The degree of crystal aggregation in organic pigments

Organic pigments are normally dispersed as fine crystals, which have a marked tendency to cluster into aggregates. Pigment crystal aggregation is technologically undesirable, for it hinders the attainment of high colour strength by reducing the effective level of subdivision of the crystals. Two methods of assessing the degree of crystal aggregation are presented, both of which compare the BET surface area, determined from nitrogen adsorption isotherms, with the mean specific surface areas of the crystals. In the first method, the specific surface areas of the crystals are determined from the principal crystal dimensions. In the second method, a refinement is proposed based on the fractal nature of the pigment crystal surfaces.