J. Soc. Cosmet. Chem., 32, 27-36 (January/February 1981) A hysteresis in heat dried hair R. CRAWFORD and C. R. ROBBINS, Colgate-Palmolive Research Center, 909 River Road, Piscataway, New Jersey 08854, j. CURRAN, Food Sciences Department, U. of Illinois, Urbana, Illinois 61801, and K. CHESNEY, A.T.&T. Long Lines, Northborough, Massachusetts 01532. Received July 22, 1980. Synopsis Hair dried with heat and equilibrated at room temperature at a moderate relative humidity will have a lower moisture content than room temperature dried hair. After heat drying, hair absorbs moisture but does not return to the room temperature dried water level until it is rewet or conditioned at a higher relative humidity and dried at room temperature, i.e. a hysteresis exists in heat drying analogous to that from chemical desiccation of hair. Heat drying also makes hair more susceptible to static charge buildup, which is related to the hysteresis, and results in increased flyaway during combing or brushing. Heat drying can also produce a short term decrease in fiber stiffness which is not related to this hysteresis but appears to be due to the direct action of heat on the fibers. INTRODUCTION Rebenfeld et al. (1) have described the effects of heating hair in buffer solutions, and Humphries et al. (2) have described a thermochemical method for evaluating human hair. In addition, Menefee et al. (3) have produced phase changes in wool by heating it to high temperatures, and D'Arey and Watt (4) have shown that the equilibrium moisture content of keratin fibers decreases with increasing temperature. Hair dryers and curling irons are capable of producing very high temperatures, therefore it is possible that in use these appliances might damage hair, however such damage has not been reported in the literature. Therefore, we undertook to investigate the effects of heat on moisture in hair and on certain important hair fiber and assembly properties. EXPERIMENTAL European dark brown hair fibers (5) were used in this investigation. Wet single fibers were allowed to dry in a constant relative humidity-temperature chamber at 55% R.H. and 22øC, and weighed using a Precision Balance (6). The fibers were then rewet and heat dried. For exaggerated conditions, a variable temperature oven was used and a 16-h drying time. For more practical conditions, hair was dried for 1 h in a simulated 27

28 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS hair dryer in which the temperature and air flow could be regulated. The simulated hair dryer was constructed by wrapping a glass tube (4 ft. long by 1 in. diameter) with nickel-chrome wire, and enclosing this in a second glass tube. The temperature was observed with a thermometer and regulated with a powerstat while air flow was regulated with a Matheson 7630 series flow meter. After heat drying, the fibers were equilibrated again to constant conditions and reweighed. Higher humidities were obtained using salt solutions (7). Wet tensile measurements (8) were made using an Instron Tensile Tester (9). Fibers were first calibrated by measuring the force required to stretch 4-in. fibers to 4.8 in. (20%) at a rate of 0.5 cm/min. After heat treatment, the fibers were retested. Bleaching and permanent waving of hair was with commercial products, following use directions. Static charge was measured by the method of Mills et al. (10). Fiber stiffness measurements were made by the method of Scott and Robbins (11). RESULTS AND DISCUSSION DRYING AND REGAIN RATES To help select equilibrium conditions for measurement of moisture in hair, rates of drying and reabsorption of moisture were investigated. The procedure used involved shampooing, rinsing, and drying the fibers to constant weight at room temperature (22øC) and 55% R.H. The hair was then fewer, and heat dried, and allowed to equilibrate at constant conditions. The rates of drying and reabsorption were measured by monitoring weight changes. We assume that weight changes represent moisture changes, and if a treatment or manipulation produces structural changes in the hair, then the fibers will not return to their original conditioned weights. Figure 1 depicts the drying rate curve (A) at 22øC and 55% R.H., and the moisture regain rate curve (B) at these same conditions for hair, after heat drying at 110øC for 16 hours. The data points in Figure 1 represent averages for 10 hair fibers. The water content (percent dry weight) is the percentage difference between the weight of the hair immediately after heat drying, which is assumed to be the dry weight, and its weight at various points along curves (A) and (B). Of course the water content of the hair expressed as Percent Dry Weight will depend on the value used for the dry weight which in turn will depend on the conditions used for drying (heat, vacuum, or desiccation) and the time of drying. Both absorption and desorption rates are rapid, and the fibers generally equilibrate within 3 hours. Moisture regain appears to be slower than the drying process, and the equilibrium moisture content is significantly lower for heat dried hair than for room temperature dried hair. Thus a major part of this investigation became concerned with trying to understand the conditions governing this phenomenon of a lower moisture content for heat dried vs. room temperature dried hair. Work with tresses revealed slower and more variable rates compared with single fibers. This may arise from changes in heat transfer caused by neighboring fibers, and their different relative orientations in different tresses (12). Because of these data, most of the subsequent experiments were run on single fibers after conditioning overnight to insure equilibrium, prior to weight measurements.