Preparation method of variable-pressure directional freeze-drying gradient cellulose aerogel

Abstract

The invention discloses a preparation method of a variable-pressure directional freeze-drying gradient cellulose aerogel, and the cellulose aerogel with a multilayer structure with alternate low / high refractive indexes is prepared by the method and can be widely applied to the field of radiation refrigeration. The preparation method comprises the following steps: 1) adding cellulose into deionized water, and stirring to obtain a cellulose solution; 2) standing and aging the prepared cellulose solution to eliminate bubbles in the solution; (3) pouring the aged cellulose solution into a pressure-variable directional freezing mold; (4) applying pressure to the cellulose solution by the compressed gas, and forming a compact freezing layer on the bottom surface of the mold; 5) after the compact layer is formed in the step 4), compressing gas in a segmented manner until the cellulose is completely frozen, 6) after the cellulose is completely frozen, inverting the mold, melting the compact layer on the surface, wiping off the molten surface layer, and putting the mold into a refrigerator for storage, and 7) putting the mold stored in the refrigerator into vacuum freeze drying equipment for freeze drying to obtain the gradient cellulose aerogel.

Description

Technical Field

[0001] This invention relates to a method for preparing cellulose aerogel, and more specifically to a method for preparing pressure swing directional freeze-drying gradient cellulose aerogel. Background Technology

[0002] Radiative cooling materials possess excellent reflectivity under the solar spectrum, reducing the temperature rise caused by sunlight. Simultaneously, the materials can radiate heat into outer space through atmospheric transparent windows (8-13 μm) to achieve a cooling effect without energy consumption. Therefore, radiative cooling is a green, clean, and pollution-free cooling method that effectively addresses both environmental and energy consumption issues. As a novel cooling technology, radiative cooling materials offer advantages such as high efficiency, energy saving, and environmental friendliness, and have broad application prospects. Traditional radiative cooling materials mostly utilize high-emissivity metallic or ceramic materials, but these materials suffer from drawbacks such as high density, high cost, and difficulty in processing, limiting their application range.

[0003] Cellulose is one of the most abundant and widely distributed natural polymers on Earth. It is inexpensive, readily available, green, and renewable, with good biodegradability and biocompatibility. The numerous COC, CC, and CH bonds it contains endow it with intrinsic optical properties of low absorption in the solar light band and high emissivity in the infrared band, resulting in excellent radiative cooling performance. Cellulose-based radiative cooling materials, as a novel type of lightweight, high-strength, and low-cost radiative cooling material, can minimize parasitic solar absorption and reduce environmental thermal gain. In the prior art, patent number ZL202310924946.6 discloses a cellulose-based radiative cooling aerogel material and its preparation method. It utilizes the multi-level aggregated structure of natural cellulose and the control of cellulose fiber size to improve the solar reflectivity of pure cellulose-based materials, but its performance still needs further improvement. Simultaneously, research has found that multi-scale porous structures can alter the solar light propagation path, thereby increasing the material's emissivity and reflectivity by increasing the number of light scattering and reflections. Therefore, developing a gradient pore structure of "high-density surface layer - low-density core layer" to prepare cellulose aerogels with alternating low / high refractive index multilayer structures is of great significance for energy conservation, consumption reduction and the development and utilization of renewable resources. Summary of the Invention

[0004] The purpose of this invention is to address the problems and shortcomings of existing technologies and provide a method for preparing gradient cellulose aerogels by pressure swing directional freeze-drying. This method uses cellulose as a raw material and constructs a gradient pore structure of a "high-density surface layer - low-density core layer" through pressure swing directional freeze-drying, preparing a multilayered cellulose aerogel with alternating low / high refractive indices. This gradient cellulose aerogel can be widely used in the field of radiative refrigeration. Furthermore, this preparation method is simple to operate, allows for precise control of freezing pressure, and enables fine-tuning of the aerogel's pore structure design.

[0005] This invention is achieved through the following technical solution: The method for preparing pressure swing directional freeze-dried gradient cellulose aerogel of the present invention includes the following steps: 1) Add cellulose to deionized water and stir to obtain a cellulose solution; 2) Allow the prepared cellulose solution to stand and age to eliminate air bubbles in the solution; 3) Pour the aged cellulose solution into a pressure swing directional freezing mold; 4) Compressed gas applies pressure to the cellulose solution, forming a dense frozen layer on the bottom surface of the mold; 5) After the dense layer in step 4) is formed, compress the gas in stages until the cellulose is completely frozen; 6) After the cellulose is completely frozen, invert the mold, melt the dense surface layer, wipe off the melted surface layer, and then store it in the refrigerator; 7) After the mold is stored in the refrigerator, it is placed in a vacuum freeze-drying device for freeze-drying to obtain gradient cellulose aerogel.

[0006] The preparation method of the pressure swing directional freeze-drying gradient cellulose aerogel of the present invention further includes the following technical solution: the cellulose is one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and bacterial cellulose dispersion, wherein the mass ratio of deionized water to cellulose is 0.3~1:50; and the stirring time is 12~24 h.

[0007] The preparation method of the pressure swing directional freeze-dried gradient cellulose aerogel described above in this invention can be further described by setting the aging time in step 2) to 12-24 h.

[0008] The preparation method of gradient cellulose aerogel by pressure swing directional freeze drying described above in this invention can be further described as follows: the pressure swing directional freezing device includes a liquid nitrogen pool, a copper column, a cylindrical mold and a piston, wherein the top surface of the copper column is flat and the area is larger than that of the cylindrical mold, and the cylindrical mold is transparent with graduations, has no bottom or top and is well sealed with the piston.

[0009] The preparation method of the pressure-switched directional freeze-dried gradient cellulose aerogel described above in this invention can be further described as follows: the volume of the compressed gas in step 4) is 0.4-0.6 of the original volume, the thickness of the dense layer is 2 mm, and the dense layer is required to prevent the cellulose solution from seeping out from the bottom when pressure is applied.

[0010] The preparation method of the pressure-switched directional freeze-dried gradient cellulose aerogel described above in this invention can be further described as follows: the pressure and segment thickness of the segmented compressed gas in step 5) are selected according to the material design requirements, and the magnitude of the applied pressure is calculated by the ratio of the compressed gas volume to the original volume.

[0011] The preparation method of pressure-switched directional freeze-drying gradient cellulose aerogel described above in this invention can be further described as follows: in step 6), after inverting the mold, the top surface of the mold is placed in a liquid nitrogen pool to prevent melting, and when wiping away the melted dense layer, it is parallel to the bottom surface of the mold to prevent damage to the lower cellulose structure.

[0012] The preparation method of the pressure swing directional freeze-drying gradient cellulose aerogel described above in this invention can be further further described as follows: the storage temperature in the refrigerator is -25 ~ -18℃, and the storage time is 12-24h; the freeze-drying time in the vacuum freeze-drying equipment is 24-48h.

[0013] Compared with the prior art, the present invention has the following advantages: (1) Compared with the existing cellulose aerogel, the present invention constructs a gradient pore structure of "high-density surface layer - low-density core layer" by pressure swing directional freeze drying, which increases the number of light scattering and reflection of cellulose aerogel, improves the emissivity and reflectivity of cellulose aerogel material, and improves the mechanical properties of aerogel, making it better applicable to practical scenarios.

[0014] (2) The process of this invention is simple, green and environmentally friendly, and the pressure control relies on the calculation of compressed air volume, which is precise and controllable. Attached Figure Description

[0015] Figure 1 Schematic diagram of the principle and structure of the pressure swing directional freeze drying device Figure 1 In the middle: 1-directional freezing mold, 2- copper pillar, 3- foam box, 4- liquid nitrogen, 5- piston Figure 2 The graph shows the compressive strength test data of the sample prepared in Example 1. Figure 3 SEM image of the gradient cellulose aerogel prepared in Example 1 Figure 4 SEM image of the gradient cellulose aerogel prepared in Example 2 Figure 5 SEM image of the gradient cellulose aerogel prepared in Example 3. Figure 6 SEM image of the gradient cellulose aerogel prepared in Example 4 Figure 7 SEM image of the gradient cellulose aerogel prepared in Example 5 Figure 8 SEM image of the gradient cellulose aerogel prepared in Example 6 Detailed Implementation

[0016] Example 1 (1) Weigh 0.3 g of carboxymethyl cellulose and add it to 50 g of deionized water. Stir for 12 h to obtain a cellulose solution; (2) Let the obtained cellulose solution stand for 12 hours to eliminate the bubbles generated during stirring; (3) Add the aged cellulose solution into the directional freezing device, compress the gas in segments with a segment thickness of 3:6:2 mm and a gas compression ratio of 0.5:1:0.5; (4) After inverting the obtained directionally frozen sample, melt the surface layer, which is about 1 mm thick. Wipe off the melted surface layer and store it in a refrigerator at -18 ℃ for 12 h. Then, freeze-dry the sample under vacuum for 24 h to obtain a gradient cellulose aerogel. Figure 3 As shown, the prepared gradient cellulose aerogel has a visible light reflectance of 90%, an infrared emissivity of 92% in the 8-13 μm band, and a compressive strength of 30 kPa.

[0017] Example 2 (1) Weigh 1 g of sodium carboxymethyl cellulose and add it to 50 g of deionized water. Stir for 24 h to obtain a cellulose solution. (2) Let the obtained cellulose solution stand for 12 hours to eliminate the bubbles generated during stirring; (3) Add the aged cellulose solution into the directional freezing device, compress the gas in segments with a segment thickness of 3:6:2 mm and a gas compression ratio of 0.8:1:0.8; (4) After inverting the obtained directionally frozen sample, melt the surface layer, which is about 1 mm thick. Wipe off the melted surface layer and store it in a refrigerator at -18 ℃ for 24 h. Then, freeze-dry the sample under vacuum for 24 h to obtain a gradient cellulose aerogel. Figure 4 As shown.

[0018] Example 3 (1) Weigh 0.3g of bacterial cellulose and add it to 50g of deionized water. Stir for 12 h to obtain a cellulose solution; (2) Let the obtained cellulose solution stand for 24 hours to eliminate the bubbles generated during stirring; (3) Add the aged cellulose solution into the directional freezing device, compress the gas in segments with a segment thickness of 3:6:2 mm and a gas compression ratio of 0.5:1:0.5; (4) After inverting the obtained directionally frozen sample, melt the surface layer, which is about 1 mm thick. Wipe off the melted surface layer and store it in a refrigerator at -25 ℃ for 12 h. Then, freeze-dry the sample under vacuum for 24 h to obtain a gradient cellulose aerogel. Figure 5 As shown.

[0019] Example 4 (1) Weigh 0.5 g of bacterial cellulose and add it to 50 g of deionized water. Stir for 24 h to obtain a cellulose solution; (2) Let the obtained cellulose solution stand for 24 hours to eliminate the bubbles generated during stirring; (3) Add the aged cellulose solution into the directional freezing device, compress the gas in segments with a segment thickness of 3:6:2 mm and a gas compression ratio of 0.5:1:0.5; (4) After inverting the obtained directionally frozen sample, melt the surface layer, which is about 1 mm thick. Wipe off the melted surface layer, store it in a refrigerator at -25 ℃ for 24 h, and then freeze-dry the sample under vacuum for 48 h; thus, a gradient cellulose aerogel is prepared. Figure 6 As shown.

[0020] Example 5 (1) Weigh 1g of bacterial cellulose and add it to 50g of deionized water. Stir for 24 h to obtain a cellulose solution; (2) Let the obtained cellulose solution stand for 24 hours to eliminate the bubbles generated during stirring; (3) Add the aged cellulose solution into the directional freezing device, compress the gas in segments with a segment thickness of 3:6:2 mm and a gas compression ratio of 0.8:1:0.8; (4) After inverting the obtained directionally frozen sample, melt the surface layer, which is about 1 mm thick. Wipe off the melted surface layer and store it in a refrigerator at -18 ℃ for 12 h. Then, freeze-dry the sample under vacuum for 60 h to obtain a gradient cellulose aerogel. Figure 7 As shown.

[0021] Example 6 (1) Weigh 1 g of bacterial cellulose and add it to 50 g of deionized water. Stir for 12 h to obtain a cellulose solution. (2) Let the obtained cellulose solution stand for 12 hours to eliminate the bubbles generated during stirring; (3) Add the aged cellulose solution into the directional freezing device, compress the gas in segments with a segment thickness of 3:6:2 mm and a gas compression ratio of 0.5:1:0.5; (4) After inverting the obtained directionally frozen sample, melt the surface layer, which is about 1 mm thick. Wipe off the melted surface layer and store it in a refrigerator at -18 ℃ for 12 h. Then, freeze-dry the sample under vacuum for 60 h to obtain a gradient cellulose aerogel. Figure 8 As shown.

Claims

1. A method for preparing pressure swing directional freeze-drying gradient cellulose aerogel, characterized in that, Includes the following steps: 1) Add cellulose to deionized water and stir to obtain a cellulose solution; 2) Allow the prepared cellulose solution to stand and age to eliminate air bubbles in the solution; 3) Pour the aged cellulose solution into a pressure swing directional freezing mold; 4) Compressed gas applies pressure to the cellulose solution, forming a dense frozen layer on the bottom surface of the mold; 5) After the dense layer in step 4) is formed, compress the gas in stages until the cellulose is completely frozen; 6) After the cellulose is completely frozen, invert the mold, melt the dense surface layer, wipe off the melted surface layer, and then store it in the refrigerator; 7) After the mold is stored in the refrigerator, it is placed in a vacuum freeze-drying device for freeze-drying to obtain gradient cellulose aerogel.

2. The method for preparing pressure-swing directional freeze-drying gradient cellulose aerogel according to claim 1, characterized in that, The cellulose is one of carboxymethyl cellulose, sodium carboxymethyl cellulose, or bacterial cellulose dispersion, wherein the mass ratio of deionized water to cellulose is 0.3~1:50; and the stirring time is 12~24 h.

3. The method for preparing pressure-swing directional freeze-drying gradient cellulose aerogel according to claim 1, characterized in that, The aging time mentioned in step 2) is 12~24 h.

4. The method for preparing pressure-swing directional freeze-drying gradient cellulose aerogel according to claim 1, characterized in that, The aforementioned variable pressure directional freezing device includes a liquid nitrogen pool, a copper column, a cylindrical mold, and a piston. The top surface of the copper column is flat and its area is larger than that of the cylindrical mold. The cylindrical mold is transparent with graduations, has no bottom or top, and is well sealed with the piston.

5. The method for preparing pressure-swing directional freeze-drying gradient cellulose aerogel according to claim 1, characterized in that, The compressed gas volume mentioned in step 4) is 0.4-0.6 of the original volume, the thickness of the dense layer is 2mm, and the dense layer must prevent the cellulose solution from seeping out from the bottom when pressure is applied.

6. The method for preparing pressure-swing directional freeze-drying gradient cellulose aerogel according to claim 1, characterized in that, The pressure and segment thickness of the segmented compressed gas mentioned in step 5) are selected according to the material design requirements, and the applied pressure is calculated from the ratio of the compressed gas volume to the original volume.

7. The method for preparing gradient cellulose aerogel by pressure swing directional freeze-drying according to claim 1, characterized in that, In step 6), after the mold is inverted, the top surface of the mold is placed in a liquid nitrogen pool to prevent melting. When wiping away the melted dense layer, the mold should be parallel to the bottom surface to prevent damage to the underlying cellulose structure.

8. The method for preparing pressure-swing directional freeze-drying gradient cellulose aerogel according to claim 1, characterized in that, The storage temperature in the refrigerator is -25 ~ -18℃, and the storage time is 12-24h; the freeze-drying time in the vacuum freeze-drying equipment is 24-48h.

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