Study On Toughening Modification Of Poly (Lactic Acid) And Toughening Mechanism

Poly(lactic acid)(PLA)is a thermoplastic polymer that can be derived from renewable natural resources.It has a number of interesting properties including biodegrability,biocompatibility,good mechanical properties and processability.For these reasons,the use of PLA can be a solution to environmental pollution。Moreover,it supplies sufficient sources for plastic materials conventionally petroleum-based.However, PLA is now not widely used because of its brittleness,thermal instability and high cost.To expand the fields of its application,PLA needs to be modified.Aiming at improving the properties of PLA,especially the impact toughness,this article selected Poly(ethylene-co-vinyl acetate)(EVA)as toughening agent to blend with PLA.The elastomers always made the modulus and thermal stability of blends decline,while Polymer/layered silicates nancomposites have the advantage of improving modulus and thermal stability of blends.So this article selected both EVA and montmorillonite(MMT)to blend with PLA.The advantages of Polymer/elastomers composite and Polymer/layered silicates nano--composite were combined to prepare PLA/EVA/MMT composites by melt blending,and the toughening mechanism of EVA and MMT was also studied.At first,in order to improve the compatibility of PLA and EVA,two different compatilizers,EVA-MAH and E-MA-GMA,were used to melt blending with PLA and EVA.The analysis of morphology and mechanical properties of blends indicated that E-MA-GMA is more effective,so it was selected as the compatilizer for PLA/EVA blends. Mechanical properties of PLA/EVA/GMA blends were investigated and the results showed that when the content of E-MA-GMA remained the same,the strength and modulus of blends decreased gradually with the increase in EVA content,but the elongation at break and the area under the stress-strain curve increased evidently.When the content of EVA remained the same,the mechanical properties of blends could not always be improved by the increasing of E-MA-GMA,and excessive amount of E-MA-GMA even impaired the mechanical properties.So the optimal ratio for PLA/EVA/GMA blend was 70/30/5.Then the impact toughness, dynamic mechanical property and thermal stability of PLA,PLA/EVA (70/30)and PLA/EVA/GMA(70/30/5)blends were investigated,and the results indicated that:(1)the addition of E-MA-GMA increased the impact strength of blends and the fracture varied from brittle to ductile with the addition of EVA and E-MA-GMA;(2)the addition of E-MA-GMA made the T_g of both PLA and EVA shift towards each other;(3)the thermal stability of blends was lowered by EVA,but it could be improved by the addition of E-MA-GMA.Based on the studies of PLA/EVA blends,PLA/EVA/MMT composites were produced through various blending technologies.At first,three PLA/(EVA/MMT)/GMA blends with different content of MMT were produced by melt mixing,in which the content of MMT was 1.5%,3% and 6%respectively.The mechanical properties and phase morphology of the blends were investigated,and the results showed that the strength,the elongation at break and the area under the stress-strain curve decreased with the increasing of MMT,and the size of EVA dispersed in the PLA matrix became bigger,so the optimal content of MMT is 1.5%.Therefore, in the following studies,the content of MMT in the PLA/EVA/MMT blends was 1.5%.The studies of PLA/EVA/MMT blends produced by different blending technologies indicated that blending technologies made no difference in strength,but the elongation at break and modulus of blends changed due to different blending technologies,and the PLA/(EVA/MMT/GMA)/GMA blend had the best mechanical properties. The mechanical properties of PLA,PLA/EVA/GMA(70/30/5)and PLA/(EVA/MMT/GMA)/GMA blends were compared,and the results showed that the elongation at break and impact strength were improved by the addition of EVA and E-MA-GMA,but the modulus decreased evidently;the addition of MMT further improved the impact strength of the blend,meanwhile,the modulus increased by 6.13%compared to that of PLA/EVA/GMA blend.The results of SEM and DMA indicated that MMT could increase the dispersion of EVA,making the EVA particles smaller,and this could further improve the impact toughness of the blend.