Radiation refrigeration coating as well as preparation method and application thereof

Abstract

The invention provides a radiation refrigeration coating as well as a preparation method and application thereof. The invention discloses a preparation method and application of a radiation refrigeration coating for constructing a surface microstructure based on a roller coating process, and aims to overcome the defects of an existing radiation refrigeration coating in the aspects of microstructure design and regulation and control, so that the overall radiation refrigeration performance of the radiation refrigeration coating is improved. According to the method, the rough microstructure with controllable height and controllable density is constructed on the surface of the coating through combination of thixotropic agent regulation and control and a roll coating process.

Description

Technical Field

[0001] This invention belongs to the field of coating technology, specifically relating to a radiation-cooling coating, a method for preparing the radiation-cooling coating, and the application of the radiation-cooling coating in the preparation of coatings. Background Technology

[0002] Since the beginning of the 21st century, against the backdrop of the energy crisis and climate change, global temperatures have continued to rise, and the impact of cooling energy consumption on electricity demand growth has become increasingly significant. Radiation cooling, as an emerging passive cooling technology, utilizes the 8-14 μm "atmospheric window" between Earth and outer space to achieve infrared radiation exchange, providing a revolutionary zero-energy solution for high-energy-consuming fields such as power communication and new energy. In power communication, this technology can effectively reduce the temperature of communication base stations and transformers, reduce equipment failure rates, and improve operational safety. In the new energy field, by cooling charging stations and energy storage cabinets, it can improve energy conversion efficiency, extend equipment life, and reduce operation and maintenance costs. Radiation cooling not only achieves passive cooling without additional energy consumption but also promotes collaborative energy conservation across multiple fields, which is of great significance for building an environmentally friendly society.

[0003] In the field of radiation-cooling coatings, constructing a reasonable coating morphology has been proven to significantly enhance solar scattering and mid-infrared emission, thereby improving cooling performance. To this end, several patents have attempted to optimize the surface morphology of the coating by controlling its pore structure or interlayer composition. For example, patent CN202210735420.9 uses an acid etching method to prepare a porous coating by introducing soluble micro / nano particles into the coating and then using acid etching to dissolve and form pores. The reflectivity reaches 0.92 in the solar spectrum wavelength range (0.25~2.5 μm), effectively reflecting sunlight. Its emissivity is as high as 0.95 in the atmospheric window wavelength range, significantly dissipating its own heat. However, this method relies on multi-step chemical processing, requires high acid resistance of the organic polymer materials, and slight errors can lead to coating swelling, embrittlement, or pore structure collapse. The process is complex and difficult to achieve stable mass production. Similarly, patent CN112375418A utilizes a high internal phase emulsion template to prepare a multi-level porous PDMS / SiO2 coating. The coating's reflectance and emissivity spectra in the 0.3–2.5 μm wavelength range show a solar reflectance as high as 0.95 and a long-wave infrared emissivity as high as 0.98. Compared to pure PDMS coatings, it achieves an average cooling reduction of approximately 4.5 °C. However, the stability of the emulsion system directly determines the uniformity of the pore structure. If demulsification occurs before polymerization, it can lead to uncontrolled pore size distribution and even overall structural collapse. This method places stringent requirements on the reaction system and process window, hindering large-scale applications. Furthermore, patent CN111690301A coordinates solar reflection and mid-infrared radiation by constructing a gradient distribution of polymer binder and inorganic powder in the vertical direction. The coating exhibits a reflectance of 0.97 in the 0.3–2.5 μm band and an emissivity of 0.98 in the 8–13 μm band, achieving an additional cooling reduction of 5.3 °C compared to coatings without a gradient structure. However, this approach requires precise control of coating thickness and component gradient, typically relying on specialized equipment such as layer-by-layer spraying or centrifugal deposition. The process involves numerous steps, poor stability, and is difficult to achieve continuous, low-cost preparation. Overall, existing technologies mostly rely on chemical reactions within the material system or complex templates to construct pores, thereby controlling the morphology of radiation-cooled coatings. However, these methods generally suffer from cumbersome processes, high costs, strong equipment dependence, and poor structural consistency, limiting their large-scale application. In contrast, directly replicating a rough texture structure mechanically during coating application is a simpler, lower-cost strategy suitable for large-area coating. For example, patent CN119869892A applies a water-based two-component polyurethane topcoat to the surface of a slide. Before the topcoat dries, a pattern is created using a sponge roller to increase the contact area of ​​the subsequent nano-silicon clear coat, enhancing interlayer adhesion. This process effectively improves the bonding strength of the composite coating.However, in practical applications, due to the low initial viscosity of polyurethane topcoat, the surface structure formed by roller coating is not stable enough, which easily leads to sagging and fading of patterns, affecting the forming stability of surface texture and the controllability of roughness.

[0004] To overcome the aforementioned shortcomings, this invention provides a radiation-cooling coating that introduces a thixotropic agent into the roller coating configuration. By controlling the shear thinning and static thickening behavior of the coating system, a balance between application fluidity and structural retention is achieved. While maintaining easy application and low cost, it effectively solves the problem of texture retention in traditional roller coating methods, demonstrating significant technical advantages and promotional value. Summary of the Invention

[0005] The primary objective of this invention is to provide a radiation-cooling coating that overcomes the shortcomings of unstable morphology and structure in existing radiation-cooling coatings.

[0006] A second objective of this invention is to provide a method for preparing the above-mentioned radiation-cooling coating.

[0007] A third objective of this invention is to provide an application of the above-mentioned radiation-cooling coating in the preparation of coatings.

[0008] To achieve the above objectives, the following technical solution is adopted: A radiation-cooling coating comprises the following components in parts by weight: 1-5 parts of radiation-functional filler dispersant, 1-5 parts of reflective-functional filler dispersant, 5-15 parts of water, 20-50 parts of film-forming substance, 10-40 parts of radiation-functional filler, 10-50 parts of reflective-functional filler, 1-12 parts of wetting agent, 1-5 parts of defoamer, 1-5 parts of thickener, and 1-5 parts of thixotropic agent.

[0009] Furthermore, the radiation cooling coating comprises the following components in parts by weight: 4 parts radiation functional filler dispersant, 1 part reflective functional filler dispersant, 10 parts water, 30 parts film-forming substance, 40 parts radiation functional filler, 10 parts reflective functional filler, 2 parts wetting agent, 1 part defoamer, 1 part thickener, and 3 parts thixotropic agent.

[0010] In this invention, the thixotropic agent is one of fumed silica, polyethylene wax, polypropylene wax, polyamide wax, and organobentonite.

[0011] Preferably, the thixotropic agent is a polyamide wax.

[0012] In this invention, the reflective filler is one or more of polytetrafluoroethylene, ZnS, ZnO, ZrO2, MgO, CaCO3, and BaSO4.

[0013] Preferably, the reflective filler is ZnO.

[0014] Furthermore, the reflective functional filler dispersant is one or more of polyvinyl alcohol, polyacrylate, sodium dodecylbenzenesulfonate, and sodium dodecyl sulfate.

[0015] In this invention, the radiation-functional filler is one or more of nanocellulose, SiO2, boron nitride, silicon nitride, polydimethylsiloxane, and CaO.

[0016] Preferably, the radiation-functional filler is silicon nitride.

[0017] Furthermore, the radiation functional filler dispersant is one or more of polycarboxylate, trimethoxysilane, and polymethacrylate.

[0018] In this invention, the film-forming substance is one or more of acrylic resin, silicone resin, polyester resin, polyvinyl chloride resin, polyethylene resin, polyurethane resin, and fluorocarbon resin.

[0019] Preferably, the film-forming substance is an acrylic resin.

[0020] In this invention, the wetting agent is one or more of the following: polyether siloxane copolymer, polyether modified polydimethylsiloxane, perfluoropolyether, fatty alcohol polyoxyethylene ether, and ethoxylated fatty acid ester.

[0021] In this invention, the defoamer is one or more of the following: tributyl phosphate, polyether-modified silicone oil, polysiloxane, polydimethylsiloxane, polypropylene glycol, and polyethylene glycol.

[0022] In this invention, the thickener is one or more of latex acrylic polymer, polyvinyl alcohol, polyacrylamide, polyethylene glycol, and copper polyethylene.

[0023] A method for preparing a radiation-cooling coating includes the following steps: (1) First, place the dispersant, water and film-forming substance in a high-speed disperser and disperse at a speed of 1000~1200 r / min for 20~30 min at room temperature to obtain a resin solution of functional filler that is easy to disperse. (2) Then, the radiation functional filler, the reflection functional filler, the wetting agent, the defoamer and the thickener are added to the resin solution and dispersed at room temperature at a speed of 1600~1800 r / min for 1~2 h to obtain the radiation cooling coating.

[0024] The application of the above-mentioned radiation-cooling coating in the preparation of coatings.

[0025] Furthermore, the application of the radiation-cooling coating in the preparation of the coating specifically includes the following steps: (1) Spray or roll a smooth base coat on the substrate surface and let it dry at room temperature for 3~8 hours; (2) The radiation cooling coating is uniformly rolled onto the smooth base layer surface using a roller with a patterned surface. After the roller coating is completed, it is placed at room temperature and allowed to dry for 3-8 hours to obtain the radiation cooling coating.

[0026] In this invention, the thickness of the smooth underlayer coating is ≥0.1 mm.

[0027] In this invention, the pattern structure on the surface of the roller is one of the following: a dot matrix, microgrooves, or a disordered rough structure.

[0028] The present invention has the following beneficial effects: (1) The radiation cooling coating of the present invention introduces a thixotropic agent into the coating, so that the system has both excellent construction rheology and structure retention ability. During the rolling process, it can effectively suppress sagging, self-leveling and texture collapse, so that the rough microstructure formed by roller replication can be stabilized and solidified, significantly increasing the scattering area of ​​sunlight, improving the radiation emissivity of the atmospheric window band and enhancing the radiation cooling effect.

[0029] (2) This invention combines thixotropic agent regulation with roller coating process to construct a rough microstructure with controllable height and density on the coating surface, which makes up for the shortcomings of existing radiation cooling coatings in microstructure design and regulation, thereby improving its overall radiation cooling performance.

[0030] (3) The hydrogen bond network in the radiation cooling coating system of the present invention is more dense and stable, which can form higher and stronger microstructure protrusions with better structure retention. By adjusting the pitch density of the pattern on the roller surface, the density of the wrinkle arrangement on the coating surface can be controlled, which can also increase the solar radiation scattering area and mid-infrared radiation area and improve the cooling performance. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the mechanism of action of the thixotropic agent of the present invention; Figure 2 This is a schematic diagram of the drum structure of the present invention; Figure 3 This is a schematic diagram showing the height of the protrusions on the coating surface of the present invention; Figure 4 This is a diagram showing the effect of the thixotropic agent-free roller coating of the present invention; Figure 5 This is a roll coating effect diagram of the underlying coating thickness of 0.08 mm according to the present invention; Figure 6 The images show the coating effects of different thixotropic agent contents according to the present invention. Figure 7 These are diagrams illustrating the effects of different roller coating methods according to the present invention; Figure 8 This is a schematic diagram of the temperature measuring device of the present invention; The following labels are used in the attached diagram: 1. Thermocouple; 2. Aluminum foil; 3. Foam box; 4. Transparent PE film; 5. Substrate; 6. Heating plate. Detailed Implementation

[0032] This invention provides a radiation-cooling coating and a method for preparing such a coating. By combining thixotropic agent regulation with roller coating, a rough microstructure with controllable height and density is constructed on the coating surface. In this invention, the thixotropic agent significantly reduces the viscosity of the coating under external forces (such as stirring, brushing, or roller coating), ensuring good flowability and film-forming properties. After application and settling, the coating rapidly recovers its high viscosity, constructing a stable hydrogen bond network structure, significantly inhibiting sagging, self-leveling, and texture collapse, thereby effectively fixing the rough microstructure formed by roller replication. This rough surface, constructed by the roller and fixed by the thixotropic agent, increases the scattering area of ​​sunlight and improves reflectivity in the visible-near-infrared band. Furthermore, the microcavity structure between the folds allows for multiple reflections and oscillations of infrared light, extending the optical path, increasing radiation absorption, and enhancing emissivity.

[0033] The basic process by which the coating prepared by the radiation-cooling coating of this invention achieves radiation cooling is as follows: On the one hand, by introducing thixotropic agents into the coating, the system possesses both excellent workability and structural retention. During roller coating, the system viscosity decreases and fluidity increases, which is beneficial for coating spread. After the external force is removed, the system viscosity rapidly recovers and rebuilds the internal hydrogen bond network, thereby effectively suppressing sagging, self-leveling, and texture collapse, allowing the rough microstructure formed by roller replication to be stably cured. This type of microstructure with a certain height not only significantly increases the scattering area of ​​sunlight, but its surface wrinkles also promote multiple oscillations of incident light in the microcavity, extending the optical path, thereby increasing the radiative emissivity in the atmospheric window band and enhancing the radiative cooling effect. Furthermore, the ratio of thixotropic agent to resin has a crucial influence on the height of the rough microstructure. As the ratio of thixotropic agent to resin increases, the hydrogen bond network in the system becomes denser and more stable, enabling the formation of higher and more structurally intact microstructure protrusions. However, when the ratio of thixotropic agent to resin exceeds a reasonable range, although the hydrogen bond network is further enhanced, the growth of microstructure height tends to saturate. In some cases, excessively high local viscosity may lead to uneven microstructure replication or local collapse, resulting in a decrease in reflectivity and emissivity, thus affecting the final radiative cooling performance.

[0034] The mechanism of action of thixotropic agents is as follows Figure 1 As shown, thixotropic agents form a three-dimensional network structure through intermolecular hydrogen bonds, enabling the coating system to possess both excellent application leveling and structure retention capabilities. Its mechanism of action mainly includes the following two interactions: Firstly, there is the wax-wax interaction: the polyamide wax particles dispersed in the system have amide bonds on their surfaces interconnected through hydrogen bonds, self-assembling to form a three-dimensional hydrogen bond network (e.g., Figure 1 (as shown in a).

[0035] Secondly, there is the wax-resin interaction: the -COOH groups on acrylic resins can also form hydrogen bonds with the amide bonds of polyamide waxes, allowing resin molecules to "bridge" and "reinforce" the existing network structure through hydrogen bonds (e.g., Figure 1 (as shown in b).

[0036] The interaction between polyamide wax thixotropic agents and acrylic resins is primarily physical, based on hydrogen bonding, forming a reversible, shear-sensitive three-dimensional network structure. This is the fundamental mechanism behind their thixotropic properties (e.g., Figure 1 (as shown in c).

[0037] Given the aforementioned synergistic effect, the number and strength of the hydrogen bond network in the system directly depend on the ratio of polyamide wax to acrylic resin. Therefore, by adjusting the ratio, the three-dimensional network structure can be precisely controlled, thereby optimizing the rheological properties and application performance of the coating.

[0038] On the other hand, by adjusting the pitch density of the pattern on the roller surface, the density of the wrinkle arrangement on the coating surface can be controlled, which can also increase the solar radiation scattering area and the mid-infrared radiation area, thereby improving the cooling performance. For example... Figure 2 The illustrated roller structure has axially extending grooves on the outer circumferential surfaces of the three rollers, with different groove pitches, resulting in three different groove distribution densities: sparse, medium, and dense. The pitch of the sparse-pitch rollers is 3.5~5 mm, the pitch of the medium-pitch rollers is 2~3.5 mm, and the pitch of the close-pitch rollers is 0.5~2 mm. In this embodiment of the invention, the pitch of the sparse-pitch rollers is approximately 4.4 mm, the pitch of the medium-pitch rollers is approximately 2.6 mm, and the pitch of the close-pitch rollers is approximately 1.2 mm. Therefore, this invention effectively solves the problem of unstable morphology and structure caused by existing radiation cooling coatings relying on internal chemical reactions or complex templates, while also offering advantages such as simple construction and low cost. This gives it significant technical advantages and broad application potential.

[0039] Example 1 A radiation-cooling coating comprises the following components in parts by weight: 4 parts radiation functional filler dispersant, 1 part reflective functional filler dispersant, 10 parts water, 30 parts film-forming substance, 40 parts radiation functional filler, 10 parts reflective functional filler, 2 parts wetting agent, 1 part defoamer, 1 part thickener, and 1 part thixotropic agent.

[0040] The dispersant for the radiation-functional filler is polycarboxylate, the dispersant for the reflection-functional filler is polyvinyl alcohol, the film-forming substance is acrylic resin, the radiation-functional filler is silicon nitride, the reflection-functional filler is ZnO, the wetting agent is polyether siloxane copolymer, the defoamer is polyether modified silicone oil, the thickener is latex acrylic polymer, and the thixotropic agent is polyamide wax.

[0041] This example provides a method for preparing a radiation-cooling coating, which consists of two layers, upper and lower, and the specific preparation method is as follows: (1) Preparation method of the primer coating: First, place 4 parts of radiation functional filler dispersant, 1 part of reflection functional filler dispersant, 10 parts of water and 30 parts of film-forming material into a high-speed disperser and disperse at room temperature and high speed for 10 min at a speed of 1000 r / min to obtain a resin solution that is easy to disperse for reflection and radiation functional fillers.

[0042] Ten parts of ZnO with a particle size of 300-500 nm, 40 parts of silicon nitride with a particle size of 800-1000 nm, two parts of wetting agent, one part of defoamer, and one part of thickener were added to a dispersible resin solution and dispersed at room temperature and high speed for 2 h at a rotation speed of 1600 r / min. The bottom layer radiation cooling coating was prepared by this method.

[0043] Preparation method of top-layer radiation cooling coating: First, place 4 parts of radiation functional filler dispersant, 1 part of reflection functional filler dispersant, 10 parts of water and 30 parts of film-forming material into a high-speed disperser and disperse at room temperature and high speed for 10 min at a speed of 1000 r / min to obtain a resin solution that easily disperses reflection and radiation functional fillers.

[0044] Ten parts of ZnO with a particle size of 300-500 nm, 40 parts of silicon nitride with a particle size of 800-1000 nm, one part of thixotropic agent, two parts of wetting agent, one part of defoamer, and one part of thickener were added to a dispersible resin solution and dispersed at high speed at room temperature for 2 h at a rotation speed of 1600 r / min. The top-layer radiation cooling coating was prepared by this method.

[0045] (2) Spray or roll a smooth base coat with a thickness of 0.2 mm onto the substrate surface. This base coat is used to effectively avoid the problem of substrate exposure due to excessive construction force during the subsequent roll coating process. Allow it to dry at room temperature for 3 to 8 hours.

[0046] (3) Adopt Figure 1 The roller with a sparse pitch pattern is used to uniformly roll the radiation cooling coating onto the surface of the underlying smooth coating. After the coating is rolled, the coating is left to cure at room temperature for 3 to 8 hours.

[0047] Example 2 The difference between this example and Example 1 is that two parts of thixotropic agent are added to the top-layer radiative cooling coating.

[0048] Example 3 The difference between this example and Example 1 is that 3 parts of thixotropic agent are added to the top-layer radiative cooling coating.

[0049] Example 4 The difference between this example and Example 1 is that 4 parts of thixotropic agent are added to the top-layer radiative cooling coating.

[0050] Example 5 The difference between this example and Example 1 is that 5 parts of thixotropic agent were added to the top-layer radiative cooling coating.

[0051] Example 6 The difference between this example and Example 3 is that a medium-pitch roller is used for roller coating.

[0052] Example 7 The difference between this example and Example 3 is that a roller with a close pitch is used for roller coating.

[0053] Example 8 The difference between this example and Example 7 is that 3 parts of radiation functional filler dispersant, 2 parts of reflection functional filler dispersant, 30 parts of radiation functional filler and 20 parts of reflection functional filler are added to the top layer radiation cooling coating.

[0054] Example 9 The difference between this example and Example 7 is that 2 parts of radiation functional filler dispersant, 3 parts of reflective functional filler dispersant, 20 parts of radiation functional filler and 30 parts of reflective functional filler are added to the top layer radiation cooling coating.

[0055] Example 10 The difference between this example and Example 7 is that 1 part of radiation functional filler dispersant, 4 parts of reflective functional filler dispersant, 10 parts of radiation functional filler and 40 parts of reflective functional filler are added to the top layer radiation cooling coating.

[0056] Comparative Example 1 The difference between this example and Example 1 is that no thixotropic agent was added to the top-layer radiative cooling coating.

[0057] Comparative Example 2 The difference between this example and Example 1 is that the thickness of the underlying smooth coating is 0.08 mm.

[0058] Performance testing (1) Test of the coating effect of different radiation cooling coatings After adding the thixotropic agent, use Figure 1 The intermediate roller applies a 0.2 mm thick base coat. After roller coating, a raised surface with a certain height is formed on the coating surface, such as... Figure 3 As shown.

[0059] The protrusion has an effective height perpendicular to the outer surface of the coating. H The effective height of the protrusion is defined by the following formula. (1) Using calipers, the height of the protrusions at nine different locations on the coating surface was selected and measured using a nine-square grid method, and the effective height of the coating protrusions was calculated using formula (1). The specific results are shown in Table 1.

[0060] In Comparative Example 1, without the addition of a thixotropic agent, a wide-pitch roller was used to coat a 0.2 mm thick undercoat. The coating effect was as follows: Figure 4 As shown.

[0061] Coatings without added thixotropic agents have a significant leveling effect, making it difficult to maintain the rough texture formed by the roller. They ultimately present a basically smooth surface morphology and cannot effectively increase the light reflection area and mid-infrared radiation area.

[0062] In Comparative Example 2, after adding one part of thixotropic agent, a roller with a wide pitch was used to coat a base layer with a thickness of 0.08 mm. The coating effect was as follows. Figure 5 As shown.

[0063] like Figure 5 As shown, if the thickness of the underlying smooth coating is too thin, the substrate may be exposed due to excessive application pressure during subsequent roller coating. Therefore, to avoid this problem, the thickness of the underlying smooth coating should be ≥0.1 mm.

[0064] In Examples 1, 3, and 5, different amounts of thixotropic agent were added, and then... Figure 2 A medium-pitch roller is used to coat a base coat with a thickness of 0.2 mm. The coating result is as follows: Figure 6 As shown.

[0065] From left to right, the thixotropic agent content is 1 part, 3 parts, and 5 parts, respectively. By introducing the thixotropic agent into the coating, the system possesses both excellent application rheological properties and structure retention capabilities. During roller coating under stress, the system viscosity decreases and the fluidity increases, which is beneficial for coating spread. After the external force is removed, the system viscosity quickly recovers and rebuilds the internal hydrogen bond network, thereby effectively inhibiting sagging, self-leveling, and texture collapse, allowing the rough microstructure formed by roller replication to be stably cured.

[0066] Based on this, the ratio of thixotropic agent to resin directly affects the protrusion height of the rough structure. When one part of thixotropic agent is added, the ratio of thixotropic agent to resin is 1:30, and the protrusion height of the microstructure is 0.17 mm. As the ratio of thixotropic agent to resin increases, the hydrogen bond network in the system becomes denser and more stable, which can form higher microstructure protrusions with stronger structure retention. When three parts of thixotropic agent are added, the ratio of thixotropic agent to resin is 1:10, and the protrusion height can reach 0.35 mm. However, when the thixotropic agent content increases to 5 parts, the ratio of thixotropic agent to resin is 1:6, and the protrusion height does not show a significant increase. The reason is that when the ratio of thixotropic agent to resin is higher than the reasonable range, although the hydrogen bond network is further enhanced, the increase in microstructure height tends to saturate, and may even lead to uneven formation or local collapse of the microstructure due to excessively high local viscosity.

[0067] In Examples 3, 6, and 7, after adding three parts of thixotropic agent, rollers with different pitches were used to coat a 0.2 mm thick undercoat. The coating effect was as follows: Figure 7 As shown.

[0068] Under the condition that the ratio of thixotropic agent to resin remains constant, the protrusion height of the microstructure on the coating surface remains consistent. By adjusting the pitch of the grooves on the surface of the roller used, the density of the protrusion arrangement on the coating surface can be independently controlled.

[0069] (2) Tests on reflectivity and emissivity of different radiation-cooled coatings The reflectivity and emissivity of the directional radiation cooling coatings obtained in Examples 1-10 and Comparative Example 1 were tested. The test methods for reflectivity and emissivity were in accordance with GB / T 2680-2021 and GB / T 30127-2013, respectively. The specific test results are shown in Table 1.

[0070] (3) Test of radiation cooling effect of different radiation cooling coatings The metal substrates coated with radiation-cooling coatings prepared in Examples 1-10 and Comparative Example 1 were placed above the opening of the temperature measuring device for radiation-cooling testing. The temperature measuring device is as follows: Figure 8 As shown in the diagram, 1 is a thermocouple, 2 is aluminum foil, 3 is a foam box, 4 is a transparent PE film, 5 is the substrate, and 6 is a heating plate. The heating plate temperature was set to 50 ℃. The aluminum foil and internal foam were used to reduce the influence of external environmental heat convection, heat conduction, and heat exchange on the test results of the coating's radiative cooling performance. The thermocouple was used to measure the internal temperature of the device. The metal substrate coated with the directional radiative cooling coating was placed at the opening above the device, and the opening was covered with a transparent PE film. The test results are shown in Table 1.

[0071] Table 1. Performance test results of different embodiments and comparative examples. The density and strength of the hydrogen bond network in the system directly depend on the ratio of the thixotropic agent to the acrylic resin (film-forming substance). Therefore, adjusting the ratio can effectively control the hydrogen bond network structure, thereby optimizing the rheological and application properties of the coating. Examples 1-5 demonstrate how this ratio is systematically changed to control the hydrogen bond network.

[0072] Experimental results show that the radiative cooling performance of the coating is mainly determined by the height of its surface protrusions and the pitch of its microstructure. Adding a thixotropic agent to the coating system enables the coating to form a stable hydrogen bond network during the application and settling process, effectively maintaining the rough microstructure formed by roller coating. This rough structure significantly increases the effective reflective area and mid-infrared radiation area, thereby improving solar reflectivity and atmospheric window emissivity.

[0073] The ratio of thixotropic agent to resin directly affects the protrusion height of the rough structure. For example, in Example 1, when one part thixotropic agent is added (thixotropic agent:resin = 1:30), the microstructure protrusion height of the coating is 0.17 mm, and the coating reflectivity and emissivity increase to 91% and 91%, respectively. The average temperature drop relative to the uncoated substrate reaches 3.8 °C. As the ratio of thixotropic agent to resin increases, the hydrogen bond network in the system becomes denser and more stable, resulting in higher and more stable microstructure protrusions after roller coating. In Example 2, when two parts thixotropic agent are added (thixotropic agent:resin = 1:15), the coating protrusion height reaches 0.24 mm, and the reflectivity and emissivity further increase to 92%, with an average temperature drop of 4.2 °C. In Example 3, when the thixotropic agent content reaches 3 parts (thixotropic agent:resin = 1:10), the coating protrusion height reaches 0.35 mm, and the reflectivity and emissivity further increase to 94%, with an average temperature drop of 4.8 °C. However, when the ratio of thixotropic agent to resin increases to a certain value, although the hydrogen bond network is still enhanced, the increase in protrusion height slows down. An excessively high ratio of thixotropic agent to resin may lead to uneven microstructure formation or local collapse, resulting in a decrease in coating reflectivity and emissivity. For example, in Examples 4-5, when the thixotropic agent content is increased to 4 parts and 5 parts, the corresponding thixotropic agent to resin ratios are 2:15 and 1:6, respectively. At this time, the protrusion height remains basically unchanged, while the reflectivity and emissivity decrease to 93%, and the average temperature drop also decreases to 4.5 ℃.

[0074] Conversely, under the same roller coating conditions, the coating without thixotropic agents exhibits significant leveling effects, making it difficult to maintain the rough texture created by the roller. It ultimately presents a generally smooth surface, failing to effectively increase the light reflection area and mid-infrared radiation area. For example, in Comparative Example 1, its solar reflectivity and emissivity are only 90%, and its average temperature reduction relative to the uncoated substrate is only 3.1 °C.

[0075] Furthermore, the pitch of the microstructure on the coating surface also has a significant impact on radiative cooling performance. At the same thixotropic agent content, as the pitch density increases, the surface texture enhances the multiple reflections and scattering of incident light, lengthening the light propagation path between the wrinkles and thus further improving the effective emissivity in the atmospheric window band. As in Example 6, when the height of the microstructure protrusions is... H Under the condition of 0.35 mm, the reflectivity and emissivity of the medium-pitch coating can be increased to 95%, and the average temperature reduction relative to the uncoated substrate can reach 5.4 ℃; the coating performance of the close-pitch structure is even more outstanding. For example, in Example 7, the reflectivity and emissivity both reach 96%, and the average temperature reduction can reach 6.2 ℃.

[0076] Finally, in Comparative Example 2, an excessively thin undercoat smooth layer can lead to substrate exposure due to excessive application pressure during subsequent roller coating processes (e.g., Figure 6 As shown in the figure, in order to avoid substrate exposure due to excessive construction pressure during subsequent roller coating, the thickness of the smooth undercoat should be ≥0.1 mm.

[0077] In summary, this invention synergistically enhances the radiative cooling performance of coatings by constructing stable rough surface microstructures and introducing thixotropic agents into the coating system. The core mechanism lies in the following: under shear force (such as stirring or brushing), the thixotropic agent reduces the coating viscosity and improves fluidity, facilitating application. After settling, the viscosity rapidly recovers, the hydrogen bond network is reconstructed and solidified into a rough texture, effectively preventing sagging and maintaining the microstructure morphology. The rough texture not only increases the light reflection and radiation area but also causes incident light to oscillate multiple times within the surface wrinkles, extending the optical path and significantly increasing the emissivity in the atmospheric window band, ultimately strengthening the radiative cooling effect of the coating. Furthermore, the ratio of thixotropic agent to resin directly affects the height of the surface microstructure. As the ratio increases, the hydrogen bond network in the system becomes denser and more stable, resulting in higher and more stable microstructure protrusions after roller coating. When the ratio of thixotropic agent to resin increases to a certain value, although the hydrogen bond network still strengthens, the increase in protrusion height slows down. Excessive thixotropic agent may lead to uneven microstructure formation or local collapse, causing a decrease in coating reflectivity and emissivity.

[0078] The above description discloses only preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Therefore, equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A radiation-cooling coating, characterized in that, It includes the following components in parts by weight: 1-5 parts of radiation functional filler dispersant, 1-5 parts of reflection functional filler dispersant, 5-15 parts of water, 20-50 parts of film-forming substance, 10-40 parts of radiation functional filler, 10-50 parts of reflection functional filler, 1-12 parts of wetting agent, 1-5 parts of defoamer, 1-5 parts of thickener and 1-5 parts of thixotropic agent.

2. The radiation cooling coating according to claim 1, characterized in that, It contains the following components in parts by weight: 4 parts of radiation functional filler dispersant, 1 part of reflection functional filler dispersant, 10 parts of water, 30 parts of film-forming substance, 40 parts of radiation functional filler, 10 parts of reflection functional filler, 2 parts of wetting agent, 1 part of defoamer, 1 part of thickener and 3 parts of thixotropic agent.

3. The radiation-cooling coating according to claim 1 or 2, characterized in that, The thixotropic agent is one of fumed silica, polyethylene wax, polypropylene wax, polyamide wax, or organobentonite.

4. The radiation cooling coating according to claim 3, characterized in that, The reflective functional filler is one or more of polytetrafluoroethylene, ZnS, ZnO, ZrO2, MgO, CaCO3, and BaSO4; the reflective functional filler dispersant is one or more of polyvinyl alcohol, polyacrylate, sodium dodecylbenzenesulfonate, and sodium dodecyl sulfate.

5. The radiation cooling coating according to claim 4, characterized in that, The radiation-functional filler is one or more of nanocellulose, SiO2, boron nitride, silicon nitride, polydimethylsiloxane, and CaO; the radiation-functional filler dispersant is one or more of polycarboxylate, trimethoxysilane, and polymethacrylate.

6. The radiation cooling coating according to claim 3, characterized in that, The film-forming substance is one or more of acrylic resin, silicone resin, polyester resin, polyvinyl chloride resin, polyethylene resin, polyurethane resin, and fluorocarbon resin; the wetting agent is one or more of polyether siloxane copolymer, polyether-modified polydimethylsiloxane, perfluoropolyether, fatty alcohol polyoxyethylene ether, and ethoxylated fatty acid ester; the defoamer is one or more of tributyl phosphate, polyether-modified silicone oil, polysiloxane, polydimethylsiloxane, polypropylene glycol, and polyethylene glycol; and the thickener is one or more of latex acrylic polymer, polyvinyl alcohol, polyacrylamide, polyethylene glycol, and copper polyethylene.

7. A method for preparing a radiation-cooling coating according to any one of claims 1-6, characterized in that, Includes the following steps: (1) First, place the dispersant, water and film-forming substance in a high-speed disperser and disperse at a speed of 1000~1200 r / min for 20~30 min at room temperature to obtain a resin solution of functional filler that is easy to disperse. (2) Then, the radiation functional filler, the reflection functional filler, the wetting agent, the defoamer and the thickener are added to the resin solution and dispersed at room temperature at a speed of 1600~1800 r / min for 1~2 h to obtain the radiation cooling coating.

8. The use of the radiation-cooling coating according to any one of claims 1-6 in the preparation of a coating.

9. The application of the radiation-cooling coating according to claim 8 in the preparation of coatings, characterized in that, The application of the radiation-cooling coating in coating preparation specifically includes the following steps: (1) Spray or roll a smooth base coat on the substrate surface and let it dry at room temperature for 3~8 hours; (2) The radiation cooling coating is uniformly rolled onto the smooth base layer surface using a roller with a patterned surface. After the roller coating is completed, it is placed at room temperature and allowed to dry for 3-8 hours to obtain the radiation cooling coating.

10. The application of the radiation-cooling coating according to claim 9 in the preparation of coatings, characterized in that, The pattern structure on the surface of the roller is one of the following: a dot matrix, microgrooves, or a disordered rough structure.

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