The degree of crystal aggregation in organic pigments

Organic pigments are normally dispersed as fine crystals, which have a mark ed tendency to cluster into aggregates. Pigment crystal aggregation is tech nologically undesirable, for it hinders the attainment of high colour stren gth by reducing the effective level of subdivision of the crystals. Two met hods of assessing the degree of crystal aggregation are presented, both of which compare the BET surface area, determined from nitrogen adsorption iso therms, 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 propose d based on the fractal nature of the pigment crystal surfaces. (C) 1999 Els evier Science Ltd. All rights reserved.