Preparation And Properties Of High Fluidity Poly(Lactic Acid)

Polylactic acid(PLA)is a kind of bio-based and biodegradable polymer with excellent biocompatibility,mechanical strength and transparency.It has broad application prospects in fiber spinning,biomedicine and packaging.Thin wall injection molding is a kind of processing method to produce thin-wall PLA products,which has the advantages of short molding cycle,thin product thickness,light weight and so on.The melt fluidity of high molecular weight PLA is difficult to meet the requirements of thin-wall injection molding,which limits its application in thin-wall injection molding.Therefore,in order to improve the melt fluidity of PLA and maintain its good mechanical properties,we designed and synthesized caprolactone lactic acid random copolymer(P(CL-co-LA))and caprolactone lactic acid hyperbranched random copolymer(HBP(CL-co-LA))as flow aids,and prepared PLA blends with high strength and melt fluidity under melt processing conditions.This work provides a new method for the preparation of high fluidity PLA materials,and provides technical support for the application of PLA in the field of thin-wall injection molding.The main contents of this paper are as follows:1)PLA/flow aids blends were prepared by melt blending.Under the same conditions,the lubricating effects of five kinds of flow aids on PLA were compared and studied.It was found that ethylene bis stearamithe(EBS)had the best lubrication effect on PLA.The effects of the amount of EBS on the lubrication effect,mechanical properties,micro morphology and crystallization behavior were investigated,and the lubrication and strengthening mechanism were discussed.The low viscosity of EBS melt can effectively reduce the interaction force between PLA chains,reduce the friction between PLA chains.When a large amount of additive is added,it has the function of external lubrication.When the content of EBS is 1 wt%,the tensile strength of PLA blends reaches to 69 MPa,which is 15% higher than that of PLA,and the melt flow rate(MFR)reaches to 28 g/10 min,which is 40% higher than that of PLA.The amide group of EBS forms hydrogen bond with the ester group of PLA,which promotes the close packing of PLA blends.Therefore,it can prove the melt fluidity of PLA and enhance PLA at the same time.When 10 wt% EBS is added,the MFR of PLA blend reaches to 83 g/10 min,which is 315% higher than that of pure PLA.However,due to the poor compatibility between EBS and PLA,excessive addition of EBS will lead to agglomeration of EBS,phase separation and decrease the tensile strength of the blends.2)Biodegradable caprolactone lactic acid linear random copolymer P(CL-co-LA)was designed and synthesized by ring opening polymerization.PLA/P(CL-co-LA)blends were prepared by melt blending with PLA and P(CL-co-LA).The chemical structure of P(CL-co-LA)can be controlled by adjusting the ratio of ?-caprolactone(?-CL)to L-lactide(L-LA).The effects of the chemical structure and addition amount of the copolymer on the morphology,crystallization behavior,mechanical properties and lubrication effect of PLA were systematically studied.It is found that the chemical structure and addition amount of P(CL-coLA)can affect the dispersibility and lubrication effect of P(CL-co-LA)in PLA matrix.When the content of P(CL-co-LA)is 1 wt%,the MFR of PLA reaches to 30 g/10 min,which is 50%higher than that of PLA,and the tensile strength reaches to 73 MPa,which is 22% higher than that of PLA.P(CL-co-LA)has good compatibility with PLA.P(CL-co-LA)can disperse into the gaps between PLA molecular chains,which promotes the compact packing of PLA,makes the density of blends increase by 9.8%.3)Biodegradable hyperbranched random copolymer of caprolactone lactic acid(HBP(CLco-LA))was designed and synthesized.The chemical structure of HBP(CL-co-LA)can be controlled by adjusting the ratio of ?-CL to L-LA.PLA/HBP(CL-co-LA)blends were prepared by melt blending with PLA and HBP(CL-co-LA).The effects of the chemical structure and amount of the hyperbranched copolymer on the morphology,crystallization behavior,mechanical properties and lubrication effect of PLA were systematically studied,and the modification mechanism was discussed.The results show that hyperbranched polymer with large free volume and low viscosity could improve the melt fluidity of PLA.When the content of HBP(CL-co-LA)was 1 wt%,the MFR reached to 35 g/10 min,which increases by 75% than that of PLA.And the tensile strength reaches to 69 MPa,which increases by 15%.With the increase of L-LA content in hyperbranched copolymers,the compatibility of hyperbranched copolymers with PLA gradually increased,the chain shielding effect gradually decreased,resulting in the MFR of blends first increased and then decreased with the increase of L-LA content.