Surface Properties and Structural Collapse of Silica in Matte Water-Based Lacquers

Abstract

An investigation into the relationship between surface microstructure and optical properties of matte water-based lacquer is presented, and a model describing this relationship in terms of the packing of silica matting agent is tested. This model assumes that, at some critical concentration, the silica structure in the film-formed lacquer is similar to that found in the dry powder. Above this critical concentration, an increase in surface roughness is expected, which is related to the reflectance of the lacquer. The lacquer surface is accessed via environmental scanning electron microscopy and atomic force microscopy, and the results are related to the model described and the reflectance measured. The reflectance is found to be inversely related to the surface roughness. The prediction for increase in surface roughness above the critical silica concentration is shown to be valid for robust precipitated silicas with large (10 μm) aggregate particles. Smaller 2 μm aggregate particles show a much reduced surface roughness. A difference in behavior is seen between fumed and precipitated silicas. Fumed silica undergoes total structural collapse at high silica concentration, with a limited quantity of silica around the surface. With precipitated silica, the collapse is much less complete, and more silica is present near the surface at high silica concentrations.