Method for producing pellets, method for producing ion exchange membrane, and pellets

Abstract

Provided is a method for producing pellets that make is possible to produce an ion exchange membrane, in which film thickness unevenness in the in-plane direction is suppressed, by means of a melt extrusion method.　In this method for producing pellets, a melt that contains a fluorine-containing polymer having a group that can be converted into an ion exchange group is extruded from a die of a melt extruder so as to obtain a strand that contains the fluorine-containing polymer, and the strand is subsequently cut so as to obtain pellets that contain the fluorine-containing polymer. The method for producing pellets includes an air-cooling treatment for blowing air to the strand extruded from the die so as to cool the strand.

Description

Pellets manufacturing method, ion exchange membrane manufacturing method and pellets

[0001] The present disclosure relates to a method for producing pellets, a method for producing an ion exchange membrane, and the pellets.

[0002] Ion exchange membranes containing fluoropolymers having ion exchange groups are used in various batteries, electrolysis processes, and processes for separating ions, etc. A known method for producing ion exchange membranes containing fluoropolymers is to use pellets of fluoropolymers having ion exchange groups or groups that can be converted into ion exchange groups as a raw material. Here, the pellets are produced, for example, by extruding a polymer melt through a die, water-cooling it to form strands, and cutting the strands with a pelletizer (see Patent Document 1).

[0003] International Publication No. 2023 / 085421

[0004] The present inventors have found that when an ion exchange membrane is formed using pellets produced from water-cooled strands with reference to the technology described in Patent Document 1, the resulting ion exchange membrane may have uneven thickness in the in-plane direction. Depending on the application of the ion exchange membrane to be produced, the uneven thickness in the in-plane direction of the ion exchange membrane may be required to be small, and therefore, reduction of the uneven thickness has been desired.

[0005] The present disclosure has been made in view of the above-mentioned problems, and an object of one embodiment of the present invention is to provide a method for producing pellets that can be used to produce, via melt extrusion, an ion exchange membrane having reduced in-plane thickness unevenness. Another object of one embodiment of the present invention is to provide a method for producing an ion exchange membrane and pellets.

[0006] The present disclosure includes the following aspects. [1] A method for producing pellets, comprising extruding a melt containing a fluoropolymer having a group convertible to an ion-exchange group through the die of a melt extruder to obtain a strand containing the fluoropolymer, and then cutting the strand to obtain pellets containing the fluoropolymer, the method comprising an air-cooling treatment of blowing air against the strand extruded from the die to cool it. [2] The method for producing pellets according to [1], wherein the group convertible to an ion-exchange group is a group convertible to a carboxylic acid type functional group or a sulfonic acid type functional group. [3] A method for producing pellets, comprising extruding a melt containing a fluoropolymer having a group convertible to an ion-exchange group through the die of a melt extruder to obtain a strand containing the fluoropolymer, and then cutting the strand to obtain pellets containing the fluoropolymer. ３ [4] A method for producing pellets according to any one of [1] to [3], wherein the linear velocity of the air blown is 5.0 to 50.0 m / sec. [5] A method for producing pellets according to any one of [1] to [4], wherein the temperature of the air blown is 40°C or less. [6] A method for producing pellets according to any one of [1] to [5], wherein the ion exchange capacity of the fluoropolymer is 0.9 meq / g resin or more and 2.00 meq / g resin or less. [7] A method for producing pellets according to any one of [1] to [6], wherein the pellets are used for producing an ion exchange membrane. [8] A method for producing an ion exchange membrane, comprising producing pellets by the method for producing pellets according to any one of [1] to [7], obtaining a precursor membrane by melt extrusion using the pellets, and using the precursor membrane to obtain an ion exchange membrane. [9] Pellets having a hydrolysis rate of less than 1.00%, produced by the method according to any one of [1] to [8].

[0007] According to one embodiment of the present invention, there is provided a method for producing pellets that can be used to produce, via melt extrusion, an ion exchange membrane having reduced in-plane thickness unevenness. Also, according to one embodiment of the present invention, there is provided a method for producing an ion exchange membrane and pellets.

[0008] 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" refers to a group that can be converted into an ion exchange group by known treatments such as hydrolysis and acidification. A "group that can be converted into a sulfonic acid functional group" refers to a group that can be converted into a sulfonic acid functional group by known treatments such as hydrolysis and acidification. A "group that can be converted into a carboxylic acid functional group" refers to a group that can be converted into a carboxylic acid functional group by known treatments such as hydrolysis and acidification. A "perfluorohydrocarbon group" refers to a hydrocarbon group in which all of the hydrogen atoms are substituted with fluorine atoms. A "perfluoroaliphatic hydrocarbon group" refers to an aliphatic hydrocarbon group in which all of the hydrogen atoms are substituted with fluorine atoms.

[0009] 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.

[0010] "Reinforcing material" refers to a material used to improve the strength of an ion exchange membrane. A material derived from a reinforcing cloth is preferred as a reinforcing material. "Reinforcing cloth" refers to a cloth used as a raw material for a reinforcing material to improve the strength of an ion exchange membrane. A numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. Furthermore, if the units for the lower and upper limits are the same, the unit for the lower limit may be omitted. In the numerical ranges described in this specification in stages, the upper or lower limit stated 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 stated in a certain numerical range may be replaced with the value shown in the examples.

[0011] [Method for producing pellets] The method for producing pellets according to the present disclosure (hereinafter also referred to as the present production method) is a method for producing pellets by extruding a melt containing a fluoropolymer having a group convertible to an ion-exchange group (hereinafter also referred to as the fluoropolymer (I')) through the die of a melt extruder to obtain a strand containing the fluoropolymer, and then cutting the strand to obtain pellets containing the fluoropolymer. Here, the present production method includes an air-cooling treatment in which air is blown onto the strand extruded from the die to cool it.

[0012] According to this production method, when an ion exchange membrane is formed by melt extrusion, the resulting ion exchange membrane exhibits reduced in-plane thickness unevenness. The details of the reason for this are unclear, but it is presumed to be due to the following reasons. When strands extruded from a die are cooled with water, the fluoropolymer comes into direct contact with water, which may cause hydrolysis of groups that can be converted into ion exchange groups. Such hydrolysis is more likely to occur on the surface of the strand (the portion that directly contacts water), and therefore, it is believed that hydrolysis occurs non-uniformly in the produced pellets. The present inventors have found that when an ion exchange membrane is formed by melt extrusion using pellets in which such hydrolysis has occurred, the resulting ion exchange membrane exhibits uneven thickness. It is presumed that the reason for this uneven thickness is that the hydrolyzed and unhydrolyzed fluoropolymer portions in the pellets have different fluidities during melt extrusion, resulting in uneven thickness during melt extrusion. On the other hand, according to this production method, the strands are cooled by air-cooling, which is thought to make it less likely for hydrolysis of groups that can be converted into ion exchange groups to occur. This is thought to make it difficult for unevenness in thickness due to differences in fluidity to occur during melt extrusion, and as a result, unevenness in thickness in the in-plane direction of the resulting ion exchange membrane is suppressed. If air is not blown, cooling is not performed sufficiently and the strands are stretched at a relatively high temperature, which is thought to deform the strands and cause fluctuations in tension, making it difficult to achieve a uniform strand thickness. If such strands are cut to produce pellets, the pellets will vary in volume, and the pellets may not be discharged at a constant rate during melt extrusion, resulting in uneven thickness in the resulting ion exchange membrane.

[0013] An example of this production method is shown below. First, a fluoropolymer having groups convertible to ion-exchange groups is fed into a melt extruder to obtain a melt of the fluoropolymer. Any known melt extruder can be used, and specific examples include a single-screw extruder, a twin-screw extruder, and a tandem extruder. The melt temperature of the fluoropolymer (I') is preferably 150 to 350°C, particularly preferably 200 to 300°C.

[0014] Next, the melt of the fluoropolymer (I') is extruded through a die at the tip of the melt extruder and cooled to obtain strands containing the fluoropolymer (I'), which are then cut into a predetermined size to give pellets containing the fluoropolymer (I').

[0015] When obtaining a strand by the above procedure, in this manufacturing method, an air-cooling treatment is performed in which air is blown onto the strand extruded from the die to cool it. Blowing air refers to a linear air velocity of 0.3 m / s or more. In order to make the strand thickness more uniform and further reduce film thickness unevenness, the linear air velocity is preferably 0.6 m / s or more, more preferably 5.0 m / s or more, even more preferably 10.0 m / s or more, and particularly preferably 20.0 m / s or more. The linear air velocity may also be 30.0 m / s or more. There is no particular upper limit to the linear air velocity, but it may be, for example, 50.0 m / s or less, or 40.0 m / s or less. In the present disclosure, the linear air velocity is measured using an anemometer, and detailed measurement methods follow the methods described in the Examples. The linear air velocity is measured in advance when the strand has not yet been extruded from the die.

[0016] The air-cooling treatment in which air is blown onto the strand is preferably carried out at least at a distance of 15 to 1 cm from the die. The distance is more preferably 10 to 3 cm, and even more preferably 8 to 5 cm. Furthermore, the range over which air is blown onto the strand in the longitudinal direction of the strand is often 10 cm or more, preferably 1 m or more, more preferably 3 m or more, and even more preferably 5 m or more. The upper limit of the range over which air is blown onto the strand is not particularly limited, but may be, for example, 20 m or less. For example, the air-cooling treatment in which air is blown onto the strand is preferably a treatment in which air is blown onto the range in the longitudinal direction of the strand, in terms of the distance range from the die. The strand may be blown onto one location, or may be divided into two or more locations. When air is blown onto the strand at two or more locations, the linear velocity of the air at each location may be the same for some or all of the locations, or may be different for each location.

[0017] When performing the air-cooling treatment, the strand conveying speed is not particularly limited, but is often 10.0 m / min or less, preferably 5.0 m / min or less, and more preferably 3.0 m / min or less. The strand conveying speed is often 0.5 m / min or more, preferably 1.0 m / min or more, and more preferably 2.0 m / min or more. The conveying speed refers to the distance traveled by one point on the strand per minute.

[0018] Furthermore, when performing the air-cooling treatment, the residence time of the strand in the region where the air-cooling treatment is performed (for example, the region where air is blown onto the strand) can also be adjusted as appropriate. The residence time can be calculated by dividing the length of the region where the air-cooling treatment is performed by the conveying speed. The residence time is often 0.5 minutes or more, preferably 1.0 minutes or more, and more preferably 2.0 minutes or more. The residence time is often 10.0 minutes or less, preferably 7.0 minutes or less, and more preferably 5.0 minutes or less.

[0019] The air blown in the cooling treatment may be humidity-controlled air. The moisture content of the air is preferably 55 g / m 2 or more, in order to further suppress the in-plane thickness unevenness of the resulting ion exchange membrane. ３ Preferably, 22 g / m or less ３ More preferably, 18 g / m or less ３ More preferably, 14 g / m or less ３ The lower limit of the moisture content of the air is not particularly limited, but is preferably 3 g / m ３ The above can be mentioned.

[0020] The temperature of the air blown in the cooling treatment may be controlled. The temperature of the air is preferably 45° C. or lower, more preferably 40° C. or lower, even more preferably 30° C. or lower, and particularly preferably 25° C. or lower. There is no particular lower limit for the air temperature, but it is preferably 5° C. or higher, more preferably 10° C. or higher, and even more preferably 15° C. or higher.

[0021] Here, the temperature of the die when the melt containing the fluoropolymer (I') is extruded through the die (hereinafter also simply referred to as the die temperature) is preferably less than 300° C., more preferably 270° C. or lower, even more preferably 260° C. or lower, particularly preferably 240° C. or lower, and most preferably 200° C. or lower. The die temperature is preferably 160° C. or higher, more preferably 180° C. or higher, from the viewpoint of facilitating extrusion of the melt of the fluoropolymer (I') through the die.

[0022] The shape of the pellets obtained by the present production method is not particularly limited, and may be, for example, any shape such as spherical (including ellipsoidal), cylindrical (for example, cylindrical), etc. The size of the pellets obtained by the present production method is not particularly limited, but for example, when the pellets are cylindrical, it is preferable that the diameter is 2 to 3 mm and the length is 2 to 3 mm.

[0023] The surface of the pellets (strands) obtained by this production method preferably has a plurality of grooves formed thereon. This can prevent the pellets from sticking together. The grooves formed on the surface of the pellets may be formed over the entire surface of the pellets, but are usually preferably formed only on the side surfaces of the pellets (surfaces other than the cut surfaces of the strands). The grooves formed on the surface of the pellets are preferably formed mainly along a direction intersecting the cut surfaces of the pellets (the flow direction of the strands).

[0024] In this production method, after cutting the strands, a roughening treatment may be performed to roughen the surfaces of the pellets. This further roughens the surfaces of the pellets, making it possible to further suppress adhesion between the pellets and obtain an ion exchange membrane with superior stability of the electrolysis voltage. A specific example of a roughening treatment method is a method in which the pellets are stirred using a mixer (e.g., a V-blender).

[0025] The pellets obtained by this production method are preferably used to produce ion exchange membranes, specific examples of uses of ion exchange membranes will be described later.

[0026] [Fluoropolymer (I')] The fluoropolymer (I') used in the present production method is a fluoropolymer having a group that can be converted into an ion-exchange group. The group that can be converted into an ion-exchange group is preferably a group that can be converted into a carboxylic acid type functional group or a group that can be converted into a sulfonic acid type functional group.

[0027] The ion exchange capacity of the fluoropolymer (I) when the groups convertible to ion exchange groups of the fluoropolymer (I') are converted into ion exchange groups is preferably 0.9 meq / g resin or more, more preferably 1.0 meq / g resin or more, even more preferably 1.1 meq / g resin or more, and particularly preferably 1.25 meq / g resin or more, from the viewpoint of being able to reduce the electrolysis voltage of an apparatus incorporating an ion exchange membrane. Furthermore, from the viewpoint of better strength of the ion exchange membrane, it is preferably 2.00 meq / g resin or less, more preferably 1.90 meq / g resin or less, and even more preferably 1.40 meq / g resin or less. Here, the fluoropolymer (I) for measuring the ion exchange capacity is obtained as follows. First, the fluoropolymer (I') that has been vacuum heat-treated at 240°C and -0.1 MPaG for 16 hours is immersed in a solution of dimethyl sulfoxide / potassium hydroxide / water = 30 / 5.5 / 64.5 (mass ratio) at 95°C for 30 minutes, and groups that can be converted into ion exchange groups in the fluoropolymer (I') are hydrolyzed to convert them to K-type ion exchange groups, and then washed with water. Thereafter, the polymer is immersed in an aqueous sodium hydroxide solution to convert the terminal groups from K-type to Na-type, thereby obtaining a fluoropolymer (I) for measuring the ion exchange capacity. The method for measuring the ion exchange capacity of the fluoropolymer (I) thus obtained is as described in the Examples section below.

[0028] The fluoropolymer (I') may be used singly or in combination of two or more. In order to better exhibit the effects of the present disclosure, the fluoropolymer (I') is preferably a fluoropolymer having a group that can be converted into a carboxylic acid type functional group (hereinafter also referred to as fluoropolymer (C')), or a fluoropolymer having a group that can be converted into a sulfonic acid type functional group (hereinafter also referred to as fluoropolymer (S')). Each fluoropolymer will be described in detail below.

[0029] (Fluorine-containing polymer (C')) The fluorine-containing polymer (C') is more preferably a copolymer of a fluorine-containing olefin and a monomer having a fluorine atom and a group that can be converted into a carboxylic acid functional group (hereinafter also referred to as fluorine-containing monomer (C')), in that the effects of the present disclosure can be more effectively exhibited. As the copolymerization method, known methods such as solution polymerization, suspension polymerization, emulsion polymerization, etc. can be used.

[0030] The fluorine-containing monomer (C') is not particularly limited as long as it is a compound having one or more fluorine atoms in the molecule, an ethylenic double bond, and a group that can be converted into a carboxylic acid functional group, and any conventionally known compound can be used. From the viewpoints of the production cost of the monomer, the reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer, the fluorine-containing monomer (C') is preferably a monomer represented by the following formula (1):

[0031] Formula (1): CF ２ =CF-(O) ｐ -(CF ２ ) ｑ -(CF ２ CFX) ｒ -(O) ｓ -(CF ２ ) ｔ -(CF ２ CFX') ｕ -A １

[0032] In formula (1), X and X′ are each independently a fluorine atom or a trifluoromethyl group. １ is a group that can be converted into a carboxylic acid type functional group. Specifically, -CN, -COF, -COOR １ (R １ is an alkyl group having 1 to 10 carbon atoms, ２ R ３ (R ２ and R ３ are each independently a hydrogen atom or an alkyl group having 1 to 10 carbon atoms. p is an integer of 0 or 1. q is an integer of 0 to 12. r is an integer of 0 to 3. s is an integer of 0 or 1. t is an integer of 0 to 12. u is an integer of 0 to 3, provided that 1≦p+s and 1≦r+u are satisfied.

[0033] Specific examples of the monomer represented by formula (1) include the following compounds, and from the viewpoint of ease of production, a compound in which p=1, q=0, r=1, s=0 to 1, t=0 to 3, and u=0 to 1 is preferred: CF ２ =CF-O-CF ２ CF ２ -COOCH ３ , C.F. ２ =CF-O-CF ２ CF ２ CF ２ -COOCH ３ , C.F. ２ =CF-O-CF ２ CF ２ CF ２ CF ２ -COOCH ３ , C.F. ２ =CF-O-CF ２ CF ２ -O-CF ２ CF ２ -COOCH ３ , C.F. ２ =CF-O-CF ２ CF ２ -O-CF ２ CF ２ CF ２ -COOCH ３ , C.F. ２ =CF-O-CF ２ CF ２ -O-CF ２ CF ２ CF ２ CF ２ -COOCH ３ , C.F. ２ =CF-O-CF ２ CF ２ CF ２ -O-CF ２ CF ２ -COOCH ３ , C.F. ２ =CF-O-CF ２ CF (CF ３ )-O-CF ２ CF ２ -COOCH ３ , C.F. ２=CF-O-CF ２ CF (CF ３ )-O-CF ２ CF ２ CF ２ -COOCH ３ The fluorine-containing monomer (C') may be used alone or in combination of two or more kinds.

[0034] Examples of fluorine-containing olefins include fluoroolefins having 2 to 3 carbon atoms and one or more fluorine atoms in the molecule. Specific examples thereof include tetrafluoroethylene (TFE), chlorotrifluoroethylene, vinylidene fluoride, vinyl fluoride, and hexafluoropropylene. Among these, TFE is particularly preferred in terms of the production cost of the monomer, reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer. One type of fluorine-containing olefin may be used alone, or two or more types may be used in combination.

[0035] In the production of the fluoropolymer (C'), in addition to the fluoromonomer (C') and the fluoroolefin, other monomers may be used. Specific examples of such other monomers include CF ２ = CFR ｆ (R ｆ is a perfluoroalkyl group having 2 to 10 carbon atoms, CF ２ =CF-OR ｆ１ (R ｆ１ is a perfluoroalkyl group having 1 to 10 carbon atoms, CF ２ = CFO (CF ２ ) ｖ CF = CF ２ (v is an integer of 1 to 3). Copolymerization of other monomers can improve the flexibility and mechanical strength of the ion exchange membrane. From the viewpoint of maintaining ion exchange performance, the content of units based on other monomers is preferably 30 mass% or less based on the total units in the fluoropolymer (C').

[0036] (Fluorine-containing polymer (S')) The fluorine-containing polymer (S') is more preferably a copolymer of a fluorine-containing olefin and a monomer having a fluorine atom and a group that can be converted into a sulfonic acid functional group (hereinafter also referred to as fluorine-containing monomer (S')), in that the effects of the present disclosure can be more effectively exhibited. As the copolymerization method, known methods such as solution polymerization, suspension polymerization and emulsion polymerization can be used.

[0037] Examples of the fluorine-containing olefin include those exemplified above, and TFE is preferred from the viewpoints of the production cost of the monomer, the reactivity with other monomers, and the 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. The content of units based on the fluorine-containing olefin relative to all units contained in the fluorine-containing polymer (S') is preferably 65 to 95 mol%.

[0038] The fluorine-containing monomer (S') may be a compound having 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 (S'), a compound represented by formula (2) is preferred in terms of the production cost of the monomer, reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer (S'). Formula (2) CF ２ =CF-L-(A) ｎ

[0039] L is an (n+1)-valent perfluorohydrocarbon group which may contain an oxygen atom. The 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 is preferably 20 or less, more preferably 10 or less.

[0040] L is preferably an (n+1)-valent perfluoroaliphatic hydrocarbon group which may contain an oxygen atom, and more preferably a divalent perfluoroalkylene group which may contain an oxygen atom, where n = 1, or a trivalent perfluoroaliphatic hydrocarbon group which may contain an oxygen atom, where n = 2. The divalent perfluoroalkylene group may be either linear or branched.

[0041] n is an integer of 1 or 2. A is a group that can be converted into a sulfonic acid type functional group. The group that can be converted into a sulfonic acid type functional group is preferably a functional group that can be converted into a sulfonic acid type functional group by hydrolysis. Specific examples of groups that can be converted into a sulfonic acid type functional group include -SO ２ F, -SO ２ Cl, —SO ２ Br. When n is 2, two A's may be the same or different.

[0042] The compound represented by formula (2) is preferably a compound represented by formula (2-1), a compound represented by formula (2-2), a compound represented by formula (2-3), or a compound represented by formula (2-4). Formula (2-1): CF ２ =CF-O-R ｆ１ -A Formula (2-2): CF ２ =CF-R ｆ１ -A

[0043]

[0044]

[0045] R ｆ１ R 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. 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. r is 0 or 1. ｆ２is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. A in the formula is as defined above.

[0046] 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.

[0047] r is 0 or 1. m is 0 or 1.

[0048] Specific examples of the compound represented by formula (2-1) 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 (CF ３ )-O-(CF ２ ) ｗ -SO ２ FCF ２ =CF-[O-CF ２ CF (CF ３ )] ｘ -SO ２ F

[0049] Specific examples of the compound represented by formula (2-2) 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

[0050] The compound represented by formula (2-3) is preferably a compound represented by formula (2-3-1). ｆ４ is a linear perfluoroalkylene group having 1 to 6 carbon atoms, and R ｆ５is 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 in the formula are as described above.

[0051]

[0052] Specific examples of the compound represented by formula (2-3-1) include the following.

[0053]

[0054] The compound represented by formula (2-4) is preferably a compound represented by formula (2-4-1).

[0055]

[0056] R in the formula ｆ１ , R ｆ２ and A are defined as above.

[0057] Specific examples of the compound represented by formula (2-4-1) include the following.

[0058]

[0059] The fluorine-containing monomer (S') may be used alone or in combination of two or more. The content of units based on the fluorine-containing monomer (S') relative to all units contained in the fluorine-containing polymer (S') is preferably 5 to 35 mol%. In addition to the fluorine-containing olefin and the fluorine-containing monomer (S'), other monomers may be used to produce the fluorine-containing polymer (S'). Examples of other monomers include those exemplified above. From the viewpoint of maintaining ion exchange performance, the content of units based on other monomers is preferably 30 mass% or less relative to all units in the fluorine-containing polymer (S').

[0060] [Pellets] Pellets obtained by the production method of the present disclosure (hereinafter also referred to as the present pellets) contain a fluoropolymer having a group that can be converted into an ion-exchange group (fluoropolymer (I')). When the present pellets are used to form an ion exchange membrane by melt extrusion, it is possible to form an ion exchange membrane in which the in-plane thickness unevenness of the resulting ion exchange membrane is suppressed. The reason for this is as described above.

[0061] The light transmittance of the pellets is preferably 30 to 60%, more preferably 30 to 50%, and even more preferably 30 to 40%. It is presumed that the pellets having a light transmittance within the above range have a roughened surface. Therefore, it is considered that the contact area between pellets is reduced, thereby preventing the pellets from sticking together. This reduces pressure fluctuations during film formation, resulting in an ion exchange membrane with excellent film thickness uniformity. The light transmittance of the pellets refers to the visible light transmittance (measured at a wavelength of 400 to 700 nm) measured using a luminous transmittance meter (manufactured by Asahi Spectroscopy, Model 304, or an equivalent device), and the specific measurement method is as follows.

[0062] First, the luminous transmittance meter is adjusted so that the visible light transmittance is 100% when the sample holder described below is not placed on the sample stage of the luminous transmittance meter. Next, a sample holder with a hole of a predetermined size (e.g., a rectangular hole 2-3 mm long and 2-3 mm wide) for fitting a pellet is placed on the sample stage, and the light intensity is adjusted so that the visible light transmittance before fitting the pellet into the hole in the sample holder is 25%. Next, a pellet of the same size as the hole in the sample holder is fitted into the hole in the sample holder, and the visible light transmittance is measured. The visible light transmittance of a pellet is measured at multiple locations on one pellet, and the arithmetic average value is calculated. For example, if the pellet is cylindrical, the pellet is fitted into the hole in the sample holder so that light is irradiated onto the side of the pellet, and the pellet is rotated 90 degrees in the circumferential direction. The visible light transmittance is measured at three locations on one pellet, and the arithmetic average value is calculated. Then, the visible light transmittance of the pellet is calculated by converting the visible light transmittance (25%) before the pellet is fitted into the hole of the sample holder to 100% (i.e., multiplying the measured visible light transmittance of the pellet by 4), and this is the light transmittance (%) of the pellet.

[0063] The degree of hydrolysis of the pellets is less than 1.00%, preferably less than 0.50%, more preferably less than 0.30%, and even more preferably less than 0.10%.

[0064] The fluoropolymer (I') contained in the present pellets is the same as the fluoropolymer (I') used in the above-mentioned present production method, and the preferred embodiments such as ion exchange capacity are also the same. The shape, size, surface condition, uses, etc. of the present pellets are also the same as those of the pellets obtained by the above-mentioned present production method.

[0065] [Ion exchange membrane] The ion exchange membrane obtained by using the present pellets (hereinafter also referred to as the present ion exchange membrane) will be explained. One example of a suitable method for producing the present ion exchange membrane is a method in which a precursor membrane containing a fluoropolymer having a group convertible to an ion exchange group (fluoropolymer (I')) is formed using the present pellets, and then the group convertible to an ion exchange group contained in the precursor membrane is converted to an ion exchange group to obtain the present ion exchange membrane containing a fluoropolymer having an ion exchange group (fluoropolymer (I)).

[0066] [Method for producing precursor film] An example of a method for producing a precursor film is an extrusion method. Specifically, the pellets are supplied to a known melt extruder for film production, and the melt of the pellets is extruded through a nozzle (e.g., a T-die) of the melt extruder to form a film, thereby obtaining a precursor film. That is, an example of a method for producing a precursor film is a melt extrusion method. The melting temperature of the pellets is preferably 150 to 350°C, and particularly preferably 200 to 300°C.

[0067] A reinforcing material may be embedded in the precursor membrane. The reinforcing material can be embedded in the precursor membrane by a known method. For example, when forming a multilayer ion exchange membrane, a method of sandwiching the reinforcing material between precursor membranes can be used. Alternatively, the reinforcing material can be embedded in the precursor membrane by coating both sides of the reinforcing material with a melt of the pellets.

[0068] Specific examples of the reinforcing material include reinforcing cloth (preferably woven cloth), fibril, and porous material, and among these, reinforcing cloth is preferred.

[0069] [Method for producing ion exchange membrane] The present ion exchange membrane containing the fluoropolymer (I) can be obtained by converting groups convertible to ion exchange groups in the fluoropolymer (I') contained in the precursor membrane into ion exchange groups. Specific examples of the method for converting groups convertible to ion exchange groups in the precursor membrane into ion exchange groups include a method of subjecting the precursor membrane to a treatment such as hydrolysis or acidification. Among these, a method of contacting the precursor membrane with an alkaline aqueous solution is preferred.

[0070] Specific examples of the method for contacting the precursor membrane with the alkaline aqueous solution include a method of immersing the precursor membrane in the alkaline aqueous solution and a method of spraying the alkaline aqueous solution onto the surface of the precursor membrane. From the viewpoint of productivity of the ion exchange membrane, the temperature of the alkaline aqueous solution is preferably 30° C. or higher and lower than 100° C., and the contact time between the precursor membrane and the alkaline aqueous solution is preferably 3 to 300 minutes.

[0071] The alkaline aqueous solution preferably contains an alkali metal hydroxide, a water-soluble organic solvent, and water. Specific examples of the alkali metal hydroxide include sodium hydroxide and potassium hydroxide, with potassium hydroxide being preferred. The alkali metal hydroxides may be used alone or in combination of two or more. In this specification, the water-soluble organic solvent refers to an organic solvent that is easily soluble in water. Specifically, an organic solvent having a solubility of 0.1 g or more in 1000 ml of water (20°C) is preferred, and an organic solvent having a solubility of 0.5 g or more is more preferred. The water-soluble organic solvent preferably contains at least one selected from the group consisting of aprotic organic solvents, alcohols, and aminoalcohols, and more preferably contains an aprotic organic solvent. The water-soluble organic solvents may be used alone or in combination of two or more.

[0072] 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.

[0073] The content of the alkali metal hydroxide in the alkaline aqueous solution is preferably 1 to 60 mass%. The content of the water-soluble organic solvent in the alkaline aqueous solution is preferably 1 to 60 mass%. When the contents of the alkali metal hydroxide and the water-soluble organic solvent are within the above ranges, the hydrolysis treatment is completed quickly, improving the productivity of the present ion exchange membrane. The content of water in the alkaline aqueous solution is preferably 39 to 80 mass%.

[0074] After the precursor membrane is brought into contact with the alkaline aqueous solution, a treatment for removing the alkaline aqueous solution may be carried out. For example, a method for removing the alkaline aqueous solution may be a method for washing the ion exchange membrane that has been brought into contact with the alkaline aqueous solution with water. After the precursor membrane is brought into contact with the alkaline aqueous solution, the resulting ion exchange membrane may be dried. Heat treatment is preferred as the drying treatment, and the heating temperature is preferably 50 to 160°C. The heating time is preferably 0.1 to 24 hours.

[0075] After converting the groups in the precursor membrane that can be converted to ion-exchange groups into ion-exchange groups, the ion-exchange membrane may be brought into contact with an aqueous solution containing potassium ions, sodium ions, or hydrogen ions to replace the counter ions (cations) of the ion-exchange groups. By replacing the cations of the ion-exchange groups with the same cations present in alkaline water, the ion-exchange membrane can be subjected to alkaline water electrolysis in an environment where the replaced cations are present, and the dimensional stability of the ion-exchange membrane is improved.

[0076] A hydrophilic layer may be formed on the surface of the precursor membrane or the present ion exchange membrane. The hydrophilic layer may be formed on at least one surface of the precursor membrane or the present ion exchange membrane. Specific examples of the hydrophilic layer include an inorganic particle layer containing inorganic particles. The inorganic particles preferably have excellent corrosion resistance against acids or alkalis and hydrophilicity. Specifically, at least one selected from the group consisting of oxides, nitrides, and carbides of Group 4 or Group 14 elements is preferred, and SiO ２ , SiC, ZrO ２ and ZrC, and ZrO ２is particularly preferred. The hydrophilic layer may contain a binder. As the binder, known binders used in known hydrophilic layers (gas release layers) can be used, such as methyl cellulose and fluorine-containing polymers having sulfonic acid groups. A specific example of a method for forming the hydrophilic layer is a method in which a solution containing inorganic particles and a binder is applied to the precursor membrane or the ion exchange membrane.

[0077] The present ion exchange membrane may be a single-layer or multi-layer structure. A multi-layer ion exchange membrane can be produced, for example, by using a precursor membrane obtained by laminating multiple layers made of a fluorine-containing polymer having groups convertible to ion-exchange groups by a co-extrusion method.

[0078] The thickness of the present 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 180 μm or less, from the viewpoint of improving current efficiency and voltage efficiency.

[0079] [Fluorine-containing polymer (I)] The fluoropolymer (I) is a fluoropolymer obtained by converting a group that can be converted into an ion-exchange group of the fluoropolymer (I') contained in the precursor membrane into an ion-exchange group. The fluoropolymer (I) is preferably a fluoropolymer having a carboxylic acid type functional group (hereinafter also referred to as fluoropolymer (C)) or a fluoropolymer having a sulfonic acid type functional group (hereinafter also referred to as fluoropolymer (S)) in terms of being able to more effectively exhibit the effects of the present disclosure. Each fluoropolymer will be described in detail below.

[0080] (Fluoropolymer (C)) The fluoropolymer (C) is preferably obtained by converting a group convertible to a carboxylic acid functional group in the above-mentioned fluoropolymer (C') into a carboxylic acid group. The fluoropolymer (C) preferably contains a unit based on a fluorine-containing olefin and a unit based on a monomer having a carboxylic acid functional group and a fluorine atom. Examples of the fluorine-containing olefin include those exemplified above. The unit based on the fluorine-containing olefin may contain one type alone, or two or more types.

[0081] As the unit based on a monomer having a carboxylic acid type functional group and a fluorine atom, a unit represented by the following formula (1C) is preferred: Formula (1C): -[CF ２ -CF((O) ｐ -(CF ２ ) ｑ -(CF ２ CFX) ｒ -(O) ｓ -(CF ２ ) ｔ -(CF ２ CFX') ｕ -COOM Ｃ ) ]- M Ｃ represents a hydrogen atom, an alkali metal, or a quaternary ammonium cation. X, X', p, q, r, s, t, and u are the same as in formula (1) above.

[0082] Specific examples of the unit represented by formula (1C) include the following units, and a compound in which p=1, q=0, r=1, s=0 to 1, t=0 to 3, and u=0 to 1 is preferred: -[CF ２ -CF(O-CF ２ CF ２ -COOM Ｃ ) ]-, -[CF ２ -CF(O-CF ２ CF ２ CF ２ -COOM Ｃ ) ]-, -[CF ２ -CF(O-CF ２ CF ２ CF ２ CF ２ -COOM Ｃ ) ]-, -[CF ２ -CF(O-CF ２ CF ２ -O-CF ２ CF ２ -COOM Ｃ ) ]-, -[CF ２ -CF(O-CF ２ CF ２ -O-CF ２ CF ２ CF ２ -COOM Ｃ ) ]-, -[CF ２-CF(O-CF ２ CF ２ -O-CF ２ CF ２ CF ２ CF ２ -COOM Ｃ ) ]-, -[CF ２ -CF(O-CF ２ CF ２ CF ２ -O-CF ２ CF ２ -COOM Ｃ ) ]-, -[CF ２ -CF(O-CF ２ CF (CF ３ )-O-CF ２ CF ２ -COOM Ｃ ) ]-, -[CF ２ -CF(O-CF ２ CF (CF ３ )-O-CF ２ CF ２ CF ２ -COOM Ｃ The unit based on a monomer having a carboxylic acid functional group and a fluorine atom may be contained alone or in combination of two or more.

[0083] The fluoropolymer (C) may contain units based on other monomers other than the units based on a fluorine-containing olefin and the units based on a monomer having a carboxylic acid functional group and a fluorine atom. Specific examples of the other monomers include those exemplified above. From the viewpoint of maintaining ion exchange performance, the content of the units based on other monomers is preferably 30 mass% or less based on all units in the fluoropolymer (C).

[0084] (Fluoropolymer (S)) The fluoropolymer (S) is preferably obtained by converting a group convertible to a sulfonic acid type functional group of the above-mentioned fluoropolymer (S') into a sulfonic acid group. The fluoropolymer (S) preferably contains a unit based on a fluorine-containing olefin and a unit based on a monomer having a sulfonic acid type functional group and a fluorine atom. Examples of the fluorine-containing olefin include those exemplified above. The unit based on the fluorine-containing olefin may contain one type alone or two or more types.

[0085] As the unit based on a monomer having a sulfonic acid functional group and a fluorine atom, a unit represented by formula (2S) is preferred. Formula (2S): -[CF ２ -CF(-L-(SO ３ M Ｓ ) ｎ )]-

[0086] In formula (2S), the definitions of L and n are the same as those in formula (2). Ｓ is a hydrogen atom, an alkali metal, or a quaternary ammonium cation. When n is 2, two M Ｓ may be the same as or different from each other.

[0087] The unit represented by formula (2S) is preferably a unit represented by formula (2S-1), a unit represented by formula (2S-2), a unit represented by formula (2S-3), or a unit represented by formula (2S-4). ２ -CF(-O-R ｆ１ -SO ３ M Ｓ )] - Formula (2S-2) - [CF ２ -CF(-R ｆ１ -SO ３ M Ｓ )]-

[0088]

[0089]

[0090] In formulas (2S-1) to (2S-4), R ｆ１ , R ｆ２ , R ｆ３The definitions of r and m are the same as those in the above formulas (2-1) to (2-4). Ｓ is a hydrogen atom, an alkali metal or a quaternary ammonium cation.

[0091] Specific examples of the unit represented by formula (2S-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. M in the formula Ｓ The definition of -[CF ２ -CF(-O-(CF ２ ) ｗ -SO ３ M Ｓ ) ]- -[CF ２ -CF(-O-CF ２ CF (CF ３ )-O-(CF ２ ) ｗ -SO ３ M Ｓ ) ]- -[CF ２ -CF(-(O-CF ２ CF (CF ３ )) ｘ -SO ３ M Ｓ )]-

[0092] Specific examples of the unit represented by formula (2S-2) include the following units: In the formula, w is an integer of 1 to 8. In the formula, M Ｓ The definition of -[CF ２ -CF(-(CF ２ ) ｗ -SO ３ M Ｓ ) ]- -[CF ２ -CF (-CF ２ -O-(CF ２ ) ｗ -SO ３ M Ｓ )]-

[0093] As the unit represented by formula (2S-3), a unit represented by formula (2S-3-1) is preferred. Ｓ The definition of is as described above.

[0094]

[0095] In formula (2S-3-1), R ｆ４ , R ｆ５ The definitions of and r are the same as in the above formula (2-3-1). Ｓ The definition of is as described above.

[0096] Specific examples of the unit represented by formula (2S-3) include the following.

[0097]

[0098] As the unit represented by formula (2S-4), a unit represented by formula (2S-4-1) is preferred. ｆ１ , R ｆ２ and M are defined as above.

[0099]

[0100] Specific examples of the unit represented by formula (2S-4-1) include the following.

[0101]

[0102] The unit based on a monomer having a sulfonic acid type functional group and a fluorine atom may be contained alone or in combination of two or more types.

[0103] The fluoropolymer (S) may contain units based on other monomers other than the units based on a fluorine-containing olefin and the units based on a monomer having a sulfonic acid functional group and a fluorine atom. Specific examples of the other monomers include those exemplified above. From the viewpoint of maintaining ion exchange performance, the content of the units based on other monomers is preferably 30 mass% or less based on all units in the fluoropolymer (S).

[0104] [Uses of Ion Exchange Membranes] Specific examples of uses of the present ion exchange membranes include various battery applications such as polymer electrolyte fuel cells, direct methanol fuel cells, redox flow batteries, and air batteries, as well as various electrolysis devices such as polymer electrolyte water electrolysis, alkaline water electrolysis, ozone water electrolysis, sodium chloride electrolysis, organic electrolysis, and chloride or oxide electrolysis. In addition to the above uses, the membranes can also be used as separators or solid electrodes in various types of electrochemical cells for selective cation transport at cell junctions. In addition to electrochemical applications, the membranes can also be 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 catalyst applications.

[0105] The present invention will be specifically described below using examples. However, the present invention is not limited to these examples. The blending amount of each component in the tables described below is based on mass. In the following, Examples 1 to 9 are working examples, and Examples 10 to 12 are comparative examples.

[0106] [Ion exchange capacity of fluoropolymer] A fluoropolymer having ion exchange groups was stored for 24 hours in a glove box flushed with dry nitrogen, 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 (meq / g resin) of the fluoropolymer.

[0107] [Production of Fluorine-Containing Polymer (S'-1)] CF ２ =CF ２ and a monomer (X1) represented by the following formula (X1) were copolymerized to obtain a fluoropolymer (S'-1) (ion exchange capacity: 1.00 meq / g resin). The blending ratio of each monomer was adjusted so that the ion exchange capacity of the fluoropolymer (S'-1) would be the above-mentioned value. CF ２ =CF-O-CF ２ CF (CF ３ )-O-CF ２ CF ２-SO ２ F (X1)

[0108] [Production of Fluorine-Containing Polymer (S'-2)] CF ２ =CF ２ and a monomer (X1) represented by the above formula (X1) were copolymerized to obtain a fluoropolymer (S'-2) (ion exchange capacity: 1.10 meq / g resin). The blending ratio of each monomer was adjusted so that the ion exchange capacity of the fluoropolymer (S'-2) would be the above value.

[0109] [Production of Fluorine-Containing Polymer (S'-3)] CF ２ =CF ２ and a monomer (X1) represented by the above formula (X1) were copolymerized to obtain a fluoropolymer (S'-3) (ion exchange capacity: 1.25 meq / g resin). The blending ratio of each monomer was adjusted so that the ion exchange capacity of the fluoropolymer (S'-3) would be the above value.

[0110] [Production of Fluorine-Containing Polymer (S'-4)] CF ２ =CF ２ and a monomer (X2) represented by the following formula (X2) were copolymerized to obtain a fluoropolymer (S'-4) (ion exchange capacity: 1.90 meq / g resin). The blending ratio of the respective monomers was adjusted so that the ion exchange capacity of the fluoropolymer (S'-4) would be the above-mentioned value.

[0111]

[0112] [Production of Fluorine-Containing Polymer (C'-1)] CF ２ =CF ２ and a monomer (Y1) represented by the following formula (Y1) were copolymerized to obtain a fluoropolymer (C'-1) (ion exchange capacity: 1.05 meq / g resin). The blending ratio of each monomer was adjusted so that the ion exchange capacity of the fluoropolymer (C'-1) would be the above value. CF ２ =CF-O-CF ２ CF ２ CF ２ -COOCH ３ (Y1)

[0113] The ion exchange capacities described in the above [Production of Fluoropolymer (S'-1)] to [Production of Fluoropolymer (S'-3)] and [Production of Fluoropolymer (C'-1)] represent the ion exchange capacities of fluoropolymers having ion exchange groups obtained when fluoropolymers (S'-1) to (S'-3) and (C'-1) are treated by the following procedure. First, a fluoropolymer having groups convertible to ion exchange groups, which has been vacuum heat-treated at 240°C and -0.1 MPaG for 16 hours, is immersed in a solution of dimethyl sulfoxide / potassium hydroxide / water = 30 / 5.5 / 64.5 (mass ratio) at 95°C for 30 minutes to hydrolyze the groups convertible to ion exchange groups in the fluoropolymer, converting them to K-type ion exchange groups, and then washed with water. Thereafter, the polymer is immersed in an aqueous sodium hydroxide solution to convert the terminal groups from K-type to Na-type, thereby obtaining a fluoropolymer having ion exchange groups for measuring the ion exchange capacity.

[0114] Example 1 Fluorine-containing polymer (C'-1) was supplied to a melt extruder for producing pellets to obtain a melt of fluoropolymer (C'-1). The obtained melt was extruded through a die heated to 190°C and cooled by blowing air to obtain strands (diameter 3.0 mm). Subsequently, the strands were cut to lengths of 3.0 mm to obtain pellets of fluoropolymer (C'-1). Cooling by blowing air was carried out under conditions where the linear velocity of the air was 2 m / sec. Air was blown over a range of 5 m toward the opposite side from the die, from the point where the strand was 10 cm after being extruded from the die. The linear velocity of the air was measured in advance before the extrusion and cooling. The linear velocity of the air was measured with an anemometer (vane anemometer, Testo 416) at the point where the strand was 10 cm after being extruded from the die, and the value used was the linear velocity of the air. The strand conveying speed was 2.5 m / min. The air blown in the above procedure was measured in advance to have a temperature of 25°C and a relative humidity of 61%. That is, the moisture content of the air was 14 g / m ３ In the table below, the moisture content of the air is expressed in g / m ３ It is written in units of .

[0115] [Measurement of Hydrolysis Rate] The obtained pellets were measured using a Fourier transform infrared spectrometer (Shimadzu Corporation, IRTracer-100) to determine the amount of groups that can be converted to ion exchange groups (-COOCH ３ ) derived from the peak (-CH ３ , 2883~3180cm －１ ) and a peak due to the ion exchange group (-COOH) (-OH, 3180 to 3450 cm －１ The area intensity of the ion exchange groups relative to the total area intensity of the groups that can be converted to ion exchange groups and the area intensity of the ion exchange groups was taken as the hydrolysis rate (unit: %). The hydrolysis rate is shown in Table 1 below.

[0116] Next, the pellets of the fluoropolymer (S'-1) were supplied to a melt extruder for film production and melted at 260°C to obtain a melt of pellets of the fluoropolymer (S'-1). The resulting melt was extruded through a T-die and formed into a film to obtain a precursor membrane made of the fluoropolymer (S'-1).

[0117] Next, the precursor membrane was immersed in a solution of dimethyl sulfoxide / potassium hydroxide / water = 30 / 5.5 / 64.5 (mass ratio) at 95 ° C for 30 minutes, and the groups in the precursor membrane that can be converted to sulfonic acid functional groups were hydrolyzed to convert them to K-type sulfonic acid functional groups, and then washed with water. The membrane was then immersed in an aqueous sodium hydroxide solution to convert the terminal groups from K-type to Na-type, and then dried to obtain an ion exchange membrane with a membrane thickness of 30 μm. The following evaluations were performed on the obtained ion exchange membrane.

[0118] [Ion Exchange Membrane Thickness Unevenness in the Plane Direction] The in-plane thickness unevenness of the ion exchange membrane obtained in the latter procedure was measured. The thickness unevenness in the plane direction of the obtained ion exchange membrane was measured at 100 arbitrary points within a 20 cm square area of ​​the obtained ion exchange membrane using a contact thickness meter, and the in-plane thickness unevenness of the ion exchange membrane was evaluated according to the following criteria. In practical terms, an A rating or a B rating is preferable, with an A rating being more preferable. A: The thickness measured at any of the 100 arbitrary points is within the range of 30 μm±2 μm at each point. B: The thickness measured at any of the 100 arbitrary points is not within the range of 30 μm±2 μm at some points, but is within the range of 30 μm±3 μm at all points. C: The thickness measured at any of the 100 arbitrary points is not within the range of 30 μm±3 μm at any point.

[0119] [Examples 2 to 9] Pellets and ion exchange membranes were prepared and the above measurements and evaluations were carried out in the same manner as in Example 1, except that one or more of the type of fluoropolymer used for producing pellets, the linear velocity of the air blown during pellet production, and the water content of the air were changed as shown in Table 1 below. The results are shown in Table 1. However, in the measurement of the hydrolysis rate, when the group that can be converted to an ion exchange group was -SO ２ In the case of F, the hydrolysis rate was determined by the following method. First, the number of ion-exchange groups was determined from the ion-exchange capacity determined by the above-mentioned method. Next, a predetermined amount of pellets was subjected to extraction treatment at 50°C for 16 hours using a 1M potassium chloride aqueous solution to obtain an extract. By the above extraction treatment, groups in the pellets that can be converted to ion-exchange groups (-SO ２ F) is hydrolyzed to form the ion exchange group (-SO ３ The polymer converted to ion exchange groups (-SO H) is extracted into the extract. The extract is titrated with an alkaline reagent to remove the ion exchange groups (-SO ３ The number of ion exchange groups (-SO H) was calculated. ３ The ratio of the number of H) was taken as the hydrolysis rate (unit: %).

[0120] [Examples 10 and 12] Pellets and ion exchange membranes were prepared in the same manner as in Example 1, except that the molten material was extruded through a die and cooled with water at 40°C or less, and the above measurements and evaluations were carried out. The results are shown in Table 1.

[0121] [Example 11] Pellets and an ion exchange membrane were prepared in the same manner as in Example 1, except that the molten material was extruded through a die and cooled in air without air blowing, and the above measurements and evaluations were carried out. The results are shown in Table 1.

[0122]

[0123] From the results shown in Table 1, in Examples 10 and 12 in which the strands were cooled with water, the hydrolysis rates were 5.00% and 1.00%, and the thickness unevenness in the in-plane direction of the resulting ion exchange membranes was large. Furthermore, in Example 11 in which cooling was performed without blowing air onto the strands, the hydrolysis rate was 0.03%, but the thickness unevenness in the in-plane direction of the resulting ion exchange membranes was large. On the other hand, in Examples 1 to 9 in which cooling was performed by blowing air onto the strands, it was confirmed that the thickness unevenness in the in-plane direction of the resulting ion exchange membranes was small. A comparison of Examples 1 and 2 with Example 3 reveals that the water content of the blown air was 22 g / m ３ It was confirmed that an ion exchange membrane with more suppressed thickness unevenness could be obtained when the linear velocity of the blown air was 5.0 to 50.0 m / sec (more preferably 10.0 to 50.0 m / sec, and even more preferably 20.0 to 50.0 m / sec).

[0124] The entire contents of the specification, claims and abstract of Japanese Patent Application No. 2023-198375 filed on November 22, 2023 are hereby incorporated by reference as the disclosure of the specification of the present invention.

Claims

1. A method for producing pellets, comprising extruding a melt containing a fluoropolymer having groups convertible to ion exchange groups through the die of a melt extruder to obtain strands containing the fluoropolymer, and then cutting the strands to obtain pellets containing the fluoropolymer, the method comprising an air-cooling treatment of blowing air against the strands extruded from the die to cool them.

2. The method for producing pellets according to claim 1, wherein the group that can be converted into an ion exchange group is a group that can be converted into a carboxylic acid type functional group or a group that can be converted into a sulfonic acid type functional group.

3. In the air cooling process, the moisture content of the air blown is 22 g / m ３ The method for producing pellets according to claim 1 or 2, wherein:

4. The method for producing pellets according to claim 1 or 2, wherein the linear velocity of the air blown is 5.0 to 50.0 m / sec.

5. The method for producing pellets according to claim 1 or 2, wherein the temperature of the air blown is 40°C or lower.

6. The method for producing pellets according to claim 1 or 2, wherein the ion exchange capacity of the fluoropolymer is from 0.9 meq / g resin to 2.00 meq / g resin.

7. The method for producing pellets according to claim 1 or 2, wherein the pellets are used for producing ion exchange membranes.

8. A method for producing an ion exchange membrane, comprising the steps of: producing pellets by the method for producing pellets according to claim 1 or 2; obtaining a precursor membrane by a melt extrusion method using the pellets; and obtaining an ion exchange membrane by using the precursor membrane.

9. Pellets produced by the method of claim 1 or 2, having a hydrolysis rate of less than 1.00%.

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