A porous, highly elastic radiation cooling film with recyclable solvent and its preparation method

Abstract

This application relates to the field of functional thin film materials technology, specifically to a porous, highly elastic radiative cooling film with recyclable solvent and its preparation method. The preparation method involves mixing a specific polymer, inorganic particles, and DMF to obtain a solution. After static cooling and ultrasonic defoaming, the solution is coated onto a hydrophobic substrate, then transferred to an aqueous solution for phase separation and drying to form a porous film. Simultaneously, the DMF in the aqueous solution is recovered for recycling. The polymer exhibits specific functional group vibrations in the 8-13µm wavelength range. The resulting film has an average pore size of 1-2µm, a thickness of 60-500µm, a visible light reflectance ≥96%, and an 8-13µm infrared emissivity ≥96%, achieving highly efficient radiative cooling. This invention features a simple and environmentally friendly process, with the DMF solvent being recyclable during preparation, making it suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of functional thin film materials technology, specifically to a solvent-recoverable porous, highly elastic radiation cooling film and its preparation method. Background Technology

[0002] The increased consumption of fossil fuels has led to greater greenhouse gas emissions, resulting in a continuous rise in global temperatures. Cooling technologies have become an urgent necessity in daily life and industrial manufacturing. According to the International Energy Agency, global electricity demand for cooling will nearly triple to 6.200 MWh by 2050. However, traditional cooling technologies typically consume fossil fuels, which in turn exacerbates global warming. Therefore, the search for new, green, fuel-free cooling technologies is urgently needed.

[0003] Radiative cooling (RC) refers to the dissipation of excess heat radiation (within the 8-13µm spectrum) into ultracold space through atmospheric inclusions. Passive diurnal radiative cooling is the phenomenon in which an object spontaneously cools its surface by reflecting sunlight and radiating heat into the cold outer space. It has become a promising research topic in recent years and is receiving increasing attention due to its advantages of not consuming additional external energy, zero pollution, safety, efficiency and cleanliness.

[0004] The way to enhance radiative cooling capacity is to maximize the reflectivity of an object's surface to solar radiation (wavelengths between 0.3-2.5µm) while simultaneously maximizing the infrared emissivity in the transparent atmospheric window band (8-13µm). Ordinary objects often lack both of these properties simultaneously; they either absorb solar radiation excessively or have weak infrared radiation capabilities in the atmospheric window. This results in the object lacking cooling capacity under direct sunlight, and its surface gradually heats up.

[0005] In 2014, Fan et al. first proposed a metallic dielectric photonic structure capable of achieving cooling power under direct sunlight, paving the way for initial passive daytime radiative cooling (PDRC) materials. Subsequently, a series of photonic crystal structures for PDRC, capable of achieving temperatures below ambient temperature under direct sunlight, emerged. However, these photonic structures are costly and complex to fabricate.

[0006] In recent years, Mandar et al. have fabricated porous PVDF-HFP films to create excellent radiative cooling materials. Their method utilizes phase separation technology, gradually separating acetone and DMF from the film in air to produce a hierarchical porous structure, achieving a radiative cooling effect. Their simpler preparation process and lower-cost polymer herald the arrival of the era of porous radiative coolers.

[0007] Phase separation technology prepares fractional porous films by volatilizing the organic solvents in the precursor solution. However, the volatilization of acetone and DMF in the air causes serious air pollution and poses a great challenge to the environment.

[0008] Dimethylformamide (DMF) is an important industrial solvent widely used in chemical, pharmaceutical, and textile industries. However, DMF is a toxic and hazardous substance that can harm human health through inhalation and skin contact. Long-term exposure or inhalation can cause eye irritation, hematopoietic or liver dysfunction. DMF in industrial wastewater is chemically stable and virtually non-biodegradable; improper treatment can cause serious environmental pollution. Therefore, it is necessary to recover DMF from waste from both environmental protection and waste reduction perspectives. Summary of the Invention

[0009] The purpose of this invention is to provide a porous, highly elastic radiation cooling film with recyclable solvent and its preparation method. It aims to solve the technical problem of improper handling of toxic gases in existing radiation cooling film preparation processes, and provides the possibility for rapid, large-scale preparation of radiation cooling films.

[0010] To achieve the above objectives, the present invention provides a method for preparing a porous, highly elastic radiation cooling film with recyclable solvent, comprising the following steps: S1. Mix the polymer, inorganic particles and organic solvent DMF, and stir at 40-55℃ for 2-5 hours at a speed of 220-280 rpm to fully crosslink the powder and liquid to obtain a mixed solution. S2. Allow the solution to stand and cool at room temperature to remove its internal stress, then place it in an ultrasonic cleaner and clean for 15 minutes. S3. After pouring the solution evenly onto the substrate, place it on a coating machine for coating. S4. The coated film, along with the substrate, is transferred to an aqueous solution. The phases are separated to form a film by utilizing the principle that polymers are immiscible with water, while DMF is miscible with water. After drying, a porous radiation-cooled film is formed. S5. Collect the aqueous solution containing DMF and recycle the DMF.

[0011] Furthermore, the polymer is a material that exhibits vibrational emission of CF, C-F3, CH, or CO functional groups in the atmospheric window 8-13µm band, and is selected from one or more of PVDF-HFP, TPU, PMMA, and polylactic acid.

[0012] Furthermore, the inorganic particles are at least one of ZrO2 or SiO2.

[0013] Furthermore, in S1, the mass ratio of the polymer, inorganic particles and DMF is 1-2:0-1:6-8.

[0014] Furthermore, in S3, the substrate is a smooth hydrophobic material selected from at least one of inorganic glass, polymer material PTFE, and metallic aluminum.

[0015] Furthermore, in S3, the coating parameters of the coating machine are: blade thickness 300-400μm, blade speed 50-100mm / min.

[0016] On the other hand, the present invention provides a solvent-recoverable porous, highly elastic radiation cooling film, wherein the radiation cooling film has a porous structure with an average pore size of 1-2µm and a thickness of 60-500µm.

[0017] Furthermore, the reflectivity R of the radiation cooling film in the visible light band is ≥96%.

[0018] Furthermore, the infrared emissivity E of the radiation cooling film in the 8~13µm atmospheric window is ≥96%.

[0019] The advantages and positive effects of the solvent-recoverable porous high-elasticity radiation cooling film and its preparation method described in this invention are as follows: This invention provides a simple and efficient method for preparing radiation cooling films. Compared with existing methods, it eliminates the use of the toxic solvent acetone and improves the preparation speed. This invention successfully achieves high reflectivity for visible light and high emissivity in the atmospheric window band. By forming the film in water, this invention locks the toxic solvent DMF in the precursor solution into the water, facilitating collection and secondary recycling, thus constructing a green and environmentally friendly integrated technology solution for solvent recovery and preparation. Furthermore, the film-forming process in water is very rapid; the gel membrane phase can be separated into a polymer membrane in just 3 minutes, making it suitable for continuous, large-scale industrial production.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the preparation process of the radiation-cooled thin film of the present invention; Figure 2 Here is a flowchart and schematic diagram of the fabrication process for radiation-cooled thin films: Figure 3 This is a schematic diagram of a scanning electron microscope (SEM) of a radiation cooling film represented by Example 4 (scale bar is 1 μm). In the figure, a is a scanning electron microscope image of the radiation cooling film, b is the EDS mapping of oxygen in the radiation cooling film, c is the EDS mapping of fluorine in the radiation cooling film, d is the EDS mapping of zirconium in the radiation cooling film, and e is the EDS mapping of carbon in the radiation cooling film. Figure 4 The figures are schematic diagrams of radiation-cooled film structures represented by Example 4. In the figures, a is a schematic diagram of the pure PVDF radiation-cooled film structure, b is a schematic diagram of the porous structure of the PVDF-HFP / ZrO2 composite film, c is a schematic diagram of the PVDF radiation-cooled film structure with added zirconium dioxide and TPU (ERCF composite film), d is a SEM microstructure of the pure PVDF radiation-cooled film (scale bar is 2 μm), e is a SEM microstructure of the PVDF-HFP / ZrO2 composite film (scale bar is 2 μm), and f is a SEM microstructure of the ERCF composite film (scale bar is 2 μm). Figure 5 The reflectance spectra of the radiation-cooled thin film at different thicknesses, as represented by Example 4; Figure 6 The reflectance spectra of the radiation-cooled thin film under different stirring times, as represented by Example 4; Figure 7 The infrared spectrum of PVDF with added TPU and ZrO2 is shown. Figure 8 The figures show the cooling performance of the radiation cooling film, represented by Example 4, on different devices, such as infrared thermal imaging data and actual temperature cooling diagrams on alloy containers, wooden school buildings, and camouflage tents. In the figures, a is a schematic diagram of the ERCF and commercial textiles wrapped in the alloy container cooling temperature device, b is a schematic diagram of the cooling temperature test device for measuring the ERCF and commercial textiles on the roof of the wooden school building, c is a schematic diagram of the cooling temperature device for ERCF and commercial textiles wrapped outside the tent, d is the real-time solar irradiance curve corresponding to the alloy container cooling temperature device, e is the real-time solar irradiance curve corresponding to the cooling temperature test device on the roof of the wooden school building, f is the real-time solar irradiance curve corresponding to the cooling temperature device outside the tent, g is a comparison diagram of the temperature distribution curves of the alloy container cooling temperature device and the ambient temperature, h is a comparison diagram of the temperature distribution curves of the cooling temperature device on the roof of the wooden school building and the ambient temperature, and i is a comparison diagram of the temperature distribution curves of the cooling temperature device outside the tent and the ambient temperature. Figure 9 The personal thermal management performance of the radiative cooling film, represented by Example 4, is shown in the figure. Figure a is a photo of people wearing commercial protective clothing and radiative cooling film under direct sunlight on a sunny day. Figure b is the solar radiation intensity data under the test conditions on a sunny day. Figure c is an optical photo of the skin covered with radiative cooling film and commercial material respectively. Figure e is the skin temperature data covered with commercial material and radiative cooling film respectively. Figure f is a thermal imaging photo of people covered with radiative cooling film and commercial material respectively. Figure 10 The stress-strain curves of the radiative cooling film at different thicknesses, represented by Example 4; Figure 11 The stress-strain curves of the radiation cooling film, represented by Example 4, under different ZrO2 contents are shown. Figure 12 The reflectance spectra of the radiation cooling film, represented by Example 4, under different stretching cycles; Figure 13 This is a statistical chart showing the mass difference of the radiation cooling film before and after stretching, as represented by Example 4. Figure 14 This is a statistical chart showing the difference in film length before and after stretching of the radiation cooling film, represented by Example 4. Figure 15 The stress-strain curves of the radiation cooling film, represented by Example 4, under different TPU contents are shown. Figure 16 This is a schematic diagram of all-weather cooling of a radiation-cooled thin film, represented by Example 4; Figure 17 This is a schematic diagram illustrating the recyclability of the radiation-cooled thin film. Figure 18 Reflectance spectra of the radiation-cooled thin film before and after recycling; Figure 19 This is a schematic diagram of a continuous preparation process. In the diagram, a is a schematic diagram of the slurry layering step, b is a schematic diagram of the aqueous phase separation step, c is a schematic diagram of the water evaporation step, and d is a schematic diagram of the finished product. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. Experimental instruments, equipment, and reagents in the following embodiments that do not specify their sources are all commercially available materials.

[0024] Unless otherwise defined or stated, all technical and scientific terms used in this invention have the same meaning as those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein can be applied to the methods of this invention. It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0025] The preparation principle and core process of the solvent-recoverable porous radiation cooling film described in this invention are as follows: Figure 1 and Figure 2 As shown.

[0026] The testing methods in this embodiment include: Reflectivity: The solar radiation reflectivity of the thin film was measured using a UH4150 UV / Vis / NIR spectrometer. The thin film was placed in the spectrophotometer, and the average reflectivity R of the thin film from 300-2500 nm was measured in 1 nm increments.

[0027] Emissivity: The infrared emissivity of the thin film was measured using a Japanese-sensor-TSS 5S-2 spectrometer with an integrating sphere. The thin film was placed in the spectrometer, and the average absorbance A of the thin film from 8 to 13 µm was measured in 1 nm increments. The infrared emissivity E of the thin film is equal to A.

[0028] SEM images: Microscopic pore size images of the thin film were obtained using a GeminiSEM500 field emission scanning electron microscope. The film was freeze-dried and fractured to obtain cross-sectional images. Morphology was then observed by adjusting the resolution in the scanning electron microscope.

[0029] Temperature change measurement: In a sunny, open-air environment with direct sunlight, a temperature sensor was placed inside the prepared radiant cooler to measure the temperature changes of the thin film and the environment, and the data were recorded and plotted.

[0030] Infrared thermal images: Infrared thermal images of the samples were provided by the Fluke R30 thermal imager.

[0031] Example 1 The preparation of a radiation-cooled thin film includes the following steps: A. Weigh out the DMF solution: PVDF-HFP: ZrO2 = 8:1:1, put it into a beaker, add a magnetic stir bar, and stir in a magnetic stirrer. Set the temperature to 40℃ and the speed to 220 rpm. Wait for 2 hours to allow the powder and liquid to fully cross-link. When the solution becomes white and viscous, it can be taken out.

[0032] B. Allow the solution to stand and cool at room temperature to remove its internal stress, then place it in an ultrasonic cleaner for 15 minutes to break up any excess air bubbles before use.

[0033] C. After the solution is evenly poured onto the glass substrate, it is placed on a coating machine for coating. The blade thickness is set to 300µm and the blade speed is 50mm / min.

[0034] After the coating is completed, the glass substrate is removed and placed in a container filled with water (25°C). After standing for about one minute, the film formation can be observed, and the film detaches from the glass substrate. Because of the presence of water inside, the film appears white at this point. The film is formed by phase separation based on the principle that PVDF-HFP is immiscible with water, while DMF is miscible with water.

[0035] A porous radiation-cooled film is obtained by removing the film from the water and drying it. The water in the container can be recycled to absorb the harmful solvent DMF in liquid phase. Its microstructure was characterized by SEM, its reflectance was characterized by a VIS-FTIR visible-near-infrared spectrophotometer, its emissivity in the 8-13µm range was measured using a Japan-sensor-TSS 5S-2 spectrometer with an integrating sphere, and its cooling performance was tested using a self-made radiation cooler.

[0036] Example 2 The preparation of a radiation-cooled thin film includes the following steps: A. Weigh out the DMF solution: TPU:PMMA = 8:1:1, put it into a beaker, add a magnetic stir bar, and stir in a magnetic stirrer. Set the temperature to 45℃ and the speed to 240 rpm. Wait for 3 hours to allow the powder and liquid to fully cross-link. When the solution becomes transparent, it can be taken out.

[0037] B. Allow the solution to stand and cool at room temperature to remove its internal stress, then place it in an ultrasonic cleaner for 15 minutes to break up any excess air bubbles before use.

[0038] C. After the solution is evenly poured onto the glass substrate, it is placed on a coating machine for coating. The blade thickness is set to 400µm and the blade speed is 100mm / min.

[0039] After the coating is completed, remove the glass substrate and place it in a container filled with water (20°C). Let it stand for about three minutes, and you can observe the formation of the film and its separation from the glass substrate. Because of the water inside, the film is transparent at this time.

[0040] A porous radiation-cooled film is obtained by removing the film from the water and drying it. The water in the container can be recycled to absorb the harmful solvent DMF in liquid phase. Its microstructure was characterized by SEM, its reflectance was characterized by a VIS-FTIR visible-near-infrared spectrophotometer, its emissivity in the 8-13µm range was measured using a Japan-sensor-TSS 5S-2 spectrometer with an integrating sphere, and its cooling performance was tested using a self-made radiation cooler.

[0041] Example 3 The preparation of a radiation-cooled thin film includes the following steps: A. Weigh out the DMF solution: TPU: polylactic acid: SiO2 = 6:1:1:1, put it into a beaker, add a magnetic stir bar, and stir in a magnetic stirrer. Set the temperature to 50℃ and the speed to 260 rpm. Wait for 4 hours to allow the powder and liquid to fully cross-link. When the solution becomes transparent, it can be taken out.

[0042] B. Allow the solution to stand and cool at room temperature to remove its internal stress, then place it in an ultrasonic cleaner for 15 minutes. Any excess air bubbles will be left unused.

[0043] C. After uniformly applying the solution to the Teflon (PTFE) substrate, place it on a coating machine for coating. Set the squeegee thickness to 400µm and the squeegee speed to 50mm / min.

[0044] After the PTFE substrate is coated, it is removed and placed in a container filled with water (30°C). After standing for about three minutes, the formation of the film can be observed, and the film separates from the glass substrate. Because of the presence of water and SiO2 inside, the film appears as a white film at this time.

[0045] A porous radiation-cooled film is obtained by removing the film from the water and drying it. The water in the container can be recycled to absorb the harmful solvent DMF in liquid phase. Its microstructure was characterized by SEM, its reflectance was characterized by a VIS-FTIR visible-near-infrared spectrophotometer, its emissivity in the 8-13µm range was measured using a Japan-sensor-TSS 5S-2 spectrometer with an integrating sphere, and its cooling performance was tested using a self-made radiation cooler.

[0046] Example 4 The preparation of a radiation-cooled thin film includes the following steps: A. Weigh out the DMF solution: PVDF-HFP:TPU:ZrO2 = 6:1:1:1, put it into a beaker, add a magnetic stir bar, and stir in a magnetic stirrer. Set the temperature to 55℃ and the speed to 280 rpm. Wait for 5 hours to allow the powder and liquid to fully cross-link. When the solution becomes transparent, it can be taken out.

[0047] B. Allow the solution to stand and cool at room temperature to remove its internal stress, then place it in an ultrasonic cleaner for 15 minutes to break up any excess air bubbles before use.

[0048] C. After the solution is evenly poured onto the aluminum substrate, it is placed on a coating machine for coating. The blade thickness is set to 400µm and the blade speed is 50mm / min.

[0049] After the coating is completed, remove the aluminum substrate and place it in a container filled with water (25°C). Let it stand for about three minutes, and you can observe the formation of the film and its separation from the glass substrate. Because of the water inside, the film appears as a white emulsion at this time.

[0050] A porous radiation-cooled film is obtained by removing the film from the water and drying it. The water in the container can be recycled to absorb the harmful solvent DMF in liquid phase. Its microstructure was characterized by SEM, its reflectance was characterized by a VIS-FTIR visible-near-infrared spectrophotometer, its emissivity in the 8-13µm range was measured using a Japan-sensor-TSS 5S-2 spectrometer with an integrating sphere, and its cooling performance was tested using a self-made radiation cooler.

[0051] Test results: Microstructure: Figure 3-4 The typical microstructure of the prepared thin film is shown, revealing a uniformly distributed porous network with an average pore size concentrated in the range of 1-2 μm. This structure forms the basis for the high reflectivity and high emissivity of the thin films in the various embodiments of this invention.

[0052] Optical performance: Figure 5 The variation in reflectance of the film, represented by Example 4, at different thicknesses is shown. The results indicate that when the film thickness is between 300 and 400 μm, the visible light reflectance is approximately 96%. Excellent reflective performance can be obtained in all examples by adjusting the coating parameters to control this thickness range. Figure 6 The reflectance of the film under different stirring times was characterized. The results show that stirring for 3 hours is the optimal condition to ensure sufficient cross-linking of the raw materials and homogeneity of the solution, at which time the film exhibits the best reflectance performance.

[0053] Chemical structure: Figure 7 Infrared spectra of TPU and PVDF-HFP were shown, demonstrating the presence of functional groups such as CF and CH, which are key to achieving high infrared emissivity (E≥96%) in the 8-13 μm atmospheric window.

[0054] Actual cooling effect: On a clear, cloudless open rooftop, a radiation cooling test device was constructed (away from the ground to avoid heat conduction). Filter foil was wrapped around the outside of a polyethylene film, and a PE film was used to cover it to shield against air convection. High-precision temperature sensors were placed both inside the cavity and in the environment to record the temperature of the cavity covered by the film and the ambient temperature in real time. The test lasted for 24 hours. Temperature was recorded using temperature sensors, and infrared thermal images of the film-covered sample and the blank sample were taken using an infrared thermal imager. Real-time solar intensity was recorded using a solar irradiance meter. Figure 8 As shown, infrared thermal imaging and temperature data are presented when the radiative cooling film, represented by Example 4, is applied to the surfaces of different devices (such as alloy containers, camouflage tents, and wooden school buildings). The results show that, compared with commercial textiles, the surface temperature of the alloy container with the radiative cooling film is lower; the indoor temperature of the wooden school building roof with the radiative cooling film is lower; and the internal temperature of the camouflage tent with the radiative cooling film is lower, demonstrating that the radiative cooling film exhibits excellent cooling effects in various application scenarios.

[0055] Figure 9The study demonstrates a comparison between the cooling effect of the radiative cooling film, as represented in Example 4, and commercial protective clothing, with human wear tests conducted under direct sunlight on a sunny day. The results show that the surface temperature of the area covered by the radiative cooling film protective clothing was significantly lower than that of the area covered by ordinary commercial protective clothing. Infrared thermal imaging revealed a significant low-temperature characteristic in the film-covered area, with no stuffy feeling, verifying the excellent application effect of the radiative cooling film.

[0056] It should be noted that the temperature of the outdoor ERCF (Example 4) is lower than that of the protective clothing because its high reflectivity causes sunlight to be reflected, thus lowering the temperature; however, the opposite is true indoors, where the high emissivity causes all the heat to dissipate quickly, resulting in a higher temperature of the ERCF under infrared thermal imaging than that of the protective clothing.

[0057] Mechanical property testing: 1) The effect of different thicknesses on tensile properties.

[0058] Experimental Procedure: Mechanical property tests of samples with different thicknesses (300μm, 400μm, and 500μm) were performed using a universal testing machine. Before testing, the film samples were cut into dumbbell-shaped specimens with dimensions of 50mm (length) × 4mm (width). The thickness of each specimen was then precisely measured using vernier calipers to ensure the accuracy of the test data. During the tensile and cyclic tensile tests, a continuous tensile speed of 10mm / min was set, and each sample was tested five times, with the average value taken to reduce error and systematically obtain the relevant mechanical property parameters of the samples.

[0059] Result: As Figure 10 As shown, the results indicate that within the tested thickness range, both the tensile strength and elongation at break of the film gradually increase with increasing film thickness. All samples of tested thicknesses exhibited excellent flexibility and tensile ductility, confirming their ability to maintain stable high elastic mechanical properties and adapt to the diverse film thickness requirements of different application scenarios.

[0060] 2) Effect of different ZrO2 concentrations on tensile properties.

[0061] Experimental Procedure: Mechanical property tests of samples with different ZrO2 concentrations (5%, 10%, 15%, and 50%) were performed using a universal testing machine. Before testing, the film samples were cut into dumbbell-shaped specimens with dimensions of 50 mm (length) × 4 mm (width). The thickness of each specimen was then precisely measured using vernier calipers to ensure the accuracy of the test data. During the tensile and cyclic tensile tests, the continuous tensile speed was set to 10 mm / min, and each sample was tested five times, with the average value taken to reduce error and systematically obtain the relevant mechanical property parameters of the samples.

[0062] Result: As Figure 11 As shown, the results indicate that within the low ZrO2 content range (5%-15%), the tensile strength of the film slightly increases with increasing ZrO2 particle content while maintaining excellent elongation at break. The sample with 15% ZrO2 content still maintains an elongation at break above 250%, demonstrating that the uniform dispersion of inorganic particles in the polymer matrix provides some reinforcement. However, when the ZrO2 content increases to 50%, the elongation at break of the film decreases significantly, and the tensile strength also decreases accordingly. Excessive inorganic particle content disrupts the continuity of the polymer porous network, leading to increased material brittleness. These results confirm that the raw material formulation of this invention ensures both excellent radiation-cooled optical properties and good elastic mechanical properties of the film.

[0063] 3) The effect of different stretching cycles on performance.

[0064] Experimental procedure: The sample was cut into strips 65mm long and 15mm wide using a universal testing machine. The strips were then fixed on a stepper machine and stretched repeatedly at a 200% amplitude each time. The reflectivity was tested after different numbers of stretches, and finally the weight and length changes were tested.

[0065] Result: As Figure 12-14 As shown, the mechanical properties of the thin film represented by Example 4 and its reflectivity under different stretching states are illustrated. The results show that after 200 cycles of cyclic stretching with varying strain amplitudes, the reflectivity spectrum of the thin film in the 500-2500 nm solar radiation band almost coincides with that before stretching, and the average reflectivity attenuation rate across the entire band is low. This confirms that the porous network structure of the thin film did not suffer irreversible damage during repeated stretching, and its core optical properties exhibit excellent deformation stability. After multiple cyclic stretching cycles, the mass of the thin film is not significantly different from its initial state, with a low mass loss rate. The stretched thin film can quickly recover to its initial length, exhibiting a low permanent deformation rate, demonstrating excellent elastic recovery capability and fatigue resistance, which can meet the application requirements under scenarios such as long-term dynamic deformation and repeated bending.

[0066] 4) Performance impact of different TPU contents.

[0067] Experimental Procedure: Mechanical property tests of samples with different TPU contents (20%, 30%, and 40%) were performed using a universal testing machine. Before testing, the film samples were cut into dumbbell-shaped specimens with dimensions of 50 mm (length) × 4 mm (width). The thickness of each specimen was then precisely measured using calipers to ensure the accuracy of the test data. During the tensile and cyclic tensile tests, a continuous tensile speed of 10 mm / min was set, and each sample was tested five times, with the average value taken to reduce errors and systematically obtain the relevant mechanical property parameters of the samples.

[0068] Result: As Figure 15 As shown, the results indicate that with the increase of TPU content, the elongation at break of the film significantly increases, and the elastic properties are greatly enhanced. Simultaneously, the addition of TPU maintains the crosslinking continuity of the polymer matrix, and the tensile strength of the film does not decrease significantly, remaining at around 1.5 MPa. This confirms that the introduction of TPU can effectively optimize the high elastic mechanical properties of the film. By controlling the proportion of TPU added, the elasticity and mechanical strength of the film can be customized without sacrificing the core optical properties of the film's radiation cooling, adapting to the application needs of different scenarios such as flexible wearables, curved surface bonding, and dynamic stretching.

[0069] All-weather performance: Figure 16 The results demonstrate the all-weather performance test of the radiative cooling film, represented by Example 4, against the surrounding environment. The test lasted for 24 hours. The results showed that during the day when there was solar radiation, the temperature of the test chamber covered by the radiative cooling film was lower than the ambient temperature; at night when there was no solar radiation, the temperature of the test chamber covered by the radiative cooling film was lower than the ambient temperature. It can be seen that the radiative cooling film achieved a stable environmental cooling effect throughout the 24-hour period, further verifying the all-weather radiative cooling performance of the film.

[0070] Solvent recovery: Figure 17 The process for recovering DMF solvent is illustrated.

[0071] Example 5 Performance testing of DMF recycled and reused to prepare radiation cooling films.

[0072] A. Using fresh DMF as a solvent, a mold was prepared according to Example 1, and the mold was recovered and recorded as the fresh DMF group. Its reflectivity was then tested. B. Using the recovered DMF as a solvent, the radiation cooling film was prepared repeatedly according to the raw material ratio and preparation process parameters of Example 1. This film was recorded as the recovered DMF group, and its reflectivity was tested. C. Compare the performance of the two groups of samples to verify the recycling effect of DMF.

[0073] The results are as follows Figure 18 As shown, it was confirmed that there was no significant difference in performance between the recycled DMF group and the fresh DMF group, and the performance degradation rate was less than 1%.

[0074] Example 6 Roll-to-roll continuous production (e.g.) Figure 19 (As shown), the specific steps are as follows: 1) Preparation of casting solution: According to the raw material ratio and process parameters in Example 4, a large-scale casting solution was prepared. After stirring, settling and ultrasonic defoaming, it was transferred to the feeding system of the roll-to-roll coating machine. 2) Continuous coating: Using PET flexible roll material as substrate, the casting liquid is uniformly coated on the surface of PET roll material through a slit coating die. The coating blade thickness is 400μm and the blade speed is 50mm / min, thus achieving continuous coating. 3) Continuous phase separation and drying: The coated wet film, together with the PET roll, is continuously introduced into a pure aqueous solution tank. The residence time in the tank is 3 minutes to complete the continuous phase separation and film formation. The film and roll are automatically separated after film formation and introduced into a subsequent drying oven. They are continuously dried with hot air at 40℃ and then wound up to obtain a porous, highly elastic radiation cooling film roll. 4) Continuous solvent recovery: The DMF-containing aqueous solution in the water tank is introduced into the vacuum distillation system. The DMF is separated and recovered in real time using the continuous vacuum distillation process. The recovered DMF is continuously transported to the casting solution preparation system to achieve closed-loop continuous production.

[0075] In summary, experimental results confirm that the porous radiation cooling film prepared using the method of this invention possesses high reflectivity, high emissivity, a porous structure, and excellent practical cooling performance. Furthermore, the preparation method provided by this invention successfully achieves the recovery and recycling of the toxic solvent DMF, meeting the requirements for green and environmentally friendly preparation.

[0076] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a porous, highly elastic radiation cooling film with recyclable solvent, characterized in that, Includes the following steps: S1. Mix the polymer, inorganic particles and organic solvent DMF, and stir at 40-55℃ for 2-5 hours at a speed of 220-280 rpm to fully crosslink the powder and liquid to obtain a mixed solution. S2. Allow the solution to stand and cool at room temperature to remove its internal stress, then place it in an ultrasonic cleaner and clean for 15 minutes. S3. After pouring the solution evenly onto the substrate, place it on a coating machine for coating. S4. The coated film, along with the substrate, is transferred to an aqueous solution. The phases are separated to form a film by utilizing the principle that polymers are immiscible with water, while DMF is miscible with water. After drying, a porous radiation-cooled film is formed. S5. Collect the aqueous solution containing DMF and recycle the DMF.

2. The preparation method according to claim 1, characterized in that, The polymer is a material that exhibits vibrational emission of CF, C-F3, CH or CO functional groups in the atmospheric window 8-13µm band, and is selected from one or more of PVDF-HFP, TPU, PMMA, and polylactic acid.

3. The preparation method according to claim 1, characterized in that, The inorganic particles are at least one of ZrO2 or SiO2.

4. The preparation method according to claim 1, characterized in that, In S1, the mass ratio of the polymer, inorganic particles and DMF is 1-2:0-1:6-8.

5. The preparation method according to claim 1, characterized in that, In S3, the substrate is a smooth hydrophobic material selected from at least one of inorganic glass, polymer material PTFE, and metallic aluminum.

6. The preparation method according to claim 1, characterized in that, In S3, the coating parameters of the coating machine are: blade thickness 300-400μm, blade speed 50-100mm / min.

7. A porous, highly elastic radiation cooling film with recoverable solvent, prepared by the method according to any one of claims 1 to 6, characterized in that, The radiation cooling film has a porous structure with an average pore size of 1-2µm and a thickness of 60-500µm.

8. The porous, highly elastic radiation cooling film according to claim 7, characterized in that, The reflectivity R of the radiation cooling film in the visible light band is ≥96%.

9. The porous, highly elastic radiation cooling film according to claim 7, characterized in that, The infrared emissivity E of the radiation cooling film is ≥96% in the 8~13µm atmospheric window.

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