Changing the Shape of Hair 174 the reaction (t 1/2 = 0.693/k). Table 2 shows half-lives for ATG and cysteamine measured by AA analysis and by SFTK. Table 2 Half-times for reduction reactions by either AA analysis or SFTK Treatment t 1/2 AA analysis (sec) t 1/2 SFTK (sec) 1 M ATG pH 9.4 210 93 1 M Cysteamine pH 7.6 394 210 1 M ATG pH 8.0 n.d. 242 1 M Cysteamine pH 8.0 n.d. 80 Comparison of ATG and cysteamine at pH levels that should produce equal concentrations of the active RS- species (9.4 and 7.6, respectively) indicated that thioglycolate reacts more readily with hair than cysteamine at equivalent RS- levels. However, at pH 8.0 reduction was faster with cysteamine because the pKa for the cysteamine thiol group is 8.6 versus 10.4 for the thioglycolate thiol group, so there is a higher concentration of the actives species present with cysteamine at pH 8. Wortman and Souren53 used chemical stress relaxation with intermittent strain pulses to try to correlate changes in tensile properties with the level of permanent set developed. After stress relaxation, strain pulses of 0.2% strain were applied. Reducing agent was added and chemical stress relaxation was followed until the decay curve leveled out. Then the hair was rinsed, treated with oxidizing agent, and rinsed again. A final extension was performed to measure the new length and final modulus of the hair. This procedure is illustrated in Figure 13. The relaxed tensile force does not increase significantly during the oxidation step, but the tensile modulus measured from the small strain pulses will increase as S-S bonds reform. The authors derived a formula (Eq. 8) for predicting recovery (the inverse of set) from parameters obtained from this experiment. Eq.8