Influence of shape and a shape factor of pigment particles on the packing ability in coating layers

The influence of shape (flaky or acicular) and a shape factor of pigment pa rticles on the properties of a coating layer has been studied. A broader sh ape factor distribution as a function of equivalent spherical particle size (sedimentation) resulted in an enhanced packing ability of the particles, yielding a lower porosity and smaller pore size as well as a higher mechani cal strength. The porosity of a coating layer based on fine clay with a low shape factor was in general higher than that of a coating based on coarse clay with a high shape factor, but the average pore size was considerably l arger for the coarser pigment. A favourable composition of an aragonite pig ment (Opacarb mix) was achieved by mixing a portion of quite large particle s with a high shape factor with a portion of small particles with a rather low shape factor. The IGT pick resistance of a coating layer prepared from this pigment was markedly higher than that of another aragonite pigment (Op acarb A60) with a narrower shape factor distribution, even though the poros ity and light scattering ability were similar in both cases. The influence of the shape factor on the gloss seemed to be small compared to the influen ce of particle size. For coatings based on different pigments, the trend in liquid permeability, evaluated by a computer algorithm (Pore-Cor), followe d the trend in median pore size rather than the trend in porosity. Empirica l relationships between glass and surface roughness, with a higher gloss co rresponding to a lower roughness, were valid only for a given particle shap e, flaky or acicular. The same applied to the derivation of the pore size f rom spectral measurements of reflectance. Comparisons could not however be made between the two shapes, probably due to fundamental differences in the way of particle packing.