A nitrogen-phosphorus modified lignin-based flame retardant as well as a preparation method and application thereof

Abstract

The application belongs to the technical field of flame retardants, and particularly relates to a nitrogen-phosphorus modified lignin-based flame retardant, a preparation method and application thereof, and the preparation method of the nitrogen-phosphorus modified lignin-based flame retardant, which comprises the following steps: adding an alkynylating reagent and an activating agent into an organic solution of lignin, heating and stirring, performing an alkynylating modification reaction to obtain alkynylated lignin; mixing the alkynylated lignin with a phosphating reagent in an organic solvent, heating and stirring, and performing a phosphating reaction to obtain phosphated lignin; mixing the phosphated lignin, an azide and a composite catalyst in an organic solvent in a nitrogen atmosphere, heating and stirring, and performing an azidation modification to obtain the flame retardant. The lignin is used as a main raw material, and high-value utilization of the lignin and improvement of the flame retardant performance of the composite material are realized.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant technology, specifically relating to a nitrogen-phosphorus modified lignin-based flame retardant, its preparation method, and its application. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Traditional flame retardants widely use halogen-containing or inorganic systems (such as aluminum hydroxide, expanded graphite, etc.). Although these materials have certain flame retardant effects, they also have obvious drawbacks: halogen-containing flame retardants produce a large amount of toxic fumes and corrosive gases when burning, causing secondary harm to human health and the environment; while inorganic flame retardants have poor compatibility with polymer matrices, and large amounts can easily lead to a decline in the mechanical and processing properties of the material.

[0004] Lignin, a byproduct of the paper industry and biomass refining, is widely available, inexpensive, renewable, and rich in aromatic ring structures. Its three-dimensional network structure and char-forming potential theoretically make it an ideal bio-based carbon source for constructing highly efficient intumescent flame-retardant systems. However, existing lignin-based flame retardants often employ single phosphorus or nitrogen element modification, resulting in low flame-retardant efficiency. Furthermore, the modified lignin exhibits poor dispersibility in polymer matrices, easily leading to a decline in material mechanical properties, thus hindering its industrial application. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a nitrogen-phosphorus modified lignin-based flame retardant, its preparation method and application, which uses lignin as the main raw material to realize the high-value utilization of lignin and improve the flame retardant performance of composite materials.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a method for preparing a nitrogen-phosphorus modified lignin-based flame retardant, comprising the following steps: An alkynylating agent and an activator are added to an organic solution of lignin, and the mixture is heated and stirred to carry out an alkynylation modification reaction, thereby obtaining alkynylated lignin. Alkylated lignin was mixed with a phosphating agent in an organic solvent, and then heated and stirred to carry out a phosphating reaction to obtain phosphated lignin. In a nitrogen atmosphere, phosphated lignin, azide, and composite catalyst are mixed in an organic solvent, heated and stirred to perform azide modification, thus obtaining a flame retardant.

[0007] Secondly, the present invention provides a nitrogen-phosphorus modified lignin-based flame retardant, which is prepared by the aforementioned preparation method.

[0008] Thirdly, the present invention provides the application of the nitrogen-phosphorus modified lignin-based flame retardant in the preparation of flame retardant materials or in improving the flame retardant properties of polymer matrices.

[0009] Fourthly, the present invention provides a method for preparing lignin-based flame-retardant gel, comprising the following steps: The prepared lignin-based flame retardant was dissolved in dimethyl sulfoxide, and then the obtained lignin-based flame retardant solution was transferred to a dialysis bag and dialyzed in deionized water to obtain a dialysate, wherein the dialysate includes water and a lignin-based flame retardant uniformly dispersed in water. The dialysate was diluted and mixed with sodium alginate to obtain a gel solution; A flame-retardant gel solution is obtained by mixing the gel solution with a matrix solution of polyvinyl alcohol or cellulose. When the matrix is ​​polyvinyl alcohol, the flame retardant gel solution is directly cast into a mold to obtain a gel; when the matrix is ​​cellulose, the flame retardant gel solution is cast into a mold, allowed to stand to degas, and then soaked in a mixed coagulation bath of CaCl2 and glacial acetic acid for 12-24 hours until it is completely gelled and solidified. After rinsing with water, the gel is obtained. After the obtained gel is freeze-cured, it is then freeze-dried to obtain a lignin-based flame-retardant gel.

[0010] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows: This invention employs a three-step modification process of alkynylation-phosphorylation-azidation to achieve nitrogen-phosphorus synergistic flame retardant modification of lignin. Compared with lignin-based flame retardants modified with a single element, the flame retardant efficiency is significantly improved. The total heat release (THR) of the composite material with the flame retardant of this invention is reduced by 11% compared with the blank sample, and the ignition time is increased from 4 s in the blank sample to 20 s, with no deflagration phenomenon.

[0011] In the preparation of lignin-based flame-retardant gel, the nanoscale dispersion of flame-retardant modified lignin is achieved through dialysis, which solves the problem of poor dispersion of traditional modified lignin in polymer matrices. It has good compatibility with matrices such as polyvinyl alcohol and cellulose and will not lead to a decrease in the mechanical properties of the material.

[0012] This invention uses alkali lignin, a byproduct of the papermaking industry, as a biomass material to prepare flame retardants. It has the advantages of being renewable, green, environmentally friendly, and pollution-free, realizing the high-value utilization of biomass resources and effectively solving the problem of lignin waste waste. Attached Figure Description

[0013] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0014] Figure 1 The total heat release (THR) curve (A) and heat release rate (HRR) curve (B) of the flame-retardant polyvinyl alcohol / sodium alginate composite flame-retardant gas gel prepared in Example 1 of this invention and the blank sample (Original) are shown, which intuitively demonstrate the effect of the flame retardant of this invention on improving the heat release performance of the material.

[0015] Figure 2 These are comparative photographs of the combustion process of the composite flame-retardant gels prepared in Example 2 (A) and Example 1 (B) of this invention at different time points in a butane flame, demonstrating the effect of the amount of ethyl azide on the flame-retardant effect. Detailed Implementation

[0016] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0017] To address the technical problems mentioned in the background art, the present invention provides a method for preparing a nitrogen-phosphorus modified lignin-based flame retardant, comprising the following steps: An alkynylating agent and an activator are added to an organic solution of lignin, and the mixture is heated and stirred to carry out an alkynylation modification reaction, thereby obtaining alkynylated lignin. Alkylated lignin was mixed with a phosphating agent in an organic solvent, and then heated and stirred to carry out a phosphating reaction to obtain phosphated lignin. In a nitrogen atmosphere, phosphated lignin, azide, and composite catalyst are mixed in an organic solvent, heated and stirred to perform azide modification, thus obtaining a flame retardant.

[0018] In this invention, alkynylating reagents such as bromopropyne are added to a lignin solution to modify the lignin. The reaction site occurs at the hydroxyl position on the lignin surface, generating alkynylated lignin containing an alkynyl ether structure.

[0019] Alkylated lignin was added to a phosphating reagent solution such as DOPO, and the alkynylated lignin was modified again. The modification site occurred at the newly introduced alkynyl position, introducing a phosphorus-containing group, and thus obtaining phosphated modified lignin.

[0020] In a nitrogen atmosphere, azides such as ethyl azide, a CuSO4 complex solution with tris(3-hydroxypropyltriazolemethyl)amine, and a sodium ascorbate solution are added to the phosphating-modified lignin solution to modify the phosphating-modified lignin. The modification site occurs at the alkynyl group, forming a flame-retardant modified lignin containing a triazole-ethyl acetate structure, thus achieving synergistic flame-retardant modification of nitrogen and phosphorus elements.

[0021] In the composite catalyst, CuSO4 provides Cu 2+Ions, serving as the metal center precursor in the reaction. Sodium ascorbate acts as a reducing agent, reacting Cu... 2+ Reduced to catalytically active Cu + Tris(3-hydroxypropyltriazolylmethyl)amine acts as a ligand with Cu + Formation of stable complexes enhances its catalytic activity and prevents Cu + Oxidative deactivation. Cu + By coordinating with alkynyl and azide groups, the activation energy of the reaction is lowered, the cycloaddition reaction process is accelerated, and nitrogen is introduced into lignin.

[0022] The surface of lignin is rich in hydroxyl groups. Through the etherification reaction of hydroxyl groups with alkynylating reagents, alkynylated lignin containing alkynyl ether structures is generated. The introduced alkynyl groups serve as highly active reaction sites and can be further introduced with nitrogen- and phosphorus-containing flame-retardant groups through chemical reactions.

[0023] The alkynyl group in alkynylated lignin can undergo an addition reaction with a phosphating agent, introducing a phosphorus-containing group into the lignin molecule to form phosphated lignin; the alkynyl group reacts with azide in Cu... + A cycloaddition reaction occurs under catalysis, introducing a nitrogen-containing triazole ring structure, ultimately achieving synergistic flame-retardant modification by nitrogen and phosphorus elements. Without the alkynylation reaction, subsequent phosphating and azidation modification steps cannot be carried out.

[0024] Alkyne modification optimizes the molecular structure of lignin through chemical modification, reducing its tendency to aggregate. Combined with subsequent dialysis, it can achieve nanoscale dispersion of flame retardants in the polymer matrix, avoiding the decline in material mechanical properties caused by poor dispersibility.

[0025] In some embodiments, the alkynylating agent is bromopropyne, propynic acid, or 3-chloropropyne.

[0026] In some embodiments, the activator is potassium carbonate, sodium carbonate, p-toluenesulfonic acid, triethylamine, or tetrabutylammonium bromide.

[0027] In some embodiments, the lignin is alkali lignin, and the solvent of the organic solution of lignin is N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, 1-methyl-2-pyrrolidone, or tetrahydrofuran.

[0028] In some embodiments, the heating temperature of the alkynylation modification reaction is 40-80 °C, and the reaction time is 6-50 h. Within this temperature range, sufficient energy can be provided to activate the etherification reaction between the alkynylating agent and the lignin hydroxyl groups, avoiding an excessively slow reaction rate that leads to excessively long reaction times; at the same time, it can prevent side reactions caused by excessively high temperatures, such as thermal decomposition of the lignin structure, thus ensuring reaction selectivity.

[0029] The heating temperature can be 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, or 80℃; the reaction time can be 6h, 7h, 8h, 9h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, 24h, 25h, 26h, 27h, 28h, 29h, 30h, 31h, 32h, 33h, 34h, 35h, 36h, 37h, 38h, 39h, 40h, 41h, 42h, 43h, 44h, 45h, 46h, 47h, 48h, 49h, or 50h.

[0030] In some embodiments, after the alkynylation modification reaction is completed, the activator is removed by filtration, and then deionized water is added to the reaction solution to precipitate the alkynylated lignin. The lignin is then separated and dried to obtain the alkynylated lignin.

[0031] In some embodiments, the phosphating agent is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), diphenylphosphine oxide (DPO), phenylphosphine acid, or dimethyl hydroxymethylphosphonate.

[0032] In some embodiments, the phosphating reaction is carried out at a temperature of 120-150 °C for 4-8 h.

[0033] The phosphating reaction temperature can be 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, or 150℃; the phosphating reaction time can be 4h, 5h, 6h, 7h, or 8h.

[0034] In some embodiments, after the phosphating reaction is complete, deionized water is added to the reaction system to precipitate the phosphated lignin, which is then separated and dried to obtain phosphated lignin.

[0035] In some embodiments, the azide is ethyl azide, 2-azidoethanol, azidoacetic acid, or sodium azide.

[0036] In some embodiments, the composite catalyst is a mixture of CuSO4 and tris(3-hydroxypropyltriazolylmethyl)amine and sodium ascorbate.

[0037] Preferably, in the composite catalyst, the mass ratio of CuSO4, the complex of tris(3-hydroxypropyltriazolylmethyl)amine, and sodium ascorbate is 1:8~9:15~17.

[0038] The mass ratio of CuSO4, the complex of tris(3-hydroxypropyltriazolylmethyl)amine, and sodium ascorbate can be 1:8:15, 1:8:16, 1:8:17, 1:9:15, 1:9:16, or 1:9:17.

[0039] In some embodiments, the reaction temperature for the azidation modification is 45-55 °C, and the reaction time is 10-12 h.

[0040] The reaction temperature for azidation modification can be 45 ℃, 46 ℃, 47 ℃, 48 ℃, 49 ℃, 50 ℃, 51 ℃, 52 ℃, 53 ℃, 54 ℃ or 55 ℃; The reaction time for azidation modification can be 10h, 11h or 12h.

[0041] Preferably, during the azide modification, the nitrogen flow rate is 0.3-1 m³ / s. 3 / h, preferably 0.3-0.8 m 3 / h. For example, it could be 0.3 m. 3 / h, 0.4 m 3 / h, 0.5 m 3 / h, 0.6 m 3 / h, 0.7 m 3 / h, 0.8 m 3 / h, 0.9 m 3 / h or 1 m 3 / h.

[0042] In some embodiments, when the alkynylating agent is bromopropyne, the phosphating agent is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and the azide is ethyl azide (ethyl azide has a density of 1.08 g / mL), the mass ratio of lignin to bromopropyne is 1~5:2~4; the mass ratio of DOPO to alkynylated lignin is 6-10:1; and the mass ratio of ethyl azide to phosphated lignin is 0.2-0.4:1.

[0043] The mass ratio of lignin to brominated propyne can be 1:2, 1:3, 1:4, 2:2, 2:3, 2:4, 3:2, 3:3, 3:4, 4:2, 4:3, 4:4, 5:2, 5:3, or 5:4. The mass ratio of DOPO to alkynylated lignin can be 6:1, 7:1, 8:1, 9:1, or 10:1; the mass ratio of ethyl azide to phosphated lignin can be 0.2:1, 0.3:1, or 0.4:1.

[0044] Preferably, the mass ratio of ethyl azide to phosphorylated lignin is 0.3-0.4:1, more preferably 0.35-0.4:1, and even more preferably 0.37-0.4:1.

[0045] Secondly, the present invention provides a nitrogen-phosphorus modified lignin-based flame retardant, which is prepared by the aforementioned preparation method.

[0046] Thirdly, the present invention provides the application of the nitrogen-phosphorus modified lignin-based flame retardant in the preparation of flame retardant materials or in improving the flame retardant properties of polymer matrices.

[0047] Fourthly, the present invention provides a method for preparing lignin-based flame-retardant gel, comprising the following steps: The prepared lignin-based flame retardant was dissolved in dimethyl sulfoxide, and then the obtained lignin-based flame retardant solution was transferred to a dialysis bag and dialyzed in deionized water to obtain a dialysate, wherein the dialysate includes water and a lignin-based flame retardant uniformly dispersed in water. The dialysate was diluted and mixed with sodium alginate to obtain a gel solution; A flame-retardant gel solution is obtained by mixing the gel solution with a matrix solution of polyvinyl alcohol or cellulose. When the matrix is ​​polyvinyl alcohol, the flame retardant gel solution is directly cast into a mold to obtain a gel; when the matrix is ​​cellulose, the flame retardant gel solution is cast into a mold, allowed to stand to degas, and then soaked in a mixed coagulation bath of CaCl2 and glacial acetic acid for 12-24 hours until it is completely gelled and solidified. After rinsing with water, the gel is obtained. After the obtained gel is freeze-cured, it is then freeze-dried to obtain a lignin-based flame-retardant gel.

[0048] The soaking time in the mixed coagulation bath of CaCl2 and glacial acetic acid can be 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h or 24h.

[0049] Dimethyl sulfoxide (DMSO) is a polar aprotic solvent with excellent solubility for nitrogen- and phosphorus-modified lignin, enabling it to uniformly disperse flame retardants into a stable solution. Its strong polarity allows it to form hydrogen bonds or dipole interactions with functional groups such as hydroxyl and ether bonds in lignin molecules, ensuring complete dissolution of the flame retardant and facilitating subsequent dialysis. DMSO is also highly miscible with water and its small molecular size allows for effective removal through a dialysis bag. During dialysis, DMSO gradually diffuses into the water, while the larger molecular weight of the flame retardant is retained and forms a nanoscale dispersion in the water, avoiding interfacial defects caused by solvent residue.

[0050] When the dialysis solution is mixed with the sodium alginate matrix, the flame-retardant modified lignin is uniformly dispersed at the nanoscale in water, which can effectively prevent agglomeration when it is compounded with the matrix, thereby improving the dispersibility and flame-retardant performance of the flame retardant.

[0051] When the matrix is ​​cellulose, the flame-retardant gel solution is cast into a mold, allowed to stand to degas, and then immersed in a mixed coagulation bath of CaCl2 and glacial acetic acid for 12-24 hours until completely gelled and solidified. CaCl2 provides Ca... 2+Ions can undergo cross-linking reactions with the carboxyl groups on the sodium alginate molecular chain. This cross-linking effect can quickly fix the gel shape, enhance mechanical strength, and prevent structural collapse during subsequent freeze-drying. A NaOH / urea system is used in the preparation of the cellulose solution, and glacial acetic acid can neutralize residual OH groups. - The pH value is adjusted to avoid the damage of nitrogen and phosphorus groups in the flame retardant caused by an alkaline environment. Under weakly acidic conditions, the hydroxyl groups on the cellulose molecular chains are protonated, weakening the electrostatic repulsion between molecules and promoting the re-aggregation of cellulose segments to form a stable gel structure.

[0052] In some embodiments, the molecular weight cutoff of the dialysis bag is 12,000 to 15,000 Da, preferably 14,000 Da.

[0053] The molecular weight cutoff can be 12,000 Da, 13,000 Da, 14,000 Da, or 15,000 Da.

[0054] Preferably, the dialysis time is 12-24 hours, and the deionized water is changed regularly during the dialysis process. The dialysis time can be 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h or 24h.

[0055] In some embodiments, the dialysate is diluted and mixed with sodium alginate, then stirred at 55-70°C to obtain a gel solution. Sodium alginate is a natural polysaccharide whose molecular chains easily aggregate through hydrogen bonds at low temperatures, resulting in a slow dissolution rate. Heating at 55-70°C can disrupt the hydrogen bonds between sodium alginate molecules, promoting rapid dissolution and stretching of the molecular chains, thus avoiding heterogeneity of the gel solution due to incomplete dissolution.

[0056] The stirring temperature can be 55℃, 56℃, 57℃, 58℃, 59℃, 60℃, 61℃, 62℃, 63℃, 64℃, 65℃, 66℃, 67℃, 68℃, 69℃ or 70℃.

[0057] After dialysis, the lignin flame retardant exists in water as nano-sized particles. Heating and stirring can enhance the uniformity of particle dispersion and prevent particle agglomeration caused by low temperature. At the same time, increasing the temperature can reduce the viscosity of the solution, improve stirring efficiency, and ensure that the flame retardant is in full contact with the sodium alginate matrix.

[0058] Preferably, the stirring temperature is 55~65℃.

[0059] In some embodiments, when the matrix is ​​polyvinyl alcohol, the matrix solution is an aqueous solution of polyvinyl alcohol, and the mass fraction of polyvinyl alcohol in the solution is 3-5%, preferably 3-4.5%, and more preferably 4%. The polyvinyl alcohol matrix solution is prepared by heating and dissolving at 90-95°C.

[0060] In some embodiments, when the matrix is ​​cellulose, the mass fraction of cellulose in the matrix solution is 3% to 5%, and the solvent system is a mixed solvent system of sodium hydroxide, urea and water, with the mass ratio of sodium hydroxide to water being 5-10:75-85 and the mass ratio of urea to water being 10-15:75-85.

[0061] When preparing the matrix solution of this cellulose, dissolve it by vigorous stirring at 1000~1500 rpm for 5~10 minutes.

[0062] The mass ratio of sodium hydroxide to water can be 5:75, 5:76, 5:77, 5:78, 5:79, 5:80, 5:81, 5:82, 5:83, 5:84, 5:85, 6:75, 6:76, 6:77, 6:78, 6:79, 6:80, 6:81, 6:82, 6:83, 6:84, 6:85, 7:75, 7:76, 7:77, 7:78, 7:79, 7:80, 7:81, 7:82, 7:83, 7:84, or 7:85. 8:75, 8:76, 8:77, 8:78, 8:79, 8:80, 8:81, 8:82, 8:83, 8:84, 8:85, 9:75, 9:76, 9:77, 9:78, 9:79, 9:80, 9:81, 9:82, 9:83, 9:84, 9:85, 10:75, 10:76, ​​10:77, 10:78, 10:79, 10:80, 10:81, 10:82, 10:83, 10:84, 10:85; The mass ratio of urea to water can be 10:75, 10:76, ​​10:77, 10:78, 10:79, 10:80, 10:81, 10:82, 10:83, 10:84, 10:85, 11:75, 11:76, 11:77, 11:78, 11:79, 11:80, 11:81, 11:82, 11:83, 11:84, 11:85, 12:75, 12:76, 12:77, 12:78, 12:79, 12:80, 12:81, 12:82, 12:83, or 12:84. 12:85, 13:75, 13:76, 13:77, 13:78, 13:79, 13:80, 13:81, 13:82, 13:83, 13:84, 13:85, 14:75, 14:76, 14:77, 14:78, 14:79, 14:80, 14:81, 14:82, 14:83, 14:84, 14:85, 15:75, 15:76, 15:77, 15:78, 15:79, 15:80, 15:81, 15:82, 15:83, 15:84, 15:85.

[0063] Cellulose molecules form a crystalline structure through numerous hydrogen bonds. NaOH, as a strong base, can penetrate into both the amorphous and crystalline regions of cellulose, electrically weakening the intermolecular forces and causing the cellulose chain segments to separate. Under alkaline conditions, the hydroxyl groups on the surface of cellulose are deprotonated, and the negatively charged molecular chains are further dispersed through electrostatic repulsion, preventing re-aggregation and forming a stable solution system.

[0064] Urea molecules can insert themselves between cellulose molecular chains, forming hydrogen bonds with hydroxyl groups to further break down the crystalline structure of cellulose and enhance its solubility. Urea can lower the freezing point of the solution, preventing cellulose from recrystallizing and precipitating at low temperatures, while maintaining the dispersed state of cellulose molecules and ensuring the stability of the solution.

[0065] In some embodiments, the mass percentage of CaCl2 in the mixed coagulation bath is 3-7 wt%, and the mass percentage of glacial acetic acid is 0.6-1.5 wt%.

[0066] The mass percentage of CaCl2 can be 3wt%, 4wt%, 5wt%, 6wt%, or 7wt%; the mass percentage of glacial acetic acid can be 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, or 1.6wt%.

[0067] In some embodiments, the freeze-curing temperature is -20 to -24°C. For example, it can be -20°C, -21°C, -22°C, -23°C, or -24°C.

[0068] In some embodiments, the freeze-drying temperature is -90 to -100°C, and the vacuum degree is below 1.0 Pa. The temperature can be -90°C, -91°C, -92°C, -93°C, -94°C, -95°C, -96°C, -97°C, -98°C, -99°C, or -100°C.

[0069] The present invention will be further described below with reference to the embodiments.

[0070] Example 1 (1) Weigh 2 g of alkali lignin using an electronic balance and dissolve it in 50 mL of N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution A. Pour solution A into a 250 mL round-bottom flask and continue stirring. Add 1.56 mL of bromopropyne and 2.51 g of potassium carbonate to the solution, heat to 75 ℃ and stir for 50 h. After cooling to room temperature, filter the potassium carbonate, add 1 L of deionized water to precipitate the lignin, allow it to stand and separate into layers, pour off the supernatant, pour the remaining solution A into a centrifuge tube, centrifuge for 10 min, pour off the supernatant again, and place the precipitate in a vacuum drying oven and vacuum dry at 85 ℃ for 12 h to obtain alkynylated lignin.

[0071] (2) Weigh 10.14 g DOPO and 1.2 g alkynylated lignin using an electronic balance and dissolve them in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution B. Heat to 150 ℃ and stir for 5 h. After cooling to room temperature, pour in 1 L of deionized water to precipitate the lignin. Allow it to stand and separate into layers. Pour off the supernatant and pour the remaining solution B into a centrifuge tube. Centrifuge the solution for 10 min and pour off the supernatant again. Place the precipitate in a vacuum drying oven and dry it under vacuum at 85 ℃ for 12 h to obtain phosphated lignin.

[0072] (3) Prepare 50 mM CuSO4·5H2O aqueous solution, 250 mM tris(3-hydroxypropyltriazolemethyl)amine aqueous solution, and 1 M sodium ascorbate aqueous solution, respectively. Mix 1 mL CuSO4·5H2O aqueous solution with 1 mL tris(3-hydroxypropyltriazolemethyl)amine aqueous solution to pre-form a complex solution C. Weigh 1 g of phosphorylated lignin using an electronic balance and dissolve it in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution D. Pour solution C into a 250 mL three-necked flask and continue stirring at 0.5 m... 3 Nitrogen gas was introduced at a flow rate of / h. 0.362 mL of ethyl azide was added dropwise to solution D using a pipette. Solution C was added dropwise to solution D using a pipette. The mixture was stirred at 500 r / min at room temperature for 5 min. 1 mL of sodium ascorbate aqueous solution was added dropwise to solution D. The mixture was heated to 50 ℃ and stirred for 12 h. After cooling to room temperature, 1 L of deionized water was added to precipitate the lignin. The mixture was allowed to stand and separate into layers. The supernatant was poured off, and the remaining solution D was poured into a centrifuge tube. The mixture was centrifuged for 10 min, and the supernatant was poured off again. The precipitate was placed in a vacuum drying oven and dried under vacuum at 85 ℃ for 12 h to obtain flame-retardant modified lignin with a yield of 87.1%.

[0073] (4) Weigh 2.1 g of flame-retardant modified lignin, dissolve it in 10 mL of dimethyl sulfoxide, and shake until completely dissolved to obtain a flame-retardant modified lignin solution. Transfer the obtained solution to a dialysis bag with a molecular weight cutoff of 14000 Da, seal it, and dialyze it in a large amount of deionized water for 24 h, changing the dialysis medium 3 times during the period, to obtain nano-flame-retardant modified lignin uniformly dispersed in water.

[0074] (5) Place the dialysis solution in a beaker, add deionized water to dilute to 100 mL, then add 4 g of sodium alginate, and stir magnetically at 60 °C to obtain flame-retardant modified lignin sodium alginate gel solution.

[0075] Weigh out polyvinyl alcohol, add deionized water to prepare a 4% (w / w) solution, heat to 95 °C and stir continuously until the polyvinyl alcohol is completely dissolved to obtain a 4 wt% polyvinyl alcohol solution.

[0076] The above polyvinyl alcohol solution and flame-retardant modified lignin sodium alginate gel solution were mixed at a volume ratio of 10:17 and stirred at room temperature until homogeneous to obtain a polyvinyl alcohol / sodium alginate flame-retardant gel solution.

[0077] (6) Take the prepared polyvinyl alcohol / sodium alginate flame retardant gel solution, pour it into the molding mold, let it stand to remove bubbles, and then freeze-cure it in a low temperature environment (-24℃). Freeze-dry the cured gel sample under the following conditions: temperature -95℃, vacuum degree below 1.0 Pa, until constant weight is obtained to obtain flame retardant polyvinyl alcohol / sodium alginate composite flame retardant gel.

[0078] The blank sample is prepared by weighing polyvinyl alcohol, adding deionized water to prepare a 4% (w / w) solution, heating to 95 °C and stirring continuously until the polyvinyl alcohol is completely dissolved, thus obtaining a 4 wt% polyvinyl alcohol solution A.

[0079] Weigh out sodium alginate, add deionized water to prepare a 4% solution, heat to 70 °C and continuously stir mechanically until the sodium alginate is completely dissolved, thus obtaining 4 wt% sodium alginate gel solution B.

[0080] Figure 1 The total heat release (THR) curve (A) and heat release rate (HRR) curve (B) of the flame-retardant polyvinyl alcohol / sodium alginate composite aerogel prepared in Example 1 of this invention, compared with the blank sample (Original), visually demonstrate the improvement effect of the flame retardant on the heat release performance of the material. Specifically, compared with the blank sample (Original), the combustion exothermic behavior of the flame-retardant composite aerogel (NPL) prepared in this invention is significantly suppressed, and its peak heat release rate (pHRR) is reduced from approximately 253.84 kW / m³. 2It dropped significantly to approximately 71.91 kW / m 2 The decrease reached 71.7%, completely eliminating the sudden strong exothermic peak during the combustion of the blank sample; the total heat release (THR) at 600 s decreased from 8.84 MJ / m 2 Reduced to 7.88 MJ / m 2 The heat release process was significantly slowed down, resulting in a 10.9% reduction in heat toxicity and fire spread risk during the combustion process. This fully demonstrates the excellent effect of the flame-retardant modification scheme of this invention on improving the flame-retardant performance of materials.

[0081] Example 2 The difference from Example 1 is that in step (3), 0.181 mL of ethyl azide is measured using a pipette and added to solution D. The rest is the same as in Example 1.

[0082] Step (3) is as follows: (3) Prepare 50 mM CuSO4·5H2O aqueous solution, 250 mM tris(3-hydroxypropyltriazolemethyl)amine aqueous solution, and 1 M sodium ascorbate aqueous solution, respectively. Mix 1 mL CuSO4·5H2O aqueous solution with 1 mL tris(3-hydroxypropyltriazolemethyl)amine aqueous solution to pre-form a complex solution C. Weigh 1 g of phosphorylated lignin using an electronic balance and dissolve it in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution D. Pour solution C into a 250 mL three-necked flask and continue stirring at 0.5 m... 3 Nitrogen gas was introduced at a flow rate of / h. 0.181 mL of ethyl azide was added dropwise to solution D using a pipette. Solution C was added dropwise to solution D using a pipette. The mixture was stirred at 500 r / min at room temperature for 5 min. 1 mL of sodium ascorbate aqueous solution was added dropwise to solution D. The mixture was heated to 50 ℃ and stirred for 12 h. After cooling to room temperature, 1 L of deionized water was added to precipitate the lignin. The mixture was allowed to stand and separate into layers. The supernatant was poured off, and the remaining solution D was poured into a centrifuge tube. The mixture was centrifuged for 10 min, and the supernatant was poured off again. The precipitate was placed in a vacuum drying oven and dried under vacuum at 85 ℃ for 12 h to obtain flame-retardant modified lignin with a yield of 84.9%.

[0083] Figure 2 These are comparative photographs of the combustion processes of the composite flame-retardant gels prepared in Examples 2 (A) and 1 (B) of this invention at different time points in a butane flame, demonstrating the effect of ethyl azide dosage on the flame-retardant effect. Figure 2It can be seen that the composite flame-retardant aerogel prepared in Example 1 has superior flame-retardant and ignition-resistant properties and self-extinguishing charring ability compared to that in Example 2. The sample in Example 2 was ignited and produced a visible flame within 6 seconds under butane flame conditions, and was completely charred and shrunken by 16 seconds, with a dense char layer and no dripping. In contrast, the sample in Example 1, under the same flame conditions, produced no visible flame within 20 seconds, with only slow charring occurring in the area in contact with the flame. The main structure of the sample remained intact, and there was no combustion spread. The results indicate that with the increase of ethyl azide content, the flame-retardant properties of the composite aerogel are significantly enhanced, effectively suppressing flaming combustion and flame propagation, and greatly improving the fire safety of the material.

[0084] Example 3 The difference from Example 1 is that in step (4), 1.05 g of flame-retardant modified lignin is weighed, and everything else is the same as in Example 1.

[0085] Step (4) is as follows: (4) Weigh 1.05 g of flame-retardant modified lignin, dissolve it in 10 mL of dimethyl sulfoxide, and shake until completely dissolved to obtain a flame-retardant modified lignin solution. Transfer the obtained solution to a dialysis bag with a molecular weight cutoff of 14000 Da, seal it, and dialyze it in a large amount of deionized water for 12 h, changing the dialysis medium twice during the process, to obtain nano-flame-retardant modified lignin uniformly dispersed in water.

[0086] Compared to Example 1, the composite aerogel prepared in Example 3 exhibited significantly reduced flame retardant and ignition resistance, as well as a significantly reduced self-extinguishing charring ability. The sample in Example 3 was ignited and produced a visible flame within 8 seconds under butane flame conditions, and by 20 seconds, large-area charring and shrinkage had occurred, with the main structure collapsing without dripping. In contrast, the sample in Example 1, under the same flame conditions, produced no visible flame within 20 seconds, with only slow charring occurring in the area in contact with the flame. The main structure of the sample remained intact, and there was no combustion spread. The results indicate that when the amount of modified lignin is only half that of Example 1, the content of nitrogen and phosphorus flame retardant components in the system is insufficient, failing to fully utilize the synergistic flame retardant effect of gas-phase quenching of free radicals and condensed-phase charring inhibition. Consequently, the composite aerogel's ability to suppress flaming combustion and flame propagation is significantly weakened, resulting in a marked decrease in fire safety performance.

[0087] Example 4 The difference from Example 1 is that steps (3), (4) and (6) are different from those in Example 1, while the rest are the same as in Example 1.

[0088] Specifically: (3) Prepare 50 mM CuSO4·5H2O aqueous solution, 250 mM tris(3-hydroxypropyltriazolemethyl)amine aqueous solution, and 1 M sodium ascorbate aqueous solution, respectively. Mix 1 mL CuSO4·5H2O aqueous solution with 1 mL tris(3-hydroxypropyltriazolemethyl)amine aqueous solution to pre-form a complex solution C. Weigh 1 g of phosphorylated lignin using an electronic balance and dissolve it in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution D. Pour solution C into a 250 mL three-necked flask and continue stirring at 0.5 m... 3 Nitrogen gas was introduced at a flow rate of / h. 0.181 mL of ethyl azide was added dropwise to solution D using a pipette. Solution C was added dropwise to solution D using a pipette. The mixture was stirred at 500 r / min at room temperature for 5 min. 1 mL of sodium ascorbate aqueous solution was added dropwise to solution D. The mixture was heated to 50 ℃ and stirred for 12 h. After cooling to room temperature, 1 L of deionized water was added to precipitate the lignin. The mixture was allowed to stand and separate into layers. The supernatant was poured off, and the remaining solution D was poured into a centrifuge tube. The mixture was centrifuged for 10 min, and the supernatant was poured off again. The precipitate was placed in a vacuum drying oven and dried under vacuum at 85 ℃ for 12 h to obtain flame-retardant modified lignin.

[0089] (4) Weigh 1.05 g of flame-retardant modified lignin, dissolve it in 10 mL of dimethyl sulfoxide, and shake until completely dissolved to obtain a flame-retardant modified lignin solution. Transfer the obtained solution to a dialysis bag with a molecular weight cutoff of 14000 Da, seal it, and dialyze it in a large amount of deionized water for 24 h, changing the dialysis medium 3 times during the process, to obtain nano-flame-retardant modified lignin uniformly dispersed in water.

[0090] (6) Take the prepared polyvinyl alcohol / sodium alginate flame retardant gel solution, pour it into the molding mold, let it stand to remove bubbles, and then freeze-cure it in a low temperature environment (-20℃). Freeze-dry the cured gel sample under the following conditions: temperature -100℃, vacuum degree below 1.0 Pa, until constant weight is obtained to obtain flame retardant polyvinyl alcohol / sodium alginate composite flame retardant gel.

[0091] Compared to Example 1, the composite aerogel prepared in Example 4 exhibited a more significant decrease in flame retardant and ignition-resistant properties and self-extinguishing charring ability. The sample in Example 4 was ignited and produced a violent and sustained open flame within 6 seconds under butane flame conditions. It was completely charred and shrunken by 15 seconds, with its original three-dimensional structure completely collapsing, and only achieved self-extinguishing after 25 seconds. In contrast, the sample in Example 1, under the same flame conditions, produced no open flame within 20 seconds, with only slow charring occurring in the area in contact with the flame. The main structure of the sample remained intact, and there was no combustion spread. The results indicate that when the amounts of ethyl azide and modified lignin are both only half that of Example 1, not only is the total content of nitrogen and phosphorus flame-retardant functional components in the system significantly reduced, but the grafting rate of flame-retardant elements in the modified lignin itself is also significantly insufficient. This fails to fully utilize the dual synergistic flame-retardant effect of gas-phase free radical quenching and condensed-phase charring inhibition, resulting in a significant weakening of the composite aerogel's ability to suppress flaming combustion and flame propagation, and a significant decrease in fire safety performance.

[0092] Example 5 The difference from Example 1 is that steps (4), (5) and (6) are different from those in Example 1, while the rest are the same as in Example 1.

[0093] Specifically: (4) Weigh 2.1 g of flame-retardant modified lignin, dissolve it in 10 mL of dimethyl sulfoxide, and shake until completely dissolved to obtain a flame-retardant modified lignin solution. Transfer the obtained solution to a dialysis bag with a molecular weight cutoff of 14000 Da, seal it, and dialyze it in a large amount of deionized water for 18 h, changing the dialysis medium 3 times during the process, to obtain nano-flame-retardant modified lignin uniformly dispersed in water.

[0094] (5) Place the dialysis solution in a beaker, add deionized water to dilute to 100 mL, then add 4 g of sodium alginate, and stir magnetically at 60 °C to obtain flame-retardant modified lignin sodium alginate gel solution.

[0095] Weigh out cellulose powder and add a cold solution of NaOH:urea:water = 7:12:81 to prepare a 4% mass fraction solution. Stir vigorously at 1500 rpm for 10 min to fully dissolve the cellulose and obtain a transparent or semi-transparent high-viscosity cellulose solution.

[0096] The above cellulose solution and flame-retardant modified lignin sodium alginate gel solution were mixed at a volume ratio of 1:1 and stirred at room temperature until homogeneous to obtain a cellulose / sodium alginate flame-retardant gel solution.

[0097] (6) Take the prepared cellulose / sodium alginate flame-retardant gel solution, pour it into a molding mold, let it stand to remove bubbles, then immerse the mold in a coagulation bath containing 5 wt% CaCl2 and 1 wt% glacial acetic acid, and let it stand at room temperature for 24 h to allow it to completely gel and solidify. Rinse it with plenty of deionized water, and then freeze-cure it at a low temperature (-24℃). Freeze-dry the solidified gel sample under the following conditions: temperature -90℃, vacuum degree below 1.0 Pa, until constant weight is obtained to obtain a flame-retardant cellulose / sodium alginate composite flame-retardant gel.

[0098] Example 6 The difference from Example 1 is that steps (3), (4), (5) and (6) are different from those in Example 1, while the others are the same as those in Example 1.

[0099] Specifically: (3) Prepare 50 mM CuSO4·5H2O aqueous solution, 250 mM tris(3-hydroxypropyltriazolemethyl)amine aqueous solution, and 1 M sodium ascorbate aqueous solution, respectively. Mix 1 mL CuSO4·5H2O aqueous solution with 1 mL tris(3-hydroxypropyltriazolemethyl)amine aqueous solution to pre-form a complex solution C. Weigh 1 g of phosphorylated lignin using an electronic balance and dissolve it in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution D.

[0100] Pour solution C into a 250 mL three-necked flask and continue stirring, adding water at a flow rate of 0.5 mL / min. 3 Nitrogen gas was introduced at a flow rate of / h. 0.181 mL of ethyl azide was added dropwise to solution D using a pipette. Solution C was added dropwise to solution D using a pipette. The mixture was stirred at 500 r / min at room temperature for 5 min. 1 mL of sodium ascorbate aqueous solution was added dropwise to solution D. The mixture was heated to 50 ℃ and stirred for 12 h. After cooling to room temperature, 1 L of deionized water was added to precipitate the lignin. The mixture was allowed to stand and separate into layers. The supernatant was poured off, and the remaining solution D was poured into a centrifuge tube. The mixture was centrifuged for 10 min, and the supernatant was poured off again. The precipitate was placed in a vacuum drying oven and dried under vacuum at 85 ℃ for 12 h to obtain flame-retardant modified lignin.

[0101] (5) Place the dialysate in a beaker, add deionized water to dilute to 100 mL, then add 4 g of sodium alginate, and stir magnetically at 60 °C to obtain a flame-retardant modified lignin sodium alginate gel solution. Weigh cellulose powder, add a cold solution with a mass ratio of NaOH:urea:water = 7:12:81 to prepare a 4% mass fraction solution, and stir vigorously at 1000 rpm for 10 min to fully dissolve the cellulose, obtaining a transparent or semi-transparent high-viscosity cellulose solution.

[0102] The above cellulose solution and flame-retardant modified lignin sodium alginate gel solution were mixed at a volume ratio of 1:1 and stirred at room temperature until homogeneous to obtain a cellulose / sodium alginate flame-retardant gel solution.

[0103] (6) Take the prepared cellulose / sodium alginate flame-retardant gel solution, pour it into a molding mold, let it stand to remove bubbles, then immerse the mold in a coagulation bath containing 5 wt% CaCl2 and 1 wt% glacial acetic acid, and let it stand at room temperature for 20 h to allow it to completely gel and solidify. Rinse it with plenty of deionized water, and then freeze-cure it at a low temperature (-22℃). Freeze-dry the solidified gel sample under the following conditions: temperature -95℃, vacuum degree below 1.0 Pa, until constant weight is obtained to obtain a flame-retardant cellulose / sodium alginate composite flame-retardant gel.

[0104] Example 7 (1) Weigh 2 g of alkali lignin using an electronic balance and dissolve it in 50 mL of N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution A. Pour solution A into a 250 mL round-bottom flask and continue stirring. Add 1.56 mL of bromopropyne and 2.51 g of potassium carbonate to the solution, heat to 75 ℃ and stir for 50 h. After cooling to room temperature, filter the potassium carbonate, add 1 L of deionized water to precipitate the lignin, allow it to stand and separate into layers, pour off the supernatant, pour the remaining solution A into a centrifuge tube, centrifuge for 10 min, pour off the supernatant again, and place the precipitate in a vacuum drying oven and vacuum dry at 85 ℃ for 12 h to obtain alkynylated lignin.

[0105] (2) Weigh 5.07 g DOPO and 1.2 g alkynylated lignin using an electronic balance and dissolve them in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution B. Heat to 150 ℃ and stir for 5 h. After cooling to room temperature, pour in 1 L of deionized water to precipitate the lignin. Allow it to stand and separate into layers. Pour off the supernatant and pour the remaining solution B into a centrifuge tube. Centrifuge the solution for 10 min and pour off the supernatant again. Place the precipitate in a vacuum drying oven and dry it under vacuum at 85 ℃ for 12 h to obtain phosphated lignin.

[0106] (3) Prepare 50 mM CuSO4·5H2O aqueous solution, 250 mM tris(3-hydroxypropyltriazolemethyl)amine aqueous solution, and 1 M sodium ascorbate aqueous solution, respectively. Mix 1 mL CuSO4·5H2O aqueous solution with 1 mL tris(3-hydroxypropyltriazolemethyl)amine aqueous solution to pre-form a complex solution C. Weigh 1 g of phosphorylated lignin using an electronic balance and dissolve it in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution D. Pour solution C into a 250 mL three-necked flask and continue stirring at 0.5 m... 3 Nitrogen gas was introduced at a flow rate of / h. 0.181 mL of ethyl azide was added dropwise to solution D using a pipette. Solution C was added dropwise to solution D using a pipette. The mixture was stirred at 500 r / min at room temperature for 5 min. 1 mL of sodium ascorbate aqueous solution was added dropwise to solution D. The mixture was heated to 50 ℃ and stirred for 12 h. After cooling to room temperature, 1 L of deionized water was added to precipitate the lignin. The mixture was allowed to stand and separate into layers. The supernatant was poured off, and the remaining solution D was poured into a centrifuge tube. The mixture was centrifuged for 10 min, and the supernatant was poured off again. The precipitate was placed in a vacuum drying oven and dried under vacuum at 85 ℃ for 12 h to obtain flame-retardant modified lignin with a yield of 84.9%.

[0107] (4) Weigh 1.05 g of flame-retardant modified lignin, dissolve it in 10 mL of dimethyl sulfoxide, and shake until completely dissolved to obtain a flame-retardant modified lignin solution. Transfer the obtained solution to a dialysis bag with a molecular weight cutoff of 14000 Da, seal it, and dialyze it in a large amount of deionized water for 20 h, changing the dialysis medium 3 times during the period, to obtain nano-flame-retardant modified lignin uniformly dispersed in water.

[0108] (5) Place the dialysate in a beaker, add deionized water to dilute to 100 mL, then add 4 g of sodium alginate, and stir magnetically at 60 °C to obtain a flame-retardant modified lignin sodium alginate gel solution. Weigh cellulose powder, add a cold solution of NaOH:urea:water = 7:12:81 (mass ratio) to prepare a 4% mass fraction solution, and stir vigorously at 1200 rpm for 8 min to fully dissolve the cellulose, obtaining a transparent or semi-transparent high-viscosity cellulose solution.

[0109] The above cellulose solution and flame-retardant modified lignin sodium alginate gel solution were mixed at a volume ratio of 1:1 and stirred at room temperature until homogeneous to obtain a cellulose / sodium alginate flame-retardant gel solution.

[0110] (6) Take the prepared cellulose / sodium alginate flame-retardant gel solution, pour it into a molding mold, let it stand to remove bubbles, then immerse the mold in a coagulation bath containing 5 wt% CaCl2 and 1 wt% glacial acetic acid, and let it stand at room temperature for 24 h to allow it to completely gel and solidify. Rinse it with plenty of deionized water, and then freeze-cure it at a low temperature (-20℃). Freeze-dry the solidified gel sample under the following conditions: temperature -95℃, vacuum degree below 1.0 Pa, until constant weight is obtained to obtain a flame-retardant cellulose / sodium alginate composite flame-retardant gel.

[0111] Example 8 The difference from Example 1 is that steps (4), (5) and (6) are different from those in Example 1, while the rest are the same as in Example 1.

[0112] Specifically: (4) Weigh 1.05 g of flame-retardant modified lignin, dissolve it in 10 mL of dimethyl sulfoxide, and shake until completely dissolved to obtain a flame-retardant modified lignin solution. Transfer the obtained solution to a dialysis bag with a molecular weight cutoff of 14000 Da, seal it, and dialyze it in a large amount of deionized water for 24 h, changing the dialysis medium twice during the process, to obtain nano-flame-retardant modified lignin uniformly dispersed in water.

[0113] (5) Place the dialysate in a beaker, add deionized water to dilute to 100 mL, then add 4 g of sodium alginate, and stir magnetically at 60 °C to obtain a flame-retardant modified lignin sodium alginate gel solution. Weigh cellulose powder, add a cold solution of NaOH:urea:water = 7:12:81 (mass ratio) to prepare a 4% mass fraction solution, and stir vigorously at 1500 rpm for 9 min to fully dissolve the cellulose, obtaining a transparent or semi-transparent high-viscosity cellulose solution.

[0114] The above cellulose solution and flame-retardant modified lignin sodium alginate gel solution were mixed at a volume ratio of 1:1 and stirred at room temperature until homogeneous to obtain a cellulose / sodium alginate flame-retardant gel solution.

[0115] (6) Take the prepared cellulose / sodium alginate flame-retardant gel solution, pour it into a molding mold, let it stand to remove bubbles, then immerse the mold in a coagulation bath containing 5 wt% CaCl2 and 1 wt% glacial acetic acid, and let it stand at room temperature for 18 h to allow it to completely gel and solidify. Rinse it with plenty of deionized water, and then freeze-cure it at a low temperature (-20℃). Freeze-dry the solidified gel sample under the following conditions: temperature -90℃, vacuum degree below 1.0 Pa, until constant weight is obtained to obtain a flame-retardant cellulose / sodium alginate composite flame-retardant gel.

[0116] Comparative Example 1 (1) Weigh 5 g of alkali lignin using an electronic balance and disperse it in 100 mL of toluene. Stir at 500 r / min at room temperature for 5 min to obtain dispersion A. Pour dispersion A into a 250 mL round-bottom flask and continue stirring. Add 1.05 g of propargyl acid and 2.85 g of p-toluenesulfonic acid monohydrate to the mixture and heat to 40 °C and reflux for 6 h. After cooling to room temperature, wash the lignin with 500 mL of anhydrous ethanol, allow it to stand and separate into layers, pour off the supernatant, pour the remaining solution A into a centrifuge tube, centrifuge for 10 min, pour off the supernatant again, and place the precipitate in a vacuum drying oven and dry under vacuum at 60 °C for 12 h to obtain alkynylated lignin.

[0117] (2) Weigh 5.07 g DOPO and 1.2 g alkynylated lignin using an electronic balance and dissolve them in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution B. Heat to 150 ℃ and stir for 5 h. After cooling to room temperature, pour in 1 L of deionized water to precipitate the lignin. Allow it to stand and separate into layers. Pour off the supernatant and pour the remaining solution B into a centrifuge tube. Centrifuge the solution for 10 min and pour off the supernatant again. Place the precipitate in a vacuum drying oven and dry it under vacuum at 85 ℃ for 12 h to obtain phosphated lignin.

[0118] (3) Prepare 50 mM CuSO4·5H2O aqueous solution, 250 mM tris(3-hydroxypropyltriazolemethyl)amine aqueous solution, and 1 M sodium ascorbate aqueous solution, respectively. Mix 1 mL CuSO4·5H2O aqueous solution with 1 mL tris(3-hydroxypropyltriazolemethyl)amine aqueous solution to pre-form a complex solution C. Weigh 1 g of phosphorylated lignin using an electronic balance and dissolve it in 50 mL N,N-dimethylformamide. Stir at 500 r / min at room temperature for 5 min to obtain solution D. Pour solution C into a 250 mL three-necked flask and continue stirring at 0.5 m... 3 Nitrogen gas was introduced at a flow rate of / h. 0.181 mL of ethyl azide was added dropwise to solution D. Solution C was added dropwise to solution D using a pipette. The mixture was stirred at 500 r / min at room temperature for 5 min. 1 mL of sodium ascorbate aqueous solution was added dropwise to solution D. The mixture was heated to 50 ℃ and stirred for 12 h. After cooling to room temperature, 1 L of deionized water was added to precipitate the lignin. The mixture was allowed to stand and separate into layers. The supernatant was poured off, and the remaining solution D was poured into a centrifuge tube. The mixture was centrifuged for 10 min, and the supernatant was poured off again. The precipitate was placed in a vacuum drying oven and dried under vacuum at 85 ℃ for 12 h to obtain flame-retardant modified lignin with a yield of 57.4%.

[0119] (4) Weigh 2.1 g of flame-retardant modified lignin, dissolve it in 10 mL of dimethyl sulfoxide, and shake until completely dissolved to obtain a flame-retardant modified lignin solution. Transfer the obtained solution to a dialysis bag with a molecular weight cutoff of 14000 Da, seal it, and dialyze it in a large amount of deionized water for 24 h, changing the dialysis medium 3 times during the period, to obtain nano-flame-retardant modified lignin uniformly dispersed in water.

[0120] (5) Place the dialysate in a beaker, add deionized water to dilute to 100 mL, then add 4 g of sodium alginate, and stir magnetically at 60 °C to obtain a flame-retardant modified lignin sodium alginate gel solution. Weigh polyvinyl alcohol, add deionized water to prepare a 4% (w / w) solution, heat to 95 °C and stir continuously until the polyvinyl alcohol is completely dissolved to obtain a 4 wt% polyvinyl alcohol solution. Mix the above polyvinyl alcohol solution with the flame-retardant modified lignin sodium alginate gel solution at a volume ratio of 1:1, and stir at room temperature until homogeneous to obtain a polyvinyl alcohol / sodium alginate flame-retardant gel solution.

[0121] (6) Take the prepared polyvinyl alcohol / sodium alginate flame retardant gel solution, pour it into the molding mold, let it stand to remove bubbles, and then freeze-cure it in a low temperature environment (-24℃). Freeze-dry the cured gel sample under the following conditions: temperature -95℃, vacuum degree below 1.0 Pa, until constant weight is obtained to obtain flame retardant polyvinyl alcohol / sodium alginate composite flame retardant gel.

[0122] Compared to Example 1, this comparative example replaced the reaction solvent and alkynylating agent in the alkynylation reaction. In Example 1, N,N-dimethylformamide was used as the reaction solvent and bromopropyne was used as the alkynylating agent; in Comparative Example 1, the reaction solvent was replaced with toluene and the alkynylating agent was replaced with propynic acid.

[0123] Since lignin cannot be dissolved in toluene and has a low degree of alkynyl substitution, it is not conducive to subsequent reactions. The ignition time of the aerogel is shortened to 2 s, and the loss of lignin is too large when washing away toluene with anhydrous ethanol. Therefore, the yield of flame-retardant lignin in this comparative ratio is not good.

[0124] Comparative Example 2 Compared to Example 1, in step (4) of this comparative example, the dialysis dispersion step of the flame-retardant modified lignin was omitted, and the flame-retardant modified lignin was directly added to the polyvinyl alcohol / sodium alginate gel system, without achieving nanoscale uniform dispersion. Everything else is the same as in Example 1.

[0125] Specifically: (4) Take 100 mL of deionized water, add 4 g of sodium alginate, and stir magnetically at 60 °C to obtain a sodium alginate gel solution. Weigh polyvinyl alcohol, add deionized water to prepare a 4% (w / w) solution, heat to 95 °C and stir continuously until the polyvinyl alcohol is completely dissolved to obtain a 4 wt% polyvinyl alcohol solution. Mix the above polyvinyl alcohol solution and sodium alginate gel solution at a volume ratio of 10:17, weigh 2.1 g of flame-retardant modified lignin and add it to the gel solution, and stir at room temperature until homogeneous to obtain a polyvinyl alcohol / sodium alginate flame-retardant gel solution.

[0126] In Comparative Example 2, because the flame-retardant modified lignin was not dialyzed, it was unevenly dispersed in the gel system and agglomeration was severe, which limited the full play of its flame-retardant performance. The ignition time of the aerogel was shortened to 5 s, and deflagration occurred during the combustion process.

[0127] Comparative Example 3 The difference from Example 1 is that the alkynylation step (1) is omitted, while all other steps are the same as in Example 1.

[0128] Because the alkynylation step was omitted, the sample in Comparative Example 3 could not provide grafting sites for phosphorus-based flame-retardant groups, nor could it achieve subsequent click chemistry crosslinking and curing. The flame-retardant components were prone to migration and loss, and the char barrier effect was greatly weakened, resulting in flame-retardant performance that was significantly inferior to that of Example 1. Under the same butane flame test conditions as Example 1, the sample was ignited in 4.5s and produced a violent and sustained open flame. The flame spread rate was significantly faster than that of Example 1. By 12s, it had completely carbonized and shrunk, accompanied by slight melting and dripping. It had no self-extinguishing ability and only left a small amount of loose and weak ash, unable to maintain its original three-dimensional structure.

[0129] Comparative Example 4 The difference from Example 1 is that the phosphating step (2) is omitted, while all other steps are the same as in Example 1.

[0130] Comparative Example 4 sample, lacking the phosphate modification step, lacks the synergistic flame-retardant effects of gas-phase free radical quenching and condensed-phase char-promoting effects of phosphorus-based flame-retardant components. Relying solely on the weak char-forming properties of the alkynylated lignin and triazole cross-linked structure, its flame-retardant performance exhibits a precipitous decline. Under the same butane flame test conditions as Example 1, the sample ignited within 3 seconds, producing a violent and sustained open flame, accompanied by significant melting and dripping. The flame rapidly spread to the entire sample strip, and by 10 seconds, it had completely charred and shrunk, with the overall structure collapsing. It lacked self-extinguishing ability, leaving only a trace amount of loose, weak ash after combustion.

[0131] Comparative Example 5 The difference from Example 1 is that the azide step (3) is omitted, while the rest is the same as Example 1.

[0132] Comparative Example 5 sample, due to the omission of the azidation step, could not trigger the alkynyl-azide click chemical reaction, thus failing to construct the triazole ring crosslinked network structure and completely lacking the nitrogen-based synergistic flame-retardant effect of the triazole ring. Phosphated lignin exhibited significantly reduced dispersibility in the matrix and was prone to agglomeration, failing to form a uniform, continuous, and dense barrier char layer during combustion. Heat and oxygen easily transferred into the material's interior, resulting in a significantly lower flame-retardant performance compared to Example 1. Under the same butane flame test conditions as Example 1, the sample ignited and produced a sustained open flame within 3 seconds, with a faster flame spread rate than in Example 1. It was completely carbonized and shrunken by 13 seconds, lacking self-extinguishing capability. The char layer formed after combustion was loose, porous, and easily fractured, unable to maintain its original three-dimensional porous structure.

[0133] Table 1 Ignition time and combustion behavior of materials in different embodiments and comparative examples.

[0134] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a nitrogen-phosphorus modified lignin-based flame retardant, characterized in that: Includes the following steps: An alkynylating agent and an activator are added to an organic solution of lignin, and the mixture is heated and stirred to carry out an alkynylation modification reaction, thereby obtaining alkynylated lignin. Alkylated lignin was mixed with a phosphating agent in an organic solvent, and then heated and stirred to carry out a phosphating reaction to obtain phosphated lignin. In a nitrogen atmosphere, phosphated lignin, azide, and composite catalyst are mixed in an organic solvent, heated and stirred to perform azide modification, thus obtaining a flame retardant.

2. The preparation method of the nitrogen-phosphorus modified lignin-based flame retardant according to claim 1, characterized in that: The alkynylating agent is bromopropyne, propynic acid, or 3-chloropropyne; Alternatively, the activator may be potassium carbonate, sodium carbonate, p-toluenesulfonic acid, triethylamine, or tetrabutylammonium bromide; Alternatively, the lignin is alkali lignin, and the solvent of the organic solution of the lignin is N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, 1-methyl-2-pyrrolidone, or tetrahydrofuran.

3. The preparation method of the nitrogen-phosphorus modified lignin-based flame retardant according to claim 1, characterized in that: The heating temperature for the alkynylation modification reaction is 40~80 ℃, and the reaction time is 6~50 h; Alternatively, after the alkynylation modification reaction is completed, the activator is removed by filtration, and then deionized water is added to the reaction solution to precipitate the alkynylated lignin. After separation and drying, the alkynylated lignin is obtained.

4. The preparation method of the nitrogen-phosphorus modified lignin-based flame retardant according to claim 1, characterized in that: The phosphating agent is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), diphenylphosphine oxide (DPO), phenylphosphine acid, or dimethyl hydroxymethylphosphonate. Alternatively, the phosphating reaction temperature is 120~150 ℃, and the reaction time is 4~8 h; Alternatively, after the phosphating reaction is complete, deionized water is added to the reaction system to precipitate the phosphated lignin, which is then separated and dried to obtain phosphated lignin.

5. The method for preparing the nitrogen-phosphorus modified lignin-based flame retardant according to claim 1, characterized in that: The azide is ethyl azide, 2-azidoethanol, azidoacetic acid, or sodium azide. Alternatively, the composite catalyst is a mixture of CuSO4 and tris(3-hydroxypropyltriazolylmethyl)amine and sodium ascorbate; Preferably, in the composite catalyst, the mass ratio of CuSO4, the complex of tris(3-hydroxypropyltriazolylmethyl)amine, and sodium ascorbate is 1:8~9:15~17; Alternatively, the reaction temperature for the azidation modification is 45~55 ℃, and the reaction time is 10~12 h; Preferably, during the azide modification, the nitrogen flow rate is 0.3-1 m³ / s. 3 / h, preferably 0.3-0.8 m 3 / h.

6. The method for preparing the nitrogen-phosphorus modified lignin-based flame retardant according to claim 1, characterized in that: When the alkynylating agent is bromopropyne, the phosphating agent is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and the azide is ethyl azide (ethyl azide density is 1.08 g / mL), the mass ratio of lignin to bromopropyne is 1~5:2~4; the mass ratio of DOPO to alkynylated lignin is 6-10:1; and the mass ratio of ethyl azide to phosphated lignin is 0.2-0.4:

1. Preferably, the mass ratio of ethyl azide to phosphorylated lignin is 0.3-0.4:1, more preferably 0.35-0.4:1, and even more preferably 0.37-0.4:

1.

7. A nitrogen-phosphorus modified lignin-based flame retardant, characterized in that: It is prepared by any one of the preparation methods described in claims 1-6.

8. The application of the nitrogen-phosphorus modified lignin-based flame retardant according to claim 7 in the preparation of flame retardant materials or in improving the flame retardant properties of polymer matrices.

9. A method for preparing a lignin-based flame-retardant gel, characterized in that: Includes the following steps: The lignin-based flame retardant of claim 7 is dissolved in dimethyl sulfoxide, and then the obtained lignin-based flame retardant solution is transferred to a dialysis bag and dialyzed in deionized water to obtain a dialysate, wherein the dialysate comprises water and a lignin-based flame retardant uniformly dispersed in water. The dialysate was diluted and mixed with sodium alginate to obtain a gel solution; A flame-retardant gel solution is obtained by mixing the gel solution with a matrix solution of polyvinyl alcohol or cellulose. When the matrix is ​​polyvinyl alcohol, the flame retardant gel solution is directly cast into a mold to obtain a gel; when the matrix is ​​cellulose, the flame retardant gel solution is cast into a mold, allowed to stand to degas, and then soaked in a mixed coagulation bath of CaCl2 and glacial acetic acid for 12-24 hours until it is completely gelled and solidified. After rinsing with water, the gel is obtained. After the obtained gel is freeze-cured, it is then freeze-dried to obtain a lignin-based flame-retardant gel.

10. The method for preparing lignin-based flame-retardant gel according to claim 9, characterized in that: The molecular weight cutoff of the dialysis bag is 12,000 to 15,000 Da, preferably 14,000 Da; Preferably, the dialysis time is 12-24 hours, and the deionized water is changed regularly during the dialysis process; Alternatively, the dialysate can be diluted and mixed with sodium alginate, then stirred at 55-70°C to obtain a gel solution; Preferably, the stirring temperature is 55~65℃; Alternatively, when the matrix is ​​polyvinyl alcohol, the matrix solution is an aqueous solution of polyvinyl alcohol, and the mass fraction of polyvinyl alcohol in the solution is 3-5%, preferably 3-4.5%, and more preferably 4%. Alternatively, when the matrix is ​​cellulose, the mass fraction of cellulose in the matrix solution is 3% to 5%, and the solvent system is a mixed solvent system of sodium hydroxide, urea and water, with the mass ratio of sodium hydroxide to water being 5-10:75-85 and the mass ratio of urea to water being 10-15:75-85. Alternatively, in the mixed coagulation bath, the mass percentage of CaCl2 is 3-7 wt%, and the mass percentage of glacial acetic acid is 0.6-1.5 wt%. Alternatively, the freeze-curing temperature is -20 to -24°C; Alternatively, the freeze-drying temperature is -90 to -100°C, and the vacuum degree is below 1.0 Pa.

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