Influence of supercritical fluid parameters on the polyimide aerogels in a high-efficiency supercritical drying process

Abstract

A semi-continuous supercritical drying process was proposed to greatly improve the drying efficiency in producing polyimide aerogels. The influence of supercritical fluid parameters such as pressure and temperature on the aerogel properties were systematically investigated, based on the vapor-liquid equilibrium diagram of the ethanol-CO2 binary system. Results showed supercritical drying efficiency was critical for the comprehensive properties of the aerogel, because solvent retention in the aerogel after drying could cause depressurization-induced shrinkage and a subsequent change in nano-structure in the vacuum drying that ensued. By continuous scCO2 flushing at 90 °C and 16 MPa with the total drying time as short as 3 h, the aerogel showed a satisfactory shrinkage as low as 11.5%, a large specific surface area as high as 344.3 m2/g. Our findings also showed that the subcritical drying condition slightly below the critical mixture pressure of the binary system could provide special interest.