Radiation cooling aerogel imitating loofah sponge and preparation method thereof

Abstract

The invention belongs to the technical field of building energy-saving materials, and relates to a loofah sponge imitating radiation cooling aerogel, which is formed by compounding cellulose nanofibers, polyvinyl alcohol and silica sol through hydrogen bond crosslinking and freeze drying self-assembly, and has a three-dimensional microfiber network imitating a natural loofah sponge continuous fiber interweaving form, the cellulose nanofibers and the polyvinyl alcohol are crosslinked through hydrogen bonds to form a network framework with a node connectivity multi-scale pore structure, and the surface of the network framework is coated with an amorphous silicon dioxide layer to form the multi-scale pore structure containing large-size pores. The multi-scale pore structure comprises micron-scale pores matched with the wavelength of sunlight, and Mie scattering and high solar reflectivity can be achieved easily. The invention also discloses a preparation method. The aerogel disclosed by the invention is rich in raw material source, renewable, biodegradable, simple in preparation process and excellent in mechanical property, and integration of various properties such as high mechanical strength, high reflection of sunlight wave bands, hydrophobicity and the like is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of building energy-saving materials, and relates to a cooling aerogel, in particular to a luffa-like radiation cooling aerogel and a preparation method thereof. BACKGROUND

[0002] At present, building refrigeration accounts for about 36% of the total global energy consumption and 39% of the global carbon emissions, and will continue to increase in the next ten years. Traditional building refrigeration relies on air conditioners that consume a large amount of non-renewable fossil energy, resulting in irreversible environmental hazards including global warming and urban heat island effect. Therefore, there is an urgent need for a sustainable alternative to traditional refrigeration technology. Passive radiative cooling (PRC) technology, which reflects excess solar radiation (0.3-2.5 μm) and emits internal heat through the atmospheric transparent window (8-13 μm) to the cold outer space (3K) to maximize heat dissipation, has emerged as a promising strategy to achieve zero-energy cooling.

[0003] Cellulose has attracted extensive attention due to its abundant source, biodegradability, rich functional groups, and adjustable morphology. Recently, a large number of cellulose-based PRC materials have been developed, including coatings, films, aerogels, etc. Among them, cellulose aerogels with high porosity are one of the promising building insulation candidates due to their ability to hinder heat transfer. Notably, cellulose aerogels have great potential in radiative cooling because their abundant C-O-C and C-O chemical bonds provide high emissivity in the atmospheric transparent window (8-13 μm), which is crucial for efficient radiative cooling.

[0004] During the preparation of cellulose aerogels, the reorganization and assembly of hydrogen bonds in cellulose molecules easily form discontinuous short fiber structures in the aerogel network, ultimately leading to reduced structural stability. Luffa sponge is a natural three-dimensional porous network material composed of a large number of continuous fibers. The close connection and entanglement between the fibers of this material enhance the overall mechanical strength and stability. This natural structure provides inspiration for solving the brittleness and instability problems of existing cellulose-based aerogels. Therefore, the present application is inspired by this natural structure and aims to develop a cellulose-based aerogel with a luffa-like continuous fiber network structure to solve the problem of brittleness and poor stability of cellulose aerogels in the prior art, while achieving efficient radiative cooling performance. SUMMARY

[0005] In view of the problems existing in the prior art, the present application aims to provide a luffa-like radiation cooling aerogel.

[0006] TECHNICAL SOLUTION

[0007] A loofah-like radiation cooling aerogel is formed by self-assembly of cellulose nanofibers, polyvinyl alcohol and silica sol through hydrogen bond crosslinking and freeze-drying. It has a three-dimensional microfiber network that imitates the interwoven morphology of natural loofah continuous fibers. The cellulose nanofibers and polyvinyl alcohol are cross-linked through hydrogen bonds to form a network framework with a multi-scale pore structure with node connectivity. The surface of the network framework is coated with an amorphous silica layer to form a multi-scale pore structure containing large-sized pores. The multi-scale pore structure contains micron-sized pores that match the wavelength of sunlight, which is conducive to achieving Mie scattering and high solar reflectivity.

[0008] In a preferred embodiment of the present invention, the cellulose nanofiber is one or more combinations of sulfonated nanocellulose, carboxylated nanocellulose, bacterial cellulose, microfibrillated cellulose, etc., preferably sulfonated nanocellulose, and the fiber diameter is 200-1000 nm and the length is 1-5 μm.

[0009] In a preferred embodiment of the present invention, the weight average molecular weight of the polyvinyl alcohol is greater than 35,000.

[0010] In a preferred embodiment of the present invention, the loofah-like radiation cooling aerogel has an average reflectivity of >95% in the sunlight band (0.3-2.5 μm), a water contact angle of 130-140°, and a density of 13-15 mg / cm 3 , mechanical strength is greater than 20 MPa.

[0011] The second object of the present invention is to disclose a method for preparing the loofah-like radiation cooling aerogel.

[0012] A method for preparing a loofah-like radiation cooling aerogel comprises the following steps:

[0013] a) mixing a cellulose nanofiber dispersion, a silica sol, and a polyvinyl alcohol solution in a volume ratio of 1:0.1 to 1:0.01 to 0.1, preferably 1:0.5:0.05, and stirring for 1 to 10 hours. After the mixture is formed, pre-freezing and freeze-drying are performed to obtain an aerogel;

[0014] b) The aerogel, water, and silane are sealed in a glass container without direct contact, and vapor-deposited at 60-100° C. for 5-10 hours to obtain the aerogel, wherein the mass-to-volume ratio of the aerogel, water, and silane is 0.1 g: 1-3 mL: 1-3 mL.

[0015] According to a preferred embodiment of the present invention, in step a), the cellulose nanofiber dispersion is prepared by dispersing 5 to 20 g of cellulose nanofibers per liter of water.

[0016] Further, the cellulose nanofiber is one or a combination of sulfonated nanocellulose, carboxylated nanocellulose, bacterial cellulose, microfibrillated cellulose, etc., with a diameter of 200-1000 nm and a length of 1-5 μm.

[0017] In the preferred disclosure of the application, in step a), the silica sol is prepared as follows: 90-110 g of wollastonite is added to each liter of water, 40-80 mL of 98 wt% H2SO4 is added dropwise, and the mixture is stirred vigorously at 30-35 ℃ for 20-40 min, filtered, and the filtrate is collected.

[0018] The filtrate is a colloidal solution containing amorphous SiO2.

[0019] The reaction equation is CaSiO3 + H2SO4→ SiO2 + CaSO4 + H2O.

[0020] In the preferred disclosure of the application, in step a), the polyvinyl alcohol solution is prepared as follows: 20-40 g of polyvinyl alcohol is added to each liter of water, and the mixture is stirred at 85-95 ℃ for 2-5 h.

[0021] In the preferred disclosure of the application, in step a), the pre-freezing process parameters are -40 to -20 ℃ for 5-15 h, preferably -20 ℃ for 10 h.

[0022] In the preferred disclosure of the application, in step a), the freeze-drying process parameters are -100 to -50 ℃ for 12-72 h, with a vacuum degree of 10-50 Pa, preferably -50 ℃ for 48 h and 30 Pa.

[0023] In the preferred disclosure of the application, in step b), the silane is any one of methyltrimethoxysilane, hexadecyltrimethoxysilane, octadecyltrichlorosilane, trifluorooctyltriethoxysilane, and hexamethyldisilazane, preferably methyltrimethoxysilane.

[0024] In the preferred disclosure of the application, in step b), the non-direct contact means that the aerogel is suspended above a container containing a mixture of water and silane, so that the aerogel is exposed to the vapor environment, avoiding the collapse of the structure caused by the liquid phase of silane.

[0025] In the present application, cellulose nanofiber and polyvinyl alcohol are used as a porous network framework, amorphous silicon dioxide is wrapped on the surface, and polyvinyl alcohol is used as a crosslinking agent to enhance the interaction between cellulose nanofiber. The obtained aerogel integrates the advantages of sustainability, excellent mechanical properties, dual-band high selectivity response and self-cleaning performance. These characteristics make the biomimetic cellulose-based aerogel a promising tough and lightweight material to replace petroleum-based polymers, thereby promoting the sustainable development of cellulose-based materials with environmental benefits.

[0026] Advantages

[0027] The application discloses a loofah-like radiative cooling aerogel and a preparation method thereof, and realizes high mechanical strength, high reflection of sunlight and hydrophobicity (self-cleaning performance), and has the advantages of performance integration. Polyvinyl alcohol and the hydroxyl groups on the surface of cellulose are chemically crosslinked, effectively limiting the movement of cellulose molecular chains, so that a stable three-dimensional network structure is formed. The pores and fibers with a size distribution of 0.3-2.5 μm in the aerogel match the wavelength of sunlight, can realize Mie scattering, and are beneficial to high solar reflectance. The aerogel is rich in raw material sources, renewable and biodegradable, has a simple preparation process and excellent mechanical properties, realizes the integration of high mechanical strength, high reflection of sunlight and hydrophobicity (self-cleaning performance) and the like. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 . SEM of the loofah-like radiative cooling aerogel prepared in Example 3, wherein a) multi-scale pores, b) high node connectivity;

[0029] Figure 2 . Water contact angle of the loofah-like radiative cooling aerogel before hydrophobic modification in Example 3;

[0030] Figure 3 . Water contact angle of the loofah-like radiative cooling aerogel after hydrophobic modification in Example 3. DETAILED DESCRIPTION

[0031] The application will be described in detail below with reference to examples, so that those skilled in the art can better understand the application, but the application is not limited to the following examples.

[0032] Example 1

[0033] A preparation method of a loofah-like radiative cooling aerogel comprises the following steps:

[0034] a) 5 g of cellulose nanofiber is dispersed in 1 liter of water to obtain a cellulose nanofiber dispersion liquid;

[0035] b) 90 g of wollastonite is added to 1 liter of water, 40 mL of H2SO4 with a concentration of 98wt% is added dropwise, 40 min of intense stirring is performed at 30℃, the filtrate is collected by filtration, and a silica sol is obtained;

[0036] c) 20 g of polyvinyl alcohol is added to 1 liter of water, 5 h of stirring is performed at 85℃, and a polyvinyl alcohol solution is obtained;

[0037] d) mixing and stirring the cellulose nanofiber dispersion, silica sol and polyvinyl alcohol solution obtained above according to a volume ratio of 1:0.1:0.01 for 1 h, and obtaining aerogel through pre-freezing and freeze-drying;

[0038] e) according to 0.1 mL of water and 0.1 mL of methyltrimethoxysilane per gram of aerogel, placing the aerogel, water and methyltrimethoxysilane in a sealed glass container without direct contact, and reacting at 60°C for 10 h to obtain a loofah-like radiative cooling aerogel.

[0039] The prepared aerogel has a reflectivity of 95.8% in a wavelength range of 300-2500 nm, a water contact angle of 130.5°, a density of 14.2 mg / cm 3 , and a mechanical strength of 22.3 MPa.

[0040] Example 2

[0041] A preparation method of a loofah-like radiative cooling aerogel, comprising the following steps:

[0042] a) obtaining a cellulose nanofiber dispersion according to 8 g of cellulose nanofiber dispersed in 1 L of water;

[0043] b) adding 95 g of wollastonite to 1 L of water, dropwise adding 50 mL of H2SO4 with a concentration of 98wt%, stirring vigorously at 32°C for 35 min, collecting the filtrate by filtration, and obtaining a silica sol;

[0044] c) adding 25 g of polyvinyl alcohol to 1 L of water, stirring at 88°C for 4 h, and obtaining a polyvinyl alcohol solution;

[0045] d) mixing and stirring the cellulose nanofiber dispersion, silica sol and polyvinyl alcohol solution obtained above according to a volume ratio of 1:0.3:0.03 for 3 h, and obtaining aerogel through pre-freezing and freeze-drying;

[0046] e) according to 0.5 mL of water and 0.5 mL of hexadecyltrimethoxysilane per gram of aerogel, placing the aerogel, water and hexadecyltrimethoxysilane in a sealed glass container without direct contact, and reacting at 70°C for 8 h to obtain a loofah-like radiative cooling aerogel.

[0047] The aerogel has a reflectivity of 97.5% in a wavelength range of 300-2500 nm, a water contact angle of 131.7°, a density of 14.8 mg / cm 3 , and a mechanical strength of 23.1 MPa.

[0048] Example 3

[0049] A preparation method of a loofah-like radiative cooling aerogel, comprising the following steps:

[0050] a) dispersing 11 g of cellulose nanofibers per liter of water to obtain a cellulose nanofiber dispersion;

[0051] b) Add 60 mL of 98 wt% H2SO4 dropwise to 105 g of wollastonite per liter of water, stir vigorously at 33°C for 30 minutes, and collect the filtrate by filtration to obtain a silica sol;

[0052] c) adding 30 g of polyvinyl alcohol per liter of water and stirring at 90°C for 3 hours to obtain a polyvinyl alcohol solution;

[0053] d) mixing the cellulose nanofiber dispersion, silica sol, and polyvinyl alcohol solution obtained above at a volume ratio of 1:0.5:0.05 and stirring for 5 h, followed by pre-freezing and freeze-drying to obtain an aerogel;

[0054] e) The aerogel, water, and octadecyltrichlorosilane were sealed in a glass container without direct contact, using 0.8 mL of water and 0.8 mL of octadecyltrichlorosilane per gram of aerogel, and reacted at 80°C for 5 h to obtain a loofah-like radiation cooling aerogel.

[0055] like Figure 1 Shown are scanning electron microscope (SEM) photos of the microstructure of the prepared aerogel, where a) shows a biomimetic network structure with multi-scale pores; b) focuses on the high node connectivity at the fiber intersections, simulating the continuous fiber entanglement structure of loofah, which is composed of a honeycomb-like porous structure and interconnected fibers, achieving high node connectivity and rigid support at the fiber intersections, providing both elasticity and compressive strength.

[0056] Figure 2 The water contact angle of the loofah-like radiation cooling aerogel before hydrophobic modification is 33.2°. Figure 3 This is the water contact angle (132.4°) of the same loofah-like radiation cooling aerogel after hydrophobic modification in step e).

[0057] The aerogel has a reflectivity of 98% in the wavelength range of 300-2500 nm, a water contact angle of 132.4°, and a density of 13.8 mg / cm 3 , the mechanical strength is 24.4 MPa.

[0058] Example 4

[0059] A method for preparing a loofah-like radiation cooling aerogel comprises the following steps:

[0060] a) dispersing 15 g of cellulose nanofibers per liter of water to obtain a cellulose nanofiber dispersion;

[0061] b) 70 mL of 98wt% H2SO4 was added dropwise, 34℃ stirring for 25 min, and the filtrate was collected by filtration to obtain a silica sol, wherein the amount of wollastonite added was 108 g per liter of water;

[0062] c) 40 g of polyvinyl alcohol was added per liter of water, and stirring was performed at 95℃ for 1 h to obtain a polyvinyl alcohol solution;

[0063] d) The cellulose nanofiber dispersion, the silica sol and the polyvinyl alcohol solution obtained above were mixed and stirred at a volume ratio of 1:0.8:0.08 for 8 h, and the aerogel was obtained by pre-freezing and freeze-drying;

[0064] e) The aerogel, water and trifluorooctyltriethoxysilane were sealed in a glass container without direct contact, and the reaction was performed at 90℃ for 3 h, to obtain a luffa-like radiative cooling aerogel, wherein the amount of water and trifluorooctyltriethoxysilane was 1.5 mL per gram of the aerogel.

[0065] The reflectivity of the aerogel in the wavelength range of 300-2500 nm was 97.1%, the water contact angle was 132.2°, the density was 14.5 mg / cm 3 , and the mechanical strength was 24.1 MPa.

[0066] Example 5

[0067] A preparation method of a luffa-like radiative cooling aerogel, comprising the following steps:

[0068] a) 20 g of cellulose nanofiber was dispersed per liter of water to obtain a cellulose nanofiber dispersion;

[0069] b) 80 mL of 98wt% H2SO4 was added dropwise, 35℃ stirring for 20 min, and the filtrate was collected by filtration to obtain a silica sol, wherein the amount of wollastonite added was 110 g per liter of water;

[0070] c) 40 g of polyvinyl alcohol was added per liter of water, and stirring was performed at 95℃ for 1 h to obtain a polyvinyl alcohol solution;

[0071] d) The cellulose nanofiber dispersion, the silica sol and the polyvinyl alcohol solution obtained above were mixed and stirred at a volume ratio of 1:1:0.1 for 10 h, and the aerogel was obtained by pre-freezing and freeze-drying;

[0072] e) The aerogel, water and hexamethyldisilazane were sealed in a glass container without direct contact, and the reaction was performed at 100℃ for 1 h, to obtain a luffa-like radiative cooling aerogel, wherein the amount of water and hexamethyldisilazane was 2 mL per gram of the aerogel.

[0073] The aerogel has a reflectivity of 96.2% in the wavelength range of 300-2500 nm, a water contact angle of 130.9°, and a density of 13.9 mg / cm 3 , the mechanical strength is 24.1 MPa.

[0074] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the description of the present invention, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A loofah-like radiation cooling aerogel, composed of cellulose nanofibers, polyvinyl alcohol, and silica sol self-assembled through hydrogen bond crosslinking and freeze-drying, has a three-dimensional microfiber network that mimics the interwoven morphology of natural loofah continuous fibers, characterized by: The cellulose nanofibers and polyvinyl alcohol are cross-linked through hydrogen bonds to form a network framework with a multi-scale pore structure having node connectivity. The surface of the network framework is coated with an amorphous silica layer to form a multi-scale pore structure containing large-sized pores. The multi-scale pore structure contains micron-sized pores that match the wavelength of sunlight, which is conducive to achieving Mie scattering and high solar reflectivity.

2. The loofah-like radiation cooling aerogel according to claim 1, wherein: The cellulose nanofibers are one or more of sulfonated nanocellulose, carboxylated nanocellulose, bacterial cellulose, and microfibrillated cellulose, preferably sulfonated nanocellulose, with a fiber diameter of 200-1000 nm and a length of 1-5 μm.

3. The loofah-like radiation cooling aerogel according to claim 1, wherein: The weight average molecular weight of the polyvinyl alcohol is greater than 35,000.

4. The loofah-like radiation cooling aerogel according to claim 1, wherein: The loofah-like radiation cooling aerogel has an average reflectivity in the sunlight band greater than 95%, a water contact angle of 130-140°, and a density of 13-15 mg / cm 3 , mechanical strength is greater than 20 MPa.

5. A method for preparing the loofah-like radiation cooling aerogel according to any one of claims 1 to 4, characterized in that: The steps include: a) mixing a cellulose nanofiber dispersion, a silica sol, and a polyvinyl alcohol solution in a volume ratio of 1:0.1 to 1:0.01 to 0.1, preferably 1:0.5:0.05, and stirring for 1 to 10 hours. After the mixture is formed, pre-freezing and freeze-drying are performed to obtain an aerogel; b) The aerogel, water, and silane are sealed in a glass container without direct contact, and vapor-deposited at 60-100° C. for 5-10 hours to obtain the aerogel, wherein the mass-to-volume ratio of the aerogel, water, and silane is 0.1 g: 1-3 mL: 1-3 mL.

6. The preparation method of the loofah-like radiation cooling aerogel according to claim 5, wherein: In step a), the cellulose nanofiber dispersion is prepared by dispersing 5 to 20 g of cellulose nanofibers in one liter of water.

7. The preparation method of the loofah-like radiation cooling aerogel according to claim 5, wherein: In step a), the silica sol is prepared as follows: 90-110 g of wollastonite is added per liter of water, 40-80 mL of 98 wt% H2SO4 is added dropwise, the mixture is vigorously stirred at 30-35°C for 20-40 min, filtered, and the filtrate is collected to obtain the silica sol.

8. The preparation method of the loofah-like radiation cooling aerogel according to claim 5, wherein: In step a), the polyvinyl alcohol solution is prepared by adding 20-40 g of polyvinyl alcohol per liter of water and stirring at 85-95° C. for 2-5 hours.

9. The method for preparing the loofah-like radiation cooling aerogel according to claim 5, wherein: In step a), the pre-freezing process parameters are -40~-20°C, 5~15h, preferably -20°C, 10h; the freeze-drying process parameters are -100~-50°C, 12~72h, and the vacuum degree is 10~50Pa, preferably -50°C, 48h, and 30Pa.

10. The method for preparing the loofah-like radiation cooling aerogel according to claim 5, wherein: In step b), the silane is any one of methyltrimethoxysilane, hexadecyltrimethoxysilane, octadecyltrichlorosilane, trifluorooctyltriethoxysilane, and hexamethyldisilazane, preferably methyltrimethoxysilane.

Images