Substrate Effects on Binder Migration in Drying Porous Coatings, 1996 Coating Conference Proceedings

A theory of drying of porous coatings after they have been consolidated from thin layers of suspensions has been developed (1). The theory differs from traditional macroscopic theories by being based on pore-level physics of the drying process. It requires only independently measurable parameters of the liquid constituents, namely vapor pressure, surface tension, viscosity, and binder diffusivity; and of the network porespace, namely connectivity and geometry of pores and liquid flow conductance in the pores. Computed outputs of the mechanism-based network model show how capillary driving force and viscous resistance compete in liquid distribution; and how diffusion and convection of soluble or suspended binder influence its distribution as it deposits in the porespace. All of the results and inferences in this paper are about consolidated wet coatings and are drawn from the theoretical model.

A substrate affects the quality of the coating in many ways. Three substrate effects are examined here. The first effect is that a non-porous substrate, such as a polymer film, prevents air from reaching any of the pores at the bottom of the coating until a path of air-filled pores from the top reaches the bottom. As a result, binder tends to deposit less in the top layer than in the case of a porous substrate surface causes through the thinner regions earlier, the coating dries faster there, and more binder migrates back into the thick regions.

The third effect examined here is that a coating suspension may penetrate into porespace of the substrate and thereby raise the amount of the coating liquid and binder that is applied to the substrate...