Investigation On Foaming And Foam Structure Of Polylactic Acid/Sodium Chloride Compounds

Polylactic acid (PLA) is a environement friendly and biocompatible material, and thus it is widely used in medical field. PLA with open-cell structure is usually made by microparticulate templating, few researchers made that with open-cell structure by gas foaming and microparticulate templating.This work aims to form open-cell structure.PLA/sodium chloride (NaCl) compounds were selected to be foamed using high-pressure vessel foaming for researching the relationship between process conditions and cell structure, two cell-opening mechanisms were proposed, meanwhile, effects of cell structure on mechanical properties were investigated by injection foaming.Firstly, rheological properties of PLA/NaCl compounds were studied. The results showed that complex viscosity and storage modulus of compounds increased linearly with NaCl content, melt strength increased gradually, but draw ratio decreased.The results of high-pressure vessel foaming of PLA/NaCl compounds showed that open-cell structures were formed, open porosity increased with foaming temperature and NaCl content. Mean cell diameter of foamed compounds increased with foaming temperature when NaCl content of were 10wt% and 20wt%, but mean cell diameter of foamed pure PLA increased firstly, and then decreased when the foaming temperature was too high, mean cell diameter of the compounds when NaCl content were 30wt% and 40wt% had the same result. Cell density of 70/30 PLA/NaCl compound was the biggest under the saturation pressure of 14MPa, when saturation pressure was in 16-20MPa, mean cell diameter of the compound increased with increasing the saturation pressure, cell density decreased and the distribution of cell size became narrow. Cell density of compounds increased with adding a small amount of NaCl because of its heterogeneous nucleation effect, but it decreased when NaCl content was too high, because draw ratio of compounds with high NaCl content was small, which made the cells collapse and merger more easily.The results of microcellular injection molding of PLA/NaCl compounds showed that open-cell structure was formed in all injection foaming samples.Cell size of the compound with 10wt% NaCl content was larger than that of 5wt% NaCl, and the Location area of open-cell was wider. Cell size increased with the increase of weight loss.High melt pressure decreased the cell size and made distribution of cells more uniform.