Modified acid dyestuff

Abstract

The present disclosure is concerned with new modifications of the dyestuff Color Index Acid Blue 324 which have significant advantages over the known form of this dyestuff. In particular, the preferred modification is much easier to isolate from an aqueous medium by filtration and the isolated product has a higher purity. The filtration proceeds much more rapidly and gives a filter residue of a much higher solids content. Furthermore, this new form of the dyestuff imparts a dramatically lower viscosity to aqueous dispersions or slurries prepared from it than are obtained using the old form of this dyestuff. These modifications are obtained by heating an aqueous dispersion or slurry of the known form of Color Index Acid Blue 324 to temperatures in excess of about 97.degree. C. and for the preferred form in the presence of a substance which reduces the surface tension between the dispersed organic dyestuff phase and the aqueous dispersing medium. The obtained dyestuff modifications display Debye-Scherrer X-ray diffraction patterns substantially different from that of the commercially available Color Index Acid Blue 324 evidencing that new crystal forms have been obtained. This is confirmed by optical microscopy which shows that the dyestuff undergoes a transformation from sharp, thin needles to either parallelograms or hexagons.