356 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Untreated I h H202 4 h H202 Bleach creme, 30 min Figure 5. SEMs of bleached hair, showing cuticle damage, hole formation, and abrasion effects. introduction of highly hydrophilic sulfonic acid groups lead to an increase in swelling, and with it to higher transport rates. One way to characterize the effect of bleaching on hair structure is, therefore, to determine the change in diffusion of dyes into the hair fiber cross section. Diffusion studies were carried out using the strongly fluorescent molecule uranin, the sodium salt of fluorescein (CI Acid Yellow 73) shown in Figure 8. The following treatment conditions were evaluated: one hour and four hours in 6% H202 and 30 minutes with bleach creme. Figure 9 shows micrographs of the cross sections of the dyed fibers and the corresponding cross-sectional scans that were used for the calculation of diffusion coefficients. The ring dyeing of the cuticular region appears to be due to the rapid penetration of the dye solution into the intercellular ,regions or the endocuticular domains. Dye concen- tration profiles (which are not shown) in the cortex in the early stages of diffusion seem to indicate Fickian kinetics, as shown by the solutions of Fick's second equation for radial diffusion in a cylindrical geometry. A typical solution is given in equation 3 (11). The diffusion coefficients are calculated using the Bessel functions. Concentration- dependent coefficients will be obtained by this procedure however, in this case, the concentration dependence of the diffusion coefficients seems to be negligible.

EVALUATION OF HAIR DAMAGE 357 (a) (b) To ba lance To balance Fw Oa Positive Negative wetting force Figure 6. Fiber-liquid interactions, showing positive and negative wetting forces. (3) Diffusion coefficients calculated at 0.1 intervals of normalized fluorescence intensity along the dye concentration profile are given in Table III for those categories for which diffusion gradients could be obtained under the chosen dyeing conditions. As seen in Figure 9, uranin diffusion into the hair after oxidation with the bleach creme is so fast that after 5.5 hours at 50øC, the fiber is completely penetrated, and the diffusion coefficient had to be determined after shorter dyeing times. The overall effect of bleaching on the structure of the hair fiber is quite significant, as indicated by the change in diffusion coefficient. After one hour of treatment with 6% H202, diffusion rates into the cortex have already doubled, and extending the oxidation time to four hours produces only a marginal increase in diffusion rates, suggesting that most of the modification had occurred within one hour or less. The increase in the rate of dye diffusion could be due to a breakdown of the barrier function of the cuticle layers, or to a change in the structure of the cortex by decreasing the crosslink density in the