The use of sulfonic acid functional pararosaniline compounds, as blue-hued pigments, is known in the art. Representative examples of the use of pigments are given in U.S. Pat. Nos. 3,582,381; 3,635,745 and 3,652,602, as well as other patents and publications.
The use of heteropoly acids as a reactant in combination with a basic dye in the form of its water-soluble chloride or sulfate salt is also known. By prior art processes, fugitive basic dyes such as Methyl Violet and Rhodamine in the form of their water-soluble chloride or sulfate salts are dissolved in water and reacted with a heteropoly acid aqueous solution to produce salts in high yield. These salts are valuable as pigments because of considerably increased lightfastness, very low solubility in water and organic solvents, good brightness and good color intensity. This process, however, cannot be applied to the water-insoluble aryl pararosanilines of this invention because these compounds and their salts are substantially insoluble in water. Alternative methods, such as dissolving the aryl pararosanilines in a common organic solvent, e.g. ethanol, acetic acid, dimethylformamide, dimethylsulfoxide, etc. before adding the heteropoly acid yield a relatively low degree of conversion to the heteropoly acid salt and as such are not commercially feasible. The new pigments and processes described herein overcome these problems by dissolving the pararosaniline compound in a special solvent, in particular a strong acid or a blend of strong acids, to improve its water solubility or dispersibility to yield a high degree of conversion to the heteropoly acid salt. The strong acids will typically have a pK.sub.a of less than about 3.0.
The pigments produced by this invention provide another advantage over the prior art in that they provide an excellent match for Indigo dye as used in the blue denim market. Blue products known in the prior art such as Phthalocyanine Blues and Dianisidine Blues, are unsatisfactory as a replacement for Indigo dye for the blue denim market. That is because the prior art blue products fail to produce satisfactory shade matches for the various standard Indigo blue colors and, also importantly, the prior art blue products will not wash and bleach out on repeated washings similarly to Indigo to meet the qualities demanded in the "blue jean" market. Within the scope of this invention, however, there are several possible ways to produce the proper shade of blue and still provide the correct magnitude of fading upon repeated washings. At least one way this has been accomplished within this invention involves replacing a portion of the water-insoluble aryl pararosaniline compound with a minor quantity of Methyl Violet basic dye, blending these dyes with a Phthalocyanine Blue and solubilizing them in a strong acid solvent prior to reaction with a heteropoly acid and thereafter co-precipitating the components as water-insoluble pigments. Optionally, darker shades of indigo can be matched by co-precipitating minor amounts of a water-insoluble black pigment together with the other pigments.