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

358 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS W (mNIm) '130 11o 90 70 50 ...... Bleach Untr 1/2h 1 h 4h 2'X10 creme Treatment Figure 7. Comparison of average water wettabilities for bleached hair. NaO-• c•O• • 0 COONa Uranin Na salt of fluorescein Figure 8. Molecular structure of uranin used in dye diffusion studies. matrix and increasing fiber swelling, or most likely to a combination of these two effects. Amino acid analysis. Cysteic acid is the major product of cystine oxidation during the bleaching of hair, and so changes in cysteic acid content have been widely studied and reported in the literature. Usually, unbleached hair contains 25-40 Ixmoles/g of cysteic acid. When the hair is bleached, the cysteic acid content increases to 200-400 Ixmoles/ g, and in severely bleached hair, up to 650 Ixmoles/g have been reported (12). A decrease of around 20% in tyrosine and methionine and of 10-15% in lysine and histidine has been reported by Robbins and Kelly (2). The results of the amino acid analysis of the untreated and bleached hair reported in