HIGH PERFORMANCE NONWOVENS IN TECHNICAL
TEXTILE APPLICATIONS
CHRISTOPHER OLARINDE OGUNLEYE
(204071852)
2013
HIGH PERFORMANCE NONWOVENS IN TECHNICAL
TEXTILE APPLICATIONS
By
Christopher Olarinde Ogunleye
Submitted in partial fulfilment of the requirements for the degree of Philosophiae Doctor in the Faculty of Science at the Nelson Mandela Metropolitan University
December, 2013
Promoter: Prof. Rajesh Anandjiwala Co-Promoter: Prof. Lawrance Hunter
DECLARATION
I, Christopher Olarinde Ogunleye (student No. 204071852) hereby declare that the thesis for Philosophiae Doctor (PhD) in Textile Science is my own work and that it has not previously been submitted for assessment or completion of any postgraduate qualification to another University or for another qualification.
Christopher Olarinde Ogunleye.
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ACKNOWLEDGEMENTS
The author would like to express his profound gratitude to the promoter, Professor Rajesh
Anandjiwala and my co-promoter, Professor Lawrance Hunter for their continous support and guidance during my studies.
I also appreciate the support given by the entire staff of CSIR and their hospitality. Thank
you all!! I am also deeply indebted to Yaba College of Technology, Lagos, Nigeria for
releasing me to undertake this study.
I am also grateful to Nelson Mandela Metropolitan University for their financial support during my study.
The author would also like to thank the technical team of Nonwoven division for their generous contribution and assistance during the project.
I also appreciate the patience of my wife, Grace, and my children. Through your support, encouragement and prayers, I am living my dreams.
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ABSTRACT
The aim of this research was to establish the optimum processing conditions and parameters
for producing nonwoven fabrics best suited for application in disposable and protective wear
for surgical gowns, drapes and laboratory coats.
Carded and crosslapped webs, of three basic weights (80, 120, and 150g/m2), from greige
(unscoured and unbleached) cotton, viscose and polyester fibres, were hydroentangled, using three different waterjet pressures (60, 100 and 120 bars), on a Fleissner Aquajet hydroentanglement machine.
An antibacterial agent (Ruco-Coat FC 9005) and a fluorochemical water repellent agent
(Ruco Bac-AGP), were applied in one bath using the pad-dry-cure technique, to impart both
antibacterial and water repellent properties to the fabrics, SEM photomicrographs indicating
that the finished polymers were evenly dispersed on the fabric surface. The effect of waterjet
pressure, fabric weight and type and treatment on the structure of the nonwoven produced,
was evaluated by measuring the relevant characteristics of the fabrics.
As expected, there was an interrelationship between fabric weight, thickness, and
density, the fabric thickness and mass density increasing with fabric weight. An increase in
waterjet pressure decreased the fabric thickness and increased the fabric density. The water
repellent and antibacterial treatment increased the fabric weight and thickness.
The antimicrobial activity of the fabrics was assessed by determining the percentage reduction in Staphylococcus aureus and Escherichia coli bacteria population. The maximum percent reduction at 24hrs contact time for both bacteria ranged from 99.5 to 99.6 % for all the fabric types.
The standard spray test ratings for the three treated fabrics ranged from 80-90%, whereas that of the untreated water repellent fabric was zero, while the contact angles for all the fabric types exceeded 90 degrees, indicating good resistance to wetting. It was found that the tensile
iii strength of the fabric in the cross-machine direction was higher than that in the machine direction, for both the treated and untreated fabrics, with the tensile strengths in both the MD and CD of the treated fabrics were greater than that of the untreated fabrics, the reverse being true for the extension at break. An increase in waterjet pressure increased the tensile strength but decreased the extension at break, for both the treated and untreated fabrics.
The finishing treatment decreased the mean pore size of all the fabrics, the mean pore size decreasing with an increase in fabric weight and waterjet pressure. An increase in waterjet pressure and fabric weight decreased the air and water vapour permeability, as did the finishing treatment, although the differences were not always statistically significant. The polyester fabrics had the highest water and air permeability.
Hence low weight fabrics of 80 g/m2, which were hydroentangled at low water jet pressures of 60 bars, were suitable for use in this study due to their higher air and water vapour permeability as well as higher pore size distribution. These group of fabrics thus meet the requirements for surgical gowns, drapes, nurses’ uniforms and laboratory coats.
Keywords: Antibacterial, water repellency, nonwovens, hydroentanglement, waterjet pressure.
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CONTENTS
DECLARATION ...... Error! Bookmark not defined.
ACKNOWLEDGEMENTS ...... ii
ABSTRACT ...... iii
CONTENTS ...... v
1 INTRODUCTION ...... 1
1.1 NONWOVEN ...... 1
1.2 NONWOVENS AND TECHNICAL TEXTILES ...... 2
1.3 NONWOVEN MANUFACTURING PROCESS...... 4
1.3.1 Web formation ...... 4
1.3.2 Fibre Bonding (consolidation) ...... 6
1.3.3 Nonwovens in the medical industry ...... 9
REFERENCES ...... 11
2.LITERATURE REVIEW ...... 13
2.1 INTRODUCTION ...... 13
2.2 DEMAND FOR PROTECTIVE CLOTHING FOR HEALTHCARE WORKERS .... 14
2.2.1 Infection control barrier fabrics for healthcare workers ...... 15
2.2.2 Mechanisms of infection transmission ...... 17
2.2.3. Criteria requirements for protective clothing ...... 18
2.2.4 Comfort of protective garments ...... 18
2.2.5 Classifications of surgical gowns, drapes and laboratory coats ...... 19
2.2.6 Characteristics of surgical gowns and drapes ...... 20
2.3.THE IMPACT OF NANOTECHNOLOGY ON PROTECTIVE GARMENTS ...... 23
2.4 FIBRES FOR NONWOVENS ...... 24
2.4.1 Fibres for hydroentanglement ...... 25
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2.5 THE HYDROENTANGLEMENT PROCESS ...... 27
2.5.1 Introduction ...... 27
2.5.2 Mechanism of hydroentangling ...... 28
2.5.3 Influence of the nozzle on hydroentanglement ...... 31
2.5.4 The influence of specific energy ...... 33
2.5.5 The effect of production speed ...... 35
2.5.6 The effect of the design of the fibre support screen ...... 35
2.6 APPLICATION OF HYDROENTANGLEMENT (SPUNLACED) FABRICS...... 38
2.6.1 Latest advances in hydroentanglement technology ...... 39
2.7 FINISHING OF NONWOVEN FABRICS ...... 40
2.7.1 Mechanical finishing ...... 41
2.7.1.1 Classification of mechanical finishes applied to nonwoven fabrics ...... 41
2.7.1.2 Shrinkage ...... 41
2.7.1.3 Wrenching ...... 42
2.7.1.4 Creeping ...... 43
2.7.1.5 Crabbing and Calendering ...... 44
2.7.1.6 Perforating and Slitting ...... 45
2.7.1.7 Splitting, Grinding, Velouring and Singeing ...... 47
2.7.2 Chemical finishing ...... 48
2.7.2.1 Pad-dry-curve process ...... 49
2.7.2.2 Low wet pick-up methods ...... 50
2.7.2.3 Foam applicators ...... 52
2.8 FUNCTIONAL FINISHES ...... 53
2.8.1 Water repellency ...... 53
2.8.1.1 Theory of wetting ...... 54
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2.8.2 Water repellent finishes ...... 56
2.8.2.1 Hydrocarbon Hydrophobes ...... 56
2.8.2.2 Silicone water repellents ...... 57
2.8.2.3 Fluorocarbon repellents ...... 58
2.8.3 Antistatic finishes...... 59
2.8.3.1 Non-durable Antistatic Agents ...... 60
2.8.3.2 Durable Antistat agents ...... 61
2.8.4 Flame Retardant Finishes ...... 62
2.8.4.1 Mechanism of Flame Retardancy ...... 63
2.8.4.2 Pyrolysis of cellulose ...... 64
2.8.4.3 Flame Retardants for cellulosics ...... 64
2.8.4.4 Flame retardants for Viscose Rayon fibres ...... 67
2.8.4.5 Flame retardants for Nylon fibres ...... 67
2.8.4.6 Flame retardants for Polyester fibres ...... 68
2.8.4.7 Flame retardants for Wool fibres ...... 69
2.8.4.8 Flame retardants for Polyester/Cotton fibres ...... 69
2.8.4.9 Flame retardants for Acrylic fibres ...... 69
2.8.5 Antibacterial Finishes ...... 70
2.8.5.1 Antibacterial finishing requirements ...... 73
2.8.5.2 Mechanisms of antimicrobial activity ...... 74
2.8.5.3 Antimicrobial finishing methods ...... 75
2.8.5.4 Antimicrobial agents and their effect on textiles ...... 76
2.8.5.4.1 Controlled-release mechanism ...... 76
2.8.5.4.2 Rechargable mechanism: ...... 77
2.8.5.4.3 Barrier-block mechanism: ...... 78
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2.8.6 Breathable barrier protection against fluid ...... 82
2.8.7 Evaluation of Antimicrobial Efficacy ...... 83
2.8.7.1 Agar Diffusion Test (Agar zone inhibition)...... 84
2.8.7.2 Suspension tests ...... 84
2.9 CHARACTERISATION OF NONWOVEN FABRICS ...... 85
2.9.1 Fabric Weight, Thickness and Density ...... 86
2.9.2 Uniformity of Fabric Weight ...... 86
2.9.3 Fibre Orientation in Nonwoven Fabrics ...... 87
2.9.4 Fabric Porosity, Pore sizes and Permeability ...... 89
2.9.5 Moisture and vapour transmission through textiles ...... 92
2.9.5.1 The diffusion process ...... 93
2.9.5.2 Absorption-Desorption process ...... 94
2.9.5.3 Convection heat flow in porous media ...... 97
2.10 MECHANICAL BEHAVIOUR OF NONWOVEN FABRICS ...... 98
2.11 PROBLEM STATEMENT ...... 102
2.12 MOTIVATION ...... 104
2.13 OBJECTIVE OF THE RESEARCH ...... 104
2.14 RESEARCH METHODOLOGY...... 105
3 EXPERIMENTAL ...... 107
3.1 MATERIALS ...... 107
3.2 METHODOLOGY ...... 107
3.2.1 Fibre preparation ...... 107
3.3 CARDING AND NEEDLE PUNCHING ...... 109
3.4 CROSS-LAPPER ...... 110
3.5 SAMPLE DESIGNATION ...... 111
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3.6 HYDROENTANGLEMENT ...... 113
3.7. FABRIC FINISHING (BARRIER PROTECTION) ...... 115
3.8 FABRIC CHARACTERIZATION ...... 116
3. 8.1 Tensile Test ...... 117
3.8.2 Scanning Electron Microscope (SEM)...... 117
3.8.3 Fabric Area Weight (Mass per unit area) ...... 117
3.8.4 Fabric thickness ...... 118
3.8.5 Air permeability ...... 118
3.8.6 Pore size distribution...... 118
3.8.7 Water vapour permeability ...... 120
3.8.8 Water repellency: spray rating test ...... 121
3.8.9 Contact angle ...... 122
3.8.10 Antibacterial test ...... 124
3.8.10.1 General procedure ...... 124
3.8.10.2 Agar Disc Diffusion ...... 125
3.8.10.3 Swatch survivability test ...... 125
4. RESULTS AND DISCUSSION ...... 127
4.1 BASIC WEIGHT UNIFORMITY ...... 127
4.2 Fabric weight, thickness and density ...... 128
4.2.1 Water jet pressure, fabric density and thickness...... 131
4.3 EVALUATION OF SCANNING ELECTRON MICROSCOPE (SEM)
PHOTOGRAPHS ...... 132
4.4 EVALUATION OF ANTIBACTERIAL ACTIVITY...... 135
4.5 EVALUATION OF TENSILE STRENGTH AND EXTENSION TEST RESULTS 139
4.5.1 Tensile strength ...... 143
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4.5.2 Statistical data analysis of tensile strength ...... 147
4.5.2.1 Tensile strength in machine direction (MD) ...... 147
4.5.2.2 Tensile strength in cross-machine direction (CD) ...... 150
4.5.3 Extension at break ...... 151
4.5.3.1 Statistical analysis on MD extension at break...... 156
4.6 EVALUATION OF WATER REPELLENCY AND WETTABILITY ...... 158
4.6.1 Water repellency ...... 158
4.6.2 Surface characterization-wettability ...... 159
4.6.2.1 Statistical analysis of contact angle ...... 165
4.6.3 Evaluation of pore size distribution ...... 165
4.6.3.1 Statistical data analysis of mean pore size ...... 168
4.6.4 Evaluation of comfort related properties-breathability ...... 169
4.6.4.1 Air permeability tests ...... 169
4.6.4.2 Statistical analysis for air permeability ...... 172
4.6.4.2 Water vapour permeability (WVP) ...... 173
4.6.4.2 Statistical analysis of water vapour permeability ...... 177
4.7 SELECTION OF MATERIALS SUITABLE FOR SURGICAL GOWNS AND
DRAPES...... 178
5. SUMMARY AND CONCLUSIONS ...... 182
5.1 RECOMMENDATIONS FOR FUTURE WORK ...... 187
REFERENCES………………………………………………………………………………18
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LIST OF TABLES
Table 1.1 Application areas of nonwoven ...... 3
Table 1.2 Overview of nonwoven manufacturing technologies ...... 8
Table 2.1 Diameter and in some cases also length of viruses and bacteria ...... 16
Table 2.3 Flamability parameters for fibres ...... 63
Table 2.5 Some commercially available biocides ...... 82
Table 2.6 Antibacterial test methods ...... 85
Table 3.1 Fibre properties ...... 107
Table 3.2 Carding processing parameters ...... 110
Table 3.3 Sample designation ...... 112
Table 3.4 Hydroentanglement parameters ...... 113
Table 3.5 Antibacterial and water repellent ...... 116
Table 3.6 Fabric tests ...... 116
Table 3.7 Evaluation of antibacterial efficiency ...... 126
Table 4.1 Physical properties of hydroentangled fabrics ...... 127
Table 4.2 Reduction in number of bacteria after 24 hrs contact time ...... 138
Table 4.3 Tensile strength and extension of untreated samples ...... 141
Table 4.4 ensile strength and extension of treated samples ...... 142
Table 4.5 ANOVA of (MD) tensile strength ...... 147
Table 4.6 ANOVA of MD tensile strength (C1) ...... 149
Table 4.7 ANOVA of MD tensile strength (C2)…………………………………………...149
Tble 4.8 ANOVA of MD tensile strength (V2) ...... 149
Table 4.9 ANOVA of MD tensile strength (P1) ...... 150
Table 4.10 ANOVA of CD tensile strength ...... 150
Table 4.11 ANOVA of CD tensile strength (C3) ...... 151
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Table 4.12 ANOVA of CD tensile strength (V3) ...... 151
Table 4.13 ANOVA of CD tensile strength (P2) ...... 151
Table 4.14 ANOVA of MD extension at break ...... 156
Table 4.15 ANOVA of CD extension at break ...... 157
Table 4.16 ANOVA of CD extension at break (C3) ...... 157
Table 4.17 ANOVA of CD extension at break ...... 157
Table 4.18 ANOVA of CD extension at break (P2) ...... 158
Table 4.19 Average standard spray tests ratings ...... 158
Table 4.20 Contact angles of the fabrics ...... 160
Table 4.21 ANOVA of contact angle ...... 165
Table 4.22 Pore size distribution of untreated and treated samples ...... 165
Table 4.23 ANOVA of mean pore size ...... 169
Table 4.24 Air permeability (AP) and water vapour permeability ...... 170
Table 4.25 ANOVA of air permeability ...... 173
Table 4.26 ANOVA of water vapour permeability ...... 178
Table 4.27 Summary of relevant fabric parameters ...... 179
Table 4.28 Effect treatment on air permeability and water vapour premeability ...... 180
Table 4.29 Selected suitable fabrics and corresponding processing parameters ...... 181
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LIST OF FIGURES
Figure 2.1(a) Schematic diagram of hydroentanglement machine…………………….…….29
Figure 2.1 (b) Major components of a hydroentanglement machine ...... 28
Figure 2.2 Hydroentaglement process……………………………………………………….30
Figure 2.3 Internal waterjet turbulence………………………………………………………31
Figure 2.4 Deflection of waterjet by conveyor belt…………………………….……………31
Figure 2.5 Images taken by a camera from water jet issued at pressures of (a) 52 bar and (b) 69 bar from the cone-up nozzle ...... 33
Figure 2.6 Effect of hydroentanglement on one or both sides ...... 35
Figure 2.7 Hydroentangled fabric (a) woven fabric (b) knitted fabric (c)………….………..37
Figure 2.8 Qualitative map of shear modulus and strength…………………………………38
Figure 2.9 Production of Evolon Apparel Fabric ...... 40
Figure 2.10 The Clupak Process…….……………………………………………………….43
Figure 2.11 The Micrex creeping process… ………………………………………………44
Figure 2.12 Pad-dry- cure process ...... 49
Figure 2.13 Macknozzle system………………………..……………………………………51
Figure 2.14 Kiss-roll applicator ...... 51
Figure 2.15 Foam applicator[101] ...... 53
Fogure 2.16 Spreading of liquid on a smooth surface ...... Error! Bookmark not defined.
Figure 2.17 N-Methylol stearamide ...... 56
Figure 2.18 Silicone repellent ...... 57
Figure 2.19 Phosphoric ester antistat ...... 61
Figure 2.20 Non-ionic antistats ...... 61
Figure 2.21 DBDPO flame retardant ...... 68
Figure 2.22 Structure of gentamicin ...... 76
Figure 2.23 The structure of Triclosan ...... 77
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Figure 2.24 The structure of MDMH ...... 78
Figure 2.25 The structure of PHMB ...... 79
Figure 2.26 The structure of chitosan ...... 80
Figure 2.27 The structure of AEM 5700 ...... 81
Figure 2.28 Production of parallel-laid, condensed, random and combined random/condensed webs in carding ...... 88
Figure 2.29 Fibre orientation ...... 88
Figure 2.30 Example of pore size distribution within a fabric ...... 90
Figure 2.31 (A) Typical ODF of nonwoven fabrics ...... Error! Bookmark not defined.
Figure 2.31 (B) The two principle directions in nonwovens-MD and CD ...... 101
Figure 3.1 Shematic layout diagram (a) the Trutzchler openning and cleaning line ...... 108
Figure 3.1(b) Schematic layout of the integrated mixer and Cleanomat cleaner ...... 108
Figure 3.2 Revolving flat card for cotton...... 109
Figure 3.3 Crosslapper ...... 111
Figure 3.4 Schematic layout diagram of the Fleissner Aquajet hydroentanglement machine with a photo of the Aquajet ...... 114
Figure 3.5 Principle of capillary Flow Porometer ...... 119
Figure 3.6 Determinationof the mean diameter of the pores ...... 120
Figure 3.7 Standard Spray Test Ratings ...... 122
Figure 3.8 Dynamic Contact Angle Analyser-Wilhelmy plate technique ...... 123
Figure 4.1 Relationship between fabric thickness and fabric weight and waterjet pressure 129
Figure 4.2 Weight of fabric before and after treatment ...... 130
Figure 4.3 Fabric thickness before and after treatment ...... 131
Figure 4.4 Changes in fabric density with increasing waterjet pressures for the different fabric types ...... 132
Figure 4.5 Scanning electron micrographs before and after antibacterial and water repellent finishes. 133
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Figure 4.6 Photomicrograps of fibre bundles hydroentangled with waterjet pressures of (a) 60 (b) 100 and (c) 120 bars. 134
Figure 4.7 Colony forming units (CFU) at time 0 and after 24 hrs contact. 137
Figurev 4.8 Comparison of the tensile strength at break of untreated and treated samples in the machine-direction (MD) and cross-machine direction (CD) ...... 143
Figure 4.9 Comparison of tensile strength at break of untreated and treatedd fabrics in MD and CD at different waterjet pressure levels……………………………………………….144
Figure 4.10 Relationship between tensile strength averaged over all the fabrics and waterjet pressure ...... Error! Bookmark not defined.
Figure 4.11 Comparison of tensile strength at break of treated and untreated fabrics in MD direction ...... 146
Figure 4.12 Comparison of extension at break in MD for treated and unterated samples ... 152
Figure 4.13 Comparison of CD extension at break for treated and untreated samples ...... 152
Figure 4.14 Relationship between extention at break averaged over all the fabric and waterjet pressure ...... 154
Figure 4.15 Comparison of the extension at break of treated and untreated fabrics in (a) MD and (b) CD directions ...... 155
Figure 4.16 Contact angle for the different fibre types (a) polyester (b) cotton (c) viscose 162
Figure 4.17 Contact angle versus waterjet pressure ...... 164
Figure 4.18 Copmarison of the mean sizes of the cotton, viscose and polyester fabrics ...... 166
Figure 4.19 Mean pore size versus waterjet pressure ...... 167
Figure 4.20 Effect of waterjet pressure, fibre type and treatment on mean pore size ...... 168
Figure 4.21 Air permeability of the various fabrics (a) polyester (b) cotton and (c) viscose 171
Figure 4.22 Effect of fabric types and weight on water vapour permeability of (a) polyster, (b) cotton and (c) viscose fibres ...... 174
Figure 4.23 Effect of watrejet pressuer and treatment on water vapour permeability ...... 175
Figure 4.24 Effect of waterjet pressure, treatment and fabric tpye/weight on water vapour permeability ...... 176
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1 INTRODUCTION
1.1 NONWOVEN The nonwoven industry had an exploratory beginning in the late 1940s, entered into a
development phase in the 1950s, followed by commercial expansion in the 1960s. Up till the
last decade, much of the world’s nonwoven industry was based in the US, Europe and Japan,
where the relevant technologies were conceived and developed some 50 years ago [1].
Large firms, such as Freudenberg, Kimberly-Clark, DuPont, Johnson and Johnson’s
Chicopee operation and Ashahi, invented nonwoven technologies and nutured them to
commercial scale to make a whole range of new materials to replace a number of traditional textile products.
Over the years, the nonwoven industry has matured and established markets by either providing cost-effective performance, as an alternative to conventional textiles, or by offering specially developed products for targeted end-users.
Opinions vary about the range of fabric to be classified as nonwoven, and the precise definition of nonwoven fabrics has long been the subject of argument and discussion [2-3].
A precise definition of nonwovens is the one which is adopted by the International Standard
Organisation-ISO 9092:1988 and the European Committee for Normalisation (CEN), EN
29092 [4], namely “ a manufactured sheet, web or batt of directionally or randomly oriented fibres, bonded by friction and/or cohesion and/or adhesion, excluding paper and products which are woven, tufted, stitch-bonded incorporating binding yarns or filaments, or not additionally needled”.
The production of nonwoven fabrics follow a much shorter process from raw material to finished products as they do not require conversion of the fibres to yarn before
the fabric is produced by weaving or knitting. The short production route is the main
1
attraction of the nonwoven sector, besides high production speeds, higher profit margins and
the general versatility of the production process.
Nonwovens can be engineered for numerous applications in a number of different areas, with new products being developed continuously. The fabrics may be designed for a limited life, single-use or for durability [5]. Based on performance, properties and end-use applications, nonwovens can be designed for high-tech applications in value-added markets.
Nonwoven fabrics can be designed and engineered to provide specific functions, such as absorbency, drape, liquid repellence, resilience, stretch, flame retardancy, softness, strength, washability, cushioning, filtering, bacteria barrier and sterility. These properties are often combined to create fabrics suited for specific functions, while achieving a good balance between usable life and cost of the product. They can mimic the appearance, texture and strength of a woven fabric and can be as bulky as required.
In combination with other materials, nonwoven fabrics provide a wide spectrum of products with diverse properties, and they are used either alone or as components, in apparel, home furnishings, hygiene and healthcare, engineering, industrial and consumer goods, geotextiles, automotive industry, agriculture, thermal and sound insulation material, which are generally classified as technical textiles. Most nonwovens are used as a hi-tech functional
item and only a few are utilized in apparel products because of its inferior mechanical
properties (bursting strength and tensile strength) and poor drape or flexibility compared to
woven fabrics. These deficiencies are, however, being improved with innovative
developments in nonwovens.
1.2 NONWOVENS AND TECHNICAL TEXTILES
Whereas most of the nonwoven industries produce products for aesthetic and decorative
purposes (fashion apparel, clothing and fashion accessories), nonwoven fabrics for technical
2 applications should have more functional properties. They are frequently used in a range of
“downstream” applications in other manufacturing and service industries.
The industry also shares a number of technologies, and has overlapping interests with other industries producing different materials, such as fibre reinforced composites, glass, plastics, films, membranes and papers. A major report on the world market for technical textiles and industrial nonwovens, by David Rigby Associates (DRA), has defined the major application areas shown in Table1.1 [6-7]:
Table 1.1: Application areas of nonwovens [6].
APPLICATIONS PRODUCTS
Hygiene Baby diapers and training pants, adult incontinence pads, sanitary napkins, tampons, cosmetic removal pads, nasalstrips, disposable underwear Wipes Disposable wipes, dusters, dishcloths, mops
Medical and surgical Surgical swabs, wound dressings, surgical gowns, masks and caps, orthopaedic casts, surgical drapes, wraps and packs, transdermal drug delivery, heat and procedure packs Protective clothing Disposable clean-room garments, laboratory coveralls, fire protective linings, thermal insulation fillings, chemical defence suits Filtration Teabags, drinks filtration, oil sorption, industrial gas filtration, respiratory filters, vacuum filter bags, odour control Interlinings and Fusible interlinings, shoulder pads, glove linings garments Shoes, leather-goods Boot and shoe lining, synthetic leather shoe uppers, shoe and coating substrates construction components, luggage and bags Upholstery, furniture Ticking, mattress pads, wadding and fillings, sheets and and bedding blankets, window blinds, quilt backings, dust covers Floor coverings Contract carpets and carpet tiles, Underlays and carpet backing fabrics, Automotive carpets and trims Building and roofing House wrap, thermal and sound insulation, roof linings, under- slating, plaster board facings, pipe wraps fabric tiles Civil engineering and Landfill membrane protectors, drainage systems, lining geosynthetics systems for reservoirs and ponds, erosion control and ground stabilization, soil separation
Additionally, the market for technical and nonwoven textiles and fibres is growing, as the industry continues to innovate and develop products for both old applications and new end
3
uses. Technology and innovation are creating the capability of these fabrics to provide high
quality and functional products. It is claimed that the global production of nonwovens
reached US$15.9 billion and approximately 110 million square metres in 2010, with
production increasing at an average annual growth rate of 7.4% per year over the past decade
[8].
The global textile industry produces a wide range of products which are supplied to most
major manufacturing sectors with technical textiles and nonwoven manufacturing regarded as
the most creative and fastest growing area of the global textile industry.
1.3 NONWOVEN MANUFACTURING PROCESS
The basic nonwoven manufacturing process involves four principal elements or phases:
Fibre selection and preparation
Web formation
Web consolidation
Finishing.
1.3.1 Web formation In all nonwoven web formation processes, fibres or filaments are either deposited onto a
surface to form a web or are condensed into a web, and fed to a conveyor. The fibres at this
stage can be dry, wet or molten-dry laid, wet laid or polymer-laid (also referred to as spun
laid and spun melt processes).
Web formation involves converting staple fibres or filaments into a 2-dimensional
(web) or a 3-dimentional batt which represents the precursor for bonding into final fabric.
Nonwoven manufacturing processes can be grouped into one of four basic technologies,
namely textile, paper, plastic and hybrid [9]. The textile technology includes garneting,
carding and aerodynamic processes to convert fibres into preferentially-oriented webs.
4
Accordingly, the fabrics produced by these systems are referred to as garneted, carded and air-laid, respectively. This technology involves forming discontinuous fibres into parallel two-dimensional layered or three dimensional random orientations, by mechanical or aerodynamic means, which are subsequently bonded mechanically, chemically or thermally.
Mechanical web formation involves the use of textile carding or garneting machinery, or components, to transform tufts of fibres or fibre blends into fibrous webs, in which individual fibres are held by cohesion. Contrary to carded or garneted nonwovens where multiple forming machines are utilized, the air laid nonwoven webs are generally formed on single machines. Mechanical consolidation can be accomplished by chemical or thermal means.
The paper technology includes dry-laid pulp and wet-laid modified paper systems designed to accommodate fibres longer than wood pulps. The fabric produced by the wet- laying method similar to conventional paper technology, is referred to as “wet-laid” nonwoven. They are manufactured with machinery similar to pulp fiberizing and paper forming (slurry pumping onto continuous screens) designed to manipulate short fibres suspended in fluid. However, processing synthetic or inorganic fibres in slurry form creates some challenges, since these fibres do not wet out readily and are difficult to disperse, and hence very high water dilutions are necessary to keep the fibres apart in the water suspension.
Very careful handling is therefore required to prevent fibre entangling and poor sheet formation. For web bonding, a number of binder types and application methods are employed, either before or after web formation, each engineered to yield specific fabric properties.
The plastic technology (extrusion-technology) employs machinery similar to those for polymer extrusion (man-made fibre spinning, film casting, extrusion coating). In polymer laid systems, fibre structures are simultaneously formed and manipulated. The textured film
5
nonwoven systems usually employ slit extrusion technology. Upon extrusion, the molten
sheets are cast onto engraved drums and may be subsequently stretched biaxialy.
The hybrid technology includes combining fabric-sheet and basic nonwoven
processes. Fabrics produced in this way are referred to as “composite” nonwovens. These are
produced with fabric laminating or joining machines (thermal calendars or embossers).
Composite nonwovens are produced by integrating components of two or more basic manufacturing systems, which provide a means of incorporating the advantages of two or
more nonwoven manufacturing systems to produce specialized nonwoven structures, with
properties unattainable by any single nonwoven process. Examples are hydroentangling a
tissue with carded rayon webs to manufacture wipes with large and small capillaries, thermo
embossing a film to high loft to yield a moisture-impermeable insulator, air-forming pulps
and textile fibres, to yield enhanced filter media, and spunbond-meltblown-spunbond
laminates for enhanced strength/surface area fabrics.
1.3.2 Fibre Bonding (consolidation)
The web consolidation process involves bonding the preferentially arranged fibres or
filaments by mechanical, chemical or thermal means. The degree of bonding largely
determines the fabric integrity (strength), porosity, flexibility, softness, and density (
thickness).
The various technologies employed for bonding nonwovens include:
Chemical bonding
Thermal bonding
Needle punching and
Spunlace (Hydroentanglement)
a) Chemical Bonding: Chemical bonding is one of the most commonly employed
processes of bonding fibrous web. The chemical binder is applied to the web and
6
cured. The most commonly used binder is latex, because it is economical, easy to
apply and very effective [10]. Several methods are used to apply the binder to the
web, which include saturation bonding, spray bonding, print bonding and foam
bonding.
b) Thermal bonding: In this process, heat is applied to bond or stabilize a web structure
made from thermoplastic fibres. The fibre itself acts as a binder, thus eliminating the
use of latex or resin. Thermal bonding is commonly employed by the cover stock
industry for baby diapers [11.12]. For point bonding, two calendar rollers are
employed. One of these rollers has an engraved pattern on its surface which leads to
local fibre-to-fibre bonding at the points of the intersection between the engraved and
smooth rollers. Both the rollers are heated internally, and the level of bonding mainly
depends on roller temperatures, besides other factors, such as nip pressure,
throughput speed, and the fibre and web properties e.g. basis weight, fibre type, fibre
mass linear density and blend level or ratio [5]. c) Needlepunching: This is a process of bonding fibrous web structures by
mechanically interlocking the fibres. The fibres are mechanically entangled by
reciprocating barbed needles through a moving batt of fibres in a needle loom. As the
web moves forward under the needle board of the loom, more fibres are progressively
entangled by the needle barbs and a coherent fabric structure is formed.
[5, 13]. d) Hydroentanglement (Spunlace): This is the process of entangling a web of loose
fibres placed on a moving porous belt, or a perforated or patterned screen, to form a
consolidated structure, by subjecting the fibres to multiple rows of fine high-pressure
water jets. Entanglement occurs when the water jet strikes the web and the fibres are
7 deflected, with the vigorous agitation within the web causing the fibre entanglement
[14]. The various stages of nonwoven production is summarised in Table 1.2 [5].
Table 1.2 Overview of nonwoven manufacturing technologies [5].
8
The majority of hydroentangled fabrics utilize dry-laid (carded or air lay) webs as a precursor. The Unichem technology however, employs wet-laid webs as precursor, before hydro entanglement, for producing nonwoven fabrics [15].
Spunlace fabrics are unique and somewhat different to other nonwoven fabrics, due to the balance achieved between strength and shear modulus. Generally speaking, spunlaced fabrics rely primarily on fibre-to-fibre friction to achieve physical integrity being characterised by relatively high strength, softness, drape, conformability and aesthetics similar to woven and knitted fabrics [16]. Nonwovens may be given one or more finishing treatments to enhance fabric performance or aesthetics [17].
1.3.3 Nonwovens in the medical industry
Nonwoven fabrics have found their way into many traditional applications, where mainly woven and knitted products were used earlier. Medical textiles are a branch of technical textiles dealing with the usage of textiles in healthcare. Medical textiles are as important as the medicine itself, because of their role in treating and preventing the medical problems of patients.
The four main classes of medical textiles are: implantable, non-implantable, extracorporeal and healthcare/hygiene and the products include textile materials used in hygiene, health and personal care, as well as in surgical applications. Healthcare and hygiene products are important in surgery. These products include surgical gowns, drapes, laboratory coats, caps etc. With the advancement in technology, the market for disposable medical textiles is expected to grow. The factor continuing to drive the growth is ongoing awareness of and concern over, Hospital-Acquired Infections (HAI), such as hepatitis and aids [18].
With the increasing sophistication in surgery, the disposable medical textiles play an important role in reducing infections. As awareness of the importance of medical fabrics
9 grew, newer designs were introduced and finally nonwoven products took the lead due to their inherent attributes [19], and the fact that they can be economically produced and tailor- made to provide the functionality required for the intended end-use [20].
It is the aim of this research, together with the current R&D drive by the academia and industry, to make a meaningful contribution to the growth of nonwoven products in the medical field.
10
REFERENCES
1) The nonwoven Handbook of INDA, Association of the Nonwoven Fabrics Industry,
(1988).
2) Horrocks, A.R. and Anand, S.C. (2007). Hand book of Technical Textiles.
3) European Disposables and Nonwovens Association, EDANA.
4) International Standard Organisation-ISO 9092:1988; BS EN 29092:1992.
5) Russel, S.J. (2007). Handbook of Nonwovens, The Textile Institute, Manchester.
6) David Rigby Associates DRA (1997). The world Technical Textile Industry and its
Market Prospects to 2005.
7) David Rigby Associates DRA (2005). Technical Textiles and Nonwovens: World
Market forecasts to 2010.
8) Rajedran, S., Anand, S.C. and Harrison, P.W. (2002). Development in Medical
Textiles: A critical Appreciation of recent developments, Textile Progress, 32, 10-13.
9) Vaugn, E.A. (2006). The relationship of Textiles, Paper and Plastic Technologies to
Emerging Nonwoven Manufacturing Processes, Journal of Industrial Textiles, 18 (2),
94-105.
10) Lunenschloss, J. and Albrecht, W. (1985). Nonwoven Bonded Fabrics, John Wiley
and Sons Inc., New York.
11) Watzl, A. (1994). Fusion Bonding, thermal bonding, and heat-setting of Nonwovens.
Theoretical fundamentals, practical experience, Market Trends, Melliand (English),
10/1994, E 217.
12) Hoyle, A.G. (1990). Thermal Bonding of Nonwoven fabrics, TAPPI Journal, pp. 85-
88.
13) Foster, J. (Oct.1992). Nonwoven Industry, pp. 44
11
14) Albrecht, W. Fuchs, H. and Kittleman, W. (2003). Nonwoven fabrics: Raw material,
Manufacture, Applications, Characteristics, Testing Processes.
15) White, C.F. (1990). Hydroentanglement Technology applied to wet formed and other
precursor webs, TAPPI Nonwoven conference, pp.177-187.
16) Spunlace Nonwoven, Perfojet (1991). December
17) Inda training Workshops on Nonwovens (Aug, 2010), Coimbatore, India, 12-13
August.
18) Wuagneux, E. (2008). Medical market maturation, Nonwoven Industry, pp. 26-33.
19) Dharmadhikary, R. (2005). Application of nonwovens in medical fabrics and the
Indian market potentials, Proceedings of the Seminar on Nonwoven Technology,
product and market potential, IIT, Delhi, 14-15, pp. 56-62.
20) Mayekar, A. (2008). Disposable Textiles-Future of India, Proceedings of
International Conference on Technical Textiles and Nonwovens, ICTN-2008, IIT
12
2. LITERATURE REVIEW
2.1 INTRODUCTION
Nonwoven fabrics are used extensively as barrier protection for workers in various hazardous occupations. Since the features desired in protective clothing vary from occupation to occupation, this review will concentrate on the healthcare and agricultural (farm) workers, with only brief mention of literature on protection for other occupations for general
information.
Nonwoven fabrics, particularly those produced by the hydroentanglement process, are versatile because a wide range of fibres can be used, and they can be processed under widely varying processing conditions.
Hydroentangled nonwoven fabrics are noted for the following characteristics [1]:
. Good drape
. High absorbency
. Soft, limp and flexible hand
. High strength without the use of binders
. Comfortable and mouldable
. Low lint formation
. Stretchable without loss in thickness
. Delamination resistance
. High bulk
Because of these unique characteristics, spunlaced fabrics, made from cotton, viscose, polyester, nylon and polypropylene, have found acceptance in applications such as medical, apparel, wipes, home furnishing and overalls for farm workers [2].
For effective barrier protection in the medical field, such as for surgical gowns, drapes, facemasks, and laboratory coats, the fabric must provide reasonable protection against all
13 anticipated exposure in occupational conditions and must not permit blood or other potentially infectious materials to pass through it or reach personal clothes, undergarments, skin, eyes, mouth or other mucous membranes of the wearer under normal conditions of use.
It is also required that the fabric must be lightweight, and breathable to provide wearing comfort.
To meet these stringent requirements, therefore, the fabric producer will have the following challenges, among others, to contend with;
. identifying the critical performance requirements of protective clothing for healthcare
workers,
. selection of suitable fibres and their preparation,
. production of high quality and uniform webs,
. selection of suitable combinations of parameters for the hydroentanglement process,
. employing suitable finishing agents to enhance the barrier protection properties of the
fabrics,
. evaluating critical fabric characteristics,
. selection of standard testing equipment, and methodologies for evaluation.
The above tasks would require in-depth understanding of the fibre-process-structure interaction which would, to a large extent, influence the protective effectiveness of the barrier properties of the fabric, among other requirements. The above tasks will be fully discussed in the following sections of this review.
2.2 DEMAND FOR PROTECTIVE CLOTHING FOR HEALTHCARE WORKERS Clinical studies have demonstrated that nonwoven fabrics are superior to their woven counterparts in controlling post operative infections [3]. These findings have led to the acceptance of nonwovens in healthcare. It is claimed that nonwovens represented over 80% of the fabrics in the U.S for all surgical procedures, while surgical personnel indicate that
14 the major reason for using nonwovens include convenience of use and superior barrier properties, surgical gowns, laboratory coats, and drapes etc [3]. Similarly, in the developing countries of Asia, Africa and South America, the demand for medical nonwovens is increasing as a result of a fast–growing, increasingly urbanised, young, and health conscious population [4]. It is anticipated that new fibres and processing technologies, as well as collaborative and multidisciplinary efforts, will further boost the market.
2.2.1 Infection control barrier fabrics for healthcare workers
Healthcare workers are at risk in terms of acquiring infections during patient-care activities, especially from blood borne pathogens released during invasive procedures. The risk of being infected with hepatitis B, hepatitis C and HIV have been well documented [5-7]. The main purpose of using gowns and drapes is to prevent the transfer of microorganisms from the skin of surgical workers and patient to the surgical wound, and thus, reduce the risk of contamination and Surgical Site Infection (SSI). Surgical apparel provides a physical barrier to the transfer of microorganisms.
The bacteria are usually transmitted through a medium referred to as carrier, such as water, alcohol, air, perspiration, etc. Surgical apparel can reduce the transfer of microorganisms, by creating a physical barrier between the infection source and a healthy individual. It has been observed that if the fabrics used in the manufacturing of the surgical gowns, drapes, or laboratory coats, have pore sizes less than the size of the microbes, then the microbes can neither enter the fabric nor come in contact with the body of the healthy person [8]. It might not be always possible, however, to design a fabric with the particular pore size to prevent microbes to enter and pass through the fabric, especially for microbes like viruses which are at the micro level in size [9].
15
The fabric designer should therefore know the diameter of common viruses and bacteria
(examples are given in Table 2.1).
Table 2.1: Diameter and in some cases also length of Viruses and Bacteria [9].
SPECIES SIZE (µm) ASSOCIATED DISEASES
Bacteriophage 0.025-0.027 Test virus used to test 2H Technology filtration efficiencies... Hepatitis Virus (HBV) 0.042-0.047 Hepatitis B
Adenovirus 0.07-0.09 Respiratory Infections
HIV 0.08-0.11 Acquired Immuno Deficiency Syndrome Filoviruses 0.08 Ebola Virus (0.79-0.97 length) Bunyaviridae 0.08-0.12 Hanta Virus
Orthomyxoviridae 0.08-0.12 Influenza A, B, and C
Coronaviridae (SARS) 0.10-0.12 SARS
Varicella-Zoster Virus 0.11-0.12 Herpes
Cytomegalovirus 0.12-0.20 Pneumonia, Hepatitis, Retinitis, Encephalitis Variola Virus 0.14-0.26 Small Pox (0.22-0.45 length) Staphylococcus aureus 1.0 Pneumonia,Osteomyelitis, Acute Endocartis Meningitis, Toxic ( 1.5-4.0 length) Shock Syndrome, Myocarditis
Bacillus Anthracis 1.0-1.5 Anthrax Infections 3.0-5.0 length Mycobacterium 1.0-1.5 Tuberculosis tuberculosis Pseudomonas aeruginosa 0.5-1.0 Endocarditis, Pneumonia, Osteomyelitis, Nosocomial Infections, Meningitis, Septicemia Serratia marcescens 0.45 Extra-intestinal Infections, Nosocomial infections
16
For surgical protection, nonwoven fabrics are required to have a special antibacterial coating.
Breathable coating/waterproof finishes are recommended to ensure an absolute barrier to viruses, as well as comfort to the wearer. For woven fabrics, special woven structures have to
be developed for this purpose. For example, twill weave fabrics are claimed to have poor
barrier properties compared to plain weave fabrics due to the large pores between adjacent
yarns at their crossing points, hence plain weaves are generally preferred [9].
2.2.2 Mechanisms of infection transmission
In order to prevent transmission of pathogens by the fabric, it is important to know how the
transmission takes place in a hospital environment [10,11]. The various ways of transmission
include contact, droplet, airborne and through a common vehicle.
Direct contact transmission can occur when there is direct contact between the body and the
contaminated surface, with the resultant physical transfer of microorganisms from an
infected person to a susceptible host, e.g. when touching blood or patient tissues with
ungloved hands. Indirect-contact transmission involves contact of a susceptible host with a
contaminated intermediate object, such as instrument, dressing or contaminated direct glove
e.g. contacting a used dental instrument or a removable patient appliance.
Droplet transmission can occur as a result of coughing, sneezing, talking etc.
Transmission takes place when droplets, containing microorganisms generated from the
infected person, are propelled a short distance through the air and deposited on the host’s
body parts, such as mucosa or mouth. An example is contact with particles or spatter of
patient body fluids.
Airborne transmission occurs through the dissemination of either droplet nuclei or dust
particles containing the infectious agents. Depending upon the environmental factors, the
pathogens are carried over a longer distance. An example is the breathing–in of very small
particles coming from patient body fluids. Common vehicle transmission applies to
17
microorganisms transmitted by contaminated items, such as device and equipment, which are
carried through different vehicles or transport systems.
2.2.3 Criteria requirements for protective clothing
The healthcare regulatory bodies consisting of The Association of Operating Room Centre of
Disease Control, Occupational Safety and Health Association (OSHA) and Association of
Operating Room Nurses (AORN) have suggested the essential criteria to be met by barrier
protection fabrics, included [12-16]:
Blood and aqueous fluid resistance
Abrasion resistant to eliminate bacteria penetration
Lint free to reduce number of particles in the air
Breathable (allow air and moisture to pass through)
Must withstand sterilization by all known methods (by steam, radiation or
gas)
Lightweight, strong, flexible and inexpensive.
For surgical gowns, drapes and laboratory coats to be considered suitable, they must satisfy
the above criteria, i.e. they should prevent the blood-borne infectious microbes from
penetrating through the fabric and the fabric should be liquid proof. At the same time, the
fabric must provide comfort to the wearer by allowing body heat and moisture to exchange
between the interior and exterior of the body. In other words, the fabric should provide
barrier protection as well as comfort.
2.2.4 Comfort of protective garments
The purpose of covering the human body is to provide protection against adverse conditions, with an adequate level of comfort [9,17]. The passage of heat and water vapour through a garment is probably the most important factor in assessing the wearer comfort of the
18
clothing. If a garment provides too great a barrier to water vapour passage, liquid moisture
is formed by condensation of water vapour and the sensation of discomfort is increased,
partly from a feeling of clamminess and partly due to clinging of wet clothing to the body
[18].
The factors contributing to thermal comfort include:
The body’s internal metabolism producing the body heat
General level of activity
External temperature
Insulating/permeable properties of fabric/clothing to moisture, water or air.
The wearing comfort of surgical garments, along with the desired protective barrier depends largely on properties, such as thermal conductivity, water vapour permeability, air permeability and water impermeability. Most of the protective garments used to prevent cross-infections, from patient to patient and from patient to medical personnel, possess barrier properties which resist microorganisms and liquid penetration but are not comfortable to wear for extended periods. The performance of hospital garments/textiles thus demands a balance between barrier and wearer comfort.
2.2.5 Classifications of surgical gowns, drapes and laboratory coats
Fabrics for healthcare workers fall essentially into two life-cycles:
. Reusable
. Disposable
Reusable products are usually made from woven fabrics. These products are laundered and
sterilized after use in order to remove stains but still retain the barrier effect. It is claimed that
approximately 20% of surgical garments are of the reusable type [3]. There is, however, the
possibility of reduced barrier protection and loss of durability after repeated use and
washing [7].
19
Disposable fabrics are for single-use only, and are generally produced from nonwoven fabrics which are suitable for medical and hygiene products. This category of nonwovens is extensively used in the United States of America where over three billion square metres of nonwoven fabrics, worth $1.15 billion, are used in various healthcare products annually [8].
It is claimed that reduced pore size, in addition to water repellent and antibacterial finishes,
would enhance barrier protection of disposable surgical gowns, drapes and laboratory coats.
Because disposable garments are used only once, any concern about damage to the barrier
caused by reprocessing does not exist, and product quality remains highly consistent
throughout the service life. Furthermore, there is no need for in-house processing, so that it facilitates a move towards instrument-focussed sterile services.
Because it is possible to dispose of contaminated textiles quickly, it eliminates laundry cost, and garments can be donned and doffed quickly in location, such as emergency rooms [19]. Also, disposable nonwoven fabrics are not as heavy as reusable ones and they are in fact cooler to wear [20], There is however, a need to focus on biodegradable products, because of tighter environmental laws on landfills relating to the handling of such disposable products.
2.2.6 Characteristics of surgical gowns and drapes
The characteristics of ideal surgical gowns and drapes have been well defined in the literature
[21-23]. Each of these characteristics may be measured by one or more of the standardized tests developed by several organizations, such as the American Society for Testing and
Materials (ASTM), The Health Industry Manufacturer’s Association and The National Fire
Prevention Association.
Although a variety of tests have been used to compare barrier qualities of fabrics, there is a controversy about which test procedure most closely imitates actual conditions found in the surgical environment. Current tests allow a broad categorization of gowns, based
20
on protection from liquids, namely, repellent or impervious (i.e. liquid proof, prevents the penetration of liquids and microorganisms).
For reuseable products, one needs to consider not only the characteristics of the new items but also those after repeated use and laundering. Maintaining manufacturer’s specifications is
easier for single-use items than for reusable products.
2.2.7 Control of microorganism by choice of fabric composition
Both reusable and single-use products are commonly reinforced to enhance their properties
and performance. For some surgical gowns and drapes, the barrier properties of a single sheet
or a layer of fabric may not be adequate for the particular application; in such cases,
additional layers of materials are often added in the form of coating, reinforcements, or
laminates. In addition, other layers may be added to improve absorbency and non slippage, or
to produce other desirable characteristics [23]. The following materials are being used in
reusable and single-use products: standard (one layer) fabric, reinforced fabric (second layer of fabric used to reinforce base material) and impervious fabric.
The barrier effectiveness of woven fabric has been improved by the development of tighter weaves and newer liquid repellent finishes. Other improvements in reusable fabric include the development of layered fabrics having a highly resistant membrane between the fabric layers. Such reusable materials provide good barrier protection on first use, but continued effectiveness of these barrier properties is dependent on the type of antimicrobial and water repellent agents employed for the finishing. Some studies have shown that the barrier effectiveness of laundered reusable fabrics is diminished over repeated cycles of reuse
and washing [9].
Liquid-proof protection is available with polyreinforced nonwovens for single–use
and some reusable, products reinforced with membranes. For situations, where resistant or
21
repellent fabric is required for reusable gowns and drapes, unreinforced nonwovens may be
chosen. Highly resistant barriers are available with single-use products made from nonwoven fabrics reinforced with a polyethylene sheet and with reusable products layered with breathable membranes.
Stronger protection requires the use of a trilaminate spunbond/meltblown/spunbond (SMS), comprising one layer of meltblown nonwoven fabric placed between two layers of spunbond fabrics. The meltblown fabric provides filteration of particulates and bacteria and fluids, while the spunbond fabrics forming the outer layers provide the necessary strength.
Gowns must provide protection against liquid strike-through at the anticipated pressures and
blood loss. With the exception of neck and head injury, plastic film reinforced gowns are
recommended for all surgical procedures when anticipated blood loss is greater than100ml
and the duration of surgery exceeds two hours. For lower blood loss, less than 100ml, and
surgery duration of less than two hours, reinforced gowns are recommended. Plastic
reinforced gowns are rcommended for use during surgical procedures conducted in the abdominal area [24].
From the foregoing, it is obvious that the fabric properties can exert a great influence on the
efficiency of the barrier protection of the fabric, and thus requires more attention.
For the protection of agricultural workers, various types of garments are employed during
spraying of pesticides and working in fields. These include chemical resistant coveralls,
suits, aprons and headgear [25-27]. Controls, such as closed cab systems and closed mixing
systems, have been demonstrated to reduce occupational hazards substantially, but such
systems are not feasible for conditions prevailing during some pesticide applications [28].
Perhaps such a control strategy must include a systematic evaluation of actual performance
under realistic exposure conditions [29].
22
The evaluation of protective clothing in the agricultural workplace is complicated by
several factors. Firstly, exposure can occur over the entire body, which requires the
simultaneous study of several types of garments, such as gloves and coveralls. Secondly, the
chemical agent is found in several physical forms, such as powder or liquid concentrates during mixing and aerosol spray during application. Thirdly, exposure to protected regions
may occur through the following routes:
Penetration-movement of the agent through fabric due to porosity
Permeation-diffusion of liquid following wetting of the fabric
Direct deposition- entrance of the agent through openings in the garment.
It has also been observed that many agricultural workers neither follow proper procedures in
using protective wear nor take sufficient precautions [30, 31], mainly due to the lack of
comfort, yet workers must be protected from dermal exposure to pesticides [32,33].
It is therefore essential that designers of protective clothing should also consider the
importance of fabric comfort as important attributes. It is claimed that water vapour
transmission and air permeability are key properties when considering garment comfort [34].
Protective garments must provide thermal and chemical/biological protection, hence test
methods and performance criteria, aimed at simultaneously maximising such competing
performance areas, are needed. Current standards rely almost exclusively on bench top test
equipment and methods to assess swatch size materials or components of protective
clothing. This approach fails to take into account clothing design issues, which have a
significant impact on wearer acceptability.
2.3.THE IMPACT OF NANOTECHNOLOGY ON PROTECTIVE GARMENTS
The textile industry is already influenced by the advent of nanotechnology. Nanotechnology research, to improve performance or to create newer functions of textile materials, is now flourishing [35]. These research endeavours are mainly focussed on using nanosize
23
substances, and developing nanostructures during manufacturing and finishing processes.
Particular attention has been paid to exercise thorough control on chemical finishing of
textile materials.
A polymeric nanofibre web can provide enhanced protection against chemical
aerosols (e.g. micro-droplets, biological and radioactive particles, stains, antistatics,
hydrophilic, antibacterial, water repellent etc.), without adding weight or thickness, while
still maintaining adequate permeability for wearer comfort. The polymeric nanofibre web can
be used as a carrier for active chemicals, which may improve the protective properties
(and/or permeability) of the substrate material [36].
Aerosol barrier properties are either already specified, or being considered for specification
in a variety of protective apparel applications. Each of these applications also has
complimentary requirements which may include other specifications, such as permeability,
resistance to penetration by specific liquids, flammability, durability of structure during
laundering, and wearer comfort.
Nanotechnology imparts more durable properties, due to the attachment of nano particles resulting from a large surface area to volume ratio. The key to achieve the desired performance is preventing aggregation of nanoparticles. These nanoparticles can be pre- engineered to adhere to textile substrates by using spray coating or electrostatic methods
[37]. Thus, the increasing interest in nanotechnology opens a floodgate of opportunities for developing new and innovative products in the textile sector, and these will be fully explored during this research work.
2.4 FIBRES FOR NONWOVENS
Nonwoven fabrics are required to meet specific functional requirements, hence the correct choice of fibre is of paramount importance. The choice of fibres is dependent on the required properties of the fabric, the cost/use ratio (cost effectiveness) and the demand for further
24 processing [38]. The fibres used in the greatest volumes in nonwoven fabrics are cotton, acetate, viscose, nylon, polypropylene and polyester. Wool and acrylics are also being used, not necessarily to replace cheaper natural fibres but to develop more exacting applications.
2.4.1 Fibres for hydroentanglement
Although basically all natural and synthetic fibres can be processed on hydroentanglement systems, cellulosic fibres (cotton, viscose, lyocell), polyester and polypropylene are more common. The criterion in favour of cellulosic fibres is their hydrophillicity.
The following fibre characteristics are considered critical for hydroentanglement [39, 40]:
Modulus: Fibres with a low bending modulus require less entangling energy than those with a high bending modulus.
Fineness: For a given polymer type, coarser fibres are more difficult to entangle than finer fibres, because of their higher bending rigidity. For polyester fibres, fineness of 1.38 to
1.65dtex appears to be optimum. The finer the fibre, the better the hydroentanglement under identical processing conditions, 4 to 6 dtex are recommended as upper limit.
Cross-section: For a given polymer type and fibre fineness, a triangular shaped fibre will have 1.4 times the bending stiffness of a round fibre. An extremely flat, oval or elliptical shaped fibre could have only 0.1 times the bending stiffness of a round fibre. The hydroentanglement process is hampered by increasing bending stiffness and smaller specific surface, while fibrillation and splitting of coarser fibres increase the bonding effect.
Length: There is no limitation on fibre length, although shorter fibres are more mobile and produce more entanglement points than longer fibres. Fabric strength, however, is directly proportional to fibre length, therefore, reasonable fibre length must be selected to strike the best balance between the number of entanglement points and fabric strength. Longer fibres, usually in the range of 20 and 60mm, act to increase the strength, because the structural formation of parallel fibre bundles is supported. Fibre producers attempt to meet these
25
demands by the development of special fibre types, which also includes suitable fibre
finishes and crimping.
Crimp: Crimp is required in staple fibre processing systems, and it contributes to fabric bulk, although too much crimp can result in lower fabric strength, due to poor fibre entanglement.
Fibre wettability: Hydrophilic fibres entangle more easily than hydrophobic fibres, because of the higher drag forces.
The influence of cotton fibre micronaire values on fabric properties has been studied [41].
Generally, low micronaire cotton is not recommended for producing hydroentangled nonwovens, because of the asssociated higher number of neps and small bundles of entangled fibres resulting in the unsightly appearance in the fabric.
Fibre rigidity and bending recovery influence the ability of the water jet to produce fibre entanglements, and therefore the structural features of hydroentangled fabrics can differ according to fibre type. For example, fabrics made from polypropylene and viscose rayon processed under the same conditions, where the specific flexural rigidity of polypropylene is higher than that of viscose rayon (0.35mN.mm/tex2), and the polypropylene fibre has higher
compression recovery than that of viscose rayon fibre [42].
In a hydroentangled fabric made from polypropylene fibres, when the water jet pressure is low, only the surface fibres are effectively bonded, and the fibres inside the fabric
are poorly entangled. Therefore, the surface is more compact than the core of the fabric. In
contrast, a fabric made from viscose fibres is more consistently bonded through the cross-
section, and the compaction achieved is greater than that in the corresponding fabric made
from polypropylene fibres.
26
2.5 THE HYDROENTANGLEMENT PROCESS
2.5.1 Introduction
Hydroentanglement, 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, although spunlace is the more popularly used term in nonwoven industry. The degree of bonding is the primary factor which influences the fabric integrity, which in turn affects strength, porosity, flexibility, softness and density.
No matter how the hydroentanglement system may vary, from one manufacturer to another, the entanglement of the fibre web requires a support surface for the web, water jet nozzles, water extraction, water circulation and filtration [43-45]. The method traditionally used for producing nonwoven web for spunlacing was the dry-laying process (card, air-laid), although recently the wet-laid process is also used. The dry-laid web is pre-wetted to remove air pockets. The interaction of the energised water with fibres in the web and the support surface increases the fibre entanglement, and induces displacement and arrangement of fibre segments in the web. In addition to providing strength by mechanical bonding, if required, structural patterns, apertures and complex three dimensional effects can be produced by the selection of appropriate configuration of the support surfaces [46]. Hydroentanglement also provides a convenient method of mechanically combining two or more webs to produce multilayer fabrics.
Fibre entanglement is achieved by the combined effects of the incident water jets and the turbulent flow created in the web which intertwines neighbouring fibres. The permeable support conveyor enables most of the de-energised water to be drawn underneath into the vacuum box for recycling and re-use. Some of the remaining water during processing continues with the web, some drains from the side of the support surface and some is
27
atomised depending on the water pressure. Normally, multiple injectors are used in sequence
to produce a fully bonded fabric.
Depending on the end-use of the fabric, suitable finishing agents, such as fire
retardant, water-repellent, antistatic and antimicrobial agents can be applied to enhance the
performance. Water circulation and contamination are critical issues in hydroentangling.
Therefore, it is required that the water used for bonding should have a neutral pH, contains
almost no particulate matter, no metallic ions, such as calcium, and no bacteria or other
microorganisms in order to prevent clogging of the orifices.
2.5.2 Mechanism of hydroentangling
Hydroentanglement fabrics derive their structural integrity by the simple entanglement of the
fibre segments. The degree of entanglement is affected by the process parameters (type of support screen, jet characteristics and conveyor belt speed) and the topographical and mechanical characteristics of the constituent fibres [47]. A schematic cross-section of a
hydroentangled machine is illustrated in Figs. 2.1 ( a) and (b) [48].
Figure 2.1 1 : A major components of a hydroentanglement machine [48]
28
Figure 2.1 (b): Major components of Honeycomb hyroentanglement machine[48]
Hydroentangling, one of the most popular methods for nonwoven production, is a process used for mechanically bonding a web of loose fibres to form a strong and compact fabric
[49-51].
The underlying mechanism in the hydroentanglement process is the exposure of the
fibres to a non-uniform spatial pressure field created by successive sets of closely–placed
high speed water jets. The impact of water jets on the fibres (in the web being entangled)
displaces and rotates them locally with respect to their neigbours. This phenomenon is
illustrated in Figs 2.2 and 2.3 [52].
29
Figure 2.2 : Hydroentaglement process [52]
During this relative displacement, some of the fibres twist around other fibres, and/or
interlock with them, to form a strong structure, due to inter fibre frictional forces. The
impinging of the water jets on the web causes the entanglement of the fibres [52-54].
As a result of the energy transfer, from the high pressure nozzle jets to the web, nonwoven with soft handle, drape and woven fabric like characteristics are produced [ 55-56].
Figure 2.3 : Internal waterjet turbulence[52]
30
The efficiency of the hydroentanglement process is attributed to the condition wherein the kinetic energy of the high-speed water jet is transferred to the fibres in such a manner that fibre to fibre entangling occurs all the way through the web thickness. This, however, requires the water jets to maintain their kinetic energy from a certain distance downstream of the nozzle. The jet however, dissipates most of the kinetic energy; primarily in rearranging the fibres inside the web and secondarily during backward deflection against the substrate as shown in Fig. 2.4 [52].
Figure 2.4 : Deflection of waterjet by conveyor belt [52]
2.5.3 Influence of the nozzle on hydroentanglement
The major requirement for a hydroentanglement nozzle is that it should efficiently transform the potential energy of the liquid flow into kinetic energy which is in turn transferred to the fibres for entanglement. This, however, requires the water jets to maintain their kinetic energy for a certain distance as they emerged from the nozzle. It is well known, however, that water jets break up into spray somewhere downstream of the nozzle, thus causing the jet flow to become a mixture of drops of water and air (cavitation), with reduced energy available for fibre entanglement [57].
31
The break up mechanism of nozzle water jets, both theoretical and experimental, has been
studied extensively over the years in many fields which utilizes such jets. These studies have
suggested that the break up behavior of the nozzle jet is determined by many factors which
may be summarized as follows [58-59]:
hydrodynamic forces (surface tension forces, viscous forces and initial disturbances)
in the liquid jet.
effect of aerodynamic interaction and nozzle internal flow resulting from cavitations
and separation of the flow inside the nozzle.
effect of jet velocity profile.
liquid turbulence at the nozzle exit.
An image showing water jet break up is shown in Fig. 2.5. The cavitation phenomenon,
which is currently receiving more attention, can be defined as the formation of vapour
bubbles in a fluid [60].
In the inlet region of the sharp edged nozzles, a low local pressure may form in the separated
region between the boundary layer of the jet and the nozzle interior walls. If the local
pressure is below the vapour pressure of the water, vapour bubbles will form, causing
cavitations, the phenomenon having been modelled and simulated as reported in the literature
[61-62].
While no single mechanism alone can be held responsible for water-jet breakup at high velocities, nozzle geometry is considered the most significant factor [63]. Hence, in order to improve the quality and uniformity of the surface texture of the hydroentangled nonwoven fabrics, the critical effect of nozzle geometry should be considered, and the nozzle should be designed to allow the formation of water jets with high-intact length. Water jet break up, resulting from cavitations, is illustrated in Fig. 2.5 [63], the asymmetric wavy appearance of second wind-induced breakup is evident in Figs 2.5 (c) and (d).
32
Figure 2.5: Images taken by a camera of water jets issued at pressures of (a) 52 bar and (b) 69 bar from the cone-up nozzle [63]
2.5.4 The influence of specific energy
The amount of energy delivered in the web is a crucial parameter which influences the fabric structure and properties, since it affects the degree of fibre entanglement [64]. Similarly, the degree of bonding, fabric properties and economic efficiency of the process are influenced by the energy transfered to the web, which can be calculated. It is normally expressed as the specific energy consumed by a unit mass of fibres in the web, (kJ/kg), and depends on the flow rate, water pressure and residence time of fibres under the jets, as given in the following formula [65]:
N 1 Cd 2 2 3 / 2 K nili di Pi (1) bmVb 4 w i1
33
Where, K is the specific energy consumed by a unit mass of the fibre web (J/kg), b is the
2 width of the web, m is the area density of the web (kg/m ), Vb is the conveyor belt velocity
th (m/s), Pi is the waterjet pressure at the i injector (Pa), Cd is the nozzle discharge coefficient,
3 th ρw is the water density (kg/m ), ni is the number of waterjets on the i injector (per metre), li is the width of the ith injector (metres)
Unfortunately, not all the energy applied to the web is directly utilized in entangling the fibres. The energy consumption required to produce a serviceable fabric depends on the physical and mechanical properties of the constituent fibres in the web, the fibre orientation, web thickness and web density.
Naturally, the energy transferred to the web is also going to have a profound effect on the physical properties of the fabric. As the target weight and density of the product increase, so does the energy requirement, to a point where web penetration can no longer be achieved without major fibre damage [65]. It is claimed that the amount of fibre entanglement is proportional to the energy transfered and it is reflected in the tensile strength, surface integrity, elongation at break and tensile modulus of the fabric [66].
Water pressure is also another parameter related to energy intake within the fabric.
Machines delivering higher water pressures are mostly used since this enables energy to be delivered into a web with fewer waterjets and therefore less water consumption [67]. It is also claimed that using a number of injectors produces longitudinal forces which play a part in determining the stress-strain profile of the fabric by increasing the fibre orientation in the machine direction (MD), while the tensile strength of the fabric, with the water jet entangling on both sides of the fabric, is much better than that with a single side or multiple treatments on one side, as shown in Fig. 2.6 [65].
34
Figure 2.6: Effect of hydroentaglement on one or both sides [65]
2.5.5 The effect of production speed
Another basic process parameter influencing the fabric is the speed of the machine. If a constant amount of energy is being delivered, the speed determines how much energy will be absorbed per unit area of the fabric. Ideally, the higher the production speed the less energy is absorbed by the fabric, thus resulting in a lower fabric, strength.
2.5.6 The effect of the design of the fibre support screen
The properties of the hydroentangled fabric are influenced by the design and pattern of the screen [66]. The commercially available screens are either flat or rotary, but with no difference in the mechanism used to achieve fibre entanglement [68]. The rotary system uses a compact machine design which enables fibre entanglement on both sides of the web.
Adequate fibre entanglement is achieved with as little as four metres of the material along the machine direction. Sometimes, in the rotary system, the fibres are driven through the screen wires, whereas in the flat screen system the wire (along with the fibres) is dragged over the
35 suction box along the machine direction, thus causing difficulties in the removal of the product.
The texture of the fabric can be either fine or coarse, depending on the mesh (count of wires per inch of the substrate), which has an important influence on the final product. It has been demonstrated that imposing the same level of energy into two identical webs on each of the two screens, the finer screen yielded a stronger product while the coarser one (20 mesh) provided a bulkier product, with more permeability but less strength. Similarly, the smaller the mesh size, the more energy is required to remove the excess water [69].
Hydroentangled fabrics exhibit a good drape, softness and handle, increased fibre entanglement leading to increased strength, without an increase in shear modulus. It has been shown that an inverse relationship exists between absorbency and the amount of hydroentangling energy used, an increase in hydroentangling energy resulting in a decrease in absorbency capacity and absorbency rate [70]. The shear modulus remains low and is virtually independent of the degree of fibre entanglement [71]. The softness of the fabric is explained by the fact that the entangled structure is more compressible than a rigidly bonded one, as well as lengthy mobility and partial re-alignment of the fibres in the thickness direction.
The absence of a binder is seen to result in a fabric with yarn-like fabric intersections composed of “pseudo-yarns” which are more interconnected than yarns in conventional fabrics because individual fibres can migrate from one pseudo-yarn to another, which tends to stabilize the intersection, as illustrated in Fig. 2.7 (A) [72]. Figures 2.7 (B) and 2.7 (C) illustrate the corresponding structures of woven and knitted fabrics, respectively.
36
(A) (B)
(C)
Fig. 2.7 : (A) Hydroentangled fabric (B) Woven fabric (C) Knitted fabric
This pseudo-yarn structure seems to be the reason for the good dimensional stability, which is also responsible for the good drape, softness, and strength/weight properties of the fabric, besides its improved pilling and abrasion behaviour. It is claimed that the strength of hydroentangled nonwoven fabrics is lower than that of woven fabrics, but higher than that of knitted fabrics, whereas the durability after washing is considerably lower than that of woven or knitted fabrics as illustrated in Fig. 2.8 [43,65].
37
Fig. 2.9: Qualitative Map of Shear Modul
Fig. 2.8: Qualitative Map of Shear Modulus and strength [65].
2.6 APPLICATION OF HYDROENTANGLEMENT (SPUNLACED) FABRICS.
Hydroentangled nonwoven fabrics are engineered fabrics offering cost effective solutions for an increasingly wide variety of applications. The applications of hydroentangled (spunlaced) fabrics can be categorised as follows [73]:
a) Hospital products-surgical gowns and drapes, operational table cover sheets,
bedsheets, towels, wound dressings, gauze, wet tissue, cotton products and pads.
b) Sanitary products-baby wipes, facial and cleaning wipes, masks, disposable
pants.
c) Household products-cleaning wipes, protection fabrics for electronics, table
cloths and napkins, curtains.
d) Industrial textiles-industrial wipes, filteration, roofing, water insulation,
protective apparel and liquid absorbents.
38
2.6.1 Latest advances in hydroentanglement technology
For decades, the only nonwoven fabrics used in apparel were as fusible interlinings, as reinforcements for shirt collars and cuffs, or front interlinings for suits. This has, however changed in recent years [74]. For example, Evolon, a spunlaced bicomponent nonwoven fabric, developed by France-based Freudenberg Evelon S.a.r.l, part of Germany-based
Freudenberg Nonwovens, has been well accepted in apparel products. The Evolon spunbonded web consists of continuous bi-component filaments, made from polyester and polyamide polymers, extruded as alternating segments in a single filament, as shown in Fig.
2.9. The filaments are drawn at a high speed and laid on a conveyor belt, and split lengthwise into single microfilament segments, using high pressure water jets that simultaneously tightly entangle and consolidate the filaments. It is claimed that Freudenberg is one of the first companies to produce high-tech nonwoven fabrics to be used in outer apparel [75]. The production process is shown in Fig. 2.9 [75]:
39
Figure 2.9: Production of Evolon Apparel Fabrics [75]
A considerable know-how in the traditional textile sectors is available which can assist nonwoven producers in adapting their products for demanding areas, such as apparel and industrial textiles. In particular, the use of advanced finishing techniques can enhance the properties of nonwovens to acceptable levels. It is also believed that nonwoven technologies, such as hydroentanglement, can offer opportunities to traditional textile manufacturers to harness the strengths of both nonwoven and conventional textile industries to create new hybrid, high-performance products that will define the future [76].
2.7 FINISHING OF NONWOVEN FABRICS
Nonwoven fabrics may be subjected to one or more finishing processes to enhance fabric performance (serviceability) or aesthetic properties, and may be carried out in one step, with web formation and consolidation or as a separate batch operation [77-79], many different methods and types of finishing equipments being used for this purpose. The performance properties include functional characteristics, such as moisture regain, moisture transport,
40 absorbency, water repellency, flame retardancy, softening, crease resistance, antistatic, and chemical and bacteria resistance.
The aesthetic properties include various attributes, such as appearance, surface texture, colour, odour, etc.
The finishing of nonwoven fabrics can be categorized into [80]:
. Mechanical processes
. Chemical processes
2.7.1 Mechanical finishing
Mechanical finishing is defined as any operation performed to improve the appearance or function of the fabric by physical manipulation. Steam or water may be applied during the physical manipulation process, chemicals, other than lubricants, not generally being used.
Fabric lustre, smoothness, and residual shrinkage are examples of the physical properties that can be altered by mechanical finishing.
2.7.1.1 Classification of mechanical finishes applied to nonwoven fabrics
The major mechanical finishing of nonwoven fabrics consist of the following [78,80]:
Shrinkage
Wrenching and Creeping compaction
Crabbing and Calendering
Perforating and Slitting
Splitting, Grinding, Velouring and Singeing
2.7.1. 2 Shrinkage
Like woven and knitted fabrics, nonwovens are also subjected to stresses during production, mainly in the longitudinal direction, with consequent distortions, which are generally removed by means of relaxation. The shrinkage process is carried out either under wet or dry 41 conditions, depending upon the type of fibres. Dry shrinkage treatments, by means of heat treatment, are suitable for nonwovens made primarily from synthetic fibres. The nonwoven fabric is fed through screen dryers, perforated drum dryers or short-loop dryers with rotating rods, the fabric being fed into the heating zone with overfeed, such that the web is fed faster to the roll than it is drawn off.
Wet shrinkage treatment is employed whenever the nonwoven fabric contains a significant amount of natural fibres. The nonwoven fabric is passed through a bath of hot water, to trigger the shrinkage, and then dried without tension, after squeezing or hydrosuction. A variation of the wet shrinkage method, which helps to save on drying energy, is steam shrinkage. By needling together two types of webs, where one shrinks and the other is shrink-proof, results in the formation of decorative fabric structures, suitable for wall and floor coverings or structured carpets.
The compaction, that accompanies the shrinkage, is utilized to achieve a higher weight per unit area, more bulk and higher strength [81-83].
2.7.1.3 Wrenching This process (Clupak process), which is similar to the sanforising process used in conventional textile processing, is aimed at improving handle and drape, by providing a fuller and softer fabric [84-85]. The machinery consists of a continuous rubber belt, about
25mm thick, with an intermediate woven layer lying on a heated, chromium-plated and polished drying cylinder, as shown in Fig. 2.10. The web is pressed against the cylinder at the first point of contact by a non-rotating clamping bar.
42
Figure 2.10: The Clupak Process [86]
The rubber belt is compacted lengthwise, which compacts the web between it and the cylinder in the same way, thus causing longitudinal compacting and crimping of the fibres in the web. The web is fed through the gap between the belt and the cylinder so as to fix it in the compacted state by drying. Hydrophilic fibres are more suitable than hydrophobic fibres for this process. Polyolefin fibres are not suitable, due to their lower moisture absorption and sensitivity to heat [86]. Webs, in which the fibres are predominantly oriented in the lengthwise direction, provide a more pronounced compacting effect than those which are cross-laid or random-laid. The degree of compacting is increased by about 20% if the moisture content is high but, if the bonding agent represents more than 50% of the fabric weight, such increases are not attainable [87-88]. Thermoplastic bonding agents assist compacting, but the heating tends to cause the web to adhere to the cylinder.
2.7.1.4 Creeping
In the Micrex process, web compaction of a web is so strong that the creeping effect is easily noticeable. The unit surface area is larger and the flexibility of the fabric is improved even further in comparison to the Clupak system [81,89]. The Micrex process, illustrated in Fig.
2.11, consists of a rotating conveyor roller with screw-shaped grooves on its surface and two guide plates, one fixed and one elastic, forming a knee lying against the cylinder. Between
43 these, the web is fed with a scraper-like compressing device, inclined at an acute angle to the surface roller as shown in Fig. 2.11.
Figure 2.11: The Micrex creeping process [89]
The web is compacted in the first gap, then raises itself from the cylinder in the relaxation zone, to be compacted again by the scraper. The process can be adjusted to produce a fine or coarse crepe, without impairment of the mechanical properties, at high production speeds of 150-200m/min, since the web is handled dry [89]. The temperature is comparable to that used in the Clupak process. This method is suitable for creeping carded webs, with predominant fibre orientation in the machine direction, as well as for spun- bonded and spunlaced products.
2.7.1.5 Crabbing and Calendering
These methods are used to improve the surface characteristics, such as smoothing and patterning of the fabrics. The processes are usually continuous and involve one or several pairs of rollers operating under pressure. The moiré (goffering) calenders are common in nonwoven fabric finishing, and they are used for compacting natural and synthetic fibre webs. This type of calendaring process can be used for both bonding and
44 finishing. Hot embossing of synthetic fibre webs, even when the fibres are longitudinally oriented, produces a remarkably strong product, due to the melting and bonding of fibres at the embossed areas. The patterns can be grid, webbed or point types [90]. The embossing is employed to achieve special effects, such as leather graining, simulated weave, plaster, brush strokes, cord and mock tiling [90]. Heated calenders are also used to manufacture laminates, by bonding various layers of different materials. Here thermoplastic fibres, layers of thread or film are placed between two layers of non-plastic web are fused together by applying heat and pressure. Such laminates are used as tablecloths, and as seat and cushion covers. Calenders are also used in the transfer printing of the bonded webs.
2.7.1.6 Perforating and Slitting
Perforating and slitting are two methods used to improve the fall or drape of adhesive and thermal bonded nonwoven fabrics [91], which are normally too stiff and consequently unsuitable for clothing. This stiffness is attributed to the bonded fibres not being free to move relative to one another, similarly to the threads in woven or knitted fabrics.
a) Perforating
The Artos method is used for perforating chemical bonded nonwoven fabric by means of
penetrating hot needles. This process not only punches holes, but also reinforces as a
result of cross-linking and condensation of the bonding agent. The Hungarian firm
Temaforg uses a similar method to perforate webs, made of synthetic fibres, to produce
bonded nonwoven fabrics which are strong and yet supple enough for building and
insulation materials [92-94].
45 b) Slitting
Three basic mechanical types of web separation or slitting methods are employed, namely, score cut slitting (crush cut slitting), razor slitting, and shear slitting [95].
Score cut slitting, also known as crush cut slitting, is the oldest form of cutting employed in slitting machinery. This technique crushes the material to sever one part from the other; therefore the knife cannot be razor sharp, because a very narrow knife edge will blunt, and chip almost immediately upon contact with the surface it is being crushed against. Hence, the knife used in score cut slitting is one that is dulled (rounded), to the ideal condition, before utilizing it for slitting. Slitting is achieved by pressing the knife against the web with sufficient pressure so as to cut through it. For best results, the web, the platen roll on which the web lies, and the knife, must all be moving at the same speed.
The method is quick to set up, thus offering a distinct advantage, but is losing favour due to the poor quality of cuts and low operating speeds.
The bust or razor slitting technique uses only one knife to slit the web, and is the most economical method for slitting the web. The set up is easily accomplished by clamping a blade in place along a bar, so that the blade penetrates through the web. While it is easy to set up and operate and adjust slit width, it is difficult to attain close slitting tolerances.
In shear slitting, there are two types of knives, namely the top and bottom knives, generally referred to as male and female, respectively. Shear slitting can be further categorized into tangent and wrap slitting. In tangential or kiss slitting, the web touches the top tangent point of the bottom knife only, whereas in wrap slitting, the web wraps around the bottom knife. Some products will not wrap, and therefore tangent slitting must be employed.
46
In wrap shear slitting, accurate slit width and close tolerances are possible through the
use of spacers for measuring and positioning of the knives. This combination of the
knives and spacers, stacked on the shaft, provides a complete support for the web,
thereby minimizing the possibility of wrinkling, bagging or sagging, which results in
excellent slit width accuracy. Shear cutting offers many advantages, namely a high speed
and the finest edge cut quality available, with less dust generation.
2.7.1.7 Splitting, Grinding, Velouring and Singeing a) Splitting
When nonwoven fabrics are substituted for leather, the thick layer of needled nonwoven
fabric is split in a way similar to the splitting of leather to make it thinner. The fabrics
used are thick, high strength, firmly bonded, closely needled and usually pre-shrunk. The
product is thin, supple, and like leather, is used for slip belts, shoe interlinings, backing
material for shoe uppers and leather bags. Splitting is done by machines, in which a
continuously rotating hoop knife is guided with great precision in the gap between two
conveyor rollers, the distance between them being dependent on the thickness and type of
fabric required [96]. b) Grinding and Velouring
Splitting is usually followed by friction calendaring or moiré calendaring and possibly
grinding and polishing, to make the surface even, thus imparting a velour or suede like
appearance to the fabric. The process is known as velouring. There are several machines
in tandem or consecutive passages on a single machine to coarsely roughen the surface
and then polish it to make it increasingly fine. After grinding, the dust is removed by
brushing or beating the fabric, or by suction. The distinctive features of such products are
their softness, elegant draping qualities and velvet-like surface [97].
47
c) Singeing
Singeing is essentially a process used to eliminate protruding fibres from needle punched
nonwoven fabrics. The process is exactly the same as traditional singeing, and is carried
out by passing the fabrics over an open flame. Important parameters include the evenness
of the flame height and intensity, the distance between the flame and the fabric, fabric
speed and singeing angle [98].
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, but a uniform moisture content should be maintained throughout the fabric. This is
followed by rapid cooling or wetting of the fabric to prevent after-burns and uneven
treatment.
2.7.2 Chemical finishing
In chemical finishing processes, the application of chemicals on a substrate is usually achieved in a bath or by coatings or by vapour deposition, to improve, or add desired properties to a textile structure. There are a vast number of chemical finishing processes, and the subject is extensively covered in many textbooks and reviews [99-101]. Chemical processing is widely used for protective textiles, for applying various finishes, such as water- repellent, oil-repellent, flame-retardant, chemical resistant, antistatic and antimicrobial, etc.
[102].
A chemical finishing process involves applying a chemical solution, using a suitable applicator, removing excess water, by squeezing, and heating the fabric to a desired temperature to activate the chemical (curing), and subsequently drying. The process is often referred to as the pad-dry-cure method. Each part of the process can influence the outcome of the treatment.
48
Traditionally, padders have been used to apply chemical finishes, but recently the cost of energy, as it relates to the cost of fabric finishing, has increased interest in developing low pick-up finishing techniques.
2.7.2.1 Pad-dry-curve process
A padder consists of a trough and a pair of squeeze rollers (mangle). The fabric passes under a submerged roller in a trough which is filled with the treatment chemicals, and then through the nip of the squeeze rollers. After application of the chemical finish, the fabric must be dried and the finish must be fixed to the fibre surface, usually by additional heating in a
“Curing” step. This is illustrated by the schematic diagram of a pad-dry-cure process in Fig.
2.12 [101].
Figure 2.12: Pad-dry- cure process [ 101]
Variants of padders are available, including single-dip, single-nip; double-dip, double-nip and horizontal configuration padders.
The wet pick-up of a chemical solution in a pad mangle is dependent on factors, such as fabric construction, machine setting (e.g. squeeze pressure, which is influenced by the composition of the rolls) and the solution or emulsion properties [103]. Therefore, for ensuring uniformity of chemical application, it is necessary to maintain a uniform nip pressure across the width of the rolls, constant bath temperature and constant fabric speed throughout the process [80].
49
While all fabrics have upper and lower wet pick-up limits, within these limits, adjustments in wet pick-up can be made by increasing or decreasing the squeezing pressure.
If the squeezing pressure is too low, excessive solution will remain on the surface of the fabric, when this is dried, excess chemical deposits in the over wet areas will result in a non- uniform application of the chemical.
2.7.2.2 Low wet pick-up methods
To avert problems associated with high wet pick-ups, various techniques, to lower pick-up by padding, have been developed. These include mach-nozzle system and kiss-roll applicators.
It has been observed that wet pick-up in the range of 75 to 100% is common in most padding techniques [80,101]. The bath contains about 10% active chemical and the rest is usually water, which must be removed by squeezing, followed by drying. Hence, with more water remaining on the fabric, the cost to dry the fabric will be high. Therefore, by using a solution with a higher concentration of active ingredients, the same quantity of active chemical can be delivered by lowering the wet pick-up, and consequently less energy will be required to dry the fabric.
With less water to evaporate, the processing machine can be run faster at significantly lower production cost. It is also claimed that uneven finish distribution, associated with high wet pick-up, due to migration of the finish to fabric surface during drying, is eliminated or drastically reduced [104]. Nevertheless, it has been observed that too low a wet pick-up can also be problematic and may lead to non uniform deposition of the chemical [105-107].
Machnozzle System: This method removes excess liquid with the aid of high pressure steam injection through the wet fabric, thus resulting in a low wet pick-up, particularly in the case of synthetic fabrics. The schematics of this system is shown in Fig. 2.13.
50
Figure 2.13 : Machnozzle system [101,106]
Kiss-roll applicator: This consists of a drum rotating in a trough containing the finishing chemicals. A layer of liquid is picked up by the drum surface. This liquid is transferred to the fabric passing over the exposed section of the drum. Within certain limits, the wet pick-up can be adjusted to meet the desired level, although uncontrolled variations in fabric speed or drum revolutions will lead to unwanted variations in fabric properties.
The kiss roll applicator is illustrated in Fig. 2.14 [101,104].
Figure 2.14 : Kiss-roll applicator [101-104]
51
2.7.2.3 Foam applicators
A foam is a collection of bubbles created when air is entrapped into a liquid. Since a bubble is a sphere of air entrapped by a thin layer or liquid, a volume of foam can be considered as a quantity of liquid diluted by air. The ratio of air volume to liquid volume is termed blow ratio
[80]. The blow ratio is a way of describing foam density, since large bubbles result in low density foams, whereas small bubbles yield higher density foams. A volume of foam will eventually break, with the bursting of the bubbles resulting in a return to the original amount of liquid, with the time it takes to revert to liquid being called “Persistence” which depends on the strength of the bubble wall. Foamed formulations which break quickly are described as metastable foams, while those that last longer are called persistent foams.
Many textile finishing agents can be foamed by incorporating a foaming agent in the formulation. The persistence of the foam can be controlled by adding water soluble macromolecules. The foam can be metered onto fabrics, and it represents another low-water application technique. Each type of foam can be metered onto fabric, although different types of applicators are required. Persistent foams can be applied with knife-coating equipment, while metastable foams require equipment where the foam is generated and applied at the same time.
In the case of persistent foam, utilizing a knife foam coater technique, the add-on is controlled by the gap setting and the density of the foam. After the foam has been metered, the fabric either passes between squeeze rolls, or passes over a vacuum slot, to break the bubbles and to allow the liquid to penetrate the fabric. Various commonly used foam applicators (one-sided and two-sided) are shown in Fig. 2.15. [101].
52
Figure 2.15 : Foam applicators[101]
Other low wet pick-up techniques include the loop transfer applicator, the engraved roll, the
Triatex MA (minimum application) system and the spray applicator. The principle of operations of these applicators are fully described elsewhere [107, 108].
2.8 FUNCTIONAL FINISHES
2.8.1 Water repellency
Water repellent finished fabrics are those which resist being wetted by water, the water drops rolling or driping off the fabric [109-111], the finish imparting hydrophobicity to the substrate.
The wetting of a fabric results from the contact of a liquid with the fabric surface under specific conditions [80], the resistance of the fabric to the liquid depending on the surface relationship between the liquid and the fabric. A fabric is repellent if the critical surface energy of the surface of the fabric is lower than the surface tension of the liquid. Fabric
53 properties which are often improved by repellent finishes include durable press, more drying and ironing, and resistance to acids, bases and other chemicals [110, 112].
2.8.1.1 Theory of wetting
Fig.2.16 : Spreading of liquid on a smooth surface [104].
The spreading of a liquid on a smooth surface is illustrated in Figure 2.16
Where:
L/V = the interface energy between liquid/vapour
S/L = the interface energy between solid/liquid
S/V = the interface energy between solid/vapour
Ө = equilibrium contact angle
The work of adhesion between the liquid and the solid, WA is given by the Duprie
Equation [104]:
WA= SN LN S / L (2)
A liquid drop on a smooth solid surface is subject to the equilibrium forces according to
Young’s Equation [80]:
SV SL LV Cos γ (3)
The relationship between the work of adhesion and the contact angle is derived by
combining the above two equations (the Young – Dupre Equation) as follows:
54
WA = LV (1 Cos ) (4)
The contact angle is a useful inverse measure of the ability of a solid to wet out. When Ө is greater than 90o, it is relatively difficult for the liquid to spread on the surface, and the substrate is generally considered as repellent to the liquid. When Ө is less than 90o, however, the liquid can spread on the surface, and the substrate is generally considered as non- repellent to the liquid.
Sometimes, the use of a single static contact angle to characterize the interaction is not adequate, as there exists a range of contact angles between a given solid and a liquid.
When the drop of liquid is being expanded, the angle represents the “advancing” contact angle, which is the maximum value. When the drop of liquid is being contracted, the angle represents the “receding” contact angle, which is the minimum value. If the liquid/solid/vapour boundary is in actual motion, the angles produced are known as Dynamic
Contact Angles, and are referred to as “advancing “ and “receding” contact angles, respectively.
Table 2.2: Surface free energy of different fibres [80].
SURFACE SOLID FREE ENERGY (mN/m) Polyamide (Nylon6.6) 46 Cotton 44 Polyester 43 PVC 39 Polyethylene 31 Polysiloxane, typical silicone oil 23-24 PTFE 18
The surface tension of water is 73 mN/m or dynes per cm [101], and therefore, water will not wet out any of the fibres listed in Table 2.2, unless a wetting agent or detergent is applied in
55 order to lower the surface tension of water to below the surface free energy of the fibre to be wetted out.
2.8.2 Water repellent finishes
There are a variety of water repellent finishes applied to textiles, including fibre reactive hydrocarbon hydrophobes, silicone water repellents and fluorochemical repellents.
2.8.2.1 Hydrocarbon Hydrophobes
The oldest, and the most economical way to impart a water repellent finish to a fabric is to coat it with paraffin wax. Solvent solutions, molten coatings and wax emulsions are different ways for applying wax to a fabric. Of these, wax emulsions are the most convenient products for finishing fabrics, applications being either by exhaustion or padding. The major disadvantage of a wax based water repellent is poor durability of the finish.
The durability of hydrocarbon based water repellents has been improved by incorporating reactive groups. The simplest of these is N-methylol stearamide (see Fig. 2.17).
Stearamide reacts with formaldehyde to form the N-methylol adduct, which is water dispersible and will either react on curing with cellulose or with crosslinking reagents included in the recipe.
`
Figure 2.17: N-Methylol stearamide
The multiple reactive sites on methylolmelamines can be utilized for synthesizing resin-forming water repellents. The reactivity of stearamide with formaldehyde can be utilized for attaching hydrophobic groups to the melamine molecule. Parts of the
N-methylol groups are used to attach the hydrophobe, some are used to add a cationic
56 site for emulsification purposes and some of the N-methylol groups are later involved in self condensation, to form a resinous coating on the fibre surface or to react with added durable press reagents.
2.8.2.2 Silicone water repellents
Methylhydrogendichlorosilanes offers a route to synthesizing a linear polysiloxane fluid with latent crosslinking potential. Hydrolysis of the dichloro groups will occur rapidly with water to form a linear polymer. When these emulsions are applied with a small amount of catalyst
(e.g. dibutyltin-dilaurate), to a fabric, the Si-H group hydrolyses to the silanol and condenses to a three-dimensional resinous polymer, making the fabric highly water repellent. The outward oriented methyl groups provide the water repellency [113]. A chemical formulation of the silicone repellent is shown in Fig.2.18
Figure 2.18: Silicone repellent
Silicone finishes are applied to fabrics, either from an organic solvent or from water as an emulsion, and are generally applied together with a durable press finish.
Schindler and Hauser enumerated some of the merits and demerits of silicone water repellents [101]. These include a high degree of water repellency at relatively low add on
(0.5-1% owf), very soft fabric handle, improved sewability and shape retention and improved appearance and handle of pile fabrics. Some modified silicone repellents can be applied by exhaustion, particularly to pressure-sensitive fabrics. Some of the disadvantages attributed to silicone repellent finishes include increased pilling in fabric and reduced repellency at excessively high concentrations. It is also claimed that a silicone finish may enhance the
57 attraction of hydrophobic dirt and that offers only moderate durability to laundering and dry cleaning. Moreover, it is found to be non-repellent to soil and oil.
2.8.2.3 Fluorocarbon repellents
Fluorochemical repellents are unique, as they confer repellency to fabrics against both water and low surface tension fluids. This property is important, because low surface tension liquids, such as blood and alcohol, often exist in the operating room. The ability of fluorochemicals to repel low surface tension liquids is attributed to their low surface energy
[115-117]. The fluorochemical finishes are organic fluorine-containing compounds, in which a majority of the hydrogen atoms are replaced by fluorine.
When these compounds are applied to fabric, followed by curing and drying, the fluorochemical tails orient themselves away from the fibres to produce a very low surface energy barrier, and the surface contact angle can be as high as 120o [118-119].
The degree of liquid repellency depends on the number of fluorines, the orientation of the fluorocarbon segment of the molecule, the add-on levels, and the distribution and coverage of fluorocarbon on the surface. A fluorocarbon based liquid repellent provides fibre surfaces with the lowest surface energies amongst all the repellent finishes in use, and both oil and water repellency can be achieved. The small spacer group can be modified to improve emulsification and solubility of the polymer [120]. General advantages include low active add-ons, less than 1% on weight of fabric, and more rapid drying of treated fabrics, while special fluorocarbons allow improved soil release during household laundering or stain resistance on nylon. Disadvantages include high cost, greying during laundering and potentially hazardous aerosols [121].
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2.8.3 Antistatic finishes Static is defined as the surface build-up of an electrical charge whenever two dissimilar surfaces contact one another. Cotton has very good antistatic properties and presents limited problems because the natural moisture content is high which provides sufficient conductivity to the fibres for dissipating accumulated charge. Some synthetic fibres, especially polyester, can sustain such a high charge density on the fibre surface that it can actually ionize the air in the vicinity, thus giving rise to a spark, which discharges the static built up. In most cases, this results in a mild shock to a person experiencing this static discharge, but where explosive gases might be present, it could even result in a disaster.
It has been observed that when two dissimilar materials are rubbed together, separation of charges occur, one material having a positive charge and the other a negative charge [101]. Hydrophilic materials, with antistatic properties, can decrease static charge accumulations by absorbing moisture from the air [122-126].
Increasing the conductivity of a textile fibre is the most suitable approach to increase the rate of static dissipation. The principal mechanism of antistatic finishes is to increase the conductivity of the fibre surface (lowering the surface resistivity) and to reduce frictional forces through lubrication. They form an intermediate layer, which is typically hygroscopic on the surface.
Nearly all the antistatic agents rely on water as the medium to transport the charged species and therefore their usefulness is dependent on atmospheric humidity. They are very effective in moist atmospheres but their effectiveness decreases as the relative humidity decreases. Common antistatic agents include hydrophilic surfactants, poly-electrolytes, long chain quaternary ammonium salts, and polyethoxylated polymers [127]. It is claimed that silver has the highest electrical conductivity of all metals, and its oxide is conductive as well as stable in air and water [128]. These properties make it suitable as an antistatic material and
59 therefore it has been widely used. [129-131]. Qiaozhen proposed that nano-silver particles not only increase the static dissipation paths, but also absorb some water on the surface, which tends to enhance the antistatic properties of the fabric [132-134]. Antistatic agents are applied to impart either durable or non-durable effects. In general, non-durable antistatic agents, like surfactants, belong to one of the following classes: Cationic, Anionic or Non- ionic.
2.8.3.1 Non-durable Antistatic Agents
These groups of antistatic agents are mainly used for mostly hygroscopic material or fibres and yarns. Commonly used antistatic agents are hygroscopic materials, including surfactants, organic salts, glycols, polyelectrolytes, polyethylene oxide compounds and esters of salts of alkylphosphonium acids.
Cationic Materials
a) The most common amongst this group are esters of phosphoric acid. The alkyl groups are derived from fatty acids and ethoxylated fatty alcohols are used to form the esters. The durability of these phosphoric acid esters increases with an increase in molecular size.
A typical example of a phosphoric ester antistat is shown in Fig. 2.19. Other cationic materials are ditallowdimethylammonium chloride and dehydrogenated tallowdimethylammonium chloride. Generally, cationic antistats have an affinity for textile fibres. They provide good lubricating properties, thus enabling them to reduce charge build- up and attract water molecules which promote charge dissipation..
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Figure 2.19: Phosphoric ester antistat
a) Non-ionic material
This group consists of ethoxilated fatty esters, alcohols and alkylamines as shown in Fig.
2.20.
Figure 2.20: Non-ionic antistats \
Observation has shown that a blend of cationic and non-ionic materials hass superior antistatic properties than when used individually. The non-ionic agent improves the water absorption while the cationic component provides electrical conductivity to the surface.
2.8.3.2 Durable Antistat agents
Polyamine and polyoxyethylene form the basis for most of the durable antistatic finishes.
The basic principle is to form a crosslinked polymer network containing hydrophilic groups.
They can be formed prior to the application to fabrics, or in-situ on the fibre surface after padding. The use of durable antistat finishes is, however, limited owing to the difficulties in achieving the perfect balance in the desired properties [80]. For example, as the number of hygroscopic units increases, moisture absorption increases and, as a result, the antistatic performance increases.
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One of the advances in antistatic agents is the series of Permolose finishes, developed by the Imperial Chemical Industries (I.C.I), which contain a block of polymers of ethylene oxide and a polyester [82]. The principle is based on treating polyester with Permolose. The polyester block of the copolymer is adsorbed by the polyester fibre, wheras the polyethylene oxide portion is incompatible with the polyester fibre, and thus remains on the fibre surface, where it attracts water and forms a conductive surface on the fibre.
Tien-Wei-Shyr et al [135] observed that a polyester fabric, treated first with nano-silver antistat followed by fluorine water-repellent, exhibited a noteworthy combination of antistatic and water-repellent properties [136]. Other wash-fast antistat agents have been described elsewhere in the literature [137].
2.8.4 Flame Retardant Finishes
The need for textile materials with reduced tendency to ignite and burn has been recognized for a long time. Whilst asbestos, as a flame resistant material, has been used in Roman times, perhaps one of the earliest significant contributions in recent history was Wyld’s patent of
1735 [138], in which he described a finishing treatment, based on aluminium, ferrous sulphate and borax, for cellulosic textiles. Almost a century later, in 1882, Gay-Lussac [139] published perhaps the first systematic study of the use of flame retardants. These ideas laid the foundation for the early theories of flame retardancy of textiles. Recent reviews have, however, highlighted considerable advances in the understanding and chemistry of flame retardants [101].
A fabric is considered to be flame retardant if it does not ignite and create a self-sustaining flame when subjected to a heat source [101]. Generally, the flame retardants help to inhibit or suppress the combustion process.
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2.8.4.1 Mechanism of Flame Retardancy
When solid materials are heated, physical and chemical changes occur at specific temperatures, depending on the chemical properties of the solid. Thermoplastic polymers soften at the glass transition temperature (Tg), and subsequently melt at the melting temperature (Tm). At some higher temperatures, above Tm, both thermoplastic and non- thermoplastic solids will chemically decompose (pyrolyze) into lower molecular weight fragments. Chemical changes begin at the pyrolysis temperature (Tp), and continue until the temperature at which combustion occurs (Tc). These four temperatures are crucial when considering the flame resistance of textile materials. Another important parameter in combustion is the Limiting Oxygen Index (LOI) [140-141]. This denotes the amount of oxygen in the fuel mix needed to support combustion, and it is used to measure the ease of extinction of a sample [142]. The higher the number, the more difficult it is for combustion to occur. A summary of the flamabiliy parameters of different fibres is given in Table 2.3
[80].
Table 2.3: Flamability parameters of various fibres [80]
Tg, oC Tm, oC Tp, oC Tc, oC Fibre Softens Melts pyrolysis combustion LOI, % Thermoplastic Wool NA NA 245 600 25 No Cotton NA NA 350 350 18.4 No Viscose NA NA 350 420 18.9 No Nylon 6 50 215 431 450 20-21.5 Yes Nylon 6.6 50 265 403 530 20-20,1 Yes Polyester 85 255 420-427 480 20-21 Yes Acrylic 100 220 290 250 18.2 Yes Polypropylene 20 165 469 550 18.6 Yes Modacrylic 80 240 273 690 29-30 Yes PTFE 126 327 400 560 95 Yes Nomex 275 375 410 500 28.5-30 Yes Kevlar 340 590 550 29 Yes PBI 400 500 500 40-42 No
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For non-thermoplastic fibres, Tp and/or Tc is lower than Tg and/or Tm. Natural fibres are not thermoplastic, therefore when subjected to a heat source, pyrolysis and combustion temperatures are encountered before the softening or melting temperatures are reached. If the molten fibre does not shrink away from the flame front, the pyrolysis and combustion temperatures are eventually reached and ignition will occur.
2.8.4.2 Pyrolysis of cellulose
When cellulose fibres are heated, the following three classes of volatile chemicals are generated at pyrolysis temperature [143-145]:
a) Flammable volatiles, e.g. alcohols, aldehydes and alkanes.
b) Flammable gases, e.g. carbon monoxide, ethylene and methane.
c) Non-flamable gases, e.g. carbon dioxide and water vapour.
These volatile products and levoglucosan can be considered as the “fuel”, when mixed with oxygen, propagating the combustion process, otherwise ignition would not occur or sustain.
The main functions of flame retardants are to inhibit and disrupt the combustion process at various stages, should a flame develop. Attempts to disrupt the combustion process can be achieved by shielding the polymer, by forming a char layer or inert gas layer, or by undergoing a reaction which cools the system. Char is a glassy protective film, which acts as a surface barrier to oxidation, and shields the base material from burning. Details of these processes are described elsewhere [145].
2.8.4.3 Flame Retardants for cellulosics
Flame retardant finishes for specific textiles are classified as non-durable, semi-durable and durable, under specified conditions [146]. A wide variety of flame retardants suitable for
64 cellulosic fabrics are available, but many of these do not retain their properties after laundering and dry cleaning, and are therefore non-durable.
1. Durable flame retardants
Durable flame retardants for cellulosic material include precondensate ammonia cure and N- methylol functional phosphorus esters [147-148]. Other versions include tetrakis
(hydroxymethyl)–phosphonium chloride (THPC) cross-linking used over the years, with the precondensate ammonia process being the most recent [143].
Fryol 76, an oligormeric phosphonate containing vinyl groups, was developed by Eisenberg and Weil [153]. The finish is applied with N-methylol acrylamide and a free radical catalyst
(potassium persulphate) by a conventional pad-dry-cure-wash procedure, producing little odour in the plant. A summary of flame retardant treatments for cotton is given in Table 2.4
[146].
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Table 2.4: Summary of flame retardant treatments for cotton[146].
2. Non-durable flame retardants
Non-durable flame retardants for cellulosics are generally water-soluble inorganic salts which provide only temporary protection, and periodic processing is required to sustain the effect. These groups include boric acid, boron derivatives, borax, and ammonium bromide
[149]. Sometimes, diammonium phosphate and phosphoric acid- (NH4)2HPO4 (DAP) are used in mixtures with ammonium sulfate and hexamethylene tetramine as a buffer. An important feature of DAP is its effectiveness in suppressing glowing [149].
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3. Semi-durable flame retardants
Unlike non-durable flame retardants, cellulosic materials, treated with semi-durable flame retardants, should withstand a limited number of washes. These include ammonium polyphosphate, and usually insoluble salts of amphoteric cations and anions-stannates, tungstates, aluminates, borates and phosphates of Zn, Al and easily reducible metallic oxides of Fe, Pb, Ti, Cr, Ce, Bi and As [146]. Direct application of insoluble salts has its limitations,
,and best results occur by internal precipitation, following application of a reagent solution
[150-153].
It is claimed that an oligomeric vinyl phosphate finish Fryol 76, applied with a conventional pad-dry-cure, gives a soft fabric handle and also uniform wrinkle recovery [154-155]. It has been established that Fyrol 76, with bromine derivatives, also finds application for fabrics made from cotton/polyester blends [156-157].
2.8.4.4 Flame retardants for Viscose Rayon fibres
Whilst the flame retardants for cotton are also suitable for rayon, additives, based on alkyl dioxaphosphorinane disulfide, have been incorporated in the viscose spinning bath for effective flame retardancy [158]. Other flame retardants incorporated into rayon fibres, during the fibre extrusion process, include decabromodiphenyl oxide (DBDPO), antimony oxide and phosphazines. Because of the high water solubility of phosphazines, it does not allow exhaustion from a dyebath, and heat treatment causes the fabric to stiffen, the finish is, however, durable to repeated laundering.
2.8.4.5 Flame retardants for Nylon fibres
Most nylon fabrics pass flammability standards, because the polymer burns at a slow rate.
Nevertheless, several finishes will enhance its flame retardancy, only additives to the
67 polymer melts having been commercialized. Phosphorus and bromine-containing compounds are the most common melt additives, while halogenated compounds, such as decabromodiphenyl oxide (DBDPO) and chlorinated paraffin combined with antimony trioxide (Sb2O3) , are also effective [159].
2.8.4.6 Flame retardants for Polyester fibres
Applications of flame retardants to polyester fibres can be achieved, either as additives to the polymer melts or as flame retardant copolymers [158]. Among other suitable flame retardants which have found commercial success, is a halogenated compound-decabromodiphenyl oxide (DBDPO). It is sold as a dispersion in water and is applied with a suitable binder to impart effective and durable flame retardancy to 100% polyester fabrics. Disadvantages associated with the product are the high add-ons needed for optimum fire retardancy, as well as the associated stiff and boardy handle, due to the level of binder needed to achieve durability [80]. The chemistry of DBDPO is shown in Fig. 2.21.
Figure 2.21: DBDPO flame retardant
Another commercial flame retardant, suitable for polyester, is a mixture of cyclic phosphate/phosphonates, used in a pad-dry-heat set process. It is claimed that this product can provide durable flame retardancy to a wide variety of polyester fibres, when applied at a
3-4% add-on [160-161]. This product is clear, water soluble and durable to repeated laundering.
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2.8.4.7 Flame retardants for wool fibres
Although, wool is an inherently flame retardant fibre, and will pass most horizontal burning tests, some flame retardant treatments are still required in order to fulfil specific flammability criteria. It has a high ignition temperature (570-6000C) and low heat of combustion
(4.9kcal/gm) [162]. The flame retardant may char the fuel, quench the combustion reaction, absorb heat or emit cooling gases or replace oxygen. Phosphorous containing flame retardants, halogenated products and metal complexes (Ziro process) are useful flame retardants for high performance wool apparel application [163-165].
2.8.4.8 Flame retardants for polyester/cotton fibres
For cotton/polyester blends, providing a suitable flame retardant is still a complex problem because of the widely different properties of the two fibres. Whereas polyester fibres normally melt and shrink away from the flame, cotton fibres are held in place by the charred backbone. In the past, many attempts have been made to provide a suitable flame retardant for a cotton/polyester blend, but this has been met with limited success due to problem associated with toxity or difficulty in maintaining desirable aesthetic and performance properties [166-168].
Flame retardants, which are active in both the condensed phase and vapour state, have been found to be very efficient for polyester/cotton blends. The performance of various phosphorous, nitrogen and antimony/halogen based flame retardants for polyester/cotton blends has been reported by various authors [169-170].
2.8.4.9 Flame retardants for acrylic fibres
Acrylic fibres, like other synthetic fibres, shrink when heated, and, once ignited, they burn vigorously, accompanied by black smoke [171-175]. Many efforts have been made to improve the flame resistance of acrylic fibres, and halogen-based bromine derivatives or
69 halogen-or phosphorous-containing co-monomers have been found to be suitable [176-178].
Self-extinguishing modacrylic fibres have been produced from vinylidene chloride and acrylonitrile copolymers or terpolymers [179-180].
A number of spinning dope additives, such as esters of antimony, tin and their oxides,
SiO2, halogenated paraffins, phosphorous and bromine compounds, are known to render flame retardancy to acrylic fibres [181-184].
2.8.5 Antibacterial Finishes It has been reported that, in about 50% of all surgical procedures performed, at least one medical worker was contaminated with blood borne pathogens [185]. Any blood-borne contamination could pose a risk of transmission of bacteria and other harmful pathogens, such as HIV and hepatitis C [186-190]. Uniforms, such as surgical gowns and laboratory coats, are often used as barriers to help eliminate or reduce the risk of infection to the health care workers, the patient and hospital visitors. Studies have shown, however, that bacteria could penetrate through such uniforms [191-192]. The uniforms of the healthcare workers, could therefore, also become potential sources of contamination, primarily as transmission vehicles. A possible solution, to reduce these types of contamination, is to apply an antibacterial finish to the fabrics used in making such uniforms so as to provide better protection to the healthcare workers and patients. It is, therefore, in the best interest of the healthcare workers to use gowns which can offer optimal protection. Clinical studies have shown that nonwoven fabrics are superior to woven fabrics in controlling post operative infections because of their outstanding bacterial filteration efficiency, due to their pore dimensions and distribution, as well as their good breathability compared to reusable woven fabrics [193-194].
These and other studies have enabled nonwovens to gain widespread acceptance in the healthcare industry [195-196]. It is also so, that single-use surgical gowns and drapes are
70 most commonly made from nonwoven fabrics or in combination with other materials, which can offer increased protection against liquid penetration [197].
Bacteria
Bacteria are single cell or unicellular organisms which appear in three forms: spherical
(cocci), rod (bacilli) and spiral (spirillum), and may be found in pairs, clusters or chains, and contain a cytoplasmic membrane and a rigid layer [198-199]. The cytoplasmic membrane is the internal structure which consists of a nucleoid and a ribosome. The nucleoid is the DNA of the cell and the ribosome transfers genetic messages to proteins. The rigid layer, also called the surface layer, consists of the capsule, cell wall and plasma membrane. The capsule protects the cell wall and maintains the overall shape of the cell. The plasma membrane is used to transport ions, nutrients and waste. Some forms of bacteria have appendages which consist of a pilus and flagellum. While the pilus allows the bacteria to attach to other cells, the flagellum provides the motility for the cell.
Types of bacteria
Bacteria can be identified as either gram-positive or gram-negative, which can be distinguished by the content and structure of their cell wall, using a staining procedure, called gram stain [199]. Purple stain confirms gram-positive bacteria, while no stain indicates gram-negative bacteria.
The gram-positive bacteria contains peptidoglycan and teichoic acids. Peptidoglycan comprises 90% of the cell wall, and is made up of amino acids and sugar [199]. Teichoic acids are responsible for the antigenic determinant of the organism. One example of gram- positive bacteria is Staphylococcus aureus, which appears in pairs, short chains, or grape- like clusters [200]. It ranges in size from 0.5µm to 0.1µm, and the temperature conducive for growth ranges from 35-40 oC. It is claimed that Staphylococcus aureus is the major cause of
71 cross-infection in hospitals, and accounts for up to 19% of total surgical infections [201].
Staphylococcus aureus can also cause boils, skin infections, pneumonia and meningitis, especially in a debilitated person [202]. It is also responsible for scaled skin and toxic shock syndromes. Other examples of gram-positive bacteria, are staphylococcus epidermis,
Corynebacterium and streptococcus faecalis.
The gram-negative bacteria are similar to gram-positive bacteria; the major difference being that the gram-negative bacteria possess an additional layer of outer membranes, attached to their peptidoglycan layer by lipoproteins [203]. A typical example of a gram-negative bacteria is klebsiella pneumoniae. It has a bacillus shape, and appears as a single pair or short chains [204]. It is transmitted through the lungs, mouth and throat, but mostly through the hands of hospital personnel. The symptoms of klebsiella pneumoniae infection include fever, difficulty in breathing, chest pain and a bloody stool [204]. Another example of gram-negative bacteria is Escherichia coli. It is also shaped like bacillus and lives in the intestine of humans. The symptoms of Escherichia coli infections are severe diarrhoea especially in children, and kidney damage [205]. Research has shown that gram-negative bacteria are harder to contain than gram-positive bacteria, because of the extra cell wall on gram-negative bacteria [206]. Murray et al [207] also confirmed that gram-negative bacteria
(eg E. Coli and Pseudomonas aeruginosa) were harder to contain than gram-positive bacterium ( eg. S. aureus) .
Spread of bacteria
Bacteria cannot move from one location to another by themselves but are usually transported by a carrier, such as blood, perspiration, alcohol, shed skin or dust [208]. The carrier in turn can be transported by either liquid or air, liquid transport providing a wet or moist transfer, whereas air transport provides a dry transport, which occurs in the air, usually through vents.
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A) Liquid transport: If the proper barrier is not used, various infections, such as from S. aureus and E. coli, can be transported from patients to healthcare workers, through liquid. It has been observed that actual liquid transfer can occur through external forces acting against healthcare workers, particularly by their processing or leaning motion against an object, such as an operating or examining room table [209].
B) Air transport: Air transport of bacteria can occur through the movement of dust laden air particles and shed skin. It has been reported that humans can shed an average of 1000 bacteria per minute, which can be a major source of contamination in an operating room
[210]. Bacteria in the air can settle on surgical instruments, which can indirectly contaminate a patient’s wound. The amount of bacteria falling from the air is dependent on the length of surgery, the number of air changes through the air vents, the turbulence in the air, the number of times the door of the operation room is opened, the heat convection from surgical lamps, and the number of people present in the room during surgery [211].
It is claimed that the most common type of bacteria that is shed in surgery is S. aureus [212].
The skin cells that can affect patients, range in size from 5 to 60 mm, their small size enabling bacteria to fall, or pass, between the pores of cotton muslin surgical fabrics [213,
214].
2.8.5.1 Antibacterial finishing requirements
An antibacterial finish is applied to reduce the spread of microorganisms, by either killing them or inhibiting their growth when in contact with the fibre surface [215].
In order to obtain the greatest benefit, an ideal antimicrobial treatment for textiles must satisfy a number of requirements [216-217]:
a) The antimicrobial treatment should be effective against a broad spectrum of bacterial and
fungal species, but at the same time it should not cause toxicity, allergy or irritation to
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the user. Antimicrobial-treated textiles need to comply with standards, in terms of
compatibility tests (cytotoxicity, irritation and sensitization) before marketing.
b) The finish should be durable to laundering, dry cleaning and hot pressing. This seems to
be the greatest challenge, when non-disposable textile products are subjected to repeated
washing during their life.
c) The antimicrobial finish should not negatively affect the quality (e.g. physical strength
and handle) or appearance of the textile.
d) The finishing should preferably be compatible with textile chemical processes, such as
dyeing, be cost effective and not contaminate the workers and the environment with
harmful substances.
e) The antimicrobial finish should not kill the resident flora of non-pathogenic bacteria on
the skin of the wearer. The skin of the wearer consists of several bacterial genera, which
are important to the health of the skin, as they lower its surface pH and produce
antibiotics to create an unfavourable environment for the growth of pathogenic bacteria
[218].
2.8.5.2 Mechanisms of antimicrobial activity
Antimicrobial agents either inhibit the growth (-static) or kill (-cidal) the microorganism.
There are two types of antibactrrial treatments for textile materials, namely, leaching type and non-leaching (bond) type [217]. The leaching type releases its antimicrobial agent in the environment to proactively attack microorganisms, while the non-leaching (bond) type contacts the organism and kills the microbes by disrupting their protective cell walls.
Certain leaching type antibacterial agents have been found to harm the environment or humans and were therefore removed from the market. These include copper naphthelate, copper-8-quinolinate and organo mercury compounds [217].
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The non-leaching type antimicrobial agents show good durability and may not cause any health problems. For example, in the ion exchange process, the release of the active substance is at a slower rate in comparison to that in direct diffusion. These include polyhexamethylene biguanide (PHMM), BiosilTM, regenerable antimicrobial agents and chitosan.
2.8.5.3 Antimicrobial finishing methods
Various methods, depending on the particular active agent and fibre type, have been developed, or are under development, to confer antimicrobial activity to textiles [219].
For synthetic fibres, the antimicrobial active agents can be incorporated in the polymer prior to extrusion or blended into the fibres during processing. Such processing provides the best durability, while the active agent is embedded in the structure of the fibre, and is released slowly during use. This method of fabrication has been adopted by some manufacturers, such as the silver-containing bioactive polyester fibres developed by Trevira, and the triclosan- containing cellulose acetate fibres manufactured by Novaceta. The conventional exhaust and pad-dry-cure processes are primarily used to apply antibacterial agents to textiles [220]. The process of incorporating the antimicrobial active agents in the polymer prior to extrusion or blended into the fibre during processing is popular for antibacterial agents such as triclosan and PHMB [221-222]. Padding, spraying and home finishing have been used for the silicone- based quaternary agent AEM 5700 [223]. Many other methods have been reported, such as the use of nanosized colloidal solution [224], nanoscale shell-core particles [225-226], chemical modification of the biocide, for covalent bond formation with the fibre [227-228], cross-linking of the active agent on the fibre using a cross-linker [229-230], and graft polymerization [231].
The sol-gel process is an emerging process for antimicrobial finishing, having been extensively employed for coating applications [232]. It is claimed that the sol-gel technology
75 can be applied to coat textiles with almost unlimited functionality, by incorporating functional agents into the sol-gel nanoparticles [233-234]. With regard to antimicrobial ability, several biocides have been encapsulated in so-gel particles, which are then coated onto textile products to provide the desired functionality [235].
2.8.5.4 Antimicrobial agents and their effect on textiles
Three major classes of antimicrobial agents are used to control, or inhibit, bacteria, namely controlled-release, regeneration and barrier-block [236]:
2.8.5.4.1 Controlled-release mechanism
The controlled–release mechanism is the most commonly used for the antibacterial agents.
The agent is released gradually in enough quantities to kill or inhibit the growth of bacteria.
Examples are gentamincin and triclosan. Gentamincin is an aminoglycoside antibiotic complex, with each component consisting of five nitrogens per mole of gentamincin base, as shown in Fig. 2.22.
Figure 2.22 : Structure of gentamincin
Gentamincin is widely used in hospitals, as an antibiotic, and it acts by inhibiting bacteria protein synthesis. Research on a dual functional finish, using gentamincin as an antibacterial agent, and a fluorochemical, as a blood repellency agent, on surgical gown fabric, showed a 98% reduction in gram-positive (S. aureus) and gram-negative (K. pneumoniae) bacteria for both 100% cotton and 55/45% woodpulp/polyester spun-laced nonwoven fabrics [237].
Triclosan is a cationic biocide and a diphenyl ether derivative, known as
76
5-chloro-2 (2,4-dichlorophenol) phenol [238]. Triclosan is a broad-spectrum antimicrobial agent, with a minimal inhibitory concentration (MIC) of less than 10ppm against many common bacteria species. It has been used in a wide array of professional and consumer products, including hand soaps, surgical soaps, toothpastes and detergents [239-240]. The structure of Triclosan is shown in Fig. 2.23
Figure 2.23 : The structure of Triclosan
During use, the antimicrobial agent migrates to the surface of the treated fabric, at a slow but sustained rate, to provide antimicrobial efficacy [241]. Triclosan interrupts the cytoplasmic membrane of the bacteria, which in turn interferes with the metabolic function of the cell. It has bacteriostatic activity against a wide range of both gram-positive and gram- negative bacteria. Some of the negative features of Triclosan are that it may cause cancer in humans and creates skin irritations [242]. Owing to such health and environmental issues, a number of leading retailers as well as governments, have banned the “unnecessary use” of
Triclosan in textiles and some other products.
2.8.5.4.2 Rechargable mechanism:
With the rechargeable mechanism an antibacterial chemical finish is applied to the fabric and is continually replenished by bleaching agents during laundering. An example is
Monomethylol-5, 5-dimethyl hydrantoin (MDMH), the structure of which is shown in Fig.
2.24.
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Figure 2.24 : The structure of MDMH
MDMH is a hydantoin derivative and is known to provide durable and regenerable antibacterial activity to cellulosic fabrics. MDMH, as antibacterial agent, covalently bonds with the fabric and, when laundered with a chlorine bleach, its antibacterial activity on the fabric is recharged [243]. The application of MDMH on 65/35% cotton/polyester, as well as on 100% cotton fabrics, exhibited a significant inhibition of both the gram-positive (S. aureus) and gram-negative (E. coli) bacteria as well as maintaining the tensile strength of both fabrics after 20 repeated washing cycles [244].
2.8.5.4.3 Barrier-block mechanism: The barrier-block mechanism inhibits bacteria through direct surface contact, the antibacterial agent bonding covalently to the fabric to form a strong and durable bond [245].
Agents, utilizing barrier-block mechanism, do not leach and present fewer problems, such as skin irritation, in comparison to the agents acting according to the controlled-release mechanism [246], they also require lower add-ons (i.e. less antibacterial agent) on the fabric
[245]. Examples of agents, with the barrier-block mechanism, include polyhexamethylene biguanide (PHMB), chitosan and AEGIS Microbe Shield (AMS).
PHMB is a hetro disperse mixture of polyhexamethylene biguanides, having an average molecular weight of approximately 2500 Da [215]. Being a potent and broad spectrum anti-bacterial agent, with low toxicity, it has been successfully used as a disinfectant in the food industry, and in the sanitization of swimming pools [248,249].
PHMB impairs the integrity of the cell membrane in its action, and its activity increases with increasing molecular weight. [247]. Resistance of bacteria to PHMB has rarely been
78 observed, particularly in comparison with bisbiguanide chlorhexidine, the activity of which has, to some extent, been resisted by bacteria. [250,251]. The structure of PHMB is shown in
Fig. 2.25.
Figure 2.25 :The structure of PHMB
Payne [252] patented a treatment of cellulosic fibres with PHMB, in which an after- treatment, with a strong organic acid, was used to increase the durability as well as to overcome fabric yellowing [252]. Payne and Yates [253] later extended the PHMB treatment to synthetic fibres, using a self crosslinkable resin and a catalyst. PHMB can also be directly exhausted onto cotton at room temperature and at a neutral pH, or applied in a pad-dry-cure process [254]. The carboxyl groups on cotton fabrics, which have originated from chemical finishing, are involved in some of these interactions [255-256]. It is recommended that
PHMB be applied at a level of 2-4% on weight of fabric (owf), for durable finishing, and at
0.25-1% owf, for disposable items [255].
A special grade of PHMB, with higher molecular weight and containing an average of 16 bioguanide activity units (Reputex), has been developed recently. The long polymer chain, not only results in higher biocidal activity, but also provides more cationic sites per molecule for possible stronger binding to the textile surface. Reputex is initially applied to cotton, or its blends, using either exhaust or pad-dry-cure processes, and more recently to polyester and nylon, under the trade name Purista [257].
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CHITOSAN is a derivative of chitin, which is the second most abundant natural polymer. Its structure is similar to that of cellulose, except one of the hydroxyl groups is replaced by an amino group, as shown in Fig. 2.26 [258].
Fig. 2.26 : The structure of chitosan
In general, it has been observed that the bacterial reduction rate increased with an increase in the molecular weight of chitosan, although the reduction was dependent on the particular bacteria strain [259-261]. The antimicrobial mechanism consists of the interactions between positively charged primary amine groups and the negatively charged residues on the surface of the microbes. Such interaction causes extensive changes in the cell surface and cell permeability, which leads to a leakage of the intracellular substance [259].
This antimicrobial activity of chitosan, coupled with its non-toxicity, biodegradability and biocompatibility, facilitates emerging applications of chitosan in food science, agriculture, medicine, pharmaceuticals and textiles [262]. The focus, for applying chitosan as an antimicrobial treatment, has remained mainly on cotton. Whereas early work indicated that the antimicrobial effect was potent against a range of microbes, the finishing was not durable [259]. To improve durability, chitosan has been crosslinked to cotton, using varying chemicals, such as dimethylol-dihydroxyethylene-urea (DMDHEU), citric acid, 1,2,3,4- butanetetracarboxylic acid ( BTCA) or glutaric dialdehyde [263-266].
Studies have shown that treated cotton fabric exhibited a higher reduction (97%) in the number of colonies of S. aureus bacteria in 100% cotton, compared to that on a 55/45% woodpulp/polyester spunlaced nonwoven fabric. Problems associated with chitosan treated fabric include poor handle and washing durability. To overcome these problems, the Swiss
80 company (Swiscofil) has now manufactured a composite fibre of chitosan and viscose
(Crabyon), which has durable antimicrobial efficacy, and is suitable for a range of textile products.
AEM 5700 produced by AEGIS Environments, is a commercial antimicrobial textile product containing quarternary ammonium compounds (QAC) as the active agent. The active substance, 3-(trimethoxysilyl) propyloctadecyl ammonium chloride (AEM 5700), formerly known as the Dow Corning 5700 Antimicrobial Agent, has a minimum inhibitory concentration (MIC) of 10-100mg/l against gram-positive and gram-negative bacteria [251].
The structure of AEM 5700 is illustrated in Fig. 2.27.
Figure 2.27 : The structure of AEM 5700
An acqueous solution of AEM 5700 is applied by padding, spraying or foam finishing. Upon drying, the non-volatile silane forms covalent bonds with the textile material, thus resulting in excellent wash durability [268-269]. It is claimed that the chemical has been commercially used on cotton, polyester and nylon fabrics [251].
Table 2.5 lists some commercially available biocides, and those under development, for the treatment of various fibres. In the application method column, “F” denotes that the particular biocide is used as finishing agent, which “I” denotes that the biocide is incorporated into the fibre during extrusion [219].
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Table 2.5: Some commercially available biocides [219]
Biocide Fibre Application Commercial Comments Method products? Silver Polyester F/I Yes Slow release, durable but silver can be depleted Nylon I Yes Wool F Yes Regenerated F Yes Cellulose QACs Cotton F Yes Covalent bonding, very (e.g.AEM- Polyester FYesdurable, possible bacteria 5700) Nylon F Yes resistance Wool F No PHMB Cotton F Yes Large amount needed, Polyester F Yes potential bacteria resistance. Nylon F Yes Triclosan Polyester F/I Yes Large amount needed, Nylon F/I Yes bacteria resistance, breaks Polypropylene I Yes down into toxic dioxin, Cellulose I Yes banned in some European acetate countries. Acrylic I Yes Chitosan Cotton F No Adverse effect on handle, Polyester F No low durability. Wool F No N-halamine Cotton F No Needs regeneration, odour Polyester F No from residual chlorine Nylon F No Wool F No Peroxiacids Cotton F No Needs regeneration, poor durability Polyester F No
2.8.6 Breathable barrier protection against fluid
The desired barrier to fluids can be achieved with the help of various polymers and treatments, such as by applying polyurethane, polyvinyl chloride (PVC), polytetrafluoroethane (PTFE) membranes, acrylics, silicones, or fluorocarbons to the fabric.
Fluorocarbons and silicones are water repellents [270]. Polyurethane offers many advantages and characteristics over other polymers, which makes it particularly suitable for applications
82 in surgical gowns. PVC lacks in certain areas, such as flexural properties, plasticizer migration, abrasion resistance, low temperature properties, and cleanability, while polyurethane, on the other hand, offers good chemical resistance, ultra violet resistance and hydrolysis resistance, excellent low temperature flexibility, and excellent flexural fatigue properties. In addition to the above, thermoplastic polyurethane (TPU) offers excellent drape properties, softness and suppleness of handle. TPU is also known to have much better moisture vapour transmission properties than PVC. The high tensile strength and elongation of TPU, combined with its superior resistance to tearing and cutting, provide exceptional toughness which makes it suitable for application in surgical gowns [271]. Also, it is a temperature-sensitive kind of polyurethane, water-vapour transport properties of which may undergo a significant increase as the body temperature rises. Particularly, by means of appropriate macromolecule design, the transition point in the water vapour transport property of temperature-sensitive polyurethane can be controlled within the range of normal room temperatures [272], although this has yet to be explored in terms of its feasibility for application in surgical gowns. PTFE-based films, with a microporous structure, are also used for surgical gowns. Gore-Tex is the most widely known manufacturer of PTFE-based membranes, the microporous structure being produced by mechanical fibrillation. The polymer film is stretched in both orthogonal directions and annealed to impart microporous rips and tears throughout the membrane to produce the pore structure.
2.8.7 Evaluation of antimicrobial efficacy
Many studies have been conductedin the laboratory, and in the operating room, to assess the efficacy of surgical gowns as barriers to microbial transmission [273-277]. These methods generally fall into two categories; the agar diffusion test and suspension test. The bacterial species Staphylococcus aureus (gram-positive) and Klebsiella pneumoniae (gram-negative) are recommended in most test methods [275]. These two species are potentially pathogenic
83 and therefore proper physical containment facilities for handling (e.g. a biosafety cabinet) are necessary. Many studies have used the innocuous Escherichia coli (gram-negative) as a test microorganism which can be cultured and handled in a standard laboratory with minimal health risk.
2.8.7.1 Agar diffusion test (Agar zone inhibition)
This test is carried out according to AATCC 147-2004 (American Association of Textile
Chemists and Colorists), JIS L 1902-2002 (Japan Industrial Standards) and SN 195920-1992
(Swiss Norm) standards [219]. The agar diffusion test is a qualitative method for quickly assessing antibacterial activity of treated textile materials against gram-positive and gram- negative bacteria. Treated material is placed in nutrient agar inoculated with test bacteria.
After inoculation, antibacterial activity is usually determined by observing inhibition zones on and around the textile material. If the antimicrobial agent can diffuse into the agar, a zone of inhibition becomes apparent and its size provides some indication of the potency of the antimicrobial activity and the release rate of the active agent. This test is, however, limited to assessment of bacteriostatic activity (inhibition of multiplication).
2.8.7.2 Suspension tests
The suspension test is carried out according to AATCC 100-2004, JIS L 1902-2002 and SN
195924-1992 standards, and provides quantitative values on the efficacy of the antimicrobial finishing, but is more time-consuming than the agar diffusion test. Typically, in this test, a small volume (e.g.1ml) of bacterial inoculums, in a growth media, is fully absorbed into the fabric samples of appropriate size, without leaving any free liquid. This ensures intimate contact between the fabric and the bacteria. Antimicrobial activity, expressed as a percentage of reduction, is calculated by comparing the size of the initial population with that after the incubation.
84
The most important antibacterial test methods, together with their main features and efficacy, are listed in Table 2.6 [219]. The results obtained from these methods depend strongly on the antibacterial additive mechanism and on the hydrophobic or hydrophilic nature of the bioactive fibres. In each analysis, the measurement of the activity of a reference sample similar in nature to the antibacterial fibre but without treatment, must be carried out.
When antimicrobial agents are used under strict application guidelines, the agents have proven effective in controlling bacterial and fungal infections in clinical settings, such as hospitals and other healthcare facilities which are susceptible to high risk of infection.
Table 2.6: Antibacterial test methods [219]. METHOD ORIGIN EVALUATION SUITABILITY
AATCC 100/2004 USA Quantitative Textiles treated with antibacterial finishes AATCC 147/2004 USA Qualitative Textiles treated with fast migrating/leaching rate agents SN 195924/1992 SWITZERLAND Quantitative Hydrophilic fibres SN 195920/1992 SWITZERLAND Qualitative Textiles treated with fast migrating/leaching rate agents AFNOR XP G FRANCE Quantitative Textiles treated with migrating 39010: 99 agents SHAKE FLASK JAPAN Quantitative Textiles with antibacterial TEST USA properties inherent to the structure. hydrophilic/hydrophobic fibres JIS L 1902/2002 JAPAN Quantitative Textiles treated with and fast migrating/leaching rate Qualitative agents.
2.9 CHARACTERISATION OF NONWOVEN FABRICS
The characterization of fabric structure, by means of quantifiable parameters, is essential for the control of the manufacturing process and the resulting properties of the nonwoven fabrics.
85
The physical properties of the nonwoven fabric depend on the nature of the filaments or the fibres, i.e. chemical composition, morphological properties, and on the fabric structure [278].
The fabric structure, in turn, depends on the web manufacturing process, i.e. spun, carded, air or water–laid, and web consolidation method, such as polymer latex, thermofusing or needling [279]. The most important parameters in terms of the fabric structure, are porosity, mean pore size, pore size distribution, specific area of any texture, and fibre Orientation
Distribution Function (ODF). Other important properties include fabric weight, thickness, density, uniformity and tensile properties, as well as water vapour transport and breathability, liquid absorbency and barrier properties.
2.9.1 Fabric Weight, Thickness and Density
Nonwoven fabric weight (or fabric mass) is defined as mass per unit area of the fabric, usually expressed in g/m2 (gsm). Fabric thickness, on the other hand, is the distance between the top and bottom fabric surfaces under a specified applied pressure, which may vary for high-loft (or compressible) fabrics. The fabric weight and thickness determine the fabric density, which has an influence on the freedom of movement of the fibres, and also on the porosity (the proportion of voids) of the nonwoven fabric. The fabric density, or bulk density, is the weight/unit volume of the nonwoven fabric, expressed in kg/m3 which can be calculated by dividing the fabric weight per unit area (g/m2) by the fabric thickness (mm).
The fabric density and the fabric porosity are important parameters, in that they influence the ease with which fluids, heat and sound can pass through the fabric.
2.9.2 Uniformity of Fabric Weight
The fabric weight and thickness usually vary in different locations along the machine- direction (MD) and cross-machine direction (CD) of a nonwoven fabric, the frequency of such variations is periodic, with a recurring wavelength, due to the mechanics of the web
86 formation and/or bonding process. Persistent variation in the fabric weight in the cross- machine direction is most commonly encountered. Variations in either fabric thickness and
/or weight per unit area ultimately determine the local variations in fabric density, fabric porosity and pore size distribution, and therefore influence the appearance, tensile properties, permeability, filteration, liquid barrier and penetration properties of the fabric. Fabric uniformity can be defined in terms of the fabric weight (or fabric density) variations, as measured at different regions of the fabric, and is normally anistropic, i.e. the uniformity is different in different directions (MD and CD) of the fabric. The index of dispersion has been used to represent the anisotropy in weight uniformity [280].
Previous studies reported a relationship between nonwoven fabric air permeability and structural characteristics, such as fabric weight, fabric thickness and fabric density [281-
284], air permeability generally decreasing with an increase in fabric thickness, weight and density. Fabric weight generally has the most significant effect on air permeability, more than fabric thickness and density and fibre diameter [285], fabric density being the second most important [284].
2.9.3 Fibre Orientation in Nonwoven Fabrics
Fibre orientation plays an important role in defining appearance, performance and processability of nonwoven fabrics. It is an important characteristic of nonwoven fabrics, since it directly influences their mechanical properties and end-use performance.
The fibres in nonwoven fabrics rarely display completely random orientation but individual fibres, mostly in-plane, are aligned in various directions, being influenced mainly by the web formation and bonding process used. The production of parallel-laid (conventional), condensed, random and combined (random/condensed) webs in carding is illustrated in Fig.
2.28. In certain nonwoven structures, the fibres can be aligned in the fabric plane and also perpendicular to it. In the two-dimensional fabric plane, however, fibre orientation is
87 measured by the angle, which defines the directional position of individual fibres in the fabric structure relative to the machine direction.
Figure 2.28: Production of parallel-laid, condensed, random and combined random/condensed webs in carding
Fibre orientation angle (βί) has also been expressed as the angle between the tangent to the fibre curl and the y-axis [286]. In the case of a cross-laid web, the y-axis can represent a cross- machine direction, as shown in Fig. 2.29 [287].
Figure 2.29 : Fibre orientation
88
The distribution of fibre orientation angles (DOA) within the nonwoven structure has been expressed in terms of cosine power functions as follows [288-289]:
y(βί) = a + b. ( Cosn (βί + α) + Cosn (βί-α)) (5) where, y(βί) is the relative frequency, at an orientation angle βί, and α is the lapping angle of the card web, a, b, and n are constants, a reflecting the ratio of randomly oriented fibres, b the peak height of the DOA curve and n controlling the peak range of the distribution curve. Equation 5 can be used to describe the fibre orientation distribution of various web structures, as shown in Fig. 2.29.
When a = 0, Equation 5 represents the theoretical distribution of a parallel laid web, in which the web is formed by laying down a number of card webs on top of one another, all in the machine direction. When b=0, the equation represents a perfectly random distribution, where the probabilities of occurrences for all the angles are the same.
When 0 < α < 90o, the equation describes the structure of a cross-laid nonwoven formed by laying down card webs or spraying a bundle of filaments on a moving lattice [289]. The anisotropy of the nonwoven structure can be adjusted by the proper lay-down of the fibre web. Generally, the value of n is determined at the maximum correlation coefficient between the modelled and experimental data sets. It is generally believed that the fibre orientation distribution function is a rapid, simple and quite useful tool for predicting the mechanical performance of nonwovens.
2.9.4 Fabric Porosity, Pore Sizes and Permeability
Thermal insulation, filter media and fluid barriers are among some of the end-use applications that are influenced by the porosity of the nonwoven fabric. The pore structure in a nonwoven fabric may be characterized in terms of the total pore volume (or porosity), and
89 the pore size and pore size distribution. Porosity is the ratio of the total volume of water thrust or forced out from a fabric sample to the volume of the fabric sample:
Porosity = volume of water/volume of fabric
= (volume of water per gram sample) (density of sample)
The porosity provides information on the overall pore volume of the porous material and is defined as the ratio of the non–solid volume (voids) to the total volume of the nonwoven fabric. In defining porosity, it is assumed that all the pore spaces are connected.
For a fabric with some of the pore spaces disconnected from the other, a term, “effective porosity”, has to be introduced, which is defined as the ratio of the connected pores to the total volume of the fabric.
It is practically impossible to have a fabric with a porosity value of 100% (totally open fabric) or 0% (i.e. completely solid without any pore).
As shown in Fig. 2.30, the distribution of pores, with respect to shape and size, is irregular.
On the pore scale (the microscopic scale), the associated flow quantities (velocity, pressure, etc.) will obviously be irregular as well.
Figure 2.30 : Example of pore size distribution within a fabric
High loft nonwoven fabrics usually have a low bulk density since they have more pore spaces compared to a compact nonwoven fabric.
90
Air permeability, is defined as the rate of air flow through a material under a differential pressure between the two fabric surfaces [290]. It is claimed that there can be no general relationship between porosity and air permeability [291], due to the latter being influenced by the capillary pressure curves and the internal surface area of the pores within the material, rather than the actual volume of the open space.
It is difficult to characterize pore size accurately, because the pore system, within a material, typically forms a very complicated pore surface, which is geometrically irregular
[291]. The term “diameter” is typically used in describing pore size, but this term is inappropriate, because pores are usually not spherical, or even tube shaped. Early research on fabric porosity relied almost exclusively on the use of mercury, in which mercury intrusion and extrusion under pressure in the evacuated specimen were measured, and calibrated, to provide a measure of pore size [292]. The surface tension of the mercury, however, is high, and therefore it requires a high pressure to force it in and out of the pores. Consequently, when this technique is used for measuring porosity of flexible materials, such as fabrics, the high pressure can distort the geometry of the pores being characterized [293-294]. The liquid extrusion method, which forms the basis of ASTM E 1294-89 for porosimetry measurement of membrane filters, has been successfully applied to evaluate the porosity of textile materials, including nonwovens [295]. In this method, neither the specimen thickness nor the complexity of the pore structure is considered in evaluating pore size, although a “tortuosity factor,” defined as the reciprocal of porosity, may be specified in the test. The tortuosity factor is a rough indication of the complexity of the flow path, or the deviation from the theoretical cylindrical flow path [296]. The increased complexity may be due to the randomness of some nonwoven structures, or the method of bonding, such as hydroentanglement.
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2.9.5 Moisture and Vapour Transmission through Textiles
Moisture permeability is generally taken as a measure of breathability. It is a critical characteristic of a textile material, indicating an ability to transfer moisture through the material. One of the main challenges of a fabric designer is to systematically design functional clothing to regulate the temperature of the human body, in relation to the ambient temperature and the level of physical activity. Thermal insulation and moisture management properties of textile materials are critical, in this respect, since they represent, amongst other things, the ability of the fabric to absorb perspiration and wick it away from the body. The human body perspires in two forms, namely, insensible (in vapour form) and sensible (in liquid form) perspiration. Therefore, for the wearer to be in a comfortable state, the clothing should allow both the types of perspiration to transmit from the skin to the outer surface
[297].
The ability of a textile material to transport perspiration, in the form of vapour, through itself and out to the exterior, is generally referred to as its breathability. Breathability is, however, only one aspect of heat and moisture management, amongst other thermo-physiological characteristics. It is also important that the liquid perspiration is absorbed, stored and then transported away in sufficient quantities, so that someone wearing the garment feels comfortable on the skin. Both thermo-physiological comfort and sensory comfort (i.e. how a textile material feels on the skin) can be objectively measured and evaluated in the laboratory.
Comfort is described as a pleasant state of psychological, physiological and physical harmony between the human being and the environment [298]. It has been noted that the processes involved in human comfort are physical, thermo-physiological, neuro- physiological and psychological [299]. The thermo-physiological comfort is associated with the thermal balance of the human body, which strives to maintain a constant body core
92 temperature of about 37 oC. A rise or fall of about ± 5 oC can be fatal, due to a deficiency or excess of heat in the body, it also being a significant factor in limiting work performance.
[300-301]. If, however, the ratio of evaporated perspiration to produced perspiration is very low, moisture will be accumulated in the inner layer of the fabric system, thus reducing the thermal insulation of the clothing, and thereby causing an unwanted loss in body heat [302].
Therefore, in both hot and cold weather and during normal and high activity levels, moisture transmission through fabrics plays a major role in maintaining the comfort of the wearer. A clear understanding of the role of moisture transmission through clothing in relation to the body comfort, is therefore essential for designing comfortable high performance fabrics for particular applications.
Brojeswari et al. have described the processes involved in moisture transmission through
textiles as follows [303]:
. diffusion of the water vapour through the layers,
. absorption, transmission and desorption of the water vapour by the fibres,
. adsorption and migration of the water vapour along the fibre surface, and
. transmission of the water vapour by forced convection.
2.9.5.1 The diffusion process
Fick [304] postulated that molecular diffusion is a random walk process, and is directly proportional to a driving force (concentration gradient) and inversely proportional to the resistance (length),this being formulated as Ficks law [304]:
dC JAX = DAB A (6) dz
Where:
JAX = the flux per unit area of species A
93
DAB = the diffusion constant of A through species B
dC A = concentration gradient of species A dz
Fickian diffusion assumes that each molecule travels along a random path, and collides with the other molecule (alike or other), but the collisions with the walls of the pore do not contribute to the diffusion process. Forhr and Lomax [305-306] investigated the diffusion of water vapour along the fibres and observed that water vapour diffuses from the inner surface of the fabric to the surface of the fibre and then travels along the interior of the fibres, and its surface, to reach the outer surface of the fabric.
At a specific concentration gradient, the diffusion rate along the textile material depends on the porosity of the material, and also on the water vapour diffusivity of the fibre.
For hydrophilic fibre assemblies, vapour diffusion is governed by a non-Fickian anomalous diffusion, which is a two stage diffusion process [307]. The diffusion process can be explained by the swelling of the fibres. Due to the affinity of the hydrophilic fibre molecules to water vapour, as it diffuses through the fibrous system, it is absorbed by the fibres, causing swelling and reducing the size of the air spaces, thus delaying the diffusion process [308,
309].
Yoou et al. investigated the effect of air permeability on water diffusion, and observed that air permeability increases with an increase in fabric porosity, with the type of finish applied
(i.e. hydrophilic or hydrophobic) to the fabric not having a great effect on the diffusion process [310]. Water vapour transmission through the fabric, however, increases with an increase in the moisture content and with the condensation of water in the fabric [311].
2.9.5.2 Absorption-Desorption process
The ability to manage heat and moisture flow has a major influence on the thermal state of the body and on human performance and perceptions of physical condition. Generally,
94 active wear fabrics are designed to promote heat and moisture flow away from the body. The extent to which absorption and desorption occur is governed by the relative humidity of the surrounding environment and the existing moisture content of the fibre. The higher the ambient humidity, the greater the absorption of water. Just as the ability of individual fibres to absorb water in vapour form is important, a fabric’s ability to allow water vapour to pass through it is also critical in managing body temperature and maintaining comfort. If transmission of moisture (in vapour or liquid form) is impeded, it can build up against the skin, creating a feeling of wetness or clamminess for the wearer.
The physical mechanism of moisture diffusion into fabrics varies for fabrics containing fibres differing in their degree of hygroscopicity. The fibre sorption properties mainly determine the evaporation process and, therefore, the heat and mass transfer by evaporation of water, diffusion of water vapour and condensation. It is important, therefore, to consider these properties when designing a fabric for a particular function.
Hydrophilic membranes are nonporous and can transmit water vapour by a molecular mechanism. The water molecules occupy a free volume between the molecular chains of the polymer and move across the membrane, without destroying the polymer when penetrating through the membrane having a nonporous structure. In the case of polymers with active hydrophilic groups, water molecules not only fill the free volume among the polymer molecular chains, but also interact with their active hydrophilic groups.
Due to the moisture gradient, water molecules move across the membrane, gradually joining the active groups. Then, they diffuse by dissolving in the polymer membrane. Upon arriving at the opposite surface of the polymer membrane, which has a lower vapour pressure, the molecules are desorbed and enter the surrounding air space as vapour.
The degree of water repellency of a fibre is determined by its surface energy, higher surface energies are found in hydrophilic fibres which are more easily wetted and can
95 promote the wicking of water along their surfaces. Hydrophilic fibres include cotton, viscose, and wool. On the other hand, hydrophobic fibres, such as polyester, acrylic and nylon all have higher surface energy values than wool but a lesser ultimate capacity for moisture absorption. Polypropylene has a similar surface energy to that of wool and could be expected to have similar wicking properties to wool. Typically, synthetic fibres rely solely on liquid water transport for their water management characteristics which is dictated by their cross sectional shapes and surface characteristics. Wool has the capacity to remove a large amount of water from the surface of the skin, in comparison to synthetic fibres, and to absorb/desorb water vapour, a process that largely prevents the need to wick liquid water away. Wool fibres generally have a hydrophobic surface and a hydrophilic interior which produce its unique moisture management properties, resulting in enhanced wearer comfort and performance. The chemical structure of the wool fibre provides the ability to absorb and desorb moisture and to gain and release heat, depending on the external and internal environment, thus affording a natural means of buffering the body’s microclimate.
The extent to which absorption and desorption occur in practice, is governed by the relative humidity of the surrounding environment and existing moisture content of the fibres; the higher the ambient humidity, the greater the absorption of water. This is an important process in maintaining the microclimate during transient conditions. A hygroscopic fabric, or fibre, absorbs water vapour from the humid air close to the sweating skin and releases it in dry air. This enhances the flow of water vapour from the skin to the environment, in comparison to a fabric which does not absorb or reduce the moisture built up in the microclimate [312-313].
In the case of absorbent fibres, such as cotton and rayon, the moisture sorption is not only dependent on regain and humidity, but also on the phenomena associated with sorption hysteresis, namely the effects of heat, dimensional changes and elastic recovery, due to the
96 reduced swelling of the fibres. During swelling, the fibre macromolecules or microfibrils are pushed apart by the absorbed water molecules, reducing the pore size between the fibres, thereby reducing the water vapour transmission through the fabric. As swelling increases, the capillaries between the fibres get blocked, thus resulting in lower wicking. The distortion caused by swelling sets up internal stresses which influence the moisture sorption process.
2.9.5.3 Convection heat flow in porous media
Convection is a mode of moisture transfer which takes place while air is flowing over a moisture layer, and is generally referred to as the forced convection method. The mass transfer in this process (hm) is controlled by the difference in moisture concentration between the surrounding atmosphere and the moisture source, and is generally expressed as
[314]:
Qm = -Ahm (Ca-Cα) (7)
Where:
Qm = mass flow by convection through area A of the fabric along direction of .
flow
Ca = vapour concentration in the air
Cα = vapour concentration in the material
The flow is controlled by the difference in concentrations (Ca-Cα) and the convective mass transfer coefficient, which depends on the fluid properties as well as on the fluid velocity. In a windy atmosphere, the convection method plays a very important role in transmitting moisture from the skin to the atmosphere [315, 316].
Evaporation and condensation also have a significant effect on moisture transmission depending on the temperature distribution in porous textiles at the time of moisture transfer
[317]. During the evaporation of liquid perspiration, latent heat is removed from the body to
97 cool it down. The role of evaporative heat transfer, in maintaining thermal balance, becomes more crucial with an increase in the surrounding atmospheric temperature. In this case, due to the low temperature gradient between the skin and the environment, conduction and convection heat transfers are reduced [318]. When a negative temperature gradient exists between the skin and the environment, evaporative heat transfer becomes the only way to reduce the body temperature. Since the latent heat of water is quite large (2500kJ/kg), even a small amount of evaporation adds significantly to the total heat flow [319-320]. Wind enhances the evaporative heat transfer and results in additional cooling, which is desirable during vigorous activity. In the steady state, the latent heat loss by water, due to evaporation is equal to the heat that comes to the water from the surrounding air, thus making it cooler.
2.10 MECHANICAL BEHAVIOUR OF NONWOVEN FABRICS
The mechanical properties of textile materials characterize their response to applied forces and deformations, which are considered as the most important technical attributes, since they contribute both to the behaviour of the material during processing as well as to the performance of the final product during its end use. It is claimed that the stress-strain behaviour of the constituent fibres and the structural parameters of nonwoven fabrics are the two major factors which influence their mechanical properties. Moreover, each type of nonwoven fabric has distinct structural characteristics, in terms of fibres and bond behaviour, which need to be considered for accurate prediction of tensile deformation [321].
Important aspects of fibre morphology include fibre orientation, curl, and thickness.
Changes in their configuration can affect the mechanical behaviour and failure mechanisms
[322]. These parameters can also be varied and controlled easily during manufacture in order to improve fabric design and performance for any particular application.
Generally, orthotropic and fibre network theories have been employed for predicting the tensile deformation of nonwoven structures. The orthotropic theory is based on classical
98 lamination theory, whereby the nonwoven is assumed to be a layered structure, defined as a laminate, each layer being known to be a lamina. The fibre network theory, however, is based on the incremental deformation principle, whereby the nonwoven is divided into numerous unit cells experiencing the same strain as that of the fabric, and the average strain developed in each fibre can be considered as equivalent to the strain experienced by the fibre which is bonded at the boundaries of a unit cell [321]. Due to the complex structure of nonwoven fabrics, researchers have devoted considerable effort towards understanding their mechanical behaviour. Studies have been conducted to investigate the tensile behaviour of the material at both micro and macro level [323-327]. Fabric properties have also been described using various theories, such as the orthotropic theory [328-329].
Adanur and Liao [323] employed a numerical method to characterize the lateral contraction of nonwoven fabric during uniaxial tensile deformation. The effects of fibre characteristics, and their dispositions, on the mechanical properties of nonwoven fabric, were also investigated, both theoretically and experimentally.
Kim and Pourdeyhimi [324] used digital image acquisition and analysis to quantify structural parameters, such as the fibre orientation distribution function, bond region strain and unit cell strain, to provide directions for establishing appropriate constitutive relations for mechanical behaviour as well as failure criteria. Kim [325], on the other hand, applied an image analysis technique, based on the Fast Fourier Transform, to quantify the fibre orientation distribution.
The results suggested that, with a typical window of processing conditions, the fibre orientation had a significant influence on the anisotropic behaviour of nonwoven fabrics. The data also suggested that mechanical anisotropy of thermally point-bonded nonwovens was likely to be governed by different load transfer mechanisms, according to the direction of the applied macroscopic tensile load [326-327]. The deformation characteristics of these materials depend on the direction of loading. The two principal directions in woven fabrics,
99 namely warp and weft, possess different mechanical properties, which enables them to be modeled as orthotropic structures. In the case of nonwoven fabrics, however, due to the distribution of fibres characterized by the orientation distribution function (ODF), the level of anisotropy is higher than that in woven fabrics. An ODF is a histogram defining the angle of fibres with respect to a reference direction. Thus, if the fibres are predominantly oriented in a given direction in a nonwoven fabric, the change in frequencies of the fibre angle in that direction will be low and the change in frequencies in the perpendicular direction will be high. Nonwoven materials have two principal directions: machine direction (MD) and cross direction (CD). MD is the flow direction of the web assembly on the conveyor during manufacturing, while CD is perpendicular to MD in the plane of conveyor, where the web assembly forms the sheet.
In order to simplify the measurement of anisotropy in nonwoven fabrics, two principal directions, namely MD and CD, are used as shown in Fig. 2.31 [326-327]. In a typical ODF curve, the density of fibre orientation is concentrated near the MD, due to the fact that, in the web-forming process, fibres are laid along the conveyor direction. Hence, the deformation behaviour of nonwoven fabrics along the MD and CD differ, with the mechanical properties being better in the MD than in the CD.
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Fig. 2.31 (A): Typical ODF of nonwoven fabrics [326-327].
Figure 2.31(B): The two principle directions in nonwovens-MD and CD [327]
Therefore, within a typical window of processing conditions, the fibre orientation has a significant influence on the anisotropic behaviour of a nonwoven fabric
Rawal et al. used experimentally determined Poisson ratios to predict the stress-strain behaviour of nonwovens, so as to compare the theoretical and experimental stress-strain curves of thermally bonded and spun bonded nonwoven structures [321].
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From the foregoing, the behaviour of a nonwoven fabric, under the application of stresses and strains, is determined by its structural parameters and fibre properties. Similarly, the deformation of a nonwoven structure, under defined mechanical loading conditions, involves several mechanisms, such as elongation, bending, buckling, and shear as well as the breakage of fibres at the micro level. The modeling of the tensile deformation of a nonwoven fabric is, however, complex due to the simultaneous existence and interaction of these basic mechanisms. Hence, it is necessary to gain a good understanding of the raw material characteristics and process parameters to achieve the desired product properties.
2.11 PROBLEM STATEMENT
Healthcare workers involved in treating and caring for injured individuals or patients are exposed to transmission of bacteria and pathogens from their patients to themselves, thereby causing a reverse contamination. The relevant bacteria (Staphylococcus aureus, Escherichia coli and Klebsiella pneumoniae) and HIV, and hepatitis B and C can pose significant risks to human health and life.
Although surgical gowns have been used as an aid in minimising contamination, due to transmission of bacteria, the rate of infection in surgical sites remains high, and scientific studies have produced mixed results in terms of their efficacy. It is on this basis, that federal agencies and professional organisations agreed to develop more effective materials to protect patients and healthcare workers from cross contamination in the clinical environment. This global challenge has stimulated intensive research and improvement in the barrier properties of protective fabrics for healthcare workers so as to reduce the risk of exposure and cross contamination. Attention is now directed to reduce the potential risk of exposure and cross contamination through the use of medical gowns, which will not only create a physical barrier between the infection source and a healthy individual, but also provide acceptable level of comfort during lengthy surgical procedures, by incorporating breathability into the
102 product (permeable to air and water vapour). To achieve the above mentioned functions, therefore, the fabric must be properly engineered (designed) to provide the required barrier, and must also be treated with a suitable antibacterial finish to kill bacteria or, at the very least, to inhibit bacterial growth on the fabric surface. Nevertheless, there exist knowledge- gaps which need to be addressed, and the following gaps have been identified, based on an extensive literature survey.
To fill a gap in the body of knowledge related to barrier (bacterial and liquid) efficacy of surgical gowns during wear, this study explored the use of greige/virgin
(unscoured/unbleached) cotton as against the use of scoured and bleached cotton which is generally believed as unsuitable because of the presence of contaminations and impurities.
Greige/virgin cotton can be a viable and promising product for incorporation into existing and new nonwoven textile and related products for many end-use applications, such as disposable/reusable products. Exploration of greige/virgin cotton fibres in this study will ultimately enhance the competitiveness which is lacking in the current bleached cotton fibres that has been a major factor and which has reduced the expected growth in the use of cotton in today’s rapidly growing market of nonwovens.
This study started with the selection of suitable raw materials through finishing and employed a commercial grade hydroentangled machine. This ensures strict monitoring of the intermediate products and prevents any undesirable faults being passed to the finished products against the usual practice of obtaining webs from manufacturers with their uncertainties with regard to uniformity.
The ultimate goal of this research work is to design an improved nonwoven fabric, starting with the fibre, which is processed into a nonwoven fabric, on an industrial hydroentanglement pilot plant, and then further processed and treated to provide adequate barrier and comfort properties suitable for surgical gowns, nurses uniforms, laboratory coats
103 and drapes for healthcare. In so doing, to reduce the risk of exposure to bacteria at a reasonable cost.
2.12 MOTIVATION
Over the years, many investigations have been conducted on the hydroentanglement process and the resultant products. Studies on the wearable and comfort properties of hydroentangled nonwoven fabrics have also been conducted, mainly in the laboratory with the associated inherent limitations. In some cases, the scope of these studies did not cover the antimicrobial and liquid repellency properties collectively, which are prime requirement for a barrier fabric, besides the comfort properties.
The scope of this study is to cover both microbial and liquid repellency aspects, as well as the comfort properties of the nonwoven fabric, using fabric type and weight and finishing treatment as variables, exploring the potential of an industrial scale pilot plant in the process.
A single-bath application of the wet finish will be employed to reduce costs. The findings from this study may assist in the development of new standards for evaluating performance of surgical gown materials, and the development of innovative gown fabrics capable of providing protection under the conditions likely to be encountered in the operating room and hospital ward.
2.13 OBJECTIVE OF THE RESEARCH
The main objective of this research is to study the effect of various processing parameters on the wear and comfort related properties of hydroentangled nonwoven fabrics, with a view to establishing the optimum processing conditions and parameters for producing fabrics best suited for application in disposable and protective wear for surgical gowns, drapes and laboratory coats.
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Specific objectives:
To identify those characteristics of hydroentangled fabrics, produced from
different fibres, which are considered significant and effective in preventing
transmission of liquid and bacteria.
To assess the efficacy of finishing agents in producing barrier protection fabrics.
To evaluate the changes in physical and functional characteristics of treated
fabrics
To quantify the antimicrobial effect of the finishing treatment.
To assess the wear comfort related properties of the fabrics.
To select the barrier protection material assessed as the most safe, effective and
economical, based on multiple evaluation methodologies.
2.14 RESEARCH METHODOLOGY
To achieve the stated objectives of the study, the following research methodology has been adopted. The research will explore the production of nonwoven webs of varying weights and comprising from three selected fibre types (cotton, viscose and polyester), using the carding and cross-laid system. Thereafter, each web will be subjected to the hydroentanglement process, using a Fleissner Aquajet hydroentangling machine, for bonding the fabric at three different levels of waterjet pressure (80, 100 and 120 bars). After this, suitable antimicrobial and water repellent agents will be applied simultaneously, by the pad- dry-cure technique, to impart resistance to bacteria and prevent liquid penetration.
For the evaluation of antimicrobial efficacy, standard tests will be conducted to evaluate the efficacy of the antimicrobial agent used, against staphylococcus aureus (Gram-positive) and
Escherichia coli, according to the AATCC 100-2004 standard. These bacteria are chosen for
105 the study because they are the most common strains found in the hospital environment. The resistance to liquid penetration will be assessed according to the ASTM E96-1984 standard.
The Scanning Electron Microscope (SEM) will be used to identify the morphological structure of the specimen and to assess the uniformity of coating of finishing chemicals. An
ESEM Quanta 200 (W) will be used to produce an image of each specimen both before and after the treatment.
Other parameters relevant to fabric comfort, such as fabric weight, thickness, tensile and elongation, porosity, water vapour permeability and contact angle, will be measured using standard test procedures. All results (antimicrobial and physical properties) obtained on the treated samples will be analysed and compared with those of the control (untreated) samples, and in terms of their suitability for surgical gowns and other healthcare clothing (nurses uniforms, laboratory coats and drapes). A statistical ANOVA package will be employed to evaluate data and calculate correlations between the variables.
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3 EXPERIMENTAL
3.1 MATERIALS
Cotton, viscose and polyester fibres were used in this study because of their proven performance in medical textiles. The properties of the fibres are shown in Table 3.1.
The fibre fineness and fibre strength were measured after conditioning for 24hrs in a standard testing atmosphere of 21oC ± 2oC and 65% ± 2% RH [330]. For each fibre type, a bundle of
1.17cm was tested with a load cell of 5kN on the Instron testing machine, from which the strength, fineness and elongation were obtained. The average of five tests per sample was recorded.
Table 3.1 Fibre properties
FIBRE FINENESS NOMINAL STRENGTH ELONGATION (dtex) LENGTH (cN/tex) (%) (mm) COTTON 1.52 35 30.5 6.3 VISCOSE 1.7 40 26.3 17.0 POLYESTER 1.6 38 45.9 29.2
Two different types of finishing agents, namely Ruco-Bac AGP for the antimicrobial and
Ruco-Coat FC 9005 water repellent treatments, respectively, were supplied by Rudolf
Chemicals (PTY) Ltd, Kwazulu Natal, South Africa.
3.2 METHODOLOGY
3.2.1 Fibre preparation
For the opening and cleaning of the cotton lint, the cotton (unbleached) was processed under mill-like conditions on a Trutzchler industrial fibre opening and cleaning line, consisting of
Kirschner opener, Multimixer and Cleanomat as fine opener, as shown in Fig. 3.1. Intimate blending was carried out in the Trutzschler Multi-Mixer MX-1 with six blending chutes, after
107 which the fibres were fed to the Cleanomat (CL-C4) to remove contamination and foreign matter, such as leaves, twigs, stones, seeds, plant debris, seed coats and dust. The opened cotton fibre was fed to a revolving flat carding machine, as shown in Fig. 3.2, for further, and more effective, cleaning. The viscose and polyester fibres were opened first and then carded, using a flat top card, to produce the fibrous web.
(a)
(b) Fig. 3.1: Schematic layout diagram of (a), the Trutzschler opening and cleaning line and (b) the Integrated Mixer and Cleanomat cleaner [331].
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Figure 3.2 : Revolving flat card for cotton [331]
3.3 CARDING AND NEEDLE PUNCHING
Each fibre type was carded and cross-lapped to form webs of 80g/m2, 120g/m2 and 150g/m2 area weight, respectively. All fibres were carded on a card consisting of a 2.5m diameter master cylinder, equipped with universal card clothing, and cross-lapped, with the consequent disentanglement, proper mixing and improvement in web uniformity. The large rotating metallic cylinder is the heart of the carding machine, and the central distributor of fibre during the process. The cylinder is partly surrounded by an endless belt and pairs of worker-stripper rollers, thus performing a dual function of carding and mixing. The doffer- stripper rollers condense and remove the fibres from the cylinder, in the form of a continuous web [332]. The process parameters of the carding machine are summarized in Table 3.2.
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Table 3.2 : Carding processing parameters
COTTON VISCOSE POLYESTER
PARAMETERS Area weight Area weight Area weight (g/m2) (g/m2) (g/m2) 80 120 150 80 120 150 80 120 150 Feeding Rate 0.30 0.40 0.62 0.50 0.50 0.5 0.5 0.50 0.55 (m/min) Vertical lap speed 2.00 2.05 1.45 2.70 2.00 1.50 1.80 1.20 0.95 (m/min) Incline Conveyor 2.20 2.20 1.40 2.70 2.00 1.50 1.80 1.20 0.95 speed (m/min) Infeed Apron 2.20 2.20 1.45 3.35 2.15 1.50 1.90 1.15 1.1 speed (m/min) Infeed Roller 2.20 2.20 1.45 3.20 2.15 1.50 1.90 1.20 1.1 speed (m/min) Needle Depth 4.0 4.0 2.00 3.00 3.00 3.0 5.00 5.00 5.00 penetration (mm) Number of strokes 232 214 242 220 238 170 212 134 126 (1/min) Output (m/min) 2.35 2.35 1.5 2.3 2.35 2.35 2.35 2.35 2.35
3.4 CROSS-LAPPER
The web from the carding machine was transported, through a conveyor belt, to a cross- lapper for producing a multiple layered fibre assembly as shown in Figure 3.3. The web was deposited on an inclined lattice as it leaves the card, and was subsequently laid in cross-wise manner on a wider lattice, moving in a direction at right angles to the original direction of laying. This cross layer is responsible for the strength of the web in the cross machine direction (CD), compared to that in the machine direction (MD), due to a re-arrangement of the fibre orientation [333].
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Figure 3.3 :Crosslapper [333]
Subsequent to cross-lapping, light needle punching was performed, at a line speed of approximately 8 mm/min, on a Dilo needle loom, in order to provide sufficient strength to the fragile web for further processing. The loom has two needle-boards, one with the straight line pattern and the other with a random pattern, the depth of needle penetration being adjusted to about 0.30mm.
3.5 SAMPLE DESIGNATION
For the purpose of identification and clarity, the samples produced are specified in Table 3.3.
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Table 3.3 Sample designation
Norminal Sample Area Jet pressure Fibre ID weight (Bar) (g/m2) C1P1 Cotton 80 60
CIP2 Cotton 80 100
C1P3 Cotton 80 120
C2P1 Cotton 120 60
C2P2 Cotton 120 100
C2P3 Cotton 120 120
C3P1 Cotton 150 60
C3P2 Cotton 150 100
C3P3 Cotton 150 120
V1P1 Viscose 80 60
V1P2 Viscose 80 100
VIP3 Viscose 80 120
V2P1 Viscose 120 60
V2P2 Viscose 120 100
V2P3 Viscose 120 120
V3P1 Viscose 150 60
V3P2 Viscose 150 100
V3P3 Viscose 150 120
P1P1 Polyester 80 60
P1P2 Polyester 80 100
P1P3 Polyester 80 120
P2P1 Polyester 120 60
P2P2 Polyester 120 100
P2P3 Polyester 120 120
P3P1 Polyester 150 60
P3P2 Polyester 150 100
P3P3 Polyester 150 120
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3.6 HYDROENTANGLEMENT
The pre-needle punched webs, from the different fibres, were produced at three different basis weights (80g/m2, 120g/m2 and 150g/m2), and subsequently hydroentangled, using three different water jet pressures (60, 100 and 120bars). A total of 27 samples were produced, each 10m long and 60cm wide. Details of the hydroentanglement process parameters are given in Table. 3.4.
Table 3.4: Hydroentanglement parameters
WATER JET PRESSURE COTTON VISCOSE POLYESTER
Nominal Nominal Nominal NO OF SAMPLE Psi Bars Area weight Area weight Area weight S 2 2 2 (g/m ) (g/m ) (g/m )
870 60 80 120 150 80 120 150 80 120 150 9
1450 100 80 120 150 80 120 150 80 120 150 9
1740 120 80 120 150 80 120 150 80 120 150 9
27 TOTAL NO OF SAMPLES
The lightly needle-punched web was processed on a Fleissner’s Aquajet hydroentanglement machine, consisting of 3 sets of jet manifolds, to produce fabrics, in a single pass, under varying waterjet pressures, as illustrated in Fig. 3.4. The speed of the line was kept constant at 10m/min. The orifice diameter was 0.10 mm and the density of the jets, 16 jets/cm. In each case, the fibre web was processed through three different manifolds, the first manifold being used only for pre-wetting of the fibre web, and the remaining two manifolds for carrying out the hydroentanglement on both sides.
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AQUAJET
Fig. 3.4: Schematic layout diagram of the Fleissner hydroentanglement machine with a photo of the Aquajet [334].
Filtered water, with a surface tension of 7.26 x 10-5 and a viscosity of 0.895 x 10-9 , was used for hydroentanglement. In this process, a curtain of high speed water jets, emanating from a series of orifices in a long stainless-steel metal strip (nozzle-strip), impinges on the
114 fibrous web, converting the loose fibrous web into a mechanically strong fabric, due to the fibre entanglement caused by the fibres curling, twisting and deflecting around each other.
The fabrics were subsequently dried by a hot air dryer, before being wound onto a roll at the front end of the production line.
3.7 FABRIC FINISHING (BARRIER PROTECTION)
In preliminary batch tests, three combinations of add-on levels of water repellent and antibacterial finish were employed in order to determine the optimum combination of the two chemicals in terms of water repellent and antibacterial properties. This preliminary test was necessary because of the large number of samples under investigation.
According to the preliminary batch tests, a combination of 25g/l (0.5%) Ruco-Bac AGP and
20g/l (2%) Ruco Coat FC 9005 was found to be the most suitable, and therefore used in all further processing.
It is claimed that the most widely used method for the antibacterial finish is the pad-dry-cure process [335]. Hence, this process was employed in this research, to provide the antibacterial and water repellent finish, the antibacterial agent (Ruco-Coat FC 9005) and water repellent fluorochemical agent (Ruco-Bac AGP), as indicated in Table 3.5, were applied in one bath.
A stock solution, containing the antibacterial (Ruco coat FC 9005) and water repellent
(fluorochemical-Ruco Bac AGP) agents, was prepared according to the instructions provided by the suppliers. The solution was stirred with an automatic stirrer so as to ensure even distribution. For finishing, the test fabric was placed on a two-roll padder and then immersed in the bath, followed by padding through squeezed rollers, to achieve a wet pick-up of 100% on the weight of fabric (owf). After padding, the fabric was dried at 120 oC for 2 minutes and cured at 150 oC for 1 minute, so as to fix the finishes.
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Table 3.5 Antibacterial and water repellent FINISHING FINISHING AGENTS FIBRES PROCESS
Water Repellent Fluorochemical Cotton, viscose and Pad-dry-cure
Ruco-coat FC9005) polyester
Antibacterial Ruco-Bac AGP Cotton, viscose and Pad-dry-cure
polyester
3.8 FABRIC CHARACTERIZATION
All fabrics were conditioned for at least 24 hours in the standard atmosphere, for testing in compliance with ASTM D1776-96 (Standard Practice for Conditioning Textiles for Testing) standards [330]. Samples were cut within 1m of the lead or tail end of the fabric roll.
Similarly, all the specimens were cut 2.5cm from the edge of each fabric, so as to avoid irregularity due to structure distortion that could occur during processing. The details of the various tests applied are given in Table 3.6.
Table 3.6: Fabric tests
FABRIC PROPERTY METHOD STANDARD
(Instrument)
Thickness Thickness gauge ASTM D 5729-97 Weight Balance ASTM D 3776-96 Water repellency Water Repellency Spray Test AATCC 22-05 Contact angle Whilhelmy plate technique ASTM D 5946-01 Porosity Capillary Flow Porometer ASTM E 1294-89 Water vapour permeability WVTR Test Method ASTM E 398-03 Air permeability WIRA Air Permeameter ASTM D737-96 Tensile strength (Dry) Instron Tester ASTM D5035-95
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3. 8.1 Tensile Test
The tensile strength and elongation of each treated as well as untreated (control) nonwoven fabric sample were determined according to ASTM D5035-95 (strip method) on a Universal
Tensile Testing Machine (INSTRON-3345), at a constant rate of extension of 120 mm/min, and using a load cell of 600N [336], 5cm wide and 2.54cm long clamps and a gauge length of 75 mm. The coefficients of variation (CV) of the strength and elongation, based on five tests per sample, were used as a measure of the structural non-uniformity of the fabrics.
3.8.2 Scanning Electron Microscope (SEM)
The Scanning Electron Microscope was used mainly to investigate the morphological structure of the specimens, and to assess the uniformity of coating of finishing chemicals on the specimens. An ESEM Quanta 200 (W) was used to provide an image of each specimen, both before and after treatment. The test sample stub was cleaned with sand paper, to remove dirt, before a specialised carbon tape was stuck to it, after which the sample was placed on the stub. The sample was then pressed with a tweezer to ensure that it remains on the stub.
The stub was then placed on the sample stage, and the surface morphology of the sample examined at an accelerating voltage of 20 kV, a current of 10µA and 1000x magnification.
3.8.3 Fabric Area Weight (Mass per unit area)
After the hydroentanglement process, the basis weight of the bonded fabric was determined in accordance with ASTM D 3776-96, Standard Test method for Mass Per Unit Area
(Weight) of woven fabric [337]. Five square samples, each measuring 20cm x 20cm, were randomly cut out from different parts of each fabric and weighed individually on a Mettler balance, to the nearest 0.001g. The average weight was determined, and reported in grams per square metre (g/m2).
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3.8.4 Fabric thickness
The samples (20cm x 20cm), used for the determination of fabric weight, were also tested for thickness, on a digital thickness (EV06B) gauge, according to the ASTM D 5729-97,
Standard Test Method for Thickness of Textile Materials [338]. A constant pressure of 1kPa, was used, this being achieved by using a 50mm diameter metal disc weighing 170g. The average of 5 readings was calculated and reported.
3.8.5 Air permeability
Air permeability tests were conducted on a WIRA Air Permeameter, as specified in the
ASTM D7371-04 standard [339], using a pressure differential of 1cm of water. Air, at 20oC ±
2oC and 65% ± 2% R.H, was drawn from the atmosphere through the test specimen by means of a suction pump, the rate of flow being controlled by means of the pass and series valves.
The rate of flow was adjusted until the required pressure drop across the fabric was indicated on a gauge, graduated from 0 to 25mm water head. When the required pressure drop, normally 1cm of water, was attained, and the indicator of the gauge was steady, the rate of flow of air was noted and recorded. Five circular specimens were tested for each sample, and the mean air flow per second was calculated, the air permeability of the fabric being expressed in cubic centimetres per second (cm3/s) at 1cm of water.
3.8.6 Pore size distribution
A capillary Flow Porometer (CFP 1100-AEXCC) was used to measure pore size characteristics, as specified in the ASTM E1294-89 standard [340]. The capillary Flow
Porometry is a liquid extrusion technique, in which the differential gas pressure and flow rates through wet and dry samples are measured [340]. The principle of the capillary Flow
Porometer is shown in Fig. 3.5
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Figure 3.5 Principle of Capillary Flow Porometer
In determinig porosity, tests were carried out on both dry and wet samples. The specimen, cleaned and free from all loose particles, was first wetted with a liquid of low surface tension (Galwick of 15.9 dyne/cm surface tension). Galwick is considered excellent for wetting because it wets samples very quickly, has a low surface tension and possesses low vapour pressures. The fabric was completely covered by the wetting fluid and placed at the bottom of the sample chamber completely covering the bottom o-ring (55-60mm diameter).
Pressure, from a non-reacting gas, was applied to one side of the fabric, and gradually increased until the pores were cleared and gas flowed through the sample at a steady rate.
The maximum pore size was determined according to the first air flow, which was characterised as the bubble point. As smaller pores are emptied, the air flow through the sample increased, and the air flow was recorded as a function of air permeability. All the results are compared with those obtained on a dry sample. The variation of flow rate with pressure, for a dry curve and a half-dry sample, to yield half of the flow rate through the dry curve at a given differential pressure, was used to determine the pore size distribution. The intersection of the wet curve and the half-dry curve gives the mean flow pressure, which corresponds to the mean diameter of the pores, as shown in Fig. 3.6 [341].
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Figure 3.6 Determination of the mean diameter of the pores [341]
Five tests were conducted for each sample, and the minimum pore diameters, the largest pore diameter, and the mean flow pore diameter were computed automatically by the instrument.
3.8.7 Water vapour permeability
The water vapour permeability was determined on a Water Vapour Permeability Tester L80-
505, according to the ASTM E 398-03 standard [342]. The method for testing water vapour permeability is based on a very sensitive capacitance sensor placed directly in the upper measuring chamber. The lower chamber is saturated with water vapour, and the permeability rate is equivalent to the transmission of water vapour from the lower to the upper chamber, through the test sample placed between the two chambers.
After calibration, the test sample was affixed to a self adhesive sample card, which was inserted in the test chamber and produced a tight seal, even around thin samples in the measuring chamber. First, the upper chamber was dried, via a purge air flow, to a defined level. The air was supplied by an internal pump, and dried by a silica gel cartridge. After
120 drying the upper chamber, the air flow was stopped, and the flow valves closed. From that point, the transmission of water vapour, through the sample, caused an increase in humidity in the upper chamber.
The instrument measures the time required for the humidity in the upper chamber to increase to a predefined upper limit. The measured time interval was then utilized to calculate the water vapour transmission rate, expressed in g/m2/day. The automatically controlled test cycle is repeated until the sample has stabilised, and equilibrium is reached, or continues until testing was manually stopped. The test results, along with the corresponding temperatures, were automatically printed. Five tests were conducted for each sample, and the average recorded, as g/m2/day.
3.8.8 Water repellency: spray rating test
Both treated and untreated (control) samples were tested for water repellency, according to
AATCC Test method 22-2005 [343]. Three square specimens (180mm x 180mm), from each sample, with the smooth and upper surfaces without wrinkles, were held taut over a 150mm diameter hoop which was mounted at 45º to the horizontal. A funnel, fitted with a standard nozzle containing 19 holes, was held 150mm above the fabric surface. Into the funnel was poured 250ml, of distilled water, at 27 ºC + 1ºC which then dripped onto the upper surface of the fabric.
After the water spray, the hoop and specimen were removed, and tapped twice against a solid object, on opposite points of the frame, with the fabric being kept horizontal to remove any large drops of water. The fabric was assigned a spray rating by comparing the appearance of the tested specimen with the standard spray test ratings, as illustrated in Fig.3.7.
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Figure 3.7 Standard Spray Test Ratings [343]
3.8.9 Contact angle
The dynamic contact angle is widely used to describe the wetting, spreading and adhesion processes between liquid and solid. Several techniques exist for determining the contact angle, the principal two being the Wilhelmy Plate method and goniometry. The dynamic contact angle measurement was adopted in this study, because the measured contact angle represented the average over the entire wetted length of the sample. This inherent averaging process, coupled with the high sensitivity of the electrobalance, makes the dynamic contact angle measurement more reproducible than when employing the conventional goniometer
[344]. Moreover, the Whilhemy plate technique has been successfully used by other
122 researchers to study the wetting and adhesion properties of various surfaces [345-347]. The
Dynamic Contact Angle (DCA), measured by means of the Wilhemy plate technique (Cahn
Radian 300 Thermo Electron Corporation), was used in this study.
The Whilhelmy balance method uses a surface tensiometer, which relies on a high-resolution electronic balance to measure the force acting on the sample (Fig. 3.8). WinDCA is a window based software program, which is used to control the DCA system, collect data and perform the data analysis. The major features of the system are the thermo Cahn thin sample holder, the built-in electronic micro-balance and the counter weight stir-up. Other features include a movable stage, on which a beaker, containing distilled water, is mounted.
Figure 3.8 : Dynamic Contact Angle Analyser-Wilhelmy plate technique [346]
After the calibration of the balance and the motor, a square nonwoven fabric sample (25mm x 25mm) of known weight and thickness was clamped, with the thermo Cahn thin film sample holder in a fixed vertical position attached to the microbalance. The equivalent weight of the sample and the sample holder was placed on the counter weight stirrup to balance the sample’s weight, so that the maximum sensitivity and range of weighing could be obtained.
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The movable stage, with the beaker containing distilled water on it, was lifted towards the suspended sample, and then kept stationary, while the test chamber was kept closed in order to protect the sample and the liquid from airborne particles and drafts. The lower edge of the sample was positioned close and parallel to the liquid surface. Once the test was initiated, the sample surface was automatically lowered and submerged in the water (22 oC), which had a surface tension of 72 dynes/s, at a pre-selected rate of 20 µm/s, until 7mm of the sample was submerged, after which the sample was withdrawn at the same velocity.
The instrument measured the force on the test surface while it entered the liquid, when it was submerged, and then retracted, representing the advancing contact angle and retracting contact angle, respectively. To prevent any possible contamination, used water was discarded and replaced after each sample measurement.
The DCA automatically calculated and captured the corresponding advancing and receding angles along with the hysteresis, as the dynamic contact angle value.
To ensure the accuracy of the data, measurements were conducted for each sample, and the average of the five equilibrium contact angle values reported.
3.8.10 Antibacterial test
3.8.10.1 General procedure
The antimicrobial activity of the coatings was evaluated in terms of the reduction in the
Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) bacteria, according to the AATCC 100-2004 standard test method.
S. aureus was grown on a yeast-dextrose agar, the composition of which was 10g/ɭ peptone,
8g/ɭ beef extract, 5g/ɭ sodium chloride, 5g/ɭ glucose, 3g/ɭ yeast extract, 15 g/ɭ agar, and distilled water of pH 6.8, while the E coli was grown on a nutrient agar (10g/ɭ peptone, 3g/ɭ yeast extract, 5g/ɭ sodium chloride and distilled water, pH 7.1) and incubated overnight at 37 oC.
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In order to acquire a mid-log phase culture (bacterial concentration of approximately 106 cells/ml), the optical density (540nm) was measured every hour, until a value of 1.0 was achieved. Upon achieving this value, the culture was diluted to 106 cells/ml, and dilution plate counts were performed in triplicate, in order to verify the number of bacterial cells.
3.8.10.2 Agar Disc Diffusion
Test specimens, 1cm in diameter, were cut and weighed, to check the weight uniformity.
Muller-Hinton agar plates (2.0g/ɭ beef extract, 17.5g/ɭ digest of Casein, 1.5g/ɭ starch, 17g/ɭ agar, distilled water pH 7.3) were used in order to perform the agar disk diffusion tests. 100µl of both E. coli and S .aureus cultures were spread onto the Mueller-Hinton agar plates, at a concentration of 106cells/ml, and were left for approximately 20 minutes, to allow the bacteria to adhere to the agar surface. The autoclaved textile samples were then aseptically transferred onto the Mueller-Hinton Agar plates, containing the culture, and were then incubated overnight, at 37oC, to detect the zone of inhibition. All experiments were performed in triplicate. For a negative control, 100µl culture was spread onto a Mueller-
Hinton Agar plate and incubated overnight at 37oC. Commercially available antibiotics
(Davies Diagnostics) were used as positive controls namely: Vancomycin (30µg) and
Gentamicin (10µg) for S. aureus and E. coli, respectively.
3.8.10.3 Swatch survivability test
The swatch survivability test was carried out according to the American Association of
Textile Chemist and Colourist technical manual (AATCC Test Method 100-2004).
Autoclaved swatches were placed in flasks containing 100ml of Mueller-Hinton broth, after which 1ml of the relevant culture (106 cells/ml) was added. These were then spread onto
Yeast- Dextrose Agar (S.aureus) and Nutrient Agar (E.coli) plates, as the 0 hr sample, and incubated at 37oC on an orbital shaker, operated at 160 rpm, for 24hrs. After 24hrs, another
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100µl sample of culture, incubated with fabric swatches, was removed, and serial dilutions were made . This was repeated after 48hrs and all experiments were performed in triplicate.
After 24hr of incubation at 37oC, the number of colonies formed on the agar plate was counted, and the number of live bacteria cells in the flask, before and after shaking, was calculated. Antibacterial efficacy was determined based on duplicated test results. Percentage bacterial reduction was calculated according to the following equation:
R= 100(B-A) / B (8)
Where,
R= % Reduction
A=the number of bacteria recovered from the innoculated specimen swatches in the
jar incubated for the desired contact period.
B=the number of bacteria recovered from the innoculated treated test specimen in the
jar, immediately after innoculation ( at “0” contact time).
Swatches of the same fabric, containing no antibacterial finish, were used as negative control in all experiments.
The antibacterial properties of the samples were quantitatively evaluated by measuring the reduction rate in the number of colonies (AATCC 100-2004), using the two bacteria:
Staphylococcus aureus (gram-positive) and Escherichia coli (gram-negative), as shown in
Table 3.7
Table 3.7 Evaluation of antibacterial efficiency Test Description Bacteria Species Method Standard Staphylococus aureus Antibacterial (gram- positive) Agar Suspension AATCC 100-2004 Escherichia coli test (gram- negative)
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4. RESULTS AND DISCUSSION
4.1 BASIC WEIGHT UNIFORMITY
The fabric weight was measured by weighing the samples and taking the average, expressed in grams per square metre (g/m2), with the uniformity in fabric weight being expressed in terms of the coefficient of variation (CV%). The results are shown in Table 4.1.
Table 4.1 Physical properties of hydroentangled fabrics Sample Fabric Waterjet Weight (g/m2) CV Thickness CV Density ID type pressure Nominal Actual (%) (mm) (%) (kg/m3) (bars) C1P1 Cotton 60 80 81.0 6.6 0.527 2.3 154 C1P2 Cotton 100 80 81.2 7.3 0.459 2.6 176 C1P3 Cotton 120 80 79.2 6.6 0.408 2.2 194 C2P1 Cotton 60 120 119.6 5.4 0.680 2.6 176 C2P2 Cotton 100 120 121.0 6.7 0.653 2.1 185 C2P3 Cotton 120 120 120.6 7.4 0.584 3.0 206 C3P1 Cotton 60 150 150.2 6.6 0.913 2.7 165 C3P2 Cotton 100 150 149.6 7.4 0.759 2.0 197 C3P3 Cotton 120 150 150.0 5.2 0.696 2.4 215 V1P1 Viscose 60 80 80.6 4.2 0.606 1. 8 132 V1P2 Viscose 100 80 80.0 6.1 0.523 1.1 153 V1P3 Viscose 120 80 77.6 4.9 0.417 1.8 186 V2P1 Viscose 60 120 117.4 5.0 0.718 1.2 164 V2P2 Viscose 100 120 122.2 3.8 0.720 1.8 170 V2P3 Viscose 120 120 119.2 4.6 0.690 2.0 173 V3P1 Viscose 60 150 150.4 4.3 0.991 1.6 152 V3P2 Viscose 100 150 148.0 5.0 0.831 2.1 178 V3P3 Viscose 120 150 150.0 3.9 0.812 1.5 184 P1P1 Polyester 60 80 81.6 3.2 0.915 1.8 89 P1P2 Polyester 100 80 79.8 4.2 0.803 0.7 99 P1P3 Polyester 120 80 82.4 4.6 0.662 1.0 124 P2P1 Polyester 60 120 120.2 3.2 1.270 1.3 95 P2P2 Polyester 100 120 122.8 3.0 1.176 1.7 104 P2P3 Polyester 120 120 119.2 2.7 0.959 1.9 124 P3P1 Polyester 60 150 150.0 3.5 1.332 1.2 113 P3P2 Polyester 100 150 149.8 4.4 1.101 1.4 136 P3P3 Polyester 120 150 150.2 3.9 1.003 1.9 150
From Table 4.1, it can be seen that the variation in weight (CV) of the fabrics produced from the cotton fibres was higher than that of the fabrics produced from the viscose and polyester
127 fibres. This may be due to the inherent variations in the properties of the cotton fibres. Also, synthetic fibres, such as viscose and polyester, are possibly more resistant to bending than the cotton fibres, which may have reduced the amount of fibre entanglement during processing and therefore also the weight variability. It is also thought that the synthetic fibres have lower fibre-to-fibre friction than the cotton fibres, which may also have reduced the amount of fibre disentanglement which usually occurs after the fibre entanglement in the process. Both of these effects may have contributed to the improved web uniformity observed in the case of the nonwoven fabrics produced from the viscose and polyester fibres.
4.2 Fabric weight, thickness and density
As can be seen from Table 4.1, there is a relationship between fabric weight, thickness and density, which is to be expected. It is observed that increase in weight leads to an increase in thickness and an increase in fabric density, for all fibre types as shown in C2P1, C3P1,
V2P1, V3P1, P2P1 and P3P1. For example, for cotton fibres, an increase in area weight from
80g/m2 to 120g/m2 (sample C2P1) resulted in an increase in thickness by 67%, from 0.408 mm to 0.680 mm, and a reduction in fabric density by 9%, from 194kg/m3 to 176kg/m3. The same phenomenon was observed in fabric V3P1 produced from viscose fibres, where the weight increased from 120g/m2 to 150g/m2 with a consequent increase in thickness by 44% from 0.690 mm to 0.991 mm, thus resulting in a decrease in density by 12%, from 173 kg/m3 to 152 kg/m3.
Similar observations to the above, were made with the fabric P3P1, produced from the polyester fibres, where an increase in basis weight, from 120g/m2 to 150g/m2 increased the fabric thickness by 38% from 0.959 mm to 1.322 mm. This resulted in a decrease in fabric density by 9%, from 124kg/m3 to 113kg/m3. The relationship between fabric weight and thickness and water jet pressure is graphically illustrated in Fig. 4.1. It can be seen that an increase in fabric weight and water jet pressure caused a decrease in fabric thickness,
128 with a consequent increase in fabric density, which ultimately produced highly consolidated fabrics.
2 1.0 80 g/m 2 120 g/m 2 0.9 150 g/m
0.8
0.7
0.6
0.5 Thickness (mm)
0.4
0.3
0.2 40 60 80 100 120 140 Water jet pressure (bar)
Figure 4.1:Relationship between fabric thickness and fabric weight and waterjet pressure
The relationship between fabric thickness and weight and waterjet pressure is important since the right combination of these parameters will eventually determine the fabric structure and physical properties of the fabric.
From Table 4.1, it can be seen that the densities of the polyester fabrics are the lowest ranging from 89kg/m3 to 150kg/m3, when compared to 154kg/m3 to 215kg/m3, for the cotton fabrics, which was the highest of the three types of fibres. It follows that, since the density of polyester is lower than that of viscose, the thickness of the polyester fabrics is higher than that of nonwoven fabrics produced from viscose fibres for identical fabric mass per unit areas. Hence, nonwoven fabrics produced from polyester fibres are bulkier than those produced from viscose fibres and consequently the structure is less dense than fabrics produced from viscose fibres.
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The density of the cotton fabrics is higher than that of the viscose fabrics, the density of the viscose fabrics ranging from 132 kg/m3 to 186kg/m3 at a similar fabric mass per unit area
(80g/m2). Consequently, nonwoven fabrics produced from polyester fibres, followed by that produced from viscose fibres, are the least dense of the three fabrics studied. The fabric weight and thickness, before and after the antimicrobial and water repellent treatments are shown in Fig. 4.2 and Fig. 4.3, respectively.
Figure 4.2 : Fabric Weight (g/m2) before and after treatment
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Figure 4.3 :Fabric thickness before and after treatment
4.2.1 Water jet pressure, fabric density and thickness.
According to the data presented on fabric density in Table 4.1, an increase in water jet pressure (bars) resulted in an increase in fabric density due to the decreased thickness of the fabric structure. This is illustrated by fabrics C3P2 and C3P3, where increasing the water jet pressure from 100 bars to 120 bars increased the fabric density from 197kg/m3 to 215kg/m3, at a fabric weight of 150g/m2. Similarly, for fabrics V1P2 and V1P3, where an increase in water jet pressure, from 60 bars to 100 bars, increased fabric density from 153 kg/m3 to 186 kg/m3, at a fabric weight of 80g/m2. For the polyester fabrics, for example, an increase in fabric density from 104 kg/m3 to 124 kg/m3 was observed (Fabrics P2P2 and P2P3) when the waterjet pressure was increased from 100 bars to 120 bars, the fabric weight being approximately constant at 120g/m2. The increase in fabric density with an increase in waterjet pressure, for the different fabric types, is illustrated in Fig. 4.4
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Figure 4.4 : Changes in fabric density with increasing waterjet pressures for the different fabric types
The decrease in fabric thickness, and associated increase in fabric density, and associated consolidation of the fabric structure, caused by the increase in water jet pressure, may be attributed to the increased turbulence at higher water jet pressure, causing increased rearrangement, shifting and entanglement of the fibres. This phenomenon was also reported in the previous work by Anandjiwala et al. [348]. For all three fibre types, covered in this study, the fabric density increased with an increase in water jet pressure, due to the increased consolidation of the fabric structures.
4.3 EVALUATION OF SCANNING ELECTRON MICROSCOPE (SEM)
PHOTOGRAPHS
Surface morphology can provide useful information about the changes in the surface of a substrate due to a chemical process. To determine any surface changes, due to the application of the antibacterial and water repellent finish, scanning electron microscopy was
132 performed on both treated and untreated fabric samples. The SEM images in Fig. 4.5 show that the surface morphology of the treated and untreated fabrics was different due to the presence of a uniform coating of the finishing agent on the fibres.
(a) (b)
(c) (d)
Fig. 4.5 : Scanning electron micrographs before (a&b) and after (c &d) antibacterial and water repellent finishes.
133
(a)
(b)
(c)
Fig. 4.6: Photomicrographs of fibre bundles hydroentangled with water jet pressures of (a) 60
(b) 100 and (c) 120 bars.
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The increased compactness of the fabrics due to the increased fibre entanglement, resulting from the higher waterjet pressures, is clearly evident in Fig.4.6, increasing the waterjet pressure, at a constant fabric mass per unit area, increasing the fabric compactness. For the cotton fabrics, an increase in waterjet pressure resulted in a more compact and paper-like appearance, while for the polyester fabrics, an increase in fabric weight produced bulkier fabric even at the highest water jet pressure. This may be partly due to the higher bending stiffness of the polyester fibres resulting in a lower fabric density.
It could also be due to possible fibre deformation which may have occurred during the hydroentanglement process. It was observed that, with increasing waterjet pressure, the cotton fabrics became harsher than the viscose and polyester fibres.
4.4 EVALUATION OF ANTIBACTERIAL ACTIVITY
The antibacterial activity, of the treated and untreated fabrics, was assessed quantitatively
(Agar diffusion method). The mechanism of the antimicrobial activity is “contact killing”
(biocidal). Antibacterial activity was expressed in terms of the percentage reduction in the microorganism after contact with the treated specimen, based upon the number of bacterial cells surviving after contact with the untreated specimen. The improvement in the antibacterial activities of the treated fabric samples may be partly due to direct adsorption and deposition of the antibacterial agent film onto, or within, the fabric structure. The antibacterial agent may interact with the bacterial peptidoglycan layer, form pits in the cell wall, change the membrane polarity to damage the membrane and thereby provide antimicrobial properties. Other possible means of providing antibacterial properties include inhibition of protein synthesis, inhibition of nucleic acid synthesis and alteration of cell wall membranes of the bacteria.
In this study, the most susceptible bacterium used was Staphylococcus aureus. The antibacterial activity of the treated fabrics against gram-positive bacteria (S.aureus) was
135 greater than that against the gram-negative bacterium (Escherichia coli). The reduced antibacterial activity against the gram-negative bacteria, is attributed to an additional outer membrane structure in the cell wall, which acts as additional barrier to the antibacterial agent.
In contrast to this, gram- positive bacteria have a simple cell wall structure, in which the cytoplasm membrane has a rigid peptidoglycan layer, composed of networks with many pores, which allow foreign molecules to enter the cell wall without any difficulty. The susceptibility of S. aureus against the antibacterial agent, used in this study, is in agreement with previous studies [349]. Photographs of bacterial colonies in contact with the samples, for 0hrs and 24hrs, respectively, are shown in Fig. 4.7.
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Fig.4.7: Colony forming units ( CFU) at time 0 and after 24 hrs contact.
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The colony forming units (CFU)/swatch, at time zero and after 24hrs, as well as percentage reduction in bacteria, (Table 4.2) illustrate the good antibacterial properties of the finish, against both gram-positive and gram-negative bacteria.
The percentage reduction in bacteria (S. aureus and E. coli), after 0hr and 24hrs contact time, is shown on Table 4.2.
Table 4.2: Reduction in number of bacteria after 24 hrs contact time
REDUCTION IN BACTERIA AFTER FIBRE 24 HRS (%) SAMPLE ID TYPE UNTREATED S. AUREUS E.COLI SAMPLE C1P1 Cotton 98.2 96.7 0 C1P2 Cotton 99.6 93.7 0 C1P3 Cotton 99.5 93.8 0 C2P1 Cotton 98.8 90.5 0 C2P2 Cotton 96.7 90.4 0 C2P3 Cotton 98.1 88.3 0 C3P1 Cotton 99.2 92.9 0 C3P2 Cotton 96.6 96.6 0 C3P3 Cotton 90.4 86.2 0 V1P1 Viscose 98.7 95.2 0 V1P2 Viscose 98.5 65.1 0 V1P3 Viscose 98.5 86.2 0 V2P1 Viscose 98.5 85.5 0 V2P2 Viscose 98.6 82.7 0 V2P3 Viscose 98.5 81.8 0 V3P1 Viscose 99.5 99.6 0 V3P2 Viscose 95.7 80.9 0 V3P3 Viscose 98.4 78.5 0 P1P1 Polyester 99.5 99.6 0 P1P2 Polyester 99.2 99.5 0 P1P3 Polyester 99.1 99.4 0 P2P1 Polyester 99.2 98.1 0 P2P2 Polyester 99.5 99.5 0 P2P3 Polyester 99.4 99.6 0 P3P1 Polyester 99.5 99.5 0 P3P2 Polyester 99.5 99.4 0 P3P3 Polyester 99.5 99.1 0
For the polyester fabrics, the reduction in S. aureus after 24hrs contact time, ranged from
99.1 to 99.5%, while the similar reduction in E. coli bacteria ranged from 99.1 to 99.6 %.
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On the other hand, for the viscose fabrics, the reduction in S. aureus and E. coli bacteria, after 24hrs contact time, ranged from 95.7 to 99.5% and 80.9 to 98.8%, respectively. For the cotton fabrics, the reduction in S. aureus bacteria ranged from 90.4 to 99.6 % while that for the E. coli bacteria ranged from 86.3 to 96.7 %, at 24 hrs contact time. Fig. 4.7 illustrates the compartative reduction in each of the bacterial strains used in this study.
The outstanding antibacterial performance of the treated polyester fabrics, as assessed by the percentage reduction in both E. coli and S. aureus bacteria, compared to that of the viscose and cotton fabrics may be due to the variation in the fabric structures. The polyester fabrics had a lower density and were bulkier and less compact than the cotton and viscose fabrics and consequently had a higher porosity, even when produced at high water jet pressures, hence offer an advantage in this respect. Generally, the antibacterial agent used in this study is considered efficient in imparting antibacterial properties on all the three fibre types covered here, the quantitative tests showing high bactericidal activities against both S. aureus and E. coli. The treated fabrics are therefore considered suitable as an effective barrier protection against these bacteria.
4.5 EVALUATION OF TENSILE STRENGTH AND EXTENSION TEST RESULTS
Tables 4.3 and 4.4 show the tensile strength and extension at break results for the untreated and treated fabrics, respectively. The mechanics of tensile failure of nonwoven fabrics, is dependent upon the degree of fibre entanglement, inter-fibre friction and the tensile and bending properties of the fibres. The applied tension has to overcome the inter-fibre cohesion resulting from the interfibre friction and entanglement [348]. Having overcome these inter- fibre frictional forces, the applied tension causes the straightening of the entangled fibres, until the fibres start to slide past each other. Once the fibres slip, the applied tension results in an easy extension, and eventually the fabric ruptures, due to the mixed modes of fibre failure and slipping [348].
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The higher extension of the polyester fabrics, particularly in the machine direction, compared to viscose and cotton fabrics, at all waterjet pressures, may be due to the higher resistance to bending of the fibres, causing less fibre entanglement and possibly also to the lower fibre-to- fibre friction of the polyester. It can also be seen that the tensile strength at break of the viscose fabrics is lower than that of the cotton and polyester fabrics. This may be due to the differences in the degree of entanglement, the compactness (density) of the fabrics as well as interfibre friction.
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Table 4.3 : Tensile strength and extension of untreated samples
Maximum Force Water jet (N) CV ( %) Extension CV ( %) Sample Fibre pressure (%) ID type (bars) MD CD MD CD MD CD MD CD C1P1 cotton 60 70 117 7.0 8.3 60 44 6.7 6.0 C1P2 cotton 100 88 140 7.9 5.9 48 36 8.5 6.8
C1P3 cotton 120 98 160 6.3 8.6 40 30 6.3 7.8 C2P1 cotton 60 96 150 5.2 5.9 55 43 7.0 7.2
C2P2 cotton 100 120 170 4.7 8.3 51 40 4.8 6.4
C2P3 cotton 120 130 206 9.6 7.9 41 35 6.3 3.5 C3P1 cotton 60 120 181 5.7 4.1 54 45 5.9 4.4 C3P2 cotton 100 140 200 8.6 5.3 45 38 6.3 4.6 C3P3 cotton 120 150 240 4.6 6.2 40 31 4.4 7.6 V1P1 viscose 60 54 98 9.2 7.7 80 51 7.05 4.5 V1P2 viscose 100 58 115 7.8 9.0 68 42 5.9 2.2 V1P3 viscose 120 61 140 6.1 3.5 52 31 3.2 8.6 V2P1 viscose 60 55 104 5.5 6.9 75 55 3.9 6.5 V2P2 viscose 100 62 140 9.3 7.3 61 49 4.5 5.6 V2P3 viscose 120 70 215 4.7 6.3 50 44 5.1 6.5 V3P1 viscose 60 120 200 10.1 5.5 65 50 6.8 5.4 V3P2 viscose 100 134 228 6.9 4.3 40 30 6.2 4.6
V3P3 viscose 120 145 237 5.4 6.7 34 20 5.1 6.7
P1P1 polyester 60 90 185 7.5 4.3 125 90 4.4 3.9
P1P2 polyester 100 163 272 6.4 5.3 115 80 5.6 2.5
P1P3 polyester 120 171 360 4.2 3.9 104 74 4.4 6.8
P2P1 polyester 60 160 231 5.6 2.5 110 86 5.3 4.3
P2P2 polyester 100 215 378 6.9 4.4 90 66 4.1 6.5
P2P3 polyester 120 236 441 7.1 3.7 80 60 3.9 5.9
P3P1 polyester 60 220 350 8.3 5.7 100 85 5.7 6.5 P3P2 polyester 100 230 431 7.7 6.3 79 60 6.5 3.9
P3P3 polyester 120 261 490 10.2 4.0 62 55 2.9 6.3
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Table 4.4 : Tensile strength and extension of treated samples Fibre Maximum Sample types Water Force CV % Extension CV % ID jet (N) % pressure MD CD MD CD MD CD MD CD (bars) C1P1 Cotton 60 84 130 6.6 5.6 46 40 5.7 775
C1P2 Cotton 100 102 156 8.4 6.8 30 28 5.0 5.3
C1P3 Cotton 120 126 205 7.9 10.7 29 24 6.8 8.2
C2P1 Cotton 60 116 162 8.4 8.4 40 35 8.0 5.6
C2P2 Cotton 100 135 198 10.2 7.2 31 29 6.7 8.2
C2P3 Cotton 120 150 249 9.9 8.2 29 36 7.3 6.9
C3P1 Cotton 60 144 200 5.6 4.1 40 35 8.2 7.8
C3P2 Cotton 100 167 229 10.1 5.3 35 30 6.8 5.5
C3P3 Cotton 120 171 280 7.1 6.2 32 24 5.8 8.8 V1P1 Viscose 60 68 113 6.5 5.8 65 48 8.5 8.02
V1P2 Viscose 100 71 124 8.1 7.7 55 33 7.5 2.6
V1P3 Viscose 120 76 183 4.4 7.0 43 28 3.1 8.4
V2P1 Viscose 60 66 122 7.9 3.5 60 49 7.2 5.3
V2P2 Viscose 100 78 150 6.9 5.4 50 35 5.2 2.5 V2P3 Viscose 120 96 240 8.4 6.7 40 30 6.8 8.5
V3P1 Viscose 60 133 217 6.3 3.2 45 37 3.8 5.4
V3P2 Viscose 100 152 248 4.7 5.0 35 25 6.6 5.3
V3P3 Viscose 120 160 265 6.92 5.42 26 22 8.5 6.7
P1P1 Polyester 60 89 200 7.42 4.55 110 74 4.4 5.0
P1P2 Polyester 100 174 290 6.94 4.78 95 70 6.5 7.9
P1P3 Polyester 120 195 385 4.29 3.25 89 66 7.4 4.1
P2P1 Polyester 60 178 242 7.25 5.32 94 78 8.6 6.0
P2P2 Polyester 100 221 400 4.93 3.16 70 60 8.5 7.5
P2P3 Polyester 120 249 460 6.78 5.53 61 50 4.0 6.7
P3P1 Polyester 60 230 370 5.24 7.11 76 60 5.2 4.9
P3P2 Polyester 100 255 450 3.56 6.17 70 50 7.5 5.3 P3P3 Polyester 120 288 510 5.71 04.18 50 40 4.6 7.0
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4.5.1 Tensile strength
From Tables 4.3 and 4.4, it is apparent that the tensile strength of the samples in the cross - machine direction (CD) is always higher than that in the machine-direction (MD), for all the samples. This is as expected, due to the anisotropic nature of the carded web, the preferential orientation and alignment of fibres in the cross–machine direction (CD) resulting in a higher strength in the cross-machine direction than that in the machine direction, as shown in Fig.
4.8.
Figure 4.8 : Comparison of the tensile strength at break of untreated and treated samples in the machine-direction (MD) and cross-machine direction (CD)
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It is apparent that the polyester fabrics exhibited the highest tensile strength, followed by the cotton, with the viscose fabrics the weakest as shown in Fig. 4.8. These differences in the tensile strength of the different fabric types are related to the fibre strength and elongation, the polyester fibre being the strongest and the viscose fibres the weakest.
It can also be seen that the tensile strength of the treated samples, both in the machine- direction and cross-machine direction, is slightly higher than that of the untreated samples, as shown in Fig. 4.9.
(a)
(b)
Fig. 4.9: Comparison of tensile strength at break of untreated and treated fabrics in (a) MD and (b) CD, at different waterjet pressure levels.
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This may be due to the interfibre friction and cohesion of the fibres due to the application of the antibacterial and water repellent finishes. For example, the tensile strength values for the untreated samples in the machine direction varies from 54N to 261N, while the corresponding values in the cross-machine direction vary from 68N to 288N. Similar values for the treated samples vary from 68N to 288N in the machine direction, and from 113N to
510N in the cross-machine direction.
The tensile strength increased with an increase in waterjet pressure for all samples in both machine and cross direction, as shown in Fig. 4.9. The increase in tensile strength, with progressive increases in waterjet pressure, is also illustrated in Fig.4.10, and supported by the photographs in Fig. 4.6.
Fig.4.10: Relationship between tensile strength, averaged over all the fabrics, and waterjet pressure.
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The photographs in Fig. 4.6 show that, as the waterjet pressure increased, the fibre entanglement increased. Examination of the structures of the fabrics, produced at different waterjet pressures, revealed that, with progressively increasing waterjet pressure, more well- defined strands of highly entangled fibres were present. The formation of these consolidated strands of entangled fibres may be responsible for the increase in fabric strength. As already mentioned, the treated fabrics are stronger than the untreated fabrics as shown in Fig. 4.11, probably due to an increase in fibre friction and cohesion, or consolidation caused by the antibacterial and water repellent treatments.
Figure 4.11 : Comparison of tensile strength at break of treated and untreated fabrics in MD direction
The maximum tensile strength, in the machine direction (MD), of the untreated polyester fabrics is the highest, at 260N, followed by that of the cotton fabrics at 150N, with that of
146 the viscose fabrics at 145N being the lowest. The corresponding values for the treated fabrics are 288N, 171N and 160N, respectively, which are 9.4%, 12.3% and 6% higher than the corresponding values for the untreated fabrics.
Similarly, the maximum tensile strengths, in the cross- machine direction (CD), of the treated fabrics are higher than the corresponding values for the untreated fabrics ( Table
4.3 and Fig. 4.8). The values for the treated polyester, cotton and viscose fibre fabrics are
510N, 280N and 265N compared to 490N, 240N, and 237N, respectively, for the corresponding untreated fabrics.
4.5.2 Statistical data analysis of tensile strength
4.5.2.1 Tensile strength in machine direction (MD)
The tensile properties were analysed, using Statistica II software in MS Excel (ANOVA), to establish the influence of possible interactions between the various parameters that can have an effect on the tensile strength at break in the machine direction (MD). The parameters studied include waterjet pressure, fibre types, and treatment. A confidence limit of 95% (P ≤ 0.05) was set for this study. The mean tensile strength at break at pressure levels of 60, 100 and 120 bars was analysed, using ANOVA, the results are summarised in
Table 4.5.
Table 4.5 ANOVA of (MD) tensile strength
Tensile strength at break SS DF MS F p MD Intercept 5121244.9 1 5121244.9 122906.9 0.00000 Water Jet pressure 84996.3 2 42498.2 1019.9 0.00000 Fibre type 888632.8 8 111079.1 2665.8 0.00000 Treatment 18338.3 1 18338.3 440.1 0.00000 Water Jet pressure*Fibre type 42909.7 16 2681.9 64.4 0.00000 Water Jet pressure*Treatment 769.2 2 384.6 9.2 0.00014 Fibre type*Treatment 1213.3 8 151.7 3.6 0.00054 Water Jet pressure*Fibre 3050.5 16 190.7 4.6 0.00000 type*Treatment Error 9000.2 216 41.7
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The ANOVA results in Table 4.5 show significant interaction between treatment, waterjet pressure, fibre type and tensile strength at break, waterjet pressure playing a major, and statistically significant, role in determining the MD breaking strength, the latter increasing with an increase in waterjet pressure. According to Fig. 4.8, the tensile strength increases with an increase in waterjet pressure, for both treated and untreated samples, with the difference being significant at the 95% (P<0.05) confidence limit.
Since the three-way interaction was significant, separate two-way ANOVA analysis was performed for each fibre type (cotton, viscose and polyester) to determine the effect of treatment on the tensile strength at break (MD) at each pressure level. This is necessary to determine which level of a particular parameter had the greater effect on the response variable. With the exception of C1, where the interaction between waterjet pressure and the treatment was significant (Table 4.6), this interaction was insignificant at 95% confidence limit for all the fibre types considered (C2, V2 and P1) as shown in Tables 4.7 to 4.9; P was
0.4490, 0.07927 and 0.09405, respectively. Waterjet pressure does not significantly affect the difference between the treated and untreated fabrics. Further analysis showed that the water jet pressure and the treatment, when treated as separate parameters, do have a statistically significant effect on the tensile strength in the machine direction (MD), the treatment increased the tensile strength in the machine direction as did an increase in water jet pressure.
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Table 4.6 : ANOVA of MD tensile strength (C1)
Include condition: "Fabric type"="C1" SS Degr. of MS F p Freedom Intercept 243414.2 1 243414.2 5560.6 0.00000 Treatment 675.0 1 675.0 15.4 0.00063 Water Jet pressure 12580.7 2 6290.4 143.7 0.00000 Treatment*Water Jet 2373.9 2 1187.0 27.1 0.00000 pressure Error 1050.6 24 43.8
Table. 4.7: ANOVA of MD tensile strength (C2)
Include condition: "Fabric type"="C2" SS Degr. Of MS F P Freedom Intercept 456580.0 1 456580.0 8177.6 0.00000 Treatment 1936.0 1 1936.0 34.7 0.00000 Water Jet pressure 4696.5 2 2348.2 42.1 0.00000 Treatment*Water Jet 92.5 2 46.2 0.8 0.44900 pressure Error 1340.0 24 55.8
Tble 4.8 ANOVA of MD tensile strength (V2)
Include condition: "Fabric type"="V2" SS Degr. Of MS F P Freedom Intercept 151940.8 1 151940.8 2940.8 0.00000 Treatment 2340.8 1 2340.8 45.3 0.00000 Water Jet pressure 2551.7 2 1275.8 24.7 0.00000 Treatment*Water Jet 291.7 2 145.8 2.8 0.07927 pressure Error 1240.0 24 51.7
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Table 4.9 : ANOVA of MD tensile strength (P1)
Include condition: "Fabric type"="P1" SS Degr. of MS F P Freedom Intercept 626407.5 1 626407.5 14129.5 0.00000 Treatment 2000.8 1 2000.8 45.1 0.00000 Water Jet pressure 59645.0 2 29822.5 672.7 0.00000 Treatment*Water Jet 231.7 2 115.8 2.6 0.09405 pressure Error 1064.0 24 44.3
4.5.2.2 Tensile strength in cross-machine direction (CD)
Similar analysis, as for the tensile strength in the MD, was carried out on the tensile strength in the cross-machine direction (CD), the results being shown in Table 4.10. Since all the effects were significant, further analysis was carried out to establish the effect of the individual parameters and interactions, a similar trend having been observed in the machine direction. The treatment (treated and untreated), as well as the waterjet pressure significantly affected the CD tensile strength, as shown in Tables 4.10 to 4.13, the interaction between treatment and waterjet pressure having no significant effect ; P being 0.67642, 0.31708 and
0.17605, respectively, as shown in Tables 4.10 to 4.13.
Table 4.10 : ANOVA of CD tensile strength
Breaking strength CD SS DF MS F P Intercept 15264258.6 1 15264258.6 117796.5 0.00000 Water Jet pressure 493975.0 2 246987.5 1906.0 0.00000 Fibre type 2562277.7 8 320284.7 2471.7 0.00000 Treatment 22118.0 1 22118.0 170.7 0.00000 Water Jet pressure*Fibre type 175142.4 16 10946.4 84.5 0.00000 Water Jet pressure*Treatment 2732.5 2 1366.2 10.5 0.00004 Fibre type*Treatment 3021.5 8 377.7 2.9 0.00415 Water Jet pressure*Fibre type*Group 10514.9 16 657.2 5.1 0.00000 Error 27989.6 216 129.6
150
Table 4.11: ANOVA of CD tensile strength (C3)
Include condition: "Fabric type"="C3" SS Degr. of MS F P Freedom Intercept 1479852.3 1.0 1479852.3 2448.9 0.00000 Treatment 6249.6 1.0 6249.6 10.3 0.00370 Water Jet pressure 25929.6 2.0 12964.8 21.5 0.00000 Treatment*Water Jet pressure 480.3 2.0 240.1 0.4 0.67642 Error 14503.2 24.0 604.3
Table 4.12: ANOVA of CD tensile strength (V3)
Include condition: "Fabric type"="V3" SS Degr. of MS F P Freedom Intercept 1604065.6 1.0 1604065.6 23118.9 0.00000 Treatment 3921.6 1.0 3921.6 56.5 0.00000 Water Jet pressure 8461.3 2.0 4230.6 61.0 0.00000 Treatment*Water Jet pressure 167.3 2.0 83.6 1.2 0.31708 Error 1665.2 24.0 69.4
Table 4.13: ANOVA of CD tensile strength (P2)
Include condition: "Fabric type"="P2" SS Degr. of MS F P Freedom Intercept 3859253.3 1.0 3859253.3 89231.3 0.00000 Treatment 2253.3 1.0 2253.3 52.1 0.00000 Water Jet pressure 242781.7 2.0 121390.8 2806.7 0.00000 Treatment*Water Jet pressure 161.7 2.0 80.8 1.9 0.17605 Error 1038.0 24.0 43.3
4.5.3 Extension at break
As shown in Tables 4.2 and 4.3, the extension at break in the cross-machine direction (CD) is generally lower than that in the machine direction, for both the untreated and treated fabrics.
This trend is also reflected in Figs 4.12 and 4.13.
151
Figure 4.12 : Comparison of extension at break in MD for treated and unterated samples
Figure 4.13 : Comparison of extension at break in CD for treated and untreated samples
152
The lower breaking extension in the CD compared to that in the MD may be due to the better entanglement, orientation, straightening and alignment of the fibres in the CD.
According to Table 4.3, the highest MD breaking extensions of the untreated samples were 125%, 80% and 60% for polyester, viscose and cotton fabrics, respectively, with the corresponding values for the treated samples 110%, 65% and 46%, respectively (Table 4.4).
The treatment reduced the maximum extension by 16%, 19% and 23% for the polyester, viscose and cotton fabrics, respectively.
Similarly, for the untreated samples (Table 4.3) the highest CD extension at break for polyester, viscose and cotton fabrics were 90%, 55% and 45%, respectively, the corresponding values for the treated samples (Table 4.4) being 78%, 49% and 40%, respectively. The treatment therefore reduced the maximum breaking extension by 18%, 6% and 9% for the polyester, viscose and cotton fabrics, respectively.
These results, therefore, show that tensile strength was increased and extension at break decreased by the treatment, possibly due to the treatment increasing the interfibre friction and cohesion. An increase in waterjet pressure generally reduced the extension at break, for both the treated and untreated fabrics, as illustrated in Fig. 4.14, the extension at break decreasing from 45% (60 bars) to 22% (120 bars) for the treated fabrics, and from
65% (60bar) to 39% (120 bar) for the untreated fabric.
153
Figure 4.14 : Relationship between extention at break averaged over all the fabrics and waterjet pressure
It was observed that extension decreased progressively as the waterjet pressure increased from 60 bars to 120 bars for both treated and untreated fabrics, as shown in Fig. 4.14.
It is apparent, therefore, that whereas tensile strength increased with increasing waterjet pressure, extension at break decreased with an increase in waterjet pressure for both the treated and untreated fabrics (Fig. 4.15).
154
(a)
(b)
Figure 4.15 : Comparison of the extension at break of treated and untreated fabrics in (a) MD and (b) CD directions.
155
4.5.3.1 Statistical analysis on MD extension at break.
The ANOVA analysis considered the main effects of waterjet pressure, fibre type and the treatment (treated and untreated fabrics), as well as interactions between these three variables. The results shown in Table 4.14. indicate that the effects of treatment, waterjet pressure, fabric type and their interactions on the extension were all significant at the 95% confidence limit.
Table 4.14: ANOVA of MD extension at break
Effective hypothesis decomposition; Std. Error of Estimate: 3.320376 SS Degr. of MS F P Freedom Intercept 988901.6 1.0 988901.6 89697.1 0.00000 Water Jet pressure 22879.6 2.0 11439.8 1037.6 0.00000 Fibre type 133076.2 8.0 16634.5 1508.8 0.00000 Treatment 13934.7 1.0 13934.7 1263.9 0.00000 Water Jet pressure*Fibre type 3072.8 16.0 192.1 17.4 0.00000 Water Jet pressure*Treatment 241.2 2.0 120.6 10.9 0.00003 Fibre type*Treatment 473.2 8.0 59.2 5.4 0.00000 Water Jet pressure*Fibre 663.0 16.0 41.4 3.8 0.00000 type*Treatment Error 2381.4 216.0 11.0
Further analysis of between subjects (fibre type) indicated that the effects of waterjet pressure and fabric treatment on the extension at break are not significant at C1
(p=10.54161) and P1 (p=54161), indicating that there was no difference in the effect of water pressure on the extension at break of the treated and untreated fabrics.
Analysis of the effects of waterjet pressure, fibre type, treatment and their interactions on the
MD extension at break showed that all significantly affected the extension at break in the machine direction as shown in Table 4.14.
Analysis of the CD extension at break produced similar results and trends as for those for the
MD extension at break, as shown in Tables 4.15 to 4.18. An increase in waterjet pressure resulted in a decrease in extension at break as also demonstrated by Fig. 4.15. In addition,
156
there was a significant difference between the extension at break of the treated and
untreated fabrics, the former being lower.
Table 4.15: ANOVA of CD extension at break
Effective hypothesis decomposition; Std. Error of Estimate: 1.534541 SS Degr. of MS F P Freedom Intercept 572043.3 1.0 572043.3 242924.8 0.00000 Water Jet pressure 14497.4 2.0 7248.7 3078.2 0.00000 Fibre type 66224.2 8.0 8278.0 3515.4 0.00000 Treatment 5368.2 1.0 5368.2 2279.7 0.00000 Water Jet pressure*Fibre type 2064.4 16.0 129.0 54.8 0.00000 Water Jet pressure*Treatment 96.2 2.0 48.1 20.4 0.00000 Fibre type*Group 789.8 8.0 98.7 41.9 0.00000 Water Jet pressure*Fibre type*Group 757.5 16.0 47.3 20.1 0.00000 Error 508.6 216.0 2.4
Table 4.16: ANOVA of CD extension at break (C3)
Include condition: "Fibre type"="C3" SS Degr. of MS F P Freedom Intercept 33737.2 1.0 33737.2 18497.4 0.00000 Treatment 600.3 1.0 600.3 329.1 0.00000 Water Jet pressure 726.1 2.0 363.1 199.1 0.00000 Treatment *Water Jet 19.2 2.0 9.6 5.3 0.01281 pressure Error 43.8 24.0 1.8
Table 4.17: ANOVA of CD extension at break (V3)
Include condition: "Fibre type"="V3" SS Degr. of MS F P Freedom Intercept 27613.6 1.0 27613.6 26023.3 0.00000 Treatment 271.6 1.0 271.6 256.0 0.00000 Water Jet pressure 2858.8 2.0 1429.4 1347.1 0.00000 Treatment*Water Jet pressure 208.8 2.0 104.4 98.4 0.00000 Error 25.5 24.0 1.1
157
Table 4.18 : ANOVA of CD extension at break (P2)
Include condition: "Fibre type"="P2" SS Degr. of MS F P Freedom Intercept 133400.0 1.0 133400.0 77111.1 0.00000 Treatment 464.4 1.0 464.4 268.5 0.00000 Water Jet pressure 3838.7 2.0 1919.4 1109.5 0.00000 Treatment*Water Jet 24.3 2.0 12.1 7.0 0.00398 pressure Error 41.5 24.0 1.7
4.6 EVALUATION OF WATER REPELLENCY AND WETTABILITY
4.6.1 Water repellency
The average ratings obtained from the standard spray rating tests are shown in Table 4.19 Table 4.19: Average standard spray test ratings
Nominal Water Untreated Treated Fabric jet sample Sample ID sample Fibre type Weight pressure Rating Rating (%) (g/m2) (bars) (%) C1P1 cotton 80 60 0 80 C1P2 cotton 120 100 0 90 C1P3 cotton 150 120 0 90 C2P1 cotton 80 60 0 80 C2P2 cotton 120 100 0 90 C2P3 cotton 150 120 0 90 C3P1 cotton 80 60 0 80 C3P2 cotton 120 100 0 90 C3P3 cotton 150 120 0 90 V1P1 viscose 80 60 0 90 V1P2 viscose 120 100 0 80 V1P3 viscose 150 120 0 90 V2P1 viscose 80 80 0 80 V2P2 viscose 120 100 0 90 V2P3 viscose 150 120 0 80 V3P1 viscose 80 60 0 90 V3P2 viscose 120 100 0 90 V3P3 viscose 150 120 0 80 P1P1 polyester 80 60 50 90 P1P2 polyester 120 100 50 90 P1P3 polyester 150 120 50 90 P2P1 polyester 80 60 50 90 P2P2 polyester 120 100 50 90 P2P3 polyester 150 120 50 90 P3P1 polyester 80 60 50 90 P3P2 polyester 120 100 50 90 P3P3 polyester 150 120 50 90
158
Table 4.19 shows that the untreated viscose and cotton fabrics are rated 0%, while the polyester fabrics are rated 50% according to the standard spray test rating. This may be attributed to the presence of hydroxyl groups in the natural and regenerated cellulose which attract water and therefore spontaneously wet the samples. The polyester samples, on the other hand, possess hydrophobic groups which repel water. The ratings of the treated fabrics are all either 80 or 90%, due to the fact that forces in the water molecules exceed those from the fabric molecules, causing spherical water droplets to be formed on the fabric.
According to the ratings obtained for all three fibre types, the fabrics can be considered suitable for the proposed function, but this will be further evaluated in terms of the wettability tests by the contact angle measurement in water.
4.6.2 Surface characterization-wettability
Contact angle, Ө, is a quantitative measure of the wetting of a solid by a liquid, and is an important parameter used to characterize wetting. Wettability studies were conducted according to the liquid-solid contact angle technique, namely, the Wilhelmy plate method.
The direct contact angle (DCA) software calculated the corresponding advancing (Өa) and receding (Өr) angles, from which the contact angle hysteresis (Өa-Өr) was derived. The average of 3 measurements was obtained for each sample, and the results are given in Table
4.20.
159
Table 4.20: Contact angles of the fabrics
Untreated samples Treated Samples Area Waterjet Sample Contact angle (degrees) Contact angle (degrees) Weight Pressure ID Advancing Receding Advancing Receding Hysteresis (g/m2) (Bars) (Өa) (Өr) (Өa) (Өr) (∆Ө) C1P1 80 60 0 0 95.9 55.5 40.1 C1P2 100 100 0 0 94.3 54.3 40.0 C1P3 150 120 0 0 96.8 55.6 41.2 C2P1 80 60 0 0 94.7 53.8 40.9 C2P2 100 100 0 0 96.0 55.5 40.6 C2P3 150 120 0 0 94.0 54.1 39.6 C3P1 80 60 0 0 93.3 53.6 39.7 C3P2 100 100 0 0 95.0 53.9 40.3 C3P3 150 120 0 0 95.9 55.4 40.5 V1P1 80 60 0 0 91.8 42.8 48.4 VIP2 100 100 0 0 92.4 44.3 48.1 V1P3 150 120 0 0 93.3 44.5 48.8 V2P1 80 60 0 0 90.4 43.7 46.7 V2P2 100 100 0 0 93.4 44.5 47.4 V2P3 150 120 0 0 93.1 44.6 48.5 V3P1 80 60 0 0 91.8 44.4 47.5 V3P2 100 100 0 0 92. 44.5 48.0 V3P3 150 120 0 0 92.6 44.1 48.5 P1P1 80 60 57.5 45.2 107.1 97.8 9.4 P1P2 100 100 62.7 47.2 109.7 98.4 11.3 P1P3 150 120 64.9 51.6 113.2 104.1 9.8 P2P1 60 60 61.7 44.3 110.3 98.9 11.1 P2P2 100 100 63.4 46.6 114.2 104.2 10.0 P2P3 150 120 68.3 46.0 116.5 106.0 10.5 P3P1 60 60 62.8 47.2 108.1 96.8 11.3 P3P2 100 100 59.5 47.0 113.6 103.0 10.0 P3P3 150 120 63.6 48.4 112.9 103.0 9.1
The contact angles of the untreated cotton and viscose samples are all indicated as zero (0), since it was not possible to measure their contact angles, due to their very high wettability, there being immediate spontaneous absorption of the water on contact with the fabric surface. The inability to measure the contact angle of untreated cotton and viscose fabrics has also been reported in the literature [350]. The high wettability of cotton and viscose fibres is due to their hydrophilicity as well as to the capillary effect of the porous fabric structure.
160
The water repellent finish increased the contact angles substantially indicating that the treatment changed the surface properties from hydrophilic to hydrophobic.
Table 4.20 and Fig. 4.16 show the advancing contact angles, and hysteresis for the various fabric samples. In all the cases, the advancing angles are greater than the receding angles.
When the advancing angle is measured, being a heterogeneous surface, as the water advances, the hydrophobic domains will pin the motion of the contact line (water front) causing an expansion, retarding the liquid from advancing over the non-homogeneous surface, thus causing an increase in the observed contact angle [350]. When the water recedes, the hydrophilic domains will hold back the contracting motion of the contact line
(water front), thereby leading to a decrease in the observed contact angle. Thus, the advancing angle tends to reflect the hydrophobic phase, and the receding angle the hydrophilic phase, resulting in a contact angle hysteresis [350]. This phenomenon explains the reasons for the differences between the values of the advancing and receding angles, as reflected in the values tabulated in Table 4.20.
From Table 4.20 and Fig. 4.16, it is apparent that the advancing contact angle is quite different for the three types of fibres studied.
161
(a)
(b)
(c)
Figure 4.16 : Contact angle for the different fibre types (a) polyester (b) cotton (c) viscose
162
The differences in the advancing and receding angles and the resultant hysteresis values,
may be attributed to the surface roughness, surface chemical heterogeneity, time-dependent
interactions of the liquid with the solid surface, swelling due to liquid penetration into the
surface region and surface reorientation of functional groups [350]. The polyester fabric
had the largest advancing contact angles, ranging from 107.1 to 116.5 degrees, followed
by the cotton fabrics which ranged from 93.3 to 96.8 degrees, those of the viscose fabrics
were the lowest, from 90.4 to 93.4 degrees.
The polyester fabrics had the lowest hysteresis, ranging from 9.1 to 11.3 degrees, followed by the cotton fabrics, from 39.6 to 40.9 degrees, with those of the viscose fabrics the highest, ranging from 46.7 to 48.8 degrees. It, therefore, follows that the polyester fabrics are the most difficult to wet and the viscose and cotton fabrics wet out very easily, these differences being largely attributed to the differences in the chemical and surface properties of the fibres.
Polyester fabrics, comprising hydrophobic fibres, have a low absorbency rate and absorbency capacity due to the fact that water intake is confined to wicking through the capillaries. Cellulosic fabrics, being hydrophilic in nature, allow liquid to be absorbed into the fibres. They also attract and hold liquid water external to the fibre, in the capillaries, and through the fabric pores, as is the case with the polyester fabrics. Consequently, the surface wetting properties of polyester are poorer than those of the cotton and viscose fabrics.
There are differences in the contact angles (advancing and receding) of cotton and viscose fabric samples, and also their hysteresis, despite the fact that they are both cellulose and have identical chemical structures. These differences might be due to the differences in their molecular orientation, the roughness along the length of the fibres and the shape of their cross sections. Cotton has more crystalline regions, with hydroxyl groups uniformly arranged along the molecular axis, whereas viscose is regenerated from natural cellulose with more
163 hydroxyl groups and less crystalline regions, but more amorphous regions, which would tend to enhance water absorption. The effect of waterjet pressure on contact angle is shown in Fig.
4.17.
Figure 4.17 : Contact angle versus waterjet pressure
Table 4.21 and Fig. 4.17 show that both waterjet pressure and fabric type significantly affect the contact angle, increasing waterjet pressure increasing contact angle. Fig. 4.17 shows that the polyester fabrics had the highest contact angles, followed by the cotton fabrics, with those of the viscose fabrics being lowest. The interaction between fabric type and waterjet pressure was, however, insignificant (P=0.963) at 95% confidence limit, as shown in Table
4.21.
164
4.6.2.1 Statistical analysis of contact angle
Table 4.21: ANOVA of contact angle
Effective hypothesis decomposition; Std. Error of Estimate: 3.677394 SS Degr. of MS F P Freedom Intercept 803766.0 1.0 803766.0 59435.9 0.00000 Fibre type 5837.1 8.0 729.6 54.0 0.00000 Water Jet pressure 94.8 2.0 47.4 3.5 0.03699 Fibre type*Water Jet pressure 95.3 16.0 6.0 0.4 0.96350 Error 730.3 54.0 13.5
4.6.3 Evaluation of pore size distribution
Table 4.22: Pore size distribution of untreated and treated samples
Sample PORE SIZE DISTRIBUTION (µm) ID Untreated Samples Treated Samples Mean pore size reduction Minimum Mean Maximum Minimum Mean Maximum (%) C1P1 6.9 37.6 104.4 6.4 35.8 84.9 3 C1P2 6.0 31.8 98.1 5.2 28.0 91.9 12 C1P3 6.7 31.2 91.5 6.1 22.8 88.0 28 C2P1 6.9 31.4 78.1 6.5 28.7 63.5 8 C2P2 7.4 30.2 83.1 6.1 22.2 59.5 27 C2P3 7.5 26.8 79.2 6.6 20.2 52.7 25 C3P1 7.1 35.2 68.3 6.3 29.1 59.1 17 C3P2 7.2 26.4 74.0 6.7 23.5 55.3 11 C3P3 6.5 26.5 65.3 6.5 22.0 49.0 17 V1P1 4.8 43.3 97.0 5.5 38.0 75.6 9 V1P2 4.4 41.8 99.2 6.1 34.5 88.8 13 V1P3 60 39.6 109.6 6.4 32.6 101.1 25 V2P1 5.7 42.0 80.4 5.7 34.1 67.6 19 V2P2 6.1 34.8 102.0 6.2 32.6 69.1 6 V2P3 7.3 34.8 81.3 7.3 26.1 67.2 29 V3P1 6.7 42.9 71.2 6.6 33.0 71.0 23 V3P2 7.5 33.7 60.5 7.8 28.0 56.4 17 V3P3 7.6 30.8 65.5 7.3 27.6 59.9 8 P1P1 4.2 82.1 172.9 5.5 77.6 141.9 9 P1P2 4.8 81.0 167.5 5.4 66.1 125.8 34 P1P3 4.9 64.2 128.4 5.6 46.4 110.8 28 P2P1 6.2 77.6 157.4 6.2 65.5 117.5 16 P2P2 6.3 55.3 109.3 5.3 45.3 97.1 18 P2P3 5.4 56.0 I09.6 6.2 39.2 87.9 35 P3P1 5.1 63.8 109.2 6.5 42.2 87.6 34 P3P2 5.8 52.7 102.7 5.4 45.9 92.2 4 P3P3 5.7 47.3 104.9 5.1 45.2 90.0 2
165
The data in Table 4.22 shows the pore size distributions of the untreated and treated samples, from which it can be seen that there are variations in the pore sizes, within and between samples. This trend is also reflected in Fig 4.18, and may be due to variations in the polyester fabrics being greater than those in the viscose and cotton fabrics at the same water jet pressure, as shown in Table 4.22. For each fibre type and fabric weight, the pore size decreased as the waterjet pressure increased.
In general, there were minimal differences in the minimum pore sizes, between and within all the samples, while there were large differences in the mean and maximum pore sizes. The mean pore sizes of the treated and untreated fabrics of the three fibre types are shown in Fig.
4.18, the polyester fabrics having the largest pore size, followed by viscose, with the pore size of the cotton fabrics the smallest.
Figure 4.18 : Comparison of the mean pore sizes of the cotton, viscose and polyester fabrics
166
The mean pore size of the untreated cotton fabrics ranged from 26.4 to 36.9 µm, whereas those of the viscose fabrics ranged from 24.7 to 35.8 µm, and those of the polyester fabrics from 47.3 to 82.1 µm. In the case of the treated fabrics, the mean pore sizes of the cotton fabrics ranged from 20.0 to 35.8µm, those of the viscose fabrics from 24.7 to 38.0 µm, with those of the polyester fabrics being the largest, ranging from 36.2 to 73.8 µm.
The treatment reduced the mean pore size significantly as shown in Figs. 4.19 and 4.20, the percentage reduction in mean pore size being the lowest for the fabrics produced with
95% confidence limit the lowest water jet pressure (60bars), and highest for the higher water jet pressures of 100 bars and 120 bars.
Figure 4.19 : Mean pore size versus waterjet pressure
The effects of fibre type and finishing treatment on mean pore size are statistically significant, as shown in Table 4.21. It can be seen, that the pore sizes of the treated fabrics were smaller than those of the untreated fabrics, as shown in Table 4.22. Similarly, the pore
167 sizes of the polyester fabrics were greater than those of the viscose fabrics, the pore sizes being the smallest for the cotton fabrics.
Figure 4.20 : Effect of waterjet pressure, fibre type and treatment on mean pore size
Comparatively, the variations and reductions in pore sizes of the treated samples may be attributed to an increased compactness and consolidation (density) of the treated fabrics. The net results are a more compact fabric determined by the degree of the intensity of the hydroentanglement process with the consequent effect on their physical properties, particularly their breathability which is vital to the functional use under consideration.
4.6.3.1 Statistical data analysis of mean pore size
Analysis of variance (ANOVA) was conducted to examine if there were any statistically significant differences, due to treatment, fibre types, and waterjet pressure.
The results of the analysis (Table 4.23) show that in fact significant differences exist in the mean pore size due to certain of the above variables. The effects of the interaction term of waterjet pressure and treatment were, however, not significant (p = 0.70844), the effects of all the other term, and their interaction, were signifant. 168
Table 4.23: ANOVA of mean pore size
Effective hypothesis decomposition; Std. Error of Estimate: 5.232419 SS Degr. Of MS F P Freedom Intercept 451532.6 1.0 451532.6 16492.4 0.00000 Water Jet pressure 6396.1 2.0 3198.0 116.8 0.00000 Fibre type 54001.5 8.0 6750.2 246.6 0.00000 Treatment 3294.6 1.0 3294.6 120.3 0.00000 Water Jet pressure*Fibre type 4519.6 16.0 282.5 10.3 0.00000 Water Jet pressure*Treatment 18.9 2.0 9.5 0.3 0.70844 Fibre type*Treatment 535.2 8.0 66.9 2.4 0.01501 Water Jet pressure*Fibre 1401.9 16.0 87.6 3.2 0.00006 type*Treatment Error 5913.7 216.0 27.4
4.6.4 Evaluation of comfort related properties-breathability
4.6.4.1 Air permeability tests
Air permeability is one of the major comfort related properties, being a measure of how easily air passes through the substrate. The passage of air through the fabric is of importance for a number of end uses, particularly for surgical gowns and drapes, there being an optimum between the fabric being too air permeable on the one hand, and not sufficiently air permeable on the other hand.
Table 4.24 gives the air permeability results for the various fabrics, air permeability values are varying, between the fabrics, including between the untreated and treated fabrics. The observed variation in permeability is most likely due to the interdependence of fabric weight, thickness, density and pore size, as observed in the present pore size results, and also, widely reported in the literature [351].
An increase in the fabric mass per unit area (g/m2) is associated with an increase in the number of fibres in the cross-sectional area, which resists the air flow through the fabric. The fabric thickness and density also influenced the air permeability, the less dense fabrics
169 generally having the higher air permeability. As the fabric density increased, the air permeability decreased, due to the reduced pore size caused by the consolidation of the web during the hydroentanglement process.
Table 4.24: Air permeability (AP) and water vapour permeability (WVP)
Nominal Waterje Sample UNTREATED SAMPLES TREATED SAMPLES Fabric t Mean Mean pore Weight pressure ID pore size AP size AP (g/m2) (bars) WVP WVP (µm) (µm) C1P1 80 60 36.9 91.2 1247.9 35.8 88.8 1243.3 C1P2 80 60 31.8 60.8 1184.4 30.4 55.7 1180.0. C1P3 80 60 31.2 50.3 1093.2 22.8 35.8 1083.6 C2P1 100 100 31.3 44.2 1055.2 28.7 37.9 1045.6 C2P2 100 100 30.2 26.7 1019.5 22.2 20.9 1000.8 C2P3 100 100 40.5 20.8 990.0 20.3 17.7 965.1 C3P1 150 120 35.2 27.5 939.7 29.1 24.0 923.6 C3P2 150 120 26.4 24.2 900.6 23.5 16.4 875.6 C3P3 150 120 26.5 23.8 837.5 22.0 15.0 814.9 V1P1 80 80 41.8 52.6 1018.7 34.5 50.8 1016.8 V1P2 80 80 39.7 49.2 1000.5 33.8 47.5 988.6 V1P3 80 80 43.3 37.9 960.7 32.6 27.5 930.7 V2P1 100 100 42.0 56.7 920.7 34.1 38.8 904.7 V2P2 100 100 34.8 35.0 873.1 32.6 31.9 850.7 V2P3 100 100 34.8 28.0 835.7 24.7 24.5 820.6 V3P1 150 150 42.9 53.3 800.7 33.0 38.1 782.7 V3P2 150 150 33.7 40.0 760.6 28.0 26.7 735.7 V3P3 150 150 30.8 30.2 765.6 27.6 28.0 700.7 P1P1 80 80 81.0 160.8 1626.7 73.8 155.8 1623.9 P1P2 80 80 82.1 134.2 1610.6 54.0 130.0 1600.7 P1P3 80 80 64.2 106.7 1585.8 46.4 71.75 1530.1 P2P1 100 100 77.6 124.2 1500.8 65.5 112.5 1480.5 P2P2 100 100 55.3 98.3 1470.5 45.3 69.7 1430.6 P2P3 100 100 56.0 60.4 1410.6 36.2 49.8 1360.6 P3P1 150 150 63.8 65.8 1340.6 42.2 36.0 1320.6 P3P2 150 150 52.7 49.0 1310.4 45.9 47.0 1285.7 P3P3 150 150 47.3 48.0 1271.7 45.2 40.7 1225.8
The treatment reduced the fabric air permeability significantly, as is apparent in Fig. 4.21, which may be attributed to increase in fabric compactness, consolidation and the density of the fabric.
170
(a)
(b)
(c)
Figure 4.21 :Air permeability of the various fabrics (a) polyester, (b) cotton and (c) viscose
The air permeability of polyester fabrics was higher than that of the viscose and cotton fabrics, at a given fabric weight and water jet pressure. This may be due to the lower fibre packing density in the polyester fabrics, offering less resistance to the passage of air
171 compared to the viscose and cotton fabrics. As could be expected, the heavier fabrics
(150g/m2) had a lower air permeability than the lighter fabrics (80g/m2).
From Table 4.22 and Fig. 4.21 it can be seen that light weight fabrics (80g/m2), that were mildly hydroentangled at a waterjet pressure of 60 bars, have a higher air permeability than the heavier fabrics (100g/m2 and 150g/m2), hydroentangled at higher waterjet pressures
(100bars and 120bars). This trend is found in all the fabrics (cotton, viscose and polyester) and is evident in Table 4.22 and Fig.4.21. Therefore, light weight fabrics (80g/m2) hydroentangled at waterjet pressure of 60 bars are most favourable for use in this study.
4.6.4.2 Statistical analysis for air permeability
Analysis of variance (ANOVA) was conducted to determine if there were any statistically significant differences in the air permeability, due to treatment, fibre type and water jet pressure and their interactions. The results of the analysis (Table 4.25) show that there were indeed significant differences due to all the above variables, the only exception being the interaction between waterjet pressure and treatment which was nonsignificant (P=0.11197).
Further analysis, involving the various fabric types, confirmed the non-significant effect of the interaction between waterjet pressure and fabric treatment on air permeability.
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Table 4.25 : ANOVA of air permeability
Effective hypothesis decomposition; Std. Error of Estimate: 1.443770 SS Degr. of MS F P Freedom Intercept 45638.1 1.0 45638.1 21894.3 0.00000 Water Jet pressure 2413.4 2.0 1206.7 578.9 0.00000 Fibre type 14777.0 8.0 1847.1 886.1 0.00000 Treatment 329.2 1.0 329.2 157.9 0.00000 Water Jet pressure*Fibre type 1205.1 16.0 75.3 36.1 0.00000 Water Jet pressure*Treatment 9.2 2.0 4.6 2.2 0.11197 Fibre type*Treatment 82.0 8.0 10.2 4.9 0.00001 Water Jet pressure*Fibre 176.0 16.0 11.0 5.3 0.00000 type*Treatment Error 450.2 216.0 2.1
4.6.4.2 Water vapour permeability (WVP)
Water vapour transmission or permeability, is the rate at which water vapour diffuses through a fabric. The moisture transport, from the skin to the outer environments through a fabric, often referred to as the breathability of the fabric, is an important factor in determining comfort. In general, the higher the rate of moisture vapour transport, the better the comfort properties, which has a direct implication on the end-use applications of the fabric. Table 4.24 shows that the water vapour permeability of both the treated and untreated polyester fabrics are higher than those of the viscose and cotton fabrics, with the water vapour permeability of the treated samples consistently slightly lower than that of the untreated fabrics for all the fabric types as also illustrated in Fig. 4.22.
173
(a)
(b)
(c) Figure 4.22 : Effect of fabric type and weight on water vapour permeability of (a) polyster, (b) cotton and (c) viscose fibres
174
The effects of waterjet pressure and finishing treatment on the water vapour permeability are shown in Fig. 4.23 from which it is apparent that the water vapour permeability decreased with an increase in waterjet pressure and is also reduced by the finishing treatment.
Figure 4.23 : Effect of waterjet pressure and treatment on water vapour permeability
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The effects of waterjet pressure, fabric type and treatment on water vapour permeability are illustrated in Fig. 4.24.
Figure 4.24 : Effect of waterjet pressure, treatment and fabric weight on water vapour permeability
It is clear that the various fabrics (untreated and treated) differ in their water vapour permeability, which can be explained by the moisture vapour transmission mechanism [352].
When vapour transmits through a textile layer, two processes are involved, namely, diffusion and sorption-desorption. Water vapour diffuses through a textile structure by simple diffusion through the air spaces between the fibres and along the fibres. At a specific concentration gradient, the diffusion rate through the textile materials depends on the porosity of the material and also on the water diffusivity of the fibres, the latter being higher for fibres with a higher moisture regain. On the other hand, water vapour permeability increases with the increase in hydrophilicity of the material. A hygroscopic material can
176 absorb water vapour from the humid air close to the skin, or in direct contact with the skin, and then releases it in less humid dry air. The speed of this process greatly influences the thermophysiological comfort, by offering quicker absorption of perspiration from the skin, and leaving it dry.
A non-hygroscopic fibre does not absorb moisture as readily and at the same rate as a hygroscopic fibre, but can spread the absorbed liquid on the outer surface of the fibre and fabric due to its wicking property.
Considering the water vapour transmission mechanism, therefore, one would expect that cotton and viscose fabrics, since they are hygroscopic fibres, should achieve higher water vapour transmission rates than the polyester fabrics, but this is not the case, as shown by our experimental results. The higher water permeability of the polyester fabrics, compared to the viscose and cotton fabrics, may be due to their higher pore size as well as their lower density.
It was also observed that increasing fabric weight and water jet pressure resulted in a decrease in water vapour permeability, due to a reduction in the pore size as shown in Table
4.22, in which the pore sizes of the cotton and viscose fabrics were made lower, with the exception of the low weight fabrics which were hydroentangled at the lower pressure of 60 bars. Hence, low weight fabrics of 80 g/m2, which were hydroentangled at a low water jet pressure of 60 bars, were suitable for use in this study due to their higher water vapour permeability.
4.6.4.2 Statistical analysis of water vapour permeability
A two-way analysis of variance (ANOVA) was conducted to examine which parameters and interactions have a statistically significant effect on water vapour transmission. The analysis showed (Table 4.25) that waterjet pressure, fibre type, and fabric treatment, as well as their interactions, all affected the water vapour transmission statistically.
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Further analysis, involving all the fibre types produced similar results, which confirmed that the higher the waterjet pressure, the lower the water vapour permeability.
Table : 4.26 ANOVA of water vapour permeability
Effective hypothesis decomposition; Std. Error of Estimate: 3.166341 SS Degr. Of MS F P Freedom Intercept 332965635.0 1.0 332965635.0 33211143.8 0.00000 Water Jet pressure 379322.9 2.0 189661.4 18917.5 0.00000 Fibre type 19485560.5 8.0 2435695.1 242944.6 0.00000 Treatment 33872.2 1.0 33872.2 3378.5 0.00000 Water Jet pressure*Fibre type 30877.8 16.0 1929.9 192.5 0.00000 Water Jet pressure*Treatment 4621.1 2.0 2310.5 230.5 0.00000 Fibre type*Group 2901.8 8.0 362.7 36.2 0.00000 Water Jet pressure*Fibre 6314.9 16.0 394.7 39.4 0.00000 type*Treatment Error 2165.6 216.0 10.0
4.7 SELECTION OF MATERIALS SUITABLE FOR SURGICAL GOWNS AND
DRAPES.
The selection of materials suitable for surgical gowns, drapes and laboratory coats were based on the antibacterial, liquid barrier and breathability properties of the fabrics. Since there was an increase in the tensile strength of the treated fabrics, there was no evidence of damage or weakness of any of the samples. A summary of the relevant fabric properties is given in Table 4.27.
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Table 4.27 : Summary of relevant fabric parameters
Nomin Antibacterial Water al Waterjet Advancing activity Air vapour Sample Fabric pressure contact (Reduction in %) permeability permeability ID weight (bars) angle (Өa) (ml/s/cm2/98Pa) (g/m2/day) (g/m2) S.aureus E.coli C1P1 80 60 95.9 98.2 96.7 1240.3 88.7 C1P2 80 100 94.3 99.6 93.7 1180.0. 55.7 C1P3 80 120 96.8 99.5 93.8 1083.6 35.8 C2P1 120 60 94.7 98.8 86.3 1045.6 37.9 C2P2 120 100 96.0 96.7 78.4 1000.8 20.9 C2P3 120 120 94.0 98.1 74.3 965.1 17.7 C3P1 150 60 93.3 75.1 89.2 923.6 24.0 C3P2 150 100 95.0 96.6 96.6 875.6 16.4 C3P3 150 120 95.9 90.4 47.4 814.9 15.0 V1P1 80 60 91.8 98.7 95.2 1016.8 50.8 V1P2 80 100 92.4 98.5 65.1 988.6 47.5 V1P3 80 120 93.3 98.6 86.2 930.7 27.5 V2P1 120 60 90.4 98.5 85.5 904.7 38.8 V2P2 120 100 93.4 98.6 82.8 850.7 31.9 V2P3 120 120 93.1 98.5 81.8 820.6 24.5 V3P1 150 60 91.8 99.5 98.8 782.7 38.1 V3P2 150 100 92.5 95.7 80.9 735.7 26.7 V3P3 150 120 92.6 99.5 78.5 700.6 28.0 P1P1 80 60 107.1 99.5 99.6 1623.9 155.8 P1P2 80 100 109.7 99.2 99.5 1600.7 130.0 P1P3 80 120 113.2 99.1 99.4 1530.1 71.8 P2P1 120 60 110.3 99.2 98.1 1480.5 112.5 P2P2 120 100 114.2 99.5 99.5 1430.6 69.7 P2P3 120 120 116.5 99.4 99.6 1360.6 49.7 P3P1 150 60 108.1 99.5 99.5 1320.6 36.0 P3P2 150 100 113.6 99.5 99.4 1285.7 47.0 P3P3 150 120 112.9 99.5 99.1 1225.7 40.7
The results in Table 4.27 show that many of the samples satisfied the barrier requirements of resistance to bacteria (S. aureus and E. Coli) and water repellence, with above 90% bacterial reduction and 90o advancing contact angle respectively. It was observed that, as expected, the air permeability and water vapour permeability were higher for the lightest fabric
(80g/m2), hydroentangled at a water jet pressure of 60 bars, both decreasing considerably with an increase in fabric weight and water jet pressure, for all fabrics.
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To further assess the suitability of the fabrics, for the intended end uses, the water vapour and air permeability obtained for the treated fabrics were compared with those for the corresponding untreated fabrics, and summarised in Table 4.28.
Table 4.28 : Effect treatment on air permeability and water vapour premeability
Air permeability (ml/s/cm2/98Pa) Water vapour permeability (g/m2/day) Sample Untreated Treated Reduction Untreated Treated Reduction ID due to due to treatment treatment(%) (%) C1P1 91.2 88.8 2.6 1247.9 1240.3 0.6 C1P2 60.8 55.7 8.4 1184.4 1179.0 0.5 C1P3 50.3 35.8 28.8 1093.2 1083.6 0.9 C2P1 44.2 37.9 14.3 1055.2 1045.6 0.9 C2P2 26.7 20.9 21.7 1019.5 1000.8 1.8 C2P3 20.8 17.7 14.8 990.0 965.1 2.5 C3P1 27.5 24.0 12.7 939.7 923.6 1.7 C3P2 24.2 16.4 32.2 900.6 875.6 2.8 C3P3 23.8 15.0 37.0 837.5 814.9 2.7 V1P1 52.6 50.8 3.4 1018.7 1016.8 0.2 V1P2 49.2 47.5 3.5 1000.5 988.6 1.2 V1P3 37.9 27.5 27.4 960.7 930.7 3.1 V2P1 56.7 38.8 31.6 920.7 904.7 1.7 V2P2 35.0 31.9 8.8 873.1 850.7 2.6 V2P3 28.0 24.5 12.5 835.7 820.6 1.8 V3P1 53.3 33.0 38.1 800.6 782.7 2.2 V3P2 40.0 26.7 33.2 760.6 735.7 3,3 V3P3 30.2 28.0 7.3 765.6 700.7 8.5 P1P1 160.8 155.8 3.1 1626.7 1623.9 0.2 P1P2 134.2 130.0 3.1 1610.6 1600.7 0.6 P1P3 106.7 71.8 32.7 1585.8 1530.2 3.5 P2P1 124.2 112.5 9.4 1500.8 1480.5 1.4 P2P2 98.3 69.7 29.1 1470.5 1430.6 2.8 P2P3 60.4 49.7 17.7 1410.6 1360.6 3.5 P3P1 65.8 36.0 45.3 1340.6 1320.6 1.5 P3P2 49.0 47.0 4.1 1310.4 1285.7 1.9 P3P3 47.9 40.7 15.0 1271.7 1225.8 3.6
It is seen from table 4.28 that mainly the less dense and lower weight fabrics produced at low waterjet pressures have lower reduction in pore size after treatment, thus resulting in higher water vapour permeability and air permeability. Taking everything into consideration, the
180 fabrics listed in Table 4.29 can be regarded as suitable for surgical gowns, drapes and laboratory coats as they meet all the requirements with regard to resistance to bacteria, liquid barrier protection and breathabiliy.
Table 4.29 : Selected suitable fabrics and corresponding processing parameters
Sample Fabric type Fabric Waterjet Advancing Antibacterial Air Water vapour ID weight pressure contact activity permeability permeability (g/m2) (bars) angle (% Reduction) (ml/s/cm2/98P (g/m2/day) (Өa) S.aureus E.coli a)
C1P1 Cotton 80 60 95.9 98.2 96.7 88.8 1240.3
C1P2 Cotton 80 100 94.3 99.6 93.7 55.7 1179.0
V1P1 Viscose 80 60 91.8 98.7 95.2 50.8 1016.8
P1P1 Polyester 80 60 107.1 99.5 99.6 155.8 1623.9
P1P2 Polyester 80 100 109.7 99.2 99.5 130.0 1600.7
P2P1 Polyester 100 60 110.3 99.2 98.1 112.5 1480.5
This study confirms that greige (unscoured and unbleached) cotton fabrics can indeed be used for disposable surgical gowns against the widely held opinion that scoured and bleached cotton fabrics are a necessity because of the wax content and other impurities which would interfere with the antibacterial and water repellent finishes. This offers cost advantage for the final product since the bleached cottons which are presently used in a few nonwoven products are considered less economical compared to other competing fibres like polyester and polypropylene.
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5. SUMMARY AND CONCLUSIONS
Currently, there is great interest in protecting healthcare workers (doctors, nurses, laboratory workers) against the spread of bacteria from patients to them, and against cross-infections resulting from exposure to bacteria and fluids that potentially contain blood-borne pathogens.
Therefore, surgical gowns should have water/liquid repellent and antibacterial properties, not only to reduce hospital-acquired infections of patients, but also to protect surgical staffs from infectious fluids. They should, however, also have sufficient breathability so as to be comfortable to wear.
In this study, the optimum processing conditions and parameters for producing hydroentangled nonwoven cotton, viscose and polyester fabrics, best suited for application in disposable and protective wear for surgical gowns, drapes and laboratory coats, have been established. Of particular importance was the investigation of greige cotton fabric (unscoured and unbleached), previously considered unsuitable, because of the possible interference of waxes and other impurities with antibacterial and water repellent finishes. This study has shown that greige cotton can, indeed, be utilized for surgical gowns, with adequate and careful cleaning and suitable combination of mechanical processing parameters. The use of single bath application for the antibacterial and water repellent chemicals, and the savings that they ensure by not having to scour and bleach the cotton fabrics, make this option a cost-saving proposition.
In order to impart antibacterial and water repellent properties, Ruco-Bac AGP and
Ruco-Coat FC9005 chemicals were applied from a single bath respectively, by the pad-dry- cure process, to 100% viscose, cotton and polyester hydroentangled nonwoven fabrics of varying basis weights and produced at different waterjet pressures. Evaluation of the antibacterial activity against Staphylococcus aureus and Escherichia coli was based on the
182 agar diffusion technique, water repellency was based on the standard spray test ratings and wettability on dynamic contact angle measurement.
Antibacterial activity against Gram-positive Staphylococcus aureus was found to be greater than that against the Gram-negative Escherichia coli. The lower antibacterial activity against the gram-negative bacteria was attributed to an additional outer membrane structure in the cell wall which acts as an additional barrier to the antimicrobial agent. In contrast to this, Gram-positive bacteria have a simple cell wall structure, in which the membrane has a rigid peptidoglycan layer, composed of networks with plenty of pores, which allow foreign molecules to enter the cell wall without any difficulty.
The colony forming units (CFU)/swatch at time zero and after a period of 24hrs, as well as percentage reduction in bacteria, showed good antibacterial properties against both the
Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria. The percentage reduction for E. aureus at 24hrs contact time ranged from 99.1 to 99.5%, for the polyester fabrics, and that for E.coli bacteria from 99.1 to 99.6%. The corresponding values for the viscose fabrics ranged from 95.7 to 99.5% and from 80.9 to 98.8%, respectively, and those for the cotton fabrics, ranged from 90.4 to 99.6 % and 86.3 to 96.7%, respectively.
The outstanding performance of the antimicrobial agents on the polyester fabrics compared to that on the viscose and cotton fabrics may be due to the differences in the fabric structures. The polyester fabrics, though hydrophobic, had a lower density and were therefore bulkier and less compact, with a higher porosity than the viscose and cotton fabrics. In general, the antimicrobial agent used in this study was considered efficient in imparting antimicrobial properties to all three fabric types studied.
The standard spray test ratings, used to assess the water repellency of the fabrics, ranged from 80 to 90%, for the three fabric types, and it could therefore be concluded that the water repellency of the fabrics was, suitable for the intended end-use.
183
The contact angles of the untreated cotton and viscose fabrics were assigned a value of zero as it was not possible to measure their contact angles, due to their high wettability owing to immediate absorption of water upon contact with the fabric surface. This is due to both the hydrophilicity of these fibres and to the capillary effect of the porous fabric structure. After the water repellent finish, however, the same fabric structure showed higher contact angle values, which indicate that the treatment had changed their surface properties from hydrophilic to hydrophobic. In all cases, the advancing angle was greater than the receding angle, the hysteresis being the difference between the advancing and receding angles. The polyester fabrics had the largest advancing contact angle, ranging from 107. 1 to 116.5 degrees, followed by the cotton fabrics ranging from 93.3 to 96.8 degrees, with those of the viscose fabrics, the lowest, from 90.4 to 93.4 degrees.
In terms of hysteresis, the polyester fabrics had the lowest values, ranging from 9.1 to 11.3 degrees, followed by cotton, which ranged from 39.6 to 40.9 degrees, with that of the viscose the highest, ranging from 46.7 to 48.8 degrees. From the above results, it follows that the polyester fabrics were the most difficult to wet, with the viscose and cotton fabrics wetting out quite easily. These differences are largely attributable to the differences in fibre structures.
According to the tensile tests on the fabrics, the strength in the cross - machine direction (CD) was always higher than that in the machine-direction (MD), for all samples.
This was as expected, due to the anisotrotropic nature of the carded web, resulting from the practice of producing nonwoven fabrics by web formation and cross-lapping, the preferential orientation and allignment of fibres, by cross lapping in the cross – machine direction (CD), providing higher strength in the cross-machine direction than in the machine direction. This difference was statistically significant.
184
It was found that the application of antibacterial and water repellent treatments increased the tensile strength and reduced the extension at break of the fabrics in both the CD and MD directions.
The lower extension at break of the treated fabrics, compared to that of the untreated fabrics may be attributed to the existence of a film on the surface of the treated fabrics, leading to increased inter-fibre friction and cohesion. It was found that an increase in the waterjet pressure increased the fabric tensile strength but reduced the extension at break,
The waterjet pressure had a great effect on the pore structure of all the fabric types, the higher the waterjet pressure, the lower the pore size, at a given fabric weight. It was found that, for a given weight, the cotton fabrics exhibited the least variation in pore size followed by the viscose fabrics, with the polyester fabrics having the largest variation in pore size. These variations are accounted for by their varying fabric densities. The treated fabrics had a smaller variation in pore size than the untreated fabrics, the difference being large for heavier fabrics produced at higher waterjet pressures, and not significant for the lighter fabrics, and those produced at the low waterjet pressures.
It was found that the air permeability and water vapour permeability of the light fabrics (80g/m2), produced at a low waterjet pressure (60 bar), were not affected by the treatment, the differences being statistically insignificant. The fabric in this group had a high air and water vapour permeability, due to their relatively low number of fibres per unit cross- sectional area readily allowing the passage of air and vapour through the fabrics.
The polyester fabrics generally had the highest rate of water vapour permeability and air permeability due to their greater porosity and bulk. An increase in fabric weight was associated with an increase in fabric thickness and density and consequently with a decrease in water vapour and air permeability. An increase in waterjet pressure decreased the thickness, air permeability and water vapour permeability, and increased the fabric density.
185
This study demonstrated the effect of fibre type, and the interaction between fibre type and waterjet pressure, on fabric tensile properties, and therefore bonding during the hydroentanglement process. Therefore, when designing functional fabric structures, it is desirable to have reliable structure-process-property relationships for hydroentangled fabrics which link the hydroentanglement process parameters to the fabric properties, to enable hydroentangled fabrics to meet the requirements of a specific product.
Since it was found that the properties of the hydroentangled fabrics depend on both the fibre properties and the waterjet pressures, a process-structure-property relationship will provide a tool to manipulate the hydroentangled fabric structure to achieve satisfactory product performance, by both eastablishing the appropriate manufacturing parameters and fibre specifications.
It is pertinent to state that among the 27 fabrics tested and evaluated, only 6 met the requirements for surgical gowns, nurses uniforms, drapes and laboratory coats, based on antibacteria barrier and breathability, air permeability, water vapour permeability, resistance to wetting (contact angle), waterjet pressure and fabric weight, as shown in Table 4.29.
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5.1 RECOMMENDATIONS FOR FUTURE WORK
1) It was found that the processing conditions, most favourable for the end-use under investigation, were a fabric weight of 80g/m2 and a 60 bar waterjet pressure. Further investigations should explore even lighter fabrics (g/m2) produced at lower waterjet pressures, using the same fibres (polyester, cotton and viscose) as here.
2) Further work on the thermal insulation properties, perhaps using thermal manikins, is also recommended, since they are essential in the area of surgical clothing, in which comfort is of paramount importance.
3) In this study, one type of gram-positive bacteria (S. aureus) and one type of gram-negative bacteria (i.e. E .coli) were examined, Many forms of bacteria, however, are found in the healthcare environment, and further studies are necessary, to determine how well the antibacterial agents used here reduce other microorganism, such as K. pneumonia.
4) There are a great number of natural antibacterial agents (e.g. extracts from tulsi leaf,
Aloe vera, pomegranate, etc.) which can be used for imparting useful antibacterial properties to textile fabrics, and although, many efforts have been made to exploit these eco-friendly natural products for textile applications, very few of the studies involved a systematic and in-depth investigation. The extraction, isolation and purification of these natural products to produce standardized products represent other challenges in terms of their application. The precise mechanism of the bactericidal action of the different natural antimicrobial agents is still largely unknown, while the attachment of the bioactive substance to different types of complex textile substrates for longer durability of the antibacterial activity, is also an area requiring thorough research.
187
5) Having established that 100% greige/virgin (unscoured and unbleached) cotton fabrics can be used for surgical gowns and related medical use, it is necessary to investigate the influence of different cotton varieties on moisture absorption, as related to fabric moisture vapour transport rate (MVTR), with a view to identifying, or even developing, new cotton varieties that have unique properties related to comfort and end-use applications, such as outdoor performance clothing. This could further enhance the competitiveness of cotton against polyester, the latter becoming increasingly popular.
6) Considering the effect of waterjet pressure on fabric properties observed in this study, further investigation is necessary to determine the minimum waterjet jet pressure that provides the highest strength for a given fabric weight, and the critical values of fabric weight/waterjet pressure at which the reverse takes place. This would be beneficial to the industry, in terms of reducing the energy cost of producing hydroentangled nonwoven fabrics to the absolute minimum.
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APPENDICES
APPENDIX A-Determination of fabric weight and thickness
SAMPLE ID WEIGHT (g/m2) THICKNESS (mm) 80 0.52 83 0.539 78 0.507 84 0.546 C1P1 80 0.52 81 0.52
78 0.411 C1P2 81 0.458 79 0.446 86 0.486 82 0.463
C1P3 84 0.433 77 0.397 85 0.438 70 0.361 80 0.412
122 0.714 118 0.69 120 0.702 C2P1 122 0.714 116 0.679
120 0.646 126 0.678 C2P2 119 0.641 124 0.668 117 0.63 120 0.581
218
122 0.591
C2P3 116 0.562 2 119 0.576 126 0.61
154 0.936 C3P1 148 0.9 150 0.912 1 152 0.924 147 0.894
152 0.771 C3P2 156 0.791 146 0.741 1 150 0.761 144 0.731
151 0.695 155 0.713 150 0.69 C3P3 146 0.694 2 148 0.69
219
APPENDIX B-Determination of mean pore size and air permeability
SAMPLE ID MEAN PORE SIZE (µm) AIR PERMEABILITY (ml/s/cm2/98Pa) 41.265 21 33.325 20.5 39.001 23 31.859 22
C1P1 33.55 20 19.057 12 46.704 10 28.024 11
C1P2 20.251 10 38.056 12
24.444 9 22.15 7 C1P3 24.21 8 21.23 10 22.19 9
26.121 9 32.21 10 29.903 8.5 C2P1 22.909 8 32.43 10
22.216 4 18.595 5.6 22.225 5.5 C2P2 22.217 5.8 23.726 5
30.054 4.6 20.61 4.4 28.024 4 20.2512 4 C2P3 20.251 4.2
220
34.294 6.2 30.389 6 26.682 7.5 C3P1 29.234 4 25.113 5
25.234 3.83 21.25 3.12 22.068 4.88 C3P2 23.961 4 25.036 3.82
34.513 3.6 17.572 4.02 16.927 3.2 C3P3 21.999 3.02 19.057 4
36.049 11 40.164 12 37.231 13 V1P1 36.442 15 40.357 10
32.752 11 31.01 12 40.382 13 33.419 10 V1P2 35.243 11
29.222 6.2 33.429 7.5 V1P3 25.262 6 39.152 6.8 35.902 6.5
221
35.0628 9 35.1832 8.2 V2P1 34.1904 9.4 32.1329 10 34.0014 10
33.8251 8 27.5252 7.5 V2P2 35.3277 8 35.8488 7.5 30.3395 7.3
23.6421 5.5 29.7129 6.3 V2P3 26.334 5.8 26.3204 4.8 24.6708 6
36.348 9 33.4234 8 V3P1 29.8212 10.2 34.684 9 30.5284 9.5
27.9512 7 27.6618 6.9 26.6174 5.6 V3P2 28.7808 6 28.8055 6.5
29.838 5.9 24.065 5.8 28.262 5.7 V3P3 29.176 5.5 26.525 5.6
80.8333 36 73.001 37 80.241 33 P1P1 72.75 36 80.968 35
222
64.817 29 60.455 33 P1P2 89.241 30 70.647 30 45.58 34
44.034 19.3 49.363 18.3 P1P3 45.604 19.5 45.604 15 44.99 14.2
83.779 29.6 78.861 28 78.817 25.8 P2P1 75.381 26.8 71.274 28
46.352 17.5 49.213 18 47.345 13.5
[ P2P2 50.389 16.2 33.352 15
40.241 12.8 38.315 11.2 42.424 11.2 P2P3 39.25 12 35.62 12.5
48.648 14.4 44.906 13.6 39.613 15.2 P3P1 40.208 12.1 38.212 12.3
223
42.158 11 47.639 11 53.036 11.5 37.916 12 P3P2 48.861 12
48.253 6.5 44.34 9 P3P3 41.065 13.8 43.505 11.5 49.041 9
224
APPENDIX C-Determination of fabric wettability
SAMPLE ID CONTACT ANGLE (Ө) P1P1 104.8 110.69 105.69
P1P2 115.92 100.5 112.63
P1P3 110.64 112.2 116.84
P2P1 114.56 100.92 115.3
P2P2 119.72 110.68 112.16
P2P3 114.8 116.45 113.3
P3P1 110.79 100.94 112.42
P3P2 110.75 115.66 112.59
P3P3 109.4 116.72 112.58
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APPENDIX B-Determination of mean pore size and air permeability
SAMPLE ID MEAN PORE SIZE (µm) AIR PERMEABILITY (ml/s/cm2/98Pa) 41.265 21 33.325 20.5 39.001 23 31.859 22
C1P1 33.55 20 19.057 12 46.704 10 28.024 11
C1P2 20.251 10 38.056 12
24.444 9 22.15 7 C1P3 24.21 8 21.23 10 22.19 9
26.121 9 32.21 10
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29.903 8.5 C2P1 22.909 8 32.43 10
22.216 4 18.595 5.6 22.225 5.5 C2P2 22.217 5.8 23.726 5
30.054 4.6 20.61 4.4 28.024 4 20.2512 4 C2P3 20.251 4.2
34.294 6.2 30.389 6 26.682 7.5 C3P1 29.234 4 25.113 5
25.234 3.83 21.25 3.12 22.068 4.88 C3P2 23.961 4 25.036 3.82
34.513 3.6 17.572 4.02 16.927 3.2 C3P3 21.999 3.02 19.057 4
36.049 11 40.164 12 V1P1 37.231 13
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36.442 15 40.357 10
32.752 11 31.01 12 40.382 13 33.419 10 V1P2 35.243 11
29.222 6.2 33.429 7.5 V1P3 25.262 6 39.152 6.8 35.902 6.5
35.0628 9 35.1832 8.2 V2P1 34.1904 9.4 32.1329 10 34.0014 10
33.8251 8 27.5252 7.5 V2P2 35.3277 8 35.8488 7.5 30.3395 7.3
23.6421 5.5 29.7129 6.3 V2P3 26.334 5.8 26.3204 4.8 24.6708 6
36.348 9 33.4234 8 V3P1 29.8212 10.2 34.684 9
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30.5284 9.5
27.9512 7 27.6618 6.9 26.6174 5.6 V3P2 28.7808 6 28.8055 6.5
29.838 5.9 24.065 5.8 28.262 5.7 V3P3 29.176 5.5 26.525 5.6
80.8333 36 73.001 37 80.241 33 P1P1 72.75 36 80.968 35
64.817 29 60.455 33 P1P2 89.241 30 70.647 30 45.58 34
44.034 19.3 49.363 18.3 P1P3 45.604 19.5 45.604 15 44.99 14.2
83.779 29.6 78.861 28 78.817 25.8 P2P1 75.381 26.8 71.274 28
46.352 17.5 49.213 18
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47.345 13.5
[ P2P2 50.389 16.2 33.352 15
40.241 12.8 38.315 11.2 42.424 11.2 P2P3 39.25 12 35.62 12.5
48.648 14.4 44.906 13.6 39.613 15.2 P3P1 40.208 12.1 38.212 12.3
42.158 11 47.639 11 53.036 11.5 37.916 12 P3P2 48.861 12
48.253 6.5 44.34 9 P3P3 41.065 13.8 43.505 11.5 49.041 9
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APPENDIX C-Determination of fabric wettability
SAMPLE ID CONTACT ANGLE (Ө) P1P1 104.8 110.69 105.69
P1P2 115.92 100.5 112.63
P1P3 110.64 112.2 116.84
P2P1 114.56 100.92 115.3
P2P2 119.72 110.68 112.16
P2P3 114.8 116.45 113.3
P3P1 110.79 100.94 112.42
P3P2 110.75 115.66 112.59
P3P3 109.4 116.72 112.58
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