High-toughness composite aerogel with efficient heat-insulating and flame-retardant properties and preparation method of high-toughness composite aerogel

Abstract

The invention provides high-toughness composite aerogel with efficient heat insulation and flame retardance and a preparation method of the high-toughness composite aerogel. The preparation method comprises the following steps: (1) preparing A-NFs; (2) adding the A-NFs into a composite aqueous solution of polyvinyl alcohol and gelatin, and dispersing; (3) adding mixed acid into the mixture obtained in the step (2), and adjusting the pH value; (4) adding a silicon source and ethanol, and stirring to form silica sol; (5) adjusting the pH value of the silica sol by using an ammonia water solution, adding ethanol, adjusting the temperature to room temperature, and continuously aging for 24-48 hours under a mechanical stirring condition; and (6) adding the aged product into a mold, and freeze-drying. Gelatin is introduced and combined with PVA, so that the toughness and compressive strength of the SiO2 aerogel are improved; firstly, the A-NFs can provide an isotropic skeleton supporting effect for the composite aerogel; boric acid can promote dehydration and carbonization of a polymer so as to prevent further combustion of PVA, and the boric acid and A-NFs synergistically enhance the overall flame retardance of the composite aerogel.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogel composite materials, and in particular to a high-strength and tough composite aerogel with efficient heat insulation and flame retardant properties and a preparation method thereof. Background Art

[0002] Aerogel materials are currently widely used in various fields, including energy storage, adsorption, catalysis, thermal insulation, oil treatment, and aerospace, due to their ultra-low density, ultra-high specific surface area, high porosity, and low nominal thermal conductivity. However, commonly used inorganic SiO2 aerogels are fragile and have poor mechanical properties. In practical applications such as thermal insulation and protective coatings, coating collapse and cracking are common. Furthermore, the internal aerogels experience pore compression and filling of open pores during the coating's drying and shrinkage, seriously affecting the coating's thermal insulation and strength. Furthermore, SiO2 aerogels experience increased thermal conductivity at high temperatures and under combustion conditions, significantly reducing their thermal insulation effectiveness.

[0003] In order to solve the defects of inorganic SiO2 aerogel itself, the existing technology has proposed the forms of inorganic / inorganic composite aerogel and inorganic / organic composite aerogel, which achieve higher mechanical strength than pure inorganic SiO2 aerogel.

[0004] Patent CN 117247268 A discloses a strong, highly insulating aerogel, its preparation method, and its application. The aerogel possesses improved mechanical strength and stiffness due to the lateral support provided by boron nitride nanotubes (BNNTs@mSiO2). Simultaneously, one-dimensional mesoporous silica offers enhanced mechanical strength. The combination of these two improves the fragility and low strength of aerogels alone. The one-dimensional nanofibers also maintain the integrity of the boundary interface structure, enhancing the toughness of the aerogel material. However, the nanofibers have limited compatibility with BNNTs@mSiO2, resulting in uneven dispersion of the reinforcement after gel formation, which in turn affects the mechanical properties and thermal conductivity of the final aerogel sample.

[0005] Patent CN 116425554 A discloses a boron nitride and silicon dioxide composite ceramic aerogel and its preparation method. In this product, silicon dioxide forms on the boron nitride aerogel fiber walls solely through electrostatic interactions, resulting in self-assembly and covalent bonding. This results in low strength. During actual use or secondary processing, the silicon dioxide nanoparticles within the aerogel easily peel off from the boron nitride fiber surface, reducing the aerogel's porosity and affecting its thermal insulation and adsorption properties. Furthermore, this aerogel preparation method does not address the fragility and poor mechanical properties common to conventional inorganic aerogels.

[0006] Patent CN 102557577 A discloses a method for preparing a silica aerogel composite material. The application uses industrially produced glass fiber or fiber wool materials to reinforce silica aerogel prepared with tetraethyl orthosilicate as the silicon source. The resulting composite material has high porosity and specific surface area, but low density, thermal conductivity, and dielectric constant. However, it should not be ignored that the glass fiber or fiber wool material has poor compatibility with the silica sol and the two are only simply physically bonded. This results in poor mechanical properties of the material, significantly reducing the thermal insulation effect in practical applications.

[0007] Kim HM et al. used mechanical blending to add hydrophobic SiO2 aerogel powder to a certain concentration of polyvinyl alcohol (PVA) aqueous solution. By heating and mechanical stirring, the solvent was evaporated at a slow rate, so that PVA was precipitated on the interface of the hydrophobic SiO2 aerogel powder, forcibly preparing SiO2 aerogel / PVA composites. While retaining the SiO2 aerogel pores, PVA was used to toughen the traditional hydrophobic SiO2 aerogel product (Kim HM, Noh YJ, Yu J, et al. Silica aerogel / polyvinyl alcohol (PVA) insulation composites with preserved aerogel pores using interfaces between the superhydrophobic aerogel and hydrophilic PVA solution[J]. Composites Part A: Applied Science and Manufacturing, 2015, 75: 39-45.). The SiO2 aerogel / PVA composite material prepared by this method has only a simple physical bond between the two substances, and the PVA material and SiO2 aerogel themselves have low compressive strength. Therefore, the compressive strength of the final product itself is still not significantly improved. Secondly, the SiO2 aerogel and PVA do not form a stable chemical bond, resulting in the partial peeling of the SiO2 aerogel from the PVA during actual application, thus affecting the final thermal insulation effect. In addition, PVA is flammable, which limits the application of the corresponding SiO2 / PVA composite aerogel and its thermal insulation coating products under high temperature or combustion conditions.

[0008] Lee H et al. prepared a flexible thermal insulation composite material by using a flexible polydimethylsiloxane (PDMS) matrix and preventing the matrix from impregnating the silica aerogel pores. First, the silica aerogel was mixed with ethanol to prevent the PDMS molecules from impregnating the silica aerogel pores. After the composite material was manufactured, the ethanol was removed. This method overcomes the brittle nature of silica aerogel, maintains its low thermal conductivity and unique porous structure, and improves the LOI value and UL-94 rating of the composite material. (Lee H, Lee D, Cho J, et al. Super-insulating, flame-retardant, and flexible poly(dimethylsiloxane) composites based on silica aerogel[J]. Composites Part A: Applied Science and Manufacturing, 2019, 123: 108-113.). However, it cannot be ignored that the final SiO2 / PDMS composite aerogel product still has a low compressive strength, which makes it easy to collapse in practical applications. Summary of the Invention

[0009] In order to solve the problems of existing SiO2 aerogels, such as low density, poor mechanical properties, high high-temperature thermal conductivity, poor high-temperature thermal insulation performance, easy filling of open pores, easy collapse and cracking of coatings, and high thermal conductivity, the present application provides a high-strength and tough composite aerogel with efficient thermal insulation and flame retardant properties and a preparation method thereof, which not only solves the problems existing in existing silicon-based aerogels, but also gives the aerogels efficient thermal insulation and flame retardant and smoke suppression functions.

[0010] A method for preparing a high-strength and tough composite aerogel with high-efficiency heat insulation and flame retardant properties, comprising the following steps: (1) acidification treatment of inorganic nanofibers; (2) adding the acidified inorganic nanofibers to a composite aqueous solution of polyvinyl alcohol and gelatin, and ultrasonically dispersing the inorganic nanofibers to obtain a uniform mixed solution; (3) adding mixed acid to the mixed solution, adjusting the pH to 2-3, and heating to 50-60°C; (4) maintaining the temperature at 50-60°C, continuously adding a mixed solution of silicon source and ethanol, and stirring to form a silica sol; (5) adjusting the silica sol to neutrality with an ammonia solution, adding ethanol, and adjusting the temperature to room temperature, and then continuing to age for 24-48 hours under mechanical stirring conditions; and (6) adding the aged product into a mold and freeze-drying it.

[0011] Preferably, the inorganic nanofibers are a mixture of one or more of carbon nanotubes (CNTs), carbon nanofibers (CNFs), zirconium oxide nanofibers (ZrO2), and silicon carbide nanofibers (SiC). The diameter and length of the inorganic nanofibers can also be selected based on the application. For example, the inorganic nanofibers have a diameter of 5 to 20 μm, preferably 10 to 15 μm, and a length of 1 to 15 mm, preferably 5 to 10 mm.

[0012] The operation of the above step (1) is to mix the inorganic nanofibers (NFs) and the acid solution, carry out an acidification reaction and then dry to obtain the acidified inorganic nanofibers (A-NFs); wherein the above acid solution is a mixed solution of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1; the mass ratio of the above inorganic nanofibers to the acid solution is (1-5):100; the temperature of the above acidification reaction is 60-80°C and the time is 1-2h.

[0013] The role of inorganic nanofibers is to form an interwoven three-dimensional skeleton and enhance mechanical properties.

[0014] Specifically, the preparation method of A-NFs includes the following steps: a) ultrasonically dispersing a certain amount of NFs in a mixed acid solution of H2SO4 and HNO3 for 0.5-1 hour, followed by a further reaction under magnetic stirring at 60-80°C for 1-2 hours. b) filtering the resulting inorganic nanofiber slurry and repeatedly rinsing with deionized water until the solution is neutral. c) drying the product to produce A-NFs.

[0015] In the above step (2), the usage ratio of polyvinyl alcohol (PVA), gelatin (GEL) and water is (2-5): (1-2): (93-97); the temperature condition for forming the above composite aqueous solution is 80-95°C.

[0016] Preferably, the PVA can be selected from one or more commercially available PVAs: 1750, 1788, 1796, 1799, 2488 and 2499. The gelatin can be selected from commercially available animal gelatin.

[0017] More preferably, the PVA is selected from commercially available 1799.

[0018] The role of polyvinyl alcohol is to act as an adhesive to cross-link the silicon source and the inorganic nanofibers, providing a supporting skeleton for the composite material.

[0019] The preparation method of the composite aqueous solution of PVA and GEL comprises the following steps: placing PVA and GEL in water, and mechanically stirring the water at 80-95° C. until the two substances are fully dissolved.

[0020] In the above step (2), the mass ratio of the acidified inorganic nanofibers to the composite aqueous solution is (0.5-2):100, and the ultrasonic dispersion time is 1-2 hours.

[0021] The mixed acid in step (3) is a mixed solution of 0.05-0.1 mol / L boric acid (BA) and 0.1 mol / L hydrochloric acid in a volume ratio of 1:1. Hydrochloric acid can promote the hydrolysis reaction of the silicon source.

[0022] In the mixed solution of the silicon source and ethanol in step (4), the mass ratio of the two is 1:(1-2.5); the silicon source is selected from tetraethyl orthosilicate (TEOS), methyl orthosilicate (TMOS), methyltriethoxysilane (MTES), and methyltrimethoxysilane (MTMS). The ethanol content has a significant impact on the network skeleton structure of the SiO2 aerogel. The increase in ethanol content reduces the concentration of the silicon source and the acid catalyst, resulting in a decrease in the silicon content of the hydrolysis product, a slowdown in the reaction rate, and a decrease in the number of Si-OH monomers contained per unit volume, which reduces the probability of collision between monomers and hinders the development of the Si-O-Si network structure chain. In addition, excessive ethanol will also cause esterification reaction, reducing the formation of Si-OH monomers in the sol. Moreover, the presence of ethanol as a hydrolysis product of the silicon source inhibits hydrolysis and polycondensation reactions, hindering the formation of Si-O-Si bonds. Therefore, if the ethanol content is too high, the silicon source is not fully hydrolyzed, the network skeleton strength of the aerogel is low, and a macroporous structure is easily formed. Therefore, the present application limits the usage ratio of silicon source to ethanol to ensure that the formed silica sol achieves better performance.

[0023] The total mass of the silicon source and ethanol is 0.6-1.4 times the total mass of the mixed solution in step (3); and the stirring time is 1-2 hours.

[0024] Preferably, the silicon source is selected from tetraethyl orthosilicate (TEOS).

[0025] In step (5), the volume of ethanol used is 30-40% of the entire system, and the mechanical stirring rate is 300-400 rpm.

[0026] The mold containing the wet gel product in step (6) is first immersed in liquid nitrogen for cooling, and then freeze-dried at a temperature of -50°C, and then continuously dried for 36-48 hours to obtain the final composite aerogel product. There is no particular restriction on the material and shape of the mold. The specific operation is: first, quickly immerse and freeze in liquid nitrogen for a few seconds to quickly freeze the wet gel, and then place it in a freeze dryer for freeze drying, and the freeze dryer temperature can be around -50°C.

[0027] The present application has a high-strength and tough composite aerogel with efficient heat insulation and flame retardant properties. In the absence of external additional conditions, there is an intermolecular hydrogen bond interaction between polyvinyl alcohol and gelatin. The acidified inorganic nanofibers A-NFs themselves can serve as the anisotropic skeleton support and catalytic carbonization of the composite aerogel. Secondly, the carboxyl and hydroxyl groups on the surface can undergo a condensation reaction with the free silanol (Si-OH) produced by the hydrolysis of the silicon source to form a stable chemical bond, further improving the toughness and rigidity. At the same time, the silicon source itself forms a SiO2 aerogel with high strength and a fine nanoporous network structure through a hydrolysis-condensation reaction under acid and alkaline two-step catalytic conditions, and PVA, gelatin and inorganic nanofibers are interspersed therein. Boric acid molecules have the ability to effectively cross-link -OH groups. When B(OH) 4- When cross-linked with PVA, borate ions penetrate into the network structure, forming a more uniform network cell structure. Many inorganic nanofiber particles attach to the cell walls, acting as cross-linking points, enhancing the intermolecular interactions in the composite aerogel, which in turn helps to improve the mechanical strength of the aerogel.

[0028] Compared with the prior art, this application has the following advantages:

[0029] (1) This application adopts the form of PVA / SiO2 organic-inorganic aerogel composite, which not only retains the advantages of low density and high porosity of SiO2 aerogel, but also solves the problem of SiO2 aerogel being fragile and having poor mechanical properties, improves the toughness of the material, and reduces the thermal conductivity in high temperature environments. It also introduces high-strength gelatin (GEL) rich in hydroxyl and amino groups on the chain and combines it with the polyhydroxy polymer PVA. The combination of gelatin and PVA improves the toughness and compressive strength of SiO2 aerogel;

[0030] (2) The surface of the A-NFs of the present application contains abundant carboxyl groups, which gives it a strong interface bonding ability. It can simultaneously interact with PVA and GEL through intermolecular hydrogen bonds and form stable chemical bonds with silica sol. At the same time, the filamentous structure of the inorganic fiber itself provides more anisotropic skeleton support for the interior of the aerogel, which further improves the toughness and rigidity of the PVA / SiO2 composite aerogel. A-NFs can first provide anisotropic skeleton support for the composite aerogel. Secondly, the abundant carboxyl groups and a small amount of hydroxyl groups on the surface can react with the silicon hydroxyl groups (Si-OH) generated by the hydrolysis reaction of the silicon source under dehydration conditions or acidic catalyst (HCl) conditions to produce Si-O-CO-NT and Si-O-NT, so that the two are cross-linked together, further improving the toughness and rigidity of the composite aerogel. In addition, NFs also have a certain catalytic carbonization effect, and its synergy with boric acid can significantly enhance the flame retardant properties of PVA.

[0031] (3) The boric acid (BA) of the present application can form a stable network structure with the PVA and GEL system through chemical cross-linking. At the same time, due to the presence of BA inside the composite aerogel during external flame combustion, it can promote polymer dehydration and carbonization, providing a glass-like coating for the aerogel, thereby preventing further combustion of the PVA. On the other hand, NFs not only have physical toughening and barrier effects, but also have a certain catalytic carbonization effect. Its synergy with boric acid can significantly enhance the flame retardancy of PVA, thereby further improving the overall flame retardancy of the composite aerogel.

[0032] (4) The preparation process of this application is simple, key raw materials are easily available, and the requirements for instruments and equipment are relatively low. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic diagram of the high-strength and tough composite aerogel structure with efficient heat insulation and flame retardant properties in this application;

[0034] Figure 2 This is a macro comparison test diagram of the combustion process of Example 3 and Comparative Example 4 in this application. DETAILED DESCRIPTION

[0035] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] Example 1

[0037] (1) The preparation method of A-CNTs is as follows: CNTs and mixed acid solution are mixed in a mass ratio of 1:50 and ultrasonically dispersed for 1 hour, and then a magnetic stirrer is used at 60°C and 400 rpm to react for 2 hours. Finally, the mixed solution is filtered, washed, and dried to obtain A-CNTs.

[0038] (2) 3.0 g of polyvinyl alcohol (PVA) and 2.0 g of gelatin (GEL) were weighed and placed in a beaker containing 95.0 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 3.0 wt.% PVA and 2.0 wt.% GEL. 0.5 g of acidified carbon nanotubes (A-CNTs) were weighed and placed in the composite aqueous solution and ultrasonically dispersed for 1 h to obtain a dispersed PVA / GEL / A-CNTs mixed solution.

[0039] (3) The PVA / GEL / A-CNTs mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.05 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 60°C.

[0040] (4) Weigh 40.0 g of tetraethyl orthosilicate (TEOS) solution and 60.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 2 h.

[0041] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 30% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 36 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0042] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 48 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-CNTs / BA / SiO2 composite aerogel product S1 with high efficiency heat insulation and flame retardant properties.

[0043] Example 2

[0044] (1) The preparation method of A-CNFs is as follows: CNFs and mixed acid solution are mixed at a mass ratio of 1.5:100 and ultrasonically dispersed for 1 h. The mixture is stirred at 60°C and 400 rpm for 2 h. Finally, the mixed solution is filtered, washed, and dried to obtain A-CNFs.

[0045] (2) 4.0 g of polyvinyl alcohol (PVA) and 1.5 g of gelatin (GEL) were weighed and placed in a beaker containing 94.5 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 4.0 wt.% PVA and 1.5 wt.% GEL. 1.0 g of acidified carbon nanofibers (A-CNFs) was placed in the composite aqueous solution and ultrasonically dispersed for 1.5 h to obtain a dispersed PVA / GEL / A-CNFs mixed solution.

[0046] (3) The PVA / GEL / A-CNFs mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.08 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 55°C.

[0047] (4) Weigh 35.0 g of tetraethyl orthosilicate (TEOS) solution and 80.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 1 h.

[0048] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 32% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 40 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0049] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 45 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-CNFs / BA / SiO2 composite aerogel product S2 with high efficiency thermal insulation and flame retardant properties.

[0050] Example 3

[0051] (1) The preparation method of A-ZrO2 is as follows: ZrO2 and mixed acid solution are mixed in a mass ratio of 4:100, ultrasonically dispersed for 2 hours, and then reacted for 2 hours using a magnetic stirrer at 60°C and 400 rpm. Finally, the mixed solution is filtered, washed, and dried to obtain A-ZrO2.

[0052] (2) 2.5 g of polyvinyl alcohol (PVA) and 1.8 g of gelatin (GEL) were weighed and placed in a beaker containing 95.7 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 2.5 wt.% PVA and 1.8 wt.% GEL. 2.0 g of acidified zirconia nanofibers (A-ZrO2) were weighed and placed in the composite aqueous solution and ultrasonically dispersed for 2.0 h to obtain a dispersed PVA / GEL / A-ZrO2 mixed solution.

[0053] (3) The PVA / GEL / A-ZrO2 mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.1 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 60°C.

[0054] (4) Weigh 38.0 g of tetraethyl orthosilicate (TEOS) solution and 75.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 1.5 h.

[0055] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 35% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 45 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0056] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 48 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-ZrO2 / BA / SiO2 composite aerogel product S3 with high efficiency heat insulation and flame retardant properties.

[0057] Example 4

[0058] (1) The preparation method of A-SiC is as follows: SiC and mixed acid solution are mixed in a mass ratio of 3:100 and ultrasonically dispersed for 2 h. The mixture is stirred at 60°C and 400 rpm for 2 h. Finally, the mixed solution is filtered, washed, and dried to obtain A-SiC.

[0059] (2) 2.0 g of polyvinyl alcohol (PVA) and 1.0 g of gelatin (GEL) were weighed and placed in a beaker containing 97.0 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 2.0 wt.% PVA and 1.0 wt.% GEL. 1.5 g of acidified silicon carbide nanofibers (A-SiC) were weighed and placed in the composite aqueous solution and ultrasonically dispersed for 2.0 h to obtain a dispersed PVA / GEL / A-SiC mixed solution.

[0060] (3) The PVA / GEL / A-SiC mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.06 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 58°C.

[0061] (4) Weigh 40.0 g of tetraethyl orthosilicate (TEOS) solution and 100.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 2 h.

[0062] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 40% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 36 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0063] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 48 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-SiC / BA / SiO2 composite aerogel product S4 with high efficiency thermal insulation and flame retardant properties.

[0064] Example 5

[0065] (1) The preparation method of A-CNFs is as follows: CNFs and mixed acid solution are mixed at a mass ratio of 2.6:100 and ultrasonically dispersed for 1.5 h. The mixture is stirred at 60 °C and 400 rpm for 2 h. Finally, the mixed solution is filtered, washed, and dried to obtain A-CNFs.

[0066] (2) 5.0 g of polyvinyl alcohol (PVA) and 2.0 g of gelatin (GEL) were weighed and placed in a beaker containing 93.0 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 5.0 wt.% PVA and 2.0 wt.% GEL. 0.8 g of acidified carbon nanofibers (A-CNFs) were weighed and ultrasonically dispersed in the composite aqueous solution for 1 h to obtain a dispersed PVA / GEL / A-CNFs mixed solution.

[0067] (3) The PVA / GEL / A-CNFs mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.1 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 52°C.

[0068] (4) Weigh 30.0 g of tetraethyl orthosilicate (TEOS) solution and 30.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 1 h.

[0069] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 33% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 36 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0070] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 48 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-CNFs / BA / SiO2 composite aerogel product S5 with high efficiency thermal insulation and flame retardant properties.

[0071] Example 6

[0072] (1) The preparation method of A-CNTs is as follows: CNTs and mixed acid solution are mixed in a mass ratio of 1:20 and ultrasonically dispersed for 2 h. The mixture is stirred at 60 °C and 400 rpm for 3 h. Finally, the mixed solution is filtered, washed, and dried to obtain A-CNTs.

[0073] (2) 3.2 g of polyvinyl alcohol (PVA) and 1.4 g of gelatin (GEL) were weighed and placed in a beaker containing 95.4 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 3.2 wt.% PVA and 1.4 wt.% GEL. 1.6 g of acidified carbon nanotubes (A-CNTs) were weighed and placed in the composite aqueous solution and ultrasonically dispersed for 2 h to obtain a dispersed PVA / GEL / A-CNTs mixed solution.

[0074] (3) The PVA / GEL / A-CNTs mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.07 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 60°C.

[0075] (4) Weigh 32.0 g of tetraethyl orthosilicate (TEOS) solution and 48.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 1.5 h.

[0076] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 34% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 40 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0077] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 36 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-CNTs / BA / SiO2 composite aerogel product S6 with high efficiency heat insulation and flame retardant properties.

[0078] Example 7

[0079] (1) The preparation method of A-SiC is as follows: SiC and mixed acid solution are mixed in a mass ratio of 4.6:100 and ultrasonically dispersed for 2 h. The mixture is stirred at 60°C and 400 rpm for 2 h. Finally, the mixed solution is filtered, washed, and dried to obtain A-SiC.

[0080] (2) 2.8 g of polyvinyl alcohol (PVA) and 1.2 g of gelatin (GEL) were weighed and placed in a beaker containing 96.0 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 2.8 wt.% PVA and 1.2 wt.% GEL. 1.3 g of acidified silicon carbide nanofibers (A-SiC) were weighed and placed in the composite aqueous solution and ultrasonically dispersed for 1.5 h to obtain a dispersed PVA / GEL / A-SiC mixed solution.

[0081] (3) The PVA / GEL / A-SiC mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.09 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 56°C.

[0082] (4) Weigh 33.0 g of tetraethyl orthosilicate (TEOS) solution and 56.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 1.5 h.

[0083] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 36% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 36 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0084] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 48 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-SiC / BA / SiO2 composite aerogel product S7 with high efficiency heat insulation and flame retardant properties.

[0085] Example 8

[0086] (1) The preparation method of A-ZrO2 is as follows: ZrO2 and mixed acid solution are mixed in a mass ratio of 3.3:100, ultrasonically dispersed for 2 hours, and then reacted for 2 hours using a magnetic stirrer at 60°C and 400 rpm. Finally, the mixed solution is filtered, washed, and dried to obtain A-ZrO2.

[0087] (2) 2.3 g of polyvinyl alcohol (PVA) and 1.7 g of gelatin (GEL) were weighed and placed in a beaker containing 96.0 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 2.3 wt.% PVA and 1.7 wt.% GEL. 0.7 g of acidified zirconia nanofibers (A-ZrO2) were weighed and placed in the composite aqueous solution and ultrasonically dispersed for 1 h to obtain a dispersed PVA / GEL / A-ZrO2 mixed solution.

[0088] (3) The PVA / GEL / A-ZrO2 mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.08 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 54°C.

[0089] (4) Weigh 34.0 g of tetraethyl orthosilicate (TEOS) solution and 66.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 2 h.

[0090] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 37% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 48 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0091] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 40 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-ZrO2 / BA / SiO2 composite aerogel product S8 with high efficiency heat insulation and flame retardant properties.

[0092] Example 9

[0093] (1) The preparation method of A-CNTs is as follows: CNTs and mixed acid solution are mixed in a mass ratio of 4.1:100 and ultrasonically dispersed for 2 h. The mixture is stirred at 60 °C and 400 rpm for 2 h. Finally, the mixed solution is filtered, washed, and dried to obtain A-CNTs.

[0094] (2) 3.9 g of polyvinyl alcohol (PVA) and 1.6 g of gelatin (GEL) were weighed and placed in a beaker containing 94.5 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 3.9 wt.% PVA and 1.6 wt.% GEL. 1.7 g of acidified carbon nanotubes (A-CNTs) were weighed and placed in the composite aqueous solution and ultrasonically dispersed for 2 h to obtain a dispersed PVA / GEL / A-CNTs mixed solution.

[0095] (3) The PVA / GEL / A-CNTs mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.06 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 57°C.

[0096] (4) Weigh 37.0 g of tetraethyl orthosilicate (TEOS) solution and 94.0 g of anhydrous ethanol solution in a beaker and mechanically stir them evenly. Then, add them dropwise to the mixed solution in step (3) and continue stirring for 2 h.

[0097] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 31% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 48 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0098] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 48 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-CNTs / BA / SiO2 composite aerogel product S9 with high efficiency heat insulation and flame retardant properties.

[0099] Example 10

[0100] (1) The preparation method of A-SiC is as follows: SiC and mixed acid solution are mixed in a mass ratio of 3.8:100 and ultrasonically dispersed for 2 h. The mixture is stirred at 60°C and 400 rpm for 2 h. Finally, the mixed solution is filtered, washed, and dried to obtain A-SiC.

[0101] (2) 4.4 g of polyvinyl alcohol (PVA) and 1.9 g of gelatin (GEL) were weighed and placed in a beaker containing 93.7 g of deionized water. The mixture was magnetically stirred at 90°C until clear and homogeneous, thereby preparing a PVA / GEL composite aqueous solution containing 4.4 wt.% PVA and 1.9 wt.% GEL. 0.9 g of acidified silicon carbide nanofibers (A-SiC) were weighed and placed in the composite aqueous solution and ultrasonically dispersed for 1 h to obtain a dispersed PVA / GEL / A-SiC mixed solution.

[0102] (3) The PVA / GEL / A-SiC mixed solution in step (2) was adjusted to pH 2 using a mixed acid solution of 0.07 mol / L BA and 0.1 mol / L HCl under mechanical stirring at 300 rpm, and then the mixed solution was heated to 52°C.

[0103] (4) Weigh 36.0 g of tetraethyl orthosilicate (TEOS) solution and 84.0 g of anhydrous ethanol solution into a beaker and mechanically stir until uniform. Then, add them dropwise into the mixed solution in step (3) and continue stirring for 2 h.

[0104] (5) The pH of the mixed solution was then adjusted to neutral with aqueous ammonia and cooled to room temperature. Finally, a 38% ethanol solution (based on the volume of the mixed solution) was added to the solution and the aging reaction was continued for 48 hours to produce a composite wet gel. The mechanical stirring rate was maintained at 300 rpm throughout the entire process.

[0105] (6) The composite wet gel product in step (5) is first cooled by immersing in liquid nitrogen, and then continuously dried for 45 hours by freeze drying to obtain a high-strength and tough PVA / GEL / A-SiC / BA / SiO2 composite aerogel product S10 with high efficiency thermal insulation and flame retardant properties.

[0106] Comparative Example 1

[0107] Comparative Example 1 differs from Example 1 in that step (2) does not contain gelatin (GEL), and the amount of deionized water used is 97 g. Other conditions are the same as in Example 1. A high-strength and tough PVA / A-CNTs / BA / SiO2 composite aerogel product D1 with efficient thermal insulation and flame retardancy is obtained.

[0108] Comparative Example 2

[0109] Comparative Example 2 differs from Example 1 in that the amount of gelatin (GEL) used in step (2) is 3.0 g, and the amount of deionized water used is 94.0 g. Other conditions are the same as in Example 1. A high-strength and high-toughness PVA / GEL / A-CNTs / BA / SiO2 composite aerogel product D2 with efficient thermal insulation and flame retardancy is obtained.

[0110] Comparative Example 3

[0111] Comparative Example 3 differs from Example 2 in that the inorganic nanofibers were not subjected to the acidification step (1). In step (2), the specific operation was as follows: 1.0 g of unacidified carbon nanofibers (CNFs) was weighed and placed in the above-mentioned composite aqueous solution and ultrasonically dispersed for 1.5 hours to obtain a dispersed PVA / GEL / CNFs mixed solution. Other conditions were the same as in Example 2. Finally, a high-strength and tough PVA / GEL / CNFs / BA / SiO2 composite aerogel product D3 with efficient thermal insulation and flame retardancy was obtained.

[0112] Comparative Example 4

[0113] Comparative Example 4 differs from Example 3 in that no acidified inorganic nanofibers are added. Step (3) involves adjusting the pH of the PVA / GEL mixed solution to 2 using a mixed acid solution of 0.1 mol / L BA and 0.1 mol / L HCl while mechanically stirring at 300 rpm, followed by heating the mixed solution to 60°C. Other conditions were the same as in Example 3. A high-strength and high-toughness PVA / GEL / BA / SiO2 composite aerogel product D4 with efficient thermal insulation and flame retardancy was obtained.

[0114] Comparative Example 5

[0115] Comparative Example 5 differs from Example 3 in that the amount of acidified zirconia nanofibers used in step (2) is 4.0 g. Other conditions are the same as in Example 3. A high-strength and high-toughness PVA / GEL / BA / SiO2 composite aerogel product D5 with efficient thermal insulation and flame retardancy is obtained.

[0116] Comparative Example 6

[0117] Comparative Example 6 differs from Example 4 in that boric acid is not used in step (3). Other conditions are the same as those in Example 4. A high-strength and tough PVA / GEL / BA / SiO2 composite aerogel product D6 with high thermal insulation and flame retardancy is obtained.

[0118] Comparative Example 7

[0119] Comparative Example 7 differs from Example 5 in that the amount of polyvinyl alcohol (PVA) used in step (2) is 8.0 g. Other conditions are the same as in Example 5. A high-strength and high-toughness PVA / GEL / BA / SiO2 composite aerogel product D7 with efficient thermal insulation and flame retardancy is obtained.

[0120] Comparative Example 8

[0121] Comparative Example 8 differs from Example 6 in that the amount of tetraethyl orthosilicate (TEOS) used in step (4) is 50.0 g. Other conditions are the same as in Example 6. A high-strength and tough PVA / GEL / BA / SiO2 composite aerogel product D8 with efficient thermal insulation and flame retardancy is obtained.

[0122] Comparative Example 9

[0123] The difference between Comparative Example 9 and Example 1 is that the materials used in Example 1 were directly mixed together and aged. As a result, the fibers were unevenly dispersed, making it difficult for the silicon source to penetrate and form a cross-linked hydrogel structure.

[0124] The various properties of the composite aerogels prepared in the above embodiments and comparative examples were tested and analyzed, and the results are shown in Table 1:

[0125] Table 1

[0126]

[0127]

[0128] According to the test results in Table 1, the composite aerogels prepared in Examples 1-10 of the present application have low thermal conductivity, high limiting oxygen index (LOI), and excellent compressive strength and tensile strength, which indicates that the composite aerogels have excellent thermal insulation, flame retardancy and high mechanical strength, and have great application prospects in practical engineering applications.

[0129] Comparative Example 1 does not contain gelatin compared to Example 1. Due to gelatin's inherent rigid structure and polyamino and polyhydroxy structures, the composite aerogel lacks a rigid flame retardant material to chemically crosslink with the PVA resin, resulting in a significant decrease in its compressive strength and limiting oxygen index. Comparative Example 2 increases the gelatin content compared to Example 1, which improves the composite aerogel's compressive strength and limiting oxygen index to a certain extent. However, it also increases the density and reduces the porosity and open pore size of the composite aerogel, resulting in a significant decrease in its thermal conductivity.

[0130] Compared with Example 2, Comparative Example 3 directly uses CNTs without acidification treatment, which makes the surface of the nanofibers lack active carboxyl groups, thereby lacking the opportunity for chemical crosslinking with other substances in the wet gel, thereby reducing the interfacial bonding strength, and the resulting composite aerogel has lower mechanical properties. Compared with Example 3, Comparative Example 4 does not add acidified inorganic nanofibers, which makes the interior of the composite aerogel lack more isotropic skeleton support, so the final composite aerogel has relatively poor mechanical properties, and the internal pores increase, resulting in a decrease in the limiting oxygen index; for details, please refer to Figure 2 Macroscopic comparison test diagram of the combustion process of Example 3 and Comparative Example 4. Compared with Example 3, Comparative Example 5 increases the dosage of acidified inorganic nanofibers. Although it improves the mechanical properties of the composite aerogel, it also further increases its density, reduces the internal porosity, and increases the thermal conductivity.

[0131] Compared to Example 4, in Comparative Example 6, boric acid was not added during the wet gel preparation process. This reduced the degree of chemical crosslinking within the composite aerogel to a certain extent, reducing its compressive strength. More importantly, boric acid provides an acid source for the entire system. During flame combustion, it promotes polymer dehydration and carbonization, providing a glassy coating on the aerogel and preventing further combustion of the PVA.

[0132] Compared with Example 5, in Comparative Example 7, an excessive amount of PVA resin was added. PVA is light, flammable, and has low compressive strength. Therefore, excessive PVA in the composite aerogel is bound to bring about the opposite effect, such as reducing the thermal insulation and flame retardant effects, and reducing the compressive strength of the composite aerogel.

[0133] Compared with Example 6, Comparative Example 8 increases the content of the silicon source (TEOS), which increases the proportion of SiO2 aerogel in the final unit volume of the composite aerogel. Correspondingly, the pore size and porosity inside the aerogel decrease, which increases the thermal conductivity and reduces the thermal insulation effect of the composite aerogel.

[0134] The above describes in detail the high-strength and tough composite aerogel with efficient thermal insulation and flame retardant properties and its preparation method. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above examples is intended only to facilitate understanding of the present invention and its core concepts, including the best mode, and to enable anyone skilled in the art to practice the invention. It should be noted that those skilled in the art will appreciate that various improvements and modifications may be made to the present invention without departing from the principles of the present invention, and such improvements and modifications fall within the scope of the claims. The scope of the present invention is defined by the claims and may include other embodiments that may be devised by those skilled in the art. If such other embodiments have structural elements that do not differ from the literal meaning of the claims, or if they include equivalent structural elements that do not differ substantially from the literal meaning of the claims, then such other embodiments are also intended to be included within the scope of the claims. Those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a high-strength and tough composite aerogel with high efficiency heat insulation and flame retardant properties, characterized in that: The following steps are involved: (1) Acidification of the inorganic nanofibers; (2) Adding the acidified inorganic nanofibers to a composite aqueous solution of polyvinyl alcohol and gelatin, and ultrasonically dispersing the mixture to obtain a uniform mixed solution; (3) Adding a mixed acid to the mixed solution, adjusting the pH to 2-3, and heating the solution to 50-60°C; (4) Maintaining the temperature at 50-60°C, continue to add the mixed solution of silicon source and ethanol to form silica sol; (5) Using an ammonia solution to adjust the silica sol to neutrality, adding ethanol, and then adjusting the temperature to room temperature, and continuing aging; (6) adding the aged product into a mold and freeze-drying it.

2. The preparation method according to claim 1, characterized in that: The operation of step (1) is to mix the inorganic nanofibers and the acid solution, carry out an acidification reaction, and then dry to obtain the acidified inorganic nanofibers; wherein the acid solution is a mixed solution of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1; the mass ratio of the inorganic nanofibers to the acid solution is (1-5):100; the temperature of the acidification reaction is 60-80°C, and the time is 1-2h.

3. The preparation method according to claim 1, wherein: In the step (2), the dosage ratio of polyvinyl alcohol, gelatin and water is (2-5): (1-2): (93-97); the temperature condition for forming the composite aqueous solution is 80-95°C.

4. The preparation method according to claim 1, wherein: In the step (2), the mass ratio of the acidified inorganic nanofibers to the composite aqueous solution is (0.5-2):100, and the ultrasonic dispersion time is 1-2 hours.

5. The preparation method according to claim 1, wherein: The mixed acid in step (3) is a mixed solution of 0.05-0.1 mol / L boric acid and 0.1 mol / L hydrochloric acid in a volume ratio of 1:

1.

6. The preparation method according to claim 1, wherein: The mass ratio of the silicon source to the ethanol in the mixed solution of step (4) is 1:(1-2.5); the silicon source is selected from tetraethyl orthosilicate (TEOS), methyl orthosilicate (TMOS), methyltriethoxysilane (MTES), and methyltrimethoxysilane (MTMS).

7. The preparation method according to claim 6, characterized in that: The total mass of the silicon source and the ethanol is 0.6-1.4 times the total mass of the mixed solution in step (3).

8. The preparation method according to claim 1, wherein: The volume amount of ethanol in step (5) is 30-40% of the entire system.

9. The preparation method according to claim 1, wherein: The freeze-drying temperature in step (6) is -50°C, and the drying time is 36-48 hours.

10. A high-strength and tough composite aerogel with high thermal insulation and flame retardant properties, characterized by: The high-strength and tough composite aerogel with efficient heat insulation and flame retardant properties is prepared by the preparation method according to any one of claims 1 to 9.

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