporation of phosphorus moieties into the polymer main chain does not diminish the thermal stability of the polymer. The presence of phosphorus in the polymer leads to a decreased flammability.

References 1. P. Georlette, J. Simons and L. Costa, ‘‘Fire Retardancy of Polymeric Materials’’, Marcel Dekker, Inc, New York, NY, 2000, p. 245–284. 2. B.A. King, in Modern Styrenic Polymers: Polystyrenes and Styrenic Copolymers, Eds., J. Scheirs, D.B. Priddy, John Wiley and Sons Inc, New York, NY, 2003, p. 685–701. 3. M. Simonson, Polym. Mater. Sci. Eng., 2000, 83, 90. 4. E. Weil, ‘‘An Attempt at a Balanced View of The Halogen Controversy,’’ Proceedings, 10th International Conference on Recent Advances in Flame Retardancy of Polymeric Materials,’’ Business Communications Company, Norwalk, CT, 1999. 5. W. Weil, Polym. Degr. Stab., 1996, 54, 125. 6. B.A. Howell and J. Uzibor, J. Therm. Anal. Cal., 2006, 85, 45. 7. B.A. Howell and J. Uzibor, J. Vinyl Addit. Tehcnol., 2006, 12, 198. 8. B.A. Howell, Y. Cui and D.B. Priddy, Thermochim. Acta, 2003, 396, 1673. 9. Y. Cui, ‘‘An Assessment of the Impact of Head-to-Head Placement on the Thermal Stability of Poly(styrene)’’, M.S. Thesis, Central Michigan University, Mt. Pleasant, MI, 2001.

CHAPTER 3

Amorphous Silicon Dioxide as Additive to Improve the Fire Retardancy of Polyamides G. SCHMAUCKS, B. FRIEDE, H. SCHREINER AND J.O. ROSZINSKI Elkem AS, Materials, P.O. Box 8126, Kristiansand, NO-4675, Norway

3.1 Introduction Fumed, precipitated and other types of silica are considered reinforcing fillers for rubber and plastics,1 but silica is not a flame retardant filler because it does not actively react through the release of water or another mechanism in fires. However, it was found that the addition of various types of silica (silica gel, fumed silica and fused silica) can significantly reduce the heat release and mass loss rate.2 It was concluded that the mechanism is based on physical processes in the condensed phase, rather than on chemical reactions. A number of papers at the Fire Retardant Polymers Meeting (FRPM) in Berlin in 2005 dealt with the effect of different silicon sources as flame retardant additives in polymer systems. Conflicting results were presented. Duquesne et al.3 reported that the addition of silica to an intumescent system did not improve the effect and that the rate of expansion was even decreased. The use of fumed silica (specific surface area 380 m2 g 1) in a textile application in combination with other halogen-free materials also did not lead to an efficient flame retardant system.4 Schmaucks et al. presented a synthetic amorphous silicon dioxide (specific surface area of only 20 m2 g 1) that showed very positive results in cone calorimeter investigations of ethylene–vinyl acetate (EVA)- and polyamide-6 Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

35

36

Chapter 3

(PA6)-based thermoplastic compounds, alone or in combination with glass fibres or aluminium trihydrate, respectively.5 Transmission electron microscopy (TEM) investigations prove that the silica particles are well dispersed, which seems to be essential to create a material that has excellent mechanical properties and high fire resistance at the same time. The evaluation of this material, SIDISTARs, was extended to other types of polyamides. The new results indicate that the efficiency of this silica as flame retardant additive depends on the polyamide type. Dispersion of the silica particles and the viscosity of the composite seem to play the most important roles. Fillers and additives used to improve the fire retardancy of polymeric materials act either by a solid-phase or a gas-phase mechanism. Some might even show both modes of action. Silica will stay intact and will not react in any fire scenario. It is not expected that it will show an effect other than the dilution of the polymer, which reduces the amount of fuel available for consumption during the fire. It might, however, in combination with other ingredients, contribute to char formation and therefore increase the residue amount and reduce the heatrelease rate (HRR) and the total heat released (THR). In a number of studies, which can be found in the literature, conflicting results have been reported.6 Primary particle size and dispersion are seen as the main reasons for the different behaviour of silica in these investigations, but to really understand these different effects it is necessary to look at the nature and morphology of the silica types used.7 A possible differentiation of silica types according to Chemical Abstract Service (CAS) Numbers is shown in Figure 3.1. Of course, as any list, this one is also disputable, but it at least shows the large number of different silica types available. As examples to illustrate the different morphologies, TEM images of precipitated silica (CAS no. 112926-00-8), pyrogenic or fumed silica (CAS no. 112945-52-5), with two different specific surface areas according to the Brunauer, Emmett and Teller (BET) method, and silica fume (SIDISTARs; CAS no. 69012-64-2), are shown in Figures 3.2–3.5. It is clear that the precipitated and the fumed silica consist of very small, irregularly formed primary particles, which are partly fused together and form agglomerates, or chains. They also have a narrow particle size distribution. The strong interactions between the small particles make it difficult, or even impossible, to disperse them as primary particles in a polymer matrix using conventional mixing technology. However, silica fume consists of spherical particles with a broad particle-size distribution, which minimises the particle interaction and allows easy dispersion down to primary particles in a polymer composite. In addition to the differences in morphology, we need to consider the chemical behaviour caused by the silanole groups at the silica surface. Precipitated silica has the largest number of silanole groups, making it very hydrophilic, hygroscopic and reactive towards silanes. Commercially available precipitated silica contains around 5% water. Fumed silica has about half the silanole group density than does precipitated silica, is still hydrophilic and contains about 2% water. The silica fume used in this investigation, however, although hydrophilic, is not hygroscopic and contains less than 0.8% water.

Synthetic Amorphous Silica (7631-86-9)

Wet

Thermal

Surface modified Silica (67762-90-7) (68611-44-9) (68909-20-6)

Amorphous Silica (7631-86-9)

Natural

Crystalline Silica

By-Products

Silica Gel (112926-00-8)

Pyrogenic Silica fumed silica (112945-52-5)

Diatomaceous earth (61790-53-2)

Fused Silica (60676-86-0)

Precipitated Silica (112926-00-8)

Electric-Arc Silica

Calcined (91053-39-3)

Silica Fume Microsilica (69012-64-2)

Plasma Silica

2) Flux-Calcined (68855-54-9)

Cristobalite (14464-46-1)

Quartz (14808-60-7)

Tridymite (15468-32-3)

Keatite (17679-64-0) Coesite(13778-38-6) Stishovite(13778-38-5) (Melanophlogite)

Figure 3.1

Amorphous Silicon Dioxide as Additive to Improve the Fire Retardancy

Silica CAS 7631-86-9

Classification of silica types according to CAS numbers. 37

38

Chapter 3

Figure 3.2

TEM image of precipitated silica (BET 190 m2 g 1) – 19.5 kx.

Figure 3.3

TEM image of fumed silica (BET 200 m2 g 1) – 19.5 kx.

Amorphous Silicon Dioxide as Additive to Improve the Fire Retardancy

Figure 3.4

TEM image of fumed silica (BET 50 m2 g 1) – 19.5 kx.

Figure 3.5

TEM image of silica fume (SIDISTARs; 20 m2 g 1) – 41.5 kx.

39

40

Chapter 3

3.2 Experimental 3.2.1 Materials The materials used are listed here and in Table 3.1. PA12 – Grilamids L20 G natural (EMS Chemie AG) PA6.6 – Durethans A30 (Lanxess AG) Glass fibres – EC10 (Glasseidenwerk Oschatz) Synthetic amorphous silica (silica fume) – SIDISTARs T120 (Elkem AS, Materials)  Surface-modified silica fume (with 3-aminopropyltriethoxysilane) – SIDISTARs T120XP.

   

3.2.2 Sample Preparation All compounds were prepared by extrusion in a co-rotating twin-screw extruder (L/D 40) with a screw diameter of 25 mm at a speed of 300 revolutions per minute (rpm) at 240–260 1C for PA6.6 and 200–220 1C for PA12, respectively, followed by granulation. The polymers were dried prior to compounding. The specimens for mechanical tests, as well as the plaques (100  100  4 mm) for cone calorimeter measurements, were prepared by injection moulding. All specimens were conditioned for 72 hours at standard climate 23/50 according to ISO 291, prior to the mechanical tests.

3.2.3 Test Methods Tensile testing was carried out according to DIN EN ISO 527 and Charpy impact test according to DIN EN ISO 179-1eA and DIN EN ISO 179-1eU, respectively. 20 mm specimens were used for the vertical burning test, according to UL94. Cone calorimeter measurements were performed according to ISO 5660. Table 3.1

List of compounds.

Materials

PA6.6 Glass fibres Silica fume Silica fume, surface modified PA12 Glass fibres Silica fume Silica fume, surface modified

Sample No. and composition in weight % (w/w) 1.1 100 0 0 0 2.1 100 0 0 0

1.2 90

1.3 80

1.4 70

10 0 0

20 0 0

30 0 0

2.2 90

2.3 80

2.4 70

10 0 0

20 0 0

30 0 0

6.1 95 0 5 0 7.1 95 0 5 0

6.2 90

6.3 80

0 10 0

0 20 0

7.2 90

7.3 80

0 10 0

0 20 0

6.4 95

6.5 90

3.1 80

3.2 80

0 0 10

10 10 0

0 20 0

4.1 80

4.2 80

4.3 90

4.4 80

10 10 0

0 20 0

10 0 10

0 0 5

5 0 5

3.3 90

3.4 80

3.5 50

10 0 10

30 20 0

4.5 50

5.1 90

5.2 80

30 20 0

0 0 10

0 0 20

5 0 5

41

Amorphous Silicon Dioxide as Additive to Improve the Fire Retardancy

3.3 Results and Discussion Results of the evaluation of silica fume addition (SIDISTARs) into PA6 and PA11, alone or in combination with varying amounts of glass fibres, have been reported previously.5 In the research reported herein, its application in PA6.6 and PA12 was investigated. In Table 3.2 the tensile modulus dependence on the silica concentration is summarised. As expected, the modulus increases with increasing amounts of the silica. The results of the impact test are given in Figures 3.6 and 3.7. Surprisingly, it was found that up to 20% of silica can be added to PA12 without any loss in impact strength, whereas an addition of only 5% to PA6.6 leads to a significant reduction in the toughness of the composite. Surface modification of the silica leads to an increase in the compound’s toughness. The fire retardancy of the composites was evaluated by cone calorimeter and the UL 94 burning test. The results of the UL 94 test are summarised in Table 3.3. In PA6.6 the addition of 20% silica leads to a reduction of the after-flame time from 140 to 96 seconds. For comparison, the addition of 20% glass fibres increases the after-flame time to 852 seconds. In PA12, 20% silica reduces the after-flame time from about 591 to 36 seconds, whereas 20% glass fibres reduce the after-flame time to 528 seconds. Cone calorimeter measurements were carried out at three different irradiations, 35, 50 and 70 kW m 2. Selected results of duplicate or triplicate measurements, respectively, are shown in Figures 3.8–3.12. Surprisingly, it was found that the effect of the silica seems to be different in PA6.6 and in PA12.

Table 3.2

Tensile modulus of silica filled polyamides. Tensile modulus according to DIN EN ISO 527-2 Et (Mpa) Standard deviation

Mixture

Composition (w/w)

Mean value

PA12 PA12/T120 PA12/T120 PA12/T120

95/ 90/10 80/20

1487 1503 1572 1792

18 13 37 70

PA66 PA66/T120 PA66/T120 PA66/T120

95/5 90/10 80/20

2942 3072 3414 3620

74 191 154 139

PA66/T120XP PA66/T120XP

95/5 90/10

3110 3389

84 82

42

Chapter 3

Charpy impact toughness acu / kJ/m2

400 350 300 250 200 150 100 50

PA12 + T120

Figure 3.6

PA66 + T120

90/10

95/5

100/0

--

80/20

90/10

95/5

100/0

--

80/20

90/10

95/5

100/0

0

PA 66 + T120XP

Charpy impact strength dependence on silica concentration.

notched impact toughness ack / kJ/m2

8 7 6 5 4 3 2 1

PA12 + T120

Figure 3.7

PA66 + T120

90/10

95/5

100/0

--

80/20

90/10

95/5

100/0

--

80/20

90/10

95/5

100/0

0

PA 66 + T120XP

Notched Charpy impact strength dependence on silica concentration.

43

Amorphous Silicon Dioxide as Additive to Improve the Fire Retardancy

Table 3.3

20 mm vertical burning test according to UL 94 V. Vertical burning behaviour according to UL 94

Polymer Sidistars T120

Sidistars T120XP

Total after-flame Materials classification timea [s]

Type

%

Glass fibre

1.1 1.2 1.3 1.4

PA66

100 90 80 70

0 10 20 30

0 0 0 0

0 0 0 0

140 66 852 775

–b V2 –b –b

2.1 2.2 2.3 2.4

PA12

100 90 80 70

0 10 20 30

0 0 0 0

0 0 0 0

591 501 528 240

–b –b –b –b

3.1 3.2 3.3 3.4 3.5

PA66

80 80 90 80 50

10 0 5 10 30

10 20 0 0 20

0 0 5 10 0

175 96 213 72 533

–b V2 –b V2 –b

4.1 4.2 4.3 4.4 4.5 5.1

PA12

80 80 90 80 50 90

10 0 5 10 30 0

10 20 0 0 20 0

0 0 5 10 0 10

724 36 87 300 62 72

–b V2 V2 –b V2 V2

Sample No.

a

Burning time after first and second ignition, five specimens. Outside classification range of UL 94.

b

In PA12 the silica behaves like an inert filler without any specific barrierforming properties. A rather high HRR indicates that the material will not be self-extinguishing in flammability testing. 20% silica in PA6.6 causes a reduction in HRR of 52% at 35 kW m 2 and 65% at 70 kW m 2. This significant increase in flame retardancy effect with increasing radiation is typical for materials that form surface barriers in a cone calorimeter. From the effects of 20% SIDISTAR T120 in PA12 and in PA6.6, it is obvious that the silica worked crucially better in PA6.6 than in PA12 (see Figure 3.12). In PA6.6 the better barrier was formed. Barrier formation is controlled by several processes during combustion, such as demixing, selforganisation, ablative re-assembly, decomposition, etc. It is assumed that the barrier formation depends on different parameters, such as the various repulsive and attractive interactions between the components, viscosity of the melt, pyrolysis temperature, bubbling due to decomposition, etc. Given these details, it is surprising that such a clear difference for SIDISTAR in these two polyamides occurred, since most of the properties for PA6.6 and PA12 are not very different. A different distribution of the silica particles in PA6.6 and in PA12 was seen as the most reasonable explanation for the observed different fire behaviours. This was investigated by TEM, and the results are shown in Figures 3.13 and 3.14.

44

Chapter 3 2500

150

2000

120

1500

90

1000

60

500

THR / MJ m-2

HRR / kW m-2

35

kW/m2

30 HRR THR

0

0 0

Figure 3.8

100

200

300 Time / s

400

600

500

HRR and THR for PA12 at 35 kW m 2.

150

2500

2000

120

1500

90

1000

60

30

500 HRR THR 0 0

Figure 3.9

100

200

300 Time / s

400

500

HRR and THR for PA12:silica fume 80:20 at 35 kW m 2.

0 600

THR / MJ m-2

HRR / kW m-2

35 kW/m2

45

Amorphous Silicon Dioxide as Additive to Improve the Fire Retardancy 2500

150

120

1500

90

1000

60

500

30 HRR THR

0 0

100

200

300 Time / s

0 600

500

400

HRR and THR for PA6.6 at 35 kW m 2.

2500

150 HRR

HRR / kW m-2

35 kW/m2

THR

2000

120

1500

90

1000

60

500

30

0 0

Figure 3.11

100

200

300 Time / s

400

500

0 600

HRR and THR for PA6.6:silica fume 80:20 at 35 kW m 2.

THR / MJ m-2

Figure 3.10

-2

2000

THR / MJ m

HRR / kW m-2

35 kW/m2

46

Chapter 3 Peak of heat release rate 3000 PA 12

PA 12/T 120 (80/20)

PA 66

PA 66/T 120 (80/20)

HRR / kW m-2

2500

2000

1500

1000

500

0 35

50

70

35

50

70

35

50

70

35

50

70

-2

External Heat Flux / kW m

Figure 3.12

HRR for PA12 and PA6.6 at three different irradiations.

Figure 3.13

TEM of PA6.6 containing 20% silica fume.

No differences in dispersion could be observed. The conclusion is therefore that reasons other than silica dispersion cause the differences in fire retardancy. When comparing the effect of 10% SIDISTAR T120 with 10% surfacemodified silica (SIDISTAR T120XP) in PA6.6, it was found that the

Amorphous Silicon Dioxide as Additive to Improve the Fire Retardancy

Figure 3.14

47

TEM of PA12 containing 20% silica fume.

unmodified silica has a more pronounced barrier effect. The peak HRR was reduced by one-third for T120, but only by one-quarter for T120XP. The surface-modified silica also showed almost no reduction in total heat evolved (THR) compared to T120. The decrease in peak HRR was accompanied by a corresponding decrease in maximum carbon monoxide (CO)- and smoke-release rate because of the reduced amount of polymer consumed by the fire.

3.4 Conclusion Synthetic amorphous silicon dioxide with a BET surface area of 20 m2 g 1 (silica fume – trade name SIDISTARs T120) shows different behaviour in terms of physical properties and fire retardancy in PA12 and PA6.6. In PA12 20% of the silica could be added without loss of impact strength, but it only shows the behaviour of an inert filler in the cone calorimeter test. In PA6.6, the addition of only 5% silica leads to a significant reduction in impact strength, but the composite possesses extraordinary flame retardancy when tested according to ISO 5660 in the cone calorimeter. The difference in behaviour cannot be explained by differences in silica dispersion.

Acknowledgement The authors thank Mrs Krajewsky and Mr Thieroff, Kunststoffzentrum, Leipzig, for the sample preparation and the mechanical and UL 94 tests, Dr Schartel of the Federal Institute for Materials Research and Testing (BAM), Berlin, for the cone calorimeter tests and Mr Seydewitz, University Halle, for the TEM investigations.

48

Chapter 3

References 1. Georg Wypych Handbook of Fillers, Plastics Design Library, Toronto New York 1999, p. 131 ff. 2. Kashiwagi et al. Journal Fire & Materials, 6, Nov./Dec. 2000, p. 277 ff. 3. Duquesne et al. Proceedings of FRPM’05, Berlin 2005, paper 2_O_1. 4. A.R. Horrocks et al. Proceedings of FRPM’05, Berlin 2005, paper 5_O_1. 5. G. Schmaucks et al. Proceedings of FRPM’05, Berlin 2005, paper 4_O_4. 6. T. Kashiwagi, A.B. Morgan, J.M. Antonucci, M.R. VanLandingham, R.H. Harris, W.H. Awad and J.R. Shields, J. Appl. Polym. Sci, 2003, 89(8), 2072–2078. 7. T. Kashiwagi, in Flame Retardant Polymer Nanocomposites, ed. A.B. Morgan and C.A. Wilkie, Wiley, Hoboken, 2007, p. 286–288.

CHAPTER 4

Use of Organosilicone Composites as Flame Retardant Additives and Coatings for Polypropylene + A. SZABO´, K. KISS AND G. MAROSI B.B. MAROSFOI, Budapest University of Technology and Economics, Department of Organic Chemical Technology, Budafoki u´t 8., H-1111 Budapest, Hungary

4.1 Introduction Organosilicone polymers and their organic–inorganic hybrid structures are promising components of flame retardant formulations. This is attributable to their superior properties, compared to polymers with an organic main chain, in terms of their thermal stability and electrical properties. Industrially, the most important silicone is polydimethylsiloxane (PDMS). Its conversion into polyborosiloxane (pBSil) via a polycondensation reaction has attracted interest in both the academic and industrial fields. The mechanical and rheological properties of pBSil have been widely studied, while their behaviour in thermooxidative atmosphere has been investigated less.1,2 These siloxane compounds may act during combustion as a shielding powder (PDMS) or as a protective ceramic layer (pBSil). The boron atom in pBSil is an essential component of ceramic layer formation, as without it the PDMS transforms into a fine silica powder.3 Advantageous synergetic actions between pBSil and ‘carbon-based’ intumescent flame retardants have also been reported – the pBSil acts as an additive that enhances the consistency of char.4 Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

49

50

Chapter 4

Recent advances in flame retardant polymeric materials have focused on the flame retardancy of polymer–clay nanocomposites (PCNs). The reducing effects of clays in PCN on heat-release rate (HRR) are well known. Moreover, the clays in combination with intumescent flame retardants have a significant influence on the char structure through their bubble-nucleating effect.5 The needle-like sepiolite clay may promote the formation of increased amounts of char of better uniformity and strength. Different boron-containing compounds, such as metal borates (zinc, aluminoand magnesium borate) and organoborates [like melamine–borate (MB)], are increasingly used in polymers, particularly for fire retardant purposes. They may act as smoke suppressants, afterglow suppressants, corrosion inhibitors or synergistic agents.6–8 Inorganic boron salts act in the condense phase by changing the degradation pathway of polymers and thus promoting the formation of char and the reduction of carbon dioxide (CO2) and carbon monoxide (CO).9,10 Although all three types (polyorganosiloxanes, PCNs and MBs) have been used as synergists in fire retardant compositions, their combination without fire-retardant active atoms (e.g. phosphorous, bromine) has not been investigated yet. In this study of pBSil, various composites that contain needlelike clay and MB were prepared and characterized as flame retardant components in polypropylene (PP). The combustion characteristics of the relevant polymer blends and composites were compared to those of pristine PP. Contact angle measurements were made, as well as the fire retardant characterisation, to determine the multifunctional character of the developed pBSil composites.

4.2.

Experimental

4.2.1 Materials PP of Moplen HP400R type was received from Basell Polyolefins, MFI: 25 (230 1C/2,16 kg). Hydroxyl-terminated PDMS) {HO–[Si(CH3)2O)n–H]} with a viscosity range of 400–650 mPa
s was received from Wacker Silicone GmbH (Wacker Silicone OEL CT 601 M). Boric acid (BA), melamine and absolute ethanol were received from Sigma Aldrich and used without further purification. The MB was prepared in our laboratory and used as an incorporated additive. For the MB preparation, first the melamine (12.6 g, 0.1 mol) was dissolved in 600 ml boiling distilled water and the BA (12.2 g, 0.2 mol) in 100 ml distilled water. After mixing these, the solution was left to cool to room temperature under constant stirring. The white flocculated MB was completely precipitated then decanted and filtered. The obtained MB was then washed with cool water and left at room temperature to dry. The needle-like clay sepiolite in organomodified form (OSEP) was supplied by Tolsa (trade-mark: Pangel B40). a,a 0 -bis(t-butylperoxy)diisopropylbenzene (trade mark Luperox

51

Use of Organosilicone Composites as Flame Retardant Additives

F90P from Elf Atochem) was used as the peroxide radical initiator. When applied, the amount was 0.5% of the pBSil content of the composites.

4.2.2 Sample Preparation The formation of a boron linkage between the PDMS chain required a treatment in boiling ethanol. The BA was dissolved in boiling ethanol, and then the PDMS added dropwise. The mixture, under continuous stirring, was heated until a transparent solution was obtained. The reaction was carried out in excess BA. To apply additives and obtain a homogeneous mixture, MB and/or OSEP was added before the removal of ethanol. The majority of ethanol was removed by heating the samples up to 140 1C and any remaining ethanol subsequently removed by evaporation in a vacuum chamber at 60 1C. The pBSil-based samples were solidified after the evaporation. The compositions of prepared samples are summarised in the Table 4.1.

4.2.3 Preparation of PP Compounds MB and OSEP that contained polyborosiloxane (pBSil–MB–OSEP) systems were introduced into PP. The amount of (pBSil–MB–OSEP) system in PP was increased by a multistep process in the mixing chamber of Brabender Plastograph. A mixing temperature of 190 1C and a mixing speed of 60 min1 were used, the samples were homogenised for 10 minutes and then melt-compressed at 200 1C.

4.2.4 Preparation of Composites with Multilayer Structures The efficiency of the pBSil–OSEP–MB system when structured as multilayers was also investigated. The upper layer was a pBSil-based composite, while the internal layer was pristine PP. The multilayer structured samples were prepared in a Colin P200E type press at 195 1C and 5.25 kPa (50 bars) for 10 minutes by compression of a 10  10  0.4 cm PP sheet and a pBSil–OSEP–MB 10  10 sheet. The thickness of the pBSil-based outer layer was 0.5 mm.

Table 4.1

Composition of pBSil and its composites with MB and/or organomodified sepiolite (OSEP).

Sample code PDMS pBSil PBSil–MB(5%) PBSil–OSEP(5%) PBSil–MB(5%)–OSEP(5%)

PDMS (wt%) 100 90.9 86.4 86.4 81.8

BA (wt%)

MB (wt%)

OSEP (wt%)

– 9.1 8.6 8.6 8.2

– – 5 – 5

– –– 5 5

52

Chapter 4

4.2.5 Characterisation The thermal stability of materials was examined by thermogravimetric analysis (TGA: Setaram Labsys TG DTA/DSC) under air atmosphere and with a heating rate of 10 1C per minute. The combustion parameters [HRR, time-to-ignition (TTI) and total heat release (THR)] of samples were determined by a mass loss calorimeter (according to ISO 13927, FTT Inc.) under a heat flux of 50 kW m2. Contact angle values were determined goniometrically by the sessile drop method using 5 ml volume of water in a Kruss Contact Angle Meter.

4.3 Results and Discussion 4.3.1 Thermo-Oxidative Stability The analyses to determine the influence of boron inclusion on the thermooxidative stability of PDMS were designed considering former experiments to determine the role of different end groups on its thermal degradation.11,12 The TGA results in Figure 4.1 show, in agreement with the literature, that the oxidative thermal degradation of PDMS takes place in two stages. In the first

Figure 4.1

TGA and differential thermal analysis (DTA) curves of PDMS and pBSil polymers and systems that contain 5 wt% OSEP and/or MB component in air, with a heating rate of 10 1C per minute.

53

Use of Organosilicone Composites as Flame Retardant Additives

Table 4.2

Characteristic TGA data of polyorganosiloxane polymers and composites. Temperature for mass loss

Sample

15%

50%

PDMS pBSil PBSil–OSEP(5%) PBSil–MB(5%) PBSil–OSEP(5%)–MB(5%)

398 415 424 431 421

542 488 498 476 477

Temperature for maximum rate of mass loss (1C) 352 – – – –

Residue at 550 1C (%)

509 481 489 472 475

48 38 36 33 30

step volatile products, mostly cyclic compounds, evolve and fragments of lower molecular weight are formed. In the second step further depolymerization and breakdown of the PDMS chain takes place. In the oxidative degradation of pBSil with respect to that of PDMS, the first degradation step completely disappears. The residue at 550 1C is smaller for pBSil, probably because boron can form volatile compounds, which are easily lost.13 The characteristic mass-loss data obtained in air atmosphere are presented in Table 4.2. The boron coupled to the polysiloxane chain of pBSil forms a network through borosiloxane linkages. The temporary B


O interchain dative bonds in polyborosiloxane also establish the linkages of a network structure. The incorporated boron favours reactions such as Si–CH3 dehydrogenation to Si–CH2 and the splitting off the methyl groups from PDMS through conversion into radicals and giving additional binding sites, as shown in Equations 4.1–4.3.14  Si  CH3 ! Sid þ dCH3

ð4:1Þ

 Si  CH3 þ dCH3 ! Si  CH2 d þ CH4

ð4:2Þ

 Sid þ dCH2  Si ! Si  CH2  Si 

ð4:3Þ

A significant enhancement in thermal stability and the disappearance of the first degradation step are observed in TG curves at low temperature (see Figure 4.1). This can be ascribed to the cross-linking structure of pBSil, which hinders the splitting of the cyclic oligomers and the formation of disadvantageous volatile products. This effect is more pronounced in the presence of MB and OSEP.

4.3.2 Combustion Characteristics of Polypropylene-Based Composites The pBSil–OSEP(5%)–MB(5%) sample was selected to prepare PP-based composites. The samples made of PP and polyborosiloxane composites are summarised in Table 4.3, where the values are given in weight percent.

54

Table 4.3

Chapter 4

The compositions of PP–(pBSil–OSEP–MB) in wt%. PBSil–OSEP–MB

Sample code NKMB060 NKMB061 NKMB062 NKMB063 NKMB073

PP 0 75 60 45 45

PBSil–OSEP–MB a

100 25 40 55 55*

pBSil

OSEP

MB

90 22.5 36 49.5 49.5

5 1.25 2 2.75 2.75

5 1.25 2 2.75 2.75

a

0.5% peroxide content.

Figure 4.2

Rate of heat release vs time curves of PP–(pBSil–OSEP–MB) composites as the percentage of pBSil–OSEP–MB was varied from 0 to 55 wt% (see Table 4.3); irradiation heat flux: 50 kW m2.

The HRR curves of the composites are shown in Figure 4.2. The amount of pBSil–OSEP–MB has a significant influence on the combustion properties. Peak HRR decreases with increasing percentage of pBSil–OSEP–MB in the composite and, furthermore, reduction in THR and effective heat of combustion (EHC) can also be observed. However, the TTI time range decreases slightly. All the quantitative results are given in Table 4.4. The lower HRR (kW
m2) and THR (MJ
m2), indicate a smaller fire hazard, but with a slightly shortened TTI (s). The curves of the PP–(pBSil–OSEP–MB composites lie between the two references, pure PP and pBSil–OSEP–MB. The shape of the curves changes

55

Use of Organosilicone Composites as Flame Retardant Additives

Table 4.4 Sample PP NKMB060 NKMB061 NKMB062 NKMB063 NKMB073

Figure 4.3

Combustion properties of PP/pBSil-OSEP-MB composites. TTI (s)

peak HRR (kW
m2)

34 22 24 25 20 20

898 132 586 441 332 230

Peak time (s) 149 62 208 227 263 256

THR (MJ
m2) 115.3 23.3 84.1 76.3 67.1 43.8

Residue (%) 0 19.8 2.3 3.7 6.1 7.9

Images of the combustion residue of (a) PP and (b) PP–(pBSil–OSEP– MB) after treatment under a cone heater (heat flux: 50 kW m2). Image (c) shows the non-carbonaceous, ceramified intumescent structure.

gradually with increasing amounts of pBSil–OSEP–MB. For pristine PP, a shoulder occurs before the maximum HRR value, while as the pBSil content of the composites is increased this shoulder gradually broadens. The presence of pBSil–OSEP–MB causes a steady-state heat release until the peak at the end of the combustion, which also decreases. Applying peroxide as radical initiator (sample NKMB073), the thermal stability and the combustion properties improved further. As shown in Table 4.4, the HRR peak reduced from 898 to 230 kW m2. The time at which this occurred increased from 146 to 256 seconds, and a remarkable decrease occurred in the THR value. Interestingly, at the beginning of combustion the composite NKMB073 exhibits a HRR as low as that of the pure pBSil–OSEP–MB reference. The peroxide probably acts by building linkages between polymer chains both within the silicone phase and at the interfaces of the two polymers, but validation of this assumption requires further experiments. The advantageous effect of the pBSil systems in PP can be explained by the formation of a non-carbonaceous, white, ceramified, intumescent structure on the surface during the combustion, which acts as a protective ceramic layer. At the end of combustion the PP leaves no residue, whereas the PP–(pBSil–OSEP– MB) samples leave a white solid coherent char-like residue. The thickness of the residue is about 3–5 centimetres, as shown in Figure 4.3.

56

Chapter 4

4.3.3 Multilayer Structure PP–(pBSil–OSEP–MB) The efficiency of the pBSil–OSEP–MB composite as a multilayer structure was also investigated, using it as a coating on the PP surface. In previous studies on boron-containing PDMS the authors found that the boron content helps to form a protective, coherent, heat-resistant ceramic layer on the polymer during combustion.4,15 These results initiated the using of polyborosiloxane composite as a coating on the PP surface. During processing we found that the pBSil–OSEP–MB composite readily forms a continuous layer on the surface of PP. The 500 mm layer formed on the PP core of 4 mm corresponds to less than the half of the lowest pBSil–OSEP– MB concentration (NKMB061) in Table 4.3. Such multilayer structures resulted in improved combustion characteristics (see Figure 4.4) – more than a 30% reduction in peak HRR and a considerably delayed peak (from 217 to 454 seconds) compared to PP. Furthermore, in this case the TTI did not decrease, but increased by 60% of the original value. The wettability study of this surface revealed that the coated layer is less adhesive than that of pristine PP, as shown in Figure 4.5. A digital camera is mounted on the contact angle meter. The contact angle of water drop on the PP surface is 901 while the coated surface has a contact angle value of 1151. The results suggest that multilayer structures may provide protection more efficiently, especially against the early degradation of the polymer, than mixtures of the same composition. Such surface layers may perform multifunctional

Figure 4.4

HRR results of the reference polymer and the multilayer structure; irradiation heat flux: 50 kW m2.

Use of Organosilicone Composites as Flame Retardant Additives

Figure 4.5

57

Photo of water drop on (a) PP coated with pBSil–OSEP–MB material and (b) uncoated PP as reference.

roles, acting both as flame retardant and as an adhesion–modifying (dust and/or graffiti releasing, self-cleaning) layer.

4.4 Conclusion The thermo-oxidative degradation of PDMS is affected by the inclusion of boron atoms. The pBSil formed by a polycondensation reaction exhibits higher stability in the initial stage of decomposition as the boron-containing units hinder the splitting of the cyclic oligomers. At higher temperatures, however, the mass loss of pBSil is larger than that of PDMS. To improve the fire retardancy of PP, pBSil was combined with MB and a fibrous clay (sepiolite). Increasing the concentration of pBSil–OSEP–MB in PP decreases the peak HRR gradually, as well as the THR value. These results suggest that the formation of a non-carbonaceous, white, ceramified, intumescent layer on the surface of PP may act similarly to the conventional carbonaceous foam, but its heat stability is greater. Further improvement was achieved by the introduction of a radical initiator into the system. This probably acts by building linkages between the polymer chains. Also, pBSil–OSEP–MB readily forms a continuous layer on the surface of PP. Applied as a surface layer it acts as a fire-retardant protecting coating and also enables the control of adhesion at the surface. This double effect of the developed coating is advantageous in construction, automotive and some other relevant industries.

Acknowledgements Supports from EU 6 Multihybrids (IP 026685-2), Nanofire (NMP3-CT 2004505637) projects, Hungarian Research Fund OTKA T049121, Fund of

58

Chapter 4

European Union and Hungarian State GVOP/3.1.1.-2004-0531/3.0, Public Benefit Association of Sciences and Sport of the Budapest University of Technology and Economics are acknowledged.

References 1. P.J. Davies and A.J. Fletcher, J. Mech. Eng. Sci., 1995, 209(6), 408. 2. A. Juhasz, P. Tasnadi and L. Fabry, Phys. Educ., 1984, 19, 302. 3. I. Ravadits, A. To´th, G. Marosi, J. Papp and S. Szabo´, Polym. Degrad. Stabil., 2001, 74, 419. + and A. Sze´p, 4. G. Marosi, A. Ma´rton, P. Anna, G. Bertalan, B. Marosfoi Polym. Degrad. Stabil, 2002, 77, 259. 5. A. Toldy, P. Anna, I. Csontos, A. Szabo´ and G. Marosi, Polym. Degrad. Stabil, 2007, 92(12), 2223. 6. http://www.borax.com. 7. S. Bourbigot, M. Le Bras and S. Duquesne, in Fire Retardancy of polymers, eds. M. LeBras, S. Bourbigot, S. Duquesne, C. Jama, C. Wilkie, The Royal Society of Chemistry, Cambridge, 2005, p. 327. 8. F. Samyn, S. Bourbigot, S. Duquesne and R. Delobel, Thermochim. Acta, 2007, 456, 134. 9. A. Dechirico, G. Audisio, F. Provasoli, M. Armanini and R. Franzese, Macromol. Symp., 1993, 74, 343. 10. A.B. Morgan, J.L. Jurs and J.M. Tour, J. Appl. Polym. Sci., 2000, 76(8), 1257. 11. G. Camino, S.M. Lomakin and M. Lageard, Polymer, 2002, 43, 2011. 12. W. Zhou, H. Yang, X. Guo and J. Lu, Polym. Degrad. Stabil., 2006, 91(7), 1471. 13. G.D. Soraru, F. Babonneau, S. Maurina and J. Vicens, J. Non-Cryst. Solids, 1998, 224, 173. 14. R. Pen˜a Alonso, F. Rubio, J. Rubio and J.L. Oteo, J. Anal. Appl. Pyrol., 2004, 71, 827. 15. G. Marosi, P. Anna, A. Ma´rton, G. Bertalan, A. Bo´ta, A. To´th, M. Mohai and I. Ra´cz, Polym. Advan. Technol., 2002, 13, 1103.

CHAPTER 5

Organomodified Ultrafine Kaolin for Mechanical Reinforcement and Improved Flame Retardancy of Recycled Polyethylene Terephthalate B. SWOBODA,a E. LEROY,a, c J.-M. LOPEZ CUESTA,a C. ARTIGO,b C. PETTERb AND C.H. SAMPAIOb a

Ecole des Mines d’Ale`s, Centre des Mate´riaux de Grande Diffusion (CMGD), 6 Av. de Clavie`res, 30319 Ale`s Cedex, France; b Laborato´rio de Processamento Mineral (LAPROM), Centro de Tecnologia – UFRGS, Av. Bento Gonc¸alves, 9500, Caixa Postal 15021-91501-970, Porto Alegre, Brasil; c Current address: Laboratoire de Ge´nie des Proce´de´s-EnvironnementAgroalimentaire (GEPEA), 37, Bd de l’Universite´, 44606 Saint-Nazaire, BP 420 Cedex, France

5.1 Introduction The development of polymer composites that contain ultrafine, delaminated or exfoliated phyllosilicates is increasing everyday. Minerals of high aspect ratio provide large interfacial areas between the mineral particles and polymer chains, which result in significant improvements in the tensile and flexural mechanical properties of the polymer composite. Barrier properties can also be

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

59

60

Chapter 5

conferred by the use of phyllosilicates in thermoplastics, which significantly influences mass transfer (limit gases and solvent permeation) and the reaction to fire because a carbonaceous and mineral protective layer caused by polymer ablation forms during combustion.1,2 Organomodified montmorillonites are the main category of phyllosilicates used in polymer composites to improve the above-mentioned properties, since they are able to intercalate with polymer chains or exfoliate inside the polymer matrix.1,2 Phyllosilicates other than organomodified montmorillonites, such as kaolinites, sepiolites attapulgites etc., are advantageous since it is possible to carry out functionalisation, in particular by reaction with the hydroxyl groups. This is not easy with montmorillonite because the hydroxyl groups are not available. Kaolinite is a 1 : 1 phyllosilicate with the general chemical formula Al2Si2O5(OH)4.3 Contrary to montmorillonite, kaolinite does not have interlayer exchangeable cations. Nevertheless, the external surface of kaolinite particles is covered with silanol groups, which are able to react with organic molecules. Moreover, depending on the origin of the mineral deposit and on the mineral processing conditions, ultrafine kaolinite can be obtained with submicronic average particle size and high aspect-ratio particle morphology, which make it suitable for the nanoscale reinforcement of a polymer matrix and improvement in barrier properties. We used unmodified kaolinite, kaolinite modified with triphenylphosphite (TPP) and a physical mixture of kaolinite and TPP to study the effect of organic modification on dispersibility in a recycled polyethylene terephthalate (PET) and, consequently, its effect on the mechanical properties and reaction to fire of the polymer composite. Recycled PET was chosen because of its intrinsically low mechanical performance and thermal stability. It is also a technological challenge to be able to upgrade the relevant properties of recycled PET compounds for use in building sector and electrical equipment. TPP was chosen for the organic modification of kaolinite for two reasons: first, its phosphorus content means it is likely to act as a flame retardant for polyesters through enhanced charring.4 This is particularly important for PET since this polyester has a particularly poor charring behaviour. Moreover, TPP is a known ‘‘chain extender’’ for PET. In the melt, the presence of TPP leads to a bridging reaction between PET chain ends [Equation (5.1), in which R1 and R2 are PET chains and Ph is a phenolic ring], which has been studied in detail by various authors,5–7 and leads to a significant increase in the average molecular mass and mechanical properties of the polymer. R1
OH þ R2
COOH þ P
ðOPhÞ3 ! R1
OCO
R2 þ HO
P
ðOPhÞ2 þ PhOH

ð5:1Þ

In the following sections, various PET compounds are characterized in order to compare the efficiency of TPP-modified kaolinite particles (for improving

Organomodified Ultrafine Kaolin for Mechanical Reinforcement

61

mechanical and reaction to fire properties) to that of unmodified kaolinite particles, ungrafted TPP and their mixtures.

5.2 Experimental 5.2.1 Materials Three different kaolin samples (named C, P and D) were obtained from different mineral deposits located in Amazonia using a pilot plant unit of the Vale do Rio Doce Company (Brazil) and were submitted to different mineral-processing operations, including size selection. TPP was purchased from Acros Organics (CAS: 101-02-0). The recycled PET (post-consumer bottles flakes) was supplied by VALORPLAST, France, (ZPET ¼ 0.76 dl g
1, in 2-chlorophenol at 25 1C, average molecular weight ¼ 26 300 g mol
1, calculated using the Mark Houwink equation (K ¼ 3.8  104; a ¼ 1.3).

5.2.2 Processing 5.2.2.1

Organic Modification of Kaolin

Kaolinite D powder was placed in a reactor at 80 1C and TPP (liquid) was progressively introduced with vigorous stirring until a kaolinite : TPP ratio of 70 : 30 (w/w) was achieved. The reactor was then sealed to avoid the evaporation of TPP. Samples were taken from the reactor after 24 hours and 48 hours for analysis. The wet powder was filtered after 72 hours of stirring, washed with tetrahydrofuran (THF) and dried at 80 1C for 12 hours. It was then used for melt processing with PET.

5.2.2.2

Polymer Compounding

The high sensitivity of PET to moisture meant we used the following protocol before melt mixing: PET flakes were dried in vacuum at 120 1C for 16 hours. These conditions were chosen on the basis of a detailed study of PET drying kinetics and sensitivity to moisture during melt processing.8 Four polymer compounds were prepared by extrusion followed by injection moulding, all containing a mineral : additive loading of 5weight percent (wt%) in mass:  PET–unmodified Kaolin D (95 : 5 w/w), called PET/K herein,  PET–TPP (95 : 5 w/w), called PET/TPP herein,  PET–unmodified Kaolin D–TPP (95 : 3.5 : 1.5 w/w), called PET/K/TPP herein,  PET–organomodified Kaolin D (95 : 5 w/w), called PET/MK herein.

62

50

100

T(°C)

25 25

200

275

150

50

50

K2

270

25

100

50

K1

260

25 50

80

Chapter 5

length (mm)

12.5

12.5

1200 mm

Legend

Thread : 33 mm

Kneading disc blocs : 90°

Thread : 25 mm

Kneading disc blocs : 45°C

Thread : 16 mm

Reverse thread : 25 mm

Conveying screw elements

Figure 5.1

Kneading screw elements

Screw configuration used for extrusion.

Pure recycled PET reference samples were prepared using the same processing conditions: blending was performed in a twin-screw co-rotating extruder Clextral BC 21 (L ¼ 1200 mm, L/D ¼ 48). Figure 5.1 shows the screw configuration designed to promote kaolin dispersion. Extrusion conditions were same for all compositions: feed rate (Q) ¼ 6 kg h
1 and screw speed (N) ¼ 350 revolutions per minute (rpm). The residence time in the extruder under these conditions was about 75 seconds. Granulation was performed after extrusion. Before injection moulding, the granules were dried in an oven at 105 1C overnight at reduced pressure [approximately 1 kPa (10 mbar)]. Injection moulding was carried out at 250–265 1C using a Sandretto Serie Otto AT 95 machine with a mould temperature of 40 1C. The specimens produced were both dog bones (according to ISO 527-2 type 1A specifications for mechanical testing) and 100  100  4 mm sheets (used for fire testing).

5.2.3 Characterization Techniques 5.2.3.1

Characterization of Unmodified Kaolins

Particle-size measurements were performed on a Cilas Laser apparatus based on light diffraction, and the specific surface area was determined using the Brunauer– Emmett–Teller (BET) method by nitrogen absorption at 77 Kelvin (
183 1C). The complete chemical composition of the samples was determined by X-ray fluorescence. The mineralogical study was carried out using a Siemens D-50 X-ray diffractometer on non-orientated samples (powder method). Finally, the particle size, and morphological and textural features of the kaolins were observed using a Quanta 200 FEG environmental scanning electron microscope (FEI Company).

Organomodified Ultrafine Kaolin for Mechanical Reinforcement

5.2.3.2

63

Characterization of Organomodified Kaolin

The percentage of TPP that remained on the kaolin powder after washing and drying was determined by calcination at 700 1C in air. In parallel, the thermo-oxidative degradation of modified kaolin was studied by thermogravimetric analysis (TGA; 5 1C min
1, 25–700 1C, samples of typically 25 mg, using a Netzsch STA 409 device) and compared to that of pure TPP and unmodified kaolin D. As for unmodified kaolin, X-ray diffraction was performed on non-orientated samples (powder method). The electrophoretical mobility (zeta potential) and the average particle sizes of pristine and modified kaolin D were measured in water and THF, using a Malvern Nanosizer NANOZS, to characterize the organophilic behaviour of modified kaolin. Finally, the grafting reaction of TPP onto a kaolin surface was investigated by Fourier transform infrared (FTIR) spectroscopy using a Bruker IFS66 spectrometer ATR mode (32 scans, resolution of 4 cm
1).

5.2.3.3

Characterization of Polymer Compounds

To study the interactions between the different components during melt mixing, blends were made using a Haake Rheomix internal mixer (in parallel to the extrusion processing) and the torque was measured as function of time (T ¼ 270 1C and rotor speed ¼ 60 rpm). PET was introduced in the mixer prior to the introduction of the other components. The morphology of polymer compounds obtained from the extrusion–injection process was observed using a Quanta 200 FEG environmental scanning electron microscope (FEI). The dispersion of kaolin particles in the PET matrix was also studied indirectly by performing dynamic rheological tests at a low shear rate at 260 1C using an ARES Rheometrics Scientific apparatus in plate–plate geometry (+ 25 mm, 1 mm gap). Flexural modulus was measured from three-point bending tests performed on a Zwick TH010 universal press according to ISO 178 Standard. TGA (Setaram) was performed under air flow (75 ml min
1) using samples of typically 30 mg in platinum pans, submitted to a temperature ramp from 25 1C to 700 1C at a heating rate of 5 1C min
1. The limiting oxygen index (LOI) was measured using a Stanton Redcroft instrument on barrels (80  10  4 mm3), according to ISO 4589 specifications. Cone calorimeter tests (ISO 5660) were performed on 100  100  4  mm3 samples placed horizontally, with an irradiance of 50 kW m
2. Only timeto-ignition (TTI), peak heat-release rate (pHRR) and the fire propagation index (FPI ¼ pHRR/TTI) values are discussed below. Results correspond to mean values obtained from three experiments for each formulation, for which a typical variation of 10% was observed. The charring behaviour of the different formulations was evaluated by placing 1 g samples at 700 1C for 4 minutes in an oven under air atmosphere. After this combustion, the mass fraction of charred residue was measured.

64

Chapter 5

Table 5.1

Chemical composition of kaolins.

Sample SiO2 Al2O3 TiO2 Fe2O3 C P D

45.4 45.1 44.9

40.0 39.7 39.1

0.63 0.89 1.6

0.55 0.54 1.1

MnO

MgO CaO Na2O K2O P2O5

o0.01 o0.1 0.01 o0.01 0.11 0.09 o0.01 0.15 0.01

0.27 0.27 0.30

0.02 0.02 0.02

0.05 0.09 0.05

PF 14.02 14.05 14.45

5.3 Results and Discussion 5.3.1 Properties of Unmodified Kaolins The chemical composition of each sample as a percentage of oxide present is given in Table 5.1. Kaolin D presents the highest level of contamination, with Fe and Ti concentrations of 1.1 and 1.6%, respectively. According to X-ray diffraction experiments, titanium dioxide in the form of anatase was the only mineralogical type associated with kaolinite, particularly for kaolin D. Figure 5.2 shows representative scanning electron microscope (SEM) micrographs of the three kaolin samples. Table 5.2 gives their particle-size distribution – the percentages correspond to volumetric concentrations of particles in determined sized ranges:  Kaolin C is characterized by well-crystallized kaolin platelets, with rectilinear edges; it presents two well-defined populations. The average size of particles is in the range 0.4–2.3 mm.  Kaolin P is composed of kaolinite platelets with an average size in the range 0.4–0.7 mm. There is also a significant population of ultrafine particles with an average size of approximately 250 nm.  Kaolin D is an ultrafine kaolin composed entirely of isolated kaolinite platelets of euhedric form with average size in the range 200–400 nm. The results obtained from BET measurements of specific surface area reveal a direct correlation with the average size of the kaolin particles. Kaolin D presents a significantly higher specific surface area (21.8 m2 g
1) than kaolins C and P, with specific surface areas of 9.9 and 12.1 m2 g
1, respectively. From these results, kaolin D was selected for modification by TPP and compounding with recycled PET.

5.3.2 Grafting of TPP onto Kaolin Surface Modified kaolin D samples obtained after treating with TPP for 72 hours, washing with THF and drying (see Section 5.2.2.1), contained 30 wt%. of organic modifier (as measured by calcination). The TGA of modified kaolin D is presented in Figure 5.3, along with those of pure TPP and unmodified kaolin D. In the temperature range studied, the unmodified kaolin only shows a first stage of mass loss below 100 1C, which corresponds to physically adsorbed

Organomodified Ultrafine Kaolin for Mechanical Reinforcement

Figure 5.2

SEM micrographs of kaolins (top to bottom: C, P and D), 50 000.

65

66

Chapter 5

Granulometric distribution of kaolins.

Table 5.2 Sample

%, o10 mm

%, o5 mm

%, o2 mm

%, o1 mm

%, o0.5 mm

%, o0.1 mm

D50 (mm)

C P D

98.52 100.00 100.00

88.64 100.00 100.00

59.66 90.66 96.03

48.32 73.74 90.35

33.30 49.19 70.25

4.58 7.26 16.29

1.13 0.51 0.32

100

Kaolin

90 80

Modified Kaolin

Mass loss (%)

70 60 50 40 30 20

TPP

10 0 0

100

200

300

400

500

600

Temperature (°C)

Figure 5.3

Thermogravimetric analysis of kaolin, TPP and modified kaolin.

water. Thereafter, the mineral is stable up to around 400 1C, which corresponds to the beginning of the dehydroxylation followed at higher temperatures by transformation into metakaolinite.3 Pure TPP and modified kaolin D both exhibit a mass loss starting around 150 1C. While pure TPP nearly reaches complete degradation, the mass loss of modified kaolin reaches a plateau at 70% of residual mass, which is in agreement with the estimate of organic modifier content by calcination. The lack of significant differences between the degradation temperatures of pure TPP and modified kaolin suggest that TPP was not intercalated between kaolin platelets. This proposition is supported by X-ray diffraction measurements, which showed that the spacing between kaolin layers remained constant at 7.3 A˚, which is the characteristic feature of pure kaolinite.9 Nevertheless, electrophoretic and granulometric measurements in water and THF (Table 5.3) clearly show that the surface properties of kaolin have been affected by the treatment, such that the unmodified kaolin shows smaller particulate size and higher Zeta potential in water than in THF. This can be attributed to the

67

Organomodified Ultrafine Kaolin for Mechanical Reinforcement

Table 5.3

Zeta potential and average particle size of kaolin and modified kaolin in water and THF solvents. Water

Solvent Average size (mm) Zeta potential (mV)

THF

Kaolin

Modified kaolin

Kaolin

Modified kaolin

0.32 40.9

2.79 0.2

0.98 18.2

0.85 48.1

hydrophilic nature of untreated kaolin particles’ surface. On the contrary, the zeta potential of modified kaolin in water is very small, and both this and a significantly increased average particle size indicate the aggregation of individual particles and thus a hydrophobic behaviour of particle surfaces. The zeta potential of modified kaolinite increased and the average particle size decreased in THF, indicating an organophilic behaviour. The FTIR analysis of kaolin powder, before modification and after 24, 48 and 72 hours of treatment with TPP (followed by THF washing and drying) shows two important changes in the absorption spectra of the modified kaolin (Figure 5.4):  a peak appears at 1193 cm
1 and can be attributed to the formation of Si–O–P chemical bonds.10  In parallel, there is an attenuation of the peaks between 3600 and 3750 cm
1 which corresponds to surface hydroxyl groups of kaolin.11 Given that the reactivity of kaolin surface hydroxyl groups is similar to that of a carboxylic acid function,12 the following reaction mechanism can be assumed: kaolin
Si
OH þ P
ðOPhÞ3 ! kaolin
Si
O
P
ðOPhÞ2 þ PhOH ð5:2Þ

5.3.3 Morphological, Rheological and Mechanical Properties of Polymer Compounds Figure 5.5 shows the torque evolution during the melt mixing in the internal mixer. The composition that contained pristine kaolin (PET–K) exhibits low torque values, comparable to those obtained for pure PET. These low values can be ascribed to the promotion of PET chain breaking by the surface hydroxyl groups present at the kaolin surface. Conversely, the PET–TPP composition has a high viscosity because of the chain-extension reaction. When both additives are introduced (sample PET–K–TPP), the influence of TPP dominates, but with much slower kinetics. Finally, for modified kaolin (PET– MK), two steps can be seen. First, a chain-extension effect seems to occur, which shows that a part of TPP fixed onto kaolin was physically adsorbed and is able to react with PET. Then, the torque increases more slowly. That the torque does not decrease suggests that the presence of TPP on the kaolin

68

Chapter 5

Transmittance (normalized)

after 72h treatment

after 48h treatment

after 24h treatment

untreated Kaolin D

700

800

900

1000 1100 Wave number (cm-1)

1200

1300

Transmittance (normalized)

after 72h treatment

3400

Figure 5.4

after 48h treatment

after 24h treatment

untreated Kaolin D

3500

3600

3700 3800 Wave number (cm-1)

3900

4000

FTIR analysis of kaolin modification by TPP.

surface (replacing surface hydroxyl groups) could limit the chain-breaking effect of kaolin. Figure 5.6 shows SEM micrographs obtained for PET–K and PET–MK compounds. On the micrographs on the left, in both cases the kaolin particles are homogeneously dispersed in the PET matrix. However, closer examination at higher magnification (micrographs on the right in Figure 5.6) reveals that in

69

Organomodified Ultrafine Kaolin for Mechanical Reinforcement 110 100 90

Torque (N.m)

80 70

PET

60

PET/K

50

PET/TPP

40

PET/K/TPP

30

PET/MK

20 10 0 0

Figure 5.5

2

4

6 mixing time (min)

8

10

12

Evolution of torque during melt mixing of polymer compounds (vertical scale corresponds to the pure PET sample, the other curves have been shifted for the sake of clarity).

the case of pristine kaolin, the aggregates are significantly bigger than those for modified kaolin. This indicates that the organophilic modification promotes the dispersion of individual kaolin platelets in the polymer. The bending modulus values are presented in Table 5.4. The best results are shown by the PET–MK sample, with a 60% increase in bending modulus compared to that of the recycled PET sample. The improved kaolin-reinforcement effect in PET–MK is also significantly better than that of pristine kaolin with the latter showing a 40% increase in bending modulus compared to pure PET. The improved mechanical performance of PET–MK with a lower mineral loading of 3.5 wt% suggests better dispersion and better compatibility of kaolin platelets with the PET matrix. It is also possible that reactions between grafted TPP and PET could have occurred in the melt, leading to chain extension and/ or coupling reactions between kaolin particles and PET chains. However, this is beyond the scope of this investigation and will be the subject of future work. PET–TPP and PET–K–TPP compounds also show significant improvements in mechanical properties, which could be due to PET chain extension and/or nanoscale reinforcement of the PET matrix by kaolin particles.

5.3.4 Thermal Stability and Reaction to Fire of Polymeric Compounds The mass-loss curves obtained in TGA experiments (Figure 5.7) show that all the samples exhibit a two-step degradation: at lower temperatures, the thermooxidation of the polymer matrix leads to both low molecular weight compounds

70

Figure 5.6

Table 5.4

Chapter 5

SEM micrographs of compounds (a) PET–K and (b) PET–MK.

Mechanical and fire performance of different polymeric compounds.

Sample

PET

PET–TPP

PET–K

PET–K–TPP

PET–MK

Bending modulus (MPa) LOI (0.5%) Cone Calorimeter TTI (s) pHRR (kW m
2) FPI (kW m
2 s
1) THR (MJ m
2) Char residue after 4 min at 700 1C in air (wt %)

2017 (272) 22.5

2866 (26)

2999 (136)

27.2

2862 (41) 26.4

26.6

3313 (86) 29.7

71 710

93 502

91 517

77 352

97 312

10.0

5.4

5.7

4.6

3.2

99 0.0

49 12.0

44 3.5

60 5.5

69 9.5

71

Organomodified Ultrafine Kaolin for Mechanical Reinforcement 100

Mass loss (%)

80

PET PET/TPP PET/K PET/K/ TPP PET/MK

60

40

20

0 330

350

370

390

410

430

450

470

490

510

530

550

570

590

T (°C)

Figure 5.7

Thermogravimetric analysis of the different polymer compounds.

and char formation. The first mass loss observed corresponds to the evaporation of these low molecular weight compounds, while the second mass loss corresponds to the degradation of the char previously formed at lower temperatures. The PET–MK compound clearly exhibits superior thermal stability, with the two mass-loss stages shifted towards higher temperatures compared to pure PET. This can be correlated with a significant increase of both TTI (+30%) in cone calorimeter tests and char residue in oven tests (Table 5.4). However, the presence of TPP has accelerated the initial mass loss compared to that of pure PET. However, enhanced char formation at higher temperatures (4500 1C) occurs in the presence of TPP. This observation can be related to the limited thermal stability of TPP, which starts to decompose at around 200 1C (see Figure 5.3) and to its flame retardant mode of action, which promotes char formation. It can be proposed that the decomposition of TPP leads to species that accelerate chain-breaking reactions, which results in the lower thermal stability of PET at lower temperatures, followed by cross-linking reactions that promote char formation at higher temperatures. The presence of pristine kaolin also accelerates the initial mass loss, probably through the promotion of chainbreaking reactions by surface hydroxyl groups on kaolin platelets. The dehydroxylation of kaolin can promote PET hydrolysis. When TPP-grafted kaolin particles are used (PET–MK), this proposed effect of surface hydroxyl groups on PET degradation is obviously limited. Nevertheless, the accelerations of the first mass loss for PET–TPP, PET–K and PET–K–TPP are not correlated with a decrease in TTI since all these compounds present higher values than pure PET (Table 5.4). The lack of a direct correlation between the thermal stability of the PET matrix in TGA experiments and the TTI in cone calorimeter tests is not surprising for such

72

Chapter 5 800 700

PET PET/K

HRR (kW/m2)

600

PET/TPP PET/K/TPP

500

PET/MK 400 300 200 100 0 0

100

200

300

400

500

600

time (s)

Figure 5.8

Typical heat release rate vs. time curves obtained in cone calorimeter experiments.

systems since additional phenomena able to retard ignition have to be taken into account. In particular, barrier effects to fuel transfer from the condensed to gaseous phases caused by char formation and/or kaolin platelets. Regarding other aspects of the reaction to fire (Table 5.4 and Figure 5.8) a significant difference can be observed between PET–MK and other samples – the former presents better performance for most of the tests. Compared to pure PET, LOI increases by 30% and pHRR decreases by 50%. However, the total time to burn has increased, hence total heat release (THR) is not much affected and it is not the best for PET–MK. For all that, it has to be taken into account that for PET–TPP and PET–K the loadings of TPP (and the associated char yield) and mineral, respectively, are higher. In addition, PET–MK offers the best combination between THR and FPI reductions. It is likely that kaolin platelets act as a reinforcement for the char formed, as observed in previous studies on recycled PET.13,14: when PET is flame retarded by systems based on red phosphorus and alumina or iron oxide, it has been shown that the introduction of talc particles or glass fibres could reinforce and stabilize the char formed, leading to a better reaction to fire. When TPP-grafted kaolin particles are used (PET–MK), the promotion of charring is strongly enhanced compared to that of PET–K–TPP (Table 5.4), even though both compounds contain the same amounts of mineral and organic additives. It can be assumed that the presence of well-dispersed modified kaolin platelets in the PET matrix leads to barrier properties that limit the evaporation of TPP and its degradation products above 200 1C. Consequently, less TPP (and degradation products) escapes to the gas phase and more remains in the condensed phase to act as a char promoter.

Organomodified Ultrafine Kaolin for Mechanical Reinforcement

73

5.4 Conclusion That ultrafine kaolins present BET surfaces up to 20 m2 g
1 as fillers for polymers, as highlighted here, is of interest. The possibility to easily modify the hydrophilic behaviour of kaolin particles to organophilic behaviour has been shown. No intercalation of polymer between the kaolin platelets was involved, contrary to the modification of bentonite or montmorillonite clays, but grafting reactions with surface hydroxyl groups of kaolin platelets were evident. In the system studied, this grafting reaction was carried out in bulk, by directly mixing the kaolin with the organic modifier. The organic modification of kaolin platelets by TTP (an additive able to act both as a char promoter and a chain extender in PET) was shown to improve their dispersion in PET and to limit the chain-breaking effects of surface hydroxyl groups (present in pristine kaolin surfaces). Moreover, incorporation of modified kaolin, at a loading level of only 5 wt%, in recycled PET significantly increased the mechanical and reaction-to-fire properties of the recycled PET.

5.5 Acknowledgements The authors thank the CAPES COFECUB for supporting this Franco-Brazilian research programme.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

J.W. Gilman, Appl. Clay Sci., 1999, 15, 31. M. Alexandre and P. Dubois, Mater. Sci. Eng.: R: Reports, 2000, 28, 1. R.E. Grim, Applied Clay Mineralogy, Mc Graw Hill, New York, 1962. S.V. Levchik, ED Weil. Polym. Int., 2005, 54, 11. S.M. Aharoni, C.E. Forbes, W.B. Hammond, D.M. Hindenlang, F. Mares, K. O’Brien and R.D. Sedgwick, J. Polym. Sci. Part A, 1986, 24, 1281. B. Jacques, J. Devaux, R. Legras and E. Nield, Polymer., 1997, 38, 5367. B. Jacques, J. Devaux, R. Legras and E. Nield, Polymer., 1996, 37, 5367. B. Swoboda, E. Leroy, J.-M. Lopez-Cuesta, Special Issue MoDeSt’06, Polym. Deg. Stab., 2007. F. Franco and M.D. Ruiz Cruz, Clay Minerals, 2004, 39, 193. B.A. Morrow and S.J. Lang, J. Phys. Chem., 1994, 98, 13319. R.L. Frost, J. Colloid Int. Sci., 2002, 1246, 164. Mineral Fillers for Rubber : kaolin clay, RT Vanderbilt Company, (2007). F. Laoutid, L. Ferry, J.M. Lopez Cuesta and A. Crespy, Fire Mater., 2006, 30, 343. F. Laoutid, L. Ferry, J.M. Lopez Cuesta and A. Crespy, Polym. Deg. Stab., 2003, 82, 357.

CHAPTER 6

Complex Micro-analysis Assisted Design of FireRetardant Nanocomposites – Contribution to the Nanomechanism + P. ANNA AND GY. MAROSI A. SZABO´, B. B. MAROSFOI, Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, 1521 Budapest, Hungary

6.1 Introduction Advancement of the methods used to produce fire retardancy is hindered by the shortcomings of the current characterization methods and insufficient understanding of the fire retardancy mechanisms. These two factors are connected with each other: better characterization would promote better understanding and mechanistic studies initiate advancements in analysis. A variety of methods is available for characterization, but none of them provide all the data required to model the combustion process adequately. Development is needed, especially of the smalland microscale analytical methods, to allow rapid screening of small experimental samples and possibly to enable high-throughput and/or in-line characterization. D. Price was the pioneer who realized, earlier than anyone else the importance of very small-scale modelling in fire testing, which became essential when the

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Complex Micro-analysis Assisted Design of Fire-Retardant Nanocomposites

75

1,2

expensive nanofillers appeared. He developed a laser pyrolysis time-of-flight mass spectroscopy (LP-ToFMS) system to understand the fundamentals of fire retardancy through analysis of the gases that evolved during a mini-fire induced by a laser with well-controlled energy.2,3 This elegant technique inspired the authors, being interested in condensed-phase fire retardant activity, to extend the method to the analysis of the condensed phase using a LP micro-Raman system.4 The Raman method is favoured at the set-up of such systems because of its applicability for in-depth measurements, its good lateral and in-depth resolution, its unique sensitivity to double bonds and physical (e.g. polymorphic) changes, and its insensitivity to water. If the fluorescence disturbs the Raman measurement a coupled attenuated total reflection infrared (ATR-IR) unit can be used. The first example of such analyses was the determination of the depth profile of fire-induced degradation of ethylene vinyl acetate (EVA) which could hardly be determined by any other methods, such as thermogravimetric analyzer coupled with Fourier transform infrared (TG-FTIR) spectroscopy used previously to examine the pyrolysis of EVA.5–7 It was indicated that on the surface of heattreated EVA the oxidation of polyethylene (PE) segments is dominant and leads to the formation of a carbonaceous surface layer, while in the deeper region scission of the acetate side-chains in vinyl acetate segments is characteristic. Even deeper, at 40 mm, melted (but not degraded) polymer could be found. Laser treatment was found to be comparable with the effect of a cone heater. Figure 6.1 shows that the measured points of different treatments fall on the same curve: 50 seconds of cone treatment, in the given experiment, corresponds to about B1 second of laser treatment, and both represent B2.5 MJ m
2. Furthermore, by adjusting the energy of the laser the rate of degradation, according to the need, can be slower or even higher. However, these preliminary experiments indicated that additional analyses were needed to predict the fire behaviour of polymeric materials. Comprehensive characterization should consider:  heat distortion temperature [influences the reproducibility of single burn item (SBI) test];  heat conductivity–temperature function (influences ignitability and heat feedback);  viscosity–temperature function (influences gas transport, dripping, spread of fire);  chemical composition of gas phase (influences ignitability and flame propagation);  temperature distribution in the gas and solid phases (influences heat feedback);  heat balance–time function of the combustion process (influences fire persistence);  ratio and rate of the condensed phase mechanism (influence barrier-layer formation);  size and kinetic energy of bubbles (influence the uniformity of the barrier layer);

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Figure 6.1

Correlation between the degradation caused by cone heater treatment and CO2 laser treatment on EVA samples.

 chemical composition and physical characteristics of the condensed phase, including the size, structure, barrier capacity, mechanical and heat resistance of char (influence the mass and heat balance). It is quite challenging that no equipment that adequately performs such a complex analysis is available, especially for micro-scale characterization. This work attempts to make progress in the characterization by combining the LP–Raman microscope system with FTIR gas analysis and micro-thermal analyser (to determine temperature-dependent deformation and heat conductivity). The use of a rheometer at high temperature for small-scale measurements of the physical resistance of the char is published elsewhere.8,9 Concerning the mechanism of fire retardancy, a better understanding is required, especially to clarify the role of nanofillers, which is still a challenge in spite of the intensive studies. The relationship between interfacial (e.g. particle– particle and polymer–particle) interactions and fire performance has not been elucidated either. The interaction at the interfaces in flame retarded polymers can be considered from different points of view:  interactions in the solid state influence mechanical, thermal and electrical properties, and heat deflection temperature;

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77

 interactions in the melt state influence processability and melt dripping;  interfacial interactions in the course of degradation influence the condensed-phase and, indirectly, the gas-phase mechanisms. Interactions in the solid state (including covalent bonds, acid–base interactions, van der Waals interactions and anchoring-type junctions) that are formed between the phases of multicomponent systems, contact area, thickness and structure of the interfacial layers may influence the temperature-dependent mechanical properties. The dimensional stability under the effect of fire strongly influences the response of material to certain types of measurement set-up, such as SBI and Federal Motor Vehicle Standard Safety n1302 (FMVSS) tests. Increased adhesion at the interfaces improves the physical stability, but high local stresses may occur at the interfacial zone because of different heat expansion of adjacent phases. An interlayer of higher thickness, formed by an elastomer, for instance, decreases the chances of formation of local stress peaks and of consequent interfacial debonding.10 Optimally selected interfacial additives may improve both the wetting (surfactant effect) and adhesion (coupling effect) of these inclusions. Coupling agents, if introduced into the appropriate interfacial zone, build chemical bridges between the phases.11,12 Reactive surfactants (RS), considered to be targeted reactive interphase (IP) modifiers, have been developed to combine the benefits of surfactants and coupling agents.13,14 Use of RS additives has resulted in improved mechanical properties in polymer blends, while in pigmented PE it contributed to higher photostability.15–17 However, better interfacial interaction does not automatically ensure better fire retardancy. The term ‘adaptive IP’ has been suggested recently to describe interlayers responding to the changes in their environment that act as a trigger signal to initiate the desirable transformations.18–20 Heat-induced IP transformations may facilitate stabilization by controlling the degradation process. Interactions in the melt state in presence of fillers, especially nanofillers of high specific surface area, increase the viscosity of the polymers as the secondary bonds form between the surface of the inclusions and the macromolecules restricting the free displacement in the direction of the shear tensor. The viscosity-increasing effect and its influence on fire retardancy have been analyzed for intumescent systems and fire retarded polymers that contain montmorillonite (MMT).21–23 The effective contact area between the phases is influenced by the value of wetting, especially at interfaces of irregular shape. Polar components of interacting phases and acid–base interactions promote maximal wetting. The kinetics of the wetting process depend on the viscosity of the melt and on the value of the shear force. The need for high viscosity to avoid melt dripping seems to be in contradiction with the processability requirements, but a material with strongly shear-rate dependent viscosity can meet both requirements. Degradation-related interactions between the polymer matrix and the included particles involve chemical reactions at the phase borders and/or physical adsorption. The catalytic or chemical effect of the active surface of the nanofillers may lead to early mass loss, while the same effect may also promote the advantageous charring process at higher temperatures. Thus, the mechanism of

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degradation can be influenced by IP modification using catalyst atoms or other active species, with different final consequences on fire retardancy from case to case.24 Adsorption at the interfaces may lead to considerable extraction of the stabilizer(s) from the polymer matrix, which results in earlier degradation compared to the pristine matrix.16,17,25 This chapter reports work that aims to make a step towards a better understanding of the complex relationship between structure and fire performance.

6.2 Experimental 6.2.1 Materials The materials used were:  Polypropylene (PP): Moplen HP400R type (Basell Polyolefins), melt flow index 25 (at 230 1C/2.16 kg).  Maleated polypropylene (PP-g-MA): Licomont AR 405 type (Clariant GmbH), acid content 37–45 (mg KOH g
1), density 0.89–0.93 g cm
3 at 23 1C.  EVA copolymer: IBUCELL K 100 with vinyl acetate content of 28% (H.B. Fuller).  Multiwall carbon nanotube: MWCNT Nanocyl 3100 type (Nanocyl S.A., Belgium), average diameter 10 nm, length 0.1–10 mm, metal oxide content o5%.  Sepiolite: Pangel S9 type (Tolsa S.A., Spain), other mineral content: 15%.  Na-MMT: Microtec type (Eurotrade Ltd., Hungary), mass loss 7.48% at 10001C. The components of the P-epoxy resin interlayer were:  Eporezit AH-16 (non-modified, resin-like reactive dilutant, epoxy equivalent 160–175; viscosity 800–1800 mPa.s at 25 1C; density 1.24 g cm
3 at 25 1C:, hydrolyzable chlorine content 1.5 mass per cent) supplied by P+M Polimer Ke´mia Kft., Hungary  TEDAP a newly synthesized phosphorus-containing reactive amine (amine number 510–530 mg KOH g
1; viscosity 400 mPa.s at 20 1C; curing time 7 days at 25 1C, 4 hours at 80 1C).26 The ratios of components in PP : PP-g-MA : P-epoxy : nanofiller systems were 60 : 20 : 15 : 5; for the carbon nanotube (CNT) the ratio was 63 : 21 : 15.8 : 0.2 because a higher ratio of CNT could not be perfectly homogenized in epoxy resin.

6.2.2 Methods Samples were prepared in a Brabender PL 2000 with a rotor speed 50 1 min
1 at 180 1C, the duration of compounding being 10 minutes.

Complex Micro-analysis Assisted Design of Fire-Retardant Nanocomposites

Figure 6.2

79

Set-up of laser pyrolysis and analysis of solid and gas phases.

The LP system (see Figure 6.2) comprises a CO2 laser (SYNRAD 48-1) and a Raman-IR microscope (from HORIBA Jobin Yvon Inc.). The beam of the CO2 laser on the sample falls within the IR range so its effect on the sample simulates the influence of fire. The LP unit is equipped with a Universal Laser Controller, which allows the operator to control the laser power using pulse width modulation (PWM). The laser was used at the standard 5 kHz frequency and the laser power was controlled. Gate control provides the ability to vary laserexposure period. The pyrolysis unit is mounted on a flange and the CO2 laser beam is focused on the sample surface using a special mirror. Before laser exposure, the sample is placed on the top of a probe, which is inserted into a position just below the Raman–ATR microscope system. The gases that evolved during the laser irradiation were evacuated and carried to IR gas cell for further analyses. The subsequent chemical changes at the surface are then monitored, either by Raman microscopy or via an ATR technique. Raman analyses to characterize the structure and interactions in the solid state were performed using a LabRam-type confocal Raman microscope (Horiba Jobin Yvon, France). The magnification used was 50 during the measurement and the excitation source was a frequency-doubled Nd–YAG laser emitting at 532 nm. A microthermal analyser of the m-TA 2990 type (TA Instruments) was used to determine the microthermal deformation and thermal conductivity values. The instrument is an AFM system into which a controllable wire is inserted, as a tip, in the AFM cantilever and used to scan along the surface of the sample (Figure 6.2). The heating rate at deformation was 5 1C s
1 in the temperature range 25–250 1C, the contact during the heating was set with the Z-piezo to the value of 20 V. The heat conductivity correlates with the power (F in mW) necessary to maintain the temperature of the probe, which was chosen to be 100 1C. The relative heat conductivity of the samples is expressed as the dimensionless value l/lair ¼ Fsample (mW)/Fair (mW), where Fair ¼ 1.01 mW. Scanning electron microscopy (SEM) analysis of the additives was made using a JEOL 5500 LV instrument.

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Heat treatment of the samples in a rheometer (before residue analysis) was performed using an AR 2000 system from TA Instruments Co. The fire performance was determined using the horizontal burning test (ISO 3795 standard) to establish the rate of flame spreading (burning) and dripping. Also investigated were the limiting oxygen index (LOI), according to ASTMD 2863, and mass loss calorimetry (Fire Testing Technology) according to ISO 13927 with heat flux 50 kW m
2, sample surface area 100 cm2, Al tray of 10  10  0.3 cm size, three replicate samples (covariance 3%), exhaust gas flow 0.024 m3 s
1.

6.3 Results and Discussion To demonstrate the applicability of the small-scale modelling and micro-analytical approach, described in the experimental part, preliminary results are presented on consecutive steps of development of a new fire retardant–PP system.

6.3.1 Nanonetwork Formation To study the nano-effects in the condensed and gas phases, PP–MWCNT systems were analyzed. In this case the degradation initiated by the laser treatment created a hole, the size of which was influenced by the duration of the treatment and the composition. Table 6.1 shows that the time-dependent growth of the hole is less when nanofillers, especially CNT, are present. Obviously, nanocomposites with a higher heat conductivity resulted in a lessfocused heating effect and thus reduced degradation in the centre of the treatment. For the PP–MWCNT system the structure of the hole was analyzed by SEM. A fine CNT network was found within the hole, which was uniform in each hole analyzed (see Figure 6.3). The SEM image confirms our earlier (Raman) Table 6.1

Depth of the hole induced by treatment (of increasing duration) with CO2 laser on the surface of different PP systems. d Sample

Time Sample

0.2 sec

0.4 sec

0.6 sec

0.8 sec

1 sec

2 sec

5 sec

PP-MWNT 3% 128 mm 356 mm 489 mm 635 mm 726 mm 1856 mm 2330 mm PP-sepiolite 3% 257 mm 532 mm 1166 mm 1747 mm 2021 mm 3825 mm 44000 mm PP 413 mm 966 mm 1233 mm 2816 mm 3752 mm 44000 mm 44000 mm

Complex Micro-analysis Assisted Design of Fire-Retardant Nanocomposites

Figure 6.3

81

Internal structure of the hole induced on the surface of a PP–3%MWCNT system by 5 second CO2 laser treatment.

observation that the residue after combustion of PP–MWCNT system consists of pure CNT.27 The network structure was expected to increase the dimensional stability and heat conductivity. For a micro-scale comparison of the heat distortion and the heat conductivity of the PP and PP–MWCNT systems the m-TA method was used. The results given in Figure 6.4 demonstrate the effect of the CNT network on the temperature-dependent deformation of the PP. Up to B150 1C the thermal dilation of the samples can be seen, and is considerably reduced in the presence of CNT. Forced penetration of the AFM tip into the materials starts when the compliance, determined by the mobility of the macromolecules, becomes high enough. In the presence of CNT the mobility of macromolecules at the interfacial region is restricted by the interfacial interaction; therefore, the deformation starts at higher temperature and takes place in a less steep manner than in the case of pristine PP. This result suggests there is less chance of distortion and dripping because of fire if CNT is dispersed in the polymer matrix. The heat conductivity can be determined by keeping the tip of the m-TA system at a constant elevated temperature. The energy needed to maintain this temperature when the tip is in contact with the material is proportional to its heat conductivity. This method allows the determination of micro-scale local differences up to the temperature of combustion, which cannot be done with other heat-conductivity measurements. The first experiments confirmed the applicability of m-TA to characterize the studied samples, as the results given in Table 6.2 correlate well with recently published heat-conductivity data of PP– CNT samples.19

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Figure 6.4

Chapter 6

The temperature-dependent deformation of PP and PP–3%MWCNT determined by the m-TA method.

Even these preliminary results clearly show the considerable increase in heat conductivity with CNT in the composites, and its opposite effect on the char residue. The introduction of 3% MWCNT into PP increased this value to thrice that of the reference PP, which suggests a good contact between the polymer matrix and the CNT of high heat conductivity. The opposite tendency was found when the heat conductivity of the residue was measured. The CNT residue (due to its voluminous, loose fibrous structure) isolates the heat 10 times better than does the char residue of an EVA (which was taken as reference in this case, as for PP no residue remains to be measured). The effects of changes to the heat conductivity on fire retardancy may be ambiguous. The increased heat conductivity of PP–CNT reduces the time to ignition (TTI); however, if it forms a heatresistant thermal barrier on the surface of material, this will slow down the degradation in the subsequent stage of combustion. This effect of increased heat conductivity explains why the CNT decreases the TTI, as reported earlier, even though the degradation of PP is shifted to higher temperature (moderately or considerably under nitrogen or air atmosphere, respectively) in the presence of CNT.27,28 As the combustion process continues, the heat barrier effect of the char, which consists of CNT residue of low heat conductivity, results in a reduced heat release rate (HRR).28 However, according to our experience the char is not consistent enough to resist the intensive bubbling in the later stage of cone heater treatment at 50 kW m
2. The photo of the residue in Figure 6.5 shows the bubbling-induced removal of the CNT from the central part of the material. The decomposition of CNT is excluded as its amount in the residue was equal to that of the introduced CNT. The entanglement of nanofibres can be preserved only if mild flame treatment conditions are applied or if binding sites are introduced.24,29

83

Complex Micro-analysis Assisted Design of Fire-Retardant Nanocomposites

Figure 6.5

Photograph of the residue of PP–3%MWCNT after treatment under cone heater at 50 kW m
2.

Table 6.2

Conductivity data and results of gas phase analysis of PP and PP + CNT systems.

PP PP + 3%MWCNT EVA (reference)

Relative heat conductivity l/lair

Integrated values of gas components in gas phase [a.u.]

reference

residue

CO2

CO

Org.

1.06 1.19 1.07


1.02 1.25

154600 148500

37.2 35.6

8.1 5.0

Note: Org. ¼ organic components; a.u. ¼ arbitrary unit

The indirect effect of the described condensed-phase changes in the gas phase was analyzed by the FTIR unit coupled to the laser pyrolyser. A moderate decrease in all the decomposition gases was found as a consequence of the transport-restriction effect of the CNT network (see Table 6.2). This moderate effect is not enough to provide real fire retardancy: the LOI value of 19 was found independantly of the composition for PP–MWCNT composites with 0.5, 1, 2, 3, 4, and 8% CNT content while the burning rate decreased and no dripping occurred above 2% MWCNT (Figure 6.6).

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Figure 6.6

Chapter 6

Rate of horizontal flame spread and dripping against the percentage of MWCNT in PP.

The PP–CNT system demonstrated the effects of inter-particle interactions, among which the maintenance of percolated structure seems to be especially important; however, the introduction of CNT could not provide fire retardancy at an industrially important level. Based on the previous results, which indicate the role of interface modification in fire retardancy, we have attempted to produce a polymer layer of fire retardant activity around nanofillers.30

6.3.2 Intumescent Polymeric Particle Formation Epoxy resin was selected as a polymeric interlayer around different nanofillers, i.e. CNT, MMT and sepiolite (fibrous clay). The selection was based on previous results.26,31 A new cross-linking agent (TEDAP), which is a reactive fire retardant, has been synthesized recently for epoxy resins.26 Detailed mechanistic studies made clear that its excellent performance is established by an optimal balance and timing of the gas-phase and solid-phase mechanisms.32 Early gas-phase action may compensate the TTI-reducing effect of CNT and clay. The laser treatment of this inherently flame retardant epoxy type resulted in a foamed structure, rather than the hole formed in PP, accompanied by an intense charring process (see Figure 6.7). Based on the considerable intumescent effect of the new fire retardant epoxy the idea to use it in combination with nanoparticles as fire retardant additives in PP emerged.

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Figure 6.7

Surface image of P-epoxy resin after 1 second treatment with CO2 laser.

Figure 6.8

Preparation method for forming PP–P-epoxy–nanofiller system.

The preparation method for the samples is described in Figure 6.8. The nanoparticles were dispersed in epoxy monomers (the high polarity of epoxy resin facilitated good dispersion) and in the course of the curing process it was mixed with a PP–MA-g-PP matrix. Surprisingly, the formed structure, shown in Figure 6.9, was the same in all cases and independent of the geometry of the nanofiller. Very small droplets were formed in a good dispersion. It is assumed that the droplet form is preferred because of the high surface energy of the epoxy resin, and the nanofillers occur within the droplets in folded and/or lamellar form. The PP nanocomposites were heat-treated in a rheometer. Here only the analysis of the residue is presented, while the rheological results will be published elsewhere. Micro-Raman comparison of the residues of PP loaded with pure MWCNT and of the system containing epoxy-coated MWCNT demonstrates the significant difference between the two materials. Figure 6.10

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Figure 6.9

Chapter 6

SEM image of the surface of a fracture of a PP–P-epoxy–sepiolite sample broken under liquid N2.

shows that, in the case of PP–MWCNT, the residue is pure MWCNT, which, according to the ratio of D (B1340 cm
1) and G bands (1500–1600 cm
1), is even purer than the original CNT. Thus in this case the combustion acts as a purification process. The presence of an epoxy resin interlayer dominates the structure of the residue, which shows the wide bands of amorphous carbon, indicating the transformation of a considerable amount of polymer into amorphous char. The PP systems that contain nanofillers coated with epoxy resin were compared by mass loss calorimetry. It is interesting to see in Figure 6.11 the slightly increased TTI of the composites that contain these coated nanofillers. This improvement is noteworthy when the decreasing effect of all the nanofillers studied is considered. It confirms the importance of a gas-phase fire retardant mechanism (of fire retardant–epoxy-resin units) in the initial stage of fireinduced degradation. The largest reduction of HRR and the most consistent char was achieved, when the active epoxy-resin interlayer was applied on MMT. Our assumption for explaining these results considers the expansion capacity of the fire retardant–epoxy-resin droplets that contain the nanofillers. The fibrous inclusions may restrict the expansion, while the MMT influences the foaming process of epoxy resin advantageously, as reported previously.33 Expansion of the tiny particulates, shown in Figure 6.9, may reach the percolation level. In conventional intumescent systems a continuous char layer is formed only after the decomposition of a considerable amount of matrix polymer. In contrast, in a ‘percolating intumescent system’ the expanding particles reach each other in the early stages of the process, which provides

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87

Figure 6.10

Raman spectra of PP–MWCNT systems, their residue and reference materials.

Figure 6.11

HRR curves of PP, PP–P-epoxy, PP–P-epoxy–sepiolite; PP–P-epoxy– MMT samples (external heat flux 50 kW m
2).

prompt protection to the underlying polymer phase.34 (A percolation model for isodimensional inclusions of polystyrene blend has been applied recently.35) Parameters that influence the percolation are the interparticle distance, volume increase and rate of action. Based on the results reported in this chapter, it is

88

Figure 6.12

Chapter 6

Scheme of percolation of intumescent particles and nanofibres.

assumed that nanotubes and/or nanofibres in the PP phase that interconnect the intumescent particles, according to Figure 6.12, would provide an even better performance. Validation of this hypothesis requires further investigation.

6.4 Conclusion The complexity of the features to be considered when fire retardancy is designed involves the need to develop complex structures, small-scale modelling and coupled analytical techniques. Treatment with a CO2 laser is an accurate tool with which to model the fire effect, while the set of micro-Raman–ATR, FTIR, mTA and HT-rheology measurements seem to be suitable for providing complex, rapid information about the changes in the gas and solid phases. Micro-scale determination of the time function to thermal balance of the combustion process requires further development. Results on PP–nanofiller systems confirm that their main fire retardancy advantages are the improved form stability at high temperatures and decreased dripping. Introducing P-containing a fire retardant, thick epoxy resin layer around nanofillers increases the TTI and reduces the HRR. This is ascribed to a balanced ratio of gas- and condensed-phase mechanisms and percolation of

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intumescent inclusions at the early stage of combustion, but these assumptions need to be validated by further analyses. The mechanistic conclusions in connection with the interfacial phenomena are:  Interfacial interaction between the nanofillers allows them to act as a physical network in the polymer matrix and so decrease the chance of deflection of the material at high temperatures and thus promote better performance in larger scale vertical fire tests.  This interaction and the formation of immobilized macromolecular interlayers around nanofillers give rise to the observed anti-dripping effect and consequently reduced spread of fire. Optimal balance between this effect and the processability requires melt of high shear sensitivity.  Fire retardant (intumescent) interfacial layers may shift the TTI and reduce the rate of heat release. The larger the expansion of the interlayer the greater the chance that rapid percolation of the intumescent particles will make the fire protection more efficient.

Acknowledgements The authors acknowledge the financial supports received through the EU-6 Framework Program (NMP3-CT-2004-505637), Multihybrids (IP 026685-2), Hungarian Research Found OTKA T049121 and Found of European Union and Hungarian state GVOP/3.1.1.-2004-0531/3.0.

References 1. D. Price, G.J. Milnes, P.J. Tayler, J.H. Scrivens and T.G. Blease, Polym. Degrad. Stab., 1989, 25(2-4), 307. 2. D. Price, G.J. Milnes, C. Lukas and T.R. Hull, Int. J. Mass.Spectrom., 1984, 60, 225. 3. D. Price, G. Fengge, G.J. Milnes, B. Eling, C.I. Lindsay and P.T. Mcgrail, Polym. Degrad. Stab., 1999, 64(3), 403. + G. Marosi, Period. Polytech. Chem., 2008 (accepted). 4. B.B Marosfoi, 5. A. Marcilla, A. Go´mez and S. Menargues, J. Anal. Appl. Pyrol., 2005, 74(1–2), 224. 6. B. Marosfoi, A. Szabo´, A. Toldy, P. Anna, Gy. Marosi, D. Tabuani and G. Camino, in Recent Advances in Flame Retardancy of Polymeric Materials XVIII, ed. M. Lewin, BCC Inc., Norwalk USA, 2006, pp. 25. 7. B.B. Marosfoi, G. Marosi, A. Szabo, B. Vajna and A. Szep, Polym. Degrad. Stab., 2007, 92(12), 2231. 8. M. Bugajny, M. Le Bras and S. Bourbigot, Fire Mater., 1999, 23(1), 49. 9. P. Anna, Gy. Marosi, I. Csontos, S. Bourbigot, M. Le Bras and R. Delobel, Polym. Degrad. Stab., 2001, 74(3), 423.

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10. Gy. Marosi, Gy. Bertalan, I. Ruszna´k and P. Anna, Colloid. Surface., 1986, 23(3), 185. 11. W. Gutowski, S. Li, L. Russell, C. Filippou, P. Hoobin and S. Petinakis, Compos. Interface., 2002, 9(1), 89. 12. W.S. Gutowski, J Adhesion, 2003, 79(5), 445. 13. Hungarian Patent 218 016 (2000). 14. Gy. Marosi, P. Anna, I. Csontos, A. Ma´rton and Gy. Bertalan, Macromol. Symp., 2001, 176, 189. 15. G. Bertalan, G. Marosi, P. Anna, I. Ravadits, I. Csontos and A. To´th, Solid State Ionics, 2001, 141–142, 211. 16. M.A. Maatoug, P. Anna, Gy. Bertalan, I. Ravadits, Gy. Marosi, I. Csontos, A. Ma´rton and A. To´th, Macromol Mater. Eng., 2000, 282, 30. 17. P. Anna, Gy. Bertalan, Gy. Marosi, I. Ravadits and M.A. Maatoug, Polym. Degrad. Stab., 2001, 73(3), 463. 18. Gy. Marosi, Gy. Bertalan, in Modification and Blending of Synthetic and natural Macromolecules, eds. F. Ciardelli, S. Penczek, NATO Science Series, Kluwer Acad. Publ. Dordrecht, 2004, 175, 1351. 19. I. Luzinov, S. Minko and V.V. Tsukruk, Progr. Polym. Sci., 2004, 29(7), 635. 20. S. Keszei, Sz. Matko´, Gy. Bertalan, P. Anna, Gy. Marosi and A. To´th, Eur. Polym. J., 2005, 41, 697. 21. S. Bourbigot and M. Le Bras, in Fire Retardancy of Polymers: The Use of Intumescence, eds. M. Les Bras, G. Camino, S. Bourbigot, R. Delobel, The Royal Society of Chemistry, Cambridge, 1998, pp. 222. 22. P. Anna, Gy. Marosi, S. Bourbigot, M. Le Bras and R. Delobel, Polym. Degrad. Stab., 2002, 77(2), 243. 23. P. Anna, Gy. Marosi, Gy. Bertalan, A. Ma´rton and A. Sze´p, J. Macromol. Sci. Phys., 2002, B41(4–6), 1321. + B. Bodzay, Gy. Marosi, J. Therm. Anal. 24. A. Szabo´, B.B. Marosfoi, Calorim., (submitted). 25. B. Mailhot, S. Morlat, J. Gardette, S. Boucard and J. Duchet J. Ge´rard, Polym. Degrad. Stab., 2003, 82(2), 163. 26. A. Toldy, N. To´th, P. Anna and Gy. Marosi, Polym. Degrad. Stab., 2006, 91(3), 585. + A. Szabo´, Gy. Marosi, D. Tabuani, G. Camino and 27. B.B. Marosfoi, S. Pagliari, J. Thermal Anal. Calorim., 2006, 86(3), 669. 28. B. Schartel, P. Potschke, U. Knoll and M. Abdel-Goad, Eur. Polym. J., 2005, 41, 1061. 29. T. Kashiwagi, E. Grulke, J. Hilding, K. Groth, R. Harris, K. Butler, J. Shields, S. Kharchenko and J. Douglas, Polymer, 2004, 45(12), 4227. 30. Gy. Marosi, P. Anna, A. Ma´rton, Gy. Bertalan, A. Bo´ta, A. To´th, M. Mohai and I. Ra´cz, Polym. Adv. Technol., 2002, 13(10–12), 1103. 31. N. Abacha, M. Kubouchi, K. Tsuda and T. Sakai, Exp. Polym. Lett., 2007, 1(6), 364.

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32. A. Toldy, A. Szabo´, Cs. Nova´k, J. Madara´sz, Gy. Marosi, Polym. Degrad. Stab., (doi:10.1016/j.polymdegstab.2008.02.011). 33. A. Toldy, P. Anna, I. Csontos, A. Szabo, Gy. Marosi, Polym. Degrad. Stab., 2007, 92(12), 2223. 34. F. Zhang, J. Zhang and Y. Wang, Exp. Polym. Lett., 2007, 1(3), 157. 35. Z. Guo, Z. Fang and L. Tong, Exp. Polym. Lett., 2007, 1(1), 37.

Nanoparticulate Fillers

CHAPTER 7

Impact of Nanoparticle Shape on the Flammability of Nanocomposites F. YANG, I. BOGDANOVA AND G. L. NELSON College of Science, Florida Institute of Technology, Melbourne, Fl, 32901, USA

7.1 Introduction There are a number of advantages of polymer–inorganic nanocomposites when compared to conventional composite materials.1–5 The surface area of nanoparticles is significantly larger than that of micro-sized particles, which provides a larger surface contact for reinforcement of polymers. The amount of nanosized additives needed is much less than those of conventional fillers to achieve the same level of reinforcement. Although polymer–inorganic nanocomposites have been studied for decades, there is limited understanding of their reinforcement and degradation mechanisms. The problems associated with controlling factors that affect the reinforcement and degradation of polymer nanocomposites are not resolved. In previous studies,6–8 the dispersion (or distribution) of nanoparticles in polymer nanocomposites was considered to be the most important factor in producing strong interfacial interactions. However, even good dispersion of nanoparticles, as achieved by many researchers, does not necessarily improve the physical properties of nanocomposites. This suggests that factors other than good dispersion of nanoparticles determine the enhancement of properties for the final composite.

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Chapter 7

To understand the mechanism of thermal degradation and flammability, factors such as the nature of polymer matrices and nano-additives, and the effect of shape, size and loading of the nano-additives on interfacial interactions in polymer nanocomposites are investigated.

7.2 Experiment 7.2.1 Preparation of Polymer–Inorganic Nanocomposites Styrons 685D polystyrene (PS) for the preparation of PS–Aerosils 90 nanocomposites, Styrons 685P for PS–Aerosils R972 nanocomposites, Styrons 685DW for PS–alumina nanocomposites, developmental grade polycarbonate resin were obtained from Dow Plastics Inc. Lexans 103 and GEs OQ 3620 110 polycarbonate (PC) were provided by GE Plastics, Inc. Nanoscale silica, Aerosils 90 (20 nm, spherical) and Aerosils R972 (16 nm, spherical – an organosilica where the surface is covered by methyl groups) were provided by Degussa Co. Aerosils 90 was pretreated with silanol-terminated polydimethylsiloxane (PDMS) and phenyltrichlorosilane (PTCS) for PS–silica nanocomposites. Aerosils R972 was used as received. Alumina hydrate (nanorods, 100 nm/10 nm, Aluminasols 200) in the form of methanol–water colloidal solution was purchased from Nissan Chemical Inc., Japan. Alumina, alpha form (40–80 nm, irregular) and alumina hydroxide (15 nm, spherical) were purchased from Nanostructured and Amorphous Materials Co., USA. Alumina whiskers (2800 nm/2–4 nm) were purchased from Sigma-Aldrich, USA. 3-Methacryloxypropyltrimethoxysilane (MAP) and methyltrimethoxysilane (MT) were purchased from Gelest Inc., USA. Polymer– inorganic nanocomposites were prepared by a single-screw extrusion method developed in the laboratory using a three-quarter inch CW Brabender Table Top Independent Extruder.5

7.2.2 Mechanical Testing of Polymer–Inorganic Nanocomposites A Tinius Olsen Series 1000UTM tensile tester with an analogue recorder was used for tensile strength, modulus and elongation at break for all materials, according to ASTM D638. The testing rate was 0.05 inch min
1.

7.2.3 Morphology Study for Polymer–Inorganic Nanocomposites The samples for transmission electron microscopy (TEM) were prepared by a Leica Ultracut UC6 Microtome. Glass knives (prepared with LKB 7800 Knifemaker) and a diamond knife (Electron Microscopy Sciences, USA) were used. The thickness of the sample is 150 nm and formvar-coated copper grids with 100 mesh (Electron Microscopy Sciences, USA) were used to support the nanocomposites sections for TEM. Tests were conducted using a Zeiss EM900 transmission electron microscope equipped with a Morada Soft Imaging System digital CCD camera, Olympus, USA.

97

Impact of Nanoparticle Shape on the Flammability of Nanocomposites

7.2.4 Thermal Degradation of Polymer–Inorganic Nanocomposites Thermal gravimetric analysis (TGA) for polymer–inorganic nanocomposites was performed using a Hi-Res TGA 2950 thermogravimetric analyzer (TA Instruments, USA). The polymer–alumina samples were pre-heated to 100 1C, equilibrated at 100 1C for 3 minutes to eliminate moisture and solvent, and then heated from 100 1C to 650 1C at 20 1C min
1. The polymer–silica samples were pre-heated to 100 1C and held for 5 minutes to eliminate solvent and moisture, followed by heating to 550 1C at 10 1C min
1. All tests were run under helium atmosphere.

7.2.5 Flammability of Polymer–Inorganic Nanocomposites Flammability of nanocomposites was tested by a horizontal burning test according to ASTM D635, vertical burning test according to ASTM D3801, oxygen index (LOI) according to ASTM D2863 and cone calorimetry. Cone calorimetry studies were performed on a Custom Scientific Cone Calorimeter instrument at 35 kW m
2, according to ASTM 1354. A modified cone-calorimetry procedure was used. Samples were made of two conjunctive strips (5  10 cm) with 1.6 mm thickness instead of 10  10  2.5 cm samples. Five tests were conducted for each sample (ASTM 1354). The error in the results is less than 5%.

7.3 Results and Discussion 7.3.1 Polycarbonate–Inorganic Nanocomposites 7.3.1.1

Morphology Studies

The TEM image of PC–alumina 40–80 nm modified with MAP is shown in Figure 7.1(a). Modified alumina nanoparticles were dispersed in PC and appeared spherical in shape. Figure 7.1(b) shows a TEM image of PC–alumina

a

3820

Figure 7.1

b

1000nm

63700 : 1

500nm

TEM of polymer/alumina a) 5 wt% PC/alumina 40–80 nm modified with MAP, b) 5 wt.% PC/alumina 100 nm modified with MAP.

98

Chapter 7

100 nm/10 nm nanoparticles modified with MAP. Alumina nanoparticles (appear as dark needles) were well dispersed in PC. As indicated from Figures 7.1(a) and 7.1(b) nanoparticles were well dispersed in the PC matrix.

7.3.1.2

Thermal Stability

Degradation mechanisms for polymer–inorganic nanocomposites are based on studies of thermal degradation and flammability, which include the effect of nanoparticles on the degradation pathway for the polymer. As reported,9 the chemical nature of nanoparticles and polymers affects the chemical degradation processes in nanocomposites. Several factors affect the thermal degradation of PC–alumina nanocomposites. It was observed10,11 that aluminium oxide can catalyze the decomposition of carbonyl-containing polymers, which results in carboxylic acid elimination. Moreover, the degradation products, free radicals, are more reactive on the alumina oxide surface. PC has main-chain carbonyl groups, which can be eliminated in heterogeneous catalysis by alumina nanoparticles and result in carboxylic acid. The thermal degradation of PC–alumina nanocomposites is expected to proceed faster as a result of alumina-catalyzed PC decomposition. The elimination of carboxylic acid from PC leads to a decrease in the PC chain length. Thermal degradation mechanisms were studied using TGA and scanning electron spectroscopy (SEM) on the chars of PC– alumina and PC–silica nanocomposites. Thermal stability is studied on the basis of temperatures at 10% and 50% weight loss. The results of TGA for PC– alumina nanocomposites are listed in the Table 7.1. PC–alumina nanocomposites with 1–3% of alumina hydrate whiskers showed a 20–21 1C increase at 10% weight loss for PC–1% alumina whisker nanocomposite. Alpha-alumina (40–80 nm, spherical) exhibited decreased thermal stability for 5 weight per cent (wt%) PC–alumina nanocomposites. On the other hand, PC–silica nanocomposites exhibit a different effect of the nanoparticles on the thermal degradation process, as is evident from Table 7.212. Silica affects the thermal degradation pathway by a restriction mechanism, which results in enhanced char formation for char-forming polymers. The higher loading of silica in PC–silica nanocomposites resulted in a larger number of restriction sites for PC chain mobility, which enhanced char formation and the thermal stability of PC–silica nanocomposites compared to those of PC. For instance, a 15 1C increase at 50% weight loss for PC–3% silica is observed. Another factor that affects the thermal stability of PC–silica nanocomposites is the trapping of degradation products, radicals, on the silica surface, which has a stabilizing effect on the thermal degradation process for PC–silica nanocomposites. To gain a better understanding of the PC–alumina and PC–silica degradation processes, SEM images of thermal degradation residues for PC–alumina and PC–silica nanocomposites were taken. Residues were prepared by heating samples to 400 1C, 460 1C and 500 1C in a helium atmosphere. Images are shown in Figure 7.2. As clearly indicated from Figure 7.2(a), alumina particles are combined in larger aggregations on the

99

Impact of Nanoparticle Shape on the Flammability of Nanocomposites

TGA of PC–alumina nanocomposites.

Table 7.1

10% loss

50% loss

Sample

T (1C)

DT (1C)

T (1C)

DT (1C)

PC (Dow, developmental resin) PC-Al-100-MAP-0.5 PC-Al-100-MAP-1 PC-Al-100-MAP-2 PC-Al-100-MAP-5 PC-Al-40-MAP-0.5 PC-Al-40-MAP-1a PC-Al-40-MAP-2 PC-Al-40-MAP-5 PC (GE OQ 3620) PC-Al-15-MAP-0.5 PC-Al-15-MAP-1 PC-Al-15-MAP-2 PC-Al-15-MAP-5 PC-Al-wh-MAP-1 PC-Al-wh-MAP-3 PC-Al-wh-MAP-5 PC-Al-wh-MT-1 PC-Al-wh-MT-3 PC-Al-wh-MT-5

460 460 461 472 465 462 469 468 435 463 481 480 485 478 475 483 479 484 477 477

0 0 1 12 5 2 9 8
25 0 18 17 22 15 12 20 16 21 14 14

501 500 505 509 508 503 508 508 497 505 512 511 510 512 507 513 510 513 511 511

0
1 4 9 8 3 8 8
3 0 7 6 5 7 2 8 5 8 6 6

a

Example of sample code structure: in PC-Al-40-MAP-1, PC abbreviates polycarbonate matrix, Al means alumina nano-additive, 40 stands for the size of nanoparticles, MAP is the silane coupling agent used, 1 is the wt% of nano-additive. Al-wh is whisker-shaped alumina nano-additive.

TGA for PC–silica nanocomposites.12

Table 7.2

10% weight loss Sample s

PC (Lexan PC-Si-16-1 PC-Si-16-3 PC-Si-16-5

a

Figure 7.2

103)

50% weight loss

T (1C)

DT (1C)

T (1C)

DT (1C)

452 456 466 456

0 4 14 4

482 492 497 491

0 10 15 9

b

SEM of PC/inorganic chars, a) PC/alumina (40–80 nm, irregular), b) PC/ silica (R972).

100

Chapter 7

surface of the materials, which indicates that the alumina particles are combined on the surface. Alumina forms a layer on the surface of nanocomposites. In contrast, examination of the residues of PC–silica samples [Figure 7.2(b)] clearly indicates char formation. No silica particles were observed on the surface of degraded PC–silica nanocomposites, which indicates that silica stabilizes PC–silica nanocomposites by staying in the body of the material.

7.3.1.3

Flammability Studies

Flammability tests for PC–inorganic nanocomposites were done to investigate the input of the degradation mechanisms on the flammability of PC–inorganic nanocomposites. Cone calorimetry results for PC–alumina nanocomposites from alumina hydrate (100 nm/10 nm, needles) and alpha-alumina (40 nm, spheres) are presented in Tables 7.3–7.6. As shown in Tables 7.3–7.6, peak heat release rate (PHRR) decreased by 30% to 45% for PC–alumina nanocomposites compared to PC, which resulted from the formation of a ‘‘barrier’’ layer of alumina on the PC–alumina nanocomposite surface. As shown on the SEM images of chars, alumina Table 7.3

Cone calorimetry of PC–MT-modified alpha-alumina (40–80 nm) nanocomposites at 35 kW m
2. PC-40PC (Dow) MT-0.5

Material PHRR (kW m
2) Time to sustained ignition (s) Time of PHRR (s) THR (MJ m
2 g–1) Average mass loss rate (g m
2 s) Sample left (%)

Table 7.4

PC-40MT-1

PC-40MT-2

PC-40MT-5

1049 111

875 (–17%) 872 (–17%) 714 (–32%) 771.6 (–26) 107 46 76 67

143 1.79 33.3

132 1.76 32.9

83 1.77 26.8

104 1.72 27.8

100 1.77 26.2

24.0

25.5

22.1

22.6

27.9

Cone calorimetry of PC– MAP-modified alpha-alumina (40–80 nm) nanocomposites at 35 kW m
2.

Material PHRR, (kW m
2) Time to sustained ignition (s) Time of PHRR (s) THR (MJ m
2 g–1) Average mass loss rate (g m
2 s
1) Sample left (%)

PC-40PC (Dow) MAP-0.5

PC-40MAP-1

PC-40MAP-2

PC-40MAP-5

1049 111

950 (–9%) 80

807 (–23%) 689 (–34%) 732 (–30%) 75 83 39

143 1.79 33.3

107 1.73 32.1

108 1.82 26.0

112 1.68 24.8

112 1.86 26.8

24.0

18.8

21.6

20.3

24.2

101

Impact of Nanoparticle Shape on the Flammability of Nanocomposites

Table 7.5

Cone calorimetry of PC–MT-modified alumina hydrate 100 nm/ 10 nm nanocomposites at 35 kW m
2. PC-100PC (Dow) MT-0.5

Material PHRR (kW m
2) Time to sustained ignition (s) Time of PHRR (s) THR (MJ m
2 g
1) Average mass loss rate (g m
2 s) Sample left (%)

Table 7.6

PC-100MT-2

PC-100MT-5

1049 111

875 (–17%) 729 (–31%) 565 (–46%) 646 (–38%) 107 89 83 47

143 1.79 33.3

131 1.76 32.9

128 1.77 26.9

123 1.73 23.1

91 1.76 23.0

24

25.5

21.2

24.0

22.5

Cone calorimetry of PC–MAP-modified alumina hydrate 100 nm/ 10 nm nanocomposites at 35 kW m
2. PC-100PC (Dow) MAP-0.5

Material PHRR (kW m
2) Time to sustained ignition (s) Time of PHRR (s) THR (MJ m
2 g
1) Average mass loss rate, (g m
2 s
1) Sample left (%)

Table 7.7

PC-100MT-1

PC-100MAP-1

PC-100MAP-2

PC-100MAP-5

1049 111

802 (–24%) 676 (–36%) 580 (–45%) 645 (–39%) 76 71 67 72

143 1.79 33.3

112 1.78 28

110 1.81 25.0

109 1.58 10.2

115 1.67 22.9

24.0

17.5

18.9

21.0

23.1

Cone calorimetry of PC–silica nanocomposites.13

Material

PC (Lexan 103)

PC-Si-16-1

PC-Si-16-3

PHRR (kW m
2) Time of PHRR (s) THR (MJ m
2 g
1) Average mass loss rate (10% to 90%) Sample left (%)

940 105 2.24 27.1 17.0

900(
4.3%) 138 1.72 27.5 23.0

803(
4.6%) 117 1.65 12.4 24.7

agglomerated on the surface of degraded PC–alumina nanocomposite residues, forming a ‘‘protective barrier’’ layer. The alumina barrier layer slows the evolution of volatile gases into the burning area and protects the bulk material from the outside heat. Notice also that reduction of PHRR is 80–100 units larger for alumina hydrate nanorods than for irregular-shaped alpha-alumina– PC nanocomposites. In contrast, PHRR decreased modestly for PC–silica nanocomposites compared to PC, because of the char-formation mechanisms, as shown in Table 7.7.13 PC–alumina nanocomposites showed shorter times to ignition than PC.

102

Chapter 7

As discussed earlier, silica formed restriction sites for PC chain mobility. Char, formed as a result of PC degradation, served as an insulator for mass and energy transfer. Not only the ‘‘barrier’’ effect, but also the catalytic effect of alumina on PC degradation is involved in the flammability mechanism for PC– alumina nanocomposites. The amount of char left is less for PC–alumina nanocomposites than for PC. In general, the total heat released (THR) for PC– alumina nanocomposites did not change with an increase of alumina concentration, which can be explained by enhanced degradation processes catalyzed by alumina. Moreover, the time to ignition (TTI) generally decreases with the increase of alumina and/or alumina hydrate loading level. Totally different trends for the THR and the sample left were observed for PC–silica nanocomposites, as indicated in Table 7.7.12 The THR decreased and the amount of sample left increased with silica concentration increase, which indicated charforming mechanisms, as well as a radical trapping mechanism, by the silica surface. The restriction effects of silica as well as free-radical trapping mechanisms change the degradation pattern for PC. Free-radical degradation products, trapped by silica, could not leave the material, and the resulting free radicals are less reactive. PC–silica nanocomposites exhibited delayed TTI compared to PC. The restriction by silica of the PC chain-mobility effect combined with the free-radical trapping effect led to char formation.

7.3.2 PS–Inorganic Nanocomposites 7.3.2.1

Thermal Stability

Several factors affect the thermal stability of PS–alumina nanocomposites. The first is the stabilizing effect of alumina nanoparticles as restriction sites at the beginning of the thermal degradation process. Second, the degradation product free radicals are more reactive on the alumina oxide surface, which negatively affects the thermal stability of PS–alumina nanocomposites. Free-radical trapping on metal oxides increases the degradation rate, while chain-mobility restriction slows it down. It is a combination effect – PC and PS differ in that the catalysis effect does not occur in PS–alumina nanocomposites. Silica does not have a catalytic effect – it traps free radicals, which leads to a less reactive substance. The PS–silica degradation mechanism is associated with restriction of the PS chain mobility by silica nanoparticles. Therefore, the increase in silica concentration can result in higher degradation temperatures for PS–silica nanocomposites in comparison to that of PS. Burning tests are done to confirm the degradation. A small increase in the degradation temperatures for 10% and 50% weight loss was observed for PS–alpha-alumina (40–80 nm, spheres) nanocomposites and PS–alumina hydrate (100 nm/10 nm). Minimal increase in thermal stability was observed for PS–alumina nanocomposites with alumina concentration increase, because of the competing effects of accelerated degradation by free radicals and enhanced thermal stability caused by the PS chain-mobility restriction by

103

Impact of Nanoparticle Shape on the Flammability of Nanocomposites

alumina nanoparticles. Less enhancement occurred for PS–alumina (AL-15MAP and AL-wh-MAP) than for the PC counterparts at 10% weight loss. As shown in Tables 7.8 and 7.9,13 the thermal stability of PS–silica (16 nm, spherical) nanocomposites increased. This was attributed to trapping of freeradical species on the silica surface, which slowed the degradation process. Values at both 10 and 50% weight loss are higher for PS–silica nanocomposites than for PS–alumina nanocomposites. From Tables 7.8–7.10 it appears that the PS–inorganic nanocomposites with spherical nano-additives of larger size show better thermal stability compared to PS and a smaller size of nanoparticles. PS–10% silica 20 nm nanocomposites exhibited a 17 1C increase at 10% weight loss, while PS–10% silica 16 nm nanocomposites showed an 8 1C increase at 10% weight loss. From Table 7.10 it appears that the spherically shaped alumina of larger size (40–80 nm, alpha-form) exhibited better thermal stability than spherically shaped alumina hydrate (15 nm) or alumina hydrate nanorods, which can be attributed to the stronger interactions of PS chains with large-size spherical nanoparticles.

7.3.2.2

Flammability

With an alumina concentration increase, the burning rates measured by the horizontal burn (HB) test decreased, which means that PS–alumina burns Table 7.8

TGA results for PS–silica (16 nm) nanocomposites.13 10% weight loss

Sample PS (STYRON PS-Si-16-1 PS-Si-16-3 PS-Si-16-10 PS-Si-16-15

Table 7.9

s

685P)

50% weight loss

T (1C)

DT (1C)

T (1C)

DT (1C)

443 449 452 451 457

6 9 8 14

473 476 479 483 490

3 6 10 17

TGA results for PS–silica (20 nm) nanocomposites.14 10% weight loss

50% weight loss

Sample

T (1C)

DT (1C)

T (1C)

DT (1C)

PS (STYRONs 685D) PS-Si-20-PTCS-1 PS-Si-20-PTCS-3 PS-Si-20-PTCS-5 PS-Si-20-PTCS-10 PS-Si-20-PDMS-5 PS-Si-20-PDMS-10 PS-Si-20-PDMS-15

389 403 404 405 406 395 400 402

14 15 16 17 6 11 13

420 425 427 427 428 424 428 430

5 7 7 8 4 8 10

104

Table 7.10

Chapter 7

TGA results for polystyrene–alumina nanocomposites. 10% weight loss

Sample s

PS (STYRON 685DW) PS-Al-100-MAP-1 PS-Al-100-MAP-3 PS-Al-100-MAP-5 PS-Al-40-MAP-1 PS-Al-40-MAP-3 PS-Al-40-MAP-5 PS-Al-15-MAP-1 PS-Al-15-MAP-3 PS-Al-15-MAP-5 PS-Al-15-MT-1 PS-Al-15-MT-3 PS-Al-15-MT-5 PS-Al-wh-MAP-1 PS-Al-wh-MAP-3 PS-Al-wh-MAP-5 PS-Al-wh-MT-1 PS-Al-wh-MT-3 PS-Al-wh-MT-5

50% weight loss

T (1C)

DT (1C)

T (1C)

DT (1C)

385 389 386 389 389 392 392 384 391 381 387 387 389 385 392 393 387 390 368

4 1 4 4 7 7 –1 6 –4 2 2 3 1 7 8 2 5 –17

404 407 406 406 409 413 414 406 411 410 407 408 409 405 412 413 409 409 402

3 2 2 5 9 10 2 7 6 3 4 5 1 8 9 5 5 –2

longer than PS. In vertical burn (VB) tests, fire travels from bottom to top. Little to no time is available in which to form a protective barrier layer. Therefore, only the degradation-accelerating effect, caused by reactive freeradical products on the alumina surface, occur, which leads to more fuel supply for the burning process. Results of OI, VB and HB tests for PS–alumina nanocomposites are listed in Table 7.11. As discussed earlier, ignition and the continued burning of a material follow different mechanisms. The degradation mechanism only applies to the continued burning of the materials, such as in cone calorimetry. The OI test is a continued burning process that characterizes the ease of extinguishment for the sample. As can be seen in Table 7.11, the concentration or shape of alumina nano-additives had no significant effect on OI. The horizontal burning rate for PS–alumina nanocomposites generally decreased with increased concentration of alumina. PS–whisker alumina nanocomposites burned slower than PS (STYRONs 685 DW), as shown by the HB test (Table 7.11). PS–alumina nanocomposites burned faster in the VB test than PS. All PS–alumina samples dripped heavily in the HB test, which removed heat from the burning samples. In contrast, the burning rates for PS– silica (20 nm) nanocomposites were increased, as can be seen from Table 7.12.16 Materials became more flame retardant when both silica and brominated PS were used – the addition of silica lowered the content of brominated PS required to achieve the same level of flame retardancy, although, silica itself is not a flame retardant additive. The OI increased slightly for PS–silica (20 nm,

105

Impact of Nanoparticle Shape on the Flammability of Nanocomposites

Table 7.11

Burning tests results for polystyrene–alumina nanocomposites.

Sample s

PS (STYRON 685DW) PS-Al-100-MAP-1 PS-Al-100-MAP-3 PS-Al-100-MAP-5 PS-Al-100-MT-1 PS-Al-100-MT-3 PS-Al-100-MT-5 PS-Al-40-MAP-1 PS-Al-40-MAP-3 PS-Al-40-MAP-5 PS-Al-40-MT-1 PS-Al-40-MT-3 PS-Al-40-MT-5 PS-Al-15-MAP-1 PS-Al-15-MAP-3 PS-Al-15-MAP-5 PS-Al-15-MT-1 PS-Al-15-MT-3 PS-Al-15-MT-5 PS-Al-wh-MAP-1 PS-Al-wh-MAP-3 PS-Al-wh-MAP-5 PS-Al-wh-MT-1 PS-Al-wh-MT-3 PS-Al-wh-MT-5

Vertical burning test

OI

HB test, burning rate, (cm min
1)

T1 (s)

T2 (s)

Notes

16.9 16.9 17.2 17.5 16.9 17.2 17.5 17.2 17.5 17.2 17.1 17.1 16.9 17.5 17.2 17.2 17.5 17.2 17.5 17.5 17.5 17.2 17.5 17.2 17.2

6.1 5.8 5.2 4.4 5.5 5.1 5.1 5.8 5.3 5.4 6.2 6.3 5.4 4.4 5.4 5.2 5.4 5.1 4.7 6.1 4.4 4.5 4.7 4.7 3.9

76 69 53 66 59 54 55 58 62 51 61 63 61 60 55 47 53 43 21 41 55 61 59 49 58

– – – – – – – – – – – – – – – – – 7 11 – – – – – –

a a a a a a a a a a a a a a a a a a a a a a a a a

a

Ignites cotton and burns up to the clamp.

Table 7.12

OI and HB results for PS–silica (20 nm) nanocomposites.14,15

Sample s

PS (STYRON 685D) PS-Si-20-PTCS-1 PS-Si-20-PTCS-3 PS-Si-20-PTCS-5 PS-Si-20-PTCS-10 PS-Si-20-PTCS-15 PS-Si-20-PDMS-5 PS-Si-20-PDMS-10 PS-Si-20-PDMS-15 15%BrPS + 85%PS 30%BrPS + 70%PS 40%BrPS + 60%PS 15%BrPS + 75%PS + 10%Si 35%BrPS + 60%PS + 5%Si 15%BrPS + 84%PS + 1%Si a

OI

HB test, burning rate, (cm min
1)

17.3 17.0 17.3 17.5 18.1 18.4 17.5 17.8 18.1 18.9 21.4 24.0 20.5 25.0 19.6

5.4 5.7 5.8 6.1 7.3 7.1 6.6 8.2 9.3 4.1 b AEB ¼ 3.5 cm b AEB ¼ 0.5 cm b AEB ¼ 6.5 cm b AEB ¼ 3 cm 4.3

Ignites cotton and burns up to the clamp. EB ¼ average extent of burning.

b

Vertical burning test T1 (s)

T2 (s)

104 92 83 68 57 – 49 53 62 8 9 1 63 2 5

– – – – – – – – – 55 5 1 – 3 50

Notes

–

a

V2 V2

a

V2

a

106

Table 7.13

Chapter 7 s

Cone calorimetry results for PS (STYRON 685P)–silica (16 nm, spherical) nanocomposites.13

Sample PHRR (kW m
2) Time of PHRR (s) THR (MJ m
2 g
1) Average mass loss rate (g m
2 s–1) Sample left (%)

PS(STYRONs 685P)

PS-Si-16-3

PS-Si-16-10

1361 69.0 3.42 39.6

1178 (–13.4%) 49.2 3.27 (–4.2%) 34.8

1048 (–22.1%) 42 2.97 (–13.1%) 30.9

0.0

3.2

12.6

spherical) nanocomposites compared to PS (STYRONs 685D). The horizontal burning rates increased with silica concentration increase for PS–silica (20 nm, spherical) nanocomposites. Of note is that PS with 3% and 5% of MT-treated whiskers in Table 7.11 are not V-2 materials, since they all burn to the clamp. Severe dripping behaviour is the cause of flame extinction in the first flame application. In cone calorimetry, each sample is subjected to a slow burning process. Therefore, there is enough time for free-radical product trapping by silica to occur, which slows the degradation process. The free-radical trapping effect, caused by silica, leads to lower PHRR and THR, as can be seen in Table 7.13.13 The amounts of sample left for PS–silica nanocomposites correspond to the concentration of silica present in the samples, which indicates that char formation did not occur.

7.3.2.3

Mechanical Properties

Results of mechanical tests indicate the strength of interfacial interactions for polymer–inorganic nanocomposites. Polymers with different stereochemistry are expected to interact with different shapes of nanoparticles with different efficacy. Here, the following simplified model is proposed for the reinforcement mechanisms in polymer–inorganic nanocomposites. The aromatic ring of polymers has a flat architecture; therefore, the area of interaction for this type of polymer with the one-dimensional additives (nanorods) is larger than with zerodimensional ones (nano-spheres). Stronger interfacial interactions are expected between aromatic polymers and one-dimensional additives. As for polymers with a flexible structure, they can entangle around spherical nanoparticles, and strong interfacial interactions will occur. PMMA–silica nanocomposites exhibited a 125% increase in tensile strength at 5% loading (Table 7.12), while PS–silica and PC–silica nanocomposites showed no improvement in tensile strength and elongation at break. In contrast, PC–alumina nanocomposites from alumina nanorods and alumina whiskers showed substantial improvement in the mechanical performance compared to PC (54% increase at 2% loading for alumina hydrate 100/10 nm) Table 7.14.

107

Impact of Nanoparticle Shape on the Flammability of Nanocomposites

Table 7.14

Mechanical properties of polymer–silica and polymer–alumina nanocomposites from extrusion.13,16

Sample

Tensile strength (MPa)

Modulus (MPa)

Elongation at break (%)

PMMA PMMA-20nm silica-5% PS(685) PS-20nm silica-5% PC (Lexan) PC-16nm silica-!% PC-16nm silica-3% PC-16nm silica-5% PC (Dow) PC-40nm alumina-1% PC-40nm alumina-2% PC-40nm alumina-5% PC-100nm alumina hydrate-1% PC-100nm alumina hydrate-2% PC-100nm alumina hydrate-5% PC (OQ 3620 110) PC-alumina whisker-1% PC-alumina whisker-3% PC-alumina whisker- 5%

16.5 37.2 33.1 31.7 56.5 57.2 58.6 57.2 59.9 79.7 76.6 50.9 81.2 92.2 61.2 65.1 84.1 82.8 78.9

178 258 557 869 304 291 323 392 194 315 322 314 350 382 418 319 393 409 433

8 17 10 10 67 62 69 36 117 78 102 25 49 30 21 60 78 44 29

Given the differences in interfacial interaction between different polymer matrixes, nanoparticle chemistry and shapes result in different flammability properties for nanocomposites. The shapes of nanofillers may greatly affect the degree of interfacial interaction between polymer matrix and nanoparticles, which in turn significantly impacts the thermal stability and flammability of nanocomposites. The largest tensile strength increase for PC is with 2% 100 nm alumina. That also has the largest reduction in PHRR at –46%.

7.4 Conclusion It was found that the chemical nature of nanoparticles and polymers played an important role in determining the mechanisms of degradation. Metal oxide nanoparticles (alumina) have the ability to catalyze the degradation of carbonyl-containing polymers as well as to trap free radicals, which accelerates the degradation process. It was found that alumina moved to the surface and formed a protective or ‘‘barrier’’ layer, which enhanced thermal stability for polymer–alumina nanocomposites. This layer slows the processes of mass and energy transfer to and from the material. Polymer–silica nanocomposites showed higher degradation temperatures, but the degradation mechanism is different for polymer–alumina nanocomposites. Since silica is not a metal oxide, it does not have a catalytic effect, and the free-radical trapping on the

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surface of silica leads to less-reactive free radicals. Moreover, silica forms restriction sites for polymer chain mobility, which slows the degradation processes for polymer–silica nanocomposites. It was found that the catalytic and free-radical trapping effects of alumina accelerated the burning process, while the formation of an alumina ‘‘barrier’’ layer enhances the flame retardancy for polymer–alumina nanocomposites. In contrast, free-radical trapping and restriction effects of silica improve flame retardancy of polymer–silica nanocomposites. Silica enhances char formation for char-forming polymers (for instance, PC), which enhances the flame retardancy of char-forming polymer–silica nanocomposites. It was shown that PC and PS matrixes have stronger interactions with a one-dimensional additive (nanorod) compared to a zero-dimensional additive (nanosphere). Thermal degradation and flammability for polymer–inorganic nanocomposites from different shapes of nano-additive exhibited quite different behaviour, as demonstrated by cone calorimetry, OI, HB and VB test results.

References 1. P. Rittigstein and J. M. Torkelson, J. Polymer Sci. Part B: Polymer Physics, 2006, 44, 2935–2943. 2. V. K. Nguyen, J. W. Lee and Y. Yoo, Sensors and Actuators B: Chemical, 2007, 120, 529–537. 3. S. Chatterjee, A. Goyal and S. I. Shah, Mater. Lett., 2006, 60, 3541–3543. 4. C. E. Powell and G. W. Beall, Current Opinion in Solid State and Materials Science, 2006, 10, 73–80. 5. F. Yang and G. L. Nelson, The 11th International Conference, ADDITIVES, 2002. 6. J. Zhu, F. M. Uhl and C. A. Wilkie, Polymer Preprints, 2001, 42, 392. 7. M. Moniruzaman and K. J. Winey, Macromolecules, 2006, 39, 5194–5205. 8. Y. W. Mai and Z. Z. Yu, ‘‘Polymer Nanocomposites’’, 2006, Woodhead Publishing Limited, Cambridge, 57–100. 9. S. C. Liufu, H. N. Xiao and Y. P. Li, Polymer Degradation and Stability, 2005, 87, 103–110. 10. A. Laachachi, M. Cochez, E. Leroy, P. Gaudon, M. Ferriol and J. M. Lopez Cuesta, Polymers for Advanced Technologies, 2006, 327–334. 11. I. C. McNeil and H. M. Mussarat, Polymer Degradation and Stability, 1995, 48, 189–195. 12. F. Yang, R. Yngard, A. Hernberg and G. L. Nelson, Proceedings of BCC 16th Annual Flame Conference, 2005, 244–255. 13. F. Yang and G. L. Nelson, Polymers for Advanced Technologies, 2006, 17, 320–326. 14. R. A. Yngard, ‘‘Preparation and characterization of flame retardant polystyrene/silica nanocomposites’’, A thesis submitted to Florida Institute

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of Technology in partial fulfillment of the requirements for the degree of master of science in chemistry, 2002, Melbourne, Florida. 15. F. Yang, R. Yngard and G. L. Nelson, J. of Fire Sci., 2005, 23, 209–226. 16. I. R. Bogdanova, ‘‘Reinforcement and degradation mechanisms in polymer/inorganic nanocomposites’’, 2007, Dissertation submitted to Florida Institute of Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry, pp. 61, 92.

CHAPTER 8

Thermal and Combustion Behaviour of Polymer–Carbon Nanofibre Composites D. TABUANI, S. PAGLIARI, W. GIANELLI AND G. CAMINO Centro di Cultura per l’Ingegneria delle Materie Plastiche, viale T. Michel 15, Alessandria, Italy

8.1 Introduction In recent years the use of graphitic nanoparticles, such as carbon nanofibres (CNFs) and nanotubes, has attracted much interest for the development of highperformance polymer composites. In particular, CNFs are an attractive alternative to nanotubes, because of their lower costs while maintaining properties similar to those of their thinner analogues. Indeed, the use of CNFs as polymer fillers is quite recent,1,2 but has developed quite fast. In particular, studies have been carried out with polypropylene (PP) in which nanofibres were found to improve thermal stability,1,3 crystallization behaviour,1,4 mechanical properties,1,4,5,6 melt viscosity,2 and electrical2 and thermal conductivity.6 Research has been devoted also to other polymer classes, such as poly(ether ether ketone)7,8, polycarbonate,6 poly(methyl methacrylate),9 epoxy10 and polystyrene (PS).11,12 With particular reference to fire retardancy, nanoadditives have recently attracted attention for their potential as fire retardants of low environmental impact, as alternatives for the more commonly used but more hazardous aromatic brominated compounds. This aspect has been developed for layered silicates,13 and also carbon nanoparticles were found to have a good potential Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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111

in the field. In particular, carbon nanotubes were found to improve significantly the fire retardant behaviour of ethylene vinyl acetate (EVA),14 PP3,15,16 and polyamide 6 (PA6)17 alone and in combination with layered silicates18. More recently this aspect was verified also for CNFs in PP, using the limiting oxygen index (LOI) as the flammability test.3 This chapter describes our investigation of the influence of the addition of CNFs on the properties of two polymer matrices, one semicrystalline and the other completely amorphous (PP and PS, respectively). Furthermore, two grades of PP are taken into account, with different melt-flow indexes (MFI), to investigate the influence of PP melt viscosity on the properties of the final composite. In the case of PS, preparation of the composites by melt blending is compared with a solution method. Two different kinds of nanofibres with different aspect ratios are considered, to evaluate the influence of this parameter on the fibre dispersability and final material properties. The morphology, thermal and combustion behaviour of the composites are evaluated. This chapter represents the first attempt to evaluate the influence of polymer characteristics, preparation conditions and nanofibre type on the final material properties.

8.2 Materials and Methods Two PP grades were used, both purchased from Basell, Italy: Moplen HP400R (MFI ¼ 26 g 10 min
1) and Moplen HP501L (MFI ¼ 6 g 10 min
1). PS (EDISTIR N 2380) was supplied by Polimeri Europa (Italy). Nanofibres, PR-24 HHT, were purchased from Pyrograf Products (USA) in two commercial grades: high density (HD) and low density (LD) standards. The HD and LD designations do not refer to the density of the individual fibre but to the bulk density of the CNFs. The bulk or aggregate density of the nanofibre is controlled by altering the intensity and duration of the process that reduces nanofibre aggregation, which also reduces the length of the individual fibres, with HD fibre having shorter lengths (50–100 mm) than the low density fibre (100–200 mm). The fibre diameter, conversely, is not influenced by the process, ranging in any case from 100 to 200 nm. Two techniques were used to prepare the composites: polymer–fibre melt blending with PP and PS, and dispersion of fibres in polymer solution in the case of PS.

8.2.1 Melt Blending Nanofibres were mechanically dispersed in the molten polymer by the shear action of a Brabender internal mixer, model PLE 67152. The mixing parameters for each matrix are reported in Table 8.1.

8.2.2 From Solution PS (4 g) was dissolved with a magnetic stirrer in 15 cm3 of tetrahydrofuran (THF, Aldrich) at room temperature and the solution obtained was mixed with

112

Table 8.1

Chapter 8

Melt processing conditions.

Matrix

Process temperature (1C)

Screw rotation speed (rpm)

Residence time (minutes)

PPHP400R

180

60

10

PPHP501L

180

60

10

PS

180

30

5

Loading (wt%) LDCNF (0.5; 1; 3) HDCNF (0.5; 1; 3) LDCNF (0.5; 1; 3) HDCNF (0.5; 1; 3) LDCNF (3; 6; 10) HDCNF (3; 6; 10)

5 cm3 of nanofibre suspension in THF (the concentration of the suspension varied depending on the filler loading), using a magnetic stirrer to break-up any nanofibre macro-agglomerates. The successive dispersion of nanofibre suspension in the polymer solution was achieved with an ultrasound bath model Uniset AC14 (power, 140 W; frequency, 22 kHz). The solvent was eliminated under vacuum in a Rotavapor, followed by vacuum oven drying (T ¼ 80 1C, P ¼ 3 mbar, t ¼ 180 minutes).

8.2.3 Characterization X-Ray (WAXRD) diffraction patterns were obtained on a ARL XTRA48 diffractometer using Cu Ka radiation (l ¼ 1.54062 A˚) on compression moulded 1 mm thick specimens. The crystallization behaviour of the PP samples was measured using a TA Q1000 instrument in hermetically sealed aluminium pans under nitrogen flow (50 cm3 min
1). Three successive runs (heating–cooling–heating) were performed at 10 1C min
1 from 30 to 250 1C, with ca. 5 mg samples. The first heating run erases the thermal history of the samples and an annealing process at 250 1C for 10 minutes followed the first heating run. A detailed differential scanning calorimetry (DSC) analysis was performed on PP nanocomposites to assess the degree of crystallinity and crystallization rate, measuring crystallization temperature (Tc), melting enthalpy (DHm) and degree of crystallization (Xc). Crystallization temperature was measured within a  0.5 1C tolerance. The degree of crystallization was calculated from the peak enthalpy area normalized to the actual polymer weight fraction according to:

Xc ¼

DHm  100 DHm0  Wpolymer

in which DHm is the theoretical value of enthalpy for 100% crystalline polymer (190 J g
1)19 and Wpolymer is the weight of the polymer fraction. Thermogravimetric analysis (TGA) was carried out both in inert (nitrogen) and in oxidizing (air) atmospheres on a TA Q500 instrument on ca. 10 mg samples, in platinum pans, with gas fluxes of 60 cm3 min
1 for sample gas

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113


1

(nitrogen or air), and 40 cm min for balance-protection gas (nitrogen) at a fixed rate (in this case 10 1C min
1) from 50 to 800 1C. From the thermograms two relevant parameters were taken to enable discussion: T5%, as the temperature at 5% of weight loss, and Tmax, as the temperature at which the maximum weight-loss rate occurred (i.e. derivative TGA, peak temperature). T5% and Tmax were measured within a  3 1C tolerance. Combustion tests were performed on a Fire Testing Technology Cone Calorimeter, with 50  50  3 mm specimens, prepared by compression moulding on a hot-plate laboratory press at 180 1C. Tests were performed at 50 kW m
2 external heat flux, to evaluate the fire properties of the composites in conditions comparable to a developed fire scenario. Specimens were wrapped in aluminium foil (except for the upper surface) and placed on a ceramic backing board at a distance of 25 mm from the cone base. The average values of three successive experiments are discussed here. Scanning electron microscopy (SEM) images were obtained by means of a LEO 1450 VP instrument on cryogenically fractured surfaces. For comparison, characterizations were also performed on pure PP and PS treated in the same way as the composites.

8.3 Results and Discussion 8.3.1 Morphology 8.3.1.1

SEM

SEM images show fairly good dispersion of nanofibres in the polymer matrices. In particular, as far as PP with HDCNF samples are concerned, nanofibre-rich areas can be identified at low magnification [Figure 8.1(a)], but single nanofibres well-adhered to the polymer matrix are also visible [Figure 8.1(b)]. Similar results are obtained with LDCNF and with PPHP501H. The dispersion of the fibres in PS is even better; indeed, these appear to be uniformly distributed in the matrix [Figure 8.2(a)], and also present a high degree of compatibility with the polymer itself [Figure 8.2(b)].

(a)

Figure 8.1

(b)

SEM micrographs of HP400R–3wt%HDCNF composites.

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(a)

Figure 8.2

(b)

SEM micrographs of PS–3wt%HDCNF composites from melt blending.

(a)

Figure 8.3

(b)

SEM micrographs of PS–3wt%HDCNF composites from solution.

The situation appears to be dramatically different when nanofibres are dispersed in PS via the solution method. The mixing technique was apparently not efficient in the disaggregation of the nanofibre bundles, which are observed in the polymer matrix even at low magnification [Figure 8.3(a)]. When observing the area in detail we could recognize the aggregates as composed only of nanofibre – the polymer, in this case, is not interacting at all with the filler.

8.3.1.2

XRD

In XRD, diffractograms for the PPHP400R–HDCNF composite, Figure 8.4, the most intense reflection peak of pristine CNF is visible at 2y ¼ 25.5. The PP matrix in the composite crystallizes with the same phase as that of the pristine polymer, as shown by comparison with XRD of pure PP (Figure 8.4). However, a strong (040) preferential orientation is observed, owing to the presence of the nanofibres, even at the lowest loading. Indeed, the significant increase of the intensity ratio between the second (040 reflection, 2y ¼ 17) and the first (110 reflection, 2y ¼ 14.3) XRD peak results from a preferential growth

Thermal and Combustion Behaviour of Polymer–Carbon Nanofibre Composites

115

2θ = 25.5

Intensity (a.u)

PPHP400R+3%HDCNF

PPHP400R+1%HDCNF

PPHP400R+0,5%HDCNF (110) (040)

6

Figure 8.4

(130) (111) (131) (041)

PPHP400R

8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 2θ

XRD diffraction pattern of HP400R–HDCNF.

of PP crystallites well-ordered along the b direction. This phenomenon was reported in the literature for other nanofillers.20–22 The hypothesis is that this preferential orientation is generated by an intimate interaction of the polymer chains with the dispersed nanofiller, so that the filler can influence the growth direction of the crystalline domains. We could not find significant differences for this aspect when considering the two PP grades and CNF types. In Figure 8.5, XRD spectra of blank PS and LDCNF-containing composites prepared by melt blending are compared. A wide peak from 2y ¼ 15 to 251 belongs to amorphous PS and the most intense reflection peak of CNF is visible in the spectra of the composites at 2y ¼ 25.5.

8.3.2 Thermal Behaviour 8.3.2.1

DSC

Temperatures of crystallization and degree of crystallinity measured during the second heating of nanocomposite materials are reported in Table 8.2. Nanofillers were observed to act as nucleation sites, as evidenced by a significant decrease in the time required by nanocomposites to crystallize during the cooling cycle. As a consequence, the nanocomposite crystallization peak endotherm temperature was higher than the value observed for the pure

116

Intensity (a.u)

Chapter 8

PS+10%LDCNF PS+6%LDCNF PS+3%LDCNF PS 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 2θ

Figure 8.5

Table 8.2

XRD diffraction pattern of PS–LDCNF composites.

Crystallization temperatures (Tc) and degree of crystallinity (Xc) for CNF–PP composites.

HP400R

Tc (increment) (1C)

Xc (variation) (%)

PPHP501L

Tc (increment) (1C)

Xc (variation) (%)

– +0.5%LDCNF +1%LDCNF +3%LDCNF +0.5%HDCNF +1%HDCNF +3%HDCNF

109 123 124 128 121 122 125

48 49 54 52 49 51 50

– +0,5%LDCNF +1%LDCNF +3%LDCNF + 0,5%HDCNF + 1%HDCNF + 3%HDCNF

111 123 124 128 121 122 124

48 54 51 50 50 51 50

(+14) (+15) (+19) (+12) (+13) (+16)

(+1%) (+5%) (+4%) (+1%) (+3%) (+ 2%)

(+12) (+13) (+17) (+10) (+11) (+13)

(+6%) (+3%) (+2%) (+2%) (+3%) (+2%)

polymer, as shown in Figure 8.6 for PPHP400R. The same behaviour was observed for PPHP501L. Data on the crystallization behaviour of both PP with HD and LD nanofibres are given in Table 8.2. No significant differences were encountered when comparing LD- and HDCNF and nor were any notable variations of Tc, found when increasing the content from 1 to 3%, which thus indicates that saturation of the crystallization nucleation sites was reached at 1% loading. This phenomenon has already been observed by Lozano et al.1 and was attributed by them to the increased number of nucleating sites, thus indicating that CNFs act as heterogeneous nucleating agents for PP. This phenomenon is in accordance with that observed from XRD spectra, which suggests a close interaction between the nanodispersed phase and the polymer chains. As evident from Table 8.2, PP crystallinity is slightly increased when there are nanofibres in the matrix; other authors have already reported such behaviour for

Thermal and Combustion Behaviour of Polymer–Carbon Nanofibre Composites

117

PP400R PP400R+0,5%HDCNF PP400R+1%HDCNF PP400R+3%HDCNF

18 16 14

Heat flow (a.u.)

12 10 8 6 4 2 0 80

100

120

140

160

Temperature (°C)

Figure 8.6

Specific heat flow (cooling step) recorded for HP400R–HDCNF composites.

similar systems1 in which vapour-grown CNF–PP composites were prepared, and the influence of degree of purification and of functionalization of the nanofibres on PP thermal and mechanical properties was determined. In the work by Lozano et al., at a concentration of 5% CNF, an increase in PP crystallinity of about 4% was observed and attributed to an enhanced mobility of the macromolecular chains, which enables a better alignment of the crystal lattice.

8.3.2.2

TGA – Polypropylene Based Materials

During heating under nitrogen, PP thermally degrades in a single step to volatile products above 350 1C (T5% 392 1C, Tmax 450 1C, dotted line in Figure 8.7) through a radical chain process propagated by carbon-centred radicals that arise from carbon–carbon bond scission.23 The thermal degradation of PP that contains CNFs takes place in a single step, as in the case of pristine PP, and shows the same temperature for a 50% weight loss. In air, above 200 1C the radical chain thermal volatilization is initiated by H abstraction from PP by oxygen.23 Volatilization begins in TGA below 250 1C (T5%), with Tmax at 324 1C (solid line in Figure 8.7; see Table 8.3), and is completed before the temperature of the initiation of the pure thermal degradation process (370–390 1C) is reached. The presence of CNF modifies the thermo-oxidative behaviour of the matrices in all the prepared nanocomposites, increasing the temperature at which 50% volatilization occurs, as shown in Figure 8.7 and Table 8.3.

118

Chapter 8 277(95%)

100

253(95%)

80 Weight [%]

PPHP400R PPHP400R+0,5% LDCNF PPHP400R+1% LDCNF PPHP400R+3% LDCNF PPHP400R nitrogen

392°C (95%)

60 40 20

Deriv. Weight [%/˚C]

0 377°C 2

450°C

324°C

1

0 100

Figure 8.7

Table 8.3

200

300

400 500 Temperature [°C]

600

700

800

TGA and differential thermogravimetric analysis (DTG) curves in air of HP400R–LDCNF composites.

Degradation temperatures for CNF–PP composites in air.

HP400R

T5% (1C)

Tmax (1C)

HP501L

T5% (1C)

Tmax (1C)

– 0.5%LDCNF 1%LDCNF 3%LDCNF 0.5%HDCNF 1%HDCNF 3%HDCNF

253 255 250 277 250 253 261

324 348 352 377 334 332 361

– 0.5%LDCNF 1%LDCNF 3%LDCNF 0.5%HDCNF 1%HDCNF 3%HDCNF

255 258 255 277 257 266 272

328 365 359 382 320 359 371

We attribute the protective action played by the nanofibres to a barrier effect against oxygen, provided by the nanofibres which organize themselves in a physical network during polymer ablation, and to an interaction of the fillers with oxygen. Moreover, this physical network could provide a mass-transport barrier towards polymer ablation. Both barrier effects lead to reduction of the observed mass-loss rate. Indeed, an interesting feature is, for both PP grades investigated and for both nanofibres, the shape of the derivative curves, i.e. the shape of the mass-loss rate. It can be seen that the pure polymer derivative curve differs significantly from those of the filled samples. At low temperatures, up to 320 1C,

Thermal and Combustion Behaviour of Polymer–Carbon Nanofibre Composites

119

the mass-loss rate of the PP in the composites is significantly lower than that of the pristine polymer (lower slope of the curve, especially for the 3% loaded samples). At above 320 1C an increase of the mass-loss rate occurs up to the maximum value (identified for the various composites by Tmax). Above Tmax an abrupt increase of the mass-loss rate is experienced by the filled samples, the overall result being a completely asymmetrical peak. We can recognize in this behaviour the efficiency of the network described above, which is high at low temperatures and decreases upon heating at higher temperatures. Above Tmax the physical rupture of this network occurs: the remaining polymer experiences a combination of high temperature and oxygen attack that makes it degrade very rapidly (the derivative curve is almost vertical above Tmax).

8.3.2.3

TGA – Polystyrene Based Materials

PS is known to degrade through a radical mechanism in nitrogen, forming styrene as a major product via b-scission of chain-end radicals; dimers, trimers and larger oligomers are also produced via intramolecular hydrogen transfer reactions (backbiting) with the yield decreasing with oligomer size.24 As far as PS composites prepared from the solution method are concerned, a low temperature peak is visible on the DTG curves (ca. 150 1C, both in nitrogen and in air), which arises from residual solvent from the preparation method. This signal is more intense for the composites than for the pure material, probably owing to the lower tendency of the solvent to evaporate from a more viscous medium or to the barrier effect towards evaporation played by the nanofibres. The nanofillers did not have an effect on the single-step weight-loss thermal degradation process of PS. Whereas under air (Figure 8.8), in which oxygen initiation decreases the temperature of weight loss (as in PP) although with a lower effect (Tmax N2 415 1C, Tmax air 374 1C), a delay in the degradation temperatures is observed with 3% of filler and further addition of nanofibre does not produce any relevant changes. The maximum degradation-rate temperature of PS becomes closer to that of the pure polymer in nitrogen, which indicates an effective protection role played by the fibres towards the degradative action of oxygen, which abstracts hydrogen and reacts with polystyryl radicals that accelerate the overall process of weight loss.24 As in the case of PP, also in PS, above a critical temperature (ca. 475 1C for PS), the degradation rate rapidly increases, probably because of the breakdown of the nanofibre protecting network. For composites prepared by melt blending, a 5 1C increase in Tmax is observed for the degradation in nitrogen. In air, in contrast, a significant increase in the degradation temperature is observed, independent of the amount of filler; such an increase is more noticeable than that observed in the samples from solution, probably because of a better miscibility given by the preparation method. Indeed, especially in the case of HDCNF, the maximum weight-loss rate temperature of the matrix in nitrogen is achieved, thus implying a very efficient protection action towards oxygen (Figure 8.9).

120

Chapter 8

Weight [%]

80

PS PS+3% LDCNF PS+6% LDCNF PS+10% LDCNF PS nitrogen

309°C(95%)

100

370°C (95%) 151°C(95%)

60 40 20 0 402°C

Deriv. Weight [%/˚C]

3,0

416°C

2,5 2,0 374°C

1,5 1,0 0,5 0,0 100

Figure 8.8

8.3.2.4

200

300

400 500 Temperature [°C]

600

700

800

TGA and DTG curves in air of PS–LDCNF prepared by the solution method.

Cone Calorimeter Test

The reaction to fire of polymer matrices and nanocomposite materials was evaluated by the cone calorimeter test and, in particular, heat release rate (HRR) patterns were recorded. The time to ignition (TTI), the maximum value of HRR plotted versus time [peak HRR (PHRR)] and the total amount of heat released during the combustion process [total heat release (THR)] are the parameters discussed here. In the case of nanocomposites based on PP it is evident, from Table 8.4 and Figure 8.10, that TTI decreases with both LD and HDCNF through an increased radiant-heat absorbance compared to that of pure PP because of the black colour of the nanocomposites. For PP composites the total heat released during combustion is maintained in the same range. To reduce appreciably the PHRR of PP (from ca. 1330 to 890 kW m
2) 3% of LDCNF has to be added to PP. Lower concentrations did not modify PHRR. HDCNFs are less efficient because, even at 3% loading, they did not affect PHRR. Similar results are obtained with PPHP501L. We attribute the effect induced by LDCNF to the assembly of a protective CNF network which lowers the combustion rate, in analogy with that described for thermo-oxidation, which does act as a barrier towards the release of combustible volatile products.

Thermal and Combustion Behaviour of Polymer–Carbon Nanofibre Composites 324°C (95%) 377°C (95%) 306°C (95%)

100

Weight [%]

80

121

PS PS+3% LDCNF PS+6% LDCNF PS+10% LDCNF PS nitrogen

60 40 20 0

Deriv. Weight [%/˚C]

3

414°C

2 375°C

411°C

1

0 100

200

300

500 600 400 Temperature [°C]

700

800

Figure 8.9

TGA and DTG curves in air of PS–LDCNF prepared by melt blending.

Table 8.4

Cone calorimeter results for nanocomposites based on PPHP400R.

HP400R

TTI (s)

PHRR (kWm–2)

THR (MJ m–2)

– +0.5%LDCNF +1%LDCNF +3%LDCNF +0.5%HDCNF +1%HDCNF +3%HDCNF

37  5 20  0 24  2 23  3 23  2 21  2 23  2

1331  40 1356  20 1266  35 891  15 1353  65 1400  20 1237  30

93  7 99  0 99  1 95  2 97  2 99  2 98  2

In all the cases described, the weight of residue obtained after the combustion process was equivalent to the weight of the employed CNF, which rules out any extensive charring effect of CNFs. For the PS system, the cone calorimeter analysis was performed on the meltblended samples at 3 and 10% loadings to evaluate the influence on the polymer fire behaviour of the lowest and highest nanofibre percentages in the composites that showed the best CNF dispersion. As shown in Figure 8.11, the two kinds of nanofibres behave differently. As far as HDCNFs are concerned, an appreciable decrease in the PHRR (about 50%) was obtained by adding 10% of filler, whereas no effect was detected at 3% loading, within the experimental error (Table 8.5).

122

Chapter 8 PPHP501L PPHP501L+0,5% LDCNF PPHP501L+1% LDCNF PPHP501L+3% LDCNF

1400

600

2

(a)

800

HRR (KW/m )

1200

1000

2

HRR (kw/m )

1200

1000

(b)

800 600

400

400

200

200

0

0 −20 0 20 40 60 80 100 120 140 160180 200 220 240 time (sec)

0 20 40 60 80 100 120 140 160 180 200 220 240 time (sec)

Figure 8.10

1600 1400 1200 1000 800 600 400 200 0

HRR vs. time: (a) HP501L–LDCNF and (b) HP501L–HDCNF composites.

(a)

0

Figure 8.11

Table 8.5

PS PS+3% LDCNF PS+10% LDCNF HRR (kW/m2)

HRR (kW/m2)

PPHP501L PPHP501L+0,5% HDCNF PPHP501L+1%HDCNF PPHP501L+3%HDCNF

1400

50

100 150 200 250 time (sec)

PS PS+3% HDCNF PS+10% HDCNF

(b)

1600 1400 1200 1000 800 600 400 200 0 0

50 100 150 200 250 300 time (sec)

HRR vs. time: (a) PS–LDCNF and (b) PS–HDCNF composites.

Cone calorimeter results for nanocomposites based on PS.

PS

PHRR (kW m 2)

TTI (s)

THR (MJ m
2)

– +3%LDCNF +10%LDCNF +3%HDCNF +10%HDCNF

1536  50 977  40 858  35 1337  40 758  30

43  1 31  3 39  2 25  4 32  2

91  1 88  1 88  4 91  1 88  3

Conversely, with LDCNF a significant decrease of the PHRR value is obtained by the addition of the lowest amount of filler (3%) and the increase of such loading (to 10%) does not bring significant further improvements on the fire behaviour of the material. Indeed, by the addition of 3% LDCNF, a decrease by 36% of the PHRR value was observed and, by increasing the LDCNF content by a factor of three (to 10%), the PHRR decreases with

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123

respect to that of pristine polymer by about 44%, that is 20–25% more than with the 3% loading.

8.4 Conclusions No differences were encountered in the miscibility of LD or HD nanofibres with PP, either of high or low molecular weight. In all cases, CNFs were found to be fairly well dispersed in the polymer and to behave as nucleating agents, increasing the polymer crystallization temperature and influencing, in the same way, a preferential orientation of PP chains along the b direction during crystallization. Significant improvements were found in PP thermo-oxidative resistance, to a greater extent in the case of LDCNF. Similarly, LDCNF shows a greater efficiency than HDCNF in reducing the PHRR of PP burning in a cone calorimeter. In PS-based materials, a noticeable effect on the properties of the composites was played by the preparation technique, mainly because of the different degree of dispersion achieved with the nanofillers; in fact, the solution method leads to an extremely poor dispersion of the CNF in the polymer. Nevertheless, also in this case, some improvements in PS thermo-oxidative behaviour were observed, yet the most interesting results were obtained by adding the filler via the meltblending method. In this case, indeed, the addition of HDCNF in particular shifted PS degradation temperatures towards those observed in nitrogen, thus proving the efficiency of the nanofibres in protecting the polymer from the degradative action of oxygen. As far as combustion tests are concerned, a significant influence of the type of filler was found on the fire behaviour of PS; LDCNFs were found to behave efficiently as fire retardants even at the lowest loading investigated (3%), while a significant decrease in the PHRR was found only by adding 10% of HDCNFs. It is clear, therefore, that specific interactions are occurring between the two polymers investigated and LDCNF, making this the most suitable filler to tune the properties of the polymer. We can hypothesise that, because LDCNF has a higher aspect ratio than HDCNF, a greater interface with the polymer is created, and it is this that is responsible for the better performance. Recently, the hypothesis of different interaction extents was confirmed for the PP matrix (in particular, for the HP400R grade) by means of rheological measurements.25

References 1. K. Lozano and E.V. Barrera, J. Appl. Polym. Sci., 2001, 79, 125–133. 2. K. Lozano, J. Bonilla-Rios and E.V. Barrera, J. Appl. Polym. Sci., 2001, 80, 1162–1172. 3. A. Chatterjee and B.L. Deoupra, J Appl Polym Sci, 2006, 100, 3574–3578. 4. X. Tong, Y. Chen and H. Cheng, J. Mater. Sci. Technol., 2005, 21, 686. 5. S. Kumar, H. Doshi, M. Srinivasarao, J.O. Park and D.A. Schiraldi, Polymer, 2002, 43, 1701–1703.

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6. E. Hammel, X. Tang, M. Trampert, T. Schmitt, K. Mauthner, A. Eder and P. Poetschke, Carbon, 2004, 42, 1153. 7. J. Sandler, P. Werner, M.S.P. Shaffer, V. Demchuk, V. Altstadt and A.H. Windle, Composites: Part A, 2002, 1033. 8. J. Sandler, A.H. Windle, P. Werner, V. Altstadt, M.V. Es and M.S.P. Shaffer, J. Mater. Sci., 2003, 38, 2135. 9. J. Zeng, B. Saltysiak, W.S. Johnson, D.A. Schiraldi and S. Kumar, Composites Part B, 2004, 35, 245. 10. Y.K. Choi, K. Sugimoto, S.M. Song, Y. Gotoh, Y. Ohkoshi and M. Endo, Carbon, 2005, 43, 2199. 11. B. Zhang, R. Fu, M. Zhang, X. Dong, L. Wang and C. Pittman, U. Mater. Res. Bull., 2006, 41, 553. 12. Y. Xu, B. Higgins and W.J. Brittain, Polymer, 2005, 46, 799–810. 13. A.B. Morgan, Polym. Adv. Technol., 2006, 17, 206–217. 14. G. Beyer, Fire Mat., 2002, 26, 291. 15. T. Kashiwagi, E. Grulke, J. Hilding, R. Harris, W. Awad and J. Douglas, Macromol Rapid Commun, 2002, 23, 761–765. 16. T. Kashiwagi, E. Grulke, J. Hilding, K. Groth, R. Harris, K. Butler, J. Shields, S. Kharchenko and J. Douglas, Polymer, 2004, 45, 4227–4239. 17. B. Schartel, P. Poetschke, U. Knoll and M. Abdel-Goad, Eur. Polym. J., 2005, 41, 1061. 18. F. Gao, G. Beyer and Q. Yuan, Polym. Degrad. Stab., 2005, 89, 559. 19. S. Lohnmayer, Die speziellen Eigenschaften der Kunststoffe, Gafenau, Expert Verlag, 1984. 20. E. Ferrage, F. Martin, A. Boudet, S. Petit, G. Fourty, F. Jouffret, P. Micoud, P. De Parseval, S. Salvi, C. Bourgerette, J. Ferret, Y. SaintGerard, S. Buratto and J.P. Fortune, J Mater Sci, 2002, 37, 1561–1573. 21. S. Radhakrishnan, P. Sonawane and N. Pawaskar, J Appl Polym Sci, 2004, 93, 615–623. 22. L. Wang and J. Sheng, Polymer, 2005, 46, 6243–6249. 23. N. Grassie and G. Scott, Polymer degradation and stabilisation, Cambridge University Press, Cambridge, 1985. 24. A. Guyot, Polym. Degrad. Stab., 1986, 15, 219–235. 25. S. Ceccia, D. Ferri, D. Tabuani, P.L. Maffettone, Rheologica Acta, 2008, 47, 425–433.

CHAPTER 9

Combination of Carbon Nanotubes with Fire Retardants: Thermal and Fire Properties of Polystyrene Nanocomposites FLORENTINA TUTUNEA AND CHARLES A. WILKIE Department of Chemistry, Marquette University, PO Box 1881, Milwaukee, WI 53201, USA

9.1 Introduction Together with numerous advantages that synthetic polymeric materials provide to society in everyday life, there is one obvious disadvantage – the flammability of many synthetic polymers.1 Polymers are used in the manufacture of not only bulk parts, but also of films, fibres, coatings and foams. Fire hazard is a combination of factors, including ignitability, ease of extinction, flammability of the volatile products generated, amount of heat released on burning, rate of heat release, flame spread, smoke obscuration and smoke toxicity, as well as the fire scenario.2–5 Polymers are used in more and more applications and specific mechanical, thermal and electrical properties are required. Flame retardants may function in the gas phase, by removing the reactive hydrogen and hydroxyl radicals, or in the condensed phase, influencing the course of pyrolysis by forming char. The char insulates the polymer from the flame front and results in fewer

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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combustible volatiles. The typical vapour-phase fire retardant is a halogen compound, most commonly an aromatic bromine compound. These function through the formation of HBr, which can react with and remove the hydrogen atoms and hydroxyl radicals which make up the flame. Among the nonreactive additives used as flame retardants is alumina trihydrate (ATH). Unlike the reactive halogen flame retardants, ATH reduces flammability by endothermically releasing water as it decomposes. Since this reaction is endothermic, it absorbs the heat of combustion, and thereby lowers the surface temperature near the resin. In addition, the evolved water dilutes the flammable gaseous reactants in the flame. The use of ATH also results in low smoke and toxic gas evolution.5 Phosphorus-containing flame retardants may function in either the vapour or condensed phase. In the vapour phase they act as radical traps similar to the process for a halogen. In the condensed phase, a phosphorus compound may change the degradation pathway of the polymer to promote char formation or the phosphorus compound may form a glassy-like layer which can act as a thermal barrier. A new class of materials, called nanocomposites, avoids one of the disadvantages of the traditional flame retardant systems, since typically the mechanical properties are enhanced. Generally the term ‘‘nanocomposites’’ describes a two-phase material with a suitable nanofiller dispersed in a polymer matrix at the nanometer (10 9) scale.6 Nanocomposites also show increased thermal stability and an improvement in flame retardancy at very low filler levels. The formation of char is responsible for these improved properties.7–9 Nanocomposite formation reduces the heat release rate (HRR), but they typically ignite more easily and all of the polymer will burn. For this reason, it is widely believed that nanocomposite formations combined with other materials may be an effective fire retardant system.10 Several papers from Kashiwagi and coworkers11–13 describe the effect of the addition of carbon nanotubes (CNTs) to polymers; the presence of CNT enhances the thermal stability of the polymer. The CNTs were at least as effective a flame retardant as organoclays and can be used at lower levels, since the char that is formed is denser. This chapter reports on the combination of the CNTs with decabromodiphenyl oxide (DECA), synergized with antimony oxide (Sb2O3), resorcinol diphosphate (RDP) and ATH. The combination of an organically modified clay along with the CNT and the flame retardants has also been examined.

9.2 Experimental 9.2.1 Materials The polymer used in this study was polystyrene (PS) with average molecular weight B230 000, and number average molecular weight B140 000, softening point 107 1C (Vicat, ASTM D 1525) and melt index 7.5 g/10 min (ASTM D

Combination of Carbon Nanotubes with Fire Retardants

127

1238, 200 1C/5 kg). The clay used was: Cloisite 20A (organically modified clay in which the surfactant is dimethyl dihalogenated tallow) and was kindly provided by Southern Clay Products. Multiwall CNTs were kindly provided by Nanocyl S.A., Belgium; DECA and ATH, Martinal OL 104 LE, were provided by Albemarle, while RDP was provided by Supresta Inc. Sb2O3 was kindly provided by Laurel Industries.

9.2.2 Preparation of Composites Nanocomposites were prepared by melt blending, using a Brabender Plasticorder at 190 1C for 10 minutes at 60 revolutions per minute (rpm) and the PS and additives were placed into the mixer at the same time. After blending, the mixture was removed and allowed to cool.

9.2.3 Instrumentation Thermogravimetric analysis (TGA) was performed on a SDT 2960 Simultaneous DTA-TGA unit from TA Instruments, under a constant nitrogen flow of 70 ml min 1; samples were run in duplicate and average values are reported. The experiments were performed at a rate ramp of 20 1C min 1 from 100 to 600 1C. The samples are first equilibrated at 100 1C then heated to 600 1C. Temperatures are considered accurate to 2 1C, while the char remaining at 600 1C is considered to be accurate to 3%. Cone calorimeter analyses were performed on an Atlas Cone 2 instrument using a cone-shaped heater, according to ASTM E 1354 at an incident flux of 35 kW m 2; the spark was continuous for 10 seconds after the sample ignited. All samples were run in triplicate and the average value is reported. The specimens for cone calorimetry were prepared by compression moulding of the sample (about 30 g) into 3  100  100 mm3 square plaques. Typical results from the cone calorimeter are reproducible to 10%, based on many thousands of samples that have been run.

9.3 Results and Discussion In this study representatives of three classes of fire retardants were used: bromine, DECA, synergized with Sb2O3 at a DECA:antimony ratio of 4:1; phosphorus, RDP; and minerals, ATH. The combination of the polymer with the flame retardant additive is considered first of all, and then combinations of the flame retardants and nanomaterials are examined.

9.3.1 Thermogravimetric Analysis The important parameters from the TGA curves are the onset temperature of the degradation, which is taken as the temperature at which 10% of the sample

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mass is lost, T10; the mid-point temperature of the degradation, T50; and the non-volatile residue at 600 1C, or char. Various loadings of ATH were melt blended with PS and the results are summarized in Table 9.1 and Figure 9.1. The temperature at which 10% mass loss occurs decreases as the amount of ATH increases, which may result from ATH decomposition, but the T50 increases, indicating enhanced thermal stability. The amount of residue is what is expected based on the ATH loading.

Table 9.1

TGA summary results for PS with fire retardants.

PS PS–15%RDP PS–30%RDP PS–7%ATH PS–14%ATH PS–21%ATH PS– 28%ATH PS–5%DECA–1.25%Sb2O3 PS–10%DECA–2.5%Sb2O3 PS–15%DECA–3.75%Sb2O3 PS–20%DECA–5%Sb2O3

Figure 9.1

T10 (1C)

T50 (1C)

Char (%)

402 391 391 396 388 388 381 382 379 376 377

426 428 436 427 429 435 439 412 406 402 401

0 1 2 5 10 14 19 2 3 5 6

TGA results for PS–ATH.

129

Combination of Carbon Nanotubes with Fire Retardants 14

Previous work in this laboratory was carried out on PS with RDP. These results are also presented in Table 9.1; the combination of RDP and PS leads to a decrease in the onset temperature and an increase in the mid-point temperatures, which may be due to the greater thermal stability of RDP. The combinations of DECA and antimony trioxide with PS are also reported in Table 9.1 and the TGA curves are given in the Figure 9.2; the onset temperature and T50 are decreased in the presence of the additives. To investigate possible synergy between nanodimensional materials and flame retardants with PS, the same loadings of fire retardants were used and an initial survey was performed at 1.5% CNT loading; the data are collected in Table 9.2 and the TGA curves are given in Figure 9.3. Except for the case of DECA, both T10 and T50 increase when both additives are present and the amount of char is greater than expected. Due to the rather considerable price of CNT, it was decided to reduce the CNT loading to 0.2 or

Figure 9.2

Table 9.2

TGA results for PS–DECA.

TGA summary results for PS–1.5%CNT–fire retardants.

PS PS–1.5%CNT PS–1.5%CNT–7%ATH PS–1.5%CNT–1.5%RDP PS–1.5%CNT–30%RDP PS–1.5%CNT–7%DECA

T10 (1C)

T50 (1C)

Char (%)

402 409 408 422 423 393

426 438 452 456 460 422

0 0 14 5 4 2

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Chapter 9

Figure 9.3

TGA results for PS–CNT–fire retardants.

Figure 9.4

TGA results for PS–0.2%CNT and PS0.5%CNT.

0.5%. As can be seen in Figure 9.4, the addition of this low amount of CNT has essentially no effect on the TGA parameters for PS. The combination of CNT with DECA and Sb2O3, reported in Table 9.3 and Figure 9.5, shows no increase in thermal stability by TGA.

131

Combination of Carbon Nanotubes with Fire Retardants

Table 9.3

TGA results for PS–CNT–DECA.

PS PS–0.2%CNT PS+0.2%CNT–5%DECA–1.25%Sb2O3 PS–0.2%CNT–10%DECA–2.5 %Sb2O3 PS–0.2%CNT–15%DECA–3.75%Sb2O3 PS–0.5%CNT PS–0.5%CNT–5%DECA–1.25%Sb2O3 PS–0.5%CNT–10%DECA–2.5%Sb2O3 PS–0.5%CNT–20%DECA–5%Sb2O3

Figure 9.5

T10 (1C)

T50 (1C)

Char (%)

402 400 377 363 372 402 375 376 377

426 424 411 400 400 425 411 405 397

0 1 2 2 6 1 2 3 5

TGA results PS–CNT–DECA.

Figure 9.6 gives the TGA curves of PS and 0.2% CNT with various loadings of ATH; the TGA results are summarized in Table 9.4. Note that the temperature for 10% degradation does not depend on ATH concentration until the ATH reaches 35%, when it drops dramatically. This is likely due to the degradation of ATH. As was previously seen with flame retardants only, the 50% mass-loss temperature exhibits a considerable increase with an increase of the ATH loading. When 1.5% CNT was used, the amount of char was much greater than expected, but at the lower amounts of CNT, the char yields simply reflect the ATH loading. This suggests that there may be some interaction between CNT and ATH at high CNT loading, but not at low loading. The type of interaction has not yet been identified and this is being actively pursued.

132

Figure 9.6

Table 9.4

Chapter 9

TGA results for PS–CNT–ATH.

TGA results for PS–CNT–ATH. T10 (1C)

T50 (1C)

PS PS–0.2%CNT PS–0.5%CNT

402 400 402

426 423 425

0 0 0

PS–0.2%CNT–7%ATH PS–0.2%CNT–14%ATH PS–0.2%CNT–21%ATH PS–0.2%CNT–28%ATH PS–0.2%CNT–35%ATH

392 400 402 395 364

426 432 436 435 442

4 10 13 15 23

PS–0.5%CNT–7%ATH PS–0.%CNT–14%ATH PS–0.5%CNT–21%ATH PS–0.5%CNT–28%ATH

399 398 394 380

430 432 437 440

7 9 15 20

9.3.1.1

Char (%)

Addition of Clay to CNT-containing Systems

Beyer has claimed some synergy when CNT and clay are combined.15 Accordingly, an organically modified montmorillonite (Cloisite 20A) and CNT were combined with ATH and the results are given in Table 9.5 and Figures 9.8 to 9.12. The addition of 2% clay to PS gives no change in T10 but a significant increase in T50, shown in Table 9.5 and Figure 9.7. The addition of clay to the ATH-CNT system shows a significant increase in both T10 and T50, shown in Figures 9.8 to 9.11; Figure 9.12 provides a summary for all of amounts of ATH.

133

Combination of Carbon Nanotubes with Fire Retardants

Table 9.5 PS PS–2%clay

TGA results for PS–clay. T10 (1C)

T50 (1C)

Char (%)

402 404

426 444

0 0

Figure 9.7

TGA results for PS–clay.

Figure 9.8

TGA results for PS nanocomposites with clay and 7% ATH.

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Chapter 9

Figure 9.9

TGA results for PS nanocomposites with clay and 14% ATH.

Figure 9.10

9.3.1.2

TGA results for PS nanocomposites with clay and 21% ATH.

Summary of the TGA Data

The addition of either 0.2 or 0.5% CNT to a PS–DECA or PS–ATH system has little effect on the TGA parameters. When 1.5% CNT is used, both T10 and T50 are increased relative to the use of the flame retardant alone. The combination of CNT and an organically modified clay with ATH leads to a significant enhancement in T50.

Combination of Carbon Nanotubes with Fire Retardants

Figure 9.11

TGA results for PS nanocomposites with clay and 28% ATH.

Figure 9.12

TGA results for PS with CNT, clay and various amounts of ATH.

135

9.3.2 Cone Calorimeter Evaluation The evaluation of the fire properties of the polymer systems uses the cone calorimeter. The parameters that are investigated with cone calorimetry are the:  time to ignition (TTI);  HRR, and especially its peak value (PHRR);

136

Chapter 9

 total heat release (THR);  specific extinction area (SEA), a measure of the amount of smoke produced during the combustion; and  mass-loss rate (MLR). Cone calorimetry provides important information on the fire properties of materials and it also provides useful information on nanocomposite formation. In work carried out in these laboratories, it has been observed that polymer– clay microcomposites do not show significant reduction in PHRR, but nanocomposites do show a significant reduction; this means that one can evaluate nanocomposite formation using the cone calorimeter.16–18 The amount of the reduction in the case of PS is up to 60%.19,20 As with the TGA work, we begin by showing the cone results for the combination of PS with only the fire retardants (Table 9.6). With the PS–RDP system, there is a 64% reduction in PHRR at 30% loading with a 55% reduction in the THR; the TTI also shows a small increase. In the ATH system, shown in Figure 9.13, the reduction in PHRR varies from 14 at 7% ATH to 53% at 28% ATH. There is not much change in the THR and the TTI was constant, except at 28% ATH where a large increase is seen. When ATH is present, these curves are similar to those reported by Schartel and Hull for thick samples, without charring at low amounts of ATH and with charring at high amounts.21 For DECA–Sb2O3, a reduction of 64% was obtained at 25% additive loading (20%DECA–5%Sb2O3), the PHRR is about the same at 15% DECA and it is lower at both 5% (26%) and 10% (43%) DECA.

Table 9.6

Cone calorimetry summary of PS–fire retardant systems.

Formulation

PHRRa (kW m 2) (% reduction)

THRb (MJ m 2)

ASEAc (m2 kg 1)

AMLRd (g s 1 m 2)

TTIe(s)

PS PS–15%RDP PS–30%RDP

1160  43 592  30 (49) 420  30 (64)

91  7 57  5 41  3

1330 1551 1852

27.4  1.1 15  1 14  1

60  0 63  1 77  6

PS–ATH7% PS–ATH14% PS–ATH21% PS–ATH28%

1002  102 (14) 787  18 (32) 608  21 (48) 550  24 (53)

90  4 93  1 89  3 84  2

1240  25 1214  12 1243  79 1309  98

23.5  0.6 20.1  0.7 13.2  0.7 9.5  0.1

37  1 37  4 37  2 51  2

821  25 (26) 662  229 (43) 487  63 (61)

52  7 34  3 27  2

1705  74 2333  74 2178  23

25.7  6.4 29.3  2.6 32.9  2.8

45  6 57  2 55  2

422  20 (64)

30  0

2127  36

33.7  1.7

55  6

PS–5%DECA–1.25%Sb2O3 PS–10% DECA–2.5%Sb2O3 PS–15%DECA– 3.75%Sb2O3 PS–20%DECA–5%Sb2O3 a

PHRR, peak heat release rate. THR, total heat released. ASEA, average specific extension area. d AMLR, average mass-loss rate. e TTI, time to ignition. b c

Combination of Carbon Nanotubes with Fire Retardants

Figure 9.13

HRR curves for PS–ATH.

Figure 9.14

HRR curves for PS–DECA.

137

The reduction in THR is 43% at the lowest DECA loading (5%) and 67% at the highest loading (20%), while the TTI shows no significant increase. The HRR curves are shown in Figure 9.14, and are similar to those of thick, noncharring samples when both DECA and Sb2O3 are present.

138

Figure 9.15

Table 9.7

Chapter 9

HRR results for PS–nanocomposites.

Cone results for PS–CNT–fire retardants.

Formulation

PHRRa (kW m 2) (% reduction)

THRb (MJ m 2)

ASEAc (m2 kg 1)

AMLRd (g/s 1 m 2) TTIe(s)

PS PS–1.5%CNT–15%RDP PS–1.5%CNT–30%RDP PS–1.5%CNT–7%ATH PS–1.5%CNT–7%DECA PS–1.5%CNT

1362 504  16 (63) 419  14 (69) 422  20 (69) 732  64 (46) 728  222 (47)

109 67  3 53  1 77  9 70  1 74  60

1507 2311  44 2483  161 1523  76 1904  105 1411  250

29.1 20.7  0.5 21.2  1.7 10.4  0.5 25.7  0.5 18.5  4.8

53 51  3 52  1 31  3 41  4 37  2

a

PHRR, peak heat release rate. THR, total heat released. c ASEA, average specific extension area. d AMLR, average mass-loss rate. e TTI, time to ignition. b

The cone results for the fire retardant systems that contain 1.5% CNT are shown in Figure 9.15 and the data are summarized in Table 9.7. The addition of 1.5% CNT to PS brings about a large reduction in PHRR, and the curves become much flatter, as well as a large reduction in THR. The reduction in THR is surprising since there is no change upon the addition of clay. The reduction in PHRR is much larger in the presence of 1.5% CNT than it is for the additive alone, except for the case of 30% RDP. When the amount of CNT used was 0.2 or 0.5%, there was no effect on the PHRR of PS, unlike the situation for 1.5% CNT; this is shown in Figure 9.16.

Combination of Carbon Nanotubes with Fire Retardants

Figure 9.16

139

HRR results for PS with 0.2 and 0.5%CNT.

This plot also shows that the dramatic decrease in the TTI at 0.2% loading (which perhaps results from an increase in melt viscosity) is independent of the reduction in PHRR, which is often ascribed to the formation of a barrier. This is a suitable topic for further investigation because it implies that there must be some interaction between the larger amount of CNT and the flame retardants, as noted earlier from TGA data. The HRR plots for the additives plus clay are given in Figure 9.17. The shape of the curves shows a drastic change compared to PS or PS–CNT. The presence of the additives makes the sample thermally thicker. The cone data of the PS–CNT–DECA system are gathered and reported in Table 9.8. For DECA alone at 10%, there is a 43% reduction, while when either 0.2 or 0.5% CNT is added, the reduction in PHRR is better than that seen with only 20% DECA – 69%. This must be contrasted to the case where 1.5% CNT was added and there was no change in the PHRR. The results when various amounts of ATH and 0.2 and 0.5% CNT are combined are shown in Table 9.9 and Figure 9.18. The reduction in PHRR is about the same for each ATH loading at both 0.2 and 0.5% CNT. The addition of the CNT causes a decrease in the PHRR, which varies between 5 and 8%. This is within the typically quoted error bars on cone measurements, so there may be no change; most likely there is no additional effect on the combination of ATH and CNT with PS. TTIs for both 0.2 and 0.5% CNT loadings decrease compared with the pure polymer, but this decrease is not a function of loading. THR is unchanged, except for those systems which contain CNT and the highest loading of ATH. Since the THR is unaffected by the addition of either

140

Chapter 9

Figure 9.17

Table 9.8

HRR results for PS nanocomposites containing DECA.

Cone results for PS–CNT–DECA.

Formulation PS PS–5%DECA– 1.25%Sb2O3 PS–5%DECA– 1.25%Sb2O3– 0.2%CNT PS–5%DECA– 1.25%Sb2O3– 0.5%CNT PS–10%DECA– 2.5%Sb2O3 PS–10%DECA– 2.5%Sb2O3– 0.5%CNT PS–20%DECA– 5%Sb2O3 PS–20%DECA– 5%Sb2O3– 0.5%CNT a

PHRRa (kW m 2) (% reduction)

THRb (MJ m 2)

ASEAc (m2 kg 1)

AMLRd (g s 1 m 2)

TTIe (s)

1208  127 821  25 (26)

105  4 52  7

1362  33 1705  74

27.7  2.1 25.7  6.4

60  1 45  6

646  8 (42)

46  3

2063  40

30.3  1.7

42  3

596  95 (47)

49  4

1959  102

29.3  1.4

37  1

662  229 (43)

34  3

2333  74

29.3  2.6

57  2

380  22 (69)

31  0

2608  45

28.7  0.6

45  4

422  20 (64)

30  0

2127  36

33.7  1.7

55  6

394  23 (67)

24  13

1961  126

28.3  2.9

41  2

PHRR, peak heat release rate. THR, total heat released. c ASEA, average specific extension area. d AMLR, average mass-loss rate. e TTI, time to ignition. b

141

Combination of Carbon Nanotubes with Fire Retardants

Table 9.9

Cone results for PS–CNT–ATH.

Formulation

PHRRa (kW m 2) (% reduction)

PS PS–0.2%CNT PS–0.5%CNT PS–2%clay

1076  41 1110  65 (0) 1013  45 (9) 971  35 (10)

PS PS–0.2%CNT–7%ATH PS–0.2%CNT–14%ATH PS–0.2%CNT–21%ATH PS–0.2%CNT–28%ATH PS–0.2%CNT–35%ATH

1233  93 926  44 761  34 579  33 486  37 397  22

PS PS–CNT 0.5%–ATH7% PS–CNT0.5%–ATH14% PS–CNT. 5%–ATH21% PS–CNT0.5%–ATH28%

THRb (MJ m 2)

ASEAe (m2 kg 1)

AMLRd (g s 1 m 2)

TTIe (s)

88  3 96  2 96  1 94  2

1113  36 1197  19 1239  11 1232  17

29.4  1.2 27.3  1.7 25.6  0.6 25.6  0.6

56  8 41  1 39  2 59  3

(25) (38) (53) (61) (68)

91  2 92  3 90  1 83  4 82  4 73  0

1122  60 1227  21 1259  25 1233  24 1205  46 1106  37

31.5  2.7 25.0  0.8 18.8  0.9 13.3  0.2 11.3  0.8 6.6  0.3

53  2 28  3 32  2 31  3 31  0 38  2

1208  127 798  20 (34) 600  16 (50) 516  22 (57) 459  20 (62)

105  4 105  3 100  0 94  5 84  1

1362  33 1422  38 1363  10 1342  29 1338  40

27.7  2.1 19.6  0.2 13.2  0.3 10.4  0.1 8.4  0.2

60  1 29  3 31  2 36  1 38  5

a

PHRR, peak heat release rate. THR, total heat released. ASEA, average specific extension area. d AMLR, average mass-loss rate. e TTI, time to ignition. b c

Figure 9.18

HRR results for PS–ATH–0.2%CNT.

142

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of these separately, this suggests that some synergistic or additive effect may be occurring; this requires further investigation.

9.3.2.1

Combination of Clay and CNT

The cone calorimeter results for the combinations of clay with CNT and ATH are shown in Table 9.10 and the heat-release plots are provided in Figures 9.19 to 9.23. For comparison, the PHRR decreases by about 50% upon the addition of 3% of this clay.22 At low ATH loadings, especially 7% ATH, there is a very large decrease in the PHRR upon the addition of the clay. As the ATH loading increases, the values of the PHRR come closer together so that at 21 or 28% ATH loading, there is essentially no difference in the PHRR in the presence or absence of the clay.

9.3.2.2

Summary of Cone Calorimetric Data

The combination of 10% DECA and 0.5% CNT gives the same value for the PHRR as is obtained when only 20% DECA is used so clearly this is an Table 9.10

Cone results for PS–fire retardants–nanocomposites. PHRRa (kW m 2) (% reduction)

THRb (MJ m 2)

ASEAc (m2 kg 1)

AMLRd (g s 1 m 2)

TTIe (s)

PS PS–0.2%CNT–7%ATH– 2%clay PS–.2%CNT0–14%ATH– 2%clay PS–0.2%CNT–21%ATH– 2%clay PS–0.2%CNT–28%ATH– 2%clay PS–0.2%CNT–35%ATH– 2%clay

1076  41 579  41 (46)

88  3 89  2

1113  36 1387  13

29.4  1.2 16.2  0.7

56  8 29  4

506  15 (53)

88  1

1353  10

11.3  0.2

31  1

397  12 (63)

81  1

1317  66

7.8  0.2

31  1

328  8 (69)

75  1

1229  48

6.1  0.2

36  2

309  12 (71)

65  0

1159  13

6.0  0.1

37  2

PS–0.5%CNT–7%ATH– 2%clay PS–0.5%CNT–14%ATH– 2%clay PS–0.5%CNT–21%ATH– 2%clay PS–0.5%CNT–28%ATH– 2%clay PS–0.5%CNT–35%ATH– 2%clay

591  14 (45)

91  1

1391  15

15.9  0

30  2

494  9 (54)

86  0

1413  11

10.9  0.2

30  1

382  9 (65)

79  2

1362  61

5.3  4.6

31  1

352  17 (67)

73  0

1242  35

7.1  0.3

37  1

317  21 (71)

68  1

1071  58

5.9  0.3

42  1

Formulation

a

PHRR, peak heat release rate. THR, total heat released. ASEA, average specific extension area. d AMLR, average mass loss rate. e TTI, time to ignition. b c

Combination of Carbon Nanotubes with Fire Retardants

Figure 9.19

HRR results for PS–fire retardants–nanocomposites.

Figure 9.20

HRR results for PS–fire retardants–nanocomposites.

143

advantageous system since it permits one to significantly decrease the halogen loading. It is surprising that at 1.5% CNT there is no change in the PHRR because of the presence of the clay. This suggests that the amount of CNT is very important. In contrast, with ATH at low loading, 0.2 or 0.5%, there does

144

Chapter 9

Figure 9.21

HRR results for PS–fire retardants–nanocomposites.

Figure 9.22

HRR results for PS–fire retardants–nanocomposites.

Combination of Carbon Nanotubes with Fire Retardants

Figure 9.23

145

HRR results for PS–fire retardants–nanocomposites.

not seem to be an effect, but at 1.5% there is a definite interaction between ATH and CNT. For this system, there does not appear to be any advantage in combining an organically modified clay with CNT when evaluated by cone calorimetry.

9.4 Conclusions The combination of CNTs at 0.2 or 0.5% with either ATH or RDP does not appear to give any significant enhancement in thermal stability as measured by TGA or in fire properties as measured by cone calorimetry compared to the use of the flame retardant alone. When 1.5% CNTs are used, there is apparently an interaction between ATH and the CNT which is manifested in a large amount of char and a greatly decreased value for the PHRR. Clearly some synergistic interaction occurs between CNT and DECA at the low, 0.2 or 0.5% CNT, loading and the reduction in the PHRR at 10% DECA–CNT is the same as that observed for 20% DECA alone. It is very clear that there is some potential advantage in combining CNT with fire retardants in some polymers. The challenge that faces us is to identify the materials which should be used, meaning the fire retardant and the polymer. In addition, the amount of material that should be added must also be ascertained through a trial-and-error process. In this work, it has been shown that there is some advantage in combining 1.5% CNT with ATH in PS. To be practical, the amount of CNT must be reduced and it has been shown that reducing it to 0.5% is too little. It may be that there are other polymers, for instance a

146

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polyolefin like polyethylene or polypropylene, that can be adequately protected by the addition of CNT with a fire retardant. Future work is required to determine this.

References 1. S.V. Levchik, in Flame retardant polymer nanocomposites, A.B. Morgan and C.A. Wilkie eds., Wiley Interscience, 2007, pp. 1–29. 2. M. Hirschler, in Fire and Polymers: Materials and Solutions for Hazard Prevention. Eds. G.L. Nelson and C.A. Wilkie, ACS Symposium Series 797, American Chemical Society, Washington, DC, 2001, pp. 293–306. 3. D.A. Purser, Polym. Int., 2000, 49, 1232. 4. D.J. Irvine, J.A. McCluskey and I.M. Robinson, Polym. Degrad. Stab., 2000, 67, 383. 5. S. Salman and D. Klempner, Plast. Eng., 1979, 35, 39. 6. G. Beyer, Polym. Adv. Tech., 2006, 17, 218. 7. G. Beyer, Polym. News, 2001, 26, 370. 8. M. Le Bras, G. Camino, S. Bourbigot and R. Delobel eds., Fire Retardancy of Polymers: The use of intumescence., Royal society of Chemistry, Cambridge, 1998. 9. J.W. Gilman, T. Kashiwagi and J. Lichtenhan, SAMPE J, 1997, 4, 40. 10. Y. Hu and L. Song, in Flame retardant polymer nanocomposites, A.B. Morgan and C.A. Wilkie eds., Wiley Interscience, 2007, pp. 191–233. 11. T. Kashiwagi, E. Grulke, J. Hilding, R. Harris, W. Awad and J. Douglas, Macromol. Rapid Comm., 2002, 23, 761. 12. T. Kashiwagi, E. Grulke, J. Hilding, K. Grith, R. Harris, K. Butler, J. Shields, S. Kharchenko and J. Douglas, Polymer, 2004, 45, 4227. 13. T. Kashiwagi, F. Du, K. Winey, K. Groth, J. Shields, S. Bellayer, H. Kim and J. Douglas, Polymer, 2005, 46, 471. 14. G. Chigwada and C.A. Wilkie, Polym. Degrad. Stab., 2003, 80, 551. 15. G. Beyer, Fire Mater., 2005, 29, 61. 16. S. Su, D.D. Jiang and C.A. Wilkie, Polym. Degrad. Stab., 2004, 83, 301. 17. S. Su, D.D. Jiang and C.A. Wilkie, Polym. Degrad. Stab., 2004, 83, 321. 18. S. Su, D.D. Jiang and C.A. Wilkie, Polym. Adv. Tech., 2004, 15, 225. 19. J. Zhu, P. Start, K.A. Mauritz and C.A. Wilkie, Polym. Deg. Stab., 2002, 77, 253. 20. M. Zanetti, G. Camino, D. Canavese, A.B. Morgan, F.J. Lamelas and C.A. Wilkie, Chem. Mater., 2002, 14, 189. 21. B. Schartel and T.R. Hull, Fire Mater., 2007, 31, 327. 22. J. Zhu and C.A. Wilkie, Polym. Int., 2000, 49, 1158.

CHAPTER 10

Significant Assessment of Nanocomposites’ Combustion Behaviour by the Appropriate Use of the Cone Calorimeter A. FINA, F. CANTA, A. CASTROVINCI1 AND G. CAMINO Centro di Cultura per l’Ingegneria delle Materie Plastiche – Politecnico di Torino, V.le T. Michel, 5-15100 Alessandria, Italy; 1 Current address: University of Applied Sciences of Southern Switzerland (SUPSI), Galleria 2, CH-6928 Manno, Switzerland

10.1 Introduction The cone calorimeter has become one of the most important tools for research and development of fire-retardant polymeric materials. Several parameters are obtained from this test,1 but the most often used is the peak of heat release rate (PHRR). However, a major problem arises from the simple comparison of PHRR values, which may be misleading when polymer materials to be compared are characterized by very different combustion behaviours. In particular, charring strongly affects the shape of the HRR curve. Char-forming materials show the PHHR at an early stage of the combustion because of the protective action of the char building up on the surface of the burning sample, whereas non-charring materials (including most of the thermoplastic polymers) show the PHHR at the end of the burning process.2–4

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

147

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Chapter 10

This is often the case when comparing pure bulk polymers, which usually show limited or no charring, and their correspondent nanocomposites, which often show the char-forming behaviour. In the past, the observed reductions in PHHR (up to 70%) for nanocomposites were directly related to improvements in the fire resistance, which now appear to be largely overestimated. This is also because combustion cone-calorimeter tests represent only a single and peculiar fire scenario, namely a well-ventilated forced combustion condition.4 The PHHR at the end of the burning phase for non-charring polymers is caused by the thermal feedback that occurs when the pyrolysis zone reaches the rear surface of the sample. During combustion, part of the heat is absorbed by the underlying material, but when the pyrolysis front approaches the base of the specimen bottom, heat is no longer removed by any remaining underlying material, which results in an increased material-heating rate. This results in a faster pyrolysis and subsequent increase of the measured HRR. This particular behaviour is determined by the standard insulated sample holder set-up in the cone calorimeter. Indeed, the large PHHR at the end of burning of non-charring materials has been demonstrated to disappear using a modified sample holder that conducts the heat away from the back of the sample by a ‘‘heat sink’’ effect.4,5 However, PHRR values for charring materials are observed just after ignition in the earlier stages of the protective char-layer build-up and usually the shape of the HRR curve is not significantly affected by thermal feedback, so that experimental set-up plays a minor role on the PHRR for these materials. Based on these observations, to evaluate fire performance in terms of PHRR for charring materials in comparison with non-charring polymer is certainly problematic. In this chapter, a method to assess the actual fire performance of nanocomposites is proposed in terms of a ‘‘steady value of HHR’’, defined as the constant rate of combustion observed in the time range between ignition and the occurrence of a thermal feedback contribution.

10.2 Experimental 10.2.1

Materials

A commercial grade of polyamide 6 from Rhodia, PA6 Technyl C206, was used (density 1.14 g cm
3, melting point 222 1C). The nanofiller used is Cloisite 30B from Southern Clay Products Inc, a natural montmorillonite modified by ionexchange with a methyl, tallow, bis(2-hydroxyethyl), quaternary ammonium chloride. The choice of components used was based on results published in the literature which show optimal nanoclay dispersions into a PA6 matrix and the formation of an exfoliated nanocomposite structure.6–9

10.2.2

Preparation and Characterization

The nanocomposite was prepared by melt processing using a Leistritz 27 co-rotating twin-screw extruder (d ¼ 27 mm, l/d ¼ 40) equipped with a low

149

Significant Assessment of Nanocomposites’ Combustion Behaviour

Temperature profile

Figure 10.1

Screw and temperature profiles used in compounding.

shear-stress screw profile (Figure 10.1), with 6 kg h
1 throughput and 100 revolutions per minute (rpm) screw speed. Polymer was dried for 4 hours at 80 1C and nanoclay for 4 hours at 100 1C to eliminate moisture before processing. Nanoclay was added at 5% weight percent (wt%), by a dedicated sidefeeder at 14 diameters length. Unfilled polymer was processed under the same conditions as the composites and a reference material. Plate specimens (100  100  8 mm) for the cone calorimeter tests were prepared by injection moulding using a Demag INT Elect 100/400 hydraulic press with the two heated plates at 60 1C using a maintenance pressure of 25 kPa (250 bar) for a cycle time of 61 s. Compounds were dried for 4 hours at 100 1C before processing to eliminate moisture. As the available injection moulding apparatus does not allow the production of specimens thicker than 8 mm, pairs of 8 mm specimens were laid one upon the other to obtain 16 mm thick samples. Improved thermal contact was achieved by gluing the two specimens together with a very small amount of cyanoacrylate glue. Composition and abbreviations for samples are given in Table 10.1 in which P stands for unfilled polymer, C for nanocomposite and the two associated figures stand for sample thickness and heat flux, respectively.

Table 10.1

Samples and imposed external heat fluxes.

Material PA6

Thickness (mm) 8 16

PA6+5 wt% Cloisite 30B

8 16

Heat flux (kW m
2)

Abbreviation

50 75 50 75 50 75 50 75

P8
50 P8
75 P16
50 P16
75 C8
50 C8
75 C16
50 C16
75

150

Chapter 10

X-Ray diffraction (XRD) patterns were obtained on a Thermo ARL XTRA48 diffractometer using Cu Ka radiation (l ¼ 1.54062 A˚) and used to evaluate the nanoclay dispersion.

10.2.3

Combustion Tests

Combustion tests were carried out on a Fire Testing Technology (FTT) cone calorimeter apparatus. The tests were performed in the horizontal position without using the retainer frame and therefore surface-dependent data were calculated using the entire specimen area (100 cm2). Specimens were wrapped in aluminium foil leaving an upper edge of 3 mm and placed on ceramic backing boards at a distance of 25 mm from the cone base (Figure 10.2). This non-standard experimental set-up was adopted to prevent dripping of molten material, which is particularly significant for the pure polymer. External heat flux was set at either 50 or 75 kW m
2. These values are comparable to conditions of medium- and well-developed fire scenarios, respectively,4,10 and lead to reasonable burning times for the largest specimen thicknesses used; lower heat fluxes would result in very time-consuming experiments. All the tests were performed at least in duplicate and the average values are reported herein, along with their deviations with respect to the average. Irradiances were measured by placing the Schmidt-Boelter heat-flux meter (Medtherm corp, mod. GWT-10-32-485A) of the cone calorimeter equipment at various distances from the cone base, in the centre of specimen position.

Figure 10.2

Specimen mounting set-up.

151

Significant Assessment of Nanocomposites’ Combustion Behaviour 6000

5000

PA6 + 5wt.% Cloisite 30B Cloisite 30B

Intensity [cps]

4000

3000

2000

1000

0 2

Figure 10.3

4

6

8 2θ angle [°]

10

12

14

XRD patterns for Cloisite 30B and PA6+Cloisite 30B nanocomposite.

10.3 Results and Discussion PA6 nanocomposite morphology was evaluated by XRD, which showed no diffraction peaks related to the nanoclay interlayer distance (Figure 10.3), suggesting exfoliation of nanoclay platelets in the PA6 matrix. This is in agreement with results from multitechnique characterization [XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), rheology, nuclear magnetic resonance (NMR)] reported elsewhere9 on an equivalent PA6 nanocomposite prepared in the same experimental conditions. Heat-release parameters obtained from cone calorimeter analyses on 8 and 16 mm specimens are discussed below, including PHRR, average heat release rate (AHRR, the average value between time to ignition and end of test, obtained with ISO5660 criterion), and total heat evolved (THE), as well as time-to-ignition (TTI) and mass loss. The discussion of other parameters obtained from the same analyses, such as smoke, CO and CO2 production, is outside the objective of this work.

10.3.1

8 mm Specimens Combustion Behaviour

Results obtained from P-8 mm and C-8 mm samples are summarized in Table 10.2. A reduction of TTI is generally observed with increasing heat flux, because of the higher heating rate on the specimen surface, e.g. for neat PA6, TTI is 84 s at 50 kW m
2 and 38 s at 75 kW m
2.

152

Table 10.2

P8
50 P8
75 C8
50 C8
75

Chapter 10

Cone calorimeter results for 8 mm thick sample. TTI (s)

Mass loss (%)

PHRR (kW m
2)

AHRR (kW m
2)

THE (MJ m
2)

84  6 38  1 54  4 30  3

100  1 100  1 97  1 97  1

1123  56 1466  124 480  14 663  24

604  18 745  19 364  20 504  17

263  1 263  2 245  1 244  4

At both heat fluxes, a significant reduction of TTI for the nanocomposite is observed as compared with that of pure PA6, with ignition at 54 s for 50 kW m
2 and at 30 s for 75 kW m
2. Both lower11,12 and higher13,14 TTI values have been previously reported in the literature for layered silicate PA6 nanocomposites compared with reference PA6 and generally show little agreement. From a general point of view, a variable trend for TTI is observed for nanocomposites compared to pure polymers, which may increase or decrease depending on the type of polymer and nanofiller, as well as on processing and testing conditions.15 The anticipated ignition observed in this work could be ascribed to the thermal instability of the quaternary alkyl ammonium nanofiller organomodifier and/or to the physical effects of nanoclay on the specimens’ surfaces. Indeed, nanoclay platelets may affect the flow of molten material prior to ignition by the well-known increase in the molten viscosity,9,16–18 thereby modifying thermal convection flow and resulting in a higher surface-heating rate. However, the mechanistic study of ignition in PA6 nanocomposites requires further studies beyond the objective of this work. Comparison between pure PA6 and nanocomposite in terms of THE shows a limited reduction for the nanocomposite at both heat fluxes, which is only slightly higher than that calculated on the basis of the nanoclay inorganic content. This means that nanoclay is not effective at preventing PA6 from combustion; nevertheless, strong effects on the HHR are induced by the nanoclay in PA6. HHR curves for 8 mm specimens are reported in Figure 10.4. Pure PA6 shows the typical plot for a non-charring polymer, at both 50 and 75 kW m
2, evidenced by the peak at the end of the burning process. Despite the curve being strongly asymmetric, no HRR plateau is observed, showing an intermediate behaviour between the typical thermally thin and thermally thick behaviours.2,4 In contrast, the nanocomposite shows a quasi steady-state combustion at both 50 and 75 kW m
2, with plateaus at ca. 440 kW m
2 and 630 kW m
2, respectively. It is furthermore clearly observable that specimens burned under the higher external heat flux (75 kW/m2) show higher HHRs over all the well-sustained combustion time range as compared with the corresponding material tested at 50 kW/m2. At the lower heat flux, the PHRR values are 1123 kW/m2 for PA6 and 480 kW/m2 for the nanocomposite; whereas at 75 kW/m2 values are increased to 1466 kW/m2 and 663 kW/m2, respectively. Based on the PHRR reductions (ca. 55% at both heat fluxes) between pure polymer and nanocomposites, one

153

Significant Assessment of Nanocomposites’ Combustion Behaviour 1600 P8-50 1400

HRR [kW/m2]

C8-50 1200

P8-75

1000

C8-75

800 630 kW/m2 600 440 kW/m2 400 200 0 0

100

200

300

400

500

600

700

800

Time [s]

Figure 10.4 Cone calorimeter HRR curves for 8 mm thick specimens at 50 and 75 kW m
2.

could predict a large improvement in fire performance for the nanocomposite compared with the pure PA6. However, the different contributions of thermal feedback in the two different materials mean that simple peak comparison is certainly misleading. The average HRR value could be considered a slightly more reliable parameter, reducing the effect of the error brought by this final peaking effect. Based on average HRR values reported in Table 10.2, a significant reduction in combustion rate is still evident when comparing pure polymer with its nanocomposite analogue, amounting to ca. 40% at 50 kW m
2 and ca. 32% at 75 kW m
2. The use of the HRR at the curve shoulder observed in intermediate-thickness specimens could, in principle, be used; however, the determination of an accurate value appears to be subjective in most cases, including the case of 8 mm thick PA6 plots here discussed. The proposal in this chapter is to evaluate the effectiveness of nanoclay in reducing HHR when transient contributions unrepresentative of the material burning are eliminated. To reach this condition, specimens with higher thickness were tested.

10.3.2

16 mm Specimens Combustion Behaviour

Results obtained from analyses of results from P-16 mm and C-16 mm samples at different heat fluxes are summarized in Table 10.3.

154

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Cone calorimeter results for 16 mm thickness sample.

Table 10.3

P16
50 P16
75 C16
50 C16
75

TTI (s)

Mass loss (%)

pkHRR (kW m
2)

avHRR (kW m
2)

Plateau HRR (kW m
2)

THE (MJ m
2)

71  3 28  1 60  2 21  1

100  1 100  1 97  1 96  1

870  9 1192  146 395  10 605  7

557  8 781  25 291  24 419  17

580  40 940  30



502  3 529  3 506  30 484  10

1200

P16-50

1000

940 kW/m2

C16-50 P16-75 C16-75

HRR [kW/m2]

800 605 kW/m2 580 kW/m2

600

400 395 kW/m2

200

0 0

Figure 10.5

500

1000 Time [s]

1500

2000

Cone calorimeter HRR curves for 16 mm thick samples at 50 and 75 kW m
2.

The effect of the nanofiller on TTI at both 50 and 75 kW m
2 follows the same trend observed for 8 mm specimens. At 50 kW/ m
2 the pure PA6 ignites at 71 s and the composite at 60 s; at 75 kW m
2 pure PA6 ignites at 28 s and the composite at 21 s. However, a general reduction of TTI in these thicker samples is observed, though no significant differences are expected when passing from intermediate thickness specimens to thermally thick ones in terms of ignition time.19 As expected, the total heat released for 16 mm specimens is double that of the correspondent 8 mm tests. The main features of the HRR plots obtained with 16 mm specimens (Figure 10.5) are the different shapes with respect of those obtained with 8 mm specimens, both for neat PA6 and the nanocomposite. P16
50 shows an HRR plateau at ca. 580 kW m
2, followed by the wellknown peak (870 kW m
2) at the end of combustion, whereas values for P16
75

155

Significant Assessment of Nanocomposites’ Combustion Behaviour
2


2

are ca. 940 kW m for the plateau and 1192 kW m for PHRR. Note that plateau values are not perfectly defined, but show a drop in the HRR curve at about half the time between ignition and flameout, probably because of the imperfect thermal contact between the two 8 mm specimens laid one upon the other. This observation shows that a single solid block is to be preferred, when such a high specimen thickness can be obtained. PHRR values observed with 16 mm PA6 specimens are significantly lower than those for the 8 mm specimens of the same material, in agreement with data previously reported for PMMA,4 possibly because of the lower thermal feedback irradiation at the end of 16 mm specimens burning due to the larger distance from the radiation source. However, measurements of irradiated heat flux from the cone heater coil show a limited decrease of incident irradiation with increasing distance from the cone base, as shown in Figure 10.6. Indeed, at (25+16) mm from the cone base, reductions are only of 3.6 kW m
2 at 50 kW m
2 on the specimen surface (25 mm from the cone base) and 4.2 kW m
2 at 75 kW m
2, this being in good agreement with results reported in literature for a lower imposed heat flux.1,4,5 The curves obtained for 16 mm nanocomposites also show other interesting features. Indeed, both at high and low heat flux, the HRR peak (395 kW m
2 for C16
50 and 605 kW m
2 for C16
75) is readily achieved after ignition, and approximately corresponds to the plateau previously observed with 8 mm specimens. However, after this first stage, a decrease in HRR is observed, because of the continuous accumulation of the nanoclay as a consequence of

75

Heat Flux [kW/m2]

70 65 60 55 50 45 40 24

Figure 10.6

26

28

34 38 36 30 32 Distance from Cone base [mm]

40

42

Decreases in imposed heat flux with distance from cone base.

156

Chapter 10

1 cm

Figure 10.7

Nanocomposite residue after combustion (top view).

polymer degradation, which produces a structured char able to slow down the feed of volatiles to the flame by the well-known barrier effect.20,21 As stated above, the PA6 nanocomposite used in this work does not lead to the formation of a continuous char layer on the specimen surface. Also, the accumulation of nanoclay in the earlier combustion stage is concentrated in spots distributed on the surface, which leads to the formation of column-like structures during combustion development (Figure 10.7). This is in good agreement with the formation of carbonaceous floccules reported by Kashiwagi et al. through the effect of convection flow and bubbling in the molten state during combustion.13 As a result of the accumulation of the inorganic nanofiller because of polymer ablation it is impossible to obtain a proper steady state of combustion for these polymer-layered nanocomposites. However, the value of the HRR peak observed just after ignition is representative of the material behaviour, and provides a meaningful index of char efficiency from the earliest stage of combustion. The results obtained for 16 mm specimens provide evidence of steady-state burning for the pure PA6. Therefore a proper comparison between PA6 and its corresponding nanocomposite in terms of HHR can be undertaken to evaluate material burning independently of the transient phenomena that occur just prior to flameout. An accurate and reliable value for the HRR reduction between pure PA6 and PA6 nanocomposite can therefore be calculated as: SteadyHRRPA6
PeakHRRPA6
nanocomposite  100 SteadyHRRPA6

ð10:1Þ

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Significant Assessment of Nanocomposites’ Combustion Behaviour

Table 10.4

HRR reductions calculated on pkHRR reduction and with the steady state method (16 mm specimens).

Heat flux (kW m
2) PeakHRRPA6
PeakHRRPA6
nanocomposite PeakHRRPA6

 100

SteadyHRRPA6
PeakHRRPA6
nanocomposite SteadyHRRPA6

 100

50

75

55%

49%

32%

36%

Using this approach, the HRR reductions for the nanocomposite compared with pure PA6 amount to about 33% at both heat fluxes used in this work. For PHRR, reductions of about 50% would have been obtained, evidence of the overestimation of HHR reduction (Table 10.4).

10.4 Conclusions Based on the experimental results reported in this chapter, the ability to evaluate properly the combustion behaviour of a polymer nanocomposite during a forced combustion test has been shown. The correct comparison between the performances of the nanocomposite and of the reference polymer, independent of transient phenomena, may be obtained by evaluating the steadystate burning rate for the reference polymer, observable as a plateau on the HRR curve. We propose to increase the specimen’s thickness to achieve steady-state burning, a method that leads to a steady-state combustion range which is not affected by external effects, such as the ‘‘heat sink’’ effect induced by other nonstandard specimen mountings, for example the use of a conductive block underneath the specimen.4,5 With the approach described here, the effectiveness of 5% nanoclay in the reduction of the HHR of PA6 was assessed and quantified to be about 33% at both 50 kW m
2 and 75 kW m
2 incident flux levels, whereas about 50% reductions are observed for PHRR values. This shows the overestimation obtained when the transient phenomena that occur in non-charring materials during the final stage of combustion in standard cone calorimeter tests are not taken into account. The proposed method to calculate the HRR reduction is not intended to provide a single parameter with which to evaluate combustion behaviour. The comprehensive assessment of nanoclay effectiveness as a fire retardant must consider other parameters from the cone calorimeter test (such as TTI, THE, effective heat of combustion, smoke and CO productions) as well as performance in standard flammability tests [e.g. UL94 test, limiting oxygen index (LOI) etc.] and other tests specific for the final application of the nanostructured material. Note also that the effectiveness in HRR reduction calculated and discussed here may not directly provide a prediction of the actual fire performance in real material applications. Specimen configuration (e.g. vertical

158

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or horizontal orientation, back-insulation) and geometry (e.g. thickness, curvature) in real applications can be significantly different from those used in the cone calorimeter tests reported here. Nevertheless, we believe that the approach discussed in this chapter is useful for preliminary research on materials, particularly when details of end-user applications are not available.

Acknowledgements This study was carried out in the Frame STRP European research program ‘‘PredFire Nano’’, n1. 013998, in the sixth Framework Program.

References 1. V. Babrauskas, in Heat Releases in Fires, ed. V. Babrauskas and S. Grayson, Elsevier Applied Science, London, 1992, p. 61. 2. R. E. Lyon, M. L. Janssens, Polymer flammability: U.S. Department of Transportation Federal Aviation Administration technical report. 1982 Technical Report DOT/FAA/AR05/14, FAA, 2005. available online at http://www.fire.tc.faa.gov. 3. V. Babrauskas, Fire Mater., 1984, 8, 81. 4. B. Schartel and T.R. Hull, Fire Mater., 2007, 31, 327. 5. B. Schartel, M. Bartholmai and U. Knoll, Polym. Degr. Stab., 2005, 88, 540. 6. T.D. Fornes, P.J. Yoon, D.L. Hunter, H. Keskkula and D.R. Paul, Polymer, 2002, 43, 5915. 7. O. Monticelli, Z. Musina, A. Frache, F. Bellucci, G. Camino and S. Russo, Polym. Degr. Stab., 2007, 92, 370. 8. H.R. Dennis, D.L. Hunter, D. Chang, S. Kim, J.L. White, J.W. Cho and D.R. Paul, Polymer, 2001, 42, 9513. 9. F. Samyn, S. Bourbigot, C. Jama, S. Bellayer, S. Nazare, T.R. Hull, A. Castrovinci, A. Fina and G. Camino, Eur. Polym. J., 2008, 44, 1642. 10. V. Babrauskas, Fire Mater., 1995, 19, 243. 11. J.W. Gilman, Appl. Clay Sci., 1999, 15, 31. 12. S. Bourbigot, E. Devaux and X. Flambard, Polym. Degr. Stab., 2002, 75, 397. 13. T. Kashiwagi, R.H. Harris, X. Zhang, R.M. Briber, B.H. Cipriano, S.R. Raghavan, W.H. Awad and J.R. Shields, Polymer, 2004, 45, 881. 14. B.N. Jang and C.A. Wilkie, Polymer, 2005, 46, 3264. 15. A. Castrovinci and G. Camino, in Multifunctional Barriers for Flexible Structure: Textile, Paper and Leather, S. Duquesne, C. Magniez, G. Camino ed., Springer Verlag, Berlin, 2007, p. 87. 16. R. Krishnamoorti and K. Yurekli, Curr. Opin. Colloid Interface Sci., 2001, 6, 464. 17. L. Incarnato, P. Scarfato, L. Scatteia and D. Acierno, Polymer, 2004, 45, 3487.

Significant Assessment of Nanocomposites’ Combustion Behaviour

159

18. T.D. Fornes, P.J. Yoon, H. Keskkula and D.R. Paul, Polymer, 2001, 42, 9929. 19. V. Babrauskas, Ignition Handbook, Fire Science Publishers, Issaquah, 2003. 20. M. Zanetti, T. Kashiwagi, L. Falqui and G. Camino, Chem. Mater., 2002, 14, 881. 21. T. Kashiwagi and J.W. Gilman, in Fire retardancy of polymeric materials, ed. A. F. Grand, C. A. Wilkie, Marcel Dekker Inc., New York, 2000, p. 353.

CHAPTER 11

Phosphorus-Based Epoxy Resin–Nanoclay Composites JIANWEI HAO, YANBING XIONG AND NA WU National Laboratory of Flame Retardant Materials, School of Materials Science and Technology, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China

11.1 Introduction Non-halogen flame retardant epoxy resins are in great demand for manufacturing printed circuit boards in electrical and electronic (E&E) equipment because recently the use of halogen flame retardants in plastic materials has raised many questions about their impact on the health and safety of both and the environment. In recent years significant attention has focused on the research on active phosphorus-containing flame retardants, nanofiller and their synergistic effect for epoxy resins.1–4 This especially worthy research has two aspects. Firstly, reactive-type phosphorus-containing flame retardants are of interest because they are able to sustain the flame retardance for a longer period, and reduce the influence on physical, mechanical and artifactual properties of polymers. Secondly, the synergistic effect between nanoclays and flame retardants can simultaneously improve the combustion performance and physical properties of the resin. Jeng et al. reported that synthesizing an epoxy resin that contained resorcinol diphosphate with 6.19 weight percent (wt%) of phosphorus resulted in epoxy resins that exhibited a high limiting oxygen index (LOI) value of 32% and high char yield of nearly 30% at 850 1C under nitrogen.5 Mauerer prepared epoxy Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

160

Phosphorus-Based Epoxy Resin–Nanoclay Composites

161

resins that contained aminophenyl phosphate by reacting tris-(3-aminophenyl) phosphate (TAPP), a cross-linker and flame retardant, with curing agents like dicyandiamide or 4,4-diaminodiphenylsulfone.6 Laminates made of this aminophenyl phosphate cross-linked epoxy resin achieved the UL-94 V-1 classification, and also met the standard requirements for water absorption, heat stability and high glass-transition temperature. Hsiue et al. reported that curing a phosphorus-containing epoxy with an aminopropylpolydimethylsiloxane (PDMS) resulted in epoxy resins that have a high LOI value of 45%, hardly achievable using solely phosphorous compounds.7 Wu et al. researched the synergistic effects of phosphorus–silicon and nitrogen–silicon in the epoxy resins of silicon-containing epoxy compounds or nanoscale colloidal silica, cured with phosphorus- and melamine-containing agents.8 Addition of silicon compounds significantly enhanced the thermal stability, LOI values and char yields of the cured epoxy resins. Levchik and Weil1,2 have given a detailed overview of the recent literature on reactive flame retardants in epoxy resins, including on 9,10-dihydro-9-oxa10-phosphaphenanthrene-10-oxide (DOPO) compounds. DOPO was reacted with quinine, naphthalene and itaconic acid, respectively, and the resultant DOPO-containing compounds or condensates could be used as reactive flame retardants for bisphenol A epoxy resin or multifunctional novolac epoxy resin.1,2,9 The rigid cyclic side-chain structure of DOPO gave the phosphoruscontaining epoxy a higher glass-transition temperature and flexural modulus, and better thermal stability than the regular tetrabromobisphenol A epoxy resin. In the work reported in this chapter, the reactive-type organophosphorus flame retardant agent, DOPO was reacted with bisphenol A epoxy resin to form a phosphorus-based epoxide (EPO-P). This (EPO-P) was mixed with and without an organophilic clay (Cloisite 30A) in organic solvent to prepare phosphorus-containing epoxy–nanoclay (EP–P–nano) composites and a phosphorus-containing epoxy resin (EP–P), respectively. It was used in combination with a 4,4 0 -diaminodiphenyl-methan (DDM, a curing agent). The evidence for EP–P–nano composite formation was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The flame retardant properties of the EP–P–nano composites were measured by LOI and a UL-94 vertical flame test. The tensile and impact strengths of the composites were tested according to the standard methods.

11.2 Experimental 11.2.1

Materials

Bisphenol A epoxide (E-51, epoxy equivalent weight ¼ 209 g mol
1) was obtained from Wuxi Resins Factory in China. DOPO was obtained from Kunshan Zhonghong Chemical Inc. Curing agent DDM was supplied by Tianjin Guangfu Institute of Fine Chemical Industry. Cloisite 30A, an organophilic clay, was supplied by Southern Clay Products Inc.

162

Chapter 11 O

CH2

CH3 CH CH2 O

CH3

O O CH2 CH

C

XO

CH2 +

CH3

O

P

C

OX

CH3

O

H

Epoxide

O X=

EPO-P

DOPO

CH2 CH

OH CH2

or

CH2 CH CH2

P

O

O

Scheme 11.1

11.2.2

The reaction of epoxide and DOPO.

Preparation of Phosphorus-Based Epoxide

E-51 and DOPO in predetermined stoichiometry ratios were mixed and stirred to form a homogeneous solution at 120 1C for 1 hour, then reacted at 160 1C for 5 hours, according to Scheme 11.1 to obtain the transparent yellow EPO–P that contained 1.0 wt% of phosphorus. The epoxy equivalent weight of EPO–P is 241.6 g mol
1, which was measured by a chemical titration method.10

11.2.3

Preparation of Phosphorus-Based Epoxy–Nanoclay Composites

The EPO–P (phosphorus content 1.0 wt%) was mixed with organophilic clay dispersed in N,N 0 -dimethylformamide, at 80 1C for 1 hour, then DDM was added and stirred at 80 1C for 2 hours to obtain the cured EP–P–nano composites. Samples of EP–P–nano composites that contained 1, 3 and 5 wt% of organophilic clay were cured at 80 1C for 2 hours and post-cured at 120 1C for 12 hours.

11.2.4

Characterization

Fourier transform infrared (FTIR) spectra of a phosphorus-containing epoxide liquid film sample were obtained with a BRUKER Vector 22 spectrometer. XRD analyses were performed using a RINT-2400 diffractometer with Cu radiation (50 kV, 40 mA). The scanning speed and step size were 0.11 min
1 and 0.11, respectively. TEM images of EP–P–nano composites were taken with a HITACHI-H-800 at an acceleration voltage of 200 kV. The LOIs were measured using a FTA instrument according to ASTM D 2863-91. The vertical burn (VB) UL-94 test was conducted using a CZF-2 type instrument according to ANSI/UL 94-2006. The tensile strengths were measured following the GB/T 1040.2-2006 standard by a DXLL-5000 test apparatus and the impact strengths following the GB/T 1843-1996 standard by a XJ-502 impact test apparatus.

163

Phosphorus-Based Epoxy Resin–Nanoclay Composites

11.3 Results and Discussion 11.3.1

Structure of EPO–P Analysis with FTIR

The infrared spectra of DOPO and the EPO–P shown in Figures 11.1 and 11.2, respectively, were analyzed to prove that the phosphorus was introduced into the molecule of epoxide. The characteristic bands corresponding to DOPO and the EPO–P are given in Table 11.1. DOPO gave adsorption at 2438 cm
1, which corresponds to P–H stretching vibrations. Figure 11.2 shows that the peak of the P–H characteristic bands disappeared in EPO–P, which indicates that the phosphorus has been bonded on the epoxide molecule.

11.3.2

Structure of EP–P–nano Composites Analysis with XRD and TEM

XRD provides information on the changes in the inter-layer spacing of the clay upon the formation of a nanocomposite. XRD results of the organophilic clay and the EP–P–nano composites with 1 wt% of phosphorus and 3 wt% of Cloisite 30A clay cured at different temperatures are shown in Figure 11.3. It can be seen that for Cloisite 30A clay, 2y ¼ 4.71 and the d-spacing is 18.8 A˚. For EP–P–nano composite samples, the peak 001 has shifted to a lower value of 2y, the d-spacing is 36.0 A˚ and the peak is broadened. This indicates that the 100

Transmittance/%

80

60

3064 =CH(Ar)

2438 P-H

1604 C=C(Ar)

40

1481 P-Ph

1208 906 P-O-Ph

20 1244 P=O 0 4000

3500

3000

2500

2000

1500

Wavenumber/cm-1

Figure 11.1

FTIR spectra of DOPO.

1000

500

0

164

Chapter 11 100

Transmittance/%

80

3054 =CH(Ar)

60

764 P-C

1609 C=C(Ar)

40

1461 P-Ph 20

1186 910.25 P-O-Ph

2873 -CH2

1241 P=O

2975 0 4000

3500

3000

2500

2000

1500

1000

500

0

Wavenumber/cm-1

Figure 11.2 FTIR spectra of EPO–P.

Table 11.1

The characteristic FTIR bands of DOPO and EPO–P. The characteristic bands (cm
1)

Specimens

P–H

P–Ph

P¼O

P–O–Ph

DOPO EPO–P

2438 Disappear

1481 1461

1244 1241

1208, 906 1188, 910

EP–P–nano composite has a mixture of intercalated and exfoliated structures, and the curing temperature has no influence on the formation of nanocomposites. TEM provides an actual image of the clay layers and helps to identify morphology of the nanocomposite. Figure 11.4(a) shows the TEM images for the mixture of EP–P and 3 wt% Cloisite 30A˚ clay (EP–P/clay), where no intercalated clay layers can be seen. The TEM image of the EP–P–nano composite (3 wt% clay) in Figure 11.4(b) indicates that a mixed intercalated and exfoliated structure has been formed for this sample.

11.3.3

Combustion Performance and Mechanical Properties

The LOI determines the flame-extinguishing properties of the polymer materials under controlled test conditions. The VB test, UL-94, determines the

165

Intensity

Phosphorus-Based Epoxy Resin–Nanoclay Composites

a - Cloisite 30 A b - 120°C cured nano - composite c - 150°C cured nano - composite d - 80°C cured nano - composite 0

2

4

6

8

10

2θ (deg)

Figure 11.3 XRD patterns of EP–P–nano composites at different curing temperature.

a

b

100 nm

Figure 11.4

100 nm

TEM images for the mixture of a) EP–P/clay and b) EP–P–nano composite.

upward burning properties of the polymer materials. LOI values and UL-94 testing results of the epoxy-clay nanocomposite without phosphorus (EP– nano) and with phosphorus (EP–P–nano) are given in Table 11.2. The phosphorus content was fixed at 1 wt% in the EP– and EP–P–nano specimens. For sample EP–nano, containing only 3 wt% clay alone and no phosphorus, the LOI value was found to be 27.8%, which is a little higher than the 25.2% of the virgin epoxy resin (EP) specimen. When clay (o5 wt%) was added into EP–P

166

Table 11.2

Chapter 11

Data of combustion performance of epoxy resin specimens.

Specimens

P (wt%)

Clay (wt%)

LOI (%)

UL-94 (3 mm)

EP EP–P EP–nano

– 1 – 1 1 1 1 1

– – 3 1 2 3 4 5

25.2 29.8 27.8 30.1 31.1 32.1 30.4 28.6

V-2 V-1 V-2 V-1 V-0 V-0 V-1 V-1

EP–P–nano

Table 11.3

Mechanical properties of epoxy resin specimens.

EP EP–P EP–P–nano, 3 wt% of clay

Tensile strength (MPa)

Elongation at break (%)

Impact strength (kJ m
2)

60.2 69.7 72.3

3.6 4.2 4.5

13.3 16.8 25.4

with 1 wt% phosphorus, the LOI values and UL-94 ratings of EP–P–nano composites increased significantly. It can be seen that the LOI and UL-94 ratings of the EP–P–nano composite that contained 3 wt% of clay are 32.1% and V-0, respectively, which are the best flame retardant properties obtained. These results show that clay or phosphorus alone cannot effectively enhance the flame retardant properties of EP. In other words, a synergistic effect in flame retardancy can be seen when both phosphorus and clay are present in EP–P– nano composite samples, which indicates synergism between phosphorus and nanoclay. This synergism can be explained on the basis that phosphorus helps in the char formation and enrichment during combustion of the resin and the nanoclays enhance the gas barrier properties of the char, hence both acting together to exhibit the synergistic effect. Notice also that when the loading level of the clay increases to 4 wt% and 5 wt%, the LOI values and UL-94 rating of EP–P–nano composites decrease. This decrease in flame retardancy could be explained because when the EP–P– nano composites contain more than 4% clay, the cross-linking density of the EP–P–nanoclay composites during curing is reduced. The other reason could be that the clay at higher concentrations is not well dispersed in the composites, which results in a reduction in the gas-barrier properties of the char layer during burning. Table 11.3 presents the data of mechanical properties for the samples of EP, EP–P and EP–P–nano composite with 3 wt.% clay. It can be seen that the tensile strength and elongation of EP–P–nano composite increased from 69.7 MPa and 4.2% in EP to 72.3 MPa and 4.5%, respectively. Especially, the

Phosphorus-Based Epoxy Resin–Nanoclay Composites

167

impact strength of EP–P–nano composite increases significantly, i.e., from 13.3 kJ m
2 in EP to 25.4 kJ m
2. It is the formation of the intercalated and exfoliated structure in the EP–P–nano composite that contains 3 wt.% clay which leads to reinforcement of the epoxy substrate. The results indicate that flame retardant, phosphorus-based epoxy resin–nanocomposites could have increased tensile and impact strengths.

11.4 Conclusion DOPO was successfully introduced into an epoxy compound with 1 wt% phosphorus. The phosphorus-containing epoxy–nanoclay composites with various organophilic clay content were prepared. The results showed that the phosphorus-based flame retardant combined with nanocomposite is an effective way to improve flame retardancy and physical properties of the epoxy resin. The LOI and UL-94 ratings of the EP–P–nano composite with 3 wt% clay were 32.1% and V-0 rating, respectively. The Notched Izod impact strength of EP–P–nano reached 25.4 kJ m
2, increasing by 50% compared to the EP–P sample.

References 1. S. Levchik, A. Piotrowski, E. Weil and Q. Yao, Polym. Degrad. Stab., 2005, 88, 57. 2. S. Levchik and E. Weil, Polym. Int., 2004, 53, 1901. 3. A. Morgan, Polym. Adv. Technol., 2006, 17, 206. 4. Y. Xiong, J. Hao, Proceeding of the International Polymer Materials Engineering Conference 2005, Shanghai, China, September 18–21, 2005. 5. R. Jeng, S. Shau and J. Lin, Eur. Polym. J., 2002, 38, 683. 6. O. Mauerer, Polym. Degrad. Stab., 2005, 88, 70. 7. G. Hsiue and Y. Liu et al., J. Appl. Polym. Sci., 2000, 78, 1. 8. C. Wu, Y. Liu and Y. Chiu, Polymer, 2002, 43, 4277. 9. C. Wang and M. Lee, Polymer, 2000, 41, 3631. 10. J. Hao, Y. Xiong and T. Zhang, Transactions of the Beijing Institute of Technology, 2006, 26, 279.

CHAPTER 12

Study of the Relationship Between Flammability and Melt Rheological Properties of Flame-Retarded Poly(Butylene Terephthalate) Containing Nanoclays S. NAZARE,a T. R. HULL,b B. BISWAS,a F. SAMYN,c S. BOURBIGOT,c C. JAMA,c A. CASTROVINCI,d A. FINAd AND G. CAMINOd a

Centre for Materials Research and Innovation, University of Bolton, Deane Campus, Bolton, BL3 5AB, UK; b Centre for Fire Hazards Science, University of Central Lancashire, Preston, PR1 2HE, UK; c Proce´de´s d’Elaboration de Reveˆtements Fonctionnels (PERF), LSPES, UMR-CNRS 8008, ENSCL, BP 90108, 59650 Villeneuve d’Ascq, France; d Politecnico di Torino Sede di Alessandria-Centro di Cultura per l’Ingegneria delle Materie Plastiche, Viale Teresa Michel 5 15100, Alessandria, Italy

12.1 Introduction Recent studies on a new class of flame retardant (FR) systems that contain nanoclay and conventional FR microparticles have shown that the threshold concentration of FR required to achieve acceptable levels of flame retardancy Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

168

169

Study of the Relationship Between Flammability and Melt Rheological Properties 1

can be significantly reduced in the presence of nanoclay. Bourbigot et al. have observed synergistic effects while incorporating nanofillers into intumescent formulations. They proposed that the reactivity of nanofillers with the intumescent FR modifies the physical behaviour of intumescent char during burning. In multicomponent polymer formulations that contain FR microparticles and inorganic nanoparticles, research has shown that the structure of the interphase (IP) strongly affects the flame retardancy and mechanical properties of the polymer system.2 The formation and structure of the IP is, however, governed by the interaction between solid–solid and solid–liquid phases. Nanoclays with different structural morphologies and organic surface treatments could interact differently with the FR microparticles, and thus result in materials with distinct physical properties. The structural morphology of the dispersed phase in the polymer strongly affects the rheological properties of the polymer system, which can sequentially alter the burning behaviour of the polymer composite.3 Therefore two different nanoclays with different structures were chosen. Cloisite 30B (CL30B) is a montmorillonite clay modified with a quaternary ammonium salt, which has a layered structure consisting of two tetrahedral silicate sheets that sandwich a central octahedral sheet. The aspect ratio of montmorillonite is very high, with a specific surface area of 750 m2 g
1. Sepiolite (SP), also a member of the same 2:1 phyllosilicate group, is a non-swelling clay with needlelike morphology. Chemically, SP is a microcrystalline-hydrated magnesium silicate with the unit cell formula of Si12O30Mg8(OH)4.8H2O.4 The SP structure consists of a magnesium octahedral sheet between two layers of silica tetrahedrons, which extend as a continuous layer with an inversion of the apical ends every six units. This inversion results in the formation of a discontinuous octahedral sheet, which allows the formation of rectangular tunnels growing in the direction of needle axis.5 The nanostructured tunnels measure approximately 0.35  1.06 nm2 in cross section and are filled with zeolitic water. The specific surface area of SP is 300 m2 g
1  10 m2 g
1 and the contact areas between the needles are both smaller than the specific surface area and contact area between the clay platelets of montmorillonite. The lower contact area between the needles facilitates dispersion of SP. Commercially, poly(butylene terephthalate) (PBT) is often rendered FR using halogen-containing additives and a synergist. However, environmental issues mean that halogenated systems are fast being replaced by additive or reactive flame retardant systems. Different flame retardant systems for PBT and thermal decomposition and combustion mechanisms of flame retarded PBT were recently reviewed by Levchik and Weil.6,7 In the present work, interactions between flame retardant microparticles and inorganic nanoparticles (of different morphologies) dispersed in PBT are examined using rheology, and changes in crystallinity and hence melting behaviour are studied using differential scanning calorimetry. Thermo-analytical studies were carried out to examine the effect of changed rheology on thermal decomposition of the polymer composites. Viscosity measurements as a function of temperature were carried out to obtain information about the interactions of the components and

170

Chapter 12

the processes that take place during heating. Finally, cone calorimetric experiments were performed to study the effects of changed melt rheological behaviour on the fire behaviour of PBT formulations.

12.2 Experimental 12.2.1

Materials

 Polymer – PBT, Celanex 2000-2 Natur, supplied by Ticona;  Nanofiller 1 – CL30B, natural montmorillonite modified with methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium chloride (MT2EtOH), supplied by Southern Clay Products, USA;  Nanofiller 2 – SP amine, surface modified with benzyl methyl dihydrogenated tallow ammonium salt, supplied by Tolsa, Spain;  FR – phosphinate salt, Exolit OP1240, supplied by Clariant, Germany.

12.2.2

Sample Preparation

Compounds were prepared by melt blending in a Leistritz ZSE 27 co-rotating intermeshing twin-screw extruder. Screw speed was set to 200 revolution per minute (rpm) and mass flux at 10 kg h
1. Screw profile and temperature profile used for compounding PBT materials are shown in Figure 12.1. The polymer was loaded in the main feed and filler added to the molten polymer by means of a gravimetric side-feeder. The extruded materials are cooled in water and then pelletized. Samples in the form of powder, films and slabs were prepared for appropriate testing. Sample description and mass percentages of various components in the formulations are given in Table 12.1.

Figure 12.1

Screw and temperature profiles for processing PBT materials.

Study of the Relationship Between Flammability and Melt Rheological Properties

Table 12.1

171

Mass percentages of various components in the formulations.

Descriptive codes

Resin (%)

FR (%)

Nanoclay (%)

PBT PBT–CL30B PBT–SP PBT–FR PBT–FR–CL 30B PBT–FR–SP

100 95 95 82 77 77

– – – 18 18 18

– 5 5 – 5 5

12.2.3

Characterization and Testing

Conventionally, nanocomposite structure(s) in a polymer matrix can be identified by monitoring the position, shape and the intensity of the basal spacing in the lower 2y region (2–101) of the X-ray diffractogram (XRD). However, the peak in the XRD of SP originates from the d-spacing between the SP tunnels and not from the separation between the needles.4 Therefore, XRD is not a suitable technique to characterize the dispersion of SP in the polymer matrix. Recently, a rheological method was developed to characterize the nanodispersion of all kinds of plate-like, fibrous or dendritic filler materials with high aspect ratios.8 This was used to characterize the nanostructures of the samples in the current study. A Polymer Laboratories DSC was used to determine the influence of the morphological structure of the nanofillers on the crystallization behaviour of PBT. The crystallinity (Xc %) for all the samples was calculated such that:

X c% ¼

DH m  100 DH f

where DHm is the enthalpy of melting and DHf is enthalpy of fusion. The theoretical value of DHf for a 100% crystalline PBT has been taken as 140 J g
1.9 Simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) was performed using an SDT 2960 TA instrument under flowing air (50 ml min
1) and at a heating rate of 10 K min
1 on 10 mg sample masses. Rheological measurements were carried out on 1 mm thick samples at 240 1C using a Dynamic Analyser Rheometer RDA II from Rheometrics. A parallel plate geometry with plate diameter 25 mm was used to conduct dynamic frequency sweep experiments. Furthermore, the changes in melt rheological behaviour of polymer composites over a temperature range close to, and above, the degradation temperature were studied in a nitrogen atmosphere. The samples were heated from 300 to 530 1C with a heating rate of 15 1C min
1. The frequency of oscillation was kept constant at 10 rad s
1 and the strain amplitude at 10%. The burning behaviour of PBT formulations was studied using cone calorimetry (Fire Testing Technology Ltd, UK). 100  100  6 mm samples were exposed to an incident heat flux of 50 kW m
2 under ambient atmosphere.

172

Chapter 12

Table 12.2

Rheological properties of PBT formulations at 0.1 rad s
1.

Samples

|Z| (Pa)

G 0 (dyn cm
2)

G00 (dyn cm
2)

Type of composite

PBT PBT–Cl30B

7.4  103 8.0  105

6.5  100 9.4  104

7.4  102 9.4  104

PBT–SP PBT–FR PBT–FR–CL30B

7.7  102 6.9  103 6.2  105

3.0  101 3.9  101 7.4  104

3.0  101 3.9  101 4.1  104

PBT–FR–SP

2.7  105

3.1  104

3.1  104

– Intercalated nanocomposite Microcomposite Microcomposite Intercalated nanocomposite Intercalated nanocomposite

12.3 Results and Discussion 12.3.1

Nanodispersion

Viscosity curves for PBT polymer and its composites are shown in Figure 12.2(a), and a summary of the rheological properties in the low frequency region (at 0.1 rad s
1) for all the formulations studied are given in Table 12.2. In Figure 12.2(a) PBT shows perfect Newtonian behaviour over all the frequency range measured, giving a shear-thinning component of Z ¼ 0.02. Addition of 5% of CL30B to the polymer matrix shows a shift to non-Newtonian behaviour in the low-frequency region and pronounced shear thinning (Z ¼ 0.67) at higher frequencies. A significant increase in the complex viscosity at lower frequencies and pronounced shear thinning in the higher frequency region at low loading levels of 5% w/w is a characteristic feature of intercalated–exfoliated nanocomposite structures.10 Characterization of PBT–CL30B as an intercalated nanocomposite based on its rheological behaviour is in agreement with the XRD results,11,12 in which the characteristic peak of CL30B at 2y ¼ 4.51 corresponding to a d-spacing of 1.88 nm has moved to a lower value of 2y ¼ 2.21, which indicates a d-spacing of 4.0 nm. XRD analyses, confirmed by TEM,11,12 show that, although the d-spacing has increased, the CL30B has still maintained its ordered platelet structure to form an intercalated nanocomposite. In contrast, PBT–SP samples that contain 5% w/w of needle-like nanofiller (SP) do not show any change in rheological properties. The viscosity curve in Figure 12.2(a) for PBT–SP shows perfect Newtonian behaviour similar to that of the pure PBT sample, which suggests that SP remains in tactoid form or does not form a percolated superstructure of well-dispersed nanoparticles. Lack of confinement of polymer chains by one-dimensional needle-like SP particles in PBT–SP samples accounts for the perfect Newtonian behaviour similar to that of pure PBT. Owing to the weak interaction between the SP particles and the PBT polymer, the tethering of polymer chains by SP is not strong enough. Moreover, the change in the yield behaviour of polymer–clay nanocomposite in molten form depends largely on the surface area of the particulates. Note that the specific surface area of montmorillonite clay is 750 m2 g
1, whereas that of SP can be less than 300 m2 g
1. The higher specific surface area of CL30B

Study of the Relationship Between Flammability and Melt Rheological Properties

173

provides greater resistance to polymer chains and hence higher viscosity, especially at lower frequencies. The montmorillonite-based CL30B forms a classic ‘‘card-house’’ structure. The polymer-layered nanocomposite structure is instrumental in imparting solid-like viscoelastic properties to PBT–CL30B samples. The rheological properties of PBT–FR in Table 12.2 do not show substantial change with respect to those of pure PBT, despite 18% w/w loading of FR. This suggests that addition of microparticles up to 18% w/w does not affect the chain movement and hence the rheological behaviour of the polymer system, whereas 5% w/w of nano-dispersed clay particles significantly affects rheological properties of the polymer nanocomposite. However, addition of FR to the PBT–SP formulation resulted in a sizeable increase in the viscosity of the PBT– FR–SP sample and a noticeable increase in the shear thinning at higher frequencies, which suggests that the FR assists in increasing compatibility between polymer chains and SP needles. SP has a very high concentration of surface silanols spaced every 0.5 nm along the length of the needles which facilitate coupling reactions with polymer, organic surfactant and/or the flame retardant. This could probably lead to diffusion of small molecules within the SP needles, and thereby assist uniform dispersion of SPs within the polymer matrix. Solidlike or pseudo solid-like viscoelastic behaviour of the PBT–FR–SP formulation, as seen in Figure 12.2 and Table 12.2, can be attributed to enhanced dispersion of SP in the presence of FR. Viscosity values for PBT–CL30B and PBT–FR–Cl30B over the whole frequency range tested are comparable (see Table 12.2), which suggests that the confined structure of CL30B within the polymer matrix and the chain stiffness of PBT limits further widening of interlayer space in the presence of FR. Furthermore, hydroxyl groups in the CL30B interlayer have two effects on PBT that contains carboxyl groups. First, it favours intercalation of PBT chains and the formation of an intercalated nanocomposite structure. Second, the enhanced interaction of ammonium cation with the silicate surface is less favourable for replacement of the surface

1000000

1000000 100000

100000

(b)

10000 G″, Pa

Iη*I,Pa.s

(a)

10000

1000 100 10

1000 0

1

10

100

1 100

Key

PBT

Figure 12.2

PBT+CL 30B

1000

10000

100000 1000000

G″, Pa

Frequency, rad/s PBT+SP

PBT+FR

PBT+FR+CL30B

PBT+FR+SP

(a) Viscosity versus frequency and (b) G 0 versus G00 plots for all PBT formulations.

174

Chapter 12

contacts by PBT chains, which thereby limits extensive intercalation and further exfoliation of CL30B in the PBT matrix.13 The shear-thinning behaviour of both the samples that contain CL30B is very similar [see Figure 12.2(a)] with a shear-thinning component of Z ¼ 0.67 for PBT–CL30B and Z ¼ 0.64 for PBT– FR–CL30B. Furthermore, the so called Cole–Cole plots (log G 0 versus log G00 ) in Figure 12.2(b) may be used to further elucidate the morphological state of such multiphase polymer systems. Note from Figure 12.2(b) that inclusion of CL30B in the pristine polymer shows a profound influence on the log G 0 versus log G00 plots and hence the morphological state as compared to the pure polymer and flame-retarded polymer. Addition of flame retardant to the PBT–SP formulations also shows an upward shift in log G00 versus log G00 plots, which suggests a change in the morphological state of the polymer system. Nanodispersion gives rise to a notable increase in the degree of heterogeneity of the polymeric system, thereby decreasing the slope of log G 0 versus log G00 plots, compared to PBT, PBT–SP and PBT–FR samples. That the log G 0 versus log G00 plots in Figure 12.2(b) differ for different samples suggests these polymer systems can be regarded as different materials from a rheological point of view. The frequency-dependent behaviour of storage and loss moduli of a polymer system is also related to its morphological state in molten form. The storage and loss moduli curves plotted as a function of frequency for PBT and its composites are shown in Figure 12.3. The frequency dependence of storage and loss moduli of PBT, PBT–FR and PBT–SP shown in Figures 12.3(a), 12.3(c) and 12.3(e) suggests that the viscoelastic behaviour of pure polymer is dominated by viscous liquid behaviour (with G 0 o G00 over all the frequency range measured and no cross-over frequency). However, for the sample PBT–Cl30B [see Figure 12.3(b)], G 0 4 G00 in the lower frequency region suggests solid-like behaviour due to physical jamming of clay platelets. The cross-over frequency is noted at 19.9 rad s
1, after which the polymer system exhibits viscous liquid behaviour. Addition of FR reduces the cross-over frequency to 6.3 rad s
1 for the PBT–FR–CL30B formulation. For PBT–FR–SP formulations, the crossover frequency is noted at the lower frequency of 3.2 rad s
1, which indicates that the interaction between needle-like particles of SP and polymer chains is lost at lower shear rates, leading to relaxation of the polymer chains and hence viscous liquid behaviour.

12.3.2

Differential Scanning Calorimetry and Thermal Analysis

Calorimetric data for pure PBT and normalized (for actual polymer content) calorimetric values for all PBT formulations are given in Table 12.3. Note that the melting temperatures have remained unchanged. However, the samples that contain SP exhibit higher crystallization temperatures (Tc) than those of pure PBT and flame-retarded PBT, both with and without CL30B. The increased temperature of crystallization for PBT–SP may result from the reduced confinement effect of the one-dimensional needle-like SP clay particles, compared to that of the two-dimensional MMT platelets.14 Furthermore, the crystallization

175

Study of the Relationship Between Flammability and Melt Rheological Properties 1.E+07 1.E+05 1.E+04 1.E+03 1.E+02

PBT G' PBT G"

1.E+01

1 10 Frequency. rad/s

1.E+03 1.E+02

PBT+CL 30B G' PBT+CL 30B G" 0.1

100

1 10 Frequency. rad/s

100

1.E+07

1.E+07 (c)

1.E+06

(d)

1.E+06

1.E+05

G' and G" Pa

G' and G" Pa

wc = 19.9

1.E+04

1.E+00 0.1

1.E+04 1.E+03 1.E+02

PBT+FR G' PBT+FR G"

1.E+01 0.1

1 10 Frequency. rad/s

1.E+05

wc = 6.3 rads/s

1.E+04 1.E+03 PBT+FR+CL 30B G' PBT+FR+CL 30B G"

1.E+02 1.E+01

1.E+00 100

1.E+00 0.1

1.E+07

1.E+07

(e)

1.E+05 1.E+04 1.E+03 1.E+02 PBT+SP G' PBT+SP G"

1.E+01

1 10 Frequency. rad/s

1.E+05 1.E+04

wc = 3.2

1.E+03 1.E+02 PBT+FR+SP G' PBT+FR+SP G"

1.E+01 1.E+00

1.E+00 0.1

Figure 12.3

Table 12.3

1 10 Frequency. rad/s

100

(f )

1.E+06 G' and G" Pa

G' and G" Pa

1.E+05

1.E+01

1.E+00

1.E+06

(b)

1.E+06 G' and G" Pa

G' and G" Pa

1.E+07

(a)

1.E+06

0.1

100

1

10 Frequency. rad/s

100

Storage modulus and loss modulus of PBT and its composites.

Calorimetric data for PBT formulations.

Samples

Tm (1C)

Tc (1C)

DHm (J g
1)

DHc (J g
1)

xc (%)

PBT PBT–Cl30B PBT–SP PBT–FR PBT–FR–CL30B PBT–FR–SP

225 224 224 225 224 225

195 193 198 190 193 203

41 48 47 43 48 56

57 69 62 59 57 65

29 35 34 31 34 40

process starts much earlier in SP-containing samples, but the enthalpy of crystallization, DHc, is smaller than that of the PBT–CL30B formulation, which suggests the formation of larger crystals with fewer nucleating sites15 in the PBT–SP sample. Nanodispersed clay platelets in PBT–CL30B provide more heterophase nuclei and a larger surface area to increase DHc, but the triggering of the crystallization is slightly delayed.16 Addition of flame-retardant microparticles reduces the temperature of crystallization of PBT–FR formulation (Tc ¼ 190 1C), compared to that of the pure polymer (Tc ¼ 195 1C). Moreover,

176

Chapter 12

(a)

Mass Loss, %

100 80 60 40 20 0 0

Figure 12.4

200 400 600 Temperature, °C

800

Temperature Difference, °C/min

inclusion of CL30B in the flame-retarded PBT slightly shifts Tc to a higher temperature, but the enthalpy of crystallization is still lower than that for the PBT–FR sample. On the contrary, addition of SP to the PBT–FR sample significantly increases Tc and DHc of the resulting PBT–FR–SP sample, which suggests early onset of crystallization in the presence of SP particles. An increase in enthalpy of crystallization may be explained by improved dispersion of SP particles in the presence of flame-retardant particles, and hence enhanced interaction between SP particles and polymer chains. The normalized values for enthalpy of melting recorded during the second heating cycles are higher for PBT formulations that contain nanofillers, which suggests that greater resistance to melting is offered by the nanofillers. Enthalpy of melting is highest for the PBT–FR–SP sample (56 J g
1), confirming that the SP is nanodispersed in the presence of FR. The percent crystallinity for PBT– FR–SP is the highest of all the samples. The increase in crystallinity can be attributed to the nanodispersed SP needles providing heterophase nuclei. One of the most important property enhancements expected from formation of a polymer nanocomposite is that of thermal stability, either in the initial stages or in the final carbonaceous residues. The degradation of pure PBT in the presence of air proceeds through a free-radical mechanism. The TGA and DTA curves for pure PBT, PBT that contains CL–30B and SP are shown in Figure 12.4. The presence of nanoclays has no impact on the thermal stability of PBT below 400 1C. The clay layers act as a mass-transport barrier to the volatile products generated during decomposition, which increases thermal stability. However, there are also catalytically active centres in the clay layers, such as those around hydroxyl groups, which might accelerate the decomposition of PBT.16 Above 400 1C both the clays improve the thermal stability of PBT and give rise to similar yields of carbonaceous char at high temperatures. Although the TGA curves for PBT–CL30B and PBT–SP show a similar trend, the DTA curves are quite different. The small exothermic peak at 279 1C for PBT–CL30B suggests decomposition of the organic modifier, whereas the organic modifier on SP is stable up to 300 1C. The DTA curve for PBT–SP shows an exothermic peak at 352 1C, which could be caused by degradation of the amine group,

12 10 8 6 4 2 0

PBT PBT+SP PBT+CL 30B

(b)

0

200 400 600 Temperature, °C

TGA and DTA responses in air for PBT-based materials.

800

177

Study of the Relationship Between Flammability and Melt Rheological Properties

followed by an endotherm that could be attributed to dehydration in which SP loses half of its coordinated water.5 The main exothermic peak for pure PBT at 417 1C, which represents the release of volatiles, is much smaller for PBT– CL30B and PBT–SP samples, which could probably result from the barrier effect of nanoclays. Inclusion of FR in the formulations that contain two different clays (not shown here) does not have any synergistic effect on the thermal stability of PBT.

12.3.3

Melt Viscosity

Viscosity versus temperature curves for PBT-based materials are given in Figure 12.5. The expanded scale within Figure 12.5 shows that the viscosity of neat PBT reduces to near to zero up to 435 1C, caused by melting and then complete decomposition of the polymer. A sharp increase in viscosity of PBT samples above 435 1C can be attributed to the presence of solid carbonaceous residue. Viscosity measurements beyond 435 1C for pure PBT were not possible for instrumental limitations. The viscous modulus of the PBT–SP formulation is greater by a factor of 10 than that of pure PBT. This increase in viscosity of the PBT–SP formulation over the temperature range 300–415 1C, despite a small (5% w/w) loading of SP, results from reinforcement of the polymer matrix by needle-like nanoparticles of SP. However, this effect of adding SP is not seen in the viscoelastic properties measured at 240 1C. This suggests that, at higher temperatures, dispersion of SP is improved, which results in an increased viscosity of PBT–SP. However, this increase in viscosity is not sufficient to prevent melt dripping of the sample when exposed to an external heat flux or flame. Above 420 1C the viscosity of PBT–SP falls to near zero, through degradation of the polymer. A sharp increase in the viscous modulus at 500 1C

Iη*I, Pa.s

25000

Iη*I, Pa.s

20000 15000

800 700 600 500 400 300 200 100 0 300

350

400

450

500

550

Temperature, °C

10000 5000 0 300

350

400

450

500

550

Temperature, °C

Figure 12.5

PBT

PBT+CL 30B

PBT+SP

PBT+FR

PBT+FR+CL 30B

PBT+FR+SP

Viscosity versus temperature curves for PBT formulations.

178

Chapter 12

could be attributed to the formation of a solid inorganic char. Note from the inset plot in Figure 12.5 that the degradation step of PBT–SP is delayed compared to those of both the pure and the flame-retarded PBT. As seen in Figure 12.5, the increased viscosity of the PBT–CL30B sample, compared to those of the PBT, PBT–SP and PBT–FR formulations, over the temperature range 300–350 1C suggests increased resistance to melt dripping. Figure 12.5 also shows that above 350 1C, the viscosity for the PBT–CL30B sample does not come close to zero until 425 1C, which suggests further resistance to melting over the temperature range 350–425 1C. Changes in viscosity with increasing temperature for PBT–FR up to 320 1C are similar to those in the pure PBT sample. However, at 335 1C, a viscosity peak appears which can be assigned to flame-retardant activity in the presence of the P-based intumescent flame retardant. This peak gradually levels to zero around 435 1C, which could be caused by the formation of phosphoric acid species from the thermal decomposition of the phosphinate. A sharp increase in viscosity and subsequent stability at higher temperatures for the PBT–FR formulation can be attributed to the enhanced formation of char in the presence of the FR. Finally, addition of 5% w/w of CL30B and SP to the PBT–FR formulation dramatically increases viscosity in the resulting PBT–FR–CL30B and PBT–FR– SP samples. A gradual decrease in viscosity values of the PBT–FR–CL30B and PBT–FR–SP formulations above 325 1C could result from the formation of phosphoric acid species, as mentioned above. Moreover, the polyphosphoric acid may react with the surfactant of the nanoclay and thereby collapse the nanostructure, and thus result in lower viscosities of PBT–FR–CL30B and PBT–FR– SP. However, the appearance of a shoulder at 360 1C (for the PBT–FR–SP formulation) and a viscosity peak at 415 1C (for the PBT–FR–CL30B formulation) suggests the formation of a porous carbonaceous char which subsequently collapses. This reduces viscosity to near zero for both PBT–FR–SP and PBT–FR– CL30B. A sharp increase in viscosity of PBT–FR–SP above 410 1C may be caused by the formation of a char that is reinforced with needle-like nanoparticles. Addition of CL30B to PBT–FR has a slightly different effect on the viscosity of the resultant formulation than does addition of SP. As seen in Figure 12.5, the reduction in viscosity is more gradual and prolonged, compared to that of the PBT–FR–SP sample. The barrier effect of nanodispersed clay platelets in the polymer matrix means the degradation step of the PBT–FR–CL30B formulation is delayed compared to that of the PBT–FR–SP formulation. The final charring process starts at 500 1C, as opposed to 410 1C, for the PBT–FR–SP sample. From the above discussion, it can be concluded that the PBT–FR–CL30B formulation might be expected to show the better fire performance because of the increased viscosity and thermal stability through the presence of nanoclay.

12.3.4

Flammability

The cone data obtained at 50 kW m
2 and given in Table 12.4 shows significant differences for various PBT formulations. Most importantly, and of more

Cone calorimetric results at 50 kW m–2 heat flux for all PBT formulations.a

Sample

PHRR TTI (s) (kW m
2)

AHRRb (kW m
2)

THRb (MJ m
2)

FIGRA (kW s
1)

Hcb (MJ kg
1)

Char residueb (%)

CO (g g
1)

CO2 (g g
1)

PBT PBT–Cl30B PBT–SP PBT–FR PBT–FR–CL30B PBT–FR–SP

64 51 44 42 37 40

229 177 191 140 110 116

138 106 115 85 66 70

1.9 2.1 3.0 1.4 1.3 1.9

22 19 21 15 13 16

27 34 37 32 41 49

0.26 0.14 0.13 0.17 0.17 0.21

2.38 1.85 2.16 1.78 0.84 1.19

a b

597 279 332 250 165 163

AHRR, average heat-release rate; FIGRA, fire growth rate; PHRR, peak heat-release rate; THR, total heat release. Values at 600 s.

Study of the Relationship Between Flammability and Melt Rheological Properties

Table 12.4

179

180

Chapter 12

significance to this work, is the time to ignition (TTI). A critical surface temperature for ignition is close to being accepted as a material property, and the time to reach this temperature (TTI) is a function of the heat transfers.17 TTI for the neat PBT is greater than the average of the nano- or FR-containing formulations. Several factors influence the ignition delay time. However, based on our rheological studies and observations, we propose a hypothesis that an increasing viscosity decreases thermal conductivity, essentially by flowing of the molten polymer, and thus results in accumulation of heat at the surface of the sample exposed to an incident heat flux. Furthermore, the thermal properties (krc) of the solid material are relatively easy to define and measure, but as the rheometric data show, most samples are somewhat molten at their ignition temperature. The increased surface temperature of the sample with higher viscosity means that this sample reaches ignition temperature more quickly than the sample with low viscosity. Based on this argument, the pure PBT would flow and bubble, and thus allow the whole sample to reach thermal equilibrium and thus increase the TTI. Once the bulk PBT reached the ignition temperature, the burning rate would be more rapid (see Figure 12.6) and lead to higher values of PHRR, FIGRA and AHRR, as seen in Table 12.4. For the samples that contain only nanofiller, the reduction in TTI can be ascribed to the increased viscosity at the ignition temperature, which results in a higher surface temperature (but a lower bulk temperature). Of the two nanofillers, the SP-containing samples show shorter TTIs than CL30B-containing formulations. This is in contrast to the above hypothesis, since the PBT–CL30B sample with higher viscosity shows an increased TTI compared to the PBT–SP sample with lower viscosity. The increased TTI in

700 600 PBT PBT+ CL 30B PBT+SP PBT+ FR PBT+FR+ CL 30B PBT+FR+SP

HRR, kW/m2

500 400 300 200 100 0 0

200

400

600

800

1000

1200

1400

Time, s

Figure 12.6

HRR as a function of time for PBT formulations.

1600

1800

2000

181

Study of the Relationship Between Flammability and Melt Rheological Properties

PBT–CL30B can be attributed to several other factors, including adsorption of volatile products on larger surface areas of clay particles and the barrier effect of the plate-like CL30B. The early ignition of PBT–SP could also be caused by catalytic degradation of the SP amine and/or less-efficient barrier properties of SP clay. With increased viscosity, HRR is decreased to give lower PHRR and lower AHRR. The higher viscosity in the presence of nanoclay may also inhibit the escape of volatile products from the burning polymer into the flaming zone, and so reduce the HRR. THR values reported at 600 s are also reduced because of slower burning of samples that contain nanofillers. Furthermore, to study the effect of changed rheological properties on the flammability of PBT composites, the relationships between the intrinsic viscosity measured at 300 1C and various cone parameters are plotted in Figure 12.7. However, to eliminate the additional effect of FR, only PBT, PBT–CL30B and PBT–SP are compared. Moreover, the presence of FR would further obscure the effect of changed viscosity on the burning behaviour of the PBT–FR– CL30B and PBT–FR–SP samples. Figure 12.7(a) suggests that the TTI is related to viscosity, but that other factors, such as nanoparticle morphology or the ability of nanoparticles to act as a barrier, must also be involved. The PHRR decreases as the viscosity increases, especially between PBT and PBT–SP, but (as discussed earlier) the higher PHRR of PBT may be the consequence of the higher overall temperature of bulk polymer, compared to the higher surface

(a)

PHRR, kW/m2

TTI, s

700 70 60 50 40 30 20 10 0

(b)

600 500 400 300 200 100 0

10

100

1000

10000 100000 1000000

10

100

1000

23

40 35 30 25 20 15 10 5 0

(d)

(c) Hc, MJ/m2

Char residue, %

10000 100000 1000000

η, Pa.s

η, Pa.s 22 21 20 19 18 10

100

1000

10000 100000 1000000

10

100

1000

η, Pa.s PBT,

Figure 12.7

10000 100000 1000000

η, Pa.s PBT+CL 30B and

PBT+SP

Relationship between (a) TTI, (b) PHRR, (c) char residue and (d) Hc.

182

Chapter 12

temperature of the nanofilled PBT samples. Figure 12.7(c) shows that the char yield appears to be independent of viscosity, and is probably dependent on the processes that occur in the later stages of burning. That the char yield does not correlate with the THR suggests some inconsistencies in the burning behaviour. Furthermore, the modest decrease in heat of combustion with increase in viscosity implies a change in the gas-phase oxidation behaviour of the volatile products. Again, this is most likely to be a consequence of the cooler bulk of the nanofilled PBT materials, which results in incomplete gas-phase combustion and greater char formation. In summary, the plots in Figure 12.5 suggest that PBT formulations with higher viscosities exhibit improved post-ignition flameretardant properties.

12.4 Conclusions Rheological measurements suggest that one-dimensional, needle-like SP has a reduced confinement effect compared to the two-dimensional platelets of CL30B. This results in perfect Newtonian viscous behaviour of PBT–SP melts. This is also confirmed by calorimetric results, in which PBT–SP samples show higher crystallization temperatures and smaller enthalpies of crystallization compared to those of PBT–CL30B. This suggests the formation of larger crystals with fewer nucleating sites. The FR acts as a compatibilizer and facilitates better dispersion of SP to give higher melt viscosity for PBT–FR–SP formulations in the lower frequency region, and pronounced shear thinning at higher frequencies. The presence of FR in PBT–FR–CL30B formulations, however, does not affect their melt rheological properties. Despite bringing about changes in melt viscosity, melting and crystallization, the introduction of the clays, CL30B and SP, does not seem to alter the thermal degradation of PBT. In terms of melting behaviour, the viscosity measurements over a temperature ramp have shown that increased viscosity in the presence of nanoclay prevents dripping and flowing of polymer. In the cone calorimetric studies, this relates to shortening the TTI and a reduction in the rate of heat release. Furthermore, PBT formulations that contain CL30B show inhibited post-ignition combustion reactions, possibly because of physicochemical adsorption of volatile degradation products on the surface of silicates with higher specific surface area compared to those of their SP analogues.

Acknowledgements The authors gratefully acknowledge the financial support from the European Union through the Sixth Framework Programme Priority 3 NMP ‘‘PREDFIRE-NANO’’ (Contract No.: STREP 013998), and thank Dr Andy Prike from the University of Sheffield for his help with the rheology experiments.

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183

References 1. S. Bourbigot, S. Duquesne, G. Fontaine, T. Turf, S. Bellayer, in Proceedings of The Eighteenth Annual BBC Conference, Stamford, Connecticut, May, 2007, 21–23. 2. S. Keszei, Sz. Matko´, Gy. Bertalan, P. Anna, Gy. Marosi and A. To´th, Eur. Polym. J., 2005, 41, 697. 3. J. Tung, R.K. Gupta, G.P. Simon, G.H. Edward and S.N. Bhattacharya, Polymer, 2005, 46, 10405. 4. A. Nohales, L. Solar, I. Porcar, C. Vallo and C.M. Go´mez, Eur. Polym. J., 2006, 42, 3093. 5. G. Tartaglione, D. Tabuani and G. Camino, Microporous Mesoporous Mater., 2008, 107(1–2), 161. 6. S.V. Levchik and E.D. Weil, Polymer International, 2004, 54, 11. 7. S.V. Levchik and E.D. Weil, A review on thermal decomposition and combustion of thermoplastic polyesters, 2004, 15(12), 691. 8. J. Zhao, A.B. Morgan and J.D. Harris, Rheological characterization of polystyrene–clay nanocomposites to compare the degree of exfoliation and dispersion, Polymer, 2005, 46, 8641. 9. G. Broza, Z. Kwiatkowska and R. K. Schulte, Processing and assessment of poly(butylene terepthalate) nanocomposites reinforced with oxidised single wall carbon nanotubes, Polymer, 2005, 46, 5860. 10. R. Wagener and T.J.G. Reisinger, Polymer, 2003, 44, 7513. 11. F. Samyn, S. Bourbigot, C. Jama, S. Bellayer, S. Nazare, T.R. Hull, A. Castrovinci, A. Fina and G. Camino, in preparation. 12. F. Samyn, S. Bourbigot, C. Jama, S. Bellayer, S. Nazare, T.R. Hull, A. Castrovinci, A. Fina and G. Camino, in preparation. 13. X. Li, T. Kang, W.J. Cho and C.S. Ha Lee, Macromol. Rapid Commun., 2001, 22, 1306. 14. S. Xie, S. Zhang, F. Wang, M. Yang, R. Se´gue´la and J.M. Lefebvre, Composites Science and Technology, 2007, 67, 2334. 15. X.Y. Tian, C.J. Ruan, P. Cui, W.T. Liu, J. Zheng, X. Zhang, X.Y. Yao, K. Zheng and Y. Li, Chem. Eng. Comm., 2007, 194, 205. 16. J. Xiao, Y. Hu, Z. Wang, Y. Tang, Z. Chen and W. Fan, Eur. Polym. J., 2005, 41, 1030. 17. B. Schartel and T.R. Hull, Fire and Materials, 31(5), 327.

CHAPTER 13

Thermal and Fire Performance of Flame-Retarded Epoxy Resin: Investigating Interaction Between Resorcinol Bis(Diphenyl Phosphate) and Epoxy Nanocomposites CHARALAMPOS KATSOULIS, EVERSON KANDARE AND BALJINDER K. KANDOLA Centre for Materials Research and Innovation, University of Bolton, BL3 5AB, UK

13.1 Introduction The flammability of polymeric materials has always been a great concern, because it limits their use in applications where fire safety is a key criterion. The use of both halogenated and non-halogenated conventional flame retardants has proved to be an effective solution to the problem, and resulted in materials with improved thermal stability and reduced flammability.1,2 However, studies have shown that the addition of high concentrations of flame retardants in a polymeric system often causes significant deterioration of the mechanical properties of the material. Moreover, environmental concerns have also been

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

184

Thermal and Fire Performance of Flame-Retarded Epoxy Resin

185

raised regarding the production of toxic gases during the combustion of polymers that contain halogenated flame retardants.1,3–5 In contrast, polymer nanocomposites have been attracting considerable attention as they offer improved thermal and fire properties not yet attained with conventional flame retardants.6–11 The most important characteristic of polymer–nanocomposites is that they can maintain their mechanical integrity while exposed to high heat-flux sources. These improvements can be achieved by using low additive contents [2–5 weight percent (wt%)] of nanoparticles such as layered silicate clays. The degree of dispersion of nanoparticles in the polymer matrix affects the enhancement in mechanical strength.12–16 Conventional flame retardants and nanoparticles when used individually are effective in improving some, but not all, of the fire-retardant and physical properties of polymers.17,18 The addition of at least two flame-retardant additives into a polymer matrix opens vast possible ways via which they can interact to result in an efficient flame-retardant system. When two or more additives are concomitantly interspersed into the polymer, their interactions may be additive, synergistic or antagonistic. Synergism means that the observed effect is greater than the additive, while an antagonistic effect is less than the additive.19 In this study the additive effect of resorcinol bis(diphenyl phosphate) (RDP) and nanoclay, Nanomer I.30E (I.30E), either individually or in combination, has been investigated. The phosphorus-containing additive RDP is an effective flame retardant; however, its low molecular weight renders it volatile, which causes partial loss from the polymer in the early stages of combustion. The reason for combining the phosphorus-containing additive RDP with the nanoclay is to have the flame retardant possibly intercalated between the nanolayers together with epoxy polymer chains so as to increase its residence time in the condensed phase during combustion. The long-term goal of this work is to develop flameretardant combinations that will be effective with respect to multiple flameretardant parameters at low concentrations.

13.2 Experimental 13.2.1

Materials

The epoxy resin, tetraglycidyl-4,4 0 -diaminodiphenylmethane (TGDDM), Araldite MY-721, and the curing agent, 4,4 0 -diaminodiphenyl sulfone (DDS), Aradur 9761, were supplied by Huntsman Corporation, Switzerland. RDP flame retardant was provided by Great Lakes, UK. The commercial grade organoclay I.30E was supplied by Nanocor Inc., while dichloromethane (CH2Cl2) was supplied by Aldrich-Sigma, UK. All the chemicals were used as received.

13.2.2

Sample Preparation and Characterization

Epoxy resin composites incorporating RDP and an organically modified clay, I.30E, were prepared by adding predetermined additive weight fractions to a

186

Chapter 13

Table 13.1

Mass percentages of various components in flame-retarded epoxy formulations. Mass (%)

Sample

Epoxy

RDP (P)a

I.30E

Epoxy Epoxy–RDP (95/5) Epoxy–RDP (90/10) Epoxy–RDP (85/15) Epoxy–RDP (80/20) Epoxy–RDP–I.30E (95/0/5) Epoxy–RDP–I.30E (90/5/5) Epoxy–RDP–I.30E (85/10/5) Epoxy–RDP–I.30E (80/15/5)

100 95 90 85 80 95 90 85 80

0(0.0) 5(0.5) 10(1.1) 15(1.6) 20(2.2) 0(0.0) 5(0.5) 10(1.1) 15(1.6)

0 0 0 0 0 5 5 5 5

a

Italicized values in parentheses represent the actual phosphorus mass fractional percent content. These values were calculated from the additive RDP weight fractions.

measured resin portion heated at 80 1C. A stoichiometric amount of the curing agent DDS (30% weight fraction with respect to epoxy formulation) and solvent CH2Cl2 (30.0 g, 0.4 mmol) were then slowly added with vigorous mechanical stirring for 30–40 minutes at room temperature to afford homogeneity. The resultant homogeneous mixture was degassed in a vacuum oven at 80 1C to evaporate the solvent and eliminate trapped air bubbles. Samples were cast in preformed aluminium dishes and cured at 180 1C for 3 hours. Details of constitutional percent loadings of respective additives are presented in Table 13.1. The morphology of the cured samples was investigated by using an X-ray diffractometer (Philips powder diffractometer) Cu Ka2 (l ¼ 1.54 A˚) radiation source at a scanning rate of 21 min
1 ranging from 21 to 601. Transmission electron microscopy (TEM) observations were also conducted using a Jeol JEM-2100 LaB6 microscope at an acceleration voltage of 200 kV with a current of approximately 100 mA.

13.2.3

Thermogravimetric Analysis

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on an SDT 2960 simultaneous DTA–TGA instrument from room temperature to 800 1C using 15  1 mg samples heated at 10 1C min
1 with air as the purge gas (flow rate, 100  5 ml min
1). The experiments were performed in triplicate and showed good reproducibility.

13.2.4

Flammability Tests

The flammability behaviours of flame-retarded resin laminates were determined via a UL-94 vertical test in accordance with protocol ASTM D 3801 with sample dimensions of 125 mm  12.5 mm  3 mm. The limiting oxygen index (LOI; Stanton-Redcroft Ltd) was determined in accordance with British

187

Thermal and Fire Performance of Flame-Retarded Epoxy Resin

standard method BS 2782; sample dimensions were 150 mm  12.5 mm  3 mm. A cone calorimeter (Fire Testing Technology Ltd, UK) was used to assess the time to ignition (TTI), heat release rate (HRR) as a function of time, total heat release (THR) and mass loss as a function of time. The test was carried out according to ISO 5660 standards, at an incident heat flux of 50 kW m
2. At least three round specimens of 56 mm diameter and 3 mm thickness were tested for each formulation.

13.3 Results and Discussion 13.3.1

XRD and TEM Analysis

XRD and TEM are techniques commonly used to identify the structural morphology of polymer nanocomposites, with observation of the extent of separation of clay layers and/or their exfoliation. The XRD patterns of the epoxy resin, epoxy–RDP–I.30E -(95/0/5) and epoxy–RDP–I.30E -(80/15/5) formulations are shown in Figure 13.1. The XRD patterns of epoxy formulations that contain 15% RDP and/or 5% I.30E by mass are similar to that of the unmodified resin. These patterns reveal the absence of basal reflections (00l ) at low angles (2–101), which suggests either complete delamination of the clay platelets (exfoliated structure) or very short-range order stacking of nanoclay platelets. The presence of the epoxy crystallographic peak centred at about 191 rules out low sensitivity of the instrument as the reason for the absence of 00l reflections from the nanoclay.

Intensity count (arb. units)

Intensity count (arb. units)

800

600

400

(c) 300

(b)

200 100

(a)

0 2

400

4

6 8 2 theta (°)

10

12

(c)

(b)

200

(a) 0 0

10

20

30

40

50

60

2 theta (°)

Figure 13.1

XRD data for (a) epoxy resin, (b) epoxy–RDP–I.30E (95/0/5) and (c) epoxy–RDP–I.30E (80/15/5) formulations. Insert shows magnified region between 2 and 101. Data are offset for clarity, but otherwise not scaled.

188

Chapter 13

(a)

(b)

tactoids

100 nm

50 nm

Figure 13.2

TEM images at low magnification (left) and high magnification (right) of an epoxy nanocomposite containing 5% I.30E clay together with 15% RDP.

TEM can be used to evaluate more directly whether some exfoliated phases exist within the polymer matrix. The low-magnification images provide information about the nanodispersion, while high-magnification images tell whether exfoliation and/or intercalation have been achieved. Figure 13.2(a) shows a TEM image of epoxy–RDP–I.30E -(80/15/5) at low magnification. The image reveals the presence of some regions that contain epoxy resin alone and dark circles that, perhaps, corresponds to clustered nanoclay particles (tactoids and agglomerates). The higher magnification image, Figure 13.2(b), reveals a nanoclay-occupied area that shows intercalated nanolayers with gallery spacings estimated at 5–10 nm. The overall picture shows that the modified nanoclay layers did not occupy the full volume with regions of epoxy visible. The absence of basal reflections at lower 2y values may result from a low degree of ordering of the nanoclay particles in the c-dimension.

13.3.2 13.3.2.1

Thermal Degradation Behaviour of Epoxy Resin and Its Composites Effect of RDP on Thermal Stability of Epoxy Resin

Figure 13.3(a) shows TG mass-loss curves for the unmodified epoxy resin and its RDP flame-retarded formulations. Thermal degradation of unmodified epoxy resin occurs primarily in three stages:20  the first a dehydration stage (100–240 1C) in which up to 3% mass loss is observed;  the second stage (240–470 1C) shows a 42% mass loss attributed to the depolymerization of polymeric chains to form a primary carbonaceous char;  the third stage is a char oxidation stage (470–680 1C) that contributes 55% mass loss and leaves no residual char at 800 1C.21,22

189

Thermal and Fire Performance of Flame-Retarded Epoxy Resin 100

Epoxy Epoxy/RDP-95/ 5 Epoxy/RDP-90/10 Epoxy/RDP-85/15 Epoxy/RDP-80/20

(a)

Mass %

80 60 40 20 0 (b)

∆ Mass %

20

Epoxy/RDP-95/ 5 Epoxy/RDP-90/10 Epoxy/RDP-85/15 Epoxy/RDP-80/20

10 0 -10 -20 50

150

250

350

450

550

650

750

Temperature (°C)

Figure 13.3 (a) TGA curves and (b) mass-difference curves between RDP flameretarded and unmodified epoxy resin versus temperature in flowing air.

Addition of RDP to the epoxy resin leads to a significant reduction in the onset degradation temperature of the resultant flame-retarded epoxy composites, measured as the temperature at which 10% mass loss occurs, T10 (Table 13.2). This suggests that the thermal stability of epoxy is compromised in the lower temperature regime after the addition of RDP. However, thermal stability for formulations that contain more than 5% RDP is improved from 425 1C upwards, as shown in Figure 13.3(a). When the epoxy resin is modified by the addition of 5% RDP, its thermal stability is only improved between 510 and 660 1C, which may be attributed to the low phosphorus content of RDP, calculated at 0.5 mass percentage of phosphorus (Table 13.1). Mass-difference curves (mass percent of epoxy–RDP samples minus mass percent of unmodified epoxy at the same degradation temperature) for epoxy and its flame-retarded composites are shown in Figure 13.3(b). The addition of 5% RDP destabilizes the epoxy resin at temperatures between 250–510 1C and beyond 660 1C, as shown by negative D mass percent values in those regions. A slight thermal stabilization is realized between 510 and 660 1C. Increasing the weight percent fraction of RDP proportionally increases the region over which

190

Table 13.2

TGA–DTA data for flame-retarded epoxy formulations. DTA 1st Stage

DTA 2nd Stage

DTA 3rd Stage

Sample

T10 (1C)

DT10 (1C)

Char at 800 1C (%)a

Epoxy Epoxy/RDP-95/5 Epoxy/RDP-90/10 Epoxy/RDP-85/15 Epoxy/RDP-80/20 Epoxy/RDP/I.30E95/ 0/5 Epoxy/RDP/I.30E90/5/5 Epoxy/RDP/I.30E85/10/5 Epoxy/RDP/I.30E80/15/5

364 337 336 330 319 362

0
27
28
34
45
2

0 0 0 0 3 0

150–475 150–450 150–450 150–450 150–450 150–450

390, 370; 343; 327; 322; 381,

463; (3, 4) (2) 417; (2, 2) (2) (1) 446; (3, 4)

450–650 450–650 450–690 450–690 450–690 450–650

560; 560; 545; 540; 547; 563;

338


26

3

150–450

327; 380; (2, 2)

450–650

562; (12)

–

–

336


28

2

150–450

335; (1)

450–650

548; (10)

–

–

329


35

2

150–450

323; (1)

450–650

555; (11)

–

–

Temp Range (1C)

Peak Pos.; (Peak Max.) (1C); (1Cmg
1)b

Temp Range (1C)

Peak Pos.; (Peak Max.) (1C); (1Cmg
1)b

Temp Range (1C)

Peak Pos.; (Peak Max.) (1C); (1Cmg
1)b

– – 690–750 690–750 690–750 –

– – 722; (2) 726; (3) 717; (4) –

(14) (11) (10) (11) (8) (14)

a

The silica content in the char has been accounted for. Peak Max. is the DTA response–signal value at the peak position, while Peak Pos. is the peak location with respect to temperature.

b

Chapter 13

191

Thermal and Fire Performance of Flame-Retarded Epoxy Resin

the flame retardant has a thermal stabilizing effect on epoxy resin. Additive fractions above 5% result in the enhanced char formation at temperatures above 450 1C. The overall stabilization effect (OSE) of RDP at specified percent loadings is calculated via integration of the area under the D mass percent versus temperature curve using the equation:

OSE ¼

800 X

ððmass % flame
retarded epoxy sampleT Þ

ð13:1Þ

T¼50


ðmass % control epoxyT ÞÞ

2000 0 -2000

5

10

15

% Additive fraction

Figure 13.4

OSE as a function of additive fraction in epoxy resin.

20

Epoxy/RDP/I.30E - 80/15/5

Epoxy/RDP - 80/20

Epoxy/RDP - 90/10

4000

Epoxy/RDP/I.30E - 95/0/5

6000 Epoxy/RDP - 95/5

OSE (%)

8000

Epoxy/RDP/I.30E - 85/10/5

10000

Epoxy/RDP - 85/15

Epoxy/RDP/I.30E - 90/5/5

where T is the temperature of degradation (T ¼ 50–800 1C) – the results are presented in Figure 13.4. A negative OSE value is observed for 5% RDP, which suggests that at this level the additive has an overall destabilization effect. The OSE values are positive and increase linearly with additive fraction for loadings at 10% and beyond. The anticipated thermal stabilization mechanism is as follows. At high temperatures, RDP degrades into phosphoric acid, which is subsequently converted into polyphosphoric acid. The polyphosphoric acids then promote cross-linking of polymer fragments to form a stable carbonaceous char network. Thus, catalytic char induction and condensed, or vapour phase, action of phosphorus may be implicated in the enhanced char formation at elevated temperatures.21–25 DTA curves of the unmodified epoxy and RDP flame-retarded samples are shown in Figure 13.5, and the extracted data are presented in Table 13.2. The DTA curve of the unmodified epoxy features a broad exothermic peak between 150 and 470 1C with spikes at 390 and 463 1C. This event is consistent with the

192

Temp. Difference (°C/mg)

Chapter 13 Epoxy Epoxy/RDPEpoxy/RDPEpoxy/RDPEpoxy/RDP-

12

95/ 5 90/10 85/15 80/20

8

4

0 50

Figure 13.5

150

250

350 450 Temperature (°C)

550

650

750

DTA curves for unmodified epoxy resin and RDP flame-retarded epoxy resin samples.

decomposition of epoxy chains to form a primary carbonaceous char. A pronounced exothermic peak is observed between 450 and 650 1C with a peak maximum at 560 1C and corresponds to char oxidation. RDP-containing epoxy resin formulations show similar, but less pronounced, features to those of the unmodified resin in the lower temperature regime, which suggests a possible chemical interaction. The exothermic peak value in the char oxidation stage is progressively suppressed with an increase in RDP content (Table 13.2). The depletion of the exothermicity is consistent with the enhanced char formation at elevated temperatures – less total heat is produced from oxidative thermal degradation of flame-retarded systems. Interestingly, exothermic features are observed between 690 and 750 1C for RDP-containing epoxy and correspond to the oxidation of the secondary char into a stable and final residue. The variations in DTA profiles between the epoxy resin and its RDP flame-retarded samples suggest a possible change in the degradation mechanism of the former. However, without performing further experimental work, such as evolved gas analysis (EGA), the authors cannot speculate on probable mechanistic alterations in the presence of RDP.

13.3.2.2

Effect of RDP and Nanoclay on the Thermal Stability of Epoxy Resin

The addition of RDP together with 5% I.30E to the epoxy resin is expected to change its decomposition behaviour. TG mass-loss profiles for the unmodified resin and its nanocomposites that contain 5% I.30E together with various amounts of RDP are shown in Figure 13.6(a). The addition of the nanoclay alone [epoxy–RDP–I.30E -(95/0/5)] does not improve the thermal stability of

193

Thermal and Fire Performance of Flame-Retarded Epoxy Resin 100

Epoxy Epoxy/RDP/I.30E - 95/ 0/ 5 Epoxy/RDP/I.30E - 90/ 5/ 5 Epoxy/RDP/I.30E - 85/10/ 5 Epoxy/RDP/I.30E - 80/15/ 5

(a)

Mass %

80

60

40

20

0 10

(b)

∆Mass %

5

0

-5 Epoxy/RDP/I.30E - 95/ 0/ 5 Epoxy/RDP/I.30E - 90/ 5/ 5 Epoxy/RDP/I.30E - 85/10/ 5 Epoxy/RDP/I.30E - 80/15/ 5

-10

-15 50

150

250

350

450

550

650

750

Temperature (°C)

Figure 13.6

(a) TGA curves and (b) mass-difference curves between RDP flameretarded nanocomposites and unmodified epoxy resin versus temperature in flowing air.

epoxy resin; in fact, the thermal degradation profile is shifted to lower temperatures relative to the unmodified resin and the effective OSE value is negative (Figure 13.4). However, the OSE value for epoxy–RDP–I.30E -(95/0/5) is less negative by a factor of two when compared to that of the epoxy–RDP (95/5) formulation. This shows that the nanoclay does not enhance the thermal stability of the epoxy resin, with the only effect seen at temperatures above 600 1C when most of the resin has already decomposed to leave silica. On the addition of 5% nanoclay together with 5, 10 and 15% RDP, the onset of thermal degradation, T10, is reduced in some cases by as much as 35 1C relative to that of the unmodified resin (Table 13.2). Similar observations were made for resin samples that contained RDP only (Table 13.2). While the flame-retarded nanoclay-containing

194

Chapter 13

formulations are destabilized in the lower temperature regime, at temperatures above 450 1C a stabilization effect is observed [Figure 13.6(b)]. Positive D mass percent values for temperatures above 450 1C suggest that the concomitant presence of RDP and the nanoclay promote char formation. The calculated OSE for epoxy resin containing 5% nanoclay and 5% RDP [epoxy–RDP–I.30E (90/5/5)], is 1.5 times higher than when RDP is used alone at 10% loading. Moreover, when used individually at 5% fractional loadings, both RDP and the nanoclay show an overall destabilization effect, as shown by negative OSE values (Figure 13.4). The combination of RDP and the nanoclay significantly improves the thermal stability of epoxy resin as measured by TGA, which suggests synergistic interactions – the combined stabilization effect is higher than a mere additive effect. However, increasing the weight fraction of RDP in the presence of clay destabilizes the resin in the lower temperature regime faster than the rate at which it promotes char formation, such that the OSE is effectively lowered relative to formulations that contain RDP alone at the same cumulative additive fractions, Figure 13.4. The combination of RDP and the nanoclay in epoxy resin is anticipated to be interactive, and thus promote the formation of char bonded structures. To evaluate synergistic and/or antagonistic interactions between RDP and the nanoclay, theoretical mass loss profiles were calculated from a linear combination of their individual profiles and compared to the experimental data. Figure 13.7 shows the calculated TG mass-loss profile of epoxy–RDP–I.30E (90/5/5) calculated from linear combination of a 1:1 mixture of epoxy–RDP (90/10) and epoxy–RDP–I.30E (90/0/10), experimental mass-loss profile and their mass difference as a function of temperature. These data suggest an

100 80 Mass %

60 1 40

∆ Mass %

3

Calculated

-1 20

Experimental

0

-3 50

150

250

350

450

550

650

750

Temperature (°C)

Figure 13.7

Calculated TG mass loss versus temperature for epoxy–RDP–I.30E (90/5/5), computed from a linear combination of a 1:1 mixture of epoxy– RDP (90/10) and epoxy–RDP–I.30E (90/0/10) ( . . . .), experimental mass-loss profile (—) and their mass difference (----) as a function of temperature.

195

Thermal and Fire Performance of Flame-Retarded Epoxy Resin Epoxy Epoxy/RDP/I.30E - 95/ 0/ 5 Epoxy/RDP/I.30E - 90/ 5/ 5 Epoxy/RDP/I.30E - 85/10/ 5 Epoxy/RDP/I.30E - 80/15/ 5

Temp. Difference (°C/mg)

12

8

4

0 50

150

250

350

450

550

650

750

Temperature (°C)

Figure 13.8

DTA curves for the unmodified epoxy resin and its RDP–nanoclay flame-retarded nanocomposites.

enhanced char formation between 350 and 550 1C and beyond 650 1C when RDP is used at low concentration (5%) together with the nanoclay as compared to when they are used individually. The exact mechanism of interaction between RDP and the nanoclay when interspersed in epoxy resin is not known; however, it is anticipated that these flame-retardant additives slow down or prevent depolymerization of the epoxy resin. DTA curves of unmodified epoxy and its RDP and nanoclay flame-retarded composites are shown in Figure 13.8. All samples exhibit a broad exothermic feature between 250 and 450 1C, with some samples showing spikes around 380 and 430 1C. A second exothermic feature that spans the temperature range 450– 650 1C and corresponds to the oxidation of primary char is observed for all samples. However, a notable feature is the depletion of the DTA exothermic response as a function of additive fraction, which suggests a change in the degradation mechanism that promotes char formation and thus possibly leads to the reduction in the amount of heat evolved.

13.3.3 13.3.3.1

Flammability Behaviour Limiting Oxygen Index

The LOI is a quantitative method via which the relative flame retardancy of materials can be evaluated. LOI is defined as the minimum oxygen concentration in an oxygen–nitrogen mixture that will just support flaming combustion. Table 13.3 and Figure 13.9 show LOI and DLOI (LOI of flame-retarded formulation minus LOI of unmodified epoxy resin) values of the unmodified

196

Chapter 13

Table 13.3

Vertical UL-94 and LOI data for flame-retarded epoxy formulations. Vertical UL-94 a

Sample

TBT (s)

Epoxy Epoxy–RDP (95/ 5) Epoxy–RDP (90/10) Epoxy–RDP (85/15) Epoxy–RDP (80/20) Epoxy–RDP–I.30E (95/0/5) Epoxy–RDP–I.30E (90/5/5) Epoxy–RDP–I.30E (85/10/5) Epoxy–RDP–I.30E (80/15/5)

88 7 3 13 15 110 23 8 16

Performance

LOI

HB V-0 V-0 V-1 V-1 HB V-1 V-0 V-0

27.8 30.2 30.2 28.6 29.6 30.0 27.8 27.8 27.5

a

Epoxy/RDP/I.30E - 80/15/5

Epoxy/RDP - 80/20

0.5

Epoxy/RDP - 85/15

Epoxy/RDP - 90/10

1.5

Epoxy/RDP/I.30E - 85/10/5

2.5

Epoxy/RDP/I.30E - 90/5/5

∆ LOI

3.5

Epoxy/RDP/I.30E - 95/0/5

4.5

Epoxy/RDP - 95/5

TBT, total burning time (s).

-0.5 5

10

15

20

% Additive fraction

Figure 13.9

DLOI (LOI of flame-retarded formulation minus LOI of unmodified resin) values vs. % additive fraction in epoxy resin.

epoxy resin and its flame-retarded formulations that contain RDP alone and, in some cases, together with the nanoclay I.30E. The LOI of the unmodified epoxy was determined to be 27.8. Addition of RDP at the 5% level increases the LOI value to 30.2. Increasing RDP to 10% had no effect, while beyond a 10% additive fraction resulted in reduced LOI values. These results suggest the existence of a threshold additive fraction beyond which increasing the phosphorus content in epoxy has no beneficial effect, in this case 1% phosphorus

Thermal and Fire Performance of Flame-Retarded Epoxy Resin

197

content. Phosphorus-containing formulations are expected to have improved flame resistance – it is understood that phosphoric and polyphosphoric acids promote char formation.26 When used alone in the epoxy resin, the nanoclay gives a higher LOI value [sample epoxy–RDP–I.30E (90/0/50), LOI ¼ 30.0) than the unmodified resin. However, when RDP is used in conjunction with the nanoclay, no improvement in LOI is observed. Despite the synergistic interactions predicted from TGA of epoxy–RDP–I.30E (90/5/5), from LOI analysis the combination of RDP and the nanoclay is not as effective as would have been expected (Figure 13.9). The authors envisage antagonistic interactions between RDP and the nanoclay in an attempt to improve flame resistance as measured by the LOI.

13.3.3.2

Vertical UL-94 Test

UL-94 ratings are used to describe the ease with which a polymeric material may be burned or extinguished. UL-94 vertical test results are presented in Table 13.3. The unmodified resin failed to satisfy the minimum requirements of UL-94 in the vertical testing mode, with a total burning time of 88 seconds and vigorous dripping leading to the ignition of surgical cotton. Epoxy resin formulations that contained 5 and 10% RDP achieved the V-0 rating. It is interesting that there is a remarkable increase in flame retardancy at low phosphorus content (5 and 10% RDP), while with the increase of phosphorus (15 and 20% RDP) diminishing returns are realized. A similar effect was observed from LOI measurements. Incorporation of nanoclay alone does not improve the flame retardancy of epoxy considerably, as measured by the vertical UL-94 test – the sample passed the horizontal burning test, but failed to satisfy the minimum requirements for the vertical test. Addition of RDP together with the nanoclay increases the flame resistance of the epoxy. Contrary to results observed in the LOI tests, flame resistance as measured by UL-94 increases with the amount of RDP when the nanoclay content is fixed at 5% w/w fraction. The presence of RDP together with nanoclay promotes charring at the surface during burning, and thus insulates the underlying polymeric material, which slows the mass-loss rate of decomposition products and reduces the tendency to drip. Dissimilar flame resistance results obtained from LOI and UL-94 measurements suggest that the action and efficiency of flame retardants in achieving their primary purpose is highly dependent on the heating conditions and environment.

13.3.3.3

Cone Calorimetry

13.3.3.3.1 Effect of RDP on Fire Performance of Epoxy Resin. The flammability behaviour of epoxy resin and its formulations that contain phosphorus and/or nanoclay has been assessed by cone calorimetry. The results, such as TTI, HHR, especially its peak value, and mass-loss rate are important to evaluate the fire safety of a material. The results obtained by cone calorimetry for RDP flame-retarded epoxy resin formulations are shown in Figure 13.10

198

Chapter 13 1000

Epoxy Epoxy/RDP Epoxy/RDP Epoxy/RDP Epoxy/RDP -

HRR(kW/m2)

800

95/ 5 90/10 85/15 80/20

600 400 200 0 0

50

100

150

200

250

Time (s)

Figure 13.10

HRR curves for the unmodified epoxy resin and RDP flame-retarded samples from cone calorimetry measurements at 50 kW m
2.

and the data are presented in Table 13.4. TTI, measured as the time required for the sample to burn with a sustained flame, which coincides with the onset of the HRR curves, is considerably reduced with the addition of RDP (Table 13.4). These results indicate that RDP-containing resins are thermally unstable and decompose at low temperatures to produce combustible volatiles, which are essential for a sustained flaming process, consistent with TG data shown in Figure 13.3(a). The presence of flame retardants may increase the viscosity of the unmodified resin, and so reduce the heat exchange between the samplesurface exposed to the radiant source and the bulk of the sample. This would result in a rapid increase in the surface temperature of the sample, and hence the time required for the volatiles to reach the pyrolysis temperature is significantly reduced.27 The HRR is considered to be a parameter of paramount importance in characterizing the intensity of a fire and is also related to mass-loss rate, and hence the fire growth rate. The peak HRR (PHRR) values of resin formulations that containing 5–15% RDP are significantly reduced, in some cases by 27%, when compared to that of the unmodified resin. However, the effect is less pronounced at 20% RDP additive fraction (Table 13.4). The observed reductions in PHRR suggest that the flame-retarded formulations burn to give a lowintensity flame. Products from the thermal decomposition of RDP promote cross-linking carbonization, which results in the formation of char that subsequently acts as a protective layer against heat and oxygen diffusion.28 The protective char thus prevents further decomposition of the epoxy resin to yield more residual char at the conclusion of the flaming combustion process. The fire growth rate index (FIGRA), defined as the ratio of PHRR values to the time at which they occur, tPHRR, indicates the burning propensity of a material,29 and these values are presented in Table 13.4. The control sample has

Cone calorimetry data for flame-retarded epoxy formulations at 50 kW m
2.a

Sample

TTI (s)

PHRR (kW m
2) (% red.)b

Epoxy Epoxy–RDP (95/5) Epoxy–RDP (90/10) Epoxy–RDP (85/15) Epoxy–RDP (80/20) Epoxy–RDP–I.30E (95/0/5) Epoxy–RDP–I.30E (90/5/5) Epoxy–RDP–I.30E (85/10/5) Epoxy–RDP–I.30E (80/15/5)

27 20 16 20 18 27 28 23 20

1076 995 (7) 786 (27) 787 (27) 841 (22) 915 (15) 1045 (3) 945 (12) 795 (26)

a

FIGRA (kW s
1)

THR (MJ m
2) (% red.)b

EHC (MJ kg
1)

AMLR (g s
1 m
2)

CY (%) @300 s

20 17 13 14 14 14 17 15 12

53 45(15) 42(21) 38(28) 41(23) 56(-6) 43(19) 47(11) 47(11)

21 20 20 20 19 21 20 21 22

22 20 18 18 22 21 19 16 16

16 23 30 42 31 15 21 22 27

% red., reduction in PHRR; AMLR, average mass loss rate, CY, char yield after 300 s. The coefficient of variations in cone calorimetry data are less than 10% for all parameters. b % reduction in PHRR and THR compared to the control sample.

Thermal and Fire Performance of Flame-Retarded Epoxy Resin

Table 13.4

199

200

Chapter 13

the highest FIGRA value of 20. On the addition of RDP, FIGRA values are reduced, in some cases by as much as 7 units, i.e. by 35%. This largely results from the reduction in PHRR, since tPHRR values are similar. The level of flame retardancy achieved with RDP alone as measured by the FIGRA index is notable. The reduction in FIGRA values with the addition of RDP suggests that the flame-retarded formulations burn with a lower propensity when compared to the unmodified resin. Low FIGRA values indicate delayed times to flashover, which allows enough time to evacuate and/or for fire extinguishers to arrive. THR values, as measured via cone calorimetry under a constant heat flux of 50 kW m
2 for RDP-containing epoxy resin formulations, are presented in Table 13.4. The control sample gave a THR value of 53 MJ m
2. Significant percent reductions, in the 15–28% range, in THR are observed when RDP is added to epoxy at 5–20% loadings. The addition of RDP at 15% gave the largest reduction in THR, B28%; however, addition of RDP at higher loadings, 20%, had a less pronounced effect with a reduction of 22% in THR observed. The lower THR values observed for epoxy-containing RDP are consistent with reduced fuel content from the flame-retarded samples during combustion (Table 13.4). The residual char yields significantly increase with RDP content; however, the trend is not the same as observed for char yields realized from TG experiments. This may be attributed to different heating conditions – slow heating rates are used for TG experiments, while sample temperatures rapidly evolve when samples are exposed to a high heat flux, as in cone calorimetry. The variation in the heating conditions dictates differences in flame-retardation mechanisms to yield dissimilar residual char trends from TG and cone calorimetry. The effective heat of combustion (EHC), which reflects the calorific value per unit mass of the specimen for the control sample, is 21 MJ kg
1 (Table 13.4). There is a slight reduction in EHC values with the addition of RDP. That the heat released per unit mass of volatiles is invariant with percent additive fraction for flame-retarded formulations (Table 13.4) suggests that the flame retardancy action of phosphorus does not involve flame inhibition and/or fuel dilution. The reduced amount of volatiles released from flame-retarded formulations, as shown by increased residual char yields, suggests that phosphorus is active in the condensed phase.

13.3.3.3.2 Effect of RDP and Nanoclay on Fire Performance of Epoxy Resin. HRR curves as a function of time, as measured by cone calorimetry, for RDP–nanoclay flame-retarded epoxy resin systems are shown in Figure 13.11 and the data are presented in Table 13.4. TTI does not change following addition of the nanoclay alone [epoxy–RDP–I.30E (95/0/5)]. Samples that contain RDP and the nanoclay exhibit shorter TTI values. However, their TTI values are always higher than when RDP is used individually at the same percent additive fraction. A slight reduction, 15%, in PHRR is observed for epoxy– RDP–I.30E (95/0/5), which suggests the nanoclay may somewhat slow down or

201

Thermal and Fire Performance of Flame-Retarded Epoxy Resin 1000

Epoxy Epoxy/RDP/I.30E Epoxy/RDP/I.30E Epoxy/RDP/I.30E Epoxy/RDP/I.30E -

HRR (kW/m2)

800

95/ 0/5 90/ 5/5 85/10/5 80/15/5

600

400

200

0 0

50

100

150

200

250

Time (s)

Figure 13.11

HRR curves for the unmodified epoxy resin and RDP flame-retarded nanocomposites from cone calorimetry measurements at 50 kW m
2.

prevent the production of volatiles by promoting carbonization. Interestingly, the reduction in PHRR is linearly dependent on the RDP additive fraction reaching a maximum value of 26% for epoxy–RDP–I.30E (80/15/5). Premature ignition and the observed reduction in fire intensity are indicated by lower PHRR for flame-retarded systems compared to that of the unmodified resin. This further underlines the proposition that the flame-retardation mechanism of phosphorus-based flame-retardant additives results from their decomposition at lower temperatures to yield by-products subsequently responsible for improving the fire resistance of the polymeric system at elevated temperatures. The fire safety of epoxy formulations that contain RDP and nanoclay was evaluated using FIGRA values (Table 13.4). Addition of 5–15% RDP together with 5% nanoclay reduces the FIGRA values, with a remarkable 40% reduction observed for the epoxy–RDP–I.30E (80/15/5) formulation. THR values as measured via cone calorimetry under a constant heat flux of 50 kW m
2 for RDP- and nanoclay-containing epoxy resin formulations are presented in Table 13.4. The sample that contains 5% nanoclay gave a THR value higher than that of the control sample (56 MJ m
2). The addition of clay alone [epoxy– RDP–I.30E (95/0/5)] reduced the PHRR values, but its combustion–flaming process is prolonged, which results in a higher THR value. When RDP and nanoclay are concomitantly added to the epoxy resin, significant reductions in THR values are observed. However, the percent reductions observed are not as pronounced as those obtained when RDP is used alone at the same loading fraction. This suggests that the presence of clay together with RDP may lead to antagonistic interaction, when compared to the unmodified resin, when the fire

202

Chapter 13

performance is evaluated via cone calorimetry. There is no change in the EHC for epoxy resin formulations that contain RDP and nanoclay. These results underline the proposition that phosphorus achieves its efficiency in flame retardation in the condensed phase. Contrary to the results obtained from TGA, the addition of RDP in juxtaposition with the nanoclay results in an antagonistic effect as the summed percent-gain in fire resistance is lower than when RDP and the nanoclay are used independently at the same loading fraction. However, these results are similar to what is observed in both LOI and UL-94. When used individually, RDP and the nanoclay were shown to increase LOI values, but did not improve the flame resistance when used together. When added at 5% loading fractions, RDP and the nanoclay achieved V-0 and HB classifications, respectively. On combining the two flame retardants to give a 10% cumulative additive fraction, the UL-94 classification obtained was V-1, which lies between HB and V-0. This suggests that there are no synergistic interactions between RDP and the nanoclay in reducing flammability as measured by UL-94. However, there were significant improvements in thermal stability for the resin mixture that contains 5% RDP and 5% nanoclay as measured by TGA. The incorporation of the nanoclay with RDP in epoxy resin did not give desirable results in some aspects of flame retardancy: LOI values were not improved with respect to that for the unmodified resin and also the UL-94 classification of V-0 was degraded to V-1 at low cumulative additive concentrations. The heavily cross-linked structure of thermosetting epoxy matrix may hinder the migration of nanoparticles to the pyrolysis surface, and thus restrict their effectiveness as a fire barrier.

13.4 Conclusions The interspersion of RDP in epoxy resin was shown to improve the thermal stability of the resin at elevated temperatures. The flame resistance, as measured by LOI and UL-94, of RDP-containing resin formulations is higher than for the control sample, but is invariant with the loading fraction. Formulations with RDP alone at 5 and 10% gave the best flame resistance in LOI and UL-94 experiments, while going beyond 10% RDP had an adverse effect. In cone calorimetry the addition of RDP at 10% results in remarkable reductions in both the PHRR and THR, and enhanced char formation. Addition of nanoclay alone did not improve the thermal stability of the resin, and neither did it improve its flame resistance as measured by cone calorimetry. In contrast, addition of clay together with RDP did not show an advantage. In fact the concomitant presence of RDP and the nanoclay in epoxy compromised thermal stability with the exception of the epoxy–RDP–I.30E (90/5/5) formulation. It also did not produce any significant improvement in the fire properties as measured by cone calorimetry and LOI. This may be attributed to possible antagonistic interactions between the constituent components. However, at

Thermal and Fire Performance of Flame-Retarded Epoxy Resin

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high concentrations of RDP (10 and 15%), UL-94 ratings of V-0 are observed. The addition of a known flame retardant, RDP, and nanoclay yielded mixed results with respect to thermal stabilization and improved fire performance. Future studies will entail the use of other types of organically modified clays in an attempt to achieve efficient thermal and fire resistance for materials based on epoxy resin.

Acknowledgements We thank Betime Nuhji, Deakin University, Australia for TEM analysis. One of the authors, Charalampos Katsoulis, would like to acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC).

References 1. G. Beyer, Nanocomposites offer new way forward for flame retardants. Plastics, Additives and Compounding, 2005, 7, 32–35. 2. S.Y. Lu and I. Hamerton, Recent developments in the chemistry of halogen-free flame retardant polymers, Prog. Polym. Sci., 2002, 27, 1661–1712. 3. G. Beyer, Nanocomposites: a new class of flame retardants for polymers, Plastics, Additives and Compounding, 2002, 4, 22–28. 4. G. Chigwada, P. Jash, D.D. Jiang and C.A. Wilkie, Synergy between nanocomposite formation and low levels of bromine on fire retardancy in polystyrenes, Polym. Degrad. Stab., 2005, 88, 382–393. 5. G. Camino and S. Lomakin, Intumescent materials, in: R.A. Horrocks, D. Price, eds Fire retardant materials, Woodhead Publishing Limited, Cambridge, UK, 2001, pp. 318–336. 6. E.T. Thostenson, C. Li and T.-W. Chou, Nanocomposites in context, Compos. Sci. Technol., 2005, 65, 491–519. 7. J. Jordan, K.I. Jacob, R. Tannenbaum, M.A. Sharaf and I. Jasiuk, Experimental trends in polymer nanocomposites – a review, Mater. Sci. Eng. A., 2005, 393, 1–11. 8. K. Putz, R. Krishnamoorti and P.F. Green, The role of interfacial interactions in the dynamic mechanical response of functionalized SWNT – PS nanocomposites, Polymer, 2007, 48, 3540–3545. 9. J. Zhang, D.D. Jiang, D. Wang and C.A. Wilkie, Styrenic polymer nanocomposites based on an oligomerically-modified clay with high inorganic content, Polym. Degrad. Stab., 2006, 91, 2665–2674. 10. G. Camino, G. Tartaglione, A. Frache, C. Manferti and G. Costa, Thermal and combustion behaviour of layered silicate–epoxy nanocomposites, Polym. Degrad. Stab., 2005, 90, 354–362. 11. P.C. Lebaron, Z. Wang and T.J. Pinnavaia, Polymer-layered silicate nanocomposites: an overview, Appl. Clay Sci., 1999, 15, 11–29.

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12. X. Kornmann, H. Lindberg and L.A. Berglund, Synthesis of epoxy-clay nanocomposites: influence of the nature of the clay on structure, Polymer, 2001, 42, 1303–1310. 13. X. Kornmann, H. Lindberg and L.A. Berglund, Synthesis of epoxy-clay nanocomposites. Influence of the nature of the curing agent on structure, Polymer, 2001, 42, 4493–4499. 14. D. Ratna, O. Becker, R. Krishnamurthy, G.P. Simon and R. Varley, Nanocomposites based on a combination of epoxy resin, hyperbranched epoxy and a layered silicate, Polymer, 2003, 44, 7449–7457. 15. A. Yasmin, J.L. Abot and I.M. Daniel, Processing of clay/epoxy nanocomposites by shear mixing, Scripta Materialia., 2003, 49, 81–86. 16. W. Liu, S.V. How and M. Pugh, Organoclay-modified high performance epoxy nanocomposites, Compos. Sci. Technol., 2005, 65, 307–316. 17. E. Kandare, G. Chigwada, D. Wang, C.A. Wilkie and J.M. Hossenlopp, Probing synergism, antagonism, and additive effects in poly(vinyl ester) (PVE) composites with fire retardants, Polym. Degrad. Stab., 2006, 91, 1209–1218. 18. B.N. Jang, M. Costache and C.A. Wilkie, The relationship between thermal degradation behavior of polymer and the fire retardancy of polymer/ clay nanocomposites, Polymer, 2005, 46, 10678–10687. 19. E.D. Weil, Additivity, synergism, and antagonism in flame retardancy, in: Flame retardancy of polymeric materials, W.C. Kuryla, A.J. Papa (ed.), Marcel Dekker, Inc, 1975, pp. 185–243. 20. E. Kandare, B.K. Kandola, J.E.J. Staggs, P. Myler. Global kinetics of thermal degradation of flame-retarded epoxy resin formulations. Polym. Degrad. Stab. 2007, 92, 1778–1787. 21. B.K. Kandola, A.R. Horrocks, P. Myler and D. Blair, Thermal characterization of thermoset matrix resins, in: G.L. Nelson, C.A. Wilkie, eds. Fire and Polymers: Materials and Solutions for Hazard Prevention. ACS Symp. Ser., Washington 2001, 797, p. 344–360. 22. B.K. Kandola, R.A. Horrocks, P. Myler and D. Blair, New developments in flame retardancy of glass-reinforced epoxy composites, J. Appl. Poly. Sci., 2003, 88, 2511–2521. 23. J. Green, Phosphorus containing flame retardants, In: Fire retardancy of polymeric materials, A.F. Grand, C.A. Wilkie, ed. Marcel Dekker, Inc. 2000, pp. 147–170. 24. F. Samyn, S. Bourbigot, S. Duquesne and R. Delobel, Effect of zinc borate on the thermal degradation of ammonium polyphosphate, Thermochimica Acta, 2007, 456, 134–144. 25. M. Jimenez, S. Duquesne and S. Bourbigot, Characterization of the performance of an intumescent fire protective coating, Surf. Coatings Tech., 2006, 201, 979–987. 26. S. Gaan and G. Sun, Effect of phosphorus and nitrogen on flame retardant cellulose: A study of phosphorus compounds, J. Anal. Appl. Pyrolysis, 2007, 78, 371–377. 27. S. Nazare´, T.R. Hull and B. Biswas, Study of relationship between rheological and flammability properties of flame retarded poly(butylene

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terephthalate) containing nanoclays. Materials in 18th Annual BCC conference on Flame Retardancy of Polymeric Materials, 21–23 May, 2007, Stamford, USA. 28. J. Deng and W. Shi, Synthesis and effect of hyperbranched (3-hydroxyphenyl) phosphate as a curing agent on the thermal and combustion behaviours of novolac epoxy resin, Eur. Polym. J., 2004, 40, 1137–1143. 29. S. Bourbigot, E. Devaux and X. Flambard, Flammability of polyamide6/clay hybrid nanocomposite textiles, Polym. Degrad. Stab., 2002, 75, 397–402.

Intumescents

CHAPTER 14

Porosity Estimates of Intumescent Chars by Image Analysis J.E.J. STAGGS Energy Resources Research Institute, University of Leeds, Leeds, LS2 9JT, UK

14.1 Introduction Intumescent chars are attractive candidates for incorporation into fire protection systems in many different scenarios, including steel protection and more general fire retardancy applications. The production of a robust, coherent, highly porous char is desirable from a thermal insulation viewpoint. Although such structures have low density by definition, they also have low thermal conductivity. This is not the complete picture, however. Consider an insulating layer of material of thickness l, thermal conductivity k, subject to an external heat flux q_ 00 on one exposed surface. The temperature drop across the layer is of the order DTBq_ 00 l=k. Thus, we see that for a given heating situation, the temperature on the unexposed face, and hence the performance of the insulator, is governed by the conduction heat transfer coefficient k0 ¼ k=l. Ideally, k should be small enough (or l large enough) to ensure that the temperature of the unexposed face remains acceptably low. This observation of the importance of k 0 is of pivotal significance to intumescent chars: their very nature is to have high volumetric expansion (large l ) and hence high porosity (which implies low k).

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

209

210

Chapter 14

Correctly assigning values for the density and specific heat capacity of a porous solid is unproblematic: if the subscript g denotes a pore property and the subscript s denotes a solid matrix property, then the composite density and specific heat capacity are given, respectively, by r ¼ jrg þ ð1
jÞrs ; c ¼ lcg þ ð1
lÞcs Ecs

ð14:1Þ

where j is the porosity (the ratio of the pore volume to the total volume) and l is the mass fraction of pores, which will be small compared with one for most chars of interest. In general, thermal conductivity is a decreasing function of porosity, but the exact dependence depends on, among other things, the shape and distribution of the pores within the char in relation to the direction of heat transfer. This point is illustrated by the direct numerical simulations shown in Figure 14.1. Here we see the results of a finite-element calculation of the effective thermal conductivity of a porous solid with two different orientations of plate-like pores. The overall porosity for both cases is fixed at 25%, with ks/kg ¼ 100, where k is thermal conductivity. In the first case the pores are aligned such that their long axes are perpendicular to the direction of heat transfer and in the second case the pores are aligned with their long axes parallel to the direction of heat transfer. The shaded contours represent temperature isotherms. Note that the case where the pores’ long axes are normal to the direction of heat transfer has the best insulating effect. The reason for this is that orientation of the low thermal conductivity pores is such that they more effectively disrupt the conductive pathway in the direction of heat transfer.

(a) k / ks = 0.31

Figure 14.1

(b) k / ks = 0.7

Direct numerical simulation to illustrate the effect of pore orientation on effective thermal conductivity, j ¼ 25%, ks/kg ¼ 100.

211

Porosity Estimates of Intumescent Chars by Image Analysis

In general it is possible to place bounds on the dependence of effective thermal conductivity on porosity. For any porous solid consisting of a solid matrix of thermal conductivity ks, with gas-filled voids of conductivity kg, it is easy to demonstrate that the effective thermal conductivity k must be between two extremes obtained by considering two thermal resistors, either in series or in parallel:1
 1 k 1 ks  1
j 1
;k¼ kj þ ð1
jÞ ks k kg

ð14:2Þ

For some cases it is possible to find explicit expressions for the dependence of k on j. For spherical voids of fixed radius and low porosity, the modified Maxwell expression1,2 k 2jð1
kÞ þ 1 þ 2k E ks 2k þ 1
jð1
kÞ

ð14:3Þ

gives reasonable results (see Figure 14.2 for an illustration). Another expression due to Bruggeman2 assumes that the voids consist of spheres with an infinite range of radii. In this case k/ks is given implicitly by the equation k=ks
1=k ðk=ks Þ1=3 ð1
1=kÞ

¼1
j

ð14:4Þ

1 0.9

(parallel)

0.8 0.7 Maxwell's approximation

k / ks

0.6 0.5

Effect of pore size & shape for fixed porosity

0.4 0.3 0.2 (series)

0.1 0 0

0.2

0.4

0.6 Porosity

Figure 14.2

Effective thermal conductivity bounds (k ¼ 10).

0.8

1

212

Chapter 14

When k is large and j is not close to one, it may be shown that the solution of Bruggeman’s expression gives
1=3
2 k 1
ð1
jÞ3=2 1 þO Eð1
jÞ1=2 þ ks k 2kð1
jÞ

ð14:5Þ

Here O(1/k)2 means terms that involve powers of 1/k of order two and higher. Hence a major step towards estimating the thermal conductivity of a char may be made if the following key facts are known:  The porosity of the char.  The shape and/or orientation of the pores with reference to the direction of heat transfer.  The distribution of pore sizes.

14.2 Pore-Finding Algorithm Consider a two-dimensional (2D) rectangular digital image of a char section that has been segmented into pure black and white. The char pores appear as collections of black pixels and the solid matrix of the char as white pixels. Let the segmented image contain a total of n black pixels. Furthermore, assume that the pixels are labelled in some consistent way and that we have a list P ¼ fPi gni¼1 of the labels of all n black pixels. In relation to the segmented image, a pore is defined as a collection of connected black pixels. Connections can be defined in one of two main ways, depending on whether a connection between two pixels is defined as a common edge or a common vertex (Figure 14.3). Common vertex connection results in fewer, larger pores than does common edge connection. Once we have decided on the connection definition, the algorithm to identify a pore is straightforward. The goal is to assemble a list of pixels p ¼ fpj gm j¼1  P that are connected and hence form a pore. This may be done efficiently as follows:  Starting with an empty pore list p, define a seed for the pore that corresponds to the first pixel label in P and add it to the pore list p.  Delete the seed from P. Common Edge Connection: Pixels 1 and 2 share a side and so are connected, but not pixel 3, so there are two pores

1 2 3

Figure 14.3

Common Vertex Connection: Pixels 1 and 2 share common vertices and pixels 2 and 3 share a common vertex, so all are connected and there is one pore.

Pixel connection definitions.

Porosity Estimates of Intumescent Chars by Image Analysis

Test Case 1: Porosity = 0.185

Figure 14.4

213

Test Case 2: Porosity = 0.26

Test cases for pore-finding algorithm.

 Search through P and add to p any members that are connected to any members of p.  Each time a connected member of P is found, delete it from P. At the end of the search, P will be depleted by the pixels comprising the pore list p, which gives a list of pixel labels that form a connected pore. This search is then repeated to find the next pore and so on until P is empty. To check the pore-finding algorithm, two negative images were prepared to show white pores of known porosity in a black matrix (Figure 14.4). In both cases the algorithm correctly identified the pores and the overall porosity. For each pore, an equivalent pore radius (EPR) and aspect ratio (AR) is defined. These correspond, respectively, to the radius of a circle with the same area as that of the pore and the ratio of the maximum height to maximum width of a pore (Figure 14.5).

14.3 Relationship Between Area Porosity and Volume Porosity We define the area porosity, j2D , as the ratio of pore area to total area as determined in a 2D section through a char, and the volume porosity, j3D , as the true porosity of the 3D char, i.e. the ratio of pore volume to total volume. Given a porous char, it is not immediately obvious that the area porosity, obtained from a section of the char, will be the same as the volume porosity. To see this, consider a single sphere circumscribed by a cube. Taking a section parallel to one of the faces of the cube, we see that the intersected sphere will produce a circle and so the resulting area porosity will be anything in the range

214

Chapter 14

EPR: circles of the same area as the pore

B

Aspect Ratio =

B A

A

Figure 14.5 Definition of EPR and AR.

Section Plane

r 2 − z2

{

r

z

3D sphere

Figure 14.6

2D Section

Difference between area and volume porosity.

from 0 to p/4, depending where the section is taken, whereas the volume porosity is always p/6 (Figure 14.6). With this in mind, consider the relationship between area and volume porosity for a porous char with a large number of pores whose centres are randomly distributed throughout the char. A thin section of thickness dz and side l through the char intersects a certain number n of pores and produces a 2D plane of intersected pore slices (Figure 14.7).  2 , where A is the average area of the The area porosity is given by j2D ¼ nA=l intersected pores. Now, since the total volume of pores intersected by the 2  it follows that the volume porosity is j3D ¼ nAdz=ðl  section is nAdz, dzÞ ¼ j2D . Hence, if the pore centres are uniformly distributed, we would expect the area

Porosity Estimates of Intumescent Chars by Image Analysis

215

3D pore

Section Plane

Figure 14.7

2D Section

Intersected pore distribution.

and volume porosities to be identical for a representative section taken through the char. To illustrate this result, Monte Carlo simulations were carried out. In the simulations, spheres were randomly allocated positions within a cube so that their (x, y, z) co-ordinates were uniformly distributed and their radii followed a given distribution. A section through the middle of the cube was taken and the area porosity computed. This was then compared with the volume porosity, and the results are shown in Figure 14.8. This figure was computed using uniformly distributed radii, although the same result was obtained for other distributions.

14.4 Relationship Between 2D and 3D Pore Distributions It is important to realise that a probability density of radii obtained from a 2D section will not be the same as the true 3D probability density. However, the key question is whether or not we can reconstruct the 3D distribution from the 2D data. To answer this, we must first investigate the relationship between the 2D and 3D distributions. Consider again a porous char with pores that consist of randomly distributed spheres within a cube of side l. Now, if the z-ordinates are uniformly distributed and the probability density function (PDF) of the radii is f ð^rÞ, where ^r ¼ r=l,

216

Chapter 14 0.3 r ~ U(0,0.025)

y=x

Volume Porosity

0.25 0.2 0.15 0.1 0.05 0 0

Figure 14.8

0.05

0.1

0.15 Area Porosity

0.2

0.25

0.3

Results of Monte Carlo simulations for randomly distributed spheres.

since r and z are independent, it may be shown3 that the PDF of radii in the 2D section f ð2DÞ ðrÞ is given in terms of the true PDF as

f

ð2DÞ

ðrÞ ¼ 2r

Z1

r

f ð^ rÞd^r ð^ r2


r2 Þ1=2

ð14:6Þ

By way of illustration, we now consider two test cases. In each test case Monte Carlo simulations were carried out whereby spheres were randomly allocated positions within a cube so that their (x, y, z) co-ordinates were uniformly distributed. The PDF of intersected sphere radii was then computed and the calculation repeated 50 times in order that an average PDF could be found and compared to an appropriate expression obtained from Equation (14.6).

14.4.1

Test Case 1 (Identical Spheres)

If the 3D spheres all have the same radius, r0 say, then it can be shown from above that the distribution of radii from a 2D section will have a PDF given by f ð2DÞ ðrÞ ¼

r ð1
r2 Þ1=2

ð14:7Þ

where r ¼ r=r0 . The graph in Figure 14.9 shows the results of the Monte Carlo simulation compared with the predicted PDF.

217

Porosity Estimates of Intumescent Chars by Image Analysis 0.35 Monte Carlo Results Prediction

0.3

Relative Frequency

0.25 0.2 0.15 0.1 0.05 0 0

0.2

0.4

0.6

0.8

1




Figure 14.9

14.4.2

Comparison between expected frequency distribution and Monte Carlo results for test case 1.

Test Case 2 (Spheres with Uniformly Distributed Radii)

If the 3D spheres have uniformly distributed radii, ^rBUð0; 1Þ, then it can be shown from above that the distribution of radii from a 2D section will have a PDF given by   pffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 þ 1
r2 f ð2DÞ ðrÞ ¼ 2r ln r

ð14:8Þ

where r ¼ r/l. The graph in Figure 14.10 shows the results of the Monte Carlo simulation, carried out as in the first test case above, and compared with the predicted PDF.

14.5 Construction of 3D Distributions from 2D Distributions We have seen above that the distribution of radii from a 2D section is not the same as the distribution of radii of the 3D spheres from which the section was taken. In particular, we have the result that for a random distribution of spheres of identical radius, the distribution of radii in a 2D section is given by the probability density, Equation (14.7). The important question here is whether or not we can construct the PDF for the 3D distribution from the PDF of

218

Chapter 14 0.08 0.07

Relative Frequency

0.06 0.05 0.04 0.03 0.02

Monte Carlo Results Prediction

0.01 0 0

0.2

0.4

0.6

0.8

1




Figure 14.10

Comparison between expected frequency distribution and Monte Carlo results for test case 2.

the 2D section. In fact, as we shall see presently, it is possible to use the relationship shown in Section 14.4 to construct the 3D distribution of pore radii from the 2D distribution. In principle, this involves the solution of a difficult Volterra integral equation of the second kind.4 In practice it is rarely possible to achieve the exact solution (especially using experimental data for the 2D distribution) and so a numerical method must be used. Suppose that the 2D distribution is given in terms of (n+1) discrete points ð2DÞ fi ¼ f ð2DÞ ðri Þ, i ¼ 0,1, . . . , n. Then we seek to construct f on the same set of points using a simple numerical rule for the integral. The only difficulty lies in evaluation of the integral in the neighbourhood of r^ ¼ r. Now, let e be small. Since 9 8rþ Z1
ð14:9Þ

rþ

r

expanding the first integral for small e gives

2r

rþ Z r

r f ð^ rÞd^ r2 ð^


r2 Þ1=2

¼ ð8rÞ1=2 f ðrÞ þ Oð3=2 Þ

ð14:10Þ

Here O(e)3/2 means terms that involve powers of e of order 3/2 and higher. So, if ri ¼ ih, where h ¼ 1/n, then using the trapezium rule to evaluate the remaining

Porosity Estimates of Intumescent Chars by Image Analysis

219

integral gives ð2DÞ

fi

1=2 Eð8r ( i hÞ fi ) n
1 X fiþ1 fn fj þ þ2 þ ri h 2 2 1=2 ðr2iþ1
r2i Þ1=2 ðr2n
r2i Þ1=2 j¼iþ2 ðrj
ri Þ

ð14:11Þ

which, assuming fn ¼ 0, may be rearranged to give fi: ð2DÞ

fi ¼

fi


Si

ð8ri hÞ1=2

8 ri hfiþ1 > > ; i ¼ n
2; > > 1=2 2 > < ðriþ1
r2i Þ " # Si ¼ n
1 > X > f f iþ1 j >r h > > : i ðr2
r2 Þ1=2 þ 2 2 2 1=2 j¼iþ2 ðrj
ri Þ i iþ1

ð14:12Þ

ð14:13Þ

and Si ¼ 0, i > n
2. To test the method, we attempt to reconstruct the 3D PDF from a 2D section obtained from a known distribution of spheres. We may then compare the reconstructed PDF with the known distribution. So, if a 2D section is generated from randomly distributed spheres with uniformly varying radius, i.e. r B U(0,l) where l is the maximum radius in the 3D distribution, then the expected 3D PDF should simply be f(r) ¼ 1, 0 r r r 1, where r ¼ r/l. Therefore, using the 2D PDF   pffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 þ 1
r2 f ð2DÞ ðrÞ ¼ 2r ln r

ð14:14Þ

the numerical method above was applied to construct the expected 3D PDF. The results are shown in Figure 14.11 and indicate clearly that the expected U(0,1) distribution for r is recovered.

14.6 Analysis of a Real Char Section The diagram in Figure 14.12 shows a segmented image of a section through a real intumescent char (supplied by International Paint Ltd), with an expansion ratio of approximately 10, along with a representative region selected for analysis. Note that in this case the char formed numerous very large pores close to the top surface – which appears in the segmented image as a dark irregular band close to the top surface. The z-direction is taken out of the plane of the figure, the x-direction is horizontal and the y-direction is vertical. The char expanded in such a way as the structure in the x- and z-directions is broadly similar.

220

Chapter 14 1.4

2D Distribution

1.2 1 PDF

Constructed 3D Distribution 0.8 0.6 0.4 0.2 0 0

0.2

0.4

0.6

0.8

1




Figure 14.11

Reconstructed 3D distribution using test data for the 2D distribution.

Figure 14.12

Sample segmented image of a char section.

221

Porosity Estimates of Intumescent Chars by Image Analysis 0.0025

Relative Frequency

0.0020

0.0015

0.0010

0.0005

0.0000 1500

2000

2500

3000

3500

4000

4500

x-ordinate of pore

Figure 14.13

Frequency distribution of x-ordinates of pores.

Individual pores were determined from the pixel data using the algorithm of Section 14.2 and the frequencies of the x-ordinates of their centroids are shown in Figure 14.13. These data indicate strongly that the x-ordinates are uniformly distributed. Given the expansion of the char, it is reasonable to assume that the char structure is similar in the x- and z-directions. It may well not be in the y-direction, because this corresponds to the main direction of heat transfer as the char was formed. Hence, it seems reasonable to deduce from the x-ordinate data that the z-locations of the pores are uniformly distributed and so the conditions of the result established in Section 14.3 apply. Hence, we may use the result of Section 14.4 to attempt to construct the 3D distribution of EPR from the 2D data. The graph in Figure 14.14 shows the relative frequencies of pore radii (triangles) from the 2D segmented image. The squares on the plot correspond to the application of the method of Section 4.5 to obtain the reconstructed 3D distribution. Note that the reconstructed 3D PDF has more noise than the 2D distribution – a direct consequence of the method – but it is still useable. Note also that a small number of very small pores were found from the image analysis, but these have been deleted as they are likely to be below the resolution of the scanner used to produce the original segmented image. A log–log plot of the 3D frequency distribution strongly suggests a PDF of the form

f ðrÞ ¼


 n
1 r
n ; r  rmin rmin rmin

ð14:15Þ

222

Chapter 14 r / max(r) 0

0.2

0.4

0.6

0.8

1

1

Relative Frequency

0.1 0.01 2D Data

0.001 0.0001 3D reconstruction 0.00001 0.000001

Figure 14.14

Comparison between empirical 2D distribution (triangles) and reconstructed 3D distribution (squares).

ln(r/r min) 0

0.5

1

1.5

2

2.5

3

0

ln(Relative Frequency)

-2 -4 -6 -8 -10 y = -3.6103x - 1.2024 -12

R2 = 0.934

-14

Figure 14.15

Confirmation of exponential distribution.

and is shown in Figure 14.15, where the exponent n has been found as approximately 3.6. The fitted frequency obtained from Equation (14.15) is shown by the solid smooth curve in Figure 14.14. Finally, the graph in Figure 14.16 shows the frequency distribution of the pore AR. This figure indicates that most pores are oblate – being compressed along the axis in the direction of heat transfer. This is actually a desirable property, as demonstrated in the introduction, since for the same porosity a char that consists of oblate, disc-like pores (where the axis of the disc is parallel to the direction of heat transfer) will have a lower thermal conductivity than a

223

Porosity Estimates of Intumescent Chars by Image Analysis 0.008

Relative Frequency

0.007 0.006 0.005 0.004 0.003 0.002 0.001 0 0

0.2

0.4

0.6

0.8

Aspect Ratio

Figure 14.16

Frequency distribution of pore AR.

char that consists of perfectly spherical pores.5,6 The Bruggeman model5,6 for oblate spheroids, flattened in the direction of heat transfer, of negligible thermal conductivity (kg E 0), may be appropriate for this type of char. Here, the composite thermal conductivity is given by an expression of the form k ¼ ð1
jÞ1=ð1
F Þ ks

ð14:16Þ

where F is a shape factor of the spheroid (F ¼ 1/3 for a sphere, 1/2 for a cylinder). Note that Equation (14.5) agrees with this for F ¼ 1/3 as k - N.

14.7 Conclusion The analysis in this chapter is concerned with interpreting details about the 3D distribution of pores within a char using data obtained from an image of a 2D section through the char. The primary reason for doing this is to obtain relevant details to enable an appropriate correlation to be used to model the thermal conductivity of the char. Also, the analysis provides a tool such that the frequency distributions of EPR may be estimated and so new correlations may be developed that assume realistic pore distributions. It transpires that if the distribution of the z-ordinates of the pores is uniform (where z is measured normal to the plane of the 2D section), then the following results are valid: 1. The volume porosity is the same as the area porosity. 2. The 2D distribution of equivalent pore radii frequencies is not the same as the 3D distribution. In fact, there is a relationship between the 2D EPR

224

Chapter 14 (2D)

distribution and the 3D equivalent pore distribution. If f (r) is the PDF of the 2D distribution, where r ¼ r/rmax (r is the EPR and rmax the maximum EPR), and f(r) is the PDF of the 3D distribution, then:

f

ð2DÞ

ðrÞ ¼ 2r

Z1

r

f ð^ rÞd^r ð^r2
r2 Þ1=2

ð14:17Þ

3. The relationship in result (2) above may be used to obtain the 3D EPR distribution from the corresponding 2D distribution. Analysis of an actual section through a char reveals that the x-ordinates of the pore centroids are uniformly distributed. By symmetry, we would broadly expect a section in the x–y plane to have a similar structure to that of a section in the y–z plane. From this, it follows that if the x-ordinates are uniformly distributed, we would expect that the z-ordinates of the centroids of the 3D pores would also be uniformly distributed. Hence, the condition for the three results above should be met in practice. Naturally, this could be confirmed by an analysis of a section taken in the y–z plane. Analysis of a 2D section of a real intumescent char leads to the conclusion that the PDF of the 3D EPR is of exponential form.

Acknowledgements The author is grateful to Drs Paul Jackson and Rachel Butler of International Paints Ltd for permission to use the segmented image of a real char section.

References 1. J.E.J. Staggs, Modelling the thermal conductivity of porous materials using thermal resistor networks, Fire Safety Journal, 2002, 37, 107. 2. I.I. Kantorovich and E. Bar-Ziv, Heat transfer within highly porous chars: a review, Fuel, 1999, 78, 279. 3. P.G. Hoel, Introduction to Mathematical Statistics. Wiley, 5th Edition, y10 (1984). 4. A.J. Jerri, Introduction to Integral Equations with Applications. Wiley, 2nd Edition, y3 (1999). 5. F. Cernuschi, S. Ahmaniemi, P. Vuoristo and T. Ma¨ntlya¨, Modelling of thermal conductivity of porous materials: application to thick barrier coatings, J. Eur. Ceram. Soc., 2004, 24, 2657. 6. B. Schultz, Thermal conductivity of porous and highly porous materials, High Temp. High Press., 1981, 13, 649.

CHAPTER 15

Efficient Modelling of Temperatures in Steel Plates Protected by Intumescent Coating in Fire J.F. YUAN AND Y.C. WANG School of Mechanical, Aerospace, and Civil Engineering, The University of Manchester, M60 1QD, UK

15.1 Introduction Intumescent coating is designed to expand and form a thick, porous charred layer when exposed to heat in fire. The charred layer insulates the underlying substrate by providing a physical barrier. The applications of intumescent coating are wide-ranging and the demands for this material have significantly increased in recent years, especially in the civil engineering area to protect building structures from fire attack. However, the mechanisms that determine the fire-resistant properties of intumescent coating are not well understood yet, because of their highly complex physical and chemical natures. This hampers the application of intumescent coating in performance-based fire engineering. At present, building fire resistance design is largely based on the ‘‘standard fire condition’’, and the thermal properties (mainly thermal conductivity) of an intumescent coating are obtained under the standard temperature–time relationship.1 However, in a natural fire condition in buildings, fire development varies depending upon such factors as the amount of

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

225

226

Chapter 15

combustible materials, ventilation and construction lining materials. For conventional fire protection materials, their thermal properties can be pre-determined because they are only temperature dependent. Unlike the conventional fire protection materials, intumescent coating will behave differently according to the applied fire condition, coating thickness and protected structures. Intumescent coating is a reactive material, so the temperature–thermal properties relationship of an intumescent coating may be entirely different under different realistic fire conditions, and any set of temperature–thermal properties relationship may only be applicable to a very narrow range of fire conditions. A key to widen the applicability of any predictive method is to ensure that it captures the essential features of intumescent coating that govern coating behaviour in fire. So far a number of researchers have developed predictive models with different degrees of complexity. At the simplest level, intumescent coating is treated in the same way as the more conventional non-reactive fire protection materials, and different effective thermal conductivity values of intumescent coating are used for different conditions. This is clearly not satisfactory because it will not be possible to extrapolate the results to different applications. At the most complex level, a few researchers2 have attempted to model the intumescence process at the microscopic level, with a detailed consideration of hydrodynamics, heat transfer and chemical reactions related to individual bubble nucleation, production, movement and burst. This method of simulation is computationally prohibitive, as it requires a large amount of input data. So far it has had limited success, being used only to demonstrate the intumescence process during the very early stage of heating. An alternative third method would be to consider only the essential features of the global chemical decomposition and physical behaviour of intumescent coating. This has been followed by a number of researchers using different assumptions.3,4 The focus of this study is on the temperatures of the protected steel, rather than the detailed behaviour of intumescent coating itself. Therefore, the primary objective was to develop a practical method to extract the basic thermal properties of intumescent coating that are independent of fire or heat. Therefore, we adopted the third approach and the framework of Di Blasi and Branca,4 but with a few revised assumptions that are pointed out in Section 15.2. According to Di Blasi and Branca, intumescent coating decomposition is modelled in three global steps and their reaction rates are governed by the Arrhenius equation. The intumescence process is simply represented by expansion of the intumescent thickness. As is shown in Section 15.3, even though the model adopted in this study may be considered simple, it still requires a large number of input data for the intumescent coating. It may be difficult to obtain all of these input data precisely. Therefore, an important objective was to identify the input parameters that will have significant influence on the calculated steel temperatures, by conducting a sensitivity study to investigate the influences of different input parameters on steel temperatures.

Efficient Modelling of Temperatures in Steel Plates

227

Figure 15.1 Illustration of energy conservation within an infinitesimal layer of intumescent coating.

15.2 Mathematical Modelling For a steel plate protected by intumescent coating exposed to fire, it is acceptable to model the assembly as a one-dimensional entity. By discretizing the intumescent coating into a number of layers, such as that shown in Figure 15.1, finite difference method (FDM) can be used to solve the corresponding equations. From energy conservation, the change in heat flow conducted through a discretized layer of intumescent coating is equal to the change in internal energy of the coating, which gives (see the section Nomenclature for the symbol usage):   @ @T @T l ¼ ðms Cs þ mg Cg Þ @x @x @t @ðxrg Þ @ms þ Cs T þ Cg T @t  @t _ @ðgTÞ @mr Cg þ Dh þ @x @t

ð15:1Þ

The left-hand side of Equation (15.1) is the conducted heat to the intumescent coating; the first term on the right-hand side (RHS) is the heat increase caused by changes in coating temperature. The second and third terms on the RHS are heat increases resulting from changes in the gas and solid masses, respectively. The fourth term on the RHS describes convective heat loss due to gas movement and the last term on the RHS is heat release from the coating. Equation (15.1) theoretically applies to the whole intumescent char; however, it is practically solved within discretized layers, as aforementioned, because of the non-linearity of coating properties throughout the thickness. To enable

228

Chapter 15

quantification of Equation (15.1), it is necessary to derive the various masschange terms, which are determined by the decomposition process. The Arrhenius law is used to describe decomposition process, giving:   Ej Kj ¼ Aj exp
; j ¼ 1; 2; 3 ð15:2Þ
ð15:3Þ

where a is the degree of conversion and n is the reaction order. It is assumed that only products of the blowing agent are respectively responsible for bubble formation and swelling: @x 1 @m2 ¼ @t rg @t

ðx  Emax x0 Þ

ð15:4Þ

The gas density may be obtained from the ideal gas law. It is assumed in this study that bubble expansion occurs fast enough for pressure inside the char to be in balance with atmospheric pressure. It is also assumed that only the gas generated in each layer of the coating contributes to swelling of this specific layer. Once a layer has reached its maximum expansion, it is assumed that the bubble volume in it does not change any more, and any excessive gaseous product will leave it to maintain atmospheric pressure. This excessive gas is assumed to flow through other layers, but not to contribute to their expansion. At this stage of the study, the maximum rate of expansion of intumescent coating cannot be predicted and is treated as an input data. It is necessary to quantify the porosity, e, which will influence both heat capacity and thermal conductivity of the intumescent coating. If movement of the solid mass is neglected, then: xs ¼ x0 ð1
0 Þ

ms m0

ð15:5Þ

229

Efficient Modelling of Temperatures in Steel Plates

The total porosity is given by: ¼

x
xs x

ð15:6Þ

The maximum expansion ratio, Emax, is probably the most important parameter in this predictive mathematical model. It determines the effectiveness of insulation of the intumescent coating. Parallel research is now being conducted to establish a reliable model to estimate this quantity. At present, the maximum expansion ratio is provided as an input value from experimental observation. With the above assumptions, volume change of a specific elementary layer is related only to the amount of gas generated within the layer. The net loss of solid mass comes from chemical decomposition of the material and solid product from the reaction. A total mass-continuity equation for gas transportation can be written as:   @g_ @ms @ xrg ¼

@x @t @t

ð15:7Þ

The formation of a multicellular char leads to a significantly reduced thermal conductivity. Rather than using the simplest parallel and serial models as in Di Blasi and Branca,4 a model that employs a theory closer to reality has been introduced into this study. For a porous material, Russell6 estimated the thermal conductivity as: l ¼ ls l g

lg ls

2

2

3 þ 1
3

2

2

3 3 ls ð
Þ þ 1
 þ 

ð15:8Þ

Radiative heat transfer cannot be neglected at high temperatures. Although the opaque solid material does not allow radiation, it takes place within the void in the coating. Separating the total heat-conduction coefficient of a gas into that of pure conduction and that of radiation gives: lg ¼ lcond þ lrad

ð15:9Þ

The thermal conductivity of gas due to pure conduction can be obtained from a standard heat-transfer textbook.7 For a porous structure with unconnected uniform spherical pores, the shape factor for radiation is 2/3. Therefore, the radiation contribution to the overall thermal conductivity of a single pore is:8 2 lrad ¼  4desT 3 3

ð15:10Þ

where d is the pore diameter. Equation (15.10) clearly shows the importance of bubble diameter on radiative thermal conductivity of the charred coating structures. In the intumescence process, it is assumed that bubble growth is

230

Chapter 15

governed by two mechanisms: initial formation during the blowing stage and gradual growth during the charring stage. Bubbles nucleate and grow rapidly during the expanding period, and the growth is considered to have a linear relationship with expansion, as the first term on RHS in Equation (15.11).

d ¼ db

  x 1 m3 þ ðdf
db Þ 1
x0 Emax m30

ð15:11Þ

Once the charring process begins, bubbles start to burst and unite. Bubbles grow slowly, but the bubble diameter will increase significantly compared with that during expansion. It is assumed4 that the bubble growth in this stage is linear to the depletion of the charring material.

15.3 Validation As part of validation of the mathematical model, Figure 15.2 compares predictions of the model with fire experiments conducted by Cagliostro et al.5 under a cone calorimeter, with an external radiative heat flux of 157 kW m
2. The back face of the test sample was thermally insulated. The material properties are listed in Table 15.1. The model was solved by use of FDM. In the FDM scheme, the coating is equally divided into 100 discrete layers. The time increment is 0.01 s. More extensive validation of the model is now being carried out by comparing the results with those of the authors’ furnace experiments. In the meantime,

Figure 15.2

Predicted and measured results of protected steel temperatures.

231

Efficient Modelling of Temperatures in Steel Plates

Table 15.1

5

Input experimental values from Cagliostro et al. (or estimated values ‘‘(*)’’ from Di Blasi and Branca4).

A1 (s–1) E1 (kJ mol–1) A2 (s
1) E2 (kJ mol
1) A3 (s
1) E3 (kJ mol
1) H1 (kJ kg
1) H2 (kJ kg
1) H3 (kJ kg
1) n10 v20 v30 Cc (kJ kg
1 K
1) Cs (kJ kg
1 K
1)

800 53.384 6.9  105 93.035 5.0 63.786 –1256(*) –1256(*) 9789 0.28 0.17 0.55 1.884 0.42

Cg (kJ kg–1 K–1) lc (Kw mK
1) ls (Kw mK
1) rc (kg m
3) rs (kg m
3) ec db (m) df (m) Emax Wg (kg mol
1) h (kW m
2) Q (kW m
2) lc0 (m) ls (m)

1.0 0.345  10
3 37.68  10
3 1400.0 7850.0 1.0 5.0  10
6(*) 325.0  10
6(*) 3.0(*) 30.0  10
3(*) 20.0(*) 157.0 0.2  10
2 0.15  10
2

Figure 15.2 indicates good agreement between the simulation and Cagliostro’s experimental results.

15.4 General Analysis of Intumescence Process Although the modelling approach adopted in this research may be considered simplistic, it is possible to capture the complex physical and chemical processes of intumescent coating decomposition and their effects on temperature developments in the protected plate and fulfil the engineering requirements. An example is provided here for illustration. In this example, the model represents the test conditions in previous work,5 in which a 1.5 mm thick steel plate is protected with a 2 mm thick intumescent coating layer, and exposed to a constant heat flux of 157 kW m
2. The material properties of coating are as in Table 15.1. Figure 15.3 shows temperature histories at the coating surface and the protected steel plate. The coating surface temperature quickly reaches a very high level, around 1200K, because of the high external heat flux. The slow substratetemperature history indicates the effectiveness of the intumescent coating. Substrate heating rate is also presented in Figure 15.3. The steel plate temperature increases rapidly initially, but the increasing rate starts to drop sharply at around 15 seconds. The sharp drop shows that the intumescent coating has started to provide a more effective insulation to the steel substrate, which is supported by rapid growth of the coating thickness (Figure 15.4). In addition to the increased coating thickness providing thermal insulation to the steel plate, the increasing coating porosity, which is a result of solid-mass depletion and coating swelling, also makes some contribution to the insulation effect of the coating. As a consequence, a reduction appears in the apparent thermal conductivity of the coating according to Equation (15.8). It can be seen that the thermal conductivity has dropped by about 40% within the first 15 seconds, and keeps decreasing until the coating has fully expanded, at around 135

232 1400

6

1200

5

1000

4

800

3

600

2

400

1

200

0

Heating Rate (K/s)

Temperature (K)

Chapter 15

-1

0 0

200

400 Time (s)

600

800

Figure 15.3 Temperature at surface and substrate (Continuous Line), and substrate heating rate (Dotted Line).

7 6 5

0.2 4 3 0.1

2 1

0

0 0

Figure 15.4

Coating Thickness (mm)

Thermal Conductivity (W/m/K)

0.3

200

400 Time (s)

600

800

Coating expansion and apparent thermal conductivity.

seconds. This is consistent with the mass loss curve in Figure 15.5, which indicates high reactant-loss rates during the first 150 seconds, when the blowing agent and inorganic acid source are dominant in chemical reactions. The results show that at around 135 seconds, when the intumescent coating has fully expanded, the substrate heating rate reaches its minimum and starts to increase. The second peak of the rate of increasing temperature of the substrate steel occurs at around 300 seconds. This increase in the heating rate is partly a

Efficient Modelling of Temperatures in Steel Plates

Figure 15.5

233

Mass-loss rate and heat of decomposition calculation.

result of increasing thermal conductivity (Figure 15.4) and also partly a result of decomposition heat released from the charring process. As shown in Figure 15.4, the thermal conductivity tends to increase after the coating has fully expanded. According to Equation (15.10), this tendency is from heat transfer through the gas phase, which in turn is largely related to radiative heat transfer. Radiative heat transfer is further related to bubble size, and the bubbles continuously grow during the expanding and charring process, as described in Equation (15.11). In contrast, Figure 15.5 presents a history of the total heat generation within the intumescent coating. It can be seen that before about 150 seconds, heat absorption (which is generally from reactions of inorganic acid source and blowing agent) is dominant. The charring process becomes dominant after 150 seconds. Therefore, heat release becomes more dominant within the coating. At around 350 seconds, heat release rate reaches a peak, which coincides with the second peak in the increase of temperature rate in the steel, as shown in Figure 15.3. After the peak, the heat release starts to drop in line with the original charring material’s depletion. Movement of the gas within the coating also has some influence on the temperature of the substrate. If the gas produced from decomposition leaves the coating, it will take a certain amount of heat. As expressed in Section 15.3, gas flowing is governed by two aspects: chemical decompositions and coating expansion. Although this effect is relatively limited compared to the others, it is included in the model to ensure accuracy.

15.5 Parametric Studies As discussed in the Section 15.4, the simplified mathematical model is able to reveal complex interactions between the various physical and chemical processes

234

Chapter 15

Figure 15.6 Effect of changing reaction kinetics of blowing agent on calculated temperature of protected steel.

in intumescent coating during reactions under fire. In contrast, even though this model can be considered simplistic, Table 15.1 shows that a large number of input data are still required to describe the physical and chemical reaction processes during decomposition of an intumescent coating. It would be difficult to obtain accurate data for all the required properties of intumescent coating in engineering applications. Therefore, it is important that sensitivity studies are carried out to identify the key parameters that should be accurately obtained. The aim of this parametric study is not to reveal how the change in values would affect the real intumescence in chemical or physical terms, but to capture those parameters which are most influential in the calculation for engineering design. The parametric studies in this section were performed by varying the values of input parameters individually, by 20% from reference case (Table 15.1). Emax was artificially set to be 20 to represent thin intumescent coatings applied in steel building structures. Temperature of steel structure is the most considered issue in structure safety design. In this parametric study, the results were examined by comparing the output steel temperatures. Average deviations in the steel temperature induced by varying the input values were calculated individually over the whole 3000 second heating period. The mean values of these deviations are displayed in Table 15.2. Parameters related to the intumescence process have comparatively more influence on the overall performance of the coating than other general material properties, such as specific heat, density and thermal conductivity of the solid coating. The results indicate that changes in the chemical kinetics of the inorganic acid source have little effect on the protected steel temperature. This is not

235

Efficient Modelling of Temperatures in Steel Plates

Table 15.2

Average deviations from mean value of steel temperature by changing different input data, heating duration ¼ 3000 seconds.

Parameter

–20%

+20%

Parameter

–20%

+20%

A1 A2 A3 E1 E2 E3 H1 H2 H3

–0.28 +0.73 –1.15 +4.13 –12.23 +6.57 +0.79 +0.52 –1.91

+0.25 –0.58 +0.88 –1.61 +13.77 –8.44 –0.77 –0.52 +2.04

v1 v2 v3 Cc P e0 Emax df ls

–0.30 +2.34 –5.48 +1.86 +3.10 –0.84 +6.91 –2.09 –0.75

+0.64 –1.93 +6.55 –1.73 –2.90 +0.85 –5.36 +1.95 +0.77

Figure 15.7

Expansion and thermal conductivity records for different E2 values.

surprising – although this acid-release process must precede other reactions, it has little to do with the intumescence process and char structures. Figure 15.6 compares simulation results of the steel substrate temperature by varying the chemical kinetics (activation energies E2) of the blowing agent. The substrate temperature histories show that the reaction kinetics values of this process have significant effects on the steel temperature, which result in differences in the steel temperature of
12.23% and +13.77%, respectively, for
20% and +20% changes in E2. The predicted results deviate from the start of the process, and the temperature plateau appears at different temperatures. This indicates variations of about 100K in the steel temperature at which its increase reaches the plateau. This variation in steel temperature behaviour is directly related to the thickness and thermal conductivity of the coating. Figure 15.7

236

Chapter 15

Figure 15.8 Effect of changing reaction kinetics of charring material on calculated temperature of protected steel.

shows that the use of a lower activation energy E2 gives a deviated calculation with a faster expansion of intumescent coating, which leads to a quicker drop in thermal conductivity. However, there is little difference in thermal conductivity of the coating after full expansion. Figure 15.8 indicates substantial effects of varying the charring kinetics. The varied input values result in differences in the calculated steel temperature by +6.57% and
8.44%, respectively, for
20% and +20% changes in E3. From Figure 15.8, it can be seen that the deviation appears more significant after the temperature plateau. The effects of changing E3 can be related to the heat of decomposition and thermal conductivity. If a lower activation energy E3 is used, then the decomposition of charring material will be modelled to happen earlier and faster than realistic test results. This would result in a stronger and sharper peak of heat of decomposition, caused by the exothermic reaction, in the prediction. Under this situation, calculated bubble growth [Equation 15.11] will also be faster, which in turn influences the thermal conductivity after full expansion. As described in Equation (15.10), the radiative component of gas thermal conductivity is inversely related to the bubble size. Figure 15.9 shows the variations of the apparent thermal conductivity with different bubble diameters. The results in Table 15.2 show that the steel temperature is affected by about 2% for a 20% change in bubble size, which indicates that it would not be necessary to determine the bubble size to great accuracy. Nevertheless, since the bubble sizes of different intumescent chars can vary widely, it is important to obtain reliable estimates of the bubble size for a specific type of intumescent coating.

Efficient Modelling of Temperatures in Steel Plates

237

Figure 15.9

Effect of bubble size variation on thermal conductivity of expanded coating.

Figure 15.10

Effect of maximum expansion coefficient on effective thermal conductivity of intumescent coating.

The expansion ratio of the coating plays the most important role in the insulation performance of intumescent coating. The expansion ratio is a key factor that affects the effective thermal conductivity, which is defined as: l x leff ¼ appx 0 . Figure 15.10 compares the effective thermal conductivity by varying the maximum expansion ratio from 20 by 20%. These variations cause
5.36% and +6.91%, respectively, differences in the predicted steel temperatures.

238

Chapter 15

15.6 Conclusions A mathematical model was constructed in this study to define the key parameters that determine intumescent coating behaviour under different fire conditions. The model was partially validated by comparing experiment and simulation results for given input values of all the necessary parameters. The general intumescence process was analyzed to obtain an understanding of the various mechanisms that govern its fire performance. A parametric study was carried out to evaluate the influence on steel temperatures of changing each individual parameter, and so identify the key parameters of the model that have significant influence on the calculated steel temperature. It was shown that the activation energies of the blowing agent and charring material and the maximum expansion coefficient are among the most important parameters. To give a robust prediction of the proposed mathematical model, it is necessary to test the validity of the model under different heating conditions and also to develop a reliable method of accurately obtaining these key parameters. Further research studies are now being conducted to resolve these issues.

Nomenclature A b C d e E Emax h H K m P Q R t T n x g_ a e l r s f c

Pre-exponential factor (s
1) Bubble size at the point full expansion reached Specific heat capacity (J K
1 kg
1) Bubble size (m) Emissivity Activation energy (kJ mol
1) Maximum expansion coefficient Convection heat transfer coefficient (W m
2) Heat of pyrolysis per unit mass of material (J kg
1) Reaction rate constant Mass (kg) Gas pressure (Pa) Heat flux (kW m
2) Universal gas constant(J mol
1 K
1) Time (s) Temperature (K) Mass fraction Ordinate along coating thickness (m) Mass flow rate of gas per unit area (kg s
1 m
2) Degree of conversion Porosity Thermal conductivity(W m
1 K
1) Density(kg m
3) Stefan–Boltzmann constant (W m
2 K
4) Final value of bubble size Coating

Efficient Modelling of Temperatures in Steel Plates

g r j ¼ 1,2,3 S

239

Gas Reactive component Inorganic source, blowing agent, charring material, respectively Solid

References 1. Y.C. Wang, U. Goransson, G. Holmstedt and A. Omrane, Proceedings of the 8th International Symposium on Fire Safety Science, Beijing, China, 2005, 235. 2. K.M. Butler, H.R. Baum and T. Kashiwag, Fire Safety Science, Proc. 5th Int. Symp., 1997, 1, 523. 3. V.S. Mamleev, E.A. Bekturov and K.M. Gibov, J. Appl. Polym. Sci., 1998, 70, 1523. 4. C. Di Blasi and C. Branca, AIChE J., 2001, 47, 2359. 5. D.E. Cagliostro, S.R. Riccitello, K.L. Clark and A.B. Shimizu, J. Fire Flammability, 1975, 6, 205. 6. H.W. Russell, J. Am. Ceram. Soc., 1935, 18, 1. 7. N.V. Tsederberg, Thermal Conductivity of Gases and Liquids, The M.I.T. Press, 1965. 8. A.L. Loeb, J. Am. Ceram. Soc., 1954, 37, 96.

CHAPTER 16

Fire Retardancy and Fire Protection of Materials using Intumescent Coatings – A Versatile Solution? S. DUQUESNE, M. JIMENEZ AND S. BOURBIGOT Laboratoire des Proce´de´s d’Elaboration de Reveˆtements Fonctionnels (PERF) UMR-CNRS 8008/LSPES; Ecole Nationale Supe´rieure de Chimie de Lille (ENSCL) BP 90108; 59652 Villeneuve d’Ascq Cedex - France

16.1 Introduction Intumescent coatings represent an important class of passive fire-proofing materials, which concern insulating systems designed to decrease heat transfer from a fire to the substrate being protected.1,2 They appear similar to a paint finish, and remain stable at ambient temperature. However, in case of a fire, coatings expand to many times their original thickness, which results in the formation of an insulating foam-like layer or ‘char’ which protects the substrate. The intumescence process results from a complex succession of chemical reactions that have to occur in the correct sequence.3–5 Upon heating, the polymeric binder begins to soften. The heat also leads to a release of an inorganic acid [for example, phosphoric acid from ammonium polyphosphate (APP)]. The acid reacts with a carbon source (such as, for example, polyol), which leads to a carbonization of the system. Gases from the decomposition of the blowing agent or of the system enable the carbonaceous material to expand. In the final stage, solidification of the foamed char, through cross-linking Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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reactions and condensation, occurs. The chemistry involved in the development of the intumescent process is a major concern when dealing with these systems. However, it is not sufficient to understand how they proceed; it is also an important task to investigate the physical aspects of the intumescent systems. For example, the mechanism of expansion that results from the low diffusion of gaseous degradation products through the carbonaceous material and/or from bubble growth has to be clearly identified. Particular attention must be paid to how this mechanism will affect the structure of the resulting char (for example, its porosity or pore-size distribution) and, as a consequence, the insulating behaviour of the system. Thus, the study of the intumescence process is a complex problem that involves many parameters. To know the effect of one parameter is not enough –relations between the parameters also need to be known. In this chapter we aim to identify and describe the main factors that affect the development of the intumescence process. The relations between these factors are also investigated and we also analyze how these factors and relationships could affect the thermal insulating properties of the charred structure. Examples from the fire protection of steel structures and of fire retardant polymers are described.

16.2 The Use of Intumescent Coatings for The Fire Protection of Steel Structures Intumescent coatings have been used in the fire protection of steel structures for more than 20 years. In this area, intumescent materials are classified as either thick or thin coatings. The first class is usually based on epoxy resins and contains agents that intumesce upon heating. They are available as solvent-free systems that allow the application of up to 8–10 mm per coat. These films are particularly suited to protection against hydrocarbon fuel fires and jet-fire scenarios. Thin intumescent films were introduced as early as the 1930s1 and are used for protection from cellulosic fuelled fires. They are generally available as solvent- or water-based systems and applied by spray or brush roller as a thin film up to 3 mm. They typically use thermoplastic acrylic-based resin systems, and they respond best by rapid intumescence at fire growth rates typical of cellulosic type fire environments. As an example, Figure 16.1 shows the increase in temperature as a function of time on the reverse side of steel plates coated with the different thermosetbased formulations according to the UL1709 test. Steel usually loses its main structural properties when the temperature increases. In most of the case, 550 1C is chosen as the failure temperature. However, in our case, for safety reasons, 400 1C was chosen as the limiting temperature. The time to reach this temperature, in the case of the steel plate covered with the thermoset resin (curve B), is close to that of the steel plate alone (curve A). This implies that the thermoset resin does not provide any protective effect. Indeed, this organic resin can initiate or propagate fire,

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Figure 16.1 Evolution of temperature as a function of time on the reverse side of a steel plate of different coatings. A, virgin steel plate; B, thermoset resin; C, thermoset resin mixed with the coated APP; D, thermoset resin mixed with boric acid; E, thermoset resin mixed with both (UL1709 standard).

because it will decompose to yield volatile combustibles when exposed to heat. For this reason flame retardants have to be added into the polymer. APP is a chain phosphate of high molecular weight. It is an interesting component because it serves as both an acid source and a blowing agent: it is a source of phosphoric acid, which speeds up the formation of carbonaceous char, and of ammonia, which improves the swelling.6–8 APP is added to the thermoset resin (curve C), and an improvement in performance is observed when coated (the time to reach 400 1C increases to 11.3 minutes compared with 5 minutes for the uncoated steel). Intumescence and charring take place, but the char falls off the plate before the end of the experiment (change of slope at 610 1C). Borax and boric acid are well-established as flame retardants and zinc borates have emerged as a replacement for antimony oxides in halogenated fire retardant polymers.1,8,9 Addition of boric acid (curve D) to the resin also leads to improved performance, the time to reach 400 1C is increased to 18.2 minutes. Development of intumescence is also observed, but the char falls off the plate (rapid change of slope at 400 1C). These falls could be explained by a loss of adherence of the coating to the plate and/or by a loss of cohesion within the char, through the effect of gravity, since the tests are carried out vertically. The best result is obtained when both the coated APP and boric acid are added to the resin (curve E). The time to reach 400 1C increases up to 29.5 minutes and the char adheres to the plate. The results show that the use of only one fire retardant additive (coated APP or boric acid) leads to a significant

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increase in the time to reach 400 1C. The problem is that the tests are carried out on vertically mounted plates and the char must adhere to the plate to provide the required protection. Coated APP or boric acid, when incorporated in the resin, does not produce chars with the required adhesion. However, the combination of the two fire retardants leads to a greater time to reach 400 1C (29.5 minutes rather than 18.2 minutes for boric acid) and the resulting intumescent char adheres strongly to the plate and exhibits a regular hemispherical shape. This demonstrates that the combination of coated APP and boric acid is required to obtain high levels of performance. To better understand these phenomena, a study of the interaction between both components has been carried out. Figure 16.2 presents 11B solid-state nuclear magnetic resonance (NMR) spectra of the boric acid and of the mixture of boric acid and coated APP heattreated at 450 1C, and compares the results with the spectra of boron oxide and borophosphate. The spectra show unambiguously that a reaction takes place between boric acid and coated APP, or between the degradation products of these compounds. The spectrum of the mixture of boric acid and coated APP heated at 450 1C exhibits a peak at diso ¼
3 parts per million (ppm), which can be assigned to crystalline borophosphate (Figure 16.3). This demonstrates that the APP and/or phosphoric acid released when coated APP degrades effectively reacts with boric acid and/or boron oxide to yield borophosphate.10,11 At 450 1C, residual boron oxide can be distinguished on the spectrum. The same mixtures were examined using 31P solid-state NMR

Boric acid 450°C Boric acid + coated APP 450°C Boron oxide Borophosphates

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Figure 16.2

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B solid-state NMR spectra of boron oxide, borophosphate and boric acid, and boric acid-coated APP mixture heat-treated at 450 1C. (11B NMR measurements were carried out at 128.3 MHz (9.4 T) at a spinning speed of 10 kHz.)

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Figure 16.3

Structure of borophosphate (BPO4).

Boric acid + coated APP HTT:95°C Boric acid + coated APP HTT: 150°C Boric acid + coated APP HTT: 250°C Boric acid + coated APP HTT:300°C Boric acid + coated APP HTT:450°C Borophosphates

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P NMR of a mixture of boric acid and coated APP treated at five different temperatures.

(not presented) and lead to the same conclusion. However, considering only the results obtained at 95 1C and 450 1C does not enable us to ascertain whether borophosphate is being formed through a reaction between boric acid and coated APP, or through the degradation products of these compounds. To determine at which temperature borophosphate is formed, the mixture of boric acid and coated APP was treated at three other temperatures – 150 1C, 250 1C and 300 1C (Figure 16.4). The peak that corresponds to borophosphate appears in the spectrum at 250 1C, i.e. when all the boric acid has turned into boron oxide and the coated APP is turning into phosphoric acid. A broad band between -18 ppm and
27 ppm is observed for the heat treatment at 300 1C, which can be assigned to polyphosphoric acid having a different level of cross-linking.

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At 450 1C, only crystalline borophosphate and boron oxide glass remain. This means the reaction that yields borophosphate is a reaction between the degradation products of boric acid and coated APP. That is, boron oxide and phosphoric acid react together to yield borophosphate. Only this reaction is possible, since the degradation reactions occur sequentially. As a consequence, the following criteria may be defined:  The release of the acid source must occur at relatively low temperatures, particularly below the decomposition temperature of the polyhydric material (in our case the coated APP and the thermoset resin).  It is necessary that the carbon source reacts with the catalyst at a lower temperature than that at which thermal decomposition occurs.  Blowing agents must decompose at the appropriate temperature and should release copious quantities of gaseous materials. The ‘‘appropriate’’ temperature will depend on the system in which they are used. Blowing should occur after the melt forms, but before the char hardens.

16.3 Fire Protection of Polyurethane Foams using Intumescent Systems Intumescent coatings may also be good candidates for the fire protection of polymers. As an example, expandable graphite (EG) or APP can be used as intumescent agent in polyurethane- (PU-) based coating for the fire protection of rigid foam. APP is an effective intumescent fire retardant for PU.12,13 Its efficiency is generally attributed to the increase of char formation through condensed-phase reaction. EG is a radically different intumescent additive that has found use in a number of flame retardant applications.14,15 In particular, its efficiency in PU foam is largely reported in the patent literature.16,17 Temperature profiles of the coated foams directly exposed to a torch flame at a temperature of 1180 1C  15 1C for 900 seconds are illustrated in Figure 16.5 for various fire retardant contents in the intumescent formulations. For all the formulations (either APP or EG), the increase in temperature occurs later when the foam is coated. For the APP-based formulations, three regions are observed: first, the temperature increases sharply up to a maximum, then the temperature decreases and finally the temperature remains constant. Such behaviour may be explained by the formation, at the beginning of the experiment, of the protective intumescent layer. Moreover, we can suggest that the maximum substrate temperature is related to the heat dissipation into the external media. With time, a regular regime of material heating is reached, and the temperature distribution at the reverse side of the sample reaches a steady state. Moreover, whatever the loading, when the steady state is obtained, the temperature tends to be constant (about 130 1C) and independent of the loading. This value is lower than that obtained with non-coated foam (180 1C), which proves the thermal insulative behaviour of the char layer.

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Figure 16.6

Picture of the reverse side for the formulation (a) PU–APP 15 weight percent (wt.%) and (b) PU–EG 15wt.%.

In the case of PU–EG, the higher the loading, the smaller the temperature rise. The fire testing clearly shows the high efficiency for low loading of the EG. In fact, the maximum temperature reached is lower than 100 1C, whereas the temperature on the surface of the sample is assumed to be equal to 800 1C. So, there is a temperature gradient of 700 1C over 2.5 cm. The insulative behaviour of the coating is thus demonstrated. Comparison of the reverse side of coated foams after 20 minutes of the experiment (Figure 16.6) shows that with a coating PU/EG, the foam is not or little affected by the degradation.

Fire Retardancy and Fire Protection of Materials using Intumescent

Figure 16.7

247

Foaming of the intumescent structure.

However, in the case of PU–APP, a browning of the reverse side of the sample that corresponds to degradation is observed. This demonstrates the superiority in terms of fire protection of EG compared to APP. When the temperature of an intumescent coating reaches a critical point, the surface begins to melt and is converted into a highly viscous liquid. Simultaneously, endothermic reactions are initiated that result in the release of inert gases with low thermal conductivity. These gases are trapped inside the viscous fluid (formation of bubbles). The result is the expansion or foaming of the coating, sometimes up to many times its original thickness, to form a protective carbonaceous char (Figure 16.7) that acts as an insulative barrier between the fire and the substrate. As a consequence, the importance of the viscoelastic properties of this layer is obvious. Investigation of the rheological parameters versus temperature or strain gives important information on the fire performance, particularly on the intumescence process of various materials. When the temperature increases and under a strain effect, the material may either deform or split. In the case of crack formation, the material rapidly degrades via thermo-oxidation because of oxygen diffusion and of mass and/or heat transfer between the virgin material and the flame. Consequently, to be effective, the charred layer has to change shape without crack formation to preserve the protective character of the carbonaceous shield. Moreover, it is also an important task to investigate the mechanical destruction of the intumescent char. If a char has good structural, morphological and heat-insulative properties, but is easily destroyed under a mechanical action, its effectiveness is totally lost. Figure 16.8 represents the expansion data in the temperature range 50–500 1C for the PU-based coating used for fire protection of the rigid foam. In the case

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Figure 16.8

Expansion of PU, PU–APP and PU–EG versus temperature.

of PU and PU–APP coatings, no expansion is observed in such degradation conditions (a heating rate of 10 1C min
1). The low decrease in the thickness of the materials (around 50%) may be explained by the influence of the low compression force used in the measurement, which leads to the partial destruction of the carbonaceous material in the case of PU and PU–APP coatings. For PU–EG formulations, the expansion starts at 210 1C, reaches a maximum at 300 1C and then slowly decreases between 300 and 500 1C. At high temperatures, the material consists of a carbonaceous layer composed of a large number of worm-like structures in the degraded matrix. This highly expanded layer can act as a thermal insulative barrier between the flame and the virgin material. In fact, the carbonaceous structure traps a large amount of gas that is a poor conductor of heat. The curve of the viscosity versus temperature for pure PU is given in Figure 16.9 – the thermogravimetric analysis (TGA) and derivative TGA curves are added to compare weight loss and changes of viscosity. The curve of the viscosity is broken into several parts. In the 100–300 1C temperature range, the viscosity of the material increases without any weight loss. This may be attributed to a phenomenon of reticulation that corresponds to thermoset materials. Then, between 300 and 370 1C the viscosity of the material decreases slightly. It then follows two different steps in which the increase in viscosity occurs with weight loss. We may explain both of these phenomena by a carbonization process. In fact, in the 370–500 1C temperature range, degradation of the PU occurs to give a polyaromatic structure.18 Similar curves for the PU–APP formulation are given in Figure 16.10. We observe that the first part of the curve is similar for PU and PU–APP formulations. The viscosity increases up to 270 1C. Then, the viscosity decreases, which corresponds to the degradation of the material, and consequently to the

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Figure 16.9 Viscosity ( . . . ) and weight loss (__) versus temperature for PU coating.

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Figure 16.10

Viscosity ( . . . ) and weight loss (__) versus temperature for PU–APP coating.

emission of gases. In fact, the presence of bubbles of gas trapped in the carbonaceous matrix may explain why the viscosity decreases. In the 290–320 1C temperature range, the viscosity increases to a maximum. This may be reasonably explained by a carbonization process. Between 320 and 365 1C, a high decrease in the viscosity begins. It may correspond to the coexistence of a liquid

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and a solid phase. In fact, when APP degrades, it yields phosphoric acid, which results in a pasty intumescent material with a significant decrease in viscosity. Moreover, we have previously18 shown that in this range of temperature P–O– C bridges between the polyaromatic species present. These bonds give some flexibility to the carbonaceous shield, which consequently leads to a decrease of the viscosity. At 365 1C, the viscosity value increases up to 410 1C and, finally, decreases with a small weight loss. We may explain such a phenomenon by a modification of the dynamic properties of the char and also by the degradation of the intumescent layer. Figure 16.11 shows the results obtained for the PU–EG formulation. The viscosity increases sharply in the first step. Two different processes occur: crosslinking and expansion of the graphite. In the temperature range 230–320 1C, the viscosity remains constant. The quantity of expanded graphite is high enough to provide slipping between the graphite layers. Between 320 and 400 1C, an increase in viscosity occurs that corresponds to the degradation of the material and thus to a carbonization process. As in the case of PU–APP, the last part of the curve corresponds to a decrease in the viscosity. The decrease occurs with weight loss, so it may be assumed that at this temperature char degradation probably starts. Comparison of the viscosity curves for the different coatings are presented in Figure 16.12. The first step of the curves is similar for PU and PU–APP; for PU–EG the viscosity increases more quickly because of the expansion of the graphite. For a fire retarded coating, the first step of charring occurs at a lower temperature than for pure PU (290 1C for PU–APP, 320 1C for PU–EG

100%

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Viscosity ( . . . ) and weight loss (__) versus temperature for PU–EG coating.

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Fire Retardancy and Fire Protection of Materials using Intumescent 200000

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Figure 16.12

Comparison of viscosity of PU, PU–APP and PU–EG versus temperature.

and 370 1C for pure PU). For PU–APP and PU–EG, there is a final softening of the char. As a consequence, the following conclusion can be drawn. For the PU–APP formulation, measurements of the viscosity showed that its range of values is lower for PU–APP than for pure PU. We may explain such behaviour by the coexistence of a liquid (phosphoric acid yields during the degradation of PU–APP) and a solid phase in the carbonaceous material. The formation of a pasty intumescent material leads to a decrease in the viscosity, which in a fire scenario (under high thermal stress) can lead to expansion of the coating system. Moreover, the viscosity of the char is low enough to accommodate the stress induced by the high internal pressure of the trapped gas and is high enough to keep this gas in the material. In the case of PU–EG formulation, the expansion is very high (around 200%). In fact, by heating EG, a voluminous vermicular product is obtained. This expanded graphite is a low-density, nonburnable, thermal insulative material that can reflect radiant heat. So, the intumescent properties of EG principally arise from the formation of a thick insulative layer which reduces heat and mass transfer between the undecomposed material and the combustion zone. Furthermore, EG can actually blow away the flame because its expansion is high and instantaneous. Measurements of the apparent viscosity showed a final softening of the char. In that temperature range, degradation of PU occurs, whereas graphite flakes are thermally stable. So, the large quantity of expanded graphite flakes allows the graphite layers to slip together, and consequently leads to a decrease in the viscosity.

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16.4 Conclusion Intumescent systems are versatile solutions since they can be used advantageously for the fire protection of polymers, steel structures and of a number of materials (textiles, wood, etc.) However, intumescent systems are very complex and the formulations of such coatings may contain more than ten components. To design high-performance systems, the role of each component has to be investigated, as does the interaction between all the components.

References 1. H.L. Vandersall, J. Fire Flammability, 1971, 1, 97. 2. S. Bourbigot, M. Le Bras, S. Duquesne and M. Rochery, Mat. Eng., 2004, 289(6), 499. 3. G. Camino, L. Costa and G. Martinasso, Polym. Degrad. Stab., 1989, 23(4), 359. 4. S. Bourbigot, M. Le Bras and R. Delobel, Carbon, 1993, 31(8), 1219. 5. R. Delobel, N. Ouassou, M. Le Bras and J.M. Leroy, Polym. Degrad. Stab., 1989, 23(4), 349. 6. C.F. Cullis and M.M. Hirschler, The Combustion of Organic Polymers, Clarendon Press; Oxford University Press, Oxford, New York, 1981. 7. T. Arthur, K. Quill, Proceedings of Flame Retardant’92, Elsevier Applied Science, London, 1992 p. 223. 8. W. Lyons, The Chemistry and Uses of Fire Retardants, Wiley-Interscience, John Wiley and sons. Inc., New York, 1970. 9. A.R. Horrocks, Polymer Degrad. Stab., 1996, 54(2–3), 143. 10. M. Jimenez, S. Duquesne and S. Bourbigot, Proceedings of the Conference on Recent Advances in Flame Retardancy of Polymeric Materials, 2005, 16, 104–118. 11. M. Jimenez, S. Duquesne and S. Bourbigot, Thermochimica Acta, 2006, 449(1–2), 16. 12. N. Grassie and M. Zulfiqar, In: Developments in Polymer Stabilisation, G. Scott, ed. Applied Science Pub., 1978, pp. 197–217. 13. M. Bugajny, M. Le Bras, S. Bourbigot, F. Poutch and J.M. Lefebvre, J. Fire Sci., 1999, 17(6), 494. 14. D.W. Krassowski, D.A. Hutchings and S.P. Qureshi, in: Proceedings of the Fire Retardant Chemicals Association, Fall Meeting, Naples, Florida, 1996, pp. 137–146. 15. F. Okisaki, in: Proceedings of the Fire Retardant Chemicals Association, Spring Meeting, San Francisco, California, 1997, pp. 11–24. 16. U. Heitmann, European Patent EP 0 450 403 A3, 1992. 17. R.W.H. Bell, US4698369, 1987. 18. S. Duquesne, M. Le Bras, S. Bourbigot, R. Delobel, G. Camino, B. Eling, C. Lindsay, T. Roels and H. Vezin, J. App. Polym Sci., 2001, 82(13), 3262–3274.

Fibres and Textiles

CHAPTER 17

Trends in Textile Flame Retardants – a Market Review R. HICKLIN, R. PADDA AND G. LENOTTE Rhodia UK Limited, Trinity Street, Oldbury West Midlands, UK, B69 4WD

17.1 Introduction Flame retardants are becoming almost ubiquitous in our daily lives; from clothing and bedding to electronics, car interiors and building materials. In this follow-up to a recent article we discuss current flame retardant trends in textiles, specifically with regard to phosphorus-containing topical treatments to cotton and cotton-synthetic fabrics.1 There are reported to be between 10 and 20 fire deaths per 1 000 000 inhabitants in the major industrial cities of the world. The number of severely injured is said to be 100 to 200 per 1 000 000 inhabitants. Every day in Europe there are about 12 fire victims and 120 people are seriously injured.2 The main areas of use for topical flame retardant finishes for textiles are protective wear, children’s sleep wear, building materials and furnishings, which may include curtain material and upholstery. For all situations the purpose of the flame retardant is to reduce the flammability of the textile and to retard the spread of fire. The ultimate objective of the flame retardant is to offer protection to lives and property. For the wearer of protective clothing, flame retardant clothing also allows more time to escape from a hazardous or life threatening situation.

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17.2 Burning Behaviour of Cotton Fabrics To better understand the mechanisms of flame retardants, some understanding of the normal combustion process of textiles is helpful. There are two processes by which fabrics composed of cotton burn. One is through flaming combustion, in which the polymer of the textile decomposes through the action of heat to form volatile organic compounds, which provide fuel for flaming. Cotton fabrics may also burn with a smouldering action, which is a slow combustion process that occurs without visible flame. If the heat of combustion increases sufficiently it is possible that any volatile gases or unburnt carboncontaining material will ignite; in which case flaming combustion will take over.3

17.2.1

Factors that Affect the Burning Behaviour of Cotton Fabrics

Several factors affect the burning behaviour of cotton fabrics: 1. Physical characteristics of the fabric – i.e. fabric construction, yarn construction, weight per unit area; 2. Environmental – oxygen available, airflow, relative humidity, temperature; 3. Chemical – inorganic and organic impurities, e.g. metals and hydrocarbons, such as grease or engine oil; 4. Ignition source; 5. Air-flow.

17.2.2

Combustion of Cotton

Combustion of cellulose is widely discussed in the literature4 and can be summarized as:  Cellulose decomposes to tarry depolymerization products, notably levoglucosan;  Then to volatile combustible products, such as alcohols, aldehydes, ketones and hydrocarbons;  Flammable gases ignite;  After flaming, the carbonized residue slowly oxidizes (smoulders) until it has been consumed.

17.3 Mechanism of Phosphorus Flame Retardants The action of a phosphorus-containing flame retardant on a cellulose-based textile may be described as follows.4 Phosphorus pentoxide and phosphoric acid are generated, which results in dehydration of the cellulose at a temperature below the point at which

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decomposition normally occurs. This promotes the formation of a stable char. The evolution of levoglucosan is inhibited by phosphorylation of the cellulose. The levels of flammable gases and tars are reduced and, once the ignition source has been removed, the textile is no longer able to support combustion. Nitrogen is considered to act as a synergist in combination with phosphorus flame retardants by promoting phosphorylation.

17.4 Classification of Flame Retardants Flame retardants may be classified in several different ways. We can, for example, consider their mode of action in the way they disrupt the combustion process. Related to this is the chemistry of the flame retardant and the fibre type to which the flame retardant needs to be applied. We can also consider the durability of the flame retardant finish – whether it is non-durable, durable or semi-durable, and this is the classification adopted in this chapter.  Non-durable – where it is possible to remove the flame retardant from the textile by wet processing, such as by water soak or wash.  Semi-durable – where the flame retardant treatment is resistant to a water soak treatment; e.g. as specified in BS 5651.  Wash durable – where the flame retardant is resistant to multiple machine washes, e.g. as specified in ISO 10528.

17.5 Flame Retardant Selection Careful selection of the flame retardant to be used is essential. Important factors include the end-use, flammability standard, durability performance, fibre composition, fabric construction and cost. The application methods and technology available at a mill are also important considerations. The effect of the flame retardant finish on the physical and aesthetic properties of the textile must also be thought through carefully. The fabric must retain a soft handle if it is for apparel and it must not present any toxicological hazards if it is going to be in contact with skin. The fabric colour must not change significantly. Equally, dye-fastness properties of the untreated textile must also be retained. Flame retardants may also be applied by incorporating the flame retardant with a binder and applying to the textile in the form of a back-coating. Back-coating techniques may be knife over air (Figure 17.1), knife over roller (Figure 17.2) or a lick roller (Figure 17.3). Full impregnation may be carried out by a simple dipping and squeezing arrangement (Figure 17.4).

17.5.1

Non-Durable Flame Retardants

A wide range of non-durable flame retardants are available on the market. Borax and boric acid are still used for cellulosic textiles, as are simple

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Figure 17.1

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Knife over air back-coating process.

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Figure 17.2

Knife over roll back-coating process.

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Figure 17.3

Lick-roller application process.

ammonium salts, such as diammonium phosphate, ammonium sulfamate and ammonium bromide. Ammonium polyphosphates are also used, sometimes in combination with urea or with ammonium bromide. More demanding end-uses and the requirement for increased production speeds have created the need for more sophisticated non-durable flame retardants based on phosphonates, which have better temperature stability. For back-coating finishes for furnishings, halogens in combination with antimony trioxide are still important, despite ecological concerns.

259

Trends in Textile Flame Retardants – a Market Review Mangle rollers

Fabric

Pad trough

Figure 17.4 Padding application process.

17.5.2

Semi-Durable Flame Retardants

In this context, semi-durable flame retardants are those which can withstand soaking or wetting with water, but will not give consistent durability to repeated washes carried out to a recognized standard. An end-use for flame retardants possessing this level of durability could be upholstery fabrics or mattress ticking. Some products based on ammonium polyphosphate and urea have been found to be suitable for application to fabrics that are required to meet BS 5852 after one water soak, as specified in BS 5651. Application of the flame retardant is carried out by a pad–dry–heat cure process. A typical solids add-on of between 10–12% is necessary.

17.5.3

Durable Flame Retardants

Durable flame retardants are used on textiles which periodically need to be subjected to washing. Examples are fabrics to be made into workwear, children’s sleepwear and bedding. The flammability test method and the wash procedure used are subject to tight specification and standardization. For example, under current EU legislation, the flame retardant test standard EN 533 for the protective wear sector requires that flame retardant fabrics meet a surface ignition test after a minimum of five wash cycles. The requirements for EN 533 index 3 being that ‘no specimen shall give flaming debris; any afterglow shall not spread from the carbonized area to any other undamaged area after the cessation of flaming; no hole formation; and the mean after-flame time of any set of six specimens shall not exceed 2.0 seconds’.

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To reflect more accurately the end-use of the garments or articles, it is commonly accepted that they are required to withstand washing to 50 washes at 75 1C and conform to flame retardant testing. Wash standards that determine the temperature, detergent composition, and wash and rinse operations (EN ISO 10528) have been generally used for testing the wash durability of protective wear flame retardant properties. This wash standard is to be replaced by EN ISO 15797, which specifies a wash cycle as one wash followed by one drying sequence and repeated five times. The two prominent chemistries used for durable topical treatments of cellulosic fabrics are both organophosphorus based, although different chemically and using different means to achieve durability on the textile. The first product is based on dialkylphosphonopropionamide. The dialkylphosphonopropionamide is co-reacted with an amino resin and an acid curing catalyst. The application to the textile is achieved by a pad–dry–heat cure process followed by an alkali scour.5 The second technology is based on tetrakis(hydroxymethyl)phosphonium chloride (THPC). A solution of the chemical is padded onto the fabric, and cured with ammonia in a specially designed cure unit to generate a highly crosslinked three-dimensional polymer. The final step of the process requires treatment with hydrogen peroxide to convert the P31 to the P51 state. This is shown in Figure 17.5. It is important for flame retardant finishes to give as little change to the physical and aesthetic properties of the textile as possible. Any change to the shade of the textile should be minimal and what change there is should be predictable and consistent. There should be minimal modification to the handle of

O CH2 P CH2 CH2 NH NH CO NH CH2 O CH2 CH P P CH2 CH2 O CH2 NH NH CO NH CH2 CH2 P CH2 O

O CH2 P CH2 CH2 NH NH CO NH CH2 O CH2 CH2 P P CH2 CH2 O CH2 NH NH CO NH CH2 CH2 P CH2 O

Figure 17.5

Structure of PROBANs polymer. PROBANs is a registered trademark of Rhodia operations.

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the fabric, and the loss of tensile and tear strength through application of the flame retardant should be kept as low as possible. Consideration should be paid to the forces applied to cotton fabrics during normal laundering operations. During washing, damage may be sustained by the cotton fibre through abrasion, which causes fibrillar damage, broken fibre ends and reduction of fibre diameter. Tumble drying may cause additional damage in terms of diagonal and horizontal cracking of the fibre. Repeated drying of the fibre may also cause collapse of the fibre structure, which results in it becoming stiffer and hence more brittle.6

17.6 Flammability Standards and Testing Standards and testing are vital to ensure the flammability performance of a fabric is satisfactory for a specific end-use. Flammability testing must be carried out in accredited test laboratories using recognized flammability test methods. It is also important to consider the ability of the test laboratory that performs the flammability test. UKAS Testing Accreditation to ISO 17025: 2005 demonstrates that the test laboratory has the necessary capability and staff competent to perform materials testing to a specific standard.7 In the case of a laboratory that carries out flammability tests on textiles to be made into garments designed to give protection in the work place, it is vital that the testing conditions, the test itself and the certification and documentation processes are strictly controlled. Flammability standards are the driving force behind the use of flame retardant finishes and development of standards and test methods that cover flammability must reflect the end-use of the textile article. The past 15 years has seen a shift in textile production to the Far East for reasons of cost. Manufacturers in developed countries are also entering into joint ventures with textile companies in the Far East. However, textiles which require a high level of regulatory, technical and innovative input continue to be manufactured efficiently and profitably in developed countries. The personal protective-equipment market especially requires technical fabrics that demand a high level of conformance to European flammability standards. Relatively short production runs and the need to develop garments and fabrics with the end users have also meant that the manufacturing base for flame resistant textiles for clothing has advantages in being closer to the point of use.8 Similarly, the US children’s sleepwear market requires exceptionally high flame retardant performance (Federal Code of Regulations CPSC 1615 and 1616) allied to soft fabric handle and bright, attractive print designs. Needless to say, garments for this market should also be made from fabric safe to be worn in direct contact with the skin of very young children.

17.7 Health, Safety and Environmental Considerations In recent years, all chemicals (including textile flame retardant chemicals) have come under intense scrutiny. Through all the (adverse) publicity that surrounds

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chemicals in the media, the consumer has been alerted to their use and presence. Some of this information has inevitably contributed to a certain amount of ‘chemophobia’ and resulted in the consumer becoming more demanding. Some of this chemophobia has been dissipated through endorsement by independent ecolabels such as Oeko-tex, whereby the endorsement confers safe toxicological and environmental profiles on the relevant chemicals. Thus, the Swiss Oeko-tex label classifies garments and textiles as ‘‘produced with special care, so as to pose no risk to health.’’ Further pressure on chemicals comes from the proposals of the Registration, Evaluation and Authorisation of Chemicals (REACH) body in the European Union (EU). The scope of REACH covers:  All activities: manufacture, import, placing on the market and use of chemical substances;  Chemical substances: on their own, in preparations and in articles.

17.8 Fibre Blends Fabrics composed of a blend of more than one fibre present challenges for flame retardant treatment, especially fabrics composed of cotton and a synthetic fibre. However, cotton synthetic blended fabrics give benefits in terms of improved appearance over a garment’s life time and better wear characteristics. On its own, a synthetic fibre will melt away from a flame, which removes the point of contact between the flame and the fabric before ignition is able to occur. When an untreated synthetic fibre, such as polyester, is interlaced with cotton fibres in the form of a yarn or a fabric, it becomes impossible for the synthetic component to melt away from the heat source. Instead it melts on to the charred cotton component and thereby increases the fuel loading of the fabric. Attempts to flame retard cotton synthetic blends from a single application bath in a one- or two-step process in which both components in the blend are treated, have been trialled using a THPC–urea flame retardant for the cotton in combination with a cyclic phosphonate ester for the polyester.9 Another means to achieve good flammability performance on cotton polyester blends is simply to apply the flame retardant to the cotton component. Fabrics that contain up to 35% polyester are possible to process using the THPC–urea–NH3 process with success to standards capable of meeting the European standards for workwear. It is also possible to incorporate an inherent flame retardant fibre into the yarn to be made into a fabric. This may either be a modacrylic or an aramid. There is still the need to apply a flame retardant to the cotton component. Antimony or halogen containing flame retardants have been applied to cotton modacrylic blends and THPC–urea to cotton aramid blends.

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17.9 Future Developments 17.9.1

Multifunctional Fabrics

More-and-more end users are requesting technical textiles with multiple properties. Flame retardant performance to a recognized standard may be required along with an antimicrobial, or the fabric may also need to give high visibility performance to meet EN 471. All of this places greater demands on the fabric manufacturer, the dyer and finisher, and the dye and chemical supplier. It is necessary to ensure that the dyer and finisher inclusion of one function in a multifunctional fabric does not compromise any of the others; for instance, the reflectance properties of a high-visibility fabric being lost during the application of a flame retardant. To achieve a high-visibility orange meeting EN 471 with flame retardant performance to meet EN 533, some manufacturers have been obliged to construct a fabric with a polyester face and cotton back. Disperse dyes are used on the polyester component to achieve the high-visibility properties.10

17.9.2

Alternative Chemistries

While developments have been made with the dialkylphosphonopropionamide and the THPC–urea processes in reducing the formaldehyde content of fabrics finished with these products, the search continues for durable flame retardants for cotton textiles which present zero formaldehyde and halogen. This goal is particularly challenging because of the requirement to apply the necessary loading of phosphorus and nitrogen to a textile to give satisfactory flame retardant performance to internationally recognized standards for apparel and bedding. High fixation efficiency is also vital to minimize effluent loadings. There is also the need for strength loss, effect on shade and effect on fabric handle to be kept to a minimum. Organophosphorus-based flame retardants have been used in combination with polycarboxylic acid cross-linking agents, and this has been found to demonstrate some durability in carpet end uses. However, its suitability for textiles needing to be washed regularly in domestic washing machines has been found to be limited.11

17.9.3

Application Technologies

Apart from developments in chemistries that involve the application of flame retardants using conventional textile-processing equipment, there has also been investigation into the use of relatively novel techniques, such as using electron beams. Here, a phosphorus-containing monomer is first applied to a textile by conventional dipping and then polymerized by the action of an electron beam.10 The limitations of such a process may lie in the relative energy of the radiation, which would only allow polymerization to occur at the

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surface of the fabric and affect the resulting physical and aesthetic properties of the fabric. Future advantages of such processes may lie with reduced water usage. Phosphorus-containing compounds have also been applied to cotton fabrics using plasma-induced graft polymerization.12

17.10 Conclusions In summary, there are ever-increasing pressures on flame retardants and chemicals in general. In addition to having to meet stringent flame retardant test standards and legislation that covers specific market sectors, textile flame retardants are required to have good toxicity and environmental profiles as well as good drape and handle properties. The finished article may (after exhaustive testing) carry an ecolabel that endorses these positive features. As noted, garment end-users are requesting more technical multifunctional fabrics. At the same time, textile finishers, equipment manufacturers and chemical suppliers are under further pressure to adapt processes to be less polluting and energy hungry, and to leave a smaller environmental footprint.

References 1. R. Padda and G. Lenotte, General Trends in textile flame retardants Speciality Chemicals Magazine, 2005, Vol. 25; No. 7, pp. 43–45. 2. A. Beard, Flame Retardants, ‘‘Frequently asked questions’’ EFRA reportThe European Flame Retardants Association, p. 4. 3. P.J. Wakelyn, N.R. Bertoniere et al., Handbook of Fiber Chemistry, Edited by M. Lewin, 2007, pp. 593–594. 4. B.K. Kandola, A.R. Horrocks, D. Price and G.V. Coleman, ‘Flame Retardant Treatments of Cellulose and their Influence on the Mechanism of Cellulose Pyrolysis’, Journal of Macromolecular Science–Reviews in Macromolecular Chemistry and Physics, 1996, C36(4), 721–794. 5. W.D. Schindler and P.J. Hauser, Chemical Finishing of Textiles, 2004, p. 107. 6. W.R. Goynes and M. Rollins, Textile Research Journal 1971, pp. 41, 226. 7. Anonymous, United Kingdom Accreditation Service web site, ‘‘The Role of UKAS’’ www.ukas.com. 8. L. Debell, Weaving a Future, Company Clothing Magazine Sept 2005, pp. 34–38. 9. J. James and R. Sujarit, US Patent 4748705A, Combined cyclic phosphonate/THPx FR process for cotton/polyester fabrics, 1988, Burlington Industries, Inc. 10. E. Clarke, Protective Fabrics and the Search for the Holy Grail, Company Clothing March 2006, pp. 44–46.

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11. A.R. Horrocks, B.K. Kandola, P.J. Davies, S. Zhang and S.A. Padbury, Developments in flame retardant textiles – a review, Polymer Degradation and Stability, Volume 88, Issue 1, April 2005, pp. 3–12, 9th European Conference on Fire Retardant Polymers. 12. P.R.S. Reddy, G. Agathian and A. Kumar, Ionizing radiation graft polymerized and modified flame retardant cotton fabric, Radiation Physics and Chemistry, Volume 72, Issue 4, March 2005, pp. 511–516.

CHAPTER 18

New and Potential Textile Flammability Regulations and Test Methods within the USA P.J. WAKELYN National Cotton Council of America (retired), 1520 New Hampshire Ave, Washington, DC 20036, USA

18.1 Introduction Virtually all common textiles can burn, potentially causing some degree of unreasonable risk to consumers. The major cause of fatalities in many fires can be directly attributed to the accidental ignition of textiles. So meaningful textile flammability standards (mandatory and voluntary), which address risk, and test methods should be in place and are very important because they have crucial safety implications in the event of a fire. In the USA, fires and burns are the fifth-leading cause of accidental injuryrelated death among children younger than 15 years. Fires that involve clothing ignition in the USA resulted in 120 fatalities annually during 2002–2004, the most recent data available, and an estimated 3947 non-fatal injuries were treated in hospitals annually during 2003–2005.1 In addition, in the USA, residential upholstered furniture fires resulted in 280 deaths and 500 injuries annually during 2000–20042 (see Table 18.1) and all causes of residential fires in the USA result in over 3000 deaths and 15 000 injuries (Table 18.2). Textile fabrics burn by two different processes. One is flaming combustion (e.g., caused by an open flame source, such as a match). Since fibres that make up fabrics are composed of large, non-volatile polymers, flaming combustion Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Table 18.1

US residential upholstered furniture: estimated average annual USA fire losses 2000–2004.

Fires Deaths Injuries Property damage

Smoking materials

Small open flame

Total

2500 260 320 $65 million

1100 30 170 $46 million

3500 280 500 $112 million

Source: US CPSC Directorate for Epidemiology.

Table 18.2

US causes of residential fires and fire losses, 2001.

Cause

Fires

Deaths

Injuries

Loss (million US $)

Incendiary/suspicious Children playing Smoking Open flame Other heat Electrical Appliances Heating Cooking Other equipment Natural Exposure Total

41 000 8000 21 000 35 000 16 000 27 000 26 000 61 000 125 000 5000 11 000 19 000 396 500

750 100 800 300 100 250 150 250 350 100 (6) (23) 3140

2000 1000 1750 2250 800 900 1000 1150 4200 250 150 200 15 575

1100 200 350 650 250 700 550 550 400 150 350 350 $5643

Source: Fire in the US, 13th edition, USFA, 2005; NFIRS, NFPA.

requires that the polymer undergo decomposition to form the small, volatile organic compounds that constitute the fuel for the flame. The combustion of polymers is a very complex, rapidly changing system that is not yet fully understood. For many common polymers, this decomposition is primarily pyrolytic with little or no thermo-oxidative character. The second is smouldering combustion (e.g., that caused by a burning cigarette or, in some cases, by radiant heat from a fire remote from the item). Smouldering is defined as a nonflaming, self-sustaining, propagating, exothermic, surface reaction that derives its principal heat from heterogeneous oxidation of the fuel (direct attack of oxygen on the surface of a solid phase fuel).3 Smouldering is a serious fire risk because it:
typically yields a substantially higher conversion of fuel into toxic compounds than does flaming (though more slowly);
is difficult to detect and extinguish in the interior of a porous material; and
provides a pathway to flaming that can be initiated by heat sources much too weak to cause a flame directly.

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Smouldering combustion involves direct oxidation of the polymer and the formation of a char (materials which decompose to give liquid products do not smoulder, e.g., thermoplastic fabrics) and other non-volatile decomposition products. Progressive smouldering (exothermic ignition not accompanied by flaming, that is self-propagating, i.e., independent of the ignition source4) is a particularly dangerous situation as it spreads slowly and can suddenly change into rapid flaming combustion.3 Research supports the concept that the transition from smoulder to flaming is basically a spontaneous gas-phase ignition reaction supported by the smoulder reaction, which acts as the source of both gaseous fuel (pyrolyzate) and the heat to support the reaction.5 The transition from smouldering to flaming combustion of flexible polyurethane (PU) foam is determined mainly by the exothermic oxidation of the residual char and the availability of oxygen inside the foam.6 Since smoulder ignition and open-flame ignition are different mechanisms, they usually require different flame retardant treatments to be used, and treatments to control open-flame ignition can adversely affect smoulder resistance.7 Flammability performance can be improved by the fabric manufacturer at the design stage to ensure a safer environment. Making a textile flame resistant is complex. Different fibres and filling materials require flame retardants. What is used and how it is used to meet the various new flame retardant standards will depend on several factors: performance, cost and meeting consumer expectations. Consumers expect textile home furnishing products and apparel to remain unchanged in terms of aesthetics, price, performance and care requirements. These requirements impact on the manufacturer’s ability to make textiles both commercially acceptable and flame resistant. The hazard and risk to the public from death, injury and property loss from fire should be balanced with the risk to human health and the environment associated with the use of flame retardant chemicals – no one wants to trade fire risk for chemical toxicity risk. The benefits for fire prevention should outweigh the risk to health and environment. Flammability standards address unreasonable risks to consumers. In the USA, there are mandatory and voluntary cigarette and open-flame ignition standards for textile products. The US Consumer Product Safety Commission (CPSC)8–14 and the California Bureau of Home Furnishings and Thermal Insulation (CA BHFTI)15–17 are developing or have already proposed or promulgated open-flame ignition and cigarette-ignition standards for upholstered furniture, mattresses, and bedclothes, and/or filled top-of-the-bed products (i.e., mattress pads, comforters, quilts, bedspreads, pillows). CPSC can consider all bedclothes (i.e., blankets and sheets in addition to filled bed products), whereas California, by statute, can only regulate filled top-of-the-bed products (i.e., mattress pads, comforters, quilts, bedspreads, pillows). CPSC has also updated and/or revised the 50+-year-old ‘‘Standard for the Flammability of Clothing Textiles’’ (i.e., ‘‘general wearing apparel’’18). The Congressional Fire Services Caucus of the US House of representatives,19 the Shriner Burn Hospitals and the National Association of State Fire Marshals (NASFM) continually raise concerns about children’s sleepwear and general wearing apparel. This despite all available data on injuries and fatalities caused by the

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ignition of children’s sleepwear, clothing textiles and general apparel (where sleepwear or clothing is the first item to ignite) indicates that the flammability standards for these textiles are doing what they are intended to do and that sleepwear and clothing textiles which comply with these standard do not present an unreasonable risk to the consumer.20 In 2007, US CPSC finalized a technical amendment to the Flammability Standards for Carpets and Rugs (16 Code of Federal Regulations 1630 and 1631).21,22 Potential regulations for ignition sources (i.e., candles and cigarette lighters) are being considered. Some other US states have legislation to adopt CA furniture flammability standards. In 2008, there is also federal and state legislation (e.g., California Assembly Bill 706) that concerns textile flammability, flame retardants (a ban on the use of brominated and chlorinated fire retardants) and for reduced ignition-propensity (RIP) cigarettes. The misleading terminology ‘‘fire safe’’ cigarettes is used by some instead of RIP cigarettes – burning cigarettes can never be safe and will always be a potential source of ignition and hazard. There are also flammability standards developed by ‘‘consensus standard setting organizations’’ – International Organization for Standardization (ISO), American Society for Testing and Materials (ASTM), American National Standards Institute (ANSI, a member body of ISO), National Fire Protection Association (NFPA), International Code Council/International Fire Codes (ICC/IFC), European Committee for Standardization (CEN), Deutsches Institut fu¨r Normung (DIN), etc. The designation of the flame resistance of a textile is test-method dependent. The test, therefore, that the material passes should always be specified when claims of flame resistance are made. As indicated, there currently are many mandatory federal and state, as well as voluntary national and international standards (smoulder and/or cigarette resistance, small and large open flame), for the flammability of soft furnishings (furniture, mattresses, bedclothes). There are component standards and composite-, large- full-scale standards. The current smoulder-test methods use a standard or specified cigarette (without a filter tip, made of natural tobacco, 85  2 mm long with a packing density of 0.270  0.02 g cm3; Pall Malls is the specified cigarette, which has become difficult to obtain). Gann23 has discussed the ignition strength of cigarettes. CPSC staff are conducting research to find a substitute standard ignition source – that is more uniform and available – to replace the standard or specified cigarette. Large open-flame standards for mattress/box springs use burners that mimic burning bedclothes (e.g., as in TB 60316), other large flame sources (e.g., an 18 kW flame for 3 minutes as in TB 12924) or a trash can full of burning paper (as in TB 12125). Small open-flame (SOF) sources usually are representative of matches, cigarette lighters and candles (e.g., a butane or propane flame for 15 or 20 seconds) and are used in upholstered furniture tests and potentially to test the flammability of bedding or filling products. For clothing textiles and general wearing apparel there is a 45 1 angle test with a SOF (1 second) and the burn rate (a measure of the ease of ignition and flame spread) is measured.26 For children’s sleepwear the test is a vertical flame test with a SOF and the distance the flame travels is measured.27,28

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There are various ways to determine compliance or pass/fail (P/F) criteria with the various standards: heat release [peak heat release rate (PHRR), heat release rate (HRR), total heat release (THR], flame spread, char length, mass and/or weight percent (wt%) loss, ease of ignition (ignitability, time to ignition), etc. USA regulatory and legislative actions that potentially affect over 653,590 MT (3 million 480 lb bales) of cotton, as well as all textile fibres, are discussed.

18.2 Mattresses and/or Foundation (Box Springs) In the USA, there are mandatory flammability standards for mattresses that address both open flame and cigarette ignition. The mattress and foundation market (excluding filling) is about 14 480 MT (66 000 bale equivalents) of cotton.

18.2.1

US CPSC

US CPSC has separate mandatory flammability standards to address openflame ignition [16 CFR 1633, Standard for the Flammability (Open Flame) of Mattresses and Mattress/Foundation Sets]12 and cigarette ignition (16 CFR 1632, Standard for the Flammability of Mattresses and Mattress Pads).29

18.2.1.1

Open Flame

In 2006, US CPSC finalized a federal flammability standard to address openflame ignition of mattresses by preventing or delaying flashover.10 The effective date was 1 July 2007. The test method uses the dual burners developed by the National Institute of Standards and Technology (NIST)30–34 on a twin-size mattress, which was designed to mimic the local heat flux imposed on a mattress and/or foundation by burning bedclothes. It consists of two T-shaped burners.12 For mattresses/ box springs the ignition source NIST Dual Flame Burner Test Protocol to be used is two T-burners: top burner heat release (HR) 19 kW (heat flux 65 kW m2), 70 seconds; side burner HR 10 kW (heat flux 45 kW m2), 50 seconds. Test criteria are that the specimen shall comply with both of the following criteria – the maximum PHHR shall not exceed 200 kW at any time in the 30 minute test; and the THR shall not exceed a maximum 15 MJ in the first 10 minutes of the test. NIST also has tested mattress and/or foundation designs of varied fire resistance with TB 603, to determine if there are size effects in the fire performance of beds33 – results suggest that there are some size-dependence considerations. The test method which was developed using a full-size mattress does not necessarily scale to smaller or larger mattress (e.g., queen size or king size).

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18.2.1.2

271

Smoulder

A US federal standard for the flammability of mattresses and mattress pads (16 CFR 1632)34 that requires mattresses and mattress pads to be resistant to cigarette ignition has been in effect since 1973. CPSC is considering revocation or amendment of this mattress and/or mattress pad flammability standard for cigarette ignition,35 because some feel it may not be necessary now that there is an open-flame standard. The federal mattress standards administered by CPSC pre-empt any state standard unless the state has requested and is granted exemption from pre-emption by CPSC.

18.2.2

CA BHFTI

Legislation (California Assembly Bill 603) directing the CA BHFTI to develop an open-flame standard for residential mattresses/box springs (foundation) and ‘‘top of the bed’’ textile products, if those products are determined to contribute to mattress fires, was signed into law in August 2001. In response to this legislation CA developed ‘‘TB 603’’.16

18.2.2.1

Open Flame

TB 603 Requirements and Test Procedure for Resistance of a Mattress/Box Spring Set to a Large Open-Flame.16 TB 603 became law on 22 February 2004 and effective on 1 January 2005, but was replaced by the US CPSC open-flame mattress standard 29 CFR 1633 on 1 June 2007.

18.2.2.2

Smoulder

TB 106, is the same as 16 CFR 1632 CPSC Standard for the Flammability of Mattresses and Mattress Pads,34 the federal cigarette (smoulder) ignition standard. The mattress industry is seeking to have this standard revoked or amended. The ignition source is 18 lighted cigarettes [nine in the bare-mattress tests and nine in the two-sheet tests; cigarettes from natural tobacco, 85  2 mm long, diameter of 7.62  0.5 mm [0.3  0.02 inches (in.)], weight 1.1  0.1 g], in a draft-protected environment on a full-scale mattress in a horizontal position. TB 26 Requirements for Record Keeping and Prototype Testing of Mattresses for Compliance with State and Federal Flammability Laws,24 is the same as the 16 CFR 1632 requirements for prototype testing and record keeping.

18.2.2.3

Open Flame Standards for Public Occupancy

TB 121 Flammability Test Procedure for Mattresses for Use in High Risk Occupancies25 (for typical institutional mattresses, not intended to be used for residential mattresses) is a full-scale test; the mattress is conditioned at 21  0.17 1C (70  5 1F), relative humidity (RH) o55%. The ignition source is a galvanized metal container with 10 double sheets of loosely wadded newspaper

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[23  28 in., B18.5  0.5 g]. The newspaper is ignited with a match and combustion is allowed to continue until either all combustion has ceased or at least 10% by weight of the mattress is consumed. The P/F criteria is the mattress fails if there is 410% weight loss in the first 10 minutes, if there is a temperature of 260 1C (500 1F) or greater at the thermocouple above the test mattress at any time during the test, or if CO concentration exceeds 1000 parts per million (ppm) at any point in the test room at any time during the test. For TB 129 Flammability Test Procedure for Mattresses for Use in Public Buildings24 the ignition source is a T-burner [205 mm (about 8 in.)] 17.8 kW flame (2050  50 kJ mol1) for 180 seconds, with side ignition. The product fails if a weight loss through combustion of Z 1.36 kg in the first 10 minutes of the test, or PHRR is Z 100 kW, or THR Z 25 MJ in the first 10 minutes of the test.

18.3 Bedclothes Since bedclothes can be part of the bed-fire scenario, standards to address the flammability of bedclothes are being considered in the USA. The bedclothes market is about 218 000 MT (1.0 million bales) if sheets and pillowcases are included [sheets and pillowcases are about 130 720 MT (600 000 bales) of cotton].

18.3.1

US CPSC

The latest statistics CPSC is using suggest that bedclothes are the first to ignite in about 80% of mattress and/or bedding fires. The burners (i.e., heat insult and/or ignition source) used in the mattress test were designed to approximate a worst-case bedclothes fire. CPSC published an advance notice of proposed rulemaking (ANPR) in 2005 requesting comments on whether a standard is needed to address the SOF ignition of bedclothes.11 It addresses all top-of-thebed products, including filled bed products (e.g., comforters, mattress pads and pillows), blanket, sheets and pillowcases – over 500 000 MT (2.3 million bales) of cotton. However, it is unlikely that CPSC would include sheets and pillowcases in their standard.

18.3.2

CA BHFTI

The CA BHFTI has determined that a flammability standard for ‘‘filled’’ topof-the-bed products (mattress pads, comforters, bed pillows, quilts, decorative pillows and padded headboards) is warranted and issued a draft standard.17 CA regulations by statute are not able to cover non-filled bedding items (e.g., blankets, sheets and pillow cases). There is a precision and bias (P & B) study underway on the test methods in draft TB 604 (Test Procedures and Apparatus for the Flame Resistance of Filled Bedclothes17) which will be completed before the CA BHFTI proposes any

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standards for bedclothes. The CA BHFTI likely will open their formal rulemaking process on open-flame ignition of filled bedding in 2008, and it could be coordinated with CPSC actions. A typical rulemaking by the CA BHFTI lasts one year. It is unlikely the CA BHFTI will finalize a flammability standard for filled textile bedding before early 2008 that would be effective one year later. The CA BHFTI revised draft standard for components (TB 604) was released October 2004 and further revised in October 2007:17 Section 1. – Flat Fill/Bedclothing Products Test: Scope – This test applies to filling materials used in bedclothing items such as comforters and bedspreads, which are designed to lie flat on or around a mattress or foundation; it also applies to mattress pads that are filled with flat filling materials and have a thickness 450 mm (2 in.). Test method: Test method is designed to measure the response of a flat bedclothing product to a small open flame, representing a match, candle or cigarette lighter. Multiple layers of flat filling materials are inserted in a case [381 mm  381 mm (15 in.  15 in.)] made of standard sheeting fabric (50% cotton/50% polyester 3.2  0.5 oz yd2, 150–200 threads/in.2, white in color, not treated with flame retardant, laundered and dried once) or of the product’s actual cover fabric. Specimen [305 mm  305 mm (12 in.  12 in.) in the thickness of use; stack number of layers to reach the thickness of about 102 mm (4 in.)]. Place specimen on a horizontal cement board that is covered with a sheet of aluminum foil on a weighing device; subject the front right hand corner to a 35 mm (1 3/8 in.) high butane gas flame, tip of the burner tube 19 mm (3/4 in.) below the corner of the specimen, for 20 sec. P/F criteria – The specimen fails if any of the following conditions are reached. Foam: average gross (fabric and fill) wt. loss % of triplicate samples exceeds 25.0% in 6.0 min. and wt. loss % of any individual specimen 430.0% in 6.0 min.; other filling materials: ave. gross (fabric and fill) wt. loss % of triplicate samples at 3.0 min. exceeds 25.0 g.; ave. gross (fabric and fill) wt. loss % of triplicate samples at 6.0 min. exceeds 30%; weight loss % of any individual specimen 435% in 6.0 min.; and a void (burn through) occurs in the outer ticking of the test specimen (if actual fabric used). Section 2. – Pillow/Cushion Products and Loose Filling Materials Test: Scope – The test applies to all pillows and bed cushions, except solid foam (molded and slabstock) pillows and pillows and cushions that weigh no more than 400 g. The test covers loose filling component materials used in other bulk items of bedclothing (such as bed-rest cushions, padded headboards, comforters and bedspreads), mattress pads containing loose fills and having a thickness 450 mm (2 in.), loose fillings including shredded PU and latex cellular foam, feathers and down, ungarnetted (loose) synthetic, natural and natural/synthetic-blend fibres, polystyrene

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beads, buckwheat hulls, etc., and synthetic and natural filling materials which are formed into a continuous fiber web consisting of battings, pads, etc. and rolled up and/or folded to form a pillow insert. Test method – The filling materials are in a case made of the standard sheeting fabric or of the ticking fabric used in the actual bedclothing product to encase the loose filling materials. The specimen is placed on a horizontal cement board on a weighing device and ignited at one corner with the same small open flame and same distance for 20 sec. as in Section 1. P/F criteria — The specimen fails if any of the following conditions is reached. The gross (fabric and fill) ave. wt. loss % of triplicate samples exceeds 25.0% in 6.0 min.; wt. loss % of any specimen exceeds 30.0% in 6.0 min.; and a void (burn through) occurs in the outer ticking of the test specimen (if actual fabric used). Section 3. – Mattress Pad Test: Scope – The test applies to all synthetic and natural filling materials that are used in mattress pads that are 450 mm (2 in.) thick. Any mattress pad weighing no more than 400 g is exempt from the test. All mattress pads with thickness 450 mm (2 in.) and containing filling material should be tested per Section 1 if they contain flat fillings; and per Section 2 if they contain loose fillings. Test method – Option A – fill component: The specimen [305 mm  305 mm (12 in.  12 in.)] of the filling material is placed between four (two on top and two on bottom) 305 mm  305 mm (12 in.  12 in.) pieces of standard sheeting fabric (see Section 1) on the test platform; place the square metal frame [305 mm  305 mm (12 in.  12 in.) 3.2 mm (1/8 in.) thick stainless steel with inside opening of 254 mm  254 mm (10 in.  10 in.)] over the top sheeting fabric; and subject the top surface of the test specimen to a 35 mm (1 3/8 in.) high gas flame oriented at 30 degrees with respect to the horizontal line and the tip of the burner at the center of the top surface for 20 sec. Option B – Actual composite test: The same except the test specimen with its own original unraveled ticking fabrics is placed between two pieces (one on top and one on the bottom) of standard sheeting fabric. P/F criteria: The specimen fails if any of the following conditions is reached. Option A – fill component test and Option B – Actual composite test: the flame burns through the bottom sheet and creates a void in the sheet or the flame creates a void of 451 mm (2 in.) in any direction in the filling material and the flame also creates a void of 451 mm (2 in.) in any direction in the standard sheeting fabric just above the bottom sheet (even if there is no void on the bottom sheet). NIST did a study on the effect of ‘‘improved’’ filled bedding textiles on the fire performance of bed assemblies.32 These textile bedding items with

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improved flammability properties provide some protection to currently available and TB 603 compliant mattresses from open-flame ignitions.32

18.4 Upholstered Furniture New mandatory standards to address the flammability of upholstered furniture are being considered in the USA. The USA upholstery and slipcover market is over 163 400 MT (725 000 480 lb cotton-bale equivalents) – cotton is about 48% of the poundage in the market (B40% of the sales value).

18.4.1

US CPSC

In response to a petition from NASFM, on 15 June 1994 CPSC published an ANPR to address risks associated with ignition of upholstered furniture by SOF sources (matches, lighters and candles). On 23 October 2003, CPSC published a new ANPR10 to address both ignition risks of upholstered furniture – SOF and cigarette ignition – in the same rulemaking. In 2005, CPSC released a draft standard with open-flame and smoulder requirements; in December 2007, CPSC released another briefing package that contained an alternative standard (Table 18.3); and in 2008 CPSC published a proposed standard containing the 2007 draft standards.

18.4.1.1

1997, 2001 CPSC Staff Draft Standard

The focus of the CPSC staff draft standard for SOF ignition, issued in briefing packages in 1997 and 2001, was upholstery fabric.8,9 CPSC Upholstered Furniture Flammability Regulatory Options in the 2001 Briefing Package9 include:
A test for the seating area and dust cover – ignition source small butane flame, 20 seconds; maximum flaming 2 minutes; maximum smoulder 15 minutes.
Alternate seating barrier – ignition source BS 5852 Crib #5 (17 g of wooded sticks, 6.5 mm square and 40 mm high); maximum flaming 10 minutes; maximum smoulder 60 minutes.

18.4.1.2

2005–06 CPSC Staff Revised Draft Standard – With Many Test Methods

CPSC staff presented revised draft standards for upholstered furniture flammability in late 2004 and 2005 and in their January 2006 options package.13 The revised draft standards provided a set of performance tests for major upholstery materials. They included requirements for cigarette and SOF ignition performance of fire barriers and filling materials. And a cigarette test but no SOF test for fabrics – except the fabric that is part of the composite in the endproduct material test and could contribute to mass loss. In summary, the CPSC

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CPSC proposed standard for upholstered furniture flammability (performance tests).14 Type 1: Manufacturer selects smoulder-resistant cover material

Material

Test description

Test requirement

Cover fabric/material (smouldering resistance)

Modified [76.2 mm (3 in.)] ASTM/UFAC seating mock-up with standard polyurethane foam (SPUF) substrate and standard cigarette ignition source

After 45 minutes: – no continued smouldering – no obvious flaming ignition (i.e., never transition from smouldering to flaming) – maximum 10% mass loss of substrate

PU foam/other filling materials

No requirements

Type 2: Manufacturer selects qualified interior barrier Material

Test description

Test requirement

Interior fire barrier (smouldering resistance)

Modified [76.2 mm (3 in.)] ASTM/UFAC seating mock-up with SPUF substrate and standard cigarette-ignition source BS 5852 seating mock-up with standard non-flame retardant PU foam substrate, standard rayon cover fabric and 240 mm (9.45 in.) per 70 seconds Flame-ignition source

After 45 minutes: – maximum 1% mass loss of substrate

Interior fire barrier (SOF resistance)

After 45 minutes: – maximum 20% mass loss of the total mock-up assembly

staff 2005–06 revised draft had no SOF test for fabrics, concentrated on foam and filling materials, and had two barrier tests.
All tests were mock-up tests: open-flame ignitions tests use the BS 5852 seating mock-up (crevice ignitions); and smoulder tests use a modified ASTM E1353/UFAC (Upholstered Furniture Action Council) seating mock-up [76.2 mm (3 in.) foam vs. 50.8 mm (2 in.) foam].
There were four ways to meet the upholstered furniture flammability test criteria – Type I, with a cover barrier; Type II, with an interior barrier; Type III, all individual materials pass the tests; and Type IV end-product materials mock-up passes the tests.

18.4.1.3

2008 CPSC Proposed Standard

In November 2007,12 CPSC again changed their approach to addressing the flammability of upholstered furniture. The new approach recognizes that

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furniture, ignited by cigarettes, accounts for about 90% of deaths and 65% of injuries from furniture fires. It also attempts to address upholstered furniture fires without requiring the use of flame retardant chemicals. The CPSC staff developed an alternative draft standard14 with a smouldering resistance test for fabrics and other upholstery cover materials, and smouldering and open flame tests for interior barriers that may be used with noncomplying cover fabrics, but it has no requirements for the PU foam or other filling materials. Thus, manufacturers can meet this performance standard by using smoulder-resistant cover fabrics or internal fire-resistant barriers to protect the furniture internal filling materials, which are the primary fuel load in upholstered furniture fires. This draft standard, which was approved on 1 February 2008 to be published as a proposed mandatory standard, defines two types of upholstered furniture:
Type I made with complying fabrics (cover fabrics or other covering materials that are smoulder resistant in accordance with the proposed cover fabric performance test) and
Type II made with complying barriers (barriers that are smoulder- and open-flame resistant in accordance with the two proposed barrier performance tests). Table 18.3 gives the proposed tests for Type 1 and Type 2 furniture. The CPSC Upholstered Furniture Smouldering Resistance standard mockup test rig (Figure 18.1), which is a modified ASTM E1353/UFAC mock-up rig, is constructed from fabric and foam to form a simulated chair with the back at a right angle to the base. This enables the ignition source to be kept in permanent contact with both back and seat throughout the test. The ignition source is a standard or specified cigarette without a filter tip, made of natural tobacco, 85  2 mm long with packing density of 0.270  0.02 g cm3 (Pall Malls) and it is placed in the crevice of the mock-up test rig (Figure 18.1). The CPSC smoulder test has 76.4 mm (3 in.) foam instead of 50.8 mm (2 in.) foam. The SPUF substrate is non-flame retardant treated PU foam. To demonstrate compliance with the 2008 US CPSC proposed performance standard for upholstered furniture, a manufacturer must test samples of each product (cover materials if it is Type 1 or barriers if it is Type 2) and demonstrate passing. The proposed standard, as mentioned earlier, places primary emphasis on the smouldering performance of cover fabrics. According to CPSC,14 most existing fabrics (84%), including predominantly thermoplastic fabrics and materials such as leather, wood and vinyl, would likely pass the fabric smouldering test without modification. However, some fabrics that are predominantly ‘‘cellulosic’’, such as certain high-cotton content fabrics, most likely will need to be used with complying barriers, be modified and/or re-engineered or treated with flame retardant chemicals to comply with the requirements. Re-engineering fabrics is not simple. To be confident that a cellulosic fabric will pass the CPSC smoulder test, which is more severe than the UFAC/ASTM 1353 test, the fabric most likely would need to be 30–50%

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Figure 18.1

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CPSC upholstered furniture test: smouldering resistance mock-up test rig (Source: US CPSC).

thermoplastic fibre (i.e., polyester). Adding polyester to a cellulosic fabric changes its characteristics. CPSC assumes textile manufacturers will not use flame retardant chemical treatments. Textile manufacturers do not want to use flame retardant chemical treatments, but many will most likely use flame retardant treatments because their customers do not want to use barriers and do not want to double upholster. Decabromodiphenyl ether (decaBDE), applied as a backcoating, would be the chemical system of choice, but it has been banned in Washington State and Maine in the USA, and is facing restrictions elsewhere.36 So research will be required to develop new flame retardant systems for cellulosic fabrics. If a fabric is at all marginal, 30 samples may have to be tested for 45 minutes to be confident that the fabric passes the test, even though there are no production testing requirements and officially only one test is required. For a small upholstered furniture manufacturer who makes 25–30 fabrics a week, this would be extremely time consuming and costly. The flame source for the barrier, 240 mm per 70 seconds flame ignition is intended to be equivalent to a BS5852 crib #5 flame insult (17 g of wooden sticks, 6.5 mm square and 40 mm high). The CPSC Upholstered Furniture Open Flame Resistance mock-up test rig for the barrier test is shown in Figure 18.2. The CPSC staff are also undertaking additional research in the areas of largescale verification testing of upholstered furniture materials and RIP cigarettes. The results of the additional research will be used in the further development of

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Figure 18.2 CPSC upholstered furniture test: open flame resistance mock-up test rig (Source: US CPSC).

a possible furniture flammability standard. The validation testing for this proposed standard, which relies on as yet unproven assumptions, will be critical in determining how effective the proposed standard test methods will be in reducing fire deaths and injuries. In addition, Underwriters Laboratories Inc. (UL) in the USA is conducting major research directed at understanding the flammability of upholstered furniture, which may also help to evaluate and refine the CPSC proposal. The CPSC, before promulgating a final rule for flammability standards for residential upholstered furniture, may consider studies by the US Environmental Protection Agency (EPA) on dermal penetration, US National Institute for Occupational Safety and Health (NIOSH) on workplace exposure and safety, and CPSC staff on durability, large-scale chair tests, laboratory roundrobin tests and others. The toxicities of potential flame retardants, which may be required in a CPSC performance standard for upholstered furniture, have been reviewed37,38 and need further review. US EPA proposed a significant new use rule (SNUR) under Section 5(a)(2) of the toxic substances control act (TSCA) to cover the polybrominated diphenyl ethers (PBDEs), pentaBDE and octaBDE – flame retardant chemicals potentially used in residential upholstered furniture.39 EPA issued this regulation to complement the phase-out of these two flame retardant chemicals. This regulation will ensure that no new manufacture or import of these two chemicals could occur after 1 January 2005, without first being subject to EPA evaluation. A SNUR allows EPA to designate any new manufacture or import as a ‘‘significant new use’’. Advance notification is required prior to commencing the new use. Thus, before the chemical can be manufactured or imported for the significant new use, the company would be required to provide advance notification to EPA under Section 5 of TSCA. This approach gives EPA the opportunity to evaluate any concerns and, if necessary, regulate future manufacture, import or uses associated with these two chemicals.

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The Polyurethane Foam Association (PFA) in the USA suggested40,41 that an effective flammability standard must be:
based on the composite performance of the finished piece, including all items of assembly;
appropriate to the risk of SOF ignition;
free from bias toward any component; and
reproducible and technically feasible. In addition, technologies used to produce flexible PU foam to meet a standard must be safe for workers, the public and the environment, and be economically feasible. To address smoulder ignition, PFA recommended a national upholstered furniture standard based on cigarette ignition, identical or similar to the UFAC standard42 (e.g., ASTM E-1353). Untreated PU foam passes this cigarette test, but flame retardant-treated foam of the density used in upholsterd furniture has problems consistently passing a cigarette test. In 2004-05 PFA members discontinued the use of pentaBDE when health concerns were raised and will likely do the same for its replacement chemicals, tris-2,3-dichloropropyl phosphate (TDCP or ‘‘chlorinated tris’’),37,38 and Firemasters 550 (a phosphorus–bromine flame retardant for flexible polyether and polyester PU foams manufactured by Great Lakes Chemical Corporation), which is ecotoxic according to an EPA study,38 because of health concerns. This greatly limits what can be used to allow PU foam to meet SOF standards (perhaps phosphorus compounds and/or melamine). Avoiding flame retardant chemical use in furniture was a major consideration for the 2008 proposed CPSC standard for upholstered furniture that places primary emphasis on the smouldering performance of cover fabrics and has no requirements for the filling materials.

18.4.2

CA BHFTI (TB 117, TB 116)

CA BHFTI announced in November 1999 that they are undertaking a full review and update of their mandatory upholstered furniture standard (TB 117).43 California Standards for Upholstered Furniture:

18.4.2.1

Smoulder/Cigarette

Test materials for TB 116 Requirements, Test Procedures and Apparatus for Testing the Flame Retardance of Upholstered Furniture44 are: 1. Cigarettes – cigarettes from natural tobacco, 85  2 mm long, diameter of 7.62  0.5 mm (0.3  0.02 in.), weight 1.1  0.1 g. 2. Furniture – the upholstered furniture tested shall be the finished product or a prototype mock-up of actual components which duplicate the design and structure of the finished product.

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In the test method, each furniture surface is tested until either (a) three cigarettes have burned their full length, (b) three cigarettes have extinguished before burning their full length or (c) one cigarette has resulted in failure. The P/F criteria are: 1. The furniture fails if any of the following occur – (a) obvious flaming combustion occurs and/or (b) a char develops 450.8 mm (2 in.) in any direction from the cigarette, measured from its nearest point. 2. Flame retardant properties must be maintained by the furniture under all normal conditions of temperature, humidity and use.

18.4.2.2

Open Flame

TB 117 Requirements, Test Procedure and Apparatus for Testing the Flame Retardance of Resilient Filling Materials Used in Upholstered Furniture40 applies to upholstery fabrics and filling material. For upholstery fabrics both surfaces of the fabric are tested to determine compliance (Class 1) with 16 CFR 1610 (451 angle test, one second surface ignition).26 For filling material, the test is, depending on the material, a vertical or one second 451 angle test. A 304.8 mm  76.2 mm (12 in.  3 in.)  normal thickness cotton batting sample has to pass a vertical flame test – 16 CFR 1610 flame source, 12 seconds flame middle bottom edge ignition. The P/F criteria are that the maximum char length shall not exceed 203.2 mm (8 in.) and the average char length of 10 specimens shall not exceed 152.4 mm (6 in.); there are no afterglow requirements.

18.4.2.3

Draft Revised TB 117

A draft revised TB 117 standard was released in February 2002 for review and comment.15 Draft revision 2/2002, TB 117 Flammability Test Procedure and Apparatus for Testing the Flame and Smoulder Resistance of Upholstered Furniture is a series of component and composite tests for filling materials and fabric with P/F criteria 4% weight loss in 10 minutes for most products. CA BHFTI indicated (March 2004) that this was not appropriate and more research was necessary for this complex standard. The CA BHFTI supports a national mandatory standard – CPSC should incorporate the best elements of the CPSC and the revised TB 117 drafts. CA is continuing to develop a revised TB 117 for a possible proposal. The Bureau has indicated that they will work closely with CPSC on their standard and do not plan to propose a separate CA standard unless CPSC takes too long to issue a standard. Legislation is also being considered in CA (AB 706) that would, by 1 March 1 2009, require the Bureau of Home Furnishings and Thermal Insulation to modify Technical Bulletins 116 and 117 with product performance standards (i.e., a composite standard) for furniture that shall achieve fire retardancy properties comparable to existing standards, sufficient to protect

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human health and safety, but without the use of brominated fire retardants and chlorinated fire retardants.

18.4.2.4

Open Flame (Public Occupancies)

For TB 133 Flammability Test Procedure for Seating Furniture for Use in Public Occupancies45 a finished product or full scale mock-up of the furniture can be tested. The ignition source is a square gas burner (250  250 mm), propane gas flow volume B13 l min1 for 80  2 seconds. The product fails if, in oxygen consumption calorimetry, the following are exceeded in a room test – maximum heat release rate (HRR) Z 80 kW, total HR Z 25 MJ in the first 10 minutes, 475% opacity at the 1.22 m [4 foot (ft)] smoke-opacity monitor, CO concentration Z 1000 ppm for 5 minutes. If a room instrument is used the product fails if there is a temperature increase of Z 93.3 1C (200 1F) at a ceiling thermocouple, of Z 10 1C (50 1F ) at the 1.22 m (4 ft) thermocouple, weight loss due to combustion Z 3 lb in the first 10 minutes (opacity and CO criteria are the same as for oxygen consumption calorimetry).

18.4.3 18.4.3.1

Upholstered Furniture Action Council Voluntary Furniture Smoulder/Cigarette Test UFAC (Voluntary) Filling/Padding Component Test Method—1990 Part A – For Slab or Garneted Materials (UFAC) (Cigarette Test)42

Vertical and horizontal panels are assembled on three specimen holders, using the UFAC Standard Type 1 mattress ticking as the cover fabric. A lighted cigarette is placed in the crevice formed by the abutment of the vertical and horizontal panels in each test assembly, and is covered by sheeting fabric. The cigarettes are allowed to burn their full lengths unless an obvious ignition occurs. Test observations are recorded. A minimum of three test specimens is required for each sample tested. The ignition source is cigarettes without filter tips made from natural tobacco, 85  2 mm (3.3  0.1 in.) long and with a packing density of 0.27  0.02 g cm3 (0.16  0.01 oz in.3) and a total weight of 1.1  0.1 g. Sheeting material is 100% cotton white bed sheeting, weight 125  28 g m2, and not treated for flame retardants, cut into 127  127 mm (5.0  5.0 in.) squares; the cover fabric is 100% cotton mattress ticking conforming to federal specification CCC-C-436E, cloth, ticking, twill, cotton: UFAC Standard Type 1 fabric.

18.4.3.2

UFAC (Voluntary) Barrier Test Method – 1990 (UFAC) (Cigarette Test)42

Vertical and horizontal panels are assembled on a small-scale test assembly using UFAC SPUF as the substrate. The Standard UFAC Type II cover fabric

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is used as the covering. The barrier material to be tested is placed between the cover fabric and the PU substrate in both vertical and horizontal panels. A lighted cigarette is allowed to burn its entire length unless an obvious ignition occurs. Test observations and measurements are recorded. A minimum of three individual specimens is required for each barrier sample tested. The ignition source is cigarettes without filter tips made from natural tobacco, 85  2 mm (3.3  0.1 in.) long and with a packing density of 0.27  0.02 g cm3 (0.16  0.01 oz in.3) and a total weight of 1.1  0.1 g. Sheeting material is 100% cotton white bed sheeting, weight 125  28 g m2, and not treated for flame retardants, cut into 127  127 mm (5.0  5.0 in.) squares; cover fabric is 100% bright regular rayon, scoured, 20/2, ring spun, basket weave construction, 271  12 g m2; UFAC Standard Type II Cover Fabric. PU foam substrate is a polyether-type PU foam that contains no inorganic fillers or flame retardants, having a density of 24.0  1.6 kg m3.

18.4.4

ASTM Standard

For ASTM E-1353: Cigarette ignition resistance, Class 1 fabrics are fabrics that pass the cigarette ignition test. Class 2 fabrics do not pass the cigarette test but furniture, when constructed with Class 2 cover fabrics, is to be assembled with Class A barriers. ASTM E-1353 is based on the UFAC voluntary standard that addresses cigarette ignition and has been subject to review by CPSC.42 The UFAC voluntary standard has been in use in the USA since the late 1970s.

18.5 Children’s Sleepwear There have been mandatory standards for children’s sleepwear for sizes 0–6x (16 CFR 1615) and 7–14 (16 CFR 1616) in the USA since the 1970’s.

18.5.1

1996 Amended Standard

CPSC amended the Children’s Sleepwear Flammability Standards (16 CFR 1615 and 1616 27,28 in September 1996 (effective 1 January 1997). The tests are vertical open flame tests with the burner flame impingement on the bottom edge of the specimen [89.6 mm (3 1/2 in.) by 254 mm (10 in.)] for 3.0  0.2 seconds. The test requirements are that the char length of each of five specimens is measured and, in general, a sample of five specimens cannot have an average char length 4177.8 mm (7.0 in.) or have more than a specified number of individual 254 mm (10 in.) char lengths. The amendments exempt sleepwear for infants, sized 9 months of age or smaller, and ‘‘tight-fitting’’ sleepwear from the vertical flammability test. Only children’s sleepwear (sizes 0–6x27 and 7–1428) is covered by the standards, which are designed to protect children, when they are up and moving around, from SOF ignition sources (e.g., match, lighter or candle) not large external flame sources (e.g., burning mattress, whole house fire or general conflagration).

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The amendments did not affect loose-fitting pyjamas, nightgowns and robes, which are the garments most likely to be involved when injuries occur. These still must be flame resistant. In 2007, CPSC estimated that over 35% of the sleepwear market is now snug-fitting cotton {about 22 000 MT [100 000 bales of 480 pounds (lbs) each] of cotton}. The Congressional Fire Services Caucus of the US House of Representatives,19 the Shriner Burn Hospitals and the NASFM continually raise concerns about the children’s sleepwear and apparel standards and would like children’s apparel for all children under seven to meet the vertical flame test of 16 CFR 1615.27 Sleepwear would be affected directly, and also playwear and underwear. About 305 000 MT (1.4 million bales) of cotton and all daywear and clothing worn by children under seven years of age could be affected.

18.5.2

New CPSC Data Collection Tool for Clothing-Related Burn Injuries to Children

To help obtain a more accurate count of burn injuries related to children’s clothing, CPSC developed the National Burn Center Reporting System (NBCRS) in 2003. It is designed to capture clothing-related burn injuries to children under 15 years old treated in burn centres in the US (92 of the 105 burn centres participate).20 Of the 253 incidents analyzed by 2007, the most frequent scenario was a child playing with a lighter, followed by a child standing too close to an outdoor fire. Nearly half of the 253 incidents involved accelerants (flammable liquids). There were no incidents involving 100% cotton ‘‘tight-fitting’’ sleepwear or infant garments sized 9 months or smaller. CPSC continued analysis of the NBCRS data has revealed no deaths or injuries attributable to the exempted infant size and tight-fitting sleepwear. In addition, CPSC has issued alerts that warn shoppers of the dangers of using loose-fitting cotton garments as sleepwear for kids.46 If CPSC continues to find no cases related to sleepwear with the new system (i.e., exemption of infant and tight-fitting sleepwear), it should help prevent legislation to repeal the 1996 amendment and to obtain more severe standards for apparel for all children under seven.

18.6 Clothing Textiles There been has a general decline in deaths caused by clothes that caught on fire, decreasing from 311 fatalities in 1980 to 129 fatalities (adjusted) in 2004, the most recent year the data were available. During 2002–2004, an average of 120 flammable clothing-related fatalities occurred annually in the USA, with higher rates for those of 65 years and older. The Standard for Flammability of Clothing Textiles (16 CFR 1610 ‘‘general wearing apparel standard’’26) was originally issued in 1953 by the US Department of Commerce under the Flammable Fabrics Act to eliminate dangerously flammable apparel from the US market. The responsibility for the

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upkeep and enforcement of the regulation was transferred to the CPSC in 1973. US CPSC has updated the standard16 because it does not reflect developments in equipment and consumer garment-care practices since it was issued in 1953.

18.6.1

US CPSC General Apparel Standard (16 CFR 1610)

The general wearing apparel standard (16 CFR 1610) classifies fabrics based on their rate and intensity of burning.26 The standard establishes three classes of flammability, sets requirements for clothing textiles and prohibits the manufacture, distribution and sale of dangerously flammable textiles for use in clothing. The test is a 451 angle test with the 16 mm (5/8 in.) gas flame applied for one second to the surface of the specimen. The specimen is allowed to burn upward until the flame burns through the stop cord to release the weight and stop the timer, or extinguishes. The test classification (three classes) is based on the time of flame spread (i.e., burn rate) and number of base burns (for raised surface apparel, e.g., fleece, chenille, terry cloth). Class 1 textiles have a flame spread time of 3.5 seconds or more for plain-surface fabrics, of 47 seconds for raised surface fabrics, or 0–7 seconds for raised surface fabrics with no ignition or melting of the base fabric (Class 1 textiles can be used for clothing). Class 2 textiles (applies only to raised fibre surfaces) have a flame spread from 4 to 7 seconds and a base fabric that ignites or melts (Class 2 can be used in clothing). Class 3 textiles have a flame spread time of o3.5 seconds for smooth-surface fabrics and o4 seconds for raised surface fabrics with a base fabric that melts or burns from other than the igniting flame (Class 3 textiles cannot be used in clothing).

18.6.2

US CPSC Updated Standard (2008)

There were problems with the test procedures and interpretation of results with the original 50+ year-old standard. Consumer garment care practices have changed significantly and modern equipment has been developed since the standard became effective in the 1950s. Some of the equipment and procedures (e.g., the laundering and dry-cleaning methods, flammability tester specifications) in the standard were obsolete, illegal, unavailable or unrepresentative of current practices. To reflect current technologies, safe laboratory practices and modern consumer-care practices, the standard required updating. In 2007,18 CPSC proposed to update the rule and finalized the updates in early 2008. The major areas updated are the refurbishing procedure and the test procedure. For refurbishing (laundering and dry cleaning):
The laundering procedure now requires an automatic washer and tumble dryer. The final amendments specify a wash temperature of 49  3 1C (120  5 1F) and reference the laundering procedure in AATCC 124-2006 Appearance of Fabrics After Repeated Home Laundering. The standard specifies the AATCC Standard Reference detergent (powder form).

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The dry cleaning procedure in the standard is similar but not identical to ASTM D1230 Standard Test Method for Flammability of Apparel Textiles Section 9.2.1.6 Option B. For the test procedure:
The revision/update of the general wearing apparel standard does not include any changes to the classification criteria of the clothing textiles standard or provide for changes to the exemptions to the standard.
The standard now allows for the use of a modern test cabinet. Diagrams and written descriptions of the critical parameters for the flammability test cabinet are included in the standard.
Terms and definitions have been added. The test procedure has been rewritten in a more logical, easy-to-follow fashion. The final rule did not reduce risks to the public of fire-related death, injury or property damage. The scope of the revised rule was limited to improvements that reflected current consumer practices and modernized test equipment, and to clarifications of several technical elements of the standard. The amendments would maintain current industry practices. Language in the revised rule for ‘‘Test sequence and classification criteria for plain surface textile fabric’’ will read the same for both the ‘‘as received or original state’’ and ‘‘after refurbishing’’ conditions. Cotton apparel under 82.2 g m2 (2.6 oz yd2), particularly raised surface fabrics, has to be tested (synthetics do not) and can require flame retardant treatment or blending with polyester (e.g., most sweatshirts contain 30–50% polyester or acrylic fibre in addition to cotton) to pass the test. The raised surface apparel market is about 261 450 MT (1.2 million 480 lbs bales) of cotton.

18.7 Carpets and Rugs There are US federal flammability standards for carpets and rugs: Standard for the Surface Flammability of Carpets and Rugs (16 CFR 163021) and Standard for the Surface Flammability of Small carpets and Rugs (16 CFR 163122). If the carpet has had a flame retardant treatment it has to be washed prior to testing. The ignition source for the test for these standards is a timed burning tablet (methenamine pill). A flattening frame is placed on the specimen and the pill is positioned in the centre of the 20.32 cm (8 in.) hole. The carpet or rug meets the acceptance criteria if the charred area does not extend to within 2.54 cm (1 in.) of the edge of the hole in the flattening frame at any point for at least seven of eight specimens. Eli Lilly stopped marketing this pill in 2002. In 2007, CPSC finalized technical amendments to provide a generic technical specification to define this ignition source.47 The ignition source is defined as ‘‘a methenamine tablet, flat, with a nominal heat of combustion value of 7180 calories/gram, a mass of

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150 mg  5 mg and a nominal diameter of 6 mm’’. This should make the ignition source more uniform and improve the precision of the test.

18.8 Ignition Sources The major SOF sources for upholstered furniture and mattress fires are matches, cigarette lighters and candles. The ignition source for the open-flame test for mattresses is considered a large open flame and is intended to represent the flame insult from the worst-case burning bedding.28

18.8.1

Cigarette Lighters

CPSC issued a final standard (16 CFR 1145) that requires multipurpose lighters to be child resistant in 1999.48 In 2001 CPSC was petitioned to adopt the ASTM F-400 voluntary safety standard for cigarette lighters as a mandatory standard and now is considering whether to formally rely on this voluntary consensus standard for cigarette lighters, Standard Consumer Safety Specification for Lighters, ASTM F-400-00.

18.8.2

Candles and Candle Accessories

Fires for which candles were the source of ignition have increased in the USA in recent years because of lifestyle changes. CPSC was petitioned by NASFM in 2004 to issue mandatory fire-safety standards for candles and candle accessories.49 In July 2006, CPSC voted to defer action on the petition and directed the staff to provide updates on the progress of voluntary standards activities. Further action on this is uncertain.

18.8.3

Matches

Child play with matches is a major cause of mattress and upholstered furniture fires that result from SOF ignition. In 1977 CPSC issued a final safety standard for matchbooks (16 CFR 1202).50 Wooden matches packaged in boxes are not covered.

18.8.4

Cigarettes

Cigarettes are the major ignition source in mattress and upholstered furniture fires. 700–900 people die each year in the USA as the result of fires caused by cigarettes, according to NFPA. One-quarter of those people killed – often including children and the elderly – are not the smoker. ‘‘RIP’’ cigarettes are considered by many to be a practical and effective way to reduce the risk of cigarette-ignited fires.51 However, the results of some research indicate that the RIP cigarettes do reduce the risk of inducing flaming ignition or progressive smouldering with materials, but the risk is clearly not eliminated.

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At least 44 US states (481% of US population) and all of Canada now have RIP cigarette requirements and other US states are considering legislation. New York State was the first state to require that cigarettes sold and manufactured in the state be RIP. In Canada, RIP cigarettes are required nationwide using the New York state standard. US CPSC does not have authority to regulate cigarettes, but if the US Congress passes legislation that requires RIP cigarettes they could. EU Member States on 30 November 2007 endorsed plans to allow only RIP cigarettes to be sold in Europe, a move which could take two or three years to come into force.52 The 27 EU nations approved a European Commission proposal which would require the tobacco industry to use fireretardant paper in all cigarettes in order to cut down on the number of sometimes fatal fires which dropped cigarettes cause each year. In 2007 and early 2008 the two largest US manufacturers, Phillip Morris USA and R.J. Reynolds Tobacco Company, announced that they already are or will manufacture all of their cigarette brands using ‘‘fire-safe’’ RIP technology,53 and both companies don’t oppose regulations that are in line with the standards 22 other US states have adopted. A RIP or so-called ‘‘fire-safe’’ cigarette (burning cigarettes can never be safe and will always be a potential source of ignition and hazard) has a reduced propensity to burn when left unattended. A number of different techniques can be used to make RIP cigarettes. A common technology used by some cigarette manufacturers is to wrap cigarettes with two or three thin bands of less-porous paper that act as ‘‘speed bumps’’ to slow down a burning cigarette. If a RIP cigarette is left unattended, the burning tobacco will reach one of these speed bumps and self-extinguish. RIP cigarettes meet an established cigarette fire-safety performance standard based on ASTM 2187-04.54 The current smoulder test method in US federal and state flammability standards uses a standard or specified cigarette (‘‘Pall Malls’’) as the ignition source. It has become difficult to obtain this standard cigarette. A substitute standard or specified ignition source, which would make the ignition source more uniform and available, as well as improve the precision of the test, is being researched by CPSC staff to replace the standard or specified cigarette. There is also discussion concerning what should be used as the ignition source if RIP cigarettes are required in most states.

18.9 Flame Retardant Chemicals The hazard and risk to the public from injury and fatality from fire has to be balanced with the risk to human health and the environment that is associated with the use of these chemicals. There is concern that some flame retardant chemicals currently in commerce are harmful to humans and the environment because they are toxic, persistent and/or bioaccumulative.36–38,55 The EU and nine US states have banned pentaBDE and octaBDE. Pentaand octaBDE are no longer produced because manufacturers have voluntarily

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stopped production. Furniture manufacturers, such as IKEA, have also stopped using parts that contain any PBDEs. In the USA, Washington State and Maine have approved bans on all PBDEs, including decaBDE.36 The Washington State law prohibits the manufacture, sale or distribution of most items that contain PBDEs as long as a safer alternative exists. In 2006 (effective 1 January 2007), Sweden banned decaBDE and several other US states (e.g., California, Maine and Illinois) are considering legislation to ban decaBDE. There has been extensive testing in the EU and USA and testing and assessment continues in both.56 California is considering legislation (Assembly Bill 706) to ban all brominated and chlorinated flame retardants, as are about 14 other US states. California had previously banned pentaBDE and octaBDE and there is another bill being considered to ban decaBDE specifically.

18.10 Summary and Conclusions US CPSC and the CA BHFTI are considering, developing or have proposed or promulgated open-flame ignition and cigarette ignition standards for upholstered furniture, mattresses and foundations, and bedclothes. These efforts will almost certainly lead to new mandatory regulations with test methods for textile products. These new and potential new regulations will require various approaches to preventing or reducing the ignition of fabrics and filling materials and could require new approaches and much new technology. Most of the emphasis for developing new flame retardant technology for textile products probably will be focused on the major fibres (see Table 18.4) and filling materials. The inherently flame resistant fibres and other specialty fibres will be important to meet flammability standards for apparel and soft furnishings. The current and potential restrictions on the use of chlorinated and brominated flame retardants mean non-halogen compounds will most likely be the main chemical systems used for the various textile applications required to meet flammability regulations in all areas except furnishings, where backcoating technologies are preferred. Table 18.4

Worldwide textile fibre utilization in 200457

Fibre Cotton Polyester Nylon Acrylic All other (including rayon, polypropylene, wool)

B 42% B 40% B 6.6% B 4.5% B 6.9%

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References 1. U.S. Consumer Product Safety Commission, Directorate of Epidemiology, General Wearing Apparel Fires – Fatalities and Emergency Department Treated Injuries, Dec 2007. 2. U.S. Consumer Product Safety Commission, Directorate of Epidemiology, ‘‘2002–2004 Residential Fire Loss Estimates’’, July 2007. 3. T.J. Ohlemiller, Smouldering Combustion, in SFPE Handbook of fire Protection Engineering, P.J. DiNenno, Ed., National Fire Protection Association, Quincy, MA, USA, Section 3/Chapter 9, 2002, p. 2–207; T. J. Ohlemiller, Progress in Energy and Combustion Science, Vol. 11, pp. 277–310, 1986; J.F. Krasny, Cigarette Ignition of Soft Furnishings – a Literature Review with Commentary, Center for Fire Safety (Technical Study Group, Cigarette Safety Act), National Bureau of Standards, June 1987. 4. J.F. Krasny, V. Babrauskas, and W.J. Parker, Fire Behavior of Upholstered Furniture and Mattresses, Technology & Engineering 2001, p. 98. [BS and ISO definition of ‘‘progressive smouldering’’]. 5. C.F. Pello and D.L. Urban, Smouldering and Transition to Flaming in Microgravity STAF Project. 2005. http://www.me.berkeley.edu/mcl/staf.html. 6. C.Y.H. Chao and J.H. Wang, Transition from smouldering to flaming combustion of horizontally oriented flexible polyurethane foam with natural convection, Combustion and Flame, 2001, 127(4) 2252; S.D. Tse, A.C. Fernandez-Pello, and K. Miyasaka, Controling Mechanisms in the Transition from Smoldering to Flaming of Flexible Polyurethane Foam, Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1999, pp. 1505–1513. 7. P.J. Wakelyn, P.K. Adair and R.H. Barker, Do Open Flame Ignition Resistance Treatments for Cellulosic and Cellulosic Blend Fabrics Also Reduce Cigarette Ignitions?, Fire and Materials, 2005, 29, 15. 8. U.S. CPSC, Regulatory Options Briefing Package on Upholstered Furniture Flammability, 28 Oct. 97. (Test Method, pp. 394–412). 9. U.S. CPSC, Briefing Package on Upholstered Furniture Flammability: Regulatory Options, 30 Oct. 01. (Test Method, pp. 11–15 & Tab C pp. 151–180). 10. U.S. CPSC, Ignition of Upholstered Furniture by Small Open Flames and/or Smouldering Cigarettes, Advance Notice of Proposed Rulemaking, 68 Federal Register p. 60629, Oct. 23, 2003. 11. U.S. CPSC, Standard to Address Open Flame Ignition of Bedclothes (16 CFR 1634), Advance Notice of Proposed Rulemaking, 70 Federal Register p. 2514, 13 Jan 05. 12. U.S. CPSC. Final Standard for the Flammability (Open Flame) of Mattresses and Mattress/Foundation Sets (16 CFR 1633), Final Rule, 71 Federal Register p. 13472, 15 Mar 06. 13. U.S. CPSC, Revised Draft Flammability Standards for Upholstered Furniture, Presentation and Personal communication Dale Ray, CPSC and revised draft 18 May 05.

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14. U.S. CPSC, Regulatory Alternatives to Address the Flammability of Upholstered Furniture, Nov 20, 2007. http://www.cpsc.gov/library/foia/ foia08/brief/ufflamm.pdf; Draft Federal Register Notice of Proposed Rulemaking to Address the Flammability of Upholstered Furniture, Jan 29, 2008. http://www.cpsc.gov/library/foia/foia08/brief/briefing.html. 15. California, (Draft 2/2002) Proposed Update of Upholstered Furniture Flammability Standard. Technical Bulletin 117, Requirements, Test Procedures and Apparatus for Testing the Flame and Smoulder Resistance of Upholstered Furniture, California Bureau of Home Furnishings and Thermal Insulation (http://www.bhfti.ca.gov/techbulletin/tb117_draft_2002.pdf). 16. California, Technical Bulletin 603, Requirements and Test Procedure for Resistance of a Mattress/Box Springs Set to a Large Open-Flame. Jan. 2004. 17. California, Draft Technical Bulletin 604, Test Procedure and Apparatus for the Open Flame Resistance of Filled Bedclothing, Oct. 2007. 18. U.S. CPSC, Standard for the Flammability of Clothing Textiles; Notice of Proposed Rulemaking. 72 Federal Register p. 8844, 27 Feb 07; Staff’s Recommendation for Final Rule to Amend the Flammability Standard for Clothing Textiles, 11 Jan 08. 19. Congressional Alarm: From the Office of U.S. Congressman Robert E. Andrews, Congressional Fire Services Institute newsletter, 27 Nov 07. (http://www.cfsi.org/newsletter/cfsi_newsletter_11_27_2007.asp#1#1). 20. P.K. Adair, National Burn Center Reporting System, Report of Incidents from June 2004 through December 2005, CPSC Memorandum January 12, 2007. http://www.cpsc.gov/LIBRARY/nbcrs05.pdf. 21. U.S. CPSC, Standard for the Surface Flammability of Carpets and Rugs, 16 Code of Federal Regulations 1630. 22. U.S. CPSC, Standard for the Surface Flammability of Small carpets and Rugs, 16 CFR 1631. 23. R .G. Gann, Robustness of Measuring the Ignition Strength of Cigarettes in with ASTM Method E2187-02b. National Institute of Standards and Technology Technical Note 1454, 1 July 03. 24. California Technical Bulletin 129, Flammability Test Procedure for Mattresses for Use in Public Buildings (techbulletin/tb129.pdftechbulletin/ tb129.pdfTechnical Bulletin 129); California Bureau of Home Furnishing and Thermal Insulation, Technical Bulletins http://www.bhfti.ca.gov/ industry/bulletin.shtml. 25. California TB 121, Flammability Test Procedure for Mattresses for Use in High Risk Occupancies (techbulletin/121.pdftechbulletin/121.pdfTechnical Bulletin 121). 26. U.S. CPSC, Standard for the Flammability of clothing textiles, 16 CFR 1610. 27. U.S. CPSC, Standard for the Flammability of children’s sleepwear: Sizes 0-618, 16 CFR 1615. 28. U.S. CPSC, Standard for the Flammability of children’s sleepwear: Sizes 7-14, 16 CFR 1616.

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29. U.S. CPSC, Standard for the Flammability of Mattresses and Mattress Pads, 16 CFR 1632. 30. T.J. Ohlemiller, Flammability of Full Scale Mattresses: Gas Burners versus Burning Bedclothes. National Institute of Standards and Technology NISTIR 7006, July 1, 2003. [Phase I]. 31. T.J. Ohlemiller, J. Shields, R. McLane, and R. G. Gann, Flammability Assessment Methodology for Mattresses. National Institute of Standards and Technology NISTIR 6497, June 2000. [Phase I]. 32. T.J. Ohlemiller and R.G. Gann, Estimating Reduced Fire Risk Resulting From an Improved Mattress Flammability Standard. National Institute of Standards and Technology Technical Note 1446, August 2002. [Phase II]. 33. T.J. Ohlemiller and R. G. Gann, Effect of Bed Clothes Modifications on Fire Performance of Bed Assemblies. National Institute of Standards and Technology Technical Note 1449, February 2003. [Phase III]. 34. T.J. Ohlemiller, A Study of Size Effects in the Fire Performance of Beds. National Institute of Standards and Technology Technical Note 1446, Jan. 2005. [http://www.fire.nist.gov/bfrlpubs/NIST_TN_1465.pdf]. 35. U.S. CPSC, Advanced Notice of Proposed Rulemaking; Possible Revocation or Amendment of Standard for the Flammability of Mattresses and Mattress Pads (Cigarette Ignition), 70 Federal Register p. 36357, 23 June 05. 36. P.J. Wakelyn, Environmentally Friendly Flame Retardant Textiles. Chapter 9, Advances in Fire Retardant Materials, Eds. D. Price and A.R. Horrocks. Woodhead Publishing Limited Cambridge, UK, 2007, pp [in press]. 37. National Academy of Sciences, Toxicological risks of selected flame-retardant chemicals, Sub-committee on Flame-retardant Chemicals of the United States, National Research Council, Washington, DC, National Academy Press, Washington, 2000. 38. U.S. EPA report, Furniture Flame Retardancy Partnership: Environmental Profiles of Chemical Flame-Retardant Alternatives for Low Density Polyurethane Foam, Vol 1, EPA 742-R-05-002A, Sep 2005. http://www. epa.gov/dfe/pubs/projects/flameret/index.htm [Design for the environment, www.epa.gov/dfe]; U.S. EPA. 2004. Furniture Flame Retardancy Publications (http://www.epa.gov/dfe/pubs/index.htm#ffr). 39. U.S. EPA, Certain Polybrominated Diphenylethers; Proposed Significant New Use Rule, 69 Federal Register p. 70404; 6 Dec 04. 40. Polyurethane Foam Association (PFA) 2005 http://www.pfa.org/forms/ penta_release.html. 41. PFA, 2007 http://www.pfa.org/Library/SOF_testing_position.pdf. 42. Upholstered Furniture Action Council (UFAC), 1990, UFAC Test Methods (http://[email protected]/testmethods.htm). 43. California Technical Bulletin 117. Requirements, Test Procedures and Apparatus for Testing the Flame Retardance of Resilient Filling Materials Used in Upholstered Furniture. (techbulletin/117.pdftechbulletin/117. pdfTechnical Bulletin 117).

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44. California TB 116 Requirements, Test Procedures and Apparatus for Testing the Flame Retardance of Upholstered Furniture. 45. California TB 133, Flammability Test Procedure for Seating Furniture for Use in Public Occupancies (techbulletin/tb133.pdftechbulletin/tb133. pdfTechnical Bulletin 133). 46. US CPSC Alerts Shoppers to Dangers of Using Loose-Fitting Cotton Garments as Sleepwear for Kids, Parents Can Help Prevent Burns by Putting Kids in Snug-fitting or Flame-resistant Sleepwear, Dec 2002 http://www. cpsc.gov/CPSCPUB/PREREL/prhtml03/03050.html. 47. U.S. CPSC, Technical Amendments to the Standard for Carpets and Rugs, Final Rule, 26 Oct 2007; 72 Federal Register p. 60765. 48. U.S. CPSC, Safety Standard for Multi-Purpose Lighters (16 CFR Parts 1145 and 1212), Final Rule, 22 Dec 99, 64 Federal Register p. 71883; Rule to Regulate Under the Consumer Product Safety Act Risks of Injury Associated With Multi-Purpose Lighters That Can Be Operated by Children (16 CFR Part 1145) 64 Federal Register pp. 71883, 22 Dec 99. 49. U.S. CPSC, Petition Requesting Mandatory Fire Safety Standards for Candles and Candle Accessories (Petition No. CP 04-1/HP 04-1), 6 April 04, 69 Federal Register p. 18059. 50. U.S. CPSC, Safety Standard for Matchbooks (16 CFR 1202), May 4, 1977, 42 Federal Register p. 22656. 51. Coalition for Fire-Safe Cigarettes, http://www.firesafecigarettes.org/ categoryList.asp?categoryID¼9&URL¼Home%20-%20The%20Coalition %20for%20Fire%20Safe%20Cigarettes. 52. EU to insist on fire-safe cigarettes, http://www.eubusiness.com/news-eu/ 1196363821.84. 53. Letter from Reynolds American to NFPA, http://www.nfpa.org/assets/ files/FSC/ReynoldsLetter.pdf; Phillip Morris USA, Legislation & Regulation, Reduced Ignition Propensity Cigarettes, 30 Jan 2008. http:// www.philipmorrisusa.com/en/legislation_regulation/ reduced_ignition_propensity.asp. 54. ASTM E 2187-04, Standard Test Method for Measuring the Ignition Strength of Cigarettes; http://firesafecigarettes.org/assets/files/NISTstandard.pdf). 55. Brominated Flame Retardants, Environmental Transport and Fate, Atmospheric Transport and Fate. 2003. Proceedings Dioxin 2003, Boston, MA, Aug. 24–29, 2003; and Studies Show Flame Retardants Breaks Down, Data Said to Refute Previous Industry Studies. BNA Daily Report for Executives, 11-24-03, p. 24. 56. US EPA, Integrated Risk Information System (IRIS); Announcement of 2008 Program, 72 Federal Register p. 72715, 21 Dec 2007. 57. World Textile Fibre Utilization, Fibre Organon, 2005, 76(7).

CHAPTER 19

Flame Retardancy of Cellulosic Fabrics: Interactions between Nitrogen Additives and Phosphorus-Containing Flame Retardants SABYASACHI GAAN,a GANG SUN,b KATHERINE HUTCHESb AND MARK ENGELHARD c a

EMPA, Lerchenfeldstrasse 5, St Gallen, CH-9014, Switzerland; b University of California, Division of Textiles and Clothing, Davis, California-95616, USA; c Environmental Molecular Science Laboratory, Interfacial and Nanoscale Science Facility, 3335, Q Avenue, Richland, Washington-99354, USA

19.1 Introduction Despite tremendous progress in the study of flame retardancy of polymeric materials, continuous investigation using novel techniques in this area is still urgently needed. Increased environmental concerns on the use of flame retardants are one of the driving forces. In addition, the existing theories of combustion process and mode of action of flame retardants are continuously being revised and new ones proposed. Recent findings by Taatjes

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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et al. regarding pyrolysis behaviour of hydrocarbons have given new insight into the complex pyrolysis process that occurs during the combustion of hydrocarbons.1 We have recently put forward a new explanation of the efficient action of phosphorus-containing flame retardants on cellulosic fibres.2,3 Phosphorus–nitrogen synergistic action in the flame retardancy of cellulosic fibres is a well-known phenomenon. Some nitrogen-containing additives, like urea (UR), cyanamides, dicyandiamide, guanidine salts, melamine and its derivatives, synergistically improve the flame retardant action of phosphorus-containing flame retardants, even though they alone have limited flame retardancy.4–7 Nitrogen compounds, such as nitriles, have been reported to have antagonistic effect on phosphorus-containing flame retardants on cellulose.8 Several theories have been proposed for the phosphorus–nitrogen syner gism on polymeric materials, especially cellulose. One theory hypothesized that the nitrogen-containing species react with phosphorus species (fire retardants) to form reactive P–N bonds which could phosphorylate cellulose more efficiently.9 The formation of P–N bonds is an undeniable fact, as it was observed from the attenuated total reflection Fourier transform infrared (ATRFTIR) spectra of charred flame retardant cotton, but the higher reactivity of P–N bonds with cellulose may not be true. The flame retardant efficacy of phosphoramidates [triethylphosphoric triamidate, melting point (m.p.) 30 1C, boiling point (b.p.) 240 1C; tri-n-propylphosphoric triamidate, m.p. 60 1C, b.p. 240 1C; tri-n-butylphosphoric triamidate, m.p. 65 1C, b.p. 340 1C] may be attributed to their higher boiling points than the analogous phosphates (phosphoric acid triethyl ester, b.p. 216 1C; phosphoric acid tripropyl ester, b.p. 252 1C; phosphoric acid tributyl ester, b.p. 289 1C). Higher boiling points of phosphoramidates make them more likely to be retained in cellulose during the combustion process and thus exhibit better a condensed-phase mechanism. Also, the presence of nitrogen additives like UR, guanidine carbonate (GC) and melamine formaldehyde (MF) would result in alkaline media during the combustion process, in which the P–N bond is more difficult to be hydrolyzed than the P–O bond.10 The P–N bond is easier to hydrolyze under acidic conditions.11 The decomposition of nitrogen additives like UR, guanidine salts and melamine derivatives leads to production of bases like ammonia.12–14 The presence of nitrogen additives improves char content and phosphorus retention on the substrate during the combustion process.15 In this work, we investigated the effect of nitrogen additives like UR, GC and MF on the fire retardant action of model phosphorus compounds like tributyl phosphate (TBP) and triallyl phosphate (TAP) with cotton cellulose as substrate. Surface analytical tools like ATR-FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to give insights into possible reaction pathways that would attribute to the synergistic interaction of TBP and nitrogen additives.

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19.2 Experimental 19.2.1

Material

Cotton fabrics (#400) were purchased from Test Fabrics Inc. (West Pittston, PA), TBP was purchased from EM Science (USA), TAP was purchased from TCI (USA), UR was purchased from J.T. Baker Chemicals (USA), GC was purchased from Acros (Pittsburgh, USA) and MF resin (Aerotex M-3) was provided by Noveon, Inc., Cleveland, Ohio, USA. The elemental analysis of Aerotex M-3 showed 19.26% nitrogen content.

19.2.2

Sample Preparation

The cotton fabric was first treated with aqueous solutions of various nitrogencontaining additives using a laboratory padder to give a wet pick-up of 100%. The treated cotton fabrics were then dried at room temperature and subjected to nitrogen analysis at the Division of Agriculture and Natural Resources (DANR) laboratory of the University of California, Davis. The predicted values of nitrogen based on the increase in weight of fabrics after the treatment (based on the difference in the weight of the fabric before and after the treatment) were very similar to the actual nitrogen content of the treated fabric obtained through elemental analysis. The fabrics were subsequently treated with TBP in carbon tetrachloride (CCl4) solution. CCl4 was chosen for the application of TBP because the nitrogen additives are insoluble in CCl4, thus ensuring retention of all nitrogen on the fabric after the second treatment. The fabrics were padded through 17.4% concentration of TBP with a wet pick-up of 100%, and dried at room temperature to remove the volatile solvent. The predicted concentration of phosphorus on the fabrics after the TBP treatment was 2%. Elemental analysis of treated cotton for phosphorus showed that the actual values were very close to the predicted ones based on the weight increase of fabric. All treated fabrics were then dried and conditioned under standard conditions (65% relative humidity and 21 1C) for 24 hours before any test.

19.2.3

Measurements

Limiting oxygen index (LOI) values of the fabrics were measured according to ASTM standard method D2863-00. The heat of combustion for treated fabrics was determined according to the ASTM D-240 method using an adiabatic bomb calorimeter. A Parr 1341 plain jacket calorimeter fitted with an 1108 oxygen combustion bomb (Parr Instrument Company, USA) was employed in the tests. The chars (residues) obtained after the LOI tests were collected, and surface morphologies of the chars (only for fabrics that contained 2% P) were analyzed using a scanning electron microscope (SEM), Philips XL30TMP (FEICO/Philips, Hillsboro, OR, USA). The ATR-FTIR spectra of char were measured in a Mattson Infinity Series spectrometer with a PIKE Veemax reflection accessory. XPS measurements were performed using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe.

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19.3 Results and Discussion 19.3.1.

Flammability of Treated Fabrics

Table 19.1 shows the LOI values of treated samples and also demonstrates the synergistic action of nitrogen additives. Cotton treated with only nitrogen additives showed no change in LOI values. There was a slight increase in LOI values (0.5–1.0%) only at a very high concentration of nitrogen additives (4–7.2% N content). The concentration of TBP in all cotton samples was controlled so as to have 2% P content and the concentration of nitrogen (UR, GC and MF) was varied from 0.9 to 7.2%. For UR and GC as nitrogen additives, the increase in nitrogen content in cotton resulted in increases in the LOI values of the treated samples. When MF was used as the additive the LOI increase was limited to lower concentrations of nitrogen (0.9–3.6% N). At higher concentrations of nitrogen the LOI value remained the same. This may be caused by an increase in the heat of combustion values for treated fabric when MF (from unreacted methylol groups) is present as additive. The heats of combustion of treated cotton with 3.6% N and 2% P for UR, GC and MF treatments were 18.0, 17.6 and 18.9
103 kJ kg1, respectively. Heats of combustion of untreated cotton and TBP-treated cotton (2% P) were 16.4 and 18.6
103 kJ kg1, respectively.

19.3.2.

Surface Morphology of Char

During the LOI tests, a remarkable difference in the char surface morphology was observed. A white–brown coating was formed on the surface of char when nitrogen-containing additives were present. The surface morphology of char could be a very important criterion which defines the efficacy of a flame retardant.3 The SEM pictures of char showed that TBP, which is not a very

Table 19.1

LOI (%) values and char content.a (CC)* (%) of treated cotton fabric as a function of various concentrations of nitrogen content with a fixed level of phosphorus.b Urea

Guanidine carbonate

Melamine formaldehyde

%P : %N on the cotton

LOI

CC

LOI

CC

LOI

CC

0 0 0 0 2 2 2 2

19.0 19.0 19.0 19.5 25.3 26.0 26.5 27.5

10.6 8.6 8.2 9.0 6.4 6.9 7.5 11.7

19.0 19.0 19.5 21.0 25.3 26.0 26.5 27.5

8.9 13.0 15.4 18.3 7.5 9.3 11.0 14.7

19.0 19.0 19.0 19.8 25.5 25.8 26.0 26.0

11.9 16.6 23.2 23.8 9.6 15.3 22.1 26.8

a

: : : : : : : :

0.9 1.8 3.6 7.2 0.9 1.8 3.6 7.2

Char content of treated cotton obtained from TGA data at 500 1C. LOI of untreated cotton was 19% and that of TBP-treated cotton (2%P) was 23.5% with CC of 1%.

b

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efficient fire retardant leaves behind a fuzzy brittle surface after the LOI tests [Figures 19.1(A1) and 19.1(A2)]. As seen from SEM images the fibres present on the surface seem to have a porous surface. The protective coating formed by TBP may not be substantial enough to insulate the underlying layers from being further damaged.3 Interesting coatings on the surface of fibres were found in the case of nitrogen-containing additives, as shown in Figures 19.1(B), 19.1(B1), 19.2(C), 19.2(C1), 19.2(D) and 19.2(D1), although the morphology (thickness and regularity) of the coatings was different for different additives. UR [Figures 19.1(B) and 19.1(B1)] and melamine treatments [Figures 19.2(D) and 19.2(D1)] gave coatings which were thin, uneven and scattered over the surface of char, whereas thicker coatings were observed for guanidine treatments. Also, for nitrogen-containing treatments, fibres underneath the charred surface seem to retain their surface morphology [Figures 19.1(B), 19.1(B1), 19.2(C), 19.2(C1), 19.2(D) and 19.2(D1)], unlike the case of char obtained from the cotton treated with TBP only [Figures 19.1(A) and 19.1(A1)]. We tested the effect of concentration of nitrogen on the char structure and found that these protective coatings become thicker and more uniform at higher levels of nitrogen content (Figure 19.3). To further substantiate the formation of these kinds of coatings we carried out similar tests with another phosphorus compound (TAP). Figure 19.4 shows similar kinds of coatings formed after the LOI tests on cotton treated with TAP and GC. The coating thus formed during the burning process could act as a protective insulation for the substrate underneath.

Figure 19.1

SEM images of char left after LOI tests from cotton treated with TBP (A and A1), and cotton treated with TBP and UR (B and B1). The fabrics contained 2% P and 3.6% N when nitrogen additive was present.

Flame Retardancy of Cellulosic Fabrics: Interactions between Nitrogen Additives

299

Figure 19.2

SEM images of char left after LOI tests from cotton treated with TBP and GC (C and C1), and cotton treated with TBP and MF (D and D1). The fabrics contained 2% P and 3.6% N when nitrogen additive was present.

Figure 19.3

SEM images of char that was left after LOI tests from the effects of the concentration of nitrogen on the coating. (M) Cotton treated with TBP (2% P) and GC (1.8% N), (M1) cotton treated with TBP (2% P) and GC (3.6% N) and (M2) cotton treated with TBP (2% P) and GC (7.2% N).

300

Chapter 19

Figure 19.4

19.3.3

SEM images of char that was left after LOI tests from cotton treated with TAP (2% P) and GC (3.6% N).

ATR-FTIR Spectra of Char Surfaces

The char obtained from the LOI test was analyzed using ATR-FTIR. Spectrum A in Figure 19.5 shows the ATR-FTIR spectrum of the char obtained from LOI tests on cotton treated with TBP only. The peaks at 1101 cm1 could be assigned to the P¼O stretching vibration. The lower wave number indicates that the phosphoryl P atom is bonded to the OH groups.16 The peak at 956 cm1 could be attributed to either P–OH stretching or P–O–C stretching.16 The broad peak at 3000–3600 cm1 could be attributed to OH stretch. Spectra B, C and D were obtained from chars of the cotton samples treated with UR, GC and MF, respectively. These spectra are very different to that obtained from the char of the TBP-treated cotton. In spectra B, C and D, the broad peaks at 1183 cm1 and 1150 cm1 could be attributed to P¼O or P¼N stretching frequency, or could be a combination of both. It is really hard to differentiate between these two bands as both fall between 1400 and 1050 cm1.17 Similarly, the sharp peak at 981 cm1 and a broad peak at 950 cm1 in spectra C and D, respectively, may be from P– NH–R or P–OH stretching.17 A small peak at 1067 cm1 in spectrum C and at 1050 cm1 in spectrum D could be attributed to PO2 symmetric stretching.17 The difference in the spectra of the chars obtained provides some clues as to the reaction products that result during burning of the treated cotton samples.

19.3.4

XPS Analysis of Char

The coatings formed on the surface of char after burning was not soluble in any solvent and hence the XPS technique was used to analyze the surfaces. Figure 19.6 shows the N1s spectra of surfaces of the chars from fabric treated with TBP–GC. Similar kinds of spectra were obtained for chars obtained from cotton treated with TBP–UR and TBP–MF. Although ATR-FTIR spectra were inconclusive, they provided clues regarding the presence of P¼N and P–N bonds, which we wanted to confirm using the XPS data. From Figure 19.6 we can see that N1s spectra do not comprise a single peak, but a peak with several shoulders. The XPS spectra obtained for different chars were deconvoluted into

1150 1068 981

1535

D

1564

3000-3500

C

301

1150 1050 951 864

Flame Retardancy of Cellulosic Fabrics: Interactions between Nitrogen Additives

B

3000-3500

A

4000

3500

3000

1183 1067 981

1546

1101

956

3000-3500

1560

Absorbance

3000-3500

2500

2000

1500

1000

Wave Numbers

Figure 19.5

ATR-FTIR spectra of char obtained from treated cotton after the LOI test. (A) TBP (2% P), (B) TBP (2% P)–UR (2% N), C TBP (2% P)–GC (3.6% N), D TBP (2% P)–MF (3.6% N).

several peaks that represent different bonds. The peak-fitting data with deconvoluted peaks and respective areas for different kinds of char are shown in Table 19.2. Table 19.3 includes the reference binding energies for several bonds that we found in the literature.18–22 It is clear from Table 19.2 that the coatings seen in SEM pictures are composed of nitrogen bonded to phosphorus with various kinds of chemistry. The majority of nitrogen seems to be bonded to phosphorus with bonds of type P¼N–P, N–(P)3 and P–NH–P. The amount of P–NH2 and P–NH41 seems to be very small as compared to other bonds discussed. It is also possible that cotton treated with TBP–UR may have small amounts of NO3 and NO2. As seen from Table 19.2, it seems that char obtained from TBP–UR has more percent fraction of N–(P)3 bonds, whereas chars from TBP–GC and TBP–MF have more imide nitrogen. There was no significant evidence to support any proposed bonding

302

Chapter 19 5400

N-(P3) P-NH-P P=N-P

5200 P-NH2 P-NH4+

c/s

5000 4800 4600 4400 4200 408

406

404

402

400

398

396

394

392

Binding Energy (eV)

Figure 19.6

N1s XPS spectra of char obtained from cotton treated with TBP–GC.

Table 19.2

Peak-fitting data for N1s XPS spectra for chars obtained from treated cotton. (% Area for respective bonds)/binding energya (eV)

Kinds of bonds

TBP–UR

TBP–GC

TBP–MF

P¼N–P N–(P)3 P–N–P (imide) P–NH2 and/or P–NH+ 4

33.1/398.4 40.8/400 22.4/401.2 3.7/403.1

34.2/398.3 32.2/399.9 27.5/400.9 6.1/402.4

36.3/398.4 27.8/399.8 29.2/400.8 6.7/402.2

a

Higher binding energy data indicate presence of NO3, NO2.

Table 19.3

Reference binding energy data.

Chemistry of chemical bond

Binding energy (eV)

¼N–(P¼N–P) –No (bonded to three P) P–NH–P (imide) P–NH+ 4 P–NH2 NO3, NO2

B397.8–398.0 B399.5 B401 B402 B402 B4403

between nitrogen and carbon, since the C1s spectra obtained for different chars (TBP–UR, TBP–GC, and TBP–MF) did not show any C¼N or C–N bonds (287.9 eV).23 All the chars show a peak centred at 284.3 eV, which is a characteristic peak for elemental or graphitic carbon.23 These XPS results certainly

Flame Retardancy of Cellulosic Fabrics: Interactions between Nitrogen Additives

303

provided additional information about the reaction mechanisms and products of nitrogen and phosphorus compounds during burning.

19.3.5

Mechanism of Formation of Surface Coating on the Char

The SEM, ATR-FTIR and XPS data indicate that the coating formed on the surface of chars after the burning process could be some kind of complex polymer made up of phosphorus, nitrogen and oxygen, which acts as a barrier to heat and flammable gasses. Phosphorus oxynitride, which has a very high thermal resistance, could be one such polymer formed during the burning process. In Schemes 19.1–19.3 we propose several mechanistic routes which could lead to the formation of these kinds of polymers. The thermal decomposition of TBP is shown in Scheme 19.1. It is has been proven in previous research that phosphates like carboxylic esters decompose at high temperatures by cis-elimination to produce acids and alkenes.24,25 Based on the new results, we thought that during the burning process TBP on cotton could undergo either thermal decomposition to produce acidic substrates or volatilize to become a fuel for the burning process (Scheme 19.1). TBP present in cotton treated with TBP only could be volatile at high temperature.3 The thermal decomposition of TBP could produce butene which also act as a fuel for combustion. The formed phosphoric substructures could further phosphorylate cellulose and catalyze its dehydration (act as a flame H O

O

P

P R

O R

O

Thermal Decomposition

O

R

O Vaporization

R cis Elimination [1]

Fuel for combustion

OH R

O P

O

R O Acidic Phosphate

Phosphorylation and Polyphosphoric acid

Scheme 19.1

O

O

Thermal decomposition of TBP.

Butene Fuel for Combustion

304

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retardant). The ability of TBP to form polyphosphoric acid and to coat the surface of the fibre during the burning process was not very efficient.3 Possible reaction mechanisms that could catalytically lead to the formation of acidic phosphate with assistance of nitrogen additives are presented in Scheme 19.2. UR, GC and MF could thermally decompose to form ammonia, an alkaline product,7 which could catalyze the decomposition of TBP to produce acidic phosphate (Scheme 19.2). The acidic phosphate thus produced by reactions 1 (Scheme 19.1), 3 and 4 (Scheme 19.2) could lead to phosphorylation of cellulose and the formation of polyphosphoric acid. This catalyzed decomposition of TBP could contribute to the synergistic action of nitrogen additive on TBP. Scheme 19.3 shows the possible reactions of nitrogen additives with

Guanidine Carbonate Urea, Melamine Formaldehyde

NH3

[2]

OH

NH3 R O

R

H

O

[3]

O

P

O R

P

O

O

O R R

OH O

R

O R

OH R

O P O

O

[4]

R

O R

P

O

O

O

Scheme 19.2

R

P

NH3, H2O ( Hydrolysis)

+ NH3 OH

Possible reaction mechanism for the production of acidic phosphates.

O NH4 NH2 [6] O P R O P HO O O + 2 R O R O Endothermic reaction

R

R

NH2 O P NH + 3 R O P O R O O Urea, R O Guanidine Carbonate, Melamine formaldehyde O

[7]

OH O P + NH3 O R O

R

Scheme 19.3

R

[8]

NH2 O P O R O

R

NH

O

P N P H OH OH Protective coating made up of P,N,O elements like Phosphorus oxynitride, phosphoramide, phosphazene N H

Proposed reactions for the formation of polymeric coating on char.

Flame Retardancy of Cellulosic Fabrics: Interactions between Nitrogen Additives

305

TBP or its decomposition products to produce phosphorus–nitrogen–oxygen polymeric species which could form an insulating coating on the surface of the char. Reaction 6 (Scheme 19.3) is a common reaction by which an organic acid and base react to form a salt, and further endothermic condensation could lead to the formation of amide and water. This reaction seems to add to the synergistic effect of nitrogen additives, as it is both endothermic and adds water to the system, which could help to retard the burning process. Reactions 7 and 8 (Scheme 3) are direct reactions of nitrogen additives with TBP or its acidic product to form amide. Finally, the amide formed from different reactions could further lead to the formation of complex polymers of phosphorus– nitrogen–oxygen by condensation reactions, which have the characteristic evidences observed from ATR-FTIR and XPS data.

19.4 Conclusion TBP and three nitrogen-containing additives, UR, GC and MF, showed synergistic flame retardant effects on cotton cellulose, and the formation of an insulating coating was observed during the burning process. Similar coatings were also observed when TAP was used as a flame retardant. Potential reaction mechanisms by which both TBP and nitrogen-containing additives might interact to catalyze the phosphorylation and dehydration of cellulose and simultaneously form polymeric coating are proposed. ATR-FTIR and XPS analyses of char surfaces revealed that the coating was composed of phosphorus-, nitrogen- and oxygen-containing species.

References 1. C.A. Taatjes, N. Hansen, A. McIlroy, J.A. Miller, J.P. Senosiain, S.J. Klippenstein, F. Qi, L. Sheng, Y. Zhang, T.A. Cool, J. Wang, P.R. Westmoreland, M.E. Law, T. Kasper and K. Kohse-Hoeinghaus, Science, 2005, 308, 1887. 2. S. Gaan S and G. Sun, J. Anal. Appl. Pyrol., 2007, 78, 371. 3. S. Gaan and G. Sun, Polym. Degrad. Stab., 2007, 92, 968. 4. G.C. Tesoro and C.H. Meiser, Text. Res. J., 1970, 40, 430. 5. F.V. Davis, J. Findlay and E. Rogers, J. Text. Inst., 1949, 40, T839. 6. S.J. O’Brien, Text. Res. J., 1968, 38, 256. 7. S. Gaan, G. Sun, Effect of Nitrogen Additives on Thermal Decomposition and Flammability of Cotton, submitted to J. Anal. Appl. Pyrolysis. 8. J.J. Willard and R.E. Wondra, Text. Res. J., 1970, 40, 203. 9. A. Granzow, Acc. Chem. Res., 1978, 11, 177. 10. J.E. Berger and E. Wittner, J Phys. Chem, 1966, 70, 1025. 11. A.W. Garrison and C.E. Boozer, J. Am. Chem. Soc., 1968, 90, 3486. 12. J. Chen and I.K. Ping, J. Mass. Spec. Soc. Jpn., 1998, 46, 299.

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13. S. Gaan, G. Sun, Abstracts of Papers, 229th ACS National Meeting, San Diego, CA, United States, March 13–17, 2005. 14. D. Feng, Z. Zhou and M. Bo, Polym. Degrad. Stab., 1995, 50, 65. 15. E.D. Weil, N. Patel, C.H. Huang, W. Zhu, Proc Beijing Int Symp/Exhib Flame Retard, 2nd edn. 1993, p. 285. 16. D. Lin-Vie, N.B. Colthup, W.G. Fateley and J.G. Grassetti, The Handbook of Infrared and Raman Characteristics Frequencies of Organic Molecules, Academic Press, Inc., Boston, Mass, USA, 1991, p. 263. 17. A.N. Pudovik, Atlas of IR Spectra of Organophosphorus Compounds, (Interpreted Spectrograms), Nauka Publishers/Kluwer Publishers, Moscow/London, 1990, p. 5. 18. H. Yung, P.Y. Shih, H.S. Liu and T.S. Chin, J. Am. Cer. Soc., 1997, 80, 2213. 19. R. Marchand. D. Agliz, L. Boukbir and A. Quemerais, J. Non-Cryst. Solids, 1988, 103, 35.3. 20. E.T. Kang and D.E. Day, J. Non-Cryst. Solids, 1990, 126, 141. 21. R.K. Brow, Y. Zhu, D.E. Day and G.W. Arnold, J. Non-Cryst. Solids, 1990, 120, 172. 22. R.K. Brow, Y.B. Peng and D.E. Day, J. Non-Cryst. Solids, 1990, 126, 231. 23. A.F. Dementjev, A. De Graaf, M.C.M. Van de Sanden, K.I. Maslakov, A.V. Naumkin and A.A. Serov, Diamond Rel. Mater., 2000, 9, 1904. 24. K.J.L. Paciorek, R.H. Kratzer, J. Kaufman, J.H. Nakahara, T. Christos and A.M. Hartstein, Am. Ind. Hyg. Assoc. J., 1978, 39, 633. 25. C.E. Higgins and W.H. Baldwin, J. Org. Chem., 1961, 26, 846.

CHAPTER 20

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres Containing Dispersed Phyllosilicate Clays and Ammonium Polyphosphate A.R. HORROCKS,a J. HICKS,b P.J. DAVIES,c A. ALDERSONa AND J. TAYLORa a

Centre for Materials Research and Innovation, University of Bolton, Bolton, BL3 5AB, UK; b Present address: 22 Jephson Road, St Judes, Plymouth, PL4 9ET, UK; c Present address: TA Instruments Ltd., Fleming Centre, Fleming Way, Crawley, RH10 9NB, UK

20.1 Introduction Of all the conventional fibres, acrylics are among the most flammable compared with the cellulosics cotton, flax and viscose in terms of representative limiting oxygen index (LOI) measurements. Commercial acrylic fibres are long-chain polymers composed of at least 85% by weight of the monomer acrylonitrile (AN) and a small amount of a second comonomer, such as methyl acrylate (MA), which acts as a plasticizing comonomer to improve processability. In addition, they often contain an anionic monomer, such as itaconic acid, to act as a site of reactivity for cationic dyes. Polyacrylonitrile-based copolymers have been successfully modified to increase their flame resistance by copolymerization with halogen-containing monomers and the well-established modacrylics Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

307

308

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typically comprise 50% by weight of comonomer such as vinylidine chloride. The presence of a high chlorine content and often a synergist, such as antimony III oxide, in modacrylic fibres is currently causing concern as the use of halogencontaining flame retardants becomes under attack on environmental grounds.1 In terms of non-halogen-containing additives for acrylic fibres, it has also been documented that phosphorus-containing additives, particularly ammonium polyphosphates (APPs), are extremely effective, although these may be introduced only when organic solvents, such as dimethylformamide, are used.2–6 However, to date no commercial flame retardant acrylic fibres have been or are in production in which phosphorus-containing agents are present. Use of aqueous salt solutions like sodium thiocyanate unfortunately may cause solubilization of previously insoluble APPs because of ion exchange in the first instance.7 An alternative method of introducing such an effective flame retardant into an aqueous salt–wet spun filament is through absorption while still in a gelled state during the extrusion bath stage. This well-established route for introducing dyestuffs into acrylic fibres8 could be applied to flame retardants and we will report on such studies in depth elsewhere, in which we show that flame retardants may be applied to filaments during their coagulation and post-coagulated (or as ‘‘never-dried’’ filament tow) stages.7,9 In addition to the possible introduction of flame retardancy via single phosphorus-containing species like APP, we have shown that in cast polyamide 6 and 6.6 films, the addition of dispersed nanoclays may enhance their performance.10,11 This chapter is the first publication of a more comprehensive study7 and describes the introduction of dispersed clays into acrylic copolymers, their extrusion into fibres, the absorption of APP when they are in their ‘‘never-dried’’ forms and the determination of their flammability using LOI.

20.2 Experimental Method and Results Two approaches were followed, namely copolymerization of the acrylic monomers in the presence of the nanoclay and blending of clays into a commercial-quality polymer solution (or extrusion dope) prior to spinning followed by spinning. The first method using in situ copolymerization is more challenging in terms of achieving the correct molecular weight averages and distributions required for use in fibre extrusion, while the second, or dopeblending method, enables conventional polymer dopes to be used.

20.2.1

Materials and Characterization

The unfunctionalized Cloisite Na1 and the quaternized methyl, dihydroxyethyl, hydrogenated tallow, ammonium ion-functionalized Cloisite 30B and similar Cloisite 20A and 93A clays were supplied by Southern Clays Inc, USA. Table 20.1 lists all the clays used with their respective properties. The monomers AN and MA were of commercial fibre-forming polymer quality and the solvent 51% w/v sodium thiocyanate (NaSCN) was of ‘‘crystal’’ quality and prepared and supplied by the former company Acordis Ltd (now

309

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

Table 20.1

Properties of organically modified clays.

Commercial clay Cloisite Na1 Cloisite 20A Cloisite 30 B

Functionalising, quaternized iona

Particle size (mm; 90% less than)

Density (g cm3)

d spacing (nm)

– (CH3)2.HT2.N1

13 13 13

2.86 1.77 1.98

1.17 2.42 1.85

13

1.88

2.36

CH2CH2OH CH3

+

N

T

CH2CH2OH

Cloisite 93A

CH3.HT2.NH1

a

HT is hydrogenated (B65% C18; B30% C16; B5% C14), anion sulfate; T is tallow (B65% C18; B30% C16; B5% C14), anion chloride in 20A and 30B, hydrogen sulfate in 93A.

no longer trading). Prepared samples of polymers were analyzed for average molecular weight in terms of intrinsic viscosity (IV) determined in the laboratories of Acordis. Here, 0.100 g dried polymer was dissolved in 51% NaSCN and the time to flow, tps, through a Type B Ostwald viscometer determined at 25 1C. IV values were determined using the equation IV ¼ 2ln(tps/ts) where ts is the time of flow of solvent.12 The flame retardant selected for treatment of all experimental filament tows and polymer samples was the low molecular weight, soluble APP product Antiblaze LR2 (Rhodia Consumer Specialities) with a nominal 53% w/w solids content. LOI values of filament tows were undertaken on 10 g samples of tow shaped to fit within the thin sample holder of a Stanton Redcroft FTB instrument according to ASTM 2863-77 (revised 1990 version). Improved reproducibility was obtained if the tow samples were pressed before mounting in the sample holder.7 Where tows were not available, pressed polymer samples (100
20
3 mm) prepared according to the method of Horrocks et al.3 were used. Thermal analysis, using a combined differential thermal analysis (DTA)– thermogravimetric analysis (TGA) TA Systems STD 2690 instrument, was undertaken on 6  0.5 mg samples under nitrogen at 101 min1 up to 900 1C and char levels were determined at 500 1C. Values were corrected for clay content by subtracting the respective clay content, if present. Phosphorus analyses were undertaken to assess the amounts of phosphorus present in each APP-treated filament tow or pressed powder sample. The method used was based on that reported by Banks,15 in which a modified molybdovanadophosphoric acid, complexing spectrophotometric technique is described. However, a more aggressive digestion was used instead of the simple perchloric acid method. This required 1 g of the sample to be converted into a char by exposure to hot concentrated sulfuric acid followed by the addition of fuming nitric acid drop-wise until the solution was a pale straw colour. Finally, both 2 ml fuming nitric and 1 ml perchloric acid were added and the resulting solution was boiled to remove the nitric acid with heating continued until the perchloric acid ceased fuming.7 In this way, all the phosphorus content present was released from the acrylic samples and oxidized to phosphate ions.

310

20.2.2

Chapter 20

In Situ Radical Polymerization of Nanocomposite Copolymers

A series of three radical polymerizations was carried out at the former Acordis laboratories, where a small-scale, continuous slurry reactor was available and enabled incorporation of both Na1 Cloisite and 30B Cloisite clays to the polymer slurry. The process used was a small-scale version of the process used commercially by Acordis. The nature of the specialized equipment, the particularly hazardous materials used and the commercial sensitivity of the process required all polymerizations be carried out by Acordis personnel, assisted by the one of the authors (JH). The polymerization system was operated in a continuous reaction mode in which fresh reagents were continually pumped into the stirred reactor and a mixture of product and unreacted starting materials allowed to overflow into a collection vessel. A characteristic of such a method of operation is the average residence time, which is defined as the effective volume of the reactor divided by the combined flow rate of raw materials (assuming that there is no change in density upon reaction). After any change in reaction conditions, it typically took three residence times for the reaction to restabilize and the overflowing product to reach equilibrium conditions. The monomers AN and MA were pre-mixed at the concentrations required to produce commercial-grade copolymer into a single feed as listed in Tables 20.2 and 20.3. Redox initiators were prepared as two separate aqueous solutions Table 20.2

Polymerization component concentrations.

Function

Component and conditions

Feed monomer

AN 2330 g, MA 123 g Feed rate ¼ 38 cm3 min1 (30.7 g min1) Oxidizing agent (O): potassium persulfate 33 g, demineralized (DM) water 10 litres pH adjusted to 2.8–3.0 with sulfuric acid Feed rate ¼ 46 cm3 min1 (or g min1 assuming unit density) Reducing agent (R): sodium metabisulfite 132 g, DM water 10 litres, ferrous ammonium sulfate 0.37 g pH adjusted to 2.8–3.0 with sulfuric acid Feed rate ¼ 46 cm3 min–1 (or g min1 assuming unit density)

Feed oxidizer

Feed reducer

Table 20.3

Polymerization I conditions.

Reactor temperature Residence time Monomer concentration Stirrer speed R:O mass ratio O:M mass ratio (where M ¼ total monomer mass)

55 1C 20 minutes 25% (of which AN 95%, MA 5%) 1200 rpm 4 0.43

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

311

and comprised the other two feeds, giving three feeds in all, each of which was pumped at pre-calculated rates into the reactor through individual dip pipes, which reached approximately 80% of the way to the bottom of the reactor. As the monomers polymerized, the resulting slurry at the reactor outlet would normally contain approximately 25% w/w of the AN–MA copolymer maximum possible yield. The overall polymerization conditions are listed in Table 20.2. The reaction in the overflowing slurry was terminated by the addition of ethylenediamine tetraacetic acid (EDTA), which strongly complexes the iron ions used as an activator for the redox initiators, and the polymer was then filtered and washed.

20.2.2.1

Polymerization I

The first polymerization (PI) was carried out to prepare three sequential batches, with the first one being initiated without the addition of nanoclay and the other two containing each of the dispersed clays. During the first batch and after equilibrium had been achieved, a small sample of control polymer was collected. The Cloisite Na1 clay was added into the feed as slurry and after re-equilibration and sampling of the second batch, the clay was changed to Cloisite 30B to prepare the third batch. To add the clay slurries to the reactor feeds, each was blended in with the reducing agent feed. This was chosen over the oxidizer feed because it had higher solids content and so would be less susceptible to absorption effects if the clay were to entrap the dissolved salt in any way. Additionally, the reducer feed had ferrous sulfate added to it and so any iron impurity present in the clay would have less effect in this feed than in the iron-free oxidizer feed. It was considered that the addition of the clay to the monomer feed would pose an unnecessary risk, as any free-radical species entrapped in the clay might have led to a violent bulk polymerization of the monomer. The Cloisite Na1 clay was initially made into a 9 : 1 (w/v) paste with water, which enabled the reducer feed to contain 1% clay by weight on monomers. The Cloisite 30B clay was made into a 5 : 1 (w/v) slurry with water and then used to make the reducer feed. The dispersions of the clays were exposed to high shear mixing prior to use, using a Silverson mixer at 5000 revolutions per minute (rpm), to break down the particle size. The Na1 clay dispersed well in the aqueous solution, so only a small amount of stirring was sufficient to keep the clay in suspension. However, the functionalized 30B clay was less hydrophilic and thus had a tendency to aggregate and form a flake-like dispersion and so continual stirring of the feed mix was necessary to maintain the dispersion. Three polymer cakes were produced at below with yields expressed with respect to monomer mass:  AN–MA control, yield ¼ 82%  Cloisite Na1 nanopolymer, yield ¼ 86%  Cloisite 30B nanopolymer, yield ¼ 79% Cakes were washed and dopes were prepared in 51% w/v NaSCN solution from all three polymer cakes to yield nominal 13% w/w concentrations.

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Chapter 20

All three dopes were observed to form easily and to be of low viscosity, indicative of low molecular weights.

20.2.2.2

Polymerization II

Building on results obtained in the first PI series, a second polymerization (PII) series was carried out using only Cloisite Na1 because of its superior dispersing properties. The aim was to achieve 5% w/w clay on monomer (i.e. 95% monomer, 5% clay) having a possible nanocomposite structure with an increased polymer molecular weight consistent with a target IV ¼ 1.5 by keeping the percentage of the monomer feed constant and reducing the initiator feed concentration by 50%. Two polymers were produced, the AN–MA control and that containing 5% w/w clay. The control polymerization, although successful, produced a conversion of only 66%, which was lower than had been obtained in the first reactions. When the clay feed was being prepared, it was observed that at the concentrations necessary to achieve the 5% level, the increase in viscosity was significant and pH sensitive, with an unacceptably high value for pumping into the reactor at pH ¼ 3. The exothermic polymerization caused an increase in reactor temperature from 55 to 63 1C over a period of less than one residence time and the viscosity of the slurry leaving the reactor also increased to the point where it was clear that the reactor was internally blocking. Small samples of polymer were collected to enable measurement of molecular weight and subsequent flammability behaviour, but equilibrium was never fully established and the reaction had to be abandoned earlier than expected. Samples of the polymers produced showed values of IV ¼ 2.24 for the control and 1.73 for the clay-containing analogue, well in excess of the target value of IV ¼ 1.5. However, while it was considered that the products were not suitable for a spinning investigation, they were retained for flammability studies because of the higher clay content of 5%.

20.2.2.3

Polymerization III

Based on the PII results, an initiator level intermediate between that used in PI and PII was selected, with both Na1 and 30B nanoclays being introduced at levels of 1 and 3% by weight on monomers. Each reaction ran well and only slight increases in exothermic intensity were observed when the clays were introduced. Yields of polymer and respective IV values are listed in Table 20.4. While these IV values were still above the target range of 1.4–1.5, the industrial sponsor was satisfied that the values achieved were suitable for fibre production. Samples of the polymers were made into spinning dopes to produce experimental fibres at the sponsor’s laboratory by stirring with a Z-blade mixer used at 2000 rpm for a period of 30 minutes. Each resulting solution was allowed to de-aerate naturally over a period of several days.

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

Table 20.4

313

Polymers produced and molecular weights from Polymerization III.

Clay type and content

Yield (%)

Molecular weight as IV

Control: none 1% Cloisite Na1 3% Cloisite Na1 1% Cloisite 30B 3% Cloisite 30B

78 89 88 85 85

1.81 1.71 1.36 2.08 1.85

20.2.3

Dope Blending of Clays

In addition to the two clays used in the polymerization work, Cloisite 93A and 20A clays were also used because, while both are modified with a greater amount of oleophilic additives than the Na1 and 30B grades and thus unsuitable for dispersion in water, they did appear to be dispersible in the polymer solution. Dispersion of all clays (1% w/w) was achieved by taking the dry powder and stirring it into a commercial quality ‘‘Courtelle’’ polymer solution collected from the former Acordis production plant in Grimsby. Again, a Z-blade mixer was used at about 2000 rpm for a period of 30 minutes before the resulting blend was allowed to de-aerate naturally over a period of several days.

20.2.4

Polymer Spinning

The polymer samples produced from PI and PIII were spun into fibre at the Acordis Coventry site, as were the copolymeric acrylonitrile (PAC) dopes blended with all four nanoclays. The schematic of the overall process is shown in Figure 20.1 and Table 20.5 summarises the various dopes extruded. The equipment was set up to produce tow comprising 3.3 decitex (dtex) filaments from each of the dopes using a jet with 1200 holes each of 63 mm diameter. A candle filter was used to protect the jet from large particulates that may have been present in the dope and, for a final protection of the jet, a small piece of filter cloth was wrapped around the back of the jet face. The speed profile of the rollers is shown in Table 20.6. Initial observations were that the filaments spun from the clays stirred into the commercial polymer dope control were of good quality and were relatively easy to spin. However, those produced from the laboratory PI samples were generally very weak and brittle and were difficult to process because of breakages that occurred on the line. The higher molecular weight filaments from PIII were similarly brittle in character. All filaments produced were collected as continuous tows on formers using the winder and the resulting tows placed as hanks in water to keep them wet. These never-dried-filaments (NDFs) were investigated for tensile properties in laboratories at the University of Bolton and Acordis.

314

Chapter 20 Steam stretch tunnel

Dope pot

3

2

5

4

1 Jet Package Spin bath

Pre-heat

Quench Wash trough 1 Wash trough 2

Candle filter

Figure 20.1 Formerly Acordis laboratory-scale spinning line. Table 20.5

Polymer samples sourced from commercial and experimental dopes used for extrusion of filaments tows.

Polymer source

Nanoclay

Concentration (%)

Production Production Production Production Production

grade (control) grade grade grade grade

– Na1 20A 30B 93A

– 1 1 1 1

Polymerization I (control) Polymerization I Polymerization I

– Na1 30B

– 1 1

Polymerization Polymerization Polymerization Polymerization Polymerization

III (control) III III III III

– Na1 Na1 30B 30B

– 1 3 1 3

Table 20.6

Roller speed profile for filament (1200 filaments per tow) production.

Godet Roller No

Roller name

Speed (m min1)

Effective stretch

1 2 3 4 5 Winder

Take up roller Pre-heat roller Steam stretch Wash trough 1 Wash trough 2 –

3 4.5 24 24 24 25

– 1.5 times 8 times –

20.2.5

Physical Characterization

Samples of each of the NDFs produced in spinning investigations were dried at room temperature in a fume cupboard or in an oven at 130 or 140 1C for 5

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

315

minutes under tension-free conditions to determine whether a post-annealing stage would improve the filament properties. Tensile testing of individual filaments was carried out where possible (see Section 20.2.5.1) in the sponsoring company’s laboratory under standard atmospheric conditions. Individual filament fineness measurements in decitex were made on a Vibroskop and then tensile tested on the Testometric Micro 350 Universal Electronic Tester. The process was repeated on 30 filaments from each sample. Experimental filament tows were tested using a Statimat tensile tester. Duplicate 20 metre samples of each tow were wound onto cones with one set being air dried and the other set being oven-dried at 80 1C overnight. For each sample, 15 replicate 100 mm specimen tow lengths were tested for tenacity and percentage elongation-at-break and the results averaged. Tow linear densities were determined gravimetrically under standard atmosphere conditions.

20.2.5.1

Filament Tensile Property Results

Table 20.7 shows the filament properties of oven-dried and air-dried samples of filaments, with and without incorporated nanoclays. While the target filament fineness was 3.3 dtex, the results show that average values were in the range 4–5 dtex. The filament linear density distributions across each 30 filament set tested were broad, as can been seen in Figure 20.2 for stirred precursor dope and in Figure 20.3 for laboratory-polymerized precursor filaments that contained 1% Cloisite 30B. All the air-dried samples, except those that contained Cloisite 20A, have smaller average linear density values than the oven-dried equivalents, probably because of shrinkage that occurred under tension-free conditions during the hot-drying process. This shrinkage after heating is also the cause of the higher extensibilities recorded for the oven-dried samples. Except for the Cloisite Na1 clay-containing sample, filaments that were oven-dried and contained clays stirred into respective precursor polymer dopes had slightly higher tenacities than the Courtelle control. Both the laboratory polymer control and Cloisite 30B nanopolymers from PI gave lower tenacity values than the Courtelle standard and stirred-in clay polymer precursors. However, the laboratory nanopolymer (Cloisite 30B) air-dried filaments showed a greater tenacity than the laboratory control, which possibly indicates the formation of a nanocomposite structure. The effects of air- versus ovendrying are not consistent and, within the standard deviation of results, both sets of results may be considered to be effectively the same. The initial Young’s moduli of the air-dried version are all approximately 50% higher in value than the oven-dried samples, again the consequence of molecular chain relaxation as a result of heating. The introduction of dispersed clays has a marginal effect on dope-stirred filament moduli (about a 10% increase), which is not as high as increases reported by other authors for fibres such as polyamide 6.11,13,14 The moduli for filaments produced from stirred precursor dopes are at least twice the values of those from PI polymers; presumably this is a consequence of the lower average molecular weights of the latter.

316

Table 20.7

Single filament testing results.a Commercial polymer: dope blended solutions Control Courtelle dope

Average linear density (dtex) SD Breaking elongation (%) SD Tenacity (cN dtex1) SD Initial modulus (N tex1) SD

a

Oven-dried filaments 4.6 4.8

1% Cloisite Na1

Experimental polymers: Polymerization I 1% Cloisite 20A

1% Cloisite 93A

Control (no clay)

1% Cloisite 30B

5.7

3.9

4.7

5.0

5.0

0.5 38.2

1.1 37.8

1.1 32.7

0.5 36.3

0.6 36.0

1.2 42.2

0.8 34.2

3.7 2.6 0.3 4.4

6.3 2.7 0.4 4.9

5.0 2.5 0.5 4.9

4.7 2.8 4.0 5.0

4.6 2.6 0.4 4.6

9.7 1.8 0.3 1.8

6.3 1.8 0.4 2.6

0.9

1.1

1.0

1.0

1.1

0.5

0.7

4.4

4.3

4.3

4.7

4.4

5.0

0.4 25.2

0.8 24.5

0.8 24.4

0.5 29.3

0.5 20.8

1.0 24.9

1.2 27.4

3.4 2.7 0.4 7.2

3.1 3.0 0.3 7.6

3.4 3.1 0.4 8.1

3.0 2.7 0.3 5.8

1.7 2.4 0.3 7.9

3.2 1.7 0.3 4.0

3.5 1.9 0.4 3.9

1.3

2.0

1.8

1.1

1.6

1.4

1.1

Air-dried 3.9

SD is the standard deviation of the 30 specimens tested.

1% Cloisite Na1 Too brittle to test

Too brittle to test

Chapter 20

Average linear density (dtex) SD Breaking elongation (%) SD Tenacity (cN dtex1) SD Initial modulus (N tex1) SD

1% Cloisite 30B

317

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

Control Courtelle Dope - oven dried 25 No. of Filaments

No. of filaments

Control Courtelle Dope - Air dried 17 16 15 14 13 12 11 10

20 15 10 5 0

1

2

3

4 5 Decitex

6

7

8

1

7

8

10 8 6 4 2

2

3

4 5 Decitex

6

7

8

1

2

20A - Air dried

3

4 5 Decitex

6

7

8

7

8

7

8

7

8

20A - oven dried 20 No. of Filaments

20 15 10 5 0 1

2

3

4

5

6

7

15 10 5 0

8

1

2

3

Decitex

4

5

6

Decitex 30B stirred - oven

30B Stirred - air dried 12

14 12 10 8 6 4 2 0

no. of filaments

No. of filaments

6

Na+ stirred - oven dried

25

10 8 6 4 2 0

1

2

3

4

5

6

7

8

1

2

3

Decitex

4

5

6

Decitex

C93A- Air dried No. of Filaments

20 No. of filaments

4 5 Decitex

0 1

No. of filaments

3

12

14 12 10 8 6 4 2 0

No. of Filaments

No. of Filaments

Na + Stirred Air dried

2

15 10 5

93A- oven dried

20 15 10 5 0

0 1

2

3

4

5

Decitex

6

7

8

1

2

3

4

5

6

Decitex

Figure 20.2 Histograms to show the spread of linear density values for each experimentally-extruded filament set from stirred solution dopes.

318

Chapter 20

No. of Filaments

No. of filaments

30B Nano Polymer - Air dried 15 10 5 0

15

2

3

4 5 6 Decitex

7

10 5 0 1

Lab Polymer Control - Air dried

10 5 0 1

2

3

4

5

6

7

30B Nano Polymer - oven dried

15

8

2

3

4 5 6 Decitex

7

8

Lab Polymer Control - oven dried No. of Filaments

No. of filaments

1

20

10 8 6 4 2 0

8

Decitex

1

2

3

4

5

6

7

8

Decitex

Figure 20.3 Histograms to show the spread of linear density values for laboratorypolymerized filament sets that contain 1% Cloisite 30B.

In spite of the higher IV values and dope viscosities of PIII polymer samples, while extrusion proved to be successful, component filaments were subjectively considered to be as brittle and difficult to handle as those that contained Cloisite Na1 clay from the previous PI samples and so were not subjected to tensile testing, either as tows or filaments. Polymer samples were, however, retained for further flame retardant absorption experiments.

20.2.5.2

Tow Tensile Property Results

While the PI, Cloisite Na1-containing laboratory-polymerized samples of filament were too brittle to be tested, it was possible to handle and test the filaments in tow form, and the tensile properties are similar to those of the other experimental tow results in Table 20.8. For stirred dope precursor tows, Table 20.8 shows that when oven-dried, all but the Cloisite 93A-containing, stirred-in clay samples have lower tenacities than the Courtelle control. Similarly, the Cloisite 30B nanopolymer (PI) gives tow with a much lower value than the laboratory polymer control, although the Cloisite Na1 analogue gives a higher tenacity than both control tows. The air-dried yarn results, while within error showing similar tenacities to the oven-dried analogues, suggest that the 93A and 20A stirred-in clay precursor tows give higher values than the Courtelle tow. Cloisite 30B- and Na1-containing tows give lower values. Both laboratory clay-containing polymeric PI tows show lower tenacities than the laboratory polymer control tow. Furthermore, the

Tow tensile properties. Commercial polymer: dope blended solutions

Linear density (tex) Tenacity (CN dtex–1) Breaking elongation (%) Linear density (tex) Tenacity (CN dtex–1) %Elongation (%)

Experimental polymers: Polymerzation I

Control Courtelle dope

1% Cloisite 30B

1% Cloisite Na1

1% Cloisite 20A

1% Cloisite 93A

Control (no clay)

1% Cloisite 30B

1% Cloisite Na1

Oven dried 196.0 1.9

263.0 1.5

237.0 0.6

232.0 1.8

210.0 2.3

218.0 1.8

232.0 1.0

174.0 2.1

14.0

4.0

14.0

14.0

12.0

10.0

11.0

258.0 1.2

200.0 2.0

210.0 2.3

224.0 2.0

200.0 2.0

174.0 1.2

196.0 1.6

8.0

10.0

12.0

13.0

10.0

7.0

9.0

10.0 Air-dried 215.0 1.9 13.0

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

Table 20.8

319

320

Chapter 20

average respective filament tenacities are higher than the tow tenacities, typical of continuous filament bundles in which the weakest filaments significantly influence the overall tow properties. This effect is also demonstrated in the reduced breaking-elongation values of tows with respect to individual filament behaviour. Again, oven-drying increases breaking elongation relative to the air-dried values. Generally, all properties determined are quite comparable to commercial polyacrylic-based filaments as reported in standard texts, with typical reported values of tenacity and elongation-at-break values in the respective ranges of 2.5–3.5 cN dtex1 and 20–30%. The lower tenacity values for the laboratorypolymerized (PI) ones reflect the low molecular weight of the polymer used. In spite of the improved IV values obtained during PIII, the molecular weight characteristics were obviously still too low for filaments with acceptably high tensile values to be obtained.

20.5.6 20.5.6.1

Preparation and Flammability Testing of Flame Retarded Experimental Tows and Polymer Samples Dope Plus Clay-Blended Tows

Three experimental NDF samples that contained Cloisite Na1 and 30B clays stirred into the commercial acrylic polymer, and the acrylic (Courtelle) control itself, were chosen from the spinning experiments to undergo treatment with Antiblaze LR2 and subsequent flame retardancy testing because of their respective higher tenacity values (Table 20.7). Based on our observations that acrylic filaments in their post-coagulated, never-dried stage may absorb flame retardant species from an aqueous medium very readily7,9, 30 g of each tow were manually immersed in 100 ml solutions of Antiblaze LR2 at nominal concentrations, PL, of 1, 3 and 6% phosphorus. These were based on the nominal manufacturer’s concentration of 53% w/w and formula of APP. In each case, the fibre hank, suspended from one end, was totally immersed in the flame retardant solution three times, with excess solution being removed by gloved finger-squeezing tightly along the length of the fibre. The fibre hank was inverted and the process repeated. The treated fibres were then oven dried at 100 1C for 20 minutes. The treated tows were then tested for LOI and fibre phosphorus contents, PF. Table 20.9 lists the results which show the expected regular increase in PF values, as phosphorus concentrations in the LR2 APP liquor, PL, increase.7,9 Similarly, LOI values of fibres increase significantly as phosphorus content increases, which may better be expressed in terms of the increase in LOI per unit concentration of phosphorus in the fibre, DLOI/PF, with respect to the LOI of the standard acrylic (Courtelle) value. The effect of added nanoclay at a given phosphorus concentration in the bath, PL, may be interpreted as the increase in LOI with respect to respective results for the control tow behaviour that contains APP only, DLOInano ¼ (LOInano – LOIFR control), where the subscript FR relates to the values for samples that contained only the flame retardant APP (Table 20.9). Both sets of derived LOI values are listed in Table 20.9.

Results of tow analysis of filament tows of blended dope–1% clay and Polymerization II samples treated with Antiblaze LR2.a

Dope-blended samples

PL, nominal (% w/w)

PF (% w/w)

DPF (% w/w)

LOI (vol%)

DLOI/PF

DLOInano (vol%)

Control (Courtelle)

0 1 3 6 0 1 3 6 0 1 3 6

0.0 1.2 3.5 6.5 0.0 1.5 4.4 6.8 0.0 1.8 4.3 6.5

– – – – 0 0.5 0.9 0.3 0 0.6 0.8 0

19.0 21.0 26.0 36.0 20.4 21.8 31.0 41.0 19.0 21.8 30.0 36.6

0.0 1.7 2.0 2.6 0.0 0.9 2.4 3.0 0.0 1.6 2.6 2.7

– – – – 1.4 0.8 5.0 5.0 0.0 0.8 4.0 0.6

Polymerization II polymer Control 0 1 3 6 Cloisite Na1 (5%) 0 1 3 6

0.0 0.5 2.3 3.1 0.0 1.4 2.4 4.8

– – – – 0 0.9 0.1 1.7

18.0 22.0 26.0 31.8 19.0 23.4 29.0 42.0

0.0 8.0 3.5 4.5 0.0 3.1 4.1 4.8

– – – – 1.0 1.4 3.0 9.2

Cloisite Na1 (%)

Cloisite 30B (%)

a

PL ¼ % w/w phosphorus in liquor; PF ¼ % P on fibre; DPF ¼ PF

nano

– PF

control

DLOInano ¼ LOInano – LOIFR

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

Table 20.9

control.

321

322

Chapter 20

The tow data sets for all three filamentshow that values of DLOI/PF rise from about 0.9–1.7 at the lowest PF values (1.2–1.6 %w/w) to as high as 2.6–3.0 at the highest PF values (6.6–6.8 %w/w). That the increase in flame retardant property is non-linear with the effective retardant concentration is typical of many polymers for which an ‘‘S-shaped’’ relationship is observed.16 However, the similar DLOI/PF values noted for PF values in the range 3.5–6.5 %w/w that resulted from exposures to APP solution concentrations of PL ¼ 3 and 6 %w/w suggest a linear relationship now exists. They also suggest that the very high LOI values ( Z 36.0) are measures of the effectiveness of APP as a flame retardant for acrylic copolymers, as noted previously.3–6 More relevant to this discussion is the further enhancement in LOI associated with the introduction of a nanoclay, designated as DLOInano in Table 20.9, which appears to be greater than similarly flame retarded tows that contain the non-functionalized Cloisite Na1 clay. This is perhaps related to the previously noted, subjectively observed, improved ease of dispersion shown by Cloisite Na1 clay in comparison to the other clays studied. The improved tensile properties of these filaments also indicate that a nanocomposite structure is present. However, closer inspection of Table 10.9 shows that the presence of each clay gives rise to greater absorbed phosphorus levels, shown as DPF [¼ (PF nano – PF control)] values. The reason why the presence of clays increases APP absorption is not obvious, but the increased phosphorus could be partly, if not wholly, responsible for the apparent increase in LOI as DLOInano. It is possible to calculate the fraction of LOI value for which each DPF value is responsible using the control APP-only value of DLOI/PF for a defined PL value in Table 20.9. Thus, LOI values determined by phosphorus contents alone may be calculated for each clay-containing sample to give LOIP ¼ LOIFR+DPF(DLOI/PF)FR. Values of LOIP, when subtracted from the actually measured LOI values for each respective clay-containing tow, yield the corrected DLOInano values, DLOInano(corr), which relate only to the effect of the clays present. These are listed in Table 20.10. The corrected values are considerably less and in some cases negative with respect to values of DLOInano in Table 20.9. It is evident that the effect of added clays is only significant when PL Z 3% w/w and PF44% w/w.

20.5.6.2

PII and PIII Polymer and Tow Samples

During the PII experiments, the samples, while having poor extrusion characteristics in spite of their acceptable molecular weights, contained 0 and 5% Cloisite Na1 and were used to investigate the effect of higher clay concentration and APP on the overall polymer flammability. 30 g sample of each polymer cake was added to 100 ml solutions of Antiblaze LR2, at nominal concentrations of 1, 3 and 6% phosphorus, for 2 minutes and then separated by filtration. After oven-drying at 80 1C for 2 hours and pressing into plaques, each was analyzed for phosphorus and tested for LOI, with results given in Table 20.9. While the results for the two control samples in Table 20.9 cannot be directly compared because of the different polymer histories, the introduction of Cloisite

323

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

Table 20.10

Corrected LOI values based on total phosphorus contents.

Dope-blended samples

PL, nominal (% w/w)

DPF (% w/w)

LOI (vol%)

LOIP

Cloisite Na1 (1%)

0

0

20.4

–

1 3 6 0

0.5 0.9 0.3 0

21.8 31.0 41.0 19.0

21.9 27.8 36.8 –

–0.1 3.2 4.2 0.0

0.6 0.8 0

21.8 30.0 36.6

22.0 27.6 36.0

–0.2 2.4 0.6

0

19.0

–

0.9 0.1 1.7

23.4 29.0 42.0

29.2 29.4 39.5

Cloisite 30B (1%)

1 3 6 Polymerization II polymer Cloisite Na1 0 (5%) 1 3 6 a

DPF ¼ PF

nano

– PF

control,

a

DLOInano (corr) (vol%) 1.4

1.0 –5.8 –0.4 2.5

LOIP ¼ LOIFR+DPF(DLOI/PF)FR, DLOInano(corr) ¼ LOInano – LOIP

Na1 clay at 5% appears to increase significantly the phosphorus take-up by the polymer, as noted in the previous set of tows of blended dope–clay that contained only 1% w/w clays. In the clay-containing PII polymer, the very high LOI value of 42.0 volume percent (vol%) is evident at lower phosphorus levels (PF ¼ 4.5%) than observed for the tow sample of blended dope–1% clay with PF ¼ 6.8% having a similar LOI (41.0 vol%). This increased flame retarding efficiency of the higher 5% clay level is also indicated by the respectively higher apparent DLOInano value of 9.2 vol%. However, if the results are corrected with regard to the contribution to LOI values from the increased PF values associated with 5% clay addition, then the true effect of clay shown in Table 10.10 is negligible, if not zero, until PL 4 3 and PF 4 4% w/w, as seen with the samples that contain 1% clay.

20.5.6.3

PIII Samples

While it is evident that high levels of flame retardancy have been achieved and that the dispersed clay and APP appear to be acting in concert, the poor water durability of the latter would remain a problem should commercial exploitation be considered. However, it was proposed that the open microstructure of the resulting acrylic fibres could contribute to this poor durability and so it was deemed necessary to consider the effects of heat on PIII samples after absorption of APP. The final drying–setting conditions chosen were air-drying and setting at 130 or 140 1C, commensurate with normal commercial filament-setting processes. Thus, 30 g of each polymer (i.e. containing 1 or 3% weight of either Cloisite Na1 or 30B clays) was mixed into nominal 1 and 3% phosphorusconcentration solutions of Antiblaze LR2. Polymer samples were treated,

324

Chapter 20

collected and dried as described above for PII samples and then assessed for LOI before and after a 30 minute, 30 1C water soak, and for percentage phosphorus levels. Selected dried samples were then heat-treated at 130 and 140 1C to fully collapse any microfibrillar voids present and hopefully increase fixation of APP. The overall results of the effect of the variables on PF and LOI are shown in Figures 20.4 and 20.5, respectively. Fixation temperatures will be expected to have little or no effect on the phosphorus-uptake values. This is seen in Figure 20.4 when comparing the PF values of the sets of tows with the same clay type and concentration and treated under the same PL conditions, but dried or heat-annealed under different conditions. For example, in Figure 20.4 for the 1% Na1 clay-containing tow treated at PL ¼ 1% w/w, PF values are 2, 2.6 and 2.0 % w/w, respectively, after air-drying or heat-annealing at 130 and 140 1C, while after treating at PL ¼ 3%w /w, PF values are 5.0, 6.0 and 8.2 % w/w. Based on this observation, the results may be simplified as presented in Table 20.11 in which PF values are the averaged values for each of the polymer samples prepared separately prior to subjecting them to air-only drying or 130 or 140 1C annealing conditions. Table 20.11 clearly shows that PF values increase with respective PL values, as expected, but the value of the latter is influenced by clay concentration and type. Generally, increasing the clay concentration from 1 to 3% decreases the respective PF values, although introduction of 1% clay increases the respective PF values with respect to the control tow when subjected to the same LR2

Air/ PL=1% 130degC/PL=1% 140degC/PL=1% Air/PL=3% 130degC/PL=3% 140degC/PL=3%

9

Polymer phosphorus content, PF%

8 7 6 5 4 3 2 1 0 No clay

Figure 20.4

1% Cloisite Na+

3% Cloisite Na+

1% Cloisite 30B

3% Cloisite 30B

Effect of drying and annealing temperature on PF values for APP-treated tows that contained Cloisite Na1 or 30B clays at 1 or 3% w/w loadings.

325

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres 40 35 30

Air/ PL=1% 130degC/PL=1% 140degC/PL=1% Air/PL=3% 130degC/PL=3% 140degC/PL=3%

LOI, vol%

25 20 15 10 5 0 No clay

1% Cloisite Na+

3% Cloisite Na+

1% Cloisite 30B

3% Cloisite 30B

Figure 20.5

Effect of drying and annealing temperature on LOI values for APPtreated tows that contained Cloisite Na1 or 30B clays at 1 or 3% w/w loadings.

Table 20.11

Characterisation of tows produced from Polymerization III.

Clay (%)

PL (%)

PF (%)

LOI after 80 1C (vol%)

LOI after 130 1C (vol%)

LOI after 140 1C (vol%)

Control

0 1 3 1 3 1 3 1 3 1 3

0 1.4 3.3 2.2 6.4 1.9 4.9 1.2 4.0 0.9 2.7

19.0 24.0 34.0 22.2 36.0 23.8 36.0 24.0 36.0 23.0 25.0

19.0 22.0 32.4 25.4 32.4 24.0 32.4 21.0 29.2 20.4 26.0

19.0 22.0 31.0 24.0 35.2 22.4 32.4 23.0 33.8 21.0 30.0

Na1 (1%) Na1 (3%) 30B (1%) 30B (3%)

solution concentration, PL. Furthermore, the 3% LR2-treated tow that contains 1% Na1 is the most effective with respect to uptake of phosphorus, which suggests that unfunctionalized clays will increase the substantivity of the acrylic tow for APP, compared to the functionalized, less polar Cloisite 30B clay (see Table 20.1).

326

Chapter 20 40

85 80 75 70 65 60 55 50 45 40

30

Char, %

LOI, vol%

35

25 20 15 10 0

(a)

2 4 6 8 Phosphorus in polymer, %

10

0

2 4 6 8 Phosphorus in polymer, %

(b)

10

PF<4%

40 LOI, vol%

35 30 25 20 15 10 55 (c)

Figure 20.6

60

65

70

75

80

Char, %

Trends for (a) LOI vs PF, (b) TGA-derived char perentage at 500 1C versus PF and (c) LOI versus char percentage for PFr4%.

It is also evident in Table 20.11 that LOI values are proportional to PF values with the 1% clay/PL ¼ 3% condition having PF ¼ 6.4% w/w yielding the highest LOI value of 36.0 vol%. This proportionality is more clearly observed in Figure 20.6(a), in which all LOI vs. PF data in Figures 20.4 and 20.5 are plotted and there is a tendency of LOI to asymptote to values in the region of about 36 vol% for PF Z 4% w/w. However, after the water soak, all air-dried samples gave LOI values in the range 21.0–22.0 vol%, 130 1C-annealed samples were in the range 21.2–21.6 vol% and 140 1C-annealed samples yielded an LOI range of 21.1–21.6 vol%. This indicated that nearly most, if not all, APP had been removed and that the post-drying treatment had no effect on the durability. With regard to residual chars from TGA at 500 1C under nitrogen, Figure 20.7 shows from an original char level of pure copolymer of 59.1%, the addition of flame retardant raises (PF 4 0.9%) values to levels generally above 60%, but in an apparently random manner with no obvious effect of added nanoclay. Plotting percentage char against polymer phosphorus content, PF [Figure 20.6(b)], also confirms that after an initial increase as a consequence of absorbed APP, there is almost a constant level independent of either increasing phosphorus level up to 4% P and then a decreasing level when PF44%. Also,

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

327

90 80 70

Char,%

60

Air/ PL=1% 130degC/PL=1% 140degC/PL=1% Air/PL=3% 130degC/PL=3% 140degC/PL=3%

50 40 30 20 10 0 No clay

Figure 20.7

1% Cloisite Na+

3% Cloisite Na+

1% Cloisite 30B

3% Cloisite 30B

Effect of drying and annealing temperature on percentage residual chars at 500 1C under nitrogen (corrected for clay content) for APPtreated tows that contained Cloisite Na1 or 30B clays at 1 or 3% w/w loadings.

the char results appear to be independent of clay type or content, as indicated in Figure 20.7. We have shown previously3 that in phosphorus-containing flame retarded acrylic copolymers, LOI increases linearly with char percentage, which indicates that condensed-phase mechanisms predominate. Plotting LOI versus char percentage of all data in this series of experiments shows no apparent correlation at all. However, if only LOI vs. char values for PFr4% are plotted, the results are shown in Figure 20.6(c) as a less random scatter, which suggests that for PF40.9%, LOI is also independent of char level within experimental error. This is surprising since both Figures 20.6(a) and 20.6(b) do show evidence that LOI and char depend upon polymer phosphorus content up to PF ¼ 4%w/w. In conclusion, it is interesting that the effects of phosphorus content are significant in determining the major flame retarding effect in the range PF ¼ 0–4% w/w, while above this the effect of increasing phosphorus reduces char while maximizing LOI at about 36% vol%. The results for both 1% clay-containing tows and 5% clay-containing PII polymers that also contain APP indicate that the effect of nanoclay becomes significant only at PF Z 4% w/w. This may explain why percentage char reduces in Figure 20.6(b), while LOI continues to slowly rise in Figure 20.6(a).

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20.6 Conclusions It is evident that the addition of nanoclays at the 1% w/w level to a prepolymerized copolymeric AN solution may give rise to a nanodispersion that may be wet extruded under simulated commercial conditions into filament tows having acceptable textile properties in terms of tensile behaviour. Furthermore, these latter were little dependent up on the clay used in terms of functionality. Since neither X-ray diffraction nor transmission electron microscopic studies were undertaken, it was not known whether definable nanocomposite structures had been achieved, although tensile Young’s moduli of clay-containing filaments were greater than the control, clay-free sample. However, when the clays were introduced at the polymerization stage, the resulting polymer solutions were more difficult to extrude and tensile properties were inferior, although the unfunctionalized Cloisite Na1-containing filaments gave slightly superior properties. The highly expanded or microvoided form of the as-spun filaments or NDF polymers also demonstrated the ability to absorb considerable amounts of a water-soluble flame retardant like APP, with LOI values as high as 36.0 vol% being achieved at phosphorus concentrations in the fibre, PF ¼ 6% w/w. The presence of clay at 1% in the tow samples of blended dope–clay showed that slight synergy with the phosphorus content present was evident and the increase in LOI as a consequence of adding the clay is clearly quantifiable. This synergy appeared to be greater for the unfunctionalized Cloisite Na1 clay with APP levels in the fibre equivalent to PF ¼ 6% w/w, which yielded an LOI value of 41.0 vol% and an increase arising from the 1% clay presence of 4.2 LOI units. On increasing the clay concentration to 5% w/w, a slightly reduced APP content (PF ¼ 4.8% w/w) yielded an even higher LOI value of 42.0 vol%, with the increase being associated with increased phosphorus absorption (DPF ¼ 1.7% w/w), equivalent to 6.7 vol%, and the clay contribution of 2.5 LOI units. For both clays, this synergy is only really significant at PF Z 4% w/ w and it is pertinent that, under these conditions, the percentage char, which previously increased with polymer phosphorus content, now starts to decrease [see Figure 20.6(b)]. Clearly this is evidence that a clay in combination with a flame retardant can enable reduced concentrations of the latter to be used to achieve the same degree of flame retardancy. We have previously demonstrated similar effects in polyamide 6 and 6.6 films,10,11 but we believe that this is the first time this effect has been observed in copolymeric acrylic filaments or, indeed, in any extruded fibre-forming polymer filaments. Unfortunately, the APP used, while being sufficiently soluble to be absorbed by as-spun filaments, is easily removed by a water soak at 30 1C, and even post-extrusion annealing cannot improve the durability. However, notwithstanding these poor results, we have demonstrated that the addition of clays to polyacrylic copolymers having fibre-forming properties not only improves tensile properties, but also may give rise to high levels of flame retardancy if a durable, phosphorus-based flame retardant is also present at lower levels than would normally be required when used alone.

Synergistic Flame Retardant Copolymeric Polyacrylonitrile Fibres

329

Acknowledgements The authors acknowledge the UK Department of Trade and Industry through its Foresight LINK initiative for financial support and to Camira Fabrics, Rhodia Consumer Specialities, Noveon, Acordis (now, unfortunately, no longer in business) and Web Processing for technical input.

References 1. A.R. Horrocks, B.K. Kandola, P.J. Davies, S. Zhang and S.A. Padbury, ‘‘Developments in flame retardant textiles – A review’’, Polym. Deg. Stab, 2005, 88(1), 3–12. 2. A.E. Standage and R.D. Matkowsky, ‘‘Thermal oxidation of polyacrylonitrile’’, Eur. Polym. J., 1971, 7, 775–783. 3. A.R. Horrocks, M.E. Hall and J. Zhang, ‘‘The flammability of polyacrylonitrile and its copolymers: 1. The flammability assessment using pressed powdered polymer sample’’, J. Fire Sci., 1993, 11, 442–455. 4. A.R. Horrocks, M.E. Hall and J. Zhang, ‘‘The flammability of polyacrylonitrile and its copolymers: 3. Effect of flame retardants’’, Fire Mater., 1994, 18, 231–241. 5. A.R. Horrocks, M.E. Hall and J. Zhang, ‘‘The Flammability of polyacrylonitrile and its copolymers: 4. The flame retardant mechanism of ammonium polyphosphate’’, Fire Mater., 1994, 18, 307–312. 6. A.R. Horrocks, M.E. Hall and J. Zhang, ‘‘The Flammability of Polyacrylonitrile and its Copolymers’’, Polym. Degrad. Stab., 1994, 44, 379–386. 7. J. Hicks, Flame retardant investigations in acrylic fibre-forming copolymers, PhD thesis, University of Bolton, Bolton, UK, 2005. 8. S. Veleva, A. Georgieva and D. Pishev, ‘‘A kinetic study on the dissolution of disperse dyes in the presence of intensifying additives’’, Coloration Technol., 2000, 116(5–6), 174–176. 9. J. Hicks, A.R. Horrocks, P.J. Davies, A. Alderson and J. Taylor, to be published. 10. A.R. Horrocks, B.K. Kandola, S.A. Padbury, ‘‘Interaction between nanoclays and flame retardant additives in polyamide 6 and polyamide 6.6 films’’, In: Le Bras M, Wilkie C A, Bourbigot S, Duquesne S, Jama C, editors. Fire Retardancy of Polymers: New Applications of Mineral Fillers’’, London: Royal Society of Chemistry; 2005, pp. 223–238. 11. A.R. Horrocks, B.K. Kandola, S.A. Padbury, ‘‘The effect of functional nanoclays in enhancing the fire performance of fibre-forming polymers’’, J. Text. Inst. (published 2005) 2003, 94 (3), 46–66. 12. R. Moncrieff, Man-made Fibres, 6th Edition, London, Butterworths, 1975, p. 49. 13. E. Giza, H. Ito, T. Kikutani and N. Okui, ‘‘Fiber structure formation in high-speed melt spinning of polyamide 6/clay hybrid nanocomposite, Polym.,’’ J. Macromol. Sci., Phys., 2000, B39(4), 545–559.

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14. C. Ibanes, L. David, M. De Boisseau, R. Sequela, T. Epicier and G. Robert, ‘‘Structure and mechanical behavior of nylon-6 fibers filled with organic and mineral nanoparticles. I. Microstructure of spun and drawn fibers’’, J. Polym. Sci., Part B: Polym. Phys., 2004, 42(21), 3876–3892. 15. M. Banks, Polymer, 1993, 34(21), 4549–4551. 16. S. Zhang and A.R. Horrocks, Rev. Prog. Polym. Sci., 2003, 28, 1517–1538.

CHAPTER 21

Flame Retardance of Polyacrylonitriles Covalently Modified with Phosphorus- and Nitrogen-Containing Groups JOHN R. EBDON,a BARRY J. HUNT,a PAUL JOSEPHb AND TARA K. WILKIEa a

The Polymer Centre, Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, S3 7HF, UK; b FireSERT, School of the Built Environment, University of Ulster at Jordanstown, Newtownabbey, County Antrim, Northern Ireland, BT37 0QB, UK

21.1 Introduction The use of phosphorus compounds as components of flame retardant additives in polymers is well-established.1 The use of additives as flame retardants, however, has disadvantages. Additives often have to be used in relatively high concentrations [typically 20–40 weight percent (wt %)] to be effective, which leads to concomitant undesirable changes in physical and mechanical properties. Also, additives may be leached or otherwise lost from the polymer during service, and thus pose a potential environmental hazard. To address some of the problems associated with the additive route to flame retardance, we have turned our attention to flame retardant strategies that involve the chemical attachment of flame retardant moieties directly to polymer backbones, i.e. a reactive strategy.

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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In previous work we studied the influence of post-polymerization phosphorylation and phosphonylation on the flame retardance of poly(vinyl alcohol), ethylene vinyl alcohol copolymers and polyethylene,2 and the effects of the incorporation of diethylvinylphosphonate comonomer units on the flame retardance of poly(methyl methacrylate) (PMMA), polystyrene (PS), polyacrylamide and polyacrylonitrile (PAN).3 More recently, we screened a variety of phosphorus-containing monomers for their effects on flame retardance when copolymerized with both styrene and methyl methacrylate.4 In all cases, significant increases in limiting oxygen index (LOI) were observed, together with increases in char yields on combustion. Reactive flame retardation of PAN has been achieved up until now through the use of comonomers that contain halogens. Copolymers of acrylonitrile (AN) that contain up to 15 mole percent (mol%) of such comonomers are referred to as ‘modacrylics’. These comonomers include vinylidene chloride, vinyl chloride, a-chloro-acrylonitrile and the corresponding bromides.5 Although adequate flame retardance has been achieved through this procedure, there is currently concern over the use of halogenated flame retardants on environmental grounds, especially with regard to the toxicities of potential combustion products. This has provided the opportunity to consider environmentally sustainable alternatives. In the work reported here, we explored some aspects of flame retardance in some copolymers of AN and acrylic acid-2[(diethoxyphosphoryl)methylamino] ethyl ester (ADEPMAE). The synthetic route to ADEPMAE is given in Scheme 21.1.

21.2 Experimental All chemicals, reagents and solvents were obtained from the Aldrich Chemical company except acryloyl chloride (Lancaster Synthesis). AN was freed from the inhibitor, 4-methoxyphenol, by passing through a column of activated basic alumina. The solvents and other reagents were purified, if necessary, by standard O

O EtO P

EtO

H Cl

+

EtO

OH

N

EtO

EtAc

CH3

O

P

base, ice, Ar

CH3

O

EtO

OH

N

EtO base, Ar, EtAc

P EtO

N CH3

P

OH

EtO O Cl

O N CH3 (ADEPMAE)

Scheme 21.1

Synthetic route to ADEPMAE.

Flame Retardance of Polyacrylonitriles Covalently

333

6

literature procedures. The detailed method of synthesis of the phosphorus- and nitrogen-containing monomer (ADEPMAE) is given here. A three-necked 500 cm3 round-bottom flask, containing 18 cm3 of triethylamine (13.1 g, 0.13 mol) and 8 cm3 of 2-(methylamino)ethanol (7.48 g, 0.1 mol) dissolved in 200 cm3 of dichloromethane, was fitted with a double-walled water condenser, a dropping funnel and an argon bubbler. The contents of the flask were stirred and purged with argon for about 30 minutes while being cooled in an ice bath. Diethyl chlorophosphate (14.4 cm3, 17.2 g, 0.1 mol) was then added slowly dropwise to the solution with stirring under a blanket of argon. A white precipitate of triethylamine hydrochloride formed. The contents of the flask were brought to room temperature and allowed to stir overnight, under a blanket of argon, to ensure the reaction went to completion. The solvent (dichloromethane) was removed by rotary evaporation, to leave a yellow oil with a white solid suspended in it. The solid was filtered off through a Buchner funnel and washed with ethyl acetate (100 cm3) to extract the oil. The ethyl acetate was then removed in vacuo to leave a pale yellow oil as the crude product. This was further distilled to yield the pure product (a colourless oil, boiling at about 80 1C), for which the yield and nuclear magnetic resonance (NMR) are:
Yield 48.92 g, 0.232 mol, 97.8%.
1H NMR (250 MHz, CDCl3): d 3.99 (m, 4H, 3J ¼ 7 Hz, CH2OP–), d 3.64 (t, 2H, 3J ¼ 5 Hz, CH2–OH), d 3.14 (m, 2H, 3J ¼ 5 Hz, NCH2–), d 2.64 (d, 3H, 3JHP ¼ 10 Hz, P–N–CH3), d 1.25 (t, 6H, 3J ¼ 7 Hz, CH3CH2O–). 31
P NMR (101 MHz, CDCl3): d 11.7. A three-necked 500 cm3 round-bottom flask, containing the required distilled precursor (24 g, 0.114 mol) and triethylamine (15.8 cm3, 11.5 g, 0.114 mol) dissolved in ethyl acetate (250 cm3), was fitted with a double-walled water condenser, a pressure-equalizing dropping funnel and an argon bubbler. Acryloyl chloride (9.24 cm3, 10.29 g, 0.114 mol) in 100 cm3 of ethyl acetate was then added slowly dropwise to the contents of the flask, under a blanket of argon, with stirring. A white precipitate of triethylamine hydrochloride formed. The contents of the flask were brought to room temperature and allowed to stir overnight, under a blanket of argon, to ensure the reaction went to completion. The solution was concentrated in vacuo to give a yellow oil with a white solid suspended in it. The solid was filtered off through a Buchner funnel and washed with ethyl acetate (100 cm3) to extract the oil. The ethyl acetate was then removed in vacuo to leave a pale yellow oil as the crude product. This was further distilled to yield the pure product:
Yield 30.84 g, 0.116 mol, B100%.
1H NMR (250 MHz, CDCl3): d 6.38 [dd, 1H, 3Jtrans ¼ 17 Hz, 2Jgem ¼ 2 Hz, (–O2C)CHQC(H)H], d 6.06 [dd, 1H, 3Jtrans ¼ 17 Hz, 3Jcis ¼ 10 Hz, (–O2C) CHQCH2)], d 5.79 [dd, 1H, 3Jcis ¼ 10 Hz, 2Jgem ¼ 2 Hz, (–O2C)CHQC(H)H], d 4.21 (t, 2H, 3J ¼ 6 Hz, CH2CH2O–), d 3.95 (q, 4H, 3J ¼ 7 Hz, CH3CH2OP–),

334

Table 21.1

Chapter 21

Preparative data for polymers based on acrylonitrile.

AN (g)

ADEPMAE (g)

P–N monomer mole fraction

Bisulfite (g)

Persulfate (g)

Yield (wt%)

10.32 13.07 12.87 8.60 12.47 8.10

0.00 0.99 2.03 1.99 3.98 3.31

0.00 0.015 0.030 0.045 0.060 0.075

1.00 1.00 1.03 1.00 1.00 1.00

0.35 0.35 0.36 0.35 0.35 0.35

88 95 89 85 75 82

d 3.28 [m, 2H, 3JHP ¼ 11 Hz, 3JHH ¼ 6 Hz, PN(CH3)CH2CH2O], d 2.65 (d, 3H, 3JHP ¼ 10 Hz, PNCH3), d 1.24 (t, 6H, 3J ¼ 7 Hz, CH3CH2OP–). 31
P NMR (101 MHz, CDCl3): d 10.37. A typical synthetic procedure for the preparation of PAN, and its copolymers, by an aqueous slurry method is given here. AN (13 cm3, 10.32 g) was placed in a three-necked round-bottomed flask, containing 250 cm3 of deionized water, which had previously been flushed with argon and maintained at 40 1C, fitted with a magnetic stirrer, a water condenser and a bubbler. The mixture was stirred for ca. 30 minutes with argon bubbling through it. Sodium metabisulfite (1.0 g) in 25 cm3 of de-ionized water was added to the reaction mixture, followed by ammonium persulfate (0.35 g in 25 cm3 of de-ionized water). The argon inlet was withdrawn from below the reaction mixture, and the polymerization allowed to proceed for 16 hours under a blanket of argon. The aqueous slurry formed was filtered through a qualitative-grade filter paper, and the polymer obtained was washed with de-ionized water to remove traces of unreacted monomer(s). The polymer was dried in a vacuum oven to constant weight before further examination (Table 21.1).

21.3 Characterization 1

H, 13C and 31P NMR spectra of the starting materials, monomers and ANbased polymers were recorded in deuterated solvents [CDCl3 or d6-dimethyl sulfoxide (d6-DMSO)] on Bruker spectrometers, operating at 250 or at 400 MHz (for protons), under ambient probe conditions. The spectra were processed using WIN-NMR software after being calibrated using the residual proton signals or the main carbon signals arising from the solvents. For 31P spectra, 85% orthophosphoric acid was employed as an external calibrant. Gas chromatograms and the corresponding mass spectra were obtained using a Perkin Elmer AutoSystem XL gas chromatograph coupled to a Perkin Elmer Turbomass spectrometer. Both electron impact (EI) and chemical ionization (CI) were employed to produce the molecular ion; in the latter case, ammonia gas was used as the soft ionization agent. Gel permeation chromatography (GPC) analyses were carried out on 0.25% weight/volume (w/v) solutions of the polymers in dimethylformamide (DMF)

335

Flame Retardance of Polyacrylonitriles Covalently

Table 21.2

Characterization data for polymers based on acrylonitrile.

Polymer system

Composition (mole fractions)

P (wt%)

LOI (%v/v)

PAN PAN/ADEPMAE PAN/ADEPMAE PAN/ADEPMAE PAN/ADEPMAE PAN/ADEPMAE

1.0/0.0 0.99/0.01 0.974/0.026 0.97/0.03 0.96/0.04 0.95/0.05

0.00 0.54 1.40 1.62 2.18 2.70

19.7 21.1 22.8 24.7 25.0 26.4

that contained 0.1% w/v lithium bromide. This solution was also used to elute the samples. The GPC instrument comprised a Polymer Laboratories LC1120 pump operating at a flow rate of 1 cm3 min1. 200 mL of solution was injected via a Rheodyne 7725 injection valve. The columns were PLgel mixed ‘B’ (3 x  30 cm) thermostatted at 70 1C in a Viscotek 300 column oven containing a refractive index detector. The system was calibrated with poly(ethylene glycol) and/or poly(ethylene oxide) standards. Polymer Laboratories ‘Cirrus’ software was used for data acquisition and analysis. Thermogravimetric analyses (TGAs) were carried out on ca. 7–15 mg samples using a Perkin Elmer Pyris 1 series thermal analysis system, both in air and in nitrogen, at a heating rate of 20 1C min1, from 30 to 700 1C. Differential scanning calorimetric (DSC) measurements were performed using a Perkin Elmer Pyris 1 system under an atmosphere of nitrogen. The main aim here was to identify the glass transition temperatures (Tg) of the various polymers, and a relatively higher heating rate of 40 1C min1, from 0 to 150 1C, was found to be the optimum to allow calculation of the glass transition temperature. Also, a heat–cool–reheat cycle was employed to eliminate any irreversible transition(s). LOIs were measured on a StantonRedcroft flammability unit on cold-pressed powder samples measuring 10  0.6  0.3 cm in conformance with ASTM-D-2863. Results of these characterization experiments are summarized in Table 21.2.

21.4 Results and Discussion The chemical structures and purities of small molecules and monomers were mainly inferred from 1H and 31P spectra. For polymers, the chemical compositions, i.e. mole fractions of different monomers present, were mainly deduced from proton spectra by comparing the integral areas of appropriately assigned signals. Limited information regarding the chemical microstructures, including monomer sequence distribution, of various copolymers were also obtained from detailed analyses of 13C and 31P spectra obtained at a higher field (400 MHz). Gas chromatography–mass spectroscopy (GC/MS) was employed mainly to ascertain the purity and molar mass of the synthesized ADEPMAE. The aqueous slurry route to PAN and its copolymers was found to be straightforward with typical yields of polymer products ranging from 75 to 95 wt%. The compositions of the copolymers were also varied (the actual mole

336

Chapter 21

fractions of the phosphorus-containing monomeric units varied from 0.01 to 0.05) by choosing different feed ratios and keeping the reaction time constant at ca. 16 hours, however with different resultant polymer yields. The redox couple employed in ths study (ammonium persulfate–sodium metabisulfite) furnishes at least four radicals for intiation.7 Furthermore, the ratio of bisulfite to persulfate has an influence on both molecular weight and possible oxidation of the product. All polymers obtained in this work were white powdery solids with little evidence of discoloration. This shows that end groups and various other possible minor structures were insignificant in triggering the intermolecular cyclization reactions of the pendent nitrile groups within the AN units of the polymers. The GPC chromatograms of the homopolymer and the copolymers generally indicated unimodal distributions of molecular weight. The Mn values ranged from 10 000 to 20 000, with polydispersity indices between four and 10. The chemical modification of PAN with the phosphorus- and nitrogen-containing comonomer had little effect on the glass transition temperature, as shown by DSC runs; the Tg of PAN being 102 1C. PAN when heated undergoes degradation to produce volatiles, which include the monomer, other nitriles, ammonia, etc. In addition, depending on the mode and/or rate of heating, intramolecular cyclization of the pendent nitrile groups occurs to form cyclic structures, ‘locking’ the nitrogens onto conjugated polyene sequences. This well-characterized cyclization scheme is the basis for a route to carbon fibres.8–10 The spontaneous thermal cyclization of PAN could also be aided by nucleophilic species.11 In this study, there were noticeable differences between the thermograms (obtained through TGA runs in both air and in nitrogen) of PAN and the copolymers with regard to the temperatures of onset of thermal degradation, slopes of the main degradation steps and the amounts of char residue obtained at 700 1C (Figure 21.1). Generally, the AN copolymers produced significantly more char compared to PAN, both in air and in nitrogen. Mass losses at lower temperatures during the TGA runs of the copolymers can be attributed to early thermal cracking of the phosphonate ester groups (Scheme 21.2). The formation of the cyclic intermediate is also entropically favoured owing to the elimination of ethene. The phosphorus acid species thus produced during the early stages of the thermal degradation can act as nucleophilic centres that promote intramolecular cyclization of the nitrile groups in the AN sequences. NMR spectra (especially 31P; see Figure 21.2) also support this mechanism. The smaller signal at dB0.0 parts per million (ppm) is indicative of phosphorus acid species and the main signal at dB11.00 ppm arises from the intact phosphorylamino groups). A similar mechanistic pathway has been shown to be operative in similarly modified systems from our previous work (Scheme 21.3).11 The condensedphase activity of the modifying groups in PAN, in bringing about the increased production of char residue, is also reflected in the significant increases in the flame retardance of these systems as gauged by the LOI values obtained. The plot of LOI values versus P content (Figure 21.3) shows an almost linear dependency of the former on the latter.

337

Flame Retardance of Polyacrylonitriles Covalently 100

90

d

80 Weight % (%)

c 70

60 b 50 a 40

30 0

100

Figure 21.1

200

300 400 Temperature (°C)

500

600

700

TGA traces of polymers in air and nitrogen recorded at a heating rate of 20 1C min1: (a) PAN in nitrogen, (b) PAN in air, (c) AN–ADEPMAE copolymer (mole fraction 0.99:0.01) in nitrogen and (d) AN–ADEPMAE copolymer (mole fraction 0.99;0.01) in air.

O

O

O

O

+ N O EtO

CH3

P OEt

HO

N

CH3

P

O OEt

O H

P

OEt

O

CH C H2

Scheme 21.2

Pyrolysis of the phosphonate ester groups to yield phosphorus acid groups.

338 -0.2011

11.0899

Chapter 21

13.0 12.0 11.0 10.0 9.0

Figure 21.2

8.0

7.0

6.0 5.0 (ppm)

4.0

3.0

1.0

0.0

-1.0 -2.0

31

P (101 MHz) NMR of copolymer of AN and ADEPMAE in d6-DMSO.

O P

2.0

O OH

OEt

N

N

N

P O OR

N

N

Scheme 21.3

N

N

N

NH N

NH N

N

N

N

N

Cyclization of PAN initiated by a nucleophilic species.11

21.5 Conclusions The chemical modification of AN polymers with ADEPMAE units possessing pendant phosphorus- and nitrogen-containing groups results in significant potential improvements in their flame retardance, shown by increased LOI values and TGA char yields. It is highly likely that the mechanism of flame retardation in these systems involves a significant condensed-phase activity initiated by the

339

Flame Retardance of Polyacrylonitriles Covalently LOI vs P contents 29

LOI (vol/vol %)

27 25 23 21 19 17 15 0

0.5

1

1.5

2

2.5

3

P (wt%)

Figure 21.3 Plot of LOI versus phosphorus content of polymers based on acrylonitrile.

precursors obtained through early thermal cracking of the phosphonate groups within the ADEPMAE units. In addition, we envisage some vapour-phase inhibitory effects of phosphorus-containing moieties (mainly oxides of phosphorus, such as PO, PO2, P2O4, etc.), since this has been shown to be the case in some polystyrenes similarly modified with phosphorus-containing comonomer units.12 Cone calorimetric measurements on these and similar systems are currently in progress and will hopefully furnish a better picture of the behaviour of these systems in real fire scenarios. Attempts are also underway to elucidate the exact mechanisms of flame retardance in some of these systems through extensive evolved gas analyses [TGA and Fourier transform infrared (FT-IR)] and through char characterization. The results of these studies will be published separately.

Acknowledgements The authors thank the EPSRC and MoD (grant no. GR/S24374/01) for financial support. Technical collaborations with Acordis UK Ltd., and Rhodia Consumer Specialities Ltd., are also gratefully acknowledged, as are inputs from collaborators at the University of Bolton, UK.

References 1. J.R. Ebdon, P. Joseph, B.J. Hunt and C.S. Konkel, In: Speciality Polymer Additives: Principles and Applications, S. Al-Malaika, A. Golovoy, C.A. Wilkie (eds.), Vol. 2, Blackwell Science, Oxford, 2001, pp. 231–57.

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2. M. Banks, J.R. Ebdon and M. Johnson, Polymer, 1993, 34, 4547. 3. M. Banks, J.R. Ebdon and M. Johnson, Polymer, 1994, 35, 3469. 4. J.R. Ebdon, D. Price, B.J. Hunt, P. Joseph, F. Gao, J.G. Milnes and L.K. Cunliffe, Polym. Degrad. Stab., 2000, 69, 267. 5. R.C. Nametz, Ind. Eng. Chem., 1970, 62, 41. 6. W.L.F. Armarego and C.L. Chai, Purification of Laboratory Chemicals, Elsevier Science, Conwall, 2003. 7. J.R. Ebdon, T.N. Huckerby and T.C. Hunter, Polymer, 1994, 35, 250. 8. T.J. Xue, M.A. McKinney and C.A. Wilkie, Polym. Degrad. Stab., 1997, 58, 193. 9. M. Surinarayanan, R. Vijayaraghavn and K.V. Raghavan, J. Polym. Sci., Polym. Chem. Ed., 1998, 36, 2503. 10. S.C. Martin, J.J. Liggat and C.E. Snape, Polym. Degrad. Stab., 2001, 74, 407. 11. P. Wyman, V. Crook, J.R. Ebdon, B.J. Hunt and P. Joseph, Polym. Int., 2006, 55, 764. 12. D. Price, L.K. Cunliffe, K.J. Bullett, T.R. Hull, J.G. Milnes, J.R. Ebdon, B.J. Hunt and P. Joseph, Polym. Degrad. Stab., 2007, 92, 1101.

CHAPTER 22

Novel Fire Retardant Backcoatings for Textiles M.A. HASSAN National Institute of Standards – Fire Protection Laboratory, Tersa St. El Haram, Giza, P.O. Box 136, Code 12211, Egypt

22.1 Introduction The accidents caused by the ignition of textiles threaten people to the risk of fire in the domestic and public environment1 so different regulatory bodies enforce regulatory standards for different applications. The fire behaviour of textiles is very complex and depends on the nature and fabric structure, including its handling. Previous studies have shown that the weight per unit area of fabric has an influence on the fire behaviour of the latter.2,3 Coatings with an acrylic binder resin are usually applied on woven or non-woven fabrics to modify their physical properties, such as hydrophobicity and impermeability, and to improve their mechanical properties, including the cohesion of the fibres, softness and resilience. The coating can also include fire retardant chemicals to modify the fire performance of the fabrics. A major requirement of any flameretardant back-coating treatment is its ability to transfer flame retardant properties from the back of the fabric to the front face, whenever the ignition source is applied. During the past 10 years many flame retardant systems have been developed and used in textile back-coatings. Antimony–bromine systems have been the most successful flame retardant systems so far for textile backcoatings4 Horrocks et al. attempted to replace this hazardous flame retardant system by phosphorus-containing species.5,6

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Polyurethane (PU) resins are widely used as coatings for fabrics to improve some properties, such as mechanical behaviour, water repellence and air impermeability. Devaux et al. used two kinds of additives, montmorillonite clay and polyhedral oligomeric silsesquioxanes (POSS) in PU to provide flame retardancy to the coated textile structure.7 The influence of an intumescent backcoating, applied to a cotton–polyester (PESFR, Trevira CS) fabric, on the thermal and fire behaviour of the system has been investigated by Drevelle et al.8 The intumescent system they used is made of an acrylic binder resin and ammonium polyphosphate (APP). The aim of this work is to investigate novel organophosphorus compounds as flame retardants for textile backcoatings and to study the effect of the number of phosphate groups attached to the organophosphorus compounds on the flame retardancy of textile backcoatings. The novel backcoating formulations were tested on cotton and polyacrlyic fabrics.

22.2 Experimental 22.2.1

Materials

Diethyl malonate was supplied by Merck, Germany. Orthophosphoric acid was supplied by S.D. Fine-Chem. Ltd, Mumbai, India. Phosphorus oxychloride was supplied by Riedel-DeHan, Germany. The 100% cotton and polyacrylic fabrics, commercial binder (MTB) polymer and polyacrylic thickener were kindly supplied by Texmar textile company, Egypt.

22.2.2 22.2.2.1

Preparation of Flame Retardant Compounds Preparation of Malonyl Phosphate (A1)

A mixture of 39.2 ml of orthophosphoric acid (0.4 M) and 32 ml (0.2 M) of diethylmalonate was refluxed for 2 hours and poured into a beaker. The resultant compound formed after cooling was filtered to give a yield of 70%. The structure of A1 has been characterized by infrared (IR) and nuclear magnetic resonance (NMR) analysis and is graphically represented in Figures 22.1 and 22.2, respectively. The IR spectrum of A1 showed the following peaks:





3405.9 cm1 1720.2 cm1 1159.9 cm1 1007.7 cm1

for for for for

the the the the

O–H stretching absorption; CQO stretching absorption; OQP stretching absorption; P–O–C stretching absorption.

The NMR spectrum for malonyl phosphate indicates a chemical shift of d 3.2 and d 4 with doublet multiplicity for the methylene protons (–CH2–), and the chemical shift d 8 can be attributed to the protons of the phosphate groups (Scheme 22.1).

Novel Fire Retardant Backcoatings for Textiles

343

Figure 22.1 IR chart of compound A1.

Figure 22.2

22.2.2.2

NMR chart of compound A1.

Preparation of Chloro-(Dimalonyl Phosphate) Phosphine Oxide (A5)

15.3 ml (0.1 M) of phosphorus oxychloride was added dropwise to 52.8 ml (0.2 M) of A1 (malonyl phosphate) into a beaker. The resultant compound formed after stirring was A5, yield 70%. The structure of A5 has been

344

Chapter 22 O

O

C O CH2 O

O

Figure 22.3

OH

O OH C

Scheme 22.1

P

P

OH

OH

Chemical structure of A1.

IR chart of compound A5.

characterized by IR and NMR analysis and is graphically represented in Figures 22.3 and 22.4. The IR spectrum of A5 showed the following peaks:





3398.9 cm1 1718.9 cm1 1160.4 cm1 1002.4 cm1

for for for for

the the the the

O–H stretching absorption; CQO stretching absorption; OQP stretching absorption; P–O–C stretching absorption.

The NMR spectrum for chloro-(dimalonyl phosphate) phosphine oxide shows a chemical shift of d 3.2 and d 3.8 with singlet multiplicity for the protons of CH– groups, whereas the chemical shift d 7.8 is related to the protons of the phosphate groups (Scheme 22.2).

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Novel Fire Retardant Backcoatings for Textiles

Figure 22.4 NMR chart of compound A5.

HO

O

O

P

OC

O HO

OH

Scheme 22.2

22.2.3

CH

OH

P

O

O C

P

O

O

C O

P

CH

C1

O

O CO

P

O

OH

OH

OH OH

Chemical structure of A5.

Preparation of Coating Paste

Various formulations prepared for backcoating fabrics are given in Table 22.1. We have used a commercial binder (MTB) polymer and polyacrylic thickener to prepare the coating paste. Different chemical components were mixed together, in the proportions given in Table 22.1, using magnetic mixer at 500 revolutions per minute (rpm) for 3 minutes. The thickener was added after 3 minutes of mixing and stirring continued at 700 rpm for 5 minutes until the required viscosity (100–150 poise) was obtained. The coating paste was applied onto the fabric using a K-bar (K control coater model 404, RK print coat

346

Chapter 22

Table 22.1

Back-coating formulations.

Back-coating

Flame-retardant (w/w %)

A1-40% A5-40% B2 B5

FR A1 (40%) FR A5 (40%) FR A1 (35%) + dextrin (15%) FR A5 (35%) + pentaerythritol (15%)

Binder (w/w %)

Thickener (w/w %)

40 40 40 40

20 20 10 10

instruments). The coating formulations were applied on cotton and polyacrylic fabrics and coated fabrics were dried and cured at 110 1C.

22.2.4

Characterization

Thermogravimetric analysis (TGA) of the flame retardant compounds (5– 10 mg) was carried out from room temperature to 750 1C under N2 atmosphere using a TGA-50 Shimadzu (Japan) analyzer. Differential scanning calorimetry (DSC) of the flame retardant compounds and different uncoated and backcoated textile samples (5–10 mg) was carried out from room temperature to 650 1C under N2 atmosphere using a DSC-50 Shimadzu (Japan) analyzer. The limiting oxygen index (LOI) was measured according to ISO 4589 by using FTA-LOI manufactured by Rheometeric Scientific Ltd. The flame spread measurements of the coated and uncoated fabrics were done according to a modified ISO 3795.9 The modification of the test method involved changing the dimensions of the sample from 7.6  33 cm to 5  15 cm. The test procedure, however, remained unchanged and as stated in the standard test method. The smoke density measurements were carried out according to ISO 5659-2 by using a smoke box apparatus manufactured by Rheometeric Scientific Ltd.

22.3 Result and Discussion 22.3.1

Thermal Characterization of A1 and A5 Organophosphorus Compounds

The thermal stability of the organophosphorus compounds A1 and A5 was studied by TGA and DSC techniques. Weight loss and temperature range of all decomposition stages during the thermal decomposition of A1 and A5 are given in Table 22.2, and the calorimetric data are given in Table 22.3. The mass-loss and heat-flow curves as a function of temperature are graphically represented in Figures 22.5 and 22.6, respectively. The malonyl phosphate A1 is completely decomposed with 99% weight loss at 750 1C. The thermal decomposition process of A1 shows four distinct weight-loss stages, as shown in Table 22.2. Over the three stages between 25 and 491 1C, compound A1 loses 41% of its

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Novel Fire Retardant Backcoatings for Textiles

Table 22.2

TGA data of new flame retardant materials. Decomposition temperature range

Flame retardant

A1 A5

Table 22.3

Stages

Initial (1C)

Peak (1C)

Final (1C)

Weight loss % at each stage

1 2 3 4 1 2 3 4

25 128 206 494 25 133 206 479

42 136 208 557 45 157 208 551

98 206 249 745 133 205 219 610

23 14 4 51 32 10 3 47

99 99

DSC data of new flame retardant materials. Decomposition temperature range (1C)

Flame retardant

Peaks Endothermic Endothermic Endothermic Endothermic Endothermic Endothermic Endothermic Endothermic

A1

A5

1 2 3 4 1 2 3 4

Onset

Peak

End

Heat (J g1)

31 137 365 590 42 153 205 589

68 178 367 593 80 183 208 593

111 225 402 597 124 200 241 598

224 82 18 12 335 13 3 10

100 90

weight loss percentage

80 70 60 A1

50

A5

40 30 20 10 0 23.83

Figure 22.5

Total weight loss %

101.5

184.7

268.1

350.9 434.4 Temperature

517.8

602

TGA comparison of new flame retardant compounds.

685.7

348

Chapter 22 30 25 20

mW

15 A5

10

A1

5 0 -5 -10 1

Figure 22.6

101

201

301 401 Temperature

501

601

701

DSC curves of new flame retardant compounds.

total weight. The main decomposition stage lies in the temperature range 494– 745 1C and shows 51% weight loss. The thermal decomposition behaviour of chloro-(dimalonyl phosphate) phosphine oxide A5 is generally similar to that of A1, except with a small difference between the weight-loss percentage at each stage of decomposition. In the first stage, the weight loss of A5 (32%) is higher than that of A1 (23%). In contrast, the weight-loss percentages of the second and third decomposition stages are lower (10 and 3%, respectively) than the corresponding weight loss of A1. The main decomposition stage for A5 is in the temperature range 479– 610 1C with a weight loss of 47%. The DSC thermogram of A1 and A5 showed that the decomposition process passes through four endothermic stages (see Figure 22.6). The first stage of decomposition of A1 requires an input of 224 J g1, while the same stage for the dimer A5 requires 335 J g1.

22.3.2

Pyrolysis Behaviour of Uncoated and Back-coated Cotton Samples

The thermal analysis study of flame retardant materials can assist in the understanding of their fire resistant mechanisms.10 The pyrolysis behaviours of uncoated and back-coated cotton samples were studied by TGA and DSC. The TGA data are given in Table 22.4 and graphically represented in Figure 22.7. The TGA data of pure cotton samples showed that the pyrolysis process has two main steps. The first step is in the temperature range 167–249 1C, with a 4% weight loss. The main second pyrolysis step lies between 268 and 452 1C,

349

Novel Fire Retardant Backcoatings for Textiles

Table 22.4

TGA data of uncoated and back-coated cotton samples.

Sample name

Stage

Uncoated cotton Cotton–A1-40% Cotton–A5 40% Cotton–B2 Cotton–B5

Decomposition temperature range (1C)

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

20–63 167–249 268–425 20–80 137–279 3287–422 20–92 152–279 282–417 20–88 162–283 285–409 20–73 167–286 289–431

Weight loss % at each stage 3 4 66 6 37 13 6 37 13 5 38 13 4 38 19

Total weight loss at 750 1C 85 71 75 76 78

120

weight loss %

100

Cotton Cotton-A140%

80

Cotton-A150%

60

Cotton-B2 Cotton- B5

40

20

0 0

Figure 22.7

100

200

300 400 500 temperature °C

600

700

800

TGA curves of uncoated and back-coated cotton samples.

with a 66% weight loss. The total weight loss at 750 1C was 85%. The backcoating formulation with the A1 compound has shown improvement in the thermal stability of cotton, especially in the main pyrolysis step. For cotton samples back-coated with 40% A1, only 13% weight is lost over a wide temperature range, from 287 to 422 1C. Cotton samples back-coated with 40% A5 also show similar thermal degradation pattern (see Table 22.4). These

350

Chapter 22

results indicate that the new back-coating formulations have increased the thermal stability of cotton samples. The DSC data of pure cotton and back-coated samples tabulated in Table 22.5 and graphically presented in Figure 22.8 complement the TGA results. The pyrolysis process of pure cotton passes through two main endothermic stages over temperature ranges of 59–324 and 324–392 1C, with the main pyrolysis step occurring between 324 and 392 1C. Most pyrolysis products are produced in this stage with L-glucose and laevo-glucosan as major decomposition products, together with combustible products like alcohols, aldehydes, ketone, furan, benzene rings and ethers.10–13 Table 22.5

DSC data of uncoated and back-coated-cotton samples. Decomposition temperature range (1C)

Sample name

Peaks

Uncoated cotton

Endothermic Endothermic Endothermic Endothermic Endothermic Endothermic Endothermic Endothermic Endothermic Endothermic

Cotton–A1-40% Cotton–A5-40%

Cotton–B2 Cotton–B5

1 2 1 2 1 2 3 4

Onset

Peak

End

Heat (J g1)

59 324 164 237 33 157 243 350 164 183

120 365 197 247 52 202 251 372 191 210

324 392 236 261 85 241 262 394 228 251

82 152 58 5 30 70 5 10 12 42

35 Cotton Cotton-B5

30

Cotton-B2

mw

25

Cotton- A1-40% Cotton-A5-40%

20 15 10 5 0 0

Figure 22.8

100

200

300 400 Temperature

500

600

DSC curves of uncoated and back-coated cotton samples.

700

Novel Fire Retardant Backcoatings for Textiles

351

Back-coating formulation A1-40% prevents the main thermal pyrolysis stage of cotton from occurring in the temperature range 300–390 1C. The backcoating formulation that contains 40% A1 lowers the temperature for the start of decomposition of cotton to 164 1C, while the second endothermic stage occurs in the temperature range 237–261 1C, with heat of decomposition 8.51 J g1. This suggests that malonyl phosphate (A1) promotes carbonization, restricts the pyrolysis reaction that can produce laevoglucosan and prevents the formation of more flammable gases. This is also supported by the absence of the main decomposition step of cotton in the DSC thermogram shown in Figure 22.8. Thus, the pyrolysis behaviour of the cotton sample has been changed by backcoating with formulations that contain this new flame retardant system. Previous workers have reported10,14,15 that the phosphate compounds decompose to produce phosphoric acid, which polymerizes into polyphosphoric acids, which catalyze the dehydration and decarboxylation of cellulose molecules. Replacing the flame retardant compound A1 by the chlorodimalonyl phosphate phosphine oxide A5 in the backcoating formulations also altered the thermal degradation behaviour of the cotton samples. An early endothermic stage in the temperature range 33–85 1C with heat of decomposition 30 J g1, shown in Table 22.5 for the cotton sample A5-40% can be related to the loss of moisture. The second and third endothermic decomposition stages give a good indication of the effectiveness of the flame retardant compound A5. The energy of decomposition of the first stage, 70 J g1, is higher than that for the respective stage of cotton sample coated with 40% A1. This means that the cotton sample coated with 40% A5 is thermally more stable than that coated with 40% A1. The mechanism of flame retardant action of the new coating formulation is through decomposition of the flame retardant material to form phosphoric acid, which effectively catalyzes the dehydration of cellulose and forms a protective layer over the cotton sample.10,15 A noticeable increase in the thermal stability of the cotton sample backcoated with B2 and B5 coatings (Table 22.1) was observed. Both of these coatings contain carbonizing agents. B2 and B5 backcoatings are composed of 35% flame retardant compound, A1 and A5, respectively, and 15% carbonizing agent. B2 contains dextrin, whereas B5 contains pentaerythritol as carbonizing agent. Incorporating the carbonizing agent into the backcoating formulations plays an important role in stabilizing the char layer, which provides good protective and thermal insulating effects. Only one endothermic peak was noticed in both cases for cotton samples coated with B2 and B5 formulations.

22.3.3

Thermal Pyrolysis Process of Uncoated and Back-coated Polyacrylic Samples

The TGA data of uncoated and back-coated polyacrylic samples is given in Table 22.6 and graphically plotted in Figure 22.9. The thermal decomposition process of uncoated polyacrylic occurs in two steps. The first decomposition stage lies

352

Chapter 22 120 Polyacrilic 100

weight loss %

Polyacrylic-A1-40% Polyacrylic-A5-40% Polyacrylic-B2

80

Polyacrylic-B5 60

40

20

0 0

100

Figure 22.9 Table 22.6

200

300 400 500 Temperature °C

600

700

800

TGA curves of uncoated and back-coated polyacrylic samples.

TGA data of uncoated and back-coated polyacrylic samples.

Sample name

Stage

Uncoated polyacrylic

1 2 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3

Polyacrylic–A1-40%

Polyacrylic–A5-40%

Polyacrylic–B2

Polyacrylic–B5

Decomposition temperature range (1C) 290–357 357–447 20–84 121–276 285–334 339–523 20–110 149–263 289–359 364–513 20–84 134–212 264–345 346–504 20–65 145–290 291–496

Weight loss % at each stage 16 18 3 8 6 25 2 9 11 25 6 6 7 11 2 10 31

Total weight loss at 750 1C 50 50

55

44

50

between 290 and 357 1C and is associated with a 16% weight loss. The second decomposition stage occurs between 357 and 447 1C, with a loss of 18% weight. In general, all back-coated polyacrylic samples in Table 22.6 show four stages of decomposition, except polyacrylic-B5, which degrades in three stages. The

353

Novel Fire Retardant Backcoatings for Textiles

Table 22.7

DSC data of uncoated and back-coated polyacrylic samples. Decomposition temperature range (1C)

Sample name

Peaks

Onset (1C)

Peak

Ends

Heat (J g1)

Uncoated polyacrylic Polyacrylic–A1-40%

Exothermic Endothermic Exothermic Endothermic 1 Exothermic 2 Endothermic 3 Endothermic Exothermic Endothermic Exothermic

311 162 292 153 292 380 156 294 177 295

324 191 313 200 309 387 170 315 201 307

337 228 336 238 338 418 193 337 240 323

113 32 108 48 134 7 16 100 25 123

Polyacrylic–A5-40% Polyacrylic–B2 Polyacrylic–B5

thermal stability and flame retardant action of the backcoatings can be seen from the weight-loss percentages during different decomposition stages of polyacrylic. The TGA data of polyacrylic samples coated by 40% A1 showed 6% weight loss within the temperature range 285–334 1C, while in case of uncoated polyacrylic sample the weight loss in this temperature range was 16%. Another indication of the thermal stability of the coated samples is the shift in the decomposition temperature of the last step. For the sample coated with 40% A5, the decomposition temperature is slightly increased. The DSC study of the back-coated polyacrylic samples supports the evidence for a flame retardant effect due to the new backcoating formulations. The DSC data for uncoated polyacrylic and back-coated polyacrylic samples is given in Table 22.7 and is graphically represented in Figure 22.10. The polyacrylic sample thermally decomposed through one exothermic stage between 311 and 337 1C, with a heat of decomposition 113 J g1. The polyacrylic sample backcoated with 40% A1 decomposed through two stages. The first endothermic stage takes place in the temperature range 162–228 1C, with heat of decomposition 32 J g1. The second stage was exothermic and occurred in the range 292–336 1C, with heat of decomposition 108 J g1. The backcoating formulation A1-40% decreased the total heat of decomposition of polyacrylic from 113 to 76 J g1, thereby suggesting higher thermal stability of the latter. Replacing A1 (malonyl phosphate) by the dimer A5 as flame retardant additive in the backcoating formulations improved the thermal stability of the polyacrylic samples. Thermal decomposition behaviour of polyacrylic samples coated with 40% A5 started with an endothermic stage within the temperature range 153–238 1C. The unexpectedly high exothermic peak with heat of decomposition 134 J g1 occurred in the temperature range 292–338 1C. Finally, an endothermic peak was observed in the temperature range 380–418 1C. The total heat of decomposition decreased to 79 J g1. For the back-coating B2 with the flame retardant system composed of 35% A1 and 15% dextrin, a decrease in the heat of decomposition associated with the exothermic peak was observed.

354

Chapter 22 30 Polyacrylic Polyacrylic- A1-40% Polyacrylic- A5-40%

25

Polyacrylic- B2

mw

20

Polyacrylic- B5

15

10

5

0 0

Figure 22.10

100

200

300 400 Temparature

500

600

700

DSC curves of uncoated and back-coated polyacrylic samples.

Table 22.8

LOI and flame spread results of backcoated cotton samples.

Flame spread

LOI

Sample

153 mm min1 Class I Class I Class I Class I

18 39 37 30 33

Uncoated cotton Cotton–A1-40% Cotton–A5-40% Cotton–B2 Cotton–B5

The value of heat of decomposition for the exothermic stage was 100 J g1, while the value for uncoated polyacrylic was 113 J g1. The flame retardant systems A1 with pentaerythritol (coating B5) did not make a significant improvement in the thermal stability of polyacrylic. This appeared clearly from the total heat of decomposition and also the heat associated with the exothermic peak.

22.3.4 22.3.4.1

Flammability Properties LOI and Flame Spread Results of Back-coated Cotton Samples

The results of LOI and the flame spread test (ISO 3795) for back-coated cotton samples are given in Table 22.8. Cotton fabric back-coated with 40% A1 formulation shows the highest LOI value of 39%, which suggests a significant improvement in the fire resistant properties of the cotton fabric. The LOI value

355

Novel Fire Retardant Backcoatings for Textiles

of 37% for cotton sample back-coated with 40% A5 is slightly lower than that of the cotton sample A1-40% with an LOI of 39%, despite the fact that A5 contains four phosphate groups. The carbonizing agents together with the phosphate compounds A1 and A5 further enhanced the fire resistance of the cotton fabric. The pentaerythritol with A5 (coating B5) achieved higher oxygen index value of 33% compared to coating B2 (LOI ¼ 30%), which contains 35% A1 and 15% dextrin. All kinds of back-coating formulation have succeeded in classifying the cotton sample as Class I fabrics according to the ISO 3795 standard. Class I implies that the flame extinguishes before reaching the first mark positioned at 38 mm from the ignited edge of the sample. Thus, the back-coating formulations used in this study that contain organophosphorus compounds A1 and A5 have rendered flammable cotton fabrics fire resistant.

22.3.4.2

LOI and Flame Spread Results of Back-Coated Polyacrylic Samples

It is known that polyacrylic fabrics are highly flammable and difficult to render flame retarded. One of the successful trials to lower the flammability properties of polyacrylic is to build a flame retardant moiety into the polyacrylic chain. The new back-coating formulation in this study has succeeded in giving the polyacrylic fabric good fire resistant properties. This is clearly evident from the LOI values of polyacrylic samples coated with 40% A1, which had a 29% LOI value (Table 22.9). The flame retardant compound A5 [chloro-(dimalonyl phosphate) phosphine oxide] at 40% loading also improved the fire retardant effect of the back–coatings, with an LOI value of 25%. The back-coating formulation B2 achieved a higher LOI value of 25% for polyacrylic in comparison with B5, with an LOI of 24%. All back-coated fabrics have achieved Class I according to the ISO standard.9 The back-coating formulations used in this study have rendered highly flammable polyacrylic fabric fire resistant.

22.3.5

Smoke Density Measurements

The specific optical density (SOD) values of uncoated and back-coated cotton and polyacrylic samples are given in Table 22.10. SOD versus time Table 22.9

LOI and flame spread results of back coated polyacrylic samples.

Flame spread

LOI

Sample

150 mm min1 High class High class High class High class

18 29 25 25 24

Uncoated polyacrylic Polyacrylic–A1-40% Polyacrylic–A5-40% Polyacrylic–B2 Polyacrylic-B5

356

Chapter 22

Table 22.10

The maximum specific optical density of uncoated and back-coated cotton and polyacrylic samples.

Specific optical density

Type of sample

127 96 54 54 86 74 62 82 73 116

Uncoated cotton Cotton–A1-40% Cotton–A5-40% Cotton–B2 Cotton–B5 Uncoated polyacrylic Polyacrylic–A1-40% Polyacrylic–A5-40% Polyacrylic–B2 Polyacrylic–B5

140 Cotton 120 Cotton-A1-40% Cotton-B5

specific optical density

100

80

Cotton-B2

60

Cotton-A5-40%

40

20

0 0

50

Figure 22.11

99

148 Time (s)

197

246

295

Smoke density curves of uncoated and back-coated cotton samples.

curves for cotton and polyacrylic fabrics are plotted in Figures 22.11 and 22.12, respectively.

22.3.5.1

Smoke Density of Back-coated Cotton Sample

Uncoated cotton samples produced large amounts of smoke giving SOD of 127. In general, all back-coating formulations lowered smoke density values, with

357

Novel Fire Retardant Backcoatings for Textiles 160

specific optical density

140

Polyacrylic-B5

120 Polyacrylic-B2

100

Polyacrylic-A5-40% 80 60

Polyacrylic Polyacrylic-A1-40%

40 20 0 0

50

Figure 22.12

99

148 Time (s)

197

246

295

Smoke density curves of uncoated and back-coated polyacrylic samples.

formulation A5-40% and B2 giving the lowest SOD of 54 and 36, respectively. This suggests that in the presence of A5 and B2 a good protective char layer is formed on the surface of the sample which prevents the further degradation of cotton. In case of sample B2 the smoke suppression effect is attributed to the role played by dextrin, as the carbonizing agent stabilizes the char protective layer. However, the addition of pentaerythritol as carbonizing agent to compound A5 resulted in an increased SOD from 54 (for A5-40%) to 86 for B5.

22.3.5.2

Smoke Density of Back-coated Polyacrylic Samples

The SOD versus time curves for different polyacrylic samples are shown in Figure 22.12. The highest smoke suppression with SOD 62 occurs for the backcoated polyacrylic containing A1. The value of SOD for the polyacrylic backcoated with 40% A5 coatings (SOD ¼ 82) is higher than the value for the uncoated polyacrylic sample (SOD ¼ 74). The highest SOD value of 116 was observed when the back-coating formulation B5 was used. In general, the SODs for the back-coated polyacrylic samples are higher than those for the uncoated polyacrylic samples, with the exception of polyacrylic A1-40% (SOD ¼ 62). The values are still within the required international code for textiles of SODo200.

22.3.6

Conclusion

Malonyl phosphate and its dimer have shown good flame retardancy for backcoating applications. The effect is apparent from the LOI values of back-coated

358

Chapter 22

cotton and polyacrylic samples. The cotton fabric coated with 40% A1 showed an LOI of 39%, whereas polyacrylic fabric coated with the same coating showed an LOI of 29%. Only the low loading percentage (40%) of the organophosphorus compounds A1 and A5 achieved Class I for both cotton and polyacrylic fabrics. Both untreated cotton and polyacrylic samples were highly flammable, with the whole sample (150 mm) being consumed by flames. In contrast, all back-coated fabric samples flame-extinguished before reaching the first mark at 38 mm, which indicates improved flame retardancy. The major achievement of the back-coatings is the change in the fire properties of polyacrylic and cotton from highly flammable to fire resistant. Increasing the number of phosphate groups in A5 has shown little or no effect on the LOI values of the back-coated fabric samples. This suggests that the fire retardant effect of organophosphorus compounds is not solely dependent on the number of phosphate groups, but on other factors which need to be identified by further research work.

References 1. J.R. Hall, In Polyurethanes EXPO 2002, API Conference; Salt Lake City, 2002. p. 217. 2. G. Edel, Bulletin Scientifique de l’Institut Textile de France, 1980, 148(24), 355. 3. Compte rendu de la journe 0 e d’information. Lyon, France: Textiles et Feu; 12 February, 1971. 4. R. Dombrowski, J. Coated Fabrics, 1996, 25, 224. 5. A.R. Horrocks, M.Y. Wang, M.E. Hall, F. Sunmomu and J.S. Pearson, Polym. Int., 2000, 49, 1079–1091. 6. M.Y. Wang, A.R. Horrocks, S. Horrocks, M.E. Hall, J.S. Pearson and S. Clegg, J. Fire Sci., 2000, 18, 265–294. 7. E. Devaux, M. Rochery and B. Serge, Fire Mater., 2002, 26, 149–154. 8. C. Drevelle, J. Lefebvrea, S. Duquesnea, M. Le Brasa, F. Poutch, M. Voutersc and C. Magniez, Polym. Degrad. Stab., 2005, 88, 130–137. 9. ISO- 3795 ‘‘Determination of fire behaviour of materials’’, 1992. 10. P. Zhu, S. Sui, B. Wang, K. Sun and G. Sun, J. Anal Appl. Pyrolysis, 2004, 71, 645–655. 11. B.K. Kandola, A.R. Horrocks, D. Price and G.V. Coleman, Journal of Macromolecular Science –Reviews in Macromolecular Chemistry and Physics, 1996, C36(4), 721–794. 12. S. Nakanishi, F. Masuko, K. Hori and T. Hashimoto, Textile Res. J., 2000, 70(7), 574–583. 13. S. Bourbigot, S. Chlebicki and V. Mamleev, Polym. Degrade. Stab., 2002, 78(1), 57–62. 14. D. Davies and A.R. Horrocks, J. Appl. Polym. Sci., 1986, 31, 1655–1662. 15. M. Bras, G. Camino, S. Bourbigot, R. Delobel, Fire Retardancy of Polymers, The Royal Society of Chemistry, 1998, p. 48.

CHAPTER 23

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance and Pigmentation on the Thermal Properties and Limiting Oxygen Index of Flame Retardant Polypropylene Fabrics C. KINDNESS,a B.K. KANDOLAb AND A.R. HORROCKSb a

Camira Fabrics Ltd. Hopton Mills, Mirfield, WF14 8HE, UK; b Centre for Materials Research and Innovation (CMRI), University of Bolton, Bolton BL3 5AB, UK

23.1 Introduction Since 1970 there has been a considerable and maintained increase in the manufacture of polypropylene fibre, with European production estimated at 2.4 million tonnes for use in textile end-uses.1 A significant proportion is polypropylene fabric used increasingly in the domestic and contract upholstery furnishings markets throughout the world. In parallel with this growth,

Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

359

360

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public awareness of textile-related fires has increased due to a number of major fires that involved significant numbers of fatalities, especially in Europe, one of the most significant being the Woolworth fire in Manchester in 1979, which involved upholstered domestic furniture. Following the investigation into the cause of this fire, changes were made to the UK fire regulations.2 Legislation within the European Community and the increased use of European architects in Hong Kong and Singapore, as well as occurrences such as the underground train fire in South Korea, have led to an increased adoption of high levels of regulation relating to safety in public places and in transportation.3 Consequently, there is a greater interest and regulatory drive in the use of fire retardant materials in public and domestic environments. At the same time there has been a gradual movement in customer demand in the contract upholstery market, especially during the past five years, from natural to synthetic fibres. Also there is more desire for effective, yet environmentally improved, flame retardants and for alternatives to the current halogenated and antimony-based flame retardant systems commonly used in polymer-based fabrics.4 These issues have caused industry to become increasingly aware of the science and technology that underpins its need to develop products which are both fire safe and match customers demands. In this chapter we describe an investigation designed to identify the important fibre, yarn and fabric structural variables that influence the burning behaviour determined using defined standard test methods, and therefore to reduce fabric flammability failures. The work centres on a proprietary halogenated flame retardant polypropylene fibre designed and developed to pass the main European flammability standards for contract upholstery while meeting the growing commercial demand for a low cost, yet high-performance product. The original commercial development process provided the background for this work and involved the manufacture and testing of a range of fabrics produced from filament yarns in a variety of colours, woven structures and area densities. Flammability performance standards were determined using methods as defined in the performance standard BS 7176,5 in this case using BS 5852 source 56 and the section NF P 92-503, the French Bruleur Electrique or M test.7 To predict the results from experimental work in an industrial development laboratory there is the need to screen new formulations to assess the performance of the final product. The major problems lie in the inability to measure absolute yarn flame retardancy and to relate this to formulation variables for flame retardant fibres. Secondly, there is the need to correlate fibre and yarn flammability data with final fabric performance as defined by standard procedures, e.g. BS 7176. The limiting oxygen index (LOI),8 although not an adopted standard for textiles, is one of the few tests that relate degree of flame retardancy to a quantitative scale. Textile fabrics usually pass a typical vertical strip test if LOI 4 26, although for polypropylene fabrics lower values may yield a pass because of the high melt-dripping tendency of this fibre compared to other conventional fibres, like

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance

361

polyester. However, if a test requires a barrier property where the fabric under test must not only burn for a restricted period, but also provide a measure of protection for an underlying flammable surface, such as foam filling as described in BS 7176, then the LOI test may not be predictive of a pass. If a given level of char-promoting behaviour can be produced with the flame retardant polypropylene formulation and this enables a pass to be obtained using the standard BS 7176, then it is highly likely that a meaningful LOI versus pass or fail relationship may be established. The unpublished test results from an industrial laboratory during the original development stage of halogenated flame retardant fabrics indicated that the fabric flammability performance was varied. The majority of early sample fabrics produced passes the BS 7176 performance standard when used over a combustion modified high resilience (CMHR) 35 kg foam, which is generally specified in the commercial environment for the particular end uses of these fabrics. However, under the radiant heat flux conditions of NF P 92-503, those fabrics with a more open, yet thicker, construction and of greater area density, at between 280 and 340 g m
2, were burnt more readily. Although the times to hole formation were relatively short at 8–10 seconds, occasional fails were observed. A pass is where the overall test performance criteria yield an M1 classification, i.e. there shall be no combustion lasting more than 5 seconds after the pilot flame is removed, no ignition points with a propagation effect or no fall of droplets. It was also intuitively felt that there were differences in behaviour related to the colour of the samples and that this might determine the degree of heat absorption and reflection and hence M test performance. For instance, black fabrics might be expected to absorb infrared (IR) energy more effectively than coloured pigmented fabrics, while those that tend to reflect IR, most likely those with a red-reflective pigment present, might be expected to have less tendency to heat up during testing. To understand the cause of the different results and ultimately to produce a predictive yarn flammability test, the characterization of currently available and archived commercial polypropylene technical yarns and fabrics was undertaken. Comparisons of physical and structural properties with their respective thermal and burning behaviour were made in an attempt to identify those features that influence fabric performance and possible failure during formal flammability testing. The fabric variables considered in this study are the yarn and fabric constructions, fabric area density, air permeability, colour shade and pigment content. An apparent colour dependence on M test performance, in which certain deeper shades appeared to be associated with a test fail, meant that for colour shade we focussed on redness and hence IR reflectance, which may influence fabric temperatures during testing and hence flammability. Since this same test causes fabric temperatures to rise, a major burning parameter may be a measure of the temperature sensitivity of LOI, a technique which has been reviewed elsewhere, along with the normal application of oxygen index measurements to textiles.9

362

Chapter 23

23.2 Experimental Methods 23.2.1

Materials

23.2.1.1

Commercial Fabrics – Yarn and Fabric Constructions

The commercial fabrics given in Table 23.1 have been selected from a range of colours of flame retardant polypropylene, with one sample used as a control made from standard polypropylene yarn (sample 1). Those fabrics produced with the ‘proprietary’ halogenated flame retardant yarns generally achieve an M1 pass to the NF P 92-503 test, while those produced from standard polypropylene fabrics usually achieve an M4 fail classification (see Table 23.3). The fabrics, with a nominally constant area density of about 260 g m
2, were manufactured from the relevant standard or flame retardant polypropylene yarns of different linear yarn densities to give upholstery fabrics in dobbyweave structures. The yarns were intermingled in a range of linear densities of 800, 1290 and 1670 decitex (dtex). These were constructed as core and effect yarns, from 330 dtex round or delta cross-section using Heberlein jets with 70 filaments, 4.8 dtex per filament, on Stahle air-texturing machines. The final yarn linear density was achieved using combinations of numbers of filaments and a range of between 20 and 30% overfeed in a core and effect structure. The air texturizing process intermingles the filaments to produce a robust yarn suitable for weaving. The 800 dtex yarns had the least amount of overfeed in the structure and had a single core and effect process. The 1290 dtex yarns had two core and one effect and the1670 dtex yarns were produced in a two core and two effect construction. These yarns had the highest texturizing input and so were the bulkiest yarns with the potential to hold the highest quantity of air within the final fabric structure. Filaments were coloured during the extrusion process via the introduction of compounded, pigmented masterbatch

Table 23.1

1 2 3 4 5 6 7 8 9 10 11 12

Commercial fabric structures.

Sample

Weave structure

Area Density (g m
2)

Warp yarn (dtex)

Weft yarn (dtex)

Dark turquoise Navy/Blue Black/White Green Black/Brown Blue Beige/Brown Green Blue Red Green Black/Brown

Crepe Dobby Plain Plain Plain Plain 2/2 hopsack 2/2 hopsack 2/2 hopsack Crepe Crepe Crepe

249 232 235 240 234 256 250 239 246 268 286 257

1290 800 1670 1670 1670 1670 800 800 800 1290 1290 1290

1290 800 1670 1670 1670 1670 800 800 800 1290 1290 1290

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance

363

in low-density polyethylene added in small percentage additions up to 6% of the total mix. The yarns were then built from a range of coloured filaments; for example, the 1670 dtex yarns could use up to four colours to produce a mixture or marl yarn effect. The final yarn colour was controlled by the amount of overfeed in the texturing process. The commercial fabrics were regularly produced through a controlled manufacturing process from production of filament, yarn texturing to the final process of weaving, and samples were selected for testing from four commercial ranges. The fabric samples were all woven on rapier dobby machines in plain weave, 2/2 hopsack, a 16-end dobby design and a six-shaft crepe weave. In these commercial fabrics the yarn construction is particular to the weave structure used. The plain-weave fabrics were woven at a sett of 15 ends cm
1  15 picks cm
1 to give an open weave and a lightweight fabric construction. The crepe-weave fabrics comprising 1290 dtex yarns were woven at 8.7 ends cm
1  9 picks cm
1 and had longer floats and produced a thick, bulky fabric with the greatest potential to trap air within the construction. The 2/2 hopsack weave was sett at 14 ends cm
1  11.5 picks cm
1 using the lowest linear density yarns with the least amount of texturing and a parallel orientation in the most highly sett fabric, with two yarns lifting together to produce a thinner yet dense construction.

23.2.1.2

Experimental Fabrics – Yarn and Fabric Constructions

To attempt to assess the effects of yarn linear density and fabric structure on flammability using experimental design methods, the fabrics in Table 23.2 were woven with nominally constant area densities of about 260 g m
2. Only two yarn colours, black and green, were chosen based on the relative extreme temperature sensitivities following high-temperature LOI testing of the commercial fabrics listed in Table 23.1 (see Section 3.2).

Table 23.2

13 14 15 16 17 18 19 20 21 22

Experimental yarn and fabric structures.

Sample

Weave structure

Area density (g m
2)

Warp yarn (dtex)

Weft yarn (dtex)

A1 Black A2 Black B1 Green B2 Green C1 Black C2 Black D1 Green D2 Green E1 Black E2 Green

Crepe Crepe Crepe Crepe 2/2 hopsack 2/2 hopsack 2/2 hopsack 2/2 hopsack Plain Plain

273 265 261 260 280 278 262 262 263 246

1290 1290 1290 1290 800 800 800 800 1670 1670

1290 1670 1290 1670 800 1290 800 1290 1670 1670

364

Table 23.3

Commercial fabric LOI DLOI and M test results. LOI (vol%) at 20 1C temperature intervals

1 2 3 4 5 6 7 8 9 10 11 12 a

M test

Sample

20 1C

40 1C

60 1C

80 1C

100 1C

120 1C

Time to DLOIa hole forma(vol%) tion (s)

Dark turquoise Navy/Blue Black/White Green Black/Brown Blue Beige/Brown Green Blue Red Green Black/Brown

19.2 26.7 24.7 27.2 26.4 26.4 26.6 27.1 25.6 23.4 26.1 21.8

19.2 26.7 24.7 27.2 26.3 26.4 26.6 27.0 25.6 23.4 26.0 21.8

19.2 26.6 24.6 27.1 25.9 26.1 26.6 26.2 25.3 23.2 26.0 21.8

19.0 26.7 24.6 26.8 25.8 25.7 26.6 26.1 25.2 23.2 25.7 21.7

18.6 26.3 24.6 26.2 25.5 25.3 26.5 25.8 24.7 23.0 24.8 21.6

18.1 26.1 24.5 26.1 25.5 25.2 26.5 25.4 24.4 22.0 24.2 21.6

1.1 0.6 0.2 1.1 0.9 1.2 0.1 1.7 1.2 1.4 1.9 0.2

10 5 5 6 5 4 7 6 6 8 5 5

Burning droplets

Distance of flame spread (mm)

M classification

BS7176 Medium Hazard

Yes No No No No No No No No No No No

220 190 190 180 190 190 180 180 170 200 180 190

M4 M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 M1

Fail Pass Pass Pass Pass Pass Pass Fail Fail Pass Pass Pass

As given by Equation (23.1).

Chapter 23

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance

23.2.1.3

365

Pigments

Commercial sensitivity means the pigment details cannot be divulged and some of the same colours have different chemical compositions. For convenience the pigments present in all the yarns used are listed as:         

Black: b1, b2 White: w1, w2, w3 Brown: br1 Red: r1, r2 Scarlet: s1 Yellow: y1, y2 Green: g1 Blue: bl1, bl2 Violet: v1

23.2.2

Flammability Testing

The commercial and experimental range of fabric samples were tested to assess their potential fire performance using the European standard flammability tests relevant to contract furnishing requirements. The NF P 92-501 to 92-507,10 which encompasses NF P 92–503, is used mainly in France, Belgium, Spain and Portugal by the building industry to certify the use of all materials in buildings for public use. The test categorizes the reaction to fire of a material with a classification from M1 to M4, with M1 a pass, M2 acceptable in certain specified uses and M3 and M4 as fails. The relevant section of the test used in these experiments is NF P 92-503. A series of tests were carried out to determine the classification. Fabric samples were subjected to a radiant heat flux followed by a flaming ignition source after 20 seconds. The ignition source is held in position under the sample for 5 seconds and then removed. This is repeated every 20 seconds until 5 minutes have elapsed. Generally, a polymeric material will melt to form a hole in the first 20 seconds or only on application of a flame. The results are calculated on a number of parameters being met – the time to hole formation, the presence of burning droplets and the distance of flame spread is less than 350 mm. Further supplementary tests are then carried out to meet the full requirements using NF P 92-505 Melting Material test. An M1 pass is given when there is hole formation, no burning droplets and the fabric has shrunk from the flaming ignition source. An M4 failure is given when there is a fall of burning droplets and combustion of the cellucotton material used in the test equipment, or there is persistent, spontaneous or consequential damage such that the average of the lengths destroyed is less than 600 mm measured from the bottom edge of the test piece. BS 7176 Medium Hazard requirement was used to determine resistance to ignition of the fabric in combination with CMHR 35 kg foam in a composite using BS5852: 2006 ignition source 5. This test determines the suitability of the fabric–foam combinations for use in public areas in the UK. It is

366

Chapter 23

recognized that in combinations with other types of foam, the performance results will be different. The LOI and high temperature LOI (using Stanton Redcroft FTA and FTB high-temperature critical oxygen index equipment, respectively), were used to measure the sensitivity of fabric index values over the range 20–120 1C, defined as: DLOI ¼ LOI20  C
LOI120  C

ð23:1Þ

Essentially, the method at room temperature used was that described in ASTM D2863-77 (and subsequent revisions) for thin materials; a 5 second ignition time was used. The high-temperature method used the same sample holder and technique, but with samples located in the FTB apparatus; samples were introduced into the chimney preheated at a defined temperature, allowed to equilibrate for 30 seconds and subjected to ignition. This temperature range was selected to simulate the effects of an external radiant heat flux on fabric ignition and burning properties present in NF P 92-503. The upper limit of 120 1C was chosen to minimize the effects of fabric shrinkage on the results.

23.2.3

Thermal Analysis

The fabric samples and the respective pigments contained within the filaments used to produce these fabrics were subjected to a TA SDT 2960 simultaneous thermal analysis instrument using differential thermal analysis (DTA)–thermogravimetric analysis (TGA) from ambient temperature to 900 1C at 10 1C min
1 heating rate under a nitrogen atmosphere (flow rate 100 cm3 min
1).11

23.2.4

Air Permeability

All the samples were tested for air permeability12 using Shirley Air Permeability Apparatus according to BS 9237:1995. Air permeability (P) of a fabric is a measure of how well it allows the passage of air through it and is expressed as the volume rate of airflow through a fabric per unit area at a standard pressure difference.

23.2.5

Colour Measurement and Pigment Analysis

The colour measurement of each fabric sample was carried out using a Datacolor System13 where ‘x’ is the measure of redness and ‘y’ is the measure of brightness. Redness was considered to be particularly relevant to this study since red pigments tend to reflect both red and near IR wavelengths, while the converse would be the case for low ‘x value’ fabrics.

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance

367

23.3 Results and Discussions 23.3.1

Commercial Fabrics

The commercial fabrics tested in Table 23.1 were selected based on previous experience of fabric flammability test performance and represented samples of normal commercial manufacture and the results from internal laboratory and external testing. The fabrics were subjected to testing for LOI at 20 1C through to 120 1C at 20 1C temperature intervals, and the results are listed in Table 23.3. Generally, for each fabric sample, the LOI value tends to reduce in magnitude as the temperature increases as expected. However, the degree of reduction may be a measure of the temperature sensitivity of the flammability of a particular sample [Equation (23.1)]. With the exception of sample 1, all samples pass the NF P 92-503 test with an M1 grade. The standard, non-flame retardant polypropylene, sample 1, has an expected low LOI value of 19.2 volume percent (vol%) and achieved an M4 fail rating and failed the BS 7176 Medium Hazard test. The flame retardant polypropylene fabrics all have higher LOI values that range from 21.8 to 27.2 The crepe weave fabrics, samples 10–12, which have the highest area densities between 257 and 286 g m
2 (see Table 23.1), have a range of LOI values at 20 1C of between 21.8 and 26.1 vol% with DLOI values of 0.2 to 1.9 vol%. The 2/2 hopsack weave fabrics, samples 7–9, with the tightest yarn and fabric constructions, have LOI values of 25.6 to 27.1 vol% at 20 1C and DLOI values that range from 0.1 to 1.7 vol%. The plain weave fabrics, samples 3–6, are the lightest in weight and of the most open construction, and have ambient LOI values from 24.7 to 27.2 vol%, similar to those of the hopsack fabrics, and respective DLOI values from 0.2 to 1.2 vol%. Two green samples, 4 and 11, compromise the same filament colour in the relevant yarn in plain and crepe woven fabrics. These have similar LOI values of 27.2 and 26.1 vol%, yet the crepe weave, sample 11, indicates a greater sensitivity of DLOI, at 1.9 compared to 1.1 vol%. Sample 8, the 2/2 hopsack weave, has a high percentage of the same green filament and shows an LOI value of 27.1 vol% with a DLOI close to that of the crepe weave structure value of 1.7 vol%. While the expectation based on openness of fabric structure would have been to see a greater difference between the results of the hopsack and plain weave samples, it appears that in the green fabrics this is not the case. The crepe weave construction would be expected to have the ability to trap more air within the fabric, while the denser and most tightly woven structures made from the finer linear density yarn in the 2/2 hopsack range would be less likely to contain entrapped air. The area density of the samples does not appear to have any obvious effect on the LOI or the DLOI performance. However, those samples with area densities below 235 g m
2 are relatively less sensitive to temperature, with DLOI values between 0.2 and 0.9 vol%, while 239 and 240 g m
2 samples are more sensitive, with DLOI values of 1.1 and 1.7 vol%. As the fabric area density

368

Chapter 23

increases, the DLOI values tend to indicate greater temperature sensitivity. The most significant observation of these results, however, is the difference in DLOI values between the fabric colours. Those samples which contain brown and black pigments in the yarns generally have DLOI values between 0.1 and 0.9 vol%, which indicates that these are relatively less sensitive to temperature, while the samples that contain green and blue pigmented yarns have higher DLOI values, between 1.1 and 1.9 vol%. The blue and green samples indicate that there is little correlation between the LOI and DLOI values and the performance of these fabrics to BS 7176 Medium Hazard. The green samples 4 (plain weave), 8 (hopsack) and 11 (crepe) have LOI values of 27.2, 27.1 and 26.1 vol%, and DLOI of 1.1, 1.7 and 1.9 vol%, respectively. While the LOI values are similar there is a sensitivity to the difference in temperature over the range 20–120 1C. The green hopsack, sample 8 has the highest LOI of all the samples tested, yet fails BS7176 Medium Hazard. The blue samples 6 (plain) and 9 (hopsack) have lower LOI values than the green samples, at 26.4 and 25.6, and sample 9, which is also a hopsack structure, has failed BS7176 Medium Hazard. However the DLOI value for both samples is 1.2, within the same range as the green samples. Sample 12, black and brown yarns combined in a crepe weave, has the lowest LOI value at 21.8 vol%, of all the flame retardant samples and a low DLOI of 0.2 vol%, yet passes the BS 7176 test.

23.3.2

Experimental Fabrics

To investigate the apparent differences in the behaviour of different fabric colours, green and black yarns only were selected to produce the experimental fabrics, as stated in Section 23.2.1 and shown in Table 23.2. These were tested for LOI values over the temperature range 20–120 1C and the results are listed in Table 23.4.

Table 23.4

Experimental fabric flammability results. LOI (vol%) at 20 1C temperature intervals

13 14 15 16 17 18 19 20 21 22 a

Sample

20 1C

40 1C

60 1C

80 1C

100 1C

120 1C

DLOIa (vol%)

A1 Black A2 Black B1 Green B2 Green C1 Black C2 Black D1 Green D2 Green E1 Black E2 Green

24.3 24.6 29.4 29.7 24.2 24.8 29.6 30.7 24.8 29.4

24.4 24.8 28.9 29.0 23.9 23.9 29.2 30.0 24.7 28.9

24.0 24.3 28.6 28.9 23.4 23.9 28.9 30.0 24.7 28.8

23.6 24.6 28.3 28.6 23.3 23.8 28.6 29.4 24.4 28.7

23.5 24.3 28.1 28.3 23.1 23.6 28.1 28.5 23.7 28.2

23.1 23.9 27.8 28.1 23.0 23.3 27.8 28.3 23.5 28.0

1.2 0.7 1.6 1.6 1.2 1.5 1.8 2.4 1.3 1.4

As given by Equation (23.1).

M result M1 M1 M1 M1 M1 M1 M1 M2 M2 M1

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance

369

These results indicate that the LOI values at 20 1C of the black samples over the range 24.2–24.8 vol% are generally lower than those of the green samples, which have a range of 29.4–30.7 vol%. The weave construction and area density have had no obvious, significant effect on the magnitude of DLOI. For example, the range of DLOI results on the 2/2 hopsack (samples 17–20) is from 1.2 to 2.4 vol%. Sample 20, green hopsack, has an LOI value of 30.7 vol% and DLOI of 2.4 vol%, the highest of all the samples tested, and yet achieved an M2 rating, because of burning droplets during the test, compared to sample 8 of the commercial fabrics which passed with an M1 rating. The area density of these samples is 262 and 239 g m
2, respectively. Sample 21, black plain weave, also achieved an M2 rating with an LOI of 24.8 vol%. Sample 5, plain weave in a black–brown combination, passed to M1 and had a higher LOI value of 26.4 vol% and a lower DLOI of 0.9 compared to 1.3 vol%. The area density of sample 5 is 234 g m
2 and that of sample 21 is 263 g m
2.

23.3.3

Air Permeability and the Effect on LOI

Little published work is available on the effect of the air permeability of woven fabrics on burning behaviour. Fabric flammability is determined not only by the fibre behaviour, but also by the physical geometry of fibres arranged in the fabric. As reviewed by Horrocks et al.,9 the area density, weight and the construction of textile fabrics can all affect their burning characteristics. Fabric area density in relation to the yarn linear density and construction will determine the pore size, or empty space, within the structure and hence influence the amount of fuel and oxygen available. Low values of fabric area density and open fabric structures could have an increased burning rate due to the amount of trapped air within the construction, whereas a heavier and multilayered construction may burn more slowly, but with an increased hazard of burning severity.2 Work by Hendrix, cited by Horrocks et al.,9 indicates that in a series of single-layered fabrics the LOI increases with fabric area density. However, other factors, such as the chemical constituent of the fibre, can also have an effect on the burning behaviour. This same review cites reports of a linear trend between air permeability and LOI in a spun wool blend yarn in a knitted structure, where LOI decreases as the air permeability increases up to a point at which the LOI also begins to increase. The area densities of all the commercial and experimental samples selected in this work are between 232 and 286 g m
2 (Tables 23.1 and 23.2) and little significant variation would be expected in the air permeability results. There may possibly be an effect on LOI at 20 1C because of the construction of the fabrics in respect of ends and picks within a given area. With respect to air permeability, the weave construction appears to have a very small effect, with the different weave structure values of the fabrics grouping together. The lowest values of permeability between 2.09 and 2.35 cm3 s
1 across both the commercial and experimental fabric samples are those for the hopsack samples. The more open plain weave fabrics have higher values in the range 2.53–2.67 cm3 s
1.

Chapter 23 (a)

33 31 29 27 25 23 21 19 17 15

LOI

LOI

370

2.2

2.3

2.4

2.5

2.6

2.7

Log (air permeability), Log A

Figure 23.1

(b)

33 31 29 27 25 23 21 19 17 15 2

2.1 2.2 2.3 2.4 Log (air permeability), Log A

2.5

Relationship between air permeability and LOI of (a) commercial fabrics and (b) experimental fabrics.

Those fabrics which fail to yield M1 ratings when tested to NF P 503 or fail to BS7176 Medium Hazard are the hopsack samples 8, 9 and 20 and plain weave sample 21. To observe the effect of air permeability on LOI, the LOI values at 20 1C of commercial fabrics are plotted against air permeability (log A) values in Figure 23.1, where it can be seen that an expected linear relationship against LOI values at 20 1C does not occur.9 The lighter sample 2 of 232 g m
2 has LOI ¼ 26.7 vol% and a log A ¼ 2.43, while the heaviest sample 11 at 286 g m
2 with LOI 26.1 vol%, has a log A ¼ 2.41. However, there is a difference in the DLOI of 0.6 and 1.9 vol%, respectively. The air permeability results of the experimental fabrics range between 2.09 and 2.46 cm3 s
1 with area density values from 246 to 280 g m
2. The plain weave samples which have the most open structure have air permeabilities between 2.33 and 2.39 cm3 s
1 and the most tightly woven hopsack fabrics have the lower results of 2.09 to 2.26 cm3 s
1, as expected. These fabrics, however, have a wide range of DLOI results from 0.7 to 2.4 cm3 s
1. However, the LOI versus log A results can be seen to separate clearly between the green and black samples. The two black crepe weave samples have LOI values of 24.3 and 24.6 vol% and DLOI values of 0.7 and 1.2 vol%, with air permeabilities of 2.33 and 2.44 cm3 s
1, respectively. In contrast, the two green crepe weave samples have LOI values of 29.4 and 30.7 vol% and DLOI ¼ 1.6 vol%, with air permeabilities of 2.44 and 2.46 cm3 s
1, respectively.

23.3.4

Effect of Colour on LOI

Previous experience had suggested that the pigments present in each fabric influence M test performance and that those fabrics that reflected IR energy to a greater extent would be less sensitive to the M test. Thus, in this study the fabrics with high red (and most likely IR) reflectivity (measured as ‘x’ reflectance) or an overall high light reflectivity (measured as ‘y’ reflectance) might be expected to show lower DLOI values and hence superior M test results. In Figures 23.2 and 23.3, DLOIs for all commercial and experimental fabrics

371

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance (a)

2

(b)

3 2.5

1.2

2

Green Black

∆ LOI

∆ LOI

1.6

1.5

0.8

1 0.4 0.5 0 0

0.2 0.4 0.6 Red reflectance, x

0

0.8

0

0.1 0.2 0.3 Red reflectance, x

0.4

Figure 23.2 Relationship between red reflectance and DLOI of (a) commercial fabrics and (b) experimental fabrics.

(a)

2.5

Green

2.5

2

2 ∆LOI

1.5

∆LOI

(b)

3

1

Black

1.5 1

0.5

0.5

0 0

Figure 23.3

0.1 0.2 0.3 Light reflectance, y

0.4

0 0.33 0.335 0.34 0.345 0.35 0.355 0.36 Light reflectance, y

Relationship between light reflectance and DLOI of (a) commercial fabrics and (b) experimental fabrics.

have been plotted as functions of their red (x) and light reflections (y) values, respectively. The black and red commercial fabrics have a higher red reflectance, particularly the latter (sample 10) and a lower light reflectance. In the results from the experimental fabrics, two distinct groups can be clearly seen in the plots of x versus DLOI. Those fabrics in the experimental fabric group which are similar in fibre and fabric structure yet different in colour have distinctly different DLOI values. The black crepe samples 13 and 14 have lower DLOI values of 0.7 to 1.2 vol% than the green crepe samples 15 and 16 which are both 1.6 vol%. In the case of the hopsack samples the black samples 17 and 18 have DLOI 1.2 and 1.5 vol%, respectively, lower than the green samples 19 and 20 with DLOI 1.8 and 2.4 vol%, respectively. The plain weave samples 21 and 22, however, have DLOI ¼ 1.3 and 1.4 Dvol%, respectively.

372

Chapter 23

The green samples have lower ’x values than the black fabrics; however, with regard to brightness, for which the green fabrics have higher ‘y’’ values, the reverse is true. It appears that temperature sensitivity of LOI as DLOI (and possibly M test performance) may be inversely related to fabric redness and hence IR-reflecting character.

23.3.5

Thermal Analysis of Fabrics and Colour Pigments

The commercial fabrics and pigments used in each fabric were subjected to simultaneous thermal analysis using a SDT 2960 simultaneous DTA–TGA instrument. The analyses were from ambient temperature to 900 1C at 10 1C min
1 heating rate in nitrogen atmosphere (flow rate 100 cm3 min
1) and the selected TGA results are given in Table 23.5. As stated in Section 2.1, the fabrics comprise a number of core and effect warp and weft yarns having different respective colour formulations. A single fabric may contain a number of the basic pigments in varying amounts, for instance sample 12 (Table 23.1), the black–brown crepe weave contains more than five pigments (br1, s1, b2, v1, bl2) in low percentage combinations. Absolute pigment concentrations are very low in the final fabrics. With 6% of colour masterbatch used in the highly saturated colours, this relates to less than 0.1% w/w of each pigment being present in the yarn, and therefore pigment behaviour may have little influence on the thermal behaviour of the final fabric. Table 23.5 shows results of thermal analysis of the fabrics and the compounded pigments [in low density polyethylene (LDPE)], which indicate that, while some pigments were particularly thermally sensitive and decomposed below 250 1C, this sensitivity was not evident from the onset of degradation data of the whole fabric. The flame retardant fabrics all have similar onset degradation temperatures (Tonset) within a range of 238 and 278 1C and a decomposition temperature of 455–468 1C. These compare with the higher onset value of 382 1C and similar decomposition values of 458 1C for the unretarded sample 1. The char residue at 500 1C of the fabrics is low in most cases, except for those that contain black pigment, for which it is around 3%. It may be surmized, therefore, that the presence of the flame retardant in the other samples is responsible for the general lowering of the onset temperature to the 238–278 1C range from 382 1C in the control. The fabrics that contain flame retardant indicate a two-stage decomposition, presented as DTG (derivative thermogravimetry) maximum in Table 23.5. Thus, the effect of the flame retardant is to decrease the Tonset value by over 100 1C. The maximum degradation temperatures (Tmax) of all the fabrics are consistent within the range 455–468 1C and are independent of flame retardant or pigment presence. The thermal behaviours of the pigments are more varied, with the onset and degradation temperatures of the blue (bl2, bl3) and brown (br1) pigments being generally higher, at around 356 to 384 1C, and maximum degradation temperatures between 474 and 486 1C, respectively. The red (r1, r2) and yellow (y1, y2) pigments have the lower onset degradation temperatures, in the region of

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance

373

200 1C, and maximum temperatures of 440 1C, approximately. The black and white pigmented LDPE compounds have onset temperatures greater than 370 1C and common maximum decomposition temperatures higher than 465 1C, which are close to pure LDPE values; hence, they show little or no sensitization of thermal degradation. The highest levels of char residue were produced from the white, probably inorganic, pigment (e.g. titanium dioxide at greater than 58%), while the black pigments (b1, b2) have between 30 and 40% char residue. The yellow (y1, y2) pigments produce char residues of 8% and 10%. (It is assumed that if a pigment decomposes, usually into volatile species, these may influence the flammability of the overall fabric, either by adding to the fuel content or by interfering with the burning and/or operating flame retardant mechanisms.) The fabric and pigment compounds have major Tmax DTG peak values in the 460 1C range for fabrics and 480 1C for compounded pigments, which probably reflects polypropylene and LDPE degradations, respectively. While the presence of those yellow and red pigments with lower Tonset values in the fabric does not appear to unduly reduce the respective fabric Tonset and Tmax values further than the expected effect of a flame retardant, they could still act as sensitizing agents for thermal degradation. However, all fabrics, except sample 1 (the control) pass the M test. DLOI values are also included in Table 23.5 to enable more direct comparisons with thermal data to be made. However, there appears to be no simple correlation between DLOI values and the presence of a pigment with a particularly low Tonset value. Those fabrics with DLOI 4 1.1 vol% are often associated with the presence of a combination of the yellow (y1), white (w2) and blue (bl2) pigments. The fabric with the lowest onset degradation temperature of 238 1C (sample 5) contains brown (br1), black (b2) and scarlet (s1) pigments, which all have high onset degradation temperatures of over 380 1C. Samples 3, 7 and 12 show the least sensitivity to temperature difference with respective DLOI values of 0.2 and 0.1 vol%. Samples 3 and 7 both contain over 0.5% of one of the white (w1 and w3) pigments, and samples 3 and 12 both contain high percentages (above 0.6%) of the black pigment (b2). It has been noted by Weil14 that pigments may have an effect on the flame retardancy, particularly those which depend on melt-flow-related mechanisms the pigment the pigment inhibits the melt dripping action.

23.4 Conclusions The current halogen-based, flame retarded polypropylene fabrics have LOI values between 23.4 and 27.2 vol% compared to 19.2 vol% for unretarded fabric. The LOI values are affected much less by environment temperatures up to 120 1C and, in some cases, show little or no change. The LOI and DLOI values do not relate to fabric air permeability in spite of the yarn and/or fabric structural variables present, although area densities are effectively constant. However, there is evidence that the DLOI values, and hence potential M test

374

Table 23.5

TGA results of commercial fabrics and pigments. Degradation temperature from DTG peaks (1C)

Char yield at 500 1C (%)

Fabric sample

DLOI (vol%)

Onset

1

Dark Turquoise

1.1

382

458

1.1

2

Navy/Blue

0.6

253

290/456

0.6

3

Black/White

0.2

268

300/467

3.6

4

Green

1.1

265

290/460

0.6

5

Black/Brown

0.9

238

250/468

2.4

6

Blue

1.2

260

295/455

2.1

7

Beige/Brown

0.1

267

298/460

0

Maximum

Pigment number w2 g1 bl1 y1 bl2 b1 r2 w1 b2 w2 bl1 y1 br1 s1 b2 bl2 r1 br1

White Green Blue Yellow Blue Black Red White Black White Blue Yellow Brown Scarlet Black Blue Red Brown

Onset

Maximum

Char Yield at 500 1C (%)

372 424 387 200 356 426 210 390 412 372 387 200 384 391 412 356 220 384

481 485 486 332 482 489 348 483 467 481 486 332 474 459 467 482 489 474

59.0 25.3 28.0 8.0 32.0 39.5 16.0 74.0 34.0 59.0 29.0 8.0 50.0 23.5 340 32.0 20.0 50.0

Degradation temperature from DTG peaks (1C)

Chapter 23

Sample number

Pigment colour in the fabric

Green

1.7

274

300/459

1.5

9

Blue

1.2

262

295/458

0.7

10

Red

1.4

276

303/463

0.9

11

Green

1.9

278

293/464

1.1

12

Black/Brown

0.2

258

292/464

3.6

White Yellow Blue White Yellow Green Blue Red Violet Scarlet Yellow White Blue Yellow Brown Scarlet Black Violet Blue

410 297 387 372 200 424 356 220 290 391 297 372 387 200 384 391 412 290 356 417

484 440 486 481 332 485 482 489 373 459 440 481 486 332 474 459 467 373 482 445

64.0 10.0 29.0 59.0 8.0 25.3 32.0 20.0 17.0 23.5 10.0 59.0 29.0 8.0 50.0 23.5 34.0 17.0 32.0 0

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance

8

w3 y2 bl1 w2 y1 g1 bl2 r1 v1 s1 y2 w2 bl1 y1 br1 s1 b2 v1 bl2 LDPE

375

376

Chapter 23

performance of the fabrics, may be colour sensitive. The black samples with ‘x’ redness reflectance values of about 0.32 have lower DLOI values than those of green fabrics, which have a lower redness reflectance of 0.26. The presence of the flame retardant in the fabrics generally lowers the onset of decomposition temperature of polypropylene from 282 1C to the 238–278 1C range. The pigments, notably the yellows and reds, appear to reduce the onset of decomposition temperature in addition to the fire retardancy effects. In summary, it is clear that while this study has not conclusively identified those parameters which might influence the flammability of fibres in the presence of an external heat flux, it has eliminated the importance of yarn linear densities and fabric constructional variables for a given area density. Furthermore, it has provided more evidence of the possible importance of pigment colour and an indication of the role of some pigments in influencing thermal degradation of the overall filaments and fabrics that contain them. A more scientifically based and detailed study would enable us to establish more fully the role of pigment colour, IR absorption behaviour and thermal stability on the thermal and burning behaviour of polymer matrix.

Acknowledgements The authors thanks the UK Department of Trade and Industry (DTI) and McCleery Technical Yarns for their kind cooperation.

References 1. H.J. Koslowski, Chem. Fibres Int., 2005, 55, 140. 2. BS 5852 – Part 1: 1979 Methods of test for the ignitability by smokers’ materials of upholstered composites for seating. 3. A.R. Horrocks and D. Price eds Fire Retardant Materials, Woodhead Publishing, Cambridge, 2001. 4. Anon., International Dyer 1997 p. 17. 5. BS 7176: 2007. Specification for resistance to ignition of upholstered furniture for non domestic seating by testing composites. 6. BS 5852:2006 Section 4 Ignition Source 5-Fire tests for furniture. Methods of test for the ignitability of upholstered composites for seating by flaming sources. 7. NF P 92-503 Fire protection. Building materials-Reaction to fire tests. Electric heater test used for flexible materials 5 mm thick or less. 8. ASTM E2079-07 Standard Test Methods for Limiting Oxygen (Oxidant) Concentration in Gases and Vapors. 9. A.R. Horrocks, M. Tunc and D. Price, The Burning Behaviour of Textiles and its Assessment by Oxygen Index Methods, Textile Progress, 1989, 18(1–3), 75–86. 10. NF P 92 501-507 December 1985 Fire protection. Classification of building interior materials according to their reaction.

Effect of Yarn, Fabric Construction and Colour in Respect of Red Reflectance

377

11. Simultaneous Thermal Analysis, SDT TA 2960, user manual 1999. 12. Air permeability BSI BS EN ISO 9237:1995 Textiles-Determination of the Permeability of Fabrics to Air. 13. Datacolor SF600 plus CT high-precision, close-tolerance, reference grade spectrophotometer. 14. E D. Weil, Synergists, Adjuvants and Antagonists in Flame Retardant Systems by Edward D. Weil, Polytechnic University, Brooklyn, NY 10522.

Fire Toxicity

CHAPTER 24

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires DAVID PURSER Hartford Environmental Research, 1 Lowlands, Hatfield, Hertfordshire, UK, AL9 5DY

24.1 Introduction Life risk from fires depends upon the probability of fire occurrence and the severity of the fire hazards. Toxic hazards in fires depend upon the mass-loss rate of the burning fuel (which, in turn, depends upon the rates of flame spread and heat release), the yields of toxic combustion products and their effects on exposed subjects.1 Fire retardant treatment of materials and products provides considerable benefits in terms of fire performance.2 The principal benefit for most products treated with fire retardants is an improved ignition resistance for heat or flame exposures up to a design level of intensity or exposure duration, thereby reducing the probability of ignition for a range of common end-use scenarios. Also, if the insult is sufficient for local ignition to occur, the treatment may result in a tendency to self-extinguish or for failure of the fire to spread from the local area of exposure. For situations such as these the toxic and environmental hazards from potential fires are essentially prevented. For situations in which the heat and/or flame exposure exceeds the performance limits of the product a sustained fire may occur. In such cases the fire retardant treatment may provide a further performance benefit by slowing the Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

381

382

Chapter 24

rate of flame spread or fire growth compared to that of an untreated product. In large-scale fire experiments it has been observed that this slowing may occur as a general slowing of the rate of fire spread and growth, effectively provide a slower t2 fire growth curve.3 Alternatively, in some situations [observed in the combustion behaviour of upholstered furniture (CBUF) project4], a given ignition source large enough to ignite an object may result in a small, steady, very slowly spreading fire. This grows almost imperceptibly over a period of many minutes, until a threshold heat-release rate is achieved beyond which a rapidly growing t2 fire curve is obtained. Performance such as this therefore reduces the rate of hazard development from fire effluents and heat, which increases the time available for an occupant to escape. The application of fire retardant systems to a wide range of products can be seen as having two kinds of overall benefits:
For situations in which products in wide general use are inherently combustible, such as clothing and furniture, the application of fire retardants reduces overall fire risk.
For other applications, such as some construction products, fire retardants can be used to provide an acceptable level of performance for products that could not be used in their untreated form for specific applications. Despite these benefits, fire retardant systems show certain limitations. As stated above, they do not provide non-combustibility in a product, but ignition resistance to a design limit. If this limit is exceeded a propagating fire may result, and in some situations rapid fire spread may occur. If a fire does occur (depending upon the fire retardant system used), the yield of common toxic fire gases from the base material [such and carbon monoxide (CO) and hydrogen cyanide (HCN)] may be increased, and the fire retardant may decompose to release additional toxic products, such as acid gases. In addition to threatening the life safety of occupants, these products may contaminate the local environment (such as a building interior) with corrosive substances or contaminants hazardous to health, or be dispersed more widely, which leads to general environmental pollution.2 A further complication is that fire retardant compounds may be toxic and may leach out from products during general use or during manufacture or disposal, which leads to environmental contamination. The challenge with respect to the development and application of fire retardant systems is therefore to ensure that the societal benefits in terms of reduced fire risk and reduced fire hazards in some situations, are not offset by increased toxic fire hazards in other situations. Neither should they be offset by unacceptable environmental contamination, either as a result of release during the product life-cycle or during fires. To achieve this it is important to apply small- and large-scale fire tests and methods of toxic hazard analysis able to provide realistic assessments of fire risk and hazards from full-scale fires. While a considerable effort is applied to product testing for ignitability and reaction-to-fire performance, less attention has been paid to the behaviour of materials and products in terms of toxic

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

383

product yields and toxic and environmental hazard development over a range of fire scenarios. In this chapter I therefore focus on the influence of fire retardants on toxic product yields in fires and their implications for the development of toxic hazard and environmental contamination.

24.2 Major Determinants of Toxic Product Yields Toxic product yields in fires depend mainly upon the elemental composition of the material and the decomposition conditions in the fire. The organic composition and the presence of fire retardants also have an influence. For these reasons it is important to measure toxic product yields from materials and products under the full range of combustion conditions likely to be encountered in end-use situations. In terms of basic fire scenarios, fire chemistry and toxic hazards it is possible to classify fires into a small set of types defined in terms of flaming behaviour, compartment ventilation and temperatures, fuel–air equivalence ratios and CO–CO2 ratios, as shown in Table 24.1 from ISO/IEC 19706:2006.5 An experimental method is therefore needed (preferably a small-scale method) that can be used to measure the yields of smoke and toxic gases from materials and products over a wide range of thermal decomposition and combustion conditions for input into fire engineering calculations, workplace hygiene and environmental impact assessments. The BS7990 tube furnace method (ISO/IEC TS19700-2006; Controlled equivalence ratio method for the determination of hazardous components of fire effluents)6,7 was developed specifically for this purpose. The method has been applied to a wide range of materials and large and full-scale experiments have been used to validate the small-scale test data.8–10 As with reaction to fire tests, there is a need to develop standard full-scale hazard tests as reference scenarios for toxic hazards. This topic is under consideration within ISO TC92/SC3, but current practice is based upon ad hoc tests designed to replicate specific case scenarios. The basic findings from the existing body of published work are that the most important types or stages of fire are:
non-flaming oxidative or smouldering decomposition;
well-ventilated flaming;
vitiated flaming (pre- and post-flashover). The main characteristics of these are:
Well-ventilated flaming provides efficient combustion, which produces mainly CO2 and water with low yields of all toxic carbon and nitrogen compounds (other than NOx) – except for materials that contain halogenated fire retardants.

384

Table 24.1

Classification of fire types from ISO/IEC 19706:2006.

Fire stage Non-flaming 1a. Self-sustained smouldering 1b. Oxidative, external radiation 1c. Anaerobic external radiation Well-ventilated flaming 2. Well-ventilated flaming Under-ventilated flaming 3a. Low-ventilated room fire 3b. Post-flashover

Heat (kW m–2)

Maximum temperature (1C)

Oxygen (%)

Fuel

Smoke

In

Out

Equivalence ratio f

n.a.

450–800

25–85

20

0–20

–

–

300–600

20

20

o1

–

100–500

0

0

441

0–60

350–650

50–500

B20

0–20

0–30

300–600

50–500

15–20

50 to 150

350–650

4600

o15

V CO a V CO2

Combustion efficiency (%)

0.1–1

50–90

o1

o0.05

495

5–10

41

0.2–0.4

70–80

o5

41

0.1–0.4

70–90

a

The use of CO to CO2 ratios (or their inverse) to characterize a fire stage is only applicable to materials which do not contain chlorine or bromine, since these elements significantly increase the CO yield even in well-ventilated fires.

Chapter 24

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

385


Under non-flaming or vitiated flaming conditions (phi41) yields of toxic carbon compounds are high [these include CO, organic irritants, benzene, isocyanate derivatives, halogenated benzene and phenols (dioxin precursors)].
Fire retardant systems that act in the vapour phase (especially halogens) reduce combustion efficiency and produce high yields of toxic carbon compounds under all conditions.
Acid gases tend to be produced at high yields under all combustion conditions. For materials that are not fire retardant under flaming combustion conditions the yields of toxic carbon compounds depend upon the fuel:air equivalence ratio (phi), with low yields of carbonaceous products under wellventilated conditions and high yields under vitiated combustion conditions. Figure 24.18 illustrates the dependency of CO yield on equivalence ratio for polymethylmethacrylate (PMMA). The left side shows data from the BS7990/ ISO17900 tube furnace compared with data obtained by Tewarson using the Factory Mutual (ASTM E2058) and BRI apparatus.11 The right side shows a validation comparison of the relationship between CO yield and equivalence ratio between tube furnace data and data from compartment fires obtained by Beyler and Gottuk, described in reference 12. For PMMA the CO yield increases by a factor of around 50 between well-ventilated and vitiated combustion conditions. The effects of different fire types on the overall yields of toxic products are illustrated in Figure 24.2. This shows 30-minute LC50 concentrations, calculated 100

0.35

CO yield (g CO/g fuel)

CO yield vc/yield wv

0.3

10

0.25 0.2 0.15 0.1 Large scale (Gottuk)

Factory mutual apparatus BSD7990 tube furnace

0.1

Figure 24.1

1

1 Equivalence ratio

Large scale (Beyler)

0.05

BS7990 Tube furnace

0 10

0

1 2 3 Equivalence ratio

4

Comparison of relationship between CO yield and equivalence ratio for PMMA in the BS7990 tube furnace, Tewarson experiments and in Gottuk and Beyler compartment fire experiments.

386

Chapter 24 180 Well ventilated flaming Small vitiated flaming Post flashover flaming Non-flaming

160 140

LC50 g/m^3

120 100 80 60 40 20 0

wood

LDPE

LSF

PVC

Figure 24.2 30-minute LC50 concentrations calculated from BS7990 tube furnace data for thermal decomposition and combustion products from three cable materials compared with those from wood (left of each pair expressed as mass loss, right as mass charge).

according to the method of Purser,13,14 from BS7990 tube furnace data for thermal decomposition and combustion products from three cable materials [low density polyethylene (LDPE), a low smoke and fume (LSF) cable material that contained aluminium hydroxide trihydrate, and a plasticized polyvinylchloride (PVC) that contained calcium carbonate] compared with those from wood.15 The results show that for all materials the least toxic condition (i.e. the condition that produces the highest LC50 concentration) is well-ventilated flaming, which tends to destroy toxic organic products, while small vitiated (i.e. pre-flashover vitiated), post-flashover vitiated and oxidative non-flaming conditions all produce somewhat more toxic atmospheres because of the higher yields of CO and irritant organic species. From a work-place hygiene and environmental perspective it has been found that atmospheres that result from non-flaming and vitiated flaming combustion tend to be rich in a wide range of toxic organic compounds (such as benzene, aldehydes and aromatics), and that halogenated materials produce high yields of dioxin and furan precursors (halogenated benzenes and phenols). Figure 24.2 also illustrates that the toxic product yield and toxic potency of a material or product can usefully be expressed in a number of different ways,

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

387

depending upon the use to which the data are to be put. The usual method used to express yield is in terms of the mass of toxic product per unit mass of material decomposed (e.g. kg CO/kg fuel mass loss). This is useful in understanding the toxicity and of the products released during a fire and for fire engineering hazard calculations. However, for the application of products of similar density it may be useful to express yield in terms of the mass of material used rather than the mass consumed (or in terms of material area or volume, depending upon the nature of the product). In Figure 24.2 the toxic potencies are expressed in terms of both mass loss and mass charge, which show significant differences for materials not fully consumed during combustion. The densities of the four materials tested were wood 0.52, LDPE 0.92, LSF 1.57 and PVC 1.55.

24.3 Influence of Different Fire Retardant Systems on Toxic Product Yields and Toxic Hazards 24.3.1

Inert and Active Fillers

With respect to toxic product yields the ideal fire retardant system acts in the solid phase and minimizes the release of organic fuel vapours and acid gases. The simplest form of fire retardancy, useful in some applications, is an inert filler, which reduces the organic content of the fuel and hence its ignitability. Further benefits are obtained if the filler also enhances the fire retardant properties by such mechanisms as water release or promotion of char formation. Figure 24.2 shows two examples of the effects of fillers in reducing toxic product yields and toxicity. The LSF cable material tested contained aluminium hydroxide trihydrate. This conveyed an improved ignitability performance for the material, which was overcome in the tube furnace tests to obtain flaming decomposition. When the toxicity of the products is expressed in mass charge terms, the toxic potency is considerably reduced compared to that when expressed in mass loss terms because of the presence of the toxicologically inert filler. This effect does not show for LDPE, which is 100% combustible, so that the mass loss and mass charge toxic potencies are almost identical. The same effect shows, to some extent, for the plasticized PVC tested, which contained a calcium carbonate filler, so that the LC50 concentrations expressed as mass charge are almost double those expressed as mass loss. In these experiments the calcium carbonate was found to exert a further beneficial effect in reducing the yield of hydrogen chloride (HCl) under some fire conditions. The effect was lost at high temperatures, at which the carbonate decomposed into calcium oxide and did not then react with the HCl. The yields of key toxic products from LDPE and LSF are compared in Table 24.2. Table 24.3 illustrates the reduction in toxic product yields and toxic potency obtained using a borax–boric acid treatment of cotton twill, which produces a glass-like layer on the solid that inhibits fuel loss when tested in the tube furnace.

388

Table 24.2

Toxic product yields (mass charge) and toxic potency of 100% organic cable material (LDPE) compared with a LSF cable material containing aluminium hydroxide. Adapted from Purser et al.15

Material and decomposition condition LDPE Non-flaming 350 1C Well-ventilated 650 1C Small vitiated 650 1C Post-flashover 825 1C LSF Non-flaming 350 1C Well-ventilated 650 1C Small vitiated 650 1C Post-flashover 825 1C

LC50 (g m3 mass charge)

LC50 (g m3 mass loss)

CO (g g1)

CO2 (g g1)

Organic carbon (g g1)

Smoke (OD g1 m2)

0.16 0.00 0.06 0.07

0.03 2.58 0.48 0.48

0.69 0.14 0.70 0.68

0.59 0.17 0.10 0.31

17 70 28 24

16 70 28 24

0.04 0.00 0.03 0.05

0.09 0.98 0.33 0.38

0.17 0.04 0.15 0.20

0.17 0.07 0.07 0.08

60 168 73 50

25 100 35 31

Chapter 24

Toxic product yields (mass charge) and toxic potency of cotton twill untreated and treated with borax–boric acid under non-flaming and flaming conditions at 400 1C and 700 1C at a mass charge concentration of 20 g m3. Adapted from Wright.16 a

Material and decomposition condition Cotton twill Non-flaming 400 1C Non-flaming 700 1C Well-ventilated 700 1C Cotton twill borax–boric acid Non-flaming 400 1C Non-flaming 700 1C a

CO (g g1)

CO2 (g g1)

Acrolein (g g1)  1000

Formaldehyde (g g1)  1000

Smoke (OD g1 m2)

LC50 (g m3 mass charge)

LC50 (g m3 mass loss)

0.21 0.33 0.04

0.75 0.70 1.61

2.10 5.83 0.00

6.25 5.24 0.00

0.16 0.06 0.00

23 14 57

21 14 55

0.11 0.24

0.36 1.06

0.47 0.82

1.76 1.51

0.01 0.00

51 25

30 22

T Wright. Environmentally friendlier flame retardant systems. PhD Dissertation, Leeds University, Leeds UK, 1997.

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

Table 24.3

389

390

24.3.2

Chapter 24

Phosphorus-based Systems

A variety of phosphorus-containing fire retardant systems are used that involve phosphorus alone or in combination with nitrogen or halogens. In terms of toxic hazard development it is important to consider individual materials or products and product combinations. For example, the fire performance of upholstered furniture often depends upon the combined performance of covering materials and the underlying foam. Both of these may be treated with different fire retardant systems to obtain the desired performance. In terms of toxic product yields, phosphorus-based systems tend to act both in the solid phase, promoting char formation, and also to some extent in the gas phase. To the extent that they promote char formation they should reduce emissions of ‘‘normal’’ toxic fire gases, and phosphorus locked into the char is not released as potentially toxic phosphorus compounds. However, gas-phase products that contain phosphorus are released during fires.2 It is also known that some phosphorus compounds used historically as fire retardants are themselves neurotoxic or carcinogenic, and certain organophosphorus compounds are know to be very highly toxic.17 It is therefore important to establish what phosphorus compounds might be released into the gas phase during combustion and what toxic effects they might have. Although some organophosphorus compounds have been detected in the combustion products from burning materials, the general finding is that most phosphorus is oxidized to phosphorus pentoxide (P2O5), which then hydrolyzes to phosphoric acid (H3PO4). This is moderately toxic (1 hour LC50 1.2 g m3), and may make some small contribution to the overall toxicity of fire effluents from treated materials. Phosphine (PH3) is another toxic compound detected in some fire effluents [highly toxic oedemogen 1 hour rat LC50 44 parts per million (ppm)].2,17 In general, there has been little evidence for significant toxic effects from materials treated with phosphorus-containing fire retardants other than those that might be predicted from the ‘‘normal’’ combustion products present. However, there was some evidence for an ‘‘unpredictable’’ increased toxic potency in experiments carried out by Kallonen on cotton treated with phosphorus-containing fire retardant compounds (see Purser et al.2). One problem is that experiments have not been designed to test for the neurotoxic effects possibly expected from organophosphorus compounds. The one serious exception is the finding that any phosphorus source when combined with a trimethylol polyol can produce a potent neurotoxic class of bicyclophosphate esters in combustion products (see Figure 24.3). In practice, this has been a potential problem only with certain turbine lubricants that contain trimethylol (see Purser et al.2).

24.3.3

Nitrogen, Melamine and Melamine–chlorinated Phosphate Systems

Other toxicity issues regarding phosphorus-based fire retardants relate to the nitrogen, halogens and antimony often used with them. Where nitrogen is present in any material a proportion is released as HCN under vitiated

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

R

H2 C C C H2 C H2

R: LD50: mg/kg i.p. (see Ref:18) Rat LC50 (see Ref:19)

O O

P

391

O

O

CH3 32

C2H5 1.0

C3H4 0.38

C4H9 1.5 ISO 0.18

HOCH2 >500

0.035 mg l–1–1 hour exposure

Figure 24.3 Bicyclophosphate esters and toxicity. i.p. is intraperitoneal injection. combustion conditions. This is an issue with polyurethane (PU) foams, which contain a significant nitrogen content, especially in melamine foams. Under well-ventilated combustion conditions HCN yields are low, but some nitrogen is released as NOx. Experimentally, most NOx is released from fires in the form of NO,9 which has a low toxic potency. This is then slowly oxidized to give NO2, but currently it is considered that because of the generally low overall NOx yields obtained and the low proportion of NO2, oxides of nitrogen are not usually a major factor in combustion product toxicity. The yield of HCN in fires has been found to increase with phi in much the same way as that of CO.9,20 This is illustrated in Figure 24.4, which shows the relationship between the efficiency of the conversion of fuel nitrogen into HCN compared to the efficiency of the conversion of fuel carbon into CO for a variety of materials that contain both carbon and nitrogen. As a general rule, it is suggested that the percentage conversion of N into HCN in fires is likely to be approximately the same as the percentage conversion of C into CO. It is considered that when the nitrogen content of any burning fuel exceeds around 1% of the total burning fuel mass, then it is likely to contribute significantly to the overall toxic hazard in terms of time to incapacitation for an exposed subject. Since toxic hazard depends upon both toxic product yields and fire growth rate it is necessary to consider both aspects in hazard modelling or to evaluate performance directly in full-scale fire tests using realistic end-use scenarios. A series of full-scale tests carried out on upholstered armchairs in a test house shows the importance of this approach. The armchairs were constructed using different combinations of PU foams [untreated high resilience (HR) foams and a melamine–chlorinated phosphate foam], with different covers, including untreated cotton and acrylic fabrics, fire retardant cotton containing 4.4% bromine and 0.9% chlorine and back-coated acrylic covers (3.6% chlorine 2.4% bromine). The results are summarized in Figure 24.5 in terms of calculated time to incapacitation for an occupant of the fire room. The general findings are that using the combustion modified foam under untreated covers has little influence on predicted time to incapacitation, but that the

392

Chapter 24 0.5

CMHR PUfoam

0.4

CN recovery (fraction)

Polyamide PIR foam 0.3

MDF Boucle (acrylic,wool,PE) Boucle-FR

0.2

Velour (acrylic,cotton,PE) Polyacrylonitrile MDF 10 or 12% O2

0.1

MDF-FR

0.0 0.0

0.1

0.2

0.3

0.4

0.5

CO recovery (fraction)

Figure 24.4 Relationship between conversion efficiency (recovery fraction) of fuel nitrogen into HCN and fuel carbon into CO for a range of nitrogencontaining polymers – BS7990 tube furnace method.

combination of either fire retardant-cotton or fire retardant-backcoated acrylic covers increases time to incapacitation by approximately two minutes. One concern regarding melamine foam systems was that the increased nitrogen content might lead to higher cyanide concentrations and shorter tenability times, but based upon these results and later work this does not appear to be the case. A further finding of relevance to both toxicity and toxic hazard testing, and to reaction to fire tests, was that the combustion pattern and yields of CO and HCN show a significant degree of interaction between the fire compartment and fuel behaviour. The yield of CO from the chairs treated with fire retardant might have been predicted to have been higher than that from the untreated chairs. However, because the untreated chairs burned more before the fires self-extinguished, the degree of vitiation in the system was greater for the chairs that are not fire retardant, and provided higher equivalence ratios and hence higher CO and HCN yields. Findings such as these emphasize the need for full-scale reference scenario tests that provide a close approximation to end-use situations.

393

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires Time to detection and effect in lounge

11 CM non FR acrylic obscuration irritancy asphyxia

21 HR nFRacr,cush

23 CM nFRacr furn

17 CM Frcot

18 CM FRDra

16 CM FRcot 0

1

2

3

4

5

6

7

Time (min)

Figure 24.5 Comparison of time to effect in burn room (lounge) for armchair with covers that are not fire retardant covers (11, 21 and 23) and fire retardant covers (17, 18 and 16). Fires were conducted in an enclosed apartment (11) or house (16–23) with the fire room door open (except for 16).

24.3.4

Halogen Acid Vapour-phase Systems

For halogenated materials and materials treated with halogen-based fire retardant systems (especially chlorine and bromine systems), inhibition of combustion in the vapour phase results in inefficient combustion. This increases the yields of toxic carbon and nitrogen compounds under all fire conditions. In addition, irritant acid gases are released, which further contribute to the effluent toxicity. The presence of the halogens somewhat reduces the organic content of the material (especially for PVC and fluoropolymers) which, to some extent, offsets the increase in yields of carbonaceous products. The conversion efficiency of polymer halogens into vapour-phase acid gases is often high (approaching 90–100%) under all combustion conditions. A result of this is that these systems tend to provide a degree of ignition and flamespread resistance up to a design level of performance. When subjected to local heating or ignition, flame spread may be slow or self-extinguishing, but if the general environment reaches high temperatures, or if the local heat source becomes large, so that fuel halogens are driven off, then the base polymer may burn more readily. This effect has been observed in experiments with a

394

Chapter 24

brominated fire retardant system. When specimens of the material were burned in the tube furnace, high yields of brominated fumes were released, which enabled the base polymer to flame. In a room corner wall-lining configuration in the (open-roofed) Single Burning Item test the material performed quite well, with a slow flame-spread rate. However, when the material was burned in a lined ISO 9705 room (which has walls and a ceiling), although it performed quite well in terms of flame spread for a period, once the fire size and upper layer temperature reach a critical stage, a sudden flashover occurred.3 Findings such as these again emphasize the importance of ensuring that systems are tested in relevant full-scale scenarios. The effects of halogens on toxic product yields and toxic potency for nonplasticized and plasticized PVC are illustrated in Figure 24.2 and Table 24.4 (tube furnace data). The general finding is relatively high yields and conversion efficiencies of fuel carbon into CO and organic products, and fuel chlorine into HCl under all fire conditions. The result, as shown in Figure 24.2, is that the toxic potency is generally somewhat higher than that for other (non-nitrogen containing) materials. Figures 24.6 and 24.7 illustrate the relationships between CO and HCN yield, and the equivalence ratios for a variety of materials tested in the BS7990 tube furnace. The results show that for materials that are not fire retardant the yields are very low under well-ventilated (low phi) combustion conditions, but increase rapidly under vitiated combustion conditions (phi 4 1). For two materials that contain significant concentrations of halogens [PVC 57% Cl and Boucle (acrylic–wool–polyester) fabric (backcoating 1% Cl, 6% Br)], the CO yields (and HCN yield for the Boucle fabric), are high under well-ventilated conditions and throughout the phi range. Other materials that contain small amounts of halogens also show somewhat increased CO and HCN yields under well-ventilated combustion conditions. Figure 24.8 shows the relationships of carbon and nitrogen conversion efficiencies to CO and HCN, versus percentage polymer chlorine and bromine content, for a variety of polymers, illustrating a relationship between halogen content and conversion efficiency irrespective of polymer type. Figure 24.9 shows that the same effect occurs for the conversion efficiencies of fuel carbon into organic products and particulates. Figure 24.10 shows the effects on toxic potency in terms of 30-minute LC50 concentrations calculated using the Purser method.13 Non-fire retardant treated materials show high values (low toxic potencies).

24.3.5

Fluoropolymers and Ultrafine Particles (Nanoparticles)

Fluoropolymers are naturally fire-retarded materials with excellent reaction to fire properties. However, they produce toxic acid gases including carbonyl fluoride, hydrogen fluoride and perfluoroisobutylene when thermally decomposed.22 Inhalation of these at sufficiently high doses results in fatal lung oedema and inflammation. The overall effect is that the toxic potency of fluoropolymer thermal decomposition and combustion products is approximately 10 times that of

Toxic product yields and toxic potencies (mass loss) of different PVC types under a range of fire conditions using DIN and Purser tube furnaces.

Material and decomposition condition PVC (non-plasticized)21,23 Non-flaming 3 W Well-ventilated 6 W PVC (plasticized)15 Non-flaming 380 1C Flaming 650 1C PVC (plastic+CaCO3)15 Non-flaming 350 1C Well-ventilated 650 1C Small vitiated 650 1C Post-flashover 825 1C

CO (g g1)

CO2 (g g1)

0.01 0.17

HCl (g g1)

Organic carbon (g g1)

Smoke (OD g1 m2)

LC50(g m3 mass charge)

0.56 0.55

0.02 0.31

0.00 2.39

0.36 0.53

0.01 0.06 0.08 0.17

0.09 1.27 0.74 0.83

0.32 0.16 0.17 0.41

LC50(g m3 mass loss) 8 7

0.44 0.12 0.33 0.29

0.76 0.29 0.30 0.34

20 8

15 7

28 34 29 18

12 22 16 9

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

Table 24.4

395

396

Chapter 24 Relationship between Phi and CO yields 0.55 0.50 0.45

Yield CO (g/g)

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.0

0.5

Boucle(acrylic,wool,PE) LDPE LDPE 850°C 10 or 12% O2 PAN PIR 850°C Polyamide 10 or 12% O2 PMMA, 10 or 12%O2 PMMA, 850°C 10 or 12% O2 plywood

1.0

1.5 Phi

2.0

Boucle - FR LDPE 10 or 12% O2 MDF PIR foam, 700°C PMMA polyamide, 850°C PMMA 850°C PVC Velour (acrylic,cotton,PE)

2.5

3.0

CMHR PU foam LDPE 850°C MDF 10 or 12% O2 PIR foam 10 or 12% O2 polyamide 6 polyamide 850°C 10 or 12% O2 Polystyrene wood

Figure 24.6 CO yields against equivalence ratio under flaming conditions for a variety of materials tested in the BS7990 tube furnace.

most common polymers. Also, under a specific set of conditions and decomposition temperatures (450–650 1C with recirculation of the fume through the hot zone), perfluorinated polymers produce high concentrations of ultrafine (nano) particles (10–150 nanometres diameter). These produce fatal lung inflammation with a very high toxic potency of around 0.015 g m3. This is approximately 1000 times the toxic potency of combustion products from wood and similar materials.22 Ultrafine particles are now known to form a significant fraction of the range of particles in most combustion-product atmospheres. They are currently of considerable interest as a form of environmental air pollution considered responsible for a significant proportion of acute cardiovascular and respiratory deaths annually in the UK, the death rate increasing proportionally to the concentration of fine particles in the atmosphere.23 This may have implications for the development of nanoclay fire retardant systems in that it may be important to ensure that combustion of these materials does not result in the release of ultrafine clay particles.

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires 0.11

397

CMHR PUfoam PIR foam, 700°C

0.10

polyamide 6 MDF

0.09

Boucle (acrylic,wool,polyester) Boucle FR

0.08

Velour (acrylic,cotton,polyester) Polyamide 6, 12 and 10%O2

Yield HCN (g/g)

0.07

MDF 12 and 10%O2 Polyamide 6850°C

0.06

Polyamide 6850°C 10 or 12%O2 r

0.05

PAN PAN 850°C

0.04

MDF 850°C

0.03 0.02 0.01 0.00 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Equivalence ratio

Figure 24.7 HCN yields against equivalence ratio under flaming conditions for a

CO and HCN recovery fraction

CO recovery fraction

variety of materials tested in the BS7990 tube furnace.

0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0 20 40 60 Chlorine or bromine content (%)

Figure 24.8

0.12 HCN recovery

0.1 CO recovery

0.08 0.06 0.04 0.02 0 0

4 6 2 8 chlorine or bromine content %

Relationship between halogen content and recoveries of HCN and CO from five different materials [PMMA, medium density fibreboard (MDF), PU foam, polyisocyanurate (PIR) foam and Boucle fire retardant fabric] under well-ventilated conditions (phio1) tested in the BS7990 tube furnace.

Chapter 24

0.25

Relationships between halogen content and recovery fraction of CO and total organics in effluent

0.2 0.15 0.1 CO recovery fraction 0.05

organic recovery fraction

0 0

10

20

30

40

% chlorine or bromine

Figure 24.9

24.3.6

50

60

Relationship between halogen content and recovery fraction of CO and particulates Fraction of fuel carbon as smoke particulates

recovery fraction of organic carbon

398

0.25 0.2 0.15 0.1

Partculate recovery fraction CO recovery fraction

0.05 0 0

10

20 30 40 % chlorine or bromine

50

60

Relationship between halogen content and recoveries of total organic carbon compounds and carbon monoxide from a six different materials (PMMA, MDF, PU foam, PIR foam, Boucle fire retardant fabric and PVC) under well-ventilated conditions (phio1).

Environmental Contamination by Dioxins and Furans from Halogenated Materials

Combustion of any material that contains chlorine or bromine produces a range of products which are not immediately hazardous to life safety in a fire, but which may contaminate a building interior or the wider environment with toxic substances hazardous to health over prolonged periods, such as polychlorinated biphenyls (PCBs), and halogenated dioxins and furans. With respect to unwanted fires there are two particular issues: 1. To what extent does the production, use and disposal of organohalogen compounds, particularly PVC, lead to general levels of environmental contamination by dioxins, dibenzofurans and phthalates, which constitute a risk to public health? 2. To what extent does the exposure of people to combustion products from PVC and other halogenated materials during and after fires in buildings constitute a risk to their long-term health? These issues are reviewed in more detail in Purser,2 but the general position is that with regard to the first point, PVC disposal does not appear to have been significant as a major factor in the background level of soil and vegetation contamination by dioxins in the past (see Figure 24.11). However a range of other major historical sources of environmental dioxin contamination has now been eliminated. This raises the question of whether halogenated materials could become a problem for the future. Just one product – plenum cable (much of it PVC) expressed in terms of millions of metres increased from around 500 to 1300 between 1991 and 1996, at an annual growth rate of 46% with a total of 2700 million metres in 2000 compared with 108 million metres in 1983

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

399

Phi vs. LC50 concentrations for some materials 100

PMMA PMMA @12 and 10% O2 wood wood @12 and 10% O2 polyamide polyamide @12 and 10% O2

90

Rat 30-min LC50 concentration (g/m3)

80

Velour fabric PU foam boucle fabric

70

Boucle-FR PVC MDF MDF@12and 10% O2

60

50

LDPE 40

30

20

10

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Phi

Figure 24.10

Calculated 30-minute LC50 concentrations versus phi for a range of materials at low phi values and higher toxic potencies under higher phi conditions. PVC and the fire retardant Boucle fabric show high toxic potencies approximately constant throughout the range.

(a 25-fold increase).2 At some point all this material will need to be disposed of and a proportion is likely to become involved in accidental or unauthorized fires. Yields of dioxins and furans are very dependent upon the combustion conditions in a fire. Low temperature (300–400 1C) vitiated fires produce high yields. An Environment Protection Agency (EPA) study showed very high yields of dioxins and furans from inefficiently burning fires that involved the burning of household waste (4.5% PVC content) in a 55 gallon oil drum. In one experiment the total yields of polychlorinated dibenzoparadioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) was 493 ng g1 waste decomposed compared with 0.0035 ng g1 waste decomposed in a modern incinerator, a

400

Chapter 24 40

400000 Grass PCDD/Fng/kg

Grass total PCDD/F

300000

30

Food pg TEQ/g lipid PVC production (tonnes x 10^5)

25 20

200000

15 10

100000

5 0 1850

1900

1950

PVC production and TEQ lake and food

35

Lake sediment pg TEQ/g

0 2000

Year

Figure 24.11

Trends in environmental dioxins compared with annual PVC production.

factor of more than 100 000.24 One household using this method could produce more dioxins than the entire output of a modern municipal waste incinerator. As stated, using the tube furnace we have found high concentrations of dioxin precursors (halogenated benzenes and phenols) when materials are combusted under vitiated conditions. It is considered that this provides a useful method to measure the yields of dioxins and other environmental contaminants from materials decomposed over a range of combustion conditions. Studies of fire scenes, such as the Dusseldorf airport fire, have revealed significant levels of dioxin contamination in soot residues. This raises issues for the health and safety of people working in such environments (such as fire investigators and fire brigade personnel), for methods of post-fire decontamination and their effectiveness, and for the health of building occupants after restoration. Table 24.5 summarizes the findings from an examination of levels of contamination at several sites and the calculations of possible uptake by exposed individuals at the scene through smoke inhalation, dust inhalation or oral intake. The basic findings were that a person near or inside such a building for one hour is unlikely to receive a significant dioxin and furan exposure dose (pgTEQ) through smoke inhalation, that there could be a minor hazard from dust inhalation but that a potentially significant dose (300–10 000  maximum acceptable daily intake of 700 pgTEQ per day if 0.1 g of soot was ingested). The WHO recommended maximum daily intake has since been decreased by a factor of 10 to 70 pgTEQ per day. These findings demonstrate that a potential hazard may exist at fire scenes and that personnel should take care to avoid dust inhalation or ingestion by the use of simple hygiene precautions (dust particulate masks and gloves – with careful hand washing after exposure).

Possible total dioxin and furan intake (pgTEQ) for a person near or inside a building during or after a fire.

Dioxin concentration of smoke particles and soot(ngTEQ g–1) Old Swedish incinerator plume (assuming 1/100 plume dilution factor) EPA household waste study 700–7000a EPA household waste study 700–7000a German fire residue maximum 200 EPA household waste study 700–7000a German fire residue maximum 200 a

Dose received(pgTEQ)

Fraction of maximum acceptable daily intake (700 pgTEQ day1)

0.05–0.40 ngTEQ m3 diluted smoke inhalation for 1 hour (1 m3)

50–400

0.07–0.57

Inhalation 100 m visibility smoke dilution 2–20 ngTEQ m3 for 1 hour (1 m3) Dust inhalation 1 mg m3 for 5 hours (5 m3) Dust inhalation 1 mg m3 for 5 hours (5 m3) Oral intake 0.1 g soot Oral intake 0.1 g soot

2000–20 000

2.9–29

3300–33 000

4.7–47

1000

1.4

70 000–700 000 20 000

100–1000 28.6

Intake

Assuming TEQ ¼ total dioxin and furan content  0.1.

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

Table 24.5

401

402

Chapter 24

Potential hazards from dioxins, furans and polycyclic aromatic hydrocarbons (PAHs) in relation to health and safety of workers at the scenes of the Dusseldorf and Lengerich fires were examined in some detail following these incidents.25–27 The findings were that the cancer risk from polyhalogenated dibenzoparadioxins (PHDDs) and polychlorinated dibenzofurans (PHDFs) was substantially lower than that from PAHs, but as already stated, both types of pollutants are strongly bound to soot and have a low bioavailability. No chronic toxic effects have been reported from individuals accidentally involved in single fire incidents and a German study has shown professional fire fighters to have no higher dioxin blood levels than the dioxin background levels in the general population.28 These findings support the conclusion that exposure at a single fire incident is unlikely to present a serious chronic health hazard from dioxins, furans and PAHs, and that simple hygiene precautions can protect professionals from the potential effects of repeated exposures.

24.4 Conclusions
Fire retardant systems produce considerable benefits in terms of reduced fire incidence and improved product performance.
They have some limitations in that they cannot achieve non-combustibility. In some situations this may lead to the release of toxic substances that affect fire safety and cause environmental contamination, while in others a rapid rate of fire growth may occur if the design limit is exceeded.
The BS7990 (ISO TS19700) tube furnace provides a small-scale test method able to measure product yields of toxic and environmental contaminants over a wide range of fire conditions for application to engineering calculations and product development.

References 1. D.A. Purser, Toxicity Assessment of Combustion Products. The SFPE Handbook of Fire Protection Engineering 3rd ed., P.J. DiNenno (ed.), National Fire Protection Association, Quincy, MA 02269, 2002, pp. 2/83 – 2/171. 2. D.A. Purser, Toxicity of fire retardants in relation to life safety and environmental hazards, In: Fire Retardant Materials, A.R. Horrocks and D. Price Ed., Chapter 3, Woodhead Publishing Ltd, Cambridge UK, 2001, pp. 69–127. 3. OPDM Building Regulations Division Project The production of smoke and burning droplets from products used to form wall and ceiling linings. BRE report 213073. 4. Fire Safety of upholstered furniture – the final report on the CBUF research programme. Ed: B. Sundstrom. Interscience Communications Ltd. London. 5. ISO/IEC 19706:2006: Guidelines for assessing the fire threat to people. 6. BS 7990:2003 Tube Furnace method for the determination of toxic product yields in fire effluents.

Influence of Fire Retardants on Toxic and Environmental Hazards from Fires

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7. ISO TS 19700:2006 Controlled equivalence ratio method for the determination of hazardous components of fire effluents. 8. D.A. Purser, (2002) ASET and RSET: addressing some issues in relation to occupant behaviour and tenability. 7th International Symposium On Fire Safety Science, Worcester Polytechnic Institute –Worcester Massachusetts, USA 16–21 June 2002. FIRE SAFETY SCIENCE – Proceedings of the seventh international symposium. International Association for Fire Safety Science, 2003, 91–102. 9. D.A. Purser and J.A. Purser, The potential for including fire chemistry and toxicity in fire safety engineering. BRE report no 202804. 28th March 2003. 10. A.A. Stec, T.R. Hull, J.A. Purser and D.A. Purser, Comparison of Toxic Product Yields from Bench-Scale to ISO Room. Fire and Materials. 2008, 32, 49–60. 11. A. Tewarson, ‘‘Generation of Heat and Chemical Compounds in Fires,’’ The SFPE Handbook of Fire Protection Engineering 3rd ed., P.J. DiNenno (ed.), National Fire Protection Association, Quincy, MA 02269, 2002, pp. 3/82–3/161. 12. W.M. Pitt, ‘‘The Global Equivalence Ratio Concept and the Formation Mechanisms of Carbon Monoxide in Fires’’, Prog. Energy Combust. Sci., 1995, 21, 197–237. 13. D.A. Purser, Interactions among carbon monoxide, hydrogen cyanide, low oxygen hypoxia, carbon dioxide and inhaled irritant gases. In Carbon Monoxide Toxicity . Ed. David G. Penney, Chapter 7 CRC Press Ltd. Boca Raton, Florida, USA, 2000, pp. 157–191. 14. BS 7899-2:1999. Code of practice for: Assessment of hazard to life and health from fire–Part 2: Guidance on methods for quantification of hazards to life and health and estimation of time to incapacitation and death in fires. 15. D.A. Purser, P.J. Fardell, J. Rowley, S. Vollam, B. Bridgeman, An improved tube furnace method for the generation and measurement of toxic combustion products under a wide Range of fire conditions. In: Proceedings of the 6th International Conference Flame Retardants ‘94, London, UK. Interscience Communications. 1994. 16. T. Wright. Environmentally friendlier flame retardant systems. PhD Dissertation. Leeds University, Leeds UK, 1997. 17. D.A. Purser, Combustion toxicology of anticholinesterases, In: Clinical and experimental Toxicology of organophosphates and carbamates, Butterworth-Heinemann, Oxford UK, 1992, pp. 386–395. 18. E. M. Bellett and J. E. Caseda, ‘‘Bicyclic phosphorus esters: high toxicity without cholinesterase inhibition’’, Science, 1973, 182, 1135–1136. 19. G. Kimmerle, A. Egen and P. Groning, et al. ‘‘Acute toxicity of bicyclophosphorus esters’’, Arch. Toxicol., 1986, 35, 149–152. 20. D. A. Purser, ‘‘Toxic product yield and hazard assessment for fully enclosed design fires involving fire retarded materials’’, Polym. Int., 2000, 49, 1232–1255.

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21. G.E. Hartzell, A.F. Grand, W.G. Switzer, Modelling of toxicological effects of fire gases: VI. Further studies on the toxicity of smoke containing hydrogen chloride. In: Advances in Combustion Toxicology, Vol. 2, Ed. G.E. Hartzell, Technomic, Lancaster PA. pp. 285–308. 22. D. A. Purser, Recent developments in understanding the toxicity of PTFE thermal decomposition products, Fire Mat., 1992, 16, 67–75. 23. Handbook on air pollution and health. Department of Health Committee on the Medical Effects of Air Pollutants. 1997 The Stationery Office, London. 24. Evaluation of emissions from the burning of household waste in barrels. Volume 1 Technical Report. United States Environmental Protection Agency Report EPA-600/R-97-134a, pp. 1–69, 1997. 25. J.H. Troitzsch, Fire gas toxicity and pollutants in fires: the role of flame retardants. FRPM07. 11th European Meeting on Fire Retardant Polymers. Bolton 3-6 July 2007. 26. Independent Experts Commission for the Minister President of the State of North Rhine Westphalia for Investigating the Consequences from the Dusseldorf Airport Fire. Report-Part I. Analysis of the 11 April 1996 Fire. Recommendations and Consequences for the Dusseldorf Airport. 14 April 1997. 27. Major Fire in a Plastics Warehouse in Lengerich, October 1992Documentation. Published by the Ministry for the Environment and the Ministry of the Interior of the State of North Rhine Westphalia and the town of Lengerich. June 1994. (In German). 28. Environmental Medical Studies on Fire Fighters. Ruhr-University Bochum and Heinrich-Heine University Dusseldorf on Behalf of the Ministry of Labour, Health and Social Affairs of the State of North Rhine Westphalia. October 1992. (In German).

CHAPTER 25

Assessment of Fire Toxicity from Polymer Nanocomposites ANNA A. STEC AND T. RICHARD HULL Centre for Fire and Hazards Science, University of Central Lancashire, Preston, PR1 2HE, UK

25.1 Introduction In the UK, dwellings have consistently been the scene of most fire related deaths (87%). Nearly 70% of dwelling fire deaths occur in the room of fire origin, often in a small, enclosed space, such as bedroom, or living or dining room, likely to restrict ventilation in a fire.1 Surveys of victims from all fires revealed that a large proportion of fatal and non-fatal casualties were overcome by smoke and toxic gases rather than by heat and burns.1 (53% by toxic gases, 15% by both burns and gases, and 21% by burns alone). Polyamide 6 (PA6) and polypropylene (PP) are two common polymers, widely used for many applications because of their low cost, low density and good thermal stability. Blending them with fire retardants (FRs) and nanoclays (NCs) to form nanocomposites can improve their mechanical and chemical properties, while reducing flammability and smoke emissions. These properties have allowed nanocomposites to be developed as a new class of flame retarded materials, alongside conventional filled composites or rigid polymers. Potentially, nanocomposites could deliver unique combinations of stronger, lighter and more versatile polymer composites, and so create a new class of products that fulfil industry and consumer demands. However, these materials have not been fully characterized and information about flammability and fire gas toxicity is incomplete. This study involves fire retarded PA6 and PP polymers Fire Retardancy of Polymers: New Strategies and Mechanisms Edited by T Richard Hull and Baljinder K Kandola r Royal Society of Chemistry 2009 Published by the Royal Society of Chemistry, www.rsc.org

405

406

Chapter 25

and nanocomposites using the steady-state tube furnace to assess the fire toxicity over a range of fire conditions.2,3

25.1.1

Fire Scenarios

Fires can be divided into a number of stages: from smouldering combustion and early well-ventilated flaming through to fully developed underventilated flaming.4 A useful concept in characterizing and predicting the gas-phase flaming combustion conditions, and the yields of products such as carbon monoxide (CO), carbon dioxide (CO2), hydrogen cyanide (HCN) and hydrocarbons, is the equivalence ratio (f), presented in Equation (25.1). f¼

Actual fuel : Air ratio Stoichiometric fuel : Air ratio

ð25:1Þ

If the amount of oxygen balances the amount of fuel exactly, then the conditions are said to be stoichiometric. This is expressed by an equivalence ratio equal to 1. Alternatively, the equivalence ratio may be higher when there is less than the stoichiometric amount of air, and the conditions are vitiated, or when the stoichiometric air requirement is exceeded, and the conditions are wellventilated. Different materials have different reactions to fire behaviour. Once a fire has started, the chemistry of combustion and the effects on the occupants depend upon the interaction between the developing fire in terms of the fuel items first ignited and the characteristics of the fire enclosure. The product yields are particularly dependent upon the composition of the polymeric material, the temperature and the ventilation conditions.5 Once the temperature of the surface is raised sufficiently (generally to around 300 1C), then a process of thermal decomposition by oxidative pyrolysis begins. The products tend to be rich in partly decomposed organic molecules (many of which are irritants), CO and smoke particulates.5 This scenario presents a particular hazard to a sleeping subject in a small enclosure, such as a closed bedroom, which can reach a lethal dose over a few hours.5 Furthermore, fire development may occur in different ways, depending on the environmental conditions, as well as on the arrangement of fuel. An early well-ventilated flaming fire is characterized by a temperature of around 600 1C and an equivalence ratio less than unity (during the early stages B0.5). This means that there is an excess of oxygen mixed with the fuel gases, and smaller yields of smoke, toxic and irritants are generated. As the fire develops, the oxygen concentration decreases, while the proportion of products of incomplete combustion (e.g. CO, HCN, organics and smoke) increases. Small vitiated (underventilated) fires, with gas temperatures around 650 1C, may grow into large vitiated fires, with higher gas temperatures of around 850 1C. Room occupants are exposed to a highly toxic effluent cocktail able to cause incapacitation and death from asphyxiation within a few

407

Assessment of Fire Toxicity from Polymer Nanocomposites

Table 25.1

Classification of the fire stages.

8

Combustion condition

Temperature(1C)

Equivalence ratio, f

CO2:CO ratio

Smouldering Well-ventilated flaming Underventilated flaming: Small vitiated fires Large vitiated fires

350 650

not applicable f o 0.75

1–5 2–20

650 825

f 4 1.5 f 4 1.5

2–20 2–20

minutes. They will also suffer from exposure to heat, with a possibility of burns. Large vitiated fires can cause ignition of all combustible materials, and in some cases are accompanied by a rapid flame spread, known as flashover. The effluent plume is similar in composition to that from a small vitiated fire – fuelrich (high f) combustion conditions, with very low oxygen concentrations and high concentrations of asphyxiant gases (CO, HCN), organic irritants and smoke particles. Since the temperatures are higher and the conditions somewhat more extreme, the yields of toxic products may be somewhat higher than for well-ventilated fires. Vitiated fires are the most common cause of death, especially in small enclosures, but may be extremely hazardous in other scenarios because a large vitiated effluent plume can rapidly fill other building spaces remote from the source of the fire. Some bench-scale fire models (such as the NF X 70-100,6 or NBS smoke chamber7) are unable to control the oxygen concentration while still supporting flaming combustion. They are only suited to replicating the developing fire, with a high combustion efficiency. To quantify the fire stage or fire condition, the equivalence ratio may be replaced by CO2:CO ratio, which can be obtained without direct sampling of the fire plume. Different ventilation conditions and fire stages are summarized in Table 25.1.

25.1.2

The Steady-state Tube Furnace (ISO 19700)

Fire toxicity, like flammability, is both scenario and material dependent. Benchscale assessment of fire gas toxicity either adopts an integrative approach, in which the material is burnt in a fixed volume of air, allowing the initially wellventilated fire condition to become underventilated to an unknown degree, or the ventilation is controlled, so that individual fire stages may be replicated. The steady-state tube furnace, also known as the Purser furnace, in which the rate of burning is controlled by the sample feed rate, allows the ventilation to be controlled without affecting the rate of burning.2,8 It is used to investigate the production of smoke and combustion gases for different fire stages, but it can also be used to study different characteristics, such as ignitability and heat release by oxygen depletion. The different equivalence-ratio conditions are created by varying the mass of sample, so as to keep the airflow constant for the range of polymers. The equivalence ratio for a particular polymer may then be varied by adjustment of

408

Chapter 25
1

the primary (furnace tube) air flow rate, up to 50 litre min , with secondary air in the effluent dilution chamber. In this way, toxic gas concentrations can be directly compared for a range of fire conditions and a range of polymers.3 During an experiment, the sample is mechanically driven into the furnace tube, at a constant and controlled flow rate, typically over a period of about 20 minutes. A fixed stream of primary air passes through the tube furnace into the mixing chamber, where it is diluted with a fixed and controlled secondary air supply. Primary air passing over the specimen supports the combustion process. Secondary air increases the volume of analyte, and helps to keep the effluents from different ventilation conditions within the same analytical range. A light and photo cell system is used to determine smoke density across the mixing and measurement chamber. The requirement in each test run is to obtain a steady state for at least 5 minutes during which the concentrations of effluent gases and particulates can be measured. On-line Fourier transform infrared (FTIR) measures each of the combustion products. Oxygen depletion is measured using a paramagnetic analyzer. Organics (unburnt and partially burnt hydrocarbons) are determined as products of incomplete combustion using a secondary oxidizer for further oxidation at 9001C in excess air, over silica wool, as the difference between secondary CO2 and primary CO and CO2 measured using a non-dispersive infrared analyzer. The toxicity of organic species in the fire effluent was quantified as a ratio of the actual organic yield to the organic yield of 10 mg l–1, which results in incapacitation, as described by Purser5 (Figure 25.1).

25.1.3

Toxic Potency of Fire Effluent

Several methods have been used to generate products to evaluate the toxic (lethal) potency of fire smoke. However, many of them fail to relate this to a particular fire scenario.9 In addition, although room- and larger-scale fire tests have been also conducted and published,10,11 only a few of these have attempted to segregate the fire stages. The ideal small-scale method to assess FTIR

Secondary air supply

Exhaust gases [

Sample boat driven in over~20 minutes Primary air supply

Figure 25.1

The steady-state tube furnace (ISO 19700).

Mixing chamber

409

Assessment of Fire Toxicity from Polymer Nanocomposites

fire toxicity must allow the toxic product yields from each fire stage to be determined, and so enable the assessment of each material under each fire condition. This appears to be the only way that the complexities of full-scale burning behaviour can be addressed using a bench-scale model. In most cases, fire effluent toxicity is expressed as the yield of each toxic product and smoke from the fire, which can then be related to toxicity using expressions for fractional effective dose (FED) and lethal concentration (LC50), and incapacitation from irritants as fractional effective concentration (FEC).12,13

25.1.3.1

Fractional Effective Dose and Lethal Concentration

One tool used to predict the toxicity of fire gases, the N-Gas model,12,14 was developed at the National Institute of Standards and Technology (NIST) to estimate FED. Despite the complexities of physiology, this assumes the contribution of each toxicant to be additive (CO, HCN, etc.), and takes the ratio of measured toxicant concentration to the concentration expected to cause death or incapacitation to 50% of the population, known as the LC50 for the particular gaseous toxicant.12,13 Equation (25.2) assumes that only the effect CO is enhanced by the increase in respiration rate caused by high CO2 concentrations, and has been modified to include the toxicants encountered in the current work. FED ¼

m½CO 21
½O2  ½HCN ½HCl ½NO2  þ þ þ þ ½CO2 
b 21
LC50;O2 LC50;HCN LC50;HCl LC50;NO2

ð25:2Þ

An alternative, the Purser model, presented in Equation (25.3) uses V CO2 , a multiplication factor for CO2-driven hyperventilation, to increasing the FED contribution from all the toxic species (not only for CO, as in N-Gas model), and incorporates an acidosis factor (A) to account for toxicity of CO2 in its own right. 

½CO ½HCN ½HCl ½NO2  FED ¼ þ þ þ þ    þ organics LC50;CO LC50;HCN LC50;HCl LC50;NO2 21
½O2   V CO2 þ A þ 21
5:4 expð0:14½CO2 Þ
1 V CO2 ¼1 þ 2 A is an acidosis factor equal to ½CO2   0:05:

 ð25:3Þ

The fire toxicity of a material can also be expressed as an LC50 for the material, which in this case is the specimen mass, M, of a burning polymeric material which would yield an FED equal to one within a volume of 1 m3. The relation to the FED from the N-Gas model is given in Equation (25.4). LC50 ¼

M FED  V

ð25:4Þ

410

Chapter 25

Table 25.2

Formulation of PA6 and PP (with 5% PPgMA) polymer materials, with and without FR and NC (in wt %).

Material

Polymer(%)

PP PP+FR PP+NC PP+FR+NC

100 70 95 65

PA6 PA6+FR PA6+NC PA6+FR+NC

100 82 95 77

Fire retardant(%)

Nanoclay(%)

30 30

5 5

18 18

5 5

where V is the total volume of diluted fire effluent in m3 at STP. The accuracy of LC50 values determined in this manner is quoted as 30% if the concentrations of all the contributing toxicants are measured and included.13 Comparing the toxic potencies of different materials, the lower the LC50 (the smaller the amount of materials necessary to reach the toxic potency), the more toxic the combustion products of the material.

25.2 Experimental 25.2.1

Materials

This work was carried out as part of the Predfire Nano Framework 6 Project (with support from European Union), which aims to develop a tool to understand and predict fire behaviour of fire retardant polymer nanocomposites. PA6 and PP, with and without FR, and NC, or both, were investigated, using the formulations presented in Table 25.2. The rigid materials used for the preparation of nanocomposites were commercial PP (PP Moplen HP500N-Basell blended with 5% PP grafted with maleic anhydride) and PA6 (Technyl S27, Rhodia). The fire retardant (FR) for PP was Exolit AP 760, and for PA6 was OP1311, both supplied by Clariant, and the NC was Cloisite 20A for PP and Cloisite 30B for PA6, both supplied by Southern Clay Products.

25.3 Results 25.3.1

Yields of Toxic Products from PA6 and PP

Mixtures of the many different gases obtained during combustion depend not only on the material being burned, but also on the conditions of burning. As the fire develops, conditions change – the temperature increases and oxygen concentration decreases, which results in different concentrations of combustion products. CO and CO2 yields were measured for both materials, together with HCN yields for PA6 materials, for a series of characteristic fire types from

411

Assessment of Fire Toxicity from Polymer Nanocomposites

40

20

0

Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V

CO2 /CO ratio

60

PA6

Figure 25.2

PA6+FR

PA6+NC

PA6+FR+NC

PP

PP+FR

PP+NC

PP+FR+NC

CO2:CO ratio for PA6 and PP with FR and NC.

smouldering to large vitiated, and are presented herein. The yields are all expressed on a mass-loss basis. The yields of toxic gases in the tube furnace are highly dependent on the decomposition and burning conditions, in particular the CO2:CO ratio, which provides a useful indicator of the combustion efficiency, or fire condition. Although it is easier to measure, the presence of halogen or other gas-phase inhibitors limit its use as a universal indicator of fire condition. In contrast, the equivalence ratio, f, is defined by the input conditions for the fire plume in terms of fuel:air ratio, which is not readily available from a naturally ventilated fire. The CO2:CO ratios for PP and PA6 are presented in Figure 25.2. PA6 and PP experiments carried out at higher temperatures (650 and 825 1C) gave significantly higher yields of CO2 and relatively low CO yields at wellventilated conditions (fo1). As the fire develops, the equivalence ratio increases, and the CO yields tend to increase while the CO2 yields decrease. This relationship between CO and CO2 yields is independent of furnace temperature over the range considered. A similar trend to that of CO is observed for PA6 with HCN. HCN yields increase with the increase of equivalence ratio, but are independent of the furnace temperature. The CO, HCN, NO and NO2 yields are presented in Figures 25.3 and 25.4. Figure 25.3 presents yields for CO for both polymers. The difference in fire toxicity for the different formulations is particularly noticeable for well-ventilated flaming conditions, where PA6 and PA6+FR+NC show lower yields of CO and the FR and NC show higher yields. It is also interesting to see a very high yield of CO for smouldering PA+NC. The CO yield for PP during smouldering is generally higher than that for PA6, especially in the presence of the fire retardant. The CO yield under wellventilated conditions is increased both by the presence of FR and of NC.

412

Chapter 25 0.5

CO yields g/g

0.4

0.3

0.2

0.0

Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V Smould'g Well-V Small-V Large-V

0.1

PA6

Figure 25.3

PA6+FR

PA6+NC PA6+FR+NC

PP

PP+FR

PP+NC

PP+FR+NC

PA6 and PP CO yields with FR and NC.

0.07 NO2

NO2,NO,HCN yields g/g

0.06

NO

HCN

0.05 0.04 0.03 0.02 0.01

PA6

Figure 25.4

PA6+FR

PA6+NC

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

0.00

PA6+FR+NC

PA6 HCN, NO and NO2 yields with FR and NC.

However, in both cases, PA6 and PP, the yields of CO and HCN under the most toxic underventilated conditions are not adversely effected by the presence of a fire retardant and/or NC. HCN, NO and NO2 yields for PA6 are presented in Figure 25.4. The higher yield of HCN for PA6+FR for well-ventilated flaming corresponds to a higher yield of CO for the same conditions. It is interesting to note

413

Assessment of Fire Toxicity from Polymer Nanocomposites

that the HCN yield increases with severity of the fire condition, whereas the CO yield levels off or even decreases.

25.3.2

Fractional Effective Dose

The toxicity of the fire effluent is estimated as a FED. For simplicity, organoirritants and NOx have been included as asphyxiants, rather than showing their contribution as incapacitants separately. The contribution of organo-irritiants is estimated using the method proposed by Purser.5 Figures 25.5 and 25.6 show the contribution of each of the main toxicants to the FED values at different fire scenarios. For the smouldering fire scenario, modelled in the tube furnace at 350 1C, the only significant contribution to the toxicity comes from NO2 (B80%). The total value of the FED is higher for well-ventilated, small and large vitiated experiments carried out at 650 1C and 825 1C. The most toxic product presented in Figure 25.5 for PA6 is HCN, which contributes approximately 60% to the FED value. Other combustion products contribute around 40% (hypoxia with NO2 around 30%, CO 8% and organic species 2%). As the ventilation becomes limited, both the CO and HCN contributions increase while the oxygen depletion, because of the make-up to 50 l min–1 with secondary air, decreases. In addition, a relationship between hypoxia and unburned hydrocarbons has been observed, as the production of both species is favoured by vitiated conditions. The increase of HCN concentration, organic species, CO and oxygen depletion are independent of the temperature of the furnace and give higher quantities with reduced ventilation.

10 CO

HCN

NO2

Hypoxia

Organics

FED

8 6 4 2

PA6

Figure 25.5

PA6+FR

PA6+NC

Contributions to FED from PA6 materials.

PA6+FR+NC

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

0

414

Chapter 25 2 CO

Hypoxia

Organics

FED

1

PP

Figure 25.6

PP+FR

PP+NC

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

Large-V

Small-V

Well-V

Smould'g

0

PP+FR+NC

Contributions to FED from PP materials.

The ratio of the FED components changes slightly with the decrease of ventilation, mainly because the increase in HCN production results in higher FED values, to give high toxicity under vitiated conditions. Conversely, the oxygen consumption and NO2 contribution lowers as the ventilation decreases. The CO and CO2 yields obtained from low density polyethylene (LDPE) under different ventilation conditions, defined by the equivalence ratio, were used to calculate the FED and are presented in Figure 25.6. For PP it can be seen that the highest yields of CO are found in smouldering combustion. However, this does not translate directly into the most hazardous fire scenario because of the slower rate of smouldering compared to that of flaming combustion. Much of the hazard that arises from smouldering is a consequence of the difficulty in detection, and hence the extent of burning, albeit over a longer time scale. Conversely, the highest yields of hydrocarbons are observed for the large underventilated fire scenario. As the ventilation becomes higher (fo0.7) the unburned hydrocarbons decrease as the oxygen depletion decreases. This effect increases, giving greater differences with the change of the equivalence ratio. The same effect is observed at higher temperatures. The total value of the FED is greater when the experiments are carried out under well-ventilated conditions and increases with fuel:oxygen ratio. CO, oxygen depletion and unburned hydrocarbons make similar contributions to the FED value. Further, apart from smouldering combustion, organic irritants are the most toxicologically significant species in the fire effluents of PP. The relatively small variations in fire toxicity for PP show that the wellventilated condition is the least toxic, and the small and large vitiated

415

Assessment of Fire Toxicity from Polymer Nanocomposites

conditions are up to five times more toxic. In contrast, for PA6 only the smouldering condition has low toxicity, the well-ventilated condition is of comparable toxicity to the vitiated condition of PP (note the different values on the FED axis), and the toxicity of the vitiated PA6 fires is around eight times those of PP or the well-ventilated PA6.

25.3.3

LC50 of Different Polymeric Materials

The toxicity of fire gases may be expressed using data obtained by exposing animals to fire gases. However, they may also be predicted from chemical analysis data using the FED models presented above. The lower the LC50 (the smaller the amount of materials necessary to reach lethality), the more toxic the burning material is. Table 25.3 presents LC50 values for the different materials. The data have been compared with different polymers using the Purser furnace apparatus.15 Larger differences are observed for the toxicity values of PA6 than for those of PP. For smouldering, PA6 is less toxic than the fire retarded or nanocomposite formulations. In the case of PP, rigid polymer and nanocomposite with a fire retardant are the most toxic for this fire scenario. Comparing these two polymers under smouldering conditions, PP is more toxic (because of its higher CO yield under well-ventilated conditions) than PA6, which has lower toxicity than formulations with FR and NC. In the case of PP, similar toxicity is observed for all formulations. For all flaming conditions, particularly underventilated tests, PA6 is more toxic than PP, resulting from the evolution of HCN. The LC50 data show that smaller amounts of PVC need to be decomposed in oxidative pyrolysis or well-ventilated fires to produce the same toxic effect as materials such as polystyrene and polyethylene. Data for polyethylene and Table 25.3

Average LC50 values (g m
3) for common materials. Fire conditions

Material

Smouldering

Well-ventilated

Small vitiated

Compared with other polymers using the Purser furnace PVC – 8.4 7.8 LDPE 32.5 45.0 21.0 PS (polystyrene) 33.1 27.2 26.8 4.0 PA6.6 92.2 44.3a This work PA6 79.2 19.4 3.5 PA6+FR 44.3 17.6 3.5 PA6+NC 60.6 14.1 3.3 PA6+FR+NC 46.7 17.1 3.4 PP 25.8 39.9 17.5 PP+FR 59.3 37.9 18.8 PP+NC 36.2 28.8 19.6 PP+FR+NC 39.9 33.9 12.6 a

NO2 data not available.

Large vitiated 8.4 22.0 33.7 2.8 2.6 3.3 2.9 3.2 15.6 18.9 13.2 10.7

416

Chapter 25

polystyrene show lower toxic potency values than those of PVC for all flaming conditions. The inference is that different components in PVC smoke adversely affect toxicity.

25.4 Conclusions The presence of fire retardants and the incorporation of NC reduce the flammability, but this work has shown that there is no significant adverse effect of these additives on the toxicity of the material studied under the most lethal fire conditions. In many cases CO, which is often assumed to be the most, or even the only, toxicologically significant fire gas is of less importance than HCN when nitrogen is present, and in some cases CO may even be less important than organo-irritants. The majority of fire deaths are attributed to the inhalation of toxic gases. The yield of the main toxic gases (CO, HCN) increases by more than a factor of 10 as the fire grows from well-ventilated to large vitiated. The toxicities of combinations of CO and HCN are likely to be additive, rather than synergistic, as the main narcosis mechanism of each toxic gas is to reduce the ability to transport oxygen to the organs. Comprehensive postmortem studies have shown that the majority of fatalities (50–80%) in fires had excessive levels of carboxyhaemoglobin, consistent with exposure to high levels of CO.16 PP is a hydrocarbon, without any hetero-elements to increase the fire toxicity. However, note that the organic contribution to FED changes significantly for PP in small vitiated tests compared with other materials. The predicted toxicities show variation of up to two orders of magnitude with changes in fire scenario. Thus, it is essential in any assessment of toxic hazard from fire to define the fire condition correctly. They also show changes of at least one order of magnitude for different materials in the same fire scenario. Finally, they show that in many cases CO, which is often assumed to be the most, or even the only, toxicologically significant fire gas, is of less importance than HCN when nitrogen is present, and in some cases CO may even be less important than organo-irritants.

References 1. United Kingdom Fire Statistics 2004, Home Office, London, 2006. 2. ISO 19703:2005 Generation and analysis of toxic gases in fire – Calculation of species yields, equivalence ratios and combustion efficiency in experimental fires. 3. BS 7990:2003 Tube Furnace method for the determination of toxic product yields in fire effluents. 4. ISO 19706:2007 Guidelines for assessing the fire threat to people. 5. D.A. Purser, SFPE Handbook of Fire Protection Engineering, National Fire Protection Association, Third Edition p. 2–83, 2002.

Assessment of Fire Toxicity from Polymer Nanocomposites

417

6. NFX 70-100, Analysis of pyrolysis and combustion gases. Tube furnace method. Part 1, Methods of analysis of gas generated by thermal degradation. Part 2, Method of thermal degradation using tube furnace. 7. ISO 5659-2:1999 Plastics-Smoke Generation-Part 2, Determination of Specific Optical Density. 8. ISO TS 19700:2006 Controlled equivalence ratio method for the determination of hazardous components of fire effluents. 9. T.R. Hull, A.A. Stec, K. Lebek and D. Price, Polym. Degrad. Stab., Vol. 92, pp. 2239–2246, 2007. 10. ISO 9705: 1993 Fire Tests – Full scale room test for surface products. 11. P. Blomqvist, A. Lo¨nnermark, Fire and Materials, Vol. 25, p. 71–81, 2001. 12. BS 7899-1:1997 Assessment of hazard to life and health from fire-Part 1. 13. BS 7899-2:1999 Assessment of hazard to life and health from fire-Part 2. 14. ISO 13344:1996 Estimation of lethal toxic potency of fire effluents. 15. A.A. Stec, PhD Thesis, University of Bolton, UK, 2007. 16. J.O. Punderson, Fire Mat., Vol. 5, p. 41, 1981.

Subject Index 16 CFR 1145 287 16 CFR 1610 281, 284–6 16 CFR 1615 282, 284 16 CFR 1616 282 16 CFR 1630 286 16 CFR 1631 286 16 CFR 1632 270, 271 16 CFR 1633 270 8 mm specimens combustion behaviour 151–3 16 mm specimens combustion behaviour 153–7 acid gases 382, 385, 393 acidic phosphates 303–4 acidosis factor 409 acrolein 389 acrylics 307–8, 320, 391–2 acrylonitrile 307, 308, 310–12, 332, 334–5 active fillers 387–9 additive effect 185, 194, 416 additive fraction 191, 194, 196, 200–2 additives, drawbacks to 331 ADEPMAE 332–5, 337–9 adsorption 78 Aerosil 96 AFM system 79, 81 air-dried filaments 315–20 air permeability 366, 369–70 alumina 10 and organoclay in ethylene–vinyl acetate 17–26 polymer–alumina nanocomposites 96, 97–105, 107

alumina trihydrate 126, 127–45 aluminium hydroxide 10, 17, 386, 387, 388 aminophenyl phosphate 161 ammonium persulfate 334, 336 ammonium polyphosphate 240 in polyacrylonitrile fibres 307–28 in polyurethane intumescents 245–51 in steel structure intumescents 242–5 in textiles 258, 259 amorphous silica 35–47 antagonistic effect 185, 197, 201–2 Antiblaze LR2 309, 320, 321, 322, 323 antimony oxide 126, 127–31, 136–7, 140, 242, 308 area density, fabrics 362, 363, 367, 369–70 area porosity 213–15 Arrhenius law 228 aspect ratio 213, 214, 222–3 ASTM E1353/UFAC 276, 277, 280, 283 attenuated total reflection Fourier transform infrared spectra 295, 300, 301 average residence time 310 azobisisobutyronitrile 30 backcoatings characterization 346 flammability 354–5 preparation of flame retardants 342–5 preparation of paste 345–6 pyrolysis behaviour 348–54 smoke density measurements 355–7 techniques 257, 258

Subject Index

for textiles 341–58 thermal characterization 346–8 bedclothes standards 272–5 children's sleepwear 261, 283–4 bending modulus 69, 70 benzophenone 29 benzpinacol 29, 30 bicyclophosphate esters 390, 391 bisphenol A epoxy resin 161–2 black fabrics 361, 368–9, 370, 371–2, 376 blowing agents 228, 232–3 decomposition 245 reaction kinetics of 234, 235–6 borates 50–7, 242 borax 242, 257, 387, 389 boric acid 242–5, 257, 387, 389 boron oxide 243, 245 borophosphates 243–5 Boucle fabric 394, 399 box springs flammability standards 270–2 British Standards BS 5651 259 BS 5852 259, 275, 276–7, 278, 360, 365 BS 7176 360, 361, 364, 365, 367–8, 370 BS 7990 383, 385, 386, 392, 394, 396, 397 BS 9237 366 bromine 28, 126, 127, 279, 394 and toxic yield 397–8 Bruggeman expression 211–12, 223 Brunauer–Emmett–Teller method 36, 38, 39 kaolins 62, 64 bubble expansion 228, 229–30, 233, 236–7 burn injuries 284 burning tests see horizontal burning test; vertical burning test cable materials 17, 24, 386, 387, 388, 398 calcium carbonate 386, 387, 395 California Bureau of Home Furnishings and Thermal Insulation (CA BHFTI) 268, 269

419 standards for bedclothes 272–5 standards for mattresses and foundations 271–2 standards for upholstered furniture 280–2 calorimetry see cone calorimetry; differential scanning calorimetry; oxygen consumption calorimetry candles 287 carbon–carbon bond, homolysis 28, 31, 34 carbon dioxide 383, 384, 388–9, 395 in toxicity assessments 409, 410–11, 414 carbon dioxide laser 79, 80–1, 85 carbon monoxide 8, 382, 384–6, 388–9, 391–2, 394–8 in toxicity assessments 409, 410–14 carbon nanofibres 110–23 carbon nanotubes 78–89 and polystyrene nanocomposites 125–46 carbon tetrachloride 296 carbonaceous char 10, 247 carbonization 7, 240, 248–9 carbonizing agents 351, 355, 357 carbonyl fluoride 394 carboxyhaemoglobin 416 carboxylic acid 98 carpets 286–7 cellulose 256, 277–8 cellulosic fabrics 294–305 see also cotton fabrics ceramic layer formation 49, 55, 56 chain extender 60, 67 chain stripping 7 Charpy impact test 41, 42 chars carbonaceous 10, 247 copolymeric polyacrylonitrile fibres 326–7 epoxy resin–resorcinol diphosphate 191, 192 fire-retardant nanocomposites 82–3 formation 7, 10–11, 126, 147–8, 390 kaolins 70, 71, 72

420

oxidation 188, 192, 195 poly(butylene terephthalate) 179, 181, 182 polypropylene fabrics 373–5 treated cellulosic fabrics 297–305 see also intumescent chars chemical action strategies 10–11 children's sleepwear 261, 283–4 chlorine 308 melamine–chlorinated phosphates 390–5 and toxic yield 397–8 chloro-(dimalonyl phosphate) phosphine oxide 343–5, 346–8, 351–3, 355–7 chromatography see gel permeation chromatography; size exclusion chromatography cigarette lighters 287 cigarette tests 268, 269, 288 bedclothes 273 mattresses or foundation 270–1 upholstered furniture 275, 280–1, 282–3 cigarettes 287–8 class 1 textiles 285, 355, 358 class 2 textiles 285 class 3 textiles 285 clays see Cloisite; kaolins; montmorillonites; nanoclays; organoclays Cloisite 127, 132, 149–51 in copolymeric polyacrylonitrile fibres 307–28 in phosphorus-based epoxy resins 161, 163–5 in poly(butylene terephthalate) 169, 170–1, 172–82 in toxicity assessments 410–16 clothing textiles 284–6 children's sleepwear 281, 283–4 protective clothing 255, 259–60 coatings formation on char 303–5 nanocomposites 12 see also backcoatings; intumescent coatings

Subject Index

Cole–Cole plots 174 colours 361–5 effect on limiting oxygen index 370–2 measurement 366 results 367–9, 376 thermal analysis 372–5 combustion cotton fabrics 256 effective heat of 199, 200 nanocomposites 147–58 tests 150, 155 textiles 266–8 see also flammability; heat of combustion; non-flaming combustion; smouldering combustion; underventilated combustion; well-ventilated combustion combustion efficiency 384, 385, 391–2, 393 combustion modified high resilience foam 361, 365 commercial polypropylene fabrics 362–3, 364, 367–8, 370, 371, 372–5 common edge connection 212 common vertex connection 212 condensed-phase fire retardancy 75, 83, 125, 126 conduction 3–4, 229 conduction heat transfer coefficient 209 cone calorimetry 147–58 alumina and organoclay in ethylene– vinyl acetates 19, 23–4, 25, 26 epoxy resin–resorcinol diphosphate 197–202 kaolins 70, 71, 72 poly(butylene terephthalate) 171, 179–82 polymer–carbon nanofibres 113, 120–3 polymer–inorganic nanocomposites 97, 100–1, 106 polystyrene–carbon nanotubes 127, 135–45 silica filled polyamides 41–2 cone heater 55, 75, 76, 82–3

Subject Index

Congressional Fire Services Caucus of the US House of representatives 268, 284 consumer expectations/demands 262, 268, 360 contamination levels, post-fire 400–2 convection 3–4 conversion efficiency 392, 393, 394 copolymeric polyacrylonitrile fibres 307–28 copolymers, in situ radical polymerization 310–13 cotton fabrics backcoatings treatment 348–51, 354–8 borax–boric acid treatment 387, 389 burning behaviour of 256 flame retardancy 294–305 laundering of 261 synthetic blends 262 toxic hazards and 391–3 Courtelle 313, 315, 317, 318, 320, 321 cross-over frequency 174, 175 crystallization poly(butylene terephthalate) composites 171, 174–6 polymer–carbon nanofibres 112, 115–16 cyclization, polyacrylonitriles 336, 337, 338 deacylation 19, 22, 26 decabromodiphenyl ether 278, 289 decabromodiphenyl oxide 126, 127– 31, 134, 136–40, 145 decay, fire 3 decitex 313, 315, 317–18, 362–3 decomposition see heat of decomposition; thermal decomposition deformation, structural 9 degradation-related interactions 77–8 dehydration 19, 21, 22, 26, 188 depolymerization 188 derivative thermogravimetry 372–5 dextrin 351, 353, 355–7 dialkylphosphonopropionamide 260, 263

421 4,4´-diaminodiphenyl-methan 161–2 4,4´-diaminodiphenyl-sulfone 185–6 dichloromethane 185–6 dichloro(phenyl)phosphine 29, 30 differential scanning calorimetry 112, 115–17 backcoatings for textiles 346–8, 350–1, 353–4, 354 covalently-modified polyacrylonitriles 335, 336 poly(butylene terephthalate) composites 171, 174–7 differential thermal analysis copolymeric polyacrylonitrile fibres 309 epoxy resin–resorcinol diphosphate 190, 191–2, 195 organosilicones 52 poly(butylene terephthalate) composites 171, 176 polypropylene fabrics 366, 372 differential thermogravimetric curves alumina and organoclay ethylene– vinyl acetates 19–20, 22 polymer–carbon nanofibres 118–20, 121 dioxins 398–402 dispersion, nanocomposites 12, 13 dope blending clays 313 plus clay-blended tows 320–2 DOPO compounds 161–4 dry cleaning 286 dual burners 270 durable flame retardants 257, 259–61 effective heat of combustion 199, 200 effluent toxicity 8–9 electron beams 263 electron microscopy see scanning electron microscopy; transmission electron microscopy end-chain scission (unzipping) 7 energy conservation 227 enthalpy of crystallization 175–6 enthalpy of melting 176

422

environmental issues 28, 110, 184–5 dioxins and furans 398–402 fire retardants and 381–402 halogens and 28, 160, 169, 185, 308, 332 textiles and 261–2 see also health and safety épiradiateur tests 18–19, 22, 23, 24, 26 epoxy resin interlayer 78, 84–6 epoxy resin–intumescent coatings 241–3 epoxy resin–nanoclays, phosphorusbased 160–7 epoxy resin–resorcinol diphosphate 184–203 flammability behaviour 195–202 flammability tests 186–7 sample preparation and characterization 185–6 thermal stability 188–95 thermogravimetric analysis 186 X-ray diffraction and transmission electron microscopy 187–8 equivalence ratio 383–5, 392, 394, 396–7, 399 in fire toxicity assessments 406, 407, 411 equivalent pore radius 213, 214, 221 ethylenediamine tetra-acetic acid 311 ethylene–vinyl acetate nanometric alumina and organoclay in 17–26 pyrolysis of 75, 76 European Standards EN 471 263 EN 533 259, 263 EN ISO 10528 260 EN ISO 15797 260 expandable graphite 245–51 expansion bubble expansion 228, 229–30, 233, 236–7 polyurethane intumescents 248, 251 volumetric 209 experimental polypropylene fabrics 363, 368–9, 370, 371 exponential distribution 222 external heat fluxes 149, 150, 152, 155

Subject Index

fabrics see cellulosic fabrics; clothing textiles; cotton fabrics; polypropylene fabrics; textiles failure temperature 241 fatalities 266, 284, 360, 396, 405, 416 Federal Code of Regulations CPSC 1615 and 1616 261 fibre blends 262 filament tows 309, 314, 315, 321–2 filaments air and oven-dried 315–20 never-dried-filaments 313, 314, 320, 328 fillers, inert and active 387–9 finite difference method 227, 230 FIPEC cable test 24 fire effluent, toxicity 8–9, 408–10 fire growth rate index 179, 198, 199, 200, 201 fire regulations 360 see also US textile flammability regulations and test methods fire retardancy and characterization methods 74–6 condensed-phase 75, 83, 125, 126 nanocomposites used for 11 using intumescent coatings 240–52 see also flame retardancy fire retardant strategies 9–12, 13 fire retardants backcoatings for textiles 341–58 benefits and drawbacks of 382 polypropylene fabrics 359–76 polystyrene–carbon nanotubes and 125–46 toxic and environmental issues 381–402 and toxic product yields 387–402 in toxicity assessments 410–16 see also flame retardants fire scenarios 406–7 fire stages 3, 5–6, 383–5, 406 classification of 407 and toxic gas production 8–9 fire statistics 266, 267, 284, 287 see also fatalities

Subject Index

fire tests 6–7 full-scale 271, 391–2, 394, 408–9 small-scale 74, 383, 403, 408–9 fire triangle 2 Firemaster® 550 280 fires and fire growth 2–7, 382 hazards from 1–2, 125, 360, 381–2 toxic and environmental hazards from 381–402 types of 383–6 flame retardancy cellulosic fabrics 294–305 covalently-modified polyacrylonitriles 331–9 epoxy resins 195–203 recycled polyethylene terephthlate 59–73 see also fire retardancy flame retardants chemicals 288–9 classification of 257 polyacrylonitrile fibres 307–28 polypropylene fabrics 359–76 selection of 257–61 test standards 259–60 textiles 255–64 see also fire retardants; phosphorus flame retardants flame spread 3–4, 5, 84 backcoatings for textiles 346, 354–5 standards and 285 flaming combustion 266–7, 268 flammability backcoatings for textiles 354–5 epoxy resin–resorcinol diphosphate 186–7, 195–202 flame-retarded tows 320–7 mattress flammability standards 270–2 poly(butylene terephthalate) 178–82 polymer–inorganic nanocomposites 97, 100–2, 103–6 polypropylene fabrics 365–6, 368–9 standards and testing 261 strained organophosphorus compounds 33 textile flammability regulations 266–89

423 treated cellulosic fabrics 297 see also combustion; cone calorimetry flashover 4, 383–4, 386, 388, 394, 395, 407 fluoropolymers 394, 396 foams see combustion modified high resilience foam; polyurethane foams formaldehyde 263, 389 foundation flammability standards 270–2 Fourier transform infrared spectroscopy 408 attenuated total reflection 295, 300, 301 kaolins 63, 67, 68 phosphorus-based epoxy–nanoclays 162, 163, 164 fractional effective concentration 409 fractional effective dose 409–10, 413–15 frequency dependence of storage and loss moduli 174, 175 frequency distribution 217, 218, 221, 222, 223 full-scale fire tests 271, 391–2, 394, 408–9 fumed silica 35, 36, 37, 38, 39 furans 398–402 gas phase ignition 268 reaction in the 10, 125 gas phase analysis 83 gel permeation chromatography 334–5, 336 glass fibres 40, 41, 43 glass transition temperature 161, 335, 336 grafting, triphenylphosphite 64, 66–7 green fabrics 367–9, 370, 371–2 growth phase 3, 4, 5–6 guanidine carbonate 295, 296–305 halogen acid vapour-phase systems 393–4, 395, 396, 397–8 halogens 126, 242, 263 banning of 289 and combustion efficiency 385 dioxins and furans from 398–402 and environment 28, 160, 169, 185, 308, 332 see also bromine; chlorine

424

hazards from fire 1–2, 125, 360, 381–2 fire retardants and 381–402 health and safety epoxy resin–resorcinol diphosphate 184, 197, 201 halogens and 160 post-fire 400, 402 steel temperature and 234 textiles and 261–2 see also environmental issues heat conductivity see thermal conductivity heat of combustion 54, 200 treated cellulosic fabrics 296, 297 heat of decomposition 233, 236, 351, 353–4 heat release rate epoxy resin–resorcinol diphosphate 198–201 fire-retardant nanocomposites 87 kaolins 72 organosilicones 54–5, 56 polyamide nanocomposites 152–7 poly(butylene terephthalate) 179, 180, 181 polymer–carbon nanocomposites 122 polystyrene–carbon nanotubes 137–8, 139, 140, 141, 143–5 silica filled polyamides 36, 43–7 see also peak heat-release rate 'heat sink' effect 148, 157 heat transfer 229 conduction heat transfer coefficient 209 in fire 3–5 porosity and 210 see also radiation high density carbon nanofibres 111, 113–115, 116–18, 121–3 high-visibility fabrics 263 horizontal burning test 80, 103–4, 105 horizontal flame spread 3–4, 84 household waste 399–400 hydrogen chloride 387, 394, 395 hydrogen cyanide 9, 382, 390–2, 394, 397, 410–14 hydrogen fluoride 394

Subject Index

identical spheres 216–17 ignitability at a distance 5 ignition 2, 3, 5, 7 gas phase ignition 268 smoulder ignition 280 see also time to ignition ignition sources 287–8 ignition temperature 7 ignition zone 4 image analysis, intumescent chars 209–24 impact strength 166–7 in situ radical polymerization 310–13 induction period 2, 3 inert fillers 387–9 infrared reflectance 361, 370 infrared spectra backcoatings for textiles 342, 343, 344 see also Fourier transform infrared spectroscopy injection moulding 149 inorganic acid 228, 232–3, 240 interior fire barrier 276, 277 International Standards Organization ISO 3795 346, 354–5 ISO 10528 257, 260 ISO 15797 260 ISO 17025 261 ISO TC92/SC3 383 ISO19700 383, 384, 407–8 interphase modifiers 77 intrinsic viscosity 309, 312 intumescence 11, 226, 240–1 general analysis of 231–3 intumescent chars analysis of 219–23 porosity estimates of 209–24 intumescent coatings behaviour of 226 fire retardancy and fire protection using 240–52 in polyurethane foams 245–51 of steel plates 225–38 in steel structures 241–5 intumescent polymeric particle formation 84–8

Subject Index

intumescent system 35 irradiances alumina and organoclays in ethylene– vinyl acetates 19, 23–4, 25 in combustion tests 150, 155 silica filled polyamides 41, 46 isothermal thermogravimetry 31 kaolins chemical composition of 64 organic modification of 61 organomodified ultrafine 59–73 knife over air back-coating process 258 knife over roll back-coating process 258 large open-flame sources 269 laser pyrolysis time-of-flight mass spectroscopy 75, 76, 79–81, 83 laundering 261, 285 LC50 385–91, 395, 399, 409–10 of different polymers 415–16 LD50 391 lick-roller back-coating process 258 light reflectance 371 limiting oxygen index test 104, 105 backcoatings for textiles 346, 354–5, 357–8 copolymeric polyacrylonitrile fibres 309, 320–2, 323–7, 328 covalently-modified polyacrylonitriles 335, 339 epoxy resin–resorcinol diphosphate 195–7, 202 fire retardant polypropylene fabrics 359–76 phosphorus-based epoxy–nanoclays 160, 161, 164–6 treated cellulosic fabrics 296, 297–300 linear density 316–18, 319, 362, 369 loss modulus 174, 175 low density carbon nanofibres 111, 113, 116, 118, 120–3 low density polyethylene 386, 387, 388 low smoke and fume material 386, 387, 388 lung oedema 394, 396

425 M test performance 361, 362, 364–70, 375 M1 classification 361, 362, 364, 365, 367, 369, 370 M2 classification 365, 369 M3 classification 365 M4 classification 362, 364, 365, 367 magnesium hydroxide 17–26 maleated polypropylene 78–89 malonyl phosphate 342–3, 346–8, 351–3, 354–7 mass charge 386, 387, 388, 389 mass-continuity equation 229 mass-difference curves 189, 193 mass loss 386, 387 mass loss curves 69, 71 epoxy resins 188–9, 192–3, 194 mass loss rate 136 intumescent coatings 228, 231–2, 233 magnesium hydroxide 19–22 polymer–carbon nanofibres 118–19 matches 287 mathematical modelling steel plate intumescent coatings 227–30 validation 230–1 mattress flammability standards 270–2 maximum expansion ratio 229, 237 Maxwell expression 211 mean inflammation period 18–19, 22, 23 mechanical reinforcement, polyethylene terephthalate 59–73 mechanical testing, polymer–inorganic nanocomposites 96, 106–7 melamine 390–5 melamine formaldehyde 295, 296–305 melamine–borate 50–7 melamine–chlorinated phosphates 390–5 melt blending 111, 112, 127 torque evolution during 67–8, 69 melt-flow indexes 111 melt state interactions 77 melt viscosity 111, 139, 177–8 metal borates 50 3-methacryloxypropyltrimethoxysilane 96–101, 104, 105 methenamine 286

426

methyl acrylate (MA) 307, 308, 310–12 methyltrimethoxysilane (MT) 96, 100–1, 104, 105 micro-analysis assisted design 74–89 microthermal analyser 79, 81, 82 'modacrylics' 332 Monte Carlo simulations 215, 216, 217, 218 montmorillonites 60, 77, 132, 148, 169 morphology cellulosic char 297–300 kaolins 67–9 nanocomposites 11 polymer–carbon nanofibres 113–14 polymer–inorganic nanocomposites 96, 97–8 multifunctional fabrics 263 multiwall carbon nanotube 78–89 N-Gas model 409 nanoclays combustion behaviour 149–57 phosphorus-based epoxy resins 160–7 in poly(butylene terephthalate) 168–82 and resorcinol diphosphate composites 185–203 in toxicity assessments 410–16 nanocomposites 11, 13, 126, 405 coating and dispersion 12 combustion behaviour 147–58 fire toxicity from 405–16 micro-analysis assisted design of 74–89 nanoparticle shape 95–108 polymer–clay 50 thermal decomposition burning behaviour 12 see also polymer–inorganic nanocomposites; polystyrene nanocomposites nanodispersion 172–4 nanofillers see nanocomposites nanomechanism 74–89 Nanomer I.30E 185–202 nanometric alumina 17–26 nanonetwork formation 80–4

Subject Index

nanoparticles and fluoropolymers 306, 394 shape 95–108 National Association of State Fire Marshals 268, 284 National Burn Center Reporting System 284 National Institute of Standards and Technology 270 neurotoxicity 390 never-dried-filaments 313, 314, 320, 328 NF P 92-503 360, 361, 362, 365, 366, 367, 370 NF P 92-505 365 nitrogen 257 in cellulosic fabrics 294–305 covalently-modified to polyacrylonitriles 331–9 toxic product yields 390–5 nitrogen oxides 391, 411–12, 413 NMR spectra 243–4 backcoatings for textiles 342, 343, 344–5 covalently-modified polyacrylonitriles 333, 334, 335, 336, 338 non-durable flame retardants 257–8 non-flaming combustion 8, 9, 383, 384, 386, 388–9, 395 oblate pores 222–3 octabrominated diphenyl ether 279, 288 Oeko-tex 262 oMMTs 17–26 open flame sources 269 open flame tests mattresses and foundations 270, 271–2 upholstered furniture 277, 278–9, 281, 282 open ventilation see well-ventilated combustion organic carbon 388, 395, 398, 414 organo-irritants 9, 406, 407, 413, 414 organoborates 50 organoclays and nanometric alumina in ethylene– vinyl acetate 17–26

Subject Index

in polystyrene–carbon nanotubes 126, 132–5, 142–5 properties of 309 organomodified sepiolate 50–7 organophosphorus compounds strained 28–34 thermal characterization of 346–8 organosilicones 49–58 oven-dried filaments 315–20 overall stabilization effect 191, 193–4 oxygen consumption 282, 408, 413–14 oxygen consumption calorimetry 282 P-epoxy resin interlayer 78, 84–6 PA6.6 40–7 PA12 40–7 padding application process 259 pad–dry–heat cure process 259, 260 parametric studies 233–7 particle-size distribution, kaolins 64, 66, 67 pass/fail criteria 270, 272–4, 281, 361–2, 364–5, 367–8 peak heat-release rate 147–8 alumina and organoclays in ethylene– vinyl acetates 19, 23–4, 25, 26 epoxy resin–resorcinol diphosphate 198, 199, 200–1 organosilicones 55, 56, 57 polyamide nanocomposites 152, 155 poly(butylene terephthalate) 179, 181 polycarbonate–alumina nanocomposites 100–1 polystyrene–carbon nanotubes 136, 138–9, 142, 145 silica filled polyamides 46 pentabrominated diphenyl ether 279, 280, 288 pentaerythritol 351, 355–7 2,4,4,5,5-pentaphenyl-1,3,2dioxaphospholane 29–34 'percolating intumescent system' 86–7, 88 perfluoroisobutylene 394 peroxide, as radical initiator 51, 55, 57 phenyltrichlorosilane 96, 103, 105

427 phosphates acidic 303–4 borophosphates 243–5 nitrogen, melamine and melaminechlorinated 390–5 see also ammonium polyphosphate; malonyl phosphate phosphine 390 phospholane 29–34 phosphonates 258, 260, 262, 337 phosphoramidates 295 phosphoric acid 178, 191, 197, 240, 242, 245, 250, 390 phosphorus 126, 127 analyses 309 covalently-modified to polyacrylonitriles 331–9 see also organophosphorus compounds phosphorus acid 336, 337 phosphorus-based epoxy resin– nanoclays 160–7 phosphorus flame retardants 256–7, 263, 264 in cellulosic fabrics 294–305 and toxic product yields 390 phosphorus oxynitride 303 phosphorus pentoxide 390 phosphorus–nitrogen synergism 295, 297–8, 304 phosphorylation 257, 303–4 phyllosilicates 59–60, 307–28 physical action strategies 10 pigments 365, 366, 372–5, 376 piloted ignition 5, 7 pixel connection definitions 212 plenum cable 398 polyacrylics 351–4, 355–8 polyacrylonitrile fibres 307–28 polyacrylonitriles, covalentlymodified 331–9 polyamides amorphous silica in 35–47 combustion behaviour 148–58 toxicity 405, 410–16 polyborosiloxane 49–57

428

polybrominated diphenyl ethers 279, 288–9 poly(butylene terephthalate) 168–82 characterization and testing 171–2 differential scanning calorimetry and thermal analysis 174–7 flammability 178–82 melt viscosity 177–8 nanodispersion 172–4 sample preparation 170–1 polycarbonate–alumina nanocomposites 96, 97–102, 107 polycarbonate–silica nanocomposites 98, 99–100, 101, 107 polychlorinated biphenyls 398 polycyclic aromatic hydrocarbons 402 polydimethylsiloxane 49, 50, 51–7, 96, 103, 105, 161 polydispersity 32 polyesters 262, 278 polyethylene terephthalate 59–73 polymer decomposition see thermal decomposition polymer nanocomposites, fire toxicity from 405–16 polymer spinning 313–14 polymer–carbon nanofibre composites 110–23 materials and methods 111–13 morphology 113–14 thermal behaviour of 115–23 polymer–clay nanocomposites 50 polymer–inorganic nanocomposites advantages and drawbacks of 95 flammability of 97, 100–2, 103–6 mechanical testing of 96, 106–7 morphology of 96, 97–8 preparation of 96 thermal stability of 97, 98–100, 102–3 polymerization I 309, 310, 311–12, 313, 318, 319 polymerization II 309, 312, 321, 322–3 polymerization III 309, 312–13, 322–7 polymers chain breaking 7 combustion 266–8

Subject Index

fire growth 2–7 hazards from fire 1–2 LC50 values 415–16 polymethylmethacrylate 385 polyphosphoric acid 178, 191, 197, 303–4 polypropylene fire-retardant nanocomposites 78–89 organosilicones in 49–58 thermogravimetric analysis 117–19 toxicity 405, 410–16 polypropylene fabrics 359–76 air permeability 366, 369–70 colour measurement and pigment analysis 366 commercial 362–3, 364, 367–8, 370, 371, 372–5 effect of colour on limiting oxygen index 370–2 experimental 363, 368–9, 370, 371 fabric structures 362, 363, 369 flammability testing 365–6, 368–9 thermal analysis 366, 372–5 polypropylene–carbon nanofibres 110–23 polystyrene 32, 119–20 polystyrene nanocomposites 125–46 polystyrene–Aerosil nanocomposites 96 polystyrene–alumina nanocomposites 96, 102–5 polystyrene–carbon nanofibre composites 111, 114, 116, 121–3 polystyrene–carbon nanotube composites 125–46 polystyrene–silica nanocomposites 96, 102–3, 105–6 Polyurethane Foam Association 280 polyurethane foams combustion of 268 intumescent systems in 245–51 toxicity of 391–2 polyurethane resins 342 polyvinylchloride 386, 387, 394, 395, 398, 399, 400 pore list 212–13 pore orientation 210

Subject Index

porosity 210–12 analysis of a char section 219–23 area porosity and volume porosity 213–15 construction of 3D distributions from 2D distributions 217–19, 220 estimates of intumescent chars 209–24 intumescent coating 228–9 pore-finding algorithm 212–13 2D and 3D pore distributions 215–17 precipitated silica 36, 37, 38 precision and bias study 272 Predfire Nano Framework 6 Project 410 primary ignition process 3 printed circuit boards 160 probability density function 215–16, 217–19, 220, 221 PROBAN® 260 progressive smouldering 268 protection goals 6 protective clothing 255, 259–60 public occupancy, open flame standards for 271–2, 282 pyrolysis 7, 8, 295 backcoatings for textiles 348–51 ethylene–vinyl acetate 75, 76 phosphonates 337 polyacrylic coatings 351–4 see also laser pyrolysis time-offlight mass spectroscopy pyrolysis zone 3, 4, 148 radiation 3–5, 229, 233, 236 radical initiator, peroxide as 51, 55, 57 radical polymerization 310–13 radii, uniformly distributed 217, 218 Raman spectra 75, 76, 79, 85, 87 random chain scission 7 reaction kinetics, blowing agents 234, 235–6 reactive flame retardation 331, 332 reactive surfactants 77 red reflectance 361, 366, 370–1, 376 reduced-ignition propensity cigarettes 269, 278, 287–8 refurbishing, clothing 285–6

429 Registration, Evaluation and Authorisation of Chemicals body 262 residential fires and fire losses 267, 405 resorcinol diphosphate 126, 127–30, 136, 138, 145, 160 and epoxy nanocomposites 184–203 rheology kaolins 67–9 poly(butylene terephthalate) composites 171, 172–4 polyurethane intumescents 247 see also viscosity rugs 286–7 safety see environmental issues; health and safety; US Consumer Product Safety Commission sample size 6 scanning electron microscopy kaolins 64, 65, 68–9, 70 polymer–carbon nanofibres 113–14 polymer–inorganic nanocomposites 98, 99 treated cellulosic fabrics 297–300 screw configuration 62 screw profiles 149, 170 segmented image, char section 220 semi-durable flame retardants 257, 259 sepiolite 50, 51, 78, 80, 84–7 in poly(butylene terephthalate) 169, 170–1, 172–82 shear thinning 173, 174 shielding powder 49 Shriner Burns Hospitals 268, 284 'significant new use' 279 silanole groups 36 silica classification of types 36, 37 in polyamides 35–47 polymer–silica nanocomposites 96, 98, 99–103, 105–7 silica fume (SIDISTAR) 36, 37, 39, 40–7 size exclusion chromatography 30, 32 small open-flame sources 269 small-scale fire tests 74, 383, 403, 408–9

430

smoke density measurements 346, 355–7 smoke inhalation 400–1 smoulder ignition 280 smoulder test methods 269, 288 mattresses and foundations 271 upholstered furniture 276, 277, 278, 280–1, 282–3 smouldering combustion 267–8, 383–4, 406–7, 413–14 LC50 values 415 SNUR rule 279 sodium metabisulfite 334, 336 sodium thiocyanate 308–9, 311 solid phase, reactions in the 10–11, 77 soot residues 400–2 specific extinction area 136 specific heat capacity 210 specific optical density 355–7 spheres identical 216–17 randomly distributed 215, 216 uniformly distributed radii 217, 218 'standard fire condition' 225 standards see British Standards; California Bureau of Home Furnishings and Thermal Insulation (CA BHFTI); European Standards; International Standards Organization; test standards; US Consumer Product Safety Commission steady state burning 148, 156–7 fire 3 steady-state tube furnace 9, 407–8, 411, 413, 415 steel plate intumescent coatings 225–38 analysis of process 231–3 mathematical modelling of 227–30 parametric studies 233–7 validation 230–1 steel plate temperature 231, 232, 234–7 steel structures, intumescent coatings in 241–5 stoichiometric conditions 406 storage modulus 174, 175

Subject Index

strained organophosphorus compounds 28–34 structural deformation 9 styrenes, organophosphorus compounds in 28–34 Styron 96, 103, 104, 105, 106 substrate heating rate 231–2 surfactants 12 synergistic effect 160–1, 166, 185 alumina and organoclays 17–26 flame retardant polyacrylonitrile fibres 307–28 TB 26 271 TB 106 271 TB 116 280–1 TB 117 280–2 TB 121 271 TB 129 272 TB 603 270, 271 TB 604 272–5 TB133 282 TEDAP 78, 84–5 temperature drying and annealing for APP-treated tows 324–5, 327 versus expansion, viscosity and weight loss 248–51 failure temperature 241 glass transition 161, 335, 336 ignition 7 intumescent coatings of steel plates 225–38 temperature profiles in compounding 149 poly(butylene terephthalate) processing 170 polyurethane intumescent systems 246 tenacity 315, 316, 318, 319, 320 tensile modulus 41 tensile strength 166 tensile testing polyacrylonitrile filaments 315–20, 328 polymer–inorganic nanocomposites 96

Subject Index

test cases pore-finding algorithm 213 spheres 216–17, 218 test standards bedclothes 273, 274 flame retardant 259–60 flammability 261 textiles 266–89 tetrakis(hydroxymethyl)phosphonium chloride 260, 262, 263 textile flame retardants 255–64 application technologies 263–4 burning behaviour of 256 classification of 257 fibre blends 262 flammability standards and testing 261 future developments 263–4 health, safety and environmental considerations 261–2 phosphorus flame retardants 256–7 selection of 257–61 textile flammability regulations and test methods 266–89 textiles fire retardant backcoatings for 341–58 see also cellulosic fabrics; clothing textiles; cotton fabrics; polypropylene fabrics thermal analysis colours 372–5 poly(butylene terephthalate) 174–7 polypropylene fabrics 366, 372–5 see also differential thermal analysis; thermogravimetric analysis thermal conductivity 81–2, 83 intumescent coatings and 229, 231–3, 235–7 and porosity 210–12, 222–3 thermal decomposition 7, 13 backcoatings for textiles 346–53 burning behaviour 12 tributyl phosphate 303–4 thermal feedback 148, 153, 155 thermal inertia 6 thermal pyrolysis, polyacrylic coatings 351–4

431 thermal stability epoxy resin–resorcinol diphosphate 188–95 kaolins 69, 71–2 organosilicones 52 poly(butylene terephthalate) composites 174–7 polymer–carbon nanofibres 115–23 polymer–inorganic nanocomposites 97, 98–100, 102–3 strained organophosphorus compounds 33 thermo-oxidative degradation alumina and organoclay in ethylene–vinyl acetates 19–26 organosilicones 52–3, 57 polyurethane intumescents 247 thermogravimetric analysis 7 alumina and organoclay in ethylene–vinyl acetates 19–23 backcoatings for textiles 346–7, 348–9, 351–3 copolymeric polyacrylonitrile fibres 309, 320 covalently-modified polyacrylonitriles 335, 336, 337 epoxy resin–resorcinol diphosphate 186, 189, 190, 193, 194, 200 kaolins 64, 66, 69, 71 organosilicones 52–3 poly(butylene terephthalate) composites 171, 176 polymer–carbon nanofibres 112–13, 117–20, 121 polymer–inorganic nanocomposites 97, 98–9, 103–4 polypropylene fabrics 366, 372–5 polystyrene–carbon nanotubes 127–35 polyurethane intumescents 248 strained organophosphorus compounds 31 thermoset resins 241–3 thick coatings 241 thin coatings 241 3D pore distributions 215–19, 220 in real char section 221, 222

432

3D sphere 213, 214, 215 time to ignition alumina and organoclays in ethylene– vinyl acetates 18, 22, 23, 24, 25, 26 epoxy resin–resorcinol diphosphate 200 kaolins 70, 71 organosilicones 55, 56 polyamide nanocomposites 152, 154 poly(butylene terephthalate) 179–81 titanium dioxide 64 torque evolution 67–8, 69 total heat evolved 152 total heat released epoxy resin–resorcinol diphosphate 199, 200, 201 organosilicones 55, 57 silica filled polyamides 36, 44–5, 47 tows filament tows 309, 314, 315, 321–2 preparation and flammability testing of 320–7, 328 tensile properties 318–20 toxic potency 386–9, 394, 395, 396, 399 fire effluent 408–10 toxic product yields 383–7, 406, 407 and different fire retardant systems 387–402 from polyamide and polypropylene 410–13 toxicity fire effluents 8–9 fire retardants and 381–402 from polymer nanocomposites 405–16 and upholstered furniture 279, 280 transmission electron microscopy epoxy resin–resorcinol diphosphate 187–8 phosphorus-based epoxy–nanoclays 162, 164, 165 polymer–inorganic composites 96 polymer–inorganic nanocomposites 97–8 silica filled polyamides 36, 38, 39, 46–7 triallyl phosphate 295, 296–305 tributyl phosphate 295, 296–305

Subject Index

triethylamine 29 trimethylol polyol 390 triphenylphosphite 60–73 tris-2,3-dichloropropyl phosphate 280 tris-(3-aminophenyl)phosphate 161 tube furnace see steady-state tube furnace 2D pore distributions 215–19, 220 in real char section 221, 222 2D section 213, 214, 215 type I upholstered furniture 276, 277, 282 type II upholstered furniture 276, 277, 282–3 UFAC voluntary furniture smoulder/cigarette test 282–3 UKAS Testing Accreditation 261 UL 94 test 41, 43 UL 1709 test 241, 242 ultrafine kaolin 59–73 underventilated combustion 8, 9, 383–6, 388–9, 394, 395 in fire toxicity assessments 406, 407, 412 LC50 values 415 Underwriters Laboratories Inc 279 uniformly distributed radii spheres 217, 218 upholstered furniture fire retardants in 390, 391–3 fires 267, 360 polypropylene used in 359–65 standards and test methods 275–83 urea 259, 262, 263, 295, 296–305 US Consumer Product Safety Commission 268, 269 bedclothes standards 272 clothing-related burn injuries 284 clothing textiles standards 285–6 mattress and foundation standards 270–1 upholstered furniture standards 275–80 proposed standard 2008 276–8 staff draft standard 1997, 2001 275 staff revised draft standard 2005-06 275–6

433

Subject Index

US Environmental Protection Agency 279, 280 US National Institute for Occupational Safety and Health 279 US textile flammability regulations and test methods 266–89 bedclothes 272–5 carpets and rugs 286–7 children's sleepwear 283–4 clothing textiles 284–6 flame retardant chemicals 288–9 ignition sources 287–8 mattresses and/or foundation (box springs) 270–2 upholstered furniture 275–83 validation 230–1 vapour-phase retardants 126 halogen acid 393–4, 395, 396, 397–8 vertical burning test 104, 105 epoxy resin–resorcinol diphosphate 196, 197, 202 phosphorus-based epoxy–nanoclays 162, 164–6 silica filled polyamides 41, 43 vertical flame spread 4 viscoelasticity 173, 174, 247 viscosity and flammability of poly(butylene terephthalate) 181–2 intrinsic 309, 312 and melt state interactions 77 melt viscosity 111, 139, 177–8 poly(butylene terephthalate) composites 172–4, 177–8 polyurethane intumescents 248–51

viscosity (viscous) modulus 13, 177 volume porosity 213–15 volumetric expansion 209 wash durable flame retardants 257, 259–61 weight loss backcoatings for textiles 346–9, 352–3 polyurethane intumescents 248–50 well-ventilated combustion 8, 383–6, 388–9, 394, 395 in fire toxicity assessments 406, 407, 411–12 LC50 values 415 wettability, organosilicones 56, 57 wood 6, 386, 387, 396 X-ray diffraction epoxy resin–resorcinol diphosphate 187–8 kaolins 64, 66 phosphorus-based epoxy–nanoclays 162, 163–4, 165 polyamide nanocomposites 151 poly(butylene terephthalate) composites 171, 172 polymer–carbon nanocomposites 114–15, 116 X-ray photoelectron spectroscopy 295, 300–3 yarns, polypropylene 361–4 Young's modulus 315 zeta potential 66–7 zinc borates 242