Continuous production of microcellular foams, characterized by cell size smaller than 10 μm and cell density larger than 10 9 cells/cm 3 , was studied using supercritical carbon dioxide (CO 2 ) as the foaming agent. Microcellular foams of polystyrene and polystyrene nanocomposites were successfully produced on a two-stage single screw extruder.
The contraction flow in the extrusion die was simulated with the FLUENT fluid dynamics computational code to predict profiles of pressure, temperature, viscosity, and velocity. The nucleation onset was determined based on the pressure profile and equilibrium solubility. It was shown that a high CO 2 concentration or a high foaming temperature induces an earlier nucleation near the die entrance. The pressure profile and the position of nucleation onset were correlated to cell nucleation and growth, which helps understand the effects of operating conditions on cell structure.
To perform the simulation, viscosity and solubility of the CO 2 /polystyrene system were characterized. Sanchez-Lacombe equation of state was applied to represent the phase equilibrium. Effects of temperature, pressure, and CO 2 content on the shear viscosity were explained using the free volume theory.
Systematic experiments were performed to verify effects of three key operating conditions: CO 2 content, pressure drop or pressure drop rate, and foaming temperature, on the foam cell structure. Experimental results were compared with simulations to gain insight into the foaming process. Studies exhibit that a higher pressure drop or pressure drop rate results in smaller cells and greater cell density. Below the CO 2 solubility, cell size decreases and cell density increases with an increase of CO 2 concentration. A high CO 2 concentration favors producing open cell foams. Die temperature affects both cell size and cell structure (open or closed).
Combining nano-clay compounding with supercritical CO 2 foaming provides a new technique for the design and control of foam structure and property. The addition of a small amount of intercalated nano-clay greatly reduces cell size and increases cell density. Once exfoliated, the nanocomposite exhibits the highest cell density and smallest cell size at the same particle concentration. Nanocomposite foams provide superior performance and synergetic effects of nano-clay and CO 2 on the polymer melt rheology were discussed.