Process model for latex film formation: Optical clarity fronts

We examined the drying behavior of latex both theoretically and experimentally. The theory extends a model for horizontal drying fronts in films with nondeformable particles to incorporate particle deformation by a capillary deformation mechanism. The pressure in the fluid, causing particle compaction, arises from flow through the packed bed to ensure evaporation from all wet areas of the film. We predicted the position of a front of volume fraction unity passing across a semi-infinite film as it dries. Experimentally, the position of the transition from a cloudy film to optical clarity was tracked visually in films comprised of either single component soft latex particles, 20°C above the glass transition, or a blend containing 35% non-deformable hard latex particles. For an initial volume fraction of 0.33, we found excellent agreement between theory and experiment. For an initial volume fraction of 0.05, the agreement is less, although still qualitative. The limitations of the model with respect to the knowledge of physical parameters and initial conditions are discussed. One major implication of the model is that deformation of soft latex particles displaces large amounts of water and, consequently, slows progression of the drying front. Harder particles and shallow initial film profiles produce more pronounced drying fronts. Dept. of Chemical Engineering, Princeton, NJ 08544. Emulsion Polymers Institute and Dept. of Chemical Engineering, Bethelhem, PA 18015.