A superhydrophobic radiative refrigeration polymer coating and method of making the same

Abstract

This invention discloses a superhydrophobic radiation-cooling polymer coating, its preparation method, and its application. The coating is white and possesses a bicontinuous micro / nano porous structure. It is prepared by a ternary copolymerization reaction of polyurethane acrylate oligomers, acrylate reactive diluents, and vinyl siloxane porogens. The method involves initiating free radical polymerization of monomers in the porogen under ultraviolet light, inducing rapid phase separation, and naturally evaporating the porogen to obtain the coating. This coating exhibits superhydrophobicity without post-treatment, virtually no absorption in the solar spectrum, and near-100% emissivity in the mid-infrared window. Furthermore, the optical properties and superhydrophobicity of the coating are minimally affected by long-term ultraviolet irradiation, thermal aging, acids, and alkalis. This coating can be applied over large areas in high-performance radiation-cooling applications.

Description

Technical Field

[0001] This application relates to the field of energy-saving technology, specifically to a superhydrophobic radiation-cooling polymer coating and its preparation method. Background Technology

[0002] Traditional refrigeration technologies achieve cooling through vapor compression and refrigerant transport, accounting for approximately 15% of global electricity consumption. This not only accelerates energy depletion but also contributes significantly to greenhouse gas emissions. To mitigate these impacts, passive radiative cooling technology has attracted widespread attention. The basic principle of this technology is that materials spontaneously radiate heat into space through the "mid-infrared atmospheric window," simultaneously reflecting sunlight to achieve cooling. Effective temperature reduction requires both high solar reflectivity and mid-infrared emissivity on the material's surface. Therefore, radiative cooling technology offers a green cooling solution that requires no electricity and has enormous application potential.

[0003] In recent years, various materials, including multilayer dielectric films, wavelength-selective metamaterials, photonic crystal thin films, inorganic particle / polymer composites, and porous polymers, have been used to prepare radiative cooling materials. Preparation methods include electron beam evaporation, roll-to-roll extrusion, electrospinning, template methods, and phase separation. Reported radiative cooling materials typically require stringent material design or preparation methods. In fact, solution-processable polymer-based coatings are more suitable for practical applications due to their advantages of easy processing and large-scale production, similar to paints.

[0004] Patent CN 201810238449.X discloses an outdoor all-weather solar reflective and infrared radiation cooling coating, which adds micron-sized metal flakes or micron-sized metal-coated spheres to the system and employs a step-by-step coating method. Therefore, the introduction of metal achieves high solar reflectivity. However, due to the use of micron-sized metal-coated reflective materials, the solar reflectivity is relatively low, and the step-by-step coating makes the on-site process more complex.

[0005] On the other hand, increasing the refractive index difference between different components in the coating can improve solar reflectivity. This is generally achieved by adding high-refractive-index inorganic particles to the coating. TiO2, ZnO, etc., have been widely used as additives in white reflective coatings, as shown in patents CN 112126287 B and CN114621613 B. However, these additives usually have narrow electronic band gaps, which inevitably leads to the absorption of ultraviolet rays (accounting for ~7% of solar radiation energy) by these inorganic particles, resulting in unsatisfactory cooling effects. Some wide-bandgap inorganic particles (with low refractive index) such as CaCO3 and BaSO4 have come into the researchers' view. However, these coatings usually require a high solid content to achieve high solar reflectivity. This greatly exceeds the critical content (40-50%) of inorganic fillers in traditional coatings, which may lead to a decrease in coating adhesion. Secondly, the preparation process of micro- and nano-inorganic particles, as well as the gradual shedding of some harmful pigments during long-term service, may cause environmental problems such as dust pollution, or pose potential health risks to the human body due to the environmental migration of nanofillers.

[0006] Furthermore, considering the long-term use of the coating in outdoor environments, its durability is crucial. Outdoor contamination on the surface of optical materials, such as rainwater, dust, and microbial accumulation, can affect the optical performance of radiative cooling materials, thus impacting their cooling effect.

[0007] In recent years, although some studies have utilized photocuring-induced phase separation technology to prepare porous radiation-cooling coatings, existing systems mostly rely on highly reactive multifunctional small-molecule monomers (such as trimethylolpropane triacrylate), and the monomer ratio window used is relatively narrow, which is not conducive to low-cost preparation. These small-molecule monomers exhibit extremely high crosslinking density during photocuring, resulting in excessively rigid polymer networks, severe volume shrinkage, and coatings that are prone to brittleness and cracking. Furthermore, they have poor adhesion to many flexible or deformable substrates, making it difficult to meet the long-term service requirements of complex outdoor environments. Therefore, there is an urgent need to develop a coating system that can maintain an excellent phase-separated porous structure while also possessing good flexibility and substrate adaptability. Summary of the Invention

[0008] To address the problems existing in the prior art, the purpose of this application is to provide a superhydrophobic radiation-cooling polymer coating and its preparation method. The preparation method described in this invention is rapid and gentle, requiring no cumbersome processes or expensive equipment, and can be mass-produced. Furthermore, the radiation-cooling coating prepared by this method exhibits good self-cleaning and weather resistance, enhancing its potential for outdoor applications.

[0009] To achieve the above objectives, the present invention provides the following technical solution: This invention discloses a superhydrophobic radiation-cooling polymer coating, the raw materials of which are composed of polyurethane acrylate oligomers, acrylate reactive diluents, vinyl siloxane porogens and initiators, and the coating is obtained by rapid phase separation induced by photopolymerization reaction.

[0010] This invention also discloses a method for preparing this coating. This method involves initiating free radical polymerization of monomers in a porogen under ultraviolet light, simultaneously inducing phase separation to obtain a bicontinuous micro / nano porous white coating. The preparation steps include: preparing a mixed solution of polyurethane acrylate oligomer / acrylate reactive diluent / porogen / initiator, stirring until the solution is uniformly clear; coating the substrate surface with the solution, then irradiating it under an ultraviolet lamp to initiate the polymerization reaction; after the coating cures, a drying process is performed to remove residual solvent, resulting in a superhydrophobic radiation-cooling polymer coating. The polyurethane acrylate oligomer and acrylate reactive diluent are monomers capable of ultraviolet light initiation, the porogen is a vinylsiloxane capable of partially participating in photopolymerization, and the initiator is a photoinitiator used in conjunction with the ultraviolet light wavelength.

[0011] Furthermore, the preferred acrylate reactive diluent is selected from at least one of butyl acrylate, lauryl acrylate, isooctyl acrylate, and octadecyl acrylate.

[0012] Further, the preferred pore-forming agent is at least one of vinyltriethoxysilane, vinyltrimethoxysilane, vinyltri(β-methoxyethoxy)silane, vinylmethyldiethoxysilane, and vinylmethyldimethoxysilane.

[0013] Further, the preferred photoinitiator is at least one of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide.

[0014] Furthermore, the mass ratio of polyurethane acrylate oligomer to acrylate reactive diluent is 1:(1~12), preferably 1:(1~4).

[0015] Furthermore, the mass ratio of monomer to porogen is 1:(2~6), preferably 1:4.

[0016] Furthermore, the content of photoinitiator is 0.5~1wt% of the total mass of photosensitive resin.

[0017] According to the present invention, vinylsiloxane acts both as a porogen to initiate phase separation and as a diluent monomer to participate in partial copolymerization.

[0018] According to the present invention, the superhydrophobic radiation-cooling polymer coating exhibits a "string of pearls" morphology (consisting of small spheres) on a microscopic scale.

[0019] According to the present invention, the pore size of the superhydrophobic radiation-cooling polymer coating is bimodal, ranging from ~200 nm to ~10 μm.

[0020] According to the present invention, the porosity of the superhydrophobic radiation-cooling polymer coating is 40-80%, preferably 60%.

[0021] According to the present invention, the thickness of the superhydrophobic radiation-cooling polymer coating is 0.2-2 μm, preferably 0.5 μm.

[0022] According to the present invention, the superhydrophobic radiation-cooling polymer coating has a solar reflectance of over 93% and a mid-infrared emissivity of over 95% for the atmospheric window; preferably, the superhydrophobic radiation-cooling polymer coating has a solar reflectance of over 96% and a mid-infrared emissivity of over 98% for the atmospheric window.

[0023] According to the present invention, the intrinsic static contact angle of the superhydrophobic radiation cooling coating is greater than 150° and the roll-off angle is less than 5°.

[0024] According to the present invention, the superhydrophobic radiation-cooling polymer coating exhibits excellent radiation-cooling effects under various weather conditions, including hazy and cloudy days. Its average temperature drop on clear nights is 13 °C, and its average temperature drop on high-humidity nights is ~9 °C, with a midday solar irradiance reaching 800 W / m². -2 The average temperature drop is ~8 °C.

[0025] According to the present invention, the superhydrophobic radiation-cooling polymer coating, after undergoing a 90-day ultraviolet aging test in a high-humidity environment (365 nm, ~2.0 mW cm⁻¹), -2 After being placed at RH ~70% or at 80 °C for 30 days, or immersed in acid / alkali (HCl solution, pH=1 or KOH solution, pH=13) for 7 days, the coating showed no yellowing, peeling, or cracking. The static water droplet contact angle of the coating remained above 150°, and the optical properties remained essentially unchanged.

[0026] According to the present invention, the superhydrophobic radiation-cooling polymer coating can be applied to various substrates and exhibits good adhesion to the substrate. The substrate can be selected from at least one of glass, metal, plastic, rubber, and cement.

[0027] Compared with existing technologies, the advantages of this invention are: unlike the use of high-refractive-index inorganic particles, this invention achieves higher solar scattering efficiency by constructing pores in the polymer. Compared with silver plating and the addition of inorganic particles, the pore construction in the coating has greater cost and performance advantages.

[0028] On the one hand, a unique ternary synergistic film-forming mechanism was developed: unlike traditional phase separation systems using small-molecule rigid monomers, this invention is the first to adopt a ternary system of 'polyurethane acrylate oligomer + acrylate reactive diluent + vinyl siloxane'. The polyurethane acrylate oligomer endows the coating with excellent flexibility, wear resistance, and strong adhesion to various substrates (metals, plastics, cement, etc.). On the other hand, the problem of internal stress cracking was overcome: research has found that if only the oligomer reacts with the porogen (as in Comparative Example 2), it will often lead to excessive network rigidity and cracking. This invention, by precisely introducing an acrylate reactive diluent (mass ratio limited to 1:(1~12)), perfectly regulates the crosslinking density and polymerization rate, ensuring effective copolymerization with vinyl siloxane while eliminating internal stress caused by volume shrinkage, thus achieving large-area defect-free film formation.

[0029] In this invention, a certain amount of vinyl siloxane can reduce the rigidity of the polymer network forming the coating, which is beneficial to film formation. If the porogen is insufficient, the polymer coating will not undergo phase separation, and the coating will be transparent and dense; if the porogen is too little, the phase separation of the resulting coating will be insufficient, resulting in insufficient porosity and a semi-transparent appearance, which cannot effectively scatter sunlight; however, too much porogen will greatly reduce the polymer network density, resulting in a large volume shrinkage effect, greater internal stress, an increased probability of silver streaks, and a significant decrease in mechanical properties after volatilization and drying, making film formation difficult.

[0030] The superhydrophobic radiation-cooling polymer coating exhibits excellent solar reflectivity, primarily due to the following reasons: 1) Air has a refractive index of 1, while the polymer has a refractive index of ~1.5. The significant difference in refractive index between the micro / nanopores and the coating interface facilitates the scattering of sunlight; 2) Moderate porosity (approximately 50-60%) is beneficial for scattering sunlight; 3) The coating contains continuous pores ranging from nanometer to micrometer scale, providing sufficient light dissipation paths to generate multiple reflections; 4) Strong Mie scattering occurs when the size of the scatterer is comparable to the wavelength of the incident light. Therefore, the coating's wide pore size range (from ~200 nm to ~10 μm) can effectively scatter incident wavelengths from 250 nm ultraviolet light to 2.5 μm near-infrared light; 5) The isotropic morphology facilitates the coating's scattering behavior, similar to a whiteboard or Lambertian body, thereby reflecting sunlight incident at any angle.

[0031] Meanwhile, all three types of monomers are intrinsically hydrophobic, resulting in a low surface energy and rich micro / nano hierarchical morphology on the coating surface, which contributes to the coating's excellent superhydrophobicity, requiring no post-treatment. Furthermore, the siloxane, acrylate, and isocyanate groups exhibit strong infrared absorption capabilities within the atmospheric window region. According to Kirchhoff's law, the emissivity of an object in thermal equilibrium equals its absorptivity; therefore, the resulting coating possesses extremely high mid-infrared emissivity. In summary, the extremely high solar reflectivity and mid-infrared emissivity give the coating excellent radiative cooling effects.

[0032] The cross-linking properties of the coating endow it with excellent weather resistance, and its optical properties and superhydrophobicity are minimally affected by long-term ultraviolet irradiation, thermal aging, acids, and alkalis. This rapid photocuring technology lays the foundation for the large-area preparation of high-performance radiation-cooled coatings and their practical applications. Attached Figure Description

[0033] Figure 1 This is a scanning electron microscope image of the superhydrophobic radiation-cooling coating described in Example 1.

[0034] Figure 2 The solar reflectance (a) and mid-infrared emissivity of the superhydrophobic radiation-cooling coating described in Example 2 are given.

[0035] Figure 3 The superhydrophobic performance characterization diagram of the superhydrophobic radiation cooling coating described in Example 3 is shown in the video screenshot of a water droplet (dyed in red) rolling off the tilted coating, demonstrating the low droplet roll-off angle (the inset is a contour diagram of the static contact angle of the water droplet).

[0036] Figure 4 The radiation cooling temperature, average midday solar irradiance, and relative humidity of the superhydrophobic radiation cooling coating described in Example 4 are shown.

[0037] Figure 5 The average solar reflectance of the superhydrophobic radiation cooling coating described in Example 5 before and after 30 days of thermal aging treatment or 7 days of immersion in an acid (pH=1) or alkali (pH=13) solution.

[0038] Figure 6 This is a contour diagram of water droplets after the superhydrophobic radiation cooling coating described in Example 5 has been subjected to various harsh environmental treatments. Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods. Example 1

[0041] Accurately weigh 2.5 g of polyurethane acrylate oligomer, 7.5 g of butyl acrylate reactive diluent, and 40 g of vinyltriethoxysilane porogen, mix them, and add 0.075 g of photoinitiator TPO. Stir at room temperature in the dark until the system presents a homogeneous, clear, and transparent mixed solution. Then, coat the solution onto a glass substrate and place it under a UV curing lamp for continuous irradiation for 300 s to initiate the polymerization phase separation. Finally, allow the cured coating to air dry at room temperature for 2 h to completely remove residual volatile porogen, thus obtaining a white polymer functional coating with a bicontinuous porous structure through in-situ curing on the substrate surface.

[0042] like Figure 1 As shown, the prepared coating possesses a uniform and continuous self-supporting hierarchical micro / nanoporous structure. Its average reflectance in the solar spectral region is 95%, and its average emissivity in the mid-infrared region of the atmospheric window is 97%. Outdoor testing demonstrates that the coating can achieve a cooling effect of 12 °C at night and 6 °C at noon. The coating exhibits superhydrophobicity without any post-treatment, with a static contact angle of 156 ± 2 ° and a roll-off angle of ~3 °, indicating that water droplets roll off very easily. Example 2

[0043] The basic steps of this embodiment are the same as those of Embodiment 1, with the core difference being the adjustment of the type of hydrophobic side-chain monomer in the reactive diluent. The specific steps are as follows: Weigh 2.5 g of polyurethane acrylate oligomer, 7.5 g of isooctyl acrylate, and 40 g of vinyltriethoxysilane, mix them, and add 0.075 g of photoinitiator TPO. Stir until the solution is clear and transparent. Then, coat the solution onto a glass substrate, apply 300 s of ultraviolet light for curing, and finally dry the coating at room temperature for 2 h to remove the liquid pore-forming medium, thus obtaining a white porous polymer coating.

[0044] The prepared coating possesses a micro-nano hierarchical porous structure. For example... Figure 2 As shown, its average reflectance in the solar spectrum is 96%, and its average emissivity in the mid-infrared region of the atmospheric window is 98%, indicating good potential for radiative cooling. Outdoor all-weather testing of the sample prepared in this embodiment showed that the coating can achieve a maximum sub-environmental cooling of 13 °C on clear nights, and a net cooling effect of 5 °C under strong direct sunlight at noon, demonstrating excellent value for passive radiative cooling engineering applications. The coating exhibits superhydrophobicity without any post-treatment, with a static water droplet contact angle of 157 ± 3° and a roll-off angle of ~4°. Example 3

[0045] The key difference in this embodiment lies in changing the type of pore-forming agent that can participate in partial copolymerization. First, 2.5 g of polyurethane acrylate oligomer, 7.5 g of butyl acrylate, and 40 g of vinyltrimethoxysilane are mixed, and 0.075 g of photoinitiator TPO is added. The mixture is stirred until the solution is clear and transparent. Then, the solution is coated onto a glass substrate and cured under 300 s of UV irradiation followed by 2 hours of room temperature evaporation drying to obtain a porous polymer film.

[0046] The prepared coating possesses a hierarchical micro / nanoporous structure. Its average reflectance in the solar spectral region is 95%, and its average emissivity in the mid-infrared region of the atmospheric window is 98%, indicating good radiative cooling potential. Outdoor testing shows that the coating can achieve a cooling effect of ~9 °C at night and ~4 °C at noon. The surface wettability of the cured porous coating was evaluated (e.g., ...). Figure 3 As shown in the figure, this porous surface exhibits an intrinsic superhydrophobic state without the need for any post-treatment with low surface energy reagents. The static contact angle of water droplets on its surface is measured to be 159±3°, and the dynamic roll-off angle is maintained at ~3°. Foreign water droplets can easily roll off at a small tilt angle and carry away surface dust. Example 4

[0047] The core difference in this embodiment lies in adjusting the physical mass ratio of the film-forming polyurethane acrylate oligomer and the reactive diluent monomer. The specific steps are as follows: A stiffness / flexibility adjusting component is added; 5g of polyurethane acrylate oligomer, 5g of butyl acrylate (mass ratio adjusted to 1:1), and 40g of vinyltriethoxysilane are mixed, and 0.075g of photoinitiator TPO is added. The mixture is stirred until the solution is clear and transparent. The solution is then coated onto a glass substrate, irradiated under a UV lamp for 300s, and finally dried at room temperature for 2 hours to obtain a white porous polymer coating.

[0048] The resulting coating possesses a multi-level micro / nanoporous structure. Its average reflectance in the solar spectrum is 96%, and its average emissivity in the mid-infrared region of the atmospheric window is 99%, demonstrating good radiative cooling potential. Figure 4 As shown, outdoor testing indicates that the coating can achieve a temperature of 9 °C during high humidity nighttime (between 22:30 and 1:30). Excluding the influence of solar radiation at night, the results reflect the material's inherent mid-infrared radiation capability, which is closely related to the coating's high mid-infrared emissivity. During the day, the ambient temperature reached a maximum of ~42 °C, and the solar irradiance reached 800 W / m² at midday. -2 At midday (11:00 - 13:00), the coating still achieved a cooling effect of ~8°C, because the coating itself only absorbs ~4% of the solar radiation energy. This porous coating also has excellent intrinsic superhydrophobic properties, with a measured static contact angle of 155±4° and a roll-off angle as low as ~4°. Example 5

[0049] This embodiment aims to investigate the in-situ processing affinity and adhesion reliability of the coating formulated in this invention on various macroscopic heterogeneous substrates commonly used in industrial and construction applications. First, 2.5g of polyurethane acrylate oligomer, 7.5g of butyl acrylate, and 40g of vinyltriethoxysilane were mixed, and 0.075g of photoinitiator TPO was added. The mixture was stirred until the solution was clear and transparent. Then, the solution was coated onto various types of substrates and irradiated under a UV lamp for 300s. Finally, the coating was dried at room temperature for 2 hours to obtain a white porous polymer coating.

[0050] The coating maintains a highly developed micro / nano-scale porous framework. Its average reflectance in the solar spectrum is 94%, and its average emissivity in the mid-infrared region of the atmospheric window is 97%. Polyacrylate is a commonly used adhesive with good adhesion to substrates, allowing the coating to be applied and cured on various substrates such as plastics, steel plates, glass, and cement. In the thermal aging test, the coating was placed in an 80 °C oven at a relative humidity (RH) of 10-30%, and its reflectance was characterized after 30 days. In the acid and alkali resistance test, the coating was immersed in acid (0.1 M HCl) or alkali (0.1 M KOH), and its reflectance was characterized after 7 days. After these two weathering resistance tests, the coating appearance showed no significant change, and the solar reflectance remained essentially unchanged. Figure 5 The coating exhibits superhydrophobicity without any post-treatment, with a static contact angle of 156±2° and a roll-off angle of ~3°, indicating that water droplets easily roll off. In UV resistance testing, the coating was irradiated with UV (365 nm, 2.05 ± 0.10 mW cm⁻²) in a high-humidity summer environment. Thermal aging and acid / alkali aging tests were performed as described above. After 90 days of UV irradiation, 30 days at 80°C, and 7 days of acid or alkali immersion, the coating still maintained its superhydrophobicity. Figure 6 ). Comparative Example 1

[0051] This comparative example aims to examine the irreplaceable structural characteristics of the main-chain resin oligomer. 2.5 g of epoxy acrylate oligomer, 7.5 g of butyl acrylate, and 40 g of vinyltriethoxysilane were weighed and mixed, and 0.075 g of photoinitiator TPO was added. The mixture was stirred until the solution was clear and transparent. The solution was then coated onto a glass substrate and irradiated under a UV lamp for 300 s. Finally, the coating was dried at room temperature for 2 h to obtain a white porous polymer coating. Due to the intrinsic hydrophilicity of the epoxy groups, even when copolymerized with butyl acrylate and vinyltriethoxysilane, a large number of strongly hydrophilic intrinsic hydroxyl / epoxy polar groups remain in the resin backbone. The resulting coating has strong surface hydrophilicity and cannot achieve a water droplet contact angle greater than 150°. Furthermore, the prepared coating cracked after a period of time. This is because the epoxy resin cured network is extremely rigid, brittle, and sensitive to volume shrinkage. During room temperature storage and solvent removal, the internal stress cannot be relaxed, resulting in large-area severe network cracking. Therefore, it cannot be used for radiative refrigeration. Comparative Example 2

[0052] This comparative example aims to investigate the synergistic effect of acrylate reactive diluents on internal stress regulation and superhydrophobic properties. First, 2.5g of polyurethane acrylate oligomer was mixed with 40g of vinyltriethoxysilane, omitting the addition of reactive diluent. Then, 0.075g of photoinitiator TPO was added, and the mixture was stirred until the solution was clear and transparent. The solution was then coated onto a glass substrate and irradiated under a UV lamp for 300s. Finally, the coating was dried at room temperature for 2 hours to obtain a white porous polymer coating. Due to the lack of reactive diluent, the prepared coating had a high crosslinking density, resulting in an excessively rigid polymer network. During the natural evaporation of the pore-forming solvent, the accompanying severe volume shrinkage generated strong internal macroscopic stress, directly leading to severe cracking, curling, and macroscopic cracking of the film upon formation. Furthermore, when a hydrophilic diluent system was introduced, adding 7.5g of hydroxyethyl acrylate as a reactive diluent failed to achieve a water droplet contact angle greater than 150°. This is because hydroxyethyl acrylate contains hydroxyl groups and is hydrophilic, failing to achieve the superhydrophobic effect required for outdoor self-cleaning. Comparative Example 3

[0053] This comparative example aims to investigate the copolymerization effect of vinyl siloxanes and the hydrophobic benefits of methoxyethoxy compounds on the coating. The pore-forming agent was changed to a non-reactive conventional volatile alcohol. First, 2.5g of polyurethane acrylate oligomer, 7.5g of butyl acrylate, and 40g of ethanol were mixed, and 0.075g of photoinitiator TPO was added. The mixture was stirred until the solution was clear and transparent. The solution was then coated onto a glass substrate and irradiated under a UV lamp for 300s. Finally, the coating was dried at room temperature for 2 hours to obtain a white porous polymer coating. Due to the lack of siloxane participation in the copolymerization, even when the polyurethane acrylate oligomer was copolymerized with butyl acrylate, it could not participate in the free radical skeleton copolymerization. As a result, a large number of low surface energy siloxane segments could not react in situ on the coating surface, leading to a significant decrease in the static water droplet contact angle of the pure polyacrylate / polyurethane composite coating after curing. It exhibited a normal hydrophobic state (contact angle less than 150°), thus completely losing its important ability for large-area outdoor self-cleaning, resistance to rainwater contamination, and optical stability. Comparative Example 4

[0054] This comparative example aims to investigate the effect of the critical mass ratio between the total amount of resin monomers and the vinylsiloxane porogen. First, 2.5 g of polyurethane acrylate oligomer, 7.5 g of butyl acrylate, and 90 g or 10 g of vinyltriethoxysilane were mixed, and 0.075 g of photoinitiator TPO was added. The mixture was stirred until the solution was clear and transparent. The solution was then coated onto a glass substrate and irradiated under a UV lamp for 300 s. Finally, the coating was dried at room temperature for 2 h. The resulting porous polymer failed to form a complete film. This is partly because the proportion of the liquid porogen in the homogeneous solution was too high, leading to excessive dilution of the active reactive network density during the crosslinking reaction. This significantly reduced the polymer network density, resulting in a large volume shrinkage effect. The generated internal stress increased the probability of silver streaks, and the mechanical properties decreased significantly after evaporation and drying, making film formation difficult. On the other hand, when the porogen was too low, the reaction-induced phase separation process was insufficient, resulting in a high crosslinking density in the prepared coating. This led to excessive rigidity of the resulting polymer network, causing the coating to crack. At the same time, due to its low porosity and incomplete pore development, the entire coating is semi-transparent or densely transparent, and cannot effectively scatter sunlight. Comparative Example 5

[0055] This comparative example aims to investigate the effect of coating thickness on optical performance. Using the same raw material formulation and preparation conditions as in Example 1, but with precise control of the coating thickness via a high-precision wet film doctor blade during the solution coating process on the glass substrate, the final porous film thickness after complete curing and removal of the pore-forming solvent was ~100 μm. Testing revealed an average reflectance of only 87% in the solar spectral region and an average emissivity of 88% in the mid-infrared region of the atmospheric window. This is because a 100 μm coating is too thin, lacking sufficient bimodal aperture light scattering capability, thus completely negating its potential for high-performance daytime passive radiative cooling applications.

Claims

1. A superhydrophobic radiation-cooling polymer coating, characterized in that, The coating is composed of polyurethane acrylate oligomers, acrylate reactive diluents, vinyl siloxane porogens, and initiators. The coating is synthesized by a rapid phase separation induced by photopolymerization. The mass ratio of the polyurethane acrylate oligomers to the acrylate reactive diluents is 1:(1~12).

2. The method for preparing a superhydrophobic radiation-cooled polymer coating according to claim 1, characterized in that, The method involves initiating free radical polymerization of monomers in a porogen under ultraviolet light, simultaneously inducing phase separation to obtain a bicontinuous micro / nano porous white coating. The polyurethane acrylate oligomer and acrylate reactive diluent are monomers capable of UV-initiated reactions. Vinylsiloxane acts as both a porogen initiating phase separation and a diluent monomer participating in partial copolymerization. The initiator is a photoinitiator selected for use with ultraviolet light wavelengths.

3. The method for preparing a superhydrophobic radiation-cooled polymer coating according to claim 2, characterized in that, The preparation steps include: preparing a mixed solution of polyurethane acrylate oligomer / acrylate reactive diluent / porogen / initiator, stirring until the solution is uniform and clear; coating the solution on the substrate surface, then placing it under a UV lamp for irradiation to initiate the polymerization reaction; after the coating is cured, removing the residual solvent through a drying process to obtain a superhydrophobic radiation-cooling polymer coating.

4. The method for preparing a superhydrophobic radiation-cooled polymer coating according to claim 3, characterized in that, The acrylate reactive diluent is selected from at least one of butyl acrylate, lauryl acrylate, isooctyl acrylate, and octadecyl acrylate.

5. The method for preparing a superhydrophobic radiation-cooling polymer coating according to claim 3, characterized in that, The pore-forming agent is at least one of vinyltriethoxysilane, vinyltrimethoxysilane, vinyltri(β-methoxyethoxy)silane, vinylmethyldiethoxysilane, and vinylmethyldimethoxysilane.

6. The method for preparing a superhydrophobic radiation-cooled polymer coating according to claim 3, characterized in that, The photoinitiator is at least one of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide.

7. The method for preparing a superhydrophobic radiation-cooled polymer coating according to claim 3, characterized in that, The mass ratio of the monomer to the porogen is 1:(2~6).

8. The superhydrophobic radiation-cooling polymer coating prepared by the preparation method according to any one of claims 1-7, wherein the coating exhibits a "string of pearls" morphology (consisting of small spheres) on a microscopic scale, has a bimodal pore size distribution, a pore size range from ~200 nm to ~10 μm, and a porosity of 40-80%.

9. The superhydrophobic radiation-cooling polymer coating according to claims 1-7, characterized in that, The coating has a thickness of 0.2-2 μm, a solar reflectivity of over 93%, and a mid-infrared emissivity of over 95% for the atmospheric window.

10. The superhydrophobic radiation-cooling polymer coating according to claims 1-7, characterized in that, The coating exhibits an intrinsic static water droplet contact angle greater than 150° and a roll-off angle less than 5°, demonstrating superhydrophobicity. It can be applied to various substrates with excellent adhesion. The coating has a short preparation cycle and can be used in high-performance radiative cooling applications.

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