Alkalis hydrolyse
protein by attacking the peptide bonds and are said to have severe damaging
effect on proteins. Hence, this process has to be carried out under controlled
condition, so as not to result in over degumming. For this process, pH should
be maintained between 9.5-10.5. Below pH 9.5, rate of degumming is too slow
causing prolonged exposure and hence mechanical damage. Above pH 10.5 there is
a danger of fibroin being attacked.
Alkalis used for
degumming are caustic (NaOH), caustic soda (Na2CO3),
sodium bicarbonate (NaHCO3), K2CO3, Na2SiO3,
trisodium phosphate, sodium phosphate, borax and ammonia. Among these caustic
soda is the most preferred alkali. Alkali is rarely used alone, since it leaves
the silk rather harsh in handle and it is recommended to use buffer system.
Hence caustic soda and sodium bicarbonate is the widely used buffer.
The optimum
concentrations are: Na 2CO3
– 1.06%
NaHCO3
– 0.84%
Non-ionic surfactant –
0.3%
Degumming can be
carried out at 100°C for 2 hours with MLR 1:40.
 |
| Silk |
Enzymes are
proteins, catalysing a specific chemical reaction, which are known as
‘bio-catalysts’. They work at atmospheric pressure and in mild conditions (e.g.
at 40°C, pH 8.0). Trypsin, papain and bacterial enzymes are the main types of
enzymes for silk degumming. These enzymes are called ‘proteases’ because they
degrade and their degradation products are polypeptides, peptides and other
substances by hydrolysis of the –CO-NH- linkage.
Proteolytic enzyme action on protein
Enzymatic degumming has the following advantage over the conventional degumming with alkaline soap.
•
It has a specific reaction thereby it may give a minimum damage to fibroin.
•
It has a lesser risk of over degumming than alkaline soap degumming,
• Weight
loss can be easily modified by adjusting the concentration of enzyme, the
reaction time
and the use of optimum pH and temperature.
•
With the enzyme method, silk is treated at low temperature (e.g. at 40°C) not
only
reducing energy costs but also preventing fibre weakness.
•
Enzyme treatment is an environmentally friendly process because enzymes are
readily
biodegrade in nature.
•
There is no soap required in enzyme degumming process.
Therefore, uneven dyeing
problem caused by metallic soap can be avoided.
Enzymatic degumming also has some economic disadvantages as:
• It
needs some pre-treatment processes, since the gum must be swollen before the
enzyme bath.
•
It is very slow reaction compared to alkaline soap degumming. Degumming of the
silk is
carried out in form of hanks as well as fabric.
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