Advances in the finishing of silk fabrics

Advances in the finishing of silk fabrics

5

A.K.R. Choudhury Gargi Memorial Institute of Technology, Kolkata, India

5.1

Introduction

A textile product does not possess its properties naturally. The finishes enhance the esthetics and/or performance of the final product. A fabric finish is applied to a fabric once it has been made to improve its appearance, feel, or other properties. The finishes can be classified in three ways: 1. Temporary/renewable/durable/permanent 2. Chemical/mechanical 3. Esthetic/functional

These will each be described in more detail below: 1. Temporary finishes are removed by washing or dry cleaning; an example is calendaring. Renewable finishes are temporary finishes that can be reapplied, such as starch, water resistant, and soil-resistant. Durable finishes are effective during the life of the product; an example is durable-press. Permanent finishes do not diminish during the life of product. 2. Chemical finishes are sometimes called wet finishes. They are applied to fabric from water or other liquid bath. Flame retardancy is an example. Resins are the most-used chemicals in wet finishes. They give durable, wrinkle- and crease-resistant finishes by padding, drying, and curing. Mechanical finishes are also called dry finishes. The fabric does not need to be wetted. These finishes involve physical treatment that changes the appearance and/or hand of the textile product. 3. Esthetic finishes: These improve the appearance and/or hand of fabric; an example is a moire´ effect. Functional finishes improve the overall performance; an example is a water-resistant finish for a raincoat. Fabric finishes are used to improve the fabric quality in some way, including: Improvement of the appearance: color, pattern, or sheen Change of the texture of the fabric: embossing, brushing, or smoothing Improvement of the feel: softer, crisper, firmer Improvement of the drape (how the fabric hangs): weighting Improvement of the wearing qualities: crease resistance, stain resistance, flammability, waterproof, etc. Modification of care requirements: easy care l

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Silk is one of the world’s most favored textile materials and has been known as the “queen of fibers” since its discovery. Commercially silk is available in four varieties: mulberry, tussah, muga, and eri. The most popular is the mulberry variety. Silk mainly Advances in Silk Science and Technology. http://dx.doi.org/10.1016/B978-1-78242-311-9.00005-7 © 2015 Elsevier Ltd. All rights reserved.

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consists of fibrous portion, fibroin and the gummy portion, sericin, and a small percentage of minerals, waxes, etc. Clothes made from silk are distinctly luxurious and have many excellent qualities, including the material’s luster, wearing comfort, fine and smooth texture, soft handle, and excellent draping quality. Therefore, it needs finishing agents to a minimum extent. Some finishing chemicals may lower rather than improve the quality of the fabric. Silk fabrics, however, have a tendency to crease easily during home laundering or when wet. This disadvantage causes considerable inconvenience in the use of silk textile materials and directly influences the world consumption of silk. Among various chemical modifications, the epoxy treatment onto silk looks promising; experimental results show an improvement in physical properties. Silk is the only natural fiber available in a continuous filament form. The unique qualities of silk in the form of high texture, fineness, strength, luster, scroopy feel, and good draping quality together with ecofriendliness in use make it the most popular textile fibers. However, these qualities in silk fabrics cannot be achieved in a raw state, as silk gum (sericin) present in the silk hides these qualities of silk fibroin filament and protects from damages that may occur during the stages of silk production. The primary reasons for the long-time use of silk have been its unique luster, durability, and tactile properties. Silk fibers are remarkable materials, displaying unusual mechanical properties: they are strong, extensible, and mechanically compressible. Silk also displays interesting thermal and electromagnetic responses, particularly in the UV range, for insect entrapment and form crystalline phases related to processing. Silk is suitable for all types of garments: from lingerie and sleepwear to suits and coats. Silk is highly valued because it possesses many excellent properties. Not only does it look lustrous and feel luxurious, but it is also lightweight, resilient, and extremely strong—one filament of silk is stronger than a comparable filament of steel. Although fabric manufacturers have created less costly alternatives to silk, such as nylon and polyester, silk is still in a class by itself.

5.2

Fundamentals of silk finishing

For silk articles, the age-old demand is a “silky handle.” The finish must take this demand into account. The finishing of silk is similar to those of other fibers and mainly includes (Sandoz, 1988): l

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Luster: discreet, not greasy Handle: soft but firm textured Fall, or draping of the material

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Silky handle and draping properties of the silk should be retained.

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The right combination and sequence of the finishing machines should be chosen depending on the silk article. The wash-and-wear trend is now becoming more important in the silk industry, which can be achieved with the use of softeners, elastomers, and synthetic resins. The drawbacks of the properties of silk should be rectified and improved by finishing techniques. Recently, there is an increasing demand for soil-resistant and flame-retardant silk. Another interesting area in this context is antimicrobial finishing in order to maintain hygiene and freshness.

For many years, finishing of silk articles was primarily a matter of the craftsmanship of the finisher. With experience, the finisher could run each batch through the various finishing machines so skillfully that the final product met the customer’s requirements. In ancient time, very few chemicals, such as acids, oils, and filling agents (gum Arabic or tragacanth), were used for finishing of silk. With the passage of time, better wear and care properties of silk have been demanded in order to compete with the performance of other fibers. Greater wear and care properties are now more important in the silk industry and are achieved by the use of softeners, elastomers, and synthetic resins. However, silk is allowed to go its own way and need not be as durable as other textile materials. The future will dictate whether we really need machine-washable silk and whether the technical problems of finishing can be solved without affecting fiber or fabric properties to a large extent.

5.3

Mechanical finishing of silk

Mechanical finishes are ecofriendly; however, the effects obtained are mostly temporary. The most important mechanical finishes are calendaring, decatizing, breaking, etc. Calendering is necessary to improve the appearance after the final wet processing is complete. There are various types of calendaring.

5.3.1

Calendering

Calendering by definition is a mechanical leveling and segmenting process for “finishing” fabrics or webs to obtain or to produce special effects. Such special effects could be flattening, luster, compacting, glazing, moire´, Schreiner, smoothing, texturing, and other embossed patterns. Calendering is a finishing process used on cloth in which fabric is passed between rollers at high temperatures and pressures. Calendering is used on fabrics such as moire´ to produce its watered effect and also on cambric and some types of sateens. The fabric is run through rollers that polish the surface and make the fabric smoother and more lustrous. Fabrics that go through the calendering process feel thin, glossy, and papery. The calendering finish is easily destroyed and does not last. Washing in water destroys it as does wear over time. Calenders are used to influence the handle and appearance of the fabric.

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For most articles, a three-bowl calendar with steel and paper rollers is sufficient. Additional calendar rolls may be needed, such as embossing or schreiner calenders (e.g., for moire´ effects). The three basic mechanical parameters of this operation, when the fabric is passed between or around the calendar rolls, are: 1. Speed (calculated on the basis of dwell time in the nip) 2. Precise pressure (PLI or pounds per lineal inch) 3. Heating (these rolls are often heated to a predetermined temperature to obtain the desired or more lasting effect)

Therefore, these three mechanical operating parameters of speed, temperature, and pressure must be controlled. Consideration must also be given to a back-up roll design and type, wear resistance of top rolls and resilient rolls, the effects of fabric finishing chemicals on rolls, web tension, and control and profile of roll temperature. The function of a rolling calender is to provide a smooth or glossy fabric surface as well as to improve hand. The basic mechanical action of this type of calender is to cause the fibers and webs to not only reshape but to also possibly flatten or deform around one another, as it also causes the fibers to nestle or more tightly stack around one another. It can process all types of cloth/webs. A three-roll calender is predominantly used on high-content cotton woven/nonwoven or impregnated fabrics. Normally, the unit comes as a three-roll calender with alternate steel and filled rolls, although it may also come with two, four, or five rolls. The intermediate resilient roll is of wool felt paper, cotton, khaki wool, and resilient wool and cotton blends, which afford the extension of the webs. The main function of the silk finishing calender is to provide a smooth fabric surface, light luster, and improved hand. The basic mechanical action for this type of calender can be the same as for the rolling but with the selection of special fiber rolls that are comparatively softer. The 78 shore D fiber rolls when loaded against a smaller diameter steel will create a very wide nip between the rolls, which will not only extend the web but will also flex any bonds from coatings or stiffness from the weaving, knitting, or yarn-forming process. It can process all types of fabrics but is used mostly for high-content cotton and coated or impregnated webs. A silk finishing calender may run at a speed of 100 yards per minute, although most nonwovens are normally run between 30 and 35 yards per minute. Nip loading is between 400 and 700 PLI (pounds per lineal inch). The wide nip flexes and releases the web, causing a disruption between the bond points, which not only serve to “soften the hand” of the web but also strengthen the web by enhancing the inter-fiber cohesion (Gunter and Perkins, 2014). The silk finishing calender has a three-roll configuration with a top and bottom filled roll and a steel middle roll. The filling is of cotton/wool blends. Some synthetic covered rolls can be used but afford far less mechanical displacement to the web. The main steel roll, which is driven by a variable speed motor and roller chain, can be heated by gas or steam to surface temperature of 175 °C. Optionally, the two auxiliary filled rolls can be driven off nip drives. The unit has a circulation system for the bearing boxes, which are required for roll heating systems over 150 °C (Gunter and Perkins, 2014).

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In most cases, silk is only calendered in a cold state, which produces a soft handle. Higher luster is obtained with hot calenders, but it must be determined in each case whether such treatment has a negative influence. In exceptional cases, such as furnishing fabrics, chintz effects are desired.

5.3.2

Palmer or felt calender

This is used for delicate fabrics such as cotton mulmul, voiles, silk, synthetic, and knitted fabrics. The machine provides a smoother handle and lustrous appearance of silk fabrics. It is mainly used for taffeta-like woven fabric and serge articles. Figure 5.1 shows the line diagram of a felt calender. The combined drying and finishing is achieved by using an iron or stainless steel cylinder of about six and a half feet diameter (C) moving together with an endless blanket (B); the fabric (A) passes in-between. The tension is controlled by small roller D. Very little tension is put on the fabric. The production of felt calendar is approximately 30–50 yards per minute.

5.3.3

Decatizing

Decatizing improves the dimensional stability of silk fabric. It removes creases, and the fabric becomes smooth. Various types of silk fabric, such as woven creˆpe, are decatized. The wool felt fabric used for decatizing of silk is not too hard, and the silk fabric is not pressed flat. Superheated steaming during decatizing gives a desired appearance and smoother feel on various fabrics. Both discontinuous and continuous types of decatizing machines are available in the marketplace. Figure 5.2 shows the principle of decatizing. The fabric to be decatized is wound with a woolen 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. Kawahara et al. (1999) covered the fabrics with the rapping cloths and wound them into a roll. The roll was steamed from the center of the roll for 5 min, and then cooled. The radial expansion of the roll by steaming was several

Figure 5.1 Schematic diagram of a felt calendar. A ¼ fabric, B ¼ blanket, C ¼ large heated cylinder, D ¼ small roller to control tension in the blanket.

A B D C A

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Wool felt

Fabric Steam inlet

Figure 5.2 Principle of decatizing.

percentages. The silk necktie fabrics with 63 denier weft yarns (manufactured by twisting 3 silk yarns of 21 denier) in doubles showed higher values of the shear and bending rigidity as well as their hysteresis compared with the fabrics with 126 denier weft yarns (manufactured by twisting 6 silk yarns of 21 denier) in singles. For the fabrics with methacrylamide grafted 126 denier weft yarns, the shear rigidity and its hysteresis decreased, but the bending rigidity of satin fabrics increased. The decatizing treatment was effective to reduce the shear rigidity and its hysteresis.

5.3.4

Breaking machine

A breaking machine is used to confer a particular type of soft handle when calendaring is insufficient. Breaking is a final treatment for fabrics with an excessively harsh handle, such as cre´pe. Breaking machines or breakers are of two types: l

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Button type: the piece is passed several times rapidly back and forth over small rollers studded with brass buttons. Knife type: the fabric is drawn over the edges of slanted knives.

The older button type has recently been replaced by knife breakers.

5.3.5

Tamponing

Silk is very sensitive to mechanical friction when it is subjected to various stages of treatment. Even if sufficient precautions are undertaken, silk fabrics are chafed in places. Chafing may originate during boiling off and then show up only after coloring or dyeing. The cause in most cases is the rubbing of the fiber/fabric surface against a harder surface. Light chafe marks look like white spots, as if the material has been sprinkled with white powder or flour. During chafing, the fiber surface gets roughened and the individual silk fibrils are split open. The nap or pelt thus formed produces a different refraction of light and generally take up a lesser quantity of dye, thereby producing a “white effect.” Chafing may be aggravated during finishing due to: 1. A dyeing temperature that is too high 2. A treatment time that is too long 3. Extreme pH values above pH 10 and below pH 4

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The dyeing temperature is the most serious factor. It is, therefore, recommended for dye-sensitive fabrics at low temperature and to prolong the treatment time somewhat to obtain good exhaustion. Depending on the degree of chafing, this damage can be removed in some cases by wiping lightly with oil, called tamponing, which pastes down the raised hairs on the fiber fibrils. In former days, faulty portions of fabric were treated by hand with a socalled tamponing cushion that carried an extremely fine film of oil. Nowadays, tamponing machines can apply a fine film of oil homogeneously on several rollers, which is in turn evenly transferred to the fabric. The correction of chafing requires considerable experience, and the fabric may have to be run more than once through the machine.

5.3.6

Shrinkage and relaxation

Horizontal screen steamers or shrinkage machines are used to produce shrinkage and relaxation of the fabric. Creˆpe-like fabrics sometime appear too flat, as they come in an insufficiently relaxed state from the previous treatment. In the screen steamer, the fabric is placed without tension (relaxed state) onto a continuous screen. Steam is passed through the screen and fabric from below. The shrinkage machines may be open or close.

5.4

Chemical finishing of silk

For many years, silk finishers used classical finishing agents and took a conservative view of modern finishing chemicals. However, as new end-use standards are set and new finishing chemicals are developed, modern products have now made headway in the silk finishing sector. Though finishes based on elastomers and synthetic resins have been developed, specialists are often critical about their use, due to the fear that the characteristics of silk, particularly its handle, may be hampered.

5.4.1

Weighting of silk

Unlike most fabric/yard goods that are sold by the yard (or meter), silk is sold to the wholesaler by weight. The first step in processing the silk fiber is “degumming the fiber,” the gum being a by-product of the production of the silk fiber by the worm. Approximately one-fifth of the weight of the silk fiber is lost in the degumming process; manufacturers felt that they had the “right” to replace this lost weight with a filler of some sort. Silk has an affinity for several metallic salts, the most common being iron, lead, and tin. It was discovered to be an easy process to return this weight lost in the degumming process by soaking the fiber in a bath of these metallic salts. This process is called weighting; by increasing the weight of the raw silk, the merchant increased his profit. Weighting with some metallic salts did improve the drapeability of the fiber. However, greedy merchants soon began adding more weight than the lost

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one-fifth; sometimes the final weight was increased tenfold (Phipson, 1898). The finishing treatment of “weighting” on silk has been commonly used for the last 300 years throughout the world. This practice was widespread in the nineteenth century and decreased somewhat in the twentieth, but is still used to some extent. The most commonly used process of weighting is tin/phosphate/silicate, where tin silicophosphate is finally formed on the fiber along with lot of intermediate and by-products. Weighting gives fullness and richness of feel and handle, whereas heavy weighting can make the fibers hypersensitive to rubbing, resulting in a rough handle. Generally, the finer and more expensive reeled silks are weighted rather than the less costly spun silks. Heavily weighted silk must be made into garment as soon as it is made. Saltwater, perspiration, and tears cause spots to be formed: the silk appears to have been eaten by acids. Sunlight also attacks weighted silk. The silk industry makes a distinction between pure-dye silk and weighted silk. In the pure-dye process, the silk is dyed and subsequently finished with water-soluble substances such as starch, glue, and gelatine. But it is not weighted. If weighting is not executed properly, it may lose much of its strength and durability. The metallic salts used for weighting silk may also cause health risks (O Ecotextiles, 2010). The weighting improves the handle and drape of the material and with some polymer weighting systems, flame-retardant and easy-care properties can be introduced. The increase in fabric weight is measured in “par,” either “below par” or “above par” and can be as high as 300% above par based on the raw weight of the silk (Prayag, 1984). Excessive weighting can be detrimental, as this diminishes the affinity of the fiber for dyestuffs and causes brittleness and a loss in fiber strength. A variety of methods has been used over the years to weight silk; these can be classified into three categories: 1. Vegetable weighting 2. Mineral weighting 3. Polymer weighting 1. Vegetable weighting The use of tanning agents is a simple operation causing improvements in hydrophobicity; however, due to their inherent color, they cannot be applied on light-colored dyed fabric or white fabrics. The method is now rarely used. 2. Mineral weighting Tin phosphate/silicate is the most popular weighting process. The application is made in several stages and careful monitoring ensures minimum undesirable side effects. The silk is soaked in stannic chloride at room temperature for about 1.5 h, hydro-extracted, and washed in cold water. Washing removes any unfixed stannic chloride and hydrolyzes fixed stannic chloride into metastannic acid. The silk is further treated with dilute disodium phosphate for 1 h at 60–70 °C, washed, and acidified to give an insoluble tin phosphate compound. The whole process is repeated until the required increase in weight is obtained. Finally, an insoluble tin silicophosphate is formed by treating silk with dilute sodium silicate at 70 °C for 1 h. Deposition of the metallic salt takes place in the amorphous regions of the fiber, with little alteration of fabric appearance.

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SnCl4 + 4H2 O ! SnðOHÞ4 + 4HCl

(5.1)

SnðOHÞ4 + Na2 HPO4 ! SnðOHÞ2 HPO4 + 2NaOH

(5.2)

SnðOHÞ2 HPO4 + 3Na2 SiO2 ! ðSiO2 ÞSnO2 + 3NaOH + Na3 PO4

(5.3)

Mineral weighted fabric can be identified by a burning text. Untreated silk melts partially and forms little char, whereas mineral weighted silk burns to leave an ash. The weighting process includes the application of 30–300% solutions of inorganic salts of aluminum, iron, lead, tin, or zinc to the fabric in order to increase the body and weight. These weighting compounds are highly acidic, lowering the pH of the solution below 3. The pH of the solutions used to treat silk during manufacturing processes is an important factor in the light stability of silk fibers. Silk is less resistant to light between pH 6 and pH 8. It has also been reported that silk shows its maximum light fastness at pH 10 (Timar-Balazsy and Eastop, 1998). Accordingly, it is obvious that weighted silks are more susceptible to photodegradation. The damage caused by strong acids to silk fibers is quite severe, and it has been shown that the effects of weighting on silk are much more deleterious under exposure to light than in darkness (Becker et al., 1987). 3. Polymer weighting Organic weighting by grafting has recently been introduced to decrease cost and to avoid deleterious effects of metallic weighting. This method of weighting is based on a treatment of methyacrylamide and ammonium persulfate. It is reported that polymer weighting of up to 50% can be achieved without impairing the fabric handle. The disadvantage of this method is that it alters the dyeing properties of the fiber; however, it is preferred as it is less harmful to the environment than mineral weighting.

5.4.2

Sandwash finishing of silk

The 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 effect is produced by treating silk under harsh conditions, when the fabric surface roughens, breaking the surface fibrils to create a peach-skin-like texture. Depending on the method used the fabric also shrinks, making it dimensionally stable. The traditional handle of silk is lost along with as much as 50% of the initial fabric strength (Hadimani et al., 1998). Sandwash finish can be produced in several ways. Silk may be dyed in overflow machines under harsh conditions, for example, for 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 fiber surface using an emerizing machine, although with this method there is a risk that whole fibrils may be broken. Uniform surface modification may be achieved by using a protein-active enzyme such as Bactosol SI (Clariant) (Engeler, 1992).

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5.4.3

Advances in Silk Science and Technology

Softeners/lubricants

Softeners and lubricants are used to improve handle, drape, cutting, sewing, etc., and to help retain the traditional feel of silk. Fabric softening is carried out during the final stages of wet processing, usually in the final wash bath. A wide variety of softeners can be applied to silk; recently, the use of multifunctional silicone softeners has become popular, particularly amino-functional polysiloxanes in micro-emulsion form (Mehra and Mehra, 1996). Silk develops extraordinary properties with the application of softeners/lubricants. Some examples are given below:

Soft, full handle finish Ceranine HCS/PNL liquid (Clariant) 10 g/l Ratifix F liquid (Clariant) 10–20 g/l Adjust pH to 5–5.5 with acetic acid, pad at room temperature and dry at 120 °C. Soft, full handle finish with improved sewability Ceranine HCS/HCL liquid (Clariant) 10 g/l Sandolube NV liquid (Clariant) 10 g/l Adjust pH to 5–5.5 with acetic acid, pad at room temperature and dry at 120 °C. Elastic nonslip finish Sandoperm FEN liquid (Clariant) Dicofix 801 liquid (Clariant) Elfugin V liquid (Clariant) Pad with 70–80% pick-up, dry, and cure for 90 s at 150 °C.

20 g/l 20 g/l 20 g/l

Absorbent finish Sandoperm FEN liquid (Clariant) Dilasoft TF (Clariant) Finish MH liquid (Clariant) Magnesium chloride Pad with 70–80% pick-up, dry, and cure for 90 s at 150 °C.

20 g/l 10 g/l 10 g/l 5 g/l

In recent years, with the popularization and application of silicone softeners in the textile industry, the silk industry widely uses organic silicon as the main body of the finishing process combined with mechanical finishing, in order to improve the performance of silk fabric. Silicone-finished silk fabric has a good level in the handle, luster, elasticity, less than 3% shrinkage, and maintains the natural style characteristic of silk. Various types of silicone finishing agents impart different degrees of smoothness in silk fabric. The film-forming silicone products provide the best flexibility. But for real silk finishing silicone, preferably using single-component, low-temperature elastic membrane species, amino-modified silicone finishing agents are used. The nonionic products include polyethoxylated esters of long chain alcohols, acids, and oils. The nonionic types are easy to mix with other finishing agents (Ye, 2014).

Advances in the finishing of silk fabrics

5.4.4

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Scroopy handle

Scroop is the ability to add a rustling sound to silk or rayon fabrics during finishing by treating with certain acids. To measure the scrooping sound of silk objectively, a sound generator was developed (Morooka and Furusato, 1976) to rub two-fiber bundles at right angles, just like violin strings. It was observed that: 1. It can generate effective frictional sound with little mechanical and outside noise. 2. The measured sound is intermittent due to stick-slip vibration. 3. The intermittent vibration contains both the string transverse vibration and the vibration of about 8 kHz frequency.

4. The scrooping sound found in silk handling also contains an intermittent vibration, which also has a frequency of about 8 kHz.

According to the prevailing fashion, the final handle of the fabric may be adjusted to harder or softer depending on the type of silk article. The fiber itself should always retain the classical feel of silk. This must be taken into account while formulating a finish liquor. In some cases, a scroopy handle finish is given as early as in the final rinsing bath after dyeing or printing. This preliminary finish is given by adding oil emulsion or Marseilles soap. In a subsequent bath, without intermediate washing, these are precipitated on the fiber surface in a finely dispersed form, with an organic acid such as tartaric, oxalic, formic, or acetic acid. It was long believed that a good handle can be produced by this double deposition. However, it is stated (Sandoz, 1988) that the addition of oil and soap do not contribute to producing the scroopy handle—it is attributed to the acid alone, and the natural scroopy handle can be restored after various treatment stages. It is believed that the acid treatment causes formation of a very fine skin on the fiber surface by reorientation of the fibroin molecules at the outermost layer of the fiber. Various acids influence the handle in different ways. Some recipes for scroopy finish (Sandoz, 1988) are given below: Treat in 1–3 g/l oil emulsion (e.g., winding oil, Clariant) or 1–3 g/l Marseilles soap solution for 15–20 min and then treat without rinsing in a second fresh bath containing 2–4 ml/l formic, acetic, lactic, or citric acid (CA) for 15–20 min at room temperature and then hydro-extract and dry. Softening treatment to improve scroopy handle is as follows: Pad fabric with 10–20 g/l cationic softener (Ceranine SG paste, Clariant) and 1 ml/l acetic acid Dry at 120 °C after padding. A few classic article-specific scroopy handle finishes are listed below: Fabric: creˆpe de chine Gum tragacanth (75:1000) Soft oil (olive oil/sulphuric acid) Glycerine Acetic acid Water to make

80 ml 160 ml 10 ml 15 ml 1000 ml

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Fabric: pongee Gum tragacanth (75:1000) Dextrin (2:1) Gum Arabic (1:1) Glycerine Tartaric acid 10% Water to make

70 ml 30 ml 25 ml 5 ml 4 ml 1000 ml

Fabric: tussah silk Dextrin (2:1) Tartaric acid 10% Water to make

40 ml 4 ml 1000 ml

5.4.5

Oil- and water-repellent finishing of silk

Traditionally, metallic acid salts or oxides, proteins, or nitrogenous compounds were used; however, the current trend is for combined polymeric finishes that offer resistance to both oil and water, based on silicones, acrylamide, acrylic acid (AA), or fluoro-chemicals. A combined treatment with a fluorocarbon derivative, a reactive resin, and a water-repellent finish produces a versatile finish (Mehra and Mehra, 1996). Grafting with the vapor of hexafluoro propylene, vinyl fluoride, vinyl chloride, and acrylonitrile produced an increase in water repellency, oil repellency, mineral acid resistance, and photo-stability (Achwal and Kaduskar, 1984). An example of such a finish is as follows: Oil- and water-repellent finish with spot protection Fluorochemical compound Sandoflur K or GTC (Clariant) 20–30 g/l Cerol EWL liquid (Clariant) (as extender) 40–60 g/l Finish MH liquid (Clariant) 5–10 g/l Zinc chloride or zinc nitrate 0.6–1.2 g/l Adjust pH to 5–5.5 with acetic acid, pad at room temperature, dry at 120 °C and cure for 4 min at 150 °C.

5.4.6

Stain repellent/release finish

Soiling generally means smearing or staining of a large surface of the fabric with dust or dirt and oil or grease, or both. A fabric gets soiled mainly by three types of mechanisms: 1. by mechanical adhesion of soil to the cloth; 2. by adhesion by electrical forces due to attraction of dust particles from air by electrically charged fiber surface;

3. by redeposition of soil during washing.

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Some of the principal reasons for discarding textile articles include the inability to remove stains. Whether on fabrics or on garments, stains are unwelcome. In fabrics, they reduce consumer appeal; pieces containing stains have to be marketed at low prices as seconds or fents. Teflon® fabric protector from DuPont (marketed by INVISTA) is a water-based fluoro-chemical finish that resists substances that are present in oil- and water-based stains. It has been studied on woven silk union fabric by Desai (2012). While many stain-resistance products are merely a surface coating for fabrics, Teflon textile finish works through sophisticated molecular engineering on the nanometer scale to protect each fiber in fabrics. When a repellent finish is applied to fabrics, it provides superior water/oil repellency and protection against spills and stains. When a stain release finish is applied, the invisible stain-release technology improves the ability for ground-in stains to be easily removed during laundering. Teflon fabric protector delivers durable, long-lasting protection by forming a molecular shield around the fibers, guarding against oil- and water-based stains, dust, and dry soil. Repellent products lower the critical surface tension of the fabric so the fabric doesn’t attract stains or soil. Oil and water bead up and roll off the fabric; spills can be easily blotted up with a clean cloth. Advanced Dual-Action Teflon® fabric protector combines stain repelling and stain release. Additional important advantages of silk blend/union fabrics are easy-care and moderately priced. The fabrics thus woven were finished with a stain repellent/release finish and the easy-care properties have seen to be enhanced. The finish applied is compatible to the antimicrobial finish applied to the fabrics. The finish applied has combined the best of two stain fighters into one finish, i.e., both stain repellency and stain release have been imparted effectively to the fabric. A spot (water) resistance finish is as follows: Cerol ZN liquid (Clariant) 20–40 g/l Ceranine HCS/HCL liquid (Clariant) 10–20 g/l Adjust pH to 5–5.5 with acetic acid, pad at room temperature and dry at 120 °C. 5.4.7

Anti-static finishing of silk

Silk has a fairly low static charge build-up property, but static charge build-up may increase if the silk is grafted with monomers used to improve certain fiber properties. Treatment with selected water-soluble vinyl monomers (e.g., N,N¢ -methylene bisacrylamide, triacryloyl polyethylene glycol methacrylate, etc.), under acidic conditions, produces an increase in antistatic and hygroscopic properties. The deposition of certain metallic particles has also been found to increase the antistatic and conductivity properties of the fiber (Achwal and Kaduskar, 1984). 5.4.8

Crease-recovery finishes

Commercially, an easy-care fabric or product is one that does not crease readily while in use, can be washed, and requires no or minimum ironing to restore its original appearance. In addition, the traditional comfort, handle, texture, drape, and color of

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the product should not be affected. Technically, an easy-care finish enhances the fabric resistance to, and recovery from, creasing without any adverse affect on other intrinsic fabric properties. Resistance to creasing depends on rigidity, and recovery from creasing depends on elasticity (Marsh, 1962). In today’s market, easy-care products are applicable to almost all clothing materials. In silk finishing, there is a gap in the market for a product that can impart true easy-care characteristics. The easy-care properties are inadequate as compared to synthetic fabrics, so a good crease-recovery (CR) finish becomes 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 catalysts. Silk has been used as a luxury fiber for many years due to its soft hand, characteristic scrooping, excellent drape, and good luster. However, it has very low fiber resilience causing wrinkling, especially when wet. This poor wet resiliency is due to silk’s lack of intermolecular chemical cross-linkages. When the fibers absorb water and swell, the salt linkages between polymers, which are responsible for the high dry crease recovery of these fibers, are broken. Silk fabrics have low wet and dry resiliency. Hence, the fabrics wrinkle easily during home laundering or when wet. One of the most important factors that influence the quality of the fabrics is the ability to recover from induced wrinkles. Wrinkles are defined as the fabric deformation based on its viscoelastic properties. Wrinkles are classified into two categories: desirable wrinkles for smartness of clothes and undesirable wrinkles occurring during wear. The crushing and creasing of textile materials is a complex effect involving tensile, flexing compressive, and torsional stresses. The tensile extension plays a relatively small part in the elastic recovery; measurements of the modulus of elasticity are little guide to the creaseability of the various fibers, for the most elastic materials in the engineering sense are apt to crease most readily. The bending elasticity seems to be of the greatest importance in the phenomenon of creasing; a crease appears when the material is distorted in such a manner that part of it is stretched beyond its small power of elastic recovery. The bending of fibers or filaments that takes place during creasing leads to an extension of the fiber on the upper surface and a compression on the under surface. The relative positions of the polymer chains may change due to the above phenomenon, resulting in the formation of creases. However, if the adjacent polymer chains are bonded by cross-links, the polymer chains can return to their original locations, preventing crease formation. The crease-resistant finish on cotton fabrics improves comfort, ease of maintenance, dimensional stability, and pilling performance. Silk has less wrinkle formation when compared to cotton and is altered by applying polycarboxylic acid (PCA) and dimethyleneurea (DMU). Various chemical methods have been examined to improve the poor performance properties of silk (Peng et al., 1993). In general, these methods may be roughly divided into three groups: grafting of vinyl monomers on silk, cross-linking through formaldehyde-containing reagents, and cross-linking through nonformaldehyde reagents.

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Grafting of silk with many different types of monomers has been extensively examined by Tsukada et al. (1993). However, this method uses organic solvent systems in many cases, and, therefore, there are some doubts for commercial application. The most likely used cross-linking agents in crease-resistant finishes have been N-methylol agents or N-methyl amides because of their efficiency and low price. Formaldehyde-free cross-linking agents for producing crease-resistant properties are of interest to replace N-methylol compounds. PCAs that are nonformaldehyde reactants are possible replacement for conventional finishing reactants. The main advantage of PCAs is that they are formaldehyde free, do not have bad odor, and produce very soft fabric hand. Nonformaldehyde di- or multi-functional cross-linking reagents included aromatic acid anhydrides, dibasic anhydrides, and epoxide. Anhydrides also require organic solvent. To obtain the optimal improvement in the crease angle of silk, about 20% of aqueous epoxide was needed. This can adversely impact hand and drape properties of the treated silk. In addition, the increase in wet crease angle for tussah silk was not very high. More recently, the butanetetracarboxylic acid (BTCA) treatment of silk was found to considerably improve the resilience of silk (Yang and Li, 1994). However, to form an anhydride molecule, as a precursor for the esterification reaction between silk and acid, a high curing temperature was needed. This can adversely affect the hand and drape of the treated silk. BTCA is expensive, and the catalyst used; i.e., sodium hypophosphite (SHP), caused some shade changes for dyed fabrics due to its reductive nature. Therefore, a search for a cross-linking agent for silk finishing must be continued. As mixed PCAs have been an interesting research area for decreasing the expense of using BTCA by reducing the required amount of BTCA in DP finishing, the finishing of BTCA mixed with CA was investigated by Yang and Hu (2006). It was observed that the wet wrinkle recovery angle (WRA) decreased when smaller amount of BTCA mixed with CA was used in the DP finishing, although the breaking strength retention did not vary too much. As CA was considered to have lower activity than BTCA (Xu and Shyr, 2000), in the same condition, CA could not have a complete crosslink with silk fabrics. To obtain better finishing results with CA, other methods should be considered. The combination of a hydroxyl-functional organophosphorous oligomer (HFPO) and 1,2,3,4-butanetetracarboxylic acid (BTCA) as a formaldehyde-free flame retardant finish was applied on silk (Guan et al., 2009). When BTCA is applied alone, most of it reacts with the hydroxyl group of silk by single ester linkage. In the presence of HFPO, BTCA is able to bond HFPO onto silk by either a BTCA “bridge” between silk and HFPO or a BTCA–HFPO–BTCA cross-linkage between two silk protein molecules. The treated silk fabric demonstrated a high level of flame retardancy with a modest loss of tensile strength. The treated fabric passed the vertical flammability test after 15 hand wash cycles. The thermal analysis data demonstrated that HFPO reduces silk’s initial thermal decomposition temperature and promotes char formation. Glyoxal is a nonformaldehyde reagent that is readily available. It has been reported that in the presence of aluminum sulfate, glyoxal provided the high reaction rates for

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cotton required in commercial processes. By using various co-reactant additives such as glycol and a-hydroxy acid, a mild curing condition such as 120 °C for 2 min may be suffice. The formation of cross-links in cotton by glyoxal was through hemiacetal formation (Welch and Peters, 1987). A similar chemistry can also be applied for the reaction between silk and glyoxal. The feasibility of using glyoxal as a crease-resistant finishing agent for silk was examined by Choi et al. (1998). Results indicated that glyoxal considerably improved the conditioned WRA of the silk in the presence of a Bronsted–Lowry acid, such as sulphuric acid and formic acid (FA) and aluminum sulfate. The presence of such strong acid did not greatly reduce the tear strength retention of the treated silk. After the treatment, the dyeability of silk with acid dye slightly decreased. The FA–CaCl2 solubility test substantiated the presence of cross-linking in the treated silk. X-ray diffraction indicated that the reaction mainly occurred in amorphous region. Thermal stability at high temperature of the treated silk was generally better than that of the untreated control. Conventional treatment is conducted with an epoxide solution in tetrachloroethlene at 60–80 °C, which is applied on an industrial scale in Japan (Shiozaki and Tanaka, 1972). In conventional epoxide finishing of silk, organic solvent haves to be used, which is hazardous to the health of the exposed workers as well as the environment. With the family of silicone epoxy cross-linking agents, crease resistance occurs because chemical cross-links occur between the silk fibroin strand and the epoxy groups (O Ecotextiles, 2010). Cai et al. (2001) used EPSIA (epoxy silicone cross-linking agent A), a new modifying agent constituted of silicone-containing epoxy cross-linking agents to improve the wet resiliency of silk, the structure of which is shown in Figure 5.3 (Cai and Qui, 2003), where R1, R2, R3 are alkyls and n is either 1 or 2. The silk fabric samples were treated with a pad–dry–steam process from an aqueous bath containing EPSIA, the dispersion agent (Invadine JFC) and the catalyst (potassium thiocyanate). The sample is then dried at 60 °C for 3.5 min, and finally steamed at 110 °C for 7 min. After chemical finishing, both the dry and wet resiliencies of the silk fabrics were distinctly increased and the finished silk showed good durability to home laundering. Results also indicated that any shrinkage in the laundering process was well controlled after the silk fabric was finished with EPSIA. Breaking strength of the fabric decreased slightly, whereas tear strength increased. Furthermore, there was a 20% decrease in bending stiffness and more than a 200% increase in abrasion resistance. These results showed that the treatment of silk with EPSIA could significantly reduce inter-fiber or inter-yarn frictional force as well as being capable of enhancing resiliency. However, there was a 12.6% reduction in moisture regain.

Figure 5.3 The structure of EPSIA (epoxy silicone cross-linking agent A).

R2 R1–– Si ––R3N(CH2––CH––CH2)CH2CH2N(CH2––CH––CH2)n R2

O

O

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Parthiban and Kumar (2009) made a comparative study on application of three cross-linking agents: glyoxal, CA, and DMDHEU. The catalysts used were aluminum sulfate, sodium hypophosphate, and magnesium chloride, respectively. The optimum concentration for treatment with glyoxal and CA were found to be 5% and 6%, respectively. The aluminum sulfate was found to be the most suitable catalyst for glyoxal. The optimum curing temperature and curing time were found to be 120 °C and 120 s. Ecofriendly crease-resistant finishes were found to have a less deleterious effect on mechanical properties, i.e., tensile strength, abrasion resistance, etc., compared with DMDHEU. The percentage of increase in crease recovery angle was found to be more in tussah fabric than in mulberry fabric (Parthiban and Kumar, 2007). Untreated silk fabric has poor easy-care characteristics. They cannot be machine washed, as they shrink. It is also impossible to remove all the creases inserted during washing by ironing. A zero-formaldehyde, easy-care finish for silk fabric was developed using a PCA finishing system, as reported in a PhD thesis submitted by Irvin (1999). Treatment of silk fabric with a PCA namely 1,2,3,4-butanetetracarboxylic acid, C8H10O8 (BTCA) (Figure 5.4a) or CA or 2-hydroxypropane-1,2,3-tricarboxylic acid, C6H8O7 (Figure 5.4b). In the presence of SHP, catalyst improved the easy-care properties of the silk fabric compared with the untreated control. The thesis confirmed that the treatment of silk fabric with selected PCA finishing systems improves the stability of silk fabric. The treated fabric had improved stability to heat, reduced swelling in solvents, and improved elastic recovery in the dry and wet states. A number of interesting mechanistic possibilities are proposed for improving the easy-care performance of silk fabrics with a PCA finishing system. It is postulated that the improvement in elastic and stability properties of the treated silk fabrics could be as a consequence of new fiber inter-chain cross-links and/or a new intra-fiber PCA polymer network being formed within the silk fibers. Three different types of possible bonds between the PCA and silk fibroin were proposed:

O

(a)

H2C

C O

OH

HC

C O

OH

HC

C O

OH

H2C

C

OH

O HO

(b)

O

OH O OH

OH

Figure 5.4 The structures of (a) BTCA (1,2,3,4-butanetetracarboxylic acid) and (b) CA.

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Ester links formed via acid and/or base catalysis with hydroxyl containing amino acids, serine, threonine and tyrosine, in silk fibroin. Amide links may also be formed between the basic amino acids, arginine, lysine and histidine, in silk fibroin and acid anhydrides formed during curing. Hydrogen bonds formed with acid, basic and hydroxy side groups and with the silk fibroin backbone. This thesis (Irvin, 1999) extends previous research with respect to PCA finishing. It proposes the use of a new analytical method to pre-determine the effectiveness of an acid/catalyst system as an easy-care finish. It recommends the use of combination acids to compete with 1,2,3,4-butanetetracarboxylic acid and proposes the use of monosodium citrate as an alternative catalyst to SHP when cured at 165 °C. A water-soluble anionic bifunctional polyurethane polymer, WPU200 was prepared from the reaction of PEG200 and hexamethylene diisocyanate at 60 °C in butanone in a stirred reactor (Hu and Jin, 2002). It is reported that WPU200 can remarkably improve the wet resiliency of silk fibers, so it is an effective wash-andwear finishing agent for silk fabrics. The WPU200 polymer has the advantages of water solubility, low reaction temperature, soft handle, and no hazards to human health. Traditional pad–dry–cure technology can be used in the wash-and-wear finishing of silk fabrics with WPU200. The optimum finishing conditions include 4% WPU200 in the finishing bath at pH 9–9.5, curing temperature of 100 °C, and curing time of 3 min. Both breaking strength and elongation of the finished silk fabric drop slightly. Whiteness and moisture regain (65% RH, 25 °C) of the finished fabric are a little lower than those of the control samples, and the stiffness is a bit higher. Drycrease recovery angle and the handle can be improved by anionic and nonionic softeners. After-treatment with various softening agents further improved the easy-care characteristics of the treated silk fabric. Treatment with a reactive silicone softener gave rise to a fabric with the greatest improvement in elastic properties and a super-soft, luxurious, but floppy handle. Treatment of silk fabrics with a BTCA/SHP finish even with a silicone softener after-treatment did not improve the elastic properties of the silk fibroin sufficiently for this finish to be recommended as a viable commercial easy-care finish for silk fabrics. However the inclusion of a fiber swelling agent FA or AA, into the PCA pad bath vastly improved the easy-care properties of the treated samples. It is proposed that the increased swelling brought about by the inclusion of a fiber-swelling agent into the pad bath wet-set silk through stabilization in its new swollen state. After-treatment with a silicone softener improved the dry elastic recovery; this should help prevent creases being formed in the dry state. The finish produced a super soft fabric with minimum ironing requirement and enhanced resistance to fabric abrasion. The treated fabric was machine-washable. Treatment of silk fabrics with a BTCA/FA/SHP or BTCA/ AA/SHP finish followed by an after-treatment with a silicone softener results in good easy-care performance. The reactions of epoxy compound (triglycidyl ether of glycerol, TEG) with silk under pad–dry–steam or pad–dry–bake conditions have been reported by Peng l

l

l

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et al. (1993). In the presence of the catalysts, thiosulfates or thiocyanates, the epoxides could rapidly react with silk. Crease-proofing has been conferred on the fabrics as a result of epoxy cross-linking. However, yellowing of silk is intensified by this process, and the bath solution containing epoxy resin and catalytic salt is rather unstable, turbiding or even precipitating after a short time storage. Epoxy resins, especially TEG and diglycidyl ether of ethylene glycol (DEE) are the effective silk finishing agents. NaH2PO4 plays a role as a buffer for the alkali released in the finishing processes catalyzed by potassium thiocyanate (chemical formula: KSCN). Two-bath pad process, i.e., padding epoxy resins and a catalyst (KSCN) in separate baths, drying and baking, is more appropriate in practice in modifying silk fabrics to have higher light resistance and excellent crease-proof, without impairing its natural luster and hydrophilicity. The fault of “water spot” on tussah silk fabric is greatly reduced by such treatments (Peng and Sun, 1995; Peng et al., 1997). A silicone-containing epoxide named as EPSIB was prepared for silk crease-resist finishing by Cai et al. (2001). Aqueous silicone-containing epoxide EPSIB was prepared by pre-polymerization of cationic emulsion and subsequent reaction with 0.5 mol of epoxy chloropropane at the optimum pH value of 9 at a temperature of 65 °C for 90 min. Two-dip-two-nip process can be applied to finish silk fabric with the liquid at 100% pick-up. Then the sample was dried at 60 °C for 2.5 min, and finally steamed at 110 °C for 5 min. Both dry and wet resiliencies of silk fabric finished with EPSIB increased with improvement of fabric dimensional stability, whereas other properties were hardly influenced. Clearly, there is a scope for considerable commercial exploitation for EPSIB in silk finishing. The performance of three nonformaldehyde cross-linking agents, namely BTCA, glyoxal, and epoxide EPSIB in crease recovery of silk fabric reported in three independent publications are compared in Table 5.1. The anti-wrinkle properties of silk fabrics dyed with four bi-functional reactive dyes and three polyethylene polyamine cross-linking dyes (crosslinker 2-chloro4,6-di(aminobenzene-4¢ -b-sulfatoethyl-sulfone)-1,3,5-s-triazine (XLC)) were investigated by Dang et al. (2010). Compared with the undyed silk fabrics, WRA of silk fabrics dyed with bi-functional reactive dyes was increased by 14.9–15.3% when the dye concentration was 0.67 g/l. With the same concentration of synthesized cross-linking dyes, the WRA of dyed silk fabrics was increased by 17.9–21.3% with high dye reactivity. The anti-wrinkle properties of silk fabrics dyed with the cross-linking dyes were as good as those finished with 1,2,3,4-butaneteracarboxylic acid. Moreover, the anti-wrinkle properties of silk fabrics were improved with the increase of the cross-linking dyes and cross-linker XLC concentration, and the handle of these silk fabrics was still satisfactory. The improvement of the WRA of the silk fabrics dyed with cross-linking dyes was about the same as those finished with glyoxal or epoxide. It may be concluded that these cross-linking dyes could lead to not only high dye fixation, but also excellent wrinkle-resistant properties of silk fabrics; in addition, it is feasible to dye and finish silk fabrics with cross-linking dyes simultaneously.

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Table 5.1 Comparative performance of nonformaldehyde cross-linking agents in crease recovery of silk Crease-recovery angle (CRA) (warp + weft) (°) Finishing agents 1,2,3,4Butaneteracarboxylic acid (BTCA) (Yang and Li, 1994) Glyoxal (Choi et al., 1998) Epoxide EPSIB (Cai et al., 2004)

Untreated silk

Treated silk

% Improvement

60 g/l BTCA, 3.3% NaH2PO2  H2O, 0.5% softener, 170 °C for 3 min

265

311

17.4

Al2(SO4)3, 130 °C for 2 min 100 g/l epoxide EPSIB, 0.5% dispersing agent JFC, 0.4% potassium thiocyanate, 130 °C for 4 min

246

316

28.4

229

280

22.3

Finishing conditions

PCAs, which are nonformaldehyde reactants, are possible replacement for conventional finishing reactants like N-methylol agents or N-methyl amides. The main advantage of polycaboxylic acids is that they are formaldehyde free, do not have bad odor, and produce very soft fabric hand (Srikrishnan and Vellingiri, 2012). Ecofriendly crease resistance finishes were carried out using synthesized PCA and DMU. The chemicals were synthesized for betted durability and were applied on cotton and silk fabrics for improving the dimensional stability of it. PCA and dimethylene urea were synthesized using sonication treatment, with ultrasound waves passing through it at 10,000 dB; centrifugation treatment is given for even dispersion of the converted particles. After centrifugation, the size particles were analyzed using Dynamic Light Scattering method. Analyzed particles were applied on the fabric using padding process and then cured. As a result, the controlled sample and the treated sampled are compared and the crease recovery angle is checked using Shirley Crease Recovery Tester. An example is given below:

Cross-linking agent Mgcl2 Citric acid Softener (combination) Wetting agent pH

40–120 g/l 10–25 g/l 0.3 g/l 40 g/l 1 g/l 4–4.5

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The crease recovery of the PCA-treated cotton and silk fabrics were better than untreated fabric. The PCA-treated cotton and silk samples showed better results in crease angle than untreated sample. Cloth weight (grams per meter, GSM), thickness, and strength of these samples were also increased (Srikrishnan and Vellingiri, 2012). An example of wash-and-wear (machine washable) finish is as follows: Sandoperm FE liquid (Clariant) Finish MH Liquid (Clariant) Elfugin V (Clariant) Sandoperm FV liquid (Clariant) Sandoperm FKN liquid (Clariant) Magnesium chloride Pad with 70–80% pick-up, dry and cure for 90 s at 150 °C.

20 g/l 10 g/l 20 g/l 1 g/l 1 g/l 5 g/l

Traditional chemical anti-crease treatments with silk are subject to increasing criticism. Fortunately, scientific development has provided alternative enzymatic processes and other methods to address this issue. On the basis of the microbial transglutaminase (MTG) catalysis principle and theory of improving crease resistance through crosslinking, MTG probably has a special effect on silk fibroin and plays a role in silk crease resistance. MTG is a type of aminoacyltransferases, which can catalyze an acyl transfer between peptide-bond glutamine (acyl-donors) and a suitable primary amine (acylacceptors) (Aeschlimann and Paulsson, 1994). They are found widely in a host of different organisms, including mammals, plants, fish, and microorganisms (Lilley et al., 1998). In most cases, the result of their reaction is forming the cross-link of proteins via intra- or inter-e (g-glutamyl) lysine isodipeptide bridges if the primary amine is the e-amino group of the peptide-bond lysine (Griffin et al., 1998). The effect of MTG on silk fabric has been investigated through the Fourier transform infrared spectroscopy (FR). The microcosmic structure of fibroin through the scanning electron microscope (SEM) was analyzed. Solo MTG treatment as well as compound treatments of MTG followed by hydrogen peroxide, protease and ultrasonic, all showed that MTG can improve the crease resistance of silk fabric. It also enhanced its tensile breaking strength or amended damage in the tensile breaking strength caused by pretreatments. Simultaneously, comparison with other treatments showed that compound treatment of MTG followed by ultrasonic exerted a better coordinated effect and conferred better performances, which made the increase of WRA by 17.4% and tensile breaking strength improve by 11.2%, respectively. At the same time, other performances were still maintained well. However, the effect of solo MTG finishing is not very ideal and needs further enhancement (Tian et al., 2008). 5.4.9

Flame-retardant finish

According to statistics, about 50% of the fires in the world are caused by textiles (Nair, 2000). Silk fabric is widely used as pajamas and domestic decoration materials for its luster, soft handle, wearing comfort, and esthetic appearance. So it is vital to be endowed

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with flame-resistance property. Silk has low flammability with LOI value about 23%, which can be attributed to its high nitrogen content (about 15–18%). But it still needs added flame resistance finish to fulfill some commercial requirements. Flame resistance finishing on silk fabrics can be traced back to 200 years ago, when silk fabrics were immersed in the mixture of borax and boric acid in solution. But this mixture is easily removed by water. In the mid-1980s, Achwal (1987) reported that silk fabrics treated with a urea phosphoric acid salt by a pad–dry process had high flame resistance with an LOI value higher than 28%; the flame resistance finish was fast to dry cleaning while not fast to washing. To this day, there is still no suitable flame retardant for silk fabrics. When fire accidents occur, the primary hazard is ignition of clothing by contact with hot surfaces or small open-flame sources. Therefore, fabric with good flameretardant character should be chosen for garments to reduce unreasonable risk of burn injuries and deaths from fires associated with clothing. Many countries have enacted flame-retardant legislation for apparel such as evening dress and pajamas. Presently, many related regulations have been enacted around the world that state the flammability performance and standardize testing methods for flammability of textiles, such as S.I.1985 No 2043, Nightwear (Safety) Regulations of the United Kingdom (HMSO, 1985), and CFR 1610 Standard for the flammability of clothing textiles in the United States of America (CPSC, 2008). The test procedure under CFR 1610 US Standard provides a method for testing and establishes three classes of flammability performance of textiles and textile products used for clothing, thereby restricting the use of any dangerously flammable clothing textiles (those designated Class 3). The test method requires that a 16 mm (5/8 in.) flame impinge on a specimen mounted at a 45° angle for 1 s. The specimen is allowed to burn its full length or until the stop thread is broken, a distance of 127 mm (5 in.). The results of several specimens are averaged and a Class designation is made based on the flammability performance and surface characteristics of the sample. Samples resulting in a designation of Class 1 or 2 meet the requirements of the Standard. If a sample fails to meet the requirements of the Standard, then the sample is designated as Class 3 (Rapid and intense burning). For plain surface fabrics, a Class 3 designation results from an average flame spread time of less than 3.5 s. For raised surface fabrics, a Class 3 designation results from an average flame spread time of less than 4.0 s and ignition or fusing of the base fabric. Flame retardancy treatment of silk fabric by immersion in a mixture of borax and boric acid solution can be traced back some 200 years (Achwal, 1987). Cross-linking by introducing N-hydroxymethyl (3-dimethyl phosphono) propionamide and trimethylol melamine (TMM) or hexamethylol melamine (HMM) onto silk fabric can improve laundering durability to 30–50 hand wash cycles, and the LOI of treated silk fabric were above 30%. But the treated silk fabrics would become stiff and yellow and had the problem of formaldehyde releases (Guan and Chen, 2006). Silk, along with other natural fibers, is rendered fire resistant and selfextinguishing by coating with a reaction product of polyhalogenated acid with a cyclic nucleus, such as chlorendic acid and thiourea. Titanium hexachloride, titanium tetrachloride and zirconyl chloride are found to give good flame retardant finish for silk (Achwal and Kaduskar, 1984); they lower the pyrolysis onset temperature and l

l

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H2C

O CH3 N

H2C

103

H 3C O

P N CH2CH2

N

N P

CH2 CH2

2

Figure 5.5 The structures of N,N¢ -ethylene-bis(p,p-bis [aziridinyl]-N-methyl phosphinamide).

increase char formation. A multifunctional finish that imparts flame resistance, dimensional stability, and easy-care properties has been reported based on N,N¢ ethylene-bis(p,p-bis[aziridinyl]-N-methyl phosphinamide) (Figure 5.5) (Mehra and Mehra, 1996). The well-known flame retardant N-methylol dimethylphosphonopropionamide (MDPA), known as “Pyrovatex CP,” is effective for cotton. Jinping and Guoqiang (2014) applied it onto silk fabrics and gained good flame resistance property but at the loss of some physical properties of silk fabrics, such as whiteness, strength, etc. They also applied a self-prepared novel reactive organophosphorus flame retardant named diethyl-2-(methacryloyloxyethyl) phosphate (DEMEP) onto silk fabrics. Only less phosphorus content can make silk fabrics gain excellent flame resistance. Silk fabrics treated with DEMEP can endure 15 laundry cycles. While MDPA can be fixed onto silk fabrics effectively only with the help of cross-linking agent, hexakis (methoxymethyl) melamine (HMM). MDPA-treated silk fabrics have better laundry endurance. Both flame retardants can cause silk fabrics to become yellow and damaged. But strength losses are negligible. DEMEP-treated silk fabrics have better elasticity and wear comfort than MDPA treated silk fabrics. 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 being treated with flame retardant has been reported in the literature. Such parameters are very imperative for garment manufacturers. Three kinds of silk fabrics with different weaving styles for apparel uses were treated with a vinyl phosphorus monomer dimethyl-2-(methacryloyloxyethyl) phosphate (DMMEP) by a graft copolymerization technique using potassium persulfate as an initiator (Figure 5.6). The treated silk fabrics can be self-extinguished after being ignited with a candle-like fire, can pass the vertical flammability test, and show some decrease in permeability. The flame-retardant silk fabrics became a little stiff, which is helpful for flat seam appearance and are more difficult to distort in fabric laying up, panel cutting, and making. The formability and sizing stability during sewing, wearing, and pressing process was improved. The flame-retardancy treatment had different CH3 O CH2

C

C

O O

CH2CH2

O P

OCH3 OCH3

Figure 5.6 The structures of dimethyl 1-2-(methacryloyloxyethyl) phosphate (DMMEP).

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affects on the drape property of silk fabrics with different weaving styles. The seam strength decreased for different weaving styles after being treated with flame retardant (Guan et al., 2013).

5.4.10

Antimicrobial finishes

The textile materials and clothing are susceptible to microbial attack because they provide the basic requirements for microbial growth (Cardamone, 2002). The materials made of cellulose and protein natural fibers provide moisture, oxygen, nutrients, and temperature for bacterial growth and multiplication. This often results in objectionable odor dermal infection, product-deterioration allergic responses, and related diseases (Thiry, 2009). The control of microorganisms (e.g., bacteria, mildews, molds, yeasts, and viruses) on textile fabrics extends to diverse areas such as hospitals, the environment, and everyday households. Neither natural nor synthetic fibers have resistance to microorganisms. Thus, various antimicrobial finishes and disinfection techniques have been developed for all types of textiles (Purwar and Joshi, 2004). The functions of antimicrobial finishes applied to textiles are: (a) to control the spread of disease and the danger of injury-caused infection; (b) to control the development of odor from perspiration, stains, and dirt on textile materials; (c) to control the deterioration of textiles, particularly fabrics made of natural fibers, caused by mildew.

Antimicrobial finishes protect the fiber from microbial attack and protect the owner against transfer of pathogenic germs. Silk is relatively resistant to attack by mildew, fungi, and other bacteria, but if stored in moist conditions, damage can occur. The fungal resistance can be improved by treatment with compounds like N-(2,2¢ dichlorvinyl)salicylamide and a 0.01–0.25% solution of benzalkonium chloride. This increases the resistance of silk to mildew during long storage (up to 2 years) even at high relative humidity (Achwal and Kaduskar, 1984). Silk is a biocompatible biopolymer applicable in biomedical and biotechnological devices. In this field, the assignment of antimicrobial activity has great importance. However a general problem related to all antimicrobial chemicals is to find the balance between high biocide activity and the requirements of safe handling, including nontoxic finish processes, nontoxicity to humans at usual concentrations, and environmental demands. Antimicrobial agents that have been used industrially in recent years include quaternary ammonium salts, metal salts solutions, and antibiotics. Unfortunately, some of these agents are toxic and ineffective, which leads to a special attention on “green chemistry” by worldwide research (Ali and EL Mohamedy, 2010). Chitin, the second most abundant biopolymer in nature next to cellulose, is a high molecular weight linear polymer of 2-acetamide-2-deoxy-D-glucopyranose units linked by 1,4-glucosidic bonds (Dutta et al., 2002). Chitosan is a deacetylated derivative of chitin, which is characterized by nontoxicity, biodegradability, and compatibility with other ingredients (Coughlin et al., 1990).

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Previous studies (Hussein et al., 1997) reported that D-glucoseamine hydrochloride (chitosan monomer) does not show any growth inhibition against several bacteria in which chitosan is effective. The antimicrobial activity of chitosan is, therefore, related to not only its cationic nature but also to its chain length. Hebeish et al. (2012) treated cotton and silk fabrics with chitosan of low, medium, and high molecular weights (210,000, 100,000, and 1800, respectively) at different concentrations. The padded samples were dried at 100 °C for 3 min, followed by curing at 150 °C for 3 min, and then washed and dried. This was followed by screen printing using a natural dye extracted from prickly pears. Thus, the obtained fabrics were monitored for nitrogen, color strength after dyeing/printing, and antimicrobial activity. The results signify that the chitosan concentration and its molecular weight determine the magnitudes of both antimicrobial activity and color strength of the printed fabric. The silk pretreated with chitosan followed by dyeing with natural dyes assumes high color values. Hosseini et al. (2013) studied the effects of chitosan on dyeability and the antibacterial properties of silk yarns against two kinds of bacteria: “Staphylococcus aureus” and “Escherichia coli” were investigated. The treated silk samples were found to have antibacterial potential due to the antibacterial property of chitosan. SEM photographs reveal the deposition of chitosan on the treated yarns. Chitosan is used in textile field, applied by wet thermal curing involving relatively high temperature with energy consumption, costs, and possible fabric degradation; moreover, the addition of toxic reagents, such as glutaraldehyde, is required as cross-linking agent (Alonso et al., 2009). In UV curing, however, radical species are generated by the interaction of UV light with a suitable photo-initiator, which induces the curing reaction of reactive monomers and oligomers at low temperature and quickly, with lower environmental impact and lower process cost than thermal process (Ferrero et al., 2008). Silk samples were prepared with a chitosan add-on of about 2%, to maintain silk hand characteristics. Antimicrobial activity test was performed according to ASTM E2149-01 method, using E. coli ATCC 8739 as microbes. Tests on treated samples revealed the antimicrobial efficiency, with complete elimination of microorganism. The best results in terms of fastness were found with 1 h impregnation at 50 °C, with 75% microorganism reduction. The partial loss of antimicrobial activity after washing was due to the loss of chitosan from fibers, as confirmed by dyeing tests with an acid dye to reveal chitosan presence on fabrics. The surface morphology of treated fabrics was examined by SEM. Acquired images revealed a good homogeneous distribution of chitosan on the fibers. It was shown that the polymer does not form a coating on the textile, but covers every single fiber without gluing them (Monica and Franco, 2014). The worldwide demand for colorants of natural origin, especially yellow or red pigments, is rapidly increasing in the food, cosmetic, and textile sectors (Akira et al., 2000). Among the species examined, common madder (Rubbia tinctorum L) and woad (Isatis tinctoria L) proved to be quite interesting sources of red (alizarin), and yellow (luteolin) dyes, respectively, either for their agronomic characteristics or dyeing properties (El-khatib and Ali, 1997). Their main disadvantage lies in the order of magnitude of their extraction yield factors (a few grams of pigment per kilogram of dried raw material). To overcome this limitation, it has been suggested that other biological sources be exploited,

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such as fungi (both molds and yeasts), bacteria, algae, and plant cultures, since appropriate selection, mutation, or genetic engineering techniques are likely to significantly improve the pigment production yields with respect to wild organisms (Elshemy, 2011). Among the several pigment-producing microorganisms described in the literature, the fungus “Thermomyces” has been thoroughly studied (Palanivel and Vellingiri, 2009). Hundred percent silk fabric was colored using the natural fungal extract “Thermomyces” and the effects of coloring behavior and fastness results were analyzed. The process parameters, like pH, temperature and time, were varied and the results were optimized. The results were examined by coloring the protein fabric samples at 2% of the fabric weight and the optimum conditions for coloration and antibacterial finishing were found to be 40 min, 30 °C, and pH 3. The process proves to be more ecofriendly in nature and can be used for medical application to develop wound dressings, face masks, bandages, etc. (Manickam and Govindarajan, 2012). 5.4.11

UV protection finishing of silk

One of the major problems with silk fibers is their susceptibility to photochemical degradation. UV-absorber organic compounds that absorb UV-radiation can impart some protection by dissipating the energy as heat. UV absorbers are organic or inorganic colorless compounds with very strong absorption in the UV range of 290–360 nm. UV absorbers incorporated into the fibers convert electronic excitation energy in to thermal energy. They function as radical scavengers and oxygen scavengers. The high-energy short-wave UVR excites the UV absorber to a high energy absorbed may then be dissipated as longer wave radiation. Alternatively, isomerization can occur and the UV absorber may then fragment into nonabsorbing isomers. Screening agents, energy transfer agents, and pigments can also be used to deactivate the fiber degradation process (Achwal and Kaduskar, 1984). Organic UV absorbers are derivatives of o-hydroxyl benzophenones, o-hydroxyphenyltriazes, o-hydroxy phenyl hydrazines. The ortho hydroxyl group in the molecule helps in absorption and to make the compound soluble in alkaline solution. Organic compounds like benzotriazole, hydro-benzophenone, and phenyltriazine can be used by normal padding or coating applications. Ortho hydroxyphenyl and diphenyltriazine derivatives have excellent sublimation fastness and self-dispersing formulation. It can be applied by the pad thermosol process and also in print pastes. The presence of organic pigments in the fibers helps in better diffusion of light from the substrate, thus providing better protection. Titanium dioxide and other ceramic materials have an absorption capacity in the UV region of 280–400 nm and reflects visible and IR rays (Sivaramakrishnan, 2014).

5.5

Conclusions

Silk materials are luxurious and have many excellent qualities, such as high luster, comfortable to wear, fine and smooth texture, high silky softness, and excellent drapeability. Therefore, it needs little finish to improve its esthetic properties. However, it lacks several functional properties. Silk materials are prone to creases and wrinkles. It

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gets easily stained. It is susceptible to microbial attack and ultraviolet light. It has high water absorbency and poor oil and water repellency. It is flammable. In spite of all these deficiencies, one must be very careful in doing physical and chemical treatments. This delicate fiber may easily lose its luster and/or softness for which it is so attractive. The processing of silk is, therefore, a very skilful job. Until recently, the processing of silk was restricted among limited 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. On the other hand, the use of chemicals is getting 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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