Acid and alkali resistant polyvinylidene fluoride filter membrane and preparation method thereof

Abstract

The invention relates to the technical field of filter membranes, in particular to a preparation method of an acid and alkali resistant polyvinylidene fluoride filter membrane, which comprises the following steps: mixing and stirring deoxidized deionized water, an emulsifier, a steric hindrance agent, a chain transfer agent and a VDF monomer in a high-pressure reaction kettle to form an emulsion, adding a micro-radiation agent to form a homogeneous emulsion polymerization system, heating, adding an initiator, and reacting for 2-4 hours to obtain the acid and alkali resistant polyvinylidene fluoride filter membrane. Adding VDF monomers in the reaction process to maintain the pressure at 3-5 MPa, relieving the high pressure after the reaction is finished, adding calcium chloride for demulsification to precipitate PVDF, and repeatedly washing, filtering and drying in vacuum to obtain unisotactic PVDF white powdery resin; the preparation method comprises the following steps: dissolving the resin and acid and alkali resistant polyurethane in N, N-dimethylacetamide under the action of an acidic inorganic salt, adding a pore-foaming agent, uniformly stirring to prepare a membrane casting solution, vacuumizing, standing, defoaming, blade-coating on a non-woven fabric to form a liquid membrane, removing part of the solvent, putting into water, performing solution phase conversion to form a membrane, cleaning and airing to obtain the acid and alkali resistant PVDF filter membrane. The filtering membrane prepared by the invention has relatively good acid and alkali resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of filter membranes, and in particular to an acid- and alkali-resistant polyvinylidene fluoride filter membrane and a preparation method thereof. Background Art

[0002] As one of the core technologies in the field of water treatment, membrane separation technology has been applied on a large scale in many application scenarios. Among them, PVDF materials are recognized as the preferred material for the preparation of ultra-microfiltration membranes for water treatment due to their excellent film-forming properties and outstanding physical and chemical properties. Various membrane products made of PVDF materials continue to play a key role in engineering practices such as municipal water supply system upgrades, deep treatment of domestic sewage, and pretreatment of seawater desalination through their unique pore size screening mechanism. However, the acid-base resistance of the fluorine-carbon bond in the molecular structure of PVDF makes it easy to undergo chain scission degradation when used in a strong alkaline environment with a pH greater than 12 and an oxidizing acid with a concentration exceeding 10% (such as concentrated sulfuric acid and nitric acid). This severely limits its application in special working conditions such as industrial wastewater treatment and separation of acidic and alkaline materials. In addition, PVDF membranes need to be chemically cleaned with 0.1-0.5 mol / L acid-base solutions and oxidants such as sodium hypochlorite during long-term use. Repeated cleaning will cause irreversible damage such as polymer main chain breakage and crystallization area destruction, resulting in a 30-50% decrease in membrane flux and a retention rate fluctuation of more than ±15%, significantly shortening its designed service life of 3-5 years.

[0003] In view of the fact that PVDF materials are not resistant to strong acids and alkalis, current research mainly focuses on three technical paths: blending modification, copolymerization modification and surface coating. In terms of blending modification, the Institute of Engineering of the Chinese Academy of Sciences uses a solution casting method to blend 2-4wt% of polytetrafluoroethylene nanofibers with PVDF. After the composite membrane is immersed in 60°C and pH = 14 for 240 hours, the tensile strength retention rate is increased from 47% of the pure PVDF membrane to 82%, but the flux recovery rate is only 76%; in the field of copolymerization modification, Advanced Materials recently reported the introduction of perfluorosulfonic acid groups into the PVDF backbone through plasma grafting. This technology allowed the membrane to maintain a molecular weight cutoff fluctuation range of ±200 Da after 500 hours of continuous operation in a nitric acid environment. In terms of surface functionalization, a 10nm thick Al2O3 / PVDF composite membrane constructed using atomic layer deposition (ALD) technology increased its membrane life from the conventional 20 times to 65 times in a sodium hypochlorite (5000ppm) cleaning cycle test, but the transmembrane pressure difference increased by 18kPa. Similar technologies have been found in patents such as CN105771690A and CN112791596A. However, these technologies still face three major technical bottlenecks in engineering applications: first, the interface compatibility between the modifier and the PVDF matrix is insufficient, and delamination and peeling are prone to occur during long-term operation. The accelerated aging test of a membrane component manufacturer showed that after 2000 hours of operation, the blended membrane had 0.5-2μm interface cracks; second, the chemical modification caused the membrane pore size distribution to widen. Dynamic light scattering (DLS) detection showed that the average pore size of the modified membrane expanded from 25nm to 38nm, causing the retention rate of 1000-5000Da substances to drop by 12-19 percentage points; third, the existing modification process cannot fundamentally eliminate the PVDF membrane's intolerance to strong acids and alkalis. Once the main PVDF material of the membrane comes into direct contact with the acid and alkali oxidants, the defluorination reaction is rapidly transmitted along the main molecular chain, causing the molecular chain to break, the membrane performance to decline, and the membrane life to reach the end. Therefore, the preparation of strong acid (pH < 2), strong alkali (pH > 12) and oxidation-resistant PVDF filtration membranes through material modification or process innovation has important engineering value for expanding its industrial application range and reducing the operation and maintenance costs of membrane systems. Summary of the Invention

[0004] The present invention solves the problems in the related art and proposes an acid- and alkali-resistant polyvinylidene fluoride filter membrane and a preparation method thereof. The filter membrane prepared by the present invention has excellent acid and alkali resistance, which changes the current situation that PVDF filter membranes for water treatment cannot be used in strong alkaline and oxidizing acid environments, and expands the application scope of PVDF filter membranes in the field of water treatment. For some separation systems where traditional PVDF filter membranes cannot be used, the filter membrane prepared by this method has obvious advantages.

[0005] In order to solve the above technical problems, the present invention is achieved through the following technical solutions: a method for preparing an acid- and alkali-resistant polyvinylidene fluoride filter membrane, comprising the following steps:

[0006] S1. Deoxygenated deionized water, emulsifier, steric inhibitor, chain transfer agent, and VDF monomer are mixed and stirred in an autoclave to form an emulsion state, and then a micro-irradiation agent is added to form a uniform emulsion polymerization system. The temperature is increased and an initiator is added. During the reaction, VDF monomer is continuously added to maintain the pressure of the system between 3 and 5 MPa. After the reaction is completed, the high pressure is released, and calcium chloride is added to break the emulsion and precipitate PVDF. After repeated water washing, filtration, and vacuum drying, anisotactic PVDF white powder resin is obtained;

[0007] S2. The anisotactic PVDF and acid- and alkali-resistant polyurethane prepared in S1 are dissolved together in the solvent N,N-dimethylacetamide under the action of an acidic inorganic salt, a porogen is added, and the mixture is stirred evenly to form a casting liquid. The casting liquid is vacuumed and allowed to stand for degassing, and then the casting liquid is scraped onto a non-woven fabric to form a liquid film. After removing part of the solvent, the liquid film is placed in water, and a film is prepared through a solution phase conversion process. The acid- and alkali-resistant PVDF filter membrane is obtained by washing and drying.

[0008] As a preferred embodiment, in step S1, the emulsifier is any one of sodium perfluorooctanoate and potassium perfluorooctanoate; the steric inhibitor is any one of 1,2-difluoroethane and 1,1-difluoroethane; the chain transfer agent is any one of dodecanethiol and 2-mercaptoethanol; the micro-radiation agent is americium-241 oxide; and the initiator is any one of tert-amyl peroxyneodecanoate and tert-butyl peroxyneodecanoate.

[0009] As a preferred embodiment, in step S1, the reaction temperature is 60-90° C., and the polymerization time is 10-14 h.

[0010] As a preferred solution, in step S2, the acid- and alkali-resistant polyurethane is any one of cast polyurethane and silicone-modified polyurethane.

[0011] As a preferred embodiment, the cast polyurethane is a polyurethane using polytetramethylene ether polyol as the soft segment, diisocyanate as the hard segment, and hydroquinone-bis(hydroxyethyl) ether as the chain extender; the silicone-modified polyurethane is a polyurethane using hydroxy silicone oil as the soft segment, isocyanate as the hard segment, and amino silicone oil as the chain extender.

[0012] As a preferred embodiment, in step S2, the acidic inorganic salt is any one of sodium bisulfite and potassium bisulfite.

[0013] As a preferred embodiment, in step S2, the mass ratio of DMAC, PVDF, polyurethane, acidic inorganic salt, and PVP is 100:(18-20):(3-5):(0.5-1):(2-3).

[0014] Another aspect of the present invention provides an acid- and alkali-resistant polyvinylidene fluoride filter membrane prepared by the above-mentioned preparation method.

[0015] PVDF is composed of many VDF monomers - (CF2-CH2) - connected together. There are three ways to connect VDF monomers. 19 The F-NMR positions are shown in the following table:

[0016] Head to tail <![CDATA[—CF2—CH2—CF2—CH2—]]> -92.5 Head to head <![CDATA[—CH2—CF2—CF2—CH2—]]> -114.2 and -116.8 Tail to tail <![CDATA[—CF2—CH2—CH2—CF2—]]> -95.8

[0017] The head-to-tail connected PVDF segments undergo a chain reaction of removing hydrogen fluoride in the presence of strong acid, alkali and oxidant (the principle is as follows Figure 1 (as shown), this chain reaction generates long chains with alternating single and double bonds in a conjugated structure. This long chain structure easily continues to react and oxidize to form carbon-oxygen double bonds, which then break. This chain reaction terminates due to heterogeneous chemical bonds at molecular segments that are connected head-to-head or tail-to-tail. Therefore, if the PVDF molecular chain contains fewer head-to-tail segments of VDF monomers and more head-to-head and tail-to-tail segments, this reaction will be difficult to occur, and even if it does occur, it will terminate quickly. The acid and alkali resistance of membranes prepared with this anisotactic PVDF will be significantly improved compared to traditional PVDF membranes.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) In the present invention, the role of the steric inhibitor is to prevent the VDF monomer from quickly connecting head to tail during the polymerization process. The steric inhibitors used in the present invention are 1,2-difluoroethane and 1,1-difluoroethane. These two substances are very similar to the VDF monomer structure and have similar physicochemical parameters. Therefore, they can be well integrated into the reaction system. The difference is that these two steric inhibitors do not have double bonds that can participate in the polymerization reaction and have relatively stable chemical properties. Under normal circumstances, VDF monomers are very easy to connect head to tail to form long molecular chains during polymerization. However, when they encounter these two steric inhibitors, the chain growth reaction of the head-to-tail connection is suspended because they cannot contact the effective reactive groups. In this way, a large number of VDF chain segments connected head to tail in the synthesized PVDF molecular chain can be avoided;

[0020] (2) In the present invention, the role of the micro-radiation agent is to provide a certain activation energy to the VDF monomer, so that when it bypasses the steric inhibitor and contacts other VDF segments containing free radicals, it can have enough energy to react with them head-to-head or tail-to-tail. Under normal circumstances, the energy required for the head-to-head or tail-to-tail chain growth reaction is higher than that for the head-to-tail reaction. The micro-radiation agent used in the present invention is americium-241 oxide (AmO2). The radiation energy of this substance is much lower than that of substances such as uranium-235, which is relatively safe and meets the radiation energy requirements of the reaction. More importantly, AmO2 is easily soluble in water and has good compatibility in a reaction system with water as the medium.

[0021] (3) In the present invention, anisotactic PVDF is blended with acid- and alkali-resistant polyurethane to form a membrane. This is mainly because the molecular weight of PVDF composed of head-to-head and tail-to-tail VDF monomers is lower than that of ordinary PVDF. If other materials with high mechanical strength are not blended, the mechanical strength of the membrane is low.

[0022] (4) In the present invention, the cast polyurethane (CPUE) and silicone-modified polyurethane used can not only improve the mechanical strength of the PVDF membrane without affecting its acid and alkali resistance, but also improve the hydrophilicity of the membrane material. Since the solubility of polyurethane in DMAC is not high and it is easy to dissolve unevenly, the present application adds acidic inorganic salts to destroy the intermolecular forces of polyurethane, so that it can be better mixed with other materials;

[0023] The present invention adds special micro-radiants and steric inhibitors during the synthesis of PVDF membrane material. Through specific processes and parameters, the number of head-to-tail connected segments of vinylidene fluoride monomers is reduced, and the number of head-to-head and tail-to-tail connected segments is greatly increased. The regularity of the PVDF macromolecular chain segments with this structure is very low. When encountering acids, bases and oxidants, the defluorination reaction cannot be conducted along the molecular chain and the chain is easily terminated. Therefore, the acid and alkali resistance of the prepared PVDF filter membrane is greatly improved compared with ordinary PVDF filter membranes. This method is stable and reliable and suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is based on the chain reaction principle of the PVDF segments connected head to tail in the presence of strong acid, alkali and oxidant to remove hydrogen fluoride;

[0025] Figure 2 It is the NMR image of Example 1 and Comparative Example 1 in the present invention. DETAILED DESCRIPTION

[0026] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0027] Example 1

[0028] A method for preparing an acid- and alkali-resistant polyvinylidene fluoride filter membrane comprises the following steps:

[0029] (1) Deoxygenated deionized water, emulsifier SPFO, steric inhibitor 1,2-difluoroethane, chain transfer agent NDM, and VDF monomer are mixed in a high-pressure reactor and stirred at high speed to form a stable emulsion state. Then, a micro-radiant AmO2 is added to form a uniform emulsion polymerization system. The temperature is raised to 70°C, and an initiator tert-amyl peroxyneodecanoate is added to decompose and generate free radicals. VDF initiates polymerization in micelles to form PVDF chains. During the reaction, VDF monomer is continuously added and the system is maintained at a high pressure of 5.0 MPa. After reacting for 10 hours, the high pressure state is released, and calcium chloride is added to break the emulsion to precipitate PVDF. Then, the residual emulsifier, initiator and other impurities are repeatedly washed with water to remove the filtered PVDF. The filtered PVDF is vacuum dried at 60°C to obtain anisotactic PVDF white powder resin.

[0030] (2) The prepared anisotactic PVDF and CPUE (polyurethane with polytetramethylene glycol as soft segment, MDI-100 as hard segment, and hydroquinone-bis(hydroxyethyl)ether) as chain extender were added to DMAC, and NaHSO3 was added to mix the two evenly. PVP was added as a porogen. The mass ratios of the several substances are shown in the table below. After being evenly dissolved, a casting solution was prepared, vacuumed, and allowed to stand for degassing. The casting solution was then scraped onto a non-woven fabric to form a liquid film. Part of the solvent was quickly evaporated in hot air, and the liquid film was then placed in tap water. After the solution phase inversion process, a film was prepared, which was then washed and dried to obtain a PVDF filter membrane.

[0031] name DMAC PVDF CPUE <![CDATA[NaHSO3]]> PVP Mass ratio 100 18 3 0.5 2

[0032] The anisotactic PVDF raw material prepared in Example 1 was tested, and the results were as follows: PVDF raw material NMR 19 F-NMR spectrum Figure 2As shown in , compared with ordinary PVDF, the prepared anisotactic PVDF has significantly increased peaks at -114.2, -116.8, and -95.8, indicating the effectiveness of this synthesis; the prepared PVDF raw material was soaked in acid with pH = 1 and alkali with pH = 14, respectively, and some was taken out every 24 hours, washed and dried, dissolved in analytical grade DMAC, and then the transmittance was measured using a UV-visible spectrophotometer. The results showed that compared with ordinary PVDF, the prepared anisotactic PVDF remained stable in DMAC solution after being soaked in acid and alkali, and the color change was small, indicating that it was relatively stable in acid and alkali.

[0033] The membrane prepared in Example 1 was tested and the results were as follows: the pure water flux at 20°C and 0.1 MPa pressure was 1000 L / m 2 h, the average pore size is 0.1 μm, the pure water bubble point is 120 kPa, and the membrane is circulated and cross-flow flushed in nitric acid solution with pH = 1, sulfuric acid solution with pH = 1, and sodium hydroxide solution with pH = 14 at 50 ° C for 48 hours. The dynamic light scattering test results show that the average pore size changes by less than 5%, and the tensile test results show that the mechanical strength of the membrane layer changes by less than 1%. There is no obvious change in the pure water bubble point and flux.

[0034] Example 2

[0035] (1) Deoxygenated deionized water, emulsifier KPFO, steric inhibitor 1,1-difluoroethane, chain transfer agent 2-mercaptoethanol (2-ME), and VDF monomer are mixed in a high-pressure reactor and stirred at high speed to form a stable emulsion state. Then, micro-radiation agent AmO2 is added to form a uniform emulsion polymerization system. The temperature is raised to 90°C, and initiator tert-butyl peroxyneodecanoate is added to decompose and generate free radicals. VDF initiates polymerization in micelles to form PVDF chains. During the reaction, VDF monomer is continuously added and the system pressure is maintained at 3.0 MPa. After 14 hours of reaction, the high-pressure state is released, and calcium chloride is added to break the emulsion to precipitate PVDF. Then, the residual emulsifier, initiator and other impurities are repeatedly washed with water to remove the filtered PVDF. The filtered PVDF is vacuum dried at 80°C to obtain anisotactic PVDF white powder resin.

[0036] (2) The prepared anisotactic PVDF and silicone-modified polyurethane (polyurethane with hydroxyl silicone oil as soft segment, IPDI as hard segment and amino silicone oil as chain extender) were added to DMAC, and KHSO3 was added to mix the two evenly. PVP was added as a porogen. The mass ratios of the several substances are shown in the table below. After being evenly dissolved, a casting solution was prepared, vacuumed and allowed to stand for degassing. The casting solution was then scraped onto a non-woven fabric to form a liquid film. Part of the solvent was quickly evaporated in hot air, and the liquid film was then placed in tap water. After the solution phase inversion process, a membrane was prepared, which was then washed and dried to obtain a PVDF filter membrane.

[0037] name DMAC PVDF CPUE <![CDATA[NaHSO3]]> PVP Mass ratio 100 20 5 1 3

[0038] The PVDF raw material prepared in Example 2 was soaked in an acid with a pH of 1 and an alkali with a pH of 14, respectively. Some was removed every 24 hours, washed and dried, and dissolved in analytical-grade DMAC. The transmittance was then measured using a UV-visible spectrophotometer. The results showed that compared to ordinary PVDF, the transmittance of the prepared anisotactic PVDF in the DMAC solution remained stable after soaking in acid and alkali, and the color change was small, indicating that it is relatively stable in acid and alkali.

[0039] The membrane prepared in Example 2 was tested and the results were as follows: the pure water flux at 20°C and 0.1 MPa pressure was 1100 L / m 2 h, an average pore size of 0.11 μm, and a pure water bubble point of 100 kPa; the membrane was respectively washed in a 50°C, pH = 1 nitric acid solution, a pH = 1 sulfuric acid solution, and a pH = 14 sodium hydroxide solution in a circulating cross-flow manner for 48 hours. The dynamic light scattering test results showed that the average pore size changed by less than 5%, and the tensile test results showed that the mechanical strength of the membrane layer changed by less than 2%. There was no significant change in the pure water bubble point and flux.

[0040] Example 3

[0041] (1) Deoxygenated deionized water, emulsifier SPFO, steric inhibitor 1,1-difluoroethane, chain transfer agent NDM, and VDF monomer are mixed in a high-pressure reactor and stirred at high speed to form a stable emulsion state. Then, a micro-radiant AmO2 is added to form a uniform emulsion polymerization system. The temperature is raised to 90°C, and an initiator tert-butyl peroxyneodecanoate is added to decompose and generate free radicals. VDF initiates polymerization in micelles to form PVDF chains. During the reaction, VDF monomer is continuously added and the system pressure is maintained at 4.0 MPa. After 12 hours of reaction, the high-pressure state is released, and calcium chloride is added to break the emulsion to precipitate PVDF. Then, the residual emulsifier, initiator and other impurities are repeatedly washed with water to remove the filtered PVDF. The filtered PVDF is vacuum dried at 70°C to obtain anisotactic PVDF white powder resin.

[0042] (2) The prepared anisotactic PVDF and CPUE (polyurethane with polytetramethylene ether polyol as soft segment, MDI-100 as hard segment, and hydroquinone-bis(hydroxyethyl)ether) as chain extender were added to DMAC, and KHSO3 was added to mix the two evenly. PVP was added as a porogen. The mass ratios of the several substances are shown in the table below. After being evenly dissolved, a casting solution was prepared, vacuumed, and allowed to stand for degassing. The casting solution was then scraped onto a non-woven fabric to form a liquid film. Part of the solvent was quickly evaporated in hot air, and the liquid film was then placed in tap water. After the solution phase inversion process, a film was prepared, which was then washed and dried to obtain a PVDF filter membrane.

[0043] name DMAC PVDF CPUE <![CDATA[NaHSO3]]> PVP Mass ratio 100 19 4 0.8 2.5

[0044] The PVDF raw material prepared in Example 3 was soaked in an acid with a pH of 1 and an alkali with a pH of 14, respectively. Some was removed every 24 hours, washed and dried, dissolved in analytical-grade DMAC, and then the transmittance was measured using a UV-visible spectrophotometer. The results showed that compared to ordinary PVDF, the transmittance of the prepared anisotactic PVDF in the DMAC solution remained stable after soaking in acid and alkali, and the color change was small, indicating that it is relatively stable in acid and alkali.

[0045] The membrane prepared in Example 3 was tested and the results were as follows: the pure water flux at 20°C and 0.1 MPa pressure was 1300 L / m 2 h, an average pore size of 0.12 μm, and a pure water bubble point of 95 kPa; the membrane was circulated and cross-flow flushed in a nitric acid solution with a pH of 1, a sulfuric acid solution with a pH of 1, and a sodium hydroxide solution with a pH of 14 at 50°C for 48 hours, and the dynamic light scattering test results showed that the average pore size changed by less than 4%, and the tensile test results showed that the mechanical strength of the membrane layer changed by less than 2%, and there was no obvious change in the pure water bubble point and flux.

[0046] Comparative Example 1

[0047] The difference from Example 1 is that the homemade anisotactic PVDF in Example 1 is replaced with a commercial ordinary PVDF, and the other aspects remain unchanged.

[0048] The NMR of PVDF used in Comparative Example 1 19 F-NMR spectrum Figure 2 As shown in , compared with the homemade anisotactic PVDF in Example 1, the peaks at -114.2, -116.8, and -95.8 are very low, and the peak at -92.5 is very high, indicating that in this PVDF, there are many head-to-tail connected VDF segments, and very few head-to-head and tail-to-tail connected segments; the PVDF raw material was soaked in an acid with pH = 1 and an alkali with pH = 14, and some was taken out every 24 hours, washed and dried, dissolved in analytical grade DMAC, and then the transmittance was measured using a UV-visible spectrophotometer. The results showed that after soaking in acid and alkali, the transmittance of the DMAC solution was greatly reduced compared to the homemade anisotactic PVDF in Example 1, and the color became darker with soaking time, indicating that it was easily degraded in acid and alkali.

[0049] The membrane prepared in Example 1 was tested, and the results were as follows: the pure water flux at 20°C and 0.1 MPa pressure was 1000 L / m 2h, an average pore size of 0.09 μm, and a pure water bubble point of 125 kPa; the membrane was circulated and cross-flow washed in a nitric acid solution with a pH of 1, a sulfuric acid solution with a pH of 1, and a sodium hydroxide solution with a pH of 14 at 50°C for 48 hours, and the dynamic light scattering test results showed that its average pore size changed by more than 120%, and the tensile test results showed that the mechanical strength of the membrane layer decreased by more than 60%, the pure water bubble point decreased to less than 30 kPa, and the flux changed by more than 300%, indicating that the filter membrane prepared by ordinary PVDF is extremely easy to degrade in strong acids and alkalis.

[0050] The above are preferred embodiments of the present invention. Those skilled in the art to which the present invention belongs can also change and modify the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, replacements or modifications made by those skilled in the art on the basis of the present invention fall within the scope of protection of the present invention.

Claims

1. A method for preparing an acid- and alkali-resistant polyvinylidene fluoride filter membrane, characterized in that: Here are the steps: S1. Deoxygenated deionized water, emulsifier, steric inhibitor, chain transfer agent, and VDF monomer are mixed and stirred in an autoclave to form an emulsion state, and then a micro-irradiation agent is added to form a uniform emulsion polymerization system. The temperature is increased and an initiator is added. During the reaction, VDF monomer is continuously added to maintain the pressure of the system between 3 and 5 MPa. After the reaction is completed, the high pressure is released, and calcium chloride is added to break the emulsion and precipitate PVDF. After repeated water washing, filtration, and vacuum drying, anisotactic PVDF white powder resin is obtained; S2. The anisotactic PVDF and acid- and alkali-resistant polyurethane prepared in S1 are dissolved together in the solvent N,N-dimethylacetamide under the action of an acidic inorganic salt, a porogen is added, and the mixture is stirred evenly to form a casting liquid. The casting liquid is vacuumed and allowed to stand for degassing, and then the casting liquid is scraped onto a non-woven fabric to form a liquid film. After removing part of the solvent, the liquid film is placed in water, and a film is prepared through a solution phase conversion process. The acid- and alkali-resistant PVDF filter membrane is obtained by washing and drying.

2. The method for preparing the acid- and alkali-resistant polyvinylidene fluoride filter membrane according to claim 1, wherein: In step S1, the emulsifier is any one of sodium perfluorooctanoate and potassium perfluorooctanoate.

3. The method for preparing the acid- and alkali-resistant polyvinylidene fluoride filter membrane according to claim 1, wherein: In step S1, the steric hindering agent is any one of 1,2-difluoroethane and 1,1-difluoroethane.

4. The method for preparing the acid- and alkali-resistant polyvinylidene fluoride filter membrane according to claim 1, wherein: In step S1, the chain transfer agent is any one of dodecanethiol and 2-mercaptoethanol, the micro-irradiation agent is americium-241 oxide, and the initiator is any one of tert-amyl peroxyneodecanoate and tert-butyl peroxyneodecanoate.

5. The method for preparing the acid- and alkali-resistant polyvinylidene fluoride filter membrane according to claim 1, wherein: In step S1, the reaction temperature is 60-90° C., and the polymerization time is 10-14 hours.

6. The method for preparing the acid- and alkali-resistant polyvinylidene fluoride filter membrane according to claim 1, wherein: In step S2, the acid- and alkali-resistant polyurethane is any one of cast polyurethane and silicone-modified polyurethane.

7. The method for preparing the acid- and alkali-resistant polyvinylidene fluoride filter membrane according to claim 6, wherein: The cast polyurethane is a polyurethane using polytetramethylene ether polyol as a soft segment, diisocyanate as a hard segment, and hydroquinone-bis(hydroxyethyl) ether as a chain extender; the silicone-modified polyurethane is a polyurethane using hydroxy silicone oil as a soft segment, isocyanate as a hard segment, and amino silicone oil as a chain extender.

8. The method for preparing the acid- and alkali-resistant polyvinylidene fluoride filter membrane according to claim 1, wherein: In step S2, the acidic inorganic salt is any one of sodium bisulfite and potassium bisulfite.

9. The method for preparing the acid- and alkali-resistant polyvinylidene fluoride filter membrane according to claim 1, wherein: In step S2, the mass ratio of DMAC, PVDF, polyurethane, acidic inorganic salt, and PVP is 100:(18-20):(3-5):(0.5-1):(2-3).

10. An acid and alkali resistant polyvinylidene fluoride filter membrane, characterized in that: Prepared by the preparation method according to any one of claims 1 to 9.

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