Method for producing liquid composition

Abstract

To provide a method for producing a liquid composition which contains a fluorine-containing polymer having a small particle diameter and is excellent in production suitability.　For a powder coating of the present invention, a solid composition containing a fluorine-containing polymer H having an ion exchange group and a first solvent is separated from a mixture obtained by subjecting a group that can be converted to an ion exchange group in a fluorine-containing polymer F having the group that can be converted to an ion exchange group to a hydrolysis treatment and an acidification treatment in a first solvent to convert the same to an acidic ion exchange group, and then the first solvent is removed from the solid composition to obtain a composition in which the content of the first solvent is 2-200 mass% with respect to the total mass of the fluorine-containing polymer H. Thereafter, the composition is mixed with a second solvent containing water and an alcohol to obtain a liquid composition containing the fluorine-containing polymer H having an ion exchange group.

Description

Method for producing liquid composition

[0001] The present invention relates to a method for producing a liquid composition.

[0002] A polymer electrolyte fuel cell is obtained, for example, by sandwiching a membrane electrode assembly between two separators to form a cell, and stacking multiple cells. The membrane electrode assembly includes an anode and a cathode each having a catalyst layer, and a solid polymer electrolyte membrane disposed between the anode and the cathode. The solid polymer electrolyte membrane is formed, for example, by impregnating a reinforcing body with a liquid composition containing a fluoropolymer having ion exchange groups and a solvent, followed by drying. The catalyst layer in the membrane electrode assembly is formed, for example, by applying a catalyst layer-forming coating liquid obtained by mixing the liquid composition with a catalyst, followed by drying. The liquid composition is produced, for example, as follows: First, the groups convertible to ion exchange groups in a fluoropolymer having groups convertible to ion exchange groups are hydrolyzed and converted to an acid form, followed by treatments such as washing with water and drying to obtain a powder of the fluoropolymer having ion exchange groups. Next, the powder of the fluoropolymer having ion exchange groups is mixed with a solvent to obtain the liquid composition (see Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2004-035864

[0004] When a liquid composition is impregnated into a reinforcing material (particularly a porous material) during the production of a solid polymer electrolyte membrane, if the particle size of the fluoropolymer having ion exchange groups in the liquid composition is large, the fluoropolymer may have difficulty entering the pores of the porous material. As a result, the fluoropolymer may not be uniformly impregnated into the reinforcing material (particularly the porous material), resulting in the formation of voids. Since the voids do not have proton transport properties, the proton transport resistance of the solid polymer electrolyte membrane increases. A membrane electrode assembly using a solid polymer electrolyte membrane with high proton transport resistance reduces the power generation performance of a fuel cell due to its low proton transport properties. Therefore, the fluoropolymer having ion exchange groups contained in the liquid composition is required to have a small particle size. Furthermore, when multiple liquid compositions are produced under the same conditions, variations in the concentration of the fluoropolymer may occur among the multiple liquid compositions, even though they were produced under the same conditions. Therefore, even when a plurality of liquid compositions are produced under the same conditions, it is required that there be little variation in the concentration of the fluoropolymer among the plurality of liquid compositions (hereinafter also referred to as excellent "suitability for production"). When the present inventors attempted to produce a liquid composition by the method described in Patent Document 1, they found that the particle size of the fluoropolymer having ion exchange groups was sometimes too large or the suitability for production was sometimes poor, and that there was room for improvement.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing a liquid composition which contains a fluoropolymer having a small particle size and has excellent suitability for production.

[0006] The present inventors have found that the above-mentioned problems can be solved by the following constitution. [1] A method for producing a liquid composition, comprising: subjecting a fluoropolymer F having groups convertible to ion exchange groups in a first solvent to hydrolysis and acid-form conversion treatment to convert the groups convertible to ion exchange groups into acid-form ion exchange groups, to obtain a mixture, from which a solid composition containing a fluoropolymer H having ion exchange groups and the first solvent is separated; then, a treatment is carried out to remove the first solvent from the solid composition to obtain a composition in which the content of the first solvent is 2 to 200 mass% based on the total mass of the fluoropolymer H; and then, mixing the composition with a second solvent containing water and an alcohol to obtain a liquid composition containing the fluoropolymer H having ion exchange groups. [2] A method for producing a liquid composition according to [1], wherein the fluoropolymer F contains units based on a compound represented by formula (1): Formula (1): CF ２ =CF-L-(A) ｎIn formula (1), L is an (n+1)-valent perfluorohydrocarbon group which may contain an etheric oxygen atom, A is a group which can be converted into a sulfonic acid functional group, and n is 1 or 2. [3] A method for producing a liquid composition according to [1] or [2], wherein the fluorine-containing polymer H has an ion exchange capacity of 0.8 to 3.0 milliequivalents / gram dry resin. [4] A method for producing a liquid composition according to any of [1] to [3], wherein the alcohol comprises at least one selected from the group consisting of ethanol, 1-propanol, 1-butanol, and 2-propanol. [5] A method for producing a liquid composition according to any of [1] to [4], wherein the liquid composition is used for producing a solid polymer electrolyte membrane. [6] The method for producing a liquid composition according to any one of [1] to [4], wherein the liquid composition is used to produce at least one of the catalyst layer in the anode and the catalyst layer in the cathode of a membrane electrode assembly having an anode having a catalyst layer, a cathode having a catalyst layer, and a solid polymer electrolyte membrane disposed between the anode and the cathode. [7] The method for producing a liquid composition according to any one of [1] to [6], wherein the content of the second solvent is 50% by mass or more and 95% by mass or less, based on the total mass of the liquid composition. [8] The method for producing a liquid composition according to any one of [1] to [6], wherein the fluoropolymer H is dispersed in the second solvent in the form of particles, and the average secondary particle diameter of the particles is 1 nm or more and less than 400 nm.

[0007] According to the present invention, there can be provided a method for producing a liquid composition which contains a fluoropolymer having a small particle size and has excellent suitability for production.

[0008] 1 is a cross-sectional view showing an example of a membrane electrode assembly;

[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 ３Here, 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.) 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. "Groups that can be converted into ion exchange groups" means groups that can be converted into ion exchange groups by treatment such as hydrolysis or acidification, and may be referred to as "precursor groups." "Groups that can be converted into sulfonic acid functional groups" means groups that can be converted into sulfonic acid functional groups by treatment such as hydrolysis or acidification. "Groups that can be converted into carboxylic acid functional groups" means groups that can be converted into carboxylic acid functional groups by known treatments such as hydrolysis or acidification.

[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 liquid composition] The method for producing a liquid composition of the present invention (hereinafter also referred to as "the present production method") comprises the steps of: step 1 of separating a solid composition comprising a fluoropolymer having ion exchange groups and the first solvent from a mixture obtained by converting, in a first solvent, groups convertible to ion exchange groups in a fluoropolymer having groups convertible to ion exchange groups into acid-type ion exchange groups through hydrolysis treatment and acidification treatment; step 2 of removing the first solvent from the solid composition to obtain a composition in which the content of the first solvent is 2 to 200 mass% based on the total mass of the fluoropolymer; and step 3 of mixing the composition with a second solvent containing water and an alcohol to obtain a liquid composition comprising the fluoropolymer having ion exchange groups. In the following description, unless otherwise specified, "fluoropolymer having groups convertible to ion exchange groups" will be referred to as "polymer F" and "fluoropolymer having ion exchange groups" will be referred to as "polymer H".

[0013] According to this production method, a liquid composition containing polymer H with a small particle size and having excellent manufacturability can be obtained. When performing the process of removing the first solvent from the solid composition obtained in step 1, it is presumed that by setting the content of the first solvent in the resulting composition to 2% by mass or more relative to the total mass of polymer H, excessive aggregation of particles containing polymer H was suppressed, resulting in a small particle size of particles containing polymer H in the liquid composition. Furthermore, when performing the process of removing the first solvent from the solid composition obtained in step 1, it is presumed that by setting the content of the first solvent in the resulting composition to 200% by mass or less relative to the total mass of polymer H, variation in the amount of first solvent contained in polymer H was suppressed, resulting in a liquid composition with excellent manufacturability.

[0014] [Step 1] Step 1 is a step of separating a solid composition containing polymer H and the first solvent from a mixture obtained by subjecting groups in polymer F that can be converted to ion exchange groups to hydrolysis and acid-form conversion treatment in a first solvent to convert them to acid-form ion exchange groups.

[0015] <Polymer F> Polymer F is not particularly limited as long as it is a polymer having a fluorine atom and a group that can be converted into an ion exchange group. However, in terms of obtaining better effects of the present invention, polymer F-1 or polymer F-2 shown below is preferred.

[0016] (Polymer F-1) Polymer F-1 is a copolymer containing units based on a fluorine-containing olefin and units based on a fluorine-containing monomer having a group that can be converted into an ion-exchange group, and is more preferably a copolymer containing units based on a fluorine-containing olefin (preferably tetrafluoroethylene) and units based on a fluorine-containing monomer having a group that can be converted into a sulfonic acid functional group (preferably a compound represented by formula (1) described below). Polymer F-1 preferably does not have a cyclic ether structure.

[0017] Examples of fluorine-containing olefins include fluoroolefins having 2 to 3 carbon atoms and one or more fluorine atoms in the molecule. Specific examples of fluoroolefins include tetrafluoroethylene (hereinafter also referred to as "TFE"), chlorotrifluoroethylene, vinylidene fluoride, vinyl fluoride, and hexafluoropropylene. Among these, TFE is preferred in terms of the production cost of the monomer, reactivity with other monomers, and excellent properties of the resulting polymer F-1. One type of fluorine-containing olefin may be used alone, or two or more types may be used in combination.

[0018] The content of units based on fluorine-containing olefin is preferably 11% by mass or more, more preferably 38% by mass or more, and preferably 59% by mass or less, more preferably 55% by mass or less, based on all units in polymer F-1.

[0019] Examples of the fluorine-containing monomer having a group that can be converted into an ion-exchange group include compounds that have one or more fluorine atoms in the molecule, an ethylenic double bond, and a group that can be converted into a sulfonic acid functional group. As the fluorine-containing monomer having a group that can be converted into an ion-exchange group, a compound represented by formula (1) is preferred in terms of the production cost of the monomer, reactivity with other monomers, and excellent properties of the resulting polymer F-1. Formula (1) CF ２ =CF-L-(A) ｎ

[0020] L is an (n+1)-valent perfluorohydrocarbon group which may contain an etheric oxygen atom. The etheric oxygen atom may be located at the terminal of the perfluorohydrocarbon group or between carbon atoms. The number of carbon atoms in the (n+1)-valent perfluorohydrocarbon group is preferably 1 or more, more preferably 2 or more, and preferably 20 or less, more preferably 10 or less. L is preferably an (n+1)-valent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom, and more preferably a divalent perfluoroalkylene group which may contain an etheric oxygen atom in the embodiment where n = 1, or a trivalent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom in the embodiment where n = 2. The divalent perfluoroalkylene group may be either linear or branched.

[0021] A is a group that can be converted into a sulfonic acid functional group. The group that can be converted into a sulfonic acid functional group is preferably a functional group that can be converted into a sulfonic acid functional group by hydrolysis. Specific examples of groups that can be converted into a sulfonic acid functional group include -SO ２ F, -SO ２ Cl, —SO ２ Br is an example.

[0022] n is 1 or 2.

[0023] The compound represented by formula (1) is preferably a compound represented by formula (1-1), a compound represented by formula (1-2), a compound represented by formula (1-3), or a compound represented by formula (1-4). ２ =CF-O-R ｆ１ -A Formula (1-2) CF ２ =CF-R ｆ１ -A

[0024]

[0025]

[0026] R ｆ１is 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.

[0027] R ｆ２ 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.

[0028] 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.

[0029] r is 0 or 1. m is 0 or 1.

[0030] The definition of A in the formula is as described above.

[0031] As the compound represented by formula (1-1) and the compound represented by formula (1-2), a compound represented by formula (1-5) is preferred. ２ =CF-(CF ２ ) ｘ -(OCF ２ CFY) ｙ -O-(CF ２ ) ｚ -SO ３ F 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 is.

[0032] Specific examples of the compound represented by formula (1-1) include the following compounds. In the formula, w is an integer of 1 to 8, and x is an integer of 1 to 5. CF ２ =CF-O-(CF ２ ) ｗ -SO ２ FCF ２ =CF-O-CF ２ CF (CF３ )-O-(CF ２ ) ｗ -SO ２ FCF ２ =CF-[O-CF ２ CF (CF ３ )] ｘ -SO ２ F

[0033] Specific examples of the compound represented by formula (1-2) include the following compounds: In the formula, w is an integer of 1 to 8. CF ２ =CF-(CF ２ ) ｗ -SO ２ FCF ２ =CF-CF ２ -O-(CF ２ ) ｗ -SO ２ F

[0034] The compound represented by formula (1-3) is preferably a compound represented by formula (1-3-1).

[0035]

[0036] R ｆ４ is a linear perfluoroalkylene group having 1 to 6 carbon atoms, and R ｆ５ represents 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 A are as described above.

[0037] Specific examples of the compound represented by formula (1-3-1) include the following.

[0038]

[0039] The compound represented by formula (1-4) is preferably a compound represented by formula (1-4-1).

[0040]

[0041] R in the formula ｆ１ , R ｆ２ and A are defined as above.

[0042] Specific examples of the compound represented by formula (1-4-1) include the following.

[0043]

[0044] The fluorine-containing monomer having a group that can be converted into an ion-exchange group may be used alone or in combination of two or more kinds.

[0045] The content of units based on a fluorine-containing monomer having a group convertible to an ion-exchange group is preferably 41% by mass or more, more preferably 45% by mass or more, and is preferably 89% by mass or less, more preferably 62% by mass or less, based on all units of polymer F-1.

[0046] Polymer F-1 may be produced using monomers other than the above monomers (hereinafter also referred to as "other monomers"). 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 30% by mass or less based on the total units of polymer F-1, in order to maintain ion exchange performance.

[0047] (Polymer F-2) Polymer F-2 is a fluoropolymer having units derived from a monomer having a cyclic ether structure and having a group convertible to an ion-exchange group, and is preferably a copolymer having units derived from a monomer having a cyclic ether structure and units derived from a fluoromonomer having a group convertible to an ion-exchange group. Specific examples of the monomer having a cyclic ether structure include monomer m11, monomer m12, monomer m21, and monomer m22.

[0048] Monomer m11 is a monomer represented by formula (m11), and a preferred embodiment of monomer m11 is represented by formula (m11-1).

[0049]

[0050] R １１ is a divalent perfluoroalkylene group which may have an ether-bonded oxygen atom. When the perfluoroalkylene group has an ether-bonded oxygen atom, the number of oxygen atoms may be one or two or more. The oxygen atom may be located between the carbon-carbon bonds of the perfluoroalkylene group, or at the carbon atom bond terminal. The perfluoroalkylene group may be linear or branched, but is preferably linear. R １２ , R １３ , R １５ and R １６ R each independently represents a monovalent perfluoroalkyl group which may have an ether-bonded oxygen atom or a fluorine atom. １５ and R １６ In view of high polymerization reactivity, it is preferable that at least one of R is a fluorine atom, and it is more preferable that both of R are fluorine atoms. １４ represents a monovalent perfluoroalkyl group which may have an ether-bonded oxygen atom, a fluorine atom, or R １１ SO ２ It is a group represented by F. When the perfluoroalkyl group has an ether-bonded oxygen atom, the number of oxygen atoms may be one or two or more. In addition, the oxygen atom may be located between the carbon-carbon bonds of the perfluoroalkyl group, or may be located at the carbon atom bond terminal. The perfluoroalkyl group may be linear or branched, but is preferably linear. In formula (m11), two R １１ If it contains two R １１ may be the same or different from each other.

[0051] Monomer m12 is a monomer represented by formula (m12), and preferred embodiments of monomer m12 include formulae (m12-1) to (m12-2).

[0052]

[0053] R ２１ is a perfluoroalkylene group having 1 to 6 carbon atoms or a perfluoroalkylene group having 2 to 6 carbon atoms and having an ether-bonding oxygen atom between the carbon-carbon bond. When the perfluoroalkylene group has an ether-bonding oxygen atom, the number of oxygen atoms may be one or more. The perfluoroalkylene group may be linear or branched, but is preferably linear. R ２２ is a fluorine atom, a perfluoroalkyl group having 1 to 6 carbon atoms, a perfluoroalkyl group having 2 to 6 carbon atoms and an ether-bonded oxygen atom between the carbon-carbon bonds, or R ２１ SO ２ It is a group represented by F. When the perfluoroalkyl group has an ether bond oxygen atom, the number of oxygen atoms may be one or two or more. The perfluoroalkyl group may be linear or branched, but is preferably linear. In formula (m12), two R ２１ If it contains two R ２１ may be the same or different from each other.

[0054] Monomer m21 is a monomer represented by formula (m21), and a preferred embodiment of monomer m21 is represented by formula (m21-1).

[0055]

[0056] R ４１ , R ４２ , R ４３ , R ４４ , R ４５ and R ４６ are each independently a monovalent perfluoroalkyl group which may have an ether-bonded oxygen atom or a fluorine atom. When the perfluoroalkyl group has an ether-bonded oxygen atom, the number of oxygen atoms may be one or two or more. The oxygen atom may be located between the carbon-carbon bonds of the perfluoroalkyl group, or at the carbon atom bond terminal. The perfluoroalkyl group may be linear or branched, but is preferably linear. R４５ and R ４６ In view of high polymerization reactivity, it is preferable that at least one of the groups is a fluorine atom, and it is more preferable that both of the groups are fluorine atoms.

[0057] Monomer m22 is a monomer represented by formula (m22), and a preferred embodiment of monomer m22 is represented by formula (m22-1).

[0058]

[0059] s is 0 or 1, and is preferably 0. ５１ and R ５２ are each independently a fluorine atom, a perfluoroalkyl group having 1 to 5 carbon atoms, or a spiro ring formed by linking together (when s is 0). ５３ and R ５４ are each independently a fluorine atom or a perfluoroalkyl group having 1 to 5 carbon atoms. ５５ is a fluorine atom, a perfluoroalkyl group having 1 to 5 carbon atoms, or a perfluoroalkoxy group having 1 to 5 carbon atoms. ５５ is preferably a fluorine atom in view of high polymerization reactivity. The perfluoroalkyl group and perfluoroalkoxy group may be linear or branched, but are preferably linear.

[0060] The content of units based on a monomer having a cyclic ether structure is preferably 30% by mass or more, more preferably 48% by mass or more, and is preferably 70% by mass or less, more preferably 63% by mass or less, based on the total units of polymer F-2.

[0061] Specific examples of the fluorine-containing monomer having a group convertible to an ion-exchange group are the same as those of the fluorine-containing monomer having a group convertible to an ion-exchange group in Polymer F-1. The content of units based on the fluorine-containing monomer having a group convertible to an ion-exchange group is preferably 20% by mass or more, more preferably 28% by mass or more, and preferably 60% by mass or less, more preferably 50% by mass or less, based on the total units of Polymer F-2.

[0062] Polymer F-2 may have units based on a fluorine-containing olefin. Specific examples of the fluorine-containing olefin are the same as those for Polymer F-1. The content of units based on a fluorine-containing olefin (particularly TFE) is preferably 0% by mass or more, more preferably 1% by mass or more, and preferably 20% by mass or less, more preferably 10% by mass or less, based on all units of Polymer F-2.

[0063] (Physical Properties of Polymer F) The TQ value of Polymer F is preferably 100° C. or higher, more preferably 120° C. or higher, and even more preferably 150° C. or higher, and is preferably 330° C. or lower, more preferably 300° C. or lower, and even more preferably 300° C. or lower. The TQ value is a value related to the molecular weight of the polymer, and is ３ The TQ value of Polymer F is determined by the method described in the Examples section below.

[0064] (Method for Producing Polymer F) As a method for producing Polymer F, known polymerization methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization can be used. Polymerization may also be carried out in liquid or supercritical carbon dioxide. The polymerization is carried out under conditions that generate radicals. Methods for generating radicals include a method of irradiating with radiation such as ultraviolet rays, gamma rays, and electron beams, and a method of adding a radical initiator.

[0065] In the method for producing polymer F, polymer F is preferably recovered by separating it from unreacted monomers, polymerization solvent, etc. The recovered polymer F may be heat-treated at 200°C to 300°C for 3 to 40 hours. This makes it possible to remove unreacted monomers, polymerization solvent, etc.

[0066] Polymer F may be in the form of pellets or powder. A method for obtaining polymer F in pellet form includes obtaining a strand containing polymer F using a melt extruder, and then cutting the strand to obtain pellets. A method for obtaining polymer F in powder form includes pulverizing polymer F using a known pulverizer. A preferred pulverization method is freeze-pulverization. The particle size of polymer F is preferably 0.1 mm or more, more preferably 0.2 mm or more, from the viewpoint of suppressing filter clogging during filtration in the hydrolysis treatment or acid-form conversion treatment described below. The particle size of polymer F is preferably 5 mm or less, more preferably 3 mm or less, from the viewpoint of excellent reaction efficiency of precursor groups during the hydrolysis treatment or acid-form conversion treatment described below. The particle size of polymer F is a value calculated from the particle size distribution measured by mechanical sieving using a stainless steel testing sieve (JIS-Z8801).

[0067] <Polymer H> Polymer H is a polymer obtained by converting groups in Polymer F that can be converted into ion exchange groups into acid-type ion exchange groups through hydrolysis treatment and acid-form conversion treatment. Polymer H is not particularly limited as long as it is a polymer having fluorine atoms and ion exchange groups, but Polymer H-1 or Polymer H-2 shown below are preferred in terms of achieving better effects of the present invention.

[0068] (Polymer H-1) Polymer H-1 is a polymer in which the groups convertible to ion-exchange groups in the above-mentioned polymer F-1 have been converted into ion-exchange groups. Polymer H-1 is a copolymer containing units based on a fluorine-containing olefin and units based on a fluorine-containing monomer having an ion-exchange group, and is more preferably a copolymer containing units based on a fluorine-containing olefin (preferably tetrafluoroethylene) and units based on a fluorine-containing monomer having a sulfonic acid functional group (preferably a compound represented by formula (2) described below). Polymer H-1 preferably does not have a cyclic ether structure.

[0069] Specific examples of units based on fluorine-containing olefin are the same as the specific examples of units based on fluorine-containing olefin in polymer F-1. The content of units based on fluorine-containing olefin relative to all units of polymer H-1 is the same as the content of units based on fluorine-containing olefin relative to all units of polymer F-1.

[0070] The unit based on a fluorine-containing monomer having an ion exchange group is preferably a unit based on a fluorine-containing monomer having a sulfonic acid functional group, and more preferably a unit represented by formula (2): ２ -CF(-L-(SO ３ M) ｎ )]-

[0071] In formula (2), L and n are defined as the same as in formula (1), and M is a hydrogen atom, an alkali metal, or a quaternary ammonium cation.

[0072] The unit represented by formula (2) is preferably a unit represented by formula (2-1), a unit represented by formula (2-2), a unit represented by formula (2-3), or a unit represented by formula (2-4). ２ -CF(-O-R ｆ１ -SO ３ M)] - Formula (2-2) - [CF ２ -CF(-R ｆ１ -SO ３ M) ]-

[0073]

[0074]

[0075] R in formula (2-1) to formula (2-4) ｆ１ , R ｆ２ , R ｆ３ The definitions of r and m are as follows: ｆ１ , R ｆ２ , R ｆ３ , r and m are the same as those defined above. The definition of M in the formula is as described above.

[0076] As the unit represented by formula (2-1) and the unit represented by formula (2-2), a unit represented by formula (2-5) is more preferred. ２ -CF(-(CF ２ ) ｘ -(OCF ２ CFY) ｙ -O-(CF ２ ) ｚ -SO ３ M)]— The definitions of x, y, z, and Y in formula (2-5) are the same as those of x, y, z, and Y in formula (1-5) above. M is as defined above.

[0077] Specific examples of the unit represented by formula (2-1) 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) ]-

[0078] Specific examples of the unit represented by formula (2-2) 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) ]-

[0079] The unit represented by formula (2-3) is preferably a unit represented by formula (2-3-1), where M is defined as above.

[0080]

[0081] R in formula (2-3-1) ｆ４ and R ｆ５ The definition of is R in formula (1-3-1). ｆ４ and R ｆ５ The definitions of r and M are as described above.

[0082] Specific examples of the unit represented by formula (2-3-1) include the following.

[0083]

[0084] As the unit represented by formula (2-4), a unit represented by formula (2-4-1) is preferred. ｆ１ , R ｆ２ and M are defined as above.

[0085]

[0086] Specific examples of the unit represented by formula (2-4-1) include the following.

[0087]

[0088] The units based on the fluorine-containing monomer having an ion-exchange group may be used alone or in combination of two or more.

[0089] The content of units based on a fluorine-containing monomer having an ion-exchange group in polymer H-1 is the same as the content of units based on a fluorine-containing monomer having a group that can be converted into an ion-exchange group in polymer F-1.

[0090] Polymer H-1 may contain units based on other monomers described above for Polymer F-1. The content of units based on other monomers in Polymer H-1 is the same as the content of units based on other monomers in Polymer F-1.

[0091] (Polymer H-2) Polymer H-2 is a polymer in which the groups convertible to ion-exchange groups in the above-mentioned polymer F-2 have been converted into ion-exchange groups. Polymer H-2 is a fluorine-containing polymer having units derived from a monomer having a cyclic ether structure and having an ion-exchange group, and is preferably a copolymer having units derived from a monomer having a cyclic ether structure and units derived from a fluorine-containing monomer having an ion-exchange group.

[0092] Specific examples of units based on a monomer having a cyclic ether structure include the unit (u11), unit (u12), unit (u21), and unit (u22). The unit (u11) is a unit based on the above-mentioned monomer m11, and a preferred embodiment thereof is the unit (u11-1). The unit (u12) is a unit based on the above-mentioned monomer m12, and a preferred embodiment thereof is the unit (u12-1) to unit (u12-2). The unit (u21) is a unit based on the above-mentioned monomer m21, and a preferred embodiment thereof is the unit (u21-1). The unit (u22) is a unit based on the above-mentioned monomer m22, and a preferred embodiment thereof is the unit (u22-1). The definitions of each group in the formula are as described above.

[0093]

[0094]

[0095]

[0096]

[0097] The content of units based on monomers having a cyclic ether structure relative to all units of polymer H-2 is similar to the content of units based on monomers having a cyclic ether structure relative to all units of polymer F-2.

[0098] Specific examples of units based on a fluorine-containing monomer having an ion exchange group are the same as those of the fluorine-containing monomer having an ion exchange group in Polymer H-1. The content of units based on a fluorine-containing monomer having an ion exchange group relative to all units of Polymer H-2 is the same as the content of units based on a fluorine-containing monomer having a group that can be converted into an ion exchange group relative to all units of Polymer F-2.

[0099] Polymer H-2 may have units based on a fluorine-containing olefin. Specific examples of the fluorine-containing olefin are the same as those for Polymer H-1. The content of units based on a fluorine-containing olefin (particularly, TFE) relative to all units of Polymer H-2 is the same as the content of units based on a fluorine-containing olefin (particularly, TFE) relative to all units of Polymer F-2.

[0100] (Physical Properties of Polymer H) The ion exchange capacity of Polymer H is preferably 0.5 meq / g dry resin or more, more preferably 0.8 meq / g dry resin or more, and even more preferably 0.9 meq / g dry resin or more, from the viewpoint of increasing proton conductivity and obtaining sufficient battery output. The ion exchange capacity of Polymer H is preferably 3.0 meq / g dry resin or less, more preferably 2.2 meq / g dry resin or less, and even more preferably 2.0 meq / g dry resin or less, from the viewpoint of increasing the molecular weight of Polymer H and being able to suppress excessive swelling in water and maintain mechanical strength. The ion exchange capacity of Polymer H is determined by the method described in the Examples section below.

[0101] <Each Treatment in Step 1> Polymer H is produced by hydrolyzing and acidifying groups in polymer F that can be converted into ion-exchange groups (precursor groups) in polymer F in a first solvent to convert them into acid-type ion-exchange groups. Here, the first solvent is a general term for solvents used in the production of polymer H. Specifically, the solvent in the alkaline aqueous solution used in the hydrolysis treatment, the water in the acidic aqueous solution used in the oxidation treatment, and the water used in the water-washing treatment correspond to the first solvent. Hereinafter, when the precursor group is a fluorosulfonyl group (-SO ２Polymer F, in which the ion exchange group is a sulfonic acid group (-SO ３ － H ＋ An example of a preferred embodiment of the method for producing polymer H will be described below, taking as an example a method for obtaining polymer H, which is

[0102] (Hydrolysis Treatment) The hydrolysis treatment is carried out, for example, by bringing the polymer F into contact with an aqueous alkaline solution, whereby the fluorosulfonyl groups of the polymer F are converted to sulfonate salts, thereby obtaining a fluorine-containing polymer having sulfonate salts.

[0103] The alkaline aqueous solution preferably contains a solvent and a basic compound. Examples of the solvent contained in the alkaline aqueous solution include water and a mixed solvent of water and a polar solvent. Specific examples of polar solvents include alcohol (methanol, ethanol, etc.) and dimethyl sulfoxide. Among these, the solvent contained in the alkaline aqueous solution is preferably a mixed solvent of water and a polar solvent. When the solvent contained in the alkaline aqueous solution is the above-mentioned mixed solvent, the content of the polar solvent is preferably 10% by mass or more, more preferably 15% by mass or more, relative to the total mass of the mixed solvent, from the viewpoint of improving the reaction rate, and is preferably 50% by mass or less, more preferably 40% by mass or less, from the viewpoint of suppressing elution of the polymer.

[0104] Specific examples of the basic compound include sodium hydroxide and potassium hydroxide. The content of the basic compound is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less, based on the total mass of the alkaline aqueous solution, in order to improve the reaction rate.

[0105] The amount of the alkaline aqueous solution used relative to the amount of polymer F used (100 parts by mass) is preferably 200 parts by mass or more, and more preferably 300 parts by mass or more, from the viewpoint of reaction efficiency, and is preferably 1,000 parts by mass or less, and more preferably 500 parts by mass or less, from the viewpoint of reducing the amount of waste liquid.

[0106] The temperature of the alkaline aqueous solution is preferably 60°C or higher, more preferably 80°C or higher, from the viewpoint of improving the reaction rate, and is preferably 120°C or lower, more preferably 100°C or lower, from the viewpoint of suppressing elution of the polymer.

[0107] The contact time (reaction time) between the polymer F and the alkaline aqueous solution is preferably 12 hours or more, more preferably 16 hours or more, from the viewpoint of allowing the reaction to proceed sufficiently, and is preferably 200 hours or less, more preferably 100 hours or less, from the viewpoint of suppressing elution of the polymer.

[0108] (Water-washing treatment 1) The method for producing polymer H preferably includes water-washing treatment 1 in which the fluoropolymer (fluoropolymer having a sulfonate) after hydrolysis treatment is washed with water. In water-washing treatment 1, the amount of water used is preferably 100 parts by mass or more, more preferably 200 parts by mass or more, relative to the amount (100 parts by mass) of the fluoropolymer having a sulfonate used, from the viewpoint of cleaning efficiency, and is preferably 1,000 parts by mass or less, more preferably 500 parts by mass or less, from the viewpoint of reducing the amount of waste liquid.

[0109] In the water washing treatment 1, the water temperature is preferably 10°C or higher, more preferably 20°C or higher, from the viewpoint of washing efficiency, and is preferably 120°C or lower, more preferably 100°C or lower, from the viewpoint of suppressing polymer elution. In the water washing treatment 1, the water washing time is preferably 20 minutes or higher, more preferably 30 minutes or higher, from the viewpoint of improving washability, and is preferably 2 hours or lower, more preferably 1 hour or lower, from the viewpoint of suppressing polymer elution. The water washing time means the water washing time per water washing.

[0110] The water washing treatment 1 may be carried out only once or may be carried out multiple times. In particular, the number of times of the water washing treatment 1 is preferably two or more, more preferably three or more, from the viewpoint of improving cleanability, and is preferably 30 or less, more preferably 20 or less, from the viewpoint of suppressing elution of the polymer.

[0111] (Acid Form Conversion Treatment) The acid form conversion treatment is carried out, for example, by bringing the fluoropolymer (fluoropolymer having sulfonate salts) after hydrolysis treatment into contact with an acidic aqueous solution. This converts the sulfonate salts into sulfonic acid groups to obtain polymer H. The acidic aqueous solution contains an acid component and water.

[0112] Specific examples of the acid component include sulfuric acid and hydrochloric acid. The content of the acid component is preferably 5% by mass or more, more preferably 8% by mass or more, relative to the total mass of the acidic aqueous solution, from the viewpoint of improving the reaction rate, and is preferably 30% by mass or less, more preferably 25% by mass or less, from the viewpoint of ease of handling the acidic aqueous solution.

[0113] The amount of the acidic aqueous solution used is preferably at least 100 parts by mass, and more preferably at least 200 parts by mass, relative to the amount (100 parts by mass) of the fluoropolymer having a sulfonate used, from the viewpoint of reaction efficiency, and is preferably at most 1,000 parts by mass, and more preferably at most 500 parts by mass, from the viewpoint of suppressing the amount of waste liquid.

[0114] The temperature of the acidic aqueous solution is preferably 40°C or higher, more preferably 50°C or higher, from the viewpoint of improving the reaction rate, and is preferably 100°C or lower, more preferably 80°C or lower, from the viewpoint of suppressing elution of the polymer.

[0115] The contact time (reaction time) between the sulfonate-containing fluoropolymer and the acidic aqueous solution is preferably 10 minutes or more, more preferably 20 minutes or more, from the viewpoint of allowing the reaction to proceed sufficiently, and is preferably 2 hours or less, more preferably 1 hour or less, from the viewpoint of suppressing elution of the polymer. The above contact time means the contact time per acid-form conversion treatment.

[0116] The acid-form treatment may be carried out only once or may be carried out multiple times. In particular, the number of times of the acid-form treatment is preferably 2 or more, more preferably 3 or more, from the viewpoint of sufficiently proceeding the reaction, and is preferably 40 or less, more preferably 30 or less, from the viewpoint of suppressing elution of the polymer.

[0117] (Water-washing treatment 2) The method for producing polymer H preferably includes water-washing treatment 2 in which the fluoropolymer (polymer H) after the oxidation treatment is washed with water. In water-washing treatment 2, the amount of water used is preferably 100 parts by mass or more, more preferably 200 parts by mass or more, relative to the amount of polymer H used (100 parts by mass) from the viewpoint of washing efficiency, and is preferably 1000 parts by mass or less, more preferably 500 parts by mass or less, from the viewpoint of reducing the amount of waste liquid.

[0118] In the water washing treatment 2, the water temperature is preferably 10°C or higher, more preferably 20°C or higher, from the viewpoint of washing efficiency, and is preferably 120°C or lower, more preferably 100°C or lower, from the viewpoint of suppressing polymer elution. In the water washing treatment 2, the water washing time is preferably 10 minutes or higher, more preferably 20 minutes or higher, from the viewpoint of improving washability, and is preferably 2 hours or lower, more preferably 1 hour or lower, from the viewpoint of suppressing polymer elution. The water washing time means the water washing time per water washing.

[0119] The water washing treatment 2 may be carried out only once or may be carried out multiple times. In particular, the number of times of the water washing treatment 2 is preferably two or more, more preferably three or more, from the viewpoint of improving cleanability, and is preferably 40 or less, more preferably 30 or less, from the viewpoint of suppressing elution of the polymer. When the acid-form conversion treatment is carried out multiple times, the water washing treatment 2 may be carried out between the acid-form conversion treatments, or may be carried out after the completion of the final acid-form conversion treatment, or may be carried out at both of these timings.

[0120] (Hydrogen Peroxide Treatment) The method for producing polymer H may include a hydrogen peroxide treatment in which polymer H is brought into contact with hydrogen peroxide after the acid-form conversion treatment (after water-washing treatment 2 if water-washing treatment 2 is included). This allows impurities contained in polymer H (such as organic substances other than polymer H) to be decomposed. Here, when the acid-form conversion treatment is performed multiple times, the hydrogen peroxide treatment may be performed between each acid-form conversion treatment, or may be performed after the final acid-form conversion treatment is completed. Hydrogen peroxide may be in the form of an aqueous solution or a gas, but is preferably an aqueous solution (aqueous hydrogen peroxide solution) from the viewpoint of handleability. When an aqueous hydrogen peroxide solution is used, the content of hydrogen peroxide is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 20% by mass or less, based on the total mass of the aqueous hydrogen peroxide solution.

[0121] The amount of hydrogen peroxide used is preferably 100 parts by mass or more, more preferably 200 parts by mass or more, relative to the amount of polymer H used (100 parts by mass) from the viewpoint of decomposition of impurities, and is preferably 1,000 parts by mass or less, more preferably 500 parts by mass or less.

[0122] When an aqueous hydrogen peroxide solution is used, the temperature of the aqueous hydrogen peroxide solution is preferably 40°C or higher, more preferably 50°C or higher, from the viewpoint of decomposing impurities, and is preferably 120°C or lower, more preferably 100°C or lower, from the viewpoint of suppressing elution of the polymer.

[0123] The contact time (reaction time) between polymer H and hydrogen peroxide is preferably 5 hours or more, more preferably 8 hours or more, from the viewpoint of promoting the decomposition of impurities, and is preferably 200 hours or less, more preferably 100 hours or less, from the viewpoint of suppressing elution of the polymer.

[0124] (Water-washing Treatment 3) The method for producing Polymer H may include Water-washing Treatment 3, in which Polymer H after the hydrogen peroxide treatment is washed with water. In Water-washing Treatment 3, the amount of water used relative to the amount of Polymer H used (100 parts by mass) is preferably 100 parts by mass or more, and more preferably 200 parts by mass or more, from the viewpoint of cleaning efficiency, and is preferably 1000 parts by mass or less, and more preferably 500 parts by mass or less, from the viewpoint of reducing the amount of waste liquid.

[0125] In the water washing treatment 3, the water temperature is preferably 10°C or higher, more preferably 20°C or higher, from the viewpoint of washing efficiency, and is preferably 120°C or lower, more preferably 100°C or lower, from the viewpoint of suppressing polymer elution. In the water washing treatment 3, the water washing time is preferably 10 minutes or higher, more preferably 20 minutes or higher, from the viewpoint of improving washability, and is preferably 2 hours or lower, more preferably 1 hour or lower, from the viewpoint of suppressing polymer elution. The water washing time means the water washing time per water washing.

[0126] The water washing treatment 3 may be carried out only once or may be carried out multiple times. In particular, the number of times of the water washing treatment 3 is preferably two or more, more preferably three or more, from the viewpoint of improving cleanability, and is preferably 20 or less, more preferably 10 or less, from the viewpoint of suppressing elution of the polymer.

[0127] (Separation Treatment) The separation treatment is a treatment for separating a solid composition containing polymer H and the first solvent from the mixture obtained through the above-mentioned treatments. Here, polymer H has liquid absorption properties. Therefore, polymer H in the mixture is in a swollen state (e.g., a gel state) after absorbing a portion of the first solvent used in the production of polymer H. In other words, the solid composition can be said to be a polymer in which polymer H is wet with the first solvent (hereinafter also referred to as a "wet polymer"). The mixture contains the solid composition and may further contain the first solvent. The first solvent is preferably water.

[0128] The content of the first solvent in the mixture is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 55% by mass or more, and is preferably 95% by mass or less, more preferably 92% by mass or less, and even more preferably 90% by mass or less, relative to the total mass of the mixture. Here, the content of the first solvent in the mixture includes not only the first solvent that is swelling polymer H (i.e., the first solvent in the solid composition), but also the first solvent that is not used to swell polymer H.

[0129] The content of the solid composition in the mixture is preferably 5% by mass or more, more preferably 8% by mass or more, and even more preferably 10% by mass or more, relative to the total mass of the mixture, and is preferably 65% ​​by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less.

[0130] The content of the first solvent in the solid composition is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, based on the total mass of the solid composition, from the viewpoint of ease of subsequent solution formation, and is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less, based on the viewpoint of shortening the drying time. The content of polymer H in the solid composition is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, based on the total mass of the solid composition, from the viewpoint of shortening the drying time, and is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less, based on the viewpoint of ease of subsequent solution formation.

[0131] The method for separating the solid composition from the mixture is not particularly limited, and examples thereof include filtration.

[0132] [Step 2] Step 2 is a step of removing the first solvent from the solid composition obtained in Step 1 to obtain a composition having a first solvent content of 2 to 200 mass % relative to the total mass of polymer H. That is, Step 2 is a step of removing at least a portion of the first solvent in the solid composition that is a wet polymer to obtain a wet polymer composition having a lower first solvent content than the wet polymer obtained in Step 1.

[0133] A method for removing the first solvent from the solid composition includes a drying treatment for drying the solid composition. Examples of the drying treatment method include natural drying, heat drying, air drying, and a combination thereof. The drying temperature is preferably 20°C or higher, more preferably 30°C or higher, from the viewpoint of improving drying efficiency, and is preferably 120°C or lower, more preferably 100°C or lower, from the viewpoint of easily controlling the amount of the first solvent removed from the solid composition. The drying time is preferably 10 minutes or longer, more preferably 20 minutes or longer, from the viewpoint of preventing uneven drying, and is preferably 72 hours or shorter, more preferably 48 hours or shorter, from the viewpoint of drying efficiency.

[0134] The composition obtained in step 2 contains polymer H and a first solvent. The content of the first solvent in the composition obtained in step 2 is 2 to 200% by mass, relative to the total mass of polymer H. The content of the first solvent in the composition obtained in step 2 is preferably 3% by mass or more, and more preferably 5% by mass or more, relative to the total mass of polymer H, from the viewpoint of further reducing the particle size of polymer H contained in the liquid composition obtained in step 3, and is preferably 180% by mass or less, and more preferably 150% by mass or less, from the viewpoint of further improving the manufacturability of the liquid composition obtained in step 3.

[0135] The treatment of removing the first solvent in step 2 is preferably carried out so that the content of the first solvent in the composition obtained in step 2 is 2% by mass or more, more preferably 3% by mass or more, relative to the content of the first solvent in the solid composition obtained in step 1. If the content is 2% by mass or more, the particle size of polymer H contained in the liquid composition obtained in step 3 can be made smaller. Furthermore, the treatment of removing the first solvent in step 2 is preferably carried out so that the content of the first solvent in the composition obtained in step 2 is 95% by mass or less, more preferably 93% by mass or less, relative to the content of the first solvent in the solid composition obtained in step 1. If the content is 95% by mass or less, the liquid composition obtained in step 3 has better manufacturing suitability.

[0136] [Step 3] Step 3 is a step of mixing the composition obtained in step 2 with a second solvent containing water and an alcohol to obtain a liquid composition containing polymer H.

[0137] The second solvent includes water and an alcohol. Specific examples of the alcohol include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, 2-methyl-2-propanol, 2,2,2-trifluoroethanol, 2,2,3,3,3-pentafluoro-1-propanol, 2,2,3,3-tetrafluoro-1-propanol, 4,4,5,5,5-pentafluoro-1-pentanol, 1,1,1,3,3,3-hexafluoro-2-propanol, 3,3,3-trifluoro-1-propanol, 3,3,4,4,5,5,6,6,6-nonafluoro-1-hexanol, and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol. Among these, ethanol, 1-propanol, 1-butanol, and 2-propanol are preferred, and ethanol and 1-propanol are more preferred, as they provide better effects of the present invention. The alcohol may be used alone or in combination of two or more. The content of the alcohol in the second solvent is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 30% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 75% by mass or less.

[0138] The content of water in the second solvent is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more, and is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less.

[0139] The amount of the second solvent used is preferably 8 parts by mass or more, more preferably 10 parts by mass or more, and is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, relative to the amount of the composition used (100 parts by mass).

[0140] The temperature of the second solvent is preferably 80° C. or higher, more preferably 100° C. or higher, and is preferably 180° C. or lower, more preferably 130° C. or lower.

[0141] The mixing time of the composition and the second solvent is preferably 1 hour or more, more preferably 2 hours or more, and is preferably 48 hours or less, more preferably 24 hours or less.

[0142] An example of the mixing method is a method of applying shear, such as stirring, to a mixture of the second solvent and polymer H. Here, the entire amounts of the second solvent and polymer H may be added all at once and the resulting mixture may be stirred, or the second solvent may be mixed with polymer H in several batches with stirring between batches. Alternatively, a mixture of polymer H to which a portion of alcohol and a portion of water have been added may be stirred, and then the remaining second solvent may be added and stirred again. Alternatively, only alcohol may be added to polymer H and stirred, and then only water may be added and stirred again. When shear is applied, the shear rate is 10 to 1000 s －１ is preferred, and 50 to 600 s －１ is more preferred.

[0143] Step 3 may include a filtration treatment for filtering the liquid composition. This allows removal of contaminated foreign matter and undissolved polymer. Examples of filtration treatment methods include filtration using a filter (e.g., a PTFE filter). In the filtration treatment, the temperature of the liquid composition is preferably 10°C or higher, more preferably 20°C or higher, and preferably 100°C or lower, more preferably 90°C or lower. The pressure in the filtration treatment is preferably 0.001 MPa from the viewpoint of shortening the filtration time, and preferably 0.2 MPa or lower, more preferably 0.1 MPa or lower, from the viewpoint of suppressing breakthrough. The filtration treatment may be a treatment that does not involve pressurization.

[0144] The liquid composition thus obtained contains polymer H and the second solvent.

[0145] Polymer H in the liquid composition is preferably dispersed in the second solvent in the form of particles containing polymer H. The average secondary particle diameter of the particles containing polymer H is preferably 1 nm or more, more preferably 5 nm or more, even more preferably 10 nm or more, more preferably less than 400 nm, even more preferably 300 nm or less. The average secondary particle diameter of the particles containing polymer H is measured by the method described in the Examples section below.

[0146] The content of polymer H is preferably 5% by mass or more, more preferably 10% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less, based on the total mass of the liquid composition.

[0147] The content of the second solvent is preferably 50% by mass or more, more preferably 60% by mass or more, and is preferably 95% by mass or less, more preferably 90% by mass or less, based on the total mass of the liquid composition.

[0148] The liquid composition obtained by the present production method is preferably used for producing a solid polymer electrolyte membrane, and is also preferably used for producing at least one of an anode catalyst layer and a cathode catalyst layer in a membrane electrode assembly described below.

[0149] [Solid Polymer Electrolyte Membrane] The solid polymer electrolyte membrane of the present invention (hereinafter also referred to as "the present electrolyte membrane") is formed using the liquid composition obtained by the above-described method, and contains polymer H. In order to further improve the strength of the present electrolyte membrane, it is preferable that the present electrolyte membrane further contains a reinforcing material. In addition, in order to further improve durability, the present electrolyte membrane may contain one or more atoms selected from the group consisting of cerium and manganese.

[0150] The polymer H contained in the present electrolyte membrane is as described above.

[0151] Specific examples of the reinforcing body include porous bodies, fibers, woven fabrics, and nonwoven fabrics. Preferred materials for the reinforcing material include polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymers, polyethylene, polypropylene, and polyphenylene sulfide, with polytetrafluoroethylene being more preferred due to its excellent chemical durability. The pore diameter of the porous body is preferably 10 to 2000 nm, and more preferably 50 to 1800 nm. The particles containing polymer H contained in the liquid composition obtained by the above method have a small particle diameter. Therefore, when a porous body is used as a reinforcing material, the particles containing polymer H are likely to enter the pores of the porous body. This can prevent voids from forming in the electrolyte membrane.

[0152] Cerium and manganese can decompose hydrogen peroxide, which is a substance that causes deterioration of the electrolyte membrane, and it is preferable that cerium and manganese exist in the electrolyte membrane as ions.

[0153] The method for producing the electrolyte membrane is not particularly limited as long as the liquid composition obtained by the above-mentioned method is used, and the electrolyte membrane can be formed, for example, by a method in which the liquid composition is applied to the surface of a substrate film or a catalyst layer and dried (casting method). Furthermore, when the electrolyte membrane further contains a reinforcing material, the electrolyte membrane can be formed by a method in which the reinforcing material is impregnated with the liquid composition and dried.

[0154] [Membrane Electrode Assembly] The membrane electrode assembly of the present invention (hereinafter also referred to as "the membrane electrode assembly") comprises an anode having a catalyst layer, a cathode having a catalyst layer, and an electrolyte membrane disposed between the anode and the cathode. The membrane electrode assembly is suitable for use in a polymer electrolyte fuel cell.

[0155] 1 is a cross-sectional view showing an example of the membrane electrode assembly. The membrane electrode assembly 10 includes an anode 13 having a catalyst layer 11 and a gas diffusion layer 12, a cathode 14 having the catalyst layer 11 and the gas diffusion layer 12, and a solid polymer electrolyte membrane 15 disposed between the anode 13 and the cathode 14 in contact with the catalyst layer 11.

[0156] The catalyst layer 11 is a layer containing a catalyst and a polymer having an ion exchange group. Specific examples of the catalyst include a supported catalyst in which platinum or a platinum alloy is supported on a carbon support. Specific examples of the polymer having an ion exchange group include polymer H and a perfluoropolymer other than polymer H having an ion exchange group.

[0157] The gas diffusion layer 12 has a function of diffusing gas uniformly in the catalyst layer 11 and a function as a current collector. Examples of the gas diffusion layer 12 include carbon paper, carbon cloth, and carbon felt. The gas diffusion layer 12 is preferably treated with polytetrafluoroethylene or the like to be water-repellent.

[0158] The solid polymer electrolyte membrane 15 is preferably the present electrolyte membrane described above.

[0159] As shown in FIG. 2 , the membrane electrode assembly 10 may have a carbon layer 16 between the catalyst layer 11 and the gas diffusion layer 12. By disposing the carbon layer 16, the gas diffusion properties on the surface of the catalyst layer 11 are improved, and the power generation performance of the polymer electrolyte fuel cell is significantly improved. The carbon layer 16 is a layer containing carbon and a nonionic fluorine-containing polymer. Examples of carbon include carbon particles and carbon fibers, and carbon nanofibers having a fiber diameter of 1 to 1,000 nm and a fiber length of 1,000 μm or less are preferred. Examples of nonionic fluorine-containing polymers include polytetrafluoroethylene.

[0160] When the membrane electrode assembly 10 does not have the carbon layer 16, the membrane electrode assembly 10 is manufactured, for example, by the following methods: A method in which a catalyst layer 11 is formed on a solid polymer electrolyte membrane 15 to form a membrane catalyst layer assembly, and the membrane catalyst layer assembly is sandwiched between gas diffusion layers 12. A method in which a catalyst layer 11 is formed on a gas diffusion layer 12 to form electrodes (anode 13, cathode 14), and the solid polymer electrolyte membrane 15 is sandwiched between the electrodes.

[0161] When the membrane electrode assembly 10 has the carbon layer 16, the membrane electrode assembly 10 is produced, for example, by the following methods: A method in which a dispersion containing carbon and a nonionic fluorine-containing polymer is applied to a substrate film and dried to form the carbon layer 16, a catalyst layer 11 is formed on the carbon layer, the catalyst layer 11 and a solid polymer electrolyte membrane 15 are bonded together, and the substrate film is peeled off to form a membrane catalyst layer assembly having the carbon layer 16, and the membrane catalyst layer assembly is sandwiched between gas diffusion layers 12. A method in which a dispersion containing carbon and a nonionic fluorine-containing polymer is applied to a gas diffusion layer 12 and dried to form the carbon layer 16, and a membrane catalyst layer assembly in which the catalyst layer 11 is formed on the solid polymer electrolyte membrane 15 is sandwiched between gas diffusion layers 12 having the carbon layer 16.

[0162] Examples of methods for forming the catalyst layer 11 include the following: A method in which a catalyst layer-forming coating liquid is applied to the solid polymer electrolyte membrane 15, the gas diffusion layer 12, or the carbon layer 16, and then dried. A method in which a catalyst layer-forming coating liquid is applied to a substrate film, and then dried to form the catalyst layer 11, and then the catalyst layer 11 is transferred onto the solid polymer electrolyte membrane 15.

[0163] The catalyst layer-forming coating liquid is a liquid in which a polymer having ion exchange groups and a catalyst are dispersed in a dispersion medium. The catalyst layer-forming coating liquid can be prepared, for example, by mixing a liquid composition containing a polymer having ion exchange groups with a catalyst dispersion. The catalyst layer-forming coating liquid may contain one or more metals, metal compounds, or metal ions selected from the group consisting of cerium and manganese in order to further improve the durability of the catalyst layer 11. The catalyst layer-forming coating liquid may also be a liquid prepared using the liquid composition obtained by the above-mentioned production method.

[0164] [Solid Polymer Electrolyte Fuel Cell] The solid polymer electrolyte fuel cell of the present invention (hereinafter also referred to as "the present fuel cell") has the above-described present membrane electrode assembly. The present fuel cell may be one in which separators having grooves formed therein to serve as gas flow paths are arranged on both sides of the membrane electrode assembly. Examples of separators include separators made of various conductive materials, such as metal separators, carbon separators, and separators made of a material mixture of graphite and resin. In the present fuel cell, power is generated by supplying an oxygen-containing gas to the cathode and a hydrogen-containing gas to the anode. The present membrane electrode assembly can also be applied to methanol fuel cells, which generate power by supplying methanol to the anode.

[0165] The present invention will be described in detail below with reference to examples. Examples 1 and 3 to 5 are working examples, and Examples 2 and 6 to 7 are comparative examples. However, the present invention is not limited to these examples. In the following, fluoropolymers having fluorosulfonyl groups will be collectively referred to as "polymer F," and fluoropolymers obtained by converting the fluorosulfonyl groups of polymer F to sulfonic acid groups will be collectively referred to as "polymer H."

[0166] [Abbreviation] TFE:CF ２ =CF ２ Monomer (m1): CF ２ = CFOCF ２ CF (CF ３ ) OCF ２ CF ２ SO ２ F Monomer (m2): Monomer represented by the following formula (m2) Monomer (m3): Monomer represented by the following formula (m3)

[0167]

[0168] [Composition] The composition of each structural unit in Polymer F is １９ The results were obtained by F-NMR and are shown in the table below.

[0169] [Ion exchange capacity] The ion exchange capacity (milliequivalents / g dry resin) of polymer F was calculated from the composition of each structural unit in polymer F determined as described above. The results are shown in the table below. The ion exchange capacity of polymer H is the same as that of polymer F, since it has the same composition as polymer F.

[0170] [TQ value] Polymer F was vacuum dried at 240°C for 16 hours. Using a flow tester (Shimadzu Corporation, CFT-500D) equipped with a nozzle having a length of 1 mm and an inner diameter of 1 mm, the vacuum-dried polymer F was melt-extruded while changing the temperature under the condition of an extrusion pressure of 2.94 MPa (gauge pressure). The extrusion amount of polymer F was 100 mm ３ The temperature at which the melting point reached 1 / second (TQ value) was determined. The results are shown in the table below.

[0171] [Water Content Relative to Polymer H in Composition Obtained in Step 2] The content (% by mass) of water (first solvent) relative to Polymer H in each composition obtained in Step 2 described below was determined by heating 1 g of the composition obtained in Step 2 at 160°C for 40 minutes, weighing the mass of the heating residue, and using the following formula. The results are shown in the table below. Water Content (% by mass) relative to Polymer H in the composition = 100 × [{mass of composition (i.e., 1 g)} - {mass of heating residue (g)}] / {mass of heating residue (g)}

[0172] [Water Content in the Composition Obtained in Step 2 Relative to the Water Content in the Solid Composition Obtained in Step 1] The water (first solvent) content A (g) in the solid composition obtained in Step 1 described below was calculated by heating 1 g of the solid composition obtained in Step 1 at 160°C for 40 minutes, and then weighing the mass of the heating residue, and using the following formula: Content A (g) = {mass of solid composition (i.e., 1 g)} - {mass of heating residue (g)} Furthermore, the water (first solvent) content B (g) in the composition obtained in Step 2 described below was calculated by heating 1 g of the composition obtained in Step 2 at 160°C for 40 minutes, and then weighing the mass of the heating residue, and using the following formula: Content B (g) = {mass of composition (i.e., 1 g)} - {mass of heated residue (g)} The ratio (mass%) of content B to content A obtained as above was calculated, and the obtained value was defined as the content of water in the composition obtained in step 2 relative to the content of water in the solid composition obtained in step 1. The results are shown in the table below.

[0173] [Evaluation of Manufacturability] Three different lots of liquid compositions were produced using the method described in Example 1. The solid content concentrations of the liquid compositions were compared between the lots, and the manufacturability was evaluated according to the following criteria. Furthermore, the manufacturability of other examples was evaluated using the same method as in Example 1, except that the liquid composition obtained in each example was used. The results are shown in the table below. In the table, "-" indicates that no evaluation was performed. (Method for calculating the solid content concentration of a liquid composition) The solid content concentration of a liquid composition was calculated by heating 1 g of the liquid composition at 160°C for 3 hours, weighing the mass of the residue, and then using the following formula: Solid content concentration (mass %) = 100 × [heated residue of liquid composition (g)] / [mass of liquid composition (i.e., 1 g)] (Evaluation criteria) ◯ (Good): The maximum difference in solid content concentration between the lots was less than 2 mass %. × (Unacceptable): The maximum difference in solid content concentration between the lots was 2 mass % or more.

[0174] [Evaluation of Particle Size] The liquid composition obtained in each example was diluted with ultrapure water to a solids concentration of 0.2% by mass, and allowed to stand for 16 hours to prepare a sample. Using a fiber optic dynamic light scattering photometer (Otsuka Electronics Co., Ltd., FDLS-3000), the scattering intensity of the sample was measured under the following conditions: temperature: 25°C, sampling time: 100 μsec, number of channels: 1024, and number of accumulations: 100. The average secondary particle size of polymer H (particles containing polymer H) in the liquid composition was calculated from the obtained autocorrelation function by cumulant analysis. Based on the obtained average secondary particle size value, the particle size of polymer H was evaluated according to the following criteria. The results are shown in the table below. In the table, "-" means that no evaluation was performed. (Evaluation Criteria) ◯ (Good): The average secondary particle size was less than 400 nm. × (Unacceptable): The average secondary particle size was 400 nm or greater.

[0175] [Example 1] Polymer F-1A containing units based on monomer (m1) and units based on TFE was obtained by referring to the method up to the hydrolysis treatment described in paragraph 0200 of JP 2015-99772 A. Polymer F-1A was placed in a hammer-type crusher with a mesh size of 3 mm together with dry ice and crushed while cooling.

[0176] <Step 1> 100 g of the pulverized polymer F-1A was hydrolyzed for 24 hours in an alkaline aqueous solution (containing 64 g of methanol, 96 g of potassium hydroxide, and 160 g of water) heated to 80°C (hydrolysis treatment). Subsequently, 105 g of the hydrolyzed polymer was washed with 230 g of water at 30°C for 1 hour, a process repeated three times (water washing treatment 1). Next, 105 g of the washed polymer was reacted with 300 g of an acidic aqueous solution (aqueous sulfuric acid solution with a sulfuric acid concentration of 8% by mass) at 60°C for 30 minutes, a process repeated five times (acid-form treatment). Subsequently, 99 g of the acid-formed polymer was washed with 300 g of water at 60°C for 1 hour, a process repeated three times (water washing treatment 2). Subsequently, 99 g of the washed polymer was reacted with an aqueous hydrogen peroxide solution (hydrogen peroxide concentration 13% by mass) at 55°C for 20 hours (hydrogen peroxide treatment). Thereafter, 99 g of the polymer after the hydrogen peroxide treatment was washed with 230 g of water at 30 ° C for 1 hour, this operation being repeated three times (water washing treatment 3). Next, 99 g of the polymer after the water washing treatment was reacted with 300 g of an acidic aqueous solution at 60 ° C (aqueous sulfuric acid solution with a sulfuric acid concentration of 8% by mass) for 30 minutes, this operation being repeated six times (acid-form treatment). Thereafter, 99 g of the polymer after the acid-form treatment was washed with 300 g of water at 60 ° C for 1 hour, this operation being repeated 11 times (water washing treatment 2). As a result, a mixture 1 containing water (first solvent) and polymer H-1A in which the fluorosulfonyl groups of polymer F-1A were converted to sulfonic acid groups was obtained. The water content in mixture 1 was 78% by mass relative to the total mass of mixture 1. Next, a solid composition 1 containing polymer H-1A and water was separated from mixture 1 by filtration using a filter (material: PFA, mesh size 313 μm, trade name "Fluororesin Mesh", manufactured by Tantore Co., Ltd.) (separation treatment). Solid composition 1 was a wet polymer in which polymer H-1A was swollen with water.

[0177] The water used in each of the above treatments had a resistivity of 18 MΩ·cm and a TOC (Total Organic Carbon) content of 50 ppb by mass.

[0178] <Step 2> Nitrogen gas (1 L / min) was passed through the solid composition 1 obtained in step 1, and 99 g of the solid composition 1 was dried at 40°C for 30 minutes to obtain composition 1 containing polymer H-1A.

[0179] <Step 3> Composition 1 weighed out so that the mass of Polymer H-1A was 20 g, 40 g of ethanol, and 40 g of water (including the water in Composition 1) were placed in a 200 mL stainless steel autoclave and stirred for 6 hours using a helical ribbon impeller at 105°C and a rotation speed of 250 rpm. After stirring, the dispersion was cooled to room temperature (25°C) and filtered through a PTFE filter (pore size 3 μm) at a pressure of 0.01 MPa to obtain Liquid Composition 1.

[0180] [Example 2] Liquid composition 2 was obtained in the same manner as in Example 1, except that composition 2 obtained by changing the drying conditions of solid composition 1 in step 2 to 90°C for 10 hours was used instead of composition 1.

[0181] [Example 3] Polymer F-2A containing units based on monomer (m2), units based on monomer (m3), and units based on TFE was obtained according to the method described in the Examples section of Japanese Patent No. 6631630. Polymer F-2A was placed in a hammer-type crusher with a mesh size of 2 mm together with dry ice and crushed while cooling.

[0182] <Step 1> 100 g of the pulverized polymer F-2A was hydrolyzed for 24 hours in an alkaline aqueous solution (aqueous solution containing 80 g of methanol, 120 g of potassium hydroxide, and 200 g of water) heated to 80°C (hydrolysis treatment). Thereafter, 105 g of the polymer after the hydrolysis treatment was washed with 300 g of water at 65°C for 30 minutes, a process repeated 20 times (water washing treatment 1). Next, 105 g of the polymer after the water washing treatment was reacted with 300 g of an acidic aqueous solution (aqueous sulfuric acid solution with a sulfuric acid concentration of 15% by mass) at 65°C for 30 minutes, a process repeated 10 times (acid-form treatment). Thereafter, 99 g of the polymer after the acid-form treatment was washed with 400 g of water at 65°C for 30 minutes, a process repeated 14 times (water washing treatment 2). The polymer after the water washing was then dried for 8 hours. Next, 99 g of the dried polymer was reacted with 450 g of an aqueous hydrogen peroxide solution (hydrogen peroxide concentration 10% by mass) at 80°C for 16 hours (hydrogen peroxide treatment). Then, 99 g of the polymer after the hydrogen peroxide treatment was washed five times with 300 g of water at 65°C for 30 minutes (water washing treatment 3). This yielded a mixture 2 containing water (first solvent) and polymer H-2A in which the fluorosulfonyl groups of polymer F-2A had been converted to sulfonic acid groups. The water content in mixture 2 was 69% by mass relative to the total mass of mixture 2. Next, solid composition 2 containing polymer H-2A and water was separated from mixture 2 by filtration using a filter (material: PFA, mesh size 313 μm, product name "Fluororesin Mesh", manufactured by Tantore) (separation treatment). Solid composition 2 is a wet polymer in which polymer H-2A has swollen with water.

[0183] The water used in each of the above treatments had a resistivity of 18 MΩ·cm and a TOC (Total Organic Carbon) content of 50 ppb by mass.

[0184] <Step 2> Nitrogen gas (1 L / min) was passed through the solid composition 2 obtained in step 1, and 99 g of the solid composition 2 was dried at 25°C for 30 minutes to obtain composition 3 containing polymer H-2A.

[0185] <Step 3> Composition 3, weighed out so that the mass of Polymer H-2A was 14 g, 43 g of 1-propanol, and 43 g of water (including the water in Composition 2), were placed in a 200 mL stainless steel autoclave and stirred using a helical ribbon impeller at 115°C and a rotation speed of 300 rpm for 6 hours. After stirring, the dispersion was cooled to room temperature (25°C) and filtered through a PTFE filter (pore size 3 µm) at a pressure of 0.01 MPa to obtain Liquid Composition 3.

[0186] [Example 4] Liquid composition 4 was obtained in the same manner as in Example 3, except that composition 4, which was obtained by changing the drying conditions of solid composition 2 in step 2 to 2 hours at 25°C, was used instead of composition 3.

[0187] [Example 5] Liquid composition 5 was obtained in the same manner as in Example 3, except that composition 5 obtained by changing the drying conditions of solid composition 2 in step 2 to 25°C for 6 hours was used instead of composition 3.

[0188] [Example 6] Liquid composition 6 was obtained in the same manner as in Example 3, except that composition 6 obtained by changing the drying conditions of solid composition 2 in step 2 to 80°C for 16 hours was used instead of composition 3.

[0189] [Example 7] Liquid composition 7 was obtained in the same manner as in Example 3, except that composition 7, which was obtained by changing the drying conditions of solid composition 2 in step 2 to 25°C for 5 minutes, was used instead of composition 3.

[0190]

[0191]

[0192] As shown in Table 2, it was demonstrated that the production method of the present invention can provide a liquid composition containing polymer H having a small particle size and having excellent production suitability (Example 1, Examples 3 to 5).

[0193] The entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2023-205520, filed on December 5, 2023, are incorporated herein by reference as part of the disclosure of the present invention.

[0194] 10 membrane electrode assembly 11 catalyst layer 12 gas diffusion layer 13 anode 14 cathode 15 solid polymer electrolyte membrane 16 carbon layer

Claims

1. A method for producing a liquid composition, comprising: subjecting a fluoropolymer F having groups convertible to ion exchange groups to hydrolysis and acidification in a first solvent to convert the groups convertible to ion exchange groups into acid-type ion exchange groups; separating a solid composition containing a fluoropolymer H having ion exchange groups and the first solvent from the mixture obtained; carrying out a treatment to remove the first solvent from the solid composition to obtain a composition in which the content of the first solvent is 2 to 200 mass% based on the total mass of the fluoropolymer H; and mixing the composition with a second solvent containing water and an alcohol to obtain a liquid composition containing the fluoropolymer H having ion exchange groups.

2. The method for producing a liquid composition according to claim 1, wherein the fluoropolymer F contains a unit based on a compound represented by formula (1). ２ = CF-L-(A) ｎ In formula (1), L is an (n+1) valent perfluorohydrocarbon group which may contain an etheric oxygen atom, A is a group which can be converted into a sulfonic acid type functional group, and n is 1 or 2.

3. The method for producing a liquid composition according to claim 1 or 2, wherein the ion exchange capacity of said fluoropolymer H is 0.8 to 3.0 milliequivalents / gram of dry resin.

4. The method for producing a liquid composition according to claim 1 or 2, wherein the alcohol comprises at least one selected from the group consisting of ethanol, 1-propanol, 1-butanol, and 2-propanol.

5. The method for producing a liquid composition according to claim 1 or 2, wherein the liquid composition is used for producing a solid polymer electrolyte membrane.

6. The method for producing a liquid composition according to claim 1 or 2, wherein the liquid composition is used to produce at least one of the catalyst layer in the anode and the catalyst layer in the cathode in a membrane electrode assembly having an anode having a catalyst layer, a cathode having a catalyst layer, and a solid polymer electrolyte membrane disposed between the anode and the cathode.

7. A method for producing a liquid composition according to claim 1 or 2, wherein the content of the second solvent is 50% by mass or more and 95% by mass or less relative to the total mass of the liquid composition.

8. A method for producing a liquid composition according to claim 1 or 2, wherein the fluoropolymer H is dispersed in the second solvent in the form of particles, and the average secondary particle diameter of the particles is 1 nm or more and less than 400 nm.

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