Non-Fickian nonisothermal model for drying of polymer coatings



As a sequel to our study on experimental measurements of trapping skinning in a poly(methyl methacrylate)/acetone coating, a mathematical model of non-Fickian diffusion that predicts trapping skinning is reported here including solvent transport due to a stress gradient that develops because of coating shrinkage. The predicted solvent concentration profile shows a sigmoidal shape and a front that moves into the coating. In the nearly constant rate period, the stress relaxes near the coating surface due to short relaxation times at high coating temperature. The stress relaxation causes negative non-Fickian flux. At higher gas flow rates, the negative non-Fickian flux causes the development of a thinner layer of low solvent concentration near the coating surface and a thicker layer of high solvent concentration inside the coating. Thus, more solvent remains in the coating dried at higher flows and trapping skinning occurs.