472 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS z x ø/ce/trtio/ Figure 9. Predicted T95v• response to TGA and GuHCI concentration at pH 10.5. Y method provides reproducible results that have been shown to correlate with in vivo performance with two commercial depilatory systems. The T95 % parameter can be ob- tained rapidly enough to allow convenient screening of a wide range of formulations. Central composite designs can be used to predict the response to multiple-treatment variables. Factors shown to increase the rate of TGA-induced stress decay as evaluated by T95 %, are: 1. Increased pH 2. Oxidative pretreatment of the hair at high pH 3. Incorporation of TGA into the water phase of an oil-in-water emulsion, thereby increasing the effective concentration

SINGLE FIBER DEPILATORY STUDIES 473 4. Guanidinium salts 5. Increased TGA concentration The rate of TGA-induced stress decay was decreased by the addition of high concentra- tions of n-propanol, triacetin, glycerin, or propylene glycol, and by incorporation of TGA into the water phase of a water-in-oil emulsion. REFERENCES (1) H. A. Yablonski and R. William, A quantitative study of the effect of depilatory solutions upon hair, J. Soc. Cosmet. Chem., 19, 699-706 (1968). (2) T. J. Elliot, Use of a laboratory model to evaluate the factors influencing the performance of depila- tories, J. Soc. Cosmet. Chem., 25, 367-377 (1974). (3) R. R. Wickerr, Kinetic studies of hair reduction using a single fiber technique,d. Soc. Cosmet. Chem., 34, 301-316 (1983). (4) R. R. Wickerr and B. G. Barman, Factors affecting the kinetics ofdisulfide bond reduction in hair,.]. Soc. Cosmet. Chem., 36, 75-86 (1985). (5) K. W. Herrman, Hair kerarin reaction, penetration, and swelling in mercaptan solutions, Trans. Faraday Soc., 59, 1663 (1963). (6) Y. K. Kamath, C. J. Dansizer, and H. D. Weigmann, Weftability of keratin fiber surfaces, J. Soc. Cosmet. Chem., 28, 273-284 (1977). (7) J. A. Maclaren, The extent of reduction of wool proteins by thiols, Aust. J. Chem., 15, 824 (1962). (8) R. Barry, in Cosmetics.' Science and Technology, Vol. 2, Balsam and Sagarin, Eds., Ch. 18, pp. 39-65 (1972). (9) J. L. Parra, J. J. Garcia Dominguez, F. Cornelles, J. Sanchez, S. Solans, C. Pelejero, and F. Balaguer, Use of microemulsions as vehicles for nucleophilic reagents in cosmetic formulations, lnt. d. Cosmet. Sd., 7, 127-141 (1985).