Stress development of latex coatings in a convection drying with the accumulation and deformation of polymer particles

Abstract

In the film formation process of latex paints, polymer particles accumulate and deform with stress relaxation and then coalesce into a continuous film. Our previous studies clarified the differences in particle-packed structure formation between drying temperatures by the accurate measurement of water content. In the present study, we investigate the mechanism of stress development for latex coating from viewpoints of particle accumulation and deformation by the simultaneous measurements of drying stress and water content. When a particle-packed layer forms on the surface side of the coating during the constant drying rate period, it is found that the stress development is proportional to the amount of the packed layer. Even for deformable particles at higher drying temperatures, the proportionality can be held, but the stress relaxation becomes noticeable at a high moisture content exhibiting a gradual decrease in the drying rate. After the complete formation of the closest-packed layer, it is revealed that the shrinkage of the layer causes additional stress development. We then proposed a model where the stress development is described by the shrinkage of the packed layer and the stress relaxation by the temperature dependency of polymer deformability.