Porous polyvinylidene fluoride based composite radiation refrigeration film and preparation method thereof

Abstract

The invention relates to the technical field of radiation refrigeration films, and discloses a porous polyvinylidene fluoride based composite radiation refrigeration film and a preparation method thereof. The method comprises the following steps: firstly, carrying out melt blending on PVDF, NaCl and ZnO nanoparticles and forming a film, then dissolving out NaCl through water washing, and constructing a continuous three-dimensional porous structure in a PVDF matrix; and immersing the obtained porous PVDF / ZnO membrane material into a methanol aqueous solution containing 2-methylimidazole, so that ZnO is subjected to a coordination reaction at the pore wall and is converted into the ZIF-8 nanocrystal in situ. In the thin film prepared by the steps, the micron-sized salt-induced pore structure, the nano-sized ZIF-8 particles and the interface of the ZIF-8 particles jointly form a multi-stage scattering network, so that the scattering ability of the thin film in the solar spectrum range of 0.25-2.5 microns can be remarkably enhanced, the high infrared emission performance of the PVDF matrix in an atmospheric window of 8-13 microns is maintained, and the thin film has high solar reflectivity and high infrared emissivity, and can be used for preparing the solar film. An excellent radiation refrigeration effect can be achieved, and the heat-dissipating material is suitable for being applied to building outer surfaces, equipment heat dissipation, individual heat management and the like.

Description

Technical Field

[0001] This invention relates to the field of radiation cooling film technology, and in particular to a porous polyvinylidene fluoride thin film suitable for radiation cooling and its preparation method. Background Technology

[0002] Passive radiative cooling technology requires no external energy input. It spontaneously dissipates heat through broadband radiation from the material surface into outer space (approximately 3K), demonstrating significant energy-saving potential in building envelopes, outdoor equipment, energy storage devices, and individual microenvironments. High-performance daytime radiative cooling materials generally need to simultaneously meet two key optical properties: firstly, they must have the highest possible reflectivity within the solar spectrum (0.25–2.5 μm) to minimize solar radiation absorption; secondly, they must have high emissivity within the atmospheric transparency window (8–13 μm) to enable the material to effectively dissipate its own heat through infrared radiation.

[0003] Polymer-based radiative cooling materials have become a research hotspot due to their advantages such as lightweight, good flexibility, large-area fabrication, and low cost. Among them, polyvinylidene fluoride (PVDF) has potential applications in radiative cooling because its molecular chain contains a large number of CF bonds, giving it a natural high infrared emission capability in the atmospheric transparency window. However, pure PVDF films have low reflectivity in the solar radiation band, making it difficult to meet the "near total internal reflection" requirement for daytime radiative cooling, thus limiting its net cooling capacity.

[0004] In existing technologies, to improve the solar reflectivity of PVDF films, the main approach is to enhance light scattering by introducing composite surface structures or adding scattering layers. For example, Chinese patent application CN202410081035.6 discloses a radiation-cooling film that introduces an outer layer composed of polymers such as polydimethylsiloxane, polyurethane, polyethylene, or polyethylene terephthalate onto the surface of the PVDF film. High-reflectivity inorganic nanoparticles such as titanium dioxide nanoparticles, alumina nanoparticles, and silica nanoparticles are incorporated into this layer, and solar light scattering is further enhanced through a micron-scale pyramid array structure. While such designs can improve the light reflectivity of the base film, their processes and structures still have certain limitations.

[0005] 1) Composite surface structures rely on micron-level array forming, which places high demands on molds and processing conditions, making them unsuitable for large-area, low-cost production;

[0006] 2) The outer layer and the PVDF base film are made of different materials. The stability of the interlayer bonding is greatly affected by the environment and operating conditions. The optical performance may degrade during long-term use.

[0007] 3) The overall material system is relatively complex, requiring multiple processing steps and the combination of various raw materials, resulting in relatively high manufacturing costs and process difficulty.

[0008] Besides the methods mentioned above, some studies have attempted to directly add high-refractive-index particulate materials to PVDF to enhance scattering. For example, Chinese patent application CN202411289214.5 discloses a porous radiation-cooling thin film filled with hollow nanospheres and its preparation method. Hollow titanium dioxide nanospheres are synthesized via a template method and then mixed with PVDF in a solution. Solvent evaporation induces phase separation to construct a composite structure rich in micro- and nano-pores. This design enhances Mie scattering through the rich refractive index interface formed by the hollow microspheres and the porous matrix, achieving a solar reflectivity of 95.8% and an infrared emissivity of 96.4%, and can cool down by approximately 12°C under direct sunlight. Although this thin film has excellent optical properties, it still has the following limitations:

[0009] 1) High filler loading leads to dispersion difficulties and agglomeration: The examples clearly indicate that 300-500mg of hollow titanium dioxide nanospheres need to be added to 1-2g of PVDF. This extremely high addition ratio makes it very difficult for the nanofiller to be uniformly dispersed in the viscous casting solution, and it is very easy to cause severe particle agglomeration, thereby destroying the uniformity of the microstructure.

[0010] 2) Decreased mechanical properties: High content of inorganic filler agglomerates often become stress concentration points inside the material. In addition, the film itself has constructed a large number of loose microporous structures to enhance scattering. This "high filling content + porous" system seriously disrupts the continuity of the PVDF matrix, resulting in a significant decrease in the tensile strength and elongation at break of the film. This makes it brittle and fragile in actual operation, and it is difficult to meet the requirements of the material's flexibility and mechanical strength in complex outdoor environments.

[0011] Therefore, existing technologies still lack a composite radiative cooling film that is structurally stable, has a simple preparation process, can be mass-produced on a large scale, has cost advantages, and can construct a three-dimensional porous structure and nanoscale scattering centers in a PVDF matrix to significantly improve the broad spectrum reflectivity of sunlight and ensure high infrared emissivity, thus meeting the needs of high-performance daytime radiative cooling. Summary of the Invention

[0012] To address the technical problems of low solar spectral reflectivity and poor structural stability of PVDF films, this invention provides a porous polyvinylidene fluoride composite radiation cooling film and its preparation method.

[0013] The specific technical solution of this invention is as follows:

[0014] As a first aspect of the present invention, a method for preparing a porous polyvinylidene fluoride composite radiation cooling film is provided, characterized by comprising the following steps:

[0015] Step S1: Add sodium chloride (NaCl) and zinc oxide (ZnO) nanoparticles to PVDF, mix well, and then melt process and press into sheets to obtain PVDF / NaCl / ZnO blend film material.

[0016] Step S2: Immerse the blended membrane material in water to dissolve NaCl, thereby forming a stable three-dimensional interconnected porous structure in the PVDF matrix to obtain a porous PVDF / ZnO membrane material.

[0017] Step S3: Immerse the porous PVDF / ZnO membrane in a 2-methylimidazole solution to allow ZnO to undergo a coordination reaction on the pore walls of the porous structure and be converted in situ to form ZIF-8, thus obtaining a porous PVDF / ZIF-8 composite radiation cooling film.

[0018] Existing PVDF films suffer from low light reflectivity and poor reflectivity stability.

[0019] To address the aforementioned problems, this invention improves the preparation method and structure of PVDF films, providing a method for preparing a porous polyvinylidene fluoride composite radiation-cooling film. This invention removes the stable pore structure formed in the PVDF material by introducing and soaking it with NaCl, resulting in a membrane material with a porous structure. This porous structure can efficiently reflect a broad spectrum of sunlight, thereby improving the cooling performance of the PVDF film. Furthermore, due to the high stability of the porous structure prepared by this invention, the PVDF film exhibits high reflectivity stability.

[0020] Furthermore, this invention involves blending ZnO nanoparticles with PVDF and NaCl, followed by in-situ growth of ZIF-8 directly in a PVDF / ZnO membrane after immersion in a 2-methylimidazole solution. This disperses and anchors the ZIF-8 nanomaterials onto the pore walls of the porous matrix, effectively introducing them into the porous PVDF matrix. This method effectively avoids the aggregation problem of ZIF-8 nanomaterials, and due to the anchoring effect of the porous structure, the ZIF-8 nanomaterials exhibit high stability. After the introduction of ZIF-8 nanomaterials, when the ZIF-8 nanoparticles accumulate on the pore walls introduced by NaCl, numerous nanoscale pores and interfaces are formed in the porous PVDF / ZIF-8 membrane. The micropores introduced by NaCl, combined with these nanoscale structures and the micro / nano structures of the particles themselves, collectively constitute a highly efficient multi-level scattering network, capable of reflecting a wide range of sunlight back to the surface to the maximum extent.

[0021] Therefore, the method described above in this invention forms a three-dimensional interconnected micron-scale porous structure in PVDF through NaCl template dissolution, providing abundant scattering interfaces for light within the film material; simultaneously, it utilizes the reaction of ZnO with 2-methylimidazole under mild conditions to generate ZIF-8, allowing ZIF-8 to grow uniformly on the pore wall surface, constructing nanoscale scattering centers, and achieving synergistic scattering at the micron-nano scale. The PVDF-based thin film prepared in this way has the advantages of high reflectivity and high stability.

[0022] The method described above in this invention has significant advantages over directly incorporating ZIF-8 or using simple filler reinforcement methods. By generating ZIF-8 through in-situ conversion of ZnO, problems such as easy decomposition, discoloration, and agglomeration of ZIF-8 during high-temperature melting processing can be effectively avoided. This allows ZIF-8 nanocrystals to be stably and uniformly anchored to the pore walls of the porous structure, significantly improving structural stability and service life.

[0023] As a preferred embodiment of the above method, in step S1, the amount of NaCl added is 80~150wt% based on the mass of PVDF, and the amount of ZnO nanoparticles added is 1~4wt%.

[0024] As a preferred embodiment of the above method, in step S1, the mixing method is: mixing at a speed of 20 to 100 rpm at a temperature of 180℃ to 220℃.

[0025] As a preferred embodiment of the above method, the blending time is 5 to 20 minutes.

[0026] As a preferred embodiment of the above method, in step S2, the soaking temperature is 45℃~65℃ and the time is 12~48 hours.

[0027] As a preferred embodiment of the above method, in step S3, the 2-methylimidazole solution is prepared by dissolving 2-methylimidazole in an aqueous methanol solution at a ratio of 0.05wt% to 2wt%.

[0028] As a preferred embodiment of the above method, the volume concentration of methanol in the methanol aqueous solution is 40-60%.

[0029] As a preferred embodiment of the above method, in step S3, the temperature of the soaking treatment is 20℃~40℃.

[0030] Further preferably, in step S3, the soaking treatment time is 20-48 hours.

[0031] As a second aspect of the present invention, a radiation-cooling thin film is provided based on the above preparation method.

[0032] As a third aspect of the present invention, based on the above preparation method and the thin film prepared therefrom, an application of the thin film in radiation cooling is provided.

[0033] Compared with the prior art, the present invention has the following technical effects:

[0034] (1) This invention constructs a stable three-dimensional interconnected porous structure within the PVDF matrix by introducing and removing NaCl. Furthermore, ZIF-8 nanomaterials are grown in situ in the PVDF / ZnO membrane by blending ZnO nanoparticles with PVDF and NaCl, followed by immersion in a 2-methylimidazole solution. This disperses and anchors the ZIF-8 material onto the pore walls of the porous matrix, effectively preventing the aggregation of ZIF-8 nanomaterials. Due to the anchoring effect of the porous structure, the ZIF-8 nanomaterials exhibit high stability. After the introduction of ZIF-8 nanomaterials, when the ZIF-8 nanoparticles accumulate on the pore walls introduced by NaCl, a large number of nanoscale pores and interfaces are formed in the porous PVDF / ZIF-8 membrane. The micropores introduced by NaCl, combined with these nanoscale structures and the micro / nano structures of the particles themselves, constitute a highly efficient multi-level scattering network, capable of reflecting a wide range of sunlight back to the surface to the maximum extent. Compared to traditional planar structures or single scattering layers, this porous structure can form a uniform, continuous and highly stable scattering interface within the material without the need for additional surface patterns or composite layers.

[0035] (2) This invention adopts industrially mature melt blending and molding technology, and uses cheap and readily available sodium chloride as a pore-forming agent, which can be removed by simple water washing. The whole process route is simple, environmentally friendly, and easy to achieve low-cost large-scale industrial production. Attached Figure Description

[0036] Figure 1 Images showing the ultraviolet-visible-near-infrared reflectance of the thin films in Examples 1 to 3;

[0037] Figure 2 Mid-infrared emissivity diagrams of the thin films of Examples 1 to 3;

[0038] Figure 3 The images show the outdoor cooling effect of the films in Examples 1 to 3. Detailed Implementation

[0039] The present invention will be further described below with reference to embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0040] In this invention, the following methods were used to characterize and test the performance of the radiation-cooling thin film:

[0041] 1. Solar Spectral Reflectance Test

[0042] The hemispherical reflectance of the thin film samples in the solar spectral range (0.25–2.5 μm) was measured using a PerkinElmer UV-Vis-NIR spectrophotometer (model: Lambda 750). During testing, reflectance spectra were obtained using a Spectralon® diffuse reflectance PTFE standard plate (SRS-99-010, Labsphere, America) to analyze the optical properties of the prepared samples under different conditions. The measured spectral reflectance data were then presented. Compared with standard AM1.5G solar spectral irradiance The total solar spectral reflectance of the material is calculated by performing a weighted integral and applying the following formula. : .

[0043] 2. Mid-infrared spectral emissivity test:

[0044] The infrared reflectance of the thin film samples in the atmospheric transparency window (8-13 μm) was measured using a Thermo Fisher Fourier transform infrared spectrometer (model: Nicoleti S50) equipped with an integrating sphere attachment. A gold-plated mirror was used as a high-reflectance reference during the test. Because the thin film samples under test are optically opaque materials (transmittance...), the infrared reflectance was measured... ≈0), according to Kirchhoff's law, its spectral emissivity With spectral reflectance Satisfying the relation: = 1 - The calculated spectral emissivity data blackbody radiation spectral intensity at 300K By performing a weighted integral, the average emissivity of the material within the atmospheric window is calculated using the following formula. : .

[0045] 3. Calculation of Net Cooling Power

[0046] When a radiation-cooled sample is placed horizontally on the ground and exposed to a clear daytime sky, it is affected by solar irradiance and downward atmospheric radiation. Net cooling power Defined as (1).

[0047] in, If the temperature of the sample is radiatively cooled, then... It is the temperature of the surrounding environment. It is the power radiated by the cooler. It is the absorption of downward atmospheric thermal radiation. It is the absorbed solar energy. This refers to the energy lost due to changes in the surrounding environment caused by conductive and convective heat. When the radiatively cooled sample reaches its equilibrium temperature, i.e. Maximum cooling temperature This will be achieved. Maximum cooling capacity is reached when the temperature of the radiatively cooled sample equals the ambient temperature. The parameters in formula (1) can be calculated using the following formula: .

[0048] in For solid angles, The solid angle is the angle between the solid angle and the direction of the normal to the sample surface. For an object at a specific wavelength and angle Emission rate at the lower level Atmospheric emissivity, The angle of sunlight direction. The nonradiative thermal coefficient, composed of heat convection and heat conduction, ranges from 0 to 9 W·m. -2 ·K -1 .

[0049] Example 1

[0050] A porous PVDF / ZIF-8 composite radiation cooling material is provided, and its preparation method is as follows:

[0051] Step S1: Mix PVDF, NaCl, and ZnO nanoparticles and mold them to obtain a blend film material. The operation is as follows:

[0052] 25g of PVDF, 25g of NaCl, and 0.25g of ZnO were mixed and then melt-blended in a Hacker torque rheometer at 200℃ and 50 rpm for 8 minutes. The resulting film was then molded at 200℃ using a tablet press to obtain a 10cm × 10cm × 0.3mm blend membrane, which is a PVDF / NaCl / ZnO ternary blend membrane.

[0053] Step S2: Soak the blended membrane material in a 50°C water bath for 24 hours to dissolve NaCl, thereby forming a porous structure in the PVDF matrix to obtain a porous PVDF / ZnO membrane material.

[0054] Step S3: Prepare a 50% (v / v) methanol-water solution, then dissolve 2-methylimidazole in the methanol-water solution to obtain a 2-methylimidazole solution. Immerse the porous PVDF / ZnO membrane material in a 0.1 wt% 2-methylimidazole solution at 25°C for 24 hours, allowing ZnO to react in situ on the pore walls of the porous structure to generate ZIF-8. After the immersion process, remove the material, wash it with water, and dry it to obtain the porous PVDF / ZIF-8 composite radiative cooling material.

[0055] Example 2

[0056] A porous PVDF / ZIF-8 composite radiative cooling material is provided. The preparation method differs from Example 1 only in that, in step S1, the ZnO nanoparticles account for 2% of the PVDF mass, i.e., 25g of PVDF, 25g of NaCl, and 0.5g of ZnO are melt-blended. Everything else is the same as in Example 1.

[0057] Example 3

[0058] A porous PVDF / ZIF-8 composite radiative cooling material is provided. The preparation method differs from Example 1 only in that, in step S1, ZnO nanoparticles account for 4% of the PVDF mass, i.e., 25g of PVDF, 25g of NaCl, and 1.0g of ZnO are melt-blended. All other steps are the same as in Example 1.

[0059] Example 4

[0060] A porous PVDF / ZIF-8 composite radiative cooling material is provided. The preparation method differs from Example 2 only in that, in step S1, NaCl accounts for 80% of the PVDF mass, that is, 25g of PVDF, 20g of NaCl, and 0.5g of ZnO are melt-blended. Everything else is the same as in Example 1.

[0061] Example 5

[0062] A porous PVDF / ZIF-8 composite radiative cooling material is provided. The preparation method differs from Example 2 only in that, in step S1, NaCl accounts for 150% of the PVDF mass, that is, 25g of PVDF, 30g of NaCl, and 0.5g of ZnO are melt-blended. Everything else is the same as in Example 1.

[0063] Comparative Example 1

[0064] The difference between this comparative example and Example 2 is that NaCl and ZnO nanoparticles are not added in step S1. The specific steps are as follows:

[0065] 10g of PVDF was molded at 200℃ using a tablet press to obtain a film sample of 10cm×10cm×0.3mm.

[0066] Comparative Example 2

[0067] The difference between this comparative example and Example 2 is that ZnO is not added in step S1; instead, 25g of PVDF and 25g of NaCl are melt-blended. The other steps are the same as in Example 2.

[0068] Comparative Example 3

[0069] The difference between this comparative example and Example 2 is that in step S1, the ZnO nanoparticles are replaced with an equal amount of ZIF-8 nanoparticles, and step S3 is omitted. The specific steps are as follows:

[0070] 25g of PVDF, 25g of NaCl, and 0.5g of ZIF-8 were mixed and then melt-blended in a Hacker torque rheometer at 200℃ and 50 rpm for 8 minutes. The resulting membrane was then molded at 200℃ using a tablet press to obtain a 10cm × 10cm × 0.3mm blend. The NaCl was then removed by immersion in a 50℃ water bath for 24 hours, yielding a porous PVDF / ZIF-8 membrane.

[0071] Comparative Example 4

[0072] The difference between this comparative example and Example 2 is that NaCl is not added in step S1; instead, 25g of PVDF and 0.5g of ZnO are melt-blended. The other steps are the same as in Example 2.

[0073] Comparative Example 5

[0074] The difference between this comparative example and Example 2 is that in step S1, the ZnO nanoparticles account for 0.8% of the PVDF mass, that is, 25g of PVDF, 25g of NaCl, and 0.2g of ZnO are melt-blended. The other steps are the same as in Example 1.

[0075] Comparative Example 6

[0076] The main difference between this comparative example and Example 2 is that in step S1, ZnO accounts for 5% of the mass of PVDF, that is, 25g of PVDF, 25g of NaCl, and 1.25g of ZnO are melt-blended. The other steps are the same as in Example 1.

[0077] Comparative Example 7

[0078] The difference between this comparative example and Example 2 is that in step S1, the NaCl nanoparticles account for 70% of the PVDF mass, that is, 25g of PVDF, 17.5g of NaCl, and 0.5g of ZnO are melt-blended. The other steps are the same as in Example 1.

[0079] Comparative Example 8

[0080] The main difference between this comparative example and Example 2 is that in step S1, NaCl accounts for 180% of the mass of PVDF, that is, 25g of PVDF, 45g of NaCl, and 0.5g of ZnO are melt-blended. The other steps are the same as in Example 1.

[0081] Performance Characterization

[0082] 1. The radiative cooling materials prepared in the examples and comparative examples were tested for ultraviolet-visible-near-infrared reflectance and mid-infrared emissivity, and the net cooling power was calculated. The results are shown in Tables 1 and 2. Figures 1 to 3 .in, Figure 1 Images showing the ultraviolet-visible-near-infrared reflectance of the thin films in Examples 1 to 3. Figure 2 The images show the mid-infrared emissivity of the films from Examples 1 to 3. Figure 3 Images showing the UV-Vis-NIR reflectance and mid-infrared emissivity of the thin films in Comparative Examples 1 to 4.

[0083] Table 1 UV-Vis-NIR Reflectance (%) Example 1 93.38 Example 2 99.10 Example 3 91.84 Example 4 92.24 Example 5 91.67 Comparative Example 1 22.91 Comparative Example 2 89.72 Comparative Example 3 63.35 Comparative Example 4 40.72 Comparative Example 5 90.32 Comparative Example 6 90.84 Comparative Example 7 68.96 Comparative Example 8 90.65 .

[0084] Table 2 Mid-infrared emissivity (%) <![CDATA[Net cooling power (W / m 2 )]]> Example 1 96.36 70.84 Example 2 96.50 118.12 Example 3 96.80 64.55 Example 4 96.11 55.05 Example 5 96.67 59.47 .

[0085] From the data in Table 1 and Figures 1 to 3 It can be known that:

[0086] (1) This invention successfully constructed a porous structure in polyvinylidene fluoride matrix by using sodium chloride template pore-forming method, which can form a high-density solid-gas scattering interface and improve the solar reflectivity of the film. Furthermore, by blending ZnO nanoparticles with PVDF and NaCl, and then soaking them in 2-methylimidazole solution, ZIF-8 nanomaterials are grown in situ directly in PVDF / ZnO film. The ZIF-8 material is dispersedly anchored on the pore walls of the porous matrix, forming a large number of pores and interfaces at the nanoscale in the porous PVDF / ZIF-8 film. The micropores introduced by NaCl, combined with these nanoscale structures and the micro-nano structure of the particles themselves, constitute a highly efficient multi-level scattering network. The synergistic effect can reflect a wide range of sunlight back to the maximum extent, greatly enhancing the scattering ability of the material for a wide range of sunlight (0.25-2.5 μm), thereby significantly improving the solar reflectivity of the material. The preparation method of this invention can effectively prepare a polyvinylidene fluoride composite radiative cooling material with both high solar reflectivity and high infrared emissivity, exhibiting excellent net cooling power.

[0087] (2) By comparing Examples 1, 4, and 5 with Comparative Examples 7 and 8, the influence of porous structure on the solar reflectivity of the product material can be obtained. Table 1 shows that when the amount of NaCl added is 80-150 wt% (based on the mass of PVDF), the solar reflectivity of the product material is relatively high, exceeding 90%. Furthermore, it can be seen that the optimal porous structure is when the addition ratio is 1:1 (Example 2). Therefore, as the initial amount of sodium chloride added increases, the porosity of the resulting porous material increases accordingly, but the solar spectral reflectivity shows a trend of first increasing and then decreasing. This indicates that there is an optimal range for the influence of porous structure on light scattering. Within a certain range, increasing the sodium chloride content helps to improve the solar spectral reflectivity; however, when the content is too high, it may lead to pore structure deterioration (such as pore collapse or excessively large pore size), which significantly reduces scattering efficiency.

[0088] (3) The comparison between Examples 1, 2, and 3 and Comparative Examples 5 and 6 aims to systematically examine the effect of ZnO addition on the final material properties based on the optimal porous structure, and to further verify the aforementioned synergistic mechanism. In Example 2, the addition of 2% (based on the mass of PVDF) of ZnO reached the optimal balance point: this ratio ensured a sufficient number of uniformly dispersed ZIF-8 scattering centers, while maintaining the integrity of the porous structure and efficient scattering characteristics to the greatest extent, thereby maximizing the synergistic effect of porous scattering and ZIF-8 nanomaterial scattering. Under the ratio of Example 2 (based on the mass of PVDF, the amount of NaCl added was 100wt%, and the amount of ZnO nanoparticles added was 2wt%), the micropores introduced by NaCl, together with the ZIF-8 nanoscale structure and the micro-nano structure of the ZIF-8 particles themselves, constituted the multi-level scattering network effect to maximize, ultimately obtaining the highest solar reflectivity and net cooling power.

[0089] Depend on Figure 1 and Figure 2 As shown in Table 1, all the samples in the examples exhibited excellent emission performance in the mid-infrared band (8~13 μm), with an average emissivity of approximately 96.5%. While the infrared emissivity showed a slight increasing trend with increasing ZnO mass fraction, the overall difference was small.

[0090] (4) The comparison between Comparative Examples 1, 2, 3, and 4 and Example 1 demonstrates the importance of adding ZIF-8 nanomaterials to the surface of the anchored microporous structure. Comparative Example 1 has neither micropores introduced by NaCl nor ZIF-8 nanomaterials, resulting in a significant reduction in solar reflectivity. Comparative Example 2 does not add ZnO, and because there is no subsequent ZnO to ZIF-8 conversion step, a multi-level successive scattering network cannot be constructed, resulting in a significant reduction in solar reflectivity. Comparative Example 4 does not add NaCl, and because there is no subsequent formation of porous structures, a multi-level successive scattering network cannot be constructed, resulting in a significant reduction in solar reflectivity. Comparative Example 3 only directly adds ZIF-8 nanomaterials, and the solar reflectivity is also significantly reduced. This shows that without the successive formation of porous structure-ZnO-porous surface ZIF-8 nanomaterials, a multi-level successive scattering network cannot be constructed, and the solar reflectivity will be significantly reduced. This further demonstrates the importance of adding ZIF-8 nanomaterials to the surface of the anchored microporous structure.

[0091] The comparative analysis above shows that the appropriate introduction of ZnO and its in-situ conversion to ZIF-8 can effectively increase the number of light scattering centers within the material and produce a positive synergistic effect with the porous structure formed by sodium chloride pores, thereby improving the overall solar reflectivity and net cooling power. This further demonstrates the importance of adding ZIF-8 nanomaterials to anchor the microporous structure surface.

[0092] 2. The films obtained in Examples 1 to 3 were tested in an actual outdoor environment to observe the change in surface temperature over time, with the ambient temperature as a reference. The results of the curves showing the change in film surface temperature over time are shown in the figure. Figure 3 .

[0093] Depend on Figure 3 The measured data show that all samples exhibited significant passive radiative cooling effects, with their surface temperatures remaining consistently lower than the ambient temperature throughout the daytime testing period. Specifically, the cooling curves of Examples 1 and 3 were quite similar, demonstrating similar cooling performance. However, the surface temperature of Example 2 remained the lowest among the three, exhibiting the largest cooling amplitude. This measured result is consistent with the theoretical net cooling power calculated based on spectral data (as shown in Table 1, Example 2 has the highest net cooling power), thus strongly verifying that the composite material prepared in this invention possesses excellent practical radiative cooling capabilities.

[0094] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0095] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a porous polyvinylidene fluoride composite radiation cooling thin film, characterized in that: The method comprises the following steps: Step S1, adding NaCl and ZnO nanoparticles into PVDF, mixing, tabletting, to obtain a PVDF / NaCl / ZnO ternary copolymer film material; Step S2, taking the ternary copolymer film material and performing immersion treatment with water to obtain a porous PVDF / ZnO film material; Step S3, taking the porous PVDF / ZnO film material and performing immersion treatment with a 2-methylimidazole solution to obtain a porous PVDF / Zn-8 film material.

2. The method for preparing a porous polyvinylidene fluoride composite radiation cooling thin film as described in claim 1, characterized in that: In step S1, the amount of NaCl added is 80-150 wt% based on the mass of PVDF, and the amount of ZnO nanoparticles added is 1-4 wt%.

3. The method for preparing a porous polyvinylidene fluoride composite radiation cooling thin film as described in claim 1, characterized in that: In step S1, the mixing method is: blending at 180-220 ℃ at a rotation speed of 20-100 rpm.

4. The method for preparing a porous polyvinylidene fluoride composite radiation cooling thin film as described in claim 3, characterized in that: The blending time is 5-20 minutes.

5. The method for preparing a porous polyvinylidene fluoride composite radiation cooling thin film as described in claim 1, characterized in that: In step S2, the temperature of the immersion treatment is 45-65 ℃, and the time is 12-48 hours.

6. The method of making a porous polyvinylidene fluoride-based composite radiative cooling film according to claim 1, wherein: In step S3, the 2-methylimidazole solution is prepared by dissolving 2-methylimidazole in a methanol aqueous solution at a ratio of 0.05 wt%-2 wt%.

7. The method of making a porous polyvinylidene fluoride-based composite radiative cooling film according to claim 6, wherein: The volume concentration of methanol in the methanol aqueous solution is 40-60%.

8. The method for preparing a porous polyvinylidene fluoride composite radiation cooling thin film as described in claim 7, characterized in that: In step S3, the temperature of the immersion treatment is 20-40 ℃, and the time is 20-48 hours.

9. The radiation-cooling film prepared by the preparation method in any one of claims 1-8.

10. The application of the film prepared by the preparation method in any one of claims 1-8 in radiation cooling.

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