Preparation And Properties Of Halogen-free Flame Retardant Epoxy Resin System

Epoxy resins are widely used in coating,adhesives,laminate and electronic industry due to their attractive characteristics of good mechanical properties,high adhesion to substrates,good chemical and corrosion resistance,superior electrical properties and excellent dimensional stability.Like most of organic polymer materials,conventional epoxy resins are combustible and their flame retardant properties are poor,which limits their application in the fields of high flame-resistance requirement.It is necessary to improve the flame retardant performances of epoxy resins in order to enlarge their applications.Currently,the applications of halogenated compounds are restrained for environmental reasons.Therefore,the development and application of halogen-free flame retardants has been researched as a popular topic in both scientific and industrial communities.In this paper,the basic idea is to combine different phosphorus-or nitrogen-containing groups with certain flame-retardant effect by means of composition and molecular designs.The resulted flame retardants were then used for flame retardant modification of epoxy resin.We hope to further enhance the flame retardant efficiency by component or group synergistic effect,and improve other properties at the same time.The effects of phosphorus/nitrogen-containing flame retardants on thermal properties,flame retardant properties and burning behaviors of epoxy resins were studied and the flame-retardant mechanisms in both gaseous and condensed phases were discussed.The main research contents of this paper were listed as follows:1.A novel reactive flame-retarded epoxy resin system was prepared by copolymerizingdiglycidyletherofbisphenol-A?DGEBA?with9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide?DOPO?,N,N'-bismaleimide-4,4'-diphenylmethane?BDM?and 4,4'-diamino-diphenyl sulfone?DDS?.Curing behavior,thermal and flame-retardant properties,burning behaviors and flame-retardant mechanism of the cured epoxy resins were studied by DSC,TGA,LOI,UL94 vertical burning test,cone calorimeter,³¹P NMR and SEM.The results indicated that DOPO group was exclusively grafted on BDM molecule and the end-cappingreactionbetweenDOPOandDGEBAwasavoided.EP/DDS/BDM/DOPO thermosets presented high glass transition temperatures of210-223°C.The TGA results disclosed that DOPO group induced the decomposition of resin matrix in advance.DOPO and BDM synergistically promoted the carbonization of resin matrix in high temperature region.The results of combustion tests indicated that the flame-retardant properties were excellent under low loading of phosphorus.EP/DDS/BDM/DOPO-10 with phosphorus content of only 0.83 wt%achieved a LOI value of as high as 34.5%and the sample passed UL94 V-0 rating;compared with the neat epoxy resin,its pk-HRR and av-HRR were decreased by 41%and 57%,respectively.The FTIR and morphology analyses indicated that BDM and DOPO promoted the formation of continuous,compact and intumescent char layer with high char yield.There was a synergistic effect between BDM and DOPO in condensed phase.In addition,DOPO suppressed the combustion intensity in gaseous phase due to its gaseous phase quenching effect.2.A reactive flame retardant?DOPO-HPM?containing maleimide and DOPO groups was successfully synthesized.Then a reactive phosphorus-nitrogen flame retardant system was prepared by copolymerizing DGEBA with DOPO-HPM,TGIC and DDS.The result indicated that Tgs of the flame retardant samples were similar to that of the neat epoxy resin?197-205°C?.The TGA data indicated that DOPO-HPM catalyzed the decomposition of resin matrix in advance.The compounded DOPO-HPM/TGIC flame retardant effectively improved the char yields in high temperature region.The results of combustion tests indicated that DOPO-HPM/TGIC system dramatically enhanced the flame retardant and smoke suppression properties.When the phosphorus content was only 1.25 wt%,P-1.25 sample achieved a LOI value of 38.5%and passed UL94 V-0 rating;compared with neat epoxy resin,its pk-HRR,av-HRR,av-EHC,THR and TSP were decreased by 61%,36%,23%,35%and 30%,respectively.The results of cone calorimeter,Py-GC/MS and char residue analysis disclosed that DOPO-HPM/TGIC system released free radicals with quenching effect and thus retarded the combustion in gaseous phase.In addition,DOPO-HPM/TGIC system prevented larger chain segments from shifting to gaseous phase,thus facilitating the carbonization of resin matrix and formation of intumescent char layer which could reduce the efficiency of heat-and oxygen-exchange.3.Triazine derivative with active maleimide group?TMT?was synthesized using a new method,and higher yield of TMT was obtained.Then a reactive phosphorus-nitrogen flame retardant system was prepared by copolymerizing DGEBA with TMT,DOPO and DDM.DOPO group was exclusively grafted on TMT molecule by controlling the adding order of components during preparation process.The T_gs of EP/DDM/TMT/DOPO thermosets were only slightly decreased?154-160°C?.The TGA data indicated that DOPO and TMT synergistically promoted the carbonization of resin matrix.The typical characteristic of EP/DDM/TMT/DOPO thermosets was the excellent flame retardant and smoke suppression performances with low loading of phosphorus.When the phosphorus content was only 0.4 wt%,the sample achieved LOI value of 34%and passed UL94 V-1 rating.When the phosphorus content was only 1 wt%,the sample achieved LOI value of as high as40.3%and passed UL94 V-0 rating;compared with the neat epoxy resin,its pk-HRR,av-HRR,av-EHC,THR and TSP were decreased by 37%,37%,32%,35%and 21%,respectively.The studies on flame retardant mechanism disclosed that there was a synergy between DOPO and TMT in both gaseous and condensed phases.During combustion,the flame retardant system released inert gases and promoted the formation of compact and intumescent char layer,exerting bi-phase flame retardant effect.4.A novel additive?DOPO-TMT?with multifunctional groups containing phosphaphenanthrene,maleimide and triazine structures was successfully synthesized.DOPO-TMT was then blended with epoxy resin to prepare flame retardant thermosets.The TGA data indicated that DOPO-TMT had good thermal stability and carbonization property.DOPO-TMT could induce the degradation of resin matrix in advance and facilitate the carbonization of resin matrix.DOPO-TMT just slightly reduced the T_gs under low loading.The combustion tests indicated that DOPO-TMT remarkably enhanced the flame retardant performance of epoxy resin.EP/DOPO-TMT-1.25 had a LOI value of 37.5%and passed UL94 V-0 rating;its pk-HRR,av-HRR,av-EHC and THR were decreased by 54%,29%,23%and 30%,respectively,compared with those of the neat epoxy resin.TGA-FTIR and Py-GC/MS analyses disclosed that DOPO-TMT released phosphorus-containing free radicals with quenching effect and nitrogen-containing inert gases during thermal decomposition.The char residue analysis indicated that all the functional groups of DOPO-TMT presented flame retardant effect in condensed phase.DOPO-TMT catalyzed the formation of compact and intumescent char layer.The good flame retardant performance of EP/DOPO-TMT thermosets was based on the synergistic flame-retardant effect between different functional group of DOPO-TMT.5.A reactive flame retardant?HMCP?containing maleimide and cyclotriphosphazene was successfully synthesized.HMCP was then blended with DGEBA to prepare flame retardant epoxy resins.The result indicated that HMCP exhibited excellent thermal stability and carbonization property;its T_5%and char yield at 800°C were 413°C and 57%,respectively.HMCP increased the T_g of epoxy resin.HMCP catalyzed the degradation of resin matrix in advance and promote the carbonization at high temperature region.The combustion tests indicated that HMCP dramatically enhanced the flame retardant property of epoxy resin.EP/HMCP-1.25achieved a LOI value of as high as 36.5%and passed UL94 V-0 rating;its pk-HRR,av-HRR,av-EHC and THR were decreased by 71%,32%,21%and 38%,respectively,compared with those of the neat epoxy resin.All the EP/HMCP samples exhibited excellent anti-dripping effect.The Py-GC/MS analysis disclosed that HMCP released PO₂,HPO₂ and phenoxyl free radicals with quenching effect.The FTIR and SEM-EDX analyses disclosed that the cyclotriphosphazene and maleimide group of HMCP exhibited flame retardant effect in condensed phase.HMCP promoted the formation of intumescent and cross-linked char layer with high char yield which was beneficial to reduce the efficiency of heat-and oxygen-exchange.HMCP exerted bi-phase flame retardant effect.