Separation membrane and method for producing same

Abstract

This separation membrane comprises: a porous resin layer which contains a polymer comprising mainly a poly(vinylidene fluoride)-based resin; and another layer. The porous resin layer is provided to a surface section. The polymer has an a value of 0.40-0.48, with the a value being determined, by approximation using a formula 1, from the radius of rotation <S2>1 / 2, which is measured using GPC-MALS (gel permeation chromatography using a multi-angle light scattering detector), and the absolute molecular weight Mw of the polymer. The weight average molecular weight of the poly(vinylidene fluoride)-based resin, as measured by GPC (gel permeation chromatography) is 300,000 or more. The porosity attributable to macrovoids is 28-80% in a region at a depth of up to 15 µm from the surface of the porous resin layer. The average pore diameter at the surface of the porous resin layer is 10-100 nm. (Formula 1): <S2>1 / 2=bMw a.

Description

Separation membrane and method for producing the same

[0001] The present invention relates to a separation membrane and a method for producing the separation membrane. The present invention also relates to a method for filtering a liquid using the separation membrane and a membrane filtration device using the separation membrane.

[0002] BACKGROUND ART In recent years, separation 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 various separation membranes used can suffer from operational stability problems if their permeability decreases due to the accumulation, adhesion, or clogging (fouling) of substances to be separated. In such cases, it becomes necessary to increase the aeration volume during aeration cleaning or the frequency of chemical cleaning, which increases operational costs. Therefore, there is a demand for separation membranes with high permeability and low fouling.

[0004] Furthermore, in water purification processes, disinfectants such as sodium hypochlorite are added to the membrane module to sterilize the permeate and prevent membrane biofouling, and the membrane itself is washed with acid, alkali, chlorine, surfactants, etc. For this reason, separation membranes must be chemically durable (resistant to chemicals).

[0005] Furthermore, in the production of tap water, accidents have been occurring since the 1990s in which chlorine-resistant pathogenic microorganisms, such as Cryptosporidium, derived from livestock manure, cannot be completely treated at water purification plants and end up contaminating the treated water. To prevent such accidents, separation membranes are required to have sufficient separation properties and high physical durability to prevent raw water from contaminating the treated water.

[0006] Separation membranes containing polymers containing polyvinylidene fluoride resins are known as separation membranes that exhibit excellent chemical resistance. For example, Patent Document 1 discloses a technique for improving low fouling and separation performance by controlling the pore size distribution of a separation membrane made of a polymer containing polyvinylidene fluoride resin. Furthermore, Patent Document 2 discloses a technique for providing a fluoropolymer membrane that combines higher porosity, higher permeability, and improved mechanical properties.

[0007] Japanese Patent No. 5310658 Publication Japanese Special Table No. 2016-510688

[0008] The inventors' studies have revealed that separation membranes made of the above-mentioned polymeric polyvinylidene fluoride resins have excellent abrasion resistance in addition to chemical resistance. However, it is known that separation membranes made of the above-mentioned polyvinylidene fluoride resins have a low porosity in the inner layer, which makes them prone to sludge adhesion, resulting in frequent fouling and reduced water permeability. Therefore, an object of the present invention is to provide a separation membrane that has low fouling and excellent water permeability while maintaining high chemical resistance and abrasion resistance.

[0009] In order to solve the above problems, the present invention is characterized by the following (1) to (10): (1) A separation membrane comprising a porous resin layer containing a polymer mainly composed of a polyvinylidene fluoride resin and another layer, the porous resin layer being disposed on a surface portion, wherein the radius of gyration <S ２ 〉 １／２ and the absolute molecular weight M of the polymer ｗ a value of a for the polymer determined by approximating the above formula 1 is 0.40 or more and 0.48 or less, the weight average molecular weight of the polyvinylidene fluoride resin measured by GPC (gel permeation chromatography) is 300,000 or more, the porosity occupied by macrovoids in a region within a depth of 15 μm from the surface of the porous resin layer is 28% or more and 80% or less, and the average pore size in the surface of the porous resin layer is 10 nm or more and 100 nm or less. ２ 〉 １／２ = bM ｗ ａ ... (Formula 1) (2) On the surface of the porous resin layer, the cross-sectional area of ​​a plane at a height of 50 nm from the reference surface is 0.015 μm ２ 0.10 μm or more ２ The average number density of the convex portions is 0.16 / μm ２(3) The separation membrane according to (1), wherein the root mean square roughness Rq per 10 μm square area on the surface of the porous resin layer is 30 nm or more and 50 nm or less. (4) The separation membrane according to any one of (1) to (3), wherein the other layer is a support. (5) A method for filtering a liquid using the separation membrane according to any one of (1) to (3). (6) A membrane filtration device using the separation membrane according to any one of (1) to (3). (7) The method for producing a separation membrane according to any one of (1) to (3), comprising: (i) a polymer solution preparation step of dissolving the polymer primarily composed of a polyvinylidene fluoride resin using a first non-solvent containing 20% ​​or more of a substance having a bound water content of 1.7 g to 3.0 g per gram, a pore-opening agent, and a solvent to obtain a polymer solution; and (ii) a porous resin layer formation step of coagulating the polymer solution in a coagulation bath containing a second non-solvent to form the porous resin layer. (8) The method for producing a separation membrane according to (7), wherein the substance having a bound water content of 1.7 g to 3.0 g per gram is glycerin. (9) The method for producing a separation membrane according to (7) or (8), wherein the polymer solution in the polymer solution preparation step contains a hydrophilic resin. (10) The method for producing a separation membrane according to any one of (7) to (9), further comprising, after the porous resin layer forming step, a heat drying step in air at a space temperature of 100° C. or higher.

[0010] According to the present invention, a separation membrane having excellent low fouling and water permeability can be provided while ensuring high chemical resistance and abrasion resistance due to the inclusion of a polymer having a specific high-molecular-weight polyvinylidene fluoride resin as a main component.

[0011] FIG. 1 is an enlarged image of a cross section of a separation membrane, illustrating the "three-dimensional network structure."

[0012] 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.

[0013] (Separation membrane) The separation membrane according to this embodiment is a separation membrane comprising a porous resin layer containing a polymer (hereinafter, sometimes referred to as a "specific polymer") whose main component is a polyvinylidene fluoride resin, and another layer, and the porous resin layer is disposed on the surface portion.

[0014] In the separation membrane according to this embodiment, the porous resin layer being disposed on the surface portion means that the porous resin layer is present on at least one surface of the separation membrane. The porous resin layer may be present on only one surface of the separation membrane, or on both surfaces. Furthermore, the porous resin layer may be present on only a portion of the surface of the separation membrane, or may constitute the entire surface.

[0015] The polyvinylidene fluoride resin refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer. Here, the vinylidene fluoride copolymer refers to a polymer having a vinylidene fluoride residue structure, and is typically a copolymer of a vinylidene fluoride monomer and another fluorine-based monomer. Examples of such a fluorine-based monomer include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene. The vinylidene fluoride copolymer may be copolymerized with ethylene or the like other than the above fluorine-based monomer, to the extent that the effects of the present invention are not impaired.

[0016] "Containing polyvinylidene fluoride resin as the main component" means that the proportion of polyvinylidene fluoride resin in the polymer constituting the porous resin layer is 50% by mass or more. In order 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 resin layer. The upper limit of the proportion is not particularly limited and may be 100% by mass. In other words, the polymer constituting the porous resin layer may consist solely of polyvinylidene fluoride resin.

[0017] The separation membrane according to this embodiment is required to use a specific polymer having a value of a of 0.40 or more and 0.48 or less. The value of a is the radius of gyration <S ２ 〉 １／２ and the absolute molecular weight M of a specific polymer ｗ Therefore, the value is determined by approximation using the following formula 1. ２ 〉 １／２ = bM ｗ ａ ... (Equation 1) where <S ２ 〉 １／２ is the radius of gyration of a particular polymer, M ｗ denotes the absolute molecular weight of a particular polymer.

[0018] When the value of a is 0.40 or more, M ｗ In contrast to <S ２ 〉 １／２ becomes moderately large, and the polymer is moderately entangled in the porous resin layer. This is presumed to rigidify the polymer and improve the abrasion resistance of the separation membrane. On the other hand, if the value of a is 0.48 or less, excessive entanglement of the polymer is prevented and the polymer density in the surface layer of the porous resin layer is homogenized. As a result, it is presumed that excellent low-fouling properties are exhibited in the separation membrane. One example of a means for setting the value of a to 0.40 or more and 0.48 or less is a method using linear PVDF (polyvinylidene fluoride). The value of a is preferably 0.41 or more and 0.44 or less.

[0019] The value of b is preferably 0.20 or less. It is presumed that a value of b of 0.20 or less allows the polymer to be moderately entangled in the porous resin layer, resulting in excellent low-fouling properties in the separation membrane. The lower limit of the value of b is not particularly limited, but is usually 0.10 or more.

[0020] 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 resin layer in a solvent. A salt may be added to the solvent in order to improve the solubility of the polymer. When measuring a polymer containing a polyvinylidene fluoride resin as a main component 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.

[0021] 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 as shown in the following formula 1 using a method commonly used in polymer research, called a conformation plot. Such a method is common, as described in, for example, "Size Exclusion Chromatography" (by Mori Sadao, Kyoritsu Shuppan Co., Ltd., first edition, 1992). The conformation plot can be approximated by plotting formula 1 as a double logarithmic graph within the range of the detector and applying the least squares method to obtain a linear approximation. <S ２ 〉 １／２ = bM ｗ ａ ...(Formula 1)

[0022] In order for the separation membrane according to this embodiment to exhibit excellent chemical resistance, the weight-average molecular weight of the polyvinylidene fluoride resin measured by GPC must be 300,000 or more. It is presumed that a weight-average molecular weight of 300,000 or more of the polyvinylidene fluoride resin appropriately reduces the amount of terminal functional groups, reduces reactivity with chemicals, and improves the chemical resistance of the separation membrane. Furthermore, the weight-average molecular weight of the polyvinylidene fluoride resin is preferably 400,000 or more, and more preferably 500,000 or more. The upper limit of the weight-average molecular weight of the polyvinylidene fluoride resin is not particularly limited, but is typically 1,500,000 or less.

[0023] The weight average molecular weight of the polyvinylidene fluoride resin can be measured by GPC equipped with a differential refractive index detector. Measurement by GPC is performed by dissolving the polymer constituting the porous resin layer in a solvent. A salt may be added to the solvent to improve the solubility of the polymer. When measuring the polyvinylidene fluoride resin by GPC, it is preferable to use, for example, NMP containing 0.1 mol / L of lithium chloride as the solvent.

[0024] The separation membrane according to this embodiment requires that the porosity of macrovoids in the region within 15 μm depth from the surface of the porous resin layer be 28% or more and 80% or less. A porosity of 28% or more narrows the region into which fouling components penetrate, including the pores, and prevents an increase in filtration resistance. A porosity of 30% or more is preferred, and 33% or more is more preferred. This further reduces the flow resistance of permeate when it flows through the porous resin layer, enabling high water permeability to be achieved. Furthermore, from the viewpoint of the strength of the porous resin layer, the porosity of macrovoids in the region within 15 μm depth from the surface of the porous resin layer is preferably 70% or less, and more preferably 50% or less.

[0025] Macrovoids and the porosity occupied by macrovoids are defined as follows. The separation membrane is stained with a 3.2 mg / mL aqueous solution of a fluorescent substance (3,3,3',3'-tetramethyl-1,1'-bis(4-sulfobutyl)indocarbocyanine sodium) for 24 hours and then washed with distilled water. Using a confocal laser microscope (Olympus Corporation; FV3000), the membrane is positioned so that the axial direction of the laser light is perpendicular to the surface of the porous resin layer. A 200 μm x 200 μm area parallel to the surface is observed at a resolution of 1024 x 1024 pixels and a magnification of 60x, covering a region from the surface to a depth of 40 μm, every 0.2 μm. The unstained regions observed in this manner can be defined as macrovoids. Furthermore, a cross-sectional image perpendicular to the surface of the porous resin layer can be extracted from a three-dimensional image created by stitching together the obtained images, and the porosity can be determined.

[0026] In order to achieve excellent low-fouling properties and water permeability, the separation membrane according to this embodiment requires that the average pore size of the surface of the porous resin layer be 10 nm or more and 100 nm or less. To achieve excellent water permeability, the average pore size of the surface of the porous resin layer is preferably 20 nm or more. Furthermore, to achieve excellent low-fouling properties, the average pore size of the surface of the porous resin layer is preferably 80 nm or less, more preferably 60 nm or less.

[0027] The average pore size of the surface of the porous resin layer is measured as follows: The surface of the porous resin layer is observed at 10,000 times 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 size of the surface.

[0028] The porous resin layer preferably has a three-dimensional network structure to further enhance low fouling properties by homogenizing the polymer density of the surface layer due to entanglement of polymers mainly composed of polyvinylidene fluoride resin. Here, the term "three-dimensional network structure" refers to a structure in which the polymers constituting the porous resin layer of the separation membrane according to this embodiment are spread three-dimensionally in a network-like manner, as shown in Figure 1. The three-dimensional network structure has pores and voids partitioned by the polymers that form the network.

[0029] The separation membrane according to this embodiment has a cross-sectional area of ​​0.015 μm on a plane at a height of 50 nm from the reference surface on the surface of the porous resin layer. ２ 0.10 μm or more ２ The average number density of the convex portions is 0.16 / μm ２ It is preferable that the average number density of the convex portions is 0.16 / μm or less. ２ By setting the average number density of the predetermined convex portions to 0.10 pieces / μm or less, damage to the surface of the porous resin layer due to the solid matter contained in the raw water to be filtered can be prevented, and high abrasion resistance can be achieved. ２ The average number density of the predetermined convex portions is preferably 0 / μm or less, and the smaller the better. ２ It is most preferable that:

[0030] The cross-sectional area of ​​the convex portions on the surface of the porous resin layer at a plane 50 nm high from the reference surface is calculated as follows. The surface of the separation membrane sample is observed in contact mode using an atomic force microscope in the atmosphere, and a 2.5 μm square area is randomly selected and measured. A reference surface is determined for the obtained image, and the cross-sectional area of ​​each convex portion on a cross section parallel to the reference surface at a position 50 nm high from the reference surface is calculated. 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.

[0031] On the surface of the porous resin layer, the cross-sectional area of ​​a plane at a height of 50 nm from the reference surface is 0.015 μm ２ 0.1 μm or more ２ The average number density of the protrusions having a cross-sectional area of ​​0.015 μm or less is calculated as follows: ２ 0.10 μm or more ２ The number of convex portions that satisfy the following conditions is counted. The total number of convex portions thus obtained is divided by the area of ​​the measurement region to calculate the average number density of the convex portions. The average number density of the convex portions is determined by measuring any five visual fields and averaging the average number densities of the convex portions in the five visual fields.

[0032] Methods for setting the average number density of the convex portions within a preferred range include using linear PVDF (polyvinylidene fluoride) and, in the production of the separation membrane, limiting the heating time in air under conditions of a space temperature of 100°C or higher to 20 minutes or less, as described below.

[0033] In the separation membrane according to this embodiment, the surface of the porous resin layer preferably has a root-mean-square roughness Rq of 30 nm or more and 50 nm or less per 10 μm square area of ​​the surface. When the root-mean-square roughness Rq is 30 nm or more, the size of the particles formed by the polymer in the porous resin layer is large, thereby increasing the interconnectivity of the porous resin layer and achieving high water permeability. When the root-mean-square roughness Rq is 50 nm or less, sludge can be prevented from adhering and remaining on the membrane surface, which can cause clogging of the membrane. The root-mean-square roughness Rq is more preferably 31 nm or more and 40 nm or less.

[0034] The root mean square roughness Rq is determined by observing the surface of a separation membrane sample in water using an atomic force microscope in contact mode, randomly selecting 10 μm square regions, taking five images from five different regions, and averaging the Rq values. To calculate the Rq value for a 10 μm square region, only images of regions within the 10 μm square image where the difference between Rq values ​​in any 10 μm × 5 μm region is 15 nm or less are used.

[0035] A method for controlling Rq within a preferred range is, in the production of a separation membrane, a method in which the heating time in air is set to 0.5 to 7 minutes under conditions of a space temperature of 100° C. or higher, as described below.

[0036] In the embodiment of the present invention, the other layer is not particularly limited as long as it is a component that can overlap with the porous resin layer to form a layer. The other layer is preferably a support. Here, the "support" is a material that supports the porous resin layer and provides strength to the separation 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 fibers, cellulose triacetate fibers, polyester fibers, polypropylene fibers, or polyethylene fibers. Among these, nonwoven fabrics are preferred because their density is relatively easy to control, they are easy to manufacture, and are inexpensive.

[0037] The thickness of the support is preferably in the range of 50 μm to 1 mm, more preferably in the range of 70 μm to 500 μm. When the thickness of the support is 50 μm or more, the strength of the separation membrane can be easily maintained, and when the thickness is 1 mm or less, sufficient water permeability can be maintained.

[0038] The thickness of the porous resin layer is preferably 50 μm or more, more preferably 80 μm or more, and even more preferably 100 μm or more. When the thickness of the porous resin layer is above the lower limit, the support is not exposed to the membrane surface, and the increase in filtration pressure due to the adhesion of contaminants to the support can be avoided, and the problem of the filtration performance not being able to be restored after washing can be avoided. The thickness of the porous resin layer is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less. When the thickness of the porous resin layer is below the upper limit, good water permeability can be maintained.

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

[0040] (Use of Separation Membrane) The separation membrane according to this embodiment can be applied to various liquid filtration methods and membrane filtration devices in the field of water treatment. In particular, it can be suitably used in the field of sewage treatment. In terms of being able to lower the operating pressure and suppress the progression of fouling, the operating conditions are as follows: pure water permeability at 25°C and 5 kPa is 0.15 m ３ / m ２ / hr or more, and ３ / m ２ / hr or more is more preferable.

[0041] (Method for Producing Separation Membrane) The separation membrane according to the present embodiment described above can typically be produced by the method described below.

[0042] The separation membrane according to this embodiment can be produced, for example, by a method including the following steps (i) and (ii): (i) a polymer solution preparation step of dissolving the polymer containing polyvinylidene fluoride resin as a main component in a first non-solvent containing 20% ​​or more of a substance with a bound water content of 1.7 g to 3.0 g per gram, a pore-opening agent, and a solvent to obtain a polymer solution; and (ii) a porous resin layer formation step of coagulating the polymer solution in a coagulation bath containing a second non-solvent to form the porous resin layer.

[0043] Furthermore, in the separation membrane according to this embodiment, when the other layer is a support, a manufacturing method can be exemplified by a method including the above steps (i) and (ii), in which in the (ii) porous resin layer forming step, a porous resin layer is formed on at least one surface of the support.

[0044] That is, first, a coating of a polymer solution containing the above-mentioned resin (a polymer mainly composed of a polyvinylidene fluoride resin), a first non-solvent containing 20% ​​or more of a substance with a bound water content of 1.7 g to 3.0 g per gram, a pore-opening agent, and a solvent is formed on the surface of the aforementioned support, and the support is then impregnated with the polymer solution. The support is then immersed in a coagulation bath containing a second non-solvent to coagulate the resin and form a porous resin layer on the surface of the support. From the viewpoint of film-forming properties, the temperature of the polymer solution is preferably selected within the range of 15 to 120°C.

[0045] The density of the support is 0.7 g / cm ３ Preferably, it is 0.6 g / cm or less, more preferably 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 resin layer and forming an appropriate composite layer of the support and the resin. On the other hand, from the viewpoint of the strength as a separation membrane, the density of the support is 0.3 g / cm or less. ３ The density referred to here is an apparent density, which can be determined from the area, thickness and weight of the support.

[0046] The non-solvent is a liquid that does not dissolve the resin. The first non-solvent in step (i) acts to control the coagulation rate of the resin and thereby control the size of the pores. The first non-solvent in step (i) can be water, alcohols such as methanol or ethanol, or a substance with high water retention. In particular, when using a polyvinylidene fluoride resin in which the value of a determined from the relationship in Equation 1 is 0.40 or more and 0.48 or less, it is preferable to use a non-solvent containing a substance with high water retention as the first non-solvent in order to achieve a porosity of 28% or more of the macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer. As a substance with high water retention, a substance with a bound water content of 1.7 g or more per gram is preferred, and examples thereof include glycerin, proline, and sodium lactate, with glycerin being particularly preferred. Substances with a bound water content of 1.7 g or more per gram tend to easily form hydrogen bonds with polymers, etc. in the polymer solution, which increases the viscosity of the polymer solution due to this property. During the formation of the porous layer by phase separation, increasing the viscosity of the polymer solution through hydrogen bonding reduces the tendency of the polymer in the polymer solution, which is primarily composed of a high-molecular-weight polyvinylidene fluoride resin, to aggregate, and before the region containing the pores is formed thickly, water flows in from the back side of the porous resin layer surface to form macrovoids, resulting in a porosity of 28% or more occupied by the macrovoids. Furthermore, to prevent excessive water absorption in the first non-solvent, the amount of bound water is preferably 3.0 g or less, and more preferably 2.6 g or less.

[0047] The bound water is water bound to a highly water-absorbent substance by hydrogen bonds, restricting molecular motion, and is distinguished from free water, which is adsorbed water and can move freely within the substance. The free water is calculated from the integrated value of the endothermic peak appearing near 0°C under nitrogen atmosphere at a temperature rise of 2°C / min from -80 to 25°C using a DSC6200 manufactured by Seiko Instruments Inc., using the following formula 2: (Amount of free water) = {(Total weight of solution) x (Enthalpy of heat of fusion) x (Molecular weight of water)} / (Enthalpy of heat of fusion of water) ... (Formula 2) The amount of bound water can be calculated from the following formula 3: (Amount of bound water) = (Total amount of water in solution) - (Amount of free water) ... (Formula 3)

[0048] A first non-solvent containing 20% ​​or more of a substance having a bound water content of 1.7 g to 3.0 g per gram refers to a non-solvent in which 20% or more of a substance having a bound water content of 1.7 g to 3.0 g per gram is contained in the non-solvent. The proportion of the substance contained in the first non-solvent is preferably 50% by mass to 100% by mass, and more preferably 60% by mass to 100% by mass, when the total amount of the first non-solvent is taken as 100% by mass.

[0049] The first non-solvent may further contain another non-solvent as a component other than the substance having a bound water content of 1.7 g to 3.0 g per gram. Examples of such another non-solvent include those similar to the second non-solvent described below. When the first non-solvent contains another non-solvent, the other non-solvent and the second non-solvent may be the same or different.

[0050] The pore-opening agent is extracted when immersed in a coagulation bath and serves to make the resin layer porous. It is preferable that the pore-opening agent be highly soluble in the coagulation bath. For example, inorganic salts such as calcium chloride and calcium carbonate can be used. Other examples include polyoxyalkylenes such as polyethylene glycol (PEG) and polypropylene glycol, water-soluble polymers such as polyvinyl alcohol, polyvinyl butyral, and polyacrylic acid, and surfactants. While any pore-opening agent can be selected, polymers containing PEG as the main component or surfactants are preferred. Among these, polymers containing PEG as the main component with a weight-average molecular weight of 10,000 or more, or surfactants having a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group are particularly preferred. Using these pore-opening agents facilitates achieving a porosity of 28% or more of macrovoids in the region within 15 μm depth from the surface of the porous resin layer. Here, "mainly composed" means that the component is contained in an amount of 50% by mass or more.

[0051] 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 monostearate (Tween 60), polyoxyethylene sorbitan monooleate (Tween 80), etc. In order to facilitate the presence of macrovoids in a region within a depth of 15 μm from the surface of the porous resin layer, polyoxyethylene sorbitan monolaurate (Tween 20), polyoxyethylene sorbitan monostearate (Tween 60), and polyoxyethylene sorbitan monooleate (Tween 80) are preferred as the surfactants.

[0052] The solvent dissolves the resin. The solvent acts on the resin and the pore-opening agent to promote their formation of a porous resin layer. 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. Of these, NMP, DMAc, DMF, and DMSO, which have high resin solubility, are preferred.

[0053] It is preferable to add a hydrophilic resin to the polymer solution in step (i) above. A hydrophilic resin refers to a resin that has a high affinity for water and dissolves in water, or a resin whose contact angle with water is smaller than that of a polyvinylidene fluoride resin. Examples of hydrophilic resins include cellulose esters such as cellulose acetate or cellulose acetate propionate, fatty acid vinyl esters, polyvinyl acetate, polyvinylpyrrolidone, acrylic acid esters or methacrylic acid esters such as ethylene oxide, propylene oxide, or polymethyl methacrylate, or copolymers of these polymers. By using a hydrophilic resin in the polymer solution, the resulting separation membrane can exhibit high water permeability.

[0054] The second non-solvent in step (ii) acts to control the rate of resin solidification and thereby control the size of pores and macrovoids. Water or alcohols such as methanol and ethanol can be used as the second non-solvent. Among these, water is preferred as the second non-solvent from the viewpoints of ease of waste liquid treatment and cost. The second non-solvent may also be a mixture containing these.

[0055] In the polymer solution preparation step, a polymer primarily composed of a polyvinylidene fluoride resin is dissolved in a first non-solvent, a pore-opening agent, and a solvent to obtain a polymer solution. In the polymer solution, the content of the polymer primarily composed of a polyvinylidene fluoride resin (specific polymer) is preferably 5 to 30% by mass, the content of the pore-opening agent is preferably 0.1 to 15% by mass, the content of the solvent is preferably 40 to 94.9% by mass, and the content of the first non-solvent is preferably 0.5 to 20% by mass. From the viewpoint of the strength of the porous resin layer and thus the separation membrane, the content of the specific polymer in the polymer solution is preferably 5% by mass or more. From the viewpoint of water permeability, the content is preferably 30% by mass or less. The content is more preferably within the range of 8 to 20% by mass.

[0056] From the viewpoint of water permeability, the content of the pore-opening agent in the polymer solution is preferably 0.1% by mass or more. On the other hand, from the viewpoint of the strength of the porous resin layer and thus the separation membrane, and from the viewpoint of preventing the pore-opening agent from remaining in the porous resin layer and leaching out during use, which could lead to a deterioration in the quality of the permeate or fluctuations in water permeability, the content is preferably 15% by mass or less. A more preferred range of the pore-opening agent content in the polymer solution is 0.5 to 10% by mass.

[0057] Furthermore, the content of the solvent in the polymer solution is preferably 40% by mass or more from the viewpoint of suppressing gelation of the polymer solution. On the other hand, from the viewpoint of the strength of the porous resin layer and, in turn, the separation membrane, the content is preferably 94.9% by mass or less. The solvent content in the polymer solution is more preferably in the range of 60 to 90% by mass.

[0058] From the viewpoint of suppressing gelation of the polymer solution, the solvent content in the polymer solution is preferably within the range of 40 to 94.8% by mass and the first non-solvent content in the polymer solution is preferably within the range of 0.5 to 20% by mass, and more preferably within the range of 40 to 94.4% by mass and the first non-solvent content in the polymer solution is preferably within the range of 0.5 to 15% by mass.

[0059] As described above, it is preferable to include a hydrophilic resin in addition to the polyvinylidene fluoride resin as the polymer dissolved in the polymer solution. When the hydrophilic resin is included, the amount of the hydrophilic resin is preferably 0.1 to 10 parts by mass per 100 parts by mass of the polymer containing the polyvinylidene fluoride resin as the main component added to the polymer solution.

[0060] In the porous resin layer forming step, the above-mentioned polymer solution is solidified in a coagulation bath containing a second non-solvent to form a porous resin layer. The coagulation bath can be a second non-solvent or a mixture containing the second non-solvent and a solvent. In the coagulation bath, the second non-solvent is preferably at least 80% by mass in order to control the coagulation rate and surface pore size of the resin and to appropriately generate macrovoids. The second non-solvent is more preferably in the range of 85 to 100% by mass. The temperature of the coagulation bath is preferably selected within the range of 15 to 80°C in order to appropriately control the coagulation rate. A more preferred temperature range is 20 to 60°C.

[0061] When forming a separation membrane in which another layer is a support, it is preferable to form a porous resin layer on at least one surface of the support in the porous resin layer forming step. The formation of a coating of the polymer solution on the support can be done by applying the polymer solution to the support or by immersing the support in the polymer solution. When applying the polymer solution, it may be applied to one side or both sides of the support. In this case, depending on the composition of the polymer solution, it is preferable to apply a polymer solution having a density of 0.7 g / cm ３ It is preferable to use a support having a temperature of 1000 to 2000°C or less, since this allows the support to be appropriately impregnated with the polymer solution.

[0062] After the porous resin layer forming step, it is preferable to provide a washing step for removing the solvent and the pore-opening agent. The washing method can be appropriately selected depending on the type of solvent and the pore-opening agent, and is not particularly limited, but an example is a method of immersing in hot water at 60 to 100°C for 1 to 10 minutes.

[0063] Furthermore, after the porous resin layer formation step, it is preferable to provide a heating and drying step in air at a space temperature of 100°C or higher. The space temperature is preferably 100°C or higher and 175°C or lower to prevent melting of the polymer mainly composed of polyvinylidene fluoride resin. Furthermore, from the viewpoint of efficient moisture removal in the membrane and heating costs, it is more preferable to set the space temperature at 130°C or higher and 150°C or lower. The heating method and conditions can be appropriately selected depending on the membrane and are not particularly limited. Heating the membrane can increase the particle size of the polymer forming the membrane and increase the root mean square roughness (Rq).

[0064] Furthermore, the heating time in the heat drying step is preferably 20 minutes or less. By setting the heating time to 20 minutes or less, the average number density of the specific convex portions described above can be reduced to 0.16 pieces / μm ２ The heating time can be set to 0.5 minutes or more and 7 minutes or less. A heating time of 0.5 minutes or more is more preferable. A heating time of 0.5 minutes or more removes moisture from the membrane surface, allowing the polymer to change uniformly due to heating. Furthermore, a heating time of 7 minutes or less can prevent the polymer from shrinking and forming large pores locally, which would otherwise deteriorate the filtration performance.

[0065] The shape of the separation membrane to be produced can be controlled by the manner of coagulation of the polymer solution in the porous resin layer forming step. When a flat separation membrane is produced, for example, a film-like support made of a nonwoven fabric, a metal oxide, a metal, or the like, coated with the polymer solution can be immersed in a coagulation bath.

[0066] When producing a hollow fiber separation membrane, the polymer solution can be discharged from the outer periphery of the double-tube spinneret and the core liquid from the center simultaneously into a coagulation bath containing a second non-solvent. The core liquid is preferably a good solvent, etc., used in the polymer solution preparation process. Alternatively, a separation membrane may be formed on the surface of a hollow fiber support made of a polymer, metal oxide, metal, or the like. Examples of methods for forming a separation membrane on the surface of a hollow fiber support made of a polymer include a method in which a solution serving as a raw material for the hollow fiber support and the polymer solution are simultaneously discharged using a triple-tube spinneret, or a method in which a polymer solution is applied to the outer surface of a hollow fiber support that has already been formed, and the coated surface is then passed through a second non-solvent in a coagulation bath.

[0067] The separation membrane according to the present embodiment can be applied to both ultrafiltration membranes and microfiltration membranes. The water to be treated by the separation membrane according to the present embodiment is not particularly limited, but is preferably used in the separation of activated sludge for biological treatment of sewage or wastewater containing suspended solids of relatively large particles.

[0068] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0069] (i) Weight-average molecular weight of polyvinylidene fluoride resin constituting the porous resin layer. The separation membrane immersed in distilled water was frozen at -20 ° C using a cryostat (Leica; Jung CM3000), and a slice of the porous resin layer on the surface was collected and vacuum-dried overnight at 25 ° C. 5 mL of 0.1 M lithium chloride-added NMP was added to 2.5 mg of the vacuum-dried porous resin layer and stirred at 50 ° C for approximately 90 minutes. The resulting polymer solution was injected into a GPC (column: Showa Denko K.K.; Shodex KF-806M φ8.0 mm × 30 cm, two columns connected in series; differential refractive index detector: Showa Denko K.K.; RI-71 sensitivity -16) under the following conditions to measure the weight-average molecular weight. The injected polymer solution eluted from the column within a range of 28.5 to 43 minutes. The weight average molecular weight of the polyvinylidene fluoride resin constituting the porous resin layer is shown in the "Weight average molecular weight of PVDF" section in the table below. Column temperature: 40°C Flow rate: 0.5 mL / min Injection amount: 0.2 mL Standard sample: Monodisperse polystyrene manufactured by Tosoh Corporation Solvent: NMP with 0.1 M lithium chloride added

[0070] (ii) a value for the polymer constituting the porous resin layer The separation membrane immersed in distilled water was frozen at -20 ° C. using a cryostat (Leica; Jung CM3000), and a slice of the porous resin layer on the surface was collected and vacuum dried overnight at 25 ° C. 5 mL of 0.1 M lithium chloride-added NMP was added to 5 mg of the porous resin layer after vacuum drying, and the mixture was stirred at 50 ° C. for about 2 hours. The obtained polymer solution was injected into a GPC-MALS (column: Showa Denko K.K.; 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 and measured. The injected polymer solution was eluted from the column in the range of 27 to 43 minutes. Column temperature: 50°C Detector temperature: 23°C Solvent: NMP with 0.1 M lithium chloride Flow rate: 0.5 mL / min Injection volume: 0.3 mL

[0071] The elution time t obtained from the RI ｉ Polymer concentration c when ｉ , the elution time t obtained from MALS ｉ The excess Rayleigh ratio R θｉ From sin ２ (θ / 2) and (K ｃｉ / R θｉ ) １／２ (Berry plot or Zimm plot; Equation 5 below), and from the value of θ→0 in the approximate equation, each elution time t ｉ Absolute molecular weight M ｗｉ was calculated. Here, K is an optical constant, and is calculated from the following formula 4. Note that dn / dc in formula 4 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 ×n ０ ２ × (dn / dc) ２ / (λ ４ ×N ０ )...(Formula 4) n ０ : Refractive index of the solvent dn / dc: Refractive index increment λ: Wavelength of incident light in vacuum N ０ : Avogadro's number Also, each dissolution time t ｉ The radius of rotation at ２ 〉 １／２ The value of was calculated from the slope of the following formula 5. ｃｉ / R θｉ ) １／２ = M ｗｉ －１／２ {1 + 1 / 6(4πn ０ / λ) ２ <S ２ 〉sin ２ (θ / 2)} ... (Formula 5)

[0072] Each elution time t calculated from the above formula 5 ｉ Absolute molecular weight M ｗｉ is taken as the x-axis, and each elution time tｉ The radius of rotation at ２ 〉 １／２ was plotted on the y-axis, and the value of a for the polymer constituting the porous resin layer was 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 linear approximation using a logarithmic graph of formula 1 and applying the least squares method. The value of a for the polymer constituting the porous resin layer is shown in the "value of a in formula 1" section in the table below.

[0073] (iii) Amount of Bound Water in First Non-Solvent The amount of bound water was calculated after determining the amount of free water. The sample after vacuum drying was dissolved in distilled water to a concentration of 10 wt %, and the amount of free water was calculated from the integrated value of the endothermic peak appearing near 0°C using a DSC6200 manufactured by Seiko Instruments Inc. under conditions of a nitrogen atmosphere, a temperature rise of 2°C / min, and a temperature rise from -80 to 25°C, using the following formula 2: (Amount of free water) = {(Total weight of solution) x (Enthalpy of heat of fusion) x (Molecular weight of water)} / (Enthalpy of heat of fusion of water) ... (Formula 2) The amount of bound water was calculated from the following formula 3: (Amount of bound water) = (Total amount of water in solution) - (Amount of free water) ... (Formula 3)

[0074] (iv) Porosity of macrovoids in the region within 15 μm depth from the surface of the porous resin layer The separation membrane was stained with a fluorescent substance (3,3,3',3'-tetramethyl-1,1'-bis(4-sulfobutyl)indocarbocyanine sodium). Using a confocal laser microscope (Olympus Corporation; FV3000), the porous resin layer surface was positioned so that the axial direction of the laser light was perpendicular to the surface of the porous resin layer. A 200 μm x 200 μm area was observed as a plane parallel to the surface, and a region from the surface to a depth of 40 μm was observed every 0.2 μm. From the three-dimensional image created by stitching together the obtained images, a cross-sectional image perpendicular to the surface of the porous resin layer was extracted, and the area S1 from the surface to a depth of 15 μm was calculated. The observed cross-sectional image was binarized into the resin structure and pores, and the area S2 of macrovoids occupying a depth of 15 μm from the surface of the porous resin layer was calculated. The porosity (%) was calculated using the following formula 6. (Porosity) = S2 / S1 × 100 (Equation 6) Five randomly selected images were measured, and the average value was used as the porosity. The porosity of macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer is shown in the "Porosity of macrovoids" section in the table below.

[0075] (v) Average pore size on the surface of the porous resin layer The surface of the porous resin layer in the separation membrane was observed using an SEM (Hitachi High-Tech Corporation; S-5500) under the following observation conditions, and the area of ​​each hole in the observation field was measured. From the area of ​​each hole, the diameter when assuming that the hole was circular was calculated as the pore size. The average value was taken as the average pore size on the surface. Acceleration voltage: 5 kV Observation magnification: 30,000 times Observation field: 4.233 μm × 3.175 μm Image processing software: ImageJ (Wayne Rasband, National Institutes of Health) Measurements were performed for any 10 fields, and the average value was taken as the average pore size. The average pore size on the surface of the porous resin layer is shown in the "average pore size" section in the table below.

[0076] (vi) Pure Water Permeability of Separation Membrane A separation membrane was cut into a circle with a diameter of 40 mm and set in a cylindrical filter holder (Ultra Holder UHP-43K, manufactured by Advantec Toyo Co., Ltd.). Distilled water was pre-permeated through the filter holder at 25°C and a pressure of 5 kPa for 5 minutes, and then the filter holder was permeated again and the permeated water was collected for 3 minutes. The amount of permeated water (m ３ ) in terms of unit time (h) and unit membrane area (m ２ The pure water permeability of the separation membrane is shown in the "Pure Water Permeability" section in the table below.

[0077] (vii) Increase in clogging filtration resistance of separation membrane and increase in clogging filtration resistance after chemical immersion A separation membrane was cut into a circle with a diameter of 40 mm, immersed in ethanol overnight, and then immersed in water for 2 hours or more, and set in a cylindrical filter holder (Ultra Holder UHP-43K, manufactured by Advantec Toyo Co., Ltd.). Activated sludge (50 g) with a solids concentration of 7,000 mg / L was placed in the filter holder, and the stirring speed was adjusted to 450 rpm. The evaluation temperature was 25 ° C., and the membrane permeation flux, converted to a daily water permeation rate (cubic meters) per square meter of membrane surface, was 3.0 m ３ / m ２ / day, activated sludge is filtered for 2 minutes, and the sludge filtration resistance calculated from the amount of permeated water in the first 5 seconds during filtration is Res Ａｘ The value was set to x. x represents the number of times that 2-minute activated sludge filtration was repeated, with x = 1 for the first filtration. Here, the filtration resistance was calculated from the following formula 7: Res = P x t x S / (μ x L) ... (Formula 7) Res: filtration resistance P: evaluated pressure t: filtration time S: membrane area μ: viscosity L: amount of filtered water After filtration was stopped, the mixture was stirred for 1 minute at a stirring speed of 450 rpm. With the separation 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 mixture was stirred for 1 minute at a stirring speed of 450 rpm. Activated sludge filtration and membrane cleaning were repeated for 2 minutes, and Res Ａ１ ~Res Ａ５ Res was measured. Ａｎ＋１ -Res ＡｎThe increase in clogging filtration resistance was calculated for values ​​of n from 1 to 4, and the average value was taken as the increase in clogging filtration resistance. The smaller the increase in clogging filtration resistance value, the better the separation membrane's anti-fouling properties can be evaluated.

[0078] The separation membrane for which the increase in clogging filtration resistance had been measured was immersed in a 12,000 mg / L aqueous sodium hypochlorite solution (100 mL) for 250 hours while maintaining the temperature at 40° C., and then immersed in water for 2 hours or more to replace the chemical solution with water. The above increase in clogging filtration resistance was measured for the obtained separation membrane, and the increase in clogging filtration resistance after chemical immersion was determined.

[0079] (viii) Root-mean-square roughness (Rq) of the surface of the porous resin layer A separation 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 this sample separation membrane was observed with an atomic force microscope (Dimension FastScan, manufactured by Bruker AXS), and the root-mean-square roughness (Rq) of the surface of the porous resin layer was calculated. The specific measurement conditions were as follows: Scanning mode: Contact mode Probe: Silicon cantilever (Bruker AXS; ScanAsyst-Fluid) Scanning range: 10 μm × 10 μm Scanning speed: 3.0 Hz Scanning resolution: 512 × 512 Measurement temperature: 25 ° C. To calculate the root mean square roughness (Rq) per 10 μm square area, only images of areas within the 10 μm square image where the difference between Rq values ​​in any 10 μm × 5 μm area is 15 nm or less were used, and the average value of any five fields of view was taken as Rq. The root mean square roughness (Rq) of the surface of the porous resin layer is shown in the "root mean square roughness (Rq)" section in the table below.

[0080] (ix) The cross-sectional area of ​​the surface of the porous resin layer at a plane having a height of 50 nm from the reference surface is 0.015 μm ２ 0.10 μm or more ２The wet separation 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 (manufactured by Bruker AXS; Dimension FastScan), and the cross-sectional area of ​​the convex portions on a plane 50 nm high from the reference surface of the surface was calculated as described above. When the cross-sectional area at this time was 0.015 μm ２ 0.10 μm or more ２ The number of convex portions that were equal to or less than 0.015 μm was counted, and the average number density was calculated. Specific measurement conditions were as follows. Scanning mode: Contact mode Probe: Silicon cantilever (ScanAsyst-Fluid manufactured by Bruker AXS) Scanning range: 2.5 μm × 2.5 μm Scanning speed: 0.5 Hz Scanning resolution: 256 × 256 Measurement temperature: 25°C Measurement was performed for any five visual fields, and the average value of the average number density of convex portions in the five visual fields was taken as the average number density of convex portions of the porous film. The cross-sectional area of ​​the surface of the porous resin layer at a plane having a height of 50 nm from the reference surface was 0.015 μm ２ 0.10 μm or more ２ The average number density of the convex portions, which is as follows, is shown in the item "average number density of predetermined convex portions" in the table below.

[0081] (x) Polystyrene latex rejection rate after abrasion Using a falling sand abrasion resistance tester (ASTM D673), 200 g of an abrasive (SiC #45) was dropped from a height of 65 cm onto the membrane surface of a separation membrane cut into a circle with a diameter of 40 mm, and the separation membrane was abraded, and then washed with ethanol. The separation membrane after abrasion was cut into a circle with a diameter of 40 mm and set in a cylindrical filter holder (manufactured by Advantec Toyo Co., Ltd., Ultra Holder UHP-43K). 50 mL of a 20 ppm dispersion of polystyrene latex microparticles 83 nm (PSt, Seradyn) was added to the filter holder, and while stirring at 500 rpm, the pressure was increased to 10 kPa using compressed air. The head difference from the liquid surface to the outlet of the tube was set to 10.5 cm, and filtration was started at a transmembrane pressure difference of 11 kPa, and the permeate was collected. The dispersion after filtration was taken as concentrated water, and the absorbance of the collected concentrated water and raw water was measured using a UV spectrophotometer (UV-2450, Shimadzu Corporation). Here, the refractive index of the permeated water was Abs. A, and the refractive index of the concentrated water was Abs. B. The polystyrene latex rejection rate was calculated using the following formula 8: (Rejection rate) = 1 - (Abs. A) / (Abs. B) (Formula 8)

[0082] (Example 1) DMF as a solvent, PEG (weight average molecular weight 20,000) as a pore-opening agent, and glycerin (amount of bound water per 1 g: 1.9 g) as a first non-solvent were added to polyvinylidene fluoride (hereinafter referred to as "PVDF") 1 (Solef6013, linear PVDF manufactured by Solvay Specialty Chemicals), and the mixture was thoroughly stirred at a temperature of 100°C to prepare a polymer solution having the following composition: PVDF: 14 mass %, DMF: 73 mass %, PEG: 9 mass %, and glycerin: 4 mass %. Then, a density of 0.6 g / cm was obtained. ３The prepared polymer solution was applied to the surface of a polyester fiber nonwoven fabric support. After application, the fabric was immediately immersed in pure water at 20°C for 5 minutes to form a porous resin layer. The fabric was then immersed in hot water at 90°C for 2 minutes to wash away the DMF solvent and the PEG pore-opening agent, forming a separation membrane in which the porous resin layer had a three-dimensional network structure. The separation membrane included a porous resin layer and a support layer, which was another layer, with the porous resin layer located on the surface. The evaluation results of the resulting separation membrane are shown in Table 1. The weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 31%, and the average pore diameter at the surface of the porous resin layer was 46 nm. The pure water permeability, which is an indicator of water permeability performance, and the increase in clogging filtration resistance, which is an indicator of low-fouling performance, both showed excellent values. Furthermore, the increase in clogging filtration resistance after chemical immersion, which is an index of chemical resistance, and the polystyrene latex rejection rate after abrasion, which is an index of abrasion resistance, showed excellent values. In each table, "-" means that the corresponding item was not measured.

[0083] Example 2 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as follows: PVDF: 11% by mass DMF: 76% by mass PEG: 9% by mass Glycerin: 4% by mass The evaluation results of the obtained separation membrane are shown in Table 1. The weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 34%, the average pore diameter at the surface of the porous resin layer was 56 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.

[0084] Example 3 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 2, except that PVDF2 (Solef6020, linear PVDF manufactured by Solvay Specialty Chemicals) was used instead of PVDF1. The evaluation results of the resulting separation membrane are shown in Table 1. The weight-average molecular weight of PVDF was 583,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 36%, the average pore diameter at the surface of the porous resin layer was 37 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.

[0085] Example 4 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as follows. The amount of bound water per 1 g of the substance contained in the first non-solvent was 0.0 g for water and 1.9 g for glycerin. That is, among the substances contained in the first non-solvent, glycerin corresponds to a substance with a bound water amount per 1 g of 1.7 g to 3.0 g, and water corresponds to another non-solvent. PVDF: 14% by mass DMF: 69% by mass PEG: 9% by mass Water: 4% by mass Glycerin: 4% by mass The results of evaluating the obtained separation membrane are shown in Table 1. The weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 35%, the average pore diameter at the surface of the porous resin layer was 39 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.

[0086] Example 5 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as follows: PVDF: 14% by mass DMF: 69% by mass PEG: 9% by mass Glycerin: 8% by mass The evaluation results of the obtained separation membrane are shown in Table 1. The weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 35%, the average pore diameter at the surface of the porous resin layer was 35 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.

[0087] Example 6 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 5, except that the first non-solvent of the polymer solution was changed to a sodium lactate solution (approximately 70%). The amount of bound water per gram of sodium lactate was 2.5 g. The evaluation results of the obtained separation membrane are shown in Table 1. The weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 34%, the average pore diameter at the surface of the porous resin layer was 38 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.

[0088] Comparative Example 1 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that water was used as the first non-solvent. The evaluation results of the resulting separation membrane are shown in Table 2. The weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 11%, the average pore diameter at the surface of the porous resin layer was 57 nm, and the increase in clogging filtration resistance was inferior to the results of the examples.

[0089] Comparative Example 2 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 2, except that water was used as the first non-solvent. The evaluation results of the resulting separation membrane are shown in Table 2. The weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 10%, the average pore diameter at the surface of the porous resin layer was 76 nm, and the increase in clogging filtration resistance was inferior to the results of the examples.

[0090] Comparative Example 3 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Comparative Example 2, except that PVDF 2 was used instead of PVDF 1. The evaluation results of the resulting separation membrane are shown in Table 2. The weight-average molecular weight of PVDF was 583,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 8%, the average pore diameter at the surface of the porous resin layer was 68 nm, and the increase in clogging filtration resistance was inferior to the results of the Examples.

[0091] (Comparative Example 4) A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Comparative Example 3, except that the composition of the polymer solution was as shown below. PVDF: 9% by mass DMF: 78% by mass PEG: 9% by mass Water: 4% by mass The evaluation results of the obtained separation membrane are shown in Table 2. The weight average molecular weight of PVDF was 583,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 26%, the average pore diameter at the surface of the porous resin layer was 84 nm, and the increase in clogging filtration resistance was inferior to the results of the examples.

[0092] Comparative Example 5 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as follows: PVDF: 8% by mass DMF: 79% by mass PEG: 9% by mass Glycerin: 4% by mass The evaluation results of the obtained separation membrane are shown in Table 2. The weight average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 45%, the average pore diameter at the surface of the porous resin layer was 109 nm, and the increase in clogging filtration resistance was inferior to the results of the examples.

[0093] Comparative Example 6 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that PVDF3 (Solef 6008, linear PVDF manufactured by Solvay Specialty Chemicals) was used instead of PVDF1. The evaluation results of the resulting separation membrane are shown in Table 2. The increase in clogging filtration resistance was excellent. However, the increase in clogging filtration resistance after chemical immersion was inferior to that of Example 1 and Example 9 described below.

[0094] (Examples 7 and 8) A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as shown below and PVDF4 (KF1300 manufactured by Kureha) was used instead of PVDF1. Cellulose acetate was added as a hydrophilic resin. PVDF: 15% by mass, DMF: 70% by mass, PEG: 9% by mass, Glycerin: 2% by mass, Water: 4% by mass, Cellulose acetate: 0.3% by mass. In Example 8, the obtained separation membrane was heated in air at a space temperature of 140°C for 3.5 minutes. The results of evaluation of each separation membrane are shown in Table 3. In Examples 7 and 8, the weight-average molecular weight of PVDF was 344,000, the value of a in the above formula 1 was 0.42, and the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 45%. The average number density of the predetermined convex portions is 0 / μm in Example 7. ２ , and 0.16 particles / μm in Example 8. ２The average pore size on the surface of the porous resin layer was 45 nm in Example 7 and 46 nm in Example 8. Furthermore, the root mean square roughness Rq of the surface of the porous resin layer was 28 nm in Example 7 and 35 nm in Example 8. The pure water permeability showed excellent values ​​in both Examples 7 and 8, but Example 8 showed a better value than Example 7. The polystyrene latex rejection rate after rubbing showed excellent values ​​in both Examples 7 and 8.

[0095] (Example 9) A separation membrane was formed in the same manner as in Example 8, except that the heating time in air at a space temperature of 140°C was 24 minutes. The evaluation results of the obtained separation membrane are shown in Table 3. The weight average molecular weight of PVDF was 344,000, the value of a in the above formula 1 was 0.42, and the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 45%. The average pore diameter on the surface of the porous resin layer was 40 nm, and the average number density of the predetermined convex portions was 0.20 / μm. ２ The root mean square roughness Rq of the surface of the porous resin layer was 53 nm. The increase in clogging filtration resistance showed an excellent value. The polystyrene latex rejection rate after rubbing was 14%, which was a lower value than Examples 7 and 8, but was still a sufficiently excellent value. The increase in clogging filtration resistance after chemical immersion showed an excellent value compared to Comparative Example 6.

[0096] Comparative Example 7 A separation membrane with a porous resin layer having a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as shown below and Solef 460 (branched PVDF) manufactured by Solvay Specialty Chemicals was used as the PVDF. Branched PVDF: 17% by mass DMF: 70% by mass PEG: 9% by mass Glycerin: 4% by mass The results of evaluating the resulting separation membrane are shown in Table 4. The weight-average molecular weight of PVDF was 730,000, the value of a in the above formula 1 was 0.31, the porosity occupied by macrovoids in the region within a depth of 15 μm from the surface of the porous resin layer was 19%, the average pore diameter on the surface of the porous resin layer was 56 nm, and the average number density of the predetermined convex portions was 0.72 / μm ２ The polystyrene latex rejection rate after rubbing was inferior to the result of Example 9.

[0097]

[0098]

[0099]

[0100]

[0101] Although the present invention has been described in detail 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 a Japanese patent application (Patent Application No. 2024-029389) filed on February 29, 2024, the entire contents of which are incorporated by reference.

Claims

1. A separation membrane comprising a porous resin layer containing a polymer whose main component is a polyvinylidene fluoride resin and another layer, the porous resin layer being disposed on the surface, wherein the radius of gyration <S ２ 〉 １／２ and the absolute molecular weight M of the polymer ｗ a value of a for the polymer determined by approximating the above formula 1 is 0.40 or more and 0.48 or less, the weight average molecular weight of the polyvinylidene fluoride resin measured by GPC (gel permeation chromatography) is 300,000 or more, the porosity occupied by macrovoids in a region within a depth of 15 μm from the surface of the porous resin layer is 28% or more and 80% or less, and the average pore size in the surface of the porous resin layer is 10 nm or more and 100 nm or less. ２ 〉 １／２ = bM ｗ ａ ...(Formula 1) 2. The cross-sectional area of ​​the surface of the porous resin layer at a plane 50 nm above the reference surface is 0.015 μm ２ 0.10 μm or more ２ The average number density of the convex portions is 0.16 / μm ２ The separation membrane according to claim 1, wherein:

3. The separation membrane according to claim 1, wherein the root mean square roughness Rq per 10 μm square area on the surface of the porous resin layer is 30 nm or more and 50 nm or less.

4. The separation membrane according to any one of claims 1 to 3, wherein the other layer is a support.

5. A method for filtering a liquid using the separation membrane according to any one of claims 1 to 3.

6. A membrane filtration device using the separation membrane according to any one of claims 1 to 3.

7. A method for producing a separation membrane according to any one of claims 1 to 3, comprising: (i) a polymer solution preparation step of dissolving the polymer containing polyvinylidene fluoride resin as a main component using a polymer containing polyvinylidene fluoride resin as a main component, a first non-solvent containing 20% ​​or more of a substance with a bound water content of 1.7 g to 3.0 g per gram, a pore-opening agent, and a solvent to obtain a polymer solution; and (ii) a porous resin layer formation step of coagulating the polymer solution in a coagulation bath containing a second non-solvent to form the porous resin layer.

8. The method for producing a separation membrane according to claim 7, wherein the substance having a bound water amount per gram of 1.7 g to 3.0 g is glycerin.

9. The method for producing a separation membrane according to claim 7, wherein in the polymer solution preparation step, the polymer solution contains a hydrophilic resin.

10. The method for producing a separation membrane according to claim 7, further comprising a step of heating and drying in air at a space temperature of 100°C or higher after the porous resin layer forming step.

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