HAIR COLORING PRODUCTS

Available in a variety of colors and shades, hair-coloring vegetable extracts and powdered metals have been used since the earliest record of human history as cosmetic agents. While Greek women were known to dye their hair black, Queen Elizabeth I dyed her hair red. Henna, a vegetable dye with an active component called lawsone (2-hydroxy-1,4-naphthoquinone), has been used throughout the centuries to give hair a red-orange tint. In addition, weak solutions of hydrogen peroxide were and currently are used as hair bleaches. A French chemist formulated the first synthetic hair dye in 1907. It was this basic formulation that the modern technology of changing hair color, accomplished through the reactions of complex synthetic organic and organometallic chemicals, is based upon.
Hair contains two natural pigments located within the inner hair cortex. These pigments are melanin, a brown-black pigment, and pheomelanin, a red-brown or yellow-brown iron-based pigment chemically similar to melanin. Melanins are biosynthesized in the hair follicle by a series of enzymatic reactions with the amino acid tyrosine. The color of hair (e.g., light blond, brown, blue-black, auburn red) depends on the amount and physical conditions of these two pigments within the strands (with redheads possessing a high concentration of pheomelanin). The absence of any pigment leads to the appearance of white or gray hair. Pigment-containing granules are usually present within the hair central cortex but also may be present in the surrounding outer hair cuticle. Dark hair (compared with light or blond hair) has both more granules and more pigment per granule.
Darkening the natural color of the hair can be accomplished through the action of temporary dyes that act similarly to a hair conditioner. Most temporary chemical dyes contain the salts of large acidic molecules (e.g., FD&C Blue No. 1) that do not easily penetrate the hair outer cuticle. A colored film forms over each hair and washes off with the action of a single shampooing. Semipermanent dyes consist of small molecular alkali bases such as nitrobenzene, azo, and anthraquinone dyes (e.g., 2-amino-4-nitrophenol [yellow], 4 -hydroxy-2-nitrodiphenylamine [orange], 1,4-diaminoanthraquinone [violet], 1,4,5,8-tetraaminoanthraquinone [blue]) that, when applied to the hair, soften and swell the cuticle cells and allow the small synthetic pigment molecules to move between the hair shaft and the outer cuticle. Because these pigments do not enter the hair shaft, some of the pigment molecules wash out with each shampooing event.
Overall, the applied hair-darkening color gradually fades over time. Specialized hair treatments that are designed to darken graying hair gradually are often based on the action of metal atom incorporation into organic molecules. A solution containing colorless lead acetate [Pb(CH3COO)2] is applied to the hair and penetrates the hair shaft to form brown-black lead sulfide (PbS) upon reaction with sulfur atoms within the cystine amino acids of the hair protein keratin. Repeated applications result in a gradual darkening of the hair as the concentration of lead sulfide increases.

Converting dark hair to a lighter shade requires first bleaching (oxidizing) the natural pigments in the hair cortex to colorless products, usually with an oxidizing agent solution such as hydrogen peroxide (H2O2), frequently followed by application of a synthetic permanent hair dye. The addition of synthetic dyes without first bleaching the hair results in hair darkening. Hydrogen peroxide, especially when used with an alkalizer such as ammonia, causes the hair cuticle scale cells to swell and separate and softens the cortex to allow the pigment granules to enter the cortex through the cuticle and hair shaft. Oxidation also prevents the synthetic dye granules from migrating out of the cortex once they have entered. A bleach “booster,” such as peroxydisulfates (persulfates), is often added to the peroxide to increase its effectiveness.
Permanent hair dyes are applied as small colorless alkali molecules that, once they have entered into the hair cortex, react with the hydrogen peroxide in a redox reaction to form giant clusters of color that shampooing will not remove. The permanent hair dye pigments are commonly applied as two separate small mobile alkali organic intermediates after bleaching, a primary intermediate and a secondary intermediate (called the coupler). While the initially applied primary intermediate is oxidized by hydrogen peroxide within the cortex, the coupler combines with the product of this redox reaction to form an immobile permanent coloring effect.
The products of these reactions probably include quinones and nitro compounds. Thus, darker shades may be formed as newly introduced dyes add to the melanin color already present, and lighter shades may be formed as the melanin is bleached and dyes induce lighter color formation. As permanent color dyes only affect the cortex of the hair, newly grown hair will have natural color as it emerges from the scalp. Permanent hair dyes are often derivatives of an aromatic amine, discovered as a hair dye in 1883, called para-phenylenediamine (H2N-C6H4-NH2), including primary intermediates such as para-aminophenol, dihydroxybenzene, 4-methylaminophenol, tetraaminopyrimidine, 2-aminomethyl-4-aminophenol, and para-aminodiphenylamine.
The para-phenylenediamine compound produces a black color, with its derivative para-aminodiphenylaminesulfonic acid used in blond formulations. Couplers are generally aromatic phenols or amines that have an available site that is subject to electrophilic interaction. Typical couplers include chemicals such as resorcinol, meta-phenylenediamine, 2,4-diaminoanisole, meta-aminophenol, 1,5-dihydroxynaphthalene, 1-naphthol, and 4-amino-2-hydroxytoluene.
A variety of colors may be produced with the combination of many primary intermediates and couplers that undergo consecutive reactions to produce a mixture of dyes leading to the final shade. Most over-the-counter permanent hair-dying kits contain separate darkened containers for oxidizing and intermediate agents and include the addition of alkalizers (e.g., ammonia, ammonium hydroxide, monoethanolamine, triethanolamine), solvents (e.g., propylene glycol, water, ethanol, carbitol), stabilizers (e.g., phosphoric acid), antioxidants (e.g., ascorbic acid, sodium sulfite, BHT), metal ion sequestrants (e.g., EDTA), viscosity-enhancing chemicals (e.g., polyacrylic acid derivatives, nonoxynol-9 and -4, hydroxyethyl cellulose, oleyl diethanolamine), conditioners (e.g., polyquaternium-6, quaternium-40, distearyl dimethylammonium chloride), preservatives (e.g.,parabens), and fragrance.

By John Toedt, Darrell Koza, and Kathleen Van Cleef-Toedt in the book' CHEMICAL  COMPOSITION OF EVERYDAY PRODUCTS', Greenwood Press, Westport, Connecticut & London, 2006, p.34-36. Adapted and illustrated to be posted by Leopoldo Costa.


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