Introduction
Silk is known as the Queen of all textile fibresbecause of its sheen and luster. It is one of the most beautiful and preciousfibres given to us by nature and has been very much overshadowed over the pastfew decades by the other natural fibres and more particularly by synthetics. Recentlyhowever its importance to textile industry has again increased. This is partlydue to the current preference for natural products and the resultant increasein demand for natural fibres, especially as silk has optimum properties interms of comfort and wearability and eco-friendly nature. The shimmeringappearance for which silk is prized comes from the fibers' triangularprism-like structure which allows silk cloth to refract incoming light atdifferent angles.
Certainly silk will never become a mass produced fibre, butit will continue to occupy its special position as a fibre for exceptionallyhigh quality garments. The silk is gaining increasing importance day by day becauseof its exclusive qualities which are rarely found in any other fibres. Silk isa natural protein fiber, some forms of which can be woven into textiles. Silkis the only natural filament that man does not have to spin before it can beused for textile fabrics. The cultivation of silk is known as Sericulture. Thenatural silk spun by silk worms in the form of cocoons is utilized only to 50%of its production because of lack in right way of processing and deficiency insericulture.
Silk is broadly divided as: a) Domestic silk or Mulberrysilk b) Wild silk.
The wild silk has 3 varieties a) Eri b) Muga c) Tussar silk.Eri is the staple fibre and others are filament. All these species rear in theforests and known as VANYA SILKS. The wild silks are the unique products of ourcountry.
The best known type of silk is obtained from cocoons made bythe larvae of the mulberry silkworm Bombyx mori reared in captivity(sericulture). "Wild silks" are produced by caterpillars other thanthe mulberry silkworm and can be artificially cultivated. Over 30 countriesproduce silk, the major ones are China (54%) and India (14%).
A variety of wild silks have been known and used in China,South Asia, and Europe since early times, but the scale of production wasalways far smaller than that of cultivated silks. They differ from the domesticatedvarieties in color and texture, and cocoons gathered in the wild usually havebeen damaged by the emerging moth before the cocoons are gathered, so the silkthread that makes up the cocoon has been torn into shorter lengths.
Commercially reared silkworm pupae are killed by dippingthem in boiling water before the adult moths emerge, or by piercing them with aneedle, 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 alsotend to be more difficult to dye than silk from the cultivated silkworm.
Silks are produced by several other insects, but only thesilk of moth caterpillars has been used for textile manufacture. There has beensome research into other silks, which differ at the molecular level. Silks aremainly produced by the larvae of insects that complete metamorphosis, and alsoby some adult insects such as webspinners.
Silk, known as Pattu in southern parts of India and Resham in Hindi/Urdu (from Persian), has a long history in India and is widely producedtoday. India is also the largest consumer of silk in the world. The traditionof wearing silk sarees in marriages by the brides is followed in southern partsof India. Silk is worn by people as a symbol of royalty while attendingfunctions and during festivals. Historically silk was used by the upperclasses, while cotton was used by the poorer classes. Today silk is mainly usedin Bhoodhan Pochampally (also known as Silk City), Kanchipuram, Dharmavaram, Mysore, etc. in South India and Banaras in the North for manufacturing garments andsarees.
"Murshidabad silk", famous from historical times,is mainly produced in Malda and Murshidabad district of West Bengal and wovenwith hand looms in Birbhum and Murshidabad district. Another place famous forproduction of silk is Bhagalpur. The silk from Kanchi is particularly well knownfor its classic designs and enduring quality. The silk is traditionallyhand-woven and hand-dyed and usually also has silver and gold threads woveninto the cloth. Most of this silk is used to make saris. The saris usually arevery expensive and vibrant in color. Garments made from silk form an integralpart of Indian weddings and other celebrations. In the northeastern state of Assam, three different types of silk are produced, collectively called Assam silk.
Chemical processing of silk
Silk is one of the strongest natural fibers but loses up to20% of its strength when wet. It has a good moisture regain of 11%. Itselasticity 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 byinsects, especially; if left dirty. Silk is a poor conductor of electricity andthus susceptible to static cling. Unwashed silk may shrink up to 8% due to arelaxation of the fiber macrostructure. So silk should either be pre-washedprior to garment construction, or dry cleaned. Dry cleaning may still shrinkthe material up to 4%. Occasionally, this shrinkage can be reversed by a gentlesteaming with a press cloth. There is almost no gradual shrinkage or shrinkagedue to molecular-level deformation. Chemical processing of silk is carried outin stages of Degumming, Bleaching, Dyeing and Finishing.
Degumming Of Silk
Composition of raw silk
Fibroin 70-80%
Sericin 20-30%
Waxy matter 0.4-0.8%
Moisture 10-11%
Carbohydrates and Starches 1.2-1.6%
Inorganic matter 0.7%
Pigment 0.2%
Under chemical examinations, silk fibre consists of two elementsi.e. inner layer called silk fibroin which forms the core of the fibre. It isinsoluble even when boiled in hot water. The outer layer of silk filament is aform of gum, or silk glue, called sericin. These two elements are present inthe fibre in the proportion of about 75 parts fibroin to 25 parts Sericin. OutsideSericin is also a thin layer of Albumen, and on the extreme outer surface is acoat of gelatine. The process of eliminating gum from raw silk is known asDegumming of silk. It is synonymous to the scouring process used forpurification of cotton and wool.
Besides sericin which is the main impurity in silk fabrics, thesilk fabric may also contain sizing agents like starch, CMC, gum tallow etc. Theseimpurities do not respond to the usual degumming treatment. In case of suchfabrics, a desizing treatment must precede the degumming treatment. Desizingof starch containing fabric can be carried out by steeping the fabric in 0.5%H2SO4 for 6-8 hours at room temperature and thoroughlywashing and then subsequently degumming.
The different ways of degumming silk are as follows:
a) Soap as a degumming agent
Soap is a good degumming agent and grey fabrics can becompletely degummed by treating with soap solution at close to boiling pointfor 1-2 hours. Neutral synthetic agents have no degumming properties. Afterdegumming the silk is thoroughly washed with water, with weak solutions ofammonium chloride or soda ash at 40 -500C for 20mins.
b) Enzymatic degumming
Proteolytic enzymes like Trypsin and Papain may be used fordegumming. All the sericin is not removed by this treatment hence a subsequenttreatment with soap solution is necessary. The enzymes preferably hydrolysepeptide bonds formed by carboxyl groups of Lysine and Arginine of silk to formlow molecular weight water soluble products which can be easily washed out.
c) Extraction with water
In order to remove sericin from raw silk the yarns must beautoclaved for long periods with water at temperature over 1000C.The degradation of silk is minimum. There might be some modification of theprotein molecule.
d) Treatment with alkalis
Alkalis have severe destructive effect on proteins. Severityof the treatment required to remove sericin from a given sample depends on thetype of silk. Degumming with soaps in the presence of alkalis is in practicesince a long time. Here soap acts as the degumming agent and the alkalis aidthe process.
Bleaching Of Silk
Natural coloring matters present in silk are associatedmainly with sericin and hence are eliminated during degumming. The naturalcolouring matter of silk can be roughly divided into yellow, green and brownpigments. However the residual pigments are adsorbed by fibroin and hence silkfabrics made from yellow raw silk after degumming are not white but have acream colour. The bleaching process may be based on reducing agents oroxidizing agents. But material bleached with reducing agents tends to reoxidiseand the original color may be restored. Hence oxidizing bleaching is mostpreferred. Predominant reducing agents used are Sulphur dioxide, sodiumhydrosulphite and sodium or zinc sulphoxylate formaldehyde. While the oxidizingagents used are potassium permanganate, sodium perborate, sodium peroxide orHydrogen peroxide. The hydrogen peroxide is the most preferred bleaching agent.The Chlorine based bleaching agents are bleaching powder, sodium hypochloriteand sodium chlorite which are generally not used since these agents tend tochlorinate the fibroin.
Optical Whitening
In order to enhance the whiteness of a fabric treatment iscarried out with an optical brightening agent. The optical brightening agentmay be applied along with bleaching agent during processing of silk.
Dyeing Of Silk
Since silk is a natural polyamide fibre it can be dyed withvarious classes of dyes as Acid, Basic, Direct, Reactive, Metalcomplex andSolubilised vat dyes. The pH of the dyebath and the temperature of dyeingshould be adjusted in such a way that slow and even adsorption of the dye takesplace from the start.
Printing of Silk
Silk is mainly printed by handblock printing and screenprinting methods. The handblock method is a slow process. The different stylesof printing are Direct, Discharge and Resist style. Hardly any other fibre canbe printed with so many different classes of dyes as silk.
The following classes of dyes can be used to print silk: Acid,Metal-complex, Direct, Reactive, Basic, Vat and Indigosols. This wide varietyis due to the fact that a high proportion of silk in the market is alwaysclaimed by Discharge printing.
Finishing Of Silk
Silk being a noble fibre care must be taken during itsfinishing so as not to affect its classic feel, scroopy handle and shimmeringappearance. During finishing silk is treated for following properties:-
- Crease resistance
- Antistatic effect
- Spot resistance( water and oil drops)
- Flame retardancy
- Dimensional stability ( with hand washing)
- Wash and wear properties.
Conclusion
Thus it is concluded that silk a lustrous, precious naturalfibre is popular for its highest position among all fibres as Queen offibres and requires careful processing so as not to affect its feel andappearance. It is worn as a symbol of royalty. Silk processed well with greatcare will fetch a great deal of exports earnings.
Theauthor is a practicing Textile Consultant and Colour Matching expert.
