Synthesis Of Bis DOPO Phosphonates And Their Flame Retardant Mechanism On Polylactic Acid

The development of green renewable materials is the main way to solve the current resource shortage and environmental pollution.Poly（lactic acid）（PLA）,bio-based polymers derived from renewable resources,regarded as a“green”polymer,have received more and more attention in recent years.However,the application of PLA was limited especially in electronics,transportation or construction fields due to its inherent flammability.Thus,rendering flame retardant performance to PLA is a significant work.9,10-dihydro-9-oxa-10-phosphaphe-nanthrene-10-oxide（DOPO）and its derivatives,as halogen-free flame retardant,have attracted a lot of attentions in recent years.In this paper,three kinds of P-O bond bridged DOPO derivatives were prepared by relatively mild Atherton-Todd reaction.Then,effects of bridged DOPO derivative on the flame-retardant properties and thermal-degradation behavior of PLA were investigated.And the flame retardant mechanicam of PLA containing the synthesized product was further analysized.In order to better study the flame retardant process and flame retardant mechanism of the bridged DOPO derivatives,one of mono DOPO derivative applied in the industry,DOPO-HQ,has been added to the PLA to systematically coMPare the difference of the flame retardant efficiency and flame retardant mechanism between mono-and bridged-DOPO derivatives.The main research contents are as follows:（1）Three kinds of bis P-O bridged DOPO derivatives,1,2-di[（9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-10-yl）oxy]ethane,1,4-di[（9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-10-yl）oxy]benzene),1,5-bis[（9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-10-yl）oxy]naphthalene,were synthesized by Atherton-Todd reaction.The structures of bridged DOPO derivatives were confirmed by infrared spectroscopy（FT-IR）,nuclear magnetic resonance(¹H NMR,³¹P NMR),mass spectrometry（MS）.Differential scanning calorimetry（DSC）test showed that the melting points of the three flame retardants were 158℃,206℃and 238℃,respectively.In addition,thermogravimetric analysis（TG）showed that all three flame retardants possess high thermal stability,and the residue increases with the increase of the aromatic groups in the bridge chain.Py-GC/MS tests showed that the degradation of three bridged DOPO derivatives produced biphenyl,dibenzofuran,biphenyl-2-phenol and other aromatic substances.The kinetic calculation results showed that all three flame retardants have higher activation energy.（2）Three kinds of bridged DOPO derivatives were added to poly（lactic acid）,and then the flame retardant properties was studied by vertical combustion（UL-94）,limiting oxygen index（LOI）and cone calorimetry.The results showed that flame retardancy of PLA was significantly enhanced after introducing three kinds of flame retardants.When EG-DOPO₂ loading was 5%,the PLA composites reached V-0 rating,resulting from the higher P content of EG-DOPO₂ than the other two product.And when the addation of HQE-DOPO₂ or TU-DOPO₂ was increased by 7.5%,PLA composite reached V-0 level.EG-DOPO₂ revealed the most obvious effect on LOI of PLA composites,when the addition amount was 15%of EG-DOPO₂,the LOI of PLA reached 36.4%.The incorporation of three kinds of bridged DOPO derivatives all prolonged the TTI,decreased the peak heat release rate（PHRR）.Characterization of char morphology by Raman spectroscopy,Energy Dispersive Spectroscopy（EDS）and scanning electron microscopy（SEM）showed that the introduction of three kinds of bridged DOPO derivatives were conducive to the formation of char layer.The highest P and the best degree of graphitization of the carbon layer of PLA/HQE-DOPO₂ were obtained,suggesting better solid phase flame retardant efficiency of HQE-DOPO₂ than both EG-DOPO₂ and TU-DOPO₂.TG-FTIR results indicated that phosphorus-containing substances were formed in the thermal degradation gases of the three flame retardants.These phosphorus-containing substances may play a good role in flame retardation during combustion.In addition,TG test results indicated that the thermal stability of PLA was significantly enhanced after introducing three kinds of bridged DOPO derivatives.Especially,under the air atmosphere,the initial degradation temperature of PLA/TU-DOPO₂ was 354℃,which was 20℃higher than pure PLA.Finally,the mechanical properties of the three composites were studied.（3）By coMParing the thermal properties of HQE-DOPO₂ and DOPO-HQ,the results showed that the introduction of the aromatic ring group in the bridged chain increased the rigidity of the molecule,endows higher thermal stability of HQE-DOPO₂than that of DOPO-HQ.The combustion behavior and thermal stability of PLA/DOPO-HQ and PLA/HQE-DOPO₂ were characterized by vertical combustion（UL-94）,limiting oxygen index（LOI）,cone calorimetry and TG.The results showed that the flame retardant efficiency of the HQE-DOPO₂ was better than that of the DOPO-HQ.When the addition of DOPO-HQ increased to 15%,the V-0 band can be achieved,and the LOI was lower 11.8%than that of PLA/HQE-DOPO₂-15%composite.In addition,the thermal stability of PLA/HQE-DOPO₂ was better than that of PLA/DOPO-HQ.However,SEM and Raman analysis of the carbon layer showed that more complete and dense char layer PLA/DOPO-HQ was formed as coMParison to PLA/HQE-DOPO₂.Therefore,DOPO-HQ displayed more obvious solid phase flame retardant effect then HQE-DOPO₂.In the gas phase,phosphorus-containing substances produced by thermal degradation of two flame retardant composites was detected by the TG-FTIR,indicating that both flame retardants showed a gas phase flame retardant effect.