Preparation method of polyvinylidene fluoride porous separation membrane, polyvinylidene fluoride porous separation membrane and filtering device

Abstract

The invention relates to a preparation method of a polyvinylidene fluoride porous separation membrane, which comprises the following steps: casting a film coating solution to obtain an initial film, exposing the initial film in a controlled air environment, and then immersing into a forming bath to obtain the polyvinylidene fluoride porous separation membrane. The film coating liquid and the forming bath both adopt non-flammable and explosive chemicals, so that the safety of the film preparation process is ensured. The exposure time of the initial film in the air is short, the system design is simpler, the cost is saved, the system efficiency is improved, meanwhile, the polymer, the pore-foaming agent and the like in the film coating liquid play a role in the film performance, and a film structure meeting the requirement is formed. The performance of flux, bubble point value and the like of the prepared polyvinylidene fluoride porous separation membrane can meet the requirements of different application scenes. The filtering device of the polyvinylidene fluoride porous separation membrane has a relatively high practical value.

Description

Technical Field

[0001] This invention belongs to the field of biopharmaceuticals, specifically relating to a method for preparing a polyvinylidene fluoride porous separation membrane, the polyvinylidene fluoride porous separation membrane, and a filtration device. Background Technology

[0002] Polyvinylidene fluoride (PVDF) membranes possess high mechanical strength, resistance to chemical solvents, heat resistance, and high stability, making them widely used in the biopharmaceutical field. A crucial step in PVDF membrane preparation is immersing the initial film in a molding bath to obtain a PVDF membrane that meets performance requirements. Highly flammable and explosive solvents are frequently used in the molding bath. For safety reasons, strict safety control of these solvents is necessary. Furthermore, fluid handling and recovery present challenges. Therefore, additional safety measures are required during membrane preparation and manufacturing to ensure production safety.

[0003] Millipore's patent US4203848 invented a standard polyvinylidene fluoride (PVDF) microporous membrane for biopharmaceutical applications. The molding bath uses a mixture of acetone and water; since acetone is a Class II flammable liquid, stringent explosion-proof measures are required to reduce manufacturing risks, thus increasing system complexity. Costar's patent WO9322034A1, and Cobot's patents CN116474570A, CN112495197A, and CN116550168A are all microporous membranes that follow similar membrane formation processes, using flammable solvents in the molding bath. Therefore, membrane formation also requires strict explosion-proof engineering controls.

[0004] Fuji's patent US4933081, Pall's patent US6146747, and GVS's patent US10981121B2 employ gas-phase induction for separation before immersion in a water coagulation bath, but limitations remain. This is because the method requires a relatively long gas-phase residence time to achieve the target pore size and flow performance of the membrane. Longer gas-phase residence times necessitate either increasing equipment length or reducing membrane fabrication speed. For example, Pall's patent US6146747 uses a longer casing within the casting line to extend the gas-phase residence time.

[0005] In Fuji patent (US4933081), the gas phase residence time determines the membrane pore size; increasing the residence time increases the pore size. In this patent, the gas phase residence time is as long as 30 seconds to achieve a microfiltration-sized pore size. Pall patent (US6146747) uses gas phase residence times ranging from 8 to 120 seconds. With increasing residence time or humidity conditions, the pore size increases. In the provided example, a longer residence time in humid air is needed to meet the water permeability requirements; in examples with shorter residence times (<8 seconds), the membrane's water permeability is negatively affected. GVS patent (US10981121B2) specifies gas phase residence times between 1 and 10 minutes to meet the membrane's water permeability requirements.

[0006] In summary, existing technologies use non-solvent or non-solvent / weak solvent mixtures in the forming bath to facilitate membrane formation. Non-solvent / weak solvent forming baths require prolonged exposure to air to meet membrane pore size and permeability requirements. Because different polymer membranes have different process requirements, producing different membrane products generally requires different equipment, increasing equipment investment. Sometimes, to maximize the utilization of existing production lines, additional investment is needed to configure different components.

[0007] In view of this, there is a need to provide a method for preparing a porous polyvinylidene fluoride (PVDF) separation membrane that can ensure both the safety of the preparation process and the efficiency of the system, and that the prepared PVDF porous separation membrane meets the required performance. Summary of the Invention

[0008] To overcome the deficiencies of the prior art, one objective of this invention is to provide a method for preparing a polyvinylidene fluoride (PVDF) porous separation membrane; another objective is to provide a PVDF porous separation membrane; and yet another objective is to provide a filtration device incorporating a PVDF porous separation membrane. To achieve the above objectives, the technical solutions adopted by this invention are as follows: One aspect of the present invention provides a method for preparing a porous polyvinylidene fluoride separation membrane, comprising the following steps: Preparation of coating solution; The coating liquid is cast into an initial thin film; The initial film was exposed to a controlled air environment for a time of less than or equal to 5 seconds; The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath is composed of a first non-solvent or a mixture of the first non-solvent and a first weak solvent. Both the first non-solvent and the first weak solvent are non-flammable and non-explosive chemicals.

[0009] Optionally, the coating liquid includes a first coating liquid and a second coating liquid.

[0010] Optionally, the coating liquid is composed of polyvinylidene fluoride, a first solvent, and a second non-solvent, wherein the first solvent and the second non-solvent are both non-flammable and non-explosive chemicals.

[0011] Optionally, the first solvent is a good solvent for polyvinylidene fluoride.

[0012] Optionally, the second non-solvent includes a porogen.

[0013] Optionally, the second non-solvent may further include a third non-solvent.

[0014] Optionally, the coating liquid comprises: Polyvinylidene fluoride: 9-14 wt% Pore-forming agent: 15-30 wt%; Third non-solvent: 10wt%.

[0015] Optionally, when casting the coating liquid into the initial film, the temperature of the coating liquid is 40°C-45°C.

[0016] Optionally, the temperature of the controlled air environment is 27-35°C and the relative humidity is 57%-98%.

[0017] Optionally, the molding bath contains 25%-100wt% of the first non-solvent.

[0018] Optionally, the first non-solvent is water, and the first weak solvent is 2-methyl-2,4-pentanediol.

[0019] Optionally, the temperature of the molding bath is 40-60°C.

[0020] Another aspect of the present invention provides a polyvinylidene fluoride porous separation membrane, which is obtained by the polyvinylidene fluoride porous separation membrane preparation method described above.

[0021] Optionally, the pore size of the polyvinylidene fluoride porous separation membrane is 0.1-5 micrometers.

[0022] Optionally, the porosity of the polyvinylidene fluoride porous separation membrane is 80%-87%.

[0023] Optionally, the bubble point value of the polyvinylidene fluoride porous separation membrane is 1-29 psi.

[0024] Optionally, when the pore size of the polyvinylidene fluoride porous separation membrane is 0.45-0.65 micrometers, the porosity of the polyvinylidene fluoride porous separation membrane is 84%-87%.

[0025] Optionally, when the pore size of the polyvinylidene fluoride porous separation membrane is 0.2 micrometers, the porosity of the polyvinylidene fluoride porous separation membrane is 82%-87%.

[0026] Optionally, when the pore size of the polyvinylidene fluoride porous separation membrane is 0.1 micrometers, the porosity of the polyvinylidene fluoride porous separation membrane is 80%-82%.

[0027] Optionally, the polyvinylidene fluoride porous separation membrane is used in a filtration device.

[0028] Another aspect of the present invention provides a filtration device including a housing having a fluid inlet and a fluid outlet, wherein a polyvinylidene fluoride porous separation membrane as described above is disposed within the housing.

[0029] The method for preparing the polyvinylidene fluoride (PVDF) porous separation membrane of the present invention involves casting an initial film using a coating solution, exposing the initial film to a controlled air environment, and then immersing it in a molding bath to obtain the PVDF porous separation membrane. Both the coating solution and the molding bath of the present invention use non-flammable and non-explosive chemicals, ensuring the safety of the membrane preparation process. The initial film of the present invention is exposed to air for a shorter time, resulting in a simpler system design, cost savings, and improved system efficiency. Simultaneously, the polymers and pore-forming agents in the coating solution of the present invention play a role in membrane performance, forming a membrane structure that meets the requirements. The flux, bubble point, and other properties of the PVDF porous separation membrane prepared by the present invention meet the needs of different application scenarios. The filtration device based on the PVDF porous separation membrane of the present invention has high practical value. Attached Figure Description

[0030] After reading the detailed embodiments of the present invention with reference to the accompanying drawings, the reader will gain a clearer understanding of various aspects of the present invention. Figure 1 SEM image of the cross-section of the polyvinylidene fluoride porous separation membrane prepared in Example 1; Figure 2 SEM image of the first surface of the polyvinylidene fluoride porous separation membrane prepared in Example 1; Figure 3 SEM image of the second surface of the polyvinylidene fluoride porous separation membrane obtained in Example 1; Figure 4 SEM image of the cross-section of the polyvinylidene fluoride porous separation membrane prepared in Example 5; Figure 5 SEM image of the first surface of the polyvinylidene fluoride porous separation membrane obtained in Example 5; Figure 6 SEM image of the second surface of the polyvinylidene fluoride porous separation membrane prepared in Example 5; Figure 7 SEM image of the cross-section of the polyvinylidene fluoride porous separation membrane prepared in Example 8; Figure 8 SEM image of the first surface of the polyvinylidene fluoride porous separation membrane prepared in Example 8; Figure 9 SEM image of the second surface of the polyvinylidene fluoride porous separation membrane prepared in Example 8. Detailed Implementation

[0031] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments, further clarifies the invention. Those skilled in the art should understand that the specific descriptions below are illustrative rather than restrictive, and should not be construed as limiting the scope of protection of the present invention.

[0032] The polyvinylidene fluoride porous separation membrane of this invention is prepared by a combination of VIPS (vacuum-phase separation) and NIPS (non-solvent-induced phase separation) processes. Specifically, VIPS involves exposing the coating liquid to a controlled air environment to induce phase separation, providing a porous structure on the upstream surface of the polyvinylidene fluoride porous separation membrane. In the controlled air environment, the interaction between moisture in the air and the coating liquid provides a porous structure on the air-side surface. NIPS involves forming the cross-section and downstream porous structure of the polyvinylidene fluoride porous separation membrane through a molding bath via the interaction of polyvinylidene fluoride, solvent, and non-solvent.

[0033] The method for preparing the polyvinylidene fluoride porous separation membrane is as follows: S1: Preparation of coating solution; The coating solution comprises polyvinylidene fluoride (PVDF), a first solvent, and a second non-solvent. Both the first solvent and the second non-solvent are non-flammable and non-explosive chemicals. The PVDF can be resins such as Kynar 761, Kynar 760, Kynar 361, and Kynar 360. The first solvent is a good solvent for PVDF, such as DMAC (dimethylacetamide), NMP (methylpyrrolidone), and DMSO (dimethyl sulfoxide). The second non-solvent includes a porogen or a mixture of a porogen and a third non-solvent. The porogen can be PVP (polyvinylpyrrolidone), and the third non-solvent can be TEG (triethylene glycol), EG (ethylene glycol), water, or a mixture thereof. Specifically, the coating solution includes: Polyvinylidene fluoride: 9-14 wt% Pore-forming agent: 15-30 wt%; wherein, the pore-forming agent is PVP; The third non-solvent: 10 wt%, of which TEG is 9 wt% and water is 1 wt%.

[0034] The coating liquid may be a polymer mixture composed of polyvinylidene fluoride, a first solvent, and a second non-solvent in a fixed ratio, or it may include a first coating liquid and a second coating liquid, wherein the first coating liquid and the second coating liquid are composed of polyvinylidene fluoride, a first solvent, and a second non-solvent in different ratios.

[0035] S2: The coating solution is cast into an initial thin film. Specifically, the coating solution is uniformly scraped into a polymer film of a specific thickness on a glass plate using a coating knife. Specifically, the temperature of the coating solution is 40°C - 45°C when casting the initial thin film. Although the formulation of the coating solution and the concentration of polyvinylidene fluoride (PVDF) play an important role in determining the pore size of the PVDF porous separation membrane, the casting process conditions are also crucial. Specifically, the temperature of the coating solution during casting affects the viscosity, thereby altering the diffusion rate of vapor and non-solvent phase separation. Generally, the higher the temperature of the coating solution, the larger the pore size and the higher the porosity of the membrane.

[0036] When the coating liquid includes a first coating liquid and a second coating liquid, the first coating liquid and the second coating liquid are co-cast during casting, forming two separate layers within the thickness of the film.

[0037] S3: Expose the initial film to a controlled air environment for at least 5 seconds. This step is a VIPS (Vacuum-to-Package Separation) process. The controlled air environment has a temperature of 27-35°C and a relative humidity of 57%-98%. The temperature and humidity during air exposure determine the exchange rate between moisture, polymer, and solvent in the air. Generally, higher temperature and humidity result in larger pore sizes and higher porosity on the film surface.

[0038] S4: Immerse the exposed initial film in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. This step is a NIPS (non-solvent phase separation) process. The molding bath consists of a first non-solvent or a mixture of a first non-solvent and a first weak solvent, both of which are non-flammable and non-explosive chemicals.

[0039] The molding bath contains 25%-100 wt% of the first non-solvent. The first non-solvent is water, and the first weak solvent is 2-methyl-2,4-pentanediol. The temperature of the molding bath is 40-60°C. Specifically, the temperature of the molding bath affects the diffusion rate between the polymer, solvent, and non-solvent; generally, a higher molding bath temperature results in a more open structure for the polyvinylidene fluoride porous separation membrane. The formulation of the molding bath, especially the resulting surface tension, also determines the interdiffusion rate between the polymer and the non-solvent; lower surface tension results in a larger pore size and higher porosity in the membrane.

[0040] The weight ratio of polyvinylidene fluoride (PVDF) in the coating solution determines, to a certain extent, the pore size of the PVDF porous separation membrane; the humidity and air temperature during air exposure determine the surface exchange rate of the initial film. The temperature of the coating solution also determines the viscosity of the solution, which in turn determines the exchange rate during air exposure and in the molding bath. During the immersion of the initial film in the molding bath, the temperature of the molding bath affects the diffusion rate between the solvent and non-solvent, as well as the composition of the molding bath; in particular, the resulting surface tension also determines the interdiffusion rate between the polymer and non-solvent.

[0041] Kynar 761 is a specific grade of PVDF because the polymer's molecular weight can play a role in membrane performance; DMAC is the primary solvent for dissolving PVDF in solution and also interacts with water in VIPS and NIPS; PVP is a porogen that washes away the solution during phase separation, thereby increasing the porosity and pore size of the PVDF porous separation membrane. TEG is a weak solvent that, due to its high hygroscopicity, contributes to increased interdiffusion in both VIPS and NIPS processes. Water slightly alters the stability of the solution, thus increasing the phase transition rate in both VIPS and NIPS processes.

[0042] The performance of the polyvinylidene fluoride porous separation membrane prepared by the aforementioned method was tested, and the test method is as follows: The polyvinylidene fluoride porous separation membrane was cut into 47 mm discs (effective diameter 39 mm). IPA (isopropanol) was used as a wetting agent, and bubble point pressure tests were performed using a pore size analyzer (Innova CFP-200A) according to ASTM F316 standard. Subsequently, the discs were re-immersed in isopropanol and subjected to flux testing by exchanging water.

[0043] The structure of the polyvinylidene fluoride porous separation membrane was tested using the following method: 1. Aperture The polyvinylidene fluoride porous separation membrane was cut into 47 mm diameter discs. IPA was used as a wetting agent, and bubble point pressure tests were performed using a pore size analyzer (Innova CFP-200A) according to ASTM F316 standard. The bubble point pressure test measures the pore size of the polyvinylidene fluoride porous separation membrane; the bubble point pressure value is inversely proportional to the pore size, with smaller pore sizes resulting in higher bubble point pressure values.

[0044] 2. Porosity Porosity is derived from the dry weight of a membrane sample with known area and thickness, and the relevant parameters of the base polymer. The specific calculation method is as follows:

[0045] in: ρ 膜 : Measuring the density of the membrane; ρ 聚合物 : The density of the polymer;

[0046] in: m: Membrane mass; A: The area of ​​the membrane; t: The thickness of the membrane.

[0047] Compare with Example 1: This comparative example is a PVDF membrane from Pall Corporation, and the performance data of the PVDF membrane is derived from patent US6146747, as shown in Table 1.

[0048] Table 1

[0049] Compare with Example 2: This comparative example is a PVDF membrane from Millipore, and the performance data of the PVDF membrane is derived from patent US6146747, as shown in Table 2.

[0050] Table 2

[0051] Compare with Example 3: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 9wt% DMAC: 61% wt% PVP: 20wt% TEG: 9wt% Water: 1 wt%.

[0052] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0053] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0054] The initial film was exposed to a controlled air environment for 10 seconds; the temperature of the controlled air environment was 32.3°C and the relative humidity was 79%.

[0055] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath is water at a temperature of 50°C.

[0056] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was immersed in isopropanol (IPA) and bubble point pressure was tested using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water.

[0057] The membrane sample was designated as Control Example 3.1. The membrane sample was tested and found to have a pore size of 0.65 micrometers, a bubble point of 7 psi, and a flux of 1022 LMH / psi.

[0058] Compare with Example 4: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 14wt% DMAC: 61% wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0059] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0060] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0061] The initial film was exposed to a controlled air environment for 30 seconds; the temperature of the controlled air environment was 30.7°C and the relative humidity was 78%.

[0062] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath consists of 30% water and 70% 2-methyl-2,4-pentanediol at a temperature of 55°C.

[0063] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0064] The membrane sample was designated as Control Example 4.1. The membrane sample was tested and found to have a pore size of 0.2 micrometers, a bubble point of 22 psi, and a flux of 344 LMH / psi.

[0065] Compare with Example 5: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 14wt% DMAC: 61% wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0066] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0067] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0068] The initial film was exposed to a controlled air environment for 60 seconds; the controlled air environment had a temperature of 27°C and a relative humidity of 75%.

[0069] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane, wherein the molding bath is water at a temperature of 55°C.

[0070] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0071] The membrane sample was designated as Control Example 5.1. The membrane sample was tested and found to have a pore size of 0.45 micrometers, a bubble point of 14 psi, and a flux of 541 LMH / psi.

[0072] Compare with Example 6: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 17wt% DMAC: 58wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0073] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0074] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0075] The initial film was exposed to a controlled air environment for 5 seconds; the temperature of the controlled air environment was 30.9°C and the relative humidity was 77%.

[0076] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath consists of 70% water and 30% 2-methyl-2,4-pentanediol at a temperature of 55°C.

[0077] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0078] The membrane sample was designated as Control Example 6.1. After testing, the membrane sample had a pore size of <0.1 micrometers, a bubble point of 48 psi, and a flux of 10 LMH / psi.

[0079] Example 1: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 9-9.5 wt% DMAC: 50-50.5 wt% PVP: 30wt% TEG: 9 wt% Water: 1 wt%.

[0080] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0081] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0082] The initial film was exposed to a controlled air environment for 5 seconds; the temperature of the controlled air environment was 31.8°C and the relative humidity was 62%.

[0083] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath is water at a temperature of 50°C.

[0084] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0085] The cross-section, first surface, and second surface of the membrane sample were imaged using a scanning electron microscope (SEM). Figure 1 The image is a cross-sectional view of the membrane sample obtained by scanning electron microscopy (SEM). Figure 2 This is a SEM image of the first surface of the membrane sample; Figure 3 This is a SEM image of the second surface of the membrane sample.

[0086] The membrane sample was designated as Example 1.1. After testing, the membrane sample had a pore size of 0.65 micrometers, a porosity of 86%, a bubble point of 7 psi, and a flux of 13801 LMH / psi.

[0087] Example 2: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 9-9.5 wt% DMAC: 50-50.5 wt% PVP: 30wt% TEG: 9 wt% Water: 1 wt%.

[0088] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0089] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0090] The initial film was exposed to a controlled air environment for 5 seconds; the temperature of the controlled air environment was 27°C and the relative humidity was 57%.

[0091] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath is water at a temperature of 50°C.

[0092] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0093] The membrane sample was designated as Example 2.1. After testing, the membrane sample had a pore size of 0.45 micrometers, a porosity of 87%, a bubble point of 9 psi, and a flux of 7368 LMH / psi.

[0094] Example 3: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 9-9.5 wt% DMAC: 50-50.5 wt% PVP: 30wt% TEG: 9 wt% Water: 1 wt%.

[0095] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0096] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0097] The initial film was exposed to a controlled air environment for 5 seconds; the temperature of the controlled air environment was 30.1°C and the relative humidity was 67%.

[0098] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath is water at a temperature of 50°C.

[0099] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0100] The membrane sample was designated as Example 3.1. After testing, the membrane sample had a pore size of 0.45 micrometers, a porosity of 87%, a bubble point of 9 psi, and a flux of 6679 LMH / psi.

[0101] Example 4: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 14wt% DMAC: 61% wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0102] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0103] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0104] The initial film was exposed to a controlled air environment for 5 seconds; the temperature of the controlled air environment was 34°C and the relative humidity was 79%.

[0105] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath consists of 50% water and 50% 2-methyl-2,4-pentanediol at a temperature of 55°C.

[0106] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was then immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0107] The membrane sample was designated as Example 4.1. After testing, the membrane sample had a pore size of 0.1 micrometers, a porosity of 80%, a bubble point of 26 psi, and a flux of 296 LMH / psi.

[0108] Example 5: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 14wt% DMAC: 61% wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0109] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0110] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0111] The initial film was exposed to a controlled air environment for 5 seconds; the controlled air environment had a temperature of 34°C and a relative humidity of 83%.

[0112] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath consists of 50% water and 50% 2-methyl-2,4-pentanediol at a temperature of 55°C.

[0113] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was then immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0114] The cross-section, first surface, and second surface of the membrane sample were imaged using a scanning electron microscope (SEM). Figure 4 This is a SEM image of the cross-section of the membrane sample; Figure 5 This is an SEM image of the first surface of the membrane sample. Figure 6 This is a SEM image of the second surface of the membrane sample.

[0115] The membrane sample was designated as Example 5.1. After testing, the membrane sample had a pore size of 0.1 micrometers, a porosity of 81%, a bubble point of 27 psi, and a flux of 256 LMH / psi.

[0116] Example 6: A coating solution is prepared, comprising Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water, in the following proportions: Kynar 761 resin: 14wt% DMAC: 61% wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0117] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the coating solution.

[0118] The coating liquid is cast into an initial thin film; specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0119] The initial film was exposed to a controlled air environment for 5 seconds; the controlled air environment had a temperature of 35°C and a relative humidity of 85%.

[0120] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath consists of 50% water and 50% 2-methyl-2,4-pentanediol at a temperature of 55°C.

[0121] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was then immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0122] The membrane sample was designated as Example 6.1. After testing, the membrane sample had a pore size of 0.1 micrometers, a porosity of 81%, a bubble point of 29 psi, and a flux of 213 LMH / psi.

[0123] Example 7: A coating solution is prepared, comprising a first coating solution and a second coating solution. Both the first and second coating solutions are composed of Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water. The first coating solution has the following proportions: Kynar 761 resin: 9wt% DMAC: 61% wt% PVP: 20wt% TEG: 9wt% Water: 1 wt%.

[0124] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the first coating liquid.

[0125] The mixing ratio of the second coating solution is: Kynar 761 resin: 14wt% DMAC: 61% wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0126] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the second coating liquid.

[0127] The first coating solution and the second coating solution are mixed at 70°C until homogeneous to obtain the coating solution.

[0128] The coating liquid is cast into an initial thin film, specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0129] The initial film was exposed to a controlled air environment for 5 seconds; the temperature of the controlled air environment was 26.9°C and the relative humidity was 98%.

[0130] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath consists of 25% water and 75% 2-methyl-2,4-pentanediol at a temperature of 55°C.

[0131] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was then immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0132] The membrane sample was designated as Example 7.1. After testing, the membrane sample had a pore size of 0.2 micrometers, a porosity of 84%, a bubble point of 17 psi, and a flux of 765 LMH / psi.

[0133] Example 8: A coating solution is prepared, comprising a first coating solution and a second coating solution. Both the first and second coating solutions are composed of Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water. The first coating solution has the following proportions: Kynar 761 resin: 9wt% DMAC: 61% wt% PVP: 20wt% TEG: 9wt% Water: 1 wt%.

[0134] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the first coating liquid.

[0135] The mixing ratio of the second coating solution is: Kynar 761 resin: 14wt% DMAC: 61% wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0136] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the second coating liquid.

[0137] The first coating solution and the second coating solution are mixed at 70°C until homogeneous to obtain the coating solution.

[0138] The coating liquid is cast into an initial thin film, specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0139] The initial film was exposed to a controlled air environment for 5 seconds; the temperature of the controlled air environment was 34.9°C and the relative humidity was 82%.

[0140] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath consists of 25% water and 75% 2-methyl-2,4-pentanediol at a temperature of 55°C.

[0141] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was then immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0142] The cross-section, first surface, and second surface of the membrane sample were imaged using a scanning electron microscope (SEM). Figure 7 This is a SEM image of the cross-section of the membrane sample. Figure 8 This is an SEM image of the first surface of the membrane sample. Figure 9 This is a SEM image of the second surface of the membrane sample.

[0143] The membrane sample was designated as Example 8.1. After testing, the membrane sample had a pore size of 0.2 micrometers, a porosity of 86%, a bubble point of 18 psi, and a flux of 514 LMH / psi.

[0144] Example 9: A coating solution is prepared, comprising a first coating solution and a second coating solution. Both the first and second coating solutions are composed of Kynar 761 resin, DMAC (dimethylacetamide), PVP (polyvinylpyrrolidone), TEG (triethylene glycol), and water. The first coating solution has the following proportions: Kynar 761 resin: 9wt% DMAC: 61% wt% PVP: 20wt% TEG: 9wt% Water: 1 wt%.

[0145] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the first coating liquid.

[0146] The mixing ratio of the second coating solution is: Kynar 761 resin: 14wt% DMAC: 61% wt% PVP: 15wt%; TEG: 9wt% Water: 1 wt%.

[0147] The Kynar 761 resin, DMAC, PVP, TEG and water are mixed at 70°C until homogeneous to obtain the second coating liquid.

[0148] The first coating solution and the second coating solution are mixed at 70°C until homogeneous to obtain the coating solution.

[0149] The coating liquid is cast into an initial thin film, specifically, the temperature of the coating liquid during casting is 40°C - 45°C.

[0150] The initial film was exposed to a controlled air environment for 5 seconds; the temperature of the controlled air environment was 32.8°C and the relative humidity was 87%.

[0151] The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath consists of 25% water and 75% 2-methyl-2,4-pentanediol at a temperature of 55°C.

[0152] A membrane sample with a diameter of 47 mm (effective diameter 39 mm) was cut. The membrane sample was then immersed in isopropanol (IPA) and a bubble point pressure test was performed using a bubble point apparatus. Subsequently, the membrane sample was re-immersed in isopropanol and subjected to flux exchange with water for a flux test.

[0153] The membrane sample was designated as Example 9.1. After testing, the membrane sample had a pore size of 0.2 micrometers, a porosity of 84%, a bubble point of 20 psi, and a flux of 395 LMH / psi.

[0154] Example 10: This embodiment compares Example 1 with Comparative Examples 1.1 and 2.1, Examples 2 and 3 with Comparative Examples 1.2 and 2.2, and Examples 4-6 with Comparative Example 1.3, as shown in Table 3: Table 3

[0155] According to Table 3, when the pore size is the same, the flux of the large-pore (0.65 μm, 0.45 μm) polyvinylidene fluoride porous separation membrane of the present invention is significantly better than that of the control example, and the flux of the small-pore (0.1 μm) polyvinylidene fluoride porous separation membrane of the present invention can also reach the same level as that of the control example.

[0156] Example 11 This embodiment compares Example 1 with Comparative Example 3.1, and Examples 2.1 and 3.1 with Comparative Example 5.1, as shown in Table 4.

[0157] Table 4

[0158] As shown in Table 4, compared with Example 1.1, the weight ratio of PVP in Comparative Example 3.1 decreased. To achieve the same pore size, the exposure time needed to be increased, which increased the system complexity and reduced system efficiency. Furthermore, the flux of Comparative Example 3.1 was significantly different from that of Example 1.1, and its performance was far inferior to that of Example 1.1. Similarly, compared with Examples 2.1 and 3.1, the weight ratio of PVP in Comparative Example 5.1 decreased. To achieve the same pore size, the exposure time needed to be increased, which increased the system complexity and reduced system efficiency. Furthermore, the flux of Comparative Example 5.1 was significantly different from that of Examples 2.1 and 3.1, and its performance was far inferior to that of Examples 2.1 and 3.1.

[0159] PVP is a pore-forming agent that is washed away during phase separation. Specifically, due to its hydrophilicity, PVP enhances the interdiffusion of polymer-nonsolvent-solvent, thereby increasing the porosity and pore size of the polyvinylidene fluoride porous separation membrane. Setting an appropriate PVP weight ratio is crucial to achieving the target pore size. Although the air exposure time in the embodiments of this invention is short, ideal performance can still be achieved by adjusting the PVP weight ratio, i.e., adjusting the ratio of the coating solution. It should be noted that the ratio of the coating solution needs to be set in conjunction with the target pore size. Generally, a higher PVP weight ratio results in a larger target pore size. For example, when the target pore size is 0.65 or 0.45 micrometers, the PVP weight ratio can be set to 30%; when the target pore size is 0.2 micrometers, the PVP weight ratio can be set to 20%; and when the target pore size is 0.1 micrometers, the PVP weight ratio can be set to 15%.

[0160] The weight ratio of polyvinylidene fluoride is also an important factor. As shown in Example 5.1, the weight ratio of Kynar 761 is relatively high, so a longer air exposure time is required to achieve the target pore size. However, the membrane flux is significantly different from that in Examples 2.1 and 3.1, indicating that setting an appropriate weight ratio of Kynar 761 is also very important.

[0161] Example 12 This embodiment compares Examples 4.1, 5.1, and 6.1 with Control Example 6.1, as shown in Table 5.

[0162] Table 5

[0163] As can be seen from Table 5, compared with Examples 4.1, 5.1 and 6.1, the temperature and humidity of Comparative Example 6.1 decreased. Therefore, Comparative Example 6.1 could not achieve the target pore size. Moreover, the flux of Comparative Example 6.1 was significantly different from that of Examples 4.1, 5.1 and 6.1, and its performance was far from meeting the requirements.

[0164] Therefore, while the formulation of the coating solution and the air exposure time play important roles in determining the pore size of the membrane, the casting process conditions are also crucial. The humidity and temperature during air exposure determine the exchange rate between moisture, polymer, and solvent in the air; generally, higher humidity and temperature result in larger pores.

[0165] Furthermore, compared with Examples 4.1, 5.1, and 6.1, Comparative Example 6.1 has a higher concentration of Kynar 761, resulting in smaller pore size and lower flux, which fails to meet the requirements.

[0166] Example 13 This embodiment compares Example 8.1 with Comparative Example 4.1, as shown in Table 6.

[0167] Table 6

[0168] As shown in Table 6, compared with Example 8.1, Comparative Example 4.1 exhibits lower temperature and humidity, and a lower PVP weight ratio. To achieve the same pore size, an increased exposure time is required, which increases system complexity, cost, and reduces system efficiency. Furthermore, the flux of Comparative Example 4.1 differs significantly from that of Example 8.1. Although Example 8.1 uses a shorter exposure time, it achieves higher membrane performance than Comparative Example 4.1.

[0169] As mentioned above, PVP is a pore-forming agent. The humidity and temperature during air exposure determine the exchange rate between moisture, polymer and solvent in the air. To achieve the target pore size and performance, it is crucial to set the appropriate PVP weight ratio, temperature and humidity. Although the exposure time of the embodiments of the present invention is short, the ideal performance can still be achieved by adjusting the ratio of the coating liquid and adjusting the temperature and humidity of the air environment.

[0170] Example 14 This embodiment compares Examples 1.1 and 4.1 with Control Example 6.1, as shown in Table 7.

[0171] Table 7

[0172] Kynar 761 is a specific grade of PVDF, and the molecular weight of its polymer plays a role in membrane performance and strength. However, increasing the weight ratio of Kynar 761 leads to a decrease in pore size, making the weight ratio of Kynar 761 crucial. As shown in Table 7, in Comparative Example 6.1, increasing the weight ratio of Kynar 761 resulted in a decrease in pore size and a sharp drop in flux.

[0173] Example 15 This embodiment provides the use of the polyvinylidene fluoride porous separation membrane described in any of Examples 1 to 9. In this embodiment, the polyvinylidene fluoride porous separation membrane is used in a filtration device. The filtration device includes a stacked filter, a cartridge filter, a capsule filter, and a spiral wound filter.

[0174] The polyvinylidene fluoride porous separation membrane can be a flat sheet membrane or a hollow fiber membrane.

[0175] Example 16 This embodiment provides a filtration device. The filtration device includes a housing with a fluid inlet and a fluid outlet, and a polyvinylidene fluoride porous separation membrane as described in any of Embodiments 1 to 9 is disposed inside the housing.

[0176] In this embodiment, the filtration device includes stacked filters, cartridge filters, capsule filters, spiral wound filters, etc.

[0177] The polyvinylidene fluoride porous separation membrane can be a flat sheet membrane or a hollow fiber membrane.

[0178] The method for preparing the polyvinylidene fluoride (PVDF) porous separation membrane of the present invention involves casting an initial film using a coating solution, exposing the initial film to a controlled air environment, and then immersing it in a molding bath to obtain the PVDF porous separation membrane. Both the coating solution and the molding bath of the present invention use non-flammable and non-explosive chemicals, and are non-solvents or mixtures of non-solvents and weak solvents, ensuring the safety of the membrane preparation process. The initial film of the present invention is exposed to air for a shorter time, resulting in a simpler system design, cost savings, and improved system efficiency. Simultaneously, the polymers and pore-forming agents in the coating solution of the present invention play a role in membrane performance, forming a membrane structure that meets the requirements. The flux, bubble point, and other properties of the PVDF porous separation membrane prepared by the present invention meet the needs of different application scenarios. The filtration device based on the PVDF porous separation membrane of the present invention has high practical value.

[0179] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. The embodiments exemplified by the present invention cannot exhaustively describe all implementation methods. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention. All documents mentioned in this invention are incorporated herein by reference as if a single document were independently incorporated by reference.

Claims

1. A method for preparing a porous polyvinylidene fluoride (PVDF) separation membrane, characterized in that, Includes the following steps: Preparation of coating solution; The coating liquid is cast into an initial thin film; The initial film was exposed to a controlled air environment for a time of less than or equal to 5 seconds; The exposed initial film is immersed in a molding bath to obtain the polyvinylidene fluoride porous separation membrane. The molding bath is composed of a first non-solvent or a mixture of the first non-solvent and a first weak solvent. Both the first non-solvent and the first weak solvent are non-flammable and non-explosive chemicals.

2. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 1, characterized in that, The coating liquid includes a first coating liquid and a second coating liquid.

3. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 1, characterized in that, The coating liquid is composed of polyvinylidene fluoride, a first solvent, and a second non-solvent, both of which are non-flammable and non-explosive chemicals.

4. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 3, characterized in that, The first solvent is a good solvent for polyvinylidene fluoride.

5. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 4, characterized in that, The second non-solvent includes a porogen.

6. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 5, characterized in that, The second non-solvent also includes a third non-solvent.

7. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 6, characterized in that, The coating liquid comprises: Polyvinylidene fluoride: 9-14 wt% Pore-forming agent: 15-30 wt% Third non-solvent: 10 wt%.

8. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 1, characterized in that, When the coating liquid is cast into the initial film, the temperature of the coating liquid is 40°C-45°C.

9. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 1, characterized in that, The controlled air environment has a temperature of 27-35°C and a relative humidity of 57%-98%.

10. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 1, characterized in that, The molding bath contains 25%-100 wt% of the first non-solvent.

11. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 1, characterized in that, The first non-solvent is water, and the first weak solvent is 2-methyl-2,4-pentanediol.

12. The method for preparing the polyvinylidene fluoride porous separation membrane according to claim 1, characterized in that, The temperature of the molding bath is 40-60℃.

13. A porous separation membrane made of polyvinylidene fluoride, characterized in that, The polyvinylidene fluoride porous separation membrane is obtained by the preparation method of the polyvinylidene fluoride porous separation membrane according to any one of claims 1 to 12.

14. The polyvinylidene fluoride porous separation membrane according to claim 13, characterized in that, The pore size of the polyvinylidene fluoride porous separation membrane is 0.1-5 micrometers.

15. The polyvinylidene fluoride porous separation membrane according to claim 13, characterized in that, The porosity of the polyvinylidene fluoride porous separation membrane is 80%-87%.

16. The polyvinylidene fluoride porous separation membrane according to claim 13, characterized in that, The bubble point value of the polyvinylidene fluoride porous separation membrane is 1-29 psi.

17. The polyvinylidene fluoride porous separation membrane according to claim 13, characterized in that, When the pore size of the polyvinylidene fluoride porous separation membrane is 0.45-0.65 micrometers, the porosity of the polyvinylidene fluoride porous separation membrane is 84%-87%.

18. The polyvinylidene fluoride porous separation membrane according to claim 13, characterized in that, When the pore size of the polyvinylidene fluoride porous separation membrane is 0.2 micrometers, the porosity of the polyvinylidene fluoride porous separation membrane is 82%-87%.

19. The polyvinylidene fluoride porous separation membrane according to claim 13, characterized in that, When the pore size of the polyvinylidene fluoride porous separation membrane is 0.1 micrometers, the porosity of the polyvinylidene fluoride porous separation membrane is 80%-82%.

20. The polyvinylidene fluoride porous separation membrane according to claim 13, characterized in that, The polyvinylidene fluoride porous separation membrane is used in filtration devices.

21. A filtration device comprising a housing having a fluid inlet and a fluid outlet, characterized in that, The outer shell is provided with a polyvinylidene fluoride porous separation membrane as described in claim 13.

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