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Saturday, March 10, 2018

Silk Fiber: Properties, Production and Applications

Silk or Silk fiber:
Silk has been a highly valued fiber for centuries due to its soft handle and lustrous nature. Silk is a natural textile fiber. It was a key material in East–West trade for millennia and the secrets of silk production was guarded closely by various Chinese dynasties before spreading to India in around AD 300. The secret and means of silk production would not reach Europe until the middle ages.

The production of silk involves the breeding and rearing of a large number of silkworms, specifically the genus Bombyx mori. This is a flightless moth that during the transition from larvae to moth produces a long continuous filament to form the cocoon. This continuous filament can then be uncoiled and formed in to a yarn of silk proteins. The as-spun silk is actually a natural bicomponent fiber consisting of two strands of fibroin embedded in a matrix of sericin. The majority of this sericin is dissolved in a bath of mildly acidic water to remove enough to separate the two strands of fibroin with just enough sericin coating to help bind the filaments together in the yarn. The individual silk filaments have a diameter around 10 μm, giving silk a very soft handle and luster. The silk filaments have a triangular shape, imparting unique light reflectance properties, which give the filament yarns inherent elasticity that allows silk fabrics to recover from stretch. The fineness and mechanical strength of silk impart excellent drape and handle characteristics, far superior than other natural materials.

The best silk is formed into continuous filament yarns combining silk strands of very long length to form a yarn known as thrown silk. These yarn types have a very smooth appearance and command the highest price. Silk is also available as staple yarn in the form of spun silk. These yarns are formed from silk that is not consistent enough for filament winding due to a flawed cocoon or from waste from silk throwing. The flawed cocoons are degummed in the same way, disentangled and then cut into staple lengths of various qualities. These staple silk fibers can be ring spun into yarns on conventional cotton and worsted processing lines. Spun silk yarns are hairier and thus less lustrous than continuous filament counterparts; the strength and yarn elasticity are also reduced and thus spun silk is available at a discount.

The chief silk producing countries are China, Korea, Japan, Thailand, India, Uzbekistan, Brazil, Iran, Italy and France. About 1 million workers are employed in the silk sector in China. Silk Industry provides employment to 7.9 million people in India, and 20,000 weaving families in Thailand. China is the world’s single biggest producer and chief supplier of silk to the world markets. India is the world’s second largest producer. Table gives the production of silk in metric tons in different countries.

Global silk production
Table: Global silk production (in metric tons)
(Image courtesy: http://inserco.org/en/statistics)
Production of silk fiber:
The eggs of silk worm are placed in well-lighted and ventilated chambers (one ounce of silk worm eggs yields as many as 36,000 silk worms and about 5 kg of raw silk). After ten days of incubation, the silk worms come out from the eggs. These are fed on chopped mulberry leaves. In just about 35 days after it is born, it grows to 10,000 times its weight at birth. When the caterpillar is about 8 weeks old, it secretes a viscous fluid from the glands in its head. This substance called fibroin is forced through 2 min channels into a side exit near its mouth. At the same time two other glands secrete a gummy liquid called sericin which passes through the same exit. The fibroin, as it emerges from the head of the silk worm, coagulates forming a twin filament coated with sericin. With this twin fine filament the caterpillar produces a protective casing known as cocoon around itself before entering the chrysalis stage of its life (see Fig-1).

Silk process flow
Fig-1: Silk process flow
In the beginning there are only few fluffy threads by means of which the worm attaches itself to the specially prepared branches of the spinning huts called chandrikas. From here onwards begins the period of transformation. It begins to change into a chrysalis and then into a moth inside the cocoon. The cocoons are stifled by steam or hot air in order to kill the chrysalis within it. The cocoon is ready for reeling.

In the production of continuous silk filament or nett silk, cocoons are collected and those that are unsuitable for reeling or those intended for supplying the next crop of eggs are removed.

Cocoons are placed in soapy water and threads reeled together to form hanks of filament. They are then degummed and twisted to get silk yarn.

The cocoons sorted for the filature (filatures are the establishments where the silk reeling is done) are treated with hot water in basins and mechanically brushed and then the outer layers of the cocoons are removed. The worker finds the end of the cocoon threads. Nearly 3–8 of these cocoons are reeled together on swift, and the material is wound in the form of a hank.

A fresh end is added to the composite thread immediately when one breaks or runs out from the cocoon.

During its passage to the swift, the filament is dried either by natural or by artificial means. The reeled silk is then taken for twisting. This is called Thrown silk. The twisted silk yarns are wound on spools, or skeins and yarns are now ready for use in weaving. For process sequence, see Fig.-2.

Process sequence of silk twisting
Fig-2: Process sequence of silk twisting
Although a cocoon can contain about 2700–4500 m of continuous filament, only approximately 900 m are suitable for filament silk. The remaining is cut into staple and is used in spun silk manufacturing.

Waste filaments from all stages of production of silk are converted into spun silk yarn (Fig.-3). This type of material lacks the lustre, fineness and general character of thrown filament silk.

Production of spun silk
Fig-3: Production of spun silk
The cocoons are handled in bales of 45 kg. The quality of the silk depends on colour, amount of spinnable fiber on the cocoon and uniformity of cocoons.

Physical properties of silk:
Silk fibers have a triangular cross section with rounded corners. This reflects light at many different angles, giving silk a natural shine. It has a smooth, soft texture that is not slippery, unlike many synthetic fibers. Its tenacity is 4.5 g/d when dry and 2.8–4.0 g/d when wet.

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 having a specific strength equal to many engineering materials. It is, in addition, an elastic and resilient fiber similar to wool but not quite as good as wool. This combination of strength and elasticity of silk was unique in the textile fibers until certain manmade fibers were developed.

Unwashed silk chiffon 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 may still shrink the chiffon 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.

Silk is a poor conductor of electricity and thus susceptible to static cling.

Chemical properties of silk:
Silk is made up of the amino acids GLY-SER-GLYALA-GLY and forms Beta pleated sheets. Inter chain H-bonds are formed while side chains are above and below the plane of the H-bond network. The high proportion (50%) of glycine, which is small, allows tight packing and the fibers are strong and resistant to stretching. The tensile strength is due to covalent peptide bonds. Since the protein forms a Beta sheet, when stretched the force is applied to these strong bonds and they do not break.

Silk is resistant to most mineral acids but will dissolve in sulfuric acid. It is yellowed by perspiration.

Uses/Application of silk:
This combination of properties, together with fineness, high degree of lustre, softness and superb drape enables silk to be converted into many beautiful types of fabrics, from delicate chiffons to heavy brocades. The fineness, regularity, strength and elasticity of silk make it suitable for fine screens for printing and parachute fabrics.

Silk’s good absorbency 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, formal dresses, high fashion clothes, negligees, pyjamas, robes, skirt suits, sun dresses and underwear.

Silk’s elegant, soft luster and beautiful drape makes it perfect for many furnishing applications. It is used for upholstery, wall coverings, window treatments (if blended with another fiber), rugs, bedding and wall hangings.

It may be interesting to note that a 5 m saree weighing about 400 g will require 1.5 million mulberry leaves weighing 150 kg for the growth of silk worms. This is the quantity of mulberry leaves which 6000 silk worms consume before they spin full cocoons to yield enough raw silk for a 5 m silk saree. In other words, it involves stifling of 6000 silk worms nestling in their cocoons. In fact, however, twice the number of silk worms is required to be reared up because of 50% mortality rate in breeding.


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