Synthesis Of Multi-element Synergistic And Environmentally Friendly Flame Retardants And Its Application In Epoxy Resin

Epoxy resins are used in a wide range of industrial applications such as coatings,laminates,circuit boards,and other electronic industries because of their excellent physicochemical properties.However,like most organic polymer resins,epoxy resins are flammable substances,which limits their application in areas with high flame retardancy requirements.Previously,halogen flame retardants were widely used because of their high flame retardant efficiency,but currently,due to higher environmental requirements,halogen flame retardants release toxic fumes when burning,which bring many risks and hidden dangers to both human and natural environment,so their use is increasingly restricted.Based on that,the development and application of halogen-free flame retardants is imperative,and has become a hot spot for research in the scientific and industrial communities.In this paper,the phosphorus-nitrogen flame retardants and biomass-based flame retardants of epoxy resin were studied based on glycidyl ether epoxy resin.The main research contents are as follows.（1）A novel DOPO-based benzimidazole derivative（DOPO-BNM）was synthesized by a two-step addition reaction using 2-aminobenzimidazole,m-nitrobenzaldehyde and DOPO as reactants and characterized by FTIR,³¹P NMR and ¹H NMR.When the phosphorus content was 0.513%,it passed the UL-94 V-0 with LOI values up to 32.5%.By characterizing and analyzing the flame retardant mechanism,the results showed that DOPO-BNM has good flame retardant properties in both gas phase and condensed phase.（2）A novel DOPO-based benzimidazole derivative（DOPO-BHM）was synthesized by a two-step addition reaction using 2-aminobenzimidazole,4-hydroxybenzaldehyde and DOPO as reactants and characterized by FTIR,³¹P NMR and ¹H NMR.When the content of DOPO-BHM in EP was 6 wt%,it passed the UL-94 V-0 with LOI values up to32.7%.By characterizing and analyzing the flame retardant mechanism,the results showed that DOPO-BHM has good flame retardant activity in both gas phase and condensed phase.Meanwhile,the mechanical properties of the modified epoxy resin were tested and found to be significantly improved.In a word,DOPO-BHM not only had good flame-retardant activity,but also can improve the mechanical properties of epoxy resin.（3）The synergistic effect of biomass chitosan（CS）on the flame retardant properties of DOPO in epoxy resins was investigated.The effects of different CS/DOPO mass ratios of the compound flame retardants on the flame retardant properties and mechanical properties of epoxy resins were investigated by directly compounding chitosan and DOPO at a certain total addition amount.At a total addition of 10 wt%and mass ratios of CS and DOPO of 1:2 and 2:1,the modified epoxy resins could successfully achieve V-0level with LOI of 33.7%and 32.5%,respectively.In addition,according to the results of cone calorimeter（CC）tests,it was shown that the flame retardant CS/DOPO had good inhibiting effects on both heat release and smoke release of the epoxy resin at a certain addition mass ratio,with the smoke suppression ability being particularly outstanding.In terms of mechanical properties,compared with EP,the tensile strength,bending strength and impact strength of EP/10%CS1/DOPO2 modified epoxy materials increased by7.4%,36.1%and 14.3%,respectively,while the tensile strength of EP/10%CS2/DOPO1 modified epoxy materials decreased by 7.4%,while the bending strength and impact strength increased by 38.4%and 23.8%.In conclusion,the addition of the compound flame retardant not only can greatly improve the fire safety of the modified materials,but also can increase the mechanical properties of the epoxy resin and expand the application range of chitosan.（4）Based on the lower thermal stability of chitosan,9-anthraldehyde and chitosan were used as reactants to obtain CSA,a chitosan derivative containing Schiff base with better thermal stability,through a simple addition reaction,which was compounded with DOPO as a flame retardant synergist to obtain a new halogen-free flame retardant.When the total addition of CSA/DOPO in EP was 8 wt%and the mass ratio of CSA to DOPO was 1:2,the modified epoxy resin could pass the UL-94 V-0 rating with LOI value up to36.4%.According to the CC tests results,the heat release rate（HRR）,total heat release（THR）,smoke generation rate（SPR）and total smoke generation（TSP）of EP/8%CSA1/DOPO2 showed significant decreases compared with pure EP,indicating that EP/8%CSA1/DOPO2 has a good inhibitory effect on both heat release and smoke release.Finally,the flame retardant mechanism of the flame retardants was further explored by Raman spectroscopy,X-ray photoelectron spectroscopy（XPS）and thermogravimetric infrared coupled instrument（TG-FTIR）.In terms of mechanical properties,the tensile strength,flexural strength and impact strength of EP/8%CSA1/DOPO2 were increased by 16.7%,52.3%and 33.3%,respectively,compared with EP.The results showed that the modified composite flame retardant system of chitosan derivatives and DOPO has good flame retardant activity in both gas phase and condensed phase,and the mechanical properties are significantly improved.（5）Glucose is a polyhydroxy biomass with abundant natural reserves and is a good carbon source.However,the thermal stability of glucose itself is poor.In this chapter,three glucose-based Schiff bases were synthesized as epoxy resin flame retardants by using chitosan,3-aminopropyltriethoxysilane and metal organic framework material（UIO-66-NH₂）to enhance their thermal stability and introducing certain nitrogen-containing gas sources,respectively,and the thermal stability,smoke suppression properties and mechanical properties of the modified epoxy resin were investigated.The structures of the three Schiff bases were determined by FTIR and XRD,and the microscopic morphologies of the products were also obtained by SEM.Three modified epoxy resins were obtained by melt blending method,in which EP/20%CG passed UL-94 V-2 rating and the other two modified epoxy resins（EP/20%Si G and EP/20%UG）both obtained UL-94 V-1 rating with LOI of 25.8%,28.3%and 27.4%,respectively.According to the CC tests analysis,the results showed that the three flame retardants had good inhibition of smoke release.In addition,the mechanical properties of the modified epoxy resins were tested and analyzed.Finally,the flame retardancy and smoke suppression mechanisms of the three flame retardants were further explored by scanning electron microscopy SEM,Raman spectroscopy,X-ray photoelectron spectroscopy（XPS）and thermogravimetric infrared coupling（TG-IR）.The mechanical property test results showed that the tensile strength and impact strength of the three modified epoxy resins are improved in different degrees,and the flexural strength was obviously improved.