Second cell growth optimized by heating mode during two-step supercritical CO2 foaming polymer process

Abstract

The batch foaming of supercritical fluids faces numerous challenges, such as poor controllability due to dynamic temperature changes during depressurization foaming, and prolonged saturation time and heterogeneous temperature distribution during temperature rising foaming, which leads to lower productivity and non-uniform cells. To overcome these challenges, a unique two-step foaming method in combination with microwave enhancement was proposed. The two-step foaming method involved depressurization foaming for cell nucleation and initial growth, followed by temperature rising foaming for subsequent cell growth, producing foams with higher expansion ratio. Meanwhile, a conduction-microwave (C-M) heating mechanism which integrates different heating schemes to enhance heating efficiency and uniformity was innovatively proposed. The effect of the microwave radiation time, power, initial environmental temperature, and content of CO2 in polymer on the foaming by C-M heating was studied. Compared to existing methods, the foaming by C-M heating produces polystyrene (PS) foam with lower density and more uniform cells.