Preparation And Properties Of PLA/(Phosphorus Containing Flame Retardant) Blends

Poly（lactic acid）（PLA）is a biodegradable polymer.It has good biocompatibility and mechanical properties and has been widely used in biomedicine,packaging and other fields.However,PLA is composed of C,H and O elements and has chain structure,so its fire resistance is very poor.Moreover,the combustion process of PLA is accompanied by violent flaming dripping,which is easy to cause secondary injury.The poor flame retardancy limits the application of PLA in many fields with fire risk.Therefore,the flame retardant modification of PLA is of great significance to broaden its application fields.In order to improve the flame retardancy of PLA without damaging its mechanical properties,a series of phosphorus containing flame retardants were designed and synthesized,and the corresponding flame-retarded PLA were prepared.The flame retardancy of the flame-retarded PLA was measured and the flame retardant mechanism was explored.The main contents of this work are as follows:First of all,using Hexachlorocyclotriphosphazene（HCCP）as raw material,the flame retardant HFPCP was synthesized by replacing the six P-Cl bonds with ethylene glycol monomethyl ether（2-Me）.The structure of the synthesized HFPCP was characterized by FT-IR,¹H NMR and ³¹P NMR.PLA/HFPCP blends were prepared by adding HFPCP to PLA through melt processing,and the flame retardancy was tested by LOI and UL-94.The test results showed that HFPCP can improve the flame retardancy of PLA obviously.When the addition amount of HFPCP reached 3 wt%,the LOI of PLA can exceed 27%and PLA can pass UL-94 V-0 rating.In addition,the results form EDS proved that HFPCP has good dispersion in PLA matrix,which is mainly related to the similarity between the alkyl segment in HFPCP and PLA.The effect of HFPCP on the thermal stability of PLA was analyzed by TGA and pyrolysis kinetics.The results showed that the initial decomposition temperature,maximum decomposition rate temperature and E_αof PLA/HFPCP were higher than those of PLA,indicating that HFPCP can improve the thermal stability of PLA.HFPCP is a small liquid molecule,which would play a plasticizing role in PLA matrix.Therefore,the tensile strength of PLA/HFPCP blend was lower than that of PLA,but the difference in notch impact strength was not obviously.Basing on the former work,we synthesized the epoxy groups-containing flame retardant HCCP-EP.The epoxy groups can react with the hydroxyl groups of PLA during melt processing,which makes HCCP-EP playing the role of a chain extender.As a result,HCCP-EP can simultaneously enhance the flame retardancy and mechanical properties of PLA.The structure of the synthesized HCCP-EP was characterized by FT-IR,¹H NMR and ³¹P NMR.PLA/HCCP-EP blends were prepared by adding HCCP-EP to PLA through melt processing and the reaction between HCCP-EP and PLA was successfully proved by FT-IR.The flame retardancy of PLA/HCCP-EP blends were tested by LOI,UL-94 and Cone calorimeter test.The test results showed that HCCP-EP can improve the flame retardancy of PLA obviously.When the addition amount of HCCP-EP reached 3 wt%,the LOI of PLA was improved from 19.5%to 27.8%and PLA can pass UL-94 V-0 rating.The flame retardant mechanism of HCCP-EP was studied by TGA,pyrolysis kinetics and TG-IR.Tensile and notch impact tests results showed that the mechanical properties of PLA/HCCP-EP blends were better than PLA（the notch impact strength of PLA/HCCP-EP-5 was about 25%higher than PLA）.Finally,in order to further improve the flame retardancy of blends,Lignin（P）was synthesized as a synergist of HCCP-HEP.Lignin（P）was synthesized by lignin,KH-550 and DPPC,and its structure was proved by FT-TR,¹H NMR,³¹P NMR and XPS.The phosphorus content of Lignin（P）was also calculated by the results from XPS.The ternary composites of PLA,HCCP-EP and lignin（P）were prepared by melt processing,and their flame retardancy were tested by LOI,UL-94 and Cone calorimeter test.The results showed that adding only 1wt%HCCP-HEP and 1 wt%Lignin（P）can improve the LOI of PLA to 28.3%and making PLA achieving UL-94 V-0 rating.Studies on flame retardant mechanism showed that Lignin（P）played an important role in the gas phase,and its effect in the condensed phase is similar to that of HCCP-HEP.The mechanical properties tests showed that the tensile strength of the composites with 1 wt%HCCP-HEP and 1 wt%Lignin（P）was 58.8 MPa.In addition,due to the nature of lignin,the addition of Lignin（P）can also improve the UV resistance of PLA obviously.This work broadened the application field of PLA and provided new ideas and technical reference for the flame retardant modification of PLA and other degradable polyesters.