Two main practices are involved in permanently changing the shape of individual fibers, i.e., permanent waves and relaxers. While different chemistry is used by these two processes, both have a similar clinical impact on the fiber.
Permanent waves, generally used to increase curls, are based on alkaline ammonium thyoglycollate. This reduces disulphide bonds in the cuticle and cortex and allows hydrogen peroxide to reform bonds in their new position. As covalent bonds adopt new positions, the extensive network of salt bridges and hydrogen bonds do so as well. Although the process and formulations are quite different, it should be remembered that thyoglycollates are also the bases for effective depilatories. When used for hair removal the reductive step is left to progress further and is not neutralized by hydrogen peroxide. There are a number of recorded cases of severe hair breakage following permanent waves, no doubt caused by poor control of the reductive step.
Straightening or relaxing techniques are designed to remove curls. These processes use high-pH sodium or guanidine hydroxide. Those that use the latter are termed no-lye relaxers. Most relaxers are used to straighten curly hair, in which case the term relaxer is something of a misnomer. It suggests the hair is in perhaps a “less stressed” state than before! In order to straighten the hair it must also be pulled straight to form its new shape, so straight hair is not a relaxed state of a curl. Relaxers or straighteners require additional tension to pull the fiber straight on already weak hair, and may also involve the use of hot irons resulting in hair that is particularly weak but far from relaxed. One should remember that high concentrations of sodium hydroxide are a useful tool for dissolving hair for analytical tests. Relaxers left on for too long can certainly cause widespread hair breakage close to the scalp as well as extensive scalp irritation.
Both permanent waves and relaxers remove the f-layer from the fiber and damage intercellular cements. In addition, changes in the extractable proteins and amino acid profiles are always evident. These combined effects result in fibers that are hydrophilic, of reduced tensile and torsional strength, are prone to tangling, and show an increased rate of weathering.
