Page 122 - Basic Principles of Textile Coloration

STRUCTURE OF WOOL FIBRES 111

size and chemical nature and project outwards from the main polymer chain.
There are six main types:
(1) non-reactive hydrocarbon groups, as in alanine;
(2) polar groups such as alcoholic or phenolic groups, as in serine and tyrosine;
(3) basic groups, as in lysine, that influence the maximum amount of acid with

which the wool combines and the absorption of anionic acid dyes (Section
1.1.2)
(4) acidic groups, as in glutamic acid;
(5) covalent crosslinking groups, as in cystine, that influence the solubility,
swelling and mechanical properties of wool;
(6) heterocyclic groups, as in proline.

The protein chains in wool are held together by hydrogen bonds. These form
between the hydrogen atom attached to the nitrogen of an amide group and the
oxygen atom in a neighbouring carbonyl group, as in nylon. In addition, the chains
are linked by ionic or salt crosslinks, by covalent bond crosslinks, and by weak
interactions between non-polar side-groups (Figure 7.2). The latter are often
called hydrophobic bonds. They arise because the association of hydrophobic non-
polar groups is energetically more favourable when they are in a polar
environment. This is analogous to the solubility of benzene in hexane, but not in

CHCH2CH2CO2 NH3(CH2)4 CH

HN Ionic bond CO

CO HN

CH3 HC CH2
NH OC

OC Covalent crosslink NH

CH CH2 S S CH2 CH
HN CO

CO HN

H2C Hydrogen bond CH CH2OH
NH OC

OC CH3 NH
CH2 CH
CH CH

CH3

Hydrophobic bond

Figure 7.2 Types of inter-chain linkages in wool