Changing the Shape of Hair 164 The cold wave process relies on two chemical reactions. First, disulfide bonds in the hair are broken by a reducing agent, while the hair is under stress, usually due to winding on curlers. Then the bonds are reformed by an oxidizing agent often referred to as a neutralizer. Figure 6 illustrates this. The hair proteins rearrange under stress during the reduction step and when the bonds are reformed during oxidation the rearrangement is stabilized leading to permanent set. In practice, less than half of the disulfide bonds in hair are broken during reduction.17 In permanent waving, reduction is performed using mercaptans of the general structure R-SH.18-23 The active species has been shown to be the thiolate anion R-S- so the reaction is highly sensitive to pH.24-32 Conversion of one mole of keratin cystine to two moles of reduced keratin cysteine requires two moles of mercaptan as illustrated by the following, with K standing for the keratin protein chain and R-S- for the mercaptan with the thiol group ionized: After the reduction step the hair is rinsed and then treated with an oxidizing agent, usually hydrogen peroxide to reform new disulfide bonds and stabilize the new configuration as illustrated in Figure 6.16,18,20,23,33 The neutralization reaction is: With thiol reducing agents, permanent set does not develop without a neutralization step.28 Either sodium or potassium bromate is occasionally used as for neutralization.17,34 The diagram in Figure 6 also illustrates the fact that not all the free –S- groups form new disulfide bonds. Some are left as free thiol groups, at least until they air oxidize and others are oxidized to cysteic acid by the neutralizer.16,17,22,35-37 This is the major source of hair damage by permanent wave treatment. See Chapter 11 of this text for more on hair damage.
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