Drying and Binder Migration in Coated Papers, 1991 Coating Conference Proceedings

The competition between binder convection, diffusion, and deposition during drying of a paper coating is examined by solving equations that describe capillarity-driven liquid flow, binder diffusion and deposition, evaporation at liquid menisci, and vapor diffusion. This is done for networks with representative pore dimensions, initially tilled with a binder solution. The external air velocity and humidity are so varied that the initial drying rate ranges from 1 to 100 kg/m2/hr and the coating dries isothermally. A range of diffusion coefficients is considered so that the binder diffusion and convection are comparable, in contrast to the limiting cases examined earlier.

The results show that at lower drying rates, binder is able to diffuse away from evaporating menisci near the surface until the smaller pores, which tend to remain liquid-filled and empty nearby larger ones, become saturated; binder distribution is governed by the distribution of pore passage sizes at and near the surface. At intermediate drying rates the influential passage size distribution reaches deeper into the coating. At higher drying rates, evaporating menisci recede more quickly into the pore space, liquid there disconnects from the drying surface, and most of the binder is deposited within the coating. Further case studies are needed to relate these results to operating experience.