Porous and composite membranes

Abstract

The present invention provides a porous film having a relief structure provided with protrusions and recesses on at least one surface. In a plane located at a height of 50 nm from a reference surface of the surface, the average number density of the protrusions having a cross-sectional area of 0.01 μm2 to 0.10 μm2 is 4.0 protrusions / μm2 or less, and in a plane located at a height of 20 nm from the reference surface of the surface, the average number density of the protrusions having a cross-sectional area of from 0.01 μm2 to 0.10 μm2 is 1.0 protrusion / μm2 or greater.

Description

[Technical Field]

[0001] The present invention relates to porous membranes and composite membranes. [Background technology]

[0002] BACKGROUND ART In recent years, porous membranes such as microfiltration membranes and ultrafiltration membranes have been used in a variety of fields, including the water treatment field (e.g., water purification or wastewater treatment), the medical field (e.g., blood purification), and the food industry.

[0003] As mentioned above, the diverse porous membranes used are prone to a phenomenon known as fouling, in which components such as suspended solids that are prevented from permeating accumulate on or within the membrane, and in some cases are adsorbed, blocking the membrane's pores. Because this phenomenon essentially reduces membrane performance, reducing and preventing fouling is extremely important. A common method for preventing membrane fouling is to make the membrane material hydrophilic. For example, Patent Document 1 discloses a technology for making the membrane material hydrophilic, thereby increasing its affinity with water and suppressing the adsorption of hydrophobic microorganisms and sediment.

[0004] Furthermore, from the viewpoint of membrane structure, it is generally believed that making the surface as smooth as possible can prevent adhesion of dirt. Patent Document 2 discloses a technology for reducing fouling by reducing surface irregularities. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 06-343843 [Patent Document 2] US Patent Application Publication No. 2003 / 36085 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the raw solution contains multiple components, such as when treating natural water such as river water or groundwater, or raw solution containing activated sludge in wastewater treatment, it is often difficult to obtain a sufficient fouling prevention effect even if the membrane material is made hydrophilic or the membrane surface is made smooth. Therefore, an object of the present invention is to provide a porous membrane that has both excellent low fouling properties and water permeability. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention is characterized by the following (1) to (17). (1) At least one surface has a concave-convex structure having convex portions and concave portions, The cross-sectional area of ​​the surface at a plane having a height of 50 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex portions is 4.0 pieces / μm 2 is as follows: The cross-sectional area of ​​the surface at a plane having a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex portions is 1.0 / μm 2 That's it, porous membrane. (2) The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex portions is 1.0 / μm 2 The porous membrane according to (1) above, which is: (3) The porous membrane according to (1) or (2) above, wherein the layer having the uneven structure has a thickness of 1 to 500 μm. (4) The porous membrane according to any one of (1) to (3) above, wherein the surface contains a polymer. (5) The porous membrane according to (4) above, wherein the polymer is a polymer containing polyvinylidene fluoride resin as a main component. (6) The porous membrane according to (5) above, wherein the polyvinylidene fluoride resin contains a branched polyvinylidene fluoride resin. (7) The radius of gyration, S, measured by GPC-MALS (gel permeation chromatograph with multi-angle light scattering detector) 2 〉 1 / 2 and the absolute molecular weight M of the polymer w The porous membrane according to any one of (4) to (6), wherein the value of a for the polymer is 0.27 to 0.39 and the value of b is 0.22 to 0.60, as determined by approximating the following formula 1 from <S 2 〉 1 / 2 =bM w a ...(Formula 1) (8) The porous membrane according to (7) above, wherein the value of a is 0.29 to 0.33, and the value of b is 0.43 to 0.50. (9) The porous membrane according to any one of (4) to (8) above, which contains a surfactant. (10) The porous membrane according to (9) above, which contains a surfactant having a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group. (11) The average pore size of the surface is 0.01 to 0.1 μm, The porous film according to any one of (1) to (10) above, wherein macrovoids having a minor axis of 60 μm or more are present. (12) The porous film according to (11) above, in which macrovoids having a minor axis of 80 μm or more are present. (13) The porous film according to any one of (1) to (12) above, which has a three-dimensional network structure. (14) A composite membrane comprising the porous membrane according to any one of (1) to (13) above and another layer. (15) The composite membrane according to (14) above, wherein the other layer is a support. (16) The porous membrane according to any one of (1) to (13) above or the composite membrane according to (14) or (15) above, which is used for ultrafiltration or microfiltration. (17) The porous membrane or composite membrane according to (16) above, which is used for membrane bioreactor activated sludge treatment. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a porous membrane that achieves both excellent low fouling properties and water permeability. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing an example of an atomic force microscope image of the porous membrane surface. [Figure 2] FIG. 2 is a diagram showing an enlarged image of a porous membrane illustrating the "three-dimensional network structure." DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, but the present invention is not limited thereto. In this specification, proportions (percentages, parts, etc.) based on mass are the same as proportions (percentages, parts, etc.) based on weight.

[0011] (porous membrane) The porous film according to this embodiment has an uneven structure with convex and concave portions on at least one surface, and the cross-sectional area of ​​the surface at a plane 20 nm above the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex portions is 1.0 / μm 2or more. Hereinafter, in this specification, a surface having such an uneven structure may be referred to as a "specific surface." The porous membrane of this embodiment is suitable for treating raw liquids such as natural water, including river water, and water containing coagulants and activated sludge. These liquids to be treated all contain a wide variety of components, and in the case of activated sludge, they contain microbial carcasses and metabolites. Among these, relatively large suspended particles of micron size are present as components that cause fouling of the porous membrane. Therefore, by providing a membrane that satisfies the above-mentioned relationship, i.e., by providing convex and concave portions on the porous membrane surface, fouling formation can be suppressed, and even if fouling does occur, the fouling can be easily removed. The porous membrane of this embodiment can reduce the contact area between relatively large particles in the raw liquid and the porous membrane, thereby preventing adhesion of suspended particles to the porous membrane surface. This enables long-term stable operation. To achieve superior low-fouling properties, the specific surface must have a cross-sectional area of ​​0.01 μm in a plane 20 nm above the reference surface. 2 ~0.10μm 2 The average number density of the convex portions is 1.5 / μm 2 It is preferable that this is equal to or greater than this.

[0012] Since the convex portion has no or few pores, in order to exhibit high water permeability, the cross-sectional area of ​​the specific surface at a plane 50 nm above the reference surface of the surface must be 0.01 μm 2 ~0.10μm 2 The average number density of the convex portions is 4.0 / μm 2 The cross-sectional area of ​​the specific surface at a plane 50 nm above the reference surface must be 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 1.0 / μm 2 In addition, it is preferable that the cross-sectional area of ​​the specific surface at a plane having a height of 50 nm from the reference surface of the surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0 / μm 2 may be.

[0013] In order to achieve higher water permeability, the cross-sectional area of ​​a specific surface at a plane 20 nm above the reference surface of the surface must be less than 0.01 μm 2 ~0.10μm 2 The average number density of the convex portions is 4.5 / μm 2 It is preferable that:

[0014] Components that cause fouling in porous membranes include those that penetrate into the pores and clog them. By providing convex and concave portions of appropriate height on the surface of the porous membrane, the flow on the surface of the porous membrane can be disrupted and fouling can be suppressed. This enables long-term stable operation.

[0015] Here, the cross-sectional areas of the convex portions on the surface at heights of 50 nm and 20 nm from the reference surface are determined from images obtained by observing the surface of a porous membrane sample in contact mode using an atomic force microscope in air and measuring a randomly selected 2.5 μm square area. For the image obtained by the above measurement, such as the example shown in Figure 1, the reference surface is determined, and the cross-sectional areas of each convex portion on a cross section parallel to the reference surface at positions 50 nm and 20 nm above the reference surface are calculated.

[0016] The reference surface is a plane defined based on the international standard ISO 25178 Surface Texture (Surface Roughness Measurement), and is the average height plane of the measurement surface in the evaluation area. The cross-sectional area of ​​each plane at a height of 50 nm and 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex portions is determined by the cross-sectional area measurement of the convex portions, which is obtained by measuring the cross-sectional area of ​​the convex portions in a cross section parallel to the reference surface at positions 50 nm and 20 nm high from the reference surface. 2 ~0.10μm 2 The number of convex portions is counted and divided by the area of ​​the measurement region to calculate the value.

[0017] In the porous membrane according to this embodiment, the thickness of the layer having a concave-convex structure is preferably 1 to 500 μm. When the thickness of the layer having a concave-convex structure is equal to or greater than the above-mentioned lower limit, damage to the porous membrane can be suppressed and dirt components can be sufficiently removed. Furthermore, when the thickness of the layer having a concave-convex structure is equal to or less than the above-mentioned upper limit, the amount of water permeation can be sufficient. The thickness of the layer having a concave-convex structure is more preferably in the range of 5 to 200 μm. Here, the layer having a concave-convex structure refers to a layer formed from the same material as the material forming the concave-convex structure. In other words, the layer having a concave-convex structure refers to a range that includes a surface having a concave-convex structure and is formed from the same material continuously with the surface. When the porous membrane is formed by solidifying a single material, for example, a polymer solution of a single composition, the thickness of the layer having a concave-convex structure is usually the same as the thickness of the porous membrane.

[0018] The porous membrane according to this embodiment may be formed from ceramics, metals, carbon, polymers, or a combination thereof. From the viewpoints of ease of handling due to weight, size, and flexibility, and economic efficiency, it is preferable that the surface (the outermost layer) contains a polymer. Specifically, the surface (the outermost layer) preferably contains 50% by mass or more of polymer, more preferably 70% by mass or more.

[0019] Examples of polymers that can be used include polyethylene resins, polypropylene resins, polyvinyl chloride resins, polyvinylidene fluoride resins, polysulfone resins, polyethersulfone resins, polyimide resins, and polyetherimide resins. The polymer is preferably a polymer primarily composed of a polyvinylidene fluoride resin. The term "polyvinylidene fluoride resin" refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer. Here, the term "vinylidene fluoride copolymer" refers to a polymer having a vinylidene fluoride residue structure, typically a copolymer of a vinylidene fluoride monomer and another fluorine-based monomer. Examples of such fluorine-based monomers include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene. The vinylidene fluoride copolymer may be copolymerized with ethylene or other fluorine-based monomers, as long as the effects of the present invention are not impaired.

[0020] The weight-average molecular weight of the polyvinylidene fluoride resin is preferably 50,000 to 1,000,000. When the weight-average molecular weight is equal to or less than the upper limit, the water permeability of the porous membrane can be improved, and when the weight-average molecular weight is equal to or greater than the lower limit, the low fouling properties of the porous membrane can be improved. When the porous membrane is used for water treatment applications where it is exposed to chemical cleaning, the weight-average molecular weight is preferably 100,000 or more, more preferably 150,000 or more. Furthermore, the weight-average molecular weight is preferably 900,000 or less, more preferably 800,000 or less.

[0021] "Containing polyvinylidene fluoride resin as the main component" means that the proportion of polyvinylidene fluoride resin in the polymer constituting the porous membrane is 50% by mass or more. To ensure high chemical resistance, the proportion is preferably 55% by mass or more, and more preferably 60% by mass or more. Examples of secondary components other than polyvinylidene fluoride resin include, but are not limited to, acrylic resins that control the hydrophilicity of the porous membrane.

[0022] In the present embodiment, the polymer preferably includes a branched polyvinylidene fluoride resin among polyvinylidene fluoride resins.Moreover, it is more preferable that the value of a for the polymer constituting the porous membrane, determined from the relationship in the following formula 1, is 0.27 to 0.39, and the value of b is 0.22 to 0.60. <S 2 〉 1 / 2 =bM w a ...(Formula 1) Here, the term "S 2 〉 1 / 2 is the radius of gyration of the polymer, and Mw is the absolute molecular weight. By including a highly branched polyvinylidene fluoride resin, the above ranges of a and b can be satisfied.

[0023] When the value of a is 0.39 or less, the absolute molecular weight M w For the radius of rotation〈S 2 〉 1 / 2 is appropriately small, and the polymer easily migrates to the surface layer of the porous membrane when the porous membrane is formed. This increases the polymer density of the surface layer of the porous membrane, which is presumably why the porous membrane exhibits excellent low-fouling properties. On the other hand, when the value of a is 0.27 or more, the polymers are moderately entangled with each other, making the polymer density of the surface layer uniform and resulting in even higher low-fouling properties. Furthermore, as the polymer density of the surface layer of the porous membrane increases, the polymer density of the inner layer decreases, which is presumably why excellent low-fouling properties and high water permeability are simultaneously exhibited. The value of a is more preferably 0.29 to 0.33.

[0024] The value of b for the above polymer is preferably 0.22 to 0.60, and more preferably 0.43 to 0.50, since the entanglement of the polymers homogenizes the polymer density in the surface layer, thereby further improving the low fouling properties.

[0025] In order to provide an uneven structure on the surface of the porous membrane, it is preferable that the value of a for the polymer is 0.27 to 0.39 and the value of b is 0.22 to 0.60.

[0026] The values ​​of a and b above are determined by the radius of gyration <S 2 〉 1 / 2 and the absolute molecular weight M w The measurement by GPC-MALS is carried out by dissolving the polymer constituting the porous membrane in a solvent. A salt may be added to the solvent to improve the solubility of the polymer. When measuring a polyvinylidene fluoride resin by GPC-MALS, it is preferable to use, for example, N-methyl-2-pyrrolidone (hereinafter referred to as "NMP") to which 0.1 mol / L of lithium chloride has been added as the solvent.

[0027] Radius of gyration〈S 2 〉 1 / 2 and the absolute molecular weight M w The values ​​of a and b can be determined by approximating the relationship between a and b as shown in the following formula 1 using a method commonly used in polymer research called a conformation plot. This method is common, as described in, for example, "Size Exclusion Chromatography" (by Sadao Mori, Kyoritsu Shuppan Co., Ltd., first edition, 1992). The conformation plot can be approximated by linearly approximating formula 1 as a logarithmic graph within the measurement range of the detector using the least squares method. <S 2 〉 1 / 2 =bM w a ...(Formula 1)

[0028] In order to increase the polymer density in the surface layer and achieve excellent low-fouling properties, the porous membrane according to this embodiment preferably has an average pore size of 0.01 to 0.1 μm on the specific surface. The average pore size on the surface of the porous membrane is more preferably 0.02 μm or more, and even more preferably 0.03 μm or more. The average pore size on the surface of the porous membrane is more preferably 0.08 μm or less, more preferably 0.06 μm or less, and even more preferably 0.04 μm or less.

[0029] The average pore size on the surface of the porous membrane is measured as follows. The surface of the porous membrane is observed at 10,000x magnification using a scanning electron microscope (hereinafter referred to as SEM), and the diameter of each hole is calculated from the area of ​​each hole, assuming that the hole is circular.The average of these values ​​can be used as the average pore diameter on the surface.

[0030] In the porous membrane according to this embodiment, it is preferable that macrovoids with a minor axis of 60 μm or more are present in order to reduce the flow resistance of permeate water when it flows through the porous membrane. Here, macrovoids refer to large voids with a minor axis of 10 μm or more that exist in the porous membrane. It is also preferable that the macrovoids exist in a region (layer) of the porous membrane that contains a polymer. The minor axis refers to the diameter in a direction parallel to the surface of the porous membrane. In this embodiment, it is preferable that the porous membrane has macrovoids with a minor axis of 70 μm or more, and more preferably macrovoids with a minor axis of 80 μm or more.

[0031] In the porous membrane according to this embodiment, in order to reduce the flow resistance of permeate water when it flows through the porous membrane and to exhibit high water permeability, it is preferable that at least a portion of the macrovoids are present in an area within 5 μm from the surface of the porous membrane. The distance from the surface to the macrovoids is more preferably within 4 μm, and even more preferably within 3 μm. Furthermore, it is more preferable that the distance from a specific surface to the macrovoids is within the above range.

[0032] In order to reduce the flow resistance of the permeate when it flows through the porous membrane, the porosity of the macrovoids in the region within 15 μm from the surface of the porous membrane according to this embodiment is preferably 15% or more, more preferably 25% or more, and even more preferably 40% or more. Furthermore, it is more preferable that the porosity of the macrovoids in the region within 15 μm from the specific surface is within the above range.

[0033] On the other hand, from the viewpoint of the strength of the porous membrane, it is preferable that the size of the macrovoids be limited to a minor axis of 300 μm or less. Also, it is preferable that the distance from the surface of the porous membrane to the macrovoids be limited to 1 μm or more from the surface. It is preferable that the porosity occupied by the macrovoids in the region within 15 μm from the surface of the porous membrane be limited to 80% or less.

[0034] In order to reduce the flow resistance of the permeate when it flows through the porous membrane, the ratio of the minor axis of the macrovoids to the average pore size on the surface of the porous membrane according to this embodiment is preferably at least 700, more preferably at least 1000. The above ratio is more preferably the ratio of the minor axis of the macrovoids to the average pore size on a specific surface. Here, the size of the macrovoids present in the porous membrane and the porosity of the porous membrane can be determined by observing a cross section perpendicular to the surface of the porous membrane using a scanning electron microscope (hereinafter referred to as "SEM").

[0035] The porous membrane according to the present embodiment preferably has a three-dimensional network structure, since the entanglement of the polymers homogenizes the polymer density in the surface layer, thereby further enhancing separation performance. Here, the "three-dimensional network structure" refers to a structure in which the polymers constituting the porous membrane according to the present embodiment are spread three-dimensionally in a network-like manner, as shown in Figure 2. The three-dimensional network structure has pores and voids partitioned by the polymers that form the network.

[0036] (composite membrane) The composite membrane according to this embodiment is characterized by comprising the porous membrane according to this embodiment and another layer. The other layer is not particularly limited as long as it is a component that can overlap with the porous membrane to form a layer. Preferably, the other layer is a support. Here, the "support" is a material that supports the porous membrane and provides strength to the composite membrane. The material of the support is not particularly limited, and may be an organic material, an inorganic material, or the like, but organic fibers are preferred because they are easy to reduce in weight. More preferably, the material is a woven or nonwoven fabric made of organic fibers such as cellulose fiber, cellulose triacetate fiber, polyester fiber, polypropylene fiber, or polyethylene fiber. Among these, nonwoven fabrics are preferred because their density is relatively easy to control, they are easy to manufacture, and are inexpensive.

[0037] If the thickness of the support is too thin, it becomes difficult to maintain the strength of the composite membrane, and if it is too thick, the water permeability tends to decrease, so the thickness is preferably in the range of 50 μm to 1 mm, and most preferably in the range of 70 to 500 μm.

[0038] The thickness of the porous membrane is preferably 50 μm or more, more preferably 80 μm or more, and even more preferably 100 μm or more. The thickness of the porous membrane is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less. If the porous membrane is too thin, the support may be exposed, and contaminants may adhere to the support, increasing the filtration pressure, or the filtration performance may not be fully restored even after washing. If the porous membrane is too thick, the water permeability may decrease.

[0039] It is preferable that a part of the resin forming the porous membrane penetrates at least the surface layer of the support and forms a composite layer with the support at least in the surface layer. By the resin penetrating into the support, the porous membrane is firmly fixed to the support by the so-called anchor effect, and the porous membrane can be prevented from peeling off from the support.

[0040] The porous membrane or composite membrane according to this embodiment has a pure water permeability of 0.15 m or less at 25°C and 5 kPa, because it can reduce the operating pressure and suppress the progression of fouling. 3 / m2 / hr or more is preferable, and 0.5m 3 / m 2 / hr or more is more preferable.

[0041] (Method of manufacturing porous membrane or composite membrane) The porous membrane or composite membrane according to the present embodiment described above can typically be produced by the method described below. The porous membrane or composite membrane according to this embodiment can be produced, for example, by a method including the following steps (1) and (2). (1) A polymer solution preparation step of dissolving a polymer in a pore-opening agent and a solvent to obtain a polymer solution, wherein the polymer preferably contains a polyvinylidene fluoride resin as a main component. (2) A porous membrane forming step in which the polymer solution is coagulated in a coagulation bath containing a non-solvent to form a porous membrane.

[0042] Furthermore, when producing a composite membrane, it is preferable to form a porous membrane on at least one surface of another layer, preferably a support, in step (2). That is, first, a coating of a stock solution (polymer solution) containing the above-mentioned resin (polymer), pore-opening agent, and solvent is formed on the surface of the support, and the stock solution is impregnated into the support. Thereafter, the support is immersed in a coagulation bath containing a non-solvent to coagulate the resin and form a porous membrane on the surface of the support. It is also preferable that the stock solution further contains a non-solvent. From the viewpoint of membrane formability, it is usually preferable to select the temperature of the stock solution within the range of 15 to 120°C.

[0043] The density of the support is 0.7 g / cm 3 Preferably, it is less than 0.6 g / cm 3 If the density of the support is within this range, it is suitable for accepting the resin that forms the porous membrane and forming an appropriate composite layer of the support and the resin. However, if the density is too low, the strength of the composite membrane tends to decrease, so the density of the support is set to 0.3 g / cm or less. 3The density referred to here is an apparent density, which can be determined from the area, thickness and weight of the support.

[0044] The pore-opening agent is extracted when the resin layer is immersed in a coagulation bath, making the resin layer porous. It is preferable that the pore-opening agent be one that is highly soluble in the coagulation bath. For example, inorganic salts such as calcium chloride and calcium carbonate can be used. Other examples that can be used include polyoxyalkylenes such as polyethylene glycol (PEG) and polypropylene glycol, water-soluble polymers such as polyvinyl alcohol, polyvinyl butyral, and polyacrylic acid, glycerin, and surfactants. Any pore-opening agent can be selected, but a polymer primarily composed of PEG or a surfactant is preferred.

[0045] In particular, when a polyvinylidene fluoride resin is used in which the value of a for the polymer determined from the relationship in the above formula 1 is 0.27 to 0.39 and the value of b is 0.22 to 0.60, it is preferable to use a polymer containing PEG as the main component and having a weight average molecular weight of 10,000 to 50,000, or a surfactant having a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group as the pore-forming agent. Such a pore-forming agent has a cross-sectional area of ​​0.01 μm2 in a plane 50 nm above the reference surface. 2 ~0.10μm 2 The average number density of the convex parts is 4.0 pieces / μm 2 or less, and the cross-sectional area on a plane 20 nm above the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 1.5 / μm 2 It is particularly preferable to use a surfactant having a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group as the pore-forming agent. Such a pore-forming agent has a cross-sectional area of ​​0.01 μm2 in a plane at a height of 50 nm from the reference surface of the surface. 2 ~0.10μm 2 The average number density of the convex parts is 1.0 / μm 2or less, and the cross-sectional area on a plane 20 nm above the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 1.5 / μm 2 This is particularly suitable for the above.

[0046] Examples of surfactants having a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group include polyethylene glycol monooleate, polyethylene glycol monostearate, polyoxyethylene sorbitan monolaurate (Tween 20), polyoxyethylene sorbitan monopalmitate (Tween 40), polyoxyethylene sorbitan monostearate (Tween 60), and polyoxyethylene sorbitan monooleate (Tween 80).

[0047] The solvent dissolves the resin. The solvent acts on the resin and the pore-opening agent to promote their formation of a porous membrane. Examples of solvents that can be used include NMP, N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, and methyl ethyl ketone. Among these, NMP, DMAc, DMF, and DMSO, which have high resin solubility, are preferred.

[0048] A non-solvent is a liquid that does not dissolve the resin. The non-solvent acts to control the rate of resin solidification and thereby control the size of pores and macrovoids. Water and alcohols such as methanol and ethanol can be used as the non-solvent. Among them, water and methanol are preferred as the non-solvent in terms of ease of wastewater treatment and cost. The non-solvent may also be a mixture containing these.

[0049] In the polymer solution preparation step, a polymer (resin), a pore-opening agent, and a solvent are preferably used to dissolve the polymer to obtain a polymer solution (stock solution). The polymer preferably contains a polyvinylidene fluoride resin as its main component. The stock solution also preferably contains a non-solvent. The stock solution preferably contains 5 to 30 wt % of resin, 0.1 to 15 wt % of pore-opening agent, 40 to 94.9 wt % of solvent, and 0 to 20 wt % of non-solvent.

[0050] If the resin content is too low, the strength of the porous membrane may decrease, while if it is too high, the water permeability may decrease. The resin content in the raw solution is more preferably in the range of 8 to 20% by weight. Furthermore, if the pore-opening agent is too low, the water permeability may decrease, while if it is too high, the strength of the porous membrane may decrease. Furthermore, if it is too high, it may remain in the porous membrane and dissolve during use, which may deteriorate the quality of the permeate or cause fluctuations in water permeability. A more preferred range for the pore-opening agent content in the raw solution is 0.5 to 10% by weight. Furthermore, if the solvent content is too low, the raw solution may easily gel, while if it is too high, the strength of the porous membrane may decrease. The solvent content in the raw solution is more preferably in the range of 60 to 90% by weight.

[0051] Adding a non-solvent to the solution is preferable because it makes it easier to uniformize the size of the pores on the surface of the porous membrane. It also makes it easier to control the size of the macrovoids. However, if the proportion of non-solvent in the solution is too high, the solution is more likely to gel. In the solution, the solvent is preferably in the range of 40 to 94.8 wt % and the non-solvent is in the range of 0.1 to 20 wt %. More preferably, the solvent is in the range of 40 to 94.4 wt % and the non-solvent is in the range of 0.5 to 15 wt %.

[0052] In the porous membrane forming step, the above-mentioned polymer solution (raw solution) is preferably coagulated in a coagulation bath containing a non-solvent to form a porous membrane. The coagulation bath can be a non-solvent or a mixture containing a non-solvent and a solvent. When a non-solvent is used in the raw solution, the non-solvent content in the coagulation bath is preferably at least 80% by weight. If the content is too low, the coagulation rate of the resin will be slow, the pore size on the surface will be large, and it will be difficult to generate macrovoids. When a non-solvent is used in the raw solution, the proportion of the non-solvent in the coagulation bath is more preferably in the range of 85 to 100% by weight.

[0053] On the other hand, when a non-solvent is not used in the raw solution, the content of the non-solvent in the coagulation bath is preferably lower than when a non-solvent is used in the raw solution, preferably at least 60% by weight. A high content of non-solvent increases the resin coagulation rate, resulting in a denser surface for the porous membrane and the formation of macrovoids within the membrane, which may lead to the formation of fine cracks on the surface of the porous membrane. In this case, the non-solvent content is more preferably in the range of 60 to 99% by weight. The pore size and macrovoid size on the surface of the porous membrane can be controlled by adjusting the solvent content in the coagulation bath. The temperature of the coagulation bath is preferably selected within the range of 15 to 80°C, since an extremely high temperature results in a too fast coagulation rate, while an extremely low temperature results in a too slow coagulation rate. A more preferred temperature range is 20 to 60°C.

[0054] When forming a composite membrane in which the other layer is a support, it is preferable to form a porous membrane on at least one surface of the support in the porous membrane forming step. The formation of a coating of the stock solution on the support can be done by applying the stock solution to the support or by immersing the support in the stock solution. When applying the stock solution, it may be applied to one side or both sides of the support. In this case, depending on the composition of the stock solution, it may be applied to a substrate having a density of 0.7 g / cm. 3 It is preferable to use a support having the following properties, since the support can be appropriately impregnated with the stock solution.

[0055] After the porous membrane formation step, it is preferable to provide a washing step to remove the solvent and pore-opening agent. The washing method can be appropriately selected depending on the type of solvent and pore-opening agent, and is not particularly limited, but an example is a method of immersing the substrate in hot water at 60 to 100°C for 1 to 10 minutes. In addition, the solvent and pore-opening agent may not be completely removed in the cleaning process. The porous membrane and composite membrane according to this embodiment may contain the above-mentioned solvent and pore-opening agent within a range that does not impair the effects of the present invention.

[0056] The porous membrane according to this embodiment can be applied to any of reverse osmosis membranes, nanofiltration membranes, ultrafiltration membranes, and microfiltration membranes. Furthermore, one or more appropriate membranes can be selected and combined depending on the size of the substances to be separated, but ultrafiltration membranes and microfiltration membranes are particularly preferred for sewage and wastewater treatment. The water to be treated with the porous membrane according to this embodiment is not particularly limited, but it is preferably used in activated sludge separation (membrane separation activated sludge treatment) for biological treatment of sewage and wastewater containing suspended solids of relatively large particles. [Example]

[0057] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples. (i) The cross-sectional area of ​​the porous film at each plane at heights of 50 nm and 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is The porous membrane or composite membrane was cut into 1 cm squares and attached to a sample stand with the surface to be measured facing up to prepare a sample. The surface of the porous membrane of this sample was observed with an atomic force microscope (Bruker AXS; Dimension FastScan), and the cross-sectional area of ​​the convex parts on each plane at heights of 50 nm and 20 nm from the reference surface was calculated as described above. When the cross-sectional area was 0.01 μm 2 ~0.10μm 2 The number of convex portions that were within the range of 0.01 to 0.1 was counted, and the average number density on each plane was calculated. The specific measurement conditions were as follows: Scanning mode: Contact mode Probe: Silicon cantilever (Bruker AXS; ScanAsyst-Air) Scanning range: 2.5μm×2.5μm Scanning speed: 0.5Hz Scanning resolution: 256×256 Measurement temperature: 25℃ Measurements were carried out for 10 arbitrary fields of view, and the average number density of the convex portions in the 10 fields of view for each plane was calculated based on the cross-sectional area of ​​0.01 μm 2 ~0.10μm 2 The average number density of the convex portions is

[0058] (ii) Thickness of the layer having the concave-convex structure of the porous membrane Samples for cross-sectional measurement of the porous membrane or composite membrane were prepared using the cryo-ultrathin sectioning method, and the thickness of the layer having an uneven structure including macrovoids in the porous membrane observed under the following observation conditions was calculated using an SEM (Hitachi High-Tech Corporation; S-5500). Accelerating voltage: 5 kV Observation magnification: 500x The measurement was carried out for 10 random fields of view, the thickness of the layer having the observed uneven structure was measured, and the average value was taken as the thickness of the layer having the uneven structure of the porous film.

[0059] (iii) a and b values ​​for the polymer constituting the porous membrane Porous or composite membranes immersed in distilled water were frozen at -20°C using a cryostat (Leica; Jung CM3000). Sections of the porous membrane (surface section of the porous membrane for composite membranes) were collected and vacuum-dried overnight at 25°C. 5 mL of 0.1 M lithium chloride-added NMP was added to 5 mg of vacuum-dried porous membrane and stirred at 50°C for approximately 2 hours. The resulting polymer solution was injected into a GPC-MALS (column: Showa Denko; Shodex KF-806M φ8.0 mm × 30 cm, two columns connected in series; differential refractometer: Wyatt Technology; Optilab rEX; MALS: Wyatt Technology; DAWN HeLEOS) under the following conditions. The injected polymer solution eluted from the column within 27 to 43 minutes. Column temperature: 50℃ Detector temperature: 23℃ Solvent: NMP with 0.1M lithium chloride Flow rate: 0.5mL / min Injection volume: 0.3mL

[0060] The elution time t obtained from the RI i Polymer concentration c i , obtained from MALS, elution time t i The excess Rayleigh ratio R θi From sin 2 (θ / 2) and (K×c i / R θi ) 1 / 2 The elution time t is calculated from the value of θ→0 in the approximate equation (Berry plot or Zimm plot; Equation 3 below). i Absolute molecular weight M Wi was calculated. Here, K is an optical constant, and is calculated from the following formula 2. Note that dn / dc in formula 2 is the amount of change in the refractive index of the polymer solution with respect to a change in polymer concentration, i.e., the refractive index increment. When the polymer to be measured is one whose main component is polyvinylidene fluoride resin and the above-mentioned solvent is used, a value of -0.050 mL / g can be applied as the refractive index increment. K=4π 2 ×n02 ×(dn / dc) 2 / (λ 4 ×N0) (Formula 2) n0: Refractive index of the solvent dn / dc: refractive index increment λ: wavelength of incident light in vacuum N0: Avogadro's number

[0061] Also, each elution time t i The radius of rotation at 2 〉 1 / 2 The value was calculated from the slope of the following formula 3. (Kc i / R θi ) 1 / 2 =M Wi -1 / 2 {1+1 / 6(4πn0 / λ) 2 <S 2 〉sin 2 (θ / 2)} (Formula 3) Each elution time t calculated from Equation 3 i Absolute molecular weight M wi is taken as the x-axis, and each elution time t i The radius of rotation at 2 〉 1 / 2 was plotted on the y-axis, and the values ​​of a and b for the polymer constituting the porous membrane were determined by approximating with the above formula 1 within the molecular weight range of 140,000 to 1,000,000 so as to fall within the measurement range of the detector. The approximation was performed by plotting the value of a and b for the polymer constituting the porous membrane on a double logarithmic graph using the least squares method.

[0062] (iv) Minor diameter of macrovoids in porous membrane The porous membrane or composite membrane was subjected to ultrathin cryosectioning to prepare a sample for cross-sectional measurement, and the minor axis of the macrovoid was calculated from the size of the macrovoid observed under the following observation conditions using an SEM (Hitachi High-Tech Corporation; S-5500). Accelerating voltage: 5 kV Observation magnification: 500x The measurement was carried out for 10 random fields of view, the minor diameters of the observed macrovoids were measured, and the average value was taken as the minor diameter of the macrovoids in the porous film.

[0063] (v) Average pore diameter of porous membrane surface The surface of the porous or composite membrane was observed using an SEM (Hitachi High-Tech Corporation; S-5500) under the following observation conditions, and the area of ​​300 randomly selected holes was measured. From the area of ​​each hole, the diameter was calculated as if the hole were circular, and the average of these was taken as the average pore size on the surface. Accelerating voltage: 5 kV Observation magnification: 10,000 times Image processing software: ImageJ (Wayne Rasband, National Institutes of Health)

[0064] (vi) Pure water permeability of porous or composite membranes The porous membrane was cut into a circle with a diameter of 50 mm and set in a cylindrical filtration holder (Ultra Holder UHP-43K, manufactured by Advantec Toyo Co., Ltd.). Distilled water was pre-permeated at 25°C under a pressure of 5 kPa for 5 minutes, and then the permeated water was collected for 3 minutes. The unit time (h) and unit membrane area (m 2 For composite membranes that have a support in addition to a porous membrane, the evaluation was performed on the entire composite membrane including the support.

[0065] (vii) Cake filtration resistance and plugging filtration resistance of porous or composite membranes The porous membrane was cut into a circle with a diameter of 50 mm, soaked in ethanol overnight, then immersed in water for at least 2 hours, and set in a cylindrical filter holder (Ultra Holder UHP-43K, manufactured by Advantec Toyo Co., Ltd.). Activated sludge (50 g) with a concentration of 7,000 mg / L was placed in the filter holder, the stirring speed was adjusted to 450 rpm, and the membrane permeation flux, converted to the amount of water permeated (cubic meters) per square meter of membrane surface per day, was measured at an evaluation temperature of 25°C. 2 / day, filter for 2 minutes, and calculate the sludge filtration resistance as R Ax The sludge filtration resistance calculated from the permeate volume for the last 5 seconds is R Bxx represents the number of times the 2-minute activated sludge filtration was repeated, and for the first filtration, x = 1. Here, the filtration resistance was calculated using the following formula 4. R=P×t×S / (μ×L) (Formula 4) R: Filtration resistance P: Evaluation pressure t: filtration time S: Membrane area μ: Viscosity L: Amount of filtered water

[0066] After filtration was stopped, the stirring speed was increased to 450 rpm for 1 minute. With the porous membrane still set in the filter holder, the remaining activated sludge solution in the filter holder was removed, the filter holder was filled with distilled water, and the stirring speed was increased to 450 rpm for 1 minute. Repeat the 2-minute activated sludge filtration and membrane cleaning. A1 ~R A5 and R B1 ~R B5 was measured. Bm -R Am The value of m was calculated from 1 to 5, and the average value was taken as the cake filtration resistance. An+1 -R An was calculated for values ​​of n from 1 to 4, and the average value was taken as the clogging filtration resistance. For composite membranes that had a support in addition to a porous membrane, the entire composite membrane including the support was evaluated. The smaller the cake filtration resistance and clogging filtration resistance, the better the porous membrane or composite membrane's low fouling properties can be evaluated.

[0067] Example 1 50% by mass of branched polyvinylidene fluoride (branched PVDF, weight-average molecular weight 730,000) and 50% by mass of linear polyvinylidene fluoride (linear PVDF, weight-average molecular weight 280,000) were mixed to prepare "PVDF." PEG20,000 (weight-average molecular weight 20,000) was added as a pore-opening agent, DMF as a solvent, and pure water as a non-solvent, and the mixture was thoroughly stirred at a temperature of 90°C to prepare a polymer solution with the following composition ratio. PVDF: 17% by weight PEG20,000: 8% by weight DMF: 72% by weight Pure water: 3% by weight

[0068] Next, density 0.6 g / cm 3 The prepared polymer solution was applied to the surface of a polyester fiber nonwoven fabric as a support. After application, the fabric was immediately immersed in pure water at 20°C for 5 minutes to form a porous membrane. The fabric was then further immersed in hot water at 90°C for 2 minutes to wash away the solvent and pore-opening agent, forming a composite membrane with a three-dimensional network structure.

[0069] The evaluation results of the obtained composite film are shown in Table 1. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 2.2 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 3.4 / μm 2 In the above formula 1, the value of a was 0.38, the value of b was 0.24, the minor axis of the macrovoid was 78 μm, and both the pure water permeability, which is an index of water permeability performance, and the cake filtration resistance, which is an index of low fouling properties, showed excellent values.

[0070] Example 2 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 1, except that the "PVDF" used in Example 1 was changed to branched PVDF. The evaluation results of the obtained composite film are shown in Table 1. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 3.0 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 4.8 / μm 2In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoids was 91 μm, and both the pure water permeability and the cake filtration resistance exhibited excellent values.

[0071] Example 3 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 2, except that PEG 10,000 (weight average molecular weight 10,000) was used as the pore-forming agent. The evaluation results of the obtained composite film are shown in Table 1. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 1.8 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 2.8 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoids was 86 μm, and both the pure water permeability and the cake filtration resistance exhibited excellent values.

[0072] Example 4 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 2, except that PEG 40,000 (weight average molecular weight 40,000) was used as the pore-forming agent. The evaluation results of the obtained composite film are shown in Table 1. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 3.9 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 4.1 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoids was 75 μm, and both the pure water permeability and the cake filtration resistance exhibited excellent values.

[0073] Example 5 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 1, except that polyoxyethylene sorbitan monooleate (Tween 80) was used as the pore-forming agent. The evaluation results of the obtained composite film are shown in Table 1. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.5 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 1.7 / μm 2 In the above formula 1, the value of a was 0.38, the value of b was 0.24, the minor axis of the macrovoid was 64 μm, and the pure water permeability, cake filtration resistance, and clogging filtration resistance all showed excellent values.

[0074] Example 6 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 2, except that polyoxyethylene sorbitan monooleate (Tween 80) was used as the pore-forming agent. The evaluation results of the obtained composite film are shown in Table 2. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.5 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 2.2 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor diameter of the macrovoids was 69 μm, and the pure water permeability, cake filtration resistance, and clogging filtration resistance all showed excellent values.

[0075] Example 7 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 2, except that polyoxyethylene sorbitan monostearate (Tween 60) was used as the pore-forming agent. The evaluation results of the obtained composite film are shown in Table 2. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.5 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 2.3 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoid was 95 μm, and the pure water permeability, cake filtration resistance, and clogging filtration resistance all showed excellent values.

[0076] Example 8 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 2, except that polyoxyethylene sorbitan monostearate (Tween 40) was used as the pore-forming agent. The evaluation results of the obtained composite film are shown in Table 2. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.4 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 2.4 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoids was 60 μm, and the pure water permeability, cake filtration resistance, and clogging filtration resistance all showed excellent values.

[0077] Example 9 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 6, except that the composition of the polymer solution was as follows: PVDF: 20% by weight Tween80: 8% by weight DMF: 69% by weight Pure water: 3% by weight The evaluation results of the obtained composite film are shown in Table 2. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 1.6 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoid was 67 μm, and the pure water permeability, cake filtration resistance, and clogging filtration resistance all showed excellent values.

[0078] Example 10 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 6, except that the composition of the polymer solution was as follows: PVDF: 14% by weight Tween80: 8% by weight DMF: 75% by weight Pure water: 3% by weight The evaluation results of the obtained composite film are shown in Table 2. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.2 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 1.1 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoid was 76 μm, and the pure water permeability, cake filtration resistance, and clogging filtration resistance all showed excellent values.

[0079] Example 11 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 6, except that the composition of the polymer solution was as follows: PVDF: 17% by weight Tween80: 6% by weight DMF: 74% by weight Pure water: 3% by weight The evaluation results of the obtained composite film are shown in Table 3. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.5 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 1.1 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoid was 65 μm, and the pure water permeability, cake filtration resistance, and clogging filtration resistance all showed excellent values.

[0080] Example 12 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 6, except that the composition of the polymer solution was as follows: PVDF: 17% by weight Tween80: 10% by weight DMF: 70% by weight Pure water: 3% by weight The evaluation results of the obtained composite film are shown in Table 3. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.5 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 2.9 / μm 2In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoid was 75 μm, and the pure water permeability, cake filtration resistance and clogging filtration resistance all showed excellent values.

[0081] Example 13 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 6, except that DMAc was used as the solvent. The evaluation results of the obtained composite film are shown in Table 3. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.2 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 3.2 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoid was 73 μm, and the pure water permeability, cake filtration resistance and clogging filtration resistance all showed excellent values.

[0082] Example 14 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 6, except that NMP was used as the solvent. The evaluation results of the obtained composite film are shown in Table 3. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 4.0 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoid was 68 μm, and the pure water permeability, cake filtration resistance, and clogging filtration resistance all showed excellent values.

[0083] (Comparative Example 1) A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 1, except that the "PVDF" used in Example 1 was replaced with linear PVDF. The evaluation results of the obtained composite film are shown in Table 4. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.5 / μm 2 In the above formula 1, the value of a was 0.42, the value of b was 0.16, the minor axis of the macrovoids was 50 μm, and the cake filtration resistance and clogging filtration resistance were inferior to the results of the Examples.

[0084] (Comparative Example 2) A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 6, except that the "PVDF" used in Example 6 was replaced with linear PVDF. The evaluation results of the obtained composite film are shown in Table 4. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.6 / μm 2 In the above formula 1, the value of a was 0.42, the value of b was 0.16, the minor axis of the macrovoids was 62 μm, and the cake filtration resistance and clogging filtration resistance were inferior to the results of the Examples.

[0085] (Comparative Example 3) A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 2, except that PEG 4,000 (weight average molecular weight 4,000) was used as the pore-forming agent. The evaluation results of the obtained composite film are shown in Table 4. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.4 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.7 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoids was 55 μm, and the cake filtration resistance and clogging filtration resistance were inferior to the results of the Examples.

[0086] Comparative Example 4 A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 2, except that PEG 100,000 (weight average molecular weight 100,000) was used as the pore-forming agent. The evaluation results of the obtained composite film are shown in Table 4. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 4.5 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 3.2 / μm 2 In the above formula 1, the value of a was 0.31, the value of b was 0.47, the minor axis of the macrovoids was 68 μm, and the pure water permeability was inferior to the results of the Examples.

[0087] (Comparative Example 5) A composite membrane having a three-dimensional network structure was formed in the same manner as in Example 6, except that the composition of the polymer solution was as follows: PVDF: 17% by weight Tween80: 2% by weight DMF: 78% by weight Pure water: 3% by weight The evaluation results of the obtained composite film are shown in Table 4. The cross-sectional area of ​​the surface at a plane 50 nm above the reference surface was 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0 / μm 2 The cross-sectional area of ​​the surface at a height of 20 nm from the reference surface is 0.01 μm 2 ~0.10μm 2 The average number density of the convex parts is 0.6 / μm 2 In the above formula 1, the value of a was 0.42, the value of b was 0.16, the minor axis of the macrovoids was 58 μm, and the cake filtration resistance and clogging filtration resistance were inferior to the results of the Examples.

[0088] [Table 1]

[0089] [Table 2]

[0090] [Table 3]

[0091] [Table 4]

[0092] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on Japanese patent applications filed on May 29, 2020 (Patent Application No. 2020-094340) and November 30, 2020 (Patent Application No. 2020-198370), the contents of which are incorporated herein by reference.

Claims

1. At least one surface has a concave-convex structure having convex portions and concave portions, The cross-sectional area of ​​the surface at a plane having a height of 50 nm from the reference surface is 0.01 μm ２ ~0.10 μm ２ The average number density of the convex portions is 4.0 pieces / μm ２ is as follows: The cross-sectional area of ​​the surface at a plane having a height of 20 nm from the reference surface is 0.01 μm ２ ~0.10 μm ２ The average number density of the convex portions is 1.0 / μm ２ That's all, the surface contains a polymer containing a polyvinylidene fluoride resin as a main component, The polyvinylidene fluoride resin includes a branched polyvinylidene fluoride resin, A porous membrane in which the value of a for the polymer is determined by approximation using the following formula 1 from the radius of gyration <S 2 > 1 / 2 measured by GPC-MALS (gel permeation chromatography equipped with a multi-angle light scattering detector) and the absolute molecular weight M w of the polymer, and the value of a is 0.27 to 0.39 and the value of b is 0.22 to 0.

60. <S 2 > 1 / 2 = bM w a ... (Formula 1)

2. The cross-sectional area of ​​the surface at a plane having a height of 50 nm from the reference surface is 0.01 μm ２ ~0.10 μm ２ The average number density of the convex portions is 1.0 / μm ２ is as follows:

2. The porous membrane according to claim 1, wherein the value of a is 0.29 to 0.33, and the value of b is 0.43 to 0.

50.

3. The porous membrane according to claim 1 or 2, wherein the layer having the uneven structure has a thickness of 1 to 500 μm.

4. The porous membrane according to any one of claims 1 to 3, comprising a surfactant.

5. The porous membrane according to claim 4, comprising a surfactant having a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group.

6. the average pore size of the surface is 0.01 to 0.1 μm; The porous membrane according to any one of claims 1 to 5, wherein macrovoids having a minor axis of 60 µm or more are present.

7. The porous membrane according to claim 6, wherein macrovoids having a minor axis of 80 μm or more are present.

8. The porous membrane according to any one of claims 1 to 7, having a three-dimensional network structure.

9. A composite membrane comprising the porous membrane according to any one of claims 1 to 8 and another layer.

10. 10. The composite membrane of claim 9, wherein the other layer is a support.

11. The porous membrane according to any one of claims 1 to 8 or the composite membrane according to claim 9 or 10, which is used for ultrafiltration or microfiltration.

12. The porous membrane or composite membrane according to claim 11, which is used for membrane separation activated sludge treatment.

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