Solid-phase stretching preparation method of fluororesin radiation cooling film with bicontinuous pore structure

Abstract

The invention relates to the technical field of radiation refrigeration, in particular to a solid-phase stretching preparation method of a fluororesin radiation cooling film with a bicontinuous pore structure, which comprises the following steps: step 1, adding a fluororesin polymer and a functional filler into an extruder, and continuously melting and blending with a green diluent under a high-temperature condition to form a homogeneous melt; step 2, continuously extruding the homogeneous melt obtained in the step 1 into a casting roller or solidification liquid with a set temperature through a mold with a set temperature to form a primary gel film; step 3, carrying out solid-phase stretching on the primary gel film continuously prepared in the step 2, and then carrying out heat setting to form a primary film; and 4, immersing the primary membrane shaped in the step 3 into an extraction liquid, removing the green diluent, and drying to obtain the fluororesin microporous membrane with a bicontinuous pore structure. And 5, feeding the fluororesin microporous membrane obtained in the step 4 into a drying oven, and carrying out annealing treatment to finally prepare the fluororesin radiation cooling membrane with the bicontinuous pore structure.

Description

Technical Field

[0001] The present invention relates to the technical field of radiative cooling, and particularly relates to a solid-phase stretching preparation method of a fluororesin radiation cooling film with a double-continuous pore structure. Background Art

[0002] With the intensification of global warming, the energy crisis and environmental pressure brought about by traditional refrigeration methods have become increasingly prominent. Currently, greenhouse gas emissions in the refrigeration field account for approximately 7% of the global total emissions. According to the "Global Cooling Watch" report released by the United Nations Environment Programme at COP28, it is estimated that by 2050, emissions in the refrigeration field will increase to 4.4 billion - 6.1 billion tons of carbon dioxide equivalent, accounting for more than 10% of the global projected emissions for that year. Traditional refrigeration technologies rely on compression technology, consume a large amount of electrical energy, and make a significant contribution to the greenhouse effect, forming a vicious cycle of "refrigeration dependence - carbon emissions - climate warming".

[0003] Passive Radiative Cooling (PRC), as an emerging cooling technology, has characteristics such as zero energy consumption and zero environmental pollution. The PRC technology realizes an all-weather cooling process by efficiently reflecting sunlight (wavelength range of 0.3 - 2.5 μm) and dissipating heat into outer space through the atmospheric window (wavelength range of 8 - 13 μm). Currently, photon structure coolers have been developed through complex and high-precision manufacturing processes (such as vacuum deposition and electron beam lithography), but these processes are costly and difficult to mass-produce.

[0004] Traditional materials such as photonic crystals or metal structures are costly and complex to process, while polymer microporous membranes have become a current research hotspot due to their adjustable pore structure, high emissivity, and low-cost advantages. However, existing preparation methods for polymer microporous membranes (such as nonsolvent phase separation) have problems such as long production cycles and complex pore structures, resulting in a decline in the flux and mechanical properties of the microporous membranes.

[0005] In addition, the polymer microporous membrane with an interpenetrating network bicontinuous pore structure has the advantages of high reflectivity, high emissivity, long service life, high porosity, good mechanical properties, etc., and is therefore particularly suitable for use as a radiative cooling membrane. However, there is currently little research on the fluororesin microporous membrane with an interpenetrating network bicontinuous structure. Asahi Kasei Corporation of Japan prepared a PVDF hollow fiber membrane by a thermally induced phase separation method in US Patents Nos. 5,022,990 and 6,299,773. In this method, PVDF, diluents (dibutyl phthalate (DBP) and dioctyl phthalate (DOP)), and hydrophobic silica are mixed at a high temperature of 250°C to form a homogeneous system, and then extruded through a twin-screw extruder and immersed in an extractant (such as trichloroethane) at 60°C. PVDF forms a gel membrane as the temperature decreases, and then its hydrophilicity is increased by soaking in an ethanol solution. Finally, the silica particles are extracted by soaking in a sodium hydroxide solution to form a porous PVDF hollow fiber membrane. Although the thermally induced phase separation method is widely used in the preparation of polymer microporous membranes, it uses toxic diluents such as N-methylpyrrolidone (NMP), dibutyl phthalate (DBP), dioctyl phthalate (DOP), and benzophenone (DPK), which impose a burden on the health of industrial workers and the environment.

[0006] Therefore, developing a green preparation method for fluororesin microporous membranes with a bicontinuous structure that has high production efficiency, is environmentally friendly, and has the potential for large-scale production has important research significance and industrial application value for applications in the field of radiative cooling. Summary of the Invention

[0007] The purpose of the present invention is to overcome the defects and deficiencies of the prior art and provide a solid-phase stretching preparation method for a fluororesin radiative cooling membrane with a bicontinuous pore structure. This preparation method can efficiently and greenly continuously produce fluororesin microporous membranes with controllable pore sizes and excellent mechanical properties.

[0008] The purpose of the present invention can be achieved through the following technical solutions: A solid-phase stretching preparation method for a fluororesin radiative cooling membrane with a bicontinuous pore structure, comprising the following steps: Step 1: Add a fluororesin polymer and a functional filler into an extruder, and continuously melt-blend them with a green diluent under high-temperature conditions to form a homogeneous melt; the total content of the fluororesin polymer and the functional filler in the homogeneous melt is 10-40% by mass percentage, and the content of the green diluent in the homogeneous melt is 60-90% by mass percentage; the temperature when the fluororesin polymer, the functional filler, and the green diluent are melt-blended is 170-300°C; Step 2: The homogeneous melt obtained from Step 1 is continuously extruded through a die with a set temperature into a casting roll or a coagulation liquid with a set temperature to form a primary gel film; the temperatures of the casting roll and the coagulation liquid are 5 - 90 °C; the draw ratio of the homogeneous melt is 1 - 15 times, and the traction speed of the primary gel film is 1 - 15 times the extrusion speed of the homogeneous melt; wherein the traction speed of the primary gel film is much greater than the continuous extrusion speed of the homogeneous melt through the die, so that the homogeneous melt is subjected to a stretching effect; Step 3: The primary gel film continuously prepared in Step 2 is subjected to solid-phase stretching and then heat setting to form a primary film; the solid-phase stretching can be unidirectional stretching or bidirectional stretching, the draw ratio is 1 - 10 times, and the temperature is 100 - 160 °C; the temperature of the heat setting is 130 - 180 °C, and the time is 30 s - 5 min; Step 4: The primary film shaped in Step 3 is immersed in an extraction liquid, and after removing the green diluent, it is dried to obtain a fluororesin microporous film with a bicontinuous pore structure; Step 5: The fluororesin microporous film obtained in Step 4 is sent into an oven and annealed at 130 - 250 °C for 0.5 - 10 min, and finally a fluororesin radiative cooling film with a bicontinuous pore structure is prepared.

[0009] As a preference, in Step 1, the fluororesin polymer is one or more of polyvinylidene fluoride, vinylidene fluoride copolymer, vinylidene fluoride - hexafluoropropylene copolymer, vinylidene fluoride - trichloroethylene copolymer, vinylidene fluoride - tetrafluoroethylene copolymer, ethylene - tetrafluoroethylene copolymer, ethylene - trichloroethylene copolymer.

[0010] As a preference, in Step 1, the functional filler is one or more of silica, zirconia, titanium dioxide, magnesium oxide, aluminum oxide, barium titanate, lead titanate, lithium niobate.

[0011] As a preference, in Step 1, the green diluent includes one or more of triacetin, triethyl citrate, tributyl acetylcitrate, triethyl phosphate, γ - butyrolactone, propylene carbonate, dioctyl sebacate, triethylene glycol, polyethylene glycol.

[0012] As a preference, in Step 1, the extruder is a twin - rotor extruder, a twin - screw extruder, a triple - screw extruder, a row - type screw extruder or a biaxial eccentric rotor extruder.

[0013] As a preference, in Step 2, the coagulation liquid is pure water or a mixed solution of the green diluent in Step 1 and pure water, wherein the mass percentage of the green diluent in the mixed solution is 5% - 50%.

[0014] As a preference, in step 4, the porosity of the fluororesin microporous membrane with a bicontinuous pore structure is 40% - 70%, and the average pore size distribution is adjustable from 40 nm to 5 μm.

[0015] As a preference, in step 4, the weight percentage of the fluororesin polymer is 70% - 100%, and the weight percentage of the functional filler is 0% - 30%.

[0016] As a preference, in step 4, the extraction liquid is ethanol or a mixed solution of ethanol and pure water, wherein the mass percentage of ethanol in the mixed solution is 5% - 70%.

[0017] Compared with the prior art, the present invention has the following advantages and beneficial effects: Compared with the prior art, the present invention uses a green diluent in combination with melt stretching and annealing treatment. The green diluent is a solvent that is environmentally friendly during polymer processing. It has low toxicity, biodegradability, renewability and complies with the principles of green chemistry. It can significantly reduce the harm to the environment and human health while ensuring process performance, and can efficiently and greenly continuously prepare a series of fluororesin radiative cooling membranes with adjustable pore structures. The surface and cross-section of the fluororesin radiative cooling membrane are uniform interpenetrating network bicontinuous pore structures, completely eliminating large finger-like pore structures and granular pore structures. This method has the characteristics of environmental friendliness, high production efficiency, low cost and simple process. The produced fluororesin radiative cooling membrane with a bicontinuous pore structure has high reflectivity and emissivity, high ion permeability, and excellent mechanical properties and thermal stability, and can be well applied to the field of radiative cooling. Description of the Drawings

[0018] Figure 1 It is the surface SEM image of the fluororesin radiative cooling membrane with a bicontinuous pore structure; Figure 2 It is the cross-section SEM image of the fluororesin radiative cooling membrane with a bicontinuous pore structure. Detailed Embodiments

[0019] The following further describes the present invention in detail with reference to the embodiments and the drawings, but the embodiments of the present invention are not limited thereto.

[0020] A method for preparing a fluororesin radiative cooling membrane with a bicontinuous pore structure in this embodiment by solid-phase stretching includes the following steps: Step 1: Add 20 parts of polyvinylidene fluoride (PVDF) and 10 parts of spherical silica (SiO2) with a particle size of 1 - 3 μm into a twin-screw extruder. Set the temperature of the feeding section of the twin-screw extruder to 190°C, and the temperatures of other sections to 220°C. Continuously melt-blend PVDF and SiO2 with 70 parts of triethyl citrate (TEC) under high-temperature conditions to form a homogeneous melt; Step 2: Continuously extrude the homogeneous melt obtained in Step 1 through a sheet die at a temperature of 170°C into a casting roll at a temperature of 25°C to form a primary gel film; the extrusion speed of the homogeneous melt is 5 m / min, and the rotational speed of the casting roll is 15 m / min; the draw ratio of the homogeneous melt is 1 - 15 times, and the traction speed of the primary gel film is 1 - 15 times the extrusion speed of the homogeneous melt; among them, the traction speed of the primary gel film is much greater than the continuous extrusion speed of the homogeneous melt through the die, so that the homogeneous melt is subjected to a stretching effect; Step 3: Unidirectionally stretch the primary gel film continuously prepared in Step 2, and then perform heat setting to form a primary film; the draw ratio is 3 times, and the temperature is 150°C; the temperature of the heat setting is 130 - 180°C, and the time is 1 min; Step 4: Immerse the primary film after heat setting in Step 3 into an extraction solution, remove the green diluent and then dry it to obtain a fluororesin microporous membrane with a bicontinuous pore structure; the extraction solution is a mixed solution of ethanol and pure water, wherein the mass percentage of ethanol in the mixed solution is 50%, and the temperature of the extraction solution is 30°C; Step 5: Send the fluororesin microporous membrane obtained in Step 4 into an oven and perform annealing treatment at 150°C for 3 min to finally prepare a fluororesin radiative cooling membrane with a bicontinuous pore structure.

[0021] The fluororesin radiative cooling membrane with a bicontinuous pore structure prepared in this example has a thickness of 60 μm, as Figure 1 and Figure 2 shown, the tensile strength is 12.4 MPa, the porosity is 53%, the average pore diameter is 1.2 microns, the solar reflectance is 94.6%, and the mid-infrared emissivity is 96.1%.

[0022] In addition to the methods mentioned in the above embodiments, in step 1, the fluororesin polymer can be one or more of vinylidene fluoride copolymer, vinylidene fluoride - hexafluoropropylene copolymer, vinylidene fluoride - chlorotrifluoroethylene copolymer, vinylidene fluoride - tetrafluoroethylene copolymer, ethylene - tetrafluoroethylene copolymer, ethylene - chlorotrifluoroethylene copolymer; the functional filler can be one or more of zirconium dioxide (ZrO2), titanium dioxide (TiO2), magnesium oxide (MgO), aluminum oxide (Al2O3), barium titanate (BaTiO3), lead titanate (PbTiO3), lithium niobate (LiNbO3); the green diluent can be one or more of glyceryl triacetate (GTA), tributyl acetylcitrate (ATBC), triethyl phosphate (TEP), γ - butyrolactone (γ - BL), propylene carbonate (PC), dioctyl sebacate (DOS), triethylene glycol (TEG), polyethylene glycol (PEG); the extruder can be a twin - screw extruder, a triple - screw extruder, a row - type screw extruder or a biaxial eccentric rotor extruder; In step 2, the coagulating liquid can be pure water, or a mixed solution of the green diluent in step 1 and pure water, wherein the mass percentage of the green diluent in the mixed solution is 5% - 50%; In step 4, the porosity of the fluororesin microporous membrane with a bicontinuous pore structure can be 40% - 70%, and the average pore size distribution can be adjusted from 40 nm to 5 μm; the weight percentage of the fluororesin polymer can be 70% - 100%, and the weight percentage of the functional filler can be 0% - 30%; the extraction liquid can be ethanol, or a mixed solution of ethanol and pure water, wherein the mass percentage of ethanol in the mixed solution is 5% - 70%. All these transformation methods are within the protection scope of the present invention.

[0023] The above - mentioned embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent should be subject to the appended claims.

Claims

1. A method for preparing a fluororesin radiation cooling film with a bicontinuous pore structure by solid-phase stretching, characterized in that, It includes the following steps: Step 1: Add a fluororesin polymer and a functional filler into an extruder, and continuously melt-blend them with a green diluent under high-temperature conditions to form a homogeneous melt; the total content of the fluororesin polymer and the functional filler in the homogeneous melt is 10-40% by mass, and the content of the green diluent in the homogeneous melt is 60-90% by mass; the temperature during the blending of the fluororesin polymer, the functional filler and the green diluent is 170-300 °C; Step 2: Continuously extrude the homogeneous melt obtained in Step 1 through a die with a set temperature into a casting roll or a coagulation liquid with a set temperature to form a primary gel film; the temperature of the casting roll and the coagulation liquid is 5-90 °C; the draw ratio of the homogeneous melt is 1-15 times, and the traction speed of the primary gel film is 1-15 times the extrusion speed of the homogeneous melt; wherein the traction speed of the primary gel film is much greater than the continuous extrusion speed of the homogeneous melt through the die, so that the homogeneous melt is subjected to a stretching effect; Step 3: Perform solid-phase stretching on the primary gel film continuously prepared in Step 2, and then perform heat setting to form a primary film; the solid-phase stretching can be unidirectional stretching or biaxial stretching, the draw ratio is 1-10 times, and the temperature is 100-160 °C; the temperature of the heat setting is 130-180 °C, and the time is 30 s-5 min; Step 4: Immerse the primary film shaped in Step 3 in an extraction liquid, remove the green diluent and then dry it to obtain a fluororesin microporous film with a bicontinuous pore structure; Step 5: Send the fluororesin microporous film obtained in Step 4 into an oven, perform annealing treatment at 130-250 °C, and the annealing time is 0.5-10 min, and finally prepare a fluororesin radiative cooling film with a bicontinuous pore structure.

2. The solid-phase stretching preparation method of a fluororesin radiation cooling film with a double-continuous pore structure according to claim 1, wherein: In Step 1, the fluororesin polymer is one or more of polyvinylidene fluoride, vinylidene fluoride copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluorochloroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer, ethylene-trifluorochloroethylene copolymer.

3. The solid-phase stretching preparation method of a fluororesin radiation cooling film with a double-continuous pore structure according to claim 1, characterized in that: In Step 1, the functional filler is one or more of silica, zirconia, titanium dioxide, magnesium oxide, aluminum oxide, barium titanate, lead titanate, lithium niobate.

4. A method for preparing a fluororesin radiation cooling film with a double-continuous pore structure by solid-phase stretching according to claim 1, characterized in that: In Step 1, the green diluent includes one or more of triacetin, triethyl citrate, tributyl acetylcitrate, triethyl phosphate, γ-butyrolactone, propylene carbonate, dioctyl sebacate, triethylene glycol, polyethylene glycol.

5. A method for preparing a fluororesin radiation cooling film with a double continuous pore structure by solid-phase stretching according to claim 1, characterized in that: In Step 1, the extruder is a twin-rotor extruder, a twin-screw extruder, a triple-screw extruder, a row-type screw extruder or a biaxial eccentric rotor extruder.

6. The solid-phase stretching preparation method of a fluororesin radiation cooling film with a double-continuous pore structure according to claim 1, characterized in that: In Step 2, the coagulation liquid is pure water, or a mixed solution of the green diluent in Step 1 and pure water, wherein the mass percentage of the green diluent in the mixed solution is 5%-50%.

7. A method for preparing a fluororesin radiation cooling film with a double-continuous pore structure by solid-phase stretching according to claim 1, characterized in that: In Step 4, the porosity of the fluororesin microporous film with a bicontinuous pore structure is 40-70%, and the average pore size distribution is adjustable from 40 nm to 5 μm.

8. A method for preparing a fluororesin radiation cooling film with a double-continuous pore structure by solid-phase stretching according to claim 1, characterized in that: In step 4, the weight percentage of the fluororesin polymer is 70 to 100%, and the weight percentage of the functional filler is 0 to 30%.

9. A method for preparing a fluororesin radiation cooling film with a double continuous pore structure by solid-phase stretching according to claim 1, characterized in that: In step 4, the extraction liquid is ethanol or a mixed solution of ethanol and pure water, wherein the mass percentage of ethanol in the mixed solution is 5% to 70%.

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