A novel phosphorus-nitrogen composite flame retardant DMM was synthesized using 9,10-dihydro-9-oxa-10-phosphafi-10-oxide(DOPO),maleic anhydride(MA)and melamine(ME)as main raw materials.The structure and thermal stability of DMM were characterized by Fourier infrared spectroscopy(FT-IR)and thermogravimetric analysis(TGA),respectively.DMM and polylactic acid(PLA)were compounded to prepare flame retardant composites.The flame retardant properties of the composites were investigated by limiting oxygen index(LOI),vertical combustion test(UL-94)and cone calorimetric test(CCT)analysis.The results show that the flame retardant performance of the composites reaches UL-94 V0 level and LOI increases from 21％to 30％when the mass fraction of DMM is 3％.The peak heat release rate(PHRR)decreases from 510 kW/m2 to 418 kW/m2,the total heat release(THR)decreases from 87 MJ/m2 to 73 MJ/m2,the total smoke release(TSR)decreases from 211 m2/m2 to 70 m2/m2,and the amount of CO2 released also decreases.Scanning electron microscopy(SEM)analysis of the char layer after combustion reveals that the PLA composite with 3％DMM produces a dense and porous char layer which effectively cut off oxygen and heat transfer.In addition,DMM releases a large amount of non-combustible gas during combustion.Consequently,DMM can play a flame retardant role in the solid and gas phases.However,the mechanical performance test results show that the addition of DMM leads to a slight decrease in the mechanical properties of PLA.
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