PVDF (Polyvinylidene Fluoride) ultrafiltration membrane as well as preparation method and application thereof

Abstract

The invention relates to the technical field of membrane separation, in particular to a PVDF (Polyvinylidene Fluoride) ultrafiltration membrane as well as a preparation method and application thereof. According to the invention, PVDF is subjected to in-situ modification through copolymerization of vinylpyridine, hydroxyl acrylate and vinylidene fluoride, so that the hydrophilicity, heat resistance and flexibility of PVDF are improved, and titanium oxide is subjected to multi-component modification, so that the dispersibility of titanium oxide and the compatibility with modified PVDF are improved; the modified PVDF, the modified titanium oxide and the pore-forming agent are used as raw materials to prepare the ultrafiltration membrane, so that the prepared PVDF ultrafiltration membrane has high water treatment flux, the mechanical property, the anti-fouling property and the stability of the PVDF ultrafiltration membrane are improved, and the prepared PVDF ultrafiltration membrane has a good water treatment effect and has a good application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, and specifically to a PVDF ultrafiltration membrane, a preparation method thereof, and an application thereof. Background Art

[0002] Ultrafiltration is a pressure-driven membrane separation technology between microfiltration and nanofiltration. Existing ultrafiltration membranes are usually made of various polymer materials, such as cellulose acetate, cellulose acetate esters, polyethylene, polysulfone, polyamide, aromatic polymers, and fluoropolymer polymer materials, etc.

[0003] Among fluoropolymer polymer materials, PVDF ultrafiltration membranes have a high removal rate of organic substances, bacteria, and viruses, and have been widely used in many fields such as water purification, sewage treatment, and biomedicine. However, the hydrophobicity of PVDF itself causes it to be easily contaminated during use, resulting in poor antifouling performance, which greatly reduces the service life and separation efficiency of the membrane.

[0004] In response to the problem of poor antifouling performance, some studies have adopted the method of in-situ modification of PVDF for improvement, which has obvious effects in improving antifouling performance. However, due to the introduction of other groups or segments into PVDF, it will instead destroy the crystallization of PVDF, affecting its chemical stability, mechanical properties, heat resistance, etc. Summary of the Invention

[0005] The purpose of the present invention is to provide a PVDF ultrafiltration membrane, a preparation method thereof, and an application thereof, so as to solve the problems of poor antifouling performance, thermal stability, and mechanical properties of PVDF ultrafiltration membranes proposed in the above background art, and to provide a PVDF ultrafiltration membrane with good antifouling performance, thermal stability, and mechanical properties.

[0006] In the first aspect, a preparation method of a PVDF ultrafiltration membrane is provided, including the following preparation steps: Step 1, prepare modified PVDF: Mix vinylpyridine, hydroxyacrylate, and vinylidene fluoride to obtain a mixed monomer, and add an initiator to the mixed monomer to carry out a polymerization reaction under heating conditions; Step 2, prepare modified titanium oxide: Disperse titanium oxide powder in an organic solvent to obtain a suspension, add a silane coupling agent to the suspension and stir and react under heating conditions, then add epoxy group-containing silicone oil and stir and react, and then add a rare earth carboxylate complex and continue to stir and react; Step 3, prepare a casting solution: Mix and heat the modified PVDF and modified titanium oxide to react, then add a pore-forming agent and an organic solvent and mix well, and then carry out kneading; Step 4, extrude and form a membrane: Extrude the casting solution under pressure and heating conditions, and then soak it in a gel solution to form a phase-separated membrane, thus obtaining the product.

[0007] In some embodiments, in the mixed monomers in Step 1, the mass fraction of vinylpyridine is 3% - 15%, the mass fraction of hydroxyacrylate is 5% - 20%, and the balance is vinylidene fluoride.

[0008] In some embodiments, the addition amount of the initiator in Step 1 is 0.5% - 2% of the total mass of the mixed monomers.

[0009] In some embodiments, the temperature of the polymerization reaction in Step 1 is 60 - 80 °C, and the time is 6 - 12 h.

[0010] In some embodiments, the addition amount of the silane coupling agent in Step 2 is 3% - 12% of the mass of titanium oxide.

[0011] In some embodiments, after adding the silane coupling agent in Step 2, the reaction temperature is 30 - 80 °C, and the reaction time is 2 - 6 h.

[0012] In some embodiments, the addition amount of the epoxy group-containing silicone oil in Step 2 is 4% - 14% of the mass of titanium oxide.

[0013] In some embodiments, after adding the epoxy group-containing silicone oil in Step 2, the reaction time is 2 - 6 h.

[0014] In some embodiments, the addition amount of the rare earth carboxylic acid complex in Step 2 is 0.1% - 0.5% of the mass of titanium oxide.

[0015] In some embodiments, after adding the rare earth carboxylic acid complex in Step 2, the reaction time is 1 - 3 h.

[0016] In some embodiments, the rare earth carboxylic acid complex in Step 2 is a rare earth citrate complex.

[0017] In some embodiments, the mass ratio of the modified titanium oxide to the modified PVDF in Step 3 is 1:5 - 1:20.

[0018] In some embodiments, the pore former in Step 3 is selected from at least one of PVP, PEG, NaCl, LiCl, methanol, ethanol, acetone, and methyl ethyl ketone.

[0019] In some embodiments, in Step 4, the pressure under the pressurization and heating conditions is 8 - 15 atm, and the temperature is 160 - 200 °C.

[0020] Second, a PVDF ultrafiltration membrane prepared by the above preparation method is provided.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: (1)The in-situ modification of PVDF is carried out by copolymerizing vinylpyridine and hydroxyacrylate with vinylidene fluoride. Flexible and hydrophilic acrylate segments are introduced onto PVDF, enhancing the toughness and hydrophilicity of PVDF. Using it to prepare ultrafiltration membranes is beneficial to improving the anti-fouling performance and mechanical properties of the ultrafiltration membranes; Rigid pyridine heterocycles are introduced onto PVDF, which is beneficial to adjusting the hydrophilic-hydrophobic balance and initially improving the problem of poor mechanical strength caused by the introduction of acrylate segments; (2)By multi-component modification of titanium oxide and adding silicone oil containing epoxy groups for modification, the hydrophilicity of titanium oxide and its compatibility with modified PVDF are improved, making the prepared PVDF ultrafiltration membranes have good consistency and avoiding the problems of uneven dispersion and affected mechanical properties caused by directly adding titanium oxide powder; (3)The epoxy groups introduced onto the obtained modified titanium oxide by adding silicone oil containing epoxy groups are convenient for reacting with the active hydroxyl groups introduced into modified PVDF. While further improving the mixing uniformity, it also increases crosslinking, which is beneficial to further improving the structural stability, heat resistance and mechanical properties of the prepared ultrafiltration membranes; (4)Adding rare earth carboxylate complexes helps to construct a more stable microstructure inside the membrane, enhance the binding force between components, improve the heat resistance, stability and mechanical properties of the prepared ultrafiltration membranes. At the same time, due to the catalytic effect of rare earth carboxylate complexes and the cooperation with the catalytic effect of titanium oxide, the anti-fouling property of the prepared ultrafiltration membranes is further improved. Description of the Drawings

[0022] Figure 1 It is a cross-sectional view of the ultrafiltration membrane prepared in Example 1 of the present invention; Figure 2 It is a SEM surface diagram of the ultrafiltration membrane prepared in Example 1 of the present invention. Detailed Embodiments

[0023] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0024] First, a method for preparing a PVDF ultrafiltration membrane is provided, including the following preparation steps: Step 1. Preparation of modified PVDF: Mix vinylpyridine, hydroxyacrylate, and vinylidene fluoride to obtain a mixed monomer. Add an initiator to the mixed monomer and carry out a polymerization reaction under heating conditions. By copolymerizing vinylpyridine and hydroxyacrylate with vinylidene fluoride, in-situ modification of PVDF is carried out, introducing flexible, hydrophilic acrylate segments, active hydroxyl groups, and rigid pyridine heterocycles onto PVDF, which not only increases the water treatment flux but also enhances the toughness, hydrophilicity, and mechanical strength of PVDF. Using it to prepare ultrafiltration membranes is beneficial to improving the anti-fouling performance and mechanical strength. Step 2. Preparation of modified titanium oxide: Disperse titanium oxide powder in an organic solvent to obtain a suspension. Add a silane coupling agent to the suspension and stir the reaction under heating conditions, then add epoxy group-containing silicone oil and stir the reaction, and then add a rare earth carboxylate complex and continue to stir the reaction. By carrying out multi-component modification of titanium oxide, adding epoxy group-containing silicone oil for modification improves the hydrophilicity, anti-fouling property, and compatibility with modified PVDF of titanium oxide, and introduces active epoxy groups. Adding a rare earth carboxylate complex helps to construct a more stable microstructure inside the membrane, enhance the binding force between components, increase mechanical strength and heat resistance, and is beneficial to improving the anti-fouling property, consistency, stability, and mechanical properties of the ultrafiltration membrane prepared using it. Step 3. Preparation of the casting solution: Mix the modified PVDF and modified titanium oxide and heat them for reaction, then add a pore former and an organic solvent and mix them evenly, and then carry out kneading. The hydroxyl groups introduced onto the modified PVDF and the epoxy groups introduced onto the modified titanium oxide react under heating conditions, increasing the crosslinking and further improving the mechanical properties and stability of the ultrafiltration membrane. Step 4. Extrusion film formation: Extrude the casting solution under pressure and heating conditions, and then soak it in a gelling solution to form a phase-separated film, thus obtaining the product.

[0025] In the preparation process of the PVDF ultrafiltration membrane of the present invention, the added modified PVDF and modified titanium oxide cooperate with each other. The prepared PVDF ultrafiltration membrane not only has a large water treatment flux but also has good anti-fouling property, mechanical properties, and stability.

[0026] In some embodiments, in the mixed monomer in Step 1, the mass ratio of vinylpyridine is 3% - 15%, and the mass ratio of hydroxyacrylate is 5% - 20%. When controlling the hydroxyacrylate within this range, if its addition amount is too low, the introduced polar groups are insufficient and the improvement of hydrophilicity is not obvious; if its addition amount is too high, it may affect the crystallization performance and mechanical properties of PVDF. When controlling the addition amount of vinylpyridine within this range, if its addition amount is too small, it is difficult to play the role of regulating surface charge and improving stability; if its addition amount is too large, it may lead to too strong hydrophilicity of the membrane and affect the retention performance.

[0027] In addition, hydroxyacrylate is a flexible chain segment. Introducing hydroxyacrylate makes the ultrafiltration membrane have better flexibility, but its introduction will destroy the crystallization performance of PVDF, resulting in insufficient strength and poor mechanical properties. The pyridine group in vinylpyridine is rigid. Introducing it can improve the strength, and at the same time its certain torsional flexibility will not greatly affect the flexibility.

[0028] In some embodiments, the addition amount of the initiator in step 1 is 0.5%-2% of the total mass of the mixed monomers, for example, 0.5%, 0.6%, 0.7%, 0.9%, 1.2%, 1.5%, 1.8%, 2%.

[0029] In some embodiments, the temperature of the polymerization reaction in step 1 is 60-80°C, and the time is 6-12h. For example, the temperature is 60°C, 65°C, 70°C, 75°C, 80°C; for example, the time is 6h, 6.5h, 7h, 7.5h, 8h, 10h, 11h, 12h.

[0030] In some embodiments, the addition amount of the silane coupling agent in step 2 is 3%-12% of the mass of titanium oxide, for example, 3%, 3.5%, 4%, 5%, 6%, 6.5%, 7%, 7.5%, 8%, 10%, 12%. The silane coupling agent is selected as a silane coupling agent containing two functional groups of both inorganic and organic affinity in the molecular structure. As an example, KH-570, KH-560, and vinyltrimethoxysilane are listed. The silane coupling agent can chemically connect the epoxy group-containing silicone oil and titanium oxide particles as a bridge and coat the surface of the titanium oxide particles, thereby improving the dispersibility of titanium oxide and its compatibility with modified PVDF.

[0031] In some embodiments, the temperature of the reaction after adding the silane coupling agent in step 2 is 30-80°C, and the reaction time is 2-6h. For example, the temperature is 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C; for example, the time is 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h.

[0032] In some embodiments, the addition amount of the epoxy group-containing silicone oil in step 2 is 4%-14% of the mass of titanium oxide, for example, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%. Controlling the addition amount within this range, if the addition amount is too large, it will affect the heat resistance; if the addition amount is too small, it will affect the modification effect, and the anti-fouling performance cannot be improved well.

[0033] In some embodiments, the reaction time after adding the epoxy group-containing silicone oil in step 2 is 2-5h, for example, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h.

[0034] In some embodiments, the function of the epoxy group-containing silicone oil in step 2 is to improve the anti-fouling performance and introduce active epoxy groups for subsequent reactions. Examples of the epoxy group-containing silicone oil include terminal epoxy silicone oil and mono-terminal epoxy silicone oil. Among them, the structural formula of the terminal epoxy silicone oil is: ; the structural formula of the mono-terminal epoxy silicone oil is: .

[0035] In some embodiments, the addition amount of the rare earth carboxylate complex in step 2 is 0.1%-0.5% of the mass of titanium oxide, such as 0.1%, 0.2%, 0.3%, 0.4%, 0.5%; the rare earth carboxylate complex can interact with titanium oxide and modified PVDF to optimize the microstructure of the ultrafiltration membrane and enhance the mechanical properties of the ultrafiltration membrane. When the addition amount is too small, the microstructure of the ultrafiltration membrane cannot be effectively optimized, resulting in limited improvement in mechanical strength and stability. In actual use, the membrane is more likely to be damaged by pressure and external force impacts; when the addition amount is too large, in addition to the high cost, it is difficult to be evenly dispersed in the reaction system and easily forms aggregates. These aggregates will not only affect the performance of the modified titanium oxide but also cause uneven dispersion when blended with modified PVDF, resulting in local defects in the ultrafiltration membrane, such as uneven pore size distribution, etc. This will reduce the rejection performance of the membrane, causing some pollutants that should have been intercepted to pass through the membrane, and at the same time, it will also affect the water flux.

[0036] In some embodiments, after adding the rare earth carboxylate complex in step 2, the reaction time is 1-3 h, such as 1 h, 1.5 h, 2 h, 2.5 h, 3 h.

[0037] In some embodiments, the rare earth carboxylate complex in step 2 is a rare earth citrate complex, and the rare earth citrate complex is at least one of cerium citrate, yttrium citrate, and lanthanum citrate; the rare earth citrate complex can interact with titanium oxide, which is more conducive to enhancing the interfacial binding force between the inorganic phase and the organic phase.

[0038] In some embodiments, the rare earth citrate complex is preferably cerium citrate, and cerium citrate can also be used as a functional additive for the ultrafiltration membrane, endowing the ultrafiltration membrane with the performance of photocatalytic decomposition of organic pollutants. Under light irradiation conditions, cerium citrate can generate highly oxidizing free radicals, decomposing the organic pollutants adsorbed on the membrane surface into carbon dioxide and water, slowing down the membrane fouling rate, and extending the service life of the membrane.

[0039] In some embodiments, the mass ratio of the modified titanium oxide to the modified PVDF in step 3 is 1:5 - 1:20, such as 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:12, 1:14, 1:16, 1:18, 1:20.

[0040] In some embodiments, for the pressurization and heating conditions in step 4, the pressure is 8 - 15 atm and the temperature is 160 - 200 °C. For example, the pressure can be 8 atm, 9 atm, 10 atm, 11 atm, 12 atm, 13 atm, 14 atm, 15 atm; for example, the temperature can be 160 °C, 165 °C, 170 °C, 175 °C, 180 °C, 190 °C, 200 °C.

[0041] In a second aspect, a PVDF ultrafiltration membrane prepared by the above preparation method is provided.

[0042] In a third aspect, an application of the above PVDF ultrafiltration membrane in water treatment is provided.

[0043] Examples Hereinafter, examples of the present application will be described. The examples described below are exemplary and are only used to explain the present application and should not be construed as a limitation of the present application. For those not specifying specific techniques or conditions in the examples, the techniques or conditions described in the literature in this field or according to the product specifications are followed. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0044] The structural formula of the epoxy group-containing silicone oil used in Examples 1 - 4 and Comparative Examples 1 - 7 is: .

[0045] The structural formula of the epoxy group-containing silicone oil used in Example 5 is:

[0046] Example 1 Preparation of PVDF ultrafiltration membrane: Step 1. Preparation of modified PVDF: Mix 2-vinylpyridine, 2-hydroxyethyl methacrylate, and vinylidene fluoride to obtain a mixed monomer. Among them, the mass fraction of 2-vinylpyridine in the mixed monomer is 5%, and the mass fraction of 2-hydroxyethyl methacrylate is 12%. Add 1% of benzoyl peroxide based on the total mass of the mixed monomer and carry out a polymerization reaction at 60 °C for 8 h; Step 2. Preparation of modified titanium oxide: Disperse titanium oxide powder in N,N-dimethylformamide to obtain a suspension with a titanium oxide powder mass content of 10%. Add 3% of KH-570 based on the mass of titanium oxide to the suspension and stir at 80 °C for 3 h. Keep the solution temperature unchanged, then add 5% of the epoxy group-containing silicone oil based on the mass of titanium oxide and stir for 5 h. Then add 0.4% of cerium citrate based on the mass of titanium oxide and continue to stir and react for 1 h. After filtration and water washing, modified titanium oxide is obtained; Step 3: Prepare the casting solution: Mix the above-mentioned modified titanium oxide and modified PVDF at a mass ratio of 1:15, react at 100 °C for 4 h, then add PVP accounting for 40% of the mass of modified PVDF and dimethylacetamide solution such that the mass content of modified PVDF after adding the solution is 20%, and mix well and then knead in a kneader at 80 °C for 6 h; Step 4: Extrusion film formation: Extrude the casting solution at a pressure of 10 atm and 160 °C, and soak it in an ethanol-water solution at 40 °C with a volume concentration of 20% for 24 h for phase inversion to obtain the product.

[0047] Example 2 Preparation of PVDF ultrafiltration membrane: Step 1: Prepare modified PVDF: Mix 4-vinylpyridine, 2-hydroxyethyl acrylate and vinylidene fluoride to obtain a mixed monomer. Among them, the mass fraction of 4-vinylpyridine in the mixed monomer is 15%, and the mass fraction of 2-hydroxyethyl acrylate is 5%. Add 0.5% of azobisisobutyronitrile based on the total mass of the mixed monomer and polymerize at 80 °C for 12 h; Step 2: Prepare modified titanium oxide: Disperse titanium oxide powder in N,N-dimethylformamide to obtain a suspension with a mass content of titanium oxide powder of 10%. Add 6% of KH-560 based on the mass of titanium oxide to the suspension and stir at 80 °C for reaction for 6 h. Keep the solution temperature unchanged, then add 10% of epoxy group-containing silicone oil based on the mass of titanium oxide and stir for reaction for 2 h, and then add 0.1% of yttrium citrate based on the mass of titanium oxide and continue to stir for reaction for 2 h. After filtration and washing with water, modified titanium oxide is obtained; Step 3: Prepare the casting solution: Mix the above-mentioned modified titanium oxide and modified PVDF at a mass ratio of 1:10, react at 80 °C for 8 h, then add PEG accounting for 50% of the mass of modified PVDF and dimethylacetamide solution such that the mass content of modified PVDF after adding the solution is 20%, and mix well and then knead in a kneader at 80 °C for 12 h; Step 4: Extrusion film formation: Extrude the casting solution at a pressure of 15 atm and 200 °C, and soak it in an ethanol-water solution at 40 °C with a volume concentration of 20% for 24 h for phase inversion to obtain the product.

[0048] Example 3 Preparation of PVDF ultrafiltration membrane: Step 1: Prepare modified PVDF: Mix 2-vinylpyridine, hydroxypropyl acrylate and vinylidene fluoride to obtain a mixed monomer. Among them, the mass fraction of 2-vinylpyridine in the mixed monomer is 10%, and the mass fraction of hydroxypropyl acrylate is 15%. Add 2% of benzoyl peroxide based on the total mass of the mixed monomer and polymerize at 60 °C for 12 h; Step 2: Preparation of modified titanium oxide: Disperse titanium oxide powder in N,N-dimethylformamide to obtain a suspension with a mass content of titanium oxide powder of 10%. Add 12% of vinyltrimethoxysilane, a silane coupling agent based on the mass of titanium oxide, to the suspension and stir at 30°C for 2 h for reaction. Keep the solution temperature unchanged, then add 14% of epoxy group-containing silicone oil based on the mass of titanium oxide and stir for 4 h for reaction. Then add 0.3% of lanthanum citrate based on the mass of titanium oxide and continue to stir for 3 h for reaction. After filtration and washing with water, modified titanium oxide is obtained; Step 3: Preparation of casting solution: Mix the above-mentioned modified titanium oxide and modified PVDF in a mass ratio of 1:5 and react at 120°C for 5 h. Then add 40% of PVP based on the mass of modified PVDF and dimethylacetamide solution with a mass content of modified PVDF of 20% after adding the solution. After mixing evenly, knead in a kneader at 80°C for 8 h; Step 4: Extrusion film formation: Extrude the casting solution at a pressure of 8 atm and 180°C, and soak it in an ethanol-aqueous solution with a volume concentration of 20% at 40°C for 24 h for phase inversion to obtain the product.

[0049] Example 4 Preparation of PVDF ultrafiltration membrane: Step 1: Preparation of modified PVDF: Mix 2-vinylpyridine, hydroxypropyl acrylate and vinylidene fluoride to obtain a mixed monomer. Among them, the mass fraction of 2-vinylpyridine in the mixed monomer is 3%, and the mass fraction of hydroxypropyl acrylate is 20%. Add 1.6% of benzoyl peroxide based on the total mass of the mixed monomer to the mixed monomer and carry out polymerization reaction at 80°C for 6 h; Step 2: Preparation of modified titanium oxide: Disperse titanium oxide powder in N,N-dimethylformamide to obtain a suspension with a mass content of titanium oxide powder of 10%. Add 8% of KH-560 based on the mass of titanium oxide to the suspension and stir at 30°C for 6 h for reaction. Keep the solution temperature unchanged, then add 4% of epoxy group-containing silicone oil based on the mass of titanium oxide and stir for 3 h for reaction. Then add 0.5% of cerium citrate based on the mass of titanium oxide and continue to stir for 2 h for reaction. After filtration and washing with water, modified titanium oxide is obtained; Step 3: Preparation of casting solution: Mix the above-mentioned modified titanium oxide and modified PVDF in a mass ratio of 1:20 and react at 100°C for 5 h. Then add 30% of PEG based on the mass of modified PVDF and dimethylacetamide solution with a mass content of modified PVDF of 20% after adding the solution. After mixing evenly, knead in a kneader at 80°C for 5 h; Step 4: Extrusion film formation: Extrude the casting solution at a pressure of 12 atm and 160°C, and soak it in an ethanol-aqueous solution with a volume concentration of 20% at 40°C for 24 h for phase inversion to obtain the product.

[0050] Example 5 Preparation of PVDF ultrafiltration membrane: Step 1: Preparation of modified PVDF: Mix 4-vinylpyridine, hydroxypropyl acrylate and vinylidene fluoride to obtain a mixed monomer. Among them, the mass fraction of 4-vinylpyridine in the mixed monomer is 8%, and the mass fraction of hydroxypropyl acrylate is 18%. Add 0.8% of benzoyl peroxide based on the total mass of the mixed monomer and carry out a polymerization reaction at 70 °C for 8 h; Step 2: Preparation of modified titanium oxide: Disperse titanium oxide powder in N,N-dimethylformamide to obtain a suspension with a mass content of titanium oxide powder of 10%. Add 10% of KH-560 based on the mass of titanium oxide to the suspension and stir at 50 °C for 4 h for reaction. Keep the solution temperature unchanged, then add 8% of epoxy group-containing silicone oil based on the mass of titanium oxide and stir for 2 h for reaction. Then add 0.3% of cerium citrate based on the mass of titanium oxide and continue to stir for reaction for 2 h. After filtration and washing with water, modified titanium oxide is obtained; Step 3: Preparation of casting solution: Mix the above-mentioned modified titanium oxide and modified PVDF at a mass ratio of 1:15 and react at 80 °C for 5 h. Then add PVP accounting for 60% of the mass of modified PVDF and dimethylacetamide solution such that the mass content of modified PVDF after adding the solution is 20%. After mixing evenly, carry out kneading at 80 °C in a kneader for 8 h; Step 4: Extrusion film formation: Extrude the casting solution at a pressure of 8 atm and 160 °C, and soak it in an ethanol-aqueous solution with a volume concentration of 20% at 40 °C for 24 h for phase inversion to obtain the product.

[0051] Comparative Example 1 The difference between the preparation method of the PVDF ultrafiltration membrane in Comparative Example 1 and that in Example 1 is that vinylpyridine is not added in Comparative Example 1.

[0052] Comparative Example 2 The difference between the preparation method of the PVDF ultrafiltration membrane in Comparative Example 2 and that in Example 1 is that hydroxyacrylate is not added in Comparative Example 2.

[0053] Comparative Example 3 The difference between the preparation method of the PVDF ultrafiltration membrane in Comparative Example 3 and that in Example 1 is that 2-hydroxyethyl methacrylate is replaced with methyl methacrylate in Comparative Example 3.

[0054] Comparative Example 4 The difference between the preparation method of the PVDF ultrafiltration membrane in Comparative Example 4 and that in Example 1 is that epoxy group-containing silicone oil is not added in Comparative Example 4.

[0055] Comparative Example 5 The difference between the preparation method of the PVDF ultrafiltration membrane in Comparative Example 5 and that in Example 1 is that the epoxy group-containing silicone oil added in Example 1 is replaced with vinyl silicone oil in Comparative Example 5.

[0056] Comparative Example 6 The difference between the preparation method of the PVDF ultrafiltration membrane in Comparative Example 6 and that in Example 1 is that no rare earth carboxylic acid complex is added in Comparative Example 6.

[0057] Comparative Example 7 The difference between the preparation method of the PVDF ultrafiltration membrane in Comparative Example 7 and that in Example 1 is that cerium citrate is replaced by cerium oxide in Comparative Example 7.

[0058] Performance Test (1) Membrane water purification performance test Prepare humic acid solutions with a concentration of 5 mg / L respectively, adjust the filtration pressure to 0.05 MPa, conduct filtration tests on the PVDF ultrafiltration membranes prepared in the examples and comparative examples, determine the removal effect of the ultrafiltration membranes on humic acid, and calculate the flux of the ultrafiltration membranes by measuring the mass of the permeate and using F = J / A*t (where F is the membrane flux; J is the sampling volume; A is the effective membrane area; t is the time).

[0059] (2) Organic pollutant removal performance test Use an ultrafiltration cup to conduct organic pollutant removal performance tests on the ultrafiltration membranes prepared in the examples and comparative examples, measure in parallel three times, record the removal rate and take the average value.

[0060] (3)Mechanical strength test The mechanical strength test of the ultrafiltration membrane is carried out by the method of tensile test to determine the mechanical strength of the ultrafiltration membrane, including the tensile strength and elongation at break of the ultrafiltration membrane; in addition, the thickness of the ultrafiltration membranes prepared in each example and comparative example is 200 μm.

[0061] Testing instrument: W56 type universal electronic testing machine.

[0062] Testing conditions: The test temperature is room temperature (about 25 °C), the tensile rate is 5 mm / min, and the results of the tensile strength and elongation at break are the average values of 5 test data.

[0063] (4)Anti-pollution ability test The thickness of the ultrafiltration membranes prepared in each example and comparative example is 200 μm.

[0064] For the ultrafiltration membrane that has been used 10 times, stir and wash it for 20 min and backwash it for 10 min. Then, conduct a water purification membrane flux test at 0.1 MPa. The anti-pollution ability calculation formula is as follows: R = F2 / F1×100%; where F1 is the membrane flux before filtration, mg / L; F2 is the membrane flux after filtration, mg / L; R is the anti-pollution ability of the ultrafiltration membrane, %.

[0065] (5)Chemical stability test At room temperature, the ultrafiltration membranes prepared in the examples and comparative examples were respectively immersed in 1 mol / L hydrochloric acid solution and 1 mol / L sodium hydroxide solution for 24 hours. After the treatment, each ultrafiltration membrane was taken out and repeatedly rinsed with deionized water to remove the residual reagents on the surface, and the change range of the water flux of the ultrafiltration membrane before and after the treatment was tested.

[0066] (6)Test results The above test results are shown in Table 1.

[0067] Table 1 It can be seen from Figure 1 and Figure 2 that the ultrafiltration membrane prepared by the present invention has a rich pore structure and good consistency. In addition, it can be seen from the results of Table 1 that the ultrafiltration membranes prepared in Examples 1-5 of the present invention have a high removal rate of humic acid and a high filtration flux. The highest filtration flux can reach 1911 L / m 2 ·h, and the highest removal rate can reach 93%; it also has a high tensile strength and elongation at break, indicating good mechanical properties; the anti-fouling ability of the ultrafiltration membrane prepared by the present invention can reach 71%, indicating good anti-fouling performance; the stability test shows that its tolerance temperature is above 85 °C, and after being immersed in acid and alkali solutions for treatment, the flux changes are all small, indicating that the ultrafiltration membrane prepared by the method of the present invention has good mechanical properties and chemical stability. In addition, by comparing Example 1 with Comparative Examples 1-7, it can be seen that changing the modified raw materials of modified PVDF and the modified raw materials of modified titanium oxide will both lead to insufficient modification, resulting in a decrease in the performance of the material, and further affecting the performance of the prepared PVDF ultrafiltration membrane.

[0068] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of a PVDF ultrafiltration membrane, characterized in that, It includes the following preparation steps: Step 1, prepare modified PVDF: Mix vinylpyridine, hydroxyacrylate and vinylidene fluoride to obtain a mixed monomer, and add an initiator to the mixed monomer to carry out a polymerization reaction under heating conditions; Step 2, prepare modified titanium oxide: Disperse titanium oxide powder in an organic solvent to obtain a suspension, add a silane coupling agent to the suspension, stir and react under heating conditions, then add a silicone oil containing epoxy groups and stir and react, and then add a rare earth carboxylate complex and continue to stir and react; Step 3, prepare a casting solution: Mix and heat the modified PVDF and modified titanium oxide to react, then add a pore-forming agent and an organic solvent and mix well, and then carry out internal mixing; Step 4, extrusion film formation: Extrude the casting solution under pressurization and heating conditions, and then soak it in a gel solution to form a phase-separated film, thus obtaining.

2. The preparation method of the PVDF ultrafiltration membrane according to claim 1, characterized in that, In the mixed monomer described in Step 1, the mass fraction of vinylpyridine is 3%-15% or / and the mass fraction of hydroxyacrylate is 5%-20%.

3. The preparation method of the PVDF ultrafiltration membrane according to claim 1, characterized in that, In Step 2, the addition amount of the silane coupling agent is 3%-12% of the mass of titanium oxide or / and the addition amount of the silicone oil containing epoxy groups is 4%-14% of the mass of titanium oxide or / and the addition amount of the rare earth carboxylate complex is 0.1%-0.5% of the mass of titanium oxide.

4. The preparation method of the PVDF ultrafiltration membrane according to claim 1, wherein, In Step 2, the rare earth carboxylate complex is a rare earth citrate complex.

5. The preparation method of the PVDF ultrafiltration membrane according to claim 1, characterized in that, After adding the silane coupling agent in Step 2, the reaction temperature is 30-80°C and the reaction time is 2-6h.

6. The preparation method of the PVDF ultrafiltration membrane according to claim 1, characterized in that, In Step 3, the mass ratio of the modified titanium oxide to the modified PVDF is 1:5-1:

20.

7. The preparation method of the PVDF ultrafiltration membrane according to claim 1, wherein, In Step 3, the pore-forming agent is selected from at least one of PVP, PEG, NaCl, LiCl, methanol, ethanol, acetone and methyl ethyl ketone.

8. The preparation method of the PVDF ultrafiltration membrane according to claim 1, characterized in that In Step 4, the gel solution is water or an organic solvent or a composite solution of water and an organic solvent, and the soaking time is 10 min-24 h.

9. A PVDF ultrafiltration membrane prepared by the method according to any one of claims 1-8.

10. An application of the PVDF ultrafiltration membrane according to claim 9 in water treatment.

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