A MODEL FOR DRYING OF VISCOELASTIC POLYMER-COATINGS

Drying of polymeric coatings often occurs under conditions where the r elaxation time of polymer molecules is significant with respect to the processing time scales. The nonequilibrium thermodynamic theory of Du rning and Tabor (1986) is applied to model 1-D drying of viscoelastic solutions with concentration-dependent physical properties. Transport of solvent to the surface of the coating occurs by viscoelastic diffus ion down the gradient of a diffusion potential with a relaxing nonequi librium contribution. Galerkin's method with finite-element basis func tions and a differential/algebraic equation system solver enable effic ient solution of this stiff nonlinear model. Predictions show that ela sticity enhances diffusion within the coating. At high Deborah numbers , however, a fall in the surface activity slows the rare of desorption . The coating thickness after a specified time under fixed total drivi ng force is the smallest at intermediate Deborah numbers, showing that a small amount of viscoelasticity actually aids in drying. This can b e interpreted as a skinning effect.