698 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS OPTIMIZATION OF TIME AND pH FOR 17% GMTG AT 50 ø C 10 Haim/Mandrel I-- 0.8. U,I 0.4 •: 0.6. MINUTES o MINUTES 2.0 4.0 6.0 8.0 10.0 pH WAVING SOLUTION Figure 9. Optimization ofdmc and pH •o• g]yccry] mono•h•og ycoJ•c. OPTIMIZING THE WAVING PARAMETERS The experiments illustrated in the above examples can be accomplished quickly and the formulation will rapidly target a few sets of conditions before beginning curl analysis or any salon testing. As an example, the data for 17% glyceryl mono- thioglycolate at three different times over the pH range of 2-9 is combined in Figure 9. Since we are trying to achieve a minimum post-yield slope in minimum time, this data would suggest that 17 % glyceryl monothioglycolate would yield satisfactory softening VARIATION OF POST YIELD SLOPE WITH TIME pH 6.5 46 ø C 10 HairslMandrel O 12% THIOGLYCOLI• ....,,,,,,•=• .b• ß 12% THIOGLYCOLI• • -'• •,•.• e12% THIOGLYCOLF ß 12% THIOGLYCOLI• o 12% GMTG • ACID • ACID + 2.1% GLYCERIN • ACID + 9.2% GLYCERIN - CACID + 17% GLYCERIN O 5 10 15 20 25 30 35 TIME (MINUTES) Figure 10. Variation of post-yield slope with time at different levels ofglycerin/ATG.

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.