Development of Crystallization in PLA during Solid-State Foaming Process using Sub-Critical CO ₂

Crystallinity of PLA is an important factor that affects its processability, mechanical strength, biodegradability, and service temperature. In this paper crystallization of PLA in solid state foaming process was studied in two main aspects: gas sorption in saturation stage, and heating as well as stretching in foaming stage. During CO ₂ saturation the equilibrium crystallinity jumped from 9.7% at 2 MPa to 26.4% at 3 MPa CO ₂ pressure. Further increases in saturation pressure did not increase the crystallinity. In the gas sorption stage of the process, it was found that the crystallization reached equilibrium at approximately the same time as the gas sorption reached equilibrium. Within the range of 2 to 5 MPa saturation pressures and 60 to 100°C foaming temperatures, a maximum crystallinity of 26.8% was observed in the foamed PLA. Crystallinity increased with the extent of cell-wall stretching during the foaming process.