Handbook of nonwovens

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Handbook of nonwovens

i

ii

Related titles: Recycling in textiles (ISBN-13: 978 1-85573-952-9; ISBN-10: 1-85573-952-6) An increasing amount of waste is generated each year from textiles (including carpets and clothing) and their production. For economic and environmental reasons it is necessary that as much as possible of this waste is recycled instead of being disposed of in landfill sites. On average approximately ten million tonnes of textile waste is currently dumped in Europe and America each year. Recycling in textiles is the first book to discuss all the issues, technology, products, processes and applications for those in the industry who are now looking for ways to recycle their textile waste.

Digital printing of textiles (ISBN-13: 978-1-85573-951-2; ISBN-10: 1-85573-951-8) The textile industry currently produces the majority of its 34 billion square metres of printed textile fabric by screen printing, but developments in the digital printing of paper are now being adapted for the textiles market. This collection gives full coverage of the current developments in digital textiles printing. It is divided into four parts covering printer and print head technologies, printer software, digital printing coloration and design and business for digital printing. It contains fundamental technical explanations along with current research.

Intelligent textiles and clothing (ISBN-13: 978-1-84569-005-2; ISBN-10: 1-84569-005-2) Intelligent textiles and clothing can be defined as those which react to exterior or physiological stimuli. This book brings together recent research in the area. The book is divided into five parts, each one containing an overview chapter followed by specific research and applications. Its main focus is on phase chromic and conductive materials. It is an essential read for anyone wanting to know more about the intelligent possibilities of textiles. Details of these books and a complete list of Woodhead titles can be obtained by: ∑ visiting our website at www.woodheadpublishing.com ∑ contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 893694; tel.: +44 (0) 1223 891358 ext. 30; address: Woodhead Publishing Limited, Abington Hall, Abington, Cambridge CB21 6AH, England)

Handbook of nonwovens Edited by S. J. Russell

CRC Press Boca Raton Boston New York Washington, DC

WOODHEAD

PUBLISHING LIMITED Cambridge, England iii

iv Published by Woodhead Publishing Limited in association with The Textile Institute Woodhead Publishing Limited, Abington Hall, Abington Cambridge CB21 6AH, England www.woodheadpublishing.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2007, Woodhead Publishing Limited and CRC Press LLC © 2007, Woodhead Publishing Limited The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN-13: 978-1-85573-603-0 (book) Woodhead Publishing ISBN-10: 1-85573-603-9 (book) Woodhead Publishing ISBN-13: 978-1-84569-199-8 (e-book) Woodhead Publishing ISBN-10: 1-84569-199-7 (e-book) CRC Press ISBN-13: 978-0-8493-2596-0 CRC Press ISBN-10: 0-8493-2596-X CRC Press order number: WP2596 The publishers’ policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elementary chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Project managed by Macfarlane Production Services, Dunstable, Bedfordshire (email: [email protected]) Typeset by Replika Press Pvt Ltd, India Printed by T J International Limited, Padstow, Cornwall, England

Contents

Contributor contact details

xi

1

1

Development of the nonwovens industry A WILSON, Nonwovens Report International, UK

1.1 1.2 1.3 1.4 1.5

Definition and classification Dry, wet and polymer-laid nonwovens Market structure and development Key companies References

1 4 10 15 15

2

Dry-laid web formation

16

A G BRYDON, Garnett Group of Associated Companies, UK (Sections 2.1–2.12) and A. POURMOHAMMADI, Consultant, Iran (Sections 2.13–2.20)

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14

Introduction Selection of raw materials for carding Opening of fibres Mixing and blending Carding: working and stripping principles Roller operations Card clothing Card and Garnett machine configurations Card feed control, weight measurement and other control systems Cross-lapping Batt drafting Vertically lapped (perpendicular-laid) web formation Airlaid web formation: raw materials and fibre preparation Airlaying technology

16 16 19 24 32 37 44 53 58 67 71 72 76 80 v

vi

Contents

2.15 2.16 2.17 2.18

Developments in airlaying Airflow and fibre dynamics in airlaying Bonding and web consolidation Physical properties and practical applications of airlaid fabrics Direct feed batt formation References

2.19 2.20 3

98 101 104 106 109 109 112

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12

Wet-laid web formation C WHITE, Consultant, France Introduction Background and historical developments Theoretical basis of wet forming Raw materials for wet-laid nonwovens Cellulose fibre preparation Man-made fibre preparation Web-forming process technology Bonding systems for wet-laid nonwovens Finishing Product applications Sources of further information References

4

Polymer-laid web formation

143

112 112 114 116 126 126 128 135 138 139 141 141

G S BHAT, University of Tennessee, USA and S R MALKAN, Synfil Technologies, USA

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16

Introduction Resins for spunbonding and meltblowing Spunbond fabric production Spunbond production systems Bonding techniques Operating variables in the spunbond process Structure and properties of spunbond fabrics Spunbond fabric applications Meltblown fabric production Meltblown characterization techniques Characteristics and properties of meltblown fabrics Meltblown fabric applications Mechanics of the spunbond and meltblown processes Composite fabrics and other extrusion processes Future trends References

143 143 149 155 157 160 168 171 172 180 184 185 186 192 195 195

Contents

5

Mechanical bonding

vii

201

S C ANAND, The University of Bolton, UK (Sections 5.1–5.8); D BRUNNSCHWEILER, Consultant, and G SWARBRICK, Foster Needle Ltd, UK (Sections 5.9–5.13); and S J RUSSELL, University of Leeds, UK (Sections 5.14–5.19)

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 5.19 5.20 5.21

Stitch bonding: introduction The Maliwatt and Malivlies stitch-bonding systems The Malimo stitch-bonding system Malipol Voltex Kunit Multiknit stitch-bonding systems Recent developments in stitch bonding Needlepunching: introduction Needle design and selection Penetration depth and other factors affecting needle use Needlepunching technology Applications of needlepunched fabrics Hydroentanglement: introduction The principles of hydroentanglement Fibre selection for hydroentanglement Process layouts Hydroentanglement process technology Applications of hydroentangled fabrics Acknowledgements References

201 202 206 214 215 216 217 220 223 226 234 240 251 255 256 264 269 275 288 294 294

6

Thermal bonding

298

A POURMOHAMMADI, Consultant, Iran

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7

Introduction Principle of thermal bonding Raw materials Calender (contact) bonding Through-air and impingement bonding Thermal radiation/infra-red and ultrasonic bonding Thermally bonded fabric structure Applications of thermally bonded fabrics References

298 299 300 305 318 322 325 327 328

Chemical bonding

330

R A CHAPMAN, Warwick Innovation Limited, UK

7.1 7.2

Introduction Chemical binder polymers

330 331

viii

Contents

7.3 7.4 7.5 7.6 7.7

Mechanism of chemical bonding Methods of binder application Drying Applications of chemically bonded nonwovens References

344 349 356 361 366

8

Nonwoven fabric finishing

368

A I AHMED, NIRI, UK

8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

Introduction Wet finishing Application of chemical finishes Lamination Mechanical finishing Surface finishing Developing technologies Fabric inspection Acknowledgements

368 369 376 385 389 394 398 399 400

9

Characterisation, testing and modelling of nonwoven fabrics

401

N MAO and S J RUSSELL, University of Leeds, UK (Sections 9.1–9.21); B POURDEYHIMI, Nonwovens Cooperative Research Centre, North Carolina State University, USA (Section 9.22)

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17 9.18 9.19

Introduction: characterisation of nonwoven fabrics Characterisation of fabric bond structure Fabric weight, thickness, density and other structural parameters General standards for testing nonwovens Measurement of basic parameters Measuring fibre orientation distribution Measuring porosity, pore size and pore size distribution Measuring tensile properties Measuring gas and liquid permeability Measuring water vapour transmission Measuring wetting and liquid absorption Measuring thermal conductivity and insulation Modelling pore size and pore size distribution Modelling tensile strength Modelling bending rigidity Modelling specific permeability Modelling absorbency and liquid retention Modelling capillary wicking Modelling thermal resistance and thermal conductivity

401 403 408 413 426 430 431 439 440 441 442 448 449 452 455 457 467 468 474

Contents

9.20 9.21 9.22 9.23

ix

Modelling acoustic impedance 478 Modelling filtration properties 483 The influence of fibre orientation distribution on the properties of thermal bonded nonwoven fabrics 492 References 502 Index

515

x


Editor

Chapter 2

Professor S. J. Russell Nonwovens Research Group School of Design University of Leeds Leeds LS2 9JT UK

Alan Brydon (main contact for Sections 2.1–2.12) Garnett Controls Ltd 3 Water Lane Bradford BD1 2JL UK

E-mail: [email protected]


Chapter 1 Mr Adrian Wilson 19 Sandal Cliff Sandal Wakefield WF2 6AU UK

Dr Ali Pourmohammadi (main contact for Sections 2.13–2.20) 3rd Floor, No. 23, 6th Street Bokharest Avenue Tehran 15146 Iran



Chapter 3 Mr Colin White Consultant in nonwovens technology Les Rossignols Chemin de la font del Sauze Vinas le Bas 34700 Lodève France E-mail: [email protected] xi

xii


Chapter 4 Professor Gajanan Bhat 434 Dougherty Engineering building University of Tenessee Knoxville TN 37996-2200 USA E-mail: [email protected]

Professor Sanjiv R Malkan President and CEO Synfil Technologies P.O. Box 31486 Knoxville TN 37930-1486 USA E-mail: [email protected]

Chapter 5 Professor S. Anand (main contact for Sections 5.1–5.8) Centre for Materials Research and Innovation University of Bolton Deane Road Bolton BL3 5AB UK E-mail: [email protected]

Professor D. Brunnschweiler (main contact for Sections 5.9.–5.13) Balderstone Lodge Commons Lane Balderstone Blackburn BB2 7LP UK

George Swarbrick Foster Needle Limited P.O. Box 7246 Wigston LE18 4WW UK E-mail: [email protected] Professor S. Russell (main contact for Sections 5.14–5.19) School of Nonwovens Research Group Design University of Leeds Leeds LS2 9JT UK E-mail: [email protected]

Chapter 6 Dr Ali Pourmohammadi 3rd Floor, No. 23, 6th Street Bokharest Avenue Tehran 15146 Iran E-mail: [email protected]

Chapter 7 Mr R. A. Chapman 3 The Wardens Kenilworth CV8 2UH UK E-mail: [email protected]


Chapter 8 Dr A. Idris Ahmed NIRI Woodhouse Lane Leeds LS2 9JT UK Email: [email protected]

Chapter 9 Dr N. Mao Nonwovens Research Group Centre for Technical Textiles School of Design Woodhouse Lane University of Leeds Leeds LS2 9JT UK E-mail: [email protected]

xiii

Professor S. J. Russell Nonwovens Research Group School of Design University of Leeds Leeds LS2 9JT UK E-mail: [email protected]

Professor Behnam Pourdeyhimi (main contact for Section 9.22) Nonwovens Cooperative Research Center The College of Textiles North Carolina State University 2401 Research Drive Raleigh NC 277769-8301 USA E-mail: [email protected]

xiv

1 Development of the nonwovens industry A WILSON Nonwovens Report International, UK

1.1

Definition and classification

In defining what a nonwoven is, there is always at least one exception that breaks the rule. This is perhaps fitting, since while being now recognised in its own right, the nonwovens industry has drawn on the practices and knowhow of many other more well-established fields of polymer and materials manufacturing with a piratical disregard and an eye to the most diverse range of end-use products. For this reason, it is possible for companies with almost nothing in common, with vastly different structures, raw materials and technologies, areas of research and development and finally, customers to be grouped together under the nonwovens ‘umbrella’. Many would define themselves by the customers they serve, as being in the medical, automotive, hygiene or civil engineering industries, for example. The term ‘nonwoven’ arises from more than half a century ago when nonwovens were often regarded as low-price substitutes for traditional textiles and were generally made from drylaid carded webs using converted textile processing machinery. The yarn spinning stage is omitted in the nonwoven processing of staple fibres, while bonding (consolidation) of the web by various methods, chemical, mechanical or thermal, replaces the weaving (or knitting) of yarns in traditional textiles. However, even in the early days of the industry, the process of stitchbonding, which originated in Eastern Europe in the 1950s, employed both layered and consolidating yarns, and the parallel developments in the paper and synthetic polymer fields, which have been crucial in shaping today’s multi-billion dollar nonwovens industry, had only tenuous links with textiles in the first place. Therefore, the nonwoven industry as we know it today has grown from developments in the textile, paper and polymer processing industries. Today, there are also inputs from other industries including most branches of engineering as well as the natural sciences. Certainly today, the nonwovens industry is reluctant to be associated with the conventional textile industry and its commodity associations nor would it want its products to be called ‘nonpapers’ or ‘nonplastics’. The term 1

2


‘nonwoven’, then, which describes something that a product is not, as opposed to what it actually is, has never accurately represented its industry, but any attempts to replace it over the years have floundered. The illusion created by this misnomer has been for some to think of nonwovens as some kind of bulk commodity, even cheap trade goods, when the opposite is often true. The nonwovens industry is highly profitable and very sophisticated, with healthy annual growth rates in double digits in certain sectors and parts of the world. It is perhaps one of the most intensive industries in terms of its investment in new technology, and also in research and development. EDANA, (The European Disposables and Nonwovens Association) defines a nonwoven as ‘a manufactured sheet, web or batt of directionally or randomly orientated fibres, bonded by friction, and/or cohesion and/or adhesion’, but goes on to exclude a number of materials from the definition, including paper, products which are woven, knitted, tufted or stitchbonded (incorporating binding yarns or filaments), or felted by wet-milling, whether or not additionally needled. To distinguish wetlaid nonwovens from wetlaid paper materials, the following differentiation is made, ‘more than 50% by mass of its fibrous content is made up of fibres (excluding chemically digested vegetable fibres) with a length to diameter ratio greater than 300’. Other types of fabric can be classified as nonwoven if, ‘more than 30% by mass of its fibrous content is made up of fibres (excluding chemically digested vegetable fibres) with a length to diameter ratio greater than 300 and its density is less than 0.40g/ m3. This definition, which forms ISO 9092:1988 and EN 29092, was most likely coined prior to the enhancement of plastic film layers which have become broadly indistinguishable from fabrics in modern multi-component or composite nonwovens. INDA, North America’s Association of the Nonwoven Fabrics Industry, describes nonwoven fabrics as ‘sheet or web structures bonded together by entangling fibres or filaments, by various mechanical, thermal and/or chemical processes. These are made directly from separate fibres or from molten plastic or plastic film.’ Nonwovens are engineered fabrics that can form products that are disposable, for single or short-term use or durable, with a long life, depending on the application. In practice, the life of a nonwoven product can be measured in seconds, minutes, hours or years but the design and engineering requirements of these fabrics are often complex and challenging regardless of the intended product life (Table 1.1). Nonwovens are engineered to provide specific functions to ensure fitness for purpose. These properties are combined to create the required functionality, while achieving a profitable balance between the expected product life and cost. Nonwoven technology also exists to approximate the appearance, texture and strength of conventional woven and textile fabrics and in addition to flat monolithic fabrics, multi-layer nonwoven composites, laminates and threedimensional nonwoven fabrics are commercially produced. In combination

Table 1.1 Examples of nonwoven product applications Hygiene

Wipes

Medical and surgical

Protective clothing

Filtration Interlinings Shoes, leather(gas and and goods and liquids) garments coating substrates

Upholstery, furniture and bedding

Floorcoverings

Building and roofing

Civil engineering and geosynthetics Landfill membrane protectors

Disposable Teabags clean-room garments

Fusible Boot and interlinings shoe and linings linings

Ticking

Contract carpets and carpet tiles

House wrap

Adult incont- Dusters inence pads

Wound dressings

Laboratory overalls

Drinks filtration

Shoulder pads

Synthetic leather shoe uppers

Mattress pads

Underlays and carpet backing fabrics

Thermal Drainage and sound systems insulation

Sanitary napkins

Dishcloths

Surgical gowns, masks and caps

Fire protective linings

Oil sorption

Glove linings

Shoe construction components

Waddings and fillings

Automotive carpets and trims

Roof linings

Lining systems for reservoirs and ponds

Tampons

Mops

Orthopaedic casts

Thermal insulation fillings

Industrial gas filtration

Luggage and bags

Sheets and blankets

Underslating

Erosion control and ground stabilisation Soil-separation

Cosmetic removal pads

Surgical High drapes, wraps visibility and packs clothing

Respiratory filters

Window blinds

Plasterboard facings

Nasal strips

Transdermal drug delivery

Vaccum filter bags

Quilt backings

Pipe wraps

Disposable underwear

Heat and procedure packs

Odour control

Dust covers

Fabric tiles (shingles)

Chemical defence suits

3

Note that the intended lifespan of a nonwoven product can be measured in seconds, minutes, hours, days, weeks, months or years depending on the particular product end-use. Compare for example the expected lifespan of a disposable wipe with a contract floorcovering. Nonwoven products may be intended for (i) single use with a short life (e.g. a teabag), (ii) single use with a long-life (e.g. a landfill protector) or (iii) multiple-use of variable life (e.g. a drinks’ machine filter).

Development of the nonwovens industry

Baby diapers Disposable Surgical and training wipes (dry swabs pants and premoistened)

4


with other materials nonwovens provide a spectrum of products with diverse chemical and physical properties. This is reflected in the large variety of industrial, engineering, consumer and healthcare goods into which nonwoven fabrics are incorporated. The conversion of nonwoven role products into finished products is a further important component step in the process and can also affect final product properties. The most common products made with nonwovens listed by INDA include: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

disposable nappies sanitary napkins and tampons sterile wraps, caps, gowns, masks and curtains used in the medical field household and personal wipes laundry aids (fabric dryer-sheets) apparel interlinings carpeting and upholstery fabrics, padding and backing wallcoverings agricultural coverings and seed strips automotive headliners and upholstery filters envelopes tags labels insulation house wraps roofing products civil engineering fabrics/geotextiles.

In Europe, EDANA1 publish detailed annual statistical tables relating to the deliveries of European-produced nonwovens in each of the various sectors and end-use categories. A breakdown showing the percentage for each end use in relation to the total weight of nonwoven deliveries is presented in Table 1.2. Hygiene is by far the largest of these categories, accounting for over 33% of European nonwovens production, followed by civil engineering/construction and building materials with 17.9%. The astonishing growth in recent years of the usage of wipes, both wet and dry, in a range of household and industrial products, is illustrated by their representing some 14.8% of nonwoven deliveries.

1.2

Dry, wet and polymer-laid nonwovens

Generally, in dividing nonwovens into three major areas – drylaid, wetlaid and polymer-laid (encompassing the spunmelt technologies of spunbond, meltblown and flashspun), it can be said that drylaid materials have their


5

Table 1.2 European-produced nonwoven deliveries by end use Classification

% of total

Hygiene Medical/surgical Wipes, personal care Wipes, other Garments Interlinings Shoe leathergoods Coating substrates Upholstery/table linen/household Floorcovering Liquid filtration Air/gas filtration Building/roofing Civil engineering/underground Automotive Others/unidentified

33.1 2.6 8.1 6.7 0.8 2.1 1.9 2.4 6.8 2.3 3.7 2.4 12.5 5.4 3.9 5.3

Source: EDANA

origins in textiles, wetlaid materials in papermaking, and polymer-laid products in polymer extrusion and plastics (remembering that there is always at least one exception to the rule). An overview of nonwoven manufacturing technologies is given in Fig. 1.1.

1.2.1

Drylaid nonwovens

The first drylaid systems owe much to the felting process known since medieval times. In the pressed felt industry, cards and web lappers were used to make a batt containing wool or a wool blend that is subsequently felted (hardened) using moisture, agitation and heat. Some of the drylaid webforming technologies used in the nonwovens industry, specifically carding and garnetting, originate from the textile industry and manipulate fibres in the dry state. In drylaid web formation, fibres are carded (including carding and cross-lapping) or aerodynamically formed (airlaid) and then bonded by mechanical, chemical or thermal methods. These methods are needlepunching, hydroentanglement, stitchbonding (mechanical), thermal bonding (sometimes referred to as thermobonding) and chemical bonding. Airlaid pulp web formation originated from the paper industry. Fabrics are formed by converting wood pulp in blends with man-made fibres into random-laid absorbent webs, using air as the dispersing medium and as the means of transferring fibres to the web-forming zone. In the traditional airlaid process, synthetic resin bonding agents were applied to the pulp web

6

Nonwovens Textile technology

Paper technology

Wetlay

Extrusion technology

Fibres 10–200 mm Web forming

Airlay

Garnetts

Cross-laying

Woollen cards

Drafting

Cotton cards

Filaments

Hybrid cards

Airlay cards

Web manipulation Spreading Scrambling

Melt blown

Spun laid

Flash spun

Crimping

Bonding Adhesive

Heat

Needlepunching

Hydroentangling

Stitch bonding

Calender/mangle Spray Foam Powder

Melt fibre (monofibre or bi-component) Powder Calender (plain or embossed) Oven (drum or lattice)

Tacking Plain needling

(Spunlace)

With or without yarn

Singeing

Coating

Multi-directional needling Textured needling

Finishing Printing

Embossing

Laminating

1.1 Overview of nonwoven manufacturing technologies (courtesy of D.B. Brunnschweiler).

Ultrasonics

Fibrillated film


Fibres 2–15 mm


7

using a spray process. Airlaid nonwovens are forecast to grow most rapidly by around 8% a year but from a very low base. Drylaid fabrics are the largest segment of the nonwovens industry and are forecast to expand by 5.3% over the next ten years.

1.2.2

Wetlaid nonwovens

Paper-like nonwoven fabrics are manufactured with machinery designed to manipulate short fibres suspended in liquid and are referred to as ‘wetlaid’. To distinguish wetlaid nonwovens from wetlaid papers, a material is regarded by EDANA as a nonwoven if ‘more than 50% by mass of its fibrous content is made up of fibres (excluding chemically-digested vegetable fibres) with a length to diameter ratio greater than 300, or more than 30% fibre content for materials with a density less than 0.40 g/cm3’. This definition excludes most wetlaid glass fibre constructions which sectors of the industry would class as nonwovens. The use of the wetlaid process is confined to a small number of companies, being extremely capital intensive and utilising substantial volumes of water. In addition to cellulose papers, technical papers composed of highperformance fibres such as aramids, glass and ceramics are produced.

1.2.3

Polymer-laid nonwovens

Polymer-laid or ‘spunmelt’ nonwovens including spunbond (spunlaid), meltblown, flash-spun, apertured films as well as layered composites of these materials, are manufactured with machinery developed from polymer extrusion. In a basic spunbonding system, sheets of synthetic filaments are extruded from molten polymer onto a moving conveyor as a randomly orientated web in the closest approximation to a continuous polymer-tofabric operation. Global spunmelt demand has grown on average by 11% per annum in recent years and it now has an estimated 25% share of the global nonwovens industry. Hygiene product components such as coverstocks, backs, distribution layers and leg-cuffs account for around 62% of spunmelt production, of which spunmelt materials account for around 65% of hygiene product components, and this is expected to rise still further to at least 72% in the coming years. Most of the first spunbonding systems were originated by fibre producers such as DuPont in the USA, Rhone-Poulenc in France and Freudenberg in Germany. DuPont is regarded as the first to successfully commercialise spunbonding with its Typar product, launched as a tufted carpet backing in the mid-1960s. The first commercial spunbonding system to be offered to the industry was the Docan system developed by the Lurgi engineering group in the 1960s. This was licensed to Corovin (now BBA) in Germany, Sodoca in France (now BBA), Chemie Linz in Austria, and Crown Zellerbach

8

Handbook of nonwovens Table 1.3 Development of Reifenhäuser Reicofil Technology, increase of specific throughput from 1986–2002 (kg per hour per metre of beam) Reicofil 1 system

Year 1986 1992

kg/hour/m of beam 50 100

1992 1995

125 145

1995 2002

150 195

2002

225

Reicofil 2 system

Reicofil 3 system

Reicofil 4 system Source: Reifenhäuser

and Kimberly-Clark in the USA. The next major step towards the global commercialisation of the spunbond process was the introduction of Reifenhäuser’s Reicofil system in 1984, which enabled many manufacturers to enter the market. The staggering increase in the productivity of spunbond machines over time is highlighted in Table 1.3.

1.2.4

Web formation

In all nonwoven web formation processes, fibres or filaments are either deposited onto a forming surface to form a web or are condensed into a web and fed to a conveyor surface. The conditions at this stage can be dry, wet, or molten – drylaid, wetlaid or polymer-laid (also referred to as spunlaid and spunmelt processes). Web formation involves converting staple fibres or filaments into a two-dimensional (web) or a three-dimensional web assembly (batt), which is the precursor for the final fabric. Their structure and composition strongly influences the dimensions, structure and properties of the final fabric. The fibre (or film) orientation in the web is controlled during the process using machinery adapted from the textile, paper or polymer extrusion industries. The arrangement of fibres in the web, specifically the fibre orientation, governs the isotropy of fabric properties and most nonwovens are anisotropic. Although it is possible to make direct measurements of the fibre orientation in a web, the normal approach is to measure the machine direction/cross direction (MD:CD) ratio of the web or more usually the fabric. This ratio of fabric properties, usually tensile strength, measured in the machine direction (MD) and cross direction (CD) reflects the fibre orientation in the fabric. Commercially, obtaining a web or a fabric with a truly isotropic structure, that is, with an MD:CD=1, is rarely achieved and technically is frequently unnecessary. Other critical fabric parameters influenced at the web formation stage are the unfinished product weight and the manufactured width.


9

Traditionally, each web-forming system was used for specific fibres or products, although it is increasingly common for similar commercial products to be made with different web formation systems. One example is in the manufacture of highloft nonwovens which can be produced with either a card and crosslapper or a roller-based airlaid system. In the hygiene industry, there is an increasing preference for the soft, staple fibre products produced by carding and hydroentanglement in favour of the alternative airlaid and thermal bonded products.

1.2.5

Web bonding

Nonwoven bonding processes can be mechanical, chemical (including latent bonding using solvents) or thermal. Hydrogen bonding is also important in bonding cellulosic webs. The degree of bonding is a primary factor in determining fabric mechanical properties (particularly strength), porosity, flexibility, softness, and density (loft, thickness). Bonding may be carried out as a separate and distinct operation, but is generally carried out in line with web formation. In some fabric constructions, more than one bonding process is used. Mechanical consolidation methods include needlepunching, stitchbonding, and hydroentangling. The latter process has grown considerably in popularity over the past few years. In respect of needlepunching, which is most commonly fed by a card and cross-lapper, the world production is in excess of an estimated 1.1 million tonnes of needlefelts of which over 72% used new fibres as opposed to reclaimed or recycled fibres. This sector represents about 35% of total nonwoven output. It is estimated that the usage of new fibres in needlefelts exceeds one million tonnes globally, and this is expected to rise by around 16% over the next ten years. Chemical bonding methods involve applying adhesive binders to webs by saturating, spraying, printing, or foaming techniques. Solvent bonding involves softening or partially solvating fibre surfaces with an appropriate chemical to provide self- or autogeneously bonded fibres at the cross-over points. Thermal bonding involves the use of heat and often pressure to soften and then fuse or weld fibres together without inducing melting.

1.2.6

Raw materials

Man-made fibres completely dominate nonwovens production, accounting for over 90% of total output. Man-made fibres fall into three classes, those made from natural polymers, those made from synthetic polymers and those made from inorganic materials. According to a study by Tecnon Ltd,2 the world usage of fibres in nonwovens production is: ∑ ∑

polypropylene 63% polyester 23%

10

∑ ∑ ∑ ∑


viscose rayon 8% acrylic 2% polyamide 1.5% other speciality fibres 3%.

The share of viscose rayon is thought to have increased due to its increased importance in the spunlace wipes market. While the tonnage of man-made cellulosics sold into European nonwovens held remarkably constant for thirty or more years and viscose rayon participated hardly at all in the massive growth of the industry and its market share by 2000 was a tenth of the 1970 figure. Viscose rayon staple fibres were, in 1966, the cheapest man-made fibre but by 2000 were around twice the price of the main synthetics without the ability to be easily spunlaid or thermally bonded. The solvent spun cellulosic fibre, Lyocell is becoming increasingly important in the nonwovens industry partly as a result of its absorbency and high wet strength. Polypropylene fibres are predominant in the nonwovens industry. Some of the reasons for this include: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

low density and specific gravity enabling lightweight fabrics to be produced low glass transition and melting temperature, which is attractive for thermal bonding inherent hydrophobicity that can be modified using fibre finishes and other treatments provides fabrics with good bulk and cover chemical stability biological degradation resistance (mildew, perspiration) stain and soil release good mechanical strength and abrasion resistance.

Polypropylene is available in a variety of grades and its surface chemistry, absorbency, mechanical properties, degradation, softness, flame retardancy and colouration are modified by auxiliary chemicals and other treatments by the fibre suppliers. Fibres having different cross-sectional configurations are also available, which affect the physical properties of resulting fabrics. The unique combination of properties offers the manufacturers of nonwovens a valuable high-performance nonwoven fibre for a competitive price.

1.3

Market structure and development

Until about the 1990s, much of the world’s nonwovens industry was based in those areas where the process technologies were conceived and developed, the USA, Europe and Japan. Many of these companies were and still remain small-scale enterprises, sometimes part of textile companies operating with a limited range of technologies often centred around carding and drylaid


11

web formation and needlepunching, chemical or thermal bonding. Meanwhile, the larger companies, such as Freudenberg, Kimberly-Clark, DuPont, Ahlstrom, Polymer Group Inc. (PGI), BBA and Asahi amongst others have been responsible for major process innovations and have nurtured them to commercial scale. A significant patent estate has also been developed to protect these developments, particularly by Kimberly-Clark. The large-scale production facilities set up by the big companies were highly capital intensive, making it too risky for smaller companies to set up production, certainly of spunlaid, wetlaid, airlaid pulp and hydroentangling businesses. The industry can still be regarded as capital intensive today, when considering that, according to the latest estimates, some 40 companies are responsible for 90% of total global nonwovens sales. When machinery builders, notably Reifenhäuser, among others, began to produce ‘turn-key’ production lines capable of making high-quality nonwoven fabrics at competitive costs, the result was further strong growth in the original three regions of the USA, Europe and Japan as new end-markets for nonwoven fabrics developed with the increased fabric supply from new nonwoven producers. At the same time, the industry began to expand globally with many new local producers. Most world regions now have nonwovens production and growth remains high, with many countries still in the early stage of industrialisation. The influence of developments in the man-made fibre industry on the technical progress and economic viability of the nonwovens industry should not be underestimated.

1.3.1

Structure of the market

The latest estimates, taking into account official INDA and EDANA figures, put the total global nonwovens production at over 3.3 million tonnes, with Western Europe accounting for around 33%, North America 31%, the AsiaPacific region 25%, and the remaining 11% produced outside these regions. The value placed on this production is somewhere between 710–11 billion. Western Europe Europe has only recently overtaken the USA as the leading nonwovens producer region as a result of multiple new installations over the past few years. According to figures released by EDANA, production of nonwovens in Europe reached a record 1,335,900 tonnes in 2004, compared to 1,025,900 tonnes in 2000. This additional production has mainly come from new developments in airlaid, spunlaid and hydroentangled nonwovens, essentially for disposable or short-life end-uses, but also from the inclusion of some companies based in countries which have only recently become part of the European Union.

12


North America North American nonwoven consumption climbed to 1,074,000 tonnes in 2003 up from 964,000 tonnes in 2000, according to industry body INDA. While expansion in the absorbent hygiene industry is expected to be modest, strong growth is expected in wipes and various airlaid pulp applications. Nonwoven consumption climbed to 1,004,000 tonnes in 2001 from 964,000 tonnes in 2000, which represents a 4.1% annual increase, according to industry body INDA. This equates to 22.2 billion square metres. INDA forecasts North American nonwovens will rise in tonnage to 1,355,000 tonnes over the next five years, representing an average annual growth of 6.3%. This forecast is consistent with the industry’s historical performance, which grew at an average rate of about 6% per year throughout the 1990s. While expansion in the absorbent hygiene industry is expected to be modest, strong growth is expected in wipes and various airlaid pulp applications. INDA adds that the square metre demand by the consumer and industrial wipes markets rose 8% during 2001 over the previous year, following many new consumer and industrial product introductions during the previous two years. Wipes accounted for retail sales of about $2 billion and the growth forecast for the wipes industry is 6–7% per year to 2006. There is growing use of the airlaid and hydroentangled (spunlaced) nonwoven technologies by this market.

Far East In 2001 total nonwovens output in Japan fell for the first time since 1997 by around 5%, to just under 300,000 tonnes, according to figures released by the Japanese Ministry of Economy, Trade and Industry (METI). The value of this production is put at Yen 190.4 billion (71.57 billion). There is a clear trend among Japanese companies of moving production to other Asian countries. Production in Korea also fell, though by just 1%, to 130,694 tonnes in 2001. Drylaid production dominates Korea’s nonwovens industry, accounting for more than 70% of total production. According to the Korean Nonwovens Industry Co-operative, there are now 262 production lines in the country, with only 16 being spunbond and/or metblown. From an output of 10,000 tonnes in 1980, China’s nonwoven production reached 350,000 tonnes in 2000 and targets 800,000 tonnes by 2010, according to figures from the China Nonwovens Technical Association (CNTA). Since making man-made fibre production a strategic target in the early 1950s, China’s share of the world man-made fibre market has grown from 0.3% in 1960 to 24% in 2000, or almost seven million tonnes, according to a study by the Chinese Academy of Engineering. The Chinese population doubled to 1.32 billion over this period. While much of the country’s production is accounted for by older needlepunch and carding technology, the country is investing heavily in


13

modern production technology. In considering the Chinese market’s potential growth, it is necessary to consider that, as a proportion of Chinese skilled worker income, Western-style disposable nappies and femcare still appear about ten times as expensive as they do in Europe or the USA. A month’s supply of Western-style femcare requires 2% of a Chinese secretary’s income, but nevertheless, this market is growing well. For nappies, the wage percentage figure is between 11 and 23% and this is too high to allow regular use at present. Overall, there are more than enough nonwovens produced in China to meet current internal market needs, so a lot is being exported at very competitive prices and it has been predicted that the biggest Chinese nonwoven companies will be starting up plants in the West within ten years. Spunbond and hydroentanglement machinery has also been developed in China at much lower cost than Western machines, and these are now available for export to the USA and Europe. Middle East The Middle East represents only between 4 and 5% of global production, most of it in Israel. There has been significant investment in new spunbond, airlaid and thermal bonding facilities in the region. Mercosur Figures show that the Mercosur countries; Argentina, Brazil, Paraguay and Uruguay produce a total of 88,000 tonnes of nonwovens per year. Of this, spunbonds are the majority, followed by carded-thermal and airlay. Mercosur imports 11,000 tpa and exports 10,000 tpa, giving a 0.4 kg per-capita consumption of nonwovens – about one-tenth of the USA figure. Growth potential is therefore enormous. For nappies, for example, Brazil has nearly ten million children under the age of two and nappy sales penetration of less than 30%. At the same time, this nappy market was said to have seen sales fall by 18% in recent years. Hygiene disposables The hygiene disposables market is by far the biggest in the nonwovens industry and its major consumer goods players, notably Procter & Gamble, Kimberly-Clark and Johnson & Johnson, understandably have a tremendous influence. Parallels can be drawn with the Tier 1, Tier 2 and Tier 3 structure of the automotive industry, where nonwovens manufacturers are sub-suppliers tied in to extensive contracts for which entire manufacturing lines, even entire plants, can be exclusively dedicated. The main three hygiene disposable product areas are nappies, feminine hygiene products and adult incontinence

14


products, and there is also considerable overlap in other fields such as medical nonwovens and wipes which are also manufactured by the consumer giants. In 1999 USA consultancy John Starr Inc. estimated the global hygiene absorbent products market to be worth $40 billion. Disposables were said to Table 1.4 The top nonwovens companies (2004)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Company

2004 Sales

Freudenberg DuPont Kimberly-Clark BBA Fiberweb PGI Nonwovens Ahlstrom Johns Manville Colbond Buckeye Technologies Japan Vilene Owens-Corning Asahi Kasei Hollingsworth & Vose British Vita Sandler Fibertex Lohmann Foss Manufacturing Toyobo Western Nonwovens Georgia-Pacific Polyfelt Mitsui Chemical Lydall Avgol SI Corporation Concert Industries Orlandi Suominen Nonwovens Pegas Textilgruppe Hof Jacob Holm Andrew Industries Unitika Propex Fabrics (formerly BP Amoco) Toray Saehan Kuraray Precision Custom Coatings KNH Companhia Providencia Lantor BV Fitesa

$1.4 billion $1.25 billion $1.15 billion $1 billion $845 million $827 million $550 million $265 million $226 million $197 million $191 million $176 million $175 million $170 million $162 million $160 million $158 million $157 million $145 million $140 million $139 million $128 million $127 million $126 million $123 million $120 million $117 million $97 million $93.5 million $93 million $91 million $91 million $89 million $85 million $82 million $80 million $71 million $70 million $65 million $60 million $53 million $37 million

Source: Nonwovens Industry


15

have now penetrated about 15% of the total available market, or 41% of the major geographic markets. Nappies and training pants amounted to $19 billion or 84 billion units, tampons, sanitary napkins and pantyliners were worth $16 billion or 160 billion units (the tampons accounting for 16 billion units) and there were 12 billion adult incontinence products sold – a market worth $5 billion. The industry consumed 36 billion square metres of coverstock, 3.3 million tonnes of pulp, 1.1 million tonnes of SAP and 500,000 tonnes of barrier film. Of a nappy maker’s total manufacturing revenue, 40% is spent on raw materials.

1.4

Key companies

According to the American magazine Nonwovens Industry, the total combined estimated sales of the top 42 nonwovens companies accounted for more than 90% of total global nonwoven sales. Within this top 42, ‘the many companies investing capital within their businesses, whether new production machinery, new plants or the acquisition of smaller companies, considerably outweighed more negative factors such as plant closings and financial troubles’. The companies are listed in Table 1.4. Each company is ranked on the basis of their 2004 sales figures, but the top five players, while encountering fluctuating fortunes, have remained at the helm for many years. The top five companies have achieved nonwoven sales of over 75 billion – approaching half of the total sales of the top 42 manufacturers. These 42 companies control 90% of the nonwovens industry, and significantly just five companies control half of that.

1.5

References

1. EDANA 2004 Nonwoven Statistics 2. Nonwoven Textiles 1997–2007 World Survey, Tecnon Ltd.

2 Dry-laid web formation A G B RY D O N Garnett Group of Associated Companies, UK (Sections 2.1–2.12) and A POURMOHAMMADI Consultant, Iran (Sections 2.13–2.20)

2.1

Introduction

The dry-laid nonwoven sector utilises carding, garnetting, airlaying and in certain specialist applications, direct feed batt formation processes to convert staple fibres into a web or batt structure that is uniform in weight per unit area.

2.2

Selection of raw materials for carding

Virtually any fibre that can be carded can be, and probably already is, used in nonwovens including both organic and inorganic fibres. As noted in Chapter 1, man-made fibres account for the majority of raw materials used in the nonwovens industry, and in the carding sector, polyester is the most widely used. This is principally because of its suitability for many product applications and comparatively low cost. Polypropylene is also important, particularly in the manufacture of heavyweight needled fabrics for durable products such as floorcoverings and geosynthetics as well as in needlepunched filtration media and lightweight thermal bonded fabrics for hygiene disposables. Viscose rayon is extensively used in the medical and hygiene sectors, principally because of its high moisture regain. The flexibility of the carding process is reflected by the diversity of staple fibre types that are utilised by the industry and includes polymers, glass and ceramic materials. Table 2.1 gives a general overview of the fibres that are carded either alone or in blends. Fundamental to the suitability of a particular fibre for dry-laid processing is its machine compatibility as well as its influence on fabric properties. There are numerous examples of new fibre developments that have been slow to develop because of processing problems, particularly during carding. Common problems are uncontrolled static electricity, low fibre-to-fibre cohesion and low fibre extension (the minimum required is 2–5%) leading to fibre breakage and poor yield. Whilst natural fibres such as cotton and wool have been carded as long as cards have been in existence, man-made fibres 16

Table 2.1 Summary of fibre properties Physical properties of textile fibres Name

Range of diameter (m)

Density (g/cm3)

Tenacity (gf/tex)

Breaking extension (%)

Moisture regain 65% r.h. (%)

Melting point (∞C)

Natural Vegetable

Cotton Flax Jute Wool Silk Viscose rayon Acetate Triacetate Nylon 6 Nylon 6.6 Polyester Acrylic

11–22 5–40 8–30 18–44 10–15 12+ 15+ 15+ 14+ 14+ 12+ 12+

1.52 1.52 1.52 1.31 1.34 1.46–1.54 1.32 1.32 1.14 1.14 1.34 1.16

35 55 50 12 40 20 13 12 32–65 32–65 25–54 20–30

7 3 2 40 23 20 24 30 30–55 16–66 12–55 20–28

7 7 12 14 10 13 6 4 2.8–5 2.8–5 0.4 1.5

Polypropylene Spandex (Lycra) Glass Asbestos

– –

0.91 1.21

60 6–8

20 444–555

0.1 1.3

– – – – – – 230 230 225 250 250 Sticks at 235 165 230

5+ 0.01–0.30

2.54 2.5

76 –

2–5 –

0 1

800 1500

Natural Animal Regenerated

Synthetic

Inorganic


Fibre type

17

18


such as polyester have evolved to improve compatibility with high-speed nonwoven carding systems. The applied forces in carding give rise to fibre breakage and permanent fibre elongation, which modifies the original fibre length distribution and in some low-temperature materials such as PVC may be subject to thermal shrinkage during the process. Whilst exceptions do exist, the general range of fibre dimensions suitable for the carding sector can be given approximately as 1–300 dtex fibre linear density and 15–250 mm mean fibre length. In practice such a range of fibre dimensions could not be satisfactorily processed on one card without modifying the card roller configuration and layout, settings and the card wire. Blending extends the range of fibre lengths and finenesses that can be processed and in certain sectors of the industry carrier fibres are used to aid processing of short, stiff or low surface friction materials. It should also be understood that the mean fibre length and the fibre length distribution as measured before carding is substantially different after carding due to fibre breakage or permanent elongation of fibres in the process. Cotton and other short-staple fibres of 50 mm and colour blends. Multi-roll openers, pickers or fine openers commonly suffice for other applications. Fine openers provide efficient in-line opening for fibres up to about 100 mm length. Such machines are arranged horizontally or vertically and are incorporated in feeding units and chutes as well as blending hoppers (see Figs 2.3 and 2.4). In chute feed systems, a pair of feed rollers presents fibre to a revolving opening roller that is clothed with either pins or coarse card wire. A secondary chute with delivery rollers that feed a finely pinned opening roller operating with a high surface speed follows this. Examples are shown in Fig. 2.4. Single roll openers are frequently suitable for opening polyester whereas a multi-roll opener may be used to open bleached cotton or viscose rayon where the tufts are more heavily entangled. One of the most important considerations in opening is the state or condition of the incoming fibre in

2.2 Bale picker – (automatic bale opener) (courtesy of Trützschler GmbH, Germany).


23

2.3 Schematic of a Fearnought Opener with pneumatic doffing (courtesy of OMMI, Italy).

terms of fibre entanglement and tuft density. Fibre entanglement is generally reduced at the expense of unwanted fibre breakage and to minimise such fibre damage, gradual opening using a sequence of opening units (rather than one single unit) is required to progressively reduce the tuft size. Based on this stepwise approach, in which a sequence of opening units is used, a theoretical optimum opening curve has been proposed (see example in Fig. 2.5). As well as the design of the feed system and the number of opening rollers used, the type of clothing, pin density or blade frequency, gauge settings and surface speeds are also varied according to the fibre opening required and the incoming tuft size. The most intensive opening is generally achieved by presenting fibre to the opener roller (or beater) via a pair of clamped feed rollers rather than by an airstream. The theoretical tuft size after each stage of opening can be estimated based on the opening roller design, feed rate and fibre linear density. It is important to recognise that decreasing the average tuft size by progressive fibre separation promotes homogeneous mixing of the different fibre components because the tufts are smaller. Also, as the tufts are reduced in size impurities are more likely to be liberated from the fibre. Clearly, it is advantageous to remove such impurities before carding, if possible, to maximise the life of the card clothing and yield.

2.3.4

Disc opener

The disc opener shown in Fig. 2.6 is remarkable in that it has only one moving part. Fibre is drawn through the system under negative pressure. As

24


4.1 5

6

4.4 4.2 4.1 4.3

6

(a)

(b)

4.1 4.2 4.3 4.4 5 6

4.1

Fully spiked roller Coarse saw tooth roller Medium saw tooth roller Fine saw tooth roller Mote knife Fixed carding segment

(c)

(d)

(e)

(f)

2.4 Opening machine variants and integrated feeding and fibre opening units; (a) multi-roll opener; (b) single roll opener; (c) universal opener; (d) blending hopper with universal opener; (e) feeding unit with single roll opener; (f) feed trunk with universal opener (courtesy of Trützschler GmbH, Germany).

the fibre enters the expansion chamber it makes contact with a high-speed rotating disc that is studded with stainless steel pins. The pins drag the fibre across a stationary, pinned plate and the opening takes place between the pins on the plate and those on the disc. Fibre then continues within the airflow, and is transported out of the machine via the exit chamber.

2.4

Mixing and blending

Different fibre types, grades or dimensions are blended either to obtain a particular combination of physical properties in the final fabric or for economic reasons to minimise cost. In some sectors, such as the manufacture of needlepunched floorcoverings, stock dyed or spun dyed fibres are blended to create specific colour and shade effects. Clearly, the mixing in such blends

Dry-laid web formation Opening degree

25

Theoretical tuft weight (g/tuft) 10–6 10–5 10–4 10–3 10–2 10–1 100 101 102 103

(a) For pinned rollers:

N=

number of fibres per minute (F/min) speed of roller (rpm) ¥ surface area of roller (cm2 ) ¥ points/cm2

For beater rollers: N=

(F/min) =

number of fibres per minute (F/min) blows per minute (B/min)

mass of fibre per minute (mg/min) ¥ 10 5 fibre linear density (mtex) ¥ average fibre length (cm)

B/min = number of blades or pins on roller ¥ roller speed (rpm) Examples: calculation of N for different rollers

1. Three bladed 1000 rpm 3 blades

2. Multibladed 1000 rpm 250 blades

3. Pinned 1560 rpm 3.7 teeth/cm2 (48,355 teeth on roller)

6 ¥ 10 8 = 200,000 3 ¥ 1000

6 ¥ 10 8 = 2400 250 ¥ 1000

6 ¥ 10 8 =8 1560 ¥ 48,355

(assuming a feed rate of 6 ¥ 108 fibres per minute) (b)

2.5 Opening sequence for fibres; (a) progressive fibre opening across a series of fibre opening units; (courtesy of Trützschler GmbH, Germany) (b) fibres per blade tooth calculation.

26


2.6 Disc opener (courtesy of J Stummer Konstruktion, Germany).

must be homogeneous throughout the entire batch to minimise shade variations. Although most fibres utilised in nonwovens are not dyed, adequate mixing is still important because of the fibre variation within bales as well as bale-tobale. Visual assessment of blending is not reliable because most of the blends appear white. Bale-to-bale variations occur in respect of crimp frequency (crimps/cm), fibre finish application level and fibre entanglement. Fused, coterminus ends and cutting problems experienced by the fibre producer are sometimes evident in bales, which can impact fibre processing performance. Clearly, the properties of a nonwoven fabric are fundamentally a function of the blend composition and it is therefore important that the blend components are consistently in proportion to minimise variations and to ensure product specifications are achieved. Poor blending leads to various processing and quality problems. When one component constitutes a small proportion of the total blend, for example 10%.


59

Nonwoven products must normally meet a minimum technical specification. Since underweight fabric is considered more undesirable to the customer than overweight, it is common practice to err on the side of caution and produce webs slightly over the target weight. Typically, this means that the average weight is above the target weight. Since the product is then sold by length, roll number or area and not by weight, the manufacturer effectively gives away fibre for free. This by itself is a motivation for using automatic weight control systems. By controlling the weight of fibre fed into the card it is possible both to improve web weight uniformity and save a significant amount of fibre, which increases production cost efficiency. Microprocessor controlled weigh-pan systems Microprocessor controlled weigh-pan systems are extensively used by most sectors of the staple fibre processing industry. Whilst much of the carding technology adopted in the nonwovens industry originated from the traditional spinning sector, the ‘Microweigh’ was the first card feed control system to be developed specifically within the nonwovens sector before being offered to the traditional industries. Such systems (Fig. 2.32) can form part of a new hopper or are retrofitted to existing installations. Initially, the feed to the weigh pan is regulated by controlling the speed of the spiked lattice to ensure a regular flow of fibre regardless of the amount of fibre in the hopper. This counteracts the well-known susceptibility of weigh-pan systems to hopper load variations and the consequent changes in weight of fibre against the spiked lattice, which results in uneven fibre flow. When the weigh pan achieves the pre-set weight, the spiked lattice is stopped and trap doors are closed above the pan. At this point, some fibre is still in mid-air, on its way into the pan. This is called ‘in-flight fibre’ and in traditional hopper feeds is a common source of irregularity. The Microweigh controls in-flight fibre by constantly monitoring in-flight values and adjusting the stop point in anticipation of a calculated weight of fibre falling into the pan when the lattice has stopped. Once the individual weight of fibre has been delivered to the weigh pan, a further control function carries out a quality check. If the weight in the pan does not exactly match the pre-set target weight, a correction is made before the fibre is allowed to enter the card. This is achieved in one of two ways. The first, and more usual option is to allocate a specific space on the card in-feed sheet for that particular weigh. For example, if the weight is detected to be 1% heavier than the target weight, the drop point is automatically adjusted to allocate 1% more space on the card feed sheet. This is made possible by dropping the contents of the weigh pan onto the in-feed sheet according to counted electronic pulses from an encoder, as opposed to the cycle cam of a traditional hopper. A light weigh is therefore dropped at a lower pulse count, whereas a heavier weigh is dropped at a higher pulse

60


2.32 Microweigh weigh pan hopper feeder (courtesy of Garnett Controls Ltd, UK).

count. Since the speed of the card in-feed sheet is constant, the result is a uniform allocation of weight per unit area on the in-feed sheet, which produces a regular feed. This function is termed ‘distance dropping’. Alternatively, fine-tuning of the individual in-feed weight is achieved by controlling the card feed roller speed. A weigh, which is above or below the pre-set weight, is allowed to drop onto the card feed sheet and is transported to the card feed rollers. Again the linear speed of the feed sheet is monitored by a signal from an encoder, which allows the microprocessor to determine the exact point where any particular weigh-pan drop will enter the feed rollers. A feed roller speed adjustment is made to correct any deviation from the target weight. Therefore, a weight that is 1% too heavy is corrected by a 1% reduction in the feed roller speed.


61

The Microweigh XLM system (Garnett Controls Ltd, UK) also incorporates a moisture control system. It is known that significant moisture loss occurs during carding. Since many fibres arriving at the card may contain combined or even interstitially held water (if they are lubricated prior to carding), any irregularities in the moisture content within a batch of fibre will lead to weight irregularities in the final product. This is particularly relevant when processing hygroscopic fibres, for example, in the production of medical products composed of polysaccharides or cellulosics. The Microweigh XLM incorporates a Streat Instruments (New Zealand) moisture measurement system within the weigh-pan assembly, which measures the moisture content of each consecutive weigh. The moisture value is subtracted from the total weight and the Microweigh bases its control functions on the dry-weight of fibre thereby eliminating the effect of moisture variations on product weight regularity. Whilst Microweigh was one of the first weight-control systems to be widely adopted, and remains one of the most precise systems available, there is a limitation on fibre throughput. The production rate of modern nonwoven cards has surpassed the production capabilities of weigh-pan systems. Despite this, such systems are in operation throughout the nonwovens industry, particularly for processing speciality fibres such as calcium alginate where the fibre physical properties do not allow, or require, the use of high-production machinery, but when maximum feed weight accuracy is essential. Volumetric chute feed systems The majority of nonwoven cards use volumetric feeds. Basic systems consist of a vertical chute arrangement into which fibre, usually fed from a spiked apron, is deposited from the top (Fig. 2.33). Most volumetric chutes incorporate a vibrating or ‘shaking’ wall to encourage the fibre to compact, thus filling air pockets and generally distributing the fibres evenly to form a continuous stream of fibre and to assist in the movement of fibre though the chute. A pair of fluted feed rollers are normally situated at the exit of the chute to continuously move the fibre onto the card feedsheet. Some chute feeds incorporate a reserve chamber and a main chute to improve feed uniformity. Volumetric chute feeds were introduced to overcome the production limitations associated with traditional weigh-pan-type systems, even before the advent of weight-control systems. The volumetric chute was designed to produce an even distribution of fibre at the card feed section than was thought possible using weigh pans. However, whilst the fibre from a chute feed appears uniform and continuous, variations in tuft density and packing density within the chute still lead to web weight variations. There are also significant differences in the way different fibre types pack under gravity in chute feeds. For these reasons, volumetric chute feed control systems were developed.

62


2.33 Volumetric feed system (courtesy of Garnett Controls Ltd, UK).

Long-term variation controllers Controllers of long-term feed weight variations usually operate by maintaining a constant average feed into the card. Their operation often relies on continuously adjusting the card feed roller speed in response to measured short-term variations in the in-feed fibre stream. Electromagnetic radiation systems Servolap, originally developed by HDB (Belgium) was one of the first volumetric weight-control systems on the market. The system measures the


63

mass of fibre on the card feed sheet by directing an isotopic ray through the material. Originally a radioactive source was used to produce the ray, which in recent years has been replaced by an X-ray generator. The emission ray from the source is directed through the full width of the fibre assembly on the card feed sheet from one side of the machine to the other. The residual radiation from the source is collected at the opposite side by a scintillation tube, which converts it to an electric signal that is inversely proportional to the density of the mass of fibre that is being conveyed. This signal is used to automatically regulate the speed of the card feed rollers to compensate for in-feed mass variations. Weigh platforms A stationary weigh platform is located between the delivery rollers of the chute and the card feed rollers. As fibre passes over the weigh platform, variations in weight are measured and recorded. The signal obtained from the measuring device is used to control the speed of the card feed roller speed. The Microchute (Garnett Controls Ltd, UK) measures over a relatively short distance and is able to detect and control short-term variation in the fibre feed. However, because of the short measuring distance, there is a degree of ‘bridging’ over the weigh platform due to the consolidation that takes place within the volumetric chute. This means that fibre on the weigh platform is to some extent supported by fibre before and after the platform. In practice therefore, these systems provide comparative information with which to control the variation but do not provide absolute weight values. Weigh-belt systems Weigh belts were introduced as long-term weight controllers allowing the user to directly input the desired production rate. The weigh belt measures over a much longer distance to minimise the ‘bridging’ effect, which occurs with weigh-platform systems. Additionally, because the belt is moving at the same speed as the fibre moving into the card, weigh belts are able to measure both speed and distance, as well as weight. A disadvantage of this type of system arises because the measuring distance is long and the system is unable to measure and control short-term variations occurring inside the weighing area. Depending on the card configuration and its mixing power, such variations introduced at the feed section can be amplified in the resultant web. Additionally, the entire assembly is comparatively heavy given the small mass of fibre that it needs to weigh. Consequently, the measuring precision of the weigh-belt system can be limited.

64


Roller weighing systems Rollaweigh (Garnett Controls Ltd) was designed to incorporate the advantages and eliminate the disadvantages of short-term regulation systems and weighbelt controllers. The original system has two distinct measuring zones (Fig. 2.34). The five-roller assembly of the Rollaweigh operates essentially like a weigh belt measuring over a longer distance, thereby minimising the ‘bridging’ effect. Moreover, because there is no belt, the weigh assembly is lighter than a weigh-belt system, which improves the resolution and weighing accuracy. The surface speed of the rollers is identical to that of the fibre as it moves into the card, therefore, the system can accurately measure distance and speed, as well as weight. The user enters the desired production rate and the system automatically maintains the desired throughput, regardless of changes in fibre characteristics. To regulate short-term variation in the in-feed fibre stream, the system incorporates a secondary control loop within the overall five-roller system. In the secondary short-term zone an additional measurement is taken over only two rollers. The short-term weight control system then adjusts the card feed roller speed in order to compensate for these shorterterm weight variations. The system may be installed as part of a new carding installation, or retrofitted to existing machinery.

2.9.1

On-line basis weight measurement

Basis weight measurements of web, batt or final fabric weight can be determined using a variety of sensor technologies including gamma backscatter, near infra-red and beta transmission. Non-contact sensors measure the output of a nonwoven line by either traversing the width of the product or by a number

2.34 Rollaweigh system (courtesy of Garnett Controls Ltd, UK).


65

of individual sensors situated across the width. The information from such systems is used to monitor product density in both the MD and CD and although the measurement is taken after the point at which automatic regulations are realistically possible, the data produced is useful for on-line qualitycontrol purposes. Web and fabric scanners are incorporated into closed loop weight-control systems that integrate operations in the entire production line. Adjustments are made to the instantaneous speeds of the card feed rollers, the card and cross-lapper to regulate both along the card as well as across the batt and fabric weight variations.

2.9.2

Cross-machine direction (CD) controllers

It is possible to control the fibre density in both the long and cross-machine directions. The Scanfeed system (Fig. 2.35) incorporates an upper feed chute within which a feed roll and opening roll are situated at the base. A constant airflow is directed through the chute and allowed to escape through outlet vents situated at the front and behind the position where fibre accumulates at the base of the chute. Fibre distribution in the chute is regulated by the airflow. If the fibre in the chute is denser at one side, or in one particular area, the air pressure correspondingly increases in this area because the packed fibre is less air permeable. Consequently, the differential pressure, which is created between areas of different density in the chute, preferentially directs the falling fibre to fill the lower density areas, which are more air

1 2 3

1. 2. 3. 4. 5. 6. 7.

4

5 6

9

10

Spring-loaded sectional tray Feed roller Opening roller Feed trunk or chute Web thickness adjustment Delivery roller Spring-loaded sectional measuring tray 8. Conveyor belt 9. Feed roll 10 Licker

8 7

2.35 Feed density control (Scanfeed, courtesy of Trützschler GmbH, Germany).

66


permeable (Fig. 2.36). At the same time, a series of spring-loaded flaps across the width of the chute provide pressure regulation as they open and close according to the thickness of the fibre passing through. Further uniformity of cross-card density is achieved by a web profile control system in the lower feed chute. At the base of the feed chute the side wall is split into a series of profiled boxes having pivoting flaps that are automatically adjusted to increase or decrease the volume available for fibre to pass. Below this point, a chute feed delivery roller is mounted above a series of spring-loaded sectional trays, which are pivoted so they can open and close according to the fibre density passing through. As fibre exits the chute feed the movement of the flaps is continuously measured and a signal is generated to control actuators on the flaps of the profile boxes. If a tray below the delivery roller opens as a result of higher fibre density passing through that section of the chute, a servo-motor automatically closes the flap on the corresponding profile box. This reduces the volume available for fibre passage and reduces the effective air volume passing through at that point thereby causing fibre to preferentially flow into less densely packed areas of the chute. This automatic regulation of fibre flow is continuous and since the distance between the measurement point and the control point is relatively short, the system provides effective short-term control of both longitudinal and cross-machine feed uniformity. The Scanfeed system can be integrated with a card in such a manner that the bottom delivery roller of the lower chute becomes the card feed roller, thus eliminating the traditional feed sheet. The measuring accuracy of the individual tray sections is claimed to be such that with the appropriate calibration, a direct correlation with web weight can be achieved without the need for an additional weight-control system. The potential for consolidating

2.36 Self-regulation of feed weight in width (Scanfeed, courtesy of Trützschler, Germany).


67

a batt produced by such a feed system directly (using, for example, hydroentanglement or needlepunching) without the use of a traditional webforming step has also been proposed. For making heavyweight products, for example quilted fabrics, the use of a volumetric hopper feeder to form a batt structure in place of the conventional card and cross-lapper has been demonstrated.

2.10

Cross-lapping

A cross-lapper (or cross-folder) is a continuous web transfer machine that normally follows a card or Garnett machine as part of an integrated web formation system. The web is layered from side to side onto a lower conveyor or bottom lattice, which runs perpendicular to the in-feed web to form a diagonally stratified batt, wadding or fleece, which typically consists of 4–>15 layers depending on requirements. Commercially, cross-lapped batt weights range from about 50 g/m2 to over 1500 g/m2 depending on fibre properties and the web weight per unit area. The ratio of the web in-feed speed to the output speed determines the laydown angle and the linear production speed is a function of both laydown width and the number of layers. The laydown width varies depending on requirements and for specialist applications such as papermakers’ felts, it may be >17 m. Therefore, crosslapping enables the production of batts much wider than the initial web fed from the carding machine which is limited to 5 dtex). Depending on composition and fabric structure, the fabrics have higher resistance to compression and elastic recovery than comparable cross-lapped and high-loft airlaid fabrics (see Fig. 2.42). To maximise the resistance to compression-recovery properties, vertical orientation of the fibre in each web fold is usually preferred instead of a slightly inclined orientation. Struto fabrics are used in a variety of applications including foam replacement materials, sound insulation in automotive interiors, thermal insulation, bedding products and air filtration. The Wavemaker system (Santex, Italy) utilises a rotary forming disc to create the web folds (Figs 2.43 and 2.44). The first rotary and reciprocating lappers originated at the University of Liberec. Whilst the rotary lapper leads to significantly higher production rates than the reciprocating version used by the Struto system, the latter produces a more pronounced z-directional fold orientation, which is approximately perpendicular to the fabric plane. The fold structures produced by rotary lappers tend to slope slightly relative to the fabric plane and therefore the resistance to compression of fabrics produced by rotary and reciprocating lappers is different. Through-air thermal bonding is used to stabilise the resulting structure. Alternatively, the Rotis system was developed for introducing fibre entanglements in Struto vertically lapped webs using revolving, barbless tubes arranged across the machine and perpendicular to the two sides of the web. These tubes are used to entangle surface fibres in discrete continuous longitudinal rows thereby connecting successive folds of

74


(a)

(b)

(c)

2.41 Cross-section of various Struto fabrics (courtesy of Struto International Inc., USA); (a) PET (heavy web) 500 g/m2; (b) black PET (standard) 500 g/m2 and; (c) (shoddy) 1050 g/m2.


75

Thickness (mm)

15 1. 2. 3. 4.

Struto Airlaid spray bonded Cross-laid spray bonded Cross-laid spray bonded and cross-laid through air bonded 5. Needle punched

10 1

5

2 3 4 5

0

% of original thickness

0

300

600 900 Load (Pa)

1200

100

1 2

Based on 25,000 loading cycles. Fabrics compressed to 50% of thickness in each loading cycle.

90 80

1. Polyurethane foam 2. Struto 3. Airlaid spray bonded

3

70 60 0

30 60 90 Recovery time (minutes)

120

2.42 Comparison of the load vs. thickness and elastic recovery behaviour of 150 g/m2 Struto fabric compared with other nonwoven materials (courtesy of Struto International Inc., USA).

3

2

1 1. 2. 3. 4. 5. 6.

5

4

Carded web Feeding discs Doffing comb Forming discs Upper oven belt Lower oven belt

6

2.43 Rotary lapper (Wavemaker, courtesy of Santex, Italy).

a

H

a

l

2.44 Comparison of folded web cross-sections produced by perpendicular-laid web formation systems.

76


web in-plane. Preformed scrims or fabrics may also be introduced from above and below to form a composite fabric structure in situ.

2.13

Airlaid web formation: raw materials and fibre preparation

Airlaying (aerodynamic or airlaid web formation) refers to a family of drylaid web formation processes used in the manufacture of disposable, singleuse products containing short, pulp fibres (including wipes, absorbent layers for incontinence products and food packaging pads) and durable products (including high-loft waddings, filtration media, interlinings, automotive components and mattress fillings) produced from longer fibres. A characteristic feature of airlaid webs is their isotropicity. In contrast to carded webs, MD:CD ratios approaching 1 may be obtained depending on fibre specifications and machine parameters. Airlaid webs are therefore frequently referred to as ‘random-laid’. Additionally, airlay processes are highly versatile in terms of their compatibility with different fibre types and specifications. This versatility partly arises from the principles of fibre transport and deposition used in airlaying as well as the variety of airlay machine designs available. Airlaying, like other technologies, has certain benefits and limitations. Among the benefits are: ∑ ∑ ∑

isotropic web properties three-dimensional structure if the basis weight is above about 50 g/m2 producing voluminous, high-loft structures with a very low density compatibility with a wide variety of generic fibre types including natural and synthetic polymer fibres, ceramics, metals including steel, carbon, melamine, aramids and other high-performance fibres.

The main limitations are: ∑ ∑ ∑

Fabric uniformity is highly dependent on fibre opening and individualisation prior to web forming. Air flow irregularity adjacent to the walls of the conduit leads to variability across the web structure. Fibre entanglement in the airstream can lead to web faults.

Depending on fibre type and fineness, airlaying is claimed to be more efficient than carding in the production of webs greater than 150–200 g/m2, where production rates of 250 kg/h/m can be achieved.1

2.13.1 Raw material specifications and fibre preparation for airlaying A variety of fibres are used by the airlaying industry. Wood pulp continues to play a major role in the pulp or short-fibre airlay industry and the characteristics of some typical pulps are now briefly discussed.


77

Wood pulp and natural fibres Wood pulp can be produced by mechanical or chemical processes. Thermomechanical pulping (TMP) involves passing wood chips between rotating plates having raised bars at high temperature and pressure. The heating softens the lignin, which is a natural phenolic resin holding the cellulose fibres together, making it possible to separate the fibres. A yield of over 90% of wood fibres can be obtained. In contrast, chemical pulping (Kraft process) dissolves the lignin using suitable chemicals such as caustic soda and sulphur under heat and pressure. The chemical pulping process produces lower fibre yield than mechanical pulping, typically 50–60%.2 Some typical wood pulp fibres currently used in airlaying are: ∑

∑ ∑

Southern Softwood Kraft. Manufactured in the southeastern USA and used in products where absorbency, softness, cleanliness and brightness are required. Softwood fibres are used to provide strength and bulk. They tend to produce less dust and lint, providing a cleaner conversion process. Scandinavian Sulfate (Kraft). Fluffs are of shorter length and coarser than American southern pines (see Table 2.2). Northern Softwood Sulfite. Used on a smaller scale for speciality products where superior formation (low fibre entanglement), softness and high brightness are required. They are mainly used in products such as airlaid tabletop covers and wipes as well as feminine hygiene pads.3

The main critical parameters that characterise wood pulp fibres are:4 ∑ ∑ ∑ ∑ ∑ ∑

wood species pulping process (mechanical or chemical process) fibre length fibre fineness fibre stiffness special treatments.

Table 2.2 Key properties of wood pulp fibres Pulp type

Main species

Fibre length (mm)

Fineness (mg/100 m)

Fibres/g (¥ 106)

Southern US (Kraft) Scandinavian (Kraft) Northwest (Sulfite) Cotton linter pulp Cold caustic extracted cellulose Cross-linked cellulose

Southern pine Spruce/pine Spruce/fir

2.70 2.06 2.08 1.8 1.8

45.6 27.0 33.0 25.0 34.0

2.6 5.0 4.2

2.3

40.0

78


Table 2.2 summarises some of the key properties of wood pulp fibres relevant to the airlaying industry. Generally the finer pulp fibres give rise to higher softness, wicking rate and printability. On the other hand, coarse and long fibres produce more resilient and bulkier fabric structures with better total absorption capacity and a higher porosity. Man-made fibres The man-made fibres used by the airlaying industry fall into two main categories, natural polymer-based fibres (e.g., regenerated cellulosic fibres, such as viscose rayon and Tencel) and synthetic polymer-based fibres (e.g., polyamide, polyester and the polyolefins). The regenerated cellulose fibres such as viscose rayon and Tencel (solvent-spun cellulose) are very hydrophilic and similar in their absorbency characteristics to wood pulp. They can hydrogen bond and are typically cut to fibre lengths of 3–12 mm. The longer fibre lengths makes them suitable for inclusion in airlaid products particularly in blends with wood pulp, as this increases the strength of the airlaid fabric. Additionally, these longer fibres contribute to higher abrasion resistance and often, a softer handle as compared to the shorter, stiff wood pulp fibres. The synthetic polymer fibres, specifically PET, PA, PP and PE are hydrophobic and are particularly effective in maintaining the bulkiness of airlaid fabrics in wet conditions. Such fibres are used in blends with wood pulp and sometimes SAP in liquid acquisition layers for nappies as well as other absorptive materials. Synthetic fibres have a high wet strength as compared to viscose rayon (which decreases in strength when wet) and can markedly increase the durability and strength of the fabric in use. The effect of fibre parameters (crimp level, fibre fineness, fibre length and fibre cross-sectional configuration) on the performance of a thermally bonded airlaid fabric was investigated by Gammelgard,5 using both Dan-web and M&J airlaying systems. The main findings of the study may be summarised as follows: ∑ ∑

∑

Finer fibres increase the tensile strength of the product. Changing from 3.3 dtex to 1.7 dtex fibres increased the tensile strength by up to 40%. The tensile strength of the airlaid web increased with decreasing crimp level. This may be attributed to the fewer bonding points available in crimped fibres. Also it was pointed out that the lower the crimp level the higher the fibre throughput. Therefore, crimp may be used to control the production capacity of an airlaid line, and should be optimised depending on the type of web formation system employed (i.e. Dan-web or M&J system). The tensile strength varied with the proportion of PE in the PP/PE bicomponent (BICO) fibre. Up to a certain point the tensile strength increased with an increase in the proportion of PE. Further increases


∑ ∑

79

affected the PP core, which became weak and broke before the thermal bonding points actually failed. The optimum proportion of PP and PE in the bicomponent fibre was claimed to be 35/65 for a concentric sheathcore bicomponent fibre or 1.7 dtex (AL-Special –C). It was established that the M&J and Dan-web lines perform differently with regard to fibre length. It was concluded that in the M&J system, selection of 3 mm fibre length optimises the production capacity whereas, 4 mm fibre length optimises the fabric tensile strength. The Dan-web line was claimed to have greater flexibility with regard to fibre length without affecting production capacity (6–8 mm fibre length) compared to the M&J line (3–4 mm fibre length).5

In textile (long) fibre airlaying, all types of synthetic fibres between 1.7 and 150 dtex linear density and staple length 40–90 mm can be processed as well as natural fibres such as cotton, wool, jute, flax, kenaf, reclaimed textile fibres, recovered wool and specialist high-performance fibres such as P84 (polyimide fibre). Superabsorbents Superabsorbent polymers (SAPs), which are available as powders, granules, beads or more recently as fibres, are increasingly being used to augment the liquid holding capacity of airlaid webs containing fluff wood pulp and other fibres. The capacity of superabsorbents (cross-linked hydrogels) to absorb fluid is several times higher than wood pulp fibres and their function is to immobilise as much fluid as possible without releasing it even when the fabric structure is compressed. The powder form is usually added to the airstream in which wood pulp fibres are suspended prior to airlaying. The fibre component that is more expensive can be blended or formed as an individual layer used in a composite web. Superabsorbent fibres are designed to absorb fluids without losing their fibrous structure and therefore retain a proportion of the dry fibre strength. On drying the fibre recovers its original form and is still absorbent. Typically, such fibres absorb 95% of their ultimate absorbent capacity in 15 seconds. One example is Oasis fibre (Technical Absorbents, UK) which is normally cut to a staple length of 6 or 12 mm for use in pulp airlaying systems. Typically, 10–40% of Oasis fibre is used in blends with woodpulp and/or staple fibres. Bonding of webs containing superabsorbent fibres is carried out using thermal bonding (assuming a thermoplastic fibre is also added to the blend) or latex (chemical) bonding. During processing of such fibres it is recommended that the relative humidity be kept below 60% and preferably 55%6 to prevent unwanted gelatinisation of the fibre. Some of the advantages of superabsorbent fibres over superabsorbent powders are derived from their

80


physical form rather than their chemical nature. The advantages are summarised below: ∑ ∑ ∑ ∑

Fibres absorb fluids faster than powder with the same absorption capacity. Fibres are integrated within the structure and do not migrate from their locations. Fabrics containing fibres are flexible and soft in contrast to the powders that are abrasive and confer a rough and harsh handle to fabrics. Fibres are easier to incorporate into the airlaid structure and are less likely to migrate from the structure during subsequent bonding and in use.

Fibre preparation In airlaying it is important to introduce opened and preferably, individualised fibres to the airstream so that a uniform web without any tangled clumps or fibre flocks can be formed. It is important to note the difference in state between opened and individualised fibres. The term ‘opened’ fibres refers to a collection of fibres that is substantially free of clumps, tangles, knots, or similar dense entanglements, but there is still significant frictional interaction between the fibres. In contrast, ‘individualised’ fibres have no substantial mechanical or frictional interaction with other fibres. Various methods for fibre opening and separation have been designed for airlaying lines. The majority of opening systems are the same as those used prior to carding. In addition, hammer mills or customised openers have been utilised. In general, opening and fibre separation can be accomplished using a clamped feeding unit consisting of a feed chamber equipped with a fine opener, a vibration chute feed with a weighing device followed by a further opening section composed of a pinned or saw-toothed roller with or without worker-stripper rollers. Typical examples of feed roller designs that can be used to separate fibres prior to airlaying are shown in Fig. 2.45. In pulp airlaying, the hammermill dominates fibre preparation procedures. A hammer mill disintegrates the feed material so it can be uniformly distributed through the forming heads. The increasing use of Sunds defibrator has increased the importance of the disc refiner. The Sunds system incorporates the use of a bale shredder, screening equipment and a disc refiner.

2.14

Airlaying technology

Airlaying involves uniformly dispersing individualised fibres in an airstream and leading this air-fibre mixture towards a permeable screen or conveyor where the air is separated and the fibres are randomly deposited in the form of a web. Fibre separation is therefore an essential part of the airlaying


(a)

(b)

(d)

81

(c)

(e)

(a) two feed rollers, (b) and (c) feed plates, (d) and (e) nose-bar and overhead feed plate. Notes: type (d) is currently used in some Spinnbau systems, DOA uses types (a), (d) and (e), whereas Laroche employs mainly type (a).

2.45 Feeding systems used in airlaying.

process and strongly influences the global and local uniformity of the final web. In the formation of lightweight webs it is particularly essential to ensure that opened, individualised fibres free from clumps and entanglements are introduced into the airstream. The fibre orientation in the final web is mainly influenced by the dynamics of the airflow in the fibre transport chamber near the landing area. In practice, this can be strongly affected by the rotation of the opening or fibre dispersing unit above the transport chamber. The following methods are used to transport fibres from the opening unit to the web forming section: ∑ ∑ ∑ ∑ ∑

free fall compressed air air suction closed air circuit a combination of compressed air and air suction systems.

The principle of airlaid web formation using a suction assisted landing area is shown schematically in Fig. 2.46. In this particular machine design, preopened fibres, which can be prepared using the feeding, mixing and opening systems described in Section 2.3, are fed to a pair of feed rollers which, in the same way as carding, are designed to grip the fibre and prevent large clumps from being drawn into the system. To ensure feed regularity, which is critical given that no long-term levelling of weight variation is possible within the airlaid forming head, automatic feed control systems of the type used by the carding industry can be applied. The rotating drum or cylinder removes fibres from the fringe presented by the feed rollers. The fibres are transported by hooking around the wire teeth on the drum and are subsequently removed by a high-velocity airstream directed over the wire

82


4

1. 2. 3. 4. 5. 6. 7.

3

Pre-made batt Feed rolls Main cylinder Air blower Suction Conveyor belt Airlaid web

1 7 2

5 6

2.46 Principle of web formation in a simple airlaying process.

teeth surface. In this way, the fibres are mixed with air and transported with it to an air permeable conveyor where the air is separated and the fibres are deposited to form the web or batt structure. Airlaying technology may be classified according to the raw materials used for processing. Using this form of classification there are two main types; the type that uses natural or man-made textile fibre (cut length >25 mm) and the type that employs short cut fibres (generally 1000 ∞C) fire protection products Table 3.7 Wet-laid product applications Wet-laid speciality papers

Wet-laid nonwovens

Wet-laid technical nonwovens

Tea bag paper

Surgical clothing and drapes Bed linen

Glass fibre roofing substrate Glass fibre mat for flooring Glass fibre mat for printed circuit boards Nonwovens for filters Wall coverings Insulation materials

Overlay papers Plug wrap paper Stencil paper Air filter papers Liquid filter papers

Table linen, cloths and napkins Towels Kitchen wipes Hygiene products Textile inserts

140


3.17 Glass/polyester wet-laid web.

3.18 Glass microfibre web for cryogenic insulation.

Wet-laid web formation

∑ ∑ ∑ ∑ ∑

141

static dissipation surfacing veils RFI shielding veils pultrusion materials heat and controlled electrical conductance materials electrolytic condenser separators.

3.11

Sources of further information

White Colin, Synthetic fibres in papermaking systems (including wetlaid nonwovens), PIRA International Leatherhead Surrey UK 1993. White Colin F, Wetlaid and short fibre airlaid nonwovens, PIRA International Leatherhead Surrey UK 1996.

3.12

References

1. Grant J, Young J H and Watson B, Paper and Board Manufacture, Technical Division, British Paper & Board Industry Federation, 1978. 2. Inagaki H, Nonwovens Asia, Miller Freeman, San Francisco, USA 1989. 3. Osbourne F H, The History of Dexter’s Long Fiber Paper Development, C.H. Dexter, Windsor Locks, Connecticut, USA 1975. 4. Parker J D, The Sheet Forming Process, Proc. TAPPI Stap No 9, TAPPI, Technical Association of the Pulp & Paper Industry USA 1972. 5. Schoffmann E, Neue Bruderhaus NoWoFormer Bulletin 6. Hardy C, Fine dtex cellulosic fibres offer significant advantages, Nonwoven Report International Yearbook 1991, No 245 p 50–52. 7. Smith J E, Cellulose acetate fibrets: A fibrillated high surface area pulp for speciality industrial applications, TAPPI Nonwovens Conference Proceedings 5–8 April 1988 p 237–243. 8. Cruz M M, Rayons in Battista O A (ed.), Synthetic Fibers in Papermaking, Interscience 1964. 9. Derwick G van Breen A W, A new polypropylene fibre for the paper industry Pira/ BPBIF/PRI Conference 28–29 November 1978 p 4.1–4.4. 10. White C F, The use of synpulps for bonding inorganic nonwoven structures, INDATEC International Nonwoven Fabrics Conference 30 May–2 June 1989 Philadelphia USA. 11. Brown H S, Casey P K and Donahue J M, Poly (Trimethylene Terephthalate) Polymer for fibers, Nonwovens World, summer 1998. 12. Haile W A and Phillips B M, Deep grooved polyester fiber for wetlay applications TAPPI Journal Vol 78 No 9. September 1995. 13. Ramirez J E and Dwiggins C F, High temperature papers made of polybenzimidazole, TAPPI Converting & Packaging December 1985. 14. Parker R B, Sulfar PPS fibers for nonwovens, TAPPI Journal May 1992. 15. Evans R E, Fibrillatable acrylic fibers for speciality nonwovens, PIRA High performance fibers and structures seminar 4 December 1990 Wilmslow UK. 16. Bailey R M and McKean W T, The characteristics of glass fibers in papermaking, TAPPI Journal August 1989. 17. Fryatt J, Production properties and applications of ceramic fibers, PIRA High performance fibers and structures seminar, 4 December 1990 Wilmslow UK.

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18. White C F and Moore G K, Ceramic and Mineral wool fibers in wetlay forming processes, TAPPI Journal December 1987. 19. Keith J M, Dispersion of synthetic fiber for wetlay nonwovens, TAPPI Journal Vol 77 No 6 June. 20. Shiffler D A, Characterising the dispersion kinetics of synthetic fibers in water, TAPPI Journal August 1985. 21. Stassen W N, Dispersing glass fibers in the wet process, TAPPI Nonwovens Conference 1983. 22. Meierhoefer A W, Wetlaid nonwovens – a survey of the fundamentals of making speciality fabrics on papermaking machinery, Nonwoven Fabrics Forum, Clemson University 19–26 June 1989 Clemson USA. 23. Schoffmann E and Schwend F, Meeting trend developments in wet forming, TAPPI Nonwovens Conference 12–16 May Marco Island Fl USA 1991. 24. Schoffmann E, The use of inclined wire formers to produce nonwoven materials for medical/health care applications, PIRA Nonwovens in Medical and Healthcare Applications seminar 10–12 November, Brighton UK 1987. 25. Magill D G, Operation and capabilities of wet forming devices for long fibered structures, TAPPI Journal January 1987. 26. Dunn M P, Multiply sheet formation on inclined wire formers, TAPPI Journal October 1988. 27. Haile W, Dean L and Gregory D, Co Polyester Polymers for binder fibers, Nonwovens World, summer 1998. 28. Ohmori A, A new water soluble synthetic fibre for nonwoven applications, EDANA INDEX 96 symposium Geneva 1996 R&D Session. 29. White C F, Hydroentanglement technology applied to wet formed and other precursor webs, TAPPI Journal, June 1990.

4 Polymer-laid web formation G S B H AT University of Tennessee, USA and S R MALKAN Synfil Technologies, USA

4.1

Introduction

Polymer-laid, spunlaid or ‘spunmelt’ nonwoven fabrics are produced by extrusion spinning processes, in which filaments are directly collected to form a web instead of being formed into tows or yarns as in conventional spinning. As these processes eliminate intermediate steps, they provide opportunities for increasing production and cost reductions. In fact, melt spinning is one of the most cost efficient methods of producing fabrics. Commercially, the two main polymer-laid processes are spunbonding (spunbond) and meltblowing (meltblown). Both are similar in principle, but the technologies used are quite different. Because of distinct differences in the structure and properties of the fabrics, these processes have grown in parallel since their inception in the late 1950s and for some applications they are also used in combination to produce bilaminates, trilaminates (e.g., SMS) and other multilayer fabrics. Advancements in polymer chemistry and extrusion technology have enabled an increasingly varied range of products to be developed based on the core spunlaid and meltblown technologies. Some of the most important research commenced in the late 1950s, and an extensive number of processing and product patents have been reported over the years. More recently, there has been a dramatic resurgence of research and development concerned with polymer-laid systems as well as an increased acceptance of the fabrics in new product areas.

4.2

Resins for spunbonding and meltblowing

In general, high molecular weight and broad molecular weight distribution polymers such as polypropylene, polyester and polyamide can be processed by spunbonding to produce uniform webs. Medium melt viscosity polymers, commonly used for production of fibers by melt spinning, are also used. In contrast, low molecular weight and relatively narrow molecular weight distribution polymers are preferred for meltblowing. 143

144


In the past decade, the use of polyolefins, especially polypropylene, has dominated the production of meltblown and spunbonded nonwovens. One of the main reasons for the growing use of polyolefins in polymer-laid nonwovens is that the raw materials are relatively inexpensive and available throughout the world. Polyolefin resins are widely used in nonwovens mainly because they offer a relatively attractive cost combined with good value and ease of use when compared to conventional resins, such as polyesters and polyamides. Moreover, continuing advances in polyolefin fiber grade resins are strengthening the olefin’s price-properties ratio, which make them more suitable for polymerlaid nonwoven applications. Commercial polyolefin technologies over the last six decades have gone through significant changes. They have gone through introduction, growth, and stabilization or maturity phases.1 The drive for technology evolution has been the industry’s desire to continuously improve control of the molecular architecture, which leads to improved polymer performance. The key developments or milestones in polyolefin technology are as follows:2 ∑ ∑ ∑ ∑

In the 1930s, ICI set a trend of making versatile plastics by introducing its high-pressure process for making polyethylene resins. In the 1950s, the discovery of stereo-regular polyolefins and the incredibly rapid development of catalysts and processes led to commercialization of crystalline isotactic polypropylene and HDPE resins. In the 1970s and through the 1990s, the invention of the low-pressure, gas phase process for making linear polyolefins started the wheels turning. In the 2000s, the introduction of a single site catalyst for making superior polyolefin resins set another technological trend.

4.2.1

Markets

Polyolefins are the most widely used resins in polymer-laid nonwovens. Table 4.1 shows the total usage of PP and PE in nonwovens.3 It is estimated that, of this total usage, about 45 to 55% is used in polymer-laid nonwovens applications. Polypropylene has the major share of the disposable diapers, sanitary product markets, and medical apparel, and is the principal fiber used in geotextiles, nonwoven furniture construction sheeting, and carpet Table 4.1 World consumption of polyolefin resins3 Region

Polypropylene (PP) in million lbs.

Polyethylene (PE)

United States Europe Japan and the Far East

815 760 150

75 40 10 mm have essentially no molecular orientation. This implies that such fibers do not contribute to meltblown web strength. It should be noted that the mass of a fiber per unit length is proportional to the square of its diameter. Therefore the mass diameter distribution of the web will be more appropriate than the number diameter distribution as a means to calculate the fiber strength realization in a meltblown web. Formation of ultrafine microfibres Wadsworth and Muschelewicz81 reported the results of a study designed to produce extremely fine meltblown fibers using 35, 300 and 700 MFR resins. The study established the optimized meltblown processing conditions to produce fine fibered webs, with less than 2 mm fiber diameters. The study found that the small orifice die (with orifice diameter in the range of 0.2–0.3 mm) resulted in statistically smaller mean fiber diameters than the die with standard size orifices (approximately 0.4 mm diameter orifices). However, the actual difference was found to be minimal, considering the fact that the cross-sectional areas of the small die holes differed by a factor greater than two. The study also reported that increased air flow rates decreased mean fiber diameters. The air gap settings did not have any noticeable effects except that much larger diameters were obtained with the standard die tip and the smaller air gap setting with 300 and 700 MFR resins. Notably higher

180


bursting strength values were achieved with the small die holes, indicating increased fiber strength with greater draw-down.

4.10

Meltblown characterization techniques

This section provides a brief review of some of the characterization techniques used to study meltblown web properties or phenomena. Mostly, meltblown webs are characterized in terms of stress-strain properties, filtration efficiency and air permeability. Many of these properties can be characterized using standard ASTM or INDA methods. However, researchers have developed new and advanced techniques to characterize web properties, such as fiber diameter, mean pore size analysis, and on-line fiber diameter measurements through light scattering and other techniques. Tsai82 proposed a mathematical relationship to characterize key web properties using an air flow technique. This mathematical relationship uses air permeability data and determines approximate fiber size and pore size. Naqwi et al.83 developed an interferometric optical technique, referred to as an adaptive phase/Doppler velocimeter (APV) for in-situ sizing of spherical and cylindrical objects as applied to spunbond and meltblown fibers. It has been suggested that this technique could be used to monitor changes in processing conditions during the meltblowing process, specifically fiber diameter. Bhat84 used sonic velocity measurements to characterize meltblown webs. The sonic velocity can be used as an indication of the overall arrangement of structural elements in the fabric. The results showed a good correlation between the measured sonic velocity values and spunbond fabric mechanical properties. Wallen et al.85 investigated the use of small angle light scattering to study transient single fiber diameter and to monitor the fiber attenuation process as a function of distance from the die during the meltblowing process. The fiber diameters were determined from the total intensity of the scattered light. The study concluded that the meltblown process is not a steady state below a certain timescale and that the fiber attenuation process is not constant with respect to time or distance from the die. Bodaghi86 has described many meltblown microfiber characterization techniques. He found that waterquenched polypropylene fibers showed a para-crystalline crystal structure whereas air-quenched fibers showed a regular monoclinic crystal structure.

4.10.1 Polymers for meltblowing It is known that the meltblowing process is highly versatile in respect of the range of polymers that can be processed. Although polypropylene is the most widely used resin in meltblowing due to its ease of processing and suitability for end-use, researchers have successfully meltblown a variety of


181

different resins including polyamide, polyester, TPX, fluoropolymers, polyphenylene sulphide, and PBT. Most of these studies have been aimed at developing specific nonwoven products for industry. In experimental studies, polyethylene is claimed to be more difficult to meltblow into fine fiber webs than polypropylene, that Nylon 6 is easier to process and has less tendency to make shot than polypropylene and that most thermoplastic polymers can be meltblown sucessfully into good finished products with few exceptions.87 Cheng and Kwalik88 described the requirements of polypropylene resins, such as melt viscosity, MWD and melting point for processing. The resin selection criteria for meltblowing, including the choice of additives, MFR, pellets or granules, resin cleanliness, and MWD are also discussed. Khan and Wadsworth89 demonstrated the feasibility of meltblowing cellulose acetate and polyvinyl alcohol resins. The aim was to produce a biodegradable web to be used in disposable consumer products. Meltblowing of fluoropolymer resins was demonstrated by Wadsworth and Fagan90. Different grades of ‘Halar’ fluoropolymer resins (Ausimont, Inc.) were meltblown. It was concluded that the higher MFR fluoropolymer resins process well and were comparable in terms of processing characteristics to high MFR PP resins. Polybutylene terephthalate (PBT) resins have been meltblown by Bhat et al.91 The higher intrinsic viscosity (IV) PBT produced higher fiber diameters than the lower IV PBT. Recycled polymers can be meltblown. The study discussed the properties of dried and undried PET meltblown webs and also blends of virgin and recycled PET. The webs produced with undried PET, which had a higher moisture content than dried PET, had a lower intrinsic viscosity as well as smaller fiber diameters, higher tenacity in the MD and lower air permeability. The blend containing the highest amount of low intrinsic viscosity polymer resulted in a thin webs containing smaller fiber diameters, lower air permeability and higher tenacity in the machine direction as compared to regular PP MB webs. Meltblown adhesive webs are commercially available for lamination applications. Meltblowing of adhesive polyester for use in lamination where strong, durable bonds are required is possible using adhesive copolyesters. The resultant adhesive webs have been used to prepare fabric-to-foam and fabric-to-fabric laminates.

4.10.2 Electrostatic charging Arguably one of the most celebrated uses of PP meltblown fabrics is in air and liquid filtration. This is mainly because PP can be readily charged to enhance the filtration efficiency. As an example, the filtration efficiency of an uncharged PP meltblown fabric ranges from 20 to 40%, while for the electrically charged web, it ranges from 80 to 99% depending on the charging technique used.92–102

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4.10.3 Theoretical studies and modeling Narasimhan and Shambaugh103 have attempted to model the meltblowing process based on a single die hole rig, with a circular air slot surrounding the meltblown spinnerette nozzle. However, the most commonly used meltblown die geometry consists of a row of spinnerettes with sheets of hot air exiting from the top and bottom sides of the die. Shambaugh continued this study104 and applied macroscopic energy balance and dimensional analysis concepts to meltblowing. These two concepts were analyzed using different die geometries. Three operating regions were identified in the melt blowing process according to the extent of the air flow rate as follows: Region I has a low gas velocity similar to a commercial melt spinning operation in so far as the fibers are continuous. Region II is unstable as the gas velocity is increased. In this region filaments break up into fiber segments and undesirable lumps. Region III occurs at a very high air velocity and involves excessive fiber breakage. The meltblown process predominately uses a low airflow rate (Region I) and is most energy efficient in this region. A monodisperse fiber distribution is claimed to require less energy to produce than a polydisperse fiber distribution. The dominant dimensionless groups in the meltblowing process are claimed to be the gas Reynolds number, the polymer Reynolds number, fiber attenuation, and the ratio of the polymer viscosity to the gas viscosity. Milligan and Haynes105 studied the air drag on monofilament fibers. The aim was to study the air drag by simulating actual meltblowing conditions. The experimental set-up closely simulated a commercial meltblowing operation. The air drag was studied as a function of fiber length, upstream stagnation pressure, air injection angle, and gravity orientation. Four series of experiments were conducted as follows: 1. Determination of air drag for a fiber of constant length over a range of air stagnation pressures. 2. Determination of air drag for a range of fiber lengths at a constant value of stagnation pressure. 3. Determination of air drag for a fiber of known length and stagnation pressure using different injection angles (15∞, 30∞, and 45∞). 4. Determination of air drag by changing the die orientation, with respect to gravity, using a 30∞ air injection angle with different fiber length and stagnation pressures. The study concluded the following: ∑

The drag increased with fiber length when all other parameters were


∑ ∑ ∑

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constant. The drag peaked at a length of 2.5 cm. This was due to large amplitude flapping of the fiber. The drag increased linearly with stagnation pressure for stagnation pressures up to 207 kPa. The drag increased with decreased air injection angle for any particular stagnation pressure. This finding was in basic agreement with the fundamental momentum consideration. Orientation of the die with respect to gravity using an injection angle of 30∞ (45∞ above the horizontal to 90∞ below the horizontal) showed no measureable difference in drag. It was concluded that the viscous and pressure forces on the filament far exceed the gravitational force for the flow conditions investigated.

Uyttendaele and Shaumbagh106 have reported various analytical studies involving mathematical modeling of meltblowing. Majumdar and Shaumbagh107 have calculated the air drag on fine filaments in the meltblowing process using a wide range of filament diameters, gas velocities (primary air velocity), and Reynolds number. Milligan and Haynes105 studied the air drag on monofilaments simulating the actual meltblowing conditions, except that the high-velocity air was at room temperature and one end of the monofilament was secured to a tensiometer. It was found from the investigation that smaller air injection angles gave larger drag with the other parameters remaining constant.

4.10.4 Minimization of energy consumption Milligan108 analyzed several design concepts to minimize energy costs in meltblowing. The pressure losses associated with the air pipework and the air heater were two of the principal sources of energy consumption. Several design rules have been presented to minimize the cost of air utilization based on Darcy’s pressure drop relation: DP/L = r.f.V2/2D

4.1

where DP = the pressure drop L = the length of the pipe D = inner diameter of the pipe P = air density f = pipe friction factor (dependent on D and pipe roughness) V = air velocity in the pipe. The design rules are: 1. The piping between the air compressor and the die assembly should be as short in length and as large in diameter as possible. 2. The air heater should be as close as possible to the die assembly. 3. The number of pipe fittings should be minimized, and all piping should be well insulated.

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The study also highlighted the importance of correctly sizing the compressor in a meltblown pilot line. The suggested design rules minimize energy cost, specifically by reducing the pressure losses associated with the air piping and efficient location of air heaters. Milligan, Wadsworth, and Cheng109 investigated the energy requirements for the meltblowing of different polymers. The studied polymers were polypropylene, linear low-density polyethylene, nylon and polyester. The energy requirements were reported in kW-HR per kg of polymer. It was apparent from the reported data that the energy cost per unit mass of the product greatly depended on the air flow rate, air temperature, polymer throughput rate, and polymer molecular weight. The results also showed that a large fraction (greater than 85% for all the materials investigated) of the energy required was associated with the hot air streams. The study also found that the difference in total energy consumed and the actual energy required at the die can be attributed to improper compressor size, compressor cooling, and heat losses from the die and piping. The study suggested that substantially lower energy consumption is possible if a meltblowing line is carefully designed and operated with the objective of minimizing energy consumption.

4.11

Characteristics and properties of meltblown fabrics

Meltblown fabric properties can be tuned depending on end-use requirements by adjusting polymer selection, process variables, bonding and finishing processes. Some of the main characteristics and properties of meltblown webs are:110 ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

Random fiber orientation. Low to moderate web strength. Generally, the web is highly opaque (high cover factor). Meltblown webs derive their strength from mechanical entanglement and frictional forces. Most meltblown webs are layered or shingled structure, the number of layers increases with increasing basis weight. Fiber diameter ranges between 0.5 and 30 mm, but the typical range is 2 to 7 mm. Basis weight ranges between 8 and 350 g/m2, typically 20–200 g/m2. Microfibers provide high surface areas for good insulator and filter characteristics. The fibers have a smooth surface texture and appear to be circular in cross-section. The fibers vary in diameter along a single fiber. Close examination of approximately 800 photomicrographs showed no


∑

185

‘fibre-ends’ (except a few near areas where ‘shot’ is present), therefore, the fibers are believed to be mostly continuous in length. The fibers show thermal branching. The exact cause of thermal branching is not known, but according to Malkan62 the branching of the fibers occurs when propagating fibers collide with other propagating fibers, which in turn, strip off portions of polymer streams as fine branches (filaments). Richardson111 pointed out that when the velocity of the liquid jet relative to air jet increases, portions of the liquid are stripped off as filaments. Bresee and Wadsworth112 stated that fiber splitting (branching) occurs when extrudate is stressed in complex ways in flight towards the collector.

4.12

Meltblown fabric applications

Owing to the inherently large fiber surface area in meltblown fabrics, applications are in filtration, insulation, and liquid absorption. The fine fiber network (and large surface area) results in some characteristic properties:113–116 ∑ ∑ ∑

enhanced filtration efficiency good barrier properties good wicking action.

4.12.1 Filter media The original development work on meltblowing was focused on the production of microfibers which could be used in high-performance filtration products. Therefore, the filtration market segment remains the largest single market for meltblown webs, representing about 30% of the total.117 The future growth is projected to be quite strong. Generally, meltblown webs are used for the more critical filtration applications where the superior filtration performance of the fine fiber network can be exploited, for example in medical applications. Applications for meltblown fabrics are: ∑ ∑ ∑ ∑ ∑

room air filter and recirculation precious metal filtration and recovery food and beverage filtration surgical mask, respiratory filtration and healthcare products water and liquid filtration (including blood and body fluids).

4.12.2 Insulation Meltblown webs provide good insulation because of the large surface area, which creates significant drag forces on air convection currents passing through the fabric. The trapping of still air as a means of providing high

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thermal insulation is a concept that has been exploited successfully in Thinsulate® (3M) for outdoor sports and leisure clothing. Ando116 has also presented applications for meltblown webs in thermal insulating media. Heat transfer through any nonwoven fabric may occur by conduction, convection or radiation. The thermal insulation can be assessed by measuring thermal resistance using heat transfer rate equations. For the purpose of simplicity, the thermal resistance for conduction is given below. Consider a dense, meltblown web of thickness L held between two plates at temperatures T1 and T2. The thermal resistance R for conduction is (steady state, one dimensional flow): R = DT/qx = L/kA

4.2

where DT is the temperature difference between the two surfaces qx is the conduction heat transfer rate qx = kADT/L L is the thickness A = cross-sectional area k = thermal conductivity of the air and fiber k = vf k f + va k a f = fiber a = air v = volume fraction.

4.12.3 Absorption Meltblown webs are widely used in oil and liquid absorption. This market is growing due to growing governmental regulations concerning spillages, contamination of ground water, and environmental cleanliness. There are many meltblown products on the market that require liquid absorption such as sanitary napkins, household and industrial wipes, oil sorbent pads and booms, amongst others. White118 has also shown the effective use of meltblown webs in food fat absorption.

4.13

Mechanics of the spunbond and meltblown processes

Spunbonding and meltblowing incorporate many engineering concepts, some of which are discussed in this section.

4.13.1 Dynamics of melt spinning process Spunbonding and meltblowing involve multi-filament fiber spinning and are an extension of the conventional fiber spinning process. In order to understand the theoretical framework of these processes, it is necessary to understand


187

the dynamics of melt spinning first. The following is a discussion on the basic equation involved in the melt spinning process based on a single filament model. The equations for multi-filament fiber spinning (related to spunbonding and meltblowing processes) are quite complex. It is beyond the scope of this book to cover multi-filament spinning but further reading is available.119,120 The dynamics of melt spinning is based on mass balance, force balance, and energy balance principles. The balance equations are now considered. Based on the principle of conservation of mass, the following mass balance relation can be written:121,122,123 W = rAV

4.3

The above equation for a cylindrically shaped filament becomes: W = r p (D/2)2 V

4.4

The overall force balance for fiber spinning can be written as follows:121–123,13 Frheo = F0 + Finert + Fdrag - Fgrav - Fsurf where Frheo F0 Finert Fdrag Fgrav Fsurf

4.5

= the rheological force in the fiber = the rheological force at the beginning of the spinline = the inertial force as the fiber is accelerated = the drag force caused by the fiber moving through a stationary fluid = the gravitational force on the fiber = the surface tension force at the fiber-air interface and is generally considered negligible compared to the magnitude of the other forces.

The individual forces may be expressed as follows:121–123,13 Frheo = W(V – V0) Fdrag = p D s f dz Fgrav = r g (p D2 / 4) dz Fsurf = (p s) /2 (D0 – D) dz where W V0 D0 sf s z

4.6

= the mass throughput rate (related to mass balance) = the average polymer velocity in the die = the die diameter = the shear stress at the fiber-air interface due to aerodynamic drag = the surface tension of polymer melt with respect to air = the distance along the fibre away from the die.

Neglecting the radial variations and the force due to surface tension, the gradient of axial tension along the spinline can be written using eqns 4.3 and 4.4 as:

188


dF/dz = W dV/dz + 1/2 ra Cd V2 p D – W g/V where D V g Cd

= = = =

4.7

the filament diameter the filament velocity the gravitational acceleration constant the drag coefficient and is defined using the equation for the shear stress at the fiber-air interface due to aerodynamic drag.

s f = 1/2 ra Cd V2

4.8

The rheological force in the spinline is related to the axial spinline stress s 11 or s zz as s11 = Frheo/(p D2/4) and szz = h (T) dV/dz

4.9

where h is the viscosity at temperature T. One of the principal unknowns in the force balance equation is the air drag force, which becomes increasingly important at high spinning speeds. Numerous theoretical investigations have focused on identifying the nature of the drag coefficient Cd, in the drag force equation. To evaluate Cd for melt spinning, Matsui124 has formulated a simple expression for C based on air Reynolds number using turbulent theory. C d = KR e–n

4.10

where Re is the air Reynolds number and is given by Re = (raVD/ma) K = 0.37 and n = 0.61 The energy balance is required to determine the fiber temperature as a function of distance from the spinneret. Heat transfer from the melt spinline to the ambient medium involves several mechanisms; radiation, free convection and forced convection. The effect of radiation is strongly dependent on the temperature of fiber. In polymer melt spinning the radiation contribution is usually negligible compared to the convective heat transfer. Heat released due to crystallization can be neglected for slow crystallizing polymers, but should be considered for fast crystallizing polymers. Neglecting radial temperature variations and including heat of crystallization, the differential energy balance equation can be written as: dT/dz = – p D h (T – Ta) / W Cp + DHf/Cp * dX/dz where

T = the fiber temperature Ta = the ambient air temperature

4.11


DHf X h Cp

= the = the = the = the

189

heat of fusion crystalline fraction heat transfer coefficient resin heat capacity.

Usually, the temperature of filament during melt spinning is determined experimentally as a function of the distance from the spinneret using an infra-red sensor. Then eqn 4.11 is used to calculate the heat transfer coefficient as a function of the distance from the spinneret.

4.13.2 Deposition ratio In spunbonding and meltblowing, the deposition of filaments on the conveyer belt is an important variable. The manner in which the filaments are laid down dictates the web geometry and hence some key fabric properties. It is difficult to quantify this step mechanistically, but a ‘deposition ratio’ (Dr) can be defined (see eqn 4.12). By manipulating the ratio, the transverse strength of the fabric can be altered. Higher ratios give higher transverse strength and lower ratios give lower transverse strength. However, the ratio has little effect on the longitudinal strength of the web. Dr = Vf /Vb

4.12

where Vf = the filament speed Vb = the conveyer belt speed. The filament speed can be calculated using the mass balance equation using the initial and final diameter of the filament. The conveyer belt speed is given by: Vb = m/(G ¥ Wc) where

4.13

m = the polymer mass throughput rate G = the desired web weight Wc = the width of the conveyer belt.

4.13.3 Polymer residence time in the extruder The polymer residence time in an extruder can be calculated as follows: t = (rmelt * v) / W where t is residence time in minutes rmelt = polymer melt density in g/cm3 v = the screw volume in cm3 W = the polymer throughput rate in g/min.

4.14

190


4.13.4 Determination of the airflow rate In meltblowing and spunbonding air plays an important role in fiber formation. It is necessary to know the air mass flow rate as well as the velocity of air in the system. The determination of airflow rate and velocity is complicated by the effect of temperature and pressure and also by the fact that the flow is likely to be unsteady or subject to disturbances. Therefore measurement conditions, instruments, and methods should be specific and reliable125. In practice, the air mass flow rate is usually determined using an orifice plate or Venturi meter. The air velocity is usually measured using an anemometer or a pitot tube. The measurement procedures are established by the American Society of Mechanical Engineers (ASME).126 More detailed descriptions of air measurement techniques are available.127,128 The following describes the determination of the air mass flow rate using orifice plates in a pipe. The orifice plate is usually flat and consists of a circular hole in the center. Its most common use is as a flow quantity measuring device. The determination of the airflow rate using an orifice is based on the use of Torricelli’s theorem and Bernoulli’s equation.129 The air mass flow rate using an orifice plate can be calculated as follows:130

Wh = 359 C F d 2 Yh w where

4.15

Wh = the air mass flow rate in lbs/hr (multiply by 1.26 ¥ 10–4 for kg/sec) C = the discharge coefficient d = the diameter of the orifice in inches (multiply by 0.0254 for m) hw = the differential pressure in H2O (multiply by 249.06 for Pa) Y = the weight of air in lbs/ft3 (multiply by 16.018 for kg/m3) entering the orifice F = the velocity of approach factor and is expressed as

F=

1 ; b= d 4 D 1–b

4.16

where d is the orifice plate diameter and D is the pipe diameter. The orifice diameter, d, is 1.335 in. (0.034 m) and the inside pipe diameter, D, is 2.9 in. (0.074 m). This gives a diameter ratio of 0.46. Evaluation of eqn 4.4 gives a velocity of approach factor, F = 1.023. Thus, eqn 4.3 becomes: Wh = 654.64 C Y h w

4.17

The specific weight of the air entering the orifice, U, can be determined from the ideal gas equation of state:


Y= P RT

191

4.18

where P is the absolute pressure in lb/ft2 (multiply by 4.88 for kg/m2), T is the absolute temperature in ∞R, and R is the gas constant of air, 53.34 ft-lb/ lbm-∞R (multiply by 3.407 ¥ 103 for kg.mol.∞R);. The absolute pressure upstream of the orifice is given by: P1 = P2 + rghw + P•

4.19

where P1 = the pressure upstream of orifice in psia (multiply by 6894 for Pa) P2 = the pressure downstream of orifice in psig (multiply by 6894 for Pa) rg = the specific weight of H2O in 0.0361 lb/in3 (multiply by 27 ¥ 103 for kg/m3) P• = the ambient pressure in psia (multiply by 6894 for Pa). The coefficient of discharge, C, is a function of B and the Reynolds number, Re, based on the inside pipe diameter. The expression for the Reynolds number is given by: Re =

YVD m

4.20

where V = the air velocity in ft/s (multiply by 0.30480 for m/s) m = the dynamic viscosity in lb/ft-s D = the inside diameter of pipe in ft. (multiply by 0.30480 for m). The viscosity of air is a function of the temperature. The following expression, derived from the Sutherland equation is useful to determine the viscosity in terms of the absolute temperature

m = 1.3183 ¥ 10 –6

T 1.5 lb in (T + 200) ft – sec

4.21

Equation 4.10 gives the air velocity in the pipe:

Wh 1 in ft 2 3600 sec p D 4.22 Y 4 The flow rate, eqn 4.5, can be converted to standard cubic feet per minute (SCFM) using a standard density, evaluated at P = 14.7 psia and T = 68 ∞F (20 ∞C). The standard density is = 0.07516 lb/ft3 (multiply by 16.018 for kg/ m3). The volumetric flow rate in SCFM (multiply by 4.72 ¥ 10–4 for m3/s); is given by: Wh 4.23 Q= in SCFM 60Ys V=

192


4.14

Composite fabrics and other extrusion processes

Spunbond and meltblown webs are often combined at the production stage to achieve a variety of composite structures for protective applications particularly in the hygiene and medical sectors. The benefits of combining spunbond and meltblown webs are: ∑ ∑ ∑

barrier to liquid permeation especially of bodily fluids in medical gowns increase in the cover of the base spunbond web barrier to penetration of particulate matter in filter applications

In the SMS composite structure, the spunbond fabric provides the strength and the abrasion resistance and the meltblown component provides the liquid and particulates barrier. The spunbond-meltblown-spunbond (SMS) concept was first introduced and patented by the Kimberly-Clark Corporation. Combining these two media has become a common practice in spunbond manufacturing and is finding rapid acceptance and integration in a variety of products. There are a variety of spunbond-meltblown structural combinations available, such as spunbond-meltblown-spunbond (SMS), spunbondmeltblown-meltblown-spunbond (SMMS), spunbond-spunbond-meltblownmeltblown-spunbond (SSMMS) and others depending on the desirable final product properties. In some applications, films, including elastomerics are combined with spunbond and meltblown fabric components at the web formation stage.

4.14.1 Coform® The Coform® process is operated by Kimberly Clark and produces meltblown webs containing wood pulp as a liquid absorbent. During the process, the wood pulp in sheet form is fiberized and the separated pulp is injected from one side onto the still tacky meltblown filaments as they travel from the die to the collector. In this way, the wood pulp adheres to the filaments as they cool. The fabrics are composed of approximatly 60–70% wood pulp and are particularly thin given their liquid absorption properties. Once the web is laid down, a preformed spunbond or meltblown fabric or film is thermally laminated to at least one side to form an absorbent composite. By changing the number and composition of these layers, a variety of products can be produced. These are used in liquid containment applications in the hygiene sector. Applications include wipes for domestic, hospital and nursing homes, incontinence, birthing, and nursing pads and fenestration areas on drapes.


193

4.14.2 Solution flash spinning Flash spinning is an alternative technique for the conversion of fiber-forming polymers into spunlaid webs using a dry spinning technique. The process, which is proprietary and is not available for commercial licensing, was developed by DuPont USA in the late 1960s. In flash spinning a polymer, typically polyethylene, is blended with a solvent (typically methylene chloride) under high temperature (about 25 ∞C or more above the boiling point of the solvent) and under high pressure. The blended solution is then released under controlled conditions to produce what is effectively an explosive reaction in which the solvent flashes off to produce a three-dimensional network of thin, continuous interconnected ribbons many of which are less than 4 mm thick. The fibrous elements are usually termed film-fibrils or plexifilaments.131 Sometimes dissolved inert gas, for example CO2, is used to increase the degree of fibrillation.25 The fibrous network is collected on a moving conveyor and is consolidated. The individual plexifilaments have high molecular orientation, leading to high strength. Tyvek® fabrics (DuPont) are produced using this method. Flash spinning is the most complex and difficult method of manufacturing spunbond fabrics because of the need to spin a heated and pressurized solution under precise conditions.21

4.14.3 Electrospinning systems Fibers having diameters in the nanometer range (1000 punches/cm2 depending on the required density, fabric weight, fibre composition and physical properties of the final product. It is desirable to punch from each side as this normally promotes a stronger more uniformly consolidated fabric. Modern finishing looms run at high punch frequencies of 1000–3000 punches/min and consequently, they tend to operate with a shorter needle penetration depth than preneedling needlelooms. Needle penetration depths are lower than in preneedling and therefore, shorter 76 mm (3 inch) needles are commonly selected. These short blade needles are stiffer than the 90 mm (31/2 inch) type and are suited to low penetration depths and high needling densities. Since preneedled fabrics are thinner than the original batt introduced to the preneedling loom, the bedplate to stripper plate gap setting is kept small and is only marginally greater than the preneedled fabric thickness. In finishing looms, it is usual to use fine gauge needles carrying small barbs. Although the number of fibres that can be carried in each of these small barbs is comparatively low, higher needle densities (punches per cm2) compensate. This approach also gives good fabric strength and produces an even fabric surface, free of the perforation patterns that large needles can introduce. One of the more versatile looms used in the finishing process is sometimes

Mechanical bonding

245

referred to as a double punch or quadpunch loom. This loom has four needleboards, two punching from the top and two punching from below. A needling line incorporating one or more quadpunch looms is more compact than a line consisiting of two or more looms punching from only the top or below. Each needleboard holds up to 8000 needles per metre of the working width. Needling from opposite sides gives greater fabric strength than needling from only one side. Both the upper and lower fabric surfaces have the same appearance using this approach. Modern four-board looms run at high speed and are arranged so that the penetration depth of each needleboard can be independently adjusted. Different needle specifications can be fitted in the upper and lower needleboards on the in-feed side to the upper and lower boards on the draw-off side if necessary to achieve desired felt characteristics. For example, if the in-feed needleboards are fitted with close barb spaced needles and the draw-off boards are fitted with regular barb spaced needles, a well consolidated fabric with a smooth surface will result. Some needlepunching factories prefer to have their finishing looms offline with only the preneedling looms being incorporated into the carding and cross-lapping line. This is common in the manufacture of filtration fabrics. The batt is preneedled and carefully wound onto rolls or ‘A’ frames and they are then transported to a finishing loom or line of finishing looms. Many filter fabrics are made up of multiple layers of preneedled fabric which are assembled and needled together in one or more passes through a finishing loom. A woven reinforcing fabric or scrim is normally incorporated during this process to increase dimensional stability of the product. Needling is also used in some applications to bondmelt blown and spunlaid webs containing filaments. For needlepunching of spunlaid webs, high line speeds are required and either high-capacity looms or multiple individual looms operating in sequence are needed to balance production. Figure 5.34 shows a three-loom line by Fehrer with two down-punch and one up-punch machines, where up to 35 m/min is claimed at a loom speed of 2,000 r.p.m., and more than 60 m/min at 3,000 r.p.m.

NL 3000

NL 3000

NL 3000/R

5.34 Multiple needleloom production line (courtesy of Fehrer GmbH).

246


5.12.3 Elliptical needlepunching The Dilo Hyperpunch system uses an elliptical needle path to enable a large advance per stroke and therefore, very high line speeds (see Fig. 5.35) in both preneedling and finish needling applications. In this system the needles move with the fabric during needle penetration and therefore the bed and stripper plates have slotted holes to allow for the needle motion. The elliptical motion is claimed to reduce drafting providing the needle penetration depth is low and to give a uniform surface finish. Synthetic leather production is one of the markets for which it is intended as well as in the needlepunching of spunlaid webs and the production of paper machine felts. The Hyperpunch system can be incorporated in high-speed structuring looms to produce rib, diagonal, diamond and hobnail patterns.

5.12.4 Inclined angle (oblique) needlepunching An early example of needling from both sides of the batt at an inclined angle was the Chatham fibrewoven process for blankets, where the needles were angled at 20∞–30∞ to the plane of the batt. In the 1960s this was a very

5.35 Principle of elliptical needlepunching (courtesy of Dilo).

Mechanical bonding

247

sophisticated machine in concept and engineering design, producing blankets with good strength and dimensional stability but output speeds were low by modern standards. A current method of achieving angled needle penetration is the Fehrer H1 system which employs a curved bed and stripper plate with a corresponding needleboard. The changing curvature varies the angle of needle penetration as the fabric passes through the needling zone, giving fibre pillars of different angular inclinations within the cross-section. Claimed effects are an improvement in the isotropy of the fabric with respect to tensile properties and a general increase in tensile strength as compared to traditional needlepunching systems. The H1 system is employed for the manufacture of geosynthetics, synthetic leather, filtration fabrics, shoe linings, automotive fabrics and papermakers’ felts.

5.12.5 Structuring needlelooms Surface textured needlepunched fabrics are produced using structuring needlelooms. Rib fabrics Preneedled fabrics may be textured to produce a looped pile using forked needles which transport fibres between lamella strips that serve as the bedplate (Fig. 5.36 and Fig. 5.37). Such structuring looms are typically down-punch machines producing rib or velour surface structures depending on the orientation of the needle fork relative to the incoming fabric. Patterning is introduced by varying the position of needles in the board and by controlling the advance per stroke. By lifting and lowering the lamella table, the height of the fibre pile is adjusted. In certain systems the needleboard position is raised or lowered to enable the pile height to be adjusted. Velour fabrics Classical velour fabrics are produced using coarse gauge fork needles orientated in the correct direction and operating in conjunction with a lamella bed plate. In the production of random velours, in place of the lamella bed plate, a continuous, moving brush conveyor is employed to produce a fine, highdensity velour finish. The design of the brush, particularly the density, brush filament diameter, height and uniformity, influence the appearance and structure of pile surface produced in the fabric. Damage or wear of the brush as a result of needlepunching produces quality problems such as pile height variation and other defects. Fine gauge fine fork needles and crown needles, sometimes in combination, are commonly used to manufacture random velour fabrics. The pile is formed in the brush conveyor and at the same time carried

248


Depth

Width

5.36 Forked needle. Fork needle Stripper plate

Lamella

5.37 Lamella strips.

forward until the finished fabric is drawn out of the brush belt by the takeup rollers. In double random velour systems, more than one needling head is positioned over a common brush conveyor to give a high pile density, and possibly to

Mechanical bonding

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introduce coloured effects by means of yarns or other material. Such yarns are tacked in to the fabric structure after being introduced from an overhead creel. An example of a random velour structuring machine (Di-lour IV) is shown in Fig. 5.38. Two needleboards have a backing felt introduced before the second needling zone. This gives a close pile and added stability to the finished product. Rib and velour fabrics with large repeat patterns with a patterned surround or border on all sides can be made on the Fehrer NL11/ Twin-SE Carpet Star® instead of using two machines in tandem. The Carpet Star® operates with two independent needle zones that are electronically synchronised to maximise pattern flexibility at high speed. Simulated oriental carpet patterns are claimed to be possible using this system. In another system known as the Dilo DiLoop RR Rug-Runner, register control is provided between two looms when complex relief patterns are produced.

5.12.6 Specialist needlelooms Continuous belts Needlepunched fabrics are produced in long continuous belt form for use in the manfacture of paper. These needlepunched papermakers’ felts are used in the pressing and drying stages of the papermaking process and have very large working widths of more than twelve metres. The felts form a wide endless belt consisting of layers of preneedled or carded web needled into a special monofilament scrim. The quality control requirements are extremely high since structural imperfections in the belt affect the quality of subsequent papermaking.

5.38 A velour structuring machine.

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Tubular fabrics Tubular needlepunched fabrics can be produced using a specially adapted needleloom design developed by Dilo. These tubes normally have an inside diameter of 25–500 mm. In some cases, tubes with diameters of only 5 mm may be produced. The Rontex S 2000 loom employs two needling units acting from opposite sides with different angles of needle penetration. A continuous needlepunched spiral is produced which may be layered with the tube wall having different types of fibre. Three dimensional linked fabrics The Laroche Napco 3D web linker produces three-dimensional fabrics (Fig. 5.39). The machine is fed with two fibrous webs (A and B) between two stripper plates and one or two spacer tables consisting of bars or tubes. As the webs pass through the machine the barbed needles drive fibres from one web to the other creating fibre bridges. Spaces between the spacer bars are used to introduce components such as wires and cables. Spacer tubes allow the insertion of powders, fluids or foams, as it is being made. Yarn and fabric punching The concept of yarn punching has been developed by Fehrer and involves needlepunching coarse yarns to increase their tensile strength. This is intended for carpet, mop and effect yarns as well as for friction-spun and open-end yarns requiring sheath-core stabilisation. The yarn is fed along a narrow Web A Web B

Composite product

5.39 The Laroche Napco 3D web linker.

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channel through which the needles pass in a reciprocating motion. Needlepunching has also found commercial application as a finishing process for woven coating substrates.

5.12.7 Fabric structure and strength There is a considerable body of published research on the complex subject of needled fabric properties. Examination of fibre migration and the formation of ‘pillars’ of fibre in the cross-section due to the action of the barbed needle are facilitated by the use of tracer fibres and optical microscopy. Some of the fibres are incorporated into these pillars along most of their length, whilst others remain predominantly in the fabric plane. The first two or three barbs engage the largest number of fibres from the upper region of the web which has the effect of tying-in these fibres to the lower surface thereby providing cohesion and a reduction in fabric thickness. Initially, as pillars are created the fabric strength increases, but after peaking the fabric strength subsequently decreases as fibres are broken and the fabric begins to perforate. It should be remembered that the presence of the structural pillars of fibre in the crosssection depends both on fibre and process factors. Few pillars are formed and therefore fabric strength increases little unless fibres are able to deform and extend whilst in contact with the barbs. The number of fibres in each pillar, their frequency and interconnection, are a function of barb dimensions, needle punch density, the depth of penetration and the advance per stroke amongst other factors. It is therefore possible to engineer the structure of a needlepunched fabric to a large extent by considering these aspects. Fibre composition, length, diameter, fibre tensile properties, fabric density and thickness are particularly important with respect to fabric properties. Fabric strength properties most frequently measured are tensile, tear and puncture. In the early days, comparisons were always made with the strength of woven fabrics, but since synthetic fibres became cheap, plentiful and available in lengths, thickness and crimp tailored to the needlepunching process, the properties of needlepunch are considered in their own right. Because needlepunched fabrics were originally viewed as cheap alternatives, they were made with relatively weak fibres such as rayon and wool, which could easily break under the action of the needle barbs therefore many studies made before nylon, polyester, polyethylene and polypropylene became available, may not be relevant to the modern industry. Similarly much work was done at what would today be considered very low speeds and it is doubtful that this is relevant to current industrial practice.

5.13

Applications of needlepunched fabrics

The applications of needlepunched fabrics are extensive and extend into many niche product areas including, for example, medical wound dressings,

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composite breather felts, capillary mattings for horticulture, fire barriers and ballistic-impact-resistant fabrics. Some of the main product applications are given below but this list is not exhaustive.

5.13.1 Geosynthetics Needled fabrics are used in civil engineering applications that require deformability, high tensile and burst strength as well as controlled permeability and weight, for road reinforcement, subsoil stabilisation, pond liners, hazardous waste containment protection liners and drainage. Staple fibre geotextiles are typically composed of polypropylene, polyester or polyamide. Where spunlaid webs are needlepunched, they are produced at high linear speed with a relatively low degree of needling. The Foster star blade needle having four barbed edges has found use in such applications where high strength geotextiles made from both staple fibre and spunlaid webs are produced. Needlepunched geosynthetic clay liners are installed for low water permeability in landfills, canals, ponds and pollution-prevention barriers in highway and airfield construction. Rolls of bentonite clay are sandwiched between two fabrics, one or both of which are nonwoven. Needling the composite locks the bentonite clay in place. The clay is the critical component as it provides extremely low hydraulic conductivity. In needling, the abrasive nature of the clay means that manufacturing conditions and needle design must be controlled to minimise needle wear. Conventional needlepunched spunlaid fabrics for geosynthetic applications are made from about 120 g/m2 to 340 g/m2. Needlepunched spunlaid fabrics are utilised in the laying of asphalt as stress-absorbing-membrane interlayers; fabrics dipped in bitumen help to improve the adhesion between layers of asphalt.

5.13.2 Filter media Fabric density and permeability are properties relevant to the filtration of gases and liquids, and depth filters are particularly suited to needlepunched fabrics because of their substantial thickness. Woven scrim reinforcement is needed in industrial bag house applications, whilst staple glass, silica or aramid fibres are utilised in high-temperature conditions. For general filtration applications, PET, PA and PP are found, but for high-temperature or corrosive environments other high-performance organic and inorganic fibres are needlepunched to make chemically or thermally stable filter fabrics including PTFE alone or in blends, polyimide, basalt and stainless steel amongst others. Electret filters are also needlepunched based on drylaid blends of staple fibres selected for their relative position in the triboelectric series. The surface of needlepunched fabrics may be coated, singed or calendered to adjust the

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surface structure and therefore both the cleaning and filtration efficiencies of the media. The fabric density may also be graduated through the fabric cross-section by adjusting needle penetration and needlepunching density, which influences filtration efficiency in use. Both roll products and tubular needlepunched fabrics are used as filtration media.

5.13.3 Synthetic leather Needled synthetic leather fabrics imitate the natural product with a densely entangled fibre construction that is impregnated with a polyurethane resin to give a smooth surface free from needle marking and with a high surface abrasion resistance. Typical production lines may have as many as eight needlelooms in sequence from preneedling to finish needling. The density is progressively increased in successive needlepunching stages. To avoid marking of the fabric and to minimise needle force, fine gauge needles with small barbs and small tip to first barb distance are selected. Sometimes, single barb needles with high needlepunching densities are employed to produce, fine high-density fabrics with a uniform surface. Only small needle penetrations are required in the process using such needles. To further increase the surface area and density of the fabric, a proportion of high-shrink thermoplastic fibres may be included in the blend, which after heating induce contraction of the fabric. Splittable bicomponent fibres have also been developed that are designed to split in needlepunching in a similar manner to that in hydroentanglement of such fibres. Non-apparel end uses of needlepunched synthetic leather are luggage, automobile seats and panels, upholstery, wall coverings, and footwear.

5.13.4 Waddings and paddings Based on fibre consumption, waddings and paddings are one of the largest single application areas for needlepunching and the fabrics are incorporated into mattresses and furniture as insulator pads (in contact with the sprung unit) as well as comfort layers to provide support, carpet underlay, sound and heat insulation for automobiles and other industrial uses. Fibre selection ranges from recycled natural and synthetic fibres, usually obtained from pulled waste clothing, jute, sisal, coir and cotton as well as virgin synthetic fibres, particularly PET, PP and acrylic.

5.13.5 Floor coverings Flat floor coverings generally consist of a face layer, a scrim and a bottom layer and are produced by preneedling, flat needling and in many cases, structuring. Needling from both sides tends to increase the wear resistance of the face

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layer. As described previously, structuring involves needlelooms with lamella bed plates in which forked needles are used, or random velour needlelooms with brush conveyor belts in which fine gauge fork needles or special crown needles are used. Needlepunched floor coverings are commonly produced from PP and blends of PP and PA, which is spun-dyed and blended prior to carding, cross-lapping and needlepunching. In some low traffic applications, where softness is needed, PET is sometimes selected. Different fibre linear densities are frequently blended to adjust the durability and compression recovery properties of the floor covering. Linear densities are about 12–20 denier and fabric weights are in the order of 300–800 g/m2. A small proportion of very coarse fibres may therefore be added to a base blend of 17 dtex fibres.

5.13.6 Automotive fabrics End uses for needled fabrics consist of decorative trim including headliners, door trims, seatbacks, boot liners, load floors and package trays. In the USA, the interior trim is composed of spun-dyed PP whereas PET is important elsewhere in the world. Fibre linear densities range from 15–18 denier in the USA, where the abrasion resistance specifications are high, to as low as 6 denier in the Far East and Japan. Other products include sound dampers, underfelts, padding, performance gaskets, seals, filters and shields. Extensive use is made of random velour fabrics in automotive applications particularly in small to medium sized cars and structured needlepunched fabrics are encountered in more expensive interiors. In moulded floor fabrics up to about 65% of the total fibre content is visible as surface pile. The quality control issues are particularly stringent and a major consideration is colour consistency and colour matching between batches even in solid shades. There is also growth in the composites field and blends of wood fibres with synthetic fibres are needled prior to resin impregnation and the formation of rigid panels. Glass fibre composites are made in a similar manner and there is ongoing development in the use of natural fibres including hemp, flax, sisal and other bast fibres in automotive composites. Composites are now fitted around the firewall, dashboard, speaker, engine bay and parcel shelf structures.

5.13.7 Insulation Both thermal and acoustic insulation fabrics are formed by needlepunching. In high-temperature applications, blown or spun ceramic fibres in batt form are needlepunched and the finished material may be up to 75 mm thick. Avoiding damage to the brittle fibres is a primary objective. High-temperature fibres are needlepunched to form thermal insulation suitable for the automotive and aerospace sectors and are subject to stringent regulation. Insulation paddings are also made by needling recycled fibres extracted from clothing and low grade wool and cotton.

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5.13.8 Blankets This was one of the earliest applications for needling, and whilst highquality natural and synthetic fibres are still used occasionally, the process is commonly found where cheap blankets made from mixtures of regenerated fibres are needed, including emergency and disposable blankets. It is common for reinforcing yarns or scrims to be incorporated into such fabrics.

5.13.9 Wipes Needling is used for some heavy-duty household and industrial wipes and polishing cloths. Use of spun dyed viscose staple blended with a thermoplastic bonding fibre is popular and many other fibres including Lyocell have been needled for personal care and industrial wipe applications. In pre-moistened wipes, needlepunched fabrics are competing in some sectors because it is possible to store larger volumes of lotion in the structure compared to a hydroentangled wipe. One application is in post-operative wipes containing anti-bacterial soap.

5.13.10 Roofing Needlepunched spunlaid fabrics composed of PET find applications as bitumencoated roofing felts because they have good puncture and tear resistance. Glass scrim reinforcement may be introduced during needling to improve dimensional stability.

5.14

Hydroentanglement: introduction

Hydroentangling, spunlacing, hydraulic entanglement and water jet needling are synonymous terms describing the process of bonding fibres (or filaments) in a web by means of high-velocity water jets. The interaction of the energised water with fibres in the web and the support surface increases the fibre entanglement and induces displacement and rearrangement of fibre segments in the web. In addition to mechanical bonding, structural patterns, apertures and complex three-dimensional effects are produced if required by the selection of appropriate support surfaces. Hydroentanglement also provides a convenient method of mechanically combining two or more webs to produce multilayer fabrics. The early work on the process of hydroentanglement has been principally attributed to Chicopee (division of Johnson and Johnson) and DuPont in the USA. The respective technical contributions of these companies are described in a series of detailed patents filed from the 1950s to the early 1970s.9,10,11 Originally, the utilisation of relatively low-pressure water jets (> 1 d 2f nrCd2 D 4 d x3 p

5.12

where R is the flexural rigidity of a fibre with a circular cross-section (Nm2).

5.16

Fibre selection for hydroentanglement

Virtually all polymeric fibres of a wide range of dimensions are compatible with hydroentanglement providing they can be first formed into a web at commercially acceptable production speeds. However, the process efficiency, fabric properties and economics of hydroentanglement vary depending on fibre selection. It is important in hydroentanglement to maximise bonding whilst minimising the energy. Fibre properties have a significant influence on the degree of bonding that can be achieved for a given energy consumption.

5.16.1 Fibre stiffness Formation of coherent fabrics with minimum energy consumption requires flexible, deformable fibres that can be readily entangled. Fibre flexural rigidity depends on its diameter, Young’s modulus, cross-sectional shape and density. Depending on fibre type some of these properties are strongly moisture-

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dependent, particularly the modulus, which is important in hydroentanglement. Fibre flexural rigidity values are frequently associated with hydroentanglement efficiency but the fibre bending deformations in hydroentanglement are believed to be three-rather than two-dimensional in nature and the torsional rigidity of the fibre should be considered.17 Viscose rayon has a low wet modulus and this partly explains the ease with which this fibre can be hydroentangled. At low pressure and energy, heavily entangled fabrics are produced from viscose rayon, whereas at the same conditions the entanglement is significantly lower in fabrics containing, for example, PP. While it is possible to hydroentangle high modulus fibres such as glass and carbon fibre, the strength realisation tends to be poor because of the limited fibre entanglement that is introduced. Attempts to increase the entanglement by increasing the applied pressure might lead to fibre damage and some fibres may be pulverised rather than entangled even at relatively low pressure. Fine fibres are therefore more flexible than coarse fibres of the same polymer composition, they hydroentangle more intensively for a given energy consumption and produce stronger fabrics. The accompanying increase in fibre surface area and the increased number of fibre intersections as the fibre diameter decreases contributes to increased fabric strength. For a given polymer type, fibre cross-sectional shape also affects entanglement. Triangularshaped fibres require more energy than do round fibres and elliptical and flat fibre cross-sections hydroentangle quite efficiently.

5.16.2 Wettability Effective hydroentanglement requires uniform and rapid wetting of the web. The choice of spin finish is particularly important to ensure proper wettingout of hydrophobic fibres such as PP and to minimise foaming during the process as the finish is removed in the waste water. The antistatic component of the finish can be particularly associated with foaming but formulations have now been developed that minimise the problems. Prior to the main injectors, pre-wetting reduces the web thickness by displacing air from the structure and therefore the web should wet-out at this stage. Due to the hydrophobic nature of synthetic fibres such as PP and PET, in particular, hydrophilic fibre finishes are utilised for synthetic fibres intended for hydroentanglement. In one example a finish containing one monoester of glycerol and a fatty acid having from six to fourteen carbon atoms is applied as an aqueous dispersion.28 The durability of hydrophilic finishes is low and they are effectively removed during hydroentanglement. This necessitates reapplication of hydrophilic agents to synthetic fibre fabrics intended for absorbent products following hydroentanglement. Durable hydrophilisation has been demonstrated by some fibre manufacturers29 and PP fibres specifically for hydroentanglement are produced.30 Treatments such as plasma discharge

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methods, used to functionalise the surface of synthetic polymers with hydrophilic groups, have been demonstrated.

5.16.3 Fibre dimensions Fibre linear density normally ranges from 1.1–3.3 dtex; common linear density and fibre length combinations are 1.7 dtex/38 mm, 3.3 dtex/50 mm and 3.3 dtex/60 mm. The fibre length and slenderness ratio also influences compatibility with preceding web-forming processes. Fine fibres have a high specific surface area for a given fabric weight leading to good fabric strength. There is also a direct relation between fibre diameter and bending stiffness, which influences entanglement efficiency. For good fabric formation, fibres must be mobile and capable of deformation and deflection in the web. While short-cut fibres hydroentangle quite efficiently, there is a positive relationship between fibre length and fabric strength within a narrow length range. When fibre fineness is constant, fabric strength increases with fibre length up to a maximum length of about 50–60 mm.16 For viscose rayon, fabric strength has been shown to increase with fibre lengths up to 51 mm.17 While crimp promotes cohesion in carded webs prior to bonding, it also influences the strength of fabrics produced by low to medium pressure hydroentanglement systems.16 As a consequence of the high resistance to compression, helically crimped fibres require higher energy inputs to obtain fabrics with acceptable strength. The effect of crimp is particularly noticeable in the hydroentanglement of fine wool, which has a high crimp frequency and requires high pressure to produce a coherent, abrasion-resistant structure.

5.16.4 Fibre types Commercially, staple fibre PET and viscose rayon are most important in hydroentanglement and may be blended with wood pulp. Partly as a consequence of the growth of hydroentangled wipes and medical products the consumption of viscose rayon has increased. The wet strength and volume of viscose rayon fabrics can be increased by blending with PET or PP. Wood pulp is widely utilised in hydroentangled fabrics as a low-cost absorbent for dry wipes, surgical gowns and drapes but only in blends or multi-layer structures. Airlaid wood pulp is blended with viscose or PET in proportions up to 50% and hydroentangled. The staple fibre matrix traps the pulp inside the fabric. Assuming appropriate water filtration is available as part of the hydroentanglement system, webs composed of linters and noils as well as bleached cotton are hydroentangled to produce absorbent products including wet wipes, medical gauzes and cosmetic pads. The fibres are usually 7–25 mm and 1.2–1.8 denier31 and a large proportion of cotton wax can be removed from unbleached fibre during hydroentanglement at comparatively low energy (below 1 MJ/kg), which increases the hydrophilicity.

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The strength of hydroentangled cotton fabrics tends to increase as the fibre fineness decreases but the fabrics have a harsher handle. One limitation of bleached cotton is that it can be difficult to card, which affects the production rate of a hydroentanglement line. The consumption of the solvent-spun fibre, Lyocell, in hydroentangled fabrics, has seen a rapid increase in recent years, particularly in absorbent products such as wipes, where the high wet strength is advantageous. Wipes containing Lyocell also have excellent pattern definition. High temperature fibres High-temperature-resistant hydroentangled fabrics have been produced for years, largely pioneered by DuPont. Meta-aramid hydroentangled fabrics are found in aerospace and protective clothing markets32,33 together with paraaramids for high-temperature protection and blends of meta and para-aramids are utilised in thermal barrier, heat-shielding materials. Hydroentanged melamine and aramid fibres have been developed for fire-retardant clothing34 and hydroentangled Basofil® fabrics are available. Provided the impact forces are low to minimise fibre damage, inorganic fibres such as glass and silica can be hydroentangled to produce pre-pregs for composites. The hydroentanglement of ceramic fibres has been discussed.35 Fibres with a high metal oxide composition including silica (SiO2) are liable to break during low-pressure hydroentanglement and fabrics produced from such fibres tend to have low integral strength because of the limited fibre entanglement that can be introduced. Splittable bicomponent (Bico) fibres The formation of microfibres in situ by splitting segmented-pie bicomponent fibres within fabrics during hydroentanglement is particularly well established in the Far East. Synthetic leather coating substrates, upholstery fabrics, highperformance wipes for applications such as optical glass cleaning are produced in this way. Carded and cross-lapped batts containing segmented-pie bicomponent fibres are hydroentangled to produce microfibres by splitting. Afterwards, fabric density can be increased by inducing thermal shrinkage of additional thermoplastic fibres blended with the bicomponent fibres in the web. The elimination of conventional leather production steps such as shrinking, splitting and grinding, leads to a significant saving in raw materials. Synthetic leather fabrics have excellent strength and durability and depending on polymer composition can be dyed and finished to develop attractive softness and handle characteristics. The cross-sectional configuration of splittable bicomponent fibres and filaments varies and the choice affects splitting efficiency and fabric properties. Rather than separating the embedded filaments in the bicomponent using

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solvents, which is the traditional practice in the manufacture of synthetic leather, or by differential thermal shrinkage of the two components, splitting during hydroentanglement relies on the low interfacial adhesion between the two polymers and the impact and shear forces delivered by the incident water jets (Fig. 5.42). The splitting efficiency of segmented-pie cross-sections improves if there is a hollow core. Rectangular striped fibre cross-sections also increase splitting efficiency because of the aspect ratio. The impact force generated by the jet is a function of the pressure and jet diameter and these must be selected to obtain uniform splitting throughout the fabric. At a fixed pressure, web weight and number of injectors, the splitting efficiency increases with jet diameter. Partial separation of some splittable bicomponent fibres can occur during high-speed carding and this can limit the production rate of both the card and hydroentanglement process. The critical pressure at which splitting is induced ranges from 50–100 bar, but varies depending on the stored strain in the individual fibres and their geometric position within the web crosssection. To achieve high splitting efficiency, water pressures up to 250–400 bar can be necessary partly because in heavyweight webs the degree of splitting varies through the cross-section. It is necessary to ensure, at least initially, that fibres are properly entangled through the fabric cross-section before splitting is complete. For these reasons, the water pressure in the first

5.42 Splitting of bicomponent fibre.

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few injectors must not induce excessive splitting but rather should entangle the fibres to introduce satisfactory bonding through the cross-section. The fineness of the fibres or filaments after splitting depends on the number of segments in the bicomponent cross-section. Typically, after splitting the fineness ranges from about 0.05–0.3 denier depending on the type of bicomponent. Experimentally, island in the sea filaments have been produced with 600–1120 individual PP filaments embedded in a soluble PVA matrix within the cross-section.36 While at present only segmented fibres are intended for splitting in hydroentanglement, hydroentangled nanofibre fabrics produced from splittable bicomponents is technically feasible and hydroentanglement of nanofibre filaments has been demonstrated.37 Fibrillating fibres Longitudinal splitting of microfibrillar fibres is induced at high impact forces which can be exploited to engineer the physical properties of fabrics. Such fibrillation during hydroentanglement can be observed in natural cellulosics including cotton and bast fibres as well as Lyocell, polynosics, polyacrylonitrile and even para-aramids if the water pressure is sufficiently high. Fibrillation usually occurs by splitting of the fibre from the outside in; the fibrils are partially exposed and project from the surface of the parent fibre and are readily entangled with neighbouring fibrils. The water pressure at which fibrillation begins varies depending on the fibre type and grade and for many commercial fabrics, hydroentanglement conditions are purposely selected to avoid the onset of fibrillation. The increase in surface area associated with mass fibrillation modifies the physical, optical and mechanical properties of the fabric. There is a decrease in fabric permeability, which is useful in the design of filter media, and an increase in the fabric opacity. Following mechanical finishing, a microfibre pile may be produced on the fabric surface, which markedly increases fabric softness. If finishing is not carried out, fibrillation tends to lead to a paper-like surface, which gives the fabric a harsh handle. Lyocell has attracted interest partly because of its propensity to fibrillate at high pressure, which can be harnessed to both increase entanglement and modify key fabric properties such as tensile strength, permeability and liquid transport. Applications for hydroentangled fibrillated Lyocell fabrics include filtration, such as cigarette filters and wipes.

5.17

Process layouts

Between the end of 1998 and 2000 alone, the market for hydroentanglement machinery increased by 50%, with the largest growth in Europe. Significant growth has also been experienced in China and the USA and the first lines for India and Pakistan have been announced. The main turn-key

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hydroentanglement machinery suppliers are Fleissner GmbH (Germany), who supply the Aquajet system and Rieter Perfojet (France) who supply the Jetlace 3000 system. These companies have been responsible for the installation of the majority of new installations since the late 1990s in the case of Fleissner GmbH and since the 1980s in the case of Perfojet. Other machinery installations have been constructed by Mitsubishi Engineering in Japan while in China, for example, installations have been developed by local engineering companies. Hydroentanglement has become increasingly cost-effective and accessible to manufacturing companies globally. The Leanjet system38 is a low cost, entry level system for manufacturers producing a narrow range of products. One or two microporous shells (MPS) or sleeves are used as support surfaces depending on the product to maximise fabric strength and minimise the energy consumption. In addition to contemporary systems, older hydroentanglement installations remain in operation which, to remain competitive, have been extensively modified and updated since their original installation. Machines over five metres wide capable of line speeds up to 300 m/min with drylaid web formation have been commissioned and higher productivity and process efficiency continue to be major drivers for development. Production rates up to 1100 kg/hr/m have been reached. In the production of lightweight hygiene fabrics line speeds of 200 m/min to over 250 m/min are in operation but not all hydroentangled fabrics are produced at such high speeds. In the production of cosmetic pads composed of cotton for example, production rates can be as low as 30–60 m/min. The water pressure, number of injectors and machine width influence the costs of the process. Consequently, minimising these while obtaining satisfactory fabric properties, particularly strength, can be advantageous providing the versatility of the line is not lost. The investment costs of an installation are claimed to approximately double for every 100 bar increase in pressure39 taking into account the costs of the water pump, electric motor, inverter and high-pressure pipework. The electrical power consumption of an injector is linked to the pressure used. Based on a 3.5 m operating width, approximately 155 kWh is consumed by an injector at 200 bar, 544 kWh at 400 bar and 1088 kWh at 600 bar.40 Over about 150 bar, the costs increase further with machine width. Commercially, there is no universal layout; the machine configuration depends on ∑ ∑ ∑ ∑

raw material properties and the method of web formation used to supply the machine fabric weight cost (particularly with respect to energy consumption) the need to introduce other components such as webs, scrims or preformed fabrics to manufacture multi-layer structures

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the required patterning or aperturing capabilities whether or not additional bonding will take place, e.g. thermal or chemical bonding the intended product application and the degree of machine versatility required.

Injectors may be arranged over the top of a flat conveyor or around the circumference of a rotary drum or cylinder. In practice, both arrangements are frequently incorporated in a modular machine in sequence. Examples of integrated machine systems are shown in Fig. 5.43. Rotary drums are favoured if, as is usual, a dual-sided treatment of the web is required although historically this has also been accomplished in flat belt systems using an overhead transfer and turn-back arrangement fitted between modules.41 Dewatering, fabric threading, floorspace requirements and pealing of the fabric from the conveyor surface are improved with the rotary system. In the flat belt system, fibre ends can partially penetrate the conveyor and when dragged over the suction slots during processing this inhibits web removal. The precursor web fed to the hydroentanglement machine largely determines the isotropy and quality of the final fabric, and in a production line web formation is often the rate-determining step. Web weights can range from 15–400 g/m2 but commercially, most are 40–150 g/m2 depending on fibre fineness. The majority of commercial fabrics remain below 100 g/m2. Technically, all generic web types can be hydroentangled including spunlaid, meltblown, carded, airlaid and wetlaid including composites of these.

5.17.1 Carding – hydroentanglement installations Commercially, drylaid systems predominate in hydroentangled fabric production. Webs are usually prepared by straight-through carding and the fibre length ranges from 25–60 mm. Parallel-laid webs tend to require higher energy to obtain adequate CD strength than do cross-laid webs. In straightthrough carding systems, one or more double doffer cards are used to supply web weights from about 20–80 g/m2. Where a card and profiling crosslapper are utilised, basis weights range from 80–400 g/m2. Web spreading using rollers is sometimes used to increase the width and to modify fibre orientation prior to hydroentanglement. In the manufacture of cotton makeup removal pads, multiple cotton cards are employed to produce web weights up to 200–250 g/m2. The design of high production nonwoven cards for producing lightweight, isotropic webs that are capable of balancing the high delivery speeds in hydroentanglement continues to be a challenge. High speed cards capable of 400 kg/hr/m (depending on fibre specifications) allow web geometry to be modified by scrambling or randomising to minimise the MD/CD ratio. However, scrambling rollers decrease the linear web speed.

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Fleissner-Aquajet One-step hydroentanglement unit

Fleissner-Aquajet Two-step hydroentanglement unit

Fleissner-Aquajet Three-step hydroentanglement unit

Fleissner-Aquajet Four-step hydroentanglement unit

Fleissner-Aquajet Multi-step hydroentanglement unit

5.43 Hydroentanglement machine configurations: one-step through to multi-step systems (courtesy of Fleissner GmbH).

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Hybrid card-airlay systems have also been introduced (see Chapter 2) that produce webs with very low MD/CD ratios. Nevertheless, regardless of the isotropy of the initial web, the resulting hydroentangled fabric can have a higher MD/CD ratio. Card widths extend to 5.1 m for producing hydroentangled fabric although trim widths may be lower at about 4.6 m.42 A line designed for the production of ‘straight-through’ carded low to medium weight fabrics of 250 m/min might consist of the following components: a worker-stripper nonwoven card, a batt compression device and a twin cylinder alternating hydroentanglement module operating with multiple injectors followed by hydroentanglement on a flat belt using two injectors. Drying is commonly by through-air single drum dryer before wind-up. If in addition to low to medium weight webs, higher weights up to 500 g/m2 are required with good isotropy, twin cards are employed. In this example, one of the cards operates with a cross-lapper to increase the versatility of the line. Following combination of the two webs, batt drafting enables modification of the MD/CD ratio to increase isotropy. Four cylinders are employed with alternating injectors before the final flat belt section, which in this example is fitted with one injector. Through-air double drum drying precedes winding up.

5.17.2 Carding and pre-formed tissue hydroentanglement installations For the production of carded web and tissue composites, the tissue is introduced onto the pre-bonded carded web between the first and second hydroentanglement modules. Carding is followed by prewetting and the web is then hydroentangled using twin-cylinder modules fitted with alternating injectors. The carded web and tissue are combined on a flat belt hydroentanglement module using two injectors to integrate the two components. Through-air drum drying and winding then follows.

5.17.3 Carding-airlaid composite hydroentanglement installations Sifted airlaid (and wetlaid) webs containing short cut fibre of 3–15 mm are hydroentangled in 100% form and in blends with wood pulp but scrims, films, textile fabrics and filaments may also be integrated into such composites between layers of webs during the process. Three-layer carded-airlaid (wood pulp)-carded (CPC) hydroentangled composite fabrics produced by combining both carded and airlaid webs are particularly suitable for wet wipe applications. Two-layer, carded-airlaid (CP) hydroentangled composites can also be produced by by-passing the second card but CPC products help to prevent dusting of

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the pulp from the wipe during use. To produce CPC composites, pulp is airlaid onto a preformed carded web composed of PET, PP or viscose rayon using a sifting air-laying process. Alternatively, pre-formed tissue may be introduced onto the web. A second carded web is laid on top of the CP layer prior to prewetting. Twin-cylinder hydroentanglement follows using alternating injectors and the composite is then fed to a flat belt hydroentanglement module fitted with two injectors. Through-air drum drying and winding up completes the installation.

5.17.4 Spunlaid hydroentanglement installations Spunlaid webs can be hydroentangled to introduce fibre entanglement and to mechanically combine webs in the production of twin-layer or mutilayer fabrics. Although high water pressures of 300–400 bar are potentially required to adequately bond spunlaid fabrics, one of the attractions is the opportunity to produce at high linear delivery speeds up to 600 m/min in widths up to 5.4 m. In an example of a spunlaid-hydroentanglement installation, a web of 10– 400 g/m 2 is compressed and hydroentangled on a multi-drum hydroentanglement installation using five injectors. The fabric is then hydroentangled on a flat belt module fitted with at least one injector. Throughair drum drying and winding up completes the line. Since spunlaid hydroentangled fabrics are free of thermo-fused regions introduced by calender bonding, the fabrics have comparatively high bulk and good tactile characteristics as well as high tear strength and a comparatively low flexural rigidity. The risk of thermal degradation of the polymer during bonding is obviated. Using a spunlaid platform followed by hydroentanglement, low to medium weight fabrics, particularly in the hygiene, filtration and geosynthetic sectors have been targeted because, for example, the polymer type, the additives used and the linear density of the filaments can be changed as required prior to hydroentanglement. Examples of spunlaid-hydroentangled fabrics include Polymer Group Inc.’s Spinlace™ fabrics produced from 0.5– 3 denier continuous filaments and Freudenberg’s Evolon® fabrics produced from splittable bicomponent filaments. The Spunjet system of Rieter Perfojet is an example of an integrated spunlaid web formation and hydroentanglement system.

5.17.5 Combination bonding Commercially, thermal or chemical bonding may be utilised after hydroentanglement to produce the final fabric. Traditionally, apertured fabrics produced at low pressure have been chemically bonded to adequately stabilise the fabric. In household wipes applications, approximately 7–27% of the fabric weight can be binder; the higher proportions being applied to viscose

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rayon fabrics. In viscose-PET blends the binder content is reduced. Rotary screen printing, for example, is used to apply a binder and pigment to the hydroentangled fabric. Fabrics may be produced using one to three lowpressure injectors operating at pressures below 80 bar; thermal or chemical bonding then follows to develop the required fabric properties. For chemical bonding, it is common to apply a low acrylic binder add-on of 2–5% owf by foam padding. This increases the modulus, tensile strength and abrasion resistance of the fabric while minimising associated increases in stiffness43 and such fabrics may be flat or apertured. The hydroentanglement-chemical bonding route is used commercially to produce linings and interlinings and some wipes products. Chemically bonded hydroentangled fabrics overcome the delamination problems associated with traditional carded-chemically bonded fabrics. Hydroentanglement followed by thermal point bonding provides a potential means to minimise energy costs and to permit high production rates since both the water pressure utilised in hydroentanglement and the temperature and pressure applied in thermal bonding are lower than would be selected if one method alone was employed. Resulting fabrics also tend to be softer than those produced by thermal bonding alone since extensive thermo-fusion of the thermoplastic fibres in the fabric is not required to develop acceptable fabric properties. The hydroentanglement of thermally bonded fabrics is also known as a means of reducing their stiffness and the separation and subsequent entanglement of continuous filaments in a thermally bonded spunbond fabric using hydroentanglement has been suggested as a route to the production of highly durable and dyeable nonwovens.44

5.18

Hydroentanglement process technology

5.18.1 Web/batt compaction and pre-wetting Pre-wetting evacuates air from the web or batt prior to hydroentanglement in order to: ∑ ∑ ∑ ∑

prevent uncontrolled disturbance of the fibre arrangement to minimise changes in the MD/CD ratio of the web prior to bonding minimise jet marking when the web is impacted by the main jets enable the web to pass between the first injector and the support surface lightly adhere the web to the conveyor to prevent slippage.

Pre-wetting must be uniform to minimise variations in the degree of bonding introduced by subsequent injectors. In practice, various methods of prewetting have been employed. A low-pressure injector can be used, but jet marking is a common problem with such an approach and it reduces the efficiency of the hydroentanglement line. Similarly, spraying systems introduce

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imperfections in the web if not precisely controlled. If properly designed, weir systems, which apply a continuous curtain of water across the web, can be effective. Compression of the web by rollers in a water bath has also been utilised. The modern approach integrates mechanical compaction of the web and pre-wetting using at least one low pressure injector. The web is sandwiched by either two permeable belts, a permeable belt and a roller or two permeable rollers depending on the machinery supplier. Using such arrangements it is possible to use higher water pressure without destroying the web and to minimise unwanted drafting of the web. In Fig. 5.44, the web is mechanically compressed between two permeable, converging conveyor belts that transport the sandwiched web towards a roller and a low pressure injector. In the case of, for example, cotton pad production, where compression needs to be minimised to maximise the porosity of the final fabric, a drum and belt system may be selected. Once at the nip point, low-pressure water jets from a single injector are directed towards the web through one of the permeable conveyor surfaces. This introduces some fibre entanglement depending on the water pressure. The excess water is removed by means of a suction slot and this must be effective to avoid a decrease in fabric strength. Interestingly, although low water pressure (50 bar, the geometry of such injectors can lead to turbulence in the lower chamber, specifically in the regions between the consecutive hole outlets in the lower chamber resulting in energy losses. This can lead to heterogeneous bonding and variations in fabric density and appearance. An improved high-pressure injector design40,51 consists of a cylindrical feed chamber inside which the high-pressure water flows through a filter and then enters a distribution region, which transports the water towards the jet strip nozzles. Inside the feed chamber there is a cartridge consisting of a perforated cylinder lined with a filter system. The pressurised water is fed down to the jet strip via a narrow rectangular slot that extends the full width of the injector. Commercially, reciprocating injectors have been introduced as a means of minimising jet marks, and high-capacity injector systems have also been developed that incorporate either two (duplex) or three (triplex) jet strips in a single injector.31

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5.18.4 Arrangement of the injectors The number of injectors fitted to commercial hydroentanglement installations varies but typically 5–8 injectors are required to provide fabrics with adequate bonding and visual uniformity assuming no additional bonding processes follow. Some installations operate with only 2–4 injectors and then follow with chemical or thermal bonding to complete fabric production. In contrast, machines with more than ten injectors have been constructed commercially that are capable of very high line speeds and jet strip changes on the run. The number of injectors and the maximum operating pressure depends partly on the line speed and the degree of versatility that is required by the roll goods manufacturer. It is possible to hydroentangle at hundreds of metres per minute (>300 m/min) provided the production can be balanced by the web formation system and sufficient energy can be transferred to the web by the injectors to produce a fabric with satisfactory properties. Although increasing the number of available injectors increases the versatility and potential line speed of the installation not all the injectors are necessarily employed to increase the degree of bonding. The final injectors may be set up to improve the visual uniformity of the fabric; relatively low pressure and fine nozzles are employed for this purpose, or to introduce embossed patterns or apertures. It is possible to produce high-strength fabrics using only a few injectors operating at high pressure. While this approach minimises production costs and simplifies the process it can lead to quality problems such as pronounced jet marking in the fabric. However, the large increase in strength after the first injector gives rise to a lower risk of drafting as the web is transferred from the cylinder to the next stage. The development of the original German Norafin process helped to establish that an alternating face and back treatment of the web by successive injectors leads to the largest increases in fabric strength. This alternating treatment is particularly important for heavyweight fabrics of 200–600 g/m2 to avoid problems of delamination. In most modern hydroentanglement systems, alternating groups of injectors (1–4 in each group) arranged in succession direct jets onto the face and reverse sides of the web in sequence. The jet pressure profile describes the position and operating pressure of each successive injector with respect to the web. The pressure is usually profiled from the entry to the exit of the machine and the smallest pressure is usually encountered at the beginning. The pressure profile affects the specific energy ratio,52 which is the ratio of the specific energy applied to the face side Kf to the total applied specific energy Kt: Specific energy ratio =

Kf Kt

5.13

Even if the specific energy ratio applied to a web is the same, the resulting

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fabric properties can be quite different depending on the pressure profile that is used. One example is the bending rigidity, which tends to vary face and back in the fabric. An example of the variation in density arising from different pressure profiles in a non-apertured fabric is illustrated in Fig. 5.45.

5.18.5 Jet strips and nozzles The water, which should be uniformly distributed inside the lower section of each injector, is forced through nozzles drilled in a thin, metal jet strip clamped to the injector, usually by hydraulic means or a self-sealing mechanism based on the water pressure in the injector. The jet strip is typically 0.6–1 mm in thickness, 12–25 mm wide and has between 1 and 3 rows of nozzles.53 The jet velocity in a high-pressure system is about 100–350 m/s and issues from nozzles with a diameter of between 80–150 mm. The spatial frequency of the nozzles is from 40–120 per 25 mm. 15 The nozzles used in hydroentanglement normally have a capillary section with straight sides that connects to a cone section (Fig. 5.46). Conventional capillary cone nozzles are formed by punching the strip. To

0.20

Profile 1 Profile 2 Profile 3

0.18

Mean density (g/cm3)

0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 Web

1 2 3 Number of injectors

20 bar (profile 1),

20 bar (profile 2),

4

50 bar (profile 3)

5.45 Example of change in fabric density with the number of injectors for different jet pressure profiles (non-apertured fabric).

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Cone section

Capillary section (a) Cone up Capillary section

Cone section

(b) Cone down

5.46 Capillary cone nozzles (the ratio of the length of the capillary section to the inlet nozzle diameter is normally about 1. Lower aspect ratios are also used to promote a constricted jet).

obtain stable, high-velocity columnar jets, nozzles are usually operated in the ‘cone-down’ rather than the ‘cone-up’ position. The capillary portion of the nozzle therefore influences the jet diameter. The energy efficiency of the process is largely dependent on the formation of a constricted jet which remains intact between the nozzle and the web. Break-up or dispersal of the jet once it emerges from the nozzle results in poor energy transfer to the web and reduces the overall efficiency of the hydroentanglement process. For

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cone-up nozzles, there tends to be greater jet instability and the water pressure more strongly influences the discharge coefficient54. Operation in the conedown position reduces the discharge coefficient and increases the velocity coefficient with a uniform jet. It also assists in the prevention of cavitation, which tends to break up the jet. The nozzle geometry, particularly around the capillary inlet, is one of the factors influencing jet break-up. The nozzle aspect ratio is also important. The flow characteristics of jets emerging from nozzles and the effect of nozzle geometry on jet stability have been extensively studied by means of computer simulations as well as experimental observations.55,56,57 Commercially, one of the limitations in hydroentanglement is the life of the jet strip, which extends to about four thousand hours at best or only one hundred hours depending on operating conditions, particularly water pressure and water composition.58 Nozzle damage due to cavitation, abrasion or chemical degradation alters nozzle geometry and resultant jet formation may be affected giving rise to instability of the jet and fabric quality problems such as variations in fabric density and texture as well as reduced energy transfer efficiency. At high pressures, such as 400 bar, stainless steel jet strips can deteriorate quickly within hours. One solution is to use nozzle inserts where a very hard material surrounds the orifice and stainless steel or a hard coating comprises the rest of the strip.59 The hardness of conventional stainless steel jet strips is about 250 shore but to increase wear resistance, which is essential in high pressure systems, modern strips constructed from new alloys have a hardness of about 1200 shore.40 In practice, the nozzle inlet diameter and the number of nozzles/m in the jet strips fitted to each consecutive injector head varies. For example, in the first few injectors, strips with relatively large nozzles (120–150 microns) are fitted to maximise impact force and fibre entanglement. Jet strips in the final injectors commonly have finer nozzles (80–100 microns), which reduce the appearance of jet marks and produce a smoother fabric surface. In practice, hydroentanglement of webs of >200 g/m2 may limit the nozzle diameter to about 100 mm to minimise flooding.

5.18.6 De-watering Suction is used to remove excess water from the support surface during hydroentanglement to prevent flooding. Flat belt systems are particularly prone to flooding. Excess water not removed by suction is allowed to drain below the machine. Flooding leads to energy losses that can cause reduced fabric strength and interference with the bonding process. Flooding produces defects in the fabric. Approximately, 100–1000 mm head of water is needed, 500 mm head of water is preferable. De-watering is further improved by mangling the fabric prior to drying by means of squeeze rollers. Roberto®

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rolls constructed from fibre are capable of drawing out water from the fabric as it passes through.

5.18.7 The water circuit and filtration The filtration system is a major cost in a hydroentanglement installation and water quality affects process efficiency. Some of the main problems associated with filtration systems are blocked jet strips leading to jet marking and variations in fabric uniformity, the high cost of frequent filter bag replacement, bacterial growth, the potential loss of sand and damage to machinery, excessive discharge of water in the backwashing of filters and the requirement to replenish sand filters. In terms of water quality a neutral pH and a low content of metallic ions, for example calcium is required. Depending on machine size, the quantity of water in the circuit is about 40–100 m3/h and the trend is to reduce the circulating water to improve process efficiency. For a 3.5 m wide machine producing fabric for wet wipes, the quantity of circulating water has been estimated to be about 100 m3/h.40 Commercially, the waste water produced in hydroentanglement is recycled and circulated back to the main high-pressure pumps. Fibre finish, fibre debris and other impurities present in the waste water must be removed by the installed filtration system. Whether or not this is practicable depends on the particular design of the filtration system and the volume of impurities removed from the fibre during the process. Self-cleaning filters are employed to minimise cost and for cotton or pulp, flotation and sand filters are selected. Filtration developments have made it possible to process a greater variety of fibre types although traditionally, sophisticated filtration systems are required for cellulosics such as cotton, pulp and viscose rayon compared to synthetics such as PET. Chemical mixing and flocculation, dissolved air flotation units and sand filters are employed. Sand filtration systems enable the removal of suspended solids and a reduction in fibre finish in the water circuit. The choice of filtration system largely governs the versatility of the hydroentanglement line in terms of the compatible fibre types as well as cost. In a system processing cellulose pulp, a closed loop arrangement consists of a flotation unit that sends water to a sand filter operating with a backwashing recovery system. The back-washed water is returned to the flotation unit, while the remaining water from the sand filter is UV sterilised and sent to a bag filtration system prior to being returned to the beginning of the process.60 For cotton, filtration systems have been designed to treat the waste water directly in sand filters without the need to use an initial flotation unit.61 A two-stage process can be adopted where back-washed water from the sand filter is sent to a flotation unit and additional sand filter before being returned to the circuit.

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5.18.8 Drying Immediately after the fabric is hydroentangled, a proportion of the water held interstitially within the hydroentangled fabric is mechanically removed by suction, which for synthetic fibres is quite effective in reducing the moisture content to well below 100%. One advantage is lower overall drying costs. For cellulosics and other hygroscopic fibres, the water content is much higher even after mechanical extraction, which places a high demand on the subsequent drying process. Drum drying, can drying and through-air flat conveyor drying are all found in operation although through-air drum drying is the most common solution.

5.18.9 Aperturing and patterning effects Apertured fabrics are produced by hydroentangling on a cylinder with surface projections or raised cross-over points referred to as ‘knuckles’ in the wire mesh around which fibres are directed and entangled. Depending on the geometry of the projections or knuckles, particularly the wall angle relative to the conveyor, fibre segments on top of the projections are displaced to adjacent regions. Because of this, the local density in the regions adjacent to the apertures is significantly higher than the global fabric density. Aperturing can also occur on support surfaces with a high open area. The support surface therefore influences the periodic structure and texture of the fabric as well as the geometry and spacing of the apertures. Analysis of the forces involved in rearranging fibres in the web into a bonded structure concluded that the work is only about 1% of the input energy.62 Normally, aperturing takes place after the web has been pre-entangled to maximise pattern definition assuming fibre segments are still mobile. Aperture definition tends to improve as fibre length decreases because fewer fibre segments bridge between the apertures that are formed during fabric formation. Water consumption can be higher in the production of apertured and patterned fabrics since large-bore nozzles of 120–150 mm are preferred to obtain good definition. A high flow rate and impact force are therefore very important in producing high-quality apertured structures. Jet strips with three rows of holes are used by some manufacturers to increase the water flow rate to the fabric. Apertured and patterned lightweight fabrics are important in disposable and short-life products in the medical (e.g., replacement gauze dressings) and hygiene industries (e.g., wipes and coverstocks). Three-dimensional patterns, ribs, logos and surface pile effects are introduced by hydroentangling a pre-entangled web on a surface containing recesses into which fibres are pushed. These embossed effects may be combined with apertures. Early in the development of hydroentanglement, embossing geometric patterns was identified as a potential way of producing textile-

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like, hydroentangled fabrics having the appearance of woven fabrics.11 Further developments have led to the adoption of more complex surface patterns produced by CAD and laser engraving on to which webs are hydroentangled.

5.18.10 Apex™ technology Apex™ technology (Polymer Group Inc., PGI) allows the introduction of complex, embossed patterns in hydroentangled fabrics in the range 50–400 g/m2. These are known as Miratec® fabrics and derivatives are Mirastretch® and Miraguard®, which have good elastic recovery and barrier properties respectively. Some of the complex structural patterns introduced in to Miratec® fabrics resemble the appearance of woven and knitted textiles. Fabrics are produced by hydroentangling webs on laser imaged three-dimensionally patterned support surfaces to enable transfer of these patterns into the fabric. To improve image definition in the fabric, the web tension should be low and there should be no differential in the imaging surface and web speeds. After hydroentanglement, polymeric binders are added to stabilise the fabric or to introduce elastic properties and finishing processes such as compressive shrinkage (e.g., compaction by Sanforising) are undertaken to further adjust the softness and drape of the fabrics. In variations to the process, blends containing fusible binder fibres avoid the need for chemical bonding after hydroentanglement, and scrims can be introduced to increase durability and pattern definition in the fabric.63 The complex patterns enable the cleaning performance of wipes and dusters to be improved, for example recessed pockets may be formed in fabrics to improve the collection of low-viscosity contaminants present on skin.64 Miratec® fabrics are designed to be durable and compatible with traditional finishing processes such as jet dyeing, rotary screen and heat transfer printing.65 Fabric applications include automotive interiors, food service towels, window dressings, upholstery fabrics, pillow covers, wall coverings, bedspreads and apparel fabrics.

5.18.11 Evolon® fabrics66,67 The Evolon ® family of hydroentangled fabrics were an outcome of Freudenberg’s Omega project, which aimed to develop technology combining the benefits of staple fibre (carded) nonwovens such as high softness, drape, bulkiness and resilience with the benefits of spunlaid fabrics, particularly the high MD and CD tensile strength. To replicate the characteristics of staple fibre nonwovens using a spunlaid platform, two bicomponent extrusion technologies can be exploited. The first relies on the differential shrinkage that can take place in an asymmetric or eccentric sheath-core bicomponent filament via quenching, drawing and heat treatment. The resulting helically

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crimped filaments increase the bulk of the fabric. The characteristic softness and textile-like handle of Evolon® fabrics principally relies on the use of segmented-pie splittable bicomponent technology. A spunlaid web is produced from splittable bicomponent filaments (normally 16 segmented pie, PET/ PA6.6 in a weight ratio of 65%/35%) of about 2 dtex, which is hydroentangled at up to 400 bar to split the filament cross-section into multiple microfilaments. These microfilaments have a linear density of 0.09–0.13 dtex. Splitting efficiency, which is claimed to be at least 97%, is maximised by the use of hollow-core segmented-pie filaments and is influenced by the nozzle diameter in the spinneret, quenching, the stretching rate and, of course, the water pressure. To maximise fabric softness the fabrics are mechanically finished using processes such as tumbling. Owing to the combination of fabric softness, high filament surface area, strength and abrasion resistance in Evolon fabrics, applications include some clothing markets in sports and activity wear (e.g., hiking, skiing, cycling) where fabric weights are in the range 100–220 g/m2. Other applications include workwear, automotive (e.g., interior trim, carpets for sound absorption), shoe components (e.g., linings, heel grips, PU coating substrates), luggage and home furnishings (e.g., bed linen). Evolon fabrics can be jet dyed and finished and are said to withstand multiple domestic washes. Additional potential applications include clean room wipes.

5.18.12 Multi-layer ‘composite’ hydroentangled fabrics Webs (as well as pre-bonded nonwoven fabrics) can be simultaneously bonded and combined during hydroentanglement to produce flexible multi-layer fabrics. Where spunbond and meltblown webs are combined in this way, the approach can be viewed as the mechanically bonded analogue of thermally bonded SM or SMS composites (see Chapter 4). Hydroentangled multi-layer fabrics are found in both disposable and durable applications although it is in absorbent wipes products that most of the development has taken place. Commercial twin-layer hydroentangled fabrics include: ∑ ∑ ∑

spunbond with airlaid pulp (SP) spunbond with carded web (SC) spunbond with wetlaid (pulp, glass or other short fibre papers). Hydroentangled fabrics having three or more layers are:

∑ ∑ ∑ ∑ ∑

spunbond-pulp-spunbond (SPS) – where the pulp may be in the form of a pre-formed roll or deposited directly by means of a sifting airlaying system carded-pulp-carded (CPC) carded-pulp-spunbond (CPS) carded-spunbond-carded (CSC) carded-net-carded (CNC).

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In addition, scrims and carded webs may be combined during hydroentanglement to modify tensile properties, particularly in lightweight fabrics. Hydroentangling fibres or pulp with elastomeric foam,68 filaments or perforated films have all been explored as a means of increasing fabric elasticity. Amongst other end uses, CPC fabrics are intended for disposable wipes, CPS in incontinence products such as nappies and both CNC and CSC in industrial wipes. SPS composites consisting of a spunbond-airlaid pulp-spunbond sandwich are intended for wet wipes as well as for absorbent medical applications. The economics of SPS fabrics are attractive because of the low cost of pulp. The pulp acts as an absorbent core and the spunlaid layers provide abrasion resistance and structural reinforcement when the pulp is wet. The spunlaid fabrics also resist linting, dusting and pilling in use. To reduce the fabric density and to increase the softness, the filaments in the two spunlaid layers can be crimped during production. The pulp in CPC, CPS and SPS fabrics is either airlaid directly onto a web or introduced as preformed cellulosic tissue (paper) before hydroentanglement. As an alternative to pulp, airlaid cotton linters are utilised. The carded components normally consist of PET, PP, viscose rayon or cotton. Hydroentangled composites are expected to have an increasingly important role in the future of hygiene and medical products and have the advantage that no thermal lamination or bonding is required. Opportunities in heavier-weight durable applications in, for example, roofing composed of PET and geosynthetics using both drylaid and spunlaid webs are evolving.

5.19

Applications of hydroentangled fabrics

While hydroentangled fabrics have become heavily associated with wipes due to the growth and diversification of this sector, their use spans a much greater variety of applications, both in single-use and durable articles. Diverse products include protective clothing particularly medical gowns, synthetic leather, filtration media, wound dressings, composites and garment linings. The utilisation of hydroentanglement to mechanically join rather than thermally laminate webs or fabrics together continues to fuel product development opportunities. There has been rapid growth in global hydroentanglement capacity with strong demand for higher productivity with wider and faster machinery. Price pressure in some consumer markets and increasing competition is intensifying demand for product differentiation and has increased interest in new market opportunities. Some of the emergent niche markets for hydroentangled fabrics in technical markets will probably not require the high production capacities of current hydroentanglement installations to meet market demand in the future.

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5.19.1 Wipes Commerically, hydroentangled fabrics for wipes have been produced since the 1970s. The soft, strong, flexible and in most cases, absorbent characteristics of the fabrics combined with increasingly attractive economics and a textilelike handle have brought hydroentanglement to the fore in this sector. One of the earliest applications was as replacements for woven gauze in products such as laparatomy and x-ray detectable sponges.69 The wipes industry is now remarkably diverse encompassing hygiene (e.g., baby wipes), personal care, facial cleansing and make-up removal, food service, industrial and household cleaning products. Technical wipes include those for the aerospace, automotive, optical and electronics industries as well as other speciality products and these include products made from splittable bicomponents. In the last five years airlaid thermal bonded wipes have been increasingly substituted by hydroentangled fabrics because of their softer handle, good strength and low thickness. Hydroentangled fabrics are also important in the impregnated wipes market alongside airlaid and thermal-bonded products. Some examples of the composition of commercial hydroentangled wipes are shown in Table 5.2.70 Hydroentangled floor wipe compositions are typically 70%/30% or 65%/ 35% viscose-PET in fabric weights of 90–100 g/m2. In floor wipes, a binder is applied, which increases the durability in the wet state; it also provides a means of adding pigments. For dusters, compositions include 75%/25% and 91%/9% PET/PP blends and fabrics with embossed 3D patterns or mock pile surfaces are produced to improve dust pick-up. Fabric weights range from 30–65 g/m2 and scrim reinforcement may be used to increase the dimensional stability. Amongst the wipes for cleanroom applications Lyocell and PET blends are produced as well as 100% PET hydroentangled wipes. 71 Hydroentangled cotton fabrics for wet wipes, gauzes and cosmetics are produced in Japan in fabric weights ranging from 30–250 g/m2. To maximise the absorbency, these pads are produced at relatively low pressure, with a Table 5.2 Examples of dry wipe products Product

Fabric weight

Composition

Baby wipe

50 g/m2 or 55 g/m2

70% viscose rayon 30% polyester

Baby wipe

55 g/m2

50% viscose rayon 50% wood pulp

Food service wipe

68–80 g/m2

80% viscose rayon 20% polyester

Swiffer® type dry wipe 68 g/m2

Carded polyester + polypropylene scrim + carded polyester composite

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maximum injector pressure of 40 bar to maximise their porosity. Therefore, whereas the fabric surfaces are well entangled to minimise linting in use, the core is lightly entangled to maximise the absorbent capacity. Localising the entanglement to the surfaces of a fabric is a strategy used in the formation of other wipes products.72 The ability to differentiate and personalise wipes products for customers helps manufacturers to avoid the effect of commoditisation. Complex patterning, the introduction of raised (three-dimensional) embossed effects in hydroentangled fabrics and multi-layer hydroentangled composites all provide a means of adding value to wipes products. To underpin the expanding technical requirements of the wipes sector and to enable further diversification, production of hydroentangled composites in which different web types are combined during hydroentanglement is a growth area. Such composites enable significant improvements in wiping performance, dimensional stability, absorption and soil cleaning according to the composition of the layers that are combined. Composites for wipes are based on combinations of carded + airlaid wood pulp, carded + scrim and spunlaid + wood pulp combinations. Carded staple fibre webs and airlaid wood pulp blends are combined to produce wipes with good absorbency in baby wipes, bodycare, food service and industrial cleaning. Hydroentangled composites containing wood pulp are particularly important for wet wipe applications. Products containing wood pulp rather than viscose rayon have been a particular focus of development. It has been a challenge to produce fabrics with the required softness and drape which do not lint or produce dust using wood pulp. To achieve this, hydroentangled composites composed of 40–45 g/m2 wood pulp sandwiched between two carded PET webs of 8–10 g/m2 were developed. In twin layer composites, the carded web weight is increased to 25 g/m2.40 To increase durability for applications such as industrial wipes, wood pulp is hydroentangled into a spunlaid web. Scrim-reinforced carded-hydroentangled fabrics are utilised in domestic wipes such as P&G’s Swiffer®. Hydroentangled biodegradable and water dispersible wipes have been developed in response to environmental concerns, particularly in Europe. Some of these materials rely on ion-sensitive cationic polymers that are applied as binders to webs or lightly bonded hydroentangled fabrics. In another approach, a hydroentangled fabric consisting of three laminated layers with a pulp core has been described in which some areas are left unbonded to promote disintegration of the structure in the sewer system.73 Hydraspun® 784 is a dispersible wet wipe composed of a latex binder-free hydroentangled fabric.74 The fabric, which is composed of a synthetic and natural fibre blend, is designed to break down under agitation in water. This fabric has a flush index (tube test-first break) of 7–8 turns depending on its weight, which is either 55 or 65 g/m2.

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Cotton pads for make-up removal are traditionally produced by bonding a drylaid batt using one of three approaches; surface impregnation with a binder, thermal bonding with bicomponent fibres and embossing (under compressive force) with engraved pattern rollers. To enable the manufacture of 100% cotton products with acceptable surface abrasion, cotton pads are also produced using hydroentanglement. Hydroentangled cotton pads and wipes are produced from 30–300 g/m2 with apertured or smooth surfaces.75 Low-pressure hydroentanglement allows bonding to be concentrated only at the surface thereby maximising fabric porosity. Hydroentangled microfibre fabrics are produced and find applications as sports towels and facial cleaning cloths.

5.19.2 Washable domestic fabrics Hydroentangled cotton fabrics for semi-durable bedsheets, napkins and tablecloths have been produced that can be washed up to ten times before disposal. Since they are cotton based, such fabrics can be dyed or printed. Impregnating hydroentangled fabrics with 0.2%–1.0% owf of polyamideamine-epichlorohydrin resin76 is claimed to enable repeated washing of hydroentangled cotton fabrics. Durable hydroentangled fabrics intended for repeated laundering have also been developed by stitching the fabrics over the top, by applying a binder or by incorporating thermally fusible binder fibres in the fabric.77 Such fabrics can also be dyed and finished. A further method of increasing the durability of fabrics for washing without the need for chemical or thermofusible binders is to hydroentangle at very high water pressure but the energy consumption is high.

5.19.3 High-temperature protective clothing Hydroentangled aramids, including blends of meta- and para-aramids, are well established as protective liners and moisture barrier substrates in firefighting garments. In addition to the high-temperature resistance derived from the polymer, the fabrics are favoured because of their softness, drape and light weight. Fabrics composed of of 50% Basofil®/50% aramid are utilised in thermal protection lining fabrics that form components in firefighting garments and such linings are commonly quilted to a woven aramid face fabric. A protective liner fabric composed of hydroentangled FR cotton and Basofil® is produced to improve comfort in high-temperature clothing. Meta-aramid fibres are commonly found in thermal protection liners and 50% Lenzing FR®/50% aramid blend hydroentangled fabrics have also been developed for fire-fighting jackets to give improved resistance to flame breakopen. In addition to fire-fighting garments, hydroentangled fire-blocking fabrics for upholstery and mattresses are produced from Type Z11 Kevlar®

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and Type E92/Kevlar blends are produced as fire blockers for aircraft seats and as thermal liners in protective clothing.78

5.19.4 Artificial leather The production of synthetic leather and coating substrates using hydroentanglement is well established particularly in the Far East, notably Japan, Korea and Taiwan. Hydroentangled fabrics are widely used as backings for PU coated synthetic leather. In addition to the traditional method of dissolving part of the matrix in a bicompnent fibre using a suitable solvent, synthetic leather articles are produced by hydroentangling splittable bicomponent fibres composed typically of 6, 16 or 36 segments in the crosssection. Such fabrics find applications as specialist wipes, particularly for cleaning glass.

5.19.5 Surgical fabrics Hydroentangled fabrics have long been favoured for surgical gowns, scrub suits, sheet and drapes for their excellent comfort and softness. In surgical gowns, infection control is paramount and spunmelts and composites containing breathable films are favoured over hydroentangled fabrics where there is a need for improved barrier protection. In surgical gowns, hydroentangled fabrics such as Softesse®, formerly known as Sontara® for medical protection garments and warming gowns, are well established and composites composed of wood pulp from paper hydroentangled onto a PET layer are produced. Disposable hot-water soluble PVA hydroentangled fabrics for surgical scrub suits, gowns and drapes have also been developed.

5.19.6 Medical gauze Traditionally, yarn-based fabrics found in wound dressings are composed of cotton or cotton blended with up to 50% viscose rayon. As an alternative, apertured hydroentangled fabrics have led to significant cost savings in this market partly because fewer layers are required. Commercially, hydroentangled gauze fabrics composed of 70% viscose rayon and 30% polyester provide high absorbency and low linting properties.

5.19.7 Linings and clothing Dyed or printed hydroentangled fabrics find use in shoe linings and clothing linings and hydroentangled fabrics have long been used in the manufacture of linings for traditional garments. Hydroentangled fabrics composed of merino wool have been developed79 and such fabrics have been incorporated

Mechanical bonding

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in performance outerwear products such as outdoor clothing. Sports shirts and vests have also been produced from hydroentangled fabrics.

5.19.8 Filtration Hydroentangled fabrics intended for air filtration, particularly bag house applications, are manufactured in addition to pleated hydroentangled media for liquid filtration. Hydroentangled PET media80 as well as fabrics composed of PTFE have been developed. As is common in needlepunched fabrics, scrim reinforced hydroentangled fabrics composed of high-temperatureresistant fibres improve dimensional stability for filtration applications. Hycofil is a pleated textile or metal scrim reinforced hydroentangled fabric for flue gas filtration composed of polyimide or aramid fibres.81

5.19.9 Automotive Hydroentangled fabrics composed of 100% polyester have been developed for backing materials for one-step injection moulding of interior trim automotive components replacing heavier-weight needlepunched fabrics. This process is used by car-part manufacturers to produce interior trim. The fabric is laminated to a knitted facing before the injection moulding process and the use of hydroentangled fabrics is claimed to improve acoustics in the car. 82 Experimentally, hydroentangled flax and hemp fabrics up to 1500 g/m2 have been produced for potential use in automotive composites.

5.19.10 Other applications The technical and durable applications for hydroentangled fabrics are developing rapidly. In the composites sector, hydroentangled PET is used as a carrier web in pultrusion products and for filament wound pipe production, and similar fabrics are found as surface veilings for fibreglass reinforced plastics. Some hydroentangled geosynthetic and construction materials are commercially produced as well as microfibre filter fabrics. The high strength to weight ratio of hydroentangled fabrics means there is potential to reduce fabric weight and obtain raw material savings in the roofing sector. Experimental thermal insulation fabrics, in which nanoporous silica gels are introduced within cavities in the cross-section of hydroentangled fabrics, have been developed using Hydrospace™ technology.83 Such fabrics have also been designed for the storage and controlled release of actives and by appropriate control of the fabric permeability; delivery can be confined mainly to one side of the fabric. Outside the nonwoven sector, hydroentanglement technology has been successfully developed to enhance the appearance and physical properties of textile fabrics in a process known as hydroenhancement

294


to control the permeability of airbag and filtration fabrics and to improve seam strength and fabric softness. Attachment of secondary carpet backings by hydroentangling webs onto the back of the carpet has also been developed as a means of simplifying the traditional production process.

5.20

Acknowledgements

The authors are indebted to a number of people for their valuable contributions, advice and support during the preparation of this chapter. They are extremely grateful to Mr Matthew Yeabsley, Karl Mayer Textile Machinery Ltd, UK, for providing the technical literature and above all for supporting a visit to Karl Mayer Malimo company at Chemnitz, Germany. His kindness and friendship is always appreciated. The authors would also like to thank the following personnel at Karl Mayer Malimo Textilmachinenfabrik GmbH, Chemnitz, Germany, for their kind cooperation and assistance in explaining their current and future stitch bonding equipment programme and for providing the literature and samples for use in this review: Mr Alexander Battel, Dr Holger Erth, Mr Axel Wintermeyer and Mr Daniel Standt. Finally, they are also grateful to Dr Monica Seegar and Mr Wolfgang Schilde of Sächsisches Textil Forschungs Institut e.V., Chemnitz, Germany, for their kindness and friendship during visits to their Institute. A great deal of information has been used here directly from the Karl Mayer literature that could not be individually referenced. The contribution of Foster Needle is gratefully acknowledged. The authors wish to thank Fleissner GmbH, of Egelsbach, Germany, and Rieter Perfojet of Grenoble, France, for their assistance and valuable contributions to this chapter.

5.21

References

1. Cotterill, P.J., ‘Production and properties of stitch bonded fabrics’, Textile Progress, The Textile Institute, 1975, 7, (2), p. 101. 2. Textile Terms and Definitions, The Textile Institute, eleventh edition, 2002, p. 333. 3. Krcma, R., Nonwoven Textiles, Manchester, Textile Trade Press, 1967 p. 156. 4. Kettenwirk Praxis, 1993 (1) E22. 5. Kettenwirk Praxis, 1994 (2) E5. 6. Anand, S.C., ‘Karl Mayer warp knitting equipment at I T M A’99, Asian Textile Journal, 1999 (9) p. 49. 7. Kettenwirk Praxis, 1994 (2) E8. 8. Kettenwirk Praxis, 2002 (2) 25. 9. Evans, F.J., US3,485,706: 1969. 10. Bunting, W.W., Evans, F.J. and Hook, D.E., US3,508,308: 1970. 11. Evans, F.J. and Shambelan, C., US3,498,874: 1970. 12. Coppin, P., ‘The Future of Spunlacing’, Nonwovens World, December 2001–January 2002, pp. 60–67. 13. http://www.nonwovens-industry.com/articles/2005/03/feature2.php

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14. Ghassemieh, E., Acar, M. and Versteeg, H.K., ‘Improvement of the efficiency of energy transfer in the hydroentanglement process’, Journal of Composite Science & Technology, 61(12), 2001, pp. 1681–1694, ISSN 0266–3538. 15. Watzl, A., New Concepts for Fiber Production and Spunlace Technology for Microdenier Bicomponent Split Fibers – From Polymer to Final Product, INDEX 99 Congress, Manufacturing Session 1, Geneva, April 1999, pp. 1–13. 16. Moschler, W., Some Results in the Fields of Hydroentanglement of Fibrous Webs and Their Thermal After-Treatment, International Nonwovens Symposium, EDANA, 1995, pp. 1–23. 17. Bertram, D., ‘Cellulosic Fibers in Hydroentanglement’, INDA Journal, 1993, vol. 5, no. 2, pp. 34–41. 18. Bunting, W.W., Evans, F.J. and Hook, D.E., US3,493,462: 1970. 19. Guerin, J.A. and Jeandron, H.T., US3,214,819: 1965. 20. Suzuki, M., Kobayshi, T. and Imai, S., US4,718,152: 1998. 21. Moschler, W., Meyer, A. and Brodtka, M., Influences of Fibre and Process on the Properties of Spunlaced Fabrics, ITB Nonwovens, Industrial Textiles, 1995, 2, pp. 26–31. 22. Zheng, H., Seyam, A.M. and Shiffler, D., ‘The Impact of Input Energy on the Performance of Hydroentangled Nonwoven Fabrics’, International Nonwovens Journal, Summer, 2003, pp. 34–44. 23. Pourdeyhimi, B. and Minton, A., ‘Structure-Process-Property Relationships in Hydroentangled Nonwovens, Part 1: Preliminary Experimental Observations’, International Nonwovens Journal, Winter, 2004, pp. 15–21. 24. Seyam, A.M. and Shiffler, D., ‘An Examination of the Hydroentangling Process Variables’, International Nonwovens Journal, Spring, 2005, pp. 25–33. 25. Pourmohammadi, A., Russell, S.J. and Hoeffele, S., ‘Effect of Water Jet Pressure Profile and Initial Web Geometry on the Physical Properties of Composite Hydroentangled Fabrics’, Textile Research Journal, 2003, 73(6), pp. 503–508. 26. Mao, N. and Russell, S.J., ‘A Framework for Determining the Bonding Intensity in Hydroentangled Fabrics’, Journal of Composites Science and Technology, 2006, 66 (1), pp. 80–91. 27. Mao, N. and Russell, S.J., ‘Towards a Quantification of the Structural Consolidation in Hydroentanglement and its Influence on the Permeability of Fabrics’, Nonwovens Research Academy Proceedings, EDANA, 2005. 28. Mathis, R., US6,190,736: 2001. 29. Yoshiharu, U. and Wakesaka, K., JP2002146630: 2002. 30. FiberVisions® Hy-Entangle WA, www.fibervisions.com 31. www.fleissner.de 32. Forsten, H.H., ‘New Sontara Spunlaced Aramid Structures’, Nonwovens Symposium, 1985, p. 251 33. www.aramid.com 34. Kelly, K.D., Hill, T.A., Lapierre, F., DeLuca, S. and DeLeon, S.D., US6,764,971: 2001. 35. Rogers, J.J., Erickson, J.L. and Sanocki, S.M., US5,380,580: 1995. 36. www.hillsinc.net/nanofiber.shtml 37. Zucker, J., WO2004092471: 2004. 38. http://www.fleissner.de/ne_25112005_e.htm 39. Noelle, F., Spunlace: Improvements which enhance production efficiencies and reduce operating costs, Technical Textiles, 2001, vol. 44, April, pp. 100–101. 40. Völker, K., ‘Advancements in Spunlacing Technology’, Nonwovens World, 2002,

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April–May, pp. 97–103. 41. Starr, J.R., ‘Water Jet Entangled Nonwovens Expanding Rapidly’, Nonwovens World, 1988, March–April, vol. 3, no. 2, pp. 62–68. 42. Anon., Nonwovens Report International, 2005, issue 6, December, p. 52. 43. Shahani, A. and Shiffler, D.A., ‘Foamed Latex Bonding of Spunlace Fabrics to Improve Physical Properties’, International Nonwovens Journal, 1999, Fall, pp. 41– 48. 44. Putnam, M., Ferencz, R., Storzer, M. and Weng, J., PCT:WO 02/05578 A1, 18/7/2002. 45. www.rieter.com 46. Vuillaume, A., ‘The Perfojet Entanglement Process’, Nonwovens World, 1987, vol. 2, no. 1, pp. 81–84. 47. Suzuki, M. and Kobayashi, T., GB2114173: 1983. 48. Noelle, F., US5,768,756: 1998. 49. Kalwaites, F., US3,033,721: 1962. 50. Ward, D.T., ITB Nonwovens & Industrial Textiles, 1997, pp. 38–42. 51. Schmit, L. and Roche, B., US6,474,571: 2002. 52. Gilmore, T.F., Timble, N.B. and Morton, G.P., ‘Hydroentangled Nonwovens made from Unbleached Cotton’, TAPPI Journal, 1997, no. 3, pp. 179–183. 53. www.nippon-nz.com 54. Ghassemieh, E., Versteeg, H.K. and Acar, M., ‘Effect of Nozzle Geometry on the Flow Characteristics of Hydroentangling Jets’, Textile Research Journal, 2003, 73(5), pp. 444–450. 55. Tafreshi, V.T. and Pourdeyhimi, B., ‘Simulating the Flow Dynamics in Hydroentangling Nozzles: Effect of Cone Angle and Nozzle Aspect Ratio’, Textile Research Journal, 2003, 73(8), pp. 700–704. 56. Tafreshi, V.T., Pourdeyhimi, B., Holmes, R. and Shiffler, D., ‘Simulating and Characterising Water Flows Inside Hydroentangling Orifices’, Textile Research Journal, 2003, 73(3), pp. 256–262. 57. Begenir, A., Tafreshi, V.T. and Pourdeyhimi, B., ‘Effect of Nozzle Geometry on Hydroentangling Water Jets: Experimental Observations’, Textile Research Journal, 2004, 74(2), pp. 178–184. 58. Fechter, A., Münstermann, U. and Watzl, A., ‘Latest Developments in Hydroentanglement’, Chemical Fibers International, 2000, vol. 60, December, pp. 587–588. 59. Fleissner, G., DE10059058: 2002. 60. www.idrosistem.com 61. www.nonwovens-industry.com/articles/2004/08/feature2.php 62. Seyam, A.M., Schiffler, D.A. and Zheng, H., ‘An Examination of the Hydroentangling Process Variables’, International Nonwovens Journal, 2005, pp. 25–33. 63. Black, S.K. and Deleon S., EP1434904: 2004. 64. Chang, K.S. and Edward, S.D., EP1454000: 2004. 65. www.pgi-industrial-europe.com 66. Groitzsch, D., Ultrafine Microfiber Spunbond for Hygiene and Medical Application, NT New Textiles, EDANA symposium, 2000. 67. Groten, R. and Riboulet, G., The Evolon Project, TUT, 2001, 41, 3rd quarter, pp. 27– 28. 68. Zlatkus, F., US6074966: 2000. 69. Mansfield, G., ‘H2O Tricks’, Textile World, 2004, February, pp. 28–31. 70. Coppin, P. ‘The Future of Spunlacing’, Nonwovens World, 2001–2002, Dec–Jan,

Mechanical bonding 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83.

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pp. 60–67. www.contecinc.com Barge, P. and Carter, N., US2004068849: 2004. Ngai M.C., EP1354093: 2003. Ahlstrom, Fiber Composites Hydraspun® 784 Dispersible Wet Wipe Leaflet, 2005. Watzl, A. and Eisenacher, J., ‘Spunlace Process for Cotton Pads and Other Products, ITB’, Nonwovens & Industrial Textiles, 2000, pp. 16–18. Vuillaume, A., Lacazale, J.C., US5393304: 1995. Putnam, M.J. and Hartgrove, H., USP6,669,799: 2003. http://www.dupont.com/nomex/spunlacepdf801.pdf http://www.technical-textiles.net/archive/htm/att_20020601.475879.htm Pearce, C.R. and DeLeon, S., WO2004073834: 2004. Schmalz, E., Hycofil: Spunlace Scrim Supported Nonwoven, TUT, 2005, 65, 2nd quarter, pp. 27–30. http://www.engineeringtalk.com/news/pgr/pgr101.html Hoeffele, S., Russell, S.J. and Brook, D.B., ‘Lightweight Nonwoven Thermal Protection Fabrics Containing Nanostructured Materials’, International Nonwovens Journal, 2005, vol. 14, no. 4, pp. 10–16.

6 Thermal bonding A POURMOHAMMADI Consultant, Iran

6.1

Introduction

Thermal bonding is successfully employed in bonding dry-laid, polymerlaid and wet-laid webs as well as multi-layer materials. The basic concept of thermal bonding was introduced by Reed in 1942. He described a process in which a web consisting of thermoplastic and non-thermoplastic fibres was made and then heated to the melting or softening temperature of the constituent thermoplastic fibres followed by cooling to solidify the bonding area. In the early development of thermal bonding, rayon fibres (the base fibre component) were blended with plasticised cellulose acetate or vinyl chloride (the binder fibre component). Typically, a carded web from a blend of base fibre and binder fibre was produced and hot calendered followed by cooling to solidify and bond the web structure. The resulting thin, strong and relatively dense product was more akin to a paper product than a textile material. Production costs for this material were very high, primarily because the available binder fibres were expensive. Its applications were limited to products requiring a smooth surface, low porosity, high strength and lower thickness. Given the product limitations and the high cost of such binder fibres, nonwoven producers continued to prefer latex bonding using chemical binders. The rising cost of energy and greater awareness of the environmental impact of latex bonding led to a change in direction. A comparison of energy consumption by various web-bonding processes is given in Fig. 6.1 which shows a considerable energy saving for the thermal bonding process.1 The high production rates possible with thermal bonding and the significant energy savings as compared to chemical bonding, due to the absence of significant water evaporation during bonding, makes the process economically attractive. In contrast to chemical bonding, the environmental impact of the process is also significantly reduced. The growing market demand for disposable and durable products spurred developments in new thermoplastic and thermoset materials in the form of powder, films, webs, hot melt compounds as well as improved production methods such as point-bonding calenders, through-air 298

Thermal bonding

299

4

Energy (KWh/Kg)

3

2

1

0 Thermal bonding

Foam bonding

Spunlace

6.1 Energy consumption of different bonding processes.1

bonding and belt bonders. This has greatly increased the diversity of products that can be manufactured by the thermal-bonding process.

6.2

Principle of thermal bonding

Thermal bonding requires a thermoplastic component to be present in the form of a homofil fibre, powder, film, web, hot melt or as a sheath as part of a bicomponent fibre. In practice, heat is applied until the thermoplastic component becomes viscous or melts. The polymer flows by surface tension and capillary action to fibre-to fibre crossover points where bonding regions are formed. These bonding regions are fixed by subsequent cooling. In this case, no chemical reaction takes place between the binder and the base fibre at the bonding sites. When binders melt and flow into and around fibre crossover points, and into the surface crevices of fibres in the vicinity, an adhesive or mechanical bond is formed by subsequent cooling. Such an adhesive bond is a physio-chemical bond at the interface of two dissimilar materials. In the thermal bonding context, a mechanical bond is formed as a result of thermal shrinkage of the bonding material, which while in the liquid state encapsulates the fibre crossover points.2 In contrast, if at the binderfibre interface both components soften or melt, inter-diffusion and interpenetration of the molecules across the interface can occur and the interface may disappear. This arises where compatible polymers are present with nearly comparable solubility parameters. Bonds formed in this way may be called cohesive bonds. Some of the main advantages of thermal bonding are as follows:

300

∑ ∑ ∑ ∑ ∑


Products can be relatively soft and textile-like depending on blend composition and bond area. Good economic efficiency compared to chemical bonding involving lower thermal energy requirements and less expensive machinery. High bulk products can be bonded uniformly throughout the web crosssection. 100% recycling of fibre components can be achieved. Environmentally friendly since no latex binders are required.

6.3

Raw materials

Thermally bonded fabrics are produced both from entirely thermoplastic materials and from blends containing fibres that are not intended to soften or flow on heating. The non-binder component may be referred to as the base fibre component and commercially, a variety of base fibre types are used. The binder fibre component normally ranges from 5–50% on weight of fibre depending on the physical property requirements of the final product.

6.3.1

Base fibres types

The base fibre contributes to key physical, chemical and mechanical properties of the fabric derived from the polymer from which it is constituted. This influences dyeing characteristics, flame resistance, tensile and attritional properties, hydrolytic resistance, biodegradability amongst many other properties. The commonly used base fibres include natural fibres (regenerated cellulosic fibres, bast, vegetable and protein fibres such as wool), synthetic fibres (polyester, polypropylene, acrylic, nylon, aramid and many others), mineral fibres (e.g., glass and silica) and metallic fibres. Sometimes the base fibre (carrier fibre) is the core of a bicomponent fibre, with the sheath component being the binder portion.

6.3.2

Binder materials

Binder components are produced in many different forms including fibre or filament (homogeneous or bicomponent; sheath/core or side-by-side type melt-bonding fibres), powder, film, low melt webs, and hot melts. The physical form of the binder affects its distribution throughout the fibre matrix which has a significant impact on fabric properties. The amount of binder also plays an important role in determining the properties of the resultant nonwoven fabric. If the binder content is more than 50% of the total blend the fabric behaves like a reinforced plastic. At a binder content of 10% the fabric is a bulky, porous and flexible structure with relatively low strength. To minimise energy costs it is desirable that binder fibres have a high melting speed, a low

Thermal bonding

301

melting shrinkage and a narrow melting point range. The most widely used thermoplastic binder polymers are given in Table 6.1. Decreasing the melt temperature of polymers, for instance PET, from 260 ∞C to 135–190 ∞C, requires the use of copolymers produced by polycondensation. The melting speed of these copolymers is very high; hence the thermal shrinkage is reasonably low. When thermoplastic fibres or powders are used as binders, their melting temperature is significantly lower than the base fibres in the web, which helps to prevent thermal degradation. In low melting temperature homopolymers, or copolymer binder fibres or powders, complete melting can occur and the polymer becomes a fluid. If the viscosity of the molten polymer is sufficiently low, it flows along the surface of the base fibres and is collected at the fibre crossover points to form bonding points in the shape of beads by subsequent cooling. In webs composed of bicomponent fibres (of the sheath/core type) the sheath polymer does not need to completely melt but softens enough to form a bond. However, if it does melt and flow, the bonding mechanism becomes similar to that of homopolymer binder fibres. The advantage of bicomponent fibres is that every crossover can be potentially bonded and also since the physical structure of the core component is not degraded, thermal shrinkage is minimised, web structure remains essentially intact and fabric strength is usually higher. Binder fibres are selected by their suitability for the different thermal bonding processes.

6.3.3

Bicomponent binder fibres

Bicomponent (‘Bico’) fibres and filaments, which are also referred to as conjugate fibres, particularly in Asia, are composed of at least two different polymer components. They have been commercially available for years; one of the earliest was a side-by-side fibre called Cantrece developed by DuPont in the mid-1960s followed by Monsanto’s Monvel, which was a self-crimping bicomponent fibre used by the hosiery industry during the 1970s. Neither of these fibres was commercially successful because of complex and expensive Table 6.1 Thermal transition points in common thermoplastic binder polymers Fibre type

Glass transition temperature (∞C)

Melting temperature (∞C)

Polyvinyl chloride (PVC) Polyamide (PA) Polyester (PET) Polypropylene (PP) Polyethylene (PE) (low density)

81 50 69 –18 –110

200–215 210–230 245–265 160–175 115

302


manufacturing processes. Later in 1986, commercially successful bicomponent spinning equipment was developed by Neumag, a producer of synthetic fibre machinery.3 Use of bicomponent fibres accelerated dramatically in the early 1990s partly because of the need to uniformly bond the entire thickness of nonwoven fabrics, which in heavy weight per unit area structures could not be satisfactorily accomplished by chemical bonding. More recently the market for bicomponent fibres has been greatly developed by Japan and Korea. Worldwide their share in bicomponent fibre production is estimated to be around 91 million kilograms annually.4 Some of the polymers used as components in bicomponent fibres are listed in Table 6.2. Common polymer combinations in bicomponent binder fibres are: ∑ ∑ ∑

Polyester Core (250 ∞C melt point) and CoPolyester Sheath (melt points of 110 ∞C to 220 ∞C) Polyester Core (250 ∞C melt point) and Polyethylene Sheath (130 ∞C melt point) Polypropylene Core (175 ∞C melt point) and Polyethylene Sheath (130 ∞C melt point).

Biocomponent technology is important in both dry-laid and spunlaid processes that produce webs for thermal bonding. Table 6.2 Polymers used in manufacturing bicomponent fibres Polymer

Notes

PET and coPET

Melt temperatures range from 110 ∞C to c. 250 ∞C. Water soluble, alkali soluble, elastomeric and biodegradable coPETs available. Polytrimethylene terephthalate, Corterra™ PET glycol Polybutylene terephthalate Polyethylene naphthalate Kromalon™ dyeable, Kromatex™ PP, SPP (syndiotactic PP) PE/PP copolymer Polylactic – melting temperature ranges from 130 ∞C to 170 ∞C. High density PE Linear low density PE PA 6 (nylon 6), PA 6,6 (nylon 6,6), PA 11 (nylon 11) PA 12 (nylon 12), PEBAX™ copolyamide Polyphenylene sulphide Polycaprolactone Polystyrene Polyvinylidene fluoride Plasticised polyvinyl alcohol Thermoplastic polyurethane Ethylene vinyl alcohol Amlon™ PAN (polyacrylonitrile)

PTT PETG PBT PEN PP PLA HDPE LLDPE PA (polyamide) PPS PCL PS PVDF PVOH TPU EVOH PAN

Thermal bonding

303

Bicomponent fibre classification Bicomponent fibres are commonly classified by the structure of their crosssection as side-by-side, sheath-core, island in the sea or segmented pie. Of these, the side-by-side and sheath-core arrangements are relevant for thermal bonding applications. Side-by-side (S/S) Two components are arranged side by side and are divided along their length into two or more distinct regions (see Fig. 6.2). The components must have good adhesion otherwise two fibres of different composition will be produced. There are several ways of producing side-by-side bicomponent fibres described in the literature.5 The geometrical configuration of side-by-side bicomponent fibres particularly asymmetry, makes it possible to achieve an additional threedimensional crimp during thermal bonding by differential thermal shrinkage of the two components, for example. This latent crimp gives rise to increased bulk stability and a softer fabric handle. The characteristics of the crimp are determined by factors such as polymer properties, the weight ratio of the two polymers and the structure of the web, which can be varied according to the method of web formation. An increase in the crimp level from 15% to >30% and in the number of crimps/cm from 6.5 to >22 can be introduced using this differential thermal shrinkage approach. Sheath-core fibres In sheath-core bicomponent fibres one of the components (the core) is fully surrounded by another component (the sheath). The arrangement of the core

(a)

(b)

(c)

(f)

(g)

(h)

(d)

(e)

6.2 Schematic diagram of side-by-side bicomponent fibre crosssections.

304


is either eccentric or concentric depending on the fabric properties required. If high fabric strength is required, the concentric form is selected whereas if bulk is required, the eccentric type is employed.6 Adhesion is not always essential for fibre integrity. A highly contoured interface between the sheath and core can provide the mechanical interlock that is desirable in the absence of good adhesion (see Fig. 6.3). One advantage of sheath-core fibres is the ability to produce a surface with the required lustre, dyeability, and handle characteristics while having a core that dominates the tensile properties. The core-sheath structure also provides a means of minimising the cost by engineering the relative proportions of the two polymer components. Commercially, the ratio of the polymer components is typically 50:50 or 30:70 but in some cases a ratio of 10:90 is used. The first industrial exploitation of bicomponent fibres involved the use of Co-PET/PET or PE/PP fibres for hygiene applications as well as high-loft waddings, wiping cloths, medical wipes and filters. The difference in the sheath-core melting temperature in PE/PP is about 40 ∞C. In Co-PET/PET bicomponents, the sheath melts at 100–110 ∞C while the core melts at 250– 265 ∞C. Bicomponent fibres are generally used in blend ratios of 10–50%, depending on the application and process parameters. A useful experimental guide is given in Table 6.3. Depending on the type of base fibres, CoPET/PET bicomponent fibres can form strong primary bonds between themselves and therefore a framework structure in which the base fibres are embedded is produced. It is also possible to modify the fibres so that secondary bonds are formed between bicomponent fibres and the base fibres. Examples are PE/PET fibres produced from Trevira.7 The marked difference between the melting temperatures of the PE sheath

(a)

(b)

(c)

(d)

(e)

6.3 Cross-sections of sheath-core bicomponent fibres. Table 6.3 A practical guide for producing nonwoven fabrics with different handle characteristics from Co-PET/PET bicomponent fibres Parameter

Bicomponent fibre content (%) Bonding temperature (∞C) Fibre fineness (dtex)

Nonwoven fabric handle Soft

Medium

Harsh

10–20 140–150 1.7–3.3

15–30 150–160 3.3–6.7

>30 160–180 >6.7

Thermal bonding

305

(125–135 ∞C) and PET core (250–256 ∞C) brings a number of advantages for PE/PET bicomponent fibres in thermal bonding of nonwovens. A wide variation in bonding temperature can be tolerated since the core component is largely unaffected by temperature variations that may inadvertently occur during the thermal bonding process. In contrast to other core-sheath combinations, the core remains stable to mechanical deformation even at high temperature after the sheath has melted, which facilitates the production of high-quality nonwovens.8 By appropriate selection of polymer composition, polymer ratio and fibre cross-sectional geometry, it is possible to engineer bicomponent fibre structures for improved economic efficiency, cost-effectiveness and functionality. Some major bicomponent fibre and technology producers in the nonwovens industry are Fibrevisions (Denmark), Fibre Innovation Technology or FIT (USA), Wellman International (Ireland), Chisso Corporation (Japan) and Hills Incorporated (USA).

6.3.4

Powder binders

Powdered polymeric binders can be applied during web or batt formation or following web formation and pre-bonding. A thermoplastic polymer with a low softening temperature is desirable that requires a short exposure to heat to melt and fuse the powder. For ease of operation the thermoplastic powder should have a low melt viscosity and the transition from melt to solid should occur over the shortest possible temperature range. Polymers such as polyethylene, low molecular weight polyamide and copolymers of vinyl chloride and vinyl acetate, are generally used. This method of thermal bonding is limited by difficulties in obtaining polymers with a suitable range of particle sizes to suit the base web. Obtaining a uniform powder distribution throughout the web is also problematic. Powder bonding is suited to lightweight webs where an open structure is required with a soft handle or in the production of reinforced, moulded products. Applications include feminine hygiene, adult incontinence, medical and automotive products, wipes, computer disks, apparel and shoe composites.9

6.4

Calender (contact) bonding

Thermal bonding relies on the use of heat energy to melt or soften one or more components of a web to achieve bonding. There are different methods of applying heat energy to the web and the heat transfer mechanism can take different forms; conduction, convection and heat radiation. The widely used methods are discussed in this section. Thermal calender bonding is a process in which a fibrous web containing thermoplastic components (fibres, powders or films) is passed continuously through a heated calender nip that is created

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by two rolls pressed against each other. Multi-nip calenders are also employed depending on the web weight and degree of bonding required. Both rolls are internally heated to a temperature that usually exceeds the melting point of the binder components in the web to ensure there is sufficient heat transfer to induce softening at the prevailing line speed. As the web passes between the calender nip, fibres are both heated and compressed. This causes the binder components of the web to become soft and tacky and induces polymer flow in and around the base fibres (if present). The fluid polymer tends to collect at fibre crossover or contact points and bonding sites are formed. Cooling leads to solidification of the polymer and bonding. Calender bonding is mainly applicable to light and medium-weight webs because the fibres in a thick web insulate heat from the interior of the structure leading to a temperature gradient and variation in the degree of bonding through the cross-section. To increase the efficiency of the process, the web may be pre-heated immediately prior to calendar bonding sometimes by infra-red heaters. Commercially, light-weight webs of 25–30 g/m2 for medical and hygiene applications and medium-weight webs of 100 g/m2 for interlining and filtration applications are thermally bonded using calendar bonding.10 The degree of bonding depends on temperature, pressure and speed, which determines the contact or dwell time. The properties of the fabric are influenced by the total bond area, which is normally expressed in percentage terms. In practice, area bonding (100%) or point bonding (embossing) is possible ( 25%. Fabrics manufactured by ultrasonic bonding are soft, breathable, absorbent and strong. Ultrasonic bonding is also suitable for manufacturing patterned composites and laminates, such as quilts and outdoor jackets.

6.7

Thermally bonded fabric structure

One of the main structural attributes determining fabric applications is fabric strength, in particular tensile strength. It is known that stronger fibres make stronger fabrics when all the other constructional factors are similar. However, in thermally bonded fabrics it is possible to make a weak fabric using strong fibre if bonding conditions are not set appropriately. The failure mode of fabrics changes with bonding conditions. For example, the tensile failures of light-weight PET (20–30 g/m2) calender bonded fabrics are explained by three main mechanisms depending on manufacturing process conditions. 1. Failure of adhesion between fibres within the bonding sites (bond point disintegration). 2. Failure of individual fibres; fibre breakage in the perimeter of bond points where they are attached to the bond points or somewhere along the side of the fibre length between bond points. 3. Fracture of bond points. The occurrence of the above mechanisms depends on how the fabrics are made. Increasing the bonding temperature changes the failure method from that of the first method to the second. Failure by the third method is expected at very high temperature and in very stiff fabrics.27

6.7.1

Effect of fibre structure on properties of thermally bonded fabrics

The published research reviewed by Dharmadhikary10 suggests that changes in fibre structure produced during point bonding impacts the properties of the fabric, and to understand these changes a comprehensive understanding

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of the structure and properties of individual polymer types is required. The role of the binder and base fibre structures is of particular importance. A study of PET area bonded fabrics with respect to the changes in binder morphology28 suggested that crystalline binder fibres produce fabrics with a higher tenacity, modulus and breaking extension than amorphous binder fibres. In terms of fabric strength, these differences may be difficult to distinguish below a critical temperature and pressure. Another study by Wei et al.29 used commercial polypropylene fibres of varying draw ratios. Fibres with a lower orientation compared with fibres with a higher orientation and microfibrillar structure, had a higher tensile strength and flexural rigidity. They suggested that this was due to the less orientated amorphous regions and lamellar crystal structure promoting fibre fusion, while the orientated microfibrillar structure was inhibiting fibre fusion during thermal bonding. Fabric shrinkage was also determined by the fibre morphology; they suggested that this resulted from molecular retraction in the amorphous regions. The highly orientated fibres also showed higher shrinkage and this resulted in increased fabric thickness. Point bonded fabric consists of a network of fibres, bonded in localised regions (bond points), by the application of heat and pressure. This results in partial melting of the crystals which is essential for the formation of bond points. Although the point bond process is simple, a clear understanding of the properties of point bonded fabrics has yet to be established and there is only limited information about the ideal fibre structures required for point bonding and the changes that occur in fibre structure and properties during bonding. The morphology of the bond points and the bridging fibres is an important influence on properties. Bond strength influences fabric strength and this has been studied by Mi et al.30 Results from their model suggest that high strength bonds, (fabric failure caused by failure of the bridging fibres) led to the strongest fabrics. The failure of a fabric will be determined both by the nature and character of the bond points and the stress/strain relationship of bridging fibres. During point bonding, depending upon the specific process variables adopted and the bridging fibres, the properties associated with the bond point differ from those of the virgin fibre. Various workers have referred to this aspect, Warner12 suggested that fibres break at the bond periphery because of the local thermo-mechanical polymer history and that the strength of point bonded fabrics is therefore controlled by the bond periphery strength. Wei et al.29 commented that the physical properties of thermally bonded fabrics are a result of the nature and quality of the bonding regions. The influences of pressure on polymer properties during point bonding are not well understood. Pressure might be expected to increase the melting point and the glass transition temperature and therefore might influence the crystallisation rate. Crystal nucleation and growth is also influenced by pressure and could produce complex interactions.

Thermal bonding

6.8

327

Applications of thermally bonded fabrics

There are now a vast number of varied end uses, almost too numerous to mention, for thermal bonded nonwoven fabrics across all sectors of the industry including both single use hygiene disposable products and durable building and construction materials. A major application is in hygiene, for example PP coverstock found in sanitary and incontinence products of 10– 30 g/m2, based on calender point bonded dry-laid or spunbond webs, the former using bicomponent fibres. Other disposables include wipes produced from airlaid short fibres, which are through-air bonded to make products in the 25–150 g/m2 weight range and through-air bonded carded wipes products of about 100–250 g/m2. Through-air bonding in this application is preferred to maximise the bulk of the fabric. Recently some thermally bonded wipes products have seen strong competition from dry-laid-hydroentangled fabrics. Wet-laid fabrics intended for tea-bag applications may be through-air bonded. Calender bonding is utilised to bond spunbond, meltblown and composites of these webs for numerous medical and hygiene applications including surgical gowns and drapes. Durable thermally bonded products produced from spunlaid webs are either through-air bonded or calendered. Those using through-air bonding in an oven are frequently based on bicomponent filament webs. Core-sheath PET-PA6 filament spunbonds have applications in carpet backing whereas core-sheath PP-PE spunbonds have applications in geosynthetics and technical textiles as air and water filtration media, horticultural products, and in clothing and footwear. Thermally bonded spunbond geosynthetic fabrics of 80–250 g/m2, find uses in various civil engineering applications. Roofing felts or carriers of about 150–350 g/m2 are an application for thermally bonded spunlaid fabrics whereas needlepunched fabrics are used as bitumen carriers in flat roofs. Thermally bonded spunlaid fabrics are installed in pitched roofs as bituminous underslating. Such fabrics are normally composed of PET partly to give better heat stability during bitumen coating. Many dry-laid filter fabrics are needlepunched, particularly those intended for high-temperature applications but others are through-air bonded in weight ranges from 100–1000 g/m2. Through-air bonded spunbond fabric of 150– 200 g/m2 is produced for carpet backing applications. A traditional thermal bond application is in the manufacture of linings and interlinings ranging in weight from 25–150 g/m2 using either calender or through-air bonding methods. In some applications, point bonding using a calender follows mechanical bonding to produce the final product. Powder and thermo-dot bonded, fusible and non-fusible nonwoven fabrics for garment interlining applications, from polyester and EVA are also made. Shoe lining fabrics of about 150–200 g/m2 composed of blended bicomponent fibres are thermally point bonded with a calender.

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Dry-laid, thermally bonded reinforcement fabrics, and substrates for electrical insulating materials, pressure sensitive tapes and filtration membranes are made from PET. Other applications include furniture and bedding components, horticultural and agricultural fabrics including crop cover, high loft waddings and paddings for thermal insulation and automotive fabrics. Spunlaid filtration fabrics stabilised by thermal bonding find applications as cabin filters in the automotive industry and are can be pleated to increase the surface area available for filtration.

6.9

References

1. Watzl A., ‘Fusion bonding, thermobonding and heat-setting of nonwoven – theoretical fundamentals, practical experience, market trends’, Melliand English Vol. 10, 1994, P. E217. 2. Batra S.K. and Pourdeyhimi B., ‘Thermal Bonding’, Nonwovens Cooperative Research Center, North Carolina State University, Raleigh, NC. USA. 3. Rave H., Schemken M. and Beck A., ‘State of the art of bicomponent staple fibre production’, Chemical fibre international Vol. 52, April 2002. 4. http://www.ifg.com/issue/june98/story3.html 5. Jeffries R., ‘Bicomponent Fibres’, Merrow Publishing Co. Ltd, 1971. BP 1048370, NAP 66-12238, Shell International Research. 6. Marcher B., ‘Tailor-made polypropylene and bicomponent fibres for the nonwovens industry’, Tappi Journal, Vol. 74, No. 12, 1991, P. 103–107. 7. Thonnessen F. and Dahringer J., ‘Trevira bicomponent fibres for nonwovens’, Chemical fibre international, Vol. 53, No. 12, 2003, P. 422. 8. Raidt P., ‘Polyester/Polyethylene bicomponent fibres for thermal bonding of nonwovens’, Index 87 Congress). 9. Hoag T.S. ‘From time-tested methods to recent innovations, bonding exhibits versatility’, Nonwovens World Vol. 4, No. 1 1989, P. 26 10. Dharmadhikary P.K., Gilmore T.F., Davis H.A. and Batra S.K., ‘Thermal bonding of nonwoven fabrics’, Textile Progress Vol. 26, 1995, P. 26. 11. Muller D.H., ‘Improvement of thermalbonded nonwovens’, Melliand Textilberichte, Vol. 70, No. 7, 1989, P. 499–502, E210. 12. Warner S.B., ‘Thermal Bonding of Polypropylene Fibres’, Textile Research Journal, Vol. 59, No. 3, 1989, P. 151–159. 13. Schwartz R.J., US Patent 4100319, 1978. 14. Haoming R. and Bhat G.S., ‘Preparation and properties of cotton-ester nonwovens’, International Nonwovens Journal, No. 12, No. 2, 2003, P. 55. 15. Kwok W.K., Crane J.P. and Gorrafa A., ‘Polyester staple for thermally bonded nonwovens’, Nonwovens Industry, 19 (6), 1988, P. 30–33. 16. Kim H.S., Pourdeyhimi B., Desai P. and Abhiraman A.S., ‘Anisotropy in the mechanical properties of thermally spot-bonded nonwovens: Experimental Observations’, Textile Research Journal, 2001, Vol. 71, No. 11, P. 965. 17. Muller D.H., ‘How to improve the thermal bonding of heavy webs’, INDA J. Nonwoven Research, 1989, Vol. 1, No. 1, P. 35–43. 18. Shimalla C.J. and Whitwell J.C., ‘Thermomechanical behaviour of nonwovens, Part I: Responses to changes in processing and post-bonding variables’, Textile Research Journal, 1976, Vol. 46, P. 405–417.

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19. DeAngelis V., DiGioacchino T. and Olivieri P., ‘Hot calendered polypropylene nonwoven fabrics’, in Proceedings of 2nd International Conference on Polypropylene Fibres and Textiles, Plastics and Rubber Institiutes, University of York, England, 1979, PP. 52.1–52. 13. 20. Mayer J.W., Haward R.N. and Hay J.N., ‘Study of the thermal effects of necking of polymers with the use of an infrared camera’, Journal of Polymer Science Polymer Physics, edn 18, 1980, P. 2169–2179. 21. Wunderlich B., Macromolecular Physics, Academic Press New York, Vol. 3, 1986. 22. Gunter D.S., ‘Calender selection for nonwovens’, Tappi Journal, January, Vol. 81, No. 1, 1998, P. 208. 23. Gunter D.S. ‘Thermal bonding utilising calender’, Tappi Journal, Vol. 77, No. 6, 1994, P 221. 24. Wuagneux E.L., ‘Full of hot air’, Nonwovens Industry, Vol. 30, No. 4, 1999, P. 52– 56. 25. Watzl A., Instruction manual of Fleissner through air bonding machinery. Internal Fleissner machine document. 26. Holman J.P., ‘Heat Transfer’, McGraw Hill, ninth edition, 2002. 27. Gibson P.E. and McGill R.L. ‘Thermally bondable polyester fibre: the effect of calender temperature’, Tappi Journal, Vol. 70, No. 12, 1987, P. 82. 28. Dantuluri S.R., Goswami B.C. and Vigo T.L. ‘Thermally Bonded Polyester Nonwovens: Effect of fibre morphology’ in Proceedings of INDA Technical symposium, 1987, P. 263–270. 29. Wei K.V., Vigo T.L. and Goswami B.C., ‘Structure-property relationship of thermally bonded polypropylene nonwovens’, Journal of Applied Polymer Science, Vol. 30, 1985, P 1523–1524. 30. Mi Z.X., Batra S.K. and Gilmore T.F., ‘Computational Model for Mechanical Behaviour of Point-Bonded Web’ First Annual Report, Nonwovens Cooperative Research Center, 1992.

7 Chemical bonding R A CHAPMAN Warwick Innovation Limited, UK

7.1

Introduction

Textile Terms and Definitions1 defines a binder as an adhesive material used to hold together the fibres in a nonwoven structure. The word ‘binder’ describes the function of a composition in the final product. The terms ‘binder’, ‘binding agent’, ‘binder composition’,‘binder system’, ‘nonwoven binder’, ‘chemical binder’ are used in the literature to describe the polymer, polymer and carrier, part-formulation or total formulation used in chemical bonding – the meaning shifts according to the context. A binder not only ‘holds fibres together’ but also affects the final properties of the nonwoven fabric including its strength (both tensile and compressive), stiffness, softness, waterproofness, breathability and flammability. The choice of binder also influences the capability of the fabric to be recycled or biochemically degraded at the end of its useful life. Chemical bonding remains popular because of the large range of adhesive binders available, the durability of the products and the broad variety of final properties that can be engineered in the fabrics. While in the early days of development natural binders such as starch and rubber were used, synthetic polymers now dominate the industry. Mostly in response to the needs for more environmentally sustainable materials, concerns about free formaldehyde, and to aid ease of disposal, there is a resurgence of interest in the biodegradeable binders derived from agricultural sources for particular applications. These include starches, pectins, oils and casein amongst others. Binders are also applied to nonwoven fabrics that are already bonded to provide additional functionality, since the binder can be mixed with active components or solids such as flame retardants and functional finishes, ceramics and metals. For example, in the manufacture of wipe products, pigments and chemical binders are commonly printed onto the surface of hydroentangled fabrics to increase the wet strength, to control wet pick-up and to improve the visual appearance of the product. It is increasingly common to use such ‘combination bonding’ procedures in which several different bonding methods are used in succession. Thermally bonded airlaid fabrics, needlepunched and 330

Chemical bonding

331

hydroentangled fabrics are frequently subject to secondary chemical bonding to modify fabric properties or appearance. Binder polymers can be dissolved in a solvent including water or they can be dispersions or emulsions. The most important binders are latices (also called latexes) of emulsion polymers. These are fine dispersions of specific polymers in water. They are applied in a number of different ways to nonwoven substrates and because their viscosity is close to that of water, they can easily penetrate thick or dense nonwoven structures by simple immersion. After application of the binder by, for example, immersion, they are dried and the water evaporates. Typically, the binder forms an adhesive film across or between fibre intersections and fibre bonding is obtained. Binders create a network of interlocked fibres, which can be throughout the fabric structure or in selected areas depending on the required end-use. The distribution of the binder in the fabric structure and its properties can be affected by the use of coagulating and crosslinking agents as well as the application method utilised. In chemically bonded fabrics, the concentration of binder on the surfaces and in the interior may not be uniform and this affects fabric stiffness, handle and the probability of delamination in some cases. The concentration of binder may be graduated in the fabric crosssection, for example it may decrease from the fabric surface towards the middle due to migration of the binder towards the surfaces during drying. Alternatively, as in some foam bonding operations, for example, the application of the binder may be purposely designed to be concentrated differently throughout the fabric cross-section. The binder system wets the fabric and following drying and/or crosslinking, forms a bonded structure. Although homopolymer emulsions can be utilised, copolymers or blends with fillers are common. Copolymers provide some tailoring of the main homopolymer properties, for example, to enable increased softness, and fillers help to reduce cost and provide additional useful properties such as improved thermal resistance, abrasion resistance, flame retardancy, water repellency or antistatic properties. Generally, fillers are an economical way of achieving such properties in contrast to changing the fibre composition. Commercially, binder systems are applied at levels between about 5% and 150% on the dry weight of fabric. A 5% binder addition is often sufficient to bond fibres at the surface. Addition levels as high as 150% are sometimes used to make stiff reinforcement components such as those found in shoes.

7.2

Chemical binder polymers

7.2.1

Introduction

Various binder polymers are used including vinyl polymers and copolymers, acrylic ester polymers and copolymers, rubber and synthetic rubber, and

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natural binders, principally starches. These are usually applied as aqueous dispersions but can be supplied as polymer solutions providing they have sufficiently low viscosity to allow penetration into the web.2 Table 7.1 gives the main types of binder in use. Acrylic thermoset resins have also been developed based on low molecular weight polyacids (polyacrylic acid) and an accelerant (sodium hypophosphite). These are intended to be formaldehyde resin alternatives and applications include glass fibre insulation.3 Commercially, latex polymers are the most commonly encountered binder systems because of the wide variety available, their versatility, ease of application and cost effectiveness.

7.2.2

Latex polymers4,5

Emulsion polymerisation An emulsion polymer is a colloidal dispersion of discrete polymer particles with a typical particle diameter of 0.01–1.0 microns in a medium such as water. Common polymers used are acrylates, styrene-butadiene copolymers, acrylonitrile-butadiene copolymers and ethylene vinyl acetate. A latex polymer is prepared by the controlled addition of several components either in a batch or a continuous monomer addition process. The components are water, monomers (the polymer building blocks), initiator (to start the polymerisation process), surfactant (to stabilise the emulsion particles as they form by preventing coalescence) and chain-transfer agent (to control the final polymer molecular weight). The role of each component will be discussed later. Process of latex formation The process starts with a distribution of monomer droplets in water, stabilised by emulsifiers that have accumulated at the interface to the water phase. Emulsifier molecules have hydrophobic and hydrophilic parts. In Fig. 7.1 the line represents the hydrophobic part of the molecule and the dot, the hydrophilic part. If the concentration of the emulsifier is above a critical value, a spheroidal collection of them form. This is called a micelle and contains about a hundred Table 7.1 Summary of the main binder types Vinyl based Acrylic esters and copolymers Polyurethane and copolymers Elastomers including silicone Thermosetting resins: epoxy, polyester, urea formaldehyde, melamine, alkyd Natural binders: starches, natural rubber, regenerated proteins

Chemical bonding Emulsification

Initiation

Particle growth

333

End product

Nucleus 5

Monomer pool

Stabilisation of nuclei by emulsifier 4 Dissolved monomer

1

Initiator radical 2

6

Chain formation 3

8

Coalescence 7

7.1 Particle formation during emulsion polymerisation (adapted from How do aqueous emulsions form? Polymer Latex GmbH4). Note: the emulsifier molecules are represented by a line and a dot.

emulsifier molecules. The hydrophilic parts project into the water producing a hydrophobic interior. The hydrophobic interior is able to accommodate other hydrophobic substances, for example monomer molecules. The initiator decomposes to form water-soluble free radicals. Nearly all of the monomer is present in the form of monomer droplets, but there is a very small proportion that is dissolved in the water. When a free radical encounters monomer molecules dissolved in the water, it reacts successively with several to form a short polymer chain. This short chain, called an oligomer radical, is no longer soluble in water. It precipitates and is stabilised by the emulsifier, which accumulates at the newly formed interface. This is now a latex particle. Provided that there is enough emulsifier available, more oligomer radicals can be stabilised and grow into latex particles. However, if there is insufficient, the insoluble oligomer radicals aggregate, presenting a smaller surface that requires less emulsifier to be stabilised. The result is that fewer but larger particles form. In addition to this process, we have to consider the emulsifier micelles. Monomer molecules diffuse into these. If an oligomer radical meets an emulsifier micelle, which contains monomer, the monomers polymerise and form another latex particle. This can occur only if the concentration of emulsifier is high enough. In other words, it is above the ‘critical micelle concentration’. Finally, it is possible for a growing oligomer radical to meet a monomer droplet and initiate polymerisation forming a latex particle. In this case the

334


latex particle would be large – about the size of the original monomer droplet. While possible, it is a rare occurrence. Now the formation of latex particles is completed and growth starts. There is a flow of monomer from the water and the monomer droplets to the latex particles where polymerisation occurs. The latex particle grows larger and rounder and it can contain hundreds or thousands of closely packed molecules in one particle. If there is a shortage of emulsifier then the growing particles do not grow as above but coalesce. As propagation proceeds, more particles are added in layers to form a larger latex particle. Binder components Monomers The monomers selected form the basic building blocks of the binder. The selection of monomers is determined by cost and the final fabric properties required. Monomers are often characterised as ‘hard’ or ‘soft’ depending on their glass transition temperature, Tg. The binder Tg influences fabric handle and the perception of softness in use. Indicative glass transition temperatures for typical monomers used for making binders vary slightly depending on the source. The values in Table 7.2 are approximate values. A calculated estimate of the Tg of any copolymer can be obtained using the Fox equation:6 1/Tg = Wx /Tgx + Wy /Tgy

7.1

where Tgx and Tgy = glass transition temperatures of polymers x and y respectively and Wx and Wy = weight fraction of polymers x and y respectively and Wx + Wy = 1. In addition to affecting the handle and bending stiffness of Table 7.2 Glass transition temperatures Monomer

Tg (∞C)

Soft Ethylene 2-ethylhexyl acrylate Butadiene n-butyl acrylate Ethyl acrylate

–120 –85 –78 –52 –22

Hard Methyl acrylate Vinyl acetate Vinyl chloride Methyl methacrylate Styrene Acrylonitrile

+9 +30 +80 +105 +105 +130

Chemical bonding

335

the final product, the choice of monomers affects the hydrophilic or hydrophobic properties of the fabric. This directly reflects the hydrophilicity of the monomers used in the assembly of the binder polymer. For example, butyl acrylate is relatively hydrophobic and vinyl acetate is relatively hydrophilic. Clearly, the wet stability of the binder is a consideration in some applications such as disposable wipes and incontinence products. It is also important in the design of single-use, water-dispersible wipes, where solubilisation of the binder may be required. Binder extensibility is inversely related with the Tg and is also influenced by the molecular weight. Surfactants Surfactants perform several functions in emulsion polymerisation of which the most important is providing latex stability both during and after polymerisation. The surfactants used are either anionic, cationic or nonionic. In emulsion polymerisation, anionic and non-ionic types are normally used.7 Typical anionic surfactants are sodium lauryl sulphate or sodium lauryl ether sulphate. The molecule contains both polar (hydrophilic) and non-polar (hydrophobic) groups. The surfactant works by stabilising latex particles using electrostatic repulsion forces to prevent particle attraction. Non-ionic surfactants, for example ethoxylated lauryl alcohol are used to improve the mechanical and freeze-thaw stability of a latex. They work by steric hindrance. The choice of surfactant affects the charge on the emulsion, the particle size, surface tension (which affects the wetting behaviour of the binder on the fibre), fibre adhesion, film formation and emulsion stability.8 The wetting behaviour is particularly important to ensure the binder is properly distributed over the fibre surfaces in the web or fabric. Initiators The initiator, which is commonly ammonium persulphate decomposes on heating to form free radicals that start the polymerisation process. Chain transfer agents Sometimes is it is desirable to limit the molecular weight of the polymer by introducing a chain transfer agent such as dodecyl mercaptan. The growing polymer radical combines with the chain transfer agent to stop chain growth. A short chain radical also forms from the chain transfer agent, which reacts with a monomer molecule to form a new polymer radical that starts to grow.

336


Buffers A buffer is used to control the pH during the polymerisation process. Some monomers may hydrolyse if the pH is not controlled. A common buffer is sodium acetate. Other additives Sometimes sodium hydroxide is added to the latex to increase the pH and improve its stability.

7.2.3

Latex polymer binder systems

The main systems are based on vinyl, acrylate (also called acrylic) and butadiene polymers. Choice depends on cost, stiffness, binder hardness and softness (which influences fabric handle), toughness, water and solvent resistance as well as ageing properties. Vinyl polymers Vinyl monomers contain carbon-carbon double bonds and form polymers of the type – [CH2–CR·CR’]n–. A range of vinyl polymers are available, of which acrylates are a subdivision as shown in Fig. 7.2. Examples of vinyl polymers include polyvinyl acetate, polystyrene and polyvinyl chloride. Vinyl homopolymers such as vinyl chloride and vinyl acetate, are hard and have strong adhesion to a wide range of fibres. Because of their hardness, they are often plasticised with internal or external plasticisers such as phthalates. Ethylene is not used as a homopolymer in binders but in a copolymer such as ethylene vinyl acetate or ethylene vinyl chloride and provides flexibility. Vinyl acetate Vinyl acetate binder polymers have a Tg of around 30 ∞C and are quite hard but tough. The hardness can be reduced using acrylates or ethylene as comonomers. The polymers are hydrophilic and tend to yellow on heating. Self-crosslinking versions provide improved stability to water. They are relatively cheap. Vinyl chloride is sometimes included to enable nonwovens to be bonded by dielectric heating because the polymer has a comparatively low softening temperature. Vinyl chloride Vinyl chloride is a hard polymer (Tg~+80 ∞C) and is therefore unsuitable for many nonwoven products. Co-polymerising with the softer acrylic monomers

Chemical bonding H

337

R

C

Vinyl monomer which when polymerised forms C repeating units to produce a vinyl polymer.

H

R¢

H

R

H

R

H

R

C

C

C

C

C

C

H

R¢

H

R¢

H

R¢

Vinyl polymer

R

R¢

Monomer

H H Cl H H H H H CH3 H H CH3 H H

H Cl Cl OCOCH3 C6 H5 CHCH2 CN COOH COOH COOC2H5 COOC4 H9 COOCH3 CONH2 CONHCH2OH

Ethylene Vinyl chloride Vinylidene chloride Vinyl acetate Styrene Butadiene Acrylonitrile Acrylic acid Methacrylic acid Ethyl acrylate Butyl acrylate Methyl methacrylate Acrylamide N-Methylol acrylamide

7.2 Vinyl monomers (adapted from Pangrazi 1992).9

reduces the hardness of the homopolymer improving its utility for nonwoven applications. These polymers are often used because of their inherent flame retardancy arising from the chlorine content. They are also thermoplastic and can be welded using dielectric heating but like vinyl acetate they tend to yellow on heating. Ethylene vinyl chloride These binder polymers can be considered similar to vinyl chloride polymers but with the ethylene monomer acting as an internal plasticiser to provide greater polymer ductility. This class of binders has a slightly broader range of stiffnesses than vinyl chloride but without the need for an external plasticiser. The vinyl chloride monomer also provides some attractive flame retardant properties and can be welded using dielectric heating. They bond well to synthetic fibres and provide good abrasion and acid resistance. Ethylene vinyl acetate (EVA) EVA polymers can be made with a wide range of softnesses. They tend to be cheaper than acrylics and have good adhesion to many synthetic fibres. They are less resistant to solvents than acrylics but provide a good combination of

338


high wet strength, excellent absorbency, durability and softness. They are often used in disposable hygiene products such as wipes and are increasingly used in the bonding of short fibre pulp airlaid fabrics used in disposable hygiene fabrics. Vinyl acetate acrylate These are mostly based on butyl acrylate. They can be regarded as being a compromise between vinyl acetate and acrylics, both in performance and cost. The vinyl acetate monomer is generally cheaper than the acrylics. The acrylic monomer decreases the sensitivity to moisture and solvents. Acrylonitrile The homopolymer is not used as a binder by itself but when the acrylonitrile monomer is used to make nitrile rubber it provides excellent resistance to solvents, oil and moisture. Styrene Styrene monomers are hard and provide stiffness and hydrophobicity. Polystyrene homopolymer is hard and brittle at room temperature and does not easily form a film. For this reason it is difficult to use as an effective binder. Acrylate polymers Polyacrylates (commonly referred to as acrylics) are a type of vinyl polymer. The most important are copolymers of acrylic acid derivatives, especially acrylic acid and methacrylic acid esters. They are made from acrylate monomers, which are esters containing vinyl groups, i.e. a group of two carbon atoms double bonded to each other, directly attached to the carbonyl atom. There are more than thirty monomers used. Their hardness and solvent resistance decreases with increasing chain length of the alcohol moiety. Polymethacrylates have higher film hardness than polyacrylates.10 Crosslinking improves their resistance to washing at the boil and dry-cleaning but they tend to be more expensive than other binders. Common examples are ethyl acrylate and butyl acrylate. To increase stiffness, these may be copolymerised with methacrylate, methylmethacrylate or styrene. For increased hydrophilicity methyl acrylate monomers are used. Specifically, for increased hydrophobicity, 2-ethylhexyl acrylate or styrene can be used.11 Styrenated acrylics are hydrophobic, tough binders, which are relatively cheap. They are used where high wet strength is needed but at some sacrifice of UV and solvent resistance.

Chemical bonding

339

Butadiene polymers Polymers based on butadiene CH2=CH·CH=CH2 usually have relatively high elasticity and toughness and have been used since the early years of the nonwovens industry. They include natural rubber latex (polyisoprene), polychloroprene, styrene butadiene rubber (SBR) and nitrile butadiene rubber (NBR). Natural rubber latex This was one of the earliest binders used in the manufacture of nonwoven fabrics and was superseded by styrene butadiene and nitrile butadiene rubbers. After drying, the temperature is increased to initiate polymer crosslinking (vulcanisation). It provides an excellent soft handle and high elasticity. Chloroprene Polychloroprene binders are unusual in that they crystallise, causing an increase in stiffness. Their resistance to organic solvents and oils is not quite as high as NBR copolymers but they are exceptionally acid resistant. Their resistance to weathering is better than NBR and SBR but their discoloration is greater and they are used for some nonwoven shoe materials. Styrene butadiene rubber (SBR) SBR binder polymers are tough, flexible and have excellent solvent resistance. Their stiffness and hardness increases with the level of styrene. They are cheaper than acrylates and nitrile rubbers (but less elastic than the latter). Crosslinking gives them excellent water resistance. Nitrile rubber These are butadiene acrylonitrile copolymers. Increasing the level of acrylonitrile in such rubbers increases the hardness. Compared to other polymers used as binders, they have low thermoplasticity and so can be sueded (or subject to intensive mechanical abrasion) without melting the fibres in the fabric. They also have high abrasion resistance and are often used to make synthetic leather.

7.2.4

Other polymer binders

Polyurethane Polyurethane (PU) based binders have been favoured for many years in the manufacture of synthetic leather nonwoven fabrics as well as products where

340


good extensibility is required. They are applied from solvent or are produced as aqueous dispersions. Today, most PU binders are actually polyesterpolyurethane copolymers. They tend to have excellent adhesion and film forming properties. Film structure is controlled by the pH of the aqueous dispersion and acid coagulation can enable the formation of microporous films used in breathable membranes and coatings. In the case of solvent systems, the PU is first dissolved in dimethylformamide (DMF) and after the fabric has been impregnated, the DMF is displaced and the PU is coagulated. During drying a dense, porous structure is formed.12 PU polymers provide soft, elastic binders and films with comparatively good resistance to hydrolysis and light fastness. Water based dispersions are increasingly favoured over solvent based PUs but they tend to have lower wet stability. PU is frequently used to produce low cost hydrophilic breathable coatings on fabrics. Traditionally, PEO is used to increase the inherent hydrophilicity of these PU materials. Phenolics Phenolic binders are occasionally used for full saturation bonding of fabrics to make durable filter fabrics and for fabrics requiring high abrasion resistance that are operated at high temperature, for example in clutch and brake pads. Epoxy resins Waterborne epoxy resins are used for bonding nonwovens when high chemical resistance, stability at high temperatures or electrical insulation properties are required. Epoxy resins are particularly important in the field of fibre reinforced composites. Obviously the chemical and mechanical properties of the constituent fibres need to be carefully selected to achieve the correct blend of overall properties.13

7.2.5

Characteristic properties of latex polymer dispersions

Suppliers typically provide data sheets indicating the binder properties as shown in Table 7.3.14

7.2.6

Minimum film forming temperature

The minimum film forming temperature is the lowest temperature at which an emulsion polymer can form a continuous film. It is usually several degrees above the glass transition temperature (Tg). An emulsion polymer comprises about 50% by weight of polymer particles in water. As the water evaporates

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341

Table 7.3 Items listed in the specifications provided by manufacturers of chemical binders Binder composition/characteristics

Notes

Monomers

Many latices are in fact copolymer systems

Crosslinking

Whether or not the system is selfcrosslinking or can be externally crosslinked

Solids content

Typically is 50% solids and can be between 30 and 60%

Average particle size

Ranges from 0.01 to 1 micron. The particle size and size distribution affects the properties of the binder and the ease of film formation

Residual monomer level

Some monomers can present a hazard to health

Ionic nature

Polymer dispersions are commonly anionic or nonionic

pH value

Is normally between 2 and 10

Viscosity

Varies between 50 and 50,000 Pa.s

Glass transition temperature (Tg)

Used as an indicator of polymer hardness and stiffness

Minimum film forming temperature

(See Section 7.2.6)

Nature of the film

For example, tacky or soft

Film mechanical properties

For example, elongation and tensile strength at break

Resistance to washing at the boil

Y/N

Resistance to dry cleaning

Y/N

Shelf life

6 months–5 years

Suitability for various methods of application

e.g., saturation, foam, spray and print

the particles move closer together and become less mobile until they touch each other. They can be imagined as an agglomeration of spherical particles packed closely together in layers. At their closest packing the level of solids is about 75%. As the water evaporates from the surface of the agglomerated spheres, it is replaced by water from lower layers. Very thin water layers form between the particles and effectively become tiny capillaries. The high capillary forces squeeze the water out, further compressing the particles together. If the polymer globules are too hard and dimensionally stable, a tightly packed heap of solid globules is produced – a powder. If the particles are soft enough, they deform under the capillary forces and become polyhedra. The remaining water is squeezed out and the polyhedra coalesce to form a film.

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The binder ‘hardness’ or ‘softness’ that affects the ease of film formation depends on how the polymer chains in the latex particles are packed together. If they have no side chains (branches), they can pack together closely and become relatively immobile, needing significant energy, for example heat energy, to separate them. These structures are ‘hard’. If the molecular chains have side branches, they cannot so uniformly pack together and remain more mobile. These are ‘soft’ structures. This polymer chain mobility depends therefore on the polymer structure and the temperature (heat energy). As the temperature is raised, the mobility of the molecules reaches a point called the minimum film forming temperature. Above this temperature the latex particles are able to merge to form a film – polymer chains on adjacent latex particles entwine and fuse the particles together. The ease of film formation can be enhanced by the use of plasticisers which facilitate the movement of the polymer molecules. Water can act as a plasticiser. If watersoluble molecular units, for example acrylic acid or methacrylic acid, are incorporated into the latex particle, they act as plasticisers. Conversely, the mobility can be impeded by crosslinking the molecular chains. The crosslinks inhibit the deformation of the globules to form polyhedra and the ability of the molecules to interpenetrate one another at the polyhedra boundaries. Although the ability to form a film is necessary for bonding using latices, not all latex binders used in a formulation need to be film forming. Sometimes a formulation will include two latices, one a high styrene latex that will not form a film in the drying process, and the other, one that is capable of film formation. The combination of the two introduces the required high stiffness into the product.

7.2.7

Functionality of latex polymers

In addition to monomers that provide the backbone of the polymer and determine the key physical properties of the binder, other specific monomers are added to provide specific functionality. These are of particular importance to the processes of coagulation and crosslinking. Coagulation and migration As an impregnated nonwoven dries, the temperature difference through the cross-section of the fabric can cause the binder polymer to migrate to the higher temperature regions. This differential migration results in a non-uniform distribution of binder where the surfaces tend to have a higher concentration of binder than the core of the fabric, which is depleted. This can lead to problems such as fabric delamination but can be beneficial in some applications such as in the manufacture of synthetic leather. In this process, the impregnated nonwoven is split through its thickness, as is natural leather, and it is important

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343

that each ‘split’ component has similar properties. Thickeners have been used to inhibit migration but they reduce penetration during impregnation and slow the process down. Some polymers can be modified so that during drying they coagulate and do not migrate. This is achieved by making them thermo-sensitive. When the binder reaches a particular temperature, the coagulation temperature, the latex particles coagulate on the fibres rather than migrating through the fabric. The ability of a binder system to be heatsensitised depends on the particular monomers and the level and type of surfactant present. Nitrile rubbers and high styrene SBR polymers can be heat sensitised. Several heat sensitising systems are known, for example, based on polyvinyl alkyl ethers, polypropylene glycols/polyacetals, divalent metal cations/amine and organopolysiloxanes. Latices tend to be increasingly unstable as the pH is reduced and so low pH aids heat sensitisation. The pH is adjusted with, for example, acetic acid or ammonia. Small amounts of some nonionic surfactant stabilisers are added. These have lower solubility in hot water than cold. They aid room temperature stability and become less stable as the temperature is raised, helping gelation.15 Migration of the binder can also occur at the drying stage due to differential temperatures in the cross-section of the fabric. Troesch and Hoffman16 commented that the binder system migrates by capillary flow during the early stages of drying. The heat causes thermosensitised dispersions to form agglomerates whose diameters are larger than the capillaries; coagulation happens in a ‘shock-like’ manner. They point out that the difference between the wet-bulb temperature of the material in drying and the coagulation temperature of the binder is crucial. For complete prevention of binder migration, the coagulation temperature must be at least 5 ∞C below the wet bulb temperature which is typically 70–80 ∞C. They describe agglomerate structure as being fine, coarse or compact; the structure is a characteristic of the binder and is only slightly influenced by the type of coagulant. The agglomerate structure of the binder can also influence fabric mechanical properties. Crosslinking Crosslinking the binder polymer can increase stiffness and waterproofness of the bonded nonwoven by providing covalent bonds between polymer chains, which reduce their mobility. The crosslinking potential of a binder system can be classified as follows: ∑ ∑ ∑ ∑

non-crosslinking crosslinkable self-crosslinking thermosetting.

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The most well-known example of crosslinking is the vulcanisation of natural latex or butadiene polymers with sulphur, an accelerator and zinc oxide. The process is complicated and the crosslinked product tends to discolour. Functional groups are introduced into the binder polymers to make selfcrosslinking systems, which are initiated by heating. Alternatively groups can be introduced which can react with a curing resin. Acrylic emulsions typically contain about 1–3% of functional groups such as amine, epoxy, carboxyl, ketone, hydroxyl and amide, associated with the copolymer backbone which react on heating to induce self-crosslinking. Crosslinkable polymers Functional monomers which contain hydroxyl or carboxyl groups can be introduced into the polymer. These can be crosslinked after impregnation using melamine formaldehyde or urea formaldehyde. Self-crosslinking polymers If N-methylol functional groups are introduced into the polymer, for example, as N-methylolacrylamide, they can react with themselves when the impregnated nonwoven is heated to form covalent bonds. The problem is that such emulsions contain free formaldehyde. This is present during the preparation, storage and use of the binder. Formaldehyde is now recognised as presenting a risk to health. As a result latex suppliers developed formaldehyde scavengers such as acetoacetamide17 and are developing formaldehyde-free binders.18

7.2.8

Formulated binder systems

The properties of binder systems are enhanced (or the cost reduced) by the addition of other materials. This is both necessary to facilitate processing and to enhance the properties of the bonded nonwoven, or to reduce cost. These additions are done just before application to the nonwoven batt, web or fabric. Examples of such auxiliaries are listed in Table 7.4. A description of the factors affecting latex stability and rheology is given by Dodge.19 When developing a new product, polymer suppliers can often provide initial formulations.

7.3

Mechanism of chemical bonding

7.3.1

Introduction

The physical properties of a bonded nonwoven, especially the strength, are determined by the fibre, the polymer, the additives and the interaction between

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Table 7.4 Auxiliary materials used in formulated binder systems Fillers

Added to reduce tackiness, cost or to reinforce; e.g., calcium carbonate and china clay (5–20% is added to reduce tackiness and 10–40% for filling purposes). Functionality of the bonded nonwoven can be improved by other filler types such as carbon black.

Flame retardants20

E.g., halogenated organics with antimony oxide, aluminium trihydrate, diammonium acid phosphate.

Antistatic agents

E.g., sodium formate.

Hydrophobic agents

Used to reduce wicking or water absorption, e.g. waxes, fluorocarbons and silicones.

Hydrophilic agents

E.g., additional surfactants (anionic and non-ionic).

Thickeners

Increase the viscosity for some processes such as knife coating or to aid foam stabilisation, e.g. polyacrylate salts, methyl cellulose or carboxymethyl cellulose.

Pigments Optical brighteners Surfactants

To improve the stability (including foams), fibre wetting and penetration into the nonwoven.

External crosslinking agents

To increase stiffness and water resistance, e.g. melamine formaldehyde.

Catalysts

To aid crosslinking.

Anti-foaming agents

E.g., silicone emulsions.

Dispersing agents

For added pigments or fillers, e.g. ammonium salts of acrylic polymers.

Other lattices

To provide additional properties, e.g. high stiffness from the use of two lattices, one to film form and one to provide high stiffness.

them – their relative spatial arrangement, surface and bulk properties. The strength of the bonded nonwoven does not derive solely from the strength of the unbonded web and the accumulated strengths of the component fibres nor the dried binder composition, but from the interaction between them. Whereas it is normal to think of adhesives bonding together two substrates, in the chemical bonding of a nonwoven fabric, there is a range of potential bonding surfaces to consider. These include: ∑ ∑

Binder polymer to fibre. Different fibres will behave differently according to their surface properties. Essentially, the binder to fibre adhesion will vary. Binder polymer to fibre finish. It is unlikely that the surface of the fibre will be free of finish or contaminants. Many surface finishes act as wetting agents for binder formulations. Some fibres have silicone

346

∑


preparations deliberately applied to inhibit wetting. Hydrophilic fibre finishes are applied to hydrophobic fibres such as polypropylene for hygiene applications, including wipes to aid wetting out and processing efficiency in hydroentanglement. A further complication is that these finishes are rarely applied uniformly. Binder polymer to added filler. Fillers such as china clay are frequently added to binders.

In some bonding situations the weight of binder in relation to the materials being bonded, is low. At high levels of binder to fibre ratio, for example 1:1, the system could be considered not as a binder sticking together the fibres to form a network but as a continuous or porous binder matrix, filled (or reinforced) with a fibrous network, and possibly an inorganic filler such as calcium carbonate or china clay. We also need to consider the cohesive properties of the binder polymer itself. Its function is not only to glue the fibres together but also to contribute to the performance of the finished product, for example, by providing toughness, stiffness or elasticity.

7.3.2

Wetting

For adhesion to occur an adhesive first needs to wet the substrate, in this case, the fibre and the binder carrier needs to spread across the surface of the fibre. This requires the fibre surface to have a higher surface energy than the binder polymer. It is important to realise that the surface of, for example, a polyester fibre will usually have an applied finish, and possibly contaminants, on it. This finish might not be continuous but present as ‘islands’. The potential bond therefore might be between binder polymer and fibre polymer, binder polymer and finish or in some cases between binder polymer and contaminants. Some fibres are treated with silicone finishes to provide water repellence or increase fabric softness and such finishes are particularly difficult to bond with existing binders. Based on the levels of binder that are most often applied, the bonded nonwoven is not a fibre-filled polymer matrix, in fact, there are large spaces between the fibres. The binder system, normally an aqueous dispersion, is free to move by capillary forces as the water evaporates. Therefore it can bridge two fibres where they come close together or touch, thus a bonded network is created and the bonded fibres are able to contribute to the overall strength of the nonwoven. The contribution that the binder makes to the overall mechanical properties of the fabric depends on: (i) binder polymer cohesion properties (i.e., bonding to itself); (ii) binder polymer adhesion properties (i.e., bonding to fibre, finish, filler, etc.) (iii) distribution of the binder and the volume of binder present in relation to the volume of fibre.

Chemical bonding

7.3.3

347

Binder polymer cohesion properties

The cohesion of a liquid is the attraction between its molecules that enables droplets and films to form. For a film to form, the polymer particles must coalesce. This happens as the carrier, water, evaporates. During evaporation, capillary forces between the emulsion particles causes them to squeeze together to form either a powder or a film. For good bonding, formation of a film is required. Smaller binder polymer particles will form a more effective film than larger particles.21 Adhesion can be defined as the intermolecular forces that hold the touching surfaces of the fibre and binder polymer together. For good adhesion, the polymer particles and carrier, i.e. water, need to wet the fibre adequately. For this to happen the binder carrier and the binder polymer need to have a lower surface energy than the fibre. Water, even at temperatures close to 100 ∞C as in a dryer, is still above the surface energy of many fibres. Therefore for wetting to occur a wetting agent usually needs to be present. Polypropylene fibres have a low surface energy (about 23 mN/m)22 and are difficult to bond. Corona and plasma treatments are sometimes used to change the chemical nature of the surface and improve wetting. Polyester fibres have a higher surface energy of about 42 mN/m. Cellulose fibres not only have a higher surface energy than both polypropylene and polyester synthetic fibres, they are also relatively porous enabling liquid to penetrate and to present a higher surface area that gives better bonding. Commercially available fibres have surface chemical finishes present. These can be left over from the fibre manufacturing process or are deliberately applied to the fibre to facilitate wetting or to modify fibre friction and electrostatic charge generation in carding prior to bonding. They can be present as continuous or discontinuous thin layers. Wetting agents are often added to aid liquid spreading. Even if the surface energy of the binder system is lower than that of the fibre, wetting can be impaired if the viscosity of the binder is high. Factors that impair film formation such as crystallinity or high Tg can hinder adhesion by reducing polymer flow.23 After the fibre surface has been wet, various interactions between the fibre and binder result in a bond. If crosslinking groups are present, further heating will increase the bonds between the binder polymer molecules and the cohesive strength of the film. Some attempts have been made to engineer binders to migrate to fibre crossover points and not coat fibres.24 Rochery et al.25 studied the interaction of fibres and binders in chemically bonded nonwovens and concluded that fibre to matrix adhesion depends on many phenomena including the fibre surface, the way the latex is processed and the choice of latex reactants.

348

7.3.4


Distribution of the binder and binder to fibre ratio

Assuming that the binder carrier wets the fibre, if there is a high density of fibre crossover points and sufficient binder, capillary forces can attract it to the crossover point to form a bond. If there are relatively few crossover points or a low level of binder, then the fibres will wet and coat with binder as the water evaporates and there is reduced opportunity for migration of the binder to a crossover point. If the binder system has been sprayed on, the spray droplets may not land near a fibre crossover point and consequently, no bonding will occur in that particular region. The physical structure of the bond points that develop therefore depends on the web structure, particularly fibre orientation and fabric density, the level of binder application, the binder flow properties and the method of application. The resulting physical structure of the bond points and their relation with surrounding fibres directly influences fabric mechanical properties including fabric bending rigidity that is related to handle. Commonly, the role of the binder is simply to bond the fibres together to achieve an increase in network strength and the binder is not intended to play a major role in dictating other properties of the final product. Nonwoven fabrics are porous materials varying in porosity from about 50% to >99% for fabric such as high loft waddings and battings. A typical polyester needlepunched fabric of 300 g/m2 might be 2 mm thick. Typically, the fibre in such a structure will occupy only about 10% of the total volume or space in the fabric; most of the product is in fact air. As the binder takes up more and more space in the fibre network, i.e. the ratio of binder to fibre is increased, the role of the latex as a binder for the fibres becomes less important. Ultimately, as the binder content increases, the nonwoven effectively becomes a fibre-reinforced polymer (FRP) composite, the properties of which are dependent on the relative proportion of fibre polymer and binder polymer present. The cohesive strength of the binder polymer then strongly influences the strength of the fabric. This situation arises in the saturation bonding of some shoe reinforcement materials. As discussed, during drying the binder system can migrate to the surfaces of the nonwoven, resulting in binder-rich surfaces and a binder-starved core. This can result in poor laminar fabric strength and is also important in the manufacture of synthetic leather where the impregnated nonwoven is split and is important that each ‘split’ has similar levels of binder. Additionally, the chemical binder concentration can vary through the fabric cross-section due to the method used to apply binder. Surface bonding and graduated binder content can be obtained depending on requirements. For certain fibres, adhesion without modification of the surface by, for example, plasma or corona treatments, is not possible. If a high level of binder polymer is used, then the fibres in the web can be in effect surrounded

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349

or encapsulated by the binder polymer without effective surface adhesion between the two.

7.4

Methods of binder application

The most common methods of applying a binder system to a drylaid web or nonwoven fabric are saturation, foam, spray and print bonding. Coating methods are also used. For wetlaid nonwovens, most of the same methods can used but bonding is applied after partial drying. For printing, the web must be dry.

7.4.1

Saturation

Saturation bonding involves the complete immersion of the web or prebonded nonwoven either in a binder bath or by flooding the nonwoven as it enters the nip of a pair of rolls. The rate at which the binder is taken up depends on its permeability and ease of wetting. Nip rolls or vacuum slots remove excess binder and regulate the applied binder concentration. This method can provide high binder to fibre levels uniformly throughout the nonwoven. Figures 7.3 and 7.4 show the basic principle of applying a binder using padding. The nonwoven is guided through the saturation bath by rollers and then passes between a pair of nip rolls to squeeze out excess liquid. Clearly, this also compresses the substrate reducing its thickness. Sometimes three rolls are used to spread the binder more evenly and give greater penetration. In some systems, the nonwoven is pressed while it is in the bath using an immersed nip. This enables air to be removed and the nonwoven to wet faster giving more even distribution. The amount of binder taken up by the nonwoven depends on its weight per unit area, the length of time in the bath, the wettability of the fibres and the nip pressure. The nip gap is usually set and maintained by applied pressure. In systems where no gap setting is required, only pressure, it is usual for one of the rolls to be rubber coated, the other usually being chrome steel. The trough and nip system is often called padding or ‘dip and squeeze’. Padders are usually either vertical or horizontal (Figs 7.3 and 7.4). In a pad machine, the binder system is usually pumped around continuously and the level and concentration kept constant. Obviously, nonwovens need to be sufficiently strong to be self-supporting when passing through the trough. Sometimes they are pre-bonded by, for example, needlepunching or thermal bonding to confer sufficient strength. It is not essential to have a bath or trough. In the horizontal flooded nip system shown in Fig. 7.5 the nonwoven passes through a pool of binder held above the rolls. Advantages over the vat method include the use of less binder and easier cleaning. Disadvantages

Handbook of nonwovens Impregnated fabric

350

Unimpregnated fabric

Binder

7.3 Horizontal padder.

include the short wetting time, which means that the method is really suitable only for lightweight highly permeable nonwovens. Other methods exist to saturate weak fabrics, for example carrying the fabric through the vat between (i) perforated screens or (ii) a perforated screen and a perforated cylinder. Saturation is not a metered system. In practice, to ensure the level of binder pick-up is correct, a few metres of saturated nonwoven is run through the machine, a sample is cut out and dried and then the pick-up level is calculated. Adjustments to the nip setting are then made to adjust the pick-up until the required level is obtained. Many of the physical properties of a saturation bonded fabric derive from the fact that all or most of the fibres are covered with a film of binder. This is particularly true of the handle and hydrophobicity or hydrophilicity, which will derive from the binder rather than the fibres.21

7.4.2

Foam bonding

In foam bonding, air is used as well as water to dilute the binder system and as the means to carry the binder to the fibres. One advantage of diluting with

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351

Impregnated fabric


Binder

7.4 Vertical padder.

air rather than water is that drying is faster and energy costs are reduced. Foam can be applied so as to remain at the surface or can be made to penetrate all the way through the fabric cross-section. Foam is generated mechanically and can be stabilised with a stabilising agent to prevent collapse during application. One or two reciprocating foam spreaders are commonly used to distribute the foam across the width of the fabric. After foam application, the substrate is passed through a nip. The foam add-on and the degree of penetration are determined by the foam density or ‘blow ratio’ and the nip setting. To minimise the amount of energy used in drying, the solids content of the binder system must be high and the foam weight low. The ratio of these two is determined by the amount of foam applied and the rate of penetration. These in turn depend on the fibre type, surface structure of the nonwoven, fibre linear density and the fabric weight per unit area. The key advantages of foam bonding are more efficient drying and the ability to control fabric softness. It is possible for the foaming process to be done in such a way that the foam structure is maintained within the foam.26 Disadvantages include the difficulty in achieving adequate foaming and in


Binder

Impregnated fabric


352

7.5 Horizontal flooded nip.

Foam or froth

Impregnated fabric

7.6 Froth padder.

controlling the process to give a uniform binder distribution. Non-stabilised foams called ‘froths’ are sometimes used. The foam is applied as in the flooded nip system or through a slot followed by a vacuum extractor (Fig. 7.6). It breaks down as it is applied and so is like saturation. As some of the ‘carrier’ is air, less drying is needed than for saturation.27 Froth application can be thought of as an alternative method of saturation bonding. The properties and uses of the fabrics are identical.21

Chemical bonding

7.4.3

353

Spray bonding

In spray bonding, binder systems are sprayed onto moving webs or prebonded nonwoven fabrics in fine droplet form. Spray bonding is used to make highly porous and bulky products such as high-loft waddings, insulation, filtration media, upholstery, absorbent and sanitary product components as well as some industrial fabrics. This is possible because the substrate does not need to pass between nip rollers. The liquid is atomised by air pressure, hydraulic pressure, or centrifugal force and is applied to the upper surfaces of the nonwoven in fine droplet form through a system of nozzles, which can be statically mounted across the machine or traverse from one side to the other. It is important that the latex has adequate shear stability. The depth of penetration of the binder into the substrate depends on the wettability of the fibres, the permeability and the amount of binder. If it is necessary to spray both sides of the substrate an additional conveyor is used, which has a second spray system. Drying is required after each spray application. The levels of binder that can be applied are typically 10–30 g/m2. If crosslinking of the binder is required, the substrate passes through a third heater. A typical spray bonding system is illustrated in Fig. 7.7. The main advantage of spray bonding is that the substrate is not compressed and the original bulk and pore structure of the incoming web or fabric is maintained. Disadvantages include lack of control over the uniformity of binder level across the surface of the nonwoven, relatively poor binder penetration, high levels of overspray and waste and the possible lack of shear stability of the binder.

7.4.4

Print bonding

Print bonding applies the binder only in predetermined areas as dictated by the pattern in the printing surface. The aim is provide adequate tensile strength Latex + air

Spray gun

Fabric To dryer

7.7 Spray bonding.

354


but to leave areas free for water absorption and permeability. By limiting the binder coverage, the handle of the fabric is also comparatively soft. Typical applications are wipes and coverstock. In wipes, the fabric may be first hydroentangled. The design of the print influences softness, liquid transport, strength and drape.8 In deciding the shape of the bonding points it is important to consider the geometry of the web in terms of fibre orientation to ensure adequate MD and CD strength is obtained. The substrate is often pre-wetted to aid printing. The two most common methods of printing used are screen printing (rotary printing) and rotogravure printing. The binder is normally thickened to a paste. In screen printing (Fig. 7.8), the binder is forced through a rotating roll that is perforated in the desired pattern. The binder is forced into the susbtrate by the pressure of the roll and the squeegee inside the roll. It is possible to impregnate both sides of the substrate with different binders by passing it between two screens rotating against each other. In a design by Stork® Brabant, the two squeegees inside the two screens are placed so that each acts as the counter pressure roller for the other screen. In rotogravure or engraved roll printing (Fig. 7.9), the binder is picked up in the grooves of the roll. The level of binder add-on depends on the engraved area, depth and level of binder solids. The excess binder is removed with a doctor blade. As the substrate passes the engraved roll, it is pressed against the surface by a counter roll, transferring the binder to the fabric. This method is suitable only for applying low levels of binder to the surface where a textile-like handle is needed. Applications include disposable clothing, coverstock and wiping cloths especially those for washing up and dusting.

Squeegee

Binder Rotary screen

7.8 Rotary screen bonding.

Rotary screen

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355

Engraved printing roll

Scraper roll

Binder

7.9 Bonding with an engraved roll.

7.4.5

Coating or scraper bonding

Another technique for applying binder at the surface is by scraper or knife coating. A scraper knife is placed above the horizontal nonwoven. The thickened binder paste (or foam) is fed upstream of the knife and forms a ‘rolling bank’ on the moving nonwoven. There are several variations of this method depending on what type of conveyor is used and the surface of the nonwoven. These are knife over air, knife over blanket and knife over roller. The degree of penetration of the binder system into the nonwoven depends on the nature of the counter surface under the nonwoven, the shape of the edge of the knife, its angle with respect to the nonwoven fabric, binder viscosity, fabric line speed and fabric wettability. In knife over air, the nonwoven is unsupported as it passes under the blade and so it is important that the nonwoven is able to withstand stretching. Although this method is often used for coating, it is rarely used for impregnation as only a low level of add-on is possible. After coating, the nonwoven passes through a nip and then to the dryer. Knife over blanket is used when an

356


intermediate level of add-on is required. The nonwoven passes over a blanket, which passes around two rollers. The method is particularly suitable for nonuniform nonwoven substrates. Knife over roller is used for relatively high levels of add-on. The substrate passes in a 90∞ pass around a roller and beneath a doctor blade. The binder is applied at the entrance to the gap. Although all of these methods are designed for coating, some binding in the depth of the nonwoven can be achieved by following the coating head with a nip to push the binder into the substrate. In reverse roll coating, the nonwoven passes between two rollers which rotate in the same direction. One applies the binder and the other provides counter-pressure. The binder add-on is determined by the gap between the rollers.

7.4.6

Solution and partial solution bonding

Solution bonding has been used for both drylaid and wetlaid webs, usually with water soluble polymers. Traditionally, to make such binders waterresistant, they are cured with melamine or urea formaldehyde. Partial solution bonding (or solvent bonding) is still used in some specific applications. A latent solvent for the fibre is first applied and is then concentrated in order to partially solvate the fibre surfaces and enable them to be fused together at their cross-over points.28 These are sometimes known as spot-welds and the process is normally initiated at elevated temperature. One of the oldest methods involved applying cyclic tetramethylene sulphone to acrylic fibres as they were fed into a carding machine followed by bonding at 115–160 ∞C. Solvent bonding of diacetate and cotton fibres using cellulose solvents has also been demonstrated. In the case of cotton, for example, interfacial bonding between fibres can be induced by zinc chloride. By applying zinc chloride to decrystallise the cellulose, subsequent washing out of the chemical leads to recrystallisation and autogeneous bonding of the fibres at the crossover points. A preferred method of applying solvents is by spraying of the web or batt prior to heating. In an alternative approach, Cerex® nylon spunlaid fabrics, originally developed by Monsanto, are autogeneously bonded using HCl gas. Solvent bonded products have been used for high loft waddings and in the case of solvent bonded diacetate for cigarette filter tips.

7.5

Drying

7.5.1

Introduction

After the binder system has been applied, the web or prebonded fabric is dried to evaporate the latex carrier (water) and allow the latex particles to bond the nonwoven. Crosslinking (external or internal) is usually carried out in the same dryer. During drying, film forming or coagulation take place as

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357

well as evaporation of the water and crosslinking (if crosslinking groups are present in the binder formulation). There are several types of dryer available of which probably the most well known are the drum dryer, flat belt dryer and stenter-based dryers. The selection of the type of dryer depends on: ∑ ∑ ∑ ∑

type and amount of bonding agent weight per unit area, strength, density and permeability of the wet nonwoven properties required in the finished product. For example, the method of drying can affect the surface finish and stiffness of the product production speed required.

The three most common drying methods used in chemical bonding rely on the following heat transfer mechanisms: convection, conduction and radiation.

7.5.2

Convection drying

In convection drying hot air is introduced to the nonwoven to heat and evaporate the water. If the nonwoven is sufficiently permeable, the hot air can be drawn through it and this is called ‘through-air’ drying. If the nonwoven is not permeable, hot air can be blown towards it from one or both sides. This is called ‘air impingement’ or ‘nozzle aeration’. In a variant of this drying method, air is directed parallel to the surface of the nonwoven. The air can be heated directly, for example, via, heat exchangers or indirectly, for example, with gas. Through-air dryers In a through-air dryer, the hot air is sucked through the material, leading to a very effective heat and mass transfer. The nonwoven is guided over a perforated conveyor surface (usually a drum or flat belt conveyor) through which heated air passes. The most common arrangement is a perforated drum and large radial fan. Air is withdrawn from the inside of the drum, heated and returned to the drum surface, this producing suction which holds the nonwoven against the drum, preventing the formation of creases. An arrangement of several drums in sequence is common. These can be arranged horizontally, or to save space, vertically. The use of several drums enables a temperature profile to be set through the dryer. For example, the first part of the arrangement might be for drying and the second part for crosslinking. As the nonwoven passes from drum to drum, the air is able to penetrate from both sides. The nonwoven travels almost tension free through the dryer. The perforated drum is designed to maximise air throughput, for example some use a honeycomb structure to maximise the permeability and open

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area. By varying the fan speed, it is possible to adjust the drying capacity of the line according to the characteristics (e.g., air permeability and weight per unit area) of the nonwoven.12 Fabric shrinkage can be achieved by overfeeding the nonwoven onto the first drum. The flow of air is designed to control the temperature at the surface to within 1 ∞C. The maximum operating temperature is typically 250 ∞C. An alternative arrangement is a conveyor dryer. This provides continuous suction through the nonwoven, unlike an array of perforated drums. The principles of through-air drying are discussed by.29,30 Air impingement dryers These dryers are also known as nozzle aeration dryers. For high density or low permeability nonwovens, including paper, air impingement dryers are used. Nozzles direct the air from one or both sides to speed up evaporation by applying turbulent airflows close to the nonwoven surface. Moist air is swept away and recirculated, with some dry air being introduced. Figures 7.10 and 7.11 show examples of single-belt and twin-belt dryers. The single-belt dryer is for drying and chemical bonding of lightweight nonwovens around 20 g/m2 and less than 3 mm thick and the double-belt system is for thicker nonwovens. The temperature of the air feeding the top and bottom can be controlled separately. A variation of the air impingement dryer is the flotation dryer (Fig. 7.12), which is often used for delicate fabric structures and is widely used for paper products. The nonwoven floats through the space between alternate nozzle arrays and very high line speeds can be achieved as compared to other systems. Figure 7.13 shows an example of a stenter dryer. The nonwoven is held at its edges by clips or pins on revolving stenter chains as it passes through a series of oven chambers. The stenter dryer can have two heater fans that can be separately controlled.

7.5.3

Conduction dryers

Conduction or contact dryers are sometimes used for thin, impermeable nonwovens because of their relatively low capital cost and high evaporative capacity. They are particularly used for nonwovens that have high steam permeability especially wet-laid webs. They usually comprise a line of revolving heated drums over which the nonwoven passes in alternate directions, giving a wrap angle that can be as high as 300∞. The surface of the nonwoven adjacent to the drum heats up and water evaporates and moves through the thickness of the nonwoven heating and evaporating in successive layers. Light nonwovens are often carried on backing felts for support. The disadvantages of contact drying compared to through-air drying include a slower heat transfer rate and an increase in the thermal insulation of the

10

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Lint filters

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Door (insulated lining)

10 Air exhaust

7.10 Single-belt dryer.

Chemical bonding

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7.11 Double-belt dryer.

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Air nozzles

7.12 Air-flotation dryer.

nonwoven as it dries.29 Sometimes a dryer is followed by a calender, hot or cold, to reduce the gauge (thickness) or smooth the surface of the impregnated nonwoven.

7.5.4

Infra-red (IR) dryers

Infra-red dryers work on the principle that water shows a marked absorption of infra-red energy, which rapidly converts into heat leading to evaporation. Infra-red dryers require low capital investment but have high running costs. They are often used in front of other dryers to pre-dry the surface. For example, they are used to prevent the first drum of a drum dryer being coated with binder and to coagulate the binder to prevent migration. They are also sometimes used after another dryer to complete crosslinking.

7.6

Applications of chemically bonded nonwovens

7.6.1

Introduction

Any new nonwoven fabric can be considered from the point of view of its architecture, i.e. its composition (fibre, binder, additives) and its structural geometry (dictated by the methods of web formation and bonding). Often more than one bonding process is used in combination. Literature covering nonwoven applications shows that although many of today’s nonwoven products had been identified some years ago, the fabric architectures have been gradually changing as new ways of improving the manufacture of nonwovens have developed. Accordingly, in many major markets dominant product architectures have not always emerged and competing manufacturers meet the same market need using different fabric structures. Examples of some of the existing nonwoven products, for which chemical bonding continues to be used are now discussed.

7.6.2

Wipes

Commercial wipes extend from light disposables including flushable products to strong, solvent-resistant, washable wipes. To provide a high volume for liquid absorption, the binder is usually applied by spray or print bonding. For

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3 Nozzle

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4 Circulating air intake chamber 5 Circulating air pressure chamber

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7.13 Stenter dryer.

10 Air exhaust


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Chemical bonding

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body care, softness and high water absorbency are needed and so absorbent fibres such as viscose rayon are used (often in a blend with polyester to improve the wet strength of the fabric) with a soft binder such as a nitrile rubber having a Tg less than –18 ∞C. These are often carboxylated so that they can be crosslinked to increase their wet strength. Acrylics are also used. An example is print coated apertured hydroentangled nonwoven fabrics, which provide softness, good wiping efficiency, absorbency and adequate wet strength. Sometimes ethylene vinyl acetate is used for disposable wipes in place of acrylics, to reduce cost. For industrial wipes crosslinkable styrene-butadiene is often used as this has good solvent resistance and handle. Ethylene vinyl acetate is also used because it is cheaper but it is not as resistant to solvents. When high resistance to solvents and oil is required, acrylonitrile is used. Flushable wipes have also been introduced, in which soluble polymer binders, for example, allow rapid breakdown of the fabric as the bond strength deteriorates in aqueous conditions. Various methods of producing flushable nonwovens rely on the appropriate choice of binder. A comprehensive list of methods to form flushable nonwovens has been given by Woodings.31 ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

hydrogen-bonded cellulose hydrogen bonded and hydroentangled cellulose man-made fibres bonded with water soluble polymers, for example starches, carboxymethyl celluloses, polyethylene oxides, polyvinyl alcohols, polyacrylates amongst others polyolefin fibres or films loaded with water soluble polymers, for example polyethylene oxide or derivatives engineered for better melt spinnability biodegradable polymers, for example PLA, blended with water soluble polymers to make fibres and fabrics fabrics made from, or pulp bonded with, water soluble fibres fabrics bonded with crosslinked water soluble polymers, for example superabsorbents in fibre or powder form fabrics bonded with bicomponent fibres having a water-soluble or hydrolysable polymer sheath fabrics bonded with soft synthetic latices, which may be incompletely cured fabrics made from cellulose/synthetic blends bonded by heat where the cellulose/synthetic thermal bond is easily disrupted by cellulose swelling laminates with water soluble films or layers bonded with water soluble adhesives thin films extruded onto flushable nonwovens, which are waterproof when the film side is wetted, but easily fragmented when both sides are wetted.

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7.6.3


Interlinings

Garment interlinings require resistance to dry cleaning, washing and yellowing, as well as a level of stiffness needed for the particular part of the garment. Acrylic binders are commonly used, for example ethyl and butyl acrylates which are self-crosslinking to give good resistance to dry cleaning. Also carboxylated butadiene methacrylates are used with a Tg of 20 ∞C and 30% butadiene. A particular example is a self-crosslinking binder based on ethyl acrylate and methacrylate; the methacrylate increases the stiffness. This has a Tg of 49 ∞C and gives a clear, colourless film. It is used for impregnating polyester nonwovens by spray or foaming. Wetlaid, carded, airlaid, hydroentangled, needled and spunbonded nonwovens have all been used in interlinings.

7.6.4

Hygiene and medical products

Many incontinence products are chemically bonded. These are often based on carded or airlaid webs and require a high rate of absorption, high capacity and some softness. Very soft grades of styrene butadiene or self-crosslinking butyl acrylates are used. Some medical products require barrier properties. Nowadays this is provided by selection of an appropriate nonwoven fabric structure, for example meltblowns and SMS composites or the insertion of continuous films. However, some products, for example head and shoe covers as well as some surgical drapes, are chemically bonded. Acrylic binders are often used as they are not degraded by sterilisation and are soft and hydrophobic. Another application is adhesive tape backings for medical applications, which are made by coating carded webs with acrylic binders.

7.6.5

Footwear

Impregnated nonwovens have been used for many years as a substitute for leather in footwear. All the leather parts of a shoe except the sole can be replaced by nonwovens, however, particular nonwoven technologies are favoured for particular components. The outer part of the shoe, the upper, is often made from microfibre nonwovens saturated with a coagulated polyurethane. These materials have excellent handle and abrasion resistance but are expensive. The toe and heel of a shoe are often stiffened by the incorporation of a nonwoven stiffener material. These are usually needlepunched fabrics saturated with carboxylated styrene butadiene. High binder to fibre levels are used to achieve high stiffness and adequate resilience. In shoemaking the stiffeners are heated in the shoe in order to shape them. The Tg of the binder has to be chosen (i) sufficiently low that it is possible to shape the stiffener in shoemaking

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without damaging the leather but (ii) sufficiently high that the stiffener does not lose its shape in the shop window or storage. Polystyrene binders are sometimes used. Toe stiffeners made with this latex can be activated for reshaping using either heat or solvent. Plasticisers or other latices have to be added to bring the film-forming temperature to below 100 ∞C. The sole and upper are attached to the insole. The two technologies used to make a suitable nonwoven insole material are (i) the manufacture of cellulose board using styrene butadiene as a binder and (ii) a polyester needlepunched fabric is saturated with a self-crosslinking styrene butadiene binder. The requirements of an insole are high perspiration absorption, stiffness and the ability to stick the upper and sole onto it. These structures are relatively porous and so provide the necessary perspiration absorption and some adhesion through mechanical bonding. The lining at the back of a high heel shoe is designed to hold the shoe on by gripping the heel. Heel linings are often polyester needlefelts saturated with nitrile butadiene rubber. This provides excellent abrasion resistance and softness. As these materials are often impregnated and dried before splitting into two, four or more ‘splits’, the latex is designed to coagulate to prevent migration.

7.6.6

Automotive

Nonwovens are used in car carpets, tray coverings, luggage compartment linings, headliners and door coverings. Needlepunched nonwovens are favoured, which are subsequently secondary bonded by chemical or thermal processes. The most common binder is styrene-butadiene although some polyvinylacetate and acrylates are used. For moulded parts to be thermoformed, the add-on is up to 60%. Saturation bonding is being replaced by foam bonding.29,30 Fibre reinforced composites consisting of glass or increasingly biocomposites incorporating bast fibre nonwovens composed of flax, jute or sisal alone or in blends are impregnated with polyester thermoset resins for use as automotive components. Door panels, parcel trays and an increasing variety of additional low-stress bearing components are being made in this way using nonwoven reinforcements. Thermoplastic PP resins are also being utilised in the manufacture of automotive components using porous drylaid and needlepunched nonwovens as the fibre component.

7.6.7

Furniture

For the bonding of fibrefill for upholstery and bedding, self-crosslinking styrene acrylic copolymers with a Tg of around 8 ∞C are used. This gives a soft, only slightly extensible and tack-free film at room temperature. It has good resistance to a wide range of chemicals and good ageing properties. It can be applied by saturation, spray or foam.

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7.6.8

Examples of other applications

Additional chemical bonding applications include the following: ∑ ∑

∑ ∑ ∑

7.7

Spray bonding of high loft airlaid batts with acrylic binders for insulation. Polyamide fibre abrasive pads for scrubbing pots, automotive and metal component finishing, etc. Batts are spray bonded with phenolic binders pre-mixed with abrasive particles of carefully selected size to prevent scratching. Cleaning cloths for the outside of aircraft consist of a needlepunched web sprayed on one side with a styrene butadiene slurry containing hydropropyl methylcellulose and nitrile rubber particles.32 Prefilters can be made from high-loft airlaid nonwovens sprayed with a crosslinkable styrene butadiene binder. Roofing membranes impregnated with bitumen can be based on needlepunched fabrics saturated with a relatively hard styrene acrylate binder.

References

1. McIntyre J E and Daniels P N, 1995, Textile Terms and Definitions, tenth edition, Manchester, The Textile Institute. 2. Pangrazi R, 1992, ‘Nonwoven bonding technologies: there’s more than one way to bond a web’, Nonwovens Industry, October, 32–34. 3. Clamen G and Dobrowolski R, ‘Acrylic Thermosets: A Safe Alternative to Formaldehyde Resins’, 2004, Nonwovens World, Vol. 13, No. 2, pp. 96–102. 4. Polymer Latex 1 refers to How do aqueous emulsions form? by Polymer Latex GmbH & Co. KG. 0/8000136/360e. 5. Polymer Latex 2 refers to How are films produced? by Polymer Latex GmbH & Co. KG. 0/8000136/366e. 6. Wang A E, Watson S L and Miller W P, ‘Fundamentals of binder chemistry’, Journal of coated fabrics, Vol.11, April, 208–225. (undated) 7. Wilson White W, 1985, ‘Functionalised styrene-butadiene latexes for non-wovens’, in Nonwovens binders: additives, chemistry and use seminar, TAPPI Notes, September 30-October 2. 8. Pangrazi R J, 1997, ‘Chemical bonding: spray, saturation, print, & foam application methods and uses’, Inda-Tec 97 Book of Papers, 6.0–6.5. 9. Pangrazi R, 1992, ‘Chemical binders for nonwovens – a primer’, INDA Journal of Nonwovens Research, Vol. 4 No. 2 Spring, 33–36. 10. Ullmann’s encyclopedia of industrial chemistry, 1996, Vol. A17 Nonwoven fabrics. 11. Morris H C and Mlynar M, 1995, ‘Chemical binders and adhesives for nonwoven fabrics’, INDA-TEC Conference 123–136. 12. Lunenschloss J and Albrecht W, 1985, Non-woven Bonded Fabrics, New York, John Wiley & Sons Inc. 13. Powell K L, 1985, ‘Waterborne epoxy resins’, in Nonwovens binders: additives, chemistry and use seminar, TAPPI Notes, September 30–October 2. 14. Mango P A, 1985, ‘Binder quality – measurement, maintenance, and effect on the

Chemical bonding

15. 16. 17. 18.

19. 20. 21. 22.

23. 24.

25.

26. 27. 28. 29. 30. 31. 32.

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end user’, in Nonwovens binders: additives, chemistry and use seminar, TAPPI Notes, September 30–October 2. Synthomer 1 refers to Heat sensitisable binders for non-woven fabrics, Synthomer Limited, Harlow, Essex. NON2 7/99. Troesch J and Hoffman G, 1975, ‘The effect of binder distribution and structure on the physical properties of nonwovens’, Paper Synthetics Conference, TAPPI, 25–35. North B and Whitley D, ‘Polymer emulsion’, European Patent Office, Pat. No. 0438284, July 1991. Schumacher K-H and Rupaner R, 1996, ‘A formaldehyde-free acrylic binder for constructing completely recyclable high performance nonwovens – reuse of fibers and binder’ Vliesstoff Nonwoven International Vol. 11 No. 6–8, 181–182. Dodge J S, 1985, ‘Colloid chemistry fundamentals of latexes’, in Nonwovens binders: additives, chemistry and use seminar, TAPPI Notes, September 30-October 2. Weil E D, 1985, ‘Flame retardants for nonwovens’, in Nonwovens binders: additives, chemistry and use seminar, TAPPI Notes, September 30-October 2. (undated) Horrocks A R and Anand S C, 2000, Handbook of Technical Textiles, Cambridge, Woodhead. Gupta B S and Whang H S, 1999, ‘Surface wetting and energy properties of cellulose acetate, polyester and polypropylene fibers’, International Nonwovens Journal, Vol. 8, No. 1 Spring. Di Stefano F V, 1985, ‘Chemical bonding of air laid webs’, Nonwovens Industry, Vol. 16, No. 6, 16 19–20, 22, 24. Blanch R M, Blanch A, Borsinger G, Lences C F and Seven M K, 2002, ‘Binders based on alpha-olefin/carboxylic acid/polyamide polymers and their ionomers’, United States Patent Application 20020077011, June. Rochery M, Fourdrin S, Lewandowski M, Ferreira M and Bourbigot S, 2002, ‘Study of fiber/binder adhesion in chemically bonded non-wovens’, 47th International SAMPE Symposium, May 12–16, 1755–1766. Parsons J, 1999, ‘Chemical binder application technology’, 30th Nonwoven Fabrics Symposium, Clemson University, 21–24 June. Mlynar M M and Sweeney E J, 1993, ‘Processing aids for resin bonded nonwoven webs’, INDA Publication, Principles of Nonwovens, 249–257. Hoyle A G, 1988, ‘Bonding as a nonwoven design tool’, TAPPI Nonwovens Conference, 5–8 April, 65–69. Watzl A, 1989, ‘The modern concept of through drying for the nonwoven and paper industries’, Nonwovens Conference, TAPPI Proceedings, 87–104. Watzl A, Production lines for nonwovens used in the automobile industry, Fleissner publication. (undated) Woodings C., (http://www.nonwoven.co.uk/reports/flushability.htm): Mann L J and Winter P M, ‘Cleaning articles and method of making’, United States Patent Application 20020173214, November 2002.

8 Nonwoven fabric finishing AI AHMED NIRI, UK

8.1

Introduction

The finishing of nonwoven fabrics is of increasing importance as producers seek to add value by increasing technical functionality, appearance or aesthetics to improve fitness for purpose. However, there are still many nonwoven products that undergo little finishing prior to the conversion and packaging stages of production. Some nonwoven finishing processes such as dyeing, padding and calendering have evolved from the traditional textile industry whereas others have their origins in paper and leather finishing. Additional procedures have developed specifically for functionalising nonwoven substrates and are rarely applied in traditional textile finishing. Accordingly, there is no standard finishing routine for nonwoven fabrics; the selection of processes and the finishing effects introduced depend on the particular end-use application. The increasing variety of both mechanical and particularly chemical finishes is providing significant new opportunities for transforming nonwoven base fabrics and widening the product offering. Traditionally, finishing is classified as either wet finishing, for example washing, chemical impregnation, dyeing and coating or dry finishing, for example calendering, embossing, emerising and microcreping. Nonwovens are also printed, flocked or combined with other fabrics, films and foils to form laminates, which combine the properties of each contributing layer. The impregnation of nonwoven fabrics with cosmetics, detergents, cleaning agents, medicaments and other lotions is a major activity, particularly in the hygiene and medical industries, and is frequenty undertaken as part of converting prior to packaging. Nonwovens are increasingly designed as applicators or delivery vehicles for chemical compositions. Depending on the finish required continuous processing is possible or a separate batch operation may be adopted. A general point regarding all wet finishing operations and dry finishing of nonwovens is the need to minimise and control operating tensions on the fabric where possible. Nonwoven fabrics, particularly when water laden or when hot in the case of thermoplastics, are extensible and are 368


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easily stretched and deformed. The fabrics tend to have comparatively poor elastic recovery and there is a risk of permanent elongation under load. If elongation cannot be recovered at a later stage of processing, for example during drying or compacting, fabric weight and density variations can arise. A further complication is that the tensile properties of nonwoven fabrics are usually anisotropic and they have high shear rigidity, which is often nearer to paper than to conventional textile materials.

8.2

Wet finishing

8.2.1

Washing (scouring)

Washing is one of the first wet processes applied to fabrics particularly where aqueous dyeing or a chemical finish is to follow. Applications include the manufacture of certain nonwoven linings such as those used in shoes that are subsequently dyed and filtration fabrics used by the food industry. The fabric is treated in an aqueous media, usually containing a detergent. Synthetic detergents have largely replaced natural soaps and non-ionics are widely used. Non-ionic detergents have good fibre compatibility and stability to variations in water supply. They are particularly stable in hard water. Nonionics are effective de-greasing agents, which can adversely affect fabric softness and anionics are selected if improved handle is required. Synthetic detergents are easily rinsed from the fabric, are relatively cheap and are supplied as liquids for ease of use. Washing can have a softening effect as contaminants are removed and strain induced during fabric formation is relaxed. Temperature, processing time, mechanical action and the addition of detergent are key factors influencing washing efficiency. The detergent creates a separating layer between the fibre and the contaminant, coating the contaminant with detergent. The contaminant collects into a globule, which detaches from the fibre. Ionic charges help prevent re-deposition of contaminants onto the fabric; the contaminant held in a scouring emulsion is usually stabilised by the presence of alkali. Removal of contaminants is also facilitated by mechanical action such as squeeze rollers, which mechanically separate the contaminants from the fabric. Washing is followed by rinsing, with attention to gradual dilution of the washing emulsion, to maintain emulsion stability. Washing machines are normally open width and continuous. The fabric may pass around guide rollers and through a series of vats containing scouring and rinsing liquors. The fabric is squeezed at intervals to promote a scouring and liquor interchange minimising transfer of liquor from the scouring to the rinsing baths. Other designs involve the use of spray bars or tangential jets, which impinge the fabric with liquor across the width of the fabric. Suction, introduced by vacuum slots, is sometimes used instead of nip rollers or in the

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scouring bath itself to transfer liquor through the fabric to promote scouring. However, even with driven rollers to promote fabric overfeed and reduce tension, these types of machine may impose too much strain for many nonwoven materials, particularly light-weights. Another favoured design includes the use of perforated drums, around which the fabric passes. Tension is minimised during transport, as the fabric is overfed onto the drums. Cleaning is achieved by sucking the washing and rinsing liquors through the drum. Washing machines are usually modular in construction and processing tensions can be further reduced by the use of relaxation zones between the scouring modules. Minimum liquor scouring reduces water use (and concentrates detergent action) and rinsing liquors are often recycled to the initial scour baths to further reduce water use, effluent waste and costs. Nonwoven fabrics composed of dope dyed fibre or bonded by hydroentanglement are normally sufficiently clean and may not require further wet processing. It is known that hydroentanglement is capable of removing a large proportion of the wax on cotton even at low specific energy inputs. Similarly, fibre finish is stripped from the fibre necessitating low foam finishes to be applied to the fibres prior to the process. Batch scouring, for example rope scouring, is not suitable for nonwoven fabrics because of the high propensity for creasing and the reduced processing loads. Solvent scouring is increasingly used in some areas of the industry, particularly for removing oil, waxes or spin finishes and contaminants that are not removed successfully or adequately by aqueous scouring. Clean fabrics reduce emissions at procedures such as heat setting. Machinery improvements have seen totally enclosed systems, which limit perchlorethylene emissions to reduced levels, within legislative requirements.

8.2.2

Coloration

Coloration is undertaken with either dyestuffs or pigments. For nonwoven materials, fabric coloration is performed in open width to avoid the creasing that results in rope dyeing. Several coloration methods are available, principally: ∑

∑

In dope dyeing (producer coloration or melt dyeing) the dye or pigments are added to the molten polymer (spinning dope) prior to melt extrusion. Dope dyeing has the disadvantage that colour commitment is made at an early stage but excellent colour fastness can be achieved. Pigments have little fibre substantivity (or solubility) and are applied along with a suitable binder resin. Pigments are often applied by printing onto the nonwoven fabric along with a binder or added to the binder resin in chemical bonding processes. The bonding agent fixes the pigment to the fibre surfaces during drying and thermal curing. Pigments must be finely ground and sufficiently well dispersed in the binder dispersion,


∑

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with even application of the binder dispersion to the web to achieve a uniform shade. The use of pigments when correctly bound to the fibre offers good non-fading properties and fastness to perspiration but wet and dry rub and crock fastness may be unacceptable. Work is continuing to develop softer binder resins to minimise deleterious effects on handle. Coloration with conventional dyestuffs can be applied to the nonwoven fabric, using either batch or continuous systems. Consideration must be given to the type of nonwoven fabric to be dyed and the nature of the fibre composition. Single fibre types dyeable with one dyestuff type is relatively straightforward, but blends present some difficulties particularly if the fibres cannot be dyed with the same type of dyestuff. Nonwoven fabrics composed of conventional fibres tend to dye to a deeper shade than woven or knitted fabrics of the same composition and have a greater accessible fibre surface area because of the high permeability and absence of twisted yarns and yarn intersections in the fabric structure.

Typical auxiliaries for coloration processes include wetting and penetrating agents, levelling and anti-frosting agents, blocking agents if applicable in the case of blends, dispersing agents, chemicals to adjust pH and pH buffers, anti-foaming agents, swelling and fixation chemicals (mainly pad batch). Adequate dyestuff fixation (fastness), dyestuff levelness (throughout the fabric and edge to edge) and shade reproducibility is achieved by optimising and reproducing procedures. Modern exhaust dyeing machines enable dyeing cycles to be controlled and reproduced batch to batch. Optimised flow rates in, for example, beam dyeing, ensure dye levelness with minimal fibre damage, thus improving fabric quality. In general, heavy or high-loft fabrics are dyed continuously, as batch dyeing systems such as beams allow small loads, which is economically unfavourable and increases the possibility of shade variation. Additionally, the lateral pressure during batch processing would reduce fabric bulk. Lightweight nonwoven fabrics, however, are dyed on batch type machinery. In batch exhaust dyeing, the fabrics are loaded into the system, followed by an initial treatment in the auxiliary chemicals. During this time adequate wetting of the fabric is achieved, the auxiliary chemicals are uniformly applied and the required pH is achieved. The dyestuffs (pre-dissolved) are then added and the dyes are gradually exhausted from the dye bath and fixed to the fibre, by controlling pH, time, temperature and chemical addition. In beam dyeing, fabrics are initially loaded under controlled batching tension onto a perforated beam (the perforations enable forced liquor circulation through the fabric). The edges are aligned so that the edges of the fabric build uniformly. It is usual practice to pre-wrap the beam with a permeable wrapper fabric to prevent marking of the beam perforations. If the beam perforations are not totally covered by the fabric or wrapper they should be blanked off to prevent channelling during dyeing. The prepared beam is placed into a chamber,

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which is sealed, allowing high-temperature pressurised dyeing on synthetic fibres. One advantage of dyeing on the beam is that at elevated temperatures there is a thermal stabilising effect, which imparts additional dimensional stability and fabric surface integrity. Dye flow directions and time influence dye levelness and must be adjusted to prevent the possibility of moiré effects. A beam dyeing system is illustrated in Fig. 8.1. Liquor flow in the beam (generally in-to-out and out-to-in flow directions are available) can lead to flattening of low density nonwovens, if not lateral collapse of the nonwoven structure. In jig dyeing, the fabric moves through the dye liquor as it passes between one roll and a second roll and reverse, when the receiving roll has filled its fabric capacity. Bowed centring bars ensure straight running of the fabric (some systems are fitted with a stagger to ensure even build up on bulky fabric edges) with immersion bars as the fabric passes through the liquor trough. Movement of the fabric aids levelling during the dyeing and fixation period. Jigs are available as atmospheric, i.e., operating around 98–100 ∞C or as HT pressurised jigs operating at high temperature with liquor to goods ratios less than 10:1 (beam dyeing requires similarly low liquor ratios). Low liquor ratios allow lower effluent but also reduced processing costs and chemicals. Sophisticated control panels display and control functions such as temperature, water levels, chemical additions and operating programs (including troubleshooting). Atmospheric jigs are suitable for most natural fibre types and generally require carriers to dye fabrics such as polyester. Heating is preferably by steam coils mounted in the trough and hood of the machine to give accurate temperature control. Live steam is also possible but this can dilute the liquor ratio.

8.1 HT Beam dyeing machine (courtesy of works photographs Thies GmbH & Co. KG).


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High temperature (HT) pressure jigs are enclosed systems allowing dyeing temperatures of around 140 ∞C. This offers the possibility of reduced dyeing times and eliminates the need for carriers. Both beam and jig techniques provide open width dyeing avoiding the problems of creases and processing marks which tend to occur if nonwovens are dyed in traditional batch rope type dyeing systems and winches. Some comment should also be made about the thermoplastic binders present in certain nonwoven fabrics. Clearly, the binder should be distributed regularly, should not soften or flow during dyeing at elevated temperature and should also possess similar affinity to the dye to enable level dyeing and fastness. Binders can vary in crystallinity compared to nonwoven fibre and this will yield unlevel dyeing. In jig dyeing, it has been found that softening of bonding agents can lead to adherence of separate layers of nonwoven fabric, which disturbs the fabric surface and makes controlled rolling and unrolling problematic. Other coloration procedures relevant to nonwoven fabrics include pad application (the padding procedure is discussed later) of dye to the fabric. Two methods are used: ∑

∑

Cold pad batch dyeing, which involves padding (immersion of the nonwoven in dye liquor followed by squeezing off the excess liquor) the fabric with dyestuff and auxiliary chemicals, followed by batching the fabric on a roll for a predetermined time to allow dyestuff fixation. The fabric batch is usually covered with impermeable sheeting to avoid drying (which reduces dye fixation) and the batch rotated during the fixation process to avoid seepage. Depending upon the dyestuff used it is also possible to heat the batch. After an appropriate time, the nonwoven fabric is open width washed to remove unfixed dyestuff and auxiliary chemicals. This procedure uses little energy for dyestuff fixation but does require batching equipment and batching space. It is mainly applicable to polyamide types. Continuous pad – steam dyeing in which the nonwoven fabric, particularly heavy or high bulk fabrics, are initially padded and then steamed or subjected to thermofixation, followed by open width washing and drying.

In chemical bonding installations, combined bonding and coloration of nonwovens is possible, for example, in foam bonding operations, where both the binder and pigment are dispersed in the foam before application to the web.

8.2.3

Printing

Nonwoven fabrics are printed for many applications, particularly in the home furnishing area including wall and floor coverings as well as tablecloths. Print bonding, in which a pigment is applied at the same time as the binder,

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is also important in the manufacture of household wipes to increase wet strength, modify appearance and to control the wet pick up of the wipe in use. Flat screen or rotary screen techniques offer wide colour and design range possibilities. The nonwoven fabric is fed continuously along the print table by a moving belt and passes either continuously (in the case of rotary printing) or intermittently (in the case of flat screen printing) below the print screen. A series of screens are used to build up the print design. Adhesive is applied to the belt to keep the fabric in position. This is important to prevent the fabric from lifting during printing, which would affect print definition. A printing paste consisting of pigment or dyestuff (together with chemical auxiliaries used in printing paste) is added (metered) to the print screen and colour applied by a blade, squeegee or magnetic rod to specific areas, as determined by the print design. Rotary and flat screen printing (rotary predominantly) remain the most important methods for printing high volumes of fabrics. Improvements to these methods include reduced set up times, increased printing speeds, low tension fabric feed, precise screen location, minimum changeover times between patterns, absolute control of squeegee pressures and evenness and individual drive of printing positions for increased precision. Online print monitoring systems located after the last screen detect and report any repetitive printing faults as they occur during rotary screen printing processes. Faults such as misfits, lint, miss colour and screen blockages can be instrumentally monitored for quality control allowing modifications to be made to reduce waste. In the application of dyestuffs thickeners include alginates, guar gum derivatives and synthetic thickeners. Thixotropic thickeners are also used which shear due to the mechanical action of the squeegee. The viscosity (dictacted by the thickener type and concentration) of the print paste is important to ensure the required definition and clarity of the printed area. Paste penetration into the fabric is important for some dyestuffs and fibre types to achieve adequate fixation of the dye and solidarity/depth of shade. Prints with poor paste penetration can have poor mechanical properties, for example, low crock (rub) fastness. Moreover, the thickening agent must be stable and maintain print definition during subsequent fixation treatments, for example steaming. Any breakdown results in flushing of the print and loss of print definition. Other additives to the print paste include wetting and levelling agents, fixing agents, anti-foam, humectants, sequestrants and antioxidants. Pigments are applied along with a suitable binder. Auxiliary chemicals include a thickener (to improve the rheology of the paste), a cross linking catalyst for the binder and softeners. The pigment is applied together with the binder in the print-bonding process. After printing and drying, the fabric is baked to cure the binder and fix the pigment. Washing is not always


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necessary but pigment printed fabrics can have a firm handle in the printed areas and may not be suitable for large areas of print. Concurrent with fixing the pigment to the fibre, binders have a secondary effect on the bonding of the nonwoven which can enhance fabric dimensional stability. Discharge prints allow the production of light colours on predominantly dark backgrounds using suitable dyestuffs. Generally, the fabric is initially dyed to the required dark shade and then overprinted with a paste containing a discharging agent to discharge some/all of the base dyed colour. The discharging paste can contain dyes stable to the discharging agent, so that as the base colour is discharged it is replaced by the stable applied dyestuff. Light and bright shades can be produced on dark grounds with sharp edge and good print definition. This can be difficult to achieve if printing light and dark shades in intricate close proximity designs (print registration) using conventional techniques. Care must be taken to optimise the discharging procedure as the discharging agents and procedures can affect fibre properties. Other print procedures include engraved roller printing and sublimation transfer techniques. With the latter, dyestuffs are transferred by sublimation from a release paper and pre-printed with the design and appropriate dyestuffs onto the nonwoven fabric. The release paper and fabric are brought together and passed around a heated drum, the design being transferred by a combination of heat and pressure. Polyester fabrics are most suitable for printing with sublimable disperse type dyestuffs. Sublimation temperatures of around 200 ∞C are used and it is important that any web resin binders are stable to the application temperature. Disperse dyestuffs sublime at different rates and penetration depending upon molecular weight and processing times reflect this. Digital ink-jet printing enables intricate computer design patterns to be transferred onto nonwoven fabric substrates. Initially developed for pattern work and shorter sampling times, full production type machinery is being developed. The print design is generated using computer software with output to a wide bed printer of sufficient width to print directly onto fabric. There is no need for roll or screen engraving and use, as with conventional printing. Various systems are available, one example is the DOD system which uses thermal or bubble-jet technology whereby the print head ejects a drop of dye at high temperature. Dyestuffs (printing inks) include reactive and acid types, disperse and pigments. Production rates are at present limited, but advances continue in print head mechanisms and print speeds, pre-treatment and aftertreatment processes, dyestuffs with improved flow and fastness properties, etc., and CAD/software systems for fabric design and reproducibility. The high porosity and variable surface structure of many nonwoven fabrics necessitates their pre-coating or thermal calendering to provide a more suitable surface for ink-jet printing.

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8.3


Application of chemical finishes

The application of topical finishes and treatments to nonwovens is a primary means of functionalising fabrics but not all these finishes are durable to wet treatment and other agencies of wear. It should be noted that masterbatch additives are now available that enable a range of functionalities to be integrated into the fibre or filament prior to web conversion, bonding and finishing. Masterbatch additives include UV absorbers, UV stabilisers and UV filters, migratory, conductive and permanent anti-stats, antimicrobials and even fragrances of different types.

8.3.1

Types of chemical finishes

A wide variety of chemical finishes are applied to nonwoven fabrics; some are dealt with below. Antistatic agents Nonwoven fabrics, particularly those composed of synthetic fibres, may be liable to clinging and the electrostatic attraction of airborne soil and dirt. Soiling is important in areas such as home furnishings including needlepunched floorcoverings. The mode of operation of antistatic agents vary. Some work by increasing the fibre conductivity by applying hydrophilic compounds to the surface, others impart a charge opposite to that normally generated, neutralising the static build up. As with all the chemical finishes, durable, semi-durable and non-durable products are available from chemical manufacturers for specific fibre types and end uses. Antimicrobial or biocidal finishes These are applied where protection is required from biological degradation resulting from the growth of undesirable organisms such as bacteria and fungi. Applications include, sportswear, insulating materials, mattress ticking and bedding components, domestic furnishings, floor and wall coverings, hygiene, woundcare and healthcare products. In the case of dust mites, the products of which are a common irritant for asthma sufferers, antibacterial finishes are designed to reduce the bacterial/fungal growth on which the dust mites feed. Anti-microbial finishes also help to prevent physical degradation caused by microbial activity (e.g., mildew) or reduce the odour emission associated with the degradation by microbial attack of perspiration. Proprietary compounds, metallic compounds containing silver or natural biopolymers such as chitosan (which can be derived from crab shells) are important in this field. Aqueous dispersions of chlorinated phenoxy-compounds containing


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pyrithione are effective against bacteria, algae, yeast and fungi and can be applied to fabrics other than those composed of polypropylene. Liquid formulations of non-ionic AOX free formulations containing isothiazolinones applied to nonwovens are effective against bacteria, fungi and yeast and have good wash stability. Padding, spraying or aqeuous coatings and foam application are utilised. Lubricants (or slip agents) Lubricants are applied to impart softness to certain nonwoven fabrics but more particularly to reduce fibre to fibre and fibre to metal friction. This is particularly important for procedures such as sewing (for interlinings) where high velocity needles penetrate the nonwoven fabric and a lubricant is needed to reduce frictional heating and the associated mechanical damage. Flameproof finishes These are designed to reduced flame propagation, afterglow and charring and the suppression of smoke emission. More recent finishes for cellulosic fabrics are based on nitrogen-phosphorous compounds, often applied with hygroscopic auxiliaries which also act to suppress flammability. Phosphorous organic compounds form the basis of the Proban R process which utilises an ammonia after cure to cross link the applied polymer in the core of the fabric. Many proprietary flameproof and smoke depressant formulations are available for a variety of fibre types and are usually applied with binders or catalysts for durability. In automotive fabrics, back coating of organic phosphorous salts is used to impart flame retardancy in combination with a polymer dispersion coating. In fabrics intended for use by the construction industry, inorganic mineral fillers act as flame retardants and can be padded or coated in conjunction with a polymer dispersion coating. Problems with flameproof finishes can include yellowing of the fabric, decreased tensile strength, and colour change. Waterproof finishes These are commonly based on silicone or fluorocarbon compounds applied as an aqueous dispersion by padding, extraction, spraying or foam and for nonwovens anionic compounds are preferred. Fluorocarbons in particular are claimed to produce a low surface tension, which inhibits wetting. They are used not only for water repellency but also oil, diesel and gasoline repellency on glass and synthetic fibres. Compared to traditional wax finishes, breathability and fabric aesthetics are largely maintained. Fluorocarbon finishes often

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require re-proofing after laundering and this is usually accomplished by pressing, to reactivate the finish. Softeners In addition to protective garments and clothing, softness is of importance in hygiene products such as sanitary coverstocks and wipes but the chemical composition is an important consideration in skin contact applications. Hydrophilic softeners have the additional effect of increasing wettability so that liquids can be distributed away from areas of high liquid concentration. Hydrophilic finishes are often known as rewetters. Stiffeners Stiffening agents and fillers are applied to add weight, firmness and bulk to fabrics or binder chemicals to add strength and abrasion resistance by spot welding of adjacent fibres. Spot welding also gives some control over dimensional stability. Polymer dispersions are applied by padding, slop padding, foam, spray and knife coating. For example, micro-dispersed anionic polystyrene copolymer is applied to needlepunched carpet fabrics as a stiffener. Self-crosslinking anionic acrylate polymers are applied to needlepunched fabrics and other nonwovens to improve dimensional stability and wash resistance. Glass fabrics and polyester spunbonds are sometimes finished with self-crosslinking anionic polystyrene acrylate dispersions and have excellent thermal resistance and dimensional stability. Thermoplastic binders are important in the manufacture of nonwoven fabrics intended for moulding operations. Other binding chemical finishes include anti-dust finishes, which bind and depress dust in applications such as mats. UV stabilisers UV (ultraviolet) light stabilisers protect polymers and adhesives from photodegradation. This may be apparent by discoloration or even chemical breakdown, resulting in a loss of polymer properties. In the case of adhesives, even if shielded between fabric layers, edges exposed to light can degrade with loss of adhesive properties. UV protection may be achieved by use of UV absorbers, which essentially absorb harmful UV radiation and protect the polymer and hindered amine stabilisers, which do not absorb UV but enable a complex reaction, protecting the polymer from chemical breakdown.

8.3.2

Methods of applying chemical finishes

There are various procedures available to apply chemical finishes. Padding and coating techniques are commonly used.


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Padding Padding involves impregnating the fabric with treatment liquor or foam by mechanically forcing it into the fabric with the aid of squeeze rollers. The fabric is squeezed to remove excess liquid to a pre-determined pick-up or add-on. Padding is suitable for fabrics containing fibres that have little substantively for the treatment chemical which could not be exhausted onto the fabric from long liquor. If the pick-up is controlled, the level of chemical add-on can be determined. Depending upon the application and fibre type, it is usual practice to aim for pick-ups below saturation levels. This is mainly because as well as treating the fibre, the interstices between fibres hold excess liquor, which is not cost-effective. During drying, excessive water removal can result in unwanted chemical migration. The situation is further complicated by wet-on-dry or wet-on-wet padding. In wet-on-dry padding, the fabric is saturated with pad liquor (in most cases unevenly at this stage). Little time is available to expel air from the fabric and replace it with treatment liquor to achieve sufficient wetting and a uniform application, during the padding process. Wetting and de-aerating or de-foaming agents can aid this process. For wet-on-wet padding the fabric is already wetted but during the process there must be sufficient interchange of liquor to permit replacement of wetting water by the treatment liquor to achieve the required chemical add-on. As with similar processes where roller nip pressure is involved (e.g., pad dyeing), uniform application of the treatment edge to edge and throughout the fabric is of paramount importance. Much work has been done regarding roller compositions and also design, for example cambered rollers, which bend when load is applied to give uniform squeezing pressure at the ends, are well known. Other sophisticated systems exist that rely on self-regulating internal pressure rolls to achieve uniform line pressure. The required application level (for wet on dry padding) can be calculated as follows.

treatment level (%) ¥

g product /litre 1000 = liquor pick up % of pad solution

8.1

Therefore, for a treatment level of 1.5% at a pad liquor pick up of 80%

1.5 ¥ 1000 = 18.8 g/l product required per litre of pad solution. 80 8.2 The pick-up can be established with a short piece of fabric, noting the pressure settings and ensuring the pick-up is even across the width of the padding nip. It is only beneficial to store excess pad liquor providing the chemical being applied has adequate shelf life. Attention should also be paid to processing

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colours; any loose dye from a previous batch will readily contaminate further batches. The pad should be thoroughly cleaned down, particularly if different treatments are to be carried out in the same pad. Padding is usually followed directly by batching, drying or curing as necessary. In another application, a foamed binder is applied to both sides of a nonwoven carrier fabric before drying and/or curing to produce cleaning cloths. Coating Coating is particularly important in the application of chemical finishes (or coating preparations) to both single use and durable nonwoven fabrics including, for example, wipes and interlinings. Coatings are generally aqueous based and may be in the form of solutions or dispersions. In nonwoven coating control of fabric, let-off is important in the minimisation of processing tensions to avoid undue stretching of the fabric. Weft straightening devices, magic eye centring and edge uncurling devices are also used. Aqueous coating lines are followed by hot air drying and curing, usually in a stenter if constant width control is required, or cans or through-air drum dryers. Economically, it is preferable to apply a coating application in one pass, providing a uniform and correct thickness of application can be achieved. However, depending on the application, it is normally necessary to build up the coating by more than one application. In this way, any defects such as holes or gaps caused by solvent evaporation from the coating surface can be avoided. However, successive layers must effectively adhere. A widely used procedure is application by rotating roller directly onto the nonwoven fabric, known as slop padding or kiss roll. Usually the slop padding roller is loaded directly with the preparation to be applied, for example partially immersed in the coating preparation (float). To achieve controlled pick-ups and also to control penetration of the coating preparation, it is possible to vary roller speed and also roller direction. These are modified to complement the solids content and viscosity of the application. Generally, roller direction gives greater penetration of the coating into the fabric. Excess is removed by a scraper and end plates act as dams. Helically wound wire metering rods (known as Meyer bars) can also be used. An excess of coating application is applied to the fabric and the application is controlled by the bar profile, as surplus is scraped off the fabric surface. Different diameter and profile bars are available to control the level of pick-up. In reverse roll coating the coating preparation is metered into the nip between two rotating rollers, a metering roller and an application roller. The nip or gap setting controls the level of application, which is then coated onto the fabric as it passes around a support roller and between the application and support rollers. A further technique involves passing the fabric over two rollers set a predetermined distance apart, between which a knife or doctor


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blade is located so as to press onto the fabric. As the fabric passes over the rollers the knife evenly spreads the coating (spreading) over the surface; application width is determined by the width of the knife used. The coating application is usually applied directly to the depression in the fabric and plates stop leakage at the edges. This method is mainly applicable to high viscosity coating applications or foams. The knife is carried in a rigid support, adjustable parameters being knife height and angle of application. Increasing tension on the fabric, regulated by the let off roller, allows knife pressure on the fabric to be modified. As tension is applied, this procedure requires fabrics of high dimensional stability. Several knife designs are available, depending upon the amount, thickness and penetration of coating application required. In general, knife designs with sharp edges leave a thin film whereas more rounded knives leave a heavier coating and slightly deeper penetration. The Zimmer Magnoroll/Magnoknife (Fig. 8.2) is a versatile system, suitable for decorative patterning and printing, finishing and coating, paste dot applications, and adhesive application. The profiled knife is made from magnetisable steel and the pressure it applies to the fabric is adjusted by means of magnetic force, which controls the application level. The magnet can be placed at different positions (Magnetsystem Plus) and is capable of applying coating weights of 40–600 g/m2. Alternatively, the system operates with a roller as the squeegee. In knife over roller systems the web or fabric passes through a gap between a knife and a support roller. Excess coating is scraped away and the procedure is useful for high viscosity coatings and high coating weights. Air knife coating utilises a powerful air jet from the air knife to remove excess preparation. Gravure coating replaces the cylindrical roller immersed in the coating

8.2 Zimmer Magnoroll/Magnoknife (adapted from Zimmer).

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preparation, by an engraved roller. Excess pick up is removed by a doctor blade, leaving a coating preparation held in engraved dots or lines. The coating preparation is transferred to the fabric as it passes through the nip between the engraved roller and a top pressure roller. Offset gravure involves depositing the application onto an intermediate roller before transferring to the substrate. Applying a coating application in discrete areas helps to reduce fabric stiffening in areas where handle and drape are considered important factors. Alternatively, coating preparations may be applied via rotary screens applying the preparation to discrete areas or overall as the preparation is extruded through holes in the screen. Screen density allows for varied application levels and the process is suitable for fabrics such as nonwovens that have uneven surfaces where knifes may cause uneven or streaky application. Hot melt coating (Fig. 8.3) is important, for example, in the application of adhesives and bonding agents. The use of melt adhesives requires only cooling zones (drying tunnels are not required) to cool the coating with little if any solvent removal. Consequently, coating speeds are not limited by drying Roller

Knife Coated or laminated fabric

Hot melt

Dosing roller

Rubber-coated counter-pressure roller

Tension roller

Fabric feed

8.3 Lacom hot melt coating/laminating (courtesy of Lacom).


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capacities. Thermoplastic melt adhesives include copolyamides, polyesters, copolyesters, polyurethanes, polyurethane, coPVCs and EVA (ethylene vinyl acetate) polymers. In the Lacom system, the thermoplastic coating polymer is melted and then metered to the coating machine for application to the fabric. This operation is linked to pressure rollers to produce the final laminate. Extrusion (slot or slot die) coating is a technique where a hot melt polymer is extruded through a slot at an angle to the substrate, the speed of the fabric controls the thickness of the applied coating. Polyethylene (usually LDPE) is commonly used for economic reasons and provides a waterresistant barrier and good heat-sealing properties, ideal for food and packaging applications. Calenders ensure good coating and adhesive contact with the substrate. In extrusion laminating, the coating acts as an adhesive between two (or more) substrates. The second layer is applied whilst the extrusion coating is hot and the layers are combined by pressure rolls. The adhesion between substrate and polymer can be improved by electrostatic corona pre-treatment, preheating of the fabric (especially high density fabrics) or co-extrusion of a tie layer. In transfer coating, application of the coating is made indirectly to the nonwoven fabric via a release carrier material in a similar manner to transfer dyeing. The coating is applied initially to a release carrier such as siliconised paper and then transferred to the nonwoven fabric at elevated heat and pressure. The coating must have sufficient adhesion to the nonwoven fabric to create a continuous film on the surface. A claimed advantage of this approach is that a smooth film of uniform thickness can be formed on the release paper prior to transfer. In powder dot coating, heat activated thermo-fusible dry powders based on polyamides, polyesters and modified ethylene compounds are applied in coating and lamination processes. Applications include clothing interlinings, seat padding and door interiors for automotive fabrics and shoe linings amongst others. The powders are normally applied by printing with either a rotary screen or direct contact using a heated engraved roller to give uniform coatings at a high coating thickness. In the Caviflex system (Fig. 8.4) screen, gravure and slot die coating are possible. Scatter (or powder) coating generally applies a random distribution of powder to the fabric. The powder is fed from hoppers that have moveable sidewalls to match the width of the fabric being processed. The powder may be dispensed by needle cylinders to meter the flow and by means of a sweeping blade. Applications include carpet backing and upholstery fabrics. Paste dot relies on a water based system in which the fusing powder is dispersed. This technique is most suited to lightweight fabrics, particularly linings and interlinings, which require coating at a low temperature to avoid fabric damage. Minimisation of drying costs can be achieved using foam applicators. Auxiliary foaming agents and stabilisers are added to the coating

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8.4 Caviflex principle with exchangeable coating modules (courtesy of Cavitec).

preparation and this is pre-foamed with air in stator mixers. The foam is piped onto and across the fabric and spread evenly and uniformly to the required add-on by a knife or roller or alternatively, application can be by rotary screen. The process is used to apply, for example, resin finishes, softeners and functional finishes. Coating uniformity depends on foam size and uniformity, stability (life) of the foam bubble and application rates. Drying to low residual moisture content is usually followed by calender pressing, which is an important step to ensure that the foam does not bulk again, (which can reduce the abrasion resistance of the foam) during curing. Advantages of foam application include low wet pick up (minimum solvent application) allowing for reduced drying and chemical costs and reduced emissions. In the Zimmer Variopress system the amount of foam applied and penetration into the fabric is determined by the speed of the toothed gears delivering from a foam bath. Applications can be direct with various nozzle shapes according to application in various configurations including floating knife or knife over roll. Pressurised foam application systems have emerged in which a ‘closed’ controlled pressure application is used. Penetration of foam is largely due to pressure within the system, and is known as the blow ratio (ratio between the liquid and air feeds). Application is via a coating slot, the aim being confined pressurised foam distribution across the width of the fabric. In this procedure fabric surface topography is claimed to be less critical in terms of achieving a uniform application. Two-sided applications are also possible.


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In solvent coating, the coating polymer is mixed with a suitable solvent providing a coating of suitable viscosity for application to the fabric. Generally, less heat is required to remove the solvent during drying when compared to aqueous systems but the emission of solvents or VOC (volatile organic compound) is increasingly scrutinised and emissions should be controlled or neutralised. Water-based solvents are less problematic in this respect but require more energy to dry.

8.4

Lamination

Joining of two (or more) pre-formed nonwoven fabrics or alternatively, nonwoven fabrics with other roll products such as films, scrims and textile materials, is important in the nonwovens industry and is frequently associated with moulding processes. The lamination of films to nonwovens, for example, is often intended to modify barrier properties (including liquids, particles and microbes and pathogens), permeability (gases, liquids and biological fluids), surface properties (abrasives, friction, appearance), dimensional stability and other mechanical properties (including elasticity, modulus and bending stiffness). Commercially, there are a large number of products in most sectors of the nonwoven industry that consist of nonwoven laminates. For the laminate to become permanently joined, either one or both of the preformed fabrics must have adhesive properties or an adhesive resin must be applied to one or both surfaces or an ‘intermediate’ adhesive scrim placed between adjacent surfaces must be used. Bonding is achieved by the application of heat and pressure. Lamination can be wet or dry. In wet lamination the adhesives are applied from a solvent or water dispersion and the adhesive is commonly applied to one substrate. Application is by spraying, slop padding, knife coating or spreading and printing, depending on the solvent system used, application level and surface penetration required. Concerning handle (for both wet and dry lamination) application of the adhesive in discrete, localised or point areas by processes such as printing produces comparatively good softness and drape in laminated fabrics. However, such localised application can lead to differential shrinkage between the laminated layers during laundering or subsequent wet treatments. Dry lamination uses thermoplastic resins including powders and melt adhesives composed of polyesters, polyamides and co-polymers (coPET), polyolefins, polyurethanes or scrims made from thermoplastic filaments or fibres that are placed between the two substrates to be joined. Certain copolymers will fuse below 100 ∞C. According to the Tg, polymers can be hard and tough or flexible, soft and extensible, depending on end use; the molten viscosity and flow characteristics are important influences in bonding. In the case of polymers little solvent, if any, is present for evaporation and so

386


a high solids bonding medium with little penetration into the fabric is achieved. Interleaving scrims or adhesive webs for lamination are usually lightweight spunbond or meltblown materials composed of extruded low melt polymers. Despite being lightweight they must have acceptable tensile strength, elasticity, hardness, porosity and bond strength to provide the required weight to performance ratio. They are based on polyamide, polyester, thermoplastic polyurethane (TPU) and polyolefin polymers. Melting points are typically between 75 ∞C and 200 ∞C. In the automotive industry the main uses are bonding, pre-coating, support backing, positioning, moulding and as slip surfaces to facilitate cutting operations. Headliners, parcel trays, door appliqués and panels, seat parts, and flooring components are produced using adhesive webs. In the manufacture of office seating, adhesive webs are used to bond moulded foam cushions to upholstery fabrics. In luggage manufacturing, the webs bond leather or vinyl fabrics to foam and in manufacturing composite structures, adhesive webs are used to position glass fibres and carbon strands in the laying-up process. In the assembly of wet filtration media, the webs are used to bond needlepunched and meltblown nonwovens. In calender (hot roll) lamination (Fig. 8.5) the film and substrate are drawn from separate rollers and heat calendered. The temperature activates the adhesive film and bonds the fabrics together. Calendar laminators typically comprise a heated three-roll stack to heat and activate the adhesive film. Good adhesion is achieved by the interaction of line speed (which determines the dwell time), operating temperature and nip pressure. Wrinkling of the film or the laminated substrate is a known problem and is due to uncontrolled let off tension. Wind-up tension and cooling rates need to be considered to prevent sticking and deformation of the laminated fabric. In the Ecosafe process high processing speeds are possible because the machine heats only the thermoplastic adhesive (previously applied by scatter coating) and the surface of the second substrate, instead of the whole package Fabric, film or web substrate

Film

Heated three-roll stacks or cans

Exit

8.5 Calender lamination.


387

of materials. The process is claimed to be environmentally friendly with low energy consumption and application to a wide variety of substrates. Extrusion laminating relies on the application of thermoplastic polymers to the nonwoven fabric, which are melt extruded as an adhesive sheet to laminate two fabrics, one either side of the extruded molten film. A pressure roller nip ensures good adhesive contact. The thermoplastic polymer is melted and pumped into a die, forming it into a thin continuous sheet. Polyethylene (particularly LDPE) is most commonly used as it is durable and easily extruded. Extrusion coating is similar but only one substrate is joined with the molten polymer. Pressure, speed and operating temperatures are important considerations. Increasing the pressure or tension on the fabric or film can increase the penetration of the extruded polymer into the substrate. The viscosity can be adjusted to compensate by varying the temperature. Following procedures include trimming and slitting the roll to the required width. With all coating and laminating procedures bond strength between the base fabric and the coating medium is important. For example, one layer is a base fabric and the other a molten film. In order for a good bond to be formed it is usually necessary for the molten film to ‘flow’ slightly into the base fabric to achieve good mechanical bonding. Alternatively, polymer intermediates or adhesives may be applied to the base fabric to ensure adhesion of the film. Flat bed lamination relies on a different principle for combining the substrates and film. The substrate and adhesive film are combined prior to entering a heated, sometimes pressurised section. Usually a plate or belt type system binds the substrates together. This is followed by cooling to solidify the adhesive. Infra-red pre-heaters increase the temperature of the fabric before lamination. Advantages of the flat bed system are claimed to be versatility of operation (production can be piece to piece panel or roll to roll) and substrate type to be laminated. Improved adhesion is claimed due to the much longer duration times in the heating and pressing zone as compared to a nip type contact. Extendable or modular tunnel systems allow laminating temperatures to be much closer to the adhesive glue-line temperature, resulting in a lower temperature during bonding and consequently, improved fabric quality. The use of thermoplastic or hot melt polymers in lamination is attractive because of the low levels of effluent and the reduced drying costs. Flame lamination (Fig. 8.6) is widely operated in the bonding of films and/or nonwoven fabrics to polyurethane foams, mainly for automotive applications such as headliners, door panels, seats, sun visors, headrests, carpets and car boot liners. The process involves passing foam over an open flame (for one-sided operation) or between two sets of flame burners for lamination to both sides (3-ply laminate). A thin layer of molten polymer is produced on the surface of the foam, which is combined with the substrate and film whilst still molten. The most commonly used foams are open-cell

388

Handbook of nonwovens End product

Pressure roller

Film

Fabric

Flame

Flame

Foam

8.6 Flame lamination.

polyester, polyether urethanes, and cross-linked polyethylene. The gas input and flame intensity, flame height, spread and nip pressure influence bond strength. One disadvantage of flame lamination is the gaseous emissions produced. In hot melt spraying, molten synthetic polymers are heated to a high temperature to achieve a sufficiently low viscosity to permit spraying onto a fabric. During spraying the resin may be atomised to enable uniform application at low application levels. This is important for permeable fabrics such as nonwovens to achieve an even coverage. Immediately after spraying the coated and non treated fabrics are brought together and bonded. The resin cures on cooling and exposure to air. Claimed advantages include no additional heating or curing step is required, no solvent is used to dissolve the resin and improved handle and drape are claimed compared to other application systems because of the minimal surface area that is coated with the polymer. Infrared heating is often used in line with laminating systems to activate and soften low melt temperature adhesives including polyethylene, prior to combining fabrics for lamination. The two fabrics are subsequently laminated using a calendar.


8.5

Mechanical finishing

8.5.1

Splitting and winding

389

To produce certain high density nonwoven fabrics with relatively low thickness it is sometimes only possible and economical to produce a thicker structure and then to split (or level) it to the required thickness. The nonwoven fabric is fed at controlled tension over a feed table and fed between feed rollers to a precisely adjusted rotating hoop knife. The two layers are then separated by different roller outfeed configurations and wound onto separate controlled tension winding beams. Mechanical adjustments enable splitting of nonwoven materials including heavily bonded needlepunched fabrics such as synthetic leather fabrics and heavyweight chemically bonded fabrics. Similarly, splitting of perpendicular-laid (Struto) fabrics has been found necessary to produce thin fabrics.

8.5.2

Perforating

Perforation of nonwoven fabrics using heated needles or modified calender rollers, for example, is useful for various purposes including increasing the vertical liquid transfer in hygiene coverstocks as well as increasing fabric softness and drape in interlinings. The vertical profile of the perforation can be adjusted. Conical profiles can be made depending on the type of perforating needle used in order to modify the drainage properties of hygiene coverstocks. In chemically bonded nonwovens rather than reducing fabric strength, the use of hot needles can beneficially promote cross-linking of the resin bonding agents. In slitting, longer perforations (or slits) are made in the fabric, the length of the slit and the distance between slits can be calculated to minimise fabric strength loss.

8.5.3

Drying

Nonwoven fabrics are subjected to tension during fabric manufacture and consequently may stretch and increase in length often with a decrease in width. This is particularly true when the webs are wet processed and are hot. If the extension is fully stabilised in bonding then the fabric should be dimensionally stable. If, however, it is not fully stabilised then shrinkage may occur in further processing. Nonwoven structures often exhibit poor elastic recovery and there is a need to remove or reduce unwanted fabric extension by the introduction of relaxation or overfeed zones, for example. Tensionless steam relaxation involves continuous overfeed onto a vibrating table and steaming without tension before cooling. Stenter frames are applicable for drying and heat setting of some nonwoven fabrics. The fabric is held at the edges by pins or clips located on chains or

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rails, which continuously transport the fabric in openwidth through the drying chambers. Minimum width tension is needed to allow width relaxation and some stenters are equipped with supporting beds to allow minimum tension processing (as used for knitted fabrics). The rails can be adjusted in or out to control width dimensions. Overfeed to allow length shrinkage may be applied where the fabric is fed to the stenter pins at a speed slightly in excess of the chain running speed, usually by rubber covered rollers. When the fabric is adjusted correctly on the pins, it is transported through a series of drying chambers. Modern stenters are equipped with optimised airflow systems to ensure uniform and cost efficient drying. Moisture meters fitted to control the residual moisture content of fabric exiting from the dryer provide automatic feedback to control drying parameters. Infra-red pyrometers measure fabric temperature, optimising dwell time for fabric quality and operating costs during heat setting or heat bonding operations. Alternatively, for thermal bonding and drying of nonwovens, forced through-air systems are used. Upper and lower nozzles allow airflow above and below the conveyor to be adjusted to control the calibration of the material. In thermal fusion ovens, flow through and overflow techniques allow the same oven to heat air permeable and impermeable materials. Control of nozzle height, temperature and air regulation is possible for powder or fibre bonding, drying and heat setting. Interest in natural and recyclable nonwoven materials composed of bast fibres, for example flax and hemp for building insulation, is a further extension of belt drying. After batt formation, the fibres are chemically impregnated to protect against mildew attack and weathering, insect damage or a flame resistant treatment is applied. The batt is dried by through-air flow and compressed as it passes through the drier. Instead of heated air, heated belts or plates are used in some machines and heat is then conducted into the fabric. Can (or drum) drying is still used for nonwovens particularly in chemical bonding installations. The fabric to be dried is fed over a series of heated cans, set vertically in a frame. As the fabric passes from one stainless steel can to the next, both the face and back of the fabric are brought into contact with the drying surface. Temperature can be graduated to ensure uniform and gradual drying and cooling. Minimisation of fabric tension and dimension control in can drying is problematic which limits the compatibility of extensible nonwoven fabrics. In hot flue dryers the fabric travels in festoon folds over rollers through a heated chamber. Drying air is channelled between the folds, but as with can drying, fabric tension may be high unless the rollers are driven to minimise tension and drag on the fabric. Alternative methods for drying include biaxial roll spreaders for thermoplastic nonwoven fabrics. The rollers are heated and by adjusting input and exit speeds, the fabric thickness and weight can be adjusted.


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Through-air drum drying and heat setting are of major importance in the nonwovens industry. Several configurations are available, including single drum, twin vertical drum or multidrum horizontal. Generally, these are modular units and the drum diameter varies depending on the required drying capacity. The fabric permeability is calculated to enable optimisation of drying efficiency. Through-air drying gives high heat and mass transfer through the fabric. The air is drawn out of the drum interior by a high-capacity radial fan passing over heating elements and is forced back to the drum. This creates a pressure difference (suction) to maintain fabric and drum contact. Tension is minimal and the fabric can be overfed from drum to drum to allow relaxation, which is advantageous for nonwovens. This mode of drying also helps to produce fabrics with good bulk and softness. High specific drying rates are claimed with minimum energy consumption and low pattern distortion. Infra-red heaters and drying ovens are important for pre-drying, localised area of thicker or higher density drying and curing. Other applications are coagulation of thermo-sensitive binders and sintering of binder powders in, for example, the production of interlinings. Infra-red sources produce high energy input in a short dwell time, allowing operating temperatures to be reached rapidly. Infra-red systems are being considered as a replacement for flame bonding to improve air quality. Further post drying relaxation may be necessary. Natural (non-compressive) relaxation can be achieved using tensionless steam relaxation tables. The fabric is continuously overfed onto the table with vibration and steaming, followed by cooling. Enclosed heads minimise but intensify the steam, giving control over steaming conditions with little exhaust required, when compared to conventional open tables.

8.5.4

Compressive finishes

Mechanical compressive shrinkage (sanforising) was originally developed for reducing shrinkage potential to very low levels in cotton fabrics. During sanforising, shrinkage is achieved by passing the pre-wetted or steamed fabric around a heated cylinder in contact with a rubber belt or blanket. The fabric is compressed in-plane as it is held in contact with the blanket (usually elastomeric) and the drying cylinder. As the blanket recovers original dimensions, the fabric is compacted. Clupak (wrenching) is similar in that a continuous rubber belt is pressed against a heated cylinder however, the compression ratio is achieved by changing the diameter of the roll. Compaction processes of this type can significantly modify fabric properties. Fabric weight increases, greater bulk and softness is observed and tensile properties may be affected. Thermoplastic binding agents can assist the compaction process but attention must be given to the nature of the binding agent to avoid problems of stickiness. Hydrophobic fibres do not necessarily compact

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efficiently because pre-wetting or steaming has little influence on fibre extension. Fibre orientation influences the ability of a fabric to compact. Cross-laid and randomly orientated fabrics are generally more difficult to compact than parallel-laid fabrics. Calender systems may also be used for compacting purposes. Two calenders are used in series, each fitted with a continuous compacting blanket. Micrex® Compressive compaction of wet-laid paper to increase its softness and elasticity is well known. The Micrex® system (Fig. 8.7) has found various applications Pressure plate Walton curve Back-up component Retarder Fabric

Primary surface Main roll

Length LO = fabric before the process

Length LC = fabric after the process

LC = 8 cm. LC

LO

LO = 10 cm.

Rewind speed 240 m/min Main roll speed 300 m/min

8.7 Arrangement of Micrex® system (courtesy of Micrex Corporation).


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in the nonwovens industry. The machine has a short mechanical compacting zone. The fabric is fed onto the main roll and conveyed into a cavity, where compaction takes place. The system is equipped with a retarder blade etched with grooves. These grooves provide numerous treatment zones into which the fabric is compacted. The fabric is usually processed dry at relatively low temperatures, although the temperature can be adjusted depending on the required tensile properties of the fabric. Normally, with thermoplastic fabrics as the temperature increases the softness decreases but the elasticity increases. Modification of fabric softness, drape, bulk and extensibility can be imparted to wet or drylaid structures, spunbonds, spray and point bonded webs and spunlaced fabrics. A three-dimensional crepe pattern of variable amplitude and wavelength is a characteristic feature of fabrics finished with the Micrex process. Marked 3D textural effects in the fabric may be formed, which significantly increase the elasticity of the fabric but reduce its length. This is referred to as microcreping. The textural effects, and increased elasticity that are achieved have found applications in the wipes industry, particularly in relation to staple fibre hydroentangled fabrics. The surface texture after microcreping is claimed to improve the cleaning efficiency of wipes. The degree of compaction can be calculated from the following formulae: Compaction C(%) =

LO – LC ¥ 100 LO

8.3

Stretch S (%) =

LO – LC ¥ 100 LC

8.4

where LO is the original measured length and LC is the compacted measured length.

8.5.5

Calendering

Calenders are extensively used for finishing of nonwovens as well as thermal bonding. Calender designs vary but the most common are the type I design where the rollers are arranged vertically in line and the type L design where the bottom roller is set slightly forward. In hot calendering, the rollers are generally oil heated. Composition roll surfaces deform and enable the line pressure to be transmitted uniformly to the fabric. They are compounded to have a range of durabilities and resilience, to withstand high temperatures and retain properties at varying temperatures and pressures. After calendering the fabrics are passed over cooling rollers to reduce stretching in the hotstate during winding. In calendering, there is generally some compaction (or closing) of the structure as the fabric is compressed, a decrease in thickness (which can result in stiffening) and, depending on the pressure used, smoothing of the fabric surface. To ensure the fabric is free from creases and runs

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straight into the calender devices such as scroll openers and curved rubber expanding rollers are fitted. For greater control of width, belt stretching systems are more commonly fitted where the fabric edges are nipped by rubber covered diverging pulleys extending the full width of the fabric. Alternatively, equalising pin frames are used. Roller bulge has been used to compensate for line pressure variations across the calender caused by bending of the rollers during operation. Generally, the diameter of the roller in the middle was greater than that at the ends. Cross axial alignment of the rollers is also a well established approach. In the HyCon calender system (Fig. 8.8), an elastic sleeve compensates for variations in deformation and allows adjustment of the line pressure. The rollers are supported by hydrostatic elements. Uniform, infinitely variable linear forces are transmitted to the sleeve contact via an oil film. By symmetrically turning on and off the supporting elements, the pressing width can be adjusted. Generally, temperatures in this type of calender are lower than stainless steel calendars. In the S-Roll (swimming roll) calender, roller deflections are compensated across the entire width of the fabric. A steel tube rotates around a fixed axle and the annular gap is separated by seals into two semi-circular chambers. The chamber facing the nip is hydraulically pressurised by oil. In this way, the external linear pressure is balanced by the internal oil pressure cushion which acts as a linear load with an infinite number of support points. S-Rolls are found in padding systems and to maximise uniformity of finish or dye applications two swimming rolls may be used. Modification of finished fabric thickness and density following thermal bonding or activation of a chemical binder in a through-air oven frequently involves use of a cold calender. Belt calendering (or drum and blanket) is a rather different mode of action. In conventional calendering, pressure is applied at the roller nip whereas in belt calendering, the nonwoven fabric is pressed against a heated drum by a tensioned blanket. The contact (bonding) time is therefore significantly longer than in conventional calendering but the pressure is reduced. The resulting fabrics are claimed to have a less papery finish than nonwovens processed by conventional calenders.

8.6

Surface finishing

8.6.1

Singeing

Singeing removes protruding fibres as the fabric passes at high speed over gas burner flames. An alternative system uses indirect singeing, where burners heat panels that radiate energy onto the fabric. This procedure is claimed to give more uniform singeing, evening out variations that occur in a gas flame with lower risk of fabric damage. Singeing of fabrics is desirable when a


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(a)

(b)

8.8 HyCon L calender system, (a) general view, (b) layout (courtesy of Küsters).

smooth, clean surface is required, for example in printing or coating or for products such as filtration media. In the case of needlepunched air filtration media composed of either PP, PE or PET fibres, singeing can lead to partial melting of surface fibres, which can modify the fabric permeability. The removal of protruding fibres also improves cake release in pulse jet cleaning and can serve to minimise pressure drop. Important parameters include the evenness of flame height and intensity, flame distance from the

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fabric, fabric speed and singeing angle. Singeing at right-angles to the fabric or at an angle is normally dependent on fibre and fabric type. For temperature sensitive fabrics singeing is performed over a water cooled roller. The moisture content should be uniform throughout the fabric to ensure even singeing performance. After singeing the fabric is rapidly cooled or wetted to prevent after burn and uneven treatment. Washing to remove singeing residues and odour usually follows.

8.6.2

Shearing

Also known as cropping or cutting, shearing is less invasive than singeing, for removing surface fibre. Compared to singeing, only partial surface fibre removal is achieved since the variations in fabric surface topography determine the cutting height. In shearing the fabric is usually brushed initially, to raise loose surface fibre before passing through a series of tensioning bars and guides to the shearing cylinder. The shearing cylinder is wound with helical blades and rotates at high speed (1000–1200 r/min). Prior to reaching the shearing cylinder, the fabric passes over an angled bed, where surface fibres are made to stand erect. These are caught by the rapidly rotating blades (usually serrated to improve contact) and cut against a stationary ledger blade, to a pre-set height. Segmented beds allow for bulky edges and the machine operates with strong vacuum to remove cut fibres (flock) to avoid clogging of the blades. The vacuum also acts across the blades to help raise surface fibre and along with oiled felts, to cool the shearing blades.

8.6.3

Flocking

Flocking produces a three-dimensional pile on the surface of a nonwoven backing fabric. Flock is short cut from synthetic fibre to a pre-determined length or in the case of natural fibres, ground into short fibres. To adhere the flock to the base fabric, the base fabric is pre-treated with an adhesive resin, which is either area coated for total cover flocking or printed into predefined areas to produce patterns. Flock landing on non-printed areas readily falls away leaving the flocked pattern firmly anchored by adhesive to the fabric base. For high-quality velvet-like finishes, electrostatic flocking is preferred where fibres are lined up vertically in an electrostatic field as they land on the backing fabric. Flocked products have many applications including interior textiles and automotive interior panels, shoes, apparel, filters, drapes or for pattern decoration effects. Mechanical flocking methods involving shaking or sprinkling mechanisms do not preferentially align the flock fibres and a randomised pile is produced on the fabric surface.


8.6.4

397

Raising

Raising includes a variety of processes that produce fibrous pile structures on the surface of pre-formed fabrics. Traditionally, the fabric is passed tangentially over a series of small rotating cylinders, arranged around a large drum (which also rotates). The small cylinders are wrapped with a fillet raising wire. Fibres are raised from the surface of the fabric to produce a dense collection of protruding ends (a pile). There are different cylinder configurations, speeds and directions (pile or counterpile). Wire types and density are chosen to enable a variety of raised surfaces to be produced. Depending on fibre type, raising can be carried out wet or dry. Fabric tension is particularly important for uniform raising and to avoid fabric damage. Stencilled raising (and shearing) machines operate by using a stencil to ‘blank off’ areas of the fabric. This produces either a raised pattern effect or a patterned shear effect, the shearing blades removing surface fibres not protected by the stencil. Sueding is a mechanical surface treatment, similar to raising except the raising wires are replaced by an emerising fillet, which abrades the fabric surface. The process breaks surface fibres and produces a dense pile with good softness and a subdued appearance. Sueding is applicable to mechanically bonded fabrics containing fine fibres particularly microfibres and fabrics containing fibres susceptible to fibrillation. A microfibrous pile can be produced from nonwovens containing split bicomponent fibres to produce a soft pile surface.

8.6.5

Polishing

Polishing improves the surface lustre in fabrics with a pile surface. Surface fibres are reorientated in one preferred direction during polishing, thereby increasing the lustre. The fabric is brought into contact with a rapidly rotating heated drum etched with deep spiralled grooves. The fabric is usually carried by a blanket, which brings it into contact with the drum. Significant improvements in handle and lustre can be achieved. For natural and hygroscopic fibres, wetting improves the effect and finish stability and reducing agents are sometimes used to further permanise the finish. Alternatively, silicone softeners are applied to the pile surface prior to polishing to accentuate softening. In cotton nonwovens, the possibility of utilising an enzymatic approach to remove surface hairs through a process of biopolishing has been investigated as a possible means of improving the surface finish.

8.6.6

Softening

Mechanical softening of fabrics (as opposed to surface softening) has seen much development in recent years. Fabrics are treated in an endless rope

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form. Basically the fabric is transported by air (pneumatic) and in some cases lightly touching belts at high speed to impact against baffle plates or grids. The fabric then relaxes in the bottom of the machine before continued treatment, until the desired finish is required. Treated fabrics have much reduced stiffness and improved handle, bulk and drape. Many systems enable steam or functional chemicals such as softening agents and enzymes to be introduced for further effects. Elsewhere, low-pressure hydroentanglement after, for example, thermal or chemical bonding has been found to improve fabric softness.

8.7

Developing technologies

8.7.1

Plasma

Plasma is an ionised gas of high energy and it is capable of chemically modifying the properties of fibre surfaces, particularly the surface chemistry. Glow discharge processes have been known for years but more recently applications have been found in the nonwoven industry. In the plasma treatment of fabrics, the surface of the polymer may be (i) partially removed or etched, (ii) materials may be deposited on to the surface and (iii) the surface may be activated by an increase in surface energy. Among the various applications of plasma processes in nonwovens, increasing the hydrophilicity of synthetic polymer surfaces particularly with respect to polypropylene fabrics for hygiene applications is well known. It has been shown that wetting out of nonwoven fabrics and the interaction of dyestuffs, pigments and chemical finishes can be improved by plasma treatments. Oxygen plasma treated PP blood filters have been produced with much higher flow rates because of improved wetting properties. Nonwovens containing synthetic fibres such as polyester and polypropylene can be treated to further increase hydrophobicity to obviate the need for a fluorochemical treatment. Environmental advantages are claimed due to low energy consumption, dry processing and no waste or effluent disposal problems.

8.7.2

Microencapsulation

Microencapsulation is a delivery vehicle for active compounds that can be applied to fabrics during finishing. Industrial, pharmaceutical and cosmetic applications have been developed. A small volume of a chemical or a particle is surrounded by a coating to produce a capsule. There are a number of possible delivery mechanisms including wall fracture and diffusion amongst others. The wall of the capsule may be permeable to allow controlled delivery of the contents once applied to the fabric, or the capsule may be designed to fracture in use to release the contents. Perfumes, cosmetic lotions,


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thermochromic inks, thermoactive phase change materials (PCMs) and antimicrobial chemicals, for example, are applied to fabrics in the form of microcapsules, usually in association with a binder resin. Another type of encapsulation involves the use of b-cyclodextrin derivatives. These products can be chemically attached to fibres and act as capsules to hold fragrances or other compounds which are slowly released.

8.7.3

Laser etching

Lasers, including copper vapour lasers have been shown to produce physical modifications to the surface of fibres in preformed fabrics. Patterning of fabrics using laser impingement methods has also been demonstrated.

8.7.4

Biomimetic finishes

The structure of biological materials and the relationship with their properties continues to inform new developments in functional finishing. Among the various examples, one that is relevant to the nonwovens industry, particularly in respect of improving the performance of medical protective fabrics, is Nanosphere® technology, which claims to simulate the surface structure of the lotus leaf. The particular nano-topography of the leaf surface combined with its hydrophobicity prevents wetting out and is claimed to render it selfcleaning. By attempting to replicate this surface structure, a self-cleaning mechanism can be conferred on the surface of fabrics. Liquids and dispersions of various viscosities have been shown not to wet out such treated surfaces and particle contaminants can be readily removed. A simulated lotus leaf structure can be produced on a hydrophobic polymer surface by depositing tiny metal or polymer particles in a ordered array. There is significant scope to mimic the nano- and micro-structural features of other biological structures as a route to establishing new functional finishes.

8.7.5

Electrochemical finishes

These finishes involve pre-metallisation of a fabric surface before an electrochemical treatment to render the surface conductive. Applications are fabric based sensors and actuators, electroluminescent fabrics and fabric electrodes.

8.8

Fabric inspection

Fabric inspection is an important and developing area relevant to nonwoven finishing. Inspection monitors can detect web faults such as holes, thick and thin areas and the distribution of fibres in a web, contamination and various

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additional marks and process-specific defects. Real-time monitoring and feedback of fabric weight variations (and coated fabric weights), thickness, moisture content, permeability and other properties are commercially available. The Mahlo QMS-10A is a modular system, which can accommodate up to four traverse assemblies, each with the facility to be attached with up to three types of sensor. These reciprocate across the product and provide a width profile. Standard interfaces enable the system to be linked to the user company’s data network and a modem can establish a worldwide line of communication with the manufacturer. The Cognex SmartView uses on-line cameras and lighting to give on-line detection, identification and visualisation of defects in nonwoven fabrics including blow-outs, calender cuts, polymer drips, fibre clumps and holes. Trends in fault and quality inspection are towards faster inspection, wider monitoring widths and closer measuring tolerances to reduce safety margins, to reduce waste and as a consequence improve end product consistency and quality. Remote diagnosis allows data to be compared between production lines within factories or within factory groups to control quality and exchange data and parameter settings. Colorimetry systems measure and control differences in colour across nonwoven webs, giving information on dyeing variations. For continuous dyeing ranges such as pad batch, many colour measuring systems will characterise colour distribution and the data enables padding settings to be adjusted to maximise dyeing quality. The SVA Lite is a compact tool that measures, analyses and controls fabric shade consistency. The system is based on a moving spectrophotometer to enable side to side and beginning to end shade variation to be characterised. The system provides a selection of shade standards to be used as references including a fabric swatch.

8.9

Acknowledgements

My sincere thanks are due to Mr Steve Myers for his expert assistance in the preparation of this chapter and to Mr Manoj Rathod of the Nonwovens Research Group at the University of Leeds who also helped to compile this chapter. The contributions of the following organisations are gratefully acknowledged: EDANA, www.nonwovens.com, www.ptj.com.pk, www.textileworld.com, www.europlasma.com, The International Dyer, Nonwovens Report International (UK), JTN Monthly, www.txm.vdma.org, Ciba and www.txm.vdma.org. Thanks are also due to Küsters (Germany), Micrex Corporation (USA), Cavitec (Switzerland), Lacom (Germany), Zimmer (Austria) and Thies GmbH and Co. (Germany).

9 Characterisation, testing and modelling of nonwoven fabrics N M A O and S J R U S S E L L University of Leeds, UK (Sections 9.1–9.21) B POURDEYHIMI Nonwovens Cooperative Research Center, North Carolina State University, USA (Section 9.22)

9.1

Introduction: characterisation of nonwoven fabrics

The physical, chemical and mechanical properties of nonwoven fabrics that govern their suitability for use depend on the properties of the composition and the fabric structure. The composition in this context refers to the fibre properties as well as chemical binders, fillers and finishes present on, between or within the fibres in the fabric. This chapter focuses on the characterisation of nonwoven materials and considers the influences of fabric structure on the physical and mechanical properties of fabrics. Standard and specialised testing methods used to determine structure and properties are presented and models designed to describe the relationship between the nonwoven structure and some important nonwoven properties are introduced. A nonwoven can be defined as a manufactured sheet, web or batt of directionally or randomly orientated fibres, bonded by friction, and/or cohesion and/or adhesion, excluding paper and products which are woven, knitted, tufted, stitch-bonded incorporating binding yarns or filaments, or felted by wet-milling, whether or not additionally needled.1 A nonwoven structure is different from some other textile structures because: 1. It principally consists of individual fibres or layers of fibrous webs rather than yarns. 2. It is anisotropic both in terms of its structure and properties due to both fibre alignment (i.e., the fibre orientation distribution) and the arrangement of the bonding points in its structure. 3. It is usually not uniform in either fabric weight and/or fabric thickness, or both. 4. It is highly porous and permeable. In addition to the constituent fibre properties and binders, if present, the structure of a nonwoven fabric is influenced by the web formation process, bonding method and fabric finishing processes. Although nonwoven fabrics 401

402


share some characteristics with textiles, paper and plastics in terms of structure, the fabric structures produced are particularly diverse and may be manipulated to obtain specific functionalities and performance characteristics. The structure and properties of a nonwoven fabric are determined by fibre properties, the type of bonding elements, the bonding interfaces between the fibres and binder elements (if present) and the fabric structural architecture. Examples of dimensional and structural parameters may be listed as follows: 1. Fibre dimensions and properties: fibre diameter, diameter variation (for example, in meltblown microfibre and electrospun nanofibre webs), crosssectional shape, crimp wave frequency and amplitude, length, density; fibre properties (Young’s modulus, elasticity, tenacity, bending and torsion rigidity, compression, friction coefficient), fibrillation propensity, surface chemistry and wetting angle. 2. Fibre alignment: fibre orientation distribution. 3. Fabric dimensions and variation: dimensions (length, width, thickness, and weight per unit area), dimensional stability, density and thickness uniformity. 4. Structural properties of bond points: bonding type, shape, size, bonding area, bonding density, bond strength, bond point distribution, geometrical arrangement, the degree of liberty of fibre movement within the bonding points, interface properties between binder and fibre; surface properties of bond points. 5. Porous structural parameters: fabric porosity, pore size, pore size distribution, pore shape. Examples of important nonwoven fabric properties are: 1. Mechanical properties: tensile properties (Young’s modulus, tenacity, strength and elasticity, elastic recovery, work of rupture), compression and compression recovery, bending and shear rigidity, tear resistance, burst strength, crease resistance, abrasion, frictional properties (smoothness, roughness, friction coefficient), energy absorption. 2. Fluid handling properties: permeability, liquid absorption (liquid absorbency, penetration time, wicking rate, re-wet, bacteria/particle collection, repellency and barrier properties, run-off, strike time), water vapour transport and breathability. 3. Physical properties: thermal and acoustic insulation and conductivity, electrostatic properties, dielectric constant and electrical conductivity, opacity and others. 4. Chemical properties: surface wetting angle, oleophobicity and hydrophobicity, interface compatibility with binders and resins, chemical resistance and durability to wet treatments, flame resistance, dyeing capability, flammability, soiling resistance.


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5. Application specific performance: linting (particle generation), aesthetics and handle, filtration efficiency, biocompatibility, sterilisation compatibility, biodegradability and health and safety status.

9.2

Characterisation of fabric bond structure

Nonwoven fabrics contain bond structures, the type, shape, rigidity, size and density of which may be characterised. The bond points can be grouped into two categories: rigid, solid bonds and flexible, elastic joints, the prevalence of which in the fabric depends on choice of manufacturing process. The bond points in a mechanically bonded fabric, for example needlepunched and hydroentangled, are formed by either interlacing of individual fibres or loose fibrous strands. These bonds are flexible and the component fibres are able to slip or move within the bonding points. By contrast, the bonds in thermally bonded and chemically bonded fabrics are formed by adhesion or cohesion between polymer surfaces, in which a small portion of the fibrous network is firmly bonded and the fibres have little freedom to move within the bond points. The bond points in thermoplastic spunbond and through-air bonded fabrics are formed by melting polymer surfaces to produce bonding at fibre cross-over points and the fibres associated with these bonds cannot move individually. In meltblown fabrics, the fibres are usually not so well bonded together as in spunbonded fabrics and in some applications, the large surface area is sufficient to give the web acceptable cohesion without need for thermal, chemical or mechanical bonding. Stitch-bonded fabrics are stabilised by knitting fibres or yarns through the web and the bonding points are flexible but connected together by these yarns and fibres. The size of the bond points is influenced by fabric manufacturing parameters, such as the size of needle barb depth in relation to the fibre diameter, punch density and number of barbs that penetrate the batt on the downstroke (needlepunching), water jet diameter, specific energy and number of injectors (hydroentanglement), the land area and bond point area, pressure and the size of adhesive particles (thermal bonding), the method of binder application, for example full saturation, spray or printing and binder viscosity (chemical bonding). The rigidity of solid bond points in most nonwoven materials can be physically characterised in terms of the measured tensile properties, for example, strength and elasticity, while the degree of bonding may be directly determined by microscopic analysis of the fabric cross-section. In mechanically bonded fabric, specifically needlepunched and hydroentangled fabric, the depth of bent fibre loops in the bonding points can be determined and based on the depth of these fibre segment loops, a simple but limited estimate of bonding intensity can be derived.2

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9.2.1


Needlepunched fabrics

Needlepunched fabrics have characteristic periodicities in their structural architecture that result from the interaction of fibres with the needle barbs. Fibre segments are reorientated and migrated from the surface of the web towards the interior of the fabric forming pillars of fibre orientated approximately perpendicular to the plane. On the fabric surface, needle marking is frequently visible, which is due to the series of punch hole locations that may be joined by reorientated fibres in the fabric plane running in the machine direction.3 On a microstructural scale, needlepunched fabrics consist of at least two different regions. The first, between the impact areas associated with the needle marks, is not directly disturbed by the needles and retains a similar structure as the original un-bonded web. The second region, the needlemarked area, contains fibre segments that are orientated approximately perpendicular to the fabric plane. Some fibres are realigned in the machine direction. This rearrangement of fibre segments induced by the process, effectively increases the structural anisotropy as compared with the original web and therefore the structure of needlepunched fabrics is not homogeneous. Both the number of needle marks and the depth of fibre penetration are related to the fabric bonding quality and fabric tensile strength. The shape and number of the holes depends mainly on the number of needles in the needle board, the size of the needles and the needle throat depth, the fibre type and dimensions relative to the barb dimensions, the advance per stroke and the punch density. The depth of needle penetration, the number of barbs that pass through the web and the distance each barb and its attached fibres travel are important variables influencing the microstructure. Previously, the effects of changes in penetration depth and the number of barbs3–5 on the fabric structure have been investigated. These experiments have demonstrated that fabric strength is influenced by any changes in barb position as the needle passes through the web. Maximum fabric tenacity for a given web may be obtained with only three barbs per apex if the depth of penetration is adjusted accordingly. Needlepunched fabrics have some fibre segments aligned in the transverse direction,4 although the majority remain aligned in-plane and the fabrics have a greater porosity and a larger number of curved inter-connected pore channels than woven fabrics. Hearle et al.3 observed that the punched loops of fibre do not protrude from the lower surface of the fabric when the needle penetration is small and the resulting fabric appearance and needle marks are illustrated in Fig. 9.1(a). A pseudo-knitted appearance resulting from linked loops of fibre tufts3 produced by the needle barbs can be detected on the fabric surface when the needle penetration is large, as illustrated in Fig. 9.1(b).


(a) Low level of needling density and low needle penetration

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(b) High level of needling density and high needle penetration

9.1 Needled fabric structures.3

9.2.2

Hydroentangled fabrics

The microstructure of hydroentangled fabrics is quite different from needlepunched fabrics in that the formation of discrete pillars of fibre in the fabric cross-section is absent. However, the incident high-speed water jets locally migrate, fibre segments, both in the transverse and in-plane machine directions. Some fibre segments impacted by the water jets are bent and formed into ‘U’ shape configurations. Bonding depends on the intertwining of fibres together within the web. Since fabrics are consolidated mainly in the areas where the water jets impact, jet marks are formed on the fabric surface, which appear as visible ‘lines’ on the jet-side of the fabric running in the machine direction. Jet marking becomes less pronounced as the number of injectors increases. Where the support surface is three-dimensional, fibres are displaced from the projections in the surface to form apertures and other structural patterns. This effectively produces local density variations in the fabric that can influence tensile and fluid flow properties as well as introducing variations in local fibre segment orientation. Therefore, even if the original web is isotropic, structural anisotropy is introduced during hydroentanglement that may be of a periodic nature. The structure of hydroentangled fabrics depends on process parameters and fibre properties. At low water jet pressure, only a small portion of fibre segments in the surface of the web are entangled and intertwined. At a high water jet pressure, some fibre segments are reorientated towards the reverse side of the web and some fibre ends project. Fibre rigidity and bending recovery influence the ability of the jet to produce fibre entanglements during hydroentanglement and therefore the structural features of hydroentangled fabrics can differ according to fibre type. An example is fabrics made from polypropylene and viscose rayon using the same process conditions. The specific flexural rigidity of polypropylene fibre (0.51 mN.mm/tex2) is higher than viscose rayon (0.35 mN.mm/tex2), and polypropylene fibre has higher

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compression recovery, bending recovery and tensile recovery6 compared to viscose rayon fibre.7 In a polypropylene hydroentangled fabric, when the water pressure is low only the surface fibres are effectively bonded, and the fibres inside the fabric are poorly entangled. The surface is therefore more compact than the fabric core. In contrast, a viscose rayon fabric is more consistently bonded through the cross-section and the compaction is greater than in the corresponding polypropylene fabric.

9.2.3

Stitch-bonded fabrics

Stitch-bonded fabrics are formed by stitching a fibrous web together by using either a system of additional yarns (filaments) or the fibres in the web using a warp-knitting action. Because the formation of a stitch-bond fabric is basically a hybrid of warp-knitting and sewing, it is reflected in the fabric structure. The fabric integrates stitching and fibrous webs, the fabric has a clear stitching pattern on at least one side of the fabric, and the stitches hold the fibres in the fibrous web together. There are three basic types of stitch-bonded fabric structure: (i) fibres bonded with the constituent fibres in which the stitches are observed on one side of the fabric (Malivlies); (ii) stitches of yarns on one surface and a projecting pile of pleated fibres on the reverse surface (Kunit); (iii) stitches of yarns on both surfaces (Multiknit, Maliwatt). The basic structural characteristics of these different types of stitch-bonded fabric structure may be summarised as follows:8 ∑

∑

∑

Malivlies. Malivlies fabrics are bonded by knitting fibres in the web rather than by additional yarns (filaments); therefore the fabric consists of staple fibres. They have a warp knitted-loop stitch pattern on one side of the fabric and the intensity of stitch bonding depends on the number of fibres carried in the needle hook. The carrying capacity depends on the dimensions of the hook and the fibre fineness. Kunit. Kunit fabric is a three-dimensional pile structure made from 100% fibres stitched using the constituent fibres in the web. Fibres on one side of the fabric are formed into stitches, while the other side of the fabric has a pile loop structure with fibres arranged with an almost perpendicular orientation with respect to the plane. The fabric has very good air permeability because of the high-loft structure, and excellent compression elasticity because of the vertical pile loop structure. Multiknit (Malimo) Multiknit constructions are formed from Kunit pile loop fabric, and both sides are formed into a closed surface by stitched loops of fibre. The two sides of the fabric are joined together by fibres orientated almost perpendicular to the plane. The fabric is stitched using the fibres in the original web rather than by additional yarns and therefore, a three-dimensional fabric composed of 100% staple fibres is formed.


∑

407

Maliwatt. Maliwatt fabrics are fibrous webs stitched through with one or two stitch-forming yarns. Both sides of the fabric have a yarn stitch pattern and the fabric weight per unit area ranges from 15 to 3,000 g/m2 with a fabric thickness of up to 20 mm, and a stitching yarn linear density in the range 44 to 4,400 dtex.

In stitch-bonded fabrics, yarn stitches are usually aligned in the fabric plane while the fibre piles or the fibre pile loops are fixed by the stitches and are generally orientated perpendicular to the stitched fabric surface. Stitchbond fabric structure is determined by the warp-knitting action applied by the machine, fibre properties and dimensions, web density and structure, stitching yarn structure, stitch density, machine gauge (number of needles per 25 mm), stitching yarn tension and stitch length. Both the stitch holes and the pile formed in the fabric surface are two unique structural characteristics of stitchbond fabrics. The number and size of the stitch holes depends on the properties of the stitching yarn, the properties of the fibrous web, machine gauge (number of needles per 25 mm), intermeshing intensity, interlacing and stitching yarn tension. The pile height, visible in certain stitch bonded fabrics ranges from 2 to 20 mm and depends on how the oscillating element is set at the stitch-bonding position. Both the stitch holes and the piles formed in the fabric surface influence fabric properties. The warp-knitting structure in stitch-bonded nonwovens has an open fabric construction and short underlaps, it is dimensionally extensible in the cross direction (CD) as well as in the machine direction (MD). To increase the fabric tensile strength in the MD a specific stitch construction is used (pillar stitch). To increase the widthwise stability, the underlaps are lengthened (e.g., satin stitch) and a three-dimensionally stable structure is achieved by combining these two types of stitch construction (e.g., pillar-satin).

9.2.4

Thermal-bonded fabrics

The types of bonding structure formed in thermal-bonded fabrics depends on the method used to introduce heat to the fibres as well as the web structure and the type of binder fibre present. In calendered thermal point-bonded fabrics, the fibres are compressed together and heat is introduced by conduction. This produces deformation of the fibres and polymer flow around the bond points. Around the immediate vicinity of the bond points, the heating of surrounding fibres can introduce interfacial bonding at the cross-over points of uncompressed fibres. This is known as secondary bonding and is particularly noticeable when bicomponent fibres are present as the binder component. In through-air thermal bonded fabrics, core-sheath bicomponent fibres are commonly utilised and convected heat introduced during the process

408


produces the interfacial bonding at the fibre cross-over points as the polymer softens and flows. There is no associated deformation of the fibres at these locations and therefore the resulting fabric density is lower as compared to a calendered thermally bonded fabric. Calendered thermal bond structures are found in a host of nonwoven materials both in monolithic and multi-layer fabrics including SMS and other spunbond-meltblown web combinations.

9.2.5

Chemically bonded fabrics

Chemically bonded fabrics are produced by the application of a resin emulsion (e.g., acrylic, polyvinyl acetate, or other suitable chemical binder) to the web that is then dried and cured. The distribution of the resin binder in the fabric is largely governed by its method of application to the web and the flow properties of the resin between fibres. Large numbers of fibres may be enveloped by a film binder connecting both fibre cross-over points and interfibre spaces and large segments of these film binders are visible in such structures that span adjacent fibres. Alternatively, the polymer may be concentrated at the fibre cross-over points producing localised bonding in these regions and either rigid or flexible bonds depending on the polymer composition of the binder.

9.3

Fabric weight, thickness, density and other dimensional parameters

The structure and dimensions of nonwoven fabrics are frequently characterised in terms of fabric weight per unit area, thickness, density, fabric uniformity, fabric porosity, pore size and pore size distribution, fibre orientation distribution, bonding segment structure and the distribution. Nonwoven fabric weight (or fabric mass) is defined as the mass per unit area of the fabric and is usually measured in g/m2 (or gsm). Fabric thickness is defined as the distance between the two fabric surfaces under a specified applied pressure, which varies if the fabric is high-loft (or compressible). The fabric weight and thickness determine the fabric packing density, which influences the freedom of movement of the fibres and determines the porosity (the proportion of voids) in a nonwoven structure. The freedom of movement of the fibres plays an important role in nonwoven mechanical properties and the proportion of voids determines the fabric porosity, pore sizes and permeability in a nonwoven structure. Fabric density, or bulk density, is the weight per unit volume of the nonwoven fabric (kg/m3). It equals the measured weight per unit area (kg/m2) divided by the measured thickness of the fabric (m). Fabric bulk density together with fabric porosity is important because they influence how easily fluids, heat and sound transport through a fabric.


9.3.1

409

Weight uniformity of nonwoven fabrics

The fabric weight and thickness usually varies in different locations along and across a nonwoven fabric. The variations are frequently of a periodic nature with a recurring wavelength due to the mechanics of the web formation and/or bonding process. Persistent cross-machine variation in weight is commonly encountered, which is one reason for edge trimming. Variations in either thickness and/or weight per unit area determine variations of local fabric packing density, local fabric porosity and pore size distribution, and therefore influence the appearance, tensile properties, permeability, thermal insulation, sound insulation, filtration, liquid barrier and penetration properties, energy absorption, light opacity and conversion behaviour of nonwoven products. Fabric uniformity can be defined in terms of the fabric weight (or fabric density) variation measured directly by sampling different regions of the fabric. The magnitude of the variation depends on the specimen size, for example the variation in fabric weight between smaller fabric samples (e.g., consecutive fabric samples of 1 m2 or 10 mm2) will usually be much greater than the variation between bigger fabric samples (e.g., rolls of fabric of hundreds of metres). Commercially, to enable on-line determination of fabric weight variation, the fabric uniformity is measured in terms of the variation in the optical density of fabric images,9 the grey level intensity of fabric images10 or the amount of electromagnetic rays absorbed by the fabric11, 12 depending on the measurement techniques used. The basic statistical terms for expressing weight uniformity in the industry are the standard deviation (s) and the coefficient of variation (CV) of measured parameters as follows: n

Standard deviation: s 2 =

S ( wi – w ) 2 i=1 n

9.1

Coefficient of variation: CV = s w

9.2

2 Index of dispersion:13 I dispersion = s w

9.3

where n is the number of test samples, w is the average of the measured parameter and wi is the local value of the measured parameter. Usually, the fabric uniformity is referred to as the percentage coefficient of variation (CV%). The fabric uniformity in a nonwoven is normally anisotropic, i.e. the uniformity is different in different directions (MD and CD) in the fabric structure. The ratio of the index of dispersion has been used to represent the anisotropy of uniformity.13 The local anisotropy of mass uniformity in a nonwoven has also been defined by Scharcanski and Dodson14 in terms of the ‘local dominant orientations of fabric weight’.

410

9.3.2


Fibre orientation

The fibres in a nonwoven fabric are rarely completely randomly orientated, rather, individual fibres are aligned in various directions mostly in-plane. These fibre alignments are inherited from the web formation and bonding processes. The fibre segment orientations in a nonwoven fabric are in two and three dimensions and the orientation angle can be determined (Fig. 9.2). Although the fibre segment orientation in a nonwoven is potentially in any three-dimensional direction, the measurement of fibre alignment in three dimensions is complex and expensive.15 In certain nonwoven structures, the fibres can be aligned in the fabric plane and nearly vertical to the fabric plane. The structure of a needlepunched fabric is frequently simplified in this way. In this case, the structure of a three-dimensional nonwoven may be simplified as a combination of two-dimensional layers connected by fibres orientated perpendicular to the plane (Fig. 9.3). The fibre orientation in such a three-dimensional fabric can be described by measuring the fibre orientation in two dimensions in the fabric plane.16 Z Fibre orientation angle

g J b

X

Y

9.2 Fibre orientation angle in three-dimensional nonwoven fabrics. Z

X

Y

9.3 Example of a simplified three-dimensional nonwoven structure.16


411

In the two-dimensional fabric plane, fibre orientation is measured by the fibre orientation angle, which is defined as the relative directional position of individual fibres in the structure relative to the machine direction as shown in Fig. 9.4. The orientation angles of individual fibres or fibre segments can be determined by evaluating photomicrographs of the fabric or directly by means of microscopy and image analysis. The frequency distribution (or statistical function) of the fibre orientation angles in a nonwoven fabric is called fibre orientation distribution (FOD) or ODF (orientation distribution function). Frequency distributions are obtained by determining the fraction of the total number of fibres (fibre segments) falling within a series of predefined ranges of orientation angle. Discrete frequency distributions are used to estimate continuous probability density functions. The following general relationship is proposed for the fibre orientation distribution in a two-dimensional web or fabric:4

Ú

p

W (a ) da = 1 ( W (a ) ≥ 0) or

9.4

0

p

S W (a ) Da = 1 ( W (a ) ≥ 0) a =0

9.5

where a is the fibre orientation angle, and W(a) is the fibre orientation distribution function in the examined area. The numerical value of the orientation distribution indicates the number of observations that fall in the direction a which is the angle relative to the examined area. Attempts have been made to fit the fibre orientation distribution frequency with mathematical functions including uniform, normal and exponential distribution density functions. The following two functions in combination with the constraints in the equation 17

18

Ú

p

W (a ) da = 1 have been suggested by

0

Petterson and Hansen respectively. Petterson: W(a) = A + B cos a + C cos3 a + D cos8 a + E cos16 a. Hansen: W(a) = A + B cos2 (2a). Fibre alignments in nonwoven fabrics are usually anisotropic, i.e. the number of fibres in each direction in a nonwoven fabric is not equal. The CD Centroid of fibre Fibre a MD

9.4 Fibre orientation and the orientation angle.

412


differences between the fibre orientation in the fabric plane and in the direction perpendicular to the fabric plane (i.e., transverse direction or fabric thickness direction) are particularly important. In most nonwovens except some airlaid structures, most of the fibres are preferentially aligned in the fabric plane rather than in the fabric thickness. Significant in-plane differences in fibre orientation are also found in the machine direction and in the fabric cross direction in nonwovens. Preferential fibre (either staple fibre or continuous filament) orientation in one or multiple directions is introduced during web formation and to some extent during mechanical bonding processes. A simplified example of an anisotropic nonwoven structure is a unidirectional fibrous bundle in which fibres are aligned in one direction only. Parallel-laid or cross-laid carded webs are usually anisotropic with a highly preferential direction of fibre orientation. Fibre orientation in airlaid structures is usually more isotropic than in other dry-laid fabrics both in two and three dimensions. In perpendicularlaid webs, such as Struto nonwovens, fibres are orientated in the direction of the fabric thickness. Spunlaid nonwovens composed of filaments are less anisotropic in the fabric plane than layered carded webs,19 however the anisotropy of continuous filament webs depends on the way in which the webs are collected and tensioned. This structural anisotropy can be characterised in terms of the fibre orientation distribution functions. This anisotropy is important because of its influence on the anisotropy of fabric mechanical and physical properties including tensile, bending, thermal insulation, acoustic absorption, dielectric behaviour and permeability. The ratio of physical properties obtained in different directions in the fabric, usually the MD/CD, is a well established means of expressing the anisotropy. The MD/CD ratio of tensile strength is most commonly encountered, although the same approach may be used to express directional in-plane differences in elongation, liquid wicking distance, liquid transport rate, dielectric constant and permeability. However, these anisotropy terms use indirect experimental methods to characterise the nonwoven structure, and they are just ratios in two specific directions in the fabric plane, which can misrepresent the true anisotropy of a nonwoven structure.

9.3.3

Fabric porosity, pore size and pore size distribution

The pore structure in a nonwoven may be characterised in terms of the total pore volume (or porosity), the pore size, pore size distribution and the pore connectivity. Porosity provides information on the overall pore volume of a porous material and is defined as the ratio of the non-solid volume (voids) to the total volume of the nonwoven fabric. The volume fraction of solid material is defined as the ratio of solid fibre material to the total volume of the fabric.


413

While the fibre density is the weight of a given volume of the solid component only (i.e., not containing other materials), the porosity can be calculated as follows using the fabric bulk density and the fibre density:

rfabric ¥ 100% rfibre

9.6

e(%) = (1 – f) ¥ 100%

9.7

f (%) =

where e is the fabric porosity (%), f is the volume fraction of solid material (%), rfabric (kg/m3) is the fabric bulk density and rfibre(kg/m3) is the fibre density. In resin coated, impregnated or laminated nonwoven composites, a small proportion of the pores in the fabric is not accessible (i.e., they are not connected to the fabric surface). The definition of porosity as shown above refers to the so-called total porosity of the fabric. Thus, the open porosity (or effective porosity) is defined as the ratio of accessible pore volume to total fabric volume, which is a component part of the total fabric porosity. The majority of nonwoven fabrics have porosities >50% and usually above 80%. A fabric with a porosity of 100% is a totally open fabric and there is no such fabric, while a fabric with a porosity of 0% is a solid polymer without any pore volume; there is no such fabric either. High-loft nonwoven fabrics usually have a low bulk density because they have more pore space than a heavily compacted nonwoven fabric; the porosity of high-loft nonwovens can reach >98%. Pore connectivity, which gives the geometric pathway between pores cannot be readily quantified and described. If the total pore area responsible for liquid transport across any distance along the direction of liquid transport is known, its magnitude and change in magnitude are believed to indicate the combined characteristics of the pore structure and connectivity.

9.4

General standards for testing nonwovens

Various testing methods and techniques have been developed for the measurement of nonwoven fabric properties. These test methods can be grouped as follows: ∑ ∑ ∑

standard test methods defined by standard authorities (e.g., ISO, EN/BS, ASTM, and ANSI) test methods established by industrial associations (e.g., INDA, EDANA, AATCC, etc.) and individual companies non-standard test techniques designed for research purposes.

The standard test methods, which are defined as orderly procedures in a reproducible environment, are designed to provide reliable measurements

414


with certain precision for use in the trading of nonwovens and their products. Industrial test methods are usually established for routine internal measurement concerned with the evaluation, benchmarking and quality control of semifinished or final end products. In addition to these standard tests, numerous techniques are available to characterise nonwoven materials for either research purposes or the monitoring of nonwoven production processes. Various national and international standard systems (ISO (BS, EN, ERT) and ASTM (ITS, AATCC) standards) exist for textiles and nonwovens. Seven major standards in Europe and North America (ISO, BS, EN with ERT, ASTM with ITS and AATCC) are summarised in this section. Many of the test standards from two nonwoven industrial organisations in Europe (EDANA) and in North America (INDA), ERT (by EDANA) and ITS (by INDA), have become part of either ISO (BS and EN) or ASTM standards. EDANA and INDA have worked together to produce a unified set of nonwoven test standards, Worldwide Strategic Partners (WSP), in 2005. A summary of the standards relating to nonwovens with reference to the five standards is given in Table 9.1.

9.4.1

Standards for nonwoven wound dressings (ISO, BP, ASTM and BS)

There are some standard testing methods for medical devices that relate to wound dressings in the ASTM, which include general practice for medical devices, 20,21 analysis of medical materials,22–26 methods for medical packages,27–29 fluid penetration,30 sterilisation and disinfection.31,32 Various standards for wound dressings have been introduced by the ASTM and BS in recent years. The BP33 has defined a series of test methods for surgical dressings. These methods include: fibre identification, yarn number, threads per stated length (unstretched, fully stretched), weight per unit area (nonadhesive dressings, adhesive dressings, weight of adhesive mass), minimum breaking load, elasticity, extensibility, adhesiveness, water-vapour permeability (tapes, foam dressings), waterproofness, absorbency (sinking time, water holding capacity), water-soluble substances, ether-soluble substances, colour fastness, content of antiseptics, content of zinc oxide in the adhesive mass, X-ray opacity, sulphated ash of surgical dressings and water retention capacity. Other standards relevant to wound dressings, such as the test of sterility,34 test of microbial contamination,35 efficacy of antimicrobial preservation36 and methods of sterilisation37 also are available. A series of standard testing methods for wound dressings was introduced in British and European Standards BS EN 13726. These methods include aspects of absorbency,38 moisture vapour transmission rate of permeable film dressings,39 waterproofness,40 conformability,41 and bacterial barrier properties.42 Other standards related to medical fabrics include the specification for spinal and abdominal fabric supports43 and the specification for the elastic


415

Table 9.1 Summary of nonwoven standards (ASTM, BS, EN, ISO, ERT, ITS, WSP)

Glossary of terms (Vocabulary)

North American (ASTM, AATCC and MIL)

EDANA and INDA (WSP, ITS and ERT ———————————————— ITS WSP ERT

Europe (ISO, BS and EN)

ASTM D123-03

ITS 1

BS ISO 11224: 2003

WSP 1.0 ERT 1.4-02

Definition

ERT 0.0-89

How to write a test method Sample and laboratory conditioning

BS EN 29092: 1992; ISO 9092: 1998

WSP 2.0 ASTM D1776-04

WSP 3.0 ERT 60.2-99

Worldwide associations

WSP 4.0 ERT Useful addresses

Sampling

WSP 5.0 ERT 130.299

List of vendors

ITS Useful WSP 6.0 Vendor’s List

Guideline test ASTM methods for D1117-01 nonwoven fabrics

ITS GL

WSP 7.0

Guidance to highloft test methods

ITS GL

WSP 8.0

Guideline test methods for evaluating nonwoven felts

ITS GL Felts

BS EN ISO 139: 2005; ISO 554: 1976

BS EN 12751: 1999; BS EN ISO 186: 2002

Safety requirements of nonwoven machinery

ISO/FDIS 11111-3

Noise emission of nonwoven machinery

BS EN ISO 9902-3: 2001

Absorption Rate of sorption and sorptive capacity Nonwoven absorption

ASTM D6651-01 ITS 10.1

WSP 10.1

ERT 10.402

BS EN ISO 9073-6: 2003

416


Table 9.1 Continued North American (ASTM, AATCC and MIL)


Rate of sorption of wiping materials

ITS 10.2

WSP 10.2

Demand absorbency

ITS 10.3

WSP 10.3

ERT 230.1-02


BS ISO 907312: 2002 (ISO 12947: 1998)

Abrasion resistance Inflated diaphragm

ASTM D3886-99

ITS 20.1

WSP 20.1

Flexing and abrasion

ASTM D3885-04

ITS 20.2

WSP 20.2

Oscillatory cylinder

ASTM D4157-92

ITS 20.3

Rotary platform, double head method

ASTM D3884-01

ITS 20.4

WSP 20.4

Modified Martindale

ASTM D4966-98

ITS 20.5

WSP 20.5

Uniform abrasion ASTM method D4158-01

ITS 20.6

(BS 5690: 1991)

Bursting strength Diaphragm

ASTM D3786-01

ITS 30.1

WSP 30.1

ITS 30.2

WSP 30.2

Surface resistivity

ITS 40.1

WSP 40.1

Decay

ITS 40.2

WSP 40.2

Nonwoven burst

ERT 80.402

(BS EN ISO 13938-1: 1999)

Electrostatic properties EN 1149-1: 1995; EN 1149-2: 1995

Binder properties Resin binder distribution and penetration

ASTM D5908-96

ITS 50.1

Appearance and integrity of highloft batting

ASTM D4770-00

ITS 50.2

Optical properties Opacity (1)

ITS 60.1

WSP 60.1

(ISO 2471: 1998)

Brightness (1)

ITS 60.2

WSP 60.2

(BS 4432-2: 1980)


417




Brightness (2)

WSP 60.3

ERT 100.1-78

BS ISO 2470: 1999

Opacity (2)

WSP 60.4

ERT 110.1-78

BS ISO 2471: 1998

WSP 70.1

ERT 140.2-99

BS EN ISO 9237: 1995; ISO/CD 9073-15: 2005

ERT 150.5-02

BS EN ISO 9073-8: 1998

Permeability Air permeability

ASTM D737-04;

ITS 70.1

Water vapour transmission (multiple tests)

ASTM D671-01

ITS 70.2

Liquid strikethrough time (simulated urine)

ITS 70.3

WSP 70.3

Water vapour transmission (Mocon) (rates of 500 to 100,000 gm/m2/day)

ITS 70.4

WSP 70.4

Water vapour transmission (Mocon) (relative humidity)

WSP 70.5

ERT New Method part 1

Water vapour transmission rate (Lyssy)

WSP 70.6

ERT New Method part 2

Repeated liquid strike-through time

WSP 70.7

ERT 153.0-02

ISO 9073-13: 2001; 04/30094395 DC

Repellency Repellency

ERT 120.2-02 ISO 811: 1981 EN 20811: 1992

Surface wetting spray test

ITS 80.1

WSP 80.1

ITS 80.2

WSP 80.2

Penetration by water (spray impact test)

ITS 80.3

WSP 80.3

Penetration by AATCC water (hydrostatic 127-98 pressure test)

ITS 80.4

(WSP 80.6)

Penetration by water (rain test)

AATCC 4294

ISO/CD 9073-17

418




Penetration by saline solution (automated mason jar test)

ITS 80.5

WSP 80.5

Water resistance (hydrostatic pressure test)

ITS 80.6

WSP 80.6

Penetration by oil (hydrocarbon resistance)

ITS 80.7

WSP 80.7

Alcohol repellency of nonwoven fabrics

ITS 80.8

WSP 80.8

Nonwovens run-off

ITS 80.9

WSP 80.9

ERT 152.1-02

WSP 80.10

ERT 151.3-02

Coverstockwetback Wetback after repeated strikethrough time Wet barrier – mason jar


ISO/CD 907316: 2005

ERT 154.0-02

BS ISO 907311: 2002

ISO 9073-14: 2001; 04/30094399 DC

(MIL-F(ITS 80.5) 36901A, section 4.3.3)

WSP 80.11

ERT 170.1-02

ASTM D5732-95

ITS 90.1

WSP 90.1

Gurley Handle-O-Meter

ITS 90.2 ITS 90.3

WSP 90.2 WSP 90.3

Drape

ITS 90.4

WSP 90.4

ERT 90.4-99

BS EN ISO 9073-9: 1998

WSP 90.5

ERT 50.602

BS EN ISO 9073-7: 1998 BS 3356: 1990

Stiffness Cantilever

Bending length

(ASTM D1197-97; ASTM D5732-95)

Blade/slot

ASTM D6828-02

Circular bend

ASTM D4032-94 (prEN ISO 13937: 1998)

Tear strength Falling-pendulum ASTM (Elmendorf) D5734-95

ITS 100.1

WSP 100.1

BS EN ISO 13937-1: 2000


419

Table 9.1 Continued

Trapezoid Tongue (single rip) Tensile Grab Seam strength Internal bond strength Strip

Ball burst

North American (ASTM, AATCC and MIL)



ASTM D5733-99 ASTM D5735-95

ITS 100.2

BS EN ISO 9073-4: 1997 BS EN ISO 13937-4: 2000

ASTM D5034-95 ASTM D1683-90a

ITS 110.1

ASTM D5035-95

ITS 100.3

ERT 70.4-99

WSP 110.1

ISO/CD 907318: 2005

ITS 110.2 ITS 110.3 219 ITS 110.4

ASTM D6797-02

Thickness Thickness of non- ASTM woven fabrics D5729-97 Highloft ASTM nonwovens D5736-95 Highloft ASTM compression D6571-01 and recovery (Measurematic) Highloft compression and recovery (plates and weights, room temperature) Highloft compression and recovery (plates and weights, high temperature and humidity) Thickness of nonwoven fabrics Weight Nonwovens mass ASTM per unit area D6242-98; (ASTM D3776-96)

WSP 100.2 WSP 100.3

WSP 110.4

ERT 20.2-89

BS EN 29073-3: 1992; ISO 9073-3: 1992

WSP 120.6

ERT 30.5-99

BS EN ISO 9073-2: 1997

WSP 130.1

ERT 40.3-90

BS EN 290731: 1992; ISO 9073-1: 1989

WSP 110.5 ITS 120.1 ITS 120.2 ITS 120.3

WSP 120.1 WSP 120.2 WSP 120.3

ITS 120.4

WSP 120.4

ITS 120.5

WSP 120.5

ITS 130.1

420





Friction Static and kinetic

ITS 140.1

Appearance/Integrity Resistance

ASTM D2724-95

ITS 150.1

WSP 150.1

Appearance and integrity of highloft batting

ASTM D4770-00

ITS 50.2

WSP 150.2

(ASTM F51)

ITS 160.1

WSP 160.1

ITS 160.2

WSP 160.2

ITS 160.3

WSP 160.3

ITS 160.4

WSP 160.4

Linting Particulate shedding (dry) Particulate shedding (wet) Fibrous debris from nonwoven fabrics

ASTM D6652-01

Fibrous debris from hydrophobic nonwoven fabrics Surface linting

WSP 400.0

ERT 220.102

BS EN ISO 9073-10:2004

ERT 300.0-84

Fibre identification Identification of fibres in textiles

ASTM D276-00

ITS 170.1

Geotextiles Geotextiles – vocabulary

ISO 10318:1990

Guidelines on durability

ISO/TR 13434:1998

Sampling

ASTM D4271-01

Mass per unit area

ASTM D5261-92

ITS 180.1 EN 965:1995

Thickness

EN 964-1:1995

Breaking (grab strength)

ASTM D5034-95

ITS 110.1

Trapezoid tear

ASTM D4533-04

ITS 180.3

Puncture strength

ASTM D4833-00el

ITS 180.4

EN ISO 12236: 1996


421



Dynamic perforation (cone drop test)


EN ISO 918: 1995

Bursting strength ASTM D3786-01

ITS 30.1

Pore size

ASTM D4751-04; (D6767-02)

ITS 180.6

EN ISO 12956: 1998

Permittivity

ASTM D4491-99a; (D5493-93)

ITS 180.7

EN ISO 11058: 1998

In-plane transmissivity

ASTM D6574-00

EN ISO 12958: 1998

Thermoplastic ASTM fabrics in roofing/ D4830-98 waterproofing

ITS 180.8

Wide-width tensile test

ITS 180.9

ASTM D4595-86

EN ISO 10319: 1996

Abrasion damage ASTM simulation D4886-88 (sliding block test)

ISO 13427: 1998

Degradable nonwoven fabrics Guide to assess ASTM the compostD6094-97 ability of nonwoven fabrics

ITS 190.1

(BS EN ISO 14855:2004)

Superabsorbent materials pH of polyacrylate (PA) powder

WSP 200.2

ERT 400.2-02

ISO 171901: 2001

Residual monomers

WSP 210.2

ERT 410.2-02

ISO 171902: 2001

Particle size distribution

WSP 220.2

ERT 420.2-02

ISO 171903: 2001

Mass loss upon heating

WSP 230.2

ERT 430.202

ISO 171904: 2001

Free swell capacity in saline, gravimetric determination

WSP 240.2

ERT 440.202

ISO 171905: 2001

422





Fluid retention capacity in saline, after centrifugation

WSP 241.2

ERT 441.2-02

ISO 171906: 2001

Absorption under pressure, gravimetric determination

WSP 242.2

ERT 442.202

ISO 171907: 20001

Permeability dependent absorption under pressure

WSP 243.1

Flow rate, gravimetric determination

WSP 250.2

ERT 450.202

ISO 171908: 2001

Density, gravimetric determination

WSP 260.2

ERT 460.202

ISO 171909: 2001

Extractables

WSP 270.2

ERT 470.202

ISO 1719010: 2001

Respirable particles

WSP 280.2

ERT 480.202

ISO 1719011: 2001

Dust in collection, Sodium atomic absorption/emission spectrometry

WSP 290.2

ERT 490.202

ISO 1719012: 2002

Bacterial filtration (ASTM F2100efficiency 04; F2101-01); MIL-M36954C-1975

WSP 300.0

ERT 180.0. (89)

Dry bacterial penetration

WSP 301.0

ERT 190.102

BS EN ISO 22612:2005 (BS EN 137951: 2002) (BS EN 13795-2: 2004)

WSP 302.0

ERT 200.102

(ISO/FDIS 22610: 2004) (ISO/DIS 22611: 2003)

WSP 310.1

ERT 210.199

EN ISO 141841: 1999

Bacterial

(ASTM F1670-03; F1671-03; F1819-04; F1862-00a)

Wet bacterial penetration

Toxicity Free formaldehyde–I (water extraction method)


423



Free formaldehyde–II (under stressed conditions)

WSP 311.0

ERT 211.199

Free ASTM formaldehyde–III D5910-96 (determination by HPLC)

WSP 312.0

ERT 212.096

Free formaldehyde–IV (in drying conditions)

WSP 313.0

ERT 213.099

Syngina method (tampons)

WSP 350.1

ERT 350.002

Ethanolextractable organotin 1

WSP 351.0

ERT 360.002

Synthetic urineextractable organotin II

WSP 352.0

ERT 361.002

WSP 401.0

New method


EN ISO 141842: 1998

Absorbent hygiene products

Other methods Lamination strength Wiping efficiency Dynamic wiping ASTM efficiency, wet D6650-01 particle removal ability, and fabric particle contribution for nonwovens used in cleanrooms Dynamic wiping ASTM efficiency of non- D6702-01 wovens not used in cleanrooms Interlinings Fusible interlinings

BS 4937-1: 1973 BS 4937-2: 1973

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properties of fabric bandages.44 The standards of test methods for medical nonwoven compresses45,46 was introduced in the British Standards recently. The Methods of Testing Surgical Dressings & Surgical Dressing Materials have been defined in AS2836.0~ AS2836.11-1998 as follows: general introduction and list of methods,47 methods for the determination of loss of mass on drying,48 identification of cotton and viscose fibres,49 determination of mass per unit area,50 determination of size,51 method for the determination of sinking time,52 determination of absorption rate and water holding capacity,53 determination of level of surface-active substances,54 determination of quantity of water-soluble substances,55 determination of the presence of starch and dextrins,56 determination of the presence of fluorescing substances57 and determination of sulfated ash content.58 The standards for testing tissueengineering medical products are also established.59–61 The testing standards related to nonwovens for medical applications are those used in the manufacture of compresses,62 finished compresses,63 uncoated nonwoven used for medical packaging,64 adhesive coated nonwoven used for medical packaging65 and packaging for terminally sterilised medical devices.66

9.4.2

Standards for air filtration

The following international and industry standards specify the filtration performance for various air filtration applications. National and international standards ISO (International Standardization Organization), IEC (International Electrotechnical Commission), CEN (European Committee for Standardization), CENELEC (European Committee for Electrotechnical Standardization), BS (British Standard), ANSI (American National Standard Institute) and ASTM (American Society for Testing Methods). Standards for specific industrial filtration products that are also available in the absence of a specific international standard ASHRAE (American Society of Heating and Refrigerating and Air-conditioning Engineers), SAE (Society for Automotive Engineers), ISIAQ (International Society of Indoor Air Quality and Climate), UL (Underwriters Laboratories), AHAM (Association for Home Appliance Manufactures), IES (Institute of Environmental Sciences). Standards for heating, ventilation, air conditioning (HVAC) BS EN 13142:2004: Ventilation for buildings. Components/products for residential ventilation. Required and optional performance characteristics.


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BS EN 13053:2001: Ventilation for buildings. Air handling units. Ratings and performance for units, components and sections. BS EN 779:2002: Particulate air filters for general ventilation. Determination of the filtration performance. BS EN 1822-1:1998 High efficiency air filters (HEPA and ULPA). Classification, performance testing, marking. BS EN 1822-2:1998, High efficiency air filters (HEPA and ULPA). Aerosol production, measuring equipment, particle counting statistics. BS EN 1822-3:1998 High efficiency air filters (HEPA and ULPA). Testing method for flat sheet filter media. BS EN 1822-4:2000 High efficiency air filters (HEPA and ULPA). Describes determination of the leakage of a filter element (scan method). BS EN 1822-5:2000 High efficiency air filters (HEPA and ULPA). Describes determination of the efficiency of a filter element. ASHRAE 52.2 Method of Testing General Ventilation Air-cleaning Devices for Removal Efficiency by Particle Size. Mil F-51068F Filters, Particulate (High- Efficiency Fire Resistant). IES RP-CC021.1 HEPA and ULPA Filter Media. IES RP-CC001.3 HEPA and ULPA Filters. Standards for healthcare and medical BS EN 13328-1:2001: Breathing system filters for anaesthetic and respiratory use. Salt test method to assess filtration performance. Nelson Laboratories: Bacterial & Virus Filtration efficiency test. Standards for automotive industry ISO/TS 11155-1: Road vehicles – Air filters for passenger compartments – Part 1: Test for particulate filtration (DIN 71460-1:2001). ISO/TS 11155-2: Road vehicles – Air filters for passenger compartments – Part 2: Test for gaseous filtration (DIN 71460-2: 2003). Standards for appliances: vacuum cleaners, room air cleaners and room air purifiers IEC 60312: Vacuum cleaners for Household use – Methods of measuring the performance (2001-11). ASTM 1977-99: Standard Test Method for Determining Initial, Fractional Efficiency of a Vacuum Cleaner System. ANSI AHAM AC-1-1988: Method for Measuring Performance of Portable Household Electric Cord-Connected Room Air Cleaners (RAC). ASTM D 6830-02 Standard Test Method for Characterizing the Pressure Drop and Filtration Performance of Cleanable Filter Media.

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ASHRAE Standard 52.1-1992, Gravimetric and Dust Spot Procedures for Testing Air Cleaning Devices Used in General Ventilation for Removing Particulate Matter. ANSI/ASHRAE Standard 52.2-1999, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size are the accepted test methods for air filter.

9.5

Measurement of basic parameters

9.5.1

Standard test method for resin binder distribution and binder penetration analysis of polyester nonwoven fabrics

WSP 150.1 (equivalent to ITS50.1 and ASTM 5908-96) is a test method designed for analysing resin binder distribution and binder penetration in polyester nonwovens. A specimen of the fabric (in full width and 0.6 m in length) is dyed (C.I. Basic Red 14) in a 60-litre solution with a concentration of 0.2% at 120 to 140 ∞F for 15 mins. After drying, the stained specimen is examined and rated for binder distribution on the nonwoven fabric surface and binder penetration through the fabric thickness by comparison to photographic rating standards on a scale of 1–5.

9.5.2

Fabric thickness67,68

Testing of nonwoven fabric thickness and fabric weight is similar to other textile fabrics but due to the greater compressibility and unevenness a different sampling procedure is adopted. The thickness of a nonwoven fabric is defined as the distance between the face and back of the fabric and is measured as the distance between a reference plate on which the nonwoven rests and a parallel presser-foot that applies a pressure to the fabric (See BS EN ISO 97032:1995, ITS 10.1). Nonwoven fabrics with a high specific volume, i.e., bulky fabrics, require a special procedure. In this context, bulky fabrics are defined as those that are compressible by 20% or more when the pressure applied changes from 0.1 kPa to 0.5 kPa. Three procedures are defined in the test standard (BS EN ISO 9703-2:1995) as summarised in Table 9.2. Three test methods, (i.e., ASTM D5729-97 (ITS 120.1), ASTM D5736-01 (ITS 120.2), ASTM D6571-01 (ITS 120.3)) (Table 9.3) are defined for the measurement of the thickness, compression and recovery of conventional nonwovens and high-loft nonwovens (it is defined, in the ASTM, as a low density fibre network structure characterised by a high ratio of thickness to mass per unit area. High-loft batts have no more than a 10% solid volume and are greater than 3 mm in thickness). Two more test standards (ITS

Area of presser foot plate

Area of lower reference plate

Measurement accuracy (mm)

Orientation of reference plate (kPa)

Pressure applied

Size of samples (mm2)

Number of test samples

2500 mm2

>19,216 mm2

+/–1.0

Horizontal, circular

0.5

2500 mm2

10

Maximum thickness up to 20 mm

2500 mm2

1000 mm2

+/–0.1

Vertical, circular/ square

0.02

130 mm ¥ 80 mm

Measurement time duration: 10 seconds

Maximum thickness from 20 mm to 100 mm

200 mm ¥ 200 mm

300 mm ¥ 300 mm

+/–0.5

Horizontal, square

0.02

200 mm ¥ 200 mm

Normal fabric Bulky fabrics


Table 9.2 Summary of testing method BS EN ISO 9703-2: 1995

427

Compression and Recovery of Highloft nonwovens*

Conventional treated or untreated fabrics, (ASTM D5729-97) Highloft nonwovens* (ASTM D5736-95, ITS 120.2)

Pressure applied (kPa)

Size of samples

Number of samples

Diameter 25.4 ± 0.02

4.14 ± 0.21

20% greater than presser foot

10

5s

300 mm by 300 mm

0.03

130 mm by 80 mm

5

9~10 s

0.03 /1.73/ 0.03

200 mm by 200 mm

5

10 s/30 min/ 5 min

1.83

200 mm by 200 mm by (min) 100 mm

Applied and removed at a series of time intervals.

10 mins to 56 hours

ASTM D6571-01

Repeated compression and recovery (Weightplate, ITS120.4; ITS120.5)

230 mm by 230 mm by 6.4 mm

* Highloft nonwovens are defined as porosity >90% and thickness >=3 mm

Test duration time

Report

Thickness, SD, CV

Percentage compression; percentage recovery Compression resistance; elastic loss; immediate recovery; long-term recovery


Thickness

Dimension of presser foot plate (mm)

428

Table 9.3 Summary of testing method ASTM D5729-97 (ITS 120.1). ASTM D5736-01 (ITS 120.2) and ASTM D6571-01 (ITS 120.3)


429

120.4, ITS 120.5) are defined for rapid measurement of the compression and recovery of high-loft nonwovens.

9.5.3

Fabric mass per unit area

The measurement of a nonwoven weight per unit area requires a specific sampling procedure, specific dimensions for the test samples, and a greater balance accuracy than for conventional textiles. According to the ISO standards (BS EN 29073-1:1992, ISO 9073-1:1989, ITS10.1), the measurement of nonwoven fabric mass per unit area of nonwovens requires each piece of fabric sample to be at least 50,000 mm2. The mean value of fabric weight is calculated in grams per square metre and the coefficient of variation is expressed as a percentage.

9.5.4

Fabric weight uniformity

Nonwoven fabric uniformity refers to the variations in local fabric structures, which include thickness and density, but is usually expressed as the variation of the weight per unit area. Both subjective and objective techniques are used to evaluate the fabric uniformity. In subjective assessment, visual inspection can distinguish non-uniform areas as small as about 10 mm2 from a distance of about 30 cm. Qualitative assessments of this type can be used to produce ratings of nonwoven fabric samples by a group of experts against benchmark standards. The consensual benchmark standards are usually established by an observer panel using paired comparison, graduated scales or similar voting techniques; these standard samples are then used to grade future samples. Indirect objective measurements of the web weight uniformity have been developed based on variations in other properties that vary with fabric weight including the transmission and reflection of beta rays, gamma rays (CO60), lasers, optical and infra-red light,69 and variation in tensile strength. With optical light scanning methods, the fabrics are evaluated for uniformity using an optical electronic method, which screens the nonwoven to register 32 different shades of grey.9,70 The intensity of the points in the different shades of grey provides a measure of the uniformity. A statistical analysis of the optical transparency and the fabric uniformity is then produced. This method is suitable for lightweight nonwovens of 10–50 g/m2. Optical light measurements are commonly coupled with image analysis to determine the coefficient of variation of grey level intensities from scanned images of nonwoven fabrics.71 In practice, nonwoven fabric uniformity depends on fibre properties, fabric weight and manufacturing conditions. It is usually true that the variation in fabric thickness and fabric weight decreases as the mean fabric weight increases. Wet-laid nonwovens are usually more uniform in terms of thickness than

430


dry-laid fabrics. Short fibre airlaid fabrics are commonly more uniform than carded and crosslaid and parallel-laid fabrics, and spunbond and meltblown fabrics are often more uniform than fabrics produced from staple fibres.

9.6

Measuring fibre orientation distribution

In modelling the properties of nonwoven fabrics and particularly in any quantitative analysis of the anisotropic properties of nonwoven fabrics, it is important to obtain an accurate measurement of the fibre orientation distribution (FOD). A number of measuring techniques have been developed. A direct visual and manual method of measurement was first described by Petterson.17 Hearle and co-workers72,73 found that visual methods produce accurate measurements and it is the most reliable way to evaluate the fibre orientation. Manual measurements of fibre segment angles relative to a given direction were conducted and the lengths of segment curves were obtained within a given range. Chuleigh74 developed an optical processing method in which an opaque mask was used in a light microscope to highlight fibre segments that are orientated in a known direction. However, the application of this method is limited by the tedious and time-consuming work required in visual examinations. To increase the speed of assessment, various indirect-measuring techniques have been introduced including both the zero span75,76 and short span77 tensile analysis for predicting the fibre orientation distribution. Stenemur78 devised a computer system to monitor fibre orientation on running webs based on the light diffraction phenomenon. Methods that employ X-raydiffraction analysis and X-ray diffraction patterns of fibre webs have also been studied.79,80 In this method the distribution of the diffraction peak of the fibre to X-ray is directly related to the distribution of the fibre orientation. Other methods include the use of microwaves,81 ultrasound,82 light diffraction methods,83 light reflection and light refraction,84 electrical measurements85,86 and liquid-migration-pattern analysis.87,88 In the last few decades, image analysis has been employed to identify fibres and their orientation,89–92 and computer simulation techniques have come into use for the creation of computer models of various nonwoven fabrics.93–96 Huang and Bressee89 developed a random sampling algorithm and software to analyse fibre orientation in thin webs. In this method, fibres are randomly selected and traced to estimate the orientation angles; test results showed excellent agreement with results from visual measurements. Xu and Ting96 used image techniques to measure structural characteristics of fibre or fibre bundle segments in a thin nonwoven fabric. The structural characteristics measured included length, thickness, curl and the orientation of fibre segments. Pourdeyhimi et al.97–100 completed a series of studies on the fibre orientation


431

of nonwovens by using an image analyser to determine the fibre orientation in which image processing techniques such as computer simulation, fibre tracking, Fourier transforms and flow field techniques were employed. In contrast to two-dimensional imaging techniques suitable only for thin nonwoven fabrics, the theory of Hilliard-Komori-Makishima101 and the visualisations made by Gilmore et al.15 using X-ray tomographic techniques have provided a means of analysing the three-dimensional orientation. Image analysis is a computer-based means of converting the visual qualitative features of a particular image into quantitative data. The measurement of the fibre orientation distribution in nonwoven fabrics using image analysis is based on the assumption that in thin materials a two-dimensional structure can be assumed, although in reality the fibres in a nonwoven are arranged in three dimensions. However, there is currently no generally accepted way of characterising the fabric structure in terms of the three-dimensional geometry. The fabric geometry is reduced to two dimensions by evaluation of the planar projections of the fibres within the fabric. The assumption of a twodimensional fabric structure is adequate to describe thin fabrics. The image analysis system in the measurement of the fibre orientation distribution is based on a computerised image capture system operating with an integrated image analysis software package in which numerous functions can be performed.102 A series of sequential operations is required to perform image analysis and, in a simple system, the following procedures are carried out:102 production of a grey image of the sample fabric, processing the grey image, detection of the grey image and conversion into binary form, storage and processing of the binary image, measurement of the fibre orientation and output of results.

9.7

Measuring porosity, pore size and pore size distribution

Porosity can be obtained from the ratio of the fabric density and the fibre density. In addition to the direct method of determination for resin impregnated dense nonwoven composites, the fabric porosity can be determined by measuring densities using liquid buoyancy or gas expansion porosimetry.103 Other methods include small angle neutron, small angle X-ray scattering and quantitative image analysis for total porosity. Open porosity may be obtained from xylene and water impregnation techniques,104 liquid metal (mercury) impregnation, nitrogen adsorption and air or helium penetration. Existing definitions of pore geometry and the size of pores in a nonwoven are based on various physical models of fabrics for specific applications. In general, cylindrical, spherical or convex shaped pores are assumed with a distribution of pore diameters. Three groups of pore size are defined: (i) the near-largest pore size (known as apparent opening pore size, or opening pore

432


size), (ii) the constriction pore size (known as the pore-throat size) and (iii) the pore volume size. Pore size and the pore size distribution of nonwoven fabrics can be measured using optical methods, density methods, gas expansion and adsorption, electrical resistance, image analysis, porosimetry and porometry. The apparent pore opening (or opening pore) size is determined by the passage of spherical solid glass beads of different sizes (50 mm to 500 mm) through the largest pore size of the fabric under specified conditions. The pore size can be measured using sieving test methods (dry sieving, wet sieving and hydrodynamic sieving). The opening pore sizes are important for determining the filtration and clogging performance of nonwoven geotextiles and it enables the determination of the absolute rating of filter fabrics. The constriction pore size, or porethroat size, is different from the apparent pore opening size. The constriction pore size is the dimension of the smallest part of the flow channel in a pore and it is important for fluid flow transport in nonwoven fabrics. The largest pore-throat size is called the bubble point pore size, which is related to the degree of clogging of geotextiles and the performance of filter fabrics. The pore-throat size distribution and the bubble point pore size can be obtained by liquid expulsion methods. However, it is found that wetting fluid, air pressure and equipment type affects the measured constriction pore size.105,106 A summary of the test methods for the determination of pore size distribution is produced in Table 9.4.

9.7.1

Dry sieving (ASTM D4751107 and BS 6906-2108)

Dry sieving involves passing spherical glass beads (or sand particles) through a nonwoven fabric to determine the fraction of bead sizes for which 5% (or 10%, 50%) or less, by weight, passes through the fabric. The apparent opening size (AOS) or O95 (or O90, O50) of the fabric is determined. However, the test accuracy for pore opening sizes smaller than 90 mm is questionable due to various problems in the testing procedure.109,110

9.7.2

Wet sieving111,112

Wet sieving is based on the dry sieving method, the primary differences are that a continuous water spray is applied to the glass beads and the test fabric during shaking. The continuous water spray reduces electrostatic charging associated with the glass bead particles; mixtures of many different glass bead sizes are used in testing rather than size fractions.

9.7.3

Hydrodynamic sieving113–115

Hydrodynamic sieving is based on hydrodynamic filtration as proposed by Fayoux.113 Glass bead mixtures are sieved through nonwovens by introducing

Table 9.4 Summary of test methods for the determination of pore size distribution Measurement of pore properties

Mechanism

Adsorption

Surface of pores

Multilayer molecular adsorption on solids

Pycnometry

Surface of pores

Calorimetry

Pore surfaces

Porosimetry

Pore volume

Porometry

Constriction pore size

Liquid – air or a set of inert gases (e.g., Helium) with known molecular size and adsorption on the sample Thermal effect of wetting liquid penetration into the pores Filling up the volume of the pores, weight or volume of gas/liquid Rate of gas flow when liquid was expulsed from pores

Particles passing through opening pores Gases passing through liquid occupied pores Pores in 2D image

Pore size and distribution


Porosity

Range of pore sizes

Specific surface Range 0.3–200 mm (0.1–1000 m2/g), pore volume Total pore volume, 0.2–1 nm pore volume, density of the solid sample

Specific surface

Range 0.5–1 nm

Porosity, pore volume and size, specific surface

Range 1 nm– 1000 mm

Filter flow pore size distribution

Range 2–1000 mm


Range 0.5–700 nm

Largest apparent opening size (AOS) Largest constriction size of pores Pore size distribution

>=90 mm N/A N/A

433

Small angle Closed pores X-rays or neutron dissipation (0–2 degrees) Sieving test Largest apparent opening size (AOS) Bubble point Largest constriction size of pores Image analysis Apparent opening size

Pore size distribution


Testing methods

434


a water flow with repeated immersion of the fabric in water. Hydrodynamic sieving is usually used to determine the O95, known as the filtration opening size (FOS), of geotextile fabrics. An example of the hydrodynamic sieving testing procedure was proposed by Mlynarek et al.114 Four testing chambers are used each consisting of a 140 mm diameter cylinder. At the base of each testing cylinder, a fabric sample is supported by two perpendicular supports, each of which is 12.7 mm wide by 55 mm long and has nine equally spaced holes with a diameter of 9 mm. The spherical glass bead mixtures used for hydrodynamic sieving tests are similar to those used in wet sieving, but their diameters range from 25 mm to 250 mm. In summary, sieving test methods (dry, hydrodynamic and wet) are:109 ∑ ∑ ∑

all based on the probability of the spherical particles of a certain diameter passing through an opening during shaking or cycles of immersion methods that provide arbitrary results, because random probability governs whether a bead meets an opening size through which it can pass limited because they measure only the largest pore sizes in the fabric.

9.7.4

Image analysis116–119

Image analysis can be used to determine the apparent opening sizes (AOS) of nonwovens, O95 and O50. Thin sections of fabric are prepared, which requires epoxy-resin impregnation of the sample, cutting, grinding, lapping and polishing. Measurements are performed following optical microscopy or SEM images are produced. The pore size distribution obtained from image analysis is different from sieving test results because in the former the pore dimensions are measured in a two-dimensional plane and measuring accuracy depends on the quality of the cross-section taken. It has been established that120 the image-based O95 pore opening sizes obtained for nonwoven geotextiles are comparable to dry sieving results based on AOS, while the image-based O50 pore opening sizes are lower than those obtained by the dry sieving test (AOS) (O50).

9.7.5

Bubble point test method121,122

Bubble point refers to the pressure at which the first flow of air through a liquid saturated fabric sample occurs and it is a measure of the largest porethroat in a sample.123 The bubble point method is based on the principle that the critical pressure of an airflow applied across the thickness of a fabric evacuates the fluid trapped in the pore with the largest pore-throat. Therefore the applied pressure must exceed the capillary pressure of the fluid in the largest pore-throat. In testing, a nonwoven fabric specimen is saturated with a liquid. The gas pressure on the upstream face of the saturated fabric is then slowly


435

increased to a critical pressure when the first air bubble passes through the largest pore-throat in the saturated fabric. Based on the Laplace equation of capillary pressure, the diameter of the largest pore-throat can then be calculated.

9.7.6

Liquid expulsion porometry123,124

Both the pore-throat size distribution and the largest pore-throat size can be determined by means of porometry, which is based on liquid expulsion. First, the relationship between the airflow rate through a liquid saturated fabric and the applied pressure when the liquid is expelled from the saturated fabric sample is determined. In the test, the airflow pressure is applied across the saturated fabric to force liquid out of the pores. With an increase in the applied pressure, the trapped liquid in the pores of the fabric is gradually forced out. According to the Laplace theory of capillary pressure, the smaller the pore diameter, the greater the applied pressure needed to overcome the capillary pressure and to push the liquid out of the pore. The relationship between the applied pressure, the pore sizes and the airflow rate through the pores can be established. However, to quantify the airflow rate through pores of different sizes, the relationship between the airflow rate through the pores in the dry fabric sample and the applied pressure should be established. By comparing the flow rates for both a dry and a saturated sample at the same applied pressure, the percentage of flow passing through pores larger than or equal to a certain size can be calculated, and the pore size distribution between the pore diameters corresponding to any pressure interval l to h from flows at l and h in terms of air flow rate (not in terms of the number of pores in the fabric) can be defined: wet flow h wet flow l ˆ – Q= Ê ¥ 100% Ë dry flow h dry flow l ¯

9.8

The pore size corresponding to the applied pressure can be determined by the following equation: d=

4s ¥ 10 6 p

9.9

where d is the pore diameter (mm); s is the surface tension (N/m) of the liquid and the contact angle between the liquid and the pore wall is assumed to be zero; p is the capillary pressure (equivalent to the applied pressure) (Pa). An example of the flow rate against applied pressure for wet and dry runs, performed on a nonwoven fabric is given in Fig. 9.5.

436

Handbook of nonwovens 1200 Wet run Dry run Half of dry run

Rate of airflow (ml/s)

1000

800

600

400

Pressure Pressure at largest at mean flow pore pore Pressure at smallest pore

200

0 0.00 0.78 0.83 1.14 1.33 1.43 1.60 2.06 2.65 3.45 Applied pressure (kPa) (a)

4.5

5.5

Pore size distribution (%)

25

20

15

10

5

0 56.28

72.32

94.41 121.99 140.32 152.17 168.43 221.69 259.97 Pore diameter (mm) (b)

9.5 Examples of differential flow pore size distributions for a nonwoven fabric measured by liquid expulsion porometry. (a) The rate of airflow against applied pressure for wet and dry runs, performed on a nonwoven fabric. (b) Differential flow pore size distribution of the nonwoven fabric.

9.7.7

Pore volume distribution and mercury porosimetry125–128

Unlike porometry where the measurement of the pore-throat size distribution is based on measurement of the airflow rate through a fabric sample, the pore


437

volume distribution is determined by liquid porosimetry, which is based on the liquid uptake concept proposed by Haines.129 A fabric sample (either dry or saturated) is placed on a perforated plate and connected to a liquid reservoir. The liquid having a known surface tension and contact angle is gradually forced into or out of the pores in the fabric by an external applied pressure. Porosimetry is grouped into two categories based on the liquid used, which is either non-wetting (e.g., mercury) or wetting (e.g., water). Each is used for intrusion porosimetry and extrusion porosimetry where the advancing contact angle and receding contact angle are applied in liquid intrusion and extrusion porosimetry respectively. Mercury has a high surface tension and is strongly non-wetting on most fabrics at room temperature. In a typical mercury porosimetry measurement, a nonwoven fabric is evacuated to remove moisture and impurities and then immersed in mercury. A gradually increasing pressure is applied to the sample forcing mercury into increasingly smaller ‘pores’ in the fabric. The pressure P required to force a non-wetting fluid into a circular cross-section capillary of diameter d is given by: P=

4s Hg cos g Hg d

9.10

where sHg is the surface tension of the mercury (0.47 N/m),130 and gHg is the contact angle of the mercury on the material being intruded (the contact angle ranges from 135∞~180∞), and d is the diameter of a cylindrical pore. The incremental volume of mercury is recorded as a function of the applied pressure to obtain a mercury intrusion curve. The pore size distribution of the sample can be estimated in terms of the volume of the pores intruded for a given cylindrical pore diameter d. The pressure can be increased incrementally or continuously (scanning porosimetry). The process is reversed by lowering the pressure to allow the mercury to extrude from the pores in the fabric to generate a mercury extrusion curve. Analysis of the data is based on a model that assumes the pores in the fabric are a series of parallel non-intersecting cylindrical capillaries of random diameters (capillary tube model).131 However, as a consequence of the nonwetting behaviour of mercury in mercury intrusion porosimetry, relatively high pressure is needed to force mercury into the smaller pores therefore compressible nonwoven fabrics are not suitable for testing using the mercury porosimetry method. Liquids other than mercury find use in porosimetry132–134 and have been commercialised.135 Test procedure is similar to that of mercury porosimetry but any liquid that wets the sample, such as water, organic liquids, or solutions may be utilised. The cumulative and differential pore volume distribution, total pore volume, porosity, average, main, effective and equivalent pore size can be obtained.

438

9.7.8


Measurement of specific surface area by using gas adsorption136–138

The number of gas molecules adsorbed on the surface of nonwoven materials depends on both the gas pressure and the temperature. An experimental adsorption isotherm plot of the incremental increases in weight of the fabric due to absorption against the gas pressure can be obtained in isothermal conditions. Prior to measurement, the sample needs to be pre-treated at an elevated temperature in a vacuum or flowing gas to remove contaminants. In physical gas adsorption, when an inert gas (such as nitrogen or argon) is used as an absorbent gas, the adsorption isotherm indicates the surface area and/ or the pore size distribution of the objective material by applying experimental data to the theoretical adsorption isotherm for gas adsorption on the polymer surface. In chemical gas adsorption, the chemical properties of a polymeric surface are revealed if the absorbent is acidic or basic. In some experiments, a liquid absorbent such as water is used in the same manner. Physical gas adsorption In physical gas adsorption, an inert gas such as nitrogen (or argon, krypton, carbon dioxide) is adsorbed on the fibre surfaces of the fabric. Usually the Brunauer-Emmett-Teller (BET)139 multilayer adsorption isotherm theory is used based on the following hypotheses: (i) gas molecules are physically adsorbed on a solid polymer surface in layers infinitely; (ii) there is no interaction between each adsorption layer; and (iii) the Langmuir theory for monolayer adsorption can be applied to each layer. The BET equation is therefore shown as follows: Ê p ˆ + 1 1 1 = Vadsorption [( p 0 / p ) – 1] Vmonolayer c Ë p 0 ¯ Vmonolayer

9.11

Ê E1 – E L ˆ

where c is the BET constant, c = e Ë RT ¯ , E1 is the heat of adsorption for the first layer, EL is that for the second and additional layers and is equal to the heat of liquefaction, p and p0 are the equilibrium and the saturation pressure of gases at the temperature of adsorption, Vadsorption is the adsorbed gas quantity (for example, in units of volume), and Vmonolayer is the monolayer adsorbed gas quantity. R is gas constant and T is temperature. There is a linear relationship of the adsorption isotherm between

1 Vadsorption [( p 0 / p )–1] and p/p0 when 0.05 < p/p0 < 0.35, and the monolayer adsorbed gas quantity Vmonolayer and the BET constant c can be obtained from the slope and the y-intercept of the straight line respectively in the plot. The


439

total surface area of the nonwoven, Stotal, and the specific surface area, S, can therefore be obtained as follows: Vmonolayer Ns M S S = total a

Stotal =

9.12 9.13

where N is Avogadro’s number (6.022 ¥ 1023), s is the adsorption crosssection of the fibre polymer material to specific gases, M is the molecular weight of the fibre polymer materials, and a is the weight of the fabric sample. Chemical gas adsorption In chemical gas adsorption a reactive gas such as hydrogen or carbon monoxide is used to obtain information on the active properties of the porous material and is frequently used in the characterisation of nano-scale pores in polymer membranes and metal materials but not usually for nonwoven fabrics containing bigger pores. Helium porosity analysis using pycnometry Helium-pycnometry gives information on the true density of solids (or skeletal density) by means of helium, which is able to enter the smallest voids or pores (up to 1 angstrom) in the surface to measure the volume per unit weight.

9.8

Measuring tensile properties

Some of the most important fabric properties governing the functionality of nonwoven materials include mechanical properties (tensile, compression, bending and stiffness), gaseous and liquid permeability, water vapour transmission, liquid barrier properties, sound absorption properties and dielectric properties. Mechanical properties of nonwoven fabrics are usually tested in both machine direction (MD) and cross-direction (CD), and may be tested in other bias directions if required. Several test methods are available for tensile testing of nonwovens, chief among these are the strip and grab test methods. In the grab test, the central section across the fabric width is clamped by jaws a fixed distance apart. The edges of the sample therefore extend beyond the width of the jaws. In the standard grab tests for nonwoven fabrics,140 the width of the nonwoven fabric strip is 100 mm, and the clamping width in the central section of the fabric is 25 mm. The fabric is stretched at a rate of 100

440


mm/min (according to the ISO standards) or 300 mm/min (according to the ASTM standards) and the separation distance of the two clamps is 200 mm (ISO standards) or 75 mm (ASTM standards). Nonwoven fabrics usually give a maximum force before rupture. In the strip test, the full width of the fabric specimen is gripped between the two clamps. The width of the fabric strip is 50 mm (ISO standard) or either 25 mm or 50 mm (ASTM standards). Both the stretch rate and the separation distance of the two clamps in a strip test are the same as they are in the grab test. The separation distance of the two clamps is 200 mm (ISO standards) or 75 mm (ASTM standards). The observed force for a 50 mm specimen is not necessarily double the observed force for a 25 mm specimen.

9.9

Measuring gas and liquid permeability

Intrinsic permeability (also called the specific permeability or absolute permeability) of a nonwoven fabric is a characteristic feature of the fabric structure and represents the void capacity through which a fluid can flow. dp The specific permeability k is defined by D’Arcy as: v = – k , where v is h dx the volumetric flow rate of the fluid in a unit flow area (m/s); h is the liquid viscosity (Pa.s); dp is the difference in hydraulic pressure (Pa); dx is the conduit distance (m) and k is the specific permeability (m2). In practical engineering applications of D’Arcy’s law, sometimes the preference is to use the permeability coefficient, K, which is also referred to as conductivity or D’Arcy’s coefficient. This characterises a fluid flowing through the porous medium at a superficial flow rate. The permeability coefficient K is defined in D’Arcy’s law as: v = K * i. Where v is the volumetric flow rate of the fluid in a unit flow area (m/s); i is the hydraulic gradient, (i.e., the differential hydraulic head per conduit distance (m/m)); and K is the permeability coefficient (m/s). The relationship between k and K is given as: k = Kh/rg (m2), where r is the liquid density (kg/m3) and g is the gravity accelerator constant (m/s2). When the liquid is water at a temperature of 20∞C, the constant becomes, k(m2) = 1.042 ¥ 10–7K(m/s). In permeability testing, the fluids used are either air or water, and the volumetric rate of the fluid flow per unit cross-sectional area are measured and recorded against specific differential pressures to obtain the air permeability or water permeability. The testing of air permeability in nonwoven fabrics is defined by the ASTM,141 ISO,142 and WSP143 (ITS and ERT) standards. The testing equipment includes the Frazier air permeability tester, the liquid expulsion porometer and the water permeability tester for geotextiles. In air permeability tests, the volumetric airflow rate through a nonwoven fabric of unit cross-sectional area at a unit differential pressure under laminar flow conditions is the fabric permeability.


441

In water permeability tests, the volumetric flow rate of water flow through a fabric of unit cross-sectional area at a unit differential pressure under laminar flow conditions is measured as the hydraulic conductivity or permittivity under standard conditions (also frequently called the permeability coefficient). Two procedures are utilised; the constant hydraulic pressure head method and the falling hydraulic pressure head methods. In the falling hydraulic head test, a column of water is introduced to the fabric to induce flow through its structure and both the water flow rate and the pressure change against time are taken. The constant head test is used when the fabric is so highly porous that the flow rate becomes very large and it is difficult to obtain a relationship of the pressure change against time during the falling hydraulic test. The intrinsic permeability may be obtained by dividing this fluid flow rate by both the fabric thickness and the viscosity of air (or water). However, the thickness of the nonwoven fabric is usually compressed by the applied pressure during permeability testing, which makes it impossible to use the nominal thickness of the fabric if an accurate assessment of specific permeability is to be obtained. In the water permeability test, the in-plane permeability of nonwoven fabrics144–146 is also defined and has been studied for many applications including RTM for composites, geotextiles and medical textiles. A test standard for measuring the in-plane permeability is defined by the ASTM for geotextiles. Adams and Rebenfeld146 developed a method to quantify the directional specific permeability of anisotropic fabrics using an image analysis apparatus that allowed flow visualisation of in-plane radial flow movement. Montgomery147 studied the directional in-plane permeability of geotextiles and gave methods for obtaining the maximum and minimum principal specific permeabilities and the resulting degree of anisotropy in the fabric. In the test, viscous liquid is forced by gas pressure to flow within a fabric sample. A mirror is positioned just below the apparatus so that the shape and the position of the radially advancing liquid front can be measured by means of an image analysis system. In this way, the local and dynamic anisotropy of liquid transport through a fabric can be evaluated and the specific permeabilities can be calculated. Capacitance methods173 have also been designed to measure the in-plane directional permeability in which separate capacitance segments are arranged radially around a central point to enable directional measurements of liquid volume to be measured in real time.

9.10

Measuring water vapour transmission148

The water vapour transmission rate through a nonwoven refers to the mass of the water vapour (or moisture) at a steady state flow through a thickness of unit area per unit time. This is taken at a unit differential pressure across the

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fabric thickness under specific conditions of temperature and humidity (g/Pa.s.m2). It can be tested by two standard methods, the desiccant method and water methods. In the desiccant method, the specimen is sealed to an open mouth of a test dish containing a desiccant, and the assembly is placed in a controlled atmosphere. Periodic weighings determine the rate of water vapour movement through the specimen into the desiccant. In the water method, the dish contains distilled water, and the weighings determine the rate of water vapour movement through the specimen to the controlled atmosphere. The vapour pressure difference is nominally the same in both methods except when testing conditions are with the extremes of humidity on opposite sides.

9.11

Measuring wetting and liquid absorption

There are two main types of liquid transport in nonwovens. One is the liquid absorption which is driven by the capillary pressure in a porous fabric and the liquid is taken up by a fabric through a negative capillary pressure gradient. The other type of liquid transport is forced flow in which liquid is driven through the fabric by an external pressure gradient. The liquid absorption that takes place when one edge of a fabric is dipped in a liquid so that it is absorbed primarily in the fabric plane is referred to as wicking. When the liquid front enters into the fabric from one face to the other face of the fabric, it is referred to as demand absorbency or spontaneous uptake.

9.11.1 Wettability and contact angle The wettability of a nonwoven fabric refers to its ability to be wetted by liquid,149 and is determined by the balance of surface energies in the interface between air, liquid and solid materials (i.e., fibres or filaments in the fabric). Wetting is concerned with the initial behaviour of the nonwoven when it is first brought into contact with the liquid,150 and involves the displacement of a solid-air (vapour) interface with a solid-liquid interface. Thus, the wettability of nonwoven fabric depends on the chemical nature of the fibre surface,151 the fibre geometry152 (especially surface roughness153), and the nonwoven fabric structure. The wettability of a fibre is determined by the fibre-liquid contact angles.154 The wettability of any fabric containing a single fibre type is the same as its constituent single fibres151 and therefore the wettability of a nonwoven fabric can be determined by fabric-liquid contact angles. However, wetting of a nonwoven fabric is a much more complex process than wetting of a fibre since simultaneously other wetting mechanisms, such as spreading, immersion, adhesion, and capillary penetration are at play.155 Because nonwovens are usually porous, heterogeneous and anisotropic in structure, the reliability of contact angle measurement is debatable, especially when the fabric is hydrophilic. Although the contact angle of nonwovens is


443

measured by either a goniometer or other indirect methods, there is no standard procedure and it is always difficult to obtain reliable measurements.156 The contact angle is usually evaluated by two types of technique: direct measurement of the contact angle by observation or optical techniques, including the goniometer and the direct imaging sessile drop method; Wetting force measurement including the Wilhelmy technique157,158 and other methods.159–161 This particular group of measurement methods does not give the contact angle (q) directly but usually requires either a force measurement or compensation of a capillary force to show g cos q (where g is the liquid surface tension that needs to be known or determined independently). Methods used in testing the wettability of other porous materials,162,163 can also provide a good reference.

9.11.2 Wettability and liquid strike time (areal wicking spot test) The areal wicking spot test method is based on the modification of two existing standards, BS3554 (1970), Determination of wettability of textile fabrics and AATCC method (79–2000), Absorbency of bleached textiles. The ‘spot’ test attempts to measure the in-plane wickability, or the capability of a liquid drop to spread over the fabric. In the test, a liquid droplet of either distilled water or, for highly wettable fabrics, a 50% sugar solution, is delivered from a height of approximately 6 mm onto a flat pre-conditioned nonwoven fabric. A beam of light illuminates the fabric to create bright reflections from the droplet surface as it contacts the fabric. The elapsed time between the droplet reaching the fabric surface and the disappearance of the reflection from the liquid surface is measured. The disappearance of the reflection is assumed to indicate that the liquid has spread over and wetted the fabric surface. The elapsed time is taken as a direct measure of the fabric wettability. The shorter the time, the more wettable the fabric. In some cases, the wetted area of the fabric at the moment reflection ceases is also recorded.164 An alternative approach is to replace the droplet by a continuous supply of liquid delivered by a capillary tube or a saturated fabric ‘wick’ in contact with the test specimen and to measure the rate of increase in diameter of the wetted region.165 For the single drop test, the results are dependent on the local fabric structure, and therefore the measurements are subject to marked variation even within the same fabric.

9.11.3 Liquid absorbency The capacity of a nonwoven fabric to retain the liquid, or the liquid absorption capacity, is defined in INDA, EDANA and ISO test methods. Contrary to the strip test to measure the amount of liquid wicking into a nonwoven fabric in

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the direction of the fabric plane, the demand absorbency test (also referred to as the demand wettability test or the transverse wicking ‘plate’ test)166,167 measures the liquid wicking into the nonwoven fabric driven by the capillary pressure in the direction of fabric thickness. In demand absorbency tests, the liquid will only enter into the fabric when the sample demands it. These tests involve contacting the dry sample with a liquid in such a way that absorption occurs under a zero or slightly negative hydrostatic head. No standards for this test method are currently available. A classic example of this type of tester166 is shown in Fig. 9.6. The device consists of a filter funnel fitted with a porous glass plate that is connected to flexible tubing and to a horizontal length of capillary glass tube. The horizontal porous plate is fed from below with water from a horizontal capillary tube, the level of which can be set so that the upper surface of the plate is filled with an uninterrupted column of test liquid and kept damp. This is often used to simulate a sweating skin surface. A disk of test nonwoven fabric is placed on the plate and held in contact with it under a defined pressure achieved by placing weights on top of it. The position of the meniscus along the capillary tube is recorded at various time intervals as water is wicked into the fabric. Given the diameter of the capillary tube, the wicking rate of the water absorbed into the fabric can be obtained. The method can be modified to integrate with an electronic balance and a computer to improve the measurement accuracy and to indicate the dynamic variance of the liquid uptake process against time. When modified and combined with the electronic balance method, the transverse porous plate method is called the gravimetric absorbency testing system (GATS).168 The GATS system is based on standard ASTM D 5802 where the amount of liquid absorbed is determined gravimetrically. The liquid introduction method is modified in the GATS system. Instead of horizontal tubing or a burette with an air bleed, the liquid source rests on top of an electronic balance via a coil spring, which has a known Hooke constant and is capable of compensating the weight loss (due to absorption of liquid by the fabric) or weight gain (due to exsorption) of the liquid source so that the liquid level can be maintained constant. The amount of liquid absorbed Weight Fabric sample

Porous plate

9.6 Instrument for measuring demand wettability.


445

is measured continuously by the electronic balance and is recorded continuously against time via a computer. Several test cells allowing different modes of contacting the absorbent sample and the liquid (including the porous plate and a point source), can be used with this equipment. Also, the system may incorporate a sample thickness measuring device which allows continuous monitoring of the change in bulk volume under a constant load. The load can be programmed to allow cyclic loading tests. A problem related to this method is that the wicking rate is strongly dependent on the applied weight on top of the test fabric, particularly for bulky nonwoven fabrics. The structure of nonwoven fabrics may change considerably under greater compression, and low compression may not give uniform porous plate-fabric contact. Another criticism of the method is that the resistance to flow imposed by the capillary tube decreases during the course of the test, as water is withdrawn from the tube, although this can be improved by replacing the capillary tube by an air bleed. A further limitation is that the hydrostatic head in some GATS systems, which is set at a low level at the start of the experiment, decreases during the test as water wicks up through the fabric sample. This can be a particular problem with thick fabrics.

9.11.4 Liquid wicking rate One-dimensional liquid wicking rate (wicking strip test) The liquid wicking rate may be measured in terms of the linear rate of advance of the liquid in a strip of nonwoven fabric in a strip test. In an upward wicking strip test, the nonwoven fabric is first conditioned at 20 ∞C, 65% relative humidity for 24 hours. A strip of the test fabric is suspended vertically with its lower end immersed in a reservoir of distilled water (or other liquid). After a fixed time has elapsed, the height reached by the water in the fabric above the water level in the reservoir is measured (Fig. 9.7).

9.7 Vertical upwards strip wicking test.

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Both the wicking rate and the ultimate height the water reaches are taken as direct indications of the wickability of the test fabric. Liquid wicking in both the machine and cross-directions of the nonwoven fabric are tested to obtain the anisotropic liquid wicking properties. The main standard test methods are: 1. BS3424-18, method 21(1988): methods for determination of the resistance to wicking and lateral leakage. 2. ERT 10.4 (2002): Nonwoven absorption. 3. ITS 10.1(1995): Standard test method for absorbency time, absorbency capacity and wicking rate. 4. WSP 10.1(2005): Three standard test methods for nonwoven absorption. 5. BS EN ISO 9073-6 (2003): Textile test methods for nonwovens– Absorption. There are some differences in these test procedures. The BS3424-18 (Method 21) specifies a very long test period (24h) and is intended for coated fabrics with very slow wicking rates. In contrast, other test methods (e.g., ITS10.1, ERT10.4, WSP 10.1 and BS EN ISO 9073-6) specify a much shorter test time (maximum 5 min) and applies to fabrics that exhibit rapid wicking. The upward wicking strip test method can be connected with a computer integrated image analyser to obtain dynamic wicking measurements or it can be modified for integration with an electronic balance to monitor the mass of water absorbed. A downward wicking strip test is also reported to enable the wicking rate and capillary pressure to be obtained.169 Other test methods include the horizontal strip test. When the strip test method is used for determining the rate of advance of the liquid front, the position of the advancing front might not be obvious because of the so-called finger-effect.170 This occurs in nonwoven fabrics, which are usually of high porosity and heterogeneous with local variations in fabric density. However, comparison of the strip test results in fabrics having large differences in fabric structure needs caution. The effect of liquid evaporation cannot be ignored in a strip test performed for a long time and the influence of the fabric structure on the gravity effect in the strip test needs to be considered. Two-dimensional liquid wicking rate The demand absorbency ‘plate’ test method can be converted into a twodimensional radial dynamic wicking measurement method168 when the liquid is introduced from a point source into the nonwoven fabric (also known as the point source demand wettability test). One example of this method is the GATS system mentioned in Section 9.11.3 when a point source liquid introduction cell was used.


447

Modified laser-Doppler anemometry (MLDA)171 is another alternative method to monitor liquid wicking in the two-dimensional fabric plane based on the Doppler principle. When a laser beam is passed through a flowing liquid, light is scattered by the particles suspended in the liquid. The scattered light is subject to a frequency shift and contains information about the velocity of the particles which can then be examined by electro-optical techniques. In order to obtain the velocity of liquid flow in a nonwoven fabric using this method, it is required that the flow medium be partly transparent and contain particles that scatter light used in the measurement. Electrical capacitance techniques have been used to monitor the liquid absorption in multi-directions in a nonwoven fabric plane.172,173 The principle of the method is based on the fact that the dielectric constant of water is about 15 to 40 times higher than that of normal fibres and fabrics and therefore the capacitance of a transducer in a measuring system will be very sensitive to the amount of liquid absorbed by a fabric. The device is computerised and is able to provide both dynamic (real time) and multidirectional measurements of the wicking rate in terms of the volume of liquid absorbed. Problems in testing nonwovens may arise from the influence of the significant geometric deformations in saturated fabrics, the liquid evaporation and limitations in the size of the capacitance transducers. Also, different types of fibrous material may have different dielectric constants, which can lead to difficulties when comparing different materials. Similar to measuring the permeability and the anisotropy of liquid transport in the nonwoven fabric plane,146 the image analysis method has been used to track the in-plane radial liquid advancing front to determine the rate of capillary spreading in the two-dimensional fabric plane. Kawase et al.174,175 used a video camera to determine the capillary spreading of liquids in the fabric plane. The apparatus used in his studies comprised a desiccator with a 200 mm diameter. The cover had an orifice for inserting a micropipette. The liquid used was n-decane which was placed at the bottom of the desiccator to minimise volatilisation of the liquid spreading in the fabrics. To aid observation of the liquid spreading in test fabrics, the n-decane was dyed with a 0.1% solution of Sudan IV or acid blue 9. The fabric was mounted on a 12.0 cm wooden ring (embroidery hoop) and placed into the desiccator along with a stopwatch. The cell was covered and the fabric was left for at least two hours. A measured amount (0.05 to 0.20 ml) of liquid was introduced onto the fabric by a micropipette. The area of the spreading liquid and the reading on the stopwatch were recorded simultaneously using a video camera. The spreading area was copied onto film, cut out, and weighed. A calibration curve was determined by recording areas of several known sizes and weighing the copied film in every experiment in order to determine the actual spreading area. The correlation coefficients of the calibration curves were reported to be higher than 0.999. With the aid of advanced image analysis software, an image analyser

448


allows capillary spreading in the fabric plane to be quantified as a distribution of brightness levels in an image. The profile of the distribution of the liquid concentration in the fabric can be obtained by calibrating the brightness or intensity values with liquid concentration levels in a fabric.

9.11.5 Liquid drainage rate (syphon test) The syphon test165,176 measures the rate of drainage under external pressure rather than the wicking rate. In this test, a rectangular strip of saturated fabric is used as a syphon, by immersing one end in a reservoir of water or saline solution and allowing the liquid to drain from the other end into a collecting beaker. The amount of liquid transmission at successive time intervals is recorded. Because the saturated fabric has a lower resistance to flow than a dry fabric, the rate of drainage is usually greater than the wicking rate.

9.12

Measuring thermal conductivity and insulation

The thermal resistance and the thermal conductivity of flat nonwoven fabrics, fibrous slabs and mats can be measured with a guarded hot plate apparatus according to BS 4745: 2005, ISO 5085-1:1989, ISO 5085-2:1990. For testing the thermal resistance of quilt, the testing standard is defined in BS 5335 Part 1:1991. The heat transfer in the measurement of thermal resistance and thermal conductivity in current standard methods is the overall heat transfer by conduction, radiation, and by convection where applicable. The core components of the guarded hot plate apparatus consist of one cold plate and a guarded hot plate. A sample of the fabric or insulating wadding to be tested, 330 mm in diameter and disc shaped, is placed over the heated hot metal plate. The sample is heated by the hot plate and the temperature on both sides of the sample is recorded using thermocouples. The apparatus is encased in a fan-assisted cabinet and the fan ensures enough air movement to prevent heat build up around the sample and also isolates the test sample from external influences. The test takes approximately eight hours including warm-up time. The thermal resistance is calculated based on the surface area of the plate and the difference in temperature between the inside and outside surfaces. When the hot and cold plates of the apparatus are in contact and a steady state has been established, the contact resistance, Rc(m2 KW–1), is given by the equation:

q – q3 Rc = 2 Rs q1 – q 2

9.14

Rs is the thermal resistance of the ‘standard’, q1 is the temperature registered


449

by thermocouple, T1, q2 is the temperature registered by T2 and q3 is the temperature registered by T3. Thus, the thermal resistance of the test specimen, Rf (m2 KW –1), is given by the equation: Rf q ¢ – q 3¢ q 2 – q 3 = 2 – Rs q1 – q 2 q 1¢ – q 2¢

9.15

where q 1¢ is the temperature registered by T1, q 2¢ is the temperature registered by T2 and q 3¢ is the temperature registered by T3. Since Rs (m2KW–1) is a known constant and can be calibrated for each specific apparatus, Rf (m2KW –1) can thus be calculated. Then the thermal conductivity of the specimen, k(Wm–1K –1) can be calculated from the equation: k=

d ( mm )*10 –3 R f ( m 2 KW –1 )

9.16

The conditioning and testing atmosphere shall be one of the standard atmospheres for testing textiles defined in ISO139, i.e., a relative humidity of 65%+/–2% R.H. and a temperature of 20 ∞C+/– 2∞C.

9.13

Modelling pore size and pore size distribution

In the design and engineering of nonwoven fabrics to meet the performance requirements of industrial applications, it is desirable to make predictions based on the fabric components and the structural parameters of the fabric. Although work has been conducted to simulate isotropic nonwoven structures in terms of, for example, the fibre contact point numbers177 and inter-cross distances,178 only the models concerned with predicting the pore size are summarised in this section.

9.13.1 Models of pore size Although it is arguable if the term ‘pore’ accurately describes the voids in a highly connective, low density nonwoven fabric, it is still helpful to use this term in quantifying a porous nonwoven structure. The pore size in simplified nonwoven structures can be approximately estimated by Wrotnowski’s model179,180 (Fig. 9.8) although the assumptions that are made for the fabric structure are based on fibres that are circular in cross-section, straight, parallel, equidistant and arranged in a square pattern. The radius of a pore in Wrotnowski’s model is shown as follows, Ê r = Á 0.075737 Ë

Tex ˆ – d f rfabric ˜¯ 2

9.17

450


d

df

2r

9.8 Wrotnowski’s model for pore size in a bundle of paralled cylindrical fibres arranged in a square pattern.

where Tex = fibre linear density (tex), rfabric is the fabric density (g/cm3) and df is the fibre diameter (m). Several other models relating pore size and fibre size by earlier researchers can also be used in nonwoven materials. For example, both the largest pore size and the mean pore size can be predicted as follows, by using, Goeminne’s equation181. The porosity is defined as e . largest pore size (2rmax): rmax =

df 2(1 – e )

mean pore size (2r) (porosity < 0.9): r =

9.18 df 4(1 – e )

In addition, pore size (2r) can also be obtained based on Hagen-Poiseuille’s law in a cylindrical tube, r=

4

8k p

9.19

where k is the specific permeability (m2) in D’Arcy’s law.

9.13.2 Models of pore size distribution If it is assumed that the fibres are randomly aligned in a nonwoven fabric following Poisson’s law, then the probability, P(r), of a circular pore of known radius, r, is distributed as follows,182


P(r) = –(2pv¢) exp(–p r2v¢)

451

9.20

where v ¢ = 0.36 , and is defined as the number of fibres per unit area. r2 In geotextiles, a series of critical pore sizes have been defined, i.e. the fabric apparent opening size (AOS, or O95) and the fabric filtration opening size (FOS). The apparent opening size (AOS, or O95) indicates the approximate largest particle that would effectively pass through the geotextile. In the dry sieving method, it is defined as the bead size at which 5% or less of the weight of the beads pass through the nonwoven fabric. Giroud110 proposed a theoretical equation for calculating the filtration pore size of nonwoven geotextiles. The equation is based on the fabric porosity, fabric thickness, and fibre diameter in a nonwoven geotextile fabric.

xed f ˘ È d –1+ Of = Í 1 (1 – e )h ˙˚ f Î 1–e

9.21

where df = fibre diameter, e = porosity, h = fabric thickness, x = an unkown dimensionless parameter to be obtained by calibration with test data to account for the further influence of geotextile porosity and x = 10 for particular experimental results, and Of = filtration opening size, usually given by the nearly largest constriction size of a geotextile (e.g., O95). Lambard183 and Faure184 applied Poissonian line network theory to establish a theoretical model of the ‘opening sizing’ of nonwoven fabrics. In this model, the fabric thickness is assumed to consist of randomly stacked elementary layers, each layer has a thickness Te and is simulated by twodimensional straight lines, (a Poissonian line network). Faure et al.185 and Gourc and Faure186 also presented a theoretical technique for determining constriction size based on the Poissonian polyhedra model. In Faure’s approach, epoxy-impregnated nonwoven geotextile specimens were sliced and the nonwoven geotextile was modelled as a pile of elementary layers, in which fibres were randomly distributed in planar images of the fabric. The crosssectional images were obtained by slicing at a thickness of fibre diameter df and the statistical distribution of pores was modelled by inscribing a circle into each polygon defined by the fibres (Fig. 9.9). The pore size distribution, which is obtained from the probability of passage of different spherical particles (similar to glass beads in a dry sieving test) through the layers forming the geotextile, can thus be determined theoretically using the following equation:185 Ê 2 + l (d + d f ) ˆ Q ( d ) = (1 – f ) Á ˜ 2 + ld f ¯ Ë (1 – f ) l= 4 , and N = T df p df

2N

e – l Nd

9.22 9.23

452


df

T

9.9 Model for constriction pore size in a nonwoven fabric consisting of randomly stacked elementary layers of fibres.

where Q(d) = probability of a particle with a diameter d passing through a pore channel in the geotextile, f = fraction of solid fibre materials in the fabric, l = total length of straight lines per unit area in a planar surface (also termed specific length) and N = number of slices in a cross-sectional image. Because of the assumption in Faure’s approach that the constriction size in geotextiles of relatively great thicknesses tends to approach zero, Faure’s model generally produces lower values.110 The use of this method is thus not recommended for geotextiles with a porosity of 50% or less.117

9.14

Modelling tensile strength

Backer and Petterson187 pioneered a fibre network theory from work on needlepunched fabrics. This estimates the tensile properties of nonwovens based on the fibre orientation, fibre tensile properties and the assumption that fibre segments between bonds are straight. Hearle and Stevenson188 expanded this theory by taking account of the effects of fibre curl. They indicated that the stress-strain properties of a nonwoven fabric were dictated by the orientation distribution of fibre segments. Later, Hearle and Ozsanlav189 developed a further theoretical model to incorporate binder deformation into the model. The FOD is an essential parameter in constructing these models. When the fibres in a nonwoven are assumed to lie in layers parallel to the two-dimensional fabric plane, the prediction of the stress-strain curve of the fabric under uniaxial extension can be established based on three models: the orthotropic models, the force analysis method in a small strain model and the energy analysis method in an elastic energy absorption model.

9.14.1 Orthotropic models of tensile strength187 Predicting fabric tensile properties based on tensile properties in different principal directions It is assumed that the deformation of a two-dimensional nonwoven fabric is analogous to that of a two-dimensional orthotropic woven fabric where


453

stress (s)-strain (e ) relationships are known for the two principal directions of the nonwoven fabric. It is assumed that the following properties are known: elastic modulus (EX, EY) in two principal directions respectively; shear modulus (G XY) between the two principal directions; and the Poissons’ ratio Ê v = 1 = e X ˆ in two principal directions. eY ¯ Ë XY v YX For a unidirectional force acting on the fabric with a small strain Ê s (q ) = E (q )ˆ , the fabric modulus, and the Poissons’ ratio in the direction Ë e (q ) ¯ q are as follows: 4 1 = e (q ) = cos q + Ê 1 – 2v XY ÁG EX EX E (q ) s (q ) Ë XY

v(q ) = –

sin 4 q ˆ 2 2 ˜ cos q sin q + E ¯ Y

e (q ) p e Êq + ˆ 2¯ Ë

(cos 4 q + sin 4 q ) Ê 1 1 1 ˆ 2 2 Á E + E – G ˜ cos q sin q – v YX EX Ë X Y XY ¯ = 4 4 2v XY ˆ cos q Ê 1 sin q 2 + Á – ˜ cos q + E EX G E Ë XY X ¯ Y

9.24 Predicting fabric tensile properties based on fibre orientation distribution and fibre properties In a nonwoven fabric, it is assumed that (i) the fibres in the fabric are straight and cylindrical with no buckling, (ii) the bond strength between fibres in the fabric is considerably higher than the fibre strength (i.e., nonwoven rupture results from fibre failure) and (iii) the shear stress and shear strain are negligible. Thus, we have the following equations for nonwoven fabric tensile properties, where the fibre orientation distribution function in the fabric is W(b)

s (q ) = E f e x

Ú

p /2

– p /2

s Êq + p ˆ = E f e x 2¯ Ë

(cos 4 b – v(q )sin 2 b cos 2 b ) W ( b )d b

Ú

p /2

–p /2

9.25

(sin 2 b cos 2 b – v (q )sin 4 b ) W ( b ) d b

9.26

454


Ú v (q ) =

p /2

(sin 2 b cos 2 b )W ( b )( d b )

–p /2

Ú

p /2

– p /2

E(q ) =

9.27 (sin 4 b ) W ( b ) d b

s (q ) e (q )

Ê p /2 Á Á cos 4 b – = Ef –p /2 Á Á Ë ¥ W (b)db

Ú

Ú

p /2

–p /2

Ú

(sin 2 b cos 2 b ) W ( b )d b p /2

–p /2

(sin 4 b ) W ( b ) d b

ˆ ˜ sin 2 b cos 2 b ˜ ˜ ˜ ¯ 9.28

When a nonwoven fabric is isotropic Ê i.e., W ( b ) = 1 ˆ , from the above p¯ Ë 1 equations, we have v(q ) = . 3

9.14.2 Force analysis method in a small strain model In force analysis in a small strain model, the following assumptions are made about the nonwoven structure. 1. The fibres are assumed to lie in layers parallel to the two-dimensional fabric plane. 2. The fabric is subjected to a small strain. 3. The nonwoven fabric is a pseudo-elastic material and Hooke’s law applies. 4. No lateral contraction of the material takes place. 5. No transverse force exists between fibres. 6. No curl is present in the fibres. The stress-strain relationship can be established by using the analysis of the components of force in the fibre elements in a nonwoven fabric is given as follows.187,188 (1 + ej)2 = (1 + e L)2 cos2 qj + [1 + eT + (1 + eL) cot qj tan b]2 sin2qj 9.29 where b is the shear undertaken by the fabric, ej is the fibre strain in the jth fibre element, eL and eT are the fabric strains in the longitudinal direction and transverse direction respectively; qj is the fibre orientation angle of the jth fibre element. If there is no shear in the fabric plane, we have (1 + ej)2 = (1 + eL)2 cos2qj + (1 + eT)2 sin2qj

9.30


455

9.14.3 Energy analysis method190 In the previous models, the fibres are assumed to be straight cylindrical rods. In fact, fibres in real nonwoven fabrics usually have various degrees of curl, thus energy analysis methods have been adopted to improve tensile property modelling. The nonwoven structure is treated as a network of energy absorbing fibrous elastic elements, where elastic energy in reversible deformation can be solely determined by changes in fibre length. The deformation geometry is defined by minimum energy criteria. The applied stresses and strain are used in the analysis rather than the applied forces and displacement used in the first method. The following assumptions are made: 1. The fabric is a two-dimensional planar sheet. 2. The sheet consists of networks of fibre elements between bond points. 3. The bond points move in a way that corresponds to the overall fabric deformation. 4. Stored energy is derived from changes solely in fibre length; (i.e., no contribution of binder, each point is freely jointed, fibres are free to move independently between bonds). When a unidirectional force is applied, we have: 1 e ¢j = 1 [(1 + e L2 )cos 2 q j + (1 – v XY sin 2 q j ) 2 sin 2 q j ] 2 – 1 9.31 Cj

sL =

N Ê cos 2 q j ˆ (1 + e L ) S m j s j Á 2 ˜ j =1 Ë C j (1 + e ¢j ) ¯ N

S mj j =1

9.32

where e ¢j = strain in the jth fibre element, eL = overall fabric strain, sj = stress in the jth fibre element sL = overall fabric stress and vXY = fabric contraction factor, which is defined as the contraction in the Y direction due to a force in the X direction, which is equal to the ratio of strain in the Y direction to the strain in the X direction. qj = orientation angle of the jth fibre element, Cj = curl factor of the jth fibre element, mj = mass of the jth fibre element and N = total number of fibre elements.

9.15

Modelling bending rigidity191

The bending rigidity (or flexural rigidity) of adhesive bonded nonwovens was evaluated by Freeston and Platt.191 A nonwoven fabric is assumed to be composed of unit cells and the bending rigidity of the fabric is the sum of the bending rigidities of all the unit cells in the fabric, defined as the bending moment times the radius of curvature of a unit cell. The analytical equations

456


for bending rigidity were established in the two cases of ‘no freedom’ and ‘complete freedom’ of relative motion of the fibres inside a fabric. The following assumptions about the nonwoven structure are made for modelling the bending rigidity. 1. The fibre cross-section is cylindrical and constant along the fibre length. 2. The shear stresses in the fibre are negligible. 3. The fibres are initially straight and the axes of the fibres in the bent cell follow a cylindrical helical path. 4. The fibre diameter and fabric thickness are small compared to the radius of curvature; the neutral axis of bending is in the geometric centreline of the fibre. 5. The fabric density is high enough that the fibre orientation distribution density function is continuous. 6. The fabric is homogeneous in the fabric plane and in the fabric thickness. The general unit cell bending rigidity, (EI)cell, is therefore as follows: ( EI ) cell = N f

Ú

p /2

–p /2

[ E f I f cos 4 q + GI p sin 2 q cos 2 q ]W (q ) dq

9.33 where Nf = number of fibres in the unit cell, Ef If = fibre bending rigidity around the fibre axis, G = shear modulus of the fibre, Ip = polar moment of the inertia of the fibre cross section, which is a torsion term and W(q) = the fibre orientation distribution in the direction, q. The bending rigidities of a nonwoven fabric in two specific cases of fibre mobility are as follows: 1. ‘Complete freedom’ of relative fibre motion. If the fibres are free to twist during fabric bending (e.g., in a needlepunched fabric), the torsion term (GIp sin2 q cos2 q) will be zero. Therefore,

( EI ) cell =

p d 4f N f E f 64

Ú

p /2

–p /2

W(q )cos 4q dq

9.34a

where df = fibre diameter, Ef = Young’s modulus of the fibre 2. ‘No freedom’ of relative fibre motion. In chemically bonded nonwovens, the freedom of relative fibre motion is severely restricted. It is assumed in this case that there is no freedom of relative fibre motion and the unit cell bending rigidity (EI)cell, is therefore: ( EI ) cell =

p N f E f d 2f h 48

Ú

p /2

–p /2

W(q )cos 4 q dq

where h = fabric thickness and df = fibre diameter.

9.34b


9.16

457

Modelling specific permeability

The specific permeability of a nonwoven fabric is solely determined by nonwoven fabric structure and is defined based on D’Arcy’s Law,192 which may be written as follows:

Dp Q= –k h h

9.35

where Q is the volumetric flow rate of the fluid flow through a unit crosssectional area in the porous structure (m/s), h is the viscosity of the fluid (Pa.s), Dp is the pressure drop (Pa) along the conduit length of the fluid flow h (m) and k is the specific permeability of the porous material (m2).

9.16.1 Theoretical models of specific permeability Numerous theoretical models describing laminar flow through porous media have been proposed to predict permeability. The existing theoretical models of permeability applied in nonwoven fabrics can be grouped into two main categories based on: capillary channel theory, for example Kozeny,204 Carman,193 Davies,194 Piekaar and Clarenburg195 and Dent.196 ii. drag force theory, for example Emersleben,197 Brinkman,198 Iberall,199 Happel,200 Kuwabara,201 Cox,202 and Sangani and Acrivos.203

i.

Many permeability models established for textile fabrics are based on capillary channel theory or the hydraulic radius model, which is based on the work of Kozeny204 and Carman.193 The flow through a nonwoven fabric is treated as a conduit flow between cylindrical capillary tubes. The Hagen– Poiseuille equation for fluid flow through such a cylindrical capillary tube structure is as follows: q=

pr 4 DP 8h h

9.36

where r is the radius of the hydraulic cylindrical tube. However, it has been argued that models based on capillary channel theory are suitable only for materials having a low porosity and are unsuitable for highly porous media where the porosity is greater than 0.8, see, for example, Carman.193 In drag force theory, the walls of the pores in the structure are treated as obstacles to an otherwise straight flow of the fluid. Drag force theory is believed to be more applicable to highly porous fibrous assemblies, such as nonwoven fabrics, where the single fibres can be regarded as elements within the fluid that cannot be displaced, see Scheidegger.213 The drag of the fluid acting on each portion of the wall is estimated from the Navier-Stokes equations,

458


and the sum of all the invididual ‘drags’ is assumed to be equal to the total resistance to flow in the fabric. Iberall199 adopted the drag force models obtained by Emersleben197 and Lamb205 and established a model of permeability for a material having a random distribution of cylindrical fibres of circular cross-section and identical fibre diameter. The model accounted for the permeability on the basis of the drag forces acting on individual elements in the structure. It was assumed that the flow resistivity of all random distributions of fibres per unit volume does not differ. The resistivity was obtained by assuming the fabric has an isotropic structure in which the number of fibres in each axis is equal and one of the axes is along the direction of macroscopic flow. Happel,200 Kuwabara,201 Sparrow and Loeffler206 and Drummond and Tahir207 have given detailed analyses of the permeability in unidirectional fibrous materials using a so-called ‘unit cell’ theory, or ‘free surface’ theory. In these models, the fibres are assumed to be unidirectionally aligned in a periodic pattern such as a square, triangular or hexagonal array. The permeability of unidirectional fibrous materials is then solved using the Navier-Stokes equation in the unit cell with appropriate boundary conditions. These models have shown good agreement with experimental results when the fabric porosity is greater than 0.5.194,200,208 Unlike capillary flow theory, drag force theory and the unit cell model demonstrate the relationship between permeability and the internal structural architecture of the material.

9.16.2 Summary of permeability models Theoretical models of permeability and empirical equations for fibrous structures are based on one of three groups of assumptions, i.e., the nonwoven fabric is homogeneous and is either isotropic, unidirectional,197,200,201,203,206,207 or anisotropic.16,211,212 There are distinct differences between the three types of permeability model. The permeability in isotropic nonwovens is identical in all directions throughout the entire structure, while the permeabilities in the three principal directions in homogeneous unidirectional nonwoven structures are obtained parallel and perpendicular to the orientation of the fibres. The permeabilities in anisotropic fabrics vary in all directions throughout the fabric structure. Various empirical permeability models for nonwoven fabrics have also been obtained. A comparison of the available models for 2D nonwovens is shown in Figs 9.10 and 9.11 and Tables 9.5, and MaoRussell equations for 3D nonwovens are summarised in Table 9.6. It is shown in Fig. 9.10 that the Kozeny equation204 and its derivations,215 which are based on capillary channel theory, agree with experimental data very well when the fabric porosity is low (0.8) (see Fig. 9.10(b)). It is also shown in the Figures that Iberall’s equation


459

Specific permeability (*10E-10)(m^2)

1 Iberall Rushton C=0.5 0.1

M_R_ISO Shen_S = 8

0.01 Davies

Iberall Rushton Shen M_R_ISO Davies

1E-3

1E-4 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 Porosity P(*100%) (a)


10000 Iberall Rushton Shen M_R_ISO Davies

1000

100

Rushton C=0.5 Shen_S = 8

10 Iberall M_R_ISO

1 Davies 0.1 0.90

0.91

0.92

0.93

0.94 0.95 0.96 Porosity P(*100%) (b)

0.97

0.98

0.99

9.10 Comparison of the existing permeability models for homogeneous isotropic two-dimensional nonwoven fabrics. (a) e = 0.30–0.90; (b) e = 0.90–1.0. (Note: In Rushton’s equation the product of the roughness factor and Kozeny constant t k0 is taken as 0.5).211,212

gives predicted permeability results that are much higher than those obtained from empirical models in low porosity fabrics (where e < 0.8) but agrees well with the empirical equations when e Æ 1 as shown in Fig. 9.11(b). In Fig. 9.10, Rushton’s equation, which is based on woven fabric, agrees well with Shen’s experimental results,209 which are based on both the transverse permeability and the in-plane permeability of needlepunched nonwoven fabrics, for porosities ranging from 0.3 to 0.8. Davies’ equation,194 which was obtained

Handbook of nonwovens Specific permeability (*10E-10)(m^2)

460

1

Emersleben 0.1 Happel_P M_R_60 M_R_45 M_R_30 Happel_V Kuwabara

0.01

Happel _P Kuwabara M_R_30 M_R_45 Happel_V M_R_60

1E-3


0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 Porosity P(*100%) (a) Happel _P Kuwabara M_R_30 M_R_45 Happel_V M_R_60

100

Happel_P M_R_60 M_R_45 M_R_30 Emersleben Happel_V

10

1 Kuwabara 0.1 0.90

0.91

0.92

0.93

0.94 0.95 0.96 Porosity P(*100%) (b)

0.97

0.98

0.99

9.11 Comparison of the existing permeability models for homogeneous unidirectional fibrous materials.211,212 (a) (P = 0.30– 0.9); (b) (P = 0.90–1.00). (Note: M–R–30, M–R–45 and M–R–60 are referred to as the directional permeabilities of unidirectional fabrics d 2 ÔÏ Ô¸ ST k (q )= – 1 f when q = p , p and q = p Ì 2 2 ˝ 32 f 6 4 3 ÓÔT sin q + S cos q ˛Ô respectively.)

from experimental results of air permeability in fabrics composed of glass fibres having a porosity of 0.70~0.99, appears to provide reasonable predictions for structures having higher porosities of 0.70~0.99. The Mao-Russell equation for isotropic fibrous structures (denoted as M_R_ISO in Fig. 9.10) shows good agreement with capillary theory at low porosity and is also in reasonable agreement with the results of empirical

Table 9.5 Existing permeability models for isotropic and unidirectional fibrous structures Permeability (m2)

Notes

Ferrandon’s theory k n¢ 213

1 = 1 cos 2 q + 1 sin 2 q k n¢ k1 k2

For anisotropic nonwoven fabrics

2

Directional permeability of 2D nonwoven fabric having unidirectional fibre alignments k(q)(M_R_Uni)212 Drag force theory

Emersleben’s equation197

d k (q ) = – 1 f 32 f

d2 k= 1 f C f

ÔÏ Ô¸ ST Ì ˝ ÔÓ T sin 2 q + S cos 2 q ˛Ô

Directional permeability of unidirectional fibrous bundles

C = 16 C II = 32 S

Happel’s model200

C ^ = – 32 T C ^ = 64 S

Kuwabara’s model201

S d2 11.2f f

Langmuir model214

k^ =

Miao210

k ^ = S d f2 9f

Ï C = –16 Ì S + T ÔÓ ST

Permeability of 3D isotropic nonwoven fabric k (M_R_ISO3D)16

Ï 2S + T ¸ C = – 32 Ì ˝ 3 ÓÔ ST ˛Ô

¸ ˝ Ô˛

Permeability of isotropic structures

461

Permeability of 2D isotropic nonwoven fabric k (M_R_ISO)211


Name of theories

462

Table 9.5 Continued Permeability (m2)

Name of theories

Notes

Hagen-Poiseuille219 Capillary channel Kozeny-Carman’s equation for theory structure of capillary channels193

pr 4 8 (1 – f ) 3 k= 1 C¢ f2 k=

C ¢ = k 0S 02

Kozeny-Carman’s equation for fibrous materials228

C¢=

Rushton’s equation for woven fabrics215

C¢=

Sullivan’s equation208 Davies’ model194

k=

1 3 2

64 f [1 + 56f ] 3

(f = 0.16 ~ 0.30) Empirical models


(4 –lnRe) 1 C = 16 3 (2 –lnRe) (1 – f )

Iberall’s model199

3 1 (1 – f ) d 2 f 2 128 f

Shen’s model209

k=

Rollin’s model182

k = 7.376*10 –6

df

f

k0 d f2 16t k 0 d f2

C ¢ = 322 xd f

Permeability of anisotropic structures

d f2

Permeability of isotropic structures

Table 9.5 Continued 1. t – Roughness factor, k0 – the Kozeny constant, x the orientation factor. Si , r is the radius of a cylindrical capillary 2. Si – Specific internal surface area, and S0 – the specific surface area where S 0 = (1 – f) tube. 3. k1 and k2 are two principal permeabilities respectively.

Ê 1 – f2 ˆ 4. S = (2 lnf – 4f + 3 + f2) and T = Á ln f + ˜ 1 + f2 ¯ Ë 5. CII and C^ are the coefficient for permeability in the direction parallel and perpendicular to the fibre orientation respectively in Happel’s equation.


Where

463

464

Table 9.6 Permeabilities in various three-dimensional nonwoven structures Fibrous structures

Directional permeability k(q )

cos 2 J cos 2 b cos 2 g 1 = + + k (J , b , g ) kX kY kZ

Ferrandon’s equation213

Generalised permeability W(a) model for simplified 3D 16 nonwoven structures

kX

Ï d f2 ÔÔ =– Ì 32f Ô zS + (1 – z ) ÔÓ

Ï d f2 ÔÔ kY = – Ì 32f Ô zS + (1 – z ) ÓÔ

kZ = –

¸ Ô Ô ˝, q = Q W (a )[T cos 2 (Q – a ) + S sin2 (Q – a )]d a Ô ˛Ô

ST

Ú

p

0

¸ ÔÔ p –Q ˝, q = 2 2 2 Ô W (a )[T sin (Q – a ) + S cos (Q ) – a )]d a Ô˛

ST

Ú

p

0

d f2 Ï ST ¸ Ì ˝ 32f Ó (1 – z )S + zT ˛

cos 2 Q – (2cos 2 Q – 1)

Ú

p

[W (a ) cos 2 (Q ) – a )]d a = 1 +

0

3D isotropic nonwoven

3D fabric with isotropic fibre alignment in the fabric plane16

Constant

W(a ) = 1 p

d Ï ¸ k = k X = k Y = k Z = – 3 f Ì ST ˝ 32 f Ó 2S + T ˛ k X = kY = –

kz = –

d f2 Ï ST ¸ 1 Ì ˝ 16 (1 – z ) f Ó S + T ˛

d f2 Ï ST ¸ Ì ˝ 32f Ó (1 – z )S + zT ˛

p

0

2

16

Ú

[W(a ) cos 2 a ]d a


Fibre orientation distribution in the fabric plane

Table 9.6 Continued

3D nonwoven fabric having layers of unidirectional fibres aligned in the fabric plane16

Fibre orientation distribution in the fabric plane Ï Ô1; W(a ) = Ì Ô0; Ó

when a = p Ô¸ 2 ˝ when a π p Ô 2˛

Directional permeability k(q)

kX = –

kz = – 3D fabric having layers of fibres aligned in two orthogonal directions in the fabric plane16

ÔÏ X ; when a = 0 Ô¸ W(a ) = Ì p ˝ 1 – X ; when a = Ô 2˛ ÓÔ

kX =

ky =

d f2 Ï ST ¸ Ì ˝ 32f Ó (1 – z )S + zT ˛

d f2 Ï ¸ ST Ì ˝, 32f Ó ((1 – z )(1 – X ) + z )S + X (1 – z )T ˛

d f2 Ï ST ¸ Ì ˝ 32f Ó ((1 – z )+ z )S + (1 – X )(1 – z )T ˛

kZ = –

Where

d f2 Ê d2 Ê ST ˆ ˆ ST kY = – f Á Á ˜ 32f Ë zT + (1 – z )S ¯ 32f Ë zS + (1 – z )T ¯˜

d f2 Ï ST ¸ Ì ˝ 32f Ó (1 – z )S + zT ˛

È 1 – f2 ˘ 1. S = – [4 f – f 2 – 3 – 2 ln f ] and T = Íln f + ˙. 1 + f 2 ˚˙ ÎÍ 2. X and z are the fraction of fibres aligned in the X and Z directions respectively, Z direction is perpendicular to the fabric plane. 3. kX, kY and kZ are the three principal permeabilities in the equation respectively.


Fibrous structures

465

466


models at high porosity. Predicted results from the M_R_ISO model are in close agreement with the empirical data from both Shen’s equation at low porosity (e = 0.3~0.8) and with Davies’ equation at higher porosity (e = 0.85~0.99). It would appear that the M_R_ISO model is applicable for both low and high porosity fibrous structures.

9.16.3 Directional permeability in anisotropic nonwovens The permeability in anisotropic fibrous materials is believed to be closely related to fibre orientation208 which influences the structural anisotropy of nonwoven fabrics. Nonwoven products are usually three-dimensional anisotropic structures and fibres in such nonwoven fabrics are frequently orientated in preferred directions, but most of the fibres are in the fabric plane and some fibre segments may be in the direction of fabric thickness. In order to simplify the calculation of permeability in needlepunched nonwovens, fibres aligned in the Z-direction in such structures are assumed to be aligned in the direction of the fabric thickness and perpendicular to the fabric plane (Fig. 9.12). It is assumed that the fibre distribution in the Z-axis is homogeneous and uniform, and that the number of fibres perpendicular to the fabric plane represents a fraction z of the total number of fibres, and assuming the fluid flow is laminar and in-plane (i.e., the flow along the Z-axis is ignored). The directional in-plane permeability in the direction q of a two-dimensional nonwoven fabric is based on the fibre orientation distribution function, k(q), and can be written as follows.16,211,212 d 2f k (q ) = – 1 32 f

Ï ÔÔ ¥Ì Ô zS + (1 – z ) ÔÓ

Ú

p

0

¸ ÔÔ ST ˝ W (a )[ T cos 2 (q – a ) + S sin 2 (q – a )]da Ô Ô˛ 9.37 Z

Z

r V Liquid flow direction

g X

Y

b

J

X

Y

9.12 Simplified models of three-dimensional nonwoven structures and directional permeability.


467

1 – f2 ˘ È , q , is the flow where S = –[4f – f2 – 3 – 2 ln f] and T = Í ln f + 1 + f 2 ˙˚ Î direction, a is the fibre orientation in each direction in the fabric plane, f is the volume fraction of the solid material, k(q) is the directional permeability of the fabric, df is the fibre diameter, W(a) is the fibre orientation distribution function and z is the fraction of fibres aligned perpendicular to the fabric plane. The permeability perpendicular to the fabric plane, kZ, can be written as:

d 2f Ï ¸ ST kz = – 1 32 f ÌÓ (1 – z ) S + zT ˝˛

9.17

9.38

Modelling absorbency and liquid retention

Liquid absorbency (or liquid absorption capacity), C, is defined as the weight of the liquid absorbed at equilibrium by a unit weight of nonwoven fabric. Thus, liquid absorbency is based on determining the total interstitial space available for holding fluid per unit dry mass of fibre. The equation is shown as follows:216

V C = A T – 1 + (1 – a ) d rf Wf Wf

9.39

where, A is the area of the fabric, T is the thickness of the fabric, Wf is the mass of the dry fabric, rf is the density of the dry fibre,Vd is the amount of fluid diffused into the structure of the fibres and a is the ratio of increase in volume of a fibre upon wetting to the volume of fluid diffused into the fibre. In the above equation, the second term is negligible compared to the first term, and the third term is nearly zero if a fibre is assumed to swell strictly by replacement of fibre volume with fluid volume.217 Thus, the dominant factor that controls the fabric absorbent capacity is the fabric thickness per unit mass on a dry basis (T/Wf). In a given fabric and fluid system, only the mean pore radius r and thickness per unit mass (T/Wf) in the above equation are not constant. The value of r is predicted by the following equation based on the assumption that a capillary is bound by three fibres, orientated parallel or randomly, and the specific volume of the capillary unit cell is equal to that of the parent fabric.218 1

È r1 r2 d n ˘2 Ê ˆ dn r= Í 1 ÁA T ¥ – 1˜ Ê 1 1 + 2 2 ˆ ˙ r2 ¯ ˚ f1 r2 + f 2 r1 ¯ Ë r1 Î 6px Ë W f for n1 = 3 – n2, n 2 =

3 f 2 d1 f1 d 2 + f 2 d1

9.40

468


where the subscripts 1 and 2 represent different fibre types and x is a constant with a value of 9 ¥ 105, d is fibre denier, r is fibre density (g/cm3) and f is the mass fraction of a fibre in the blend (f1 + f2 = 1).

9.18

Modelling capillary wicking

Liquid wicking in nonwovens can be studied as a steady state fluid flow in a porous media, although in many practical situations the liquid is in fact an unsteady state flow where the nonwoven fabric is not uniformly and completely saturated. There is usually a saturation gradient in the medium along the direction of flow and this saturation gradient changes with time as the absorption process continues. Wicking processes can be divided into four categories: 1. Pure wicking of a liquid without diffusion into the interior of the fibres. 2. Wicking accompanied by diffusion of the liquid into the fibres or into a finish on the fibre surface. 3. Wicking accompanied by adsorption by fibres. 4. Wicking involving adsorption and diffusion into fibres.

9.18.1 Capillary pressure (Laplace’s equation) Liquid wicking into a nonwoven fabric is driven by the capillary pressure in the void spaces between adjacent fibres in the fabric. It is known that capillary pressure in a cylindrical capillary tube is given by Laplace’s equation, Pcap =

2s cos g r

9.41

where r is the radius of the capillary tube, g is the contact angle between the liquid and the capillary tube surface and s is the surface tension of the liquid. Nonwoven fabrics containing capillary pores having an average diameter 2r are frequently modelled as an equivalent system of parallel cylindrical capillary tubes having the same diameter 2r.

9.18.2 Hagen-Poiseuille equation From a consideration of the laws of hydrodynamic flow through capillary channels, Poiseuille219 first deduced the relation between the volume of fluid flowing through a narrow tube and the pressure difference across its ends. The Hagen-Poiseuille equation for a laminar fluid flow through a cylindrical capillary tube is as follows:

dh r 2 D P = dt 8h h

9.42


469

where h is the distance through which the fluid flows in time t, and h is the viscosity of the fluid.

9.18.3 Lucas-Washburn equation Based on Poiseuille’s equation, Lucas220 and Washburn131 calculated the distance along which turbulence occurs and, by converting the volume-flow in Poiseuille’s equation into linear-flow in uniform cylindrical tubes, they developed the Lucas-Washburn equation as follows: 1

h = Ct 2

9.43

where h is the distance through which the fluid flows in time t and C is a constant related to both the liquid properties and nonwoven fabric structure. A typical example of the application of the Lucas-Washburn equation is in liquid absorption in the upwards vertical strip test, in which the capillary pores in a nonwoven fabric are modelled as a series of parallel capillary tubes vertically supported and liquid is absorbed from one end upwards into the tube. The upward driving pressure is as follows: DP = Pcap – rgh

9.44

where Pcap is the capillary pressure in the tube, h is the rising height of liquid in the tube, g is the acceleration of gravity and, r is the liquid density. Substituting equation 9.44 and Laplace’s equation into Hagen-Poiseuille’s equation, the rising height of the liquid capillary flow is as follows: 2 dh Ê r s cos g – r r g ˆ = dt 8h ¯ Ë 4hh

9.45

The solution of this equation is:

t= –

r grh ¸ 16hs cos g 8hh Ï – 3 2 2 log e Ì1 – s 2 cos g ˝˛ r 2 rg r r g Ó

9.46

To obtain a simplified form of the relationship between t and h, Laughlin221 rewrote the above equation as follows: bt = – hm log e ÏÌ1 – h ¸˝ – h hm ˛ Ó

9.47

r 2 rg rs cos g where hm = a with a = and b = b 4h 8h 222 By using Taylor’s expansion, and when the gravity effect is negligible (e.g. h 0.4.

where Stk = and

Enhanced diffusion due to interception of diffusing particles268 2

E Dr =

1.24 R 3 1

for Pe > 100

9.91

( KuPe ) 2 Gravitational settling269 EG @ (1 + R)G for VTS and U0 in the same direction. EG @ –(1 + R)G, for VTS and U0 in the opposite direction. EG @ –G2, for VTS and U0 in the orthogonal direction. where

G=

rd d p2 Cc g VTS = U0 18hU 0

Electrostatic attraction194 1

ˆ ' – 1ˆ 2 Ê q2 Eq = Ê Á 2 Ë ' + 1 ¯ Ë 3phd p d f U 0 ( 2 – ln Re) ˜¯

9.92

Where ' is the dielectric constant of the particle and q is the charge on the particle. Filter efficiency of nonwoven filters having multiple fibre components When particles of multiple sizes are filtered by a fabric consisting of fibres

490


having the same diameter, the filter efficiency of the fabric can then be obtained from the above model of collection efficiency for a single fibre, E, by sub-dividing the size range of the particles into several sub-ranges. Ej, Ej are obtained for each sub-range j of average particle diameter dpj from the above equations for a single fibre. The filter efficiency Y is then calculated by the following equations. n ˆ Y = 1 – S aj Ê j Ë n0 ¯ j

where

4f E j h Ê n ˆ = exp Ê Á Ë n0 ¯ j Ë p (1 – f ) d f e f

9.93

ˆ Ê n ˆ ˜ ; Ë n ¯ and aj are the number of ¯ 0 j

Filter efficiency (%)

penetrations and mass fraction of the jth size range of the particles respectively. In respect of the prediction of filter efficiency in a nonwoven filter containing fibres of the same diameter, it is evident that poor filter efficiency will be observed in filtering particles of certain sizes and is unavoidable. This is illustrated in Fig. 9.15 where the variation in filter efficiency with particle size in an air flow is evident. In Fig. 9.15, it is evident that for very small particles less than dp1 in diameter, the primary filtration mechanism is diffusion. For particles between dp1 and dp2, the filter is less efficient as the particles are too large for diffusion effects and too small for a large interception effect. For particles of diameter above dp2, the filter is very efficient because the interception, along with inertial impaction effects, is predominant during filtration. The relatively low filter efficiency for particles of diameters between dp1 and dp2 is unacceptable but inevitable in cases where particles are of multiple

Diffusion regime

dp 1

Diffusion and interception regime

Inertia and interception regime

dp 2

Diameter of particles (mm)

9.15 Filter efficiency of a nonwoven fabric against the particle size in an air flow.


491

sizes in the fluid being filtered. To design a nonwoven filter with high filter efficiency, nonwoven fabrics composed of fibres of more than one diameter are common. For example, high-efficiency and HEPA filters consist of fibre diameters ranging from 0.65 to 6.5 microns, usually in three nominal diameter groups.272 If a nonwoven filter is composed of multiple fibre components, the filter efficiency when dealing with a fluid containing particles of multiple diameters will be as follows:

n Y = 1 – S a j Ê ˆ where j Ë n0 ¯ j

9.94

Ê 4f S E j ( d f )h ˆ Ê n ˆ = exp Á d f ˜ Ë n0 ¯ j Á p (1 – f ) d f e f ˜ ¯ Ë Ej(df) = ED(df)j + ER(df)j + EDr(df)j + EI(df)j + EG(df)j + Ee(df)j where Ê n ˆ and aj are the number of penetrations and mass fraction of the Ë n0 ¯ j jth size range of the particles respectively and Ej(df) is the collection efficiency of a single fibre having a diameter of df against a particle of diameter dpj.

9.21.3 Pressure drop in dry air filtration The pressure drop across a dry fibrous filter, DP0, can be predicted using the expression developed by Davies.259

D P0 =

U 0 hh (64 f 1.5 + (1 + 56f 3 )) d 2f

9.95

9.21.4 Wet-operating filters273 For nonwoven fabrics intended for mist filtration or the filtration of liquid particles, the specified collection efficiencies can be obtained with various combinations of filter thickness, fibre diameter, packing density and gas velocity. For a specified efficiency of 90%, the required filter thickness varies according to the approximate relation h = 5f–1.5 d 2.5 f . The corresponding pressure drop at constant filtration efficiency is insensitive to df but varies approximately according to the relation DPwet µ f0.6U0.3 when f > 0.01.

492


9.21.5 Liquid filtration Particle capture in liquid filtration is much more complicated and less efficient than in air filtration. For example, captured particles may easily re-enter the liquid flow, and this particle re-entrainment may be one of the key factors responsible for the low filter efficiency of liquid filter media compared to air filtration. The pressure drop through the fabric is determined using the following expression: DP = DPH–P + DPB–P

9.96

where DPH–P is the pressure drop for a Hagen-Poiseuille fluid, and DPB–P is the pressure drop due to the particle flow resistance. This can be calculated based on the assumption that the captured bed of particles consists of spherical particles274 in the equations DPH – P =

f hhU A ( OA ) 2

9.97

DPB– P =

khhU (1 – e ) 2 d p2 e 2

9.98

where U is the face velocity, A is the filter area, and OA is the open filter area for flow, f is the correction factor for the clean filter fraction as determined experimentally, h is the filter thickness, dp is the mean particle diameter, k is the Carman-Kozeny constant, and e is the dynamic porosity of the filter containing deposited particles. The filter porosity, e, should be calculated by considering the total area of the filter and the area covered by the deposited particles, thus e will change with time.

9.22

The influence of fibre orientation distribution on the properties of thermal bonded nonwoven fabrics

Nonwovens, regardless of the process utilised, are assemblies of fibres bonded together by chemical, mechanical or thermal means. In most nonwovens, the overwhelming majority of fibres are planar x, y stacks of fibres having little or no orientation through the plane (z direction). Some airlaid processes make an attempt to create a third dimension in the orientation of webs they produce. It may be argued that needlepunching and perhaps hydroentangling also result in some fibres lying in the z direction. However, the ratio of fibres in the z direction is a small fraction of the total number of fibres and the planar x-y orientation is still responsible for the performance of the nonwoven. It may be argued therefore, that the x, y planar fibre orientation is the most important structural characteristic in any nonwoven. Clearly, the properties


493

of a nonwoven fabric will depend on the nature of the component fibres as well as the way in which the fibres are arranged and bonded.275–280 Modelling and predicting the performance of nonwovens cannot be separated from the fibre orientation distribution and the structure anisotropy it brings about. Equally important is the so-called basis weight uniformity of a nonwoven. This refers to the degree of mass variation in a nonwoven normally measured over a certain scale. Local variation of the mass results in an unattractive appearance, but more importantly will lead to, and potentially dictate, the failure point of a nonwoven. For example, tensile failure may be initiated and propagated first in areas that are fibre-poor (regions with low mass), or barrier properties are lost because of the existence of fibre-poor regions in the fabric. Another characteristic of a nonwoven may be the extent to which the fibre diameter varies in a nonwoven. This is particularly important in spunbonded and meltblown structures where the fibre diameter variation may come about as a result of roping (fibres sticking together to form bundles) or because of the process, which leads to thick and thin places along the length of the fibres. This becomes significant at the micro scale and can lead to failure in a similar manner to the variations in basis weight. In addition to weight variation, the fibre orientation distribution function is of particular importance in governing fabric properties and the directional variation in properties within a fabric. The rest of this section discusses fibre orientation and its role on performance by examining some case studies.

9.22.1 Fibre orientation distribution The definition of Folgar and Tucker275 best describes the fibre orientation distribution function (ODF) in a nonwoven. The orientation distribution function (ODF) y is a function of the angle a. The integral of the function y from an angle a1 to a2 is equal to the probability that a fibre will have an orientation between the angles a1 and a2. The function y must additionally satisfy the following conditions: y(a + p) = y(a)

Ú

p

y ( a )da = 1

9.99

0

The peak direction mean is at an angle a given by:276 N

a = 1 tan –1 2

S f ( a i ) sin 2a i i=1 N

S f ( a i ) cos 2a i i=1

while the standard deviation about this mean is given by:276

9.100

494

Handbook of nonwovens N È ˘ s ( a ) = Í 1 S f ( a i )(1 – cos 2( a i – a )) ˙ i=1 2 N Î ˚

1/2

9.101

Anisotropy is often described as the ratio of the maximum to the minimum frequency of the ODF. For uni-modal distributions, in the range 0 to 180 degrees, the degree of anisotropy can also be characterised by the width of the orientation distribution peak given above. These definitions have to be reinterpreted for bimodal distributions in the range 0 to 180 degrees such as are obtained from cross-lapped webs or for crimped fibre webs viewed at short segment lengths. A more general approach would be to use the socalled cos2 anisotropy parameter, Ht, given by281 Ht = 2·cos2 fÒ – 1, where

· cos 2 f Ò =

Ú

p /2

– p /2

9.102

f t (f ) cos 2 (f ) df

The average cos2 anisotropy parameters can range between –1 and 1. A value of 1 indicates a perfect alignment of the fibres parallel to a reference direction and a value of –1 indicates a perfect perpendicular alignment to that direction. A uniform ODF (random ODF) would yield a zero value. It is customary to set the reference direction to the machine direction. More appropriately, the peak direction should be used as the reference instead of the machine direction. A direct experimental method for measuring fibre orientation extends back several decades when orientation was measured manually.280 Other indirect methods explored since include short span tensile analysis,282–284 microwaves (used primarily for paper),285 ultrasound,286 diffraction methods281 and more recently, image analysis287,288 methods. In a series of publications, the present author evaluated various optical means and methods for determining the fibre orientation distribution in nonwovens.289–294 There are various commercial systems now available for measuring fibre orientation distribution.295

9.22.2 The influence of the production method on anisotropy Today, nonwovens are made by a variety of processes, alone or together. The final structure and its anisotropy is therefore, a function of the ODF, the bonding and the layering of various webs to form a consolidated web. Orientation anisotropies are induced by various nonwoven processes. Most thermally bonded nonwoven fabrics are made by hot calendering a carded web of short staple fibres. A typical thermal bonding line has an opening section, a carding section, and a subsequent calender-bonding section. The


495

opening and carding processes have a significant impact on the orientation of the resultant web. The primary goal of the opening section is to separate fibres and provide a uniform feed to the cards. Openability is affected by fibre crimp and finish level. Each of these properties must be carefully controlled if the opener is to provide a uniform batt to the card. A high crimp value provides more cohesion, but it also makes the fibre opening more difficult. A low-crimp fibre opens easily and yields a high-quality web, but it is more difficult to process. The opening properties of the fibre must be balanced with its cohesive properties to have an efficient bonding line. The carding process, by nature, imparts a high degree of orientation to the fibres in the machine direction. The main cylinder and the workers in the card align the fibres parallel to the machine direction. Inadequate opening of the fibres creates a blotchy, non-uniform fabric that has a tendency to break easily during processing. A fabric formed from a web with fibres mostly aligned in the machine direction is expected to have high strength in the machine direction and relatively low strength in the cross direction. Other properties follow the same pattern. To improve the cross-direction strength requires the rearrangement of the fibres so as to have a higher degree of orientation in the cross direction. This can be achieved by several mechanical methods. One method involves stretching the web in the cross direction prior to the consolidation or bonding step. When the web is stretched in the cross direction, fibres are pulled away from the machine direction and realigned in the cross direction. Of course, the web must be cohesive enough to prevent too much fibre slippage, which could tear the web. The second method of imparting cross direction orientation to fibres involves a randomising doff mechanism at the exit of the card. This randomising is accomplished by buckling the web as it is doffed. Another method commonly employed is a cross-lapper that takes a card feed and cross-laps it into a uniform batt before consolidation or bonding. Most crosslapped webs have a bimodal fibre orientation distribution. The ODF in the wet lay process can also have a machine direction dependency. Here, the ODF can be adjusted by controlling the relative throughput and the speed of the belt. Unlike the systems above, most airlay systems have a tendency to create a more randomised web. The spunmelt spunbonded and meltblown variety of webs also often have a machine direction dependency. Some spunbonded products also have a bimodal distribution. Here, the aspirator and the laydown system are responsible for the laydown of the webs. What is perhaps significant is that most nonwovens are anisotropic and machine direction dependency and the web anisotropy typically increases with machine (belt) speed. This also implies that the properties of most nonwovens are also anisotropic. Also significant is that the ODF is typically symmetrical around the machine or cross directions. The symmetry is lost at any other direction.

496


9.22.3 The role of ODF on mechanical performance When a simple tensile deformation is applied along a direction around which the initial orientation distribution is symmetric, it will remain symmetric through the deformation process. However, if it is applied along a different direction, the symmetry could be lost with respect to the initial symmetry direction, but develop progressively with regard to the test direction. The changes in ODF that occur as a result of fabric strain can be followed by the following three average anisotropy parameters and an asymmetry parameter. Overall average anisotropy parameter, Ht, given below Ht = 2·cos2 fÒ – 1, where

· cos 2 f Ò =

Ú

p /2

– p /2

9.103


We define a left-quadrant average anisotropy parameter, H tL , as H tL = 2 ·cos 2 f Ò L –1,

where

· cos 2 f Ò L

Ú =

0

9.104


– p /2

Ú

0

– p /2

f t (f ) df

and a right-quadrant average anisotropy parameter, H tR , as:

H tR = 2 · cos 2 f Ò R – 1,

where

· cos f Ò R 2

Ú =

p /2

9.105


0

Ú

p /2

f t (f ) df

0

We can therefore, define an asymmetry parameter, At( m ) , as: ÊÊ At( m ) = 4 Á Á ËË

Ú

p /2

Ê –Á Ë

0

0

Ú

ˆ f t (f )df ˜ ·cos 2 f sin 2 f Ò R ¯

– p /2

ˆ ˆ f t (f )df ˜ ·cos 2 f sin 2 f Ò L ˜ ¯ ¯

9.106

Each of the average anisotropy parameters can range between –1 and 1. A value of 1 indicates a perfect alignment of the fibres parallel to a reference direction and a value of –1 indicates a perfect perpendicular alignment to that direction. A uniform ODF (random ODF) would yield a zero value. The


497

asymmetry parameter, At( m ) , will govern the magnitude of the moment that can arise around the tensile test direction and also its direction, with A < 0 and A > 0 leading to clockwise and anticlockwise moments, respectively. The factor, 4, has been introduced in the definition of At( m ) only to limit its range from –1 to 1. These limiting values represent conditions that would lead, respectively, to maximum clockwise and anti-clockwise moments when a tensile stress is applied along the reference (test) direction. Let us examine the behaviour of a carded, calendered nonwoven under tension in various directions. Tensile testing was performed at 0 degrees (machine direction), ± 34 degrees (bond pattern stagger angles), and 90 degrees (cross direction). The choice of these three specific test directions was based on the goal of exploring the anisotropic mechanical properties of the fabric and the requirement that the repeating unit of the bond pattern is easily identifiable with respect to the test direction. The nonwoven sample strips, 25.4 mm (1 in) wide, were tested at a gauge length of 101.6 mm (4 in). The tensile tests were carried out at a 100%/min extension rate. Five strips were tested at each angle; the average values are used in the plots. From the images digitised during tensile testing at 0∞, +34∞, 90∞, and –34∞ directions, the fibre orientation distribution function (ODF) and the shear deformation angle of the unit cell were measured. The deformation parameters are described in Fig. 9.16. The ODF was measured from a series of such images captured at regular intervals of deformation in each test direction. a = 0.50 mm b = 1.01 mm c = 2.26 mm d = 1.51 mm q = 34 58 spots/cm2

Machine direction

She

Unit cell height, c

ar a ngl e

0∞

q

90∞

Bond height, a

Unit cell width, d

9.16 Unit cell

Bond width, b

498


The ODF results are summarised in Figs 9.17–9.20. The loading direction is defined with respect to the sample axis (i.e., orientation angle). As may be noted from Fig. 9.18, when the samples are tested in the cross direction (90∞), the fibres reorientate significantly and the dominant orientation angle changes from its initially preferred machine direction towards the loading direction. In the case of samples tested in the machine direction (0∞), where the initially preferred orientation coincides with the loading direction, the deformation-induced effect is, as expected, primarily to increase this preference of fibres (Fig. 9.18). Because of the anisotropy of the initial structure, it is expected that when the samples are tested in different directions, the relative contributions to the Load direction Machine direction

12

50

9

40

6

ang

90

0

tion

60

nta

(%

in ra

10

30

Orie

–30

–60

3

)

30 20

St

Frequency (%)

15

0

le

9.17 Reorientation when tested in cross direction.

Load direction Machine direction

12

50

9

angl

30

0 ion

ra

0

90

ntat

60

Orie

10

–30

–60

3

in

30 20

(%

6

)

40

St

Frequency (%)

15

e

9.18 Reorientation when tested in machine direction.


499

Load direction Machine direction

12

50

9

40

6

ang

30

90

tion

60

nta

0

(%

in ra

–30

–60 Orie

10

)

30 20

3

St

Frequency (%)

15

0

le

9.19 Reorientation when tested in +34∞ direction. Load direction Machine direction

12

50

9

60

30

ra

0

90

ntati

0

–30

–60 Orie

10

in

30 20

3

(%

6

)

40

St

Frequency (%)

15

on a ngle

9.20 Reorientation when tested in –34∞ direction.

total deformation from structural reorientations and fibre deformations would be different. The reorientations due to the test deformations imposed at 34 degrees and –34 degrees also show similar changes in the dominant orientation angle (Figs 9.19 and 9.20), but of a much smaller magnitude than that obtained at 90 degrees. When the samples are tested in the cross direction, the nonwoven structure undergoes significant reorientation before the fibres themselves are strained. This is reflected in a high failure strain. In this case, reorientation is due to bending of fibres at their interfaces with the bonds. This would obviously lead to highly localised stress concentrations and high shear stresses at the fibre-bond interface, leading to a relatively low failure stress. In contrast, if

500


the samples are tested in the machine direction, which is the direction of initial preferred orientation, there can be only a limited extent of fibrereorientation facilitated deformation of the nonwoven material. This is reflected in a low strain but high stress at failure, occurring predominantly due to tensile failure of the fibres. If the bonding is optimal, failure can be initiated at the fibre-bond interface, or any other position in the path of the fibres that traverse between bonds. As can be seen in Fig. 9.21, the samples tested in the 34 degree and –34 degree directions fall between the two cases of ‘low stress–high strain’ and ‘high stress–low strain’ failure along the cross and machine directions, respectively. Also, the failures are dominated by shear when the fabrics are tested at 34 degrees and –34 degrees. The fracture edges are shown for each case in Fig. 9.21. As expected, failure tends to propagate along the dominant orientation angle. The propensity for shear deformation along the direction of preferred fibre orientation is clearly manifested in these tests. The unit-cell shear deformation results are shown in Fig. 9.22. It is clear that application of a macroscopic tensile strain produces a significant shear deformation along the initially preferred direction in fibre ODF, except when the two directions are either parallel or normal to each other. The degree of asymmetry in the structure is shown in Fig. 9.23. As may be noted, the moments are greatest when the test is performed in directions other than the two principal directions (machine and cross).

9.22.4 Concluding remarks with respect to ODF A fundamental link that can serve to identify the appropriate structural parameters, and to establish relationships between them and properties of

30 0∞

Load (N)

25 20

–34∞ 34∞

15 10

90∞

5 0 0

20

40

60 80 Extension (%)

100

120

9.21 Stress strain behaviour and fracture surfaces.


501

Shear angle (degrees)

150 0∞ 34∞ 90∞ –34∞

100 50 0 –50

–100

0

10

20 30 40 Fabric strain (%)

50

60

9.22 Shear angle as a function of strain.

Asymmetry parameter

0.10 0∞ 34∞ –34∞ 90∞

0.05

0.00

–0.05

–0.10 0

10

20 30 40 Fabric strain (%)

50

60

9.23 Asymmetry as a function of strain.

interest, pertains to quantitative relationships between macroscopic stress fields, or deformation parameters, and the consequent structural changes. It has been shown clearly that the fabric performance is a function of its structure or the manner in which the fibres are arranged within the structure. It has also been revealed that, while failure can follow different modes, it is likely to be dictated, under most conditions, by shear along the preferred direction of fibre orientation. Regardless of bonding conditions (a most important processing parameter), the structural changes brought about in the structure and the microscopic deformations are driven by the initial orientation distribution function (ODF) of the fibres and are similar for all structures with the same initial ODF. The bonding conditions only dictate the point of failure. The magnitude of the moment during the deformation process that can arise around the tensile test direction and also its direction can be determined by the asymmetry parameter. It is confirmed that from the asymmetry parameter

502


values the moments are greatest when the test is performed in directions other than the two principal directions (machine and cross).

9.23

References

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22. ASTM F561-97 Practice for Retrieval and Analysis of Implanted Medical Devices, and Associated Tissues. 23. ASTM F619-79 (1997) Standard Practice for Extraction of Medical Plastics. 24. ASTM F997-98a Standard Specification for Polycarbonate Resin for Medical Applications. 25. ASTM F1251 Terminology Relating to Polymeric Biomaterials in Medical and Surgical Devices. 26. ASTM F1855-00 Standard Specification for Polyoxymethylene (Acetal) for Medical Applications. 27. ASTM F1585-00 Standard Guide for Integrity Testing of Porous Barrier Medical Packages. 28. ASTM F1886-98 Standard Test Method for Determining Integrity of Seals for Medical Packaging by Visual Inspection. 29. ASTM F1929-98 Standard Test Method for Detecting Seal Leaks in Porous Medical Packaging by Dye Penetration. 30. ASTM F1862-00a Standard Test Method for Resistance of Medical Face Masks to Penetration by Synthetic Blood (Horizontal Projection of Fixed Volume at a Known Velocity). 31. ASTM E1766-95 Standard Test Method for Determination of Effectiveness of Sterilization Processes for Reusable Medical Devices. 32. ASTM E1837-96 Standard Test Method to Determine Efficacy of Disinfection Processes for Reusable Medical Devices (Simulated Use Test). 33. Appendix XX, Methods of Test for Surgical Dressings (A~T), British Pharmacopoeia 1993, A214. 34. Appendix XVIA, Test of sterility, British Pharmacopoeia 1993, A180. 35. Appendix XVIB, Test of microbial contamination, British Pharmacopoeia 1993, A184. 36. Appendix XVIC, Efficacy of antimicrobial preservation, British Pharmacopoeia 1993, A191. 37. Appendix XVIII, Methods of sterilisation, British Pharmacopoeia 1993, A197. 38. BS EN 13726-1:2002. Test methods for primary wound dressings. Part 1. Aspects of absorbency. 39. BS EN 13726-2:2002. Test methods for primary wound dressings. Part 2. Moisture vapour transmission rate of permeable film dressings. 40. BS EN 13726-3:2003. Test methods for primary wound dressings. Part 3. Waterproofness. 41. BS EN 13726-4:2003. Test methods for primary wound dressings. Part 4. Conformability. 42. BS EN 13726-5:2003. Test methods for primary wound dressings. Part 5. Bacterial barrier properties. 43. BS 5473:1977 Specification for spinal and abdominal fabric supports. 44. BS 7505:1995 Specification for the elastic properties of flat, non-adhesive, extensible fabric bandages. 45. BS EN 1644-1:1997 Test methods for nonwoven compresses for medical use: Nonwovens used in the manufacture of compresses. 46. BS EN 1644-2:2000 Test methods for nonwoven compresses for medical use: Finished compresses. 47. AS 2836.0-1998 defined the Methods of Testing Surgical Dressings & Surgical Dressing Materials in the following areas: General Introduction & List of Methods.

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Calculation of aerosol deposition in model filters in the range of maximum penetration. Ann Occup. Hyg, 12, pp. 1–8. Friedlander SK, Theory of aerosol filtration, Ind. Engng Chem., 30, pp. 1161– 1164, 1958. Friedlander SK, Aerosol filtration by fibrous filters, in Biochemical and Biological Engineering (edited by Blakebrough), Vol. 1, Chap 3, Academic Press, London, 1967. Steckina IB and Fuchs NA, Studies on fibrous aerosol filters I: Calculation of diffusional deposition of aerosols in fibrous filters, Ann. Occ. Hyg., 1966, 9, pp. 59–64. Kirsch AA and Chechuer PV, Diffusion deposition of aerosol in fibrous filters at intermediate Peclet numbers. Aerosol Science and Technology, 4(1), 11–16, 1985. Hinds WC, Aerosol Technology: Properties, behaviour and measurements of airborne particles, John Wiley and Sons, New York, 1999. Lee KW and Gieseke JA, Note on the approximation of interceptional collection efficiencies, J. Aerosol Sci., 1980, 11, pp. 335–341. Yeh, HC and Liu BYH, 1974, Aerosol filtration by fibrous filters, J. Aerosol Sci. 5, 191–217. Vaughan NP and Brown RC, Observations of the microscopic structure of fibrous filters, Filtration & Separation, 9, pp. 741–748, 1996. Stenhouse, JIT, 1975, Filtration of air by fibrous filters, Filtration and Separation, 12 (May/June), pp. 268–274. Bird RB, Steward WE and Lightfood EN, 2002, Transport Phenomena, John Wiley and Sons, pp. 196–200. Folgar F, Tucker III, C, J. of Reinforced Plastics and Composites, 3, 98–119, (1984). Kim, HS, Deshpande A, Pourdeyhimi B, Abhiraman AS and Desai P, Characterizing Structural Changes in Point-Bonded Nonwoven Fabrics during Load-Deformation Experiments, Text. Res. J., 71(2), 157–164 (2001). Kim HS, Pourdeyhimi B, Abhiraman AS and Desai P, Angular Mechanical Properties in Thermally Point-Bonded Nonwovens, Part I: Experimental Observations, Text. Res. J., to appear. Lee SM and Argon AS, The Mechanics of the Bending of Nonwoven Fabrics, Part I: Spunbonded Fabric (Cerex), J. Text. Inst., No. 1, 1–11, (1983). Lee SM and Argon AS, The Mechanics of the Bending of Nonwoven Fabrics, Part II: Spunbonded Fabric with Spot Bonds (Fibretex), J. Text. Inst., No. 1, 12–18, (1983). Lee SM and Argon AS, The Mechanics of the Bending of Nonwoven Fabrics, Part III: Print-Bonded Fabric (Masslinn), J. Text. Inst., No. 1, 19–30, (1983). Pourdeyhimi B, Dent R, Jerbi A, Tanaka S and Deshpande A, Measuring Fibre Orientation in Nonwovens, Part V: Real Fabrics, Text. Res. J., 69, 185–92, (1999). Lee SM and Argon AS, The Mechanics of the Bending of Nonwoven Fabrics, Part IV: Print-Bonded Fabric with a Pattern of Elliptical Holes (Keybak), J. Text. Inst., No. 1, 31–37, (1983). Patel SV and Warner SB, Modeling the Bending Stiffness of Point Bonded Nonwoven Fabrics, Text. Res. J., 64(9), 507–513 (1994). Pourdeyhimi B and Xu B, Characterizing Pore Size in Nonwoven Fabrics: Shape Considerations, Int. Nonwovens J., 6, (1), 26–30, (1994).

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285. Pourdeyhimi B, Ramanathan R and Dent R, Measuring Fibre Orientation in Nonwovens, Part II: Direct Tracking, Text. Res. J., 66, 747–753, (1996). 286. Pourdeyhimi B, Dent R and Davis H, Measuring Fibre Orientation in Nonwovens, Part III: Fourier Transform, Text. Res. J., 67, 143–151, (1997). 287. Guceri SI, Gillespie JW and Ravi Shanker, Polymer Engineering and Science, Vol. 31, 3, (1991), p. 161. 288. http://www.allasso-industries.com 289. Xuan-chao H and Bresee R, Characterizing Nonwoven Web Structure Using Image Analysis Techniques, Part III: Web Uniformity Analysis. INDA Journal of Nonwovens Research, 5, (3) 28–38, (1994). 290. Drouin B, Gagnon R, Cheam C and Silvy J, A New Way for Testing Paper Sheet Formation, Composite Science and Technology, 61, 389–393, (2001). 291. Kallmes OJ, Techniques for Determining the Fiber Orientation Distribution Throughout the Thickness of a Sheet, Tappi, 52, 482, 1969. 292. Kallmes OJ and Corte H, Formation and Structure of Paper, 1, Technical section British Paper and Board Maker’s Association, William Clowes & Sons, Ltd., London, (1962), pp. 13–46. 293. Boeckerman P, Meeting the Special Requirements for on-line Basis Weight Measurement of Lightweight Nonwoven Fabrics, Tappi, 166–172, 1992. 294. Weigert RG, The Selection of an Optimum Quadrant Size for Sampling the Standard Crop of Grasses and Forbes, Ecology, (43), 125–129, (1962). 295. Pourdeyhimi B and Kohel L, Area Based Strategy For Determining Web Uniformity, Text. Res. J., 72 (12), 1065–1072, (2002).

Index

AATCC standards system 414, 415–23 abrasive pads 366 abrasion resistance 416 absorption see liquid absorption acetoacetamide 344 acoustic impedance 478–83 acrylate polymers 338 acrylic fibres 10, 123, 124 acrylic thermoset resins 332 acrylonitrile 338 adaptive phase/Doppler velocimeter (APV) 180 additives 19, 132 adhesion 347 adhesive webs 386 Ahlstrom 11, 14, 134 air drag 182–3, 188 air filtration filter efficiency in dry air filtration 486–91 pressure drop 491 standards 424–6 see also filtration air flotation dryer 358, 361 air impingement dryers 357, 358, 359, 360, 361, 362 air jetting systems 321–2 air knife coating 355, 381 air mass flow rate 182 determination of 190–1 air permeability testing 440 air Reynolds number 188 air velocity 178–9, 190, 191 aircraft cleaning cloths 366 airflow dynamics 101–4 Airlace 106 airlaying 5–7, 76–108 airflow 101–4 benefits and limitations 76 bonding and web consolidation 104–6

carding-airlaid composite hydroentanglement installations 273–4 combined with other web formation methods 101, 102 developments 98–101, 102 fibre dynamics 104 fibre preparation 80, 81 physical properties of airlaid fabrics 106–7 product applications and markets 107–8 raw materials 76–80 technology 80–98 Airweb system 91 aluminium hydroxide binder 137–8 aluminium sulphate 137 ammonium persulphate 335 anionic surfactants 335 anisotropic liquid wicking 472–3 anisotropy 168–9, 412, 494 influence of production method on 494–5 permeability models for anisotropic nonwovens 458, 461–2, 466–7 role of fibre orientation distribution on mechanical performance 496–500 anti-foaming agents 345 antimicrobial finishes 376–7 antistatic agents 345, 376 aperturing 285–6 Apex technology 286 apparent (opening) pore size 431–2, 451 appliances, standards for 425–6 apron batt feeding 241, 243 AquaJet 106 Arachne 201–2 aramid fibres 122 area bonding 158, 306 areal wicking spot test 443 Asahi 11, 14

515

516

Index

Ason-Neumag system 150, 157, 164–5 ASTM standards 414, 415–23 nonwoven wound dressings 414–24 asymmetry parameter 500, 501 atactic polypropylene 145 atmospheric jigs 372 automotive applications 171, 219, 254, 293, 365 automotive industry air filtration standards 425 back angle 45, 48–9 Backer and Petterson model 170 bacterial filtration 422 bacterial penetration 422 bale breakers 20–1 bale pickers 21–2 barbed needles 223, 224, 226 barb designs 227 barb dimensions and shape 230–2 barb spacing 227–30 barb wear 236–8 basis weight uniformity 493 batch exhaust dyeing 371 batch scouring 370 batt compaction 225, 275–7 batt drafters 71–2 batt feeding 241, 243 BBA 11, 14 beam dyeing 371–2 belt calendering 317–18, 394 bending rigidity 264–5, 418 modelling 455–6 bentonite clay 252 b-cyclodextrin derivatives 399 biaxial roll spreaders 390 biaxial stitch-bonding machines 209–10, 211, 221 bicomponent fibres/filaments (bicos) 149, 159 splittable 149, 267–9 in thermal bonding 301–5 binder distribution test method 426 binder penetration analysis 426 binder to fibre ratio 348–9 binders 416 chemical bonding binder polymers 330, 331–44 bonding surfaces 345–6 cohesion properties 347 formulated binder systems 344, 345 methods of application 349–56 distribution of 348–9, 426 printing 374–5 thermal bonding 300–5 binder fibres 300–5 powder binders 305

biocidal finishes 376–7 biomimetic finishes 399 biopolymers 120 birefringence 164–5 blankets 255 blended composite web airlaying system 85 blending 24–32 blending hoppers 29, 30 bond points 403 bonding 1, 6, 9 airlaying 104–6 characterisation of fabric bond structure 403–8 chemical see chemical bonding degree of bonding 261–4 mechanical see mechanical bonding spunbonding 157–60 fabric properties 169 temperature and pressure 161 thermal see thermal bonding wet-laid nonwovens 135–8 bonding surfaces 345–6 BP 414 branches (side chains) 342 bridging effect 63 Britton et al model 170 Brownian motion 485, 488, 490 Brunauer-Emmett-Teller (BET) multilayer adsorption isotherm theory 438–9 brush conveyors 247–9 BS standards 414, 415–23 nonwoven wound dressings 414–24 bubble point 432 test method 434–5 buffer zones 31 buffers 336 bundle separation devices 166–7 bursting strength 416 butadiene polymers 339 Bywater Machine Company 54 cal-rod system 316 calender bonding 6, 158, 305–18 area bonding 158, 306 belt calendering 317–18 effect of process parameters 308–15 methods of heating calender rolls 316–17 point bonding 158, 306–8, 325–6 calender lamination 386 calendering 393–4, 395 calibrating unit 320–1 Caliweb 219 camel back 67–8 can (drum) drying 390 Cantrece 301–2 capacitance techniques 447

Index capillary channel theory 457, 458–66 capillary cone nozzles 281–3 capillary pressure 442, 468, 470–3 directional 472–3 two-dimensional models of 470–2 capillary wicking 443–8 modelling 468–74 capital intensity 11 card clothing 44–53 card configurations 57, 58 carding 6, 32–67 card clothing 44–53 combined with airlaying 102 control systems 58–67 Garnett machines 53–4 hybrid carding-airlaying systems 82–3, 90–1 interaction between card rollers 35–7 machines 54–8 roller operations 37–44 selection of raw materials for 16–19 thermal bonding line 494–5 working and stripping principles 32–7 carding-airlaid composite hydroentanglement installations 273–4 carding-hydroentanglement installations 271–3 carding-preformed tissue hydroentanglement installations 273 Carpet Star 249 catalysts 147, 345 Caviflex coating system 383, 384 cellulose bonding 135 cellulose fibres 118–19 layered structure 118, 119 preparation for wet-laying 126 see also wood pulp centrifugal spinning systems 194–5 ceramic fibres 125, 137–8 Cerex nylon spunlaid fabrics 356 chain transfer agents 332, 335 characterisation see fabric properties chemical bonding 6, 9, 298, 330–67 airlaying 104–5, 107, 108 bond structure of chemical-bonded nonwovens 408 chemical binder polymers 330, 331–44 drying 356–61 hydroentanglement with 274–5 mechanism of 344–9 binder polymer cohesion properties 347 distribution of the binder and binder to fibre ratio 348–9 wetting 346

517

methods of binder application 349–56 product applications 361–6 spunbond process 159–60 wet-laid nonwovens 135–6 chemical finishes 376–85 methods of application 378–85 types of 376–8 chemical gas adsorption 439 chemical properties 402 see also under individual properties chemical pulping 77 Chicopee 255 airlaying system 85–6 China 12–13 chloroprene 339 circular dies 176 civil engineering 171, 252, 327 Clapeyron effect 311 classification of nonwovens 2–4 cleaning systems 31–2 cleanliness of resin 147 clearer roll 41 close barb (CB) spacing 228–30 clothing 292–3 Clupak process 138 CNC (carded-net-carded) fabrics 287, 288 coagulation 342–3 coating 6 chemical bonding 355–6 chemical finishes 380–5 coefficient of variation 409 Coform process 192 Cognex SmartView 400 cohesion 347 cohesive bonds 299 cold pad batch dyeing 373 coloration 370–3 colorimetry systems 400 combination bonding 274–5, 330–1 combs 44 compaction 225, 275–7 degree of 393 ‘complete freedom’ bending rigidity 456 composite fabrics blended composite airlaying process 85 carded web and tissue composites 273 carding-airlaid composite hydroentanglement installations 273–4 glass composite preforms 209–10, 211 multi-layer composite hydroentangled fabrics 287–8 nappy composites 99, 107 spunbond and meltblown webs 192 compression resistance 73, 75 compressive finishes 391–3 computational fluid dynamics (CFD) 103, 279

518

Index

concentric sheath-core fibres 303–4 condensed web 43 condensing rollers 43, 51, 58 conduction, heat 310–11, 474 conduction dryers (contact dryers) 358–61 conductivity, thermal see thermal conductivity constant hydraulic head test 441 constriction pore size (pore throat size) 432, 451, 452 contact angle 442–3 contact bonding see calender bonding contact dryers 358–61 contact time see residence time continuous belts 249 continuous pad – steam dyeing 373 control systems 58–67 convection 474–5 convection drying 357–8 convergent forming 134, 135 conveyor dryer 358 copolymers 331 core-sheath bicomponent fibres 159, 302, 303–5 cotton 17, 18, 266–7 cotton cards 6, 18 cotton pads/wipes 291 CPC (carded-pulp-carded) fabrics 287, 288 CPS (carded-pulp-spunbond) fabrics 287, 288 crimp 18–19, 78, 266, 303 crimping 6 critical micelle concentration 333 cropping (shearing) 396 cross-flow air 178–9 cross-lapping (cross-folding) 6, 67–71 cross-machine direction (CD) controllers 65–7 cross weft insertion 208, 209, 221 crosslinkable polymers 344 crosslinking 342, 343–4 crowding factor 104 crown needles 234, 235 crowning 315 cryogenic insulation 139, 140 crystalline structure 165–6, 178 CSC (carded-spunbond-carded) fabrics 287, 288 Curlator Corporation 82, 83 cut glass strand 125 cylinder 33, 34 wire angles 47–9 wire profile 52 cylinder mould machines 128, 130–2 Dan-Web 92 airlaying system 78–9, 95–7

D’Arcy’s law 440, 457, 472 D’Arcy’s pressure drop relation 183 Davies model 457, 459–60, 462, 488 deep grooved polyester fibre 122 deflection, roller 314–15 deformation-induced heating (DIH) 311 degradable nonwovens 421 degree of bonding 261–4 Delany-Bazley equations 483 Delta card 55 demand absorbency tests 443–5 density fabric see fabric density fibre linear density 266 deposition 154–7 deposition ratio 189 depth filters 484 desiccant method 442 detergents 369 de-watering 283–4 Dexter 113–14 diameter, fibre 56, 162, 163, 164–5, 493 die block assembly 153–4 differential thermal shrinkage 303 diffusion 473–4 filtration and 485, 488, 489, 490 particle diffusion coefficient 486 digital ink-jet printing 375 Dilo DiLoop RR Rug-Runner 249 Di-lour IV 249 dilution ratio 96, 102 dimensional parameters 402, 408–13 direct feed batt formation 109 direct interception 484–5, 488–9, 490 directional capillary pressure 472–3 directional permeability 461, 464–5, 466–7 disc opener 23–4, 26 discharge prints 375 dish feed arrangement 41–2 disintegrator 97 dispersing agents 345 dispersion, index of 409 distance dropping 59–60 DOA airlaying system 88–9 Docan system 7, 150, 156 DOD system 375 dodecyl mercaptan 335 doffer rollers 33, 37–9, 50 double doffers 38–9, 58 wire covering 44–53 Doffmaster system 44 dope dyeing 370 double-belt dryer 358, 360 double doffers 38–9, 58 double-layer Hydroformer 130 double punch loom 245 double reduction needle 226, 228

Index double-sided air jetting systems 321–2 down-striking licker-in 40–1 drafting 6, 71–2, 224–5 drag coefficient 188 drag force theory 457–66 drainage rate test 448 drawing 154–7 operational variables 161–6 drilled injectors 279 drum and conveyor surfaces 277–8 drum drying (can drying) 390 drum-forming airlaying technology 92, 95–7 dry air filtration 486–91 dry-laid nonwovens 4–7, 16–111 airlaying see airlaying batt drafting 71–2 carding see carding cross-lapping 67–71 direct feed batt formation 109 mixing and blending 24–32 opening of fibres 19–24, 25, 26 selection of raw materials for carding 16–19 vertically lapped web formation 72–6 dry lamination 385–6 dry sieving 432 dry spinning 151 drycleaning 420 drying chemical bonding 356–61 hydroentangled fabrics 285 mechanical finishing 389–91 dual rotor airlaying system 83 dumb-bells 126–7 DuPont 7, 11, 14, 150, 151, 255 dust-holding capacity (filter capacity) 486 dyestuffs 370–1 see also coloration eccentric sheath-core fibres 303–4 Ecosafe process 386–7 EDANA (European Disposables and Nonwovens Association) 2, 11 standards 414, 415–23 elastic recovery 73, 75 electrical heating system 316 electrochemical finishes 399 electro-hot liquid roll 316 electromagnetic radiation systems 62–3 electrospinning (ES) systems 193–4 electrostatic charging 166, 181 electrostatic properties 416 elliptical needlepunching 246 embossing 6, 307–8 Emersleben permeability model 457, 458, 460, 461 emulsifiers 332–3

519

emulsion polymerisation 332–4 EN standards 414, 415–23 energy analysis method 455 energy balance 188–9 energy consumption bonding processes 298, 299 minimising 183–4 engraved rollers chemical bonding 354–5 thermal bonding 306–8, 312, 313 enhanced point wire 53 entanglement, in airlaying 103 entanglement completeness 262–3 entanglement frequency 262–3 epoxy resins 340 ERT standards 414, 415–23 ethoxylated lauryl alcohol 335 ethylene vinyl acetate (EVA) 337–8 ethylene vinyl chloride 337 Europe 4, 5, 10, 11 European standards 414, 415–23 Evolon fabrics 149, 160, 274, 286–7 exhaust dyeing machines 371 external crosslinking agents 345 extrusion coating 383 extrusion laminating 383, 387 extrusion spinning 151–2, 193–5 extrusion technology 6 fabric density 408–12 measuring 429 and thermal conductivity 477 variation and jet strips 281 fabric hand 170 fabric inspection 399–400 fabric properties 401–514 airlaid nonwovens 105–6 dimensional parameters 408–13 effect of fibre structure on properties of thermally-bonded fabrics 325–6 fabric bond structure 403–8 meltblown nonwovens 184–5 modelling see modelling needlepunching 251 spunbonding 168–71 testing see testing fabric strength 177–8, 261–2, 266, 416, 418–19 tensile strength see tensile strength fabric structure 401–2 characterisation of fabric bond structure 403–8 measurement of basic structural parameters 426–30 mechanism of hydroentanglement and 259–61 thermal bonding 325–6

520

Index

fabric thickness see thickness, fabric fabric weight see weight falling hydraulic head test 441 fanning unit 154–5 Far East 12–13 Fearnought fibre opener 22, 23 feed control systems 58–67 feed distribution 153–4 feed rollers airlaying 80, 81 card feed rollers 40–1 Fehrer airlaying systems 86–8 H1 system 247 felt 5 felting needles 223, 224 design and selection 226–34 Ferrandon permeability model 461, 464 fibre diameter 56, 162, 163, 164–5, 493 fibre distributor 95 fibre dynamics 104 fibre finish 19, 265–6, 347 bonding between binder polymer and 345–6 fibre length airlaying 78, 79, 82 carding 18 and fabric strength 266 see also short-staple fibres; textile fibres fibre linear density 266 fibre lubrication 28 fibre openers 22–3, 24, 25 fibre orientation angle 410, 411 fibre orientation distribution (FOD) 410–12 and directional permeability in anisotropic nonwovens 466–7 measurement 430–1 mechanical performance and 496–500 prediction of fabric tensile properties based on 453–4 role in properties of thermal-bonded nonwovens 492–502 fibre stacking theory 103 fibres 9–10 for airlaying 76–80 preparation 80, 81 bonding between binder polymer and 345 effect of fibre structure on properties of thermal-bonded fabrics 325–6 identification of 420 mixing and blending for dry-laid nonwovens 24–32 opening of 19–24, 25, 26 physical properties 17, 120, 121, 125 selection for carding 16–19 selection for hydroentanglement 264–9

fibre dimensions 266 fibre stiffness 264–5 fibre types 266–9 thermal bonding 300–5 base fibre types 300 bicomponent binder fibres 301–5 wet-laid nonwovens 116–25 fibres per tuft calculation 25 fibrillation 123, 124, 126, 269 filament formation and drawing 161–6 filament laydown 166–8 filament spinning, drawing and deposition systems 154–7 fillers 331, 345, 346 film-fibrils 193 filter capacity 486 filter efficiency 485 in dry air filtration 486–91 based on a single fibre collection efficiency 488–9 nonwoven filters having multiple fibre components 490–1 filter quality performance (filter quality coefficient) 485–6 filtration air filtration see air filtration filtration mechanisms and evaluation of filter performance 484–6 hydroentangled fabrics used for 293 in hydroentanglement installations 284 meltblown nonwovens 185 modelling filtration properties 483–92 needlepunched nonwovens 245, 252–3 wet forming 116, 117 filtration opening size 451 finish, fibre see fibre finish finishing 6, 368–400 chemical finishing 376–85 developing technologies 398–9 fabric inspection 399–400 lamination 385–8 mechanical finishing 389–94 surface finishing 394–8 wet finishing 369–75 wet-laid nonwovens 138 flame lamination 387–8 flame retardants 345 flameproof finishes 377 flash spinning 6, 193 flat bed airlaying 93–5 flat bed lamination 387 flat finish needlepunching 241, 244–5 flat wire machines 114–16, 128–9 Fleissner 270 Aquajet Multi-step hydroentanglement unit 272 flexural rigidity see bending rigidity

Index floating 315 flocking 396 flooding 283 floor coverings 253–4 floor wipes 289 flotation filters 284 flow box 114–16 fluid handling properties 402 see also under individual properties fluorocarbon finishes 377–8 flushable wipes 363 foam application coating systems 383–4 foam bonding 6, 299, 350–2 foam substitute 219 footwear 364–5 force analysis method 454 force balance 187–8 fork needles 233–4 form drag 175 formaldehyde 344 formed barbs 230, 231 formulated binder systems 344, 345 Fort James Corporation 92 Fox equation 334 fracture surfaces 500 free blade area 45, 46–7 Freudenberg 11, 14, 150 Fricke equation 478 friction 420 Friederlander equation 488 front angle 45, 47–8 froth padder 352 furniture 365 fusion bonding see thermal bonding Garnett-Bywater 54 Garnett machines 53–4 garnetting (Europe) 53–4 gas adsorption 438–9 gas heating system 316 gas permeability see permeability gauze, medical 292 geotextiles 252, 293, 420–1 Giroud equation 451 glass composite preforms 209–10, 211 glass fibre 125, 136, 140 glass transition temperature 334 Goeminne’s equation 450 grab test 439–40 gravimetric absorbency testing system (GATS) 444–5 gravure coating 382 ‘green strength’ binder 137 Grindstaff and Hansen model 170 H1 system 247 Hagen-Poiseuille equation 457, 462, 468–9

521

Hagen-Poiseuille’s law 450 hammer mills 80 Happel permeability model 457, 458, 460, 461 hard monomers 334 HDPE resin 146 healthcare and medical air filtration standards 425 Hearle et al model 170 heat of deformation 311 heat sensitisation 343 heat transfer mechanisms 310–11, 474–5 heating systems 316–17 heating, ventilation, air conditioning (HVAC) standards 424–5 helium-pycnometry 439 high-loft nonwovens 9 airlaying system 87–8 high-performance organic fibres 122 high-temperature fibres 267 high-temperature (HT) pressure jigs 373 high-temperature protective clothing 291–2 Honshu Paper 92 TDS system 97–8 horizontal flooded nip system 349–50, 352 horizontal padder 349, 350 hot flue dryers 390 hot melt coating 382–3 hot melt spraying 388 hot oil heating system 316–17 hybrid carding-airlaying systems 82–3, 90–1 Hycon calender system 394, 395 Hydraspun 784 wipe 290 hydrodynamic sieving 432–4 hydroenhancement 293–4 hydroentangled woodpulp fabrics 99 hydroentanglement (spunlacing) 6, 9, 255–94, 299 bond structure of hydroentangled nonwovens 405–6 bonding of airlaid nonwovens 106, 107–8 combined airlay and hydroentanglement technology 98–9 fibre selection for 264–9 principles 256–64 degree of bonding 261–4 jet impact force 258–9 mechanism of hydroentanglement and fabric structure 259–61 specific energy 257–8 process layouts 269–75 process technology 275–88 product applications 288–94 spunbonding 160 wet-laid nonwovens 136–7

522

Index

hydroentanglement intensity 263–4 Hydroformer 129–30 hydrogen bonding 135 hydrophilic agents 345 hydrophilic fibre finishes 265–6 hydrophobic agents 345 hydropulper 126 hygiene products 171–2, 327, 364, 423 market 13–15, 107 hyperspeed card 55–7

isotropic nonwovens 458, 459, 461–2 ITS standards 414, 415–23

Iberall permeability model 457, 458, 459, 462 ideal gas equation of state 190–1 IFB process 98 image analysis 169, 430–1, 434 liquid wicking rate 447–8 impaction, inertial 484, 489 impingement bonding 321–2 impingement dryers 357, 358, 359, 360, 361, 362 in-flight fibre 59 inclined angle needlepunching 246–7 inclined wire formers 129–30 INDA (North American Association of Nonwoven Fabrics Industry) 2, 12 standards 414, 415–23 index of dispersion 409 individualised fibres 80 industrial test methods 413–14 inertial impaction 484, 489 infra-red bonding 322–4 infra-red drying 361, 391 initiators 332, 335 injection card 55, 56 injectors 278–81 arrangement of 280–1 operation 278–80 ink-jet printing 375 inorganic binder systems 137–8 inorganic fibres 17, 120, 123–5 preparation for wet-laying 127 inspection, fabric 399–400 insulation 139, 140, 185–6, 254, 293 measurement 448–9 integrated forming and bonding (IFB) 98 interception, direct 484–5, 488–9, 490 interlinings 364, 423 interlocking wire 52 Intor system 204 intrinsic permeability see specific permeability inverted airlaying systems 101, 102 ISO standards 414, 415–23 nonwoven wound dressings 414–24 ISOjet system 276–7 isotactic polypropylene 145

K coefficient 56–7 K12 airlaying system 86, 87 K21 airlaying system 86–7 key companies 14, 15 kick-up 230, 231 Kim and Pourdeyhimi model 171 Kimberly-Clark 11, 13, 14 kiss roll (slop padding) 380 knife coating 355–6, 380–1 knife over air coating 355, 381 knife over blanket coating 355–6 knife over roller coating 356, 381 knock-over sinkers 204 knuckles 285 Komori and Makishima model 170 Korea 12 Kozeny-Carman equations 462 Kozeny permeability model 457, 458 Kraft process 77 Kroyer, Karl 92, 93 Kunit system 216–17, 218, 219, 406 Kuwabara permeability model 457, 458, 460, 461

Japan 10, 11, 12 jet impact force 258–9 jet marking 260–1, 276–7, 405 jet rebound effect 259 jet strips 281–3 jig dyeing 372–3 Johnson & Johnson 13

Lacom hot melt coating system 382, 383 lamella strips 247, 248 laminar flow 101 lamination 6, 385–8 land area 45, 50 Langmuir permeability model 461 Laplace theory of capillary pressure 435, 468 Laroche airlaying system 89–90 Laroche Napco 3D web linker 250 laser etching 399 latex bonding see chemical bonding latex bonding airlaying (LBAL) 104–5, 107, 108 latex polymer binder systems 336–9 latex polymers 137, 331, 332–9 binder components 334–6 data sheets of binder properties 340, 341 emulsion polymerisation 332–4 functionality 342–4 minimum film forming temperature 340–2

Index lay down, filament 166–8 LDPE resin 146 LDS system 44 LLDPE resin 146 Leanjet system 270 Liba Maschinenfabrik 202, 222 licker-in 40–1, 51 linear density, fibre 266 linings 292–3, 327, 364, 423 linting 420 liquid absorption 186, 415–16 measurement 442–8 modelling liquid absorbency 467–8 liquid drainage rate 448 liquid expulsion porometry 435–6 liquid filtration, modelling 492 liquid heating system 316 liquid permeability see permeability liquid retention, modelling 467–8 liquid strike time 443 logs/sticks 126–7 long-term variation controllers 62 long web path cross-lappers 69 lubricants (slip agents) 377 lubrication, fibre 28 Lucas-Washburn equation 469–70 Lutravil system 150, 157 Lyocell 10, 120–1, 267, 269 Lystil OY process 134 M & J Fibretech 78–9, 92, 93 machine direction/cross direction (MD:CD) 8, 42, 412 machine settings 175–9 magnetic hump 28 Mahlo QMS-10A system 400 Malifol technique 214 Malimo stitch-bonding system 206–14, 221 cross weft insertion 208, 209 Malifol technique 214 manufacture of glass composite preforms 209–10, 211 multiaxial weft insertion 210–13 parallel weft insertion 209 recent developments in biaxial stitchbonding 221 Schusspol technique 213 Malipol system 214–15 Malivlies system 205–6, 220, 406 Maliwatt system 202–5, 220, 407 man-made fibres 78–9, 82 wet-laid nonwovens 119–25 preparation of fibres 126–7 manufacturing technologies 4–5, 6 Mao-Russell model 459, 460–6, 470–3 market structure 10–15 mass balance 187

523

maximum fabric strength (MFS) 261 mechanical bonding 9, 201–97 airlaying 106 hydroentanglement see hydroentanglement needlepunching see needlepunching spunbond process 160 stitch-bonding see stitch-bonding mechanical finishing 389–94 mechanical properties 402 see also under individual properties medical products 172, 292, 364 melt flow rate (MFR) 174 melt spinning 143, 151 dynamics of process 186–9 melt viscosity 147 meltblowing 6, 143, 172–91 characterisation techniques 180–4 characteristics and properties of meltblown fabrics 184–5 combined with airlaying 102 composite fabrics 192 fabric production 172–80 mechanics of process 186–91 process technology 173–80 product applications 185–6 resins for 143–9 melting, polymer 152–3 melting point 146 Mercosur 13 mercury porosimetry 436–7 metal detection 27–8 metallic wire card clothing 44–53 metallocene-based resins 147–8 metering pump 153 Mi and Batra model 170 Miao permeability model 461 micelles 332–3 Micrex system 138, 392–3 Microchute 63, 64 microencapsulation 398–9 microfibres 172 see also meltblowing microperforated sleeves 278 microprocessor controlled weigh-pan systems 59–61 Microweigh system 59–61 XLM system 61 Middle East 13 migration 342–3 MIL standards 415–23 minimum film forming temperature 340–2 Miraguard 286 Mirastretch 286 Miratec fabrics 286 mixed polymers 149 mixing fibres 24–32

524

Index

modelling 449–92 absorbency and liquid retention 467–8 acoustic impedance 478–83 bending rigidity 455–6 capillary wicking 468–74 filtration properties 483–92 meltblowing process 182–3 pore size and pore size distribution 449–52 prediction of spunbonded nonwoven fabric properties 170–1 role of fibre orientation distribution on properties of thermal-bonded nonwovens 492–502 specific permeability 457–67 tensile strength 452–5 thermal resistance and thermal conductivity 474–8 modified laser-doppler anemometry (MLDA) 447 moisture content 28–9 moisture control 61 molecular weight 161 molecular weight distribution (MWD) 147, 161, 164 monomers 332–3, 334–5 Monvel 301–2 moulded webs 99–100 multiaxial stitch-bonding 210–13, 222–3 multi-board needlepunching machines 240 multi-bonding airlaying (MBAL) 105–6 multi-hopper systems 27 Multiknit system 216, 217–19, 406–7 multi-layer composite hydroentangled fabrics 287–8 multi-layer nappy composites 99, 107 multimixers 30 multiple dies 175–6 multi-roll openers 22, 24 mungo 54 nanofibres 193–4 Nanosphere technology 399 nappy composite fabric 99, 107 natural fibres 17, 77–8, 82 wet-laid nonwovens 118–19 preparation of fibres 126 see also cellulose fibres; wood pulp natural polymer-based fibres 78–9 natural rubber latex (polyisoprene) 339 needle arrangement 239–40 needle gauge 232 needle rotation 238–9 needle types 232–4, 235 needleboard changeovers 239 needlepunching 6, 9, 71, 223–55 airlaying 106, 108

basics of operation 225 batt formation 224 bond structure of needlepunched nonwovens 404–5 drafting 224–5 needle design and selection 226–34 penetration depth and other factors affecting needle use 234–40 product applications 251–5 spunbonding 160 technology 240–51 needlepunching machine sequences 240–1, 242 nip pressure 310–11, 313 nitrile butadiene rubber (NBR) 339 ‘no freedom’ bending rigidity 456 Nomex 291 non-ionic detergents 369 non-ionic surfactants 335 nonwoven wound dressing standards 414–24 nonwovens industry 1–15 European-produced nonwovens 4, 5 global production 11 key companies 14, 15 market structure and development 10–15 Norafin process 280 North America 12 standards 414, 415–23 Northern Softwood Sulphite 77 Novonette system 307–8 nozzle aeration dryers 357, 358, 359, 360, 361, 362 nozzles 281–3 Oasis fibre 78 oblique needlepunching 246–7 off–line processing variables 161 offset gravure 382 oligomer radicals 333 on-line basis weight measurement 64–5 on-line processing variables 161 one-dimensional liquid wicking rate 445–6 open flow box 114–16 open porosity (effective porosity) 413, 431 opening of fibres 19–24, 25, 26 airlaying opened fibres 80 thermal bonding line 494–5 opening pore size 431–2, 451 optical brighteners 345 optical properties 416–17 organic synthetic fibres 120 orientation angle 410, 411 orientation distribution function (ODF) see fibre orientation distribution orifice plates 190–1

Index orthotropic models of tensile strength 452–4 Osborne, F.H. 113 packaging 172 pad machine 349, 350, 351 padding/pad machines application of chemical binders 349, 350, 351 application of chemical finishes 379–80 coloration 373 paddings 253 paper technology 6 papers 113–14 wet-laid speciality papers 139 papyrus 113 parallel capillary pore models 479 parallel fibre models 479–82 parallel web 42, 43 parallel weft insertion 209, 221 parametric studies 173–80 partial solution bonding (solvent bonding) 9, 159–60, 356 partially oriented yarns (POYs) 166 particle diffusion coefficient 486 paste dot coating 383 patterning effects 285–6 Peclet number 486–7 penetration depth 234–6, 404–5 Perfojet 270, 277 perforated conveyor aprons 70 perforated conveyor through-air bonding 321 perforated drum through-air bonding 318, 319–21 perforation 389 permeability 417 directional permeability 461, 464–5, 466–7 modelling specific permeability 457–67 summary of permeability models 458–66 testing 440–1 water vapour transmission testing 441–2 perpendicular-laid web formation 72–6 phenolic binders 340 physical gas adsorption 438–9 physical properties 402 airlaid fabrics 106–7 fibres 17, 120, 121, 125 spunbond fabrics 168–70 see also under individual properties pigments 174–5, 345, 370–1 printing 374–5 see also coloration pile height 407 pillar stitch 407 pitch 45, 49 plasma treatment 398

525

plasticisers 342 plexifilaments 193 point bonding 158, 306–8, 325–6 point density 49 point profile 45, 49–2 polishing 397 polyacrylates 338 polyacrylonitrile (PAN) fibres 10, 123, 124 polyamide fibres 10, 122–3 resin 148 polydispersity 174 polyester fibres 9, 16, 17, 122, 123 resin 148 polyethylene (PE) fibres 78–9 resins 144–8 polymer feed distribution 153–4 Polymer Group Inc. (PGI) 11, 14 polymer-laid nonwovens 4–5, 7–8, 143–200 combining spunbonding/meltblowing with airlaying 108 composite fabrics 192 future trends 195 mechanics of spunbond and meltblown processes 186–91 meltblowing see meltblowing resins 143–9 spinning processes 151–2, 193–5 spunbonding see spunbonding polymer melting 152–3 polymer throughput 161 polymers chemical binder polymers 330, 331–44 for meltblowing 180–1 thermal conductivity 476 polymethacrylates 338 polyolefins fibres 121 resins 144–8 see also polyethylene; polypropylene polypropylene (PP) fibres 9, 10, 16, 17, 78–9 resins 144–8, 180–1 polyurethane (PU) binders 339–40 resins 148 polyvinyl alcohol (PVA) 136 pond height, flow box 115 pore connectivity 413 pore size 412–13 measurement 431–9 modelling 449–50 pore size distribution 412–13 measurement 431–9 modelling 450–2

526

Index

pore throat size (constriction pore size) 432, 451, 452 pore volume distribution 436–7 pore volume size 432 porometry 435–6 porosimetry 436–7 porosity 412–13 measurement 431–9 powder bonding 305 powder coating (scatter coating) 383 powder dot coating 383 preformed webs 99–100 preneedling 241–4 pressure drop 485 in dry air filtration 491 liquid filtration 492 pressurised foam application systems 384 pre-wetting 275–7 print bonding 353–5, 373–4 printing 6, 373–5 Proban R process 377 process variables calender bonding 308–15 spunbonding 160–8 Procter & Gamble 13 product applications 2–4, 5 airlaying 107–8 chemical bonding 361–6 hydroentangled fabrics 288–94 meltblown fabrics 185–6 needlepunching 251–5 spunbond fabrics 171–2 thermal bonding 327–8 wet-laid nonwovens 139–41 product lifetime 2, 3 profiling cross-lappers 71 protective clothing 291–2 PTAT 149 PTT 149 pulp, wood see wood pulp punch density 236 pycnometry 439 quadpunch loom 245 qualitative assessment 429 quench air rate 161 quench air temperature 161, 162–3 Radfoam process 132–4 radiation, thermal 475 thermal radiation bonding 322–4 rag tearing (rag grinding) 54 raising 397 Rando Machine Corporation 82 Rando-webber 83–4 random-laid nonwovens see airlaying random condensed web 43

random pitch wire 52–3 random web 42, 43 randomisers 42–3 raw materials 9–10 airlaying 76–80 carding 16–19 polymer-laid nonwovens 143–9 thermal bonding 300–5 wet-laid nonwovens 116–25 rayons 149 see also viscose rayon reciprocating lapper 72–3 recycled polymers 181 reduction needles 226, 228 Reemay 150 regular barb (RB) spacing 227–8, 229 Reifenhauser 8, 11, 150 Reicofil system 8, 150, 156–7 relaxation 391 repellency 417–18 residence time (contact time) calender bonding 308, 312, 313 polymer residence time in extruder 189 resin melt flow rate (MFR) 174 resins 143–9 resistance to compression 73, 75 reverse roll coating 356, 380 Reynolds number 191 RFX system 150 rib fabrics 247 roll calendering 305–17 Rollaweigh 64 roller batt feeding 241, 243 roller deflection 314–15 roller draft airlaying system 100–1 roller train cards 57 roller weighing systems 64 roller width 314 rollers, carding 33 interaction between 35–7 roller operations 37–44 Rollin’s permeability model 462 roofing products 255, 327, 366 room air cleaners/purifiers 425–6 ropes 126–7 rotary lapper 73, 75 rotary screen bonding 354 rotary screen coating 382 rotary tacker 241, 243 Rotiformer 130–2 Rotis system 73–6 row width 45, 49 Rushton permeability model 459, 462 S-Roll calender 394 S-TEX system 150 sand filters 284

Index sanforising 391–3 satellite rollers 33 satin stitch 407 saturation bonding 349–50, 351 saturation rate 474 SBAL process (airlaid-spunlace combination) 106 scale of production 10–11 SCAN-e-JET system 31–2 Scandinavian Sulphate 77 Scanfeed system 65–7 scatter (powder) coating 383 Schuhmeister equation 477 Schusspol technique 213 scouring 369–70 scramblers 6, 42–3 scraper bonding 355–6 scrim reinforced fabrics 240 seating fabric 219 secondary bonding 407 self-crosslinking polymers 344 self-emptying bins 29 Servolap 62–3 shear deformation 500, 501 shearing (cropping) 396 sheath-core bicomponent fibres 159, 302, 303–5 Shen permeability model 459, 462 shoddy 54 short-staple fibres 18 airlaying 82, 91–8, 103–4 short web path cross-lappers 69–70 shot 176 side-by-side (S/S) bicomponent fibres 159, 303 side chains (branches) 342 sieving test methods 432–4 Sigmaformer 132, 133 singeing 6, 394–6 single-belt dryer 358, 359 single-board needlepunching machines 240 single fibre collection efficiency 486 filter efficiency bsed on 488–9 single-layer Hydroformer 129–30 single reduction needle 226, 228 single roll openers 22, 24 skewing 315 slice 114 opening requirements for low consistency operation 116, 117 slice velocity 115–16 slop padding (kiss roll) 380 slot coating (slot die coating) 383 slot die 167 slot-type injectors 279–80 small strain model 454

527

SMS (spunbond-meltblown-spunbond) composite structure 192 sodium hydroxide 336 sodium lauryl ether sulphate 335 sodium lauryl sulphate 335 soft monomers 334 softeners 378 softening 397–8 solution bonding 356 solvent bonding (partial solution bonding) 9, 159–60, 356 solvent coating 385 solvent scouring 370 solvent spun cellulose (Lyocell) 10, 120–1, 267, 269 sonic velocity 180 Sontara 256 sound energy absorption coefficient 479, 481–2 sound propagation 478–83 Southern Softwood Kraft 77 specialist needlelooms 249–51 specific energy 257–8, 280–1 specific permeability 440 modelling 457–67 specific surface area 438–9 spin finish 265–6 Spinlace fabrics 274 Spinnbau airlaying system 90–1 spinnerets 154 spinning, drawing and deposition systems 154–7 spinning techniques 151–2, 193–5 spiralling effect 52 splittable bicomponent fibres 149, 267–9 splitting 389 spray bonding 6, 353 spray systems 28 SPS (spunbond-pulp-spunbond) fabrics 287, 288 spunbonding 6, 7–8, 143, 149–72 airlaying combined with 102 bonding techniques 157–60 composite fabrics 192 fabric production 149–55 die block assembly 153–4 extrusion spinning 151–2 filament spinning, drawing and deposition 154–5 polymer melting 152–3 fabric structure and properties 168–71 mechanics of process 186–91 operating variables 160–8 material variables 160–1 operational variables 161–8 product applications 171–2 production systems 155–7

528

Index

resins for 143–9 spunbond hydroentanglement installations 274 SpunJet system 274 spunlacing see hydroentanglement standard deviation 409 standard test methods 413–26 see also testing star blade needles 233 Star Former 98 steam heating system 316 steel rollers 312–14 Stefan-Boltzmann law 323 stencilled raising machines 397 Stenhouse equation 488 stenter dryers 358, 362, 389–90 stiffeners 378 stiffness see bending rigidity stitch-bonding 1, 6, 9, 201–23 bond structure of stitch-bonded nonwovens 406–7 Kunit system 216–17, 218, 219, 406 Malimo system 206–14, 221 Malipol system 214–15 Malivlies system 205–6, 220, 406 Maliwatt system 202–5, 220, 407 Multiknit system 216, 217–19, 406–7 recent developments 220–3 Voltex system 215–16 stitch holes 407 Stokes number 487 straining 484 strength, fabric see fabric strength; tensile strength stress-strain behaviour 454, 499–500 strip test 440 stripper 32–7 principles of stripping 35 structural parameters 402 measurement of 426–30 see also fabric structure structuring needlepunching machines 241, 247–9 Struto system 72–3, 74, 75 styrenated acrylics 338 styrene butadiene rubber (SBR) 339 sublimation transfer printing 375 suction-assisted web handling 55 sueding 397 Sullivan permeability model 462 Sunds system 80 superabsorbent polymers (SAPs) 79–80, 421–2 support surface 277–8 surface finish of steel roller 315 surface finishing 394–8 surfactants 332, 335, 345

surgical fabrics 292 SVA Lite 400 swelling 126 syndiotactic polypropylene 145 synthetic fibres 17, 78–9 see also man-made fibres synthetic leather 253, 292 synthetic wood pulp (SWP) fibres 121–2 syphon test 448 tacking 241–4 take-up speed 161 TDS (totally dry system) 97–8 tear strength 418–19 technical nonwovens, wet–laid 139 technical wipes 289 teeth 40 point density 49 point profile 45, 49–2 tooth depth 45–7 temperature bonding temperature 309–10 glass transition temperature 334 minimum film forming temperature 340–2 polymer/die temperature 161 quench air temperature 161, 162–3 tenacity 177–8 Tencel (Lyocell) 10, 120–1, 267, 269 tensile strength 78–9, 325, 419 hydroentanglement 261–2 modelling 452–5 needlepunched fabrics 251 tensile testing 439–40 role of fibre orientation distribution on fabric properties 496–500 tensioned wire mesh 319, 320 testing 413–49 basic structural parameters 426–30 fibre orientation distribution 430–1 gas and liquid permeability 440–1 general standards for 413–26 porosity, pore size and pore size distribution 431–9 tensile properties 439–40 thermal conductivity and insulation 448–9 water vapour transmission 441–2 wetting and liquid absorption 442–8 textile fabrics, thermal conductivity of 476 textile fibres, airlaying 78–9, 82, 101–3 textile technology 6 thermal bonding 6, 9, 298–329 bond structure of thermally-bonded nonwovens 407–8 calender bonding 6, 158, 305–18 fabric structures 325–6

Index hydroentanglement with 274–5 impingement bonding 321–2 principle of 299–300 product applications 327–8 raw materials 300–5 role of fibre orientation distribution in fabric properties 492–502 spunbonding 146, 158–9 thermal radiation bonding 322–4 through-air bonding 318–21 ultrasonic bonding 324–5 wet-laid nonwovens 136 thermal bonding airlaying (TBAL) 105, 107, 108 thermal conductivity 310 measurement 448–9 modelling 474–8 thermal fusion ovens 390 thermal insulation see insulation thermal radiation see radiation, thermal thermal resistance 186 measurement 448–9 modelling 474–8 thermomechanical pulping (TMP) 77 thermoplastic binders 300–1, 373 thermoplastic polyurethane (TPU) 148 Thibeau hybrid card-airlay machine 91 thickeners 345, 374 thickening 116, 117 thickness 408–12, 419 testing 426–9 Thinsulate 185 three-dimensional linked fabrics 250 three-dimensional web preforms and moulds 99–100 through-air bonding 158, 318–21 principle 318–19 through-air dryers 357–8, 391 tip width 45, 50 tissue and carded web composites 273 tooth depth 45–7 see also teeth total porosity 413 toxicity 422–3 transfer coating 383 transfer rollers 39–40 transport chamber 101 transverse wicking plate test 443–5 triangular blade needles 232 tricot lapping 202–4 tubular fabrics 250 tuft size 23, 25 Turbo Lofter system 57, 90–1 turbulent flow 101–3 twin-layer inclined wire former 129 two-dimensional liquid wicking rate 446–8

529

two-dimensional models of capillary pressure 470–2 Typar 150, 158 Tyvek 150, 193 ultrafine microfibres 179–80 ultrasonic bonding 6, 159, 324–5 underlap movement 207 Unicharm system 277 unidirectional nonwovens 458, 460, 461 uniformity, fabric weight 409, 429–30, 493 unit cell models 458 United States (USA) 10, 11, 12 up-striking licker-in 41 upwards vertical strip test 445–6, 469–70 US patent 3535187 85 UV stabilisers 378 vacuum cleaners 425–6 velour fabrics 247–9 Venturi gaps 157, 168 vertical padder 349, 351 vertically lapped web formation 72–6 vibrating screen 93–4 Viledon M 150 vinyl acetate 336 vinyl acetate acrylate 338 vinyl chloride 336–7 vinyl polymers 336–8 viscose rayon 10, 16, 17, 120, 265, 266 viscosity 191 Voltex system 215–16 volumetric chute feed systems 61–2 Voronina models 482–3 vulcanisation 339, 344 waddings 253 warp yarn racking device 208 washable domestic fabrics 291 washing 369–70 waste recycling 54 water circuit 284 water method for water vapour transmission 442 water permeability testing 441 water spraying 176 water vapour transmission testing 441–2 waterproof finishes 377–8 Wavemaker system 73, 75 web bonding see bonding web compaction 225, 275–7 web detachment systems 44 web formation 8–9 dry-laid see dry-laid nonwovens polymer-laid see meltblowing; polymerlaid nonwovens; spunbonding wet-laid see wet-laid nonwovens

530

Index

web manipulation 6 carding 42–3 web weight 56 weigh-belt systems 63 weigh-blenders 27 weigh platforms 63 weight fabric weight 408–12, 419 uniformity 409, 429–30, 493 on-line basis weight measurement 64–5 profiles 71 web weight and fineness 56 weight measurement control systems 58–67 wet finishing 369–75 wet-laid nonwovens 4–5, 6, 7, 112–42 background and historical development 112–14 bonding systems 135–8 combining wet-laying with airlaying 102 fibre preparation 126–7 finishing 138 product applications 139–41 raw materials 116–25 theoretical basis 114–16 web-forming process technology 128–35 wet lamination 385 wet-on-dry padding 379 wet-on-wet padding 379 wet-operating filters 491 wet sieving 432 wet spinning 120, 152 wettability 265–6, 442–3 wetting 346, 347 measuring 442–8 wetting agents 347 white-water circuit 130 wicking 442

areal wicking spot test 443 measurement 443–8 modelling 468–74 one-dimensional liquid wicking rate 445–6 two-dimensional liquid wicking rate 446–8 wicking strip tests 445–6, 469–70 winding 160, 389 wipes 4, 5, 255 airlaid nonwovens 108 chemically bonded nonwovens 361–3 hydroentangled nonwovens 289–91 wiping efficiency 423 wire angles 45, 47–9 wire covering, card 44–53 wire depth 45–6 wire foundation 50–2 wood pulp 76–8, 82, 103–4, 118–19, 266 airlaying technology for 91–8 hydroentangled wood pulp fabric 99 see also cellulose fibres wool fabric 477 worker 32–7, 38, 50 interaction with cylinder and wire angles 47–8 principle of working 34–5 Worldwide Strategic Partners (WSP) standards 414, 415–23 wound dressings, standards for 414–24 Wrotnowski’s model 449–50 yarn punching 250–1 Young’s modulus 177–8 Zimmer Magnoroll/Magnoknife 381 Variopress system 384

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Applications of Nonwovens in Technical Textiles (Woodhead Publishing Series in Textiles)

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Handbook of Inorganic Chemicals

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