| Epoxy resins have excellent mechanical properties,insulation performance,chemical stability and belong to flammable materials.It is difficult to meet the need of advanced fields for the existing halogen-free flame retardant epoxy resins due to low flame retardancy and poor mechanical properties.Therefore,it is urgent to develop new kinds of epoxy resins with both highly efficient flame retardancy and high mechanical performance simultaneously for the wider application.Highly efficient reactive flame retardant is chosen to be covalently bonded to the cured epoxy resin without sacrificing other properties.On this basis,the reinforced materials are introduced into the cured epoxy resin to further improve the mechanical and other properties.5,10-Dihydro-phenophosphazine-10-oxide(DPPA)is a novel flame retardant with phosphorus-containing azine ring and an active P-H bond in its structure,which shows higher flame retardantcy than the commercially-used halogen-free flame retardant9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO).It is expected that epoxy resins possess both improved flame retardancy and mechanical properties by incorporating DPPA and carbon nanotubes in a proper way.Based on the above ideas,a reactive DPPA-based flame retardant was synthesized and covalentlylinked to the epoxy resins.The thermal stability,mechanical properties,and flammability behaviors of the cured products,as well as the flame retardant mechanism were investigated.Subsequently,carboxylated carbon nanotubes(MCNTs-COOH)were introduced into the epoxy resins to prepare a series of flame retardant epoxy composites reinforced by carbon nanotubes.The flammability behavior,thermal stability and mechanical properties were investigated,and the reinforcing and toughening mechanism was also analyzed.(1)A novel halogen-free flame retardant 10-(hydroxy(4-hydroxyphenyl)methyl)-5,10-dihydrophenophosphazinine-10-oxide(HB-DPPA)was synthesized by one-step method using 4-hydroxybenzaldehyde and DPPA as raw materials.The structure of HB-DPPA was characterized and confirmed by 1H NMR,FTIR and HRESI-MS.The HB-DPPA was used as co-curing agent of 4,4’-diaminodiphenylmethane(DDM)for curing diglycidyl ether of bisphenol A(DGEBA),and a series of cured epoxy resins with different phosphorus contentwere prepared.The flammability and thermal stability of samples were evaluated by thermogravimetric analysis(TGA),differential scanning calorimetry(DSC),dynamic mechanical analysis(DMA)and limiting oxygen index(LOI),vertical flame test,cone calorimetry test.The results showed that HB-DPPA did not affect the main process of curing reaction of DDM/DGEBA.When the content of HB-DPPA was 2.0 wt%,(that is 0.18 wt% of phosphorus),the cured epoxy resin EP-2 achieved UL-94 V-0 rating with LOI of 29.3%.Compared with EP-0(without HB-DPPA),the peak heat release rate(PHRR),average heat release rate(MHRR)and total heat release(THR)of EP-2 were decreased by 9.1%,8.7% and13.4%,respectively,while the residual weight was significantly increased by 80%.In addition,the tensile and flexural properties of EP-2 and EP-0 were very close.Furthermore,TGA-FTIR,scanning electron microscopy(SEM)and FTIR were employed to analyze the gas and condensed phase products from thermal degradation,providing insight into the degradation mechanism.The results indicated that HB-DPPA mainly exerted its flame-retardant effect through improvement in the condensed phase,forming the protective char layers with orderly,dense and multilayered structure,which inhibit the exchange of gas and energy during combustion,and thus reduce the combustion.(2)Carboxylated carbon nanotubes MWNTs-COOH were added as reinforcing and toughening materials to obtain MWNTs/HB-DPPA/DDM/DGEBA composites.The addition of carbon nanotubes further enhanced the flame retardancy of the epoxy resin.When the content of MWNTs-COOH was 1wt%,PHRR 、 MHRR and THR of cured flame-retarded epoxy resins CD-EP-1.0 were 469 kW/m2,131 kW/m2 and 91 MJ/m2,respectively,decreased by 14.6%、10.9% and 7.1% compared with EP-0,respectively.The LOI of CD-EP-1.0 could reach to 31.5%.After TGA test,the carbon residue rate of CD-EP-1.0 reached 24.9%,increased by 20.2% compared with EP-2,improved by 40.6% compared with C-EP-1.0(containing 1 wt% carbon nanotubes without HB-DPPA),increased by 56.6% compared with EP-0,was especially 15.2% more than the supraposition of the increment in EP-2 and C-EP-1.0 compared with EP-0,respectively,indicating the synergistic effect between MWCNTs-COOH and HB-DPPA on the flame retardancy of epoxy resins.(3)The carbon nanotubes improved significantly the mechanical properties of cured epoxy resins.The tensile and flexural strength of CD-EP-1.0 were 90.1±1.1 MPa and134.3±1.2 MPa,respectively,increased by 28.0% and 22.3%,respectively,compared with EP-2,and increased by 23.1% and 18.1%,respectively,compared with EP-0.The flexural modulus of CD-EP-1.0 was 4.1±0.1 GPa,increased by 24.2% and 32.3%,respectively,compared with EP-2 and EP-0.The elongation and fracture energy of CD-EP-1.0 displayed an obvious increment of 47.9% and 136.8% compared with EP-2,and 39.2% and 109.3%compared with EP-2,respectively. |
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