HEAT DRIED HAIR 33 Table VI Weight Changes in Damaged Hair After Heat Drying and Rewetting After Heat • Hair Type Drying % Weight Change After Resoak And R.T. Drying Untreated - 1.853 -- 0.283 Permanent Waved 2 - 1.903 - 0.663 Bleached 2 - 2.103 _ 0.583 •These fibers were all heat dried to 110øC for one hour in a simulated hair dryer. 2See experimental section for procedure. 3Change significant at o• = 0.05 level. PERMANENT WAVING AND BLEACHING The data in Table VI show that a similar heat drying hysteresis exists for both permanent waved and bleached hair. There was no significant difference in the size of the hysteresis gap among the three hair types. All hair types, on resoaking, fail to return to the original room temperature dried weights suggesting that permanent structural changes have been produced in the fibers, and this is greater but not significantly different for the damaged hair compared to the chemically altered hair. FLYAWAY The effects of relative humidity on statioinduced flyaway were demonstrated by Mills et al. (10) who showed that flyaway decreases with increasing relative humidity. This effect is due to increased moisture binding by the hair at higher relative humidities. This in turn decreases the electrical resistance of the fibers and thus lowers their capacity to acquire a static charge. Therefore, the decrease in moisture content of hair produced by heat drying should have an adverse effect on flyaway. Table VII summarizes the data from an experiment which shows that heat drying hair significantly increases its ability to acquire a static charge. The increase immediately after heat drying was expected, however, the large difference 24 h after heat drying Table VII Heat Drying and Flyaway Treatment Or Relative Conditioning of Tresses Static Charge 1 Day 1--Shampoo and Dry at -- 55% R.H. and 22øC. Day 2--24 Hours After Shampooing 7.6 Day 3--Immediately After Heat Drying 37.92 1 Hour at 50øC in Simulated Dryer Day 4--24 Hours After Heat Drying 22.02 •Data are static charge peak heights from osciltograph readings of charge on comb using the method of Mills et al. (7) Control tresses were run days 2, 3, and 4, and their readings did not change significantly. All readings were done at 55% R.H. 2Static Charge value different from Day 2 value at o• = 0.05 level.

34 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS suggests that the heat drying induced hysteresis, even hours after drying, may play a significant role in the chargeability of the hair, and thus produce a greater tendency for flyaway hair after heat drying vs. room temperature drying. However, since the hysteresis is eliminated by high humidity, the flyaway difference should also be eliminated by high humidity. TENSILE PROPERTIES The tensile properties of heat dried hair were examined from two points of view. First, we wanted to determine if the hysteresis in moisture binding affects the tensile properties of human hair, and second, we wanted to determine if cumulative heat drying might adversely damage the hair, which might show up as a decrease in the tensile properties. To determine if the hysteresis affects the tensile properties, we decided to first examine the dry tensile properties of hair fibers immediately after heat drying at 50øC for it h in a simulated dryer. The results of this experiment do not show a significant effect for a single low temperature heat drying temperature, suggesting that the hysteresis in moisture binding does not affect the dry tensile properties of hair. To determine if tensile damage might be produced by cumulative heat drying, we treated precalibrated hair fibers of cosmetically altered and unaltered hair 25 times, shampooing and heat drying in a simulated hair dryer, at both 50øC and it00øC, and evaluated these fibers for changes in the wet load-elongation properties. The results of this study are summarized in Table VIII. These data show no significant effects at 50øC, however, at it00øC significant but small decreases in the wet tensile properties were observed. Thus, we conclude that normal heat drying of human hair does not produce meaningful tensile damage, although excessive heating can damage the hair. FIBER STIFFNESS The wool fiber literature (22) shows that all parameters of load elongation, with the exception of extensibility, decrease with increasing temperature. Since the Hookean Table VIII Wet Tensile Damage from Heat Drying Hair Type Drying Temperature % Change In Force to 20% Extension Unaltered 22øC 0 50øC -0.2 100øC --4.3 Bleached 22øC +0.3 50øC -- 0.5 1 O0 øC - 2.1 Permanent waved 22øC + 0.9 50øC +0.5 100oc -6.7 •Change significant at oz = 0.05 level.