Preparation method of a multi-level pore structure radiative cooling film material

Abstract

The application provides a preparation method of a multi-level pore structure radiation refrigeration film. Hollow inorganic microspheres are embedded in a porous film by a non-solvent phase separation method to construct a multi-level pore structure. The multi-level pore film has high reflectivity (>92%) and emissivity (>84%), can reflect solar radiation and generate infrared radiation, and realizes refrigeration effect. Meanwhile, the multi-level pore structure greatly reduces the thermal conductivity (<30 mW / mk) of the film, can weaken the heat exchange / heat conduction between the material and the outside world, and realizes effective radiation refrigeration in a high-temperature environment. It is worth noting that in the preparation of the hollow inorganic microspheres, inorganic materials with ultraviolet absorption performance are selected, which can effectively improve the weather resistance and stability of the reflectivity of the polymer film, and avoid the yellowing phenomenon of the film caused by long-term irradiation of outdoor ultraviolet light. The film material obtained by the method has excellent refrigeration effect, strong long-term stability and wide application field, can realize energy saving and emission reduction and environmental protection.

Description

Technical Field

[0001] The invention relates to a method for preparing a multi-level porous structure radiation cooling thin film material, and particularly to the design and preparation of a novel material for use in radiation cooling and thermal insulation. Background Technology

[0002] Due to climate change, the frequency of extreme weather events (such as heat waves and high temperatures) has been increasing in recent years, causing catastrophic impacts on human health, infrastructure, and the environment. Traditional cooling technologies (such as air conditioning) are energy-intensive and lead to increased greenhouse gas emissions, exacerbating climate change. Therefore, developing sustainable and efficient cooling technologies has become crucial. Radiative cooling technology is an effective method with high cooling capacity, requiring no additional energy supply and being environmentally friendly during use, thus showing promise in addressing the global climate crisis. Radiative cooling achieves its effect by emitting infrared radiation into space, lowering the surface temperature of an object. For effective cooling, materials must have high emissivity in the atmospheric window zone (8-15 μm) and their surfaces must reflect solar radiation (0.3-2.5 μm). By radiating infrared radiation into space and reflecting solar radiation, objects can achieve highly efficient cooling.

[0003] However, conventional radiative cooling materials lack low thermal conductivity, rendering them ineffective in high-temperature environments and exhibiting poor thermal insulation and cooling performance. Furthermore, existing radiative cooling primarily utilizes polymer-based materials, which are susceptible to accelerated corrosion from ultraviolet (UV) radiation. Prolonged outdoor use exposes these materials to UV radiation, causing not only yellowing of the surface but also reduced physical and chemical stability, leading to decreased cooling performance and shortened lifespan. Therefore, developing radiative cooling materials with low thermal conductivity and high durability is crucial for achieving long-term stable cooling performance, extending material lifespan, and coping with harsh environmental conditions. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a multi-level porous structure radiation-cooling thin film material. This thin film material has low thermal conductivity, enabling radiation cooling in high-temperature environments and maintaining long-term stability in outdoor environments.

[0005] This invention proposes a method for preparing a hierarchical porous structure radiation cooling thin film material. The raw materials for this thin film material include: (a) at least one hollow inorganic microsphere with a particle size of 10-500 nm, (b) at least one organic resin with a pore size of 500 nm-50 µm, (c) a solvent, and (d) optional additives. The weight percentages of each component are as follows: hollow inorganic microspheres 1-20 wt%, organic resin 10-50 wt%, and optional additives 0-10 wt%. The remainder is solvent, and the total weight of the raw materials is 100%. The raw materials (a)-(d) above are blended, and hollow inorganic microspheres of 10-500 nm are embedded in porous organic resin with a pore size of 500 nm-50 µm by a non-solvent phase separation method to prepare a radiation cooling thin film material with a hierarchical pore structure. The pore structure of different levels can effectively reflect radiation of specific wavelengths, increase the specific surface area of ​​the thin film material, improve the reflectivity of the thin film material to sunlight (>92%) and the emissivity to atmospheric windows (>84%), and increase the radiation cooling efficiency of the thin film material. The hierarchical pore structure can increase the internal cavity volume of the thin film, so that its thermal conductivity is lower than 30 mW / mk.

[0006] The specific steps for preparing thin film materials are as follows:

[0007] (1) Preparation of hollow inorganic microspheres / organic resin composite: Mix 10-50g of organic resin and 5-450g of solvent, stir at 0-60℃ until a homogeneous solution is obtained to obtain an organic resin solution; mix 0.5-100g of hollow inorganic microspheres, 15-500g of organic resin solution and 0-50g of non-essential additives, stir at room temperature for 10-20 minutes to obtain hollow inorganic microspheres / organic resin composite;

[0008] (2) Preparation of hierarchical porous radiation cooling thin film material: Using a non-solvent phase separation method, the hollow inorganic microspheres / organic resin composite obtained in step (1) is poured into a groove mold and left to stand at room temperature for 0.1-12 hours. Then, the entire mold is immersed in water and left for 0.1-100 hours for phase separation. The film is then removed from the mold and dried at 0-120℃ for 0.1-100 hours to obtain the hierarchical porous radiation cooling thin film material. The hollow inorganic microspheres have a particle size of 10-500 nm, and their structure is shown in the figure. Figure 1 .

[0009] In this invention, hollow microspheres are combined with porous resin to construct a hierarchical porous structure. The porous resin is obtained using a solvent-free phase separation method, which involves placing the resin solution in a solvent-free (water) environment. Since organic solvents and solvents are immiscible, resin molecules aggregate to form cores and gradually deposit. As the polymers deposit, a porous structure is formed, and after drying, a porous film is obtained.

[0010] In this invention, the organic resin is one or more of the following: polytetrafluoroethylene resin, polyvinylidene fluoride resin, polycarbonate resin, polyurethane resin, silicone resin, polyethylene resin, polypropylene resin, amino resin, polyester resin, or acrylic resin.

[0011] In this invention, the solvent is one or more of the following: alcohol solvents, amine solvents, benzene solvents, ether solvents, alcohol-ether solvents, ketone solvents, ester solvents, or hydrocarbon solvents. Furthermore, the solvent, in non-limiting embodiments, is any one of methanol, ethanol, isopropanol, n-butanol, propylene glycol, propylene glycol methyl ether, dimethylformamide, propylene glycol butyl ether, propylene glycol methyl ether acetate, propylene glycol butyl ether acetate, benzene, toluene, xylene, ethylene glycol methyl ether, acetone, pentanone, ethyl acetate, or butyl acetate.

[0012] In this invention, hollow microspheres are combined with porous resin to construct a hierarchical porous structure, wherein the hollow inorganic microspheres are prepared by hard template method, soft template method or template-free method.

[0013] Furthermore, the hard template method uses a solid template with a specific shape and porous structure as a template, deposits material on the template surface, and then removes the template by dissolving or etching, leaving hollow microspheres to form hollow microspheres.

[0014] Furthermore, the soft template method uses a soluble template (also known as a template agent) to form template microspheres in a solution, then deposits material on the surface of the template microspheres to form a thin shell, and finally forms a hollow microsphere structure by dissolving or ablating the template microspheres.

[0015] Furthermore, the template-free method involves controlling the self-assembly and aggregation of materials under specific conditions. The material spontaneously forms a core in solution, and then a thin shell is gradually deposited on the core surface, ultimately forming a hollow microsphere structure.

[0016] In this invention, the hollow inorganic microspheres are hollow inorganic metal oxide microspheres or hollow inorganic non-metal oxide microspheres with a particle size of 10-500 nm. They have ultraviolet shielding function to ensure that the film material has good weather resistance and aging resistance.

[0017] Furthermore, inorganic metal oxides and inorganic non-metal oxides with ultraviolet shielding properties include zinc oxide, zirconium oxide, titanium dioxide, silicon dioxide, and nitrogen trioxide.

[0018] This invention employs a new radiative cooling principle, utilizing the temperature difference between the Earth's surface and outer space to transfer thermal radiation. Without considering non-radiative heat transfer, the net cooling power of an object on the Earth's surface equals its radiative cooling power minus the power difference between atmospheric radiation and solar radiation absorbed by the object. Radiative cooling materials typically require high reflectivity in the solar wavelength range (0.3-2.5 μm) and in the atmospheric window wavelength range (8-13 μm) to increase the material's net cooling power and achieve the effect of radiative cooling.

[0019] In this invention, the multi-level porous thin film material has an ultra-low thermal conductivity of less than 30 mW / mk, which can achieve a high-efficiency cooling effect in high-temperature environments by reducing heat exchange / conduction with the external environment.

[0020] The beneficial effects of this invention are as follows:

[0021] (1) The hierarchical porous structure radiation cooling thin film material prepared by this invention has excellent radiation cooling effect. The organic resin itself has high emissivity, and the hierarchical porous structure formed by the use of non-solvent phase separation method and the introduction of hollow microspheres greatly improves the reflectivity of the film. Therefore, the film can efficiently reflect most of the solar radiation and radiate infrared radiation, achieving excellent cooling effect.

[0022] (2) The multi-level porous thin film material prepared by the present invention has an ultra-low thermal conductivity of less than 30 mW / mk, which can effectively reduce the thermal gain generated by the thin film material from the surrounding environment through heat conduction / heat exchange, and achieve a high-efficiency cooling effect in a high-heat environment.

[0023] (3) The hierarchical porous structure radiation cooling thin film material of the present invention has excellent weather resistance and long-term stability. The prepared hollow microspheres have excellent ultraviolet absorption capacity, effectively preventing resin aging caused by long-term outdoor ultraviolet radiation. The reflectivity of the composite hierarchical porous film can remain stable, ensuring that the film has long-term stable radiation cooling performance in outdoor environments.

[0024] (4) The multi-level porous structure radiation cooling thin film material of the present invention is applicable to many fields, such as buildings, automobiles, and electronic devices. This film does not require an additional energy supply, but relies solely on infrared radiation from the environment for cooling, thus possessing the potential for sustainable energy applications. Therefore, it provides an innovative solution for energy conservation, emission reduction, and environmental protection. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a multi-level porous structure radiation cooling thin film material, where: a) is a hollow microsphere, and b) is a porous resin;

[0026] Figure 2These are SEM images of the porous polyurethane film in Example 1, where: a) surface SEM image, b) cross-sectional SEM image;

[0027] Figure 3 These are SEM and TEM images of the hollow titanium dioxide microspheres in Example 2, where: a) SEM image, b) TEM image;

[0028] Figure 4 These are SEM images of the multi-level porous TiO2-TPU film in Example 2, where: a) surface SEM image, b) cross-sectional SEM image;

[0029] Figure 5 This is a schematic diagram of the radiation cooling test device in Example 3. Implementation

[0030] The present invention will be further illustrated below through specific embodiments. It should be noted that those skilled in the art can make several modifications and improvements without departing from the principle of the present invention, and these should also be considered to fall within the protection scope of the present invention.

[0031] Example 1

[0032] Preparation of a porous polyurethane film

[0033] Porous polyurethane films were prepared using a solvent-free phase separation method. 10g of TPU particles and 90g of dimethylformamide were mixed and stirred at 20°C until a homogeneous polyurethane solution was obtained. An appropriate amount of the polyurethane solution was poured into a grooved mold and allowed to stand at room temperature for 10 minutes. Then, the entire mold was immersed in water to begin phase separation. After 5 hours of phase separation, the film was removed from the mold and dried at 20°C for 24 hours to finally obtain a porous polyurethane radiation cooling film.

[0034] Figure 2 (a) is a SEM image of the surface morphology of the porous polyurethane film. Figure 2 (b) is a SEM image of the cross-sectional morphology of the porous polyurethane film. The SEM image of the film surface morphology shows that the polyurethane film obtained through solvent-free phase separation exhibits a porous structure with pore sizes ranging from 0.5 to 10 μm. The SEM image of the cross-sectional morphology further confirms that the porous structure permeates the entire film.

[0035] Example 2

[0036] Preparation of a porous polyethylene film

[0037] Porous polyethylene films were prepared using a solvent-free phase separation method. 10g of TPU particles, 30g of PE particles, and 160g of ethyl acetate were mixed and stirred at 40℃ until a homogeneous polyethylene solution was obtained. A suitable amount of the polyethylene solution was poured into a mold with a groove and allowed to stand at room temperature for 40 minutes. Then, the entire mold was immersed in water to initiate phase separation. After 10 hours of phase separation, the film was removed from the mold and dried at 40℃ for 48 hours, finally obtaining a porous polyethylene radiation cooling film. The pore size of the film ranged from 10-20 μm, and the porous structure permeated the entire film. Example 3

[0038] Preparation of a porous polytetrafluoroethylene film

[0039] Porous polyethylene films were prepared using a solvent-free phase separation method. 45g of PVDF particles and 105g of acetone were mixed and stirred at 60℃ until a homogeneous polytetrafluoroethylene (PTFE) solution was obtained. A suitable amount of the PTFE solution was poured into a grooved mold and allowed to stand at room temperature for 60 minutes. The entire mold was then immersed in water to initiate phase separation. After 20 hours of phase separation, the film was removed from the mold and dried at 60℃ for 80 hours, ultimately yielding a porous PTFE radiation cooling film. The pore size of the film ranged from 40-50 μm, and the porous structure permeated the entire film. Example 4

[0040] Hollow titanium dioxide microspheres were prepared using the soft template method.

[0041] 0.2 g of Triton and 0.2 g of sodium dodecylbenzenesulfonate were dissolved in 90 g of deionized water to form an aqueous phase. In the oil phase, 2.5 g of n-hexane and 2.5 g of tetrabutyl titanate were dissolved in 12 g of ethanol. The oil phase was added dropwise to the aqueous phase at room temperature while stirring. After the reaction was complete, the synthesized hollow titanium dioxide microspheres were collected by centrifugation (8000 rpm, 10 min), washed three times with deionized water and ethanol, and then dried in a vacuum oven at 40 °C for 12 hours.

[0042] Figure 3 (a), Figure 3 (b) are SEM and TEM images of the hollow titanium dioxide microspheres, respectively. According to the SEM image, the prepared hollow titanium dioxide microspheres are spherical with a particle size of about 100-300 nm; according to the TEM image, the prepared hollow titanium dioxide microspheres have a hollow structure. Example 5

[0043] Hollow silica microspheres prepared using the hard template method

[0044] 0.5 g of polystyrene microspheres (40-80 nm) were dispersed in 50 g of ethanol. Tetraethyl orthosilicate was slowly added dropwise to the ethanol at 45 °C. Simultaneously, a small amount of hydrochloric acid was added to adjust the pH to 4. After reacting for 5 h, the microspheres were washed repeatedly with deionized water and dried. The dried silicate microspheres were then placed in ethanol or dimethyl thionamide and subjected to ultrasonic treatment to completely remove the hard template from the interior of the silicate microspheres, yielding hollow silica microspheres with a particle size of 50-100 nm. Example 6

[0045] Hollow zirconia microspheres prepared by template-free method

[0046] 2g of zirconium oxide nitrate was dissolved in 40g of ethanol to form a transparent zirconium oxide precursor solution. The zirconium oxide precursor solution was then treated in an ultrasonic cleaner for 5 hours, causing the zirconium oxide precursor to form gel microspheres under ultrasonic treatment. After ultrasonication, the gel microspheres were washed repeatedly with deionized water to remove unreacted precursors and byproducts. Finally, the gel microspheres were dried in an oven to obtain hollow zirconium oxide microspheres with a particle size of 300-500nm. Example 7

[0047] Preparation of hierarchical porous TiO2-TPU hybrid films

[0048] 95 wt% of the polyurethane solution from Example 1 was mixed with 5 wt% of the hollow TiO2 microspheres from Example 4 and thoroughly ultrasonically dispersed to obtain a hollow TiO2 microsphere / TPU composite. 10 g of the obtained hollow TiO2 microsphere / TPU composite was poured into a grooved mold and allowed to stand at room temperature for 10 minutes. Then, the entire mold was immersed in water to begin phase separation. After 5 hours of phase separation, the film was removed from the mold and dried at 20°C for 24 hours to finally obtain a hierarchical porous TiO2-TPU radiation cooling film with a thermal conductivity of 15 mW / mK.

[0049] Figure 4 (a) is a SEM image of the surface morphology of the hierarchical porous TiO2-TPU film. Figure 4 (b) is a SEM image of the cross-sectional morphology of the hierarchical porous TiO2-TPU film. (The image is obtained through...) Figure 4 (a) It can be seen that the TiO2-TPU film obtained by non-solvent phase separation exhibits a porous structure with a pore size range of 8-10 μm on the surface. Hollow TiO2 microspheres can also be observed embedded in the porous structure, forming a hierarchical porous structure that runs through the entire film. Figure 4 (b)). Example 8

[0050] Preparation of hierarchical porous SiO2-PE hybrid films

[0051] 90 wt% of the polyethylene solution from Example 2 was mixed with 10 wt% of the hollow silica microspheres from Example 5 and thoroughly ultrasonically dispersed to obtain a hollow SiO2 microsphere / PE composite. 10 g of the obtained hollow SiO2 microsphere / PE composite was poured into a grooved mold and allowed to stand at room temperature for 40 minutes. The entire mold was then immersed in water to begin phase separation. After 10 hours of phase separation, the film was removed from the mold and dried at 60°C for 48 hours, ultimately obtaining a hierarchical porous SiO2-PE radiation cooling film with a thermal conductivity of 25 mW / mK. Example 9

[0052] Preparation of hierarchical porous ZrO2-PVDF hybrid films

[0053] 85 wt% of the polytetrafluoroethylene solution from Example 3 was mixed with 15 wt% of the hollow ZrO2 microspheres from Example 6 and thoroughly ultrasonically dispersed to obtain a hollow ZrO2 microsphere / PVDF composite. 10 g of the obtained hollow ZrO2 microsphere / PVDF composite was poured into a grooved mold and allowed to stand at room temperature for 60 minutes. The entire mold was then immersed in water to begin phase separation. After 20 hours of phase separation, the film was removed from the mold and dried at 40°C for 80 hours, finally obtaining a hierarchical porous ZrO2-PVDF radiation cooling film with a thermal conductivity of 21 mW / mK.

[0054] As shown in Table 1, the hierarchical porous structure formed by the introduction of hollow microspheres can further improve the Mie scattering of the film and obtain a higher solar reflectivity. Since the polymer itself has high emissivity, and the hollow microspheres, due to their unique hollow structure, can produce reflection and scattering phenomena, the emissivity of the film can be further enhanced.

[0055] To test the aging resistance of the films prepared in Examples 4-6 and 7-9, they were simultaneously placed outdoors for observation. The reflectance was measured using a Hitachi U-4100 UV-Vis-NIR spectrophotometer at the initial stage, after one week, two weeks, three weeks, and four weeks. Table 2 shows that the porous polymer films exhibited significant yellowing after four weeks of outdoor exposure, with a marked decrease in reflectance. This is because some components of the polymer material are oxidized when exposed to sunlight. The films with added hollow microspheres showed significantly reduced yellowing while maintaining high reflectance. This is because TiO2, SiO2, and ZrO2 possess good photostability, resisting UV radiation and photo-oxidation. UV radiation is one of the main factors causing polyurethane yellowing, and the photostability of TiO2, SiO2, and ZrO2 can effectively reduce the decomposition and oxidation of polyurethane materials under UV irradiation, thereby slowing down the yellowing rate.

[0056] To evaluate the radiative cooling performance of hierarchical porous films, the following methods were employed: Figure 5The apparatus simultaneously performed continuous outdoor temperature measurements on the films of Examples 7-9. The testing period was from 10:00 AM to 2:00 PM, with an average solar irradiance of 904.28 W / m². 2 As shown in Table 3, when the internal temperature of the device reaches its highest point of 55°C, the multi-level porous hybrid films of Examples 7-9 all exhibit excellent radiative cooling performance, with a cooling effect of 18-22°C.

[0057] Table 1 shows the reflectivity of the thin films prepared in Examples 4-9 in the solar radiation band and the emissivity in the atmospheric window band.

[0058] Table 2 shows the reflectance of the films prepared in Examples 4-9 after outdoor exposure for different time periods.

[0059] Table 3 shows the readings of the test device and Examples 7-9 when the temperature reached its highest point during the cooling test.

[0060] Table 1: Reflectivity and emissivity of the thin films prepared in Examples 4-9 in the solar radiation band and the atmospheric window band.

[0061] Example 4 Example 7 Example 5 Example 8 Example 6 Example 9 Reflectivity (%) 90 95 88 92 92 96 Emission rate (%) 82 84 83 84 85 86

[0062] Table 2: Reflectance of the films prepared in Examples 4-9 after outdoor exposure for different time periods

[0063] initial One week Two weeks Three weeks all around Example 4 90% 89% 87% 85% 84% Example 5 88% 86% 84% 83% 80% Example 6 92% 90% 88% 87% 87% Example 7 95% 94% 93% 93% 93% Example 8 92% 91% 90% 90% 90% Example 9 96% 95% 95% 95% 95%

[0064] Table 3: Readings of the testing device and Examples 7-9 at the highest temperature point during the cooling test.

[0065] Inside the device Example 7 Example 8 Example 9 Temperature (°C) 55 35 37 33

Claims

1. A method for preparing a multi-level porous structure radiation-cooling thin film material, characterized in that: The specific steps are as follows: (1) Preparation of hollow inorganic microspheres / organic resin composite: Mix 10-50g of organic resin and 5-450g of solvent, and stir at 0-60℃ until a homogeneous solution is obtained to obtain an organic resin solution; mix 0.5-100g of hollow inorganic microspheres, 15-500g of organic resin solution and 0-50g of non-essential additives, and stir at room temperature for 10-20 minutes to obtain hollow inorganic microspheres / organic resin composite; the hollow inorganic microspheres are hollow inorganic metal oxide microspheres or hollow inorganic non-metal oxide microspheres with a particle size of 50-100nm and have ultraviolet shielding function; (2) Preparation of multi-level porous structure radiation cooling thin film material: The hollow inorganic microspheres / organic resin composite obtained in step (1) are poured into a groove mold by non-solvent phase separation method and left to stand at room temperature for 0.1-12 hours. Then the entire mold is immersed in water and left for 0.1-100 hours for phase separation. Then the film is taken out of the mold and dried at 0-120℃ for 0.1-100 hours to obtain the multi-level porous structure radiation cooling thin film material. The thermal conductivity of the thin film material is less than 30mW / mk. The pore structure of different levels effectively reflects radiation of specific wavelengths, increases the specific surface area of ​​the thin film material, improves the reflectivity of the thin film material to sunlight and the emissivity to atmospheric windows, and increases the radiation cooling efficiency of the thin film material.

2. The preparation method according to claim 1, characterized in that: The organic resin is one or more of the following: polytetrafluoroethylene resin, polyvinylidene fluoride resin, polycarbonate resin, polyurethane resin, silicone resin, polyethylene resin, polypropylene resin, amino resin, polyester resin, or acrylic resin.

3. The preparation method according to claim 1, characterized in that: The solvent is one or more of the following: alcohol solvents, amine solvents, benzene solvents, ether solvents, alcohol-ether solvents, ketone solvents, ester solvents, or hydrocarbon solvents.

4. The preparation method according to claim 1, characterized in that: Hollow inorganic microspheres were prepared using hard template method, soft template method or template-free method.

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