The aim of this section is to look at the changes induced in the hair fiber by coloring processes and not the detailed chemistry of hair coloring. However, a brief overview gives a greater understanding of how and why the fundamental structure of the fiber is affected. Permanent hair colors result in a change to the natural color of hair that, although subject to fading, will only be completely lost when the hair is cut or re-colored. This class of products also includes products designed to lighten, or bleach, the natural color of hair. Permanent colorants typically contain three components: two for coloring and one for conditioning. To change the natural color of hair the following steps must occur: (i) remove or lighten the natural hair color, i.e., with bleach melanin, and (ii) form new colors (dye couples) within the hair cortex. To achieve the color, low pH hydrogen peroxide (developer) is mixed with high pH dyes (tint). The activated or alkaline hydrogen peroxide both bleaches melanin in the cortex and develops the new colors from the tint. In particular, blonde shades are not simply achieved by bleaching melanin and they require the formation of new dye colors to offset red and brass tones left by the incomplete degradation of melanin.
As alkaline hydrogen peroxide diffuses through the fiber it encounters transition metal ions that occur naturally in hair, as well as those acquired from the environment, e.g., copper absorbed from tap water. Rapid degradation of peroxide forms the highly damaging and nonspecific hydroxyl radical.
A fundamental challenge to the colorant formulator is accessing and decolorizing melanin that occurs only in the cortex while minimizing damage to the rest of the fiber. This presents two problems. First, alkaline peroxide must travel through the cuticle and will cause damage en route. Second, the path of the peroxide is not specific to melanin. Due to the relatively low concentration of melanin in Caucasian hair there is a greater chance that the peroxide will interact with the hair structure and damage the proteins in the cortex than that it will interact with melanin. As a result, permanent hair colors cause measurable damage to the tensile and torsional properties of the hair fi ber. Of greater importance is the effect on the hair surface. All unmodified cuticle cells are covered in a covalently bound fatty acid on the outer aspect of the cell. The fatty acid 18-methyl eicosanoic acid, termed the f-layer (4), is readily cleaved by perhydrolysis, which changes the surface of the cell from hydrophobic to hydrophilic. The consequence of this change is two-fold. First, when the hair is wet the hair fibers are held tightly together by films of water. This makes the hair difficult to detangle. Second, many conventional silicone-based conditioners, which are typically hydrophobic in nature, fail to deposit on the hair and as a result provide little or no protection.
Fortunately, the level of damage is relatively low and can be managed as is evidenced by the numerous examples of individuals with long hair who have used colorants multiple times. But it is essential for patients to understand that the hair is changed by the coloring process and, while its appearance is enhanced its properties are altered. Failure of patients to increase conditioning, which will counteract these changes, will lead to rapid weathering and breakage and is often typified by trichorrhexis nodosa. The patient must be encouraged to reduce the frequency of coloring and the amount of styling, while greatly increasing the amount of conditioning.
