COSMETICS AND BATHROOM PRODUCTS:PERMANENT WAVING OF HAIR

PERMANENT WAVING OF HAIR

The desire of individuals to improve the natural appearance of their hair can be dated back to ancient Egyptian and Greco-Roman societies. Such dissatisfaction continues to be the driving factor in the development of modern permanent waving by the cosmetics industry. The first permanent waving systems were pioneered in the early 1900s and included a nearly nine-hour procedure using permanent waving machines and various solutions. In the 1920s, permanent waving as a chemical method was invented. In the 1930s, scientists demonstrated that disulfide bonds that give proteins their spatial three-dimensional structure could be severed at ambient temperature and slightly alkaline pH with the action of sulfides or mercaptans, and by the 1940s, the use of thioglycolic acid as a “cold wave” treatment revolutionized the “perm” industry and provided a foundation for the chemistry of modern permanent waving methods.

Hair is composed primarily of the protein keratin. As a protein macro- molecule, keratin exists as multiple amino acid chains linked together by forces that include hydrogen bonds, salt bridges, disulfide linkages, and hydrophobic interactions. While hydrogen bonds between protein chains may be disrupted by water and salt bridges destroyed by alterations in pH, disulfide linkages may be broken and restored when hair is permanently waved or straightened. In keratin, the amino acid cystine dominates the polymer, and the sulfur-sulfur bonds of cystine residues provide hair strength by connecting parallel strands of protein.

Chemical modification may be achieved by wetting the hair alone, thereby protonating and breaking the hydrogen bonds that hold the hair proteins in place. Drying the hair while held in a straight or curlier shape than before wetting sets the new hairstyle temporarily by the presence of newly realigned polymeric chains. A permanent wave change is achieved by applying chemicals to the hair that break not only the hydrogen bonds but also the disulfide bonds. These disulfide bonds are then reconstructed when the hair is in curlers, allowing the hair to permanently take on the curled shape.

When the hair is placed in curling rollers, permanent wave lotions containing a mercaptan reducing agent (e.g., thioglycolic acid [HS- CH2COOH]) that breaks the disulfide bonds are applied. This agent ef- fectively transfers the hydrogen from its own sulfur to the cystine units and cleaves them into cysteines. Thioglycolic acid is usually applied in a suitable pH buffer (usually slightly basic), together with a hair cuticle softener. This process is usually accompanied by a slight smell of rotten eggs. The hair is then treated with a mild oxidizing agent (e.g., a dilute solution of hydrogen peroxide [H2O2]) that neutralizes the effect of thioglycolic acid and allows the covalent disulfide linkages to be re- formed between free cysteine bases to form cystine-cystine bonds. The newly reorganized strands are now in their new set “curled” positions to provide hair with a different shape than before the chemical treatment. These reorganized disulfide bonds remain intact whether the hair is sub- sequently wet or dry. However, since the oxidation process does not result in the reformation of all of the disulfide linkages cleaved during the reduction step, permanent chemical treatment may result in hair weakening and loss of tensile properties. Permanent wave curls naturally grow out as new hair is formed. Other types of “thio-free” permanent wave solutions contain active reducing agents such as ethanolamine sulfite, cysteamine HCl, ammonium thiolactate, or monoethanolamine thiolactate. An alkaline compound such as ammonium hydroxide usually neutralizes such thio-free wave solutions. Other types of neutralizing agents used in permanent wave formulations include perborates, ammonium persulfate, and sodium or potassium bromate.