896 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table IX Mechanical Properties of Oxidized Fibers Yield Breaking Stress, g/den Stress, g/den Time, Fiber Treatment Min Dry Wet Dry Wet Breaking Extension, Dry Wet Untrcated control 3% H20•, pH 10, 35øC 3% H•O•, pH 10, 35øC 3% H•O•, pH 10, 35øC 0.5% CHaCOOOH, 25øC (then 0.1M NH4OH, 30 •nin) ... 1.10 0.41 1.92 1.68 40.1 51.4 30 1.08 0.36 1.84 1.61 41.6 56.5 90 1.07 0.28 1.79 1.05 44.2 58.1 180 1.07 0.20 1.43 0.75 42.7 57.6 10 1.08 0.36 1.86 1.58 41.0 53.5 20 1.02 0.30 1.78 1.30 43.5 56.5 30 1.02 0.27 1.74 1.20 41.8 58.2 60 0.99 0.20 1.72 1.10 46.1 59.1 Mechanical Properties of Bleached Hair The disulfide bonds contribute greatly to the wet strength of keratin fibers, which decreases almost linearly with the cystine content. On the other hand, the strength of the dry fibers is not appreciably affected by the breakdown of covalent cross links, being dependent largely on the main chain length and interchain hydrogen bonding. When viewed from this standpoint, the changes in mechanical proper- ties of hair keratin brought about by bleaching can be satisfactorily in- terpreted in terms of the oxidative attack on the disulfide bonds alone. Thus, we observe (Table IX) a steady decrease in wet modulus with in- creased time of bleaching and virtually no change in either the modulus or ultimate strength of dry fibers. The latter observation supports the view that the extent of the main chain scission during the bleaching process is negligible it was shown by Elod (32) that the breakdown of 1% of the peptide bonds in keratin brings about an almost 20% loss in the dry strength of the fiber. The dry breaking extension is slightly affected by bleaching. Cer- tainly, no evidence of the brittleness often referred to is found. This is not changed by varying the rate of straining from 1 to 50 in./min. The apparent retention of dry strength by the bleached fibers does not result from a decrease in regain. On the contrary, the regain of the fiber increases with th? extent of bleaching (Table X) over a wide range of tested humidities. Bearing in mind the charge rearrangement in- volving the cysteic and carboxylic acid residues, it would appear that this increase in regain may be directly related to the ionization of the

HAIR BLEACHING 897 Table X Effect of Bleaching on Moisture Absorption by Hair Regain (%) at Relative Humidity of: Sample 17% 41% 61% 72% 84% 94% Intact 5.4 8.7 13.5 15.9 19.2 23.9 Bleached 30 rain 5.6 9.3 13.9 16.1 19.5 25.0 Bleached 120 rain 6.0 9.8 14.4 16.6 20.1 25.9 Bleached 240 rain 6.5 10.1 14.8 17.5 20.5 28.7 carboxylic groups and by depressing such an ionization a return to re- gain values close to those of intact fiber could be attained. This indeed is the case. When bleached hair is soaked briefly in acid, then rinsed in deionized water to remove any bound acid and its regain is redeter- mined, the value obtained is almost identical with that of untreated hair. The wet mechanical properties were measured with fibers immersed in pH 7 buffer (Fig. 6). However, unlike the untreated hair, the hydra- tion of bleached hair is strongly pH dependent and thus should mani- fest itself accordingly in the mechanical performance of wet fibers. Let 1.0 '•]•C"•'• • •'• reduced 0.9 0.8 • oxidized 0.7 _: o.6 F-- 0.5 I 0.4 0.3 • I I I I I I I 0.0 2 3 4 5 6 7 8 pH Figure 6. ElFoct of pH on yield index of oxidized and reduced hair