Study On Polylactic Acid Foam Beads Via Supercritical CO₂

Polylactic acid?PLA?has good mechanical properties and biocompatibility.It is derived from renewable materials that are completely biomaterials,and its performance is comparable to that of petroleum-based polyolefins.Its foaming materials have both the original properties and the original properties.Because of its light weight,sound insulation,and shock absorption,it has a wide range of applications in tissue materials,medical materials,packaging materials,and 5G materials.It is hailed as the most promising alternative to petroleum-based polyolefin materials in the future.PLA is a semi-crystalline polymer with poor toughness,low melt strength,slow crystallization rate,and narrow foaming temperature window.These characteristics easily cause defects such as co-foaming and cell collapse in the foaming process,which is not conducive to foaming.The hot air method was used instead of the liquid medium to explore,and the thermal crystallization,capillary rheometer,polarizing microscope and scanning electron microscope were used to analyze the melt crystallization properties,rheological properties and thermal stability of PLA foam beads of different brands.A series of comparative analyses were performed with the cell morphology.The results show that in the range of processing temperature of 85-100?and saturation pressure of 12-20MPa,with the increase of foaming pressure and temperature,the foaming ratio of foaming beads increases;the PLA of 2003D due to d-lactic acid High content,lower crystallinity,smaller molecular weight,melting point and melt viscosity are lower than PLA of 4032D.Therefore,the former has a larger cell diameter under the same process conditions,and can maintain cell stability While getting higher magnification.Using supercritical CO₂ as the foaming agent,the autoclaved foaming method was used,and the solid phase medium was used instead of the liquid phase medium.The effects of foaming process parameters and different foaming media on the cell structure of the PLA foam samples were investigated.The one-to-one correspondence between the cell foaming process parameters,the foaming medium and the cell structure realizes a series of regulation of the cell structure.The results show that when nano-CaCO₃ is used as the foaming medium,the microcellular structure,binary cell structure,"honeycomb"cell structure,and hollow cells can be obtained by increasing the foaming pressure without changing the foaming temperature.These four different cell structures realize the regulation of different shapes and sizes of the foamed cells,and can obtain hollow foamed beads with a maximum expansion ratio of 51.4times at 100^oCand 20MPa.A high-melt-strength polylactic acid was prepared by solid-phase grafting of polyfunctional acrylate?SR-444 NS?with PLA,and a high-melt-strength polylactic acid blend was prepared by a kettle method using supercritical CO₂ as a blowing agent.Expanded beads.With the increase of SR-444 NS content,the degree of branching entanglement increases,the melt strength is higher,the cell density increases,the expansion ratio increases,and the crystallization performance is enhanced,thereby enhancing the foaming performance.The results show that the foaming effect is the best when the monomer SR-444 NS content is 1.6%and 2.0%.Under the process conditions of 95^oC and 20MPa,while maintaining the micropores,the maximum foaming ratio is increased from 4.67 times to 19.23 times.