Method for producing dried ion exchange membrane

Abstract

The present invention provides a method for manufacturing a dried ion exchange membrane that can manufacture a water electrolysis device having suppressed liquid leakage.　The method for producing a dried ion exchange membrane according to the present invention is a method in which a wet ion exchange membrane, which was obtained as a result of an ion exchange membrane that includes a fluoropolymer having ion exchange groups being wetted using a liquid medium, is dried to remove the liquid medium, thereby obtaining a dried ion exchange membrane, wherein the wet ion exchange membrane is dried from a state where the amount of the liquid medium adhering to the surface of the wet ion exchange membrane is 40 g / m2 to 600 g / m2.

Description

Method for producing dry ion exchange membrane

[0001] The present disclosure relates to a method for producing a dry ion exchange membrane.

[0002] Ion exchange membranes are used in various batteries, electrolysis processes, and processes for separating ions, etc. Patent Document 1 describes a method for separating ions by using an acid type ion exchange group (-SO ３ The membrane containing the fluorine-containing polymer having —SO ３ The document discloses a method for obtaining a dry ion exchange membrane containing a fluoropolymer having an H group. Such an ion exchange membrane is installed in a tank of a water electrolysis device in the form of a membrane electrode assembly in which an anode, an ion exchange membrane (electrolyte membrane), and a cathode are laminated in this order. Here, a gasket is usually attached to the peripheral edge of the membrane electrode assembly to prevent leakage of the electrolyte solution such as water to the outside of the water electrolysis device and to prevent damage to the membrane electrode assembly.

[0003] Japanese Patent Application Laid-Open No. 2005-060516

[0004] The present inventors have found that when a membrane electrode assembly is manufactured using a dry ion exchange membrane as described in Patent Document 1 and a gasket is attached to the periphery of the membrane electrode assembly, the assembly is placed in a tank of a water electrolysis device, and when an electrolytic solution (e.g., water) is supplied into the tank, leakage of the liquid to the outside of the water electrolysis device may occur.

[0005] The present disclosure has been made in view of the above-described circumstances, and an object of one embodiment of the present invention is to provide a method for manufacturing a dry ion exchange membrane that can manufacture a water electrolysis device in which the occurrence of liquid leakage is suppressed.

[0006] The present disclosure includes the following aspects: [1] A method for producing a dried ion exchange membrane, in which a wet ion exchange membrane containing a fluorine-containing polymer having ion exchange groups is wetted with a liquid medium, and the wet ion exchange membrane is dried to remove the liquid medium, thereby obtaining a dried ion exchange membrane, wherein the amount of the liquid medium adhering to the surface of the wet ion exchange membrane is 40 g / m ２ Above, 600g / m ２[2] A method for producing a dried ion exchange membrane, wherein the wet ion exchange membrane is dried from a state in which the amount of the liquid medium adhering to the surface of the wet ion exchange membrane is 50 g / m or less. ２ Above, 300g / m ２ [3] The method for producing a dried ion exchange membrane according to [1], wherein the drying temperature is 60 to 150°C. [4] The method for producing a dried ion exchange membrane according to any of [1] to [3], wherein the drying is carried out from a state in which the content of the liquid medium in the wet ion exchange membrane is 60% by mass or more and 200% by mass or less, based on the total mass of the dry ion exchange membrane. [5] The method for producing a dried ion exchange membrane according to any of [1] to [4], wherein the ion exchange capacity of the fluoropolymer is 0.90 to 2.05 meq / g dry resin.

[0007] [6] The method for producing a dry ion exchange membrane according to any one of [1] to [5], wherein the fluoropolymer contains a unit represented by formula (1): Formula (1) -[CF ２ -CF(-L-(SO ３ M) ｎ ))]--In formula (1), L represents a (n+1)-valent perfluorohydrocarbon group which may contain an etheric oxygen atom, M represents a hydrogen atom, an alkali metal, or a quaternary ammonium cation, and n represents 1 or 2. [7] The method for producing a dry ion exchange membrane according to any one of [1] to [6], wherein a reinforcing material is contained inside or on a surface of the wet ion exchange membrane. [8] The method for producing a dry ion exchange membrane according to [7], wherein the reinforcing material contains at least one reinforcing thread selected from the group consisting of reinforcing threads made of polytetrafluoroethylene, reinforcing threads made of a tetrafluoroethylene-perfluoroether copolymer, reinforcing threads made of polyphenylene sulfide, reinforcing threads made of polyether ether ketone, reinforcing threads made of nylon, and reinforcing threads made of polypropylene. [9] The method for producing a dry ion exchange membrane according to any one of [1] to [8], wherein the dry ion exchange membrane is used as an electrolyte membrane contained in a membrane electrode assembly of a water electrolysis apparatus.

[0008] According to one embodiment of the present invention, it is possible to provide a method for producing a dry ion exchange membrane that can produce a water electrolysis device in which the occurrence of liquid leakage is suppressed.

[0009] The definitions of the following terms apply throughout the present specification and claims unless otherwise specified. An "ion exchange group" is a group that can exchange at least a portion of the ions contained in this group with other ions, and examples thereof include the sulfonic acid functional group and carboxylic acid functional group shown below. A "sulfonic acid functional group" is a sulfonic acid group (-SO ３ Here, the form of the sulfonate group is, for example, (—SO ３ － ) Ma ＋ , (-SO ３ － ) ２ Mb ２＋ , and (-SO ３ － ) ３ Mc ３＋ (However, Ma ＋ is an alkali metal ion or a quaternary ammonium cation, and Mb ２＋ is a divalent metal ion, Mc ３＋ is a trivalent metal ion.) When there are two ligands, the number of ion exchange groups is counted as two, and when there are three ligands, the number of ion exchange groups is counted as three. "Carboxylic acid type functional group" means a carboxylic acid group (-COOH) or a carboxylic acid salt group. Here, the form of the carboxylic acid salt group can be, for example, (-COO － ) Ma ＋ , (-COO － ) ２ Mb ２＋ , and (-COO － ) ３ Mc ３＋ (However, Ma ＋ is an alkali metal ion or a quaternary ammonium cation, and Mb ２＋ is a divalent metal ion, Mc ３＋is a trivalent metal ion.) Note that when there are two ligands, the number of ion exchange groups is counted as two, and when there are three ligands, the number of ion exchange groups is counted as three. A "precursor membrane" is a membrane containing a polymer having a group that can be converted into an ion exchange group. A "group that can be converted into an ion exchange group" means a group that can be converted into an ion exchange group by known treatments such as hydrolysis treatment and acidification treatment. A "group that can be converted into a sulfonic acid functional group" means a group that can be converted into a sulfonic acid functional group by known treatments such as hydrolysis treatment and acidification treatment. A "group that can be converted into a carboxylic acid functional group" means a group that can be converted into a carboxylic acid functional group by known treatments such as hydrolysis treatment and acidification treatment.

[0010] A "unit" in a polymer refers to an atomic group derived from one molecule of a monomer formed by polymerization of the monomer. The unit may be an atomic group formed directly by the polymerization reaction, or may be an atomic group in which part of the atomic group is converted into a different structure by treating the polymer obtained by the polymerization reaction. In the following, units derived from individual monomers may be referred to by the name of the monomer followed by "unit" in some cases.

[0011] A numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. In the numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples.

[0012] [Method for producing dried ion exchange membrane] The method for producing a dried ion exchange membrane according to the present disclosure (hereinafter also referred to as "the present production method") is a method for producing a dried ion exchange membrane in which an ion exchange membrane containing a fluoropolymer having ion exchange groups (hereinafter also referred to as "fluoropolymer (I)") is wetted with a liquid medium, and the wet ion exchange membrane is dried to remove the liquid medium, thereby obtaining a dried ion exchange membrane, and the amount of water adhering to the surface of the wet ion exchange membrane is 600 g / m ２The wet ion exchange membrane is dried from the following state.

[0013] Using the dried ion exchange membrane obtained by this production method, a water electrolysis device with reduced liquid leakage can be produced. While the details of the reason for this are unclear, it is presumed to be due to the following reasons. The wet ion exchange membrane is wet with the medium used for hydrolysis of the fluoropolymer having groups convertible to ion exchange groups, the medium used for counterion exchange, and the water used for rinsing after these processes. In this case, the liquid medium in an amount exceeding the saturated water absorption of the membrane (such as the precursor membrane described below) used to produce the wet ion exchange membrane is not absorbed into the membrane but remains attached to the membrane surface. Therefore, the liquid medium may remain attached to the surface of the wet ion exchange membrane. If the wet ion exchange membrane is dried with too much liquid medium attached to its surface, uneven drying occurs, resulting in a non-uniform thickness of the dried ion exchange membrane. Furthermore, if the wet ion exchange membrane is dried with too little liquid medium attached to its surface, dimensional changes occur before drying, resulting in a non-uniform thickness of the dried ion exchange membrane. As a result, it is thought that the dried ion exchange membrane is prone to wrinkles when wound into a roll. When a membrane electrode assembly is manufactured using a wrinkled dried ion exchange membrane and a gasket is attached to the periphery of the membrane electrode assembly, a gap may be formed between the gasket and the membrane electrode assembly due to the wrinkles in the dried ion exchange membrane. When a membrane electrode assembly with such a gap is applied to a water electrolysis device and the supply of electrolyte is started, the electrolyte leaks from the gap. As a result, it is presumed that the electrolyte leaks to the outside of the water electrolysis device.

[0014] <Wet ion exchange membrane> The wet ion exchange membrane used in the present production method is an ion exchange membrane wetted with a liquid medium. The fluoropolymer (I) contained in the wet ion exchange membrane contains the liquid medium and is in a swollen state.

[0015] The content of the liquid medium in the wet ion exchange membrane is preferably 20% by mass or more, more preferably 25% by mass or more, even more preferably 30% by mass or more, and particularly preferably 60% by mass or more, based on the total mass of the dry ion exchange membrane, from the viewpoint of suppressing uneven drying during roll transport, improving the flatness of the membrane, and suppressing the occurrence of wrinkles during transport. The content of the liquid medium in the wet ion exchange membrane is preferably 200% by mass or less, more preferably 180% by mass or less, even more preferably 160% by mass or less, and particularly preferably 140% by mass or less, based on the total mass of the dry ion exchange membrane, from the viewpoint of excellent drying efficiency. Here, in this specification, when a liquid medium is attached to the surface of the wet ion exchange membrane, the content of the liquid medium in the wet ion exchange membrane mentioned above also includes the amount of liquid medium attached to the surface. The content of the liquid medium in the wet ion exchange membrane is determined by the method described in the Examples section below.

[0016] When the wet ion exchange membrane is dried (i.e., at the start of drying), the amount of the liquid medium adhering to the surface of the wet ion exchange membrane is 600 g / m ２ or less, and the effects of the present disclosure are more excellent, so 400 g / m ２ Preferably, 300 g / m or less ２ More preferably, 250 g / m or less ２ More preferably, 200 g / m or less ２ The following is particularly preferred: When the wet ion exchange membrane is dried (i.e., at the start of drying) as described below, the amount of liquid medium adhering to the surface of the wet ion exchange membrane is 40 g / m or less, from the viewpoint of being able to suppress deformation (e.g., curling) of the wet ion exchange membrane due to rapid drying. ２ More than 50 g / m ２ More preferably, 100 g / m or more ２ The amount of the liquid medium adhering to the surface of the wet ion exchange membrane can be determined by the method described in the Examples section below.

[0017] The thickness of the wet ion exchange membrane is preferably 30 μm or more, more preferably 40 μm or more, from the viewpoint of maintaining a certain strength, and is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 250 μm or less, from the viewpoint of improving current efficiency and voltage efficiency.

[0018] The wet ion exchange membrane preferably has a certain degree of surface roughness. In particular, the wet ion exchange membrane preferably contains a reinforcing material, which will be described later, inside or on the surface. When the wet ion exchange membrane contains a reinforcing material, the reinforcing material generates slight irregularities on the surface of the wet ion exchange membrane. This makes it easier for the liquid medium to be retained on the surface of the wet ion exchange membrane, and makes it easier to appropriately adjust the amount of liquid medium adhering to the surface of the wet ion exchange membrane.

[0019] The contact angle of water on the surface of the wet ion exchange membrane is preferably 50 to 100°, more preferably 55 to 85°, from the viewpoint of easily retaining an appropriate amount of liquid medium on the surface of the wet ion exchange membrane. It is thought that if the wet ion exchange membrane contains a reinforcing material, the contact angle tends to be small, and if it does not contain a reinforcing material, the contact angle tends to be large. The contact angle is determined by the method described in the Examples section below.

[0020] (Liquid medium) The liquid medium may be any medium capable of swelling the fluoropolymer (I), and examples thereof include organic solvents and water. The liquid medium may be either an organic solvent or water, or a mixture of both. The organic solvent may be used alone or in combination of two or more. Examples of organic solvents capable of swelling the fluoropolymer (I) include water-soluble organic solvents. In this specification, the water-soluble organic solvent refers to an organic solvent that is easily soluble in water, and specifically, an organic solvent having a solubility of 0.1 g or more in 1,000 ml of water (20°C) is preferred, and an organic solvent having such a solubility of 0.5 g or more is more preferred. The water-soluble organic solvent preferably contains one or more selected from the group consisting of aprotic organic solvents, alcohols, and aminoalcohols, and more preferably contains at least an aprotic organic solvent. The water-soluble organic solvent may be used alone or in combination of two or more.

[0021] Specific examples of aprotic organic solvents include dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, and N-ethyl-2-pyrrolidone, with dimethyl sulfoxide being preferred. Specific examples of alcohols include methanol, ethanol, isopropanol, butanol, methoxyethoxyethanol, butoxyethanol, butylcarbitol, hexyloxyethanol, octanol, 1-methoxy-2-propanol, and ethylene glycol. Specific examples of aminoalcohols include ethanolamine, N-methylethanolamine, N-ethylethanolamine, 1-amino-2-propanol, 1-amino-3-propanol, 2-aminoethoxyethanol, 2-aminothioethoxyethanol, and 2-amino-2-methyl-1-propanol.

[0022] (Fluoropolymer (I)) From the viewpoint of excellent voltage efficiency, the ion exchange capacity of the fluoropolymer (I) is preferably 0.90 milliequivalents / gram dry resin (also referred to as meq / g) or more, more preferably 1.00 meq / g or more, even more preferably more than 1.05 meq / g, and particularly preferably 1.10 meq / g or more. From the viewpoint of the balance between current efficiency and voltage efficiency, the ion exchange capacity of the fluoropolymer (I) is preferably 2.05 meq / g or less, more preferably 1.50 meq / g or less, and particularly preferably 1.25 meq / g or less.

[0023] Examples of the fluoropolymer (I) include a fluoropolymer having a sulfonic acid type functional group (hereinafter also referred to as "fluoropolymer (S)") and a fluoropolymer having a carboxylic acid type functional group (hereinafter also referred to as fluoropolymer (C)). The fluoropolymer (I) may be one type, or two or more types may be laminated or mixed and used. Examples of the wet ion exchange membrane include an ion exchange membrane containing a fluoropolymer (S) and an ion exchange membrane in which the fluoropolymer (S) and the fluoropolymer (C) are laminated in layers. An ion exchange membrane containing a fluoropolymer (S) is preferred, and from the perspective of applicability to water electrolysis, an ion exchange membrane containing only the fluoropolymer (S) as the fluoropolymer (I) is more preferred. Below, embodiments of the fluoropolymer (S) will mainly be described in detail.

[0024] The fluorine-containing polymer (S) preferably contains a unit based on a fluorine-containing olefin and a unit having a sulfonic acid functional group and a fluorine atom. Examples of the fluorine-containing olefin include fluoroolefins having 2 to 3 carbon atoms and having one or more fluorine atoms in the molecule. Specific examples of the fluoroolefin include tetrafluoroethylene (hereinafter also referred to as "TFE"), chlorotrifluoroethylene, vinylidene fluoride, vinyl fluoride, and hexafluoropropylene. Among these, TFE is preferred from the viewpoints of the production cost of the monomer, reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer (S). One type of fluorine-containing olefin may be used alone, or two or more types may be used in combination.

[0025] As the unit having a sulfonic acid type functional group and a fluorine atom, a unit represented by formula (1) is preferred. ２ -CF(-L-(SO ３ M) ｎ )]-

[0026] L is an (n+1)-valent perfluorohydrocarbon group which may contain an etheric oxygen atom. The etheric oxygen atom may be located at a terminal of the perfluorohydrocarbon group or between carbon atoms. The (n+1)-valent perfluorohydrocarbon group preferably has 1 or more carbon atoms, more preferably 2 or more carbon atoms, and preferably 20 or less, more preferably 10 or less.

[0027] L is preferably an (n+1)-valent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom, more preferably a divalent perfluoroalkylene group which may contain an etheric oxygen atom, where n = 1, or a trivalent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom, where n = 2. The divalent perfluoroalkylene group may be either linear or branched.

[0028] M is a hydrogen atom, an alkali metal, or a quaternary ammonium cation, and multiple Ms may be the same or different. n is 1 or 2.

[0029] The unit represented by formula (1) is preferably a unit represented by formula (1-1), a unit represented by formula (1-2), a unit represented by formula (1-3), or a unit represented by formula (1-4). ２ -CF(-O-R ｆ１ -SO ３ M)] - Formula (1-2) - [CF ２ -CF(-R ｆ１ -SO ３ M) ]-

[0030]

[0031]

[0032] R ｆ１ is a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

[0033] R ｆ２is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

[0034] R ｆ３ is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

[0035] r is 0 or 1. m is 0 or 1. The definition of M in the formula is as described above.

[0036] As the unit represented by formula (1-1) and the unit represented by formula (1-2), a unit represented by formula (1-5) is more preferred. ２ -CF(-(CF ２ ) ｘ -(OCF ２ CFY) ｙ -O-(CF ２ ) ｚ -SO ３ M)]—x is 0 or 1, y is an integer from 0 to 2, z is an integer from 1 to 4, and Y is F or CF ３ M is as described above.

[0037] Specific examples of the unit represented by formula (1-1) include the following units. In the formula, w is an integer of 1 to 8, and x is an integer of 1 to 5. The definition of M in the formula is as described above. -[CF ２ -CF(-O-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF(-O-CF ２ CF (CF ３ )-O-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF(-(O-CF ２ CF (CF ３ ))ｘ -SO ３ M) ]-

[0038] Specific examples of the unit represented by formula (1-2) include the following units. In the formula, w is an integer of 1 to 8. The definition of M in the formula is as described above. -[CF ２ -CF(-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF (-CF ２ -O-(CF ２ ) ｗ -SO ３ M) ]-

[0039] The unit represented by formula (1-3) is preferably a unit represented by formula (1-3-1), where M is defined as above.

[0040]

[0041] R ｆ４ is a linear perfluoroalkylene group having 1 to 6 carbon atoms, and R ｆ５ is a single bond or a linear perfluoroalkylene group having 1 to 6 carbon atoms which may contain an oxygen atom between the carbon atoms. The definitions of r and M are as described above.

[0042] Specific examples of the unit represented by formula (1-3-1) include the following.

[0043]

[0044] As the unit represented by formula (1-4), a unit represented by formula (1-4-1) is preferred. ｆ１ , R ｆ２ and M are defined as above.

[0045]

[0046] Specific examples of the unit represented by formula (1-4-1) include the following.

[0047]

[0048] The unit having a sulfonic acid type functional group and a fluorine atom may be used alone or in combination of two or more.

[0049] The fluoropolymer (I) may contain units based on other monomers other than the units based on fluorine-containing olefin and the units having a sulfonic acid functional group and a fluorine atom. Specific examples of other monomers include CF ２ = CFR ｆ６ (However, R ｆ６ is a perfluoroalkyl group having 2 to 10 carbon atoms, CF ２ =CF-OR ｆ７ (However, R ｆ７ is a perfluoroalkyl group having 1 to 10 carbon atoms, CF ２ = CFO (CF ２ ) ｖ CF = CF ２ (wherein v is an integer of 1 to 3.) The content of units based on other monomers is preferably at most 30 mass % based on all units in the fluoropolymer (I) from the viewpoint of maintaining ion exchange performance.

[0050] The content of the fluoropolymer (I) is preferably from 30 to 90% by mass based on the total mass of the wet ion exchange membrane.

[0051] The wet ion exchange membrane may have a single layer structure or a multilayer structure. In the case of a multilayer structure, for example, a mode in which a plurality of layers containing a fluoropolymer (S) and having different ion exchange capacities or units are laminated is exemplified. Furthermore, as described above, a layer of a fluoropolymer (C) may be laminated.

[0052] The wet ion exchange membrane may contain a reinforcing material inside or on its surface. That is, the wet ion exchange membrane may be in an embodiment containing the fluorine-containing polymer (I) swollen with a liquid medium and a reinforcing material. The reinforcing material is preferably a member derived from a reinforcing fabric (preferably a woven fabric). In addition to the reinforcing fabric, fibrils and porous bodies can also be mentioned as reinforcing materials. The reinforcing fabric is preferably composed of warp yarns and weft yarns, and the warp yarns and weft yarns are preferably orthogonal to each other. The reinforcing fabric is preferably composed of reinforcing yarns and sacrificial yarns.

[0053] The reinforcing yarn is preferably made of a material that does not dissolve when the reinforcing fabric is immersed in an alkaline aqueous solution (for example, a 32% by mass aqueous solution of sodium hydroxide). Specifically, the reinforcing yarn is preferably at least one type of reinforcing yarn selected from the group consisting of reinforcing yarns made of polytetrafluoroethylene (hereinafter also referred to as PTFE), reinforcing yarns made of tetrafluoroethylene-perfluoroether copolymer (hereinafter also referred to as PFA; as perfluoroether, perfluoroalkyl vinyl ether is preferred), reinforcing yarns made of polyphenylene sulfide (hereinafter also referred to as PPS), reinforcing yarns made of polyether ether ketone (hereinafter also referred to as PEEK), reinforcing yarns made of nylon, and reinforcing yarns made of polypropylene.

[0054] The sacrificial yarn is a yarn that dissolves at least a portion of in the operating environment of the device containing the ion exchange membrane, and is preferably a yarn made of a material that dissolves in an alkaline aqueous solution when the reinforcing fabric is immersed in the alkaline aqueous solution. The sacrificial yarn may be a monofilament consisting of one filament or a multifilament consisting of two or more filaments. The sacrificial yarn maintains the strength of the ion exchange membrane during handling, such as during production of the wet ion exchange membrane and installation of the ion exchange membrane in a battery, but dissolution of the sacrificial yarn in the operating environment of the battery reduces the resistance of the membrane.

[0055] The wet ion exchange membrane may have an inorganic particle layer containing inorganic particles and a binder on its surface. The inorganic particle layer is preferably provided on at least one surface of the wet ion exchange membrane, and more preferably on both surfaces. When the wet ion exchange membrane has an inorganic particle layer, the hydrophilicity of the ion exchange membrane is improved, and the ionic conductivity is improved.

[0056] (Method for producing a wet ion exchange membrane) The wet ion exchange membrane can be obtained by a known method, for example, as described in International Publication No. 2018 / 070444. That is, the method includes a polymerization step of polymerizing a monomer having a group that can be converted into an ion exchange group (preferably a sulfonic acid functional group) to obtain a fluoropolymer having a group that can be converted into an ion exchange group (preferably a sulfonic acid functional group), a membrane formation step of forming the fluoropolymer into a membrane of a fluoropolymer having a group that can be converted into a sulfonic acid functional group (hereinafter also referred to as a "precursor membrane"). Then, a hydrolysis step of hydrolyzing the group that can be converted into an ion exchange group (preferably a sulfonic acid functional group) in the precursor membrane to convert it into an ion exchange group (preferably a sulfonic acid functional group) to form an ion exchange group (preferably a sulfonic acid functional group). After the hydrolysis step, a step of converting the counter ion of the ion exchange group (preferably a sulfonic acid functional group) to hydrogen, sodium, potassium, or the like (for example, an acid-form treatment step) may be included depending on the application. When the wet ion exchange membrane is a laminate, multiple precursor membranes may be prepared and then hydrolyzed after lamination. In addition, when the wet ion exchange membrane contains a reinforcing material, the reinforcing material may also be laminated between the precursor membranes when laminating the precursor membranes.

[0057] After hydrolysis or counter ion exchange, the ion exchange membrane is usually wet with the medium used for the hydrolysis or counter ion exchange of the fluoropolymer, or with the water used for subsequent washing. The ion exchange membrane in this wet state may be used as a wet ion exchange membrane as it is and dried. However, if the amount of the liquid medium adhering to the surface of the wet ion exchange membrane is 40 g / m or less, the amount of the liquid medium adhering to the surface of the wet ion exchange membrane may be 40 g / m or less. ２ More than 600g / m ２It is preferable to carry out a treatment to adjust the wet state of the ion exchange membrane after hydrolysis or counter ion exchange so that the wet ion exchange membrane can be dried from a state below 0.25. Specifically, when the wet state of the ion exchange membrane after hydrolysis or counter ion exchange is insufficient, it is preferable to carry out a treatment to attach an aqueous medium to the surface of the ion exchange membrane, such as a treatment to spray the aqueous medium onto the surface of the ion exchange membrane or a treatment to immerse the ion exchange membrane in an aqueous medium. On the other hand, when the wet state of the ion exchange membrane after hydrolysis or counter ion exchange is excessively high, it is preferable to carry out a treatment to remove the aqueous medium attached to the surface of the ion exchange membrane, such as a treatment to wipe off the aqueous medium attached to the surface of the ion exchange membrane.

[0058] <Drying Method> In this manufacturing method, the amount of the liquid medium adhering to the surface of the wet ion exchange membrane is 600 g / m ２ The wet ion exchange membrane is dried from the state described below to obtain a dry ion exchange membrane.

[0059] The drying of the wet ion exchange membrane may be natural drying or may be heat drying using a known drying device, but drying by heating is preferred from the viewpoint of drying efficiency.When heating is performed during drying of the wet ion exchange membrane, the drying temperature (heating temperature) of the wet ion exchange membrane is preferably 60°C or higher, more preferably 80°C or higher, and even more preferably 90°C or higher from the viewpoint of drying efficiency, and is preferably 150°C or lower, more preferably 120°C or lower, and even more preferably 105°C or lower from the viewpoint of further suppressing the occurrence of wrinkles in the ion exchange membrane due to rapid drying.In particular, the drying temperature of the wet ion exchange membrane is preferably a temperature higher than the softening point of the fluoropolymer (I), more preferably a temperature 10°C or higher than the softening point of the fluoropolymer, and even more preferably a temperature 20°C or higher than the softening point of the fluoropolymer (I), from the viewpoint of reducing the residual internal stress during drying.The softening point of the fluoropolymer (S) is measured by the method described in the Examples section below. The drying temperature of the wet ion exchange membrane means the surface temperature of the wet ion exchange membrane.

[0060] The drying of the wet ion exchange membrane is preferably carried out by a method of drying while restraining the periphery of the wet ion exchange membrane (for example, a method of fixing the periphery of the wet ion exchange membrane by clamping it with a metal frame or a method of fixing the periphery of the wet ion exchange membrane by inserting needles into it), a method of applying a load around the periphery of the wet ion exchange membrane, or the like, in order to suppress dimensional changes of the ion exchange membrane before and after drying and further suppress the occurrence of wrinkles.

[0061] The present production method may be carried out by a roll-to-roll method, in which a long roll of a wet ion exchange membrane is unwound, and after each step of the present production method is carried out, a dry ion exchange membrane is wound up into a roll.

[0062] This manufacturing method may include a conveying step. In particular, it is preferable to include a conveying step after producing the wet ion exchange membrane and before the drying step. In the conveying step, the wet ion exchange membrane is preferably conveyed between each step by a belt conveyor or the like. The conveying speed is, for example, preferably 0.0320 m / s or less, more preferably 0.0167 m / s or less, even more preferably 0.0116 m / s or less, and particularly preferably 0.0083 m / s or less. The conveying speed is, for example, preferably 0.0020 m / s or more, more preferably 0.0040 m / s or more. The conveying speed refers to the distance (m) that a point on the film moves per second when a film-like object is conveyed in the longitudinal direction of the film. The conveying temperature may be, for example, 0°C or higher and 100°C or lower, and preferably 10°C or higher and 50°C or lower. The conveying temperature refers to the temperature of the environment in which the film is conveyed (e.g., room temperature if indoors). The amount of liquid medium adhering to the surface of the wet ion exchange membrane can also be appropriately adjusted by adjusting the transport conditions. In particular, adjusting the transport speed is preferable to the wiping method described above in that it is easy to adjust the amount of liquid medium adhering to the surface of the wet ion exchange membrane evenly and uniformly, and wrinkles are less likely to occur in the dried ion exchange membrane.

[0063] The dry ion exchange membrane obtained by this production method preferably has a liquid medium content of 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less, based on the total mass of the dry ion exchange membrane. The lower limit is 1% by mass.

[0064] The thickness of the dried ion exchange membrane obtained by this production method is preferably 20 μm or more, more preferably 40 μm or more, from the viewpoint of maintaining a certain strength, and is preferably 300 μm or less, more preferably 200 μm or less, from the viewpoint of improving current efficiency and voltage efficiency. The standard deviation of the thickness of the dried ion exchange membrane obtained by this production method is preferably 10 μm or less, more preferably 5 μm or less, from the viewpoint of improving the flatness of the membrane and further suppressing the occurrence of wrinkles that occur during transportation. The lower limit of the standard deviation of the thickness of the dried ion exchange membrane is usually 0.1 μm.

[0065] The dried ion exchange membrane obtained by this production method is suitable for use as an electrolyte membrane included in a membrane-electrode assembly of a water electrolysis device (solid polymer water electrolysis device), and can also be used for other applications. Specific examples of other applications include various battery applications such as solid polymer fuel cells, direct methanol fuel cells, redox flow batteries, and air batteries, as well as alkaline water electrolysis, ozone water electrolysis, salt electrolysis, organic electrolysis, and various electrolysis devices for chlorides or oxides. In addition to the above applications, the membrane is also used as a separator or solid electrode in various types of electrochemical cells for selective cation transport at cell junctions. In addition to electrochemical applications, the membrane is also used in sensor applications such as various gas sensors, biosensors, light-emitting devices, optical devices, and organic sensors, as well as for carbon nanotube solubilization, actuators, and catalysts.

[0066] The present invention will be described in detail below with reference to examples. Examples 1 to 3 are working examples, and Examples 4 and 5 are comparative examples. However, the present invention is not limited to these examples.

[0067] [Thickness of each film] The thickness of each film was measured using a magnified image (for example, 100x) of the cross section of each film taken with an optical microscope (product name "BX-51", manufactured by Olympus Corporation), and the arithmetic mean value of the thicknesses of 20 arbitrary points on each film was taken as the thickness of each film. If the surface of each film was uneven, the thickness of 10 concave portions on each film and the thickness of 10 convex portions on each film were measured, and the arithmetic mean value of the thicknesses of a total of 20 points was taken as the thickness of each film. However, if the convex portions contained threads that constituted the woven fabric, the thickness of the convex portions was taken as the value obtained by subtracting the thickness of the threads present in the convex portions.

[0068] [Ion exchange capacity of fluoropolymer] The fluoropolymer was placed in a glove box filled with dry nitrogen for 24 hours, and the dry mass of the fluoropolymer was measured. Thereafter, the fluoropolymer was immersed in a 2 mol / L aqueous sodium chloride solution at 60°C for 1 hour. The fluoropolymer was washed with ultrapure water and then taken out, and the solution in which the fluoropolymer had been immersed was titrated with a 0.1 mol / L aqueous sodium hydroxide solution to determine the ion exchange capacity of the fluoropolymer.

[0069] [Softening Point of Fluoropolymer] The softening point of the fluoropolymer was measured using a dynamic viscoelasticity measuring device according to the following procedure. First, dynamic viscoelasticity measurement was carried out using a dynamic viscoelasticity measuring device (DVA-225, manufactured by IT Measurement & Control Co., Ltd.) under the conditions of sample width: 5.0 mm, grip length: 15 mm, measurement frequency: 1 Hz, heating rate: 2°C / min, and tensile mode. Next, tan δ (loss tangent) was calculated from the ratio (E" / E') of loss modulus E" to storage modulus E', and a tan δ-temperature curve was prepared. The peak temperature between -100 and 300°C was read from the prepared tan δ-temperature curve, and this value was taken as the softening point.

[0070] [Content of Liquid Medium in Wet Ion Exchange Membrane] The content of liquid medium in the wet ion exchange membrane was calculated using the following formula based on the mass of 0.6 g of the wet ion exchange membrane after drying at 90°C for 16 hours and the mass of the wet ion exchange membrane before drying: Content of liquid medium in wet ion exchange membrane (mass %) = 100 × {(mass of wet ion exchange membrane before drying) - (mass of membrane after drying)} / (mass of membrane after drying).

[0071] [Amount of liquid medium adhering to the surface of a wet ion exchange membrane] The amount of liquid medium adhering to the surface of a wet ion exchange membrane was determined by wiping the surface of the wet ion exchange membrane (size: length 1 m, width 1 m, thickness 30 to 200 μm) with filter paper (circular qualitative filter paper No. 2, manufactured by ADVANTEC) immediately before drying until the aqueous medium adhering to the surface could no longer be visually confirmed, and calculating the amount of liquid medium (g / m) adhering to the surface of the wet ion exchange membrane using the following formula based on the mass of the filter paper before wiping off the aqueous medium and the mass of the filter paper after wiping off the aqueous medium. ２ ) = {(mass of filter paper after wiping off the aqueous medium) - (mass of filter paper before wiping off the aqueous medium)} / surface area of ​​wet ion exchange membrane

[0072] [Water Contact Angle of Wet Ion Exchange Membrane] An electrolyte membrane was immersed in ion-exchange water at 23°C for 60 minutes. The water on the surface of the wet electrolyte membrane was wiped off with filter paper, and 2 μL of ion-exchange water was dropped onto the electrolyte membrane at room temperature. The waiting time before measurement was set to 100 mS, and the contact angle was measured without curvature correction. The contact angle was measured using a PCA-11 manufactured by Kyowa Interface Science Co., Ltd. and determined by the sessile drop method (θ / 2 method). For each example, the measurement was performed five times, and the arithmetic average of the five measurements was used.

[0073] [Standard deviation of thickness of dry ion exchange membrane] The standard deviation (μm) of the thickness of the dry ion exchange membrane was determined from the thicknesses of 20 points of the dry ion exchange membrane obtained by the measurement method of "thickness of each membrane" above. However, when the surface of the dry ion exchange membrane was uneven, the standard deviation (μm) of the thickness of the dry ion exchange membrane was determined from the thicknesses of 10 recessed points on the dry ion exchange membrane.

[0074] [Wrinkles in dried ion exchange membrane] The dried ion exchange membrane wound into a roll obtained in each example described below was unwound, and the number of wrinkles 10 mm or longer in length was visually counted in a region of the dried ion exchange membrane (length in the width direction: 1 m, length in the flow direction: 1 m). The number of wrinkles was evaluated as A when it was 0 to 3, B when it was 4 to 7, C when it was 8 to 10, and D when it was 11 or more. In practice, a grade of C or higher (A, B, or C) is preferred.

[0075] [Evaluation of Liquid Leakage in Water Electrolysis Device] (Production of Membrane Electrode Assembly) CF ２ =CF ２ and a monomer (X) described below were copolymerized, and the resulting polymer (ion exchange capacity: 1.10 meq / g dry resin) was converted to an acid form through hydrolysis and acid treatment. The resulting polymer was dispersed in a water / ethanol solvent of 40 / 60 (mass%) at a solids concentration of 26.0% to obtain a dispersion (hereinafter also referred to as "Dispersion Y"). Ethanol (18.06 g) and Zeorola-H (manufactured by Zeon Corporation) (10.58 g) were added to the resulting dispersion Y (33.0 g), and the mixture was mixed for 5 minutes at 2200 rpm using a planetary centrifugal mixer (Thinky, Awatori Rentaro). Ethanol (46.44 g) and water (75.75 g) were added to the mixed composition (54.06 g), and a mixture having a specific surface area of ​​100 m containing 74.8 mass% iridium was further mixed. ２ 40.0 g of an iridium oxide catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was added to the mixture. The resulting mixture was processed in a planetary bead mill (rotational speed: 300 rpm) for 90 minutes to obtain an anode catalyst ink with a solids concentration of 22 mass %. The anode catalyst ink was applied to an ETFE sheet so that the iridium concentration was 1.0 mg / cm. ２ The anode catalyst layer decal was obtained by applying the coating with an applicator so that the coating became smooth, followed by drying at 80° C. for 10 minutes and then heat treating at 150° C. for 15 minutes.

[0076] Water (59.4 g) and ethanol (39.6 g) were added to 11 g of a supported catalyst ("TEC10E50E" manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) in which 46% by mass of platinum was supported on carbon powder, and the mixture was mixed and pulverized using an ultrasonic homogenizer to obtain a catalyst dispersion. To the catalyst dispersion, a mixture (29.2 g) of dispersion Y (20.1 g), ethanol (11 g), and Zeorola-H (manufactured by Zeon Corporation) (6.3 g) was premixed and kneaded. Furthermore, water (3.66 g) and ethanol (7.63 g) were added to the resulting dispersion and mixed for 60 minutes using a paint conditioner to obtain a cathode catalyst ink with a solids concentration of 10.0% by mass. The cathode catalyst ink was applied to an ETFE sheet using a die coater, dried at 80°C, and further heat-treated at 150°C for 15 minutes, resulting in a platinum content of 0.4 mg / cm. ２ As a result, a cathode catalyst layer decal of 1000 nm was obtained.

[0077] The side of the anode catalyst layer decal on which the catalyst layer was present was placed opposite one side of the dried ion exchange membrane in each example, and the side of the cathode catalyst layer decal on which the catalyst layer was present was placed opposite the other side of the dried ion exchange membrane. The anode catalyst layer, the dried ion exchange membrane, and the cathode catalyst layer were bonded together by hot pressing under conditions of a pressing temperature of 150°C and a pressure of 3 MPa for 10 minutes. After the temperature was lowered to 70°C, the pressure was released and the membrane was removed. The ETFE sheets of the anode catalyst layer decal and the cathode catalyst layer decal were peeled off, and an electrode with an area of ​​16 cm was obtained. ２ A membrane electrode assembly of 1000 .mu.m was obtained.

[0078] (Production of water electrolysis device) The obtained membrane electrode assembly was heat-treated at 150°C for 15 minutes, and then the peripheral edge of the membrane electrode assembly was sandwiched between a plurality of subgaskets made of PPS (polyphenylene sulfide) and PEN (polyethylene naphthalate) in a stacked state, and the assembly was set in a water electrolysis evaluation jig EH50-25 (manufactured by Greenlight Innovations, Inc.), thereby obtaining a water electrolysis device.

[0079] (Evaluation Method) First, to fully hydrate the solid polymer electrolyte membrane and both electrode ionomers, pure water with a conductivity of 1.0 μS / cm or less, a temperature of 80°C, and atmospheric pressure was supplied to the anode and cathode sides at a flow rate of 50 mL / min for 12 hours. The cathode side was then purged with nitrogen. After the nitrogen purging, pure water with a conductivity of 1.0 μS / cm or less, a temperature of 80°C, and atmospheric pressure was supplied to the anode side at a flow rate of 50 mL / min. While the generated gas pressure on the cathode side was kept at atmospheric pressure, a current density of 2 A / cm was applied using a Kikusui Electronics PWR1600L DC power supply. ２ The battery was operated for 300 hours at 100°C. The battery that did not leak during the 300 hours of operation was evaluated as A, and the battery that leaked was evaluated as B.

[0080] [Production of Fluorine-Containing Polymer (S'-1)] CF ２ =CF ２ and a monomer (X) represented by the following formula (X) were copolymerized to obtain a fluoropolymer (S'-1) (ion exchange capacity: 1.25 meq / g): ２ =CF-O-CF ２ CF (CF ３ )-O-CF ２ CF２ -SO ２ F (X)

[0081] The ion exchange capacity described in the above [Production of Fluoropolymer (S'-1)] represents the ion exchange capacity of the fluoropolymer obtained when the fluoropolymer (S'-1) is hydrolyzed by the procedure described below.

[0082] [Production of Film α1] The fluoropolymer (S'-1) was molded by melt extrusion to obtain a film α1 (thickness: 60 μm) made of the fluoropolymer (S'-1).

[0083] [Production of Woven Fabric A1] 18.6 denier PFA yarns were used as warp and weft yarns, and plain weaving was performed so that the density of the PFA yarns was 100 threads / inch to obtain Woven Fabric A1. The basis weight of Woven Fabric A1 was 16.3 g / m ２ It was.

[0084] [Examples 1 to 5] In Examples 1, 2, 4, and 5, PET film / film α1 / woven fabric A1 / film α1 / PET film were stacked in this order. In Example 3, woven fabric A1 was not used, and PET film / film α1 / film α1 / PET film were stacked in this order. The stacked members of each example were heated and pressed using a roll press at a temperature of 200 ° C and a linear pressure of 40 kg / cm, and then the transfer substrates (PET films) on both sides were peeled off at a temperature of 50 ° C to obtain a precursor film. The precursor film was transported to a solution of dimethyl sulfoxide / potassium hydroxide / water = 30 / 5.5 / 64.5 (mass ratio) at 95 ° C for 30 minutes, and groups in the precursor film that can be converted to sulfonic acid functional groups were hydrolyzed (hydrolysis step) to convert them to K-type sulfonic acid functional groups, and then washed with water. The resulting membrane was then transferred to 1 M sulfuric acid at 40 to 50°C, where the terminals of the sulfonic acid functional groups were converted from K-type to H-type (acid-form conversion treatment step), yielding a wet ion exchange membrane. While the ends of the resulting wet ion exchange membrane were fixed, the wet ion exchange membrane was dried at a temperature (105°C) above the softening point of the fluoropolymer (drying step), yielding a dried ion exchange membrane. Using the resulting dried ion exchange membrane, leakage from a water electrolysis device was evaluated. Furthermore, the resulting dried ion exchange membrane was wound into a 150 m roll and evaluated for wrinkles. In each example, the amount of aqueous medium adhering to the surface of the wet ion exchange membrane at the start of the drying step was adjusted to the value shown in Table 1 depending on the conveying speed in the conveying step from after the acid-form conversion treatment step to before the drying step. The conveying speed in Example 5 was used as the standard, and the conveying speed in each example compared to that of Example 5 is shown in Table 1. The conveying distance was 5 m, the temperature of the conveying environment was room temperature (25°C), and the conveying time in Example 1 was 5 minutes. The standard deviation of the thickness of the dry ion exchange membrane in Example 1 was 5 μm, and the standard deviation of the thickness of the dry ion exchange membrane in Example 4 was 15 μm.

[0085]

[0086] As shown in Table 1, the amount of the liquid medium adhering to the surface of the wet ion exchange membrane was 40 g / m ２ More than 600g / m ２It was confirmed that when a wet ion exchange membrane was dried from the below-mentioned state and the resulting dried ion exchange membrane was applied to a water electrolysis device, liquid leakage from the water electrolysis device could be suppressed.

[0087] The entire contents of the specification, claims, and abstract of Japanese Patent Application No. 2023-207544, filed on December 8, 2023, are incorporated herein by reference as part of the disclosure of the present invention.

Claims

1. A method for producing a dried ion exchange membrane, comprising drying an ion exchange membrane containing a fluorine-containing polymer having ion exchange groups, which is wetted with a liquid medium, to remove the liquid medium and obtain a dried ion exchange membrane, wherein the amount of the liquid medium adhering to the surface of the wet ion exchange membrane is 40 g / m ２ Above, 600g / m ２ The method for producing a dried ion exchange membrane comprises drying the wet ion exchange membrane from the following state:

2. The amount of the liquid medium adhering to the surface of the wet ion exchange membrane is 50 g / m ２ Above, 300g / m ２ The method for producing a dry ion exchange membrane according to claim 1, wherein:

3. The method for producing a dried ion exchange membrane according to claim 1 or 2, wherein the drying temperature is 60 to 150°C.

4. A method for producing a dried ion exchange membrane as described in claim 1 or 2, wherein the drying is performed when the content of the liquid medium in the wet ion exchange membrane is 60 mass % or more and 200 mass % or less relative to the total mass of the dried ion exchange membrane.

5. The method for producing a dry ion exchange membrane according to claim 1 or 2, wherein the ion exchange capacity of the fluoropolymer is 0.90 to 2.05 milliequivalents / gram of dry resin.

6. The method for producing a dry ion exchange membrane according to claim 1 or 2, wherein the fluoropolymer contains a unit represented by formula (1). ２ -CF(-L-(SO ３ M) ｎ In formula (1), L is an (n+1) valent perfluorohydrocarbon group which may contain an etheric oxygen atom, M is a hydrogen atom, an alkali metal or a quaternary ammonium cation, and n is 1 or 2.

7. The method for producing a dry ion exchange membrane according to claim 1 or 2, wherein a reinforcing material is contained inside or on the surface of the wet ion exchange membrane.

8. The method for producing a dry ion exchange membrane according to claim 7, wherein the reinforcing material comprises at least one type of reinforcing thread selected from the group consisting of reinforcing threads made of polytetrafluoroethylene, reinforcing threads made of tetrafluoroethylene-perfluoroether copolymer, reinforcing threads made of polyphenylene sulfide, reinforcing threads made of polyether ether ketone, reinforcing threads made of nylon, and reinforcing threads made of polypropylene.

9. The method for producing a dry ion exchange membrane according to claim 1 or 2, wherein the dry ion exchange membrane is used as an electrolyte membrane contained in a membrane electrode assembly of a water electrolysis device.

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