KR101757859B1 - Dual-layer hollow fiber membrane containing nanoparticles and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a porous inner supporting layer having macropores; And a porous outer selective layer containing nanoparticles having fine pores, and a method for producing the same.
According to the present invention, it is possible to provide various categories (such as a microfiltration membrane (MF), an ultrafiltration membrane (UF) or a nanofiltration membrane (NF)) having excellent water permeability and virus removal rate without peeling of the support layer and the selective layer through the triple spinning nozzle Layer hollow fiber membrane containing nanoparticles of the present invention can be applied to a water purification apparatus.

Description

TECHNICAL FIELD [0001] The present invention relates to a hollow fiber membrane containing nanoparticles,

TECHNICAL FIELD The present invention relates to a nanoparticle-containing double layer hollow fiber membrane and a method of manufacturing the same, and more particularly, to a porous double layer hollow fiber membrane having a porous inner supporting layer having macropores; And a porous outer selective layer containing nanoparticles having fine pores, and a method for producing the same.

In general, most of the water purification equipment is composed of nonwoven fabric filter, activated carbon and separation membrane, and the most important purification function is the separation membrane. These membranes can be roughly divided into microfiltration membranes (MF), ultrafiltration membranes (UF), nanofiltration membranes (NF), and reverse osmosis membranes (RO) depending on the pore size. In recent water purification systems, reverse osmosis membranes and ultrafiltration membranes have.

The reverse osmosis membrane is a method of concentrating the impurities in the water by moving the concentration of the impurities in the water from the higher side to the lower side by forcibly applying the pressure. It has a high removal rate of the impurities, but all the harmful components as well as the beneficial minerals are filtered, There are many disadvantages. On the other hand, the ultrafiltration membrane can be operated by water pressure of tap water rather than an artificial high-pressure pump. However, depending on the size of pores, there are some limitations on filtering viruses, bacteria or pathogenic microorganisms.

Recently, nanofiltration membranes have been attracting attention because they have pores smaller than the pores of the ultrafiltration membranes and can filter almost even organic matter such as viruses, bacteria, or pathogenic microorganisms to purify drinking water from contaminated water.

The nanofiltration membrane is a separation membrane located between the reverse osmosis membrane and the ultrafiltration membrane when it is divided into pore sizes, whereas the reverse osmosis membrane removes the smallest dissolved molecules having a size of less than 0.0001 μm, whereas the nanofiltration membrane filters dissolved molecules having a diameter of 0.001 μm . Since the pore size of the nanofiltration membrane is larger than that of the reverse osmosis membrane, the nanofiltration membrane process is essentially a low-pressure reverse osmosis membrane process, because it can be operated at lower pressure conditions than the reverse osmosis membrane process and can also filter out some salts including organic substances. In other words, the purity of the filtered water does not represent the level of ultrapure water required for semiconductor processing or the pharmaceutical industry, so nanofiltration membranes are used for applications where high efficiency reverse osmosis membrane processes are not required. Like the reverse osmosis membrane, these nanofiltration membranes can remove not only organics but also bacteria and viruses almost completely without producing by-products during the separation process. Therefore, nanofiltration membranes can be effectively used for water purification, which is essential to ensure the stability of supply of public drinking water that uses surface water or ground water as raw water, which must remove pesticides and other organic pollutants contained in raw water.

Examples of the polymer material used in the water treatment process using the nanofiltration membrane include polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, polyacrylonitrile, polyethylene, polypropylene, cellulose acetate or polyvinylidene fluoride. For example.

On the other hand, the separator can be divided into asymmetric membranes having uniform pores and uniformly symmetric membranes according to their structures. In particular, the asymmetric membranes can increase the removal performance of the membranes in the selective layer while securing the mechanical strength of the membranes in the support layer There is an advantage in that there is active research.

As a prior art related to the nanofiltration hollow fiber membrane having such an asymmetric structure, there is known a manufacturing technique of a nanofilter hollow fiber membrane having a support layer of a sponge structure and a polyamide layer coated as a separation layer along the periphery of the support layer. To increase the bonding force between the support layer and the separating layer by introducing a hydrophilic radical to form a solid and uniform polyamide coating layer, thereby achieving the effect of achieving stain resistance, chemical resistance and high salt rejection rate The separation layer (coating layer) may be peeled off when used for a long period of time in the water treatment process, thereby deteriorating separation performance (Patent Document 1).

In connection with the separation membrane containing nanoparticles, in preparing a separation membrane by interfacial polymerization on a porous support, a metal oxide sol is added to an aqueous solution to increase dispersibility, thereby forming a polyamide active layer, Inorganic composite membrane having a high water permeation rate has been known. However, there is a possibility that the porous support and the polyamide active layer may peel off when used for a long period of time. The nanoparticle-containing separation membrane prepared herein is a flat membrane, The structure of the hollow fiber membrane is fundamentally different, and commercialization of the hollow fiber membrane is limited as compared with that of the hollow fiber membrane, and the removal performance of viruses and the like comparable to the hollow fiber membranes has not been disclosed (Patent Document 2).

Therefore, the present inventors paid attention to a hollow fiber membrane rather than a flat membrane in terms of commercialization, and unlike a conventional hollow fiber membrane in which a hollow fiber was formed through a conventional double nozzle in manufacturing an asymmetric hollow fiber membrane, When the support layer and the porous outer selective layer containing nanoparticles having micropores are formed at the same time, a high virus removal rate and water permeability can be achieved due to the nanoparticles that function as an adsorbent without peeling of the support layer and the selective layer , A microfiltration membrane (MF), an ultrafiltration membrane (UF) or a nanofiltration membrane (NF), and it can be applied to a water purification apparatus. It came.

Patent Document 1: Japanese Laid-Open Patent Publication No. 10-2014-0003296 Patent Document 2. Registered Patent Publication No. 10-1451304

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide a nanoparticle-containing double layer hollow fiber membrane excellent in water permeability and virus removal rate without peeling of a support layer and a selective layer, will be.

In order to accomplish the above object, the present invention provides a porous inner supporting layer having macropores; And a porous outer selective layer containing nanoparticles having fine pores.

The hollow fiber membrane is characterized by being a microfiltration membrane (MF), an ultrafiltration membrane (UF) or a nanofiltration membrane (NF).

The ultrafiltration membrane has a large pore size of 0.1 to 100 μm.

The ultrafiltration membrane has a fine pore size of 0.01 to 1 μm.

Wherein the inner support layer is selected from the group consisting of polysulfone, polyethersulfone, polyacrylonitrile, polyamide, polyimide, polyetherimide, polyetheretherketone, polyvinylidene fluoride, cellulose acetate, and cellulose triacetate. It is characterized by one.

The nanoparticles are characterized by any one selected from the group consisting of alumina, silica, zirconia, titania, zeolite, and carbon nanotubes.

Wherein the outer selection layer is selected from the group consisting of polysulfone, polyethersulfone, polyacrylonitrile, polyamide, polyimide, polyetherimide, polyetheretherketone, polyvinylidene fluoride, cellulose acetate, and cellulose triacetate And is characterized by any one of them.

The hollow fiber membrane has a thickness of 100 to 400 mu m.

The present invention also relates to a method for preparing an internal dope solution, comprising the steps of: I) obtaining an internal dope solution by mixing a first polymer material, a solvent and an additive; II) obtaining an external dope solution by mixing a second polymer material, a solvent, an additive and nanoparticles; III) supplying and discharging the inner and outer dope solution together with the bore solution to the triplet spinning nozzle; And IV) contacting the discharged dope solution with a coagulating liquid to form a hollow fiber membrane.

Wherein the first polymeric material is selected from the group consisting of polysulfone, polyethersulfone, polyacrylonitrile, polyamide, polyimide, polyetherimide, polyetheretherketone, polyvinylidene fluoride, cellulose acetate, and cellulose triacetate And is characterized by any one selected.

Wherein the second polymeric material is selected from the group consisting of polysulfone, polyethersulfone, polyacrylonitrile, polyamide, polyimide, polyetherimide, polyetheretherketone, polyvinylidene fluoride, cellulose acetate, and cellulose triacetate And is characterized by any one selected.

The solvent of step I) or step II) is selected from the group consisting of N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO) And is characterized by any one selected.

The additive in step I) or step II) is a hydrophilic polymer.

Wherein the hydrophilic polymer is polyvinylpyrrolidone or polyethylene glycol.

The nanoparticles are characterized by any one selected from the group consisting of alumina, silica, zirconia, titania, zeolite, and carbon nanotubes.

And the content of the first polymer material in the inner dope solution is 15 to 25 wt%.

The content of the second polymer material in the external dope solution is 10 to 20% by weight.

The bore solution may be any one selected from the group consisting of water, isopropanol, ethylene glycol, glycerol, dimethoxy ethanol, diethoxy ethanol, butoxy methanol, dimethoxybutylene oxide, diglycidyl dimethyl ether, .

In the step III), the discharge amount of the internal dope solution is 1.0 to 15.0 g / min, the discharge amount of the external dope solution is 1.0 to 15.0 g / min, and the discharge amount of the bore solution is 1.0 to 10.0 g / min.

The coagulating solution in the step iv) is characterized by any one selected from the group consisting of water, methanol, ethanol, isopropanol, pentane, hexane, and mixtures thereof.

According to the present invention, it is possible to provide various categories (such as a microfiltration membrane (MF), an ultrafiltration membrane (UF), or a nanofiltration membrane (NF)) having excellent water permeability and virus removal rate without peeling of the support layer and the selective layer through the triple spinning nozzle Layer hollow fiber membrane containing nanoparticles of the present invention can be applied to a water purification apparatus.

1 is a cross-sectional view of a triple spinning nozzle for making a double layer hollow fiber membrane according to the present invention;

Hereinafter, the double-layer hollow fiber membrane according to the present invention and the method for producing the same will be described in detail.

The present invention relates to a porous inner supporting layer having macropores; And a porous outer selective layer containing nanoparticles having fine pores.

In addition, the nanoparticle-containing double layer hollow fiber membrane according to the present invention can be provided in various categories ranging from a microfiltration membrane (MF), an ultrafiltration membrane (UF), or a nanofiltration membrane (NF) by controlling the pore size.

In particular, in the case of an ultrafiltration membrane, the size of the macropores is 0.1 to 100 탆. If the size of the macropores is less than 0.1 탆, the size of the pores is too small, It is preferable to control the size of the macropores in the above range because the pore size is too large to reduce the removal rate.

The size of the micropores in the ultrafiltration membrane is in the range of 0.01 to 1 占 퐉. When the size of the micropores is less than 0.01 占 퐉, the size of the pores is too small to reduce the water permeability, It is preferable to control the size of the micropores in the above range because the size of the pores is too large and the performance of the selective layer (removal layer) deteriorates.

In addition, the inner support layer serves as a support for the external selective layer so that the double-layer hollow fiber membrane can be used in the device industry in addition to the water purifier to withstand high pressures. Therefore, the inner support layer must have excellent mechanical properties. The material is preferable. It is therefore possible to use polyether sulfone (PSF), polyethersulfone (PES), polyacrylonitrile (PAN), polyamide (PA), polyimide (PI), polyetherimide Any one selected from the group consisting of vinylidene fluoride (PVDF), cellulose acetate (CA), and cellulose triacetate (CTA) can be used, and particularly commercialized polysulfone or polyethersulfone can be more preferably used .

In addition, the outer selection layer contains nanoparticles. Since the nanoparticles are contained in the outer selection layer, the functions of adsorption and filtration can be simultaneously performed, thereby increasing the virus removal rate. The nanoparticles are not particularly limited as long as they are inorganic particles including nano-sized metal oxide particles, but any one selected from the group consisting of alumina, silica, zirconia, titania, zeolite, and carbon nanotube (CNT) have.

In addition, the outer selection layer substantially functions as a selective separation layer. Examples of the polymer material forming the outer selection layer include polysulfone (PSF), polyethersulfone (PES), poly (PA), polyimide (PI), polyetherimide (PEI), polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), cellulose acetate Cellulose triacetate (CTA) may be selected from the same or different.

In addition, the double-layer hollow fiber membrane of the present invention has a thickness of 100 to 400 탆. If the thickness of the hollow fiber membrane is less than 100 탆, the mechanical strength may be lowered. If the thickness exceeds 400 탆, It is more preferable to control the thickness of the hollow fiber membrane within the above range.

The present invention also relates to a method for preparing an internal dope solution, comprising the steps of: I) obtaining an internal dope solution by mixing a first polymer material, a solvent and an additive; II) obtaining an external dope solution by mixing a second polymer material, a solvent, an additive and nanoparticles; III) supplying and discharging the inner and outer dope solution together with the bore solution to the triplet spinning nozzle; And IV) contacting the discharged dope solution with a coagulating liquid to form a hollow fiber membrane.

Examples of the first polymer material in the step I) include polysulfone (PSF), polyethersulfone (PES), polyacrylonitrile (PAN), polyamide (PA), polyimide (PI), polyetherimide Any one selected from the group consisting of polyether ether ketone (PEEK), polyvinylidene fluoride (PVDF), cellulose acetate (CA), and cellulose triacetate (CTA) Ether sulfone is more preferably used.

As the second polymer material in the step II), the first polymer material of the step I) may be a polysulfone (PSF), a polyethersulfone (PES), a polyacrylonitrile (PAN), a polyamide (PA) Any one selected from the group consisting of polyethersulfone (PI), polyetherimide (PEI), polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), cellulose acetate (CA), and cellulose triacetate May be the same or different.

1 is a cross-sectional view of a triple spinning nozzle for producing a double layer hollow fiber membrane according to the present invention.

In order to produce the double-layered hollow fiber membrane according to the present invention, two kinds of dope solutions of the internal dope solution and the external dope solution are required as the first step of the spinning process. As the solvent of the step I) or II) (150 DEG C or more) solvent is preferably used. If the boiling point is low, defects may be generated in the hollow fiber selected layer due to rapid evaporation of the organic solvent during the hollow fiber spinning process. If the boiling point is too high, evaporation of the organic solvent does not occur while the spinning solution passes through the air, Can not be obtained. Thus, a representative polar aprotic solvent such as N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), and dimethylsulfoxide (DMSO) One may be used, or a mixture thereof may be used.

In addition, by using the hydrophilic polymer as the additive in the step I) or II), the bonding force between the inner supporting layer formed from the inner dope solution and the outer selective layer formed from the outer dope solution is improved, Thereby contributing to an increase in water permeability. As the hydrophilic polymer, polyvinyl pyrrolidone or polyethylene glycol can be preferably used.

The nanoparticles in the step II) are not particularly limited as long as they are inorganic particles including nano-sized metal oxide particles, but any one selected from the group consisting of alumina, silica, zirconia, titania, zeolite and carbon nanotubes And the nanoparticles are contained in the external selection layer formed from the external dope solution, so that the function of adsorption and filtration can be performed at the same time, thereby increasing the virus removal rate.

If the content of the first polymer material in the inner dope solution is less than 15% by weight, it is preferable that the content of the first polymer material in the inner dope solution is in the range of 15 to 25% by weight. The mechanical strength of the inner supporting layer to be performed can be lowered and the shrinkage of the inner supporting layer caused by the spinning process can not be prevented. When the amount exceeds 25% by weight, the resistance against the flow of the fluid having selectively passed through the separation layer is minimized There is a problem.

If the content of the second polymer material in the external dope solution is less than 10% by weight, the viscosity of the dope solution is low, and if the content of the second polymer material in the external dope solution is less than 10% The pore size of the prepared hollow fiber membrane is increased to decrease the virus removal rate. If the amount exceeds 20% by weight, it is difficult to obtain a homogeneous phase dope solution, and an external selective layer The content of the second polymer material in the external dope solution is preferably adjusted to 10 to 20% by weight.

Next, the inner and outer dope solutions are supplied to and discharged from the triple spinning nozzle together with the bore solution. First, the inner and outer dope solutions are respectively transferred to a storage tank and left in a 50 ° C oven for 12 hours to remove bubbles. , And the bore solution is supplied to the triple spinning nozzle through a liquid transfer pump (HPLC pump), respectively, through a gear pump. At this time, the bore solution acts as an internal coagulant to initiate a phase transition between the dope solutions and contribute to the formation of a hollow fiber. It is preferable to use water, isopropanol, ethylene glycol, glycerol, dimethoxyethanol, diethoxyethanol, , Dimethoxybutylene oxide, diglycidyl dimethyl ether, and mixtures thereof. It is more preferable to use a mixture with water.

In the step III), the discharge amount of the inner and outer dope solution and the discharge amount of the bore solution are adjusted by using a gear pump and a liquid transfer pump (HPLC pump), respectively, so that secondary elongation does not occur during spinning. It is preferable to control the discharge amount of the external dope solution in the range of 1.0 to 15.0 g / min, and the discharge amount of the bore solution in the range of 1.0 to 10.0 g / min. Then, the discharged dope solution is brought into contact with the coagulating solution to solidify to form a hollow fiber membrane. The coagulating solution may be any of those containing a non-solvent such as water, methanol, ethanol, isopropanol, pentane, Or a mixture thereof.

Finally, the hollow fiber membrane thus formed is washed and dried by a conventional method to prepare a double layer hollow fiber membrane.

Hereinafter, specific examples will be described in detail.

(Example 1) Production of nanoparticle-containing double layer hollow fiber membrane

15 g of polysulfone (PS) [Udel ^® ] was slowly added to 70 g of N-methylpyrrolidone (NMP) in a 1 L round bottom flask equipped with a stirrer, 10 g of polyvinylpyrrolidone (PVP) After sufficiently stirring for 12 hours to obtain a homogeneous phase solution, 5 g of alumina was added as nanoparticles and further stirred for 1 hour to obtain an external dope solution. On the other hand, an internal dope solution was obtained with a composition of 20 g of polyethersulfone (PES) [Victrex ^® ], 75 g of NMP and 5 g of PVP. The outer and inner dope solutions were each transferred to a storage tank and allowed to stand in an oven at 50 ° C for 12 hours to remove bubbles. The inner and outer dope solution and the bore solution (water) were supplied and discharged to the triple spinning nozzle through the gear pump and the HPLC pump, respectively, by using the hollow fiber membrane production apparatus having the triple spinning nozzle shown in Fig. The discharged spinning solution was contacted with a coagulation bath filled with water to complete the phase transition to form a hollow fiber membrane, followed by washing and drying according to a conventional method to prepare a double layer hollow fiber membrane.

(Examples 2 to 10) Preparation of nanoparticle-containing double layer hollow fiber membrane

A double-layer hollow fiber membrane was prepared in the same manner as in Example 1, except that the composition of the external dope solution was changed as shown in Table 1 below.

Hollow fiber membrane External dope solution composition The polymer (g) Solvent (g) Additive (g) Nanoparticles (g) Example 2 PSF 15 NMP 65 PVP 10 Alumina 10 Example 3 PSF 15 NMP 70 PVP 10 Zeolite 5 Example 4 PSF 15 NMP 65 PVP 10 Zeolite 10 Example 5 PSF 15 NMP 65 PVP 10 Zirconia 10 Example 6 PSF 15 NMP 65 PVP 10 Titania 10 Example 7 PSF 15 NMP 70 PVP 10 Silica 5 Example 8 PSF 15 NMP 65 PVP 10 Silica 10 Example 9 PSF 15 NMP 70 PVP 10 CNT 5 Example 10 PSF 15 NMP 65 PVP 10 CNT 10

(Comparative Example 1) Production of a single-layer hollow fiber membrane containing no nanoparticles

Using only one dope solution obtained by sufficiently stirring the composition of 20 g of polyethersulfone (PES) [Victrex ^® ], 75 g of NMP and 5 g of PVP at 50 ° C. for 12 hours, a conventional hollow fiber spinning apparatus A single layer hollow fiber membrane was prepared under the same conditions as the spinning conditions described in Example 1. [

( Comparative Example 2 ) Non-nanoparticle-free Single layer  Manufacture of hollow fiber membranes

Using only one dope solution obtained by sufficiently stirring for 12 hours at 50 ° C in a composition of 20 g of polysulfone (PS) [Udel ^® ], 75 g of NMP and 5 g of PVP, a conventional hollow fiber spinning device A single-layer hollow fiber membrane was prepared under the same conditions as the spinning conditions described in Example 1.

( Test Example ) Water permeability  And virus removal rate

A hollow fiber membrane module having a diameter of 50 mm and a length of 80 mm was prepared to measure the water permeability and bacterial and virus removal rates of the hollow fiber membranes prepared in Examples 1 to 10 and Comparative Examples 1 and 2. The water permeability was measured at a pressure of 1.0 bar per minute, and the water permeability was calculated by the following equation.

Flux = L / m ² h

(L: permeation volume, m ² : effective membrane area, h: permeation time)

Bacteria and viruses were removed using K. terrigena, MS ₂ colophage. The culture medium used for the liquid culture of the host was a modified TYG liquid medium. The composition of the medium was 1 g of bastotrypton 10 g of yeast extract, 1 g of glucose, 8 g of NaCl, 0.3 g of CaCl ₂ .2H ₂ O and 11 g of distilled water. For 5 minutes. Bacteria and viruses were amplified to 10 ¹¹ PFU / ml using a soft agar layer method and diluted to 10 ⁶ PFU / ml in a feed tank and used as test water. The bacterial and viral counts were calculated by the standard plaque forming method and the removal rates were calculated. The results are shown in Table 2 below.

Hollow fiber membrane Water permeability (L / m ² h bar) Removal rate bacteria virus Example 1 687 log 6 (99.9999%) 99.99% Example 2 714 log 6 (99.9999%) 99.99% Example 3 703 log 6 (99.9999%) 99.99% Example 4 725 log 6 (99.9999%) 99.99% Example 5 702 log 6 (99.9999%) 99.99% Example 6 692 log 6 (99.9999%) 99.99% Example 7 680 log 6 (99.9999%) 99.99% Example 8 669 log 6 (99.9999%) 99.99% Example 9 679 log 6 (99.9999%) 99.99% Example 10 681 log 6 (99.9999%) 99.99% Comparative Example 1 587 log 6 (99.9999%) 92.40% Comparative Example 2 567 log 6 (99.9999%) 93.40%

As shown in Table 2, the nanofiber-containing double-layer hollow fiber membranes prepared in Examples 1 to 10 exhibited significantly higher water permeability than the nanoparticle-free single layer hollow fiber membranes prepared in Comparative Examples 1 and 2 In particular, the nanoparticle-containing double-layered hollow fiber membrane prepared in Example 4 contributed to increase the permeability of the nanoparticles of the external dope solution, so that the highest water permeability was obtained. The double layer hollow fiber membranes prepared from Examples 1 to 10 and Comparative Examples 1 and 2 of the present invention showed excellent removal performance with a bacterial removal rate of not less than log 6 (99.999%). However, It can be seen that the nanoparticle-containing double-layer hollow fiber membranes prepared from Examples 1 to 10 are significantly higher than the single-layer hollow fiber membranes containing no nanoparticles prepared from Comparative Examples 1 and 2.

Therefore, according to the present invention, the microfiltration membrane (MF), the ultrafiltration membrane (UF), and the ultrafiltration membrane (UF), which have excellent water permeability and virus removal efficiency without peeling of the inner support layer and the external selective layer, Or nanofiltration membranes (NF), and it is possible to apply the nanoparticle-containing double layer hollow fiber membranes to water purification apparatuses.

Claims (20)

A porous inner supporting layer of a polyethersulfone material having macropores of 0.1 to 100 mu m in size; And
A polysulfone material containing alumina nanoparticles having micropores of 0.01 to 1 μm in size Layer outer ultrafiltration hollow fiber membrane comprising a porous outer selective layer. delete delete delete delete delete delete The dual-layer ultrafiltration hollow fiber membrane according to claim 1, wherein the hollow fiber membrane has a thickness of 100 to 400 μm. I) mixing the polyethersulfone, the solvent and the additive to obtain an internal dope solution;
II) mixing the polysulfone, the solvent, the additive and the alumina nanoparticles to obtain an external dope solution;
III) supplying and discharging the inner and outer dope solution together with the bore solution to the triplet spinning nozzle; And
IV) The discharged dope solution is brought into contact with the coagulating solution to form a porous inner support layer of polyethersulfone material having macropores of 0.1 to 100 mu m size and a polysulfone material containing alumina nanoparticles having micropores of 0.01 to 1 mu m size And forming a hollow fiber membrane including a porous outer selective layer of the porous outer membrane. delete delete 10. The method of claim 9, wherein the solvent of step I) or step II) is selected from the group consisting of N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide And a mixture of two or more of the above components. [Claim 11] The method according to claim 9, wherein the additive in step I) or step II) is a hydrophilic polymer. 14. The method of claim 13, wherein the hydrophilic polymer is polyvinylpyrrolidone or polyethylene glycol. delete The method of claim 9, wherein the content of the first polymer material in the inner dope solution is 15 to 25 wt%. 10. The method of claim 9, wherein the content of the second polymeric material in the external dope solution is 10 to 20% by weight. 10. The method of claim 9, wherein the bore solution comprises water, isopropanol, ethylene glycol, glycerol, dimethoxy ethanol, diethoxy ethanol, butoxy methanol, dimethoxybutylene oxide, diglycidyl dimethyl ether, Lt; RTI ID = 0.0 > 1, < / RTI > 10. The method of claim 9, wherein in the step III), the discharge amount of the inner dope solution is 1.0 to 15.0 g / min, the discharge amount of the outer dope solution is 1.0 to 15.0 g / min, and the discharge amount of the bore solution is 1.0 to 10.0 g / min Lt; RTI ID = 0.0 > of: < / RTI > The method according to claim 9, wherein the coagulating solution in step IV) is any one selected from the group consisting of water, methanol, ethanol, isopropanol, pentane, hexane, and mixtures thereof. .

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