Fabrication of poly(lactic acid) foam with high expansion ratio and oriented cellular structure by restricting cold crystallization.

The foaming behavior of semi-crystalline polymers is significantly affected by their crystallization. To achieve high expansion ratio of poly(lactic acid) (PLA) foams, we thought its cold crystallization should be restricted. Therefore, we used a short soaking time of CO to fabricate high-expansion PLA foams. Dynamic mechanical analysis of unfoamed PLA showed that only one rubbery plateau was observed owing to complete cold crystallization under a soaking time of 40 min at 10 MPa. The crystal morphology demonstrated that a short soaking time of 3 min could restrict the cold crystallization of PLA. Owing to plasticization of CO, PLA crystallization of was accelerated at low temperatures (40-80 °C) but hindered at high temperatures (80-130 °C) at 10 MPa. Foaming results showed that under a soaking time of 3 min, a high expansion ratio exceeding 10 was achievable over a wide foaming temperature range of 90 to 115 °C because more amorphous regions were preserved at 10 MPa. In addition, the prepared foams presented an oriented cellular structure. Compared with the isotropic foam, the anisotropic foam exhibited higher compressive strength and heat resistance. The prepared PLA foams have good application prospects in the fields such as cushioning, packaging, and construction.