A porous PVDF radiative cooling material, a preparation method and application thereof

Abstract

The application discloses a kind of porous PVDF radiative cooling materials and preparation method and application thereof, at 20~28 ℃ temperature, the PVDF / PEO nanofiber membrane prepared using electrospinning process is immersed in the mixed solution of deionized water and ethanol (the volume ratio of ethanol is 55~70%) and is handled to obtain porous PVDF radiative cooling material;The content of beta crystal form in the prepared porous PVDF radiative cooling material is not higher than 90.5%;The absorbance of the porous PVDF radiative cooling material with the thickness of 150 μm in the wavelength range of ultraviolet-visible light is 0.01~0.02, the average reflectivity is 95~99.9%, the average emissivity in the wavelength range of 8~13 μm is 88~92%, and the cooling amplitude measured in indoor simulation cooling test and outdoor cooling performance test is 4~10 ℃.The method of the application can solve the problem that the color of the fiber changes after pore forming treatment, thereby greatly reducing the cooling effect.

Description

Technical Field

[0001] This invention belongs to the field of radiation cooling materials technology, specifically relating to a porous PVDF radiation cooling material and its preparation method. Background Technology

[0002] With economic development, total energy consumption has gradually increased, leading to excessive carbon emissions and increasingly severe air pollution, which directly affects climate change. In the coming decades, climate change may lead to extreme heat waves, greatly increasing the global demand for cooling energy.

[0003] Against this backdrop, passive radiative cooling, as an effective and renewable method to reduce cooling energy demand, offers a viable solution to alleviate the growing global cooling needs. Radiative cooling is widely used in various fields, including energy-efficient buildings, photovoltaic cooling, energy harvesting, and personal thermal management. The advantage of passive radiative cooling is that it can achieve cooling without any additional energy input. This passive cooling mechanism has the potential to dissipate excess heat from the Earth into space, potentially slowing glacial melting, reducing water evaporation to protect the lives and property of coastal residents and meet daily water needs, and even reducing energy consumption for personal cooling.

[0004] In recent years, "personal thermal management" has become a research hotspot for scholars as a more efficient and energy-saving solution, with continuous innovation in its materials and structures. To ensure the wearability of textiles, such as moisture permeability and breathability, this problem can be solved by modifying the surface and internal structure of textile fibers. The multi-scale disordered porous structure of fibers helps achieve high transparency of infrared radiation and backscattering of visible light in the human body, and pores of different scales can maximize the reflection of solar radiation and enhance the dissipation of heat radiation from the human body. At the same time, the porous structure of fibers can also significantly improve the water absorption and breathability of textiles. Therefore, constructing fibers with porous structures can be an effective solution for personal cooling.

[0005] Due to its inherent properties, PVDF exhibits excellent chemical stability, possessing advantages such as acid and alkali resistance, oxidation resistance, and aging resistance. Furthermore, its mid-infrared emissivity reaches as high as 87%, and its ultraviolet reflectivity can exceed 90% when the fiber thickness is 100 μm, endowing PVDF nanofiber membranes with excellent cooling performance. Currently, there is a wealth of research data on PVDF-based radiative cooling materials. However, most of these materials suffer from several drawbacks, such as large fiber thickness, poor moisture and air permeability, unstable fiber structure, and low ultraviolet reflectivity.

[0006] Meanwhile, since existing processes for preparing porous PVDF nanofiber membranes are mostly used for adsorption and filtration, color changes are not considered. For example, the porous PVDF nanofiber membrane prepared in Reference 1 (Preparation of porous PVDF nanofibers by electrospinning and its application in oil absorption [J]. Journal of Materials Science and Engineering, 2022, 40(01): 148-153.) has a finished product image, which can be seen with the naked eye as pale yellow; the processing temperature range of PVDF prepared in Reference 2 (Transparent PVDF film and special material. Jiaxing Gaozheng New Material Technology Co., Ltd., Zhejiang Province, 2019-05-22.) is relatively narrow, and the thermo-oxidative stability during the processing is poor, making it very easy to yellow.

[0007] In reality, changes in fiber color can adversely affect reflectivity in the ultraviolet-visible wavelength range. Therefore, it is necessary to propose a solution to address the problem that changes in fiber color affect the reflectivity in the ultraviolet-visible wavelength range of porous PVDF nanofiber membranes, thus leading to a decrease in cooling effect. This would enable the development of a highly efficient radiative cooling material. Summary of the Invention

[0008] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0009] In view of the problems existing in the above and / or prior art, the present invention is proposed.

[0010] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing porous PVDF radiative cooling material.

[0011] To solve the above-mentioned technical problems, the present invention provides the following technical solution: including,

[0012] PVDF and PEO are dissolved in DMF, stirred and mixed, and then degassed by ultrasonication to obtain a PVDF / PEO solution.

[0013] PVDF / PEO nanofiber membranes were prepared by electrospinning PVDF / PEO solution.

[0014] After vacuum drying, the PVDF / PEO nanofiber membrane is immersed in a mixed solution of deionized water and ethanol for pore formation treatment, thus obtaining a porous PVDF radiation cooling material.

[0015] In the mixed solution of deionized water and ethanol, the volume percentage of ethanol is 55% to 70%.

[0016] In a preferred embodiment of the preparation method of the porous PVDF radiative cooling material of the present invention, the mass fraction of the PVDF / PEO solution is 8-14%, and the mass ratio of PVDF to PEO is 1:1-5.

[0017] In a preferred embodiment of the preparation method of the porous PVDF radiation cooling material of the present invention, the stirring temperature is 75-85°C and the stirring time is 6-8 hours.

[0018] In a preferred embodiment of the preparation method of the porous PVDF radiation cooling material of the present invention, the ultrasonic power of the ultrasonic degassing is 60-100W and the ultrasonic time is 30-60min.

[0019] In a preferred embodiment of the preparation method of the porous PVDF radiative cooling material of the present invention, the electrospinning process is characterized by a spinning voltage of 14–22 kV, a spinning flow rate of 0.1–0.9 mL / h, and a spinning distance of 18–21 cm.

[0020] In a preferred embodiment of the preparation method of the porous PVDF radiation cooling material of the present invention, the vacuum drying temperature is 55-65°C and the time is 10-12 hours.

[0021] In a preferred embodiment of the preparation method of the porous PVDF radiation cooling material of the present invention, the pore-forming treatment temperature is 20-28°C and the treatment time is 2-8 hours.

[0022] Another objective of this invention is to provide a porous PVDF radiative cooling material.

[0023] As a preferred embodiment of the porous PVDF radiative cooling material of the present invention, the porous PVDF radiative cooling material has a pore size range of 20 nm to 4 μm and a β crystal content of <90.5%.

[0024] As a preferred embodiment of the porous PVDF radiative cooling material of the present invention, the porous PVDF radiative cooling material has the following characteristics: when the thickness of the porous PVDF radiative cooling material is 150 μm, the absorbance in the ultraviolet-visible wavelength range is 0.01 to 0.02, the average reflectance is 95 to 99.9%, and the average emissivity in the wavelength range of 8 to 13 μm is 88 to 92%.

[0025] Another objective of this invention is to provide an application of porous PVDF radiative cooling material in the preparation of thermally comfortable fabrics and building materials with hydrophobic properties.

[0026] Beneficial effects of this invention:

[0027] (1) The method of the present invention can solve the problem that the fiber color changes after the pore forming process, resulting in a significant reduction in the cooling effect. The porous PVDF radiation cooling material can achieve high ultraviolet reflectivity and reflectivity with a small material thickness.

[0028] (2) If the porous PVDF radiation cooling material prepared by the method of the present invention is used as a fabric, its multi-level porous structure (including micron pores between fibers and nanopores on a single fiber) can improve the thermal comfort and wearability of the fabric. If it is used in the outer layer of a building, the radiation cooling material has good hydrophobic properties. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0030] Figure 1 This is a scanning electron microscope image of the PVDF / PEO nanofiber membrane prepared in Example 4 of the present invention.

[0031] Figure 2 This is a scanning electron microscope image of the porous PVDF / PEO nanofiber membrane prepared in Example 4 of the present invention.

[0032] Figure 3 The image shows the DTG diagrams of the PVDF / PEO nanofiber membrane and the porous PVDF / PEO nanofiber membrane prepared in Example 4 of this invention.

[0033] Figure 4 This is a reflection data diagram of the porous PVDF / PEO nanofiber membrane prepared in Example 4 of the present invention.

[0034] Figure 5 This is an infrared emission data diagram of the porous PVDF / PEO nanofiber membrane prepared in Example 4 of the present invention.

[0035] Figure 6 This is a contact angle diagram of the porous PVDF / PEO nanofiber membrane prepared in Example 4 of the present invention.

[0036] Figure 7 The images show scanning electron microscope (SEM) images of porous PVDF nanofibers with different treatment solution ratios according to the present invention.

[0037] Figure 8 The infrared spectra of porous PVDF nanofibers under different treatment liquid ratios according to the present invention are shown.

[0038] Figure 9The UV-Vis absorption spectra of porous PVDF nanofiber membranes obtained at different treatment temperatures when the treatment solution is deionized water and anhydrous ethanol in a volume ratio of 1:2.

[0039] Figure 10 The morphology of porous PVDF nanofiber membranes obtained at different treatment temperatures when the treatment solution is pure deionized water is shown in the following images: (A) SEM image a. 25℃; b. 40℃; c. 60℃; d. 80℃; (B) Optical image.

[0040] Figure 11 The images show the surface morphology of the fiber before and after pore formation when the treatment solution is pure deionized water. Detailed Implementation

[0041] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0042] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0043] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0044] The testing / calculation methods used in this invention are as follows:

[0045] β-crystal content in porous PVDF radiative cooling materials:

[0046] According to the literature (Research progress on performance optimization of PVDF piezoelectric films [J]. Piezoacoustics and Acousto-optics, 2021, 43(4): 542-549.), the calculation formula is:

[0047]

[0048] In the formula: X α and X β The α and β crystal forms are respectively located at 764 cm⁻¹ -1 and 840cm -1 Crystallinity at K; α =6.1×10 4 cm 2 / mol, K β =7.7×104 cm 2 / mol, where A represents the absorption coefficients of the α and β crystal form absorption peaks, respectively; α and A β The α and β crystal forms are respectively located at 764 cm⁻¹ -1 and 840cm -1 The absorption peak intensity at that location;

[0049] Average reflectance:

[0050] In this experiment, the UV3600 UV-Vis-NIR spectrophotometer was used to measure the UV reflectance of the nanofiber membrane, and PTFE was used as a diffuse reflectance standard white plate.

[0051] The temperature measurement method in this invention is as follows:

[0052] The prepared membrane or standard cotton is placed under an artificial light source, and the temperature of the area covered by the membrane or standard cotton is tested. The temperature sensor is placed under the membrane or standard cotton to measure the temperature directly.

[0053] Indoor simulated cooling test:

[0054] Apply 1000W / m respectively 2 2000W / m 2 3000W / m 2 The irradiance of the light source;

[0055] Outdoor simulated cooling test:

[0056] The test was conducted on a sunny, windy day in summer and another in winter. In the outdoor cooling performance test, during the summer season, the average solar intensity was 2539 W / m². 2 When the test season is winter, the average solar intensity is 978 W / m². 2 ;

[0057] Unless otherwise specified, all raw materials used in this invention are commercially available in the art. Specifically:

[0058] The correspondence between the raw materials used in this invention and their Chinese names is as follows:

[0059] PVDF: Polyvinylidene fluoride

[0060] PEO: Polyethylene oxide

[0061] DMF: N,N-dimethylformamide

[0062] The manufacturer of PVDF is Arkema, with a value of Mw = 700,000; the manufacturer of PEO is Shanghai McLean Biochemical Technology Co., Ltd., with a value of Mw = 300,000.

[0063] This invention uses a pure white cotton fabric that has been desized, bleached, and contains no finishing agents as a control cotton, with a fabric weight of (110±5) g / m². 2 Whiteness: Y 10 =89±2, W 10 =78±3,T 10 =1±1.

[0064] Example 1

[0065] This embodiment provides a method for preparing porous PVDF radiative cooling material, specifically as follows:

[0066] 1) Mix PVDF and PEO at a mass ratio of 1:2 and dissolve them in DMF. Stir and mix at 75°C for 8 hours, and then degas at an ultrasonic power of 80W for 30 minutes to obtain a PVDF / PEO mixed solution with a mass fraction of 10%.

[0067] 2) PVDF / PEO mixed solution was electrospun with a spinning voltage of 18kV, a spinning flow rate of 0.5mL / h, and a spinning distance of 18cm to obtain PVDF / PEO nanofiber membrane.

[0068] 3) After vacuum drying the PVDF / PEO nanofiber membrane at 55°C for 12 hours, it is immersed in a mixed solution of deionized water and anhydrous ethanol with a volume ratio of 1:2. The membrane is then subjected to pore-forming treatment at 25°C for 4 hours to obtain a porous PVDF nanofiber membrane, which is the porous PVDF radiation cooling material of this embodiment.

[0069] Example 2

[0070] The difference between this embodiment and Embodiment 1 is that the mass ratio of PVDF to PEO in step 1) is adjusted to 1:1, specifically:

[0071] 1) Mix PVDF and PEO in a mass ratio of 1:1 and dissolve them in DMF. Stir and mix at 75°C for 8 hours and then degas at an ultrasonic power of 80W for 30 minutes to obtain a PVDF / PEO mixed solution with a mass fraction of 10%.

[0072] 2) PVDF / PEO mixed solution was electrospun with a spinning voltage of 18kV, a spinning flow rate of 0.5mL / h, and a spinning distance of 18cm to obtain PVDF / PEO nanofiber membrane.

[0073] 3) After vacuum drying the PVDF / PEO nanofiber membrane at 55°C for 12 hours, it is immersed in a mixed solution of deionized water and anhydrous ethanol, wherein the volume ratio of deionized water to anhydrous ethanol is 1:2. The membrane is then subjected to pore-forming treatment at 25°C for 4 hours to obtain a porous PVDF nanofiber membrane, which is the porous PVDF radiation cooling material of this embodiment.

[0074] Example 3

[0075] The difference between this embodiment and Embodiment 2 is that the stirring temperature in step 1) is adjusted to 80°C, the stirring time is adjusted to 7 hours, and the volume ratio of deionized water to anhydrous ethanol in step 3) is 1:1. Specifically:

[0076] 1) Mix PVDF and PEO in a mass ratio of 1:1 and dissolve them in DMF. Stir and mix at 80°C for 7 hours, and then degas at an ultrasonic power of 80W for 30 minutes to obtain a PVDF / PEO mixed solution with a mass fraction of 10%.

[0077] 2) PVDF / PEO mixed solution was electrospun with a spinning voltage of 18kV, a spinning flow rate of 0.5mL / h, and a spinning distance of 18cm to obtain PVDF / PEO nanofiber membrane.

[0078] 3) After vacuum drying the PVDF / PEO nanofiber membrane at 55°C for 12 hours, it is immersed in a mixed solution of deionized water and anhydrous ethanol, wherein the volume ratio of deionized water to anhydrous ethanol is 1:1. Pore forming is performed at 25°C for 4 hours to obtain a porous PVDF nanofiber membrane, which is the porous PVDF radiation cooling material of this embodiment.

[0079] Example 4

[0080] The difference between this embodiment and Embodiment 3 is that the volume ratio of deionized water to anhydrous ethanol in step 3) is adjusted to 1:2, specifically:

[0081] 1) Mix PVDF and PEO in a mass ratio of 1:1 and dissolve them in DMF. Stir and mix at 80°C for 7 hours, and then degas at an ultrasonic power of 80W for 30 minutes to obtain a PVDF / PEO mixed solution with a mass fraction of 10%.

[0082] 2) PVDF / PEO mixed solution was electrospun with a spinning voltage of 18kV, a spinning flow rate of 0.5mL / h, and a spinning distance of 18cm to obtain PVDF / PEO nanofiber membrane.

[0083] 3) After vacuum drying the PVDF / PEO nanofiber membrane at 55°C for 12 hours, it is immersed in a mixed solution of deionized water and anhydrous ethanol, wherein the volume ratio of deionized water to anhydrous ethanol is 1:2. The membrane is then subjected to pore-forming treatment at 25°C for 4 hours to obtain a porous PVDF nanofiber membrane, which is the porous PVDF radiation cooling material of this embodiment.

[0084] Figure 1 This is a scanning electron microscope image of the PVDF / PEO nanofiber membrane obtained in step 2) of this embodiment. Figure 2 The image shows a scanning electron microscope (SEM) image of the porous PVDF / PEO nanofiber membrane prepared in step 3). It can be seen that the nanofiber membrane formed a porous structure after the pore-forming treatment.

[0085] Figure 3 The DTG plots of the two show that the PVDF / PEO nanofiber membrane first experienced weight loss at 370℃, reaching a peak at 400℃, corresponding to the decomposition of PEO in the sample, with a weight loss of 51.31%, and the decomposition products were mainly H2O, CO and CO2; the second mass loss occurred at 465℃, corresponding to the decomposition of PVDF, with the remaining 8.58% being carbon slag.

[0086] The first decomposition temperature of the porous PVDF nanofiber membrane was 360℃, with a weight loss of 8.65%, proving that most of the PEO inside the fiber was removed after phase separation; the second decomposition of PVDF was carried out at 460℃, proving that the amount of PEO remaining in the porous PVDF nanofiber membrane was extremely small.

[0087] Figure 4 The image shows the reflectance data of the porous PVDF nanofiber membrane fabricated in this embodiment in the ultraviolet-visible range at a thickness of 150 μm. Figure 5 This is an infrared emission data diagram in the range of 8–13 μm when the thickness is 150 μm. Figure 6 The contact angle of the porous PVDF radiative cooling material is shown to be 138°, indicating that the porous PVDF radiative cooling material has good hydrophobic properties.

[0088] The relevant properties of the PVDF radiative cooling materials prepared in Examples 1 to 4 were measured, and the results are shown in Table 1.

[0089] Table 1

[0090]

[0091]

[0092] (Note: The results in the simulated cooling test represent the temperature difference between PVDF radiative cooling material and ordinary cotton material. '-' indicates that the temperature of PVDF radiative cooling material is lower than that of ordinary cotton material.)

[0093] As can be seen from Table 1, the porous PVDF radiation cooling material of the present invention has a good cooling effect under different light source irradiance, and the cooling range increases with the increase of light intensity.

[0094] Example 5

[0095] This embodiment provides a method for preparing porous PVDF radiative cooling material, specifically as follows:

[0096] 1) Mix PVDF and PEO at a mass ratio of 1:3 and dissolve them in DMF. Stir and mix at 85℃ for 6 hours, and then degas at an ultrasonic power of 80W for 40 minutes to obtain a PVDF / PEO mixed solution with a mass fraction of 8%.

[0097] 2) PVDF / PEO mixed solution was electrospun with a spinning voltage of 14kV, a spinning flow rate of 0.1mL / h, and a spinning distance of 19cm to obtain PVDF / PEO nanofiber membrane.

[0098] 3) After vacuum drying the PVDF / PEO nanofiber membrane at 60°C for 11 hours, it is immersed in a mixed solution of deionized water and anhydrous ethanol, wherein the volume ratio of anhydrous ethanol is 55%. The membrane is then subjected to pore-forming treatment at 20°C for 8 hours to obtain a porous PVDF nanofiber membrane, which is the porous PVDF radiation cooling material of this embodiment.

[0099] Example 6

[0100] This embodiment provides a method for preparing porous PVDF radiative cooling material, specifically as follows:

[0101] 1) Mix PVDF and PEO at a mass ratio of 1:4 and dissolve them in DMF. Stir and mix at 85°C for 6 hours and then degas at an ultrasonic power of 80W for 50 minutes to obtain a PVDF / PEO mixed solution with a mass fraction of 12%.

[0102] 2) PVDF / PEO mixed solution was electrospun with a spinning voltage of 20kV, a spinning flow rate of 0.6mL / h, and a spinning distance of 20cm to obtain PVDF / PEO nanofiber membranes.

[0103] 3) After vacuum drying the PVDF / PEO nanofiber membrane at 62°C for 11 hours, it is immersed in a mixed solution of deionized water and anhydrous ethanol, wherein the volume ratio of anhydrous ethanol is 60%. The membrane is then subjected to pore-forming treatment at 23°C for 6 hours to obtain a porous PVDF nanofiber membrane, which is the porous PVDF radiation cooling material of this embodiment.

[0104] Example 7

[0105] This embodiment provides a method for preparing porous PVDF radiative cooling material, specifically as follows:

[0106] 1) Mix PVDF and PEO at a mass ratio of 1:5 and dissolve them in DMF. Stir and mix at 85℃ for 6 hours, and then degas at an ultrasonic power of 80W for 60 minutes to obtain a PVDF / PEO mixed solution with a mass fraction of 14%.

[0107] 2) PVDF / PEO mixed solution was electrospun with a spinning voltage of 22kV, a spinning flow rate of 0.9mL / h, and a spinning distance of 21cm to obtain PVDF / PEO nanofiber membrane.

[0108] 3) After vacuum drying the PVDF / PEO nanofiber membrane at 65°C for 10 hours, it is immersed in a mixed solution of deionized water and anhydrous ethanol, wherein the volume ratio of anhydrous ethanol is 70%. The membrane is then subjected to pore-forming treatment at 28°C for 2 hours to obtain a porous PVDF nanofiber membrane, which is the porous PVDF radiation cooling material of this embodiment.

[0109] The relevant properties of the PVDF radiative cooling materials prepared in Examples 5-7 were measured, and the results are shown in Table 2.

[0110] Table 2

[0111]

[0112]

[0113] (Note: The results in the simulated cooling test represent the temperature difference between PVDF radiative cooling material and ordinary cotton material. '-' indicates that the temperature of PVDF radiative cooling material is lower than that of ordinary cotton material.)

[0114] As can be seen from Table 2, the porous PVDF radiation cooling material of the present invention has a good cooling effect under different light source irradiance, and the cooling range increases with the increase of light intensity.

[0115] Comparative Example 1

[0116] The difference between this comparative example and Example 1 is that the mixed solution in step 3) is changed to a pure anhydrous ethanol solution, while all other process parameters are the same as in Example 1. This yields a porous PVDF radiative cooling material with a β-crystal content of 88.47%.

[0117] Comparative Example 2

[0118] The difference between this comparative example and Example 1 is that the volume ratio of deionized water to anhydrous ethanol in the mixed solution in step 3) was adjusted to 1:1. All other process parameters remained the same as in Example 1. This yielded a porous PVDF radiative cooling material with a β-crystal content of 90.93%.

[0119] Comparative Example 3

[0120] The difference between this comparative example and Example 1 is that the volume ratio of deionized water to anhydrous ethanol in the mixed solution in step 3) was adjusted to 2:1. All other process parameters remained the same as in Example 1. This yielded a porous PVDF radiative cooling material with a β-crystal content of 91.26%.

[0121] Comparative Example 4

[0122] The difference between this comparative example and Example 1 is that the mixed solution in step 3) is adjusted to be a pure deionized aqueous solution, and the porous PVDF radiation cooling material of this comparative example is prepared with a β crystal content of 91.52%.

[0123] It can be seen that when the content of deionized water in the system is high, the content of β-crystal form is also relatively high. At this time, the membrane structure is unstable, and if only deionized water is used, high-temperature treatment is required. The treated PVDF membrane will yellow, affecting the cooling effect. When the ethanol content is high, the treated PVDF membrane will swell, affecting its use.

[0124] Figure 7 The image shows scanning electron microscope (SEM) images of porous PVDF nanofibers with different treatment solution ratios, as shown in Comparative Example 1 and Comparative Example 4. Figure 7 f and Figure 7 b) It can be seen that if the treatment solution is a single deionized water or anhydrous ethanol solution, large-scale adhesion will occur between the fibers, especially when treated with pure anhydrous ethanol. The fibers swell, become coarse, and curl, resulting in extremely poor orientation. With the mixing of deionized water and anhydrous ethanol solutions, such as... Figure 7 c. Figure 7 d、 Figure 7 As shown in e, the fiber morphology is improved when the treatment solution is deionized water and anhydrous ethanol at a volume ratio of 1:2. Figure 7 As shown in e, the fibers are arranged regularly and have the best orientation.

[0125] Figure 8 The infrared spectra of porous PVDF nanofiber membranes under different treatment solution ratios are shown below; where ETOH is anhydrous ethanol and DI is deionized water. The curves from top to bottom are the infrared spectra of porous PVDF nanofiber membranes of Comparative Example 1, Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, and Example 4, respectively.

[0126] Comparative Example 5

[0127] The difference between this comparative example and Example 1 is that the temperature of the pore-forming process in step 3) is adjusted to 40°C, while the other process parameters are the same as in Example 1. The porous PVDF radiation cooling material of this comparative example is obtained with an absorbance of 0.054.

[0128] Comparative Example 6

[0129] The difference between this comparative example and Example 1 is that the temperature of the pore-forming process in step 3) is adjusted to 60°C, and the porous PVDF radiation cooling material of this comparative example is obtained with an absorbance of 0.257.

[0130] Comparative Example 7

[0131] The difference between this comparative example and Example 1 is that the temperature of the pore-forming process in step 3) is adjusted to 80°C, and the porous PVDF radiation cooling material of this comparative example is obtained with an absorbance of 0.445.

[0132] like Figure 9 As shown, comparing the absorbance of Example 1 with that of Comparative Examples 5, 6, and 7 reveals that higher pore-forming temperatures result in greater absorbance and darker fiber color. Heat accelerates the oxidation process, leading to the formation of a peroxide structure, which easily generates free radicals, causing the white material to discolor. With increasing temperature, the overall color of the treated nanofiber membrane changes and deepens. This is due to the effect of heat, which, in addition to activating oxidation, can also cause the breaking of -CC- bonds, leading to yellowing of the material.

[0133] This invention can form pores at a lower temperature, reducing the possibility of PVDF yellowing without damaging the fiber morphology. The processing temperature range of this invention is 20-28°C, with the optimal processing temperature being 25°C. If the temperature is too low (e.g., below 20°C), a longer processing time is required. However, as the processing time increases, the arrangement between fibers gradually becomes disordered, and the bending degree of individual fibers increases, the orientation becomes worse, and the fibers gradually loosen and bend, affecting the cross-linking between fibers and damaging the morphology of the fiber membrane.

[0134] Comparative Example 8

[0135] The difference between this comparative example and Examples 4 and 5-7 is that only pure deionized water was used to prepare the porous PVDF material. The porous PVDF nanofiber membranes obtained at different processing temperatures are as follows: Figure 10 As shown.

[0136] Figure 10 (A) shows the surface morphology of nanofibers at different processing temperatures. As shown in the figure, the fibers swell and their diameter gradually increases with increasing temperature.

[0137] Figure 10 (B) shows optical images of nanofibers at different processing temperatures. As the temperature increases, the overall color of the processed nanofiber membrane changes and deepens. This is because heat accelerates the oxidation process, leading to the formation of peroxide structures, which easily generate free radicals, causing the white material to discolor. Besides activated oxidation, it can also cause the breaking of -CC- bonds, resulting in yellowing. Simultaneously, the temperature during processing affects whether PVDF forms double bonds; yellowing and discoloration are external manifestations of double bond formation. Therefore, controlling the temperature during processing reduces the likelihood of yellowing in porous PVDF.

[0138] And such Figure 11 The images show the surface morphology of the fiber at pore formation temperatures of <60℃ (left) and >60℃ (right), respectively. It can be seen that when the temperature is <60℃, the fiber membrane fails to form pores. When the temperature is >60℃, although the fiber membrane forms pores, the pore formation effect is poor, and uneven defects are visible on the surface. Therefore, the pore formation scheme using a pure deionized water system in the prior art inevitably requires increasing the pore formation temperature, which in turn reduces the membrane reflectivity and affects the membrane's cooling capacity.

[0139] In summary, this invention provides a porous PVDF radiation cooling material, its preparation method, and its application. A PVDF / PEO nanofiber membrane prepared by electrospinning is immersed in a mixed solution of deionized water and ethanol (ethanol volume percentage 55-70%) at a temperature of 20-28°C to obtain a porous PVDF radiation cooling material. This achieves pore formation at low temperatures, reduces the possibility of PVDF yellowing, and does not damage the fiber morphology and structure.

[0140] The porous PVDF radiative cooling material prepared by this invention has a β-crystal content of no more than 90.5%; the porous PVDF radiative cooling material with a thickness of 150 μm has an absorbance of 0.01–0.02 in the ultraviolet-visible wavelength range, an average reflectance of 95–99.9%, and an average emissivity of 88–92% in the 8–13 μm wavelength range. The cooling effect measured in indoor simulated cooling tests and outdoor cooling performance tests is 4–10℃. The method of this invention can solve the problem of fiber color change after pore-forming treatment, which leads to a significant reduction in cooling effect.

[0141] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a porous PVDF radiative cooling material, characterized in that: include, PVDF and PEO are dissolved in DMF, stirred and mixed, and then degassed by ultrasonication to obtain a PVDF / PEO solution. PVDF / PEO nanofiber membranes were prepared by electrospinning PVDF / PEO solution. After vacuum drying, the PVDF / PEO nanofiber membrane is immersed in a mixed solution of deionized water and ethanol and subjected to pore formation treatment at 20~28℃. The volume percentage of ethanol in the mixed solution is 55%~70%, and the treatment time is 2~8h, thus obtaining a porous PVDF radiation cooling material with β crystal content <90.5%. The porous PVDF radiation cooling material has a pore size range of 20 nm to 4 µm. When the thickness of the porous PVDF radiation cooling material is 150 µm, the absorbance in the ultraviolet-visible wavelength range is 0.01 to 0.02, the average reflectance is 95 to 99.9%, and the average emissivity in the wavelength range of 8 to 13 μm is 88 to 92%.

2. The preparation method of the porous PVDF radiative cooling material as described in claim 1, characterized in that: The PVDF / PEO solution has a mass fraction of 8-14%, wherein the mass ratio of PVDF to PEO is 1:1-5.

3. The preparation method of the porous PVDF radiative cooling material as described in claim 1, characterized in that: The mixing temperature is 75~85℃, and the mixing time is 6~8h.

4. The preparation method of the porous PVDF radiative cooling material as described in claim 1, characterized in that: The ultrasonic power for ultrasonic debubbling is 60~100W, and the ultrasonic time is 30~60min.

5. The preparation method of the porous PVDF radiative cooling material as described in claim 1, characterized in that: The electrospinning process uses a spinning voltage of 14~22 kV, a spinning flow rate of 0.1~0.9 mL / h, and a spinning distance of 18~21 cm.

6. The preparation method of the porous PVDF radiative cooling material as described in claim 1, characterized in that: The vacuum drying temperature is 55~65℃, and the time is 10~12h.

7. The application of the porous PVDF radiative cooling material prepared by any one of the preparation methods described in claims 1 to 6 in the preparation of thermally comfortable fabrics and building materials with hydrophobic properties.

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