Calcined Kaolin For Catalytic Preparation Of Poly L-lactic Acid

Poly lactic acid(PLA)is a kind of green polymer materials which is produced from renewable resources.This polymer can be degraded to carbon dioxide and water.It has attracted much attention from both industrial and academic fields since it can be put to a variety of uses,including biomedical,packaging and textile industry.The synthesis of PLA can be carried out through either the ring-opening polymerization of lactide or the direct polycondensation of the L-lactic acid monomer.PLA is commonly produced by ROP process which is highly successful in producing PLA with high molecular weight and good mechanical properties.However,the cost of producing PLA using ROP method is high due to the complicated purification process of the lactide.In contrast,PLA can be synthesized by simpler and cheaper direct melt polycondensation and this method has now attracted increasing interest.Kaolin is a kind of natural mineral materials.In recent years,with its low cost and wide source,kaolin have been widely used in ceramics,drug slow-releasing fields and so on.Kaolin calcinated in high temperature loses the hydroxy groups and the content of silica,aluminium trioxide increase.These make the calcined kaolin a new type of mineral nanometer catalyst since the reactivity points increase.In polymerization of poly lactic acid,stannous(II)octoate is widely recognised as one of the most effective catalysts,but so far,at home or abroad,there was not report in the literature about calcined kaolin as a catalyst for the preparation of PLA.In this paper,PLA with high molecular weight was successfully produced through the ring-opening polymerization of lactide which catalyzed by the new type of mineral material calcined kaolin.Then modified melt polycondensation of L-lactic acid was conducted with a binary calcined kaolin/Sn(Oct)2 catalyst system to synthesize high molecular weight poly lactic acid(PLA).The effect of the reaction time and temperature,catalyst amount on the molecular weight and yield of PLA has been investigated.The weight-average molecular weight of the poly lactic acid reached 48800 when degree of vacuum,reaction temperature,reaction time,calcined kaolin and Sn(Oct)2 content is 3000Pa,170?,8h,0.03wt%and 0.067wt%respectively.Hydroxyl terminated poly lactic acid was synthesized by melt polycondensation of PLA added a reserve amount of 1,4-butanediol.Then the chain extender 4,4'-diphenylmethane diisocyanate(MDI)was added and reacted with the poly lactic acid.As a result,the weight-average molecular weight of the poly lactic acid improved from 44700 to 74300.The microstructure and thermal properties of PLA have been characterized by FT-IR,1H NMR and DSC respectively.In conclusion,this paper mainly studied the direct melt polycondensation of L-lactic acid.The mild reaction conditions,simple equipments and processes make the poly lactic acid industrialized production possible.Calcined kaolin was used completely or partially to replace the traditional Sn(Oct)2 and catalyzed the polycondensation of poly lactic acid.This method with good catalytic efficiency,not only was safe,cheap,but also won the outstanding performance of the PLA.