Role of high-density polyethylene in the crystallization behaviors, rheological property, and supercritical CO2 foaming of poly (lactic acid)

A simple and effective methodology to improve the crystallization behaviors, rheological property, and supercritical CO2 foaming of poly (lactic acid) (PLA) using high-density polyethylene (HDPE) as modifier was proposed. PLA was blended with various content of HDPE through a melt blending method. The results demonstrated that with the blending of PLA and HDPE, the crystallization behaviors of PLA and HDPE were improved simultaneously as well as the rheological property of PLA got better gradually. The morphology of HDPE dispersion phase in the PLA/HDPE blends was irregular and its average size became larger gradually with the content of HDPE increasing. Then resultant PLA/HDPE blends were foamed using supercritical CO2 as physical blowing agent in the autoclave. The cellular morphology evolution of PLA/HDPE blending foams had a relationship with the content of HDPE. When the content of HDPE increased from 0 wt% to 5 wt%, complex cellular structure (CCS) appeared in the PLA/HDPE blending foams. With the content of HDPE more than 10 wt%, the CCS disappeared gradually. The interface between PLA and HDPE could be acted as heterogeneous nucleation points for both the crystallization of PLA and the cell nucleation of PLA/HDPE blends. Finally, the influence of foaming temperature on the foaming behaviors of pure PLA and PLA/HDPE blends was also investigated and the foaming mechanism on pure PLA foam and PLA/HDPE blending foams with various blending ratio was presented.