| Epoxy resins are widely used in many fields due to the excellent cohesive force,thermal stability,insulation,chemical resistance,and high mechanical properties.However,their high flammability has strongly limited the wider application,it is necessary to prepare epoxy resin with good flame retardancy.The research of phosphorus-containing halogen-free flame retarded epoxy resins have received a lot of attention in recent years,owing to lower toxicity and higher efficacy in flame retardancy.To achieve a satisfactory flame retardant effect,more than 1 wt% phosphorus content is needed to be introduced into epoxy systems.Nevertheless,thermal stability and mechanical properties of cured epoxy resins may be dramatically deteriorated due to the incorporation of too much thermolabile phosphorus group,and it is impossible to satisfy the requirements of high-end fields.Therefore,it is necessary to investigate the efficient flame retardant to minimize the negative effects on other properties of cured epoxy resins by reducing content of flame retardant.It is conducive to improving the mechanical properties and thermal stability of phosphorus-containing flame retarded epoxy resins by means of introducing rigid and heat-resistant group into curing systems.Triazine and benzene ring have the advantages of strong rigidity and good heat resistance.It is expected that the incorporation of 5,10-dihydro-phenophosphazine-10-oxide(DPPA)and triazine or benzene ring endows cured epoxy resins with outstanding flame retardancy,thermal and mechanical properties.Based on the above ideas,two DPPA derivatives were synthesized and introduced into epoxy resins to prepare halogen-free flame-retardant cured epoxy resins.The flame retardancy,mechanical properties and thermal stability of cured epoxy resins were investigated.(1)A novel flame retardant DPPA derivative containing triazine named TRIDPPA was synthesized from DPPA,cyanuric chloride and 4-aminobenzaldehyde.The structure of TRIDPPA was characterized and confirmed by 1H NMR,13 C NMR,31 P NMR,Fourier transform infrared spectroscopy(FTIR)and elemental analysis.A series of halogen-free flame-retardant cured epoxy resins were prepared by introducing TRIDPPA into epoxy matrix.The flame retardancy and thermal stability of samples were evaluated by differential scanning calorimetry(DSC),dynamic mechanical analysis(DMA),thermogravimetric analysis(TGA),and limiting oxygen index(LOI),vertical flame test,cone calorimetry test.The results showed that the introduction of TRIDPPA obviously enhanced char yield and endowed the cured epoxy resins with excellent flame retardancy.When the content of TRIDPPA was 1 wt%,where the phosphorus content was as low as 0.08%,cured epoxy resins of EP-TD1.0 passed UL-94 V-0 and achieved a LOI value of 31.5%,and its glass transition temperature(Tg)and initial degradation temperature(T5%)were 170.1℃(DMA)and 366.1℃,respectively.Peak of heat release rate(PHRR),mean heat release rate(MHRR)and total heat release(THR)of EP-TD1.0 were reduced by 14.6%、21.7%、10.9%,compared with unmodified cured epoxy resins.The mechanical properties of cured epoxy resins were improved by the incorporation of TRIDPPA,in which the tensile strength and flexural strength of EP-TD1.0 were increased by 6.9% and 9.6%,respectively.Moreover,The tensile modulus and flexural modulus were increased by 18.8%和 16.1%,respectively.(2)A DPPA derivative containing 5 active hydrogen atoms named DPPA-NH2 was synthesized from DPPA,aniline and 4-aminoacetophenone.A series of halogen-free flame-retardant cured epoxy resins were prepared by introducing DPPA-NH2 into epoxy matrix.DPPA-NH2 endowed epoxy resins with excellent flame retardancy at the low loading.The cured epoxy resins EP-DN2.0 passed UL-94 V-0 and achieved a LOI value of 33.3% when DPPA-NH2 content is 2 wt%.Tg and T5% of EP-DN2.0 were 164.1℃(DMA)and 368.5℃,respectively.The tensile strength and flexural strength of EP-DN2.0 were 85.2 MPa and 128.3 MPa,increased by 14.3 MPa and 13.1 MPa,respectively.The tensile modulus and flexural modulus of EP-DN2.0 were 3.4 G and 3.5 GP,increased by 0.3 GPa and 0.3 GPa,respectively,compared with unmodified cured epoxy resins. |
