DEPILATORY ACTION VIA THERMOMECHANICAL ANALYSIS 289 WEIGHT TRAY UPPER SHAFT TRANSDUCER SAMPLE HOLDER LOWER PROBE HAIR STRANDS Figure 1. DuPont 943 Thermomechanical Analyzer with fiber tension accessory. formulations containing no depilatory active were run to account for any dimensional changes unrelated to the depilatory active ingredient. Observed Ta's for controls were greater than 60 minutes therefore, no corrections were applied. RESULTS AND DISCUSSION In order for this method to be useful in the product development process, it must be able to precisely measure the time at which significant hair weakening occurs when the hair is exposed to solutions of different depilatory actives. Of equal importance, it must be insensitive to differences in formulation physical properties. The instrumental pa- rameter which controls the elongation of the hair bundle is the force on the lower probe assembly. This force should be minimized to give a breakage time long enough to be reproducibly measured, but large enough to overcome the eftkcts of formulation viscosity which will impede the probe's motion. Therefore, instrumental response was observed by deter- mining T a as a function of added weight for a highly viscous (37,500 cps) commercial cream depilatory. Weights of 2, 5, and 10 grams were used, resulting in Ta's of 34.2, 15.8, and 15.2 minutes, respectively. Since no appreciable decrease in Ta was observed for weights above 5g, it was decided that all further determinations would be made at that weight. The precision of the method was determined by making T a measurements for a com- mercial cream depilatory. The measurements were made over a period of three days.

290 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TMA • o ! 0.0 2.0 4.• i i 6.0 8. 10.0 I 0 Time (min) Figure 2. Depilation time determination for depilatory solution. 1•,.0 '•8.0 •8.0 2(•.0 22•0 TMA -2 -1 xxx\x/ Td i ß i i i i ß ß i i i 0.0 21.0 4."0 I 6.10 8.• 10.•) 12:0 1•1.0 16.0 8.0 0.0 2•.0 Time (min) Figure 3. Depilation time determination for commercial depilatory lotion.