Page 140 - Basic Principles of Textile Coloration

REFERENCES 129

extrusion of protein solutions and their coagulation, has not been commercially
successful, mainly because of the poor durability of the filaments produced.

Silk filaments are obtained by unwinding the cocoons of the silk worm. The raw
fibre consists of twin filaments of a protein called fibroin with a coating of a
protein gum called sericin. The silk filaments are degummed using soap solution at
or near the boil. Different degrees of degumming are used for different qualities of
silk. Silk filaments are often weighted by treatment with tin salts, phosphates and
silicates and may therefore contain appreciable amounts of insoluble inorganic
material. In all processing, the silk filaments must be protected from breakages by
using smooth machinery and placing them in permeable bags.

The amino acid composition of fibroin is quite different from that of keratin.
Fibroin has only four major amino acid components – glycine, alanine, serine and
tyrosine – containing long sequences of alternating glycine and alanine residues. It
has fewer amine and acid side-chains and contains no cystine. Silk only absorbs
about 150 mmol kg–1 of acid compared to about 820 mmol kg–1 for wool.
Compared to wool, silk is somewhat less sensitive to alkalis but more sensitive to
acids. Treatment with dilute mineral acid solutions causes shrinkage and is used to
produce crepe effects. Organic acids harden the fibre surface and increase the
scroop, the crunching noise made when the fabric is crumpled. The protein chains
in fibroin are fully extended in the b-configuration and silk therefore has lower
elasticity than wool. It is, however, sufficiently resilient to give fabrics that retain
their shape and resist wrinkling. It is still considered the luxury fibre.

REFERENCES

1. J A Rippon, ‘The Structure of Wool’, in Wool Dyeing, D M Lewis, Ed (Bradford: SDC, 1992).
2. D M Lewis, Ed, Wool Dyeing (Bradford: SDC, 1992) 4–5.
3. G F Wood, ‘Raw Wool Scouring, Wool Grease Recovery and Scouring Wastewater Disposal’, in

Handbook of Fiber Science and Technology, Vol. I, Part A, M Lewin and S B Sello, Eds (New York:
Marcel Dekker, 1984).
4. D M Lewis, ‘Ancillary Processes in Wool Dyeing’, in Wool Dyeing, D M Lewis, Ed (Bradford: SDC,
1992).
5. Standard Test Method for Alkali-Solubility of Wools, Test Method D 1283–85, Annual Book of ASTM
Standards, Section 7, Textiles, ASTM, Conshohocken, PA, USA, 1996.
6. J G Cook, Handbook of Textile Fibres. I. Natural Fibres (Shildon, UK: Merrow Publishing, 1984).