A white composite polyimide radiative cooling film and a preparation method and application thereof

Abstract

The application discloses a kind of white composite polyimide radiative cooling film preparation and application, comprising: in monomer 2,2 '-bis (trifluoromethyl) -4,4 '-diamino diphenyl, 2,2 '-bis (trifluoromethyl) -4,4 '-Diamino diphenyl, 4,4 '- (4,4 '-isopropylidene diphenyloxy) bis (phthalic anhydride) and 4,4 '- (hexafluoroisopropylidene) di-phthalic anhydride, after being fully stirred under ice water bath Reaction forms polyamide acid solution, polyimide polymer is prepared by imidization to polyamide acid solution;The prepared polyimide polymer is redissolved, and is mixed with different proportions 3-aminopropyl triethoxysilane modified zinc oxide particles, stirring is obtained at room temperature White dispersion;After dispersion is cast into film, it is placed in vacuum oven and dried by heating, and finally white composite polyimide radiative cooling film is obtained.The white composite polyimide radiative cooling film prepared by the application has high solar reflectivity and mid-infrared emissivity optical properties, and can achieve efficient temperature control on spacecraft.

Description

Technical Field

[0001] This invention belongs to the field of polymer membrane material preparation technology. More specifically, this invention relates to a white composite polyimide radiation cooling film, its preparation method, and its application. Background Technology

[0002] Since the beginning of the 21st century, aerospace technology has become a crucial support for humankind's daily communication, astronomical observation, meteorological monitoring, and space exploration. With ever-increasing application demands, the development of aerospace technology has become increasingly urgent. In this process, the quality and lifespan of spacecraft are particularly critical, especially for long-term operational space stations and satellites. Thermal management is one of the core challenges ensuring the reliable operation of spacecraft. The thermal management system of a spacecraft's outer surface, exposed to the extreme environment of space, must continuously withstand intense cosmic radiation and extreme temperature cycles; therefore, efficient thermal control is paramount. Because space is a high vacuum, heat exchange between the spacecraft and the external environment can only be achieved through radiation; other heat transfer methods are ineffective. Therefore, it is imperative to employ special radiation cooling materials such as radiation cooling films or coatings to effectively block overheating caused by solar radiation while simultaneously dissipating excess heat generated inside the spacecraft into space through radiation.

[0003] The optical properties of radiation-cooled materials must meet the following requirements: within the solar radiation spectrum range (R... solar High reflectivity is achieved in the mid-infrared band (0.4–2.5 μm) to minimize heat input; simultaneously, high emissivity is maintained in the mid-infrared band (MIR, 3–25 μm), i.e., high emissivity according to Kirchhoff's radiation law, to achieve effective heat dissipation. To achieve the ideal balance between excellent solar reflection and high heat dissipation performance, meticulous optimization of the material system and structural design is required.

[0004] To address the thermal management requirements of spacecraft, traditional radiative cooling materials (such as quartz glass and polytetrafluoroethylene) are no longer sufficient to meet the demands of next-generation missions due to their low heat dissipation efficiency and poor adaptability to the space environment. While current spacecraft thermal control coatings primarily employ organic polymer varnishes containing oxidizing pigments or potassium silicate (K2SiO3)-based inorganic coatings, both systems exhibit significant performance defects. Although K2SiO3-based coatings possess excellent weather resistance, their inherent limitations, such as high porosity, low fracture toughness, and weak adhesion strength, severely restrict their reliability under complex mechanical loads. Simultaneously, traditional organic binders, represented by acrylic resins and polymethyl methacrylate (PMMA), are prone to photodegradation and yellowing under long-term strong ultraviolet radiation and atomic oxygen erosion, leading to a significant increase in solar absorptivity over time and ultimately causing thermal control failure. Therefore, existing binder systems have become a key bottleneck restricting the performance improvement of thermal control coatings for long-life spacecraft. To overcome this limitation, there is an urgent need to develop special polymer matrices with excellent heat resistance, radiation stability, and mechanical strength to replace traditional varnishes and K2SiO3 binders, meeting the stringent environmental requirements of deep space exploration. Polyimide (PI) films have been widely used in the aerospace field due to their excellent radiation stability, flexibility, mechanical and thermal properties, and lightweight characteristics. However, traditional PI films are brownish-yellow due to the presence of intramolecular and intermolecular charge transfer complexes (CTCs), resulting in low visible light transmittance and high solar absorptivity. Their characteristic of converting some incident sunlight into heat contradicts the fundamental goal of radiative cooling technology: maximizing visible light reflection to reduce heat input. In contrast, colorless polyimide (CPI) films significantly improve optical performance, exhibiting high transparency and extremely low solar absorptivity, thus becoming an ideal candidate material for polymer matrices in flexible radiative cooling films (RCF).

[0005] Based on this, a series of white polyimide-based radiation cooling films will be designed and prepared using CPI as the polymer matrix and zinc oxide (ZnO) particles as the functional filler. The goal is to synergistically achieve high solar reflectivity and high-to-medium infrared emissivity. This research can provide a new option for spacecraft radiation cooling materials, and has significant theoretical and engineering application value for improving the thermal control capabilities and service reliability of spacecraft in extreme thermal environments. Summary of the Invention

[0006] One object of the present invention is to solve at least the above-mentioned problems and / or defects, and to provide at least the advantages described below.

[0007] To achieve these and other advantages of the present invention, the present invention provides a white composite polyimide radiation cooling film comprising a blend of polyimide polymer and modified particles; wherein the modified particles are modified ZnO particles (m-ZnO); and the mass ratio of the polyimide polymer to the modified particles is 70~50:30~50.

[0008] Preferably, the thickness of the white composite polyimide radiation cooling film is 200~250 μm.

[0009] A method for preparing a white composite polyimide radiation cooling film as described above includes the following steps: Step 1: Under nitrogen protection, the monomer 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was added to the reaction vessel, followed by N,N'-dimethylacetamide. The mixture was stirred in an ice-water bath. Subsequently, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride were added, and the reaction was continued in an ice-water bath to obtain a polyamic acid solution. Step 2: Add a mixed solution of acetic anhydride and pyridine to the polyamic acid solution and stir the reaction at room temperature. After stirring, pour the homogeneous reaction mixture very slowly into a large amount of methanol. The resulting white fibrous precipitate is repeatedly washed with methanol, collected by filtration, and dried under vacuum to obtain white fibrous polyimide polymer (CPI). Step 3: Add ZnO particles to a mixed solution of ethanol and water, then slowly add 3-aminopropyltriethoxysilane dropwise, stir at room temperature, filter the dispersion, and repeatedly wash the filter cake with ethanol. Place the resulting white solid in a vacuum oven and heat it to dry to obtain modified ZnO particles. Step 4: Redissolve the white fibrous polyimide polymer in N,N'-dimethylacetamide, add modified ZnO particles, and stir continuously at room temperature to obtain a white dispersion. Cast the dispersion onto a glass plate covered with water-based acrylic to form a film, place it in a vacuum oven to heat and dry, and after the film cools to room temperature, immerse it in deionized water for demolding. Then wash the water-based acrylic with deionized water and dry it to finally obtain a white composite polyimide film.

[0010] Preferably, in steps one to four, a magnetic stir bar is added during the stirring process, and the rotation speed of the magnetic stir bar is 500~600 r / min. -1 .

[0011] Preferably, in step one, the ratio of monomers 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, and N,N'-dimethylacetamide is 0.64~1.28 g:0.62~1.24 g:0.36~0.72 g:7.5~15 mL.

[0012] Preferably, in step one, the stirring time in the ice-water bath is 30-60 min, and the reaction time in the ice-water bath is 24-36 h.

[0013] Preferably, in step two, the volume ratio of acetic anhydride to pyridine is 1.9 mL : 0.8 mL, and the volume ratio of the mixed solution of acetic anhydride and pyridine to the polyamic acid solution is 2.7 mL : 7.8 mL.

[0014] Preferably, in step three, the ratio of zinc oxide particles, mixed solution, and 3-aminopropyltriethoxysilane is 5.0~10.0 g: 100~200 mL: 0.10~0.20 mL, and the mixed solution is ethanol and water in a ratio of V... 乙醇 V 水 The modified ZnO particles are composed of a 99:1 mixture with a particle size of 0.3~10 μm. They are soaked in deionized water for 1~3 h and dried in a vacuum oven at a temperature of 40~50 ℃ for 24~36 h.

[0015] Preferably, in step four, the ratio of white fibrous polyimide polymer, modified ZnO particles, and N,N'-dimethylacetamide is 1.0 g : 1.0~2.3 g : 3.2~3.6 mL; the drying temperature in the vacuum oven is 80~90 ℃, the drying time is 16~20 h, the soaking time in deionized water is 12~18 h, and the soaking temperature is 60~80 ℃.

[0016] An application of the white composite polyimide radiation cooling film as described above, wherein the white composite polyimide radiation cooling film is used for thermal management of spacecraft surfaces.

[0017] The present invention has at least the following beneficial effects: (1) This invention employs a molecular structure design and monomer ratio control strategy to synthesize a colorless and transparent polyimide film (CPI) using a trifluoromethyl diamine 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB), a trifluoromethyl dianhydride 4,4'-(hexafluoroisopropylidene) phthalic anhydride (6FDA), and a dianhydride 4,4′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride) (BPADA) with a non-coplanar structure and dual ether bonds. This effectively inhibits the formation of charge transfer complexes (CTCs) within or between molecular chains, reduces light absorption, and improves the transmittance, yellowing resistance, and flexibility of the CPI. This lays a good foundation for the whiteness and flexibility of the subsequently prepared white composite polyimide radiation cooling film.

[0018] (2) Surface modification of white inorganic filler ZnO particles with 3-aminopropyltriethoxysilane (KH550) significantly improves the interfacial compatibility between ZnO particles and the CPI matrix, thereby greatly increasing the ZnO particle loading and promoting the formation of a dense composite network, endowing the composite film with high reflectivity to solar radiation. Simultaneously, multi-scale ZnO particles with a particle size mainly distributed around 0.3 μm are selected to match their size to the solar spectral band, inducing a strong Mie scattering effect and further enhancing reflectivity. Furthermore, ZnO, as a wide-bandgap semiconductor material, has high thermal conductivity, which facilitates the rapid transfer of heat accumulated inside the material to the surface and efficient release to the external space through radiation. These synergistic effects enable the prepared radiative cooling film to possess high solar reflectivity, high-mid-infrared emissivity, and excellent mechanical properties, thus meeting the stringent requirements for efficient thermal control in spacecraft.

[0019] (3) Polyimide polymers are synthesized by chemical imidization. This process can quickly complete the imidization reaction at a lower temperature, effectively avoiding prolonged high-temperature heat treatment, thereby inhibiting the formation of CTCs between molecular chains, significantly reducing the yellowing degree of the film, and improving its optical transparency. The improved transparency is beneficial to enhancing the whiteness of the white composite radiation cooling film and ultimately improving its reflectivity to solar radiation.

[0020] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0022] Figure 1 This is a macroscopic image of the white composite polyimide radiation cooling film of the present invention; Figure 2 This is a schematic diagram of the synthesis route of the white composite polyimide radiation cooling film of the present invention; Figure 3 The infrared spectra of the white composite polyimide radiation cooling films of ZnO, m-ZnO, CPI, Example 2, Comparative Example 1 and Comparative Example 2 in this invention are shown. Figure 4 The XRD patterns of ZnO and m-ZnO in this invention are shown. Figure 5 The XRD patterns are of the white composite polyimide radiation cooling films of m-ZnO, Examples 1 to 3, Comparative Examples 1 and 2 in this invention. Figure 6 The solar reflectance diagrams are for the white composite polyimide radiation cooling films of Examples 1-3, Comparative Examples 1 and 2 of the present invention. Figure 7 The mid-infrared emissivity diagrams are for the white composite polyimide radiation cooling films of Examples 1-3, Comparative Examples 1 and 2 of the present invention. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0025] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof. Example 1 like Figure 2 As shown, this embodiment provides a method for preparing a white composite polyimide radiation cooling film, including the following steps: Step 1: Add TFMB (0.64 g, 2 mmol) and N,N'-dimethylacetamide (DMAc) (3.6 mL) to a 50 mL three-necked flask equipped with a magnetic stirrer and nitrogen protection. Incubate in an ice-water bath (0–5 °C) at a speed of 600 r / min. -1 Stir until TFMB is fully dissolved, then add BPADA (0.62 g, 1.2 mmol) to the solution and continue stirring at 600 rpm. -1 Stir for 30 min. After stirring, add DMAc (3.5 mL) and 6FDA (0.36 g, 0.8 mmol) to the above solution and react for 24 h to form a polyamic acid solution (PAA). Step 2: Add acetic anhydride / pyridine (V) dropwise to the PAA solution obtained in Step 1. 乙酸酐 V 吡啶 A mixed solution of 1.9 mL:0.8 mL was prepared and continuously rotated at 600 r / min. -1 After vigorous stirring for 12 h, the chemical imidization process was completed. The homogeneous reaction mixture was then very slowly poured into a large amount of ethanol. The resulting white fibrous precipitate was repeatedly washed with ethanol, filtered, collected, and dried at 80 °C for 12 h under vacuum to obtain CPI polymer. Step 3: In a 250 mL round-bottom flask equipped with a magnetic stirrer, add 5.0 g of ZnO granules, and then add ethanol and water (V). 乙醇 V 水 In a mixed solution of 99 mL:1 mL, KH550 (0.1 mL) was added, and the mixture was heated at room temperature and a rotation speed of 600 r / min. -1The mixture was stirred for 6 h. After the reaction was fully stirred, the mixture was filtered and the filter cake was repeatedly washed with ethanol. The solid was then dried under vacuum at 50°C for 24 h to obtain m-ZnO particles. Step 4: Redissolve 1.0 g of the dried white fibrous CPI polymer obtained in Step 2 in DMAc (3.2 mL) and stir at room temperature for 12 h. Then add 1.0 g of m-ZnO particles obtained in Step 3 to the above CPI solution and stir at room temperature and a stirring speed of 600 r / min. -1 A white dispersion was obtained by continuous stirring for 3 hours. The white dispersion was cast into a film on a glass plate covered with water-based acrylic acid. The film was kept at 80 °C under vacuum for 10 hours. After the film cooled to room temperature, it was immersed in deionized water for demolding. The water-based acrylic acid was then washed with deionized water and dried to obtain a white composite polyimide film.

[0026] The white composite polyimide radiation cooling film prepared in this embodiment is designated as Cm-50ZnO / CPI.

[0027] Example 2 like Figure 2 As shown, this embodiment provides a method for preparing a white composite polyimide radiation cooling film, including the following steps: Step 1: Add TFMB (0.64 g, 2 mmol) and DMAc (3.6 mL) to a 50 mL three-necked flask equipped with a magnetic stirrer and nitrogen protection. Incubate in an ice-water bath (0–5 °C) at a speed of 600 r / min. -1 Stir until TFMB is fully dissolved, then add BPADA (0.62 g, 1.2 mmol) to the solution and continue stirring at 600 r / min. -1 Stir for 30 min. After stirring, add DMAc (3.5 mL) and 6FDA (0.36 g, 0.8 mmol) to the above solution and react for 24 h to form PAA solution. Step 2: Add acetic anhydride / pyridine (V) dropwise to the PAA solution obtained in Step 1. 乙酸酐 V 吡啶 A mixed solution of 1.9 mL and 0.8 mL was prepared and continuously rotated at 600 r / min. -1 After vigorous stirring for 12 h, the chemical imidization process was completed. The homogeneous reaction mixture was then very slowly poured into a large amount of methanol. The resulting white fibrous precipitate was repeatedly washed with methanol, filtered, collected, and dried at 80 °C for 12 h under vacuum. Step 3: In a 250 mL round-bottom flask equipped with a magnetic stirrer, add 5.0 g of ZnO granules, and then add ethanol and water (V). 乙醇 V 水 In a mixed solution of 99 mL:1 mL, KH550 (0.1 mL) was added, and the mixture was heated at room temperature and a rotation speed of 600 r / min. -1 The mixture was stirred for 6 h. After the reaction was fully stirred, the mixture was filtered and the filter cake was repeatedly washed with ethanol. The solid was then dried under vacuum at 50°C for 24 h to obtain m-ZnO particles. Step 4: Redissolve 1.0 g of the dried white fibrous CPI polymer obtained in Step 2 in DMAc (3.2 mL), stir at room temperature for 12 h, then add 1.5 g of m-ZnO particles obtained in Step 3 to the above CPI solution and stir at room temperature and 600 r / min. -1 The mixture was stirred continuously for 3 hours to obtain a white dispersion. The white dispersion was then cast into a film on a glass plate covered with water-based acrylic acid. The film was kept at 80 °C under vacuum for 10 hours. After the film cooled to room temperature, it was immersed in deionized water for demolding. The water-based acrylic acid was then washed with deionized water and dried to obtain a white composite polyimide film.

[0028] The white composite polyimide radiation cooling film prepared in this experiment is designated as Cm-60ZnO / CPI.

[0029] Example 3 like Figure 2 As shown, this embodiment provides a method for preparing a white composite polyimide radiation cooling film, including the following steps: Step 1: Add TFMB (0.64 g, 2 mmol) and DMAc (3.6 mL) to a 50 mL three-necked flask equipped with a magnetic stirrer and nitrogen protection. Incubate in an ice-water bath (0–5 °C) at a speed of 600 r / min. -1 Stir until TFMB is fully dissolved, then add BPADA (0.62 g, 1.2 mmol) to the solution and continue stirring at 600 r / min. -1 Stir for 30 min. After stirring, add DMAc (3.5 mL) and 6FDA (0.36 g, 0.8 mmol) to the above solution and react for 24 h to form PAA solution. Step 2: Add acetic anhydride / pyridine (V) dropwise to the PAA solution obtained in Step 1. 乙酸酐 V 吡啶 A mixed solution of 1.9 mL and 0.8 mL was prepared and continuously rotated at 600 r / min. -1After vigorous stirring for 12 h, the chemical imidization process was completed. The homogeneous reaction mixture was then very slowly poured into a large amount of methanol. The resulting white fibrous precipitate was repeatedly washed with methanol, filtered, collected, and dried at 80 °C for 12 h under vacuum to obtain CPI polymer. Step 3: In a 250 mL round-bottom flask equipped with a magnetic stirrer, add 5.0 g of ZnO granules, and then add ethanol and water (V). 乙醇 V 水 In a mixed solution of 99 mL:1 mL, KH550 (0.1 mL) was added, and the mixture was stirred at room temperature and a rotation speed of 600 rpm. -1 The mixture was stirred for 6 h. After the reaction was fully stirred, the mixture was filtered and the filter cake was repeatedly washed with ethanol. The solid was then dried under vacuum at 50°C for 24 h to obtain m-ZnO particles. Step 4: Redissolve 1.0 g of the dried white fibrous CPI polymer obtained in Step 2 in DMAc (3.2 mL) and stir at room temperature for 12 h. Then add 2.34 g of m-ZnO particles obtained in Step 3 to the above CPI solution and stir at room temperature and a stirring speed of 600 r / min. -1 The mixture was stirred continuously for 3 hours to obtain a white dispersion. The white dispersion was then cast into a film on a glass plate covered with water-based acrylic acid. The film was then kept at 80 °C under vacuum for 10 hours. After the film cooled to room temperature, it was immersed in deionized water for demolding. The water-based acrylic acid was then washed with deionized water and dried to obtain a white composite polyimide film.

[0030] The white composite polyimide radiation cooling film prepared in this experiment is designated as Cm-70ZnO / CPI.

[0031] Comparative Example 1 This comparative example provides a method for preparing a white composite polyimide radiation cooling film, including the following steps: Step 1: Add TFMB (0.64 g, 2 mmol) and DMAc (3.6 mL) to a 50 mL three-necked flask equipped with a magnetic stirrer and nitrogen protection. Incubate in an ice-water bath (0–5 °C) at a speed of 600 r / min. -1 Stir until TFMB is fully dissolved, then add BPADA (0.62 g, 1.2 mmol) to the solution and continue stirring at 600 r / min. -1 Stir for 30 min. After stirring, add DMAc (3.5 mL) and 6FDA (0.36 g, 0.8 mmol) to the above solution and react for 24 h to form PAA solution. Step 2: Pour the PAA solution obtained in Step 1 onto a dry and clean glass plate to cast a film. Then, maintain the film in an oven at 80°C for 10 h, at 100°C for 1 h, at 110°C for 1 h, at 140°C for 1 h, at 170°C for 1 h, at 200°C for 2 h, at 270°C for 1 h, and at 300°C for 1 h. This thermal imidization process yields a CPI film. After the film cools to room temperature, immerse it in deionized water for demolding, wash it with deionized water, and then dry it in an oven at 60°C for 2 h. Step 3: In a 250 mL round-bottom flask equipped with a magnetic stirrer, add 5.0 g of ZnO granules, and then add ethanol and water (V). 乙醇 V 水 In a mixed solution of 99 mL:1 mL, KH550 (0.1 mL) was added, and the mixture was stirred at room temperature for 6 h. After stirring the mixture thoroughly, the mixture was filtered and the filter cake was repeatedly washed with ethanol. The solid was then dried under vacuum at 50 °C for 24 h to obtain m-ZnO particles. Step 4: Redissolve 1.0 g of the dried CPI membrane obtained in Step 2 in DMAc (3.2 mL) and stir at room temperature for 12 h. Simultaneously, add 2.34 g of m-ZnO granules obtained in Step 3 to the above CPI solution and stir at room temperature and a stirring speed of 600 r / min. -1 The mixture was stirred continuously for 3 hours to obtain a white dispersion. The white dispersion was then cast into a film on a glass plate covered with water-based acrylic acid. The film was kept at 80 °C under vacuum for 12 hours. After the film cooled to room temperature, it was immersed in deionized water for demolding. The water-based acrylic acid was then washed with deionized water and dried to obtain a white composite polyimide radiation cooling film.

[0032] The white composite polyimide radiation cooling film prepared in this experiment is designated as Tm-70ZnO / CPI.

[0033] Compared to Example 3, Comparative Example 1 used a thermal imidization method to prepare the CPI film, while Example 3 used a chemical imidization method to prepare the CPI solid. The chemical imidization method is completed quickly at a lower temperature, avoiding a prolonged high-temperature process and reducing the chance of CTCs forming in the molecular chains at high temperatures. This suppresses yellowing, improves transparency, and ultimately enhances the whiteness of the white composite polyimide radiation cooling film, thus affecting its solar reflectivity performance. In contrast, the thermal imidization method, through a gradient heating process, requires maintaining a high temperature for a longer period. The high temperature and the presence of oxygen (even under nitrogen protection, trace amounts of oxygen may still exist) can cause thermal oxidation and discoloration on the surface or inside the film. Furthermore, the molecular chains are more prone to close packing at high temperatures, enhancing intramolecular and intermolecular CTC effects, leading to a darker film color and decreased transmittance, thereby further affecting the solar reflectivity performance of the white composite polyimide radiation cooling film.

[0034] Comparative Example 2 This comparative example provides a method for preparing a white composite polyimide radiation cooling film, including the following steps: Step 1: Add TFMB (0.64 g, 2 mmol) and DMAc (3.6 mL) to a 50 mL three-necked flask equipped with a magnetic stirrer and nitrogen protection. Incubate in an ice-water bath (0–5 °C) at a speed of 600 r / min. -1 Stir until TFMB is fully dissolved, then add BPADA (0.62 g, 1.2 mmol) to the solution and continue stirring at 600 r / min. -1 Stir for 30 min. After stirring, add DMAc (3.5 mL) and 6FDA (0.36 g, 0.8 mmol) to the above solution and react for 24 h to form PAA solution. Step 2: Add acetic anhydride / pyridine (V) dropwise to the PAA solution obtained in Step 1. 乙酸酐 V 吡啶 A mixed solution of 1.9 mL and 0.8 mL was prepared and continuously rotated at 600 r / min. -1 After vigorous stirring for 12 h, the chemical imidization process was completed. The homogeneous reaction mixture was then very slowly poured into a large amount of methanol. The resulting white fibrous precipitate was repeatedly washed with methanol, filtered, collected, and dried at 80 °C for 12 h under vacuum to obtain CPI polymer. Step 3: Redissolve 1.0 g of the dried white fibrous CPI polymer obtained in Step 2 in DMAc (3.2 mL), stir at room temperature for 12 h, and simultaneously add 1.5 g of pure zinc oxide particles to the above CPI solution. Stir at room temperature and 600 r / min. -1A white dispersion was obtained by continuous stirring for 3 hours. The white dispersion was then cast into a film on a glass plate covered with water-based acrylic acid. The film was then kept at 80 °C under vacuum for 10 hours. After the film cooled to room temperature, it was immersed in deionized water for demolding. The water-based acrylic acid was then washed with deionized water and dried to obtain a white composite polyimide film.

[0035] The white composite polyimide radiation cooling film prepared in this experiment is designated as C-60ZnO / CPI.

[0036] The chemical structures of KH550, m-ZnO, CPI, Examples 1-3, and Comparative Examples 1-2 were characterized using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray diffraction (XRD) spectroscopy. Figure 3 The ATR-FTIR spectra of KH550, m-ZnO, CPI, Experimental Examples 1-3, and Comparative Examples 1-2 are shown. All spectra are within 2940 cm⁻¹. -1 and 2840 cm -1 The peak at 1080 cm⁻¹ exhibits a characteristic CH stretching vibration peak. For KH550, this peak is at 1080 cm⁻¹. -1 The absorption peak at 954 cm⁻¹ is attributed to the stretching vibration of the Si-O-Si bond, a typical characteristic of the siloxane network formed after the hydrolysis and condensation of the silane coupling agent. Compared with the original KH550, the absorption peak at 954 cm⁻¹ in the m-ZnO spectrum is significantly higher. -1 The Si-OC stretching vibration band disappears at 890 cm⁻¹. -1 A new absorption peak appears at 500 cm⁻¹. This new peak originates from the Si-O-Zn chemical bond formed by the condensation reaction between KH550 and the hydroxyl groups on the ZnO surface. Simultaneously, a new absorption peak appears at 500 cm⁻¹. -1 The Zn-O stretching vibration peaks that appeared further confirmed the success of the ZnO particle surface modification.

[0037] For a pure CPI membrane, 1790 cm -1 and 1720 cm -1 The nearby absorption peaks are attributed to the asymmetric and symmetric stretching vibrations of the C=O bond in the imide ring, respectively. (1368 cm⁻¹) -1 and 717 cm -1 The peak at [location] corresponds to the CN stretching and bending vibrations within the imide ring. Additionally, at approximately 1237 cm⁻¹... -1 The characteristic absorption peak at 500 cm⁻¹ is attributed to the stretching vibration of the CF bond, confirming the successful introduction of the fluorinated structural unit. The spectra of Experimental Examples 1-3 and Comparative Examples 1-2 all retain the characteristic absorption peak of CPI, and the peak at 500 cm⁻¹ is also present. -1A new Zn-O peak appeared at approximately 1080 cm⁻¹, indicating that ZnO was introduced without disrupting the chemical structure of the polyimide matrix. Notably, compared to Comparative Example 2, the spectra of Examples 1-3 and Comparative Example 1 showed significant spectral changes. At approximately 1080 cm⁻¹... -1 A strong and broad absorption peak was observed, attributed to the overlap of stretching vibrations of Si-O-Si and Si-O-Zn bonds, confirming the formation of a covalent cross-linked network between the inorganic ZnO and the organic CPI matrix. In summary, these results confirm the successful preparation of a series of white composite polyimide radiation cooling films.

[0038] Figure 4 The XRD spectra of ZnO and m-ZnO are shown. All diffraction peaks in the figure can be easily attributed to ZnO with an orthorhombic crystal structure (PDF:98-000-0483). It can be seen that the phase composition and structure of ZnO did not change after surface treatment with silane coupling agent KH550.

[0039] Figure 5 The XRD spectra of CPI, m-ZnO, Experimental Examples 1-3, and Comparative Examples 1-2 are shown. The main diffraction peak of CPI is located at approximately 20°, corresponding to its amorphous ordered structure. For Experimental Examples 1-3 and Comparative Examples 1-2, the intensity changed to some extent. Except for the peak at 20°, all other peaks correspond to the peaks of m-ZnO. No other new or stray peaks were observed, indicating that the ZnO crystal form was not destroyed during preparation and that the crystal phase stability was good. Compared to pure CPI, Experimental Examples 1-3 and Comparative Examples 1-2 showed peak broadening and decreased crystallinity at 20°, attributed to the interfacial interaction between a large number of ZnO particles and CPI. The uniform dispersion of ZnO particles in the matrix restricted the orderly arrangement of CPI molecular chains, making it difficult to form an ordered crystal structure and thus hindering its crystallization process. Among them, the characteristic peak intensity of ZnO in Comparative Example 2 was significantly lower than that in Experimental Examples 1-3 and Comparative Example 1, and the peak shape was slightly broadened, indicating that the ZnO particles were poorly dispersed in the CPI matrix and there was a large amount of agglomeration, which ultimately led to a decrease in peak intensity and a decrease in apparent crystallinity.

[0040] The optical properties of the white composite polyimide radiation cooling films prepared in each embodiment and comparative example were tested using the following methods: The optical properties of the thin film were measured using a UV / Vis / NIR spectrophotometer and a Fourier transform infrared spectrometer equipped with a gold-plated integrating sphere. The wavelengths measured were 0.4–2.5 μm and 3–25 μm, respectively, with the film thickness controlled between 200 and 250 μm. The average solar reflectance of the film was calculated using the following formula (1-1): in, λIt is the wavelength of sunlight, ranging from 0.3 to 2.5 μm; I solar (λ) It is the global solar intensity spectrum (AM0). R solar (λ) This is the spectral reflectance of the film. The formula for calculating the average mid-infrared emissivity is shown in equation (1-2) below: in, This represents the spectral thermal emissivity of the surface in the range of 3~25 μm.

[0041] The solar reflectance of the white composite polyimide radiation cooling film of Examples 1-3, Comparative Example 1, and Comparative Example 2 was tested as follows: Figure 6 As shown, the average solar reflectance measurements were: 77.9% (Example 1), 81.2% (Example 2), 86.6% (Example 3), 80.9% (Comparative Example 1), and 83.9% (Comparative Example 2). For Examples 1-3, the reflectance of samples with different filler contents significantly increased with the increase of m-ZnO particle content. This is because the particle size of m-ZnO particles matches the wavelength range of the solar spectrum (especially visible and near-infrared light), which can induce a strong Mie scattering effect. Moreover, as a white inorganic filler, it has extremely low absorption in the visible light band, providing excellent intrinsic reflectivity for the composite film. At the same time, with the increase of ZnO particle content, the number of scattering units inside the composite film increases, the scattering path of light inside the film is lengthened, and the probability of multiple, disordered scattering is greatly increased, significantly enhancing the reflection effect and ultimately exhibiting superior solar reflectance. It should be noted that the average solar reflectance of Example 2 is lower than that of Comparative Example 2. This may be because the unmodified ZnO particles agglomerate in the CPI matrix. The relatively large particles help to build a multi-scale structure in the film, thereby achieving backscattering of light more effectively and ultimately improving the solar reflectance.

[0042] The mid-infrared emissivity of the white composite polyimide radiation cooling films of Examples 1-3, Comparative Examples 1 and 2 was measured using a Fourier transform infrared spectrometer equipped with a gold-plated integrating sphere (results are shown in Figure 1). Figure 7 The average mid-infrared emissivity in the infrared atmospheric window (3-25 μm) The percentages of the m-ZnO filler were 89.1% (Example 1), 89.2% (Example 2), 90% (Example 3), 90.3% (Comparative Example 1), and 90% (Comparative Example 2), respectively. The test results show that in Examples 1-3, as the mass fraction of m-ZnO filler increased, the interface ratio between ZnO and the CPI matrix increased, leading to an increase in the number of vibrations of polar functional groups. Therefore, the infrared emissivity in the samples gradually increased from 89.1% to 90%. With a fixed filler content, Comparative Example 2, with the same ZnO particle doping amount, showed better infrared emissivity than Experimental Example 2. This is because the interface between unmodified ZnO and the CPI matrix is ​​poorer, resulting in more interphase gaps and microscopic defects, which promotes the radiative loss of infrared energy. When the filler ratio remains unchanged and the thermal imide process is used for preparation, the mid-infrared emissivity of the membrane sample is 90.3%. Compared with the chemical imide method with mild reaction conditions, high-temperature thermal curing can easily cause the polyimide molecular chains to become loosely packed, increasing the number of micropores and random structures in the membrane, which further improves the mid-infrared emissivity.

[0043] It is worth noting that, at the same composite level of 60 wt%, Comparative Example 2 exhibits superior optical performance compared to Experimental Example 2, but its ZnO doped particle content cannot be further increased. Figure 1 In the C-61ZnO / CPI mixture, increasing the content to 61% resulted in film rupture. Only surface-modified ZnO particles could achieve higher filler contents. When the modified filler content increased to 70 wt%, Experimental Example 3 exhibited the best optical performance among all samples. However, 70 wt% was also the maximum composite level achievable by this modified system; further increasing the filler content led to a sharp decline in the mechanical properties of the film, and even cracking failure. Figure 1 (Cm-71ZnO / CPI). Therefore, the optimization of optical performance is essentially a trade-off between filler dispersibility, interface effects, and mechanical properties, and 70 wt% is the critical threshold under this comprehensive balance.

[0044] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0045] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A white composite polyimide radiation cooling film, characterized in that, It includes a blend of polyimide polymer and modified particles; the modified particles are modified ZnO particles; the mass ratio of the polyimide polymer to the modified particles is 70~50:30~50.

2. The white composite polyimide radiation cooling film as described in claim 1, characterized in that, The thickness of the white composite polyimide radiation cooling film is 200~250 μm.

3. A method for preparing a white composite polyimide radiation cooling film as described in any one of claims 1 to 2, characterized in that, Includes the following steps: Step 1: Under nitrogen protection, the monomer 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was added to the reaction vessel, followed by N,N'-dimethylacetamide. The mixture was stirred in an ice-water bath. Subsequently, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride) and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride were added, and the reaction was continued in an ice-water bath to obtain a polyamic acid solution. Step 2: Add a mixed solution of acetic anhydride and pyridine to the polyamic acid solution and stir the reaction at room temperature. After stirring, pour the homogeneous reaction mixture very slowly into a large amount of methanol. The resulting white fibrous precipitate is repeatedly washed with methanol, collected by filtration, and dried under vacuum to obtain white fibrous polyimide polymer. Step 3: Add zinc oxide particles to the mixed solution of ethanol and water, and then slowly add 3-aminopropyltriethoxysilane dropwise. Stir the reaction at room temperature, filter the mixed dispersion, and wash the filter cake repeatedly with ethanol. Place the resulting white solid in a vacuum oven and heat it to dry, to obtain 3-aminopropyltriethoxysilane modified zinc oxide particles, i.e. modified ZnO particles. Step 4: Redissolve the white fibrous polyimide polymer in N,N'-dimethylacetamide, add modified ZnO particles, and stir continuously at room temperature to obtain a white dispersion. Cast the dispersion onto a glass plate covered with water-based acrylic to form a film, place it in a vacuum oven to heat and dry, and after the film cools to room temperature, immerse it in deionized water for demolding. Then wash the water-based acrylic with deionized water and dry it to finally obtain a white composite polyimide film.

4. The method for preparing the white composite polyimide radiation cooling film as described in claim 3, characterized in that, In steps one through four, a magnetic stir bar is added during the stirring process, and the magnetic stir bar rotates at a speed of 500-600 r / min. -1 .

5. The method for preparing the white composite polyimide radiation cooling film as described in claim 3, characterized in that, In step one, the ratio of monomers 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 4,4'-(4,4'-isopropylidenediphenoxy)bis(phthalic anhydride), 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, and N,N'-dimethylacetamide is 0.64~1.28 g:0.62~1.24 g:0.36~0.72 g:7.5~15 mL.

6. The method for preparing the white composite polyimide radiation cooling film as described in claim 3, characterized in that, In step one, the stirring time in the ice-water bath is 30-60 min, and the reaction time in the ice-water bath is continued for 24-36 h.

7. The method for preparing the white composite polyimide radiation cooling film as described in claim 3, characterized in that, In step two, the volume ratio of acetic anhydride to pyridine is 1.9 mL : 0.8 mL, and the volume ratio of the mixed solution of acetic anhydride and pyridine to the polyamic acid solution is 2.7 mL : 7.8 mL.

8. The method for preparing the white composite polyimide radiation cooling film as described in claim 3, characterized in that, In step three, the ratio of zinc oxide particles, mixed solution, and 3-aminopropyltriethoxysilane is 5.0~10.0 g: 100~200 mL: 0.10~0.20 mL, and the mixed solution is ethanol and water in a ratio of V... 乙醇 V 水 The modified ZnO particles are composed of a 99:1 mixture with a particle size of 0.3~10μm. They are soaked in deionized water for 1~3 h, dried in a vacuum oven at a temperature of 40~50 ℃ for 24~36 h.

9. The method for preparing the white composite polyimide radiation cooling film as described in claim 3, characterized in that, In step four, the ratio of white fibrous polyimide polymer, modified ZnO particles, and N,N'-dimethylacetamide is 1.0 g : 1.0~2.3 g : 3.2~3.6 mL; the drying temperature in the vacuum oven is 80~90 ℃, the drying time is 16~20 h, the soaking time in deionized water is 12~18 h, and the soaking temperature is 60~80 ℃.

10. An application of the white composite polyimide radiation cooling film as described in claim 1, characterized in that, The white composite polyimide radiation cooling film is used for thermal management of spacecraft surfaces.

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