Polyvinylidene fluoride porous film as well as preparation method and application thereof

Abstract

The preparation method comprises the following steps: dissolving PVDF (Polyvinylidene Fluoride) in acetone, adding a small amount of Tween 80, carrying out oil bath at 60 DEG C, and uniformly mixing until the solution is transparent, so as to prepare a polymer solvent system; and standing and defoaming the obtained solution, pouring into a mold, and naturally volatilizing by controlling the humidity to obtain the polymer porous membrane with the porous microsphere net structure. According to the method, phase separation is induced through solvent evaporation, and synchronous construction of micron holes and nanospheres of the PVDF porous membrane is achieved. Compared with a traditional phase separation membrane preparation technology, the method has the advantages that the pore size of the PVDF porous membrane can be greatly expanded, controllable preparation of micron / nano pores of the PVDF porous membrane is realized, and the prepared porous membrane has more excellent reflectivity and cooling performance.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer porous films, and specifically relates to a polyvinylidene fluoride porous film and a preparation method and application thereof. Background Art

[0002] At present, the global energy supply pressure is constantly increasing, especially the traditional refrigeration technology based on air compression, which brings comfort to people but also puts a huge burden on the power system. The development of new and more energy-saving refrigeration technology is an urgent issue. In recent years, radiative refrigeration technology has received widespread attention. This technology uses the natural heat sink of the universe and the selective permeability of the earth's atmosphere to discharge the heat of ground objects into the external space in the form of spontaneous radiation, which has two significant advantages: zero energy consumption and zero carbon emissions. Usually, the radiative heat gain of objects mainly comes from solar radiation in the 0.3-2.5μm band, while objects need to dissipate heat to outer space through the atmospheric window in the 8-13μm band. Therefore, radiative refrigeration materials need to have high reflectivity in the 0.3-2.5μm solar light band to prevent objects from gaining heat; at the same time, they need to have high emissivity in the 8-13μm infrared light band to reduce the temperature.

[0003] There are many methods for preparing porous materials. In view of the special requirements of radiation cooling on the spectral behavior and pore structure of materials, the methods that have been adopted mainly include the breathing image method, stretching method, template method, electrospinning method and phase separation method. Among them, the related products prepared by the breathing image method and the stretching method are rarely used directly for radiation cooling, but they can also achieve optical regulation. The template method is one of the most commonly used methods for preparing porous materials. It can achieve strict control of the pore morphology of the material through the selection of the template. However, for porous materials used for radiation cooling, how to completely and efficiently and environmentally remove the template without affecting the mechanical properties of the polymer matrix is ​​still an urgent problem to be solved. The high-porosity porous membrane prepared by electrospinning can meet the dual requirements of radiation cooling for pore morphology and softness, and has been practically applied in the field of radiation cooling, but electrospinning has high requirements for equipment and environment, and the strength of the produced porous membrane is relatively low due to the limitations of electric field and voltage. The phase separation method is a method of dissolving a polymer in a solvent, and using evaporation, cooling, immersion in a non-solvent and other means to change the thermodynamic state of the solution to induce phase separation, thereby forming pores. The phase separation method does not require high-cost templates, has a wide range of applications, and is flexible to operate. However, as far as radiation cooling is concerned, it is currently difficult to achieve precise control of the aperture, which means that there is still much room for improvement in the spectral selectivity of the material.

[0004] Fluoropolymers are considered to be good substrates for radiative refrigerators because the intrinsic vibration frequencies of the carbon-hydrogen bonds and carbon-fluorine bonds they contain are in the "atmospheric window" band range, which makes them have intrinsic high emissivity in the infrared thermal radiation band. At present, there are many methods for preparing porous materials containing fluoropolymers. In 2018, Mandal et al. used controlled evaporation-induced phase separation to prepare hierarchical porous polyvinylidene fluoride-hexafluoropropylene (P(VDF-HFP)) monolayer membranes with acetone as solvent and water as non-solvent. Wu et al. added tetraethyl orthosilicate (TEOS) to a PVDF / DMSO mixed solution by thermally induced phase separation. While TESO acts as a pore-forming agent, it can also reduce the phase separation rate to obtain a PVDF porous material with a combined morphology of large tubular pores and small circular pores. García et al. prepared P(VDF-HFP) hollow fiber membranes by non-solvent-induced phase separation, and found that the large pores in the membrane gradually transformed into sponge-like pores as the polymer concentration increased. Wang et al. mixed tetraethyl orthosilicate (TEOS) with PVDF / DMF solution and prepared a porous composite membrane frame by adjusting the spinning direction, which showed excellent cooling performance while also having flexibility and strength. Although there are many reports on the preparation of fluorinated polymer porous materials, there are few reports on how to prepare fluorinated porous materials with regular microstructures. The solvent evaporation induced phase separation method (SEIPS) is to dissolve the polymer in a solvent and a non-solvent to form a uniform and stable casting solution, which is then scraped to obtain a wet film, in which the solvent evaporates first, and the concentration of the non-solvent and the polymer increases, resulting in phase separation. Finally, the polymer solidifies and precipitates into a film, and forms a porous film after complete evaporation. SEIPS is simple to operate, has low equipment requirements, and requires mild conditions. It can achieve precise control of the pore size, and the prepared porous material has a relatively regular microstructure, which makes it widely used in the preparation of porous materials.

[0005] Polyvinylidene fluoride (PVDF) has become one of the most widely used polymer porous materials due to its excellent mechanical strength, chemical stability, radiation resistance, heat resistance and other characteristics. PVDF porous film materials have been widely favored in the field of radiation refrigeration due to their simple preparation process and significant radiation refrigeration effect. However, due to its semi-crystalline nature, the microstructure of PVDF porous film materials is less regular and difficult to control, resulting in uneven radiation refrigeration performance. In order to solve this problem, we used the solvent evaporation induced phase separation method to prepare a PVDF porous film material with a relatively regular microstructure. Its structure is composed of many microspheres with a size of micrometers. We also studied the morphology of PVDF porous film materials, the relationship between porosity and reflectivity, and the influencing factors of the microstructure of PVDF porous film materials, and focused on characterizing the radiation refrigeration performance of the prepared PVDF porous film materials as radiation refrigeration materials. Summary of the invention

[0006] Technical problem to be solved: The present invention provides a method for preparing a porous polyvinylidene fluoride film and its application. The preparation method overcomes the shortcomings of irregular and difficult-to-control pores of current polymer porous materials, realizes wide-size scattering, and effectively improves the cooling performance of the material. The present invention uses polyvinylidene fluoride as a polymer, acetone as a solvent, and Tween 80 as a surfactant to stabilize the pores, and adopts a solvent evaporation-induced phase separation method to prepare a polymer porous membrane with a porous microsphere network structure.

[0007] Technical solution: A method for preparing a porous polyvinylidene fluoride film, comprising the following steps: Step 1, dissolving polyvinylidene fluoride (PVDF) in acetone, and adding Tween 80, wherein the mass concentration of PVDF is 2.5%-7.5%, and the mass fraction of Tween 80 is 1%-5%, and heating in an oil bath at 55-60°C for 10-15 minutes until the color of the polymer solvent system changes to light yellow and transparent; Step 2: standing the solution obtained in Step 1 for degassing and then pouring it into a mold, and naturally drying it by controlling the humidity to obtain a polymer porous membrane with a porous network structure; Step 3: immersing the above-mentioned polymer porous membrane in ethanol, removing Tween 80, and drying at room temperature to obtain a polymer porous membrane with a porous network structure.

[0008] Preferably, the mass concentration of PVDF in the above step 1 is 5%.

[0009] Preferably, the solution in step 2 is placed in a static degassing condition of heating to 30° C. and maintaining for 2 hours; the natural drying condition is an air environment with a temperature of 25° C., a humidity of 70%, and a phase separation time of 15 minutes.

[0010] Preferably, in the above step 3, the temperature of the ethanol is 25° C., the soaking time is 24 h, and the drying time at room temperature is 12 h.

[0011] The polyvinylidene fluoride porous film is prepared by the above method.

[0012] The polyvinylidene fluoride porous film is used in manufacturing daytime radiation cooling materials for outdoor facilities.

[0013] A cooling material for outdoor facilities is made from the above-mentioned polyvinylidene fluoride porous film.

[0014] Beneficial effects: (1) The method of the present invention achieves the pore adjustment of the polyvinylidene fluoride porous film and its radiation cooling application by inducing phase separation through solvent evaporation. Compared with the traditional porous membrane material preparation technology, this strategy can greatly broaden the pore size of the polyvinylidene fluoride porous film and achieve the simultaneous construction of micron pores and nanospheres in the polyvinylidene fluoride porous film. Compared with the traditional phase separation membrane making technology, this strategy can greatly broaden the pore size of the polyvinylidene fluoride porous film and achieve the controllable preparation of micron / nano pores of the polyvinylidene fluoride porous film, so that the prepared porous film has better reflectivity and cooling performance.

[0015] (2) The PVDF in the present invention contains a large number of CF bonds, which makes it have excellent weather resistance. The polarity of F atoms is relatively small, which makes the PVDF molecule highly inert and not easy to react with other substances, thereby maintaining its stability.

[0016] (3) The present invention selects a base material that matches the 8-13μm atmospheric transparent window band to achieve radiation cooling. When the infrared band of the material basically overlaps with the blackbody radiation 8-13μm of the surface object, it can transfer heat to the outer space with a temperature of 3K in the form of infrared radiation. The vibration of the functional group is the main reason for the absorption and emission of infrared by the polymer. Among them, the CF wavelength range of PVDF is 8-13μm, which can emit its own heat. Specifically, the porous microsphere mesh structure of the porous polyvinylidene fluoride film is beneficial to improve the scattering of sunlight and reduce the heating of the porous membrane by sunlight. The porous polyvinylidene fluoride film is rich in CF functional groups and has high emission in the atmospheric transparent window, realizing efficient radiation cooling. In addition, the chemical inertness of the CF functional group also maintains the stability of the PVDF porous membrane in various climates and environments. The above properties work synergistically to achieve the daytime radiation cooling effect of the PVDF porous membrane in a complex environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a scanning electron microscope photograph of the pore structure of the porous polyvinylidene fluoride film prepared in Example 1.

[0018] Figure 2 This is a scanning electron microscope photograph of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 1.

[0019] Figure 3 This is a scanning electron microscope photograph of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 2.

[0020] Figure 4 This is a scanning electron microscope photograph of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 3.

[0021] Figure 5This is a scanning electron microscope photograph of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 4.

[0022] Figure 6 The reflectivity curves of the polyvinylidene fluoride porous films prepared in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3.

[0023] Figure 7 Schematic diagram of the radiation cooling test device for the polyvinylidene fluoride porous film prepared in Example 1.

[0024] Figure 8 The following is a comparison curve of the radiation cooling effect of Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3.

[0025] Fig. 9 It is a comparison curve of the radiation cooling effect of Example 1 and Comparative Example 4. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0027] PVDF (molecular weight 50w) was purchased from Arkema, France; acetone was purchased from Sinopharm Chemical Reagent Co., Ltd.; Tween 80 was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0028] Example 1

[0029] Step 1: Dissolve 2.5 g of PVDF in 47.5 g of acetone, add 0.5 g of Tween 80, continue heating in a 60° C. oil bath, and stir magnetically for 30 minutes to obtain a transparent light yellow PVDF solution with a mass concentration of 5%.

[0030] Step 2: Heat the solution in a 30°C oil bath for 1 hour to defoam, then pour it into a glass mold and place it in a room temperature and 70% humidity environment to evaporate for 10 minutes to obtain a PVDF film.

[0031] Step 3: Place the completely evaporated PVDF porous film in ethanol at room temperature, soak it for 24 hours, take it out, and dry it at room temperature for 12 hours to prepare a polyvinylidene fluoride porous film.

[0032] Comparative Examples 1, 2, and 3

[0033] The preparation was carried out according to the preparation method of Example 1, except that the drying was carried out at room temperature under humidity environments of 30%, 50% and 90%, respectively, to prepare Comparative Examples 1, 2 and 3.

[0034] Comparative Example 4

[0035] The preparation was carried out according to the preparation method of Example 1, except that the surfactant Tween 80 was not added to the solution, and the comparative example 4 was prepared directly by the solvent evaporation-induced phase separation method.

[0036] Step 1: Dissolve 2.5 g of PVDF in 47.5 g of acetone, heat continuously in a 60° C. oil bath, and stir magnetically for about 30 minutes to obtain a colorless, transparent, and clear solution with a mass concentration of 5% PVDF.

[0037] Step 2: Heat the solution in a 30°C oil bath for 1 hour to defoam, then pour it into a glass mold and place it in a room temperature and 70% humidity environment to evaporate for 10 minutes to obtain a PVDF film.

[0038] Step 3: Place the completely evaporated PVDF porous film in ethanol at room temperature, soak for 24 hours, take it out and dry it at room temperature for 12 hours to prepare a polymer film.

[0039] Figure 1 This is a scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Example 1. It can be seen that the porous film with a mass concentration of 5% prepared by the solvent evaporation induced phase separation method is stacked with microspheres with a particle size of about 1μm. The addition of Tween 80 surfactant makes the water evenly distributed in the system during the pore formation process, which improves the regularity of the porous film. At the same time, the improvement of the roughness of the pore wall is conducive to the improvement of reflection. The polyvinylidene fluoride porous film prepared under this condition has the best radiation cooling effect.

[0040] Figure 2 The scanning electron microscope photo of the pore structure of the porous polyvinylidene fluoride film prepared in Comparative Example 1 shows that in a low humidity environment of 30%, the porous film with a mass concentration of 5% prepared by the solvent evaporation induced phase separation method has smaller pore diameters and spherical diameters, regular pore diameters, and is close to dense. The radiation cooling effect of the membrane is average.

[0041] Figure 3 The scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 2 shows that under a low humidity environment of 50%, the porous membrane with a mass concentration of 5% prepared by the solvent evaporation induced phase separation method has a pore size that is improved compared to Comparative Example 1, and the pore size is regular. The radiation cooling effect of the membrane is improved compared to Comparative Example 1.

[0042] Figure 4The scanning electron microscope photo of the pore structure of the porous polyvinylidene fluoride film prepared in comparative example 3 shows that under a high humidity environment of 90%, the porous membrane with a mass concentration of 5% prepared by the solvent evaporation induced phase separation method has a greatly improved pore size relative to Example 1, the pore size changes greatly and is unevenly distributed, and the ball diameter is slightly increased. The radiation cooling effect of the membrane is reduced relative to Example 1.

[0043] Figure 5 This is a scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 4. When the surfactant Tween 80 is not added, the porous film with a mass concentration of 5% prepared by the solvent evaporation induced phase separation method has uneven pore size distribution and size, and the ball size is not uniform. The radiation cooling effect of the membrane is average.

[0044] Figure 6 The reflectivity curves of the film materials prepared in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3 are shown. It can be seen that the reflectivity of Example 1 is the highest, which can reach 98.4%. This is mainly because the polyvinylidene fluoride porous film microspheres prepared in a 70% humidity environment are relatively large, and the pore size distribution and size are the most regular; while the reflectivity of Comparative Examples 1 and 2 prepared in 30% and 50% humidity environments can only reach about 90%, and the reflectivity increases with the increase of humidity during preparation; and Comparative Example 3 prepared in a 90% humidity environment has uneven pore size distribution and size, and the ball diameter does not change much compared with Example 1.

[0045] Figure 7 The schematic diagram of the radiation cooling test device of the porous polyvinylidene fluoride film prepared in Example 1 is shown. The device consists of a polystyrene film cavity wrapped in aluminum foil, and the cavity is covered with the porous polyvinylidene fluoride film prepared in Example 1. Thermocouples are placed above the sample and in the polystyrene foam cavity wrapped in tin foil, respectively, to record the temperature changes of the outside air and the cavity in real time.

[0046] Figure 8 The temperature change curve of the radiation cooling test device of the membrane material prepared in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3 under the actual environment from 10 o'clock to 14 o'clock. It can be observed that there is a 5℃-9℃ difference between the device covered with the polyvinylidene fluoride porous film and the outside air. As the preparation humidity increases, the radiation cooling effect of the polyvinylidene fluoride porous film becomes more obvious, and the cooling effect is best at 70% humidity. At 90% humidity, due to the high humidity, the ball diameter and pore size become uneven, and the cooling effect decreases.

[0047] Fig. 9The temperature change curves of the radiation cooling test device of the polyvinylidene fluoride porous film prepared in Example 1 and Comparative Example 4 under the actual environment from 10 o'clock to 14 o'clock. It can be observed that the cooling effect of Example 1 with the addition of the surfactant Tween 80 is better, because the addition of the surfactant Tween 80 improves the pore size distribution and the regularity of the size.

Claims

1. A method for preparing a polyvinylidene fluoride porous film, characterized in that: The method comprises the following steps: step 1, dissolving polyvinylidene fluoride (PVDF) in acetone, and adding Tween 80, wherein the mass concentration of PVDF is 2.5%-7.5%, and the mass fraction of Tween 80 is 1%-5%, and heating in an oil bath at 55-60°C for 10-15 minutes until the color of the polymer solvent system changes to light yellow and transparent; step 2, standing the solution obtained in step 1 for degassing and then pouring it into a mold, and naturally drying it by controlling the humidity to obtain a polymer porous membrane with a porous network structure; step 3, immersing the above polymer porous membrane in ethanol, removing Tween 80, and drying at room temperature to obtain a polymer porous membrane with a porous network structure.

2. The method for preparing the polyvinylidene fluoride porous film according to claim 1, characterized in that: The mass concentration of PVDF in step 1 is 5%.

3. The method for preparing the polyvinylidene fluoride porous film according to claim 1, characterized in that: The solution in step 2 is placed to degas under the conditions of heating to 30° C. and maintaining for 2 hours; the natural drying conditions are in an air environment with a temperature of 25° C., a humidity of 70%, and a phase separation time of 15 minutes.

4. The method for preparing the polyvinylidene fluoride porous film according to claim 1, characterized in that: In step 3, the temperature of the ethanol is 25° C., the soaking time is 24 h, and the drying time at room temperature is 12 h.

5. A polyvinylidene fluoride porous film obtained by the method according to any one of claims 1 to 4.

6. Use of the polyvinylidene fluoride porous film according to claim 5 in manufacturing daytime radiation cooling materials for outdoor facilities.

7. A cooling material for outdoor facilities, characterized in that: It is made from the polyvinylidene fluoride porous film as claimed in claim 5.

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