Preparation method of biomass-based flame-retardant aerogel

Abstract

The invention discloses a preparation method of biomass-based flame-retardant aerogel, which comprises the following steps: preparing phosphorylated lignin (PL) by adopting a mechanochemical method, and introducing lignocellulose (PDA-LCF) with a dopamine coating into the system to form a P-N synergistic flame-retardant system. Furthermore, green and renewable sodium alginate (SA) is adopted as a gel matrix to prepare the composite aerogel material with fireproof performance and thermal insulation capacity. Benefited from the characteristics of lignin, the prepared biomass-based flame-retardant aerogel has excellent flame retardance and heat insulation capacity, and has huge potential in industrial application.

Description

Technical Field

[0001] The present invention relates to the technical field of composite flame-retardant thermal insulation aerogels, and particularly relates to a preparation method of a biomass-based highly flame-retardant thermal insulation aerogel. Background Art

[0002] In the construction industry, petroleum-based foams, including polyurethane (PU), expanded polystyrene (EPS), and phenolic foam boards, are commonly used as thermal insulation materials for industrial and commercial applications due to their light weight and excellent thermal insulation properties. Unfortunately, despite the many advantages of the organic polymer matrix, its high flammability imposes significant limitations on industrial applications, resulting in fires continuing to cause incalculable losses to human life and property every year. In addition, the use of petroleum-based thermal insulation boards brings serious environmental pollution and sustainability challenges. The non-degradability of foam plastics made from petroleum exacerbates the energy and environmental crises, and the pollution caused by waste polymer plastics has become a widely recognized social problem. Therefore, there is an urgent need to develop green and safe new thermal insulation materials using biomass energy.

[0003] Aerogels are low-density, high-porosity materials with important application potential in the field of thermal insulation materials. In addition, aerogels made from biomass energy can fundamentally solve the environmental pollution problems related to petroleum foams due to their inherent advantages, including eco-friendliness, economic benefits, and biodegradability. In recent years, many researchers have developed a series of biomass composite aerogels using biomass raw materials, including cellulose, chitin, and gelatin. However, similar to traditional petroleum-based insulation foam boards, most biomass-based aerogels also pose potential fire hazards. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of a biomass-based flame-retardant composite aerogel. The present invention prepares phosphorylated lignin (PL) by a mechanochemical method, and at the same time introduces lignocellulose with dopamine coating (PDA@LCF) into the system to form a phosphorus-nitrogen (P-N) synergistic flame-retardant system. In addition, a composite aerogel material is prepared using green and renewable sodium alginate (SA) as the gel matrix, and this material has excellent fire resistance and thermal insulation properties. This invention opens up a new path for the rational design of sustainable flame-retardant thermal insulation aerogel materials.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A preparation method of a biomass-based flame-retardant composite aerogel, characterized by comprising the following steps:

[0007] (1) Phosphorylation modification of lignin: The pretreated prehydrolysis lignin, phosphorus pentoxide (P2O5), and urea were mixed in a zirconia tank and ground with zirconia balls at a speed of 200 - 250 rpm for 6 - 8 h; then, the obtained mixture was uniformly dispersed in deionized water and dialyzed using a dialysis tube. After that, the obtained mixture was freeze-dried to obtain phosphorylated lignin (PL);

[0008] (2) Preparation of lignocellulose fibers with dopamine coating (PDA@LCF): The pretreated lignocellulose fibers (LCF) were immersed in a dopamine (PDA) precursor solution for 24 - 48 h to obtain lignocellulose fibers with a polydopamine coating loaded on the surface;

[0009] (3) Preparation of biomass-based flame-retardant aerogel: Phosphorylated lignin (PL) and lignocellulose fibers with dopamine coating (PDA@LCF) were uniformly dispersed in an alkaline aqueous solution with pH = 10 - 12, and then sodium alginate (SA) was added. The mixture was stirred at 150 - 200 rpm for 3 - 6 h, placed in a special mold, and unidirectionally frozen with liquid nitrogen. Subsequently, it was freeze-dried at -40 °C and 0 - 0.1 Pa for 2 - 3 days to obtain a lignin composite aerogel.

[0010] Furthermore, in step (1), the ratio of the pretreated prehydrolysis lignin, phosphorus pentoxide (P2O5), and urea is 4 - 6:4 - 6:1 - 2.

[0011] Furthermore, in step (1), the diameter of the zirconia balls is 5 - 10 mm.

[0012] Furthermore, in step (1), the cut-off molecular weight of the dialysis tube is 900 - 1000 Da.

[0013] Further, in step (1), the method for preparing the pretreated prehydrolyzed lignin is as follows: First, dissolve the prehydrolyzed lignin in a 1,4-dioxane solution, stir at 200-250 rpm for 4-5 h, and then concentrate the volume of the lignin mixed solution to 1 / 3-1 / 4 of the original volume through a rotary evaporator to obtain a lignin concentrate; drop the obtained lignin concentrate into acidic water with pH = 2.0-2.5, let it stand in a refrigerator at 4-5 °C for 24-48 h, vacuum filter to remove the supernatant, and continuously rinse with deionized water; freeze-dry the obtained precipitate to obtain purified prehydrolyzed lignin (L). Among them, the 1,4-dioxane solution is an aqueous solution of 1,4-dioxane with a volume fraction of 80-90%, the ratio of prehydrolyzed lignin to the 1,4-dioxane solution is 10-20 g: 200-400 mL, and the ratio of prehydrolyzed lignin to acidic water with pH = 2.0-2.5 is 10-20 g: 1-2 L; the acidic water with pH = 2-2.5 is an aqueous hydrochloric acid solution with pH = 2-2.5.

[0014] Further, in step (2), the method for preparing the pretreated lignocellulose fibers is as follows: First, mechanically disperse softwood pulp board into water to obtain fiber pulp, and then filter the fiber pulp to obtain wet fibers (lignocellulose fibers). Secondly, treat the wet fibers with a simple mechanical ball milling method to facilitate dispersion and obtain dispersed lignocellulose fibers (i.e., pretreated lignocellulose fibers (LCF)). Among them, the softwood is coniferous wood, for example, the dosage of lignocellulose is 4.0-6.0 g. Further, the method of mechanically dispersing the softwood pulp board into water to obtain fiber pulp is specifically as follows: Cut the coniferous wood pulp board into pieces, soak it in water for 12-24 h, and then obtain fiber pulp through mechanical crushing. The method of treating wet fibers with a mechanical ball milling method is specifically as follows: Load the lignocellulose into a zirconia jar, use zirconia balls to grind at 350-400 rpm for 1-2 h to obtain pretreated lignocellulose fibers (LCF). Among them, the diameter of the zirconia balls is 5-10 mm.

[0015] Further, store the wet fibers in a refrigerator at 4-5 °C.

[0016] Further, in step (2), the method for preparing the dopamine (PDA) precursor solution is as follows: First, dissolve tris(hydroxymethyl)aminomethane (Tris) in water, gradually add a 0.1-0.15 mol / L HCl solution until the pH reaches 8.5-9.0; then add dopamine hydrochloride and mix to obtain a PDA precursor solution. Among them, the ratio of tris(hydroxymethyl)aminomethane, water, and dopamine hydrochloride is 0.4-0.6 g: 100-150 mL: 0.4-0.6 g.

[0017] Further, the concentration of the HCl solution is 0.1 - 0.15 mol / L HCl solution.

[0018] Further, in step (3), the mass ratio of phosphorylated lignin (PL), lignocellulose fibers with dopamine coating (PDA@LCF), and sodium alginate (SA) is 0.3 - 4.0:1.0 - 2.0:2.0 - 4.0. Further still, the mass fraction of phosphorylated lignin in phosphorylated lignin, lignocellulose fibers with dopamine coating, and sodium alginate is 10 - 40%, such as 10%, 20%, 30%, and 40%.

[0019] Further, in step (3), the ratio of phosphorylated lignin (PL) to the alkaline aqueous solution with pH = 10 - 12 is 0.5 - 4.0 g:50 - 100 mL; the alkaline aqueous solution with pH = 10 - 12 is sodium hydroxide aqueous solution with pH = 10 - 12.

[0020] Further, in step (3), the specific operation method of unidirectional freezing with liquid nitrogen is as follows: First, prepare a stainless - steel container filled with liquid nitrogen, then place the bottom of the material at the upper liquid level of the liquid nitrogen and keep it for 0.5 - 1 h to obtain a unidirectionally frozen and formed material. The present invention also relates to protecting the biomass - based flame - retardant aerogel prepared by the above - mentioned preparation method.

[0021] As one of the most abundant aromatic natural polymers, lignin contains many active sites on its macromolecular chain, including phenolic and aliphatic hydroxyl groups. Therefore, it is suitable for the functionalization of the flame - retardant segment.

[0022] In order to improve the flame - retardant performance of lignin - based flame retardants, the present invention introduces phosphorus (P) / nitrogen (N) into the molecular structure by chemical methods to form a P - N synergistic flame - retardant system. Phosphorus - functionalized lignin not only reduces the heat release rate during the combustion process of polymer materials but also reduces the total smoke production rate during combustion. Further, green and renewable sodium alginate (SA) is used as the gel matrix to prepare a composite aerogel material with fire - prevention performance and heat - insulation ability. Benefiting from the inherent properties of lignin, the prepared biomass - based flame - retardant aerogel has excellent flame - retardant performance and heat - insulation ability and has great potential in industrial applications. In short, this technology puts forward an innovative design concept for the development of the next - generation fire - prevention and heat - insulation materials.

[0023] The beneficial effects brought by the technical solution provided by the present invention are:

[0024] The present invention uses biomass-based materials to prepare high-performance flame-retardant and heat-insulating aerogel materials. It is found that the addition of phosphorylated lignin enhances the flame-retardant performance of the aerogel materials, thereby reducing the heat release rate and smoke generation. At the same time, PDA@LCF is introduced as a nitrogen source to construct a P-N synergistic system, effectively improving the flame-retardant efficiency of the materials. Subsequently, it can be used as an effective fire prevention solution for various substrates, providing great application potential in construction, commercial insulation and related fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of lignin modification provided by an embodiment of the present invention;

[0026] Figure 2 It is an FT-IR and XPS data diagram of a lignin sample provided by Embodiment 2 of the present invention (wherein, Figure a is the FT-IR spectrum, and Figure b is the XPS spectrum);

[0027] Figure 3 It is a preparation flow chart of the flame-retardant composite aerogel of the present invention;

[0028] Figure 4 It is a TG and DTG curve of different aerogel samples provided by Embodiments 1-4 of the present invention (wherein Figure a is the TG curve. Figure b is the DTG curve);

[0029] Figure 5 It is a flame-retardant performance diagram of the flame-retardant composite aerogel provided by Embodiments 1-4 (wherein, Figure a is a schematic diagram of vertical combustion, Figure b is a vertical combustion diagram of the material, Figure c is a heat release curve diagram of different aerogel samples, and Figure d is a bar chart of the limiting oxygen index and vertical combustion of different materials);

[0030] Figure 6 It is an SEM image of different aerogel samples after combustion provided by Embodiments 1-4 of the present invention and an EDS energy spectrum diagram of the aerogel sample after combustion provided by Embodiment 2 (where the left figure is the SEM image and the right figure is the EDS energy spectrum diagram);

[0031] Figure 7 It is a bar chart of the heat insulation performance of different aerogel samples provided by Embodiment 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] In order to further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following combines the drawings and preferred embodiments to describe in detail the preparation method and specific implementation manner, structure, characteristics and effects of the biomass-based flame-retardant composite aerogel proposed according to the present invention.

[0033] The present invention proposes a preparation method for a biomass-based flame-retardant composite aerogel, which includes the following steps:

[0034] 1) Purification treatment of prehydrolyzed lignin: First, dissolve 10 - 20 g of prehydrolyzed lignin in 200 - 400 mL of 1,4 - dioxane solution (80 - 90%), stir at 200 - 250 rpm for 4 h, and then concentrate the volume of the lignin mixed solution to 1 / 3 - 1 / 4 of the original volume through a rotary evaporator. Drop the obtained lignin concentrate into 1 - 2 L of hydrochloric acid aqueous solution with pH = 2, let it stand in a refrigerator at 4 - 5 °C for 24 - 48 h, filter off the supernatant under vacuum, and continuously rinse with deionized water. Freeze - dry the obtained precipitate to obtain purified prehydrolyzed lignin (L).

[0035] 2) Phosphorylation modification of prehydrolyzed lignin: Mix 4 - 6 g of pretreated prehydrolyzed lignin, 4 - 6 g of P2O5, and 1 - 2 g of urea in a 250 mL zirconia jar, and grind with zirconia balls with a diameter of 5 - 10 mm at a speed of 200 - 250 rpm for 6 - 8 h. Then, evenly disperse the obtained mixture in 1 - 2 L of deionized water. Then, purify the mixed solution using a dialysis tube with a molecular weight cut - off of 900 - 1000 Da. Finally, obtain phosphorylated lignin (PL) by freeze - drying;

[0036] 3) Preparation of PDA@LCF: First, mechanically disperse softwood pulp board in water to obtain fiber pulp, and then filter the wet fibers (lignocellulose fibers) obtained from the fiber pulp and store them in a refrigerator at 4 °C. Further, the method of mechanically dispersing softwood pulp board in water to obtain fiber pulp is as follows: Cut the softwood pulp board into pieces, soak it in water for 12 - 24 h, and then obtain fiber pulp by mechanical crushing. Secondly, obtain more dispersed fibers by a simple mechanical ball - milling method, specifically: Load 4.0 - 6.0 g of lignocellulose (wet fibers) into a 250 ml zirconia jar, and grind with zirconia balls with a diameter of 5 - 10 mm at 350 - 400 rpm for 1 - 2 h to obtain pretreated lignocellulose fibers (LCF). Immerse the pretreated lignocellulose fibers (LCF) in the PDA precursor solution for 48 h to obtain lignocellulose fibers with a polydopamine coating on the surface; among them, the preparation method of the PDA precursor solution is as follows: First, dissolve 0.4 - 0.6 g of tris(hydroxymethyl)aminomethane (Tris) in 100 - 150 mL of water, gradually add 0.1 - 0.15 mol / L HCl solution until the pH reaches 8.5 - 9.0; then add 0.4 - 0.6 g of dopamine hydrochloride and mix evenly to obtain the PDA precursor solution.

[0037] (4) Preparation of biomass-based flame-retardant aerogel: 0.3 - 4.0 g of PL and 1.0 - 2.0 g of PDA@LCF were uniformly dispersed in 50 - 100 mL of sodium hydroxide aqueous solution with pH = 10 - 12. Then, 2.0 - 4.0 g of sodium alginate (SA) was added, and the mixture was stirred at 150 - 200 rpm for 4 - 6 h. It was placed in a special mold and unidirectionally frozen with liquid nitrogen. Subsequently, it was freeze-dried at -40 °C and 0 - 0.1 Pa for 2 - 3 days to obtain a lignin composite aerogel. Among them, the specific operation method of unidirectional freezing with liquid nitrogen was as follows: First, a stainless-steel container filled with 200 ml of liquid nitrogen was prepared, and then the bottom of the material was placed at the upper liquid level of the liquid nitrogen and kept for 0.5 - 1 h to obtain a unidirectionally frozen and formed material.

[0038] In the following examples, P2O5, dopamine hydrochloride, Tris, etc. were all purchased from Anyoji, and all reagents were of analytical grade. The prehydrolyzed lignin was the lignin separated from the eucalyptus prehydrolysis liquor and was provided by Jining Mingsheng New Materials Co., Ltd.

[0039] Example 1

[0040] This example presents a preparation method of a biomass-based flame-retardant composite aerogel, including the following steps:

[0041] 1) Purification treatment of prehydrolyzed lignin: First, 20 g of prehydrolyzed lignin was dissolved in 400 mL of 1,4-dioxane solution (85%), and the mixture was stirred at 250 rpm for 4 h. Then, the volume of the lignin mixed solution was concentrated to 1 / 3 of the original volume by a rotary evaporator. The obtained lignin concentrate was dropped into 2 L of hydrochloric acid aqueous solution with pH = 2, and it was left to stand in a 4 °C refrigerator for 24 h. The supernatant was removed by vacuum filtration, and it was continuously rinsed with deionized water. The obtained precipitate was freeze-dried to obtain purified prehydrolyzed lignin (L).

[0042] 2) Phosphorylation modification of prehydrolyzed lignin: 6.0 g of purified prehydrolyzed lignin (L), 6.0 g of P2O5, and 2.0 g of urea were mixed in a 250 mL zirconia jar, and zirconia balls with a diameter of 10 mm were used to grind the mixture at a speed of 250 rpm for 8 h. Then, the obtained mixture was uniformly dispersed in 2 L of deionized water. Then, the mixed solution was purified using a dialysis tube with a cut-off molecular weight of 1000 Da, and continuous dialysis was carried out for 48 h. Finally, phosphorylated lignin (PL) was obtained by freeze-drying;

[0043] 3) Preparation of PDA@LCF: First, cut the softwood pulp board into pieces of paper (about 2 - 3 cm in size), soak it in water for 24 h, then obtain fiber pulp through mechanical crushing, and then filter the fiber pulp to obtain wet fibers (lignocellulose fibers), and store the wet fibers in a refrigerator at 4 °C. Load 6.0 g of lignocellulose into a 250 ml zirconia jar, and grind it with zirconia balls with a diameter of 10 mm at 400 rpm for 2 h. Finally, obtain pretreated lignocellulose (LCF). Immerse the pretreated lignocellulose fibers (LCF) in the PDA precursor solution for 48 h to obtain lignocellulose fibers with a polydopamine coating on the surface (PDA@LCF); among them, the preparation method of the PDA precursor solution is as follows: First, dissolve 0.4 g of tris(hydroxymethyl)aminomethane (Tris) in 100 mL of water, gradually add 0.1 mol / L HCl solution until the pH reaches 8.5; then add 0.4 g of dopamine hydrochloride and mix evenly to obtain the PDA precursor solution.

[0044] 4) Preparation of biomass-based flame-retardant aerogel: 0.6 g of PL and 2.0 g of PDA@LCF are evenly dispersed in 100 mL of sodium hydroxide aqueous solution with a pH of 12. Then add 4.0 g of sodium alginate (SA), stir at 150 rpm for 3 h, put it into a 10 cm × 10 cm × 1 cm mold, and perform unidirectional freezing molding with liquid nitrogen. Subsequently, freeze-dry it at -40 °C and 0.1 Pa for 2 days to obtain a lignin composite aerogel, labeled as PDA@LCF / SA-PL10, where "10" represents that the lignin content is 10%. Among them, the specific operation method of unidirectional freezing with liquid nitrogen is as follows: First, prepare a stainless steel container filled with 200 ml of liquid nitrogen, then place the bottom of the material at the upper liquid level of the liquid nitrogen and keep it for 0.5 - 1 h to obtain a unidirectionally freeze-molded material.

[0045] Example 2

[0046] This example proposes a preparation method of lignin-based thermosetting resin, including the following steps:

[0047] 1) Purification treatment of prehydrolyzed lignin: First, dissolve 10 g of prehydrolyzed lignin in 400 mL of 1,4-dioxane solution (85%), stir at 250 rpm for 4 h, and then concentrate the volume of the lignin mixed solution to 1 / 3 of the original volume through a rotary evaporator. Drop the obtained lignin concentrate into 2 L of hydrochloric acid aqueous solution with a pH of 2, let it stand in a refrigerator at 4 °C for 24 h, vacuum filter to remove the supernatant, and continuously rinse with deionized water. Freeze-dry the obtained precipitate to obtain purified prehydrolyzed lignin (L).

[0048] 2) Phosphorylation modification of prehydrolyzed lignin: 6.0 g of purified prehydrolyzed lignin (L), 6.0 g of P2O5 and 2.0 g of urea were mixed in a 250 mL zirconia jar, and ground with zirconia balls with a diameter of 10 mm at a speed of 250 rpm for 8 h. Then, the obtained mixture was evenly dispersed in 2 L of deionized water. Then, the mixed solution was purified using a dialysis tube with a molecular weight cut-off of 1000 Da and dialyzed continuously for 48 h. Finally, phosphorylated lignin (PL) was obtained by freeze-drying;

[0049] 3) Preparation of PDA@LCF: First, the softwood pulp board was cut into pieces of paper (size about 2 - 3 cm), soaked in water for 24 h, and then fiber pulp was obtained by mechanical crushing. Then, the fiber pulp was filtered to obtain wet fibers (lignocellulose fibers), and the wet fibers were stored in a refrigerator at 4 °C. 6.0 g of lignocellulose was loaded into a 250 ml zirconia jar and ground with zirconia balls with a diameter of 10 mm at 400 rpm for 2 h. Finally, the pretreated lignocellulose (LCF) was obtained. The pretreated lignocellulose fibers (LCF) were soaked in the PDA precursor solution for 48 h to obtain lignocellulose fibers with a polydopamine coating on the surface; the preparation method of the PDA precursor solution is as follows: First, 0.4 g of tris(hydroxymethyl)aminomethane (Tris) was dissolved in 100 mL of water, and gradually added to a 0.1 mol / L HCl solution until the pH reached 8.5; then 0.4 g of dopamine hydrochloride was added and mixed evenly to obtain the PDA precursor solution.

[0050] 4) Preparation of biomass-based flame-retardant aerogel: 4.0 g of PL and 2.0 g of PDA@LCF were evenly dispersed in 100 mL of an aqueous sodium hydroxide solution with a pH of 12. Then 4.0 g of sodium alginate (SA) was added and stirred at 150 rpm for 3 h. It was placed in a mold of 10 cm × 10 cm × 1 cm and unidirectionally freeze-molded with liquid nitrogen. Subsequently, it was freeze-dried at -40 °C and 0.1 Pa for 2 days to obtain a lignin composite aerogel, labeled as PDA@LCF / SA-PL40, where "40" indicates that the lignin content is 40%. Among them, the specific operation method of unidirectional freezing with liquid nitrogen is as follows: First, a stainless steel container filled with 200 ml of liquid nitrogen was prepared, and then the bottom of the material was placed at the upper liquid level of the liquid nitrogen and kept for 0.5 - 1 h to obtain the unidirectionally freeze-molded material.

[0051] Example 3

[0052] This example presents a preparation method of a lignin-based thermosetting resin, including the following steps:

[0053] 1) Purification treatment of prehydrolyzed lignin: First, dissolve 10 g of prehydrolyzed lignin in 400 mL of 1,4-dioxane solution (85%), stir at 250 rpm for 4 h, and then concentrate the volume of the lignin mixed solution to 1 / 3 of the original volume through a rotary evaporator. Drop the obtained lignin concentrate into 2 L of hydrochloric acid aqueous solution with pH = 2, let it stand in a 4°C refrigerator for 24 h, filter off the supernatant under vacuum, and continuously rinse with deionized water. Freeze-dry the obtained precipitate to obtain purified prehydrolyzed lignin (L).

[0054] 2) Phosphorylation modification of prehydrolyzed lignin: Mix 6.0 g of purified prehydrolyzed lignin (L), 6.0 g of P2O5, and 2.0 g of urea in a 250 mL zirconia jar, and grind with zirconia balls with a diameter of 10 mm at a speed of 250 rpm for 8 h. Then, uniformly disperse the obtained mixture in 2 L of deionized water. Then, purify the mixed solution using a dialysis tube with a molecular weight cut-off of 1000 Da and continuously dialyze for 48 h. Finally, obtain phosphorylated lignin (PL) by freeze-drying;

[0055] 3) Preparation of PDA@LCF: First, cut coniferous pulp board into paper pieces (size about 2 - 3 cm), soak in water for 24 h, then obtain fiber pulp by mechanical crushing, and then filter the fiber pulp to obtain wet fibers (lignocellulose fibers), and store the wet fibers in a 4°C refrigerator. Load 6.0 g of lignocellulose into a 250 ml zirconia jar, and grind with zirconia balls with a diameter of 10 mm at 400 rpm for 2 h. Finally, obtain pretreated lignocellulose (LCF). Immerse the pretreated lignocellulose fibers (LCF) in the PDA precursor solution for 48 h to obtain lignocellulose fibers with a polydopamine coating on the surface; The preparation method of the PDA precursor solution is as follows: First, dissolve 0.4 g of tris(hydroxymethyl)aminomethane (Tris) in 100 mL of water, gradually add it to 0.1 mol / L HCl solution until the pH reaches 8.5; then add 0.4 g of dopamine hydrochloride and mix evenly to obtain the PDA precursor solution.

[0056] 4) Preparation of biomass-based flame-retardant aerogel: 1.5 g of PL and 2.0 g of PDA@LCF were uniformly dispersed in 100 mL of sodium hydroxide aqueous solution with pH = 12. Then 4.0 g of sodium alginate (SA) was added, and the mixture was stirred at 150 rpm for 3 h. It was placed in a mold of 10 cm × 10 cm × 1 cm and unidirectionally freeze-casted with liquid nitrogen. Subsequently, it was freeze-dried at -40 °C and 0.1 Pa for 2 days to obtain a lignin composite aerogel, labeled as PDA@LCF / SA-PL20, where "20" represents the lignin content of 20%. Among them, the specific operation method of unidirectional freezing with liquid nitrogen is as follows: First, prepare a stainless-steel container filled with 200 ml of liquid nitrogen, then place the bottom of the material at the upper liquid level of the liquid nitrogen and keep it for 0.5 - 1 h to obtain the unidirectionally freeze-casted material.

[0057] Example 4

[0058] This example proposes a preparation method of lignin-based thermosetting resin, including the following steps:

[0059] 1) Purification treatment of prehydrolyzed lignin: First, 10 g of prehydrolyzed lignin was dissolved in 400 mL of 1,4-dioxane solution (85%), and the mixture was stirred at 250 rpm for 4 h. Then, the volume of the lignin mixed solution was concentrated to 1 / 3 of the original volume by a rotary evaporator. The obtained lignin concentrate was dropped into 2 L of hydrochloric acid aqueous solution with pH = 2, and left standing in a 4 °C refrigerator for 24 h. The supernatant was removed by vacuum filtration and continuously rinsed with deionized water. The obtained precipitate was freeze-dried to obtain purified prehydrolyzed lignin (L).

[0060] 2) Phosphorylation modification of prehydrolyzed lignin: 6.0 g of prehydrolyzed lignin (L), 6.0 g of P2O5 and 2.0 g of urea were mixed in a 250 mL zirconia jar, and zirconia balls with a diameter of 10 mm were used to grind at a speed of 250 rpm for 8 h. Then, the obtained mixture was uniformly dispersed in 2 L of deionized water. Then, the mixed solution was purified using a dialysis tube with a molecular weight cut-off of 1000 Da and continuously dialyzed for 48 h. Finally, phosphorylated lignin (PL) was obtained by freeze-drying;

[0061] 3) Preparation of PDA@LCF: First, cut the softwood pulp board into pieces of paper (about 2 - 3 cm in size), soak it in water for 24 h, then obtain fiber pulp through mechanical crushing, and then filter the fiber pulp to get wet fibers (lignocellulose fibers), and store the wet fibers in a refrigerator at 4 °C. Put 6.0 g of lignocellulose into a 250 ml zirconia jar, and grind it with zirconia balls with a diameter of 10 mm at 400 rpm for 2 h. Finally, obtain pretreated lignocellulose (LCF). Immerse the pretreated lignocellulose fibers (LCF) in the PDA precursor solution for 48 h to obtain lignocellulose fibers with a poly-dopamine coating on the surface; the preparation method of the PDA precursor solution is as follows: First, dissolve 0.4 g of tris(hydroxymethyl)aminomethane (Tris) in 100 mL of water, gradually add it to a 0.1 mol / L HCl solution until the pH reaches 8.5; then add 0.4 g of dopamine hydrochloride and mix evenly to obtain the PDA precursor solution.

[0062] 4) Preparation of biomass-based flame-retardant aerogel: 2.6 g of PL and 2.0 g of PDA@LCF are evenly dispersed in 100 mL of sodium hydroxide aqueous solution with a pH of 12. Then add 4.0 g of sodium alginate (SA), stir at 150 rpm for 3 h, put it into a mold of 10 cm × 10 cm × 1 cm, and perform unidirectional freezing molding with liquid nitrogen. Subsequently, freeze-dry it at -40 °C and 0.1 Pa for 2 days to obtain a lignin composite aerogel, labeled as PDA@LCF / SA-PL30, where "30" represents the lignin content of 30%. Among them, the specific operation method of unidirectional freezing with liquid nitrogen is as follows: First, prepare a stainless steel container filled with 200 ml of liquid nitrogen, then place the bottom of the material at the upper liquid level of the liquid nitrogen and keep it for 0.5 - 1 h to obtain a material with unidirectional freezing molding.

[0063] Comparative Example 1

[0064] The present invention provides a method for preparing a blank aerogel sample, including the following steps:

[0065] Preparation of blank aerogel sample: Disperse 4.0 g of sodium alginate evenly in 100 mL of water, then stir at 150 rpm for 3 h, put it into a mold of 10 cm × 10 cm × 1 cm, and perform unidirectional freezing molding with liquid nitrogen. Subsequently, freeze-dry it at -40 °C and 0.1 Pa for 2 days to obtain a lignin composite aerogel, labeled as SA. Among them, the specific operation method of unidirectional freezing with liquid nitrogen is as follows: First, prepare a stainless steel container filled with 200 ml of liquid nitrogen, then place the bottom of the material at the upper liquid level of the liquid nitrogen and keep it for 0.5 - 1 h to obtain a material with unidirectional freezing molding.

[0066] Comparative Example 2

[0067] The present invention provides a method for preparing a PDA@LCF aerogel sample without adding lignin but only adding PDA@LCF, comprising the following steps:

[0068] Preparation of the aerogel sample: 4.0 g of sodium alginate and 2.0 g of PDA@LCF prepared in Example 2 were uniformly dispersed in 100 mL of aqueous solution, and then stirred at 150 rpm for 3 h. It was placed in a mold of 10 cm×10 cm×1 cm and freeze-molded unidirectionally with liquid nitrogen. Subsequently, it was freeze-dried at -40°C and 0.1 Pa for 2 days to obtain a lignin composite aerogel, denoted as PDA@LCF / SA. Among them, the specific operation method of unidirectional freezing with liquid nitrogen was as follows: First, a stainless steel container filled with 200 ml of liquid nitrogen was prepared, and then the bottom of the material was placed at the upper liquid level of the liquid nitrogen and kept for 0.5 - 1 h to obtain a unidirectionally freeze-molded material.

[0069] Table 1 Main flame retardant performance parameters of the aerogel samples prepared in Examples 1 - 2 and Comparative Examples 1 - 2

[0070]

[0071] The present invention first phosphorylates the hydroxyl groups of lignin by a solvent-free mechanochemical method ( Figure 1 ), enhancing the gas-phase free radical capture efficiency of lignin at the combustion interface, thereby improving the flame retardant performance of the material. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) were used to study the lignin samples before and after modification (L before modification and PL after modification), and it was confirmed that the lignin modification was successful ( Figure 2 a and b). It should be noted that the FT-IR spectra of L and PL showed characteristic peaks of G / S-type lignin, and new peaks of P=O and P-O-H / P-O-C were observed at 1013 and 961 cm -1 respectively ( Figure 2 a). In addition, the survey spectra of XPS showed that PL presented a P2p peak signal, which also indicated that P species were successfully introduced into the lignin ( Figure 2 b). Interestingly, all the PL peaks had slight shifts, and the peak of C-O was enhanced. This was because P elements were introduced in a covalent (C-O-P) manner.

[0072] Generally speaking, incorporating external flame retardants of phosphorus (P) and nitrogen (N) into materials is an effective way to achieve high flame retardancy. This is due to the inherent synergistic effect of the P-N flame retardant system, which effectively enhances the flame retardant performance of the material. In addition, phosphorus and nitrogen are considered to be the two most environmentally friendly flame retardant elements at present. Therefore, we used PL containing phosphate groups and nitrogen-rich PDA@LCF as external flame retardants and incorporated them into the sodium alginate matrix to prepare an aerogel thermal insulation material with excellent flame retardant performance. The schematic diagram of the preparation process and the obtained composite aerogel samples are respectively as Figure 3As shown. As expected, the prepared lignin composite aerogel has a vertically oriented microstructure. This anisotropic cell wall structure enhances the thermal insulation ability of the material when utilized laterally ( Figure 3 ). In addition, the thermal degradation behavior of the aerogel was evaluated by thermogravimetric analysis (TGA) ( Figure 4 ). Specifically, the sample without PL showed poor thermal stability, with the maximum degradation rate occurring at approximately 305 °C and a carbon residue of approximately 45%. However, with the addition of PL, the thermal stability of the composite aerogel was significantly improved. It is worth emphasizing that as the PL content increases, the thermal stability of the composite material also gradually increases. When reaching the decomposition temperature of 800 °C, the carbon residue rate of the 40% lignin composite material was observed to be as high as 56%. More importantly, as the PL content increases, the distribution of its DTG peaks shifts ( Figure 4 b). With the addition of lignin, the initial decomposition temperature (T i ) increases to 256 °C, and the peak maximum decomposition temperature (T max ) also changes accordingly. Among them, the T max of the 40% lignin composite material is 326 °C, which is 77 °C higher than that of pure SA. Therefore, it can be concluded that single PDA@LCF cannot improve the thermal stability of the material. After adding a high concentration of phosphorylated lignin (PL) to the system, the synergistic effect between the phosphate groups of PL and nitrogen-rich PDA@LCF significantly enhances the char-forming ability of the material, thereby improving the thermal stability of the material.

[0073] It is worth noting that one of the most important properties of thermal insulation foams is their fire resistance. Among them, the flame retardancy of the composite aerogel was tested by the limiting oxygen index (LOI) and vertical burning test, and the results are as Figure 5 shown. To evaluate the flame retardant properties of the lignin composite aerogel, a 20 s vertical burning test was carried out ( Figure 5 a, 5b). It is worth noting that the PDA@LCF sample has poor flame retardancy due to its single nitrogen source and lack of necessary flame retardant components. To improve the flame retardancy of the material, PL was used as the primary flame retardant, and its content was controlled at 10 - 40 wt%. In addition, vertical burning tests were carried out for different lignin contents ( Figure 5b). The results show that with the increase in the PL content, the flame retardancy exhibits varying degrees of change. When the PL content is 10 wt% and 20 wt%, the flame retardancy is poor. However, when the lignin content increases to 30 wt%, the flame retardancy of the composite aerogel is improved, and the flame retardant effect is the best when the PL content is 40 wt%. LOI is a key parameter for evaluating flame retardancy, representing the minimum oxygen concentration (vol / vol%) required to maintain the combustion of the material for 3 minutes or consume a 5-cm-long sample. Using this parameter, the flame retardancy of the PDA@LCF-PLX composite aerogel was evaluated. In this case, the LOI values of the SA and PDA@LCF composites are relatively low, 17.5% and 17.7% respectively ( Figure 5 d). However, after adding phosphorylated lignin (PL) to the sodium alginate matrix, the LOI increases significantly, and its value increases proportionally with the PL content. When the PL content is 40 wt%, the maximum LOI value of this composite material is 35.6%. These results are consistent with the vertical burning test, further confirming that PL is an effective flame retardant component in the material. In addition, we also compared with commercial thermal insulation materials (PU and PS), and the results show that the flame retardancy of the samples prepared with a higher lignin content is significantly better than that of commercial thermal insulation materials.

[0074] The cone calorimeter was used to simulate real fire conditions to evaluate the heat release behavior of the composite aerogel at the combustion interface during combustion Figure 5 Figure c shows the heat release rate (HRR) curves of SA, PDA@LCF, and PDA@LCF-PL40 at a heat flux density of 35 kW / m2. The comprehensive data are shown in Table 1. Among them, the HRR curve of pure SA shows a rapid rise to the peak, then a slight decrease, and finally drops to the minimum and extinguishes within about 100 s. The first peak (P1-HRR) and the second peak (P2-HRR) are 30.3 kW / m 2 and 26.6 kW / m 2 . These results indicate that pure SA has the lowest flame retardancy and exhibits a typical carbonization process during combustion. In contrast, although the HRR curve of PDA@LCF also shows an obvious rise, the P1-HRR only decreases by 16%. This shows that a single nitrogen source additive is not sufficient to significantly improve the flame retardancy. It should be noted that compared with pure SA, the parameters of PDA@LCF / PL10 decrease slightly by 35.2% and 22.3% respectively. When the lignin content increases to 40%, these parameters decrease significantly, and the P1-HRR and P2-HRR decrease by 56.9% and 30.9% respectively. This phenomenon can be attributed to the demonstration of typical HRR characteristics related to the combustion of the material during the test. The formation of the surface protective carbon layer causes the HRR to gradually decrease after P1-HRR until the volatile substances escape through the gaps in the carbon layer, resulting in the appearance of P2-HRR.

[0075] It is important to study the synergistic flame retardancy mechanism of PDA@LCF and PL. To achieve this goal, we characterized the microscopic morphology of the residual carbon in the aerogel using scanning electron microscopy (SEM), and then analyzed the flame retardant effect of phosphorus-nitrogen (P-N) compounds in the condensed phase. Figure 6 SEM images of the materials at different resolutions are shown. It was observed that the density of the residual carbon layer in PDA@LCF / SA was slightly higher than that of pure SA, but there were still a large number of micropores. This indicates that its carbonization ability was not significantly enhanced, allowing combustible molecules after pyrolysis to escape into the gas phase. This escape contributed to the energy release during the combustion process and promoted the formation of smoke, ultimately limiting the improvement of the flame retardant performance. When PL was added to the sodium alginate matrix, the carbonization ability increased with the increase in the PL content. Especially when the PL content increased to 40%, the carbon layer became denser and the pores were almost invisible, effectively acting as a barrier to block the release of heat and smoke.

[0076] Building insulation materials not only require excellent flame retardant performance but also effective insulation. To demonstrate the potential application prospects of the composite aerogel as an insulation material, we compared the actual insulation performance of different samples with that of a polyurethane (PU) insulation material. The change in the surface temperature of the samples placed on a 300 °C hot plate within 30 minutes was monitored using a thermal infrared imaging device ( Figure 7 ). The size of all samples was 5 cm × 5 cm × 2 cm. Among them, pure SA was quickly heated to about 100 °C within 5 minutes and then gradually increased, finally reaching about 110 °C after 30 minutes. At the same time, at 30 minutes, the surface temperature of the commercial PU was about 3 °C lower than that of pure SA. The addition of PL improved its insulation performance to a certain extent. In addition, with the increase in the PL content, the insulation performance was improved, and the optimal PL content was 40 wt%. In contrast, the temperature remained below 90 °C after heating for 15 minutes and reached 95 °C after 30 minutes. Thus, a higher PL content can effectively improve the insulation performance of the aerogel.

[0077] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of a biomass-based flame-retardant aerogel, characterized in that, It includes the following steps: (1) Phosphorylation modification of lignin: Mix the pretreated prehydrolyzed lignin, phosphorus pentoxide, and urea in a zirconia jar, and grind them with zirconia balls at a speed of 200 - 250 rpm for 6 - 8 h; then, disperse the obtained mixture in deionized water and perform dialysis using a dialysis tube to obtain a purified mixed solution; afterwards, freeze-dry the purified mixed solution to obtain phosphorylated lignin; (2) Preparation of lignocellulose with a dopamine coating: Immerse the pretreated lignocellulose fibers in a dopamine precursor solution for 24 - 48 h to obtain lignocellulose fibers with a polydopamine coating loaded on the surface; (3) Preparation of biomass-based flame-retardant aerogel: Disperse phosphorylated lignin and lignocellulose with a dopamine coating in an alkaline aqueous solution with pH = 10 - 12, then add sodium alginate, stir at 150 - 200 rpm for 3 - 6 h, place it in a mold, and unidirectionally freeze it with liquid nitrogen; subsequently, freeze-dry it at -40 °C and 0.1 Pa for 2 - 3 days to obtain a lignin composite aerogel.

2. The preparation method according to claim 1, wherein, In step (1), the preparation method of the pretreated lignin is as follows: First, dissolve the prehydrolyzed lignin in a 1,4-dioxane solution, stir at 200 - 250 rpm for 4 - 5 h, then concentrate the volume of the lignin mixed solution to 1 / 3 - 1 / 4 of the original volume through a rotary evaporator to obtain a lignin concentrate; drop the obtained lignin concentrate into acidic water with pH = 2 - 2.5, let it stand in a refrigerator at 4 - 5 °C for 24 - 48 h, vacuum filter to remove the supernatant, wash it with deionized water, and freeze-dry the obtained precipitate to obtain purified prehydrolyzed lignin.

3. The preparation method according to claim 1, characterized in that, In step (1), the ratio of the pretreated prehydrolyzed lignin, phosphorus pentoxide, and urea is 4 - 6:4 - 6:1 - 2; the diameter of the zirconia balls is 5 - 10 mm; the cut-off molecular weight of the dialysis tube is 900 - 1000 Da.

4. The preparation method according to claim 1, characterized in that, In step (2), the method for the pretreated lignocellulose fibers is as follows: First, mechanically disperse softwood pulp boards in water to obtain fiber pulp, and then filter the fiber pulp to obtain wet fibers; secondly, treat the wet fibers by mechanical ball milling to obtain the pretreated lignocellulose fibers.

5. The preparation method according to claim 4, characterized in that The 1,4-dioxane solution is a 1,4-dioxane aqueous solution with a volume fraction of 80 - 90%, and the ratio of prehydrolyzed lignin to the 1,4-dioxane solution is 10 - 20 g:200 - 400 mL, and the ratio of prehydrolyzed lignin to acidic water with pH = 2.0 - 2.5 is 10 - 20 g:1 - 2 L.

6. The preparation method according to claim 4, characterized in that, The method for mechanically dispersing softwood pulp boards in water to obtain fiber pulp is specifically as follows: Cut the softwood pulp boards into pieces, soak them in water for 12 - 24 h, and then obtain fiber pulp through mechanical crushing; the method for treating wet fibers by mechanical ball milling is specifically as follows: Load the wet fibers into a zirconia jar, grind them with zirconia balls at 350 - 400 rpm for 1 - 2 h to obtain the pretreated lignocellulose fibers; among them, the diameter of the zirconia balls is 5 - 10 mm.

7. The preparation method according to claim 1, wherein The method for preparing the dopamine precursor solution is as follows: First, dissolve tris(hydroxymethyl)aminomethane in water, add an HCl solution until the pH reaches 8.5 - 9.0; then add dopamine hydrochloride and mix to obtain the PDA precursor solution; wherein, the ratio of tris(hydroxymethyl)aminomethane, water and dopamine hydrochloride is 0.4 - 0.6 g: 100 - 150 mL: 0.4 - 0.6 g.

8. The preparation method according to claim 7, characterized in that, The HCl solution is an HCl solution with a concentration of 0.1 - 0.15 mol / L.

9. The preparation method according to claim 1, wherein In step (3), the mass ratio of phosphorylated lignin, lignocellulose fibers with a dopamine coating, and sodium alginate is 0.3 - 4.0: 1.0 - 2.0: 2.0 - 4.0; the ratio of phosphorylated lignin and an alkaline aqueous solution with pH = 10 - 12 is 0.5 - 4.0 g: 50 - 100 mL; the specific operation method of unidirectional freezing with liquid nitrogen is: place the bottom of the material at the upper liquid level of liquid nitrogen and keep it for 0.5 - 1 h to obtain a unidirectionally freeze - formed material.

10. The biomass - based flame - retardant aerogel prepared by the preparation method according to any one of claims 1 - 9.

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