Radiation refrigeration film, preparation method and application in fruit and vegetable logistics preservation

Abstract

The application belongs to the technical field of food preservation and new material, and discloses a radiation refrigeration film, which is composed of the following components and in the following weight fractions: 2 parts of polyvinyl alcohol, 1.4 parts of glycerol, 10 parts of nano titanium dioxide or nano barium sulfate or nano silicon dioxide particles, and 20 parts of deionized water. The polyvinyl alcohol has good flexibility and processability, is easy to manufacture large-area film or complex shape, and is suitable for different application scenarios, especially has good adhesion and practicability in the field of postharvest preservation of fruits, so as to reduce the potential harm of degradable materials to the environment and meet the requirements of sustainable development. The film has excellent spectral properties, and can improve the quality of fruits and vegetables and prolong the shelf life of fruits and vegetables when applied in the preservation of fruits and vegetables in logistics.

Description

Technical Field

[0001] This invention belongs to the fields of food preservation and new materials technology, and in particular to a radiation cooling film, its preparation method, and its application in the logistics and preservation of fruits and vegetables. Background Technology

[0002] Refrigeration is a crucial method for food preservation, delaying spoilage by lowering temperatures. Common low-temperature control methods include cold storage and freezers, which utilize refrigeration systems for cooling. Traditional refrigeration methods typically rely on large equipment and consume energy. Radiative refrigeration, an emerging technology, uses specific materials or structures to radiate heat into outer space, achieving passive cooling without energy consumption. This technology brings new possibilities to the field of food preservation and may become one of the important refrigeration methods in the future. Radiative refrigeration is a process that uses thermal radiation to transfer heat from an object to its external environment. Thermodynamics shows that when hot and cold objects participate in radiative exchange, heat spontaneously flows from the hot body to the cold body. Therefore, the huge temperature difference between Earth (approximately 300K) and the cold universe (3K) drives objects on Earth to emit thermal energy into the vast outer space, resulting in radiative refrigeration. Based on this principle, researchers have developed various efficient radiative refrigeration materials, such as multilayer nanofilms with high solar reflectivity and high mid-infrared emissivity, and polymer-based composite materials.

[0003] The intrinsic properties and structural composition of radiative cooling materials are key factors in improving cooling efficiency. The rapid development of nanophotonics and metamaterials has provided new avenues for realizing spectrally modulated devices. By introducing periodic nanostructures to modulate spectral properties, researchers can simultaneously enhance the solar reflectivity and infrared emissivity of thin films. Furthermore, thin film materials adapted to different scenarios are under development, such as colored films, transparent films, and flexible coatings, to meet the needs of architectural design, agricultural covering, and portable electronic devices. Despite the significant potential of radiative cooling technology, it still faces many challenges, such as high production costs, limited lifespan, and performance limitations in humid or cloudy environments. To achieve widespread application, future research will focus on developing more economical and durable materials, optimizing manufacturing processes, and integrating energy storage technologies to improve adaptability. With continuous technological breakthroughs, radiative cooling is expected to bring revolutionary impacts to fields such as building energy conservation, agricultural preservation, and industrial heat dissipation.

[0004] Currently, the relevant technical solutions and their shortcomings in the existing technology are as follows:

[0005] 1. Technical solution of existing technology one

[0006] The fabrication of a superhydrophobic, self-cleaning, temperature-adaptive radiation-cooling thin film involves multiple collaborative steps. First, a tungsten-doped vanadium dioxide suspension is prepared, and uniform particles are obtained through grinding and ultrasonic dispersion. Next, a PU-PDMS sol is prepared, which is then cross-linked and degassed to form a transparent, bubble-free sol. Finally, a fluorine-modified nano-titanium oxide colloid is prepared, surface-modified, cleaned, dried, and dispersed in a solvent. Subsequently, a PE film is activated and softened by heating, and an embedded microstructure array is formed using template imprinting, followed by spraying with the tungsten-doped vanadium dioxide suspension for curing. In the encapsulation stage, the PE film is encapsulated with an Ag / PET polymer composite film, and the stacking order is adjusted to facilitate subsequent operations. Finally, PU-PDMS sol and fluorine-modified nano-titanium oxide colloid are sequentially sprayed onto the film surface, and cured by microwave heating to form a superhydrophobic, self-cleaning coating, completing the film fabrication.

[0007] The shortcomings of existing technology 1:

[0008] Multi-step process: This technology involves multiple processes, such as grinding and dispersing tungsten-doped vanadium dioxide, imprinting microstructure arrays, and spraying and curing multilayer materials. The process is cumbersome and has high requirements, resulting in long production cycles and low efficiency.

[0009] Dependence on high-precision equipment: It requires precision equipment such as high-pressure spray guns, microwave devices, and cylindrical rolling mills. The procurement and maintenance costs of these devices are high, making them unsuitable for low-cost large-scale production.

[0010] Environmental shortcomings: The radiation refrigeration raw material polydimethylsiloxane (PDMS) has poor biodegradability, is unstable when heated and easily decomposes, producing toxic and harmful volatile substances, making it difficult to expand its application in the agricultural field.

[0011] 2. Technical solution of existing technology one

[0012] The preheated film substrate is heat-pressed using an printing roller to form a raised structure on its surface, and the printing temperature is dynamically adjusted to match the substrate's movement speed. Nanoparticles are then filled between the raised structures using an electrostatic spray gun, and excess particles are removed by extrusion and a scraper to complete the filling. A protective film is then applied to the surface of the raised structures, and the entire film structure is formed by heat pressing with a hot roller.

[0013] The shortcomings of existing technology 2:

[0014] Nanoparticle filling inhomogeneity: The electrostatic spraying and filling of nanoparticles may be affected by equipment status and operating conditions, resulting in uneven filling and thus affecting the radiation performance of the film.

[0015] Expensive equipment: It requires high-precision equipment such as printing rollers, electrostatic spraying devices, and hot press roller assemblies, resulting in high initial investment costs and complex equipment maintenance.

[0016] By comparison, the present invention patent application is fundamentally different from the aforementioned patent publications. Summary of the Invention

[0017] The purpose of this invention is to overcome the shortcomings of the prior art and provide a radiation cooling film, a preparation method, and its application in the preservation of fruits and vegetables.

[0018] The technical solution adopted by this invention to solve its technical problem is:

[0019] A radiation-cooling thin film, the composition and weight parts of which are as follows:

[0020] 2 parts polyvinyl alcohol;

[0021] 1.4 parts glycerin;

[0022] 10 parts of nano-titanium dioxide, nano-barium sulfate, or nano-silica particles;

[0023] 20 parts deionized water.

[0024] Furthermore, the titanium dioxide is TiO2 nanoparticles with a diameter of 550 nm.

[0025] The method for preparing the radiation-cooling thin film as described above includes the following steps:

[0026] First, polyvinyl alcohol is dissolved in deionized water, heated to 90°C and stirred for 1 hour to form a homogeneous solution. Then, glycerol is added as a plasticizer and stirring is continued to reduce the viscosity. Next, nano-titanium dioxide, nano-barium sulfate, or nano-silica particles are added, and the temperature is kept constant within the range of 50°C-90°C while stirring to ensure uniform mixing. Finally, a radiation cooling film is obtained.

[0027] Furthermore, the uniform mixing specifically involves uniformly combining inorganic micro / nano particles and polyvinyl alcohol through a chemical dissolution and mixing or physical blending step.

[0028] Furthermore, the radiation cooling film is obtained using a Roll-to-Roll (R2R) process, which specifically involves uniformly coating a TiO2-PVA mixed solution onto a substrate. Commonly used substrates include glass, plastic, silicone, polystyrene, and aluminum. By controlling the coating speed and the amount of raw materials added, gravure coating is used to obtain the film, which precisely controls the film thickness, has high uniformity, and is suitable for large-scale production.

[0029] The method for preparing the radiation-cooling thin film as described above includes the following steps:

[0030] First, polyvinyl alcohol is dissolved in deionized water, heated to 90°C and stirred for 1 hour to form a homogeneous solution. Then, glycerol is added as a plasticizer, and stirring is continued to reduce the viscosity. Next, nano-titanium dioxide, nano-barium sulfate, or nano-silica particles are added, and the temperature is kept constant within the range of 50°C-90°C while stirring to ensure uniform mixing. Finally, the mixture is placed at -20°C for freeze-thaw cycles to form a uniform gel film, ultimately obtaining a radiation cooling film with excellent performance.

[0031] The method for preparing the radiation-cooling thin film as described above includes the following steps:

[0032] First, polyvinyl alcohol is dissolved in deionized water to form a homogeneous solution. Then, glycerol is added as a plasticizer, and stirring is continued to reduce the viscosity. Next, nano-titanium dioxide, nano-barium sulfate, or nano-silica particles are added, and the temperature is kept constant within the range of 50℃-90℃ while stirring to ensure uniform mixing. Finally, a radiation cooling film is obtained.

[0033] Furthermore, the uniform mixing specifically involves uniformly combining inorganic micro / nano particles and polyvinyl alcohol through a chemical dissolution and mixing or physical blending step.

[0034] Furthermore, the radiation cooling film is obtained using a Roll-to-Roll (R2R) process, which specifically involves uniformly coating a TiO2-PVA mixed solution onto a substrate. Commonly used substrates include glass, plastic, silicone, polystyrene, and aluminum. By controlling the coating speed and the amount of raw materials added, gravure coating is used to obtain the film, which precisely controls the film thickness, has high uniformity, and is suitable for large-scale production.

[0035] The application of radiation-cooling films as described above in the preservation of fruits and vegetables.

[0036] The advantages and positive effects of this invention are as follows:

[0037] 1. The method of this invention simplifies the process and reduces costs. The composite process of polyvinyl alcohol substrate and titanium dioxide nanoparticles is simple, avoiding complex multilayer structures and cumbersome equipment dependence, while improving production efficiency and significantly reducing manufacturing costs.

[0038] 2. This invention contains a mixture of polyvinyl alcohol and titanium dioxide used as a radiative cooling material, which is environmentally friendly and biocompatible. All three raw materials used have good biocompatibility, are non-toxic and harmless, and are biodegradable or recyclable, avoiding the use of solvents or other difficult methods in traditional approaches.

[0039] 3. The polyvinyl alcohol in this invention possesses excellent flexibility and processability, making it easy to manufacture large-area films or complex shapes, suitable for various application scenarios. It exhibits particularly good adhesion and practicality in post-harvest fruit preservation, thus mitigating the potential environmental hazards of materials and meeting the requirements of sustainable development. This film also possesses excellent spectral characteristics, enabling its application in fruit and vegetable logistics preservation to improve fruit and vegetable quality and extend shelf life.

[0040] 4. This invention uses polyvinyl alcohol (PVA) as a substrate and incorporates titanium dioxide (TiO2) nanoparticles and glycerol. Through material selection and design optimization (using virtual simulation technology to analyze the difference between the refractive index of the nanoparticles and the refractive index of the substrate, with the scattering efficiency factor as a metric), this invention not only solves the problems of traditional radiation cooling films in terms of production cost, environmental friendliness, spectral performance, and applicability, but also successfully extends its application to the field of postharvest fruit preservation. Under the same conditions, it delays the softening of fruits and vegetables, extends the shelf life of fruits, and reduces the economic value of postharvest waste of fruits and vegetables, providing a new direction and potential for the practical promotion and commercialization of this technology.

[0041] 5. In this invention, spectral modulation enhances the cooling effect. By selecting raw materials with large differences in refractive index through simulation, titanium dioxide nanoparticles have excellent spectral modulation capabilities. Combined with a polyvinyl alcohol substrate, they can effectively reflect sunlight and enhance infrared radiation heat dissipation, significantly improving the radiative cooling performance of the film and ensuring excellent cooling effect in high-temperature environments. Attached Figure Description

[0042] Figure 1 The diagram shows the multi-particle scattering efficiency factor of the TiO2-PVA thin film prepared in Example 1 of this invention; the left diagram is the dimensionless scattering coefficient diagram (Mie scattering effect of a single particle), and the right diagram is the scattering coefficient diagram (simulating the Mie scattering effect of multiple particles).

[0043] Figure 2 The image shows the nanoparticle size (left), polymer dispersion coefficient (middle), and potential diagram (right) of the TiO2-PVA solution system prepared in Example 1 of this invention.

[0044] Figure 3 These are transmission electron microscope (TEM) images of the thin films prepared in Example 1 of this invention; the left image is an internal structure diagram of the TiO2-PVA thin film, and the right image is an internal structure diagram of the TiO2-PVA solution; the scale bar is 100 nm.

[0045] Figure 4 The spectral characteristics of the radiation-cooling thin film prepared in Example 1 of this invention are shown in the solar radiation band (left) and mid-infrared thermal radiation band (right).

[0046] Figure 5The Fourier transform infrared (FTIR) spectrum of the radiation-cooling thin film prepared in Example 1 of this invention is shown below.

[0047] Figure 6 This is an indoor simulated cooling experiment diagram of the radiative cooling film prepared in Example 1 of this invention;

[0048] Figure 7 The image shows a physical diagram (left) and an application diagram (right) of the radiation-cooling thin film prepared in Example 1 of this invention.

[0049] Figure 8 This is a diagram showing the effect of applying the radiation cooling film prepared in Example 1 of this invention to postharvest winter peaches;

[0050] Figure 9 This is a photograph of the polyvinyl alcohol (PVA) film prepared in Comparative Example 2 of this invention; wherein, the polyvinyl alcohol exhibits agglomeration when stirred at room temperature.

[0051] Figure 10 This is a photograph of the composite large film (TiO2:PVA mass ratio of 1:1) based on polyvinyl alcohol (PVA) and titanium dioxide (TiO2) nanoparticles prepared in Comparative Example 3 of this invention. Detailed Implementation

[0052] The present invention will be further described below with reference to the embodiments. The following embodiments are descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0053] The various experimental operations involved in the specific embodiments are all conventional techniques in the field. For parts not specifically annotated in this document, those skilled in the art can refer to various commonly used reference books, scientific and technological documents or related instructions and manuals prior to the filing date of this invention to carry out the operations.

[0054] A radiation-cooling thin film, the composition and weight parts of which are as follows:

[0055] 2 parts polyvinyl alcohol;

[0056] 1.4 parts glycerin;

[0057] 10 parts of nano-titanium dioxide, nano-barium sulfate, or nano-silica particles;

[0058] 20 parts deionized water.

[0059] Preferably, the titanium dioxide is TiO2 nanoparticles with a diameter of 550 nm.

[0060] The method for preparing the radiation-cooling thin film as described above includes the following steps:

[0061] First, polyvinyl alcohol is dissolved in deionized water, heated to 90°C and stirred for 1 hour to form a homogeneous solution. Then, glycerol is added as a plasticizer and stirring is continued to reduce the viscosity. Next, nano-titanium dioxide, nano-barium sulfate, or nano-silica particles are added, and the temperature is kept constant within the range of 50°C-90°C while stirring to ensure uniform mixing. Finally, a radiation cooling film is obtained.

[0062] Preferably, the uniform mixing specifically involves uniformly compounding inorganic micro / nano particles and polyvinyl alcohol through a chemical dissolution and mixing or physical blending step.

[0063] Preferably, the radiation cooling film is obtained using a Roll-to-Roll (R2R) process, which specifically involves uniformly coating a TiO2-PVA mixed solution onto a substrate. Commonly used substrates include glass, plastic, silicone, polystyrene, and aluminum. By controlling the coating speed and the amount of raw materials added, gravure coating is used to obtain the film, which precisely controls the film thickness, has high uniformity, and is suitable for large-scale production.

[0064] The method for preparing the radiation-cooling thin film as described above includes the following steps:

[0065] First, polyvinyl alcohol is dissolved in deionized water, heated to 90°C and stirred for 1 hour to form a homogeneous solution. Then, glycerol is added as a plasticizer, and stirring is continued to reduce the viscosity. Next, nano-titanium dioxide, nano-barium sulfate, or nano-silica particles are added, and the temperature is kept constant within the range of 50°C-90°C while stirring to ensure uniform mixing. Finally, the mixture is placed at -20°C for freeze-thaw cycles to form a uniform gel film, ultimately obtaining a radiation cooling film with excellent performance.

[0066] The method for preparing the radiation-cooling thin film as described above includes the following steps:

[0067] First, polyvinyl alcohol is dissolved in deionized water to form a homogeneous solution. Then, glycerol is added as a plasticizer, and stirring is continued to reduce the viscosity. Next, nano-titanium dioxide, nano-barium sulfate, or nano-silica particles are added, and the temperature is kept constant within the range of 50℃-90℃ while stirring to ensure uniform mixing. Finally, a radiation cooling film is obtained.

[0068] Preferably, the uniform mixing specifically involves uniformly compounding inorganic micro / nano particles and polyvinyl alcohol through a chemical dissolution and mixing or physical blending step.

[0069] Preferably, the radiation cooling film is obtained using a Roll-to-Roll (R2R) process, which specifically involves uniformly coating a TiO2-PVA mixed solution onto a substrate. Commonly used substrates include glass, plastic, silicone, polystyrene, and aluminum. By controlling the coating speed and the amount of raw materials added, gravure coating is used to obtain the film, which precisely controls the film thickness, has high uniformity, and is suitable for large-scale production.

[0070] The application of radiation-cooling films as described above in the preservation of fruits and vegetables.

[0071] Specifically, the relevant testing and preparation methods are as follows:

[0072] Example 1

[0073] This experiment aimed to improve the cooling and preservation effects of PVA films. A suitable combination of polymer and nanoparticles was selected through finite-domain difference (FDTD) simulation. The results are as follows: Figure 1 As shown, TiO2 nanoparticles possess high scattering efficiency and scattering coefficient, especially in the ultraviolet and visible light bands (0.2-0.8 μm), making them highly efficient light-shielding materials. They can significantly enhance the reflectivity of the film to sunlight and its ultraviolet shielding ability, thereby improving cooling effects and fruit preservation performance. In contrast, SiO2 nanoparticles and CaCO3 nanoparticles have lower dimensionless scattering coefficients, indicating that their light scattering effect is lower than that of TiO2 nanoparticles.

[0074] A method for preparing a composite large film based on polyvinyl alcohol (PVA) and titanium dioxide (TiO2) nanoparticles specifically includes the following steps:

[0075] Weigh 2g of freshly purchased polyvinyl alcohol (PVA) and add it to 20g of deionized water. Heat the mixture to 90℃ and stir at 500rpm for 1 hour until the PVA is completely dissolved, forming a homogeneous polymer solution. Add 1.4g of glycerol as a plasticizer to the mother liquor and continue stirring at 500rpm for 1 hour to ensure uniform dispersion of the plasticizer. A decrease in the viscosity of the solution was observed, enhancing the flexibility of the subsequent film. Add 10g of titanium dioxide (TiO2) nanoparticles with a diameter of 550nm to the mother liquor, maintain the temperature at 90℃, and continue stirring at 500rpm for 1 hour to ensure thorough mixing of the titanium dioxide nanoparticles with the polymer matrix, forming a homogeneous composite solution (hereinafter referred to as TiO2-PVA solution). Take 5g of the uniformly mixed composite solution and pour it into a 15cm diameter glass petri dish. Shake to ensure the film spreads evenly on the surface of the petri dish, achieving the desired thickness and smoothness. Through the above steps, a large thin film with excellent properties, namely the radiation-cooling thin film (hereinafter referred to as TiO2-PVA film), can be obtained, which is suitable for various subsequent applications. A photograph of the prepared sample is shown below. Figure 7 As shown.

[0076] The relevant tests and results are as follows:

[0077] The TiO2-PVA solution (prepared as described above: weigh 2g of freshly purchased polyvinyl alcohol (PVA) and add it to 20g of deionized water. Heat the mixture to 90℃ and stir at 500rpm for 1h until the polyvinyl alcohol is completely dissolved, forming a homogeneous polymer solution. Add 1.4g of glycerol as a plasticizer to the mother liquor and continue stirring at 500rpm for 1h to ensure uniform dispersion of the plasticizer. A decrease in the viscosity of the solution was observed, enhancing the flexibility of the subsequent film. Add 10g of titanium dioxide (TiO2) nanoparticles with a diameter of 550nm to the mother liquor, maintain the temperature at 90℃, and continue stirring at 500rpm for 1h to ensure thorough mixing of the titanium dioxide nanoparticles with the polymer matrix, forming a homogeneous composite solution, which is the TiO2-PVA solution.) exhibits electrostatic repulsion, resulting in overall uniformity and stability. In the TiO2-PVA solution system, the TiO2 nanoparticles are small and uniformly distributed. Due to electrostatic repulsion, the solution is generally stable, making it suitable for preparing high-performance films. Figure 2 Transmission electron microscopy (TEM) images of the thin film reveal an internal layered, entangled structure. This structure not only enhances the film's mechanical strength but also strengthens its light-scattering ability, thus improving its reflectivity in the solar wavelength range. Figure 3 In the solar radiation band (0.3-2.5μm), the film has an average reflectivity of 0.92, effectively shielding solar thermal radiation; in the mid-infrared atmospheric window band (8-13μm), the emissivity is as high as 0.95, demonstrating significant thermal radiation capability and achieving excellent radiative cooling effect. Figure 4 Fourier transform infrared spectroscopy revealed that PVA exhibits CO stretching vibrations, with a stretching vibration peak in the atmospheric window range (769-1250 cm⁻¹), displaying a significant absorption peak, further verifying the film's efficient thermal radiation performance. Figure 5 In an indoor simulated cooling experiment, a radiative cooling film covered a beaker and was irradiated by a regular xenon lamp. The temperature under the film was 20°C lower than the ambient temperature, while the temperature difference under a regular xenon lamp was as high as 20°C, demonstrating its excellent cooling capability. Figure 6 The physical images show that the radiation cooling film has a diameter of 15cm and a thickness of 0.02cm, exhibiting good flexibility and suitability for applications such as food preservation and packaging materials, demonstrating high practicality. Figure 7 ).

[0078] TiO2-PVA film and PVA film (The preparation steps of PVA film are as follows: Weigh 2g of freshly purchased polyvinyl alcohol (PVA) and add it to 20g of deionized water. Heat the mixture to 90℃ and stir at 500rpm for 1h until the polyvinyl alcohol is completely dissolved to form a homogeneous polymer solution. Add 1.4g of glycerol as a plasticizer to the mother liquor and continue stirring at 500rpm for 1h to ensure that the plasticizer is evenly dispersed. Take 5g of the homogeneous composite solution and pour it into a glass petri dish with a diameter of 15cm. Shake to ensure that the film is evenly spread on the surface of the petri dish to achieve the required thickness and smoothness, thus obtaining the PVA film.) Compared with the preservation of winter peaches, it has many advantages, such as Figure 8 The image shows the effect of its application on post-harvest winter peaches. Polyvinyl alcohol-nano titanium dioxide film can effectively slow down the weight loss and firmness reduction of fruits and vegetables, slow down their ripening, and extend their shelf life. The formula is: ΔE = [(ΔL) + (Δa) + (Δb)]¹ / ², where ΔL (brightness difference), ΔA (red-green difference), and ΔB (yellow-blue difference) are used for color matching, etc. The ΔE value represents the total color difference and is an important indicator for assessing the degree of difference between two colors. The smaller the ΔE value, the closer the two colors are; the larger the ΔE value, the more obvious the color difference. From... Figure 8 The results show that TiO2 nanoparticles possess excellent light scattering and reflection capabilities in the ultraviolet and visible light bands, effectively reducing light penetration, minimizing UV damage to the pigments in winter peaches, lowering the surface temperature of fruits and vegetables, helping to maintain the bright color of winter peaches, slowing down the respiratory metabolic rate of winter peaches, and reducing softening. After 6 days of storage, the softening trend analysis showed that the hardness of the peaches wrapped with TiO2-PVA film was 7.87±0.59N, significantly higher than the weight loss rate of the control group (5.46±0.34N) (p<0.05). Regarding color changes, the a* value (redness) of the peaches wrapped with TiO2-PVA film was 1.73, while the a* value of the control group was 10.20. Simultaneously, the addition of TiO2 enhanced the film's shielding effect against solar radiation and reduced the driving force of water evaporation. The addition of nanoparticles may have altered the film's microstructure, reducing water vapor permeability and thus effectively inhibiting water loss. After 6 days of storage, the weight loss rate of the winter peaches wrapped in TiO2-PVA film was 15.75±1.04%, which was significantly lower than the weight loss rate of 25.79±1.72% of the control group (p<0.05).

[0079] Comparative Example 1

[0080] A method for preparing polyvinyl alcohol (PVA) thin films, which uses solution casting to prepare high emissivity thin films with atmospheric windows, specifically includes the following steps:

[0081] Weigh 2g of freshly purchased polyvinyl alcohol (PVA) and add it to 20g of deionized water. Heat the mixture to 90°C and stir at 500rpm for 1 hour until the PVA is completely dissolved, forming a homogeneous polymer solution. Add 1.4g of glycerol as a plasticizer to the mother liquor and continue stirring at 500rpm for 1 hour to ensure uniform dispersion of the plasticizer. A decrease in solution viscosity was observed, enhancing the flexibility of the subsequent film. Pour 5g of the homogeneous composite solution into a 15cm diameter glass petri dish and agitate to ensure the film spreads evenly on the surface, achieving the desired thickness and smoothness. Through these steps, a film with moisturizing properties and high emissivity spectral characteristics can be obtained, suitable for various subsequent applications.

[0082] The relevant tests and results are as follows:

[0083] FT-IR ( Figure 5 Tests showed that PVA and TiO2-PVA films exhibited significant stretching vibration peaks at 1090 cm⁻¹ and 1045 cm⁻¹, which are typically associated with the stretching vibrations of CO or CC. These characteristic peaks are determined by the presence of hydroxyl (-OH) and carbon-oxygen (CO) bonds in the PVA molecule; the vibrations of these functional groups exhibit strong absorption characteristics in the infrared region. These groups readily form broad absorption bands, especially in the ultraviolet and visible light bands (part of the solar band). Enhanced molecular absorption reduces the film's reflection of incident light, while its low refractive index and surface smoothness further suppress reflection behavior; therefore, PVA films have low reflectivity. The preservation effect of PVA films on winter peaches is significantly lower than that of TiO2-PVA films. Figure 8 To improve its reflectivity in the solar spectrum, additional nanoparticles need to be added. After two days of storage, the weight loss of the winter peach wrapped in PVA film was 14.38±1.81%, which was significantly higher than that of TiO2-PVA film (p<0.05).

[0084] Comparative Example 2

[0085] A method for preparing polyvinyl alcohol (PVA) films specifically includes the following steps:

[0086] Weigh 2g of newly purchased polyvinyl alcohol (PVA) and add it to 20g of deionized water. Heat the mixture to 90℃ and stir at 1000rpm for 1 hour. Agglomeration was observed in the solution. Figure 9 As shown, from Figure 9Analysis revealed that high-speed stirring significantly increased the shear force in the solution, causing excessive stretching and re-entanglement of the PVA polymer chains under intense stirring conditions. Due to the strong hydrogen bonding between PVA molecules, this entanglement further led to localized aggregation of molecular chains. The process parameters in Example 1—a lower stirring rate—allowed for more complete unfolding and dissolution of the PVA molecular chains, reducing chain entanglement and resulting in a more homogeneous solution system, which is beneficial for film formation using solution casting.

[0087] Comparative Example 3

[0088] A method for preparing a composite large film based on polyvinyl alcohol (PVA) and titanium dioxide (TiO2) nanoparticles specifically includes the following steps:

[0089] Weigh 2g of freshly purchased polyvinyl alcohol (PVA) and add it to 20g of deionized water. Heat the mixture to 90℃ and stir at 500rpm for 1 hour until the PVA is completely dissolved, forming a homogeneous polymer solution. Add 1.4g of glycerol as a plasticizer to the mother liquor and continue stirring at 500rpm for 1 hour to ensure uniform dispersion of the plasticizer. A decrease in solution viscosity was observed, enhancing the flexibility of the subsequent film. Add 20g of titanium dioxide (TiO2) nanoparticles with a diameter of 550nm to the mother liquor, maintain the temperature at 90℃, and continue stirring at 500rpm for 1 hour to ensure thorough mixing of the titanium dioxide nanoparticles with the polymer matrix, forming a homogeneous composite solution. Take 5g of the uniformly mixed composite solution and pour it into a glass petri dish to form a film. The radiation-cooled film is shown below. Figure 10 As shown.

[0090] With increasing TiO2 content, the film's ductility decreases significantly, leading to reduced flexibility and increased brittleness. This degradation in mechanical properties limits the film's processability and practical applications, causing it to be unable to withstand tensile or bending stresses during operation, affecting production line continuity and finished product quality. Therefore, it is necessary to optimize the TiO2 content while balancing its impact on mechanical and optical properties to ensure the film possesses sufficient toughness to meet the requirements of industrial production.

[0091] Meanwhile, by comparing Example 1, Comparative Example 1, and Comparative Example 2, it can be seen that polyvinyl alcohol and titanium dioxide have a synergistic effect in this invention, which can synergistically improve the relevant properties of the prepared radiation-cooling film. In particular, when the mass ratio of polyvinyl alcohol to titanium dioxide is 1:5, it can significantly synergistically improve the relevant properties of the prepared radiation-cooling film.

[0092] Meanwhile, by comparing Example 1 and Comparative Examples 1 to 3, it can be seen that there is a synergistic effect between the amount of titanium dioxide added in this invention, which can synergistically improve the relevant properties of the prepared radiation-cooling film. In particular, when the mass ratio of polyvinyl alcohol to titanium dioxide is 1:5, it can significantly synergistically improve the relevant properties of the prepared radiation-cooling film.

[0093] Although embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the scope of the invention is not limited to the contents disclosed in the embodiments.

Claims

1. A radiation-cooling thin film, characterized in that: Its composition and weight percentages are as follows: 2 parts polyvinyl alcohol; 1.4 parts glycerin; 10 parts of nano-titanium dioxide, nano-barium sulfate, or nano-silica particles; 20 parts deionized water; The titanium dioxide is TiO2 nanoparticles with a diameter of 550 nm; The method for preparing the radiation-cooling thin film as described above includes the following steps: First, polyvinyl alcohol is dissolved in deionized water, heated to 90 °C and stirred for 1 hour to form a homogeneous solution; then, glycerol is added as a plasticizer and stirring is continued to reduce the viscosity; next, nano-titanium dioxide, nano-barium sulfate, or nano-silica particles are added, and the temperature is kept constant within the range of 50 °C-90 °C while stirring to ensure uniform mixing; finally, a radiation-cooling film is obtained. The uniform mixing process specifically involves uniformly combining inorganic micro / nano particles and polyvinyl alcohol through a chemical dissolution and mixing or physical blending step. Alternatively, the method for preparing the radiation-cooling thin film as described above includes the following steps: First, polyvinyl alcohol is dissolved in deionized water, heated to 90 °C and stirred for 1 hour to form a homogeneous solution. Then, glycerol is added as a plasticizer, and stirring is continued to reduce the viscosity. Next, nano-titanium dioxide, nano-barium sulfate, or nano-silica particles are added, and the temperature is kept constant within the range of 50 °C-90 °C while stirring to ensure uniform mixing. Finally, the mixture is placed at -20 °C for freeze-thaw cycles to form a uniform gel film, ultimately obtaining a radiation cooling film with excellent performance.

2. The application of the radiation cooling film as described in claim 1 in the preservation of fruits and vegetables.

Images