Preparation method of large-flux and anti-pollution PVDF microfiltration membrane

Abstract

The invention discloses a preparation method of a large-flux and anti-pollution PVDF (polyvinylidene fluoride) micro-filtration membrane, which is used for solving the technical problem that the flux and the pollution resistance of the PVDF micro-filtration membrane in the prior art need to be further improved, and specifically comprises the following steps: putting carboxyl modified PVDF, acetone, N, N-dimethylformamide and an auxiliary additive into a reaction kettle, uniformly mixing, standing for defoaming, and carrying out vacuum drying to obtain the PVDF micro-filtration membrane. A membrane casting solution is obtained; modified nano silicon dioxide and trans-4-hydroxycinnamic acid serve as reinforcing materials, a carboxyl modified PVDF base membrane is prepared, carboxyl of the carboxyl modified PVDF base membrane is activated through an activating solution, the carboxyl is chemically bonded with amino of modified nano gel balls containing amphoteric molecules in a modifying solution, and the PVDF micro-filtration membrane is obtained. The flux of the PVDF micro-filtration membrane is improved, and the mechanical property and the pollution resistance of the PVDF micro-filtration membrane are also improved.

Description

Technical Field

[0001] The present invention relates to the technical field of filter membrane processing, and specifically relates to a preparation method of a high-flux and anti-pollution PVDF microfiltration membrane. Background Art

[0002] Membrane separation technology is a method in which one or more components in a mixture are selectively permeated through a specific driving force while other components are blocked. This technology is widely used in the fields of separation, purification, and concentration of substances. According to the size difference of the substances to be separated, the separation membranes can be divided into microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes. Among them, PVDF microfiltration membranes are commonly used for separating oily wastewater. The separation process does not use harmful chemical agents, meets environmental protection requirements, can reduce pollutant emissions, has less impact on the environment, and compared with traditional oil-water separation methods, the microfiltration membrane separation process is relatively simple, the operation is relatively automated, and there is no need for a large amount of chemical reagents or complex equipment. However, in practical applications, its flux and anti-fouling performance are often insufficient to meet the requirements of efficient separation.

[0003] The main reasons affecting the flux of PVDF microfiltration membrane for oil-water separation are as follows: The high electronegativity of fluorine atoms in the PVDF molecular chain leads to extremely low surface energy, forms weak hydrogen bond interactions with water molecules, shows strong hydrophobicity, significantly increases the water penetration resistance. At the same time, the regular arrangement of molecular chains forms high crystalline regions, restricting the free volume and pore connectivity of the amorphous region, and hindering the diffusion channel of water molecules. At the same time, the microfiltration membrane is greatly affected by pollutants in the oil-water mixture. Especially, the oil droplets in oily wastewater will cause the membrane pore channels to be blocked, resulting in a decrease in the hydrophilicity of the membrane surface, thus significantly reducing the flux. To increase the hydrophilicity of the PVDF microfiltration membrane, during the preparation of the PVDF microfiltration membrane, hydrophilic substances are added to it for co-blending to form a membrane. However, the interfacial compatibility between the hydrophilic substance and PVDF is poor, resulting in a decrease in the mechanical properties of the microfiltration membrane, and there are few active functional groups in PVDF, making it difficult to carry out efficient graft modification. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of a high-flux and anti-pollution PVDF microfiltration membrane, which is used to solve the technical problem that the flux and anti-pollution performance of the PVDF microfiltration membrane in the prior art need to be further improved.

[0005] The purpose of the present invention can be achieved by the following technical solutions: A preparation method of a high-flux and anti-pollution PVDF microfiltration membrane, including the following preparation steps:

[0006] S1. Put carboxyl-modified PVDF, acetone, N,N-dimethylformamide, and auxiliary additives into a reaction kettle, mix evenly, and stand for defoaming to obtain a casting solution;

[0007] S2. Cast the casting solution on a glass plate, level it with a scraper to form a film, and obtain a carboxyl-modified PVDF base film through post-treatment;

[0008] The preparation reaction principle of the carboxyl-modified PVDF base film is as follows:

[0009] During the film-forming process, the porogen in the base film is washed away by ethanol and deionized water to form a microporous carboxyl-modified PVDF base film.

[0010] S3. Place the carboxyl-modified PVDF base film in the activation solution and immerse it for 1 - 2 h to obtain an activated microfiltration base film;

[0011] The preparation reaction principle of the activated microfiltration base film is as follows:

[0012] During the reaction, the imine group of N-cyclohexyl-N-(3-dimethylaminopropyl)carbodiimide and the hydroxyl group of N-hydroxysuccinimide can react with the carboxyl groups on the surface of the carboxyl-modified PVDF base film to generate highly reactive intermediates that can react with amino groups. 4-Morpholineethanesulfonic acid is used as a buffer to maintain the stability of the intermediates, and an activated microfiltration base film is obtained.

[0013] S4. Immerse the activated microfiltration base film in the modification solution, heat it to 40 - 50 °C, keep it warm for reaction for 10 - 12 h, and obtain a PVDF microfiltration membrane through post-treatment.

[0014] The preparation reaction principle of the PVDF microfiltration membrane is as follows:

[0015] During the reaction, the amino group in the modification solution and the ester group of the activated microfiltration base film undergo an amidation reaction to form a chemical bond, and a PVDF microfiltration membrane with surface-modified amphiphilic molecules is obtained.

[0016] Furthermore, in step S1, the dosage ratio of the carboxyl-modified PVDF, acetone, N,N-dimethylformamide, and auxiliary additive is 15 - 20 g: 220 - 300 mL: 20 - 25 mL: 2 - 5 g; the auxiliary additive is composed of a porogen, an antioxidant, and an antibacterial agent in a mass ratio of 2:0.5:0.5. The porogen is one or both of polyethylene glycol and polyvinylpyrrolidone, the antioxidant is one or more of polyaniline, polydopamine, and butylphenyl toluene, and the antibacterial agent is one or both of silver nanoparticles and titanium dioxide; in step S2, the film-forming operation includes: placing the leveled glass plate in a coagulation bath composed of deionized water and ethanol, heating it to 25 - 35 °C, and the coagulation time is 2 - 5 min to obtain a film. The post-treatment steps include: after coagulation, washing the film with ethanol 1 - 2 times, transferring it to a drying oven at 40 - 50 °C, and drying it to constant weight to obtain an activated microfiltration base film;

[0017] Further, in step S3, the bath ratio is 1:30 - 35, and the activation solution is composed of N-cyclohexyl-N-(3-dimethylaminopropyl)carbodiimide, N-hydroxysuccinimide, 4-morpholineethanesulfonic acid, and deionized water in a dosage ratio of 0.1 - 0.2 g:1 - 1.5 g:2 - 3 g:25 - 30 mL; in step S4, the bath ratio is 1:30 - 35, and the post-treatment steps include: after the reaction is completed, the membrane is washed 1 - 2 times with ethanol and deionized water, transferred to a drying oven at a temperature of 40 - 50 °C, and dried to a constant weight to obtain the PVDF microfiltration membrane.

[0018] Further, the preparation method of the carboxyl-modified PVDF is as follows: N,N-dimethylformamide and PVDF are placed in a reaction kettle protected by a nitrogen atmosphere, heated to 70 - 80 °C, benzoyl peroxide is added, stirred for 20 - 30 min, a solution of trans-4-hydroxycinnamic acid and modified nano-silica are added, and the reaction is carried out under insulation for 1 - 2 h, and then post-treatment is performed to obtain carboxyl-modified PVDF.

[0019] The reaction principle for the preparation of carboxyl-modified PVDF is as follows:

[0020] During the reaction process, under heating conditions, benzoyl peroxide decomposes into free radicals, and these free radicals attack the olefin double bonds of PVDF, trans-4-hydroxycinnamic acid, and modified nano-silica, and a free radical polymerization reaction occurs to obtain carboxyl-modified PVDF.

[0021] Further, the solution of trans-4-hydroxycinnamic acid is composed of trans-4-hydroxycinnamic acid and N,N-dimethylformamide in a dosage ratio of 1 - 2 g:100 - 150 mL, and the dosage ratio of N,N-dimethylformamide, PVDF, benzoyl peroxide, the solution of trans-4-hydroxycinnamic acid, and modified nano-silica is 30 - 50 mL:2 - 3 g:0.10 - 0.15 g:10 - 15 mL:0.5 - 1 g. The post-treatment steps include: after the reaction is completed, when the reaction solution cools to room temperature, ethanol is added to precipitate the product, suction filtration is carried out, the filter cake is washed 1 - 2 times with ethanol, transferred to a drying oven at a temperature of 50 - 60 °C, and dried to a constant weight to obtain carboxyl-modified PVDF.

[0022] Further, the modified nano-silica is prepared by the following steps:

[0023] A1. Ammonia water, ethanol, and deionized water are placed in a reaction kettle, heated to 30 - 40 °C, stirred for 5 - 10 min, tetraethyl orthosilicate is added, heated to 70 - 80 °C, and the reaction is carried out under insulation for 8 - 10 h to obtain nano-silica sol;

[0024] The reaction principle for the preparation of nano-silica sol is as follows:

[0025] During the reaction process, tetraethyl orthosilicate is hydrolyzed into silanol under the action of deionized water. Under the alkaline catalysis of ammonia water, condensation occurs between silanol groups to obtain nano-silica sol.

[0026] A2. Place the nano-silica sol in a reaction kettle, heat it up to 30 - 40 °C, add dilute hydrochloric acid to adjust the pH = 7 ± 0.5, add 3-butenyltriethoxysilane solution, heat it up to 50 - 60 °C, keep the temperature for reaction for 2 - 4 h, and perform post-treatment to obtain modified nano-silica.

[0027] The reaction principle for the preparation of modified nano-silica is as follows:

[0028] During the reaction process, the three siloxane bonds of 3-butenyltriethoxysilane are hydrolyzed into silanol under the action of deionized water. In a neutral environment, to avoid self-condensation of 3-butenyltriethoxysilanol, 3-butenyltriethoxysilanol further undergoes a condensation reaction with the silanol groups of the nano-silica sol to obtain modified nano-silica modified with a silane coupling agent.

[0029] Furthermore, in step A1, the dosage ratio of ammonia water, ethanol, deionized water, and tetraethyl orthosilicate is 0.5 - 1 mL : 15 - 20 mL : 4 - 6 mL : 15 - 20 g; in step A2, the dosage ratio of the nano-silica sol and 3-butenyltriethoxysilane solution is 15 - 20 g : 100 - 120 mL, and the 3-butenyltriethoxysilane solution is composed of ethanol and 3-butenyltriethoxysilane with a dosage ratio of 40 - 50 mL : 1 - 2 g. The post-treatment steps include: after the reaction is completed, perform suction filtration, wash the filter cake with deionized water and ethanol for 1 - 2 times, transfer it to a drying oven at a temperature of 55 - 65 °C, and dry it to a constant weight to obtain modified nano-silica.

[0030] Furthermore, the modified solution is prepared by the following steps:

[0031] B1. Place potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride, potassium chloride, and deionized water in a reaction kettle, heat it up to 35 - 45 °C, stir for 15 - 30 min, add sodium hydroxide solution to adjust the pH = 7.0 ± 0.5 to obtain a buffer solution;

[0032] The reaction principle for the preparation of the buffer solution is as follows:

[0033] Potassium dihydrogen phosphate, as a weak acid, provides protons, and disodium hydrogen phosphate, as a conjugate base, accepts protons. The two form a buffer pair. Sodium chloride and potassium chloride provide ionic strength to maintain the electrolyte balance of the solution, which helps to promote the stability of the buffer solution. Sodium hydroxide adjusts the pH of the solution to be maintained in a neutral environment to obtain a buffer solution.

[0034] B2. Place the modified nano-gel beads and the buffer solution in a reaction kettle, mix them evenly to obtain the modified solution.

[0035] Further, in step B1, the dosage ratio of potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride, potassium chloride and deionized water is 0.2 - 0.5 g: 1.5 - 1.6 g: 8 - 10 g: 0.2 - 0.3 g: 1000 mL, and the concentration of the sodium hydroxide solution is 1 - 5 wt%; in step B2, the dosage ratio of the modified nanogel spheres and the buffer solution is 5 - 10 g: 100 - 120 mL.

[0036] Further, the modified nanogel spheres are prepared by the following steps:

[0037] C1. Put deionized water, 3 - [N,N - dimethyl - [2 - (2 - methylprop - 2 - enoyloxy)ethyl] ammonium] propane - 1 - sulfonate inner salt, allylamine hydrochloride and N,N'-methylenebisacrylamide into a reaction kettle, stir for 10 - 15 min to obtain a mixed salt solution;

[0038] C2. Put polysorbate monooleate, sorbitan monolaurate and cyclohexane into a reaction kettle, stir for 10 - 15 min, add the mixed salt solution, and stir for 15 - 30 min to obtain a mixed emulsion;

[0039] The reaction principle for the preparation of the mixed emulsion is as follows:

[0040] During the reaction, the non - ionic surfactants polysorbate monooleate and sorbitan monolaurate can reduce the surface tension between the two phases of cyclohexane and the mixed salt solution to obtain a stable mixed emulsion.

[0041] C3. Put the mixed emulsion into a reaction kettle protected by a nitrogen atmosphere, add azobisisobutyronitrile, heat up to 35 - 45 °C, keep the temperature for reaction for 20 - 22 h, add tetrahydrofuran, stir for 6 - 8 h, and perform post - treatment to obtain the modified nanogel spheres.

[0042] The reaction principle for the preparation of the modified nanogel spheres is as follows:

[0043] During the reaction, azobisisobutyronitrile thermally decomposes under heating conditions to generate free radicals. These free radicals attack the olefin double bonds of 3 - [N,N - dimethyl - [2 - (2 - methylprop - 2 - enoyloxy)ethyl] ammonium] propane - 1 - sulfonate inner salt, allylamine hydrochloride and N,N'-methylenebisacrylamide in the mixed emulsion to undergo free - radical polymerization reaction. N,N'-methylenebisacrylamide acts as a cross - linker, providing bisacrylamide groups to react with two polymer chains to form covalent cross - link points and construct a three - dimensional network structure. The oxygen atom of tetrahydrofuran can react with the free radicals in the system to terminate the reaction, and the modified nanogel spheres are obtained.

[0044] Further, in step C1, the dosage ratio of deionized water, 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium] propane-1-sulfonate inner salt, allylamine hydrochloride, and N,N'-methylenebisacrylamide is 100-120 mL: 7-8 g: 2-3.5 g: 4-5 g; in step C2, the dosage ratio of polysorbate monooleate, sorbitan monolaurate, cyclohexane, and mixed salt solution is 4-5 g: 4-5 g: 90-110 mL: 40-50 mL; in step C3, the dosage ratio of the mixed emulsion, azobisisobutyronitrile, and tetrahydrofuran is 50-60 mL: 1-1.5 g: 30-35 mL. The post-treatment step includes: after the reaction is completed, suction filtration is carried out, the filter cake is washed 1-2 times with tetrahydrofuran, transferred to an oven at a temperature of 50-60 °C, and dried to constant weight to obtain modified nanogel spheres.

[0045] The present invention has the following beneficial effects:

[0046] 1. A high-flux and anti-fouling PVDF microfiltration membrane prepared by the present invention uses modified nano-silica and trans-4-hydroxycinnamic acid as reinforcing materials to prepare carboxyl-modified PVDF. The carboxyl-modified PVDF-based membrane is prepared by the phase inversion method using carboxyl-modified PVDF and auxiliary additives. Then, the carboxyl groups of the carboxyl-modified PVDF-based membrane are activated by an activation solution, and a chemical bond is formed with the amino groups of the modified nanogel spheres containing amphiphilic molecules in the modification solution to obtain a PVDF microfiltration membrane. The present invention prepares nano-silica by the sol-gel method and then modifies it with a silane coupling agent containing olefins to obtain modified nano-silica. Modified nano-silica, trans-4-hydroxycinnamic acid, and PVDF undergo free radical polymerization to obtain carboxyl-modified PVDF. The carboxyl-modified PVDF-based membrane is prepared by the phase inversion method. The surface of the nano-silica contains abundant silanol groups, which can form weak hydrogen bonds with the fluorine atoms of PVDF, improving the polarity of the membrane. When performing water-oil separation, the nano-silica adsorbs water molecules through weak hydrogen bonds to form a hydration layer, reducing the water contact angle, increasing the flux of the PVDF microfiltration membrane, and reducing the adsorption of hydrophobic pollutants. The nano-silica can also serve as a heterogeneous nucleation point to disrupt the regular arrangement of PVDF molecular chains, reduce the crystallinity, and increase the proportion of amorphous regions. Moreover, the nano-silica can bind to the PVDF chain through physical entanglement and interfacial hydrogen bonds, restricting the molecular chain slip and improving the mechanical strength of the PVDF microfiltration membrane.

[0047] 2. The present invention prepares a carboxyl-modified PVDF-based membrane through a free radical polymerization reaction. To improve the carboxyl activity of the carboxyl-modified PVDF-based membrane, the PVDF-based membrane is placed in an activation solution for carboxyl activation. The activation solution converts the carboxyl group into a highly active intermediate through a reaction, improving the grafting rate of the carboxyl-modified PVDF-based membrane and the modified nanogel spheres, and further enhancing the flux and anti-pollution ability of the PVDF microfiltration membrane.

[0048] 3. The present invention also prepares modified nanogel spheres by introducing amphiphilic molecules through a free radical polymerization reaction. 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium] propane-1-sulfonate inner salt contains both quaternary ammonium cations and sulfonic acid anions, which are amphiphilic molecules. One molecule of amphiphilic molecule can bind to 7-8 water molecules simultaneously. Compared with hydroxyl and carboxyl groups, it has higher hydrophilicity. When the PVDF microfiltration membrane is used for oil-water separation, the nanogel microspheres have a certain spatial structure compared with single hydrophilic polymers. They can not only form a relatively high hydrophilic density in the local area, but also obtain a higher hydration layer thickness after binding with water due to the existence of their own three-dimensional structure, effectively enhancing the resistance of the membrane surface to oil stains. At the same time, the cationic groups can interact with polar molecules in the oil droplets to inhibit the aggregation of oil droplets, while the anionic groups can enhance the wettability of water, thus promoting the smooth progress of the oil-water separation process and improving the anti-pollution ability of the PVDF microfiltration membrane. Specific embodiments

[0049] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work belong to the scope of protection of the present invention.

[0050] Example 1

[0051] This example provides a preparation method of carboxyl-modified PVDF for a high-flux and anti-pollution PVDF microfiltration membrane, including the following steps:

[0052] Weigh: 50 mL of ammonia water, 150 mL of ethanol, and 40 mL of deionized water and place them in a reaction kettle. Heat up to 30 °C, stir for 5-10 min, add 150 g of tetraethyl orthosilicate, heat up to 70 °C, and keep the reaction for 8 h to obtain nano-silica sol;

[0053] Weigh: 400 mL of ethanol and 10 g of 3-butenyltriethoxysilane and mix them evenly to obtain a 3-butenyltriethoxysilane solution for standby;

[0054] Weigh: 150 g of nano-silica sol was placed in a reaction kettle, heated to 30 °C, adjusted to pH = 7.1 with dilute hydrochloric acid, 1000 mL of 3-butenyltriethoxysilane solution was added, heated to 50 °C, and kept for reaction for 2 h. After the reaction was completed, suction filtration was carried out, and the filter cake was washed once with deionized water and ethanol, transferred to a drying oven at 55 °C, and dried to constant weight to obtain modified nano-silica;

[0055] Weigh: 10 g of trans-4-hydroxycinnamic acid and 1000 mL of N,N-dimethylformamide were mixed evenly to obtain a trans-4-hydroxycinnamic acid solution for standby;

[0056] Weigh: 300 mL of N,N-dimethylformamide and 20 g of PVDF were placed in a reaction kettle protected by a nitrogen atmosphere, heated to 70 °C, 1 g of benzoyl peroxide was added, stirred for 20 min, 100 mL of trans-4-hydroxycinnamic acid solution and 5 g of modified nano-silica were added, and kept for reaction for 1 h. After the reaction was completed, the reaction solution was cooled to room temperature, ethanol was added to precipitate the product, suction filtration was carried out, and the filter cake was washed once with ethanol, transferred to a drying oven at 50 °C, and dried to constant weight to obtain carboxyl-modified PVDF.

[0057] Example 2

[0058] This example provides a method for preparing carboxyl-modified PVDF for a high-flux and anti-fouling PVDF microfiltration membrane, including the following steps:

[0059] Weigh: 7 mL of ammonia water, 15 - 20 mL of ethanol and 50 mL of deionized water were placed in a reaction kettle, heated to 35 °C, stirred for 7 min, 170 g of tetraethyl orthosilicate was added, heated to 75 °C, and kept for reaction for 9 h to obtain nano-silica sol;

[0060] Weigh: 450 mL of ethanol and 15 g of 3-butenyltriethoxysilane were mixed evenly to obtain a 3-butenyltriethoxysilane solution for standby;

[0061] Weigh: 170 g of nano-silica sol was placed in a reaction kettle, heated to 35 °C, adjusted to pH = 7.3 with dilute hydrochloric acid, 1100 mL of 3-butenyltriethoxysilane solution was added, heated to 550 °C, and kept for reaction for 3 h. After the reaction was completed, suction filtration was carried out, and the filter cake was washed twice with deionized water and ethanol, transferred to a drying oven at 60 °C, and dried to constant weight to obtain modified nano-silica;

[0062] Weigh: 15 g of trans-4-hydroxycinnamic acid and 12500 mL of N,N-dimethylformamide were mixed evenly to obtain a trans-4-hydroxycinnamic acid solution for standby;

[0063] Weigh: 400 mL of N,N-dimethylformamide and 25 g of PVDF and place them in a reaction kettle protected by a nitrogen atmosphere. Heat to 75 °C, add 1.2 g of benzoyl peroxide, stir for 25 min, add 125 mL of trans-4-hydroxycinnamic acid solution and 7 g of modified nano-silica, keep the temperature for reaction for 1.5 h. After the reaction is completed, wait for the reaction solution to cool to room temperature, add ethanol to precipitate the product, filter by suction, wash the filter cake with ethanol twice, transfer it to a drying oven at 55 °C, and dry to constant weight to obtain carboxyl-modified PVDF.

[0064] Example 3

[0065] This example provides a preparation method of carboxyl-modified PVDF for a high-flux and anti-pollution PVDF microfiltration membrane, including the following steps:

[0066] Weigh: 10 mL of ammonia water, 200 mL of ethanol and 60 mL of deionized water and place them in a reaction kettle. Heat to 40 °C, stir for 10 min, add 200 g of tetraethyl orthosilicate, heat to 80 °C, and keep the temperature for reaction for 10 h to obtain nano-silica sol;

[0067] Weigh: Mix 500 mL of ethanol and 20 g of 3-butenyltriethoxysilane evenly to obtain a 3-butenyltriethoxysilane solution for standby;

[0068] Weigh: Place 200 g of nano-silica sol in a reaction kettle, heat to 40 °C, add dilute hydrochloric acid to adjust the pH = 7.5, add 1200 mL of 3-butenyltriethoxysilane solution, heat to 60 °C, and keep the temperature for 4 h. After the reaction is completed, filter by suction, wash the filter cake with deionized water and ethanol twice, transfer it to a drying oven at 65 °C, and dry to constant weight to obtain modified nano-silica;

[0069] Weigh: Mix 20 g of trans-4-hydroxycinnamic acid and 1500 mL of N,N-dimethylformamide evenly to obtain a trans-4-hydroxycinnamic acid solution for standby;

[0070] Weigh: Place 500 mL of N,N-dimethylformamide and 30 g of PVDF in a reaction kettle protected by a nitrogen atmosphere. Heat to 80 °C, add 1.5 g of benzoyl peroxide, stir for 30 min, add 150 mL of trans-4-hydroxycinnamic acid solution and 10 g of modified nano-silica, keep the temperature for reaction for 2 h. After the reaction is completed, wait for the reaction solution to cool to room temperature, add ethanol to precipitate the product, filter by suction, wash the filter cake with ethanol twice, transfer it to a drying oven at 60 °C, and dry to constant weight to obtain carboxyl-modified PVDF.

[0071] Example 4

[0072] This embodiment provides a method for preparing a modified liquid for a high-flux and anti-pollution PVDF microfiltration membrane, comprising the following steps:

[0073] Weigh: 1000 mL of deionized water, 70 g of 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium] propane-1-sulfonate inner salt, 20 g of allylamine hydrochloride, and 40 g of N,N'-methylenebisacrylamide, place them in a reaction kettle, stir for 10 min to obtain a mixed salt solution;

[0074] Weigh: 40 g of polysorbate monooleate, 40 g of sorbitan monolaurate, and 900 mL of cyclohexane, place them in a reaction kettle, stir for 10 min, add 400 mL of the mixed salt solution, and stir for 15 min to obtain a mixed emulsion;

[0075] Weigh: 500 mL of the mixed emulsion, place it in a reaction kettle protected by a nitrogen atmosphere, add 10 g of azobisisobutyronitrile, heat up to 35 °C, keep the temperature for reaction for 20 h, add 300 mL of tetrahydrofuran, stir for 6 h, carry out suction filtration, wash the filter cake with tetrahydrofuran once, transfer it to an oven at 50 °C, and dry it to constant weight to obtain modified nano-gel beads;

[0076] Weigh: 2 g of potassium dihydrogen phosphate, 15 g of disodium hydrogen phosphate, 80 g of sodium chloride, 2 g of potassium chloride, and 10000 mL of deionized water, place them in a reaction kettle, heat up to 35 °C, stir for 15 min, and add sodium hydroxide solution to adjust the pH = 7.0 to obtain a buffer solution;

[0077] Weigh 50 g of the modified nano-gel beads and 1000 mL of the buffer solution, place them in a reaction kettle, and mix evenly to obtain a modified liquid.

[0078] Example 5

[0079] This embodiment provides a method for preparing a modified liquid for a high-flux and anti-pollution PVDF microfiltration membrane, comprising the following steps:

[0080] Weigh: 1100 mL of deionized water, 75 g of 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium] propane-1-sulfonate inner salt, 30 g of allylamine hydrochloride, and 45 g of N,N'-methylenebisacrylamide, place them in a reaction kettle, stir for 10 min to obtain a mixed salt solution;

[0081] Weigh: 45 g of polysorbate monooleate, 45 g of sorbitan monolaurate, and 1000 mL of cyclohexane, place them in a reaction kettle, stir for 12 min, add 450 mL of the mixed salt solution, and stir for 20 min to obtain a mixed emulsion;

[0082] Weigh: Place 550 mL of the mixed emulsion in a reaction kettle protected by a nitrogen atmosphere, add 12 g of azobisisobutyronitrile, heat up to 40 °C, keep the temperature for reaction for 21 h, add 325 mL of tetrahydrofuran, stir for 7 h, carry out suction filtration, wash the filter cake with tetrahydrofuran twice, transfer it to an oven at 55 °C, and dry it to constant weight to obtain modified nano-gel beads;

[0083] Weigh: Place 4 g of potassium dihydrogen phosphate, 15.5 g of disodium hydrogen phosphate, 90 g of sodium chloride, 2.5 g of potassium chloride and 10000 mL of deionized water in a reaction kettle, heat up to 40 °C, stir for 25 min, add sodium hydroxide solution to adjust the pH = 7.2 to obtain a buffer solution;

[0084] Weigh 70 g of the modified nano-gel beads and 1100 mL of the buffer solution and place them in a reaction kettle, mix them evenly to obtain a modified solution.

[0085] Example 6

[0086] This example provides a method for preparing a modified solution for a high-flux and anti-pollution PVDF microfiltration membrane, which includes the following steps:

[0087] Weigh: Place 1200 mL of deionized water, 80 g of 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium] propane-1-sulfonate inner salt, 35 g of allylamine hydrochloride and 50 g of N,N'-methylenebisacrylamide in a reaction kettle, stir for 15 min to obtain a mixed salt solution;

[0088] Weigh: Place 50 g of polysorbate monooleate, 50 g of sorbitan monolaurate and 1100 mL of cyclohexane in a reaction kettle, stir for 15 min, add 500 mL of the mixed salt solution, and stir for 30 min to obtain a mixed emulsion;

[0089] Weigh: Place 600 mL of the mixed emulsion in a reaction kettle protected by a nitrogen atmosphere, add 15 g of azobisisobutyronitrile, heat up to 45 °C, keep the temperature for reaction for 22 h, add 350 mL of tetrahydrofuran, stir for 8 h, carry out suction filtration, wash the filter cake with tetrahydrofuran twice, transfer it to an oven at 60 °C, and dry it to constant weight to obtain modified nano-gel beads;

[0090] Weigh: Place 5 g of potassium dihydrogen phosphate, 16 g of disodium hydrogen phosphate, 100 g of sodium chloride, 3 g of potassium chloride and 10000 mL of deionized water in a reaction kettle, heat up to 45 °C, stir for 30 min, add sodium hydroxide solution to adjust the pH = 7.5 to obtain a buffer solution;

[0091] Weigh 100 g of the modified nano-gel beads and 1200 mL of the buffer solution and place them in a reaction kettle, mix them evenly to obtain a modified solution.

[0092] Example 7

[0093] This embodiment provides a method for preparing a high-flux and anti-pollution PVDF microfiltration membrane, comprising the following steps:

[0094] S1. Prepare a carboxyl-modified PVDF base membrane

[0095] Mix polyethylene glycol, polyaniline, and titanium dioxide evenly according to a mass ratio of 2:0.5:0.5 to obtain an auxiliary additive for standby;

[0096] Weigh: 150 g of carboxyl-modified PVDF, 2200 mL of acetone, 200 mL of N,N-dimethylformamide, and 20 g of the auxiliary additive, place them in a reaction kettle, mix evenly, and let stand to remove bubbles to obtain a casting solution;

[0097] Cast the casting solution on a glass plate, level it with a scraper, place the leveled glass plate in a coagulation bath composed of deionized water and ethanol, heat it to 25 °C, with a coagulation time of 2 min, wash the membrane once with ethanol, transfer it to a drying oven at 40 °C, and dry it to a constant weight to obtain a carboxyl-modified PVDF base membrane.

[0098] S2. Prepare a PVDF microfiltration membrane

[0099] Weigh: 1 g of N-cyclohexyl-N'-(3-dimethylaminopropyl)carbodiimide, 10 g of N-hydroxysuccinimide, 20 g of 4-morpholineethanesulfonic acid, and 250 mL of deionized water, mix evenly to obtain an activation solution for standby;

[0100] Place the carboxyl-modified PVDF base membrane in the activation solution, and immerse it for 1 h according to a bath ratio of 1:30 to obtain an activated microfiltration base membrane;

[0101] Weigh: Immerse the activated microfiltration base membrane in the modification solution, heat it to 40 °C, keep it warm and react for 10 h, wash the membrane once with ethanol and deionized water, transfer it to a drying oven at 40 °C, and dry it to a constant weight to obtain a PVDF microfiltration membrane.

[0102] Example 8

[0103] This embodiment provides a method for preparing a high-flux and anti-pollution PVDF microfiltration membrane, comprising the following steps:

[0104] S1. Prepare a carboxyl-modified PVDF base membrane

[0105] Mix polyethylene glycol, polyaniline, and titanium dioxide evenly according to a mass ratio of 2:0.5:0.5 to obtain an auxiliary additive for standby;

[0106] Weigh: 170 g of carboxyl-modified PVDF, 2700 mL of acetone, 220 mL of N,N-dimethylformamide, and 35 g of auxiliary additive, place them in a reaction kettle, mix evenly, let stand to remove bubbles, and obtain a casting solution;

[0107] Pour the casting solution onto a glass plate, scrape it flat with a scraper, place the scraped glass plate in a coagulation bath composed of deionized water and ethanol, heat it to 30 °C, the coagulation time is 4 min, wash the membrane with ethanol twice, transfer it to a drying oven at 45 °C, and dry it to constant weight to obtain a carboxyl-modified PVDF base membrane.

[0108] S2. Preparation of PVDF microfiltration membrane

[0109] Weigh: 1.5 g of N-cyclohexyl-N-(3-dimethylaminopropyl)carbodiimide, 12 g of N-hydroxysuccinimide, 25 g of 4-morpholineethanesulfonic acid, and 270 mL of deionized water, mix evenly to obtain an activation solution for standby;

[0110] Place the carboxyl-modified PVDF base membrane in the activation solution, according to a bath ratio of 1:32, immerse it for 1.5 h to obtain an activated microfiltration base membrane;

[0111] Weigh: Immerse the activated microfiltration base membrane in the modification solution, heat it to 45 °C, keep it warm and react for 11 h, wash the membrane with ethanol and deionized water twice, transfer it to a drying oven at 45 °C, and dry it to constant weight to obtain a PVDF microfiltration membrane.

[0112] Example 9

[0113] This example provides a method for preparing a high-flux and anti-pollution PVDF microfiltration membrane, which includes the following steps:

[0114] S1. Preparation of carboxyl-modified PVDF base membrane

[0115] Mix polyethylene glycol, polyaniline, and titanium dioxide evenly according to a mass ratio of 2:0.5:0.5 to obtain an auxiliary additive for standby;

[0116] Weigh: 200 g of carboxyl-modified PVDF, 3000 mL of acetone, 250 mL of N,N-dimethylformamide, and 50 g of auxiliary additive, place them in a reaction kettle, mix evenly, let stand to remove bubbles, and obtain a casting solution;

[0117] Pour the casting solution onto a glass plate, scrape it flat with a scraper, place the scraped glass plate in a coagulation bath composed of deionized water and ethanol, heat it to 35 °C, the coagulation time is 5 min, wash the membrane with ethanol twice, transfer it to a drying oven at 50 °C, and dry it to constant weight to obtain a carboxyl-modified PVDF base membrane.

[0118] S2. Preparation of PVDF microfiltration membrane

[0119] Weigh: 2 g of N-cyclohexyl-N-(3-dimethylaminopropyl)carbodiimide, 15 g of N-hydroxysuccinimide, 30 g of 4-morpholineethanesulfonic acid and 300 mL of deionized water, mix them evenly to obtain an activation solution for standby;

[0120] Place the carboxyl-modified PVDF-based membrane in the activation solution, and soak it for 2 h according to a bath ratio of 1:35 to obtain an activated microfiltration-based membrane;

[0121] Weigh: Immerse the activated microfiltration-based membrane in the modification solution, raise the temperature to 50 °C, keep the temperature for reaction for 12 h, wash the membrane twice with ethanol and deionized water, transfer it to an oven at 50 °C, and dry it to a constant weight to obtain a PVDF microfiltration membrane.

[0122] Comparative Example 1

[0123] The difference between this comparative example and Example 8 is that in step S1, when the carboxyl-modified PVDF was prepared, trans-4-hydroxycinnamic acid was not added.

[0124] Comparative Example 2

[0125] The difference between this comparative example and Example 8 is that in step S1, when the carboxyl-modified PVDF was prepared, modified nano-silica was not added.

[0126] Comparative Example 3

[0127] The difference between this comparative example and Example 8 is that in step S2, the carboxyl-modified PVDF-based membrane in step S1 was used to replace the activated microfiltration-based membrane.

[0128] Comparative Example 4

[0129] The difference between this comparative example and Example 8 is that in step S2, the activated microfiltration-based membrane was used as the PVDF microfiltration membrane.

[0130] Performance test:

[0131] Refer to the standard HY / T 053-2001 "Microporous Membrane" to test the pure water flux of the PVDF microfiltration membranes prepared in Examples 7-9 and Comparative Examples 1-4 under a pressure of 0.01 Mpa;

[0132] Refer to HY / T 065-2002 "Polyvinylidene Fluoride Microporous Membrane" to test the porosity of the PVDF microfiltration membranes prepared in Examples 7-9 and Comparative Examples 1-4 and the porosity after the sample was subjected to 5 times of oil-water separation;

[0133] Refer to the standard HY / T 213-2016 "Determination Method for Tensile Strength at Break of Hollow Fiber Ultra / Microfiltration Membrane" to test the tensile strength and elongation at break of the PVDF microfiltration membranes prepared in Examples 7-9 and Comparative Examples 1-4;

[0134] The water contact angles of the PVDF microfiltration membranes prepared in Test Examples 7-9 and Comparative Examples 1-4 were measured with reference to the standard HY / T 266-2018 "Contact Angle Method for Testing the Surface Hydrophilicity of Hollow Fiber Ultrafiltration Membranes under External Pressure". The specific data are shown in Table 1.

[0135] Table 1. Performance test data of each sample

[0136]

[0137]

[0138] Data analysis:

[0139] By comparing and analyzing the data in Table 1 above, the pure water flux of the PVDF microfiltration membrane prepared in the present invention is 82.35 L·m -2 ·h, the initial porosity is 82.13%, the porosity after 5 times of oil-water separation is 78.44%, the tensile strength is 11.51 Mpa, the elongation at break is 402.13%, and the water contact angle is 17°;

[0140] By comparing and analyzing the data of Comparative Example 1 and Test Example 8, it can be found that the pure water flux, the porosity after 5 times of oil-water separation, and the water contact angle of the PVDF microfiltration membrane prepared in Comparative Example 1 decreased significantly. This shows that in the present invention, carboxyl-modified PVDF is obtained by free radical polymerization of modified nano-silica, trans-4-hydroxycinnamic acid, and PVDF. When carboxyl groups are introduced into the PVDF membrane, the hydrophilicity of the membrane can be greatly improved. The membrane with enhanced hydrophilicity can better interact with water molecules, thereby improving the wettability of the membrane to water, reducing the contact angle when water flows through the membrane, and increasing the anti-fouling rate and the flux of the membrane;

[0141] By comparing and analyzing the data of Comparative Example 2 and Test Example 8, it can be found that the pure water flux, the porosity after 5 times of oil-water separation, the tensile strength, the elongation at break, and the water contact angle of the PVDF microfiltration membrane prepared in Comparative Example 2 decreased significantly. This shows that in the present invention, nano-silica is prepared by the sol-gel method and then modified with a silane coupling agent containing olefins to obtain modified nano-silica. The carboxyl-modified PVDF-based membrane is prepared by the phase inversion method. The surface of the added nano-silica is rich in silanol groups, which can form weak hydrogen bonds with the fluorine atoms of PVDF, improving the polarity of the membrane. During oil-water separation, nano-silica adsorbs water molecules through weak hydrogen bonds to form a hydration layer, reducing the water contact angle and increasing the flux of the PVDF microfiltration membrane, and reducing the adsorption of hydrophobic pollutants. Moreover, nano-silica can be combined with PVDF chains through physical entanglement and interfacial hydrogen bonds, restricting the slip of molecular chains and improving the mechanical strength of the PVDF microfiltration membrane;

[0142] By comparing and analyzing the data of Comparative Example 3 and Example 8, it can be found that the pure water flux, porosity and water contact angle of the PVDF microfiltration membrane prepared in Comparative Example 3 decreased significantly after 5 times of oil-water separation. This shows that the carboxyl-modified PVDF-based membrane prepared by the present invention through free radical polymerization reaction. To improve the carboxyl activity of the carboxyl-modified PVDF-based membrane, the PVDF-based membrane was placed in an activation solution for carboxyl activation. The activation solution converts the carboxyl group into a highly active intermediate through a reaction, improving the grafting rate of the carboxyl-modified PVDF-based membrane and the modified nanogel spheres, and further improving the flux and anti-pollution ability of the PVDF microfiltration membrane;

[0143] By comparing and analyzing the data of Comparative Example 4 and Example 8, it can be found that the pure water flux, porosity and water contact angle of the PVDF microfiltration membrane prepared in Comparative Example 4 decreased significantly after 5 times of oil-water separation. This shows that the present invention prepares modified nanogel spheres by introducing amphiphilic molecules through free radical polymerization reaction. 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium] propane-1-sulfonate inner salt contains both quaternary ammonium cations and sulfonic acid anions, which are amphiphilic molecules. One molecule of amphiphilic molecule can bind to 7-8 water molecules at the same time. Compared with hydroxyl and carboxyl groups, it has higher hydrophilicity. When the PVDF microfiltration membrane is used for oil-water separation, the nanogel microspheres have a certain spatial structure compared with single hydrophilic polymers. It can not only form a relatively high hydrophilic density in the local area, but also obtain a higher hydration layer thickness after binding with water due to the existence of its own three-dimensional structure, effectively improving the resistance of the membrane surface to oil pollution. At the same time, the cationic group can interact with the polar molecules in the oil droplets to inhibit the aggregation of oil droplets, while the anionic group can enhance the wettability of water, thus promoting the smooth progress of the oil-water separation process and improving the anti-pollution ability of the PVDF microfiltration membrane.

[0144] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments. Obviously, many modifications and changes can be made according to the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and utilize the present invention well. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A method for preparing a high-flux, anti-pollution PVDF microfiltration membrane, characterized in that: The method comprises the following preparation steps: S1. placing carboxyl-modified PVDF, acetone, N,N-dimethylformamide and auxiliary additives in a reaction kettle, mixing them evenly, standing them to remove bubbles, and obtaining a film casting solution; S2, casting the film-casting solution on a glass plate, scraping it flat with a scraper to form a film, and post-treating it to obtain a carboxyl-modified PVDF base film; S3, placing the carboxyl-modified PVDF base membrane in an activation solution and immersing it for 1-2 hours to obtain an activated microfiltration base membrane; S4, immersing the activated microfiltration base membrane in the modified liquid, heating to 40-50°C, keeping the temperature for reaction for 10-12h, and post-treating to obtain a PVDF microfiltration membrane.

2. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 1, characterized in that: In step S1, the amount ratio of the carboxyl-modified PVDF, acetone, N,N-dimethylformamide and auxiliary additives is 15-20g:220-300mL:20-25mL:2-5g; in step S3, the activation solution is composed of N-cyclohexyl-N-(3-dimethylaminopropyl)carbodiimide, N-hydroxysuccinimide, 4-morpholineethanesulfonic acid and deionized water in an amount ratio of 0.1-0.2g:1-1.5g:2-3g:25-30mL, and the bath ratio is 1:30-35; in step S4, the bath ratio is 1:30-35.

3. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 1, characterized in that: The preparation method of the carboxyl-modified PVDF is as follows: N,N-dimethylformamide and PVDF are placed in a reactor protected by a nitrogen atmosphere, the temperature is raised to 70-80° C., terephthaloyl peroxide is added, stirred for 20-30 minutes, trans-4-hydroxycinnamic acid solution and modified nano-silica are added, the temperature is kept for reaction for 1-2 hours, and the carboxyl-modified PVDF is obtained by post-treatment.

4. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 3, characterized in that: The trans-4-hydroxycinnamic acid solution is composed of trans-4-hydroxycinnamic acid and N,N-dimethylformamide in a dosage ratio of 1-2g:100-150mL, and the dosage ratio of N,N-dimethylformamide, PVDF, terephthaloyl peroxide, trans-4-hydroxycinnamic acid solution and modified nano-silica is 30-50mL:2-3g:0.10-0.15g:10-15mL:0.5-1g.

5. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 3, characterized in that: The modified nano silicon dioxide is prepared by the following steps: A1. Place ammonia water, ethanol and deionized water in a reaction kettle, heat to 30-40°C, stir for 5-10 minutes, add ethyl orthosilicate, heat to 70-80°C, and keep warm for 8-10 hours to obtain nano-silica sol; A2. Place the nano-silica sol in a reaction kettle, heat it to 30-40°C, add dilute hydrochloric acid to adjust the pH to 7±0.5, add 3-butenetriethoxysilane solution, heat it to 50-60°C, keep it warm for 2-4h, and post-treat to obtain modified nano-silica.

6. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 5, characterized in that: In step A1, the amount ratio of ammonia water, ethanol, deionized water and tetraethyl orthosilicate is 0.5-1mL:15-20mL:4-6mL:15-20g; in step A2, the amount ratio of the nano-silica sol and the 3-butenetriethoxysilane solution is 15-20g:100-120mL, and the 3-butenetriethoxysilane solution is composed of ethanol and 3-butenetriethoxysilane in a ratio of 40-50mL:1-2g.

7. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 1, characterized in that: The modified liquid is prepared by the following steps: B1. Place potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride, potassium chloride and deionized water in a reaction kettle, heat to 35-45° C., stir for 15-30 min, add sodium hydroxide solution to adjust the pH to 7.0±0.5, and obtain a buffer solution; B2. Place the modified nanogel balls and buffer solution in a reaction kettle, mix them evenly, and obtain a modified solution.

8. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 7, characterized in that: In step B1, the dosage ratio of potassium dihydrogen phosphate, disodium hydrogen phosphate, sodium chloride, potassium chloride and deionized water is 0.2-0.5g:1.5-1.6g:8-10g:0.2-0.3g:1000mL, and the concentration of the sodium hydroxide solution is 1-5wt%; in step B2, the dosage ratio of the modified nanogel spheres and the buffer solution is 5-10g:100-120mL.

9. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 7, characterized in that: The modified nanogel spheres are prepared by the following steps: C1. Deionized water, 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium]propane-1-sulfonic acid inner salt, allylamine hydrochloride and N,N'-methylenebisacrylamide are placed in a reaction kettle and stirred for 10-15 minutes to obtain a mixed salt solution; C2, placing polysorbate monooleate, sorbitan monolaurate and cyclohexane in a reaction kettle, stirring for 10-15 minutes, adding a mixed salt solution, stirring for 15-30 minutes, and obtaining a mixed emulsion; C3. Place the mixed emulsion in a reactor protected by a nitrogen atmosphere, add azobisisobutyronitrile, raise the temperature to 35-45°C, keep the temperature for reaction for 20-22 hours, add tetrahydrofuran, stir for 6-8 hours, and post-treat to obtain modified nanogel spheres.

10. The method for preparing a high-flux, anti-pollution PVDF microfiltration membrane according to claim 9, characterized in that: In step C1, the amount ratio of deionized water, 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium]propane-1-sulfonic acid inner salt, allylamine hydrochloride and N,N'-methylenebisacrylamide is 100-120mL:7-8g:2-3.5g:4-5g; in step C2, the amount ratio of polysorbate monooleate, sorbitan monolaurate, cyclohexane and mixed salt solution is 4-5g:4-5g:90-110mL:40-50mL; in step C3, the amount ratio of the mixed emulsion, azobisisobutyronitrile and tetrahydrofuran is 50-60mL:1-1.5g:30-35mL.

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