Chemicals reactions occurring in curing treatment for permanent hair straightening using thioglycolic and dithiodiglycolic acids

Abstract

A curing method for hair straightening has been practically applied for decade. The straightening process consists of three step reactions: reduction followed by washing, heat treatment and oxidation. Two different reduction systems were used: one is a system containing thioglycolic acid (TGA) only and the other is a bicomponent system containing TGA and dithiodiglycolic acid (DTDG). Reaction mechanism was proposed on the basis of amino acid analysis of the hair samples obtained at various treatment steps. New equilibrium reactions were assumed to be established in the reduced fiber after washing and we could explain successfully the experimental results. As far as TGA is used for the reduction, a sever problem for the damage of hair arises from the formation of the mixed disulfide groups. There were important findings on this subject that (1) when the TGA-only system was used, the mixed disulfide groups were formed in the fiber and the amount of the groups incorporated was dependent on the concentration of TGA, (2) in the heat-treatment step at 180°C for 3 sec, the mixed disulfide groups were wholly converted into the thiol groups of cysteine residues, and (3) when the bicomponent system was applied, only the thiol groups were linked to the protein chains even under a hot permanent conditions at 45°C for 15 min. The role of DTDG is not only a reduction-regulating agent, but also a thiol-reforming agent, in other words, DTDG acts as an anti-damaging agent.