Page 131 - Basic Principles of Textile Coloration

120 PROTEIN FIBRES

alkaline conditions. In fact, wool rapidly dissolves in boiling 2% aqueous NaOH
solution, whereas cotton is totally unaffected by this treatment. Alkaline solutions
rapidly loosen the surface scales and penetrate into and attack the cortex. In wool
processing, it is essential that the pH of any solution does not exceed a value of
10.0 to 10.5, particularly if the temperature is above 50 °C. Dilute ammonia
solution at about pH 10 is a relatively safe choice since wool can withstand
treatment under these conditions for 30 min at temperatures up to 90 °C. When
using weak alkalis such as ammonia and sodium carbonate in scouring, care is
always essential. Wool is even damaged by extended treatment in boiling water.
The fibres become weaker, less resilient and yellow.

Wool is considerably less sensitive to hot dilute solutions of acids. Cotton can
be completely dissolved by hydrolysis in acid solutions that cause minimal damage
to wool. This is used to advantage in the carbonising process in which wool is
impregnated with dilute sulphuric acid solution and dried. This converts all
cellulosic material into brittle hydrocelluloses that can be removed from the wool
by beating. More concentrated acid solutions, or prolonged treatment with hot
dilute acid solutions, will, however, cause hydrolysis of the protein. During dyeing,
in the presence of acids, there is invariably some degree of damage to the wool
fibres, which manifests itself as a loss of strength and abrasion resistance. Alkali
damaged wool is even less resistant to acids.

7.3.4 Setting of wool

When damp wool is stretched and dried, as in ironing, the fabric retains the shape
it has been given under tension. This dimensional stability arises because hydrogen
bonds are broken as the keratin chains are stretched under the influence of water
and heat. The intermolecular hydrogen bonds between amide groups are replaced
by hydrogen bonds with water. As drying proceeds, new hydrogen bonds then form
between the protein molecules in their new positions, which stabilises the new
structure. The effect is not permanent and the material may shrink on wetting as
the hydrogen bonds are again broken.

The effect is called setting and, in the above case, temporary setting, because of
the lack of permanence. The same effect occurs when wet human hair is wound
on a curler and allowed to dry. More permanent setting of the dimensions of wool
fabrics and articles occurs on treatment with hot water or steam over a more
prolonged period. This has a chemical action on the wool. For example, in the
decatising process, wool fabric, wound on a roller under tension with a cotton