1. Soc. Cosmet. Chem., 22, 339-348 (May 27, 1971) Chemical Aspects Human Hair of Bleaching CLARENCE ROBBINS, Ph.D.-* $ynopsis--A review of HAIR BLEACHING, which describes reactions of bleaching agents with hair proteins and with hair pigments, is presented. The major emphasis is on the CHEMICAL ASPECTS of bleaching with special attention given to the oxidative degradation of the DISULFIDE BONDS in hair. [NTRODUCTION During the past 20 years a great deal of progress has been made in determining the composition of amino acid residues (1-5) and hydrol- yzates (6, 7) of oxidized keratin fibers, and in the isolation and analysis of fragments formed from the chemical degradation of natural pigments (8-12). Although many aspects are still controversial, general features of the structures of hair and of hair pigments, and of the reactions occur- ring during bleaching, may be described in terms of chemical structures and reaction mechanisms. The objective of this paper is to review and to describe some of this work in the language of physical-organic chemistry. Thus, the following discussion contains a brief description of current bleaching compositions followed by a review of the chemistry concerned with the oxidative degradation of hair proteins and hair pigments. REAGENTS USED IN HAm BLEACHING Modern bleaching preparations contain hydrogen peroxide as the primary oxidizing agent with salts of persulfate added as "accelerators" (13). The pH of these systems is generally in the range of 9.0 to 11.0, and stabilizers (e.g., sequestrants) are generally added to minimize decom- * Colgate-Palmolive Research Center, 909 River Road, Piscataway, N. J. 08854. 339

340 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS position of the peroxide. Additional details concerning bleaching for- mulae may be found in the book edited by Sagarin (14). ThE REACTION OF BLEACHING AGENTS WITH THE PROTEINS OF HUMAN HAIR The primary purpose in the bleaching of human hair is to lighten the hair however, due to the severe reaction conditions required for destruc- tion of the pigment chromophore, side reactions with the hair proteins occur simultaneously. Zahn (6) demonstrated that the reaction of oxidiz- ing agents with the proteins of human hair occurs primarily at cystine. Robbins and Kelly (7) have shown that rather small amounts of degrada- tion occur to the amino acid residues of tyrosine, threonine, and methio- nine during severe bleaching however, in accord with Zahn, the main site of attack is at the disulfide bonds of the cystyl residues in the fibers. These authors have also shown that whereas 15 to 25% of the disulfide bonds in human hair are degraded during "normal" bleaching, as high as 45% of the cystine crosslinks may be broken during severe "in practice" bleaching. Although a description of the kinetics of the reaction of human hair with bleaching agents could not be found, there is evidence to suggest that at least the oxidative cleavage of the disulfide bonds is diffusion con- trolled. Harris and Brown (15) have shown, by reduction and methyla- tion of keratin fibers, that the wet tensile properties decrease almost linearly with the disulfide content. Alexander et al. (16) have arrived at this same conclusion after studying wool fiber oxidized with peracetic acid. A similar phenomenon has been observed, by this author, for hair that had been oxidized with alkaline hydrogen peroxide. These observa- tions lead to the conclusion that the percentage loss of various parameters of the wet tensile properties of hair, that occur during bleaching, e.g., the 20% index (17), is a measure of the per cent cleavage of cystine cross- links. Edman and Marti (18) have described the change in the 20% index of hair fibers as a function of treatment time in 6% hydrogen peroxide, at 32øC, using a 25:1 solution:hair ratio at pH 9.52. Their data, plotted in Fig. 1, versus the square root of the time provides a straight line from which an approximate diffusion coefficient may be calculated. An equa- tion developed by Crank (19) which describes diffusion from a stirred so- lution of limited volume into a cylinder of infinite length was used in this instance.