Porous membrane and method for producing 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

Porous membrane and method for producing the same

[0001] The present invention relates to a porous membrane for use in the fields of water treatment, pharmaceutical manufacturing, food industry, fermentation, etc.

[0002] In recent years, porous membranes such as microfiltration membranes and ultrafiltration membranes have been used in a variety of fields, including water treatment (such as water purification or wastewater treatment), medical treatment (such as blood purification), and the food industry. In the food industry, recent filtration stock solutions (liquids to be filtered) contain organic substances with small molecular weights, such as enzymes, proteins, and polysaccharides. Filtration stock solutions containing organic substances tend to clog porous membranes, making them difficult to filter. A porous membrane capable of efficiently filtering such stock solutions is needed. For these applications, porous membranes must be resistant to fouling and chemicals.

[0003] For stain resistance, it is important to make the pores on the surface fine so that organic matter does not penetrate inside the porous membrane, and to improve hydrophilicity to prevent adhesion of organic matter. For chemical resistance, when organic matter adheres, it is necessary to clean it with chemicals such as sodium hypochlorite, and it is important that the porous membrane is made of components that do not change its structure even after immersion in chemicals.

[0004] For the above reasons, it is preferable that the resin constituting the porous membrane contains a hydrophobic polymer and a hydrophilic polymer, and Patent Documents 1 and 2 disclose, for example, porous membranes containing a polyvinylidene fluoride resin and a polyvinylidene fluoride resin and a cellulose resin.

[0005] Furthermore, when repeated filtration is performed in the pharmaceutical manufacturing, food industry, fermentation, and other fields, sterilization must be performed before filtration to prevent contamination, which can lead to a decrease in product quality and productivity. Examples of sterilization methods include heat-based sterilization such as dry heat sterilization and steam sterilization, electromagnetic wave-based sterilization such as gamma ray sterilization, and chemical sterilization using ethylene oxide gas. Among these sterilization methods, steam sterilization is particularly preferred from the standpoint of ease of use and safety for the human body. Therefore, it is important that porous membranes containing hydrophobic and hydrophilic polymers have heat resistance so that the structure and performance of the porous membrane, especially its organic matter removal performance, do not change even when repeatedly subjected to high-temperature steam sterilization.

[0006] International Publication No. WO 2023 / 054228 International Publication No. WO 2012 / 063669

[0007] However, in the porous membranes containing hydrophobic polymers and hydrophilic polymers, the membrane structure is significantly deformed by steam sterilization, and the heat resistance is insufficient. Specifically, Patent Document 1 reports an example of a porous membrane containing polyvinylidene fluoride resin and cellulose resin, which has chemical resistance and stain resistance. However, the porous membrane disclosed in Patent Document 1 has a change in the surface pore structure due to high-temperature steam sterilization, and the organic substance blocking ability is reduced. This is presumably because the interaction between the polymer chains constituting the porous membrane is small and the mobility of the polymer chains is high, so that when the pores shrink during steam heating, the surrounding pores are pulled, and the pores expand, resulting in a pore diameter of 20 nm or more.

[0008] In Patent Document 2, a porous membrane mainly composed of a polyvinylidene fluoride resin is brought into contact with steam in advance to suppress subsequent thermal changes. However, the method described in Patent Document 2 does not have heat resistance, and therefore merely enlarges the pores in advance, but the pores are relatively large and cannot prevent the intrusion of organic matter, and there is a problem in that high organic matter blocking properties cannot be obtained even for a filtrate containing organic matter with a small molecular weight.

[0009] The present invention aims to provide a porous membrane that is heat resistant in addition to stain resistance and chemical resistance, and to provide a method for producing a porous membrane that combines a hydrophobic polymer with a cellulose-based resin with a high degree of saponification as a hydrophilic polymer.

[0010] In order to solve the above problems, the present invention provides a porous membrane and a method for producing a porous membrane having the following configuration: (1) A porous membrane whose main component is a hydrophobic polymer, which contains a cellulose-based resin on a surface, one surface being surface A and the other surface being surface B, the average pore size (hereinafter referred to as surface pore size) of surface A being smaller than the average pore size of surface B, the surface pore size of surface A being 5 nm or more and 50 nm or less, and the contact angle of surface A with water being 10° or more and 40° or less. (2) The hydrophobic polymer contains at least one thermoplastic resin selected from the group consisting of polyvinylidene fluoride resins, polyethersulfone resins, and polysulfone resins, and the relationship between the absorption wavelength (1034 cm) of the pyranose ring of the cellulose-based resin and the following peak intensity Ix derived from the hydrophobic polymer, as measured on surface A by an ATR-IR method (attenuated total reflection spectroscopy), is: －１ The porous membrane according to (1), wherein the intensity ratio (Is / Ix) of the peak intensity Is derived from the polyvinylidene fluoride is 0.6 or more and 1.5 or less. <Absorption wavelength of hydrophobic polymer> In the case of a polyvinylidene fluoride-based resin, the absorption wavelength of the hydrophobic polymer is 881 cm -１ In the case of polyethersulfone-based resins, the 1578 cm -１ In the case of polysulfone-based resins, the 1580 cm -１ (3) The hydrophobic polymer is a polyvinylidene fluoride resin, and the absorption wavelength of polyvinylidene fluoride (881 cm) measured on the surface A by an ATR-IR method (attenuated total reflection measurement method) is -１ ) relative to the absorption wavelength (1034 cm ) of the pyranose ring of the cellulose-based resin. －１(4) The porous membrane according to (1), wherein the intensity ratio (Is / Ix) of the peak intensity Is derived from the absorption wavelength (1034 cm ) of the pyranose ring of the cellulose-based resin measured on the surface A of the porous membrane by an ATR-IR method (attenuated total reflection spectroscopy) is 0.6 or more and 1.5 or less. －１ The absorption wavelength (1744 cm) of the acetyl group of the cellulose-based resin relative to the peak intensity Is derived from －１ The porous membrane according to any one of (1) to (3), wherein the intensity ratio (Iso / Is) of the peak intensity Iso derived from the above-mentioned 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 molecular weight cutoff 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 outermost surface of a layer having one outermost surface is surface A, and the other outermost surface of a layer having the other outermost surface is surface B, and the Is / Ix ratio measured by ATR-IR (attenuated total reflectance spectroscopy) at surface B is 0.6 or less. (7) The porous membrane according to (6), wherein the layer having the other outermost surface of the porous membrane composed of a plurality of layers has 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 solidifying the polymer solution in a non-solvent to form a porous membrane, wherein the step (A) contains a hydrophobic polymer and a cellulose resin; the non-solvent in step (B) contains 90 to 100% by weight of water and the temperature of the non-solvent is 6 to 60°C; and a step (C) of contacting at least one surface of the formed porous membrane with an alkaline solution after steps (A) and (B). (10) The method for producing a porous membrane according to (9), wherein the surface has a smaller average pore size (hereinafter referred to as surface A), and surface A is treated in step (C). (11) The method for producing a porous membrane according to (9) or (10), wherein the hydrophobic polymer in step (A) is a polyvinylidene fluoride resin as a main component; the alkaline solution in step (C) contains a 0.0001N to 0.01N sodium hydroxide aqueous solution; and the formed porous membrane is immersed in the alkaline solution. (12) A method for producing a porous membrane having multiple layers, according to any one of (9) to (11), 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 a porous structure, the outermost surface of the formed porous membrane being surface A, and surface A is treated in step (C) of contacting the porous membrane with the alkaline solution. (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 sugars, proteins, and reaction products thereof.(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).

[0012] According to the present invention, a porous membrane can be provided which is heat resistant in addition to being stain resistant and chemical resistant, and which can ensure high organic substance blocking properties even for a filtrate containing organic substances with low molecular weight.

[0013] 1 is a schematic diagram showing the concept of the filtration state of the porous membrane of the present invention, and FIG. 2 is a schematic diagram showing the concept of the filtration state of a conventional porous membrane.

[0014] The following describes in detail embodiments of the present invention, but the present invention is not limited thereto. In this specification, "mass" and "weight" are synonymous. Furthermore, a stock solution containing organic matter with a low molecular weight is a stock solution in which the weight-average molecular weight of the organic matter contained in the stock solution is 15,000 Da to 40,000 Da. Examples of organic matter include proteins and polysaccharides. These stock solutions have traditionally been difficult to filter. However, by using the porous membrane of the present invention as a separation membrane, it is possible to filter with high efficiency stock solutions containing organic matter with an average molecular weight of 15,000 Da to 40,000 Da, such as enzymes, proteins, and polysaccharides.

[0015] "Heat-resistant" in the present invention means that the structure and properties of the porous membrane do not change even when exposed to high temperatures. In the present invention, this means that the change in organic substance blocking ability before and after steam sterilization is small. A membrane that does not have heat resistance has a large change in organic substance blocking ability before and after steam sterilization, and the organic substance blocking ability deteriorates after steam sterilization.

[0016] In conventional porous membranes containing polyvinylidene fluoride resin and cellulose resin, steam sterilization tends to deteriorate the organic substance blocking ability. When organic substance blocking ability deteriorates, organic substances with small molecular weights penetrate into the porous membrane, reducing filtration efficiency. This is thought to be because the polymer chains that make up the porous membrane are mobile, making the micropores smaller and the coarse pores larger when steamed.

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

[0018] <Regarding the Porous Membrane> The porous membrane according to an embodiment of the present invention is a porous membrane containing a hydrophobic polymer and a cellulose-based resin, with the surface in contact with the liquid to be treated designated surface A and the other surface designated surface B, and the average pore size (hereinafter referred to as the average surface pore size) of surface A being 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 making the pore size of the surface in contact with the liquid to be treated smaller than that of the other surface, it is possible to prevent organic matter from penetrating into the pores and to increase the permeability of the permeated liquid. When the raw filtrate containing organic matter with a low molecular weight is used, a surface pore size of 5 to 50 nm prevents contaminant components and substances to be removed in the raw filtrate from penetrating into the porous membrane, thereby achieving high fouling resistance. The surface pore size will be described later.

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

[0020] The contact angle is measured as a static contact angle using a contact angle meter, as described below. The contact angle in the present invention is determined by measuring the static contact angle with the water surface using an air-bubble method, in which air bubbles are brought into contact with the porous membrane surface while it is immersed in water, and the static contact angle is then measured.

[0021] FIG. 1 shows a schematic diagram of the filtration of a raw liquid, which has been considered difficult to filter using the porous membrane of the present invention. FIG. 1 is a conceptual schematic diagram showing a portion of the cross section of a porous membrane. The filtration direction FL is the direction from surface A to surface B. In the figure, porous membrane 101 has a surface layer 102 and an inner layer 103. Surface A of surface layer 102 has numerous fine pores with a pore size of 5 to 50 nm (shown between the fine mesh lines in FIG. 1). The pore size of inner layer 103 is larger than that of 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, which reduces the mobility of the polymer chains even when subjected to high-temperature steam sterilization using water vapor. Therefore, the change in pore size is small, and fine organic matter 201 does not penetrate the pores.

[0022] Figure 2 shows a schematic diagram of a previously known porous membrane filtering a difficult-to-filter raw solution. The porous membrane of Patent Document 1 exhibits weak interactions between polymer chains, resulting in high mobility of the polymer chains during steam sterilization. This results in a large change in pore size, causing the fine pores to become finer and the coarse pores to become coarser. In Figure 2, coarse pores are present on the surface that comes into contact with the liquid being treated (the partially coarse meshed areas in Figure 2). Therefore, fine organic matter 201 easily penetrates the pores, resulting in reduced organic matter blocking.

[0023] Examples of hydrophobic polymers contained in the porous membrane include polysulfone-based resins, polyethersulfone-based resins, and polyvinylidene fluoride-based resins, but polyvinylidene fluoride-based resins are preferred. In the present invention, the term "polyvinylidene fluoride-based resin" refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer. Here, the term "vinylidene fluoride copolymer" refers to a polymer having a vinylidene fluoride residue structure. Polymers having a vinylidene fluoride residue structure are typically copolymers of vinylidene fluoride monomers and other fluorine-based monomers. Examples of such fluorine-based monomers include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene. The vinylidene fluoride copolymer may also be copolymerized with ethylene or other fluorine-based monomers, as long as the effects of the present invention are not impaired.

[0024] The peak intensity Ix of the hydrophobic polymer measured by the ATR-IR method (total reflection measurement method) on the surface A of the porous film (if the absorption wavelength of the hydrophobic polymer is polyvinylidene fluoride, X = 881 cm -１ , for polyethersulfone, X = 1578 cm -１ , for polysulfone, X = 1580 cm -１ ) relative to the peak intensity Is (S = 1034 cm) derived from the pyranose ring of the cellulose-based resin －１When the ratio (Is / Ix) is 0.6 to 1.5, the proportion of cellulose-based resin on the membrane surface is high, and excellent heat resistance can be obtained. Furthermore, it is preferable that surface A is mainly composed of a hydrophobic polymer and contains a large amount of cellulose-based resin within an appropriate range. It is preferable that Is / Ix is 0.6 to 1.2, and more preferably Is / Ix is 0.6 to 1.0. Ｘ It is particularly preferable that the ratio is 0.6 to 0.8. The ATR-IR method can analyze a depth of up to 5 μm from the outermost surface. Therefore, it is possible to observe the composition of the compounds on the surface that comes into contact with the liquid to be treated. Surface A has good chemical resistance because it is mainly composed of hydrophobic polymers.

[0025] When the hydrophobic polymer is a polyvinylidene fluoride resin, three types of crystal structures are known for polyvinylidene fluoride resin: α-type, β-type, and a very small amount of γ-type. It is generally known that when polyvinylidene fluoride resin is exposed to chemicals or stressed by heat or the like, the ratio of α-type, β-type, and γ-type changes. On the other hand, among the peak intensities representing polyvinylidene fluoride resin, the peak at 881 cm －１ is a peak intensity common to all crystals and does not change with chemical treatment. Furthermore, in the chemical structure of cellulose-based resins, the pyranose rings are 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] In addition, the peak intensity (Is) derived from the pyranose ring of the cellulose-based resin relative to the peak intensity (Iso) derived from the acetyl group of the cellulose-based resin measured on surface A by the ATR-IR method (total reflection measurement method) is －１) is preferably 0.6 or less, since the ratio of acetyl groups in the cellulose-based resin on the membrane surface to hydroxyl groups is high, resulting in excellent heat resistance. Since the higher the ratio of hydroxyl groups converted to acetyl groups, the better the heat resistance. Therefore, the ratio is preferably 0.5 or less, more preferably 0.2 or less, and even more preferably 0.1 or less. As mentioned above, pyranose rings are not affected by the saponification treatment, while acetyl groups are converted to hydroxyl groups by the saponification treatment, resulting in a decrease in their ratio. In other words, a state in which there are fewer acetyl groups relative to the peak intensity derived from the pyranose ring in the cellulose-based resin indicates a high ratio of hydroxyl groups in the cellulose-based resin. The lower limit of Iso / Is can be set arbitrarily. From the viewpoint of production efficiency, Iso / Is is preferably 0.01 or more. Within this range, a porous membrane can be efficiently produced while maintaining excellent heat resistance and chemical resistance.

[0027] A porous membrane with a molecular weight cutoff of 10,000 to 200,000 is preferred because it prevents organic matter to be removed and concentrated in the filtrate from penetrating into the porous membrane, making it more likely to exhibit high organic matter blocking properties. The porous membrane's molecular weight cutoff is more preferably 10,000 to 60,000, and particularly preferably 10,000 to 30,000. In this case, the molecular weight cutoff is defined as the molecular weight at which a removal rate of 90% is achieved when several types of dextran aqueous solutions are filtered. The dextran aqueous solution was prepared by mixing 500 ppm each of Fluka dextrans with average molecular weights of 1500 Da, 6000 Da, 15000-25000 Da, and 40000 Da, and Aldrich dextrans with average molecular weights of 60000 Da and 200000 Da. The solution to be filtered was subjected to cross-flow filtration at a membrane surface linear velocity of 0.9 m / sec, and the filtrate and the water to be filtered at this point were sampled and subjected to GPC measurement to calculate the removal rate.

[0028] A surface pore diameter of 5 to 50 nm on the surface A of the porous membrane prevents contaminants in the filtrate and substances to be removed from penetrating into the porous membrane, which is preferable because it tends to exhibit high fouling resistance. Furthermore, it maintains high organic substance blocking properties and tends to exhibit heat resistance. The surface pore diameter is more preferably 12 nm or less, more preferably 8.0 nm or less. The surface pore diameter is the diameter of the pores present in the surface when observing the surface of the porous membrane. To determine the surface pore diameter of the porous membrane, an image obtained by SEM observation of the surface of the porous membrane is binarized using the free software "ImageJ." When binarizing, Subtract Background is set to 1 pixel, and then Create Background is selected as the Threshold (binarization threshold), and the condition: RenyiEntropy is selected. In the obtained binarized image, the area of ​​each pore is calculated by selecting Area in Analyze Particles, and the diameter calculated assuming each pore is a circle is used as the surface pore size. The average surface pore size is calculated by averaging the sizes of more than 1,000 pores.

[0029] When the standard deviation of the surface pore size of the surface A of the porous membrane is 10.0 nm or less, many pores tend to shrink uniformly during steam treatment, maintaining high organic substance blocking properties and easily exhibiting heat resistance. The standard deviation is preferably 5.0 nm or less, 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 be composed of a single layer or multiple layers. When composed of multiple layers, it is preferable that the layer in contact with the liquid to be treated (the layer including the surface A) contains a large amount of cellulose-based resin, and that the layer including the surface B has an Is / Ix ratio of 0.6 or less. The layer in contact with the liquid to be treated contains a large amount of cellulose-based resin, which is preferable because it is likely to exhibit heat resistance in the region that blocks organic matter. Furthermore, the layer including the surface B contains less cellulose-based resin, and is thermally deformed and the pore size increases, resulting in a fine 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 permeated liquid. In other words, the average pore size of the surface A of the porous membrane of the present invention is smaller than the average pore size of the surface B, and the surface pore size of the surface A is 5 to 50 nm.

[0031] <Regarding the method for producing the porous membrane> The shape of the porous membrane of the present invention can be a flat plate, hollow fiber, tubular, etc., and an appropriate shape can be selected and used depending on the type of filtration device used and the properties of the raw liquid to be filtered.

[0032] The porous membrane of the present invention can be obtained by a manufacturing method comprising step (A) dissolving a polymer in a solvent to obtain a polymer solution, followed by step (B) solidifying the polymer solution in a non-solvent to form a porous membrane, and step (C) contacting the porous membrane with an alkaline solution, wherein the non-solvent used in step (B) contains 90 to 100 wt % water and the temperature of the non-solvent is 6 to 60°C. Step (A): obtaining a polymer solution and step (B): forming a porous membrane are non-solvent-induced phase separation methods, which contain a hydrophobic polymer as the main component and a cellulose-based resin, and can obtain surface pores with a pore size of 5 to 12 nm. That is, the manufacturing method of the porous membrane of the present invention comprises step (A) dissolving a polymer resin in a solvent to obtain a polymer solution, and step (B) solidifying the polymer solution in a non-solvent to form a porous membrane, wherein step (A) contains a hydrophobic polymer and a cellulose resin, and step (B) the non-solvent contains 90 to 100 wt % water and the temperature of the non-solvent is 6 to 60°C. Then, following steps (A) and (B), step (C) is carried out in which the porous membrane is brought into contact with an alkaline solution, whereby the acetyl groups in the cellulose resin on surface A of the porous membrane are converted to hydroxyl groups, and hydrogen bonds are formed in the polymer chains, resulting in excellent heat resistance.

[0033] The polymer used in step (A) is preferably a hydrophobic polymer or a cellulose acetate resin, with polyvinylidene fluoride resins and cellulose acetate resins being particularly preferred. In the present invention, the term "polyvinylidene fluoride resin" refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer. Here, the term "vinylidene fluoride copolymer" refers to a polymer having a vinylidene fluoride residue structure. A polymer having a vinylidene fluoride residue structure is typically a copolymer of a vinylidene fluoride monomer and another fluorine-based monomer. Examples of such fluorine-based monomers include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene. The vinylidene fluoride copolymer may be copolymerized with ethylene or other fluorine-based monomers, as long as the effects of the present invention are not impaired.

[0034] The weight-average molecular weight of the polyvinylidene fluoride resin may be appropriately selected depending on the required strength and water permeability of the porous membrane. Generally, as the weight-average molecular weight increases, the water permeability decreases, and as the weight-average molecular weight decreases, the strength decreases. Therefore, the weight-average molecular weight is preferably 50,000 or more and 1,000,000 or less. In particular, when the treatment stock solution is highly turbid and clogging substances adhering to the porous membrane need to be removed by chemical washing and the highly turbid treatment stock solution needs to be repeatedly filtered, the weight-average molecular weight is preferably 100,000 or more and 700,000 or less. Furthermore, when chemical washing is performed multiple times, the weight-average molecular weight is preferably 150,000 or more and 600,000 or less.

[0035] The cellulose-based resin in the present invention is not particularly limited as long as it has a cellulose ester as a molecular unit in the main chain and / or side chain, 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. Ethylene, methyl methacrylate, and methyl acrylate are particularly preferred because they are inexpensive and easily incorporated into the main chain and / or side chain. Known polymerization techniques such as radical polymerization, anionic polymerization, and cationic polymerization can be used as the introduction method. Furthermore, homopolymers containing essentially only cellulose ester as a molecular unit are known for their inexpensive availability and ease of handling. 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 a polymer even in a low-temperature range of 60°C or less. 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 more preferable that the good solvent accounts for 40% by weight or more of the solvent, and particularly preferably 60% by weight or more. Inclusion of a large amount of good solvent is preferable because polymer chains spread in the polymer solution and the viscosity of the solution can be easily controlled within an appropriate 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 a high temperature up 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, and low-molecular-weight polyethylene glycol, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, other chlorinated organic liquids, and mixtures thereof. The polymer concentration (wt %) in the polymer solution is preferably equal to or higher than the entanglement concentration in order to control the viscosity of the solution within an appropriate range. More specifically, it is preferably 10 to 40 wt %, more preferably 12 to 30 wt %, and particularly preferably 15 to 25 wt %.

[0038] The porous membrane formation step (B), in which a polymer solution is solidified in a non-solvent to form a porous membrane, is a so-called non-solvent-induced phase separation process. When the polymer solution comes into contact with the non-solvent, the polymer cannot be completely dissolved in the solvent, and phase separation occurs into a polymer-rich phase and a solvent-rich phase. Each phase coarsens while coalescing with the surrounding identical phase.

[0039] Step (C) is a step of contacting the porous membrane produced in steps (A) and (B) with an alkaline solution. The porous membrane is saponified by immersing it in an alkaline solution at a predetermined temperature for a predetermined time. 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 1,440 minutes while stirring. "Room temperature" refers to a range of 15 to 35°C. Here, with regard to the sodium hydroxide concentration, immersion temperature, and immersion time, if the concentration is high, the immersion temperature can be lowered or the immersion time can be shortened. Furthermore, if the concentration is low, it is preferable to increase the immersion temperature or extend the immersion time. The present invention does not depend on the saponification conditions. Therefore, the alkaline solution includes an alkaline aqueous solution, and the alkaline solution includes sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonia (NH ３ From the viewpoint of efficiently and stably carrying out the saponification reaction, the alkaline solution preferably has a pH of 10 or higher, and more preferably a pH of 11 or higher.

[0040] As described above, it is preferable that at least surface A has many hydroxyl groups and many hydrogen bonds are formed, and this can be achieved by contacting only surface A with an alkaline solution of pH 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 so as to be in contact with the liquid to be treated, since components contained in the raw filtrate are less likely to penetrate into the porous membrane, and high permeability can be maintained for a long period of time. The other layer is not particularly limited as long as it is a component that can be overlapped with the layer containing surface A to form a layer, but it is preferable that it is a porous structure with high breaking strength. In order to increase the breaking strength, the breaking strength (breaking strength per unit area) is preferably 3 MPa or more, more preferably 10 MPa or more. In addition, when the composite membrane having other layers is in the form of a hollow fiber, the breaking strength of the other layer is preferably 2900 N or more, more preferably 7800 N or more.

[0042] A membrane filtration device using the porous membrane of the present invention obtained as described above can be used to filter liquids. Examples of membrane filtration devices include, but are not limited to, those equipped with a raw water tank, a booster pump, a module consisting of several thousand to several tens of thousands of the porous hollow fiber membranes of the present invention, a filtrate tank, and a backwash pump. Examples of methods for filtering liquids include, but are not limited to, using the membrane filtration device to filter raw liquid such as industrial wastewater at an operating pressure of 10 kPa to 1 MPa, and removing organic matter and the like contained in the raw water.

[0043] The liquid filtration method using the porous membrane of the present invention is suitable for filtering liquids containing sugars, proteins, and their reaction products. Examples of such liquids include polysaccharides, gelatin, Maillard reaction compounds, etc. 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. With the absorbance at 420 nm and Brix in this range, Maillard reaction compounds can be efficiently removed. A Brix of 10 or more ensures that the material to be filtered is not too dilute and can be efficiently filtered. A Brix of 75 or less is preferred 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, a Brix of 50 or less is preferred, with 30 or less being particularly preferred. An absorbance at 420 nm of 5.0 or less is preferred, particularly from the viewpoint of the solubility of Maillard reaction compounds. When the absorbance at 420 nm is 0.01 or more, the Maillard reaction compounds are not too diluted and can be filtered efficiently.

[0044] The absorbance of the sugar solution at 420 nm can be measured using a spectrophotometer. Brix is ​​calculated by measuring the refractive index using a saccharometer. A sugar solution having 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 adjusted to fall within preferred ranges by diluting or concentrating the solution.

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

[0046] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. First, measurement methods and evaluation methods are described below.

[0047] (i) Method for measuring contact angle of porous membrane The static contact angle can be measured using a Drop Master DM500 manufactured by Kyowa Interface Science Co., Ltd. The static contact angle was measured by contacting an air bubble (approximately 2 μL) with the porous membrane surface while it was immersed in pure water. The contact angle with the water surface was calculated from the contact angle obtained from the tangent of the air bubble in contact with the surface. Using pure water, the measurement water temperature was 20° C., and three air bubbles were measured, and the contact angle was calculated from the average value.

[0048] (ii) Measurement of the ratio of cellulose resin to hydrophobic polymer The measurement sample was a porous membrane that had been vacuum dried for 12 hours. The measurement was carried out 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 an IRTracer-100 manufactured by Shimadzu Corporation, and a MicromATR Vision and a single reflection diamond disk manufactured by the same company as accessories for total reflection. The measurement conditions were a resolution of 8 cm －１ The frequency was set to 100 Hz, and the number of scans was set to 64. The obtained spectrum was expressed as absorbance, and multi-point baseline correction was performed. LabSolutions-IR manufactured by Shimadzu Corporation was used for analysis.

[0049] In the obtained spectrum, the peak intensity Ix (X: for polyvinylidene fluoride, X = 881 cm) derived from the hydrophobic polymer -１ , for polyethersulfone, X = 1578 cm -１ , for polysulfone, X = 1580 cm -１ ) and 1034 cm －１ The peak intensity (I) derived from the pyranose ring of cellulose-based resin ＳThe ratio of the hydrophobic polymer to the cellulose-based resin was calculated from the above data using the following formula. Measurements were carried out at two or more points, and the average value was used. Cellulose-based resin / hydrophobic polymer ratio = I Ｓ / Ix Formula (1) (iii) Measurement of the ratio of acetyl groups to pyranose rings in a cellulose-based resin The measurement sample used was a porous membrane that had been vacuum dried for 12 hours. The measurement was carried out 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 an IRTracer-100 manufactured by Shimadzu Corporation, and a MicromATR Vision and a single-reflection diamond disk manufactured by the same company as accessories for total reflection. The obtained spectrum was expressed in terms of absorbance, and multi-point baseline correction was performed. LabSolutions-IR manufactured by Shimadzu Corporation was used for the analysis. In the obtained spectrum, the peak at 1034 cm －１ The peak intensity (Is) due to the pyranose ring of the cellulose-based resin appears at the position －１The saponification ratio of the cellulose-based resin was calculated using the following formula from the peak intensity (Iso) derived from the acetyl group of the cellulose-based resin appearing in the graph. Measurements were performed at two or more points, and the average value was used. Saponification ratio = acetyl group / pyranose ring ratio = Iso / Is (Equation (2)). (iv) Method for measuring the molecular weight cutoff of a porous membrane. A dextran aqueous solution was prepared by mixing Fluka dextran with average molecular weights of 1500 Da, 6000 Da, 15000-25000 Da, and 40000 Da with Aldrich dextran with average molecular weights of 60000 Da and 200000 Da, each at 500 ppm. The resulting dextran solution was subjected to crossflow filtration at a membrane surface linear velocity of 0.9 m / s, and the filtrate and the water being filtered at that time were sampled. Each sampled solution was subjected to GPC measurement using a Tosoh Technosystems HLC-8320 and an RI detector (double flow system). Measurement conditions included purified water as the eluent, a Tosoh TSKgel G4000PWxl column, a temperature of 40°C, and a flow rate of 0.5 mL / min. The molecular weight cutoff was calculated by dividing the value of the dextran solution after filtration by the value of the dextran solution before filtration by the removal rate, and the molecular weight at which the removal rate was 90% was defined as the molecular weight cutoff (Da).

[0050] (V) Regarding the surface structure (pore size, standard deviation) of the surface portion The porous membrane was vacuum dried at 25 ° C for 12 hours and then observed at a magnification of 30,000 to 100,000 times using an SEM (Hitachi High-Technologies Corporation; S-5500). The image obtained by observing the surface of the porous membrane using an SEM was binarized using the free software "ImageJ". When binarizing, after setting Subtract Background to 1 pixel and selecting Create Background, the condition: RenyiEntropy was selected as the Threshold (binarization threshold). In the obtained binarized image, the area of ​​each hole was determined by selecting Area in Analyze Particles, and the diameter calculated assuming each hole to be a circle was used as the surface pore size. When calculating the average surface pore size, the pore sizes of more than 1,000 holes were averaged. Similarly, the standard deviation of each surface pore size was determined. Measurements were carried out at two or more points, and the average value was used.

[0051] (Vi) Method for Evaluating the Filterability of a Porous Membrane after Steam Treatment in 125°C Water for a Liquid Containing Sugars, Proteins, and Their Reaction Products. Examples of sugars, proteins, and their reaction products include polysaccharides, gelatin, and Maillard reaction compounds. A gelatin aqueous solution was selected as the protein-containing solution. A 10% gelatin aqueous solution (molecular weight: 30,000 to 300,000 Da, turbidity: 23 NTU) was supplied to a porous membrane at 25°C so that the transmembrane pressure difference was 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 calculated when the amount of gelatin in the unfiltered solution was taken as 100%. The amount of gelatin in the permeate was calculated by measuring absorbance at 292 nm. Here, the cross-flow linear velocity is the value obtained by dividing the flow rate of the unfiltered solution in a direction perpendicular to the filtration direction by the cross-sectional area of ​​the flow path. The transmembrane pressure difference is the difference between the pressure on the raw filtrate side and the pressure on the permeate side across the porous membrane. The standard for acceptable change in heat-resistant rejection is a value of 0.5 or more obtained by dividing the organic substance rejection after heat treatment by the organic substance rejection before heat treatment. When the change in organic substance rejection before and after heat treatment is expressed as a percentage, a rejection rate of 50% or more is considered excellent, and a rejection rate of 15% or more is considered sufficient.

[0052] A sugar solution containing Maillard reaction compounds was selected as a solution containing a sugar-protein reaction product. The sugar solution (absorbance at 420 nm of 0.01 to 5.0, Brix: 10 to 70%) was supplied to a porous membrane at 60°C to achieve a transmembrane pressure difference of 80 kPa and subjected to cross-flow filtration at a cross-flow linear velocity of 0.8 m / sec. The absorbance at 420 nm of the permeate was measured, and the rejection rate of Maillard reaction compounds was calculated when the glycoprotein content of the unfiltered solution was taken as 100%. The absorbance at 420 nm of the sugar solution was measured using a spectrophotometer (Shimadzu Corporation: UV-2450). The Brix value was calculated by measuring the refractive index using a saccharimeter (Atago Co., Ltd.: Pocket Saccharimeter PAL-1). As in the above, when the change in organic substance rejection before and after the heat treatment was expressed as a percentage, 70% or more was considered to be an excellent rejection rate, and 50% or more was considered to be a sufficient rejection rate.

[0053] Example 1 A porous membrane was produced by the following method. 38% by mass of PVDF (KF1300, manufactured by Kureha Corporation, 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-tube nozzle while accompanying an 85% by mass aqueous γ-butyrolactone solution as a hollow-portion-forming liquid. The discharged support membrane stock solution was solidified in a cooling bath placed 30 mm below the nozzle and containing an 85% by mass aqueous γ-butyrolactone solution at 20°C, to produce a hollow fiber-shaped porous structure having a spherical structure.

[0054] 12% by mass of PVDF1 (Arkema; ​​Kynar (registered trademark) 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 were mixed and stirred at 120 ° C. for 4 hours to prepare a polymer solution having the composition shown in Table 1. Next, the polymer solution was uniformly applied to the outer surface of the hollow fiber porous structure at 30 m / min (thickness 50 μm). 1 second after application, the porous structure to which the polymer solution had been applied was immersed in a coagulation bath of distilled water at 30 ° C. for 10 seconds to coagulate, forming a porous membrane. The surface portion of the porous membrane from the surface to a thickness of 10 μm was denser than the interior. Next, as step (C), the obtained porous membrane was immersed in a 0.01 N NaOH aqueous solution at room temperature for 24 hours. The evaluation results of the obtained porous membrane are shown in Table 1.

[0055] The static contact angle of the obtained porous film was 36°, Is / Ix was 0.85, Iso / Is was 0.20, and the molecular weight cutoff was 23,000. Furthermore, the surface pore size was 8.0 nm, with a standard deviation of 2.5 nm. Furthermore, as a result of underwater steam treatment at 125°C, the molecular weight cutoff was 18,000, confirming that the structure was maintained. Before steam treatment, the gelatin rejection rate was 70.2%, while after steam treatment, the rejection rate was maintained at 76.7%. The organic substance rejection rate after heat treatment divided by the organic substance rejection rate before heat treatment was 109%, which was an acceptable level of heat resistance. The results are shown in Table 1. The rejection 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 rejection rate in the sugar solution (absorbance at 420 nm: 0.06, Brix: 61%) was 70.3%, both of which were high rejection rates.

[0056] (Comparative Example 1) In the film production 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 static contact angle of the obtained porous film was 44.4 °, Is / Ix was 0.87, Iso / Is was 0.63, and the molecular weight cutoff was 40,000. Furthermore, the surface pore size was 8.8 nm, with a standard deviation of 3.0 nm. Furthermore, as a result of underwater steam treatment at 125 ° C, the molecular weight cutoff was 50,000, and the structure was not maintained. Before steam treatment, the gelatin rejection rate was 65.7%, while after steam treatment, it was 30.0%, and the rejection rate could not be maintained. The value obtained by dividing the organic substance rejection rate after heat treatment by the organic substance rejection rate before heat treatment was low at 46%, and the heat resistance was poor.

[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 were low inhibition rates. The results are shown in Table 2.

[0058] (Example 2) A porous membrane was obtained by the same method as in Example 1, 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 membrane are shown in Table 1. The surface portion of the porous membrane, from the surface to a thickness of 10 μm, was denser than the interior. The static contact angle of the obtained porous membrane was 35.0°, Is / Ix was 0.87, Iso / Is was 0.18, and the molecular weight cutoff was 18,000. Furthermore, the surface pore size was 7.4 nm with a standard deviation of 2.1 nm. Furthermore, as a result of underwater steam treatment at 125°C, the molecular weight cutoff was 17,000, confirming that the structure was maintained. Before steam treatment, the gelatin rejection rate was 80.2%, while after steam treatment, the rejection rate was maintained at 82.5%. The value obtained by dividing the organic substance rejection rate after heat treatment by the organic substance rejection rate before heat treatment was 102%, indicating acceptable heat resistance.

[0059] (Example 3) A porous film was obtained by the same film formation method 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 portion of the porous film from the surface to a thickness of 10 μm was denser than the interior. The static contact angle of the obtained porous film was 34.7°, Is / Ix was 0.76, Iso / Is was 0.08, and the molecular weight cutoff was 16,000. Furthermore, the surface pore size was 6.9 nm, with a standard deviation of 2.0 nm. Furthermore, as a result of underwater steam treatment at 125°C, the molecular weight cutoff was 15,000, confirming that the structure was maintained. Before steam treatment, the gelatin rejection rate was 90.2%, while after steam treatment, the rejection rate was maintained at 93.5%. The value obtained by dividing the organic substance rejection rate after heat treatment by the organic substance rejection rate before heat treatment was 104%, which was an acceptable level of heat resistance.

[0060] (Example 4) A porous membrane was obtained in the same manner as in Example 1, except that the NMP in the polymer solution was replaced with DMF and the distilled water in the coagulation bath was set to 40°C. The evaluation results of the porous membrane are shown in Table 1. The surface portion of the porous membrane from the surface to a thickness of 10 μm was denser than the interior. The static contact angle of the obtained porous membrane was 32.6°, Is / Ix was 0.99, Iso / Is was 0.28, and the molecular weight cutoff was 55,000. Furthermore, the surface pore size was 11.4 nm, with a standard deviation of 4.1 nm. Furthermore, as a result of underwater steam treatment at 125°C, the molecular weight cutoff was 45,000, confirming that the structure was maintained. Before steam treatment, the gelatin rejection rate was 26.2%, while after steam treatment, the rejection rate was maintained at 30.3%. The value obtained by dividing the organic substance rejection rate after heat treatment by the organic substance rejection rate before heat treatment was 115%, which was an acceptable level of heat resistance.

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

[0062]

[0063]

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

Claims

1. A porous membrane whose main component is a hydrophobic polymer, which contains a cellulose-based resin on its surface, one surface being surface A and the other surface being surface B, wherein the average pore diameter (hereinafter referred to as the surface pore diameter) of surface A 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 contains at least one thermoplastic resin selected from the group consisting of polyvinylidene fluoride resins, polyethersulfone resins, and polysulfone resins, and the absorption wavelength (1034 cm ) of the pyranose ring of the cellulose resin relative to the following peak intensity Ix attributed to the hydrophobic polymer, as measured on the surface A by an ATR-IR method (attenuated total reflection IR method): －１ 2. The porous membrane according to claim 1, wherein the intensity ratio (Is / Ix) of the peak intensity Is derived from ... <Absorption wavelength of hydrophobic polymers> In the case of polyvinylidene fluoride resins, the absorption wavelength is 881 cm -１ In the case of polyethersulfone resin, 1578 cm -１ In the case of polysulfone-based resins, the 1580 cm -１ Let's say.

3. The hydrophobic polymer is a polyvinylidene fluoride resin, and the absorption wavelength of polyvinylidene fluoride (881 cm ) measured on the surface A by an ATR-IR method (attenuated total reflection spectroscopy) -１ ) relative to the absorption wavelength (1034 cm ) of the pyranose ring of the cellulose-based resin. -１ 2. The porous membrane according to claim 1, wherein the intensity ratio (Is / Ix) of the peak intensity Is derived from ...

4. The absorption wavelength (1034 cm ) of the pyranose ring of the cellulose-based resin measured on the surface A of the porous film by an ATR-IR method (attenuated total reflection measurement method) -１ The absorption wavelength (1744 cm) of the acetyl group of the cellulose-based resin relative to the peak intensity Is derived from －１ 3. The porous membrane according to claim 1, wherein the intensity ratio (Iso / Is) of the peak intensity Iso derived from the ion beam is 0.01 or more and 0.6 or less.

5. 3. The porous membrane according to claim 1, wherein the molecular weight cutoff of the porous membrane is 10,000 to 200,000.

6. The porous film according to claim 1 or 2, wherein the porous film is composed of a plurality of layers, one outermost surface of a layer having one outermost surface is surface A, and the other outermost surface of a layer having the other outermost surface is surface B, and the Is / Ix ratio measured at surface B by an ATR-IR method (attenuated total reflection infrared (ATR)-IR method) is 0.6 or less.

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

8. 3. The porous membrane according to claim 1, wherein the surface A is a 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 solidifying the polymer solution in a non-solvent to form a porous membrane, wherein the step (A) comprises a hydrophobic polymer and a cellulose resin; the step (B) comprises a non-solvent containing 90 to 100% by weight of water, and the temperature of the non-solvent is 6 to 60°C; and a step (C) of contacting at least one surface of the formed porous membrane with an alkaline solution after steps (A) and (B).

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

11. 11. The method for producing a porous membrane according to claim 9 or 10, wherein the step (A) uses a polyvinylidene fluoride resin as a main component as the hydrophobic polymer, the step (C) contains a 0.0001N to 0.01N aqueous sodium hydroxide solution as the alkaline solution, and the formed porous membrane is immersed in the alkaline solution.

12. 11. 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 a porous structure, and the outermost surface of the formed porous membrane becomes surface A, and in step (C) of contacting the porous membrane with the alkaline solution, surface A is treated.

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

14. 14. The method for filtering a liquid according to claim 13, wherein the liquid is a sugar solution containing sugar, protein, and a reaction product thereof.

15. 15. The method for filtering a liquid according to claim 14, wherein the sugar solution has an absorbance of 0.01 to 30 at a wavelength of 420 nm and a Brix of 10 to 75.

16. A membrane filtration device comprising the porous membrane according to claim 1 or 2.