Influence of pressure-induced temperature drop on the foaming behavior of amorphous polylactide (PLA) during autoclave foaming with supercritical CO2

Abstract

Autoclave technology is widely used for the fabrication of cellular structures based on thermoplastic polymers. In this work, the autoclave process is investigated focusing on thermodynamic phenomena during the expansion of supercritical fluids. The depressurization with supercritical carbon dioxide (scCO2) leads to rapid adiabatic expansion, which affects pressure drop rate (PDR) and temperature inside the vessel. Analysis of density versus saturation temperature and pressure allows identification of a "valley" bounded by three limits to low densities. First two are caused by (i) low viscosity for stabilization and (ii) thermodynamically suppressed nucleation and expansion at low pressures. At high pressures and low temperatures, the expansion of the medium leads to a temperature drop (iii), which increases resistance of the polymer to stretching. Morphological examination showed significant influence of the initial pressure drop rate on the nucleation density. Size and integrity of cells can be adjusted by selective modification of foaming conditions.