Multifunctional Barriers for Flexible Structure: Textile, Leather and Paper (Springer Series in Materials Science)

Springer Series in

materials science

97

Springer Series in

materials science Editors: R. Hull

R. M. Osgood, Jr.

J. Parisi

H. Warlimont

The Springer Series in Materials Science covers the complete spectrum of materials physics, including fundamental principles, physical properties, materials theory and design. Recognizing the increasing importance of materials science in future device technologies, the book titles in this series ref lect the state-of-the-art in understanding and controlling the structure and properties of all important classes of materials. 88 Introduction to Wave Scattering, Localization and Mesoscopic Phenomena By P. Sheng 89 Magneto-Science Magnetic Field Effects on Materials: Fundamentals and Applications Editors: M. Yamaguchi and Y. Tanimoto 90 Internal Friction in Metallic Materials A Reference Book By M.S. Blanter, I.S. Golovin, H. Neuh¨auser, and H.-R. Sinning 91 Time-dependent Mechanical Properties of Solid Bodies By W. Gr¨afe 92 Solder Joint Technology Materials, Properties, and Reliability By K.-N. Tu 93 Materials for Tomorrow Theory, Experiments and Modelling Editors: S. Gemming, M. Schreiber and J.-B. Suck 94 Magnetic Nanostructures Editors: B. Aktas, L. Tagirov, and F. Mikailov 95 Nanocrystals and Their Mesoscopic Organization By C.N.R. Rao, P.J. Thomas and G.U. Kulkarni 96 GaN Electronics By R. Quay

97 Multifunctional Barriers for Flexible Structure Textile, Leather and Paper Editors: S. Duquesne, C. Magniez, and G. Camino 98 Physics of Negative Refraction and Negative Index Materials Optical and Electronic Aspects and Diversified Approaches Editors: C.M. Krowne and Y. Zhang 99 Self-Organized Morphology in Nanostructured Materials Editors: K. Al-Shamery and J. Parisi 100 Self Healing Materials An Alternative Approach to 20 Centuries of Materials Science Editor: S. van der Zwaag 101 New Organic Nanostructures for Next Generation Devices Editors: K. Al-Shamery, H.-G. Rubahn, and H. Sitter 102 Photonic Crystal Fibers Properties and Applications By F. Poli, A. Cucinotta, and S. Selleri 103 Polarons in Advanced Materials Editor: A.S. Alexandrov 104 Transparent Conductive Zinc Oxide Basics and Applications in Thin Film Solar Cells Editors: K. Ellmer, A. Klein, and B. Rech

Volumes 40–87 are listed at the end of the book.

S. Duquesne C. Magniez G. Camino (Eds.)

Multifunctional Barriers for Flexible Structure Textile, Leather and Paper

With 153 Figures, 5 in Color and 44 Tables

123

Dr. Sophie Duquesne ENSCL – PERF/LSPES – UMR 8008 BP 90108, 59652 Villeneuve d’Ascq Cedex, France E-mail: [email protected]

Dr. Carole Magniez Institute Franc¸ais du Textile et de l’Habillement (IFTH) rue de la recherche, 59650 Villeneuve d’Ascq, France E-mail: [email protected]

Professor Dr. Giovanni Camino Politecnico di Torino Sede di Alessandria Centro di Cultura per l’Ingegneria delle Materie Plastiche Viale Teresa Michel 5, 15100 Alessandria, Italy E-mail: [email protected]

Series Editors:

Professor Robert Hull

Professor Jürgen Parisi

University of Virginia Dept. of Materials Science and Engineering Thornton Hall Charlottesville, VA 22903-2442, USA

Universit¨at Oldenburg, Fachbereich Physik Abt. Energie- und Halbleiterforschung Carl-von-Ossietzky-Strasse 9–11 26129 Oldenburg, Germany

Professor R. M. Osgood, Jr.

Professor Hans Warlimont

Microelectronics Science Laboratory Department of Electrical Engineering Columbia University Seeley W. Mudd Building New York, NY 10027, USA

Institut f¨ur Festk¨orperund Werkstofforschung, Helmholtzstrasse 20 01069 Dresden, Germany

ISSN 0933-033X ISBN 978-3-540-71917-5 Springer Berlin Heidelberg New York Library of Congress Control Number:

2007927159

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Foreword

The FLEXIFUNBAR project is an IP SME addressing research issues on emerging technologies for the production of new flexible structures (paper, leather, technical textiles for applications in transport, medical, security and clothing). It will clearly support the development of new knowledge based added value products in textile, leather and paper industries. The work plan contains work packages tackling simultaneously integrated areas of research and scaling up (nanostructures, materials research for barrier effects, new production processes). With a critical mass of 45 partners FLEXIFUNBAR project is coordinated by an SME, the PME DUFLOT, manufacturer of technical non wovens for protection and insulation (having 2 ultramodern plants in France). It clearly serves multisectoral needs and industrial leadership is ensured in several work packages (Annebergs, Europlasma, Libeltex, Patraiki, Amkey management). The strength relies also on the adequacy of the part of the project dedicated to textile perfectly in line with the measures identified in the Communication on the future of the Textiles and Clothing sector adopted in October 2003 by the Commission to improve the competitive position of the Textiles and Clothing sector. Based on the very high number of partners a particular strong management structure has been adopted (governing board, exploitation committee, scientific committee) providing SMEs to have a decisive role and the majority vote in the decision making structure of the project. This pragmatic management structure is also well adapted to the nature of the project (integration of different activities, different disciplines, stakeholders coming from different industrial sectors). IPR issues addressed in the consortium agreement towards SMEs benefits are clearly defined. Brussels, June 2007

Odile Demuth Program Officer CE

Preface

Everyone relies on barrier structures in one form or another, to protect against extraneous environments such as fire, noise, thermal extremes and microorganisms; to shield from electrostatic or electromagnetic fields; or to filter dust and other matter. In order to meet the technical and economic requirements demanded from this diverse range of applications, barrier structures should ideally be flexible, have multifunctional properties and be easily fabricated at an acceptable cost. Materials used for this purpose are generally based on paper, leather and natural or synthetic textiles, generally modified to enhance their functionality and resulting service properties. Within Europe, products from this sector are, in the main, produced by high-tech SME companies. However, as a consequence of strong foreign competition and imports, particularly in the field of textiles, European industry is facing a familiar scenario: to survive, it must become more cost efficient and increasingly innovative in the development of high performance barrier products. This challenge forms the underlying driver for the Integrated Project termed FLEXIFUNBAR: “Multifunctional barrier for flexible structure; textile, leather and paper”, funded through the 6th European Framework Programme over the period 2004-2008. In this context, the programme partners have proposed fourteen themes concerned with different approaches to functionalising flexible barrier structures, which define the scope of this book. Its aim is to give a complete overview of the present state of the art of these materials, including methods for barrier fabrication and their evaluation. For the first time, this book provides a multidisciplinary approach to the subject, covering a number of industrially relevant topics including: barriers to fire; enhanced antibacterial properties; shielding from electrostatic, electromagnetic and acoustic waves; and means for preventing odour. Particular consideration is also given to developments and opportunities from using nanomaterials and fabrication technologies, together with advanced techniques for characterising their structure and properties.

VIII

Preface

Acknowledgements We gratefully acknowledge Prof. J. Catrysse (KHBO, Dept. IW&T, Oostende, Belgium) ; Pr. G. Chase (Microscale Physiochemical Engineering Center, University of Akron, United States) ; Pr. G. Dennler (Linz Institute for Organic Solar Cells, Linz, Austria) ; Pr. M. Fonseca de Almeida (Universidade do Porto, Porto, Portugal) ; Pr. G. Gallone (Universita’ degli di Pisa, Italy) ; Dr. T. Gambichler, (Department of Dermatology, Ruhr University Bochum, Bochum, Germany) ; Dr. S. Giraud (ENSAIT, GemTEX, Roubaix, France) ; Pr. R. Hull (University of Bolton, UK) ; Pr. T. Kashiwagi (NIST, Gaithersburg, United States) ; Dr. J. Levalois-Gr¨ utzmacher (ETH Honggerberg, Zurich, Switzerland) ; Pr. B. Mahltig (GMBU e. V., Arbeitsgruppe Funktionelle Schichten, Dresden, Germany) ; Pr. C.M. Melo de Pereira (Universidade do Porto, Porto, Portugal) ; Dr A. Sarkar (Imperail College, London, UK) ; Dr. B. Schartel (DAM, Berlin, Germany) ; Dr. T. Steigmaier (ITV, Denkendorf, Germany) ; Pr. J. Tiller (Freiburger Materialforschungszentum (FMF), Freiburg, Germany) ; Pr. G.F. Ward (Nonwoven Technologies, Inc., Alpharetta, GA, United States) ; Pr. C. Wilkie (Marquette University, United States) ; Pr. Z. Yan (Lund University, Sweden) for their extensive and helpful work as referees. We acknowledge the European Commission for their financial support of the FLEXIFUNBAR Project (NMP2-CT-2004-505864). Villeneuve d’Ascq, Alessandria, June 2007

S. Duquesne, C. Magniez, G. Camino

Contents

Part I Mono-Functional Barrier Effects - Review 1 The Application of Fire-Retardant Fillers for Use in Textile Barrier Materials P.R. Hornsby . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Fire-Retardant Fillers and Limitations for Textile Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Mechanism and Application of Conventional Fire-Retardant Fillers . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Scope of Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Flame Retardancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Smoke Suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 Synergism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Nano-Size Fire-Retardant Fillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Antimicrobial Functionalisation of Textile Materials E. Heine, H.G. Knops, K. Schaefer, P. Vangeyte, and M. Moeller . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Functionalisation of Fibre Material by Application of Effective Substances . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Antimicrobial Finishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Medical Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Functionalised Fibres: Application of Effective Agents During Melt Spinning . . . . . . . . . . . . . . . 2.2.4 Use of Silver as an Antimicrobial Agent in Textile Functionalisation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Requirements for Antimicrobial Agents on Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Antimicrobial Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 3 3 5 5 6 10 11 14 19 19 23 24 24 24 24 25 26 26 27

X

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2.3 2.4

Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Micro-Encapsulation of Antimicrobial Agents for Hygienic Functionalisation of Textiles . . . . . . . . . . . . . . . . . . . . . . 2.5 Antimicrobial Peptides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Antimicrobial Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 Antimicrobial Finishing Methodologies . . . . . . . . . . . . . . . . . 2.6.2 Antimicrobial Polymers and Their Effect . . . . . . . . . . . . . . . 2.7 Chitin and Chitosan Derivatives as Antimicrobial Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Synthesis of Chitin Derivatives as Antimicrobial Agents for Textiles . . . . . . . . . . . . . . . . . . . 2.8 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35 36 37 37

3 Intumescent Flame-Retardant Treatments for Flexible Barriers R. Kozlowski, D. Wesolek, M. Wladyka-Przybylak, S. Duquesne, A. Vannier, S. Bourbigot, and R. Delobel . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Mechanisms of Intumescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Flame-Retarded Natural Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Cotton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Flame-Retarded Synthetic Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Poly(Ethylene Terephtalate) . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Polypropylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Polyamide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39 39 41 43 43 51 51 53 57 58 59

4 Protection Against Electrostatic and Electromagnetic Phenomena S. Nurmi, T. Hammi, and B. Demoulin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Protection Against Electrostatic Phenomena . . . . . . . . . . . . . . . . . . . 4.1.1 Static Electricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Fundamental Principles of Electrostatics . . . . . . . . . . . . . . . 4.1.3 Electrostatic Charging and Textile Type Materials . . . . . . . 4.1.4 Charging Mechanisms in Textiles . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Charge Dissipation Mechanisms of Textiles . . . . . . . . . . . . . 4.1.6 Electrostatic Control Fabrics and Garments . . . . . . . . . . . . 4.1.7 Review of Techniques on Fibrous Materials for Protection Against Electrostatic Discharge . . . . . . . . . . 4.2 Protection Against Electromagnetic Phenomena . . . . . . . . . . . . . . . . 4.2.1 Electromagnetic Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Basic Electromagnetic Principles . . . . . . . . . . . . . . . . . . . . . .

27 27 29 29 30 31 34

63 63 63 64 67 68 69 71 72 74 74 75

Contents

4.2.3 Uniform Plane Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Electromagnetic Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.5 Flexible Electromagnetic Shields . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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77 78 81 82

Part II New Technologies for Barrier Effects 5 Fire-Retardant Mechanisms in Polymer Nano-Composite Materials A. Castrovinci and G. Camino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5.2 Overview of Commercially Available Nano-Fillers . . . . . . . . . . . . . . . 88 5.2.1 Three Dimension Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 5.2.2 Two Dimension Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.2.3 One Dimension Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 5.2.4 Synthetic 3D or 2D Nano-Fillers . . . . . . . . . . . . . . . . . . . . . . 93 5.3 Structure of Nano-Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 5.4 Combustion Behaviour of Polymer Nano-Composites . . . . . . . . . . . . 95 5.5 Mechanism of Nano-Composites Combustion . . . . . . . . . . . . . . . . . . . 97 5.5.1 Barrier Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.5.2 Charring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 5.6 Fire-Retardant Nano-Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 5.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 6 Cold Plasma Technologies for Surface Modification and Thin Film Deposition C. Jama and R. Delobel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 6.1 Classification of Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 6.2 Cold Plasma Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 6.3 Applications of Cold Plasma Technology . . . . . . . . . . . . . . . . . . . . . . 112 6.3.1 Functionalisation of Organic and Inorganic Polymeric Surfaces . . . . . . . . . . . . . . . . . . . . . . 112 6.3.2 Plasma-Assisted Thin Films Deposition . . . . . . . . . . . . . . . . 113 6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 7 Nano-Fibres for Filter Materials K. Schaefer, H. Thomas, P. Dalton, and M. Moeller . . . . . . . . . . . . . . . . . 125 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 7.2 Principle of Electrospinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 7.2.1 Practical Electrospinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 7.2.2 Nano-Fibres Produced by Electrospinning form Polymer Solutions or Melts . . . . . . . . . . . . . . . . . . . . . . 130 7.2.3 Electrospraying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

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Application of Nano-Fibres or Nano-Webs as Filter Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 7.4 New Developments in Electrospinning . . . . . . . . . . . . . . . . . . . . . . . . . 135 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 8 The Development of Non-Wovens T. Le Blan, M. Vouters, C. Magniez, and X. Normand . . . . . . . . . . . . . . . 139 8.1 Definition of Non-Wovens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 8.2 Raw Materials for Non-Wovens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 8.2.1 Fibres and Filaments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 8.2.2 Other Raw Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 8.3 Web-Forming Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 8.3.1 Drylaid Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 8.3.2 Spunlaid and Meltblown Technologies . . . . . . . . . . . . . . . . . . 144 8.3.3 Other Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 8.4 Bonding Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 8.5 Web Conversion and Finishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 8.6 Barrier Effect in Non-Wovens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 8.6.1 Regularity and Homogeneity of Materials . . . . . . . . . . . . . . . 149 8.6.2 Saving of Raw Materials with Equal or Superior Performances . . . . . . . . . . . . . . . . . . 149 8.6.3 Functionalisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 8.6.4 Characterisation/Standardisation . . . . . . . . . . . . . . . . . . . . . . 149 8.6.5 Mechanical Performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 8.6.6 Lifetime Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 8.6.7 Comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 9 Mechanical Models and Actuation Technologies for Active Fabrics: A Brief Survey of the State of the Art F. Carpi, M. Pucciani, and D. De Rossi . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 9.2 Knitted Fabrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 9.3 Mechanical Behaviour of Weft-Knitted Fabrics . . . . . . . . . . . . . . . . . 153 9.3.1 Geometrical Identification of the Yarn Loop . . . . . . . . . . . . 153 9.3.2 Rheological Models and Constituting Elements . . . . . . . . . . 153 9.3.3 Potential Energy as an Approach to Describe Non-Linear Mechanical Properties of Fabrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 9.4 Different Approaches to Describe the Mechanical Behaviour of Weft-Knitted Fabrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 9.4.1 Load–Extension Behaviour of Weft-Knitted Fabrics . . . . . . 159 9.4.2 A Theoretical Analysis Based on the Elastic Theory . . . . . 160 9.5 Woven Fabrics with Barrier Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

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9.6

Technologies for Actuation of Fabrics . . . . . . . . . . . . . . . . . . . . . . . . . 163 9.6.1 Shape Memory Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 9.6.2 Shape Memory Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 9.6.3 Future Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 9.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 Part III Modelling 10 Pyrolysis Modelling Within CFD Codes P. Van Hees and J. Axelsson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 10.2 Description of Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 10.3 Additional Changes or Additions in the Model . . . . . . . . . . . . . . . . . 175 10.4 Sensitivity Analysis of the Physical Flame Spread Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 10.4.1 Influence of the Pyrolysis Temperature on the Results . . . 175 10.4.2 Influence of Heat of Pyrolysis on the Results . . . . . . . . . . . . 176 10.4.3 Influence of the Heat of Combustion on the Results . . . . . . 176 10.4.4 Influence of the Char Density on the Results . . . . . . . . . . . . 178 10.4.5 Influence of the Specific Heat on the Results . . . . . . . . . . . . 178 10.4.6 Influence of the Thermal Conductivity on the Results . . . . 178 10.4.7 Influence of the Number of Iterations and Thickness of Numerical Strips on the Results . . . . . . . 179 10.4.8 Influence of Ignition Temperature for Non-Charring Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 10.4.9 Final Evaluation and Procedure to Define Material Parameters . . . . . . . . . . . . . . . . . . . . . . . . 182 10.5 Verification of the Physical Flame Spread . . . . . . . . . . . . . . . . . . . . . . 184 10.5.1 Verification with Cone Calorimeter Test Data . . . . . . . . . . . 184 10.5.2 Verification with a Stand-Alone Flame Spread Model . . . . 186 10.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 11 Life-Cycle Assessment Including Fires (Fire-LCA) P. Andersson, M. Simonson, and H. Stripple . . . . . . . . . . . . . . . . . . . . . . . . 191 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 11.2 Life-Cycle Assessment: The Basic Concept . . . . . . . . . . . . . . . . . . . . . 192 11.3 Methodology: An Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 11.3.1 The Risk Assessment Approach . . . . . . . . . . . . . . . . . . . . . . . 196 11.3.2 The Fire-LCA System Description . . . . . . . . . . . . . . . . . . . . . 196 11.4 Fire-LCA Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 11.4.1 Goal and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 11.4.2 Special Fire Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

XIV

Contents

11.4.3 Statistical Fire Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 11.4.4 Replacement of Burned Materials . . . . . . . . . . . . . . . . . . . . . 204 11.4.5 Data Inventory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 11.4.6 Competences Needed to Conduct a Fire-LCA Analysis . . . 208 11.5 Evaluation of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 11.6 Computer Modelling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 11.7 Simplified Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 11.7.1 Background Minimisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 11.7.2 Parameter Minimisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 11.7.3 Scenario Minimisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 11.8 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 11.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 12 Modelling of Euroclass Test Results by Means of the Cone Calorimeter P. Van Hees and J. Axelsson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 12.1 Description of Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 12.1.1 Principles of Prediction Model . . . . . . . . . . . . . . . . . . . . . . . . 216 12.1.2 Burning Area Growth Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 12.1.3 Criteria for Flame Spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 12.1.4 Calculation of Heat Release Rate . . . . . . . . . . . . . . . . . . . . . . 219 12.1.5 Correction for Cone Calorimeter Data Obtained at Other Heat Flux Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 12.2 Sensitivity Study of Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 12.2.1 Influence of HRR Threshold and Ignition Time . . . . . . . . . . 220 12.2.2 Influence of Backing Board . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 12.2.3 Shiny Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 12.3 Guidance and Description Testing Protocol . . . . . . . . . . . . . . . . . . . . 222 12.4 Comparison and Discussion of Simulation Results . . . . . . . . . . . . . . 224 12.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Part IV Applications of Multifunctional Barriers 13 Characterisation of Barrier Effects in Footwear R.M. Silva, V.V. Pinto, F. Freitas, and M.J. Ferreira . . . . . . . . . . . . . . . . 229 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 13.2 Upper Part: Leather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 13.2.1 Leather Tanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 13.2.2 Water Resistance Barrier Effect . . . . . . . . . . . . . . . . . . . . . . . 232 13.2.3 Flame Resistance Barrier Effect . . . . . . . . . . . . . . . . . . . . . . . 236 13.2.4 Micro-Organisms Resistance Barrier Effect . . . . . . . . . . . . . 244

Contents

XV

13.3

Rubber Outsoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 13.3.1 Flame Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 13.3.2 Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 13.4 Complete Footwear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 13.4.1 Water Resistance Barrier Effect . . . . . . . . . . . . . . . . . . . . . . . 254 13.4.2 Flame Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 13.4.3 Thermal Resistance Barrier Effect . . . . . . . . . . . . . . . . . . . . . 259 13.4.4 Chemical and Micro-Organism Resistance Barrier Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 13.4.5 Other . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 14 Filtration Technologies in the Automotive Industry E. Jandos, M. Lebrun, C. Brzezinski, and S. Capo Canizares . . . . . . . . . . 269 14.1 Basic Filtration Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 14.1.1 What is Filtration? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 14.1.2 Characteristics of Contaminant Particles . . . . . . . . . . . . . . . 269 14.1.3 Mechanisms of Capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 14.2 Filtration Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 14.2.1 Engine Air Intake Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 14.2.2 Technical Features of a Fibrous Medium . . . . . . . . . . . . . . . 280 14.2.3 Filtration Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 14.2.4 Filtration Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 14.3 Test Methodologies: Standards and Benches . . . . . . . . . . . . . . . . . . . 285 14.3.1 Requirements of Air Filter Media . . . . . . . . . . . . . . . . . . . . . . 285 14.3.2 Material Related Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 285 14.3.3 Filter Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 14.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

List of Contributors

P. Andersson SP Swedisch National Testing and Research Institute Fire Technology Box 857, 50115 Bor˚ as, Sweden J. Axelsson SP Swedisch National Testing and Research Institute Fire Technology Box 857, 50115 Bor˚ as, Sweden S. Bourbigot Laboratoire Proc´ed´es d’Elaboration de Revˆetements Fonctionnels LSPES/UMR-CNRS 8008 Ecole Nationale Sup´erieure de Chimie de Lille 59650 Villeneuve d’Ascq, France C. Brzezinski Centre Technique de Lens ZAC de la Croisette Mecaplast Group 62300 Lens, France G. Camino Centro di Cultura per l’Ingegneria delle Materie Plastiche (CDCMP) Politecnico di Torino Allesandria Branch via T. Michel 5 15100 Alessandria, Italy

S. Capo Canizares Centre Technique de Lens ZAC de la Croisette Mecaplast Group 62300 Lens, France F. Carpi University of Pisa School of Engineering Interdepartmental Research Centre “E. Piaggio” via Diotisalvi 2 56100 Pisa, Italy A. Castrovinci University of Applied Sciences of Southern Switzerland Galleria 2, CH-6928 Manno, Switzerland P. Dalton DWI an der RWTH Aachen e.V. Pauwelsstr. 8 52056 Aachen, Germany R. Delobel Centre de Recherche et d’ Elude sur les Proc´ed´es d’ Ignifugation des Mat´eriaux Polym`eres (CREPIM)

XVIII List of Contributors

Parc de la Porte Nord Av. C. Colomb 62700 Bruay-la-Buissi´ere, France B. Demoulin University of Lille 1, Telice Group DHS IEMN, Bˆ at. P3 F-59655 Villeneuve d’Asq cedex France S. Duquesne Laboratoire Proc´ed´es d’Elaboration de Revˆetements Fonctionnels LSPES/UMR-CNRS 8008 Ecole Nationale Sup´erieure de Chimie de Lille 59650 Villeneuve d’Ascq, France D. De Rossi University of Pisa School of Engineering Interdepartmental Research Centre “E. Piaggio” via Diotisalvi 2 56100 Pisa, Italy M.J. Ferreira Centro Tecnol´ogico do Cal¸cado Rua de Fund˜ oes, Devesa Velha 3701-121 S˜ ao Jo˜ao Madeira, Portugal F. Freitas Centro Tecnol´ogico do Cal¸cado Rua de Fund˜ oes, Devesa Velha 3701-121 S˜ ao Jo˜ao Madeira, Portugal T. Hammi University of Lille 1, Telice Group DHS IEMN, Bˆ at. P3 F-59655 Villeneuve d’Asq cedex France E. Heine DWI an der RWTH Aachen e.V. Pauwelsstr. 8 52056 Aachen, Germany

P.R. Hornsby School of Mechanical and Aerospace Engineering Queen’s University Belfast BT9 5AH, UK C. Jama Laboratoire Proc´ed´es d’Elaboration de Revˆetements Fonctionnels LSPES/UMR CNRS 8008 Ecole Nationale Sup´erieure de Chimie de Lille 59650 Villeneuve d’Ascq, France E. Jandos Centre Technique de Lens ZAC de la Croisette Mecaplast Group 62300 Lens, France H.G. Knops DWI an der RWTH Aachen e.V. Pauwelsstr. 8 52056 Aachen, Germany R. Kozlowski Institute of Natural Fibres ul. Wojska Polskiego 71B60-630 Poznan, Poland M. Lebrun Centre Technique de Lens ZAC de la Croisette Mecaplast Group 62300 Lens, France T. Le Blan Institut Fran¸cais du Textile et de l’Habillement 2 rue de la Recherche, BP637 59656 Villeneuve d’Ascq cedex France C. Magniez Institut Fran¸cais du Textile et de l’Habillement 2 rue de la Recherche, BP637 59656 Villeneuve d’Ascq cedex France

List of Contributors

M. Moeller DWI an der RWTH Aachen e.V. Pauwelsstr. 8 52056 Aachen, Germany X. Normand Institut Fran¸cais du Textile et de l’Habillement 2 rue de la Recherche, BP637 59656 Villeneuve d’Ascq cedex France S. Nurmi VTT P.O. Box 1300 33101 Tampere, Finland V.V. Pinto Centro Tecnol´ogico do Cal¸cado Rua de Fund˜ oes, Devesa Velha 3701-121 S˜ ao Jo˜ao Madeira, Portugal M. Pucciani University of Pisa School of Engineering Interdepartmental Research Centre “E. Piaggio” via Diotisalvi 2 56100 Pisa, Italy K. Schaefer DWI an der RWTH Aachen e.V. Pauwelsstr. 8 52056 Aachen, Germany

XIX

H. Stripple IVL Swedisch Environmental Research Institute Box 5302, 40014 Gothenburg, Sweden H. Thomas DWI an der RWTH Aachen e.V. Pauwelsstr. 8 52056 Aachen, Germany P. Vangeyte DWI an der RWTH Aachen e.V. Pauwelsstr. 8 52056 Aachen, Germany A. Vannier Laboratoire Proc´ed´es d’Elaboration de Revˆetements Fonctionnels LSPES/UMR-CNRS 8008 Ecole Nationale Sup´erieure de Chimie de Lille 59650 Villeneuve d’Ascq, France M. Vouters Institut Fran¸cais du Textile et de l’Habillement 2 rue de la Recherche, BP637 59656 Villeneuve d’Ascq cedex France

R.M. Silva Centro Tecnol´ogico do Cal¸cado Rua de Fund˜ oes, Devesa Velha 3701-121 S˜ ao Jo˜ao Madeira, Portugal

D. Wesolek Institute of Natural Fibres ul. Wojska Polskiego 71B60-630 Poznan, Poland

M. Simonson SP Swedisch National Testing and Research Institute Fire Technology Box 857, 50115 Bor˚ as, Sweden

M. Wladyka-Przybylak Institute of Natural Fibres ul. Wojska Polskiego 71B60-630 Poznan, Poland

Part I

Mono-Functional Barrier Effects - Review

1 The Application of Fire-Retardant Fillers for Use in Textile Barrier Materials P.R. Hornsby

Summary. Available fire retardant fillers are reviewed with reference to their mechanism of action, both as fire retardants and smoke suppressant additives. Means for enhancing their efficiency are considered using flame retardant synergists and nanoparticulate filler variants of magnesium hydroxide, hydrotalcite and boehemite.

1.1 Introduction There has been a trend in recent years, driven principally by environmental and safety considerations, towards increasing use of halogen-free fire-retardant systems, including hydrated fillers, such as magnesium and aluminium hydroxides. Their application in textile fibres, however, is limited by the need for high filler levels to confer adequate fire protection and their large particle size, which is generally of the same order as the diameter of the polymer fibre to which they are added. Both these factors significantly limit the spinnability and tenacity of such compositions. This review will consider current fire-retardant fillers available, their characterisation, application to different polymer types, current understanding of their mechanism of action as fire retardants and smoke suppressants and means for improving their efficiency. This includes combination with other fire retardants as synergists. Emphasis will be given to issues specifically relating to their use in polymer fibres and means for potentially overcoming drawbacks, which may mitigate their application in textile barrier structures. To this end, discussion will also focus on nano-scale variants of these materials with, in particular, magnesium hydroxide, hydrotalcite and boehemite.

1.2 Fire-Retardant Fillers and Limitations for Textile Use Particulate fillers can strongly influence the combustion characteristics of a polymer, including its resistance to ignition, and the extent and nature of

4

P.R. Hornsby

smoke and toxic gas emission products. This may result from simple dilution of the combustible fuel source, slowing down the diffusion rate of oxygen and flammable pyrolysis products and changing the melt rheology of the polymer, thereby affecting its tendency to drip. However, depending on the nature of the filler, the heat capacity, thermal conductivity and emissivity of the polymer composition may also change, giving rise to heat transfer and thermal reflectivity effects, which can also slow the rate of burning. In general, fillers cannot be classed as totally inert in relation to their effect on polymer combustion, however some, notably metal hydroxides, hydrates and carbonates can confer additional flame retardancy and smoke suppressing qualities being in widespread use for this purpose. These undergo endothermic decomposition, which cools the solid, or condensed phase, and release gases (water and/or carbon dioxide), which dilute and cool flammable combustion products in the vapour phase. The inorganic residue remaining after filler decomposition may also be highly significant in providing a thermally insulating barrier between the underlying polymer substrate and external heat source, in addition to contributing to overall smoke suppression. In this connection, materials, which are currently used or have potential for use as fire-retardant fillers, are listed in Table 1.1, together with relevant thermal properties and gaseous products evolved on decomposition. In addition to their fire-retarding efficiency, to be commercially exploitable, these fillers should ideally be inexpensive, colourless, non-toxic, free from conductive contaminants, and readily available. Being thermally unstable they must have a sufficiently high decomposition temperature to withstand thermoplastics melt processing temperatures. However, to achieve maximum fire-retarding effect, thermal decomposition should generally occur near the onset of polymer degradation, with subsequent release of flammable volatiles. When used in load-bearing situations, the presence of the filler generally has an adverse effect on strength and toughness of the composite, which can be limited by judicious formulation, and principally through the use of surface treatments. The implications of filler on polymer viscosity and mechanical properties are exacerbated by the high filler levels normally required to achieve acceptable resistance to combustion. These aspects are not considered in this review. The size and shape of the filler particles are also important considerations. Filler particle size and the need to use high addition levels to confer adequate fire retardancy create particular limitations on their potential use in textile barrier structures, both in terms of their processability and ultimate physical properties.

1 The Application of Fire-Retardant Fillers

5

Table 1.1. Current and potential fire-retardant fillers [1] Candidate material (common names and formula) Nesquehonite [MgCO3 · 3H2 O] Alumina trihydrate, aluminium hydroxide [Al(OH)3 ] Basic magnesium carbonate, hydromagnesite [4MgCO3 · Mg(OH)2 · 4H2 O] Sodium dawsonite [NaAl(OH)2 CO3 ] Magnesium hydroxide [Mg(OH)2 ] Magnesium carbonate subhydrate [MgO · CO2(0.96) H2 O(0.30) ] Calcium hydroxide [Ca(OH)2 ] Boehemite [AlO(OH)] Magnesium phosphate octahydrate [Mg3 (PO4 )2 · 8H2 O] Calcium sulphate dihydrate, gypsum [CaSO4 · 2H2 O]

Approximate onset of decomposition (◦ C)

Approximate enthalpy of decomposition (kJ g−1 × 103 )

70–100

1,750

71

39

180–200

1,300

34.5

34.5

220–240

1,300

57

19

38

240–260

Not available

43

12.5

30.5

300–320

1,450

31

31

0

340–350

Not available

56

9

47

430–450

1,150

24

24

0

340–350 140–150

560 Not available

15 35.5

15 35.5

0 0

60–130

Not available

21

21

0

Volatile content (%w/w) Total H2 O

CO2 32 0

1.3 Mechanism and Application of Conventional Fire-Retardant Fillers 1.3.1 Scope of Application Substantial industrial use is made of the principal fire-retardant fillers, aluminium hydroxide (ATH), magnesium hydroxide (MH) and, to a lesser extent, hydromagnesite/huntite mixtures. Whereas there is widespread application of ATH in elastomers, thermosetting resins and thermoplastics, its use is generally limited to polymers processed below 200◦ C. MH is stable to temperatures above 300◦ C, however, permitting incorporation in polymers such as polypropylene, polyamides and polyketones, in addition to certain elastomers, where increased thermal stability is essential. Its use in thermoplastic polyesters is limited by its tendency to catalytically decompose the

6

P.R. Hornsby

polymer during processing [2], whereas unlike ATH, in unsaturated polyester resins MH acts as chain extender adversely affecting resin rheology. Although it has been shown that this effect can be ameliorated by using maleic acid coated grades of MH filler, the long-term stability of these systems is still questionable [3]. Hydromagnesite (usually found in combination with huntite) has intermediate thermal stability, decomposing between 220 and 240◦ C [4, 5]. Mixtures of these minerals are used in wire and cable applications, due to their higher thermal resistance than ATH and lower cost compared to MH. They have also been considered for use in ethylene–propylene copolymers [6] and PVC formulations, where reduced smoke and acid gas emission are requirements [7]. In thermosets, there is widespread use of ATH in unsaturated polyester resin moulding compounds, for example for automotive parts, epoxy and phenolic resin formulations, especially in electrical applications, and cross-linked acrylic resins where flame retardancy is a key requirement [2]. In thermoplastics and elastomers, applications for ATH have been found in rigid poly(vinyl chloride), high, low and linear low density polyethylene, ethylene–propylene rubber, ethylene–propylene–diene cross-linked rubbers, ethylene–ethyl acrylate copolymers, and ethylene-vinyl acetate copolymers [8–11]. Although it is claimed that ATH can also be used in polypropylene, the limited thermal stability of this filler generally necessitates special compounding and processing measures, which has inhibited its large-scale application in this polymer [12]. In this connection, a modified ATH has been reported with apparent thermal stability up to about 350◦ C and claimed to be suitable for use in many engineering resins [13]. A major use for both ATH and MH is in low smoke, halogen-free wire, cable and conduit applications, where there has been significant commercial activity [14–18]. 1.3.2 Flame Retardancy The relative performance of hydrated fire-retardant fillers in polymers strongly depends on the nature and origin of the filler type and the chemical characteristics of the host polymer, in particular, its decomposition mechanism. In this regard, specific interactions may exist between certain polymers and fillers, which influence their mechanism of action [19]. However, compared to alternative fire retardants, including phosphorousbased intumescent and halogen-containing formulations, hydrated fillers are relatively ineffective, requiring addition levels of up to 60% by weight to achieve acceptable combustion resistance [20]. For example, with polypropylene, 60% by weight would be required to achieve an oxygen index in excess of 26%. At the same addition level in polyamide 6, however, an oxygen index of nearly 70% can be obtained [21]. Although this might seem more than enough to suppress ignition with this polymer, polyamides are prone to dripping on decomposition. This can be the determining factor when flammability is assessed by the UL94 test procedure widely used in industry for screening

1 The Application of Fire-Retardant Fillers

7

purposes. Increasing filler level tends to raise the viscosity of the decomposing polymer, inhibiting its tendency to drip [22]. The following contributing effects may combine to determine the overall mechanism of fire-retardant fillers. Thermal Effects from Filler As mentioned earlier, a characteristic of hydrated fire-retardant fillers is that they undergo endothermic breakdown. Differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) have been widely applied to study their thermal decomposition [23]. Comparing magnesium hydroxide grades, wide differences have been reported in the magnitude of their decomposition endotherm and decomposition temperature [24]. In addition to inherent characteristics of the filler types, the apparent decomposition behaviour may also be influenced by the analytical procedure adopted [23]. This includes sample size, rate of heating, rate of inert gas flow rate and degree to which the pan is sealed. Furthermore, it has also been reported that different grades of magnesium hydroxide may degrade at different rates, dependent on filler morphology and/or surface area [25]. The heat capacity of these fillers and in particular, their strong endotherm can strongly influence the input of heat required for polymer decomposition and release of combustible volatiles [21]. This effect has been modelled using a heat balance approach which can be applied to the whole combustion process [23]. By this means it can be shown that at sufficiently high filler levels, hydrated fillers can also reduce the mass burning rate by inhibiting the rates of heat transfer from the flame to the underlying matrix, causing the flame to extinguish due to fuel starvation [26]. Hence reductions in applied heat flux or increased surface heat losses will lead to a decrease in the mass burning rate of the polymer, as has been reported for polypropylene/aluminium hydroxide composites [27]. Forced combustion studies provide a method for measuring rates of heat transfer through a fire-retardant polymer composition exposed to an ignition source at its outer surface. In studies involving thermal breakdown of polypropylene, magnesium and aluminium hydroxides decompose to their respective oxides, which together with any carbonaceous char produced, provide an effective thermal barrier, reducing heat transmission to the underlying substrate [28]. Similar behaviour has been observed with other polymer types, including modified-polyphenylene oxide (PPO), polybutylene terephthalate (PBT) and acrylonitrile–butadiene–styrene copolymer (ABS) [20]. Microscopic analysis of the oxide/char residue formed on combustion of magnesium hydroxide-filled polypropylene has revealed an oxide morphology similar in form to the parent hydroxide [29]. In this example, hexagonal platelets appear to align predominantly in the same plane and in some cases overlap, which contrasts with large aggregated structures derived from hydroxide particles formed from association of small crystallites. There is some

8

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evidence of increased crystal growth and that the coherency of the oxide particles contributes to the stability of the decomposition residue observed from combustion products arising from oxygen index tests. Although the magnitude of possible inter-particle attractions arising from oxide residues is unknown, the strength of agglomerates containing magnesium hydroxide pseudomorphs has been estimated to be 50 MN m−2 , arising from physio-chemical association between magnesium oxide and water [30]. Dilution of Combustible Polymer The presence of up to 60% by weight of fire-retardant filler results in around 35% by volume reduction of combustible polymer (in the case of magnesium hydroxide). In studies using polypropylene compositions containing different grades of magnesium hydroxide, magnesium oxide and glass beads, values of heat release rate (HRR) were determined by cone calorimetry [29]. It was found that rates of heat release were significantly reduced, after allowing for the volume dilution of each of these fillers. However in this regard, magnesium oxide was far more effective than the glass beads, even though both are nominally considered to be inert fillers. This suggests that even with thermally stable and nominally inert fillers, particle geometry, surface chemistry and perhaps thermal conductivity, have an active role in influencing fire retardancy. Filler Polymer Interaction TGA and DSC can provide useful information concerning the nature of filler/polymer interaction, together with their relative decomposition temperatures, when used in combination with evolved gas analysis (EGA) and on-line FTIR techniques. It was demonstrated that thermal breakdown of magnesium hydroxide exerts a significant prodegrative action on polyamide 6 (PA-6) and polyamide 66 (PA-66) which has been attributed to water release and resulting hydrolysis of the polymer chain [31]. Evolved gases released from both filled and unfilled PA compositions were shown to be water, carbon monoxide, carbon dioxide, ammonia and various hydrocarbon fragments. PA-6 compositions were found to be significantly more fire retardant than corresponding formulations made using PA-66 and in PA-66, polymer degradation occurred before magnesium hydroxide breakdown, whereas there was much greater overlap in thermal decomposition of PA-6 and this filler. Despite the high oxygen index values obtained through introduction of magnesium hydroxide into polyamides, achieving a VO rating according to the UL94 test, strongly depends on their tendency to drip during combustion. It has been shown that different magnesium hydroxide filler variants influence the rheological behaviour of thermally decomposing polyamides in different ways and hence their resistance to dripping [32]. In general, plate-like filler particles operate more effectively in this regard [33].

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Several comparisons exist on the relative efficiency of magnesium and aluminium hydroxides in the same polymer type. One study, reported for polyethylene, showed that at an equivalent additive loading, these fillers gave the same oxygen index [34]. However in ethylene-vinyl acetate copolymer (EVA) with 30% vinyl acetate content, magnesium hydroxide yielded an oxygen index of 46%, whereas using aluminium hydroxide, this was measured as 37%. From non-isothermal thermo-gravimetric analysis, it was suggested that in this polymer, water release is delayed from aluminium hydroxide, whereas this is accelerated from magnesium hydroxide, possibly arising from acetic acid evolved from the polymer. In studies on the ignition and incandescence of filled polymers, both ATH and MH were found to increase the self-ignition temperature of an EVA copolymer, with magnesium hydroxide being more effective [35]. Using TGA, in these systems it was concluded that the solid-state afterglow effects observed were due to oxidation of carbonaceous residues. Vapour Phase Action The release of water and/or inert gas into the vapour phase on decomposition of hydrated fillers, also contributes to the overall fire retardation mechanism. Although little detailed analysis has been undertaken in this area, it is generally considered that water release into the vapour phase exerts a beneficial effect through dilution and cooling of volatiles produced on polymer degradation [21]. Effects of Filler Particle Size and Morphology It has often been observed that different grades of the same fire-retardant filler can give significantly different effects, despite apparent similarities in their endothermic decomposition or release of inert gas. Whilst this may, in part, be an outcome of the flammability test procedure applied, distinct particle size and particle morphology effects have been reported. These factors also have a significant bearing on the mechanical properties and melt rheology of polymer composites containing hydrated fillers. In relation to flammability, however, it has been shown using the UL94 vertical burn test that the effectiveness of magnesium hydroxide in polypropylene increased with decreasing particle size [36]. Similarly, in studies involving PMMA modified with ATH, fine grades (< 1 µm) gave markedly higher oxygen index values than coarser (45 µm) grades, particularly at filler loadings above 50% by weight [23]. ATH is reported to be less thermally stable as the particle size increases [37]. Early in the decomposition process the alumina produced is very reactive, readily combining with water vapour to rehydrate to ATH. In larger particles, water escaping nearer the centre of the particle has a larger diffusion path, giving more time to react with alumina formed near the surface of the decomposing particle. During this process boehmite

10

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or pseudo-boehmite is formed and, being a partial decomposition product, is more stable than ATH, decomposing at about 450◦ C. In relation to the effects of particle size on thermal stability, it has been shown that there is a greater transition from gibbsite to pseudo-boehmite as the particle size increases [38]. These observations on particle size effects are especially significant in the context of nano-sized hydrated fillers, considered later, and whether at this scale, further improvements in efficiency are achievable. The thermal stability of magnesium hydroxide is also influenced by particle morphology. Spherical particles have been shown to decompose more slowly than platy structures, when heated isothermally at 390◦ C [21]. 1.3.3 Smoke Suppression Since most fatalities in fires arise from smoke-related effects, it is important to consider the influence of hydrated fillers on smoke emission during polymer combustion. An early study discussed the effects of calcium carbonate, ATH and MH fillers on smoke production from styrene butadiene (SBR) foams [39]. It was evident that all the fillers reduced soot formation relative to unfilled foam with the hydrated fillers being more effective than the calcium carbonate, which was considered to act merely as matrix diluent. ATH and MH were found to give enhanced char formation and promotion of solid-state cross-linking as opposed to pyrolytic degradation. The occurrence of afterglow, after extinction of the flame, was noted with MH and attributed to slow combustion of carbon residues. There have been a number of other reports demonstrating the smoke suppressing tendencies of hydrated fillers in various polymers including ethylene–propylene–diene elastomers [40], polypropylene [20], polystyrene [41], modified polyphenylene oxide, polybutylene terephthalate and ABS [28]. Not only do these hydrated fillers reduce overall levels of smoke released, but they can delay the onset of smoke evolution, potentially allowing more time for escape from the vicinity of a fire [21]. Although there has been extensive analysis of the composition and formation of soot from polymers undergoing combustion [42, 43], only limited work has been published on the mechanism of smoke suppression using hydrated fillers. It seems likely however, that the process is a consequence of the deposition of carbon onto the oxide surface, produced on decomposition of the hydrated filler [20]. Volatilisation of carbonaceous residue as carbon oxides subsequently occurs, which do not contribute to the obscuration effects of smoke. On hydroxide decomposition, these oxides have high surface areas [20] and being catalytically active [44], can promote both carbon deposition and subsequent oxidation processes [45]. The reduced combustion rate arising from the effects of the fire-retardant filler will also play a part in lowering the rate of smoke evolution and also improving oxygen to fuel ratios, further limiting the obscuration effect [23]. The role of evolved water from hydroxide decomposition is of interest, since water can also oxidise carbon. In this connection, smoke yields from

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polypropylene compounds containing magnesium hydroxide and magnesium oxide were compared [20]. These results showed little difference in levels of smoke evolution, suggesting that water has limited effect on the smoke suppression mechanism. These data are supported by CO emissions from burning ABS, which again demonstrate little distinction between oxide and hydrated forms of this magnesium compound [28] and also by the fact that the socalled water–gas oxidation reaction occurs at temperatures and pressures well in excess of those normally found at the burning surface of a polymer [46]. The afterglow or incandescence effect mentioned earlier, commonly observed following combustion of polymers containing hydrated fillers has been studied in EVA copolymer [35]. Using a heated quartz reactor purged with air, oxidation, self-ignition and incandescence were monitored as a function of temperature and filler loading. With magnesium and aluminium hydroxides, self-ignition temperature was raised progressively with increasing filler level, whereas the onset temperature for incandescence decreased. It was concluded that afterglow was due to catalytic oxidation of carbonaceous residues by surface active oxides produced from filler decomposition, with magnesium oxide showing greater activity [47]. 1.3.4 Synergism The challenge to improve the efficiency of hydrated fillers as fire retardants and thereby enable reductions in filler levels, has prompted much interest into the use of co-agents or synergists. As shown in Table 1.2 and in the following examples, significant improvements in overall performance can be achieved by this approach, although the mechanisms of interaction are frequently unclear. Combinations of MH and ATH can give improved performance when used together [48, 49], due to the increased range of endothermic reaction (180–400◦ C) and release of water in the vapour phase. The different metal oxides produced on dehydration may also contribute to this effect. ATH and red phosphorus (3–5%) have also been used in synergistic mixtures with to increase fire retardancy and enable lower filler loadings [50]. The addition of melamine and novolac (∼1%) to PP/MH mixtures has been found to reduce the burning time and give a UL94 VO rating at lower filler levels (30–50%) as opposed to a more usual value of around 60%, allowing the formulation to be mechanically more flexible. The novolac causes a structurally stabilising effect above the melting point of PP. Thermal evidence suggests that a novolac magnesia gel may be formed [51]. Metal hydroxides in combination with various formulations of siliconcontaining compounds have been used to reduce the amount of additive required to achieve a required level of flame retardancy in a variety of polymeric materials, including polyolefins [52,53]. Systems, which have been used, contain a combination of reactive silicone polymers, a linear silicone fluid or gum and a silicone resin, which is soluble in the fluid, plus a metal soap, in

12

P.R. Hornsby Table 1.2. Examples of synergists for metal hydroxides

Co-additives Antimony trioxide

Hydrated filler(s) Polymer(s) Effect(s) ATH, MH PVC (flexible), Reduced Polyolefins, EVA overall filler level/reduced smoke Antimony ATH PVC (flexible) Reduced trioxide/zinc overall filler borate level/lower smoke Borate ATH EVA Enhanced compounds flammability (zinc resistance at low borate/calcium co-additive borate) additions, increased char promotion MH/ATH ATH, MH PVC Reduced flammabicombinations lity, wider range of endotherm and water release, enhanced oxide thermal barrier (?) Molybdenum ATH, MH PVC Reduced flammacompounds bility and smoke (molybdenum emission, oxide/molybincreased char date promotion salts) Red ATH, MH – Reduced overall phosphorus filler levels, suppression of phosphine formation by metal hydroxide, coloured formulations, low co-additive additions SiliconATH, MH Polyolefins Enhanced containing flammability compounds resistance/reduced (organosmoke, silicones) improved processability and physical properties, handling issues

References [47, 60, 61]

[65]

[48, 49]

[50]

[52, 53]

1 The Application of Fire-Retardant Fillers Polyacrylonitrile fibres

ATH, MH

Transition metal oxides (nickel oxide/cobalt oxide)

ATH, MH

Metal nitrates (copper nitrate/iron nitrate)

ATH

Melamine

ATH, MH

Tin compounds ATH, MH (zinc stannate/zinc hydroxystannate)

Nano-clays

ATH, MH

Polyolefins

Char promotion, reduced filler levels can be pigmented Polyolefins Reduced overall filler levels, colour limitations, possible adverse toxicity effects EVA Enhanced flammability resistance with low co-additive additions PP Improved fire retardancy, reduced afterglow PVC, Cl-Rubbers, Enhanced EVA flammability resistance/ reduced smoke especially with ZH/ZHS-coated filler variants EVA Lower heat release rates/ reduced smoke emission used in combi- nation with tin compounds

13

[64]

[54]

[55]

[51, 60]

[57, 58]

particular magnesium stearate. However, there is little insight into how these formulations work. Some recent work has shown that required loading levels of metal hydroxides to flame retard polyolefins, could be reduced by addition of transition metal oxides as synergistic agents. For example, combination of 47.6% MH modified with nickel oxide in PP gave a UL94-VO flammability rating which would require ∼55% of unmodified MH [54]. These systems, however, can only be used where colour of the product is not important. The addition of metal nitrates to improve the flame retardancy of metal hydroxides and EVA has been reported [55]. Synergistic behaviour was observed by addition of 2% of copper nitrate to EVA containing only 33% ATH, in which the oxygen index was raised from 19.9 to 30.0%. The flammability properties of an intumescent fire-retardant PP formulation with added MH has been investigated [56]. The results show that the intumescent flame retardant ammonium polyphosphate-filled PP has superior

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flammability properties but gives higher CO and smoke evolution. The addition of MH was found to reduce smoke density and CO emissions, in addition to giving superior fire resistance. PP filled with ammonium polyphosphate, pentaerythritol and melamine has improved flammability performance, without reducing its mechanical properties [56]. In halogen-containing polymers, zinc hydroxystannate- or zinc stannatecoated hydrated fillers can give significantly improved flame resistance and lower smoke emission compared with uncoated fillers [57, 58]. The efficiency of the coated fillers was found to be superior to simple admixtures of these components, reflecting improved dispersion and possible synergism in these systems. Addition of silane cross-linkable PE copolymer to PE/metallic hydroxide systems can significantly improve the flame retardant properties of these materials allowing lower filler levels to be used [59]. The combination of melamine with hydrated mineral fillers can improve the fire retardancy behaviour of PP, eliminating at the same time the afterglow phenomenon, associated with these fillers used in isolation [60]. Similarly in EVA copolymer, antimony trioxide used in combination with metal hydroxides has been reported to reduce incandescence [47]. Chlorinated and brominated flame retardants are sometimes used in combination with metal hydroxides to provide enhanced fire-retardant efficiency, lower smoke evolution and lower overall filler levels. For example, in polyolefin wire and cable formulations, magnesium hydroxide in combination with chlorinated additives was reported to show synergism and reduced smoke emission [61]. A natural mineral filler, containing mainly huntite and hydromagnesite, has been used, together with a blend of antimony trioxide (Sb2 O3 ) and decabromodiphenyl oxide (DPDPO) to reduce the flammability of an ethylene–propylene copolymer [62]. The addition of very small amounts of fine carbon fibre [63] or polyacrylonitrile fibres [64] can reduce the level of inorganic hydroxide required to achieve UL94-VO flammability ratings in polyolefin compounds. These secondary additives are thought to function as char promoters. The addition of low levels (∼3%) of zinc borate to metal hydroxides can give synergistic effects [65]. For example in an EVA/MH formulation, LOI was found to increase from 39 to 43%, together with a significant reduction in heat release rate. Solid-state NMR of carbon in the residues showed that polymer fragments were in the char layer. It was suggested that zinc borate slows the degradation of the polymer, creating a vitreous protective physical barrier to combustion.

1.4 Nano-Size Fire-Retardant Fillers Nano-particulate fillers have been shown to significantly increase the properties of polymers using only small levels of additive, typically between 3 and

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5% by weight, which is far below that normally required from conventional micron-sized fillers to achieve a similar effect. In this regard, most emphasis has been given to silicate layer nano-composites, which after intercalation and exfoliation within the polymer structure, can yield large increases in mechanical properties, reduced gas and vapour transmission, and decreased flammability under certain test conditions. A large volume of work has been published on these additives, which is beyond the scope of this review. Similarly, polyhedral oligomeric silsesquioxanes (POSS), functionalised nanostructured chemicals with a silicon–oxygen core cage, have been applied in fire-retardant formulations. These materials are not generally considered as conventional fire-retardant fillers since they do not undergo endothermic thermal decomposition and will not be considered further. However, consideration is given here to hydrated nano-fillers with potential use as fire-retardant inclusions in textile fibres. The real challenge is to achieve acceptable fire retardancy at sufficiently low addition levels without unduly compromising melt spinnability and ultimately physical properties in the polymer fibre. To this end, there are a number of reports which consider the synthesis and application of magnesium hydroxide nano-particles as flame retardants in polymers [66–68]. Magnesium hydroxide nano-particles with different morphological structures of needle-, lamellar- and rod-like nano-crystals have been synthesised by solution precipitation reactions of magnesium chloride in the presence of water soluble polymer dispersants [69]. The size and morphologies of the magnesium hydroxide nano-crystals were controlled by the reaction conditions, in particular temperature, alkaline solution injection rate and reactant concentration. Needle-like morphologies were produced having dimensions of 10 × 100 nm2 , the laminar particles were made around 50 nm in diameter and an estimated 10 nm in thickness, with rod-like particles formed being 4 µm in length and 95 nm in diameter. TGA–DTA analysis of the lamellar crystals gave a pronounced weight loss between 250 and 396◦ C with a corresponding endothermic peak near to 354◦ C ascribed to decomposition of magnesium hydroxide. The overall weight loss for the MH was 30.1%. A comparison of LOI and tensile strength (TS) data for EVA/MH composites using three different types of MH, nano-size filler and micron-sized filler, showed that tensile strength for the larger particles decreased with increasing filler level, whereas values for the nano-MH/EVA composite increased. LOI results for the nano-MH were also superior, especially at high filler levels. The enhancement in fire retardancy seen using this nano-scale fillers was attributed to a more compact char structure creating more effective gas barrier properties [70]. However, it should be noted that in this work high nano-MH filler levels were still required to achieve reasonable resistance to ignition in common with more conventional MH fillers. Nanotubes of magnesium hydroxide have been synthesised by a solvothermal reaction from basic magnesium chloride and ethylenediamine solvent [71]. These were reported to have diameters of 80–150 nm, a wall thickness

16

P.R. Hornsby

of 30–50 nm and lengths of 5–10 µm. However, their use as polymer fire retardants was not considered. Nano-magnesium hydroxide and three forms of micro-magnesium hydroxide filler (all commercially available in China) were mixed with EPDM rubber and the mechanical properties and fire resistance of these composites determined [72]. The particle size of the micro-MH used was around 2.5 µm and the nano-MH had a hexagonal sheet-like structure with dimensions of 100 nm width by <50 nm thick. Thermal analysis on the fillers showed that the nanofiller had a lower decomposition temperature and a larger endotherm. The LOI of these composites were very similar although it was noted that the nano-filled material showed less tendency to drip and gave a more coherent char residue. The HRR from the nano-composite was substantially lower than the micro-filled systems (peak HHR 259 relative to 329–346 kW m−2 ) and the time to ignition increased to from 81–89 to 95 s. It should be noted, however, that filler levels for all the composites was again high with test samples containing between 60 and 100 phr of additive. A further study considered the use of nano-magnesium hydroxide on the mechanical and flame retarding properties of polypropylene [73]. The nanofiller was synthesised from MgCl2 and polyethylene glycol solution by an in situ technique yielding a particle size of 25 nm. Substantial improvements in mechanical properties were obtained by the addition of only small amounts of nano-magnesium hydroxide. For example the addition of 12 wt% of filler gave a 433% increase in Young’s modulus and a 35% improvement in flame retardancy (expressed as a burning rate). However, no comparison was given with conventional micron-sized magnesium hydroxide filler. Nano-magnesium hydroxide particles have been made by a phase transfer reaction with a mean particle size of about 10 nm and a core–shell structure comprising a magnesium hydroxide inner region surrounded by an alkyl chain [74]. This filler was combined in EVA at a 5 wt% loading and its fire properties compared with conventional 1 µm-sized filler at addition levels of 5, 30 and 60 wt%. It was shown that the peak HRR of the EVA/n-MH composite was reduced by 60% compared to pure EVA even though only 5 wt% of n-MH was present. The n-MH also causes a significant reduction in smoke (specific extinction area) although LOI was not significantly affected. It was evident that the n-MH significantly outperformed µ-MH at equivalent filler loadings. Magnesium–aluminium-layered double hydroxides (LDHs) are also increasingly being studied. In particular use of hydrotalcite as a filler in polymer compounds is of particular interest because of its layered structure and high ion-exchange capacity. It is a form of hydrated magnesium– aluminium (Mg–Al) hydroxycarbonate with lamellar structure and general formula Mg6 Al2 (OH)16 CO3 · 4H2 O. The structure can potentially be chemically modified to allow intercalation of polymer chains and subsequent exfoliation into nano-platelets in a similar manner to silicate layer nano-fillers, for example based on montmorillonite.

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DSC of hydrotalcite shows three peaks at 207, 291 and 416◦ C with corresponding decomposition enthalpies of 356 J g−1 associated with the first peak and 594 J g−1 from the second and third peak [75]. The weight loss begins at a very low temperature (50◦ C), which is attributed to water loss, but overall its thermal behaviour in terms of heat absorption capacity and mass loss is comparable with that of magnesium and aluminium hydroxide and therefore might be expected to confer similar fire retarding properties to polymers. In this regard, additions of only 30% by weight of hydrotalcite in unplasticised PVC gave an LOI value of 29 and UL94 V0 rating [76]. However, it should be noted that in this study the initial hydrotalcite particles, made by chemical co-precipitation, were found to be very coarse with an average size of around 200 µm. This dimension was reduced substantially after treatment with a titanate coupling agent and by shearing with the polymer on a two-roll mill. A comparison has also been given of the fire retarding performance in EVA of a commercially available hydrotalcite, with median particle size of 2.26 µ m and surface area 17 m2 g−1 , aluminium and magnesium hydroxides with similar particle sizes [75]. It was demonstrated that the most significant flame retardancy effect in terms of slowest heat release rate, lowest evolved gas temperature and longest ignition time, was in EVA filled with 50 wt% hydrotalcite. This was attributed to the wide temperature range (200–500◦ C) for water loss and associated heat absorption, a delay in deacetylation of the EVA due to inter-layer water loss at about 207◦ C and possible intercalation of acetate anions in the layer structure. It has also been reported that the inter-layer spacing in hydrotalcite particles can be increased by intercalation with organic molecules, such as citric acid [77]. Results were given for linear burning rates from a UL-94 horizontal burn test comparing nano- and micro-hydrotalcite modified epoxy resin. At 5 wt% addition level the micro-composite gave a reduction in burn rate of unmodified epoxy resin from around 22 to 18 mm min−1 . Whereas with the apparent nano-composite variant, also at 5 wt%, the burn rate was less than 5 mm min−1 . At an optimum addition level of only 1.5%, nano-size magnesium– aluminium LDHs have been shown to enhance char formation and fire-resisting properties in flame retarding coatings, based on an intumescent formulation of ammonium polyphosphate, pentaerythritol and melamine [78]. The coating material comprised a mixture of acrylate resin, melamine formaldehyde resin and silicone resin with titanium dioxide and solvent. It was reported that the nano-LDH could catalyse the esterification reaction between ammonium polyphosphate and pentaerythritol greatly increasing carbon content and char cross-link density. SiO2 -coated nano-LDH particles have been prepared by a sol gel process with a film thickness of about 5 nm through the formation of Mg–O–Si and Al–O–Si bonds [79]. Thermal analysis showed that both coated and uncoated nano-LDH particles showed three stages of mass loss between 40 and 700◦ C.

18

P.R. Hornsby

The uncoated material gave two endothermic peaks at 244 and 430◦ C with corresponding heat absorption capacities of 412 and 336 J g−1 , respectively. However, the coated nano-LDH exhibited only one endothermic peak at 243◦ C with a heat absorption capacity of 221 J g−1 . The influence of these fillers as polymer flame retardants was not reported. Boehemite, AlO(OH), is a partly decomposed aluminium hydroxide in which two-thirds of the water is removed. It undergoes endothermic decomposition, commencing around 400◦ C, which is considerably higher than the temperature of breakdown of aluminium hydroxide. Results from this study yielded a heat absorption of 612 J g−1 relative to 1,190 J g−1 for Al(OH)3 [75]. With a high theoretical residue of 85% it could potentially act in the condensed state by formation of an insulating layer, but is not considered to have high fire retarding qualities. In this study, the boehemite had a median particle size of 0.6 µm and surface area of 17 m2 g−1 . Colloidal boehemite nano-rods have been included in a polyamide-6 matrix to yield a homogeneous dispersion by in situ polymerisation [80]. At weight fractions up to 9%, improvements in the Young’s modulus of the composite and changes in the crystalline morphology of the PA-6 matrix were observed, although fire properties were not reported. Some potentially relevant work to the field of fire-retardant textiles concerns the attachment of magnesium hydroxide nano-particles onto multi-wall carbon nanotubes (MWCNT) [81]. These were prepared from water-in-oil emulsions specifically for conversion into MgO to functionalise and preserve the mechanical and electrical properties of the carbon nanotubes, although not for fire-retardant purposes. However, although more speculative, this work may be of interest as it has been reported [82] that combinations of MWCNT and micron-sized ATH in EVA function as very efficient fire retardants through enhanced char formation and coherency. A method for synthesising nano-aluminium hydroxide with an ATH core and alkyl hydrocarbon chain shell structure has been described [83]. In composites prepared using EVA, mechanical properties of the nano-filled material was almost the same as a conventional 1 µm-sized ATH variant, however, the heat release rate of the former composition, was markedly lower. In this work 10 phr of filler was used. Magnesium hydroxide sulphate hydrate (MHSH) whiskers have been used with and without micro-encapsulated red phosphorus synergist as fire retardants in low density polyethylene [84]. The MHSH whiskers were shown to degrade endothermically with release of water in a two-step process with DTG peaks at 301 and 411◦ C. The additive was shown to be an effective fire retardant and smoke suppressant but only at high filler levels. For example to achieve a UL-94 V-0 rating required 60% by weight loading, similar to that for magnesium hydroxide. However, these fibres appear to have length dimensions in the micron range, which combined with their relatively inefficient fire retarding efficiency, makes their useful application in textile fibres unlikely.

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19

1.5 Conclusions The application of fire-retardant fillers for polymers is dominated by two principal types, aluminium and magnesium hydroxides, of which the former finds much greater commercial use. However, many filler variants exist within these general filler categories, which differ in fire retarding efficiency depending on their particle characteristics, notably, their size, surface area and morphology. Relative to many alternative fire retardants available, a distinguishing feature of hydrated fillers is the need to incorporate high filler loadings to achieve acceptable fire retardation and enhanced smoke suppression. Being halogen-free, they are generally considered to be environmentally acceptable. Fire retardancy is primarily effected through condensed phase action, involving endothermic decomposition, water release and oxide residue formation, inhibiting thermal feedback. Smoke suppression is less well understood, but is strongly influenced by the presence of high surface area oxides generated on filler decomposition. Various synergists can be used in combination with hydrated fillers to enhance flame retarding efficiency thereby lowering the overall levels of filler required to achieve a specific performance requirements. To function in textile fibres, hydrated filler particle size must be reduced, ideally to nano-dimensions to minimise their effect on spinnability and fibre tenacity. A review of literature on hydrated nano-fillers has shown some potential candidate materials, especially those based on magnesium hydroxide and hydrotalcite. In some reports these were found to give high flammability resistance to polymers, but much work is still needed to fully characterise and optimise their effects and to establish their commercial viability.

References 1. R. Rothon, in Particulate-Filled Polymer Composites, 2nd edn., ed. by R. Rothon (Rapra Technology, Shawbury, 2003), p. 271, Chap. 6 2. W.E. Horn, in Fire Retardancy of Polymeric Materials, ed. by A.F. Grand, C.A. Wilkie (Dekker, Basel, 2000), Chap. 9 3. W.Z.A. Wan Hanafi, P.R Hornsby, Plast. Rub. Compos. Process. Appl. 19, 175–184 (1993) 4. R. Bolger, Flame Retardant Materials in Industrial Minerals (1996), p. 29 5. K.-L. Govanucci (ed.), Functional Filler News (20 November 1995), p. 1 6. B. Toure, J.-M.L. Cuesta, A. Benhassaine, A. Crespy et al., Int. J. Polym. Anal. Charact. 2, 193 (1996) 7. C.C. Briggs, B. Bhardwaj, M. Gilbert, Proceedings of 5th BPF International Fillers Conference, Manchester, UK, 19–20 May 1992, p. 9.51 8. I. Sobolev, E.A. Woychesin, in Handbook of Fillers for Plastics, ed. by H.S. Katz, J.V. Milewski (Van Nostrand Reinhold, New York, 1987), Chap. 16 9. G.S. Kirschbaum, Proceedings from Flame Retardants’94, British Plastics Federation, London (Interscience Communication, London, 1994), p. 169

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10. S.C. Brown, M.J. Herbert, Proceedings from Flame Retardants’92, Plastics and Rubber Institute, UK (Elsevier Applied Science, Amsterdam, 1992), p. 100 11. B. Szablowska, J. Pelica, in Proceedings from Flame Retardants’96, ed. by S.J. Grayson, British Plastics Federation, London (Interscience Communication, London, 1996), p. 241 12. S.C. Brown, K.A. Evans, E.A.Godfrey, New Developments in Alumina Trihydrate, Proceedings from BPF/PRI Conference Flame Retardants’87, London, 1987, p. 11 13. J.M. Stinson, W.E. Horn, Flame Retardant Performance of a Modified Aluminium Hydroxide with Increased Thermal Stability, Proceedings from Society of Plastics Engineers 52nd Annual Technical Conference (ANTEC’94), Part 3 (1–5 May 1993), pp. 2829–2833 14. J. Jow, D. Gomolka (Union Carbide Chemicals) Flame-Retardant Compositions Useful for Cable Insulating and Jacketing. US Patent 5482990A (1996) 15. E. Sezaki, M. Akami, H. Endo, Flame-Retarded Thermoplastic Elastomer Composition. European Patent 0331358 (1989) 16. Y. Yamamoto, M. Tanmachi, Thin Fire-Resistant Electrically Insulated Wire. Japanese Patent 06215644 (1994) 17. Hitachi Cable, Fire-Resistant Electrically Insulating Polyolefin Compositions. Japanese Patent 04253748 (1992) 18. Hitachi Cable, Manufacture of Halo-Free Flame-Retardant Polyolefin-Covered Electric Wires. Japanese Patent 07122140 (1995) 19. P.R Hornsby, Macromol. Symp. 108, 203 (1996) 20. P.R Hornsby, C.L. Watson, Plast. Rub. Compos. Process. Appl. 11, 45–51 (1989) 21. P.R Hornsby, Fire Mater. 18, 269 (1994) 22. P.R Hornsby, J Wang, K Cosstick, R. Rothon, G. Jackson, G. Wilkinson, Progr. Rub. Plast. Technol. 10(3), 204–220 (1994) 23. R.N Rothon, in Particulate Filled Polymer Composites, ed. by R.N. Rothon (Longman, London, 1995), Chap. 6 24. P.R Hornsby, C.L. Watson, IOP Short Meetings Series No. 4 (Institute of Physics, London, 1997), p. 17 25. P.R Hornsby, A Mthupha, Mechanism of Fire Retardancy in Magnesium Hydroxide Filled Polypropylene, Proceedings from Society of Plastics Engineering Annual Technical Conference (ANTEC’93), New Orleans, 9–13 May 1993, pp. 1954–1956 26. A. Tewarson, in Flame Retardant Polymeric Materials, ed. by M. Lewin, S.M. Atlas, E.M. Pearse, (Plenum, New York, 1982) 27. I. Spilda, M. Kosik, A. Blazej, J. Appl. Polym. Sci. 31, 589 (1986) 28. P.R Hornsby, and C.L. Watson, Polym. Degrad. Stab. 30, 73–87 (1990) 29. P.R. Hornsby, A. Mthupha, Plast. Rub. Compos. Process. Appl. 25, 347–355 (1996) 30. K. Itatani, J. Mater. Sci. 23, 3405 (1988) 31. P.R Hornsby, J. Wang, R. Rothon, G. Jackson, G. Wilkinson, and K. Cosstick, Polym. Degrad. Stab. 51, 235–249 (1996) 32. P.R. Hornsby, J. Wang, G. Jackson, R.N. Rothon, G. Wilkinson, K. Cosstick, Analysis of Fire Retardancy in Polyamides Modified with Magnesium Hydroxide Filler, Proceedings from Society of Plastics Engineers Annual Technical Conference (ANTEC’94), San Francisco, 1–5 May 1994, pp. 2834–2839 33. J. Wang, Mechanism of Flame Retardancy in Polyamides Containing Magnesium Hydroxide, Ph.D. Thesis, Brunel University (1994)

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34. J. Rychl´ y, K. Vesely, E. Gal, M. Kummer, J. Jancar, L. Rychla, Polym. Degrad. Stab. 30, 57 (1990) 35. L. Delfosse, C. Baillet, A. Brault, D. Brault, Polym. Degrad. Stab. 23, 337–347 (1989) 36. S. Miyata, T. Imahshi, H. Anabuki, J. Appl. Polym. Sci. 25, 415 (1980) 37. M. Hancock, R. Rothon, in Particulate-Filled Polymer Composites, ed. by R. Rothon (Longman, Harlow, 1995), Chap. 2 38. N.R. Dando, T.R. Clever, A. Pearson, J.M. Stinson, P.L. Kolok, E.S. Martin, Aluminium Trihydroxide (ATH) as a Filler for Polymer Composites: Improvements in Thermal Stability by Controlled Precipitation, Proceedings from 50th Annual Technical Conference, Composite Institute, Society of Plastics Industry, Session 1-D, 1995, pp. 1–4 39. D.F. Lawson, E.L. Kay, D.T. Roberts, Rub. Chem. Technol. 48, 124 (1975) 40. M. Moseman, J.D. Ingham, Chem. Technol. 51, 970 (1978) 41. M.M. Hirschler, T.R. Thevaranjan, Eur. Polym. J. 21(4), 371 (1985) 42. J. Lahaye, Polym. Degrad. Stab. 30, 111–121 (1990) 43. I. Jagoda, G. Prado, J. Lahaye, Combust. Flame 37, 261 (1980) 44. O.V. Krylov, Catalysis by Non-Metals (Academic, London, 1970) 45. D.W. McKee, in The Catalyzed Gasification Reactions of Carbon in Chemistry and Physics of Carbon, vol. 16 (of this series), ed. by P.L. Walker, P.A. Thrower (Dekker, New York 1981) 46. P.L. Walker, M. Shelef, R.A. Anderson, in Catalysis of Carbon Gasification in Chemistry and Physics of Carbon, vol. 4, ed. by P.L. Walker Jr. (Edward Arnold, London, 1968) 47. C. Baillet, L. Delfosse, Polym. Degrad. Stab. 30, 89–99 (1990) 48. G.S. Kirschbaum, Proceedings of the 1995 Fall Conference of the Fire Retardant Chemical Association, Rancho Mirage, CA, 29 October–1 November 1995 (Technomic Publishing, Lancaster, 1995), p. 145 49. G.S. Kirschbaum, Kunststoffe 79(11), 62–64 (1989) 50. H. Staendeke, Proceedings of the Spring Conference of the Fire Retardant Chemical Association, Technomic Publishing, Grenelefe, FL, 20–23 March 1988, p. 32 51. E.D. Weil, M. Lewin, H.S. Lin, J. Fire Sci. 16, 383–404 (1998) 52. M.J. Chavez, D.J. Romanesco, Proceedings of the 1995 Fall Conference of the Fire Retardant Chemical Association, Technomic Publishing, Rancho Mirage, CA, 29 October–1 November 1995, p. 169 53. M.S. Huber, Proceedings of the Spring Conference of the Fire Retardant Chemical Association, Technomic Publishing, New Orleans, FL, 25–28 March 1990, p. 237 54. T. Imahasi, A. Okada, T. Abe, US Patent 5,583,172 (1996) 55. W. Zhu, E.D. Weil, J. Appl. Polym. Sci, 56, 925–933 (1995) 56. S.H. Chiu, W.K. Wang, Polymer 39, 1951–1955 (1998) 57. R.G. Baggaley, P.R. Hornsby, R. Yahya, P.A. Cusack, A.W. Monk, Fire Mater. 21, 179 (1997) 58. P.A. Cusack, P.R. Hornsby, J. Vinyl Addit. Technol. 5, 21–30 (1999) 59. J.T. Yeh, H.M. Yang, S.S. Huang, Polym. Degrad. Stab. 50, 229–234 (1995) 60. G. Bertelli, P. Goberti, R. Marchini, G. Camino, M.P. Luda, Combined Melamine/Mineral Fillers as Fire Retardants for Polypropylene, Proceedings from 6th European Meeting on Fire Retardancy of Polymeric Materials (F.R.P.M.’97), Lille, France, 1997, p. 34

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2 Antimicrobial Functionalisation of Textile Materials E. Heine, H.G. Knops, K. Schaefer, P. Vangeyte, and M. Moeller

Summary. Textile materials can be exposed to contamination with microbes (bacteria, fungi, algae) during production, usage or storage. The microbial attack of textiles leads to quality losses due to changes of colour and appearance or to reduction in strength and can result in unpleasant odour formation. Moreover, since microbes absorb to textiles there is a risk of contamination and infection. This knowledge led to the development of antimicrobial finishing agents for textiles, in particular for textiles with special usage, e.g. socks, underwear, sports wear, medical textiles, technical textiles, geo-textiles or other materials. In hygienic functionalisation of textiles different concepts exist. Textiles are either finished with antimicrobial substances or those substances are introduced into the fibre bulk during fibre production, the latter in the case of synthetic fibres. Among others active substances from the cosmetic sector, metal ion-containing compounds like zinc–pyrithion, and metal or metal ions like silver are used. In this paper, a survey on the existing antimicrobial agents and finishing procedures is given. The antimicrobial properties of silver are well known for a long time. They are used for various purposes, however, only few applications exist in the textile sector. Commercial products are based on silver-coated fibres or ceramic supports that release silver ions. We consider the deposition of colloidal, i.e. nano-particulate, elementary silver on different carrier materials and their application onto or into the fibre as innovative approach. The large surface of these particles should result in an effective abatement of bacteria together with minimum requirement for the expensive noble metal. Furthermore, carrier materials can be used that have themselves antimicrobial effectiveness, like, e.g. zinc oxide or surface-modified, polymeric nano-spheres. Polymeric materials are non-volatile, chemically stable and do not permeate through the skin of man or animal (non-released material). Antibacterial properties are often related to the introduction of onium functions like ammonium, mimicking polypeptides (natural antibacterial macromolecules) or phosphonium groups. Chitosan-based polymers are known to provide some antimicrobial effectiveness to textiles, in particular if the naturally occurring chitosan is modified with quaternary ammonium groups. These polymers can be obtained by polymerisation of monomer bearing such function or by grafting them on pre-made polymer.

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2.1 Introduction Antimicrobial functionalisation of textiles aims on the one hand to protect the material from microbe derived destruction, and on the other hand to protect human beings from contamination. Hygienic functionalisation of fibres is performed either during melt spinning or in finishing processes. In the first method to be active the effective substances have to migrate to the fibre surface, in the latter functionalisation can be performed at the fibre, yarn or fabric/fleece stage also in combination with other finishing processes. On the garment sector it is the hygienic aspect that counts and for the technical textiles it is the material protecting aspect [1]. For the latter topic it has to be considered that not only the fibre material itself but also the residual auxiliaries or finishing substances build a substrate for microbes [2].

2.2 Functionalisation of Fibre Material by Application of Effective Substances 2.2.1 Antimicrobial Finishing Microbial growth on textiles leads to odour development, mildew growth derived discolouration up to the loss of functional properties (elasticity and tenacity) [3]. For that reason already in the seventeenth century ship cloth was conserved by tanning with iron salt solutions (brown colour). The use of hygienically effective substances today is related to body tight worn garment and sports textiles, mattresses and socks. Especially cellulosic fibres and in the first place cotton are target fibres for antimicrobial functionalisation since they retain water and nutrients and are therefore more prone to microbial attack than synthetic fibres; as effective substances, e.g. Mg-hydroxyperoxyacetate and Mg-dihydroperoxide are used [4]. In a regenerative functionalisation step, pre-stages of the effective agent (monomethylol-5,5-dimethylhydantoin) are applied to cotton and are activated or regenerated during washing (chlorine bleach). Halogenated hydantoins are normally used as swimming pool disinfectants [5]. In other processes, bifunctional agents are used to enhance permanency and efficacy of the metal salts applied [6,7]. The permanency of a zinc complex of pyrithion on cotton was achieved by the application of urea-containing solution [8]. By spraying a polyurethane hot-melt, antimicrobially effective agents are permanently attached to textile materials [9]. Polycationic molecules that bind to cotton by strong interaction belong to the new generation of effective agents in comparison to heavy metals and formaldehyde [10]. 2.2.2 Medical Textiles Approximately 9% of all patients during medical treatment are infected with microbes since microbes penetrate through textile material or through a

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leakage of the seams of plastic material. By the kind of weaving it is aimed at a certain barrier effect and additionally these materials are finished with antimicrobially effective agents like Ag, Cu and Zn, quaternary ammonium components, oxidising agents (chlorine, chloramines, H2 O2 , iodine) and phenolic components. The hygienic efficacy is targeted to the matrix (the microbially attacked textile) and not to the surroundings, like, e.g. human skin. The inactivation mechanisms are related to the inhibition of metabolic enzymes (antibiotics), the interference with enzymatic function (metal ions), the damage of membranes (quaternary compounds), the reaction with functional groups in or at micro-organisms (oxidising agents) and the penetration of cell membranes and influence on metabolic enzymes (phenolic compounds like, e.g. Triclosan). Hospital textiles were treated with chitosan and fluoropolymers both to resist against microbes and body fluids (especially blood). The application of chitosan leads to higher bending and shearing energies compared to untreated material which are decreased again by the treatment with fluorine polymers [11]. Especially chitosan is suitable for being used in the medical application field since it is effective without causing any bacterial resistance. Target objects here are filter materials for air condition devices which loose their function by microbial attack.

2.2.3 Functionalised Fibres: Application of Effective Agents During Melt Spinning Compared with the antimicrobial finishing, during melt-spinning larger amounts of effective substances are required since they are homogeneously distributed in the fibres and have to diffuse to the fibre surface. The efficiency is dependent on the concentration of the effective agent within the fibres, the thermal resistance, the stability of the chemical and physical bondings between the fibres and the additives, the solvent and surfactant resistance and the migration potency within the fibres to the surface [12]. Polypropylene, e.g. has a relatively low extrusion temperature and is thus suitable to take up antimicrobial substances (“Permafresh”: yarns for mattresses and floor coverings) [13]. “Amicor” is a hygienically effective acrylic fibre [14], Amicor AB contains Triclosan, being used in cosmetics industry; during the first exposure surface adhering Triclosan is released, thereafter the effective agent is slowly released during wash and wear by abrasion. “Bactekiller” is a polyester fibre containing zeolithes (aluminum silicates) with incorporated metal cations which are introduced during melt spinning [15]. Silver ion-containing zeolithes are incorporated into polyamide fibres during spinning (“Livefresh N Neo”) [16]. As an alternative to the zeolithes titan silicates were introduced into fibres during spinning with particle sizes <1 µm; the hygienic function is retained after 20 times washing [11]. Also into acetate fibres (“Silfresh”) or

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modal fibres (“Modal fresh”), agents are incorporated during spinning (e.g. Triclosan) [17, 18]. A process in between finishing and melt spinning is the incorporation of antimicrobially effective substances into the outer parts of the fibres during dyeing. Compared to melt spinning a lower amount of substances is needed and the permanency is higher compared with the antimicrobial finishing process [19].

2.2.4 Use of Silver as an Antimicrobial Agent in Textile Functionalisation Silver-containing textile products on the market to a large extent base on silver salts bound to zeolites (“BiotecTM Silver” (Trevira), “Sanitised Silver”), where the release of silver is a controlled release only under defined parameters or on products where the fibres are completely covered by silver (“Pady CareTM ”, PA and Lycra). Compared with the application of silver salts the latter provides the possibility of a continuous release of very low but longterm effective amounts of silver ions depending on the solubility of the silver salts generated at the surface, the corrosion of the metallic silver and the wettability (of the surrounding material, e.g. the coating, etc.). The effect will be increased by enlarging the active surface, i.e. by the application of silver nanoparticles [20, 21]. Commercially available colloidal silver solution is applied onto textile material by padding, leading to antimicrobial textiles of more or less washing stability. Anson Nanotechnology Group Co. Ltd (ANG) delivers yarns made from 100% cotton being impregnated with nano-silver (0.25% nano-silver). Anson Nano-silver products are described to contain nano-silver of 25 nm particle size and to guarantee antimicrobial effect for 50 washing cycles. AgPureTM nano-silver (rent a scientist GmbH) originally was developed for the medical sector, e.g. to render heart catheters resistant from bacterial growth. The effective concentrations amount to around 0.01% silver. The product is advertised also for being used in paints, filters, fibres, coatings, etc. Ultra-Fresh Silpure (Thomson Research Associates, Canada) is also among the new generation of nano-silver-based antimicrobial textile finishings. Depending on the application, the nano-silver is applied via padding or the suspension is introduced during extrusion.

2.2.5 Requirements for Antimicrobial Agents on Textiles Requirements for antimicrobial agents on textiles concern safety (producer and user), simple mode of application, wash and heat fastness and applicability without negative effects on the textile properties.

log CFU/mL

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control

microbiostatic agent

microbicidal agent time

Fig. 2.1. Changes in microbial growth after addition of an antimicrobial agent (time of application is labelled by an arrow ); CFU colony forming unit

2.2.6 Antimicrobial Effect The antimicrobial effect can be divided into two different categories: microbiostatic and microbicidal (Fig. 2.1): (1) Microbiostatic effect: if an antimicrobial agent leads to the inhibition of the proliferation of a microbial population and thus, hinders microbial growth it is called microbiostatic. (2) Microbicidal effect: antimicrobial agents leading to the elimination or death of the cells are called microbicidal.

2.3 Testing Up to now no general testing system for antimicrobial surfaces, e.g. fibres, yarns or fabrics exists. Partially that might be related to the fact that there are no general requirements: e.g. materials in steady contact to liquids, materials which shall be effective in humid or dry surroundings; under dry condition, adhesion and stability of micro-organisms play a key role. For this reason at DWI test methods are developed and/or applied depending on the demands or specifications of the antimicrobial system.

2.4 Micro-Encapsulation of Antimicrobial Agents for Hygienic Functionalisation of Textiles At DWI Aachen, a research project was performed aiming at a permanent hygienic functionalisation of textile materials by using micro- and sub-microparticles containing antimicrobial agents. The active ingredients should be

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released over a defined period of time leading to a nearly permanent antimicrobial finishing. Further to known systems, e.g. from cosmetics or pharmaceutics area (“controlled drug release”) different polymeric systems were used for particle synthesis. Ideally the core material is released diffusion controlled leading to lower exposure of human compared to direct application of the active agents onto the fibres. For the micro-encapsulation system especially alternative agents are of high interest, which are eventually unstable in situ; by the incorporation into carrier systems they are stabilised and become suitable for industrial application. The biggest practical demand of the new concept of the development of controlled release systems for textiles is the durability or regenerativity. One possibility to enhance the permanency of the finishing is covalent attachment of the vesicles to the fibres. Particles made from polymethylmethacrylate (PMMA) or polymethylmethacrylate-co-methacrylic acid (PMMA-co-MAA) were produced by using the emulsification/solvent evaporation method. Triclosan as a model antimicrobial agent was incorporated into these particles and was tested both in situ and encapsulated by using the agar diffusion test. Figure 2.2 shows nutrient agar with the inoculated test micro-organism Escherichia coli. Filter paper saturated with defined amounts of Triclosan (left in situ, right microencapsulated) was placed onto inoculated nutrient agar plates. The nonencapsulated antimicrobial agent diffuses into the surroundings and causes great inhibition zones whereas the encapsulated Triclosan is slowly released; depending on the amount of encapsulated Triclosan the antimicrobial effect can be limited to the substrate itself without diffusion into the surrounding area [22]. Escherichia coli (gram-) 1

1

4

4

2

2

3

3

Fig. 2.2. Agar diffusion test with different concentrations of Triclosan in situ, and encapsulated in PMMA micro-particles with E. coli as test micro-organism (left: 1 reference, 2 0.5 µg, 3 5 µg, 4 10 µg; right: 1 reference, 2 0.5 µg, 3 5 µg, 4 25 µg encapsulated)

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2.5 Antimicrobial Peptides Antimicrobial peptides are naturally occurring antibiotics. They are part of the innate immune system of vertebrates and exhibit direct antimicrobial function. In the past 20 years more than 700 antibacterial peptides have been identified (http://www.bbcm.univ.trieste.it). Antimicrobial peptides (AMP) are molecules having antimicrobial activity and being composed of less than 100 amino acids. All AMP have two properties in common: due to the existence of cationic amino acids (arginine, lysine) at neutral pH values they are positively charged and secondly the three-dimensional-structure exhibits a hydrophobic and a hydrophilic side [23]. These structural properties are important for the mechanism of action. The positive net charge of the peptides leads to interactions with negative charges on the surface of the micro-organisms, especially of the lipopolysaccharides. These chargedependent interactions lead to a relative specific action of antimicrobial peptides against microbial cell membranes since the latter are built up of more negatively charged molecules compared with eukaryotic cell membranes. Due to the amphipathic character antimicrobial peptides integrate into the membranes and build up together with several peptides membrane channels leading finally to a deletion of the membrane potential and consequently to cell death [24]. Other mechanisms of action concern specific interactions with receptor molecules or interactions with intracellular molecules. Cathelicidin, e.g. binds to intracellular receptors. Structural properties

Typical AMP

Linear, alpha-helical peptides without cysteine Peptides with disulphide bridges

Magainine (frog)

Peptides with unusual high content of one or two specific amino acids

Protegrine (cathelicidin, pig), betadefensin, alpha-defensin (mammals) Indolicidin (bovine)

The knowledge on mechanism of action can be used and transferred to the development of synthetic antimicrobial polymeric systems.

2.6 Antimicrobial Polymers Polymeric materials are an interesting issue to protect yarns, fibres, fabrics or any textiles against biological attack. An increasing demand on antibacterial and antifungal textiles is observed in many different fields like: socks, underwear clothes, household textiles, sportswear, bedding and hospital textiles. The need for avoiding microbial invasion comes from three issues:

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– Hygienic problems: allergy, nosocomial infections – Physical properties: decrease of mechanical properties, comfort – Aesthetic appearance: odours development, colour change, moisture To fulfil these requirements, the antimicrobial finishing needs to be durable (resistant to washing, body fluids, disinfection), selective (without side effect on the users, producers or environment), easy to apply, non-damaging for the fabric, compatible with process and other finishing, and, as much as possible, avoiding to induce bacteria resistance [25]. However, the majority of antimicrobial finishes function by the controlledrelease mechanism. The applied active compounds continually produce active germicidal species that have to be regenerated by different techniques like bleaching agent during laundering, or exposure to UV light which would break some strategic covalent bond. In addition of a problem of durability because of a “limited” reservoir of species to be delivered, the permanent leaching of active compounds into the surrounding can either affect the environment or even the users or the producers. The main characteristics of macromolecules compared to the usual antimicrobial agents (silver nano-particles, antibiotics, Triclosan, etc.) is their high molecular weight which imparts them to be non-volatile, chemically stable and find it difficult to permeate through the skin of a man or an animal. Polymeric materials allow then antimicrobial and antifungal properties with a non-release system, favourable for the surroundings: user health and environment, and showing good durability. In some applications, a regular wiping of the dead cells or organic components on the surface helps for a long lasting antimicrobial performance. In addition, the mode of action of the polymeric material is usually by contact and permeation of the cytoplasmic membrane of the bacteria and therefore do not lead to bacterial resistance like antibiotics can do. 2.6.1 Antimicrobial Finishing Methodologies In the frame of antimicrobial polymers, antimicrobial polymer finishing for textiles is a particular situation, as the textile is a macromolecule as well. This latter can be a natural (cotton, wool, silk, linen, etc.) or a synthetic one (polyester, polyamide, etc.). However, the situation of small molecules like quaternary ammonium salts, halogenated derivatives, metallic substances or others, either covalently or electrostatic linked or immobilised onto the macromolecular fibres will not be considered as an antimicrobial polymer. These kinds of systems will not be regarded hereafter. During their life time, textiles are confronted to various constraints, indeed, such as washing, drying, ironing, body fluids, microbial attacks. The long lasting character will then be an important parameter for the quality and the efficacy of the antimicrobial finishing. Compared to treatment with substances of low molecular weight with permanent controlled release, the macromolecules are already of more permanent solution as explained earlier, nevertheless

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the contact between the fibres and the polymer can be more intimate or not. Different binding concepts exist: (a) Textile fibres are treated with antimicrobial polymers after production as an after-treatment. The adherence of the finishing will vary with the method used. On the one hand, it will be relatively low for impregnation methods like: coating, padding, pad-dry-cure, film casting, padding, spraying or dipping methods. However, a better fixation of the antimicrobial polymer can be achieved by addition of a binder or a plasticiser. For example, Kim et al. carry out the finishing of cotton fabrics with a quaternary derivative of chitosan by padding, the use of binder such as dimethyloldihydroxyethylene urea (DMDHEU) or polyurethane resin, improve the resistance to washing [26]. On the other hand, the antimicrobial polymer can be covalently attached to the fibres. This can be achieved either by grafting the monomer and subsequent polymerisation of this latter, or by polymerisation directly onto the fibre as done by Kanazawa et al. [27] with 3-(trimethoxysilyl) propyltrialkyl phosphonium chloride onto cotton fibres. The use of a crosslinking agent [28], or an organic reaction between functionalities borne by the polymer and the fibre like the reaction between polyethylenimine and hydroxyl or amino groups onto cotton, nylon or wool, as described by Lin et al. [29, 30] allow a permanent linkage of the antimicrobial species. A technique such as plasma process is an easy way to improve the number and nature of functionalities along the fibres [31, 32]. (b) The antimicrobial macromolecules are introduced into the fibre bulk during fibre production. This second situation is possible in the case of synthetic fibres and leads to a rather high cohesion between the fibre and the polymer. Three situations arise: either the polymer and the fibre are co-extruded (mixed before spinning) or they are blended (mixed after spinning), or the fibre is only composed with the antimicrobial polymer. This latter can only be possible when antimicrobial polymer can be spun. The three situations are reported by Ottersbach and Kossmann [33, 34] for acrylic, acrylamide, methacrylic and methacrylamide polymers with woven or non-woven substrates. 2.6.2 Antimicrobial Polymers and Their Effect Antibacterial properties are often related to the combination of surface located onium functions like ammonium or phosphonium groups, hydrophobic tails and the three-dimensional structure of the polymer. These polymers can be obtained by polymerisation of monomers bearing such function or by grafting them on pre-made polymer. The degree of activity against bacteria, algae or fungi is differentiated either by the term “-cidal” when they lead to the death of the microbes,

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or the term “static” when the polymers inhibit the microbial growth without much destruction. At the present stage of the study, the mode of action of polycationic antimicrobial compounds is to target the cytoplasmic membranes of bacteria. The mechanism usually takes place in the next six steps process: (1) (2) (3) (4) (5) (6)

Adsorption onto the bacterial cell surface Diffusion through the cell wall Binding to the cytoplasmic membrane Disruption of the cytoplasmic membrane Release of cytoplasmic constituents such as K+ ion, DNA and RNA Death of the cell

Nevertheless in addition to this process, a second mechanism of antimicrobial activity is proposed for chitosan. It involves the binding of chitosan to the DNA of the bacteria to inhibit mRNA synthesis. This implies a low degree of polymerisation to allow the chitosan to permeate into the cell. The production and analysis of antimicrobial polymers are widely described in the scientific and patent literature [35]. However, the application of polymers, i.e. non-release systems, as antimicrobial material on textiles is not broadly exploited in textile industry up to date. To the best of our knowledge, only a few kinds of antimicrobial polymers are used to obtain antimicrobial textile fibres. Some polymers are produced directly as fibres while others are used as finishing treatment on textile fibres. On the market, mainly chitosan polymers are applied on textile material to obtain an antimicrobial effect, Fuji spinning for example sales ChitopolyTM . Chitosan is a derivative of chitin, the most common biopolymer besides cellulose, derived from the cell walls of fungi but also from the exo-skeleton of crustaceae and insects. It combines a lot of attractive properties: hydrophilicity even if it can only be solubilised in weak organic acids, a polycationic character, antimicrobial activity and biodegradability. Chitosan being a particular natural macromolecule, and widely applied onto textiles, will be further mentioned in Sect. 2.7. Commercial Antimicrobial Polymers and Fibres Antimicrobial textiles reported by companies often include polymer fibres impregnated with antimicrobial substances of low molecular weight. For example TeritalTM saniwear from Montefibre SpA consist of inorganic compounds (Ag+ salt) added to polyester fibres mixed to the melt polymer before spinning [36], or UltrafreshTM from Thomson Research Associates contain organics (Triclosan, Tributyltin, etc.) or inorganic substances embedded in the polymer fibres. All these commercially available antimicrobial fibres have been listed elsewhere [25]. Organosilane bearing ammonium groups are commercialised by Aegis Microbe ShieldTM (Devan Chemicals NV) under the trade AegisTM . It consists

2 Antimicrobial Functionalisation of Textile Materials

33

of a 3-(trihydroxysilyl)propyldimethyloctadecyl ammonium chloride which is polymerised onto the fibre surface (PET, cotton) and to itself. In addition, polyhexamethylene guanidine (or biguanide) appears as suitable antimicrobial polymer for textile finishing, that are produced by Daiwa [37] and Avecia [38] and have been applied onto cotton with the help of a binder [39]. Patents In addition, some other polymers – acrylates or methacrylate derivatives – are presented as fibres or finishing with antimicrobial properties. Clariant patented polymers like 3-methacryloylaminopropyl- trimethylammoniumchloride for cotton or polyamide fibres. The finishing allows a reduction of around log 3 CFU ml−1 (colony forming unit) of Staphylococcus aureus depending on the concentration applied [40]. Creavis presents polymer derivatives of 2-tert.butylaminoester-methacrylic acid or dimethylaminopropyl-methacrylamide to obtain woven [34] and non-woven textiles [33, 41]. The fibre can either being made completely from antimicrobial polymers, or co-extruded, coated, grafted or blended. The treated textile presents a reduction of log 4 or 5 CFU ml−1 of Pseudomonas aeruginosa. Worley et al. [42–44] presented acrylate and methacrylate derivatives including N -halamine compounds. This N -containing 5-membered ring can link a halogen atom which can be released and act as biocide. It is therefore an antimicrobial polymer with a release effect. The activity of coated fabrics has been shown with a reduction of log 4 CFU ml−1 against Salmonella enteritidis. The same group has studied the N -halamines siloxanes as antimicrobial finishing with the same principle of halogen release. Treated cotton swatches were challenged with S. aureus and E. coli. A reduction of log > 5 CFU ml−1 was observed but with a delay of at least 30 min [45]. They recently presented some other results of this system showing a broad activity spectrum and refreshable possibilities [46]. To the best of our knowledge, no real commercial application has been observed with these products so far. Papers Few other antimicrobial polymers have been reported for textile finishing in the scientific literature. Kanazawa et al. [27] have studied organosilane modified with phosphonium groups, by direct polymerisation onto cotton fibres as explained earlier. A reduction of log 7 CFU ml−1 is observed after 30 min contact time with quaternised phosphonium bearing long alkyl chains (at least eight carbons). Alkylated polyethylenimine was considered as antimicrobial agent by the group of Klibanov [29]. The polymer was immobilised on different kind of fibres: cotton, wool, nylon or polyester by organic reaction is then N -alkylated to obtain a quaternary ammonium function. The activity was

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tested against bacteria and fungi resulting in a reduction of log 2 CFU ml−1 . Exhaustive washing was performed to check the fastness of the activity. No significant variation was observed with the polymer of high molecular weight. With a similar strategy, the antimicrobial treatment with a water-repellent finishing on cotton fabrics was combined. A thin coating allows to retain some antimicrobial activity [30].

2.7 Chitin and Chitosan Derivatives as Antimicrobial Agents Chitin is a renewable polymer which has only been used in the textile sector for a short time. Chitin is the second most often biopolymer after cellulose, it is the skeleton substance of the insects and the crustacean. Chitin is a polysaccharide based on amino sugars, i.e. β-(1 → 4) glycosidic bound N -acetyl-d-glucosamine residues with an approximate molecular weight of about 400,000. On a commercial scale, chitin is won by extraction from the shells of crustacean. Chitin is only poorly soluble in diluted acids and bases as well as in organic solvents. The deacetylation product of chitin, the chitosan, exhibits a markedly better solubility in aqueous acids and alcohols (Scheme 2.1). Chitosan is a naturally occurring, non-toxic, biologically degradable polymer. It possesses antimicrobial effectiveness due to its amino group in C-2 position of the glucosamine unit. Chitosan can bind via its cationic form to the anionic cell wall of microbes and can like this inhibit their growth. Due to its hemostatic, antibacterial and fungicide properties chitosan can be used for finishing of wound dressings in the medical technology. Up to now, no allergic reactions of chitosan are known. The good film-forming properties of chitosan are used in the coating of textile fibres. Chitosan films exhibit high water retention capacity, good adhesion, chelating properties, biodegradability and biocompatibility. Furthermore, chitosan can be spun. Chitosan fibres and synthetic fibres coated with chitosan layers were produced by several Japanese CH2OH

CH2OH

O O

OH

H

O O O

OH

H

n H

HN COCH3

Chitin

O n

H

NH2

Chitosan

Scheme 2.1. Structures of chitin and chitosan

2 Antimicrobial Functionalisation of Textile Materials

35

companies. Textiles from chitosan fibres possess moisture controlling, antibacterial and fungicide properties which were used for the production in sports clothes and socks. Several substances based on chitin or chitosan derivatives were patented and published as antibacterial agents for varying applications. Plenty of publications and patents on the application of chitin and chitosan derivatives for the cosmetic, pharmaceutical, food and textile sector are known: – In 1989, Morita and Nakada [47] patented non-wovens which were finished with chitosan salts or quaternary chitosan salts as moist cleaning cloth for sterilising and deodorising the hands and the skin. – In 1991, Furuta et al. [48] patented antibacterial fibres with good washing fastness which were treated with a coating which contained quaternary ammonium salts, chitosan and a fixing agent. – In 2000, Abe [49] patented antibacterial fibres which were finished with N -3-trimethylammonium-2-hydroxypropyl-chitosan. Abe treated fibres which contained OH, NH2 and/or COOH groups. The molecular weight of the used chitosan was in the range of 30,000–50,000. Abe applied polyethylene glycol diglycidylether as fixing agent. – In 2001, Daly and Manuszak-Guerrini [50] patented N -3-trimethylammonium-2-hydroxypropyl-chitosan as antimicrobial, pharmaceutical and as preserving agent for cosmetics. – In 2002, Yamaguchi [51] patented the synthesis of alkylammoniumchitosan iodine and its application as antibacterial agent. 2.7.1 Synthesis of Chitin Derivatives as Antimicrobial Agents for Textiles Chitin is a biopolymer with high molecular weight which is poorly soluble in water, thus it has to be derived for application in textile finishing which is performed usually from aqueous media. In principal, textiles can be finished with chitin or with chitosan (i.e. the deacetylation form of chitin). To achieve a sufficient application of chitin or chitosan onto textiles, either the molecular weight of chitin has to be reduced so that the modified chitin derivative can penetrate into interior parts of the fibre or the solubility of chitin has to be increased so that a film surrounding the fibre can be formed during the finishing operation. In the Ph.D. Thesis of Al-Bahra [52, 53], chitin was used as starting material for the synthesis of antimicrobial agents for textile finishing. At first, chitosan, the deacetylated form of chitin, was depolymerised with sodium nitrite to produce chitooligosaccharides. The degree of polymerisation was determined by colorimetry. Quaternary ammonium salts are known as antimicrobial agents since 1935 [54]. On the basis of this knowledge, chitin and chitosan were provided with quaternary ammonium groups in the second approach to obtain improved

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effectiveness against bacteria and better water solubility. For this, chitosan was reacted with diethylaminoethyl groups at the hydroxy and amino groups. The synthesised product was characterised with the help of IR-, Raman-, NMR- and two-dimensional-NMR-spectroscopy as well as by elemental analysis and TGA. The degree of substitution was determined by means of NMR-spectroscopy, elemental analysis and conductometry. Chitosan has two functional groups which are available for modification. In the Ph.D. work of Al-Bahra chitosan was coupled with diethylamino groups (DEAE) either at the primary hydroxyl groups, at the amino groups or at both of them. The synthesised products were then quaternised with ethyliodide, dimethylsulphate or diethylsulphate. The structure of the products was confirmed by IR-, Raman- and NMR-spectroscopy. The degree of quaternisation was determined with the help of NMR-spectroscopy, elemental analysis and XPS. The synthesised chitosan derivatives were applied onto wool and cotton fabrics. Polycarboxylic acid was used to fix the chitosan derivatives on cotton fabrics. The amount of the fixed derivatives on the fabrics was determined by means of elemental analysis. Furthermore, the chitosan derivatives were subjected to antimicrobial tests in solution. TEAE-chitosaniodide and MDEAE-chitosanmethylsulphate show very good activity against Vibrio fischeri, Bacillus subtilis and Escherichia coli.

2.8 Applications A wide range of textile products is now available on the market comprising yarns such as: cotton, linen, silk, wool, PET, polyamide for many different applications: – – – – –

Garments: socks, underwear, sportswear, working clothes Indoor: mattresses, floor covering, bedding, household, furnishing, curtain Outdoor: tent, shoe linings Technical: fabrics, metal fibres, glass wool, geo-textiles, paper Hospital/medical: clothes, masks, sheets, wound healing, bandages, tissue engineering

Despite the large amount of literature on these polymers for other purposes and the answer given by polymeric material on the fastness and toxic issues, only a few kind of antimicrobial polymers are used in textile processing so far. Chitosan, polybiguanide and organosilane mainly are the polymers used till now. The requirements, as explained earlier, are probably limiting factors but there is a great challenge to develop a non-expensive and simple technology to be applicable in textile industry.

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2.9 Future Prospects (a) Regarding the development of the polymer synthesis and the huge literature on antimicrobial polymers in general, polymers with a better efficiency are on the way. By comparison with what the nature delivered, some parameters such as the three-dimensional structure and the hydrophilic– hydrophobic balance, emerge as key factors. A better understanding of its interaction with the microbes should help. Peptides mimicking polymers, for example, show a very high antimicrobial activity [55, 56]. (b) The intrinsic activity of the polymer is not the only point; an easy way to bring it onto any kind of fibre without loosing the properties neither of the fibre nor of the polymer is an important step. Plasma treatment or new linker development are promising technologies. (c) Finally a crucial point will be to combine antimicrobial polymers with others to impart textiles a multi-functional character. Some studies already try to undertake these objectives as water- or blood-repellent properties [30, 57], antistatic [58] drug release for wound healing for example [59, 60].

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Anon., Text. Praxis Int. 35(2), 167–171 (1980) L. Yan, Int. Dyer 43(2), 31 (1997) M. Bossard, Textilveredlung 34(1/2), 16–19 (1999) T.L. Vigo, G.F. Danna, W.R. Goynes, Text. Chem. Col. 31(1), 29–33 (1999) A. Weinberg, Off. Gazette US Patent Tradem. Office Patents 1999, 1218/1-USP 5856248 (1995) N.A. Ibrahim, M.H. Abo-Shosha, M.A. Gaffar, Colourage 45/7, 13–19, 30 (1998) C.E. Morris, C.M. Welch, Text. Res. J. 53(12), 725–728 (1983) Anon., Med. Text. 3–4 December 1997 J. Payne, J. Soc. Dyers Colour 113, 48–50 (1997) S. Lee, J.-S. Cho, G. Cho, Text. Res. J. 69(2), 104–112 (1999) R. Stevanato, R. Tedesco, Man-Made Fiber Yearbook 30–32 (1999) D.T. Ward, Int. Text. Bull. 1, 44–45 (1999) D. Service, Man-Made Fiber Yearbook 33–38 (1999) Anon., Techn. Textilien 42, 88 (1999) Anon., Jpn Text. News 438, 81–82 (1991) T. Kawata, Chem. Fib. Int. 48, 38–43 (1998) Anon., Man-Made Fiber Yearbook 22–23 (1998) Anon., Med. Text. 4 (1997) S. Rahbaran, Chem. Fib. Int. 49, 491–493 (1999) H.J. Lee, S.Y. Yeo, S.H. Jeong, J. Mat. Sci. 38, 2199–2204 (2003) H.J. Lee, S.H. Jeong, Text. Res. J. 74(5), 442–447 (2004) H.-G. Knops, N. Wyrsch, E. Heine, H. H¨ ocker, DWI Rep. 126, 530–534 (2003) R.E.W. Hancock, Lancet 349, 412–422 (1997)

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24. R. Bals, Medizinische Klinik 95, 496–502 (2000) 25. T. Ramachandran, K. Rajendrakumar, R. Rajendran, IE (I) Journal-TX 84, 42 (2004) 26. Y.H. Kim, H.M. Choi, J.H. Yoon, Text. Res. J. 68, 428 (1998) 27. A. Kanazawa, T. Ikeda, T. Endo, J. Appl. Polym. Sci. 52, 641 (1994) ¨ 28. T. Oktem, Color. Technol. 119, 241 (2003) 29. J. Lin, S. Qiu, K. Lewis, A. Klibanov, Biotechnol. Bioeng. 83, 168 (2003) 30. J. Lin, S. Murthy, B. Olsen, K. Gleason, A. Klibanov, Biotechnol. Lett. 25, 1661 (2003) 31. M. Lin, Master’s Thesis, Chung Yuan University, China (2002) 32. M. Risbud, E. Karamuk, J. Mayer, J. Mater. Sci. Lett. 21, 1191 (2002) 33. P. Ottersbach, B. Kossmann, DE10127513A1 34. P. Ottersbach, B. Kossmann, DE10122753A1 35. T. Tashiro, Macromol. Mater. Eng. 286, 63 (2001) 36. G. Salvio, Chem. Fib. Int. 51, 34 (2001) 37. S. Son, H. Ju, K. Kitamura, T. Otsuki, T. Suyama, JP2002038373 38. S. Li, A. Brandon, US 20030118786 A1 39. P.J. Hauser, M. Tariq, J. Rajan, AATCC Rev. 4, 24 (2004) 40. G. Crass, U. Falk, M. Glos, DE 10302174A1 41. P. Ottersbach, B. Kossmann, DE10136456A1 42. S.D. Worley, M.W. Eknoian, Y. Li, US006162452 43. S.D. Worley, M.W. Eknoian, Y. Li, US005902818 44. S.D. Worley, M.W. Eknoian, Y. Li, US006469177 45. S.D. Worley, M.W. Eknoian, Y. Li, WO03106466 46. J.F. Williams, J. Suess, J. Santiago, Y. Chen, J. Wang, R. Wu, S.D. Worley, Surf. Coat. Int. B: Coat. Trans. 88, 35 (2005) 47. I. Morita, H. Nakada, JP 01025821 (1989), 3 p 48. T. Furuta, K. Kamermaru, Y. Kijima, JP 03051369 (1991), 4 p 49. M. Abe, JP 200219605 (2000), 5 p 50. W.H. Daly, M.A. Manuszak-Guerrini, US 6306835 (2001), 7 p 51. T. Yamaguchi, JP 2002088102 (2002), 12 p 52. M.M. Al-Bahra, K. Sch¨ afer, H. H¨ ocker, DWI Rep. 125, 405–409 (2002) 53. M.M. Al-Bahra, Synthesis of Chitin Derivatives as Antimicrobial Finishing Agents for Textiles, Ph.D. Thesis, RWTH Aachen (2004) 54. T.L. Vigo, in Handbook of Fiber Science and Technology: vol. II, Chemical Processing of Fibers and Fabrics, Functional Finishes Part A, ed. by M. Lewin, S.B. Sello (Dekker, New York, 1983), p. 367 55. A. Statz, R. Meagher, A. Barron, P. Messersmith, J. Am. Chem. Soc. 127, 7972 (2005) 56. G.N. Tew, D. Liu, B. Chen, W.F. DeGrado, Proc. Natl Acad. Sci. USA 99, 5110 (2002) 57. S. Lee, J.S. Cho, G. Cho, Text. Res. J. 69, 104 (1999) 58. C.W. Nam, Y.H. Kim, S.W. Ko, J. Appl. Polym. Sci. 74, 2258 (1999) 59. S.Y. Lin, K.S. Chen, L.R. Chu, Biomaterials 22, 2999 (2001) 60. K. Chen, Y. Ku, C. Lee, H. Lin, F. Lin, T. Chen, Mater. Sci. Eng. C 25, 472 (2005)

3 Intumescent Flame-Retardant Treatments for Flexible Barriers R. Kozlowski, D. Wesolek, M. Wladyka-Przybylak, S. Duquesne, A. Vannier, S. Bourbigot, and R. Delobel

Summary. Intumescent flame-retardant systems are known from several years in coating for the protection of building structure against fire. More recently, this concept was applied to polymer and to flexible structure such as textiles. The aim of this paper is to review the use of the intumescent flame-retardant process for flexible materials. In a first part, the mechanism of intumescence is described. It is pointed out that limited factors to apply this process to textiles are the durability of the systems, the stiffness due to surface deposit of the treatment and the spinnability of intumescent formulations for the bulk treatment. In a second and in a third part, recent developments to overcome those problems are presented, respectively, for natural and synthetic fibers.

3.1 Introduction The fires of dwellings constitute about 60% of total number of fires in buildings [1, 2]. For instance, in UK 72,000 of the 116,000 building fires corresponds to dwelling buildings in 1999 [2]. The death toll in those fires is 60–85% of all deaths in fires [2, 3]. Also, about 80% of all accidents in fires occurred in fires of dwellings [1, 2]. Flammable interior furnishings and decorative materials (particularly textiles) are among the main fire hazards in dwellings. In fire conditions, they can constitute so-called first ignited material and contribute to the development of fire. Fire statistics show that about 20% fires of dwellings in the UK is caused by textiles (mainly upholstered furnishings and beddings) as the first ignited material [1]. These textiles are responsible for about 50% of deaths in these fires. However, due to their other qualities, they have to be used in dwellings and public buildings.

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Fig. 3.1. Example of intumescent coating for wood

To improve the behaviour of fabrics in fire conditions, better flame retardants are developed, which improve thermal resistance of materials to a very high extent, increase their ignition temperature, reduce combustion rate and decrease the amount of heat released. One of the most efficient ways of flame and fire retardancy of combustible materials is the application of the intumescent concept. Intumescent materials begin to swell and then to expand when heated beyond a critical temperature. The result of this process is a foamed cellular charred layer on the surface which protects the underlying material from the action of the heat flux or the flame (Fig. 3.1) [4]. Fire protection of flammable materials by an intumescence process is known for several years. Flame retarding polymers or textiles by intumescence are essentially a special case of a condensed phase mechanism [5–7]. Intumescent systems interrupt the self-sustained combustion of the polymer at its earliest stage, i.e. the thermal degradation with evolution of the gaseous fuels. The intumescence process results from a combination of charring and foaming of the surface of the burning polymer. The resulting foamed cellular charred layer which density decreases with temperature, insulates perfectly the covered material from excessive rise of temperature, oxygen penetration and thus protects it against thermal decomposition, playing the major role in stopping development of burning process. This paper reviews how intumescent can be used to flame retard textiles. The first part deals with the mechanisms of intumescence. Then, in a second and in a third part, several innovative approaches to achieve durable intumescent systems, respectively for natural and synthetic fibers are presented. Developments of new formulations for textile finishing as well as development of inherent intumescent fibers are described.

3 Intumescent Flame-Retardant Treatments for Flexible Barriers

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3.2 Mechanisms of Intumescence The protection mechanism of intumescent systems is based on the formation of a charred layer acting as a physical barrier which slows down heat and mass transfer between gas and condensed phases. Intumescent polymeric systems decompose and form upon heating large amount of thermally stable carbonaceous residue. A large improvement of flammability properties can be achieved using intumescent systems in bulk polymers and in coatings. Thermal protection is the main purpose of intumescent materials; heat transfer is limited by the formation of the intumescent shield. Swelling is crucial to the fire protective capabilities and a fundamental understanding of the mechanisms that cause expansion is important. Temperature gradients and heat transfer play a crucial role in intumescent behaviour. In particular, the effect of the growing bubbles on the temperature field cannot be neglected (Fig. 3.2). To make the intumescent flame-retardant efficient, a proper selection of components is essential, namely char formers, carbonising, dehydrating substances and modifiers allowing to obtain a maximum degree of carbonisation and thus an efficiency of the protective char. Also it is very important to select proper binder resins. The required components for intumescent coating production are shown in Table 3.1. The proper selection of these components in intumescent flame-retardant system and their dispersion degree have fundamental influence not only on the velocity of carbon layer formation, its structure or resistance to thermal factors but also on the quality of obtained product. For intumescence occurring with the subsequent formation of a thick carbonised and porous layer several distinct reactions must occur in the proper sequence [4–10]: – At the first stage, the decomposition of dehydrating agent occurs, followed by the formation of acid – Acid reacts further with hydroxyl groups of carbonising substance – esterification to form thermally unstable ester

α=0

0<α<1

α→1

Fig. 3.2. Schematic representation of the development of intumescence (α: conversion degree)

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Table 3.1. The basic and essential components of intumescent flame-retardant system [8–14] Function of components

Compounds

Carbonising substance

Polyhydric alcohols (erythritol and its oligomers (pentaerythritol, pentaerythritol dimer and trimer, arabitol, sorbitol, inositol), saccharides (glucose, maltose, arabinose) and polysaccharides (starch dextrin, cellulose), polyhydric phenols (rezorcinol)

With a considerable number of carbon atoms, thermal decomposition of which results in the formation of carbonaceous material having a large number of hydroxyl groups, able to be esterification with acids Dehydrating agent Substance releasing during its thermal decomposition an acid which esterifies hydroxyl groups

Foam forming substance Releases large quantities of nonflammable gases during its thermal decomposition, thus forming foamed structure of carbonaceous layer

Phosphoric acid, its ammonium, aminic salt and esters (ammonium phosphate and polyphosphate, melamine and urea phosphate tributyl phosphate), boric acid and its derivatives (borax, ammonium borate) Nitrogen or halogen compounds such as melamine and its phosphoric salts, urea, dicyandiamide, guanidine and its derivatives, glycine, chlorinated paraffins

Binder resin

Amino, epoxy, acrylic, polyacetic-vinyl and polyurethane resins

Solvents, stabilisers, etc.

Specific, chemical compounds depending on the kind of resin

– Further temperature increase decomposes esters with the formation of carbon, free acid, water, carbon dioxide – The decomposition of ester is accompanied by the decomposition of foamforming agents resulting in the formation of significant amounts of nonflammable gases – Liberated gaseous products cause swelling of carbonaceous material (created as a result of the ester decomposition), thus form a thick, porous, insulating layer – Gelification of the foamed layer that insulates the combustible material from the heat and oxygen access occurs It seems very likely that intumescent systems will not only complement the traditional flame retardants but also will replace them due to their numerous applications. Flame retardancy of textiles may be achieved using surface treatment (of the fabrics or of the fibers) or by the use of inherently flame-retarded synthetic fibers or by a combination of those methods.

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A recently developed greater interest in intumescent coatings and their application to fabrics as a flexible effective heat barrier has been observed [1,12,15–17]. Definitely any enhancement of the char barrier in terms of thickness, strength and resistance to oxidation, improves the flame and heat barrier performances of textiles [1]. The fire barrier is more and more widely used and includes direct application of an intumescent system to the reverse surface of flammable fabric. Back-coatings appear to be one of the most versatile methods for flame-retardant textile especially for furnishing fabrics and nonwovens. Back-coating methods are used where aesthetics of the front face are important (furnishing fabrics, curtains). Recent developments of intumescent fire protective barrier for natural and synthetic fibers are described in the following parts.

3.3 Flame-Retarded Natural Fibers 3.3.1 Cotton The research into the using of intumescent systems to fibers and fabrics has demonstrated that their application to the surface of flame-retardant cellulosics significantly increases fire barrier performance when exposed to temperatures as high as 1, 200◦ C for short periods of time [18]. It was reported that the intumescent systems based on resin binder with ammonium and melamine phosphate can improve fire barrier properties of flame-retarded viscose and cotton fabrics even to the level reached by high performance fibers such as aramids [19]. Recent research focuses on possibilities to use them for other fibers, especially wool [20]. Another area of research is developing systems for protecting vibrating or changing shape systems, barriers especially made of textile materials (e.g. fabrics and non-woven). The research is carried out to obtain high resistance to ignition of these materials while sustaining their flexibility [21–24]. Recent studies on flame-retardant amino resins have been carried out at the Institute of Natural Fibers (INF). New amino resins of increased flame resistance were obtained by condensation reaction with phosphorus compounds. Resins prepared in such a way played the role of both film-forming agent and carbonising agent (source of carbon). Compounds used in the condensation, namely urea, melamine and dicyandiamide can show intumescent properties as well. Moreover, phosphate compounds, well known for their fireproofing properties, played additionally the role of dehydrating agent and catalyst. To strengthen foam structure, carbonising agents, such as dextrin, starch and pentaerythritol, were added to resins. Ammonium polyphosphate has also been investigated. It is widely used in intumescent systems due to its high dehydrating ability and its way of thermal degradation which results in the formation of ammonia and polyphosphoric acid.

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Such intumescent flame-retardant formulations were halogen-free compounds. Intumescent formulations were applied to cotton fabric of plain weave and with surface mass of 180 g m−2 . Flame-retarded samples were evaluated for ability to swelling, easiness of application, coating appearance, fabric flexibility (manual evaluation). The effectiveness of flame retardancy was evaluated according to Limiting Oxygen Index method (LOI) [25], cone calorimeter method [26] and according to the method for interior materials applied in vehicles PN-ISO 3795 [27]. Thermal analyses (TG and DTA) were also performed on those materials. Fabrics covered by the intumescent coating were subjected to flammability tests, which were performed before and after three times repeated soaking in water. The modified amino resin with phosphate compounds UDPD, applied in the amount of 36 wt% (64.8 g m−2 ), turned out favourable for fabrics. It is characterised by a very high ability to intumesce, transparent coating, high stability on storage. UDPD formulation caused further slight stiffening of the fabric. Particular attention should be paid to the formulation based on the ammonium polyphosphates with borate compounds PPAB, applied in the amount of 19 wt% (34.2 g m−2 ) which effectively fireproofs a fabric even at a low amount applied and at the same time does not affect touch and flexibility of the fabric. High flame-retarding effectiveness of intumescent compounds UDPD and PPAB was confirmed by oxygen index LOI (Fig. 3.3). LOI index for fabrics subjected to flame retardation with formulations based on PPAB, applied in the amount of 19% is over 60%. In this case, however, three times repeated soaking resulted in FR washing out which is also reflected by the reduction in the value of oxygen index. It is worth to add that in spite of a considerable washing out, the value of oxygen index is still equal to about 30 vol.-%. The high LOI values for the flame-retarded fabric with formulation UDPD points out a good flame retardancy and a higher resistance to soaking than in the case of PPAB application.

LOI [%]

60 50

untreated fabric

40

PPAB treated fabric

30

PPAB treated fabric after soaking UDPD treated fabric

20

UDPD treated fabric after soaking

10 0

Fig. 3.3. Flammability of flame-retarded cotton fabrics tested by the LOI method

200

200

160

160

HRR [kW/m2]

HRR [kW/m2]

3 Intumescent Flame-Retardant Treatments for Flexible Barriers

120 80 40 0

45

120 80 40 0

0

40

80

120 160 time [s]

200

240

0

untreated fabric PPAB treated fabric PPAB treated fabric after soaking

40

80

120 160 time [s]

200

240

untreated fabric UDPD treated fabric UDPD treated fabric after soaking

Fig. 3.4. Heat release rate (HRR) for flame-retarded cotton fabrics, before and after soaking. Heat Flux 35 kW m−2 cone calorimeter test ISO 5660 Table 3.2. Flammability of flame-retarded cotton fabrics before and after soaking Specimen

Amount applied (%)

PN-ISO 3795

Cone calorimeter method; heat flux 35 (kW m−2 )

Flammability TTI Degree −1 (mm min ) (s) Untreated UDPD PPAB

No soaking After soaking No soaking After soaking

– 36 21 19 9

0 0 0 0

17,8 ∞ ∞ ∞ 10.97

HRRmax

HRRav

MLRav

(kW m−2 )

(kW m−2 )

(g s−1 m−2 )

169.96 21.89 21.43 12.75 78.49

37.98 10.67 10.86 5.09 22.45

9.92 3.54 3.04 3.59 3.39

Fabrics including flame retardent intumescent agents also show a high resistance to ignition in conditions of the cone calorimeter test, i.e. they do not ignite when exposed to heat flux of 35 kW m−2 (Fig. 3.4). Other flammability parameters (Table 3.2) such as maximum and average heat release rate (HRRmax and HRRav ) total heat released (THR) and mass loss rate (MLRav ) confirm higher flame-retarding effectiveness of PPABcontaining formulation compared to the one based on amino resins. A special attention is paid to the UDPD formulation which did not ignite after three times soaking (Table 3.2). Its combustibility parameters measured after three times soaking, are comparable to those of non-soaked fabric. The tested fabrics before soaking as well as after soaking met the requirement of the test PN-ISO 3795. In the test conditions, they did not ignite. The results in Table 3.3 show that the fabrics flame retarded with PPAB meet the standard PN-ISO 3795, both before and after repeated cleaning by using the spray-suction system (Figure 3.7). The samples did not ignite in the test conditions. The fabrics also showed a very high resistance to ignition in the conditions (Fig. 3.5) of cone calorimeter test (Table 3.3 and Fig. 3.6).

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Table 3.3. Flammability of flame-retarded cotton fabrics as fire barrier before and after multiple cleaning Code

PN-ISO 3795 Cone calorimeter method; heat flux 35 kW m−2 Amount Flammability TTI HRRmax HRRav THR MLRav applied degree (%) (mm min−1 ) (s) (kW m−2 ) (kW m−2 ) (MJ m−2 ) (g s−1 m−2 )

No 18 cleaning 10× PPAB Cleaning 15× Cleaning 20× cleaning

0

∞

14.53

7.07

0.80

3.00

0

∞

17.59

7.80

0.86

3.07

0

∞

17.92

8.20

0.92

3.47

0

∞

18.03

8.14

0.96

3.82

Fig. 3.5. Flammability of PPAB flame-retarded cotton fabrics (as a fire barrier in the upholstery), before and after multiple cleaning, tested acc. to PN-ISO 3795, before and after multiple cleaning

The samples did not ignite when exposed to heat flux of 35 kW m−2 . The flammability parameters such as HRRmax , HRRavr , MLRavr are also point to the fact that multiple washing does not affect significantly flammability of the fabrics. The high effectiveness of PPAB flame retardant after multiple cleaning was also confirmed by the results of LOI method (Fig. 3.7). The fabric protected with PPAB after 20 cleaning operations still shows a very high value of LOI index amounting to 50.0%. The results of tests conducted according to the PN-ISO 3795 (Table 3.4) show that the UDDP and PPMel formulation (based on melamine polyphos-

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100

HRR [kW/m2]

80 PPAB [0x] 60

PPAB [10x] PPAB [15x]

40

PPAB [20x] 20 0 0

10

20

30

40

50

60

70

80

90

100 110

time [s]

Fig. 3.6. Heat release rate (HRR) from flame-retarded cotton fabrics as fire barrier at heat flux 35 kW m−2 , before and after multiple cleaning. Heat Flux 35 kW m−2 cone calorimeter test ISO 5660

60

LOI [%]

50 40 30 20 10 0

cotton

no

10x

15x

20x

cleaning multiplicity Fig. 3.7. Flammability of flame-retarded cotton fabrics tested by the LOI method. The effect determination of water multiple cleaning

phate and natural rubber latex) have considerably reduced the burning intensity of the covering fabric. The application of intumescent systems contribute to significantly decreasing of fabrics flammability degree. However, the flame combustion of the fabric from the unprotected side (plush side) was observed. It was found that the cleaning increases the effect of flame-retardant treatment (lower values of indices are observed). It may be concluded that washing causes penetration of flame retardant to upper layers of the fabric and that increased number of washing operations decreases the flammability degree (Table 3.4 and Fig 3.8). The fabric treated with the PPMel formulation undergoes selfextinguishing during the test and the flammability degree is better compared to UDPD.

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Table 3.4. Flammability of flame-retarded decorative fabrics, tested by the LOI method and acc. to PN-ISO 3795, before and after multiple cleaning Code

Amount PN-ISO 3795 Remarks Applied (%) Flammability Degree (mm min−1 ) Untreated – 77 No cleaning 28.9 26 UDPD 10× cleaning 23 15× cleaning 5 20× cleaning 3 Burning of the sample from its web side No cleaning 35.4 14 PPMel 10× cleaning 0 15× cleaning 0 20× cleaning 0

LOI (vol%)

21.4 24.6 24.6 26.1 26.1

24.6 24.6 24.6 26.1

Fig. 3.8. Flammability of decorative fabric with PPMel back-coatings (as a covering fabric in the upholstery), tested according to PN-ISO 3795, before and after cleaning

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The results of the LOI test (Table 3.4) point to similar extent of flame retardancy when using UDPD and PPMel. The intumescent formulation caused the increase of 3 vol.-%. However it has increased of 5 vol.-% after 15 and 20 washing operations compared to the LOI index of untreated fabric. The intumescent systems UDPD and PPMel reduced the flammability of the decorative fabric, however, the unprotected plush side of the fabric had a significant effect on an increase in flammability parameters (Fig. 3.8). Presented results indicate high efficiency of intumescent flame-retardant systems for fabrics while maintaining their good flexibility and elasticity. In the case of using compositions based on modified amino resin with phosphate compounds (UDPD) only slight stiffening of the fabrics was observed, therefore possible ways of its softening are investigated. However, it was concluded that the high ability to swelling of the intumescent formulations, especially (UDPD), do not form the intumescent carbonised foamed coating on the protected fabrics when exposed to flame or heat. Processes undergoing during the combustion of the fabrics, lead to changes of reactions happening between substances, which are responsible for foam formation, carbonisation and dehydratation. This might be the cause of the change in velocity of carbon layer formation with the subsequent modification of the thermal resistance. Other researchers refer that as expected, the back-coatings containing intumescents promoted higher levels of char formation, but these did not reflect their performance to match tests where most of them fails [28]. Another area of research concerns the application to flame-retarded polyurethane (PU) and polyurea (PUe) with micro-encapsulated ammonium phosphate for textile coating [29, 30]. Textile coating with polyurethane (PU) resins provides to fabric properties such as abrasion resistance, water repellance, leather aspect, etc. The addition of a flame retardant in PU coatings is necessary to improve the fire behaviour of those materials. Polyurethanephosphate combinations are known to form flame-retardant (FR) intumescent systems [31]. The intumescent formulation is not permanent because of the water solubility and migration of the phosphate. This problem might be solved by the technique of micro-encapsulation. Micro-encapsulation [32–34] is a process of enveloping microscopic amounts of matter (solid particles, droplets of liquids or gas bubbles) in a thin film of polymer which forms a solid wall. This core/shell structure allows the isolation of the encapsulated substance from the immediate surroundings and thus protects it from any degrading factors such as water [30]. Giraud, Bourbigot et al. have synthesised micro-capsules containing diammonium hydrogen phosphate (DAHP) with a polyurethane and polyurea shell [29, 30, 35, 36]. Micro-capsules are obtained by two different techniques, micro-encapsulation by interfacial polymerisation (IP) [36] and microencapsulation by evaporation of solvent (ES) [37]. In the previous work [29], they demonstrated that the concept of using IP micro-capsules of phosphate in a polyurethane coating was efficient in providing flame retardancy for cotton fabrics. The expected advantages of this

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new concept of encapsulated FR agent is that micro-capsules should be easily incorporated and compatibilised in a polymeric matrix and should provide a permanent FR effect while being an efficient FR intumescent formulation for many materials. They showed that the thermal stability of commercial PU for textile coating is improved by the loading of encapsulated DAHP, in particular with a ratio 60/40 PU-µDAHP. In comparison with encapsulated DAHP, the thermal stability of commercial PU with neat DAHP is more significant because the DAHP quantity is higher in formulations with neat DAHP than in formulations with encapsulated DAHP for a same loading. They studied heat and fire resistance of cotton coated by FR PU containing different ratio of neat or encapsulated DAHP. The thermal degradation of cotton is delayed to higher temperature than for cotton coated by virgin commercial PU. Nevertheless in contrast to neat DAHP formulations, encapsulated DAHP formulations did not really show an increase of the thermal stability. Indeed contrary to neat DAHP formulations for the same loading, the quantity of DAHP for encapsulated DAHP formulations is not enough to develop interactions both with cotton and commercial PU. Moreover encapsulated DAHP is less available than neat DAHP to develop char with cotton. The results obtained with cone calorimeter as fire model confirm thermal analyses. The coatings with neat DAHP and encapsulated DAHP give a significant flame-retardant effect with regard to virgin PU coating. But in comparison with encapsulated DAHP formulations, neat DAHP formulations present a stronger enhancement of fire retardant properties. The cotton coated by the formulation 60/40 PU-µDAHP presents several elements proving the formation of a more efficient intumescent system than other PU-µDAHP formulations. In their next research two types of micro-capsules of di-ammonium hydrogen phosphate with polyetherpolyurethane (IP micro-capsules) and polyesterpolyurethane (ES micro-capsules) shells, respectively, were evaluated as intumescent FR agents in commercial polyurea coatings for textile fabrics. The reaction to fire of cotton fabrics coated by FR polyurea loaded with neat or micro-encapsulated DAHP was studied by cone calorimetry. The coatings with 20% of IP or ES micro-capsules show decreases in propensity to spread flame with regard to virgin polyurea coating. Their decreases of RHR (rate of heat release) peaks are similar to that for a coating with 20% of DAHP only. But in comparison with encapsulated DAHP formulations, neat DAHP formulations present smaller total heat evolutions, and thus a greater enhancement in fire resistance. Micro-encapsulated DAHP does not develop a strong enough intumescent shield to resist heat and flame stresses and develops cracks in the intumescent structure. Future work is in progress to increase the encapsulation yield of DAHP [38]. The micro-capsules of DAHP with PU shell have been studied as the intumescent flame-retardant component in different polymers [39]. Despite the above listed research on the production of intumescent flame retardants and methods of their manufacture, there is still an actual need to

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obtain more effective solutions, facilitating more efficient fire protection and thermo-insulation of intumescent compositions, minimising their cost while maintaining their high quality.

3.4 Flame-Retarded Synthetic Fibers Synthetic fibers, such as polyester, polypropylene or polyamide are difficult to flame retard since the production of fibers (spinning) is incompatible with some parameters of the flame retardancy requirement such as for example, high additive levels (it is widely admitted that intumescent systems have to be incorporated at a higher level than 20% to achieve fire retardancy of interest [15]) or large particle size of intumescent additives (ammonium polyphosphate presents particle size varying form a few micron to 20 µm [40]). Due to those problems, flame retardancy of fibers or fabrics is mainly achieved via textile finishing or by making inherent flame-retardant polymers (copolymerisation, grafting on polymer chain, etc.). 3.4.1 Poly(Ethylene Terephtalate) Poly(ethylene terephatlate) (PET) is an important class of polyester for the production of engineering plastics and fibers. PET fibers represent 70% of all synthetic fibers which find commercial interest. However, PET is a highly flammable material and exhibits a high dripping when burning. The use of phosphorus based flame retardant for polyester is known from several decades [41]. The phosphorus is mainly chemical bonded to the polymer to obtain durable fire-retardant properties. Incorporating a comonomeric phosphinic acid unit HO–P( O)(R)–X–COOH into the PET chains where R is hydrogen or alkyl and X an alkyl groups leads to the well known and high efficient Trevira CS. In that case, the incorporation of phosphorus into the polymer does not promote char formation and evidence of gas phase mechanism has been demonstrated [42]. The co-polymerisation of phosphorus and/or nitrogen–phosphorus containing monomers to developed inherent flame-retardant PET fibers has been recently reviewed [41] and is not presented in this paper, since the generally proposed mechanism of action cannot be classified as intumescent system. There is very few literature reporting the use of intumescent systems in polyester and even less for polyester fiber. Ma et al. [43] synthesised intumescent phosphate-polyester co-polymers from spiro-cyclic pentaerythritol di(phosphate acid monochloride) (SPDPC). Because of the new monomer being introduced, the structure and properties of the polyester were altered. The oxygen index (OI) of the co-polymer increases with the increasing phosphate content. The flame-retardant mechanism was possibly attributed to intumescence according to analysis of the scanning electron microscopy (SEM) of the co-polymer chars.

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Fig. 3.9. The expandable textile structure

Expandable graphite offers a great opportunity to obtain the fire flexible barrier. Used as a filling agent, it is able to expand significantly during heating and certain amounts of water vapour and traces of nitrogen-containing gases are released at the same time [20, 44]. A porous layer made of expanded graphite lamellae, having excellent heat insulating properties, is formed as a result. Because of these properties it is used in the back-coating methods and the needle punching method for obtaining flame-retardant textile materials. The expandable flame-retardant textile materials were produced by needle punching method, included expandable graphite particulate, a non-woven and sandwich structural fabrics with polyester fibers [45]. Figure 3.9 shows the expandable textile structure [45]. Due to high intumescent ability of graphite, tests were begun on increasing the efficiency of intumescence of flame-retarded acrylic resins, which form thin, flexible coatings of good bending and tearing strength and resistance to ageing and using it as a filling agent in modified intumescent amino resins. Expandable graphite flake was used: Grafguard 160-80A (UCAR Carbon Company Inc.). The preliminary results concerning the using of the formulation based on acrylic derivatives and expandable graphite shown that the flame-retarded fabrics are characterised by high resistance to flames and by a pleasant touch and flexibility, but the structure of formed foam and its adhesion to substrate is discontent. Another inconvenience of formulations is the necessity of applying relatively large amounts of the intumescent material, which results in difficulties in the application. Moreover, relatively long time of expandable graphite glowing was also observed. Caz´e et al. [46] evaluate the efficiency of an intumescent coating based on a polyvinyl acetate binder and filled with phosphorus–nitrogen compound called Aflamman and supplied by Thor Company on polyester fabric. They demonstrate that those systems have an intumescent behaviour in the presence of a fire. These systems allow an increase in time to ignition and a decrease in extinction time of polyester fabric. Moreover, the application of these formulations on textile fabrics does not radically modify the fabric’s handle. Cotton-PET blends are also of a considerable economic importance. However, despite great efforts made over several decades, no commercially acceptable solution has been developed [47]. The difficulty of this problem is

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attributed to the great contrast between the chemical and physical properties of cotton and PET. Indeed, the cotton degrades at lower temperature than PET with charring and as a consequence acts as ignition source. Since charring occurs, PET, instead of dripping is maintained by the degraded cotton and so the combustion cycle is not stopped. Phosphorus-containing back-coating including some intumescents have been formulated by Horrocks et al. [48, 49]. It was demonstrated that while back-coatings containing intumescents promoted higher levels of char formation, this did not reflect their fire-retardant performance. The performances of back-coating are dependent on their ability to liquefy by melting and/or decomposition to products that diffuse to the front face of the fabric. Drevelle et al. [50] investigates the influence of an intumescent back coating, applied to a cotton/PESFR (Trevira CS) fabric on thermal and fire resistance behaviour. The intumescent system was made of an acrylic binder resin and ammonium polyphosphate (APP). The pure acrylic binder resin coated on fabric leads to a decrease in the thermal and fire resistance properties of the virgin materials while when APP was added to the binder resin, thermal stabilisation and increase in the fire resistant properties were noted. 3.4.2 Polypropylene Polyolefins are increasingly used in building (flooring), transport and electrical applications (cables and wires) due to their low cost, properties and easy process. In particular, the use of polypropylene as fibers for general textile applications has progressed rapidly since its introduction. A large range of characteristics can be imparted to olefin fibers with variations in the polymer, by use of different process conditions or by addition prior to or during melt spinning of additives, in particular of fire-retardant (FR) additives. The flame retardancy of polypropylene fiber has been recently reviewed [51]. Phosphorus compounds having flame-retardant properties in polypropylene may be both inorganic and organic. It is also common for them to be used in the presence of halogen or nitrogen-containing compounds and especially those that generate intumescent char-forming characteristics. Polypropylene fibers may be treated with intumescent flame-retardant finishes and back-coating. Patent EP1300506 [52] describes a solution relating to a textile substrate with improved resistance to heat and fire with a fire-retardant coating which consists of a polymer binder and an intumescent composition, which is additionally comprised of a small quantity of one or more synergic agents in the coating which act under the influence of heat to produce a fine uniform layer on top of the deep alveolar layer produced by the intumescent compound. The synergic agents are aluminium hydroxide, magnesium hydroxide, bohemite, titanium oxide, sodium silicate, zeolites, low melting point glass, clay nano particles, borosilicate products. . . The synergic agent is used at 0.2–3wt.-% with respect to the polymer binder.

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Magniez et al. [53] pointed out the differences in fire behaviour of polypropylene textile structures in comparison with plastics. Fire behaviour was validated on cone calorimeter equipment. This study shows how after reducing the rate of heat release, which is one of the parameters of the FR regulations, polypropylene fibers can be used for flame-retardant materials. The method based on the curves of rate of heat release difference estimates and classifies the association of textile fibers and binder resin. The exploitation of this methodology permits to understand the fire behaviour of the textile products, to orient the research on binder resin and intumescent concept and to elaborate a predictive toolkit for the optimisation of fire proofing textiles. A major drawback of intumescent back-coating is that even the most water insoluble intumescent systems cannot survive textile laundering [54]. To avoid this problem new formulations have to be developed. Recent research carried out at PERF [55, 56] deals with the development of intumescent back-coating for polypropylene non-woven presenting higher durability than traditional back coating. Polyurethane is used as binder as well as charring agent [57] in an intumescent back coating containing APP and melamine. The non-woven polypropylene (465 g m−2 ) is coated with fireretarded polyurethane presenting a thickness of around 500 µm (Fig. 3.10). The add-on is fixed at 80%. Table 3.5 reports the mechanical and the FR performance of the untreated and of the back-coated non-woven. It clearly appears that the fire performance of the back-coated sample are greatly improved when compared with the virgin non-woven. It is possible to reach an LOI value of approximately 32% and the burned surface is sharply reduced. These fire-retardant properties are kept also after 72 h immersion in water. Moreover, the mechanical properties of the PP non-woven are slightly affected by the fire-retardant treatment. Another approach to flame retard PP fiber is to develop inherent intumescent fibers adding flame-retardant additives during the extrusion process. This solution appears to be the ideal solution for achieving fibers with good overall performance.

(a)

(b)

Fig. 3.10. SEM pictures of virgin non-woven PP (a) and back coated non-woven PP (b)

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Table 3.5. FR performance and mechanical properties of PP non-woven and back coated PP non-woven

LOI (1) Burned surface (2) Burned surface after washing (3) Tensile strength (4) Elongation at break (4) Tear strength (5)

PP

Back-coated PP

18 vol% 92% 92% 140 daN 78% 80 daN

32 vol% 5% 16% 162 daN 72% 100 daN

(1) ASTM D2863/77 (2) NF G07-184 using a bunsen burner as ignition source (3) NF G07-184 using a bunsen burner as ignition source after immersion of samples in water during 72 h at 20◦ C with agitation (4) ISO EN 9073-3 (5) EDANA 70.4-99

Fig. 3.11. SEM picture of intumescent PP filaments

Ammonium polyphosphate in combination with polyamide 6 in PP are known to act as intumescent system [58]. The spinnability of PP/APP/ PA6/PP-g-MA-based systems has been investigated [55]. Compounding of polymeric matrixes with the FR additives was performed using a twin-screw extruder. The extrudate was then cooled in air and pelletised before processing melt filaments using a single screw extruder equipped at the end of a roll that stretches the filaments. Filaments of about 20 dTex are obtained (Fig. 3.11). The influence of the crystalline form of APP on both FR properties and spinnability have been evaluated. It is observed that the LOI of intumescentbased PP formulation including crystalline forms I and II of APP are very close (respectively, 28 and 27 vol%). Mechanical properties of intumescent PP fibers are sharply decrease compare with pure PP (Fig. 3.12). However, this decrease in properties is limited when form crystalline II of APP is used. This reduction of the tensile properties of the filaments limits the commercial development of such formulation in the textile industry.

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Elongation at break (%dTex)

35 30 25 20 15 10 5 0 PP

Intumescent PP Intumescent PP including APP including APP crystalline form I crystalline form II

Fig. 3.12. Elongation at break of PP and intumescent PP

Fig. 3.13. Reaction between melamine phosphate (MP) and pentaerythritol (PER)

To overcome problems associated with the incorporation of particles into PP, synthesis of intumescent-flame-retardant masterbatch by reactive extrusion has been reported [59]. Mixtures of melamine phosphate (MP) and pentaerythritol (PER) were prepared in a high-speed mixer. The obtained master batch was then blended with PP into a twin-screw extruder. The reaction occurring between MP and PER during the process has been demonstrated by FTIR leading to the formation of a “spiro”-structure presented in Fig. 3.13. The good fire performance of such system associated with a good water resistance and their capabilities of forming fiber-like structure have been reported. Finally, the combination of nano-particles with intumescent system has been proposed to improve the fire retardancy of polymers and in particular of polyolefins [60, 61]. The synergistic effect achieved when nano-particles are combined with intumescent systems could lead to a decrease of the level of additives required to achieved acceptable FR properties.

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300

RHR (KW/m2)

250 200 150 100 50 0 0

20

40

60

80

100

120

140

160

180

200

Time (s)

Fig. 3.14. Heat release rate vs. time for PP/APP/PA6 (thin line) and PP nano/ APP/PA6 nano (thick line)

As an example, the use of clay-nanocomposite matrixes in PP/APP/PA6 formulations leads to a decrease and to a delay of the two characteristic PHRR (peak of heat release rate) of intumescent system (Fig. 3.14). Mechanism of action is not completely elucidated but it may be assumed that reaction takes place between the nano-filler and the phosphate to thermally stabilise the charred structure. It slows down the degradation of the material and then the evolution of flammable molecules. This reaction does not significantly modify the expansion of the intumescent coating or even decreases it but it permits the reinforcement of the char strength and avoids the formation of cracks. The char looks like a foam with evenly dispersed close cells ensuring the limitation of heat transfer between the flame and the substrate. It may also be proposed that the high degree of particle dispersion causes changes in combustion processes as well as decomposition of intumescent coating by modifying the carbon structure into small-cell one, which improves thermoinsulating properties, conductivity and heat convection of foam formed [62]. 3.4.3 Polyamide Little was published dealing with the use of intumescent systems for nylon in textile applications. The effectiveness of various binary metal–ammonium phosphates (BMAPs) and melamine and its salts in nylon 6 was reported by Levchick et al. [63, 64]. The fire-retardant effect of these compounds was mostly attributed to polymer mass retention and intumescent-layer-protection mechanisms. However, no evaluation of the spinnability of those formulations

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Fig. 3.15. Chemical formula of SPDPC, CPPC and CDPPC

has been investigated. As far as we know, it exists no effectively durable flame retardant to incorporate into fiber of linear polyamides. Based on the work performed by Ma et al. in PET and previously reported [43], Horrocks et al. [54, 64, 65] demonstrate that polyol phosphoryl chlorides in polyamide fiber confer inherent intumescence. The polyamides (nylon 6 and 66) phosphorylated by spiro-cyclic pentaerythritol phosphoryl chloride (SPDPC), cyclic 1,3-propanediol phosphoryl chloride (CPPC), and cyclic 2,2diethyl-1,3-propanediol phosphoryl chloride (CDPPC) can yield phosphorus levels up to 0.7% (wt/wt) since phosphorylation can only occur at the amine end group (Fig. 3.15). It is demonstrated that the presence of substituted 1,3-propanediol phosphonate moieties significantly increases polyamide char formation above 500◦ C and scanning electron microscopy indicates that the residual char has an intumescent structure. The same group investigates the effect of selected phosphorus-based additives into nylon 6 and nylon 6.6 polymer films in the presence and absence of nano-clay. No spinnability testing has been carried out.

3.5 Conclusion This paper has reviewed how the intumescent concept could be used to flame retard textile. It is highlighted that the low durability of intumescent systems requires the development of innovative formulations to overcome this problem. The use of amino resins in combination with phosphorus compounds could lead to durable treatment for cotton. On the other hand, polyurethane based back-coating intumescent formulations have been developed to flame retard polypropylene non-woven. Selection of the appropriate formulations could lead to durable products with no modification of the stiffness of the material. To overcome the problem associated to the durability, inherent

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flame-retardant fibers have been developed, using the co-polymerisation approach or adding additives in the molten polymer. Finally, the combination of nano-particles with intumescent systems to improve the fire retardancy of polymer and in particular of polyolefin has been proposed.

References 1. A.R. Horrocks, Fire Retardant Materials (Woodhead Publishing Limited, Cambridge, 2001), p. 128 2. U.K. Fire Statistics London, Home Office, HOSB 20/00 (2000) 3. M.J. Karter Jr., NFPA 2000 United States Fire Loss, September 2001 4. H.L. Vandersall, J. Fire Flammability 2, 97 (1971) 5. G. Camino, L. Costa, L. Trossarelli, Polym. Degrad. Stab. 12, 213 (1985) 6. R. Delobel, M. Le Bras, N. Ouassou, F. Alistiqsa, J. Fire Sci. 8(2), 85 (1990) 7. S. Bourbigot, M. Le Bras, S. Duquesne, M. Rockery, Macromol. Mater. Eng. 289, 499–511 (2004) 8. J.W. Lyons, The Chemistry and Uses of Fire Retardants (Wiley, London, 1970), pp. 122–142 9. H.L. Vandersall, J. Fire Flammability 2, 97–140 (1971) 10. M. Kay, A.F. Price, I. Lavery, J. Fire Retardant Chem. 6(2), 69–91 (1979) 11. W.A. Rains, Fire Retardant Coatings. Handbook of Fire retardant Coatings and Fire Testing Services (Technomic Publishing Company, Lancaster, 1994), pp. 1–4 12. B.K. Kandola, R.A. Horrocks, Polym. Degrad. Stab. 54, 289–303 (1996) 13. M. Wladyka-Przybylak, R. Kozlowski, D. Wesolek, The Thermal Characteristic of Different Intumescent Coatings, Proceedings of the 5th Arab International Conference on Materials Science, Alexandria, Egypt, 22–25 March 1998, pp. 45–58. 14. G. Camino, S. Lomakin, Fire Retardant Materials (Woodhead Publishing Limited, Cambridge, 2001) 15. A.R. Horrocks, Polym. Degrad. Stab. 54, 143–154 (1996) 16. H.V. Landin, Flexible Fire Barrier Felt. US Patent 5830319 (1998) 17. T.W. Tolbert, J.P. Panela, J.S. Dugan, J.E. Hendrix, Fireproof Fabrics, US Patent 333174 (1993) 18. A.R. Horrocks, S.C. Anand, B.J. Hill, Fire and Heat Resistant Materials. US Patent 5645926 (1997) 19. A.R. Horrocks, B.K. Kandola, in: Fire Retardancy of Polymers - The Use of Intumescence, ed. by M. Le Bras, G. Camino, S. Bourbigot, R. Delobel (Royal Society of Chemistry, London, 1998) pp. 343–362 20. A.R. Horrocks, P.J. Davies, Fire Mater. 24, 151–157 (2000) 21. R. Kozlowski, B. Mieleniak, M. Muzyczek, D. Wesolek, M. Wladyka-Przybylak, Project: New Surface Modified Flame Retarded Polymeric Systems to Improve Safety in Transportation and Other Areas. Acronym: FLAMERET. Contract No: G5RD-CT 1999-00120 (2000–2003) 22. R. Kozlowski, B. Mieleniak, M. Muzyczek, A. Kubacki, Fire Mater. 26, 243–246 (2002) 23. R. Kozlowski, D. Wesolek, M. Wladyka-Przybylak, Nat. Fibres 45, 149–156 (2001)

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24. R. Kozlowski, D. Wesolek, M. Wladyka-Przybylak, FR Treatment of Fabrics With Intumescent Systems, 2nd International Conference on Polymer Modification, Degradation and Stabilisation – MoDeSt 2002, Budapest, Hungary, 30 June–4 July, 2002 25. PN-76/C–89020, Plastics. Determination of Ignitability by the Method of Limiting Oxygen Index (LOI) 26. ISO 5660, Fire Test Reaction to Fire (Cone Calorimeter Method) 27. PN-ISO 3795:1996, Road Vehicles, Tractors and Machinery for Agriculture and Forestry – Determination of Burning Behavior of Interior Materials 28. A.R. Horrocks, M.Y. Wang, M.E. Hall, F. Sunmonu, J.S. Pearson, Polym. Int. 49(10), 1079–1091 (2000) 29. S. Giraud, S. Bourbigot, M. Rochery, I. Vroman, L. Tighzert, R. Delobel, Polym. Degrad. Stab. 77, 285–297 (2002) 30. S. Giraud, S. Bourbigot, M. Rochery, I. Vroman, L. Tighzert, R. Delobel, F. Poutch, Polym. Degrad. Stab. 88, 106–113 (2005) 31. M. Bugajny, M. Le Bras, S. Bourbigot, F. Poutch, J.M. Lefebvre, J. Fire Sci. 17, 494 (1999) 32. C. Thie, Encyclopedia of Polymer Science and Engineering, Microencapsulation, 2nd edn. (Wiley, New York, 1987) 33. A. Kondo, in Microcapsules Processing and Technology, ed. by J.W. Van Valkenburg (Dekker, New York, 1982) 34. P.B. Deasy, Microencapsulation and Related Drug Processes (Dekker, New York, 1984) 35. S. Giraud, S. Bourbigot, M. Rochery, I. Vroman, L. Tighzert, R. Delobel, J. Ind. Text. 31, 11 (2001). S. Giraud, Microencapsulation d’un diisocyanate et d’un phosphate d’ammonium – Application: e laboration d’un systeme polyurethane monocomposant a ` propri´ et´ees retardatrices de flame pour l’enduction textile, Ph.D. Thesis, Universit´e des Sciences et Technologies de Lille (2002), pp. 113–140 (www.univ-lille1.fr/bustl-grisemine/pdf/ extheses/50376-2002-311-312.pdf) 36. R. Arshady, J. Microencapsul. 6, 13–28 (1989) 37. R. Arshady, Polym. Eng. Sci. 30, 915–924 (1990) 38. D. Saihi, I. Vroman, S. Giraud, S. Bourbigot, React. Funct. Polym. 64(3), 127–138 (2005) 39. S. Giraud, A. Castrovinci, I. Vroman, G. Camino, S. Bourbigot, Thermal Stability and Fire Performance of Polypropylene Loaded with Microencapsulated Ammonium Phosphate, 10th European Meeting on Fire Retardancy and Protection of Materials, BAM, Berlin, Germany, 7–9 September 2005 40. www.specialchem4polymers.com/sf/budenheim/index.aspx?id=ammonium 41. S.V. Levchik, E.D. Weil, Polym. Int. 54(1), 11–35 (2005) 42. J.D. Conmey, M. Day, D.M. Wiles, J. Appl. Polym. Sci. 29, 911–923 (1984) 43. Z. Ma, W. Zhao, Y. Liu, J. Shi, J. Appl. Polym. Sci. 63, 1511–1515 (1997) 44. G. Camino, S. Duquesne, R. Delobel, E. Berend, C. Lindsay, T. Roels, Mechanism of Expandable Graphite Fire Retardant Action in Polyurethanes, 220th ACS National Meeting, Washington, DC, 20–24 August 2000 45. S.C. Yao, H.E. Chen, S.C. Yeh, The Expandable Flame-Retardant Textile Materials, vol. 1, Proceedings of the 5th Asian Textile Conference, 1999, p. 1160 46. C. Caze, E. Devaux, G. Testard, T. Reix, in Fire Retardancy of Polymers: The Use of Intumescence, ed. by M. Le Bras, G. Camino, S. Bourbigot, R. Delobel (Royal Society of Chemistry, London, 1998), pp. 363–375

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47. M. Lewin, Polym. Degrad. Stab. 88, 13–19 (2005) 48. R. Horrocks, M.Y. Wang, M.E. Hall, J.S. Pearson, J. Ind. Text. 29(3), 206–239 (2000) 49. R. Horrocks, M.Y. Wang, M.E. Hall, F. Sunmonu, J.S. Pearson, Polym. Int. 49, 1079–1091 (2000) 50. C. Drevelle, J. Lefebvre, S. Duquesne, M. Le Bras, F. Poutch, M. Vouters, C. Magniez, Polym. Degrad. Stab. 88(1), 130–137 (2005) 51. S. Zhang, A.R. Horrocks, Prog. Polym. Sci. 28, 1517–1538 (2003) 52. C. Magniez, R. Delobel, F. Poutch, Textile Substrate with Improved Fire Resistance, EP1300506 (2003) 53. C. Magniez, A. Dubois, M. Vouters, R. Delobel, F. Poutch, J. Ind. Text. 32(4), 255–266 (2003) 54. A.R. Horrocks, B.K. Kandola, P.J. Davies, S. Zhang, S.A. Padbury, Polym. Degrad. Stab. 88, 3–12 (2005) 55. C. Drevelle, Conception and Development of Fire Retardant Systems for Synthetic Fibers. Ph.D. Thesis (2005) 56. S. Duquesne, C. Drevelle, S. Bourbigot, R. Delobel, F. Poutch, Influence of the Fireproofing Method on the Fire Retardant Performance of Intumescent Polypropylene Non-Woven, 17th Annual BCC Conference on Flame Retardancy – Recent Advances in Flame Retardancy of Polymeric Materials, Stamford, CT, 21–24 May 2006 57. G. Montaudo, E. Scamporrino, C. Puglisi, D. Vitalini, J. Appl. Polym. Sci. 30, 1449–1460 (1985) 58. S. Bourbigot, M. Le Bras, C. Siat, in Recent Advances in FR of Polymeric Materials, vol. 8, ed. by M. Lewin (Business Communications, Norwall, 1998), pp. 146–160 59. Q. Wang, Y. Chen, Y. Liu, H. Yin, N. Aelmans, R. Kierkels, Polym. Int. 53, 439–448 (2004) 60. S. Bourbigot, M. LeBras, F. Dabrowski, J. Gilman, T. Kashiwagi, Fire Mater. 24, 201–208 (2000) 61. S. Duquesne, S. Bourbigot, M. Lebras, C. Jama, R. Delobel, in Fire Retardancy of Polymers – New Applications of Mineral Fillers, ed. by M. Lebras, C. Wilkie, S. Bourbigot, C. Jama (RSC Publications, London, 2005), pp. 239–247 62. R. Kozlowski, D. Wesolek, M. Wladyka-Przybylak, Flammability of Intumescent Fire Retardant System Based on Nano-Modifiers. Proceedings of the 15th BCC Conference on Flame Retardancy, Recent Advances in Flame Retardancy of Polymeric Materials, Holiday Inn Select, Stamford, CT, 22–25 May 2005 63. S.V. Levchik, A.I. Balabanovich, G.F. Levchik, L. Costa, Fire Mater. 21(2), 75–83 (1997) 64. G.F. Levchik, S.V. Levchik, A.F. Selevich, A.I. Lesnikovich, A.V. Lutske, L. Costa, in Fire Retardancy of Polymers. The Use of Intumescence ed. by M. Le Bras, G. Camino, S. Bourbigot, R. Delobel (Royal Society of Chemistry, London, 1998), pp. 280–289 65. A.R. Horrocks, S. Zhang, Text. Res. J. 74, 433–441 (2004)

4 Protection Against Electrostatic and Electromagnetic Phenomena S. Nurmi, T. Hammi, and B. Demoulin

Summary. Electrostatics (also known as static electricity) is the branch of physics that deals with the forces exerted by a static (i.e. nonchanging) electric field upon charged objects. Electrostatics involves the buildup of charge in objects due to contact between mostly-nonconductive surfaces. These charges are generally built up through the flow of electrons from one object to another. These charges then remain in the object until a force is exerted that causes the charges to balance: e.g., the familiar phenomenon of a static ‘shock’ is caused by the neutralization of charge built up in the body from contact with nonconductive surfaces. Electromagnetism is the physicsHYPERLINK “http://en.wikipedia.org/wiki/ Physics”\o “Physics” of the electromagnetic field: a field which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles. The magnetic field is produced by the motion of electric charges, i.e. electric current. The magnetic field causes the magnetic force associated with magnets.

4.1 Protection Against Electrostatic Phenomena 4.1.1 Static Electricity Static electricity occurs commonly in industry and in daily life. Many of the effects are harmless and either pass completely unnoticed or are simply a nuisance, but static electricity can also give rise to a hazardous situation. Hazards caused by electrostatic charge include: ignition and/or explosion, electric shock in combination with another hazard, e.g. fall and trip and electric shock giving rise to injury or death. Many electronic components are known to be at risk to damage from electrostatic discharges (ESD) unless they are handled within an ESD Protected Area (EPA). In the EPA electrostatic fields and sources of ESD are controlled to keep ESD risks to an insignificant level. In addition static electricity introduces operational problems during manufacturing and handling processes, e.g. by causing articles to adhere to each other or by attracting dust [1].

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4.1.2 Fundamental Principles of Electrostatics Charge All electrostatic effects are caused by forces between electric charges. The charge is a property of certain fundamental particles, most importantly protons (positive) in all nuclei and electrons (negative) located around the nuclei. All materials are made of positively charged atomic nuclei and negative electrons. The charge of a proton is positive and the charge of an electron is negative. If the charge effects of a proton and an electron are equal, they cancel each other in close proximity. Opposite charges attract each other and like charges repel each other. Under some circumstances the electrical effects of the charges are not balanced and then a static electrical charge exists at that location. The charge of an electron and a proton has the same numerical elementary charge value e (Fig. 4.1) e = 1.609 × 10−19 C(Coulomb) Moving charges form electrical currents. One coulomb of charge has passed if 1 A has flowed for 1 s. 1 C = 1 A × 1 s. Contact Charge The most important source of electrostatic charge is contact charging. Contacts and separations between two materials result in a transfer of charge between the surfaces and charge separation happens. In contact electrons move from one material to the other and in separation charge separation takes place. Induction Charge When an uncharged isolator (B) is placed in the electric field of an object (positively charged insulator A), the voltage on an object (B) will change under the influence of a nearby charged object (A) and its electric field (Fig. 4.2).

Fig. 4.1. Like and unlike charges [2]

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Fig. 4.2. Electrostatic induction [2]

E

+q

r Fig. 4.3. Field from point charge [2]

Electric Field The charges make themselves known through the forces by which they interact. These forces form an electric field. If the force on the charge Q is F, then the field strength E is defined by (Fig. 4.3) F = QE

(4.1)

In the electric field like polarity charges repel and unlike polarity charges attract each other. If a charge q is located within a very small region (a point charge) in a distance r it will create an electric field E given by E=

q . 4πεr2

(4.2)

From a small charged object the strength of the electric field often decreases with distance r. The unit is volts per metre, (V m−1 ).

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Voltage (Potential) Voltage is a property of a point in an electric field. It means that we measure potential energy in a point. The voltage of a point is the energy which is needed to move the charge from the point to ground. If the charge is moved from point A to point B the field do a work (energy) WAB given by


B

E · da = Q[VA − VB ],

WAB = Q

(4.3)

A




B

VA − VB =

E · da.

(4.4)

A

The unit volt (V) is equivalent to joules per coulomb (Fig. 4.4). Electrostatic Discharges Under normal circumstances air is a good insulator. If, however, an electric field strength exceeds a certain value (about 3 MV m−1 , 3 kV mm−1 ) the insulating property of air weakens and an electrostatic discharge occurs. The type of discharging depends on different things, among others the nature of the material through which it develops and on geometry. Corona Discharge Corona discharge happens typically on conductors with sharp points. The electric field increases above the breakdown field locally at the sharp surface and charge will discharge. The field strength at the edge is typically about 3 MV m−1 .

E Q

A

F

da

B

Fig. 4.4. Charge Q placed in an electric field E [2]

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Spark Discharge The best known type of electric discharge is the spark. It happens between two conductors, which have a high voltage difference between them. Brush discharge If the charges are not arranged in form of one single layer of one polarity on a non-conducting surface but in form of a double layer of charges of opposite polarity on the opposite surfaces of a non-conducting material in form of a sheet, propagating brush discharges may occur. The energy density in a brush discharge is higher than in corona discharge and it may be enough to ignite flammable gases, liquids or powders [3]. Charge Decay If an electric field is established in a material containing mobile charge carriers, the positive charges will flow in the direction of the field and negative charges in the opposite directions (Fig. 4.5). The force F on a carrier with the charge q exposed to a field with the field strength E is F = qE. (4.5) 4.1.3 Electrostatic Charging and Textile Type Materials The problems arising from the development of electrostatic charge on fibres, fabrics and other textile assemblies are many and varied. In general they are more serious with fibres having a relatively low moisture regain and at low relative humidities, where even the most moisture absorbent fibres can give rise to high and persistent static charges. Because the ordinary fibres used in textile materials are insulators or exhibit rather low electrical conductivity, E V

F

q

Fig. 4.5. Mobile ions in the electric field [2]

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charge once generated dissipates only with difficulty. Electrostatic charge is a surface phenomenon and textile fibres have exceptionally high surface-tovolume ratios. Their susceptibility to electrostatic charge is particularly high. The susceptibility is higher, the lower the atmospheric humidity is. Fibres with relatively high moisture regain, such as wool and cotton, are less likely to develop a high electrostatic charge at intermediate values of atmospheric humidity, but at low relative humidity they are at least as prone to charge development as fibres with low moisture regain. In dry conditions cotton is quite resistive and dissipates charge with difficulties [4]. At high relative humidity charging of textiles is not usually a big problem. The problems begin during textile manufacturing and processing where charge may accumulate on the textile material during handling and in other points of the textile product logistic chain, such as transportation. The whole subject of problems arising from electrostatic charging of persons in a work environment has become of increasing importance not just because of the nuisance value to the individuals but also because of the effect that electrostatic discharges may have on sensitive telecommunication, computers and semiconductor assembly operations. Most of the work on damage to micro-electronic devices by electrostatic discharges has concentrated upon discharges from the human body. It is known that some clothing generate as high electrostatic charge levels that may cause risks to electrostatic sensitive components. To prevent damages personnel in industry wear so called ESD protective coats on their own cloths. Also it is possible that charge induced on the body by charged clothing may cause ignition hazards from spark discharges. 4.1.4 Charging Mechanisms in Textiles In the textile field the most important source of electrostatic charge is contact charging. In practice it is better known as triboelectric charging. This occurs when any two materials contact each other or are rubbed together. Charge separates across the contact boundary leaving one surface with a net positive charge and the other surface with a net negative charge. When the materials are moved apart, some of the charge remains on the surface of each material. If both materials are good electrical conductors, metals for instance, charge will re-combine under the influence of the electric field that is established between the separating surfaces. In this case very little charge, if any, is left on each surface. If one of the surfaces is a non-conductor, then re-combination of charge cannot occur within the short time taken to separate the surfaces and so a significant amount of charge may be left on the two materials. Repeated contacts and separations lead to further transfer of charge until an equilibrium value is reached. The amount of charge generated during abrasion depends on a number of factors. A lot of empirical data to predict the amount of charge generated has been made and many difference indicative triboelectric series created. Materials are ranked in a triboelectric series according to how they

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Table 4.1. Triboelectric series [6] Positive (+)

Negative (−)

Wool Hercosett woll Nylon 6.6 Nylon 6 Silk Regenerated cellulose Cotton Poly(vinyl alcohol) (PV Alc) Chlorinated wool Cellulose triacetate Calcium alginate Acrylic Cellulose acetate Polytetrafluoroethylene (PTFE) Polyethylene Polypropylene Poly(ethylene terephthalate) Poly(1,4-butylene terephthalate) Modacrylic Chlorofibre

charge when rubbed against other materials in the series. A material will normally charge positively when rubbed against another lower down in the series, and negatively when rubbed against a material higher up. The series only show trends but not the amounts of charge generated [5, 6] (Table 4.1). Induction charging is the production of an electric charge on a conductive body by the proximity of an already charged body without contact between the two bodies. The induced charge is opposite in sign from the inducing charge. Charge induction is important as a means of transferring charge from textiles to the surfaces, such as the human body or charge dissipative fibres and fabrics. Charging by ion or electron bombardment method of charging is usually effected by creating a corona discharge. This results from raising fine points, such as fibre ends, or fine wires of a conductive material, to an electric potential high enough to cause electrical breakdown of the local atmosphere. The lower the radius of curvature of the point or wire, the lower the potential needed for electrical breakdown. Electrostatic charge can also be produced on a textile material by direct contact with a highly charged conductive electrode. 4.1.5 Charge Dissipation Mechanisms of Textiles If the electrical resistance of a material is low enough, any charge which is generated on its surface can be dissipated by either spreading across or through the material thereby reducing the local charge density, or by moving to earth

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via a suitable connection. These electrical conduction processes have been relied on for many years as the main mechanism for controlling static electricity. Some materials used to control static electricity have inherent conductive properties. There are three mechanisms by which static dissipation occurs and by which the charge on a heterogeneous dissipative fabric will be neutralised (Fig. 4.6). (a) If the fabric is grounded, the charge on or near the conducting element of the conducting thread will be conducted to earth. (b) The charge on the insulating base fabric induces a charge of opposite polarity in grounded conductive threads leading to partial neutralisation of the total charge. The phenomenon can also be understood as an increase in the capacitance caused by the grounded threads which lower the effective potential to be observed. Charge of one polarity (opposite to that on the base fabric) remains on the conductive thread, whilst charge of the other polarity is conducted to earth. The field from the charge on the base fabric is coupled to the opposite polarity charge on the conductive thread and so its effect is neutralised. Another way of looking at this mechanism is to consider it as a type of capacitive loading. By bringing an earthed conductor close to the charge on the base fabric, its capacitance is increased and hence its potential is decreased. The internal structure of the fabric, especially the spacing of the grounded conductive threads, have the strongest influence on the effective potential observed on the fabric through the induction mechanism. (c) Partial neutralisation of charges on the base fabric may also occur by the air ions formed in corona discharges, if the corona onset field strength in the region is exceeded. The fineness and cross-sectional shape of the threads will have a big influence on the corona onset field strength. It should be pointed out that the corona mechanisms do not require grounded threads but surface charge density should be relatively high to initiate the corona.

Induction

+

Conduction

−

+ + +

− +

+

+ −

−

+ + +

Corona

+

Fig. 4.6. Three charge dissipation mechanisms [7]

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The only mechanism of the three that is related to the resistivity of the material is conduction. Conduction and induction rely on the conductive threads being earthed. Corona dissipation mechanism does not require the conductive threads to be earthed [5, 7]. 4.1.6 Electrostatic Control Fabrics and Garments Materials used to control the build up of static electricity should satisfy at least one of two basic requirements: they should not easily charge, or if they do, they should be capable of dissipating charge safely or at a rate which is fast in comparison to the rate of charging. The main purpose of a protective garment in the electronic industry is to shield sensitive devices in the production phase from static electricity generated in the normal clothing worn by the operator. Garments on which high levels of static electricity can be generated are one of the causes of electrostatic discharge damages. Protective performance of a garment is highly reduced or even completely lost if the garment is not grounded. Grounding is typically realised by a direct collar or sleeve contact to the grounded operator. Improper use of the garment may easily disconnect the ground path. Therefore alternative grounding methods could be considered or continuous ground connection monitoring systems developed to guarantee the desired function of the protective clothing. Grounding of a electrostatic dissipative (surface conductive) garment can be accomplished by several means, through a conductive wrist cuff in direct contact with the skin of a grounded body, through a separate ground cord directly attached to the garment, through a wrist strap direct connection with adapter and through a contact with, e.g. conductive chair grounded via conductive floor (person is seated) or conductive footwear via an conductive floor (person is standing) [7]. Electrostatic shielding of charge enclosed by the garment, suppression of electrostatic field outside the garment due to grounded operator body, and, in some cases, shielding of radiated EMI pulse are also important features of a good ESD garment. Some composite fabric structures would give better performance than others. Another question is related to electrical safety in those works where that is relevant. Personnel safety requirements can be contradictory to ESD protection requirements [7]. To protect from electrostatic discharge garments need to be grounded either through a direct contact with the wearer’s skin or by alternative means such as being electrically connected to a wrist strap. Grounding is not needed if garments are manufactured from low charging textile materials. Besides the covering garments also middle and under garments shall be considered and chosen in such a way that they do not increase ESD risks. It is important that ESD protective garment sleeves cover the end of the inner garment sleeves [8]. The aim in material development is to build-up the balance between charge generation and charge dissipation. This means conductive elements, such as chemical additions, fibres, yarns added to the structures. Modern electrostatic discharge protective fabrics are today heterogeneous composite fabrics where

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a grid or stripes of conductive yarns are presented inside an insulating matrix of cotton, polyester, polyamide or mixtures of these materials. The conductive yarns are more and more frequently made by composites, i.e. by a mixture of conductive and insulating fibres (core conductive fibres, sandwich type fibres, etc.). All the latter elements lead to very heterogeneous fabrics for garments. There are several variations in both fabric and yarn structures. 4.1.7 Review of Techniques on Fibrous Materials for Protection Against Electrostatic Discharge Reduction of electrostatic charging can be applied in different parts of the logistic chain of the product, e.g. during processing textile fibres (spinning), during weaving, knitting and finishing processes and during final end use of the product depending on the product type, target and environment. Natural fibre materials such as cotton may be sufficiently conductive to allow the rapid dissipation of static charge, but such properties are dependent on moisture regain from the atmosphere. Adsorption of atmospheric moisture can also be used to improve the conductivity of synthetic fibres such as nylon or polyester, which would otherwise be quite insulating. [5] Finishing Treatments Durable antistatic finishing treatments of fibres or fabrics require that the electrical properties of the surface will be modified in a way that is not seriously affected by the subsequent use of the material, particularly by washing and abrasion. If a finishing agent is applied to the surface it should be strongly attached and if the surface is chemically modified that modification should remain unaffected by subsequent use. Following finishing methods for reduction of electrostatic charging of textile fibres and fabrics are in use, antistatic finishes with surface affinity, cross-linking finishes, surface hydrolysis treatments and craft polymerisation [6]. Emulsions of water-insoluble liquid quaternary ammonium compounds, specially trioctylmethylammonium chloride, long chain alkyl quaternary ammonium compounds, diglycidyl ether with tetraethylenepentamine, polyethylene oxide, potassium butyl phosphate, copolymer of polyethylene terephthalate and polyoxyethylene and ethoxylated alkanesulphonate are typical antistatic finishes with surface affinity. They improve the surface hydrophilicity and wettability alike their effectiveness to decrease electrostatic surface charging and charge dissipation is dependent on moisture regain in the surrounding atmosphere. Moisture regain of the fibre may also increase. The graft polymerisation methods have been applied to fibres and fabrics. These include chemical grafting such as gamma-ray, electron and ultraviolet irradiation techniques in the presence of polymerisable monomers and plasma treatment with and without the presence of polymerisable materials.

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Special Fibres A large numbers of specialised fibres with better electrostatic properties have been developed. There are the fibres that are modified forms of standard fibres and designed to replace the standard fibres completely or almost completely in appropriate end-uses. Besides the fibres that are designed to be incorporated at a low concentration into a blend with standard fibres. The former are usually fibres whose conductive properties have been enhanced by incorporating moisture-absorbing additives during their manufacturing process. Their low charging activity is therefore humidity dependent. The latter are modified forms of standard fibres such as bicomponent fibres or alternatively fibres of chemical structures that are not otherwise used in textiles such as metal and carbon based. Their electrical conductivity is higher than that of fibres with which they are blended and they are independent from humidity [6]. Humidity dependent fibres improve electrostatic properties by increasing water absorption and the conductivity is based on ionic conduction. There are some general approaches of the production types of different fibres [6]: – Polymer copolymerisation with a moisture-absorbing reactant by copolyaddition, copolycondensation or graft polymerisation. – Polymer is co-extruded with a moisture-absorbing polymer to form a bicomponent fibre (sheath-core, side-side, matrix-fibril). – Polymer is co-extruded with a moisture-absorbing non-polymeric compound. Conductive Fibres In the market there are two main structural types of fibres; surface conductive and core conductive. Sometimes it is also seen the term partially surface conductive fibre and it means the fibre structure where conductive component is partially on the surface and partially embedded in the fibre (Fig. 4.7). Metal Containing Fibres Metal fibres is a large group of different types of conductive fibres whose conductivity is based on stainless steel wire, metal alloys, metal oxides and metal salts. Stainless steel fibres are totally metal and they are cut to staple fibre length. The commercial products include among others Bekinox, Brunsmet and Naslon. Conductive fibres containing metals can be constructed as a core

Surface conductive fibres

Core conductive fibres

Fig. 4.7. Cross-sections of carbon-based conductive fibres [5]

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conductive fibre, a sheath-core fibre or surface conductive fibre. The conductive element may be silver, nickel, copper, alumina, cobalt or metallic alloy. The commercial products include among others X-Static, Flectron and Texmet. Fibres containing conductive metal oxides can be constructed as surface conductive, sheath or sheath-core fibres, concentric core or eccentric core fibres. Metal oxide particles are embedded into fibre material. Conductive element can be stannic oxide, zinc oxide, aluminium oxide, tin oxide, titanium dioxide and antimony oxide. The commercial products include among others Belltron 632, Belltron 638 and Megana-E. Fibres containing conductive metal salts are produced by chemically forming the salt within the surface layer of the fibre. Metal salts are often copper sulphide, copper iodide or nickel sulphide. The commercial products include among others R-Stat, Thunderon, Nitril-Static, Bemberg, Conflex C, Conflex V and T-25 [6]. Carbon Containing Fibres Nearly all conductive fibres are based on conductive carbon made by the acetylene process. Various kinds of conductive fibres have been produced using such carbons as additives. The earliest products were made by coating the fibres with a resin containing a high concentration of carbon. Existing products are mainly based on incorporation of the carbon into the fibre material. The whole fibre may be loaded to a high concentration with carbon or carbon can be incorporated into the core of a sheath-core bicomponent fibre. In some fibres carbon is incorporated into one component of a side-side or modified side-side bicomponent fibre. Carbon can also be incorporated into the sheath of a sheath-core bicomponent fibre at extrusion or into the fibrils of a matrix-fibril bicomponent fibre. The commercial products include among others Viscostat, Resistat, Antron II, Antron III, Nomex Delta A, Negastat, No-Shock and Belltron [6]. Conductive Polymers Containing Fibres Coating and impregnation of conventional fibres with conductive polymers and production of fibres from conductive polymers alone or in blends with other polymers are possible. Development work has been based mainly on polyaniline, polypyrrole and polythiophen structures [6].

4.2 Protection Against Electromagnetic Phenomena 4.2.1 Electromagnetic Compatibility Numerous sources of electromagnetic emissions in our environment produce electromagnetic waves that can cause interference in electronic and electrical devices. The natures of these sources are various: lightning, electric motors, digital computers, cellular phones. Electromagnetic compatibility (EMC)

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means the ability of such devices to function in such environment without producing interference and without being susceptible to emitted electromagnetic fields. Then electromagnetic emissions have to be reduced as possible and simultaneously there is a growing need for suitable materials, which can act as shields against electromagnetic waves. Actually shields like copper or other metallic features are used as electromagnetic screens. However, recently, electromagnetic shields based on flexible materials have been developed for many applications, especially for protective clothing. 4.2.2 Basic Electromagnetic Principles Maxwell’s Equations Maxwell’s equations are the basis of the electromagnetic phenomena. These equations are therefore necessary to our understanding of how electrical systems can cause interference and how they can be disturbed by external sources [9, 10]. Faraday’s Law The Faraday’s law states that the line integral of the electric field around a closed contour C, which corresponds to the electromotive force, is equal to the time rate of change of the magnetic flux through the area S bounded by that contour
  · dl = − d  · dS  E B (4.6) dt S C  is the electric field intensity vector (volts per metre, V m−1 ) E  is the magnetic flux density vector (tesla, T) B This law shows that a time changing magnetic field will induce an electric field. It can be expressed in a differential form, which is more useful, by applying Stokes’s theorem:    =∇×E  = ∂B  curl E (4.7) ∂t    , the curl operator applied to the electric field, which is equal with curl E to the result of the vector product between the nabla operator ∇ (or “del” operator) and the electric field. Ampere’s Law A time changing magnetic field can induce an electric field as we have seen with Faraday’s law. Ampere’s law states that a time changing electric field will generate a magnetic field. This law is expressed in integral form by

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 · dl = H

C

+ d J · dS dt S


 · dS,  D

(4.8)

S

 is the magnetic field intensity vector (ampere per metre, A m−1 ), J where H  is the is the current density vector (ampere per square metre, A m−2 ) and D −2 electric flux density vector (coulombs per square metre, C m ). This constitutes the dual of the Faraday’s law. The right-hand side of the expression (4.8) represents the magnetomotrice force, which corresponds to the sum of the total conduction current and the total displacement current that penetrates the surface S.
 J · dS, (4.9) Ic = S
d  · dS.  ID = D (4.10) dt S  acts like a With this law we can notice that a time-changing electric flux D   current J in producing a magnetic field H. The differential form of this law is given by     =∇×H  = J + ∂ D . curl H ∂t

(4.11)

Gauss’ Laws Gauss’ laws consist of two laws: the first one is the Gauss’ law for the electric field and the second is the Gauss’ law for the magnetic field. The integral form of the Gauss’ law for electric field is

 · dS  = ρv dv. D (4.12) S

v

ρv is the volume free charge density (coulombs per cubic metre, C m−3 ). This  out of the closed surface S equals equation states that the flux of the vector D the total quantity of electric charges enclosed by the surface. Applying the divergence theorem to (4.12), we obtain the differential form  = ρv . ∇·D

(4.13)

The Gauss’ law for the magnetic field can be expressed in the integral form by
 · dS  = 0. B (4.14) S

The magnetic flux through any closed surface S is zero. This equation is a statement of the experimental absence of magnetic charges (absence of magnetic monopole: trying to divide a permanent magnet, the new pieces have

4 Protection Against Electrostatic and Electromagnetic Phenomena

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always N–S poles at the opposite ends). This implies that all magnetic field lines form loops, which never begin, or end. Applying the divergence theorem to (4.14) we obtain the differential form of the Gauss law for magnetic field:  = 0. ∇·B

(4.15)

Summary of the Equations

Name Faraday’s law of induction Amp` ere’s law Gauss’ law for electric field Gauss’ law for magnetic field

Integral form



C


· d
l = E

C


· d
l = H

S


· dS
= D

S


· dS
=0 B

  

d − dt

 

S v

Differential form

 S

 


=
=∇×E curl E


· dS
B


+ J
· dS

d dt

 S


· dS
D

ρv dv

 

∂
B ∂t


=∇×H
= J
+ curl H

 ∂D ∂t


= ρv ∇.D
=0 ∇·B

The presented vectors exist in material media. Under certain conditions (depending on the medium), we may assume that these field vectors are linearly related as  = εE,  D  = µH,  B

(4.16) (4.17)

where ε, µ are, respectively, the permittivity and the permeability of the medium. 4.2.3 Uniform Plane Waves In the case of plane wave propagation, which can be considered as the simplest type of propagation, the electric field and the magnetic field are contained in infinite parallel planes normal to the direction of propagation (Fig. 4.8). This can be achieved by using an electromagnetic source at large distance r compared with the wavelength of the electromagnetic wave (far-field region). When this source is cast far away, the plane wave area increases infinitely and  and H  are independent of location in behaves purely uniform that means E each plane. Indeed, if the electromagnetic wave comes from a sinusoidal source with a phase angle frequency ω, the component of the electric field in the x direction can be expressed, using Maxell’s equations by Ex = Ex0 e−γz .

(4.18)

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Fig. 4.8. Typical sinusoidal plane wave

In this expression appears the propagation constant γ, which is related to the wave phase angle frequency ω and the speed of light in vacuum c ω γ=j . c

(4.19)

This relation is only valid for lossless media; the wave suffers no attenuation as it propagates through the medium. We can see the magnitude of field strength is independent of z whereas the phase angle depends of this geometrical variable. By using Maxell’s equation  The relation between Ex we can state similar relations for magnetic field H. and Hy is of a simple form Ex µ0 = = Zw , (4.20) Hy ε0 where µ0 and ε0 are the absolute magnetic permeability and the absolute electric permittivity, respectively, Zw is called wave impedance. This impedance may be connected with the electromagnetic power radiated from the RF source and its numerical value for lossless media is: Zw = 120π = 377Ω.

(4.21)

4.2.4 Electromagnetic Shielding Electromagnetic shields are used to produce attenuations of electromagnetic waves coming from external RF sources. Then shielding material acts like a barrier against electromagnetic fields, working on a very large frequency range which can be 50 Hz (60 Hz) for AC supply sources, 100 kHz–30 MHz for long wave, middle wave and short wave broadcasting stations, 100 MHz–500 MHz

4 Protection Against Electrostatic and Electromagnetic Phenomena

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for FM and TV transmitters, 900 MHz and few GHz for mobile phones and radar sources. A metallic enclosure that completely encloses an electronic device can be a simple way to realise a good electromagnetic shield. Mechanisms of Shielding Assuming static (DC) electric and magnetic fields, a purely electrical conductive material like copper for example is supposed to be a perfect screen against electric field but fully transparent against magnetic field. However, for a time changing incident wave, many physical mechanisms will contribute to fields attenuation. The first is reflection of the wave at the area between free space and the enclosure barrier; indeed, part of incident wave is reflected at the surface (Fig. 4.9) while other part of the wave penetrates throughout the material and is attenuated when crossing up to opposite side (absorption loss). For an incident plane wave, which penetrates a good conductive medium (a good conductor must satisfy the condition: σ >> ωε) perpendicularly to its surface, absorption loss may be characterised by the depth of penetration so called skin depth δ and given by: 1 . (4.22) δ= πµσf That means over a distance of δ amplitude of the wave will be reduced by 30% at a frequency f . Therefore if the thickness of the medium is much greater than the skin depth the wave amplitude that travels through this material is reduced dramatically. Furthermore, we can notice that combination of multiple reflections and transmissions of this wave occurs within the material at both side areas of that material which causes multiple travels of the wave and contributes to even more attenuated merging fields (Fig. 4.9). Then the total field transmitted throughout the shield will be the sum of many transmitted waves after multiple reflections. Moreover, the time changing magnetic field induces currents on the enclosure, these induce magnetic field inside the enclosure with opposite phase incident

reflected

transmitted

σ, µ, ε

transmitted

transmitted

Fig. 4.9. Multiple reflections phenomenon

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angle, so the resulting magnetic field decreases inside when the frequency increases. Shielding Effectiveness Usually efficiency of electromagnetic shields is given by the shielding effectiveness, which represents the ratio between the electric (or magnetic) field strength without enclosure and the resulted electric (or magnetic) field strength inside the enclosure. According to that definition, we may express the shielding effectiveness for electric field with dB (decibels) unit by:



Ei

(4.23) SE = 20 log10



. Et In term of magnetic field, the shielding effectiveness becomes



Hi

SH = 20 log10



. Ht

(4.24)

Typical behaviours of SE and SH vs. the frequency are given in Fig. 4.10. We notice that below a given frequency ft electric and magnetic field behave as explained before. Above ft , shielding effectiveness for electric field increases to join the shielding effectiveness for magnetic field at higher frequencies. This characteristic frequency ft corresponds to the condition where the skin depth δ becomes smaller than shield thickness t δ≤t

then ft =

1 . πµσt2

(4.25)

This phenomenon appears when the enclosure dimension becomes oversized compared to the wavelength. Attenuation

SE

oversized behaviour SH

ft

frequency

Fig. 4.10. Evolution of electric and magnetic shielding effectiveness vs. frequency

4 Protection Against Electrostatic and Electromagnetic Phenomena

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We can notice that apertures in enclosures reduce dramatically the shielding effectiveness. Indeed, these apertures will change current and charges distributions at the enclosure surface area producing leakage of magnetic and electric field inside. 4.2.5 Flexible Electromagnetic Shields Shields are traditionally made of metallic materials. However new lightweight and low-cost materials increasingly tends to replace high conductive metals. Recently there is an increase of investigations on potential harmful biological effects of body exposition with electromagnetic waves. Therefore the need of protections for human beings has grown. As for antistatic applications, materials have to exhibit higher electrical conductivity than traditional flexible materials like textile for example. The expression of skin depth (4.22) prove that increase of conductivity reduce skin depth and improve shielding properties. However, high shielding effectiveness not necessarily means high surface conductivity, indeed flexible shields do not have to be so conductive as metals like copper, but we need higher conductivities than for antistatic applications. Textile-Based Flexible Electromagnetic Shields Concerning textile materials, specific fabrics used for EMC shielding can be obtained with similar processes, which are used for antistatic materials, but with higher electrical conductivities. Indeed, those fabrics are mainly metal coated and metal interwoven fabrics. The whole fabric can be made of metal fibres or filaments, or it can be made of conventional non conductive yarns with a metal grid [11, 12]. Composite materials are also increasingly used for shielding. Mainly polymer-matrix composites with conductive fillers are used. The nature of fillers is often metallic, but it can also be carbon fillers [13]. The applications for such electromagnetic shields are protective clothing for workers exposed to high frequency electromagnetic fields, protection of rooms for military buildings, protection of electronic devices, medical applications. An example of medical application is given within the reference [14,15] concerning protective suits which aim is to reduce electromagnetic exposition of people with pacemakers. Inherently Conductive Polymers The recent discovery of inherently conductive polymers or intrinsically conducting polymers (IPCs) has allowed development of a large number of application areas, in particular antistatic applications and electromagnetic shielding [16, 17].

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N n

Polyaniline

H

n

Polypyrrole

S

n

Polythiophene

Fig. 4.11. Examples of some polymers

Indeed, conventional polymers are known as electrical insulators. However, the structure of this new class of polymer (Fig. 4.11), which have extended delocalised bonds (π-systems) that creates a band structure similar to silicon, allows high electrical conductivities after doping. The electronic conductivity of these materials is similar to those of semiconductors and metals. For example, polyacethylene doped with AsFe5 has conductivity greater than 105 S cm−1 whereas the conductivity of copper is of the order of 106 S cm−1 . Flexible shields with conductive polymers can be obtained by coating or as composite materials. Low Frequency Field Shielding High conductive materials and high value of dielectric constant provide good shielding effectiveness for electromagnetic fields at high frequency. However, for lower frequencies, attenuation of magnetic field H is very difficult. At such frequencies, used materials must have a high value of magnetic permeability. This is traditionally obtained using ferromagnetic materials. Recently, conductive polymers appear to be also good candidates for low frequency shielding thanks to their intrinsic properties, especially doped polyaniline.

References 1. Safety of Machinery, Guidance and Recommendations for the Avoidance of Hazards Due to Static Electricity (Cenelec, Brussels, 1999). p. 69 (R044-001:1999) 2. N. Jonassen, Fundamentals of Electrostatics. Staha Seminar Proceedings, Tampere, 28 October 1999 (VTT, Tampere, 1999). p. 50 3. M. Glor, Electrostatic Ignition Hazards Associated with Flammable Substances in the Form of Gases, Vapors Mists and Dusts. Staha Seminar Proceedings, Tampere, 12 February 1999 (VTT, Tampere, 1999), p. 14 4. J. Paasi, S. Nurmi, R. Vuorinen, S. Strengell, P. Maijala, J. Electrostat. 51–52, 429–434 (2001) 5. P. Holdstock, Static Control Materials: Test Methods & Requirements. Staha Seminar Proceedings, Tampere, 28 October 1999 (VTT, Tampere, 1999), p. 18

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6. I. Holme, J.E. McIntyre, Z.J. Shen, Electrostatic Charging of Textiles, Textile Progress, vol. 28, number 1 (The Textile Institute, Manchester, 1998), p. 92 (ISBN 1 8708 12875) 7. J. Paasi, S. Nurmi, T. Kalliohaka, G. Coletti, F. Guastavino, L. Fast, A. Nilsson, P. Lemaire, J. Laperre, C. Vogel, J. Haase, T. Peltoniemi, G. Reina, A. B¨ orjesson, J. Smallwood, Electrostatic Testing of ESD-Protective Clothing for Electronics Industry, Electrostatics 2003, Edinburgh, 23–27 March 2003, Institute of Physics Conference Series No. 178 (Institute of Physics, Bristol 2004). p. 239–246 8. EN 61340-5-2, Electrostatics Part 5-2: Protection of Electronic Devices from Electrostatic Phenomena – User Guide (Cenelec European Committee for Electrotechnical Standardization, Brussels, 2001), p. 109 9. C.R. Paul, Introduction to Electromagnetic Compatibility, Wiley Series in Microwave and Optical Engineering (Wiley, New York, 1992) 10. P. Degauque, J. Hamelin, Electromagnetic Compatibility (Oxford University Press, Oxford, 1993) 11. M. Koch, Applications of Electrically Conductive Textiles, 14th International Zurich Symposium and Technical Exhibition on Electromagnetic Compatibility, Zurich, February 2001 12. M. Koch, Protection Properties of Advanced Textile Shields Determined in Frequency and Time Domain, 15th International Zurich Symposium and Technical Exhibition on Electromagnetic Compatibility, Zurich, February 2003 13. D.D.L. Chung, Carbon 39, 279–285 (2001) 14. R. Haug, J. Mouton, B. Demoulin, L. Kone, M. Sauvage, Stimucoeur 29(3) (2001) 15. T. Yajima, K. Yamada, S. Tanaka, J. Artif. Organs 5, 175–178 (2002) 16. Y. Wang, X. Jing, Polym. Adv. Technol. 16, 344–351 (2005) 17. S.K. Dhawan, N. Singh, D. Rodrigues, Sci. Technol. Adv. Mater. 4, 105–113 (2003)

Part II

New Technologies for Barrier Effects

5 Fire-Retardant Mechanisms in Polymer Nano-Composite Materials A. Castrovinci and G. Camino

Summary. An overview of the characteristics of nano-particles and of the nanocomposites structure is reported. During combustion, the nano-fillers, accumulating on the surface of the burning polymer, build up an inorganic barrier that reduces the heat and mass transfer between the polymer and the flame. Besides the formation of a protective barrier on the surface of the burning polymer, a catalytic charring action is proposed for the mechanism of fire retardancy of nano-fillers. The evolution of the nano-composites morphology on heating is also discussed.

5.1 Introduction A general approach in condensed phase fire retardancy involves the formation on heating of a protective layer on the surface of the polymeric material which could prevent heat and mass transfer between the burning polymer and the flame. Diffusion of flammable volatiles towards the flame is slowed down together with a decrease of the rate of heat transfer from the flame to the polymer, leading to decrease of combustion rate below self-sustaining conditions. Nano-fillers in polymer nano-composites are ideal candidates to provide such surface protection to polymer matrices. In the last decade, methods have been developed to prepare and characterise nano-fillers of different chemical compositions and aspect ratios with nano-size in either three dimensions (3D: particles as silica, polyhedral oligomeric silsesquioxanes – POSS, etc.), two dimensions (2D: tubes as carbon nanotubes – CNTs, needle-like clays, whiskers, etc.) or one dimension (1D: lamellar inorganics as phillosilicates/clays, hydrotalcites, phosphates, etc.) (Fig. 5.1). The dimension(s), which is(are) not in the nano-size range, can reach the micron with an aspect ratio that is about 1.000 in two and mono-nano-size fillers.

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Fig. 5.1. Nano-fillers aspect ratio

5.2 Overview of Commercially Available Nano-Fillers Examples of the commercially available nano-fillers which can be used in flame retardancy are: – – – – – –

Polyhedral oligomeric silsesquioxanes (POSS) Carbon nanotubes (CNTs) Needle-like clays Clay-type layered silicates Nano-boehmites Layered double hydroxides (LDHs)

This list does not include all the nano-fillers available on the market; however, it gives an overview of different nano-fillers, grouped on the basis of their aspect ratio. 5.2.1 Three Dimension Particles Polyhedral Oligomeric Silsesquioxanes POSS are a class of organo-silicic compounds, of nanometre size (0.7–50 nm), with cage frameworks with different degrees of symmetry, whose general formula is (RSiO1,5 )n where n is an even number and R is an organic group [1,2]. Each silicon atom is bound to one-and-a-half oxygen (sesqui-) and to a hydrocarbon (-ane), making these compounds half way between silica (SiO2 ) and silicones (R2 SiO)n . Some examples of commercial POSS are reported in Fig. 5.2.

5 Fire-Retardant Mechanisms in Polymer Nano-Composite Materials R O

Si O

R Si

R

Si

O

O

R

R

O

Si

O

O Si R

Si

O O

O

O

O O

OSi

R R

R

R

O O Si

O

Si

R

O O

R

Si

Si R

R

O O Si O O

Si R

O Si O O

R

O O Si

Si O

R Si

O

R

O Si

O Si

R

O

O

Si R

Si

O

O

R

O Si R O

O O

R Si

Si

O

Si

Si R

Si

O

Si

O O

R

R

R Si

89

O

R

Si R

Fig. 5.2. POSS molecular structures

Physically, POSS molecules can be considered the smallest surface modified silica particles available, which on dispersion in a polymer matrix, make it possible to relate material properties to silica molecular modification. In comparison with silica and clays, where the modification is generally made post-synthesis (e.g. via ion-exchange procedure in clays), the synthetic procedures for POSS can lead to chemically well defined and crystalline hybrid nano-fillers, by one-pot preparations. POSS may have organic pendant groups adapted to reach the desired affinity with the polymer matrix or capable of co-polymerising during polymer synthesis or to be grafted to the polymer during processing, to become part of the polymer structure. The tuning of the chemical and affinity properties of POSS can be made starting from a mixture of differently functionalised precursors. Furthermore, POSS can be molecularly modified incorporating several transition metals including Ti, Zr, V, Mo, Cr, Hf, Al, Sn, Sb, Y and Nd in the polyhedral cage [3, 4]. These metal–POSS bear a potential catalytic centre, with geometrically well-defined characteristics [5]. In the field of hybrid and nano-composite materials, POSS were successfully used to improve polymer properties, increasing use temperature, oxidation resistance and improving mechanical properties, as well as reducing polymer flammability and viscosity during processing [6,7]. Neat POSS were used as low dielectric constant materials, new resists for electron beam lithography materials, high temperature lubricants and catalysis [8]. 5.2.2 Two Dimension Particles Carbon Nanotubes Since their discovery [9–11], carbon nanotubes have attracted a great interest, being a new crystalline carbon structure, and a large number of studies were performed to determine their chemical and physical properties, also using computed aided molecular dynamic simulations. A carbon nanotube can be described as a rolled up graphite sheet. Figure 5.3 shows an image, obtained

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Fig. 5.3. Carbon nanotubes in a polymer matrix [16]

with transmission electron microscopy (TEM), of a network of entangled nanotubes surrounded by a polymer. Carbon nanotubes have high Young’s modulus and axial tensile strength [12], but they are also very flexible [13] so that they can be knotted tightly without breakage and exhibit a plastic behaviour, in contrast with conventional carbon fibres. It has been verified that carbon nanotubes improve thermal and flammability properties of a number of polymers [14, 15]. A large variety of CNTs with a unique variability of properties (transport, mechanical, optical, chemical, field emission and adsorption) can be produced. Different production routes are used: high temperature evaporation of a solid carbon feedstock through resistive heating in an electric arc (electric arcdischarge process); high temperature laser evaporation; medium temperature catalytic decomposition of light hydrocarbon (methane up to benzene) by pyrolysis or chemical vapour deposition (CVD) process. All these processes (and others such as plasma CVD, high-pressure CO conversion HiPco) produce single-wall CNTs (SWNT) or multi-wall CNTs (MWNT) which have different structures and properties; every process has its own advantages/disadvantages concerning quantities, up-scale possibilities and production costs. CNTs show outstanding mechanical properties: stiffness, strength and resilience exceeds any current material [17]. CNTs also posses superior thermal and electrical properties: thermal stability up to 2,800◦ C in vacuum (this

5 Fire-Retardant Mechanisms in Polymer Nano-Composite Materials

91

y

x

Si4+ = tetrahedra

Mg2+ = octahedra

Structural Holes

Water

Fig. 5.4. Sepiolite structure

property is particular useful in the case of safety cables), thermal conductivity about twice as high as diamond, electric current-carrying capacity 1,000 times higher than copper wires [18]. These unique properties offer great opportunities for the development of new advanced nano-composite materials for applications in various technological fields [19, 20]. Needle-Like Clays Figure 5.4 shows the structure of Sepiolite, a needle-like clay. Generally this kind of clay shows an hole diameter of 0.3–0.6 nm, a cross-section of 20–50 nm, a length of 1.5–2 µm. Two examples of needle-like clays are Sepiolite (Mg silicate) and Attapulgite (Al/Mg silicate). 5.2.3 One Dimension Particles Layered Clay-Type Silicates The well-known structure of phyllosilicates is reported in Fig. 5.5. Their crystalline structure consists of a two-dimensional layer obtained by blending two tetrahedral silica laminas with metal atoms (i.e. Mg for talc and Al for mica) to form a corresponding octahedral metal oxide lamina (Fig. 5.5). Each layer is separated from its neighbours by a van der Waals gap called a gallery or inter-stratum. These galleries are usually occupied by cations that counterbalance the negative charge generated by the isomorphous substitution of the atoms forming the crystal (Mg2+ in the place of Al3+ in montmorillonite or Li+ instead of Mg2+ in hectorite). These alkaline and alkaline-earth metal cations are normally hydrated [21].

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Layered Double Hydroxides “Layered double hydroxides (LDHs), also known as anionic clays or hydrotalcite-like or hydrotalcite-type compounds, are a family of layered materials which have attracted much attention in recent years. [. . . ] They are formed by layers of M(OH)6 octahedral sharing edges [. . . ]; when in brucite, (Mg(OH)2 ), a partial Mg2+ /Al3+ substitution takes place, the positive charge of the layers is balanced by anions located, together with water molecules, in the inter-layer region. The mineral hydrotalcite (Fig. 5.6) has the formula [Mg6 Al2 (OH)16 ]CO3 · 4H2 O[. . .]. The nature of the layer cations can be changed in a wide range: main group cations (e.g. Mg2+ , Ca2+ , Al3+ , Ga3+ or In3+ ) or transition metal cations, such as V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, generally in the divalent or trivalent state [. . . ]”, can combine to form LDHs layers with excess of positive charge. “Also the inter-layer anions can be very different: halides, oxoanions, oxometalates, polyoxometalates, coordination compounds and organic anions are used to compensate the layers positive charge. The nature of the layer Si4+ = tetrahedra

Mn+ + nH2O

Al3+, Mg2+ = octahedra

Fig. 5.5. Structure of phyllosilicate

Fig. 5.6. Schematic representation of the structure of Mg–Al hydrotalcites

5 Fire-Retardant Mechanisms in Polymer Nano-Composite Materials

93

cations and of the inter-layer anions somewhat determines the specific use of these materials for specific applications or devices” [22]. Calcinated hydrotalcites can be used for catalysis as support for an active centre [15, 23, 24] or as catalytic centre themselves [25–27]. Modified hydrotalcites can also be used as “tuneable” lasers, fluorescent and non-linear optics devices [28]. Also applications in pharmaceutical formulations, to modify bioavailability of drugs, were found [27]. 5.2.4 Synthetic 3D or 2D Nano-Fillers Nano-Boehmite Synthetic boehmites (AlOOH) can be produced as 3D or 2D nano-fillers. Differently from natural alumina fillers, synthetic boehmites are free from transition metal cation impurities. This is important to prevent undesirable problems with polymer stabilisation. Boehmites are readily modified with organic carboxylates, sulphonates and phosphonates. This organophilic modification is the key to achieve very effective de-agglomeration of the primary nano-particles during polymer processing. In addition, size, shape and surface properties are readily tailored.

5.3 Structure of Nano-Composites The morphology characteristics of nano-composites explain their properties which are surprising since they cannot be extrapolated from macro- and microcomposites as a function of the inorganic particles size. Thus a number of mechanical and physical properties are improved by relatively small weight amount of nano-fillers (typically < 10%). In the case of 3D nano-fillers (e.g. POSS molecules), the molecules can be dispersed in clusters which can reach few microns of diameter (microcomposite), or dispersed as single molecules in the polymer matrix, i.e. a nanocomposite material. The dispersion depends on factors such as the nature of the organic group R [29], and also the preparation process. In the case of 2D nano-fillers such as CNTs-nano-composites, a good dispersion of the nano-fillers corresponds to a uniform network-like distribution of nanotubes (Fig. 5.3), avoiding the presence of aggregates [30]. In the case of 3D and 2D nano-fillers, the dispersion of the nano-filler cannot be easily assessed apart from use of TEM or FESEM which only explore limited samples areas. Indeed, wide angle X-ray diffraction (WAXRD) widely used for example in the case of clays is useless, and other approaches to be combined with TEM based on rheology measurements and solid-state NMR are under development [31]. In early works, terms such as “exfoliated” or “delaminated” and “intercalated” (Fig. 5.7) were attributed to nano-composite structures on the basis of WAXRD and TEM [21, 32].

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Exfoliated

100 nm

Intercalated

100 nm

Fig. 5.7. Exfoliated and intercalated morphology of lamellar nano-composites by transmission electron microscopy

Further extensive work on lamellar nano-composites (1D nano-fillers) indicates that their properties most often cannot be rationalised only on the basis of these two characterisation methods used at room temperature [33, 34]. Indeed, morphologies obtained in the case of lamellar nano-composites are generally a mixture of dispersions ranging from ordered intercalated to disordered exfoliated through a complex mixture of order degrees. In these cases, WAXRD only can assess the intercalation situation while TEM describes nano-size local structures. Moreover, XRD reflection, which is the most XRD technique used, can only examine 0.5-mm thick of the outer part of the nanocomposite. A multi-scale morphology assessment is now being developed in which bulk properties such as rheology, transmission XRD, NMR, etc. contribute to the whole material morphology description. Typical morphologies obtained in the case of lamellar nano-composites are shown in Fig. 5.7. It is clearly evident that the interface contact between inorganic and organic phase is increased to the extent that the inorganic phase is now present almost exclusively as an “interphase”, i.e. the material resulting from molecular interactions between organic and inorganic species which, in traditional composites, is limited to the surface of the inorganics whereas in nano-composites it involves the whole inorganic material (nano-composite = interphase materials). Because of this “interphase” characteristics of nano-composites, a number of mechanical and physical properties are improved by relatively small weight amount of nano-fillers (typically < 10%). In the case of layered nano-composites (≤5% wt), generally they exhibit: – – – – –

Increasing of the elastic modulus Modified thermal degradation Decreasing of the thermal expansion coefficient Lower combustion heat release rate Reducing gas permeability

5 Fire-Retardant Mechanisms in Polymer Nano-Composite Materials

– – – – –

95

Increasing solvent resistance Enhancing ionic conductivity Optical transparency Condensed phase catalysis Easy recyclability

5.4 Combustion Behaviour of Polymer Nano-Composites To assess combustion behaviour of polymer nano-composites, cone calorimeter test, performed accordingly to ISO 5660-1, ISO 5660-2 and ISO TR 5660-3 standards, is widely used. A 100 × 100 mm2 sample is exposed to the radiant flux of an electric heater which can provide heat fluxes to the specimen up to 100 kW m−2 . An electric spark plug is used for the piloted ignition [35]. A number of combustion parameters can be evaluated by this test as a function of heat exposure time, among which the time to ignition (TTI), heat PBT PBT + 3wt.% phenyl POSS PBT + 10wt.% phenyl POSS

a) 2000

1608 kW/m2

PE PE + 5wt.% LDH

1600

2014 kW/m

1400

1500

1491 kW/m2 (−26%)

1000

1144 kW/m2 (−43%)

500

HRR [kW/m2]

HRR [kW/m2]

b) 1800 2

1200 1000

825 kW/m2 (−49%)

800 600 400 200

0

0 0

50

100

150

200

250

0

50

100

Time [s]

c)1400

1260 kW/m2

150

200

250

300

Time [s] PE PE + 1wt.% MWCNT

PA6 PA6 + 5wt.% Cloisite 30B

d) 2000

1808 kW/m2

1800

1200 1000

680 kW/m2 (−46%)

800 600

HRR [kW/m2]

HRR [kW/m2]

1600 1400 1200 1000

800 kW/m2 (−55%)

800 600

400

400 200

200 0

0 0

50

100

150

Time [s]

200

250

0

50

100

150

200

250

300

350

Time [s]

Fig. 5.8. HRR curves from cone calorimetric tests. At 35 kW m−2 : (a) PBT (solid line), PBT + 3 wt% POSS (filled diamond ), PBT + 10 wt% POSS (open diamond ) [37]; (b) PE (solid line), PE + 5 wt% of hydrotalcite (open circle). At 50 kW m−2 : (c) PE (solid line), PE + 1 wt% of MWCNT (filled triangle); (d) PA6 (solid line), PA6 + 5wt% Cloisite 30B (open circle)

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Table 5.1. Time to ignition (TTI) and peak of heat release rate (pkHRR) of polymeric nano-composites TTI (s)

pkHRR (kW m−2 )

pkHRR reduction (%)

82 ± 8 71 ± 7 73 ± 7 69 ± 7

1, 197 ± 120 796 ± 80 844 ± 84 849 ± 85

– 34 29 29

Polypropylene (PP) [39] PP + 10 wt% Al-POSS [39] 2D particles Polymethylmethacrylate (PMMA) [30] PMMA + 1 wt% SWNT [30]

56 ± 6 37 ± 4

1, 100 ± 110 624 ± 62

– 43

NA NA

1, 300 ± 130 610 ± 61

– 53

PP [40] PP + 5 wt% Sepiolite [40] PP [14] PP + 1 wt% MWCNT [14] 1D particles Poly-1-butene (PB) [41] PB + 5 wt% clay [41] Textile fabric of polyamide 6 (PA6) [42] Textile fabric of PA6 + clay [42] PP [43] PP + 3 wt% clay [43] PP + 8 wt% clay [43] PP + 12 wt% clay [43] Polyethylene (PE) [43] PE + 3 wt% clay [43] PE + 8 wt% clay [43] PE + 12 wt% clay [43] poly(ethylene-co-vinyl acetate) (EVA) [44] EVA + 5 wt% clay [36] PA6 [44] PA6 + 5 wt% clay [44]

36 ± 4 29 ± 3 NA NA

1, 619 ± 162 834 ± 83 3, 100 ± 310 510 ± 51

– 56 – 61

34 ± 3 34 ± 3 70 ± 7 20 ± 2 52 ± 5 48 ± 5 49 ± 5 52 ± 5 71 ± 7 75 ± 7 72 ± 7 64 ± 6 60 ± 6 60 ± 6 NA NA

2, 011 ± 201 1, 412 ± 141 400 ± 40 250 ± 25 1, 897 ± 190 1, 577 ± 158 1, 309 ± 131 1, 160 ± 116 1, 892 ± 189 1, 614 ± 161 1, 415 ± 141 1, 044 ± 104 2, 510 ± 251 560 ± 56 2, 000 ± 200 700 ± 70

– 30 – 38 – 17 31 40 – 15 25 45 – 78 – 65

Material

3D particles Poly(vinyl ester) (PVE) [38] PVE + 3 wt% of POSS [38] PVE + 5 wt% of POSS [38] PVE + 10 wt% of POSS [38]

NA not available.

release rate (HRR) and the peak of HRR (pkHRR) are the most relevant data because they are associated with the fire behaviour of polymer materials. Polymer nano-composites invariably exhibit a reduction of the pkHRR which can reach 80%, with respect to neat polymers [36]. Examples of comparison of HRR curves of nano-composites and corresponding polymer matrix are reported in Fig. 5.8, and data of pkHRR and TTI for a number of nanocomposites with respect to neat polymers are reported in Table 5.1.

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Although nano-composites show lower pkHRR compared to neat polymers, the total heat release (i.e. the area under the HRR curve) is generally the same for polymer and nano-composite. However, the release of heat by the nanocomposites over a longer period shows that nano-composites burn at a lower rate than the corresponding neat polymer [21]. Whereas, a variable trend for TTI is observed for nano-composites as compared to neat polymers which may increase or decrease depending on type of polymer and nano-filler (Fig. 5.8). A number of proposals have been made to interpret reduction of TTI in nano-composites when it occurs, such as thermal instability of nano-filler organic modifier (e.g. quaternary alkyl ammonium in organo-clays) and/or of polymer-nano-particles compatibiliser (e.g. PP-g-MA in PP-based nano-composites), enhanced radiant heat absorption (e.g. black colour of CNT). None of the interpretations supplied so far in the literature result, however, in a general rule because the same type of nano-filler may reduce or increase TTI depending on type of polymer matrix.

5.5 Mechanism of Nano-Composites Combustion The combustion behaviour of polymer nano-composites is due to a twofold mechanism brought about by the nano-fillers: a physical barrier effect and a chemical charring catalytic action that are discussed below. 5.5.1 Barrier Effect During combustion, the polymer ablation rapidly leads to the accumulation of a network of floccules mainly made of nano-particles [45], whatever the chemical composition and aspect ratio are [46], combined with a relatively small fraction of carbonaceous char [29, 36, 47–57]. This thermally stable ceramic surface layer is able to act as a thermal shield by surface re-irradiation and as a barrier to heat and oxygen transfer from flame to the material and of degradation products from the material to the flame. Thus, the overall rate of flame feeding by combustible products from polymer pyrolysis and thermo-oxidation is decreased. Hence, rate of combustion and of heat release are decreased accordingly. The barrier action towards gases diffusion of crystallising or sintering nano-particles forming the surface layer on degrading polymer material in combustion is easily proved by comparing thermal degradation in air of an oxygen-sensitive polymer such as PP or EVA and corresponding nanocomposites. Indeed, oxygen of air behaves like a bi-radical and can perform chemical initiation of thermal degradation of polymers creating carbon radicals through hydrogen abstraction as shown in Scheme 5.1 for polyolefins, pathway 2. In oxygen-sensitive polymers, the temperature required for detectable rate of oxygen chemical initiation (path 2 in Scheme 5.1) is much lower than that required by thermal initiation, e.g. C–C bond scission in

98

A. Castrovinci and G. Camino H H

H H C C. .C C

1

H H

H H

C C

C C

H H

H H

N2

2

O2

H H

VOLATILES

H H

a H H

H a H

C C . H

C

C + . OOH

Hb

H H

b CH2 CH

CH CH

+ H2O2

O2 . OOH +

CH2 CH

CH CH CH

CH2 CH

.

CH CH

+ .OOH

polyene aromatics

polyaromatics +

CH

CH CH CH

CH

CH

Scheme 5.1. Volatilisation vs. charring in polyolefins thermal degradation

polyolefins, occurring, for example, on heating in inert atmosphere (path 1 in Scheme 5.1). In-chain carbon radicals created by oxygen initiation can evolve through two competing routes. One involves monomolecular carbon–carbon chain bond scission (route a in Scheme 5.1) leading to chain end carboncentred radicals which propagate polymer degradation to volatile products as for thermally initiated radicals (e.g. by path 1 in Scheme 5.1). The second alternative route involves further bimolecular hydrogen abstraction (route b in Scheme 5.1) leading to in-chain carbon–carbon double bonds. This is the well-known oxidative dehydrogenation process widely used, for example, for olefins industrial production. When the temperature of polymers heated in air is progressively increased such as in thermo-gravimetry or in polymer burning, monomolecular chain scission overwhelms dehydrogenation. This is due to the fact that the rate of hydrogen abstraction is relatively low due to the low rate of diffusion of oxygen into the polymer and hence low oxygen concentration available for the bimolecular dehydrogenation reaction. Thus, in these conditions, oxygen initiated thermal volatilisation occurs through the same radical chain propagation process with formation of similar mixture of saturated and unsaturated hydrocarbons, as in thermally initiated degradation although at a much lower temperature. This can be seen by comparing, for example, thermal degradation of a mixture of PP and maleic anhydride grafted polypropylene (MA g-PP) as compatibiliser (PP/MA g-PP) in nitrogen and in air (Fig. 5.9) on heating at 10◦ C min−1 . This relatively low heating rate is useful to time resolve thermal phenomena occurring in polymer combustion since it has been shown that the behaviour in polymer combustion can be reasonably simulated by thermogravimetry [58–61]. In air, thermal volatilisation of PP/MA g-PP begins by

5 Fire-Retardant Mechanisms in Polymer Nano-Composite Materials

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PP/MA g-PP TGA

100

Weight [%]

80

N2 Air

60 40

Weight Loss Derivative [%/⬚C]

20 0 2,4

DTG

445⬚C 300⬚C

2,0 1,6 1,2 0,8 0,4 0,0 200

400

600

Temperature [⬚C]

Fig. 5.9. Thermo-gravimetry (TG) and derivative curve (DTG) (heating ramp 10◦ C min−1 ) of a PP/MA g-PP in nitrogen (solid line) and in air (filled square)

chemical initiation at 220◦ C and is concluded at 315◦ C at which temperature thermal initiation does not yet produce volatile species as shown by the weight loss curve in nitrogen atmosphere. Figure 5.10 shows that the volatilisation of a PP/MA g-PP-clay nanocomposite on heating at 10◦ C min−1 in air occurs with a maximum rate (DTG max) at a temperature by 114◦ C larger than that of corresponding neat PP/MA g-PP. This is a clear demonstration of the barrier effect of nano-clay cumulating on the surface of the degrading material which hinders diffusion of oxygen towards PP. This slows down oxygen chemical initiation which is the only source of radicals propagating the volatilisation at these temperatures. The pronounced asymmetry of the derivative curve (DTG) indicates that, with increasing of the temperature, the rate of volatilisation increases in spite of the temperature antagonistic effect on oxygen diffusion, because the rate constant for chemical initiation increases. As far as polymer combustion is concerned, barrier to inbound oxygen diffusion to the polymer may be relevant in absence of flame (e.g. pre-ignition stage or direct solid oxidation in glowing), whereas in flaming combustion the

100

A. Castrovinci and G. Camino PP/MA g-PP nanocomposite PP/MA g-PP 100

Weight [%]

80

Air 10⬚C/min

60 40 20

Weight loss derivative [%/⬚C]

2,0 300⬚C

416⬚C

1,5 1,0 0,5 0,0 200

400

600

Temperature [⬚C]

Fig. 5.10. Thermo-gravimetry (TG) and derivative curve (DTG) in air (heating ramp 10◦ C min−1 ) of PP/MA g-PP (filled square) and PP/MA g-PP + 10% of organically modified fluorohectorite nano-composite (open circle)

concentration of oxygen reaching the surface of the degrading polymer is likely to be negligible in terms of contribution to combustible volatiles formation by chemical initiation [60–62]. On the other hand, the barrier role to gas diffusion of coalescing nano-particles in nano-composites combustion, demonstrated for oxygen, is likely to affect combustion by slowing down the outbound diffusion of volatile polymer degradation products issued by polymer thermal degradation either thermally or chemically initiated. The rate of surface ceramisation (for example, in the case of clays reassembly to pristine crystal structure of natural clay) and therefore effectiveness in barrier properties build up is likely to depend on the nano-composite morphology. However, it has been pointed out [33] that the morphology of the nanocomposite at room temperature may not be the same as that on the surface of the nano-composite exposed to heat of the flame. In fact, in such a complex scenario, many factors – such as low viscosity of the degrading matrix, physical turbulence of the molten polymer due to bubbling of gaseous thermal decomposition products evolving towards the flame, mobility of nano-particles, etc.

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– can strongly modify the pristine morphology of the nano-composite skin that is involved in the combustion process. For example, it has been shown that, in the case of poly(ethylene-co-vinyl acetate) (EVA) nano-composites, a temperature-induced structural rearrangement of the intercalated organomodified clay occurs in the range between 75 and 350◦ C [33]. The surface overall protection effect is proposed as the main mechanism of fire retardancy for a large number of nano-fillers, including 3D nano-particles, such as POSS [37, 48]; 2D nano-fillers, such as CNTs [46]; and 1D lamellar nano-fillers, such as clays [29,36,46,47,49–56]. In some cases, the assessed gas barrier properties of the ceramised protective surface layer produced during nano-composites combustion was shown to be ineffective in reducing HRR, depending on type of nano-composite. It might, therefore, be that thermal re-irradiation of radiant heat from polymer to air (heat transfer barrier), which measurements are not yet available in literature, could also be a relevant contribution to combustion behaviour of nano-composites. 5.5.2 Charring Partial charring of burning polymer induced by the presence of nano-fillers is a typical result of nano-composites combustion particularly evident in the case of polymer matrices which do not produce charred residue when burned by themselves. Figure 5.11a shows that PA6 burns without charring, whereas 5 wt% clay catalyses extensive charring in the resulting nano-composites (Fig. 5.11b). Extensive charring of the polymer matrix is the ultimate target in fire retardancy since it would allow the use of the material at service temperature whereas, in the presence of an accidental source of heat, the material would (a)

(b)

Fig. 5.11. Combustion char residue of PA6 (a) and PA6 + 5 wt% of clay (nanocomposite) (b)

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100

Weight [%]

80 60

PP/MA g-PP PP/MA g-PP nanocomposite

40 20 0 0

100

200

300

400

500

600

700

800

900

Time [min]

Fig. 5.12. TG curves in isothermal conditions (230◦ C, air) of a PP/MA g-PP (open circle) and of a PP/MA g-PP + 10% of organically modified fluorohectorite (nano-composite) (filled square)

rapidly char limiting production of combustible volatiles. This is a most difficult task in linear polymers, such as polyolefins which thermally decompose quantitatively to volatile combustible products. A possible route to charring could be provided by chemical initiation of thermal degradation by hydrogen abstraction as, for example, in thermal oxidative degradation shown in Scheme 5.1, route 2 successively leading to dehydrogenation, aromatisation, charring (Scheme 5.1, pathway 2b). The effective role of oxygen in polyolefins charring is, for example, shown when PP is isothermally heated in air at a relatively low temperature, however, high enough for oxygen initiated thermal degradation (e.g. 230◦ C) [63] (Fig. 5.12), in which conditions probability of occurrence of route 2b (Scheme 5.1) leading to charring is enhanced in the competition with volatilisation (pathway 2a in Scheme 5.1) as compared to dynamic heating (e.g. Fig. 5.10). Indeed, it can be seen that when isothermal PP decomposition is concluded, about 10% charred material is produced [63] whereas 2–3% char is produced in dynamic heating which is burned above 350◦ C (Fig. 5.10). Unfortunately, in flaming combustion, concentration of oxygen on polymer surface and heating rate are such that charring by thermal oxidative dehydrogenation is negligible as shown by thermo-gravimetry analysis of epoxy resin [63], which was proved to simulate polymer behaviour in combustion [58–61]. It was suggested that polymer additives or fillers, capable of catalysing oxidative dehydrogenation or to supply oxygen at polymer thermal decomposition temperature, could improve fire retardancy of polymers by inducing charring through enhancement of thermal oxidation [64].

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From this point of view, nano-composites, charring shown to take place in nano-composites combustion, indicate that nano-fillers can catalyse oxidative dehydrogenation of PP (Scheme 5.1, route 2b) at the expenses of competing volatilisation (Scheme 5.1, route 2a) leading to charring, although limited, even in presence of a very small amount of oxygen and in unfavourable heating rate conditions of polymer combustion. This result encourages to develop nano-fillers bearing catalytic centres with tailor-made activity towards charring reactions which are specific of each polymer chemical structure. For example, it was shown that, in the case of epoxy resins, charring occurs through a molecular rearrangement of the degrading cross-linked structure [63].

5.6 Fire-Retardant Nano-Composites The effectiveness of nano-fillers as flame retardants for polymers is most evident in fire tests characterised by horizontal burning fire model, such as the cone calorimeter. Whereas, from an industrial point of view, tests like UL94 [65] or oxygen index (OI), in which the fire model involves vertical combustion, often represent the final goal for commercialisation of flame-retarded polymer materials. Ranking of nano-composites in these standards is often penalised by their combustion behaviour. However, even if polymer nanocomposites generally do not reach the most desirable V-0 ranking in UL94 test, they show a much lower combustion rate and material consumption, with respect to neat polymers. Indeed, the presence of nano-fillers increases polymer melt viscosity, eliminating dripping and the flame slowly reaches the clamps of the sample holder which makes material ranking impossible (n.c.) in UL94 test. Burning is, however, confined to the specimen surface, where a thin layer of carbonaceous residue is left behind by combustion. Besides, specimen bulk is not interested by combustion. Thus, nano-composites make a step forward towards reduction of fire hazard for polymers because they avoid flame spreading by flaming dripping and reduce the rate of combustion. For the first time in polymer fire-retardant history, we have polymer materials available in which fire hazard is reduced with simultaneous improvement of other polymer material properties and using an environmentally friendly technology. Whereas most of the fire retardants used now have to be loaded to polymers in relatively large amount (10–70% by wt) with negative effects on polymer physical and mechanical properties and on environmental issues. To comply with end user’s tests intended to ensure specific applicationsrelated combustion behaviours, a proper design of nano-fillers chemistry based on above mechanisms may provide the appropriate solution. At present, the highly recommended risk reduction policy in fire-retardant regulations foresees

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a reduction of fire retardants loading as a first desirable step. This is already possible by adding fire retardants to nano-composites (Tables 5.2 and 5.3). Table 5.2 shows the effect of clay, decabromodiphenyl oxide (DB) and antimony trioxide (AO) combination on PP combustion behaviour [66]. Indeed, with 5 wt% clay, the pkHRR is reduced by 45% and TTI is increased from 40 to 55 s, but LOI is unaffected and in UL94 the specimen slowly burns to the top, leading to not rated ranking (NR). On the other hand, addition of 28 wt% of DB and AO to MA g-PP does not modify HRR of MA g-PP in cone calorimeter, although it increases TTI from 40 to 65 s, whereas it increases OI from 19 to 24. However, UL94 V-2 rating of the polymer matrix, which is due to polymer dripping, is not modified. The same DB–AO addition level to the nano-composite further decreases pkHRR to 200 kW m−2 , keeps LOI at the level reached with DB–AO (23) while V-0 rating is attained with very low combustion times without dripping. It is possible that these results could be kept with substantial reduction of halogenated additive concentration. Effectiveness of nano-filler fire-retardant combination has been demonstrated also in the case of halogen-free flame-retardant systems. For example, Table 5.3 shows the effect of clay combined with phosphorus-based flame retardant in an epoxy resin (ER). The flame retardant was added as a phosphorus-containing monomer (2,2-bis(3-diethyloxyphosphonyl-4-hydroxyphenyl)propane, bisP) Table 5.2. TTI and pkHRR of flame-retarded MA g-PP nano-composites [66] Material

TTI (s)

MAg-PP MAg-PP + 5 wt% clay MAg-PP + 22 wt% DB + 6 wt% AO MAg-PP + 22 wt% DB + 5 wt% AO + 5 wt% clay a

pkHRR

pkHRR reduction (kW m−2 ) (%)

LOI

UL94

(%) Rating t1 a (s)

t1 a (s)

40 ± 4 55 ± 6 65 ± 7

600 ± 60 330 ± 33 620 ± 62

– 45 –

19 19 24

V2 NR V2

5 – 20

3 – 20–3

65 ± 7

200 ± 20

67

23

V0

8

2

t1 and t2 are combustion time after first and second ignition, respectively. Table 5.3. TTI and pkHRR of flame-retarded ER nano-composites [63] Material

ER ER + 10 wt% clay ER + 5 wt% bisP ER + 5 wt% bisP + 10 wt% clay

TTI

pkHRR

(s)

(kW m−2 )

pkHRR reduction (%)

35 ± 4 35 ± 4 40 ± 4 48 ± 5

2, 030 ± 203 1, 250 ± 125 1, 440 ± 144 645 ± 65

– 38 30 68

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co-polymerised with diglycidyl ether of bisphenol-A (DGEBA) [63]. When clay alone is added to the epoxy prepolymer, the pkHRR reduction with respect to ER is 35%. The flame-retardant bisP decreases the pkHRR by 38% with respect to neat resin. Combination of the flame retardant with the clay reduces the pkHRR by 68% with respect to ER. Further examples reported in the literature concern combination of clays with intumescent flame retardants [67] and successful use of combination of clays [68,69] and carbon nanotubes [69] with alumina trihydrate in production of fire-retardant electrical cables.

5.7 Conclusions The combustion mechanism of nano-composites depends on both a physical action (surface ceramisation) and on a chemical action (e.g. catalysed polymer charring). As a result, nano-composites burn at a much lower rate than the pure polymer. Degree of nano-particles dispersion in the polymer matrix is the key factor in combustion behaviour of nano-composites. In this respect, it has to be noticed that the relevant morphology is that at the surface of burning polymer: i.e. observed low temperature morphology may change on heating because of several factors such as mobility of the nano-filler in molten matrix, diffusivity of polymer chain fragments, polarity induced in polymer by surface oxidation, decomposition of organic modifier, turbulence induced by volatiles diffusing towards the material surface. At the present state of the art, flame extinguishment in nano-composites combustion is not yet generally obtained without complementary action of fire retardants. In nano-composites, the amount of fire retardant required to comply with end user’s tests is likely to be lower than that required by the polymer matrix. This result is in line with reduction of environmental impact while current studies are carried out with the target of obtaining flame extinguishment in nano-composites combustion without further addition of fire retardants, for example, by increasing rate and amount of charring of the polymer, induced by nano-particles.

Acknowledgements The authors thank Daniela Tabuani, Federica Bellucci, Giuseppina Tartaglione, Sara Pagliari, Alberto Frache, Alberto Fina, Sergio Bocchini and Walter Gianelli for their helpful collaboration.

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63. G. Camino, G. Tartaglione, A. Frache, C. Manferti, P. Finocchiaro, L. Falqui, in ACS Symposium Series 922, ed. by C.A. Wilkie, G.L. Nelson (American Chemical Society, Washington, 2005) 64. S.V. Levchik, G.F. Levchik, G. Camino, L. Costa, A.I. Lesnikovich, Die Angewandte Makromolekulare Chemie 245, 23–35 (1997) 65. J. Troitzsch, Plastics Flammability Handbook (Hanser, Munich, 2004), pp. 532–536 66. M. Zanetti, G. Camino, D. Canavese, A.B. Morgan, F.J. Lamelas, C.A. Wilkie, Chem. Mater. 14, 189–193 (2002) 67. S. Bourbigot, M. Le Bras, S. Duquesne, M. Rochery, Macromol. Mater. Eng. 289, 499–511 (2004) 68. G. Beyer, J. Fire, Science 23, 75–87 (2005) 69. G. Beyer, Fire Mater. 29, 61–69 (2005)

6 Cold Plasma Technologies for Surface Modification and Thin Film Deposition C. Jama and R. Delobel

Summary. Plasma technologies offer a wide spectrum of possible treatments of materials. The main advantages of these technologies in comparison to the conventional wet chemistry approaches are: they are dry processes. By cold plasmas it is possible to modify the superficial functional characteristics of any materials. This contribution aims to provide a brief overview on non-equilibrium plasma technologies which are surface modification processes which result in surface material layers that retain the inherent advantages of the substrates while providing controlled surface modification. In this paper, we will also present some aspects of barrier films using organosilicon thin coatings on polymeric substrates to protect pharmaceuticals and food products from oxygen. These coatings proved also to be efficient barriers towards diffusion of other small penetrants such as moisture, as also aroma losses. The versatility of these coatings has led to new applications including fire retardant coatings acting as thermal and mass transfer barriers and opens considerable potential for advancing future technologies.

6.1 Classification of Plasmas The plasma state can be considered as a gaseous mixture of oppositely charged particles with a roughly zero net electrical charge. Sir William Crooks suggested the concept of the “fourth state of matter” (1879) for electrically discharged matter and Irving Langmuir first used the term “plasma” to denote the state of gases in discharge tubes. The plasma technology for industrial processes essentially uses two different types of plasma. The first one, named “thermal plasma”, is produced at high pressure (>10 kPa) by means of direct or alternating current (DC–AC) or radio frequency (RF) or microwave (MW) sources. These devices, known as torches, produce plasmas that are characterised by very high temperatures of electrons and heavy particles, both charged and neutral, and they are close to maximal degrees of ionisation (100%). The produced plasma is mainly utilised to destroy toxic–harmful substances or, as in the case of the plasma spray, to produce coatings of thick films. The second type of plasma, named “cold or non-equilibrium plasma”,

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is characterised by the electron temperature higher than the ion temperature. Cold plasmas are composed of low temperature particles (charged and neutral molecular and atomic species) and relatively high temperature electrons, and they are associated with low degrees of ionisation (10−4 – 10%). It is produced under vacuum conditions using low power RF, MW or DC sources. The interactions of the plasma particles on the materials produce the modification of the surfaces to add different functional properties with respect to the bulk material.

6.2 Cold Plasma Technology Cold plasma technologies offer efficient routes for the modification of natural polymeric raw materials. The main advantages of plasma technologies in comparison to the conventional wet chemistry approaches are: they are dry processes, can be developed in a wide pressure range, alter only the very top layers of the plasma exposed surface leaving the bulk properties of the substrates unchanged and they are energy efficient [1, 2]. By cold plasmas, it is possible to modify the superficial functional characteristics of any organic materials, since the plasma gas is substantially at room temperature. Some properties of surface that could be modified with the superficial treatments of the materials are listed in Table 6.1. The treatment processes show peculiar advantages, low environmental impact, competitive costs and particularly the possibility to modify the surface properties of any materials (also inactive). The process consists of the excitation/ionisation of gaseous products (under vacuum and at room temperature) produced by DC, MW or RF energy. These discharges are initiated and sustained through electron collision processes under the action of the specific electric or electromagnetic fields. Accelerated electrons (energetic electrons) induce ionisation, excitation and molecular fragmentation processes leading to a complex mixture of active species,

Table 6.1. Some properties of plasma superficial treatments of the materials Surface properties that could be modified

Field of application

Generation of low friction surfaces Improvement of biocompatibility Increase of corrosion resistance Increase of wear resistance Increase of scratch resistance Increase of depth of dyeing Decrease of wetting Decrease of fluids permeability Aesthetical and functional increase Increase of wetting

Biomedical (tools for endoscopy) Biomedical (orthopaedic prothesis) Mechanical Mechanical (cutting tools) Optic (lens for glasses, contact lens) Textile industry Textile industry Food packaging industry Decorative components industry Paper industry

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which will undergo, depending on the specific plasma mode, recombination processes in the presence or absence of the plasma. The energetic particles of the plasma, through collisions with the material surface (placed in the plasma zone), break the chemical bonds producing free radicals on the surface. These are subjected to additional reactions that depend on the type of plasma gas used. The result is the generation of surfaces that have very different properties with respect to the material bulk. The effect of the plasma on the surface depends on the gas chemistry and on the plasma parameters. If electrons with specific energy distribution functions initiate and control all the processes in glow discharges, it might appear obvious that similar electron energy distribution environments created using different power sources (DC, RF, MW, etc.) should initiate similar chemistries; and consequently, the type of the plasma would be of less importance for the generation of specific processes. However, due to the electrode, antenna and reactor geometries, their chemical nature, their relative positions in the reaction chambers, plasma non-uniformities can be highly variable. Therefore, proper selection and control of plasma parameters are necessary for efficient approaches to specific applications. The type of modification depends on the pre-treatment and composition of the substrate, on the type and the quantity of reactive gas, on the total reactor pressure, on the applied power and on the process time. Cold plasma processes involve both gas phase and surface reaction mechanisms. The gas phase reaction mechanisms involve the interaction of neutral and charged plasma-created species, including atoms, molecules, free radicals, ions of either polarity, excited species, electrons and photons. The competition between the deposition, grafting, functionalisation processes and “destructiveinteraction” of plasma species (etching, degradation) will control the intensities and the predominance of ablation, surface functionalisation and thin film deposition reactions. During the last two decades, the plasma process developments were in the following directions: – Plasma-enhanced chemical vapour deposition (PECVD) of thin films – Cross-linking and surface functionalisation of polymeric materials – Etching of inorganic or polymeric substrate surfaces PECVD process involves the dissociation of starting materials and reorganisation of the resulting neutral and charged molecular fragments on the surfaces located inside or outside of the plasma zone. When the starting materials are common monomers, the recombination processes are more complex due to the development of simultaneous conventional polymerisation reactions along with the fragment recombination mechanisms initiated by the plasmacreated and surface-attached active species (e.g. ions, free radicals). Remote plasma and pulsed plasma processes allow recombination mechanisms in the absence of the plasma state in time or in space, and are characterised by significantly minimised dissociation processes. These approaches do not change dramatically the nature of molecular fragmentation processes; however, they

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limit significantly the intensities of plasma-induced degradation mechanisms, observed in some cases for polymeric substrates. Owing to the high reactivity of plasma species, surface functionalisation reactions of even the most inert polymeric substrates can be conveniently achieved. These mechanisms usually involve non-polymerising gas plasmas, which generate active molecular fragments that covalently attach during the plasma reactions to the activated substrate surfaces.

6.3 Applications of Cold Plasma Technology 6.3.1 Functionalisation of Organic and Inorganic Polymeric Surfaces Cold plasma techniques are very promising approach for the surface functionalisation of polymeric materials. It has been demonstrated that efficient surface modification reactions can be carried out with cold plasmas even on very inert substrate surfaces including, Teflon, polyethylene and polypropylene [1–21]. The mechanisms of surface functionalisation of polymeric substrates are different from the gas-phase processes. While electrons play the most important role in the plasma state, positive ions play a significant role in the surface chemistry. Interactions of plasma species with polymers can induce bond cleavages. Free radical and unsaturated bond development can result in cross-linking of polymeric layers. As an example, the modification of the wetting characteristics of application on a material is reported. The plasma is used to increase or decrease the wetting (measured by the decrease or the increase of the contact angle on the surface of different liquids including water). Therefore, a surface can be transformed from hydrophobic to hydrophilic and vice versa. The treatment time changes from some seconds to minutes in function of the material. Figures 6.1 and 6.2 show the experimental results from the works published by Bonizzoni [19]. An other example concerns the development of painting technology using plasma surface modification technology for automobile parts [20]. The surface of polyolefin bumpers were treated by microwave plasma for enhancing their wettability and adhesion properties between paint layer and plastic substrates. The tests including adhesion, impact, cold resistant impact and water-resistant impact tests were performed according to the ISO 2409 standard conditions. These tests show, for samples painted without using a primer, good adhesion of the polyolefin surface using microwave power below 500 W. Consequently, by using the plasma process, it is possible to avoid the use of a primer. Large volume plasma reactors (several cubic m3 ) for the automotive industry were developed using remote nitrogen plasma reactor [21]. The treatment quality is homogeneous in the whole reactor, and optimum adhesion quality is obtained after a treatment duration of only 30 s. Nitrogen

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Fig. 6.1. Water absorption on a non-absorbing paper

Fig. 6.2. Water repulsion on an absorbing paper

plasma treatment of different kinds of polymer surface allows the grafting of nitrogenated and oxygenated functions. The incorporation of these functions improves the wettability and adhesion qualities of the polymer surface. The ageing of the treated samples in open air for a period of up to 1 year showed no influence on either the adhesion quality or the ratio of the grafted functions. 6.3.2 Plasma-Assisted Thin Films Deposition Thin films deposited by plasma vapour deposition techniques are used in a broad variety of applications. The combination of substrate materials with functional and protective coatings offers a number of key advantages

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over alternative bulk materials, such as light weight, complex shape and design freedom, transparency or tailored optical characteristics, and also costeffectiveness. The numerous application fields include dielectric, anti-reflection coatings multi-layers for optical components, transparent-conducting coatings, high refraction index and high permittivity coatings, and films for flat panel displays, solar cells and other opto- and micro-electronic devices. In the last few years, a great deal of work has been produced in the field of plasma deposition of organo-silicon compounds (SiOx ) [22–26]. The first target of these studies was the production of SiO2 -like films devoted to micro-electronics applications. However, beyond their dielectric properties, plasma-deposited SiO2 -like materials are characterised by other very important properties such as good hardness, resistance to chemicals and abrasion, good biocompatibility and low gases permeability [27, 28]. The main advantages of these films compared to metallic films are their optical transparency, recyclability and suitability for micro-waving [29]. Such properties make thin films of these materials suitable for a great number of applications ranging from the production of anti-scratch coatings for ophthalmic lenses protection to barrier films for food and pharmaceutical packaging. Common deposition techniques for SiOx films are based on physical vapour deposition (PVD) or PECVD. PVD processes comprise evaporation or sputtering of a solid precursor (Si, SiO and SiO2 ). In contrast, PECVD processes use volatile organo-silicon precursors, which become excited and partially decomposed in the plasma [30]. Therefore, PECVD opens up a chemical pathway to precisely control polymerisation and deposition process by means of external plasma parameters. Thus, compared to PVD methods, it can yield strong chemical bonding [31, 32]. Moreover, it enables three-dimensional coating, while PVD techniques are restricted to line-of-sight deposition. Although SiO2 -like thin films can be deposited from silane-containing feeds, the use of organo-silicon monomers is by far preferred because of their cheapness and ease to handle. A great number of safer monomers are utilised both in the field of organosilicon thin film deposition research and industrial production. Reviews on SiOx barrier film deposition on flexible polymeric webs are given in [29,33,34]. With regard to application, deposition on paper-based substrates [35] has been reported. In the future, improved multi-barrier film systems for moisture protection of organic-based display technologies will be an emerging field of application [36], where the barrier effect of coatings will still have to be improved by orders of magnitude. In the next paragraphs, we will focus on barrier films using organo-silicon thin coatings on thermoplastic substrates, which have emerged as an alternative to metallised plastics, to protect pharmaceuticals and food products from oxygen. These systems proved also to be efficient barriers towards ingression of other small penetrants such as moisture, as also aroma losses. The versatility of this deposition technology has led to new applications including fire-retardant coatings acting as thermal and mass transfer barriers and opens considerable potential for further applications.

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Oxygen Barrier Lamendola and d’Agostino [37] use hexamethyldisiloxane/oxygen (O2 / HMDSO) and hexamethyldisilazane/oxygen (O2 /HMDSN) RF plasmas for deposition of organo-silicon and SiO2 -like thin films with high barrier properties. The films have been deposited on 12-µm thick polyethyleneterephtalate A thick siloxane (PET) substrates. O2 and H2 O vapour permeability of 500-˚ films has been evaluated by means of a MOCOM permeameter. H2 O vapour and O2 gas transmission rates (GTRs) of 12 µm, HMDSO–O2 and HMDSN– O2 , plasma-coated PET substrates are reported in Fig. 6.3a,b, respectively. In both cases, GTRs sharply decrease by increasing the O2 to monomer ratio in the feed. When O2 to monomer ratios ranging from 10 to 20 are utilised, excellent O2 and H2 O vapour diffusion barrier layers were obtained (O2 GTR < 0.5 cc (m2 day atm.)−1 on conventional polymers used for food packaging. The lowest permeability values have been obtained for 500-˚ A thick films with SiO2 -like stoichiometry. This findings were explained by the high monomer fragmentation conditions giving materials with highly cross-linked structure. Oxygen permeabilities, PO2 , of a variety of PECVD-coated polymers are reported in Table 6.2. Additional information may be found in the work of Ryder [38] on commercial polymers used for food packaging. The oxygen permeation of the coated polymer is typically two orders of magnitude lower than that of the uncoated polymer. Theses results give evidence that silicon oxide (SiOx ) possesses excellent barrier properties. The coating acts as simple defective blocks to oxygen transport, and that the dominant transport mechanism was permeation through the polymer substrate, followed by flux through available defects in the coating. This is in contrast to the behaviour of water molecules that are believed to interact and react with deposited coatings.

Fig. 6.3. O2 and water vapour gas transmission rates (GTRs) of films deposited from (a) O2 /HMDSO and (b) O2 /HMDSN plasmas

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Polymer Polyethylene Polyamide Polycarbonate Polypropylene Polyethyleneterephthalate

Thickness Permeability, PO2 × 1016 References (nm) (cm3 (STP) cm cm−2 s−1 Pa−1 ) 40 40 100 – – 12 – –

85 0.07 3 2 0.15 0.15 0.12 0.04

[39] [39] [40] [41] [42] [43] [44] [45]

Mass Transfer Barrier in Liquid Medium In several applications, such as foodstuffs, cosmetics and packaging, polymer or rubber materials are in contact with liquid surrounding the medium. In this case, chemical agents can migrate from the material into the liquid. When pharmaceutical rubber cups are in contact with distilled water, the liquid penetrates into the material, and chemical agents such as Zn2+ migrate into the distilled water (or into a pharmaceutical liquid phase) with the following results (1) the surrounding medium is contaminated by the chemical additives and (2) the material shows a deterioration of mechanical properties. Tetramethyldisiloxane (TMDS) or its mixture with oxygen was used to deposit polymeric layers on a rubber discs at ambient temperature using a cold remote nitrogen plasma (CRNP) process. Four films were deposited using CRNP at constant TMDS flow rate (80 sccm) and different dioxygen flow rates (ΦO2 ) – TMDS1: ΦO2 = 0 sccm; TMDS2: ΦO2 = 5 sccm; TMDS3: ΦO2 = 10 sccm and TMDS4: ΦO2 = 15 sccm. The deposited films appear to be efficient against chemical agents diffusion from the discs to a distilled water surrounding phase [46]. Figure 6.4 shows the extraction profiles for coated discs at ambient temperature for a material containing 2.5% weight in ZnO. It shows that the barrier efficiency is enhanced when oxygen is added to the TMDS monomer. The higher is the oxygen flow, the lower is the extracted Zn2+ concentration. It is very well known that oxygen addition to monomers increases deposition rate, the films deposited without any oxygen addition are thinner and the Zn2+ extraction is then easier. The barrier efficiency is increased for films deposited from a TMDS/O2 mixture. The extracted quantity of Zn2+ after 30 days of immersion in distilled water at ambient temperature is 70% lower in comparison to the uncoated ones. Figure 6.5 shows the effect of the temperature of the liquid phase on the Zn2+ diffusion kinetic for discs coated with TMDS4. The migration rate is accelerated by increasing the temperature. However, for coated discs, the barrier efficiency is still preserved for extraction at ambient temperature and at 40◦ C during period tested (30 days).

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Fig. 6.4. Evolution of the extraction profiles for coated discs containing 2.5% weight in ZnO at ambient temperature with dioxygen flow rate (ΦO2 ) – TMDS1: ΦO2 = 0 sccm; TMDS2: ΦO2 = 5 sccm; TMDS3: ΦO2 = 10 sccm and TMDS4: ΦO2 = 15 sccm

Fig. 6.5. Effect of the temperature of the liquid phase on Zn2+ diffusion kinetic for discs coated with TMDS4

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In the next paragraph, the results reported are related to both mass and thermal transfer barrier performance. The films deposited from TMDS– oxygen mixtures using remote nitrogen plasma reactor show an application to improvement of fire retardancy performance of thermoplastic. Mass and Thermal Transfer Barrier Coatings: Application to Fire Retardancy Mass and thermal barrier coatings were also developed to improve the fire retardancy properties of polymers [47–50]. This plasma process allows preserving mechanical and physical properties of the polymer and concentrating fire-retardant properties at its surface, where ignition occurs. An example is presented here, where flame-retardant properties of polyamides can be improved, thanks to a film obtained from CRNP-assisted polymerisation of tetramethyldisiloxane monomer pre-mixed with oxygen. The experimental setup is shown in Fig. 6.6. A nitrogen flow was excited by a micro-wave discharge produced in a quartz tube. By a continuous pumping, excited species were led to the reactor chamber where the CRNP appeared like a yellow afterglow. The CRNP is free of charged particles (substrate damages are avoided) and temperature is approximately ambient. The monomer (TMDS), pre-mixed with oxygen, was injected in the CRNP, through a coaxial injector. The specific gravity of the coatings is approximately equal to 1.9 g cm−3 and the deposition rate is equal to 40 ˚ A s−1 . Figure 6.7 shows FTIR

Fig. 6.6. Experimental setup

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Fig. 6.7. FTIR spectrum of a film deposited on silicon (1-µm thick)

spectrum of a 1-µm thick deposited film. The main groups are Si(CH3 )x and Si–O–Si. Asymmetric and symmetric ν(CH3 ) bands are located at 2,960 and 2,910 cm−1 , respectively [51], the δ(CH3 ) ones appear, respectively, at 1,410 and 1,250 cm−1 [52, 53]. ρ(CH3 ) and ν(Si–C) bands are in the range 900–700 cm−1 [53, 54]. The asymmetric ν(Si–O–Si) band, located in 1,200–1,000 cm−1 range [52, 55], is the strongest one. This chemical group is provided by the monomer; the deposited film has a polysiloxane-like structure. Flame-retardant performances of the films deposited on polyamide 6 (PA6) and on polyamide 6 clay nano-composite (PA-6 nano) were evaluated using limiting oxygen index and cone calorimeter measurements. Limiting oxygen index (LOI) tests were performed using a Stranton Redcroft Instrument according to the ASTM D 2863/77 norm [56]. This test allows determination of the minimal oxygen rate, in an oxygen–nitrogen mixture, assuring the combustion of a sample vertically settled (standard size: 100 × 10 × 3 mm3 ). LOI values vs. the film thickness were studied. Results are shown in Table 6.3. Whatever the film thickness, PA-6, coated PA-6 and PA-6 nano have the same LOI value (21 ± 1%). The LOI value of the coated PA-6 nano is strongly improved: it sharply increases as soon as a film thickness equal to 0.6 µm. A maximum value, equal to 48%, is obtained for 1.5 µm. Cone calorimeter measurements were obtained with a Stranton Redcroft cone calorimeter according to the ASTM E 1354-90a norm [57]. Samples (standard size: 100 × 100 × 3 mm) were exposed under a 35 kW m−2 external heat flux which represents the heat flux found in the vicinity of solid-fuel ignition source. Conventional data, such as rate of heat release (RHR), ignition time, total heat evolved (THE), volume of smoke production (VSP), CO rate

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Table 6.3. LOI values vs. the thickness of the film deposited on PA-6 and PA-6 nano-samples

LOI (%)

PA-6 PA-6 nano

0

0.6

21 22

22 45

Thickness of the film deposited (µm) 1.1 1.5 2.1 3.2 5.3 9.6 22 47

22 48

22 47

22 43

22 43

22 42

18.1 22 42

Table 6.4. Cone calorimeter measurements for virgin and coated samples Samples

PA-6

Coated PA-6

PA-6 nano

Ignition time (s) 66 ± 3 67 ± 11 98 ± 2 1, 053 ± 30 967 ± 70 699 ± 34 RHR peak (kW m−2 ) THE (kJ) 1, 346 ± 70 829 ± 39 949 ± 45 CO peak (ppm) 253 ± 1 127 ± 15 94 ± 5 Total CO 14, 564 ± 370 7, 961 ± 939 11, 011 ± 1, 398 emission (ppm s−1 ) 4.4 ± 0.4 2.5 ± 0.1 3.0 ± 0.2 VSP peak (103 m3 s−1 ) Total VSP 0.228 ± 0.017 0.134 ± 0.013 0.312 ± 0.034 emission (m3 ) Residual weight 1.0 ± 0.2 1.9 ± 0.2 4.0 ± 0.3 (%)

Coated PA-6 nano 96 ± 2 623 ± 10 900 ± 23 82 ± 5 10, 944 ± 880 2.7 ± 0.5 0.304 ± 0.013 4.2 ± 0.2

of combustion gases and residual weights, can then be obtained. Table 6.4 shows cone calorimeter results obtained for virgin and coated PA-6 and PA-6 nano-samples. The film thickness was equal to 1.5 µm. Though clay incorporation (2 wt%) to PA-6 does not improve the LOI value, it leads to a decrease of every peaks (RHR: 34%; CO: 63%; VSP: 32%) and of total quantities of energy (29%) and of CO (24%). The PA-6 nano-combustion is delayed for 50 s in comparison to the PA-6 one (Fig. 6.8), is slightly slowed down and leads to a residual mass of 4% (1% for PA-6). These results show that, thanks to clay incorporation, a protective coating is formed at the polymer surface during the combustion, reducing mass and heat transfers between the flame and the polymer. The coating deposited on PA-6 does not allow to reduce significantly neither the LOI value (which remain equal to 22%) nor RHR peak. However, it leads to a decrease of the THE (38%), CO (50%) and VSP (43%) peaks and of total quantities of CO (45%) and of VSP (41%). Coated PA-6 nano leads to very good flame-retardant properties. In comparison to PA-6, the LOI is drastically improved. The ignition time is increased for 30 s. RHR, CO, CO2 and VSP peaks are decreased (41, 68, 51 and 39% respectively) as well as total CO2 and CO quantities (44 and 25%). The combustion is delayed for

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Fig. 6.8. Evolution of the residual weight during the combustion for virgin and coated PA-6 and PA-6 nano

50 s (as for PA-6 nano) and slowed down. The corresponding residual mass is equal to 4%. The comparison between flame-retardant properties of virgin and coated PA-6 and PA-6 nano, evaluated in standard conditions, shows that significant results are obtained with the combined use of a clay addition (2 wt%) and of an organo-silicon coating. In comparison to virgin PA-6, the fire-retardant performances of the coated PA-6 nano are characterised by an increase of its LOI (130%) and a decrease of the RHR peak (41%) and of the THE (33%): the advantage of this process is a resulting simultaneous improvement of these three parameters. During the combustion, the structure of the polymer leads to the formation of a surface protective layer which action is reinforced by the coating. This carbonaceous- and silica-like layer acts as a barrier limiting mass and heat transfers between the flame and the polymer and slows down the toxic gases emission produced by polymer combustion.

6.4 Conclusions Cold plasma-assisted processes have been receiving increasing interest since the 1980s. Cold plasma technologies are surface modification processes which result in surface material layers that retain the inherent advantages of the substrates while providing more exact film chemistry control, and as a result they have potential in many applications. The plasma contains several reactive species such as electrons, atomic or molecular ions, atoms or molecules

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energetically excited. Cold plasma reactive species can then promote surface functionalisation reactions or generate organic or inorganic thin layers as a result of recombination of radicals or molecular fragments species on the surfaces. This article illustrates the continuing interest in achieving controlled surface modification under plasma conditions, and the potential of plasmachemistry for future technologies. Cold plasma modification process and its application area are considered. It is shown that this method is very effective for the enhancement of adhesive or non-adhesive properties of a wide range of polymeric materials used in different fields of technology. Organo-silicon thin films deposited by plasma vapour deposition techniques are presented and their application in the manufacture of efficient barriers towards ingression of even small penetrants such as oxygen and moisture is shown. The versatility of this deposition technology has led to new applications including fire-retardant coatings.

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7 Nano-Fibres for Filter Materials K. Schaefer, H. Thomas, P. Dalton, and M. Moeller

Summary. Textile materials are used for a variety of dry and wet filtration processes allowing either the increase of the purity of the material filtered or the recovery of solid particles. Typical examples for textile-based filtration processes are air filtration, process filtration (e.g. solid–liquid separation), industrial effluent treatment or dehydration of sewage sludges. Current conventional textile filters consist of natural or human-made fibres with diameters ranging from a few single to a few ten microns. Small fibres are well known to provide better filter efficiency which is related to the increase in surfacearea-to-weight ratio. For this reason, nano-fibre filter media enable new levels of filtration performance for several applications in different environments ranging from industrial and consumer to defence filtration processes. Nano-fibres with diameters between 100 nm and 3 µm are readily accessible by the electrospinning process. Electrospinning uses a high electrical field to draw a polymer solution (or melt) from the tip of a capillary to a collector. By applying voltages of approximately 10–50 kV, fine jets of the solution (or melt) can be drawn to a grounded or oppositely charged collector. The evaporating solvent (or cooling of the melt) results in fibres that are collected and formed into nano-fibre mats with adjustable fibre diameters mainly based upon solution viscosity and electrical field strength. A broad range of polymers ranging from natural and synthetic organic to inorganic polymers can be electrospun from the solution or melt allowing the generation of tailored nano-fibre webs for various applications. Furthermore, the nano-fibre webs may be used as carrier material for subsequent fixation of various substances to fibre surfaces as well as for their direct implementation into the fibre. This increases the possibilities for production of, e.g. hygienic functionalised filters or of temperature stable filters with catalytic activity. Hygienic filters produced from cationic polymers or with incorporated silver can reduce the contamination of air or water filters with bacteria while temperature stable filters, which can be obtained from SiO2 -precursor or silica hybrid materials and which are loaded with metal/metal oxide nano-particles, are destined for air pollution control.

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7.1 Introduction Raw materials for non-wovens are generally natural or human-made fibres with diameters ranging from about 3 to 50 µm. New levels of performance can be enabled by nano-scaled fibres in all fields of application demanding a high surface-area-to-weight ratio, e.g. filtration and catalysis. Nano-fibres with diameters between 100 nm and 3 µm can be made by the electrospinning process. The technique of electrospinning has been known from the work of Formhals [1] since 1934 but received relatively little attention until recently. In 1971, Baumgarten [2] performed studies on the electrospinning of acrylic micro-fibres; he obtained fibres with diameters of 500–1,000 nm. With increasing interest in nano-technology and motivated by the reviving work of Reneker’s research group electrospinning has gained exponential research interest in the last few years (Fig. 7.1) [3–6]. Since 1990s, the research groups of Reneker, Vancso, Greiner and Wendorff investigate the electrospinning in detail [3–14]. During the last decade, extensive investigations on the electrospinning process have been conducted from different viewpoints like aspects of theoretical simulation [15, 16], fibre formation mechanism, influencing factors for fibre size and morphology [17] and applications [18, 19]. A wide variety of polymers (natural, synthetic, organic and inorganic polymers) have been electrospun from the solution and melt phase allowing the generation of tailored nano-fibre webs for various areas of application, e.g. filtration [18], reinforcement in composite materials [7], protective clothing [21] or biomedical uses [22–24].

Fig. 7.1. Increase in papers on the electrospinning in the last decade [20]

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7.2 Principle of Electrospinning Electrospinning (or electrostatic spinning) uses a high-voltage electrical field (10–50 kV) to draw a polymer solution or melt from the tip of a capillary to a collector (Fig. 7.2). When the electric forces at the surface of a polymer solution or melt overcome the surface tension, an electrically charged fine jet is formed which can be drawn to a grounded or oppositely charged collector. The evaporating solvent (or cooling of the melt) creates fibres that are collected and formed into nano-fibre mats. Electrospun fibres are continuous in length, their diameter ranges from under 3 nm to over 50 µm depending on the electrospinning conditions. The smallest possible polymer fibre must contain one polymer molecule [5]. The fibre diameter of fibres formed during electrospinning is influenced by: System parameters – Polymer properties Molecular weight, structure and poly-dispersity of the polymer, concentration, melting point and glass transition point – Solution properties Solvent, volatility, viscosity, conductivity, surface tension, presence of further additives (e.g. salts) Process parameters – Ambient parameters Solution temperature, humidity, atmosphere, air velocity in the electrospinning chamber – Equipment parameter Voltage, field strength, electrode distance and arrangement, flow rate, delivery volume, needle diameter

Pressure gauge

Solution

Pump

HV 0–50 kV Taylor cone

Ground Electrode

substrate

Fig. 7.2. Setup for electrospinning from polymer solutions

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The formation of fibres in the electrospinning process is mainly influenced by the following forces: – Surface tension – Electrical-repellent force derived from electrical charged polymer droplets – Visco-elastic force coming from the polymer Higher polymer concentrations typically result in larger fibre diameters, an increase of the electrical field strength leads to a decrease of the fibre diameter. The fibre diameter shall be consistent and controllable; the fibre surface shall be defect-free or defect-controllable. However, in practical electrospinning experiments often inhomogeneous fibres with defects and beads can occur. Splitting of the jet can occur, which results in finer fibres. Advantages of nano-fibres: – Fibre diameter: <3 nm to >50 µm – High surface area to volume ratio (→ high specific surface) – High aspect (length to diameter) ratio – High bending performance – Flexible surface functionalities – Ability to control pore size in non-woven fabrics – Possibility to insert special functionality A further advantage of electrospinning compared to conventional solvent spinning is that water can be used as solvent. Water-soluble fibres have to be cross-linked, e.g. by thermal or by chemical cross-linking [13, 20, 25]. These advantages result in great application potentials of nano-fibres in broad fields such as separation, adsorption, filtration, catalysis, fibrereinforced composites, tissue engineering, wound dressings, drug delivery systems, sensors, cleaning tissues, protective textile and other [18–29]. Nano-fibres can be spun from polymer solutions or from polymer melts. Larrondo and Manley [30–32] were the first to carry out and report on melt electrospinning experiments. Working with PE and PP in the early 1980s, they successfully formed fibres with diameters only as small as the tens of microns range. Electrospinning from polymer melts has the advantage that no solvents are needed which have to be removed by evaporation. However, the melting temperature of the polymers is an important influencing factor for the applicability of the procedure to produce nano-fibres. In general, nano-fibres which are produced by melt electrospinning have a higher fineness than those electrospun from solutions, achieving nano-dimensions by melt electrospinning is non-trivial (Scheme 7.1). At DWI, a working group is using melt electrospinning for the production of nano-fibres or nano-webs for biomedical applications like scaffolds for tissue engineering, in vitro neuron interactions with oriented electrospun fibres or others [33–36].

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Electrospinning from

Polymer solutions Restrictive parameters

Polymer melts Restrictive parameters

• Solubility of polymers

Melting point of polymers

• Suitable solvents with

Viscosity of melts

regard to viscosity,

etc.

volatility, toxicity etc. ⇒ Requirements to polymers and solvents

⇒ Requirements to the equipment to reach the right temperature

Results in finer (nanofibres)

Results in coarser

and more homogeneous

(approx. 1 µm) and less

nanofibres.

homogeneous nanofibres.

Scheme 7.1. Comparison of electrospinning from solutions or from polymer melts

7.2.1 Practical Electrospinning Typical electrospinning equipment consists of three components: a highvoltage source, a spinneret (or nozzle) and a collector (Fig. 7.2). The polymer solution or melt is applied into a syringe (or a spinneret) which is equipped with a piston and a stainless steel capillary serving as electrode and pushed through by a pump with a defined flow rate. The spinneret is connected with the high-voltage source and applies high voltage to the polymer. This results in the formation of a polymer drop at the end of the spinneret. Under higher voltage the drop changes its shape and turns into a conic form (Taylor cone) (Figs. 7.2 and 7.3) [30–32, 37, 38]. At a defined voltage, the surface tension of the polymer cone at the tip of the spinneret starts to elongate and stretch so that a charged jet is formed. The jet moves in loops bending and whipping towards the electrode with opposite polarity or to the grounded target (Fig. 7.3). Recent experiments demonstrate that the rapidly whipping fluid jet is an essential mechanism of electrospinning [39, 40]. Different collection systems are known [35]. For the usually produced nonwoven mats metal plates are used as counter electrode and collection system of the nano-fibres or nano-webs. However, for special applications further grounded collectors were developed (Fig. 7.4) [36].

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Fig. 7.3. Schematic presentation of the electrospinning process [41]

Fig. 7.4. Different electrospinning collection systems: (a) single plate configuration, (b) rotating drum, (c) triangular frame placed near single plate, (d) parallel dual plate and (e) dual-grounded ring configuration [36]

7.2.2 Nano-Fibres Produced by Electrospinning form Polymer Solutions or Melts In Figs. 7.5–7.10, nano-non-wovens or nano-fibres which were spun from polymer solutions (here: poly(vinyl alcohol) (PVA) in water or polycaprolactone (PCL) in chloroform/ethanol – 3/1, v/v) or from polymer melts (here: a blend of poly(ethylene oxide-block-ε-caprolactone) (PEO-PCL) and PCL) are shown. The melt electrospinning was performed at a temperature of 85◦ C applying the rotating drum collection system (Fig. 7.4b). High voltages of 30 kV were applied during the electrospinning of PCL and 17 kV for the spinning of PVA solutions. Nano-fibres with average finenesses of about 300–600 nm were produced by electrospinning of PVA or PCL solutions, some very fine fibres with fibre diameters of approximately 100–300 nm were found in electrospun PCL nano-fibres. The nano-fibres which were obtained after melt electrospinning had fibre diameters of about 1 µm.

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Fig. 7.5. Nano-non-woven obtained by melt electrospinning of a blend PEO–PCL and PCL

20 µm

10 µm

Fig. 7.6. Nano-fibres produced by electrospinning from aqueous PVA solutions (average fibres in the range of 300–600 nm fineness)

7.2.3 Electrospraying Electro-driven jets of polymeric fluids undergo instabilities causing either breaking of the jet into droplets (electrospraying) [42–44] or splitting into finer jets resulting in the production of superfine fibres (electrospinning). Both processes are mechanistically similar with the exception that in electrospinning high molecular weight polymers and chain entanglement in more concentrated polymer solutions stabilise the initial jet towards spraying (Figs. 7.10 and 7.11). Electrospraying can be used for the production of multi-functional

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100 µm

10 µm

Fig. 7.7. Nano-fibres produced by electrospinning from PCL solutions (PCL chloroform/ethanol solution) (average fibres in the range of 300–600 nm fineness and the fine fibres 100–300 nm)

Fig. 7.8. Nano-fibres produced by electrospinning from melts at 85◦ C of a blend of PEO–PCL and PCL. Collection times are 1 min (left figure) and 6 h (right figure). The average fibres are approximately 1 µm, however with long collection times, larger fibres are observed. Such impurities are commonly observed for both melt and solution electrospinning

Fig. 7.9. Nano-fibres electrospun from melts of a blend of PEO–PCL and PCL onto a conventional PET non-woven

7 Nano-Fibres for Filter Materials Mainly spraying

Spraying/spinning

10 µm

10 µm

2 % PVA

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Spinning

10 µm

4 % PVA

6 % PVA

Fig. 7.10. Electrospraying or electrospinning in dependence on the PVA concentration in solution

200 µm

200 µm

20 kV

10 kV

200 µm

30 kV

Fig. 7.11. Influence of voltage on the particle size obtained during electrospraying of a non-polymeric organic compound

materials, too. The formation of droplets in the electrospraying process is caused by breaking up of the jet due to Rayleigh instability [45]. Functional fibre coatings can be obtained by electrospraying, whereas, nano-fibre webs for implementation into non-wovens are produced by electrospinning.

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7.3 Application of Nano-Fibres or Nano-Webs as Filter Media The large surface area of nano-fibre webs allows rapid adsorption of dust and other particles from air such as micro-organisms or pollen as well as hazardous molecules. The latter necessitates reactive sites in the polymer or catalytically active additives allowing chemical binding or decomposition of hazardous substances, respectively. Besides fineness and resulting large specific surface area of nano-fibre webs, their high porosity and small pore size contribute further to their high adsorption and filtration efficiency. Pore size and porosity of filter media are determined by the diameter of fibres used for production of filter media. For filter media very thin webs consisting of just a few nano-fibre diameters thickness are effective. The thickness of the nano-web can be less than 1–5 µm [46]. While the thinness of the nano-web provides high permeability to flow, the nano-web has limited mechanical properties that preclude the use of conventional web handling and filter pleating equipment. The small fibre diameter of nano-fibres and the thin nano-web layer result in high filter efficiency with minimal pressure drop increases. Furthermore, nano-fibre filter media have demonstrated longer filter lifetimes than conventional filtering materials. The technical requirements for filters are a balancing of the three major parameters of filter performance: filter efficiency, pressure drop and filter lifetime. An improvement in one category generally means a corresponding sacrifice in another category. It was shown that the proper use of nano-fibres can provide marked improvements in both filtration efficiency and lifetime, while having a minimal impact on pressure drop [46]. Nano-fibre webs can be applied onto various substrates, e.g. onto conventional non-wovens, too. These substrates can be selected to provide appropriate mechanical properties to allow pleating, filter fabrication, durability in use, and in some cases, filter cleaning [46, 47]. In the beginning of the 1980s, Freudenberg and Weinheim started to apply the electrostatic spinning for the development of non-wovens by arrangement of electrospun fibres between a support layer and a preliminary filter in a sandwich-like structure [48–50]. Donaldson Company, Inc. has been using electrospinning technology to make fine fibres for more than two decades [18,19]. Donaldson produces UltraWebTM nano-fibres with sub-half-micron diameters for air filtration in commercial, industrial and defence applications [46]. Nano-fibre filter media make new levels of filtration performance possible in several transportation applications including internal combustion engines, fuel cells and cabin air filtration. According to Luzhansky, Donaldson produces about three pounds of nylon or over 10,000 m2 nano-fibres per day [20, 51]. Greiner and Wendorff developed together with Hollingsworth & Vose GmbH/JC Binzer Mill, Hatzfeld/Germany, the so-called NanowebTM , i.e. a super-filter which is produced by electrospinning of nano-fibres onto a base

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material [20]. NanowebTM can be used for air filtration, e.g. for filtering pollen or other particles from the air [52]. The big advantage of NanowebTM is, beside of the optimisation of the filter capacity, the absolutely negligible materials usage. In Liberec, the company Elmarco developed in co-operation with the Technical University of Liberec modified electrospinning technology called “Nano-spider” which is based on electrospinning from non-water-based polymer solutions [53–55]. Elmarco presented a pilot line at INDEX 05 in Geneva/Switzerland, to the non-woven industry [55]. The nano-fibre materials of Elmarco are developed for wide use in medical, biological and technical fields. Apart from using synthetic polymers bearing special functionalities or specific add-ons to the spinning solution, chemical and biological functionality can also be achieved from natural polymers accessible from waste materials. For example the chitin-derivative chitosan is known to provide antimicrobial effectiveness [56] or keratin fibres are known for their propensity in binding air polluting substances by nucleophilic addition, e.g. formaldehyde [57]. This was the basis for us to investigate natural polymers like chitosan and wool keratins during electrospinning [25]. Keratins isolated as S -sulpho-keratins cannot only be electrospun but also allow the reformation of cystine bridges and thus the fibre stabilisation after reductive removal of the protection group. Chitosanbearing nano-fibres or nano-fibres post-coated with chitosan can reduce microbial growth and are potentially interesting for air filtration uses [25]. Fibre formation with lower molecular weight proteins as well as chitosan needs the addition of interfering polymers (e.g. PEO) to disturb the rigid association of chitosan molecules caused by hydrogen bonding. Co-spinning of bio-polymers and water-soluble polymers requires the use of cross-linkers for fibre stabilisation [25].

7.4 New Developments in Electrospinning Actual R&D work on electrospinning is focusing on precise control over fibre size and morphology by changing the process parameters, modelling of the electrospinning process, the development of new structures and functionalities of nano-fibres and the development of practical applications of electrospun fibres. The working group of Greiner and Wendorff developed a co-electrospinning procedure enabling the production of core–shell nano-fibres with specialty properties [13, 14, 20, 58]. Other specialty nano-fibres produced by electrospinning are nanotubes or fibres with very porous surface structure [14]. Another possibility is the incorporation of nano-particles/micro-spheres into nano-fibres to achieve special functionality (Fig. 7.12) [14, 59].

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Fig. 7.12. Nano-fibre with incorporated micro-spheres [59]

Big interest in electrospinning of nano-fibres exists in the area of biomedical applications [33–36]. Yet 1980, ICI patented a “product comprising electrostatically spun fibres” produced from polyurethane melts which were intended to be used as vascular prosthesis [60]. Recently, portable electrospinning equipment was developed which can be applied for wound healing [61].

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41. G. M¨ uller, Herstellung von wasserbest¨ andigen organischen und anorganischen Nanofaservliesen mittels Elektrospinning/Production of Water-Resistant Organic and Inorganic Nanofibre Nonwovens by Means of Electrospinning, Diploma Thesis, RWTH Aachen (2004) 42. M. Cloupeau, B. Prunet-Foch, J. Electrost. 22, 135 (1989) 43. M. Cloupeau, B. Prunet-Foch, J. Aerosol Sci. 25, 1021 (1994) 44. A. Jaworek, A. Krupa, J. Aerosol Sci. 30, 873 (1999) 45. L. Rayleigh, Phil. Mag. 14, 184 (1882) 46. T. Grafe, M. Gogins, M. Barris, J. Schaefer, R. Canepa, Filtration 2001 International Conference and Exposition of the INDA (Association of the Nowovens Fabric Industry), Chicago, IL, 3–5 December 2001 47. K. Graham, M. Ouyang, T. Raether, T. Grafe, B. McDonald, P. Knauf, Proceedings of 15th Annual Technical Conference & Expo of the American Filtration & Separations Society, Galveston, TX, 9–12 April 2002 48. A. Weghmann, Nonwovens Ind. 24 (1982) 49. A. Weghmann, Schriftenreihe des Deutschen Wollforschungsinstitutes 85, 314–331 (1981) 50. K. Schmidt, Melliand Textilber. 61(6), 495–497 (1980) 51. D. Luzhansky, Quality Control in Manufacturing of Electrospun Nanofiber Composites, INDA/TAPPI, Baltimore, MD, 16–18 September 2003 52. http://www.hovo.com 53. http://www.elmarco.cz and http://www.nanospider.cz/aplikace.php? kategorie=1&h... 54. J. Oldrich, F. Sanetrnik, D. Lukas, V. Kotek, L. Martinova, J. Chaloupek, Process and Apparatus for Producing Nanofibres from Polymer Solution by Electrostatic Spinning; WO 2005024101 A1 (2004) 55. Exhibition at INDEX 05, Geneva, Switzerland, 12–15 April 2005 56. S. Hirano, Biotechnol. Annu. Rev. 2, 237 (1996) 57. G. Wortmann, R. Sweredjuk, G. Zwiener, F. Doppelmayer, F.J. Wortmann, DWI Rep. 124, 378 (2001) 58. Z. Sun, E. Zussman, A. Yarin, J.H. Wenndorff, A. Greiner, Adv. Mater. 22, 1929 (2003) 59. R.M. Pereira Paz, Studien zur kontrollierten Freisetzung biologisch aktiver Substanzen aus resorbierbaren Nano- und Mikrosph¨ aren/Investigations on the Controlled Release of Biologically Active Compounds from Resorbable Nano- and Microspheres, Ph.D. Thesis, RWTH Aachen (2004) 60. A. Bornat, R.M. Clarke, Setting a Product Comprising Electrostatically Spun Fibres, EP 11437 19800528 (1980) 61. D.J. Smith, D.H. Reneker, A.T. McManus, H.L. Schreuder-Gibson, Ch. Mello, M.S. Sennett, Electrospun Fibers and an Apparatus Therefore, US Patent 6.753.454 B1 (2004)

8 The Development of Non-Wovens T. Le Blan, M. Vouters, C. Magniez, and X. Normand

Summary. Non-woven are widely used for manufacturing products with barrier properties, due to their easiness to process are relatively low cost of final products. This chapter provides description of different processes available for manufacturing non-woven, parameters influencing on their final barrier properties and proposal of ways to improve their barrier effects.

8.1 Definition of Non-Wovens Non-wovens are webs, batts or matts made of staple fibres or filaments whose origin can be natural (vegetal, animal and mineral) or human-made. The fibres or filaments are laid randomly or with a preferential orientation and then bonded by mechanical, thermal or chemical ways. According to ISO 9092, the definition of non-wovens is “A manufactured sheet, web or batt or directionally or randomly oriented fibres, bonded by friction, and/or cohesion and/or adhesion excluding paper and products which are woven, knitted, tufted, stitch-bonded incorporating yarns or filaments, or felted by wet milling, whether or not additionally needled”. The fibres may be of natural or human-made origin. They may be staple fibres, continuous filament or be formed in situ. In the case of wetlaid nonwovens and wetlaid papers, a material will be considered as a non-woven if it fulfils one of the two following conditions: either more than 50% by mass of its fibrous content is made up of fibres with a length to diameter ratio greater than 300, or more than 30% by mass of its fibrous content is made up of fibres with a length to diameter ratio greater than 300 and its density is less than 0.40 g cm−3 . If we compare non-wovens with traditional textiles, the main difference identified is based on the fact that woven or knitted fabrics are made from yarns. The way the yarns are interlaced will provide the main characteristics of the fabrics: strength, elongation, design, etc. In the case of non-wovens, there is no yarn. The fibres are laid directly and bonded. Regarding to the

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production of a fabric or a knitted material, the non-woven process is thus simpler and quicker than the traditional textile way: the spinning step does not exist.

8.2 Raw Materials for Non-Wovens 8.2.1 Fibres and Filaments As previously stated, the basic materials for the manufacturing of non-wovens are fibres or filaments. The distinction between fibre and filament is only based on the length of these materials. A fibre has a finite length when the one of a filament is considered as infinite. A fibre and a filament may have exactly the same chemical nature. Fibres or filaments are classified in two main groups: natural and human-made materials. The natural fibres are commonly classified according to their origin: vegetal, animal or mineral. For vegetal fibres, several kinds exist but, for all of them, the main component is a carbohydrate molecule-named cellulose. Individual cellulose molecules are arranged in a crystalline structure that forms itself layers to build up the fibre structure. One of the most interesting property of this material will be its ability to absorb water, i.e. hydrophilic. Table 8.1 gives a brief description of vegetal fibres. In animal fibres, the main known is wool. This fibre and other animal hairs are proteins constituted with amino acids. The wool fibre is notably covered with scales and is naturally crimped: these two characteristics give it insulation properties. Because of its price and of the necessity to clean it, wool is hardly ever used in non-wovens. Silk or spider are protein filaments but with no application at this moment in non-wovens. Table 8.1. Description of vegetal fibres Wood fibres

Cotton

Bast fibres

Wood pulp fibres are obtained from trees. The characteristics of fibres will depend on the species of trees that are used and on the process to extract them. Their cellulose content will range between 45 and 98%. Wood fibres are short (several mm) and will be used in wetlaid or airlaid processes to form webs. Approximately 20 million tons of cotton are harvested each year and are widely used in textile industry. The cotton fibre contains about 90% of cellulose and had a length around 30 mm. It is usable in the drylaid technologies alone or in blend with other fibres. These fibres are extracted from leaves or stems of plant as flax, jute, hemp, kenaf and ramie. These are mainly coarse and stiff fibres that are processed through drylaid technologies.

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Concerning human-made fibres, a lot of them have been developed during for about hundred years. Here are presented the ones that are often used in non-wovens manufacturing. Human-made fibres are classified in, first, artificial fibres. They are made by using a natural existing polymer and processing it to produce fibres. The natural polymer that is widely regenerated to produce fibres is cellulose. Various processes exist to transform cellulose into different fibres as viscose, Lyocell, Modal, acetate and triacetate. Nowadays, some other natural polymers are exploited to produce fibres as, for example, alginate (from seaweed) or collagen but in marginal amount. Mineral fibres could also be considered as artificial fibres. Most important ones are glass, ceramic, metallic, carbon and rockwool fibres. The second category is synthetic fibres for which the polymer will be synthesised through a chemical process and then arranged to form fibres. They constitute the big majority of fibres consumed in non-wovens industry. Until now, the polymers that are used to produce the fibres come from petrochemistry through different processes. The most usual polymers that are processed are polyethylene, polypropylene, polyester, polyamide, aramide, acrylic, elasthane and polyvinylic alcohol. Today some polymers are synthesised from renewable resources as vegetal and the most known today is the polylactic acid (PLA). In some cases, some fibres are designed with two polymers. These fibres are called bi-component fibres and present different sectional arrangement (Fig. 8.1). As raw materials, fibres need to be characterised to process them on the best way. The main characteristics of fibres are: – Chemical nature. – Fineness: it is expressed in dTex or denier. When the fineness is below 1 dTex, the fibre may be called micro-fibre. – Length: for most fibres, the length is given in mm. For filament, as it is considered as infinite, length is not expressed. – Sectional shape: in case of human-made fibres, the sectional shape can be designed for specific purpose. – Tenacity and elongation.

A Core and Sheath

B

Side by side

Islands in a sea

Fig. 8.1. Examples of bi-component fibres

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Table 8.2. Re-partition of consumption of fibres in Europe in 2003 (source: CIRFS, EDANA) Polyester Polypropylene Other synthetics fibres Cellulosic fibres Cotton Others fibres

30% 43% 2% 12% 3% 10%

– Crimp: the number of crimps cm−1 and the amplitude of the crimp may be measured. The re-partition of the consumption of fibres for Europe in 2003 is presented in Table 8.2. 8.2.2 Other Raw Materials Other raw materials used are for the strengthening of the web obtained and the functionalisation of the non-woven. One of the way to bind fibres in a web is to use chemical binders. These binders will be first applied on the web and will then stick fibres, thanks to chemical and thermal reaction. Most of the binders are latex binders as acrylates, styrene butadiene and vinyl derivatives. To give specific properties to the non-wovens or for processing purposes, additives may be introduced. These may be powders: fusing powders, superabsorbent powders and carbon powders. The additives may be added through an aqueous or solvent solution way: dyestuff, hydrophilic or hydrophobic agent, fire-retardant agent, perfumes, etc.

8.3 Web-Forming Technologies The process to manufacture non-wovens is traditionally divided in two steps. First the web forming allows to arrange the fibres in a web, which is followed by a bonding step to link the fibres together to give cohesion to the web. These two steps are made in a continuous processing. There are four main technologies to form a web: 1. 2. 3. 4.

Drylaid technologies Spunlaid and meltblown technologies Wetlaid technologies Short fibres airlaid technologies

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Spunlaid / Meltblown; 41% Drylaid; 47%

Wetlaid; 4% Airlaid; 8%

Fig. 8.2. Re-partition of worldwide production of non-woven according technologies (source: EDANA)

In 2004, Fig. 8.2 presents the re-partition of the worldwide production according to the technologies. 8.3.1 Drylaid Technologies Drylaid systems have been designed from technologies that were originally developed for textile and more precisely for spinning industries. The basic raw materials are various staple fibres as polyester, polypropylene, viscose, cotton. The range of fineness is commonly between 1 and 15 dTex and the length from 30 to 100 mm. The fibres will be selected to reach the properties targeted for the web. The drylaid system includes two consecutive phases: preparation of fibres and web forming. The objectives of the preparation phases are: – – – –

To To To To

open the compressed fibres blend the different fibres to reach the maximum homogeneity clean fibres, especially for natural fibres feed regularly the web-forming machine

There are various preparation machines based on mechanical principals that can differ in function notably of the fibres to process. These machines are called bale-breaker, opener, mixers, etc. Special machines are also designed to prepare recycled materials. The web can be formed through a carding operation (Fig. 8.3). The carding is a mechanical process consisting in opening tufts of fibres, blending them and producing a web. The principle relies on the teasing of fibres through cylinders equipped with clothing and with given differential speed to allow

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Web (next step : strenghtening)

Staple fibre

Carding

Fig. 8.3. Carding processing

the transfer from one to another. Additional cylinders allow a fluffing of fibers and a re-cycling in the process. The quality of the carding will rely on the evenness of the web and the absence of defects as neps of fibres. In a carded web, according to the design of the card, fibres are more or less oriented which gives to the web-specific characteristics in terms of resistance to pulling. So carded webs will be called parallel or random webs. A card delivers generally webs under 80 gm−2 . To get a heavier web or to change the orientation of fibres, a cross-lapper can be used. This machine enables to get a web constituted with several layers of the carded web. Webs can be formed too through aerodynamic way. In this case, after being mechanically open and blended, fibres are sucked and then blown on the forming surface (conveyor or perforated cylinder). This process is rather used for heavy webs and presents the advantage to get a more homogenous distribution of fibres orientation than for carding webs. This way is called “airlaid” and is described in Sect. 8.3.3. Main non-wovens elaborated with drylaid technology are: – – – – – – – – – –

Synthetic leathers Substrates for coating (shoes, etc.) Filtration Geo-textiles Building and roof (insulation) Automotive (carpet, acoustic and thermal insulation, filters, etc.) Wadding for clothes and furnishing Rug Medical applications (surgery, cleaning pieces, etc.) Cleaning products (wipes for baby, cosmetics, home and industrial cleaning, etc.)

8.3.2 Spunlaid and Meltblown Technologies These two processes are usable only with thermoplastic polymers that can been spun. Most common ones are polyester and polypropylene. The process will start with chips of these polymers.

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Extruder

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Polymer extrusion

Cooling, stretching

Mixing fibres To bonding

Fig. 8.4. Spunbond processing (front and rear view )

Cold air

Hot air Polymer

Extrusion

Web linking

Fig. 8.5. Meltblown technology

The spunlaid process integrates the following steps (Fig. 8.4): – – – –

Extrusion to melt the chips Spinning of filaments Drawing of filaments to reach the targeted fineness distribution Lay down of filaments on a forming surface (conveyor)

So the produced webs are made with filaments with an homogenous distribution of fibres orientation which will give these webs good mechanical properties. In meltblown process (Fig. 8.5), pellets of polymer are also melted and spun as in spunlaid process. The fundamental difference between the two technologies remains in the intensity of the drawing of filaments. In meltblown, the filaments produce through the spinning phase will be drawn until breakage producing a web with very fine fibres ranging from 0.5 to 10 µm. The main advantages of meltblown web rely on the fact that it is constituted with very fine fibres distributed equally in each direction. Nevertheless, its mechanical performances (pulling, abrasion) will be generally poor in comparison with spunlaid webs.

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Main non-wovens elaborated with spunlaid or meltblown technologies technology are found in: – – – – – – – –

Filtration Geo-textiles Building and roofing (insulation) Automotive (filters) Electronics Agriculture (winter protection) Packaging Disposal protective clothing for industry and medical applications (coveralls, shoes protection, etc.) – Absorbent products for oil (meltblown) 8.3.3 Other Technologies In the non-woven, elaboration remains the wetlaid and the airlaid technologies. Wetlaid technologies came directly from paper-making technologies. The raw material will be cellulosic fibres as wood pulp and a wide variety of other fibres. The main characteristic of these fibres will be their short length between 2 and 20 mm. The process consists in a dispersion, as most homogeneously as possible, the blend of fibres in water, to flow the fibres solution onto a forming wire and to extract water through the forming wire to lay fibres in a web form. Because of the size and fineness of fibres, the webs will look very uniform and sometimes very similar to paper. Main products elaborated with this technology are currently: – – – – – –

Wall paper Filtration Substrates for coating Roof insulation Automotive (filters) Cleaning products (home and industrial cleaning)

Airlaid technologies (Fig. 8.6) use same kind of raw materials as wetlaid and notably short fibres as wood fibres. This process consists in getting a homogenous suspension of fibres in air and then to filter this suspension through a forming wire. Fibres retained by the wire will form the web. As for wetlaid, the webs will look very uniform.

8.4 Bonding Technologies The aim of bonding is to give the targeted cohesion to the web which has been produced at the web-forming step. It may be performed through thermal, mechanical or chemical bonding.

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Air in

Feeding of fibres in drums

Air out (vaccum)

Fig. 8.6. Airlaid technology

Thermal bonding is based on the ability of fibres or additives (powders) to fuse when heated. Synthetic fibres as polyester, polypropylene, polyethylene have this property and are called thermo-fusible fibres. On the contrary, cellulosic fibres do not melt and fuse. So a web may be thermobonded only if it contains a sufficient amount of thermo-fusible components (fibres or powders). The heating of the web can be achieved on one hand with hot air heating: heat is brought by a flow of hot air that goes through the web or on its surface. Hot air ovens are used in this purpose. This technology allows to get a thick web. On the other hand, heating can be achieved with hot cylinders under pressure between which the web passes, and make the thermo-fusible components fusing. Machines commonly used are calenders. Because of the pressure applied to the web during bonding, the webs will be flat. Mechanical bonding consists in entangling fibres together. It can be achieved, thanks to three main technologies: 1. Needlepunching: barbed needles enter and leave vertically in the web, hooking fibres across and entangling them. Needle-punched non-wovens are generally heavy webs above 100 g m−2 . 2. Hydro-entanglement: the entanglement of fibres is obtained through the mechanical action of high-pressure water jets which act as very fine needles. The web must then be dried. This technology allowed to process light webs. 3. Stitch bonding: the entanglement of fibres is made, thanks to knitting needles (with hook) which go through the web and pull bundles of fibres when they go back. There is no binding threads. Presence of long fibres facilitates this process which is dedicated to heavy webs. For chemical bonding, a binder is added to the web and acts as a glue by sticking fibres together. The binder is added in a liquid or solid form to the web by impregnation, spraying, printing, powder scattering, etc. After being introduced in or on the web, the adherence of the binder to the fibres is obtained through a chemical reaction or heat. Then the web must be dried when the binder is brought, thanks to an aqueous solution.

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8.5 Web Conversion and Finishing After being produced, the non-woven web may undergo some conversion or finishing operation intended to give it specific characteristics or properties. These processes are very numerous and are made off-line. Here are some common conversion processes: – – – – –

Slitting Folding Coating Laminating of several webs Spraying or impregnation of chemical agents (hydrophilic, flame retardant, perfumes, etc.) – Printing

8.6 Barrier Effect in Non-Wovens Non-wovens are involved alone or in association with other materials in numerous products which provide barrier effects. Some examples in different economic sectors are presented below: – Building: thermal or sound insulation, roofing or ground watertightness, air or water filtration – Transports: thermal or sound insulation, air or gas filtration, flame retardancy, magnetic barrier – Medical: protective clothing against micro-organisms or blood, protective packages, filtration of air for sterile atmosphere – Geo-textiles or agro-textiles: drainage layers, light protection, pollution protection The barrier performance of a non-woven will depend on several parameters among them: – The density of the web involving the surface weight and the thickness of the web: for some barrier effects, the lowest density will be necessary (thermal insulation for example), for others a maximal density will be searched (liquid proofing for example). – The porosity of the web: for a giving density, the porosity can be influenced by the geometric parameters of fibres (fineness, length, sectional shape) and their arrangement in the web (fibres orientation). – The homogeneity of distribution of fibres in the web: the more homogeneous the web is, the better is the control of parameters as porosity or permeability and thus the control of barrier effect. – The chemical nature of its components: some barrier effects are reached, thanks to chemical processes. For example, additives as antibacterial or flame-retardant agents will support the barrier effect. These chemicals may be present in raw materials or added on the web.

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To improve the barrier effect or to design new ones, the main developments to carry out in the non-wovens area will concern. 8.6.1 Regularity and Homogeneity of Materials Because of raw materials and technologies used for their manufacturing, nonwovens have structures relatively heterogeneous. Thus, it is not rare to get non-woven webs whose density of fibres can vary until 10%. In case of some barrier applications, these variations are critical. To avoid this problem and answer to the specifications, the manufacturers of non-wovens can increase the basis weight to ensure a minimal density and reach the targeted barrier effect. Other solutions are, thus, to imagine and to test. Among them, the design and processing of thin fibres that give a better cover are of great interest. 8.6.2 Saving of Raw Materials with Equal or Superior Performances The cost of raw materials represents very often a major part of the total cost of a non-woven. To remain competitive on their market, producers of non-wovens look constantly for reducing the consumption of raw materials while maintaining or improving the performance of their products. This concern implies the research and development of new materials or manufacturing technologies. Besides, in the case of filters for example, the reduction of consumption of raw materials answers also to environmental considerations. 8.6.3 Functionalisation Whatever is the sector of use, non-wovens are still little functionalised. It is also the case in the sector of barrier effect. However, to face the evolution of the legislation or the requirements of consumers, the request for products offering one or several specific functionalities is more and more strong. Manufacturers apply themselves to design and introduce on the market non-wovens with additional properties (antibacterial, hydrophilic, flame retardant, etc.) to answer accurately to the requests and distinguish themselves from competitors. 8.6.4 Characterisation/Standardisation There are several methods of characterisation of barrier effects non-wovens to check if they are appropriate to their use. Sometimes these methods are not totally in adequacy with the real use of the product (for example: use of pollutants for the tests which are not representative of filtered pollutants during the life of the filter). Moreover, it may be difficult to obtain a reliable correlation between the results obtained through a test method and the real performance of the product in situation of use. The adaptation of the current methods and their standardisation should be thus useful.

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8.6.5 Mechanical Performances The use of non-wovens may be limited because of their mechanical properties (burst, tear breaking, delaminating). So it is necessary to improve them through different ways: selection of raw material, control of the orientation of fibres, bonding process, reinforcement with other materials (threads, screens, films, etc.). 8.6.6 Lifetime Improvement Numerous barrier effect products have a limited lifetime as filtration media for example. For cost reasons or to reduce maintenance operations, private or economic users would like the lifetime of these products to be increased or to be able to re-use them after decontamination. On an other hand, this objective will match with the policy of durable development which implies reduction of raw material, re-cyclability of products and minimal environmental impact of waste. These parameters have to be taken into account since the design phase of products. 8.6.7 Comfort Concerning products which will be worn and assuming a barrier effect (masks, medical clothing, protective clothing, etc.) improvements aim at making these products more comfortable and ergonomic. Main properties involved in this area are the touch, drape, elasticity, suppleness, breathability.

References 1. X. Normand, Les non-tiss´ees: de la fibre aux produits fonctionalis´ es, Training Program, Institut Fran¸cais du Textile et de l’Habillement, 2003 2. EDANA (European Disposables and Nonwovens Association), Training Program 3. http://www.edana.org 4. Man-Made Fiber Yearbook, August 2005

9 Mechanical Models and Actuation Technologies for Active Fabrics: A Brief Survey of the State of the Art F. Carpi, M. Pucciani, and D. De Rossi

Summary. This chapter presents a survey of reported mechanical models of fabrics, as well as of technologies currently available to confer actuation properties to fabrics. The modelling of base mechanics of fabrics is here reviewed as a useful tool for a proper design of future new compliant actuators capable of being integrated into a textile substrate. The embedding of such devices into fabrics is studied to confer them actuation properties, functional to one or more barrier effects, such as active filtration. To this aim, state-of-the-art technologies for actuation of fabrics are reviewed and discussed in the light of such new applications.

9.1 Introduction Fabrics are systems belonging to our daily life, used by everyone as an interface between our own skin and the external environment. Despite the “traditional” use of such interfaces as a thermal barrier against temperature variations, several other types of barrier-like properties can be conferred to fabrics for different purposes. Fire-retardant, anti-bacterial, chemical, electrostatic and electromagnetic barrier effects are just few examples, which are today focusing several efforts for their development. Active filtration is one of the barrier effects being currently object of considerable attention. A filtering operation is defined here as active if it can be modulated and controlled on demand, by adopting an external energy source. In this respect, a fabric will be here considered to show active filtration properties when it is able to respond to an external input, such as an electrical signal, by showing a variation of its capability of filtering a definite substance. As an example, a fabric with active gas filtration properties may present a variable gas permeability that can be electrically modulated. A driving of the filtration properties by means of input variables of electrical type can be considered as the most preferred, since electrical signals are the most practical for control purposes. Such a specification induces investigations aimed at assessing the possibility of integrating into fabrics electromechanical actuating components, i.e. devices capable of

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transducing input electrical energy into mechanical energy. This effect may be exploited to electrically modulate the textile structure of the fabric. In this context, this chapter is aimed at analysing the state of the art of actuation technologies potentially suitable to confer active properties to fabrics. Moreover, it is aimed also at reviewing the tools currently available for simplified descriptions of the basic mechanics of textiles, to be exploited for the design of future new actuators capable of being embedded into a textile substrate. This second aspect is quite relevant and will be the object of the first part of the chapter, followed by a discussion on the actuation technologies.

9.2 Knitted Fabrics Most of the studies about a modelling of fabric mechanics deal with textile structures of the knitted type. Knitted fabrics are usually divided in two categories: weft fabrics (Fig. 9.1a) and chain fabrics (Fig. 9.1b). These reference terminologies (weft and chain) derive from the traditional webbing of orthogonal threads, where the threads disposed in the sense of the width of the woven one are defined “weft”, while the threads disposed in the sense of the length are defined “warp” or “chain”. The singular stitches of weft-knitted fabrics are transversally connected with traits of yarn called “inter-stitches”. On the contrary, the singular stitches of chain-knitted fabrics are connected longitudinally. Knitted fabrics significantly differ from woven and non-woven fabrics in structural and, therefore, mechanical properties. Knitted fabrics can undergo large deformations under small applied forces during bending (draping), shearing and extension, and they can withstand a considerable load when are extended.

Fig. 9.1. Structures of (a) a weft-knitted fabric and (b) a chain- or warp-knitted fabric

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9.3 Mechanical Behaviour of Weft-Knitted Fabrics This section reviews results recently achieved [1] to model the mechanical behaviour of knitted fabrics in quasi-static deformation from an initially relaxed state to an extended state. The problem of extension of a knitted fabric is complicated by the combination of non-linear properties derived from both the characteristics of the structure and the properties of the constituent yarns. In the model which is going to be taken into consideration here, only the in-plane behaviour is considered. Moreover, only the effective properties of the yarn that can be experimentally derived or predicted by means of some models are considered. On the contrary, the micro-mechanism of yarn deformation is not considered. A yarn is assumed to behave as an elastic rod with linear bending and torsional properties and non-linear, time-independent tensile properties. Furthermore, it is assumed that some geometrical properties of the fabric and geometrical/mechanical properties of the yarn are given. A precise modelling of fabric deformation requires a full boundary analysis of complex systems of fabric deformation. This in turn requires the definition of the complex deformation of a small part of the fabric, i.e. its unit cell. Several attempts to solve this problem by considering the deformation of each yarn within the fabric structure have been reported [2,3]. However, such a kind of analysis is limited by the impossibility of sub-dividing the whole problem into a series of problems for the smaller elements. The finite element approach, introduced in [1] by considering a system of unit cells, allows to consider larger finite elements for faster solutions and smaller elements for higher precisions. 9.3.1 Geometrical Identification of the Yarn Loop The definition of the initial form of the yarn loop is the first step to perform an analysis of the mechanical properties of a fabric. Yarns of a knitted fabric follow a complex 3D path, which is affected by a number of factors. A drawing of a plain-knitted fabric is presented in Fig. 9.2, where, according to the textile terminology, the OX-direction is the “course” direction, while the OY -direction is the “wale” direction and O is the origin of the reference system (Fig. 9.2b). The parameters Akn and Bkn represent, respectively, the fabric-loop dimensions in the wale and course directions and are considered to be the main periods of the knitted structure. The fabric thickness is in the Z-direction. The loop dimension Hy requires a direct measurement of individual loops. 9.3.2 Rheological Models and Constituting Elements Following the removal of a fabric sample from the knitting machine, it relaxes. Nevertheless, some residual stresses remain acting on the sample. These stresses cause in-plane instability. Normally, a flat-knitted sample tends to curl around itself from the edges towards the centre. During the process of stress

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Fig. 9.2. (a) Drawing of a knitted structure and (b) fabric dimensions (adapted from [1])

relaxation, the fabric sample changes its shape and dimensions in three directions and its structure becomes roughly stable. Owing to the time-dependent mechanical properties of yarns, stresses in a fabric tend to be balanced by internal friction. Nevertheless, it is assumed here that all yarns are slack and hence no friction force acts in an initially slack fabric. The basic feature of a knitted structure is that, subject to strain in one direction, it achieves a considerable lateral contraction in the other in-plane dimension. The Poisson’s ratio for a continuous and incompressible material is 1/2. Experimental values of the Poisson’s ratio for a knitted structure have been reported up to 0.63. This means that the behaviour of such fabrics could be classified as close to that of an incompressible isotropic material. On the other hand, the fabric is soft enough to allow considerable volume change. To describe the micro-mechanism of fabric deformation, a hierarchical subdivision of the fabric into a series of elements can be considered, as described in [1]. The surface of the fabric can be sub-divided into elementary cells (unit cells) and each cell can be considered as a system of elementary elements (constituting elements). Sampling analysis can be used to divide the unit cells into several elements and define the properties for each element separately. Thus, the properties of the horizontal and vertical parts of the loop, contact zones, etc., are separately considered. Each element of unit cell can be considered with respect to kinematics and force conditions that determine the behaviour of the unit cell as a single entity. Each unit cell can be considered with respect to conditions providing deformations of a system of a unit cells as a whole sample. Having been arranged together, unit cells determine the behaviour of a sample with integrated mechanical and geometrical properties inherited from the single unit cell and from each of its constituent elements. One of the possible rheological models of a fabric material is represented in Fig. 9.3.

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Fig. 9.3. A possible rheological model of an elastic material (adapted from [1])

Fig. 9.4. Rheological scheme of the cell element: (a) initial state and (b) in-plane deformation (adapted from [1])

One of the factors that prevent cells from collapsing is the yarn-to-yarn compression. By assuming a yarn cross-section with known compression rigidity, it is possible to depict the cell as shown in Fig. 9.4 [1]. The unit cell is represented by a closed yarn loop with pulleys. The centre of each pulley is free to move in the plane XY . The springs account for the yarn-to-yarn compression and extension rigidities in the vertical and horizontal directions, Cv and CH , respectively. The yarn in Fig. 9.4 is free to slide over the pulley and hence yarn-length re-distribution occurs in the deformed cell. Boundary conditions (BCs) are also very important when large displacements are considered, as shown in Fig. 9.5. Thus, a “hinge”-like BC (Fig. 9.5c) restricts the edge points from movement in the direction transverse to that of the applied stress. This makes the system much more rigid than that shown in Fig. 9.5b, where the system with

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Fig. 9.5. Different boundary conditions for uni-axial extensions of a fabric sample: (a) clamped edge, (b) hinged-trolley edge and (c) hinged edge (adapted from [1])

“hinge-trolley”-like BC is represented. The “clamped edge”-like BC (Fig. 9.5a) does not considerably differ from the “hinge”-like BC, owing to the low value of the yarn-bending rigidity. When a uni-axial load is applied to the sample, the boundary conditions in combination with the cell-compression properties (Fig. 9.4) restrict the lateral contraction. In the model proposed in [4], the mechanical behaviour of each element is modelled by using an energy-minimisation technique for the behaviour of the yarn in corresponding parts of the structure, as described in Sect. 9.3.3. 9.3.3 Potential Energy as an Approach to Describe Non-Linear Mechanical Properties of Fabrics A complex behaviour under tensile load is shown by knitted structures. In fact, loops change their shape and dimensions. Initially, curved yarns become straight, elongated and compressed in the interlacing regions. Yarn-length re-distribution among loops takes place. This is complicated by the nonlinearity of the mechanical properties of the yarns. A reasonable approach to such a complex problem is to consider the potential energy of the structure as a function of its deformation [4]. This approach enables a model of each structural element to be developed independently and then included in the system. Total Potential Energy The scalar function E(q) gives the total potential energy of the mechanical system and can be expressed through the vector of the virtual external forces F and that of the coordinates q of the system as [4] E(q) = Π(q) − F q,

(9.1)

where Π(q) defines the potential energy of the system. Mechanical Properties of a Unit Cell To obtain the mechanical properties of a unit cell, it is necessary to determine the effective properties of each element. In particular, each part of the cell

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should be replaced by an element with the same mechanical properties. A contact zone can be represented by a “helix element” and the free part of a yarn loop can be replaced by a “spring” with non-linear properties [4]. The effective relative deformation εE and corresponding effective rigidity DE characterise the mechanical properties of the spring. The effective deformation εE can be defined as the relative deformation of a chord connected to the two ends of the element. The mechanical properties of the loop’s part that is not involved in mutual contact (free zone) can be represented as the stretching of a pre-bent rod, as shown in Fig. 9.6. In particular, it is possible to divide the process of pre-bent yarn deformation into two phases: straightened (only bending deformation takes place) and tensioning (only tensile deformation takes place), as in Fig. 9.6. Mutual Yarn Compression (“Side” and “Height” Elements) Let us consider two yarns with compressible cross-sections, as depicted in Fig. 9.7. In some special cases of fabric deformation, a measure of the compression can be obtained with good approximation by using the side length. Respective constitutional elements are side elements. The use of side elements has been reported to cause method instability or very slow convergences [4]. To solve

Fig. 9.6. Two phases of deformation of a rod initially bent and then subject to a force Fx (adapted from [4])

Fig. 9.7. Mutual compression of two yarns (adapted from [4])

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Fig. 9.8. Arbitrary deformation of a unit cell (adapted from [4])

this problem, a height element has been adopted, resulting useful to take into account arbitrary deformations of the unit cell, as in Fig. 9.8 [4]. Yarns’ Contact Zone (“Helix” Element) The behaviour of a yarn in contact with another is very complex. Unevenness of the yarn cross-section makes it virtually impossible to determine the exact bounds of a contact zone. Moreover, friction forces in the contact cause additional difficulties for stress analysis. Taking into account the fact that a yarn has non-linear properties makes the problem even harder to solve. To approach this problem, the following assumptions have been used [4] (1) the initial arc length of each contact zone is determined as in [1]; (2) the 3D path of the yarn axis in the contact zone is approximated by a 3D helix; (3) the helix makes a half-turn around its axis; (4) during deformation of the fabric, the 3D path of the yarn axis remains helical, with the diameter changing owing to compression of the yarns in the contact zone; (5) during deformation of the fabric, the helix changes its arc length owing to forces acting at the ends of the yarns involved in the contact; (6) the mechanical properties in the contact zone are associated with the unit cell vertices, so that the orientation of the helix axis is not critical and (7) two helices, which represent two yarns in the contact zone, have an identical form. An identical deformation of both the helices is assumed. Following such assumptions, the constituent element that represents the yarn in the contact zone is referred to as the helix element (Fig. 9.9). The elements of the unit cell are schematically illustrated in Fig. 9.10. The description provided so far defines the basic features of a model, which has been proposed to describe the load–extension behaviour of knitted fabrics [4]. The model is based on a representation of the textile structure as a 2D mesh of unit cells made of specific constitutive elements. The mechanical properties of the constitutive elements are derived by using a method for minimising the potential energy of deformation. This method permits to derive that the model of any constitutive element can be developed independently of before being included into the model of the whole system. Alternative approaches to this method are reported in Sect. 9.4.

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Fig. 9.9. A model of the contact zone: the unit vectors t (tangent), n (normal) and b (bi-normal) represent a natural-basis vectors of the helix (adapted from [4])

Fig. 9.10. Schematic illustration of the rheology of the constituent elements of a unit cell (adapted from [4])

9.4 Different Approaches to Describe the Mechanical Behaviour of Weft-Knitted Fabrics 9.4.1 Load–Extension Behaviour of Weft-Knitted Fabrics This section is aimed at reporting a different approach for a modelling of the mechanical behaviour of weft-knitted fabrics, as presented in [5]. The mechanical properties of weft-knitted fabrics are strongly related to the fabric structure, yarn properties and fabric direction. For a particular testing direction, the tensile behaviour of the fabric is highly non-linear. The examination of the stress–strain characteristic shows a two-stage deformation process, reported in Fig. 9.11. In particular, the deformation process can be divided into two stages.

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Fig. 9.11. Typical load–extension characteristic of a weft-knitted fabric tested in the wale-wise direction. Step 1 : stretching of the curved yarns up to the critical stretch state (yarns are straightened and not elongated). Step 2 : elongation of the straightened yarns up to the breaking point (adapted from [5])

In a first stage, the deformation of the knitted fabric is mainly due to the straightening of the curved yarns. The yarns slip with friction in the interlacing regions, while the diameter of the yarn continuously decreases because of local compression effects. This process continues up to the “critical stretch state”, which is a hypothetical state of deformation. From the mechanical point of view, in this initial stage of deformation, the fabric behaves like a structure rather than a continuous material. As the deformation is non-linear, the Hook’s law does not apply. In the second stage, the load is transferred directly to the yarn. When the load increases, the cross-section of the fabric becomes more compact. Although a small structural effect of the fabric still exists, this may be ignored as it is less important in the deformation process. Some practical observations can be made (1) to increase the stiffness of knitted fabrics, and therefore their capacity to resist deformation from applied loads, pre-tensioning techniques or the introduction of straight yarns in various directions are required and (2) to increase the resilience of knitted fabrics, and therefore their capacity of absorbing energy, a relaxed stretchable loop structure is required. 9.4.2 A Theoretical Analysis Based on the Elastic Theory A 3D model based on the classic elastic theory is here reported and it is used to predict the load–extension curves of a plain weft-knitted fabric in the course-wise and wale-wise directions, as presented in [6].

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Fig. 9.12. Schematisation of (a) loop structure, (b) forces and torque applied onto a quarter of the loop (adapted from [6])

Elastic Model The model adopts the following assumptions (1) the plain-knitted fabric is made of frictionless, inextensible, incompressible and naturally straight filament yarns, which can be considered as a homogeneous elastic rod; (2) the knitted fabric is formed by planar loop structures, all loops within the same fabric keep an identical configuration; (3) no plastic deformation of the yarn is assumed to take place when the fabric is knitted from a straight yarn; (4) two loops at adjacent courses interlock in such a way that the yarns of the two loops are fully in contact at the cross-regions; (5) the distance between the interlocking points B–B  of two neighbouring loops is equal to the diameter of the yarn, as shown in Fig. 9.12 and (6) the reaction forces R produced in the loop-interlacing region, due to yarn contact, are simplified as a concentrated force (Fig. 9.12). These reaction forces act at the loop-interlocking points B and B  and along a line perpendicular to the yarn axis. It is possible to demonstrate [6] that the force P is related to the torque T by the following relation: T = −P (sin γ tan β + cos γ) .

(9.2)

Comparison Between Theoretical Calculations and Experimental Results The load–extension curves related to a direction parallel to the wale-wise and course-wise directions for a given knitted fabric have been theoretically predicted according to the proposed model and compared with experimental results [6]. A good agreement has been found between theoretical and experimental values, as presented in Fig. 9.13. It has been reported [6] that, apart from the prediction of uni-axial tensile properties, such a model may also be used to calculate the bi-axial tensile properties of plain-knitted fabrics.

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Fig. 9.13. Comparison between theoretical and experimental load–extension curves of a knitted fabric in the case of (a) an extension in the wale-wise direction and (b) an extension in the course-wise direction (adapted from [6])

9.5 Woven Fabrics with Barrier Effect This section reports some designs of woven fabrics with channel-like structures and solutions for “sealing” woven fabrics to confer them a mechanical barrier effect, as presented in [7]. The investigation of this type of effect is particularly relevant to tailor filtration properties of fabrics. Fabrics with barrier function can be designed by adopting solutions capable of implementing a tightening of the thread structure. This can be achieved by programmable increases of the thread density, so that to decrease the channel cross-sections. A decrease in the dimensions of the free spaces between threads is possible with a change of the configuration of the woven fabric structure. As an example, starting from the square structure reported in Fig. 9.14, a decrease in the deflection of one thread system and a consequent increase in the deflection of the other system can lead to a reduction of the free spaces. Figure 9.15 presents a variation of the structural model of the woven fabric of Fig. 9.14, obtained by changing the thread configuration with a stretching of one thread system. Another method for closing free spaces between threads is based on the introduction of additional components in the free spaces themselves (Fig. 9.16). Such methods to modulate on demand the barrier function of a textile substrate may be implemented, for instance, with the use of active threads having actuating functions. This would make the overall system intrinsically capable of modifying its specific structure, depending on the necessity. More generally, actuating elements endowed in textiles may provide them with adaptive properties of functional barrier, by exploiting even other principles to modulate the textile structure. Accordingly, the availability of textilecompatible actuating technologies represents the key issue to be addressed for such a purpose. In this respect, Sect. 9.6 presents a state of the art of existing technologies which have a role in this field.

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Fig. 9.14. Schematic drawing of a woven fabric with square structure, made of non-deformable threads of circular cross-section and plain weave (adapted from [7])

Fig. 9.15. Model of the woven fabric presented in Fig. 9.14 after changing the thread configuration (adapted from [7])

Fig. 9.16. Model of a woven fabric with an inter-thread channel filler (adapted from [7])

9.6 Technologies for Actuation of Fabrics So far, very few technologies have been demonstrated to be able to confer actuation properties to fabrics. They most significant rely on materials with shape memory effect, in the form of either alloys or polymers. A material

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presents the shape memory effect if, once deformed in a permanent state at a low temperature, it is able to recover (“remember”) its original shape, following its heating above a certain transition temperature. This ability is known as the one-way shape memory effect. Accordingly, shape memory alloys (SMAs) or polymers are adaptive materials capable of converting thermal energy into mechanical work, by changing shape, dimensions, stiffness, natural frequency, damping and other static and dynamical properties. Possible applications of these materials to fabrics are separately reported in Sects. 9.6.1–9.6.3. 9.6.1 Shape Memory Alloys SMAs exhibit the shape memory effect (Fig. 9.17) as a consequence of the mechanical properties of their crystal lattice structure. In particular, they show two stable phases: a low temperature phase called martensite phase and a high temperature phase called austenite phase. While a SMA is in the martensite phase, it can be distorted into a prescribed shape and then it can recover its primary form by the reverse transformation upon heating up to a critical temperature Af (Fig. 9.17), whose value can be tailored, depending on both the type of material and its processing. Nitinol is the most diffused SMA. It consists of a well-known alloy of Nickel and Titanium. The temperature increase can be achieved by exploiting the Joule’s effect, through electrical currents flowing in the conductive SMAs. This feature is useful in practical applications, since it permits an electrical activation and control of the material. For detailed information about the electrical driving of SMAs, we refer the interested reader to the ample-related literature. So far, some examples of insertion of wire-shaped SMAs within textile substrates have been reported, as described below. Application to Textiles and Fabrics SMA wires have been integrated into textile substrates, such as curtains, to enable their invigoration on demand [8–10]. Such a functionality has been

Fig. 9.17. Principle of shape memory effect (adapted from [8])

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Fig. 9.18. Smart shirt with sleeves undergoing a thermal memory effect (adapted from [11])

conceived for different purposes, such as aesthetic features, privacy needs or adaptation to environmental conditions. As an example, a curtain with intrinsic actuating functions would be able to respond to the environmental conditions of a room by sensing the surrounding temperature. The woven textile may adapt its own structure, from a “closed” state to an “open” one and vice versa, so that to modulate its insulation properties with respect to light or air flows. As another application of SMAs for textiles, they have been used in adaptable fabrics with thermal management capabilities [11]. The company D’Appolonia has presented a smart shirt with woven Nitinol yarns. This cloth is “self-ironing” and sensitive to environmental changes. In particular, the sleeves of this shirt are able to short when it becomes too warm, as presented in Fig. 9.18. The temperature necessary to trigger such shape memory effect has been fixed above 37◦ C. The sleeves have been developed to shorten as soon as the room temperature becomes a few degrees hotter, behaving like a thermostat strip when its heated and cooled. 9.6.2 Shape Memory Polymers Shape memory polymers (SMPs) offer, in comparison with SMAs, greater deformation capabilities, lower forces and easier shaping procedures. Their transition temperatures and mechanical properties can be varied in a wide range with small changes in their chemical structure and composition. Their memory effect is based on two key structural features: triggering segments that have a thermal transition temperature within the range of interest and cross-links that determine the permanent shape of the sample. Depending on the type of cross-links, SMPs can be either thermoplastic elastomers (which soften when heated, and harden when cooled) or thermosets (which solidify after being heated and cooled and cannot be re-melted). Polyurethane SMPs have shape recovery temperatures which can be tailored from approximately −30 to 70◦ C. A Mitsubishi subsidiary commercialises a segmented polyurethane called Diaplex [12], used to manufacture a sort of active “breathing” clothing with

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Fig. 9.19. Temperature-dependent permeability of the Diaplex shape memory polymer: (a) before activation and (b) after activation (adapted from [12])

barrier function. According to the manufacturer, when the ambient temperatures rises, the non-porous micro-structure of the material opens up to allow heat and humidity transport, as depicted in the drawing of Fig. 9.19. This mechanism takes advantage of thermal vibrations, which occur when the temperature rises above a pre-determined activation point. As a result of this motion, micro-pores are created in the polymer membrane, varying its permeability and thus allowing water vapour and body heat to be exchanged. 9.6.3 Future Developments Despite these few examples, no successes towards an effective and comfortable embedding of actuating functions into textiles have been substantially reported so far. In this respect, it is opportune to underline that SMA fibres are basically metallic wires, which inevitably stiffen the textile substrate, decreasing the comfort of the wearable system. Furthermore, the shape memory effect relies on heat diffusion across the material: this determines response speeds limited by the time constant of the diffusion process. Finally, the presence of hysteresis can be responsible for a tendency to thermal saturation, which negatively affects the actuation performance. For these reasons, different solutions for the embedding of efficient actuating functions into fabrics are demanded. Electro-active polymer (EAP)-based actuators may be employed for such a purpose. In this regard, we are investigating the feasibility of using actuating devices based made of dielectric elastomers with suitable planar configurations. This type of materials, belonging to the EAP family, can be used for electromechanical actuation, according to a simple principle of operation. The elementary form of such a device consists of two parallel compliant electrodes separated by a dielectric elastomer, which is deformed by the application of a high electric field between the electrodes. The thickness of the elastomer decreases while its surfaces expand [13,14]. Silicone rubbers are being tested as dielectric elastomers capable of high-strain wearable actuators. Dielectric elastomers possess several advantages: actuation strains up to the order of 100%, fast response times (down to tens of milliseconds) and generated stresses up to the order of 1 MPa. The price for achieving such performances is represented by the very high driving electric fields needed (order of 100 V µm−1 ) [14].

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Fig. 9.20. Schematic drawing of a textile fabric substrate with deposited dielectric elastomer planar actuators

We are currently developing the integration into fabrics of planar dielectric elastomer actuators. The idea is to combine the compliance of the actuator itself with that of suitable elastic fabrics, to be able to modify their shape or dimensions. For this purpose, we are using Lycra/cotton textiles as substrates for the deposition of layers of dielectric elastomer, as shown in Fig. 9.20. Several materials, deposition methods and actuating configurations are under evaluation to identify the best performing combination for the application of interest. This approach may provide a viable means to confer elementary actuating functions to fabrics for simple and low-force actuation tasks. These types of systems are going to be described in details in future communications.

9.7 Conclusions A survey of state-of-the-art models of basic mechanics of fabrics has been presented in this chapter. These models and approaches have been reviewed to collect useful tools to be used for a proper design of future new compliant actuators capable of being integrated into a textile substrate. The embedding of actuating functions into textiles is studied as a means to confer them functional barrier effects, to be mainly employed for active filtrations. In this respect, state-of-the-art technologies suitable for such a purpose have been reviewed. They consist of SMAs and polymers used in the form of wires and membranes. Despite the relevance of such solutions, new necessary approaches have been identified as possible candidates to endow fabrics with compliant

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electromechanical actuating devices. They may enable future developments of functionalised fabrics with active filtration properties as a barrier effect.

References 1. A.U. Loginov, S.A. Grishanov, R.J. Harwood, J. Text. Inst. 93(3), 218–238 (2002) 2. R.B. Hepworth, G.A.V. Leaf, J. Text. Inst. 67, 241–248 (1976) 3. S. De Jong, R. Postle, J. Text. Inst. 68, 307–315 (1977) 4. A.U. Loginov, S.A. Grishanov, R.J. Harwood, J. Text. Inst. 93(3), 239–250 (2002) 5. M. De Ara´ ujo, R. Fangueiro, H. Hong, AUTEX Res. J. 3(3), 111–123 (2003) 6. M. De Ara´ ujo, R. Fangueiro, H. Hong, AUTEX Res. J. 3(4), 166–172 (2003) 7. J. Szosland, AUTEX Res. J. 3(3), 103–110 (2003) 8. F. Boussu, J. Petitniot, H. Vinchon, AUTEX Res. J. 2(1), 1–7 (2002) 9. Y.Y.F. Chan, G.K. Stylios, Designing Aesthetic Attributes with Shape Memory Alloy for Woven Interior Textiles, Technical Document, Heriot-Watt University, RIFLEX Institute, Galashiels, Scotland, 2003 10. Y.Y.F. Chan, R.C.C. Winchester, T.Y. Wan, G.K. Stylios, The Concept of Aesthetic Intelligence of Textile Fabrics and Their Application for Interior and Apparel, Technical Document, Heriot-Watt University, RIFLEX Institute, Galashiels, Scotland, 2002 11. S. Carosio, A. Monero, Smart and Hybrid Materials: Perspectives for Their Use in Textile Structures for Better Health Care, Proceedings of International Workshop: New Generation of Wearable Systems for e-Health: Towards a Revolution of Citizens’ Health and Life Style Management?, Lucca, Italy, 2003, pp. 271–280 12. Diaplex website: http://www.diaplex.com 13. R.E. Pelrine, R.D. Kornbluh, J.P. Joseph, Sens. Actuator A 64, 77–85 (1998) 14. R. Pelrine, R. Kornbluh, Q. Pei, J. Joseph, Science 287, 836–839 (2000)

Part III

Modelling

10 Pyrolysis Modelling Within CFD Codes P. Van Hees and J. Axelsson

Summary. This chapter focuses on the description of a pyrolysis model which can be used for implementation within computational fluid dynamics (CFD) codes. The pyrolysis model is available from the literature and includes both thermal heat transfer in the solid phase as well as chemical kinetics. Besides the description of the model, the way how to obtain input parameters is discussed in this chapter and their sensitivity is shown. Finally validation results of the pyrolysis model are given.

10.1 Introduction The physical flame spread model described is the model developed by Yan at Lund University [1]. In this chapter, a short description of the physical flame spread model is given. More detailed information can be found in [1]. It gives an example of a pyrolysis model which was later incorporated in at least two CFD codes. The model is based on a one-dimensional numerical heat transfer model which uses a standard numerical solver for the heat conduction equation. Each numerical heat conduction strip is then divided in a number of sub-strips to which a simple pyrolysis model is applied. The pyrolysis model is explained in paragraph 10.2. The input parameters with respect to thermal and pyrolysis model are: – – – – – –

Ignition temperature (K), only of interest for non-charring materials Pyrolysis temperature (K) Heat of pyrolysis (J kg−1 ) Heat of combustion (J kg−1 ) Virgin density (kg m−3 ) Char density (kg m−3 )

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– Specific heat (J kg−1 K−1 ) – Thermal conductivity (W m−1 K−1 ) The next paragraph 10.2 will explain more in detail the model while paragraph 10.3 of this chapter discusses the changes made at SP to be able to use the model efficiently for future use [2].

10.2 Description of Model One of the possibilities of implementing a flame spread model into a CFD code is the direct use of cone calorimeter data for each cell at the surface of the material. However the problem here is that the net heat flux input will vary substantially during the simulation because of the fire growth or decay. It is hence difficult to change the mass loss rate in each cell depending on the actual net heat flux into the cell calculated within the CFD code. For this reason Yan developed a flame spread code that allows a more flexible approach. A brief explanation is given in this paragraph. For further details one is referred to the corresponding literature in [1]. This method differs from the cone data input method mainly in its way of providing the heat release rate (HRR) for the elements of the solid fuel. The one-dimensional transient heat conduction equation is solved numerically here, but with pyrolysis and charring included. The heat conduction equation can now be written as    

 ∂ m ˙ HG,T − HG,Tp ∂ ∂T ∂(ρH) +m ˙  (Hpy + H) + = k , (10.1) ∂t ∂x ∂x ∂x where

∂m ˙  ∂ρ = ≥0 ∂t ∂x representing the mass loss rate of the pyrolysing material per unit volume. The third term is the energy required to heat the vaporised gas as it flows to the solid surface. This term will be zero for non-charring material and has no important effect in this study, and thus it is ignored here (but it can be very easily included). Hpy is the heat of reaction of the pyrolysis process, and can be calculated by the difference in total enthalpy of virgin material and volatile products, i.e.  
m ˙  = −

∗ ∗ ∗ − Hvir,T = Hvol,T − Hpy = Hvol,T p p p

∗ Hvir,T + o

Tp

cp dT

.

(10.2)

To

It is worth pointing out that Hpy is a material constant and is different from the heat of gasification ˙ total = Hg = q˙net /m

[hc (Tg − Tx=0 ) + Rflux ] , m ˙ total

(10.3)

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which is a local and transient value and changes considerably during the pyrolysis process. For the thermally thick vaporising material, at steady state, Hg =

∗ Hvol,T p

−

∗ Hvir,T o




Tp

= Hpy +

cp dt.

(10.4)

.

(10.5)

To

Equation (10.1) can be rewritten as ∂T ∂ +m ˙  Hpy = ρcp ∂t ∂x

∂T k ∂x



The material will start to pyrolyse only when its temperature reaches the pyrolysis temperature, Tp , and it will then keep this temperature until completely pyrolysed. Thus we have

 ∂ ∂T ∂T = (10.6) ρcp k when T ≤ Tp or ρ ≡ ρchar , ∂t ∂x ∂x

 ∂ ∂T (10.7) m ˙  Hpy = k when T ≥ Tp and ρ > ρchar . ∂x ∂x The following is the detail of the numerical solution of (10.5), which can be discretised as

 Tn+1 − Tn Tn − Tn−1 Tn − Tn  δx + m ˙ δx Hpy = k − ρcp , (10.8) ∆t δx δx where the prime indicates the previous time step and m ˙ δx is the mass loss rate per unit area of the δx thick strip. If we define k(n+1)(n) k(n)(n−1) , Aw = , δx δx Ap = Ac + Aw + ρcp δx/∆t, Su = ρcp δxTn /∆t − m ˙ δx Hpy ,

(10.10) (10.11)

Ap Tn = Ac Tn+1 + Aw Tn−1 + Su .

(10.12)

Ac =

(10.9)

then (10.8) becomes

Since the conductivity, k, is generally a function of temperature, it is consequently a function of x and it is not necessary for Ac to be equal to Aw . It was found by Yan [1] that to obtain a reasonable result for the mass loss rate, a very fine grid was required. However, this very fine grid is unnecessary and very expensive for the temperature solution. This inconsistency is overcome by defining a reasonably coarser grid for the temperature solution and refining the grid into a second grid to determine the mass loss rate, as shown in Fig. 10.1.

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T1

T2

T4

Char layer (if appropriate)

Pyrolysing zone

T5

Virgin material

Fig. 10.1. Temperature solution node and grid refinement (N = 5, M = 10)

The temperature of the refined node, m, of the coarser node, n (we will denote this node as node (n, m) later), Tn,m is obtained by interpolation, as shown in Fig. 10.1, assuming a linear distribution between Tn and Tn+1 . From (10.12), for an arbitrary small node (n, m), the energy available for pyrolysis can be approximated as Hn,m = max [0.0, Ap (Tn,m − Tp ) /M ] .

(10.13)

On the other hand, the mass of the volatisable material remaining in the node (n, m), which may have been completely, partially or not at all pyrolysed, is generally given by massvol = where

δx min {ρvir − ρchar , max [0.0, (ρn,m − ρchar )]} , M

(10.14)

ρm = M ρ − (m − 1) ρchar − (M − m) ρvir .

The mass loss rate from node (n, m) is thus finally determined by m ˙ n,m = min {Hn,m /Hpy , massvol /∆t} .

(10.15)

The overall pyrolysis rate can be obtained by summation over all the nodes and expressed as   m ˙ n,m = min (Hn,m /Hpy , massvol /∆t). (10.16) m ˙ = n

m

n

m

The HRR is represented by Q = mH ˙ c,

(10.17)

where Hc is the heat of combustion related to the gaseous fuel produced. Generally, during flaming combustion, it has been shown that Hc is approximately constant.

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10.3 Additional Changes or Additions in the Model The original model developed by Yan was mainly written for the use of homogeneous materials such as PMMA and particle board. One of the disadvantages was the difficulty with the model when using composite materials. As the model was also being used in the FIPEC project [3] involving cables and composite material tests, it was necessary to modify and improve the programme so that both multiple combustible and non-combustible materials could be used. Also a flexible input routine was written to facilitate the input of the material data and the numerical properties [2]. Most of these changes were software related. Another change which was added recently by Yan [4,5] in his code was the introduction of chemical kinetics by implementing an Arhenius function.

10.4 Sensitivity Analysis of the Physical Flame Spread Model To understand the model as good as possible, a number of sensitivity analyses were performed. This is extremely important for later use of the model within the CFD code. In the following paragraphs different graphs give an overview of the influence of specific material parameters on the modelling results for a specific material. Time and HRR scales are adapted to show more clearly the changes observed. It should be noted that this sensitivity analysis has been performed on a specific set of input data and that only specific trends are shown. The material simulated for this study was a 3 mm PVC plaque with a heat flux level of 75 kW m−2 . This plaque was simulated as a composite of two 1.5 mm plaques and with a copper plate underneath the combustible material. As mentioned earlier the original model of Yan had to be adapted to deal with composite materials. The changes were mainly in changing variables into arrays and to add additional loops. The physics of the model was not changed. In addition, a flexible input routine was written to facilitate the change of the different parameters without compiling the programme each time one single parameter had to be changed. The standard input levels are given in Table 10.1. The table also gives an example of the new developed flexible input file. 10.4.1 Influence of the Pyrolysis Temperature on the Results As can be seen in Fig. 10.2, a reduction of the pyrolysis temperature results in a delay of ignition time and peak HRR time and a reduction of the peak HRR.

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Table 10.1. Overview of standard input parameters for the sensitivity analysis Parameter(s)

Value

Number of faces and number of materials (-) Flux levels (kW m−2 ) Time steps, iterations and number of strips (-) Number of combustible layers and number of non-combustibles layers (-) Ignition temperaturea (K) Pyrolysis temperature material 1 (K) Pyrolysis temperature material 2 (K) Heat of pyrolysis material 1 (MJ kg−1 ) Heat of pyrolysis material 2 (MJ kg−1 ) Heat of combustion material 1 (MJ kg−1 ) Heat of combustion material 2 (MJ kg−1 ) Density char material 1 (kg m−3 ) Density char material 2 (kg m−3 ) Virgin density material 1 (kg m−3 ) Virgin density material 2 (kg m−3 ) Virgin density material 3 (kg m−3 ) Thickness material 1 (m) Thickness material 2 (m) Thickness material 3 (m) Specific heat material 1 (J kg−1 K−1 ) Specific heat material 2 (J kg−1 K−1 ) Specific heat material 3 (J kg−1 K−1 ) Thermal conductivity material 1 (W m−1 K−1 ) Thermal conductivity material 2 (W m−1 K−1 ) Thermal conductivity material 3 (W m−1 K−1 )

4/3 75 1,000/25/10 2/1 650 593 593 3.0 3.0 14 14 100 100 1,000 1,000 9,000 0.0015 0.0015 0.0007 1,000 1,000 380 0.5 0.5 400

a

It should be noted that the ignition time in the model is only taken into account for non-charring materials and determines the change in the boundary conditions on the surface. Hence changes in the ignition temperature were not studied for this case.

10.4.2 Influence of Heat of Pyrolysis on the Results In Fig. 10.3, it can be seen that the heat of pyrolysis mainly affects the peak HRR. An increase of the heat of pyrolysis reduces the peak HRR and flattens the curves. Rather limited influence on ignition time is observed as the temperature governs this one. 10.4.3 Influence of the Heat of Combustion on the Results As can be expected, a change in heat of combustion results only in a linear change of the actual HRR output as it is directly linked to the mass loss rate (Fig. 10.4).

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Fig. 10.3. Analysis of the influence of the heat of pyrolysis on the results

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Fig. 10.4. Analysis of the influence of the heat of combustion on the results

10.4.4 Influence of the Char Density on the Results Varying the char density results in a small change of the peak HRR and also a transposition of the decay period of the HRR curve. The values of char densities are those which can be expected as realistic with the corresponding virgin density (Fig. 10.5). 10.4.5 Influence of the Specific Heat on the Results Even with a considerable change, the main influence of the specific heat is the ignition time (Fig. 10.6). 10.4.6 Influence of the Thermal Conductivity on the Results The parameter mostly influencing the HRR curve is the thermal conductivity. An example of this is given in Fig. 10.7. A reduction of this parameter results first in a shorter ignition time and higher peak HRR. At very small numbers of thermal conductivity, the ignition time is still reduced but the peak HRR decreases again. Unfortunately the thermal conductivity is one of the parameters which is very difficult to determine in fire simulations as it is strongly temperature dependent and changes considerably if the material undergoes transformations such as intumescing, charring, melting, etc.

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200 100 kg/m3 200 kg/m3 300 kg/m3

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Fig. 10.6. Analysis of the influence of the specific heat on the results

10.4.7 Influence of the Number of Iterations and Thickness of Numerical Strips on the Results With a separate analysis, the influence of the number of iterations on the numerical model and the thickness of the strips were investigated. In the discussion of his thermal flame spread model, Yan only mentions the importance

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Fig. 10.7. Analysis of the influence of the thermal conductivity on the results

of time steps and number of sub-strips. However it could be observed that the number of strips and the number of iterations can have an influence especially when using small numerical strips for the numerical thermal heat conduction solver. From experience it is advised not to reduce the thermal strips to less than 0.5 mm. With this value a number of iterations about 20 are more than sufficient (case of n = 10 in Fig. 10.8). When the thickness of the strip for the thermal heat conduction solver is less than 1 mm, the number of iterations needs to be increased to 100. Some more study is required to investigate this phenomenon and to determine an automatic convergence criterion. 10.4.8 Influence of Ignition Temperature for Non-Charring Materials Another important input parameter for the model is the change in addressing the boundary conditions from charring to non-charring materials. It was observed that for non-charring materials the ignition temperature is used for the change in boundary conditions. This means that before the ignition temperature is reached, the net flux to the surface is composed of the incident heat flux minus radiation and convection losses. At the moment of ignition, the convection losses are zero while the total heat flux is increased by the flame heat flux. Radiation losses are equal to the radiation losses from a surface at a temperature equal to pyrolysis temperature. This means that the pyrolysis temperature should be chosen equal or lower to the ignition temperature or else a sudden change in boundary conditions results in an abrupt change of

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Fig. 10.9. Influence of ignition temperature for non-charring materials

heat release as can be seen in Fig. 10.9. Some caution is hence appropriate and a further investigation of this change in boundary condition is desirable. For charring material the actual calculated heat release is used as the trigger for a change in boundary conditions. Here the calculated surface

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temperature is used for the surface radiation losses during the whole simulation time. 10.4.9 Final Evaluation and Procedure to Define Material Parameters From the previous sections, it can be concluded that especially the thermal conductivity, heat of pyrolysis and pyrolysis temperature have a strong influence on the simulation results with respect to ignition time and heat release curve. The heat of combustion is only a linear parameter, which is used in the model for linking the mass loss and the HRR. On the other hand, the difference between charring and non-charring materials is also reflected in the change in boundary condition. While this is correct with respect to how the radiation losses are calculated, it is more questionable by the way the ignition temperature is introduced in the calculation model for non-charring materials. Since a char density of 5 kg m−3 is used to make the difference between a charring and non-charring material with respect to change of boundary conditions, the user of the model is warned for a big change in results if low char densities would be used. This can clearly been seen in Fig. 10.10. While for many materials such as PMMA, many of the input parameters can be found in literature; a simple procedure was developed by using mainly cone calorimeter test data. 800 700

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Fig. 10.10. Difference between a simulation with small char density and a simulation as non-charring material

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The procedure is as follow: 1. Cone calorimeter tests on the material at specific heat flux levels to obtain the HRR curve as a function of time. 2. Cone calorimeter ignition tests on some additional heat fluxes to determine the ignition times. 3. Determination of kρc and Tig (ignition temperature) by means of ignition test results of the material at different heat flux levels. This is done by using thermally thin or thick ignition models, e.g. the model developed by Janssens [6] which can even be used for both cases. Then assume a specific heat value (the influence of c is rather limited why an estimated guess with help of literature data can be made). Then determine the density of the material and calculate the thermal conductivity. With the same ignition model, a value for the ignition temperature can be obtained. As explained earlier this only has an influence for non-charring materials. 4. Determination of the char density (if appropriate) by checking the remaining mass after the cone calorimeter tests. When several flux levels are used an average value can be obtained. 5. Determine the heat of combustion as the effective heat of combustion obtained in the cone calorimeter tests. 6. Optimise by means of changing the pyrolysis temperature and heat of pyrolysis the HRR curve of the simulation so that it corresponds as good as possible with the measured data. As start values, literature values for the envisaged material can be used or data from other test methods can be used if available (e.g. thermo-gravimetric analysis, TGA) for pyrolysis temperatures. If necessary small changes in the other input parameters can be allowed to fine tune the HRR curve and the ignition time. 7. Checks should not only be performed at one heat flux level but also at different heat flux levels especially for the ignition time. Experience with this rather simple procedure shows that about 10–15 simulations with the pyrolysis model of the cone calorimeter tests have to be performed to obtain the parameters needed for both the thermal and pyrolysis model. By means of the flexible input file this process can be performed simultaneously for different flux levels or for different changes in one specific input parameter when keeping the other input parameter constant. An automatic process by means of curve fitting is mostly desirable in the future. Due to lack of resources this has not yet been performed. However, this should be considered as a high priority item if a user-friendly technique is desirable.

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10.5 Verification of the Physical Flame Spread 10.5.1 Verification with Cone Calorimeter Test Data Tests with PMMA A check with PMMA has been performed with the modified code based on the material data given in Yan’s Ph.D. thesis. However, the simulation was performed with mineral wool under the PMMA simulating the exact thermal boundary condition as in the real test. Yan used namely adiabatic boundary conditions at the backside of the sample. The test used for comparison was one of the PMMA tests in the pre-RR within WP8 of the FIPEC project [3]. The results of the simulation are given in Fig. 10.11. It can be seen that the ignition time and the constant HRR levels are predicted very well. The fact that the last peak is predicted less well can be explained as follows. PMMA burns as a liquid pool fire and at the end of the test the area is changing causing a different heat release curve. It can be concluded that the modified model predicts even better the cone calorimeter test on PMMA than the original code developed by Yan. This is mainly due to the fact that a more flexible composite input module is possible which allows introduction of the correct boundary conditions on the back of the PMMA. PMMA 20 mm, Mineral Wool 20 mm 50 kW heat flux 1000 Model Experiment

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Fig. 10.11. Simulation of a PMMA cone calorimeter test at 50 kW m−2

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Fig. 10.12. Simulation of a particle board test at 50 kW m−2

Tests with Particle Board Similarly, a simulation of a 12-mm thick particle board with ceramic wool was performed. Figure 10.12 gives the results for a cone calorimeter test at 50 kW m−2 . Also here the difference between simulation and test results is acceptable considering the complex behaviour of burning particle board. The second peak is not included in the simulation. To predict this second peak, it is clearly necessary to have the possibility to include a temperature-dependent thermal conductivity of the char. Even here simulations are slightly better compared to Yan’s examples mainly due to the introduction of a composite particle board–ceramic wool. Tests with a PVC Plaque In the FIPEC project, the procedure given in paragraph 10.4.9 was used to determine the input parameters for the pyrolysis model. In Fig. 10.13 the result of the optimisation for a PVC material at 75 kW m−2 is given. As can be seen the material simulation is quite good. Only the decay period is different. This can most likely be explained by the fact that the thermal conductivity of the char is identical to the thermal conductivity of the virgin materials. In future versions it is necessary to include a better representation of the thermal

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Fig. 10.13. Prediction of a 3 mm PVC plaque and comparison with the cone calorimeter test results at 75 kW m−2

conductivity of the char to better predict the behaviour of materials with a highly active char during the combustion process. Ignition times for different heat fluxes and peak heat release for other heat flux levels are within 10% of the measured values. With the pyrolysis model a procedure for obtaining the composite behaviour was developed, which allows a first prediction of cable behaviour by means of material test results of the different cable components. Further information can be found in the final report of the FIPEC project [3]. 10.5.2 Verification with a Stand-Alone Flame Spread Model Description of the Stand-Alone Flame Spread Model The stand-alone flame spread model was developed to investigate if there was a possibility to simulate upward vertical flame spread in a simple way using the physical flame spread model alone without combining it with a CFD code. The first step is to obtain all relevant physical parameters for the physical flame spread model according to 10.4.9. The strategy is then to divide a panel of a certain material or composite into discrete, uniform cells or faces along an upward x-axis. Each face and material is sub-divided in depth into a number of strips. A gas burner is then simulated by exposing a number of faces at the lower end of the panel to an external heat flux. The above faces are also exposed to a flux according to an exponential profile q˙ = q0 e−5.0(x−xf0 ) ,

(10.18)

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where q0 is the burner flame flux and xf0 is the section of the panel directly exposed by the burner flames. For each face the physical flame spread model is then called with each respective external heat flux and the material physical parameters. It returns heat release and temperature of each face, which is used to determine ignition and the position of the pyrolysis front. The flame length, xf , is coupled to the total heat release from all faces and to the pyrolysis length by xf =

xp + kf Q˙ n , 2

(10.19)

where xp is the position of he pyrolysis front, kf the flame height constant and Q˙ the sum of heat release from all faces. The exponent n is taken as 2/3 for wall flames [1] and kf as 0.054 kW−2/3 . All faces exposed by the flames are assigned a constant external flux while the faces above the flame are assigned an external flux according to the profile given in (10.18). Results The stand-alone flame spread model has been tested for two cases: a 12 mm particle board and a 3 mm PVC wire. Both cases have also been tested experimentally in the FIPEC project according to a test method developed for the project and using a standardised IEC method (IEC 60332-3). Particle Board The particle board was experimentally tested in the IEC 60332-3-10 chamber as a 4 m high and 0.3 m wide panel vertically mounted on a ladder. A premixed-flame propane burner with a 20 kW output was applied to the lower part of the panel and flame spread was recorded visually and on video. As the cone calorimeter experiments used are not exactly from this type of particle board, some uncertainty of the physical parameters should be considered. A cone experiment would make it possible to optimise the physical parameters to achieve a better simulation. The most important observation is that the model predicts correctly the fact that pyrolysis front stops and does not continue to the top of the 4 m rig, see Fig. 10.15, which was also observed in the test. During the flame spread experiments the back of the boards ignited at a later phase. This cannot be predicted by the model for the moment and is thus ignored here. PVC Wire In the experiment the PVC wire was mounted on a vertical ladder in 4-m lengths. As for the particle board a 20 kW pre-mixed-flame was applied to the lower part of the wires. The heat release was measured and flame spread recorded visually.

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Fig. 10.15. Simulated pyrolysed length in the two cases

In the simulation the wire was represented by a 1.8 mm PVC panel, the thickness corresponding to the amount of combustible material mounted in the experiment. This is of course a very rough approximation of a wire but the

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result is certainly reasonable as can be seen in Fig. 10.14. In the experiment the flame spread to the top in about 100 s and in the model the top was reached in 130 s, see Fig. 10.15. The results prove that the pyrolysis model is capable of covering a lot of the physical phenomena occurring during the flame spread process. It should be noted that for cables an additional dimension in the numerical heat transfer model might be necessary to simulate the longitudinal heat transfer in the copper conductor.

10.6 Conclusion Use of CFD models for the prediction of smoke and fire spread in, e.g. buildings, transport modes, etc. is become more popular with the increased use of fire performance-based codes. To predict the fire growth, there is a need for advanced pyrolysis models. In this chapter a specific pyrolysis model has been presented together with a method which allows also the determination of the input parameter for the model. Also verification of the model has been discussed.

References 1. Z. Yan, Numerical Modeling of Turbulent Combustion and Flame Spread, Department of Fire Safety Engineering, Lund University, TVBB-1018, Lund, 1999 2. H. Tuovinen, P. Van Hees, J. Axelsson, B. Karlsson, Implementation of a physical flame spread model in the SOFIE CFD model, vol. 32, Brandforsk Project, SP Report 1999, Bor˚ as, 2000, pp. 307–971 3. S. Grayson, A. Green, P. Van Hees, J. Axelsson, U. Vercelotti, H. Breulet, Final Report of the FIPEC project (Fire Performance of Electrical Cables), SMT4-CT96-2059 Project, ISBN 0-9532312-5-9 (Interscience Communications, London, 2000), 410 pp 4. Z. Yan et al., Validation of CFD Model for Simulation of Spontaneous Ignition in Bio-mass Fuel Storage, Proceedings of the IAFSS conference in Bejing 2005 (Interscience Communications, London, 2006) 5. Z. Yan, in Transport Phenomena in Fires, ed. by B. Sunden, M. Faghri, Wit Press, UK, 2007 ISBN 978-1845641603 6. M. Janssens, in Determining Flame Spread Properties from Cone Calorimeter Measurements, Heat Release in Fires (Elsevier Science, New York, 1992)

11 Life-Cycle Assessment Including Fires (Fire-LCA) P. Andersson, M. Simonson, and H. Stripple

Summary. Traditional life-cycle assessments (LCAs) of consumer products such as computers, furniture, etc. do not consider the environmental impact of fires involving such products. In so doing, LCA practitioners ignore any benefit from increased resistance to fire through the use of additives as a potential counter-weight to environmental costs of including said chemical. Conventional LCA models include additives and more complex production processes in consumer products only as a cost, i.e. the environmental benefit of the additive is not taken into account. Recently a Fire-LCA model was developed that also includes fires and their impact on the environment. This chapter describes how to perform a Fire-LCA.

11.1 Introduction Environmental issues are a vital part of our society and the ability to perform accurate estimates and evaluations of environmental parameters is a vital tool in any work to improve the environment. Initially, environmental studies were mainly focussed on the various emission sources, such as factory chimneys, exhaust gases from vehicles, effluents from factories, etc. However, in the 1980s it became apparent that a simple measurement of an emission did not provide a full picture of the environmental impact of a specific product or process. The emissions from a chimney, for example, only reflect one of several process steps in the production of a specific product. To fully describe the environmental impact of a product or activity, the entire process chain has to be described including raw material extraction, transports, energy and electric power production, production of the actual product, the waste handling of the product, etc. There was, therefore, an obvious need for a new methodology and an analytical tool able to encompass this new situation. The tool that was developed during this period (end of 1980s and 1990s) was: Life-Cycle Assessment (LCA). However, the LCA methodology also needs continuous improvements to incorporate new aspects and processes. An LCA typically describes a process during normal operation and abnormal conditions such as accidents are left

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out of the analysis, usually due to lack of a consistent methodology or relevant data. For example, LCA data for power production usually assume normal conditions without any accidents. Provisions for certain accidents in the analysis of the life-cycle could be included provided these could be specified in sufficient detail and occurred with sufficient regularity to make their inclusion relevant. In traditional LCA models a higher fire performance is only included as a change in energy and material consumption and no account is taken of the positive effect of higher fire performance in the form of fewer and smaller fires. The emissions from fires contribute to the environmental impact from products and should be included in a more complete evaluation of the environmental impact of a product where the fire performance is an important parameter. In cases where the fire performance is not a critical product performance characteristic (e.g. underground piping) one should not include this in the product LCA. This chapter describes a methodology for the incorporation of fires into an LCA. Fires occur often enough for statistics to be developed providing necessary information on material flows in the model. A model has been specifically developed to allow for this inclusion and will be referred to as the Fire-LCA model. The Fire-LCA method was originally developed by SP and IVL [1, 2] and they have since applied the model to three different case studies [3–11]. The guidelines given in this chapter are based on the experience gained during development of the model and its application to the case studies and the work conducted in a recent Nordtest project where the guidelines were developed [12]. The Fire-LCA model is generally applicable, provided that appropriate additions and changes are made whenever a new case is studied. To date, the examples that have been analysed are related to building contents and not to building materials. Therefore, guidelines are more fully developed for building contents applications although this does not exclude their application to building materials.

11.2 Life-Cycle Assessment: The Basic Concept LCA is a versatile tool to investigate the environmental impact of a product, a process or an activity by identifying and quantifying energy and material flows for the system. The use of a product or a process involves much more than just the production of the product or use of the process. Every single industrial activity is actually a complex network of activities that involves many different parts of society. Therefore, the need for a system perspective rather than a single object perspective has become vital in environmental research. It is no longer enough to consider just a single step in the production. The entire system has to be considered.

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The LCA methodology has been developed to handle this system approach. An LCA covers the entire life-cycle from “the cradle to the grave” including crude material extraction, manufacturing, transport and distribution, product use, service and maintenance, recycling and final waste handling such as incineration or landfill. In an LCA a mathematical model of the system is designed. This model is a representation of the real system with various approximations and assumptions. With LCA methodology it is possible to study complex systems where interactions between different parts of the system exist to provide as complete a picture as possible of the environmental impacts of, for example, a production process. Applications for an LCA can be many and some are listed below, divided into internal and external use for an organisation: Internal – Knowledge generation – Strategic planning – Forecasting – Development of environmental strategies – Environmental improvement of the system – Design, development and optimisation of products or processes – Identifying critical processes for the system – Development of specifications, regulations or purchase routines – Environmental audit – Waste management – Environmental management systems (EMAS, ISO 14000) External – Environmental information – Environmental labelling – Environmental audit of companies An LCA usually evaluates the environmental situation based on ecological effects and resource use. In a few cases the work environment has also been included. A traditional LCA does not cover the economic or social effects. International standards for LCA methodology have been prepared by the International Organisation for Standardisation (ISO). The following standards are available today: Principles and framework (ISO 14040) [13] Goal and scope definition and inventory analysis (ISO 14041) [14] Life-cycle impact assessment (ISO 14042) [15] Life-cycle impact interpretation (ISO 14043) [16] Generally the method can be divided into three basic steps with the methodology for the first two steps relatively well established while the third step (Impact assessment) is more difficult and controversial. The first two steps

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Goal and scope definition

Inventory analysis

Interpretation

Impact Assessment

Fig. 11.1. The main phases of an LCA according to the ISO standard [13]

are usually referred to as the life-cycle inventory (LCI) and can be applied separately without the following impact assessment. In addition to the different steps in the procedure there can also be an interpretation phase. The three basic steps are shown in Fig. 11.1. The goal and scope definition consists of defining the study purpose, its scope, system boundaries, establishing the functional unit, and establishing a strategy for data collection and quality assurance of the study. Any product or service needs to be represented as a system in the inventory analysis methodology. A system is defined as a collection of materially and energetically connected processes (e.g. fuel extraction processes, manufacturing processes or transport processes) which perform some defined function. The system is separated from its surroundings by a system boundary. The entire region outside the boundary is known as the system environment. The functional unit is the measure of performance, which the system delivers. The functional unit describes the main function(s) of the system(s) and is thus a relevant and well-defined measure of the system. The functional unit has to be clearly defined, measurable, and relevant to input and output data. Examples of functional units are “unit surface area covered by paint for a defined period of time”, “the packaging used to deliver a given volume of beverage” or “the amount of detergents necessary for a standard household wash”. It is important that the functional unit contains measures for the efficiency of the product, durability or lifetime of the product and the quality/performance of the product. In comparative studies, it is essential that the systems are compared on the basis of equivalent functional unit.

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Other important aspects to consider in the goal and scope definition include: – Whether the LCA is complete or if some component is excluded from the study – Which type of environmental impact is considered in the study – A description of other important assumptions In the inventory analysis the material and energy flows are quantified. The system consists of several processes or activities, e.g. crude material extraction, transport, production and waste handling. The different processes in the system are then quantified in terms of energy use, resource use, emissions, etc. Each sub-process has its own performance unit and several in- and outflows. The processes are then linked together to form the system to analyse. The final result of the model is the sum of all in- and outflows calculated per functional unit for the entire system. The most difficult part and also the most controversial part of an LCA is the impact assessment. So far, no standard procedure exists for the implementation of an entire impact assessment. However, the ISO standard covers the so-called Life-Cycle Impact Assessment (LCIA) [15], where different impact categories are used and recommendations for Life-Cycle Interpretation [16]. Transparency of the LCA model is, however, important and inventory data must also be available in addition to aggregated data. Several methods/tools have been developed for impact assessment and the tools can usually be integrated with different LCA computer softwares. The modern tools today usually include a classification and characterisation step where the different parameters, e.g. emissions are aggregated to different environmental classes such as acidification, climate change or eutrophication. There are of course also possibilities for direct evaluation/interpretation of the different emissions or environmental classes.

11.3 Methodology: An Overview The LCA methodology that has been used in this guideline is based on traditional LCA methodology. This methodology is described in the ISO standard 14040-series [13–16] and other documents from different countries in Europe and the USA. Different documents have been published in different countries but the basic theories are relatively similar. In the Nordic countries for example the “Nordic Guidelines on Life-Cycle Assessment” (1995) has been published as a guideline, not a standard [17]. The LCA model including fires has been called the “Fire-LCA” model and will be referred to as such henceforth.

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11.3.1 The Risk Assessment Approach In a conventional LCA the risk factors for accidental spills are excluded. In the LCA data for the production of a chemical, for example, only factors during normal operation are considered. However, there can also be, for example, emissions during a catastrophic event such as an accident in the factory. Those emissions are very difficult to estimate due to a lack of statistical data and lack of emission data during accidents. The same type of discussion exists for electric power production in nuclear power plants. In the case of the evaluation of normal household fires, the fire process can be treated as a commonly occurring activity in the society. The frequency of fire occurrences is relatively high (i.e. high enough for statistical treatment) and statistics can be found in most countries. This implies that it is possible to calculate the different environmental effects of a fire if emission factors are available. Statistical fire models can be set up for other types of fires but the uncertainty in the statistical fire model will increase as the statistical data is more limited. The fundamental function of a better fire performance is to prevent a fire from occurring or to slow down the fire development. Improving a product’s fire performance will thus change the occurrence of fires and the fire behaviour. By evaluating the fire statistics available with and without different types of fire performance improvements, the environmental effects can be calculated. The benefits of a higher fire performance must be weighed against the “price” society has to pay for the production and handling of possible additives and/or other ways of production. The LCA methodology will be used to evaluate the application of higher fire performance in society. In this way a system perspective is applied. 11.3.2 The Fire-LCA System Description An LCA model should be able to describe the LCA system as defined in Sect. 11.4.1. In this case, it should be able to describe the entire life-cycle of a product with different fire performance. Schematically the LCA model proposed for a Fire-LCA can be illustrated as in Fig. 11.2. The model is essentially equivalent to a traditional LCA approach with the inclusion of emissions from fires being the only real modification. In this model a functional unit is characterised from the cradle to the grave with an effort made to incorporate the emissions associated with all phases in the unit’s life-cycle. Thus, the model includes production of material for the product to be analysed, as well as the production of the additives if applicable. If possible the model should be designed in such a way that the fire performance can be varied. Furthermore, the model should include production, use and waste handling of the product during its lifetime. During the lifetime of the products to be analysed, some products will be involved in different types of fires. The Fire-LCA model will therefore

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Crude material preparation 0 or X % FR in material Material production

Fire retardant production

Fire extinguishing Decontamination processes Recycling processes

Production of primary product

Replacement of primary products

B%

A% Use of primary product

Fire of secondary products

D%

C% Ash Incineration

Landfill

Fire of primary products

Replacement of secondary products

Fire of secondary products Fire of primary products

Ash Replacement of primary products

Landfill Fire = Primary product production A+B+C+D=100 %

= Primary product use = Waste handling = Fire processes = Product replacement processes due to fires

Fig. 11.2. Schematic representation of the LCA model

include modules to describe the fire behaviour for the different types of fires. Fire statistics are used to quantify the amount of material involved in the different types of fires. In addition, the model should also include modules for handling the production of replacement materials that are needed due to the shortening of lifetime that the fires have caused. If possible the model should also include modules for the handling of the fire extinguishing process and the decontamination process. A wealth of statistics is available concerning fires from a variety of sources (such as, Fire Brigades and Insurance Companies). Differences between countries and between different sources of data in the same country provide

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information concerning the frequency of fires and their size and cause. The use of these fire statistics is discussed in more detail in Sect. 11.4.3.

11.4 Fire-LCA Guidelines The choices below provide the framework for a Fire-LCA. They should not be seen as insurmountable boundaries but as guidelines. Typically the system boundaries may be defined in different ways and the effect of this definition can be important for our understanding of the model. 11.4.1 Goal and Scope The aim of the model is to obtain a measure of the environmental impact of the choice of a given level of fire safety. Implicit in this model is the fact that to obtain a high level of fire safety some fire performance improvement measures need to be taken, these could be for example the addition of flame retardants (FRs), or a fire extinguishing system, or to change the design of the product. To assess the environmental impact of the different levels of fire safety, it will be necessary to compare at least two examples of the same functional unit: one with lower fire safety and one with higher fire safety. The model does not necessarily aim to obtain a comprehensive LCA for the chosen functional unit. In other words only those parts of the model that differ between the different versions of the product will be considered in detail. All other parts can be studied in sufficient detail to obtain an estimate of the size of their relative contribution. Functional Unit The functional unit should include the actual function of the product or service to be analysed. It is also important that the functional unit contains measures for the efficiency of the product, durability or lifetime of the product and the quality/performance of the product. In a Fire-LCA model where the fire performance of a product or a process is evaluated the actual function of the fire mitigation system could be how well the fire mitigation works or the number of fire occurrences for a given fire mitigation system. However, it can be very difficult to find relevant measures for such an approach. Experience from previous applications of the model has shown it is appropriate in a Fire-LCA to follow the life-cycle of the product whose fire performance is studied, as the functional unit during its entire lifetime. Thus, in this case one or a number of products are chosen as the functional unit. A practical method chooses the number of products originally produced at the factory and then follows the products throughout their lifetime. In many cases it can be practical to choose a relatively large number of products

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that, e.g. represent the European or a specific country production during a year. Functional units that have been used in previous studies have been, e.g. one million TV sets or one million sofas. In comparative studies such as the Fire-LCA, it is also essential that the systems are compared on the basis of equivalent functional unit. System Boundaries A schematic model of a Fire-LCA has already been described in Fig. 11.2. The figure shows the main components of the model and thus also the system boundary. The main parts to be included in the model should be as follows: – Production of materials and fuels to be used in the product production. – Production of the fire mitigation system. – Production of the product to be analysed (defined as the primary product). – Use of the primary product. – Waste handling of the primary product including: Landfill Incineration Recycling – Fire modules describing: – Fires starting at primary product and spread to surrounding products (defined as secondary products). These fires are called primary fires. – Fires starting at secondary product and spread to primary product. These fires are called secondary fires. – Waste from fire activities including: – Demolition – Decontamination – Landfill, incineration, recycling – Additional production of primary products for replacement of primary products that have been lost in fires. – Production of secondary products for replacement of secondary products that have been lost in primary fires spreading to secondary products. – Fire extinguishing activities. – Landfill fires in the land-filled materials. This represents a comprehensive list of the processes involved in fires. In practice it is sometimes not possible to include all of the above activities. According to standard practice no account should be taken of the production of infrastructure such as construction of plants for production of chemicals, etc. or impact due to personnel. Concerning the features of the model that are specifically related to fires the system boundaries should be set such that they do not appear contrived. In general, it is realistic that we assume that material that is consumed in a fire would be replaced. Where possible, one should rely on literature data to ascertain the size of such contributions. In lieu

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of such data an estimate of the contribution should be made based on experience of similar systems. In the case of small home fires, which are extinguished by the occupant without professional help, the mode of extinguishment is not included due to the difficulty in determining the extinguishing agent. In cases where the fire brigade is called to a fire, transport and deployment should be included as realistically as possible. In the case studies performed so far using the Fire-LCA model, however, neither extinguishment activities nor landfill fires have been assessed. Note that while some discussion can be made of the potential for death or injury due to the fires included in the model, the Fire-LCA model cannot take toxicity into account directly. Thus, information concerning the toxicity of the fire gases, the ventilation conditions in the fire and the ability for escape of potential fire victims is not included. Parameters to be Considered: Resources, Energy, Emissions and Waste A Fire-LCA study follows the same criteria as a traditional LCA study concerning the parameters to be considered in the analyses. Thus, the parameters used are based on: – – – –

Energy use Resource use Emissions Waste

In the case of fire the emissions are of greatest interest. A wide variety of species are produced when organic material is combusted. The range of species and their distribution is affected by the degree of control afforded in the combustion process. Due to its low combustion efficiency a fire produces much more unburned hydrocarbons than does controlled combustion. In the case of controlled combustion one would expect that carbon dioxide (CO2 ) and water (H2 O) emissions would dominate. In a fire, however, a wide variety of temperature and fuel conditions and oxygen availability produce a broader range of chemical species, such as CO, polycyclic aromatic hydrocarbons (PAH), volatile organic compounds (VOC), particles, and dibenzodioxins and furans, etc. Exactly which species should be considered depends on the materials involved in the evaluated product, for instance if the product does not include any bromine in itself or during the production cycle then brominated species can be excluded. Other Model Parameters and Scenarios Formation An LCA model contains not only information concerning resource uses and different emissions but also of, e.g. different types of fire protection, waste

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handling procedures or recycling scenarios. In LCA models there is also information concerning different transports and generation of electric power for the various modules. In most applications of an LCA it is common to propose a variety of scenarios and to investigate the effect of the choices involved. In many cases the different scenarios chosen reflect the waste handling used today and anticipated waste handling in the future. Other scenarios might reflect use of different statistical fire models. Indeed, due to a lack of detail in much of the available fire statistics it will often be necessary to postulate a number of best and worst case fire models to determine the robustness of the results. 11.4.2 Special Fire Considerations In the Fire-LCA model, the terms “primary fires” and “secondary fires” have special meaning that may differ from the terminology used elsewhere. Thus, they are defined here as follows: – Primary fires. Primary fires are fires starting in the primary product, i.e. the functional unit. These fires can spread to also involve the entire room or the entire building. – Secondary fires. Secondary fires are fires starting in some item other than the functional unit which spread and ultimately involve the functional unit. In the Fire-LCA model, fires are included as a possible end of life scenario before the normal end of life, i.e. the fire shortens the lifetime of the product. The products that end their lives in this way can either start the fire themselves or be consumed in a fire that has originated elsewhere. The case where the product starts the fire is referred to as a “primary fire” in this model and this fire can then spread to involve other items. Fires, which originate from other items are referred to as “secondary fires”. The primary fires have been divided into four categories in the case studies conducted so far, i.e.: 1. Small fire in product only, results in no emissions, i.e. only replacement of the product. 2. Larger fire involving the product only, results in product replacement and inclusion of fire emissions from the burning product. 3. Fire involving entire room, results in fire emissions from the room (including the product) and replacement of both the product and room contents. 4. Fire involving the entire dwelling or building, results in emissions from burning the entire dwelling or building and replacement of the entire dwelling or building. This grouping is probably appropriate for most fires in building contents, but more primary fire categories could be added if one has a statistical model

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detailed enough to support this, e.g. if significant emissions from a small fire such as that defined in point 2 require the replacement of more products than the functional unit the cost of such replacement may be included. Changes probably needs to be done if building materials, industrial fires, etc. are evaluated. There is only one category of secondary fires. Emissions from burning the product and the replacement of the product should be included for the secondary fires. All other material involved in secondary fires is not included in the environmental load of this occurrence. The emissions in this case are the emissions from the product alone, in many cases burning in a flashed over room. 11.4.3 Statistical Fire Model The number of products that are involved in the different types of fires constitutes the fire model. The fire model should preferably be based on fire statistics but could also, if there are no statistics available, be based on some hypothesis and perhaps comparison to other similar products where statistics are available. The fire statistics that are used to develop the fire model must be detailed. One must be able to determine the number of primary and secondary fires each year. In addition one must be able to estimate the size of these fires, i.e. the number of fires that grow to involve the rest of the room and/or the rest of the building. Fire statistics tend only to include fires that are large enough for the fire brigade to be summoned. In many cases small fires are extinguished by people nearby and the fire brigade is not called. These fires are, however, often reported to insurance companies as part of an insurance claim. Therefore statistics from insurance companies should also be included in construction of the fire model. The number of fires differs in many cases significantly between different countries. This depends on method of reporting the statistics together with cultural and possibly geographical differences. In addition, the number of fires in a country change over time due to changes in regulations or in lifestyle, e.g. proportion of smokers, use of certain equipment such as smoke detectors, etc. Therefore care must be taken when choosing which statistics are used to construct the fire model. In addition, there are always stochastic differences between different years and thus the calculations should not be based on statistics from a single year. The Fire-LCA model is suitable for investigating the effect of different fire regulations. In this case there are three possibilities: (a) Comparisons are made within one country, where the regulations have been made stricter. (b) Comparisons are made between two countries, where regulations are different, i.e. one country has stricter rules than the other.

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(c) Comparisons are made within one country where stricter regulations are proposed but have not yet been implemented. In case (a) statistics exist for the time period before the stricter rules were applied and after. Usually, when a change in regulations is implemented all old equipment is not thrown away but is changed gradually and products with higher fire performance will coexist with products with a lower fire performance for a period of time. The rate at which products are changed into newer products depends on the lifetime of the product. The most commonly used distribution of lifetime is the exponential distribution with the survival function R(t) R(t) = e−λt = e−t/lifetime , where t is the time. The percentage of the product in use based on the old regulations is calculated from the survival function and that percentage is then used to calculate the number of fires that would occur if all products where based on the old regulations and if all were based on the new rules. If market evaluations are available which show how many old products and new products that are currently in use then these numbers can be used instead of the ones obtained from the above survival function calculation. One can also use different survival functions for different categories of people, etc. to include effects such as, e.g. elderly people updating their equipment more seldom and being more prone to fires. This of course requires that the statistics is detailed enough to accommodate these different categories. In case (b) it is important to choose statistics from two countries that are culturally and construction-wise as similar as possible and to carefully investigate the differences that exist in the method of reporting statistics and lifestyle of people in the countries chosen. In case (c) no statistics are available. In this case one has to use the statistics from the country in which the regulations are about to be changed and then estimate what the statistics would be if the regulations were adopted. This estimation can be made from experiments where one tests ignitability and flame spread properties of the product constructed based on the new regulation compared to those for the product according to the old regulations, or experience from previous regulations on similar products, if such exist. Another application of the Fire-LCA model is to compare two products with different fire performance regardless of the regulations. This places extra demands on the details available in the statistics. In this case, one must be able to distinguish the different types of the product in the statistics. If this is not possible then one has to estimate the fire frequencies in the different types of the product. However, if the two types of the product can be assumed to have the same fire performance then one uses the same fire frequencies for both types. In cases when a series of assumptions have been made to set up the statistical fire model it is prudent to run the Fire-LCA analysis using several statistical fire models to conduct a sensitivity analysis. Further, it is imper-

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ative that the assumptions are clearly defined in the model presentation to facilitate a critical evaluation of the results. 11.4.4 Replacement of Burned Materials A fire can be considered to be a process where the lifetime of a product is shortened. Thus, the product has to be replaced earlier than the average. This results in an increased production with a corresponding increase in energy use and emission release. Lifetime distributions often follow the bath tub curve with many faults in the beginning of a products lifetime due to manufacturing faults and then again many faults when the product is approaching the natural end of its life-cycle. As an average, however, a 50% reduction of the lifetime can be assumed if no information is available on the fire distribution over a products lifetime. To reflect this reduction of lifetime it is assumed that only 50% of the burned material is replaced. The replacement of burned material should not only comprise the actual product to be analysed but all material involved in the fire, for example material in the room or in the house that is also involved in primary fires. From the fire statistics the number of fires in the particular product and the fire spreading are derived. This information gives the number of products and surrounding materials to be replaced due to the fires. In the model, this will result in an increased amount of products produced that are analysed and also a production of replaced materials due to fire spread beyond the functional unit in the LCA. Thus, the model must include LCA production modules for production of a house and the interior materials that are involved in the fires. In the previous Fire-LCA studies performed by IVL and SP the fire spread beyond the functional unit has been divided into room fires and house fires (entire house) due to the organisation of the fire statistics. Thus, LCA modules for production of a house and interior materials have been included in the FireLCA model. The amount of interior materials reflects in these cases an ordinary house and also the materials in the room fire tests. In the cases conducted thus far LCA data for a house with 121 m2 has been used [18] for the replacement of the building material of the entire dwelling. A typical/standard lounge room area of 16 m2 has been assumed (assuming that a three-room flat has an area of 80 m2 and dividing this with three rooms + kitchen + bathroom). For the room fire case an area allocation has been used. The room fire replacement of building materials contribution has thus been assumed to be 16/121 of the house. The same approach has been used for replacement of building contents and interior material. It is assumed that a typical room contains the same proportion of wood, paper, textiles, PVC, PUR and polyethene as the entire dwelling [19], and no special calculations have been made for atypical rooms such as the kitchen or laundry. The amount of the different materials used for the design of the fire room experiments and replacement of

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Table 11.1. Amount of different types of burnable material in a dwelling and room, respectively (kg) Material Wood Paper Textiles PVC PUR Polyethene

Dwelling (121 m2 )

Room (16 m2 )

2,780 720 720 240 240 100

368 95 95 32 32 13

burned materials is listed in Table 11.1. These have been calculated assuming a fire load of 720 MJ m−2 [20, 21] which corresponds to a material density of 40 kg m−2 floor area assuming an average heat of combustion value 18 MJ kg−1 .1 11.4.5 Data Inventory In the inventory analysis the material and energy flows are quantified. An important aspect in the inventory analysis is the model resolution. The model resolution can be expressed as the smallest unit that can be resolved in the analysis. The resolution requirements are determined by the ultimate evaluation. In many cases the evaluation includes an evaluation of different fire protection systems, e.g. use of different FRs. This requires a high resolution of the composition of the product materials, usually plastic materials. The resolution and quality of the model must be so high that the composition of the materials can be varied and the result can be evaluated. This can usually be considered as a high resolution for an LCA. Furthermore, all relevant emissions have to be covered as well as the use of raw materials and energy resources. Material, Product Production and Product Use In a Fire-LCA it is useful to divide the different materials used in a product into FRs and other materials used in the product. This distinction makes it easy to analyse and vary the different types of fire protection systems. The two groups can also be aggregated and classified in the LCA model to simplify the analysis. It is also advised that the model is designed in such a way that the composition/concentration of the FR can be varied. Other materials can be handled in the same way as in a traditional LCA. 1

The combustible material used in the room experiments in the TV study has later proved to be less than 40 kg m−2 but should be 40 kg m−2 to reflect the values obtained in the CIB work [21].

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The different materials and components are then put together in a production process to form the final product to be analysed. In some cases the design of a product can include fire protection. This can result in a more difficult product to produce. This aspect can be included in the product production module. Otherwise, the production can be handled as in a traditional LCA. Sometimes the use of a product can include environmental aspects such as use of electric power of a TV set during its lifetime. This also has to be included in the model and this is included in the Product Use Module. Waste Handling The waste handling procedures may have a major influence of the overall result of an analysis. The type of fire performance improvement system used must be reflected in the waste modules. The influence of, e.g. an FR on all the different waste handling alternatives must be included. Thus, the resolution requirements for the model are also high for the waste modules. The calculation of data for the waste handling modules is usually difficult and requires some estimation in cases where full data is not available. The calculations may also include allocation difficulties. Fire Emission Data Literature data on fire emissions can be used if available. The emissions should be detailed and preferably include, e.g. CO, CO2 , HCN, NH3 , HCl, NOx , HBr, VOC, PAHs, isocyanates, chlorinated and brominated dioxins and furans. However, if the products evaluated do not, for example, contain any phosphorous then the phosphorous containing species can be excluded. Similarly, if the product contains any specific additives then these must be included in the measurement together with possible products when this additive burns, for example, if the product contains any brominated FR then the specific FR in question and brominated dioxins and furans should be considered. If literature data is not available then experimental data should be obtained. The fire experiments should provide as realistic input data as possible to the Fire-LCA model. Preferably one should conduct at least one test for each type of fire to obtain a good estimate of the emissions from the fires. Fire experiments and the analyses needed to measure the emissions are, however, costly and therefore the number of experiments must be optimised. The fire experiments should give required input to the primary fires (confined to the functional unit, confined to the room of origin or confined to the house of origin) and the secondary fires (in this case relating to the emissions from the functional unit only). Primary Fire, Product Only For the case of a primary fire that only involves the product an experiment should be set up such that the entire product is consumed in the fire. Typical

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ignition sources for primary fires are cigarettes, matches and candles. It is often not possible to ignite a product with a high fire performance using a small flame. However, the statistics may indicate that these fires occur. These fires can usually be explained in that there is some other material involved in the fire as well, for instance a blanket or cushion in a sofa. In those cases the primary fire experiments need to be conducted using a larger ignition source such as larger burner or a pool fire. In these cases the emissions from the ignition source (essentially only CO2 ) should be subtracted. Primary Room and House Fire For the primary fire that spreads to involve the room an experiment should be set up where an entire room is consumed in a fire, which starts in the product evaluated. The room should preferably reflect a typical room in which one usually finds the product. If, for instance, a TV or video is evaluated then the room should be a living room. If one evaluates a fridge then the room should be a kitchen. In some cases it is difficult to determine the surroundings of a product. Take for instance, a washing machine; this can either be situated in the bathroom, the kitchen or a laundry. The contents of these rooms differ somewhat but there are some similarities, i.e. there are no upholstered furniture or bookshelves, there are several machines present, i.e. one or several of: tumble dryer, dishwasher, stove, fridge, etc. This makes it possible to construct a model environment. For the entire house/dwelling case a similar approach is preferred, i.e. a fire experiment starting in the product evaluated spreads to involve the entire dwelling. Fire experiments involving an entire dwelling are, however, usually too expensive or logistically difficult to conduct due to the large heat release rate. Instead one must extrapolate from the room experiment. In the studies conducted thus far, to estimate the emissions from a full house fire from experimental data for room fires, the emissions from a full room experiment has been presented as emissions per square metre and scaled up to the full area of the model house. The basis for this is that the material content is approximately the same in all types of rooms, i.e. amount of plastic, wood, etc. as presented in Table 11.1, and that the room experiment is designed accordingly. The scale up is done on an area basis using an area of 16 m2 for a typical room and 121 m2 as a typical house. In the case studies conducted to date the product evaluated has been situated in a living room. The fire load has been chosen as 40 kg m−2 , which corresponds to a fire load of 720 MJ kg−1 [21]. The fire emissions from the building materials in the room and dwelling have not been included in the analysis. Secondary Fires For the secondary fires the emissions from the product in a burning room should be measured. It is not possible to distinguish the amount of emissions due to the product and the amount due to the other burning items if one

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measures the emissions from a burning room. Instead one has to set up an experiment where the product is subject to similar radiation and heat as in a burning room. This requires large burners and that the product is contained in an empty room. But care must be taken so that the walls of the room do not give any emission during the experiment. Reducing the Number of Experiments In many cases one cannot run the full set of four experiments per product (three primary and one secondary) described above due to budget limitations. If the number should be reduced then one has to make sure that the types of fires that are most common according to the statistical fire model set up are simulated most accurately and the types of fires not simulated are estimated from the data obtained from the experiments. For instance, primary fires are more common for products with lower fire performance than for those with higher fire performance while the number of secondary fires is the same independent of fire performance. This means that in many cases it is more important to simulate the secondary fire for the product with higher fire performance and the primary fires for the product with lower fire performance. How to estimate the types of fires that are not directly simulated in the experiments differs depending on the type of experiments that have been conducted [12]. 11.4.6 Competences Needed to Conduct a Fire-LCA Analysis Since a Fire-LCA analysis involves several different aspects it is usually not possible for one person to conduct such a study. The people involved in the process must have competence within LCA, fire statistics and other statistics, fire experiments, emission data sampling and analysis and detailed knowledge of the production processes for the product evaluated is essential. Therefore a group must be formed to cover all these areas of expertise.

11.5 Evaluation of Results The most difficult, and also the most controversial, part of an LCA is the Impact Assessment. No single standard procedure exists for the implementation of impact assessment although generally different methods are applied and the results compared. In the valuation phase, the different impact classes, e.g. acidification, climate change and eutrophication are weighed against each other. This can be done qualitatively or quantitatively. Several evaluation methods have been developed. The methods that have gained most widespread acceptance are based on either expert/verbal systems or more quantitatively methods based on valuation factors calculated for different types of emissions and resources.

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Due to the fact that many important emission species from fires (i.e. dibenzodioxins, furans, PAH, etc.) are either not dealt with in detail or not available at all, these methods are not suitable for an objective interpretation of environmental impact in the Fire-LCA application. Thus, to date a qualitative comparison method has been found to be most beneficial. In some cases, the LCA analysis is followed by an interpretation phase where the results are analysed. This phase provides an opportunity for the discussion of the results in terms of safety aspects. The fact that people may die in fires and that products with higher fire performance cause a reduction in the number of fire deaths cannot be included explicitly in the LCA. This can, however, be discussed together with the results of the LCA analysis to provide a context for their interpretation and a connection to the reality of fire safety. In the Fire-LCA studies conducted to date the emphasis has been on emissions to air while emissions to water and soil have only been discussed briefly. The evaluation has been based on comparisons between the different cases, i.e. high and low fire performance or cable type for different emissions such as CO, CO2 , HCN, PAH, Dioxins, NOx , HCl, Antimony, HBr, hydrocarbons, Phosphorous and PBDEs and energy resource use. In addition the environmental effect of two species, i.e. PAH and chlorinated dibenzo-dioxins, has been discussed based on a comparison between these two species and their cancer risk. The comparison is based on the assignment of “Unit Risk Factors” (URF) which have been defined according to epidemiological studies [22]. Using this “unit risk” model one can compare the risk that a person exposed to the same quantity of different substances over his/her lifetime would have to develop cancer. Although this model is not directly applicable to the Fire-LCA studies it does provide a method by which the PAH and PCDD/F emissions can be reduced to a common denominator to make a coarse comparison between their relative importance. This relative importance is of interest due to the fact that TCDD/TBDD equivalents typically receive most attention as environmental toxins while, in many applications of the Fire-LCA model, PAH may actually pose the greatest environmental danger. One should, further, keep in mind that while the LCA model is based on information from single fire experiments the emission results are not point emissions but total emissions over the whole life-cycle of the product evaluated. Thus, the application of a general exposure model is not entirely inappropriate. The application of the Unit Risk Factor model requires that the PAH emissions be reduced to a single toxicity equivalence factor in essentially the same manner as for the TCDD and TBDD equivalents. These single toxicity equivalence factors are then compared. In the case of PAH the most toxic species to which all other species are reduced is benzo(a)pyrene, or BaP [23]. This species has been defined as the most toxic species and assigned a toxic equivalence factor of 1 in the same way that 2,3,7,8-tetrachloro-dibenzodioxin (2,3,7,8-TCDD) is defined as the most toxic of the polychlorinated dibenzodioxins and furans. All other species are then assigned toxic equivalence factors relative to BaP, allowing the calculation of BaP-equivalents. BaP is 20

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times less carcinogenic than the species 2,3,7,8-TCDD, the unit risk factor is 0.07 µg m−3 for BaP and 1.4 µg m−3 for 2,3,7,8-TCDD (the species).

11.6 Computer Modelling Methods Different computer software solutions for LCA calculations exist. Generally the software can be divided into two different groups: – Specific Life-Cycle Assessment programs (KCL-ECO, LCA Inventory Tool, SimaPro, etc.) – General calculation programs such as different spread sheet programs (Excel, etc.) In addition to the different LCA calculation programs several database structures for storage of LCA data and meta-data exist. Experience from previous Fire-LCA projects have shown that the use of a specific LCA software is a great advantage compared to other more general calculation software. Specific LCA software is a versatile tool for performing LCA studies. With LCA software you can easily build complex LCA system models and calculate results for the system. Such software can handle processes as well as transports and material flows between modules. Flows can be “feedback connected” and it is therefore easy to handle material recycling processes. LCA software is basically a program for solving linear equation systems. Non-linear processes can usually not be calculated in these programs. If necessary, non-linear processes can be calculated separately in other programs and inserted as constants. Specific LCA programs usually also contain modules for impact assessment calculations often with options for the calculation of classification and characterisation data. It is also possible to include sensitivity analysis and different valuation methods based on valuation factors such as Ecoscarcity, the Effect Category Method and the EPS-system.

11.7 Simplified Approach The full Fire-LCA (indeed any full LCA) requires considerable effort with the determination and collection of suitable LCI data being the most time consuming part of the study. In some cases a simplified approach could be preferable to the full LCA model approach to save time and money. Several simplification alternatives exist that can still provide an indication of the relative environmental impact of, for example, a certain FR treatment relative to that of the fires one avoids through the construction of high fire performance products. Common to all simplifications suggested here is that they only provide relevant information if they are used as a part of a comparison, i.e. between two alternative design approaches to the same product.

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11.7.1 Background Minimisation In this approach, all parts of the model that are the same in the two design approaches used in the comparison are excluded. This approach has the advantage that LCI requirements are generally significantly reduced. The main disadvantage, however, is that while one obtains interesting information concerning the relative importance of the specific design choices made one cannot obtain any indication of the relevance of these differences in the context of the total environmental impact of the product during its life-cycle. This is perhaps best illustrated by considering a simplistic and figurative example of a comparison between products A and B where emission of PAH for those parts of the model that differ only shows that product A emits 10 units of PAH while product B emits only 1 unit PAH (a factor between products A and B of 10:1). Should one include the full LCA data, however, one finds that products A and B have the same PAH emission (within the certainty of the model) as the background PAH emission from all the similar parts of the model is 106 units, reducing the factor between products A and B to 1:1. 11.7.2 Parameter Minimisation In both traditional LCA models and the Fire-LCA model one tends to include as many parameters as possible to obtain as detailed a treatment of the product as possible. One includes information concerning both CO, CO2 , PAH, acid gases, organic species, energy consumption, etc., emissions to air, water and soil. It is not unusual to have over 1,000 variables with a similar number of linear equations describing their interaction. Of these thousands of variables only very few are typically included in the final analysis of the environmental impact of the product design choices. One could potentially reduce the number of species included in the LCI to those species one knows, from experience, are most important in Fire-LCA applications. Experience from applications of the Fire-LCA model to date suggests that large organic species appear to be typically most important in this model. Similarly, if one is most interested in the emissions to air then one could reduce the extent of the model by considering only emissions to the air and not those to water and soil. 11.7.3 Scenario Minimisation In the applications of the Fire-LCA model conducted to date several scenarios have been investigated. The different scenarios include present day and future waste handling, different degrees of recycling and different interpretations of the fire statistics. These scenarios were chosen to investigate the result depending of the assumptions made in the model. To save time one can minimise the number of scenarios and investigate, e.g. only one scenario.

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11.8 Limitations While the Fire-LCA tool provides a good starting point for a holistic interpretation of a realistic life-cycle of a product including information concerning the probability that the product may be involved in a fire it does not provide information concerning, for example, the effect of the toxicity of chemicals used in the product, number of lives saved, costs associated with the different cases or the societal effect of manufacturing practice.

11.9 Conclusions Fire-LCA is an LCA method that incorporates fires as one possible end of life scenario. It was developed by SP and IVL to be able to assess life-cycle aspects of the fire performance of a product. The guidelines provided in this chapter are based on experience gained during the development and application of the Fire-LCA model. A great deal of input data is needed to conduct a Fire-LCA study. Very little fire emission data is reported in the literature. Only recently have detailed characterisation of fire emissions been conducted on a more regular basis in some laboratories. Much data is confidential. However, as the number of fireLCA studies and research on fire emissions increase, such data will become more readily available. It can also be difficult to find production data for some materials, although this problem is common to both Fire-LCA and traditional LCA applications. While the Fire-LCA tool provides a good starting point for a more holistic interpretation of a realistic life-cycle of a product including information concerning the probability that the product may be involved in a fire it does not provide information concerning, for example, the effect of the toxicity of chemicals used in the product, number of lives saved, costs associated with the different cases or the societal effect of manufacturing practice. The FireLCA concept would pose a much more powerful tool if these aspects could be included. This requires that a multivariate analysis method be developed which would potentially assist decision makers to fully evaluate all consequences of a change in regulations, the introduction of a new production method, a new product, etc. Full application of such a model would also require a significant amount of research into the toxicology of many of the emissions analysed within each model application.

References 1. M. Simonson, A. Boldizar, C. Tullin, H. Stripple, J.O. Sundqvist, The Incorporation of Fire Considerations in the Life-Cycle Assessment of Polymeric Composite Materials: A Preparatory Study. SP Report, 1998, p. 25

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2. M. Simonson, H. Stripple, The Incorporation of Fire Considerations in the Life-Cycle Assessment of Polymeric Composite Materials: A Preparatory Study, Interflam, 1999, pp. 885–895 3. M. Simonson, P. Blomqvist, A. Boldizar, K. M¨ oller, L. Rosell, C. Tullin, H. Stripple, J.O. Sundqvist, Fire-LCA Model: TV Case Study, SP Report, 2000, p. 13 4. M. Simonson, H. Stripple, Proceedings of the IEEE International Symposium on Electronics and the Environment, 2000 5. M. Simonson, H. Stripple, Flame Retardants 2000, 2000, pp. 159–170 6. M. Simonson, C. Tullin, H. Stripple, Chemosphere 46, 737–744 (2002) 7. M. Simonson, P. Andersson, L. Rosell, V. Emanuelsson, H. Stripple, Fire-LCA Model: Cables Case Study, SP Report 2001, p. 2 (available at http://www.sp. se/fire/br reports.HTM) 8. M. Simonson, P. Andersson, V. Emanuelsson, H. Stripple, Fire and Materials 27, 71–89 (2003) 9. P. Andersson, M. Simonson, L. Rosell, P. Blomqvist, H. Stripple, Fire-LCA Model: Furniture Case Study, SP Report, 2003, p. 22 10. P. Andersson, M. Simonson, P. Blomqvist, H. Stripple, Flame Retardants 2004, 2004, pp. 15–26 11. P. Andersson, P. Blomqvist, L. Rosell, M. Simonson, H. Stripple, The Environmental Effect of Furniture, Interflam, 2004, pp. 1467–1478 12. P. Andersson, M. Simonson, C. Tullin, H. Stripple, J. Olov Sundqvist, T. Paloposki, Fire-LCA Guidelines, SP Report, 2004, p. 43 13. Environmental Management – Life Cycle Assessment – Principles and Framework, ISO 14040:1997 14. Environmental Management – Life Cycle Assessment – Goal and Scope Definition and Inventory Analysis, ISO 14041:1998 15. Environmental Management – Life Cycle Assessment – Life Cycle Impact Assessment, ISO 14042:2000 16. Environmental Management – Life Cycle Assessment – Life Cycle Impact Interpretation, ISO 14043:2000 17. L.-G. Lindfors, K. Christiansen, L. Hoffman, Y. Virtanen, V. Juntilla, O.-J. Hanssen, A. R¨ onning, T. Ekvall, G. Finnveden, Nordic Guidelines on LifeCycle Assessment, Nord (Nordic Council of Ministers, Copenhagen (1995), p. 20 ohus 18. M. Erlandsson, Environmental Declaration, Villa v¨ axa D548, 121 m2 Myresj¨ ¨ AB, Tr¨ atek Milj¨ odeklarationer 9604038, TRATEK, 1996 19. B. Persson, M. Simonson, M. M˚ ansson, Emissions from Fires to the Atmosphere, Utsl¨ app fr˚ an br¨ ander Till atmosf¨ aren, SP Rapport, 1995, p. 70 (available in Swedish only) 20. O. Pettersson, S.E. Magnusson, J. Thor, Fire Engineering Design of Steel Structures, Publ. 50 (Swedish Institute of Steel Construction, Stockholm, 1976) (Swedish Edition 1974) 21. P. Thomas, Fire Safety J. 10(2), 101–118 (1986) 22. E.J. Spindler, Chemische Technik 49(4), 193–196 (1977) (available in German only) 23. I. Nisbet, P. LaGoy, Regul. Toxicol. Pharmacol. 16, 290–300 (1992)

12 Modelling of Euroclass Test Results by Means of the Cone Calorimeter P. Van Hees and J. Axelsson

Summary. An important part of introduction of the construction productive directive (CPD) is the introduction of the so-called Euroclasses for building products. The Euroclasses define the different fire classes within the European harmonised system. The major fire test method in this system for wall and ceiling linings, the so-called SBI test (EN 13823), is, however, an intermediate fire test and requires larger test specimens. This makes it difficult for industry to develop new innovative materials as large amounts of samples are necessary to run the test, which is also more cost expensive. In this chapter, a product development tool for this method will be discussed. The product development tool is a simulation model based on small-scale test data obtained in the cone calorimeter tests. First the model will be explained in this chapter. The next step is to show the validation of the model and give guidance on how to use the model for building materials.

12.1 Description of Model The model described in this chapter simulates the SBI test method results [1] by means of cone calorimeter results [2]. The SBI test method is the major method within the system of Euroclasses for wall and ceiling linings [3]. The model is available as a user-friendly software package. The calculation model [4, 5] presented here uses ignition time as well as the complete heat release rate (HRR) curve from the cone calorimeter. In principle, results from a single small-scale test are used to predict the first part of the HRR curve in the SBI and hence the FIGRA(SBI) index. Other models [6, 7] exist but for this project the above-mentioned model was used. The calculation model, based on the cone tools model [8] for the room corner, is described in detail below in sections containing principles, area growth, criterion for flame spread and HRR, respectively.

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12.1.1 Principles of Prediction Model Three major assumptions have been made in the prediction model of HRR in the SBI test: 1. The burning area growth rate and the HRR are decoupled. 2. The burning area growth rate is proportional to the ease of ignition, i.e. the inverse of the time to ignition in small scale. 3. The history of the HRR per unit area at each location in the SBI test is the same as in small scale. 12.1.2 Burning Area Growth Rate The fire spread can follow two different routes as shown in Fig. 12.1. All products start to spread along route I. A product is assumed to continue to spread along route III if the calculated sustained flame height is at least 1.5 m, which is equal to the height of the test sample. Otherwise the product is assumed to spread along route II. The calculation of flame height will be outlined below. Within the different flame spread regimes, the burning area growth rate of a product depends on ignitability, i.e. time to ignition in the cone calorimeter. Once the flame spread rate is determined, the HRR is calculated assuming that products always give the same HRR per unit area as a function of time in small scale as in the SBI test. In other words all parts of the tested product are assumed to burn in the same way in the SBI as in small scale. This is of course a simplification. The HRR depends more or less on the actual heat flux level received by the product as a function of time. However, the experience so far of the model shows that the errors average out and can be included in the empirical constants. 0.8 0.7

Area [m2]

0.6 III

0.5 0.4 0.3

II

0.2

I

0.1 0

0

100

200

300

400

500

600

Time [sec.]

Fig. 12.1. Burning area curve modelled for the SBI

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The flame spread of the product is described by an S-shaped curve, which is a function of time. The curve represents a step response of a second-order system. In the beginning of the test, the product ignites at one point on the test sample. This ignition time is assumed to be a time equal to half of the ignition time found in the cone calorimeter at 40 kW m−2 . Immediately after ignition, the area growth rate of the product is slow. The area growth rate will then accelerate, depending on the time to ignition in the cone calorimeter, until the involved area gets close to its maximum value. Then the area growth rate slows down again. The area growth rate is described by the following function      t t t − ign t − ign 2 2 exp − , (12.1) A(t) = Amax 1 − 1 + tign tign where Amax is the maximum area involved and tign is time to ignition in the cone calorimeter. In the beginning of the test, all products are assumed to follow the same area growth function. However, if the sustained flame height reaches the top of the test specimen, which is 1.5 m, then the maximum area in the area growth function changes. This is the only parameter that is changed when changing from one flame spread regime to another. The sustained flame height is a function of the calculated total heat release in the test as explained in the section about criteria for flame spread. The area growth function and the different values for the maximum area are empirically chosen. However, they agree very well with those observed during the SBI round robin test series at both SP and Danish Institute of Fire Technology (DIFT). The maximum area is assumed to be 0.35 m2 for the products, which do not have a sustained flame height of 1.5 m. This area is roughly equal to the area behind the burner flames. For products where the sustained flame height exceeds 1.5 m, the maximum area is 0.60 m2 . This maximum area is chosen based on the configuration of the SBI test. The burner has a side length of 250 mm and is positioned at a distance of 40 mm from the test specimen. If the flames were spreading to the top of the test specimen in the entire width of the burner, the maximum area should be 0.87 m2 . However, since the burner is triangular, the thickness of the flame varies. During the tests, it was observed that the flame leaned into the corner and in no way has a width identical to the width of the burner in its entire height. Using these areas as maximum areas for flame spread, the model gives good agreement with observations during tests. Some products will spread the flames more than 0.6 m2 before reaching their first peak in HRR. These are products with an extremely short ignition time in the cone calorimeter, or thermoplastic products, which create pool fires before reaching their maximum heat release. The model does, therefore, not give correct results for these types of products when it comes to peak heat

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release. But as will be shown later, the FIGRA(SBI) index for these products is predicted quite good by the model which is due to a good estimation of the initial inclination of the heat release curve. 12.1.3 Criteria for Flame Spread As shown in Fig. 12.1, the flames will spread either over a small area or over a larger part of the test sample. The criterion used in this model, to decide what the maximum area of the flame spread will be, is the sustained flame height in the corner. In the original cone tools model, the criteria used to determine if flames were spreading over a larger area than what was initially involved, was an assumed surface temperature. This assumed surface temperature depended on the temperature of the combustion gasses passing over the surface and on the thermal response of the product surface. This approach agrees well with the fact that there is a hot gas layer under the ceiling in the room corner. However in the SBI test, there is no ceiling and hence no hot gas layer to heat the product. The temperature of the surface of the product in the SBI test depends primarily on the radiation and convective heat transfer from the flame. Using the flame height to determine the maximum area over which the flames will spread is based on observations from SBI tests, and on the assumption that the part of the product behind the sustained flame will receive a heat flux from the flame sufficient to ignite that part of the product. In the SBI test the product will first ignite in the corner behind the burner flames. The involved area will then spread mostly upwards behind the flame from the burner. Depending on the burning behaviour of the product, the heat release from the burner and the product can be high enough to create a flame in the corner, which will have a sustained height equal to, or higher, than the height of the test sample. If this is the case the upward flame spread will continue to the top of the test specimen. The flame height in a wall corner geometry is given as [9]

where

H = 3Q˙ ∗2/3 , D

(12.2)

Q˙ ∗ = Q˙ total (D5/3 × 1, 110),

(12.3)

D is the diameter of the burner, which was assumed 150 mm considering that the burner is triangular. Using these expressions gave the criteria that the total heat release Q˙ total shall be greater than 59 kW if the flames shall spread to the top of the specimen. This criterion also agrees well with what was observed in the SBI tests.

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12.1.4 Calculation of Heat Release Rate The total heat release from the SBI test is obtained by summing up the contributions from each part of the total burning area and the burner Q˙ total = Q˙ product + Q˙ burner ,

(12.4)

where Q˙ burner is constant at 30 kW while Q˙ product varies with time as the fire spreads, the involved area A(t) increases as described above and the burning intensity at each position is time dependent. Q˙ product is obtained by adding the contributions from burning parts which have started to burn at various times. The HRR of the specimen at each location is then assumed to go through the same history as was measured in bench-scale, i.e. the cone calorimeter. Q˙ product is calculated using the Duhamel’s integral
t ˙ ˙ )q˙ (t − τ )dτ , Qproduct = A(τ (12.5) bs 0  is the heat where A˙ is the time derivation of the burning area, t is time, q˙bs release per unit area as recorded in the cone calorimeter and τ is a dummy variable. The following very simple numerical solution to the Duhamel’s integral is the approach used in this model  N −i Q˙ product = ∆Ai q˙bs , (12.6)

where ∆Ai is the incremental burning area growth at the time increment i, N −i is the HRR per unit area after (N − i) time increments as recorded and q˙bs in the cone calorimeter. 12.1.5 Correction for Cone Calorimeter Data Obtained at Other Heat Flux Levels The model has been developed to use cone calorimeter at a heat flux level of 50 kW m−2 . To be able to use the model also with cone results different from the preset value, a correction was introduced for both the ignition time and the HRR level. The correction is based mainly on fine-tuning the results tignCorr = tignCone (ConeFlux/SBIFlux), HRRCorr = HRRCone (SBIFlux/ConeFlux)0.5 , where tignCorr is the corrected ignition time used in the model, tignCone the ignition time in the cone calorimeter test, HRRCorr the corrected HRR, HRRCone the HRR in the cone calorimeter test, ConeFlux the flux level in the cone calorimeter test and SBIFlux the corresponding reference flux for the coneSBI model being 40 kW m−2 . It is understood that for ignition the correction is based on thermally thin theory but this has shown to give the best simulation results. The exponent for the HRR correction was determined in a similar way.

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12.2 Sensitivity Study of Model 12.2.1 Influence of HRR Threshold and Ignition Time The main input parameters to the model are the ignition time and the heat release curve. The HRR curve is automatically registered by a computer, but the ignition time is obtained from visually observing the experiment. In the project also a HRR threshold value was used to investigate whether it is possible to run this as an alternative. This is especially interesting for material with heavy flashing behaviour (FR materials) and for materials with low HRR levels where maybe even no ignition occurs. From the results in Table 12.1, it can be seen that a threshold of 10 kW m−2 can be used as alternative for a visual ignition time. Using 50 kW m−2 as a threshold gives mainly lower FIGRA values, i.e. results with a better classification. However, the use of a Table 12.1. Results of five materials with visual ignition time and HRR threshold values as input for the ignition time Material Cone test Ignition criterion

FIGRAa FIGRA0.2 FIGRA0.4 THR

CP1

Test 1

CP1

Test 2

CP2

Test 1

CP2

Test 2

CP3

Test 1

CP3

Test 2

CP4

Test 1

CP5

Test 1

383 254 383 255 185 255 79 NA1 79 47 NA1 47 62 NA1 59 46 NA1 34 NA2 NA1 27 NA2 NA1 19

Visual = 7 s HRR = 50 kW m−2 HRR = 10 kW m−2 Visual = 10 s HRR = 50 kW m−2 HRR = 10 kW m−2 Visual = 11 s HRR = 50 kW m−2 HRR = 10 kW m−2 Visual = 14 s HRR = 50 kW m−2 HRR = 10 kW m−2 Visual = 15 s HRR = 50 kW m−2 HRR = 10 kW m−2 Visual = 17 s HRR = 50 kW m−2 HRR = 10 kW m−2 Visual = None HRR = 50 kW m−2 HRR = 10 kW m−2 Visual = None HRR = 50 kW m−2 HRR = 10 kW m−2

274 176 274 197 135 197 67 NA1 67 43 NA1 43 44 NA1 41 36 NA1 27 NA2 NA1 25 NA2 NA1 0

77 58 77 79 63 79 32 NA1 32 27 NA1 27 0 NA1 0 14 NA1 11 NA2 NA1 17 NA2 NA1 0

0.61 0.61 0.61 0.68 0.68 0.68 0.58 NA1 0.58 0.61 NA1 0.61 0.45 NA1 0.45 0.53 NA1 0.53 NA2 NA1 1.1 NA2 NA1 0.72

NA1 not applicable since HRR is lower than the threshold (values would be zero). NA2 not applicable since no visual ignition occurred (values would be zero). a Without threshold level of THR, only HRR > 3 kW.

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HRR threshold value should also be done after studying the actual HRR curve. This can be done in the cone tools software package before the calculations are performed. From our experience it is also advisable to run a small sensitivity study on the ignition time to investigate whether it has a great influence on the result. If so it can be advisable to run at another flux level. This is mainly the case for materials with short ignition times. 12.2.2 Influence of Backing Board Figures 12.2 and 12.3 give the difference between a sample preparation with and without the standard backing board used in the SBI. It can be seen that this improves the quality of the simulation, especially in the second part of the SBI curve. It is hence advisable to use as often as possible a backing board or substrate identical to the one that will be used in the SBI test. 12.2.3 Shiny Materials The total heat flux towards the specimen in case of the cone calorimeter consists mainly of radiation (more than 90%). This means that materials with a shiny surface such as M4 and insulation material 2 in Table 12.2 will reflect a large part of the incident heat flux from the cone heater. In the SBI test, however, the radiation will be lower than in the cone calorimeter as a larger part of the incident heat flux is based on convection. Moreover, the materials will be sooted very fast and hence receive more radiation energy due

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Fig. 12.2. Simulation of particle board (M22) with backing board

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Fig. 12.3. Simulation of particle board (M22) without backing board

to an increase of the surface emissivity. This could be observed for the two above-mentioned materials. Without a sooted or painted surface the materials did not ignite at a heat flux level of 50 kW m−2 (insulation material 2) or showed a very high ignition time (M4) resulting, respectively, in a > B and C classification.

12.3 Guidance and Description Testing Protocol The following guidance can be given when preparing test specimens in the cone calorimeter: 1. Materials should by preference be tested at 50 kW m−2 unless very short ignition times (less than 5 s) are observed. In this case a lower heat flux level can be chosen. 2. The preparation of the sample should closely follow the mounting as in the SBI test. So it is advisable to run the materials in the cone calorimeter with the backing boards described in the SBI standard. 3. Shiny materials, e.g. materials with aluminium foil facing, should also be tested with the surface sooted or blackened by paint (with limited

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Table 12.2. Summary of simulation results for the cone-SBI model Material

M01 M02a M03 M04 M05a M06 M07a M08 M09 M10 M11 M12 M13 M14 M15 M16 M19 M20 M21 M22 M23 M24 M25 M26a M27 M28 M29 M30 Eurefic3 Ceiling P1 Ceiling P2 Ceiling P3 Ceiling P4 Ceiling P5 Insulation 1 Insulation 2 Insulation 3 a

FIGRA0.2 (mW s−2 ) 28 262 1, 554 2, 109 1, 212 0 428 37 147 675 60 592 47 96 0 335 10 361 6 473 430 450 421 734 42 21 153 2, 236 476 236 55 34 25 0 1, 404 767 448

Simulation too severe.

FIGRA0.4 (mW s−2 ) 6 262 1, 554 2, 109 1, 212 0 428 24 114 658 35 592 28 88 0 335 7 361 3 473 430 450 421 734 36 3 127 2, 227 476 78 30 6 17 0 1, 052 747 448

THR (MJ)

0.5 0.6 19.0 0.6 26.0 0.4 0.6 0.35 1.1 6.0 0.35 23.9 0.35 4.9 0.1 0.6 0.35 0.6 0.35 33.6 0.6 31.0 36.7 35.0 0.35 0.35 1.45 0.6 3.2 0.65 0.60 0.49 1.1 0.59 7.6 4.5 16.7

Euroclass Euroclass according to according to simulation test result ≥B D E E E ≥B D ≥B C D B D ≥B ≥B ≥B D ≥B D ≥B D D D D D ≥B ≥B C E D C ≥B ≥B ≥B ≥B E D D

≥B >B E E D ≥B >B ≥B C D B D ≥B ≥B ≥B D ≥B D ≥B D D D D E ≥B ≥B C E D C ≥B ≥B ≥B ≥B E D D

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combustibility, e.g. heat flux metre paints). The results may in this case be more conservative, but will allow a better overall prediction. 4. If very short ignition times (less than 5 s) are obtained at the cone heat flux level, it can be advisable to reduce the heat flux level in the cone calorimeter.

12.4 Comparison and Discussion of Simulation Results The data given here are all SBI RR materials (except cables and pipes), one Eurefic material (used as market place material in the SBI project), five ceiling panels and three insulation materials. More information on the type of materials is available in the literature [10]. From the results in Table 12.2 it can be seen that a satisfactory prediction tool has been developed. The marked materials are those where a wrong classification is obtained. In two cases the materials are melting products (M2 and M7). Here some more research is needed to try to improve the model if possible. In the two other cases, the results are so-called borderline results (M5 and M26). The results show that an easy-to-use model is available which is of interest for industry in their product development. Figures 12.4–12.7 give a number of examples of simulations which are related to paper, leather and textile applications. As it can be seen there is a good prediction of the HRR in the SBI apparatus. 40 35 data 30 hrr (kW)

25 20 15 10 Conetools 5 0 0

100

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400

500

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Fig. 12.4. Simulation of a textile wall covering on gypsum plaster board

12 Modelling of Euroclass Test Results 18 16 14 data hrr (kW)

12 10 Conetools 8 6 4 2 0 0

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Fig. 12.5. Simulation of a paper wall covering on gypsum plaster 90 80 70

data

hrr (kW)

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40 30 20 10 0

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Fig. 12.6. Simulation of a paper wall covering on particle board 16 14

hrr (kW)

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Fig. 12.7. Simulation of a textile wall covering on silicate board

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12.5 Conclusions The implementation of Euroclasses for wall and ceiling linings introduced a new intermediate scale test method called SBI method (single burning item). Since the method requires larger samples, product development will be difficult for industry if no easier way can be found. In this chapter, a small-scale test method, the cone calorimeter, together with a mathematical model has been presented. The model gives satisfactory prediction results for a wide number of building materials and can also be used for textiles, paper and leather wall and ceiling covers.

References 1. EN 13823, Reaction to Fire Tests for Building Products. Building Products Excluding Floorings Exposed to the Thermal Attack by a Single Burning Item, CEN, February 2002 2. ISO 5660, Fire Tests – Reaction to Fire – Rate of Heat Release from Building Products, International Standards Organisation (ISO), 1991 3. EN 13501-1, 2001 E. Fire Classification of Construction Products and Building Elements – Part 1: Classification Using Test Data from Reaction to Fire Tests, European Committee for Standardization (CEN), Brussels, Belgium, February 2002 4. B. Messerschmidt, P. Van Hees, U. Wickstr¨ om, Prediction of SBI (Single Burning Item) Test Results by Means of Cone Calorimeter Test Results, Interflam Proceedings, Interscience Communications, London, 1999, pp. 11–22 5. P. Van Hees, The Need for of a Screening Method for the Major Euroclass Methods, Flame Retardants, Conference Proceeding, 2002 6. T. Hakkarainen, Correlation Studies of SBI and Cone Calorimeter Test Results, Interflam Proceedings, Interscience Communications, London, 2001, pp. 519–530 7. A. Steen Hansen, P. Hovde, Prediction of Smoke Production Based on Statistical Analyses and Mathematical Modelling, Interflam Proceedings, Interscience Communications, London, 2001, pp. 113–124 8. U. Wickstr¨ om, U. G¨ oransson, J. Fire Mater. 16, 1992 9. B. McCaffrey, Flame Height, The SFPE Handbook of Fire Protection Engineering, 2nd edn., Chap. 2-1, NFPA publications, 1998. 10. P. Van Hees, A. Steen Hansen, Development of a Screening Method for the SBI and Room Corner Test Based on the Cone Calorimeter, vol. 11, Nordtest Project 1479-00, SP Report, Bor˚ as, 2002

Part IV

Applications of Multifunctional Barriers

13 Characterisation of Barrier Effects in Footwear R.M. Silva, V.V. Pinto, F. Freitas, and M.J. Ferreira

Summary. Footwear is designed to provide comfort, pleasure and protect feet from hard and rough surfaces, as well as climate environmental exposure and in some cases aggressive conditions like protective footwear. Actually, consumers expectations and needs demand development of footwear that integrates fashion, emotional desires and real multifunctional performance namely barrier effect to water and other liquids, thermal insulation, fire resistance or microorganisms resistance. Regarding barrier effects upper materials and outsoles are the most important contributors in footwear because they are directly in contact with environmental and aggressive external conditions protecting the foot. In the past, leather was used for every part of the shoe, but today it is largely confined to the upper. Relatively to outsoles, actually, they are based in a large range of materials raging from elastomers as natural and synthetic rubbers to thermoplastic rubbers and polyurethanes. Specific compositions and functions of those materials provide different barrier effects for each application. A synthetic review of the barrier effects related with the two major components of shoes (upper and outsoles) and with complete footwear is presented. The proof of new concepts namely multifunctional barrier footwear requires suitable testing methodologies to be chosen and implemented. Actual methodologies and standards for the characterization of defined footwear materials, components and complete footwear are described.

13.1 Introduction Consumers and industrial activities increasingly demand goods and services that are tailored to their specific needs and tastes. Footwear industry followed these trends which lead to a wide variety of materials (textile, plastics, rubber or leather) and products from casual footwear to technical products as protective footwear. Footwear may be defined as: “all articles with applied soles design to protect or cover the foot. . . ” [1], normally consisting of an upper exterior part with lining and a sole with a heel. The footwear parts, according to EN ISO 20345:2004(E), are given in Fig. 13.1.

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Key 1 2 3 4 5

Facing Tongue Collar Upper Vamp lining

6 7 8 9 10

Insock Toecap Outsole Cleat Penetration-resistant insert

11 12 13 14

Insole Heel Quarter Vamp

Fig. 13.1. Parts of footwear adapted from [2] Table 13.1. Materials randomly used in footwear parts Upper 1 2 3 4 13

Material

Facing Tongue Collar Leather, textile, synthetic Upper Quarter

Sole 8 9 12 10 7

Material

Outsole Cleat Rubber, polyurethane, Heel thermoplastic R PR insert Metal, Kevlar Toecap

The materials normally used for the production of the footwear parts are schematised in Table 13.1. Regarding safety, protective and occupational footwear for professional use is very important to control the barrier effects that influence the complete footwear performance. There are several methodologies for evaluation of the barrier effects. This chapter intent to make a brief review of the available methodologies for footwear barrier effects evaluation. A special attention will be addressed for the applications and methods related to the evaluation of the waterproof, antimicrobial, antibacterial and thermal behaviour barrier effects.

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13.2 Upper Part: Leather In Europe, leather is the most commonly used material for the upper part of shoes and for this reason will be addressed in this section. Animal skin constitutes a barrier between the organism and the external world. That function of the skin is decisive for its histological and physiological properties. Skin is one of the principal adaptation organs of the organism, almost impermeable to water, aqueous solutions and pathogenic micro-organisms. Nerve ends contained in it are receptors of touch (pressure), heat, cold and pain stimuli. From the physical point of view, skin reveals a specific viscoelastic behaviour permitting to avoid small injuries (because of its suppleness) and adaptation to change in shape and size of the body (in view of its stretching and shrinking ability). From the chemical point of view, the components of skin reveal a lower adaptation feature. The dye contained in the epidermis (or in the hair coat) protecting the organisms against the effects of electromagnetic radiation, can serve as an example [3]. In looking for a covering material for himself, his hut and food, primitive man turned either to large leaves from plants or to the skins of the animals he killed. The latter are usually chosen for clothing as they were bigger, stronger and warmer. However, they had three main defects [4]: (1) They are damp. (2) If left in a wet and especially warm climate they soon started to putrefy. (3) Dried skins lost their and softness, and became very hard brittle. Hides and skins are turned into leather by “tanning”. There are many ways of tanning, but all of them cause the following changes in the raw hide or skin: (1) Tanned skin does not putrefy, even after drying and wetting. (2) On drying, the tanned skin does not become a hard, brittle material, but remains flexible and workable. Chosen of tannage method is largely concerned with how soft or hard, tight or stretchy, the resultant leather should be. A definition of leather is “a material which is resistant to putrefaction and enzymatic destruction and after repeated wetting drying returns back to its former soft characteristics”. This has been valid for a long time, and this is what the tanning process is all about. Everything in the leather making process – from the preliminary work in the beamhouse, through the tanning, re-tanning, fatliquoring/softening and finishing process – must be oriented to this objective [5]. However, there are many definitions, and another general description that is appropriate is: “a material formed from a network of collagen fibres of hides and skins, prepared by appropriate chemical and physical processes to the properties necessary for its final use” [6]. Tannage therefore has to change the properties of collagen, either by chemical reaction or by covering the fibres against outside influences.

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Making of leather had its origin in the antiquity of man as an empirical development. It is generally conceded to have been the first manufacturing process of man. Today leather through its many uses has become a most essential commodity of man [7]. 13.2.1 Leather Tanning Leather industry may be regarded as a bridge between productions of the hide as a by-product of the food industry and its manufacture into shoes and wearing apparel, for which its provides a basic raw material. Technologies and skills involved in the production of meat and those required in the production of usable goods from leather are widely different. Production of leather is a long and complicated process, and certainly not one which can be embarked upon successfully without specialisation skills. Tanning is the process of converting unstable raw hides into leather, with adequate strength properties and resistance to various biological and physical agents. Tanning is a process of introducing a tanning agent into the hide. This is accompanied by introduction of additional cross-link into collagen, which binds the active groups of the tanning agents to functional groups of the protein (COO− and NH+ 3 ). However, not all tanning procedures give rise to clearly defined bonds. To what extent the resistance to micro-organisms is a criterion of tanning remains an open question. It is true that tanned leather is very resistant to putrefaction, although that is largely a matter of humidity. In tanning industry, it is possible to find several types of tannage: – – – – – – –

Chrome tannage Aluminium tannage Titanium tannage Zirconium tannage Vegetable/synthetic tannage Aldehyde tannage Oils tannage

Chrome tanning is the most common type of tanning in the world. Chrome tanned leathers are characterised by top handling quality, high hydro-thermal stability, user-specific properties and versatile applicability. Waste chrome from leather manufacturing, however, poses a significant disposal problem [8]. 13.2.2 Water Resistance Barrier Effect Water resistance of leather is an important property to several applications, like footwear and clothing. To improve this property several leather making process and leather surface modifications has been applied.

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Leather Water Resistance Collagen fibres that make leather are hydrophilic and to obtain leather water resistance several approaches can be made [9]: 1. Sealing the leather with an impermeable layer of polymer. 2. Closing the spaces between the leather fibres with water-repellent substances such as greases – closed waterproof. 3. Creating a hydrophobic net around the fibres without filling spaces – open waterproof. Hydro-phobisation is a leather modification to make it water resistant and water-tight with reduced water uptake. Leather can be made waterproof using modern tanning methods. There are four main methods [10]: – Grease impregnation is a long established system, and gives a special look and feel to the leather. The grease needs to be re-applied many times during wear. Attaching the soles with adhesives is very difficult! – Silicone impregnation works quite well at first but requires constant treatment to retain its properties, a spray can be found in most footwear retail and outdoor shops. – Fluorcarbon impregnation during the fatliquoring process in the tannery is very effective. – Coating the leather. A foil or thin laminate of waterproof synthetic can be attached to be surface of the leather by adhesive. Silicones may be applied from hydro-carbon solvents on the dry leather by dipping or spraying or a silicone emulsion may be applied in the drum on the wet leather by a fatliquoring technique and the emulsifying agent discharged subsequently by multi-valent metal ions such as aluminium, zirconium, etc. Silicones have very high interfacial tensions relative to water and these are not very temperature sensitive. Fluorcarbons are applied from solvent solutions and have equally high water repellency and also oil repellency. Sealing the leather with a waterproof coating, i.e. a heavy polymer finish, could be one approach, but this detracts from natural appearance of the leather and reduces water vapour permeability (breathability), which is one of the leather’s key advantages. If the waterproofing agent has been applied only to the surface, the barrier it creates is likely broken once the leather is flexed, thus allowing water to penetrate. So it is not only important to obtain good penetration into the centre of the leather, but also that waterproofing should be evenly distributed and coat the fibres to reduce their natural hydrophilic tendency. The choice of waterproofing system is dependent on the degree of water resistance required, the purpose for which the leather is intended and price.

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R.M. Silva et al. Table 13.2. Standard methods for leather water resistance evaluation

Standard

Methods

EN ISO 20344: 6.13: 2004(E) ISO 5403:2002 EN 13518:2001 DIN 53338 IUP 10: 2000 ASTM D 2099: 2000

Determination of leather water resistant in dynamic conditions – Bally Penetrometer

Determination of the dynamic water resistance of shoe upper leather by the Maeser water penetration tester ISO 2417 IULTCS/IUP 7:2002 Determination of leather water resistance in IUP 7:2000 static conditions DIN 53330 Draft IUP 45: 2002 Determination of the water penetration pressure of leather

Test Methods To evaluate the water resistance of upper leather material several standards methods can be used. In Table 13.2 the mainly used test methods for static and dynamic conditions are presented. Standards EN ISO 20344:6.13: 2004(E) [11], ISO1 5403: 2002 [12], EN2 13518: 2001 [13], DIN3 53338 [14], IUP4 10: 2000 [15] specify a method for determining the dynamic water resistance of leather using a Bally Penetrometer. Figure 13.2 presents the Bally Penetrometer test method which is applicable to all flexible leathers but is particularly suitable for leathers intended for footwear uppers. In these tests a piece is formed into the shape of trough and flexed whilst partially immersed in water. The time taken for water penetrates through the test piece is measured. The methods also allow the percentage mass of water absorbed and the mass of water transmitted through the test piece to be determined. The Bally Penetrometer as well as the more stringent Maeser test simulates the dynamics shoe upper leather is subjected to during each step. 50,000 Maeser flexes, taken over a period of about 8 h, is the equivalent of more than 70 km of treading in water (Fig. 13.3). Although the test procedures seem very similar, they are not interchangeable. The test method described in standard ASTM5 D 2099:2000 [16] covers the determination of the dynamic water resistance of shoe upper leather by 1 2 3 4

5

ISO – International Standard Organisation. EN – European Standard. DIN – Deutsches Institut f¨ ur Normung. IUP – International Union of Leather Technologists and Chemists Societies – Physical test methods. ASTM – American Society for Testing and Materials.

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Fig. 13.2. Bally penetrometer test method R by Zipor, email: [email protected]) (courtesy of Pegasil

Fig. 13.3. Maeser test method R by Zipor, email: [email protected]) (courtesy of Pegasil

the Maeser water penetration tester. It is applicable to all types of shoe upper leather. The flex imparted to the leather is similar to the flex given the vamp of the shoe in actual wear. Certain waterproof processes can cause contamination of the stainless steel balls. When this happens, visual inspection is recommended. This test method does not apply to wet blue. Water resistance of leather can be measured in static conditions using the standard ISO 2417 IULTCS 6 /IUP 7:2002 [17], DIN 53330 and IUP 7:2000 [18]. For this test, a leather test piece is immersed in water for a defined time from 30 min to 24 h. Although test conditions seem to be the least stressing ones, this is the only test method where the cut face is in contact with water. 6

IULTCS – International Union of Leather Technologists and Chemists Societies.

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R.M. Silva et al. Table 13.3. Guidelines for leather water resistance tests

Test Method

Standard

CTCP guidelines

Bally water permeability

EN ISO 20344: 6.13: 2004(E) DIN 53338 EN 13518: 2001 IUP 10: 2000 ISO 5403: 2002

Water absorption after 60 min – Max 30% Water penetration after 60 min – Max 0.2 g

Maeser water permeability ASTM D 2099: 2000

Medium resistance – 1.000– 5.000 cycles Good resistance – 5.001– 10.000 cycles Excellent resistance – superior to 10.001 cycles

IUP 45: 2002 [19] standard describes a method for determining the water penetration pressure of leather. A sample of leather is clamped over a container of water with the surface of the leather in contact with the water. The water pressure is raised at a specified rate and the pressure required to force droplets of water through the leather is measured. In Table 13.3 the specifications to evaluate the water resistance of leather normally used in footwear constructions for some of the referred methods, according to CTCP7 guidelines are presented. 13.2.3 Flame Resistance Barrier Effect Flaming combustion can be roughly divided into physical and chemical processes taking place in each of three separate phases: gas, mesophase and condensed, which are schematised in the left- and right-hand side of Fig. 13.4, respectively. Chemical processes are responsible for the generation of flammable volatiles while physical changes, such as melting and charring, can markedly alter the decomposition and burning characteristics of a material [20]. The most important physical and chemical processes taking place in each of the phases during the flaming combustion are described in Table 13.4. Thermal Behaviour of Leather When heat is applied to leather, physical and chemical changes undergo in this material leading to undesired transformations. An important change is the thermal degradation, that according to American Society for Testing and Materials (ASTM) [22] is defined as “a process whereby the action of heat 7

CTCP – Centro Tecnol´ ogico do Cal¸cado de Portugal.

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Fig. 13.4. Representation of physical and chemical processes occurring in flaming combustion. (1) Fuel rich limit; (2) Combustion zone; (3) Fuel lean limit. Adapted from [21] Table 13.4. Important processes occurring in flaming combustion Processes Physical –

–

Chemical

Energy transport by radiation and – convection between the gas phase (flame) and the mesophase; – Energy loss from the mesophase by mass transfer and conduction into – the solid.

Thermal degradation of the material in a thin layer surface; Mixing of volatile pyrolysis products with air, by diffusion; Combustion of the fuel–air mixture in a combustion zone.

or elevated temperature on material, product, or assembly causes a loss of physical, mechanical, or electrical properties”. Thermal degradation of hydrated collagen, or a sort of leather, in the temperature range 20–400◦ C occurs through two successive processes accompanied by mass loses, related with [23, 24]: – Dehydration – Thermo-oxidative degradation The first process is endothermic and takes place in the temperature range of 25–125◦ C. It is attributed to collagen dehydration and is characterised by shrinkage of the leather when heated at a defined temperature. The second process is exothermal and consists of the decomposition and thermo-oxidation

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of dry material. Some volatile products with low molecular mass are released during this process. Thermal properties of collagen and collagen-based materials as leather depend on procedure used for obtaining the collagen, the operating conditions of tanning, the water content and the deterioration resulted by natural or artificial aging. Tanning, which is mainly a cross-linking process, introduces reactive sites in leather that increase thermo-oxidative rate. Similarly, the oxidative reactivity of polymeric carbon materials increases with the increased degree of substitution obtained by cross-linking [23]. Water content is related with the hydration degree of collagen. Humidity changes during the time in saturated water vapours were followed by Budrugeac et al [24] and maximum hydration capacity and time for reaching hydration equilibrium were established, as represented in Fig. 13.5. Deterioration of leather is related with the denaturation of the collagen through it hydrothermal stability, of which shrinkage is the macroscopic manifestation, together with the temperature at which the phenomenon occurs. Denaturation is defined as a transition from the triple helix to a randomly coiled form, taking place in the domains between the cross-links. The bonds which stabilise the superhelix are hydrogen, hydrophobic, van der Waals bonds and interactions between oppositely charged residues on side chains. All these non-covalent bondings break down on heating [25]. Fire Retardants As described in previous issue, it is impossible to make leather completely resistant to charring and decomposition when exposed to flame or to high

Fig. 13.5. Time dependence of the humidity of collageneous matrices in saturated water vapour conditions [24]

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temperature, but a degree of flame resistance can be achieved. There are basically four treatment methods to produce fire-retardant materials [26]: – – – –

Chemical change Pressure impregnation Coating Impregnation

For leather, since it is a very porous material, normally treatment is achieved through impregnation by dipping or spraying. According to the flame-retardant type they are classified as “durable” or “non-durable”. Non-Durable Flame Retardants The nondurable flame retardants are water soluble inorganic salts. Their low cost in achieving effective flame retardancy assures their use in application not subject to leaching. In the production of upholstery leather where exposure to aqueous leaching in normal product use is most unlikely, the non-durable flame retardants are becoming increasingly important. The main non-durable flame retardant used in textile applications are listed in Table 13.5. Durable Flame Retardants Durability in the context of this discussion of flame retardants is a concept intended to describe only the ability of the flame retardant to withstand leaching with water and, to some extent, dry-cleaning solvents. The most successful durable flame retardant has employed a urea-phosphate treatment. Other compounds have sometimes been combined with the urea phosphate, i.e. cynamide, ammonium sulphamate, chlorinated paraffin wax or antimony oxide, as well as stabilisers for outdoor military applications. A major step in Table 13.5. Typical non-durable flame retardants Typical non-durable flame retardants Borax Boric acid Ammonium borate Diammonium phosphate Sodium phosphate dodecahydrate Ammonium sulphamate Ammonium bromide Sodium phosphate Ammonium molybdate Sodium tungstate Zinc chloride Sodium stannate

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the development of durable flame-retardant treatments for cotton rayon, and other cellulose fabrics was the discovery of Tetrakis (hydroxymethyl) phosphonium compounds (THP), which has great applications in these flameretardant finishing markets (Table 13.6). Despite of the development of the THP flame retardants the global market of flame retardancy in commercial goods has a large number of compounds that could be considered as alternatives. Brominated flame retardants are one of the most important and represent 15% of the global flame-retardant market. Flame Resistance Test Methods and Characterisation Methodologies Flammability of leather must be tested to demonstrate compliance with governing regulation. Laboratorial tests on samples with a small ignition flame have traditionally been used to assess the flammability performance of clothing materials. There are several flammability test methods for sheet materials but, generally, reaction-to-fire tests involve supporting a cut sample on a frame and applying a flame from a burner, between 3 and 15 s, depending on the standard. These same tests may have similar applications to leather. The test methods that could be used for experimental evaluation of leather flammability are schematised in Table 13.7. Information from leather manufacturers indicate that ignition tests as the cabinet method (ALCA8 method 50) or the flame resistance test for textile fabrics (ISO 15025:2000) are the most commonly used for fire resistance 8

ALCA – American Leather Chemical Association.

Table 13.6. Durable flame retardants Chemical name

Producer

Pyrovatex Cp THP chloride THP sulphate Decabromodiphenyoxide and Antimony trioxide

Ciba-Geigy Albright & Wilson Albright & Wilson White Chemical Corp.

Table 13.7. Standard methods for leather flame resistance determination Standard

Methods

ISO 15025: 2000

Protective clothing – protection against heat and flame – method of test for limited flame spread Cone calorimeter Measuring the minimum oxygen concentration to support candle-like combustion of plastics (oxygen index) Fire resistance of leather

ISO 5660-1:2002 ASTM D 2863: 2000 ALCA method E50

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Fig. 13.6. Flame resistance test method, according to ISO 15025:2000

determination of leather materials (Fig. 13.6). But Oxygen Index Method is probably a more informative and useful test. Standard ISO 15025:2000 [27] describes a method for the measurement of limited flame spread properties of vertically oriented textile fabrics and industrial products, when subjected to a small defined flame. Specimens are oriented vertically, and a defined flame from a specified burner is applied to the specimen surface for 10 s, first horizontally and after evaluation to the bottom edge of specimens. Afterflame and afterglow times are recorded. Other issues must be evaluated as whether any flaming reaches the upper edge, the occurrence of debris, flaming debris (if applicable), or whether a hole develops. Cone calorimeter is one of the most used small-scale fire test methods for the prediction of fires and for fire test results. This method is described in ISO 5660-1:2002 [28]. Specimens with an area of 100 × 100 mm are positioned on a load cell, and expose to an adjustable heat flux from 10 to 100 kW m−2 . Heat release rate (HRR) due to combustion is determined using an oxygen consumption methodology, which is derived from the observation that the net heat of combustion is directly related to the amount of oxygen required for combustion. Lost mass of the test specimen is also recorded as well as time to ignition and smoke production [29]. Oxygen Index Test Method (LOI) test method described in standard ASTM D 2863:2000 [30] was designed to overcome specific drawbacks to previous combustion methods. This test is based upon the use of a specific mixture of oxygen and nitrogen which is fed through a glass-bead bed into a glass chimney. Sample is placed vertically and ignited at the upper end with a natural gas flame. The end-point is that the sample burns for 3 min or up to a defined mark at 50, 75 or 100 mm. A severe disadvantage of LOI is the lack of correlation with heat release results, which are described in literature as the most important descriptor of the fire behaviour [31].

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American Leather Chemists Association test method ALCA E50 uses a vertical Bunsen burner test and a 45-degree micro-burner test for light leather, especially upholstery leather. The primary objective of this test is to evaluate afterflame, afterglow, char length and weight loss of the leather test specimen. Specimen must be weighed to the nearest 0.01 g and the thickness measured nearest 0.001 in. Results are determined by measuring the time from the removal of the burner flame (a 12-s burning time) until the afterflame and afterglow have ceased. Finally the vertical char length is measured and the weight loss determined. However, phase determination of the burning process involved with particular chemical action on leather in terms of flame retardancy demands mechanistic information that only can be obtained with more sophisticated thermal-analysis equipment, but the operation of this type of equipment is many times more expensive and complex then previous test methods [32]. To evaluate and quantitatively measure leather fire-retardant properties several methodologies can be applied. Differential Scanning Calorimetry Differential Scanning Calorimetry (DSC) is a thermal analysis technique that measures the heat flow to the sample that is required to maintain a temperature equivalent to a reference cell. DSC allows heat flow determination, over the employed temperature range, and to obtain enthalpy changes which illustrate endothermic or exothermic conditions, since at DSC constant pressure conditions heat flow is equivalent to enthalpy changes. In an endothermic process, such as most phase transitions, heat is absorbed and heat flow to the sample is higher than that to the reference. In an exothermic process, such as crystallisation, some cross-linking processes and oxidation reactions the opposite is true and heat flow is negative. These enthalpy changes are caused by phase changes or chemical reactions. In DSC test procedure, both sample and reference material are kept at the same temperature during the linear temperature program, and the heat of reaction is measured as the difference in heat input required by the sample and the reference material. The system is calibrated using standard materials, such as high purity metals (indium, lead, tin and zinc at 99.99%), and high purity organic compounds (as cyclopentane, cyclohexane, n-alkanes and hexatriacontane) with well-defined melting temperatures and heats of fusion [33]. Manich et al. [34] determined the oxidation temperature of water saturated fatliquored leather, as produced in tanning industry. They achieved that nonisothermal DSC gives a fast and reliable method of testing the oxidability of the press-cut fatliquored leather samples, through the determination of onset temperature of thermo-oxidation and the oxidation energy released between 195 and 265◦ C.

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Thermo-Gravimetric Analysis The desire for a quantitative analytical laboratory test that correlates fire behaviour or flame test performance with material properties has been the motivation to relate thermo-gravimetric analyses to flammability. To date, TGA is the most commonly used thermal decomposition method. In TGA experiments, the sample is brought quickly up to the desired temperature (isothermal procedure) and the weight of the sample is monitored during the course of thermal decomposition. However, although thermo-gravimetric studies give important information about the decomposition process, they are incapable of simulating themselves the thermal effects due to large amounts of material burning and supplying energy to the decomposition materials at different rates. Determination of flammability relies therefore on a single thermal stability parameter (e.g. char yield or thermal decomposition temperature) to relate the chemical composition of a material to its fire or flame test performance (e.g. char yield vs. limiting oxygen index). Individually, these thermal stability parameters have found limited success as material descriptors of flammability and their interrelationship in the context of flaming combustion has remained obscure until recently, when it was shown that a particular combination of thermal stability and combustion parameters could correlate fire behaviour [35]. Thermal Insulation Test Methods The heat resistance of leather depends on its insulating properties and its resistance to high temperatures. To have good heat-resisting properties, a material must be made of a substance that does not readily conduct heat. Air is a very poor heat conductor and so a material that contains many air spaces is a better insulator than a solid. Leather has both these advantages: the fibres do not readily conduct heat, and they are interlaced with air spaces. The thermal insulating power is a primary property of leathers. The thermal insulating power is expressed with thermal resistance that is directly proportional to the thickness and reverse proportional to the thermal conductivity of sample. Thermal resistance is a measure of material’s resistance to transferring heat through its thickness. This property has long been known to be a critical factor in influencing foot comfort. Several methods have been developed to determine the thermal insulating properties of products made of leather (Table 13.8). European standard EN ISO 6942: 2002 [36] specifies two complementary methods (methods A and B) for determining the behaviour of materials for heat protective clothing subjected to heat radiation. In method A a specimen is supported in a specimen holder and is exposed to a specific level of radiant heat for a specific time. After exposure, a visual assessment of any changes in

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R.M. Silva et al. Table 13.8. Standard methods for leather thermal insulation evaluation Standard

Method

EN ISO 6942: 2002

Evaluation of materials and materials assemblies when exposed to a source of radiant heat EN 13519: 2001 Footwear – test methods for uppers – high temperature behaviour ASTM D 2214: 2002 Determination of thermal conductivity of leather using the Cenco-Fitch apparatus

the material after the action of the heat radiation is performed. For method B a specimen is supported in a specimen holder and is exposed to a specific level of radiant heat. Times for temperature rising of 12 and 24◦ C in the calorimeter are recorded and expressed as radiant heat transfer indexes (t12 and t24 ). Transmitted heat flux density (Qc ) and incident heat flux density (Qo ), in kW m−2 , are calculated. Heat transmission factor (TF (Qo )) is also calculated from the ratio of the transmitted to the incident heat flux density. The levels of incident heat flux density should be chosen from the following levels: – Low level: 5 and 10 kW m−2 – Medium level: 20 and 40 kW m−2 – High level: 80 kW m−2 The European Standard EN 13519: 2001 [37] specifies a test method for determining the effect of heat on the tensile strength of uppers or complete upper assembly irrespective of the material, to assess the suitability for the end use. In this test method the specimens are pressed between to hot rigid surfaces for a predetermining time. The effect of this heat treatment on breaking strength and elongation is then determined in accordance with EN 13522:2002. Another method that could be used in leather thermal conductivity determination is the test method presented in standard ASTM D 2214:2002 [38], using a Cenco-Fitch apparatus. This standard method allows the quantitative determination of the thermal conductivity of leather. The measured parameters are the area, the thickness and the temperature difference between the two sides of a leather specimen. This test method is not limited to leather, but may be used for any poorly conductive material such as rubber, textiles, and cork associated with the construction of shoes. Specimens up to 13 mm thick may be run. This test method does not apply to wet blue. 13.2.4 Micro-Organisms Resistance Barrier Effect Leather Micro-Organism Resistance Leather is particularly susceptible to the actions of micro-organisms and will be stained and weakened by them. As a by-product, fungi can produce organic acids that will corrode and etch inorganic materials.

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Mould growth contributes to the deterioration by increasing the humidity and preventing the leather from drying. Direct effects of mould growth include damage to the grain of the leather and stains that cannot be removed without damaging the grain. Often the first indication that micro-organism problem exists is a characteristic musty odour. A careful visual examination will generally locate stains that are clearly visible as pigmentations on a surface. Another means of detection is by the use of ultraviolet (UV) light. Under UV light, a micro-organism growth will appear luminescent [39]. In the leather industry there has been a continued interest in the development of new antifungal compounds and especially compounds which have the dual behaviour of being bactericide and fungicide [40]. Different antifungal agents used in leather industry are presented in Table 13.9. Degradation of leather results from the activity macro- and microorganisms on raw hides, during the leather manufacture and also during storage of finished leathers and products. Test Methods Bio-deterioration is an important factor impairing aesthetic, functional and other properties of leather. It takes place particularly under conditions of high relative humidity that enable bacteria, actinomycetes or fungi to grow. Micro-mycetes, or moulds, belong to the most dominant group of microorganisms responsible for the degradation of bio-polymers and other organic materials. The methods used for the evaluation of bio-deterioration caused by micro-mycetes have many variations. In a recently article Orlita [41] proposed three basic methods for bio-deterioration evaluation described as: – Naturally contaminated or artificially inoculated samples are incubated in a temperature-controlled chamber maintained at 28–37◦ C and 90–100% relative humidity (RH). After a period of time, usually 4–8 Table 13.9. Antifungal agents used in leather industry in last 20 years [41] Phenolics CMC – para-chloro-meta-cresol OPP – ortho-phenylphenol TCP – 2,4,6-trichlorophenol Heterocyclic compounds TCMTB – 2-(thiocyanomethylthio) benzothiazole OITZ – 2-n-octylisothiazolin-3-one BMC2 – benzimidazolyl-methylcarbamate MBTP – 2-mercaptobenzothiazol sodium pyrithione Others DIMTS – diiodomethyltolyl-sulphone

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weeks, the degree of growth is evaluated, its intensity characterising the degree of resistance or susceptibility to the micro-organisms being tested. The degree of deterioration is evaluated by assessing physical or chemical properties. – Samples are buried in soil, where they are exposed to a complex biocoenosis of soil and climatic factors such as rain, temperature change, etc. Bio-deterioration is assessed in terms of physical and mechanical properties. – Samples are placed on agar medium lacking a carbon source and incubated under optimum conditions for 28–56 days. The mineral requirements of the test organisms can be met by using Czapek–Doxagar (CDA) minus its carbohydrate component. Also the mould test [42] could be used to determine the micro-organisms growth. A sample is divided into its individual materials, which are laid on an agar–agar substrate for micro-organisms, at a constant temperature of 30–37◦ C, and stored in a damp atmosphere for seven days. After incubation, a semi-quantitative evaluation grades them as zero, light, strong or extreme contamination with mould. Two methods normally used to test textile materials [42] can be used to test the antimicrobial finishes in leather and are presented in Table 13.10. Standard method ASTM D 4576:2001 [43], “Standard Test Methods for Mould Growth Resistance of Wet Blue” is used for the determination of mould growth resistance of wet blue subject to storage and shipping requirements and intended for use in leather manufacturing. This test method may not be suitable to evaluate fungicides that are inactivated by proteins. In this test, wet blue test specimens are surrounded by but not covered with agar, inoculated and incubated (Fig. 13.7). After various incubation periods, mould Table 13.10. Tests to evaluate antimicrobial finishes in leather Semi-quantitative DIN EN ISO 20645: 2005 AATCC 147:2004

Quantitative DIN EN 1276: 2002–05 ASTM E 2149-2001 AATCC 100: 2004 ASTM E 2180: 2001

This agar diffusions test establishes the minimal inhibitory concentration necessary to prevent the growth of a specific indicator strain. Various concentrations of antimicrobial substances are sprinkled on to filters or samples placed on homogenous agar cultures. After incubation, the germ free area around the filters or samples is measured. In this test (Challenge test), samples with and without antimicrobial substance are treated with a specific test germ suspension. The fluid is immediately washed off one portion of the test bed, whilst incubation is allowed to take place on the remaining samples before they are also washed off. The amount of germs on each sample can then be compared to quantify the effectiveness of the antimicrobial finish.

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x

x

247

x

Fig. 13.7. Specimen with inoculum location shown (x) [43]

growth is rated as a percentage of the wet blue surface covered by mould. Then the resistance to mould growth of the wet blue test specimen is determined by comparison with wet blue of known resistance characteristics (the control) that is tested simultaneously.

13.3 Rubber Outsoles Rubber, thermoplastics (PVC – polyvinyl chloride, TPU – thermoplastic polyurethane soles, TR – thermoplastic rubber), leather, polyurethane are used for footwear soles. Taking in consideration the physical and barrier properties of these materials, rubber is in general the more performing and will therefore be detailed in this section. All European rubber production is of synthetic rubber. According to International rubber study group, in 2004 European production of synthetic rubber was of 2,871 thousand tonnes, representing 24.1% of worldwide production. There are several different rubber types, being styrene-butadiene rubber (SBR) the most widely used representing more than 50% of the European total production, followed by butadiene rubber (BR), ethylene-propylenediene rubber (EPDM) and acrylonitrile butadiene rubber (NBR) who represent about 30% of production. For example, in Germany, around 3% of the synthetic rubber is used for sole material production [44]. Despite of other materials as leather, thermoplastic rubber, polyurethane are also used for the production of outsoles, rubber outsoles still be the one which present better physical properties and suitable resistance for barrier effects. These characteristics make rubber outsoles the main material used in the production of outsoles for safety footwear. 13.3.1 Flame Resistance When polymers, as rubber, are subjected to heating or burning conditions, several processes start to occur, as random chain scission, chain stripping, cross-linking or charring. Due to their combustible properties, a special attention is needed for their fire-safety, regarding end-use product applications. To

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achieve the desired fire performance several modifications in their composition must be performed in different aspects as ignition, flame spread, heat production as well as smoke and gases production. Burning behaviour of rubber compounds depends on the combustibility of the individual formulation components (e.g. plasticisers, fillers and processing additives). Flammability is also strongly influenced by the type and the level of cross-linking. According to these, two strategies could be used for minimising the problem: first strategy involves preventing or, at least, minimising the likelihood of ignition. Since, in practice, it is not possible to completely eliminate ignition, the second strategy involves managing the impact of a subsequent fire [21]. 13.3.2 Test Methods Flame Resistance There is no universal agreement on the definition of flammability tests and how they are different from fire tests. Fire safety codes and regulations are generally based on two strategies: – Action in ignition – Managing fire To estimate fire behaviour of products under real conditions, small and intermediate scale tests determine some key parameters as: ignitability, heat release, flame spread, smoke production, charring rate and mechanical properties that are considered to be representative of real fire conditions. This broad group of parameters that are analysed bring some problems in comparing results from different test methods that are mainly [45]: – – – – –

Parameters that not correlate Different fire exposure levels Different types of exposures Limitations of applicability Behaviour of joints and fixings

To give a general idea of methods applied in the evaluation and quantitative determination of the parameters that represent rubber fire-retardant properties, a description of the most accurate and currently used methods is presented (Table 13.11). European standard EN 15090:2006 specifies the minimum requirements and test methods for the performance of three types of footwear for use by fire-fighters for general-purpose rescue, fire rescue and hazardous materials emergencies. Test pieces shall be taken from the whole footwear but samples of the material, as rubber outsoles, may be used if noted in the report. Regarding flame resistance, EN 15090:2006 describes a flame resistance test method in accordance with EN ISO 15025:2000. After exposure to flame

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Table 13.11. Standard methods for rubber flame resistance determination Standard

Methods

EN 15090: 2006 UL 94

Footwear for fire-fighters Flammability of plastic materials for parts in devices and appliances SS 162222: 1986 Rubber and thermoplastic elastomer – Evaluation of flame resistance

for 10 s samples shall neither flame for more than 2 s (afterflame time) nor glow more than 2 s (afterglow time). Samples should be tested so that the minimum distance to top of the burner is 17 mm and the angle between the test piece and the horizontal plane 45◦ . Underwriters Laboratories Inc. Test Method UL 94 is intended to be used solely to measure and describe the flammability properties of materials, used in devices and appliances, in response to heat and flame under controlled laboratory conditions. The actual response to heat and flame of materials depends upon the size and form, and also on the end-use of the product incorporating the material. Assessment of other important characteristics in the end-use application includes, but is not limited to, factors such as ease of ignition, burning rate, flame spread, fuel contribution, intensity of burning, and products of combustion [46]. Swedish Standards Institute Standard SS 162222 describes a methodology for evaluation of flame resistance of rubber and thermoplastic elastomers and is the only standard that is directed to rubber and thermoplastic materials flame resistance determination, but the principles are the same as the previous two standards. This standard is very similar to ISO/R 1326:1970 and the apparatus similar to ALCA E50. Sample is positioned vertically inside a box and a Bunsen burner with a diameter of 1 cm is applied for 12 s. Results are determined by measuring the time from the removal of the burner flame until extinction of the afterflame and afterglow. As in leather test methods, apparatus of the referred standards are very similar. The main differences lie on the burning time, and the energy of the flame, being the results very subjective and of difficult comparison. Thus, the desire for a quantitative analytical laboratory test that correlates fire behaviour or flame test performance with rubber properties lead to the application of other techniques and methodologies, described in next items: Cone Calorimetry As referred, most polymers as including the manufactured on commercial scale, are inflammable materials. Therefore, the studies on polymer flammability and methods of its retardation have been for years carried out in many research centres. Rybi´ nski et al. [47] determined the flammability of NBR by the method of cone calorimeter. Specimens, with a dimension of 100 × 100 mm were

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tested in horizontal position, according to ISO 5660-1:2002 [28], with a radiant heat flux density of 35 kW m−2 . During testing the following parameters were recorded: initial specimen mass, ignition time, specimen mass during testing, exhaust gas temperature and pressure, O2 , CO2 and CO concentrations in tested exhaust gas, as well as extinction coefficient, final specimen mass and test length. They verified that cone calorimeter parameters characterise the behaviour of butadiene–acrylonitrile copolymers under fire conditions, above described, and show a considerably hazard as compared to that of the commonly used polymers such as polyethylene or polypropylene. Also, Lyon et al. [48] used a fire calorimeter operating on the oxygen consumption principle to measure the mass loss rate, smoke generation, heat release rate and total heat release of polyurethane and polyphosphazene rubbers containing 20% expandable graphite. A forced flaming combustion was performed at a coldwall external radiant heat flux of 50 kW m−2 , according to standard method ASTM 1354-90. Duplicate samples of each formulation having approximate dimensions of 10 × 10 × 0 : 4 ± 0.1 cm were cut from full density rubber sheets and tested for heat release rate, total heat release, smoke and CO2 and CO yield. Results indicate that in flaming combustion a polyphosphazene rubber had a four times lower peak heat release rate than polyurethane rubber. The addition of expandable graphite flakes to these rubbers reduces their peak heat release rate by a factor of seven for polyurethane and of five for polyphosphazene rubbers. Differential Scanning Calorimetry As a synthesis of the described previously, DSC monitor heat effects associated with phase transitions and chemical reactions as a function of temperature. A special case in which the temperature of a phase transition is of great importance in polymers is the glass transition temperature, Tg . This is not a true phase transition but this is the temperature at which the polymer is converted from a glassy solid state to an elastic phase, the reason from which polymers are called elastomers [49]. Pruneda et al. [50] performed the thermal characterisation of NBR/PVC blends by means of DSC. They determine Tg values for nitrile rubber (NBR) and NBR/PVC blends of −62 and −57◦ C, respectively, verifying that NBR/PVC blend has a lower Tg , which is a consequence of the fact that result because PVC and NBR being a miscible system. Also Yehia and collaborators [51], used the DSC method to evaluate the compatibility of some technically polymer blends, namely butadiene rubber (BR) with natural rubber (NR), NR/NBR and polychoroprene rubber (CR) with NBR. In another work from Janowska [52], DSC measurements were carried for assessing thermal properties of nitrile rubber before and after their swelling in solvents such as benzene, toluene and dimethylformamide. It was verified that a slight increase in the glass transition temperature, Tg , due to the nitrile rubber cross-linking manifests itself in the cooling process.

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Thermo-Gravimetric Analysis Thermal stability of polyurethane, vinyl-substituted silphenylene-siloxane polymers, phenoxy-p-ethylphenoxy polyphosphazene (PZR), expandable graphite polyurethane rubber and expandable graphite polyphosphazene rubber was determinate by Lyon et al. [48] using thermo-gravimetric analysis (TGA). Weight loss vs. temperature was recorded at a constant heating rate of 10–20 k min−1 , under nitrogen. Also, Castrovinci et al. [53] used TGA-FTIR for analysing the thermal degradation of SBR. They verified that degradation of styrene and butadiene blocks proceeded by random scission of polymer chains in a broad temperature range from 350 to 540◦ C. Pyrolysis–Gas Chromatography Rubbers are frequently filled with opaque materials like carbon black, making them difficult to analyse by spectroscopy. Furthermore the cross-linking make them insoluble in most the organic solvents many of the traditionally used for organic analysis making difficult or impossible to analyse the rubber components [54]. Pyrolysis is a popular technique to study rubbers because of the ease with which a complex polymer product may be introduced into an instrument like a gas chromatograph. In pyrolysis–gas chromatography (py–GC) sample is subjected to elevated temperatures sufficient to break bonds, degrading the molecules. The key in analytical pyrolysis is to select a temperature at which samples degrade to produce decomposition volatile products. Most of analytical pyrolytic systems work is done using set points between 500 and 800◦ C [55]. Decomposition products are injected into the carrier gas flow to the GC column. A detector placed at the end of column will respond to the composition gases and if the separation is successful, the detector output will be a series of peaks. Recently, two standards were presented for the rubber analysis by pyrolytic gas: ISO 7270-1:2003 and ISO 7270-2:2005. In ISO 7270-1:2003 a method for the identification of polymers, or blends of polymers, in raw rubbers and in vulcanised or unvulcanised compounds from programs, obtained under the same conditions, is specified. This method applies first and foremost to single polymers and just allows qualitative identification of single rubbers or blends with exceptions discussed later. When the pyrogram indicates a characteristic hydro-carbon, the method is also applicable to blends. The method may be also applicable to other types of polymer, but this must be verified by the analyst in each particular case. ISO 7270-2:2005 specifies the principles and procedures for determining, by pyrolysis and subsequent gas chromatography, the styrene (STY)/butadiene (BD)/isoprene (IP) ratio in copolymers, or blends of homopolymers and/or copolymers, in raw rubbers or vulcanised or unvulcanised compounds.

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The use py–GC with mass spectrometric detection (MS) is routinely used for the characterisation and analysis of polymers, including cross-linked rubbers. For example, Hiltz [56] has described a py–GC method for the identification of NBR. The method is based on the identification of compounds in the pyrolysate that can be attributed to areas of the copolymer rubber where acrylonitrile and butadiene molecules are adjoined. Also, Phair and Wampler [57] reported an overview of py–GC–MS results for a wide variety of rubber and rubber-like materials, including polyisoprene, polybutadiene, SBR copolymers and polydimethylsiloxane. On the other end Choi [58] used py–GC to investigate the differences in the rubber composition of the bound and compounded rubber in detail and for comparing characteristics of pyrolysis pattern of SBR with different micro-structures (different ratios of styrene and different butadiene units: 1,2, cis-1,4 and trans-1,4) [59]. Thermal Insulation Test Methods As described previously a material that has good heat resistance properties, must be a material that is a poor heat conducer. This property protects industrial workers or fire fighters that may be exposed to relatively low heat intensity over a long period of time or in some cases to high heat intensity for very short periods of time. The test methods for the heat resistance determination of outsole materials are schematised in Table 13.12. The standard test method EN ISO 20344:8.7:2004(E) [11] describe a method for the visual assessment of the effects on soling materials of short-term contact with a hot surface (Fig. 13.8). This method enables the assessment of the suitability of soiling materials for footwear which is used in situations where brief contact with hot objects is likely. Test piece is placed in a platform below with its wear side uppermost, and then it is covered with aluminium foil to prevent contamination of the heated bit. When the bit temperature exceed 300◦ C is necessary to switchoff the heating block and allow the temperature fall to 300◦ C, with the bit still resting on its insulating support. Then the insulating support is moved aside and the bit centrally on the test piece is immediately placed, so its sides are parallel to the side of the test specimen. Test piece is removed from the support after being left in position for 60 s without switching the heating block. Foil is removed to allow the test piece cool at least 10 min and then examined the surface that had been heated. Table 13.12. Standard methods for resistance to hot surface contact Standard

Methods

EN ISO 20344:8.7: 2004(E) Determination of resistance to hot contact ISO 188-8.1: 1998 Accelerated ageing in air and heat resistance test NFPA 1971-6.8: 2000 Test methods: Conductive heat resistance test two

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Fig. 13.8. Resistance to hot contact test method R by Zipor, email: [email protected]) (courtesy of Pegasil

Standard method ISO 188-8.1: 1998 [60] describes a method to test the accelerated ageing in air and heat resistance of rubber, vulcanised rubber and thermoplastics. In this method, the test pieces are subjected to the same temperature as they would experience in service and, after definite periods, appropriate properties are measured and compared with those of unaged rubber. The oven is heated to operating temperature and the test pieces are placed inside. When using a cell-type oven, only one rubber or compound should be placed in each cell. Test pieces must be stationary, free from strain, freely exposed to air on all and not exposed to light. US National Fire Protection Association Standard NFPA 1971-6.8: 2000 [61] describes a heat resistance test method for protective footwear sole. In this method, specimens are preconditioned and placed in an iron plate. The plate should be heated to a temperature of 500◦ C, and this temperature is maintained for 30 s. Other In this issue some standards related with methodologies for determination of other barrier effects in rubber outsoles that have not been yet described are presented. Slip resistance For the evaluation of slip resistance of footwear parts (footwear sole, heel and related materials) ASTM F695-01 [62] is presented. In this standard, a

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footwear outsole and a flooring surface are brought in contact under a predefined force. To quantitatively measure the resistance to relative movement in contact (slip resistance), a dimensionless property is determined, according to test data: coefficient of friction. Resistance to Liquids The action of a liquid on a vulcanised rubber may result in: absorption of the liquid, extraction of soluble constituents from the rubber or a chemical reaction with the rubber. Those effects can profoundly alter physical and chemical properties. International Standard ISO 1817:2005 [63] describes the methods necessary for determination of the properties considered representatives of the described effects, namely: – Change in mass, volume and dimensions – Extractable matter – Change in hardness and tensile stress–strain properties after immersion and after immersion and dying

13.4 Complete Footwear 13.4.1 Water Resistance Barrier Effect Footwear Water Resistance Penetration of water through leather from the outside to the inside of the shoe depends mainly on the wettability of leather fibres, which varies with the tannage. Vegetable-tanned leather soaks up water because the fibres are relatively easy to wet, but chrome-tanned fibres resist water. Average upper leather is usually more water resistant than the shoe construction because water most often enters at the leather edger through the upper stitching. There is an increasing demand for waterproof leather for shoes, garments or bags in the market. Unfortunately, leather itself is inherently hydrophilic, and most re(tanning) agents and fatliquors are good dispersants, foes of any sort of water repellency. And following the general trend, the demand for the quality of water resistance is rising while simultaneously ecological considerations limit the number of available chemical products. Where a high water resistance is required, such as in tramping boots, the leather is usually stuffed with fats and oils in the tanning processes although silicon-based materials give similar effects. Public requirements of water resistance in everyday footwear are very moderate and are usually met by treated leathers. Actually, the application of waterproof membranes between the upper and lining has been a good approach to obtain water resistance footwear. For years though the possibility to providing waterproof footwear in a range of different materials and constructions was a problem waiting to be solved. Various ideas

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Table 13.13. Standard methods for whole footwear water resistance evaluation Standard

Method

EN ISO 20344:5.15.1:2004(E) EN 13073:2001

Determination of resistance to water for whole footwear – a pair of footwear is worn whilst a measured number of paces is walked over a surface flooded with water to a measured depth. The extent of water entry is determined by inspection. Machine method Gore Centrifugal Tester A test piece of known mass or volume is immersed in water for a known period of time and the volume of water absorbed measured.

EN ISO 20344:5.15.2:2004(E) Gore-Tex
Footwear ISO 2417:2002 (E)

were used, one being the plastic sack or lining which was incorporated in ladies boots for winter use. These fully prevented the ingress of water, providing the membrane was not breached but equally, prevented the dissipation of foot moisture. This permeable membrane technology sow its first apparel applications in outdoor clothing such as ski and foul weather gear and it is only in more recent years that it has been applied to footwear. It was initially a slow starter but now appears to be gaining ground strongly. The commonest membranes are micro-cellular plastics, either in foil form (for laminating to textiles) or coatings cost directly onto a fabric. Most are based on polyurethanes (PUs), although web-like PTFE is widely used. The other type comprises solid, hydrophilic polymers, again as foils or direct coatings. PU is again the commonest, although polyester is also found as a foil. These generally tend to be thinner than micro-cellular types. Test Methods Different methods can be applied to test water resistance of whole footwear; Table 13.13 presents the test methods normally used. Standards EN 13073:2001 [64], EN ISO 20344:5.15.1:2004(E) [11] and specify a test method for the determination of the water resistance of footwear, irrespective of the material. In this test a pair of footwear is worn while a measured number of paces are walked over a surface flooded with water to a measured depth. The extent of water entry is determined by inspection. After 100 trough lengths step out the footwear it is carefully removed and the inside is examined visually and by touch for signs of water penetration. If any penetration has occurred, the position and extension should be recorded on diagrams for each boot or shoe (Fig. 13.9). The standard EN ISO 20344:5.15.2:2004 (E) [11] describes another type of method to test the whole footwear. In this method the whole

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Fig. 13.9. Trough test method

Fig. 13.10. Machine test method R by Zipor, email: [email protected]) (courtesy of Pegasil

footwear in a defined depth of water is subjected to the mechanical action of rotating wetted brushes (Fig. 13.10). The extent of water penetration is determined by examination. The shoe is fixed in a rectangular metal plate with a fixed jaw at one extremity and a sliding jaw at the other to adjust to the height of shoe. Then two brushes situated one on either side of the test piece, that are adjusted to the size of shoe, describes a backwards and forwards motion over the whole length of the test piece. The horizontal motion of each brush is completed by rotational movement, the direction of which changes at the end of each horizontal cycle. The direction of rotation of each brush is the same as the corresponding backwards or forwards motion. The number of brushes and injectors should be adjusted considering the type of test that is being tested (Table 13.14). The horizontal distance between the two brushes system should be adjusted so that the whole surface of the footwear upper is contacted by the bristles. And the water tank should be filled until the water level is 20 mm

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Table 13.14. Spraying parameters Footwear (according to EN ISO 20345:2004(E))

Number of water injectors

Number of brushes

1 1 2 3 3

1 1 1 2 2

Low shoe Ankle boot Half kneeboot Knee height boot Thigh boot

Table 13.15. Guidelines for footwear water resistance tests Standard

Guidelines

EN ISO 20344:5.15.1:2004 (E) The total area of water penetration after 100 trough lengths shall be not greater than 3 cm2 . EN ISO 20344:5.15.2:2004 (E) Water penetration shall not occur before 15 min

above the top surface of the test piece support. The constant level device should be adjusted to maintain this depth. The guidelines established for the standard EN ISO 20344:5.15.1.:2004 and EN ISO:5.15.2.:2004 are summarized on Table 13.15. R Footwear method is another test that can be used to determine Gore-Tex the water resistance of footwear. This method use a machine that works like a centrifuge, where is placed absorbent paper in the shoe support and on absorbent paper shoe with water inside. Then the machine initiates a predefined cycle of 90 min, at the end of the test any water should be observed inside the shoe. While the penetration of liquid water should be prevented, water vapour should pass the leather as freely as possible, or at least be absorbed, to enable good acclimatisation of the shoe interior. These two possibilities to get rid of excess water vapour are reflected in the definition foe the water vapour permeability rate, which is the most suitable experimental value for the specification of wearing comfort. The standard method EN ISO 20344:6.6:2004(E) [11] is used to test the water vapour permeability (Fig. 13.11). In this method the test piece is fixed over the opening of a jar, which contains a quantity of solid desiccant. This unit is placed in a strong current of air in a conditioned atmosphere. The air inside the container is constantly agitated by the desiccant, which is kept in movement by the rotation of the jar. The jar is weighted to determine the mass of the moisture that has passed through the test piece and has been absorbed by the desiccant. The standard EN ISO 20344:6.7:2004(E) [11] is able to determine the water vapour absorption (Fig. 13.12). In this method an impermeable material

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Fig. 13.11. Water vapour and permeability (EN ISO 20344:6.6:2004(E)) R by Zipor, email: [email protected]) (courtesy of Pegasil

Fig. 13.12. Water vapour absorption (EN ISO 20344:6.7:2004(E))

and the test piece are clamped over the opening of a metal container, which holds 50 ml of water, during 8 h. Test piece is then weighted immediately and the water absorption determined by the mass difference before and after the test. When the leather is used as upper material and tested in accordance with standard EN ISO 20344:6.6:2004(E) and EN ISO 20344:6.8:2004(E) the water vapour permeability shall be not less than 0.8 mg cm−2 h−1 and the water vapour coefficient shall be not less than 15 mg cm−2 . Another method that could be used to determine the water vapour R PM47 [65] permeability and absorption is the standard method SATRA (Fig. 13.13). method is used to determine the amount of water vapour an assembly or a single material will absorb and transmit through its structure in a specified time. In this test an assembly of circular test specimens is clamped across the open end of a vertical cylindrical chamber containing a specified volume of warm water. After a set time the mass of water absorbed by, transmitted through, the test specimens is measured. The test is designed to give a measure of the ability of a material to remove perspiration from the skin. The water absorption should at least 3.5 mg cm−2 h−1 .

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R Fig. 13.13. Water vapour permeability and absorption test method (SATRA PM47)

13.4.2 Flame Resistance Demands in safety at work for workers directly exposed to flames require the development of protective footwear with flame resistance properties. To evaluate the degree of protection introduced by this barrier effect, there are available a few test methods for whole footwear. Two test methods used for the evaluation of the flame resistance of complete footwear are schematised in Table 13.16. For whole footwear flame resistance evaluation applies the EN 15090:2006 procedure, as described previously. Additionally in this situation, assessment of the state of the footwear afterflame exposure must be performed. Footwear for fire-fighters shall be failed if there is: deep cracking affecting half of the upper material thickness, deformations or burns in upper material, cracks higher than 10 mm long and 3 mm deep in outsoles, upper/sole separation of more than 15 mm long and 5 mm wide or cleat height in flexing area lower than 1.5 mm. US National Fire Protection Association Standard NFPA 1971-6.5:2000 [61] describes a flame resistance test method for whole boots protective footwear. In this method three complete footwear items are tested. Specimens are mounted in the support assembly and a dimensioned flame is applied during 12 s at defined angles. Afterflame and afterglow times are determined also as burn-through, melting or dripping. 13.4.3 Thermal Resistance Barrier Effect Thermal Insulation A great deal of attention has been paid recently to leathers designed for production of protective footwear worn under “hot” conditions. During winter the body seldom generates enough heat to create perspiration with the result that there is no cooling effect [66]. In this case the millions of tiny voids existing within the matrix of leather fibres provide thermal insulation [67]. The amount of thermal insulation required from a material depends on final intended use. Items assigned for cold climate need to be good insulators, while summer wear should encourage heat to dissipate.

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Table 13.16. Standard methods for whole footwear flame resistance evaluation Standard

Methods

prEN ISO 15090: 2006 NFPA1971-6.5: 2000

Footwear for fire-fighters Flame resistance test four

Table 13.17. Standard methods for whole footwear thermal insulation Standard

Method

EN ISO 20344:5.13:2004 (E) EN 12784:1999 EN ISO 20344:5.12:2004 (E)

Determination of insulation against cold in whole footwear Determination of insulation against heat in whole footwear Heat and thermal shrinkage resistance test: Specific requirements for testing footwear

NFPA 1971 6-6.14:2000

Effective heat insulation plays a major part in many footwear applications: from industrial boots to carpet slippers. As the outer material of a shoe is generally defined by factors such as fashion or durability, the lining usually has to provide the insulation. Since no fibre or polymer can match the insulation potential of still air, the primary requisite for any warm lining must be ability to trap air within its structure. Pile fabrics and foams are therefore favourite materials. However, when the lining is compressed, some air will be removed so compressibility of the material is also important. Test Methods The protective footwear sometimes is subject to very high or very low temperatures, so it is necessary to evaluate the capability of footwear to resist to these two extremes situations. Determination of insulation against cold and determination of insulation against heat is an essential point for protective footwear. Several standards methods are presented in Table 13.17. The determination of insulation against cold according to test method described in EN ISO 20344:5.13:2004(E) [11] is used for protective footwear worn on cold surfaces (Fig. 13.14). The footwear is filled with metal ball bearings and conditioned at 23◦ C. It is then placed on a copper plate in a cold box at −17◦ C and left for 30 min. A temperature sensor placed inside the footwear on the insole records the fall in temperature. In this type of test there is no heating supply and the temperature difference is continually changing so it is not possible to determine a conventional insulation value. The recorded result is simply the fall in temperature after 30 min. A small decrease indicates high insulation properties. Following the specification of standards EN ISO 20345:2004(E) [2], EN ISO 20346:2004(E) and EN ISO 20347:2004(E), for safety, protective and occupational footwear,

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Fig. 13.14. Determination of insulation against cold (EN ISO 20344:5.13:2004(E)) R by Zipor, email: [email protected]) (courtesy of Pegasil

respectively, the temperature decrease on the upper surface of the insole shall be not greater than 10◦ C. Another standard that could be used for the measurement of insulation against cold of footwear is the standard EN 12784:1999 [68]. The apparatus used in this standard is similar to the apparatus used in EN ISO 20344:2004(E) the only difference between this two methods is the temperature of the cold box during the test (in this case −20◦ C). Occupational applications, from bread making to foundry work often require hot items to be moved by hand. Hot contact test are therefore important to determine that protective clothing and footwear provide adequate heat transmission. Determination of insulation against heat according to test method described in EN ISO 20344:5.12:2004(E) [11] is used for protective footwear worn on hot surfaces (Fig. 13.15). The footwear is filled with metal ball bearings and conditioned at 23◦ C it is then placed in a plate at 150◦ C or 250◦ C depending on the properties claimed by tested footwear. The recorded result is simply the increase in temperature after 30 min. A small increase indicates high insulation properties. Following the specification of standards EN ISO 20345:2004(E) [2], EN ISO 20346:2004(E) [69] and EN ISO 20347:2004(E) [70], for safety, protective and occupational footwear, respectively, the temperature increase on the upper surface of the insole shall be not greater than 22◦ C. The principle of the hot contact test is to apply a heated cylinder incorporating a thermal sensor to a piece of material and to monitor the temperature rinse of a sensor placed behind the sample. US National Fire Protection Association Standard NFPA 1971 6-6.14:2000 [61] describes a heat and thermal shrinkage resistance test method for whole boots protective footwear. In this method, footwear specimens shall be size 9. Three specimens shall be filled

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Fig. 13.15. Determination of insulation against heat (EN ISO 20344:5.12:2004 (E)) R by Zipor, email: [email protected]) (courtesy of Pegasil

with dry vermiculite. Specimen marking and measurements shall be conducted in accordance with the procedure specified in AATCC 135. 13.4.4 Chemical and Micro-Organism Resistance Barrier Effect Footwear Odour and Test Methods The chemical and physical tests made on shoes and their component materials are well known. Less is known about microbial testing, although this also provides information on the quality of the materials and the resulting shoes. Micro-organism cover bacteria, viruses, yeast, fungi (mould), algae and other microscopic life forms. Testing can identify and isolate different germs in or on a product and establish the degree of contamination. One concern about micro-organism in footwear is allied with the odour generated during the use of shoes. Feet can smell as the foot sweats and it is trapped inside footwear. It is the interaction of these two factors along with bacteria that cause the smell. Feet have more sweat glands than any other part of the body, so they can sweat profusely which cannot evaporate due to being enclosed in footwear. The bacteria produce isovaleric acid which is what causes the odour. The equation that sweat equals smell is inaccurate: sweat itself has no discernible scent. However, when it reacts with bacteria that are naturally present all over our skin, including on our feet, an unpleasant odour is released. Bacteria multiply more quickly in warmer weather, which explains why foot and shoe odour is often more a problem in summer than the winter. Several approaches can be adopted to avoid the problem: – Keep the foot cool and dry – Reduces the humidity – Reduces the debris which is food for micro-organisms

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– Using antimicrobial treatments which inhibit the growth of bacteria and fungi Another approach could be remove the moisture after it has formed. Many products could be used to absorber the volatile species (e.g. active carbon). To test the odour in footwear materials we can adapt the standard method GMW3205:2000, Test Method for Determining the Resistance to Odour Propagation of Interior Materials. This test method normally shall be used to determine the odour propagation of automotive interior materials when subjected to elevated temperatures and high humidity. To test the footwear components, it is necessary perform some modifications as the temperature used that it is too high. Resistance Against Chemicals and Micro-Organisms Test Methods Other aspect-relevant issues are the footwear resistance against microorganisms using the sterilisation test and test the footwear resistance against chemical products, using the degradation and permeation test. The standard EN 13832-1:2006 [71] specifies the test method for footwear to protect the user against chemicals and/or micro-organisms and defines terms to be used. Inside this standard we have three test methods: degradation test; permeation test and sterilisation test. In degradation test the test pieces are placed in a vessel for degradation resistance of footwear components with a chemical chosen for the test to depth. Then the apparatus should be maintained at the standard laboratory temperature 23 ± 2◦ C for 23 ± 1 h. After this, the liquid is removed and the test piece released. Any surplus liquid should be removed from the surface of the test piece, the test piece should be washed with a large amount of water using a wash flask and the test piece is dried by wiping with absorbent paper or a textile fabric which does not deposit lint. The basic physical properties of the footwear component (upper and sole) are checked before and after contact with chemicals. The standard EN 13832-2:2006 [72] specifies the requirements for footwear highly protective against chemicals. The levels of requirements are: – For tear resistances (upper and soles), the materials are acceptable if after degradation they met the level of performance of EN ISO 20345:2004(E). – For elongation at break (upper), results after degradation shall be between 80 and 120% of the initial value. – For hardness (sole), the minimum value should be 30 Shore A and the maximum value not greater than the initial value plus 10 Shore A. In permeation test a simple flow-through, two compartment permeation cell, of standard dimensions, is used to measure quantitatively the permeation of chemicals through footwear materials. Breakthrough time is measured and used as a measure of protection.

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The resistance of a footwear material to permeation by solid or liquid chemical is determined by measuring the breakthrough time of the chemical through the footwear material. In the permeation test apparatus the footwear material separates the test chemical from the collecting medium. The collecting medium, which can be a gas or a liquid, is analysed quantitatively for its concentration of the chemical and thereby the amount of that chemical that permeated the barrier as a function of time after the initial contact with the footwear material. The standard EN 13832-3:2006 [73] specifies the requirements for footwear highly protective against chemicals. The levels of performance are defined as follows: – – – – – –

Level Level Level Level Level Level

0: 1: 2: 3: 4: 5:

permeation occurs before 120 min permeation happen between 121 and 240 min permeation happen between 241 and 480 min permeation happen between 481 and 1,440 min permeation happen between 1,441 and 1,920 min no permeation after 1,921 min.

The sterilisation test is used to test the footwear resistance against microorganisms. In the sterilisation test the basic physical properties of the footwear component (upper and sole) are checked before and after sterilisation. The sample (pair of footwear) is subjected to autoclave to a period of 70 min at 121◦ C. The boot should be removed at the earliest possible time after the 35 min period has been completed. The boots should be cooled to ambient temperature after they have been removed from the autoclave. And its necessary record any physical changes that are apparent as a result of the treatment. After the final set of heat has been completed, the boots should be reconditioned for at least 24 hours in the controlled climate laboratory before proceeding to the testing phase. The requirements for footwear protection against micro-organisms presented in EN 13832-3:2006 [73] are defined as follows: After the sterilisation procedure: – Whole footwear shall not leak – Tear resistance (upper and sole) – The materials are acceptable if after sterilisation they meet: 80% of the level of performance of EN ISO 20345:2004(E). Some methods used to test textiles, fibres and fabrics can be applied to test the resistance against micro-organisms of footwear component materials. Other test methods that have been used in the evaluation of footwear microorganism’s resistance in are presented in Table 13.18.

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Table 13.18. Antimicrobial tests for footwear components [74] Test title

Description

AATCC-100-1998 (USA)

Quantitative assessment of antibacterial Training Shoes finishes on textiles – measures the degree of anti-bacterial activity Quantitative assessment of fibres and fab- Training Shoes rics with inherent antibacterial properties (static and cidal) Soil Buriel Test Shoes liners Severe test conditions

JIS L 1902–1998 (Japan) BS EN ISO 11721, 2001

Materials tested

13.4.5 Other Slip Resistance International and European standards ISO/TR 11220:1993(E) [75] and EN 13287:2004(E) [76] specify a method for the determination of the slip resistance of footwear for professional use. In these methods the slip resistance, expressed as the coefficient of friction of the footwear, is determined by placing the footwear on the testing surface (floor), with glycerine present as lubricant, applying a given load and either moving the footwear horizontally in relation to the surface or moving the surface in relation to the footwear. The frictional forces are measured and the dynamic coefficient of friction is calculated. Acknowledgements Ricardo Moreira da Silva and Vera Vaz Pinto would like to acknowledge Funda¸c˜ao para a Ciˆencia e Tecnologia for financial support of their Ph.D. grants, SFRH/BDE/15525/2004 and SFRH/BDE/15537/2005.

References 1. Directive 94/11/EC of 23 March 1994, Labelling of the Materials Used in the Main Components of Footwear 2. European Standard/International Standard Organizations International Standard Organization EN ISO 20345:2004(E), Personal Protective Equipment – Safety Footwear (2004) 3. K. Bienkiewicz, Physical Chemistry of Leather Making (Robert E. Krieger, Malabar, FL, 1983) 4. J.H. Sharphouse, Leather Technician’s Handbook (Leather Producer’s Association, England, 1971) 5. H. Wachsmann, World Leather 30–32 (2004) 6. G. Reich, Leder H¨ autemarkt 1–2 (2003)

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7. F. O’Flaherty, W.T. Roddy, R.M. Lollar, The Chemistry and Technology of Leather (Robert E. Krieger, Malabar, FL, 1978) 8. J. Ludvik, Chrome Management in the Tanyard, United Nations Industrial Development Organization (2000) 9. Leather Business Unit, Waterproofing Without Chrome or Other Metal Salts, World Leather, 2002, pp. 37–40 10. R. Beeby, Making Waterproof Footwear, World Footwear, 1996, pp. 14–22 11. European Standard/International Standard Organizations EN ISO 20344: 2004(E), Personal Protective Equipment – Test Methods for Footwear (2004) 12. International Standard Organization ISO 5403:2002, Leather – Physical and Mechanical Tests – Determination of Water Resistance of Leather (2002) 13. European Standard (E) 13518:2001, Footwear – Test Methods for Uppers – Water Resistance (2001) 14. Deutsches Institut f¨ ur Normung DIN 53338, Testing of Leather; Determination of the Behaviour Against Water Under Dynamic Stress in the Penetrometer 15. International Union of Leather Technologists and Chemists Societies – Physical Test Methods IUP 10:2000, Water Resistance of Flexible Leathers (2000) 16. American Society of Testing and Materials ASTM D 2099:2000, Standard Test Method for Dynamic Water Resistance of Shoe Upper Leather by the Maser Water Penetration Tester (2000) 17. International Standard Organization ISO 2417:2002(E) IULTCS/IUP 7, Leather – Physical and Mechanical Test – Determination of the Static Absorption of Water (2002) 18. IUP 7:2000, Measurement of static absorption of water (2000) 19. International Union of Leather Technologists and Chemists Societies – Physical Test Methods IUP 45, Measurement of Water Penetration Pressure (2002) 20. C.L. Beyler, M.M. Hirschler, SFPE Handbook of Fire Protection Engineering, 3rd edn., NFPA, 1995, pp. 110–131, Chaps. 1–7 21. R.E. Lyon, M.L. Janssens, Polymer Flammability, U.S. Department of transportation – Federal Aviation Administration, DOT/FAA/AR-05/14, May 2005 22. American Society of Testing and Materials ASTM E176-99, Standard Terminology of Fire Standards (1999) 23. P. Budrugeac, V. Trandafir, M. G. Albu, J. Therm. Anal. Calorim. 72(2), 581–585 (2003) 24. P. Budrugeac, L. Mil, V. Bocu, F.J. Wortman, C. Popescu, J. Therm. Anal. Calorim. 72(3), 1057–1064 (2003) 25. C. Chahine, Thermochim. Acta 365(1–2), 101–110 (2000) 26. K. Donmez, W.E. Kallenberger, J. Am. Leather Chem. Assoc. 87, 1–19 (1992) 27. International Standard Organization ISO 15025:2002, Protective Clothing – Protection Against Heat and Flame – Method of Test for Limited Flame Spread (2000) 28. International Standard Organization ISO 5660-1:2002, Reaction-to-Fire Tests – Heat Release, Smoke Production and Mass Loss Rate – Part 1: Heat Release Rate (Cone Calorimeter Method) (2002) 29. R.H. White, M.A. Dietenberger, Cone Calorimeter Evaluation of Wood Products, 15th Annual BCC Conference on Flame Retardancy, Stamford, 2004 30. American Society of Testing and Materials ASTM D 2863:2000, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support CandleLike Combustion of Plastic (Oxygen Index) (2000)

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31. E.D. Weil, M.M. Hirschler, N.G. Patel, M.M. Said, S. Shakir, Fire Mater. 16, 159–167 (2002) 32. K. D¨ onmez, W.E. Kallenberger, J. Am. Leather Chem. Assoc. 86, 93–106 (1991) 33. Pyris Hardware for Windows, Cap2: DSC7, PerkinElmer Instruments, 2002 34. A.M. Manich, S. Cuadros, J. Cot, J. Carilla, A. Marsal, Thermochim. Acta 429(2), 205–211 (2005) 35. R.E. Lyon, R.N. Walters, J. Anal. Appl. Pyrolysis 71(1), 27–46 (2004) 36. EN ISO 6942:2002, Protective Clothing – Protection Against Heat and Fire Method of Test: Evaluation of Materials and Material Assemblies When Exposed to a Source of Radiant Heat (2002) 37. European Standard EN 13519:2001(E), Footwear – Test Methods for Uppers – High Temperature Behaviour (2001) 38. American Society of Testing and Materials ASTM D 2214:2002, Standard Test Method for Estimating the Thermal Conductivity of Leather with the CencoFitch Apparatus (2002) 39. A. Jordan, National Park Serv. 13(4), 1–4, (1993) 40. G.A. Rajkumar, N. Arunasri, T. Annamalai, M. Swamy, P.T. Perumal, J. Soc. Leather Chem. 81(5), 204–206, 1997. 41. A. Orlita, Int. Biodeteriorat. Biodegrad. 53, 157–163 (2004) 42. M. W¨ urtz, P.F.I. Pirmasens, Microbiological Test on Shoes and materials, Footwear Technology, March–April 2004 43. American Society of Testing and Materials ASTM D 4576-01, Standard Test Methods for Mold Growth Resistance of Wet Blue (2001) 44. OECD Emission Scenario Document, Additives in the Rubber Industry, Umweltbundesmt, Berlim, 2003 45. E. Mikkola, Polym. Int. 49, 1222–1225 (2000) 46. http://ulstandardsinfonet.ul.com/scopes/0094.html 47. P. Rybi´ nski, G. Janowska, M. Helwig, W. D¸abrowski, K. Majewski, J. Therm. Anal. Calorim. 75, 249–256 (2004) 48. R.E. Lyon, L. Speitel, R.N. Walters, S. Crowley, Fire Mater. 27, 195–208 (2003) 49. J.L. Laird, G. Liolios, TA techniques for the Rubber Industry, Rubber World, 13–19 January 1990 50. F. Pruneda, J.J. Su˜ nol, F. Andreu-Mateu, X. Colom, J. Therm. Anal. Calorim. 80(1), 187–190 (2005) 51. A.A. Yehia, A.A. Mansour, B. Stoll, J. Therm. Anal. 48, 1299–1310 (1997) 52. G. Janowska, P. Rybi´ nski, J. Therm. Anal. Calorim. 78, 839–847 (2004) 53. A. Castrovinci, G. Camino, C. Drevelle, S. Duquesne, C. Magniez, M. Vouters, Eur. Polym. J. 41(9), 2023–2033 (2005) 54. T.P. Wampler, J. Anal. Appl. Pyrol. 71(1), 1–12 (2004) 55. T.P. Wampler, J. Chromatogr. A 842, 207–220 (1999) 56. J.A. Hiltz, J. Anal. Appl. Pyrol. 55(2), 135–150 (2000) 57. M. Phair, T.P. Wampler, Rubber World, 215, 30–34 (1997) 58. S.-S. Choi, J. Anal. Appl. Pyrol. 55(2), 161–170 (2000) 59. S.-S. Choi, J. Anal. Appl. Pyrol. 62(2), 319–330 (2002) 60. International Standard Organization ISO 188:1998(E), Rubber, Vilcanized or Thermoplastic – Accelerated Ageing and Heat Resistance Tests (1998) 61. International NFPA 1971, Standard on Protective Ensemble for Structural Fire Fighting, 2000

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62. American Society of Testing and Materials ASTM F695-01, Standard Practice for Ranking of Test Data Obtained for Measurement of Slip Resistance of Footwear Sole, Heel, and Related Materials (2001) 63. International Standard Organization ISO 1817:2005, Rubber, Vulcanized – Determination of the Effect of Liquids (2005) 64. European Standard EN 13073:2001(E), Footwear – Test Methods for Whole Shoe – Water Resistance (2001) 65. SATRA Technology Centre SATRA
PM47:1997, Water Vapour Permeability and Absorption (1997) 66. T. Bosch, A.M. Manich, A.J. Long, J. Soc. Leather Technol. Chem. 84(6), 263–265, 2000. 67. New England Tanners Club, Leather Facts, 3rd edn. 1994 68. European Standard EN 12784:1999(E), Footwear – Test Methods for Whole Shoe – Thermal Insulation (1999) 69. European Standard/International Standard Organizations EN ISO 20346: 2004(E), Personal Protective Equipment – Protective Footwear (2004) 70. European Standard/International Standard Organizations EN ISO 20347: 2004(E), Personal Protective Equipment – Occupational Footwear (2004) 71. European Standard EN 13832-1:2006, Footwear Protecting Against Chemicals. Part 1: Terminology and Test Methods (2006). 72. European Standard EN 13832-2:2006, Footwear Protecting Against Chemicals. Part 2: Requirements for Footwear Resistant to Chemicals Under Laboratory Conditions (2006). 73. European Standard EN 13832-3:2006, Footwear Protecting Against Chemicals. Part 3: Requirements for Footwear Highly Resistant to Chemicals Under Laboratory Conditions (2006) 74. http://www.shirleytech.com/pdf/micro-article-300404.pdf 75. Technical Report ISO TR 11220:1993(E), Footwear for Professional Use – Determination of Slip Resistance (1993) 76. European Standard EN 13287:2004(E) Personal Protective Equipment – Footwear – Test Method for Slip Resistance (2004)

14 Filtration Technologies in the Automotive Industry E. Jandos, M. Lebrun, C. Brzezinski, and S. Capo Canizares

Summary. The filtration in the automotive industry is diverse. Many filters are used either for the filtration of air or liquid in the tank, engine or cabine. This paper will focus on air filtration and more specifically on engine air filtration. After a brief presentation of the basic filtration principles, the filtration technologies used in this field of the automotive industry will be reviewed. Then, in a last part, the testing methodologies will be described.

14.1 Basic Filtration Principles 14.1.1 What is Filtration? Filtration in vehicles is diverse. Many filters are fitted today in cars as shown in Table 14.1 and in Fig. 14.1. Air filtration consists in separating and capturing particles of any nature in air. The level of filtration efficiency is determined according to the application field or use of the clean air obtained (hygiene, cleanness, manufacturing, security and so on). Many vehicles are fitted with depth filters. Depth filtration is always the most economical method when there is a low concentration of particles to be separated. The purpose of the filter elements used is to separate particles (the solid phase) from fluids (the continuous phase), i.e. gases and liquids. Filtration differs from simple dust removal because of the very low concentrations of pollutant and the small size of the particles. 14.1.2 Characteristics of Contaminant Particles The contaminants are impurities coming from numerous sources and consists of organic and mineral dusts, particles of abraded metal, and soot from incomplete combustion (Figs. 14.2, 14.3 and 14.4). They do not, however, appear only as solid particles, but may also be of liquid form, thus necessitating for example, the filtration of droplets of oil from the blow-by gas in crankcase ventilation or droplets of water out of diesel fuel.

Tank venting

Crankcase ventilation

Engine intake air

Engine exhaust gas

Air filtration Cabin air

Braking system air

Coolant

Gasoline engine

Table 14.1. Air and liquid filtration required in vehicles

Diesel engine

Gearbox oil, hydraulic oil

Liquid filtration Engine oil

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Cabin air filter

diesel fuel filter

engine air filter (synthetic)

engine air filter (cellulosic)

271

Fig. 14.1. Example of filters available in the automotive industry (Mecaplast)

Ashes

mist Bacteria Pollen Oil mist Tobacco Smoke Smokes Dust 0,01

0,1

1 ( µm )

10

Fig. 14.2. Breakdown of air pollutants particle size

100

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Fig. 14.3. Examples of particles that can be found in the air: mineral dust, metal particles and fibres (Source: Sofrance)

100% 90%

91,700%

80% 70% 60% 50% 40% 65%

30% 20% 10% 0%

1% 6,800%

2%

4%

1,100% 20% 8% 0 to 0,5µm 0,250% 0,145% 0,5 to 1µm 0,005% 1 to 3µm 3 to 5µm 5 to 10µm 10 to100µm

Percentage (weight) Percentage (in number)

Fig. 14.4. Repartition of the particles in the air

Dust removal Filtration

Concentration of particles in the air

Particle size

>30 mg m−3 <30 mg m−3

>20 µm <20 µm

Only 0.005% of the particles have a diameter superior to 10µm. 0.4% of the particles represent 93% of the total weight (Fig. 14.4; Tables 14.2 and 14.3). 14.1.3 Mechanisms of Capture There are at least seven mechanisms by which a filter can capture particles. All of these mechanisms are combined in a filter at any given time and may

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Table 14.2. Size breakdown of organic and mineral road contaminants

Mineral (%) Organic (%)

<2,5 µm

2,5
>15 µm

9% weight 50 50

80% weight 81 19

11% weight 65 35

Table 14.3. Weight breakdown of road contaminants Carbon

75%

Sulphur Additives Tyres Earth dusts Non-identified

1% 9% 1% 11% 3%

Fig. 14.5. Direct interception of particles

change as operating conditions change. The mechanisms of particle capture are listed below. Direct Interception Interception of a particle occurs by this method when a particle approaches a media obstruction a distance equal to or less than the particle radius. If the particle “runs into” a physical barrier, it becomes captured (Fig. 14.5). Bridging One single particle may be too small to be directly intercepted or blocked by the filter medium. However, two particles hitting the obstruction at the same time may stick together and be deposited (Fig. 14.6). Particles form a bridge across a pore by hitting the pore simultaneously, or by adhering to each other earlier in the process and then becoming deposited. Bridged particles may not clog the opening completely, thus creating a smaller pore that is more difficult to pass through. The gradual accumulation of particles on the filter medium is known as the formation of a filter cake. This cake creates a finer matrix for subsequent interception.

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Fig. 14.6. Bridging mechanism of particles capture

Fig. 14.7. Sieving mechanism of particles capture

Sieving Similar to bridging, sieving is a specialised case of direct interception. Sieving occurs when the opening or pore in the medium is more constrictive than the diameter of the particle (Fig. 14.7). The particle is simply too large to pass through the pore. Sieving may occur on the surface of the filter or through the pore. Sieving may occur on the surface of the filter or throughout the depth of the medium. Inertial Impaction Inertial impaction is based on the scientific principle of inertia, stating that a moving object will continue to move in a straight line unless acted on by an outside force (Fig. 14.8). As particles flow through a filter, they may encounter an obstruction and become captured while the fluid flows around the barrier. Due to the inertia of the particle, it continues to move in a straight line and becomes impacted. Fluid viscosity also greatly affects inertial impaction.

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Fig. 14.8. Internal impaction mechanism of particles capture

Fig. 14.9. Diffusion interception mechanism of particles capture

Fluids that are highly viscous exert greater drag on particles, reducing the chances of inertial impaction. Gases, on the other hand, have extremely low viscosity, enhancing inertial impaction to the point of being a primary mechanism of capture in gas filtration. Diffusion Interception The mechanism of diffusion interception is attributable to the fact that molecules are in constant random motion (Fig. 14.9). This motion enhances the opportunity for a particle to become intercepted by the filter medium. Diffusion interception is more prevalent in particles that are 0.1–0.3 µm in size, since small particles are most affected by molecular bombardment. Diffusion interception is primarily found in gases due to their inherently low viscosity and high degree of molecular mobility. Electrokinetic Effects Electrical charges may be present on the filter medium and/or on the particles. Particle deposition can occur due to attractive forces between charges or induced forces due to the proximity of the particle to the medium. Some manufacturers purposely alter the surface of the filter medium to enhance electrokinetic capture. The fibres are called “electret fibres”. An example of such medium is given in Sect. 14.2, where an electrostatically charged medium shows a higher initial efficiency than a non-charged medium.

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Fig. 14.10. Bridging mechanism of particle capture

Gravitational Settling Particles have mass and are therefore affected by gravity. It is possible that a particle may leave the fluid streamlines and settle in the same fashion as sediment in a settling tank. Particles may be deposited within a filter medium or in the up-stream chamber of a filter housing. This mechanism is related to the gravitational settling (Fig. 14.10).

14.2 Filtration Technologies 14.2.1 Engine Air Intake Filters Engine needs air, fuel and oil that must be cleaned of any particles that could damage mechanical parts. Particle size ranges from less than 1 to 50µm, such particles are a potential source of wear. Filtration on fibrous media is the most widely used in air filtering process. The filter media used in vehicles generally are depth filters. Fibrous media can be either cellulosic media or synthetic non-woven media. Cellulosic Media The world market of cellulosic media is presented in Fig. 14.11 and example of filters based on this type of media are illustrated in Fig. 14.12. Crude paper is obtained by wet process. The paper is then impregnated with different kinds of resins that bring specific properties: Figure 14.13 presents an SEM picture that illustrates the structure of cellulosic media: 1. Transformation of the cellulosic medium to obtain a filtering element is a continuous process: cutting of paper roll to the appropriate width

14 Filtration Technologies in the Automotive Industry – – – –

Phenolic resin Acrylic resin Epoxy resin Polyvinyl acetate resin

– – – – – – –

277

Mechanical strength Water resistance Oil resistance Fuel resistance Fire retardancy Visual aspect Pleatability and stability of the pleats obtained

Rest of the world 11% North America 44%

Asia 19%

Europe 26%

Fig. 14.11. World market of cellulosic medias (Total of 1,80,000 ton in 2003/Source: Holling worth&Vose)

OPEL VECTRA

SKODA FABIA / VW POLO

PSA XSARA / 206

AUDI A4

PSA 106 Saxo

Fig. 14.12. Examples of Mecaplast filters based on cellulosic media

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Fig. 14.13. SEM picture of cellulosic medium (Source: Mecaplast)

Fig. 14.14. Picture of embossing pattern (Source: Mecaplast)

2. Humidification 3. Heating 4. Embossing (necessary to maintain pleats in the right position in the final product) (Fig. 14.14) 5. Glue deposit (brings air-tightness to the filter) 6. Pleating (necessary to obtain the best filtering surface) 7. Cooling 8. Cutting 9. Printing of serial number 10. PU foam seal deposit

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Synthetic Non-Woven The use of synthetic media is a new trend in automotive filtration (Fig. 14.15). A good mechanical resistance and enhanced filtration properties are the principle advantages of this new generation of filters. What is more, a filter element made of high-efficiency non-woven material takes up 35% less space than a paper filter of the same dust holding capacity and fineness (Figs. 14.16 and 14.17). other countries 18% Japan 11%

North America 37%

Western Europe 34%

Fig. 14.15. World market of non-woven medias (Total of 2 694 000 Tons in 2003/ Source: EDANA and Hollingsworth&Vose)

Fig. 14.16. Synthetic engine air filters

Fig. 14.17. Synthetic cabin air filters (Source: Freudenberg)

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Fig. 14.18. SEM picture of synthetic Non-woven (cross-section + side). Thermobonding inside of the medium is clearly visible

Non-woven can be obtained by: – Wet process (similar to paper industry process) – Melt process (melt blown, spun bond) – Dry process (carded and needle punched webs) The webs are consolidated by: – Mechanical process. Needle punching or hydro-entanglement – Physico-chemical process. Thermo bonding (use of bicomponent fibres) or padding (Fig. 14.18) Manufacturing of synthetic filter element requires the following step: 1. Cutting of roll to the appropriate width 2. Heating 3. Embossing (necessary to maintain pleats in the right position in the final product) 4. Glue deposit (brings air-tightness to the filter) 5. Pleating (necessary to obtain the best filtering surface) 6. Cooling 7. Cutting 8. Printing of serial number 9. PU foam seal deposit 14.2.2 Technical Features of a Fibrous Medium Pore size of the filter is the most important consideration when choosing a cartridge. Pore size is dependent upon the following: (a) Fibre diameter. As fibre diameter decreases, mean pore size decreases. In other words, in order to get a finer filter, use thinner fibres.

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(b) Porosity. Porosity is the ratio of the void volume to the total volume of a filter medium. Porosity decreases the mean pore size and makes the filter finer. However, decreasing porosity also increases the resistance to flow of the cartridge, consequently increasing the overall differential pressure. (c) Thickness of the filter media. As filter medium becomes thicker, mean pore size decreases and as layers of medium are added to a cartridge, the pores become smaller. However, as it is the case with porosity, adding layers to the medium increases the resistance to flow and, consequently, the overall differential pressure. The fibrous medium is a barrier placed inside the air intake line of the engine, it thus introduces a differential pressure (difference of pressure before and after the filter). The differential pressure should be the lowest as possible in order to maintain the performance of the engine. Designing a fibrous filter is a juggling between fibre diameter, porosity and thickness of filter medium. 14.2.3 Filtration Process After the initial particle separation on the filter surface, a filter cake will be formed by the subsequently captured particles. Owing to the particle surface properties and its momentum, two types of cake formation may be observed. Filter Cake Formation: Dust Holding Capacity Uncohesive particles approaching the filter medium combined with a high particle momentum – due to a large particle size and/or a high particle density and/or a high particle velocity – are expected to form a densely packed filter cake as depicted in Fig. 14.19. These particles are able to compact the already existing powder bulk constantly under impact (Fig. 14.19). The separation of particles takes place on the surface of the individual fibres deep inside the structure of the medium. At the beginning of the filtration process, individual particles initially settle on the surface of the fibres (Fig. 14.20). Over time, the density of the accumulation increases, allowing dendrite-like formations to develop. In this case the inter-particle bonds are strong enough to capture an approaching particle at its first point of contact with already collected particles and to retain the “open” channel-like bulk structure of the filter cake. Bridging of two “dendrites” may be observed with this type of cake build-up as indicated in following pictures. These filter cakes have obviously a lower resistance-to-flow than the densely packed cake type. The dendrites or cake diminish the volume of pores available for filtration. By contrast, the differential pressure at the same flow rate increases. After a certain period of service, the capacity of the filter is exhausted, necessitating its replacement. The increase in the differential pressure in a filter as a function of operating time is shown schematically in Fig. 14.21. The gradual initial

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Fig. 14.19. Particles settled on the media

air

dendritic bridging

dendritic growth

Fig. 14.20. Particles settled on the fibres Differential pressure

t1

∆P max

∆P0 Dust load or time

Fig. 14.21. Schematic description of the increase of differential pressure of a depth filter as a function of dust load or time. The filter element should be changed at t1

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Table 14.4. Comparison between dust holding capacity of cellulosic and synthetic media Filter medium

Specific dust holding capacity (g m−2 )

Paper Non-woven

190–220 900–1,100

Weight (g m−2 ) 100–120 230–250

rise in differential pressure is typical of the behaviour of depth filters. Only after a certain period of time, when a high proportion of pores are clogged with particles, the differential pressure rises steeply. To prevent the engine from losing power, the filter element should be changed at the latest when the predefined maximum pressure drop is reached. This maximum differential pressure is laid down in the car manufacturer’s specification. It is possible to designate the amount of dust that the element is capable to hold before a predetermined maximum differential pressure is reached. The specific dust holding capacity defines the filter service interval. This materialrelated value provides the basis for the design of the filter (Table 14.4). Particles of different sizes or sources have different effects on filter media. A high number of small particles (e.g. soots) will generally clog an air filter element more quickly than coarse particles of sand and dust. 14.2.4 Filtration Efficiency Definition A variety of methods are used to evaluate the efficiency of filters. The filtration efficiency represents the proportion of particles trapped by the filtration process. A distinction is made between grade (or fractional) efficiency and overall filtration efficiency. – Grade efficiency relates to individual particle diameters or categories of particle sizes (fractions). – Overall efficiency describes the filter effect for all particles by the filter. For example: Grade efficiency (3–5µm) = 87% means that 87% of the particles with a diameter between 3 and 5µm are separated by the filter. The initial filtration efficiency is another important characteristic. This gives the efficiency of a new filter medium or element. In the case of depth filters without electrostatic charges, it is generally lower than the efficiency of a laden filter medium (Fig. 14.22). The grade efficiency is a function of particle size. Figure 14.23 reports the grade efficiency vs. particle size in the case of an air filter element consisting of synthetic fibres. The decrease in efficiency at approximately x = 0.5 µm

284

E. Jandos et al. 100 95 90 efficiency (%)

85 80 75 70 65 60

Efficiency (%) classic synthetic media

55

Efficiency (%) electrostatically charged media

50 0

5

10

15

20

25

30

35

40

time (min)

Fig. 14.22. Comparison of efficiencies between an electrostatically charged filter and a non-charged media Grade efficiency (%) 100

50

Particle size (µm) 0

0,5

1

5

Fig. 14.23. Grade efficiency curve as a function of particle size

indicates that the filtration mechanisms (diffusion, inertia effect) which predominate in air filtration are not fully developed. Virtually 100% of larger particles (>6 µm) are separated as a result of interception and inertia effects. The β-Value The differences between high efficiency filters is often not obvious, the β-value is thus increasingly being used to compare the performance of filter elements.

14 Filtration Technologies in the Automotive Industry

285

Table 14.5. Relation between β-value and the grade efficiency β-value

Grade efficiency (%)

1 2 5 10 20 50 100 200 500 1,000 5,000

0 50 80 90 95 98 99 99.5 99.8 99,9 99.98

This is defined as the number N1 of particles up to certain size xj upstream the filter, divided by the measured number N2 of particles of the same size interval downstream of the filter (Table 14.5). β(xj) =

(N1 ≥ xj) upstream (N2 ≥ xj) downstream

14.3 Test Methodologies: Standards and Benches 14.3.1 Requirements of Air Filter Media Modern air filter media are expected to comply with the filtration values laid down in the relevant specifications for dust holding capacity and filtration efficiency under all operating conditions. The medium filter must keep a high stability under pulsating forces and not allow any dust to permeate. 14.3.2 Material Related Parameters The characteristics of media are defined by various data listed in Table 14.6: 14.3.3 Filter Parameters The filtration properties of a filter are determined under standardised conditions laid down in ISO 5011 at a temperature of 23±5◦ C and relative humidity of 55±15% (Figs. 14.24 and 14.25). The filtration efficiency (expressed in %) can be determined by two complementary methods. First, the filtration efficiency is calculated from the relationship between the increase in weight of the filter medium and the weight of dust injected. This

286

E. Jandos et al. Table 14.6. Characteristic data and standards defined for filtration media

Data

Unit

Standard

Grammage Thickness Air permeability Size of pore Resistance to tearing and bursting Bending Flammability Elongation at break Tensile strength

G/m2 mm L m−2 s−1 µm kPa

ISO 536; EN 29073-1 ISO 534; EN ISO 9073-2 ISO 9237; DIN 53887 BS 6410 DIN 53113

Rating K1, F1; or A to F % N/5cm

DIN 53864 DIN 53438; or FMVSS 302 EN 29073-3 EN 29073-3

Fig. 14.24. Schematic layout of ventilation bench

method uses standardised dust (PTI Coarse/Fine, see Fig. 14.26) in order to obtain consistent comparable information on the retaining capacity of different filter media. Second, detailed information on the filtration efficiency of filter media is obtained from measuring the filtration efficiency as a function of particle size. The dust holding capacity (expressed in g) is determined as follows: a certain amount of dust is directed onto a filter element until the pressure drop due to the element increases by a predefined value ∆P (e.g. 20 mbar). This ensures that the air filter positioned in the intake duct does not affect engine performance. The dust holding capacity measured in the laboratory is correlated with the results of on road trials to calculate the permitted period of use of the filter in the vehicle. The laboratory investigations are reinforced by regular practical tests using fleets of vehicles and stationary ambient air test rigs to determine this important information. These test eventually show the progression of differential pressure at a constant airflow rate as a function of time. An example is given in the graph hereafter (Fig. 14.27).

14 Filtration Technologies in the Automotive Industry

Dust feeding

Filter element

Absolute filter

Air flow

Fig. 14.25. Picture of the ventilation bench (Mecaplast)

Fig. 14.26. Particle size distribution of Coarse and Fine ISO dusts

287

288

E. Jandos et al. 50

Pressure drop (mbar)

40

Filter 1 30

Filter 2 20

10

Running time (hours)

0 0

500

1000

1500

2000

2500

3000

3500

Fig. 14.27. Pressure drop curves vs. running time for two different filters. Measured at constant flow rate during clogging test. Filter 1 shows a quick increase in pressure drop in comparison with filter 2. Filter 2 has a good dust holding capacity

14.4 Conclusion Filtration is a complex process influenced by a great number of parameters. The design of an air filter relies on a good knowledge of technical requirements in terms of dust holding capacity and efficiency. This governs the choice of an appropriate medium defined by its thickness, fibre size, or porosity. According to growing demand from car manufacturer, the medium should also repel water when the vehicle is running in bad weather, and also show a low flammability in case of back fire or ingestion of burning items. New trends in automotive filtration will emphasise the importance of life service interval, recyclability, and high efficiency.

References 1. C. Brzezinski, L. Criquet, E. Jandos, M. Lebrun, Mecaplast Filtration Workshop, 2003 2. D.B. Purchas, Handbook of Filter Media (Elsevier Advanced Technology, 1996) 3. T. Christopher Dickenson, Filters and Filtration Handbook, 4th edn. (Elsevier Advanced Technology, 1997) 4. Automotive Filtration, Basics and Examples of Air, Oil and Fuel Filtration – Filterwerk Mann & Hummel GMBH, 2002 5. http://www.gopani.com

Index

actuation, 151, 152, 163, 166, 167 air filter, 271, 279, 283, 285, 286, 288 airlaid, 140, 142, 144, 146, 147 aluminium hydroxide, 5, 7, 9, 11, 17, 18 antimicrobial agents, 23, 26, 27, 30, 34, 35 antimicrobial effect, 26–28, 32 antimicrobial functionalisation, 24 attenuation, 82 barrier coatings, 118 barrier effects, 230, 253 carbon containing fibres, 74 carding, 143, 144 CFD, 171, 172, 175, 186, 189 charge, 63–72, 76 charge decay, 67 charging mechanisms, 68 cold plasma, 110–112, 121 conductive fibres, 72, 73 conductive polymers containing fibres, 74 cone calorimeter, 172, 182–187, 215–217, 219, 221, 222, 224, 226 diffusion barrier, 115 drylaid, 140, 142–144 electric field, 65, 67, 68, 75–77, 79–81 electromagnetic compatibility, 74 electrospinning, 125–131, 133–136 electrospraying, 131, 133

electrostatic discharges, 63, 66, 68 euroclass, 215, 223, 226 fields attenuation, 79 filtration, 269, 270, 272, 275, 276, 279, 281, 283–286, 288 fire emissions, 201, 206, 207, 212 fire retardancy, 40, 51, 56, 59, 118 Fire-LCA, 191, 192, 195, 196, 198–206, 208–212 flame retardancy, 88 footwear, 229, 230, 232–234, 236, 244, 247, 249, 252–255, 257, 259–265 guidelines, 192, 195, 198, 212 hydrated fillers, 3, 6, 7, 9–11, 14 hydro-entanglement, 147 hydrotalcite, 3, 16, 17 intumescence, 39–41, 51, 52, 58 leather, 229–238, 240, 242, 244–246, 249, 254, 257, 259 magnesium hydroxide, 5, 7–9, 15–17, 19 mechanical models, 151 medium, 273–276, 278, 280, 281, 283, 285, 288 methodologies, 230, 240, 242, 249, 253 microbes, 23–25, 31, 34, 37 nano-filler, 87–89, 93, 97, 101, 103–105 nano-non-wovens, 130 natural fibers, 43

290

Index

needlepunching, 147 non-woven, 139–144, 146–150 oxygen barrier, 115 porosity, 281, 288 pyrolysis, 171–177, 180, 182, 183, 185–187, 189 rubber, 229, 230, 244, 247, 249, 250, 252–254 safety, 230, 247, 259–261 SBI test, 215–219, 221

shape memory effect, 163–166 shielding material, 78 spunbond, 145 static electricity, 63, 70, 71 statistical fire model, 196, 201–203, 208 surface modification, 112, 122 synthetic fibers, 39, 40, 42, 43, 51 textile, 23–33, 35, 37, 39, 40, 42, 43, 54, 151–153, 158, 162, 164–167 thin film, 111, 113, 114 wetlaid, 139, 140, 142, 146

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