| Polylactic acid (PLA) is a biodegradable thermoplastic polyester, which has a wide range of applications in the medical materials, packaging films and other fields because of its good biocompatibility, biodegradability, mechanical strength and light transmittance andother properties.However, its drawbaks, such as high brittleness, thermal instability, slow crystallization rate, poor heat resistance etc. limit the practical application of PLA products.In order to improve its ductility and toughness, PLA was modified by castor oil (CO) in this paper. Due to poor compatibility between PLA and CO, as well as drastical reduction in melt viscosity of PLA with the introduction of CO, the mechanical performances and processability of PLA/CO blend was unsatisfactory. Hence, a multicomponent epoxy compound (ADR) as chain extender was introduced to improve the properties of PLA/CO blends. It was found that the addition of ADR led to a significant increase in the viscosity of the PLA/CO blends and a decrease in CO domains' size. As a result, the tensile ductility of PLA/CO blend showed a greater improvemriet. The effects of the addition of ADR on the isothermal melt crystallization and isothermal cold crystallization of PLA/CO blends were systematically studied. The crystallinity of PLA/CO/ADR blends was further improved by heat treatment, and finally a set of PLA/CO/ADR blends with high heatresistance and high tensile toughness were obtained.(1) Effects of CO on the rheological behavior, micro structure and mechanical properties of PLA were studied.The effects of heat treatment on the crystallinity, tensile properties, heat distortion temperature 'and micro structure of the composites were investigated.The relationship of crystallinity and tensile toughness and heat resistance of the samples were established. It was found that the addition of CO led to a decrease in the melt viscosity and melt torque value of PLA. With the increase of the content of CO from 0 to 10%, the elongation at break of PLA/CO blends increased from 5.5% to 126.3%. When the content of CO increased to 15%, the elongation at break fall to 76.7%.The tensile strength of PLA/CO blends decreased significantly with the increase of CO content. SEM images of brittle section of the sample showed that CO formed a large number of dispersed phases in PLA matrix. SEM showed that the PLA blends had obvious plastic deformation with the addition of CO. After heat treatment, the crystallinity of the samples increased from the lower crystallinity (<8%) to more than 40%. VST of the samples increased from about 65 ? to above 150?, and the heat resistant performance was improved significantly.And the tensile strength of the samples increased with the increase of crystallinity, but the elongation at break obviously reduced. This is because the crystal structure has a large rigidity, hindering the yield of the deformation caused by the blends.(2) Effects of the addition of epoxy chain extender ADR on the processing torque,melt flow rate index, thermal performance of PLA/CO blends were studied. The results showed that the addition of ADR increased the torque value of PLA, and torque value was greater with the larger amount of ADR.The MFR of the blends reduced with the addition of ADR, ADR can significantly increased viscosity of the blends.It was found that the addition of ADR led to a decrease in CO domains' size. SEM observations showed that the drawing energy increased with the increase of ADR. The elongation at break of specimens increased significantly with increase of the content of ADR. Elongation at break of the PLA/CO/ADR(95/5/1.0) blends was as high as 159.9%,and the tensile strength was 47.1 MPa. However, the elongation at break of the PLA/CO/ADR(95/10/1.0) blends was up to 182%. However, the strength reduced to 39.2 MPa. It was obvious that PLA/CO/ADR(95/5/1.0) blend showed a high tensile elongation with a lower loss on its tensile strength. DSC results showed the Tg decreased slightly and the Tcc shifted to high temperature with the addition of ADR, indicating that ADR inhibited cold crystallinity of blends. The Xc of blends was very low in quenched state, close to amorphous state, and Xc reduced due to chain extension reaction of PLA chains with the addition of ADR.(3) Effects of ADR addition on the isothermal melt crystallization and isothermal cold crystallization behavior of PLA/CO blends were studied. Crystallization kinetics was analyzed by the avrami equation. In addition, PLA/CO/ADR samples with high crystallinity were obtained by isothermal annealed treatment. Effects of crystallinity on mechanical properties, microstructure and heat resistant performance of the blends were investigated.From the isothermal melt crystallization kinetics, it was found that CO led to an increase in the melt crystallization rate of PLA. At the temperature lower than 105?, t1/2 increased as Tc decreased from 105 to 80?. And in the lower temperature range, under the same Tc, t1/2 increased with the increase of ADR, and the crystallization rate decreased.And in the higher temperature range above 105?, t1/2 increased with the increase of Tc.The isothermal crystallization process of PLA/CO/ADR blends belongs to homogeneous nucleation mechanism.The results showed that the cold crystallization temperature Tc was in the range of 80 to 90 ?, and the cooling capacity increased with the increase of Tc. At the same Tc, CO made the cold crystallization ability of PLA enhanced, t1/2 decreased. While the cold crystallinity of the blends decreased, t1/2 increased with the addition of ADR.Xc of the annealed samples increased significantly, close to 40%. The Xc decreased as the addition of ADR increased due to the extended chain/branched reaction of PLA at the presence of ADR, which damaged the regular structure of linear macromolecules of PLA.VST of the blends increased from 60 ? to above 150?, and the elongation at break of samples remained at a high level.As a result,with a proper annealed treatment and the addation of ADR,some PLA/CO blends with excellent tensile performance and heat resistance were obtained successfully. |
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