Printing and finishing of silk fabrics

Printing and finishing of silk fabrics 8.1

8

Introduction to printing of silk

Silk fabric, because of its relative rarity, unique lustre, softness, and drape, has over thousands of years been considered the most valuable and prized fibre available. Recent fashion trends have shifted to ‘natural’ clothing, which has resulted again in an increased demand for silk fabrics, particularly printed articles. Exact estimates of printed silk are difficult to give but figures in the region of 150 million square meters are of the right order. Compared with other fibres, discharge printing holds a far higher share of the production (around 50–60%). Printing on silk has been practiced for centuries and the recently increasing popularity of printed silk products has led to questions as to whether silk goods can meet today’s requirements with respect to current fashions, printing techniques, wear behaviour, and easy-care properties (Anon, 1981). Printed silk has always formed a high proportion of the total output of the silk industry. Articles such as ties, headscarves, and ‘haute couture’ dresswear have traditionally been printed with a range of designs and colourings. Generally, because of the exclusiveness of the finished article and the incorporation of dry cleaning labels, little regard to fastness performance has been given. This lack of fastness performance is particularly true with discharge styles, where the range of illuminant dyes provides extremely poor fastness levels.

8.2

Methods of printing

At the turn of the century, silk was generally printed by means of manually engraved copper rollers or handmade blocks. Roller printing has since lost its original significance and is employed only on a very small scale. For the production of extremely fine longitudinal stripes and very fine outlines, the copper roller is still the method of choice. The larger part of silk is, at present, printed on by means of flat screen on flat bed printing machines. The mechanization of the hand screen-printing technique, for instance, by the introduction of automatic printing trolleys, extension of tables up to 100 m long, and improved drying conditions within the unit, has permitted a more rational mode of operation. Exclusive designs on silk call for a high number of different colours, for example, for scarves and shawls where 50 or more colours are to be applied. Thus for these articles, continuous methods of printing find little use. The rotary screen-printing technique is seldom used for printing on silk, which is partly due to the limited yardages produced from each design. The printing is more or less the same as it is applied for other fibres. Considering the higher price of silk prints, customer demands Silk. https://doi.org/10.1016/B978-0-08-102540-6.00008-5 © 2019 Elsevier Ltd. All rights reserved.

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for quality standards, namely, fineness and sharpness of motifs and levelness of areas, are rigorous. An essential prerequisite for good printing results is effective fixation of the fabric on a suitable printing table. Glues such as thermoplastic or permanent adhesives should be selected to meet the requirements of the printing technique. In the case of handling extremely thin fabrics including chiffon, voile, and georgette, it is currently the usual practice to attach two to three layers on top of each other on the table by means of pins, or to glue the fabric onto the blanket of the screenprinting table.

8.2.1 Composition of printing paste A large proportion of silk is printed in the form of exclusive designs. In keeping with the high value of prints on silk, the requirements for sharpness, definition of motifs, and levelness within the printed areas are particularly high. Good results can be achieved by choosing the proper printing paste. Silk can be printed in direct, discharge, and resist styles of printing. The general composition of print paste includes the following essential ingredients: l

l

l

l

l

l

Dyestuff Thickeners Hydrotropic agents such as solvents, solution aids, and humectants pH-controlling agents Oxidizing agents for reduction protection Defoaming agents

8.2.1.1 Dyes used Silk can be printed with acid, metal-complex, direct, basic, reactive, and natural dyes. Azoic, vat, and solubilized vat dyes as well as pigment colours can also be used but to a limited extent.

8.2.1.2 Thickeners Printing is considered as localized dyeing therefore the use of thickener is essential in printing. The thickener provides viscosity to the print paste to allow the application of dye in the form of design and prevents the spreading of dye beyond the printed portions thus maintaining the sharpness of the design. The suitability of the thickener in printing with different types of dyes depends on the compatibility of the thickener with the chemicals used in the printing paste. Acid and metal-complex dyes on silk are applied from an acidic medium. Therefore thickeners stable to acidic pH should be used. Basic dyes are also printed in an acidic medium. However, anionic thickener would react with basic dyes leading to dye precipitation and difficulty in the removal of thickener during washing-off. In silk printing, the thickener should not show alkaline reaction. Therefore, if

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necessary, thickeners should be neutralized with non-volatile organic acids to avoid faulty prints. The most commonly used thickeners for silk printing are modified locust bean and guar gum thickeners. These are available in the market under the trade names Meypro gum and Indalca. These thickeners are stable under acidic pH and therefore most commonly used for printing of silk with acid, metal-complex, and direct dyes. Sodium alginate is the only thickener which is suitable for printing of reactive dyes on silk. It is available under the trade names Manutex, Algogel, and others. Sometimes, in order to obtain improved flow properties and level prints, conventional thickeners are used in combination with oil-in-water emulsion thickeners. Emulsion thickeners can be prepared by mixing a small quantity of water (15–20%) with kerosene or mineral turpentine oil in the presence of a suitable emulsifying agent under high-speed stirring. Emulsion thickeners are most commonly used for pigment printing. However, due to health reasons, pollution control, and non-availability of kerosene, emulsion thickeners are being replaced with polyacrylate-based synthetic thickeners for pigment printing.

8.2.1.3 Hydrotropic agents: Solvents, solution aids, and humectants Hydrotropic agents are water-soluble compounds that increase the water solubility of other compounds. Hydrotropic agents are short-chain aromatic sulfonates and include chemicals like benzyl sulphanilate (Sodium Salt B) and compounds containing carbonyl groups (urea formaldehyde) or those containing OH groups (glycerine, glycols, phenols). Many compounds like urea, glycols, and phenols have multiple functions as solvents, hygroscopic agents, swelling agents, and so on (Miles, 1994).

8.2.1.4 Solvents Solvents improve the solubility of dyes and prevent the aggregation of dyes in the print paste to ensure level prints with high colour yield. Commonly used solvents include diethylene glycol and thiodiethylene glycol. Thiodiethylene glycol is a classic solvent. Most dyes dissolve in it very well, especially when it is used in combination with urea. It is important to note that the total amount of solvent should not be too high in order to maintain the sharpness of the prints, especially during steaming.

8.2.1.5 Hygroscopic agents The function of hygroscopic agents in printing paste is to take up a controlled amount of moisture during steaming to allow fixation of dyes. Glycerine, diethylene glycol, and urea are generally used as hygroscopic agents. The quantity of hygroscopic agents used depends on the quantity of steam in terms of moisture content.

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8.2.1.6 pH-controlling agents Acid, metal-complex, and direct dyes require acidic conditions during steaming for dye fixation. To achieve better fixation, organic acids (e.g. tartaric acid) or their ammonium salt (ammonium tartrate) are used. In some cases, ammonium sulphate can be used. Reactive dyes are fixed on silk under alkaline conditions. Mild alkalis like sodium bicarbonate, sodium carbonate, or sodium acetate are used when print fixation is carried out by steaming. Sodium silicate is used in the pad-batch method of dyeing.

8.2.1.7 Reduction protection Certain dyes, especially those containing azo groups, are susceptible to the reducing influences of thickener and or fibre itself during steaming, resulting in loss of colour value. In the case of acid and metal-complex dyes, 5–10 g/kg sodium chlorate (1:2) is used in the print paste as reduction protection for sensitive dyes under unfavourable steaming conditions. In the case of reactive dyes, sodium salt of m-nitrobenzene sulphonic acid (Resist Salt H) is used as a mild oxidizing agent.

8.2.1.8 Defoaming agents Foaming that is formed during mechanical agitation of print paste can be reduced by adding defoaming agents to the print paste. Silicone defoamers are quite effective and popular. Perminal KB (ICI) is a yellowish-brown liquid that is an aqueous emulsion of sulphated sperm oil and pine oil and is anionic in nature. Emulsified pine oil and hydrocarbons can also be used as defoaming agents.

8.3

Silk printing styles

8.3.1 Direct style of printing Printing on silk fabrics using the direct style of printing can be done by using acid, metal-complex, direct, basic, and reactive dyes. Before printing, it is extremely important to prepare the silk fabrics. The basic preparatory processes are degumming, scouring and bleaching, and optical whitening. Degumming and scouring are normally carried out in an alkaline medium using soap and soda for 1 to 2 h at boil. The bleaching is carried out with either oxidizing or reducing agents or both. The addition of a suitable brightening agent is advised in the final bleach bath to improve whiteness of the fabric. Generally, screen-printing is done. Block printing is also carried out on a limited scale. The printing sequence essentially consists of: Print ! Dry ! Steam fixation ! Wash ! Dry

8.3.1.1 Printing with acid, metal-complex, and direct dyes These are the most important classes of dyes for silk printing. The general printing recipe used is presented in Table 8.1

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Table 8.1 General printing recipe x 50 50 y 400–600 z 1000

g g g g g g g

Acid, metal-complex or direct dye Urea solvent Boiling water Thickener paste Acid donor Total

8.3.1.2 Urea and solvent Urea serves the purposes of dye dissolving and dye fixation. However, if the steam is too moist, the amount of urea must be reduced or even eliminated. Complete dye solubilization in a limited quantity of water used in print paste is ensured by incorporation of a suitable solvent. Thiodiethethylene glycol is a classic solvent. Most dyes dissolve in it, especially when it is used in combination with urea. Solvents are dye specific. Excessive concentration of urea and solvent should be avoided to prevent the flushing of prints during steaming.

Thickeners Modified locust bean and guar gum thickeners are most suitable. These are available in the market under various trade names such as Meypro gum, Indalca AGBV, Indrez AGBV, and others. Thickeners with high solid content are preferred for outlines and small motifs to maintain sharpness, whereas thickeners with low solid content are used for larger areas for levelling effects and reducing the possibility of crack marks after printing.

Acid donor To achieve better dye fixation during steaming, free non-volatile organic acids such as tartaric acid, or acid-liberating salt like ammonium tartarate (e.g. 50–60 g, 1:2 per kg), are used. In some cases ammonium sulphate can also be used.

Preparation of print paste Usually the dye, urea, and solvent are mixed and boiling water is poured over the mixture. In some cases the solution must be boiled again and stirred to ensure complete dye solubility. It is advisable to stir the solution into the thickener paste as quickly as possible. The acid donor and other auxiliary products, such as reduction protective agents, anti-coking agents, and defoamers, are added after the print paste has cooled down through bolting cloth (used for screen making) before the printing operation.

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Printing, print fixation, and washing The silk fabric is usually printed by normal or automatic flatbed screen-printing operation. After printing, the fabric is dried at low temperature to remove moisture applied through the print paste. This is necessary to prevent the printed goods from markingoff during the subsequent steaming operation. After drying the prints are fixed either continuously in a Festoon steamer for 30–60 min at 102°C or in a star steamer at atmospheric pressure at 102–104°C. The duration of steaming is adjusted according to the depth of colour and the print coverage and the type of steamer used. In India and China primitive type of steaming arrangements are used at the cottage-industry level. Therefore each printing unit has to standardize steaming conditions to get satisfactory results.

Printing of silk with reactive dyes Metal-complex dyes produce prints on silk with satisfactory wash fastness. However, acid, direct, and basic dyes are poor in wet fastness because the binding forces between dye and fibre are based on electrostatic, van der Waals, and hydrogen-bond interactions only. The demand for machine-washable silk prints has promoted interest in the use of reactive dyes. The advantage of these dyes over other dye classes is that they offer a wide range of bright shades with excellent fastness properties. With the increase in consumer awareness for better wash fastness (machine washability) printing of silk with reactive dyes holds a good promise. A typical recipe for monochlorotriazine reactive dyes is presented in Table 8.2. After printing and drying, the fabric is steamed under atmospheric conditions at 102°C for 10–15 min. To attain good fastness properties, reactive prints must be washed-off at higher temperature than acid or metal-complex dyes. Care must be taken so that no mechanical damage is inflicted on the fabric during washing.

8.3.2 Discharge style of printing It is estimated that more than 50% of the silk fabric produced is discharge printed, and discharge printing of silk is one area where little technical knowledge has been published. Specialist discharge printers are found in Italy, France, West Germany, the Table 8.2 Reactive dye printing recipe x 100 y 500 30 20 25 1000

Parts Parts Parts Parts Parts Parts Parts Parts

Reactive dye (Procion H) Urea Water Manutex F (10% paste) (low viscosity sodium alginate thickener) Matexil P-AL (mild oxidizing agent) Matexil WA-KBN (combined wetting and anti-foaming agent) Sodium bicarbonate

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United Kingdom, China, South Korea, Japan, Thailand, and India although many designers, students, and others attempt to produce their own lengths for their collections. Detailed methods are rarely published either by the printer (for obvious reasons) or by the dyestuff manufacturers. In this style, the silk is first dyed with direct or acid dyes and is then printed with a discharge printing paste containing thickener, a reducing agent such as Rongalite C, and a whitening agent such as ZnO or TiO2. During subsequent steaming, reduction of azo chromophore of the ground dye takes place, with decolourization at the printed portion; a white discharge print around the dyed background is obtained. Alternatively, for the colour discharge, the discharge paste is mixed with non-dischargeable illuminating colour, such as the vat dye, which can get fixed while the ground is discharged. Discharge printing on silk is a delicate process. Ensuring good results requires a great deal of experience. There are many discharge printing processes available, such as (1) discharges based on sodium, calcium, or zinc sulphoxylate formaldehydes together with discharge-resistant direct, acid, metal-complex, or basic dyes on fabrics dyed with dischargeable, acid, metal-complex or direct dyes or reactive dyes; (2) discharges with tin salt with the same dye classes as in the former; and (3) discharges based on sodium sulphoxylate formaldehyde with vat dyes on silk dyed with reactive dyes. The most popular discharging agents used include sodium formaldehyde sulphoxylate (Rongalite C), zinc sulphoxylate formaldehyde, calcium sulphoxylate formaldehyde, zinc dust, sodium disulphide, and tin chloride (Chu and Provost, 1987). The following precautions must be taken while discharge printing of silk: 1. Proper selection of dye for dyeing before discharge printing is essential. It is required to make sure that the dye used for dyeing is dischargeable. 2. White discharge paste should be neutral or slightly acidic to avoid the degradation of silk during steaming. 3. When vat dye is used for colour discharge effect, the print paste used is alkaline. Quantity of alkali should be taken according to the experience. Otherwise, there is danger of silk degradation during steaming resulting in strength loss or the tearing of silk at the printed portion. 4. From the point of view of preserving the strength of silk, non-dischargeable acid or basic dyes are recommended for colour discharge. Since the discharge paste has mild acidic pH that is not harmful to silk, degradation can be prevented. 5. Steaming is the most critical operation during discharge printing. The whole success of the discharge printing depends on the uniform steaming of the fabric. The present practice of generating steam by heating water underneath the container in which the fabric sits in the form of layers is not satisfactory and the reproducibility of results is not guaranteed. 6. Using a star ager along with the steam generated in a boiler is recommended for satisfactory results.

A typical white discharge paste includes 5% urea, 4% diethylene glycol (DEG), 15% reducing agent such as Rongalite C, 7% zinc oxide or titanium dioxide as a whitening agent, and 1% fluorescent whitening agent. The remainder of the paste is made of thickener and water. For colour discharge, a discharge-resistant dye could be used along with about 4% glycerine or 3% DEG, 2% urea, 12% zinc dust, 6% sodium bisulphite, a thickener such as British gum, and water (Teli, 2015).

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In case of vat discharge, vat dye as an illuminating colour can be used. Vat-dyeready paste is used along with 4% sodium carbonate, 12% Rongalite C, 2% coconut oil, 2% glycerine, thickener, and water. The prints are then dried and steamed for 20 min under pressure of 20 psi. This is followed by cold rinsing and treatment with 5 g/L hydrogen peroxide (35%) at 50°C to get the parent vat dye by oxidation. Even acid, direct, and cationic dyes that are resistant to discharge can be selected as an illuminant. Printing can be carried out with 4% of such dye, 4% glycerine, 12% Rongalite C, and thickener. The prints can be steamed under pressure of 20 psi for about 15 min (Teli, 2015). Pigments can also be used as illuminant colours in discharge printing on silk. In this case, nondischargeable pigment is mixed with 5% zinc sulphoxylate formaldehyde, 2% (1:2) tartaric acid–water mixture, and 90% stock paste. This stock paste can be based on kerosene oil emulsion thickener or synthetic thickener. In case of the former, about 8% carboxyl methylcellulose thickener, 1% emulsifying agent, 5% urea, 10% acrylic binder, and about 12% water are taken; about 65% kerosene is slowly added in constant stirring for about 20 min to prepare such print paste. Once printed, the fabric is dried and steamed for 5 min in saturated steam (Teli, 2015).

8.3.3 Resist style of printing In this style of printing, the colour is prevented from getting fixed at the printed portion. In the case of chemical resist on reactive ground, silk could be first printed with citric acid and then treated with a reactive dye by nip padding, so that at the printed portion, due to the presence of citric acid, the reactive dye is prevented from getting fixed on the silk. There is also a physical resist, which can be obtained using a batik printing technique or a tie-dye technique. The former involves the application of wax on the fabric followed by the crushing of the waxed fabric and treatment of the same with cold brand reactive dyes or azo dyes solution. The latter involves tying the silk fabric at various portions in different forms and then dyeing it. After dyeing, the wax (first case) or threads used for tying (second case) are removed and the fabric shows a localized dyeing pattern or printed effect.

8.3.3.1 Mechanical resist style Batik (wax) style Batik and tie-dye are two popular methods based on mechanical resist. Batik work is found in India, South East Asia, Europe, and Africa, but it reached its highest development in Java. In principle, the batik process consists of applying hot molten wax onto fabric in predetermined areas. When the fabric is creased or folded during the subsequent dyeing operation, very fine cracks are produced at random through which dye can penetrate thus producing a characteristic design with a crackle effect. For printing on silk, very fine and intricate designs are best produced on lightweight fabrics. When selecting fabric, care must be taken that it is free from any permanent finishing treatment such as resin. Prior to batik printing, the cloth should be washed in

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boiling water containing soap and soda to remove any oil and water-soluble finish. The cloth is then pressed to remove creases. Normally two types of waxes (i.e. paraffin and beeswax) are melted together and used for batik. Paraffin wax is hard and brittle, whereas beeswax is soft and pliable. If paraffin alone is used, it gives poor adhesion to fabric and it may flake off from the fabric during dyeing. This leads to the dye reaching portions of the fabric meant to remain undyed, or a smudged effect. The addition of beeswax gives better adhesion. The proportions vary between 1:1 and 3:1 paraffin to beeswax. Selection of dye class is very important in batik printing. Only those dyes which can be applied at room temperature are suitable for dyeing. If hot dyes are used, there is a danger of melting the fabric. Dyes such as cold brand reactive dyes and azoic dyes are most suitable. In the case of azoic colours, the first step involves the application of a coupling component (naphthol) to the waxed fabric, followed by development of colour by treatment with a diazo component (diazo fast bases or salts). This class of dye produces deep and bright shades.

Tie-dye Tie-dye is a resist dyeing process. It consists of knotting, binding or tying, pleating, folding, and sewing certain parts of the silk cloth in such a way that when it is dyed, the dye cannot penetrate into these areas. No specific equipment is required for tie-dye; therefore the process can be carried out at home or in small dyeing houses without much difficulty. The sequence of operations for tie-dye is as follows: 1. Prepare the fabric before dyeing by knotting, binding, pleating, folding, sewing, or some combination of these. 2. Prepare the dye solution and check its colour strength. 3. Wet the tied fabric with water, squeeze it out, and dye at suitable conditions for the required length of time 4. Squeeze out and dry the fabric without removing the ties 5. Tie the fabric up again at different places before dyeing with a second colour. 6. Repeat the tie-dye process for each colour application 7. Rinse the fabric with water and dry it after the final dyeing 8. Untie the fabric, rinse again if necessary, then dry and iron (Chavan, 1993).

8.3.3.2 Chemical resist In chemical resist, the nature of the chemical is such that it will not allow the fixation of dyestuff at the printed portions hence producing a white resist effect. The resist effect is obtained by incorporating suitable dyes (whose fixation is not affected by the resist agent) in the resist paste colour. This technique consists of printing with the resist paste, drying the fabric, nip-pad dyeing the ground shade, drying, steaming, washing, and drying and finishing. The choice of the resisting agent depends on the class of dye used for the ground shade. Chemical-resisting agents include a wide variety of chemical compounds, such as acids, alkalis, various salts, and oxidizing and reducing agents.

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8.4

Silk

Pigment printing of silk

The pigments do not have a specific affinity for any of the fibres; thus their application on the fabric is based on the use of a binder and emulsion thickener or a synthetic thickener. Since pigment printing is very easy to carry out, and its print fixation is done by curing, it offers a lot of flexibility and there is no need for washing. The binders used for pigment printing are to be properly selected so that the final print has a very transparent and highly flexible thin film of the binder, entrapping the pigment, which gets fixed with the fabric. This results in brighter prints without adversely affecting the fabric’s softness. Pigment printing, which was earlier restricted to cotton only, is now virtually used on all fibres and fibre combinations. Silk has a good receptivity for a large number of dyes, such as acid, direct, and indigo. But the demand for washable prints has ignited a new interest in the use of pigment dyes for printing (Chattopadhya and Bhadra, 1997). Permanent printing of pigment is very important from a practical point of view for the manufacturer as well as consumer. Pigment emulsion colours are known for their brilliant shades, but the achievement of optimum colourfastness to washing, dry cleaning, and crocking is still a matter of dispute (Bishnoi and Balakrishnaiah, 1999).

8.5

Fancy prints on silk

8.5.1 Metallic prints: Gold, silver and copper prints These prints are used as an economic substitute for gold and silver woven brocades. Gold prints are obtained by using bronze powder, which is an alloy of copper, tin, and zinc. Silver prints are obtained by using aluminium powder, whereas copper prints are obtained by using copper powder. The most important requirement for printing is the particle size of the metallic powder, so that they can easily pass through the screen of appropriate mesh. The particle size should be in the range of 0.5–5 μm. Fortunately, all three powders (bronze, aluminium, and copper) are commercially available in the desired particle sizes. Silver and copper powders each produce a single shade. However, by incorporating a small quantity of colour (preferably transparent colour) beautiful silver-coloured print effects can be produced. Bronze powder is available with 2–3 tonal variations of gold. These prints are usually produced on dark grounds. The concentration of the metal powder required usually varies between 15% and 25% depending on the covering power of the powder, which in turn depends on the particle size. The smaller the particle size, the greater the covering power. In earlier days, metallic prints were obtained by mixing a 20–25% concentration of metallic powder with synthetic varnish to make a smooth paste, which was then printed in the form of design using screen of appropriate mesh. Fixating the print was accomplished by either air-drying, during which polymerization of synthetic varnish takes place by air oxidation, or by dry heat or steaming.

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The bronze and copper powders are sensitive to ammonia and sulphur. Therefore any compound containing sulphur or liberating ammonia should be avoided. For this reason, the use of ammonium acid-liberating salts or urea in the print paste should be avoided. In presence of sulphur or ammonia the bronze and copper powders turn black and thus lose the charm of the prints.

8.5.2 Pearl prints: Silver and gold pearl Although satisfactory metallic prints are obtained using aluminium and bronze powder, they lack in shine. This problem is overcome by using silver and gold pearl powders. These powders essentially consist of titanium dioxide coated with pearlescence. A silver pearl effect can also be obtained by using finely ground lead carbonate dispersed in a suitable medium. However, with lead being toxic, its use is not recommended. Silver pearl powders are available in different grades varying in particle size and shine effect. The gold pearl powder is free from the danger of blackening that is normally associated with bronze powder. The method of application is similar to the printing of metallic powders using aqueous binder systems. Coloured pearl effects obtained by using pigment colour along with silver pearl powder are more attractive than those produced by using aluminium powder. The incorporation of colour pigment is normally not recommended with bronze, copper, and gold pearl powders.

8.5.3 Iridescent prints These prints give a two-tone effect depending on the angle at which the prints are viewed. Light pink, green, and blue iridescent pigments are available in the form of powders of very low particle size (0.5–1 μm) which can be used directly like normal pigment powders for printing. Iridescent pigments can be directly printed onto white fabric. However, in the case of dyed fabrics (deep shades) it is necessary first to print a white base (usually khadi) followed by top printing with iridescent colour print paste. The print paste is usually made with polyvinyl acetate or acrylic-based adhesive. A concentration of 5–10% of iridescent pigment colour is required to obtain a satisfactory effect. A conventional binder system can also be used for making the print paste. However, the binder concentration required is usually 20–25% in order to obtain satisfactory fastness properties.

8.5.4 Glitter prints These prints differ from metallic (gold, silver, copper) or pearl prints in the sense that the particle size of the glitter powder is much bigger (250–500 μm) than that of metallic or pearl powders. The effect of glitter prints is therefore more prominent compared to metallic or pearl prints. Two types glitter powders are available in the market: (a) Metallized polyester film glitter – the colour of this glitter powder is not resistant to solvents (b) Anodized aluminium glitter

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The glitter powders are available in variety of shades. They have hiding power and therefore can be used on white and dark grounds. The effect is much better on dark grounds, however.

8.5.5 Luminescent prints These prints are like traffic indicator boards, where they look like normal pigment prints during daylight. However, in dark, when light falls on them through headlights of a vehicle, they glow. These prints are obtained by incorporating luminescent colours in the print paste.

8.5.6 Thermocolour printing In this type of printing finely ground liquid crystals are incorporated into the print paste. These liquid crystals change colour with temperature. Thus the printed design shows a variation of colours depending on the temperature. This type of printing is also sometimes called ‘magic print’ because the design is not visible in normal sunlight when the temperature is higher. However, the design becomes invisible in shade at cooler temperatures. The depth of the colour increases with the decrease in temperature. The colours are sensitive even to body temperature.

8.5.7 Foil printing Gold, silver, and multicolour foils on release paper are available. The method of foil printing essentially consists of printing a blank print paste which would remain tacky even on drying. After printing, the foil (on release paper) of the desired colour is brought in contact with the design and suitable heat and pressure is applied for a short time. During this operation the foil from the release paper adheres to the underneath blank paste which was applied in the form of design. Thus we get the transfer of foil onto the fabric in the form of design.

8.5.8 Foam or puff printing Foam printing was introduced in India in 1988. During the early days of this technique, there were problems with foam cracking and uneven surfaces due to poor quality of the foam binder. However, with improvement in foam binders, these problems no longer occur. A foam binder consists of an opaque polymer emulsion of printable viscosity containing a blowing agent. The blowing agent decomposes on application of heat, liberating nitrogen gas. This gas is responsible for raising polymeric film on the fabric, which was applied in the form of design. Thus the method essentially consists of printing the foam binder paste onto the fabric, drying at 90–100°C and curing at 150–160°C for 15–30 s. During the curing stage, the blowing agent present in the print paste decomposes and the liberated gas raises the

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polymeric film by 1/8–1/400 at the printed portions. The overall effect is that the print acquires an embroidered look. Raised coloured effects can be obtained by incorporating normal pigment colours in the print paste. However, it is customary to use fluorescent pigments instead of normal pigments to obtain this effect. The fluorescent pigments are available in powder form. It is recommended to first make a smooth paste of the fluorescent colours with a pigment-printing binder (e.g. acramine SLN) and then mix them with the foam binder. This results in a uniform print with raised effect. Direct mixing of fluorescent pigments with the foam binder results in non-uniform prints.

8.6

Transfer printing of silk

Although 100% polyester is the ideal substrate for sublimation transfer printing, the industry has been eager to adopt this technique for natural fibres because of the advantages associated with it. It is a simple, pollution-free technique with a reduced number of steps involved, it has low capital and space requirements, and it ensures quality printing. Disperse dyes are the only class of dyes that can sublime at high temperatures (180–240°C) for use in sublimation transfer printing, but the lack of affinity of these dyes for natural fibres means this process is not a straightforward one for silk. The literature on transfer printing of silk is limited. A printing process called ‘Silk O Print’ was developed by Wolfgang Mehi, Albert Amon, SICPA Holding SA in 1984 (Wolfgang et al., 1984). In this process, the silk is first pre-treated at l80°C using a special carrier coated paper and overprinted with transfer printing paper for polyester containing sublimable disperse dyes. This process was used for printing silk wraps, but the shades were rather dull.

8.6.1 Sublimation transfer printing Sublimation transfer printing involves the heat transfer at high temperatures (180–230°C) of disperse dyes to the substrate. Silk naturally has little affinity for disperse dyes, so for this method to work, silk has to be chemically modified to improve its substantivity towards disperse dyes. This is done by grafting with vinyl monomers, or acylation of silk with some compounds. Good transfer printability can be obtained by these methods. However, due to the high temperatures involved in printing silk, yellowing is unavoidable. Chavan and Nalankilli (1991) have imparted an affinity for disperse dyes using a synthesized resin precondensate of melamine, formaldehyde, and polyethylene glycol. This pad-dry application simultaneously cures and transfers print onto silk and results in a crease-resistant finish. Heat transfer printing is a kind of transfer printing method that has been used for polyester fabrics for many years. It is a process of the sublimation and diffusion of volatile disperse dyes when the fabric is held in contact with the transfer paper and heated to a high temperature (Datye, 1980). Natural fibre fabrics cannot be printed using this method owing to their lack of affinity towards disperse dyes. However,

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reactive dyes can be fixed on natural fibres by virtue of covalent bond formation and excellent colourfastness properties. Thus the application of reactive dyes in transfer printing is very interesting and can be applied to natural fibre fabrics such as silk. To achieve this, a wet transfer printing method is generally employed, which demands the fabrics to be printed in a moistened state and must be subjected to a wet pre-treatment (Chung and Town, 2011). The humidity content of fabric influences the amount of dye transferred and the clarity of coloured patterns significantly, while the steady humidity content is difficult to obtain considering the different fabric structures and the external environment. To avoid this problem, as a most novel printing method, dry transfer printing has been researched and developed. In this printing process, reactive dyes are adhered from a transfer paper surface onto a dry surface via a heat-pressing process and then fixed on fibres via steaming (Liu and Chen, 2011). Accordingly, the preparation of quality transfer paper is the most pivotal step to achieve dry transfer printing.

8.7

Printing of silk using natural dyes

Natural dyes obtained from different parts of plant and animal residues and some minerals are non-toxic, non-allergic, biodegradable, and eco-friendly (Kale et al., 2009; Gahlot et al., 2009). The fabrics dyed or printed with natural dyes have natural colours and odours (Parvinzadeh and Kiumarsi, 2008). Natural dyes have been used since ancient times (Mihalick and Donnelly, 2006) and they are still popular today. As is done in dyeing, the printing of silk with natural dyes can be done on premordanted fabric or mordant, and the dye can be taken together in the printing paste. Although in the first case, the ground is sometimes likely to be tinted with the spreading of the print, as the dye from the prints may get fixed on the mordanted ground. However, whether the mordant and the dye are taken together, sharp prints on white backgrounds can be obtained. When the thickener or natural dyes have issues of compatibility with metal mordants, pre-mordanted and dried fabric are also used for printing. Subsequently, prints are steamed at 102°C for 30 min and washed with non-ionic detergent, 2 g/L in lukewarm water, and again rinsed with cold water. Many artisans put the petals of flowers in the folds of pre-mordanted and dried silk fabric; after steaming, the attractive printed effects of such flower petals are obtained, which are unique and difficult to imitate (Teli, 2015). Rekaby et al. (2009) studied the printing of natural fabrics such as silk with natural dyes from alkanet and rhubarb by using a pigment-printing technique. Results show that the highest colour-strength (K/S) value was obtained by using Meypro gum as a thickener. The K/S increases rapidly as the concentration of the natural dye powder in the printing paste increases from 10 to 40 g/kg printing paste. Moreover, results show that the printed goods, which were fixed via steaming, have relatively higher colour strength than their corresponding samples fixed via thermodynamics. The best results were obtained by using metal mordants at a concentration of 20 g/kg printing paste. The colourfastness results range between very good and excellent.

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Recent developments in printing of silk

Silk is an expensive and niche fabric sometimes used for special occasions. It requires extra care. Most of the time, dry-cleaning is recommended for coloured products so that there is no bleeding of the colour. Due to its exceptional lustre, soft feel, and royal appearance, this ‘queen of fibres’ will continue to be the most sought-after material for formal wear. However, the fastness of various colours (synthetic or natural) on silk has always been limited; therefore one of the driving factors for future research will be the development of silk/dye systems that can withstand aqueous machine wash. Due to the advent of digital printing technology, short-lot sizes of silk fabric could be printed in order to manufacture high-value customized products. It is also known that as it is a proteinic fibre, silk is attacked by microbes, resulting in its tendering. Hence antibacterial finishing agents for silk will also be a vital area of active research. However, such finishes have limited durability to washing. Therefore certain metal-complex dyes that are antibacterial in nature have been developed; when silk is dyed/printed with such dyes, the silk fabric shows durable antibacterial property against Gram-positive and Gram-negative bacteria (Teli, 2015).

8.9

Specialty chemicals for silk printing

Silks are made attractive by a series of processing operations, especially during printing. Numerous effects call for specialized techniques which give rise to fancy products. Different styles of printing, namely, direct, resist, discharge, and a combination of all these, give rise to a varied mixture of results highly appreciated by silk users. Modifications of these procedures and the use of partial discharge or resist results in half-tone effects, which are also widely accepted. The standard chemicals used in printing include the following: l

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pH-controlling agents: acids and bases Mild oxidizing agents: sodium m-nitrobenzene sulphonate Wetting: sulphonated fish/castor oil Defoaming: pine oil, others Dye-fixing agents: cationic dye-fixing preparations Detergents: Ethoxylated fatty products Reducing agents

8.10

Printing on tasar fabrics

Bihar, India is a large producer of tasar or tussah silk. The region also produces mulberry silk. Cottage industries in Bihar include sericulture, hand weaving, printing, and finishing. Bhagalpur in Bihar is well known for the printing of silk sarees and bed covers. Prior to printing, the fabrics are washed in a locally produced, iron-free soap. Printing is done with blocks smeared with colour. The blocks are produced by local artisans using their own designs. The dyes used for printing are acid, metal-complex

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acid, or direct dyes. The printing pastes are stored in special wooden containers. Printing is done on a heavy table. After printing, the fabrics are dried, steamed, and wrapped in unbleached cotton. Printed mulberry silk fabrics are popular due to the exclusiveness of designs and colouristic effects that can be achieved. But this is less in the case of tasar silk because of its inherent colour and a lack of available technological knowledge in the trade. However, the success depends largely on proper pre-treatment such as desizing and bleaching.

8.10.1 Direct printing For direct printing of tasar, acid, metal-complex, direct, and reactive dyes are normally used. Printing may be carried out by screen- or block-printing method. In the case of block printing, the thickener used is Arabic gum. The printed fabric is dried under mild conditions to retain a good printed mark and prevent the goods from marking off during subsequent processes. Steaming is carried out in saturated steam for 45–120 min depending on the steamer used. Washing is carried out under mild alkaline conditions with a standard detergent to prevent the re-adsorption of washed dye onto the fabric. The fabric is neutralized and dried at low temperature.

8.10.2 Ironing Finally, the finished product goes for ironing and packaging.

8.11

Introduction to finishing of silk

Silk is prized for its lustre, sheen, and hand. Its popularity is mainly due to the finishing it is subjected to. Clothes made from silk are distinctly luxurious and have many excellent qualities, including the material’s lustre, wearing comfort, fine and smooth texture, soft handle, and excellent draping quality. The final handle of silk is the most important selling feature of this fibre. The drawbacks of the properties of this fibre are rectified and improved by finishing techniques. The importance of this fibre requires that the finished silk product expresses its typical character in both its optical appearance and its silky, scroopy handle.

8.11.1 Basics of silk finishing The finish must take into account demand of silky handle and draping of the material. Finished results depend on the proper combination of the silk article and the order in which the finishing machines are selected (Perkins, 1996). At present, the wash-and-wear finishing trend is becoming more important in the silk industry. This quality can be achieved with the use of softeners, elastomers, and

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synthetic resins. Finishing operations serve to improve the functional performance of the fabric and rectify any problems with the fibre. In addition to wash-and-wear finishes, current interest is in creating soil-resistant, flame-retardant silk. Another interesting area in this context is antimicrobial finishing in order to maintain hygiene and freshness.

8.12

Mechanical finishing

8.12.1 Calendering The main function of calendering is to provide a smooth fabric surface, light lustre, and improved hand. This is the technique used to influence the handle and appearance of the fabric. In most cases, silk is only calendered in the cold stage, which produces a soft handle. With hot calendaring higher lustre is obtained, but it has to be determined in each case in order to prevent negative influence. The Palmer or felt calender machine (Fig. 8.1) is one type of unit used for delicate fabrics such as cotton mulmul, voiles, silk, synthetics, and knits. The machine provides a smoother handle and lustrous appearance to silk fabrics. It is mainly used for taffeta-like wovens with a smooth character and for serge fabrics (e.g. pongee, twill). The combined drying and finishing is achieved by using an iron or stainless steel cylinder with a diameter of about 6.50 moving together with an endless blanket; the fabric passes in between small rollers that exert light tension on the fabric. A felt calender produces approximately 30–50 yards per minute.

Back grey

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Fig. 8.1 Felt calender.

Heated roller

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8.12.2 Decatizing Decatizing mainly imparts dimensional stability to textile fabrics. It also removes creases and smooths the fabric. Various types of silk fabric, such as woven crepe, are decatized. Here the principle involved is controlled relaxation of strains stored in a fabric. Fabric along with a felt are rolled in open width onto a perforated cylinder and subjected to superheated steam. Here it is important that the wool felt used should not be so hard that the silk fabric is not pressed flat. The discontinuous decatizing machine (Fig. 8.2) is the commonly used machine today, but continuous decatizing equipment is also found in which felt calenders might be used. The fabric to be decatized is wound with a woollen felt fabric (as the external layer) on a perforated stainless steel roller. Steam is passed from the inside of the perforated roller through the wound layers for specific time, say 30 min. The fabric is rewound by changing the upper surface of the fabric as the inner layer and steamed again.

8.12.3 Stentering The main objectives of stentering are: l

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Drying the goods by means of hot air. Bringing them to the desired width. Producing the desired feel in finished goods by the suitable application of heat in conjunction with or without special motions applied to the cloth.

The goods are stretched upon a stenter in such a way as to straighten out the fabric and produce the necessary width with a minimum of mechanical strain. The goods are then dried by means of hot air. In this way, the fixing of the dressing occurs while the cloth has the desired width and form. Further, by regulating the temperature and therefore the rate of drying, combined with a motion known as swissing, the nature of the final ‘feel’ and the elasticity can be controlled.

Wool felt

Fabric Steam inlet

Fig. 8.2 Discontinuous decatizing machine.

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In the past, a stenter operating with hot air was sufficient to dry the fabric, but today it requires curing frames with temperatures up to 150°C since various elastomers and resins are being used.

8.12.4 Tamponing Tamponing (Fig. 8.3) is a treatment that improves silk’s appearance. Silk is sensitive to the mechanical friction it is subjected to in the various stages of treatment. Even after taking precautions, it is likely that the silk fabric may develop chafe marks. Tamponing helps to smooth out these irregularities. Tamponing is achieved by applying an extremely fine film of oil evenly on both sides of the fabric. Thus the chafe marks become less visible. Earlier, only faulty portions of fabric were treated by hand with a tamponing cushion. Today, tamponing machines can apply a fine film of oil homogeneously on several rollers, which is in turn evenly transferred to the fabric. Correcting chafing requires considerable experience, and the fabric may have to be run through the machine more than once.

8.12.5 Breaking finish A breaking machine (Fig. 8.4) is used to impart a particular soft handle mainly when calendaring is not sufficient. Breaking is specially indicated as a final treatment for fabrics with an excessively harsh handle, such as crepe. Two types of machines are available: 1. Button Here the piece is passed several times rapidly back and forth over small rollers studded with brass buttons (Fig. 8.4). The button breaking machine has recently been replaced by the knife breaking machine. 2. Knife Here the fabric is drawn over the edges of slanted knives.

Tamponing oil

Fabric Rolls covered with woolen felt

Fig. 8.3 Tamponing machine.

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Wooden rolls with spirals of brass buttons

Fig. 8.4 Button breaking machine.

8.12.6 Shrinkage and relaxation Screen steamers or shrinking machines are used for shrinking and relaxing fabric. Crepe-like fabrics often appear too flat. The goods coming from previous treatments are many times in an insufficiently relaxed state. In the screen steamer, the fabric is laid with steam from below, on which the fabric can relax. A conventional open, horizontal screen steamer is shown in Fig. 8.5.

Fabric Steam pipes

Continuous wire screen

Fig. 8.5 Conventional open horizontal screen steamer.

8.13

Chemical finishing

8.13.1 Weighting During the degumming process cultivated silk loses up to 25% of its weight. From a commercial point of view, it is advantageous to replace some of the lost weight and at the same time achieve a desirable change in the ‘hand’ of fabric. Both results can be effectively obtained by weighting providing it is not carried to extremes. Weighting, also called charging, is carried out in order to compensate the loss of weight incurred by degumming. The weighting medium should be incorporated in to the fibre structure and be wash-proof. If water-soluble substances, for example, dextrin, are applied, this is called ‘loading’. Loading does not lead to a permanent improvement in the fibre

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properties but merely produces an increase in weight. During the weighting process, silk absorbs up to 300% of foreign substances. The weight loss during pre-treatment of silk can be restored or even increased in the processed fabric by treatment with chemicals. These chemicals help in improving drapability, and there are instances where weight increases of up to 400% have been achieved. The weight loss caused by degumming is compensated by weighting, which is measured in ’par’, either ‘below par’ or ‘above par’. Numerous procedures help in giving weighting, such as the following: l

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Vegetable weighting Mineral weighting Mineral/vegetable weighting Weighting by grafting

Metallic weighting can often be accompanied by deleterious changes in the mechanical properties of silk arising from acid hydrolysis and oxidation effects during treatment, storage, and exposure to light. A variety of metallic salts have been used over the years, but in practice the tin/phosphate/silicate process has been the most common. Silks with up to 15% added weight are often referred to as ‘pure silk’. Kamat (1989) explained all the processes involved in weighting and the most commonly used tin/phosphate/silicate process. The grafting technique and use of methylacrylamide monomers seem promising. Tin weighting involves a number of stages that require careful monitoring to ensure minimum undesirable side effects. The uptake of metal involves a complex series of chemical and physical reactions that are not fully understood. Two views have been put forward regarding the possible mechanisms, namely, simple adsorption or chemical combination. In silk weighting, the material is first soaked in a stannic chloride solution at room temperature for approximately 1.5 h during which time the increase in weight of silk attains a maximum value of around 10%. Subsequent mixing with cold water results in the removal of uncombined stannic chloride and the hydrolysis of combined stannic chloride to insoluble metastannic acid. SnCl4 + 3HOH ! H2 SnO3 + 4HCl Next, the silk is steeped in a dilute solution of disodium phosphate at 60–70°C to give the following: H2 SnO3 + Na2 HPO4 ! Na2 SnO3 + H3 PO4 followed by: Na2 SnO3 + Na2 HPO4 + HOH ! SnðONaÞ2 HPO4 + 2NaOH Another rinse with water results in further hydrolysis SnðONaÞ2 HPO4 ! SnðOHÞ2 HPO4 + 2NaOH

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to give an insoluble tin phosphate compound of molecular weight 249. If the whole process is repeated, a further 10% increase in weight based on the original weight of the fabric can be achieved. Next, the silk is treated with dilute aluminium sulphate to neutralize any residual alkalinity and to precipitate aluminium hydroxide in the fibre. Finally, an insoluble tin silico-phosphate of high molecular weight is formed by treating the material with dilute sodium silicate. Because the deposition of metallic salt occurs within the fibre, the appearance of the silk is not appreciably altered. It has been shown that deposition takes place in the amorphous regions (Yamama et al., 1985). Silk treated with organic solvents and dried showed an increase in tin weighting that was ascribed to an increase in ‘voids’ in the amorphous region. It is reported that weighting may be achieved by using a mixture of water-soluble precondensates of mono- and dimethylol thiourea at 100 gpl levels to give a weight increase of nearly 20%. Such treatment gives silk better washing properties and makes it resistant to creasing. By increasing the fabric weight, the weighting process does indirectly reduce the cost of the treated fabric, but excessive weighting can cause the following problems: (a) Diminished affinity of the fibre for dyestuffs (b) Decreased fibre strength and occurrence of brittleness

It must, however, be pointed out that the durability of silk is minimized as the degree of weighting increases. Thus the weighting of silk can, if the silk has been weighted within reasonable limits, produce a fullness and richness of feel and handle that cannot be obtained with degummed material. Heavy weighting on silk can partially damage its characteristics or quality, making it hypersensitive to rubbing, giving it a rough handle. Other methods of weighting include the use of organic compounds in an effort to reduce the cost and deleterious effects of metallic weighting. The use of synthetic tanning agents based on dioxy-diphenyl propane and dioxy-diphenyl sulfphone has been recently reported.

8.13.2 Softening Silk develops its properties by the application of softeners/lubricants. These help to improve hand, drape, cutting, and sewing, all the desirability qualities characteristic of silk. Generally, softening of fabrics is popular towards the final process of wet processing. As consumer demands increase, yarns, cords, twines, and so on are all taken for softening at some stage of yarn manufacture (Kamat, 1988). In the case of silk, a lot of importance is placed on ‘scroop’, which is a peculiar cracking sound produced when the fabric is rubbed or squeezed by hand. This is not a natural property of silk but an acquired one. It is acquired by dilute acetic acid or tartaric acid treatment. Lactic acid at 10 gpl for 5–10 min is also recommended. Specialized softeners consisting of the emulsions of fatty alcohols with their sulphated counterparts also achieve this desirable effect.

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Numerous products give good softening results. Silicones and their blends have established themselves firmly in the arena of softeners. Many functional groups of manipulated products are available and useful for softening silk as well. The current trend is to use the epoxy derivatives of silicones to obtain softness, crease-resistance, and non-yellowing. Auxiliary compounds used in softening compositions include the following: 1. Oil, waxes, and fats 2. Polymers: polyacrylic acid salts or polyacrylamides of varying molecular weights 3. Esters: stearates, oleates, palmitates, fatty alcohol sulphates, phosphates, amide-modified esters, glycerides 4. Ethers: polyoxyalkalene glycols and polyether containing at least a fatty group 5. Quaternary ammonium compounds: quaternary fatty amine and fatty amine ethoxylates, such as dimethyl ammonium chloride and distearyl dimethyl ammonium chloride 6. Amides: fatty acid amides, such as stearamide octadecyl ethylene urea 7. Silicones and modified silicone fluids 8. Emulsifiers: anionic, non-ionic, cationic, and amphoteric 9. Other: latexes, different blends.

8.13.3 Scroopy finish As previously mentioned, the classic feel of silk, which is termed scroopy handle, is the peculiar crackling sound produced when silk fabric is rubbed together. In the case of silk, scroop is quite important. It is not a natural property of silk but an acquired one. Scroop can be adjusted to be either harsher or softer depending on market trends. Adjustment usually occurs prior to the actual finishing process by adding oil emulsions or Marseilles soap, which are precipitated on the surface of the fibre with an inorganic acid such as tartaric, oxalic, formic, or acetic acid in a fine dispersion. Investigations have shown that it is the acid alone that contributes to the fabric scroop by forming a fine skin on the fibre surface produced by the reorientation of fibroin molecules at the outermost layer of the fibre (Anon, 1989a,b). Conventionally, scroop is acquired by dilute acetic acid or tartaric acid treatment. Lactic acid at 10 gpl for 5–10 min is also recommended. Specialized softeners consisting of the emulsions of fatty alcohols with their sulphated counterparts also impart this desirable effect (Maruthi, 1983a,b). The latest trend is to use fatty saturated C18-20 groups which are modified to give such an effect. The modern day scrooping compounds do not affect the storage of silk and remain effective under changing weather conditions.

8.13.4 Crease recovery finish The easy-care properties of silk are inadequate as compared to synthetic fibres, so a good crease recovery finish is quite desirable for silk. Suitable resin precondensates are used to achieve this effect. These products are such that they either react with one another or cross-link the fibroin backbone to form water-insoluble products under the action of heat and catalyst. Many types of N-methylol derivatives of nitrogenous compounds are used for this purpose. Ethylene urea and glyoxal are found to give good crease proofing

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when used with a metal salt catalyst and alkyl ethylene urea as additives (Batty, 1967). Crease-resistant agents based on urea-formaldehyde precondensates can also increase the weight of silk (Achwal and Kuduskar, 1984). It is found that even at a concentration of 150 g/l with ammonium thiocynate as a catalyst, the handle of the fabric is not affected and the fibre is also protected from photo-chemical degradation. It is also found that dimethylol ethylene urea, silicone acrylate softener, and a catalyst give both dry and wet crease resistance on silk. Improved tear strength and better abrasion resistance are also obtained. Epoxides also give good results on silk. The diglycidyl ether of ethylene glycol (DEE) and the triglycidyl ether of glycerol (TEG) are effective epoxies to impart crease-resistant properties on silk. These are catalysed with thiocyanate cyanide and solvents like tolurene, ethanol, and isopropanol. The silk-epoxide reaction is found to be mainly with phenolic hydroxyl groups of tyrosine residues. They also result in weight gain of about 15–20%. Both wet and dry crease-resistant properties are obtained (Yang and Li, 1992). Dimethylol ethylene carbamide has been reported to increase the weight of silk and improve its dry and wet crease resistance, tear strength, elasticity, and resistance to photochemical degradation without affecting the whiteness of the fabric. In another process, the goods are treated with a solution of urea, thiourea, and an aliphatic alicyclic or aromatic compound having at least two OH groups, dried up to 10% moisture content, and treated with gaseous formaldehyde at 100°C. Epoxides also give good crease-resistant results on silk. Dimethylol ethylene carbamide can help increase silk weight, increase dry and wet crease resistance, improve tear strength, increase elasticity and resistance to photochemical degradation, and obtain a better degree of whiteness. Urethane resins with or without the addition of modified urea formaldehyde resins have also been recommended for wash-and-wear silk fabrics (Seafimov et al., 1970). These give good crease-resistant properties and 2–4 times better abrasion resistance with little or no reduction in tensile or tear strength. They are applied by either a solvent or an aqueous emulsion system.

8.13.5 Oil-repellent finish Generally, water- and oil-repellent finishes are applied in conjunction with each other. The conventional auxiliaries needed for this effect are as follows: l

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Waxes, metal acid salt, and oxides Proteins and nitrogenous compounds Silicones Fluorochemicals

The fluorochemicals produce a versatile finish; they are fluorinated polyacrylates such as:

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They are normally used in conjunction with wax, pyridium salts, or other resin-type water repellents which act as extenders and pad-bath stabilizers and in some cases act to improve water-repellency. Grafting natural silk with vapour of hexafluoropropylene, vinyl fluoride, vinyl chloride, and acrylonitrile increases its water-repellency, oil-repellency, mineral acid resistance, and photostability (Sahai, 1975).

8.13.6 Antifungal and antimildew finishing The antimicrobial finishes have two different purposes: either to protect the fibre from microbial attack or protect the user against transfer of pathogenic germs. In the case of silk, its mildew resistance may be good as compared to cotton, but its storage stability and mildew resistance are not up to par. Silk’s fungal resistance can be improved by impregnating it with compounds like N-(2.2-dichlorovinyl) salicylamide, 0.01–0.25% solution of benzalkonium chloride (e.g. dodecyl benzyl trimethyl ammonium chloride), and dodecyl trimethyl ammonium chloride (Holme, 1993). This treated silk is found to have better mildew resistance than untreated silk during long storage. Phenolic compounds are widely used as they have optimum activity and can be incorporated into polymeric films which can be deposited from aqueous emulsions. In order to get antimicrobial finishes with high fasteners, reactive agents with reactive systems similar to those for reactive dyes, for example, monochloro and dichlorotriazene, trichloro pyrimidine, vinyl sulphone, and acrylamide, are quite useful.

8.13.7 Flame retardancy To meet the flame-retarding requirements of silk, a reaction product of polyhalogenated acids with a cyclic nucleus, such as chlorendic acid and thiourea, is used to impart self-extinguishing properties. Titanium hexachloride, titanium tetrachloride, and zirconyl chloride can make silk flame-retardant. Titanium hexachloride adsorption onto silk fabric from citric acid solution increases with citric acid concentration, time, and temperature of treatment. The titanium chloride lowers pyrolysis and onset temperature, and increases char formation. Titanium tetrachloride is more effective than zirconyl chloride for making silk flame-retardant. The flame resistance of the fibrous material increased with treatment with an anionic complex of zirconium, titanium, or tungsten. This treatment is carried out under acidic conditions and hence could be coupled with dyeing of silk with acidic dyes. It is reported that certain phosphorus-containing preparations may be used to impart flame resistance along with resistance to shrinkage and creasing. Such preparations include N,N0 -ethylene bis (P,P-bis [aziridinyl]-N-methyl phosphinamide.

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Treatment with such compounds in the presence of zinc fluoroborates gives good flame-resistant properties along with wash fastness, crease-resistance, and shrinkcontrol characteristics. General-purpose flame-retardants include the following: 1. Alkyl and aryl phosphate, phosphonates and phosphites and polyphosphonates 2. Phosphazenes, phosphonyl or thionophosphonyl amides or ester amides 3. Halogenated alkyl or aryl phosphonates or polyphosphonates, halogenated alkyl or aryl phosphates, phosphites or phosphozenes 4. Poly (vinyl or vinylidene halides) as latexes, poly (halogenated acrylate) latexes, emulsions or dispersions of alkyl or aryl halides, halogenated paraffins 5. Colloidal antimony pentoxide

Developing a durable flame retardancy and formaldehyde-free process for silk fabric is still a challenge. Recently, a series of phosphorous-based flame-retardants have been successfully synthesized and applied to silk fabrics to produce durable and formaldehyde-free effects (Yang et al., 2012). However, little research on the properties of apparel and sewing ability for silk fabrics after treatment with flameretardants has been reported in the literature. Such parameters are important for garment manufacturers to know.

8.13.8 Photostabilization One of the biggest drawbacks to silk is its susceptibility to photochemical tendering. This is caused by ultraviolet radiation. For this purpose, chemicals known as UV absorbers have been found to be useful. UV absorbers are organic compounds that absorb UV radiations mainly responsible for causing degradation and give protection by dissipating the energy as heat. Other materials used for protection are screening agents such as pigments and energy-transfer agents, which deactivate the degradation process. The oldest technique consisted of treatment with thiourea, which was known to retard the photochemical reduction of silk and the resultant loss of tensile strength. Other compounds that can be used include ammonium thiocyanate, tannic acid, and formates. Today, the trend is shifting towards UV absorbers. The other UV absorbers and stabilizers available for formulation on silk are as follows: l

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Derivates of 2-hydroxyl benzophenone Derivatives of 2-(2-hydroxy phenyl)-benzotriazole Phenyl esters and salicylates Substituted cinnamic acid derivatives P-aminobenzoates Polymeric and polymerizable absorbers Hindered-amine light stabilizers (HALS) like bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate Stabilizers like Ni-chelate complexes and other salts Miscellaneous: Thermal antioxidants based on hindered phenols and esters of trivalent phosphorous, which also function as UV absorbers

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These UV absorbers can be applied by normal exhaustion method from the dye-bath, either as ionic dyes or as disperse colours. Selection of the type and concentration of UV stabilizers depends upon many factors, such as the composition of the substrate, thickness of the sample, colour requirements, processing conditions, and expected service life. UV absorbers, HALS, and phosphonites are generally used at a concentration of around 1%. However, thermal antioxidants generally are applied at levels of 0.2–0.5% on the weight of silk. Nickel chelates have the drawback of imparting a greenish colour to the fibre. However, the fibre can be dyed with dyes that can complex with nickel and can thus mask the greenish shade. The concept of UV stabilizers is catching on fast and it is time that we introduce such products for our own domestic consumption, especially for silk. It is up to the processor to choose from several different classes of compounds used to retard this light-induced polymer degradation, including UV absorbers, hindered amines, nickel chelates, hindered phenols, and aryl esters.

8.13.9 Antistatic finish The static-charge build-up on silk is only moderate, but when silk is grafted with monomers to modify certain properties the problem of static charge increases. The static charge of styrene-grafted silk increases with an increase in styrene content; the increase being nominal up to 40% styrene build-up but rapid beyond this concentration. This is checked by the antistatic composition, which is rightly classified as stat dissipatants and stat resistants (Usmanov et al., 1977). The static-dissipating agents function by increasing the hygroscopicity of the polymer species. In the case of silk, certain water-soluble vinyl monomers (e.g. N,N0 -methylene bisacrylamide, tri acryl hexahydrotriazine, methyloxy polyethylene glycol methacrylate, etc.) and treatment with certain acids give good stat-dissipatant effect. Further, conductivity is found to increase by depositing certain metallic particles (Mehra and Mehra, 1990; Maruthi, 1983a,b; Achwal and Kaduskar, 1984). This has opened up a large and new market of microelectronics for silk. Also, the stat-dissipatant effect is obtained by rendering silk fibre conductive by coating it with metallic silver; in fact, a coating of 1 mm in diameter on silk gives it properties that could be used in microelectronic circuitry (Yee, 1981). The stat-resistants, on the other hand, function by reducing fibre-fibre, fibre-metal friction and as the action is mainly of lubrication, certain basic lubricants like mineral oil and butyl stearate are used. The antistatic composition is obtained by these spin finish components which invariably consist of a lubricant, emulsifier, antistatic component, and other additives like antioxidants of corrosion resistants (World Textile Abstracts, 1977).

8.13.10 Silicone finishes Normally, urea formaldehyde-based chemicals were needed to improve the low stress mechanical properties of textile goods. Due to some inherent drawbacks in the chemicals, their use is restricted. Alternatively, silicones of a different chemical nature were used to study the properties of silk fabrics. The silicone-treated fabrics were

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tested for mechanical, comfort, and aesthetic properties. Results have shown improvement in handling, crease recovery, strength, drapability, and other tests when compared to normal, unfinished fabrics. The silicones are normally available in emulsion form. The required amount of silicone emulsion on the weight of the fabric to the pad-bath and required quantity of water is also added to it. The fabric is padded with the emulsion and care should be taken to retain 70–100% pick-up. The pH of the bath must be maintained between 5 and 6. The padded material is immediately cured in the stretched condition at a temperature of 80–90°C for 2–3 min. The initially cured fabric was cured at a temperature of 120–130°C for 2 min in a stretched manner. Silicones are based on the following chemical groups: 1. 2. 3. 4. 5. 6. 7.

Fatty amides combined with dimethyl polysiloxanes Microemulsion with amino functional group Microemulsion based on silicone fluid having amino hydroxyl and epoxy end groups Precatalyzed emulsion polymer of reactive silicone fluid Reactive microemulsion of an elastomeric-modified polysiloxane Softener based on microemulsion technology Organo-functional silicone elastomer

8.13.11 Waterproof finish The treatments for waterproofing are based on wax/metal salts. Modern-day usage has shifted towards the zircon-based salts, whereas the traditional aluminium salts/wax salts formulations still prevail. Silicones containing the methyl hydrogen polysiloxanes cured with a catalyst give the requisite levels of waterproofing. Chromolan, along with silicones, can do the job as well. Chromolan is a chromium-containing preparation and a finish of the by-gone era. It is used only in systems where chromium salts are acceptable.

8.13.12 Sandwash finishing A sandwash finish produces a machine-washable fabric with a soft velvet-like feel. The sandwashing process itself gives rise to a rough handle, but this is modified using softeners to produce the sandwash feel. The feel is produced by treating the silk under harsh conditions; this roughens the fabric surface by breaking the surface fibrils to create a peach-skin-like texture. Depending on the method used, the fabric also shrinks; this makes it dimensionally stable. The traditional handle of silk is lost along with as much as 50% of the initial fabric strength (Engeler, 1992). There are several ways in which this sandwash finish can be produced. Silk can be dyed on overflow machines under harsh conditions, for example, for a longer time or at higher temperatures, either directly during dyeing or as a pre-wash treatment. The process can be carried out in a washing machine, sometimes with the addition of pumice stones or small pieces of expanded clay. The disadvantage of these methods is inadequate uniformity of the finish and the risk of chafe marks. Other methods include mechanical raising of the fibre surface using an emerizing machine, although with this method there is a risk that whole fibrils may be broken. Surface modification using a

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protein-active enzyme such as Bactosol SI (Clariant) is claimed to be capable of producing a uniform effect (Hadimani et al., 1998)

8.14

Finishing of tasar fabrics

Finishing is an important factor for marketing of the finished commodities. Tasar finishing is generally conducted by two techniques to improve the cover, feel, lustre, and look of the fabric. There are two types of finishing as follows: 1. Kundi finishing (i.e. beating with a wooden hammer) 2. Calender finishing

Kundi finishing is very common and mostly done for all export varieties of tasar fabrics manufactured for the domestic market. Calender finishing is only applicable if a continuous length of a fabric is available, and as such, calender finishing is very rare in the tasar silk industry.

8.14.1 Kundi finishing Kundi is an indigenous finishing practice for silk fabrics. After bleaching, the cloth is washed well in cold water and treated in a finishing bath. The following is a sample bath recipe for 50 saree pieces.

Kalatek J Stabilizer C Arrowroot powder Glycerine & T.R.O. Temperature

1200 g 50 g 50 g Little amount Room temperature

After treating the silk material in the finishing bath at room temperature, the silk is dried and then moistened by sprinkling. Then, approximately 10 sarees are folded into a packet and wrapped in a thick cotton cloth. The bundle is placed on a wooden block and vigorously beaten by two persons from two sides with the help of hammer for about 15–20 min before ironing. Later the silk material is folded and sent for packing.

8.14.2 Calender finishing This finishing method uses the same bath recipe previously described, after which the material is passed through the steam-heated calenders at a slow speed and then folded and packed. The calender finish is generally a mechanical type finish and produces a high degree of lustre on tasar silk fabrics. The aim of finishing silk is to reveal its lustre, handle, drapability, and so on. Tasar silk is not usually finished; however, depending upon specific requirements, either

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chemical or mechanical, the fabric can either be soaked in or sprayed with the finishing chemicals and squeezed manually.

8.14.3 Mechanical finishing Dried tasar fabric can be moistened and wrapped with cotton/silk fabric and hammered manually by two wooden hammers alternately to impart a soft handle and lustre. This process is known as Kundi finishing and can be compared with a button or knife breaking machine finish, where fabric is passed several times rapidly back and forth over small rollers studded with brass buttons or slanted knives. Tasar silk fabrics can be calendered on a two-bowl calendering machine for improving handle and appearance.

8.15

Future trends

With the recent increase in the printing of natural fibres, silk is again gaining popularity. Apart from the traditional industry of printing scarves, ties, and evening gowns, there is an increasing trend towards more silk in the ‘young’ fashion areas. Dyers and printers of silk have, for obvious reasons, kept their knowledge (based on many years of experience) secret, and little information on the practical processing of silk has appeared in the technical literature.

8.16

Summary

Silk has remained the ‘queen of fibres’ over the years. Its properties have made it a highly sought-after apparel and furnishing fabric. Because of the enormous amount of manual labour required in the production of silk, it has always been expensive. In recent years, synthetic fibres have been produced that compete with the desirable properties of silk, such as lustre, hand, and drape, at a far lower cost. This has resulted in the stagnation of growth of this fibre, but has increased its usage for specialty and high-quality luxury items. The introduction of polyester microfibres at 0.5 denier do seem to imitate silk, but the niche carved out by silk over the centuries will continue to fascinate silk users. The chemicals that enhance the properties of silk are the auxiliaries used in its processing. A series of value-added properties are imparted by the use of softeners and lubricants, crease-recovery finishes for easier care, flameretardants for overall statutory requirements, oil-repellent and antistatic finishes, and antimicrobial protective finishes. A finishing method that holds a lot of promise for silks in the future is one that imparts photochemical protection. UV absorbers can accomplish this and it is time for silk processors to begin incorporating these versatile compounds in their processing recipes. Adding UV absorbers will allow for commanding a higher price and lead to greater acceptance in the international markets. Limited availability of silk worldwide would give a ‘Handle with Care’ label to this wonder fibre. As weighting is

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one of the processes available for silk processing, chemical add-on is highly desirable, but however, selection of the right chemicals in appropriate quantities result in the product for a specific end use and the finish will stay on for a long time. Future thrust will be on items that conform to ‘Green’ standards and are environmentally safe. The processing of silk is, therefore, a very skilful job. Until recently, the processing of silk was limited to small-scale skilful processors, and classical finishing methods were in vogue. However, in recent years, diversified uses of silk materials have compelled processors to search for newer processes and chemicals. However, the use of chemicals is becoming more and more restricted due to objections from an environmental point of view. In spite of this, many newer chemicals have proved to be promising for the finishing of silk.

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