Porous radiation cooling film and preparation method thereof

Abstract

According to the porous radiation cooling film provided by the invention, polyurethane is used as a matrix, cellulose acetate and a phosphorus flame retardant are doped, the thickness of the film is 150-200 microns, the pore diameter of the film is 0.2-2.3 microns, and the porosity is 50-62%. The CA is added into the PU material to improve the overall radiation cooling performance of the material, by adjusting the mass ratio of the CA, it is found that the mechanical property of the PU-CA is reduced and the optical property is remarkably improved along with the increase of the mass ratio of the CA, and the flame retardance of the material is greatly improved by adding the DOPO; by adjusting the proportion of the polyurethane, the cellulose acetate and the DOPO, the advantages of the polyurethane, the cellulose acetate and the DOPO are well fused, meanwhile, the defects of the polyurethane, the cellulose acetate and the DOPO are well overcome, the flame retardance is achieved, the solar reflectivity is high by 98.2%, the infrared emissivity is high by 94.3%, the mechanical performance is excellent, and the tensile strength reaches 13.2 MPa.

Description

Technical Field

[0001] The present invention relates to the technical field of functional film materials, and in particular to a porous radiation cooling film and a preparation method thereof. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance understanding of the overall background of the invention and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to those skilled in the art.

[0003] Passive radiation cooling technology has become a research hotspot for replacing traditional refrigeration methods due to its energy-saving and environmentally friendly characteristics. The surface of the radiation cooling material has extremely high reflectivity in the solar spectrum of 0.3 to 2.5 μm, and has high emissivity in the infrared band of 8 to 13 μm in the atmospheric transparent window, which can achieve zero energy consumption and pollution-free cooling.

[0004] At present, porous radiation cooling materials have attracted widespread attention due to their easy processing, low cost and excellent cooling performance. So far, many porous radiation cooling materials have been applied, such as porous polyvinylidene fluoride-co-hexafluoropropylene PVDF-HFP, porous polytetrafluoroethylene, and porous polymethyl methacrylate PMMA. Generally, most porous materials improve their radiation cooling performance by increasing the porosity, but the increase in porosity often reduces the mechanical properties of the material. In addition, most of these radiation cooling materials are not flame retardant and cannot be used in some special cases. Therefore, it is necessary to prepare porous radiation cooling materials with excellent mechanical properties and flame retardancy.

[0005] Polyurethane (PU) is widely used in various fields, including construction, clothing, and daily necessities, due to its excellent mechanical properties and stability. In recent years, radiative cooling polyurethane-based materials have been widely reported. The pores in these polyurethanes are prepared using various methods, such as phase separation, particle blending, and electrospinning. However, the PU-based materials prepared by these methods do not exhibit high radiative cooling performance and lack flame retardancy. Summary of the Invention

[0006] In order to overcome the above problems, the present invention provides a porous radiation cooling film and a preparation method thereof.

[0007] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0008] The first aspect of the present invention is a porous radiative cooling film, which is composed of a polyurethane matrix doped with cellulose acetate and a phosphorus-based flame retardant, and has a thickness of 150-200 μm;

[0009] The pore size of the film is 0.2-2.3 μm, and the porosity is 50-62%;

[0010] Preferably, the pore size of the film is 0.8-1 μm and the porosity is 58%.

[0011] In one or more embodiments, the phosphorus-based flame retardant is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide.

[0012] In one or more embodiments, the polyurethane is obtained by reacting the following raw materials: component A constituting a hard segment structure and component B constituting a soft segment structure;

[0013] The component A comprises isophorone diisocyanate and 1,4-bis(2-hydroxyethoxy)benzene as a chain extender;

[0014] The component B comprises polycarbonate diol and OP550 as a chain extender;

[0015] The molar ratio of polycarbonate diol, 1,4-bis(2-hydroxyethoxy)benzene, isophorone diisocyanate and OP550 is 3-15:15-27:36:6.

[0016] A second aspect of the present invention provides a method for preparing the porous radiative cooling film according to the first aspect, comprising the following steps:

[0017] (1) preparing polyurethane material, comprising the following steps:

[0018] The polycarbonate diol, OP550, and 1,4-bis(2-hydroxyethoxy)benzene are vacuum dried, the vacuum-dried polycarbonate diol and OP550 are placed in a reactor, a solvent and a catalyst are added, and then isophorone diisocyanate is slowly added dropwise to the reactor containing the polycarbonate diol and OP550, the temperature is raised to react, and then 1,4-bis(2-hydroxyethoxy)benzene and the catalyst are sequentially added to react, and a polyurethane material is obtained after drying;

[0019] (2) Preparing a PU@CA radiative cooling film, comprising the following steps:

[0020] Dissolving polyurethane in a mixed solvent of acetone and N,N-dimethylformamide, mixing uniformly, adding cellulose acetate and a phosphorus-based flame retardant, and heat-treating the mixture while stirring;

[0021] Then it is poured into a mold, the resulting film is dried, then placed in a water bath, and finally dried to obtain the PU@CA radiative cooling film.

[0022] In one or more embodiments, in step (1), the molar ratio of polycarbonate diol, 1,4-bis(2-hydroxyethoxy)benzene, isophorone diisocyanate, and OP550 is 3-15:15-27:36:6. The ratio of the polyurethane soft segment to the hard segment affects the mechanical properties of the polyurethane. Within this range, the resulting polyurethane has good mechanical properties.

[0023] Preferably, the molar ratio of polycarbonate diol, 1,4-bis(2-hydroxyethoxy)benzene, isophorone diisocyanate, and OP550 is 15:15:36:6, 10:20:36:6, 5:25:36:6, or 3:27:36:6. Within this usage range, the mechanical properties of the obtained polyurethane are better.

[0024] More preferably, the molar ratio of polycarbonate diol, 1,4-bis(2-hydroxyethoxy)benzene, isophorone diisocyanate and OP550 is 5:25:36:6. In order to further improve the mechanical properties of the polyurethane, the mechanical properties of the obtained polyurethane are best within this range.

[0025] In one or more embodiments, in step (1), the vacuum drying is carried out at a vacuum degree of 2.5 MPa and at 100-120° C. for 2-3 hours, preferably at 110° C. for 2 hours. When preparing the polyurethane material, polycarbonate diol, OP550, and 1,4-bis(2-hydroxyethoxy)benzene are vacuum dried because isophorone diisocyanate reacts easily with water.

[0026] In step (1), the temperature of the reactor is raised to 70-90° C. and the reaction is carried out for 0.5-1 h, preferably 80° C. and the reaction is carried out for 0.5 h.

[0027] In step (1), 1,4-bis(2-hydroxyethoxy)benzene and a catalyst are added in sequence to react at a temperature of 70 to 90° C. for 6 to 7 hours, preferably at 80° C. for 6 hours.

[0028] In step (1), the drying step is performed at 60° C. for 3 h.

[0029] In one or more embodiments, in step (1), the solvent is N,N-dimethylformamide. N,N-dimethylformamide is used during the reaction to reduce viscosity.

[0030] In step (1), the catalyst is dibutyltin dilaurate.

[0031] In one or more embodiments, in step (2), the volume ratio of acetone to N,N-dimethylformamide is 1-4:1-2. The volume ratio of acetone to N,N-dimethylformamide affects the pore size and porosity of the PU@CA film. Different pore sizes have different scattering efficiencies. Within this range, the resulting PU@CA film has a pore size of 0.2-2.3 μm and a porosity of 50-62%.

[0032] Preferably, the volume ratio of acetone to N,N-dimethylformamide is 1:1, 3:2, 3:1 or 4:1.

[0033] More preferably, the volume ratio of acetone to N,N-dimethylformamide is 3: 1. Pores with a size of 0.8-1 μm have the highest scattering efficiency for wavelengths of 400-780 nm. Within this range, the pore size of the film is 0.8-1 μm and the porosity is 58%.

[0034] In one or more embodiments, in step (2), the phosphorus-based flame retardant is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide;

[0035] In step (2), the mass ratio of polyurethane, cellulose acetate, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 3.3-5.1:0.9-2.7:0.48-1.08. Within this range, the resulting PU@CA film exhibits good radiative cooling performance, mechanical properties, and flame retardancy.

[0036] Preferably, the mass ratio of polyurethane, cellulose acetate, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 4.5:1.5:0.78, 3.3:2.7:0.78, 3.9:2.1:0.78, 5.1:0.9:0.78, 4.5:1.5:0.48, or 4.5:1.5:1.08. Within this range, the resulting PU@CA film exhibits improved radiative cooling performance, mechanical properties, and flame retardancy.

[0037] The preferred mass ratio of polyurethane, cellulose acetate, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 4.5:1.5:0.78. This range is ideal for further improving the radiative cooling, mechanical, and flame retardancy properties of the PU@CA film.

[0038] In one or more embodiments, in step (2), the heat treatment is performed by heating the mixture to 60-80° C. and stirring for 8-10 minutes, preferably 60° C. and stirring for 10 minutes, to rapidly dissolve the cellulose acetate and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide.

[0039] In step (2), the drying is carried out at 60-70° C. for 15-20 minutes, preferably at 60° C. for 20 minutes. The acetone is volatilized by drying, so that the material network is initially formed.

[0040] In step (2), the water bath is kept for 20-30 minutes, preferably 20 minutes. The non-benign solvent water is used to displace DMF to form a porous structure.

[0041] In step (2), the drying temperature is 60-70° C., preferably 60° C. The drying removes moisture to produce the final material.

[0042] The third aspect of the present invention provides the use of the porous radiative cooling film described in the first aspect or the porous radiative cooling film prepared by the preparation method described in the second aspect, for use in radiative cooling materials and building materials.

[0043] The beneficial effects of the present invention are:

[0044] The present invention constructs a porous radiation cooling film, and realizes a porous structure by volatilizing a low-boiling point solvent and replacing it with a high-boiling point solvent. The porous structure enhances reflectivity and emissivity through internal scattering. The volume ratio of the low-boiling point solvent to the high-boiling point solvent affects the size and distribution of the pores. The low-boiling point solvent evaporates to form initial pores, and the high-boiling point solvent is replaced by the non-benign solvent water to optimize the pore distribution and connectivity. The structure evolves from "coarse pores" to "fine pores", achieving a pore size of 0.2-2.3μm and a porosity of 50-62% in the film; CA is added to the PU material to improve The material's overall radiative cooling performance is high. CA's high refractive index and good ductility increase the scattering interface within the composite, resulting in more sunlight being reflected and less absorbed. CA's extinction coefficient in the solar wavelength range is negligible, and its addition does not significantly increase absorption, but rather enhances reflectivity through scattering. The CO-C bonds in CA exhibit a distinct vibrational peak in the mid-infrared region of 8-13 μm, increasing the composite's overall mid-infrared emissivity and thus enhancing its thermal radiation capacity. Therefore, the combination of the porous structure and CA synergistically enhances radiative cooling performance. Due to the excellent mechanical properties and structural stability of the PU in the material, the PU@CA film exhibits remarkable flexibility. Even after repeated crumpling 100 times, the film's reflectivity only decreases by 2%, demonstrating excellent durability and adaptability to more complex applications. Thanks to the porous spectral response, the film achieves both aesthetically pleasing color and cooling performance. The addition of DOPO significantly improves the material's flame retardancy. DOPO decomposes at high temperatures to produce phosphorus-containing free radicals. These active substances effectively capture the H· and OH· free radicals produced during combustion, interrupting the combustion chain reaction and significantly reducing the flame propagation rate. DOPO decomposes to produce inert gases, which reduce oxygen concentration and the ratio of combustible gas to oxygen, further inhibiting flame spread. Furthermore, the porous radiative cooling film prepared using this method effectively combines the advantages of polyurethane, cellulose acetate, and DOPO by adjusting the ratio of these three materials, while significantly improving their defects. This results in a film with high mechanical properties, good radiative cooling performance, and flame retardancy.

[0045] The porous radiative cooling film provided by the present invention has a high solar reflectivity of 98.2%, a high infrared emissivity of 94.3% and excellent mechanical properties, with a tensile strength of 13.2 MPa. In addition, in the outdoor temperature test of the material, 900W / m -2 Under sunlight radiation, PU@CA can effectively cool down the environment by 15.6℃, with a cooling efficiency of 148W / m 2 In addition, good flame retardancy and color-binding properties are conducive to its application as a building material, achieving high efficiency and energy saving. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0047] Figure 1 This is the SEM image of the PU@CA radiative cooling film prepared in Example 7;

[0048] Figure 2 This is a flow chart for preparing PU@CA of the present invention;

[0049] Figure 3 This is the Fourier infrared spectrum of the polyurethane prepared in Example 3;

[0050] Figure 4 The stress-strain curves of the polyurethanes prepared in Examples 1-4 are shown;

[0051] Figure 5 SEM images and pore size distribution diagrams of the PU@CA radiative cooling films prepared in Examples 5-8; Figure 5 a is the SEM image and pore size distribution of the PU@CA radiative cooling film prepared in Example 5; Figure 5 b is the SEM image and pore size distribution of the PU@CA radiative cooling film prepared in Example 6; Figure 5 c is the SEM image and pore size distribution of the PU@CA radiative cooling film prepared in Example 7; Figure 5 d is the SEM image and pore size distribution of the PU@CA radiative cooling film prepared in Example 8;

[0052] Figure 6 Graphs showing the reflectivity and stress-strain curves of the PU@CA radiative cooling films prepared in Examples 5-8; Figure 6 f is the reflectivity graph of the PU@CA radiative cooling film prepared in Examples 5-8; Figure 6 g is the stress-strain curve of the PU@CA radiative cooling film prepared in Examples 5-8;

[0053] Figure 7 The figure is a simulation calculation diagram of the scattering efficiency of different apertures in different bands;

[0054] Figure 8 The stress-strain curves and reflectivity graphs of the PU@CA radiative cooling films prepared in Example 7, Examples 9-13, and Comparative Example 4 are shown; Figure 8 a is a stress-strain curve of the PU@CA radiative cooling films prepared in Example 7 and Examples 9-11; Figure 8 b is a stress-strain curve of the PU@CA radiative cooling films prepared in Example 7, Example 12, Example 13, and Comparative Example 4; Figure 8 c is the reflectivity graph of the PU@CA radiative cooling films prepared in Example 7 and Examples 9-11; Figure 8 d is the reflectivity graph of the PU@CA radiative cooling films prepared in Example 7, Example 12, Example 13, and Comparative Example 4;

[0055] Figure 9 The reflectivity and emissivity graph of the PU@CA radiative cooling film prepared in Example 7, the characterization graph of the mechanical properties of the film, and the white object graph; Figure 9 h Characterization diagram of the mechanical properties of the PU@CA radiative cooling film prepared in Example 7; Figure 9 i White physical image of the PU@CA radiative cooling film prepared in Example 7; Figure 9 j is the reflectivity and emissivity graph of the PU@CA radiative cooling film prepared in Example 7;

[0056] Figure 10 Comparison of stress and reflectivity of the PU@CA radiative cooling film prepared in Example 7 and other materials;

[0057] Figure 11 This is a study on the daytime radiative cooling performance of the PU@CA radiative cooling film prepared in Example 7; Figure 11 a is a schematic diagram of the temperature test for practical daytime radiative cooling performance evaluation; Figure 11 b is the actual temperature picture of practical daytime radiation cooling performance evaluation; Figure 11 c is the real-time temperature change of the material on a sunny day; Figure 11 d is the solar radiation intensity and humidity conditions during the temperature test; Figure 11 e is the theoretical cooling power of PU@CA; Figure 11 f is the infrared thermal image of the film and white paper under sunlight;

[0058] Figure 12 To explore the flame retardant properties of PU@CA; Figure 12 a is the limiting oxygen index diagram of Example 1 and Example 12 at different DOPO ratios; Figure 12 b is the flame retardant test graph of PU@CA of Example 7 and Comparative Example 4;

[0059] Figure 13 The PU@CA radiative cooling film prepared in Example 7 has stable structure and color rendering; Figure 13 a is the folding resistance test of the film; Figure 13 b is the reflectivity after folding; Figure 13 c is a photo of color PU@CA; Figure 13 d is the reflectivity of colored PU@CA. DETAILED DESCRIPTION

[0060] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0061] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0062] Raw materials and reagents

[0063] Polycarbonate diol (PCDL, Mn = 2000) was purchased from Wuhan Lanbai Pharmaceutical Chemical Co., Ltd. Isophorone diisocyanate (IPDI) was purchased from Ron Reagent (China). 1,4-Bis(2-hydroxyethoxy)benzene (HQEE), cellulose acetate (CA), and N,N-dimethylformamide (DMF) were provided by Shanghai MacLean Biochemical Co., Ltd. Exolit OP550, analytical grade, was purchased from Clariant Chemicals Co., Ltd. Dibutyltin dilaurate (DBTDL) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was purchased from Shanghai MacLean Biochemical Co., Ltd. Acetone was purchased from Yantai Far East Fine Chemical Co., Ltd.

[0064] Example 1

[0065] The preparation of polyurethane (PU) material includes the following steps:

[0066] Polycarbonate diol PCDL, OP550, and 1,4-bis(2-hydroxyethoxy)benzene HQEE were vacuum dried and dried at 110°C for 2h. 15mmol of PCDL and 6mmol of OP550 after vacuum drying were placed in a reactor, and 10mL of N,N-dimethylformamide DMF and 50μL of dibutyltin dilaurate DBTDL were added. Then, 36mmol of isophorone diisocyanate IPDI was slowly added dropwise to the reactor with polycarbonate diol PCDL and OP550. The reactor temperature was raised to 80°C and reacted at this temperature for 0.5h. Then, 15mmol of 1,4-bis(2-hydroxyethoxy)benzene HQEE and 50μL of dibutyltin dilaurate DBTDL were added in sequence to react for 6h, and then dried at 60°C for 3h to obtain a polyurethane PU material.

[0067] Example 2

[0068] Polycarbonate diol PCDL, OP550, and 1,4-bis(2-hydroxyethoxy)benzene HQEE were vacuum dried and dried at 110°C for 2h. 10mmol of PCDL and 6mmol of OP550 after vacuum drying were placed in a reactor, and 10mL of N,N-dimethylformamide DMF and 50μL of dibutyltin dilaurate DBTDL were added. Then, 36mmol of isophorone diisocyanate IPDI was slowly added dropwise to the reactor with polycarbonate diol PCDL and OP550. The reactor temperature was raised to 80°C and reacted at this temperature for 0.5h. Then, 20mmol of 1,4-bis(2-hydroxyethoxy)benzene HQEE and 50μL of dibutyltin dilaurate DBTDL were added in sequence for reaction. The reaction was carried out for 6h, and then dried at 60°C for 3h to obtain a polyurethane PU material.

[0069] Example 3

[0070] Polycarbonate diol PCDL, OP550, and 1,4-bis(2-hydroxyethoxy)benzene HQEE were vacuum dried and dried at 110°C for 2h. 5mmol of PCDL and 6mmol of OP550 after vacuum drying were placed in a reactor, and 10mL of N,N-dimethylformamide DMF and 50μL of dibutyltin dilaurate DBTDL were added. Then, 36mmol of isophorone diisocyanate IPDI was slowly added dropwise to the reactor with polycarbonate diol PCDL and OP550. The reactor temperature was raised to 80°C and reacted at this temperature for 0.5h. Then, 25mmol of 1,4-bis(2-hydroxyethoxy)benzene HQEE and 50μL of dibutyltin dilaurate DBTDL were added in sequence for reaction. The reaction was carried out for 6h, and then dried at 60°C for 3h to obtain a polyurethane PU material.

[0071] Example 4

[0072] Polycarbonate diol PCDL, OP550, and 1,4-bis(2-hydroxyethoxy)benzene HQEE were vacuum dried and dried at 110°C for 2h. 3mmol of PCDL and 6mmol of OP550 after vacuum drying were placed in a reactor, and 10ml of N,N-dimethylformamide DMF and 50μL of dibutyltin dilaurate DBTDL were added. Then, 36mmol of isophorone diisocyanate IPDI was slowly added dropwise to the reactor containing polycarbonate diol PCDL and OP550. The reactor temperature was raised to 80°C and reacted at this temperature for 0.5h. Then, 27mmol of 1,4-bis(2-hydroxyethoxy)benzene HQEE and 50μL of dibutyltin dilaurate DBTDL were added in sequence for reaction. The reaction was carried out for 6h and then dried at 60°C for 3h. After drying, a polyurethane PU material was obtained.

[0073] Polycarbonate diol is used as the soft segment of polyurethane, and the rigid structured HQEE is used as a chain extender to improve the mechanical strength of polyurethane. Phosphorus-containing Exolit OP550 is introduced into the molecular chain to improve flame retardancy.

[0074] In order to verify the successful synthesis of polyurethane, the material prepared in Example 3 was tested by Fourier transform infrared spectroscopy. Figure 3 As shown, the isocyanate (-N=C=O) peak is detected at 2230-2270 cm -1 The disappearance of the isocyanate peak in the FTIR spectrum proves the successful synthesis of PU.

[0075] By adjusting the ratio of soft segment raw materials to hard segment raw materials, the effects of different components on the mechanical properties of polyurethane were explored. A series of PUs were synthesized by adjusting the molar ratio of PCDL and HQEE, such as Figure 4 As shown in the figure, as the molar ratio of PCDL to HQEE decreases, the stress first increases and then decreases. When the molar ratio of PCDL to HQEE is 1:5, the stress is 35.5 MPa. Therefore, the PU-based radiative cooling material prepared in Example 3 was selected as the raw material for the PU@CA radiative cooling films of Examples 5-13 and Comparative Examples 1-7.

[0076] Example 5

[0077] The preparation of PU@CA radiative cooling film includes the following steps:

[0078] 4.5 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio of 1:1) and mixed well. 1.5 g of cellulose acetate and 0.78 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0079] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0080] Example 6

[0081] The preparation of PU@CA radiative cooling film includes the following steps:

[0082] 4.5 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio 3:2), mixed well, and then 1.5 g of cellulose acetate and 0.78 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0083] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0084] Example 7

[0085] The preparation of PU@CA radiative cooling film includes the following steps:

[0086] 4.5 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio 3:1), mixed well, and then 1.5 g of cellulose acetate and 0.78 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0087] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0088] Example 8

[0089] The preparation of PU@CA radiative cooling film includes the following steps:

[0090] 4.5 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio 4:1), mixed well, and then 1.5 g of cellulose acetate and 0.78 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0091] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0092] Example 9

[0093] The preparation of PU@CA radiative cooling film includes the following steps:

[0094] 3.3 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio 3:1), mixed well, and then 2.7 g of cellulose acetate and 0.78 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0095] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0096] Example 10

[0097] The preparation of PU@CA radiative cooling film includes the following steps:

[0098] 3.9 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio 3:1) and mixed well. 2.1 g of cellulose acetate and 0.78 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0099] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0100] Example 11

[0101] The preparation of PU@CA radiative cooling film includes the following steps:

[0102] 5.1 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio 3:1), mixed well, and then 0.9 g of cellulose acetate and 0.78 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0103] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0104] Example 12

[0105] The preparation of PU@CA radiative cooling film includes the following steps:

[0106] 4.5 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio 3:1), mixed well, and then 1.5 g of cellulose acetate and 0.48 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0107] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0108] Example 13

[0109] The preparation of PU@CA radiative cooling film includes the following steps:

[0110] 4.5 g of polyurethane was dissolved in 20 ml of a mixed solvent of acetone and DMF (volume ratio 3:1), mixed well, and then 1.5 g of cellulose acetate and 1.08 g of flame retardant DOPO were added. The mixture was heated to 60 °C and stirred for 10 minutes.

[0111] Then it is poured into a mold, and the obtained film is dried at 60°C for 20 minutes, then in a water bath for 20 minutes, and finally dried at 60°C to obtain the PU@CA radiation cooling film.

[0112] Comparative Example 1

[0113] Compared with Example 7, the volume ratio of acetone to DMF was 5:1, and other conditions were the same.

[0114] Comparative Example 2

[0115] Compared with Example 7, the volume ratio of acetone to DMF is 1:3, and other conditions are the same.

[0116] Comparative Example 3

[0117] Compared with Example 7, no CA was added, and other conditions were the same.

[0118] Comparative Example 4

[0119] Compared with Example 7, DOPO was not added, and other conditions were the same.

[0120] Comparative Example 5

[0121] Compared with Example 7, the mass ratio of PU, CA and DOPO is 2.7:3.3:0.78.

[0122] Comparative Example 6

[0123] Compared with Example 7, the mass ratio of PU, CA and DOPO is 5.7:0.3:0.78.

[0124] Comparative Example 7

[0125] Compared with Example 7, the mass ratio of PU, CA and DOPO is 4.5:1.5:1.38.

[0126] The present invention prepares PU@CA films through a simple and rapid solvent-nonsolvent exchange method using two solvents with different boiling points (acetone: 56°C, N,N-dimethylformamide (DMF): 152.8°C). The rapid evaporation of acetone at 60°C leads to the initial formation of the material skeleton, and the remaining DMF is replaced by water. As a result, the PU@CA film quickly precipitates in water, forming a porous structure.

[0127] like Figure 7 As shown in the figure, by simulating the scattering efficiency of different pore sizes, it was found that the pores with a size range of 0.8-1μm have the highest scattering efficiency for wavelengths of 400-780nm. The size and distribution of pores have a significant impact on the mechanical and optical properties of the material PU@CA film, such as Figure 5As shown in Figure 5, when PU@CA films were prepared by mixing acetone with DMF at different volume ratios of 1:1, 3:2, 3:1, and 4:1, the pore size and porosity decreased with decreasing DMF ratio.

[0128] like Figure 6 As shown in Figure 5, in the reflectivity test of films with different solvent ratios, as the DMF ratio decreases, the reflectivity first increases and then decreases. It is observed that the reflectivity first increases, reaches a maximum value at a ratio of 3:1 between acetone and DMF, and then decreases as the DMF ratio decreases. Furthermore, the reflectivity in Comparative Examples 1 and 2 is lower than that in Example 1.

[0129] Regarding mechanical properties, such as Figure 6 As shown in Figure g, the tensile strength decreases with increasing DMF ratio. Furthermore, the tensile strength in Comparative Example 1 is higher than that in Example 1, while the tensile strength in Comparative Example 2 is lower than that in Example 1. Considering the reflectivity and tensile strength of the film, a film prepared with acetone and DMF in a ratio of 3:1 was selected for further study.

[0130] The ratio of PU to CA is closely related to the overall performance of the material. To explore the effect of CA and ensure that the PU@CA film has excellent mechanical properties, tensile tests were conducted. The DOPO content remained unchanged at 0.78g and the ratio of PU to CA was adjusted. The study found that as the CA content decreased from 2.7g to 0.9g, the stress of the porous material increased from 6.9MPa to 14.2MPa, and the strain increased from 25% to 41% ( Figure 8 a), indicating that the addition of CA impairs the tensile strength of the material. For Comparative Example 3, the porous material without CA had a stress of 20 MPa and a strain of 110%. For Comparative Example 5, when the CA content was 3.3 g, the porous material had a stress of 5 MPa and a strain of 15%. For Comparative Example 6, when the CA content was 0.3 g, the porous material had a stress of 18 MPa and a strain of 90%.

[0131] Then, the content of PU and CA remained unchanged, the PU content was 4.5g, and the CA content was 1.5g, and the DOPO content in the material was adjusted. Through the tensile test, it was found that as the DOPO content increased from 0 to 1.08g, the tensile strength decreased from 14.9MPa to 7.8MPa ( Figure 8 b) The DOPO content of Comparative Example 7 was 1.38 g, and the tensile strength was 5.3 MPa.

[0132] Next, we explored the effects of CA and DOPO on the optical properties of the material. The DOPO content remained constant at 0.78 g and the ratio of PU to CA was adjusted. The study found that as the CA content decreased, the reflectivity first increased, but when the CA content was too low, the reflectivity also dropped sharply ( Figure 8c). The reflectivity of the porous material in Comparative Example 3 without adding CA was 9.5%, the reflectivity of Comparative Example 5 with a CA content of 3.3g was 98.5%, and the reflectivity of Comparative Example 6 with a CA content of 0.3g was 32%. Then, the PU and CA contents remained unchanged, with the PU content being 4.5g and the CA content being 1.5g, and the DOPO content in the material being adjusted. As the DOPO content increased, the reflectivity of the film first increased and then decreased ( Figure 8 d) The DOPO content of Comparative Example 7 was 1.38 g, and the reflectivity was 90.5%.

[0133] From the above, it can be seen that the ratio of polyurethane, cellulose acetate and DOPO is closely related to the overall performance of the material, especially the optical and mechanical properties. By adjusting the ratio of polyurethane, cellulose acetate and DOPO, the advantages of the three materials of polyurethane, cellulose acetate and DOPO are well integrated, and the defects of polyurethane, cellulose acetate and DOPO are greatly improved, so that the film has high mechanical properties, good radiative cooling performance and flame retardancy. Among them, when the mass ratio of polyurethane, cellulose acetate and DOPO in Example 7 is 4.5:1.5:0.78, the PU@CA film prepared in Example 7 has higher solar reflectivity (98.2%) and tensile strength (13.2 MPa).

[0134] According to the ASTM G173 global solar spectrum standard, the visible light band (400-780nm) accounts for about 50% of the solar radiation intensity. Therefore, adjusting the absorption of the visible light band is an important step in achieving good cooling performance.

[0135] like Figure 9 As shown, the PU@CA radiative cooling film prepared in Example 7 has high solar reflectivity (98.2%), high infrared emissivity (94.3%) ( Figure 9 j), while the PU@CA film can lift an object 30,000 times its own weight ( Figure 9 h).

[0136] like Figure 10 As shown in the figure, a comparison of the stress and reflectivity of the PU@CA radiative cooling film prepared in Example 7 and other material films shows that the PU@CA film prepared in the present invention has higher solar reflectivity (98.2%) and tensile strength (13.2 MPa), showing significant advantages and greater practicality.

[0137] Study on the Daytime Radiative Cooling Performance of PU@CA Film

[0138] Cooling temperature difference and net cooling power are important parameters for evaluating the radiative cooling performance of materials. The cooling temperature of the porous structure under direct sunlight was tested. The temperature test equipment is shown in the figure below. Figure 11As shown in a, an insulating foam box is covered with aluminum foil, which has a mirror-reflective surface to prevent heat conduction. Thermocouples are placed in prefabricated hollow slots in the foam box. The radiative cooling capacity PU@CA is evaluated based on the cooling temperature difference (ΔT = TSample-TBare), where TSample and TBare are the internal box space temperatures with and without sample coverage, respectively. Figure 11 b shows a photo of the field test. Figure 11 As shown in c, the maximum solar irradiance at noon on the test day is 900W / m 2 , humidity is 15%. Real-time recording of cooling temperature. The results show that the temperature of PU@CA can reach 15.6℃ at noon ( Figure 11 d), the theoretical net cooling power (Pnet) is calculated to be 148 W / m using equations S1-S5 in the Supporting Information. 2 ( Figure 11 e). Temperature PU@CA was tested on the film and white paper under sunlight using an infrared thermal imager. After 15 minutes of testing, a cooling effect of 6.4°C can be clearly observed ( Figure 11 f). This indicates that PU@CA has great potential in the field of building cooling. Meanwhile, the porous material prepared in Comparative Example 3 without adding CA has almost no cooling performance.

[0139] Study on the flame retardant properties of PU@CA

[0140] Considering that the proportion of flame retardants will affect the flame retardancy, mechanical properties and optical properties of the material, two materials containing different proportions of flame retardants were tested for limiting oxygen index (LOI). The test results show that when the mass of DOPO in Example 12 is 0.48g, the LOI is 23%, and when the mass ratio in Example 7 is increased to 0.78g, the LOL rises to 26% ( Figure 12 a). By conducting a vertical combustion test, it was observed that the film of Comparative Example 4 without DOPO addition burned violently 5 seconds after ignition and completely burned out after 15 seconds. In contrast, the film of Example 7 did not ignite even after being exposed to the flame of an alcohol lamp for 15 seconds, indicating that the PU@CA film has good flame retardancy ( Figure 12 b). This makes the film safer in practical applications and broadens its potential application scenarios.

[0141] Structural stability and color rendering

[0142] Due to the excellent mechanical properties and structural stability of PU in the material, PU@CA film exhibits excellent flexibility ( Figure 13 a), can adapt to more complex application environments. After 20, 50 and 100 cycles of arbitrary folding and flattening, the reflectivity of the porous material decreased by 0.9%, 1.5% and 2% respectively ( Figure 13b), showing excellent structural stability. Thanks to the spectral response of micropores and nanopores, it can achieve both beautiful colors and cooling performance. By coating with yellow, blue, green and red dyes ( Figure 13 c) PU@CA films exhibit a colorful appearance, and in most cases the reflectivity of the dyed materials exceeds 90% ( Figure 13 d). This indicates that the porous structure can effectively reflect sunlight, while the colored material can compensate for the absorbed portion of solar radiation. The above results indicate that PU@CA will promote its application in practical radiative cooling scenarios.

[0143] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A porous radiation cooling film, characterized in that: It is made of polyurethane as the matrix mixed with cellulose acetate and phosphorus flame retardant, and the thickness of the film is 150-200μm; The pore size of the film is 0.2-2.3 μm, and the porosity is 50-62%; Preferably, the pore size of the film is 0.8-1 μm and the porosity is 58%.

2. The porous radiative cooling film according to claim 1, wherein: The polyurethane is obtained by reacting the following raw materials: component A constituting a hard segment structure and component B constituting a soft segment structure; The component A comprises isophorone diisocyanate and 1,4-bis(2-hydroxyethoxy)benzene as a chain extender; The component B comprises polycarbonate diol and OP550 as a chain extender; The molar ratio of polycarbonate diol, 1,4-bis(2-hydroxyethoxy)benzene, isophorone diisocyanate and OP550 is 3-15:15-27:36:

6.

3. The method for preparing a porous radiative cooling film according to claim 1 or 2, wherein: (1) preparing polyurethane material, comprising the following steps: The polycarbonate diol, OP550, and 1,4-bis(2-hydroxyethoxy)benzene are vacuum dried, the vacuum-dried polycarbonate diol and OP550 are placed in a reactor, a solvent and a catalyst are added, and then isophorone diisocyanate is slowly added dropwise to the reactor containing the polycarbonate diol and OP550, the temperature is raised to react, and then 1,4-bis(2-hydroxyethoxy)benzene and the catalyst are sequentially added to react, and a polyurethane material is obtained after drying; (2) Preparing a PU@CA radiative cooling film, comprising the following steps: Dissolving polyurethane in a mixed solvent of acetone and N,N-dimethylformamide, mixing uniformly, adding cellulose acetate and a phosphorus-based flame retardant, and heat-treating the mixture while stirring; Then it is poured into a mold, the resulting film is dried, then placed in a water bath, and finally dried to obtain the PU@CA radiative cooling film.

4. The preparation method according to claim 3, wherein In step (1), the molar ratio of polycarbonate diol, 1,4-bis(2-hydroxyethoxy)benzene, isophorone diisocyanate and OP550 is 3-15:15-27:36:6; Preferably, the molar ratio of polycarbonate diol, 1,4-bis(2-hydroxyethoxy)benzene, isophorone diisocyanate and OP550 is 15:15:36:6, 10:20:36:6, 5:25:36:6 or 3:27:36:6; More preferably, the molar ratio of polycarbonate diol, 1,4-bis(2-hydroxyethoxy)benzene, isophorone diisocyanate and OP550 is 5:25:36:

6.

5. The preparation method according to claim 3, wherein In step (1), the vacuum drying is carried out at a vacuum degree of 2.5 MPa and at 100-120° C. for 2-3 h, preferably at 110° C. for 2 h; In step (1), the temperature of the reactor is raised to 70-90°C and the reaction is carried out for 0.5-1h, preferably 80°C and the reaction is carried out for 0.5h; In step (1), 1,4-bis(2-hydroxyethoxy)benzene and a catalyst are added in sequence to react at a temperature of 70 to 90° C. for 6 to 7 hours, preferably at 80° C. for 6 hours; In step (1), the drying step is performed at 60° C. for 3 h.

6. The preparation method according to claim 3, wherein In step (1), the solvent is N,N-dimethylformamide; In step (1), the catalyst is dibutyltin dilaurate.

7. The preparation method according to claim 3, wherein In step (2), the volume ratio of acetone to N,N-dimethylformamide is 1-4:1-2; Preferably, the volume ratio of acetone to N,N-dimethylformamide is 1:1, 3:2, 3:1 or 4:1; More preferably, the volume ratio of acetone to N,N-dimethylformamide is 3:

1.

8. The preparation method according to claim 3, wherein In step (2), the phosphorus-based flame retardant is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide; In step (2), the mass ratio of polyurethane, cellulose acetate and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 3.3-5.1:0.9-2.7:0.48-1.08; Preferably, the mass ratio of polyurethane, cellulose acetate and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 4.5:1.5:0.78, 3.3:2.7:0.78, 3.9:2.1:0.78, 5.1:0.9:0.78, 4.5:1.5:0.48 or 4.5:1.5:1.08; Preferably, the mass ratio of polyurethane, cellulose acetate and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 4.5:1.5:0.

78.

9. The preparation method according to claim 3, wherein In step (2), the heat treatment is to heat the mixture to 60-80°C and stir for 8-10 minutes, preferably 60°C and stir for 10 minutes; In step (2), the drying is performed at 60-70°C for 15-20 min, preferably at 60°C for 20 min; In step (2), the water bath is 20-30 min, preferably 20 min; In step (2), the drying temperature is 60-70°C, preferably 60°C.

10. Use of the porous radiative cooling film according to claim 1 or 2 or the porous radiative cooling film prepared by the preparation method according to any one of claims 3 to 9 in radiative cooling materials and building materials.

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