Wool Fibres:


The wool fibre is composed of the protein keratin, whichconsists of long polypeptide chains built from eighteen different amino acids.Most of these acids have the general formula H2N.CHR.COOH, in whichR is a side chain of varying character. The chain structure is of the type:



And at intervals bridges derived from the amino acid cystineconnect the chains. Some of the side chains end in amino groups and others incarboxyl groups; internal salts are therefore formed and the



Molecules are bound together by electrovalent linkages. Themolecules of keratin are very large, with and average molecular weightestimated at about 60,000.


The wool fibre is readily destroyed by alkali, butwithstands acid conditions fairly well; some hydrolysis of peplide linkagesoccurs on prolonged boiling with acids, however. The carboxylic acid and aminogroups in the keratin molecule confer affinity for basic and acid dyes. Basicdyes are now little used on wool since their fugitive properties render themunsuitable for such and expensive and durable fibre. Acid dyes, however, areextensively used, and the general characteristics of this large class and therelated mordant and pre-metallised azo dyes are now described.


Since the bonds between dye anions and amino groups in thewool fibre are easily broken and re-formed, dyes attached in this way areliable to migrate. This property is advantageous, in that level dyeing isreadily attained, but it leads to low fastness to wet treatments, and anyundyed wool present during washing becomes stained. These characteristics arechiefly apparent in dyes of low molecular weight, and fastness to washing is ingeneral much better in more complex dyes. The larger dye molecules areevidently attached the fibre by some means other than the ionic bonds mentionedabove, and it is believed that they are held by non-polar van der Waals forcesexerted between hydrophobic dye anions and hydrophobic regions of the woolfibre, their strength being proportional to the area of contact.


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About the Author


The authoris associated with Pakistan Council of Scientific & Industrial Research(PCSIR) Lab complex, Karachi as a Senior Research Associate.