Porous hollow fiber membrane and method for producing 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. [Solution] The porous hollow fiber membrane of the present invention is characterized in that, when the inner or outer surface having the smaller local thickness average diameter of the pores is defined as one surface, at a position 5.0 μm in the membrane thickness direction from the one surface, Yt is the tortuosity in the membrane thickness direction and Zt is the tortuosity in the fiber axis direction, Yt / Zt is 1.1 or more.

Description

[Technical Field]

[0001] The present invention relates to a porous hollow fiber membrane and a method for producing the 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 flat porous membrane with a specified tortuosity in three dimensions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 54-145379 [Patent Document 2] Patent No. 7103715 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°C (625LMH·atm·25°C), the rejection rate of dextran with a molecular weight of 40,000 was 24%, and the water permeation rate, which is particularly important for membrane performance, was low. Furthermore, the balance between water permeation rate and rejection performance was poor, resulting in insufficient membrane performance. Patent Document 2 discloses a porous membrane that achieves similar values ​​for tortuosity in the X, Y, and Z directions, and focuses on obtaining an isotropic porous membrane. Obtaining an isotropic porous membrane is common practice and does not suggest the present invention. Furthermore, the tortuosity is adjusted by stretching, which is a different technique from the control of porous membrane structure during the phase separation process of the present application.

[0006] 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. [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 characterized in that, when one of the inner and outer surfaces has a smaller local thickness average diameter of pores as one surface, at a position 5.0 μm in the film thickness direction from the one surface, Yt is the tortuosity in the film thickness direction, and Zt is the tortuosity in the fiber axis direction, Yt / Zt is 1.1 or more. [2] The porous hollow fiber membrane according to [1], wherein the Yt / Zt is 2.0 or more. [3] The porous hollow fiber membrane according to [1] or [2], wherein the local thickness average diameter of the pores on the one surface is 10 nm or less. [4] The porous hollow fiber membrane according to any one of [1] to [3], wherein the local thickness average diameter of pores on the other surface, which is different from the one of the inner surface and the outer surface, is 12 nm or more. [5] The porous hollow fiber membrane according to any one of [1] to [4], wherein the ratio of the Local Thickness average diameter of pores on the other surface, which is one of the inner surface or the outer surface and is different from the one surface, to the Local Thickness average diameter of pores on the one surface (Local Thickness average diameter of pores on the other surface / Local Thickness average diameter of pores on the one surface) is 1.75 or more. [6] The porous hollow fiber membrane according to any one of [1] to [5], wherein the distance from the outer surface to the tip of the macrovoid is 12 μm or more. [7] The porous hollow fiber membrane according to any one of [1] to [6], wherein, at the position 5.0 μm from the one surface in the membrane thickness direction, Yt / Xt is 1.3 or more, where Xt is the circumferential tortuosity. [8] The porous hollow fiber membrane according to [7], wherein the Yt / Xt is 2.0 or more. [9] The porous hollow fiber membrane according to any one of [1] to [8], wherein the Yt is 4.5 or more.

[10] The porous hollow fiber membrane according to any one of [1] to [9], wherein the Yt is 6.2 or more.

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

[10] , wherein the porosity at the position 5.0 μm from the one surface in the membrane thickness direction is 40% or more.

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

[11] , wherein the volume ratio of isolated pores at the position 5.0 μm from the one surface in the membrane thickness direction is 0.10% or less.

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

[12] , wherein the porosity of the outer surface is 5.00% or more.

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

[13] , which contains at least one macrovoid.

[15] The porous hollow fiber membrane according to any one of [1] to

[14] , which is mainly composed of polysulfone or polyethersulfone.

[16] The porous hollow fiber membrane according to any one of [1] to

[15] , which is used for producing ultrapure water.

[17] A method for producing a porous hollow fiber membrane, characterized in that the ratio of the draw ratio to the relative energy difference on one surface side is 0.25 or more.

[18] The method for producing a porous hollow fiber membrane according to

[17] , wherein the draw ratio of the one surface side is 1.10 or more.

[19] The method for producing a porous hollow fiber membrane according to

[17] or

[18] , wherein the relative energy difference on the one surface side is 4.0 or less.

[20] The ratio of free-running humidity to free-running time is 200g / m 3 The method for producing a porous hollow fiber membrane according to any one of

[17] to

[19] , wherein the membrane has a flow rate of 1 / sec or more. [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. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a diagram illustrating the XYZ directions in a porous hollow fiber membrane. [Figure 2] FIG. 2 is a diagram illustrating the outer surface and inner surface of a porous hollow fiber membrane, the distance from the outer surface to the tip of a macrovoid, and the location of the macrovoid layer. [Figure 3] 1 is an SEM image (400x magnification) of the cross section of the membrane of Example 1. [Figure 4] 1 is a cross-sectional SEM image (10,000x) of the vicinity of one surface of Example 1. [Figure 5]1 is a cross-sectional SEM image (10,000x) of the vicinity of one surface of Example 2. [Figure 6] 1 is an example of a manufacturing apparatus for manufacturing a porous hollow fiber membrane. [Figure 7] FIG. 1 is a simplified diagram of a spinneret discharge surface. [Figure 8] FIG. 1 is a diagram illustrating a method for calculating a local thickness average diameter. [Figure 9] 1 is a three-dimensional image of Example 1 taken at a position 5.0 μm from one surface. [Figure 10] 10 is a three-dimensional image of Example 2 taken at a position 5.0 μm from one surface. [Figure 11] 1 is an image that serves as a basis for image analysis for calculating the curvature ratio in Example 1. 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] In the porous hollow fiber membrane of this embodiment, when the surface having the smaller local thickness average diameter of the pores on the inner or outer surface is defined as one surface, at a position 5.0 μm in the membrane thickness direction from the one surface, Yt / Zt is 1.1 or more, where Yt is the tortuosity in the membrane thickness direction and Zt is the tortuosity in the fiber axis direction. The Yt / Zt is preferably 2.0 or more, and may be 10.0 or less. The porous hollow fiber membrane of this embodiment has an inner surface and an outer surface ( FIG. 2 ). Here, the inner surface and the outer surface may refer to the inner surface and the outer surface, respectively, in a cross section of the porous hollow fiber membrane cut along a plane perpendicular to the fiber axis direction (sometimes referred to herein as a “membrane cross section”). In this specification, the inner or outer surface having a smaller Local Thickness average diameter of pores is referred to as “one surface,” and the inner or outer surface different from the one surface is referred to as “the other surface.” In the porous hollow fiber membrane of this embodiment, the circumferential direction is the X-axis, the radial direction and membrane thickness direction are the Y-axis, and the fiber axis direction is the Z-axis (Fig. 1). Specifically, in a cylindrical (i.e., tubular) hollow fiber membrane, the extension direction of the hollow tubes surrounded by the inner surface is the fiber axis direction (i.e., the Z-axis direction), the direction perpendicular to the fiber axis direction from the inner surface to the outer surface or from the outer surface to the inner surface is the membrane thickness direction (i.e., the radial direction, Y-axis direction), and the circumferential direction of the hollow fiber membrane in a cross section perpendicular to the Z-axis of the hollow fiber membrane (i.e., membrane cross section) and perpendicular to the Z-axis and Y-axis is the circumferential direction (i.e., the X-axis direction).

[0013] In the porous hollow fiber membrane of the present embodiment, the local thickness average diameter of the pores on the one surface is preferably smaller than the local thickness average diameter of the pores on the other surface.

[0014] In the porous hollow fiber membrane of this embodiment, at a position 5.0 μm from the one surface in the membrane thickness direction, Yt / Zt is preferably 1.1 or more, where Yt is the tortuosity in the membrane thickness direction and Zt is the tortuosity in the fiber axis direction. During filtration through a porous hollow fiber membrane, the liquid to be filtered flows primarily in the Y-axis direction. If Yt / Zt is 1.1 or more, the tortuosity in the Y-axis direction is relatively higher than that in the Z-axis direction, thereby improving the capture of the substances to be filtered. In a porous hollow fiber membrane having a small pore size, i.e., a membrane structure having a gradient structure in which the pore size increases in the membrane thickness direction from the surface where the dense layer is present, a high Yt / Zt at a position 5.0 μm from the surface has little effect on the water permeation resistance, resulting in a high level of balance between the capture performance of the substances to be filtered, i.e., the blocking performance and the water permeability. For example, a gradient structure is a structure in which the cross-sectional pore size increases in the membrane thickness direction after equally dividing the relevant portion from the surface. Here, the pore size in the dense layer is approximately 30 nm or less, preferably 2.0 nm or more. The position from the surface having a structure in which Yt / Zt is 1.1 or more is preferably a position having a pore size 10 times or more the surface pore size. If the pore size is 10 times or more, it is sufficiently small compared to the water permeation resistance at the surface pore size, and the impact on water permeability is small. Furthermore, when a macrovoid layer is present at the position of 5.0 μm, the tortuosity of the pore structure portion between the macrovoids contributes to the blocking performance, so it is preferable that the tortuosity of this portion is within the above range.

[0015] The tortuosity represents the degree of detour of the path of the liquid to be filtered through the porous hollow fiber membrane and is defined as the ratio of the detour distance along the path to the linear distance between the endpoints of the path. In a porous hollow fiber membrane, it generally represents the ratio of the length of the flow path penetrating from one surface to the opposite surface to the membrane thickness. For example, the tortuosity at a position 5.0 μm in the membrane thickness direction from either the inner or outer surface is calculated by analyzing a portion of the porous hollow fiber membrane and calculating the tortuosity of the analyzed portion. For example, the tortuosity in the Y-axis direction can be calculated from the Y-axis length of the analyzed portion and the Y-axis component length of the path the liquid to be filtered passes through the analyzed portion.

[0016] At a position 5.0 μm from the one surface in the film thickness direction, when the tortuosity in the circumferential direction is Xt, if Yt / Xt is 1.3 or more, the tortuosity in the Y-axis direction is relatively higher than that in the X-axis direction, and the capture ability of the filtered substance can be improved. It is preferably 2.0 or more and may be 10.0 or less.

[0017] In the porous hollow fiber membrane of this embodiment, the tortuosity Yt is preferably 4.5 or more. If it is 4.5 or more, the distance that the substance to be filtered passes through the membrane becomes longer, and there is a higher possibility that the substance will be adsorbed to the core of the polymer constituting the porous hollow fiber membrane or trapped in the fine pores, thereby achieving higher blocking performance. It is preferably 6.2 or more, and may be 10.0 or less.

[0018] In the case of filtration using a porous hollow fiber membrane, the above-mentioned effects of Yt / Zt and Yt / Xt can be obtained both in the case of external pressure filtration, in which filtration is performed from the outer surface side to the inner surface side, and in the case of internal pressure filtration, in which filtration is performed from the inner surface side to the outer surface side. Furthermore, the porous membrane may have the above structure not only on one surface but also on the other surface, or may have the above structure on both surfaces.

[0019] In the porous hollow fiber membrane of this embodiment, the local thickness average diameter of the pores on the one surface is preferably 10 nm or less. Here, the Local Thickness diameter refers to the distribution of the diameter size of the largest circle that can be arranged in the pore portion. When there are perfectly circular holes and irregularly shaped holes with the same pore area, the Local Thickness diameter of the irregularly shaped holes is smaller than the Local Thickness diameter (=diameter of the perfect circle) of the perfectly circular holes, so that only smaller particles can pass through the irregularly shaped holes, resulting in higher blocking performance. A diameter of 10 nm or less can exhibit high blocking performance, making it suitable for use in producing ultrapure water in semiconductor manufacturing, where it is necessary to block fine impurities. A diameter of 9.0 nm or less is preferable.

[0020] In the porous hollow fiber membrane of this embodiment, the local thickness average diameter of the pores on the other surface, which is the surface different from the one of the inner surface and the outer surface, is preferably 12 nm or more. If it is 12 nm or more, the water permeation resistance on the large pore diameter side can be reduced, thereby improving the water permeability. It is preferably 13 nm or more, and may be 50 nm or less.

[0021] In the porous hollow fiber membrane of this embodiment, the ratio of the Local Thickness average diameter of the pores on the one surface to the Local Thickness average diameter of the pores on the other surface (Local Thickness average diameter of pores on the other surface / Local Thickness average diameter of pores on one surface) is preferably 1.75 or more. Alternatively, it may be 5.0 or less. When the ratio is 1.75 or more, a gradient occurs in the cross-sectional direction of the porous hollow fiber membrane, so that the necessary blocking performance is exerted on one surface, and by reducing the water permeation resistance on the other surface, the balance between blocking performance and water permeability can be improved.

[0022] The porous hollow fiber membrane of this embodiment preferably has at least one macrovoid, more preferably a macrovoid layer, inside the membrane between the inner and outer surfaces, which reduces the filtration resistance inside the membrane and improves water permeability. The porous hollow fiber membrane of this embodiment preferably has a dense layer on at least the membrane surface having the smallest pore size layer of either the inner or outer surface, and has at least one macrovoid layer inside the membrane. The membrane surface having the smallest pore size layer refers to the inner or outer surface with the smallest local thickness average diameter of the pores. As shown in Figure 2, macrovoids refer to large pores that extend over several micrometers in the membrane thickness direction and are discontinuous with the surrounding pores. A macrovoid layer refers to a layer in which multiple macrovoids exist in the circumferential direction. 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 2). A macrovoid layer refers to a layer in which multiple 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.

[0023] When observed from the cross section of a porous hollow fiber membrane, it often has a conical or cylindrical shape (Figure 2). This reduces the filtration resistance inside the membrane and makes it possible to improve water permeability. It is even more preferable for it to have two or more macrovoid layers. By having two or more macrovoid layers and an intermediate layer between the macrovoid layers, it is possible to have a trapping layer not only on the dense layer on the surface but also inside the membrane, making it possible to capture and block substances to be filtered in multiple stages.

[0024] The distance from the outer surface of the porous hollow fiber membrane of this embodiment to the tip of the macrovoid is preferably 12 μm or more. When the distance from the outer surface to the tip of the macrovoid is 12 μm or more, the membrane has sufficient strength against external pressure during external pressure filtration and suppresses a decrease in water permeability due to compression of the distance from the outer surface to the tip of the macrovoid. Preferably, the distance is 16 μm or more. The distance from the inner surface and the outer surface to the tip of the macrovoid is preferably more than 5 μm, and more preferably 10 μm or more. The distance from the outer surface to the tip of the macrovoid may be the distance of the shortest line segment parallel to the film thickness direction connecting the outermost end point of the macrovoid to a point on the outer surface (Figure 2).

[0025] The porous hollow fiber membrane of this embodiment preferably has a water permeation rate of 80% or more during external pressure filtration at 0.50 MPa compared to the water permeation rate during external pressure filtration at 0.10 MPa. To achieve a water permeation rate retention rate of 80% or more, the thickness of the outer surface (distance from the outer surface to the tip of the macrovoid) is preferably 12 μm or more. When the porous membrane has a hollow fiber membrane structure, external pressure filtration, which can ensure a large filtration area, is mainly used. Furthermore, in the production of ultrapure water for semiconductors, which requires the suppression of the generation of fine impurities, impurities may also be generated from the container containing the porous hollow fiber membrane. Therefore, an external pressure filtration method in which the filtrate passes through the inner surface of the porous hollow fiber membrane is desirable to minimize contact with the container. In the production of ultrapure water for semiconductors, filtration is often performed at a pressure difference of approximately 0.20 to 0.40 MPa. To maintain a stable and constant water permeation rate over the long term, it is desirable to be able to suppress a decrease in water permeation rate even at high pressures. Therefore, according to the present invention, it is possible to obtain not only a porous hollow fiber membrane with a good balance between water permeability and blocking performance, but also a porous hollow fiber membrane with durability that allows stable use even under high pressure load for a long period of time, more preferably 85% or more.

[0026] In conventional technology, it has been impossible to achieve a high level of balance between the water permeation rate retention, water permeability, and blocking performance. In other words, in order to increase the water permeation rate retention, the distance from the outer surface to the tip of the macrovoid must be increased, resulting in a decrease in water permeability. Furthermore, in order to increase the decreased water permeability, the pore size must be increased, which results in a decrease in blocking performance. The porous hollow fiber membrane of this embodiment is an unprecedented porous membrane that achieves a high level of balance between the water permeation rate retention, water permeability, and blocking performance by controlling the tortuosity at a position 5.0 μm from one surface in the membrane thickness direction.

[0027] In the porous hollow fiber membrane of this embodiment, the porosity at the position 5.0 μm from the one surface in the membrane thickness direction is preferably 40% or more. When the porosity is 40% or more, high water permeability is achieved. Preferably, the porosity is 47% or more.

[0028] In the porous hollow fiber membrane of this embodiment, the volume ratio of isolated pores at the position 5.0 μm from the one surface in the membrane thickness direction is preferably 0.10% or less. If it is 0.10% or less, it suggests that there are few independent pores and that pores are highly connected, resulting in high water permeability. Preferably, it is 0.098% or less.

[0029] In the porous hollow fiber membrane of this embodiment, the pore size at the position 5.0 μm from the one surface in the membrane thickness direction is preferably 100 nm or more. A pore size of 100 nm or more is sufficiently larger than the pores on one surface of the porous hollow fiber membrane of the present application. Therefore, the effect of a high Yt / Zt ratio at this position on the water permeation resistance is small, resulting in a high level of balance between the ability to capture filtered substances, i.e., blocking ability, and water permeability.

[0030] The porous hollow fiber membrane of this embodiment preferably has a pure water permeability of 820 LMH or more and a dextran T40 rejection of 5.0% or more. A pure water permeability of 820 LMH or more enables efficient production of ultrapure water for semiconductor manufacturing, and a dextran T40 rejection of 5.0% or more enables reliable removal of fine particles of several nanometers in size. Inherently, there is a trade-off between water permeability and rejection in porous membranes, and overcoming this trade-off is extremely difficult. Porous membranes produced by conventional technology have a poor balance between water permeability and dextran T40 rejection, resulting in membranes with a pure water permeability of 820 LMH or more but a dextran T40 rejection of less than 5.0%, or a dextran T40 rejection of 5.0% or more but a pure water permeability of less than 820 LMH. Because this enables efficient production of ultrapure water for semiconductor manufacturing, a pure water permeability of 820 LMH or more is preferred, with 820 LMH or more being more preferred, and 850 LMH or more being even more preferred. Since this improves the ability to remove fine particles of a few nanometers in size, the T40 rejection rate is preferably 5.0% or more, more preferably 30.0% or more, and even more preferably 40.0% or more.

[0031] In the porous hollow fiber membrane of this embodiment, the porosity on the outer surface side is preferably 5.00% or more. If it is 5.00% or more, the load of the material to be filtered per pore can be reduced, and a decrease in water permeability due to clogging can be suppressed. It is preferably 8.00% or more and 20.0% or less, and more preferably 10.0% or more and 15.0% or less.

[0032] The porosity on the inner surface side is preferably 0.200% or more. If it is 0.400% or more, the load of the material to be filtered per hole can be reduced, and a decrease in water permeability due to clogging can be suppressed. It is preferably 0.500% or more and 10.0% or less, and more preferably 0.500% or more and 7.00% or less.

[0033] The porous hollow fiber membrane of this embodiment preferably has an inner diameter of 0.30 mm or more. When filtration is performed using a porous 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 0.30 mm or more, and more preferably 0.40 mm or more.

[0034] The porous hollow fiber membrane of this embodiment preferably has a membrane thickness of 0.10 mm or more and 1.0 mm or less. A membrane thickness of 0.10 mm or more provides practical strength, while a membrane thickness of 1.0 mm or less reduces the resistance of the liquid to be filtered when passing through the membrane, allowing high water permeability to be achieved. A membrane thickness of 0.15 mm or more and 0.60 mm or less is preferred.

[0035] The porous hollow fiber membrane of this embodiment preferably has an outer diameter of 0.50 mm or more and 5.0 mm or less. If it is 0.50 mm or more, the membrane area on the side of the liquid to be filtered during external pressure filtration can be increased. If it is 5.0 mm or less, it is possible to store more porous hollow fiber membranes in a container that stores the porous hollow fiber membranes. Preferably, it is 0.60 mm or more and 2.0 mm or less.

[0036] The porous hollow fiber membrane of this embodiment contains, as the polymer component constituting the membrane, for example, a polyolefin such as polyethylene or polypropylene, a fluororesin such as polyvinylidene fluoride, an ethylene-vinyl alcohol copolymer, polyamide, polyetherimide, polystyrene, polyketone, a polysulfone-based material such as polysulfone or polyethersulfone, polyvinyl alcohol, polyphenylene ether, polyphenylene sulfide, cellulose acetate, polyacrylonitrile, or the like as a main component. Among these, it is preferable that the resin contains polysulfone and / or polyethersulfone as the main component, and it is more preferable that the resin contained in the porous hollow fiber membrane is only polysulfone and / or polyethersulfone.

[0037] Here, "contains as a main component" means that the polymer component accounts for 50% by weight or more in terms of 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.

[0038] Although the following description focuses on polysulfone, the polymer component for constituting the porous hollow fiber membrane of this embodiment is not limited to this, and is not particularly limited as long as the polymer component exhibits a certain degree of compatibility with the solvent described below (hereinafter, sometimes referred to as a good solvent) and can uniformly prepare a membrane-forming stock solution before membrane formation. The polymer component for constituting the porous hollow fiber membrane may be used alone or in combination of two or more types.

[0039] The polysulfone may be a polysulfone (PSf) having a repeating unit represented by the following formula (1), or a polyethersulfone (PES) having a repeating unit represented by the following formula (2). Formula (1): [ka] Formula (2): [ka]

[0040] The polysulfone 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.

[0041] The concentration of the polymer component (e.g., polysulfone and / or polyethersulfone) in the membrane-forming solution is preferably 10% by weight to 50% by weight, more preferably 15% by weight to 35% by weight, and even more preferably 15% by weight to 30% by weight, from the viewpoints of membrane strength, permeability, and membrane formability.

[0042] The weight average molecular weight (Mw) of the polysulfone is not particularly limited, but is preferably from 10,000 to 500,000, and more preferably from 30,000 to 300,000. In addition, polysulfones are not limited to those with a single molecular weight, and polysulfones with different molecular weights may be mixed. In this embodiment, the weight average molecular weight (Mw) can be measured by gel permeation chromatography (GPC) using a standard resin with a known molecular weight as a reference.

[0043] (Application) The porous hollow fiber membrane of this embodiment can be suitably used for producing ultrapure water. The porous hollow fiber membrane of this embodiment provides a porous hollow fiber membrane that has a good balance between water permeability and blocking performance and is durable enough to be used under high pressure in the process of producing ultrapure water for semiconductor washing. 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.

[0044] (Manufacturing method) In the method for producing a porous hollow fiber membrane of this embodiment, the ratio of the draw ratio to the relative energy difference on one surface side is 0.25 or more. The production method is preferably the production method of this embodiment described above.

[0045] 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.

[0046] The good solvent used in the membrane-forming solution can be a wide variety of solvents, such as 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. However, in the case of polysulfone-based polymers, amide solvents such as NMP, DMF, and DMAc are preferred, with NMP being more preferred. Furthermore, 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 before membrane formation.

[0047] It is preferable to add a desired amount of poor solvent or non-solvent to the membrane-forming solution. A poor solvent or non-solvent has a low ability to dissolve the polymer components required to form the porous hollow fiber membrane, while a good solvent 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 the poor solvent or non-solvent. 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 or non-solvent added to the membrane-forming solution that allows the membrane-forming solution to exist as a homogeneous, single-phase system without becoming cloudy. Examples of poor solvents or non-solvents used in the membrane-forming solution include water, polyalkylene glycol compounds, alcohols such as ethanol, glycerin, polyvinylpyrrolidone, etc., and examples of inorganic compounds include calcium chloride, magnesium chloride, lithium chloride, barium sulfate, etc. In this embodiment, polyalkylene glycol compounds are preferred, and examples of polyalkylene glycols that can be used include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, glycerin, trimethylolpropane, polytetraethylene glycol, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, and ethylene glycol methyl ether derivatives such as triethylene glycol monomethyl ether, and propylene glycol derivatives such as propylene glycol monomethyl ether. In this embodiment, those with a molecular weight similar to that of tetraethylene glycol are 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% by weight to 40% by weight, and more preferably 10% by weight to 30% by weight, relative to 100% by weight of the membrane-forming solution. If the content exceeds 40% by weight, it becomes difficult for the film-forming solution to exist as a homogeneous system, which may lead to a decrease in spinnability due to a decrease in spinnability and structural defects after film formation.

[0048] The following describes an example in which polysulfone is used as the polymer component. A uniform membrane-forming solution is prepared by mixing and dissolving polysulfone, a good solvent, and a poor solvent in a desired composition. A non-solvent may be added as needed. 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 an internal coagulation liquid (a mixture of one or more of the above-mentioned good solvents, poor solvents, or non-solvents used to form the inner surface structure of the hollow fiber membrane; sometimes referred to as the "core liquid"). The resulting solution passes through an idling section, the temperature and humidity of which are controlled by, for example, controlling the temperature of a surrounding tube such as a cylindrical container, and then introduced into the coagulation liquid (a mixture of one or more of the above-mentioned good solvents, poor solvents, or non-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, poor solvent, or non-solvent, and 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 with low surface tension, such as hot water or ethanol, which does not change the membrane structure of the porous membrane, and then water is replaced to obtain a wet porous membrane with water filling the pores of the porous membrane again. Hereinafter, this series of operations to obtain a wet porous membrane from this dry porous membrane is referred to as "hydrophilization treatment."

[0049] The membrane-forming solution is obtained by dissolving a polymer component, a good solvent, a poor solvent, or a non-solvent at a constant temperature while stirring. The temperature is preferably higher than room temperature, 30°C or higher and 80°C or lower. 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 lower.

[0050] 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.

[0051] 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.

[0052] In this embodiment, the composition of the internal coagulation liquid (i.e., core liquid) used when forming the hollow fiber membrane is important for controlling the porous membrane structure. It is preferable that the liquid be composed of the above-mentioned good solvent and the above-mentioned poor solvent or nonsolvent 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 internal coagulation liquid slows the coagulation of the polymer component, promoting the formation of a porous membrane structure through phase separation and thereby enlarging the pores formed. Increasing the amount of poor solvent or nonsolvent accelerates the coagulation process, thereby reducing the pore size (control of the local thickness average diameter is also possible). A poor solvent and a nonsolvent may also be mixed. Water is commonly used as the nonsolvent. When water is mixed with a good solvent or a poor solvent, the concentration of the good solvent or poor solvent is preferably 20 wt% or more and 80 wt% or less. A concentration of 20 wt% or more moderately slows the coagulation process, enabling desirable pore formation. A concentration of 80 wt% or less can prevent excessive pore enlargement and ensure blocking performance. The content is preferably 20 wt% or more and 70 wt% or less.

[0053] A double-tube cylindrical spinneret, for example, is used as a spinneret for discharging a membrane-forming solution for producing a hollow fiber-shaped porous membrane (Figure 7). In Figure 7, 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 7 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 7, 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 is represented by A, the diameter of the inner spinneret, which is the core liquid line, is represented by a, the outer diameter of the porous hollow fiber membrane is represented by B', and the inner diameter of the porous hollow fiber membrane is represented by B. When the ratio of the inner diameter (A) of the annular slit at the outlet of the double-tube cylindrical nozzle to the inner diameter (B) of the porous hollow fiber membrane is defined as the inner draw ratio (A / B) (Equation (3)), in this embodiment, the inner draw ratio is preferably 1.05 or more. Formula (3): Inner draw ratio = (inner diameter A of annular slit (mm)) / (inner diameter B of porous hollow fiber membrane (mm))

[0054] Furthermore, when the ratio of the outer diameter (A') of the annular slit to the outer diameter (B') of the porous hollow fiber membrane is defined as the outer draw ratio (A' / B') (Equation (4)), in this embodiment, the outer draw ratio is preferably 1.05 or more. Formula (4): Outer draw ratio = (outer diameter of annular slit A' (mm)) / (outer diameter of porous hollow fiber membrane B' (mm))

[0055] 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 allows the porous structure near the inner surface to be elongated in the fiber length direction, resulting in the production of an oriented porous hollow fiber membrane. As the internal draw ratio increases, the pores become more elongated, and the local thickness average diameter tends to decrease. To produce a membrane with a pore size suitable for removing fine particles, it is important to appropriately set the composition of the internal coagulation liquid and the internal draw ratio. To produce a porous 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. Unlike the draft ratio, which is a common parameter in spinning, the draw ratio is a parameter that can control the pore structure near the surface of the porous membrane. It may also be 1.50 or less.

[0056] 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, and the desired local thickness average diameter can be obtained.

[0057] 6 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 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 or nonsolvent concentration in the free run section, the free run section may be enclosed in a cylindrical tube, a gas with 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. 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 perspective of controlling the progression of phase separation in the free run section, the passage time through the free run section (free run time) is preferably 0.10 seconds to 3.0 seconds, more preferably 0.10 seconds to 2.5 seconds, and even more preferably 0.12 seconds to 1.5 seconds. The temperature of the free running portion may be 15 to 90°C.

[0058] The humidity in the free running area is 11g / m 3 More than 90g / m 3 The greater the amount of poor solvent or non-solvent in the free-running section, i.e., the higher the humidity, the earlier the phase separation of the membrane-forming solution discharged from the spinneret occurs, making it possible to enlarge the pores on the outer surface. 3 If the thickness is less than this, excessive phase separation can be prevented and the pore size can be adjusted to an appropriate size.

[0059] In the method for producing a porous hollow fiber membrane of this embodiment, the ratio of free running humidity to free running time (free running humidity / free running time) is 200 g / m 3 / sec or more 1000g / m 3 / sec or less is preferable. 3 / sec or more, the distance from the outer surface to the tip of the macrovoid can be increased. The detailed mechanism is unknown, but when the ratio of free-running humidity to free-running time is high, moisture absorption into the film-forming solution per unit time is efficient, promoting coagulation, while at the same time strengthening the entanglement of the polymer and slowing down the rate of disentanglement. Since macrovoids are originally formed by the growth of the polymer dilute phase toward the inside of the film thickness, the growth of the dilute phase is delayed, and moisture absorption progresses during this time, forming nuclei for phase separation, forming numerous holes, and increasing the distance from the outer surface to the tip of the macrovoid. 1000g / m 3 If the flow rate is less than 350 g / m, excessive phase separation can be suppressed and the pore size can be adjusted to an appropriate value. 3 / sec or more 800g / m 3 / sec or less, more preferably 400g / m 3 / sec or more 600g / m 3 / sec or less.

[0060] In the method for producing a porous membrane of this embodiment, the relative energy difference (RED) between the polymer component on one surface and the liquid having coagulation ability is preferably 4.0 or less, more preferably 3.6 or less. Here, the liquid having coagulation ability refers to the internal coagulation liquid when one surface is the inner surface of the porous hollow fiber membrane, and refers to the vapor component scattered in the free-running section when the other surface is the outer surface. The vapor component scattered in the free-running section refers to water vapor or solvent vapor. When the RED is 4.0 or less, the porous hollow fiber membrane of this embodiment can have a tortuosity due to the effect of the draw ratio described below, with coagulation appropriately delayed.

[0061] 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

[0062] In the method for producing a porous hollow fiber membrane of this embodiment, the ratio of the draw ratio on one surface side to RED is preferably 0.25 or more. If it is 0.25 or more, the polymer forming the porous membrane is in a moderately coagulated state, and the pores are elongated in the fiber length direction, thereby increasing the ratio of the tortuosity Yt in the membrane thickness direction to the tortuosity Zt in the fiber axis direction, although the detailed mechanism is unknown. A value of 0.30 or more is desirable.

[0063] In this embodiment, the composition and temperature of the coagulation liquid are important for controlling the porous membrane structure. The composition of the coagulation liquid preferably comprises the above-mentioned good solvent and the above-mentioned poor solvent or non-solvent for the polymer component that forms the porous membrane. A mixture of multiple components may also be used. Generally, increasing the amount of good solvent in the coagulation liquid slows the progress of coagulation of the polymer component, promoting the formation of a porous membrane structure through phase separation and thereby enlarging the pores formed. Increasing the amount of poor solvent or non-solvent has the effect of accelerating the progress of coagulation and thereby reducing the pores formed. 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 or non-solvent in the coagulation liquid is preferably 20% to 100%, more preferably 30% to 100%, and even more preferably 40% to 100%.

[0064] The temperature of the coagulation liquid is preferably 15°C or higher and 90°C or lower, more preferably 20°C or higher and 85°C or lower, and even more preferably 25°C or higher and 85°C or lower. The coagulation liquid is not limited to a single-stage bath, and may be installed in multiple stages of two or more stages. When multiple stages of baths are used, the type of coagulation liquid is not limited to one type. [Example]

[0065] 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.

[0066] The evaluation methods will be explained below, followed by a description of the manufacturing methods and evaluation results of the examples and comparative examples.

[0067] (1) 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, for example, a microscope, and the inner and outer diameters were determined using the following equations (5) and (6), respectively. The membrane thickness [mm] was determined using equation (7). The average values ​​of n = 10 were used as the inner and outer diameter values, with two significant digits. 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

[0068] (2) Pure water permeability [LMH] A hollow fiber membrane approximately 10 cm long and wetted with water was prepared, one end sealed, and a syringe needle 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. The amount of pure water permeating to the outer surface was measured, and the pure water permeability was determined using the following equation (8). Here, the effective membrane length refers to the net membrane length excluding the portion where the syringe needle was inserted. The pure water permeability under each condition was calculated as the average of n=4. Formula (8): Pure water permeability [LMH] = (60 [min / hr] x permeate volume [L]) / (π x inner diameter [m] x effective membrane length [m] x measurement time [min])

[0069] (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))

[0070] (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 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 (Local Thickness average diameter measurement of membrane surface) Image analysis was performed using ImageJ. 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 images were then subjected to threshold processing (Image-Adjust-Threshold: maximum entropy method (MaxEntropy selected) to binarize the pore and polymer skeleton regions. The resulting binarized image (the part corresponding to the polymer skeleton is a black image with brightness 0, and the part corresponding to the pore region is a white image with brightness 255) was analyzed using Analyze-LocalThickness-LocalThickness (masked, calibrated, Silent) was applied to obtain a Local Thickness image. Analyze-Histgram was applied to the obtained Local Thickness image to obtain the Local Thickness diameter [nm] of the pores observed in the observation field. The average value of the Local Thickness diameter of each pore was taken as the Local Thickness average diameter [nm] at the SEM image acquisition site. This procedure 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 hollow fiber porous membrane, and the average value of five fields of view in each of the six regions (30 fields of view in total) was taken as the Local Thickness average diameter of each surface. The value was calculated to the first decimal place. In the present invention, the shape of the pores present in the porous hollow fiber membrane is not necessarily a perfect circle. Since the Local Thickness diameter evaluates the distribution of the diameter size of the largest circle that can be arranged in the pore portion, it is thought that it can directly evaluate the particle size that can be blocked in terms of the membrane's blocking performance. For example, particles with a size larger than the Local Thickness diameter cannot pass through the pore and are removed. Figure 8 shows an outline of the method for calculating the Local Thickness average diameter.

[0071] (5) Surface pore diameter measurement (Inner surface porosity measurement, outer surface porosity measurement) The area occupied by the portions corresponding to the pores in the binarized image obtained by the above method was taken as the porosity of each surface. Up to the third decimal place is valid.

[0072] (6) 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 was then used as an 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, 1000x, 10000x (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. (Distance from the outer surface to the tip of the macrovoid) In the 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 points per field of view, and the arithmetic mean value 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 closest to the outer surface of each macrovoid observed near the outer surface side of the hollow fiber membrane.

[0073] (7) Permeability retention rate at external pressure of 0.50 MPa (compressive strength) 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 for 30 seconds was measured repeatedly until the pressure reached 0.30 MPa. The amount of water permeated was measured at each measurement pressure in the same manner as above. After 0.30 MPa, the pressure was increased at 0.05 MPa intervals and the amount of water permeated for 30 seconds was measured repeatedly, and the amount of water permeated was measured sequentially up to a measurement pressure of 0.95 MPa. The permeate volume measured at each measurement pressure was determined from the external pressure filtration permeate volume using equation (10). The external pressure flux retention at 0.50 MPa was calculated using equation (11). Equation (10): External pressure filtration permeability [LMH] = (60 [min / hr] x permeate volume [L]) / (π x inner diameter [m] x effective membrane length [m] x measurement time [min] x measurement pressure [MPa] x 10 [bar / MPa]) Equation (11): Flux retention at 0.50 MPa = (external pressure filtration permeability [LMH] at 0.50 MPa) / (external pressure filtration permeability [LMH] at 0.10 MPa) × 100

[0074] (8) Calculation method for tortuosity, pore size, porosity, and volume ratio of isolated pores To fit the tip of a dry hollow fiber membrane within the observation field, a razor was used to trim the membrane to a thickness of 100 μm square from the surface of interest. High-resolution imaging X-ray CT measurements were then performed at SPring-8 (BL24-XU). The experimental setup was based on the following paper (Kagoshima, Y., Aoki, S., Kakuchi, M., Maezawa, H., & Ando, ​​M. (1990) Image formation of periodic objects illuminated by undulator radiation in a zone plate microscope. Jpn. J. Appl. Phys, 29, L172.). The measurement conditions were: photon energy 8 keV, pixel size 30 nm / pixel, image size 2048 pixels × 2048 pixels, exposure time 0.8 sec, defocus 0.5 mm, and 400 projection images. The filtered back projection method was used to calculate 3D reconstructions from the CT dataset. In addition, for noise reduction using sparse modeling, 1200 2D projection images were denoised using a total variation filter. Specifically, the weight was fixed at 0.001 in denoise-chambolle, a denoising process of the TotalVariation filter in scikit-image, an open source image processing library for the Python programming language. How to extract the analysis area The center of the cube was 5 μm from the edge of the sample (corresponding to the surface of the porous hollow fiber membrane) in the membrane thickness direction, and a cube with a side length of 160 voxels (4.8 μm) was extracted with N=1 from a location where the influence of X-ray scattering was not present, and used as the analysis location. ·Binarization conditions The CT reconstruction images were binarized using Otsu's method with ImageJ. Specifically, two-dimensional images were extracted from a three-dimensional image of a cube with a side length of 160 voxels, and each image was binarized using Otsu's method. - Calculation method for pore size, tortuosity, porosity, and volume ratio of isolated pores To calculate the pore size, the three-dimensional image was binarized using Otsu's method, and the distribution of the average value of the maximum diameter of spheres that can fit into three-dimensional space was calculated using LocalThickness in BoneJ, a plug-in software for ImageJ. The porosity was calculated by dividing the number of pore voxels in the three-dimensional image binarized by Otsu's method by the total number of voxels (a cube with one side of 160 voxels). The volume ratio of isolated pores was calculated by using the ImageJ plugin ParticleAnalyser to calculate the number of voxels of isolated pores in the resin (after binarization, white is the resin and black is the pore, and "isolated" means that the pore, or black part, is surrounded by resin, or white), on a 3D image binarized by Otsu's method, and dividing this by the total number of voxels (a cube with one side measuring 160 voxels). For reference, Figure 11 shows an image extracted from a 3D image with isolated pores removed. To calculate the tortuosity, a three-dimensional image excluding isolated pores in the resin was subjected to expansion and contraction processing using Morpholib J, an ImageJ plug-in software, with the opening processing performed on a sphere with a radius of 2 voxels and the closing processing on a sphere with a radius of 2 voxels. The tortuosity in the circumferential direction, film thickness direction, and fiber axis direction was calculated for the image using simulations.tortuosity_fd in the Python module porespy.

[0075] 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 by heating 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, the temperature of which was controlled at 60°C. At the same time, an internal coagulation solution consisting of 30 wt% tetraethylene glycol and 70 wt% water was extruded through the inner spinneret. The spinneret used 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 coagulation solution of water at 43°C for desolvation and solidification. The idle humidity during this process was 64 g / m. 3 The membrane was then passed through a water washing bath at 65°C and wound onto a take-up roll at 20 m / min. The draw ratio / RED on one surface side was 0.34. The membrane was cut into 23 cm pieces in the longitudinal direction, 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 wt% glycerin and 70 wt% 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 0.60 mm and an outer diameter of 1.0 mm. The performance of the obtained membrane is shown in Table 1. In Example 1, the surface with the smaller average local thickness of the pores (i.e., one of the surfaces) was the inner surface.

[0076] Example 2 The internal coagulation liquid was 45% by weight of tetraethylene glycol and 55% by weight of water, and a spinneret with an annular slit outer diameter of 1.30 mm, an inner diameter of 0.50 mm, and an inner spinneret diameter of 0.40 mm was used. The idle temperature was 35°C and the idle humidity was 30 g / m 3 A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the coagulation liquid temperature was 35° C. In Example 2, the surface having the smaller Local thickness average diameter of the pores (i.e., one of the surfaces) out of the inner surface and the outer surface was the outer surface.

[0077] (Comparative Example 1) The internal coagulation liquid was water, and the spinneret had an outer diameter of 1.30 mm, an inner diameter of 0.60 mm, and an inner diameter of 0.40 mm. The idle temperature was 43°C and the idle humidity was 51 g / m 3 A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that in Comparative Example 1, the surface having a smaller Local thickness average diameter of pores (i.e., one of the surfaces) out of the inner surface and the outer surface was the inner surface.

[0078] (Comparative Example 2) The composition of the membrane-forming solution was 19% by weight of polysulfone (Udel P-3500 manufactured by Solvay), 20% by weight of tetraethylene glycol (manufactured by Nippon Shokubai), and 61% by weight of NMP (manufactured by Mitsubishi Chemical). The internal coagulation liquid was water. The annular slit had an outer diameter of 1.30 mm, an inner diameter of 0.50 mm, and an inner diameter of 0.40 mm. The solution temperature was 40°C, the idle time was 0.87 seconds, the idle temperature was 30°C, and the idle humidity was 25 g / m. 3 A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the coagulation liquid temperature was set to 30° C. In Comparative Example 2, the surface with the smaller Local thickness average diameter of the pores (i.e., one of the surfaces) out of the inner surface and the outer surface was the outer surface.

[0079] [Table 1] [Industrial Applicability]

[0080] The porous film 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 characterized in that, when one of the inner and outer surfaces has a smaller pore local thickness average diameter, at a position 5.0 μm in the film thickness direction from the one surface, Yt is the tortuosity in the film thickness direction and Zt is the tortuosity in the fiber axis direction, Yt / Zt is 1.1 or more.

2. The porous hollow fiber membrane according to claim 1, wherein the Yt / Zt is 2.0 or more.

3. 2. The porous hollow fiber membrane according to claim 1, wherein the local thickness average diameter of the pores on said one surface is 10 nm or less.

4. 2. The porous hollow fiber membrane according to claim 1, wherein the other surface of the inner surface or the outer surface, which is different from the one surface, has a local thickness average diameter of 12 nm or more.

5. The porous hollow fiber membrane according to claim 1, wherein the ratio of the local thickness average diameter of the pores on the other surface, which is a surface different from the one surface of the inner surface or the outer surface, to the local thickness average diameter of the pores on the one surface (local thickness average diameter of the pores on the other surface / local thickness average diameter of the pores on the one surface) is 1.75 or more.

6. 2. The porous hollow fiber membrane according to claim 1, wherein the distance from the outer surface to the tip of the macrovoid is 12 μm or more.

7. 2. The porous hollow fiber membrane according to claim 1, wherein, at the position 5.0 μm from the one surface in the membrane thickness direction, Yt / Xt is 1.3 or more, where Xt is a circumferential tortuosity.

8. The porous hollow fiber membrane according to claim 7, wherein the Yt / Xt is 2.0 or more.

9. The porous hollow fiber membrane according to claim 1, wherein the Yt is 4.5 or more.

10. The porous hollow fiber membrane according to claim 1, wherein the Yt is 6.2 or more.

11. 2. The porous hollow fiber membrane according to claim 1, wherein the porosity at the position 5.0 μm from the one surface in the membrane thickness direction is 40% or more.

12. 2. The porous hollow fiber membrane according to claim 1, wherein the volume ratio of isolated pores at the position 5.0 μm from the one surface in the membrane thickness direction is 0.10% or less.

13. 2. The porous hollow fiber membrane according to claim 1, wherein the porosity of the outer surface is 5.00% or more.

14. 10. The porous hollow fiber membrane of claim 1, comprising at least one macrovoid.

15. 2. The porous hollow fiber membrane according to claim 1, which is mainly composed of polysulfone or polyethersulfone.

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

17. A method for producing a porous hollow fiber membrane, characterized in that the ratio of the draw ratio to the relative energy difference on one surface side is 0.25 or more.

18. The method for producing a porous hollow fiber membrane according to claim 17, wherein the draw ratio on the one surface side is 1.10 or more.

19. The method for producing a porous hollow fiber membrane according to claim 17, wherein the relative energy difference on the one surface side is 4.0 or less.

20. The ratio of free-running humidity to free-running time is 200 g / m ３ The method for producing a porous hollow fiber membrane according to claim 17, wherein the permeability is 1 / sec or more.

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