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| Issue date:27/05/2010 |
| ATA Journal for Asia on Textile & Apparel - May 2010 Issue |
| Source:Journal for Asia on Textile & Apparel |
| by Vijaya Shanbhag |
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| Often dubbed as the queen of textiles, silk is regarded as a precious fibre given by nature with sheen and luster properties. It has been overshadowed in past decades by synthetics and other fibers. More recently, however, silk has regained its popularity in the stage of the global textiles partly because of the current preference for apparel made of natural fabrics. |
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| Silk cocoons (Photo: Aurora Silk) |
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Silk fabrics naturally offer good comfort and wearability. The shimmering appearance for which silk is prized comes from the fibers' triangular prism-like structure, which allows silk cloth to refract incoming light at different angles.
In terms of marketing positioning, it is understandable that silk is a niche fiber for quality apparel, rather than being a kind of massively produced fibre.
Manufacturing of silk apparel
A natural protein fiber, silk is the only natural filament that man does not have to spin before it can be used for textile fabrics.
The cultivation of silk is known as sericulture. The natural silk spun by silk worms in the form of cocoons is utilized only to 50% of its production due to a lack of correct processing and deficiency in sericulture.
Silk naturally offers a glossy appearance, soft drape and noble feel to a garment
(Photo: Banana Republic) |
Silk is broadly divided as domestic silk (or mulberry silk) and wild silk. Wild silk in India is often forest-grown and known as Vanya silk. There are three varieties in wild silk: Eri, Muga and Tussar silk. Eri silk is a staple fibre while the other two are filaments.
The best-known type of silk is obtained from cocoons made by the larvae of the mulberry silkworm Bombyx mori reared in captivity (sericulture). Over 30 countries produce silk, and major producing countries include China (54%) and India (14%). A variety of wild silks have been known and used in China, South Asia, and Europe.
Since the early times, the production scale of wild silk has been smaller than that of cultivated silk, and it differs from the domesticated varieties in color and texture. Cocoons gathered in the wild usually are often damaged by the emerging moth before the cocoons are gathered, so the silk thread that makes up the cocoon has been torn into shorter lengths.
Commercially reared silkworm pupae are killed by dipping them in boiling water before the adult moths emerge, or by piercing them with a needle, allowing the whole cocoon to be unraveled as one continuous thread. This permits a much stronger cloth to be woven from the silk. Wild silks also tend to be more difficult to dye than silk from the cultivated silkworm.
Silks can be produced by several other insects, but only the silk of moth caterpillars has been used for textile manufacture. There has been some research into other silks, which differ at the molecular level. Silks are mainly produced by the larvae of insects that complete metamorphosis, and also by some adult insects such as webspinners.
Woven on handlooms, silk textiles often use complicated patterns in various colors and styles. A single thread filament is too thin to use on its own, so a manufacturer often combine many threads to produce a single thicker, usable fiber. This is done by hand-reeling the threads onto a wooden spindle to produce a uniform strand of raw silk, and it takes nearly 40 hours to produce a half kilogram of silk.
Local silk processing operations use a reeling machine to produce a uniform strand of raw silk. Meanwhile, there are three grades of hand-reeled silk threads: two fine grades are ideal for lightweight fabrics and a thick grade for heavier material. The silk fabric is then soaked in hot water and bleached before dyeing in order to remove the natural yellow coloring of silk yarn.
In some cases skeins of silk thread are immersed in large tubs of hydrogen peroxide. Once washed and dried, the silk is woven using a traditional hand operated or machine loom.
Elegance, soft luster and beautiful drape of silk makes it ideal for a number of furnishing applications, from upholstery, wall coverings, curtains, rugs to beddings and scarves. Before the emergence of synthetic fibres, silk was used in technical textiles, such as parachutes and artillery gunpowder bags. Until roughly the World War I, early bulletproof vests were made from silk. Further, a special manufacturing process once made it suitable as non-absorbable surgical sutures. Silk cloth was also used as a material on which to write.
Today, silk is mainly found in apparel and home textiles. Good absorbency of silk makes it comfortable to wear in warm weather and while active. Its low conductivity keeps warm air close to the skin during cold weather. It is often used for clothing such as shirts, blouses, high fashion clothing, lounge wear and intimate wear.
Chemical processing of silk
Silk is one of the strongest natural fibers but loses up to 20% of its strength when wet. It has a good moisture regain of 11%. Its elasticity is moderate to poor, if elongated even a small amount it remains stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty.
Silk is a poor conductor of electricity and thus susceptible to static cling. Unwashed silk may shrink up to 8% due to a relaxation of the fiber macrostructure. So silk should either be pre-washed prior to garment construction, or dry cleaned. Dry cleaning can still shrink the material up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. There is almost no gradual shrinkage or shrinkage due to molecular-level deformation. Chemical processing of silk is carried out in stages of degumming, bleaching, dyeing and finishing.
Degumming
Under chemical examinations, silk fibre consists of an inner layer — silk fibroin — which is the core of the fibre. This part is insoluble even when boiled in hot water. The outer layer of silk filament is a form of gum, or silk glue, called sericin. These two elements are present in the fibre in the proportion of about 75 parts fibroin to 25 parts Sericin. Outside Sericin is also a thin layer of Albumen, and on the extreme outer surface is a coat of gelatine.
The process of eliminating "gum" from raw silk is known as degumming of silk. It is synonymous to the scouring process used for purification of cotton and wool.
Composition of raw silk |
Besides sericin which is the main impurity in silk fabrics, the silk fabric may also contain sizing agents like starch, CMC, gum tallow etc. These impurities do not respond to the usual degumming treatment. In case of such fabrics, a desizing treatment must precede the degumming treatment. Desizing of starch containing fabric can be carried out by steeping the fabric in 0.5% H2SO4 for 6-8 hours at room temperature and thoroughly washing and then subsequently degumming.
The different ways of degumming silk are as follows:
• Soap as a degumming agent
Soap is a good degumming agent and grey fabrics can be completely degummed by treating with soap solution at close to boiling point for one to two hours. Neutral synthetic agents have no degumming properties.
After degumming the silk is thoroughly washed with water, with weak solutions of ammonium chloride or soda ash at 40 –50°C for 20 mins.
• Enzymatic degumming
Proteolytic enzymes like Trypsin and Papain may be used for degumming. All the sericin is not removed by this treatment, hence a subsequent treatment with soap solution is necessary. The enzymes preferably hydrolyse peptide bonds formed by carboxyl groups of Lysine and Arginine of silk to form low-molecular-weight and water-soluble products, which can be easily washed out.
• Extraction with water
In order to remove sericin from raw silk, the yarns must be autoclaved for long periods with hot water at temperature over 100°C.The degradation of silk is minimum. There might be some modification of the protein molecule.
• Treatment with alkalis
Alkalis have severe destructive effect on proteins. Severity of the treatment required to remove sericin from a given sample depends on the type of silk. Degumming with soaps in the presence of alkalis is in practice since a long time. Here soap acts as the degumming agent and the alkalis aid the process.
Bleaching
Natural coloring matters present in silk are associated mainly with sericin, and hence are eliminated during degumming. The natural colouring matter of silk can be roughly divided into yellow, green and brown pigments. However, the residual pigments are adsorbed by fibroin; therefore, silk fabrics made from yellow raw silk after degumming are not white but have a cream colour. The bleaching process may be based on reducing agents or oxidizing agents. Material bleached with reducing agents tends to reoxidise and the original color may be restored. Oxidizing bleaching is thus most preferred. Predominant reducing agents used are sulphur dioxide, sodium hydrosulphite and sodium or zinc sulphoxylate formaldehyde. While the oxidizing agents used are potassium permanganate, sodium perborate, sodium peroxide or hydrogen peroxide. The hydrogen peroxide is the most preferred bleaching agent.
Chlorine-based bleaching agents such as bleaching powder, sodium hypochlorite and sodium chlorite are generally not used since these agents tend to chlorinate the fibroin.
In order to enhance the whiteness of a fabric treatment is carried out with an optical brightening agent. The optical brightening agent may be applied along with bleaching agent during processing of silk.
Dyeing
Since silk is a natural polyamide fibre, it can be dyed with various classes of dyes as acid, basic, direct, reactive, metalcomplex and soluble vat dyes.
The pH of the dyebath and the temperature of dyeing should be adjusted in such a way that slow and even adsorption of the dye takes place from the start.
Printing
Silk is mainly printed by handblock printing and screen printing methods. The handblock method is a slow process. Various printing can be used such as the direct, discharge and resist style.
Hardly any other fibre can be printed with so many different classes of dyes as silk. Acid, metal-complex, direct, reactive, basic, vat and indigosols can be used with silk, often by discharge printing.
Finishing
Last but not the least, silk being a noble fibre should be taken care during its finishing to avoid adverse impact on its classic feel, scroopy handle and shimmering appearance. During finishing, silk is treated for an array of properties, e.g. crease resistance, antistatic effect, spot resistance( water and oil drops), flame retardancy, dimensional stability (with hand washing), and more.
Ms Vijaya Shanbhag is a textile and chemical technologist and consultant based in Mumbai, India
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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