DYEING AND FINISHING OF SILK: TREATMENT OF SILK TO IMPROVE WRINKLE RECOVERY AND DYE RESIST BEHAVIOUR
Thesis is submitted to The University of New South Wales for the degree of Master of Science
by I Nyoman Supriyatna
School of Fibre Science and Technology The University of New South Wales 1991 DECLARATION
I certify that the work described in this thesis was performed by me in the school of Textile Technology at the University of New South Wales, and has not been submitted previously for any other university degree or award.
I.N. Supriyatna.
i ACKNOWLEDGEMENTS
I would like to express my sincere gratitude thanks to my supervisor DR. S.K. David for her patient guidance during the course of this thesis.
I also would like to thank to Associate Professor M. T. Pailthorpe for his advice and encouragement.
My thanks are also due to other postgraduate students and the staff of the school of Textile Technology for their assistance in many ways.
Further, I would like to thanks to my wife, N.M. Widiari, my daughter, P. Tania Sari and my parents for their encouragement.
Finally, I am pleased to acknowledge to the Commonwealth Government of Australia for the provision of an AIDAB fellowship and to the Indonesian Government through Ministry of Industry for granting the necessary study leave.
ii ABSTRACT
This thesis is divided into two sections. The first section examines the wrinkle recovery behaviour of silk fabrics with respect to fabric structure, crosslinking treatments and surface polymer application. Changes in wrinkle recovery were measured with the Thermobench and Shirley Crease Recovery methods. It was found that the weave-type of the silk fabrics had a significant effect on their wrinkling behaviour. A crosslinking treatment utilising butane tetracarboxylic acid resulted in only marginal improvement in dry wrinkle recovery, while surface polymer application with silicone elastomers resulted in more significant improvements in dry wrinkle recovery. Both types of treatments were found to contribute to improvements in wet wrinkle recovery behaviour of the treated fabric.
The second section of this thesis describes the dye resist behaviour of silk fabric treated with the reactive compounds, sulphamic acid and Sandospace R, with respect to the uptake of acid, metal complex and reactive dyestuffs. Sulphamic acid treated silk exhibited excellent dye resist behaviour toward all three classes of dyes, even at low treatment levels of this resist agent. High treatment levels of Sandospace R were required, however, for a significant resist effect to be conferred on the treated silk. The breaking strength,
iii yellowness index and subjective handle of the treated fabrics were compared.
iv CONTENTS
page
Chapter 1 The Silk Fibre
1.1 Introduction 2
1.2 Chemistry of Silk 5
1.2.1 Morphology of Silk 5
1. 2. 2 Chemical Composition of Silk 5
1.2.3 Physical Properties of Silk 7
1. 2. 4 Chemical Properties of Silk 9
Chapter 2 Introduction to Wrinkling Behaviour 11
2.1 The Phenomenon of Wrinkling/Creasing 12
2.1.1 General 12
2.1.2 Mechanism of Wrinkle Recovery 12
2.1.3 Factors Affecting Wrinkle Recovery 14
2.1.3.1 Temperature and Relative Humidity 15
2.1.3.2 Fibre, Yarn and Fabric Parameters 16
2.1.3.3 Effect of Ageing/Annealing 17
2.1.3.4 Laboratory Wrinkling Test Methods 18
2.2 Wrinkling Resist Finishing Agents 20
2.2.1 Historical Development of Wrinkle Resist
Agents 20
2.2.2 Comparison of Wrinkle/Crease Resist
Agents Used for Cotton, Wool, and Silk 23
2.3 Objective Measurement of Fabric Handle 29
2.4 Aim of The Present Work 30
Chapter 3 Wrinkling of Silk: Materials and Methods 32
V 3.1 Materials 33
3.1.1 Silk Fabric 33
3.1.2 Chemicals 33
3.1.2.1 Butane Tetracarboxylic Acid 33
3.1.2.2 Silicone Polymers 33
3.1.2.3 Catalyst 34
3. 2 Experimental Methods 34
3.2.1 Bleaching of Silk Fabrics 34
3.2.2 Crease Resist Finishing 34
3.2.2.1 Treatment of Silk With Butane
Tetracarboxylic Acid (BTCA) 34
3.2.2.2 Treatment of Habutai Silk With
Ultratex ESU and EMJ 35
3.2.3 Crease Recovery Measurements 35
3.2.3.1 Thermobench Method 35
3.2.3.2 Shirley Crease Recovery Test
Method 36
3.2.3.2.1 Dry Crease Recovery 36
3.2.3.2.2 Wet Crease Recovery 37
3.2.4 Fabric Breaking Strength Measurements 37
3.2.5 Fabric Whiteness 37
3.2.6 Weight Gain 38
3.2.7 Laundering 38
Chapter 4 Wrinkling of Silk: Results and Discussion 39
4.1 Wrinkle Recovery of Silk Fabric-Various Weave
Types 40
vi 4.2 Treatment of Unbleached Twill Silk Fabric With
BTCA 41
4.2.1 Weight Gain 41
4.2.2 Wrinkle Recovery 42
4.3 Treatment of Various Bleached Fabrics With BTCA 44
4.4 Changes in Wet and Dry Wrinkle Recovery of
Bleached Habutai Silk Treated With a Variety of
Finishing Agents 45
4.5 Low Stress Mechanical Properties of Silk Fabric
Treated With BTCA, ESU and EMJ Finishing
Treatments 49
4.5.1 General 49
4.5.2 Bending Properties 49
4.5.3 Shear Properties 51
4.5.4 Compression and Surface Characteristics 52
4.5.5 Tensile Properties 54
4.5.6 Handle Evaluation 54
4.6 Conclusions 55
Chapter 5 Introduction to Dye Resist Effects 60
5.1 General 61
5.2 Dyeing of Silk 63
5.2.1 Acid Dyes 63
5.2.2 Metal Complex Dyes 65
5.2.3 Reactive Dyes 66
5.3 Reactive Dye Resist Agents 67
5.3.1 General 67
5.3.2 Sulphamic Acid 68
vii 5.3.3 Sandospace R 70
5.4 Aim of The Present Work 71
Chapter 6 Dye Resist Behaviour of Silk: Materials and
Method 73
6.1 Materials 74
6.1.1 Fabric 74
6.1.2 Chemicals 74
6.2 Methods 74
6.2.1 Treatment With Sulphamic Acid 75
6.2.1.1 Determination of Free Acid 75
6.2.1.2 Determination of Bound Acid 75
6.2.2 Treatment With Sando space R 76
6.2.3 Dyeing 76
6.2.3.1 Acid Dyeing 77
6.2.3.2 2:1 Metal Complex Dyeing 77
6.2.3.3 Reactive Dyeing 77
6.2.4 Resist Effect Evaluation 78
6.2.5 Test Methods
6.2.5.1 Weight Gain 79
6.2.5.2 Yellowness Index 79
6.2.5.3 Breaking Strength Retention 79
Chapter 7 Dye Resist Behaviour of Silk: Results and
Discussion 81
7.1 Treatment of Silk With Sulphamic Acid 82
7.1.1 Weight Gain, Yellowness Index and
Strength Loss 82
viii 7.1.2 Free and Bound Acid on Sulphamic Acid
Treated Silk 82
7.1.3 Dye Resist Effects of Sulphamic Acid
Treated Silk 84
7.2 Treatment of Silk With Sandospace R 86
7.2.1 Weight Gain, Yellowness Index and
Strength Loss 86
7.2.2 Dye Resist Effects of Sandospace R
Treated Silk 87
7.3 Conclusion 88
Chapter 8 Conclusions 90
Bibliography 93
ix CHAPTER ONE
THE SILK FIBRE 1.1 INTRODUCTION
Silk is considered to be a luxury fibre owing to its fine, soft handle, drape and comfortable next-to-skin qualities during wear. Silk fabric can also be very easily dyed or printed with a range of dyestuffs and thus lends itself well to fashion demands. One major limitation of this unique fibre, however, relates to the very poor fastness properties of bright coloured dyestuffs used in the dyeing and printing of silk fabrics. Another entirely different drawback associated with fabric usage arises from the tendency of silk fabrics to develop unsightly wrinkles during wear. As silk articles have generally been regarded as delicate and exclusive in quality, little attention has been given to these limitations associated with silk fabric usage.
A vast amount of research has been carried out in order to develop wrinkle resistant finishes for cotton, and more recently wool, whereas, relatively little work has been done on silk. Traditionally, formaldehyde utilising chemical treatments have been successful on silk, however, as a result of the rapid growth of prohibitive legislation concerning formaldehyde usage, non-formaldehyde treatments are now being explored for all fibre types.
2 In the first part of this thesis, chemical treatments that have been shown to be successful on cotton or wool, with respect to improving the wrinkle resistance of these fibre types, were applied onto silk fabric. The potential for both crosslinking treatments and surface polymer applications were separately investigated with respect to the wrinkle recovery behaviour imparted to the treated silk fabrics.
Multicolour effects that are often necessitated by fashion demands are generally achieved by fabric printing rather than dyeing; consequently, a large proportion of silk fabric is printed, either by direct or discharge methods.
The most commonly used dyestuffs are acid and metal complex dyes although the bright shades within these two ranges generally exhibit poor fastness properties. Moreover, very little published information is available on the printing or dyeing of silk, which remains very much a trade secret of printers and dyers specialising in this exclusive fibre.
An alternative to discharge or direct printing is offered through resist printing (or dyeing). This procedure requires the resist agent, which serves to retard the uptake of dyestuff, to be initially printed onto the fabric surface. The fabric may then be subsequently printed or
3 dyed in order to achieve tone-on-tone effects. Resist methods are well established in the printing of cellulosic and wool textile fabrics, and offer an attractive alternative permitting the use of a wider range of dyestuffs than those applicable to discharge styles, and with better fastness properties.
In the second part of this thesis dye resist agents that are commercially applied to wool fabric were applied to silk, and their capacity to retard the uptake of acid, metal complex, and reactive dyestuffs was examined.
The work described in this thesis, therefore, relates to two separate areas of research. Chapters 2, 3 and 4 deal with the wrinkling behaviour of silk fabric, while Chapters 5, 6 and 7 examine the dye resist effects imparted to silk fabric by the application of reactive resist agents which are already known to achieve dye resist effects on wool.
4 TABLE 1.1 CHEMICAL COMPOSITION OF WOOL AND SILK
Wool Keratin Silk Fibroin ( %) (%)
Amino acids 34 77 (non polar)
Basic side 10 3.4 chains
Acidic side 24 2.7 chains
Hydroxyl 22 17 groups
Sulphur 10 0.3 Fig.1.1 Longitudinal and cross sections of raw silk
Fig.1.2 Longitudinal and cross sections of degummed silk 1.2 CHEMISTRY OF SILK
1.2.1 Morphology of silk
Raw silk consists of two filaments of the protein fibroin, coated and cemented together by a second protein, sericin.
Several pairs of these filaments are formed into silk yarn by twisting and folding, and the silk yarn is usually woven or knitted before the sericin gum is removed by degumming in aqueous soap solution. Degummed silk filaments are fine, uniform and have a subdued lustre. Under the microscope, the cross section of silk fibres appear as pairs of irregular triangles, and longitudinally the filaments are fine and without any visible internal markings (Figs.1.1 and 1. 2) .
1.2.2 Chemical composition of silk
Silk fibre is composed of the protein fibroin, and exhibits some significant differences in its amino acid composition compared to the keratin protein of wool. An approximate indication of the chemical differences between fibroin and keratin is given in Table 1.1 [1].
Silk fibroin consists of a variety of amino acid residues, of which glycine, alanine and serine generally comprise more than half the total amino acid content [2]. Sericin,
5 TABLE 1.2
AMINO ACIDS IN FIBROIN OF BOMBYX MORI
Side chain of Moles of acid Amino acid the amino acid per 10 5 g
1. Glycine H - 567.2 2. Alanine CH3 - 385.7 3. Leucine (CH3)2CHCH2 - 6.2 4. Isoleucine CH3CH2 CH (CH3) - 6.9 5. Valine (CH3)2 CH - 26.7 6. Phenylalanine C6H5CH2 8.0 7. Serine CH20H- 152.0 8. Threonine CH3CH(OH) 12.5 9. Tyrosine HOQjH4CH2- 62.3 10. Aspartic acid HOOCCH2 - 17.6 11. Glutamic acid HOOCCH2CH2 - 11.8 12. Arginine NH 2 C(NH)NH(CH2)3- 5.6 13. Lysine NH2(CH2)4 - 3.8
14. Histidine N --CH 1.9 II ~C-CH2 - CH-- NH/
15. Praline CH2 --CIJi 5.1 I I CH2 CH-COOH
~NH/
16. Tryptophan ....._---CHi- 2.5 1 / / ···H-N c==O··· "'/ c=o ··· H-N "'/ R-C-H H-C-R \ "/ N-H ··· O=C / ···O==C N-H··· \ "/
Fig.1.3 Hydrogen bonding in silk. Adjacent chains run in opposite directions
I
C=O "'
Fig. 1.4 A Salt linkage between two polymers on the other hand, is far richer in polar amino acid residues with serine, glycine and aspartic acid accounting for approximately two-thirds of the total amino acid content of sericin [2]. Silk fibroin and sericin contain only a very small proportion of cystine, 0.23% of the total amino acid content, which is mostly destroyed during the degumming operation [2]. The almost complete absence of cystine in silk fibroin is one of the most important factors that distinguishes silk fibroin from wool keratin.
The amino acid analysis of Bombyx Mori fibroin by Schroeder and Kay [3] is given in Table 1.2.
X-Ray diffraction studies on Bombyx Mori silk have shown that it consists of extended polypeptide chains, lying parallel to the fibre axis, and are linked by hydrogen bonds between N-H and CO groups of adjacent chains. The adjacent chains are oppositely oriented as shown in Figure 1. 3, and the sheets are held together by hydrophobic interactions [4].
The small amino acid groups within the crystalline regions of the fibre allow close approach of parallel polypeptide chains, so that hydrogen-bonding may occur. Consequently,
H-bonding between adjacent chains, and salt-links formed between carboxyl and amine end groups play a very important role in maintaining the conformation and integrity of the
6 fibroin molecule [5] (Figures 1.3 and 1.4).
A series of amino acid sequencing studies carried out by several researchers [6-10] has led to the proposal of a three phase structure for the silk fibroin molecule. Phase
1 comprises the crystalline regions of fibroin (60% of the molecule) and consists of a 59-residue sequence: gly-ala gly-ala- (ser-gly (ala-gly) n> 8 -ser-gly-ala-gly-tyr, where n has a mean value of 2. Phase 2 consists of tetra and octa peptides, composed essentially of glycine, tyrosine, alanine, and valine, and comprises approximately 30% of the fibre. Phase 3 constitutes the remaining 10% and is largely amorphous, containing acidic amino acids and those with bulky side chains [11]. It is estimated that silk fibroin is approximately 65% crystalline and 35% amorphous
[ 12] .
1.2.3 Physical Properties of Silk
The silk filament is strong compared to other natural fibres due to the highly crystalline nature of its polymer system. The tensile strength and breaking extension of silk are 0.38 Newton/tex and 23.4%, respectively [2]. When the silk fibre is wet it loses some strength as a result of the disruption of hydrogen-bonds by water molecules (12].
7 Silk has a moderately high degree of elastic recovery, although due to its high degree of crystallinity, silk is considered to be more plastic than elastic [ 12] . The stress-strain behaviour of silk has been studied in detail
[13,14]. When tension is applied, initial deformation occurs in the amorphous regions of the polymer system, and complete elastic recovery results on release of the tension. However, if the tension is increased, interchain bonds are broken to the extent that polymer chains slide past each other, and do not return to their original position on release of the tension. If silk is stretched to near breaking extension (20% strain), the elastic recovery is approximately 33%. The elastic recovery is also known to vary depending on the moisture content of the fibre [2], where for small strains (1%), elastic recovery decreases with increasing humidity; however, for larger strains (>5%) elastic recovery increases with increasing moisture content.
Silk has a moisture regain of 9-10% at 65% RH, and is thus considered to be an absorbent fibre. It is, however, less absorbent than wool because of its more crystalline polymer system. Moisture absorption occurs within the more amorphous regions of the polymer system, hence fibres with a higher degree of crystallinity will allow penetration of only a limited number of water molecules into their polymer
8 network.
Silk is more sensitive to heat than wool. Peptide bonds, salt linkages and hydrogen bonds tend to deteriorate when heated to temperatures in excess of 100°c [12]. The presence of disulphide crosslinks in wool is considered to be at least partly responsible of its ability to withstand heat better than silk [12].
1.2.4 Chemical Properties of Silk
Silk fibroin is susceptible to peptide chain hydrolysis in solutions of acid or alkali. Unlike wool, the lack of disulphide crosslinks in silk renders the fibre much more prone to acid hydrolysis than wool. Concentrated acids and alkalis will completely hydrolyse silk, with the least degradation occurring between pH 4 and 8. Viscosity measurements in cuprammonium hydroxide indicate that the hydrolysis in alkali is greater than that in acid.
Dilute organic acids such as tartaric and citric acids are often used on a commercial basis for producing a rustling effect known as "scroop" [2].
The degradation of silk by oxidising agents such as hydrogen peroxide and peracids, is thought to originate at the side-chains of tyrosine, the amino-terminal residues
9 and the peptide bonds [15]. These oxidation reactions are not well understood, and the degree of degradation increases with increasing concentration of the oxidising agent, leading to significant strength loss of the fibre.
Peracids are generally more degradative than hydrogen peroxide. Hydrogen peroxide is most commonly used for the bleaching of silk.
Weighting with metal salts, such as stannic chloride, is carried out in order to supplement the loss in weight occurring during the degumming of silk. This procedure also imparts better handle and drape to silk fabric [16]. The weighting reaction is as follows:
SnC1 4 + 4HOH Sn (OH) 4 + 4HC1
----Sn (OH) 2 HP0 4 + 2NaOH
Silk is very sensitive to damage by sunlight, where exposure for 100 days to strong sunlight, can result in a loss of 95% of the original tensile strength [ 1 7] . Weighted silk is particularly sensitive to sunlight degradation because of its high metal content.
10 CHAPTER TWO
INTRODUCTION TO WRINKLING BEHAVIOUR
11 Polyester
Aged wool
45/65 wooVpolyester
De-aged wool
Nylon
Silk
Cotton
Viscose
Linen
0 W ~ W W Wrinkle recovery (%)
Fig. 2.1 Wrinkle recovery of textile fabrics 2.1 THE PHENOMENON OF WRINKLING/CREASING
2.1.1 General
The terms "creasing" and "wrinkling" are synonymously used, therefore both these terms deserve clarification. While "creasing" often refers to the deliberate "setting" of creases into a garment for aesthetic purposes, "wrinkling" always refers to the undesirable residual bending deformation of the fabric which occurs during wear. Hence, the phrase" ... the crease is the one that so quickly comes out while the wrinkle's the one that stays", aptly defines the difference between the two terms [18]. As the problem at hand involves the unsightly development of wrinkles during wear, the term "wrinkling" will be used most often, however, the common usage of the term "creasing" when referring to both cotton and silk fabrics renders the additional usage of this term unavoidable.
2.1.2 Mechanism of Wrinkle Recovery
Wrinkle or crease recovery can be defined as the ability of a fabric to spontaneously recover from a bending deformation [19,20]. A comparison of the wrinkle recovery of various textile fabrics under mild deformation conditions is presented in Figure 2.1 [18].
12 'I I I 'I I I 'I •I I I ' ' ' t ' I I ' t + I I + I t • • • • • • • ' • • • • • • • • Before
After
Fig.2.2 Schematic of a cross-linked structure before and after bending In order to explain the poor wrinkle recovery behaviour of cellulosic textiles, a simplistic model of deformation on a molecular level may be presented. When a fabric is placed under stress, polymer chains will undergo deformation to the extent that interchain forces within the molecular system may become strained and disrupted. Under prolonged stress, the interchain forces may break and rejoin at new positions, such that when the stress is removed, recovery of the polymer chains to their original positions is restricted by the new positioning of the interchain forces.
Such a condition can be readily envisaged with hydrogen bonding interactions occurring in cellulosic textiles, and in this circumstance the textile will exhibit poor wrinkle recovery (Figure 2.2) [21].
In a review of the wrinkling behaviour of cotton, Tovey
[20] postulated that, while extension due to the stretching of interchain bonds is recoverable, extension due to slippage of fibre elements is not recoverable. If slippage occurs, the fibrils are considered to become well aligned and continuous in their new configuration so that they no longer return to their original configuration.
Exceptionally good wrinkle recovery is exemplified at the other extreme by polyester, where the rigid molecular chains resist permanent deformation and polymer slippage
13 t - . -I • •• , ,. • I i \ v'. .' '', ,l • 1', . . ., •' • I • •
,. I ·,," l ' J i '
l Fig.2.3 Elastic rods embedded in matrix does not occur to a significant extent under moderate wrinkling conditions (Figure 2 • 2) • Under these circumstances the interchain forces are considered to be stretched without actually breaking, so that when the stress is removed they return the polymer network to its original configuration [21].
In order to attempt to explain wrinkling behaviour on an all encompassing level, which takes into account the various factors that affect wrinkling such as ambient conditions, fibre, yarn and fabric parameters, a two-phase viscoelastic model for wrinkling has been invoked [22-24].
This model is now widely accepted and consists of elastic rods embedded in a viscoelastic matrix (Figure 2.3). The extent of wrinkle recovery is governed by the internal elastic forces in the fibre as well as external assisting forces (i.e. fabric parameters), which together overcome the interfibre frictional forces. These elastic restoring forces will be constrained if molecular links within the fibre have been rearranged during the deformation process.
2.1.3 Factors Affecting Wrinkle Recovery
The various factors affecting wrinkle recovery have been extensively reviewed in the literature [18,20,25-27]; only those factors pertaining to this work are discussed here.
14 2.1.3.1 Temperature and Relative Humidity
The moisture regain and temperature of apparel fibres are influenced by ambient environmental conditions as well as the microclimate of the wearer. Wilkinson et al. [28] investigated the temperature and RH conditions which can exist adjacent to the body when a person is seated, and found that the temperature can increase up to 35°c and the
RH up to 90%. Studies carried out by Sorenson and Hog [25] conclusively showed that the extent of wrinkling is more severe if the temperature and/or regain of the fabric is increased during or immediately prior to the wrinkling deformation. Clearly in wear situations, during sitting, the temperature and regain at the back, or at the seat, will increase leading to a greater degree of wrinkling in these regions.
The influence of temperature and humidity on wrinkle recovery is also comparatively seen with polyester, where this hydrophobic fibre undergoes much smaller changes in regain under wear conditions. Hence, the wrinkle recovery changes to a lesser extent compared with fibres that have higher regain values, such as cotton and silk. Obviously, the fabric temperature and regain play a very important role in the wrinkling behaviour of textiles.
15 2.1.3.2 Fibre, Yarn and Fabric parameters
Wrinkling is primarily dependent upon the resiliency of the fibre in question, that is, the ability of a fibre type to absorb energy/work without permanent deformation [19]. The higher the resiliency of the fibre the better the wrinkle recovery. The resiliency of a fibre is primarily affected by the viscoelastic properties of the fibre type.
The effect of yarn structure on wrinkle recovery is known to be small [29], while the effect of yarn twist is variable and thus uncertain [29]. One important factor, however, is that in general, coarser yarns have been found to contribute to better wrinkle recovery behaviour [30].
Plain weaves wrinkle more easily than twills because the latter have more open weave structures than the former which are more tightly woven. Choe et al. [31] showed that a heavy density silk fabric gave better wrinkle recovery than one of light density. Leeder [27] has pointed out that fabric thickness is the most important structural parameter affecting wrinkle recovery, for the following reasons:
1. The radius of curvature increases with increase in
fabric thickness, therefore the more rounded
wrinkles in thicker fabrics make them appear less
16 wrinkled.
2. The strain imposed on fibres within a thicker fabric assembly is less, resulting in lower stress relaxation.
3. Thicker fabrics have greater resistance to sharp
folds, resulting in less noticeable wrinkles.
2.1.3.3 Effect of Ageing/Annealing
The term "ageing" refers to the storage of a fabric under normal ambient conditions (65% RH, 20°C) for a long period of time. During the process of ageing "molecular rearrangements take place within the fibre under the influence of sorbed water" such that a more stable lower energy state is attained. The phenomenon of ageing is common to all polymeric materials stored below their glass transition temperature [32, 33]. The factors influencing the ageing of wool have been studied extensively [34-44]. A very important property of aged wool is its high wrinkle recovery (Figure 2.1). However, the effect is not permanent and is removed by steam pressing, wetting, or by large changes in temperature or moisture content of the fabric
[28]. This latter reversal of the effect is referred to as "deageing". Improvement in the wrinkle recovery of wool
17 through ageing has attracted much interest in recent years, as the effect is much larger than that produced by chemical treatment or changes in yarn and fabric parameters.
Annealing is the process of accelerated ageing under conditions of high temperature and constant regain, followed by slow cooling. The result is very similar to that of ageing, where macromolecular rearrangement takes place via labile hydrogen bonds, which results in large improvements in wrinkle recovery [45]. Besides the requirement for high temperature, the regain of the fabric must be high enough to allow the required molecular rearrangements to take place. Very high regain conditions can, however, cause too much molecular mobility, and result in lower wrinkle recovery.
2.1.3.4 Laboratory Wrinkling Test Methods
As stated in Section 2.1.3.1, during wear situations the temperature and regain of a fabric will increase during the wrinkling stage (sitting) and decrease during the recovery period (standing). Therefore, laboratory tests which attempt to simulate these conditions are recommended in order to obtain results that can be related more closely to wrinkling during actual wear.
18 Three options are available for testing, namely, wear trials, fixed deformation and random deformation tests, of which the latter two are utilized for routine laboratory testing. Wear trials [ 18, 4 6] which provide the most realistic option for the assessment of wrinkling during wear are, however, too expensive and time consuming to be carried out on a routine basis. This method of testing is therefore not considered in detail here.
In fixed deformation testing, the fabric is deformed in a specific direction under a given load, and the conditions of temperature and humidity may or may not be varied during the deformation and recovery stages. These wrinkle recovery measurements are relatively more reproducible compared to other testing methods and are also considered to largely depend on the viscoelastic properties of the fibre in question, rather than factors such as yarn or fabric parameters. Examples of this method of testing include the
CSIRO Multiple Pleat Test [37), the Monsanto Test [47), the
Shirley Crease Recovery Test [48) and the IWS Thermobench
Test [49).
Random deformation tests allow the fabric to deform in a way that is influenced by fabric parameters i.e. thickness, weave, etc.), and thus more closely simulate actual wear situations. This test is, however, not
19 reproducible in a routine manner (34,46].
In a recent review of wrinkle recovery testing, Smuts (29] has stated that no one specific set of conditions in laboratory testing simulate wear conditions closely. Furthermore, tests which involve wrinkle insertion and recovery under the same ambient conditions are misleading as actual wear conditions continually change. Thus wrinkle recovery testing under standard atmospheric conditions
(20°c, 65% RH), using various test methods, does not satisfactorily differentiate between fabrics which have variable wrinkle recovery characteristics during wear (29]. In another study Abbot (26] found that wrinkle recovery is improved when the specimen is creased and recovered at a temperature of 20°c and 65% RH.
It can be concluded that the wrinkle recovery of any fabric is markedly dependent upon changes in atmospheric conditions during testing, and hence these conditions must be monitored closely, and maintained consistently for reproducible results to be obtained and rendered meaningful.
20 I I I I -N -CO -N -CH2 -N -CO -N -CH2 -N -CO - I I CH2 CH2 loss of water I I CO - and formaldehyde - -N -CO -N -CH2 -N -CO -N -CH2-N -CO - NH _ CH 0H (acid. heat) I I I 2 CH2 CH2 CH2 I I I - N - CO - N - CH2 - N - CO - N - CH 2 - N - CO - I I
Fig.2.4 Self condesation of dimethylol urea to form a hard insoluble resin
HOHiC, /Hi()H ~ //c \ N N HOHiC'. I 11 f H20H ~--c c--N, HOH2C ~ / CH20H N
Fig.2.5 Melamine formaldehyde (hexamethylol derivatives) 2.2. WRINKLE RESIST FINISHING AGENTS
2.2.1. Historical Development of Wrinkle Resist Agents
The first successful wrinkle/crease resist finishing agents were self-crosslinking resins developed for cellulosic textiles, involving the use of agents made from urea and formaldehyde. The earliest patent dates as far back as 1918
[50], and subsequently a commercial process was established in 1926 [50] involving the use of a urea-formaldehyde precondensate (N,N'-dimethylol urea). The chemical reactions taking place are complex and involve self condensation of dimethylolurea to form an insoluble resin on the fabric surface (Fig.2.4). In addition, reaction of the resin with the hydroxyl groups of the cellulose polymer occurs, resulting in the formation of crosslinks within the cellulose polymer network. In the late 1930's, amino resins based on the precondensates of cyanamide or dicyanamide with formaldehyde (e.g. melamine formaldehyde, Figure 2.5) were also introduced for cellulosic textiles [51].
These resins greatly improved the crease recovery of cellulosic materials, however, they were severely disadvantaged in three respects. Resin application drastically reduced the abrasion resistance of the fabrics, increased bleaching damage of the fabric, and in particular, the resins have a strong tendency to release
21 Fig.2.6 DMDHEU
HOHC-CHOH HOHC-CHOH I I I I HOHiC -N N -cH20H "-.~/ 0 crosslink
Fig.2.7 DMDHEU form crosslinking with cellulose formaldehyde under certain conditions of use. This led to their gradual replacement by other types of reactive crosslinking agents, introduced in the 1950's.
The reactive compounds developed were typically N hydroxymethyl and N-alkoxymethyl derivatives of acyclic and cyclic ureas, carbamates, carboxylic acid amides and aminotriazines [52). Of these, dimethylol-dihydroxy ethylene urea (DMDHEU, Figure 2.6) has been the most commonly used agent. All of these agents differ from the resins discussed above in that they tend not to undergo self-condensation, but instead react primarily with the hydroxyl groups of cellulose to form cross links
(Figure 2.7). These reactive crosslinking agents offered a soft hand and excellent resistance to chlorine bleach damage, however, the abrasion resistance and strength retention of the treated fabrics was significantly reduced. More importantly, the liberation of formaldehyde, although reduced compared to other formaldehyde based resin treatments, still posed a serious problem for clothing workers during processing, as well as during fabric wear.
More recently, therefore, intense research has shifted toward the development of low-formaldehyde or preferably "formaldehyde free" crosslinking agents as wrinkle resist
finishes, as discussed in the next Section.
22 2.2.2 Comparison of Wrinkle/Crease Resist Agents Used
for Cotton, Wool and Silk
The majority of wrinkle resist agents used in textile finishing have been developed for cotton. The terms "easy care", "durable press", "permanent press" and "crease resistant" are all commonly used for cotton textiles, and are synonymous terms referring to the improved ability of the cotton fabric to recover from deformation during wear and after washing. Generally, the finishing treatment is carried out after scouring, bleaching and dyeing, and involves a pad-dry-cure process. The fabric is impregnated with a solution of the crosslinking agent and catalyst, dried and then cured at high temperature, generally in the range of 120-180°c.
The mechanism of wrinkle resistance primarily depends on the introduction of crosslinks within the fibre which presumably serves to retard slippage between polymer chains, thereby preventing permanent deformation under stress (see Section 2.1.2).
As mentioned in the previous Section, N hydroxy(alkoxy)alkyl compounds have been mostly employed as crosslinking agents for cotton (e.g. DMDHEU,
Figure 2.6). The crosslinking reaction is acid catalysed,
23 0 11
NH~c" NH
Fig.2.8 4,5 Dialkoxy Ethyleneurea involving cleavage of the C-0 bond of the hydroxy (alkoxy) methyl compound and nucleophilic attack of the hydroxyl group of cellulose on the activated methylene carbon. The scheme for the reaction is shown in Figure 2.7. A Lewis acid, commonly magnesium salts (e.g. MgC1 2 ), is generally used as catalyst. The extent of wrinkle resistance conferred on the fabric depends on the reactivity of the catalyst used and the curing conditions. These factors have been reviewed extensively by Petersen [52].
Within this general group of crosslinking agents the N hydroxymethyl derivatives (e.g. DMDHEU) contain small amounts of free formaldehyde and further, during fabric impregnation and curing, formaldehyde is released and transferred to the fabric. During subsequent use the formaldehyde may then be liberated via hydrolysis reactions.
N-Alkoxymethyl compounds, however, release less formaldehyde than the N-hydroxy analogs. Other crosslinking agents such as 4,5-dialkoxy-ethylene urea (Figure 2.8) have been developed for formaldehyde-free finishing, and are produced by reacting substituted ureas with glyoxal. In the presence of alcohol and buffer salts these agents undergo crosslinking reactions with cellulose.
24 TABLE 2.1 EFFECT OF VARIOUS CHEMICAL TREATMENTS ON THE WRINKLE RECOVERY OF WOOL [59]
Treatment % Wrinkle Recovery
Untreated 56
Mercuric salts 70
Formaldehyde 59
Resorcinol/formaldehyde 60
Benzoquinone 65
Synthappret BAP 58 COOH COOH I I HzC -CH - CH -CHz I I COOH COOH
Fig.2.9 Butane Tetracarboxylic acid The most recent development in non-formaldehyde wrinkle resist finishing of cotton involves the introduction of ester crosslinks into cellulose, utilising polycarboxylic acid compounds. Polycarboxylic acids, having between 3 and
6 carboxyl groups per molecule, were found to be the most effective, and of these the most successful was BTCA (Figure 2.9). The carboxylic groups are capable of forming anhydride rings when heated to a high temperature; crosslinking then occurs through a base catalysed esterification of the hydroxyl groups of cellulose fibres by these reactive cyclic anhydride intermediates [53]. The catalyst used is usually an alkali metal phosphate, and a high curing temperature (160-180°C) is required. This agent, BTCA, is purported to confer wrinkle resistance on a par with DMDHEU, with improved strength retention, and of course, zero-formaldehyde release [53,54].
Deaged wool has a much higher wrinkle recovery than cotton.
Unlike cotton, the low wrinkle recovery of which can be significantly improved by numerous crosslinking agents, chemical modifications to wool for the purpose of improving wrinkle recovery have not been achieved on a commercially viable level. Some of these treatments are shown in Table
2. 1 [ 55 J • Annealing in the presence of resorcinol and formaldehyde can result in "permanent" improvements in wrinkle recovery, after deaging. This process is considered
25 to increase the stability of the annealed state by introducing cross links into the stable molecular arrangement formed during the annealed state [55). Another treatment involves the use of heavy metal salts, such as mercuric acetate, which also results in a significant increase in wrinkle recovery [ 55]; the mechanism responsible for reducing the wrinkle recovery in this case is unknown. Both the above treatments result in large increases in weight gain, however, and are unsuitable for reasons of impractical processing conditions and high levels of toxicity.
A different approach is to reduce the regain of the wool fabric and thus reduce the tendency of the fabric to wrinkle under conditions of high humidity. Chemical treatment with benzoquinone is considered to improve wrinkle recovery for this reason, although once again a high weight gain is required [55). Finally, surface elastomer applications with either Synthappret BAP or silicone elastomers are used for the purpose of shrinkproofing and softening wool, respectively. Ultratex
ESU and EMJ are self-crosslinking silicone polymers used as durable finishes to achieve multiple effects. These
include improvement of stretch, wash and wear, dimensional stability, crease shedding properties and sewability of woven and knitted fabrics of all fibres types [56]. The
26 CH3 CH3 CH3 CH3 I I I I H3C-siO SiO SiO Si -CH3 I I I I CH R CH 3 CH3 X I 3 NH2 y
Fig.2.10 Aminofunctional Silicone Polymer most durable silicone polymers are those of organofunctional silicones based on a polydimethyl siloxane polymer which has been modified with amine, epoxide, or alcohol functional groups (Figure 2.10). These self crosslinking polymers may require an external catalyst which can be incorporated into the preparation, and is activated at high temperature.
The silk fibre has relatively poor crease shedding properties compared to most other fibre types, except for cotton. In general, attempted crease resist processes for silk have included formaldehyde-based resin precondensates as for cotton, which impart varying degrees of wrinkle resistance depending on the processing conditions.
One very successful treatment with respect to a large improvement in wrinkle recovery involves a gas-phase treatment patented by Kanebo Ltd in 197 0 [ 5 7] . In this process silk fabric is initially impregnated with a solution containing a dihydroxy compound (aliphatic or aromatic), or alternatively, urea or thiourea; additionally, an inorganic catalyst (e.g. ammonium chloride, magnesium chloride or zinc nitrate) is present.
The fabric is dried to less than 10% moisture regain and then treated with gaseous formaldehyde in a treatment
chamber heated to 100-120°c. The process was considered to
27 introduce crosslinks between functional groups in the silk fibre as well as form condensation products that "fill-in" the fibrous structure in a uniform manner, thereby effecting improved wrinkle recovery. It was considered essential that the moisture content of the fabric was <10% and that the temperature was at least 100°c. This was necessary in order to maintain the formaldehyde in a gaseous state, thus effecting adequate penetration of formaldehyde into the fibrous structure, rather than forming only a surface coating of polymer. In that respect, the treatment is somewhat analogous to the annealing/crosslinking treatments carried out on wool. Although the process is claimed to impart >85% wrinkle recovery to silk fabric without deterioration of the soft handle, one severe drawback for current implementation is the use of formaldehyde.
Continued interest in crosslinking reactions in silk fibroin has resulted in a number of studies being carried out using epoxides [58, 59]. Tanaka et al. [59] investigated the crease recovery of fibres and fabrics treated with 1,2-bis(2,3-epoxy-1-propoxy)ethane in a solution of tetrachloroethylene. The epoxide modified silk was found to have a lower moisture regain and improved crease recovery compared with untreated fabric.
28 In another study, silk fabrics of varying moisture content were treated with ethylene glycol diglycidyl ether, having
2 epoxide functionalities, in a solution of tetrachloroethylene [60]. It was found that the extent of wrinkle recovery achieved was dependent upon the water content of the treated fabric. The lower the water content, the larger the dry crease recovery, while the higher the water content the larger the wet crease recovery.
Other crosslinking reactions explored include those with l-fluoro-2,4-dinitrobenzene [61], 4,4-difluoro-3,3- dinitrodiphenyl-sulphone [ 62] and bi functional reactive dyes [63]. In all cases the solubility of silk in alkali decreases, indicating formation of crosslinks within the molecular structure of silk fibroin, presumably between the hydroxy-functional tyrosine and/or serine residues. Similar studies carried out on wool, however, indicate that wool keratin has more abundant reactive sites available for crosslinking [63].
In a recent comparison between wool and silk [ 55] , the application of a surface elastomer (e.g. Ultratex) was also found to increase the wrinkle recovery of silk by approximately 10%.
29 2.3. OBJECTIVE MEASUREMENT OF FABRIC HANDLE
The measurement of low stress mechanical properties is the most appropriate way of assessing tailorability, handle and the appearance retention of fabrics treated with finishing agents.
An explanation of these low stress measurements is provided in Section 4.5, and discussed in relation to trends observed in the wrinkle recovery of the treated silk fabrics in this work.
2.4. AIM OF THE PRESENT WORK
As discussed in Section 2.2.2, no one process exists to date that successfully imparts adequate wrinkle resistance to silk fabric without having a major drawback with respect to toxicity or loss of the luxurious and highly desirable handle of the silk fibre.
The aim of the present work is to explore the potential of the non-formaldehyde wrinkle resist treatments that have been used on either cotton or wool, for use on silk fabric.
One crosslinking treatment chosen was that involving use of butane tetra-carboxylic acid (BTCA). This crosslinking
30 agent has been very successfully applied to cotton [64,
65], and while silk fibroin has few sites available for crosslinking, various crosslinking treatments have been known to improve the crease recovery of silk at least marginally, as discussed in Section 2.2.2. A different approach involves the application of a surface elastomer which has been shown in one study to increase the wrinkle recovery of silk fabric [55].
In this work, both the wet and dry wrinkle recovery of silk fabric treated with BTCA and silicone elastomers are compared. In addition, the extent of crosslinking taking place under conditions of annealing with formaldehyde is examined, and discussed with respect to the fibres that can be successfully crosslinked under annealing conditions in order to their improve wrinkle recovery.
Measurements were carried out using the Thermobench method, utilising the distinct advantage of crease insertion at
35°c, and recovery at 20°c, 65% RH. The Shirley Crease
Recovery method was also used in order to compare both dry and wet wrinkling behaviour.
31 CHAPTER THREE
WRINKLING OF SILK: MATERIALS AND METHODS
32 3.1 Materials 3.1.1 Silk Fabric
Chinese silk fabric of various weave types was purchased from the Australian Silk Wholesalers, Sydney. The weights of the silk fabrics as follows, Shantung:91 g/m2 ; Satin:77 g/m2 ; Crepe de chine:68 g/m2 ; Twill:41 g/m2 ; Habutai:32 g/m2.
3.1.2 Chemicals 3.1.2.1 Butane Tetracarboxylic acid
1, 2, 3, 4-Butane Tetracarboxylic Acid ( BTCA ) is a non formaldehyde cross linking agent which has been used for the purpose of improving the crease resistance of cotton. BTCA is an ester type crosslinker of solid form and white in colour. It is easy to dissolve in a cold water. It was purchased from Aldrich Chemical Company, and has molecular weight of 234.16.
3.1.2.2 Silicone polymers
Ultratex ESU and EMJ are cationic silicone polymers which contain a self crosslinking agent. These polymers are supplied by Ciba - Geigy as aqueous emulsions.
33 3.1.2.3 Catalyst
Sodium hypophosphite, which is used as a catalyst in the treatment with BTCA, was purchased from Ajax Chemicals.
3.2 Experimental Methods 3.2.1 Bleaching of silk fabrics
Silk fabrics were bleached using the following recipe. 15 ml/1 Hydrogen Peroxide (30%) 3 g/1 Tetra Sodium Pyrophosphate 1 g/1 Ethylene Diamine Tetra Acetic Acid
Liquor ratio 1 : 70
The fabrics were treated for 3 hours at a temperature of
70°c followed by a thorough rinsing in cold water.
3.2.2 Crease Resist Finishing
3.2.2.1 Treatment of silk with Butane Tetra Carboxylic
Acid (BTCA) [64, 65]
Bleached or unbleached silk fabrics were treated according to a pad - dry - cure system. The fabric was immersed in an aqueous solution containing BTCA and sodium hypophosphite as catalyst. The concentrations of both these reagents were varied from 2%, 4%, and 6% w/v. The fabric was then passed through squeeze rollers (pad
34 mangle), with the nip pressure set to give a wet pick up
110 - 130% on weight of fabric. The fabric was predried
at 85°c for 5 minutes and then cured at 180°c for 90
seconds. The fabric was then rinsed in running water (50°C)
for 30 minutes and finally dried to constant weight.
3.2.2.2 Treatment of Habutai silk with Oltratex ESO and
EMJ
Bleached silk fabric was treated according to a pad - dry -
cure system. The fabric was immersed in an aqueous solution
containing Ultratex silicone elastomer. The concentration
of Ultratex ESU was varied as 2%, 6%, and 10% w/v, while
concentration of Ultratex EMJ was varied as 1%, 2%, and 3%
w/v. The fabric was then passed through squeeze rollers and
the process repeated to give a wet pick up of 90 - 100% on
weight of fabric. The fabric was dried at 110°c for 5
minutes and then cured at 160°c for 3 minutes. Finally, the
fabric was rinsed with cold water and then dried to
constant weight.
3.2.3 Crease Recovery Measurements
3.2.3.1 Thermobench Method [66]
Prior to the measurement of wrinkle recovery, the fabric
35 was steam pressed for 10 seconds and vacuum pressed for 10 seconds. Then the fabric was conditioned for 24 hours at
65% RH, 20°c. This fabric preparation sequence was carried out every time in order to minimise variation between measurements due to changes in fabric regain. Cut specimens of 2 x 3 cm in size, folded lengthwise, were placed in sealed plastic bags within the Thermobench apparatus, at
35°c for 15 minutes under the 2 kg load. The load was removed and the samples allowed to recover at 65% RH, and
20°c for 15 minutes. The angle of recovery at each end of the lengthwise samples was measured using a wrinkle recovery protractor and expressed as the percentage wrinkle recovery based on a 180° recovery angle. Each measurement quoted is an average of 6 readings. All wrinkle recovery values are within an error limit of ± 5% (absolute).
3.2.3.2 Shirley Crease Recovery Test Method 3.2.3.2.1 Dry Crease Recovery
Prior to measuring the recovery angle each fabric sample was steam pressed for 10 seconds and vacuum pressed for 10 seconds. The fabric was then conditioned for 24 hours at
65% RH, 20°c. Cut specimens 1 x 2 inch in size, 5 parallel to the warp and 5 parallel to the weft were folded precisely in half to give a crease 1 inch long. The fabric fold was inserted between two glass plates and a 2 kg mass
36 placed on top for exactly 1 minute. At the end of this time
the mass was removed and the specimen transferred using
tweezers to the tester. The recovery angle was measured
after exactly 5 minutes. All recovery angle values are within an error limit of+
5% (absolute).
3.2.3.2.2 Wet Crease Recovery
The method used was identical to that used for the
measurement of dry crease recovery, except that before
testing, the specimens were wetted out with water containing 0.05% wetting agent and then pressed between
filter papers to thoroughly remove excess water.
3.2.4 Fabric Breaking Strength Measurements
The fabric breaking strength was measured by the methods
described in Australian Standard AS 2 0 0 1 . 2 . 3-1 9 8 8
Method A-Ravelled Strip Test. The rate of extension was 100
mm/min on the Instron meter.
3.2.5 Fabric Whiteness
Fabric whiteness was evaluated by comparing the whiteness
(WIE 313) (CIE Lab color scale) value of treated fabrics
37 to that of untreated fabrics using the Spectrogard Color System.
3.2.6 Weight Gain
Weight gain was measured based on the dry weight of the fabric before and after treatment.
3.2.7 Laundering
The laundering method ( hand wash) employed was AATCC
Test Method 88C - 1989. Samples were washed with AWTA standard detergent (MS-65) at 40°c for 2 minutes.
38 CHAPTER FOUR WRINKLING OF SILK: RESULTS AND DISCUSSION
39 TABLE 4.1 % WRINKLE RECOVERY FOR VARIOUS TYPES OF UNTREATED SILK FABRICS
Type of fabric Untreated
Shantung(weft) 26
Shantung(warp) 42
Satin(back) 36
Satin(face) 36
Crepe de chine 37
Twill 33
Habutai 23 4.1 WRINKLE RECOVERY OF SILK FABRIC - VARIOUS WEAVE
TYPES
Smuts [29] has reviewed the influence of yarn/fabric parameters on the wrinkle recovery of fabrics. It is generally accepted that with high recovery fibres (e.g. polyester, aged wool) the high elastic recovery is almost constant over a range of strains and hence the frictional constraints due to the type of fabric weave is negligible. However, with poor recovery fibres (e.g. cotton, silk), significant improvements in wrinkle recovery can be obtained by changes in fabric parameters. The wrinkle recovery of various types of silk fabrics, as measured by the Thermobench method [66] is shown in Table 4.1. The satin, crepe and twill fabrics have wrinkle recovery values in the range 33-37% for fabric weights from 41-77 g/m2 • The Habutai plain weave fabric has the lightest weight
(32g/m2 ) and exhibits a significantly lower wrinkle recovery value of 23%. The heaviest Shantung fabric (91 g/m2 ) shows a large variation in the wrinkle recovery measured in the warp and weft direction.
Generally, for poor recovery fibres such as silk, plain weave fabrics have been found to exhibit the poorest wrinkle recovery [67-81], and thus the wrinkling behaviour
of the Habutai fabric, combined with its light weight, is
40 TABLE 4.2 % WEIGHT GAIN OF UNBLEACHED TWILL SILK FABRICS
Concentration Concentration Weight gain of of ( %) BTCA (%) catalyst (%)
2 4.4
4 4.7 2 6 4.6
2 6.0
4 7.0 4 6 6.9
2 8.6 6 4 9.7
6 8.3 a consistent observation. The large variation in wrinkle
recovery of the warp (42%) and weft (26%) yarns of the
relatively heavier Shantung fabric is consistent with the much coarser warp yarns exhibiting better elastic recovery.
Yarn coarseness has been considered to influence the
wrinkle recovery to a greater extent than fabric mass [30).
Conversely, during deformation of the weft yarns, recovery
is more restricted as a result of the crease being formed
parallel to the direction of the relatively coarse, heavy
warp yarns.
4.2 TREATMENT OF UNBLEACHED SILK TWILL FABRIC WITH BTCA
4.2.1 Weight Gain
In this treatment, unbleached silk twill was treated with
various concentrations of BTCA (2%, 4% and 6%) and various
concentrations of catalyst (sodium hypophosphite, 2%, 4%
and 6%). The weight gain of the treated fabrics is seen to
increase significantly with increasing concentration of
BTCA (Table 4.2) Increasing the concentration of catalyst,
however, at constant BTCA concentration, has no
significant effect on the weight gain.
The high weight gains achieved are surprising considering
that BTCA is primarily a crosslinking agent, and indicates
41 TABLE 4.3 % WRINKLE RECOVERY OF UNBLEACHED TWILL SILK FABRICS
Concentra Concentration % Wrinkle Recovery tion of of catalyst BTCA (%) (%) Warp Weft Average
Untreated 33 33 33
2 47 49 48 2 4 46 47 46
6 41 43 42
2 49 48 49
4 46 50 48 4 6 46 51 49
2 48 52 50 6 4 50 51 51 that a substantial amount of this reagent acts as a "fill in", deposited on the surface of the fabric.
4.2.2 Wrinkle Recovery
Untreated silk twill fabric was found to have a wrinkle recovery of approximately 33%. This measurement was repeated many times and was found to be consistent and reproducible, according to the method described in Section 3.2.3.1. On treatment with 2% BTCA, the wrinkle recovery increases significantly to an average value of 45%, with little variation caused by increasing the catalyst concentration from 2% to 6% (Table 4.3). The improvement in wrinkle recovery appears to level off, however, as increasing the BTCA concentration from 2 to 6% results in only a marginal increase in wrinkle recovery (45 to 50%). Hence, an initial application of BTCA to give a weight increase of approximately 4-5% results in a large improvement in wrinkle recovery, while larger increases in weight gain of 8-10% appear to give only marginally further
improvement in wrinkle recovery.
Chapman [ 82] proposed that the improvement in wrinkle
recovery of fabrics treated with finishing agents is
influenced by the formation of interfibre bonds. These interfibre bonds are considered to reduce the frictional
42 TABLE 4.4 % WRINKLE RECOVERY OF UNBLEACHED SILK TWILL FABRICS AFTER 3 LAUNDERING CYCLES (CONCENTRATION OF CATALYST 2-6%)
Concentration % Wrinkle % Wrinkle of BTCA ( %) Recovery before Recovery laundering Average
Untreated 33 33
2 46 47
4 49 42
6 50 40 TABLE 4.5 WHITENESS AND STRENGTH RETENTION OF UNBLEACHED SILK TWILL FABRICS AFTER TREATMENT WITH BTCA
Concentration Whiteness % Strength of BTCA ( %) (WIE) Retention
Untreated 76 100
2 62 95
4 62 94
6 61 96 component of wrinkling and give rise to an additive elastic restoring moment which assists recovery of the viscoelastic components within the fabric from bending deformations.
The BTCA finishing treatment was found to be quite stable to washing for lower application levels. However, the wrinkling behaviour of the washed fabric deteriorated for the higher application levels of BTCA (Table 4.4). Evidently, some detachment of the finish from the fibre surface occurs to the extent that the restoring forces of the interf ibre bonds are no longer effectively coupled [82]. Hence, in general, the improvement in wrinkle recovery resulting from 3-4% weight gain is more reproducible and durable.
Although the whiteness of the treated fabric is less than that of untreated fabric, the decrease is only marginal, and the treated fabric appears to visually retain a good white (Table 4.5). The drop in strength retention is also only marginal. This latter result is in sharp contrast to the finishing of cotton fabric with BTCA which results in only 50-60% breaking and tear strength retention. This level of strength retention in the case of cotton is nevertheless considered to be of an acceptable standard for durable press finishing.
43 TABLE 4.6 WRINKLE RECOVERY OF VARIOUS TYPE OF BLEACHED SILK FABRICS
Type of Untreated Bleached Bleached treated fabrics only with 6% BTCA,6% catalyst
Shantung 26 28 41 (weft)
Shantung 42 43 58 (warp)
Satin 36 42 46 (back)
Satin 36 44 46 (face)
Crepe de 37 41 47 Chine
Twill 33 41 47
Habutai 23 39 44 4.3 TREATMENT OF VARIOUS BLEACHED SILK FABRICS WITH BTCA
Table 4.6 gives the wrinkle recovery results for various silk fabrics that have been bleached and then treated with BTCA after bleaching. A substantial increase in wrinkle recovery is seen for the Habutai fabric after bleaching with hydrogen peroxide. Furthermore, the handle of this fabric was distincly softer after bleaching. This difference in handle may possibly result from the application of a "scroopy" finish (e.g. citric or lactic acid) on the fabric which generally imparts a slight stiffening effect for better fabric drape. Alternatively, a less extensive degumming operation would allow more sericin to remain on the fibre, again for the purpose of retaining some stiffness and better drape.
A staining test with C.I. Direct Blue 22 (83), carried out on both bleached and unbleached fabrics showed no difference in the level of staining, indicating that the amount of sericin (stains darker) on the fibre was the same in both cases. Hence, a "scroopy" finish is offered as an explanation for the slight stiffness of the unbleached fabric, which is removed during the bleaching operation. The removal of this finish evidently reduces a substantial degree of interfibre friction during recovery of the fabric from deformation.
44 With the exception of the Shantung fabric, the BTCA finishing treatment gives rise to only a marginal increase (3-6%) in wrinkle recovery for all the treated fabrics. This is in sharp contrast to the increase obtained in the case of the unbleached fabric (twill), and indicates that once the stiffening effect of the "scroopy" finish has been removed, the formation of interfibre bonds by the application of BTCA has little effect on the wrinkle recovery behaviour. The high weight gain in each case confirms that the cross linked finishing agent is substantive to the silk fabric, although the finish does not appear to impart improved recovery from deformation.
The treatment of the Shantung fabric with BTCA does, however, result in a significant increase (10-15%) in wrinkle recovery of both warp and weft yarns, and the large difference in the recovery behaviour between the two yarn directions is still evident after finishing. In the case of this heavier, thicker fabric construction, evidently a greater degree of interfibre bonding occurs which effectively couples and contributes to an overall improvement in elastic recovery.
45 TABLE 4.7 PROPERTIES OF BLEACHED HABUTAI SILK FABRICS AFTER TREATMENT WITH VARIOUS FINISHING AGENTS
DCRA WCRA Weight Whiteness Strength Treatment (W+F) (W+F) gain Index Retention ( %) (WIE) (%)
Bleached 301 215 - 75 100 only
2% BTCA, 305 258 3.7 65 103 6% cata- lyst
6% BTCA, 317 255 8.2 67 105 6% cata- lyst
6% ESU 330 252 2.9 64 68
10% ESU 326 255 3.8 65 82
3% EMJ 327 241 1. 7 70 79
Untreated, 294 227 -- - annealed
Annealed, 306 219 -- - 5% Resorcinol and 9% para formaldehy de Footnote : The wrinkle recovery of unbleached Habutai was DCRA, 282; WCRA, 225. The wrinkle recovery of annealed cotton fabric in the presence of 5% Resorcinol and 9% para formaldehyde was: DCRA, 285 (215); WCRA, 227 (165). DCRA and WCRA of untreated cotton is given in parenthesis. 4 . 4 CHANGES IN WET AND DRY WRINKLE RECOVERY OF BLEACHED
BABOTAI SILK TREATED WITH A VARIETY OF FINISHING
AGENTS
In this series of experiments the Shirley Crease Recovery Tester was used to measure the wet and dry crease recovery of treated silk fabric after treatment with BTCA, and silicone elastomer systems such as Ultratex ESU and EMJ (Table 4.7). The aim of the investigation was to compare the influence of each particular treatment on the wet and dry recovery behaviour of the treated fabric. In order to rule out the influence of weave structure on changes in the wrinkle recovery during finishing, the plain weave Habutai fabric was chosen. Furthermore, because of the variable effect of the "scroopy" finish on wrinkle recovery, bleached fabric was used.
On treatment with BTCA, the dry crease recovery does not change significantly, consistent with the result obtained for bleached Habutai in Table 4.6. However, the wet crease recovery increases, indicating that the "smooth drying" property of the fabric after washing is improved. This presumably indicates an increased tendency of the finished fabric to shed water.
Application of the reactive aminofunctional silicone
46 polymers, Ultratex ESU and EMJ results in increases in both the dry and wet wrinkle recovery of the silk fabric. A weight gain as low as 1.7% in the case of EMJ contributes
to a significant improvement in wrinkle recovery behaviour. These elastomeric polymers therefore exhibit greater resiliency during recovery from deformation. The application of aminofunctional silicones to cotton [84],
wool [55) and silk [55) has been shown to increase the dry wrinkle recovery of the treated fabrics. The effect is only
marginal for cotton and less economically viable compared
to that obtained with inexpensive crosslinking treatments.
For wool, high application levels are required in order to
significantly improve wrinkle recovery. In the case of silk, however, low application levels have been shown to improve wrinkle recovery by 10% as measured by the Thermobench method [66]. The results of this work show that for both silicone elastomers, ESU and EMJ, the wet and dry
wrinkle recovery values of treated Habutai silk increase even for weight gains as low as 1.7%, as in the case of
Ultratex EMJ. Thus, the hydrophobic characteristics of
these silicone polymers also results in an increased
tendency to shed water and thus improve wet wrinkle recovery.
For comparison, bleached Habutai silk was subjected to an
annealing treatment in the presence of p-f ormaldehyde
47 (Table 4.7). As can be seen from the data, neither the wet nor dry wrinkling properties of the fabric change significantly as a result of the treatment. This is in sharp contrast to the result obtained during the treatment of cotton and wool fabrics under annealing conditions in the presence of crosslinking agents [85). In this work, a comparison was carried out with cotton fabric, and it was found that the annealing/crosslinking treatment under identical conditions to that carried out for silk fabric, gave rise to a large increase in both the dry and wet wrinkle recovery of cotton. This latter finding is consistent with that reported for cotton and also for wool
[85), in that these fibre types have the ability to undergo a siginificant degree of molecular rearrangement within their respective polymer networks to the extent of forming a more stable molecular state during annealing. This state is further rendered "permanent" towards changes in humidity and temperature by the introduction of crosslinks. Under the same conditions, which promote the formation of crosslinks in cotton, however, silk fabric does not undergo similar "permanent" changes in molecular structure that improve either the dry or wet wrinkle recovery. Even the small degree of crosslinking that might be expected to take place between the hydroxyl groups of tyrosine and/or serine residues of silk fibroin, evidently does not produce any significant improvement in wrinkle recovery. Further, the
48 TABLE 4.8 THE SIXTEEN PARAMETERS DESCRIBING FABRIC MECHANICAL PROPERTIES
I. Tensile 1. LT Linearity of load-extension curve (-)
2 • WT Tensile energy ( gf . cm/ cm2 )
3. RT Tensile resilience ( % )
II. Shear 4. G Shear rigidity (gf/cm.degree)
5. 2HG Hysteresis of shear force at 0.5 degrees (shearangle) (gf /cm)
6 . 2HG5 Hysteresis of shear force at 5 degrees (gf/cm)
III. Bending 7 • B Bending rigidity (gf .cm2 /cm)
8. 2HB Hysteresis of bending moment (gf .cm/cm)
IV.Compress- 9 . LC Linearity of compression- ion thickness curve (-)
10. we Compressional energy
( gf . cm/ cm2 )
11. RC Compressional resilience ( % )
V. Surface 12. MIU Coefficient of friction (-) charac- 13. MMD Mean deviation of MIU (-) teristics 14. SMD Geometrical roughness (micron)
VI. Fabric 15. w Fabric weight per unit area
cons- (mg/ cm2 ) truction 16. T Fabric thickness (mm) TABLE4.9 THE MECHANICAL PROPERTIES OF SILK TREATED WITH VARIOUS FINISHING AGENTS (WARP ONLY)
LC we RC To B 2HB RB Treatment (J/rrf) (%) (mm) ()lNm) (mN) (nrl)
Untreated 0.05 0.05 76 0.17 3.24 0.14 21.2
2%ESU 0.05 0.05 62 0.17 1.18 0.04 16.7
2%EMJ 0.05 0.07 64 0.18 1.08 0.04 18.2
2%BTCA 0.05 0.04 77 0.16 2.94 0.08 13.3
2HG Treatment G 2HG5 RS LT WT RT (N/m) (N/rn) (N/m) (J/rrf) (%)
Untreated 11.58 0.07 0.16 0.19 0.57 7.7 68
2%ESU 9.84 0.07 0.13 0.24 0.53 8.4 63
2%EMJ 10.66 0.09 0.17 0.24 0.55 7.7 69
2%BTCA 10.57 0.07 0.12 0.18 0.63 7.2 65
EMT Treatment MIU MMD SMD (%) (pm)
Untreated 5.3 0.33 0.015 1.75
2%ESU 6.5 0.19 0.015 1.07
2%EMJ 5.7 0.17 0.010 1.66
2%BTCA 4.7 0.20 0.022 2.84 polymer system of silk is far more crystalline than that of wool and hence changes in molecular structure as a result of annealing are slight, such that the viscoelastic properties of the silk fibre change little or not at all
during annealing.
4.5 LOW STRESS MECHANICAL PROPERTIES OF SILK FABRIC TREATED WITH BTCA, ESU AND EMJ FINISHING TREATMENTS
4.5.1 General
The KES-F system for the objective measurement of fabric
handle consists of four machines used to measure the
bending, shear, tensile, compressional and surface
properties of fabrics. These measurements are expressed in
the form of 16 parameters (Table 4.8) which characterise
fabric properties relating to handle, tailoring performance
and drape. The system has been used extensively to evaluate
the low stress mechanical properties imparted to finished
wool fabrics as a result of finishing treatments. The KES-F
data obtained for the treatment of bleached Habutai silk
with BTCA, Ultratex ESU and EMJ (2% w/v) is given in Table
4. 9.
49 4.5.2 Bending Properties
The bending rigidity (B) is defined as the force required to deform a fabric, while the residual curvature (RB) is the amount of unrecovered bending strain left in a fabric after a bend-recovery cycle. These measurements are normally a function of fabric weight and thickness, and while the former measurement determines the suppleness of the fabric (low bending rigidity) with respect to sewability, the latter can indicate the extent of recovery after wrinkling (i.e. the lower the residual curvature, the greater the restoring couple after deformation).
As can be seen from the data in Table 4.9, the bending rigidity of silk fabric treated with the silicone elastomers is greatly reduced, while that treated with BTCA is not significantly changed compared with untreated fabric. This presumably reflects the supple, elastic nature of the silicone polymer systems. The residual bending curvature of fabrics in all three cases is lower than that of untreated fabric, particularly for the BTCA treated fabric. This result for BTCA treated fabric is somewhat surprising considering the trend observed in the actual wrinkle recovery measurement (Table 4.7).
In contrast to the results obtained for silk fabric, wool
50 (plain-weave) fabrics treated with silicone elastomer
systems do not exhibit a significant change in either bending rigidity or residual curvature (86], despite the level of improvement seen in wrinkle recovery (55]. The application of a more rigid polyurethane polymer (Synthappret BAP) to wool is, however, known to increase the bending rigidity by at least 3-fold (86] and imparts a noticeably stiffer handle to treated wool fabric; this
polymer system, which is known to effectively form
interfibre bonds and improve wrinkle recovery, only
marginally reduces the residual bending curvature of treated fabric (82].
Hence trends observed in the residual bending curvature are not a reliable reflection of fabric wrinkle recovery.
4.5.3 Shear Properties
The shear rigidity (G) provides a measure of the resistance to rotational movement of warp and weft threads in a fabric
when subjected to low levels of shear deformation. A high
shear rigidity thus represents a high interaction between
warp and weft yarns during deformation, which is influenced
by the extent of interfibre bonding. The residual shear angle (RS) is a measure of the extent to which a fabric recovers from shear deformation.
51 The shear properties of the treated silk fabrics (Table
4.9) do not change significantly from that of the untreated fabric. In each case the shear hysterisis of silk fabric is very small compared with that of other fibre types.
Generally, the shear properties of finished wool fabrics tend to mirror the bending properties. In the case of wool treated with silicone elastomers, the shear properties do not change significantly, whereas with the application of
Synthappret BAP a 4-fold higher shear rigidity is obtained, as expected [86]. By comparison, the shear properties of silk are relatively unchanged, while the bending properties are altered by the finishing treatments with BTCA, and silicone elastomers.
4.5.4 Compression and Surface Characteristics
The compressional properties of a fabric are important for the assessment of fabric handle. These properties are measured by increasing the lateral pressure on the fabric from O. 5 gf / cm2 to 50 gf / cm 2 • While the initial fabric thickness (T 0 ) and compressional energy (J/m2 ) required is much the same for all the treated fabrics compared with untreated fabric, the compressional resilience varies. The compressional resilience (RC) provides a measure of the percentage energy recovery from lateral deformation.
52 Kim and Lee [87) have attempted to correlate the low stress mechanical properties of wool fabric to wrinkle recovery behaviour, and found that the properties most closely related to that of wrinkle recovery are the compressional resilience and the frictional coefficient of the fabric. As these regression coefficients are determined from
inverse regression analysis, the smaller the compressional resilience and frictional coefficient of the fabric, the better the wrinkle recovery [87). The significantly lower
values obtained for the compressional resilience (RC) of
the silicone elastomer treated silk fabrics in this work, is thus consistent with the better dry wrinkle recovery
obtained for these fabrics compared with BTCA treated or bleached silk fabric.
The frictional coefficient (MIU) for the treated silk fabrics is somewhat more variable, where the differences
obtained between values determined in the warp and weft
directions render interpretation difficult.
The surface contour (SMD), which is also closely related to fabric handle, measures the variation in the surface
geometry of a fabric, as indicated by roughness or
smoothness. Once again some variation is observed between
values determined in the warp and weft directions of the
silk fabric. A general trend does, however, show that BTCA
53 treated silk exhibits a greater degree of variation along the fabric surface (i.e. roughness) compared with untreated or silicone polymer treated silk fabric.
4.5.5 Tensile Properties
The extensibility (EMT) measures the ability of a fabric to extend under load in the warp and weft directions, and the tensile resilience (RT) provides a measure of the percentage energy recovery of the fabric after the measurement of extensibility.
The tensile resilience is considered to be less closely related to the ability of a fabric to recover from wrinkling than compression resilience; indeed, no particular trend in tensile resilience of the treated fabrics is observed that can be related to wrinkle recovery.
4.5.6 Handle Evaluation
The handle of Habutai silk treated with BTCA, Ultratex ESU and EMJ systems were evaluated by using the regression analysis developed by Kawabata [88]. Silk fabric is suitably considered in the category of summer clothing, and the Shari (crispness) and Koshi (stiffness) values
54 TABLE 4.10
HAND EVALUATION OF HABUTAI SILK AFTER TREATED WITH VARIOUS FINISHING AGENTS
TREATMENT KOSHI SHARI (STIFFNESS) (CRISPNESS)
UNTREATED 0.6 0.5
2 % ESU 1. 3 0.9
g. 2 0 EMJ 1.5 1. 9
2% BTCA,6% 1.5 0.9 CATALYST evaluated for this category are given in Table 4.10. Both the stiffness and crispness values increase for silk fabric treated with any of the above reagents, however, the silicone polymer treated fabrics are indeed very soft to touch. Hence analysis with respect to a subjective handle assessment is necessary in conjuction with this data in order to render the hand evaluation data meaningful.
4.6 CONCLUSIONS
For poor recovery fibres such as silk, fabric construction is seen to have a significant influence on wrinkle recovery, where substantial variation occurs between the lightweight plain weave Habutai (23%) and the heavier
Shantung having much coarser float warp yarns (42% wrinkle recovery) .
A significant increase in wrinkle recovery on bleaching of the lightweight plain weave Habutai fabric has been attributed to removal of a "scroopy" finishing treatment, which results in a more flexible fabric. Slight increases are also observed for the other fabric structures, although the effect is most pronounced for the Habutai weave. A staining test carried out with C.I. Direct Blue 22 (83] in order to differentiate between the sericin content of the
55 unbleached and bleached fabrics shows no difference between the two. Therefore, it is unlikely that there is a difference in the extent of sericin on the fabrics which may possibly influence the wrinkle recovery.
Treatment of silk fabric with crosslinking agents has met with limited success with respect to improvement in wrinkle recovery. The results of this work show that treatment with
BTCA, a successful non-formaldehyde crosslinking agent for cotton, gives only marginal improvement in the wrinkle recovery of bleached silk fabric. This is presumably a reflection of the limited penetration of the crosslinking molecule into the highly crystalline molecular structure of silk fibroin, and the limited number of reactive sites
(hydroxyl and amino groups) available for crosslinking.
Despite the high weight gain achieved, solution application of crosslinking agents give rise to variable and limited improvement in dry wrinkle recovery. The wet wrinkle recovery, however, is more readily improved by a variety of solution treatments (e.g. dibasic anhydrides [89], epoxides [59], ethylene glycol diglycidyl ether [60]), and the results of this work show that treatment with BTCA also has potential to improve the crease shedding properties of the wet fabric after washing. In addition, low levels of applied BTCA are found to be adequately wash fast.
56 The application of BTCA does, however, improve the wrinkle recovery of unbleached fabric. Thus the formation of interfibre crosslinks as well as surface deposition by BTCA improves the restoring couple of the fabric coated with a
"scroopy" finishing treatment. The very low wrinkle recovery of unbleached Habutai is thus increased substantially by BTCA treatment to the same level as that of bleached fabric treated with BTCA.
Treatments utilising solutions of crosslinking agents or gas phase reactions with formaldehyde have, in general, exemplified the difficulty in achieving a significant number of crosslinks within the structure of fibroin which can impart improvement in wrinkle recovery. This difficulty is considered to be due to the highly crystalline nature of fibroin combined with the limited number of hydroxyl and amino groups available to participate in crosslinking.
Crosslinking treatments for silk will therefore continue to provide limited success unless carried out with reagents in the gas phase, where penetration is achieved with small molecules other than formaldehyde.
The application of silicone surface elastomers to silk results in a noticeable improvement in both dry and wet wrinkle recovery. These reactive polymers are known to form interfibre bonds within the yarn which effectively couple
57 to provide an elastic restoring force after a bending deformation [82). These surface polymer systems are evidently more effective as finishing agents for silk than crosslinking treatments as they do not necessarily require reactive sites on the fibre. Since they are self crosslinking, soft, and elastic in character, the interfibre bonds formed as a result of surface polymer deposition contribute to an added restoring force after fabric deformation. Thus, these surface polymers are suitable for the improvement in wrinkle recovery for all fibre types, although it must be noted that large application levels (>15%) are often required. In the case of silk, however, smaller application levels resulting in 3-4% weight gain are sufficient to provide a significant improvement in wrinkle recovery with little or no loss of the desirable attributes of fabric handle.
The trend observed in the compressional properties of the treated silk fabrics is similar to that observed for wrinkle recovery; this correlation is consistent with that found by other workers [ 85] for wool fabric. While the shear hysterisis of silk fabric is very small compared to other fibre types, little variation is seen between the shear behaviour of the treated silk fabrics. The bending properties indicate that the silicone fabrics exhibit reduced resistance to bending compared with the BTCA
58 treated or untreated fabrics. However, the residual curvature of bending does not appear to correlate with the extent of wrinkle recovery for these treated silk fabrics.
The surface properties indicate that the treatments do not significantly alter the "smoothness" and thus desirable handle of the treated silk, although the silicone elastomer treatments are noticeably softer than BTCA treated fabric by subjective feel.
59 CHAPTER FIVE
INTRODUCTION TO DYE RESIST EFFECTS
60 5.1 GENERAL
Attractive coloured patterns on textile fabrics are generally accomplished by direct, discharge or resist printing methods. The latter two processes offer the most
varied multicoloured affects, although they differ markedly
in their respective application techniques. Discharge
styles are achieved by initially applying the dyestuff of
the required ground shade onto the fabric and then
printing the fabric with a reducing agent that discharges
the dye within the printed area. Resist printing (or
dyeing) involves the initial application of the resist
agent onto the undyed fabric, prior to the application of
dyestuffs. During subsequent dyeing the resist agent serves
to retard the diffusion of the dyestuffs into the fabric
within the applied area.
Discharge or direct printing is usually carried out on silk
fabric, despite the wide variety of chemical resist methods
used on cellulosic and wool textiles. The reason for this
perhaps stems from the very delicate and unique handle of
silk fabric which may deteriorate significantly during the
application of chemicals required for resist printing or
dyeing. A critical comparison of the importance of these
various methods for silk textiles is difficult due to the
very limited amount of published information on the
61 printing or dyeing of silk. This area remains very much a trade secret of printers and dyers specialising in this exclusive fibre.
Some early methods trialed on silk which claim to produce dye resist effects include the treatment of silk with sulphuryl chloride [ 90 J; mixtures of gum combined with immersion in formic acid [91); or acetylation/alkylation
reactions [ 92 J • None of these methods have appeared to have been utilised, presumably because of the inadequate dye
resist obtained and/or detrimental effects to the fabric handle.
More recently, reactive resist agents have been developed and applied to wool fabric, which successfully achieve
resist effects during printing or dyeing. Two commercially
utilised reactive resist agents are sulphamic acid and Sandospace R (see Section 5.3.2 and 5.3.3). The latter of these two has been applied to silk fabric also, although
no systematic comparison has been published with respect
to the extent of resist achieved for acid, metal complex and reactive dyestuffs.
In this work these two resist agents were separately
applied to silk fabric and the dye resist effects achieved
62 Azo
Cl. Acid Red 29
Triphenylmethane
Cl. Acid Violet 19 Anthraquinoid O NI½ II I OJNa I I
II 0
Cl. Acid Blue 25
Fig. 5.1. Acid dyes towards acid, metal complex and reactive dyestuffs were
comparatively assessed.
5.2 DYEING OF SILK
Silk may be dyed with the same dyestuffs used in the dyeing of wool, namely: acid dyes, metal complex dyes (premetallised dyes) and reactive dyes. Acid and
premetallised dyes have been widely used in silk dyeing
although, more recently, interest has increased regarding the application of reactive dyes onto silk, as these dyestuffs offer a wide range of bright shades with superior fastness properties [93].
5.2.1 Acid Dyes
Acid dyes are classified into three chemical types, viz; azo, anthraquinoid and triphenylmethane. Examples of these
dyes are given in Figure 5.1. Acid dyes derive their name
from the fact that they are applied under acidic
conditions. All acid dyes contain the sodium salt of a
sulphonate group (D-S03- Na+) attached to the dye molecule.
The sulphonate group is ionized even at low pH, and hence
is capable of forming salt links with the protonated amino groups of a protein fibre. These salt links are stronger
than hydrogen bonds and confer dye substantivity to the
63 fibre [94].
Acid dyestuffs may be divided into three groups according to their respective application conditions.
1. Acid levelling dyes: These dyes are of small to moderate molecular size, monosulfonic, and commonly applied from a
strongly acidic dyebath (pH 2.5 - 4) containing sodium
sulphate. The small molecular size of the dye leads to a
low affinity of the dye for the fibre, resulting in more
even dye uptake. These dyes are used primarily for dyeing
apparel, knitted goods, carpet and upholstery [95]. They
exhibit only a low to moderate fastness to washing and wet
treatments. Fixation can be improved by fixing the dyed
fabric with tannic acid or synthetic fixers [95, 96].
2. Acid Milling (weakly acid) dyes: These dyes require a
weakly acidic dyebath (pH 3.5 5) containing sodium
sulphate. They are of higher molecular weight than
levelling dyes, and maybe disulfonated or polysulfonated.
Hence, they have a slightly greater affinity to the fibre
than acid levelling dyes, resulting in decreased levelling
action with a tendency to form spots or streaks [95]. The
wet fastness properties of these dyestuffs vary from
moderate to good.
64 Hi) Hi) Hi() ~1/
S°-3Na
Cl.Acid Red 214
Fig. 5.2 1:1 Metal Complex dye
SOzCH3 -N=N~ 0 ! 0.,...... "-er/ 6--N=No----- f '---o H3COzS
Cl.Acid Violet 78
Fig. 5.3 2:1 Metal Complex dye 3. Acid Supermilling dyes; These dyes are applied from neutral or weakly acidic dyebaths (pH 4.5 - 6.5). They are generally of large molecular size, and have very high
affinity for the fibre. Therefore, care needs to be
exercised in their application in order to obtain a level
dyeing. They do, however, confer very good wet fastness properties.
5.2.2 Metal Complex Dyes
The time consuming application of chrome dyes to textile
fabric led to the development of metal complex or
premetallised dyestuffs. In these dyes the metal is
incorporated into the dyestuff by the manufacturer, hence
providing more simplified and economical dyeing procedures
[93]. The dyestuffs contain one metal atom, either chromium
or cobalt, generally complexed with selected azo acid dyes. Either one or two molecules of a monoazo dye containing two
hydroxy groups, located o,o' to the azo linkage, chelate
to the central metal ion [93].
There are two types of metal complex dyes viz
1. 1:1 metal complex dyes: these dyes generally contain
mono- or disulfonic acid solublising groups (Fig.5.2) and
require strongly acidic conditions for application. These
65 dyestuffs are rarely used in the dyeing of silk and will not be discussed further.
2. 2:1 metal complex dyes: two dihydroxy azo dye molecules
complex with one metal atom, either chromium or cobalt
(Fig. 5. 3) . In general, 2: 1 metal complex dyes do not
contain -S03H or -COOH groups. Instead, the solubilising
groups of this dye class are: -S02CH 3 and -S02NH 2 groups,
and the dye molecule carries an overall negative charge
which is not localized but distributed over the entire dye
molecule [96]. These dyes are applied from a neutral to
slightly acidic dyebath (pH 5 - 7), have good level dyeing
properties, and provide excellent light and wash fastness.
The dye-fibre interactive forces in this case are
considered to be largely hydrophobic.
5.2.3 Reactive Dyes
Reactive dyestuffs have more recently received increased
attention due to the wide range of brilliant colours within
this dyestuff range that possess very good wet and light
fastness properties compared to that obtained with acid or
metal complex dyes. Reactive dyestuffs differ from all
other classes of dyes, in that they contain a reactive group which is capable of forming a covalent bond with
nucleophilic -SH, -NH 2 and -OH groups of textile fibres.
66 Monochloro difluoro pyrimidine
Drimalan type
Dichlorotriazine
Procion Yellow MX-R
Monochlorotriazine
Procion Brilliant Blue HGR
Fig. 5.4. Reactive dyes Acryloylamino
Cl.Reactive Blue 19
Vinyl Sulfone
Cl.Reactive Yellow 17
Fig. 5.4. Reactive dyes During the reactive dyeing of silk, fixation is difficult to achieve under neutral conditions due to the small number
of reactive -NH 2 groups, and almost complete absence of -SH groups within this fibre (relative to that in wool) . Reactive dyeing of silk is therefore more easily carried out at pH 8-9, allowing the hydroxyl functional groups to participate to a large extent as reactive dye sites, analogous to the reactive dyeing of cellulosic fibre. This has been confirmed in the dyeing of silk with a
difluorochloropyrimidine reactive dye, where Xia [83] showed that reaction preferentially occurred with the hydroxyl groups of tyrosine residues.
Reactive dyes are classified according to their reactive groups [95], as shown in Fig .5.4.
5.3 REACTIVE DYE RESIST AGENTS
5.3.1. General
Dye resist agents can be classified into two main groups
depending on the technique used to achieve the resist
effect [97], viz:
1. Mechanical resist agents: These resist agents (e.g.
resins, thickeners, and pigments, such as china clay, and the oxides of zinc or titanium) simply form a physical
67 0 0 H"' H"' 11 11 - N S-QH H-N+ S-0 11 H/ 11 H/ 0 0
Acid form Zwitter ion form I II
Fig.S.S Sulphamic acid
N~l @-NH_/ N HOS \ // 3 N~l
Fig.5.6 Sandospace R barrier between the fabric and the dyestuff. Batik printing
of cotton is a classic example of a mechanical resist in which wax is applied to the fabric prior to the dyeing process. The waxing and dyeing procedure can be repeated many times in order to achieve various coloured effects on
the fabric.
2. Chemical resisting agents: These agents include a wide
variety of chemical compounds, such as acids, alkalis,
various salts, oxidising and reducing agents which serve
to retard dye fixation into the fibre. The merits of these
various resist agents has been dicussed in several reviews
[1, 17, 97-98]. Only the recently developed reactive resist
agents, sulphamic acid and Sandospace R, pertinent to this
work are discussed here.
5.3.2 Sulphamic acid
Sulphamic acid is a stable white crystalline solid, non
hygroscopic, non volatile, colourless, and odourless, with
a pH range similar to hydrochloric and nitric acids [99].
Sulphamic acid is soluble in water and exists in solution
either in the acid form (I) or in the zwitterion form (II)
as shown in Figure 5.5 [100].
As early as 1955 Sandoz [101] patented a process involving
68 the reaction of sulphamic acid with protein fibres, which conferred a high degree of resist to acid dyestuffs. The reaction mechanism for wool fibre was subsequently investigated by Elliot [102], Lewin [103], and more recently by Cameron and Pailthorpe [104]. The latter researchers also established the optimum conditions for promoting reaction between sulphamic acid and the wool fibre. The reactions proposed to take place are as follows:
( 1)
( 2)
The work of Cameron and Pailthorpe [104] showed that direct reaction between cysteine and sulphamic acid did not occur, as evidenced by the lack of change in thiol content between treated and untreated wool. Further, for an 8. 5% weight gain of bound sulphamic acid, only a small proportion
(0.5%) was determined to be due to the reaction of sulphamic acid with the -NH 2 groups of amino acid residues (eqn. 1). Thus, by difference, it was concluded that the predominant reaction taking place was that between sulphamic acid and the hydroxyl-containing amino acid residues (eqn. 2). The presence of serine-0-hydrogen sulphate was in fact detected in amino acid hydrolysates of sulphamic acid treated wool [ 104] . The reaction is
69 therefore considered to be similar to that taking place between sulphuric acid and wool during "over carbonising" [105, 106]. Although silk fibroin has few nucleophilic
amino acid residues compared to wool keratin, the hydroxyl
group content of silk is substantial; serine, 12.6%,
threonine, 1. 5%, and tyrosine, 10. 6%. The corresponding
values for wool are 9. 5%, 6. 6%, and 6. 1%, respectively.
Hence, a significant extent of reaction can be expected to
take place between sulphamic acid and silk.
5.3.3 Sandospace R
Sandospace R (Fig.5.6) is a dichloro-s-triazinyl reactive
compound and may be regarded as a colourless reactive dye.
This compound was patented by Sandoz initially as a powder
which when applied to wool confers dye resist effects. The
powder product was withdrawn because of skin irritation
problems, and has since been re-introduced as a paste. As
in the case of reactive dyeing, this molecule can be
exhausted onto wool or silk under appropriate conditions
that promote reaction between the dichlorotriazinyl group
and nucleophilic residues in wool or silk. Exhaustion may
be carried out at 80-90°C with minimal fibre damage, and
therein lies the advantage of utilising this dye resist
agent over sulphamic acid. The latter requires a very high
curing temperature of 150-160°c.
70 The mechanism by which a dye resist effect is conferred is
considered to be similar for both sulphamic acid and Sandospace R treated wool fibres [98]. The anionic
sulphonate group is considered to provide a resist effect toward acid dyes, while covalent bond formation between the
resist agent and nucleophilic residues on the fibre serves
to block the fibre reactive sites necessary for the
covalent binding of reactive dyes.
5.4 AIM OF THE PRESENT WORK
Both sulphamic acid and Sandospace Rare established dye
resist agents for wool. While sulphamic acid provides
superior resist behaviour over that of Sandospace R,
fixation of the latter agent may be carried out at lower
temperatures and is therefore potentially less damaging to the fibre. Moreover, the conventional exhaustion treatment with Sandospace R may be more easily incorporated into a
normal processing line for fabric dyeing.
Sulphamic acid has been shown to react primarily with the
hydroxyl residues of wool and hence can be expected to react with silk which has a significant proportion of these
nucleophilic residues. The treatment of silk with sulphamic
acid has not been investigated to date. However, Sandospace
R has been shown to confer some retardance of reactive dye
71 fixation on silk. This molecule behaves essentially like a colourless reactive dye, and can react with the fibre under conditions that promote reactive dye exhaustion. However a systematic investigation of the dye resist effects shown toward acid, metal complex and reactive dyes has not been carried for silk fabric.
Therefore, in this work, both of these reactive resist agents were applied to silk fabric, and their respective capacity to resist the fixation of acid, metal complex and reactive dyes was compared.
72 CHAPTER SIX
DYE RESIST BEHAVIOUR OF SILK: MATERIALS AND METHODS
73 Acid dye : Coomassie Navy Blue type
2:1 Metal complex dye: PM-4
2 _(r;\_ N F N=N-fe NH.../ ~ H03 S SOJH Cl V F
Reactive dye : Drimalan type Reactive dye: Lanasol type
Fig.6.1 Structure of dye types used in the dyeing process 6.1 Materials
6.1.1 Fabric
Plain weave Habutai silk obtained from The Australian Silk
Wholesalers, Sydney, Australia was used in all of these
experiments. The fabric was bleached with hydrogen peroxide
as described in Section 3.2.1.
Plain weave thermoset wool was obtained from Yarra Falls,
Ltd. Melbourne.
6.1.2 Chemicals
Sulphamic acid was purchased from Ajax Chemicals. Chemical
auxiliaries used in the treatment of silk with sulphamic
acid were urea, and Lissapol TN 450, purchased from Ajax
Chemicals and ICI, respectively. Sandospace R in paste form
was supplied by Sandoz Australia.
Three different classes of dyes were used in these
experiments viz; acid dyes, 2:1 metal complex dyes, and
reactive dyes obtained from ICI, Ciba Geigy and Sandoz,
respectively. The structures are shown in Figure 6.1. The
structure of PM-4 and PM-5 were revealed to the author on
the condition that the trade name would be kept secret.
74 6.2 Methods 6.2.1 Treatment with Sulphamic acid [104)
Silk fabric was immersed in a solution containing sulphamic acid varying in concentration from 1 to 15%, 20% urea and 0.1% Lissapol TN 450. The fabric was squeezed in a pad mangle set to give a wet pick up of 100%. The fabric was dried at 80°c for 5 minutes and then cured at 160°c for
5 minutes. Finally the fabric was rinsed thoroughly with cold water and dried at room temperature.
Treatment of wool fabric with sulphamic acid was carried out in an identical manner [104].
6.2.1.1 Determination of free acid
Treated samples were placed in a 500 ml flask, to which 250 ml of 0.4% pyridine-water was added. The flask was stoppered and shaken well overnight. Then 50 ml aliquots were titrated with 0.01 N sodium hydroxide using phenolphthalein as indicator.
6.2.1.2 Determination of bound acid
Samples were hydrolysed using 6N HCl for 4 hours at the boil. The hydrolysates were filtered into 100 ml volumetric
75 flasks and made up to volume with distilled water. Then 25 ml aliquots were pipetted into beakers, diluted to 100 ml,
and brought to the boil, whereupon 10 ml of 10% barium
chloride was added. The barium sulphate precipitate was
collected in tared No. 4 sintered crucibles and dried at
105°c overnight.
6.2.2 Treatment with Sandospace R [107]
Sandospace R was applied to silk fabric by aqueous
exhaustion. The bath was set with the required stock
solution of Sandospace R varying in concentration from 2
to 20% (owf), with 5 g/1 sodium bicarbonate (pH 8). The
initial temperature was maintained at 40°c for 15 minutes,
then raised to 80°c over 30 minutes, and maintained at this
temperature for further 10 minutes. Sodium sulphate (10
g/1) was then added and the temperature maintained for 60
minutes. Finally, the fabric was rinsed thoroughly in warm
water.
6.2.3 Dyeing
Dyeing of treated and untreated silk fabric was carried out
in the same dyebath, with the percent additives being
calculated on the basis of the combined weights of the
treated and untreated silk fabrics.
76 6.2.3.1 Acid dyeing
The bath was set with 2% dye, 2% formic acid (90%), and 5%
sodium sulphate, pH 4 - 4.5, for a liquor ratio of 1:100.
(All percentages were based on weight of fabric) . The
dyeing process was started at 50°c. After 15 minutes the
temperature was raised to 90°c over a 30 minute period, and maintained for 60 minutes. At this stage complete
exhaustion of the dye had occurred. Wool dyeing was carried
out in an identical manner.
6.2.3.2 2:1 Metal Complex dyeing
The bath was set with 2% dye, 4% ammonium sulphate, and 2%
acetic acid (80%), pH 4 - 4.5, for a liquor ratio of 1:100.
The dyeing was identical to that of dyeing with acid dyes,
and all percentages were based on weight of fabric. Wool
dyeing was carried out in an identical manner.
6.2.3.3 Reactive dyeing
The dyeing process employed was that of Ball et al. [63].
The dyeing recipe used was: 2% (o.w.f) dye, 110 g/1 sodium
sulphate, and 2.5 g/1 sodium carbonate, liquor ratio of
1:100. The dyeing process was started at 40°c. The bath was
set with one-third of sodium sulphate and the required of
77 volume dye stock solution and water. The silk fabric was
then added. After 15 minutes, the temperature was raised
to 90°c over a period of 30 minutes. At this stage the
remaining sodium sulphate was added in 2 batches at 30 minutes and 45 minutes. After a total period of 75 minutes
sodium carbonate was added. The temperature of the dyebath
was maintained at 90°c for a further 15 minutes. The final
dyebath pH was about 9.0.
Wool dyeing was set with 2 % dye and 2% (owf) Albegal B for
a liquor ratio of 1:100, at pH 4.5. The dyeing process was
started at 20°c. After 15 minutes, the temperature was
raised to 40°c over a period of 15 minutes, and maintained
at 40°C for 10 minutes. Then the temperature was raised to
65°c over a period of 10 minutes, maintained at 65°c for 20
minutes, and finally increased to 90°c over a period of 10
minutes. The dyebath was maintained at 90°c for a further
20 minutes. The final dyebath pH was about 6.0.
6.2.4 Resist Effect Evaluation
The extent of resist achieved was quantified using the
following equation:
(K/S) ut - (K/S) t
Percentage resist= ------x 100% (K/S)ut
78 where ut refers to untreated silk and,
t refers to treated silk.
K/S is the Kubelka Munk factor proportional to the amount of dye in the fabric calculated from the reflectance values of the samples, obtained at the wavelength of maximum absorption on the Spectrogard Coler System.
6.2.5 Test Methods
6.2.5.1 Weight gain
Fabric weight gain was based on the dry weight of the samples before and after treatment.
6.2.5.2 Yellowness Index
Yellowness indices (YID 1925) (CIE Lab color scales) value, were evaluated by comparing the treated fabrics to untreated fabrics using the Spectrogard Coler System.
6.2.5.3 Breaking Strength Retention
The breaking strength of treated and untreated samples was determined according to Australian Standard ( AS 2001.2.3-
1988 ) Method A-Ravelled Test. Rate of extension was 100
79 mm/rnin on the Instron Meter.
80 CHAPTER SEVEN
DYE RESIST BEHAVIOUR OF SILK: RESULTS AND DISCUSSION
81 TABLE 7.1 WEIGHT GAIN, YELLOWNESS, AND STRENGTH RETENTION OF SOLPHAMIC ACID TREATED SILK
Cone. of Weight Yellowness Strength Sulphamic gain (%) Index retention acid ( % ) (YID) ( % )
Untreated - 1 100
1 1. 9 17 86
3 3.5 22 88
5 4.6 24 88
10 7.2 23 88
15 9.1 23 89
TABLE 7 .2 FREE AND BOUND ACID OF SULPHAMIC ACID TREATED SILK
Cone. of Weight Free Acid Bound Acid Sulphamic gain ( % ) ( % ) ( % ) acid ( % )
1 1. 9 0.29 1. 8
3 3.5 0.33 -
5 4. 6 0.45 -
10 7.2 0.61 -
15 9.1 0.80 8.4 7.1 TREATMENT OF SILK WITH SULPBAMIC ACID
7.1.1 Weight Gain, Yellowness Index and Strength Loss
The weight gain of silk treated with sulphamic acid increases markedly with increasing concentration of applied sulphamic acid (Table 7.1). During the padding procedure, sulphamic acid is presumably absorbed by the fibre, resulting in higher weight gain than excepted for the lower treatment levels. The high weight gains achieved indicate that a signficant degree of reaction occurs within the fibre. These weight gains are comparable to those obtained for similar treatment levels of sulphamic acid on wool
[108).
Sulphamic acid treated silk, even for low treatment levels, has a much higher yellowness index than untreated silk
(Table 7 .1) . The very high curing temperature necessary for reaction thus results in a considerable increase in fabric yellowness. A strength loss of 11-14% is also observed, although importantly, the subjective handle of the silk fabric is not noticeably changed to any extent.
7.1.2 Free and Bound Acid On Sulphamic Acid Treated Silk
For both low and high treatment levels of sulphamic acid,
82 the determined amounts of total sulphate on the fibre were found to correlate well (<5% error) with the weight gains obtained in each case (shown in Table 7.2 for two treatment levels).
Unreacted sulphamic acid, after curing (in the form of free sulphamic acid) may be extracted from the fabric with 4% pyridine-water and determined by titration. In each case the free acid contribution to the total weight gain was less than 10% (Table 7.2). One exception was observed for the lowest treatment level of sulphamic acid, where 0.29% free acid was determined for 1.9% fabric weight gain (that is, 15% of the weight gain is due to the presence of free acid). Hence, the significant weight gains obtained for reaction of sulphamic acid with silk fibre are a direct result of covalently bound sulphamic acid within the fibre.
Complete saturation of the serine, threonine and tyrosine residues of silk would lead to approximate increases in fabric weight of 6%, 0.4%, and 2%, respectively. Neither threonine-O-S03H nor tyrosine-O-S03H were detected in the reactions of sulphamic acid [109) or sulphuric acid [110) with wool, however, the high weight gains of >8% (for 10-
15% sulphamic acid treatments) indicate that tyrosine and threonine residues of silk probably also participate in the
83 100 ------,-, , , , , ~ ,,,, I 80 · · · · · · · · · · · · · · · · · · · · · · • · · · · · · · · · ·······I············•··· I I I I I I I 60 ...... ,...... ,_ I I ...... ,,,~ I ..... I r;JJ .. .. I r;JJ ..,., ..., ..., I ·-Q) .._ ..._ ...., I ~ ..,...... ,. I 40 ...... :..1····························
20 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · • · · ·
0 ....______._ ___ _..._.
3 5 10 15 Concentration of sulphamic acid (%)
Fig.7.5 Resist effect of sulphamic acid treated silk and wool dyed with Lanasol Red 6G; silk -- , wool . . .. 100------,, , , , , , , I I I 80 · • · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·.,. · · · · · · · · · · · · · · · · I I I I
I I 60 ••••••••••••••••••••••••••••••••• .I• •••••••••••••••••••••• ,_ I I ~ I '-' ....c:l'l c:l'l I ·-(!) I ~ I 40 ...... , ., ...... , , , , , , , , 20 : .: .: .: .-: .-: .-: ."". · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·
0 ....____ _._ ___ _._ ___ _._ ___ _._. 3 5 10 15 Concentration of sulphamic acid ( % )
Fig. 7 .4 Resist effect of sulphamic acid treated silk and wool dyed with Drimalan Red F-2BL; silk -, wool - - -· 100 ,---======-i-----
80 ,- · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · .. · · · , ------, , , , 60 - ...... •..•.., , ...... , , , , ... , ~ , ~ , ·-~ , , ~ 40 ...... ______· ·,, · ..... ~. · .. · .. · · · .. · ... · · · ...... · · ·
20 -························································
Q I I I I I 3 5 10 15 Concentration of sulphamic acid (%)
Fig.7.3 Resist effect of sulphamic acid treated silk and wool dyed with Irgalan Red 2GL KWL 200%; silk wool -- -. 1oor----======-,
~ , , , , 80 - · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ,· ·, .,. · · · · · · · , , I I I I ~ 60 -··· · ...... / ...... ~ I ~ I ·- 40 ... · · · · · · · · · · · · · · · · · · · · ·······.I.··························I , --- , , --- , , , , 20 -· ...... 0 I I I I I 3 5 10 15 Concentration of sulphamic acid (%) Fig. 7.2 Resist effect of sulphamic acid treated silk and wool dyed with the PM-5 dye; silk-, wool -- -. 100 ------, 80 , , , , , , , , ········································r············ I I I I I I I 40 · · · · · · · · · · · · · · · · · · · · · · · · · · · · • · · · · ./. · · · · · · · · · · · · · · · · · · · I I I I I I I ...... '...... 20 I ... I ...... -...... - ... -- o------3 5 10 15 Concentration of sulphamic acid(%) Fig.7.1 Resist effect of sulphamic acid treated silk and wool dyed with the PM-4 dye; silk - , wool -- -. TABLE 7.3 RESIST EFFECT OF SOLPHAMIC ACID TREATED SILK DYED WITH ACID DYES Dyes type Coomassie Navy Lanasyn Grey Blue 2RN 140 BLRN 200% Cone. of Sulphamic % resist effect % resist effect acid (%) 1 98.25 97.71 3 99.64 99.24 5 99.77 99.38 10 99.82 99.53 15 99.80 99.51 reaction of silk with sulphamic acid. 7.1.3 Dye Resist Effects of Sulphamic Acid Treated Silk As expected, the treatment of silk with sulphamic acid confers an excellent resist toward acid dyes (Table 7.3), similar to that observed in the case of wool [111]. The introduction of an anionic sulphonate group (ionized even at low pH) into the silk fibre by reaction with sulphamic acid is clearly responsible for reducing the affinity of (anionic) acid dyestuffs. Treated silk fabric was also dyed with three different 2:1 metal complex dyes. The resist effect obtained for varying treatment levels of sulphamic acid is shown in Figs. 7.1 - 7.5. In addition, the dye resist behaviour for wool treated with sulphamic acid was carried out for comparison purposes. For all three metal complex dyes a far superior resist is obtained for silk treated with sulphamic acid than for wool, particularly for low treatment levels of sulphamic acid. Dye resist effects for metal complex dyes on wool are, in general, difficult to achieve as a result of the largely hydrophobic character of these dyestuffs, which are consequently substantive within the hydrophobic regions of 84 the wool fibre. Therefore, unlike anionic acid dyestuffs, 2: 1 metal complex dyes do not require the presence of protonated amino groups (-NH/) in order to become substantive to the fibre. At high treatment levels of sulphamic acid (>10%), however, the increased anionic character of the wool fibre presumably reduces the affinity of these weakly anionic dyestuffs. By comparison, the increase in ionic character of silk resulting from reaction with sulphamic acid, even for very low treatment levels, is apparently sufficient to diminish the hydrophobic interactions necessary for dye substantivity. This difference presumably stems from the relatively more crystalline nature of silk compared with wool. The reaction of sulphamic acid within the limited amorphous regions of silk increases the anionic character of these regions sufficiently, so as to significantly retard the uptake of these weakly anionic, and large dye molecules. The differences observed between the three metal complex systems may be due to the relative molecular size of the dye systems. Significant differences in dye resist effects are also observed for the reactive dyeing of wool and silk treated with sulphamic acid (Fig.7.4 and 7.5). The most important nucleophilic residues for reactive dye binding in the wool 85 TABLE 7.4 WEIGHT GAIN, YELLOWNESS INDEX, AND STRENGTH RETENTION OF SANDOSPACE R TREATED SILK Cone. of Weight YID Strength Sandos pace R gain retention (%) (%) ( %) Untreated - 1 100 2 0.2 2 98 6 0.7 2 97 10 1.1 3 98 15 1. 6 3 98 20 2.2 3 98 fibre are considered to be the -NH 2 groups of basic amino acid residues. In silk, however, these residues are far more limited in proportion than in wool, and therefore the dyeing of silk with reactive dyes is necessarily carried out at pH 8-9, where reaction occurs predominantly at the hydroxyl-containing residues. The treatment of silk with sulphamic acid would be expected to effectively block a large proportion of these residues by the formation of bound sulphate (Silk-O-S03H) . Hence, a significant dye resist effect can be expected at low treatment levels, as is observed. In the case of wool, the reactive dye sites (-NH2 groups) are less effectively blocked except at the higher treatment levels of sulphamic acid. 7.2 TREATMENT OF SILK WITH SANDOSPACE R 7.2.1 Weight Gain, Yellowness Index and Strength Loss Exhaustion of Sandospace Rat pH 8 will result in reaction at the hydroxyl residues of the fibre, as in the reactive dyeing of silk. However, these mild treatment conditions of pH and temperature do not confer extensive reaction, as evidenced by the very low fabric weight gain (2%) obtained even for high treatment levels (20%) (Table 7. 4) . The treatment of silk with Sandospace R is therefore far more limited in the extent of reaction than in the case of sulpharnic acid. Virtually no strength loss, increase in 86 lOOr------, ~ 80 ······················;·/·-::':' ...... ::. / .···· / ...... / ...... · ~- ~- :-. ~- :-. :-...... -- . ~ 60 ...... -I . -...... '~-~ :"". '-' I .· .··,, I .·· , .· .· , , I .. , , 40 ···I··· · :-:·. :,·.-: · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · I .,.:,, I "._I 20 · · ,. · ... · ...... · · 0 ...... ______....______...... 2 6 10 15 20 Concentration of Sandospace R (% owf) Fig.7.6 Resist effect of Sandospace R treated silk; Coomassie Navy Blue 2RN 140-,PM-4---, PM-5 · ·, Drimalan Red F -2BL - - . yellowness index or loss of fabric handle was observed. 7.2.2 Dye Resist Effects of Sandospace R Treated Silk The dye resist effect observed toward acid, metal complex and reactive dyestuffs is shown in Fig.7.6. As expected, the resist effect toward all these dye classes increases with an increasing level of treatment with Sandospace R. For a practical level of treatment, however, the resist effect provided by Sandospace R is inferior to that conferred by sulphamic acid. Comparing the Sandospace Rand sulphamic acid treated silk fabrics, for a weight gain of approximately 2%, similar levels of dye resist behaviour are observed for all three dye classes. This is consistent with both these reactive molecules conferring resist effects essentially via the same mechanism of reactive site blocking combined with anionic dye repulsion. A somewhat diminished resist effect is obtained for the metal complex dye systems in the case of Sandospace R treatments, perhaps as a result of the hydrophobic benzene ring in Sandospace R, which may increase the potential for hydrophobic dye fibre interaction. For all three dye classes, an application level of Sandospace Rat least as high as 10% is necessary in order to confer any significant dye resist effect. 87 7.3 CONCLUSION The treatment of silk with sulphamic acid confers excellent dye resist behaviour toward acid, metal complex (2:1), and reactive dyestuffs. At the lower treatment levels (<10%) reduction in breaking strength of the treated fabric is minimal (10-15%), and there is no significant loss of the soft and flexible fibre handle. A high yellowness index is, however, a drawback, although when dyed to achieve tone-on tone effects, the shades obtained are not diminished in brightness. Even at very low treatments levels, tone-on tone effects on silk fibre can be achieved successfully with a wide range of dyestuffs. The most striking difference between the resist effects for sulphamic acid treated silk compared to those for treated wool is observed for metal complex dyes. This is considered to be due to the less amorphous nature of the molecular system of silk, where the introduction of an anionic moiety has a more pronounced effect on diminishing the substantivity of these large, and weakly anionic dye molecules. Sandospace R reacts to a much lesser extent with silk fibre than sulphamic acid and thus provides only marginal dye resist effects. However, tone-on-tone effects may nevertheless be achieved for weight gains of 1-2%, and the desirable properties of silk such as whiteness, breaking 88 strength and the soft fabric handle are not adversely affected. 89 CHAPTER EIGHT CONCLUSIONS 90 Efforts to improve the wrinkle recovery of silk fabric by solution treatments has met with limited success. Silk has a very limited number of nucleophilic groups, and hence crosslinking treatments that improve the wrinkle recovery of cotton or wool, by limiting polymer slippage, have only a marginal effect on silk. Surface polymer deposition can provide more significant improvements in wrinkle recovery, and at the present time offers the best option, considering fabric handle, treatment practicalities and environmental concerns. An important finding from this work is that while the dry crease recovery of silk is difficult to alter, improvement in wet crease recovery is more easily achieved, thus imparting a better "smooth-drying" property to the treated silk fabric. Furthermore, the treatment of silk to produce a "scroopy" handle appears to exacerbate the problem of poor wrinkling behaviour, particularly in the case of a light weight plain weave fabric. The achievement of multicoloured effects on silk fabric is an extremely important property relating to the popularity of this fibre in fashionable apparel. The work described in this thesis has shown that the treatment of silk with sulphamic acid produces excellent dye resist effects towards the uptake of acid, metal complex and reactive 91 dyestuffs. The ability to utilise reactive dyes in achieving coloured effects is a valuable advantage, as the washfastness of bright colours within this class is generally superior to those of acid and metal complex dye classes commonly used on silk. The handle and strength of the treated fabric is not adversely affected, and the yellowness index is sufficiently low, so as not to dull the pale shades obtained. Resist printing or dyeing with sulphamic acid is therefore an important finding, offering an alternative to the currently used discharge methods which are brought with problems for the achievement of multicoloured effects on silk fabric. In conclusion, highly crystalline nature of silk permits only limited penetration by chemicals serving to modify fibre properties. This characteristic is a disadvantage when seeking to improve the wrinkle recovery of silk, but it is used to advantage is achieving dye resist effects, particularly with sulphamic acid as the resisting agent. 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