A curing method for permanent hair straightening using thioglycolic and dithiodiglycolic acids

Abstract

A successful process of permanent hair straightening was achieved by applying a hot iron press technique to hair treated with an alkaline solution containing thioglycolic acid (TGA) and dithiodiglycolic acid (DTDG). Supercontraction of the hair was determined on the basis of fiber length. The effectiveness of straightening was evaluated as a function of supercontraction, which was dependent on the concentration ratio of DTDG to TGA, pH, time, and heat-treatment temperature. A range of around 5% to 8% supercontraction was found to be necessary to achieve successful permanent hair straightening, while at relatively higher contraction levels the smoothness of the fiber surface tended to be lost. About 89% of the initial content of cystine was retained in straightened hairs, and about 11% was converted to cysteic acid. X-ray diffraction and high-pressure differential scanning calorimetry were used to estimate the amount of α-crystallites. The relationship between the melting enthalpy and the degree of supercontraction was discussed in relation to the stability in the straight configuration. Important conclusions include the observations that supercontraction of less than 10% is caused by randomization of the α-helix and that permanency related to hair straightening is a result of the irreversible transformation of the α-crystal into amorphous phase.