Porous super-hydrophobic and oleophobic colored radiation refrigeration coating and preparation method thereof

Abstract

The invention relates to the field of composite materials, and discloses a porous super-hydrophobic and oleophobic colored radiation refrigeration coating and a preparation method thereof.The porous super-hydrophobic and oleophobic colored radiation refrigeration coating comprises a porous radiation refrigeration layer serving as a bottom layer and a super-hydrophobic and oleophobic colored porous radiation refrigeration layer serving as a top layer; the porous radiation refrigeration layer contains hollow SiO2-TiO2 core-shell particles and PVDF-HFP (Polyvinylidene Fluoride-Hexafluoropropylene) particles; the super-hydrophobic and oleophobic colored porous radiation refrigeration layer contains pigment, hollow SiO2-TiO2 core-shell particles and PVDF-HFP particles, and is subjected to alkali treatment and low-surface-energy silane coupling agent modification treatment. The porous super-hydrophobic and oleophobic colored radiation refrigeration coating not only has an excellent radiation refrigeration effect, but also has super-hydrophobic and oleophobic properties and colors.

Description

Technical Field

[0001] The present invention relates to the field of functional materials, and in particular to a porous superhydrophobic and oleophobic color radiative cooling coating and a preparation method thereof. Background Art

[0002] With the acceleration of climate change and global warming, humans have been facing an increasing demand for refrigeration electricity. Traditional electric refrigeration technologies such as air conditioners are increasingly causing huge energy consumption and strong greenhouse effects, especially during peak summer hours. As an alternative, passive daytime radiative cooling has emerged as a promising energy-free cooling technology that minimizes sunlight absorption by reflecting radiation in the solar spectral region (0.3 - 2.5 μm) and emits infrared thermal radiation through the atmospheric transparent window (8 - 13 μm) to the cold outer space (3K) without energy input.

[0003] Generally, cooling is achieved by using metal mirrors or white materials with high solar reflectance. However, their broadband reflectance at visible light wavelengths limits their use in real life. For aesthetic and application reasons, white is usually not suitable as a coating for buildings or other objects. In addition, the white or silver glare generated by these designs can harm the human eye. Currently, colored radiative coolers have been explored to solve this problem. In colored radiative coolers, dyes or pigments will result in high absorption in the entire solar spectrum, and a part of visible light (Vis, 0.4 to 0.74 μm) is selectively absorbed to display the desired color and additional absorption at near-infrared (NIR, 0.74 to 2.5 μm) wavelengths, which will increase the heat load. To solve this problem, the most important thing is to reduce unnecessary solar absorption, while maintaining the desired color, and increasing the emissivity in the mid-infrared region.

[0004] Patent CN118755327A discloses a one-step method for preparing a colored radiative cooling coating and its application. It directly modifies inorganic particles with inorganic fluorescent dyes, acrylic resin emulsion, hollow glass microspheres and additives, and can establish an efficient energy transfer effect, thereby effectively converting the absorbed sunlight directly into visible emission light to produce a specific color. Patent CN117416115A discloses a colored radiative cooling coating based on a transparent composite film and a preparation method thereof. It prepares a colored radiative cooling coating by using a transparent resin, barium sulfate particles, hollow structure dielectric microspheres, a light stabilizer, and an ultraviolet absorber. Although the above-mentioned invention patents have high solar reflectance and high infrared emissivity, they both use high dielectric medium particles and high infrared emissive resins as the emission layer, and do not explore the relationship between the internal structure of the radiative cooling layer and sunlight emission, infrared emission, and color, lacking research work on particle structure.

[0005] In addition, these radiative cooling materials are limited by poor weather resistance in practical applications. As a material for outdoor applications, it is inevitably affected by pollutants such as dust or other particles, resulting in solar reflectivity and infrared emissivity. At the same time, when the surface is wetted by rainwater or covered by accumulated water, the radiative cooling ability will decrease. Patent CN116102928B discloses a preparation method of a radiative cooling coating with superhydrophobic properties, which uses polydimethylsiloxane and polyvinylidene fluoride as hydrophobic functional monomers to obtain a radiative cooling coating with superhydrophobic properties. However, relying solely on hydrophobic or superhydrophobic characteristics, stubborn pollutants such as bird droppings and grease continue to pose challenges to sustainable self-cleaning. Therefore, the development of superamphiphobicity has become imperative. Summary of the Invention

[0006] Aiming at the contradiction between the selective absorption of color pigments and high solar reflectivity in the radiative cooling coating in the prior art and the problem of easy pollution of the coating, the present invention provides a porous superhydrophobic and oleophobic color radiative cooling coating and a preparation method thereof. The porous superhydrophobic and oleophobic color radiative cooling coating of the present invention not only has excellent radiative cooling effect, but also has superhydrophobic and oleophobic properties and colors.

[0007] The specific technical solution of the present invention is as follows:

[0008] First, a porous superhydrophobic and oleophobic color radiative cooling coating is provided, which includes a porous radiative cooling layer as the bottom layer and a superhydrophobic and oleophobic color porous radiative cooling layer as the top layer.

[0009] Specifically: the porous radiative cooling layer contains hollow SiO2-TiO2 core-shell particles and PVDF-HFP particles; the superhydrophobic and oleophobic color porous radiative cooling layer contains pigments, hollow SiO2-TiO2 core-shell particles and PVDF-HFP particles, and is treated by alkali treatment and low surface energy silane coupling agent modification.

[0010] The prior art has not covered the influence of the differences between the hollow core and the shell in a radiative cooling coating on the light scattering of different parts of the core-shell particles, which leads to some different properties and affects the reflection of sunlight. Through previous research, the present invention has found that core-shell particles with different materials and sizes have different absorption and scattering properties under visible light, which in turn affect the radiative cooling effect. The hollow SiO2-TiO2 core-shell particles and PVDF-HFP particles in the coating of the present invention form a micro-nano pore structure, which can produce a synergistic effect to achieve efficient passive radiative cooling. The pore structures of different scales maximize the scattered sunlight that is not absorbed by color, thus achieving excellent radiative cooling effect; while the top layer contains pigments on the basis of the bottom layer, which can selectively absorb the visible wavelengths complementary to the desired color; the present invention uses an environmentally friendly perfluoropolyether silane coupling agent for surface modification, combined with the rough structure of the top layer, and the two work together to endow it with good water and oil repellency properties.

[0011] Preferably, the hollow SiO2-TiO2 core-shell particles include hollow SiO2 particles and a TiO2 shell layer coated on their outer surface; the inner diameter of the hollow SiO2 particles is 400-800 nm; the thickness of the TiO2 shell layer is 150-250 nm.

[0012] The present invention simulates the scattering efficiency of hollow SiO2 with different inner diameters at a specific wavelength through finite-difference time-domain simulation. When the inner diameter of the hollow SiO2 particles is about 600 nm, the refractive index of the particles is different from that of air, resulting in the incident light being dissipated in the hollow particles and scattered in the direction opposite to the incident light, showing a backward scattering phenomenon, and the reflectivity in the visible light spectrum is relatively high. When the inner diameter is too small (less than 400 nm), the incident light is scattered at different angles in the cavity, and some residual light can still penetrate the particles, so the reflectivity in the visible light spectrum is relatively low. When the inner diameter is too large (more than 800 nm), the increase in reflectivity of the scattering effect compared to the hollow SiO2 particles with an inner diameter of 600 nm is not significant. Therefore, the hollow SiO2 particles with an inner diameter of 600 nm exhibit the best scattering efficiency, effectively scattering the incident solar radiation and enhancing the reflectivity in the visible light spectrum.

[0013] On the other hand, when the thickness of the TiO2 shell layer is 150-250 nm, it is found through simulation of the SiO2-TiO2 core-shell particles that the backward electric field is much stronger than that of the hollow SiO2 particles. This is also because of the high refractive index of TiO2, which leads to a further enhancement of the backward scattering. When the shell layer thickness is too small, Rayleigh scattering occurs preferentially, and its scattering intensity is inversely proportional to the fourth power of the incident light wavelength, and the scattering of short-wavelength light is stronger, which will reduce the proportion of short-wavelength light in the reflected light. Conversely, when the shell layer particle size is too thick, the enhancement of backward scattering is not significant.

[0014] Preferably, the micron pores of the porous superhydrophobic and oleophobic color radiative cooling coating are 2 - 8 μm, and the nano pores are 200 - 800 nm.

[0015] When the pore size is close to or larger than the wavelength of incident light, the scattered light intensity is asymmetric in all directions, and most of the incident light is scattered along the forward direction. That is to say, when the micron pores in the porous superhydrophobic and oleophobic color radiative cooling coating of the present invention are 2 - 8 μm (optimal 5 μm), sunlight of all wavelengths can be effectively scattered, and the nano pore diameter of 200 - 800 (optimal 300 nm) can strongly scatter shorter visible light wavelengths, further enhancing the scattering performance. When the micro - nano size of the pores further increases, it leads to high optical back - scattering of sunlight, resulting in a matte white appearance. When the micro - nano size of the pores is small, the scattering performance will decrease.

[0016] Preferably, the low - surface - energy silane coupling agent includes one or more of perfluorobutyltrimethoxysilane, tetraethoxysilane, perfluorohexyltrimethoxysilane, and perfluoropolyethertrimethoxysilane.

[0017] Preferably, the thickness of the porous superhydrophobic and oleophobic color radiative cooling coating is 100 - 400 μm.

[0018] Secondly, a preparation method of a porous superhydrophobic and oleophobic color radiative cooling coating is provided, which includes the following steps:

[0019] 1) Add CTAB and ammonia water to the ethanol aqueous solution, stir, add TEOS, continue to stir, centrifuge, dry, and calcine to obtain hollow SiO2 particles; disperse them in the ethanol aqueous solution to obtain a dispersion, add the mixed solution of TBOT and ethanol to the dispersion, stir under acidic conditions, centrifuge, dry, and calcine to obtain hollow SiO2 - TiO2 core - shell particles.

[0020] 2) Mix the hollow SiO2 - TiO2 core - shell particles, PVDF - HFP particles, acetone, and water evenly to obtain a bottom - layer precursor solution; coat it on the substrate and dry to obtain a porous radiative cooling layer.

[0021] 3) Take another bottom - layer precursor solution, add pigments to obtain a top - layer precursor solution, coat it on the porous radiative cooling layer, dry, perform alkali treatment, surface modification with a low - surface - energy silane coupling agent, and dry to obtain a superhydrophobic and oleophobic color porous radiative cooling layer.

[0022] Due to its unique chemical structure, PVDF-HFP contains non-reactive functional groups in its molecular chain and has stable chemical properties. In the present invention, base-catalyzed dehydrofluorination is used to generate new reactive hydroxyl groups, endowing it with hydrophilicity. Subsequently, a low surface energy silane coupling agent is grafted onto the surface by a surface grafting method. In the low surface energy silane coupling agent, methoxy groups are hydrolyzed into silanol groups in water, and then react with the hydroxyl functional groups on the surface of the PVDF-HFP membrane. The grafting occurs through consecutive condensation reactions between -OH and Si-O- groups, endowing it with hydrophobic and oleophobic properties.

[0023] Preferably, in step 1), during the preparation of hollow SiO2 particles, the mass ratio of CTAB, ammonia water, ethanol aqueous solution (the volume ratio of ethanol to water is 1:0.5 - 2), and TEOS is 0.1 - 0.2:1:50 - 100:1 - 4.

[0024] CTAB can self-assemble through stirring in ethanol aqueous solutions with different ratios to form micelles of different sizes. When CTAB is added to the water-ethanol-ammonia mixed solution, CTAB begins to hydrolyze and generate cationic cetyltrimethylammonium, and then self-assembles to form micelles and is evenly distributed in the solution. Among them, the relative amount of ethanol is crucial. An increase in its content will cause an increase in the particle size of CTAB micelles, resulting in an increase in the size of the prepared hollow SiO2 particles. The present invention finds that under the above ratios, hollow SiO2 particles meeting the target size of the present invention can be obtained.

[0025] Preferably, in step 1), in the dispersion liquid, the volume ratio of water to ethanol is 1:0.5 - 3.

[0026] Preferably, in step 1), the mass ratio of the hollow SiO2 particles to TBOT is 1:4 - 6.

[0027] The pH of the acid is 4 - 6. The acid is provided by one or more of nitric acid, hydrochloric acid, and acetic acid.

[0028] The temperature of the first calcination is 500 - 600 °C, and the time is 4 - 8 h; the temperature of the second calcination is 800 - 1000 °C, and the time is 2 - 4 h.

[0029] Preferably, in step 2), in the bottom precursor solution, the mass ratio of hollow SiO2-TiO2 core-shell particles, PVDF-HFP particles, acetone, and water is 0.5 - 0.7:1:7 - 8:1 - 2, and most preferably 0.5:1:8:1.

[0030] The present invention uses the non-solvent induced phase separation method to form a porous coating, and prepares different porosity ratios by regulating the content of the solute PVDF-HFP. Pores are constructed according to the different boiling points of acetone and water, resulting in different volatilization rates, and by regulating the content of acetone and water, different pore structures are prepared. The present invention finds that under the above ratios, a coating with micro-nano pore sizes meeting the objectives of the present invention can be obtained.

[0031] Preferably, in step 2), the substrate is a thin film, fabric, leather, glass, wall, wood, aluminum sheet or iron sheet.

[0032] Preferably, in step 3), the conditions for alkali treatment are: the concentration of the sodium hydroxide aqueous solution is 10-40 wt%, the treatment temperature is 40-80 °C, and the treatment time is 0.5-4 h.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] (1) The present invention prepares a hollow core-shell dielectric particle with a high refractive index according to the Mie scattering theory of the core-shell structure, obtains a hollow SiO2 particle with strong backscattering, and then coats it with TiO2 to further enhance the backscattering.

[0035] (2) The present invention constructs a porous PVDF-HFP coating with high reflectivity and high emissivity, and then combines it with strongly backscattering hollow core-shell particles, which can effectively solve the problem of selectively absorbing visible light by colored pigments and achieve full-color coverage in the visible light range. It not only meets the requirements of architectural aesthetics but also has excellent radiative cooling effects.

[0036] (3) The colored radiative cooling coating of the present invention endows it with superhydrophobic and oleophobic properties without losing the physical properties of the coating itself, avoiding the influence of dust, oil-water or other particulate pollutants on the solar reflectivity and infrared emissivity, and greatly improving its outdoor application value.

[0037] (4) The solar reflectivity of the colored radiative cooling coating is ≥90%, and the average emissivity in the atmospheric transparent window is ≥90%. Description of the Drawings

[0038] Figure 1 are the SEM and TEM images of the hollow SiO2 particles in Example 2;

[0039] Figure 2 are the SEM and TEM images of the hollow SiO2-TiO2 core-shell particles in Example 2;

[0040] Figure 3 is the actual temperature of the colorful radiative cooling coating under sunlight at 35 °C;

[0041] Figure 4is the contact angle of water and oil for each radiative cooling coating. Detailed implementation manners

[0042] The present invention will be further described below in conjunction with embodiments.

[0043] Embodiment 1:

[0044] (1) 0.16 g of CTAB and 1 mL of ammonia water were added to a mixed solution of 50 mL of deionized water and 30 mL of ethanol. The mixture was stirred in a 35 °C water bath for 30 min, then 1 mL of TEOS was added, and further stirred for 24 h. After centrifugation and drying, it was calcined at a high temperature of 550 °C for 6 h to obtain hollow silica. Subsequently, 0.5 g of the above hollow silica was dispersed in 50 mL of ethanol and 2 mL of aqueous solution, and a mixed solution of 3 g of TBOT and 2 mL of ethanol was added to the above dispersion. After stirring at room temperature for 24 h under the condition of adjusting the pH = 5 with nitric acid, centrifugation and drying were carried out, and it was calcined at a high temperature of 900 °C for 3 hours to obtain hollow SiO2-TiO2 core-shell particles with an inner diameter of about 200 nm and a TiO2 layer thickness of about 200 nm.

[0045] (2) 0.5 g of the above hollow SiO2-TiO2 core-shell particles, 1 g of PVDF-HFP particles, 8 g of acetone and 1 g of water were mixed and stirred evenly to obtain a bottom-layer precursor solution. The bottom-layer precursor solution was coated on the substrate by brush coating and dried to obtain the bottom layer.

[0046] (3) In the same way, 0.3 g of pigments (red, yellow, blue) was mixed with the bottom-layer precursor solution to obtain a top-layer precursor solution. The top-layer precursor solution was sprayed on the bottom layer and dried. The top layer was placed in a 30 wt% sodium hydroxide solution and treated at 70 °C for 2 h. Subsequently, the surface of the coating was modified with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent, and dried to form the top layer, and finally a porous superhydrophobic and oleophobic color radiative cooling coating (micropore diameter 4 - 6 μm, nanopore diameter 200 - 400 nm, thickness about 400 μm) was obtained.

[0047] The porous superhydrophobic and oleophobic color radiative cooling coating was completed by testing the reflectance of the composite film in the solar spectrum band (0.3 - 2.5 μm) with an ultraviolet spectrometer with a special accessory metal integrating sphere and the emissivity in the mid-infrared band (8 - 13 μm) with an infrared spectrometer.

[0048] Embodiment 2:

[0049] (1) Add 0.16 g of CTAB and 1 mL of ammonia water into a mixed solution of 30 mL of deionized water and 50 mL of ethanol. Stir the mixture in a 35 °C water bath for 30 min, then add 1 mL of TEOS, stir further for 24 h, centrifuge and dry, and calcine at 550 °C for 6 h to obtain hollow silica. Subsequently, disperse 0.5 g of the above hollow silica in 50 mL of ethanol and 2 mL of aqueous solution, add a mixed solution of 3 g of TBOT and 2 mL of ethanol to the above dispersion, stir at room temperature for 24 h under the condition of adjusting the pH = 5 with nitric acid, centrifuge and dry, and calcine at 900 °C for 3 h to obtain hollow SiO2-TiO2 core-shell particles with an inner diameter of about 600 nm and a titanium dioxide layer thickness of about 200 nm. Figure 1 SEM and TEM images of the hollow SiO2 particles in Example 2; Figure 2 SEM and TEM images of the hollow SiO2-TiO2 core-shell particles in Example 2.

[0050] (2) Mix 0.5 g of the above hollow SiO2-TiO2 core-shell particles, 1 g of PVDF-HFP particles, 8 g of acetone and 1 g of water evenly and stir to obtain a bottom precursor solution, coat the bottom precursor solution on the substrate by brushing, and dry to obtain the bottom layer.

[0051] (3) Mix 0.3 g of pigments (red, yellow, blue) with the bottom precursor solution in the same way to obtain a top precursor solution, spray the top precursor solution on the bottom layer and dry; place the top layer in a 30 wt% sodium hydroxide solution and treat it at 70 °C for 2 h. Subsequently, modify the surface of the coating with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent, and dry to form the top layer, and finally obtain a porous superhydrophobic and oleophobic colored radiative cooling coating (micropore diameter 4 - 6 μm, nanopore diameter 200 - 400 nm, thickness about 400 μm).

[0052] Example 3:

[0053] (1) Add 0.16 g of CTAB and 1 mL of ammonia water into a mixed solution of 20 mL of deionized water and 60 mL of ethanol. Stir the mixture in a 35 °C water bath for 30 min, then add 1 mL of TEOS, stir further for 24 h, centrifuge and dry, and calcine at 550 °C for 6 h to obtain hollow silica. Subsequently, disperse 0.5 g of the above hollow silica in 50 mL of ethanol and 2 mL of aqueous solution, add a mixed solution of 3 g of TBOT and 2 mL of ethanol to the above dispersion, stir at room temperature for 24 h under the condition of adjusting the pH = 5 with nitric acid, centrifuge and dry, and calcine at 900 °C for 3 h to obtain hollow SiO2-TiO2 core-shell particles with an inner diameter of about 800 nm and a titanium dioxide layer thickness of about 200 nm.

[0054] (2) Mix 0.5 g of the hollow SiO2-TiO2 core-shell particles, 1 g of PVDF-HFP particles, 8 g of acetone and 1 g of water evenly and stir to obtain the bottom precursor solution. Coat the bottom precursor solution on the substrate by brushing and dry to obtain the bottom layer.

[0055] (3) Mix 0.3 g of pigments (red, yellow, blue) with the bottom precursor solution in the same way to obtain the top precursor solution. Spray the top precursor solution on the bottom layer and dry. Immerse the top layer coating in a 30 wt% sodium hydroxide solution and treat it at 70 °C for 2 h. Subsequently, modify the surface of the coating with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent, and dry to form the top layer, finally obtaining a porous superhydrophobic and oleophobic colored radiative cooling coating (micron pore size 4-6 μm, nano pore size 200-400 nm, thickness about 400 μm).

[0056] Example 4:

[0057] (1) Add 0.16 g of CTAB and 1 mL of ammonia water to a mixed solution of 30 mL of deionized water and 50 mL of ethanol. Stir the mixture in a 35 °C water bath for 30 min, then add 1 mL of TEOS, stir further for 24 h, centrifuge and dry, and calcine at 550 °C for 6 h to obtain hollow silica. Subsequently, disperse 0.5 g of the above hollow silica in 50 mL of ethanol and 2 mL of aqueous solution, add a mixed solution of 2 g of TBOT and 2 mL of ethanol to the above dispersion, stir at room temperature for 24 h under the condition of adjusting the pH = 5 with nitric acid, then centrifuge and dry, and calcine at 900 °C for 3 h to obtain hollow SiO2-TiO2 core-shell particles with an inner diameter of about 600 nm and a titanium dioxide layer thickness of about 100 nm.

[0058] (2) Mix 0.5 g of the hollow SiO2-TiO2 core-shell particles, 1 g of PVDF-HFP particles, 8 g of acetone and 1 g of water evenly and stir to obtain the bottom precursor solution. Coat the bottom precursor solution on the substrate by brushing and dry to obtain the bottom layer.

[0059] (3) Mix 0.3 g of pigments (red, yellow, blue) with the bottom precursor solution in the same way to obtain the top precursor solution. Spray the top precursor solution on the bottom layer and dry. Immerse the top layer coating in a 30 wt% sodium hydroxide solution and treat it at 70 °C for 2 h. Subsequently, modify the surface of the coating with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent, and dry to form the top layer, finally obtaining a porous superhydrophobic and oleophobic colored radiative cooling coating (micron pore size 4-6 μm, nano pore size 200-400 nm).

[0060] Example 5:

[0061] (1) 0.16 g of CTAB and 1 mL of ammonia water were added to a mixed solution of 30 mL of deionized water and 50 mL of ethanol. The mixture was stirred in a water bath at 35 °C for 30 min, then 1 mL of TEOS was added, and further stirred for 24 h. After centrifugation and drying, it was calcined at 550 °C for 6 h to obtain hollow silica. Subsequently, 0.5 g of the above hollow silica was dispersed in 50 mL of ethanol and 2 mL of aqueous solution. A mixed solution of 3 g of TBOT and 2 mL of ethanol was added to the above dispersion. After stirring at room temperature for 24 h under the condition of adjusting the pH = 5 with nitric acid, it was centrifuged and dried, and calcined at 900 °C for 3 h to obtain hollow SiO2-TiO2 core-shell particles with an inner diameter of about 600 nm and a titanium dioxide layer thickness of about 250 nm.

[0062] (2) 0.5 g of the above hollow SiO2-TiO2 core-shell particles, 1 g of PVDF-HFP particles, 8 g of acetone and 1 g of water were mixed and stirred evenly to obtain a bottom-layer precursor solution. The bottom-layer precursor solution was coated on the substrate by brushing and dried to obtain the bottom layer.

[0063] (3) In the same way, 0.3 g of pigments (red, yellow, blue) were mixed with the bottom-layer precursor solution to obtain a top-layer precursor solution. The top-layer precursor solution was sprayed on the bottom layer and dried. The top layer was placed in a 30 wt% sodium hydroxide solution and treated at 70 °C for 2 h. Subsequently, the surface of the coating was modified with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent and dried to form the top layer, and finally a porous superhydrophobic and oleophobic colored radiative cooling coating (micropore diameter 4 - 6 μm, nanopore diameter 200 - 400 nm) was obtained.

[0064] Example 6:

[0065] (1) 0.16 g of CTAB and 1 mL of ammonia water were added to a mixed solution of 30 mL of deionized water and 50 mL of ethanol. The mixture was stirred in a water bath at 35 °C for 30 min, then 1 mL of TEOS was added, and further stirred for 24 h. After centrifugation and drying, it was calcined at 550 °C for 6 h to obtain hollow silica of a certain size. Subsequently, 0.5 g of the above hollow silica was dispersed in 50 mL of ethanol and 2 mL of aqueous solution. A mixed solution of 3 g of TBOT and 2 mL of ethanol was added to the above dispersion. After stirring at room temperature for 24 h under the condition of adjusting the pH = 5 with nitric acid, it was centrifuged and dried, and calcined at 900 °C for 3 h to obtain hollow SiO2-TiO2 core-shell particles with an inner diameter of about 600 nm and a titanium dioxide layer thickness of about 200 nm.

[0066] (2) Mix 0.5 g of the hollow SiO2-TiO2 core-shell particles, 1 g of PVDF-HFP particles, 7 g of acetone and 2 g of water evenly and stir to obtain the bottom precursor solution. Coating the bottom precursor solution on the substrate by brushing and drying to obtain the bottom layer.

[0067] (3) Mix 0.3 g of pigments (red, yellow, blue) with the bottom precursor solution in the same method to obtain the top precursor solution. Spray the top precursor solution on the bottom layer and dry. Immerse the top layer coating in a 30 wt% sodium hydroxide solution and treat at 70 °C for 2 h. Subsequently, modify the surface of the coating with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent, and dry to form the top layer. Finally, a porous superhydrophobic and oleophobic color radiative cooling coating is obtained (micropore diameter 3-5 μm, nanopore diameter 200-300 nm, thickness about 400 μm).

[0068] Control group 1:

[0069] Red, yellow, and blue PVDF-HFP emulsions purchased on the market are sprayed on the substrate to obtain a color radiative cooling coating. Subsequently, immerse the top layer coating in a 30 wt% sodium hydroxide solution and treat at 70 °C for 2 h to obtain a hydrophobic color radiative cooling coating. Then, modify the surface of the coating with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent and dry to obtain a superhydrophobic and oleophobic color radiative cooling porous coating.

[0070] Control group 2:

[0071] Mix 1 g of PVDF-HFP particles, 8 g of acetone and 1 g of water evenly and stir to obtain the bottom precursor solution. Coating the bottom precursor solution on the substrate by brushing and drying to obtain the bottom layer. Then, mix 0.3 g of pigments (red, yellow, blue) with the bottom precursor solution in the same method to obtain the top precursor solution. Spray the top precursor solution on the bottom layer and dry. Immerse the top layer coating in a 30 wt% sodium hydroxide solution and treat at 70 °C for 2 h. Subsequently, modify the surface of the coating with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent and dry to obtain a superhydrophobic and oleophobic color radiative cooling coating.

[0072] Control group 3:

[0073] SiO2 and TiO2-based and commercial white paint purchased on the market are mixed and coated on the substrate as the bottom layer. Subsequently, use red, yellow, and blue commercial coatings as the top layer to prepare a color radiative cooling non-porous coating.

[0074] Control group 4:

[0075] (1) Add 0.16 g of CTAB and 1 mL of ammonia water to a mixed solution of 30 mL of deionized water and 50 mL of ethanol. Stir the mixture in a water bath at 35 °C for 30 min, then add 1 mL of TEOS, and further stir for 24 h. Centrifuge and dry, and calcine at 550 °C for 6 h to obtain hollow silica with an inner diameter of about 600 nm.

[0076] (2) Mix 0.5 g of the above hollow silica, 1 g of PVDF-HFP particles, 8 g of acetone and 1 g of water evenly and stir to obtain a bottom precursor solution. Coating the bottom precursor solution on the substrate by brushing and drying to obtain the bottom layer.

[0077] (3) Mix 0.3 g of pigments (red, yellow, blue) with the bottom precursor solution in the same way to obtain a top precursor solution. Spray the top precursor solution on the bottom layer and dry. Immerse the top layer in a 30 wt% sodium hydroxide solution and treat at 70 °C for 2 h. Subsequently, modify the surface of the coating with 1 g of tetraethoxysilane and 1 g of perfluoropolyether trimethoxysilane coupling agent and dry to form the top layer, and finally obtain a porous superhydrophobic and oleophobic color radiative cooling coating.

[0078] Performance Test

[0079] Detect the solar reflectance and mid-infrared emissivity of the coatings in each example and each control group, and the results are shown in Table 1-3.

[0080]

[0081]

[0082]

[0083] Since the pigments are selectively absorbed in the visible light (Vis, 0.3 to 0.74 μm) part, the solar reflectance at the near-infrared (NIR, 0.74 to 2.5 μm) wavelength is investigated. It can be seen that in Example 2 (near-infrared reflectance > 80%, mid-infrared emissivity > 0.9), the coating temperature can be reduced to 6.6 °C, a decrease of 28.4 °C relative to the ambient temperature (35 °C). This is due to the construction of a radiative cooling coating by the multi-layer micro-nano hollow structure and high refractive index dielectric particles with high solar reflectance and the high infrared emissivity of C-C and C-F in PVDF-HFP. The radiative cooling of the present invention not only has the effect of radiative cooling, but also has superhydrophobic and oleophobic (water contact angle ≥ 150°, salad oil ≥ 140°) properties, improving its outdoor use value and bringing new social and economic benefits.

[0084] In addition, from Table 1-3 and Figure 3It can also be seen that when the inner diameter of the hollow SiO2-TiO2 core-shell particles is small (Example 1, about 200 nm), the incident light will undergo backward scattering at different angles in the cavity, reducing the solar reflectivity and resulting in a lower actual temperature drop. When the inner diameter of the hollow SiO2-TiO2 core-shell particles is large (Example 3, about 800 nm), it has little effect on the solar reflectivity and mid-infrared emissivity, but the preparation cost increases. When the TiO2 shell layer is thin (Example 4, about 100 nm), the solar reflectivity and mid-infrared emissivity decrease.

[0085] It can be seen from Figure 4 that when the particle size of the hollow SiO2-TiO2 is large, it can provide a significant micro-nano rough structure in the radiative cooling coating, and its synergistic effect with the low-surface-energy side chain endows it with good hydrophobic and oleophobic properties. When the particle size is small (Example 1), when the coating encounters oil, the contact angle of the oil is smaller than that in other cases, but it can still reach the oleophobic state.

[0086] The raw materials and equipment used in the present invention are all common raw materials and equipment in the art unless otherwise specified; the methods used in the present invention are all conventional methods in the art unless otherwise specified.

[0087] The above are only the preferred embodiments of the present invention, and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent transformations made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A porous superhydrophobic and oleophobic color radiative cooling coating, characterized in that: It includes a porous radiative cooling layer as the bottom layer and a superhydrophobic and oleophobic color porous radiative cooling layer as the top layer; The porous radiative cooling layer contains hollow SiO2-TiO2 core-shell particles and PVDF-HFP particles; The superhydrophobic and oleophobic color porous radiative cooling layer contains pigments, hollow SiO2-TiO2 core-shell particles and PVDF-HFP particles, and is treated by alkali treatment and modification with a low surface energy silane coupling agent.

2. The porous superhydrophobic and oleophobic color radiative cooling coating according to claim 1, characterized in that: The hollow SiO2-TiO2 core-shell particles include hollow SiO2 particles and a TiO2 shell layer coated on the outer surface thereof; The inner diameter of the hollow SiO2 particles is 400-800 nm; The thickness of the TiO2 shell layer is 150-250 nm.

3. The porous superhydrophobic and oleophobic color radiative cooling coating according to claim 1 or 2, characterized in that: The micron pores of the porous superhydrophobic and oleophobic color radiative cooling coating are 2-8 μm, and the nano pores are 200-800 nm.

4. The porous superhydrophobic and oleophobic color radiative cooling coating according to claim 1 or 2, characterized in that: The low surface energy silane coupling agent includes one or more of perfluorobutyltrimethoxysilane, tetraethoxysilane, perfluorohexyltrimethoxysilane and perfluoropolyethertrimethoxysilane.

5. The porous superhydrophobic and oleophobic colored radiative cooling coating according to claim 1 or 2, wherein: The thickness of the porous superhydrophobic and oleophobic color radiative cooling coating is 100-400 μm.

6. A method for preparing a porous superhydrophobic and oleophobic color radiative cooling coating as described in any one of claims 1-5, characterized in that It includes the following steps: 1) Add CTAB and ammonia water to an ethanol aqueous solution, stir, add TEOS, continue to stir, centrifuge, dry, calcine to obtain hollow SiO2 particles; Disperse it in an ethanol aqueous solution to obtain a dispersion, add a mixed solution of TBOT and ethanol to the dispersion, stir under acidic conditions, centrifuge, dry, calcine to obtain hollow SiO2-TiO2 core-shell particles; 2) Mix the hollow SiO2-TiO2 core-shell particles, PVDF-HFP particles, acetone and water evenly to obtain a bottom layer precursor solution; coat it on a substrate and dry to obtain a porous radiative cooling layer; 3) Take another bottom layer precursor solution, add pigments to obtain a top layer precursor solution, coat it on the porous radiative cooling layer, dry, subject it to alkali treatment, surface modification with a low surface energy silane coupling agent, and dry to obtain a superhydrophobic and oleophobic color porous radiative cooling layer.

7. The preparation method according to claim 6, characterized in that: In step 1), In the process of preparing hollow SiO2 particles, the mass ratio of CTAB, ammonia water, ethanol aqueous solution and TEOS is 0.1-0.2:1:50-100:1-4; In the dispersion, the volume ratio of water to ethanol is 1:0.5-3; The mass ratio of the hollow SiO2 particles to TBOT is 1:4-6; The pH of the acidity is 4-6; The acidity is provided by one or more of nitric acid, hydrochloric acid and acetic acid; The temperature of the first calcination is 500-600 °C, and the time is 4-8 h; the temperature of the second calcination is 800-1000 °C, and the time is 2-4 h.

8. The preparation method according to claim 6, characterized in that: In step 2), in the bottom layer precursor solution, the mass ratio of hollow SiO2-TiO2 core-shell particles, PVDF-HFP particles, acetone and water is 0.5-0.7:1:7-8:1-2.

9. The preparation method according to claim 6 or 8, characterized in that: In step 2), the substrate is a thin film, fabric, leather, glass, wall, wood, aluminum sheet or iron sheet.

10. The preparation method according to claim 6, characterized in that: In step 3), the conditions for alkali treatment are as follows: the concentration of the sodium hydroxide aqueous solution is 10-40 wt%, the treatment temperature is 40-80 °C, and the treatment time is 0.5-4 h.

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