Silk
Silk
Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoon of the larvae of the mulberry silkworm Bombyx mori reared in captivity (sericulture). The shimmering appearance of silk is due to the triangular prism-like structure of the silk fiber, which allows silk cloth to refract incoming light at different angles, thus producing different colors.
Properties of the silk
*It is versatile and very comfortable.
*It absorbs moisture.
*It is cool to wear in the summer yet warm to wear in winter.
*It can be easily dyed.
*It retains its shape and is relatively smooth.
*It has a poor resistance to sunlight exposure.
*It is the strongest natural fiber and is lustrous.
Production process of silk
*Sericulture: Cultivation of the silkworm is known as sericulture. The breeding nurturing and care of silk warms.
*Sorting of cocoons: The cocoons are sorted according to color, size, shape and texture.
*Softening the sericin: After the cocoons have been sorted they are put through a series of hot and cold immersions as the sericin must be softened to permit the unwinding of the filament as one continuous thread.
*Reeling: Process of unwinding cocoons.
*Throwing: A process of winding and doubling silk.
Types of silk
Four different types of silk thread may be produced from this procedure:
1. Crepe
Crepe is made by twisting individual threads of raw silk, doubling two or more of these together, and then twisting them again.
2.Tram
Tram is made by twisting two or more threads in only one direction.
3.Thrown singles
Thrown singles are individual threads that are twisted in only one direction.
4. Organzine
Organzine is a thread made by giving the raw silk a preliminary twist in one direction and then twisting two of these threads together in the opposite direction.
Chemical Composition of Silk
Silk gum or sericin → 22% to 25%
Silk or fibroin → 62% to 67%
Water → 10 % to 11%
Salt → 1 % to 1.5%
The macro structure of silk
Silk is very fine, regular, translucent filament.
Length → 600m long, average 300m
Diameter → 12µm to 30 µm
Length width ratio → 2000 : 1
Color → Bright yellow
Physical Properties of Silk
Specific gravity → 1.34
Moisture regains → 11%
Strength → Tenacity → 4.3 g/d (dry)
Elasticity → Breaking extension → 23.4%
Resiliency → Moderate
Abrasion resistance → Good
Dimensional stability → Good
Degumming
* A major undesirable constituent part of silk is ‘silk gum’ or ‘sericin’ about 25% of total mass. Sericin is removed by degumming. Degumming is the process of removing the sericin or silk gum from silk. Removing the gum improves the sheen, color, hand and texture of the silk. Because the gum can serve as a protective layer, it is typically left on the silk until it is ready to dye. In some cases, the fabric is woven to completion and then degummed, to protect the yarn from abrasion on the loom. The process involved to remove these impurities is called ‘degumming’ or ‘scouring’ of silk.
* Sericin is insoluble in water. It is comparatively easily hydrolyzed, whereby the long protein molecule of sericin, is broken down into smaller fractions, which are easily dispersed or solubillsed in hot water.
Typical Recipe for Silk Degumming
*Wetting agent………………….= 0.5-1.0 g/l
*Sequestering agent…………….= 1.0-2.0 g/l
*Antifoaming agent……………...= 0.5-1.0 g/l
*Natural soap……………………= 15.0-30.0 g/l
*TSP/Soda ash…………………..= 1.0-3.0 g/l
*Detergent……………………….= 1.0-3.0 g/l (not mandatory if use soda)
*Temperature…………………….= 90-95°C
*Time…………………………… .= 90-120 min
*pH………………………………= 9.5-10.5
*M:L………………………………= 1:10
Working Procedure (silk)
*Set the bath with substrate at room temperature and add soap and other necessary auxiliaries.
*If necessary add trisodium phosphate (TSP) or soda ash to maintain desired pH 9-10.
*Raise the temperature to 95° and run for 2 hours for optimum removal of sericin gum.
*Cool down to 70°C and then drop the bath.
*Wash it with warm and cold water successively.
The Silk Polyme
The silk polymer is linear, fibroin polymer. It differs from the wool polymers as follows:
*Silk is composed of 16 different amino acids compared with 20 amino of the wool polymer.
*The silk polymers are not composed of any amino acids containing sulphar. Hence, the polymer system of silk does not contain any disulphide bonds.
*The silk polymer occurs only in the beta – configuration.
Why silk filament is strong?
The silk filament is strong. This strength is due to its linear, beta – configuration polymers and very crystalline polymer system. These two factors permit many more hydrogen bonds to be formed in a much more regular manner.
Uses of Silk fiber
Silk’s 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, ties, blouses, formal dresses, high fashion clothes, lingerie, pajamas, robes, dress suits, sun dresses and kimonos.
THE END
