| Polylactic acid (PLA) is a completely biodegradable polyester material produced in recent years by natural products such as corn and wheat. Due to its excellent biocompatibility, high mechanical strength and dimensional stability, PLA has been becoming a hot research and development in recent years. However,its shortcomings,such as high brittleness, slow crystallization rate and poor heat resistance greatly hinder the large-scale application of PLA.In this paper, an efficient nucleating agent TMC300 was used to improve the crystallization ability and heat resistance of PLA. At the same time, the fully biodegradable elastomer PBAT and a reactive compatibilizer - multiple epoxy compound (ADR) were introduced to study the influence of ADR and PBAT feeding method and nucleating TMC300 on the heat resistance and impact toughness of PLA.In addition, a green and efficient plasticizer-supercritical carbon dioxide was used to research the synergistic effect of supercritical carbon dioxide assisted blending and ADR on enhancements of the compatibility and toughness of PLA/PBAT blends.Moreover, the crystallinity and heat resistance of PLA/PBAT/ADR saMples were enhanced by using annealing treatment.Finally, a series of PLA/PBAT/ADR blends with high impact toughness and high heat resistance were obtained by using the two methods mentioned above.The main research contents and results are as follows:(1) Effect of nucleating agent TMC300 on improvement of crystallization ability of PLA was researched. The crystallization behavior of PLA under non-isothermal crystallization was studied by differential scanning calorimetry (DSC).The isothermal crystallization kinetics of PLA/TMC300 blends was studied in detail. It was found that the isothermal crystallization rate of PLA showed a drastic increase with the introduction of TMC300. And the optimal loading of TMC300 was determined to be 1 phr. After the isothermal crystallization, the crystallinity of PLA/TMC300 (100/1.0) specimen increased from 4.9% for quenched PLA specimen to 42.0%, and vicat softening point increased from 64.3? to 160.6?. Inaddition, it was found that tensile strength of the PLA/TMC300 specimen also showed a slightly enhancement.(2) Influences of ADR loadings and feeding methods (one-step and two-step methods) on the rheological properties, impact toughness and microstructure of PLA/PBAT (60/40) blends were studied in detail. Furthermore, the nucleating agent TMC300 was introduced to improve the crystallization rate and crystallinity of the blends. Finally, PLA/PBAT/ADR/TMC300 blends with high toughness and heat resistance were obtained.With the increase of ADR loading, the impact toughness of PLA/PBAT/ADR blends showed an enhancement. Moreover, compared with the one-step method, the two-step method further enhanced the reactive compatibilization effect of ADR on the interface between PLA matrix and PBAT, leading to stronger interfacial adhension and more refined PBAT domains. As a result, the PLA/PBAT/ADR blend prepared by using two-step method showed higher impact toughness than that obtained by using one-step method.It was found that the above-mentioned blends had a slow crystallization rate,resulting in a low crystallinity and vicat softening point (VST). And then, the nucleating agent TMC300 was added to increase the crystallization rate and crystallinity of PLA. And the influences of TMC300 and ADR on the crystallization behaviors of PL A/P BAT blends were researched. The obtained results showed that the addition of TMC300 greatly improved the crystallization rate of the blends due to the efficient heterogeneous nucleating effect of TMC300. However, the addition of ADR increased the melt viscosity, as well as reduced the segmental mobility of PLA/PBAT blends. Hence, it was found that the increase of melt viscosity reduced the crystallization ability of PLA matrix to a certain extent according to the obtained DSC and melt index results.Effects of crystallization temperature on the crystallinity, crystal structure,impact toughness and VST were studied. It was found that the impact toughness and VST of the PLA/PBAT/ADR blends prepared by using two-step method were significantly improved when fully crystallized at lower temperatures (80 0C) for 15 min. For example, notched impact strength and VST of quenched PLA/(PBAT/ADR)(60:40: 1.0) specimen were 74.48 kJ·m-2 and 78.4?, respectively. And notched impact strength and VST of crystallized PLA/(PBAT/ADR)/TMC300 (60:40:1.0:1.0)specimen were 74.26 kJ·m-2 and 129.6 ?, respectively. It can be pointed to that the stronger interfacial adhesion, finer PBAT domains obtained by using the two-step method and smaller crystals produced under the lower crystallization temperature could dissipate impact stress effectively. As a result, the crystallized PLA/(PBAT/ADR)/TMC300 specimens obtained using the two-step method and fully crystallized at 80 °C showed an excellent heat resistance with high impact toughness.As contrast, while PLA/PBAT/ADR specimens prepared using the one-step method had crystallized fully at 80? for 15min, their nothched impact strength showed a decrease to a certain extent. It could be attributed to a weaker reactive compatibilization effect of ADR on the interface between PLA and PBAT with the one-step method.When the PLA/PBAT/ADR/TMC300 samples prepared with one-step or two-step method had fully crystallized at a relatively high crystallization temperature(130?) for 15min, it was found from POM observation that the formed spherical crystals showed a very large size. The fully developed large spherulites squeezed each other, and caused an excessive rigidity and internal stress in the PLA/PBAT/ADR samples. It would make the PLA matrix difficult to yield, and easy to produce cracks.As a result, the impact toughness of PLA/PBAT/ADR blends crystallized fully at 130? decreased drasticlly.(3) By using a supercritical carbon dioxide assisted internal mixer, the synergistic enhancement and toughening effect of supercritical carbon dioxide and chain extender ADR on PLA/PBAT blends was studied, and the properties of the blends were further improved by heat treatment. As a result, a set of PLA/PBAT/ADR blends with high toughness and high heat resistance was obtained.It is confirmed by the torque heological behavior that the introduction of supercritical carbon dioxide (SCO2) can reduce the viscosity of the blending system during the preparation of the blends and play the role of physical plasticizer and compatibilizer.By SEM observation on microstructure of the samples, it is confirmed that the introduction of ADR can play the role of reaction compatibilizer, strengthening the interface adhesion, and refining the phase size. The notched impact strength increased from 8.76 kJ/m2 to 12.87 kJ/m2 At the same time, the addition of supercritical carbon dioxide further strengthened the compatibilization of ADR, and the phase size further reduced, and the notched impact strength increased up to 36.23 kJ/m2 significantly.After annealed treatment at 80? for 1h, VST of the fully crystallized PLA/PBAT/ADR specimens with high toughness increased from about 70 for the quenched specimens up to 149 ?. |
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