Indo dyes

Hair dyes must meet a number of conditions related to their end use. Color can be assessed by colorimetry [49], The limits of precision are set by the substrate on which the measurement is performed. Studies on test subjects are difficult because of the uneven natural hair color and the background color of the scalp. Tresses are hard to prepare at a constant quality level. Measurements on wool cloth give reproducible results, but for oxidation dyes the shades are not identical to those produced on hair. Colorimetric methods are therefore useful only for comparative measurements on the same object, for example, in lightfastness tests. Because hair must be redyed after four to six weeks due to growth, the fastness required of hair dyes is generally less than that needed for textiles. However, stability is still a problem with many indo dyes (see Section 5.4.3). Some of them... 

As a general rule, the higher the electron-donating capacity of the coupling agent (especially unsubstituted carbocyclic w-couplers), the higher the absorbance maximum of the indo-dye formed. In the presence of couplers, di-and monoimines react to produce indo-dyes. 

Oxidation PPD gives benzoquinone imines as a result of oxidation. The imines react rapidly with the couplers (another chemical material in the formulation) and/ or an oxidized PPD to produce indo dyes. The most frequent couplers are 2, 4-diaminoanisole (blue forming coupler), resorcinol (green brown), metami-nophenol (magenta/brown) and 1-naphtol (purple blue color). The most commonly used oxidant is hydrogen peroxide. Free ammonia is present to promote the oxidation reaction and the pH of the mixture on the dyed area is about 9.5 [3]. 

Table 6-3 summarizes a scheme for formation of oxidation dyes and provides some examples of the types of dyes that have been isolated from these reactions. This scheme shows that a dye precursor (e.g., p-phenylenediamine) is oxidized to its corresponding diiminium ion (IX), and this active intermediate condenses with an electron-rich dye coupler, forming a dinuclear product that is oxidized to an indo dye. This reaction may stop at the dinuclear dye stage, or additional condensation-oxidation... 

Table 6-3. Scheme and examples of indo dyes formed in oxidative hair dyeing.

The diimine (XXII) has been described as a vital intermediate in oxidative hair dyeing [23]. More recently, Corbett [3] has described the protonated diimine (IX) as the reactive species that actually attacks dye-coupUng agents, ultimately forming indo dyes. 

Therefore, although diimines may form in these interactions, they are not necessary intermediates for forming the di-, tri-, and polynuclear indo dyes that have been shown to form. For example, compound (XXIII), an... 

Mechanisms may also be written involving Wurster salts (XXV) to provide the di-, tri-, and polynuclear indo dyes formed in these reactions. Lee and Adams [28] have generated the Wurster salt of p-phenylenediamine by electrochemical oxidation in buffered media. Above pH 6, the radical stability decreases rapidly, indicating the low stability of these species under hair-dyeing conditions therefore, the diiminium ion is probably the active intermediate in actual hair dyeing. 

Dinuclear species is oxidized to a dinuclear indo dye. If the 4 position of the indo dye is blocked (bears a substituent other than hydrogen), the reaction tends to stop at this step. 

Dye precursor or another molecule of indo dye may add by 1,4 addition across the indo dyes, forming a trinuclear or polynuclear species (see Table 6-3). 

Oxidation of trinuclear or polynuclear species to higher indo dyes. Steps 4 and 5 may be repeated, forming higher polymeric dyes. 

Scheme 10.6 Pathway proposed for the formation of dinuclear, 10.24, and trinuclear indo dyes, 10.25, in the oxidative coupling involved in permanent hair dyeing. X, Y and Z may be independently O or NH.

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