Utilization of CO2 as a physical blowing agent for foaming of high temperature sulfone polymers

Abstract

The present work adopts the one-step batch foaming technique to develop microcellular foams from polysulfone (PSU) and polyethersulfone (PES) using supercritical carbon dioxide (CO2) as a physical blowing agent. The results illustrates that the high temperature polymers can successfully generate highly uniform cellular structures with CO2, exhibiting cell densities in the order of 1010 cells/cm3 but limited volume expansion ratios of up to 4 times. The cell densities of both systems, PSU/CO2 and PES/CO2, were observed to be highly affected by the foaming pressure, showing significant enhancement of over 5 orders of magnitude with an increase in pressure from 3.4 to 13.8 MPa. On the other hand, the expansion ratio showed a strong dependence on the foaming temperature. Especially, the foams produced at pressures below 13.8 MPa exhibited a unique double peak behavior consisting of two maximum expansion ratios obtained at two different temperatures. In the temperature range of the first expansion peak, nanostructures were observed on the cell walls of the foams. The surficial features appeared in the form of a nanofibril network and gradually disappeared as the temperature approached the region of the second peak. Based on the observation, the unique double peak behavior may be correlated to the presence of the nanostructures. The work also encompassed measurement of CO2 solubility and high pressure thermal analysis of the two polymers, which were incorporated in the discussion of the distinctive foaming behavior.