Enhanced Nonisothermal Crystallization and Heat Resistance of Poly(l-lactic acid) by d-Sorbitol as a Homogeneous Nucleating Agent

We report on enhancing crystallization and heat resistance of poly(l-lactic acid) (PLLA) by d-sorbitol as a small molecule nucleating agent via melt blending. During the reheating process, the cold crystallization disappeared and the crystallinity of nucleated PLLA exceeded 50%. The heat deflection temperature of PLLA was elevated from 56 to 132 °C by simply increasing the mold temperature (90 °C) without an additional annealing treatment. We also observed the polymorphic crystals of PLLA during melt crystallization, i.e., the coexistence of hexagonal and lenticular crystals, along with their various geometrical aggregates in addition to plenty of conventional spherulites. On the basis of the fact that the nonisothermal crystallization temperature of PLLA (110 °C at a cooling rate of 10 °C/min) was higher than the melting point of d-sorbitol (about 93 °C), we speculated that d-sorbitol promoted the crystallization of PLLA through a homogeneous nucleation mechanism.

Crystallization Morphology Regulation on Enhancing Heat Resistance of Polylactic Acid

To expand the use of polylactic acid (PLA) in high-temperature environments, crystallization morphology regulation was studied to enhance the heat resistance of PLA. PLA crystallinity was controlled using heat treatment and nucleating agent (zinc phenylphosphonate, brand TMC). The heat deflection temperatures of PLAs with same crystallinities considerably varied using different treatments. The crystallization morphology of PLA (4032D) and PLA/TMC composites was studied using X-ray diffraction (XRD) and polarized optical microscopy. XRD test results show that TMC can improve the crystallization rate and heat treatment can enhance the crystallinity and thickness of PLA, suggesting that the crystallization morphology improved after heat treatment. Nucleating agents can increase the crystallinity of PLA but cannot improve its crystallization morphology. The findings indicate that at the same crystallinity, PLAs exhibit improved crystallization morphology and high heat resistance; these results can provide guidance for improving the heat resistance of PLAs and facilitate the design of new nucleating agents.

Development of Poly (Lactide Acid) Foams with Thermally Expandable Microspheres

This study presents the investigation of different content of thermally expandable microsphere (EMS) type of a physical blowing agent added to polylactic acid (PLA). The effects of the different doses of EMS, processing temperatures, and d-lactide content of the polylactic acid were analyzed for foam properties and structures. We characterized the different PLAs and the physical blowing agent with different testing methods (gel permeation chromatography, rotational rheometry, isothermal thermogravimetric analysis, and thermomechanical analysis). The amounts of the foaming agent were 0.5, 1, 2, 4, 8 wt%, and processing temperatures were 190 °C, 210 °C, and 230 °C. The foam structures were produced by twin-screw extrusion. We used scanning electron microscopy to examine the cell structure of the foams produced, and carried out morphological and mechanical tests as well. The result of extrusion foaming of PLA using different amounts of EMS shows that an exponentially decreasing tendency of density reduction can be achieved, described by the following equation, ρ(x)=1.062∙e-x7.038+0.03 (R² = 0.947) at 190 °C. With increasing processing temperature, density decreases at a lower rate, due to the effect that the microspheres are unable to hold the pentane gas within the polymer shell structure. The d-lactide content of the PLAs does not have a significant effect on the density of the produced foam structures.