Porous multilayer structure self-cleaning radiation refrigeration film and preparation method thereof

Abstract

The invention relates to a porous multilayer structure self-cleaning radiation refrigeration film and a preparation method thereof. The film comprises a film layer and a battery diaphragm, the thin film layer is prepared from titanium dioxide particles, titanium dioxide aerogel and polymer filler through a solvent exchange method. The film provided by the invention has extremely high reflectivity in a visible light wave band (0.3-2.5 mu m), and shows remarkable emissivity in an atmospheric window wave band (8-13 mu m). Meanwhile, the film has strong hydrophobic performance, realizes excellent self-cleaning capability, can effectively cope with various natural environment conditions, and has good application prospects.

Description

Technical Field

[0001] This invention belongs to the field of thin film materials technology, and specifically relates to a porous multilayer self-cleaning radiation cooling thin film and its preparation method. Background Technology

[0002] Radiative cooling is a technology that utilizes the principle of thermal radiation to cool objects. Its basic mechanism involves the selective radiation of long-wave infrared heat while minimizing the absorption of heat from solar radiation. In the solar spectrum, the object's surface should possess high reflectivity to effectively reduce the heat impact from ultraviolet, visible, and near-infrared light. Within the atmospheric window (8-13 μm), the material needs high emissivity to ensure that thermal radiation can effectively penetrate the atmosphere and dissipate into outer space (approximately 3 Kelvin), thereby achieving a cooling effect. Furthermore, to ensure the material's durability in outdoor applications, higher requirements must be placed on its manufacturing process and self-cleaning capabilities.

[0003] CN119391022A discloses a durable radiative cooling thin film material and its preparation method. This method is based on a composite polymer design, combining PVDF HFP with ZnO@SiO2 nanoparticles to form a composite polymer, which is then sprayed onto the film surface to obtain a durable radiative cooling thin film. However, this film currently employs a single-layer design, leaving room for improvement in reflectivity. Furthermore, relying solely on the polymer's properties limits its environmental resistance and self-cleaning capabilities. CN115449252B discloses a radiative cooling coating and its preparation method. This coating is prepared by combining a resin emulsion (20-35 parts), functional additives (1-5 parts), and a solvent (10-30 parts), thereby enhancing the synergistic effect between fillers and exhibiting good solar reflection and radiative cooling properties. However, it is worth noting that the preparation process of this coating is relatively complex, which may lead to increased production costs and limit its practical application. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a porous, multilayer self-cleaning radiation-cooling thin film and its preparation method. This film exhibits extremely high reflectivity in the visible light band (0.3–2.5 μm) and significant emissivity in the atmospheric window band (8–13 μm). Simultaneously, this film possesses strong hydrophobic properties, achieving excellent self-cleaning ability, effectively coping with various natural environmental conditions, and has promising application prospects.

[0005] This invention provides a porous, multilayer self-cleaning radiation-cooling thin film, including a thin film and a battery separator; the intermediate layer of the thin film is prepared by solvent exchange method using titanium dioxide particles, titanium dioxide aerogel and polymer filler.

[0006] Preferably, the polymer filler includes one of polyurethane, polydimethylsiloxane, polylactic acid, and polymethyl methacrylate.

[0007] More preferably, the polyurethane is a water-based polyurethane with a solid content of 30%-50%.

[0008] The present invention also provides a method for preparing a porous multilayer self-cleaning radiation cooling film, comprising the following steps: (1) mixing silica aerogel and titanium dioxide particles with an organic solvent and ultrasonically dispersing them to obtain a uniform mixture; (2) adding polymer filler to the mixture obtained in step (1) and stirring at room temperature to obtain a casting solution; wherein the mass percentage of silica aerogel and titanium dioxide particles in the casting solution is 10%-25%;

[0009] (3) Cast the casting solution into the mold and place the battery separator on top of the mold. Place it at room temperature to allow the casting solution to form a preliminary film.

[0010] (4) The film obtained in step (3) is solvent exchanged with deionized water, and then the film is dried and taken out to obtain a porous multilayer self-cleaning radiation cooling film.

[0011] Preferably, the organic solvent in step (1) includes one of N,N-dimethylformamide, acrylic acid carbonate, and cyclopentyl methyl ether.

[0012] Preferably, the ultrasonic dispersion time in step (1) is 10-30 min.

[0013] Preferably, the polymer filler in step (2) has a mass percentage of 50%-80% in the casting solution.

[0014] Preferably, the stirring speed in step (2) is 100-1000 rad / min and the time is 10-60 min.

[0015] Preferably, the drying temperature in step (4) is 40-60℃ and the drying time is 5-10h.

[0016] Preferably, in step (4), the moisture in the film is removed after drying, thereby forming a porous structure inside the film.

[0017] The silica aerogel (SiO2) used in this invention is a widely used inorganic material with good thermal and chemical stability. Due to its wide bandwidth (approximately 8.4 eV) and small maximum absorption wavelength (0.148 μm), silica absorbs very limited energy from sunlight, effectively reducing heat absorption. Furthermore, silica exhibits good transparency in optical and electronic applications, enabling it not only to reflect sunlight but also to effectively dissipate heat.

[0018] The titanium dioxide particles (TiO2) used in this invention are a commonly used photocatalyst and pigment with excellent optical properties. Its wide bandgap is approximately 3 eV, and its maximum absorption wavelength is 0.413 μm, effectively reflecting visible and near-infrared light from sunlight. The high refractive index of titanium dioxide allows it to significantly improve reflectivity in composite materials, providing additional thermal management advantages for radiative cooling materials. Furthermore, titanium dioxide also exhibits good UV resistance, extending the material's lifespan.

[0019] Beneficial effects

[0020] (1) The thin film of the present invention exhibits extremely high reflectivity in the visible light band (0.3-2.5 μm) and significant emissivity in the atmospheric window band (8-13 μm). At the same time, the thin film has strong hydrophobic properties, achieves excellent self-cleaning ability, can effectively cope with various natural environmental conditions, and has good application prospects.

[0021] (2) The thin film layer of the present invention uses titanium dioxide particles and silica aerogel as additives to enhance the radiation cooling capacity through the complementary properties of the two particles in different wavelength bands.

[0022] (3) The thin film layer of the present invention is prepared by solvent exchange method. By exchanging organic solvent with water, pores are created inside the polymer filler to improve the reflectivity of the thin film and thus improve the cooling effect of the radiation cooling film.

[0023] (4) The thin film of the present invention adopts a multi-layer structure. The battery separator has many pore structures inside. At the same time, the battery separator has good stability and durability. The whole structure not only increases the radiation cooling effect of the material, but also increases the mechanical strength and durability of the material.

[0024] (5) The thin film preparation method of the present invention is simple, efficient and economical, which is conducive to the practical application of radiation cooling thin films. Attached Figure Description

[0025] Figure 1 This is a graph showing the reflectance of thin films with different compositions and structures in Example 1 of the present invention.

[0026] Figure 2 This is an emissivity curve of thin films with different compositions and structures in Example 1 of the present invention.

[0027] Figure 3 This is a physical image of the temperature testing device in Embodiment 1 of the present invention.

[0028] Figure 4 The results are temperature test results for thin films with different compositions and structures in Example 1 of this invention.

[0029] Figure 5 This refers to the temperature difference in cooling of thin films with different compositions and structures in Example 1 of the present invention.

[0030] Figure 6 This is the Fourier transform infrared spectrum of the thin film in Embodiment 2 of the present invention.

[0031] Figure 7 The image shows the reflectance curves of a thin film with two types of particles of different mass fractions added in Embodiment 2 of the present invention.

[0032] Figure 8 The graph shows the emissivity of a thin film with two types of particles of different mass fractions added in Example 2 of the present invention.

[0033] Figure 9 ah is a scanning electron microscope (SEM) image of the thin film of the present invention.

[0034] Figure 10 ac is a schematic diagram of the process for testing the self-cleaning ability of the thin film of the present invention. Detailed Implementation

[0035] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0036] Example 1

[0037] This embodiment provides a method for preparing a porous, multilayer self-cleaning radiation-cooling thin film, comprising the following steps:

[0038] (1) Mix silica aerogel and titanium dioxide particles with 10 ml of N,N-dimethylformamide and disperse by ultrasonication to obtain a uniform mixture;

[0039] (2) Add 20 ml of waterborne polyurethane (solid content 40%) to the mixture obtained in step (1) and stir at 600 rad / min for 30 minutes at room temperature to obtain casting solution;

[0040] (3) Cast the casting solution into the mold and place the battery separator on the top of the mold. Let it stand at room temperature for 24 hours to allow the casting solution to form a preliminary film.

[0041] (4) The film obtained in step (3) is immersed in a mold containing deionized water for 1 hour, and then dried in an oven at 50°C for 8 hours. After that, the film is removed from the mold to obtain a porous multilayer self-cleaning radiation cooling film.

[0042] In this process, the mass fraction of both silica aerogel and titanium dioxide particles in the casting solution is 10%, and the resulting film is denoted as ASTPU-10%.

[0043] To verify the performance of the above-mentioned thin film, this embodiment also prepared:

[0044] ① No particles added: No silica aerogel or titanium dioxide particles are added during the preparation process. The water-based polyurethane mixture in step (2) is directly poured into the mold. No multilayer structure is used. The resulting film is denoted as PU.

[0045] ② Add only silica aerogel: Replace the silica aerogel and titanium dioxide particles, which each account for 10% by mass during the preparation process, with silica aerogel accounting for 10% by mass. Do not use a multilayer structure. The resulting film is denoted as SPU-10%.

[0046] ③ Add only titanium dioxide particles: Replace the silica aerogel and titanium dioxide particles, which each account for 10% by mass during the preparation process, with titanium dioxide particles accounting for 10% by mass. Do not use a multilayer structure. The resulting film is denoted as TPU-10%.

[0047] ④ Simultaneous addition of silica aerogel and titanium dioxide particles: During the preparation process, silica aerogel and titanium dioxide particles, each accounting for 10% by mass, are added, but a multilayer structure is not used. The resulting film is denoted as STPU-10%.

[0048] The optical properties of the above-mentioned thin film samples were characterized, and reflectance curves were plotted, such as... Figure 1 As shown in the figure, by comparing the curves, it was found that the film with two particles added has a higher reflectivity than the film with a single particle, and the reflectivity of the film with a dual-particle multilayer structure is significantly improved, reaching 80%. This indicates that by simultaneously adding two types of particles and adopting a multilayer structure, the reflectivity of the radiation-cooling film is significantly improved. This further suggests that the cooling effect of this film is expected to be further improved.

[0049] The emissivity of the above-mentioned thin film samples was characterized by optical properties, and emissivity curves were plotted, such as... Figure 2As shown in the figure, the comparative results show that the emissivity of each film is relatively similar, all above 95%. This indicates that the polymer (polyurethane) used has high emissivity characteristics and is an excellent choice for radiation-cooled polymer substrates.

[0050] The testing apparatus used in the temperature testing experiment is as follows: Figure 3 As shown, this test aims to measure the outdoor radiative cooling performance of various thin film samples. The basic frame of the testing equipment is a polystyrene foam box. To isolate the surrounding heat conduction, the surface of the foam box is covered with a layer of aluminum foil. Multiple cavities are set in the top of the foam box, and a K-type thermocouple is placed in each cavity to collect temperature data of different samples in real time. In addition, a K-type thermocouple is installed in one of the cavities to record the ambient temperature as a control group. To minimize the influence of heat convection and heat conduction on the experimental results during the test, a layer of high-transmittance PE film is fixed above the cavities. The entire device is placed on a metal frame covered with a sunshade, 1 meter high, to reduce the influence of direct sunlight on the test.

[0051] The experiment was conducted under clear weather conditions, with the test chamber placed horizontally. Radiation-cooling films with no sample, no particles, single particles, two particles, and two-particle multilayer structures were compared. Temperature changes from 14:25 to 15:00 were recorded during the test. Figure 4 The temperature results within the test chamber are shown, with thermocouples monitoring the sub-ambient temperature (i.e., the temperature in the cavity without the sample film) and the temperatures of various types of films. According to... Figure 4 The temperature test data showed that the sub-ambient temperature of the dual-particle multilayer structure film was 8°C lower on average than that of the blank sample, and the cooling effect was significant compared with other control group samples. Figure 5 The temperature difference during the temperature test is shown to more intuitively reflect the cooling effect of the film. Based on the above temperature tests, it can be concluded that, in daytime tests, the dual-particle multilayer structure film exhibits a significantly improved radiative cooling effect compared to other films.

[0052] Example 2

[0053] The three main materials in Example 1—polyurethane, silica, and titanium dioxide—have highly complementary absorption peaks in the atmospheric window band, which significantly improves the emissivity of the thin film. The Fourier transform infrared spectrum is shown below. Figure 6 As shown.

[0054] Referring to the preparation method of Example 1, in the preparation process of this example, silica aerogel and titanium dioxide particles with a mass fraction of 15%, 20% and 25% respectively were added, but a multilayer structure was not used. The resulting films are denoted as STPU-15%, STPU-20%, and STPU-25%. At the same time, the film with a mass fraction of 20% silica aerogel and titanium dioxide particles was prepared into a multilayer structure, and the resulting film is denoted as ASTPU-20%.

[0055] Similarly, a film without added inorganic particles was used as a control group to study the changes in reflectivity and emissivity of different film materials. The optical properties of the film samples with different mass fractions were characterized, and reflectivity curves were plotted, as shown below. Figure 7 As shown, comparisons revealed that the film with a 20% mass fraction exhibited higher reflectivity compared to other films. Increasing the mass fraction significantly improved the film's reflectivity. However, when the particle mass fraction reached 25%, slight cracks appeared on the sample surface, leading to a decrease in reflectivity. Therefore, considering all factors, a 20% mass fraction of silica aerogel and titanium dioxide particles was the optimal choice. By simultaneously increasing the mass fraction of inorganic particles, the reflectivity of the radiation-cooled film was significantly improved, reaching a maximum of 92%. In Example 1, it was demonstrated that the multilayer structure contributes to improving the film's reflectivity, and the results for ASTPU-20% showed a reflectivity exceeding 95%.

[0056] The emissivity of the above-mentioned thin film samples was characterized by optical properties, and emissivity curves were plotted, as shown below. Figure 8 As shown in the figure. The comparison results show that the emissivity of each film is relatively similar, all above 95%. This also indicates that the polymer (polyurethane) used has excellent emissivity characteristics and is an ideal choice for radiation-cooled polymer substrates, further confirming the conclusions of Example 1.

[0057] Example 3

[0058] Solvent exchange is a commonly used material preparation technique, especially in the preparation of thin films or nanomaterials. This method achieves control over the morphology and properties of materials by altering the properties of the solvent. This invention utilizes the exchange of an organic solvent (N,N-dimethylformamide, DMF) with water in a polymer to induce a porous structure in the polymer.

[0059] By immersing the thin film in water, replacing the original DMF solvent, and then removing the moisture during drying, a porous structure is formed within the film. This porous structure significantly increases the specific surface area of ​​the film, providing more surface for radiation and heat exchange. This helps improve the film's emissivity, enabling it to radiate heat more effectively. The porous structure optimizes the absorption characteristics of specific wavelengths of light through multiple reflections and scatterings, especially within the atmospheric window range. This allows for effective absorption and radiation of infrared radiation, enhancing the cooling effect. The porous structure typically possesses good mechanical properties and flexibility, making the film less prone to breakage or deformation during use, thus maintaining its functionality.

[0060] This embodiment conducted detailed tests on the internal morphology and structure of each thin film, such as... Figure 9 As shown, Figure 9 a and Figure 9 Image e is a scanning electron microscope (SEM) image of silica aerogel. The two images show the porous microstructure of the aerogel, differing only in magnification. Figure 9 'a' represents a lower magnification, showing the macroscopic distribution of the overall pores, while Figure 9 The higher magnification (e) provides clearer details, revealing the microscopic details of the pores. Both results indicate that the aerogel has a loose, porous scaffold structure, fully demonstrating its extremely high porosity. Figure 9 b and f are also SEM images of the same group of TiO2 particles, obtained by different magnifications. Figure 9 b reflects the larger-scale morphology of the particles, which are spherical or subspherical, with uniform particle size distribution and smooth particle surfaces. Figure 9 f is a scan image at a higher magnification, revealing the microscopic texture and aggregation of the particle surface. These small, uniformly dispersed nanoparticles facilitate subsequent uniform dispersion within the polyurethane matrix. Figure 9 c and 9g are SEM images of pure PU films without the addition of inorganic nanoparticles. Figure 9 c shows that the surface of pure PU is relatively flat, with fewer pores and a relatively dense structure. Figure 9 g shows that the prepared sample, which has undergone porous structure regulation, has significantly increased pore size and uniform pore distribution. Figure 9 d and 9h are SEM images of composite materials with two inorganic particles (such as SiO2 or TiO2) added to the PU film. Figure 9 d shows that the inorganic particles are well dispersed in the polymer matrix, with the particles uniformly coated or embedded in the pores, and the overall pore structure is well maintained. Figure 9 h reveals the complexity of the pore network and the formation of multi-scale pore structures in composite materials, providing a microstructural basis for improving the performance of materials in radiation refrigeration.

[0061] Example 4

[0062] Figure 10 The diagram illustrates the self-cleaning ability test process of the film prepared in Example 1. First, the film is placed in a petri dish, and a few drops of ink are dropped on top of the sample, such as... Figure 10 As shown in Figure a. Next, tilt the film in the petri dish and use a dropper to drip water over the sample. You can see the ink on the sample surface slide off with the water droplets, as shown in Figure a. Figure 10 As shown in b, as the ink completely slides off, the film achieves self-cleaning, leaving the surface smooth and clean as before. Figure 10 As shown in c.

Claims

1. A porous, multilayer self-cleaning radiation-cooling thin film, characterized in that: It includes a thin film layer and a battery separator; the thin film layer is prepared by solvent exchange of titanium dioxide particles, titanium dioxide aerogel and polymer filler.

2. The porous multilayer self-cleaning radiation-cooling thin film according to claim 1, characterized in that: The polymer filler includes one of polyurethane, polydimethylsiloxane, polylactic acid, and polymethyl methacrylate.

3. A method for preparing a porous, multilayer self-cleaning radiation-cooling thin film, comprising the following steps: (1) Mix silica aerogel and titanium dioxide particles with an organic solvent and disperse by ultrasonication to obtain a uniform mixture; (2) Add the polymer filler to the mixture obtained in step (1) and stir at room temperature to obtain a casting solution; wherein, The mass percentage of both silica aerogel and titanium dioxide particles in the casting solution is 10%-25%. (3) Cast the casting solution into the mold and place the battery separator on top of the mold. Place it at room temperature to allow the casting solution to form a preliminary film. (4) The film obtained in step (3) is solvent exchanged with deionized water, and then the film is dried and taken out to obtain a porous multilayer self-cleaning radiation cooling film.

4. The preparation method according to claim 3, characterized in that: The organic solvent in step (1) includes one of N,N-dimethylformamide, acrylic acid carbonate, and cyclopentyl methyl ether.

5. The preparation method according to claim 3, characterized in that: The ultrasonic dispersion time in step (1) is 10-30 min.

6. The preparation method according to claim 3, characterized in that: The stirring speed in step (2) is 100-1000 rad / min, and the time is 10-60 min.

7. The preparation method according to claim 3, characterized in that: The drying temperature in step (4) is 40-60℃ and the time is 5-10h.

Images