Polymer porous membrane and manufacturing method therefor

Abstract

One or more embodiments of the present invention relate to a polymer porous membrane, wherein a hydrophobic polymer is configured from a hydrophobic polymer including 20 mass% or more of a constituent unit or units derived from one or more halogen-including monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide, and the ratio Eo / Ei of the porosity Eo of a first layer that is at a depth of less than 500 nm from a first surface in a dry condition and the porosity Ei of a second layer that is adjacent to the first layer in the depth direction and is at a depth of 500-1000 nm from the first surface is 0.5 or greater. Consequently, provided is a polymer porous film wherein even in a dry state, the film does not crack, and the characteristics of the film can be maintained.

Description

Polymer porous membrane and method for producing the same

[0001] The present invention relates to a polymeric porous membrane containing a hydrophobic polymer, which can be suitably used in water treatment such as wastewater treatment and water purification, and to a method for producing the same.

[0002] Membrane filtration using water treatment membranes is widely used for water treatment to remove impurities contained in seawater, river water, and industrial water. Examples of water treatment membranes (also called filtration membranes) include polymer-based membranes. For example, Patent Document 1 describes a hollow fiber membrane containing constituent units derived from vinyl chloride monomer. Patent Document 2 describes a polymer water treatment membrane made of a vinylidene chloride copolymer containing 50 to 99% by mass of constituent units derived from vinylidene chloride monomer. Patent Document 3 describes a porous membrane containing a fluororesin such as vinylidene fluoride resin.

[0003] Japanese Patent Publication No. 2016-93789, Japanese Patent Publication No. 2013-132579, Japanese Patent No. 7569866

[0004] On the other hand, when polymers containing constituent units derived from vinyl chloride monomers or vinylidene chloride monomers are hydrophobic polymers, drying after film formation can irreversibly change properties such as water permeability, or cause cracking and damage to the film. Therefore, hydrophilization is performed after film formation by glycerin substitution treatment. However, glycerin substitution treatment increases the water absorption rate of the polymer porous membrane, and foaming occurs due to the reaction of the water absorbed by the polymer porous membrane with the potting agent (such as urethane) used during module formation, which can lead to module leakage. In addition, because the polymer porous membrane becomes heavier due to water absorption, the membrane may sag under its own weight during potting, changing the module shape.

[0005] To solve the above-mentioned problems, the present invention provides a polymer porous membrane that does not crack even in a dry state and can maintain its membrane properties.

[0006] One or more embodiments of the present invention relate to a polymer porous membrane containing a hydrophobic polymer, wherein the hydrophobic polymer contains 20% by mass or more of constituent units derived from one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide, and the ratio Eo / Ei of the porosity Eo of a first layer having a depth of less than 500 nm from a first surface to the porosity Ei of a second layer adjacent to the first layer in the depth direction and having a depth of 500 to 1000 nm from a first surface is 0.5 or more.

[0007] One or more embodiments of the present invention relate to a method for producing a polymer porous membrane, comprising the step of contacting a polymer solution containing a hydrophobic polymer, a porosity-opening agent, and a good solvent with a coagulation solution containing water to allow it to coagulate, wherein the hydrophobic polymer contains 20% by mass or more of constituent units derived from one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide; the porosity-opening agent is an ionic compound having a molecular weight of 1000 or less, which dissociates into anions and cations when dissolved in water; and the good solvent is one or more selected from the group consisting of dimethyl sulfoxide, dimethylacetamide, dimethylformamide, and acetone.

[0008] According to one or more embodiments of the present invention, it is possible to provide a polymer porous membrane that does not crack even when dried and maintains its film properties. Furthermore, according to the manufacturing method of one or more embodiments of the present invention, it is possible to obtain a polymer porous membrane that does not crack even when dried and maintains its film properties.

[0009] The inventors of the present invention have diligently studied how to prevent cracking of a polymer porous membrane when it dries and maintain the performance of the membrane, using a hydrophobic polymer containing constituent units derived from one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide. As a result, they found that in the polymer porous membrane, by setting the ratio Eo / Ei (hereinafter also simply referred to as Eo / Ei) of the porosity Eo of the first layer, which is less than 500 nm deep from the first surface when the moisture content is less than 10% by mass, to 0.5 or more, the polymer porous membrane does not crack when it dries, specifically when the moisture content is less than 10% by mass, and the change in the performance of the membrane, such as water permeability, is small even after drying (moisture content less than 10% by mass) compared to before drying (moisture content of 10% by mass or more).

[0010] Furthermore, the inventors of the present invention have found that when producing a polymer porous membrane using a non-solvent-induced phase separation method with a hydrophobic polymer containing constituent units derived from one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide, a polymer porous membrane with an Eo / Ei of 0.5 or higher can be obtained by using specific good solvents, coagulation solutions, and porosity opening agents. Specifically, in the step of solidifying a polymer solution containing a hydrophobic polymer, a porosity opening agent, and a good solvent by contacting it with a coagulation solution, a polymer porous membrane with an Eo / Ei of 0.5 or higher can be obtained by using a coagulation solution containing water, using one or more selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylacetamide, dimethylformamide, and acetone as the good solvent, and using an ionic compound with a molecular weight of 1000 or less that ionizes when dissolved in water and separates into anions and cations as the porosity opening agent. The reason for this is presumed to be as follows. (1) In the process of solidifying a polymer solution containing a hydrophobic polymer and a good solvent by contacting it with a solidification solution, when the rate of desorption of the good solvent, such as DMSO, from the polymer solution is lower than the rate of inflow of water from the solidification solution into the polymer solution, the polymer solution swells by absorbing water during solidification, making it possible to create a solidified structure without a skin. On the other hand, when the rate of desorption of the good solvent, such as DMSO, from the polymer solution is higher than the rate of inflow of water from the solidification solution into the polymer solution, the volume of the polymer solution contracts due to the desorption of the good solvent, such as DMSO, and a dense skin layer is created. (2) When an ionic compound that dissociates into anions and cations when dissolved in water is used as a porosity-opening agent, the solubility of the ionic compound in water is greater than that in a good solvent such as DMSO, and the rate of desorption of the good solvent such as DMSO from the polymer solution is lower than the rate of water inflow from the coagulation solution into the polymer solution. As a result, the moment the polymer solution containing the hydrophobic polymer, the ionic compound, and the good solvent comes into contact with the coagulation bath containing water, water is drawn into the polymer solution, creating a skinless structure, and a polymer porous membrane with an Eo / Ei of 0.5 or more is obtained, that is, a polymer porous membrane that does not crack even when dried and can maintain its properties. Furthermore, if the molecular weight of the ionic compound is 1000 or less, the rate of water inflow from the coagulation solution into the polymer solution becomes faster, making it easier to exert the effect.

[0011] Furthermore, in the porous polymer membrane of the present invention, by using a hydrophobic polymer containing vinyl chloride and one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide (excluding fluorine-containing monomers), that is, by using a hydrophobic polymer that is not a fluorine-containing resin, fouling resistance is improved compared to when fluororesins such as vinylidene fluoride resin are used, and there is no risk of generating organofluorine compounds (PFAS) that are a concern for adverse effects on the human body, as is the case when fluororesins such as vinylidene fluoride resin are used.

[0012] In this specification, "hydrophobic polymer" means a polymer containing 50% by mass or more of constituent units derived from hydrophobic monomers, and having a solubility in water at 25°C of less than 1000 mg / kg. In this specification, "first surface of polymer porous membrane" means the inlet-side surface of the polymer porous membrane that is being filtered (hereinafter also referred to as the membrane inlet surface). In this specification, "wet state" means a state in which the water content of the polymer porous membrane is 10% by mass or more, and "dry state" means a state in which the water content of the polymer porous membrane is less than 10% by mass. In the following, unless otherwise specified, "dry state" is used.

[0013] In this specification, when a numerical range is indicated by "~", the numerical range includes both endpoints (upper and lower limits). For example, the numerical range "X~Y" includes both endpoints, X and Y. Furthermore, when multiple numerical ranges are described in this specification, the range shall include numerical ranges formed by appropriately combining the upper and lower limits of different numerical ranges. Furthermore, when multiple upper and lower limits of a numerical range are described separately in this specification, the range shall include numerical ranges formed by appropriately combining the upper and lower limits.

[0014] In one or more embodiments of the present invention, a "porous membrane" refers to a membrane having a large number of micropores inside, with these micropores interconnected, allowing gas or liquid to pass from one surface to the other. In one or more embodiments of the present invention, the internal structure of the porous membrane can be confirmed by observing a cross-section parallel to the thickness direction of the porous membrane with a scanning electron microscope. In the polymer porous membrane of one or more embodiments of the present invention, it is preferable that the hydrophobic polymer has a three-dimensional network structure and has one or more through-holes that penetrate from the first surface to the surface on the opposite side in the thickness direction. This improves the filtration performance during water treatment.

[0015] The polymer porous membrane contains a hydrophobic polymer. In the polymer porous membrane, the content of the hydrophobic polymer is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 85% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more. The polymer porous membrane may consist of 100% by mass of the hydrophobic polymer.

[0016] The hydrophobic polymer may contain 20% by mass or more of constituent unit A derived from one or more halogen-containing monomers (hydrophobic monomers) selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide, and is not particularly limited, but from the viewpoint of chemical resistance, it is preferable to contain 25% by mass or more of constituent unit A, preferably 30% by mass or more, more preferably 35% by mass or more, even more preferably 40% by mass or more, and even more preferably 45% by mass or more. In the following, unless otherwise specified, halogen-containing monomer means one or more selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide.

[0017] The hydrophobic polymer may consist only of constituent unit A, but from the viewpoint of strength, it is preferable to include constituent unit B derived from acrylonitrile (hydrophobic monomer) in addition to constituent unit A. From the viewpoint of fouling resistance, the hydrophobic polymer preferably contains 15% by mass or more of constituent unit B, preferably 20% by mass or more, more preferably 25% by mass or more, even more preferably 30% by mass or more, even more preferably 35% by mass or more, even more preferably 40% by mass or more, and even more preferably 45% by mass or more. The hydrophobic polymer may also contain 80% by mass or less, 75% by mass or less, 70% by mass or less, 65% by mass or less, 60% by mass or less, or 55% by mass or less of constituent unit B.

[0018] From the viewpoint of chemical resistance and fouling resistance, the hydrophobic polymer preferably contains 20 to 85% by mass of constituent unit A and 15 to 80% by mass of constituent unit B, more preferably 25 to 80% by mass of constituent unit A and 20 to 75% by mass of constituent unit B, even more preferably 30 to 70% by mass of constituent unit A and 30 to 70% by mass of constituent unit B, even more preferably 35 to 65% by mass of constituent unit A and 35 to 65% by mass of constituent unit B, even more preferably 40 to 60% by mass of constituent unit A and 40 to 60% by mass of constituent unit B, and even more preferably 45 to 55% by mass of constituent unit A and 45 to 55% by mass of constituent unit B. The hydrophobic polymer preferably contains a total of 50% by mass or more of constituent unit A and constituent unit B, which are constituent units derived from hydrophobic monomers.

[0019] In addition to constituent units A and B, the hydrophobic polymer may also contain constituent unit C derived from a vinyl monomer (hydrophilic monomer) having an ionic substituent, from the viewpoint of fouling resistance. The hydrophobic polymer preferably contains 10% by mass or less of constituent unit C, and may contain 0.1 to 9% by mass, 0.2 to 8% by mass, 0.3 to 7% by mass, 0.4 to 6% by mass, 0.5 to 5% by mass, or 1 to 3% by mass. More specifically, the hydrophobic polymer preferably contains 20 to 85% by mass of constituent unit A, 15 to 80% by mass of constituent unit B, and 0 to 10% by mass of constituent unit C; more preferably contains 29.5 to 70% by mass of constituent unit A, 29.5 to 70% by mass of constituent unit B, and 0.5 to 5% by mass of constituent unit C; and even more preferably contains 39 to 60% by mass of constituent unit A, 39 to 60% by mass of constituent unit B, and 1 to 3% by mass of constituent unit C.

[0020] The vinyl monomer having the ionic substituent is not particularly limited, and various vinyl monomers having different ionic substituents can be used. The ionic substituent may be anionic or cationic. Furthermore, the ionic substituent can be of the strongly electrolytic type, which has a degree of ionization of 99% or more in water, or of the weakly electrolytic type, which has a degree of ionization of less than 99% in water. The strongly electrolytic type anionic substituent is not particularly limited, but examples include sulfonic acid groups. The weakly electrolytic type anionic substituent is not particularly limited, but examples include carboxylic acid groups and phosphoric acid groups. The strongly electrolytic type cationic substituent is not particularly limited, but examples include ammonium groups, phosphonium groups, and onium groups such as sulfonium groups. The weakly electrolytic type cationic substituent is not particularly limited, but examples include primary to tertiary amino groups, pyridyl groups, and imino groups.

[0021] The vinyl monomer having the ionic substituent is preferably a vinyl monomer having a strongly electrolytic anionic substituent, and more preferably a vinyl monomer containing a sulfonic acid group. The vinyl monomer containing a sulfonic acid group is not particularly limited, but examples include allyl sulfonic acid, methallyl sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, styrene sulfonic acid, 2-methyl-1,3-butadiene-1-sulfonic acid, and salts thereof (sodium salts, etc.).

[0022] The vinyl monomer having the ionic substituent may be used individually or in combination of two or more.

[0023] The hydrophobic polymer may contain constituent units derived from other monomers in addition to constituent units A, B, and C. The other monomers may be hydrophobic monomers or hydrophilic monomers, and are preferably hydrophilic monomers such as unsaturated carboxylic acids such as (meth)acrylic acid and their salts; esters of unsaturated carboxylic acids such as methyl (meth)acrylate and glycidyl (meth)acrylate; and vinyl esters such as vinyl acetate and vinyl butyrate. In the hydrophobic polymer, the content of constituent units derived from other monomers may be 20% by mass or less, 10% by mass or less, or 5% by mass or less.

[0024] In the polymer porous membrane, the ratio Eo / Ei of the porosity Eo of the first layer to the porosity Ei of the second layer in a dry state (moisture content less than 10% by mass) is 0.5 or more. This suppresses membrane cracking in a dry state (moisture content less than 10% by mass) and makes it easier to suppress the difference in membrane performance, such as water permeability, between a polymer porous membrane in a wet state with a moisture content of 10% by mass or more and a polymer porous membrane in a dry state with a moisture content of less than 10% by mass. The Eo / Ei is preferably 0.55 or more, more preferably 0.6 or more, even more preferably 0.65 or more, and even more preferably 0.7 or more. From the viewpoint of reducing the difference in membrane performance between a dry state and a wet state, the Eo / Ei is preferably 5 or less, more preferably 4.5 or less, even more preferably 4 or less, even more preferably 3.5 or less, even more preferably 3 or less, even more preferably 2.5 or less, and even more preferably 2 or less. More specifically, the Eo / Ei ratio may be 0.5 to 5, 0.55 to 4.5, 0.6 to 4, 0.6 to 3.5, 0.6 to 3, 0.65 to 2.5, or 0.7 to 2. In this specification, the porosity Eo of the first layer and the porosity Ei of the second layer in the dry state (moisture content less than 10% by mass) of the polymer porous membrane are calculated by freezing and cleaving the polymer porous membrane in a direction perpendicular to the first surface (membrane inflow surface) and using the image of the obtained cross-section. Specifically, this can be measured as described in the examples.

[0025] In the polymer porous membrane, the porosity Eo of the first layer in a dry state (water content less than 10% by mass) is preferably 1 to 40%, more preferably 2 to 35%, even more preferably 3 to 30%, even more preferably 4 to 30%, even more preferably 5 to 30%, even more preferably 6 to 30%, even more preferably 7 to 25%, and even more preferably 8 to 20%, from the viewpoint of reducing the difference in membrane performance between the dry state and the wet state.

[0026] In the polymer porous membrane, the porosity Ei of the second layer in the dry state (water content less than 10% by mass) is preferably 2 to 80%, more preferably 3 to 70%, even more preferably 4 to 60%, even more preferably 5 to 50%, even more preferably 6 to 40%, even more preferably 7 to 30%, even more preferably 8 to 25%, and even more preferably 9 to 20%, from the viewpoint of reducing the difference in membrane performance between the dry state and the wet state.

[0027] The aforementioned porous polymer membrane has a water permeability (also referred to as initial water permeability) H in a wet state with a water content of 10% by mass or more, in order to reduce the difference in membrane performance between dry and wet states. W And, the permeability H in a dry state with a moisture content of less than 10% by mass. D Ratio H D / H W It is preferably 0.5 to 2.0, more preferably 0.6 to 1.8, even more preferably 0.7 to 1.6, even more preferably 0.8 to 1.4, and even more preferably 0.9 to 1.2. In this specification, the permeability H of a polymer porous membrane in a wet state with a water content of 10% by mass or more. W And, the water permeability H of a polymer porous membrane in a dry state with a water content of less than 10% by mass. D This can be measured and calculated as described in the examples.

[0028] The polymer porous membrane is not particularly limited, but from the viewpoint of filtration performance and water permeability, for example, it is preferably 0.01 to 1 mm thick, and more preferably 0.05 to 0.5 mm thick. If the thickness is 0.01 mm or more, defects such as cracks are less likely to occur and filtration performance is easily improved, and if the thickness is 1 mm or less, water permeability is easily improved.

[0029] In the polymer porous membrane described above, from the viewpoint of excellent water permeability after treating water containing impurities in membrane filtration, the average pore diameter of the first surface in a dry state (water content less than 10% by mass) is preferably 3 to 160 nm, more preferably 3 to 100 nm, still more preferably 13 to 90 nm, even more preferably 20 to 90 nm, and even more preferably 30 to 90 nm. Further, in the polymer porous membrane described above, from the viewpoint of excellent water permeability after treating water containing impurities in membrane filtration, the porosity of the first surface in a dry state (water content less than 10% by mass) is preferably 0.5 to 30%, more preferably 0.5 to 20%, and still more preferably 1 to 15%. In the present specification, the average pore diameter and porosity of the first surface of the polymer porous membrane in a dry state (water content less than 10% by mass) are calculated using an image of the surface of the first surface (the inflow surface of the membrane), and specifically can be measured as described in the Examples.

[0030] From the viewpoint of excellent water permeability after treating water containing impurities in membrane filtration, when an aqueous solution of 50 ppm sodium alginate obtained by dissolving sodium alginate in a 0.5 mM aqueous sodium hydrogen carbonate solution is passed through the polymer porous membrane for 30 minutes, the water permeability (hereinafter also referred to as water permeability after fouling) is 100 L / (m 2 ・atm・h) or more, preferably 110 L / (m 2 ・atm・h) or more, more preferably 120 L / (m 2 ・atm・h) or more, still more preferably 140 L / (m 2 ・atm・h) or more, even more preferably 200 L / (m 2 ・atm・h) or more, and particularly preferably 200 L / (m・atm・h) or more. In the present specification, the water permeability after fouling can be specifically measured as described in the Examples.

[0031] From the viewpoint of excellent initial water permeability, the initial water permeability of the polymer porous membrane is 1000 L / (m 2 ・atm・h) or more, preferably 1100 L / (m 2・atm・h) or more, more preferably 1200 L / (m 2 ・atm・h) or more, even more preferably 1500 L / (m 2 ・atm・h) or more, still more preferably 1800 L / (m 2 ・atm・h) or more, still more preferably 2000 L / (m 2 ・atm・h) or more, still more preferably 2500 L / (m 2 ・atm・h) or more, still more preferably 3000 L / (m 2 ・atm・h) or more is particularly preferred. In the present specification, the initial water permeability can be specifically measured as described in Examples.

[0032] The shape of the polymer porous membrane is not limited, and any shape can be selected according to specific applications and purposes, for example, hollow fiber shape, flat membrane shape, spiral shape, pleated shape, and tubular shape. Among these, hollow fiber shape and flat membrane shape are preferable from the viewpoints of easy production, low cost, and usability at low pressure.

[0033] The polymer porous membrane does not undergo membrane cracking in a dry state. For example, when the membrane is bent at 90° with the first surface facing inward in the dry state and the bent portion is flattened again, it is preferable that there is no curled-up portion of the membrane surface within a range of 5 mm × 2 mm.

[0034] In one or more embodiments of the present invention, the polymer porous membrane can be produced by a non-solvent induced phase separation method. Specifically, the method includes a membrane forming step of bringing a polymer solution containing a hydrophobic polymer, a pore-forming agent, and a good solvent into contact with a coagulation liquid containing water to coagulate the polymer. As the hydrophobic polymer, the hydrophobic polymers described above can be used.

[0035] The pore-forming agent is an ionic compound that ionizes when dissolved in water and separates into anions and cations. This makes it possible to obtain a polymer porous membrane in which the ratio Eo / Ei of the porosity Eo of the first layer to the porosity Ei of the second layer in a dry state (water content less than 10% by mass) is 0.5 or more.

[0036] In the ionic compound, the anion is preferably a monovalent anion from the viewpoint of pore control, and may be a monovalent inorganic anion or a monovalent organic anion. Examples of monovalent inorganic anions include halide ions and nitrate ions. Examples of monovalent organic anions include acetate ions. From the viewpoint of pore control, the anion is more preferably one or more selected from the group consisting of halide ions, acetate ions, and nitrate ions, and even more preferably one or more inorganic anions selected from the group consisting of halide ions and nitrate ions. Examples of halide ions include fluoride ions, chloride ions, bromide ions, and iodide ions.

[0037] In the ionic compound, the cation is preferably an alkali metal ion, an alkaline earth metal ion, or an ammonium ion, from the viewpoint of pore control. Examples of alkali metal ions include lithium ions, sodium ions, and potassium ions. Examples of alkaline earth metal ions include calcium ions, magnesium ions, and strontium ions. From the viewpoint of pore control, it is more preferable that the cation is one or more selected from the group consisting of sodium ions, calcium ions, and ammonium ions.

[0038] The ionic compound has a molecular weight of 1000 or less. This makes it possible to obtain a polymer porous membrane in which the ratio Eo / Ei of the porosity of the first layer to the porosity of the second layer in a dry state (water content of less than 10% by mass) is 0.5 or more. In the ionic compound, from the viewpoint of pore control, the molecular weight of the ionic compound is preferably 500 or less, more preferably 450 or less, and even more preferably 400 or less.

[0039] The ionic compound can be used as an aqueous solution. The concentration of the aqueous solution of the ionic compound is not particularly limited, but may be, for example, 4 to 30% by mass, or 5 to 25% by mass. From the viewpoint of use as an aqueous solution, it is preferable that the ionic compound is one or more selected from the group consisting of sodium chloride, lithium chloride, calcium chloride, and calcium nitrate. The aqueous solution of the ionic compound may be prepared using a buffer such as a phosphate buffer. The pH of the phosphate buffer is not particularly limited, but may be, for example, pH 2 to 12, or pH 5 to 9.

[0040] The good solvent is one or more selected from the group consisting of dimethyl sulfoxide, dimethylacetamide, dimethylformamide, and acetone. This makes it possible to obtain a polymer porous film in which the ratio Eo / Ei of the porosity of the first layer to the porosity of the second layer in a dry state (water content less than 10% by mass) is 0.5 or more. The good solvent is preferably one or more selected from the group consisting of dimethyl sulfoxide, dimethylacetamide, and dimethylformamide, and more preferably dimethyl sulfoxide.

[0041] The polymer solution can be prepared, for example, by mixing the hydrophobic polymer, an aqueous solution of the ionic compound, and a good solvent. In the polymer solution, the concentration of the hydrophobic polymer can be appropriately selected according to the pore size and water permeability of the target polymer porous membrane, and is not limited, but is preferably 5 to 30% by mass, more preferably 8 to 20% by mass. When the concentration of the hydrophobic polymer is within the above range, the processability during film formation and the strength of the resulting polymer porous membrane are improved.

[0042] From the viewpoint of easily obtaining a polymer porous film in which Eo / Ei satisfies the above range, the polymer solution preferably contains 3 to 30 parts by mass of the ionic compound, more preferably 4 to 25 parts by mass, and even more preferably 5 to 15 parts by mass of the ionic compound per 100 parts by mass of the hydrophobic polymer.

[0043] Furthermore, when preparing the polymer solution, various film-forming aids such as lubricants and stabilizers may be used in combination as needed, within a range that does not hinder the effects of the present invention. The amount of these film-forming aids added can be arbitrarily adjusted to suit the physical properties of the desired polymer porous membrane, as long as it does not hinder the effects of the present invention and allows for film formation.

[0044] The coagulation solution used during film formation contains water. This makes it possible to obtain a polymer porous film in which Eo / Ei satisfies the above-mentioned range. From the viewpoint of easily obtaining a polymer porous film in which Eo / Ei satisfies the above-mentioned range, the coagulation solution used during film formation preferably contains 20% by mass or more of water, more preferably 50% by mass or more, even more preferably 70% by mass or more, and particularly preferably 90% by mass or more. The coagulation solution used during film formation may contain the above-mentioned good solvent in addition to water, but it may also be a water bath consisting of 100% by mass of water.

[0045] The temperature of the coagulation solution is not particularly limited and may be, for example, 20 to 80°C or 30 to 75°C.

[0046] The method for producing the polymer porous membrane may include a water washing step after the film formation step. The water washing step can remove good solvents from the polymer porous membrane. In the water washing step, for example, water at 5°C or higher can be used, and the water temperature may be 10 to 80°C or 20 to 70°C.

[0047] The moisture content of the wet polymer porous membrane obtained after the washing step may be 10% by mass or more, 20% by mass or more, 30% by mass or more, or 40% by mass or more. Alternatively, the moisture content of the wet polymer porous membrane obtained after the washing step may be 90% by mass or less, 10 to 90% by mass, 20 to 80% by mass, 30 to 70% by mass, or 40 to 60% by mass. In this specification, the moisture content of the wet polymer porous membrane can be measured and calculated as described in the examples.

[0048] The above manufacturing method may further include a drying step after the washing step. The drying step can yield a dry polymer porous membrane with a moisture content of less than 10% by mass. The moisture content of the dry polymer porous membrane may be 9% by mass or less, 8% by mass or less, 7% by mass or less, 6% by mass or less, 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, or 1% by mass or less. Alternatively, the moisture content of the dry polymer porous membrane may be 0.1% by mass or more, and may be 0.1 to 9% by mass, 0.1 to 8% by mass, 0.1 to 7% by mass, 0.1 to 6% by mass, 0.2 to 5% by mass, 0.2 to 4% by mass, 0.2 to 3% by mass, 0.2 to 2% by mass, or 0.2 to 1% by mass. Drying may be performed, for example, by heating air. The temperature of the heated air may be 30 to 80°C or 40 to 60°C. The drying time can be, for example, 1 to 5 hours, or 2 to 3 hours.

[0049] More specifically, the polymer porous membrane can be obtained, for example, by applying the copolymer solution prepared as described above to a support using an applicator, then coagulating and washing it with water as described above. If necessary, drying may be performed after washing with water. As the support, a filter paper made of cellulose fibers can be used. In such a polymer porous membrane, the side opposite the support such as the filter paper becomes the inflow side of the material to be filtered, and the surface of the polymer porous membrane opposite the support such as the filter paper becomes the first surface of the polymer porous membrane.

[0050] The aforementioned porous polymer membrane can be suitably used to remove impurities contained in various treated waters, such as river water and industrial water. For example, it can be used as a filter material in water purifier cartridges or membrane units for wastewater treatment.

[0051] The present invention will be described in detail below based on examples. However, the present invention is not limited to these examples.

[0052] (Example 1) Copolymer 1, comprising 48.0% by mass of constituent units derived from vinyl chloride, 50.5% by mass of constituent units derived from acrylonitrile, and 1.5% by mass of constituent units derived from sodium styrenesulfonate, was synthesized by emulsion polymerization. 60 g of copolymer 1, 25 g of an aqueous solution of sodium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (sodium chloride concentration 20% by mass) as a porosity-opening agent, and 415 g of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed to obtain a copolymer solution. The obtained copolymer solution was applied to filter paper (Advantec Toyo No. 2 qualitative filter paper) using an applicator with a gap of 200 μm, solidified in a 74°C water bath (coagulation solution), and then washed with 20°C water to obtain a polymer porous membrane.

[0053] (Example 2) 60 g of copolymer 1, 25 g of an aqueous solution of lithium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (lithium chloride concentration 10% by mass) as a porosity-opening agent, and 415 g of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed to obtain a copolymer solution. The obtained copolymer solution was applied to filter paper (Advantec Toyo No. 2 qualitative filter paper) using an applicator with a gap of 200 μm, solidified in a 60°C water bath (coagulation solution), and then washed with 20°C water to obtain a polymer porous membrane.

[0054] (Example 3) 60 g of copolymer 1, 25 g of an aqueous solution of sodium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (sodium chloride concentration 10% by mass) as a porosity-opening agent, and 415 g of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed to obtain a copolymer solution. The obtained copolymer solution was applied to filter paper (Advantec Toyo No. 2 qualitative filter paper) using an applicator with a gap of 200 μm, solidified in a 40°C water bath (coagulation solution), and then washed with 20°C water to obtain a polymer porous membrane.

[0055] (Example 4) 50 g of copolymer 1, 25 g of an aqueous solution of sodium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (sodium chloride concentration 10% by mass) as a porosity-opening agent, and 425 g of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed to obtain a copolymer solution. The obtained copolymer solution was applied to filter paper (Advantec Toyo Co., Ltd. No. 2 qualitative filter paper) using an applicator with a gap of 200 μm, solidified in a 60°C water bath (coagulation solution), and then washed with 20°C water to obtain a polymer porous membrane.

[0056] (Example 5) 60 g of copolymer 1, 25 g of an aqueous solution of calcium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (calcium chloride concentration 10% by mass) as a porosity-opening agent, and 415 g of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed to obtain a copolymer solution. The obtained copolymer solution was applied to filter paper (Advantec Toyo No. 2 qualitative filter paper) using an applicator with a gap of 200 μm, solidified in a 25°C water bath (coagulation solution), and then washed with 20°C water to obtain a polymer porous membrane.

[0057] (Comparative Example 1) 65 g of copolymer 1, 204.5 g of polyethylene glycol 200 (manufactured by Fujifilm Wako Pure Chemical Industries) as a porosity-opening agent, and 230.5 g of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries) were mixed to obtain a copolymer solution. The obtained copolymer solution was applied to filter paper (Advantec Toyo No. 2 qualitative filter paper) using an applicator with a gap of 200 μm, solidified in a 40°C water bath (coagulation solution), and then washed with 20°C water to obtain a polymer porous membrane.

[0058] (Comparative Example 2) 55 g of copolymer 1, 209 g of polyethylene glycol 200 as a porosity-opening agent, and 236 g of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries) were mixed to obtain a copolymer solution. The obtained copolymer solution was applied to filter paper (Advantec Toyo No. 2 qualitative filter paper) using an applicator with a gap of 200 μm, solidified in a 50°C water bath (coagulation solution), and then washed with 20°C water to obtain a polymer porous membrane.

[0059] The initial moisture content (moisture content in the wet state), moisture content after drying (moisture content in the dry state), porosity, initial water permeability (water permeability in the wet state), water permeability after drying (water permeability in the dry state), number and size of film cracks after drying (dry state), average pore diameter and porosity of the first surface, and water permeability after fouling of the polymer porous membranes obtained in the examples and comparative examples were measured and evaluated as follows. In the polymer porous membranes obtained in the examples and comparative examples, the surface of the polymer porous membrane opposite the filter paper side is the first surface. The results are shown in Table 1 below. Table 1 below also shows the concentration of copolymer 1 in the polymer solution (polymer solution), the type and amount of porosity opening agent, and the temperature of the coagulation solution. In Table 1 below, the amount of porosity opening agent is parts by mass per 100 parts by mass of copolymer 1.

[0060] (Initial moisture content) First, a polymer porous membrane was cut into a 5 cm x 3 cm section, and the surface water was wiped off with a Kimwipe. The mass of the sample (W0) was then measured. Next, the sample was dried in a 120°C forced-air constant-temperature incubator for 1 hour. The mass of the dried sample (W1) was then measured, and the moisture content was calculated using the following formula, which was defined as the initial moisture content: Moisture content (mass%) = 100 × [(W0 - W1) / W1]

[0061] (Moisture content after drying) After drying the polymer porous membrane in a 40°C forced-air constant-temperature incubator for 3 hours, a 5 cm x 3 cm sample was cut out and used as the sample. Except for this, the moisture content was measured and calculated in the same manner as for the initial moisture content, and this was recorded as the moisture content after drying.

[0062] (Porosity) After drying a polymer porous membrane in a 40°C forced-air constant-temperature oven for 3 hours, it was cut into 5 cm x 1 cm sections to create samples. The samples were immersed in liquid nitrogen and, while frozen, folded to create fracture surfaces. Scanning electron microscope (Hitachi High-Tech, S-4800) was used to acquire 50,000x magnified images of the film fracture surfaces at 1280 x 960 pixels. Two fracture surfaces were created for each sample, and images were acquired and analyzed for each fracture surface. WinROOF2018 (Mitani Corporation) was used for image analysis. After setting the image resolution and length per pixel in WinROOF2018, the analysis range was specified. For the first layer, where the depth from the film surface (first surface of the film) is less than 500 nm, and for the second layer, where the depth is from 500 to 1000 nm, the analysis range was specified as a 300 nm square area for each layer. After checking the luminance histogram within the specified analysis range, the image brightness was adjusted so that the average luminance value for the entire analysis range was between 95 and 105. To enhance the color of the image, the color density distribution within the analysis range was transformed and adjusted so that 99.95% of the range extended across the entire image. The adjusted image was converted to a monochrome image, the brightness was set to +20 and the contrast to +50, and then black and white binarization was performed in the range of 0 to 60. After five morphological processes—(1) degeneracy (1 time), (2) isolated point removal (bright points), (3) exclusive expansion (1 time), (4) closing, and (5) hole filling—the area of ​​the measured part (the total area of ​​all black parts corresponding to voids) was calculated, and the porosity Eo of the first layer and the porosity Ei of the second layer were determined using the following formula. Porosity of the first layer Eo (%) = (Area of ​​measurement site / Area of ​​analysis range) × 100 Porosity of the second layer Ei (%) = (Area of ​​measurement site / Area of ​​analysis range) × 100

[0063] (Average pore size and porosity) After drying the polymer porous membrane in a 40°C constant temperature oven for 3 hours, the pores on the membrane surface (first surface) of the polymer porous membrane were observed using a scanning electron microscope (Hitachi High-Tech S-4800). Images of the membrane surface (first surface) were acquired at 5,000x and 50,000x magnification. If the estimated pore size was less than 50 nm, the 50,000x image was used to calculate the average pore size and porosity. If the pore size was 50 nm or more, the 5,000x image was used. WinROOF2018 (Mitani Corporation) was used for image analysis. After setting the image resolution and length per pixel in WinROOF, the analysis range was specified. The analysis range was 10 μm square for the 5,000x image and 1 μm square for the 50,000x image, selecting areas without defects or other abnormalities and with an average pore state. After checking the luminance histogram within the specified analysis range, the image brightness was adjusted so that the average luminance value for the entire analysis range was between 95 and 105. To enhance the color of the image, the color density distribution within the analysis range was transformed and adjusted so that 99.95% of the range extended throughout the entire image. The adjusted image was converted to a monochrome image, the brightness was set to +20 and the contrast to +50, and then black and white binarization was performed in the range of 0 to 60. After five morphological processes—(1) degeneracy (1 time), (2) isolated point removal (bright points), (3) exclusive expansion (1 time), (4) closing, and (5) hole filling—the area of ​​each measurement site (black parts corresponding to voids) was calculated. After calculating the area, measurement sites overlapping on the boundary line of the analysis range were deleted, and the equivalent circle diameter was calculated based on the area of ​​the remaining individual measurement sites. <Method for calculating average hole diameter> The average of the equivalent circle diameters of each measurement site within the entire analysis range was used as the average hole diameter. <Method for Calculating Open Area Ratio> [Formula 1] Open area ratio (%) = {Sum of the areas of individual measurement points / Area of ​​the analysis range} × 100%

[0064] (Initial water permeability) A porous polymer membrane (water content of 10% by mass or more) is cut into a 4 cm x 4 cm section, with an effective membrane area of ​​9 cm². 2 It was set in a flat membrane module (3 cm x 3 cm). The first surface was the inlet surface to be filtered, and pure water was passed through it under conditions of a flow rate of 34 mL / min and a pressure of 0.01 MPa (0.1 atm). The permeability was calculated using the following formula 2, and the initial permeability (i.e., H) was determined. W) was assumed. In the following equation 2, V represents the volume of pure water that permeates through the membrane (L), and A represents the effective area of ​​the membrane (m²). 2 ) is shown, where P represents water pressure (atm) and Δt represents measurement time (h). [Equation 2] Permeability (L / m 2 ・atm・h)=V / (A×P×Δt)

[0065] (Water permeability after drying) A polymer porous membrane (water content of 10% by mass or more) was dried in a 40°C forced-air constant-temperature oven for 3 hours, then cut into 3 cm x 3 cm sections to obtain a sample (water content of less than 10% by mass), with an effective membrane area of ​​9 cm². 2 Using a (3cm x 3cm) flat membrane cross-flow module, with the first surface as the inlet surface to be filtered, pure water was passed through for 30 minutes under conditions of a flow rate of 34 mL / min and a pressure of 0.01 MPa (0.1 atm). Then, the permeability was measured in the same way as for the initial permeability and the permeability was calculated, and the permeability after drying (i.e., H) was calculated. D )

[0066] (Confirmation of the number and size of film cracks after drying) A polymer porous membrane (water content of 10% by mass or more) was dried in a 40°C forced-air constant-temperature oven for 3 hours. Then, a 3 cm x 3 cm section was cut out as a sample (water content of less than 10% by mass). The sample was folded at a 90° angle with the first surface facing inward, and the folded part was flattened again. An image (25x magnification) was acquired using a 3D shape measuring machine (KEYENCE VR-5000). Next, areas where the film surface was peeled up within a 5 mm x 2 mm area were identified as film cracks, and the number of film cracks and the size of each crack were measured. For the measurement, the image was printed on A4 size paper, the number and size of the cracks were measured, the size of the scale bar was measured, and the size of the crack was calculated from the ratio of these values.

[0067] (Water permeability after fouling) The water permeability after fouling was measured using a dead-end module and the following procedure was followed: A polymer porous membrane (water content of 10% by mass or more) was cut into a 3 cm x 3 cm section, the surface water was wiped off with Kimwipes, and the sample was prepared with an effective membrane area of ​​9 cm². 2A flat membrane module measuring 3 cm x 3 cm was set with the first surface facing the inlet side of the filtration target. A 50 ppm sodium alginate aqueous solution (test solution), prepared by dissolving sodium alginate in a 0.5 mM sodium bicarbonate aqueous solution, was passed through the membrane at a flow rate of 5 mL / min using a dead-end method for 40 minutes. The pressure P on the membrane was measured over time, and the permeability after 30 minutes was calculated using the above-mentioned formula 2, which was then used as the permeability after fouling.

[0068]

[0069] As can be seen from the data in Table 1 above, the polymer porous membranes of the examples in which the Ei / Eo ratio was 0.5 or higher in a dry state with a low moisture content (less than 10% by mass) did not experience membrane cracking in the dry state with a low moisture content (less than 10% by mass), and the membrane performance in the dry state with a moisture content of less than 10% by mass, specifically the change in water permeability, was also small compared to the wet state with a moisture content of 10% by mass or more.

[0070] On the other hand, the polymer porous membranes of Comparative Examples 1 and 2, which had a low moisture content (less than 10% by mass) and an Ei / Eo of less than 0.5 in a dry state, showed membrane cracking in the low moisture content (less than 10% by mass) dry state. The membrane performance, specifically the water permeability, in the dry state with a moisture content of less than 10% by mass was significantly different from that in the wet state with a moisture content of 10% by mass or more.

[0071] Furthermore, a comparison of the examples and comparative examples shows that by using an ionic compound with a molecular weight of 1000 or less as a porosity agent, a polymer porous membrane with an Ei / Eo ratio of 0.5 or higher can be obtained. Also, a comparison of Examples 1, 2, 4, and 5 with Example 3 shows that when the porosity of the first surface of the polymer porous membrane in a dry state is 0.5% or higher, the filtration performance after treating water containing impurities in membrane filtration is excellent.

[0072] The present invention is not particularly limited, but preferably includes, for example, the following embodiments: [1] A polymer porous membrane composed of a hydrophobic polymer, wherein the hydrophobic polymer contains 20% by mass or more of constituent units derived from one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide and vinylidene bromide, and the ratio Eo / Ei of the porosity Eo of a first layer having a depth of less than 500 nm from the first surface to the porosity Ei of a second layer adjacent to the first layer in the depth direction and having a depth of 500 to 1000 nm from the first surface is 0.5 or more when the water content of the polymer porous membrane is less than 10% by mass. [2] The polymer porous membrane according to [1], wherein the porosity Eo of the first layer having a depth of 1 to 40% when the water content of the polymer porous membrane is less than 10% by mass. [3] The polymer porous membrane according to [1] or [2], wherein the Eo / Ei is 5 or less when the water content of the polymer porous membrane is less than 10% by mass. [4] The polymer porous membrane according to any one of [1] to [3], wherein the porosity Ei of the second layer is 2 to 80% when the water content of the polymer porous membrane is less than 10% by mass. [5] The water permeability H when the water content of the polymer porous membrane is 10% by mass or more. W The permeability H of the polymer porous membrane when the water content is less than 10% by mass is as follows: D Ratio H D / H W[1] to [4], wherein the polymer porous membrane is 0.5 to 2.0. [6] The polymer porous membrane according to any one of [1] to [5], wherein the average pore diameter of the first surface is 3 to 160 nm when the water content of the polymer porous membrane is less than 10% by mass. [7] The polymer porous membrane according to any one of [1] to [6], wherein the porosity of the first surface is 0.5 to 30% when the water content of the polymer porous membrane is less than 10% by mass. [8] The polymer porous membrane according to any one of [1] to [7], wherein the hydrophobic polymer contains 35 to 65% by mass of constituent units derived from the halogen-containing monomer and 35 to 65% by mass of constituent units derived from acrylonitrile. [9] A method for producing a polymer porous membrane, comprising the step of contacting a polymer solution containing a hydrophobic polymer, a porosity opening agent, and a good solvent with a coagulation solution containing water to coagulate, wherein the hydrophobic polymer contains 20% by mass or more of constituent units derived from one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide, the porosity opening agent is an ionic compound having a molecular weight of 1000 or less, which dissociates into anions and cations when dissolved in water, and the good solvent is one or more selected from the group consisting of methyl sulfoxide, dimethylacetamide, dimethylformamide, and acetone, a method for producing a polymer porous membrane.

[10] The method for producing a polymer porous membrane according to [9], wherein the polymer solution contains 3 to 30 parts by mass of the ionic compound per 100 parts by mass of the hydrophobic polymer.

[11] The method for producing a polymer porous membrane according to [9] or

[10] , wherein the anion is one or more selected from the group consisting of halide ions, acetate ions, and nitrate ions.

Claims

1. A polymer porous membrane containing a hydrophobic polymer, wherein the hydrophobic polymer contains 20% by mass or more of constituent units derived from one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide, and the ratio Eo / Ei of the porosity Eo of a first layer having a depth of less than 500 nm from the first surface to the porosity Ei of a second layer adjacent to the first layer in the depth direction and having a depth of 500 to 1000 nm from the first surface is 0.5 or more.

2. The polymer porous membrane according to claim 1, wherein, when the water content of the polymer porous membrane is less than 10% by mass, the porosity Eo of the first layer is 1 to 40%.

3. The polymer porous membrane according to claim 1, wherein the Eo / Ei is 5 or less when the water content of the polymer porous membrane is less than 10% by mass.

4. The polymer porous membrane according to claim 1, wherein, when the water content of the polymer porous membrane is less than 10% by mass, the porosity Ei of the second layer is 2 to 80%.

5. Permeability H of the polymer porous membrane when the water content is 10% by mass or more. W The permeability H of the polymer porous membrane when the water content is less than 10% by mass is as follows: D Ratio H D / H W The polymer porous membrane according to claim 1, wherein the coefficient is 0.5 to 2.

0.

6. The polymer porous membrane according to claim 1, wherein, when the water content of the polymer porous membrane is less than 10% by mass, the average pore diameter of the first surface is 3 to 160 nm.

7. The polymer porous membrane according to claim 1, wherein the porosity of the first surface is 0.5 to 30% when the water content of the polymer porous membrane is less than 10% by mass.

8. The porous polymer membrane according to claim 1, wherein the hydrophobic polymer contains 35 to 65% by mass of constituent units derived from the halogen-containing monomer and 35 to 65% by mass of constituent units derived from acrylonitrile.

9. A method for producing a polymer porous membrane, comprising the step of contacting a polymer solution containing a hydrophobic polymer, a porosity-opening agent, and a good solvent with a coagulation solution containing water to coagulate, wherein the hydrophobic polymer contains 20% by mass or more of constituent units derived from one or more halogen-containing monomers selected from the group consisting of vinyl chloride, vinylidene chloride, vinyl bromide, and vinylidene bromide, the porosity-opening agent is an ionic compound having a molecular weight of 1000 or less, which dissociates into anions and cations when dissolved in water, and the good solvent is one or more selected from the group consisting of dimethyl sulfoxide, dimethylacetamide, dimethylformamide, and acetone.

10. The method for producing a polymer porous membrane according to claim 9, wherein the polymer solution contains 3 to 30 parts by mass of the ionic compound with respect to 100 parts by mass of the hydrophobic polymer.

11. The method for producing a polymer porous membrane according to claim 9, wherein the anion is one or more selected from the group consisting of halide ions, acetate ions, and nitrate ions.

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