A model for evaporative casting of polymeric membranes incorporating convection due to density changes

A new model is developed for the evaporative or dry-casting process for polymeric membrane formation that incorporates convective transport owing to density changes; the latter effect has been ignored in all prior evaporative casting models. Densification inevitably occurs during the evaporative casting process owing to the removal of solvent that in turn causes the polymer molecules to assume a more compact configuration. The model predictions for the cellulose-acetate (CA)/acetone/water system indicate that the convective contribution to the mass-transfer flux can be 35% of that due to diffusion during the early stages of evaporative casting. The convective flux contribution can have a marked effect on the properties of the functional layer since the latter is formed during the initial stage of the membrane casting process. The predictions of this new model agree quantitatively with real-time evaporative casting data for the onset and duration of phase separation, instantaneous casting-solution mass, and temperature of the liquid/gas interface for the CA/acetone/water system.