A method for preparing a near-infrared high-reflection radiative cooling coating

Abstract

This invention relates to a method for preparing a near-infrared high-reflectivity radiation-cooling coating. This method uses KH-570 to treat Al... 2 O 3 Modification treatment yields K-Al 2 O 3 Then, it is added to fluorinated ethylene acrylate (FEP) to improve its dispersibility in the fluorinated ethylene acrylate, thereby improving the reflectivity of the radiation-cooling coating in the solar radiation band and its emissivity and scattering efficiency in the atmospheric window band. The radiation-cooling coating of the present invention is easy to prepare, has a significant cooling effect, and exhibits high reflectivity in the solar radiation band and high emissivity in the mid-infrared band.

Description

A method for preparing a near-infrared high-reflectivity radiation-cooling coating Technical Field

[0001] This invention relates to the field of radiation cooling technology, specifically a near-infrared high-reflectivity radiation cooling coating with self-cleaning capabilities. Background Technology

[0002] In recent years, increased energy consumption for cooling and heating has led to a rise in global carbon dioxide levels, subsequently exacerbating the "urban heat island" effect. In summer, with air conditioning in use, greenhouse gas emissions account for 10% of total emissions and 15% of total electricity consumption. In the solar energy sector, a major challenge for solar photovoltaic modules is the loss of optical efficiency and spectral absorption caused by the covering glass. This can lead to a reduction in output power of approximately 3% to 50% due to dust accumulation. Daytime radiative cooling technology relies primarily on reflection from the solar spectral bands and thermal radiation from atmospheric window bands. This technology has the potential to significantly improve building temperature control, thereby reducing the need for other components such as fans and air conditioners.

[0003] During long-term operation, especially in humid regions, the performance of PDRC materials can be severely affected. There is an urgent need to develop energy-efficient cooling methods and materials with self-cleaning capabilities.

[0004] In order to achieve good cooling effect, daytime radiative cooling materials should have high reflectivity in the solar radiation band (0.3-2.5μm) and high emissivity in the atmospheric window band (8-13μm). However, radiative cooling technology is limited by the following problems: (1) In the early days, in order to pursue high reflectivity, a polymer was coated on the surface of silver and aluminum to improve emissivity. However, under long-term working conditions, the surface oxidizes and the cooling performance decreases. Due to the presence of reflective metal, it will cause "light pollution". (2) In order to obtain high-performance radiative cooling materials, some researchers have adopted methods such as electrospinning, etching and deposition of multilayer thin films. However, the preparation process is relatively complex and it is difficult to achieve large-scale preparation. (3) Due to the dust covering the surface of the radiative cooling material, the micro-nano structure of the surface is damaged, which seriously limits the radiative cooling performance. (4) Therefore, it is necessary to explore the application mode of coatings or paints to make radiative cooling materials with self-cleaning properties applicable to various scenarios.

[0005] Some current studies, such as CN 108250873 A (Outdoor All-Weather Solar Reflection and Infrared Radiation Cooling Coating), add micron-sized metal-plated sheets or micron-sized metal-plated spheres to the coating system and use a layered coating method. The metal-plated sheet structure achieves high solar reflectivity and high infrared radiation through the coating direction, thereby achieving a passive cooling effect. Another example is CN 114933731 A (Polymer-Based Radiation Cooling Material with Graded Microspheres and Pores and its Preparation Method), which describes a polymer-based radiation cooling material with graded microspheres and pores in the form of a single-layer structure (film, sheet, or coating). It uses a polymer material as a substrate, with large-diameter microspheres, small-diameter microspheres, and micropores uniformly distributed within the substrate. These patents simply add different fillers to different polymers, aiming to improve reflectivity by adding different inorganic fillers. However, due to the uneven dispersion of fillers in the polymer, the dispersibility of fillers in the polymer is not considered, resulting in a limitation of low scattering efficiency in the radiation cooling coating. Summary of the Invention

[0006] The purpose of this invention is to address the limitations of current technologies by providing a method for preparing a near-infrared high-reflectivity radiation-cooling coating. This method uses KH-570 to modify Al2O3 to obtain K-Al2O3, which is then added to fluorinated ethylene acrylate (FEP) to improve its dispersibility within the FEP, thereby increasing the reflectivity of the radiation-cooling coating in the solar radiation band and its emissivity in the atmospheric window band. The radiation-cooling coating of this invention is easy to prepare, exhibits a significant cooling effect, and demonstrates high reflectivity in the solar radiation band and high emissivity in the mid-infrared band.

[0007] The technical solution of this invention is as follows:

[0008] A method for preparing a near-infrared high-reflectivity radiation-cooling coating, the method comprising the following steps:

[0009] (1) Preparation of K-Al2O3: Add silane coupling agent to ethanol aqueous solution, then use glacial acetic acid to adjust the pH value of the solution to 3.5-5.5, and let it stand for 10-15 minutes; then add Al2O3 particles to it and perform ultrasonic treatment for 10-30 minutes; then heat in a water bath at 60-80℃ for 5-8 hours, and after centrifugation, washing and drying, obtain modified Al2O3 particles, namely K-Al2O3.

[0010] In step (1), the mass ratio of KH-570 to Al2O3 is 6:(19-94);

[0011] The silane coupling agent is KH-570; the particle size of the Al2O3 particles is 0.2-0.4 μm.

[0012] The drying process involves drying in a vacuum drying oven at 50-70°C for 18-24 hours; the ethanol-water solution has an ethanol-to-water mass ratio of 6-12:1.

[0013] (2) Preparation of the mixture: FEP, K-Al2O3 and deionized water are mixed and magnetically stirred for 3-5 hours to obtain a suspension;

[0014] The mass ratio of FEP:K-Al2O3:deionized water is 1:1:(0.8-2.2).

[0015] (3) Film formation: A pure FEP pre-cured layer with a thickness of 30-50μm is sprayed onto the substrate surface using a spray gun, and then the mixture obtained in step (2) is sprayed onto the pre-cured layer. Finally, it is cured at 340-350℃ for 3-5 minutes to obtain a near-infrared high-reflectivity radiation cooling coating; the thickness of the radiation cooling coating is 200μm-300μm.

[0016] In step (3), the spraying pressure of the spray gun is 0.6-0.8 MPa, the diameter of the nozzle is 0.8-1.2 mm, and the distance between the spray gun and the substrate is 12-17 cm.

[0017] The substrate can be glass, wood, plastic or metal.

[0018] The coating exhibits excellent reflectivity of 95.62% to 96.03% in the 300–2500 nm wavelength range; at the same time, it has a high emissivity of 95.81% to 96.34% in the atmospheric window region (wavelength 8–13 μm); the coating also has superhydrophobic properties in various humid environments, with a water contact angle of 158.6° to 159.3°.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] This invention prepares a thin layer of pure FEP pre-cured layer on the substrate surface to improve the bonding force between the radiation cooling material and the substrate, and then deposits it on the substrate by spraying to prepare a radiation cooling coating with high reflectivity; wherein deionized water is used as solvent, FEP achieves high emissivity, and K-Al2O3 improves the reflection of solar radiation and enhances the emissivity of the atmospheric window band.

[0021] Specifically, this is reflected in;

[0022] 1. The radiation-cooling coating has an average reflectivity of 96.03% in the solar radiation band (0.3-2.5μm) and an emissivity of 96.34% in the atmospheric window band (8-13μm); it achieves an average temperature drop of 4.5℃ in autumn. It also exhibits self-cleaning properties and has a contact angle of 159.3°.

[0023] 2. The radiation cooling coating is a single-layer coating, which has a simple preparation process, low cost, good performance, and can be prepared over a large area.

[0024] 3. The radiation cooling coating has good self-cleaning properties, reducing the impact of pollutants on it.

[0025] 4. The radiation cooling coating has good particle dispersion on its surface, which improves the optical performance of the coating. It can be applied to ordinary buildings and various solar panels. Attached Figure Description

[0026] Figure 1 is a low-magnification SEM image of the radiation-cooling coating prepared in Example 1.

[0027] Figure 2 is a high-magnification SEM image of the radiation-cooled coating obtained in Example 1.

[0028] Figure 3 shows the coating temperatures measured during the day for different KH-570 mass fractions in Examples 2-5.

[0029] Figure 4 shows the reflectance spectra of fillers with different contents in Examples 6-10 in the solar radiation band.

[0030] Figure 5 shows the emission spectra of fillers with different contents in Examples 6-10 in the mid-infrared band.

[0031] Figure 6 shows the reflectance spectra of samples of different thicknesses in Examples 11-15 in the solar radiation band.

[0032] Figure 7 shows the emission spectra of samples with different thicknesses in the mid-infrared band in Examples 11-15.

[0033] Figure 8 is a graph showing the measured daytime temperatures of different coatings in Example 16.

[0034] Figure 9 is a measured daytime radiation cooling coating-outdoor temperature diagram in Example 16.

[0035] Figure 10 is a measured daytime power diagram from Example 16.

[0036] Figure 11 is a static contact angle diagram in Example 17.

[0037] Figure 12 is a diagram of the self-cleaning process in Example 17. Detailed Implementation

[0038] Specific embodiments of the present invention are given below. These specific embodiments are only used to further illustrate the present invention and do not limit the scope of protection of the claims of this application.

[0039] The coating is mainly prepared using the following materials: fluorinated ethylene acrylate (Dow Chemical Company), alumina (Shanghai Aladdin Biochemical Technology Co., Ltd.), and glass (ordinary glass).

[0040] In the performance test, emissivity was measured using a Fourier transform infrared spectrometer with a gold-plated integrating sphere, INVENIO R, Bruker GmbH, Germany; reflectivity was measured using a Lambda 750s ultraviolet-visible-near-infrared spectrophotometer, PerkinElmer, USA.

[0041] Example 1

[0042] The specific preparation steps are as follows:

[0043] (1) Preparation of K-Al2O3: 3g of silane coupling agent (KH-570) was pre-dispersed in 60mL of ethanol-water solution (ethanol to water mass ratio of 9:1). The pH of the mixed solution was then adjusted to 4 using glacial acetic acid, and the solution was allowed to stand at room temperature for 15 minutes. Al2O3 particles (particle size 0.2-0.4μm) were added to the pre-dispersed solution of KH-570, wherein the mass ratio of Al2O3 to KH-570 was 44:9 (KH-570 mass fraction was 18%), and ultrasonic treatment was performed for 20 minutes to decompose some of the original physical groups of Al2O3, thereby improving the dispersion of Al2O3 particles. The above mixed solution was heated in a 75℃ water bath for 6 hours to fully modify its surface. Once the reaction was complete, the modified Al2O3 could be obtained by centrifugation. Then it was washed four times with ethanol and then placed in a vacuum drying oven at 65°C for 20 hours to obtain dried modified Al2O3 particles (K-Al2O3).

[0044] (2) Preparation of coating mixture: The mass ratio of FEP:K-Al2O3:deionized water is 1:1:2 (K-Al2O3 accounts for 25%). FEP, K-Al2O3 and deionized water are mixed and stirred magnetically for 4 hours to obtain a uniform and stable suspension.

[0045] (3) Coating film formation: The above mixture is deposited on the glass substrate with a spray gun to form a 30-50 μm thick pre-cured layer to improve the bonding force between the radiation cooling material and the substrate; then: the spray gun is connected to an air compressor with a nozzle diameter of 1 mm and the distance between the spray gun and the substrate is controlled to be 12 cm during spraying. By controlling the spraying time and the amount of coating, a coating with a thickness of about 300 μm is obtained; then the deposited layer is placed in an oven at 340℃ for 3 min to cure.

[0046] Microstructure of K-Al2O3

[0047] The morphology of the particles was characterized using a field emission scanning electron microscope (Hitachi S-4800). Gold was sputtered for 65 seconds before testing. Figure 1 shows the SEM image of the radiation-cooled coating (5000x magnification), and Figure 2 shows the high-magnification (10000x) SEM image of the radiation-cooled coating. Both low-magnification and high-magnification images show a reduction in agglomeration, which essentially disappears. This indicates reduced agglomeration, more dispersed particles, improved scattering efficiency and reflectivity, better optical performance of the radiation-cooled coating, and a more effective cooling effect.

[0048] Examples 2-5: Effects of samples with different KH-570 mass percentages on radiative cooling performance

[0049] The other steps are the same as in Example 1, except that the total mass of Al2O3 and KH-570 remains unchanged, and the mass fraction of KH-570 in Example 1 is changed from 18% to 0%, 6%, 18%, and 24%, respectively.

[0050] This experiment modified Al2O3 with different mass fractions of KH-570 (0wt%, 6wt%, 18wt%, 24wt%), and tested the cooling performance of the prepared radiative cooling coating using a laboratory-assembled test chamber. The device consisted of a polystyrene foam frame covered with aluminum foil and high-density polyethylene. The test was conducted in Tianjin on October 23, 2023. Figure 3 shows the temperature test results for different mass fractions of KH-570 (0wt%, 6wt%, 18wt%, 24wt%), a radiative cooling coating thickness of 300μm, and an Al2O3 mass fraction of 26% during the period from 10:00 AM to 2:00 PM. It is evident that the temperature reduction effect improves with increasing KH-570 mass fraction. At a KH-570 mass fraction of 18wt%, the average temperature reduction reached 6.1℃. Compared to Al2O3 without KH-570 modification, a KH-570 mass fraction of 6wt%–24wt% is preferred.

[0051] Examples 6-10: Effects of different K-Al2O3 mass percentages on radiative cooling performance

[0052] The other steps are the same as in Example 1, except that in step (3), the coating thickness is controlled to be 200 μm.

[0053] In addition, with the total mass of FEP, K-Al2O3 and deionized water remaining constant and the mass ratio of FEP to K-Al2O3 always being 1:1, the mass percentage of K-Al2O3 was changed from 25% in Example 1 to 2%, 10%, 18%, 26% and 32%, respectively.

[0054] The reflectance spectra of the samples in the 0.3–2.5 μm wavelength range were measured using a UV-Vis-NIR spectrophotometer equipped with a BaSO4 integrating sphere, as shown in Figure 4. We can see that when the mass percentage of K-Al2O3 is 2%, the reflectance of the coating is very low, reaching only 66.08%. When the mass percentage of K-Al2O3 is 26%, it reaches 92.82%. When the mass percentage of K-Al2O3 is 18%, it reaches 90.76%. When the mass percentage of K-Al2O3 is 32%, it reaches 92.94%.

[0055] The emissivity of the sample in the 2.5–25 μm band was measured using a Fourier transform infrared spectrometer equipped with a gold-plated integrating sphere via the reflectance method, as shown in Figure 5. We can see that the coating exhibits high infrared emissivity in both the 8–13 μm and 2.5–25 μm bands. When the mass percentage of K-Al₂O₃ is 26%, the emissivity of the coating in the 8–13 μm and 2.5–25 μm bands are 96.34% and 90.37%, respectively.

[0056] This shows that, compared to the low optical performance of 2% and 10%, K-Al2O3 mass fractions of 18%, 26%, and 32% show better results.

[0057] Examples 11-15: Effects of Samples of Different Thicknesses on Radiation Cooling Performance

[0058] The other steps are the same as in Example 1, except that the mass percentage of K-Al2O3 in the single-layer radiation cooling coating is controlled to be 26%. First, a pure FEP pre-cured layer with a thickness of 30-50 μm is prepared on the substrate surface using a spray gun, and then coatings with thicknesses of 50 μm, 100 μm, 200 μm, 300 μm, and 400 μm are prepared.

[0059] The reflectance spectrum of the sample in the 0.3–2.5 μm band was measured using a UV-Vis-NIR spectrophotometer equipped with a BaSO4 integrating sphere, as shown in Figure 6. We can see that the reflectance of the coating gradually increases in the 0.3–2.5 μm band with increasing thickness, rising from 93.03% to 96.31%. This is because the increased uniformity of the K-Al2O3 particles with increasing thickness enhances the scattering of sunlight, resulting in more sunlight being reflected.

[0060] The emissivity of the sample in the 2.5–25 μm band was measured using a Fourier transform infrared spectrometer equipped with a gold-plated integrating sphere via the reflectance method, as shown in Figure 7. We can see that as the thickness of the emitting layer increases, the sample's radiation performance in the mid-infrared band remains at a very high level, with emissivity consistently above 90%.

[0061] This shows that, compared to the low optical performance of 50μm and 100μm, the optimal thickness range for K-Al2O3 is 200μm-400μm.

[0062] Example 16: Radiative Cooling Temperature and Radiative Refrigeration Power Test

[0063] The cooling performance of the prepared radiation-cooling coating was tested using a laboratory-assembled test chamber. The apparatus consisted of a polystyrene foam frame covered with aluminum foil and high-density polyethylene. The test was conducted in Tianjin on October 24, 2023. Figure 8 shows the temperature test results of a radiation-cooling coating with a thickness of 300 μm and a K-Al₂O₃ content of 26%, a commercial cooling film, and blank glass, between 10:00 AM and 2:00 PM. These three samples are designated as Sample 1, Sample 2, and Sample 3. The average temperature reductions for Sample 1, Sample 2, and Sample 3 were 4.9 °C, 1.5 °C, and -0.05 °C, respectively. This demonstrates that Sample 1 exhibits superior cooling performance compared to Sample 2.

[0064] The performance of the prepared radiation-cooling coating was tested using a laboratory-assembled device. The power testing apparatus consisted of three parts: a power supply, a feedback control temperature control system, and a polystyrene temperature measuring device. The test was conducted in Tianjin from 11:00 AM to 12:00 PM on November 22, 2023. As shown in Figure 9, under direct sunlight, a temperature difference was generated between the coating and the ambient temperature. Heating elements were used to raise the temperature of the coating to match the outside temperature. As shown in Figure 10, the average radiation-cooling power of the coating was measured to be 81.2 W / m². 2 .

[0065] Example 17 Self-cleaning test of radiation-cooled coating

[0066] Figure 11 shows the permeability and contact angle of the PDRC material (Optical Contact Angle Meter OCA20, Beijing Audlino Instruments Co., Ltd.). In the contact angle test, measurements were taken every 5 minutes for a total of 15 minutes. The droplets remained almost stationary and maintained their shape, indicating low permeability. The mass of the liquid also affects the contact angle on the coated surface. Due to gravity, the contact angle of the sample decreased. However, the droplets did not diffuse to the surface but instead aggregated. This indicates that the PDRC material has extremely low surface energy and excellent hydrophobic properties, enabling it to resist various liquid contaminations and erosion. Figure 12 shows a tinplate coated with a radiation-cooled coating, used for a self-cleaning performance test. Sand was effectively washed away by water, leaving almost no residue.

[0067] Matters not covered in this invention are common knowledge.

Claims

1. A method for preparing a near-infrared high-reflectivity radiation-cooling coating, characterized in that the method includes the following steps: (1) Preparation of K-Al2O3: adding a silane coupling agent to an ethanol aqueous solution, then adjusting the pH of the solution to 3.5~5.5 using glacial acetic acid, and letting it stand for 10-15 minutes; then adding Al2O3 particles to it, And perform ultrasonic treatment for 10-30 minutes; then heat in a water bath at 60-80℃ for 5-8 hours, centrifuge, wash and dry to obtain modified Al2O3 particles, namely K-Al2O3; wherein, the mass ratio of KH-570 to Al2O3 in step (1) is 6: (19-94); the silane coupling agent is KH-570; (2) Preparation of the mixture: mix fluorinated ethylene acrylate, K-Al2O3 and deionized water, and stir magnetically for 3-5 h to obtain the mixture; wherein, the mass ratio of fluorinated ethylene acrylate: K-Al2O3: deionized water = 1:1: (0.8-2.2); (3) Film formation: spray a layer of pure fluorinated ethylene acrylate pre-cured layer with a thickness of 30-50μm on the substrate surface with a spray gun, and then spray the mixture obtained in step (2) on the pre-cured layer, and finally spray 340-350 o Curing at C for 3-5 minutes yields a near-infrared high-reflectivity radiation-cooling coating; the thickness of the radiation-cooling coating is 200 μm-400 μm.

2. The method for preparing the near-infrared high-reflectivity radiation-cooling coating as described in claim 1, characterized in that: In step (3), the spraying pressure of the spray gun is 0.6-0.8 MPa, the diameter of the nozzle is 0.8-1.2 mm, and the distance between the spray gun and the substrate is 12-17 cm.

3. The method for preparing the near-infrared high-reflectivity radiation-cooling coating as described in claim 1, characterized in that: The substrate can be glass, wood, plastic or metal.

4. The method for preparing a near-infrared high-reflectivity radiation-cooling coating as described in claim 1, characterized in that the coating exhibits excellent reflectivity of 95.62%~96.31% in the 300~2500nm wavelength band; and simultaneously possesses high emissivity of 95.81~96.34% in the wavelength range of 8~13μm; the water contact angle of the coating is 158.6°~159.3°.

5. The method for preparing the near-infrared high-reflectivity radiation-cooling coating as described in claim 1, characterized in that: In step (1), the mass ratio of ethanol to water in the aqueous ethanol solution is 6~12:1; the particle size of the Al2O3 particles is 0.2-0.4μm.

6. The method for preparing the near-infrared high-reflectivity radiation-cooling coating as described in claim 1, characterized in that the drying is carried out in a vacuum drying oven at 50-70°C for 18-24 hours.

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