Multifunctional Barriers for Flexible Structure Textile Leather and Paper

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

All rights reserved. No part of this book may be reproduced in any form, by photostat, microfilm, retrieval system, or any other means, without the written permission of Kodansha Ltd. (except in the case of brief quotation for criticism or review.) This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specif ically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microf ilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. Springer is a part of Springer Science+Business Media. springer.com © Springer-Verlag Berlin Heidelberg 2007 The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specif ic statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Data prepared by SPi using a Springer LATEX macro package Cover: eStudio Calamar Steinen Printed on acid-free paper

SPIN: 11901150

57/3180/SPi

543210

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

Contents

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

XI

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

XII

Contents

7.3

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

Contents

XIII

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

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

P.R. Hornsby

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].

1 The Application of Fire-Retardant Fillers

9

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

P.R. Hornsby

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

1 The Application of Fire-Retardant Fillers

11

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

14

P.R. Hornsby

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

1 The Application of Fire-Retardant Fillers

15

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 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”,

110

C. Jama and R. Delobel

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

6 Cold Plasma Technologies

111

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

112

C. Jama and R. Delobel

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

6 Cold Plasma Technologies

113

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

114

C. Jama and R. Delobel

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.

6 Cold Plasma Technologies

115

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

116

C. Jama and R. Delobel Table 6.2. Oxygen permeability of PECVD-coated polymers

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).

6 Cold Plasma Technologies

117

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

118

C. Jama and R. Delobel

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

6 Cold Plasma Technologies

119

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

120

C. Jama and R. Delobel

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

6 Cold Plasma Technologies

121

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

122

C. Jama and R. Delobel

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.

References 1. N. Inagaki, Plasma Surface Modification and Plasma Polymerisation. Interactions Between Plasma and Polymeric Materials (Technomic Publishing, Lancaster, 1996), Chap. 2, pp. 22–28 2. R. d’Agostino, Plasma Deposition, Treatment and Etching of Polymers (Academic, New York, 1990) 3. C. Jama, O. Dessaux, P. Goudmand, J. Surf. Sci. 352–354, 490–494 (1996) 4. C. Jama, O. Dessaux, P. Goudmand, L. Gengembre, J. Grimblot, Surf. Sci. 352–354, 893–897 (1996) 5. C. Jama, O. Dessaux, P. Goudmand, L. Gengembre, J. Grimblot, J. Surf. Interf. Anal. 18(11), 751–756 (1992) 6. C. Jama, J.-D. Quensierre, L. Gengembre, V. Moineau, O. Dessaux, P. Goudmand, J. Grimblot, Surf. Interf. Anal. 27(7), 653–658 (1999) 7. O. Dessaux, P. Goudmand, C. Jama, Surf. Coat. Technol. 100–101(1–3), 38–44 (1998) 8. F. Arefi, P.M. Rahmati, V. Andre, J. Amouroux, J. Appl. Polym. Sci.: Appl. Polym. Symp. 46, 33–60 (1990) 9. P.M. Rahmati, F. Arefi, J. Amouroux, Surf. Coat. Technol. 45, 369–378 (1991) 10. A.G. Shard, J.P.S. Badyal, Polym. Commun. 32(7), 217–219 (1991) 11. A.G. Shard, J.P.S. Badyal, Macromolecules 25(7), 2053–2054 (1992) 12. O.D. Greenwood, S. Tasker, J.P.S. Badyal, J. Polym. Sci. A: Polym. Chem. 32, 2479–2486 (1994) 13. J. Hopkins, J.P.S. Badyal, J. Phys. Chem. 99(1), 4261–4264 (1995) 14. G. Akovali, Z.M. Rzaev, D.G. Mamadov, Eur. Polym. J. 32(3), 375–383 (1996) 15. J. Hopkins, S.H. Wheale, H.P.S. Badyal, J. Phys. Chem. 100, 14062–14066 (1996) 16. E. Kiss, J. Samu, A. Toth, I. Bertoti, Langmuir 12, 1651–1657 (1996) 17. S.D. Lee, M. Sarmadi, F. Denes, J.L. Shohet, Plasmas Polym. 2(3), 177–198 (1997) 18. S.D. Lee, S. Manolache, M. Sarmadi, F. Denes, Polym. Bull. 43, 409–416 (1999) 19. G. Bonizzoni, E. Vassallo, Vacuum 64(3–4), 327–336 (2002)

6 Cold Plasma Technologies

123

20. C.-K. Jung, I.-S. Bae, S.-B. Lee, J.-H. Cho, E.-S. Shin, S.-C. Choi, J.-H. Boo, Thin Solid Films 506–507, 316–322 (2006) 21. B. Mutel, C. Jama, O. Dessaux et al., Vide: Science, Technique et Applications (1995), vol. 275(Suppl.), 10th International Colloquium on Plasma Processes, 1995, pp. 126–129 22. J.A. Selamoglu, D.E. Mucha, D.A. Ibbotson, D.L. Flamm, J. Vac. Sci. Technol. B 7(6), 1345 (1989) 23. K. Ray, C.K. Maiti, S.K. Lahiri, N.B. Chakrabarti, J. Vac. Sci. Technol. B 10(3), 1139 (1992) 24. C.S. Pai, C.P. Chang, J. Appl. Phys. 68(2), 793 (1990) 25. Y. Kageyama, Y. Taga, Proceedings of 8th ISPC, Tokyo, Japan, 1987, p. 1073 26. F. Fracassi, R. d’Agostino, P. Favia, M. van Sambeck, Plasma Sources Sci. Technol. 2, 106 (1993) 27. N. Inagaki, S. Kondo, M. Hirata, H. Urushibata, J. Appl. Polym. Sci. 30, 3385 (1989) 28. T. Okuhara, J.M. White, Appl. Surf. Sci. 29, 223 (1987), Chap. 3 29. G.L. Czeremuszkin, M.R. Wertheimer, A.S. Da Silva Sobrinho, Plasmas Polym. 6(1/2), 107 (2001) 30. M. Ohring, The Materials Science of Thin Films (Academic, Boston, 1992) 31. J.A. Thornton, J.E. Greene, in Handbook of Deposition Technologies for Films and Coatings – Science, Technology and Applications, ed. by R.F. Bunshah (Noyes Publications, Park Ridge, NJ, 1994), p. 55 32. E.M. Liston, L. Martinu, M.R. Wertheimer, J. Adhes, Sci. Technol. 7(10), 1091 (1993) 33. H. Chatham, Surf. Coat. Technol. 78(1–3), 1 (1996) 34. Y. Leterrier, Prog. Mater. Sci. 48(1), 1 (2003) 35. A. Gruniger, Ph. Rudolf von Rohr, Surf. Coat. Technol. 174, 1043 (2003) 36. A. Sugimoto et al., IEEE J. Sel. Top. Quant. Electron. 10(1), 107 (2004) 37. R. Lamendola, R. d’Agostino, Pure Appl. Chem. 70(6), 1203–1208 (1998) 38. L.B. Ryder, Plast. Eng. 41 (1994) 39. J.T. Felts, A.D. Grubb, J. Vac. Sci. Technol. A 10, 1675 (1992) 40. M. Walther, M. Heming, M. Spallek, Surf. Coat. Technol. 80, 200 (1996) 41. L. Agres, Y. Segui, R. Delsol, P. Raynaud, J. Appl. Polym. Sci. 61, 2015 (1996) 42. J.E. Klemberg-Sapiepha, L. Martinu, O.M. Kuttel, M. Wertheimer, Proceedings of 36th Society of Vacuum Coaters Annual Conference, SVC, 1993, p. 445 43. M. Izu, B. Dotter, S.R. Ovshinski, Proceedings of 36th Society of Vacuum Coaters Annual Conference, SVC, 1993, p. 333 44. A.G. Erlat, R.J. Spontak et al., J. Phys. Chem. B 103, 6047 (1999) 45. N. Inagaki, S. Tasaka, H. Hiramatsu, J. Appl. Polym. Sci. 71, 2091 (1999) 46. C. Jama, K. Asfardjani, O. Dessaux, P. Goudmand, J. Appl. Polym. Sci. 64(4), 699–705 (1997) 47. A. Qu´ed´e, B. Mutel, P. Supiot, C. Jama, O. Dessaux, R. Delobel, Surf. Coat. Technol. 180–181, 265–270 (2004) 48. A. Quede, J. Cardoso et al., J. Mater. Sci. 37(7), 1395–1399 (2002) 49. A. Qu´ed´e, C. Jama, P. Supiot, M. Le Bras, R. Delobel, O. Dessaux, P. Goudmand, Surf. Coat. Technol. 151–152, 424–428 (2002) 50. I. Errifai, C. Jama, M. Le Bras, R. Delobel, L. Gengembre, A. Mazzah, R. De Jaeger, Surf. Coat. Technol. 180–181, 297–301 (2004)

124

C. Jama and R. Delobel

51. Standard test method for measuring the minimum oxygen concentration to support candle-like combustion of plastics (Oxygen Index), ASTM D 2863/77 (1977), Philadelphia 52. Standard test method for heat and visible smoke release for materials and products using an oxygen depletion calorimeter, ASTM E 1354-90a (1990), Philadelphia 53. L.L. Tedder, G. Lu, J.E. Crowel, J. Appl. Phys. 69, 7073 (1991) 54. D.R. Anderson, Analysis of Silicones. Infrared, Raman and Ultraviolet Spectroscopy (Wiley, New York, 1974), Chap. 10, p. 247 55. C. Rau, W. Kulisch, Thin Solid Films 249, 28 (1997) 56. F. Callebert, P. Supiot, K. Asfardjani, O. Dessaux, P. Goudmand, P. Dhamelincourt, J. Laureyns, J. Appl. Polym. Sci. 52, 1595 (1994) 57. P.G. Pai, S.S. Chao, Y. Takagi, G. Lucovsky, J. Vac. Sci. Technol. A 4, 689 (1986)

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.

126

K. Schaefer et al.

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]

7 Nano-Fibres for Filter Materials

127

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

128

K. Schaefer et al.

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: 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].

7 Nano-Fibres for Filter Materials

129

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].

130

K. Schaefer et al.

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.

7 Nano-Fibres for Filter Materials

131

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

132

K. Schaefer et al.

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

133

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.

134

K. Schaefer et al.

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

7 Nano-Fibres for Filter Materials

135

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].

136

K. Schaefer et al.

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].

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

A. Formhals, US Patent 1-975-504 (1934) P.K. Baumgarten, J. Colloid Interf. Sci. 36, 71 (1971) J. Doshi, D.H. Reneker, J. Electrostat. 35, 151 (1995) J.N. Doshi, G. Shrinivasan, D.H. Reneker, Polym. News 20, 206 (1995) D.H. Reneker, I. Chun, Nanotechnology 7, 216 (1996) X. Fang, D.H. Reneker, J. Macromol. Sci.: Phys. B 36, 169 (1997) M.M. Bergshoef, G.J. Vancso, Adv. Mater. 11, 1362 (1999) H. Hou, J. Zeng, A. Reuning, A. Schaper, J.H. Wendorff, A. Greiner, Macromolecules 35, 2429 (2002) M. Bognitzki, H. Hou, M. Ishaque, T. Frese, M. Hellwig, C. Schwarte, A. Schaper, J.H. Wendorff, A. Greiner, Adv. Mater. 12, 637 (2000) M. Bognitzki, W. Czado, T. Frese, A. Schaper, M. Hellwig, M. Steinhart, A. Greiner, J.H. Wendorff, Adv. Mater. 13, 70 (2001) M. Bognitzki, T. Frese, J.H. Wendorff, A. Greiner, Polym. Prepr. (ACS, PMSE) 82, 45 (2000) M. Bognitzki, T. Frese, M. Steinhart, A. Greiner, J.H. Wendorff, A. Schaper, M. Hellwig, Polym. Eng. Sci. 41, 982 (2001) R. Dersch, A. Greiner, M. Steinhart, J.H. Wendorff, DWI Rep. 127 (2003) R. Dersch, M. Steinhart, U. Boudriot, A. Greiner, J.H. Wendorff, Polym. Adv. Technol. 16, 276 (2005)

7 Nano-Fibres for Filter Materials

137

15. C.C. Rutledge, M.Y. Shin, S.B. Warner, A. Buer, M. Grimler, S.C. Ugbolue, National Textile Center Annual Report: November 2000, M98-D01 (2000) 16. D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, J. Appl. Phys. 87, 4531 (2000) 17. J.M. Deitzel, J. Kleinmeyer, D. Harris, N.C.B. Tan, Polymer 42, 261 (2001) 18. T. Grafe, K. Graham, Proceedings of the International Nonwovens Technical Conference (Joint INDA-TAPPI Conference), 24–26 September 2002, Atlanta, 2002 19. T.H. Grafe, K.M. Graham, Nonwovens in Filtration, 5th International Conference, Stuttgart, Germany, March 2003 20. A. Greiner, Lecture on “Electrospinning for Biomedical Applications” at DWI Aachen, Germany, 2005 21. H. Schreuder-Gibson, P. Gibson, K. Senecal, M. Senett, J. Walker, W. Yeomans, D. Ziegler, P.P. Tsai, Adv. Mater. 34, 44 (2002) 22. L. Huang, R.A. McMillan, R.P. Apkarian, B. Pourdeyhimi, V. Conticello, E.L. Chaikof, Macromolecules 33, 2989 (2000) 23. G. Verreck, I. Chun, J. Rosenblatt, J. Peeters, A. Van Dijck, J. Mensch, M. Noppe, M.E. Brewster, J. Control. Release 92, 349 (2003) 24. A. Frenot, I.S. Chronakis, Colloid Interf. Sci. 8, 64 (2003) 25. H. Thomas, E. Heine, R. Wollseifen, C. Cimpeanu, M. M¨ oller, Int. Nonwovens J. 14(3), 12 (2005) 26. P. Gibson, H. Schreuder-Gibson, D. Rivin, Colloids Surf. A: Physicochem. Eng. Asp. 187–188, 469 (2001) 27. M. Jacobsen, Nonwovens Industry (1991), pp. 36–41; cf. also: Chemiefasern/ Textilindustrie 39/91, 868 (1989) 28. E.R. Kenawy, G.L. Bowlin, K. Mansfield, J. Layman, D.G. Simpson, E.H. Sanders, G.E. Wnek, J. Control. Release 81, 57 (2002) 29. J. Zeng, Meso- and Nano-Scaled Polymer Fibers and Tubes Fabrication, Functionalization, and Characterization. Ph.D. Thesis, University Marburg (2003) 30. L. Larrondo, R. St. John Manley, J. Polym. Sci.: Polym. Phys. Ed. 19, 909 (1981) 31. L. Larrondo, R. St. John Manley, J. Polym. Sci.: Polym. Phys. Ed. 19, 921 (1981) 32. L. Larrondo, R. St. John Manley, J. Polym. Sci.: Polym. Phys. Ed. 19, 933 (1981) 33. D. Grafahrend, Darstellung von Polyethylenglycol-block-Polyester als Basis f¨ ur elektrogesponnene Nanofasern/Synthesis of polyethyleneglycol-block-polyesters for electrospun nanofibres, Diploma Thesis RWTH Aachen (2004) 34. K. Feil, Melt Electrospinning of Scaffolds for Tissue Engineering, Diploma Thesis, RWTH Aachen (2005) 35. P. Dalton, T. Kuenzel, D. Klee, M. Moeller, J. Mey, Eur. Cell Mater. 7(1), 52 (2004) 36. P.D. Dalton, D. Klee, M. M¨ oller, Polymer 46, 611 (2005) 37. G.I. Taylor, Proc. R. Soc. Lond. Ser. A 280, 383 (1964) 38. G.I. Taylor, Proc. R. Soc. Lond. Ser. A 313, 453 (1969) 39. M. Shin, M.M. Hohman, M.P. Brenner, G.C. Rutledge, Appl. Phys. Lett. 78, 1149 (2001) 40. M.M. Hohman, M. Shin, G. Rutledge, M.P. Brennera, Phys. Fluids 13(8), 2201 (2001)

138

K. Schaefer et al.

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

140

T. Le Blan et al.

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.

8 The Development of Non-Wovens

141

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

142

T. Le Blan et al.

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

8 The Development of Non-Wovens

143

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

144

T. Le Blan et al.

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.

8 The Development of Non-Wovens

Extruder

145

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.

146

T. Le Blan et al.

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.

8 The Development of Non-Wovens

147

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.

148

T. Le Blan et al.

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.

8 The Development of Non-Wovens

149

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.

150

T. Le Blan et al.

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

152

F. Carpi et al.

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

9 Mechanical Models and Actuation Technologies

153

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

154

F. Carpi et al.

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.

9 Mechanical Models and Actuation Technologies

155

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

156

F. Carpi et al.

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

9 Mechanical Models and Actuation Technologies

157

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])

158

F. Carpi et al.

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.

9 Mechanical Models and Actuation Technologies

159

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.

160

F. Carpi et al.

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].

9 Mechanical Models and Actuation Technologies

161

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.

162

F. Carpi et al.

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.

9 Mechanical Models and Actuation Technologies

163

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

164

F. Carpi et al.

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])

9 Mechanical Models and Actuation Technologies

165

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

166

F. Carpi et al.

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].

9 Mechanical Models and Actuation Technologies

167

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

168

F. Carpi et al.

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)

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 )

172

P. Van Hees and J. Axelsson

– 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)

10 Pyrolysis Modelling Within CFD Codes

173

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.

174

P. Van Hees and J. Axelsson T3

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.

10 Pyrolysis Modelling Within CFD Codes

175

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.

176

P. Van Hees and J. Axelsson

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).

10 Pyrolysis Modelling Within CFD Codes

177

300 600 ⬚C 550 ⬚C 500 ⬚C 450 ⬚C

250

HRR (kW/m2)

200

150

100

50

0 0

50

100

150

200

Time (s)

Fig. 10.2. Analysis of the influence of the pyrolysis temperature on the results 700 3 x 106 2 x 106

600

1 x 106 HRR (kW/m2)

500

0,5 x 106

400 300 200 100 0 60

80

100

120

140

Time (s)

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

178

P. Van Hees and J. Axelsson 500 14 MJ/kg 20 MJ/kg 30 MJ/kg 40 MJ/kg

HRR (kW/m2)

400

300

200

100

0 0

50

100

150

200

250

300

350

400

Time (s)

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.

10 Pyrolysis Modelling Within CFD Codes

179

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

150 HRR (kW/m2)

400 kg/m3 Virgin density 1000 kg/m3 100

50

0 0

100

200

300

400

500

Time (s)

Fig. 10.5. Analysis of the influence of the char density on the results 200 1000 kJ/kg 750 kJ/kg 500 kJ/kg 250 kJ/kg

HRR (kW/m2)

150

100

50

0 20

30

40

50

60

70

80

90

100

Time (s)

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

180

P. Van Hees and J. Axelsson 300 0,5 W/mK 0,25 W/mK 0,125 W/mK 0,01 W/mK

250

HRR (kW/m2)

200 150 100 50 0 −50 0

50

100

150

200

Time (s)

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

10 Pyrolysis Modelling Within CFD Codes

181

300 n=20 it=100 n=20 it=50 n=20 it=20 n=20 it=10 n=10 it=10

250

HRR (kW/m2)

200

150

100

50

0 0

50

100

150

200

250

300

Time (s)

Fig. 10.8. Analysis of the influence of the number of iterations and thickness of numerical strips on the results 800 700

HRR (kW/m2)

600 500

Tig >tpyr Tig=Tpyr

400

Tig 3 kW.

12 Modelling of Euroclass Test Results

221

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

140

120 hrrSBIsim(kW) HRRSBI

100

80

60

40

20

0 0

30

60

90

120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600

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

222

P.V. Hees and J. Axelsson

140

hrrSBIsim(kW)

120

HRRSBI 100

80

60

40

20

0 0

30

60

90

120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570 600

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

12 Modelling of Euroclass Test Results

223

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

224

P.V. Hees and J. Axelsson

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

200

300 time (s)

400

500

600

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

100

200

300 time (s)

400

500

600

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

data

hrr (kW)

60 50 Conetools

40 30 20 10 0

0

100

200

300

400

500

600

700

time (s)

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

hrr (kW)

12 10

data

8 6 4 Conetools

2 0

0

100

200

300 time (s)

400

500

600

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

225

226

P.V. Hees and J. Axelsson

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

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.

230

R.M. Silva et al.

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.

13 Characterisation of Barrier Effects in Footwear

231

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.

232

R.M. Silva et al.

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.

13 Characterisation of Barrier Effects in Footwear

233

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.

234

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.

13 Characterisation of Barrier Effects in Footwear

235

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.

236

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.

13 Characterisation of Barrier Effects in Footwear

237

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

238

R.M. Silva et al.

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]

13 Characterisation of Barrier Effects in Footwear

239

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

240

R.M. Silva et al.

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

13 Characterisation of Barrier Effects in Footwear

241

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].

242

R.M. Silva et al.

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.

13 Characterisation of Barrier Effects in Footwear

243

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

244

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.

13 Characterisation of Barrier Effects in Footwear

245

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

246

R.M. Silva et al.

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.

13 Characterisation of Barrier Effects in Footwear

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

248

R.M. Silva et al.

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

13 Characterisation of Barrier Effects in Footwear

249

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

250

R.M. Silva et al.

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.

13 Characterisation of Barrier Effects in Footwear

251

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.

252

R.M. Silva et al.

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

13 Characterisation of Barrier Effects in Footwear

253

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

254

R.M. Silva et al.

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

13 Characterisation of Barrier Effects in Footwear

255

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

256

R.M. Silva et al.

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

13 Characterisation of Barrier Effects in Footwear

257

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

258

R.M. Silva et al.

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 .

13 Characterisation of Barrier Effects in Footwear

259

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.

260

R.M. Silva et al.

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,

13 Characterisation of Barrier Effects in Footwear

261

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

262

R.M. Silva et al.

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

13 Characterisation of Barrier Effects in Footwear

263

– 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.

264

R.M. Silva et al.

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.

13 Characterisation of Barrier Effects in Footwear

265

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)

266

R.M. Silva et al.

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)

13 Characterisation of Barrier Effects in Footwear

267

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

268

R.M. Silva et al.

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

270 E. Jandos et al.

14 Filtration Technologies in the Automotive Industry

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

272

E. Jandos et al.

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 20 µm