STUDIES OF CONVECTIVE-TRANSPORT IN EVAPORATIVE CASTING OF DENSE POLYMER-FILMS

Although several models have been developed to describe the evaporativ e casting of dense polymer films, none of them has included the convec tive transport terms which arise owing to the densification which occu rs. In this paper we first describe a new finite element solution to t he binary nonisothermal evaporative casting process which is used to c onfirm the predictions of the finite difference solution recently deve loped by Shojaie et al. (J. Mater. Process. Manu. Sci., 1 (1992) 181). This comparison then establishes that systematic deviations between e xperimental measurements and the model predictions of Shojaie et al. d o not arise because of any inaccuracies in the numerical solution meth odology. We then explore whether these deviations arise owing to omiss ion of the convective transport terms by Shojaie et al. and all other modeling efforts. We employ a scaling analysis to demonstrate that con vective transport effects can be significant in the evaporative castin g process. The convective transport terms then are incorporated into t he nonisothermal evaporative casting model of Shojaie et al. for the c ellulose acetate/acetone system. The model predictions both with and w ithout convective transport are compared to real-time gravimetric and surface-temperature data. This study indicates that ignoring convectiv e transport can result in differences as large as 40% in the model pre dictions. The model predictions incorporating convective transport are in quantitative agreement with real-time data at short evaporation ti mes, but progressively deviate at longer times. This deviation is thou ght to be due to the use of an equilibrium equation-of-state for the s olution density as a function of concentration.