Multi-layer flexible radiation cooling composite diaphragm

Abstract

The invention discloses a multi-layer flexible radiation cooling composite diaphragm which structurally comprises a flexible base layer film, and a SiO2 inorganic coating, a first layer of polyvinylidene fluoride (PVDF), a nano-silver layer deposited by magnetron sputtering and a second layer of PVDF which are sequentially coated to form a base layer film / SiO2 / PVDF / Ag / PVDF sandwich structure. The SiO2 layer is prepared through a sol-gel method, the surface roughness is smaller than 2 nm, and the visible light transmittance is larger than 95%; the thickness of the silver layer is 50 + / -5 nm, and the total thickness of SiO2 / PVDF / Ag does not exceed 150 nm. According to the composite diaphragm, ultraviolet reflection, visible light high transmission (greater than 92%) and near-infrared high reflection (greater than 88%) are realized through spectrum selective regulation and control, and meanwhile, SiO2 and PVDF synergistically enhance the infrared emissivity of an atmospheric window wave band (8-13 [mu] m). The flexible PET base material is combined with the PVDF interlayer design to restrain silver layer oxidation and interface stripping, the recovery rate is larger than 95% after 1000 times of bending circulation, and the NIR reflectivity attenuation is smaller than or equal to 5%. The composite diaphragm disclosed by the invention has the advantages of high light transmission, efficient cooling, excellent flexibility and long-term stability, and is suitable for flexible scenes such as agricultural greenhouses and the like.

Description

Technical Field

[0001] The invention relates to the technical field of cooling diaphragm preparation, in particular to a multi-layer flexible radiation cooling composite diaphragm. Background Art

[0002] With the increasing demand for smarter agricultural facilities and energy conservation, radiative cooling membranes, which passively cool the environment without the need for active energy, are showing significant application value in greenhouse coverings, building curtain walls, and other applications. However, existing cooling membranes still face the following technical challenges: First, traditional membranes often utilize a single-layer functional coating or a simple stacked structure (such as a metal reflective layer or polymer film), making it difficult to achieve precise, layered control across the entire wavelength range. For example, some membranes reduce solar heat input through full-band reflection strategies, but their visible light transmittance is typically less than 80%, resulting in reduced crop photosynthetic efficiency. Other membranes, while having a visible light transmittance greater than 90%, have a near-infrared reflectance less than 70%, failing to effectively block solar thermal radiation and causing temperature imbalances within the greenhouse. Second, most existing membranes are designed based on rigid substrates (such as glass or hard plastic), making them difficult to conform to curved greenhouse structures. Frequent deformation or prolonged outdoor exposure can lead to problems such as silver oxidation and delamination, resulting in optical performance degradation. Furthermore, the infrared emissivity of existing membranes in the atmospheric window band is generally less than 85%, limiting their ability to dissipate heat through radiation. The above problems indicate that there is an urgent need to develop a composite cooling membrane that combines spectral domain regulation, high flexibility and long-term stability to solve the conflicting needs of light transmission and cooling, while meeting the coordinated optimization of mechanical properties and durability in complex application scenarios. Summary of the Invention

[0003] The embodiments of the present application solve the technical problems of the conflicting requirements of light transmittance and cooling performance of cooling membranes and poor durability in the prior art by providing a multi-layer flexible radiation cooling membrane. The composite membrane achieves the excellent performance of high light transmittance, high reflectivity, excellent flexibility and long-term stability of the radiation cooling membrane through multi-layer collaborative design and band-specific spectral selective regulation.

[0004] The present application provides a multi-layer flexible radiation cooling membrane, comprising: Flexible base film, Spin coating the SiO2 inorganic coating on the base film; A first polyvinylidene fluoride (PVDF) layer coated on the SiO2 inorganic coating; a silver layer (Ag) deposited on the polyvinylidene fluoride (PVDF) layer by magnetron sputtering; A second polyvinylidene fluoride (PVDF) layer is coated on the silver layer, and the multilayer flexible radiation cooling membrane forms a multilayer sandwich structure of base film-SiO2 layer-PVDF-Ag-PVDF, and the multilayer sandwich structure achieves a photosynthetic active radiation PAR transmittance greater than 92% and a near-infrared reflectivity greater than 88%.

[0005] Preferably, the SiO2 is prepared by a sol-gel method, and the surface roughness of the SiO2 inorganic coating is less than 2 nm, and the visible light transmittance is greater than 95%.

[0006] Preferably, the total thickness of the SiO2 coating, the first PVDF layer and the Ag layer does not exceed 150 nm, wherein the thickness of the Ag layer is 50±5 nm and the Ag layer is a discontinuous structure.

[0007] Preferably, the flexible base film is PET.

[0008] Preferably, an antioxidant is added to the second polyvinylidene fluoride (PVDF) layer, and the antioxidant is Irganox 1010.

[0009] Preferably, the infrared emissivity of the composite membrane in the atmospheric window band is greater than 90%.

[0010] A technical solution provided in the embodiments of the present application has at least the following technical effects: 1. The synergistic effect of multiple layers of composite coatings achieves selective spectral modulation across wavelengths. The SiO2 layer (high refractive index) selectively reflects UV light (200–400 nm), transmits visible light (400–700 nm), and inhibits UV damage to plants, while maintaining a visible light transmittance of >95%. The silver layer reflects near-infrared light (700–2500 nm) through plasmon resonance, blocking over 85% of solar thermal radiation and significantly reducing greenhouse temperatures. The polyvinylidene fluoride (PVDF) layer and SiO2 synergistically enhance infrared emissivity (>90%) in the atmospheric window band (8–13 μm), dissipating heat within the greenhouse into space through radiation. The multi-layered structure of the composite membrane achieves band-specific modulation while maintaining the visible light transmittance required for photosynthesis while achieving efficient cooling (NIR reflectance >88%). This overcomes the conflict between transmittance and cooling performance associated with conventional cooling membranes, which reflect light across the entire wavelength range.

[0011] 2. This embodiment utilizes a PET flexible substrate combined with a PVDF / Ag / PVDF sandwich structure. The PVDF layer provides interfacial buffering through molecular chain entanglement. The silver layer embedded within the PVDF not only inhibits silver oxidation but also prevents silver fracture and interfacial delamination. After 1000 bending cycles (radius of curvature 5 mm), the substrate recovery rate exceeded 95%, the silver layer oxidation spot coverage was less than 3%, and the NIR reflectivity attenuation was ≤5%. This demonstrates that the composite membrane maintains excellent optical stability even under complex deformation, making it suitable for the dynamic deformation requirements of curved greenhouses.

[0012] 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 composite membrane's long-term serviceability in high-temperature and high-humidity environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a diagram of the working principle of the composite diaphragm in Example 1 of this application. DETAILED DESCRIPTION

[0014] 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

[0015] (1) Preparation of SiO2 inorganic coating by sol-gel method Add 20.8 g of tetraethyl orthosilicate and 18.4 g of anhydrous ethanol to a beaker. Slowly add a mixture consisting of 1.89 g of deionized water and 8 drops of hydrochloric acid under constant temperature magnetic stirring. Stir at room temperature for 4.5 hours. Then, add 0.2 g of a photoinitiator such as Irgacure 184 and stir for an additional 0.5 hour to form a SiO2 sol. The sol is stable for more than 24 hours, with a viscosity change of <5%.

[0016] 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%.

[0017] 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. Induce 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.

[0018] (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.

[0019] 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.

[0020] 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.

[0021] (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.

[0022] 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.

[0023] The prepared composite membranes were subjected to spectral performance testing. PAR transmittance and NIR reflectance were measured using a UV-Vis-NIR spectrophotometer (PerkinElmer Lambda 950). 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%.

[0024] The results show that the PAR transmittance of the composite membrane can stably reach more than 90%, and the NIR reflectivity can reach 88–92% after optimization, meeting the synergistic needs of greenhouse cooling and photosynthesis.

[0025] The prepared composite diaphragm 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 up 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.

[0026] 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 membrane has good environmental adaptability and is suitable for long-term outdoor applications such as agricultural greenhouses.

[0027] The mechanical properties of the prepared composite diaphragm were tested, and 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 of the flexible substrate.

[0028] 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

[0029] 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.

[0030] 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.

[0031] 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. Multi-layer flexible radiation cooling composite membrane, characterized in that: include Flexible base film, Spin coating the SiO2 inorganic coating on the base film; A first polyvinylidene fluoride (PVDF) layer coated on the SiO2 inorganic coating; a silver layer (Ag) deposited on the polyvinylidene fluoride (PVDF) layer by magnetron sputtering; A second polyvinylidene fluoride (PVDF) layer is coated on the silver layer, and the multilayer flexible radiation cooling membrane forms a multilayer sandwich structure of base film-SiO2 layer-PVDF-Ag-PVDF, and the multilayer sandwich structure achieves a photosynthetic active radiation PAR transmittance greater than 92% and a near-infrared reflectivity greater than 88%.

2. The composite diaphragm according to claim 1, wherein The SiO2 is prepared by a sol-gel method, the surface roughness of the SiO2 inorganic coating is less than 2 nm, and the visible light transmittance is greater than 95%.

3. The composite diaphragm according to claim 1, wherein The total thickness of the SiO2 coating, the first PVDF layer and the Ag layer does not exceed 150 nm, wherein the thickness of the Ag layer is 50±5 nm and the Ag layer is a discontinuous structure.

4. The composite diaphragm according to claim 1, wherein The flexible base film is PET.

5. The composite diaphragm according to claim 1, wherein An antioxidant is added to the second polyvinylidene fluoride (PVDF) layer, and the antioxidant is Irganox 1010.

6. The composite diaphragm according to claim 1, wherein The infrared emissivity of the composite membrane in the atmospheric window band is greater than 90%.

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