Preparation,Properties And Flame-retardant Mechanism Of Flame-retardant Epoxy Resins Based On DOPO-based Nitrogenous Compounds

Epoxy resin possesses outstanding advantages such as high thermal stability,good insulation property,excellent adhesion property and superior mechanical property,which make it widely used in adhesives,coatings,electronic materials and composites.However,its highly flammable nature has severely restricted its further applications in the fields requiring flame resistance.Therefore,it is imperative to improve the flame retardancy of epoxy resin.Recently,9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO)-based nitrogenous compounds(DPIs)by covalently bonding DOPO and imine,have captured tremendous interest toward researchers.These compounds,as cocuring agents incorporated into epoxy matrices,have endowed epoxy resins with improved flame retardancy.Nevertheless,there are still several problems left for the investigation about the flame-retardant epoxy resin based on DPI: 1)the flame-retardant mechanism of DPI has not yet been clarified;2)the flame-retardant efficiency of DPIs reported was unsatisfactory;3)the research about the flame-retardant synergism between DPI and nanofiller has also not been carried out.In this dissertation,three types of DPIs,namely DOPO-based phenolic amine(DAP),DOPO-based nitrogenous oligomer(PDAP)and DOPO-based triazole compound(DTA),were successfully synthesized by the way of molecular design.These DPIs serving as co-curing agents of 4,4’-diaminodiphenylmethane(DDM),incorporated into bisphenol A type epoxy matrices,were used to endow epoxy resins with improved flame retardancy.The thermal stabilities,mechanical properties and flame-retardant properties,along with the flame-retardant mechanism of modified thermosets were investigated.Moreover,the nano organo-montmorillonite(MT)as a synergistic agent,coupled with PDAP,was used to improve the flame retardancy of epoxy resin.The flame-retardant synergism was also investigated in detail.The main research work of this dissertation was summarized as follows:(1)The DAP was synthesized by covalently bonding DOPO and imine(SB)obtained from the condensation of p-phenylenediamine with salicylaldehyde.Moreover,its analogue(AP)via the addition reaction between diethyl phosphite and SB,was also synthesized.Their chemical structures were characterized by FTIR,1H-NMR,13 CNMR and 31P-NMR.Both DAP and AP,as co-curing agents introduced into epoxy matrices,were used to improve the flame retardancy of epoxy resins.The thermal stabilities,mechanical properties and flame-retardant properties of modified thermosets were studied by TGA,DMA,LOI and UL-94 tests.The flame-retardant mechanisms of modified thermosets were investigated by FTIR,Py-GC/MS,SEM,Raman and elemental analysis.The results showed that the addition of DAP or AP decreased the glass transition temperature(Tg)and 5 wt% mass loss temperature(T5%),and increased the char yield at elevated temperature of thermoset.With the addition of 10 wt% DAP,the modified thermoset showed an LOI value of 36.1% and V-0 rating in UL-94 test.In the condensed phase,the thermal degradation of thermoset modified with DAP produced phosphorus-based acids,which could esterify and dehydrate the decomposed epoxy matrix to promote the char formation.In the gas phase,the thermal degradation of modified thermoset generated phosphorus-based radicals and nonflammable gases,which could quench free radicals and dilute flammable gases respectively.Moreover,the thermosets modified with DAP showed unique blowing-out effect during combustion.A compact char layer with honeycombed cavity inside could be rapidly formed on the surface of burning matrix,which accumulated pyrolytic gases containing considerable phosphorus-based free radicals and nonflammable gases.When the amount of pyrolytic gases surpassed the holding capacity of the char layer,the pyrolytic gases were instantly released to blow out the flame.However,the thermoset modified with 10 wt% AP only achieved an LOI value of 25.7% and no rating in UL-94 test.The inferior flame retardancy was possibly assigned to the mismatching of charring process with gas emission process during combustion.(2)The PDAP was synthesized via the addition reaction between DOPO and imine(PSB)obtained from the condensation reaction of terephthalaldehyde and pphenylenediamine.The chemical structure of PDAP was characterized by FTIR,1HNMR,31P-NMR and elemental analysis.The PDAP serving as co-curing agent of DDM,was employed to prepare epoxy thermoset with improved flame retardancy.The thermal stabilities,flame-retardant properties and mechanical properties of modified thermosets were studied by TGA,LOI measurement,UL-94 test,DMA and tensile test.The flame-retardant mechanism was investigated by FTIR,Py-GC/MS,SEM and Raman.The addition of PDAP reduced the T5% and Tg,and raised the char yield at elevated temperature of thermoset.With the incorporation of 7 wt% PDAP,the modified epoxy thermoset achieved an LOI value of 35.3% and V-0 rating in UL-94 test.Interestingly,all modified thermosets showed blowing-out effect during UL-94 test,which was attributed to the intense release of pyrolytic gases containing phosphorus-based free radicals and nonflammable gases.The flame-retardant mechanism was attributed to the quenching effect of phosphorus-based free radicals and diluting effect of nonflammable gases in the gas phase,and the barrier effect of phosphorus-rich char layers with polyaromatic structures in the condensed phase.Furthermore,the incorporation of PDAP decreased the elongation at break,and increased the tensile strength of epoxy thermoset.(3)The MT,combined with PDAP,was used to improve the flame retardancy of epoxy resin.The thermal stabilities,flame-retardant properties and mechanical properties of modified thermosets were investigated by TGA,LOI test,UL-94 test,DMA and tensile test.The flame-retardant synergism between MT and PDAP was studied by FTIR,Py-GC/MS,SEM,Raman and elemental analysis.The results showed that the incorporation of MT decreased the T5%,and increased the char yield at elevated temperature of EP/PDAP4(thermoset modified with 4 wt% PDAP).With the increase of MT content in EP/PDAP4,the Tg of thermoset initially decreased and then increased.It revealed that 0.5 wt% MT combined with 4 wt% PDAP showed synergistic effect on enhancing the flame retardancy of thermoset.The corresponding thermoset achieved an LOI value of 35.5% and V-0 rating in UL-94 test.Moreover,it showed intense blowing-out effect during combustion.The flame-retardant synergism between MT and PDAP was ascribed to the formation of silicoaluminophosphate(SAPO)originating from the reaction between MT and PDAP during thermal degradation.The SAPO serving as a solid acidic catalyst,coupled with the acid sites from the decomposition of organomodifier in MT,could promote the oxidative dehydrogenation crosslinking charring process.The addition of excessive MT resulted in the diminished blowing-out effect during combustion,which was adverse to the further improvement of flame retardancy.Besides,the incorporation of MT reduced the elongation at break,along with the tensile strength of thermoset.(4)The DTA was prepared by reacting DOPO with imine(TSB)resulting from the condensation of terephthalaldehyde with 3-amino-1H-1,2,4-triazole.Its chemical structure was characterized by FTIR,1H-NMR,13C-NMR,31P-NMR and elemental analysis.The DTA as co-curing agent of DDM,was used to prepare flame-retardant epoxy thermoset.The thermal stabilities,mechanical properties and flame-retardant properties of modified thermosets were investigated by TGA,tensile test,LOI and UL-94 tests.Moreover,the flame-retardant mechanism was also investigated by FTIR,PyGC/MS,SEM,Raman and elemental analysis.The results suggested that the addition of DTA reduced the T5%,and raised the char yield at elevated temperature of thermoset.DTA possessed superior flame-retardant efficiency.With the addition of only 4 wt% DTA,the resultant thermoset achieved an LOI value of 34.8% and V-0 rating in UL-94 test.The flame during combustion was blew out based upon the quenching effect of phosphorus-based radicals and diluting effect of nonflammable gases.The pyrolysis of modified thermoset produced phosphorus-based radicals and nonflammable gases,which could exert flame-retardant effect in the gas phase by quenching active radicals and diluting flammable gases respectively.The phosphoric or phosphonic acid derivatives could be generated during the early degradation of DTA moiety,and then converted to polyphosphoric acid derivatives.These phosphorus-based acids could promote the formation of phosphorus-rich char layers,thus exerting flame-retardant effect in the condensed phase.Moreover,the addition of DTA also resulted in the decrease of elongation at break and tensile strength of thermoset.