Chapter 5 181 The other is to use alkali “relaxers” to relax the curl. Reductive (permanent waving) chemistry is not as effective for straightening hair as it is for curling hair. If we want to get a curl of a certain diameter and the best we can do is ~ 50% set, all we have to do is wind the curl tighter, at half the diameter of the desired final curl, and we can achieve the desired result. This approach is not available for straightening hair. We can try to hold the hair perfectly straight during treatment, but we can’t go beyond that. Standard thioglycolate straighteners are good for taking the wave out of wavy hair but will not work to straighten really tightly curled or “kinky” hair. Alkali based relaxers are the products primarily used to straighten African hair types, commonly known as “Afro” hair. Thioglycolate hair straighteners are very similar in their formulation and mechanism of action to the permanent waves discussed above. The only significant difference in formulation is that they are usually in the form of thick creams to help hold the hair straight during treatment.72 Neutralizers may use either hydrogen peroxide or sodium bromated, as with permanent waves. Alkali based hair relaxers come in two varieties, those based on sodium hydroxide (NaOH) and the so-called “no lye” relaxers. Relaxers based on NaOH use concentrations in the range of 2–3% depending on whether they are base or “no base.” The term “no base relaxer” may be confusing to a chemist since all relaxers are highly basic. No base relaxers have lower NaOH concentrations and thus do not require that a protective “base” (usually containing petrolatum) be applied to the scalp prior to relaxer treatment. No lye relaxers have two components, a relaxer cream containing calcium hydroxide and an “activator” containing guanidine carbonate. The two are mixed right before treatment to produce the active ingredient guanidine hydroxide. (For a detailed review of various hair relaxers, which is outside the scope of the present writing, Ali Seyd gives example formulations for thioglycolate, lye and no lye relaxers.72 Mechanisms of hair relaxers: Relatively little work on the mechanism of hair relaxers is available in the literature.