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CHAPTER 7
Protein fibres
7.1 INTRODUCTION
Animal hair consists of complex proteins. The hair of sheep and goats is
particularly important for textiles; wool is, by far, the major animal fibre in
quantity, but cashmere and mohair are significant for their market value. This
class of fibres also includes the fine double filament secreted by the silk worm. The
entire range of protein fibres accounts for only about 6% of world fibre
consumption, and the proportion is even less in many developed countries where
synthetic fibres are readily available. Quality wools, fibres such as cashmere, and
particularly silk, however, denote luxury and are priced accordingly.
Proteins are complex polyamides, also called polypeptides, produced by the
biological polycondensation of a mixture of a specific type of amino acid with a
variable side-group R (Scheme 3.3). The amino acid composition, and the
sequence of the amino acid units along the polymer chain, characterise the
structure of a protein molecule. Protein molecules, however, can adopt a variety of
elaborate spatial arrangements. These include sheets of extended protein
molecules laying side by side, or cord-like arrangements of individual helical
molecules wound around each other. In globular proteins, such as enzymes, the
protein molecules adopt even more complex, but characteristic, configurations. In
living tissue, proteins are frequently associated with other types of biomolecules in
complex cellular structures. It is only in fairly recent times that the detailed
morphology (form and structure) of wool fibres, and the way in which it influences
the dyeing process, has become clearer.
7.2 STRUCTURE OF WOOL FIBRES [1]
7.2.1 Keratins and wool proteins
Proteins are polymers of a- or 2-aminocarboxylic acids. They have an amino group
attached to the carbon atom next to the carboxylate carbon atom. Apart from
glycine (aminoacetic acid), the 2-carbon atom always has a substituent other than
hydrogen. Thus, the four different groups attached to the 2-carbon atom satisfy
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