The wool fibre is composed of the protein keratin, which consists of long polypeptide chains built from eighteen different amino acids. Most of these acids have the general formula H2N.CHR.COOH, in which R is a side chain of varying character. The chain structure is of the type:
And at intervals bridges derived from the amino acid cystine connect the chains. Some of the side chains end in amino groups and others in carboxyl groups; internal salts are therefore formed and the
Molecules are bound together by electrovalent linkages. The molecules of keratin are very large, with and average molecular weight estimated at about 60,000.
The wool fibre is readily destroyed by alkali, but withstands acid conditions fairly well; some hydrolysis of peplide linkages occurs on prolonged boiling with acids, however. The carboxylic acid and amino groups in the keratin molecule confer affinity for basic and acid dyes. Basic dyes are now little used on wool since their fugitive properties render them unsuitable for such and expensive and durable fibre. Acid dyes, however, are extensively used, and the general characteristics of this large class and the related mordant and pre-metallised azo dyes are now described.
Since the bonds between dye anions and amino groups in the wool fibre are easily broken and re-formed, dyes attached in this way are liable to migrate. This property is advantageous, in that level dyeing is readily attained, but it leads to low fastness to wet treatments, and any undyed wool present during washing becomes stained. These characteristics are chiefly apparent in dyes of low molecular weight, and fastness to washing is in general much better in more complex dyes. The larger dye molecules are evidently attached the fibre by some means other than the ionic bonds mentioned above, and it is believed that they are held by non-polar van der Waals forces exerted between hydrophobic dye anions and hydrophobic regions of the wool fibre, their strength being proportional to the area of contact.
About the Author
The author is associated with Pakistan Council of Scientific & Industrial Research (PCSIR) Lab complex, Karachi as a Senior Research Associate.