Preparation method of cellulose / nano-zinc oxide composite aerogel

Abstract

The application discloses a preparation method of cellulose / nano-zinc oxide composite aerogel and belongs to the field of functional materials. The preparation method comprises the following steps: modifying nano-zinc oxide by using a silane coupling agent, dissolving cellulose in pre-cooled sodium hydroxide and urea aqueous solution to obtain a cellulose solution; adding the modified nano-zinc oxide particles into the cellulose solution under stirring to obtain a cellulose / nano-zinc oxide mixed solution, then performing directional freezing, replacing the formed gel with ethanol, washing the gel with deionized water to remove excessive solvent, until the pH value is 7, obtaining a cellulose / nano-zinc oxide composite hydrogel, and placing the hydrogel into a freeze-drying machine to perform drying, thereby obtaining the cellulose / nano-zinc oxide composite aerogel. The prepared aerogel has ordered directional pore structures in the interior, and has the advantages of good mechanical properties, hydrophobic properties and heat insulation performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of functional materials, and particularly relates to a preparation method of cellulose / nano-zinc oxide composite aerogel. BACKGROUND

[0002] Thermal insulation materials play a crucial role in modern architecture and industry. With the continuous growth of global energy demand and the increasing awareness of environmental protection, high-efficiency thermal insulation materials have become a key means to reduce energy consumption and carbon emissions. In the field of thermal insulation materials, petroleum-based foamed plastics such as polyurethane, expanded polystyrene, and mineral wool are widely used due to their excellent thermal insulation performance. However, these materials have drawbacks such as high flammability, non-renewability, and non-biodegradability, leading to increasing waste pollution problems and restricting their sustainable development. In contrast, cellulose aerogels have high porosity, low density, large adsorption capacity, excellent thermal insulation performance, and natural environmental sustainability. These characteristics make them have wide application potential in fields such as adsorption, thermal insulation, and energy storage. The pores filled with air and extremely low thermal conductivity of cellulose aerogels effectively hinder heat conduction and convection, giving them excellent thermal insulation capacity.

[0003] Cellulose aerogels still face some challenges in practical applications. First, their mechanical properties are insufficient, lacking sufficient toughness and compression resistance, and they are prone to collapse under external forces. Second, the cellulose molecular chain is rich in hydrophilic hydroxyl groups, making cellulose aerogels prone to moisture absorption in high humidity environments, leading to an increase in total thermal conductivity and a decrease in thermal insulation performance. These mechanical property deficiencies and high moisture absorption greatly limit the widespread application of cellulose aerogels in thermal insulation fields. After cellulose forms a stable gel system through regenerative or non-regenerative methods, the liquid phase components in the gel need to be replaced with air through further drying processes to obtain aerogels. Traditional aerogel drying preparation methods, such as atmospheric drying and supercritical drying, often face problems such as complex process, high cost, and unstable aerogel structure, limiting their large-scale application. SUMMARY

[0004] [TECHNICAL PROBLEM]

[0005] The technical problem to be solved by the present application is the insufficient mechanical strength, complex preparation process, high cost, and excessive moisture absorption of cellulose aerogels in thermal insulation applications.

[0006] [TECHNICAL SCHEME]

[0007] In order to solve the above problems, the application provides a preparation method of cellulose / nano zinc oxide composite aerogel thermal insulation material, which takes cellulose as raw material, modifies nano zinc oxide, and performs directional freezing on cellulose solution and modified nano zinc oxide to obtain cellulose / nano zinc oxide composite hydrogel, and then performs freezing in liquid nitrogen, and then transfers the frozen hydrogel to a vacuum freeze dryer to perform drying, so as to obtain cellulose / nano zinc oxide composite aerogel. The method has low cost, simple process and controllable aerogel structure.

[0008] The application provides a preparation method of cellulose / nano zinc oxide composite aerogel thermal insulation material, and specifically includes the following steps.

[0009] (1) Nano zinc oxide is added into anhydrous ethanol and ultrasonic stirring is performed to obtain a nano zinc oxide suspension, and then γ-methacryloxypropyl trimethoxysilane is added into the suspension to perform reaction, and finally centrifugal separation, washing and drying are performed to obtain modified nano zinc oxide particles.

[0010] (2) Sodium hydroxide and urea are added into water, stirring is performed until they are dissolved, and then pre-freezing is performed to obtain a pre-cooled alkali solution, and then cellulose is added into the pre-cooled alkali solution to perform stirring and reaction, so as to obtain a cellulose solution.

[0011] (3) The modified nano zinc oxide particles are added into the cellulose solution, and stirring is performed to mix them, so as to obtain a cellulose / nano zinc oxide mixed solution.

[0012] (4) The cellulose / nano zinc oxide mixed solution is placed in a directional freezing device for treatment, and then taken out and left to stand, and then washing is performed, so as to obtain a cellulose / nano zinc oxide composite hydrogel.

[0013] (5) The cellulose / nano zinc oxide composite hydrogel is subjected to freeze drying to obtain a cellulose / nano zinc oxide composite aerogel.

[0014] In an embodiment of the application, in step (1), the mass ratio of nano zinc oxide to anhydrous ethanol is 1:20-30.

[0015] In an embodiment of the application, in step (1), the mass ratio of nano zinc oxide to γ-methacryloxypropyl trimethoxysilane is 10-20:1.

[0016] In an embodiment of the application, in step (1), the reaction temperature is 80-100℃, and the reaction time is 10-12h.

[0017] In an embodiment of the application, in step (2), the mass ratio of sodium hydroxide, urea, water and cellulose is 7:26:67:3.

[0018] In one embodiment of the present application, in step (2), the pre-freezing temperature is -18 to -12℃, and the pre-freezing time is 3-5h.

[0019] In one embodiment of the present application, in step (2), the cellulose is added to the pre-cooled alkali solution, and the reaction is stirred at 1-3℃ for 3-5h.

[0020] In one embodiment of the present application, in step (3), the mass ratio of the modified nano-zinc oxide particles to cellulose is 1:(5-10), and the stirring time is 1-3h.

[0021] In one embodiment of the present application, in step (4), the temperature of the directional freezing is -196℃, and the time is 20-30min.

[0022] In one embodiment of the present application, in step (4), the room temperature is kept for 5-8h.

[0023] In one embodiment of the present application, in step (5), the temperature of the freeze-drying is -55℃, and the time is 24-48h.

[0024] The present application provides the cellulose / nano-zinc oxide composite aerogel prepared by the above-mentioned method.

[0025] The present application provides the application of the above-mentioned cellulose / nano-zinc oxide composite aerogel in the field of thermal insulation materials.

[0026] Beneficial effects:

[0027] (1) The present application constructs a stable organic modified layer on the surface of nano-zinc oxide particles by grafting modification, reduces the attraction and aggregation tendency between particles, prevents the precipitation or aggregation of the particles in the matrix, and realizes the uniform dispersion of nano-zinc oxide.

[0028] (2) The NaOH-urea solution is a non-derivatized solvent for cellulose, especially under low temperature conditions, the hydrogen bond breaking ability of urea is enhanced, which can more effectively break the hydrogen bonds between cellulose molecules, thereby facilitating the solvent-cellulose interaction and the effective dissolution of cellulose, and the low temperature also helps to maintain the stability of the solution and prevent the recrystallization and precipitation of cellulose. Under the condition of continuous low temperature, the outer layer of cellulose will be dissolved first, and the viscosity of the dissolved cellulose will gradually increase slowly at low temperature, and then urea and NaOH will diffuse to the core of cellulose, thereby completely dissolving the cellulose. Compared with the expensive solvents and processing agents used in traditional methods, the raw material cost and process of the low-temperature sodium hydroxide-urea solution are more competitive.

[0029] (3) In the directional freezing process, the cellulose gel precursor gradually forms ice crystals during the freezing process, and the ice crystal framework formed plays a role in supporting the gel structure, reducing the shrinkage and crack formation of the gel, and can maintain the structural integrity and stability of the aerogel, thereby obtaining a directional pore structure, a nanoscale pore size distribution and an ordered microstructure, so that the aerogel has a large specific surface area and porosity.

[0030] (4) The modified nano-zinc oxide is attached to the inside and surface of the cellulose aerogel, so that the stress area is increased when an external force is applied, thereby effectively dispersing the stress and avoiding stress concentration, and significantly improving the overall compressive strength and rigidity. At the same time, the surface roughness of the aerogel is increased, the hydrophobic group is introduced, and the hygroscopicity of the aerogel is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is a schematic diagram of the mechanism of the directional freezing device used in the present application;

[0032] In the figure, 1 is a liquid nitrogen container, 2 is liquid nitrogen, 3 is a heat preservation shell, 4 is a copper column, 5 is a PP tubular mold, and 6 is a mixed solution; the copper column is vertically installed at the lower end of one end of the PP tubular mold and extends into the liquid nitrogen container, the mold is placed in the liquid nitrogen container, the liquid nitrogen container is located in the heat preservation shell, one end of the copper column is placed in the liquid nitrogen, and a temperature gradient is formed on the surface of the circular copper column;

[0033] Figure 2 is a SEM image of the cross section of the cellulose / nano-zinc oxide composite aerogel prepared in Example 1(a) and Comparative Example 1(b);

[0034] Figure 3 is a SEM image of the cross section of the cellulose / nano-zinc oxide composite aerogel prepared in Example 2(a) and Comparative Example 2(b);

[0035] Figure 4 is a stress-strain curve of the cellulose / nano-zinc oxide composite aerogel prepared in Example 2 and Comparative Example 2;

[0036] Figure 5 is a SEM image of the surface of the cellulose / nano-zinc oxide composite aerogel prepared in Example 3(a) and Comparative Example 3(b);

[0037] Figure 6 is a polarized light microscope image of the cellulose dissolution state in the solution of Example 4(a) and Comparative Example 4(b);

[0038] Figure 7 is a surface temperature-time curve of the aerogel prepared in Example 3 and Comparative Example 5;

[0039] Figure 8Infrared thermal imaging images of aerogels prepared for Example 3 and Comparative Example 5;

[0040] Figure 9 Water contact angle of aerogels prepared for Example 3(a) and Comparative Example 5(b). DETAILED DESCRIPTION

[0041] The technical solutions of the present application are further described below in combination with the drawings and specific examples:

[0042] In the present application, unless specified, the raw materials and equipment used can be purchased from the market or commonly used in the art. The methods in the examples, unless specified, are conventional methods in the art.

[0043] Mechanical performance test steps:

[0044] The compression strength of the composite aerogel was measured using a CTM2050 electronic universal material testing machine to evaluate its mechanical performance. Using a compression clamp with a diameter of 100 mm, the composite aerogel sample was placed in the middle of the compression disc, and then the test was started. The pre-load force was set to 0.005 N, and the compression rate was 0.5 mm / min.

[0045] Thermal insulation performance test steps:

[0046] The thermal insulation performance of the composite aerogel was visualized using a FOTRIC 240M infrared thermal imager. The aerogel was placed in a heat source with a set temperature of 80℃ for 10 min, the surface temperature change of the aerogel was recorded, and the infrared thermal image was taken.

[0047] Hydrophobic performance test steps:

[0048] The composite aerogel was attached to a glass slide, 5 μL of droplet was dropped on the surface of the composite aerogel using the automatic injection system of the contact angle measuring instrument, the water contact angle was analyzed by the contact angle measuring system, and the change of the water contact angle was recorded.

[0049] Example 1

[0050] A preparation method of cellulose / nano-zinc oxide composite aerogel, the specific steps are:

[0051] (1) Preparation of modified nano zinc oxide particles: Nano zinc oxide was added to anhydrous ethanol, the mass ratio of nano zinc oxide to anhydrous ethanol was 1:30, and uniform nano zinc oxide suspension was obtained by using ultrasonic treatment and vigorous stirring. To the suspension, 10:1 of γ-methacryloxypropyltrimethoxysilane relative to the mass of nano zinc oxide was added, and the reaction was stirred in an oil bath for 10 h at a temperature of 80°C. The reaction mixture was ultrasonically centrifuged, the product was collected, washed with anhydrous ethanol, and then placed in an oven for drying to obtain modified nano zinc oxide particles.

[0052] (2) Preparation of cellulose solution: Sodium hydroxide and urea were added to deionized water and stirred until completely dissolved to form a clear and transparent strong alkaline solution. Then, the solution was placed in the freezer compartment of a refrigerator (pre-cooling temperature was -18°C) for pre-cooling for 3 h to prepare a pre-cooled alkali solution containing urea and sodium hydroxide. Cellulose was quickly added to the pre-cooled alkali solution and uniformly stirred at 2°C for 3 h to finally obtain a cellulose solution; wherein the mass ratio of sodium hydroxide, urea, water, and cellulose was 7:26:67:3.

[0053] (3) Preparation of cellulose / nano zinc oxide mixed solution: Modified nano zinc oxide particles were slowly added to the cellulose solution under stirring, the mass ratio of modified nano zinc oxide particles to cellulose was 1:5, and the mixture was uniformly stirred for 1 h to obtain a cellulose / nano zinc oxide mixed solution.

[0054] (4) Preparation of cellulose / nano zinc oxide composite hydrogel: The cellulose / nano zinc oxide mixed solution was poured into a directional freezing device, the directional freezing temperature was -196°C, and the time was 20 min. After taking out, it was placed at room temperature for 8 h. The formed gel was exchanged with ethanol to form a hydrogel, and then washed with deionized water to remove excess solvent until pH=7 to obtain a cellulose / nano zinc oxide composite hydrogel.

[0055] (5) Preparation of cellulose / nano zinc oxide composite aerogel: The obtained cellulose / nano zinc oxide composite hydrogel was placed in a freeze dryer, the temperature of freeze drying was -55°C, and the drying time was 48 h to obtain a cellulose / nano zinc oxide composite aerogel.

[0056] Example 2

[0057] A method for preparing a cellulose / nano zinc oxide composite aerogel, the specific steps are as follows:

[0058] (1) Preparation of modified nano zinc oxide particles: Nano zinc oxide was added to anhydrous ethanol, the mass ratio of nano zinc oxide to anhydrous ethanol was 1:20, and ultrasonic treatment and vigorous stirring were used to obtain a uniform nano zinc oxide suspension. Then, 10:1 (mass ratio) of γ-methacryloxypropyltrimethoxysilane was added to the suspension, and the reaction was stirred in an oil bath at 90°C for 11 h. The reaction mixture was ultrasonically centrifuged, the product was collected, washed with anhydrous ethanol, and then dried in an oven to obtain modified nano zinc oxide particles.

[0059] (2) Preparation of a cellulose solution: Sodium hydroxide and urea were added to deionized water and stirred until completely dissolved to form a clear and transparent strong alkaline solution. Then, the solution was pre-cooled in the freezer compartment of a refrigerator (pre-cooling temperature: -18°C) for 3 h to prepare a pre-cooled alkali solution containing urea and sodium hydroxide. Next, cellulose was quickly added to the pre-cooled alkali solution and uniformly stirred at 2°C for 4 h to finally obtain a cellulose solution, with a mass ratio of sodium hydroxide, urea, water, and cellulose being 7:26:67:3.

[0060] (3) Preparation of a cellulose / nano zinc oxide mixed solution: Modified nano zinc oxide particles were slowly added to the cellulose solution under stirring, with a mass ratio of modified nano zinc oxide particles to cellulose being 1:5, and the mixture was uniformly stirred for 1 h to obtain a cellulose / nano zinc oxide mixed solution.

[0061] (4) Preparation of a cellulose / nano zinc oxide composite hydrogel: The cellulose / nano zinc oxide mixed solution was poured into a directional freezing device, and directional freezing was performed at -196°C for 30 min. After being taken out, the formed gel was allowed to stand at room temperature for 8 h, and then exchanged with ethanol to form a hydrogel. The hydrogel was washed with deionized water to remove excess solvent until the pH reached 7, and a cellulose / nano zinc oxide composite hydrogel was obtained.

[0062] (5) Preparation of a cellulose / nano zinc oxide composite aerogel: The cellulose / nano zinc oxide composite hydrogel was placed in a freeze dryer, and freeze drying was performed at -55°C for 48 h to obtain a cellulose / nano zinc oxide composite aerogel.

[0063] Example 3

[0064] A method for preparing a cellulose / nano zinc oxide composite aerogel, the specific steps of which are as follows:

[0065] (1) Preparation of modified nano zinc oxide particles: Nano zinc oxide was added to anhydrous ethanol, and the mass ratio of nano zinc oxide to anhydrous ethanol was 1:30. Ultrasonic treatment and vigorous stirring were used to obtain a uniform nano zinc oxide suspension. To the suspension, 20:1 of γ-methacryloxypropyltrimethoxysilane relative to the mass of nano zinc oxide was added, and the reaction was stirred in an oil bath at 100°C for 12 h. The reaction mixture was ultracentrifuged, the product was collected, washed with anhydrous ethanol, and then placed in an oven for drying to obtain modified nano zinc oxide particles.

[0066] (2) Preparation of cellulose solution: Sodium hydroxide and urea were added to deionized water and stirred until completely dissolved to form a clear and transparent strong alkaline solution. Then, the solution was placed in the freezer compartment of a refrigerator (pre-cooling temperature -18°C) for pre-cooling for 3 h to prepare a pre-cooled alkali solution containing urea and sodium hydroxide. Next, cellulose was quickly added to the pre-cooled alkali solution and uniformly stirred at 1°C for 5 h to finally obtain a cellulose solution with a mass ratio of sodium hydroxide, urea, water, and cellulose of 7:26:67:3.

[0067] (3) Preparation of cellulose / nano zinc oxide mixed solution: Modified nano zinc oxide particles were slowly added to the cellulose solution under stirring, and the mass ratio of modified nano zinc oxide particles to cellulose was 1:10. The mixture was uniformly stirred for 1 h to obtain a cellulose / nano zinc oxide mixed solution.

[0068] (4) Preparation of cellulose / nano zinc oxide composite hydrogel: The cellulose / nano zinc oxide mixed solution was poured into a directional freezing device, and the directional freezing temperature was -196°C for 30 min. After being taken out, the formed gel was placed at room temperature for 6 h to exchange with ethanol to form a hydrogel. The hydrogel was washed with deionized water to remove excess solvent until pH=7 to obtain a cellulose / nano zinc oxide composite hydrogel.

[0069] (5) Preparation of cellulose / nano zinc oxide composite aerogel: The obtained cellulose / nano zinc oxide composite hydrogel was placed in a freeze dryer, and the freeze-drying temperature was -55°C for 48 h to obtain a cellulose / nano zinc oxide composite aerogel.

[0070] Example 4

[0071] A method for preparing a cellulose / nano zinc oxide composite aerogel, the specific steps are as follows:

[0072] (1) Preparation of modified nano zinc oxide particles: Nano zinc oxide was added to anhydrous ethanol, and the mass ratio of nano zinc oxide to anhydrous ethanol was 1:30. Ultrasonic treatment and vigorous stirring were used to obtain a uniform nano zinc oxide suspension. To the suspension, 20:1 of γ-methacryloxypropyltrimethoxysilane relative to the mass of nano zinc oxide was added, and oil bath stirring was performed for 10 h at a temperature of 100°C. The reaction mixture was ultracentrifuged, the product was collected, washed with anhydrous ethanol, and then placed in an oven for drying to obtain modified nano zinc oxide particles.

[0073] (2) Preparation of a cellulose solution: Sodium hydroxide and urea were added to deionized water and stirred until completely dissolved to form a clear and transparent strong alkaline solution. Then, the solution was pre-cooled in a freezer compartment (pre-cooling temperature of -18°C) for 3 h to prepare a pre-cooled alkali solution containing urea and sodium hydroxide. Next, cellulose was quickly added to the pre-cooled alkali solution and uniformly stirred at 1°C for 4 h to finally obtain a cellulose solution with a mass ratio of sodium hydroxide, urea, water, and cellulose of 7:26:67:3.

[0074] (3) Preparation of a cellulose / nano zinc oxide mixed solution: The modified nano zinc oxide particles were slowly added to the cellulose solution under stirring, and the mass ratio of modified nano zinc oxide particles to cellulose was 1:10. The mixture was uniformly stirred and allowed to react for 1 h to obtain a cellulose / nano zinc oxide mixed solution.

[0075] (4) Preparation of a cellulose / nano zinc oxide composite hydrogel: The cellulose / nano zinc oxide mixed solution was poured into a directional freezing device, and directional freezing was performed at a temperature of -196°C for 20 min. After being taken out, the formed gel was allowed to stand at room temperature for 7 h, and then exchanged with ethanol to form a hydrogel. The hydrogel was washed with deionized water to remove excess solvent until the pH was 7, and a cellulose / nano zinc oxide composite hydrogel was obtained.

[0076] (5) Preparation of a cellulose / nano zinc oxide composite aerogel: The obtained cellulose / nano zinc oxide composite hydrogel was placed in a freeze dryer, and freeze drying was performed at a temperature of -55°C for 48 h to obtain a cellulose / nano zinc oxide composite aerogel.

[0077] Comparative Example 1

[0078] The preparation method and Example 1 were consistent, except that in step (2), the mass ratio of sodium hydroxide, urea, water, and cellulose was 7:26:67:2.

[0079] Figure 2SEM images of the cross-section of the cellulose / nano-ZnO composite aerogels prepared in Example 1(a) and Comparative Example 1(b); the cross-section of the cellulose / nano-ZnO composite aerogel prepared in Example 1 (cellulose content of 3%) showed a compact and uniform pore structure, while the cross-section of the cellulose / nano-ZnO composite aerogel prepared in Comparative Example 1 (cellulose content of 2%) showed a relatively loose and irregular porous structure, which was due to the low cellulose content, resulting in a decrease in the strength of the gel skeleton and leading to the collapse of part of the pore structure.

[0080] Comparative Example 2

[0081] The preparation method was consistent with that of Example 2, except that in step (4), the cellulose / nano-ZnO mixed solution was directly placed in a refrigerator for freezing (temperature of -12°C, time of 12 h), and after being taken out, it was left to stand at room temperature for 8 h. The formed gel was exchanged with ethanol to form a hydrogel, and then washed with deionized water to remove excess solvent until the pH was 7, to obtain a cellulose / nano-ZnO composite hydrogel.

[0082] Figure 3 SEM images of the cross-section of the cellulose / nano-ZnO composite aerogels prepared in Example 2(a) and Comparative Example 2(b); the cross-section of the cellulose / nano-ZnO composite aerogel prepared in Example 2 (directional freezing) showed a uniform and ordered three-dimensional layered porous structure, and the layers were further connected to each other through fibrous substructures. The directional freezing process guided the growth direction of ice crystals, so that the cellulose and nano-ZnO formed a directional pore structure after freeze-drying, thereby enhancing the overall strength and stability of the aerogel. The cellulose / nano-ZnO composite aerogel prepared in Comparative Example 2 (direct freezing) showed a random and porous microstructure, because the formation of ice crystals in the direct freezing process had no directionality. Such structure had no layered arrangement, and the pore size was uneven, resulting in a loose structure.

[0083] Figure 4 Stress-strain curves of the cellulose / nano-ZnO composite aerogels prepared in Example 2 and Comparative Example 2; compared with the cellulose / nano-ZnO composite aerogel prepared in Comparative Example 2, Example 2 showed a higher compressive strength. This was mainly due to the fact that the cellulose formed a stable and dense three-dimensional network structure through hydrogen bonding, thereby enhancing the overall strength of the aerogel. In addition, the multilayered pore structure of the aerogel endowed it with excellent damping performance. When an external force was applied to the composite aerogel, the multilayered structure could effectively dissipate the external force along the pore path layer by layer.

[0084] Comparative Example 3

[0085] The preparation method and example 3 are consistent, the only difference is that step (1) is omitted, the nano zinc oxide is not modified, in step (3), the nano zinc oxide particles are slowly added to the cellulose solution under stirring, the mass ratio of nano zinc oxide particles to cellulose is 1:10, and the cellulose / nano zinc oxide mixed solution is obtained by uniformly stirring and fully mixing for 1h.

[0086] Figure 5 The SEM images of the surface of the cellulose / nano zinc oxide composite aerogel prepared for example 3(a) and comparative example 3(b); the surface nano zinc oxide of example 3 (modified nano zinc oxide) shows uniform dispersity and small particle size. The particle size of the surface nano zinc oxide of comparative example 3 (nano zinc oxide) is large, and it is in a cluster state and unevenly distributed. After modification, an organic coating layer is gradually formed on the surface of the zinc oxide, which significantly reduces the agglomeration tendency between the particles, and greatly improves the dispersity in the matrix.

[0087] Comparative example 4

[0088] The preparation method and example 4 are consistent, the only difference is that in step (2), the pre-freezing and continuous low-temperature stirring process is omitted during the cellulose dissolution process, and the cellulose is directly added to the prepared sodium hydroxide-urea solution to obtain a cellulose solution, and the mass ratio of sodium hydroxide, urea, water and cellulose is 7:26:67:3.

[0089] Figure 6 The polarizing microscope images of the cellulose dissolution state in the solutions of example 4(a) and comparative example 4(b); example 4 (pre-freezing + continuous low-temperature stirring) significantly promotes the dissolution of cellulose in the sodium hydroxide and urea solution. At 0h, both of them are not dissolved, after 2h, most of example 4 is dissolved, while only a small part of comparative example 4 is dissolved. After 4h, the cellulose of example 4 is almost completely dissolved, while comparative example 4 is still not completely dissolved. It is shown that the freezing and continuous low-temperature stirring treatment significantly promotes the dissolution of cellulose in the sodium hydroxide and urea solution, so that the cellulose is almost completely dissolved in a short time. Without these treatments, the dissolution degree of cellulose is low, and the dissolution effect is obviously not as good as that of example 4.

[0090] Comparative example 5

[0091] A preparation method of a cellulose aerogel, the specific steps are as follows:

[0092] The difference from the above example 3 is that no nano zinc oxide particles are added.

[0093] (1) Preparation of cellulose solution: sodium hydroxide and urea were added to deionized water, stirred until completely dissolved, forming a clear and transparent strong alkaline solution. Then, the solution was placed in the freezer compartment of the refrigerator (pre-freezing temperature was -18°C) for pre-freezing for 3h, to prepare a pre-cooled alkali solution containing urea and sodium hydroxide. Then, cellulose was quickly added to the pre-cooled alkali solution, and stirred uniformly at 1°C for 5h, to finally obtain a cellulose solution, with a mass ratio of sodium hydroxide, urea, water, and cellulose of 7:26:67:3.

[0094] (2) Preparation of cellulose hydrogel: the cellulose solution was poured into a directional freezing device, with a directional freezing temperature of -190°C and a time of 30min. After taking out, the formed gel was placed at room temperature for 6h, and then exchanged with ethanol to form a hydrogel. The hydrogel was washed with deionized water to remove excess solvent, until the pH was 7, to obtain a cellulose hydrogel.

[0095] (3) Preparation of cellulose aerogel: the obtained cellulose hydrogel was placed in a freeze dryer, with a freeze-drying temperature of -55°C and a drying time of 48h, to obtain a cellulose aerogel.

[0096] Figure 7 Surface temperature-time curve of the aerogel prepared for Example 3 and Comparative Example 5; within the initial 0-30s, the surface temperature of the aerogel rapidly increased, and then gradually stabilized, finally reaching thermal equilibrium. The final stable temperature of the cellulose aerogel prepared in Comparative Example 5 was significantly higher than that of the composite aerogel prepared in Example 3, which was loaded with nano-zinc oxide and prepared under the same cellulose content conditions. The composite aerogel prepared in Example 3 exhibited more excellent heat insulation effect.

[0097] Figure 8 Infrared thermal imaging image of the aerogel prepared for Example 3 and Comparative Example 5; infrared thermal imaging further showed that the aerogel prepared in Comparative Example 5 and the aerogel prepared in Example 3 had significant differences in heat preservation performance. The heat source covered by the aerogel showed obvious color difference in the imaging. Among them, the color change speed of the aerogel covered area in Example 3 was slower, indicating that its heat preservation and insulation effect was more significant, and the heat preservation and insulation time was also significantly longer.

[0098] Figure 9 Water contact angle of the aerogel prepared for Example 3(a) and Comparative Example 5(b). The contact angle of the cellulose / nano-zinc oxide composite aerogel prepared in Example 3 was more than 120°, and the water droplets were spherical, showing strong hydrophobicity. This indicates that the surface of the aerogel of Example 3 has good hydrophobic performance, while the contact angle of the cellulose aerogel prepared in Comparative Example 5 is close to 0°, and the water droplets completely spread on the surface of the aerogel, showing strong hydrophilicity. The modified nano-zinc oxide increases the surface roughness of the aerogel, introduces hydrophobic groups, reduces the moisture absorption of the aerogel, and has good hydrophobic performance.

[0099] The above embodiments are not intended to limit the scope of the present application, and the described steps are not intended to limit the order of execution. Those skilled in the art will make obvious modifications to the present application in light of the prior art, and such modifications fall within the scope of the claims of the present application.

Claims

1. A method for preparing a cellulose / nano-zinc oxide composite aerogel, characterized in that, Includes the following steps: (1) Add anhydrous ethanol to nano zinc oxide and sonicate, then add γ-methacryloxypropyltrimethoxysilane to react at a temperature of 80-100℃ for 10-12h; finally centrifuge, wash and dry to obtain modified nano zinc oxide particles. (2) Add sodium hydroxide and urea to water, stir until dissolved, and then pre-freeze to obtain a pre-cooled alkaline solution. Then add cellulose to the pre-cooled alkaline solution to react and obtain a cellulose solution. The mass ratio of sodium hydroxide, urea, water and cellulose is 7:26:67:

3. Add cellulose to the pre-cooled alkaline solution and stir to react at 1-3℃ for 3-5 hours. (3) Add the modified zinc oxide nanoparticles to the cellulose solution to obtain a cellulose / zinc oxide mixed solution. Then, place the cellulose / zinc oxide mixed solution in a directional freezing device for treatment, take it out and let it stand, and exchange it with ethanol to form a hydrogel. Wash it to obtain a cellulose / zinc oxide composite hydrogel. (4) The cellulose / nano zinc oxide composite hydrogel was freeze-dried to obtain cellulose / nano zinc oxide composite aerogel.

2. The method for preparing a cellulose / nano-zinc oxide composite aerogel according to claim 1, characterized in that, In step (1), the mass ratio of nano zinc oxide to anhydrous ethanol is 1:20-30.

3. The method for preparing a cellulose / nano-zinc oxide composite aerogel according to claim 1, characterized in that, In step (1), the mass ratio of nano zinc oxide to γ-methacryloxypropyltrimethoxysilane is 10-20:1; the reaction temperature is 80-100℃ and the reaction time is 10-12h.

4. The method for preparing a cellulose / nano-zinc oxide composite aerogel according to claim 1, characterized in that, In step (2), the pre-freezing temperature is -18 to -12℃ and the pre-freezing time is 3 to 5 hours.

5. The method for preparing a cellulose / nano-zinc oxide composite aerogel according to claim 1, characterized in that, In step (3), the mass ratio of modified nano zinc oxide particles to cellulose is 1:(5-10).

6. The method for preparing a cellulose / nano-zinc oxide composite aerogel according to claim 1, characterized in that, In step (3), the directional freezing temperature is -196℃ and the time is 20-30 min.

7. The cellulose / nano zinc oxide composite aerogel prepared by any of the methods described in claims 1-6.

8. The application of the cellulose / nano zinc oxide composite aerogel according to claim 7 in the field of thermal insulation materials.

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