PERMANENT WAVING AND PERM CHEMISTRY 125 been published on bond breaking, relatively little is written on this reformation process. It is again suggested that this step may be seen as somewhat trivial because bond reformation is recognized to occur to some extent by the so-called air oxidation, namely, this process ap- pears to begin during rinsing of the perm solution from the head, wherein the hair becomes saturated with large amounts of aerated water. Nonetheless, there is obviously the desire to perform this important function properly, and consequently, treatment with an oxidizing agent is prudent. Most often this is accomplished using hydrogen peroxide. j 2 2 2 2K - SH H O K - S-S- K 2H O (20) Wortmann and Souren (31) used a method somewhat analogous to the SFTK approach to observe the recovery in mechanical properties of reduced hair on rinsing with water and treatment with hydrogen peroxide. In performing this more thorough “neutralization” treatment, there is increased likeli- hood for a degree of “overoxidation” whereby cysteine is converted to the corresponding sulfonic acid (i.e., cysteic acid). In short, there is a reduction in the number of strength- supporting cystine disulfi de bonds and a concomitant decrease in tensile properties. This topic of hair damage resulting from perm treatments will be discussed shortly. REVERSION DUE TO SULFIDE–DISULFIDE BOND INTERCHANGE The permanency of permanent waves merits some discussion, that is, it is well-recognized and relatively commonplace for the induced curls to relax somewhat from the initial freshly permed state. The culprit for this occurrence is generally considered to be sulfi de– disulfi de bond interchange, through which a rearrangement of the newly formed internal bonds takes place to release stress within the S-S bond network. A schematic of this pro- cess is given in Figure 21. This occurrence is sometimes speculated as a reason why thiol-based relaxers are not es- pecially effective on Afro hair, namely, a more dramatic change in conformation results in additional internal stress and an increased tendency for this interchange. The creation of more permanent lanthionine bonds under highly basic conditions may therefore repre- sent more enduring chemistry for this challenging hair type. HAIR DAMAGE ASSOCIATED WITH THE PERMING PROCESS As documented throughout this article, the perm process involves the rather drastic process of breaking down a signifi cant portion of hair’s internal structure, followed by subsequent Figure 21. Schematic representation of sulfi de–disulfi de bond interchange.

JOURNAL OF COSMETIC SCIENCE 126 Figure 22. Schematic representation of a damage–effi cacy curve. reformation with fi bers anchored in a new conformation. However, perhaps not surpris- ingly, it is not possible to put everything back exactly the way it was. Specifi cally, there will be some depletion in disulfi de cystine content, a commensurate increase in cysteic acid, and a subsequent decrease in tensile properties. It has been shown how the aggres- siveness of the perm process can be altered by many formulation variables, for example, concentration of the reducing agent, oxidation potential of the reducing agent, pH of the formulation, the presence of the oxidized form of the reducing agent, and also possibly the ability for the active to diffuse into the hair. The capability to cleave and reform a higher number of disulfi de bonds would instinctively be expected to yield a stronger perm (at least perhaps up to some upper limit). However, the ability to effectively reform all these bonds would also seem to become more diffi cult as the extent of reaction pro- gresses, and the hair is broken down further. These ideas are realized in practice, where, for example, thioglycolate-based perms gener- ally yield tighter, true to rod-shaped curls than a cysteamine-based perm, but at the same time, the hair is left in a more compromised state. This leads to the concept of a damage– effi cacy trade-off curve whereby more aggressive, more effi cacious treatments and conditions are obtained at the expense of higher damage (see Figure 22). Manufacturers will often offer different variants under a given product line that provide levels of activity commen- surate with the end goals of consumers. In marketing such treatments, it is not uncom-