Degumming
is at the heart of wet processing of raw silk. The main purpose of the
degumming process are to remove the Sericin from the fibre, to remove some
impurities (e.g. waxes, fats, mineral salts) affecting both the dyeing and
printing processes, to make the fibre highly absorbent for dyes and chemicals
and to reveal the lustre of fibroin and to improve the appearance of the fibre.
The fact that the raw silk contains two components fibroin and Sericin, which
covers the filaments. Sericin contains some impurities, for example, waxes,
fats, mineral salts and pigments. Sericin has the same amino acid residues, as
fibroin but the proportions contained in both components are quite different.
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| Silk-Cocoon |
As a result of this, the degumming process must be carefully carried out on
silk in the appropriate conditions otherwise the fibroin may be damaged. The pH
range from 4 to 8 is normally safe for fibroin and it has been found that
alkaline conditions are less harmful to fibroin than acid conditions. In
contrast to fibroin, the solubility of Sericin is very high at pH values
between 1.5 and 2 and between 9.5 and 10.5. The Sericin is removed from the
fibre but the fibroin must not be damaged
Table
- Composition of raw silk
Fibroin
|
70-80%
|
Sericin
|
20-30%
|
Carbohydrates
|
0.7%
|
Wax materials
|
0.4-0.8%
|
Inorganic
matter
|
0.6%
|
Natural
pigments
|
0.2%
|
Water
at room temperature does not dissolve silk but silk is highly susceptible to
dissolution in boiling water. For complete removal of Sericin, in case of
cultivated varieties of silk, it is necessary to extract the silk yarn with
water at 120°C for 4 hours.
For this reason, this process gives a risk of fibroin being damaged when the
time of treatment is prolonged. This process needs large autoclaves to treat
the fibre in silk industry. A further disadvantage is that this process gives
incomplete degumming and sometimes soap or synthetic detergent must be added to
improve the degumming effect. Therefore this process is very difficult to
control and now it is not used in silk industry in order to remove Sericin from
silk.
Different
soaps like olive oil, palm oil can be used for degumming. Marseilles soap, an
olive oil soap, is an outstanding soap for degumming because of its high degree
of hydrolysis which gives better lustre. For example, this process may be
carried out using 10 – 20 g/l soap at 92 – 98 °C
for 2- 4 hours adjusted pH to 10.2 – 10.5 in order to react effectively upon
the sericin.
The
degumming action of the soap is due to alkali formed, which forms a chemical
bond with Sericin and produce soda salt, on the hydrolysis of the soap. The
Sericin, in the form of soda salt, is separated by soap and dissolved in water
due to the emulsification action of soap. The quantity and type of soap
required degumming generally depends upon the nature and type of silk.
Disadvantage
of soap degumming are
The
process requires soft water. The metallic ions such as Ca and Mg combine with
soap and produce insoluble metallic soap, which deposits on fibre and reduces
the lustre of fabric. Combination of soap and alkali accelerate the process.
As
a result of the high temperature, this process tends to attack both sericin and
fibroin because of the sensitive nature of fibroin itself and chemical
similarity of fibroin and sericin.
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