Pretreatment Process of Silk

Pretreatment of Silk:Means any treatment, which is done before actual (dyeing and printing) process. To prepare a silk yarn for dyeing and silk fabrics for dyeing and printing, it is necessary to partially or completely remove sericin, as well as natural oils and organic impurities. Depending on the percentage of sericin removed during scouring (sericin is present in raw silk in a ratio between 20 % to 25 %), the end-product is defined as unscoured (used only for shirts and suits), `souple' or degummed.

1. Degumming/Scouring
2. Bleaching

Degumming of Silk:
The process of eliminating “Gum (sericin)” from raw silk is known as degumming of silk. Degumming of silk involves mainly the removal of sericin from the fibroin. 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 solubilized in hot water. Hydrolysis of proteins can be carried out by treatment with acids, alkalis and enzymes. Acids are non-specific and tend to attack vigorously. Alkalis also attack both, sericin and fibroin. However, the variation in the rate of hydrolysis is large enough to control the reaction.

Silk

The degumming with soaps in the presence of mild alkalis like soda ash is practiced. Degumming with alkalis is a function of pH, temperature and duration of treatment. The pH should be kept at the leve1 of 9.5 to 10.5. If the level is below 9.5, then the process of removing sericin will be slow. If the pH is over 10.5, the weight loss will be greatly increased.

The degumming loss in this process is usually 20-25%. In certain cases, entire silk gum is not removed, but only sufficient amount is removed to make the silk soft and lustrous and workable in dyeing and bleaching. This is known as “Soupling” in which only 10% to 15 % of the gum is removed. In addition to removing the soil and additives used while weaving silk, scouring removes any sericin (gum) that remains on the silk. Often a quantity of the natural gum has been allowed to remain on the silk fibre to give it additional body and to make it easier to handle in spinning and weaving.

Although for raw silk fabrics the gum is retained purposely to provide body or produce a different texture, most silk fabrics are degummed as a part of the finishing process. The resultant fabric has a much softer hand and a whiter appearance. Raw silk is sometimes given a very mild scouring for the purpose of softening the fibre. This is called as "Ecru silk” in which only 2 to 5% in weight of silk gum is removed. Ecru silk can be prepared by simply washing the raw silk in hot water without the use of soap. This is used mainly for warp; hence the gum is left purposely. 

Bleaching of Silk: 
The silk being spun by silkworm contains natural colouring matter tinted with yellow, yellow -green and brown pigments. During degumming the removal of sericin from the silk results in dull white to lightly tinted material. Since some of the sericin is closely held by fibroin, complete elimination of the colour by degumming is not possible. During bleaching these natural colouring matters are decolorized /removed to produce pure white material. An efficient bleaching process must ensure pure whiteness and level dyeing properties and non- degradation of the material. The bleaching of silk is based on the use of either reducing agents or oxidizing agents.

Some of the important reducing agents used for bleaching are:

1. Sodium hydrosulphite (Hydrose)
2. Sulphur dioxide
3. Sodium/Zinc sulphoxylate Formaldehyde

The above reducing agents at time tend to de-oxidize original colour may be restored in the bleached material.

The popular oxidizing agents used for bleaching of silk are:

1. Hydrogen peroxide
2. Potassium permanganate
3. Sodium perborate
4. Sodium peroxide

The chlorine-based agents such as bleaching powder are not generally used, as they tend to chlorinate the silk fibroin. Hydrogen peroxide is most commonly used for bleaching.