Effect of Crystallization and Foaming Behavior on the Mechanical Properties of Expanded Poly(lactic acid) Bead Foams

Biodegradable expanded poly(lactic acid) (EPLA) foam exhibits advantages such as low density and favorable mechanical properties, and it is an ideal alternative to conventional petroleum-based expanded polystyrene (EPS) foam. In this work, the influence of double-melting crystallization on interface adhesion and the mechanical properties of bead foams was systematically investigated. First, PLA bead foams were fabricated using a lab-scale autoclave system employing carbon dioxide (CO2) as the physical blowing agent. By adjusting the foaming conditions, PLA bead foams with double melting peaks were produced, and all PLA bead foams exhibited high-temperature melting peak crystal (crystallinity of 7.7–19.6%) and low-temperature melting peak crystal (crystallinity of 18.7–26.9%), respectively. Then, the PLA bead foams were molded by a steam-chest molding machine under the same conditions. The effects of the crystallization behavior and foaming behavior on the adhesion and mechanical properties of PLA bead foams were studied, and the interfacial bonding mechanism of PLA bead foams was further investigated and analyzed. Moreover, the compression properties of EPLA samples were compared with those of the EPS sample. These research results showed that good interfacial bonding and cell morphology were beneficial to improving the compression performance of EPLA, and the maximum yield strength of EPLA could reach 535.1 kPa, which was higher than that of EPS (67.3 kPa). This investigation into the double melting peaks and foaming behavior of PLA provides insights that could be extended to the molding of other polymeric bead foams.