A high-strength passive daytime radiation cooling film and its preparation method

Abstract

This invention discloses a high-strength passive daytime radiation cooling film and its preparation method, belonging to the field of functional polymer materials and thermal management technology. The film has a bilayer composite structure, including an upper reflective layer and a lower emitting layer. This invention employs an integrated process of melt blending-lamination-biaxial stretching-induced phase separation-extraction pore-fixing. Through a process design of stretching followed by extraction and a hierarchical biaxial stretching strategy, it achieves highly oriented molecular chains to enhance mechanical properties while precisely constructing a multi-level structure with both nanopores and micropores. The resulting film exhibits an average reflectivity >93% in the solar radiation band (0.25–2.5 μm), an average emissivity >95% in the atmospheric window (8–14 μm), a thermal conductivity <0.1 W / (m·K), and a tensile strength >30 MPa. This invention solves the technical bottleneck of achieving high reflectivity, high emissivity, low thermal conductivity, and high strength simultaneously in radiation cooling materials, and can be widely applied in building energy conservation, outdoor equipment thermal management, and electronic device heat dissipation.

Description

Technical Field

[0001] This invention belongs to the field of functional polymer materials and thermal management technology, specifically relating to a high-strength passive daytime radiation cooling film and its preparation method. Background Technology

[0002] With global climate change and rising energy demand, energy consumption issues in areas such as building cooling, electronic heat dissipation, and transportation thermal management are becoming increasingly prominent. Traditional compression refrigeration technology is energy-intensive and emits a lot of carbon, exacerbating the urban heat island effect. Passive daytime radiative cooling (PDRC) technology achieves high reflectivity in the solar wavelength range (0.25–2.5 μm) and high infrared emission in the atmospheric window (8–14 μm) through optical manipulation of materials, dissipating heat into outer space via radiation to achieve zero-energy cooling. Ideal PDRC materials should possess high solar reflectivity, high infrared emission, and low thermal conductivity.

[0003] Current radiation cooling materials have limitations. For example, metal films have high reflectivity but low infrared emissivity and high thermal conductivity; multilayer or photonic crystal structures have good optical properties but are complex to manufacture and costly; porous polymers have low thermal conductivity but random pore structures, making it difficult to balance optical and mechanical properties. In porous materials, pore size affects light scattering: small pores produce Rayleigh scattering, while large pores produce Mie scattering. Single-scale pores cannot cover the entire wavelength of sunlight, so constructing pore structures where nanometers and micrometers coexist can improve reflection efficiency. However, porous structures often weaken mechanical strength, and maintaining mechanical properties under high porosity is key. Ultra-high molecular weight polyethylene (UHMWPE) has high molecular weight, low thermal conductivity, chemical corrosion resistance, and excellent mechanical properties. If a multi-scale porous structure is constructed in its matrix and molecular chain orientation is induced, it is expected to achieve a combination of high reflectivity, high emissivity, low thermal conductivity, and high strength.

[0004] Therefore, developing a radiation-cooled thin film material that combines excellent optical, thermal, and mechanical properties with simple processing is of great significance for promoting the engineering application of this technology. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a high-strength passive daytime radiation cooling film and its preparation method. This film achieves a balance between high reflectivity of broad-spectrum sunlight, high emissivity of atmospheric windows, extremely low thermal conductivity, and excellent mechanical properties through a bilayer functional structure design, precise construction of multi-scale pores, and high orientation of UHMWPE molecular chains, fundamentally solving the core bottleneck of performance imbalance in radiation cooling materials. Another objective of this invention is to provide a method for preparing the aforementioned high-strength passive daytime radiation cooling film. This method innovatively adopts an integrated process route of melt blending-lamination-biaxial stretching-induced phase separation-extraction-pore fixation. In particular, through the design of a pre-stretching and post-extraction process, the high orientation of molecular chains in a gel state enhances mechanical properties, while the precise control of phase separation scale through a graded stretching process achieves the coexistence of nanopores and micropores, providing a structural basis for efficient broad-spectrum scattering.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a high-strength passive daytime radiation cooling film with a multi-scale porous structure, which is a two-layer composite structure with a connected multi-level porous network, including an upper reflective layer located on the light-facing side and a lower emitting layer located on the back-facing side or near the heat source side. The upper reflective layer comprises a super UHMWPE framework, highly reflective nanoparticles uniformly dispersed in the framework, and a three-dimensional interconnected hierarchical porous structure penetrating the framework; the mass of the highly reflective nanoparticles is 1%-20% of the mass of UHMWPE. The lower emission layer comprises a UHMWPE framework, high emissivity nanoparticles uniformly dispersed in the framework, and a three-dimensional interconnected hierarchical porous structure penetrating the framework; the mass of the high emissivity nanoparticles is 1%-20% of the mass of UHMWPE; the hierarchical porous network of the lower emission layer and the hierarchical porous network of the upper reflective layer are interconnected at the interface. The hierarchical porous structure is composed of nanoscale pores (main pore size range of 10 nm-100 nm) and microscale pores (main pore size range of 0.1 μm-10 μm); the upper reflective layer and the lower emitting layer are fused together by hot pressing to form an integral composite structure; the nanopores and micropores are uniformly distributed in the upper and lower layers, and the volume fraction ratio of nanopores to micropores is preferably 1:2 to 1:5; The film has an average reflectivity greater than 93% in the solar radiation band (0.25-2.5 μm), an average emissivity greater than 95% in the atmospheric window band (8-14 μm), a thermal conductivity less than 0.1 W / (m·K), and longitudinal (MD) and transverse (TD) tensile strengths of not less than 30 MPa.

[0007] Preferably, the thickness of the film is 50-500 μm, wherein the thickness of the upper reflective layer accounts for 30%-70% of the total thickness of the film. By adjusting the thickness ratio of the two layers, the light absorption efficiency and thermal management can be optimized.

[0008] Preferably, the UHMWPE has a viscosity-average molecular weight of 1 million to 3 million. Too low a molecular weight makes it difficult to form a stable entangled network during stretching to achieve high orientation, while too high a molecular weight leads to difficulties in melt processing.

[0009] Preferably, the high-reflectivity nanoparticles are one or more of rutile titanium dioxide (TiO2), zinc oxide (ZnO), and barium sulfate (BaSO4), with an average particle size of 100-500 nm. This particle size range is designed to achieve efficient backscattering in the visible-near-infrared band. The high-emissivity nanoparticles are one or more of carbon nanotubes (CNTs), silicon carbide (SiC), boron nitride (BN), and silicon dioxide (SiO2), with an average particle size of 10-50 nm. Among them, carbon nanotubes (especially multi-walled carbon nanotubes) not only enhance infrared emissivity, but their high thermal conductivity also helps to rapidly transfer heat to the framework surface for radiation at the nanoscale.

[0010] This invention further provides a method for preparing the above-mentioned high-intensity passive daytime radiation cooling film, which includes the following steps: S1: Mix the high-temperature oily dispersant with UHMWPE powder and preheat and swell it at a temperature higher than the melting temperature of UHMWPE (100-120 ℃) ​​to allow the dispersant to fully penetrate into the interior of UHMWPE; then add the swollen mixture and high-reflectivity nanoparticles into a mixer and melt-blend them at a temperature of 130-160 ℃ to form a uniform gel-like blend; the gel blend is then hot-pressed or roll-pressed and cooled to obtain the upper reflective layer gel preform. S2: Mix the high-temperature oily dispersant with UHMWPE powder, preheat and swell at 100-120 ℃, then add it to the internal mixer with high emissivity nanoparticles, melt and blend at 130-160 ℃ to form a uniform gel blend, and obtain the lower emissivity layer gel preform after hot pressing or roll forming and cooling. S3: The upper reflective layer gel preform obtained in S1 and the lower emitting layer gel preform obtained in S2 are stacked and bonded together, and placed in a hot press. The hot pressing is carried out at a temperature higher than the melting point of UHMWPE (140-160 ℃) to allow the molecular chains at the interface of the two layers to diffuse and fuse with each other, forming an integrated double-layer composite gel film.

[0011] S4: The bilayer composite gel film obtained in S3 is preheated to the stretching temperature (110-135 ℃), and then subjected to graded biaxial stretching on a biaxial stretching device: First, a lower stretching rate is used for the first stretching to induce the formation of nanoscale microphase separation regions; then, a higher stretching rate is used for the second stretching to further expand some of the nanoscale phase separation regions into micrometer-scale phase separation regions. After stretching to a predetermined ratio (3×3 to 6×6), heat setting is performed under tension (temperature 110-125 ℃, time 5-60 seconds) to fix the orientation of the molecular chain structure and the multi-level phase separation morphology.

[0012] S5: The bilayer composite gel membrane with a multi-stage phase separation structure, after treatment in S4, is immersed in an organic solvent (such as n-hexane, xylene, decahydronaphthalene, etc.) to fully extract and remove the high-temperature oily dispersant, forming pores. Then, it is dried to remove the solvent, finally obtaining the high-strength passive daytime radiation cooling film.

[0013] In the above preparation method, the high-temperature oily dispersant in S1 and S2 can be the same or different. The high-temperature oily dispersant is selected from small molecules or oligomers such as white oil, paraffin oil, petrolatum, and dioctyl phthalate that can form a thermodynamically compatible system with UHMWPE. By mass, the high-temperature oily dispersant in S1 and S2 comprises 50-150 parts, and the UHMWPE comprises 5-20 parts. The mass of the functional nanoparticles (high-reflectivity nanoparticles in S1 or high-emissivity nanoparticles in S2) is 1%-20% of the mass of UHMWPE in the corresponding step.

[0014] In the above preparation method, the graded biaxial stretching is key to controlling the hierarchical porous structure. Preferably, the stretching rate of the first stretching is 1% / s-10% / s, with a stretching ratio of 1.5×1.5 to 3×3; the stretching rate of the second stretching is 20% / s-100% / s, and the total stretching ratio eventually reaches 4×4 to 8×8. This step-by-step stretching method can precisely control the kinetics of phase separation and is a core technical means to achieve the synergistic coexistence of nanopores (dominantly Rayleigh scattering) and micropores (dominantly Mie scattering).

[0015] The thin film of the present invention, or the thin film prepared by the aforementioned method, exhibits high reflectivity in the solar radiation band and high infrared emissivity in the atmospheric window band, while also possessing low thermal conductivity and high mechanical strength. The thin film can be applied to the exterior surface of buildings, outdoor equipment, or electronic devices for passive daytime radiative cooling. Typically, but not limited to, the thin film can be applied to the exterior wall or roof surface of a building, with the upper reflective layer facing the direction of sunlight incidence, to achieve passive daytime radiative cooling.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention achieves the unified performance of PDRC materials with "three highs and one low" (high reflectivity > 93%, high emissivity > 95%, high strength > 30 MPa, and low thermal conductivity < 0.1 W / m·K) through the synergistic design of a dual-layer functional structure (high reflectivity on the upper layer and high emissivity on the lower layer) and a multi-level porous structure (nanopores + micropores). The nanopores in the upper reflective layer strongly reflect short-wavelength blue-violet light through Rayleigh scattering, while the micropores efficiently reflect long-wavelength visible and near-infrared light through Mie scattering. The synergistic effect of the two covers the entire solar spectrum, significantly improving the reflection efficiency. The lower emissive layer ensures that heat is efficiently released in the form of infrared radiation.

[0017] (2) This invention differs from the traditional approach of first forming pores in porous membranes and then stretching or stretching-assisted pore formation. By performing high-ratio biaxial stretching in a gel state containing a dispersant, the UHMWPE molecular chains are first fully oriented, extended, and crystallized under stress, providing the final product with ultra-high mechanical strength far exceeding that of conventional porous materials; the oriented molecular chain network forms a regular framework structure after subsequent extraction, further improving the stability of the structure. This process sequence is key to ensuring both high strength and high porosity (see Comparative Example 2 for comparison).

[0018] (3) The present invention adopts a graded stretching strategy to precisely control the thermally induced phase separation process. By inducing the formation of a nanoscale phase separation region through low-rate pre-stretching, and then expanding part of the region through high-rate stretching to form a micrometer-scale phase separation region, the precise control of the pore size is achieved, providing the optimal structural basis for broadband light scattering (see Comparative Example 3 for comparison).

[0019] (4) This invention reveals the intrinsic relationship between film thickness and optical performance. Within the preferred thickness range (50-500 μm), appropriately increasing the thickness can increase the scattering path length of light, thereby improving the reflectivity of sunlight; however, excessive thickness will increase thermal resistance and affect the penetration of infrared radiation. By adjusting the thickness ratio of the upper and lower layers, the overall optical performance and heat conduction path of the film can be optimized, achieving predictability from material design to device performance.

[0020] (5) The gel-state hot-pressing composite process is adopted, which enables the upper and lower layers to diffuse and entangle with each other at the molecular level, eliminating the risk of delamination caused by interface defects in traditional multilayer materials. The interconnected double-layer multi-level porous network ensures smooth transfer of water vapor or heat without interface blockage.

[0021] (6) The mixing, hot pressing, biaxial stretching and extraction processes used in this invention are all mature technologies in the field of polymer material processing. The equipment is highly versatile and easy to realize continuous roll-to-roll production, and has extremely high industrial transformation value. Attached Figure Description

[0022] Figure 1 This is a SEM image of the cross-section of the high-intensity passive daytime radiation cooling film prepared in Example 1 of the present invention. The nanopores and micropores in the skeleton can be clearly distributed in the image.

[0023] Figure 2 The graph shows the reflectance of the thin film prepared in Example 1 of this invention in the solar spectral band (0.25-2.5 μm).

[0024] Figure 3 This is an emissivity curve of the thin film prepared in Example 1 of the present invention in the atmospheric window band (8-14 μm).

[0025] Figure 4 This is a comparison graph of the temperature change curves of the thin film prepared in Example 1 of the present invention and the temperature change curves under outdoor experimental conditions. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and comparative examples. However, it should be understood that the embodiments of this invention are not limited thereto, and all technologies implemented based on the above content of this invention fall within the scope of this invention. Unless otherwise specified, the parts in the following embodiments are all parts by weight, and the test methods, unless otherwise specified, are conventional methods.

[0027] Example 1 S1: Preparation of the upper reflective layer gel preform 80 parts of white oil (high-temperature oily dispersant) and 10 parts of UHMWPE powder with a viscosity-average molecular weight of 3 million were added to a high-speed mixer and preheated at 110°C for 2 hours to allow the white oil to fully swell and integrate into the UHMWPE. Then, the swollen mixture was added together with rutile TiO2 nanoparticles (average particle size 200 nm, added at 5% of the mass of UHMWPE) to a Banbury mixer and mixed at 140°C and 60 rpm for 15 minutes to obtain a uniform white gel-like blend. While still hot, the gel blend was placed on a flat vulcanizing apparatus and hot-pressed at 150°C and 10 MPa for 5 minutes using a 200 μm mold frame, followed by holding the pressure and cooling to room temperature to obtain the upper reflective layer gel preform.

[0028] S2: Preparation of the lower emission layer gel preform 80 parts of white oil and 10 parts of UHMWPE powder with a viscosity-average molecular weight of 3 million were preheated and swollen at 110 °C for 2 hours. The swollen mixture was then added together with multi-walled carbon nanotubes (CNTs, average diameter 15 nm, length 1-5 μm, added at 5% of the mass of UHMWPE) into a mixer and kneaded at 140 °C and 60 rpm for 15 minutes to obtain a uniform black gel-like blend. Using the same hot-pressing process as S1, and with a 200 μm mold frame, the lower emission layer gel preform was prepared.

[0029] S3: Laminated Composite The upper reflective layer gel preform obtained in S1 and the lower reflective layer gel preform obtained in S2 are stacked neatly (with the upper reflective layer facing up), placed in a flat vulcanizing apparatus, and hot-pressed again for 5 minutes at 150 ℃ and 10 MPa using a 400 μm mold frame, so that the molecular chains of the two layers can interpenetrate and fuse at the interface. Then, cold-pressing is performed to obtain an integrated bilayer composite gel film.

[0030] S4: Staged biaxial stretching-induced phase separation and heat setting The bilayer composite gel film obtained in S3 was cut into 10 cm × 10 cm samples and placed on the fixture of a biaxial tensile testing machine. It was heated to 125 ℃ and held for 3 minutes. Subsequently, staged stretching was performed. The first stage (induced nanophase separation): biaxial stretching was performed simultaneously at a low rate (5% / s), achieving a stretch ratio of 2.5 × 2.5. This stage primarily elongated some of the dispersant-enriched regions into nanoscale micro-regions. The second stage (induced micron-phase separation): biaxial stretching was then continued at a higher rate (30% / s), ultimately achieving a total stretch ratio of 5.0 × 5.0. This stage further expanded some of the nanoscale micro-regions into micron-scale phase-separated regions.

[0031] After stretching, while maintaining tension, the temperature is lowered to 120 ℃ for heat setting for 30 seconds to fix the orientation structure of the molecular chains and the multi-level phase separation morphology, thus obtaining a stretched and oriented bilayer composite gel membrane.

[0032] S5: Extraction and Pore Fixation The stretched and oriented bilayer composite gel membrane obtained in S4 was immersed in a sealed container containing n-hexane (extractant) and extracted at 50 °C for 6 hours, with fresh n-hexane replaced once during the extraction to thoroughly remove white oil. The membrane was then removed and dried in a vacuum drying oven at 60 °C for 4 hours to finally obtain a high-strength passive daytime radiation-cooled membrane.

[0033] The performance of the thin film obtained in this embodiment was tested: Microstructure: SEM observation showed ( Figure 1The film cross-section exhibits a distinct bilayer structure, with a tight bond between the upper and lower layers and no delamination. The film contains nanopores of 10-80 nm and micropores of 0.2-5 μm, which are interconnected, forming a typical hierarchical porous structure. The porosity, calculated by gravimetric analysis, is 63%.

[0034] Optical performance: Tested using a UV-Vis-NIR spectrophotometer (equipped with an integrating sphere), the average reflectance of the thin film in the 0.25-2.5 μm wavelength range was 94.8%. Figure 2 The average emissivity of the thin film in the 8-14 μm band was 96.5% as measured by Fourier transform infrared spectroscopy. Figure 3 ).

[0035] Thermal properties: The thermal conductivity of the thin film was 0.068 W / (m·K) as measured by a thermal constant analyzer.

[0036] Mechanical properties: According to GB / T 1040.3 standard, the longitudinal (MD) tensile strength of the film is 45.6 MPa and the transverse (TD) tensile strength is 43.2 MPa.

[0037] Outdoor measurement: On a sunny midday in Ningbo (solar irradiance approximately 950 W / m²) 2 The film was placed over the simulated wall surface, and the temperature beneath the film was measured to be 4.7°C lower than the ambient temperature and 6.2°C lower than the surface covered by Comparative Example 1 (unstretched). Figure 4 ).

[0038] Example 2 Following the raw material ratios and process steps of Example 1, the thickness ratio of the upper and lower layers in the final double-layer film was changed only by adjusting the mold frame thickness of the pre-formed film in S1 and S2. Two samples were prepared: Sample A: Upper reflective layer thickness: lower reflective layer thickness = 1:2 (total thickness 400 μm).

[0039] Sample B: Upper reflective layer thickness: lower reflective layer thickness = 2:1 (total thickness 400 μm).

[0040] Performance testing: Sample A: Due to the thicker lower layer and higher heat capacity, and the relatively thinner upper reflective layer, the solar reflectivity is 92.5%. However, the abundant emissive material and porous structure of the lower layer result in an emissivity as high as 97.0%. The thermal conductivity is 0.072 W / (m·K). The mechanical properties are slightly reduced (MD: 40.2MPa) due to the slightly thinner load-bearing frame.

[0041] Sample B: The upper reflective layer is thicker, extending the light scattering path and increasing the solar reflectivity to 95.5%. However, the excessively thick upper layer slightly affects the penetration of infrared radiation from the lower layer, reducing the emissivity to 94.8%. The thermal conductivity is 0.065 W / (m·K). It exhibits optimal mechanical properties (MD: 48.5 MPa). This example demonstrates that by adjusting the ratio of the two-layer thickness, the thin film performance can be fine-tuned according to specific application scenarios (such as maximizing reflection or maximizing emission).

[0042] Example 3 Following the process of Example 1, only the average particle size of TiO2 nanoparticles in the upper reflective layer was changed to prepare three samples of 100 nm, 300 nm, and 500 nm, respectively, and their effects on reflective performance were investigated.

[0043] Performance testing: 100 nm TiO2: Average solar reflectance of 93.5%. Smaller particle size is more effective for short-wavelength scattering.

[0044] 300 nm TiO2: Average solar reflectance of 95.2%. This particle size is closer to the peak wavelength of the solar spectrum, resulting in significant Mie scattering and optimal reflectance.

[0045] 500 nm TiO2: Average solar reflectance is 93.8%. Larger particle size has a better effect on long-wavelength reflection, but slightly reduces short-wavelength reflection.

[0046] This embodiment demonstrates that high-reflectivity particles with a preferred particle size range of 100-500 nm can complement the hierarchical porous structure constructed in this invention, jointly achieving broadband high reflectivity.

[0047] Comparative Example 1 After preparing the bilayer composite gel membrane according to steps S1-S3 of Example 1, the biaxial stretching step S4 was skipped, and extraction was performed directly in step S5. Although the resulting film had a porous structure, the pores were unevenly distributed and lacked molecular chain orientation.

[0048] Performance testing: Solar reflectance is only 81.5%, emissivity is 91.0%, and thermal conductivity is 0.098 W / (m·K). Tensile strength is extremely low, only 9.2 MPa (MD), and it is brittle, making it unsuitable for use as a self-supporting thin film. This demonstrates that biaxial stretching is crucial for forming a regular pore structure and improving reflectivity and mechanical strength.

[0049] Comparative Example 2 After preparing the bilayer composite gel membrane according to S1-S3 of Example 1, the white oil was first removed by extraction in S5 to obtain a nascent porous membrane. This porous membrane was then subjected to biaxial stretching (5.0×5.0) at the same temperature (125°C).

[0050] Performance testing: The film exhibited significant brittleness during stretching, ultimately making it impossible to obtain a complete sample. Tests showed that the pre-generated pore structure became stress concentration points under tensile stress, leading to pore wall rupture, structural collapse, and severe degradation of both mechanical and optical properties. This comparative example vividly demonstrates the inventiveness of the unique process sequence of this invention: stretching to induce orientation and phase separation, followed by extraction to define the pores.

[0051] Comparative Example 3 A bilayer composite gel membrane was prepared using the raw materials from Example 1 and steps S1-S3. In step S4, instead of graded stretching, the membrane was stretched to 5.0 × 5.0 in a single step at a constant rate (20% / s) at 125 °C.

[0052] Performance Testing: SEM observation showed that the obtained thin film pore structure was mainly composed of micron-sized pores (0.5-15 μm), with very few nano-sized pores. The average solar reflectance was 87.6%, far lower than the 94.8% of Example 1. This is because the lack of Rayleigh scattering of short wavelengths by nanopores resulted in a dip in the reflectance spectrum in the blue-violet region. The tensile strength was 38.5 MPa, slightly lower than that of Example 1, indicating that while uniform stretching can achieve orientation, hierarchical stretching is more conducive to the full expansion of the entangled network. This comparative example demonstrates that hierarchical stretching is key to constructing nano- to micron-level hierarchical porous structures and achieving broadband high reflectance.

[0053] Comparative Example 4 The process was followed as in Example 1, but TiO2 nanoparticles were not added in S1 (upper layer pure UHMWPE / white oil gel), and CNTs were not added in S2 (lower layer pure UHMWPE / white oil gel). Subsequent steps were the same as in Example 1.

[0054] Performance testing: Although the obtained film possesses a hierarchical porous structure and high strength (MD: 44.1 MPa), its solar reflectivity is only 82.3% (due solely to pore scattering), and its atmospheric window emissivity is only 89.5% (due to insufficient intrinsic emission of UHMWPE). Outdoor measurements showed a cooling rate of only 1.8 ℃. This comparative example demonstrates that high-reflectivity and high-emissivity nanoparticles are indispensable components for achieving high-performance PDRC.

[0055] In summary, this invention, through a dual-layer functional design, a multi-level porous structure, and a unique pre-stretching followed by extraction and graded stretching process, successfully prepared a radiation-cooling thin film possessing ultra-high solar reflectivity, ultra-high infrared emissivity, ultra-low thermal conductivity, and excellent mechanical strength. The results of the various embodiments and comparative examples fully demonstrate the systematic nature, innovativeness, and non-obviousness of the technical solution of this invention.

[0056] The above description is only a portion of preferred embodiments of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-strength passive daytime radiation cooling film with a multi-scale porous structure, characterized in that, The film is a two-layer composite structure with a connected multi-level pore network, including an upper reflective layer on the light-facing side and a lower emitting layer on the back-facing side or near the heat source side. The upper reflective layer comprises a UHMWPE framework, highly reflective nanoparticles uniformly dispersed in the framework, and a three-dimensional interconnected hierarchical porous structure penetrating the framework; the mass of the highly reflective nanoparticles is 1%-20% of the mass of the UHMWPE. The lower emission layer includes a UHMWPE framework, high-emissivity nanoparticles uniformly dispersed in the framework, and a three-dimensional interconnected hierarchical porous structure penetrating the framework. The mass of the high-emissivity nanoparticles is 1%-20% of the mass of the UHMWPE. The hierarchical porous structure in both the upper reflective layer and the lower emitting layer is composed of both nano-sized pores and micro-sized pores, wherein the pore size of the nano-sized pores is 10 nm–100 nm and the pore size of the micro-sized pores is 0.1 μm–10 μm; the nano-pores and micro-pores are uniformly distributed in the upper and lower layers, and the volume fraction ratio of nano-pores to micro-pores is 1:2 to 1:5; The upper reflective layer and the lower emitting layer are fused together by hot pressing to form an integrated composite structure; The film has an average reflectivity greater than 93% in the solar radiation band of 0.25–2.5 μm, an average emissivity greater than 95% in the atmospheric window band of 8–14 μm, a thermal conductivity less than 0.1 W / (m·K), and longitudinal and transverse tensile strengths of not less than 30 MPa.

2. The high-strength passive daytime radiation cooling thin film with a multi-scale porous structure according to claim 1, characterized in that, The high reflectivity nanoparticles are one or more of rutile titanium dioxide, zinc oxide, and barium sulfate, with an average particle size of 100-500 nm; the high emissivity nanoparticles are one or more of carbon nanotubes, silicon carbide, boron nitride, and silicon dioxide, with an average particle size of 10-50 nm.

3. The high-strength passive daytime radiation cooling thin film with a multi-scale porous structure according to claim 1, characterized in that, The thickness of the film is 50-500 μm, wherein the thickness of the upper reflective layer accounts for 30%-70% of the total thickness of the film.

4. The high-strength passive daytime radiation cooling film with a multi-scale porous structure according to claim 1, characterized in that, The viscosity-average molecular weight of the UHMWPE is between 1 million and 3 million.

5. A method for preparing a high-strength passive daytime radiation-cooled thin film with a multi-scale porous structure as described in any one of claims 1-4, characterized in that, Includes the following steps: S1: Mix high-temperature oily dispersant with UHMWPE powder, preheat and swell at 100-120 ℃, then add it to a mixer with high reflectivity nanoparticles and melt-blend at 130-160 ℃ to form a uniform gel blend, which is then hot-pressed or roll-pressed and cooled to obtain the upper reflective layer gel preform. S2: Mix the high-temperature oily dispersant with UHMWPE powder, preheat and swell at 100-120 ℃, then add it to the internal mixer with high emissivity nanoparticles, melt and blend at 130-160 ℃ to form a uniform gel blend, and obtain the lower emissivity layer gel preform after hot pressing or roll forming and cooling. S3: The upper reflective layer gel preform obtained in S1 and the lower emitting layer gel preform obtained in S2 are stacked and bonded together, placed in a hot press, and hot-pressed at 140-160℃ to allow the molecular chains at the interface of the two layers to diffuse and fuse with each other, forming an integrated double-layer composite gel film. S4 preheats the bilayer composite gel film obtained in S3 to 110-135 ℃, and then performs graded biaxial stretching on a biaxial stretching device: first, a lower stretching rate is used for the first stretching to induce the formation of nanoscale microphase separation regions; then, a higher stretching rate is used for the second stretching to expand some of the nanoscale phase separation regions into micrometer-scale phase separation regions; after stretching to a predetermined ratio, heat setting is performed at 110-125 ℃ for 5-60 seconds under tension to fix the molecular chain orientation structure and multi-level phase separation morphology. S5: The bilayer composite gel membrane treated by S4 is immersed in an organic solvent to extract and remove the high-temperature oily dispersant. After drying, the high-strength passive daytime radiation cooling film with multi-scale porous structure is obtained.

6. The method according to claim 5, characterized in that, The high-temperature oily dispersant is selected from one or more of white oil, paraffin oil, petrolatum, and dioctyl phthalate, with a mass fraction of 50-150 parts, UHMWPE with a mass fraction of 5-20 parts, and high reflectivity nanoparticles or high emissivity nanoparticles with a mass fraction of 1%-20% of the mass of UHMWPE.

7. The method according to claim 5, characterized in that, In the graded biaxial stretching, the stretching rate of the first stretching is 1% / s-10% / s, and the stretching ratio is 1.5×1.5 to 3×3; the stretching rate of the second stretching is 20% / s-100% / s, and the total stretching ratio reaches 4×4 to 8×8.

8. The method according to claim 5, characterized in that, The organic solvent in step S5 is n-hexane, xylene, or decahydronaphthalene.

9. An application of a high-strength passive daytime radiation cooling thin film with a multi-scale porous structure as described in any one of claims 1-4, characterized in that, The film is applied to the exterior surface of a building, the surface of outdoor equipment, or the surface of electronic devices for passive daytime radiative cooling.

10. The application according to claim 9, characterized in that, The film is applied to the exterior wall or roof surface of a building, with the upper reflective layer facing the direction of sunlight incidence, to achieve passive daytime radiative cooling.

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