Vinylidene fluoride copolymer as well as preparation method and application thereof

Abstract

The invention relates to a vinylidene fluoride copolymer as well as a preparation method and application thereof, and the preparation method of the vinylidene fluoride copolymer comprises the step of polymerizing polyvinylidene fluoride containing carbon-carbon double bonds and urethane acrylate to prepare the vinylidene fluoride copolymer. According to the preparation method of the vinylidene fluoride copolymer, the synthetic route is simple, and the reaction endpoint is definite; the prepared vinylidene fluoride copolymer is used for preparing a polyvinylidene fluoride membrane, and the hydrophilicity of the membrane can be effectively improved.

Description

Technical Field

[0001] The invention relates to a vinylidene fluoride copolymer, in particular to a polyvinylidene fluoride film with strong hydrophilicity. Background Art

[0002] Ultrafiltration membrane is an important member of membrane separation and is increasingly widely used in various fields such as medicine and health, scientific research, industrial production, wastewater treatment and recycling. Polyvinylidene fluoride itself has good thermal stability and chemical stability, and is an ideal membrane separation material, accounting for a large proportion of ultrafiltration membranes. Polyvinylidene fluoride itself has strong hydrophobicity, which leads to low flux and large membrane operation resistance. In addition, organic pollutants adsorbed on the membrane surface during use will cause membrane pollution. Membrane pollution will shorten the service life of ultrafiltration membranes, increase hydraulic cleaning, and thus increase the cost of chemical cleaning and membrane component replacement. In order to reduce operating costs, it is necessary to control membrane pollution, and improving the hydrophilicity of ultrafiltration membranes is one of the important methods to increase the water flux of membranes and control membrane pollution.

[0003] The prior art discloses some methods for modifying polyvinylidene fluoride membranes. For example, CN102240510A discloses a method for preparing a super-hydrophilic polyvinylidene fluoride membrane, which first synthesizes a copolymer containing maleic anhydride, adds it to a casting solution, and after the membrane is formed, immerses it in a solution containing aminosilane for reaction modification to form a surface having a certain hydrophilic component and an organic-inorganic hybrid membrane. However, the later cross-linking modification time is relatively long, and the durability of the hybrid membrane needs to be verified.

[0004] CN103418254A discloses a method for hydrophilic modification of polyvinylidene fluoride membranes. First, a polyethylene glycol methacrylate-methyl methacrylate random copolymer is synthesized, and then the polyvinylidene fluoride membrane is immersed in the random copolymer solution, so that a hydrophilic modifier is adsorbed on the surface of the polyvinylidene fluoride membrane to improve the hydrophilicity of the membrane. However, simple physical adsorption easily causes the problem of loss of hydrophilic substances during use.

[0005] CN110511642A discloses a method for hydrophilically modifying polyvinylidene fluoride microporous filter membrane, which uses polymethyl methacrylate to blend, and then grafts acrylic acid and alcohol monomers to the surface of the formed microfiltration membrane under the action of an initiator to form a hydrophilic layer to improve the hydrophilicity. However, since alcohol monomers do not contain carbon-carbon double bonds, it is easy to cause a low grafting rate and the presence of side reactions such as esterification reactions between acids and alcohols. Summary of the invention

[0006] To overcome at least one defect of the above-mentioned prior art, in a first aspect, an embodiment of the present invention provides a method for preparing a vinylidene fluoride copolymer, comprising polymerizing polyvinylidene fluoride containing carbon-carbon double bonds with polyurethane acrylate to prepare a vinylidene fluoride copolymer.

[0007] In a second aspect, an embodiment of the present invention provides a vinylidene fluoride copolymer, comprising the following polyurethane acrylate structural units and polyvinylidene fluoride structural units:

[0008]

[0009] Wherein, R1 and R2 are each independently methyl or hydrogen, PU represents a polyurethane chain, and PVDF represents a polyvinylidene fluoride chain.

[0010] In a third aspect, an embodiment of the present invention provides a polyvinylidene fluoride film, comprising a vinylidene fluoride copolymer prepared by the above preparation method or the above vinylidene fluoride copolymer.

[0011] In a fourth aspect, an embodiment of the present invention provides a method for preparing the above-mentioned polyvinylidene fluoride membrane by an immersion precipitation phase inversion method.

[0012] The method for preparing a vinylidene fluoride copolymer according to one embodiment of the present invention has a simple synthesis route and a clear reaction endpoint; the prepared vinylidene fluoride copolymer is used to prepare a polyvinylidene fluoride film, which can effectively improve the hydrophilicity of the film. DETAILED DESCRIPTION

[0013] Typical embodiments that embody the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention can have various changes in different embodiments without departing from the scope of the present invention, and the descriptions therein are essentially used as illustrations rather than to limit the present invention.

[0014] An embodiment of the present invention provides a method for preparing a vinylidene fluoride copolymer, comprising polymerizing polyvinylidene fluoride containing carbon-carbon double bonds with polyurethane acrylate to prepare the vinylidene fluoride copolymer.

[0015] In one embodiment, the weight average molecular weight of the vinylidene fluoride copolymer is 70,000 to 310,000, for example, 80,000, 100,000, 120,000, 150,000, 180,000, 200,000, 220,000, 250,000, 280,000, or 300,00

[0016] In one embodiment, the mass ratio of polyvinylidene fluoride containing carbon-carbon double bonds to polyurethane acrylate can be (10-30):(0.1-5), for example, 10:0.5, 10:1, 10:2, 10:3, 10:4, 20:0.5, 20:1, 20:2.5, 20:3, 30:0.5, 30:1, 30:2, 30:4.

[0017] In one embodiment, polyurethane acrylate is prepared by reacting polyurethane containing isocyanate groups with hydroxyl-containing acrylic esters, wherein the hydroxyl-containing acrylic esters include one or more of hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxyethyl acrylate, and hydroxypropyl acrylate. Wherein, an isocyanate group may be included at each end of the polyurethane chain, and the hydroxyl group in the hydroxyl-containing acrylic ester will react with the isocyanate group to form polyurethane acrylate.

[0018] In one embodiment, the weight average molecular weight of the polyvinylidene fluoride containing carbon-carbon double bonds may be 70,000 to 300,000, for example, 80,000, 100,000, 120,000, 150,000, 180,000, 200,000, 220,000, 250,000, or 280,000

[0019] In one embodiment, the preparation method of polyurethane acrylate comprises:

[0020] reacting a polyether polyol with a polyisocyanate to obtain a polyurethane containing an isocyanate group; and

[0021] Polyurethane acrylate is prepared by reacting polyurethane containing isocyanate groups with acrylic ester containing hydroxyl groups.

[0022] In one embodiment, the temperature of the reaction of the polyether polyol and the polyisocyanate may be 70 to 90° C., such as 75° C., 80° C., or 85° C. The molar ratio of the polyether polyol to the polyisocyanate may be 1:2 to 2:3. Furthermore, in the reaction of the polyether polyol and the polyisocyanate, the reaction is controlled by monitoring the content of the isocyanate group in the system, and the reaction endpoint is when the content of the isocyanate group remains unchanged.

[0023] In one embodiment, the polyether polyol may be polyethylene glycol and / or polypropylene glycol; the number average molecular weight of the polyether polyol may be 200 to 1000, and further may be 400 to 600, for example, 200, 400 or 600.

[0024] In one embodiment, the polyisocyanate includes one or more of toluene diisocyanate, diphenylmethane diisocyanate, and isophorone diisocyanate.

[0025] In one embodiment, the number average molecular weight of the polyurethane containing isocyanate groups is 500 to 3000, for example, 600, 800, 1000, 1200, 1500, 1800, 2000, 2200, 2500, 2800.

[0026] In one embodiment, the molar ratio of the polyurethane containing isocyanate groups to the acrylic ester containing hydroxyl groups may be 1:2 to 3:4, for example, 1:2 or 2:3.

[0027] In one embodiment, the reaction temperature of the polyurethane containing isocyanate groups and the acrylic ester containing hydroxyl groups can be 70-90° C., such as 75° C., 80° C., 85° C. Further, in the reaction of the polyurethane containing isocyanate groups and the acrylic ester containing hydroxyl groups, the reaction is controlled by monitoring the content of isocyanate groups in the system, and the reaction endpoint is reached when the content of isocyanate groups is 0.

[0028] In one embodiment, the polyurethane containing isocyanate groups and the acrylic ester containing hydroxyl groups can react under the action of a first catalyst, and the first catalyst can be dibutyltin dilaurate and / or stannous octoate. The amount of the first catalyst can be 0.1-1wt%, such as 0.2wt%, 0.5wt%, 0.8wt%, and the amount of the above catalyst is based on the total mass of the polyurethane containing isocyanate groups and the acrylic ester containing hydroxyl groups.

[0029] In one embodiment, polyvinylidene fluoride containing carbon-carbon double bonds is obtained by subjecting polyvinylidene fluoride to alkali treatment, wherein the alkali treatment comprises: reacting polyvinylidene fluoride at 60 to 80° C. under the action of an alkaline compound and a second catalyst, wherein the polyvinylidene fluoride removes part of HF to form carbon-carbon double bonds, thereby obtaining polyvinylidene fluoride containing carbon-carbon double bonds. Further, the suspension obtained after the reaction can be soaked in water (e.g., deionized water), then filtered, and repeatedly treated until the pH of the supernatant is 7, and then the obtained powder is dried.

[0030] In one embodiment, the reaction temperature of the alkali treatment can be 60-80°C, for example 65°C, 70°C, 75°C; the reaction time of the alkali treatment can be 30-80 minutes, for example 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, 65 minutes, 70 minutes, 75 minutes.

[0031] In one embodiment, the second catalyst may be one or more of tetrabutylammonium bromide, n-tetrabutylammonium hydrogen sulfate, tert-butyl benzoyl peroxide, and tetrabutylammonium hydroxide.

[0032] In one embodiment, the alkaline compound and the solvent may be mixed to form an alkaline compound solution, and then the alkaline compound solution may be mixed with polyvinylidene fluoride and the second catalyst for reaction. Further, based on the total mass of the materials in the alkali treatment reaction system, the mass content of the alkaline compound solution may be 70-90%, such as 75%, 80%, 85%, 89%; the mass content of polyvinylidene fluoride may be 10-30%, such as 15%, 20%, 25%, 29%; the mass content of the second catalyst may be 0.1-1%, such as 0.2%, 0.5%, 0.8%; wherein the total mass of the materials in the alkali treatment reaction system may be the sum of the mass of the alkaline compound, the solvent, polyvinylidene fluoride, and the second catalyst.

[0033] In one embodiment, the alkaline compound may include sodium hydroxide and / or potassium hydroxide; and the solvent used for the alkaline treatment may be ethanol or an aqueous solution of ethanol.

[0034] In one embodiment, the concentration of the alkaline compound in the alkaline compound solution may be 0.5 to 5 mol / L, such as 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L. The alkaline compound solution may be an ethanol solution of potassium hydroxide.

[0035] In one embodiment, the reaction of polyvinylidene fluoride containing carbon-carbon double bonds and polyurethane acrylate is carried out under the action of an initiator, and the initiator may be benzoyl peroxide and / or azobisisobutyronitrile.

[0036] In one embodiment, based on the total mass of the materials of the reaction system of polyvinylidene fluoride containing carbon-carbon double bonds and polyurethane acrylate, the mass content of polyvinylidene fluoride containing carbon-carbon double bonds can be 10-30%, for example, 15%, 20%, 25%; the mass content of polyurethane acrylate can be 0.1-5%, for example, 0.5%, 1%, 2%, 2.5%, 3%, 4%; the mass content of organic solvent can be 50-80%, for example, 55%, 60%, 65%, 70%, 75%; the mass content of initiator can be 0.1-1%, for example, 0.2%, 0.5%, 0.8%; wherein, the total mass of the materials of the reaction system of polyvinylidene fluoride containing carbon-carbon double bonds and polyurethane acrylate can be the sum of the masses of polyvinylidene fluoride containing carbon-carbon double bonds, polyurethane acrylate, organic solvent and initiator.

[0037] In one embodiment, the organic solvent may be one or more of dimethylformamide, dimethylacetamide, and N-methylpyrrolidone.

[0038] In one embodiment, the reaction of polyvinylidene fluoride containing carbon-carbon double bonds and polyurethane acrylate is carried out at a temperature of 60 to 80°C, such as 65°C, 70°C, or 75°C; the reaction time can be 6 to 12 hours, such as 8 hours, 10 hours, or 12 hours; and the reaction can be carried out in a nitrogen atmosphere.

[0039] One embodiment of the present invention provides a vinylidene fluoride copolymer prepared by the above-mentioned preparation method.

[0040] One embodiment of the present invention provides a vinylidene fluoride copolymer, comprising the following polyurethane acrylate structural units and polyvinylidene fluoride structural units:

[0041]

[0042] Wherein, R1 and R2 are each independently methyl or hydrogen, PU represents a polyurethane chain, and PVDF represents a polyvinylidene fluoride chain.

[0043] In one embodiment, the polyurethane acrylate structural unit is obtained by polymerization of the aforementioned polyurethane acrylate, and the polyvinylidene fluoride structural unit is obtained by polymerization of the aforementioned polyvinylidene fluoride containing carbon-carbon double bonds.

[0044] In one embodiment, the mass ratio of the polyvinylidene fluoride structural unit to the polyurethane acrylate structural unit can be (10-30):(0.1-5), for example, 10:0.5, 10:1, 10:2, 10:3, 10:4, 20:0.5, 20:1, 20:3, 30:0.5, 30:1, 30:2, 30:4.

[0045] One embodiment of the present invention provides a polyvinylidene fluoride film, comprising the above-mentioned vinylidene fluoride copolymer.

[0046] One embodiment of the present invention provides a method for preparing the above-mentioned polyvinylidene fluoride membrane by an immersion precipitation phase conversion method.

[0047] In one embodiment, the immersion precipitation phase transformation method includes: configuring a plurality of polymers and solvents into a casting solution; spraying the casting solution from a spinneret, and then rapidly immersing the casting solution in a non-coagulating bath, wherein the solvent diffuses into the coagulating bath, and the non-solvent diffuses into the interior of the membrane, and after the solvent and the non-solvent are exchanged to a certain extent, the membrane is solidified.

[0048] In one embodiment, a method for preparing a polyvinylidene fluoride film comprises:

[0049] Providing a casting solution with a temperature of 70 to 90° C.; and

[0050] The casting solution is formed into membrane fibers through a spinneret and then placed into a coagulation bath to form a membrane through phase separation;

[0051] Wherein, the casting solution includes the above-mentioned vinylidene fluoride copolymer.

[0052] In one embodiment, the temperature of the casting solution may be 75°C, 80°C or 85°C.

[0053] In one embodiment, the casting solution includes a plurality of polymers and a solvent, the plurality of polymers include modified / unmodified polyvinylidene fluoride, the modified / unmodified polyvinylidene fluoride includes the above-mentioned vinylidene fluoride copolymer and polyvinylidene fluoride, and the solvent may be dimethylacetamide. Further, the mass content of the modified / unmodified polyvinylidene fluoride in the casting solution is 10 to 30%, for example, 15%, 20%, 25%.

[0054] In one embodiment, the modified / unmodified polyvinylidene fluoride includes a vinylidene fluoride copolymer and polyvinylidene fluoride, and the mass content of the vinylidene fluoride copolymer in the modified / unmodified polyvinylidene fluoride can be 10-30%, preferably 20-30%, for example, 15%, 25%; the mass content of the polyvinylidene fluoride in the modified / unmodified polyvinylidene fluoride can be 70-90%, preferably 70-80%.

[0055] In one embodiment, the multiple polymers include modified / unmodified polyvinylidene fluoride, and the mass content of modified / unmodified polyvinylidene fluoride in the multiple polymers is 40-65%, for example, 45%, 50%, 55%, and 60%; further, the multiple polymers also include polyvinyl pyrrolidone and polyethylene glycol, and the mass content of polyvinyl pyrrolidone in the multiple polymers is 8-10%, for example, 8.5%, 9%, and 9.5%; the mass content of polyethylene glycol in the multiple polymers is 25-35%, for example, 28%, 30%, and 32%.

[0056] In one embodiment, the weight average molecular weight of polyvinylidene fluoride in the plurality of polymers may be 70,000 to 300,000, for example, 80,000, 100,000, 120,000, 150,000, 180,000, 200,000, 220,000, 250,000, or 280,000; the weight average molecular weight of polyvinyl pyrrolidone may be 10,000 to 160,000, for example, 20,000, 50,000, 80,000, 100,000, 120,000, or 150,000; the weight average molecular weight of polyethylene glycol may be 200 to 600, for example, 400.

[0057] In one embodiment, the casting solution preparation process includes: mixing a plurality of polymers and a solution and stirring the mixture for 8 hours, heating the mixture to 70-90° C., and stirring the mixture into a uniform casting solution.

[0058] The polyvinylidene fluoride membrane according to one embodiment of the present invention can be used as an ultrafiltration membrane.

[0059] In one embodiment, the membrane flux of the polyvinylidene fluoride membrane can be 550 to 830 L / m 2 h, for example 600L / m 2 h, 620L / m 2 h, 650L / m 2 h, 700L / m 2 h, 750L / m 2 h, 800L / m 2 The contact angle between the polyvinylidene fluoride film and water may be 59 to 68°, such as 60°, 62°, or 65°.

[0060] The preparation method of a vinylidene fluoride copolymer according to one embodiment of the present invention uses a polyurethane acrylate monomer as a hydrophilic modifier of polyvinylidene fluoride, and grafts it to the polyvinylidene fluoride molecular chain. Since the polyethylene glycol segment and the carbamate group contained in the monomer have strong hydrophilicity, they can form hydrogen bonds with water, thereby enhancing the wetting ability of water on the membrane surface, so that the hydrophilicity of the membrane prepared by using the copolymer is enhanced. In addition, compared with the blending modification of the prior art, the grafting modification improves the compatibility of the hydrophilic modifier and polyvinylidene fluoride and will not cause loss during the subsequent membrane preparation and use process, so that the hydrophilic modification effect is more durable.

[0061] The following is a further description of a polyvinylidene fluoride film according to an embodiment of the present invention in conjunction with an example. The weight average molecular weight of the polyvinylidene fluoride used in each example and comparative example is about 300,000.

[0062] Example 1

[0063] Preparation of polyurethane acrylate monomer

[0064] Add 200g of polyethylene glycol-200 and 500.52g of diphenylmethane diisocyanate into a three-necked flask, start stirring, heat to 80°C, keep warm for reaction, measure the isocyanate content every 30 minutes, and when the isocyanate content remains unchanged, the reaction ends; cool the reaction product to room temperature, add 260g of hydroxyethyl methacrylate and 1.92g of dibutyltin dilaurate, start stirring, heat to 85°C, measure the isocyanate content every 30 minutes, and reach the reaction end when the isocyanate content is 0, thereby obtaining a polyurethane acrylate monomer.

[0065] Alkali treatment of polyvinylidene fluoride

[0066] In another three-necked flask, add 800 g of 1 mol / L potassium hydroxide ethanol solution, 200 g of polyvinylidene fluoride powder and 3 g of tetrabutylammonium bromide, and react at 60°C for 60 min under stirring; soak the obtained suspension in deionized water, and then filter it with suction, repeat the process several times until the pH value of the powder supernatant is 7, and dry the obtained powder to obtain alkali-treated polyvinylidene fluoride containing carbon-carbon double bonds.

[0067] Preparation of vinylidene fluoride copolymer

[0068] In another three-necked flask, 200 g of alkali-treated polyvinylidene fluoride powder containing carbon-carbon double bonds, 800 g of dimethylacetamide, 25 g of the above-prepared polyurethane acrylate monomer and 3 g of benzoyl peroxide initiator were added, and the mixture was reacted in a nitrogen atmosphere under stirring for 12 h; methanol was added to the obtained reaction solution to precipitate the generated copolymer from the solution, and then the mixture was thoroughly washed with deionized water and dried to obtain a vinylidene fluoride copolymer.

[0069] Preparation of polyvinylidene fluoride membrane

[0070] Mix 100g of the above-mentioned vinylidene fluoride copolymer powder with 900g of polyvinylidene fluoride powder, 150g of polyvinyl pyrrolidone-K30, 500g of polyethylene glycol-400 and 3350g of dimethylacetamide, heat to 85°C under stirring, and stir for 8 hours to form a uniform casting liquid. The casting liquid is formed into membrane fibers through a spinneret and then immersed in a coagulation bath for phase separation and membrane formation.

[0071] Example 2

[0072] Preparation of polyurethane acrylate monomer

[0073] Add 400g of polyethylene glycol-400 and 348g of diphenylmethane diisocyanate into a three-necked flask, start stirring, heat to 75°C, keep warm for reaction, measure the isocyanate content every 30 minutes, and when the isocyanate content remains unchanged, the reaction ends; cool the reaction product to room temperature, add 232g of hydroxyethyl acrylate and 3.92g of dibutyltin dilaurate, start stirring, heat to 80°C, measure the isocyanate content every 30 minutes, and reach the reaction end when the isocyanate content is 0, thereby obtaining a polyurethane acrylate monomer.

[0074] Alkali treatment of polyvinylidene fluoride

[0075] In another three-necked flask, add 800 g of 1.5 mol / L potassium hydroxide ethanol solution, 200 g of polyvinylidene fluoride powder and 4 g of tetrabutylammonium hydroxide, and react at 65°C for 65 min under stirring; soak the obtained suspension in deionized water, and then filter it with suction, and repeat the process several times until the pH value of the powder supernatant is 7, and dry the obtained powder to obtain alkali-treated polyvinylidene fluoride containing carbon-carbon double bonds.

[0076] Preparation of vinylidene fluoride copolymer

[0077] In another three-necked flask, 200 g of alkali-treated polyvinylidene fluoride powder containing carbon-carbon double bonds, 800 g of dimethylacetamide, 25 g of the above-prepared polyurethane acrylate monomer and 4 g of benzoyl peroxide initiator were added, and the mixture was reacted in a nitrogen atmosphere under stirring for 12 h; methanol was added to the obtained reaction solution to precipitate the generated copolymer from the solution, and then the mixture was thoroughly washed with deionized water and dried to obtain a vinylidene fluoride copolymer.

[0078] Preparation of polyvinylidene fluoride membrane

[0079] Mix 200g of the above-mentioned vinylidene fluoride copolymer powder with 800g of polyvinylidene fluoride powder, 150g of polyvinyl pyrrolidone-K30, 500g of polyethylene glycol-400 and 3350g of dimethylacetamide, heat to 85°C under stirring, and stir for 8 hours to form a uniform casting liquid. The casting liquid is formed into membrane filaments through a spinneret and then immersed in a coagulation bath for phase separation and membrane formation.

[0080] Example 3

[0081] Preparation of polyurethane acrylate monomer

[0082] Add 600g of polyethylene glycol-600 and 500.52g of diphenylmethane diisocyanate into a three-necked flask, start stirring, heat to 80°C, keep warm for reaction, measure the isocyanate content every 30 minutes, and when the isocyanate content remains unchanged, the reaction ends; cool the reaction product to room temperature, add 260g of hydroxyethyl methacrylate and 8.16g of stannous octoate, start stirring, heat to 80°C, measure the isocyanate content every 30 minutes, and reach the reaction end when the isocyanate content is 0, thereby obtaining a polyurethane acrylate monomer.

[0083] Alkali treatment of polyvinylidene fluoride

[0084] In another three-necked flask, add 800 g of 2 mol / L potassium hydroxide ethanol solution, 200 g of polyvinylidene fluoride powder and 3.5 g of tetrabutylammonium hydrogen sulfate, and react at 60° C. for 80 min under stirring; soak the obtained suspension in deionized water, and then filter it with suction, repeat the process several times until the pH value of the powder supernatant is 7, and dry the obtained powder to obtain alkali-treated polyvinylidene fluoride containing carbon-carbon double bonds.

[0085] Preparation of vinylidene fluoride copolymer

[0086] In another three-necked flask, 400 g of alkali-treated polyvinylidene fluoride powder containing carbon-carbon double bonds, 1600 g of dimethylacetamide, 60 g of the above-prepared polyurethane acrylate monomer and 10 g of benzoyl peroxide initiator were added, and the mixture was reacted in a nitrogen atmosphere for 12 h under stirring; methanol was added to the obtained reaction solution to precipitate the generated copolymer from the solution, and then the mixture was thoroughly washed with deionized water and dried to obtain a vinylidene fluoride copolymer.

[0087] Preparation of polyvinylidene fluoride membrane

[0088] Mix 300g of the above-mentioned vinylidene fluoride copolymer powder with 700g of polyvinylidene fluoride powder, 150g of polyvinyl pyrrolidone-K30, 500g of polyethylene glycol-400 and 3350g of dimethylacetamide, heat to 85°C under stirring, and stir for 8 hours to form a uniform casting liquid. The casting liquid is formed into membrane fibers through a spinneret and then immersed in a coagulation bath for phase separation and membrane formation.

[0089] Example 4

[0090] Preparation of polyurethane acrylate monomer

[0091] Add 400g of polyethylene glycol-400 and 348g of diphenylmethane diisocyanate into a three-necked flask, start stirring, heat to 75°C, keep warm for reaction, measure the isocyanate content every 30 minutes, and when the isocyanate content remains unchanged, the reaction ends; cool the reaction product to room temperature, add 232g of hydroxyethyl acrylate and 3.92g of dibutyltin dilaurate, start stirring, heat to 80°C, measure the isocyanate content every 30 minutes, and reach the reaction end when the isocyanate content is 0, thereby obtaining a polyurethane acrylate monomer.

[0092] Alkali treatment of polyvinylidene fluoride

[0093] In another three-necked flask, add 800 g of 1.5 mol / L potassium hydroxide ethanol solution, 200 g of polyvinylidene fluoride powder and 4 g of tetrabutylammonium hydroxide, and react at 65°C for 65 min under stirring; soak the obtained suspension in deionized water, and then filter it with suction, and repeat the process several times until the pH value of the powder supernatant is 7, and dry the obtained powder to obtain alkali-treated polyvinylidene fluoride containing carbon-carbon double bonds.

[0094] Preparation of vinylidene fluoride copolymer

[0095] In another three-necked flask, 200 g of alkali-treated polyvinylidene fluoride powder containing carbon-carbon double bonds, 800 g of dimethylacetamide, 25 g of the above-prepared polyurethane acrylate monomer and 4 g of benzoyl peroxide initiator were added, and the mixture was reacted in a nitrogen atmosphere under stirring for 12 h; methanol was added to the obtained reaction solution to precipitate the generated copolymer from the solution, and then the mixture was thoroughly washed with deionized water and dried to obtain a vinylidene fluoride copolymer.

[0096] Preparation of polyvinylidene fluoride membrane

[0097] Mix 100g of the above-mentioned vinylidene fluoride copolymer powder with 900g of polyvinylidene fluoride powder, 150g of polyvinyl pyrrolidone-K30, 500g of polyethylene glycol-400 and 3350g of dimethylacetamide, heat to 85°C under stirring, and stir for 8 hours to form a uniform casting liquid. The casting liquid is formed into membrane fibers through a spinneret and then immersed in a coagulation bath for phase separation and membrane formation.

[0098] Example 5

[0099] Preparation of polyurethane acrylate monomer

[0100] Add 400g of polyethylene glycol-400 and 348g of diphenylmethane diisocyanate into a three-necked flask, start stirring, heat to 75°C, keep warm for reaction, measure the isocyanate content every 30 minutes, and when the isocyanate content remains unchanged, the reaction ends; cool the reaction product to room temperature, add 232g of hydroxyethyl acrylate and 3.92g of dibutyltin dilaurate, start stirring, heat to 80°C, measure the isocyanate content every 30 minutes, and reach the reaction end when the isocyanate content is 0, thereby obtaining a polyurethane acrylate monomer.

[0101] Alkali treatment of polyvinylidene fluoride

[0102] In another three-necked flask, 1600 g of 1.5 mol / L potassium hydroxide ethanol solution, 400 g of polyvinylidene fluoride powder and 8 g of tetrabutylammonium hydroxide were added, and the mixture was reacted at 65° C. for 65 min under stirring; the obtained suspension was immersed in deionized water, and then filtered, and the process was repeated several times until the pH value of the powder supernatant was 7, and the obtained powder was dried to obtain alkali-treated polyvinylidene fluoride containing carbon-carbon double bonds.

[0103] Preparation of vinylidene fluoride copolymer

[0104] In another three-necked flask, 400 g of alkali-treated polyvinylidene fluoride powder containing carbon-carbon double bonds, 1600 g of dimethylacetamide, 50 g of the above-prepared polyurethane acrylate monomer and 8 g of benzoyl peroxide initiator were added, and the mixture was reacted in a nitrogen atmosphere for 12 h under stirring; methanol was added to the obtained reaction solution to precipitate the generated copolymer from the solution, and then the mixture was thoroughly washed with deionized water and dried to obtain a vinylidene fluoride copolymer.

[0105] Preparation of polyvinylidene fluoride membrane

[0106] Mix 300g of the above-mentioned vinylidene fluoride copolymer powder with 700g of polyvinylidene fluoride powder, 150g of polyvinyl pyrrolidone-K30, 500g of polyethylene glycol-400 and 3350g of dimethylacetamide, heat to 85°C under stirring, and stir for 8 hours to form a uniform casting liquid. The casting liquid is formed into membrane fibers through a spinneret and then immersed in a coagulation bath for phase separation and membrane formation.

[0107] Comparative Example 1

[0108] Take 1000g of polyvinylidene fluoride powder, 150g of polyvinyl pyrrolidone-K30, 500g of polyethylene glycol-400 and 3350g of dimethylacetamide, mix them, heat them to 85°C under stirring, stir them for 8 hours to form a uniform casting liquid, and then form the casting liquid into membrane fibers through a spinneret and immerse them in a coagulation bath for phase separation and membrane formation.

[0109] Comparative Example 2

[0110] 800g of polyvinylidene fluoride powder, 200g of polymethacrylate, 150g of polyvinyl pyrrolidone-K30, 500g of polyethylene glycol-400 and 3350g of dimethylacetamide were heated to 85°C under stirring conditions and stirred for 8 hours to form a uniform casting solution. The casting solution was formed into membrane filaments through a spinneret and then immersed in a coagulation bath for phase separation and membrane formation.

[0111] The polyvinylidene fluoride membranes prepared in Examples 1 to 5 and Comparative Examples 1 to 2 were measured for membrane flux and contact angle. The membrane flux was measured according to the "Test Method for Hollow Fiber Ultrafiltration Membrane" (GB / T 32360-2015), the operating pressure was 0.1 MPa, and the test was performed at room temperature to obtain the pure water permeation per unit time and unit membrane area. The static contact angle of a pure water droplet on the membrane surface was measured using a contact angle meter to obtain the contact angle value. See Table 1 for specific results.

[0112] Table 1

[0113] Examples Membrane flux / LMH Contact angle / degree Example 1 550 68 Example 2 620 62 Example 3 830 59 Example 4 580 61 Example 5 730 60 Comparative Example 1 350 79 Comparative Example 2 420 72

[0114] The difference between Example 2 of the present invention and Comparative Examples 1 and 2 is that: in the preparation of the polyvinylidene fluoride membrane, Example 2 uses 200g of polyvinylidene fluoride copolymer and 800g of polyvinylidene fluoride, while Comparative Example 1 uses 1000g of polyvinylidene fluoride, and Comparative Example 2 uses 200g of polymethacrylate and 800g of polyvinylidene fluoride. Further combined with the results of Table 1, it can be seen that in the preparation of the polyvinylidene fluoride membrane, by using the polyvinylidene fluoride copolymer instead of polyvinylidene fluoride and polymethacrylate, the membrane flux of the prepared polyvinylidene fluoride membrane can be significantly improved, and its hydrophilicity can be improved.

[0115] In Examples 1 to 5 of the present invention, polyvinylidene fluoride membranes are prepared by using vinylidene fluoride copolymers, so that the membrane flux can be increased to 550-830, the contact angle is reduced to below 70°, and the hydrophilicity of the membrane is significantly improved.

[0116] Further, the difference between Example 2 of the present invention and Examples 4 and 5 is mainly that: in the preparation of the polyvinylidene fluoride membrane, the vinylidene fluoride copolymer used in Example 4 accounts for 10% of the total mass of the vinylidene fluoride copolymer and polyvinylidene fluoride, the vinylidene fluoride copolymer used in Example 2 accounts for 20%, and the vinylidene fluoride copolymer used in Example 5 accounts for 30%. According to the results in Table 1, as the proportion of the vinylidene fluoride copolymer increases, the membrane flux of the prepared membrane also increases, and the hydrophilicity of the membrane becomes better. Therefore, based on the total mass of the vinylidene fluoride copolymer and polyvinylidene fluoride, the mass content of the vinylidene fluoride copolymer is preferably 10 to 30%, and more preferably 20 to 30%.

[0117] In addition, when the amount of vinylidene fluoride copolymer is the same, Example 4 uses polyethylene glycol with a larger molecular weight as the raw material for synthesizing polyurethane acrylate monomers, and the obtained film is better than Example 1. Example 3 uses polyethylene glycol-600 with a larger molecular weight as the raw material for synthesizing polyurethane acrylate. When the amount of vinylidene fluoride copolymer is the same, the performance of the film obtained by Example 3 is significantly better than that of Example 5. The above results show that the larger the molecular weight of polyethylene glycol, the better the hydrophilic modification effect. Therefore, the number average molecular weight of polyethylene glycol and / or polypropylene glycol used to prepare polyurethane acrylate is preferably 400 to 1000, and more preferably 400 to 600.

[0118] Unless otherwise defined, the terms used in the present invention have the meanings commonly understood by those skilled in the art.

[0119] The embodiments described in the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Those skilled in the art may make various other substitutions, changes and improvements within the scope of the present invention. Therefore, the present invention is not limited to the above embodiments, but is only limited by the claims.

Claims

1. A method for preparing a vinylidene fluoride copolymer, comprising polymerizing polyvinylidene fluoride containing carbon-carbon double bonds with polyurethane acrylate to prepare a vinylidene fluoride copolymer.

2. The preparation method according to claim 1, wherein The mass ratio of the polyvinylidene fluoride containing carbon-carbon double bonds to the polyurethane acrylate is (10-30):(0.1-5); and / or, The weight average molecular weight of the polyvinylidene fluoride containing carbon-carbon double bonds is 70,000 to 300,000; and / or, The reaction temperature of the polyvinylidene fluoride containing carbon-carbon double bonds and the polyurethane acrylate is 60 to 80° C.; and / or, The polyurethane acrylate is prepared by reacting polyurethane containing an isocyanate group with an acrylic ester containing a hydroxyl group, and the acrylic ester containing a hydroxyl group includes hydroxyethyl methacrylate and / or hydroxyethyl acrylate.

3. The preparation method according to claim 2, wherein The preparation method of the polyurethane acrylate comprises: Reacting polyether polyol with polyisocyanate to obtain the polyurethane containing isocyanate group; and The polyurethane containing isocyanate groups is reacted with the acrylic ester containing hydroxyl groups to obtain the polyurethane acrylate; and / or, The number average molecular weight of the polyurethane containing isocyanate groups is 500 to 3000; and / or, The reaction time of the polyvinylidene fluoride containing carbon-carbon double bonds and the polyurethane acrylate is 6 to 12 hours.

4. The preparation method according to claim 3, wherein The molar ratio of the polyurethane containing isocyanate groups to the acrylic ester containing hydroxyl groups is 1:2 to 3:4; and / or, The polyether polyol comprises polyethylene glycol and / or polypropylene glycol; and / or, The number average molecular weight of the polyether polyol is 200 to 1000, further 400 to 1000; and / or, The polyisocyanate includes one or more of toluene diisocyanate, diphenylmethane diisocyanate, and isophorone diisocyanate.

5. The preparation method according to claim 1, wherein The polyvinylidene fluoride is subjected to an alkali treatment to obtain the polyvinylidene fluoride containing carbon-carbon double bonds, wherein the alkali treatment comprises: reacting the polyvinylidene fluoride under the action of an alkaline compound and a second catalyst at 60 to 80°C.

6. The preparation method according to claim 5, wherein: The second catalyst is selected from one or more of tetrabutylammonium bromide, n-tetrabutylammonium hydrogen sulfate, tert-butyl benzoyl peroxide, and tetrabutylammonium hydroxide; and / or, The alkaline compound comprises sodium hydroxide and / or potassium hydroxide; and / or, The reaction time of the alkali treatment is 30 to 80 minutes.

7. A vinylidene fluoride copolymer comprising the following polyurethane acrylate structural units and polyvinylidene fluoride structural units: in, R1 and R2 are each independently methyl or hydrogen, PU represents a polyurethane chain, and PVDF represents a polyvinylidene fluoride chain.

8. A polyvinylidene fluoride film, comprising the vinylidene fluoride copolymer prepared by the preparation method according to any one of claims 1 to 6 or the vinylidene fluoride copolymer according to claim 7.

9. A method for preparing the polyvinylidene fluoride membrane according to claim 8, wherein the membrane is prepared by an immersion precipitation phase inversion method.

10. The preparation method according to claim 9, comprising: Providing a casting solution with a temperature of 70 to 90°C; as well as The casting liquid is formed into membrane filaments and then placed into a coagulation bath to form a membrane through phase separation; The casting solution comprises a plurality of polymers and a solvent, the plurality of polymers comprises modified / unmodified polyvinylidene fluoride, and the modified / unmodified polyvinylidene fluoride comprises 10-30 wt % of the vinylidene fluoride copolymer and 70-90 wt % of polyvinylidene fluoride.

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