Pretreatment of Silk - degumming process
Degumming of silk in water
Cultivated varieties of silk can be degummed by extraction with water at 120~ for about 2 h and repeating the process three to four times. In this case degradation of silk is minimum, but use of pressure equipment is essential. However, some modification of the protein molecule does take place.
Degumming of silk with alkali and acid
Silk degumming in aqueous solutions of acids and alkalies is greatly influenced by pH and temperature. An alkaline reaction at a pH > 9 and acid reaction at pH < 2.5 ensure a rapid elimination of sericin completely after 30 min of treatment. The temperature should not exceed 95~ to avoid weakening of the fibre. The
degree of hydrolysis of fibrion also depends on pH of the bath.
Degumming of silk with soap
Degumming is effected by careful boiling-off in soap baths which should be slightly alkaline. The fibre is treated with soap solution to give a pH around 10.0 at 90-95~ for 1.5-2 h. In order to avoid lien soap deposits and resulting stains, the water should be properly softened. Sequestering agent may be added in the bath to correct the hardness of water. In case of white and delicate varieties a two bath method may be employed. The second degumming bath will consist of 50% quantity of soap taken for the first degumming bath and the duration of treatment may be divided equally between 40-45 min for each bath.
Neutral soap like Merseille soap or olive oil soap have no degumming property as the free alkali present in them are very negligible. However, Merseille soap (8 g/l) can be used in presence of non-ionic tenside (fatty alcohol polyglycol ether) (1-3 g/l), polyphosphate (1-3 g/l) (for softening the water) and soda (1 g/l) at 95~ for 1-2 h with a material to liquor ratio of 1:30. Here also a two bath method can be resorted to. Non-ionic tenside accelerates the degumming process and facilitates rinsing out the soap. Tussah silk is usually prewetted with boiling water before degumming and then degummed in strong alkaline bath .
Degumming of silk with synthetic detergents
Degumming with synthetic detergent is increasingly replacing soap. The main draw backs are soap is expensive, great quantity of soap is required and longer treatment time. A large number of synthetic detergents are available in the market and their selection is very important particularly on the requirement of soft feel as that achieved with soap. Non-ionic product like nonyl phenol ethoxylated compound
in the pH range of 11.2-11.5 for a treatment time of 40 min at 95~ is found to remove the gum and maintain the strength of the yam.
Enzymatic degnmming of silk
Enzyme which can hydrolyse the sericin is classified as proteolytic enzymes . The proteolytic enzymes cleave the peptide/amide linkages and convert them into amino acid. Mainly there are three types ofproteolytic enzymes such as zinc protease (e.g. carboxy peptidase A), serine protease (Chymotrypsin, Trypsin, Thrombin) and thiol protease (acts as cystine residue in the protein). The function of proteolytic enzymes in their degree of degumming depends on the pH of the bath and the optimum activity is found to be different at different pH for different enzymes.
Usually enzymatic desizing of silk is a two stage process. In pre-degumming stage, the cloth is treated with a solution containing soda-ash (1 g/l) at 95~ for 20 min at a liquor ratio of 30. In the second stage, further degumming is carried out by treatment in a solution containing protein enzyme (0.06-0.1 g/l), non-ionic detergent (1 g/l) for 30 min at 55 to 60~ It is usually impossible to achieve full degumming of silk by enzymes. A short-time treatment in a third bath containing soda-ash or soap may be given for the removal of the remaining sericin.
6 Foam degumming
The degumming of silk can also be carried out by foam method. The silk skiens are subjected to the action of foam from a boiling soap bath. Skiens are actually hung above the soap solution and the foam action has a tendency to dissolve and eliminate sericin. This method, however, is not popularly adopted.
7 Washing of degummed silk
After boiling-off, the silk is thoroughly washed with lot of water at 50-60~ containing lml/1 ammonia for 15-20 min. This is then followed by 1 to 2 cold rinsing baths. Half-boiled silk should not be subjected to hot soap baths and alkaline liquors over 30~ because this would act on the silk gum still present. Following washing and drying silk is given further mechanical treatment in the form of stretching, beating or glossing. Scroop is imparted to the silk fabric by treatment with 2-5 g/1 of 30% acetic acid at room temperature for 15-30 min. When silk is delivered in dyed condition, scrooping is done after dyeing. Both degumming and partial degumming are only suitable for reeled silk (greige). These finishing processes are insignificant where schappe and bourette silks are concerned, since these are freed from their gum in the early stages of preparation before spinning .