Effects of nano-/micro-sized additives and the corresponding induced crystallinity on the extrusion foaming behavior of PLA using supercritical CO2

Highlights: • Polylactide (PLA) extrusion foaming was developed using supercritical CO_2. • Nanoparticles enhanced PLA's slow crystallization via crystal nucleation mechanism. • Nanoparticles & enhanced crystallization significantly improved PLA's foaming. • Varying crystallization kinetics in extruder significantly affected PLA's foaming. This study investigated how polylactide's extrusion foaming, using supercritical CO₂, depends on nano-/micro-sized additives (i.e., talc, nanosilica, & nanoclay). It also explored the crystallinity that these additives induced in the PLA/CO₂ fluid mixture during the foaming process. The expansion ratio and cell density of the PLA foams were enhanced due to the presence of well-dispersed nanoparticles and large number of induced crystals through these nanoparticles. In contrast, the more effective role of talc on the crystallization kinetics of PLA increased the melt strength of PLA too much during foaming and reduced the die pressure. This resulted in less uniform final foam morphology. Further, applying different temperature profiles along the second extruder-heat exchanger-die varied the crystallization kinetics of PLA, which influenced the die pressure, and subsequently, the foaming of PLA. The die pressure dropped significantly at a distinct temperature profile where the crystallization kinetics was the fastest and thereby induced less uniform final cell morphology. Below this temperature profile, slower crystallization kinetics was dominated more by crystal nucleation. Therefore, the increased die pressure, caused by reduced crystallization rate, and the large number of crystals with a slow growth rate, contributed to the increased final cell density and the foam morphology's uniformity.