porous hollow fiber membrane

Abstract

An object of the present invention is to provide a porous hollow fiber membrane that has a good balance between water permeability and blocking performance, is durable enough to be repeatedly used over a long period of time, and is resistant to deterioration of removal performance due to defects. [Solution] The porous hollow fiber membrane of the present invention has a minimum pore size layer on either the inner or outer membrane surface, and is characterized in that the average local thickness diameter of the pores on the membrane surface having the minimum pore size layer is 4.0 nm or more and 10.0 nm or less, and the orientation strength of the membrane surface having the minimum pore size layer is 1.45 or more and 2.00 or less.

Description

[Technical Field]

[0001] The present invention relates to a porous hollow fiber membrane. [Background technology]

[0002] Currently, the use of porous membranes is essential for producing ultrapure water for use in the semiconductor and pharmaceutical manufacturing industries. Since the development of transistors and integrated circuits (ICs), semiconductors have become increasingly miniaturized through linear scaling, which reduces the wiring width and spacing within chips in order to reduce power consumption and improve high-speed operation (performance). Along with this miniaturization of semiconductors, the size of substances that affect semiconductor manufacturing yields has also become smaller every year. This has created a demand for ultrapure water for semiconductor cleaning, where the size and concentration of dissolved substances are controlled to be as small as possible. Furthermore, in recent years, there has been an increasing need for improved permeability in the ultrapure water production process in order to improve energy efficiency and reduce environmental impact. Therefore, porous membranes with high permeability and high particle removal performance for dissolved substances on the nanometer scale are required.

[0003] Patent Document 1 describes a porous porous membrane with pores of 1 to 10 nm on the inner and outer surfaces, and a water permeability of 15 m 3 / m 2 Patent Document 2 discloses a polysulfone-based resin hollow fiber membrane having a three-layer structure consisting of an inner surface skin layer, an outer surface skin layer, and a single void layer sandwiched between both surface skin layers, and a total membrane thickness of 100 μm or less. Patent Document 3 discloses a hollow fiber semipermeable membrane having dense layers on the inner and outer surfaces of the membrane, which has high reliability in fractionation even if a defect occurs in one of the dense layers. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 54-145379 [Patent Document 2] Japanese Patent Application Publication No. 58-132111 [Patent Document 3] Japanese Patent Application Publication No. 62-201602 Summary of the Invention [Problem to be solved by the invention]

[0005] The membrane performance disclosed in the example of Patent Document 1 is a water permeability of 15 m 3 / m 2 At 625LMH·atm·25℃, the rejection rate of dextran with a molecular weight of 40,000 was 24%, resulting in a poor balance between water permeability and rejection performance, and insufficient membrane performance. The performance of the membrane disclosed in the example of Patent Document 2 is 22m 3 / m 2 The membrane has a temperature of 917LMH·atm·25℃ and a cut-off molecular weight of 35,000, providing a good balance between water permeability and blocking performance. However, the membrane thickness is set to 100μm or less to improve the balance between water permeability and blocking performance, and the small outer diameter / inner diameter ratio means that the membrane has low mechanical strength, making it difficult to use for long periods of time. The membrane disclosed in Patent Document 3 has dense layers on the inner and outer surfaces in case a defect occurs in the dense layer on one surface, and the two dense layers combined provide high removal performance. However, in the case of a membrane that ensures removal performance with two layers, there is a problem in that removal performance is halved if a defect occurs in one layer on one side.

[0006] The problem to be solved by the present invention is to provide a porous hollow fiber membrane that has a good balance between water permeability and blocking performance, is durable enough to be repeatedly used over a long period of time, and is resistant to a decrease in removal performance due to defects. [Means for solving the problem]

[0007] As a result of extensive research, the present inventors have found that the above problems can be solved by providing a porous hollow fiber membrane with a specific configuration, and have thus completed the present invention.

[0008] That is, the present invention is as follows. [1] A porous hollow fiber membrane having a minimum pore size layer on either the inner or outer membrane surface, wherein the average local thickness of pores on the membrane surface having the minimum pore size layer is 4.0 nm or more and 10.0 nm or less, and the orientation strength of the membrane surface having the minimum pore size layer is 1.45 or more and 2.00 or less. [2] The porous hollow fiber membrane according to [1], having a surface pore size ratio represented by the following formula of 1.2 or more and 12.5 or less: Surface pore size ratio = (average local thickness diameter of pores on the surface of a membrane not having a minimum pore size layer) / (average local thickness diameter of pores on the surface of the membrane having a minimum pore size layer) [3] The porous hollow fiber membrane according to [1] or [2], wherein the average local thickness of the pores on the membrane surface not having the minimum pore size layer, of either the inner surface or the outer surface, is 50.0 nm or less. [4] The porous hollow fiber membrane according to any one of [1] to [3], wherein the membrane surface not having the minimum pore size layer, of either the inner surface or the outer surface, does not have pores with a Local Thickness diameter of 100.0 nm or more. [5] The porous hollow fiber membrane according to any one of [1] to [4], wherein the average local thickness diameter of pores on the membrane surface not having the minimum pore size layer, of either the inner surface or the outer surface, is 20.0 nm or less. [6] The porous hollow fiber membrane according to any one of [1] to [5], which has at least one macrovoid between the inner surface and the outer surface. [7] The porous hollow fiber membrane of any one of [1] to [6], wherein the membrane surface having the smallest pore size layer is the inner surface. [8] A porous hollow fiber membrane according to any one of [1] to [7], having a water permeability of 750 LMH or more. [9] A porous hollow fiber membrane according to any one of [1] to [8], having a dextran T40 rejection rate of 30.0% or more.

[10] The porous hollow fiber membrane according to any one of [1] to [9], having a compressive strength of 0.30 MPa or more.

[11] A porous hollow fiber membrane of any one of [1] to

[10] , whose main component is a polysulfone polymer.

[12] A porous hollow fiber membrane according to any one of [1] to

[11] , having an inner diameter of 0.400 mm or more.

[13] A porous hollow fiber membrane according to any one of [1] to

[12] , in which (outer diameter) / (inner diameter) is greater than 1,400.

[14] A porous hollow fiber membrane according to any one of [1] to

[13] , which is used for producing ultrapure water. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a porous hollow fiber membrane that has a good balance between water permeability and blocking performance, is durable enough to be used repeatedly over a long period of time, and is resistant to deterioration of removal performance due to external factors. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an SEM image (400x magnification) of the cross section of the membrane of Example 1, showing the outer surface, inner surface, and macrovoid layer of the membrane. [Figure 2] 1 is an SEM image (5000x magnification) of the cross section of the membrane of Example 1. [Figure 3] 1 is an SEM image (50,000x magnification) of the membrane surface (inner surface) having the minimum pore size layer of Example 1. [Figure 4] 1 is an SEM image (50,000x magnification) of the membrane surface (inner surface) having the minimum pore size layer of Comparative Example 1. [Figure 5]1 is an SEM image (50,000x magnification) of the membrane surface (outer surface) having the minimum pore size layer of Comparative Example 2. [Figure 6] 1 is a 512×512 pixel cropped image of the center portion of an SEM image (magnification: 50,000) of the membrane surface (inner surface) having the minimum pore size layer in Example 1. [Figure 7] This is a power spectrum image obtained by subjecting Figure 6 to fast Fourier transform processing. [Figure 8] This shows the method for measuring the average local thickness diameter. [Figure 9] The figure shows the structure of the double-tube cylindrical nozzle and the internal draw ratio. [Figure 10] 1 is a schematic diagram of a membrane production facility. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described. The present invention is not limited to the following embodiment, and various modifications can be made within the scope of the gist of the present invention.

[0012] [Porous hollow fiber membrane] The porous hollow fiber membrane of this embodiment has a minimum pore size layer on only one of the inner surface and the outer surface, and the average local thickness diameter of the pores on the membrane surface having the minimum pore size layer is 4.0 nm or more and 10.0 nm or less, and the orientation strength of the membrane surface having the minimum pore size layer is 1.45 or more and 2.00 or less. The membrane surface having the smaller average local thickness diameter is considered to be the membrane surface having the minimum pore size layer. By providing the smallest pore size layer on either the inner surface or the outer surface, it is possible to halve the possibility that the particle removal performance of the membrane will be reduced when a defect occurs. By setting the average local thickness diameter of the pores on the membrane surface having the smallest pore size layer to 4.0 nm or more and 10.0 nm or less, and setting the orientation strength of the membrane surface (preferably the inner surface) having the smallest pore size layer to 1.45 or more and 2.00 or less, a membrane with an excellent balance between water permeability and removal performance can be obtained. In this specification, the term "membrane cross section" may refer to a cross section of a porous hollow fiber membrane cut in a plane perpendicular to the axial direction. In addition, the term "porous hollow fiber membrane" may also be simply referred to as "hollow fiber membrane."

[0013] An excellent balance between water permeability and removal performance means a water permeability of 750 LMH or more and a dextran T40 rejection of 30.0% or more. A water permeability of 750 LMH or more makes it possible to efficiently produce ultrapure water for semiconductor manufacturing, and a dextran T40 rejection of 30.0% or more makes it possible to remove fine particles of a few nanometers in size. Membranes made using conventional technology have a poor balance between water permeability and dextran T40 rejection, and only membranes with a water permeability of 750 LMH or more but a dextran T40 rejection of less than 30.0% or a dextran T40 rejection of 30.0% or more but a water permeability of less than 750 LMH could be obtained. Since this enables efficient production of ultrapure water for semiconductor manufacturing, the water permeability is preferably 750 LMH or more, more preferably 800 LMH or more, and even more preferably 850 LMH or more. The water permeability can be measured by the method described in the Examples below. Since this improves the ability to remove fine particles of a few nanometers in size, the T40 rejection is preferably 30.0% or more, more preferably 35.0% or more, and even more preferably 40.0% or more. The dextran T40 rejection (sometimes referred to as "T40 rejection" in this specification) can be measured by the method described in the Examples below.

[0014] The Local Thickness diameter is the distribution of the diameter size of the largest circle that can be arranged in the pore portion. When there are circular holes and irregularly shaped holes with the same pore area, the Local Thickness diameter of the irregularly shaped holes will be smaller than the Local Thickness diameter of the circular holes (i.e., the diameter of the perfect circle), so only smaller particles can pass through the irregularly shaped holes, resulting in higher blocking performance. Since the blocking performance is higher as the Local Thickness diameter is smaller, it is preferably 10.0 nm or less, more preferably 9.0 nm or less, and even more preferably 8.0 nm or less. If the Local Thickness diameter is too small, the water permeability will decrease, so it is preferably 4.0 nm or more, more preferably 5.0 nm or more, and even more preferably 6.0 nm or more. The local thickness diameter can be adjusted, for example, by the inner draw ratio, the concentration of the core liquid, and the like.

[0015] The orientation strength was determined by the method described in "NONDESTRUCTIVE DETERMINATION OF FIBER ORIENTATION DISTRIBUTION OF PAPER SURFACE BY IMAGE ANALYSIS" (Nordic Pulp and Paper Research Journal, 21(2), 253-259(2006)) by Emae et al. That is, the method for determining paper fiber orientation by Fourier image analysis described in the above literature was applied. Scanning electron microscope (SEM) images of the inner surface of the membrane were Fourier transformed to create a power spectrum. The average amplitude (square root of power) of each point was calculated radially for a central angle of 0 to 180 degrees in this power spectrum, and an angular distribution diagram of the average amplitude was created. The major axis / minor axis ratio of an ellipse (D thin line) approximated from this diagram was used as the orientation strength. When comparing the permeability of circular pores with elliptical pores having a minor axis the same length as the radius of the circle, the particle size that can be captured is the same, but the elliptical pores have a larger area and a higher water permeability. A higher orientation strength provides a better balance between water permeability and removal performance, so the orientation strength is preferably 1.45 or more, and more preferably 1.50 or more. If the orientation strength is too high, there is a concern that the membrane structure will be excessively oriented and the membrane strength will decrease, so the orientation strength is preferably 2.00 or less. The orientation strength can be adjusted, for example, by the inner draw ratio.

[0016] The porous hollow fiber membrane of this embodiment preferably has a surface pore size ratio, represented by the following formula, of 1.2 or more. Surface pore size ratio = (average local thickness diameter of pores on the surface of a membrane without a minimum pore size layer) / (average local thickness diameter of pores on the surface of a membrane with a minimum pore size layer) This makes it possible to have the removal performance of the membrane mainly depend on the membrane surface having the minimum pore size layer, rather than having the removal performance ensured by both the inner and outer surfaces, and therefore prevents the removal performance of the membrane from being significantly impaired even if a defect occurs on the membrane surface that does not have the minimum pore size layer. Further, as an additional effect, the membrane surface without the smallest pore size layer is expected to serve as a preliminary capture layer. By setting the surface pore size ratio to 1.2 or more and performing filtration from the membrane surface without the smallest pore size layer toward the membrane surface with the smallest pore size layer, large particles can be captured on the membrane surface without the smallest pore size layer, and small particles can be captured on the membrane surface with the smallest pore size layer. If the surface pore size ratio is less than 1.2, a large number of fine particles will be captured on the membrane surface without the smallest pore size layer, which may cause clogging at the membrane surface without the smallest pore size layer, making it difficult to use the membrane for a long period of time. The surface pore size ratio is preferably 1.2 or more, more preferably 1.3 or more. If the surface pore size ratio is too large, all fine particles contained in the water will be captured on the membrane surface with the smallest pore size layer, which may cause clogging at the membrane surface with the smallest pore size layer, making it difficult to use the membrane for a long period of time. Therefore, the surface pore size ratio is preferably 12.5 or less, more preferably 5.0 or less, and even more preferably 2.0 or less.

[0017] In the porous hollow fiber membrane of this embodiment, the average local thickness diameter of the pores on either the inner or outer membrane surface, which does not have a minimum pore size layer, is preferably 50.0 nm or less. Ultrapure water for semiconductor cleaning is required to be free of fine particles of several nm to several tens of nm in size, and the quality of ultrapure water is controlled using a particle detector for 50.0 nm or 20.0 nm. If the average local thickness diameter of the pores on the membrane surface not having a minimum pore size layer is greater than 50.0 nm, all of the fine particles contained in the water will be captured on the membrane surface having the minimum pore size layer, and clogging of the membrane surface having the minimum pore size layer may make it difficult to use the membrane for a long period of time. By making the average local thickness diameter of the membrane surface that does not have a minimum pore size layer 50.0nm or less, by carrying out filtration from the membrane surface that does not have a minimum pore size layer to the membrane surface that has a minimum pore size, the fine particles of 50.0nm or more can be captured on the membrane surface that does not have a minimum pore size layer, and the fine particles of less than 50.0nm can be captured on the membrane surface that has a minimum pore size layer, so that the load on the membrane surface that has a minimum pore size layer is reduced and the membrane can be used for a long period of time.The average local thickness diameter of the membrane surface that does not have a minimum pore size layer is preferably 50.0nm or less, more preferably 20.0nm or less.

[0018] The porous hollow fiber membrane of this embodiment preferably does not have pores with a Local Thickness diameter of 100.0 nm or more on either the inner or outer membrane surface, which does not have a minimum pore size layer. If the membrane surface without a minimum pore size layer has pores with a Local Thickness diameter of 100.0 nm or more, large particles will penetrate into the membrane through the pores, and both large and small particles will be captured on the inner membrane surface, which may cause clogging on the membrane surface with the minimum pore size layer, making it difficult to use the membrane for a long period of time. In order to capture large particles on the membrane surface without a minimum pore size layer and small particles on the membrane surface with a minimum pore size layer, it is desirable that the membrane surface without a minimum pore size layer does not have pores with a Local Thickness diameter of 100.0 nm or more.

[0019] The porous hollow fiber membrane of this embodiment preferably has at least one macrovoid inside the membrane between the inner and outer surfaces. This reduces the filtration resistance inside the membrane and improves water permeability. As shown in Figure 1, a macrovoid refers to a large pore that extends over several micrometers or more in the membrane thickness direction (i.e., from the inner surface to the outer surface) and is discontinuous with the surrounding pores. In particular, it is more preferable for the porous hollow fiber membrane to have a macrovoid layer in which macrovoids are arranged in layers in the membrane cross section (Figure 1). The macrovoid layer refers to a layer in which a plurality of these macrovoids exist in the circumferential direction. The macrovoids are pores having a membrane length of 25 μm or more measured across the cross section of the porous hollow fiber membrane. Pores that communicate from the inner surface to the outer surface are not included in the macrovoids. The circumferential length of the macrovoids measured across the cross section of the porous hollow fiber membrane is not particularly limited, but may be 10 μm or more. Furthermore, the porous hollow fiber membrane of this embodiment may have a skin layer on at least the membrane surface having the smallest pore size layer out of the inner and outer surfaces.

[0020] In the porous hollow fiber membrane of this embodiment, the membrane surface having the smallest pore size layer is preferably the inner surface. In the manufacturing process of a hollow fiber membrane module, defects due to the inclusion of minute foreign matter in the membrane forming solution occur with equal probability on the inner and outer surfaces, but defects due to external factors such as contact are more likely to occur on the outer surface. If the smallest pore size layer is provided on the inner surface, the removal performance of the membrane will not be reduced even if defects due to external factors occur on the outer surface of the membrane. Therefore, it is preferable to provide the smallest pore size layer on the inner surface.

[0021] The porous hollow fiber membrane of this embodiment preferably has a compressive strength of 0.30 MPa or more. When the hollow fiber membrane is used for producing ultrapure water, it may be used for a long period of time at a filtration pressure of 0.30 MPa, so the compressive strength is preferably 0.30 MPa or more, and more preferably 0.60 MPa or more. The compressive strength can be measured by the method described in the Examples below.

[0022] The porous hollow fiber membrane of this embodiment preferably contains, as the polymer component constituting the membrane, for example, polyolefin, polyamide, polyimide, polyester, polyketone, polyvinylidene fluoride, polyhexafluoropropylene, polymethyl methacrylate, polyacrylonitrile, or a polysulfone-based polymer as the main component, and more preferably contains a polysulfone-based polymer as the main component. Here, "containing as the main component" means that the polymer component accounts for 50% by weight or more, calculated as the solid content. The content of the polymer component in the porous hollow fiber membrane is preferably 60% by weight or more, and more preferably 70% by weight or more. Although the following description focuses on polysulfone-based polymers, the polymer components for constituting the porous hollow fiber membrane of this embodiment are not limited to these, and are not particularly limited as long as the polymer components exhibit a certain degree of compatibility with the solvents described below (hereinafter, these may be referred to as good solvents) and can uniformly prepare a membrane-forming stock solution before membrane formation. The polymer components for constituting the porous membrane may be used alone or in combination of two or more types.

[0023] The polysulfone polymer is preferably polysulfone (PSf) having a repeating unit represented by the following formula (1) or polyethersulfone (PES) having a repeating unit represented by the following formula (2). Formula (1): [ka] Formula (2): [ka] The polysulfone polymer may contain a functional group or a substituent such as an alkyl group in the structure of formula (1) or formula (2), and the hydrogen atoms of the hydrocarbon skeleton may be substituted with other atoms or substituents such as halogens. The concentration of the polymer component (for example, the above-mentioned polysulfone polymer) in the membrane-forming solution is preferably 10 to 30% by weight, more preferably 15 to 25% by weight, from the viewpoints of membrane strength, permeability, and membrane formability.

[0024] The porous hollow fiber membrane of this embodiment preferably has an inner diameter of 0.400 mm or more. When filtration is performed using a hollow fiber membrane, pressure loss occurs when a liquid flows through the hollow portion. The greater the pressure loss in the hollow portion, the lower the filtration flow rate, which is undesirable. To reduce pressure loss, the inner diameter is preferably 0.400 mm or more, more preferably 0.500 mm or more, and even more preferably 0.550 mm or more.

[0025] The porous hollow fiber membrane of this embodiment preferably has an outer diameter / inner diameter ratio of greater than 1.400. The larger the outer diameter / inner diameter ratio, the greater the compressive strength. To achieve a compressive strength of 0.30 MPa or greater, the outer diameter / inner diameter ratio is preferably 1.400 or greater, and more preferably 1.600 or greater.

[0026] In the porous hollow fiber membrane of this embodiment, the minimum pore size layer is preferably present on either the outer surface or the inner surface, rather than inside the membrane.

[0027] The membrane thickness of the porous hollow fiber membrane of this embodiment is preferably 0.080 mm or more, and more preferably 0.100 mm or more.

[0028] (Application) The porous hollow fiber membrane of this embodiment can be suitably used for producing ultrapure water. The object of the present embodiment is to provide a porous hollow fiber membrane that has a good balance between water permeability and blocking performance in the process of producing ultrapure water for semiconductor cleaning, is durable enough to be used repeatedly over a long period of time, and is resistant to a decrease in removal performance due to defects.

[0029] The porous hollow fiber membrane of the present embodiment is preferably used for filtering from a membrane surface that does not have a minimum pore size layer toward a membrane surface that has a minimum pore size, and more preferably used for filtering from the outer surface toward the inner surface.

[0030] (Manufacturing method) A method for producing the porous hollow fiber membrane of this embodiment will be described. Examples of a method for producing the porous hollow fiber membrane of this embodiment include a method in which a membrane-forming solution is flowed into the outer nozzle (i.e., the annular slit portion) of a double annular nozzle, a core liquid is flowed into the inner nozzle (i.e., the center portion), and each is discharged from the double annular nozzle, and passed through an air gap, a coagulation bath, a water washing tank, etc. to form the membrane into a hollow fiber.

[0031] A wide variety of good solvents can be used for the membrane-forming solution, including N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide, ε-caprolactam, and 2-pyrrolidone (2P), as long as they are compatible with the polymer components that form the porous hollow fiber membrane. For polysulfone-based polymers, amide solvents such as NMP, DMF, and DMAc are preferred, with NMP being more preferred. As with the polymer components, good solvents that dissolve the polymer components that form the porous hollow fiber membrane may be used alone or in combination, as long as a uniform membrane-forming solution can be prepared prior to membrane formation.

[0032] It is preferable to add a desired amount of poor solvent to the membrane-forming solution. A poor solvent has a low ability to dissolve the polymer components that constitute the porous hollow fiber membrane, while a good solvent is a solvent that exhibits a certain degree of compatibility. Although the detailed mechanism is unknown, the viscosity and tolerance of the membrane-forming solution can be adjusted by changing the type and amount of poor solvent added. By controlling the phase separation rate and time, the pore size of the final porous hollow fiber membrane can be controlled. Here, the tolerance refers to the maximum amount of poor solvent added to the membrane-forming solution that allows the membrane-forming solution to exist as a single-phase homogeneous system without becoming cloudy. Examples of poor solvents used in the membrane-forming solution include water and polyalkylene glycol compounds, with polyalkylene glycol compounds being preferred. In this embodiment, the polyalkylene glycol used may be ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, glycerin, trimethylolpropane, polytetraethylene glycol, ethylene glycol methyl ether derivatives such as ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, and triethylene glycol monomethyl ether, or propylene glycol derivatives such as propylene glycol monomethyl ether. In this embodiment, a polyalkylene glycol with a molecular weight similar to that of tetraethylene glycol is preferred. The optimal addition ratio of polyalkylene glycol varies depending on the polymer concentration, the good solvent used, and the type of polyalkylene glycol, but is preferably 5 to 40 wt%, and more preferably 10 to 30 wt%, relative to 100 wt% of the membrane-forming solution. If the addition ratio exceeds 40 wt%, it becomes difficult for the membrane-forming solution to remain homogeneous, which may lead to reduced spinnability and structural defects after membrane formation.

[0033] The following describes an example using a polysulfone-based polymer as the polymer component. A uniform membrane-forming solution is prepared by mixing and dissolving a polysulfone-based polymer, a good solvent, and a poor solvent in a desired composition. The resulting membrane-forming solution is degassed and simultaneously discharged from the annular slit and center of a double-tube cylindrical spinneret (sometimes simply referred to as a spinneret) together with a core liquid (a mixture of one or more of the aforementioned good solvents and poor solvents used to form the inner surface structure of the hollow fiber membrane). The resulting solution passes through a temperature-controlled free-running section and is then introduced into a coagulation liquid (a mixture of one or more of the aforementioned good solvents and poor solvents used to form the hollow fiber membrane structure) to form the hollow fiber membrane. The formed hollow fiber membrane is then desolvated in a water washing tank to extract the good solvent and poor solvent. The membrane is then cut to an appropriate fiber length, heat-treated for heat fixation, treated with glycerin for dry storage, and then dried. The dried hollow fiber membrane is immersed in a solvent such as hot water or ethanol, which has a low surface tension and does not change the membrane structure of the porous hollow fiber membrane, and then the water is replaced to obtain a wet porous hollow fiber membrane with the pores filled with water again. Hereinafter, this series of operations for obtaining a wet porous hollow fiber membrane from a dry porous hollow fiber membrane is referred to as "hydrophilization treatment."

[0034] The membrane-forming solution is obtained by dissolving a polymer component, a good solvent, and a poor solvent at a constant temperature while stirring. The temperature is preferably higher than room temperature, 30 to 80°C. Compounds containing tertiary or lower nitrogen (NMP, DMF, DMAc, etc.) are oxidized in air, and heating facilitates further oxidation. Therefore, the membrane-forming solution is preferably prepared in an inert gas atmosphere or under reduced pressure. Examples of inert gases include nitrogen and argon, and nitrogen is preferred from the viewpoint of production costs. The reduced pressure may be 10 kPa or less.

[0035] It is preferable to degas the membrane-forming solution to prevent thread breakage during spinning and to suppress the formation of pinholes, which are structural defects. The degassing process can be performed as follows: The pressure in a tank containing the uniformly dissolved membrane-forming solution is reduced to 2 kPa and the solution is allowed to stand for 16 hours. To improve degassing efficiency, the solution may be stirred during degassing.

[0036] It is preferable to remove foreign matter from the membrane-forming solution before it is discharged from the spinneret. Removal of foreign matter can prevent thread breakage during spinning, suppress pinhole formation, and appropriately control the membrane structure. In order to prevent foreign matter from entering through the packing of the membrane-forming solution tank, it is preferable to install a filter before the membrane-forming solution is discharged from the spinneret.

[0037] In this embodiment, the composition of the core liquid used in forming the hollow fiber membrane is important for controlling the porous membrane structure, and it is preferably composed of the above-mentioned good solvent and poor solvent for the polymer component that forms the porous hollow fiber membrane. A mixture of multiple components may also be used. Generally, increasing the amount of good solvent in the core liquid has the effect of slowing the progress of coagulation of the polymer component and promoting the formation of a porous membrane structure through phase separation, thereby enlarging the pores formed. Increasing the amount of poor solvent has the effect of accelerating the progress of coagulation, thereby reducing the pores formed (it is also possible to control the local thickness diameter). To ensure the appropriate progress of coagulation and obtain a suitable porous membrane structure in this embodiment, the relative energy difference (RED) between the polymer component and the core liquid is preferably 2.5 or greater, and more preferably 2.8 or greater. Here, "relative energy difference (RED)" is the ratio of the interaction radius R0 when the smallest sphere containing a group of solvents with high solubility for the solute is formed to the distance Ra between the solute and the solvent, as expressed by equation (3). If the HSP value of the solvent is RED<1, it means that it is inside the sphere, indicating high solubility in the polymer component. On the other hand, if RED>1, it indicates low solubility in the polymer component. The RED value is used as an index for evaluating solubility. Formula (3): Relative Energy Difference (RED) = Ra / R0

[0038] (double-tube cylindrical nozzle) A double-tube cylindrical spinneret, for example, is used as a spinneret for discharging the membrane-forming solution for producing a porous hollow fiber membrane (Figure 9). In Figure 9, 1 represents the membrane-forming solution line of the double-tube cylindrical spinneret 3, and 2 represents the core liquid line. Also, 4 represents the porous hollow fiber membrane discharged from the spinneret 3. The top diagram in Figure 9 is an enlarged schematic diagram of the vicinity of the discharge port of the double-tube cylindrical spinneret 3, the middle diagram is an enlarged schematic diagram of the dotted line in the top diagram, and the bottom diagram is a schematic diagram of the cross section of the porous hollow fiber membrane 4 discharged from the spinneret 3. Also in Figure 9, the outer diameter of the annular slit, which is the membrane-forming solution line, is represented by a, the inner diameter of the annular slit by b, the diameter of the inner spinneret, which is the core liquid line, by c, and the inner diameter of the porous hollow fiber membrane by d. When the ratio of the inner diameter (b) of the annular slit at the outlet of the double-tube cylindrical nozzle to the inner diameter (d) of the porous hollow fiber membrane is defined as the inner draw ratio (b / d) (Equation (4)), in this embodiment, the inner draw ratio is preferably 1.05 or more. Formula (4): Inner draw ratio = (inner diameter of annular slit b (mm)) / (inner diameter of porous hollow fiber membrane d (mm)) For example, increasing the internal draw ratio means increasing the inner diameter of the annular slit while keeping the inner diameter of the porous hollow fiber membrane fixed, or decreasing the inner diameter of the porous hollow fiber membrane while keeping the inner diameter of the annular slit fixed. Increasing the internal draw ratio stretches the porous structure near the inner surface in the fiber length direction, resulting in the production of a porous hollow fiber membrane with high orientation strength. Furthermore, as the internal draw ratio increases, the pores become more stretched, which tends to result in a smaller local thickness diameter. To obtain a membrane with a pore size suitable for removing fine particles, it is important to appropriately set the composition of the core liquid and the internal draw ratio. To produce a porous hollow fiber membrane with a moderate orientation, the internal draw ratio is preferably 1.05 or more, more preferably 1.10 or more, and even more preferably 1.15 or more. It may also be 1.50 or less. The above-mentioned effect is not limited to the inside of the spinneret. It can also be achieved by flowing the coagulation liquid outside the membrane-forming solution, for example, by using a triple spinneret. Furthermore, in the case of flat membranes, it can also be achieved by adjusting the distance between the doctor blade and the roll that casts the membrane-forming solution and the roll rotation speed to bring the membrane-forming solution into contact with the coagulation liquid while the draw effect remains near the coagulation liquid contact surface. The draw effect is a hydrodynamic effect that occurs when fluids such as the membrane-forming solution and the core liquid flow through the flow path in the spinneret used to shape the porous hollow fiber membrane. Therefore, the draw effect can be achieved regardless of the composition of the membrane-forming solution, which affects pore formation through thermodynamic and kinetic effects. In other words, regardless of the type and combination of polymers, solvents, and additives that make up the membrane-forming solution, by preparing a uniform membrane-forming solution and selecting a coagulation liquid with appropriate coagulation properties for that membrane-forming solution, phase separation while the draw effect remains can be achieved, for example, using the above-mentioned method, thereby achieving the desired orientation strength and local thickness diameter.

[0039] 10 is a schematic diagram of an apparatus used to produce a porous hollow fiber membrane of this embodiment. A porous hollow fiber membrane 8 discharged from a double-pipe cylindrical spinneret 5 passes through an empty run section 6 and a coagulation liquid 7. In this embodiment, the term "free run section" refers to the gap between the double-tube cylindrical spinneret and the coagulation liquid. The distance and temperature of the free run section are important for controlling the porous hollow fiber membrane structure. To stabilize the temperature, good solvent concentration, and poor solvent concentration of the free run section, the free run section may be enclosed in a cylindrical tube or the like, a gas having a constant temperature and humidity may be supplied to the free run section, or the air in the free run section may be exhausted using an exhaust duct or the like. Increasing the distance of the free run section can lengthen the progression of phase separation in the free run section, thereby allowing for a larger pore size on the outer surface. Increasing the temperature of the free run section reduces the viscosity of the membrane-forming solution in the free run section, accelerating the progression of phase separation and allowing for a larger pore size on the outer surface. From the viewpoint of controlling the progression of phase separation in the free run section, the distance of the free run section is preferably 5 to 500 mm, more preferably 10 to 400 mm, and even more preferably 15 to 300 mm. The temperature of the free running portion may be 15 to 90°C.

[0040] In this embodiment, the composition and temperature of the coagulation liquid are important for controlling the porous hollow fiber membrane structure. The composition of the coagulation liquid is composed of the above-mentioned good solvent and poor solvent for the polymer component that forms the porous hollow fiber membrane. A mixture of multiple components may be used. In general, increasing the amount of good solvent in the coagulation liquid slows the progress of coagulation of the polymer component, promoting the formation of the porous hollow fiber membrane structure by phase separation and thereby enlarging the formed pores. Increasing the amount of poor solvent has the effect of accelerating the progress of coagulation and thereby reducing the formed pores. Because the coagulation liquid comes into contact with the outer surface of the hollow fiber, the composition of the coagulation liquid is particularly useful for controlling the pore size of the outer surface. The concentration of the poor solvent in the coagulation liquid is preferably 20% by mass or more and 100% by mass or less, more preferably 30% by mass or more and 100% by mass or less, and even more preferably 40% by mass or more and 100% by mass or less.

[0041] Increasing the temperature of the coagulation liquid makes it possible to increase the amounts of good solvent and poor solvent supplied from the coagulation liquid to the free-running section. The greater the amount of poor solvent in the free-running section, the faster phase separation occurs in the membrane-forming solution discharged from the spinneret, which has the same effect as increasing the free-running distance, making it possible to enlarge the pores on the outer surface. The temperature of the coagulation liquid is preferably 15 to 90°C, more preferably 20 to 85°C, and even more preferably 25 to 80°C. [Example]

[0042] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. The measurement methods used in these examples are as follows. All measurements below were carried out at 25°C unless otherwise specified.

[0043] [evaluation] The evaluation methods will be explained below, followed by a description of the manufacturing methods and evaluation results of the examples and comparative examples.

[0044] (1) Measurement of inner and outer diameters [mm] The hollow fiber membrane was thinly sliced ​​perpendicular to the longitudinal direction of the membrane using a razor or similar tool, and the major and minor inner diameters and major and minor outer diameters of the membrane cross section were measured using a microscope, for example, to determine the inner and outer diameters, respectively, using the following equations (5) and (6). The membrane thickness [mm] was determined using equation (7). Values ​​were rounded to three decimal places. Formula (5): Inner diameter [mm] = (inner major diameter [mm] + inner minor diameter [mm]) / 2 Formula (6): Outer diameter [mm]=(Outer major axis [mm]+Outer minor axis [mm]) / 2 Formula (7): Film thickness [mm] = (outer diameter [mm] - inner diameter [mm]) / 2

[0045] (2) Water permeability [LMH] A hollow fiber membrane approximately 10 cm long and wetted with water was prepared, one end of which was sealed, and a syringe needle was inserted into the hollow at the other end using a sealing needle. Pure water at 25°C was injected into the hollow through the syringe needle at a pressure of 0.1 MPa, and the amount of pure water permeating to the outer surface was measured, and the pure water permeability was determined using the following formula (8). Here, the effective membrane length refers to the net membrane length excluding the part where the syringe needle was inserted. Formula (8): Water permeation rate [LMH] = (60 [min / hr] x permeation rate [L]) / (π x inner diameter [m] x effective membrane length [m] x measurement time [min])

[0046] (3) Dextran T40 Rejection Rate [%] A 0.1% aqueous solution of dextran with an average molecular weight of 40,000 (Dextran T40, manufactured by Sigma-Aldrich) was prepared in purified water at 25°C. A hollow fiber membrane moistened with water was cut to a length of 20 cm, and the aqueous solution was supplied to the hollow fiber membrane at an average flow rate of 1 m / sec and an average filtration pressure of 0.075 MPa. One minute after the start of filtration was used as the membrane replacement time, and the filtrate was sampled for one minute starting one minute after the start of filtration. The dextran concentrations of the filtrate and the raw aqueous solution were measured using a digital refractometer DX-5000i-Plus manufactured by Atago Co., Ltd. The rejection rate R [%] was calculated using the formula (9) based on the dextran concentration Cp of the filtrate and the concentration Cf of the raw aqueous solution. The values ​​were rounded to one decimal point. Formula (9): Rejection rate R=100×(1-(Cp / Cf))

[0047] (4) Surface pore diameter measurement (Preparing the observation sample) A dry porous hollow fiber membrane was prepared. If the porous hollow fiber membrane was wet, it was freeze-dried to prepare a dry porous hollow fiber membrane. The porous hollow fiber membrane was fixed to a sample stage with carbon paste to prepare a sample for observation of the outer surface. The porous hollow fiber membrane was cut parallel to the fiber length using a razor, and the inner surface was exposed and fixed to the sample stage with carbon paste to prepare a sample for observation of the inner surface. Using an ion sputter (MC1000, Hitachi High-Tech Corporation), a platinum coating was applied under conditions of a discharge current of 15 mA, a sample distance of 30 mm, a discharge time of 30 seconds, and a vacuum of approximately 7 Pa to prepare the observation sample. (Surface SEM observation) The specimen was irradiated with an electron beam under the following conditions, and the contrast was adjusted automatically within the field of view for photography. Under the above conditions, SEM images of the inner and outer surfaces of the hollow fiber porous membrane were obtained. However, these SEM images are focused on a smooth cross section and do not contain distortion of the resin skeleton structure due to charge-up, abnormal contrast, or obvious foreign matter adhering to the porous hollow fiber membrane surface. [Observation conditions] SEM: Hitachi High-Technologies Corporation SU8000 scanning electron microscope Acceleration voltage: 1 kV Working distance: 1.5 mm Observation magnification: 50,000 times (device display magnification) Detector: SE detector (secondary electron image) Image size: 860 x 1280 pixels (Measuring the average local thickness diameter of the membrane surface) ImageJ was used for image analysis. The SEM images used for image analysis were previously trimmed to remove labels that would interfere with image analysis, such as scale bars and magnification information. First, calibration was performed using Analyze-Set Scale. Then, filtering was performed using Plugins-Bilateral Filter (spatial radius = 3, range radius = 50). The filtered SEM image was subjected to threshold processing (Image-Adjust-Threshold: maximum entropy method (MaxEntropy selected) to binarize the pores and polymer skeleton. The resulting binarized image (the part corresponding to the polymer skeleton is a black image with brightness 0, and the part corresponding to the pores is a white image with brightness 255) was subjected to Analyze-LocalThickness-LocalThickness (masked, calibrated, silent) to obtain a LocalThickness image. The obtained LocalThickness image was subjected to Analyze-Histgram to obtain the Local Thickness diameter [nm] of the pores observed in the observation field. The average value of the LocalThickness diameter of each pore was taken as the average Local Thickness diameter [nm] at the SEM image acquisition site. This operation was performed in the same way for any five fields of view on the inner and outer surfaces present in each of the six equally divided regions in the circumferential direction of the porous hollow fiber membrane, and the average value of five fields of view in each of the six regions (30 fields in total) was used as the average Local Thickness diameter of each surface. The average Local Thickness diameter of the inner or outer surface of the porous hollow fiber membrane was determined to be the minimum pore size layer. The average Local Thickness diameter of the inner or outer surface was determined to be the smaller of the two. In the present invention, the shape of the pores in the porous hollow fiber membrane is not necessarily a perfect circle. The Local Thickness diameter evaluates the distribution of the diameter size of the largest circle that can be arranged in the pore portion, and therefore it is thought that the particle size that can be blocked can be directly evaluated in terms of the blocking performance of the membrane. For example, particles with a size larger than the Local Thickness diameter cannot pass through the pore and are removed.

[0048] (Inner surface porosity measurement, outer surface porosity measurement) The ratio [%] of the area occupied by the portions corresponding to the pores in the binarized image obtained by the above method was taken as the pore ratio of each surface. Up to the third decimal place is valid.

[0049] (5) Observation of membrane cross-sectional structure A wet porous hollow fiber membrane was prepared and immersed in liquid nitrogen to freeze it. If the porous hollow fiber membrane was dry, it could be wetted by impregnating it with a liquid such as ethanol. The membrane was then cleaved in the transverse direction while still frozen and dried in a vacuum dryer to obtain a membrane cross-section for SEM observation. The sample was fixed to a commercially available SEM sample stage using conductive carbon tape so that the cleaved surface was the observation surface. The fixed membrane cross-section observation sample was coated with platinum using an ion sputter (MC1000, Hitachi High-Tech Corporation) under conditions of a discharge current of 15 mA, a sample distance of 30 mm, a discharge time of 40 seconds, and a vacuum of approximately 7 Pa. This gave the observation sample. (SEM observation of film cross section) An electron beam was irradiated onto the observation sample under the conditions below, and the contrast was adjusted automatically within the field of view to be photographed. Cross-sectional SEM images of porous hollow fiber membranes were obtained under the conditions below. The observation magnification for determining the presence or absence of a macrovoid layer in a porous hollow fiber membrane can be determined so that the entire membrane thickness is within the field of view. The observation magnification for determining the location of the smallest pore size layer in a porous hollow fiber membrane is determined so that a region of a specified thickness is continuously cut out in the membrane thickness direction, and a pore size profile can be obtained in the membrane thickness direction. However, these SEM images are images in focus on a smooth cross section, and do not contain distortion of the resin skeleton structure due to charge-up or abnormal contrast. [Observation conditions] SEM: Hitachi High-Technologies Corporation SU8000 scanning electron microscope Accelerating voltage: 3 kV Working distance: 8.0 mm Observation magnification: 400x 1000 times 5000 times (Confirmation of macrovoid layer) The above-mentioned SEM image of the cross section of the film at 400 magnifications was visually inspected to see if a macrovoid layer existed inside the film. (Confirmation of the smallest pore size layer) The 5000x magnification SEM images of the membrane cross sections were visually inspected to determine whether the minimum pore size layer was present on the inner or outer surface of the membrane. For membranes in which the minimum pore size layer was present on either the inner or outer surface, the average local thickness diameter of the membrane surface was measured, and the surface with the smaller average local thickness diameter was determined to be the surface with the minimum pore size layer. (Distance from outer surface to tip of macrovoid) In five fields of view of the cross-sectional SEM image (1000x magnification) obtained above, the distance from the outer surface to the tip of the macrovoid was measured at five locations per field of view, and the arithmetic mean of these measurements was defined as the distance from the outer surface to the tip of the macrovoid. Here, the tip of the macrovoid refers to the part of each macrovoid observed near the outer surface of the hollow fiber membrane that is closest to the outer surface. The value was rounded to four decimal places.

[0050] (6) Orientation strength measurement The orientation strength was determined by analyzing the orientation in the following steps (A) to (D). (A) 50,000-magnification SEM images of the membrane surface having the minimum pore size layer, each showing the center part of five fields of view. n ×2 n It was cropped to the size of a pixel. (B)2 n ×2 n Each power spectrum image was obtained by Fourier transforming each of the surface SEM images cut into pixels. (C) From each of the power spectrum images, approximate ellipses were calculated for the angular distribution diagram of each average amplitude. (D) The following orientation intensity was determined based on each of the approximate ellipses, and the average value of five fields of view was taken as the orientation intensity of the membrane surface having the smallest pore size layer. Orientation strength: Ratio of major axis length to minor axis length in an approximate ellipse The orientation analysis method using the steps (A) to (D) above is described in a document by Toshiharu Enmae et al. ("Method for determining fiber orientation in paper using Fourier image analysis," Abstracts of Research Presentations at the 26th Annual Meeting of the Japanese Society for the Preservation and Restoration of Cultural Properties, pp. 44-45, 2004), and can be performed using the method described in that document. However, the following provides a supplementary explanation of the steps (A) to (D) above. In the step (A), a fast Fourier transform (FFT) process is performed on the image, and the central part of each of five fields of view of a 50,000-magnification SEM image of the membrane surface having the smallest pore size layer is divided into two parts. n ×2 n Cut out to pixel. In the step (B), 2 n ×2 n A Fourier transform was performed on each surface SEM image cut into pixels. This Fourier transform can be performed using a fast Fourier transform (FFT). In the power spectrum image obtained by this Fourier transform, spots appear at locations corresponding to the wavenumber and direction of the periodic structure. In step (C), the angular distribution map of the mean amplitude (power spectrum pattern) is obtained by displaying each power spectrum image obtained in step (B) in polar coordinates. Since the angular distribution map of the mean amplitude appears as a shape approximating an ellipse, in step (C), an approximation ellipse of each angular distribution map of the mean amplitude is obtained. In step (D), based on each approximate ellipse (major axis length, minor axis length, angle in the minor axis direction) obtained in step (C), the angle of the minor axis direction of the approximate ellipse with respect to the positive direction of the X axis was calculated as the orientation angle (°), and the ratio of the major axis length / minor axis length in the approximate ellipse was calculated as the orientation strength, and the average value of five fields of view was taken as the orientation strength of the membrane surface having the smallest pore size layer. The orientation analysis according to steps (A) to (D) can be performed using known image analysis software. For example, by using "ImageJ (V.1.54j)" (https: / / imagej.net / ij / ) and "Non-destructive Paper Surface Fiber Orientation Analysis Program FiberOri8single03.exe (V.8.03)" (downloadable from http: / / www.enomae.com / FiberOri / index.htm), the orientation analysis according to steps (A) to (D) can be easily performed and the orientation strength can be determined. Below, we will explain the procedure for determining the orientation strength using "ImageJ (V.1.54j)" and "Non-destructive Paper Surface Fiber Orientation Analysis Program FiberOri8single03.exe (V.8.03)". First, a 50,000x magnification SEM image of the membrane surface containing the smallest pore size layer of a porous hollow fiber membrane is imported into "ImageJ." The central portion of the image is then cut out to 512 x 512 pixels. The 512 x 512 pixel cutout surface SEM image is then imported into "Non-destructive Paper Surface Fiber Orientation Analysis Program FiberOri8single03.exe." A fast Fourier transform (FFT) is then performed on the surface SEM image to create a power spectrum image. Next, the orientation intensity is calculated based on the power spectrum image, and the orientation intensity value is output. The value is rounded to two decimal places. The orientation due to the internal draw ratio is thought to be expressed not only in the pores but also in the resin part of the film. The orientation strength measurement method of the present application evaluates the orientation of the film including not only the pores but also the resin part of the film, and is therefore thought to evaluate the orientation of the film more directly than methods that evaluate the aspect ratio of only the pores.

[0051] (7) Compression resistance A porous hollow fiber membrane approximately 10 cm long and wetted with water was prepared and subjected to external pressure filtration at 40°C. First, the filtration pressure was increased to 0.10 MPa, and the amount of pure water passing through the inner surface of the porous hollow fiber membrane was measured for 30 seconds. Next, the pressure was increased at 0.10 MPa intervals and the amount of water permeated was measured for 30 seconds repeatedly until the pressure reached 0.30 MPa. The amount of water permeated was measured in the same manner as above at each measurement pressure. After 0.30 MPa, the pressure was increased at 0.05 MPa intervals and the amount of water permeated was measured for 30 seconds repeatedly, until the measurement pressure reached 0.95 MPa. The measurement pressure at which the amount of water permeated was the highest was taken as the compressive strength of the porous hollow fiber membrane. The value was rounded to two decimal places.

[0052] (8) Porosity of the entire membrane The porosity of the entire membrane can be determined by equation (10). The value is rounded to three decimal places. Equation (10): Porosity [%] = 100 × ((wet weight [g] - dry membrane weight [g])) / water specific gravity [g / cm 3 ])×(1 / membrane volume [cm 3 ]) Here, a wet membrane refers to a membrane in which the pores of the porous hollow fiber membrane are filled with pure water, but the hollow portion is empty. Specifically, a 10-20 cm porous hollow fiber membrane is prepared, and if the membrane is dry, the above-mentioned hydrophilization treatment is performed to obtain a wet membrane. Then, one end of the wet porous hollow fiber membrane is held by hand and shaken vigorously about five times, and then the other end is held by hand and shaken vigorously about five times again to remove the water from the hollow portion, thereby obtaining a wet membrane. A dry membrane can be obtained by measuring the weight of the wet membrane and then drying it in an oven, for example, at 60°C, until it reaches a constant weight. The membrane volume can be determined by equation (11). Equation (11): Membrane volume [cm 3 ]=(π / 4)×(outer diameter 2 [cm 2 ]-inner diameter 2 [cm 2 ])×Membrane length [cm] If the weight of a single membrane is too small and the weight measurement error becomes large, multiple membranes can be used.

[0053] Example 1 17 wt% polysulfone (Udel P-3500, manufactured by Solvay), 25 wt% tetraethylene glycol (manufactured by Nippon Shokubai), and 58 wt% NMP (manufactured by Mitsubishi Chemical) were dissolved at 65°C for 16 hours to obtain a homogeneous membrane-forming solution. This membrane-forming solution was extruded through the annular slit of a double-tube cylindrical spinneret maintained at 60°C. Simultaneously, a core solution consisting of 30 wt% tetraethylene glycol and 70 wt% water was extruded through the inner spinneret. The annular slit had an outer diameter of 1.30 mm, an inner diameter of 0.70 mm, and an inner spinneret diameter of 0.50 mm. The membrane-forming solution extruded through the annular slit passed through a 50-mm idle section set at 50°C and then introduced into a 43°C water coagulation bath for desolventization and solidification. It then passed through a 65°C water wash bath and was wound onto a take-up roll at 20 m / min. The internal draw ratio was 1.08, and the relative energy difference (RED) was 3.47. The membrane was cut into 23 cm pieces longitudinally, immersed in room temperature water for 16 hours, and then subjected to hot water treatment at 90°C for 3 hours. Subsequently, the membrane was treated in a glycerin solution consisting of 30% by weight of glycerin and 70% by weight of water at 90°C for 3 hours, and then dried at 70°C for 16 hours to obtain a glycerin-treated porous hollow fiber membrane. The obtained hollow fiber membrane was immersed in ethanol for 2 hours and then washed with water to obtain a hollow fiber porous membrane with an inner diameter of 0.649 mm and an outer diameter of 1.002 mm. The performance of the obtained membrane is shown in Table 1.

[0054] Example 2 A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the core liquid was a solution consisting of 45% by weight of tetraethylene glycol and 55% by weight of water, the coagulation liquid temperature was set to 35°C, the internal draw ratio was set to 1.15, and the fiber diameters of the obtained hollow fibers were set to an inner diameter of 0.606 mm and an outer diameter of 0.998 mm.

[0055] (Comparative Example 1) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the core liquid was water, the inner diameter of the annular slit of the double-tube cylindrical spinneret was 0.60 mm, the internal draw ratio was 1.01, and the free-running section was 43°C, resulting in a hollow fiber with an inner diameter of 0.596 mm and an outer diameter of 0.976 mm. Because lowering the internal draw ratio increases the pore size on the inner surface, we attempted to reduce the pore size on the inner surface by using water as the core liquid. However, there was a poor correlation between the water permeation rate and the dextran T40 rejection, and the dextran T40 rejection was less than 30%.

[0056] (Comparative Example 2) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the core liquid was a solution consisting of 45% by weight of tetraethylene glycol and 55% by weight of water, the temperatures of the free running section and the coagulation liquid were set to 35°C, the inner diameter of the annular slit of the double-tube cylindrical spinneret was set to 0.50 mm, the inner draw ratio was set to 0.84, and the fiber diameters of the obtained hollow fiber were set to an inner diameter of 0.596 mm and an outer diameter of 0.997 mm.

[0057] (Comparative Example 3) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the composition of the membrane-forming solution was 19 wt% polysulfone (Udel P-3500 manufactured by Solvay), 20 wt% tetraethylene glycol (manufactured by Nippon Shokubai Co., Ltd.), and 61 wt% NMP (manufactured by Mitsubishi Chemical Corporation), the core liquid was water, the temperature of the double-tube cylindrical spinneret was 40°C, the free-running distance was 290 mm, the free-running section temperature and the temperature of the coagulation liquid were 30°C, the inner diameter of the annular slit in the double-tube cylindrical spinneret was 0.50 mm, and the inner draw ratio was 0.83, resulting in an inner diameter of 0.604 mm and an outer diameter of 1.005 mm.

[0058] [Table 1] [Industrial Applicability]

[0059] The porous hollow fiber membrane of the present invention has industrial applicability in that it can be suitably used in industrial fields such as semiconductor manufacturing.

Claims

1. A porous hollow fiber membrane having a minimum pore size layer on either the inner or outer membrane surface, wherein the average local thickness of pores on the membrane surface having the minimum pore size layer is 4.0 nm or more and 10.0 nm or less, and the orientation strength of the membrane surface having the minimum pore size layer is 1.45 or more and 2.00 or less.

2. 2. The porous hollow fiber membrane according to claim 1, wherein the surface pore size ratio represented by the following formula is 1.2 or more and 12.5 or less. Surface pore size ratio = (average local thickness diameter of pores on the surface of a membrane not having a minimum pore size layer) / (average local thickness diameter of pores on the surface of the membrane having a minimum pore size layer)

3. 2. The porous hollow fiber membrane according to claim 1, wherein the average local thickness of pores on the membrane surface not having the minimum pore size layer, of either the inner surface or the outer surface, is 50.0 nm or less.

4. 2. The porous hollow fiber membrane according to claim 1, wherein the membrane surface not having the minimum pore size layer, of either the inner surface or the outer surface, does not have pores with a local thickness diameter of 100.0 nm or more.

5. 2. The porous hollow fiber membrane according to claim 1, wherein the average local thickness of pores on the membrane surface not having the minimum pore size layer, of either the inner surface or the outer surface, is 20.0 nm or less.

6. 2. The porous hollow fiber membrane of claim 1, having at least one macrovoid between the inner surface and the outer surface.

7. 2. The porous hollow fiber membrane according to claim 1, wherein the membrane surface having the smallest pore size layer is the inner surface.

8. 2. The porous hollow fiber membrane according to claim 1, having a water permeability of 750 LMH or more.

9. 2. The porous hollow fiber membrane according to claim 1, having a dextran T40 rejection rate of 30.0% or more.

10. The porous hollow fiber membrane according to claim 1, having a compressive strength of 0.30 MPa or more.

11. The porous hollow fiber membrane according to claim 1, which is mainly composed of a polysulfone-based polymer.

12. 2. The porous hollow fiber membrane according to claim 1, wherein the inner diameter is 0.400 mm or more.

13. 2. The porous hollow fiber membrane according to claim 1, wherein the ratio of outer diameter to inner diameter is greater than 1.

400.

14. The porous hollow fiber membrane according to claim 1, which is used for producing ultrapure water.

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