HEAT DRIED HAIR 35 Table IX Hair Drying and Fiber Stiffness Treatment Or Stiffness Index • Conditioning of Fibers (mm) Shampoo and Dry at 55% -- R.H. and 22øC 24 Hours After Shampooing 6.78 Immediately After Heat 4.852 Drying, 1 Hour at 50øC in Simulated Dryer. 24 Hours After Heat Drying 6.33 Step 1 Step 2 Step 3 Step 4 'Hanging fiber method of Scott and Robbins (8) used to evaluate stiffness index. Readings taken at 55% R.H. 2Stiffness value different from Step 2 value at c• = 0.05 level slope and limit decrease with increasing temperature, fiber stiffness should also decrease. However, since heat drying decreases the moisture content of hair and hair fiber stiffness increases with decreasing moisture content (11) the effects of heat drying on hair fiber stiffness are not predictable. Therefore we decided to examine the effects of heat drying on hair fiber stiffness, a fundamental fiber property. The data are summarized in Table IX. Step 3 represents the stiffness index immediately after removing the fibers from the simulated hair dryer. This point shows a significant and substantial decrease in fiber stiffness indicating that increased temperature is dominant in governing the stiffness properties. After re-equilibration at 55% R.H. and room temperature (24 h), the fibers return to their "normal" stiffness level in spite of the moisture binding hysteresis. This effect suggests that when heat appliances such as hot combs and styling irons are used on hair, the fibers are more pliable because of the heat. Therefore they conform more readily in styling operations. However, after removing the heat source and equilibration to room temperature hair fiber stiffness returns to normal, unaffected by the hysteresis in moisture binding and in this respect is a contrast to the flyaway effects. CONCLUSIONS Hair that has been heat dried and then equilibrated at room temperature and moderate humidity will contain less moisture than it would if it were dried at room temperature, i.e. a hysteresis exists in heat drying similar to that found in chemical desiccation. However, heat dried hair will return to its room temperature dried moisture level if it is resoaked in water or conditioned at higher humidities. Heat drying will increase the tendency of hair to acquire a static charge during combing and this charge buildup seems to be related to the heat drying hysteresis. Heat drying can also produce a short term decrease in fiber stiffness. This decrease in fiber stiffness is related to the action of heat on the fibers, but is not related to the hysteresis. Heat drying at moderate temperature has no effect on the tensile properties, although small changes in the tensile properties were detected in hair dried at very high temperature.

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