METHOD FOR PERMANENT HAIR STRAIGHTENING 385 reformed in the subsequent oxidation step. Major chemical reactions occurring in each step of the present treatment are represented by the following equations, equations 3-10. Reduction step: KSSK + RSH k• KSH + KSSR KSSR + RSH k• KSH + RSSR (3) (4) where K is the keratin chain, KSSK is the cystine disulfide linkage in keratin, RSH is the reducing agent, KSH is the cysteine residue, and KSSR is the mixed disulfide. The reaction of equation 3 seems to predominate in this reducing system, since, as discussed above, about one half of the cystine disulfides is transformed into mixed disulfides. The occurrence of equation 4 may be dependent on the concentration of reducing agent in the proximity of the mixed disulfide group. In a solid-state reaction, the rate of the reaction of equation 4 can be expected to decrease as a result of the ionic repulsion between carboxylate ion fixed on the keratin chain and thioglycolate ions. Heat treatment step: KSSR + KSH -• KSSK + RSH (5) KSSR + RSH -• KSH + RSSR (6) Equation 5 is the reverse reaction of equation 3, and the reaction may proceed at a lower concentration of reducing agent (RSH), producing cystine linkages. However, the ana- lytical results suggest that no production of cystine residues is involved (the second column in Table II), and therefore, equation 5 may be ruled out. Equation 6, which is essentially the irreversible reaction of equation 4, is considered as the predominant reaction that leads to production of cysteine residues by the reaction of mixed disulfide with the reducing agent remaining in the fibers even after washing with water before heat treatment. In this respect, further study is needed to demonstrate the amount of reducing agent remaining in the fibers. It is important to note that to obtain a high yield of cystine linkages, the mixed disulfide groups must have been converted into cysteine residues before oxidation. Oxidation step: 2KSH --- KSSK (7) KSH --- KSO3H (8) KSSK --- 2KSO3H (9) KSSR --- KSO3H + RSO3H. (10) The reactions expressed by equations 7 and 8 predominate in the oxidation step, as clearly indicated by the previous discussion. Equations 9 and 10 are likely to be com- paratively minor reactions for the case of sodium bromate as a milder oxidizing agent (9). Table III shows the variation of cystine and cysteic acid contents at different heat treatment temperatures. The hair sample used in this experiment contains a higher amount of cystine than the sample shown in Table II. It is emphasized that an almost perfect recovery of cystine linkages is achieved through the oxidation reaction after heat treatment at the higher temperature of 220øC. On the contrary, the recovery level is considerably lower at the heat treatment condition of 180øC, although this makes possible a perfect reformation of disulfide linkages in the fiber with a much lower cystine

386 JOURNAL OF COSMETIC SCIENCE Table III CyS and CySO3H Contents of the Heat-Treated Samples at Different Temperatures Using TGA-Only System Heat treatment 1/2CyS + temperature CyS CySO3H CySO3H Samples (øC) (p mol/g) (p mol/g) (p mol/g) Untreated -- 464 24 952 Reduced and oxidized • -- 143 74 359 Reduced, heat-treated, and oxidized 2 180 359 87 805 Reduced, heat-treated, and oxidized 2 220 458 74 990 Reduction: 7% TGA, pH 9.20 45øC, 15 min. Oxidation: 7% NaBrO3 35øC, 15 min. Reduction and oxidation conditions are the same as described above. content such as the hair sample shown in Table II. These results suggest that the mobility of the keratin chain is an important factor for the reformation of cystine links at the final oxidation step. The chain mobility will be affected by either the water content or the cross-link density of hair. Under constant water content as in the present case, the chain mobility is associated with the latter. The higher the cross-link density, the shorter the chain length between the disulfide cross-links and the lower the number of chain segments. For hair with a higher cystine content such as the present sample, the chain mobility would be expected to be lower, and as a result, the reformation of cystine links would be suppressed during the oxidation treatment performed at 35øC for 15 min. The fact that almost perfect recovery of cystine linkages was observed at the heat treatment temperature of 220øC suggests the occurrence of the structure change affect- ing the chain mobility of hair. With respect to the network structure change during heat treatment, interchange reactions may occur more readily at higher heat temperatures, as represented by equation 11: KS•S2K + KS3H • KS•S3K + KS2H (11) It can be presumed that during heat treatment at the higher temperature of 220øC, the cross-link density in the hair decreases for the transformation of intermolecular cross- links into intramolecular linkages through interchange reactions, facilitated by heat treatment that gives rise to a high degree of lateral swelling for the keratin fiber (10). This makes possible the reformation of disulfide bonds at a higher yield, since there is an increase in the number of collisions between the cysteine groups on highly mobile keratin chains in the lower cross-linked network. The mobility of keratin chains may also be enhanced by the main chain scission resulting from hydrolytic reaction of peptide bonds at higher heat-treatment temperatures. Further study is needed to characterize such degradation of keratin molecules, which is closely related to hair damage. Analytical results of amino acids from the sample obtained by using the bicomponent system with various concentrations of TGA and DTDG show that no substantial change occurs in the content of the amino acids other than cystine and cysteic acid (Table I). These are similar to the previous results obtained from the TGA system without DTDG.