Solid-state foaming of biodegradable polyesters by means of supercritical CO2/ethyl lactate mixtures: Towards designing advanced materials by means of sustainable processes

In this work, a clean and sustainable approach for the design and manufacture of biodegradable foams with controlled pore structures is reported. Porous polycaprolactone (PCL), polylactic acid (PLA) and poly(L-lactide/caprolactone) (PLC) homo- and co-polymers have been prepared by a supercritical solid-state foaming technique. The process has been carried out at low temperature, in the range of 35–40 °C, and at the pressure of 20 MPa. Supercritical CO2 (scCO2), pure or with a 0.2% molar fraction of ethyl lactate (EL), has been selected as the blowing agent. Foaming has been induced by a pressure quench approach and by using either a low (0.5 min) or a high (6 min) depressurization time. Results showed that blowing agent uptake and foaming was strongly affected by the thermal properties and crystalline structure of the polymers. Furthermore, adding a small amount of EL to scCO2 decreased the plasticization temperature of the polymeric materials, finally improving foams uniformity and polymer foaming. Hence, homogeneous foams could be manufactured avoiding the use of toxic chemicals and at a temperature as low as 35 °C for PLA and PLC, and as low as 40 °C for the more crystalline PCL.