Structure And Properties Of Supercritical CO₂ Foamed Polylactic Acid

In 2020,the National Development and Reform Commission and various cities have issued a policy of"plastic restriction and plastic ban",which clarified that conventional petroleum-based non-degradable plastics can no longer be used in some fields,bringing a spring of development for fully biodegradable plastics.As a fully biodegradable plastic,polylactic acid（PLA）has attracted much attention due to its excellent biocompatibility and physical properties;its physical foaming material has low density and light weight on the basis of its original superior performance.Advantages,as well as special functions such as sound insulation and heat insulation,shock absorption and wave absorption,have huge application potential in packaging,construction,tableware materials and tissue supports,and are increasingly becoming a substitute for traditional petroleum-derived polymer foams that are widely used in packaging.However,in the application of foaming materials,polylactic acid has the defects of low melt strength and slow crystallization rate,which lead to the collapse and merger of cells during the foaming process,inhibiting the production of high-quality foam.Therefore,it is meaningful to study the melt strength and crystallization properties of PLA under supercritical CO₂ to improve the foaming properties and control the cell morphology.In this paper,PLA microcellular foamed material was prepared by supercritical CO₂ autoclave foaming method using PLA as raw material.In the experimental part,the processing properties of two grades of PLA were investigated,the influence of material parameters on its rheological and crystalline properties was clarified,and its foaming properties were evaluated.PLA1 and polybutylene adipate-terephthalate（PBAT）were used to prepare PLA blended composite materials by melt extrusion.The foaming temperature,saturation pressure and saturation time were reasonably adjusted to obtain PLA foamed materials.The crystallization signal was characterized by differential scanning calorimetry.On this basis,the modifier and the composite material were selected for blending to prepare the modified material and its foamed material,and the effects of different process conditions on the foaming behavior and mechanical properties of the material were studied.The main work and conclusions of this paper are as follows:1.Molecular weight has a great influence on the rheological properties of PLA.Compared with PLA2,PLA1 has more high-molecular-weight moieties,so PLA1 has higher melt strength,and at the same time,it is easier to build an entangled network and disentangle during the process of melt stretching.During the cooling crystallization process,the folding of PLA1 molecular chains into the crystal lattice requires a larger activation energy,a higher melting point,and a larger crystallinity.PLA1 is beneficial for widening the processing temperature range and providing higher melt strength during foaming.2.PLA1 was used as raw material to prepare PLA/PBAT foaming precursor by melt blending method,and supercritical CO₂ was used as foaming agent to prepare PLA/PBAT foam by intermittent foaming.The effects on the foaming properties of PLA/PBAT foams were systematically characterized by SEM,DSC,density meter and universal tensile testing machine.The experimental data show that by adjusting the foaming temperature and saturation pressure within the same sufficient saturation time,the magnification of the foamed material can be controlled to be 15-30times,and the average cell size can be changed in the range of 10-70μm.When the foaming pressure or temperature is constant,and the temperature or pressure is increased,the cell size will show a growing trend,and the cell density will be the opposite;when the foaming temperature and pressure are constant,the foaming ratio and cell density will increase with the saturation time.lengthened and reduced.According to the DSC data of the foamed materials,there are double melting peaks under certain process condition.In the saturation stage of the foaming process,the PLA crystallization continues to improve,forming a high-temperature melting peak;while the induction of the low-temperature melting peak occurs during the pressure relief cooling during the foaming process.stage.At the same time,we found that the total crystallinity of all samples increased slightly,with a maximum of 39.3%;and the crystallinity of the high-temperature melting peak showed a regular change with the change of the foaming process conditions.Finally,combined with the foam parameters,it is concluded that the content of high melting point crystals is inversely proportional to the cell size;it is proportional to the cell density.The degree of perfection of PLA crystals has a strong correlation with foam parameters.3.In order to further improve the foaming performance of PLA materials,PLA and nucleating agents were blended to prepare modified foaming precursors and foaming materials.The single factor variable method was used to study the effect of nucleating agent content on the modified foaming precursors.rheological and crystalline properties,followed by characterization of foaming properties.The results show that the nucleating agent reduces the melt strength of PLA and improves the crystallization kinetics.The foaming performance of PLA foam material has a strong dependence on the melt strength,and the foaming ratio increases with the increase of melt viscosity.The foaming density increases with the increase of nucleating agent content.In the follow-up foam compression mechanical performance test,it is found that the compressive stress-strain curve of the foam showed a typical s-shape,with a linear elastic zone,a plateau zone and a compaction zone.The contribution of the deformation mainly comes from the shrinkage of the cell walls.For PLA foams with different relative densities,as the relative density decreases,the compressive strength and modulus of the foam decrease.