Modeling Liquid Distribution and Binder Migration in Drying Porous Coatings

Sean Xiangyang Pan, a Ph.D candidate in Chemical Engineering and Materials Science, is completing a thesis on porous coatings and solute (binder) migration. Pan is part of Professor L.E. Scriven's research group which is doing research on the computational aspects of continuum and network theories of fluid physics and transport in films and porous media structures. Pan is using a network model to examine the physics of liquid flow and binder movement in porous coatings. Pan's research is primarily on paper coatings but is broad enough to be of interest in the coating of ceramics and other useful materials.

Porous coatings are made up of large particles that do not fuse completely and binders--polymers such as latex--which form the bonds between these larger particles in the coating. This project examines the patterns of drying and the distribution of binder in drying paper coatings, both of which are controlled by the pore-level physics of meniscus action, water flow, vapor diffusion, binder transport, and binder deposition. The way in which water distributes in a coating affects the distribution of the binder, which in large part determines important characteristics such as printability and how ink is distributed on the paper. Pan has used supercomputer simulations of a small, disorderly, three-dimensional sample of a paper coating to understand the pore-level physics involved. The simulation required solution of the many equations that describe the pore-by-pore physics as an aqueous solution or suspension of a single nonvolatile binder is dried.

The results on pore emptying show the competition of capillary forces and viscous resistance to flow, which in turn depends on the drying rate. This competition is characterized by a useful ratio, or dimensionless number. Binder tends to deposit at evaporating menisci wherever they pause. The lower the drying rate, and the higher the binder's mobility, the more it tends to concentrate with the liquid before it deposits. This competition is also characterized by a useful ratio, a second dimensionless number. The two delineate regimes of binder distribution. It appears that in ordinary practice it is difficult to alter the course of liquid distribution but not that of binder distribution.