OIL FILMS AND MOISTURE ABSORPTION ON HAIR 141 equilibrium has not been reached. This is possible since we use the same condition (dm/dt = 0.00015 % for 20 min) for deciding the reaching of equilibrium for the un­ treated and the oil-treated hair. This will be discussed further in a subsequent section. We note that the coconut and sunflower oil-treated samples show comparable diffusion coefficients while that of the mineral oil-treated sample is much lower. The possible reason is that, because of penetration into the fiber, the vegetable oil films left on the surface are thinner, for the same amount of oil applied, compared to the film of mineral oil, which does not penetrate into the hair. Thicker films give greater resistance to diffusion than thinner films. EFFECT OF TEMPERATURE The coconut oil-treated hair was subjected to a low level of heating to see its effect on absorption of oil from the film into the fiber. As can be seen from Figure 5, the sorption-desorption isotherms for oil-treated hair before and after application of heat are essentially identical, indicating the negligible effect of heat on oil absorption and the subsequent sorption-desorption of water vapor. EFFECT OF THICK OIL FILMS The thicker-surface oil films were formed by treating the hair samples with larger 30 25 20 .5 15 C'G 10 5 I-+-- coc-heat -+- control -+-- coconut I untreated coconut coc & heat 0 �--....... --....... --"""T"""--"""T""--......------"""'T"----r----------. 0 10 20 30 40 50 RH(%) 60 70 80 90 100 Figure 5. Water vapor isotherms for untreated hair sample and coconut oil-treated hair samples before and after exposure to heat. Effect of heat seems to be negligible (isotherms are essentially coincident).

142 JOURNAL OF COSMETIC SCIENCE amounts of oils. The effect of coating thickness on sorption isotherms for mineral and coconut oil-treated samples is shown in Figure 6. For both of the oils, the additional increase in coating thickness further reduced the amount of moisture sorbed by the fibers. As was the case for the thin oil coatings, the isotherms for the excess amounts of coconut and mineral oil are close together, with slightly higher moisture regain occur­ ring in the coconut oil sample. This probably suggests that coconut oil film is thinner because of its ability to penetrate hair. Since mineral oil does not penetrate the hair, the film would be thicker however, the differences in the sorption behavior are smaller because the differences in the two films are very small because of comparable viscosities. The sorption diffusion coefficients for the coconut oil with two different coating thick­ nesses are plotted in Figure 7. With increasing coating thickness, the diffusion coeffi­ cients are decreased. The same trend appeared for the mineral oil sample (not shown here). While the sample with the thin coconut oil coating retains the general shape of the untreated plot, the sample with the thick coating has a flat sorption diffusion coefficient plot. EFFECT OF REMOVING THE OIL In order to determine if the reduction in moisture pickup and the slowing of moisture diffusion rates by the coconut oil are affected by the oil penetrated into the fiber, we performed the following experiment. As described in the Experimental section, the oil 30 -------------------------------------, 1---ccntrcl --¼-0.1 ml/g ccc ....._ 0.1 ml/g min -+-1.2 ml/g ccc -¾-1.2 ml/g min I 25 20 - � C 15 ·ca 10 5 0 10 20 30 40 50 60 70 80 90 100 Target RH (%) Figure 6. Water vapor sorption isotherms for hair samples with varying thicknesses of surface oil films. Samples with thicker films absorb less water vapor compared to thinner films. Mineral oil-treated hair consistently absorbs less water vapor compared to coconut oil-treated hair, in both cases.