Study On The Behavior Of Polylactic Acid Foaming Regulated By Supercritical Carbon Dioxide Induced Crystallization

Poly?lactic acid?microcellular foamed material?PLA-MCF?has received extensive attention due to its biodegradability and green and non-polluting preparation process.At the same time,it can replace petroleum-based foamed materials and has great development potential.However,due to its low melt strength,PLA is difficult to control the cell structure,which limits its application.Studies have shown that the crystal structure formed by PLA under supercritical CO₂ can improve its melt strength and help improve foaming performance.Due to the presence of supercritical CO₂ induced crystal formation,the solubility of supercritical CO₂ in the PLA matrix is reduced,and it also affects the nucleation and growth of the cells.Therefore,it is of great significance to study the crystallization behavior of PLA under the action of supercritical CO₂ to improve the foaming performance and regulate the cell morphology.In this paper,PLA was used as the raw material to induce its crystallization through supercritical CO₂ at different temperatures and different immersion pressures.Under the same conditions,PLA foam was prepared by supercritical CO₂ intermittent foaming.The influence of crystal morphology,crystallinity and crystal form on cell structure under different conditions was studied.At the same time,the formation mechanism of various crystal forms and the correlation between crystallization and foaming behavior under the action of supercritical CO₂ were deeply discussed,which provided a theoretical basis for the design of special cell structures.The results showed that PLA can induce the formation of four crystal morphologies under the action of supercritical CO₂,which were ring-banded spherulite,transition spherulite,mixed spherulite and ordinary spherulite.In the low-temperature and high-pressure region,it was easier to form annulus spherulites,showing obvious structures of alternating concentric ridges and concentric valleys,while in a high temperature region?130°C?,it was easier to form a general spherulite structure that was scattered in the radial direction.At moderate temperatures?100?¹20??,it was easier to form mixed spherulites with ordinary spherulites in the middle and ring-banded spherulites on the outside.At the same time,the crystallinity of supercritical CO₂ induced PLA isothermal crystallization decreased with the increase of isothermal crystallization temperature and immersion pressure,and under the action of supercritical CO₂,the transition from?crystal form to??crystal form was induced.The formation mechanism of these three special crystal morphologies may be due to the different plasticizing effects of supercritical CO₂ and the different degrees of mutual transformation of ordinary spherulites and ring-banded spherulites during the growth of spherulites.The results of the PLA cell structures showed that foaming at 16 MPa/80?⁹0?induced the formation of ring-banded spherulites and transition spherulites,respectively,leading to the formation of cell structures elongated in the radial direction.Foaming at16 MPa/100?¹20?induced the formation of mixed spherulites,leading to the formation of a uniform flower-like cell structure with a few open cells.However,foaming at 16 MPa/130?,irregular ordinary spherulites and low crystallinity resulted in the formation of merged and collapsed cell structures.In addition,increasing the soaking pressure was beneficial to increase the expansion ratio,reducing the cell size and cell density,and led to a partially open cell structure.However,under different foaming conditions,the compressive strength tended to increase first and then decrease.When foamed at 16 MPa/100?,the maximum compressive strength was 13.87 MPa.