Porous membrane and method for manufacturing a porous membrane

Abstract

Conventional porous membranes in which a hydrophobic polymer and a cellulose-based resin are combined experience major deformation of the membrane structure due to steam sterilization, and do not have sufficient heat resistance. Provided as a heat-resistant membrane is a porous membrane in which a hydrophobic polymer and a cellulose-based resin with a high degree of saponification are combined. A porous membrane that solves the aforementioned problem has a hydrophobic polymer as a main component thereof, wherein the porous membrane includes a cellulose-based resin on the surface thereof, and when one face is treated as a surface A and the other face is treated as a surface B, the average pore diameter (hereinafter, surface pore diameter) of the surface A is smaller than than average pore diameter of the surface B, the surface pore diameter of the surface A is 5 to 50 nm, and the water contact angle of the surface A is 40° or less.

Description

[Technical Field]

[0001] This invention relates to porous membranes used in fields such as water treatment, pharmaceutical manufacturing, food processing, and fermentation. [Background technology]

[0002] In recent years, porous membranes such as microfiltration membranes and ultrafiltration membranes have been used in a variety of fields, including water treatment (water purification or wastewater treatment), medical fields (blood purification, etc.), and the food industry. In the food industry, recent filtration stocks (filtrates) contain organic substances with small molecular weights, such as enzymes, proteins, and polysaccharides. Filtration stocks containing organic substances have been difficult to filter because porous membranes are prone to clogging. Therefore, there is a need for porous membranes that can efficiently filter such stocks. For these applications, porous membranes need to have resistance to fouling and chemicals.

[0003] Regarding stain resistance, it is important to make the surface pores finer to prevent organic matter from penetrating the interior of the porous membrane, and to improve hydrophilicity to suppress the adhesion of organic matter. Regarding chemical resistance, it is necessary to clean the membrane with chemicals such as sodium hypochlorite when organic matter adheres to it, so it is important that the porous membrane is composed of components that do not change in structure even after immersion in chemicals.

[0004] For the reasons stated above, it is preferable that the resin constituting the porous membrane includes both hydrophobic and hydrophilic polymers. Patent documents 1 and 2 disclose, for example, polyvinylidene fluoride-based resins and porous membranes containing both polyvinylidene fluoride-based resins and cellulose-based resins.

[0005] Furthermore, in fields such as pharmaceutical manufacturing, food processing, and fermentation, when repeated filtration is performed, it is necessary to sterilize the material before filtration to prevent contamination and avoid a decline in product quality and productivity due to bacterial contamination. Sterilization methods include heat treatment such as dry heat sterilization and steam sterilization, electromagnetic wave treatment such as gamma ray sterilization, and chemical sterilization using ethylene oxide gas. Among these sterilization methods, steam sterilization is particularly suitable from the standpoint of the simplicity of the sterilization equipment and safety for human health. Therefore, it is important that porous membranes containing hydrophobic and hydrophilic polymers have heat resistance so that the structure, performance, and especially the organic matter removal performance of the porous membrane do not change even when subjected to repeated high-temperature steam sterilization. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2023 / 054228 [Patent Document 2] International Publication No. 2012 / 063669 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, conventional porous membranes containing hydrophobic and hydrophilic polymers have suffered significant deformation of the membrane structure due to steam sterilization, resulting in insufficient heat resistance. Specifically, Patent Document 1 reports an example of a porous membrane containing polyvinylidene fluoride resin and cellulose resin that possesses chemical resistance and stain resistance. However, the porous membrane disclosed in Patent Document 1 underwent a change in its surface pore structure due to high-temperature steam sterilization, resulting in a decrease in its ability to block organic matter. This is presumed to be because the interaction between polymer chains constituting the porous membrane is small, and the mobility of the polymer chains is high. As the pores shrink during steam heating, the surrounding pores are pulled, causing them to expand and resulting in a pore diameter of 20 nm or more.

[0008] Patent Document 2 describes a method in which a porous membrane mainly composed of polyvinylidene fluoride resin is brought into contact with steam beforehand to suppress subsequent thermal changes. However, the method described in Patent Document 2 does not have heat resistance, so it merely causes pre-emptive pore expansion, and the pores are relatively large, making it impossible to prevent the intrusion of organic matter. As a result, high organic matter blocking performance cannot be obtained even for filtration stocks containing organic matter with small molecular weights.

[0009] The present invention aims to provide a porous membrane that has heat resistance in addition to stain resistance and chemical resistance, and to provide a method for producing a porous membrane by compounding a hydrophobic polymer with a highly saponifiable cellulose-based resin as a hydrophilic polymer. [Means for solving the problem]

[0010] To solve the above problems, the present invention provides a porous membrane and a method for manufacturing a porous membrane, comprising the following configurations. (1) A porous membrane mainly composed of a hydrophobic polymer, wherein the surface contains a cellulose resin, one side is designated as surface A and the other side as surface B, the average pore diameter of surface A (hereinafter referred to as surface pore diameter) is smaller than the average pore diameter of surface B, the surface pore diameter of surface A is 5 nm or more and 50 nm or less, and the contact angle with water on surface A is 10° or more and 40° or less. (2) The hydrophobic polymer comprises at least one thermoplastic resin selected from the group consisting of polyvinylidene fluoride resins, polyethersulfone resins, and polysulfone resins, and the absorption wavelength of the pyranose ring of the cellulosic resin (10³⁴ cm) relative to the following peak intensity Ix originating from the hydrophobic polymer, measured on surface A by ATR-IR (total internal reflection) method. -1 The porous membrane according to (1), wherein the intensity ratio (Is / Ix) of the peak intensity Is derived from ) is 0.6 or more and 1.5 or less. <Absorption wavelength of hydrophobic polymers> In the case of polyvinylidene fluoride resins, the absorption wavelength is 881 cm², which is derived from polyvinylidene fluoride. -1 In the case of polyethersulfone resins, 1578 cm is derived from the polyethersulfone.-1 In the case of polysulfone resins, 1580 cm² is derived from polysulfone. -1 Let's assume that. (3) The hydrophobic polymer is a polyvinylidene fluoride resin, and the absorption wavelength of polyvinylidene fluoride (881 cm) measured on surface A by ATR-IR (total internal reflection) is -1 The absorption wavelength (1034 cm) of the pyranose ring of the cellulose resin relative to the peak intensity Ix derived from ) -1 The porous membrane according to (1), wherein the intensity ratio (Is / Ix) of the peak intensity Is derived from ) is 0.6 or more and 1.5 or less. (4) The absorption wavelength of the pyranose ring of the cellulose resin, measured on the surface A of the porous membrane by ATR-IR (total internal reflection measurement), is 10³⁴ cm⁻¹. -1 The absorption wavelength (1744 cm) of the acetyl group in cellulose resin relative to the peak intensity Is derived from ) -1 A porous membrane according to any of (1) to (3), wherein the intensity ratio (Iso / Is) of the peak intensity Iso derived from ) is 0.01 or more and 0.6 or less.

[0011] (5) The porous membrane according to any one of (1) to (4), wherein the fractional molecular weight of the porous membrane is 10,000 to 200,000. (6) The porous membrane according to (1) to (5), wherein the porous membrane is composed of a plurality of layers, one of the outermost surfaces of the layer having one outermost surface is surface A, the other outermost surface of the layer having the other outermost surface is surface B, and the Is / Ix measured by ATR-IR (total reflection measurement) on surface B is 0.6 or less. (7) The porous membrane according to (6), wherein the other outermost layer of the porous membrane composed of the plurality of layers is a porous structure. (8) The porous membrane according to (1) to (7), wherein surface A is the surface that comes into contact with the liquid to be treated. (9) A method for producing a porous membrane, comprising: a step (A) of dissolving a polymer resin in a solvent to obtain a polymer solution; and a step (B) of coagulating the polymer solution in a non-solvent to form a porous membrane. In step (A), the polymer solution contains a hydrophobic polymer and a cellulose resin. In step (B), the non-solvent contains 90 to 100% by weight of water, and the temperature of the non-solvent is 6°C to 60°C. After steps (A) and (B), the method further includes a step (C) of bringing at least one surface of the formed porous membrane into contact with an alkaline solution. (10) The method for producing a porous membrane according to (9), wherein the surface is a surface with a smaller average pore diameter (hereinafter referred to as surface A), and surface A is subjected to the treatment of step (C). (11) In step (A), the hydrophobic polymer is mainly composed of a polyvinylidene fluoride resin. In step (C), the alkaline solution contains an aqueous sodium hydroxide solution with a concentration of 0.0001N to 0.01N, and the formed porous membrane is immersed in the alkaline solution. The method for producing a porous membrane according to (9) or (10). (12) A method for producing a porous membrane having a plurality of layers. In step (B) of coagulating the polymer solution in a non-solvent to form a porous membrane, the porous membrane is formed on the surface of a porous structure, and the outermost surface of the formed porous membrane becomes surface A. In step (C) of bringing the surface into contact with the alkaline solution, surface A is treated. The method for producing a porous membrane according to any one of (9) to (11). (13) A method for filtering a liquid using the porous membrane according to any one of (1) to (8). (14) The method for filtering a liquid according to (13), wherein the liquid is a sugar solution containing sugar, protein, and their reaction products. (15) The method for filtering a liquid according to (14), wherein the sugar solution has an absorbance at a wavelength of 420 nm of 0.01 to 30 and a Brix of 10 to 75. (16) A membrane filtration device comprising the porous membrane according to any one of (1) to (8).

Advantages of the Invention

[0012] According to the present invention, it is possible to provide a porous membrane that has heat resistance in addition to stain resistance and chemical resistance, and can ensure high organic matter blocking performance even for a stock solution to be filtered containing organic matter with a small molecular weight.

Brief Description of the Drawings

[0013] [Figure 1] It is a schematic diagram showing the concept of the filtration situation of the porous membrane of the present invention. [Figure 2] It is a schematic diagram showing the concept of the filtration situation by a conventional porous membrane.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited thereto. In this specification, "mass" and "weight" are synonymous. Further, a stock solution to be filtered containing organic matter with a small molecular weight means a stock solution in which the weight average molecular weight of the organic matter contained in the stock solution to be filtered is from 15,000 Da to 𝐷𝑎. Organic matter is, for example, protein or polysaccharide. These are stock solutions that have conventionally been difficult to filter, but by using the porous membrane of the present invention as a separation membrane, it becomes possible to filter efficiently from a stock solution to be filtered containing organic matter such as enzymes, proteins, and polysaccharides with an average molecular weight of from 15,000 Da to 40,000 Da.

[0015] "Having heat resistance" in the present invention means that the structure and properties of the porous membrane do not change even when the porous membrane is exposed to high temperatures. In the present invention, it means that the change in the organic matter blocking performance before and after steam sterilization is small. A membrane without heat resistance has a large change in the organic matter blocking performance before and after steam sterilization, and the organic matter blocking performance deteriorates after steam sterilization.

[0016] Conventional porous membranes containing polyvinylidene fluoride resins and cellulose resins tended to deteriorate in their organic matter blocking performance after steam sterilization. When organic matter blocking performance deteriorates, small molecular weight organic substances can penetrate the porous membrane, reducing filtration efficiency. This is thought to be because the polymer chains constituting the porous membrane are highly mobile, and steam treatment tends to cause the micropores to become smaller and the coarse pores to become larger.

[0017] Therefore, after diligent research, the inventors discovered that by substituting carbonyl groups in the cellulose resin within the porous membrane with hydroxyl groups through a saponification reaction, numerous hydrogen bonds are formed between the polymer chains of the cellulose resin, thereby reducing the mobility of the polymer chains of the hydrophobic polymer containing the cellulose resin. In particular, the dispersion of the cellulose resin within the hydrophobic polymer in the porous membrane strengthens the hydrogen bonding force between the polymer chains of the hydrophobic polymer and the cellulose resin. To disperse the cellulose resin within the hydrophobic polymer, it is preferable to mix the hydrophobic polymer and the cellulose resin from the raw materials. By reducing the mobility of the cellulose resin using the method described above, the mobility of the polymer chains of the entire porous membrane containing the hydrophobic polymer is reduced. Therefore, even when steam treatment is performed, the polymer chains of the entire porous membrane are less likely to move, making it possible to reduce changes in pore size and maintain the inhibition rate of organic matter. The porous membrane with improved heat resistance of the present invention will be described below.

[0018] <About porous membranes> The porous membrane according to the embodiment of the present invention is a porous membrane comprising a hydrophobic polymer and a cellulose-based resin, wherein the surface in contact with the liquid to be treated is designated as surface A, and the other surface as surface B, and the average pore size of surface A (hereinafter referred to as average surface pore size) is smaller than that of surface B. That is, the average surface pore size of surface A must be smaller than that of surface B, and the contact angle of surface A with water must be 40° or less. By having a smaller pore size on the surface in contact with the liquid to be treated than on the other surface, it is possible to prevent organic matter from entering the pores and to improve the permeability of the permeate. When targeting a filtration stock solution containing organic matter with a small molecular weight, a surface pore size of 5 to 50 nm is required to prevent fouling components and substances to be removed from the filtration stock solution from entering the porous membrane, thereby obtaining high fouling resistance. The surface pore size will be described later.

[0019] A contact angle of 40° or less on surface A indicates that, as described above, in a porous membrane containing a hydrophobic polymer and a cellulose-based resin, the amount of hydroxyl groups on the surface in contact with the liquid being treated increases, resulting in the formation of numerous hydrogen bonds. When numerous hydrogen bonds are formed on the surface in contact with the liquid being treated, it becomes possible to maintain the pore size that blocks organic matter even after steam sterilization, improving the heat resistance of the porous membrane. A contact angle of 40° or less on surface A with water is an indicator of excellent heat resistance. Here, a smaller contact angle on surface A with water is preferable because it reduces the movement of polymer chains, and 35° or less is more preferable. Furthermore, regarding the lower limit of the contact angle, 10° or more is preferable from the viewpoint of converting the amount of hydroxyl groups while the cellulose-based resin is dispersed in the hydrophobic polymer within the porous membrane.

[0020] The contact angle is measured using a contact angle meter, as described later. In this invention, the contact angle is determined by the underwater bubble method, where bubbles are brought into contact with the porous membrane surface while it is submerged in water, the static contact is measured, and the static contact angle with respect to the water surface is determined from the obtained contact angle.

[0021] Figure 1 shows a schematic diagram of filtering a raw liquid that has been considered difficult to filter using the porous membrane of the present invention. Figure 1 is a conceptual schematic diagram showing a part of the cross-section of the porous membrane. The filtration direction FL is the direction from surface A to surface B. In the figure, the porous membrane 101 has a surface layer 102 and an inner layer 103. On surface A of the surface layer 102, there are many fine pores, and the pore size is 5 to 50 nm (shown between the lines of the fine mesh in Figure 1). The inner layer 103 has a larger pore size than the surface layer 102. Organic matter 201 contained in the liquid to be filtered is larger than the pore size of surface A and cannot penetrate the porous membrane. The porous membrane of the present invention has strong interactions between polymer chains, so even when high-temperature steam sterilization is performed with water vapor, the mobility of the polymer chains can be reduced, and therefore the change in pore size is small, and fine organic matter 201 cannot penetrate into the pores.

[0022] Figure 2 shows a schematic diagram of filtering a highly difficult-to-filter raw material using a porous membrane known to date. In the porous membrane of Patent Document 1, the interaction of polymer chains is weak, so when steam sterilization is performed the polymer chains become highly mobile, and therefore the pore size changes significantly, resulting in the phenomenon where fine pores become finer and coarse pores become coarser. In Figure 2, coarse pores are present on the surface in contact with the liquid being treated (the areas with a partially coarse mesh in Figure 2). Therefore, fine organic matter 201 can easily penetrate into the pores, and the ability to block organic matter decreases.

[0023] Examples of the hydrophobic polymer contained in the porous membrane include polysulfone resins, polyethersulfone resins, and polyvinylidene fluoride resins, and it is preferable to contain polyvinylidene fluoride resins. The polyvinylidene fluoride resin in the present invention refers to a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride. Here, the copolymer of vinylidene fluoride refers to a polymer having a vinylidene fluoride residue structure. The polymer having a vinylidene fluoride residue structure is typically a copolymer of a vinylidene fluoride monomer and other fluorine-based monomers or the like. Examples of such fluorine-based monomers include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, or chlorotrifluoroethylene. In the above copolymer of vinylidene fluoride, ethylene or the like other than the above fluorine-based monomers may be copolymerized as long as the effects of the present invention are not impaired. <oooo139> The ratio (Is / Ix) of the peak intensity Is (S = 1034 cm -1 If it is polyethersulfone, X = 1578 cm -1 If it is polysulfone, X = 1580 cm -1 ) derived from the pyranose ring of the cellulose-based resin to the peak intensity Ix of the hydrophobic polymer (if the absorption wavelength of the hydrophobic polymer is polyvinylidene fluoride, X = 881 cm -1 ) measured by the ATR-IR method (total reflection measurement method) on the surface A of the porous membrane is 0.6 to 1.5, so that the proportion of the cellulose-based resin on the membrane surface is large and excellent heat resistance can be obtained. Furthermore, it is preferable that the surface A mainly contains a hydrophobic polymer and contains a large amount of the cellulose-based resin within an appropriate range. It is preferable that Is / Ix is 0.6 to 1.2, more preferably Is / Ix is 0.6 to 1.0, and particularly preferably Is / I X is 0.6 to 0.8. In the ATR-IR method, the depth within 5 μm from the outermost surface can be analyzed. Therefore, the composition state of the compound on the surface in contact with the liquid to be treated can be seen. Since the surface A mainly contains a hydrophobic polymer, the chemical resistance is good.

[0025] When the hydrophobic polymer is a polyvinylidene fluoride resin, three crystalline structures are known for polyvinylidene fluoride resins: α-type, β-type, and a γ-type, which is present in very small amounts. It is generally known that when polyvinylidene fluoride resins are exposed to chemicals or subjected to stress by heat, the ratio of α-type, β-type, and γ-type changes. On the other hand, among the peak intensities representing polyvinylidene fluoride resins, 881 cm⁻¹ is the highest. -1 This peak intensity is common to all crystals and does not change with chemical treatment. Furthermore, even in cellulose-based resins, the pyranose ring in its chemical structure is not affected by chemical treatment. Therefore, the ratio and state of the hydrophobic polymer and cellulose-based resin on surface A can be evaluated.

[0026] Furthermore, the peak intensity Iso(S0), derived from the acetyl group of the cellulose resin, is compared to the peak intensity Is derived from the pyranose ring of the cellulose resin, as measured by ATR-IR (total internal reflection) on surface A. The absorption wavelength of Iso(S0) is 1744 cm². -1 A ratio of (Iso / Is) of 0.6 or less is preferable because it increases the rate at which acetyl groups in the cellulosic resin on the film surface are converted to hydroxyl groups, resulting in excellent heat resistance. Since a higher rate of conversion to hydroxyl groups improves heat resistance, a ratio of 0.5 or less is preferable, more preferably 0.2 or less, and even more preferably 0.1 or less. As mentioned above, the pyranose ring is not affected by saponification, while the acetyl group is converted to hydroxyl groups by saponification, and its proportion decreases. In other words, a state in the cellulosic resin where there are few acetyl groups relative to the peak intensity derived from the pyranose ring indicates a high proportion of hydroxyl groups in the cellulosic resin. Furthermore, the lower limit of Iso / Is can be set arbitrarily. From the viewpoint of production efficiency, an Iso / Is of 0.01 or higher is preferable. Within this range, porous films can be efficiently manufactured while having excellent heat resistance and chemical resistance.

[0027] A molecular weight cutoff of 10,000 to 200,000 for the porous membrane is preferable because it prevents organic substances, which are the target of removal and concentration in the filtration stock solution, from penetrating the porous membrane, thus easily exhibiting high organic substance blocking properties. A molecular weight cutoff of 10,000 to 60,000 for the porous membrane is more preferable, and 10,000 to 30,000 is particularly preferable. In this case, the molecular weight cutoff is defined as the molecular weight at which a 90% removal rate is achieved when several types of dextran aqueous solutions are filtered. Dextran aqueous solutions were prepared by mixing 500 ppm each of dextran from Fluka with average molecular weights of 1500 Da, 6000 Da, 15000-25000 Da, and 40000 Da, and dextran from Aldrich with average molecular weights of 60000 Da and 200000 Da, and filtering them at a membrane linear velocity of 0.9 m / sec. The filtrate and the filtered water at that point were sampled, and GPC measurements were performed to calculate the removal rate.

[0028] The surface pore diameter of surface A of the porous membrane being 5 to 50 nm is preferable because it prevents contaminants and substances to be removed from the filtration solution from penetrating into the porous membrane, thus easily exhibiting high fouling resistance. It also maintains high resistance to organic matter and easily exhibits heat resistance. A surface pore diameter of 12 nm or less is more preferable, and 8.0 nm or less is more preferable. Surface pore diameter refers to the diameter of the pores located within the plane when the surface of the porous membrane is observed. To determine the surface pore diameter of a porous membrane, the image obtained by SEM observing the surface of the porous membrane is binarized using the free software "ImageJ". When binarizing, create a background with 1 pixel in Subtract Background, and then select the condition: RenyEntropy in Threshold (binarization threshold). In the obtained binarized image, select Area in Analyze Particles to find the area of ​​each pore, and the diameter calculated assuming each pore is a circle is taken as the surface pore diameter. When determining the average value of the surface pore diameter, the pore diameters of 1,000 or more pores are averaged.

[0029] The standard deviation of the surface pore size of the initial surface A of the porous membrane is 10.0 nm or less. This allows many pores to shrink uniformly during steam treatment, maintaining high resistance to organic matter and exhibiting heat resistance. The standard deviation is preferably 5.0 nm or less, and more preferably 2.5 nm or less. The lower limit of the standard deviation is 1.0 nm or more.

[0030] The porous membrane of the present invention may consist of a single layer or multiple layers. When it consists of multiple layers, it is preferable that the layer in contact with the liquid to be treated (the layer containing surface A) contains a large amount of cellulose resin, and that the Is / Ix ratio in the layer containing surface B is 0.6 or less. A large amount of cellulose resin in the layer in contact with the liquid to be treated makes it easier to exhibit heat resistance in the region that blocks organic matter, which is preferable. Furthermore, in the layer containing surface B, the amount of cellulose resin is small, and it deforms with heat, causing the pore size to expand, resulting in a fine structure on the surface in contact with the liquid to be treated and a coarse structure in the other layers, which is preferable from the viewpoint of improving the permeability of the permeate. In other words, the average pore size of surface A of the porous membrane of the present invention is smaller than the average pore size of surface B, and the surface pore size of surface A is 5 to 50 nm.

[0031] <About the manufacturing method of porous membranes> The porous membrane of the present invention can be in the form of a flat plate, hollow fiber, or tubular shape, and an appropriate shape can be selected and used depending on the type of filtration device used and the properties of the liquid to be filtered.

[0032] The porous membrane of the present invention can be obtained by a manufacturing method comprising the steps of (A) dissolving a polymer in a solvent to obtain a polymer solution, (B) forming a porous membrane by solidifying the polymer solution in a non-solvent, and (C) contacting the porous membrane with an alkaline solution, wherein the non-solvent used in step (B) contains 90 to 100% by weight of water, and the temperature of the non-solvent is 6°C to 60°C. Steps (A) (obtaining a polymer solution) and (B) (forming a porous membrane) are non-solvent-induced phase separation methods, and the porous membrane can be obtained by mainly comprising a hydrophobic polymer and a cellulose-based resin, with a surface pore size of 5 to 12 nm. In other words, the method for manufacturing the porous membrane of the present invention comprises the steps of (A) dissolving a solution polymer resin in a solvent to obtain a polymer solution, and (B) forming a porous membrane by solidifying the polymer solution in a non-solvent, wherein step (A) contains a hydrophobic polymer and a cellulose resin, and step (B) contains 90 to 100% by weight of water as the non-solvent, and the temperature of the non-solvent is 6°C to 60°C. Then, following steps (A) and (B), step (C) brings the porous membrane into contact with an alkaline solution. This process converts the acetyl groups in the cellulose resin on surface A of the porous membrane into hydroxyl groups, and hydrogen bonds are formed in the polymer chain, resulting in excellent heat resistance.

[0033] The polymer used in step (A) is preferably a hydrophobic polymer and a cellulose acetate resin, but polyvinylidene fluoride resin and cellulose acetate resin are particularly preferred. In the present invention, polyvinylidene fluoride resin refers to a homopolymer or copolymer of vinylidene fluoride. Here, a copolymer of vinylidene fluoride refers to a polymer having a vinylidene fluoride residue structure. A polymer having a vinylidene fluoride residue structure is typically a copolymer of vinylidene fluoride monomer and other fluorinated monomers. Examples of such fluorinated monomers include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, or trifluoroethylene chloride. In the above-mentioned copolymer of vinylidene fluoride, ethylene or other elements other than the above-mentioned fluorinated monomers may be copolymerized to an extent that does not impair the effects of the present invention.

[0034] Furthermore, the weight-average molecular weight of the polyvinylidene fluoride resin can be appropriately selected depending on the required strength and water permeability of the porous membrane. Generally, as the weight-average molecular weight increases, water permeability decreases, and as the weight-average molecular weight decreases, strength decreases. For this reason, a weight-average molecular weight of 50,000 to 1,000,000 is preferable. In particular, when the treatment stock solution is highly turbid and it is necessary to remove clogging substances adhering to the porous membrane by chemical washing and repeatedly filter the highly turbid treatment stock solution, a weight-average molecular weight of 100,000 to 700,000 is preferable. Furthermore, when chemical washing is performed multiple times, a weight-average molecular weight of 150,000 to 600,000 is preferable.

[0035] Furthermore, the cellulose-based resin in this invention is not particularly limited as long as it has cellulose ester as a molecular unit in the main chain and / or side chains, and other molecular units may also be present. Examples of molecular units other than cellulose ester include alkenes such as ethylene and propylene, alkynes such as acetylene, vinyl halides, vinylidene halides, methyl methacrylate, and methyl acrylate. In particular, ethylene, methyl methacrylate, and methyl acrylate are readily available at low cost and are easily introduced into the main chain and / or side chains, so they are preferred for use. Known polymerization techniques such as radical polymerization, anionic polymerization, and cationic polymerization can be used as introduction methods. In addition, homopolymers that consist substantially only of cellulose ester as molecular units are known. Homopolymers are readily available at low cost and are easy to handle, so they are preferred for use. Examples of such homopolymers include cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate.

[0036] The solvent preferably contains a good solvent. Here, a "good solvent" refers to a solvent that can dissolve 5% by weight or more of the polymer even in a low temperature range of 60°C or below. Examples of good solvents include N-methyl-2-pyrrolidone (hereinafter "NMP"), 2-pyrrolidone (hereinafter "2P"), ε-caprolactam (hereinafter "ε-CL"), dimethyl sulfoxide (hereinafter "DMSO"), dimethylacetamide (hereinafter "DMAc"), dimethylformamide (hereinafter "DMF"), methyl ethyl ketone, acetone, tetrahydrofuran, tetramethylurea or trimethyl phosphate, glycerin, or a mixture thereof. It is even more preferable that the good solvent constitutes 40% by weight or more of the solvent, and particularly preferable that it constitutes 60% by weight or more. A high content of the good solvent is preferable because it allows the polymer chains to expand in the polymer solution, making it easier to control the viscosity of the solution within a suitable range.

[0037] Here, the "non-solvent" in step (B) refers to a solvent that does not dissolve or swell the polymer even when heated to its boiling point. Examples of non-solvents include aliphatic hydrocarbons such as water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, pentanediol, hexanediol, or low molecular weight polyethylene glycol, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, or other chlorinated organic liquids or mixed solvents thereof. The concentration (by weight) of the polymer in the polymer solution is preferably above the entanglement concentration in order to control the viscosity of the solution within a suitable range, more specifically, 10 to 40% by weight is preferred, 12 to 30% by weight is even more preferred, and 15 to 25% by weight is particularly preferred.

[0038] The porous film formation process in step (B), which involves solidifying the polymer solution in a non-solvent to form a porous film, is a process of forming a porous film by so-called non-solvent-induced phase separation. When the polymer solution comes into contact with a non-solvent, the polymer cannot completely dissolve in the solvent, and phase separation occurs into a phase containing a large amount of polymer and a phase containing a large amount of solvent. Each phase then coarses as it merges with the surrounding identical phases.

[0039] Step (C) is a step in which the porous membrane produced in steps (A) and (B) is brought into contact with an alkaline solution. By immersing the porous membrane in an alkaline solution at a predetermined temperature for a predetermined time, the porous membrane is saponified. In this embodiment, for example, the porous membrane produced in steps (A) and (B) is immersed in a 0.0001 to 1.0 N aqueous sodium hydroxide solution at room temperature to 60°C for 10 to 1440 minutes while stirring. Room temperature means a range of 15 to 35°C. Here, regarding the concentration of sodium hydroxide, immersion temperature, and immersion time, if the concentration is high, the immersion temperature can be lowered or the immersion time can be shortened. Conversely, if the concentration is low, it is preferable to raise the immersion temperature or lengthen the immersion time. The present invention is not dependent on saponification conditions. Therefore, the alkaline solution includes alkaline aqueous solutions, and examples of alkaline solutions include sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonia (NH3), etc., but is not limited to these. From the viewpoint of carrying out the saponification reaction efficiently and stably, the alkaline solution is preferably pH 10 or higher, and more preferably pH 11 or higher.

[0040] As described above, it is preferable that at least a large number of hydroxyl groups are present on surface A and that many hydrogen bonds are formed. Alternatively, surface A may be exposed to an alkaline solution with a pH of 10 or higher for a short period of time.

[0041] The porous membrane of the present invention may further comprise other layers. In this case, it is preferable that surface A is arranged in contact with the liquid to be treated, as this makes it difficult for components contained in the filtration stock to penetrate into the interior of the porous membrane, and allows for the maintenance of high permeability performance over a long period of time. The above-mentioned other layers are not particularly limited as long as they are components that can overlap with the layer including surface A to form a layered structure, but it is preferable that they are porous structures with high breaking strength. To increase the breaking strength, the breaking strength (breaking strength per unit area) is preferably 3 MPa or more, and more preferably 10 MPa or more. If the composite membrane comprising the other layers is hollow fiber-like, the breaking strength of the other layers is preferably 2900 N or more, and more preferably 7800 N or more.

[0042] Liquids can be filtered using the porous membrane of the present invention obtained as described above. The membrane filtration device may include, but is not limited to, a raw water tank, a booster pump, a module composed of several thousand to tens of thousands of porous hollow fiber membranes of the present invention, a filtered water tank, a backwash pump, etc. As for the method of filtering liquids, for example, raw liquids such as factory wastewater can be filtered using the above membrane filtration device at an operating pressure of 10 kPa to 1 MPa to remove organic matter and other substances contained in the raw water, but is not limited to these.

[0043] The liquid filtration method using the porous membrane of the present invention is suitable for filtering liquids containing sugars, proteins, and their reactants. Examples of such liquids include polysaccharides, gelatin, and Maillard reaction compounds. In particular, when filtering a sugar solution containing Maillard reaction compounds, it is preferable that the absorbance at a wavelength of 420 nm is 0.01 to 30 and the Brix is ​​10 to 75. When the absorbance at 420 nm and Brix are within this range, Maillard reaction compounds can be efficiently removed. If the Brix is ​​10 or higher, the material to be filtered is not too diluted, and filtration can be performed efficiently. If the Brix is ​​75 or lower, it is preferable from the viewpoint of the solubility of the material to be filtered in an aqueous solution. From the viewpoint of the permeate flow rate during filtration, the Brix is ​​preferably 50 or lower, and particularly preferably 30 or lower. If the absorbance at 420 nm is 5.0 or lower, it is particularly preferable from the viewpoint of the solubility of Maillard reaction compounds. If the absorbance at 420 nm is 0.01 or higher, the Maillard reaction compounds are not too diluted, and filtration can be performed efficiently.

[0044] The absorbance of the above sugar solution at 420 nm can be measured using a spectrophotometer. The Brix value is calculated by measuring the refractive index using a refractometer. A sugar solution satisfying an absorbance of 0.01 to 30 at a wavelength of 420 nm and a Brix of 10 to 75 can be prepared, for example, by dissolving raw sugar. Furthermore, the absorbance and Brix at a wavelength of 420 nm can be brought within a desirable range by diluting or concentrating the solution.

[0045] The steam sterilization assumed in this invention is based on the standards described in JIS K 3605 (1992), where the standard steam sterilization conditions are 121°C for 20 minutes. In industrial steam sterilization, the temperature and time may be further increased for safety reasons. [Examples]

[0046] The present invention will be specifically described below with reference to examples, but the present invention is not limited in any way by these examples. First, the measurement method and evaluation method are shown below.

[0047] (i) Method for measuring the contact angle of a porous membrane The static contact angle can be measured using the Drop Master DM500 manufactured by Kyowa Interface Science Co., Ltd. Static contact was measured by bringing a bubble (approximately 2 μL) into contact with the porous membrane surface while it was submerged in pure water. The contact angle relative to the water surface was determined from the contact angle obtained from the tangent of the bubble in contact with the surface. Pure water was used, the measurement temperature was set to 20°C, and the contact angle was determined from the average value of three bubbles measured.

[0048] (ii) Measurement of the ratio of cellulose resin to hydrophobic polymer The measurement sample used was a porous membrane that had been vacuum-dried for 12 hours. Measurements were performed in an atmosphere adjusted to a temperature of 25°C and a humidity of 40%. An ATR-IR spectrum was obtained by irradiating the surface of the porous membrane with infrared light using a Shimadzu IRTracer-100, and Shimadzu's MicromATR Vision and a one-fold reflection diamond disk as accessories for total internal reflection. The measurement conditions included a resolution of 8 cm. -1 The settings were adjusted, and the number of scans was set to 64. The obtained spectra were expressed as absorbance, and multi-point baseline correction was performed. Shimadzu LabSolutions-IR was used for the analysis.

[0049] In the obtained spectrum, the peak intensity Ix (X: if polyvinylidene fluoride, then X = 881 cm) originates from the hydrophobic polymer. -1 If it is polyethersulfone, then X = 1578 cm -1 If it's polysulfone, then X = 1580 cm -1 ) and 1034cm -1 The peak intensity (I) appears, which originates from the pyranose ring of the cellulose resin. S The ratio of hydrophobic polymer to cellulose resin was calculated using the following formula. Measurements were taken at two or more points, and the average value was used. Ratio of cellulose resin to hydrophobic polymer = I S / Ix...Formula (1) (iii) Measurement of the ratio of acetyl groups to pyranose rings in cellulose resins The measurement sample was a porous membrane that had been vacuum-dried for 12 hours. Measurements were performed in an atmosphere adjusted to a temperature of 25°C and a humidity of 40%. Using a Shimadzu IRTracer-100, and Shimadzu MicromATR Vision and a one-fold reflection diamond disk as accessories for total internal reflection, infrared light was irradiated onto the surface of the porous membrane to obtain ATR-IR spectra. The obtained spectra were expressed as absorbance and multi-point baseline correction was performed. Shimadzu LabSolutions-IR was used for analysis. At 1034 cm⁻¹ in the obtained spectra, -1 The peak intensity (Is) originating from the pyranose ring of the cellulose resin appearing at this position, and 1744 cm -1 The saponification ratio of the cellulose resin was calculated using the following formula based on the peak intensity (Iso) derived from the acetyl groups of the cellulose resin that appeared in the measurement. Measurements were taken at two or more points, and the average value was used. Saponification ratio = ratio of acetyl groups to pyranose rings = Iso / Is... Equation (2) (iV) Method for determining the molecular weight cutoff of porous membranes As dextran aqueous solutions, solutions were prepared by mixing Fluka dextran with average molecular weights of 1500 Da, 6000 Da, 15000-25000 Da, and 40000 Da, and Aldrich dextran with average molecular weights of 60000 Da and 200000 Da, each at a concentration of 500 ppm. The resulting dextran solutions were cross-flow filtered at a membrane linear velocity of 0.9 m / s, and the filtrate and the filtered water at that point were sampled. GPC measurements were performed on each sampled solution using an HLC-8320 from Tosoh Techno Systems Co., Ltd., with an RI detector (double-flow type). For the measurement conditions, pure water was used as the eluent, TSKgel G4000PWxl from Tosoh Corporation was used as the column, the temperature was 40°C, and the flow rate was 0.5 mL / min. For calculating the fractional molecular weight, the value of the dextran solution after filtration was used as the removal rate compared to the value of the dextran solution before filtration, and the molecular weight at which the removal rate was 90% was defined as the fractional molecular weight (Da).

[0050] (V) Surface structure of the surface (pore size, standard deviation) After vacuum drying the porous membrane at 25°C for 12 hours, it was observed using a SEM (Hitachi High-Technologies Corporation; S-5500) at magnifications of 30,000 to 100,000 times. The SEM images of the porous membrane surface were binarized using the free software "ImageJ". For binarization, the background was created by setting 1 pixel in Subtract Background, and then the condition: RenyEntropy was selected for Threshold (binarization threshold). In the obtained binarized image, the area of ​​each pore was determined by selecting Area in Analyze Particles, and the diameter calculated assuming each pore was a circle was defined as the surface pore diameter. When calculating the average surface pore diameter, the diameters of pores of 1,000 or more were averaged. Similarly, the standard deviation of each surface pore diameter was calculated. Measurements were performed at two or more points, and the average value was used.

[0051] (Vi) Method for evaluating the filterability of a porous membrane after 125°C steam treatment of a liquid containing sugars, proteins, and their reactants. Sugars, proteins, and their reactants include, for example, polysaccharides, gelatin, and Maillard reaction compounds. As a protein-containing solution, an aqueous gelatin solution was selected. A 10% aqueous gelatin solution (molecular weight: 30,000-300,000 Da, turbidity: 23 NTU) was supplied to a porous membrane at 25°C to achieve a transmembrane pressure differential of 30 kPa, and cross-flow filtration was performed at a cross-flow linear velocity of 1.0 m / sec. The amount of gelatin in the permeate was measured, and the gelatin rejection rate was determined when the amount of gelatin in the filtration stock solution was set to 100%. The amount of gelatin in the permeate was calculated by measuring the absorbance at 292 nm. Here, the cross-flow linear velocity is the value obtained by dividing the flow rate of the filtration stock solution in the direction perpendicular to the filtration direction by the cross-sectional area of ​​the flow path. The transmembrane pressure is the difference between the pressure on the filtration stock solution side and the pressure on the permeate side across the porous membrane. The passing criteria for the change in inhibitory properties with heat resistance was a value of 0.5 or higher obtained by dividing the organic inhibitory properties after heat treatment by the organic inhibitory properties before heat treatment. Furthermore, when the rate of change in organic inhibitory properties before and after heat treatment is expressed as a percentage, a rate of 50% or higher was considered excellent inhibitory properties, and a rate of 15% or higher was considered sufficient inhibitory properties.

[0052] As a solution containing the reaction products of sugar and protein, a sugar solution containing Maillard reaction compounds was selected. The sugar solution (absorbance at 420 nm: 0.01-5.0, Brix: 10-70%) was supplied to a porous membrane at 60°C to achieve a transmembrane pressure differential of 80 kPa, and cross-flow filtration was performed at a cross-flow linear velocity of 0.8 m / sec. The absorbance at 420 nm in the permeate was measured, and the rejection rate of Maillard reaction compounds was determined, with the glycoprotein content of the filtrated solution set to 100%. The absorbance at 420 nm of the sugar solution was measured using a spectrophotometer (Shimadzu Corporation: UV-2450). Brix values ​​were calculated by measuring the refractive index using a refractometer (Atago Corporation: Pocket Refractometer PAL-1). Similarly, when the rate of change in organic rejection before and after heat treatment was expressed as a percentage, a rejection rate of 70% or more was considered excellent, and a rejection rate of 50% or more was considered sufficient.

[0053] (Example 1) The porous membrane was prepared by the following method: 38% by mass of PVDF (Kureha Corporation; KF1300, weight-average molecular weight 350,000 Da) and 62% by mass of γ-butyrolactone were mixed and dissolved at 160°C to prepare a support membrane stock solution. This support membrane stock solution was discharged from a double-walled nozzle while an 85% by mass γ-butyrolactone aqueous solution was added as a hollow-forming liquid. The discharged support membrane stock solution was solidified in a cooling bath containing an 85% by mass γ-butyrolactone aqueous solution at 20°C, placed 30 mm below the nozzle, to produce a hollow fiber porous structure with a spherical structure.

[0054] A polymer solution with the composition ratios shown in Table 1 was prepared by mixing 12% by mass of PVDF1 (Arkema; ​​Kynar® 710, weight-average molecular weight 180,000 Da), 4.8% by mass of cellulose diacetate CDA (Eastman; CA-398-3), 2.4% by mass of cellulose triacetate CTA (Eastman; CA-436-80S), 64.7% by mass of NMP, 12.1% by mass of ε-caprolactam ε-Cl, and 4.0% of glycerin Gly, and stirring at 120°C for 4 hours. Next, the polymer solution was uniformly applied to the outer surface of the hollow fiber porous structure at a rate of 30 m / min (thickness 50 μm). One second after application, the porous structure coated with the polymer solution was immersed in a coagulation bath of distilled water at 30°C for 10 seconds to coagulate and form a porous film. The porous membrane was denser on the surface up to a thickness of 10 μm than in the interior. Next, the obtained porous membrane was immersed in a 0.01 N NaOH aqueous solution at room temperature for 24 hours as step (C). The evaluation results of the obtained porous membrane are shown in Table 1.

[0055] The obtained porous membrane had a static contact angle of 36°, an Is / Ix ratio of 0.85, an Iso / Is ratio of 0.20, and a molecular weight cutoff of 23,000. Furthermore, the surface pore size was 8.0 nm, with a standard deviation of 2.5 nm. After steam treatment in water at 125°C, the molecular weight cutoff became 18,000, confirming that the structure was maintained. The gelatin inhibition rate was 70.2% before steam treatment, while it was maintained at 76.7% after steam treatment. The value obtained by dividing the organic substance inhibition rate after heat treatment by the organic substance inhibition rate before heat treatment was 109%, indicating that the heat resistance was at an acceptable level. The results are shown in Table 1. The inhibition rate of Maillard reaction compounds in the sugar solution after steam treatment (absorbance at 420 nm: 1.50, Brix: 10%) was 85.3%, and the inhibition rate in the sugar solution (absorbance at 420 nm: 0.06, Brix: 61%) was 70.3%, both of which showed high inhibition rates.

[0056] (Comparative Example 1) In the film formation of Example 1, after forming the porous film, step (C) was not performed and the film was not immersed in an alkaline aqueous solution. The evaluation results of the porous film are shown in Table 2. The obtained porous film had a static contact angle of 44.4°, an Is / Ix ratio of 0.87, an Iso / Is ratio of 0.63, and a molecular weight cutoff of 40,000. Furthermore, the surface pore size was 8.8 nm and the standard deviation was 3.0 nm. In addition, after steam treatment in water at 125°C, the molecular weight cutoff became 50,000, and the structure was not maintained. The gelatin inhibition rate was 65.7% before steam treatment, but after steam treatment it was 30.0%, and the inhibition rate could not be maintained. The value obtained by dividing the organic matter inhibition rate after heat treatment by the organic matter inhibition rate before heat treatment was low at 46%, indicating poor heat resistance.

[0057] The inhibition rate of Maillard reaction compounds in the sugar solution after steam treatment (absorbance at 420 nm: 1.50, Brix: 10%) was 46.7%, and the inhibition rate of Maillard reaction compounds in the sugar solution (absorbance at 420 nm: 0.06, Brix: 61%) was 39.0%, both of which showed low inhibition rates. The results are shown in Table 2.

[0058] (Example 2) In Example 1, a porous film was obtained by similarly fabricating the film, except that the concentration of ε-caprolactam in the polymer solution was changed and the distilled water in the coagulation bath was set to 40°C. The evaluation results of the porous film are shown in Table 1. The surface of the porous film was denser from the surface up to a thickness of 10 μm than the interior. The obtained porous film had a static contact angle of 35.0°, an Is / Ix of 0.87, an Iso / Is of 0.18, and a molecular weight cutoff of 18000. Furthermore, the surface pore size was 7.4 nm and the standard deviation was 2.1 nm. In addition, after steam treatment in water at 125°C, the molecular weight cutoff became 17000, confirming that the structure was maintained. The gelatin inhibition rate was 80.2% before steam treatment, while it was maintained at 82.5% after steam treatment. The value obtained by dividing the organic substance inhibition rate after heat treatment by the organic substance inhibition rate before heat treatment was 102%, indicating that the heat resistance was at an acceptable level.

[0059] (Example 3) In Example 1, a porous film was obtained by forming a film in the same manner as in Example 1, except that the alkaline solution was changed to a 0.1N NaOH aqueous solution. The evaluation results of the porous film are shown in Table 1. The surface of the porous film was denser from the surface up to a thickness of 10 μm than the interior. The obtained porous film had a static contact angle of 34.7°, an Is / Ix of 0.76, an Iso / Is of 0.08, and a molecular weight cutoff of 16000. Furthermore, the surface pore size was 6.9 nm and the standard deviation was 2.0 nm. In addition, after steam treatment in water at 125°C, the molecular weight cutoff became 15000, confirming that the structure was maintained. The gelatin inhibition rate was 90.2% before steam treatment, while it was maintained at 93.5% after steam treatment. The value obtained by dividing the organic substance inhibition rate after heat treatment by the organic substance inhibition rate before heat treatment was 104%, indicating that the heat resistance was at an acceptable level.

[0060] (Example 4) In the film formation of Example 1, a porous film was obtained by forming the film in the same manner as in Example 1, except that NMP in the polymer solution was changed to DMF and the distilled water in the coagulation bath was set to 40°C. The evaluation results of the porous film are shown in Table 1. The surface portion of the porous film was denser from the surface up to a thickness of 10 μm than the interior. The obtained porous film had a static contact angle of 32.6°, an Is / Ix of 0.99, an Iso / Is of 0.28, and a molecular weight cutoff of 55000. Furthermore, the surface pore size was 11.4 nm and the standard deviation was 4.1 nm. In addition, after steam treatment in water at 125°C, the molecular weight cutoff was 45000, confirming that the structure was maintained. The gelatin inhibition rate was 26.2% before steam treatment, while it was maintained at 30.3% after steam treatment. The value obtained by dividing the organic substance inhibition rate after heat treatment by the organic substance inhibition rate before heat treatment was 115%, indicating that the heat resistance was at an acceptable level.

[0061] (Example 5) In the film formation of Example 1, a porous film was obtained by forming a film in the same manner as in Example 1, except that NMP in the polymer solution was changed to DMF and the distilled water in the coagulation bath was set to 60°C. The evaluation results of the porous film are shown in Table 1. The surface portion of the porous film was denser from the surface up to a thickness of 10 μm than the interior. The obtained porous film had a static contact angle of 32.7°, an Is / Ix of 0.97, an Iso / Is of 0.44, and a molecular weight cutoff of 110,000. Furthermore, the surface pore size was 17.1 nm and the standard deviation was 7.6 nm. In addition, after steam treatment in water at 125°C, the molecular weight cutoff was 98,000, confirming that the structure was maintained. The gelatin inhibition rate was 15.1% before steam treatment, while it was maintained at 16.5% after steam treatment. The value obtained by dividing the organic substance inhibition rate after heat treatment by the organic substance inhibition rate before heat treatment was 112%, indicating that the heat resistance was at an acceptable level.

[0062] [Table 1]

[0063] [Table 2] [Explanation of Symbols]

[0064] 101 Porous membrane 102 Surface layer 103 Inner layer 201 Organic matter FL Flowline A Surface A B Surface B

Claims

1. A porous membrane mainly composed of a hydrophobic polymer, wherein the surface contains a cellulose resin, one side is designated as surface A and the other side as surface B, the average pore diameter of surface A (hereinafter referred to as surface pore diameter) is smaller than the average pore diameter of surface B, the surface pore diameter of surface A is 5 nm or more and 50 nm or less, and the contact angle with water on surface A is 10° or more and 40° or less.

2. The hydrophobic polymer comprises at least one thermoplastic resin selected from the group consisting of polyvinylidene fluoride resins, polyethersulfone resins, and polysulfone resins, and the absorption wavelength of the pyranose ring of the cellulosic resin (1034 cm) is measured on surface A by ATR-IR (total internal reflection) method for the following peak intensity Ix derived from the hydrophobic polymer. －１ The porous membrane according to claim 1, wherein the intensity ratio (Is / Ix) of the peak intensity Is derived from ) is 0.6 or more and 1.5 or less. <Absorption wavelength of hydrophobic polymers> In the case of polyvinylidene fluoride resins, the absorption wavelength is 881 cm², which is derived from polyvinylidene fluoride. -１ In the case of polyethersulfone resins, 1578 cm is derived from the polyethersulfone. -１ In the case of polysulfone resins, 1580 cm is derived from polysulfone. -１ Let's assume that.

3. The hydrophobic polymer is a polyvinylidene fluoride resin, and the absorption wavelength of polyvinylidene fluoride (881 cm) measured on surface A by the ATR-IR method (total internal reflection measurement) is -１ The absorption wavelength (1034 cm) of the pyranose ring of the cellulose resin relative to the peak intensity Ix derived from ) -１ The porous membrane according to claim 1, wherein the intensity ratio (Is / Ix) of the peak intensity Is derived from ) is 0.6 or more and 1.5 or less.

4. The absorption wavelength (1034 cm) of the pyranose ring of the cellulose resin, measured by ATR-IR (total internal reflection) on the surface A of the porous membrane, is measured. -１ The absorption wavelength (1744 cm) of the acetyl group in cellulosic resin relative to the peak intensity Is derived from ) －１ The porous membrane according to claim 1 or 2, wherein the intensity ratio (Iso / Is) of the peak intensity Iso derived from ) is 0.01 or more and 0.6 or less.

5. The porous membrane according to claim 1 or 2, wherein the fractional molecular weight of the porous membrane is 10,000 to 200,000.

6. The porous membrane is composed of multiple layers, where one outermost surface of the layer having one outermost surface is surface A, and the other outermost surface of the layer having the other outermost surface is surface B. The hydrophobic polymer comprises at least one thermoplastic resin selected from the group consisting of polyvinylidene fluoride resins, polyethersulfone resins, and polysulfone resins. The porous membrane according to claim 1 or 2, wherein the ratio of the peak intensity Is / Ix between the peak intensity Ix derived from the hydrophobic polymer, measured on the surface B by the ATR-IR method (total internal reflection), and the peak intensity Is derived from the absorption wavelength (1034 cm⁻¹) of the pyranose ring of the cellulosic resin, is 0.6 or less. <Absorption wavelengths of hydrophobic polymers> For polyvinylidene fluoride resins, the absorption wavelength is 881 cm⁻¹ derived from polyvinylidene fluoride; for polyethersulfone resins, it is 1578 cm⁻¹ derived from polyethersulfone; and for polysulfone resins, it is 1580 cm⁻¹ derived from polysulfone.

7. The porous membrane according to claim 6, wherein the other outermost layer of the porous membrane composed of the plurality of layers is a porous structure.

8. The porous membrane according to claim 1 or 2, wherein the surface A is the surface that comes into contact with the liquid to be treated.

9. A method for producing a porous membrane, comprising the steps of (A) dissolving a polymer resin in a solvent to obtain a polymer solution, and (B) solidifying the polymer solution in a non-solvent to form a porous membrane, wherein step (A) includes a hydrophobic polymer and a cellulose resin, step (B) includes 90 to 100% by weight of water as the non-solvent and the temperature of the non-solvent is 6°C to 60°C, and step (C) includes contacting at least one surface of the formed porous membrane with an alkaline solution of pH 10 or higher, wherein step (C) is measured on the surface by ATR-IR (total internal reflection) and the intensity ratio (Iso / Is) of the peak intensity Is derived from the absorption wavelength of the acetyl group of the cellulose resin (1744 cm⁻¹) to the peak intensity Is derived from the absorption wavelength of the pyranose ring of the cellulose resin (1034 cm⁻¹) is 0.01 or more and 0.6 or less.

10. The method for manufacturing a porous membrane according to claim 9, wherein the surface is a surface with a smaller average pore diameter (hereinafter referred to as surface A), and surface A is subjected to the treatment of step (C).

11. A method for producing a porous membrane according to claim 9 or 10, characterized in that step (A) mainly consists of a polyvinylidene fluoride resin as a hydrophobic polymer, and step (C) contains a 0.0001 N to 0.01 N aqueous sodium hydroxide solution as an alkaline solution, and the formed porous membrane is immersed in the alkaline solution.

12. A method for producing a porous membrane having multiple layers, wherein in step (B) of solidifying the polymer solution in a non-solvent to form a porous membrane, the porous membrane is formed on the surface of the porous structure, the outermost surface of the formed porous membrane becomes surface A, and in step (C) of contacting the alkaline solution, surface A is treated, as described in claim 9 or 10.

13. A method for filtering a liquid using a porous membrane according to claim 1 or 2.

14. The method for filtering a liquid according to claim 13, characterized in that the liquid is a sugar solution containing sugar, protein, and a reaction product of sugar and protein.

15. The liquid filtration method according to claim 14, characterized in that the sugar solution is a sugar solution that satisfies the requirements of having an absorbance of 0.01 to 30 at a wavelength of 420 nm and a Brix of 10 to 75.

16. A membrane filtration apparatus comprising a porous membrane according to claim 1 or 2.

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