Hair Anionic Dyes

The general wetfastness properties of acid dyes are normally enhanced in comparison to those of the disperse dyes due to the fact that the hair protein forms additional salt linkages with the anionic dyes. Moreover, the lightfastness is in most cases better than with disperse dyes. The application technique and killing pretreatments for acid dyes are similar to those for disperse dyes. 

If the metal complex dyes carry two sulfo groups, they are suitable for dyeing suede with hair reservation. For full penetration on suede, dyeing starts at a weakly alkaline pH of 8 (mostly adjusted with ammonia), and anionic dye-penetration agents such as naphthalenesulfonic acid-formaldehyde condensate. Finally, the dyes are fixed with an organic acid, preferably formic acid, at a pH of ca. 3.5. 

Anionic dyes are usually azo dyes and are employed only in special cases. Because the skin is more basic than the hair, these dyes are adsorbed preferentially on the skin, so that contact with the dyeing product leads to severe scalp staining. For this reason, the mainly anionic food colorings are not employed as hair dyes. Attempts to mitigate these disadvantages by using appropriate vehicles (see Seetion 5.4.4) have not been successful. 

Metal-complex dyes are of minor importance. They can be used as such or can be formed on the hair, although the metal salt treatment entails problems. Brilliant fashion colors (pink, green, etc.) are mosdy obtained with anionic dyes, which are often food colorants. Other methods for dye formation in situ [44] or with reactive dyes have not been accepted because of toxicological concerns. 

Similar isoelectric points for hair and wool fiber are to be expected because chemical compositions of the cuticle are similar and because both fibers show similar dye-staining characteristics. Cuticle from both fibers stains more readily with cationic dyes than with anionic dyes [96], whereas the cortex stains readily to anionic dyes [121]. 

Grit, M. Hair treatment agent containing quaternary ammonium compound and anionic dye or UV absorber. Ger. Offen. DE 19907260, 2000 Chem. Abstr. 2000,133, 182715. 

These facts play an important factor in the dyeing behavior. The isoelectric point of hair is at a pH of ca. 5, whereas untreated collagen has its isoeletric point at a pH of ca. 6, which varies with the kind of tannage. For pure chrome suede it is close to pH 7 or about pH 6 after slightly anionic retanning. Consequently, for good dye fixation the acidification at the end of a dyeing is around pH 3 for fur hair and ca. pH 4 for chrome-tanned suede. 

Hall [61] as far back as 1937 first proposed intercellular diffusion. It has recently been clearly demonstrated by Feeder et al. [62] for metal complex dyes. Feeder has shown that a large cationic dye, rhodamine B (479Da) triphenyl pyrazine, a neutral molecule (311 Da) and the high-molecular-weight anionic oligomeric Synthappret BAP (>3,000Da) all penetrate hair through the intercellular route. 

Figure 5-8. Light micrograph of a split hair stained with red 80 dye after shampooing with sodium lauryl sulfate. Dye does not stain the hair with anionic adsorbed.

Liquid water at room temperature can penetrate across the entire fiber in less than 15min and in less than 5min at 92°F [76], whereas more than 6h is required for single hbers to equilibrate in a humid atmosphere, and even longer for a hber assembly. Dyes like methylene blue (MW 320) and orange II (MW 350) generally require over an hour to penetrate through the cuticle layers to the cortex. Similar penetration times would be expected for typical anionic and cationic surfactants used in shampoos and hair conditioners. 

In neutral dyeing or surfactant-hair interactions, competition of cations with hydrogen ions must play a role. When the concentration of hydrogen ions is low and cations of low affinity are present, the adsorption of anion is influenced by the concentration and affinity of cations for hair. If the cation affinity is high enough so that it is adsorbed, a counterion must accompany it to maintain electrical neutrality. In the presence of low-affinity cations... 

Despite its convenience, today the dye method is seldom used for quantitative assessment of deposited polycation. It is suspected that one of the main reasons is that the presence of coadsorbed anionic surfactant (as would be the case in ordinary shampooing practice) could seriously affect the development of color. [This is a possible inference from the data registered for one such system (57).] However, it retains its usefulness as an indicator of polycation substantivity from simple aqueous solution (especially if coupled with hair fiber sectioning). 

Uses Permanent conditioning protein and bodying agent for cream rinses, shampoos, conditioners, perms, naii care prods. foam buiider/stabiiizer in shampoos, conditioners, styiing products, hair dyes, perms, faciai cieansers, and shower gels Features Retains a cationic nature even in high pH systems compat. with anionic, cationic and nonionic surfactants... 

Uses Surfactant, emulsifier, solubilizer in detergent formulations for dry cleaning, chlorated detergents and degreasing agents for automotive parts emulsifier for hair dyes, permanents and bleaches, lotion, deodorants solubilizers for bath oils Features Moderate foaming power reduces surface tension does not precipitate in the presence of ions in hard water maintains chemical, physical stability in alkaline or acid environments compat. with non-ionic, anionic, cationic surfactants... 

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