Extruded PLA/clay nanocomposite foams blown with supercritical CO ₂

The effects of nanoclay (Cloisite 30B) on the foamability of polylactide (PLA) were investigated in continuous extrusion foaming using supercritical CO₂ as the blowing agent. PLA samples containing 0-5 wt% of nanoclay were prepared. The X-ray diffraction and transmission electron microscopy images showed a high degree of exfoliation of clay nanoparticles within PLA. A single-screw tandem extruder was used to produce foams with 5 wt% and 9 wt% supercritical CO₂. The crystallization behavior of the samples was analyzed using regular and high-pressure differential scanning calorimeters and using a rotational rheometer under small amplitude oscillatory shearing. In the presence of dissolved CO₂, clay, and shear action, the PLA crystallization kinetics was significantly enhanced. The foamed results showed that both the cell density and the expansion ratio were greatly promoted with increased clay content and the dissolved CO₂, as well as by the possibly nucleated crystals. By further use of Cloisite 20A nanoclay particles with poor disperse-ability in PLA, we also proved that a high degree of dispersion significantly promoted the cell density and the expansion ratio of the PLA nanocomposites. Further, by varying the temperature profile within second extruder of the tandem-line, it was confirmed that the more rapid crystallization along the second extruder was responsible for the enhanced cell density and expansion ratio. The final crystallinity of the foamed samples was also enhanced at higher expansion ratios due to the strain induced crystallization.