Green Method to Widen the Foaming Processing Window of PLA by Introducing Stereocomplex Crystallites

Abstract

Polylactide (PLA) microcellular foams with well-defined cell morphology were successfully fabricated from poly­(l-lactide) (PLLA) and poly­(d-lactide) (PDLA) blends through a supercritical carbon dioxide (CO2) foaming method. The gaps between the melting temperature of homocrystallites (T m(HC)) and stereocomplex crystallites (T m(SC)) were utilized to improve PLA’s melt foaming behaviors (i.e., cell nucleation and cell morphology) owing to the existence of exclusive SC crystallites when the foaming temperature fell in between T m(HC) and T m(SC). Thanks to the enhanced melt strength, which resulted from the introduction of SC crystallites, the cell morphology transformed from elliptical to nearly circular and the cell collapse and coalescence reduced when foamed at high temperatures and pressures. The wider foaming processing window is favorable for actual industrial manufacture such as the continuous extrusion foaming process. In addition, PLA foams possessing more SC crystallites exhibit higher heat resistance, which further broadens their applications in high-temperature conditions.

Tailoring of advanced poly(lactic acid)‐based materials: A review

“…On the other hand, the average cell diameter decreased and cell density increased with D‐isomer content for pressures under 12–24 MPa. For these samples with relatively high melt strength, the diameter decreased due to the more nucleating points, which caused less CO 2 usage for cell growth 64 . A shift of T m to lower value for two different PLA types (PLA 1 and PLA 2 ) was identified with increasing CO 2 pressures up to 20 and 30 MPa, indicating a plasticizing effect of CO 2 on the polymers 109 .…”

“…Processes of PLA melt blending and molding require high operating temperatures above the polymer melting point (up to 200°C) and/or high pressures 16,19,22,24,39,56–68 . After being melted, PLA is shaped into the desired form by molding (with or without a mechanical press) and cooled down to stabilize its dimensions 23 …”

“…Supercritical CO 2 ‐foaming process was employed for the preparation of PLA foams with different morphologies 8,42,64,107–114 . Schematic presentation of the scCO 2 ‐assisted foaming unit can be seen in Figure 6a.…”

“…When foaming pressure was risen to 20 MPa, the cell morphology changed from elliptical to circular until the content of D‐isomer was risen to 3%. Further pressure increase to 24 MPa lead to the appearance of the elliptic pores in the pure PLA 1 that disappear in all samples with D‐isomer 64 . For pure PLA 1 sample, which has poor melt strength, the average cell diameter became large with the pressure increase from 12 to 20 MPa due to cell coalescence.…”

“…Frerich 108 reported that increase of pressure from 20 to 30 MPa at 130°C increased compressive modulus of PLA 6 foam from 61 to 83 MPa and compressive strength from 9 to 11 MPa. Effect of pressure (12–24 MPa) on the PLA foams with different amount of D‐isomer (1%–15%) obtained by blending PLA 1 and PLA 18 was also studied 64 . For samples foamed at 150°C under 12 and 16 MPa, the elliptic cells are observed in the pure PLA, 1 while the circular cells appeared in samples with 1%–15% of D‐isomer.…”