NUCLEATION AND GROWTH IN MICROCELLULAR MATERIALS - SUPERCRITICAL CO2 AS FOAMING AGENT

Bubble growth is a phenomenon encountered in several commercially impo rtant processes. A mathematical model presented here describes the gro wth of bubbles during phase separation of an initially homogeneous pol ymer-supercritical fluid mixture, triggered by a sudden pressure drop at constant temperature. It is a modification of the viscoelastic mode l of Arefmanesh and Advani (1991) in which the polymer is treated as a single relaxation-time Maxwell fluid. Since properties of the polymer -fluid mixture vary with the amount of fluid absorbed in the polymer ( as a function of fluid pressure), the model needs to be used evaluatin g system properties as functions of temperature and pressure. The visc osity of polymer/fluid mixture, density of the mixture, diffusivity of CO2 in the mixture, and relaxation time for poly(methyl methacrylate) swollen by supercritical carbon dioxide are, therefore, predicted as functions of CO2 pressure and temperature using appropriate model equa tions at each step of the bubble growth simulation. The model predicts well the trends in equilibrium cell size vs. saturation pressure and temperature.