88—A SPECTROPHOTOMETRIC STUDY OF YELLOWING IN WOOL FABRIC

Abstract

The differential spectrophotometric method of measuring colour in wool fabric reveals that the fabric yellows much more slowly when heated in air than in aqueous solution. Yellowing by heating in buffer solutions increases with pH value and is also markedly affected by the nature of the buffer ions, glycine and related nitrogen compounds causing least yellowing and pyrophosphate most when compared at pH 9·0. Hydrothermal yellowing is partly prevented by prior heating of the fabric in 0·1M KCN. A green-yellow shade is produced in dry wool fabric when exposed to ultraviolet irradiation over a desiccant, λ_max occurring at about 6300 Å in addition to the usual λ_max at 3200 Å. On exposure to the moist air, the absorption peak at 6300 Å and a minimum in the curve at 3800 Å disappear, and the optical density at 3200 Å increases to a higher value than is obtained by irradiation in moist air. The optical density at 3200 Å due to irradiation increases as rapidly in wool fabric which has been dyed or impregnated with fluorescent brightening agent as in the untreated material. The yellow shade in wool produced either by heating in pH 8 buffer or U. V. irradiation, is partly discharged by immersion in dilute acid solution. Amino-acid analysis of wool fabric after yellowing, and exposure of individual amino acids and peptides to aqueous heating and irradiation treatments, suggest that cystine is the amino acid chiefly responsible for the discolouration process.