Modelling coating structure development : Monte-Carlo deposition of particles with irregular shapes

There is an increasing interest in enhancing coated paper performance by combining the unique properties of various pigments, i.e. developing multi-pigment coating formulations. An understanding of the synergy between pigments of different sizes and shapes is therefore becoming critical. The literature on this subject is rather empirical and fragmentary. In this report, we introduce an improved 3D Monte-Carlo deposition model to simulate coating structure development for irregularly shaped particles. The model relies on the minimization of the total energy of the particle system, which is defined as a sum of three terms: a pair interaction that accounts for collisional interactions, a gravitational potential energy, and a drag potential energy that takes into account the drainage rate of the colour. The model predicts that pigments with higher aspect ratios give a more porous structure and a smoother surface when applied at high solids content. Coating at low solids content (<45-55%) is shown to produce “looser” and more disordered structures, especially in the case of platelet pigments like clay. These results agree with practical and experimental observations. When blending clays with calcium carbonates, the model predicts the occurrence of maximum pore volume and surface micro-roughness at specific mixing ratios. The location of these maxima varies significantly with the particle’s aspect ratio, the relative size of the blended pigments, and the solids content of the suspension. With further experimental validation, this model has the potential of being used as a tool for designing and optimizing coating formulations