HUMAN HAIR MOISTURIZATION WITH COSMETIC PRODUCTS 313 diffusion coeffi cient, and t is time. For example, the time taken for a water molecule to reach the center of a 100-μm hair at 40% RH and 25°C (D = 4.5 × 10-9 cm2/s) will be 2,780 s (~0.75 h). This shows that the time required to attain equilibrium (2–3 h) in these experiments is quite adequate for the saturation of the sample. EFFECT OF CROSS-LINKING Table III shows the hysteresis data for the heat-treated and untreated hair from Figure 5. From Table III, we obtain an HR value of 1.44 (31.44/21.88) for heat-treated hair. This value is close to 1.45, based on actual area measurement. This shows that heat- treated hair is 44% better in retaining moisture compared with the untreated hair. Because hair fi ber undergoes cross-linking at these temperatures, diffusion rate is consid- erably reduced, locking-in the moisture. Although the fi ber is cross-linked, during sorption, the restraining effect on diffusion is less because of swelling. However, during desorption, because of cross-linking and fi ber shrinkage, diffusion rate is drastically reduced, thus retaining the sorbed water. Quantitatively, cross-linking with heat treatment alone, or in combination with multifunctional low molecular weight compounds capable of penetrating the hair, has a better effect of locking-in moisture than rinse-off conditioning actives. This can be seen clearly by comparing the HR values in Tables II and III. Figure 6. Sorption a nd desorption diffusion coeffi cients of heat-treated and untreated hair.

JOURNAL OF COSMETIC SCIENCE 314 PRODUCT EVALUATION Determining HR values for hair treated with a product with respect to the correspond- ing untreated hair can be useful in evaluating the moisturizing potential of the product. Although the earlier results with rinse-off conditioners were not favorable, it is possi- ble to formulate products which show signifi cant moisturizing capability. To demon- strate the possibility of using this approach, two different hair samples treated with two different products (compositions confi dential) along with the corresponding con- trol sample of hair were subjected to the DVS experiment as aforementioned at 25°C. Sorption–desorption isotherms of these samples are shown in Figures 7–9. Comparing the areas of the hysteresis loops, we can almost “see” the effects. Figure 7. Sorption–d e sorption isotherms of hair with product 1. Table III Hysteresis Data of Heat-Treated (120°C for 1 h) and Untreated Hair RH (%) Untreated Hi Heat-treated Hi 10 2.50 3.00 20 2.43 4.00 30 2.50 4.25 40 2.75 4.00 50 2.94 3.69 60 2.94 3.56 70 2.56 3.50 80 2.13 3.38 90 1.13 2.06 ¦Hi 21.88 31.44