Theoretical aspects of self-stratification

Abstract

In order to predict whether a system of two polymers or resins in a common solvent mixture will ‘self-stratify’, two predictive models, one based on UNIFAC and the other on surface tension relationships, have been developed. The UNIFAC model can predict phase behaviour, vapour pressure, evaporation rates and surface tensions for systems containing two polymers or resins in a solvent mixture. Comparisons between experimental and calculated data are made for acrylic/acrylic, epoxn/acrylic and alkyd/acrylic resin combinations. The prediction of phase separation during solvent evaporation from a thin film is satisfactory in most cases, especially when the difference in molecular weight between the two polymers is not too large. For epoxy/acrylic resin combinations, the calculated evaporation profiles are in reasonable agreement with observed values. The evaporation rate decreases with increasing epoxy molecular weight, but is almost independent of the acrylic resin and the resin ratio. The tendency of epoxy/acrylic resin combinations to undergo self stratification is predicted to increase as the molecular weight of the epoxy resin increases. The degree of phase separation is increased as the molecular weight of the acrylic resin is increased. These predictions agree with experimental observation but for acrylic/alkyd resin combinations, no phase separation is predicted which is not in agreement with experiment. This is probably because the model cannot allow for the broad molecular weight distribution of the alkyd resin. A second predictive model based on surface energy variation with concentration of two polymer solution phases in contact has been developed. This model can be used to predict which resin combinations will give rise to self-stratifying systems given the surface energy/concentration relationship of the pure resins in solution and assuming the systems have phase separated. For solutions of epoxy resins combined with a second resin the model correctly predicts the observed tendency for the resins to stratify provided the system phase separates at some point.