THE KERATINIZATION PROCESS 



251 



0-30% is completely reversible. Beyond this point the fibre may recover 

 its length after stretching, but it is thereafter easier to stretch, showing that 

 irreversible damage has occurred. The smaller the index the greater the 

 reaction between the fibre substance and the reagent. 



>. 10 



■S 



Fig. 104. Stress— strain curves of wool fibres in water at various tempera- 

 tures. From Peters and Woods (1956) by permission. 



Salt Linkages 



All the keratins contain a high proportion of residues of the diamino and 

 dicarboxylic acids, and in wool these are approximately equal in amount 

 (p. 219). In the neutral condition when both types of group are ionized: 



P.COO- +H 3 N.P (P = polypeptide chain) 



there is the possibility of electrostatic attraction between the two which 

 could contribute to the cohesion of the material. This electrostatic force is 

 referred to as a " salt-link". Experimentally the probable existence of such 

 salt link combinations is demonstrated by numerous experiments made by 



