Preparation And Foaming Properties Of High Heat-Resistant Polylactic Acid Composites

Environmentally friendly materials have been studied extensively to handle the plastic pollution caused by petroleum-based materials.Polylactic acid（PLA）is one biodegradable polymer material with excellent biocompatibility and mechanical properties,which has great potential applications in food packaging,biomedicine,and agriculture.However,the application of PLA in fields such as engineering plastics and high-temperature environments has been seriously hindered by its brittleness,low melt strength,and poor heat resistance caused by slow crystallization rate.Therefore,toughening,heat resistance and melt strength modification of PLA are very important.In this thesis,a special nucleating agent（TMC）was used to improve the crystallinity of PLA,the toughness of PLA was improved by melt blending polybutylene terephthalate（PBAT）,and a reactive chain extender（ADR）was used to improve the compatibility of PLA and PBAT and melt strength,finally,PLA-based composites with both high heat resistance and toughness were prepared by supercritical CO₂ foaming technology.The main studies are as follows:The nucleating agent TMC was used for the co-blending modification of PLA.The results show that the crystallinity of PLA was increased from 2.79%to 35.05%and the semi-crystallization time was shortened from 3.78 min to 0.70 min when the mass fraction of TMC was 1%,and a certain content of TMC could induce PLA lamellar to form dense“shish-kebab”structure.The Vicat softening temperature of pure PLA increased from 57.4℃ to 149.2℃,which indicated that the material had better heat resistance and thermal stability.PLA-PBAT/ADR/TMC composites with excellent heat resistance and toughness were prepared by melt blending toughening of PLA using PBAT and modifying PLA-PBAT blends by ADR and TMC.The results showed that the PLA-PBAT/ADR/TMC composites had better interfacial compatibility between the two phases,and the elongation at break of pure PLA increased from（5±0.8）%to（176.42±9.16）%,and the crystallinity increased from 2.79%to 34.75%,which significantly improved the heat resistance of PLA,and the Vicat softening temperature reached 122.1℃,which was about 2.1 times that of pure PLA（57.4℃）.A corresponding increase in the complex viscosity and energy storage modulus of the composite with the addition of ADR and TMC could be observed over the entire frequency range,indicated an increase in the melt strength of the composite,which facilitated subsequent foaming of the composite.The foaming behavior of PLA-based foams was investigated by supercritical CO₂foaming technology.When the foaming temperature was 120℃,the expansion rate and cell diameter of PLA/1%TMC reached the maximum value,and the resistance to thermal deformation was greatly enhanced.When the soaking temperature was much lower than the melting temperature,spherical crystals and"shish-kebab"nanoporous structures were observed in PLA and PLA/1.0%TMC foams,respectively.As the foaming temperature increased,the unmelted completely crystals act as physical cross-linking points,which also contribute to the high melt strength.In combination with the chain expansion reaction of ADR,PLA-PBAT/ADR/TMC foam showed the largest cell diameter and expansion ratio of 19μm and 8.9,respectively,and the heat resistance of PLA-PBAT/ADR/TMC foam was also significantly improved.Thus,the composite foams with excellent heat resistance and toughness were obtained to broaden their applications as biodegradable disposable hot food tableware.