Studies On Autoclave Foaming Of Polylactic Acid And Polylactic Blends

Polylactic acid (PLA) is a kind of important biodegradable polymer material of good biocompatibility and biodegradability. Microcellular polymer foams gain more and more interests recently due to their high performance and low density. It is a promising way to improve the mechanical properties of PLA and expand its applications by preparing PLA foams. In this study, the autoclave foaming technology was used to achieve the well-defined PLA foams through studying the foaming behavior of PLA, PLA/PPC and PLA/PMMA blends.First, linear PLA (L-PLA) and long chain branching PLA(B-PLA) foams were prepared by autoclave foaming technology using CO2as the physical blowing agent. B-PLA foams exhibited a high expansion ratio of about40and cell density of105-6cells/cm3, whereas L-PLA foams only had the highest expansion ratio of29.8and cell density of103-6cells/cm3. When PLA resins were induced crystallization by CO2saturation, the in situ formed crystal domains supplied nucleating sites to enhance cell nucleation and acted as physical cross-linking points to stabilize cell structure. Thus the prepared L-PLA foams presented the highest expansion ratio of37.4and cell density of106-7cells/cm3.Then, the amorphous PMMA was selected to study the effect of blending on the foaming behavior. When introducing little PMMA, the crystallinity of the blends increased and the solubility of CO2declined sharply, which resulted in that the blend could not be foamed. But when the PMMA content was30%, because of the solubility of CO2in PMMA was higher than PLA, the cell nucleation sites increased so that the expansion of blend improved to53.3. The prepared PLA/PMMA foams had well-distributed cell with good cell morphology. Thanks to the elastic property of PLA/PMMA bead foams, they could be moulded using the "two procedure foaming process". This provided a basic study for PLA bead foams that was used for the lunch boxes or the cup and so on, which expanded application of the PLA.Finally, the PPC was selected to be blended with PLA on the basis of the effect of crystallization behavior on the foaming behavior. Because of the plasticization of PPC to PLA, the effect of the gas barrier property of PPC on the foaming behavior of PLA was studied. It was found that the crystallinity of the blend did not change obviously with the adding of PPC, but the melt strength was weakened so that increased the cell size. When the PPC content was10wt%, expansion ratio improved to53.3, which was the twice about the pure PLA. The gas barrier property was improved through introducing PPC, which could slow down the diffusion rate of CO2, thus enhancing the nucleation of PLA foams and extending the transfer time of PLA/PPC sample during the foaming process.