Radiation cooling composite coating co-deposition preparation method based on spectral selectivity regulation and control

Abstract

The invention discloses a codeposition preparation method of a radiation cooling composite coating based on spectral selectivity regulation and control. The method comprises the following steps: preparing a SiO2 inorganic coating through a sol-gel method, taking tetraethoxysilane as a precursor, carrying out catalytic hydrolysis in an ethanol solvent through hydrochloric acid, adding a photoinitiator to form sol, carrying out spin coating, and carrying out ultraviolet curing; the SiO2 coating is coated with a polyvinylidene fluoride (PVDF) layer; depositing a nano-silver layer on the surface of the PVDF layer through a magnetron sputtering method; and finally, covering the silver layer with another layer of PVDF solution to form a PVDF / Ag / PVDF sandwich structure. According to the method, through multi-layer composite design, ultraviolet reflection, visible light high transmission (gt, 92%) and near-infrared high reflection (gt, 88%) are achieved, meanwhile, the flexibility and environmental stability of the coating are enhanced, and the method is suitable for flexible scenes such as agricultural greenhouses.

Description

Technical Field

[0001] The present invention relates to the technical field of coating preparation, and in particular to a method for co-deposition preparation of a radiation cooling composite coating based on spectral selective regulation. Background Art

[0002] With global warming and the increasing demand for constant temperature in agriculture, radiative cooling technology has attracted significant attention in areas such as agricultural greenhouses and building energy conservation, due to its ability to passively cool the environment without requiring additional energy. Traditional radiative cooling materials typically rely on single coatings or simple structures, such as metal reflective layers or polymer films. While these materials can reflect or transmit light in specific wavelengths (such as the infrared), they lack the ability to precisely control the ultraviolet, visible, and near-infrared regions. For example, some materials employ a full-band reflection strategy, which reduces heat input but severely reduces visible light transmittance (e.g., <80%), leading to reduced plant photosynthesis efficiency. Other materials, while highly transparent in the visible, have insufficient near-infrared reflectivity (e.g., <70%), making them ineffective at blocking solar thermal radiation. Furthermore, existing coatings often utilize rigid substrates (such as glass or hard polymers), making them unsuitable for curved greenhouse structures. Long-term exposure to outdoor environments can lead to problems such as silver oxidation and interfacial delamination, resulting in optical performance degradation. These technical solutions fail to achieve spectrally selective control through multi-layer collaborative design and lack comprehensive optimization of material flexibility and environmental stability. Therefore, there is an urgent need to develop a radiative cooling coating with high light transmittance, high reflectivity, excellent flexibility and long-term stability to meet the dual needs of agricultural greenhouses for photosynthetic efficiency and cooling performance. Summary of the Invention

[0003] The embodiments of the present application solve the technical problem of the full-band reflection contradiction of the radiation cooling coating in the prior art by providing a co-deposition preparation method of the radiation cooling composite coating based on spectral selective regulation, and achieve the technical effects of high light transmittance, high reflectivity, excellent flexibility and long-term stability of the radiation cooling coating through spectral selective regulation.

[0004] The present invention provides a method for preparing a composite coating by co-deposition of radiative cooling based on spectral selective regulation, comprising the following steps: S01 Preparation of SiO2 Inorganic Coating by Sol-Gel Method Using tetraethyl orthosilicate as a precursor, hydrolyzing it in an ethanol solvent by hydrochloric acid catalysis, stirring it evenly, adding a photoinitiator in the later stage of stirring, stirring it evenly to form a sol with a three-dimensional network structure, spin-coating the sol on a film, and using ultraviolet curing to form an inorganic SiO2 coating; S02 Coating of polyvinylidene fluoride (PVDF) layer First, a polyvinylidene fluoride (PVDF) solution is prepared, and then the polyvinylidene fluoride (PVDF) solution is coated on the inorganic SiO2 coating in step S01, and the polyvinylidene fluoride (PVDF) layer is formed after drying; S03 Deposition of silver layer In step S02, a nanosilver layer is deposited on the surface of the polyvinylidene fluoride (PVDF) layer by a physical method; S04 Covering of protective layer The silver layer in step S03 is covered with another layer of polyvinylidene fluoride (PVDF) solution to form a "polyvinylidene fluoride (PVDF) / Ag / polyvinylidene fluoride (PVDF)" sandwich structure.

[0005] Preferably, the mass ratio of ethyl orthosilicate: ethanol: water is 11:9.7:1, and the mass ratio of hydrochloric acid to the total SiO2 sol is 0.1% to 0.15%.

[0006] Preferably, the hydrolysis catalyzed by hydrochloric acid needs to be stirred for 5 hours to form a uniform SiO2 sol.

[0007] Preferably, the photoinitiator is added after stirring for 4.5 hours, and the mass of the photoinitiator is 1% based on the mass of tetraethyl orthosilicate.

[0008] Preferably, a surfactant is added to the SiO2 sol, preferably Triton X-100, accounting for 0.1% by mass of the SiO2.

[0009] Preferably, an antioxidant is added to the other layer of polyvinylidene fluoride (PVDF) solution in step S04, and the mass of the antioxidant accounts for 0.5% of the total mass of the polyvinylidene fluoride (PVDF) solution.

[0010] Preferably, the polyvinylidene fluoride (PVDF) solution is prepared by dissolving polyvinylidene fluoride (PVDF) powder in a mixed solvent of DMF and acetone to form a transparent solution, wherein the volume ratio of DMF to acetone in the mixed solvent of DMF and acetone is 7:3.

[0011] Preferably, in the steps S02 and S04, the polyvinylidene fluoride (PVDF) layer is annealed at 80° C. for 10 minutes after coating.

[0012] A technical solution provided in the embodiments of the present application has at least the following technical effects: 1. Utilizing a multi-layer composite coating process, the SiO2 layer (high refractive index) selectively reflects ultraviolet light (200–400 nm), transmits visible light (400–700 nm), and inhibits UV damage to plants. The silver layer reflects near-infrared light (700–2500 nm) through plasmon resonance, blocking over 85% of solar thermal radiation. The polyvinylidene fluoride (PVDF) layer and the SiO2 synergistically enhance infrared emissivity (>90%) in the atmospheric window band (8–13 μm), dissipating heat from the greenhouse into space through radiation. This multi-layer structure, through band-specific control, achieves efficient cooling (NIR reflectivity >88%) while maintaining the visible light transmittance (>92%) required for photosynthesis. This overcomes the conflict between transmittance and cooling performance associated with traditional materials due to full-band reflection.

[0013] 2. This example utilizes a flexible PET substrate combined with a PVDF / Ag / PVDF sandwich structure. The PVDF layer provides interfacial buffering through molecular chain entanglement, while the silver layer embedded within the PVDF inhibits oxidation. After 1000 bending cycles (at a curvature radius of 5 mm), the substrate recovery rate exceeded 95%, the silver layer showed less than 3% coverage of oxidation spots, and the NIR reflectivity attenuation was ≤5%. These results demonstrate that the coating maintains excellent optical stability despite complex deformation, making it suitable for flexible applications such as curved greenhouses.

[0014] 3. This application's SiO2-PVDF-Ag multi-coating system achieves synergistic benefits through interfacial chemical bonding (hydrogen bonding between PVDF and SiO2) and physical coating (a PVDF interlayer protects the silver layer). Accelerated aging testing (85°C / 85% RH, 100 h) demonstrated NIR reflectivity retention of >96.5%, PAR transmittance retention of >97%, and oxidation product coverage of only 4.1%. The adhesion level met ASTM D3359 standards, demonstrating the coating's long-term serviceability in high-temperature and high-humidity environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a working principle diagram of the composite coating in Example 1 of the present application; Figure 2 This is a colorless and transparent solution of polyvinylidene fluoride (PVDF) in Example 1 of the present application. DETAILED DESCRIPTION

[0016] In order to better understand the above technical solution, the above technical solution will be described in detail below with reference to the accompanying drawings and specific implementation methods. Example 1

[0017] (1) Preparation of SiO2 inorganic coating by sol-gel method Add 20.8 g of ethyl orthosilicate and 18.4 g of anhydrous ethanol into a beaker, and slowly add a mixed solution consisting of 1.89 g of deionized water and 8 drops of hydrochloric acid under constant temperature magnetic stirring. After stirring at room temperature for 4.5 hours, add 0.2 g of a photoinitiator such as Irgacure 184, and stir for another 0.5 hour to form a SiO2 sol.

[0018] Adding the photoinitiator too early (e.g., after 2 hours) can cause partial crosslinking of the sol, leading to a sudden increase in viscosity (viscosity > 200 cP). Adding the photoinitiator too late (e.g., after 5 hours) can cause the sol to gel and the photoinitiator to disperse unevenly. Experiments have shown that adding the photoinitiator after 4.5 hours can achieve a UV-cured coating with a pencil hardness of 4H and a transmittance > 95%.

[0019] Add 0.04 g of a surfactant, such as Triton X-100, to the SiO2 sol to reduce the surface tension of the sol and minimize coating defects. Spin coat the sol onto a PET film with a thickness of 100 μm. Spin coating parameters: 3000 rpm, 30 s. Spread the sol evenly before spin coating. Initiate crosslinking by UV light to avoid high-temperature treatment. UV curing parameters: UV wavelength 365 nm, irradiation intensity 10 mW / cm 2 , and cured for 5 minutes to form an inorganic SiO2 coating with a surface roughness of <2 nm.

[0020] (2) Coating of polyvinylidene fluoride (PVDF) layer First, N,N-dimethylformamide DMF and acetone were mixed in a volume ratio of 7:3 to prepare a DMF / acetone mixed solvent for later use.

[0021] 0.9 g of polyvinylidene fluoride (PVDF) powder was dissolved in 11.1 g of a DMF / acetone mixed solvent to prepare a polyvinylidene fluoride (PVDF) solution. The resulting mixture was stirred at 40°C for 1 hour to form a colorless transparent solution.

[0022] Then, the above-mentioned polyvinylidene fluoride (PVDF) solution is coated on the inorganic SiO2 coating in step (1), and annealed at 80°C for 10 minutes after coating. Annealing at 80°C for 10 minutes can not only eliminate solvent residues, but also increase the crystallinity of polyvinylidene fluoride (PVDF) from 45% to 60% (DSC test), thereby enhancing mechanical strength without affecting light transmittance. After drying, a polyvinylidene fluoride (PVDF) layer is formed.

[0023] (3) Deposition of silver layer A nanosilver layer was deposited on the polyvinylidene fluoride (PVDF) layer in step (2) by magnetron sputtering. The process parameters were as follows: high-purity Ag target (99.99%), DC power 80 W, argon flow rate 25 sccm, working pressure 0.8 Pa, substrate temperature 25°C, and sputtering time 3 min. An island structure with a thickness of 50±5 nm (coverage <50%) was formed to reduce visible light reflection. The layer was then covered with a second layer of polyvinylidene fluoride (PVDF) solution to form a "polyvinylidene fluoride (PVDF) / Ag / polyvinylidene fluoride (PVDF)" sandwich structure.

[0024] Among them, 0.5% (based on the mass of the polyvinylidene fluoride PVDF solution) of an antioxidant (such as Irganox 1010) is added to the second layer polyvinylidene fluoride PVDF solution to delay the oxidation of the silver layer.

[0025] The prepared composite coatings were subjected to spectral performance testing. PAR transmittance and NIR reflectance were measured using a UV-Vis-NIR spectrophotometer (PerkinElmer Lambda 950). The test conditions were a wavelength range of 300-2500 nm and an incident angle of 8°. Three to five samples were prepared from the same batch and averaged. The measured PAR transmittance was >92%, reaching a maximum of 93%, and the NIR reflectance was >88%, reaching a maximum of 92%.

[0026] The results show that the PAR transmittance of the composite coating can stably reach above 90%, and the NIR reflectivity can reach 88–92% after optimization, meeting the synergistic needs of greenhouse cooling and photosynthesis.

[0027] The prepared composite coating was subjected to an accelerated aging test at 85°C / 85% RH for 100 hours to observe the oxidation of the silver layer. The following technical indicators were tested. Three samples were set in each group and the average value was taken to reduce the error. Unaged samples were also tested at the same time for data comparison.

[0028] Table 1 Accelerated aging test results Technical indicators Test Method Test results NIR reflectivity retention UV-Vis-NIR 96.5% PAR transmittance retention rate UV-Vis-NIR 97% Oxidation product coverage SEM / XPS 4.1% Adhesion grade Cross-cut method (ASTM D3359) Slight peeling The results show that the composite coating has good environmental adaptability and is suitable for long-term outdoor applications such as agricultural greenhouses.

[0029] The mechanical properties of the prepared composite coating were tested. The average of 5 groups of data was taken and the standard deviation was calculated to ensure the reliability of the results. The bending test was performed for 1000 cycles with a curvature radius of 5 mm to verify the coating adhesion to the flexible substrate.

[0030] Test results: The PET substrate has no permanent creases or breaks (recovery rate after bending >95%). SEM observation of crack density shows local microcracks in the SiO2 layer (length <50 μm), no visible cracks in the polyvinylidene fluoride (PVDF) layer, and no breaks in the silver layer. However, there is slight peeling at the interface (oxidation spot coverage <3%), meeting the requirements of flexible applications. The NIR reflectivity decreases by ≤5%, and the PAR transmittance decreases by ≤2%. The decreases in NIR reflectivity and PAR transmittance are controllable, and the optical performance is stable and excellent. Example 2

[0031] A parameter optimization comparison experiment was conducted on the preparation of the SiO2 sol in Example 1. The other parameters and steps remained unchanged, but 18.4 g of anhydrous ethanol was replaced with 15.1 g of anhydrous ethanol. Local gelation occurred during the stirring process.

[0032] 18.4 g of anhydrous ethanol was changed to 22.7 g of anhydrous ethanol, and local precipitation occurred during the stirring process.

[0033] Therefore, when the ratio deviates from 11:9.7:1 (e.g., 11:8:1 or 11:12:1), the sol is prone to localized gelation or precipitation during stirring. At the optimized ratio, the sol can be stored stably for over 24 hours with a viscosity change of <5%. Furthermore, at this ratio, ethanol as a solvent effectively dilutes the ethyl orthosilicate, slowing the hydrolysis rate and preventing excessive hydrolysis that could lead to uneven sol formation or gelation. The addition of hydrochloric acid catalyst further optimizes the pH of the hydrolysis reaction, ensuring a controllable hydrolysis process. Example 3

[0034] A comparative environmental stability test under the same conditions (85°C / 85% RH, 100 hours) was conducted on the polyvinylidene fluoride (PVDF) sandwich structure and the composite coating with an antioxidant in Example 1. Compared with the traditional Ag / PET coating, the composite coating in Example 1 had a silver layer oxidation coverage of 4.1% and a slight peeling adhesion rating, while the traditional Ag / PET coating had a silver layer oxidation coverage of 25% and a significant peeling adhesion rating. This demonstrates that the polyvinylidene fluoride (PVDF) sandwich structure and antioxidant significantly inhibit silver layer oxidation, resulting in superior adhesion.

[0035] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0036] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A co-deposition preparation method for a radiation cooling composite coating based on spectral selective regulation, characterized in that: The following steps are involved: S01 Preparation of SiO2 Inorganic Coating by Sol-Gel Method Using tetraethyl orthosilicate as a precursor, hydrolyzing it in an ethanol solvent by hydrochloric acid catalysis, stirring it evenly, adding a photoinitiator in the later stage of stirring, stirring it evenly to form a sol with a three-dimensional network structure, spin-coating the sol on a film, and using ultraviolet curing to form an inorganic SiO2 coating; S02 Coating of polyvinylidene fluoride (PVDF) layer First, a polyvinylidene fluoride (PVDF) solution is prepared, and then the polyvinylidene fluoride (PVDF) solution is coated on the inorganic SiO2 coating in step S01, and the polyvinylidene fluoride (PVDF) layer is formed after drying; S03 Deposition of silver layer In step S02, a nanosilver layer with a thickness of 50±5 nm is deposited on the surface of the polyvinylidene fluoride (PVDF) layer by magnetron sputtering; S04 Covering of protective layer The silver layer in step S03 is covered with another layer of polyvinylidene fluoride (PVDF) solution to form a "polyvinylidene fluoride (PVDF) / Ag / polyvinylidene fluoride (PVDF)" sandwich structure.

2. The preparation method according to claim 1, wherein The mass ratio of the tetraethyl orthosilicate: ethanol: water is 11:9.7:1, and the mass ratio of hydrochloric acid to the total SiO2 sol is 0.1% to 0.15%.

3. The preparation method according to claim 1, wherein The stirring in step S01 is performed for 5 hours to form a uniform SiO2 sol.

4. The preparation method according to claim 3, wherein After stirring for 4.5 hours, a photoinitiator was added, wherein the mass of the photoinitiator was 1% based on the mass of ethyl orthosilicate.

5. The preparation method according to claim 1, wherein The surfactant added to the SiO2 sol is Triton X-100, which accounts for 0.1% of the mass of SiO2.

6. The preparation method according to claim 1, wherein In the step S04, an antioxidant is added to the other layer of polyvinylidene fluoride (PVDF) solution, and the mass of the antioxidant accounts for 0.5% of the total mass of the polyvinylidene fluoride (PVDF) solution.

7. The preparation method according to claim 1, wherein The polyvinylidene fluoride (PVDF) solution is prepared by dissolving polyvinylidene fluoride (PVDF) powder in a mixed solvent of N,N-dimethylformamide (DMF) and acetone to form a transparent solution, wherein the volume ratio of DMF to acetone in the mixed solvent of N,N-dimethylformamide (DMF) and acetone is 7:

3.

8. The preparation method according to claim 1, wherein In the steps S02 and S04, the polyvinylidene fluoride (PVDF) layer is annealed at 80° C. for 10 minutes after coating.

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