Effect of Pigment Size and Shape Distribution on the Cohesion of Pigmented Coatings, 1998 Coating Conference Proceedings

The influence of the size, the size distribution and the shape of a pigment on the out-of-plane cohesive strength of pigmented coatings was investigated systematically. At equivalent mean size, a broader distribution gave higher strength. At a similar size distribution, a larger size gave a higher cohesive strength at a given latex content. And at equivalent size and size distribution, isometric Calcite gave a much higher strength than acicular Aragonite. However, most of the observed effects could be explained when considering the differences in void fraction of these coatings, pointing to the overwhelming importance of coating density, although the general inverse relationship between strength and void fraction - including size, size distribution, and shape - was poor.

A model of closed-packed spheres was developed to calculate void fraction, binder requirement and cohesive strength. Calculated void fraction agrees with the literature. The model predicts no influence of pigment size on binder requirement and cohesive strength: experimental results confirm that, at constant void fraction, the effect is indeed minor.