160 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS already dyed portion of hair. To minimize this effect, a dye intermediate can be used that will partially reduce and solubilize the old dye so that both new and old dye will be free to redistribute evenly throughout the fiber. Dyes for human hair must not easily rub off on linens or dissolve in a sham- poo or even change color when exposed to other cosmetics. Of course, temporary color rinses, on the other hand, are expected to rinse out, but still should not stain clothing. This requires that only those dyes be used that can be firmly locked into the fiber. In cases where the dye was too intense or not quite the color the woman wanted, or where the woman has decided to make a radical change in hair color, the dye must be strippable (or removable), preferably by a mild solubilization, usually a reduction, and rinsing out. Finally, we add to all these restrictions that the dye selected must be natural and appealing in color. Needless to say, compromises must be made to find dyes that even reasonably approach all these restrictions. It is interesting to see, now, how these many restrictions have reduced the choice of dyes from the thousands available in the textile industry to just a few suitable for use on human hair. It is convenient to consider the available dyes by the type of reaction they undergo with human hair. Covalent bonding produces the most irreversible chemical binding. Dyes containing one or two active chlorides in a cyanuryl ring, for example, will split out HC1 with active hydrogen in keratin. Under proper conditions, vinyl dyes also will react with similar sites to form covalent bonds. How- ever, because these dyes cannot be removed except by severe oxidation of the hair, they are eliminated for use on heads, for removal is mandatory if a woman wants to drastically lighten or alter the color developed in her hair. Ionic bonding is commonly employed in the textile art. Basic dyes form rather firm salt linkages with carboxylic groups of hair, and, being positively charged, will penetrate human hair at reasonable rates since the hair is oppositely charged, but unfortunately, the basic dyes tend to stain the scalp. Acid dyes, on the other hand, are bound rather well to keratin, also by salt linkages, yet are less prone to stain the skin. Large textile acid dyes, which can be put into a swollen fiber at high temperatures, are quite well locked in at room temperature, but the smaller acids that will penetrate hair at body temperature are not bonded strongly enough to be classified truly permanent nevertheless, they can be used as temporary color rinses or hair tints. There are other dyes or pigments important to both the textile and the hair color field which can be classified under the general category of ionic bonding. Most metals, for example, are firmly bound to keratin, including human hair, partially by ionic bonds and partially by coordination bonds, and many dyes which do not in themselves firmly bind to hair will strongly chelate to these metals. This constitutes

BASIC ELEMENTS OF DYEING HUMAN HAIR 161 a class of mordant dye which has been set aside in cosmetics because many of the metals tend to form off-shades when they come in contact with other cosmetic chemicals, such as waving lotions or other dyes. With mordant dyes, natural hair shades are difficult to produce, and stripping also is difficult. Another special class of bonding, which is partly ionic and partly covalent, is the formation of heavy metal sulfides. Adsorbed lead, bis- muth, mercury and other metals all tend to form sulfides slowly as hydrogen sulfide is released by the keratin fiber. Mercaptides of cysteine also slowly form by cleavage of cystine. The action is accelerated by the presence of moisture and sunlight. Lead and silver are probably the most commonly used metals. The great advantage of dyeing with metallic salts is that the color develops slowly over a period of weeks, suggesting a natural restoration of color, but metallic salts, again, tend to form off-shades with other cosmetics and they are nearly impossible to strip off. Another type of binding to keratin substrate is through hydrogen bonds, van der Waals forces, or, generally, secondary bonds. All classes of dyes are adsorbed on their substrate partially through secondary bonds. In the textile field this type of bonding can become exclusively sufficient, especially with large molecular weight dyes. To dye synthetic ester fibels in an organic medium, dyes are chosen that have a very low solubility in water so that the dye, once in the fiber, can be removed with water only with great difficulty. In this case the results can be classified as a solid solution. On human hair, however, it is difficult to bind dyes with sec- ondary bonds alone because the secondary bond is seldom strong enough in itself to lock in the dye, and the molecule which is large enough to be per- manently bound diffuses too slowly at 25-35øC. to penetrate human hair in a reasonable length of time. Another major classification of dyes could be intitled "Insolubilized Dyes." They are classified as such because they are locked into the fiber by precipitation. Secondary bonding and ionic bonding enter into the partitioning of the dye between solvent and fiber, but these bonds are not strong enough to accomplish thorough fixing by themselves. This class includes many dyes, among which are the most popular dyes now being used for human hair. These dyes are commonly referred to as oxidation dyes because the color and fixation are brought about by oxidation. A small water-soluble molecule like phenylenediamine or analogue thereof is diffused into the hair with modifiers such as aminophenols and catechols, which couple with oxidation into large insoluble chromophoric molecules that are incapable of desorbing from the fiber. It is speculated that oxidation dyes also may form some reversible covalent bonds with the amino groups in the hair. Since the dye molecule grows in size after it penetrates the fiber, intermediates can be quite small so that they penetrate very rapidly at body temperature, and experience has shown that they give a minimal