Radiation cooling coating with improved solar reflectance

Abstract

The present invention relates to a radiation cooling paint with improved solar reflectance, which is composed of ceramic microparticles that function as pigments, polymer resins that function as binders, and solvents, forms a paint coating layer on a substrate, maximizes the reflection of incident sunlight and minimizes absorption, at the same time, prevents the inflow of energy from incident sunlight by maximizing the emission of long-wave infrared rays corresponding to 8 to 13 μm and increasing the energy release based on the emission of the long-wave infrared rays, thereby exerting a radiation cooling function, and the volume of pores formed inside the paint coating layer can be 3% or more and 50% or less in order to increase the reflection of incident sunlight without reducing the emission of long-wave infrared rays.

Description

[0001] This invention is a divisional application of the invention patent application filed on June 7, 2023, with application number 202380013754.8 and invention title "Radiation Cooling Coating with Improved Solar Reflectivity". Technical Field

[0002] This invention relates to a radiation cooling coating with improved solar reflectivity, and more specifically, to a radiation cooling coating that prevents energy from incident sunlight from flowing in by increasing the reflectivity of incident sunlight and by increasing the energy release through long-wave infrared radiation. Background Technology

[0003] Typically, energy must be used for cooling. For example, widely used cooling devices such as refrigerators and air conditioners utilize the principle of converting electrical energy (compressor) into mechanical energy to compress the refrigerant and then absorbing the heat generated when the compressed refrigerant expands.

[0004] In other words, energy must be used to achieve cooling, which moves heat from a low temperature area to a high temperature area.

[0005] However, radiation cooling is a new technology that cools without consuming energy by exchanging heat with the outside of the Earth's atmosphere rather than around the cooled body through a spontaneous process called infrared radiation, which does not use energy.

[0006] That is, radiation-based heat exchange from the cooler to the outside of the Earth's atmosphere is achieved through thermal radiation (infrared radiation), which is a spontaneous process that requires no energy.

[0007] When you get close to a hot object, you can feel the heat even if the surrounding air is not hot. This is due to the conduction of heat energy through radiation.

[0008] Thermal radiation is a method of heat transfer that can occur without direct contact with a heat source or through a medium. It achieves heat transfer by releasing electromagnetic waves, thus requiring no medium and conducting at the speed of light to astronomical distances.

[0009] All objects above absolute temperature (0K) possess thermal energy and emit thermal radiation. The amount of radiation emitted is determined by the object's temperature, surface area, and surface properties.

[0010] The core of zero-energy radiative cooling is to avoid absorbing and maximize the reflection of incident sunlight, effectively releasing the heat energy of an object out of the Earth's atmosphere.

[0011] Therefore, the energy of an object is released as long-wave infrared radiation with wavelengths of 8μm to 13μm (also known as the atmospheric window region), which is not absorbed by the Earth's atmosphere.

[0012] Infrared rays with wavelengths outside of 8μm to 13μm are absorbed by carbon dioxide, water vapor, and other substances in the Earth's atmosphere as they pass through it, and no heat transfer occurs between objects on the Earth's surface and outer space.

[0013] For radiative cooling, infrared radiation should be emitted effectively; therefore, the material should possess properties that can effectively absorb the infrared radiation to be emitted.

[0014] In particular, for certain materials to achieve a lower temperature than the surrounding environment through radiative cooling during the daytime when there is sunlight, the absorption, reflection, transmission, and radiation of light should be well controlled independently in each wavelength band.

[0015] In most cases, the main heat source is incident sunlight, which arrives in the form of ultraviolet (UV), visible light, and near-infrared rays. In order to achieve daytime radiative cooling, the surface should reflect and not absorb the incident sunlight (UV, visible light, and near-infrared rays) to the greatest extent possible to block the heat caused by sunlight. The heat it has should be able to radiate out well in the form of infrared rays.

[0016] If more heat energy than the incoming sunlight can be emitted in this way, it can be cooled to a lower temperature than the surrounding environment without consuming energy.

[0017] To give a simple example, on a sunny day, the interior temperature of a black car, which easily absorbs light, rises easily, but the temperature of a white car, which is relatively less absorbent and more reflective of light, rises relatively less.

[0018] If a car's surface does not absorb and reflects light in the ultraviolet-visible-near-infrared wavelength band to the maximum extent—that is, incident sunlight—while simultaneously emitting more energy from the car than the energy that would flow in due to the inability to 100% reflect sunlight by emitting infrared rays of 8μm to 13μm that are not absorbed by the Earth's atmosphere, then the car's temperature can be cooled to a lower temperature than the surrounding temperature.

[0019] All objects radiate their own energy outward in the form of light, and in this case, the wavelength of the emitted light is determined by the surface temperature of the object.

[0020] The reason why the sun emits light in the ultraviolet, visible, and near-infrared wavelength range is that the sun's surface temperature reaches 6000℃.

[0021] Objects with a surface temperature of tens of degrees Celsius (°C) emit long-wave infrared radiation with wavelengths ranging from several to tens of micrometers (e.g., 5 μm to 100 μm) to the outside.

[0022] If a material that suppresses or re-reflects long-wave infrared radiation is applied to the surface of an object, it will have a heat-insulating effect by reducing heat loss caused by long-wave infrared radiation.

[0023] This phenomenon has been applied to winter clothing. Similarly, if a surface is prone to infrared radiation, the object is prone to heat release through thermal radiation.

[0024] In addition to nitrogen, oxygen, and argon, Earth's atmosphere also contains small amounts of water vapor and carbon dioxide. Water vapor and carbon dioxide gases suppress radiation to the outside by absorbing a portion of the long-wave infrared light reflected outward from Earth.

[0025] A prime example is the "greenhouse effect." The higher the concentration of carbon dioxide in the Earth's atmosphere, the more it hinders the release of long-wave infrared radiation emitted by the Earth into the universe, causing the Earth's temperature to rise because heat cannot be released from the Earth into the universe.

[0026] However, long-wave infrared radiation in the 8μm to 13μm wavelength band, commonly known as the atmospheric window, is not absorbed by the Earth's atmosphere and is easily emitted outwards.

[0027] For reference, the temperature of outer space is close to 0K, -270℃. The radiation of long-wave infrared radiation from the Earth's surface, which has a surface temperature of tens of degrees Celsius, into space is a natural phenomenon of heat movement.

[0028] If a material releases its heat energy well in the long-wave infrared band of 8μm to 13μm, known as the atmospheric window region, it is more likely to induce radiative cooling.

[0029] To achieve sufficient solar radiation reflection, existing coating-based radiation cooling materials (devices) require thick coatings and low binder content (high ceramic particle content).

[0030] To prepare radiation-cooling coatings, ceramic microparticles with appropriate physical properties should be selected by choosing particle size and maximizing light scattering.

[0031] The material selected is one with a high refractive index in the solar band gap and a high extinction coefficient (k) in the atmospheric window region.

[0032] Even if Mie scattering is maximized by adjusting the size of ceramic microparticles and selecting materials with high refractive index in the incident solar band gap (ultraviolet-visible-near-infrared (UV-vis-NIR)), the radiation cooling capacity and hiding power of radiation-cooling coatings are still limited due to restricted light scattering when the coating thickness is low. Summary of the Invention

[0033] Technical issues

[0034] The purpose of this invention is to provide a radiation cooling coating that forms bubbles inside the coating film to reduce the coating film thickness required to achieve radiation cooling performance, reduces the coating operability and ease of application, and thereby enables more active light scattering within the radiation cooling coating.

[0035] The object of the present invention is to provide a radiation cooling coating that exhibits excellent radiation cooling performance even at a small thickness, without requiring the coating to be applied to a thick thickness, thereby providing excellent coating operability.

[0036] The purpose of this invention is to provide a radiation cooling coating that exhibits excellent radiation cooling performance even with increased binder content due to effective light scattering caused by bubbles, thereby improving the durability of the coating layer by increasing the binder content.

[0037] The purpose of this invention is to provide a radiation cooling coating that has high cooling radiation capacity regardless of day or night. When applied to outdoor structures or buildings, it minimizes the absorption of incident sunlight even during the day when sunlight is intense, and effectively maintains the heat release through long-wave infrared radiation to improve radiation cooling performance.

[0038] The purpose of this invention is to provide a radiation cooling coating for solving the problem of temperature rise in data centers, communication equipment, or relay equipment when they are installed outdoors due to internal heat accumulation.

[0039] Technical solution

[0040] The radiation cooling coating of one embodiment of the present invention is composed of ceramic microparticles acting as pigments, polymer resin acting as binders, and solvent. After being applied to a substrate, it forms a coating film layer. The coating film layer reflects incident sunlight to the maximum extent and minimizes absorption. At the same time, it prevents energy from the incident sunlight from flowing in and increases energy release based on the emission of long-wave infrared rays equivalent to 8μm to 13μm by maximizing the emission of long-wave infrared rays, thereby performing a radiation cooling function. In order to increase the reflection of incident sunlight without reducing the emission of long-wave infrared rays, the volume of bubbles formed inside the coating film layer can be more than 3% and less than 50%.

[0041] In the aforementioned coating film layer, the aforementioned ceramic microparticles and the aforementioned polymer binder are homogeneously mixed together to form the aforementioned bubbles on the surface of the ceramic microparticles, thereby forming a combination of the aforementioned ceramic microparticles and the aforementioned bubbles.

[0042] The aforementioned composite increases the reflection of incident sunlight at at least one of the interfaces between the ceramic particles and the bubbles and between the bubbles and the polymer adhesive without reducing the long-wave infrared radiation. As the bubble volume increases, it can reduce at least one of the following: the thickness of the coating film and the content of the ceramic particles.

[0043] The aforementioned ceramic microparticles may include at least one of titanium dioxide (TiO2), aluminum oxide (Al2O3), hexagonal boron nitride (h-BN), zirconium dioxide (ZrO2), silicon dioxide (SiO2), calcium carbonate (CaCO3), barium sulfate (BaSO4), magnesium oxide (MgO), yttrium trioxide (Y2O3), yttrium-stabilized zirconium oxide (YSZ), beryllium oxide (BeO), manganese oxide (MnO), zinc oxide (ZnO), silicon carbide (SiC), and aluminum nitride (AlN), and may include at least one polymer microparticle of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and ethylene-tetrafluoroethylene copolymer (ETFE).

[0044] The size of the ceramic particles and the bubbles mentioned above can be from 0.1 μm to 5 μm.

[0045] The ceramic microparticles were selected by taking into account the refractive index and extinction coefficient of the incident sunlight and the extinction coefficient of the long-wave infrared radiation.

[0046] The aforementioned polymer resin may include at least one of polyurethane resin, alkyd resin, acrylate resin, polyvinyl chloride (PVC), polyethylene (PE), acrylic resin, dipentaerythritol hexaacrylate (DPHA), and fluoropolymer resin.

[0047] The weight ratio of the ceramic microparticles to the polymer resin is x:1, where x can be from 0.15 to 3.

[0048] The thickness of the coating film can be less than 300 μm.

[0049] The radiation cooling coating of one embodiment of the present invention may further include at least one additive selected from dispersing agent and photoinitiator to improve the workability of the coating.

[0050] The effects of the invention

[0051] The present invention can provide a radiation cooling coating that forms bubbles inside the coating film to reduce the coating film thickness required to achieve radiation cooling performance, reduces the coating operability and coating difficulty of the radiation cooling coating, thereby enabling more active light scattering inside the radiation cooling coating.

[0052] The present invention can provide a radiation cooling coating that exhibits excellent radiation cooling performance even at small thicknesses, without requiring the coating to be applied to a thick thickness, thereby providing excellent coating operability.

[0053] The present invention can provide a radiation cooling coating that exhibits excellent radiation cooling performance even with increased binder content due to effective light scattering caused by bubbles, thereby improving the durability of the coating layer by increasing the binder content.

[0054] The present invention can provide a radiation cooling coating that has high cooling radiation capacity regardless of day or night. When applied to outdoor structures or buildings, it minimizes the absorption of incident sunlight even during the day when sunlight is intense, and effectively maintains the heat release through long-wave infrared radiation to improve radiation cooling performance.

[0055] This invention provides a radiation cooling coating for solving the problem of temperature rise in data centers, communication equipment, or relay equipment when they are installed outdoors due to internal heat accumulation. Attached Figure Description

[0056] Figure 1 This diagram is used to illustrate the concepts of radiation cooling devices and radiation cooling coatings.

[0057] Figure 2 and Figure 3 A diagram illustrating a radiation-cooling coating that improves the solar reflectivity according to an embodiment of the present invention.

[0058] Figure 4a and Figure 4b This is a diagram illustrating the optical properties of a radiation-cooling coating according to an embodiment of the present invention.

[0059] Figure 5 An electron microscope image of a radiation cooling coating used to illustrate an embodiment of the present invention. Detailed Implementation

[0060] Hereinafter, various embodiments of this specification are described with reference to the accompanying drawings.

[0061] The embodiments and the terms used herein should not be construed as limiting the technology described herein to a particular implementation, but should be construed as including various modifications, equivalents and / or alternatives to the related embodiments.

[0062] In the description of the various embodiments described below, detailed descriptions of known functions or structures will be omitted when it is determined that such detailed descriptions may unnecessarily obscure the essence of the invention.

[0063] Furthermore, the terms used below are defined with reference to functionality in various embodiments and may vary depending on the intent or conventions of the user, operator, etc. Therefore, their definitions should be determined based on the entire contents of this specification.

[0064] In connection with the description of the accompanying drawings, similar structural elements may use similar reference numerals.

[0065] If the context does not explicitly state otherwise, the singular expression may include the plural expression.

[0066] In this specification, “A or B” or “at least one of A and / or” can include all possible combinations of the listed items.

[0067] Expressions such as “first,” “second,” “firstly,” or “secondly” can be used to distinguish one structural element from others, regardless of the order or importance of the related structural elements, rather than to limit the related elements.

[0068] When referring to a (e.g., first) structural element as being “connected (functionally or communicatively)” or “joined” with other (e.g., second) structural elements, it can mean that the aforementioned structural element is directly connected to the other aforementioned structural elements or is connected through other structural elements (e.g., a third structural element).

[0069] In this specification, the term “configured to” may be used interchangeably with terms such as “suitable for”, “capable of”, “modified to”, “prepared to”, “capable of”, “designed to”, etc., depending on the context.

[0070] In some cases, "the device is configured as ~" can mean that the device, together with other devices or accessories, "can be ~".

[0071] For example, the statement “processor configured (or set) to perform A, B and C” can refer to a dedicated processor (e.g., an embedded processor) used to perform the relevant operations or a general-purpose processor (e.g., a central processing unit (CPU) or application processor) that executes one or more software programs stored in a memory device to perform the relevant operations.

[0072] Furthermore, compared to the exclusive logic or "exclusive or", "or" represents the inclusive logic or "inclusive or".

[0073] That is, unless otherwise defined or explicitly stated in the context, the expression “x using a or b” represents any of the natural inclusive permutations.

[0074] The terms “…part”, “…device”, etc. used below refer to units that perform at least one function or operation, which can be implemented by hardware or software or a combination of hardware and software.

[0075] Figure 1 This diagram is used to illustrate the concepts of radiation cooling devices and radiation cooling coatings.

[0076] Figure 1 In relation to the radiation cooling coating of the present invention, a radiation cooling device formed using a conventional radiation cooling coating that achieves radiation cooling performance is illustrated.

[0077] Reference Figure 1 An example is shown of a radiation cooling device 100 manufactured using radiation cooling coatings from the prior art.

[0078] The radiation cooling device 100 includes a coating film layer 120 formed on a substrate 110. The coating film layer 120 is formed based on a radiation cooling coating composed of ceramic microparticles 121 and 122 that act as pigments, a polymer resin that acts as a binder, and a solvent.

[0079] Although radiation-cooling coatings are limited by the fact that light scattering only occurs at the interface of polymeric compounds with different refractive indices than ceramic particles, various types of radiation-cooling devices have been studied.

[0080] Initially, a radiation cooling device in the form of a multilayer thin film deposited on a substrate was proposed. In order to reflect incident sunlight, a silver (Ag) thin film was deposited on the substrate, and a multilayer thin film of a material that is transparent to incident sunlight and can absorb and emit long-wave infrared light well was stacked on it to form the device.

[0081] Furthermore, a radiation cooling device in the form of a polymer film is proposed, in which a silver film for sunlight reflection is deposited on one side of the polymer film and ceramic microparticles for long-wave infrared radiation are dispersed inside the film.

[0082] Both of these devices use specular reflection, which uses thin films of metals such as silver to reflect incident sunlight like a mirror, in order to reflect the incident sunlight.

[0083] Alternatively, white scattering, which emits all wavelengths of incident sunlight and has a white appearance that is not like a mirror, can be used to replace specular reflection, achieving radiative cooling by reflecting all incident sunlight without absorbing it.

[0084] In particular, white scattering reflection does not use expensive silver films, which not only reduces manufacturing costs but also avoids the performance degradation caused by the deterioration of silver films, thus extending product lifespan and making it more suitable for the manufacture of radiation cooling devices.

[0085] To effectively induce white scattering reflection, ceramic microparticles of similar size to the wavelength to be reflected are needed, as well as an adhesive material to connect such microparticles. Polymer materials are very suitable as adhesive materials.

[0086] Polymers are highly competitive because they are easy to mass-produce, inexpensive, and have a wide range of adjustable properties.

[0087] Therefore, if polymers are used to construct zero-energy radiative cooling devices, a variety of products can be manufactured at low cost, and they also have many advantages due to their good processability.

[0088] Many polymer resins and ceramic microparticles have high emissivity in the atmospheric window region of 8μm to 13μm, making them transparent to incident sunlight and not easily absorbed.

[0089] White radiation-cooled devices can be fabricated by combining only such materials, and can be realized as radiation-cooled devices in the form of a "coating" that combines polymer resin and ceramic microparticles.

[0090] That is, the general form of coatings composed of various polymer resins and ceramic particles dissolved and dispersed in a solvent.

[0091] If such a coating is applied to various surfaces to form a coating film, and the resulting coating film reflects incident sunlight to the maximum extent while minimizing absorption, and maximizes the emission of long-wave infrared rays from 8μm to 13μm for radiative cooling, then such a coating is a radiative cooling coating.

[0092] Compared to various radiative cooling devices, the advantage of radiative cooling coatings is that they can be applied to any surface to form a film, which then radiates and cools the surface, thus enabling a variety of applications.

[0093] Figure 2 and Figure 3 A diagram illustrating a radiation-cooling coating that improves the solar reflectivity according to an embodiment of the present invention.

[0094] Figure 2 and Figure 3An example of a radiation-cooling coating in an embodiment of the present invention is a coating film layer in which air bubbles are formed and light scattering is enhanced by the air bubbles.

[0095] Figure 2 An example is given of a radiation cooling device formed using a radiation cooling coating according to an embodiment of the present invention, in which only one type of ceramic particle is generated as a composite.

[0096] on the other hand, Figure 3 An example is shown in a radiation cooling device formed using a radiation cooling coating according to an embodiment of the present invention, in which multiple ceramic particles form a composite.

[0097] For example, the composite enhances the reflection of incident sunlight at at least one interface between the ceramic microparticles and the bubble and between the bubble and the polymer binder without reducing long-wave infrared radiation, and can reduce at least one of the following as the bubble volume increases: the thickness of the coating film and the content of ceramic microparticles.

[0098] For example, the binder can be a combination of ceramic particles and bubbles.

[0099] Reference Figure 2 In one embodiment of the present invention, the radiation cooling device 200 is formed of a radiation cooling coating.

[0100] The radiation cooling coating consists of ceramic microparticles that act as pigments, polymer resins that act as binders, and solvents. After being applied to the substrate 210, it forms a coating film layer 220.

[0101] The ceramic particles may include first ceramic particles 221 and second ceramic particles 222.

[0102] The ceramic microparticles may include at least one of titanium dioxide, aluminum oxide, hexagonal boron nitride, zirconium dioxide, silicon dioxide, calcium carbonate, barium sulfate, magnesium oxide, yttrium oxide, yttrium-stabilized zirconium oxide, beryllium oxide, manganese oxide, zinc oxide, silicon carbide, and aluminum nitride, and may include at least one polymer microparticle of polyvinylidene fluoride, polytetrafluoroethylene, and ethylene-tetrafluoroethylene copolymer.

[0103] The first ceramic particle 221 and the second ceramic particle 222 can be different substances among the aforementioned ceramic particle materials.

[0104] For example, the size of ceramic particles can range from 0.1 μm to 5 μm.

[0105] The ceramic microparticles can be selected by considering their refractive index and extinction coefficient for incident sunlight, as well as their extinction coefficient for long-wave infrared radiation.

[0106] In the case of radiation-cooled coatings, in order to effectively reflect incident sunlight, ceramic particles that scatter the light and polymeric compounds that bind these ceramic particles together are required.

[0107] In this case, the greater the refractive index between the ceramic particles and the polymer, the more it promotes light scattering, thus more effectively causing scattering and reflection.

[0108] For effective radiative cooling, more than 90% of the incident sunlight should be reflected, the concentration of ceramic particles that scatter and reflect the incident light should be high, and the thickness of the coating layer formed by the ceramic particles and polymer admixture should be above the specified thickness.

[0109] If the coating layer formed by ceramic particles and polymer admixture is uniformly present in bubbles 223 of similar size to the ceramic particles, light is refracted due to the high refractive index difference at the boundaries between the bubbles 223 and the polymer admixture, and between the bubbles and the ceramic particles, thereby promoting light scattering. This can reduce the thickness of the coating layer used to reflect 90% of the incident sunlight, and also reduce the concentration of ceramic particles.

[0110] Generally, the lower the content of ceramic particles, the better it is for preparing coatings, and the thinner the coating film, the better.

[0111] According to one embodiment of the present invention, the coating film layer 220 reflects incident sunlight to the maximum extent and minimizes absorption, while preventing energy from incident sunlight from flowing in and increasing energy release through the radiation of long-wave infrared rays equivalent to 8μm to 13μm by maximizing the radiation of long-wave infrared rays, thereby performing a radiative cooling function.

[0112] Furthermore, in order to increase the reflection of incoming sunlight without reducing long-wave infrared radiation, the volume of bubbles 223 formed inside the coating film layer 220 can be more than 3% and less than 50%.

[0113] According to one embodiment of the present invention, the coating film layer 220 treats the ceramic particles to be hydrophilic or hydrophobic according to the solvent, and homogenizes them together with the polymer binder to form bubbles 223 on the surface of the ceramic particles, thereby forming a combination 224 of the second ceramic particles 222 and the bubbles.

[0114] Regardless of whether hydrophilic or hydrophobic treatments are performed, ceramic microparticles can form bubbles 223.

[0115] According to one embodiment of the present invention, in the coating film layer 220 of the radiation cooling device 200, when the bubble 223 is above a certain volume, it can promote the scattering of incident sunlight to reflect all the light. However, if the volume fraction of the bubble is too large, it will reduce the mechanical properties of the coating film layer.

[0116] In the radiation cooling device 200 formed by radiation cooling coating, light scattering occurs at the interface between ceramic particles and polymer composite, the interface between ceramic particles and bubbles, and the interface between bubbles and polymer composite. Compared with the case without bubbles, the thickness of the coating layer used for a specified amount of light scattering reflection or the required content of ceramic particles can be reduced.

[0117] Reference Figure 3 In one embodiment of the present invention, the radiation cooling device 300 is formed of a radiation cooling coating.

[0118] The radiation cooling coating consists of ceramic microparticles that act as pigments, polymer resins that act as binders, and solvents. After being applied to the substrate 310, it forms a coating film layer 320.

[0119] The ceramic particles may include first ceramic particles 321 and second ceramic particles 322.

[0120] The ceramic microparticles may include at least one of titanium dioxide, aluminum oxide, hexagonal boron nitride, zirconium dioxide, silicon dioxide, calcium carbonate, barium sulfate, magnesium oxide, yttrium oxide, yttrium-stabilized zirconium oxide, beryllium oxide, manganese oxide, zinc oxide, silicon carbide, and aluminum nitride, and may include at least one polymer microparticle of polyvinylidene fluoride, polytetrafluoroethylene, and ethylene-tetrafluoroethylene copolymer.

[0121] The first ceramic particle 321 and the second ceramic particle 322 can be different substances among the aforementioned ceramic particle materials.

[0122] For example, the size of ceramic particles can range from 0.1 μm to 5 μm.

[0123] The ceramic microparticles can be selected by considering their refractive index and extinction coefficient for incident sunlight, as well as their extinction coefficient for long-wave infrared radiation.

[0124] According to an embodiment of the present invention, in the radiation cooling coating used to form the coating film layer 320, the polymer resin may include at least one of polyurethane resin, alkyd resin, acrylate resin, polyvinyl chloride, polyethylene, acrylic resin, dipentaerythritol hexaacrylate, and fluoropolymer resin.

[0125] The weight ratio of ceramic microparticles to the polymer resin is x:1, where x can be from 0.15 to 3.

[0126] According to one embodiment of the present invention, the radiation cooling coating can be a radiation cooling coating that further enhances light scattering through bubbles 323.

[0127] For example, the size of bubble 323 can be from 0.1 μm to 5 μm.

[0128] As an example, the coating film layer 320 is formed by mixing bubbles 323 with a size similar to ceramic particles to promote light reflection, and high light reflection and radiative cooling performance can be achieved even by reducing the content of ceramic particles in the radiative cooling coating.

[0129] That is, radiation cooling coatings form a coating film by mixing bubbles of similar size to ceramic particles, thereby achieving high light reflection and radiation cooling performance even with a reduced ceramic particle content.

[0130] In one embodiment of the present invention, light scattering of the radiation cooling coating occurs at the interface between ceramic particles and polymer composite, the interface between ceramic particles and bubbles 323, and the interface between bubbles 323 and polymer composite. Compared with the absence of bubbles 323, the thickness of the coating layer used for a specified amount of light scattering reflection or the required content of ceramic particles can be reduced.

[0131] Mixtures of polymer particles such as polyvinylidene fluoride, polytetrafluoroethylene, and ethylene-tetrafluoroethylene copolymer, and ceramic particles such as titanium dioxide, alumina, hexagonal boron nitride, zirconium dioxide, silicon dioxide, calcium carbonate, barium sulfate, magnesium oxide, yttrium oxide, yttrium-stabilized zirconium oxide, beryllium oxide, manganese oxide, zinc oxide, silicon carbide, and aluminum nitride, as well as polymer resins such as polyurethane resin, fluoropolymer resin, polyethylene resin, polyacrylate resin, polydimethylsiloxane (PDMS), and polyvinyl chloride, do not absorb and effectively reflect incident sunlight (ultraviolet-visible light-near-infrared rays). They can have high absorption (emissivity) in the entire atmospheric window region from 8μm to 13μm, thus possessing radiative cooling function.

[0132] Moreover, the mixture is homogenized as a solvent, thus giving it the form of a coating that can be easily applied to a variety of surfaces.

[0133] Polymer particles and ceramic microparticles have different refractive indices than polymer resins, which reduce the absorption of incident sunlight and increase reflection by scattering incident light.

[0134] The substrate 210 or substrate 310 can be the surface of an outdoor device such as a data center, communication equipment, or relay equipment that generates internal heat.

[0135] Therefore, the present invention can provide a radiation cooling coating for solving the problem of temperature rise in data centers, communication equipment or relay equipment when they are installed outdoors due to internal heat accumulation.

[0136] Ceramic microparticles include not only single particles, but also core-shell particles composed of different types of ceramic materials or hollow particles with empty interiors.

[0137] Compared to the case where only ceramic particles and polymer binders (combinations) are formed, when bubbles are added, light scattering occurs not only at the interface between ceramic particles and polymer binders, but also at the interfaces between polymer binders and bubbles, and between ceramic particles and bubbles, thereby further promoting light scattering.

[0138] The refractive index of ceramic microparticles is approximately 2.0 or higher, the refractive index of polymer binders is 1.4 to 1.6, and the refractive index of bubbles is 1.0, thus enabling more effective light scattering by bubbles.

[0139] The mechanical properties, gloss, drying properties, and dispersibility of polymer (ceramic) particles of coating films can be improved by adding photoinitiators, thermal initiators, or dispersants to polymer resins such as polyurethane resin, fluoropolymer resin, polyethylene resin, polyacrylate resin, polydimethylsiloxane, and polyvinyl chloride.

[0140] Therefore, in order to improve the workability of the coating, the radiation cooling coating may also contain at least one additive from the dispersant and photoinitiator.

[0141] According to one embodiment of the present invention, the thickness of the coating film layer 320 can be formed to be less than 300 μm.

[0142] Increasing the content of polymer binder, i.e. reducing the content of ceramic particles, has the problem of reducing sunlight reflection and increasing sunlight transmission.

[0143] However, generally, the higher the content of polymer binder, the better the workability of the coating and the more aesthetically pleasing the surface of the coating film.

[0144] Reducing the minimum coating thickness required to achieve radiative cooling performance means that it is necessary to reduce the operability and ease of application of radiative cooling coatings.

[0145] Therefore, light scattering should be made more active within the radiation-cooled coating.

[0146] In order to achieve sufficient reflection of incident sunlight in a coating film layer 320 with reduced thickness, the radiation cooling device 300 of one embodiment of the present invention can trigger additional light scattering in addition to the scattering by the ceramic particles and polymer binder of the existing radiation cooling coating.

[0147] Therefore, in one embodiment of the present invention, a radiation cooling coating distributes bubbles of similar size to ceramic particles inside the coating film layer 320 by forming a first binder 324 and a second binder 325, thereby causing light scattering between the bubbles and the ceramic particles, and between the bubbles and the polymer binder.

[0148] According to one embodiment of the present invention, the first composite 324 may be a composite of the first ceramic particles 321 and the bubbles 323, and the second composite 325 may be a composite of the second ceramic particles 322 and the bubbles 323.

[0149] The refractive index of the bubbles is 1.0, which is significantly different from the refractive index of the polymer binder and ceramic particles. Light scattering occurs very effectively around the bubbles, providing sufficiently high solar reflectivity and low solar transmittance even in a thin coating film layer 320.

[0150] Even if a small amount of air bubbles are present, it will not affect the emissivity of the atmospheric window region at all.

[0151] More specifically, when air bubbles 323 are present in the paint film layer 320, they are reflected away because light cannot reach the depth of the paint film layer 320 due to active light scattering.

[0152] However, in the absence of bubbles, light scattering is reduced, allowing light to reach deeper into the coating layer, where it encounters more absorbing particles, increasing absorption and thus reducing reflection.

[0153] Compared with existing radiation cooling coatings or commercially available heat insulation (heat blocking) coatings, the radiation cooling coating of one embodiment of the present invention reduces the absorption of incident sunlight, maximizes reflection, and promotes the emission of infrared rays from 8μm to 13μm, thereby having superior radiation cooling performance.

[0154] In order to increase the reflection of incident sunlight and reduce absorption, the coating film layer 320 is uniformly distributed with a first binder 324 containing bubbles 323 and a second binder 325.

[0155] Under the influence of air bubbles present inside the coating film layer 320, light scattering is promoted, increasing the reflection of incident light. Light cannot reach the deep part of the coating film layer, and scattering reflection also forms on the upper part of the coating film layer, thereby reducing absorption.

[0156] In order to generate bubbles inside the coating film layer 320, in the case of a water-soluble coating with water as the solvent, a hydrophobic (oleophilic) surface is formed on the surface of the ceramic particles. When the ceramic particles are mixed with the polymer binder and solvent to homogenize, bubbles are formed on the surface of the ceramic particles.

[0157] Similarly, in the case of oil-based coatings with oil as the solvent, a hydrophilic (oleophobic) surface is formed on the surface of ceramic particles. When the ceramic particles are mixed with polymer binders and solvents to homogenize, bubbles are formed on the surface of the ceramic particles.

[0158] In this way, the manipulation of the surface of ceramic particles can be performed on some or all of the ceramic particles, and can be achieved on some or all types of ceramic particles. The concentration of bubbles can be adjusted in this way.

[0159] Typically, ceramic microparticles are naturally hydrophilic, and their surface properties can be modified to hydrophobic by treating them with a solution containing stearic acid.

[0160] To effectively scatter and reflect incident sunlight, radiation-cooling coatings can be made of materials with a refractive index that differs greatly from that of the polymer resin that acts as a binder.

[0161] That is, materials with high refractive index values ​​have high bandgap energy values, and therefore materials that are transparent to incident sunlight are selected.

[0162] Therefore, the present invention can provide a radiation cooling coating that forms bubbles inside the coating film to reduce the coating film thickness required to achieve radiation cooling performance, reduces the coating operability and ease of application of the radiation cooling coating, thereby enabling more active light scattering inside the radiation cooling coating.

[0163] Furthermore, the present invention can provide a radiation cooling coating that exhibits excellent radiation cooling performance even at a thin thickness, and can be applied without a thick thickness, thereby having excellent coating operability.

[0164] Figure 4a and Figure 4b This is a diagram illustrating the optical properties of a radiation-cooling coating according to an embodiment of the present invention.

[0165] Figure 4a The invention will be illustrated by comparing it with the prior art through the optical properties of a radiation-cooling coating according to an embodiment of the invention, specifically those related to reflectivity.

[0166] Reference Figure 4a The curve 400 compares the reflectance of sample 401, which is a radiation cooling coating with a porous coating layer based on the present invention, with that of sample 402 in the prior art.

[0167] Figure 4b The invention will be illustrated by comparing it with the prior art through the optical properties of a radiation-cooling coating according to an embodiment of the invention, which are related to absorption.

[0168] Reference Figure 4bThe graph 410 compares the absorption rate of sample 411 of the radiation cooling coating based on the present invention, which forms a coating film layer in a porous manner, with that of sample 412 of the prior art.

[0169] Related to curves 400 and 410, yttria-stabilized zirconia (YSZ) microparticles with a particle size of 0.4 μm to 0.6 μm were mixed with a polytetrafluoroethylene polymer binder to prepare a radiation cooling coating. One sample was prepared by low-speed stirring at 1000 rpm, and another sample was prepared by high-speed stirring at 2000 rpm.

[0170] Graphs 400 and 410 show the results of measuring the optical properties after coating two radiation-cooled coating samples prepared in this way onto a substrate. The samples subjected to low-speed stirring correspond to samples 401 and 411, which are relevant to this invention, while the samples subjected to high-speed stirring correspond to samples 402 and 412, which are relevant to the prior art.

[0171] As shown in graphs 400 and 410, it can be confirmed that compared with samples 402 and 412, samples 401 and 411 exhibit high reflectivity and low absorptivity in all regions of incident sunlight.

[0172] It can be confirmed that in samples 401 and 411, the presence of bubbles promotes light scattering and increases reflection. In samples 402 and 412, there are relatively few bubbles, so light scattering cannot occur effectively. This reduces the reflection caused by light scattering and allows light to penetrate deeper into the coating film layer, thus showing an increased absorption rate.

[0173] Therefore, the present invention can provide a radiation cooling coating that exhibits excellent radiation cooling performance even with increased binder content due to effective light scattering caused by bubbles, thereby improving the durability of the coating layer by increasing the binder content.

[0174] Furthermore, the present invention can provide a radiation cooling coating that has high cooling radiation capacity regardless of day or night. When applied to outdoor structures or buildings, it minimizes the absorption of incident sunlight even during the day when sunlight is intense, and effectively maintains the heat release through long-wave infrared radiation to improve radiation cooling performance.

[0175] Figure 5 This is an electron microscope image illustrating a radiation cooling coating according to an embodiment of the present invention.

[0176] Figure 5 An electron microscope image of a radiation cooling coating according to an embodiment of the present invention is shown.

[0177] Reference Figure 5 The first electron microscope image 500 shows agglomerates of about 20 micrometers observed in the case of a radiation-cooled coating containing bubbles prepared by low-speed stirring.

[0178] On the other hand, in the second electron microscope image 510, the aforementioned agglomerates were not observed in the de-bubbling specimen prepared by high-speed stirring.

[0179] As shown in the first electron microscope photograph 500 and the second electron microscope photograph 510, which are high-magnification electron microscope images, the coating sample prepared by high-speed stirring to remove bubbles shows fewer bubbles, corresponding to empty spaces behind particles, compared to the sample prepared by low-speed stirring that contains bubbles.

[0180] It can be confirmed that both samples show similar atmospheric window reflectance of 94% to 95%, while the sample with low-speed stirring has high reflectance of incident sunlight, showing a high radiative cooling capacity of over 100W.

[0181] The ceramic microparticles of the radiation cooling coating according to an embodiment of the present invention have a sufficiently high refractive index and a low extinction coefficient in sunlight, and a high extinction coefficient in the atmospheric window region, thereby achieving high solar reflection, low solar transmittance, and high atmospheric window region radiation even at a low thickness.

[0182] For example, radiation cooling power can be increased by using radiation cooling coatings with a refractive index of 1.7 or higher and a bandgap of 5 eV or higher at a visible light wavelength of 550 nm.

[0183] For example, radiation cooling coatings can be in powder form.

[0184] To improve the workability of radiation cooling coatings, various additives (such as dispersants, photoinitiators, etc.) can be added.

[0185] The performance of the coating layer can be improved by adding a transparent topcoat on top of the coating layer formed by radiation-cooled coating.

[0186] An undercoat or intermediate coat can be added to the underside of the coating film formed by radiation-cooled coating to improve adhesion to the substrate.

[0187] The radiation cooling coating of one embodiment of the present invention can replace existing coatings and be applied to buildings, containers, antenna boxes, cooling towers, oil (water) pipes, automobiles, safety helmets, etc., and can be applied to all product groups that require cooling.

[0188] Furthermore, since the composition of radiation cooling coatings is similar to that of existing coatings, it can be achieved by changing some of the constituent materials.

[0189] In the above specific embodiments, the structural elements included in the present invention are expressed in singular or plural forms in the disclosed specific embodiments.

[0190] However, the singular or plural expression is chosen appropriately in the situation shown for ease of explanation. The above embodiments are not limited to singular or plural structural elements. Structural elements expressed in plural can be composed of singular elements, and structural elements expressed in singular can also be composed of plural elements.

[0191] On the other hand, although the description of the invention is related to specific embodiments, it is obvious that various modifications can be made without departing from the scope of the technical concept contained in the various embodiments.

[0192] Therefore, the scope of the present invention should not be limited to the illustrated embodiments, but should be determined by the scope of protection of the appended claims and equivalents.

Claims

1. A radiation cooling coating, characterized in that, It consists of ceramic microparticles that act as pigments, polymer resins that act as binders, and solvents. After being applied to a substrate, it forms a coating film. The aforementioned coating film layer maximizes the reflection of incident sunlight and minimizes absorption. Simultaneously, it maximizes the emission of long-wave infrared radiation equivalent to 8μm to 13μm to prevent energy inflow from the incident sunlight and increases energy release based on this long-wave infrared radiation, thereby achieving a radiative cooling function. To increase the reflection of incident sunlight without reducing the long-wave infrared radiation, the volume of bubbles with a size similar to the aforementioned ceramic microparticles formed within the coating film layer is 3% to 50%. In the aforementioned coating film layer, the ceramic microparticles and the aforementioned polymer binder are homogeneously mixed to form bubbles on the surface of the ceramic microparticles, thereby forming a composite of the ceramic microparticles and the aforementioned bubbles. The aforementioned bubbles are located at the interface surrounding the surface of the ceramic particles dispersed in the aforementioned polymer binder. The ceramic-bubble interface composite distributed throughout the entire coating film layer is defined by bubbles surrounding at least a portion of the surface of the aforementioned ceramic microparticles. The aforementioned composite increases the reflection of incident sunlight at the interfaces between the ceramic particles and the polymer binder, between the ceramic particles and the bubbles, and between the bubbles and the polymer binder, without reducing the long-wave infrared radiation. As the bubble volume increases, it reduces at least one of the following: the thickness of the coating film and the content of the ceramic particles. The aforementioned conjugate includes a first conjugate and a second conjugate. The aforementioned ceramic particles include first ceramic particles and second ceramic particles. The first ceramic microparticle and the second ceramic microparticle have different wavelength-dependent optical properties. The first composite material is a combination of the first ceramic microparticle and the bubble. The second composite is a combination of the second ceramic microparticle and the bubble. The size of the aforementioned ceramic particles and bubbles ranges from 0.1 μm to 5 μm.

2. The radiation cooling coating according to claim 1, characterized in that, The aforementioned ceramic microparticles include at least one of titanium dioxide, aluminum oxide, hexagonal boron nitride, zirconium dioxide, silicon dioxide, calcium carbonate, barium sulfate, magnesium oxide, yttrium oxide, yttrium-stabilized zirconium oxide, beryllium oxide, manganese oxide, zinc oxide, silicon carbide, and aluminum nitride, and at least one of polymer microparticles selected from polyvinylidene fluoride, polytetrafluoroethylene, and ethylene-tetrafluoroethylene copolymer.

3. The radiation cooling coating according to claim 2, characterized in that, The ceramic microparticles were selected by taking into account the refractive index and extinction coefficient of the incident sunlight and the extinction coefficient of the long-wave infrared radiation.

4. The radiation cooling coating according to claim 1, characterized in that, The aforementioned polymer resin includes at least one of polyurethane resin, alkyd resin, acrylate resin, polyvinyl chloride, polyethylene, acrylic resin, dipentaerythritol hexaacrylate, and fluoropolymer resin.

5. The radiation cooling coating according to claim 1, characterized in that, The weight ratio of the above-mentioned ceramic microparticles to the above-mentioned polymer resin is x∶1, where x is 0.15 to 3.

6. The radiation cooling coating according to claim 1, characterized in that, The thickness of the coating film layer is less than 300 μm.

7. The radiation cooling coating according to claim 1, characterized in that, To improve the workability of the coating, it also contains at least one additive from the dispersant and photoinitiator.

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