KR101025755B1 - Ultrafiltration membranes with improved water permeability and mechanical strength and manufacturing method thereof - Google Patents

Abstract

The present invention relates to an ultrafiltration membrane and a method of manufacturing the same, while maintaining the properties of the ultrafiltration membrane produced by a phase transfer method and having excellent mechanical strength.

The ultrafiltration membrane of the present invention has a structure consisting of an asymmetric surface layer having no porosity and a gradual increase in pore size depending on its position within the film thickness, and a porous layer formed in the form of a spherical particle under the surface layer. As a result, it is possible to suppress the loss of mechanical strength and improve fouling resistance, and due to the porous layer in the form of a spherical particle formed at the bottom thereof, the water permeability and the mechanical strength can be simultaneously satisfied. In particular, the present invention can manufacture the ultrafiltration membrane through the change of temperature and humidity conditions in the air gap, the transport speed of the support, the solution temperature, etc., the manufacturing method of the present invention is a simple production without additional processes such as stretching It is useful for next-generation high efficiency separation process industry, including highly purified water treatment and water treatment processes, as it provides an ultrafiltration membrane with excellent water permeability and promotes excellent mechanical strength without seriously reducing the rejection rate at a low cost with a process and low cost. Can be utilized.

Ultrafiltration membrane, hollow fiber membrane, asymmetric surface layer, spherical particle porous layer

Description

ULTRAFILTRATION MEMBRANES WITH IMPROVED WATER PERMEABILITY AND MECHANICAL STRENGTH AND MANUFACTURING METHOD THEREOF}

The present invention relates to an ultrafiltration membrane and a method of manufacturing the same, which maintain the properties of the ultrafiltration membrane prepared by the phase-transfer method and have excellent mechanical strength, and more particularly, the asymmetric surface layer and porous formed in the form of a spherical particle under the surface layer. According to the layer, the present invention relates to an ultrafiltration membrane and a method for manufacturing the same, which simultaneously satisfy the permeability and mechanical strength.

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Membrane separation technology is an advanced separation technology that can almost completely separate and remove the material to be treated in wastewater / purified water according to the pore size of the membrane and the surface charge of the membrane. Its application range is gradually expanding to production, advanced treatment and recycling of wastewater, and also clean production process related to the development of zero discharge system, and it is becoming one of the core technologies that will be noticed in the 21st century.

In particular, the ultrafiltration membrane is commonly referred to as a separator having an average pore size of 0.1 micron or less, and is applied to various fields including wastewater treatment, water production, food, electric, electronic, semiconductor, and medical industries.

In recent years, as interest in water quality and drinking water increases and the standards thereof become stricter, the use of ultrafiltration membranes has gradually expanded. The general method of preparing the ultrafiltration membrane is mainly a phase transfer method using phase separation (phase transition) through interdiffusion with a solvent / nonsolvent component by precipitating a solution containing a polymer, a solvent, and other organic additives in a nonsolvent coagulation bath. It is used. Since the 1960s, many attempts have been made to treat large volumes of water and wastewater with a smaller membrane area. The ultrafiltration membrane is characterized by consisting of a plurality of porous layers including a surface layer of a predetermined thickness from the membrane surface, and a macropores from below the surface layer, and has a desirable rejection ratio from such a structure, and excellent water permeability due to the fractional molecular weight and high porosity At the same time, it is very useful in terms of water treatment efficiency.

However, the ultrafiltration membrane has a high permeation resistance due to the symmetrical structure of the surface layer, and does not have high resistance to fouling, and due to the macropores, the ultrafiltration membrane is not preferable in terms of durability. Therefore, the development of technology for producing a membrane having excellent mechanical strength while having excellent permeability and removal efficiency of the ultrafiltration membrane is still at a limited level.

Membranes prepared through the phase transition method can be classified into symmetrical and asymmetrical structures. Symmetrical structure refers to the case where the pore size is evenly distributed across the cross-section of the membrane, and the asymmetrical structure refers to a structure composed of a surface layer having a very small average pore size and a porous layer having a relatively large average pore size compared to the surface layer. .

When the asymmetric structure is precisely subdivided, a membrane whose average pore gradually changes depending on the position of the entire cross section is called an asymmetric membrane.

However, most of the ultrafiltration membranes prepared by the phase transition method include a porous layer consisting of a surface layer and a macropore to remove a specific component at the same time, there is a characteristic that there is no gradual change in pore size depending on the position in the surface layer. At this time, the formation of the surface layer is indispensable in that the removal efficiency of the film is determined by the surface layer. However, the surface layer is easily exposed to external shock and thus is easily damaged.

On the other hand, the pore formation of a certain size on the surface acts as a prefilter has the advantage of reducing the plugging (Plugging). On the other hand, since the formation of macropores reduces the mechanical properties of the membrane, it is preferable to avoid them in most cases, but techniques for suppressing the formation of such macropores are not easy.

As an example of the manufacturing method of the ultrafiltration membrane, US Pat. No. 6,024,872, a polyvinylidene fluoride (hereinafter referred to as "PVDF") resin is coated on a polyester reinforcing material (BRAID), a surface layer and an average pore size of 0.04㎛ Disclosed is a method for preparing an ultrafiltration membrane having a macroporous structure. Korean Patent Laid-Open Publication No. 2006-22866 discloses a four-layer structure comprising spinning a spinning solution containing polyvinylpyrrolidone and an inorganic salt in PVDF resin, passing through a dehumidification chamber and forming a double active layer and a macroporous support layer on the inside and outside surfaces of the membrane. A method for producing a porous hollow fiber membrane is disclosed.

In addition, Korean Patent Laid-Open Publication No. 2005-18624 discloses a method of manufacturing a porous membrane having both a three-dimensional network structure and a spherical structure. The method comprises simultaneously forming both a three-dimensional mesh structure and a spherical structure from one resin solution, a method of forming a mesh structure later on either side of the porous membrane having a spherical structure, and simultaneously forming two or more resin solutions from the spinneret. A method of producing a porous membrane by discharging is described. In addition, the method changes the composition of a cooling liquid composed of a solvent, a poor solvent, and a non-solvent to solidify the discharged spinning solution. A manufacturing method is disclosed.

In addition, Korean Patent Publication No. 2008-45275 discloses a hollow fiber membrane having a porous multilayer structure and a manufacturing method thereof using a triple nozzle and an extruder, and Korean Patent Publication No. 2008-57637 discloses a humidity in a closed air gap region. It discloses a method for producing a microfiltration membrane having an anisotropic structure by controlling the.

From the prior art, the ultrafiltration membrane is dependent on variables such as temperature and humidity, additive composition in solution, and other coagulating solution changes in the manufacturing method thereof, and it is possible to manufacture a separator having various performances by controlling these variables. Nevertheless, the ultrafiltration membranes prepared by the general phase-transfer method have a porous layer containing macropores, which are disadvantageous in terms of mechanical properties, or are mostly made of a membrane without a gradual change in pore size even in a structure without macropores. It is known to manufacture ultrafiltration membranes without macropores at the same time.

Accordingly, the present inventors have tried to provide the ultrafiltration membrane with durability while maintaining the properties of the ultrafiltration membrane prepared by the phase transition method, and have no macroporosity under certain conditions and have an anisotropic surface layer and at the same time permeability and mechanical strength By forming a structure composed of spherical particles which is very advantageous to the membrane, it provides a ultrafiltration hollow fiber membrane with excellent water permeability with very low water permeation resistance without significant reduction of exclusion rate through simple manufacturing process and low cost without additional processes such as drawing. By this, the present invention was completed.

It is an object of the present invention to provide an ultrafiltration membrane capable of simultaneously satisfying permeability and mechanical strength by a phase transition method.

Another object of the present invention is to provide a method for producing an ultrafiltration membrane made of asymmetric surface layer and a porous layer formed in the form of spherical particles under the surface layer through a controlled manufacturing method under specific conditions.

The present invention provides an ultrafiltration membrane composed of an asymmetric surface layer having no porosity and having a gradual increase in pore size depending on its position within the film thickness, and a porous layer formed in the form of spherical particles under the surface layer.

The porous layer of the present invention is formed into a spherical particle shape of 0.1 ~ 10.0㎛ size, the ultrafiltration membrane of the present invention has a tensile strength of 37 ~ 45 N / ㎡, and the water permeability of 450 ~ 1200L / m 2 hr.

The present invention provides a method for producing an ultrafiltration membrane. More specifically,

1) Manufacturing process of dope solution which melt | dissolves the composition which consists of PVDF-type resin, a solvent, and a pore forming agent at 40-150 degreeC temperature, 2) The outer tube of the double nozzle which maintained the said dope solution at 40-150 degreeC temperature And then transported at a constant speed to discharge the hollow support while passing the air gap, and 3) coating the dope solution on the support and 3) discharging or depositing the coated dope solution on the support to the coagulation solution. It consists of the process of manufacturing a hollow fiber membrane.

Among the dope solutions of the present invention, the PVDF resin is selected from the group consisting of PVDF homopolymer, polyvinylidene hexafluoropropylene (PVDF-HFP) copolymer, and polyvinylidene fluoride chlorotrifluoroethylene (PVDF-TCFE) copolymer Use at least one species.

In the dope solution of the present invention, the pore-forming agent is any one selected from the group consisting of polyvinylpyrrolidone (PVP), polyethyloxazoline (POZ) and polyethylene glycol (PEG), the solvent is N-methyl At least one selected from the group consisting of pyrrolidone, dimethylacetamide, dimethyl sulfoxide and dimethylformamide is used.

In this case, the dope solution of the present invention contains 10 to 65% by weight of the polyvinylidene fluoride-based resin and the pore-forming agent in 35 to 90% by weight of the solvent, the polyvinylidene fluoride-based resin and pore-forming It is preferable that the mixing composition of the agent consists of a weight ratio of 1: 0.5 to 2.0.

In the manufacturing method of the present invention, the feed rate of the support in step 2) is carried out at 0.1 to 5.0 m / min, the air gap atmosphere is adjusted to a temperature less than 40 ℃, humidity 40% or more.

In the manufacturing method of the present invention, step 3) is a step of extracting a solvent using water of 30 to 70 ° C. or less as a coagulation solution.

The manufacturing method of the present invention may further perform an extraction step of removing the pore-forming agent present in the membrane by using the extractant in the hollow fiber membrane prepared in step 3).

The present invention can provide an ultrafiltration membrane having excellent mechanical strength while maintaining the properties of the ultrafiltration membrane produced by the phase transfer method.

The present invention can provide an ultrafiltration membrane comprising a porous surface formed in the form of a spherical particle under the asymmetric surface layer and the surface layer from the manufacturing method controlled under specific conditions.

Thus, the manufacturing method of the present invention can provide an ultrafiltration membrane having excellent water permeability with low water permeation resistance without a significant reduction in exclusion rate through a simple manufacturing process and low cost without additional processes such as stretching.

Furthermore, by forming an asymmetric surface layer having no porosity and having a gradual increase in pore size depending on the position within the film thickness, it is possible to suppress the loss of mechanical strength in the long term and improve pollution resistance, which is useful for water and wastewater processes. Can be.

Hereinafter, the present invention will be described in detail.

The present invention provides an ultrafiltration membrane composed of an asymmetric surface layer having no porosity and a gradual increase in pore size depending on its position within the film thickness, and a porous layer formed in the form of spherical particles under the surface layer [ FIG. 1 ].

In the ultrafiltration membrane structure of the present invention, due to the asymmetric surface layer, it is possible to suppress the loss of mechanical strength and improve the stain resistance in the long term, and at the same time, due to the porous layer formed in the form of spherical particles, Can be satisfied. With this structure, the ultrafiltration membrane of the present invention has a tensile strength of 37 to 45 N / m 2 and a water permeability of 450 to 1200 l / m 2 hr.

At this time, the porous layer can be confirmed that the distribution is formed in the form of spherical particles of 0.1 ~ 10㎛ size.

The present invention provides a method for producing an ultrafiltration membrane consisting of an asymmetric surface layer and a porous layer formed in the form of a spherical particle under the surface layer from a controlled manufacturing method under specific conditions. More specifically, the manufacturing method of the ultrafiltration membrane of the present invention 1) a process for preparing a dope solution for dissolving a composition consisting of polyvinylidene fluoride resin, a solvent and a pore-forming agent at a temperature of 40 ~ 150 ℃,

2) The dope solution is transferred to an outer tube of a double nozzle maintained at a temperature of 40 to 150 ° C., and then transported at a constant speed to discharge the hollow support, and at the same time through an air gap, to dope the solution onto the support. Coating process,

3) A process of manufacturing a hollow fiber membrane by discharging or precipitating the coated dope solution on the support to the coagulation solution.

In addition, the method of manufacturing an ultrafiltration membrane of the present invention may further perform a 4) washing and drying process of the manufactured hollow fiber membrane.

Hereinafter, the manufacturing method of the hollow fiber membrane of this invention is demonstrated for each step.

1) Manufacturing process of dope solution

The dope solution of the present invention is composed of a PVDF resin, a solvent, and a pore-forming agent, and is prepared by dissolving uniformly at 40 to 150 ° C. without forming precipitates or suspended solids.

The PVDF resin is at least one selected from the group consisting of PVDF homopolymer, polyvinylidene hexafluoropropylene (PVDF-HFP) copolymer, and polyvinylidene fluoride chlorotrifluoroethylene (PVDF-TCFE) copolymer. It is.

The pore-forming agent used in the dope solution of the present invention is any one selected from the group consisting of polyvinylpyrrolidone (PVP), polyethyloxazoline (POZ) and polyethylene glycol (PEG), the solvent is N-methylpy One or more mixed forms selected from the group consisting of rolidone, dimethylacetamide, dimethylsulfoxide and dimethylformamide are used.

The dope solution of the present invention preferably contains from 35 to 90% by weight of the solvent, and 10 to 65% by weight of the PVDF-based resin and pore-forming agent, more preferably solvent 60 to the total weight of the dope solution. 15 wt% to 40 wt% of the PVDF resin and the pore-forming agent in the range of 85 wt% to 85 wt%. The weight ratio of the PVDF-based resin and the pore-forming agent in the dope solution is preferably 1: 0.5 to 2.0.

At this time, when the mixing content of the PVDF-based resin and the pore-forming agent is less than 10% by weight, the coating is uneven due to the low viscosity, and when it exceeds 65% by weight, it is difficult to prepare a uniform film due to the high viscosity. In addition, when the weight ratio between the PVDF-based resin and the pore-forming agent is less than 0.5, there is a disadvantage in that the water permeability decreases, and when larger than 2, mechanical properties such as peel strength are caused.

2) Coating process of dope solution on support

In the hollow fiber membrane manufacturing process of the present invention, the dope solution is transferred to an outer tube of a double nozzle maintained at a temperature of 40 to 150 ° C., and then a hollow support and a dope are formed into and out of the double nozzle within an air gap under a predetermined condition. The step of coating the dope solution on the support by simultaneously discharging the solution independently and simultaneously. In this case, the hollow support may be selected from a polyester, polyamide or polypropylene material in a similar form to a straw, more preferably to use a polyester.

The air gap is a semi-closed region between the nozzle discharge portion and the coagulating liquid, and is formed at a constant temperature and a humidification condition. At this time, the temperature is maintained at 40 ℃ or less, the humidity is preferably maintained at a relative humidity of 40% or more, more preferably 60% or more. When the temperature in the air gap is 40 ° C. or higher or the relative humidity is 40% or less, a porous layer having macropores is formed.

In the coating process of the present invention, the support is preferably transported to the inner tube of the double nozzle at a speed of 0.1 to 5.0 m / min, in this case, if the transport speed of the support is less than 0.1 m / min, the coating speed is low to improve the productivity There is a problem, the isotropic surface layer is easily formed, which is not suitable for ultrafiltration applications, and when the feed rate exceeds 5 m / min, the coating thickness becomes thin, which is disadvantageous for improving mechanical strength.

3) solidification process

In the coagulation process of the present invention, a hollow fiber membrane is prepared by discharging or depositing a coating on a support to a coagulation solution maintained at 30 to 70 ° C., whereby the coagulation solution is preferably water.

At this time, when the temperature of the coagulation solution exceeds 70 ℃, shrinkage of the coating layer occurs, the water permeability is reduced, if kept below 30 ℃, it takes a lot of time to extract the solvent.

4) washing drying process

In the present invention, a washing process may be performed to remove the remaining solvent inside and outside the separator. As a preferred washing liquid, water is used, and the washing time is not particularly limited, but is preferably performed at least 1 day or more and 3 days or less.

After immersing the hollow fiber membrane prepared in the present invention in an aqueous solution containing 50% by weight of glycerin, it may further comprise a step of drying in the air, wherein the immersion and drying period is not particularly limited, but is carried out in less than one day desirable.

Furthermore, the manufacturing method of the present invention may further perform an extraction step of removing the pore-forming agent present in the membrane using a conventional extracting agent in the prepared hollow fiber membrane within a range that does not make the pore size too large. .

Hereinafter, the present invention will be described in more detail with reference to Examples.

This embodiment is intended to illustrate the present invention in more detail, and the scope of the present invention is not limited to these examples.

≪ Example 1 >

After slowly adding N, N-dimethylacetamide (DMAC) to PVDF (Solvay, Mw: 304,000) and pore-forming agent, the mixed weight of polyvinylpyrrolidone (ISP, Mw: 60,000) is 25% by weight. These were mixed and the uniform dope solution was prepared at 120 degreeC.

Then, after removing the bubbles contained in the prepared uniform dope solution by using a vacuum pump, the dope solution using a gear pump with a double nozzle that is 1.9mm in internal diameter, 2.5mm in outer diameter and maintained at 80 ℃ Transferred. Subsequently, after discharging the support and the coating solution of the polyester material transported at a rate of 0.5m / min into an air gap set to a temperature 35 ℃, humidity 70% and length 50 cm, the coagulation solution is continuously precipitated in water at room temperature A hollow fiber membrane coated on the support was prepared.

The hollow fiber membrane which passed the coagulation liquid was continuously wound through a winding bobbin and washed for 48 hours in a water washing tank to remove more organic solvent remaining. The completely washed hollow fiber membrane was immersed in 50% by weight aqueous solution of glycerin for 24 hours, and then dried at room temperature. The hollow fiber membrane having three inner diameters of 0.6 mm and an outer diameter of 1.8 mm was used as an effective length of 170 cm, thereby increasing the membrane area. A membrane module of 0.04 m 2 was prepared.

Comparative Example 1

A hollow fiber membrane was prepared in the same manner as in Example 1 except that the air gap was changed to a temperature of 20 ° C., a humidity of 40%, and a length of 50 cm in Example 1.

Experimental Example 1

1. Measurement of Permeability

By using pure water at room temperature for the manufactured hollow fiber membrane module and using a peristaltic pump (peristaltic pump) in accordance with the change of TMP (Trans membrane pressure) in an outside-in, a vacuum means applied to one side of the module After measuring the amount of water filtered through a certain time, the permeability was measured by converting the calculated slope into the unit membrane area.

2. Measurement of Exclusion Rate

Bovin Serum Albumin, BSA (Aldrich, Mw 66,000) was dissolved in pure water at room temperature to prepare a 1,000 ppm aqueous solution. By supplying a pressure of 1.0 kg / ㎠ to the BSA aqueous solution on one side of the module prepared in the dead-end (a dead-end) solution and the concentration of BSA dissolved in the initially supplied raw water UV spectroscopy (Varian, Cary-100). Then, the relative ratio of the absorption peak measured at the wavelength of 278nm was calculated by the following equation, to determine the exclusion rate.

3. Measurement of tensile strength

Tensile strength was measured using a tensile tester (LLOYD), and the average value of the sample having a holding distance of 100 mm was measured three times at a crosshead speed of 50 mm / min. Both ends of the sample (10 mm in length) before the measurement were potted using a tubular mold and then measured under the same conditions as the tensile strength measurement method.

As a result of comparing the results of Table 1 and the cross-sectional photographs of FIGS. 1 and 2, it was confirmed that the water permeability of the hollow fiber membrane was clearly improved due to the increase in humidity in the air gap. In the case of Figure 2, the structure of the membrane is composed of a porous layer formed of a large pore in the surface layer, the lower portion of the spherical particles. In this case, as shown in Table 1, the results of low permeability and tensile strength.

On the other hand, in the case of Figure 1 of the hollow fiber membrane prepared in Example 1, it is a structure composed of a porous layer made of spherical particles in the lower portion, having asymmetric surface layer without forming macropores in the surface layer. The hollow fiber membrane of the present invention prepared with such a structure has a significant increase in permeability and a significant increase in mechanical strength without a significant decrease in rejection rate.

<Examples 2-3

Instead of the nozzle temperature 80 ° C carried out in Example 1, by changing the nozzle temperature to 60 ° C and 100 ° C, except that the discharge and coating process is carried out under the same conditions as in Example 1 A hollow fiber membrane was prepared, and the results are shown in Table 2 below.

As shown in Table 2, it was confirmed that the increase in the nozzle temperature increases the permeability of the membrane due to the increase in the air temperature difference in the air gap.

<Examples 4-5>

The hollow fiber membrane was manufactured under the same conditions as in Example 1, except that the polyester support transfer speed was changed to 0.3 m / min and 1.0 m / min in Example 1, and the results are as follows. Table 3 shows.

From the results of Table 3, as the transport speed of the support decreases, the humidity exposure effect is increased to confirm the result of the water permeability is further improved.

As described above, the present invention provides an ultrafiltration membrane comprising an asymmetric surface layer having no porosity and a gradual increase in pore size depending on its position within the film thickness and a porous layer formed in the form of spherical particles under the surface layer. The water permeability and the mechanical strength can be simultaneously satisfied while maintaining the characteristics of the filtration membrane.

The present invention has improved permeability and removal rate by forming a porous layer of spherical particles in the asymmetric surface layer and the lower portion of the macroporous structure through the change of temperature and humidity conditions in the air gap, the transport speed of the support, the solution temperature, etc. Provided is a method for producing an ultrafiltration membrane consisting of a hollow fiber membrane.

The manufacturing method of the present invention provides an ultrafiltration membrane having excellent water permeability with low water permeability without a significant reduction in exclusion rate through a simple manufacturing process and low cost without additional processes such as drawing, and thus, has been becoming increasingly stringent. It can be used for next generation high efficiency separation process industry including water treatment process.

While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.

1 is an entire cross-section and a partially enlarged cross-sectional photograph of a hollow fiber membrane according to an embodiment of the present invention,

2 is an entire cross-sectional view and a partially enlarged cross-sectional photograph of a hollow fiber membrane according to a comparative example of the present invention.

Claims (12)

Asymmetric surface layer without macropores and Ultrafiltration membrane, characterized in that consisting of a porous layer formed in the form of spherical particles below the surface layer. The ultrafiltration membrane according to claim 1, wherein the porous layer is formed in the form of a spherical particle having a size of 0.1 to 10 µm. The ultrafiltration membrane according to claim 1, wherein the ultrafiltration membrane has a tensile strength of 37 to 45 N / m 2 and a water permeability of 450 to 1200 l / m 2 hr. 1) manufacturing process of dope solution which dissolves the composition which consists of polyvinylidene fluoride system resin, a solvent, and a pore forming agent at 40-150 degreeC temperature, 2) The dope solution is transferred to an outer tube of a double nozzle maintained at a temperature of 40 to 150 ° C., and then transported at a constant speed to discharge the hollow support, and at the same time through an air gap, to dope the solution onto the support. Coating process, 3) A method for producing an ultrafiltration membrane, comprising a step of discharging or depositing a coated dope solution on a support to a coagulation solution to produce a hollow fiber membrane. The method of claim 4, wherein the polyvinylidene fluoride resin of step 1) is polyvinylidene fluoride (PVDF) homopolymer, polyvinylidene hexafluoropropylene (PVDF-HFP) copolymer and polyvinylidene fluoride chloro Method for producing the ultrafiltration membrane, characterized in that at least one member selected from the group consisting of trifluoroethylene (PVDF-TCFE) copolymer. The method of claim 4, wherein the pore-forming agent of step 1) is any one selected from the group consisting of polyvinylpyrrolidone (PVP), polyethyloxazoline (POZ) and polyethylene glycol (PEG). Method for producing an ultrafiltration membrane. The method of claim 4, wherein the solvent of step 1) is at least one member selected from the group consisting of N-methylpyrrolidone, dimethylacetamide, dimethyl sulfoxide, and dimethylformamide. . The ultrafiltration membrane according to claim 4, wherein the dope solution of step 1) contains 10 to 65% by weight of the polyvinylidene fluoride resin and the pore-forming agent in 35 to 90% by weight of the solvent. Manufacturing method. The method of claim 8, wherein the mixing composition of the polyvinylidene fluoride resin and the pore-forming agent is in a weight ratio of 1: 0.5 to 2.0. The method of claim 4, wherein the feed rate of the support in step 2) is 0.1 to 5.0 m / min. The method of claim 4, wherein the air gap atmosphere in step 2) is adjusted to a temperature of less than 40 ℃, humidity 40% or more. The method of claim 4, wherein the solvent is extracted using water of 30 to 70 ° C. or less in the step 3) as a coagulation solution.

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