HEAT DRIED HAIR 31 hair. If heating to lower temperatures produces irreversible changes, they are too small to be detected in our experimental system. This difference in the moisture content of room temperature and heat dried hair is reminiscent of the hysteresis in the moisture binding by human hair and wool fiber demonstrated by Speakman nearly 50 years ago (13). In this classical study, Speakman demonstrated that the moisture content of human hair, by dehydration from the wet state, is higher than by regain after drying over a chemical desiccant such as P20 5 in vacuum. Similar hysteresis phenomena have been demonstrated for high polymers (14) and by other proteins such as wool fibers (15) and casein (16). Several explanations have been offered to explain this hysteresis phenomenon (14, 17-20). One of these, by Urquhart, suggests that the primary water binding sites are the polar groups of the fiber, which can also bond to each other via intrachain hydrogen bond crosslinks, so when a fiber is completely dry, intrachain crosslinks are at a maximum, and only a few free polar sites remain. On the other hand, when a fiber is completely hydrated, intrachain hydrogen bond crosslinks are at a minimum. Therefore, on absorption of moisture, the free polar sites hydrate first followed by rupture of the hydrogen-bonded crosslinks to free polar groups which are hydrated. Thus, on absorption, the number of active or free groups available for moisture binding is lower at all stages than it is during desorption, which accounts for the lower amount of moisture on absorption. These data, and Tables I and I1, clearly show that a hysteresis occurs by heat drying hair, and that the size of this moisture binding gap depends on the temperature, i.e. on the extent of drying. Speakman (13) reported a hysteresis of about 2 percent at 55% R.H. via desiccation and our results under our most severe conditions of drying are in line with this value. Watt (21) has shown that the hysteresis will disappear for single wool fibers under certain conditions of desorption. In view of this, we decided to determine what factors might affect the size, or existence of, this heat drying hysteresis, and what physical properties it might influence. We elected to first examine the effects of changing humidities, primarily to higher humidities, since resoaking in water eliminates the hysteresis. EFFECTS OF HIGHER AND VARYING HUMIDITIES Hair fibers were shampooed, rinsed, conditioned at 55% R.H. (22øC), and weighed. These same fibers were then rewet and heat dried at 90øC for one hour. Ninety degrees Celsius was selected because it gave the largest hysteresis which was reversible. Several of these fibers were then conditioned for 16 hours at 55% R.H., several at 65% R.H., some at 75% and the remaining at 85% R.H. The final part of this experiment involved re-equilibrating the fibers at the starting 55% R.H. and reweighing them. The results summarized in Table IV show that the hysteresis is still present in the fibers conditioned at 55% R.H., but the fibers conditioned at higher humidities do not show a weight loss hysteresis. In fact these fibers show a weight gain which is not significant after conditioning at 65% R.H. but is significant after conditioning at 75% and 85% R.H. The reason for this weight increase is not clear at this time. Since the weight pick up is so small (.26% to .33%) we elected not to investigate this apparent anomaly although we conclude that higher humidities have the same effect as soaking hair in water by eliminating the heat drying hysteresis. This also suggests that hair problems

32 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table IV Effect of Increased Humidity % R.H. % Weight Change 55 -- 1.26 • 65 q- O.26 75 q- 0.29 • 85 q-0.33 • •Change significant at ot = o.o5 level. associated with this heat drying hysteresis will be eliminated when the hair encounters higher humidity for several hours. The effects of shorter exposures to higher humidity were not measured. PERCENT MOISTURE The values expressed in Tables I and II show small weight changes in hair from loss of moisture. These values are based on changes in the total weight of hair. Perhaps a more realistic way to. express these changes is as changes in the weight of moisture of the hair relative to equilibrium moisture content at 55% R.H. Our data from Figure 1 shows the moisture content of hair to be 11.5% of its dry weight. Using this figure, we calculated the equilibrium moisture content of hair at 55% C.H. and room temperature to be 10.3% of its total weight. The values in Table V show the percentage change in the actual amount of moisture in the hair calculated from the weight changes in Table II relative to this equilibrium moisture content: % Weight Change Due To Heat % Change in Total Moisture of hair = x 100. 10.3% Subsequent results on physical properties of hair confirm that this is a more realistic estimate of the magnitude of the moisture changes in hair due to heat drying. Table V Percent Moisture Loss from Heat Drying Percent Change • Drying In Total Weight In Total Temperature Of Hair 2 Moisture of Hair 2 40øC - 0.63 - 6.1 50øC - 0.66 - 6.4 70øC - 0.48 - 4.6 90øC -- 1.00 -- 9.7 110øC - 1.85 - 17.9 •Values represent the difference between the percent weight loss due to heat drying (Table 2 Values) and the percent moisture content (10.3%) calculated from Figure 1. 2Change significant at o• = 0.05 level.