A polyvinylidene fluoride porous film and a preparation method and application thereof

Abstract

The application discloses a kind of polyvinylidene fluoride porous film and its preparation method and application, PVDF is dissolved in acetone, and a small amount of Tween 80 is added, and is mixed uniformly to transparent in oil bath 60 DEG C, and polymer solvent system is prepared;After the solution obtained is placed and deaerated, it is poured into mold again, and the porous polymer porous film of porous microspheres network structure is obtained by controlling humidity and being naturally evaporated dry.The method of the application realizes the synchronous construction of PVDF porous film micron hole and nanometer ball by solvent evaporation induced phase separation, compared with the traditional phase separation film forming technology, the strategy can greatly widen the pore size of PVDF porous film, realizes controllable preparation of the micron / nanopore of PVDF porous film, so that the porous film prepared has more excellent reflectivity and cooling performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of polymer porous membranes, and particularly relates to a polyvinylidene fluoride porous film and a preparation method and application thereof. BACKGROUND

[0002] At present, the global energy supply pressure is rising, especially the traditional refrigeration technology based on air compression, which brings people comfort but also causes a huge burden to the power system. Developing a new type of energy-saving refrigeration technology is an urgent issue. In recent years, radiation refrigeration technology has received extensive attention. This technology uses the natural heat sink of the universe and the selective permeability of the earth's atmosphere to emit the heat of ground objects to the outside space in the form of spontaneous radiation, having two significant advantages of zero energy consumption and zero carbon emission. The heat of an object is mainly obtained from the 0.3-2.5 μm band of solar radiation, and the object is cooled by the 8-13 μm band of atmospheric window to outer space. Therefore, the radiation refrigeration material needs to have high reflectivity in the 0.3-2.5 μm band of solar light to prevent the object from being heated, and at the same time, has a high emissivity in the 8-13 μm band of infrared light 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 the material, the methods that have been adopted mainly include breath figure method, stretching method, template method, electrospinning method and phase separation method. Among them, the breath figure method and the stretching method are less directly used for radiation cooling, but they can also achieve optical control. The template method is one of the most commonly used methods for preparing porous materials, which can strictly control the pore morphology of the material by selecting the template, but for radiation cooling porous materials, how to completely and efficiently remove the template without affecting the mechanical properties of the polymer matrix is still a problem to be solved. The high-porosity porous film prepared by electrospinning can meet the dual requirements of pore morphology and softness for radiation cooling, and has been applied in the field of radiation refrigeration. However, electrospinning has high requirements for equipment and environment, and the strength of the porous film produced is relatively low due to the limitation of electric field and voltage.

[0004] Fluoropolymers are considered as good substrates for radiative coolers due to the intrinsic vibration frequencies of the carbon-hydrogen and carbon-fluorine bonds contained in them, which fall within the range of the atmospheric window, making them intrinsically highly emissive in the infrared thermal radiation band. Currently, there are many methods for preparing fluoropolymer porous materials. In 2018, Mandal et al. used the controlled evaporation-induced phase separation method to prepare a hierarchical porous polyvinylidene fluoride-hexafluoropropylene (P(VDF-HFP)) monolayer film using acetone as the solvent and water as the non-solvent. Wu et al. used the thermal-induced phase separation method to add tetraethyl orthosilicate (TEOS) to the PVDF / DMSO mixed solution. TESO acts as a pore-forming agent while also reducing the phase separation rate, resulting in a combination of large tubular pores and small circular pores in the PVDF porous material. García et al. used the non-solvent-induced phase separation method to prepare P(VDF-HFP) hollow fiber membranes, and found that as the polymer concentration increased, the large pores in the membrane gradually changed to sponge-like pores. Wang et al. mixed tetraethyl orthosilicate (TEOS) with a PVDF / DMF solution and prepared a porous composite membrane frame by adjusting the spinning direction, which exhibited excellent cooling performance while also having flexibility and strength. Although there are many reports on the preparation of fluoropolymer porous materials at present, there are few reports on how to prepare fluoropolymer porous materials with regular microstructure. 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, then the wet film is obtained by casting. The solvent is first volatilized, the concentration of the non-solvent and the polymer is increased, leading to phase separation, and finally the polymer is solidified and precipitated into a film. After complete volatilization, a porous material is formed. SEIPS is simple to operate, requires small equipment, and has mild conditions. It can precisely control the pore size and has a relatively regular microstructure, making 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, and heat resistance. PVDF porous thin film materials have been widely favored in the field of radiative cooling due to their simple preparation process and significant radiative cooling effect. However, the microstructure of PVDF porous thin film materials is irregular and difficult to control due to their semi-crystalline nature, resulting in inconsistent radiative cooling performance. To solve this problem, we used the solvent evaporation-induced phase separation method to prepare PVDF porous thin film materials with a regular microstructure. The structure is composed of many microspheres with a size of microns. We studied the relationship between the morphology, porosity, and reflectivity of the PVDF porous thin film materials, as well as the influencing factors of the microstructure of the PVDF porous thin film materials. We focused on characterizing the radiative cooling performance of the prepared PVDF porous thin film materials as radiative cooling materials. SUMMARY

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

[0007] Technical scheme: a preparation method of polyvinylidene fluoride porous film, comprising the following steps: step 1, dissolving polyvinylidene fluoride (PVDF) in acetone and adding Tween 80, the mass concentration of PVDF is 2.5%-7.5%, the mass fraction of Tween 80 is 1%-5%, heating in 55-60℃ oil bath for 10-15 minutes until the color of the polymer solvent system becomes light yellow transparent; step 2: after the solution obtained in step 1 is deaerated, it is poured into a mold, and a porous network structure polymer porous membrane is obtained by controlling the humidity and naturally evaporating; step 3: soaking the above polymer porous membrane in ethanol to remove Tween 80, and drying at room temperature to obtain a porous network structure polymer porous membrane.

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

[0009] Preferably, the deaeration condition of the solution in step 2 is heating to 30℃ for 2h; the natural evaporation condition is air environment, the temperature is 25℃, the humidity is 70%, and the phase separation time is 15min.

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

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

[0012] The application of the above polyvinylidene fluoride porous film in manufacturing outdoor facility daytime radiation cooling materials.

[0013] An outdoor facility cooling material prepared from the above polyvinylidene fluoride porous film.

[0014] Beneficial effects: (1) The method of the present application realizes the pore adjustment of polyvinylidene fluoride porous film and its radiation cooling application by solvent volatilization induced phase separation. Compared with the traditional porous membrane material preparation technology, the strategy can greatly widen the pore size of polyvinylidene fluoride porous film, and realize the synchronous construction of microporous and nanospheres of polyvinylidene fluoride porous film. Compared with the traditional phase separation film preparation technology, the strategy can greatly widen the pore size of polyvinylidene fluoride porous film, realize the controllable preparation of microporous / nanoporous of polyvinylidene fluoride porous film, and make the prepared porous film have more excellent reflectivity and cooling performance.

[0015] (2) In the present application, PVDF contains a large number of C-F bonds, which has excellent weather resistance. The polarity of F atom is small, so that the PVDF molecule has strong inertness and is not easy to react with other substances, thereby maintaining its stability.

[0016] (3) In the present application, the matrix material matched with the 8-13 μm atmospheric transparent window band is selected to realize radiation cooling. When the infrared band of the material is basically overlapped with the black body radiation of 8-13 μm of the ground object, the heat can be transferred to the outer space with a temperature of 3K in the form of infrared radiation. The vibration of functional groups is the main reason for the absorption and emission of infrared of polymer, and the C-F wavelength range of PVDF is in 8-13 μm, which can emit its own heat. Specifically, the porous microsphere reticular structure of polyvinylidene fluoride porous film is beneficial to improve the scattering of sunlight and reduce the heating of the porous film by sunlight. The C-F functional groups in the polyvinylidene fluoride porous film have high emission in the atmospheric transparent window, realizing high-efficiency radiation cooling. In addition, the chemical inertness of C-F functional groups also maintains the stability of PVDF porous film in various climates and environments. The above-mentioned properties synergistically realize the daytime radiation cooling effect of PVDF porous film in complex environment. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Example 1.

[0018] Figure 2 The scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 1.

[0019] Figure 3 The scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 2.

[0020] Figure 4 The scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 3.

[0021] Figure 5Scanning electron microscope picture of the pore structure of the polyvinylidene fluoride porous film prepared for Comparative Example 4.

[0022] Figure 6 Reflectivity curve of the polyvinylidene fluoride porous film prepared for Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3.

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

[0024] Figure 8 Comparison curve of the radiation cooling effect of Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3.

[0025] Figure 9 Comparison curve of the radiation cooling effect of Example 1 and Comparative Example 4. DETAILED DESCRIPTION

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

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

[0028] Example 1

[0029] Step 1: 2.5g PVDF was dissolved in 47.5g acetone, and 0.5g Tween 80 was added. The mixture was continuously heated in a 60℃ oil bath and magnetically stirred for 30 minutes to obtain a 5% PVDF transparent light yellow solution.

[0030] Step 2: The solution was heated at 30℃ in an oil bath for 1h to remove bubbles, then poured into a glass mold, and placed in a normal temperature 70% humidity environment to evaporate for 10min to obtain a PVDF film.

[0031] Step 3: The completely evaporated PVDF porous film was placed in ethanol at room temperature, soaked for 24h, then taken out and dried at room temperature for 12h to obtain a polyvinylidene fluoride porous film.

[0032] Comparative Examples 1, 2 and 3

[0033] According to the preparation method of Example 1, Comparative Examples 1, 2 and 3 were prepared, except that the normal temperature drying was carried out in a 30%, 50% and 90% humidity environment, respectively.

[0034] Comparative Example 4

[0035] Prepared according to the preparation method of Example 1, except that the solution was not added with surfactant Tween 80, and directly induced phase separation by solvent evaporation, to prepare Comparative Example 4.

[0036] Step 1: 2.5 g PVDF was dissolved in 47.5 g acetone, continuously heated in a 60°C oil bath, and magnetically stirred for about 30 minutes to obtain a 5% PVDF colorless transparent clear solution.

[0037] Step 2: After the solution was heated at 30°C in an oil bath for 1 h to remove bubbles, it was poured into a glass mold and placed in a normal temperature 70% humidity environment to evaporate for 10 min to obtain a PVDF film.

[0038] Step 3: The completely evaporated PVDF porous film was placed in ethanol at room temperature, soaked for 24 h, and then taken out and dried at room temperature for 12 h to prepare a polymer film.

[0039] Figure 1 The scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Example 1 can be seen that the 5% porous film prepared by solvent evaporation induced phase separation is stacked by microspheres with a particle size of about 1 μm. The addition of Tween 80 surfactant makes the water uniformly distributed in the system during the pore forming process, improving the regularity of the porous film. At the same time, the improvement of the roughness of the pore wall is beneficial to the improvement of the 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 polyvinylidene fluoride porous film prepared in Comparative Example 1 can be seen that in a 30% low humidity environment, the 5% porous film prepared by solvent evaporation induced phase separation has smaller pore size and regular pore size, close to dense. The radiation cooling effect of the film is general.

[0041] Figure 3 The scanning electron microscope photo of the pore structure of the polyvinylidene fluoride porous film prepared in Comparative Example 2 can be seen that in a 50% lower humidity environment, the 5% porous film prepared by solvent evaporation induced phase separation has a relatively improved pore size compared to Comparative Example 1, and the pore size is regular. The radiation cooling effect of the film is improved compared to Comparative Example 1.

[0042] Figure 4The scanning electron microscope image of the pore structure of the polyvinylidene fluoride porous film prepared for Comparative Example 3 can be seen that in a 90% high humidity environment, the pore size of the porous film prepared by the solvent evaporation induced phase separation method with a mass concentration of 5% is relatively large compared with Example 1, the pore size varies greatly and is unevenly distributed, and the ball size is slightly increased. The radiation cooling effect of the film is relatively decreased compared with Example 1.

[0043] Figure 5 The scanning electron microscope image of the pore structure of the polyvinylidene fluoride porous film prepared for Comparative Example 4 can be seen that in the absence of the surfactant Tween 80, the pore size distribution and size of the porous film prepared by the solvent evaporation induced phase separation method with a mass concentration of 5% are uneven, and the ball size is not uniform. The radiation cooling effect of the film is general.

[0044] Figure 6 The reflectivity curve of the film material prepared for Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3 can be seen that the reflectivity of Example 1 is the highest, which can reach 98.4%, mainly because the microspheres of the polyvinylidene fluoride porous film prepared in a 70% humidity environment are relatively large, and the pore size distribution and size are the most regular; the reflectivity of Comparative Example 1 and Comparative Example 2 prepared in a 30% and 50% humidity environment can only reach about 90%, and the reflectivity increases with the increase of the humidity during preparation; and the pore size distribution and size of Comparative Example 3 prepared in a 90% humidity environment are uneven, and the ball size changes little compared with Example 1.

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

[0046] Figure 8 The temperature change curve of the radiation cooling test device of the film material prepared for Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3 in the actual environment from 10 o'clock to 14 o'clock. It can be observed that there is a 5-9°C difference between the device covered with the polyvinylidene fluoride porous film and the external air, and the radiation cooling effect of the polyvinylidene fluoride porous film is more obvious with the increase of the preparation humidity, and the cooling effect is the best in a 70% humidity environment. In a 90% humidity environment, the ball size and pore size are uneven due to the too high humidity, and the cooling effect is decreased.

[0047] Figure 9The temperature change curve of the radiation cooling test device of the polyvinylidene fluoride porous film prepared for 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 surfactant Tween 80 is better, which is due to the addition of surfactant Tween 80 to improve the pore size distribution and the regularity of the size.

Claims

1. A method for preparing a porous polyvinylidene fluoride (PVDF) film, characterized in that, The process includes the following steps: Step 1: Dissolve polyvinylidene fluoride (PVDF) in acetone and add Tween 80. The PVDF mass concentration is 5%, and the Tween 80 mass fraction is 1%~5%. Heat in an oil bath at 55~60℃ for 10~15 minutes until the polymer solvent system turns pale yellow and transparent. Step 2: Let the solution obtained in Step 1 stand to remove bubbles, then pour it into a mold and allow it to dry naturally by controlling the humidity to obtain a porous polyvinylidene fluoride film with a porous network structure composed of micron-sized microspheres. Step 3: Immerse the above-mentioned porous polyvinylidene fluoride film in ethanol to remove Tween 80, and dry it at room temperature to obtain the porous polyvinylidene fluoride film with the porous network structure.

2. The method for preparing a porous polyvinylidene fluoride film according to claim 1, characterized in that, In step 2, the solution is allowed to stand and degas under the following conditions: heated to 30°C and held for 2 hours; the natural drying conditions are: air environment, temperature 25°C, humidity 70%, and phase separation time 15 minutes.

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

4. The polyvinylidene fluoride porous film prepared by any one of claims 1-3.

5. The application of the polyvinylidene fluoride porous film of claim 4 in the manufacture of daytime radiative cooling materials for outdoor facilities.

6. An outdoor facility cooling material, characterized in that, It is prepared from the polyvinylidene fluoride porous film as described in claim 4.

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