| Phenolic foam(PF)is expected to replace highly flammable polyurethane and polystyrene foams as insulation materials due to its excellent flame retardant properties.However,the molecular structure of PF's methylene cross-linked rigid benzene ring makes it brittle and friable,which hinders its large-scale promotion and application.In this study,the biomass resource tung oil was used as a raw material,and its carbon-carbon double bond was used to react with Friedel-Crafts of phenol to embed the flexible alkyl chain of tung oil into the rigid molecular structure of phenolic resin to achieve the purpose of toughening.However,the flammability of the alkyl chain leads to a decrease in the flame retardancy of the modified PF.For this purpose,a series of reactive tougheners of tung oil-based diethyl phosphate,nitrogen-phosphorus and silicon-phosphorus cooperative flame-retardant were designed and synthesized to prepare a series of flame retardant tung oil-based phenolic foam with enhanced toughness.1.Tung oil-toughened phenolic foams(TPF-x,x represents the amount of tung oil added to phenol,the same below)were prepared.The mechanical and flame-retardant properties of TPF-x were then studied.Compared with pure PF,TPF-x had enhanced mechanical strength.The compressive and flexural strengths of the toughened foam TPF-6 were maximized to 0.278 MPa and 0.450 MPa,which were increased by 68.8%and 86.7%,respectively.However,the thermal stability and flame retardancy of TPF-x decreased with the increase of tung oil content.The residual mass(800?)and Tmax(DTG peak temperature)of TPF-x were significantly less than pure PF.The limiting oxygen index(LOI)values of TPF-9 and TPF-12 were 25.2%and 23.8%,respectively,which was lower than the flame-retardant level.The ignition time(TTI),peak heat release rate(p HRR)and residual mass of TPF-12 were 15 s,46.30 k W/m2 and 0.84%,respectively,far worse than those of pure PF's 56 s,38.19 k W/m2 and 7.8%.Nevertheless,TPF-12 showed good smoke suppression performance,with a total smoke release(TSR)of 7.77m2/m2,a decrease of 72.2%.2.Flame-retardant tung oil based diethyl phosphate(TAP)was synthesized by aminolysis and substitution reactions from tung oil.TAP toughened phenolic foams(TAPPFx)were then prepared.The mechanical properties of TAPPFx were better than pure PF,and TAPPF6 had the highest compressive(0.243 MPa)and flexural(0.416 Mpa)strengths,which was increased by51.9%and 73.3%,respectively.The pulverization rates of TDPPFx were lower than that of pure PF,and the pulverization rate of TAPPF12 was the smallest,which was reduced by 38.5%.The Tmax of TAPPFx was comparable to pure PF,and the residual mass was slightly increased.The LOI values of TAPPFx increased with the increase of TAP content,the maximum is 41.2%.The p HRR and total heat release(THR)of TDPPF18 were minimized to 28.36 k W/m2 and 16.57MJ/m2,which were reduced by 25.9%and 10.2%,respectively.These results show that the introduction of flame-retardant phosphorus eliminates the hidden danger of alkyl chain flammability,and allows TAPPFx to obtain enhanced thermal stability,mechanical and flame-retardant properties.3.A reactive flame retardant of tung oil-based phosphate ester containing triazine ring(TAPC)was synthesized.TAPC toughened phenolic foams(PF-x)were then prepared.The compressive and flexural strengths of PF-10 were maximized to 0.265 MPa and 0.57 MPa,which were 65.6%and 137.5%higher than pure PF,respectively.PF-10 also had the lowest brittleness,with a pulverization rate of 11.8%,a decrease of 53.1%.The thermal stability of PF-x were enhanced,and the residual mass at various temperatures were higher than that of pure PF.The flame-retardant and smoke-suppressant properties of PF-x were enhanced with the increase of TAPC content.The p HRR,THR and TSR of PF-15 were 27.81 k W/m2,17.36 MJ/m2 and 9.73m2/m2,respectively,a decrease of 27.2%,5.9%and 67.7%.The phosphate ester and triazine ring in TAPC play a synergistic role,promoting the formation of a dense and continuous protective carbon layer,which can protect against oxygen and heat,and effectively limit the transfer of soot.Moreover,the triazine ring still can decompose and release water vapor and nitrogen-containing non-combustible gas,which dilutes the flammable volatiles and oxygen on the surface of the foam,and makes the carbon layer expand thicker,thereby enhancing the flame-retardant and smoke-suppressant properties of the foam.4.To enhance the flame-retardant effect of tung oil-based phosphate ester toughener in condensed phase,a phosphorus-containing tung oil based siloxane was(TDK)synthesized.TDK toughened phenolic foams(TDKPFx)were then prepared.The introduction of TDK improved the micro-morphology and mechanical properties of the foams.TDKPFx had a honeycomb-like cell structure,more uniform and regular cells,higher cell density,smaller average pore size and narrower pore size distribution range,compared with pure PF.TDKPF10 had the highest compressive and flexural strengths,which was increased by 65.6%and 137.5%,respectively.On the one hand,the C=C bonds in TDK undergone Friedel-Crafts reaction with phenol;on the other hand,the siloxane was hydrolyzed and undergone polycondensation with the hydroxyl group of the phenolic resin,and finally the flexible alkane chains and silicone chains were embedded in the rigid molecules structure of the phenolic foam,the purpose of toughening was achieved.As the amount of TDK increasing,the LOI values of TDKPFx increased,and the maximum LOI value of TDKPF15 is 41.7%.The p HRR and THR of TDKPF15 were reduced by 15.5%and22.2%,respectively.The TSR values of TDKPF5,TDKPF10 and TDKPF15 were reduced by55.3%,75.8%and 69.2%,respectively.Characterization of residual char by EDS,FT-IR,Raman spectroscopy and XPS found that phosphorus and silicon in the foams synergistically promote the carbonization of the polymer,forming a higher graphitization and dense hybrid char layer rich in polyphosphate and SiO2,thereby enhancing the flame-retardant and smoke-suppressant properties of the foams. |
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