Bleaching Of Silk
Natural coloring matters present in silk are associated mainly with sericin and hence are eliminated during degumming. The natural colouring matter of silk can be roughly divided into yellow, green and brown pigments. However the residual pigments are adsorbed by fibroin and hence silk fabrics made from yellow raw silk after degumming are not white but have a cream colour.
The bleaching process may be based on reducing agents or oxidizing agents.
Predominant reducing agents used are Sulphur dioxide, sodium hydrosulphite and sodium or zinc sulphoxylate formaldehyde.
The material bleached with reducing agents tends to reoxidise and the original color may be restored. Hence oxidizing bleaching is most preferred.
The oxidative agents used are
- sodium perborate,
- sodium peroxide
- Hydrogen peroxide.
2 to 4 hours at 70°C
or 1 to 2 hours at 80 to 85°C,
With 10 to 15 ml/l hydrogen peroxide (35% solution),
and 1 g/l EDTA (30% solution)
and 3 g/l sodium pyrophosphate to pH 9,
followed by thorough rinsing.
The Chlorine based bleaching agents such as bleaching powder, sodium hypochlorite and sodium chlorite ,are generally not used since these agents tend to chlorinate the fibroin.