HEAT DRIED HAIR 29 n 20 16 "' 14 ,=. 12 r- i0 z 8 6 2 0 ,of A 50 I00 150 200 250 300 350 TIME IN MINUTES Figure 1. Desorption of water from hair after shampooing (A) and absorption after heat drying (B). Steps A and B were measured at 22øC and 55% R.H. Heat drying was for 16 h at 110øC. TEMPERATURE AND HEAT DRYING Commercial hair dryers were found to generate temperatures in excess of 100øC and air flow rates around 40 1/min. Of course, with this type of appliance, the hair encounters such conditions for only brief periods of time, at more reasonable conditions, hair dryers may produce temperatures of up to 50øC for very long periods of time. To help define experimental conditions for later work and to determine what parameter(s) to explore under more pragmatic experimental conditions, we elected to run the following preliminary experiments. Preconditioned preweighed hair was rewet and heat dried for prolonged periods of time at temperatures ranging from 70øC to 110øC. After heat drying, the fibers were reconditioned and reweighed. The data are summarized in Table I. As in the rate study, the fibers did not return to their original conditioned weight. This suggests that a structural change, related to the binding of moisture, is produced by heat drying under these conditions. To confirm this conclusion, the material driven off by heat drying was captured in a dry ice-acetone trap. Analysis of the trapped material detected only water. Our next step was to determine if a similar phenomenon might occur under milder conditions of heat drying. These conditions involved rewetting and then heating preconditioned hair fibers for 1 hour, at several temperatures, in a simulated hair dryer with controlled air flow at a rate of 40 1/min. The results of this experiment are summarized in Table II and show significantly lower moisture contents after heat

30 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Weight Changes in Hair • Heated Under Exaggerated Conditions Temperature % Weight Change 2 70oc -- 0.793 90øC - 1.443 110øC -- 1.943 •Chemically unaltered hair used. 2percent weight change = (B -- A)/A x 100 where: A = equilibrium weight after drying at 22øC B = equilibrium weight after heat drying 3Change significant at c• = 0.05 level. Table II Weight Changes in Hair • Heated in a Simulated Hair Dryer Temperature % Weight Change 2 40oc _ 0.633 50oc _ 0.663 70oc _ 0.483 90øC - 1.003 110oc -- 1.853 •Chemically unaltered hair used. 2percent weight change = (B -- A)/A x 100 where: A = equilibrium weight after drying at 22øC B = equilibrium weight after heat drying •Change significant at c• = 0.05 level. Table III Weight Changes After Resoaking Heat Dried Hair Weight Change After Heat After Resoaking Temperature Drying and R. T. Drying 40øC - 0.62 • + O. 13 50øC - 0.66 • - 0.20 70øC -0.48 • -0.11 90øC -- 1.00 • -- 0.19 110oC - 1.85 • -0.28 ' •Change significant at a = 0.05 level. drying at all temperatures compared to room temperature drying. Therefore, one may conclude that structural alterations are produced in hair by "normal" heat drying. We then decided to re-examine these same hair fibers by resoaking in dilute surfactant solution for 1 h and allowing the fibers to come to constant weight to determine if these structure changes were permanent. The data from this experiment are summa- rized in Table III and show that heat drying produces only temporary changes in the bonding of moisture in hair in all cases except at 110øC, i.e. changes are reversed by shampooing. Apparently heating to 110øC or above produces irreversible changes in