Semipermanent hair dyes are designed for use on natural, unbleached hair to cover gray, add highlights, or rid hair of unwanted tones. Semipermanent dyes are longer-lasting than temporary dyes since they are retained in the hair shaft by weak polar and van der Waals attractive forces. A typical semipermanent dye will last through 6–10 shampooings. Usually, 10 to 12 dyes are mixed to obtain the desired shade, which must be darker than the natural hair color. Thus, in the cosmetic industry, semipermanent dyes are known as suitable for staying “on shade.” There are several different types of semipermanent hair dyes based on the derivation of the dye: textile dyes, vegetable dyes, and synthetic hair stains.
Semipermanent hair colorings derived from textile dyes are popular with both men and women. Since human hair is basically a textile, dyes for wool and natural fi ber cloths are well suited for adaptation to hair dyeing. The dyes used include the nitroanilines, nitrophenylenediamines, nitroaminophenols, azos, and anthraquinones. Sometimes these dyes are combined with henna, botanically known as Lawsonia alba, to create a “natural” vegetable dye. However, most of the currently marketed vegetable dyes use a small amount of synthetic henna, combined with traditional semipermanent dyes, to achieve the desired hair color. These dyes are commonly available as shampoos and mousses that are applied to wet, freshly shampooed hair and rinsed in 20 to 40 minutes.
A newer, longer-lasting form of the semipermanent dye, known as a demipermanent hair coloring, usually lasts through 10–12 shampooings. This is due to enhanced dye penetration into the hair shafts facilitated by the addition of small amounts of ammonia. As might be expected, demipermanent dyes are more damaging to the hair shafts than semipermanent dyes.
