Polyvinyl alcohol-based radiative cooling porous material and preparation method thereof

Abstract

This invention discloses a polyvinyl alcohol-based radiation-cooled porous material and its preparation method, belonging to the field of new materials. The material is prepared from a foaming system comprising an aqueous solution of polyvinyl alcohol, hydrophilic silica, a shrinkage-reducing agent, a foam stabilizer, polyethyleneimine, epichlorohydrin, and sodium dodecyl sulfate, through mechanical stirring foaming, heating curing, and atmospheric pressure drying. This invention effectively suppresses defects such as volume shrinkage, internal cracking, and skin separation in porous materials under atmospheric pressure drying conditions through the synergistic effect of the shrinkage-reducing agent and the foam stabilizer, avoiding the complex process of traditional freeze-drying. The porous material possesses both a high solar reflectivity of 93%–98% and an average infrared emissivity of over 97% within the 8–13 μm atmospheric window, enabling a cooling effect 10–20°C lower than ambient temperature without consuming additional energy, thus possessing significant practical application value.

Description

Technical Field

[0001] This invention belongs to the field of new materials and relates to a polyvinyl alcohol-based radiation-cooled porous material and its preparation method. Background Technology

[0002] With rapid global population growth and sustained socio-economic development, per capita energy consumption and cooling demands are rising daily. While traditional refrigeration technologies (such as vapor compression refrigeration, absorption refrigeration, and thermoelectric refrigeration) can achieve significant cooling effects, they generally suffer from high energy consumption and reliance on refrigerants. The extensive use of fossil fuels and refrigerants will trigger environmental problems such as global warming, air pollution, and ecological damage, seriously threatening human survival and sustainable social development. Therefore, developing efficient and low-energy-consumption new refrigeration technologies has become a current research hotspot.

[0003] Radiative cooling is a passive cooling method that requires no energy input. It works by radiating heat from the Earth's surface into outer space through atmospheric windows, thus achieving cooling. In recent years, researchers have developed various radiative cooling materials, such as radiative cooling coatings, photonic crystal emitters, and radiative cooling fabrics. While these materials can achieve passive cooling, they generally suffer from drawbacks such as complex structures and the release of volatile organic compounds during preparation, limiting their large-scale application.

[0004] Therefore, developing a porous material with a simple structure, environmentally friendly preparation process, and excellent daytime radiative cooling performance is of great significance for promoting the practical application of radiative cooling technology. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing radiation cooling materials, such as complex structural design, cumbersome preparation process, and easy release of volatile organic compounds, by providing a method for preparing polyvinyl alcohol-based radiation cooling porous materials through atmospheric pressure drying. The aim is to construct a porous structure with both high solar reflectivity and high infrared emissivity in the atmospheric window through a simple mechanical stirring foaming, heating curing, and atmospheric pressure drying process, thereby achieving efficient daytime passive radiation cooling.

[0006] This invention utilizes a mechanical stirring foaming method to uniformly disperse nano-silica in a polyvinyl alcohol aqueous solution. Epichlorohydrin acts as a crosslinking agent to stabilize the porous material's skeletal structure. A shrinkage-reducing agent slows down the water evaporation rate, preventing the porous material from cracking due to capillary forces during atmospheric pressure drying. A foam stabilizer enhances the stability of the foam system, preventing foam cracking due to austenitic curing during heated polymerization. More importantly, the synergistic effect of the foam stabilizer and shrinkage-reducing agent, through rheological-capillary force synergistic regulation and interface-anti-austenitic curing synergistic effects, effectively overcomes defects such as volume shrinkage, cracking, and peeling caused by atmospheric pressure drying, achieving a 1+1>2 effect while ensuring the uniform dispersion of the nanofiller and the material's high reflectivity. Under high-speed stirring, air is introduced into the system to form a uniform and stable foam slurry. After crosslinking, curing, and atmospheric pressure drying, a polyvinyl alcohol-based radiation-cooling porous material with both high solar reflectivity and high infrared emissivity in the atmospheric window is finally obtained.

[0007] The technical solution of this invention is implemented as follows: A polyvinyl alcohol-based radiation-cooling porous material is prepared by mechanical stirring and foaming followed by atmospheric pressure drying, using a foaming system composed of polyvinyl alcohol aqueous solution, hydrophilic silica, shrinkage reducing agent, foam stabilizer, sodium hydroxide, polyethyleneimine, epichlorohydrin, and foaming agent. Furthermore, the polyvinyl alcohol aqueous solution is obtained by mixing polyvinyl alcohol and deionized water at a mass ratio of 0.5 to 1:10; Preferably, the polyvinyl alcohol has an average degree of polymerization of 1700 and a degree of alcoholysis of 99%. Furthermore, the shrinkage reducing agent is one of polyethylene glycol 200, polyethylene glycol 400, dimethyl sulfoxide, and glycerol; Furthermore, the foam stabilizer is one of xanthan gum, guar gum, gelatin, carrageenan, pectin, and sodium alginate; Furthermore, the polyethyleneimine is branched and has a number-average molecular weight of 70,000.

[0008] This invention provides a polyvinyl alcohol-based radiation-cooled porous material and its preparation method, comprising the following steps: S1: Mix 100-1000 parts by weight of polyvinyl alcohol aqueous solution, 10-200 parts by weight of shrinkage reducing agent and 10-100 parts by weight of nano silica and disperse evenly to obtain a mixture; S2: Add 1-10 parts by weight of foam stabilizer to the mixture described in step S1 and stir to disperse evenly. Then, add 10-100 parts by weight of polyethyleneimine, 10-100 parts by weight of epichlorohydrin, and 1-10 parts by weight of sodium dodecyl sulfate in sequence. Disperse the components evenly by ultrasound and adjust the pH of the system to 10 to obtain the foaming precursor liquid. The mass ratio of foam stabilizer to shrinkage reducer is 0.5-1:10. S3: Stir the foaming precursor liquid described in step S2 at a speed of 500~1800 r / min for 3~10 minutes to form a stable foam slurry; S4: Place the foam slurry in a mold and seal it. React it in an oven at 60~80℃ for 6~8 hours to allow it to solidify and crosslink. S5: The cured and cross-linked material is dried in a forced-air oven at 50~70 ℃ under normal pressure for 8~10 hours to obtain the polyvinyl alcohol-based radiation-cooled porous material.

[0009] Compared to existing technologies that require complex steps such as freezing and freeze-drying, the method for preparing polyvinyl alcohol-based radiation-cooled porous materials by atmospheric pressure drying provided by this invention has significant simplification advantages. This method only requires sequential mechanical stirring and foaming, heating and curing, and atmospheric pressure drying, greatly shortening the production cycle, reducing operational difficulty and preparation costs, while significantly improving production efficiency and demonstrating promising prospects for industrial application.

[0010] The mechanical stirring foaming, heating curing, and atmospheric pressure drying of this invention achieve efficient preparation through the synergistic effect of multiple components: silica increases the reflectivity of sunlight and the infrared emissivity of the atmospheric window, enhancing the cooling effect; shrinkage-reducing agents slow down the rate of water evaporation, preventing the porous material from cracking due to capillary forces during atmospheric pressure drying; foam stabilizers stabilize the foam structure, preventing foam cracking due to austenitic curing during heating polymerization; more importantly, the synergistic effect of foam stabilizers and shrinkage-reducing agents, through rheological-capillary force synergistic regulation and interface-anti-austenitic curing synergistic effect, effectively overcomes the defects such as volume shrinkage, cracking, and peeling caused by atmospheric pressure drying, achieving a 1+1>2 effect and ensuring the structural integrity and optical properties of the material; polyethyleneimine is used to promote the crosslinking reaction of epichlorohydrin and polyvinyl alcohol, thereby strengthening the porous framework; sodium dodecyl sulfate is used as a foaming agent to reduce the surface tension of the system, promote air introduction, and form rich and uniform foam. Under the combined effect of the above components, the present invention does not require complex processes such as freeze drying or supercritical drying. It can obtain a polyvinyl alcohol-based radiation-cooled porous material with complete structure and excellent performance simply by mechanical stirring and foaming, heating and curing and drying at normal pressure.

[0011] The present invention also provides a polyvinyl alcohol-based radiation-cooling porous material synthesized by the above method and its application in daytime radiation cooling.

[0012] Thanks to the porous structure and chemical composition of the prepared porous material, both contribute to its excellent optical properties, enabling it to efficiently reflect sunlight and emit thermal radiation into outer space. Specifically, the polymer-air interface formed by the porous structure effectively scatters sunlight. Since polyvinyl alcohol has weak absorption in the ultraviolet-visible-near-infrared band, the porous structure significantly enhances the reflection of energy in this band. Simultaneously, silica, as a wide-bandgap inorganic material, lacks sufficient photon energy to excite electron transitions in this band, and there is a significant difference in refractive index between silica, the polymer, and air. Therefore, adding silica creates numerous multiple scattering interfaces within the material, further enhancing its solar reflectivity. These characteristics result in highly efficient scattering of sunlight at the silica-polymer-air interface, leading to high solar reflectivity.

[0013] On the other hand, the bending and stretching vibration peaks of the CO bond in polyvinyl alcohol, the stretching vibration peak of Si-O-Si in silica, and the asymmetric stretching vibration peak of the COC bond in epichlorohydrin are all located within the atmospheric transparency window (8~13 μm). The vibrations of these chemical bonds resonate with broadband infrared radiation, producing strong absorption and emission. Furthermore, multi-molecular vibrational coupling can further enhance the infrared absorption and emission effects, thereby endowing the material with high infrared emissivity.

[0014] In summary, the beneficial effects are as follows: The polyvinyl alcohol-based radiation cooling porous material prepared by this invention minimizes the absorption of solar radiation heat through high reflectivity of sunlight, and maximizes the radiation of heat to the cold outer space through high infrared emissivity in the atmospheric window band. Thus, it achieves efficient daytime radiation cooling without consuming additional energy and has important practical application value. Attached Figure Description

[0015] Figure 1 These are digital photographs of the polyvinyl alcohol-based radiation-cooled porous material prepared in Example 1 and the porous materials prepared in Comparative Examples 1-5 after foaming, heating and polymerization, drying at normal pressure, and their interiors. Figure 2 This is a graph showing the difference in solar reflectance between the polyvinyl alcohol-based radiation-cooled porous material prepared in Example 1 and the porous materials prepared in Comparative Examples 1, 5, and 6. Figure 3 This is a graph showing the difference in infrared emissivity between the polyvinyl alcohol-based radiation-cooled porous material prepared in Example 1 and the porous material prepared in Comparative Example 1. Figure 4 These are scanning electron microscope images of the polyvinyl alcohol-based radiation-cooled porous material prepared in Example 1 and the porous material prepared in Comparative Example 1. Figure 4 (a) is a scanning electron microscope image of the porous material prepared in Example 1. Figure 4 (b) is a scanning electron microscope image of the porous material prepared in Comparative Example 1; Figure 5 This is a diagram and schematic of the device used for outdoor testing of daytime radiative cooling. Figure 6 The images show the daytime cooling effect of the polyvinyl alcohol-based radiation cooling porous material prepared in Example 1 and the porous material prepared in Comparative Example 1, as well as the solar irradiance during the test, measured in an outdoor testing device. Figure 7 It shows the curves of wind speed and relative humidity changing over time on the day of the test; Figure 8 The graph shows the average and maximum temperature differences between the polyvinyl alcohol-based radiation-cooled porous material prepared in Example 1 and the porous material prepared in Comparative Example 1 during outdoor testing and the environment. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to the embodiments shown in the accompanying drawings, but this does not constitute any limitation on the present invention.

[0017] Example 1: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 500 parts by weight of polyvinyl alcohol aqueous solution, 60 parts by weight of silica, and 50 parts by weight of polyethylene glycol 200 were stirred and dispersed evenly; (2) 5 parts by weight of xanthan gum, 50 parts by weight of polyethyleneimine, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at 1800 r / min for 5 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 65°C for 8 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 65°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0018] Example 2: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 800 parts by weight of polyvinyl alcohol aqueous solution, 100 parts by weight of silica, and 80 parts by weight of polyethylene glycol 400 were stirred and dispersed evenly; (2) 4 parts by weight of carrageenan, 60 parts by weight of polyethyleneimine, 30 parts by weight of epichlorohydrin and 7 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 1300 r / min for 6 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 80°C for 6 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 50°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0019] Example 3: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 300 parts by weight of polyvinyl alcohol aqueous solution, 30 parts by weight of silica, and 60 parts by weight of dimethyl sulfoxide were stirred and dispersed evenly; (2) 5 parts by weight of guar gum, 40 parts by weight of polyethyleneimine, 15 parts by weight of epichlorohydrin and 6 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 1000 r / min for 8 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 70°C for 7 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 50°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0020] Example 4: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 1000 parts by weight of polyvinyl alcohol aqueous solution, 50 parts by weight of silica, and 50 parts by weight of polyethylene glycol 200 were stirred and dispersed evenly; (2) 6 parts by weight of gelatin, 40 parts by weight of polyethyleneimine, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 500 r / min for 10 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 50°C for 8 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 40°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0021] Example 5: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 400 parts by weight of polyvinyl alcohol aqueous solution, 80 parts by weight of silica, and 80 parts by weight of polyethylene glycol 400 were stirred and dispersed evenly; (2) 5 parts by weight of sodium alginate, 50 parts by weight of polyethyleneimine, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 1500 r / min for 10 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 50°C for 6 hours to react. After the reaction was completed, the cured product was taken out and dried at constant weight in an oven at 80°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0022] Comparative Example 1: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 500 parts by weight of polyvinyl alcohol aqueous solution and 50 parts by weight of polyethylene glycol 200 were stirred and dispersed evenly; (2) 5 parts by weight of xanthan gum, 50 parts by weight of polyethyleneimine, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 1800 r / min for 5 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 65°C for 8 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 65°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0023] Comparative Example 2: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 500 parts by weight of polyvinyl alcohol aqueous solution, 60 parts by weight of silica, and 50 parts by weight of polyethylene glycol 200 were stirred and dispersed evenly; (2) 5 parts by weight of xanthan gum, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 1800 r / min for 5 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 65°C for 8 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 65°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0024] Comparative Example 3: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 500 parts by weight of polyvinyl alcohol aqueous solution and 60 parts by weight of silica were added; (2) 5 parts by weight of xanthan gum, 50 parts by weight of polyethyleneimine, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 1800 r / min for 5 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 65°C for 8 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 65°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0025] Comparative Example 4: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 500 parts by weight of polyvinyl alcohol aqueous solution, 60 parts by weight of silica, and 50 parts by weight of polyethylene glycol 200 were stirred and dispersed evenly; (2) 50 parts by weight of polyethyleneimine, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 1800 r / min for 5 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 65°C for 8 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 65°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0026] Comparative Example 5: Example 1: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 500 parts by weight of polyvinyl alcohol aqueous solution, 60 parts by weight of silica, and 200 parts by weight of polyethylene glycol 200 were stirred and dispersed evenly; (2) 5 parts by weight of xanthan gum, 50 parts by weight of polyethyleneimine, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at a speed of 1800 r / min for 5 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 65°C for 8 hours. After the reaction was completed, the cured product was taken out and dried at constant weight in a forced-air oven at 65°C under normal pressure to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0027] Comparative Example 6: Example 1: A polyvinyl alcohol-based radiation-cooled porous material was prepared by the following steps: (1) 500 parts by weight of polyvinyl alcohol aqueous solution, 60 parts by weight of silica, and 50 parts by weight of polyethylene glycol 200 were stirred and dispersed evenly; (2) 5 parts by weight of xanthan gum, 50 parts by weight of polyethyleneimine, 20 parts by weight of epichlorohydrin and 5 parts by weight of sodium dodecyl sulfate were added to the mixture obtained in (1) and ultrasonically dispersed evenly; (3) the pH of the foaming precursor liquid obtained in (2) was adjusted to 10 using sodium hydroxide, and the mixture was stirred at 1800 r / min for 5 min using a magnetic stirrer until a stable foam slurry was finally formed; (4) the foam slurry obtained in (3) was sealed and placed in an oven at 65°C for 8 hours. After the reaction was completed, the cured product was taken out to obtain the radiation-cooled polyvinyl alcohol-based porous material.

[0028] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0029] The performance tests involved in the embodiments of this invention are performed using the following methods: 1. Digital photographs of Example 1 and Comparative Examples 1-5 after foaming, heating and polymerization, and drying under normal pressure, as well as their interiors. Figure 1 These are digital photographs of the polyvinyl alcohol-based radiation-cooled porous material prepared in Example 1 and the porous materials prepared in Comparative Examples 1-5, wherein... Figure 1 (a1) is a photograph of Example 1 after foaming. Figure 1 (a2) to (a6) are photographs of Comparative Examples 1 to 5 after foaming; Figure 1 (b1) is a photograph of Example 1 after heating and polymerization. Figure 1 (b2) to (b6) are photographs of Comparative Examples 1 to 5 after heating and polymerization; Figure 1 (c1) is a front view of Example 1 after drying at atmospheric pressure. Figure 1 (c2) to (c6) are front views of Comparative Examples 1 to 5 after drying at atmospheric pressure; Figure 1 (d1) is a top view of Example 1 after atmospheric pressure drying. Figure 1 (d2) to (d6) are top views of Comparative Examples 1 to 5 after drying at atmospheric pressure; Figure 1 (e1) is an internal view of Example 1. Figure 1(e2) to (e6) are internal diagrams of Comparative Examples 1 to 5. As shown in the diagrams, Comparative Example 2 failed to form after heating. This indicates that without polyethyleneimine, the system lacks cross-linking catalysts, filler dispersion fails, foam interface elasticity is insufficient, and system viscosity is low. This results in the wet foam lacking cross-linked framework support, and the foam film rapidly ruptures and collapses after heating and dehydration, ultimately failing to form. Comparative Example 3 cracked after atmospheric pressure drying. This indicates that without shrinkage reducer, the water evaporation rate during atmospheric pressure drying is too fast, stress cannot be released, leading to stress concentration on the foam wall, ultimately causing the sample to crack. Comparative Example 4 exhibited skin separation after atmospheric pressure drying. This indicates that without foam stabilizer, the system... Low viscosity and weak interfacial bonding cause the surface layer to harden and shrink first during normal pressure drying, followed by the inner layer, eventually leading to interfacial tearing and peeling. Comparative Example 5 showed a decrease in foam volume, stickiness after heating, and the appearance of microcracks and large pores inside. This indicates that when the mass ratio of foam stabilizer to shrinkage reducer is not within the range of 0.5 to 1:10, their synergistic effect fails. Excessive polyethylene glycol 200 will cause the system viscosity to be too high, resulting in large pores, loose structure, and foam collapse. Excessive free components migrate to the surface, making the sample sticky and producing precipitates. At the same time, the material's heat resistance is reduced, making it prone to deformation during high-temperature drying, and it will also weaken optical reflection performance.

[0030] 2. Reflectivity Test Figure 2The graph shows the difference in solar reflectance between the polyvinyl alcohol-based radiation-cooled porous material prepared in Example 1 and the porous materials prepared in Comparative Examples 1, 5, and 6. The graph overlays the atmospheric mass 1.5 solar spectrum to correspond to the main energy distribution of solar radiation in the ultraviolet to near-infrared band. For reflectance testing, a SHIMADZU UV-3600PLUS ultraviolet-visible-near-infrared spectrophotometer from Shimadzu Corporation, Japan, was used to measure the reflectance of the porous materials prepared in Example 1 and Comparative Examples 1, 5, and 6. As shown in the graph, the wavelength range for this reflectance test was 0.3-2.5 nm. The reflectance of Example 1 after being dried at normal pressure was 77.56%, 94.66%, and 91.33% in the ultraviolet, visible, and near-infrared bands, respectively, with a total average reflectance of 92.56%. This demonstrates that Example 1, after being dried at normal pressure, possesses excellent high reflectance across the entire spectrum of sunlight. The reflectance of Example 1 in all three bands and the total average reflectance are higher than those of Comparative Example 1, indicating that the addition of silica effectively improves reflectance. The reflectance of Example 1 is higher than that of Comparative Example 5, suggesting that excessive shrinkage agent negates its synergistic effect with the foam stabilizer, resulting in a looser texture. The loose foam structure is not conducive to the reflection of sunlight, and excessive shrinkage-reducing agents can coat the silica filler, which also reduces the solar reflectivity of porous materials. The significant difference in reflectivity between Example 1 and Comparative Example 6 indicates that water, polyvinyl alcohol, and inorganic fillers in the wet foam have similar refractive indices, resulting in weak light scattering. Furthermore, the loose structure and high translucency of the wet foam allow incident light to easily penetrate, leading to a total average reflectivity of only 72.64%. After atmospheric pressure drying to remove moisture, the pores are filled with air, forming numerous multiple scattering interfaces, increasing the total average reflectivity to 92.56%. In summary, Example 1 requires atmospheric pressure drying and exhibits high reflectivity when the foam stabilizer and shrinkage-reducing agent have a synergistic effect.

[0031] 3. Emissivity Test Figure 3 The graph shows the difference in infrared emissivity between the polyvinyl alcohol-based radiation-cooling porous material prepared in Example 1 and the porous material prepared in Comparative Example 1, with atmospheric window spectra superimposed. For emissivity testing, a Thermo Scientific Nicolet iS50 Fourier transform infrared spectrometer was used to measure the emissivity of the porous materials prepared in Example 1 and Comparative Example 1. As shown in the graph, the wavelength range of this emissivity test was 2.5-25 μm. Within the atmospheric window range of 8-13 μm, both Example 1 and Comparative Example 1 exhibited high emissivity. Example 1 showed an average emissivity of 98.16% within this range, while Comparative Example 1 showed an average emissivity of 97.14%, indicating that both Example 1 and Comparative Example 1 possess the ability to efficiently emit heat into the cold outer space.

[0032] 4. Scanning electron microscope image (SEM) Figure 4 Scanning electron microscope (SEM) images of the polyvinyl alcohol-based radiation-cooled porous material prepared in Example 1 and the porous material prepared in Comparative Example 1 are shown. The morphology of the porous materials prepared in Example 1 and Comparative Example 1 was analyzed using a ZEISS EV0 MA15 scanning electron microscope (Carl Zeiss Microscopy GmbH, Germany) with an accelerating voltage of 20.0 kV. Before testing, the samples underwent surface gold sputtering treatment. Figure 4 (a) is a scanning electron microscope image of the porous material prepared in Example 1. Figure 4 (b) is a scanning electron microscope image of the porous material prepared in Comparative Example 1. It can be observed that the porous materials prepared in Example 1 and Comparative Example 1 both have continuous pore structures, and from... Figure 4 (a) It can be seen that nano-silica is uniformly distributed on the pore framework of Example 1, forming a large number of silica-polymer-air interfaces, indicating that nano-silica was successfully added and thereby enhanced the reflectivity of the porous material.

[0033] 5. Radiative Cooling Test Figure 5 This is a schematic diagram of a device for testing daytime radiative cooling. The device uses low-thermal-conductivity polystyrene foam as its external support frame, which also serves as insulation. The exterior is covered with aluminum foil, which acts as both an incident light reflector and a thermal barrier to block surrounding heat conduction, thus reducing interference from outdoor solar radiation. Furthermore, the foam filling inside the device further reduces heat exchange between the interior and the environment. During testing, Example 1 and Comparative Example 1 were placed in the groove of this testing device. To minimize the impact of ambient heat convection on the radiative cooling measurement results, a highly transparent polyethylene film was placed over the foam box to eliminate heat convection interference caused by unpredictable or unstable airflow. To ensure the accuracy of temperature measurement of the porous material, thermocouples were fixed inside the material for real-time temperature data acquisition. The radiative cooling performance of the porous material was ultimately evaluated through temperature changes. The solar irradiance intensity, ambient temperature, and sample temperature monitoring data during the experiment are as follows: Figure 6 As shown, the wind speed and humidity monitoring results are as follows: Figure 7 As shown, this ensures the reliability of the test data. The temperature difference between Example 1, Comparative Example 1, and the environment during the test is as follows: Figure 8 As shown, by Figure 8 It can be seen that, during the test, the temperature of Example 1 was able to achieve an average cooling effect of 8.7 ℃ and a maximum of 12.0 ℃ compared to the ambient temperature. This indicates that when the emissivity is similar, Example 1, which has a higher reflectivity, can reflect sunlight more efficiently to reduce energy absorption, thereby achieving a higher cooling effect.

[0034] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a polyvinyl alcohol-based radiation-cooled porous material, characterized in that, Includes the following steps: S1: Mix 100-1000 parts by weight of polyvinyl alcohol aqueous solution, 10-100 parts by weight of shrinkage reducing agent and 10-100 parts by weight of nano silica and disperse evenly to obtain a mixture; S2: Add 1-10 parts by weight of foam stabilizer to the mixture described in step S1 and stir to disperse evenly. Then, add 10-100 parts by weight of polyethyleneimine, 10-100 parts by weight of epichlorohydrin, and 1-10 parts by weight of sodium dodecyl sulfate in sequence. Disperse the components evenly by ultrasound and adjust the pH of the system to 10 to obtain the foaming precursor liquid. The mass ratio of foam stabilizer to shrinkage reducer is 0.5-1:

10. S3: Stir the foaming precursor liquid described in step S2 at a speed of 500~1800 r / min for 3~10 minutes to form a stable foam slurry; S4: Place the foam slurry in a mold and seal it. React it in an oven at 60~80℃ for 6~8 hours to allow it to solidify and crosslink. S5: The cured and cross-linked material is dried in a forced-air oven at 50~70 ℃ under normal pressure for 8~10 hours to obtain the polyvinyl alcohol-based radiation-cooled porous material.

2. The method for preparing a polyvinyl alcohol-based radiation-cooled porous material as described in claim 1, characterized in that, The polyvinyl alcohol-based radiation-cooling porous material is composed of nano-silica and a polymer network. The polyvinyl alcohol-based radiation-cooling porous material has a large number of silica-polymer-air interfaces. The solar reflectance of the porous material is 93%~98%, and the infrared emissivity is above 97%.

3. The method for preparing a polyvinyl alcohol-based radiation-cooled porous material as described in claim 1, characterized in that, The polyvinyl alcohol-based radiation-cooled porous material is 10-20°C cooler than the ambient temperature under direct sunlight.

4. The method for preparing a polyvinyl alcohol-based radiation-cooled porous material as described in claim 1, characterized in that, The polyvinyl alcohol aqueous solution is obtained by mixing polyvinyl alcohol and deionized water at a mass ratio of 0.5 to 1:

10. The average degree of polymerization of polyvinyl alcohol is 1700 and the degree of alcoholysis is 99%.

5. The method for preparing a polyvinyl alcohol-based radiation-cooled porous material as described in claim 1, characterized in that, The shrinkage reducing agent is one of polyethylene glycol 200, polyethylene glycol 400, dimethyl sulfoxide, and glycerol.

6. The method for preparing a polyvinyl alcohol-based radiation-cooled porous material as described in claim 1, characterized in that, The foam stabilizer is one of xanthan gum, guar gum, gelatin, carrageenan, pectin, and sodium alginate.

7. The method for preparing a polyvinyl alcohol-based radiation-cooled porous material as described in claim 1, characterized in that, The polyethyleneimine is branched and has a number average molecular weight of 70,000.

8. A polyvinyl alcohol-based radiation-cooled porous material, characterized in that, It is prepared by the method described in any one of claims 1-7.

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