494 JOURNAL OF COSMETIC SCIENCE WHAT'S NEW IN HAIR COLORING Keith Brown, Ph.D. Permanent hair dyes color hair by the formation of dyes within the hair cortex from colorless dye precursors. This chemical reaction is initiated by oxidation and results in the combining of at least two single ring aromatics to produce multi-ring dyes. The smaller precursors can readily penetrate the hair but, once formed, the larger dye is essentially locked inside the hair and is therefore stable to removal by water or shampoo. During the last 50 years, the detailed chemistry of these complex reactions has been studied and it is now generally well understood and published. Reactions occur in three consecutive steps. Initial oxidation of suitably substituted, readily oxidized primary intermediates gives electron deficient imino compounds which then react rapidly with the electron rich couplers to form leuco dye intermediates. The third step is oxidation of the leuco dye to the final dye. This chemistry produces a dye containing two precursor units (one from the primary intermediate and one from the coupler). In some cases reactions involving addition of further precursor units to the dye can occur and results in dye consisting of at least three precursor units. These two dye types have different applications in hair dyeing, but both chemistries are essential for successful shade development. In practice, formulations contain several primary intermediates and couplers, and the final shades results from a series of reactions where couplers compete for the various imines giving a mixture of dyes which contribute to the desired shade. In addition, much is now known of the relationship between dye structural characteristics and the quality of the color, as well as the specific and preferred applications for the various multi-ring dyes. Although these dyes are essentially stable to removal by shampoo, they may undergo color changes or fading due to chemical reactions occurring within the hair caused by various external factors such as exposure to weather or additional cosmetic treatments. Studies have established the chemical reactions that cause such changes and it appears that pH is the critical factor in most fading reactions. Application of this chemical understanding has now allowed for the development of new dye precursors with specific and targeted properties involving both color and performance improvements. In particular dyes with brighter and more distinct colors have been developed from novel primary intermediates and couplers, and significant improvements in dye stability towards shampoo and perspiration have been achieved by manipulating structural features of both primary intermediates and couplers. All of these developments have appeared in commercial products in recent years.

2004 ANNUAL SCIENTIFIC SEMINAR 495 Permanent dye products, as described above, are by far the most popular type of hair colorant in the US, but there is still a significant market for less permanent dyes based on direct (intrinsically colored) dyes. These products, usually termed semipermanents, do not need chemistry to develop color, but rely on transfer of color from the application medium to hair by diffusion. Since no chemical changes are involved, the process is reversible, and color is relatively easily removed by shampoo. An understanding of the relationship between dye structure and color, and its effects on dye properties has been critical in developing dyes with improved coloring and stability towards shampoo removal for these products.