Page 129 - Basic Principles of Textile Coloration

118 PROTEIN FIBRES

7.3.2 Wool properties

Wool fibres are hygroscopic and the most hydrophilic of textile fibres. The
standard regain is around 16–18% water and clearly significant when wool is sold
by weight. The actual regain of purified wool is quite sensitive to traces of residual
impurities and to any chemical modification of the protein fibre.

Despite the high regain, wool does not feel damp. Wool is a very warm fibre and
is ideal for undergarments in contact with the skin. The absorption of water from
perspiration by wool fibres is exothermic and releases heat. On drying, the rate of
evaporation from the wool in contact with the body is so slow that there is no
cooling effect.

Although wool is hydrophilic, the fibres may be difficult to wet out because of
the scaly barrier and thorough wetting usually requires hot water and often a
wetting agent. On water absorption, hydrogen bonds with water molecules replace
those between amide groups, and salt linkages break as the ionic groups become
strongly solvated by water molecules. The fibre therefore becomes weaker. These
types of intermolecular force are so predominant in dry or conditioned wool that
they can mask the effects of broken peptide and disulphide bonds on the
mechanical properties. For this reason, testing of the mechanical properties to
evaluate protein damage is often conducted on wet wool.

Wool fibres are elastic and resilient. Wool fabrics therefore do not crease easily,
have good crease recovery, and wool garments fit well. Wool fibres are much less
rigid than those of cotton. The elastic recovery of wool fibres is 65% for 20%
extension and almost 100% for short extensions. The elasticity is related to the
reversible deformation of the helical a-keratin molecules, which act rather like
springs. The natural crimp of the fibres contributes to the elasticity of wool as the
fibres return to their wavy form after deformation. The crimp also stabilises low
twist woollen yarns by holding the fibres together. Such yarns trap air, and when
used in garments, providing an insulating barrier to loss of body heat.

Like all proteins, wool is a sensitive biopolymer. On extended exposure to light
and air, it will gradually deteriorate in quality, often yellowing considerably. For
this reason, it is very difficult to maintain the quality white of bleached wool. It is
also not very stable to dry heat and yellows readily on over-heating during drying.

7.3.3 Effects of acids and alkalis on wool

Aqueous solutions of acids and alkalis initially influence wool by changing the
degree of dissociation of carboxylic acid and ammonium ion groups in the fibre.