Polyvinylidene fluoride resin and preparation method thereof

Abstract

The invention relates to the technical field of macromolecules, in particular to polyvinylidene fluoride resin and a preparation method thereof.The preparation method comprises the following steps that S1, water, a dispersing agent, an initiator and a polyvinylidene fluoride monomer are added into a closed reactor; s2, the closed reactor is heated to the reaction temperature, a polar comonomer solution is supplemented into the closed reactor through an intermittent method, the pressure in the closed reactor is controlled to change between the first pressure and the second pressure in a reciprocating mode, and the polyvinylidene fluoride resin is prepared. The possibility of simultaneously preparing a homopolymerized block and a copolymerized block is realized by combining a batch-type method for supplementing a polar comonomer and a pressure swing reaction process, so that not only is the polymerization uniformity improved, but also the prepared polyvinylidene fluoride resin has excellent thermal stability and high adhesive property.

Description

Technical Field

[0001] The present invention relates to the technical field of polymers, and particularly relates to a polyvinylidene fluoride resin and a preparation method thereof. Background Art

[0002] Polyvinylidene fluoride (PVDF) resin refers to a homopolymer of vinylidene fluoride (VDF) or a copolymer of VDF and a small amount of other vinyl monomers, and its repeating unit is —CH2—CF2—. PVDF resin combines the characteristics of fluororesins and general resins and has excellent comprehensive properties. It is a high-performance thermoplastic known for its excellent chemical resistance, thermal stability, and physical durability.

[0003] The polyvinylidene fluoride resin has the following characteristics: (1) Chemical resistance: PVDF shows excellent resistance to a variety of chemicals, acids, and solvents; (2) Thermal stability: PVDF has a high melting point and good thermal stability, and can maintain its performance in high-temperature environments; (3) Electrical insulation: PVDF is famous for its excellent electrical insulation performance and is commonly used in electrical and electronic applications; (4) Mechanical strength: PVDF has good mechanical strength and toughness and can withstand mechanical stress and impact; (5) UV resistance: PVDF has good resistance to ultraviolet (UV) radiation and is suitable for outdoor applications. The polyvinylidene fluoride resin is widely used in the fields of chemical industry, electrical and electronics, membrane technology, medical equipment, and lithium batteries.

[0004] In the prior art, in order to improve the bonding strength of polyvinylidene fluoride, Solvay disclosed a linear semi-crystalline copolymer in Chinese Patent Document CN101679563A. The polymerization process is the copolymerization of vinylidene fluoride monomer and hydrophilic (meth) acrylic acid monomer. This patent improves the bonding strength of polyvinylidene fluoride resin by introducing a polar copolymer monomer, but the resulting copolymer resin has poor thermal stability and limited improvement in bonding strength. In addition, in the industry, a copolymer resin formed by copolymerizing vinylidene fluoride monomer and hydrophilic (meth) acrylic acid monomer is directly mixed into the homopolymer resin. The uneven distribution of the polar copolymer monomer in the resulting resin composition leads to low bonding strength, and there are also problems of poor thermal stability. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defects of low bonding strength and poor thermal stability of polyvinylidene fluoride resin in the prior art, so as to provide a polyvinylidene fluoride resin and a preparation method thereof.

[0006] For this purpose, the present application provides a preparation method of a polyvinylidene fluoride resin, including the following steps: Step S1: Add water, a dispersant, an initiator, and a vinylidene fluoride monomer to a closed reactor; Step S2: Heat the closed reactor to the reaction temperature, and intermittently add a polar comonomer solution into the closed reactor to control the pressure in the closed reactor to reciprocate between a first pressure and a second pressure, thereby obtaining a polyvinylidene fluoride resin; wherein, the first pressure is M1, the second pressure is M2, 9 MPa ≤ M1 ≤ 14 MPa, the difference between the first pressure and the second pressure is M1 - M2, 0.1 MPa ≤ M1 - M2 ≤ 1.2 MPa, and the polar comonomer has the structural formula shown in the following formula (I): R1R2C=CR3—COOR4 (I); wherein, R1, R2, and R3 are the same or different, and R1, R2, and R3 independently selected from H or a linear or branched C1-C20 hydrocarbon group; R4 is selected from H, a linear or branched C1-C20 hydrocarbon group, or a hydroxyl-substituted linear or branched C1-C20 hydrocarbon group.

[0007] Further, R1, R2, and R3 are the same or different, and R1, R2, and R3 independently selected from H or a linear or branched C1-C3 alkyl group; R4 is selected from H, a linear or branched C1-C8 alkyl group, or a hydroxyl-substituted linear or branched C1-C8 alkyl group.

[0008] Further, R1 and R2 are both H, R3 is selected from H or methyl; R4 is selected from H, a hydroxyl-substituted ethyl group, butyl group, or octyl group.

[0009] Further, the polar comonomer is selected from one or more of acrylic acid, methacrylic acid, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, and butyl acrylate.

[0010] Further, in Step S2, the total addition amount of the polar comonomer is 0.2 wt% - 5 wt% of the mass of the vinylidene fluoride monomer; and / or, the mass percentage of the polar comonomer in the polar comonomer solution is 0.4 wt% - 8 wt%; and / or, the solvent of the polar comonomer solution includes water.

[0011] Further, the dispersant is selected from one or more of hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, polyvinyl alcohol, and polyethylene glycol; and / or, the initiator is selected from one or more of diethyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, benzoyl peroxide, and tert-butyl peroxypivalate; and / or, the addition amount of the dispersant is 0.04 wt% - 0.4 wt% of the mass of the vinylidene fluoride monomer; and / or, the addition amount of the initiator is 0.05 wt% - 0.42 wt% of the mass of the vinylidene fluoride monomer; and / or, the mass of water is 1 - 3 times the mass of the vinylidene fluoride monomer.

[0012] Further, in the step S1, the step of adding a fluorinated comonomer and / or a chain transfer agent is further included in the reactor.

[0013] Further, the fluorinated comonomer is selected from fluorinated olefins; and / or, the chain transfer agent is selected from one or more of ethyl acetate, dimethyl carbonate, methyl ethyl carbonate, acetone, and methyl propionate; and / or, the addition amount of the fluorinated comonomer is 0.05 wt% - 0.42 wt% of the mass of the vinylidene fluoride monomer; and / or, the addition amount of the chain transfer agent is 0 - 3 wt% of the mass of the vinylidene fluoride monomer.

[0014] Further, the fluorinated comonomer is selected from one or more of vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, hexafluoroisobutene, perfluorobutylethylene, and pentafluoropropylene.

[0015] Further, in the step S2, the step of replenishing the polar comonomer solution into the closed reactor by the batch method specifically includes replenishing the polar comonomer solution into the closed reactor, stopping the replenishment when the pressure in the closed reactor rises to the first pressure, allowing the material to undergo a polymerization reaction to reduce the pressure in the closed reactor, and replenishing the polar comonomer solution again when the pressure in the closed reactor drops to the second pressure; then repeating the above steps until the addition of the polar comonomer solution is completed and the reaction ends; and / or, 8 MPa ≤ M2 ≤ 13 MPa; and / or, in the step S2, the reaction temperature is 45 - 60 °C; and / or, after the reaction ends, the steps of washing, filtering, and drying the reactants are further included.

[0016] Further, the washing temperature is 50 - 80 °C.

[0017] Further, the drying temperature is 80 - 100 °C.

[0018] The closed reactor can be a conventional reactor in the art, such as a high-pressure reaction kettle.

[0019] The closed reactor starts to react when heated to the reaction temperature.

[0020] The present application also provides a polyvinylidene fluoride resin prepared by the preparation method described in any one of the above.

[0021] The technical solution of the present invention has the following advantages: 1. The preparation method of polyvinylidene fluoride resin provided by this application includes the following steps: Step S1: Add water, dispersant, initiator and vinylidene fluoride monomer into a closed reactor; Step S2: Heat the closed reactor to the reaction temperature, and use an intermittent method to supplement the polar copolymer monomer solution into the closed reactor to control the pressure in the closed reactor to reciprocate between a first pressure and a second pressure, and obtain polyvinylidene fluoride resin; wherein, the first pressure is M1, the second pressure is M2, where 9 ≤ M1 ≤ 14 MPa, the difference between the first pressure and the second pressure is M1 - M2, and 0.1 MPa ≤ M1 - M2 ≤ 1.2 MPa; by using an intermittent method to supplement the polar copolymer monomer solution, the copolymerization reaction of vinylidene fluoride monomer under variable pressure is realized. During the polymerization process, the polar copolymer monomer solution shown in the general formula (I) is intermittently supplemented into the reaction kettle to make the reaction pressure reciprocate within the above specific range. During the addition stage of the polar copolymer monomer solution, the polar copolymer monomer unit is connected to the main chain of vinylidene fluoride to obtain a copolymer block, thereby improving the bonding strength of the resin. During the stage when the addition of the polar copolymer monomer solution stops, more vinylidene fluoride homopolymer blocks are prepared, thereby improving the thermal stability of the resin. Therefore, the possibility of simultaneously preparing homopolymer blocks and copolymer blocks is realized by combining the intermittent method of supplementing the polar copolymer monomer with the variable pressure reaction process, which not only improves the uniform distribution of the polar copolymer monomer and the bonding strength of the polyvinylidene fluoride resin, but also significantly improves the thermal stability of the polyvinylidene fluoride resin.

[0022] In addition, it is found through research that when the difference M1 - M2 between the first pressure and the second pressure is too small, the polar copolymer monomer solution will be added too frequently, and the degree of product modification will be too large, resulting in insufficient thermal stability of the product. When M1 - M2 is too large, the polymerization will be mainly homopolymerization, and the product modification will be insufficient, resulting in the bonding performance not meeting the requirements. In addition, when the first pressure M1 is too small, the reaction pressure will be too low and the monomer concentration will be insufficient, resulting in limited molecular chain growth and low product molecular weight, and both the bonding strength and thermal stability cannot meet the requirements. When M1 is too large, the reaction pressure will be too high, the monomer concentration will be too high, and the reaction rate will be too fast, and the product with a moderate molecular weight cannot be obtained either, and both the bonding strength and thermal stability cannot meet the requirements. By controlling the first pressure between 9 MPa and 14 MPa in this application and combining the control of the difference M1 - M2 between the first pressure and the second pressure within 0.1 MPa - 1.2 MPa, the replenishment cycle of the polar monomer is moderate, so that the relative amounts of the homopolymer molecules and modified molecules prepared are relatively optimal, and the product has both high bonding strength and good thermal stability, and the comprehensive performance is excellent. Detailed implementation mode

[0023] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0024] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be purchased commercially.

[0025] 1. Main sources of raw materials Vinylidene fluoride: Wanhua Chemical Group Co., Ltd., industrial products; Polyvinyl alcohol (alcoholysis degree 80%): Yingzhan Chemical, industrial products; Hydroxypropyl methylcellulose: Merck Chemical, analytical grade; Benzoyl peroxide: Aladdin, analytical grade; Diethyl peroxydicarbonate: Aladdin, analytical grade; Ethyl acetate: Sigma, analytical grade; Dimethyl carbonate: Sigma, analytical grade; Acrylic acid: Wanhua Chemical Group Co., Ltd., industrial products; Hydroxyethyl acrylate: Inokane, industrial product; Hexafluoropropylene: Hunan Yuanchuang Company, industrial products; Chlorotrifluoroethylene: Hunan Yuanchuang Company, industrial product.

[0026] 2. Main test methods The polyvinylidene fluoride resin produced in each embodiment or comparative example was subjected to the following performance tests: (1) Bond strength According to the IS04624 standard (adhesion pull-off test), the bonding strength of the polyvinylidene fluoride resin after being prepared into an electrode is measured. The preparation process of the electrode is as follows: at room temperature, 1g of the polyvinylidene fluoride resin produced in the embodiment or the comparative example is dissolved in 50g of NMP under mechanical stirring. 2g of conductive carbon black and 30g of lithium cobaltate (LiCoO2) are added under stirring, and fully mixed to ensure uniformity. The mixture is then degassed under vacuum and spread on aluminum foil with a scraper, and finally the aluminum foil spread with the electrode material mixture is placed in a vacuum oven and heated to 60°C for 12 hours to prepare an electrode sheet. The bonding strength of the electrode material on the electrode sheet is measured using an INSTRON tensile tester.

[0027] (2) Thermal stability Weigh approximately 200 g of polyvinylidene fluoride resin powder. Using a twin-screw extruder, extrude and pelletize it under the conditions that the temperature of each barrel section is stable at 200 °C and the screw speed is 50 r / min to obtain polymer pellets. Weigh an appropriate amount of the polymer pellets and prepare them into a 4-mm thick plate by a molding method. Heat it in an oven at 250 °C for 1 hour. Then, test it with a Hunter Lab CQX3576 yellowness meter, and characterize the degree of yellowing with the yellow index (YID1925). The lower the YID index, the better the high-temperature stability.

[0028] (3)Random distribution The portion of randomly distributed second monomer units represents the percentage between the average number (%) of second monomer sequences (the sequences include those between two repeating units derived from the first monomer) and the total average number (%) of second monomer units:

[0029] The first monomer is a vinylidene fluoride monomer. The second monomer is a polar comonomer.

[0030] When each second monomer repeating unit is separated, that is, included between two repeating units of the first monomer, the average number of second monomer sequences is equal to the total average number of second monomer units. At this time, the portion of randomly distributed second monomer units is 100%.

[0031] The total average number of second monomer units in the polymer can be determined by acid-base titration of carboxyl groups. Weigh 0.5 g of the polymer sample into acetone at a temperature of 70 - 80 °C. Dropwise add 5 mL of water with sufficient stirring to avoid precipitation of the polymer. Then titrate it with a 0.1 N NaOH solution until complete neutralization. The titration endpoint is approximately -270 mV. Based on the measured acid equivalent, the second monomer content is determined, and further the total average number of second monomer units (i.e., the mole percentage of second monomer units) is determined.

[0032] The average number (%) of second monomer sequences can be determined by 19 F-NMR. It has been found that the signal (the bold part in the following chemical formula) related to the CF2 part of the vinylidene fluoride unit adjacent to the separated second monomer unit corresponds to a peak at -94.8 ppm. According to the ratio of the intensity of the polar monomer signal to all the peaks of this spectrum, determine the average number of second monomer sequences per 100 first monomer units (i.e., the mole percentage of second monomer sequences), and thus finally determine the portion of randomly distributed second monomer units as the ratio between the mole percentage of second monomer sequences and the mole percentage of second monomer units.

[0033] Example 1 This example provides a method for preparing polyvinylidene fluoride resin, which includes the following steps: (1) Add 22.4 g of the polar copolymerizable monomer acrylic acid to 950 g of deionized water, mix evenly to obtain an acrylic acid solution for standby. Add 2100 g of deionized water to a 5 L high-pressure reactor equipped with a stirrer. After starting the stirring, add 2.8 g of the dispersant hydroxypropyl methylcellulose, 16.8 g of the chain transfer agent ethyl acetate, 3.5 g of the initiator diethyl peroxydicarbonate, 60.2 g of the fluorine-containing copolymerizable monomer hexafluoropropene, and then add 1400 g of the vinylidene fluoride monomer to obtain a mixed solution.

[0034] (2) Raise the temperature of the reactor to 53 °C. After starting the reaction, add the acrylic acid solution to the reactor. When the pressure in the reactor reaches 13.2 MPa (the first pressure, denoted as M1), stop adding. Continue the reaction, and the pressure in the reactor drops. When the pressure in the reactor is as low as 12.2 MPa (the second pressure, denoted as M2), add the acrylic acid solution again. Repeat the above steps of adding and stopping the addition of the acrylic acid solution to make the pressure in the reactor fluctuate reciprocally between 12.2 MPa and 13.2 MPa until the addition of the acrylic acid solution prepared in step (1) is completely finished and the reaction ends. Cool down the reactor and discharge the material. Use deionized water as a detergent to wash the reaction product at 65 °C. After the washing is completed, filter the reaction product and add it to an oven at 95 °C. After drying, obtain the polyvinylidene fluoride resin product.

[0035] Example 2 This example provides a method for preparing polyvinylidene fluoride resin, which includes the following steps: (1) Add 63 g of the polar copolymerizable monomer hydroxyethyl acrylate to 950 g of deionized water, mix evenly to obtain a hydroxyethyl acrylate solution for standby. Add 1540 g of deionized water to a 5 L high-pressure reactor equipped with a stirrer. After starting the stirring, add 0.84 g of the dispersant hydroxypropyl methylcellulose, 5.6 g of the chain transfer agent ethyl acetate, 5.74 g of the initiator benzoyl peroxide, 109.2 g of the fluorine-containing copolymerizable monomer hexafluoropropene, and then add 1400 g of the vinylidene fluoride monomer to obtain a mixed solution.

[0036] (2) Raise the temperature of the reactor to 47 °C. After starting the reaction, add the hydroxyethyl acrylate solution to the reactor. When the pressure in the reactor reaches 9.8 MPa, stop adding. Continue the reaction, and the pressure in the reactor drops. When the pressure in the reactor is as low as 9.5 MPa, add the hydroxyethyl acrylate solution again. Repeat the above steps of adding and stopping the addition of the hydroxyethyl acrylate solution to make the reaction pressure fluctuate reciprocally between 9.5 MPa and 9.8 MPa. After the addition of the hydroxyethyl acrylate solution is completely finished, the reaction ends. Cool down the reactor and discharge the material. Use deionized water as a detergent to wash the reaction product at 52 °C. After the washing is completed, filter the reaction product and add it to an oven at 92 °C. After drying, obtain the polyvinylidene fluoride resin product.

[0037] Example 3 This embodiment provides a method for preparing polyvinylidene fluoride resin, which comprises the following steps: (1) Add 4.2 g of polar comonomer acrylic acid to 950 g of deionized water and mix evenly to obtain an acrylic acid solution for standby. Add 2380 g of deionized water to a 5 L high-pressure reactor equipped with a stirrer. After starting the stirring, add 5.3 g of dispersant polyvinyl alcohol (degree of alcoholysis 80%), 36.4 g of chain transfer agent ethyl acetate, 0.98 g of initiator diethyl peroxydicarbonate, 25.2 g of fluorine-containing comonomer trichloroethylene, and then add 1400 g of vinylidene fluoride monomer to obtain a mixed solution.

[0038] (2) Raise the temperature of the reactor to 58 °C. After starting the reaction, supplement the acrylic acid solution to the reactor. When the pressure in the reactor reaches 13.6 MPa, stop supplementing and continue the reaction. The pressure in the reactor drops. When the pressure in the reactor is as low as 12.4 MPa, supplement the acrylic acid solution again. Repeat the above steps of supplementing and stopping the addition of the acrylic acid solution to make the reaction pressure reciprocate between 12.4 MPa and 13.6 MPa. After all the acrylic acid solution is supplemented, the reaction ends. Cool down the reactor and discharge the material. Use deionized water as a detergent to wash the reaction product at 78 °C. After the washing is completed, filter the reaction product and add it to an oven at 83 °C. After drying, a polyvinylidene fluoride resin product is obtained.

[0039] Example 4 This embodiment provides a method for preparing polyvinylidene fluoride resin, which comprises the following steps: (1) Add 30.8 g of polar comonomer hydroxyethyl acrylate to 950 g of deionized water and mix evenly to obtain a hydroxyethyl acrylate solution for standby. Add 2240 g of deionized water to a 5 L high-pressure reactor equipped with a stirrer. After starting the stirring, add 2.4 g of dispersant hydroxypropyl methylcellulose, 21 g of chain transfer agent dimethyl carbonate, 2.5 g of initiator diethyl peroxydicarbonate, 50.4 g of fluorine-containing comonomer trichloroethylene, and then add 1400 g of vinylidene fluoride monomer to obtain a mixed solution.

[0040] (2) Raise the temperature of the reactor to 51 °C. After starting the reaction, supplement the hydroxyethyl acrylate solution to the reactor. When the pressure in the reactor reaches 12.5 MPa, stop supplementing and continue the reaction. The pressure in the reactor drops. When the pressure in the reactor is as low as 11.9 MPa, start supplementing. Repeat the above steps of supplementing and stopping the addition of the hydroxyethyl acrylate solution to make the reaction pressure reciprocate between 11.9 MPa and 12.5 MPa. After all the hydroxyethyl acrylate solution is supplemented, the reaction ends. Cool down the reactor and discharge the material. Use deionized water as a detergent to wash the reaction product at 60 °C. After the washing is completed, filter the reaction product and add it to an oven at 95 °C. After drying, a polyvinylidene fluoride resin product is obtained.

[0041] Example 5 This example provides a method for preparing polyvinylidene fluoride resin, which is basically the same as that in Example 1, except that in step (1), the fluorine-containing comonomer hexafluoropropylene is not added to the mixed solution.

[0042] Example 6 This example provides a method for preparing polyvinylidene fluoride resin, which is basically the same as that in Example 1, except that in step (1), butyl acrylate of the same mass is used instead of acrylic acid to prepare a butyl acrylate solution, and in step (2), "supplementing acrylic acid solution" is replaced by "supplementing butyl acrylate solution".

[0043] Example 7 This example provides a method for preparing polyvinylidene fluoride resin, which is basically the same as that in Example 1, except that in step (1), the chain transfer agent ethyl acetate is not added to the mixed solution.

[0044] Comparative Example 1 This comparative example provides a method for preparing polyvinylidene fluoride resin, which includes the following steps: (1) Add 2100 g of deionized water to a 5 L high-pressure reactor equipped with a stirrer. After starting the stirring, add 2.8 g of dispersant hydroxypropyl methylcellulose, 16.8 g of chain transfer agent ethyl acetate, 22.4 g of polar comonomer acrylic acid, 3.5 g of initiator diethyl peroxydicarbonate, 60.2 g of fluorine-containing comonomer hexafluoropropylene, and then add 1400 g of vinylidene fluoride monomer to obtain a mixed solution.

[0045] (2) Raise the temperature of the reactor to 53 °C. After starting the reaction, supplement deionized water to the reactor. Stop supplementing when the pressure in the reactor reaches 13.2 MPa, and start supplementing deionized water again when the pressure in the reactor is lower than 12.2 MPa. Repeat the above steps of supplementing and stopping the addition of deionized water to make the pressure in the reactor reciprocate between 12.2 MPa and 13.2 MPa until all 950 g of deionized water is supplemented and the reaction ends. Cool the reactor and discharge the material. Wash the reaction product with deionized water as a detergent at 65 °C. After the washing is completed, filter the reaction product and add it to an oven at 95 °C. After drying, a polyvinylidene fluoride resin product is obtained.

[0046] Comparative Example 2 This comparative example provides a method for preparing polyvinylidene fluoride resin, which includes the following steps: (1) Add 22.4 g of the polar copolymer monomer acrylic acid to 950 g of deionized water and mix evenly to obtain an acrylic acid solution for standby. Add 2100 g of deionized water to a 5 L high-pressure reaction kettle with stirring. After starting the stirring, add 2.8 g of the dispersant hydroxypropyl methylcellulose, 16.8 g of the chain transfer agent ethyl acetate, 3.5 g of the initiator diethyl peroxydicarbonate, 60.2 g of the fluorine-containing copolymer monomer hexafluoropropene, and then add 1400 g of the vinylidene fluoride monomer to obtain a mixed solution.

[0047] (2) Raise the temperature of the reaction kettle to 53 °C. After starting the reaction, add the acrylic acid solution to the reaction kettle to maintain the pressure in the reaction kettle at 13.2 MPa until the addition of the acrylic acid solution prepared in step (1) is completed and the reaction ends. Cool down the reaction kettle and discharge the material. Use deionized water as the detergent to wash the reaction product at 65 °C. After the washing is completed, filter the reaction product and add it to an oven at 95 °C. After drying, obtain the polyvinylidene fluoride resin product.

[0048] Comparative Example 3 It is basically the same as Example 1, except that the second pressure in step (2) is adjusted to 11.7 MPa.

[0049] Comparative Example 4 It is basically the same as Example 1, except that the second pressure in step (2) is adjusted to 13.15 MPa.

[0050] Comparative Example 5 It is basically the same as Example 1, except that the first pressure in step (2) is adjusted to 15 MPa and the second pressure is adjusted to 14 MPa.

[0051] Comparative Example 6 It is basically the same as Example 1, except that the first pressure in step (2) is adjusted to 8 MPa and the second pressure is adjusted to 7 MPa.

[0052] Comparative Example 7 Take 50 g of the polyvinylidene fluoride resin sample prepared in Comparative Example 1 and another 50 g of the polyvinylidene fluoride resin sample prepared in Comparative Example 2, mix them evenly to obtain a physical blend sample.

[0053] The properties of the polyvinylidene fluoride resins obtained in the above examples and comparative examples are shown in Table 1 below: Table 1 Performance Results of Examples and Comparative Examples

[0054] The results show that compared with Comparative Examples 1-7, the bonding strength and thermal stability of the polyvinylidene fluoride resins prepared in Examples 1-7 of this application are significantly improved, especially in Example 1.

[0055] Obviously, the above embodiments are merely examples for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. And the obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A method for preparing polyvinylidene fluoride resin, characterized in that, It includes the following steps: Step S1: Add water, a dispersant, an initiator, and vinylidene fluoride monomer into a closed reactor; Step S2: Heat the closed reactor to the reaction temperature, and use an intermittent method to replenish a polar comonomer solution into the closed reactor to control the pressure in the closed reactor to reciprocate between a first pressure and a second pressure, thereby obtaining a polyvinylidene fluoride resin; wherein, the first pressure is M1, the second pressure is M2, where 9 MPa ≤ M1 ≤ 14 MPa, the difference between the first pressure and the second pressure is M1 - M2, 0.1 MPa ≤ M1 - M2 ≤ 1.2 MPa, and the polar comonomer has the structural formula shown in the following formula (I): R1R2C=CR3—COOR4 (I); wherein, R1, R2, and R3 are the same or different from each other, and R1, R2, and R3 are independently selected from H or a linear or branched C1-C20 hydrocarbon group; R4 is selected from H, a linear or branched C1-C20 hydrocarbon group, or a linear or branched C1-C20 hydrocarbon group substituted with a hydroxyl group.

2. The preparation method of the polyvinylidene fluoride resin according to claim 1, characterized in that, R1, R2, and R3 are the same or different from each other, and R1, R2, and R3 are independently selected from H or a linear or branched C1-C3 alkyl group; R4 is selected from H, a linear or branched C1-C8 alkyl group, or a linear or branched C1-C8 alkyl group substituted with a hydroxyl group.

3. The preparation method of the polyvinylidene fluoride resin according to claim 2, wherein, R1 and R2 are both H, R3 is selected from H or methyl; R4 is selected from H, a hydroxyl-substituted ethyl group, a butyl group, or an octyl group.

4. The preparation method of the polyvinylidene fluoride resin according to claim 1, wherein, The polar comonomer is selected from one or more of acrylic acid, methacrylic acid, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, butyl acrylate; and / or, in step S2, the total addition amount of the polar comonomer is 0.2 wt% - 5 wt% of the mass of the vinylidene fluoride monomer; and / or, the mass percentage of the polar comonomer in the polar comonomer solution is 0.4 wt% - 8 wt%; and / or, the solvent of the polar comonomer solution includes water.

5. The preparation method of the polyvinylidene fluoride resin according to any one of claims 1-4, characterized in that, The dispersant is selected from one or more of hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, polyvinyl alcohol, polyethylene glycol; and / or, the initiator is selected from one or more of diethyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, benzoyl peroxide, tert-butyl peroxypivalate; and / or, the addition amount of the dispersant is 0.04 wt% - 0.4 wt% of the mass of the vinylidene fluoride monomer; and / or, the addition amount of the initiator is 0.05 wt% - 0.42 wt% of the mass of the vinylidene fluoride monomer; and / or, the mass of water is 1 - 3 times the mass of the vinylidene fluoride monomer.

6. The preparation method of the polyvinylidene fluoride resin according to any one of claims 1-4, characterized in that, In step S1, the step of adding a fluorinated comonomer and / or a chain transfer agent is further included in the reactor.

7. The preparation method of the polyvinylidene fluoride resin according to claim 6, characterized in that, The fluorinated comonomer is selected from fluorinated olefins; and / or, the chain transfer agent is selected from one or more of ethyl acetate, dimethyl carbonate, methyl ethyl carbonate, acetone, and methyl propionate; and / or, the addition amount of the fluorinated comonomer is 0.05 wt% - 0.42 wt% of the mass of the vinylidene fluoride monomer; and / or, the addition amount of the chain transfer agent is 0 - 3 wt% of the mass of the vinylidene fluoride monomer.

8. The preparation method of the polyvinylidene fluoride resin according to claim 7, wherein The fluorinated comonomer is selected from one or more of vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, hexafluoroisobutene, perfluorobutylethylene, and pentafluoropropylene.

9. The preparation method of the polyvinylidene fluoride resin according to any one of claims 1-4, characterized in that, In the step S2, the method of intermittently adding the polar comonomer solution into the closed reactor specifically includes adding the polar comonomer solution into the closed reactor, stopping the addition when the pressure in the closed reactor rises to the first pressure, allowing the material to undergo a polymerization reaction to reduce the pressure in the closed reactor, and adding the polar comonomer solution again when the pressure in the closed reactor drops to the second pressure; then repeating the above steps until the addition of the polar comonomer solution is completed and the reaction ends; and / or, 8 MPa ≤ M2 ≤ 13 MPa; and / or, in the step S2, the reaction temperature is 45 - 60 °C; and / or, after the reaction ends, it further includes the steps of washing, filtering, and drying the reactants.

10. A polyvinylidene fluoride resin prepared by the preparation method according to any one of claims 1 - 9.

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