Heat-yellowing of Wool and Silk

Abstract

PREVIOUS work on the heat-yellowing of wool has been summarized by Howitt¹. Neither the amino-acids responsible for the phenomenon, nor the reactions involved, have been elucidated, and an attempt has, therefore, been made to solve these problems by studying the action of heat on untreated and chemically modified wools. The general principle was to modify different side-chains and cross-linkages in turn, and then to compare the extent of yellowing of the untreated and chemically modified wools after heating for 24 h in a slow stream of dry air or nitrogen at 150° C, the yellowness indices of the samples being determined by a procedure similar to that of Norton and Nicholls². Using iodinated wool and wool methylated with diazomethane it was shown that the tyrosine side-chains of wool play little part in heat-yellowing under the foregoing conditions; but no satisfactory way of discovering the part played by the hydroxylic side-chains of serine and threonine could be devised. As silk is rich in these amino-acids, and as methylation with dimethyl sulphate and alkali is permissible with this material³, the action of heat on untreated and methylated silk was examined. The results are given in Table 1.