Page 136 - Basic Principles of Textile Coloration

WOOL PROCESSING 125

therefore swells and becomes softer. Chlorination also generates some water-
soluble peptides from the proteins in the cuticle, which also contribute to
softening the scales. The anti-shrink effect of chlorination may not be permanent
if these proteins are gradually removed from the fibre surface on washing and the
scales harden.

A number of other so-called subtractive or destructive wool shrink-resist
processes are known. These also modify the scale structure. In particular, the
process based on treatment of wool with permonosulphuric acid (H2SO5) merits
mention. It will be of increasing importance as concern for pollution by
organochlorine compounds increases.

The most successful, permanent anti-shrink treatments for wool combine mild
chlorination with cationic polymer deposition on the fibre surface. The chlorine-
Hercosett method for continuous treatment of wool tops (combed wool sliver) is
particularly well known. The process is also applicable to wool garments. Mild pre-
chlorination with hypochlorite and acid prepares the wool fibre surface for the
cationic polymer. An anti-chlor treatment with sodium bisulphite eliminates
residual chlorine and reacts with residual acid to form sulphurous acid. The
Hercosett polymer is a polyamide with cationic azetidinium groups introduced by
reaction of secondary amino groups with epichlorhydrin (3-chloropropene oxide)
(Figure 7.6). The polymer is applied to wool from aqueous solution and is
substantive to the anionic wool surface. During drying, the azetidinium groups are
able to react with thiol groups in the wool and with free secondary amino groups
in other parts of the polyamide chain. This insolubilises and fixes the resin to the
fibre.

Hercosett-treated wool has different dyeing properties than wool and
chlorinated wool. Under neutral or acidic conditions, the deposited polymer
retains a pronounced cationic nature. This arises from residual azetidinium groups
and the protonation of the tertiary amino groups from their hydrolysis (2,3-
dihydroxypropanamine groups). Anionic acid dyes have higher substantivity for
the treated wool and high initial rates of absorption because of their attraction to
the cationic polymer coating. Level dyeing requires considerable care. Under
washing conditions, these quaternary 2,3-dihydroxypropanamine groups lose their
protons, becoming neutral, so that the ionic attraction holding the dye molecules
is lost. There is therefore a substantial decrease in the washing fastness of most
anionic dyes, except reactive dyes that have covalently bonded to the protein.

A variety of other polymer deposition processes is used for producing shrink-
resistant wool. These act by coating the fibre scales and reducing the frictional