PERMANENT WAVING: POST-YIELD SLOPE 699 in 10 min between pH 4 and 6.5. This has been verified on both hair tresses in our laboratories and on patrons in our test salon. THE EFFECT OF FORMULATION ADDITIVES The completed permanent wave is generally not just a mercaptan solution. Additives are commonly used to increase the tightness of the curl, further decrease the process- ing time, or to improve the feel of the perreed hair. Any such additives can have an ef- fect on the rate of permanent wave processing. Glyceryl monothioglycolate solutions are mixed at the time of processing since this ester hydrolyzes in water to yield glycerin and thioglycolic acid. If the hydrolysis were rapid, then the same waving results should be obtained from equivalent amounts of glycerin and thioglycolic acid. This hydrolysis is of course not instantaneous. As an example, consider the addition of glycerin to a waving solution of 12% thioglycolic acid which has been adjusted with ammonia to pH 6.5 and compared to thioglycolate itself and glyceryl monothioglycolate as shown in Figure 10. While thioglycolic acid at 12% processes slowly under these conditions, and glyceryl monothioglycolate at 12 % processes only slightly faster, the addition of glycerin to the formula at levels greater than 2% retards the decay of the past-yield slope with time. The overall rate for slope decay decreases with increasing glycerin concentration. The 9.2% glycerin level is approximately equimolar with 12% thioglycolic acid and the results differ from those obtained with glyceryl monothioglycolate, which further sup- ports the fact that hydrolysis is not appreciable during the time of waving. It is likely 0.80 UJ 0.70 • 0.60 .-I o UJ 0.40 0.20 VARIATION OF THE DEGREE OF CLEAVAGE WITH TIME 12%GMTG pH 6.5 ' I ' I ' ' 500 C I ,3.0 M urea I •1.5 M urea No urea I ' I 10 15 20 25 30 TIME (MINUTES) Figure 11. Variation of degree of cleavage with time at different levels ofurea/GMTG.

700 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS that the effect of glycerin is to alter the pentration rate of the mercaptan into the fiber by altering fiber swelling under these conditions. As a second example, consider the effect of the addition of urea to a 12% glycerol monothioglycolate soluton. Urea is known to alter the secondary and tertiary structure of proteins, predominantly through hydrogen bond breakage. This would conceivably open the structure of the fiber to more rapid penetration of waving agent. This experi- ment was conducted for three levels of urea as shown in Figure 11. While all levels of urea increased the rate of post-yield slope decay relative to 12 % gly- ceryl monothioglycolate (approximately 50% cleavage in 20 min), they all also increased the equilibrium cleavage value (62% for 1.5M urea to 78% for 6M urea). This suggests that the effect of urea has been to open areas of the fiber that were pre- viously inaccessible to the mercaptan. While this at first might suggest more effective perming, high degrees of keratin disulfide cleavage are generally damaging and when tresses were examined utilizing these solutions, the results were entirely unsatisfac- tory. The effect of additives on the rate of perming and degree of cleavage can be assessed by post-yield slope analysis though the ultimate evaluation of other important perming parameters must be done by conventional methods to evaluate cosmetic appeal and hair damage. CONCLUSIONS The post-yield slope evaluation can provide the formulator with important information about formula parameters. We have demonstrated its use in estimating the time of processing, rate of penetration and degree of cleavage. We have further presented examples showing the effects of mercaptan structure, mercaptan concentration, pH of waving and formulation additives. The ultimate evaluation of a permanent wave depends upon the degree of curling that is achieved, its lasting qualities and the cosmetic appeal of the hair. These factors are best observed by more conventional methods of evaluation. The evaluation method that has been presented can be used to guide the formulation through early formula- tion steps with a minimum of laborious tress evaluation. ACKNOWLEDGMENTS The authors would like to express their appreciation to Donna Garza for technical assistance, and to Dodie Keithley and Dayna Banakas for their salon expertise and patience. REFERENCES (1) H. D. Weigmann, L. Rubenfeld and C. Dansizer, A transition temperature in wool fibers under stress in relation to structure, Text. Res. J., 35,604-611 (1965). (2) L. Rubenfeld, H. D. Weigmann and C. Dansizer, Temperature dependence of the mechanical properties of human hair in relation to structure, J. Soc. Cosmet. Chem., 17, 525-538 (1966).