Investigation On Preparation Of Eugenol-based Flame Retardant Epoxy Resin And Its Flame Retardant Mechanism

Because of excellent comprehensive properties,epoxy resin is widely applied in the field of adhesives,electronic printed circuit boards,packaging materials and so on.At present,more than 90%of commercial epoxy resins are petroleum-based bisphenol A(BPA)epoxy resins prepared from petroleum-based BPA,which disturbs human endocrine system and even causes cancer.Therefore,the development of a harmless bio-based epoxy resin can meet the dual needs of environment and health.However,bio-based epoxy resin still has the problem of low heat resistance and high flammability.First,biomass platform compounds,eugenol and hexachlorocyclotriphosphazene were used as raw materials to design and synthesize a six-arm structure eugenol-based cyclotriphosphazene epoxy monomer(EHEP).NMR and MALDI-TOF were employed to prove the successful synthesis of this monomer.The corresponding epoxy resin(EHEP-D230)was prepared with EHEP cured by polyetheramine(D230).Compared with commercial epoxy resin(E51-D230)through the test of DSC,DMA,tensile test,TGA,limiting oxygen index test,and vertical combustion,we found that:EHEP-D230 has a higher optimal curing temperature(144 vs.130?),better Tg(119 vs.85?),tensile strength(56.7 vs.64.2MPa),and carbon residue rate(39%vs.8%),limiting oxygen index(31%vs.21%)and flame retardant grade(V-0 vs.No Grade).The cone calorimetry test exhibited that the peak value of heat release rate,total heat release and total smoke production of EHEP-D230 during combustion are reduced by 66%,65%and 78%respectively,and it showed typical intumescent flame retardant characteristics.Additionally,SEM,EDS surface scan,FTIR,XPS and solid ³¹P-NMR were used to analyze the EHEP-D230 char.It was found that the char exhibited a unique"phosphorus aggregation-honeycomb"intumescent structure and the ligaments among the vesicles of char were the main accumulation area of phosphoric acid(or phosphate).Based on the principle of"similarity and compatibility",we preliminarily proposed and explained the possible formation mechanism of this"phosphorus aggregation-honeycomb"char.Second,we designed and synthesized 2,2'-bis-eugenol bisglycidyl ether(EP4)as a basic compound,and a series of epoxy monomers with biaromatic or biphenyl structures,allyl and methoxy groups were designed and synthesized to be comparative model compounds.The influence of various structural units in epoxy monomer(allyl and its number,methoxy,benzene ring bridging group and the relative positions of each group)on the monomer viscosity,the optimal curing temperature and the properties of cured epoxy resin were investigated.The results showed that,the viscosity of epoxy monomer would decrease with the increase of the amount of allyl,while the introduction of methoxy would increase the viscosity of the corresponding monomer.The low viscosity epoxy resin is conducive to the impregnation of substrate and mold filling,which can be applied to a wide range of molding processes.When the allyl group were in the ortho position of glycidyl ether group,the increase range of the optimal curing temperature is larger,which could raise the requirements for curing equipment.However,from another point of view,longer the operation time would endow the resin a good machinability.The results of DMA showed that,although the introduction of allyl groups would reduce the Tg(Tg1)of the resin at a range from 40 to 76?,a higher Tg(Tg2,maximum Tg2 reached 390?)could be obtained after crosslinking of the allyl group at high temperature.Tg1 and Tg2 would decrease as the length of the bridging group increases(Tg1decreasing about-25?,Tg2 about-80?),while the methoxy group has little effect on the two Tgs;TGA results showed that,the introduction of allyl groups increased the thermal degradation temperature of the resin(up to+38?).Although the introduction of the methoxy group reduced the pyrolysis temperature of the resin(about-10?),it increased the char yield of the resin(about+5%).The pyrolysis temperature of the resin(about-3%)and the char yield(at least-7%)decreased due to the growth of the bridge bond.In addition,with the help of LOI and UL94,it was found that the introduction of allyl group and crosslinking was beneficial to the flame retardancy of the resin,while methoxy group and bridging group did harm to the flame retardancy.Py-GC/MS and the second-order FTIR were employed to characterize the allyl cross-linked network.Allyl groups might react through cationic mechanism at high temperature to produce four different structures.Among them,the 2,3-dihydro-2-methylbenzofuran(DHMBZF)structure was a five-membered ring structure formed by the addition of a phenolic hydroxyl group and an ortho allyl group,which was helpless for increase of Tg2.The performance of the epoxy resin could be adjusted by the time of the second curing time,and the oligomer had more advantages in performance control than the allyl epoxy resin prepared by the monomer.