Chapter 5 165 In step A, the cysteine disulfide bond in hair keratin is broken leading to the formation of a mixed keratin/mercaptan disulfide bond and one free keratin-S-. In step B, the mixed disulfide is cleaved, leading to the formation of the mercaptan disulfide and the second free keratin SH. The overall equilibrium constant, Ka, for the reactions in A and B above is: Eq. 3 There are two factors that mobilize the keratin structure so that that permanent set can be achieved. The first is, obviously, the breaking of the disulfide bonds. However, not all disulfide bonds are broken, and free keratin sulfhydril groups may contribute to stress relaxation through the process of sulfide-disulfide interchange, as illustrated in Figure 7.38,39 Feughelman reports that high levels of permanent set can be obtained at low levels of S-S bond reduction by using heat to activate sulfhydril-disulfide interchange.38 However, sulfhydril-disulfide interchange is a double-edged sword. Free –S- groups remaining after the neutralization step can cause loss of set. This is the reason Figure 6 Schematic of the chemistry of permanent waving. The hair is under stress when reduced. Protein flow occurs during waving. The new configuration is stabilized by oxidation to form new bonds. Not all reduced -SH groups form new bonds.