Structure of silica in matt water-based lacquer

Abstract

A model for the structure of silica matting agent in water-based lacquers is presented. It is assumed that, during film formation, the air that lies between the silica particles in the dry powder is replaced by polymer. At a critical concentration this leads to a silica structure similar to that of the dry powder. We assume the bulk volume occupied by the dry silica powder (silica and air) equals the dried lacquer film (polymer, silica, and residual air). Since the silica structure in the dry powder percolates, the silica in the dried lacquer is tested for percolation. Experimentally, a percolation threshold is found close to the critical concentration predicted by the model. Two further silica structures are also seen under different conditions. At low silica concentrations, the silica particles are suspended in the lacquer matrix and isolated. Above the percolation threshold, where the bulk volume of the dry silica exceeds that of the film, the silica structure can undergo collapse. This is caused by volume reduction in the lacquer as water evaporates. The lacquer is imaged with confocal laser scanning microscopy, to produce three-dimensional images of the bulk of the 50-microm film. Resolution is enhanced with image reconstruction via deconvolution. Computational image analysis is then used to investigate the structure quantitatively.