A multi-stage hierarchical network super-insulating aerogel and its preparation method

Abstract

The present invention discloses a multi - hierarchical network aerogel with heat insulation and its preparation method, belonging to the technical field of aerogel composite materials. Based on the sol - gel method and physical adsorption method, the present invention obtains a multi - hierarchical network hybrid aerogel heat - insulation material containing a long - range continuous micron - pore sponge skeleton, a macroporous polymer cross - linked skeleton and a mesoporous silica network through supercritical carbon dioxide drying. Among them, using the chitosan polymer long chain as a growth template and the aldehyde - based resin as a cross - linker, a polymer gel skeleton is cross - linked to encapsulate the in - situ formed silica aerogel, thereby forming a macropore - mesopore hierarchical structure at the nanoscale; by impregnating and adsorbing the sol mixture into a commercial sponge carrier, the encapsulation of the polymer network is realized, solving the problem that traditional aerogel materials are prone to powdering and fragmentation under stress, and obtaining a multi - hierarchical network aerogel material with adiabatic characteristics, excellent mechanical strength and self - extinguishing characteristics that can be prepared on a large scale.

Description

Technical Field

[0001] The present invention relates to a heat-insulating aerogel composite material, and more particularly, to a multi-hierarchical network heat-insulating aerogel and a preparation method thereof. Background Art

[0002] Aerogel materials have extremely low density and extremely high specific surface area, which greatly limit heat transfer from convection and conduction, and thus have extremely low thermal conductivity. They are currently the best heat-insulating materials and have broad application prospects and great demands in the fields of fire protection, aerospace, military, and energy. However, the production cost and fragile mechanical properties of aerogels limit their large-scale application. The nano-porous network of silica aerogel can inhibit the free movement of gas molecules, and at the same time, the loose particle assembly mechanism can ensure low heat conduction efficiency, and it is considered to have the best heat-insulating performance. However, this fine heat-insulating structure also limits the development of its mechanical enhancement and low-cost, large-scale manufacturing processes.

[0003] Introducing reinforcing phases such as glass fiber or polymer fiber substrates is the existing idea to solve the practical problems of aerogels. However, these methods lack nano-scale structural design and are difficult to solve the mechanical instability of composite aerogel heat-insulating materials intrinsically caused by the macroscopic separation of the reinforcing phase and the aerogel phase. Summary of the Invention

[0004] Aiming at the problem of mismatch between the large-scale cost, mechanical properties and heat-insulating performance requirements of existing aerogel heat-insulating materials, the purpose of the present invention is to provide a multi-hierarchical network heat-insulating aerogel and a preparation method thereof, which use the combination of nano-pores and low-density skeletons for heat insulation to solve the contradiction between the mechanics and thermal conductivity of aerogels.

[0005] To achieve the above object, the present invention provides a multi-hierarchical network heat-insulating aerogel, which is obtained through a simple continuous production process. The composite aerogel with a super-large pore - large pore - mesopore multi-hierarchical porous skeleton structure is composed of a sponge material, a polymer gel skeleton, and a silica aerogel. Among them, the super-large pore sponge material skeleton provides mechanical support, and an organic-inorganic hybrid aerogel is uniformly distributed among its pores; the organic-inorganic hybrid aerogel includes a macroporous organic polymer gel skeleton formed by cross-linking chitosan and resin, and a mesoporous inorganic silica aerogel filled in the polymer gel skeleton.

[0006] The key to the aerogel of the present invention lies in the construction of a triple skeleton: the skeleton of the supermacroporous sponge material restricts the stress slip of the aerogel particles and provides mechanical support, and the pore size is preferably about 100 to 500 micrometers; in the sponge material, the polymer and mesoporous silica are uniformly blended, and the macroporous polymer skeleton is a polymer skeleton cross-linked by chitosan-resin, and the pore size is preferably about 100 to 300 nm; the mesoporous silica encapsulated by in-situ cross-linking has a pore size preferably of 10 to 20 nm.

[0007] First, the present invention uses the long chain of chitosan polymer with amino functional groups as the growth template and aldehyde resin as the cross-linking agent to form a cross-linked gel skeleton under heating conditions, encapsulating the in-situ formed silica aerogel, thereby forming a macroporous-mesoporous hierarchical structure at the nanoscale. Further, the sol mixture is impregnated and adsorbed on a commercial sponge carrier to realize the encapsulation of the polymer network, solving the problem that the traditional aerogel material is prone to powdering and fragmentation under stress, and obtaining a multi-hierarchical network adiabatic aerogel material with adiabatic characteristics, excellent mechanical strength and self-extinguishing characteristics that can be prepared on a large scale.

[0008] Specifically, the preparation method of the multi-hierarchical network adiabatic aerogel includes the following steps:

[0009] Step S1: Take a certain mass of chitosan, uniformly mix it with water, ethanol, and acetic acid in a certain proportion, and stir for a certain time to obtain a transparent weakly acidic chitosan solution A;

[0010] Step S2: Take a certain mass of silicon source precursor, add it to the aforementioned weakly acidic chitosan solution A, and stir to fully hydrolyze the silicon source precursor in it to form silica sol, obtaining a mixed solution B;

[0011] Step S3: Slowly add a certain amount of aqueous solution of thermosetting resin prepolymer to the above mixed solution B and stir evenly to obtain a mixed solution C;

[0012] Step S4: Pour the mixed solution C onto the hydrophilic sponge material and submerge the sponge material, and use an air pump to evacuate to negative pressure to complete the impregnation of the solution and the elimination of bubbles, obtaining a mixture D;

[0013] Step S5: Place the mixture D in a sealed container, raise the temperature to the resin polymerization temperature, cool to room temperature after sufficient reaction, and obtain a wet gel block;

[0014] Step S6: Place the wet gel block in absolute ethanol for solvent replacement to remove water and free unreacted substances, obtaining an ethanol gel;

[0015] Step S7: Place the ethanol gel block in an autoclave and use liquid carbon dioxide for solvent replacement to remove ethanol;

[0016] Step S8: Keep the autoclave sealed and slowly heat it up (so that the temperature and pressure inside the autoclave meet the requirements of exceeding the supercritical temperature of carbon dioxide by 31 °C and the supercritical pressure of 7.38 MPa, while ensuring safety), so that carbon dioxide enters the supercritical state; then keep the temperature unchanged and slowly open the valve to reduce the pressure until the atmospheric pressure state is reached, and a multi-stage hierarchical network aerogel can be obtained.

[0017] In the above step S1, the chitosan preferably uses a chitosan raw material with a deacetylation degree ≥ 85% and a viscosity between 50 and 00 mPa·s. Preferably, the pH value of the weakly acidic chitosan solution A is between 2 and 3, and the concentration of chitosan is 1% - 2.5% by mass fraction.

[0018] In the above step S2, the silicon source precursor preferably is an alkoxysilane, such as common organosilicon precursors like tetramethyl orthosilicate, tetraethyl orthosilicate, tetraisopropyl orthosilicate, methyltrimethoxysilane, methyltriethoxysilane, etc.

[0019] In the above step S3, the thermosetting resin prepolymer preferably is an aldehyde resin prepolymer such as a phenolic resin prepolymer, a melamine resin prepolymer, a furfural resin prepolymer, etc. In the mixed solution C, the mass fraction of each component preferably is water:ethanol:acetic acid:chitosan:silicon source precursor:resin prepolymer = 100:100:(1 - 2):(10 - 50):(10 - 50).

[0020] In the above step S4, the sponge material can select common commercial sponge materials, such as foamed melamine resin sponge, foamed polyurethane sponge, melamine resin sponge, etc.

[0021] In the above step S5, the temperature for heating the gel is 80 - 160 °C to polymerize and crosslink the resin prepolymer.

[0022] In the above step S6, preferably, place the wet gel block in absolute ethanol at 50 - 70 °C for solvent replacement to accelerate the replacement speed.

[0023] In the above step S7, preferably, keep the temperature inside the autoclave between 8 - 25 °C and the pressure between 8 - 12 MPa.

[0024] In the above step S8, preferably, heat to make the temperature inside the autoclave reach 40 - 70 °C and the pressure remain between 8 - 12 MPa.

[0025] Based on the sol-gel method and physical adsorption method, the present invention obtains a hierarchical network hybrid aerogel thermal insulation material containing a long-range continuous micro-porous sponge skeleton, a macroporous polymer cross-linked skeleton, and a mesoporous silica network through supercritical carbon dioxide drying. The preparation method has a simple process and only requires basic raw materials. The finally obtained adiabatic aerogel consists of a triple hierarchical porous network: the continuous sponge skeleton with super-large pores acts as a reinforcing phase to restrict the stress slip of aerogel particles and provides sufficient mechanical strength; the macroporous polymer skeleton in-situ cross-links and encapsulates the mesoporous silica porous structure to achieve hybrid mechanical enhancement at the nanoscale and maintain adiabatic performance. This multi-hierarchical network adiabatic aerogel has a uniform structure, adjustable pore size, low density, a thermal conductivity lower than that of dry air, and excellent mechanical strength. The supporting force can exceed one atmospheric pressure under 20% compression; and it does not powder under compression, has compression and rebound characteristics, and can be cut and formed; the preparation process has strong continuity, the product can be self-supporting, and has the potential for large-scale production. Description of the Drawings

[0026] Figure 1 It is a physical photograph of the aerogel plate obtained in Example 1, showing its self-supporting strength and the potential for large-size preparation.

[0027] Figure 2 It is a micrograph of the aerogel obtained in Example 1, showing its hierarchical pore structure.

[0028] Figure 3 It is the XRD diffraction pattern of the three aerogels in Example 1, showing their homogeneous characteristics at the nanoscale.

[0029] Figure 4 It is the isothermal nitrogen adsorption curve (a) and the pore size distribution fitting result (b) of the aerogel obtained in Example 1, showing the mesoporous structure characteristics of the aerogel. Detailed Description of the Invention

[0030] The present invention will be described in detail below with reference to the embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the protection scope of the present invention.

[0031] The following illustrates the implementation manners of the present invention through specific specific examples. Those skilled in the art can easily understand the advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0032] Example 1:

[0033] 2 g of chitosan was uniformly mixed with 100 g of water, 100 g of ethanol and 2 g of acetic acid, and stirred for a certain time until a transparent light yellow solution was obtained. 10 g of tetraethyl orthosilicate was added, and stirring was continued for 60 minutes to allow it to hydrolyze sufficiently to form silica sol, obtaining a uniform solution. 10 g of melamine resin prepolymer was slowly added to the above mixed solution and stirred until dissolved. Then, the mixed solution was poured onto a commercial melamine resin sponge board and immersed therein, and air was pumped through an air pump and a negative pressure was maintained to complete the impregnation of the solution and the elimination of bubbles, obtaining a packaged gel precursor. It was placed in a sealed high-pressure container, heated to 120 °C and kept warm for 10 hours, and then cooled to room temperature to obtain a crosslinked and cured wet gel block. The wet gel block was placed in an anhydrous ethanol solution at 50 °C for solvent replacement to remove water and free unreacted substances, obtaining an ethanol gel. The ethanol gel block after solvent replacement was placed in an autoclave, and solvent replacement was carried out using liquid carbon dioxide to remove ethanol. Keeping the autoclave sealed, it was slowly heated to raise the temperature in the autoclave to 50 °C and the pressure to 10 MPa, and carbon dioxide entered the supercritical state. After maintaining the high temperature and high pressure state for 1 hour, while keeping the temperature constant, the valve was slowly opened to reduce the pressure until the normal pressure state was restored, and a multi-stage hierarchical network adiabatic aerogel could be obtained. The obtained aerogel material has a hierarchical porous structure from millimeters to nanometers, as Figure 2 shown. The composite aerogel is flat as a whole and has no obvious macroscopic cavities; the sponge skeleton with a pore diameter of about 200 μm penetrates into the hybrid aerogel block and is divided into micron blocks; each aerogel block also includes a skeleton with a pore diameter of more than 100 nm composed of a chitosan-resin skeleton, and silica aerogel with a smaller pore diameter (about 10 - 20 nm) filled in the pores of the skeleton. The measured density is 97 mg / mL, and the thermal conductivity measured by the hot wire method is 0.026 W / mK, which is lower than the thermal conductivity of air (0.0278 W / mK), can be cut into different shapes, and has a certain compressive strength and resilience.

[0034] Silica aerogel was obtained by directly heating, hydrolyzing, gelling, replacing and supercritically drying in a mixed solution of water, ethanol and acetic acid with only tetraethyl orthosilicate added; pure organic resin aerogel was obtained by adding only chitosan and resin prepolymer in a mixed solution of water, ethanol and acetic acid, heating, gelling, replacing and supercritically drying; and an organic-inorganic hybrid composite aerogel combining the two was obtained according to the normal process. The powder diffraction characterization of these three materials was carried out using an X-ray diffractometer, as Figure 3As shown, it is found that both silica aerogel and resin aerogel are amorphous structures, and at the same time, the positions of their bulging peaks are significantly different; while the composite aerogel is also an amorphous structure and only has a bulging peak with a peak position between the inorganic and organic phases. This indicates that the resin and silica in the composite aerogel are in a homogeneous blend state at the nanoscale and no phase separation has occurred between the two. This organic-inorganic hybrid state enables the material to overcome the brittleness of conventional silica aerogel and the high thermal conductivity defect of conventional organic aerogel.

[0035] Take the multi-hierarchical network adiabatic aerogel sample of Example 1 to conduct 77 K isothermal nitrogen adsorption and desorption experiments, and use the BET method to fit the data points to obtain the mesoporous specific surface area of the aerogel, and conduct DFT theoretical calculations on the data points to obtain the pore size distribution and pore volume of the aerogel. The results are as Figure 4 shown, which characterizes that the aerogel sample has typical mesoporous distribution characteristics and a large specific surface area.

[0036] Example 2:

[0037] Mix 2 grams of chitosan evenly with 100 grams of water, 100 grams of ethanol and 2 grams of acetic acid, and stir for a certain time until a transparent light yellow solution is obtained. Add 10 grams of tetraethyl orthosilicate and continue to stir for 60 minutes to fully hydrolyze it to form silica sol, obtaining a homogeneous solution. Slowly add 10 grams of melamine resin prepolymer to the above mixed solution and stir to dissolve. Then, pour the mixed solution onto a commercial melamine resin sponge board and immerse it. Complete the impregnation of the solution and the elimination of bubbles by pumping air with an air pump and maintaining negative pressure to obtain a packaged gel precursor. Place it in a sealed high-pressure container, heat it to 80 degrees Celsius and keep it warm for 10 hours, and then cool it to room temperature to obtain a cross-linked and cured wet gel block. Place the wet gel block in an anhydrous ethanol solution at 50 degrees Celsius for solvent replacement to remove water and free unreacted substances, obtaining an ethanol gel. Place the ethanol gel block after solvent replacement in an autoclave and use liquid carbon dioxide for solvent replacement to remove ethanol. Keep the autoclave sealed, slowly heat up to make the temperature in the autoclave reach 50 degrees Celsius and the pressure reach 10 MPa, so that carbon dioxide enters the supercritical state. After maintaining the high temperature and high pressure state for 1 hour, while keeping the temperature unchanged, slowly open the valve to reduce the pressure until the normal pressure state is restored, and a multi-hierarchical network adiabatic aerogel can be obtained. The obtained aerogel material has a hierarchical porous structure from millimeters to nanometers, the measured density is 158 mg / mL, the thermal conductivity measured by the hot wire method is 0.032 W / mK, it can be cut into different shapes, and has a certain compressive strength and resilience. The stress exceeds 1 atmosphere when compressed by 5%.

[0038] Example 3:

[0039] 3 g of chitosan was uniformly mixed with 100 g of water, 100 g of ethanol, and 2 g of acetic acid, and stirred for a certain period of time until a transparent pale yellow solution was obtained. 10 g of tetraethyl orthosilicate was added, and stirring was continued for 60 minutes to allow it to fully hydrolyze to form silica sol, obtaining a homogeneous solution. 10 g of phenolic resin prepolymer was slowly added to the above mixed solution and stirred until dissolved. Thereafter, the mixed solution was poured onto a commercial melamine resin sponge board and immersed therein, and the solution impregnation and bubble elimination were completed by pumping air with an air pump and maintaining a negative pressure, obtaining a packaged gel precursor. It was placed in a sealed high-pressure container, heated to 160 °C and kept warm for 24 hours, and then cooled to room temperature to obtain a crosslinked and cured wet gel block. The wet gel block was placed in an anhydrous ethanol solution at 50 °C for solvent replacement to remove water and free unreacted substances, obtaining an ethanol gel. The ethanol gel block after solvent replacement was placed in an autoclave, and solvent replacement was carried out using liquid carbon dioxide to remove ethanol. Keeping the autoclave sealed, the temperature in the autoclave was slowly raised to 50 °C and the pressure reached 10 MPa, and carbon dioxide entered the supercritical state. After maintaining the high temperature and high pressure state for 1 hour, while keeping the temperature constant, the valve was slowly opened to reduce the pressure until the normal pressure state was restored, obtaining a multi-hierarchical network adiabatic aerogel. The obtained aerogel material has a hierarchical porous structure from millimeters to nanometers, the measured density is 106 mg / mL, the thermal conductivity measured by the hot wire method is 0.025 W / mK, it can be cut into different shapes, and has a certain compressive strength and resilience. At the same time, due to the composition of the resin material, it has self-extinguishing ability.

[0040] Example 4:

[0041] 2 g of chitosan was uniformly mixed with 100 g of water, 100 g of ethanol and 2 g of acetic acid, and stirred for a certain time until a transparent light yellow solution was obtained. 20 g of tetraethyl orthosilicate was added, and stirring was continued for 60 minutes to allow it to fully hydrolyze to form silica sol, obtaining a homogeneous solution. 10 g of furfural resin prepolymer was slowly added to the above mixed solution and stirred until dissolved. Thereafter, the mixed solution was poured onto a commercial hydrophilic polyurethane open-cell sponge board and immersed therein, and air was pumped through an air pump and a negative pressure was maintained to complete the impregnation of the solution and the elimination of bubbles, obtaining a packaged gel precursor. It was placed in a sealed high-pressure container, heated to 100 °C and kept warm for 12 hours, and then cooled to room temperature to obtain a crosslinked and cured wet gel block. The wet gel block was placed in an anhydrous ethanol solution at 50 °C for solvent replacement to remove water and free unreacted substances, obtaining an ethanol gel. The ethanol gel block after solvent replacement was placed in an autoclave, and solvent replacement was carried out using liquid carbon dioxide to remove ethanol. Keeping the autoclave sealed, the temperature in the autoclave was slowly raised to 40 °C and the pressure was raised to 10 MPa, and carbon dioxide entered the supercritical state. After maintaining the high temperature and high pressure state for 1 hour, while keeping the temperature unchanged, the valve was slowly opened to reduce the pressure until the normal pressure state was restored, and a multi-hierarchical network adiabatic aerogel was obtained. The obtained aerogel material has a hierarchical porous structure from millimeters to nanometers, the measured density is 121 mg / mL, the thermal conductivity measured by the hot wire method is 0.029 W / mK, it can be cut into different shapes, and has a certain compressive strength and flexibility.

[0042] Example 5:

[0043] 2 g of chitosan was uniformly mixed with 100 g of water, 100 g of ethanol and 2 g of acetic acid, and stirred for a certain time until a transparent light yellow solution was obtained. 10 g of tetraethyl orthosilicate was added, and stirring was continued for 60 minutes to allow it to fully hydrolyze to form silica sol, obtaining a uniform solution. 30 g of melamine resin prepolymer was slowly added to the above mixed solution and stirred until dissolved. Then, the mixed solution was poured onto a commercial melamine resin sponge board and immersed, and air was pumped out by an air pump and negative pressure was maintained to complete the impregnation of the solution and elimination of bubbles, obtaining a sealed gel precursor. It was placed in a sealed high-pressure container, heated to 160 °C and kept warm for 12 hours, and then cooled to room temperature to obtain a crosslinked and cured wet gel block. The wet gel block was placed in an anhydrous ethanol solution at 50 °C for solvent replacement to remove water and free unreacted substances, obtaining an ethanol gel. The ethanol gel block after solvent replacement was placed in an autoclave, and solvent replacement was carried out using liquid carbon dioxide to remove ethanol. Keeping the autoclave sealed, it was slowly heated to raise the temperature in the autoclave to 50 °C and the pressure to 10 MPa, and carbon dioxide entered the supercritical state. After maintaining the high temperature and high pressure state for 1 hour, while keeping the temperature unchanged, the valve was slowly opened to reduce the pressure until the normal pressure state was restored, obtaining a multi-level hierarchical network adiabatic aerogel. The obtained aerogel material has a hierarchical porous structure from millimeters to nanometers, the measured density is 178 mg / mL, the thermal conductivity measured by the hot wire method is 0.034 W / mK, it can be cut into different shapes, and has a high compressive strength and does not powder during the compression process.

[0044] The above embodiments are only illustrative of the principles and effects of the present invention, and are not used to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes completed by those skilled in the art without departing from the spirit and technical idea disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A multi - stage hierarchical network insulating aerogel, characterized in that It is a composite aerogel composed of a sponge material, a polymer gel framework, and silica aerogel, with a supermacroporous-macroporous-mesoporous multi-hierarchical porous framework structure. Among them, a uniformly distributed organic-inorganic hybrid aerogel is filled in the supermacroporous sponge material framework; the organic-inorganic hybrid aerogel includes a macroporous polymer gel framework formed by crosslinking chitosan and resin, and mesoporous silica aerogel filled in the polymer gel framework; wherein, the pore diameter of the supermacroporous sponge material framework is 100-500 microns; the pore diameter of the macroporous polymer gel framework is 100-300 nm, and the pore diameter of the mesoporous silica aerogel is 10-20 nm.

2. The preparation method of the multi-hierarchical network adiabatic aerogel according to claim 1, comprising the following steps: 1) Uniformly mix chitosan with water, ethanol, and acetic acid, and stir to obtain a transparent weakly acidic chitosan solution A; 2) Add a silicon source precursor to the weakly acidic chitosan solution A, and stir to fully hydrolyze the silicon source precursor to form silica sol, obtaining a mixed solution B; 3) Slowly add an aqueous solution of a thermosetting resin prepolymer to the mixed solution B and stir evenly to obtain a mixed solution C; 4) Pour the mixed solution C onto a hydrophilic sponge material and immerse the sponge material, and use an air pump to evacuate to negative pressure to complete the impregnation of the solution and the elimination of bubbles, obtaining a mixture D; 5) Place the mixture D in a sealed container, heat up to the resin polymerization temperature, cool to room temperature after sufficient reaction, and obtain a wet gel block; 6) Place the wet gel block in absolute ethanol for solvent replacement to remove water and free unreacted substances, obtaining an ethanol gel; 7) Place the ethanol gel block in an autoclave and use liquid carbon dioxide for solvent replacement to remove ethanol; 8) Keep the autoclave sealed, slowly heat up, and make carbon dioxide enter the supercritical state; then keep the temperature constant, slowly open the valve to reduce the pressure until the atmospheric pressure state, and obtain the multi-hierarchical network adiabatic aerogel.

3. The preparation method according to claim 2, characterized in that, In step 1), the chitosan is chitosan with a deacetylation degree ≥ 85% and a viscosity between 50-00 mPa·s; the pH value of the weakly acidic chitosan solution A is 2-3, and the concentration of chitosan therein is 1%-2.5% mass fraction.

4. The preparation method according to claim 2, characterized in that, In step 2), the silicon source precursor is an alkoxysilane; in step 3), the thermosetting resin prepolymer is an aldehyde resin prepolymer.

5. The preparation method according to claim 4, characterized in that, In step 2), the silicon source precursor is selected from one or more of tetramethyl orthosilicate, tetraethyl orthosilicate, tetra-isopropyl orthosilicate, methyltrimethoxysilane, and methyltriethoxysilane; in step 3), the thermosetting resin prepolymer is selected from one or more of phenol formaldehyde resin prepolymer, melamine resin prepolymer, and furfural resin prepolymer.

6. The preparation method according to claim 2, characterized in that, In the mixed solution C described in step 3), the mass fractions of each component are water:ethanol:acetic acid:chitosan:silicon source precursor:resin prepolymer = 100:100:(1-2):(10-50):(10-50).

7. The preparation method according to claim 2, characterized in that, In step 4), the sponge material is a foamed melamine resin sponge, a foamed polyurethane sponge, or a melamine resin sponge.

8. The preparation method according to claim 2, characterized in that, Step 5): Heat to 80 - 160 °C to polymerize and crosslink the resin prepolymer; Step 6): Place the wet gel block in absolute ethanol at 50 - 70 °C for solvent replacement.

9. The preparation method according to claim 2, characterized in that, In Step 7), the temperature inside the autoclave is maintained between 8 - 25 °C, and the pressure is maintained at 8 - 12 MPa; In Step 8), heat to make the temperature inside the autoclave reach 40 - 70 °C, and the pressure is maintained at 8 - 12 MPa to make carbon dioxide enter the supercritical state.

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