Reflectometry of drying latex paint

The optical properties of pigmented coatings containing titanium dioxide, calcined and hydrous kaolin, and ethylene-vinyl acetate latex at various pigment volume concentrations were measured during drying using the technique of polarized light reflectometry. Gloss (at two acceptance angles), effective refractive index (RI), microroughness, and backscattered intensity were obtained simultaneously as a function of time. It is shown that the gloss response depends on measurement geometry and the combined effects of several sample physical properties and therefore requires caution in interpretation. The combination of RI, microroughness and backscatter provides more direct information on physical transitions during drying and fits with a three-stage description of drying. Differences in drying due to pigment aspect ratio are relatively small, but the platy kaolin appears to extend the period required for solids immobilization and also prolongs measurable development of surface microstructure during film formation.