Study On The Flame Retardant Properties And Flame-retardanding Mechanism Of Poly(Lactic Acid) Composites Containing Phosphaphenanthrene Derivatives

Polylactic acid(PLA) has been widely used in various fields because of its excellent mechanical properties and biodegradability.However,PLA is a flammable material with extremely low limiting oxygen index.And the melting droplet phenomenon of PLA is serious during the combustion process.Therefore,it is of great significance to enhance the flame retardancy of PLA.Flame retardant of phosphaphenanthrene derivatives have been applied extensively owing to their environmental friendliness,lasting flame retardancy and good compatibility with matrix.In this paper,two kinds of phosphaphenanthrene derivatives with different structures were synthesized and added into PLA.Effects of the flame retardants on the thermal stability,combustion behavior and mechanical properties of PLA were studied,and the flame retardant mechanism was discussed.The main research contents are as follows:1.Two kinds of flame retardant including maleic anhydride-9,10-dihydro-9-oxa-10-phosphophene-10-sulfide(MAH-DOPS)and hexa(4-(9,10-di-hydro-9-oxa-10-phosphophene-10-sulfide)-hydroxymethylphenoxy)cyclotriphosph-azene(HAP-DOPS)derivatives were synthesized.The structures of the two flame retardants were characterized by Fourier infrared spectroscopy(FT-IR),nuclear magnetic resonance(NMR)and high resolution mass spectrometry(HR MS).The thermal stability of the two flame retardants was estimated by thermogravimetric analyzer(TGA).The results show that MAH-DOPS and HAP-DOPS have been successfully synthesized.They have good thermal stability under nitrogen atmosphere.Moerover,the residues at 700?of MAH-DOPS and HAP-DOPS is 19.07%and 54.14%,respectively.2.PLA/MAH-DOPS and PLA/HAP-DOPS were prepared by melt blending The chemical structure,thermal stability,flame retardant and mechanical properties of the two flame retardant composites were investigated by FT-IR,TGA,LOI,vertical combustion(UL-94),CONE calorimetry(CONE)and mechanical properties test.Moreover,Kissinger method,Flynn-Wall-Ozawa method and Coast-Redfern method were used to analyze the non-isothermal thermal decomposition kinetic data of PLA and flame retardant composites.In addition,the apparent morphology and elemental composition of the residue after CONE test were analyzed by scanning electron microscopy(SEM)and energy dispersive X-ray spectroscopy(EDS).The gaseous substances produced during thermal decomposition were measured by thermogravimetric infrared spectroscopy(TG-IR).Furthermore,flame retardant mechanism of the two derivatives in PLA composites was analyzed.The results show that both the addition of MAH-DOPS and HAP-DOPS reduces the initial decomposition temperature(T_5%)and the maximum thermal weight loss rate of the PLA composites,and the residuces at 700?of the two flame retardant composites increase.The activation energy(E)and pre-exponential factor(A)of the two flame retardant composites are lower than those of pure PLA.Both the thermal decomposition mechanism functions of PLA and PLA/MAH-DOPS are in accordance with g(?)=[-ln(1-?)]^3/4,with the reaction order of n=3/4.And the thermal decomposition mechanism are belongs to random nucleation and subsequent growth reaction.However the thermal decomposition mechanism function of PLA/HAP-DOPS is consistent with g(?)=1-(1-?)³,with the reaction order n=3.The results show that MAH-DOPS and HAP-DOPS display good flame retardant efficiency for PLA.PLA/MAH-DOPS and PLA/HAP-DOPS are both reached to UL-94 V-0 grade when the phosphorus content is 0.47%and 0.67%,respectively.The corresponding LOI value is 28.1%and 26.6%,respectively.CONE test results show that MAH-DOPS and HAP-DOPS decrease the ignition time(TTI),total heat release(THR)and effective heat of combustion(EHC)of PLA composites.However,the total smoke release(TSR)of flame retardant PLA composites are increased significantly.This indicates that both MAH-DOPS and HAP-DOPS are play a main flame retardant role in the gas-phase.SEM results show that the char layer formed by the addition of MAH-DOPS and HAP-DOPS become more dense.CONE,TG-IR and residues analyses show that MAH-DOPS and HAP-DOPS decompose and release phosphorus and sulfur containing substances,which have both gaseous and condensed flame retardant effects.Additionally,MAH-DOPS and HAP-DOPS have little effect on the mechanical properties of the two flame retardant composites.