Daytime radiation cooling film for vertical surface as well as preparation method and application of daytime radiation cooling film

Abstract

The invention discloses a daytime radiation cooling film for a vertical surface and a preparation method and application of the daytime radiation cooling film, and belongs to the technical field of film preparation. The daytime radiation cooling film for the vertical surface contains a porous polyethylene (nanoPE) film, nano VO2, nano ZrO2 and polytetrafluoroethylene; according to the invention, the porous polyethylene film is prepared by adopting a thermally induced phase separation process, the porous polyethylene film has high emissivity in the wave band of 8-13 microns and high reflectivity in the wave band of 0.5-2.0 microns, and the cooling mode does not need to consume any electric energy or other energy sources, so that the cost is reduced, and the production efficiency is improved. And no pollutant or greenhouse gas is generated, so that the method is a completely renewable and environment-friendly cooling mode, and the problem of global warming can be effectively relieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of film preparation, and in particular relates to a daytime radiation cooling film for a vertical surface and a preparation method and application thereof. Background Art

[0002] As we all know, in the summer in the south, the high temperature indoors and in cars is unbearable. People use various cooling methods to achieve indoor and car cooling to make it more comfortable in the hot summer. The most common refrigeration equipment is air conditioning, electric fans, etc. However, these methods not only consume a lot of energy, but also consume more than 15% of the global electricity. With economic development and global warming, this proportion will increase. However, the most widely used refrigeration technology is still the traditional gas-liquid compression refrigeration technology that has been used for nearly a hundred years. This technology has the disadvantages of high energy consumption, high noise and the production of "greenhouse gases". So far, the development of new refrigeration technology has become an urgent need for the development of today's society. Is there a cooling technology that can effectively reduce the temperature without consuming any energy?

[0003] Radiative cooling is an effective way to solve global warming and reduce energy consumption due to its completely passive cooling mechanism. The cooling system does not require any energy input. It is a technology that uses the heat radiation emitted by the object itself to dissipate heat. All objects emit electromagnetic waves of a certain wavelength according to their own temperature, which is thermal radiation. If the thermal radiation emitted by an object can penetrate the atmosphere and escape directly into outer space, the object will lose heat, thereby reducing the temperature. The temperature of outer space is close to absolute zero (3K), and it can be regarded as a huge heat sink that can absorb any thermal radiation from the earth's surface. Daytime radiative cooling technology can emit most of the solar radiation (0.3-2.5μm) and dissipate the heat of the object to the outside world through the atmospheric window (3-8μm), so it is considered to be a perfect alternative. To date, most research on daytime radiative cooling has focused on surfaces that can directly face the sky (i.e., planar radiative cooling), such as the roof of a building. Traditional radiative cooling technology mainly relies on the blackbody radiation characteristics of omnidirectional radiation, but this characteristic limits its scope of application. Especially in the actual scenario of vertical surfaces, the cooling capacity of traditional equipment is greatly reduced due to the inability to effectively radiate the low-temperature sky. In practical applications, there are still many objects that need to be cooled, such as the exterior walls of buildings, vehicle bodies, and textiles, most of whose exterior surfaces are vertical. If vertical surfaces such as building exterior walls and cars are cooled by daytime radiation, it will help reduce dependence on traditional air-conditioning systems and promote the development of building energy conservation and automotive thermal management. This means that the development of a daytime radiation cooling technology suitable for vertical surfaces is of great practical significance.

[0004] Although some international research teams have tried to control the spectrum or angle of thermal radiation in recent years, the daytime sub-ambient (referring to the state where the temperature of an object or surface is lower than the ambient temperature under daytime conditions) radiation cooling of vertical surfaces still faces huge challenges. When thermal radiation is used for vertical surfaces (such as walls, clothing, vehicle sides, and other practical scenarios), the field of view of the device facing the low-temperature sky is significantly reduced. At the same time, it has to absorb a large amount of heat from the ground (especially in the summer when cooling is urgently needed, the surface temperature is much higher than the ambient temperature), surrounding objects, and the downward radiation of the non-transparent window band of the atmosphere, which makes its sub-ambient radiation cooling fail. In addition, the atmospheric transmittance decreases with the increase of the zenith angle, and the atmospheric transmittance in the normal direction of the vertical surface is the lowest, which limits its cooling power to only about ~40Wm -2 , which is less than half of the horizontal surface. The vertical surface not only absorbs direct sunlight, but also receives sunlight reflected from the ground, so the requirements for sunlight reflectivity and the angle and spectral characteristics of infrared thermal radiation are more stringent and complex. Although some international research teams have tried to regulate the spectrum or angle of thermal radiation in recent years, daytime sub-ambient radiative cooling on vertical surfaces still faces huge challenges. Summary of the invention

[0005] In order to solve the above technical problems, the present invention proposes a daytime radiation cooling film for vertical surfaces and a preparation method and application thereof.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] One of the technical solutions of the present invention:

[0008] The present invention provides a method for preparing a daytime radiation cooling film for a vertical surface, comprising the following steps:

[0009] Adding polytetrafluoroethylene (PTFE), a curing agent, VO2 and ZrO2 into ethyl acetate, stirring evenly to obtain a suspension, immersing a porous polyethylene (nanoPE) film into the suspension, and obtaining the daytime radiation cooling film for a vertical surface after curing;

[0010] The pore size of the porous polyethylene film is 0.3 μm-1 μm.

[0011] Due to the porous structure, the density of the polyethylene film is lower than the density of the suspension, that is, by controlling the pore size of the polyethylene film, its mass is lower than the mass of the suspension, so that the top layer of the daytime radiation cooling film is a porous polyethylene film. The top layer is a porous polyethylene film, which has a strong scattering effect on ultraviolet-visible light (UV-VIS) and almost no scattering effect on mid-infrared (mid-IR). The daytime radiation cooling film used for vertical surfaces of the present invention has a cooling effect of about 15°C on magnesium-aluminum plates, a cooling effect of about 9°C on sunny days, and a cooling effect of about 8°C on cloudy days. The daytime radiation cooling film used for vertical surfaces of the present invention has broad application prospects in the field of refrigeration.

[0012] The preparation method of the porous polyethylene film is as follows: polydimethylsiloxane and SiO2 are uniformly mixed to obtain a suspension, the suspension is mixed with polyethylene, and a single-layer extrusion method is used to extrude the polyethylene composite sheet, and the polyethylene composite sheet is immersed in a water bath. After the water bath immersion, the polyethylene composite sheet is subjected to a biaxial stretching treatment to obtain the porous polyethylene film.

[0013] As an example, biaxial stretching is carried out in a biaxial stretching machine.

[0014] The invention adopts a thermally induced phase separation process to prepare a porous polyethylene film, and controls the biaxial stretching ratio to achieve the pore size of the porous polyethylene film, so that the pore size is between 0.3 μm and 1 μm.

[0015] The mass ratio of polydimethylsiloxane to SiO2 is 2:3-3:2; and / or

[0016] The mass ratio of the suspension to polyethylene is not higher than 1:9, preferably 1:9.

[0017] The particle size of the SiO2 is 10 μm-50 μm.

[0018] The water bath immersion temperature is 150-200° C., and the time is 1-3 minutes.

[0019] The mass ratio of the polytetrafluoroethylene, curing agent, VO2, ZrO2 and ethyl acetate is 1:6:5:120:10.

[0020] The particle sizes of VO2 and ZrO2 are 20nm and 200nm respectively.

[0021] The curing method is thermal curing, preferably thermal curing at 100-120° C. for 3-4 hours, more preferably thermal curing at 100° C. for 3 hours.

[0022] The second technical solution of the present invention:

[0023] The present invention also provides a daytime radiation cooling film for vertical surfaces prepared according to the above method, wherein the top layer of the daytime radiation cooling film for vertical surfaces is a porous polyethylene film, the thickness of the porous polyethylene film is not higher than 0.5 μm, and the thickness of the daytime radiation cooling film for vertical surfaces is 1 μm-3 μm.

[0024] The third technical solution of the present invention:

[0025] The present invention also provides the use of the daytime radiation cooling film in the daytime radiation cooling of vertical surface objects, wherein the vertical surface objects include walls, clothes or vehicle bodies.

[0026] The fourth technical solution of the present invention:

[0027] The present invention also provides an automobile body film, which is prepared from the daytime radiation cooling film.

[0028] The shape and size of the daytime radiant cooling film can be customized according to the specific shape and needs of the car, and the daytime radiant cooling film can be directly attached to the car body (when attached, the porous polyethylene film side faces outward) to achieve the best cooling effect.

[0029] Different from the existing automobile bodies made of steel plates, aluminum plates and magnesium-aluminum alloy sheets, the automobile body film prepared by the daytime radiation cooling film of the present invention can effectively reduce the temperature inside the car, reduce the frequency of using air conditioners, save energy and reduce carbon dioxide emissions. At the same time, the automobile body film of the present invention has weather resistance and anti-aging properties, which can ensure stability in long-term use.

[0030] Compared with the prior art, the present invention has the following advantages and technical effects:

[0031] (1) The daytime radiation cooling film for vertical surfaces of the present invention is a super hydrophobic radiation daytime cooling film that does not consume any electrical energy or other energy and does not produce any pollutants or greenhouse gases. It is a completely renewable and environmentally friendly cooling method that can effectively alleviate the problem of global warming.

[0032] (2) The daytime radiation cooling film for vertical surfaces of the present invention has high emissivity in the 8 μm-13 μm band and high reflectivity in the 0.5 μm-2.00 μm band.

[0033] (3) The experiment was conducted in the hot summer in Hunan Province (108°47′-114°15′E, 24°38′-30°08′N), with a solar radiation intensity of 1025 W / m 2 Up to 1160W / m 2Under direct sunlight, the temperature of the untreated magnesium-aluminum alloy sheet rises rapidly to 70°C, while the temperature of the magnesium-aluminum alloy sheet attached with the daytime radiation cooling film for vertical surfaces of the present invention is about 55°C. Under this condition, the cooling temperature of the daytime radiation cooling film for vertical surfaces of the present invention is about 15°C.

[0034] (4) In the sunny summer, the total radiation is 300-500MJ / m 2 In the range, the interior temperature of the car body film prepared by the daytime radiation cooling film for vertical surfaces of the present invention (hereinafter referred to as "film-coated car") is about 27°C, while the interior temperature of the original car is about 36°C, indicating that the cooling effect of the film is 9°C. In the cloudy summer, the radiation intensity is 350MJ / m 2 -500MJ / m 2 The internal temperatures of the coated car body and the original car were 19°C and 27°C respectively, indicating that the coated car body still has an 8°C cooling effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0036] Figure 1 The present invention is a flow chart for preparing a daytime radiation cooling film for a vertical surface;

[0037] Figure 2 is a scanning electron microscope image of the daytime radiation cooling film for vertical surfaces in Example 1 at low magnification (×500);

[0038] Figure 3 is a scanning electron microscope image of the daytime radiation cooling film for vertical surfaces in Example 1 at a high magnification (×2000);

[0039] Figure 4 is the water contact angle of the daytime radiation cooling film for vertical surfaces in Example 1;

[0040] Figure 5 is the reflectivity of the daytime radiation cooling film for vertical surfaces in Example 1;

[0041] Figure 6 is the emissivity of the daytime radiation cooling film for vertical surfaces in Example 1;

[0042] Figure 7Schematic diagram of a cooling experiment of a magnesium-aluminum alloy sheet with the daytime radiation cooling film for a vertical surface in Example 1 attached thereto, wherein the PE film represents a polyethylene film, the thickness of the magnesium-aluminum film is 0.8 μm, the magnesium-aluminum sheet represents a magnesium-aluminum alloy sheet, the cooling film represents the daytime radiation cooling film for a vertical surface in Example 1, and the foam thickness is 8 μm;

[0043] Figure 8 The curve of the change of sunlight intensity over time of the magnesium-aluminum alloy sheet to which the daytime radiation cooling film for the vertical surface in Example 1 is attached;

[0044] Fig. 9 The temperature variation curves of the magnesium-aluminum alloy sheet and the magnesium-aluminum alloy sheet attached with the daytime radiation cooling film for the vertical surface in Example 1 over time;

[0045] Fig.10 A curve showing the change of sunlight intensity over time on a sunny day in summer for a car with the daytime radiation cooling film for vertical surfaces in Example 1 attached to the car body;

[0046] Fig.11 The temperature inside the car is plotted over time for a car with the daytime radiation cooling film for vertical surfaces in Example 1 attached to the car body (the temperature inside the car with the cooling film) and a car without any treatment (the temperature inside the car) on a sunny summer day;

[0047] Fig.12 A curve showing the change of sunlight intensity over time on a cloudy summer day for a car with the daytime radiation cooling film for vertical surfaces in Example 1 attached to the car body;

[0048] Fig.13 The graphs are the temperature inside the car over time of a car with the daytime radiation cooling film for vertical surfaces in Example 1 attached to the car body (the temperature inside the car with the cooling film) and a car without any treatment (the temperature inside the car) on a cloudy day. DETAILED DESCRIPTION

[0049] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0050] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0051] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0052] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0053] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0054] By analyzing the effects of atmospheric transparency and solar radiation, making an object cooler than its surroundings in sunlight depends on the reflectivity and emissivity of the material. Emissivity is a measure of a material's ability to emit infrared energy, usually between 0.00 and 1.00. Reflectivity refers to the ability to reflect electromagnetic waves from the surface of a material, also between 0.00 and 1.00. The closer a material's emissivity is to 1.00, the more it absorbs infrared energy and only emits its own infrared energy. Reflectivity is the opposite. Materials with high reflectivity and high infrared emissivity (especially between 8 and 13 μm wavelengths) can achieve a cooling effect under sunlight. These materials can achieve cooling by reflecting sunlight and emitting infrared radiation without absorbing the heat of sunlight. To achieve radiative cooling, the material of the object must have two characteristics: one is the ability to emit strong thermal radiation within the atmospheric transparent window, and the other is the ability to reflect most of the sunlight. These two characteristics are often contradictory, because generally speaking, the emissivity and absorptivity of an object are proportional. Therefore, the present invention provides a daytime radiative cooling film with a special structure to achieve a balance and optimization of these two characteristics.

[0055] In the present invention, an eco-friendly, simple and efficient super hydrophobic radiation film is proposed. The daytime radiation cooling film for vertical surfaces of the present invention contains porous polyethylene (nanoPE) film, nano VO2, nano ZrO2 and polytetrafluoroethylene. From the perspective of microstructure design, the present invention designs a daytime radiation cooling film with a top layer of porous polyethylene film. The present invention adopts a thermally induced phase separation process to prepare a porous polyethylene film, and the method is as follows: polydimethylsiloxane and SiO2 are uniformly mixed at a mass ratio of 2:3-3:2 to obtain a suspension, wherein the particle size of the SiO2 is between 10μm and 50μm; then the suspension is mixed with polyethylene at a mass ratio of 1:9, and the suspension is melt-blended, extruded and formed in an extruder in a single-layer extrusion method to obtain a polyethylene composite sheet; then the polyethylene composite sheet obtained is immersed in a water bath, the water bath temperature is controlled at 150-200°C, and the time is 1-3min; finally, the polyethylene composite sheet is subjected to a biaxial stretching treatment by a biaxial stretching machine, and the pore size of the porous polyethylene film is achieved by controlling the biaxial stretching ratio, the pore size of the porous polyethylene film is between 0.3μm and 1μm, and the thickness of the porous polyethylene film is not higher than 0.5μm.

[0056] The present invention is a method for preparing a daytime radiation cooling film for a vertical surface, and the preparation process is as follows: Figure 1 As shown, the method comprises the following steps: adding polytetrafluoroethylene, a curing agent, VO2 (particle size of 20 nm) and ZrO2 (particle size of 200 nm) to ethyl acetate in a mass ratio of 10:1:6:5:120, stirring the obtained suspension for 2 hours to make the substances uniformly mixed, immersing a porous polyethylene film in the suspension for 10 minutes, and obtaining a daytime radiation cooling film for a vertical surface after thermal curing at 100°C for 3 hours. The thickness of the daytime radiation cooling film for a vertical surface is 1 μm-3 μm. Due to the porous structure, the density of the polyethylene film is lower than that of the suspension, that is, by controlling the pore size of the porous polyethylene film, its mass is lower than that of the suspension, thereby forming a design in which the top layer is a porous polyethylene film. The top layer is a porous polyethylene film, which has a strong scattering effect on ultraviolet-visible light (UV-VIS) and almost no scattering effect on mid-infrared (mid-IR). The daytime radiation cooling film for vertical surfaces of the present invention has a cooling effect of about 15°C on the magnesium-aluminum plate, a cooling effect of about 9°C on sunny days, and a cooling effect of about 8°C on cloudy days. The daytime radiation cooling film for vertical surfaces of the present invention has broad application prospects in the field of refrigeration.

[0057] The shape and size of the daytime radiant cooling film can be customized according to the specific shape and needs of the car, and the daytime radiant cooling film can be directly attached to the car body (when attached, the porous polyethylene film side faces outward) to achieve the best cooling effect.

[0058] Different from the existing automobile bodies made of steel plates, aluminum plates and magnesium-aluminum alloy sheets, the automobile body containing the automobile body film prepared by the daytime radiation cooling film of the present invention can effectively reduce the temperature inside the car, reduce the frequency of use of air conditioners, save energy and reduce carbon dioxide emissions. PTFE itself has excellent weather resistance and anti-aging properties. In its molecular structure, fluorine atoms replace hydrogen atoms in polyethylene to form a tight "fluorinated" protective layer. This structure makes PTFE have extremely high chemical stability and low surface energy, and can resist the erosion of ultraviolet rays, oxygen and moisture. In addition, PTFE has extremely high stability to ultraviolet rays, and its mechanical properties will not decrease significantly even after being exposed to the outdoors for many years. The curing agent plays a role of crosslinking and strengthening in the process of preparing the daytime radiation cooling film. Through the action of the curing agent, the PTFE molecular chain can be closely combined with other components (such as VO2 and ZrO2) to form a more stable network structure, which can improve the mechanical strength and chemical resistance of the daytime radiation cooling film, thereby enhancing its anti-aging performance. VO2 and ZrO2 as fillers can significantly improve the weather resistance and anti-aging properties of the film. VO2 has high hardness and good chemical stability, which can enhance the wear resistance and scratch resistance of the film; ZrO2 has excellent thermal stability and chemical inertness, and can remain stable in high temperature and corrosive environments. The addition of these two fillers can effectively improve the weather resistance and anti-aging properties of the film. Ethyl acetate as a solvent can effectively dissolve PTFE and other components during the preparation process to ensure that each component is evenly dispersed. The evenly dispersed components can form a more uniform film structure, thereby improving the overall performance of the daytime radiation cooling film, including weather resistance and anti-aging properties. Therefore, the automobile body film of the present invention has weather resistance and anti-aging properties, which can ensure stability in long-term use.

[0059] All raw materials used in the embodiments of the present invention are purchased from the market. As an example, the magnesium-aluminum alloy sheet used is purchased from Dalian Deqiang Aluminum Co., Ltd., China, with a thickness of 1.5 mm; polytetrafluoroethylene (PTFE) is purchased from Shandong Longhui Chemical Co., Ltd.; ethyl acetate is purchased from Dalian Bono Chemical Co., Ltd., China; the curing agent is industrial-grade triethylenetetramine, purchased from Jinan Huijinchuan Chemical Co., Ltd.; VO2, ZrO2 and polyethylene film (PE film, thickness of 0.3 μm) are all purchased from Suzhou Yuante New Materials Co., Ltd., China; magnesium-aluminum film is purchased from Northeast Light Alloy Co., Ltd.

[0060] It should be pointed out that the matters not described in detail in the present invention are conventional operating means in the art and are not the focus of the present invention.

[0061] The technical solution of the present invention is further illustrated by the following embodiments.

[0062] Example 1

[0063] Preparation of porous polyethylene film: polydimethylsiloxane and SiO2 are uniformly mixed in a mass ratio of 2:3 to obtain a suspension, wherein the particle size of SiO2 is 30μm; then the above suspension is mixed with polyethylene in a mass ratio of 1:9, and a single-layer extrusion method is used to melt-blend and extrude it in an extruder to obtain a polyethylene composite sheet; then the polyethylene composite sheet obtained is immersed in a water bath, and the water bath temperature is controlled at 180°C for 2 minutes; finally, the polyethylene composite sheet is biaxially stretched by a biaxial stretching machine, and the pore size of the porous polyethylene film is achieved by controlling the biaxial stretching ratio, and the pore size is 0.5μm. The thickness of the porous polyethylene film in this embodiment is 0.5μm.

[0064] Preparation of daytime radiation cooling film for vertical surfaces: polytetrafluoroethylene, curing agent, VO2 (particle size of 20 nm) and ZrO2 (particle size of 200 nm) were added to ethyl acetate, the mass ratio of ethyl acetate, polytetrafluoroethylene, curing agent, VO2 and ZrO2 was 10:1:6:5:120, the obtained suspension was fully stirred for 2 hours to make these substances evenly mixed, the porous polyethylene film prepared above was immersed in the suspension and kept for 10 minutes. After thermal curing at 100°C for 3 hours, a daytime radiation cooling film with a thickness of 1 μm for vertical surfaces was obtained.

[0065] The scanning electron microscope images of the daytime radiation cooling film for vertical surfaces in this embodiment at low magnification and high magnification are shown as follows: Figure 2 and Figure 3 As shown, from Figure 2 From the low-power microstructure surface morphology, we can see that the surface of the cooling film is uneven, loose and porous, forming a rough plane with particles of different sizes. These are formed by the aggregation of nanoparticles. Holes of different sizes can also be clearly seen on the plane, which is the top polyethylene film. Figure 3 From the high-magnification microstructure surface morphology, it can be seen that the diameter of the large particles is about 20μm, and the diameter of the smaller particles is about less than 1μm. It is obvious that the surface of the cooling film is sparse and porous, and the largest holes are about 10μm.

[0066] The water contact angle of the daytime radiation cooling film for the vertical surface in this embodiment is as follows: Figure 4 As shown, it can be seen that nano-VO2 particles and nano-ZrO2 particles are aggregated and embedded in polytetrafluoroethylene, forming a micro / nano-scale rough structure together.

[0067] The reflectivity and emissivity of the daytime radiation cooling film used for the vertical surface in this embodiment are respectively as follows: Figure 5 and Figure 6 As shown, it can be seen that it has high emissivity in the 8μm-13μm band and high reflectivity in the 0.5μm-2.0μm band.

[0068] Example 2

[0069] Preparation of porous polyethylene film: polydimethylsiloxane and SiO2 are uniformly mixed in a mass ratio of 3:2 to obtain a suspension, wherein the particle size of SiO2 is 50μm; then the above suspension is mixed with polyethylene in a mass ratio of 1:9, and a single-layer extrusion method is used to melt-blend and extrude it in an extruder to obtain a polyethylene composite sheet; then the polyethylene composite sheet obtained is immersed in a water bath, and the water bath temperature is controlled at 200°C for 1 min; finally, the polyethylene composite sheet is biaxially stretched by a biaxial stretching machine, and the pore size of the porous polyethylene film is achieved by controlling the biaxial stretching ratio, and the pore size is 0.3μm. The thickness of the porous polyethylene film in this embodiment is 0.5μm.

[0070] Preparation of daytime radiation cooling film for vertical surfaces: polytetrafluoroethylene, curing agent, VO2 (particle size of 20 nm) and ZrO2 (particle size of 200 nm) were added to ethyl acetate, the mass ratio of ethyl acetate, polytetrafluoroethylene, curing agent, VO2 and ZrO2 was 10:1:6:5:120, the obtained suspension was fully stirred for 2 hours to make these substances evenly mixed, the porous polyethylene film prepared above was immersed in the suspension and kept for 10 minutes. After thermal curing at 100°C for 4 hours, a daytime radiation cooling film with a thickness of 1 μm for vertical surfaces was obtained.

[0071] Example 3

[0072] Preparation of porous polyethylene film: polydimethylsiloxane and SiO2 are uniformly mixed in a mass ratio of 2:3 to obtain a suspension, wherein the particle size of SiO2 is 10μm; then the above suspension is mixed with polyethylene in a mass ratio of 1:9, and a single-layer extrusion method is used to melt-blend and extrude it in an extruder to obtain a polyethylene composite sheet; then the polyethylene composite sheet obtained is immersed in a water bath, and the water bath temperature is controlled at 150°C for 3 minutes; finally, the polyethylene composite sheet is biaxially stretched by a biaxial stretching machine, and the pore size of the porous polyethylene film is achieved by controlling the biaxial stretching ratio, and the pore size is 1μm. In this embodiment, the thickness of the porous polyethylene film is 0.5μm.

[0073] Preparation of daytime radiation cooling film for vertical surfaces: polytetrafluoroethylene, curing agent, VO2 (particle size of 20 nm) and ZrO2 (particle size of 200 nm) were added to ethyl acetate, the mass ratio of ethyl acetate, polytetrafluoroethylene, curing agent, VO2 and ZrO2 was 10:1:6:5:120, the obtained suspension was fully stirred for 2 hours to make these substances evenly mixed, the porous polyethylene film prepared above was immersed in the suspension and kept for 10 minutes. After thermal curing at 120°C for 3 hours, a daytime radiation cooling film with a thickness of 1 μm for vertical surfaces was obtained.

[0074] Taking the daytime radiation cooling film for vertical surfaces in Example 1 as an example, its performance was tested. The specific method and results are as follows:

[0075] The experiment was conducted in the hot summer in Hunan Province (108°47′-114°15′E, 24°38′-30°08′N), with a solar intensity of 1025W / m 2 Up to 1160W / m 2 The experimental schematic diagram is shown in Figure 7 The curve of the change of sunlight intensity over time of the magnesium-aluminum alloy sheet with the daytime radiation cooling film for vertical surface in Example 1 is shown as follows: Figure 8 As shown, the temperature variation curves of the magnesium-aluminum alloy sheet and the magnesium-aluminum alloy sheet attached with the daytime radiation cooling film for the vertical surface in Example 1 are as follows: Fig. 9 As shown, under direct sunlight, the temperature of the untreated magnesium-aluminum alloy sheet quickly rises to 70°C, while the temperature of the magnesium-aluminum alloy sheet attached with the daytime radiation cooling film for vertical surfaces of Example 1 is about 55°C. Under this condition, the cooling temperature of the daytime radiation cooling film for vertical surfaces of Example 1 is about 15°C.

[0076] In the sunny summer, the total radiation is 300-500MJ / m 2 In the range, the curve of sunlight intensity variation over time on a sunny day in summer for a car with the daytime radiation cooling film for vertical surface in Example 1 attached to the car body is as follows: Fig.10 As shown; the curve of the temperature inside the car over time is as follows: a car with the daytime radiation cooling film for the vertical surface in Example 1 attached to the car body and a car without any treatment (original car) on a sunny summer day Fig.11 As shown, the interior temperature of the coated car of Example 1 (the material of the car body is magnesium-aluminum alloy plate) is about 27°C, while the interior temperature of the original car is about 36°C, indicating that the cooling effect of the film is 9°C.

[0077] On a cloudy summer day, the radiation intensity is 350MJ / m 2 -500MJ / m 2The curve of the change of sunlight intensity over time on a cloudy summer day for a car with the daytime radiation cooling film for vertical surface in Example 1 attached to the car body is as follows: Fig.12 As shown; the curve of the temperature inside the car over time on a cloudy day for a car with the daytime radiation cooling film for the vertical surface in Example 1 attached to the car body and a car without any treatment is as follows Fig.13 As shown, the internal temperatures of the coated car body and the original car are 19°C and 27°C respectively, indicating that the coated car body still has a cooling effect of 8°C.

[0078] There is no significant difference in the daytime radiant cooling film performance for vertical surfaces between Examples 2-3 and Example 1.

[0079] The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for preparing a daytime radiation cooling film for a vertical surface, characterized in that: The following steps are involved: Adding polytetrafluoroethylene, a curing agent, VO2 and ZrO2 into ethyl acetate, stirring evenly to obtain a suspension, immersing a porous polyethylene film into the suspension, and obtaining the daytime radiation cooling film for a vertical surface after curing; The pore size of the porous polyethylene film is 0.3 μm-1 μm.

2. The method for preparing a daytime radiation cooling film for a vertical surface according to claim 1, characterized in that: The preparation method of the porous polyethylene film is as follows: polydimethylsiloxane and SiO2 are uniformly mixed to obtain a suspension, the suspension is mixed with polyethylene, and a single-layer extrusion method is used to extrude the polyethylene composite sheet, and the polyethylene composite sheet is immersed in a water bath. After the water bath immersion, the polyethylene composite sheet is subjected to a biaxial stretching treatment to obtain the porous polyethylene film.

3. The method for preparing a daytime radiation cooling film for a vertical surface according to claim 2, characterized in that: The mass ratio of polydimethylsiloxane to SiO2 is 2:3-3:2; and / or The mass ratio of the suspension to polyethylene is not higher than 1:

9.

4. The method for preparing a daytime radiation cooling film for a vertical surface according to claim 3, characterized in that: The particle size of the SiO2 is 10 μm-50 μm.

5. The method for preparing a daytime radiation cooling film for a vertical surface according to claim 2, characterized in that: The water bath immersion temperature is 150-200° C., and the time is 1-3 minutes.

6. The method for preparing a daytime radiation cooling film for a vertical surface according to claim 1, characterized in that: The mass ratio of the polytetrafluoroethylene, curing agent, VO2, ZrO2 and ethyl acetate is 1:6:5:120:

10.

7. The method for preparing a daytime radiation cooling film for a vertical surface according to claim 1, characterized in that: The curing method is thermal curing.

8. A daytime radiation cooling film for vertical surfaces, characterized in that: According to the preparation method according to any one of claims 1-7, the top layer of the daytime radiation cooling film for vertical surfaces is a porous polyethylene film, the thickness of the porous polyethylene film is not higher than 0.5 μm, and the thickness of the daytime radiation cooling film for vertical surfaces is 1 μm-3 μm.

9. Application of the daytime radiation cooling film according to claim 8 in daytime radiation cooling of vertical surface objects, characterized in that: The vertical surface objects include walls, clothes or vehicle bodies.

10. An automobile body film, characterized in that: Prepared from the daytime radiation cooling film according to claim 8.

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