Page 138 - Basic Principles of Textile Coloration

WOOL PROCESSING 127

7.4.3 Bleaching wool

The natural colour of white wool varies from white to pale yellow. The yellow tint
is usually a sign that the protein has undergone some reaction on exposure to air
and light. To eliminate the yellow tint, or if dyeing bright pastel shades, wool must
be bleached. Although bleaching of wool produces a white fibre, the effect is never
permanent. On further exposure to air and light, the sensitive protein slowly reacts
and again reverts to its more usual yellowish tint.

Wool cannot be bleached with sodium hypochlorite solutions, as for cotton,
since it is extensively damaged to the point at which it even dissolves in the
solution. The traditional method of bleaching moist wool used sulphur dioxide gas.
This simple and cheap process was called stoving but is now obsolete.

Today, the preferred bleaching agent is hydrogen peroxide. Bleaching is usually
carried out in weakly alkaline solution containing a stabiliser and a sequestrant, as
this gives the best quality white fibres. Even after carefully controlled alkaline
bleaching of wool, the goods should not be boiled as the bleached wool is quite
sensitive to yellowing. Dyeing of pale shades should be conducted at temperatures
not exceeding 80 °C. In some cases, the wool material is simply soaked in hydrogen
peroxide solution and dried. If bleaching is used to brighten a dyed fabric, and alkali-
sensitive dyes are present, neutral or acidic peroxide bleaching is possible.

7.4.4 Moth-proofing wool

Wool has rather poor resistance to mildew and bacteria that grow when moist wool
is stored over an extended period. The larvae of a variety of moths, and some
beetles, also eat wool. The adult moth lays its eggs in the wool, the larvae hatch,
feed on the wool, and then metamorphose into adult moths, thus continuing the
cycle. Considerable damage to the wool usually results.

In the past, it was common practice to store woollen articles with mothballs.
These contain chemicals, such as naphthalene or p-dichlorobenzene, that are
slightly volatile and have a characteristic odour. These chemicals are deterrents
but do not really ‘moth-proof’ the fibres. To do this, the wool must be treated with
polychlorinated organic compounds developed specifically for wool, or with
insecticides generally used in agriculture, or mixtures of the two (Figure 7.7). The
major application of these agents is for wool yarn in carpets.

Moth-proofing agents are usually applied during dyeing, some, such as Mitin FF
(Figure 7.7), acting rather like colourless acid dyes. They have adequate fastness
to washing and light to maintain their moth-proofing effect over the life of the