Capillary forces and stress development in drying latex coating

In latex coating a layer of aqueous suspension of micron- or submicron scale polymer particles is deposited on a substrate, and then dried and transformed into continuous, adhering polymer film. Of the stages of its transformation from colloidally stable suspension to continuous film: namely consolidation, compaction, particle contacts formation, and coalescence this research focuses on the role of capillary forces in particle compaction and stress development.; The theoretical model developed and employed in the course of this work is based on the following scenario: during drying particles consolidate and compact into porous network driven by water surface tension; the particles packs generate menisci---at the top layer in wet stage and around particle contacts as pendular rings in moist stage---which provide capillary pressure and surface tension forces; the particles deform and flatten against each other under capillary forces and adhesion forces; as the distance between flattened surfaces lessens stabilizing layers rupture, so that interdiffusion can proceed to weld the particles together into a continuous layer.; In this work the wet and moist stages of particle compaction are identified and analyzed theoretically. FEM calculations of air-water menisci allowed to calculate and study the capillary forces and their role in deformation of latex particles and development of stresses in the whole coating. In the wet stage each particle flattens at the downward contacts and expands laterally so that its volume is conserved. The flattening results from the balance between compression and the particle elastic reaction and is described by Hertz's solution. For the moist stage a new solution is constructed for an axisymmetric contact of two elastic sphere with distributed stress on their surfaces outside their initial point contact. The main result is that on each contact there is a local distribution of both compressive and tensile stresses, however an average normal stress in coating is zero, and an average in-plane stress is present and can be either compressive or tensile.