Synopsis

A single-fiber method of investigating reduction kinetics in hair has been applied to the study of hair depilation. The time required to reduce the tensile stress upported by a hair by 95%, (T95%), has been shown to correlate to in vivo hair removal by two commercial depilatory products. The reaction of hair with thioglycolic acid (TGA) under depilating conditions was enhanced by increasing pH, by adding guanidine salts, and by oxidative pretreatment. Reduction with TGA was slowed by the addition of n-propranol, triacetin, glycerin, and propylene glycol. Emulsion state also affected reduction rate with TGA. Illustrative of varying pH effects with different reactants, sodium dihydrolipoate was found to react faster than TGA at pH 10.5 but was slower than TGA at pH 11.5, while ammonium sulfide was slower than TGA at 10.5 and faster at pH 11.5. A central composite design experiment was used to calculate a response surface for the effects of TGA concentration, pH, and guanidine hydrochloride concentration on hair reduction rate. An equation with good fit to the data was developed for prediction of responses to changes in the variables over the ranges studied