| Thermal insulation material market has a broad prospect.The self-flame retardant phenolic foam is very popular and is widely used in construction,petrochemical pipelines and transportation.However,as the country has increasingly strict requirements on the flame retardancy of thermal insulation materials,phenolic foam has become The flame retardancy of organic thermal insulation materials needs to be further improved.The improvement of the flame retardancy of phenolic foam generally uses additive flame retardants.The added amount is large and affects other properties of the foam.In addition,the current halogen-based flame retardants cause great harm to the environment and promote the development of halogen-free flame retardants,especially multi-element synergistic flame retardant has become the development direction of halogen-free flame retardants,such as nitrogen and phosphorus synergistic flame retardant.As a renewable resource with abundant reserves,lignin has a natural phenolic structure and is considered an ideal substitute for phenol.In this study,lignin was first degraded in the presence of molten salt hydrate,and the content of phenolic hydroxyl groups and lignin side chain carbonyl groups increased.The side chain carbonyl groups were used for Schiff base reaction and carbon-nitrogen double bond addition reaction,and successfully introduced nitrogen and phosphorus flame retardant.Element,prepared a reactive lignin-based flame retardant,used it to partially replace phenol to prepare a flame-retardant foamed resin,and further prepared a flame-retardant lignin-based phenolic foam.The main research content and conclusions are as follows:（1）Indulin alkali lignin（IL）was depolymerized by molten salt hydrate,and the structure of lignin was characterized before and after depolymerization by modern analytical methods.It was found that molten salt hydrate can effectively break the ether bond of lignin.Reduce the molecular weight of lignin,depolymerization is accompanied by partial demethylation process,and improve the reactivity of lignin.GPC analysis showed that the molecular weight distribution curve of lignin after depolymerization obviously shifted to the low molecular weight end,the average molecular weight decreased from 1921 to 1386,and the polydispersity index（PDI）decreased from 6.35 to 5.71.The XPS spectra of IL and depolymerized lignin（DL）are similar.Their C ls peaks can be fitted to the peaks at 284.8 e V and 286.43 e V,corresponding to the C atom（C-OH）and ether connected to the hydroxyl group of lignin.Bond the C atom or the C atom of the methoxy group（C-OR）.After depolymerization,the peak at 284.8 e V increases and the peak at 286.43 e V decreases,which can also prove that the content of hydroxyl groups in depolymerized lignin increases and the content of methoxy groups decreases.The total weight loss in the whole thermogravimetric process of IL and DL under N2 and air atmosphere is basically the same,which proves that depolymerization does not change the thermal stability of lignin.（2）Using depolymerized lignin（DL）as raw material,Schiff base nitrogen-containing lignin（N-DL）was prepared by ultrasonic method,and then further reacted with DOPO to synthesize nitrogen-phosphorus modified DL（NP-DL）.NP-DL is a new type of expanded lignin-based nitrogen and phosphorus flame retardant.The chemical structure of lignin was characterized by FT-IR,1H NMR and 31P NMR,and the results showed that the NP-DL successfully introduced N and P elements.GPC,elemental analysis and XPS analysis carried out qualitative and quantitative analysis on the introduction of lignin-based flame retardant N and P elements.The N content of N-DL was significantly higher than that of DL,from 1.04%to 4.27%.NP-DL The content of P is 1.1%and there are more characteristic peaks of N and P elements than DL in XPS analysis.The flame retardant properties of NP-DL were analyzed by TGA,TGA/FT-IR and TGA/MS.It can be seen from TGA that the Tmax of NP-DL is 578.5 oC,which is about 100 oC higher than that of DL.The carbon residues at 600 oC and 800 o C are increased by 1285.5%and 362.6%,respectively,compared with DL,indicating that lignin has a higher functional modification.Thermal stability.TGA/FT-IR and TGA/MS indicate that NP-DL is thermally degraded to produce phosphorus-containing compounds:OP+with m/z=47,H3PO with m/z=50,O2P+with m/z=63 and HO2P+with m/z=64,phosphorus-containing compounds can capture free radicals in the gas phase and inhibit combustion.（3）Use DL and NP-DL as raw materials to replace part of phenol and formaldehyde to prepare lignin-modified phenolic resin under alkaline conditions.And through FT-IR,TGA,GPC,rheological properties,viscosity characterization methods to characterize the structure and performance of the modified resin.The reaction process of NP-DLPRx was analyzed,and FT-IR,GPC,and rheological analysis were used to confirm this process.FT-IR analysis showed that the polymerization between NP-DLPRx and PR was completed by the formation of methyl ether and aliphatic ether bridges.GPC shows that as the amount of lignin increases,the molecular weight and dispersion coefficient of the resin tend to increase as a whole.At the same time,the thermal stability of PR,DLPRx and NP-DLPRx under N2 and air atmosphere is studied.The carbon residue rate of NP-DLPR15 at 800°C under N2atmosphere is 11.63%higher than that of PR;under air atmosphere,it is increased by 170.93%.The overall carbon residue of NP-DLPRx is higher than that of PR and DLPRx,indicating that nitrogen and phosphorous elements improve the thermal stability of the resin and form More charcoal residue.（4）Designed and synthesized a flame-retardant modified phenolic foam with a phenolic molar ratio of 1:1.7 containing nitrogen,phosphorus and lignin,which passed FT-IR,microscopic morphology,mechanical properties,thermal conductivity,thermal stability,and critical oxygen index（LOI）,Cone calorimetry and other characterization methods to study the performance of the foam.Analyze the structure and elemental composition of carbon residue,and explore the reasons for the flame retardancy of nitrogen and phosphorus-containing lignin-based phenolic foam.The cell morphology and mechanical properties of the prepared DLPFx and NP-DLPFx are better than pure PF.NP-DLPF10 has the smallest average pore size,100.32μm,and the narrowest pore size distribution,20-130μm.Compared with pure PF,the micropores in the modified foam are more and relatively concentrated.The compressive strength and flexural strength of NP-DLPF10 reach the maximum,which are 63.3%and 300%higher than pure PF（compressive strength of 0.158 MPa,flexural strength of 0.205 MPa）,respectively.The thermal conductivity of NP-DLPFx is smaller than that of pure PF,the thermal stability is better,and the LOI is increased.The thermal conductivity of pure PF is0.0330 W/m·K,and the thermal conductivity of DLPFx and NP-DLPFx is generally lower than that of pure PF.The final carbon residue rate of DLPFx and NP-DLPFx is higher than that of pure PF,indicating that both lignin-based phenolic foam DLPFx and NP-DLPFx have good thermal stability.As the amount of phenol substitution by NP-DL increases,the LOI of the foam shows an increasing trend.The LOI value of NP-DLPF15 is the highest,reaching 52%,which is an increase of 26.8%compared to pure PF.The cone calorimetry results show that the flame retardant effects of gas and condensed phases exist at the same time when NP-DLPFx burns.The H2O and NH3 generated by the pyrolysis of NP-DLPFx dilute the oxygen content,and the phosphorus-containing degradation products gradually release PO groups,and the gas phase hydrogen or hydroxide radicals are captured to inhibit combustion.In the condensed phase,the thermal decomposition product phosphate also promotes the formation of the carbon layer,which reduces the heat release and smoke release of foam combustion.The p HRR values of NP-DLPF10（22.96 k W/m2）and NP-DLPF15（19.87 k W/m2）are respectively 2.17%and15.30%lower than those of pure PF.The p RSR values of DLPF15（0.026 m2/s）and NP-DLPF10（0.024 m2/s）were 66.23%and 68.83%lower than those of pure PF,respectively.Then,SEM,FT-IR,XPS and Raman characterization of the residual carbon after the cone calorimeter test were carried out,and the reasons of flame retardancy and smoke suppression of NP-DLPFx were studied.It was found that phosphate,a thermal decomposition product of NP-DLPFx,promotes the formation of a carbon layer with a high degree of graphitization.The dense carbon layer acts as a barrier between the foam matrix and the flame,which effectively isolates the material from hot oxygen,thereby inhibiting the release of combustible gases.Therefore,it is judged that NP-DLPFx has excellent flame retardant and smoke suppression properties. |
