BASIC ELEMENTS OF DYEING HUMAN HAIR By D. L. U•D•P. WOOa* Presented September 15-16, 1960, Seminar, Chicago THE SUBJECT of hair coloring is one of increasing appeal to men as well as women. Many million dollars were spent on color products last year, and today even the barber schools are preparing for a large increase in demands by men for hair color treatments. To formulate dyes to meet this cosmetic demand may seem a simple ex- trapolation of the textile dyeing art because keratin, be it wool or human hair, is such a reactive chemical substrate that it can be dyed by practi- cally every class of dye known. In fact, there are thousands of dyes avail- able for coloring textile keratins, yet only a few can be used on human hair. This paper shows why the cosmetic chemist is so limited. The basic ele- ments of textile dyeing are outlined, the problems peculiar to dyeing of human hair are pointed out, then the type or reactions that dyes undergo with their substrate are listed, and finally, it is shown how the problems peculiar to human use have so greatly restricted the choice of cosmetic dyes. There are three basic elements of dyeing. The dye must diffuse from the body of the solution some finite distance into the fiber then, in cases where dye intermediates are used, the intermediate must be altered chem- ically to develop the end color and finally the dye must be bound to the fiber so that it does not rinse out. The chemistry of dye intermediates was treated extensively by Kass at this Seminar (5) and elsewhere (6) there- fore, attention is given here principally to diffusion and reaction of the dye in the fiber. When a typical commercial dyeing solution is applied to hair, or to any other fiber, the dye molecules begin to penetrate. The concentration of dye then decreases in the interface or solution immediately surrounding the fiber. When the bulk of fabric or limitations on stirring prevent direct re- placement of dye, the partially exhausted interface must be resupplied by relatively slow diffusion of dye molecules from the body of the dye solution to the interface. Obviously, dye must reach the interface rapidly if dyeing is to be achieved in a reasonable length of time. * The Toni Co., Chicago 54, I11. .4uthor's Note: The objective in delivering this paper was to provide at the Seminar an in- troductory background to the subject of hair coloring. More detailed recent review papers on the reactivity of hair keratin and the elements of hair coloring have appeared elsewhere (1-4). 155

156 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Many items must be accounted for in any complete treatment however, the more important factors in diffusion from solution to interface are these: 1. All other things being equal, the higher the concentration of dye in solution, the more dye will reach the fiber in a given time. Some dyes ac- tually are used in concentration above solubility in the form of a dispersion which dissolves and replenishes the bath as it is exhausted. Hydrophilic groups also are synthesized into the molecules to increase their solubility and hence their concentration at the interface. 2. The rate at which any particle moves in a solution is called that par- ticle's diffusion constant in that particular solution. The larger the parti- cle, the slower it diffuses. The size of dye particles frequently exceeds that suggested by molecular weight because aggregation of dye molecules into micelles is so common. Hydration also increases the effective size of the particle in solution. The addition of dispersing agents greatly increases diffusion rate by reducing the size of the dissolved entities. 3. Viscosity of the solution plays a simple part in diffusion rate. The more viscous the dye solution, the slower is the movement of the molecules therein. 4. Very important to the textile dyer is temperature. Diffusion through the solution depends upon the random motion of the solvent molecules which bombard the larger dye solute entities. The higher the temperature, the more rapid and energetic is the bombarding, and hence the more rapid is the diffusion of the dyes to the fiber. From the dye-solution interface, dye molecules must diffuse into the fiber itself. The pore size of wool or human hair usually is considered compa- rable to, or smaller than, the molecular size of the dyes used to color it. The energy in the fiber causes its network to vibrate, and occasionally random oscillations with the help of energy supplied by bombarding water molecules momentarily will open holes large enough for a dye molecule to penetrate. Without the optimizing of many factors, the process is im- practically slow. Some of the factors affecting diffusion within the fiber are these: 1. Molecular weight or particle size is very important The diffusion rate varies over many orders of magnitude with the range of molecular sizes employed for dyeing. Associated molecules which are much too large to penetrate sometimes can be broken down by a change in pH, solvent or dispersing agent. 2. Fibers tend to carry a net electrostatic charge, especially near ionizable groups. Keratin normally is negatively charged. Therefore, negatively charged dye ions tend to be repelled and penetrate slowly. The distance through which these fiber charges can effectively repel or restrict movement can be decreased by increasing the ionic strength of the dye solution. Some salts also may compete for sites of adsorption so that less