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

Bubble growth is a phenomenon encountered in several commercially important processes. A mathematical model presented here describes the growth of bubbles during phase separation of an initially homogeneous polymer-supercritical fluid mixture, triggered by a sudden pressure drop at constant temperature. It is a modification of the viscoelastic model 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 evaluating system properties as functions of temperature and pressure. The viscosity 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 equations at each step of the bubble growth simulation. The model predicts well the trends in equilibrium cell size vs. saturation pressure and temperature.