In-situ visualization of PLA crystallization and crystal effects on foaming in extrusion

We studied poly(lactic acid)'s strain-induced crystallization behavior during a tandem extrusion process using visualization techniques. Our visualization results showed that, despite its inherently slow crystallization kinetics in the quiescent condition, PLA crystallites were quickly formed inside the die at a temperature even above the melting temperature (Tm), as the melt temperature was decreased. Their rapid formation at a high temperature above the Tm peak, was due to the strain-induced crystallization. We also studied how the strain rate and the use of supercritical CO2 affected PLA crystallization. A significant increase in the PLA crystallization kinetics was caused by a sudden increase in the screw speeds of the extruders with a fixed barrel temperature. The crystallization enhancement, which was caused by the increased flow rate, became more pronounced at lower temperatures due to the higher degree of molecular orientation. Adding the CO2 shifted the crystallization temperature to a lower processing temperature due to its plasticization effect, but it accelerated the crystallization kinetics as well. We also observed a correlation between the content of the induced crystallites inside the extruder and the cellular structure of the extruded foams. This clearly explained the decrease in the optimal processing temperature for foaming with an increased CO2 content.