WO2025110152A1 - Method for producing pellets, method for producing ion exchange membrane, and pellets - Google Patents

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

Pellet manufacturing method, ion exchange membrane manufacturing method and pellet

This disclosure relates to a method for producing pellets, a method for producing ion exchange membranes, and pellets.

2. Description of the Related Art Ion exchange membranes containing fluorine-containing polymers having ion exchange groups are used in various batteries, electrolysis processes, and processes for separating ions and the like.
As a method for producing an ion exchange membrane containing a fluoropolymer, there is known a method using pellets of a fluoropolymer having an ion exchange group or a group that can be converted to an ion exchange group 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).

International Publication No. 2023/085421

The present inventors, referring to the technology described in Patent Document 1, formed an ion exchange membrane using pellets produced from water-cooled strands, and found that the resulting ion exchange membrane sometimes had uneven thickness in the in-plane direction.
Depending on the application of the ion exchange membrane to be manufactured, there are cases where the unevenness in the in-plane thickness of the ion exchange membrane is required to be small, and therefore there has been a demand for reducing the unevenness in the thickness.

The present disclosure has been made in consideration of the above 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 a melt extrusion method, an ion exchange membrane in which in-plane thickness unevenness is suppressed.
Another object of one embodiment of the present invention is to provide a method for producing an ion exchange membrane and a pellet.

The present disclosure includes the following aspects.
[1] A method for producing pellets, comprising extruding a melt containing a fluoropolymer having a group convertible into an ion-exchange group through a die of a melt extruder to obtain a strand containing the fluoropolymer, and then cutting the strand to obtain pellets containing the fluoropolymer, comprising the steps of:
The method for producing pellets includes an air-cooling treatment in which air is blown onto the strand extruded from the die to cool it.
[2] The method for producing pellets according to [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] The method for producing pellets according to [1] or [2], wherein in the air-cooling treatment, the water content of the air blown is 22 g/ ^m3 or less.
[4] The 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] The method for producing pellets according to any one of [1] to [4], wherein the temperature of the air blown is 40° C. or lower.
[6] The method for producing pellets according to any one of [1] to [5], 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 any one of [1] to [6], wherein the pellets are used for producing an ion exchange membrane.
[8] Producing pellets by the method for producing pellets according to any one of [1] to [7],
A precursor membrane is obtained by melt extrusion using the pellets,
A method for producing an ion exchange membrane, comprising obtaining an ion exchange membrane using the precursor membrane.
[9] Pellets having a hydrolysis rate of less than 1.00%, produced by the method according to any one of [1] to [8].

According to one embodiment of the present invention, there can be provided a method for producing pellets that can be used to produce, via a melt extrusion method, an ion exchange membrane in which thickness unevenness in the in-plane direction is suppressed.
According to one embodiment of the present invention, a method for producing an ion exchange membrane and a pellet can also be provided.

The following definitions of terms apply throughout the specification and claims, unless otherwise stated.
The term "ion exchange group" refers to a group capable of exchanging at least a portion of the ions contained in this group for other ions, and examples thereof include the following sulfonic acid type functional groups and carboxylic acid type functional groups.
The term "sulfonic acid functional group" refers to a sulfonic acid group (-SO ₃ H) or a sulfonate group. Examples of the form of the sulfonate group include (-SO ₃ ^- )Ma ⁺ , (-SO ₃ ^- ) _2Mb ²⁺ , and (-SO ₃ ^- ) _3Mc ³⁺ (where Ma ⁺ is an alkali metal ion or a quaternary ammonium cation, Mb ²⁺ is a divalent metal ion, and 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.
The term "carboxylic acid type functional group" refers to a carboxylic acid group (-COOH) or a carboxylate salt group. Examples of the form of the carboxylate salt group include ( ^-COO- )Ma ⁺ , (-COO-) _2Mb2 ⁺ , and ^{( -COO-} ) _3Mc3 ⁺ (where Ma ⁺ is an alkali metal ion or a quaternary ammonium cation, Mb2 ⁺ is a divalent metal ion, and Mc3 ⁺ 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.
A "precursor membrane" is a membrane that includes a polymer having groups that can be converted to ion-exchange groups.
The term "group that can be converted into an ion-exchange group" refers to a group that can be converted into an ion-exchange group by a known treatment such as hydrolysis or acidification.
The term "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 a known treatment such as hydrolysis treatment or acidification treatment.
The term "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 a known treatment such as hydrolysis or acidification.
The term "perfluorohydrocarbon group" refers to a hydrocarbon group in which all of the hydrogen atoms have been replaced with fluorine atoms.
The term "perfluoroaliphatic hydrocarbon group" means an aliphatic hydrocarbon group in which all of the hydrogen atoms have been replaced with fluorine atoms.

The term "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 processing the polymer obtained by the polymerization reaction. In the following, units derived from individual monomers will sometimes be referred to by the name of the monomer with "unit" added.

The term "reinforcing material" refers to a material used to improve the strength of the ion exchange membrane. As the reinforcing material, a material derived from a reinforcing cloth is preferable.
"Reinforcing fabric" refers to a fabric used as a raw material for a reinforcing material for improving the strength of an ion exchange membrane.
A numerical range expressed using "to" means a range including the numerical values described before and after "to" as the lower and upper limits. In addition, when the units of the lower and upper limits are the same, the unit for the lower limit may be omitted. 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. In addition, 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.

[Method of manufacturing 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 comprising extruding a melt containing a fluoropolymer having a group convertible to an ion-exchange group (hereinafter also referred to as 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 against the strand extruded through the die to cool it.

According to the present production method, when an ion exchange membrane is formed by melt extrusion, the resulting ion exchange membrane has less uneven thickness in the in-plane direction. Although the details of the reason for this are not clear, it is presumed to be due to the following reasons.
When the strands extruded from the 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 strands (parts that come into direct contact with water), and therefore it is believed that hydrolysis occurs non-uniformly in the produced pellets.
The present inventors have found that when such hydrolyzed pellets are used to form an ion exchange membrane by melt extrusion, the resulting ion exchange membrane has an uneven thickness. The reason for this uneven thickness is presumably that the hydrolyzed fluoropolymer portion and the non-hydrolyzed fluoropolymer portion in the pellets have different fluidity during melting, causing uneven thickness during melt extrusion.
On the other hand, according to the present production method, since the strands are cooled by air-cooling, it is considered that hydrolysis of groups that can be converted to ion-exchange groups is unlikely to occur. As a result, it is considered that thickness unevenness due to differences in fluidity is unlikely to occur during melt extrusion, and as a result, thickness unevenness in the in-plane direction of the obtained ion-exchange membrane is suppressed.
If air is not blown, cooling is not sufficient and the strands are stretched at a relatively high temperature, so that the strands are likely to deform and the tension is likely to fluctuate, making it difficult to maintain a constant strand thickness. If such strands are cut to produce pellets, the pellets will vary in volume, and the pellets may not be extruded at a constant rate when melted, resulting in uneven thickness in the in-plane direction of the obtained ion exchange membrane.

An example of this manufacturing method will be described below.
First, a fluoropolymer having a group that can be converted into an ion-exchange group is fed into a melt extruder to obtain a melt of the fluoropolymer.
The melt extruder may be a known device, and specific examples thereof include a single screw extruder, a twin screw extruder, and a tandem extruder.
The melting temperature of the fluoropolymer (I') is preferably from 150 to 350.degree. C., particularly preferably from 200 to 300.degree.

Then, the molten fluoropolymer (I') is extruded from a die at the tip of the melt extruder and cooled to obtain strands containing the fluoropolymer (I'), which are then cut to a predetermined size. In this way, pellets containing the fluoropolymer (I') are obtained.

When obtaining the 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 means that the linear velocity of the air is 0.3 m/sec or more, and in terms of making the strand thickness more uniform and reducing the unevenness in the film thickness, the linear velocity of the air is preferably 0.6 m/sec or more, more preferably 5.0 m/sec or more, even more preferably 10.0 m/sec or more, and particularly preferably 20.0 m/sec or more. The linear velocity of the air may be 30.0 m/sec or more. The upper limit of the linear velocity of the air is not particularly limited, but may be, for example, 50.0 m/sec or less, and may be 40.0 m/sec or less.
In the present disclosure, the linear velocity of the air is measured by an anemometer, and the detailed measurement method is in accordance with the method described in the Examples. Note that the linear velocity of the air is measured in advance in a state where the strand has not yet been extruded from the die.

The air-cooling treatment by blowing air is preferably carried out at least at a point that is 15 to 1 cm away from the die, more preferably 10 to 3 cm, and even more preferably 8 to 5 cm.
The range over which air is blown onto the strands is often 10 cm or more in the longitudinal direction of the strands, 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 strands is not particularly limited, but may be, for example, 20 m or less.
For example, the air cooling process in which air is blown onto the strand to cool it is preferably performed by blowing air onto the strand in the longitudinal direction thereof, since this is the distance range from the die.
The portion where air is blown onto the strands may be one portion, or may be divided into two or more portions. When air is blown onto the strands at two or more portions, the linear velocity of the air at each portion may be the same in some or all of the portions, or may be different from each other.

In 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 above-mentioned conveying speed refers to the distance that one point of the strand moves in one minute.

In addition, when performing the air-cooling treatment, the residence time of the strand in the area where the air-cooling treatment is performed (for example, the area where air is blown onto the strand) can also be appropriately adjusted. The residence time can be calculated by dividing the length of the area 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, and is often 10.0 minutes or less, preferably 7.0 minutes or less, and more preferably 5.0 minutes or less.

The air blown in the cooling process may be humidity-conditioned air.
The moisture content of the air is preferably 55 g/ ^m3 or less, more preferably 22 g/m3 or less, even more preferably 18 g/m3 or ^less , and particularly preferably 14 g/m3 or ^less , in order to further suppress unevenness in the thickness of the ion exchange membrane in the in-plane direction of the membrane obtained. The lower limit of the moisture content of the air is not particularly limited, but may be 3 g/ ^m3 or ^more .

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

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 less, even more preferably 260°C or less, particularly preferably 240°C or less, and most preferably 200°C or less.
The die temperature is preferably 160° C. or higher, more preferably 180° C. or higher, since this facilitates extrusion of the melt of the fluoropolymer (I') from the die.

The shape of the pellets obtained by the present production method is not particularly limited, and may be, for example, any shape such as a sphere (including an ellipsoid) or a columnar shape (for example, a cylindrical shape).
The size of the pellets obtained by this production method is not particularly limited, but for example, when the pellets are cylindrical, they preferably have a diameter of 2 to 3 mm and a length of 2 to 3 mm.

The surface of the pellets (strands) obtained by this production method preferably has a plurality of grooves formed thereon, which makes it possible to prevent the pellets from sticking together.
The grooves formed on the surface of the pellet may be formed over the entire surface of the pellet, but it is usually preferred that they are formed only on the side surface of the pellet (surfaces other than the cut surfaces of the strands).
The grooves formed on the surface of the pellet are preferably formed mainly along a direction intersecting the cut surface of the pellet (the flow direction of the strand).

In the present production method, after cutting the strands, a roughening treatment may be carried out to roughen the surfaces of the pellets, which makes the surfaces of the pellets more rough, and can further suppress adhesion between the pellets, thereby obtaining an ion exchange membrane with better stability in the electrolysis voltage.
A specific example of the surface roughening treatment method is a method in which pellets are stirred using a mixer (for example, a V blender).

The pellets obtained by this manufacturing method are preferably used to manufacture ion exchange membranes. Specific examples of uses for ion exchange membranes are described below.

[Fluorine-containing polymer (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.

The ion exchange capacity of the fluoropolymer (I') when the groups convertible to ion exchange groups of the fluoropolymer (I) are converted to 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 reducing the electrolysis voltage of a device incorporating an ion exchange membrane. Also, from the viewpoint of superior 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') which has been vacuum heat-treated at 240°C and -0.1 MPaG for 16 hours is immersed in a solution of dimethylsulfoxide/potassium hydroxide/water = 30/5.5/64.5 (mass ratio) at 95°C for 30 minutes to hydrolyze groups which can be converted into ion exchange groups in the fluoropolymer (I') and convert them into 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 the fluoropolymer (I) for measuring the ion exchange capacity.
The ion exchange capacity of the thus obtained fluoropolymer (I) can be measured by a method as described later in the Examples section.

The fluoropolymer (I') may be used alone or in combination of two or more kinds.
The fluoropolymer (I') is preferably a fluoropolymer having a group which can be converted into a carboxylic acid type functional group (hereinafter also referred to as fluoropolymer (C')), or a fluoropolymer having a group which can be converted into a sulfonic acid type functional group (hereinafter also referred to as fluoropolymer (S')), in terms of enabling the effects of the present disclosure to be more effectively exhibited.
Each of the fluoropolymers will be described in detail below.

(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 type functional group (hereinafter also referred to as fluorine-containing monomer (C')), since the effects of the present disclosure can be more effectively exhibited.
The copolymerization method may be a known method such as solution polymerization, suspension polymerization, or emulsion polymerization.

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.
The fluorine-containing monomer (C') is preferably a monomer represented by the following formula (1) from the viewpoints of the production cost of the monomer, the reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer.

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

In formula (1), X and X' are each independently a fluorine atom or a trifluoromethyl group. A ¹ is a group that can be converted into a carboxylic acid type functional group. Specific examples include -CN, -COF, -COOR ¹ (R ¹ is an alkyl group having 1 to 10 carbon atoms), and -COONR 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.

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 ,
CF ₂ =CF-O-CF ₂ CF ₂ CF ₂ -COOCH ₃ ,
CF ₂ =CF-O-CF ₂ CF ₂ CF ₂ CF ₂ -COOCH ₃ ,
CF ₂ =CF-O-CF ₂ CF ₂ -O-CF ₂ CF ₂ -COOCH ₃ ,
CF ₂ =CF-O-CF ₂ CF ₂ -O-CF ₂ CF ₂ CF ₂ -COOCH ₃ ,
CF ₂ =CF-O-CF ₂ CF ₂ -O-CF ₂ CF ₂ CF ₂ CF ₂ -COOCH ₃ ,
CF ₂ =CF-O-CF ₂ CF ₂ CF ₂ -O-CF ₂ CF ₂ -COOCH ₃ ,
CF ₂ =CF-O-CF ₂ CF(CF ₃ )-O-CF ₂ CF ₂ -COOCH ₃ ,
_CF2 =CF-O- _{CF2CF ( CF3} )-O- _{CF2CF2CF2 - COOCH3} .
The fluorine-containing monomer (C') may be used alone or in combination of two or more kinds.

The fluorine-containing olefin may be a fluoroolefin having 2 to 3 carbon atoms and having one or more fluorine atoms in the molecule. Specific examples thereof include tetrafluoroethylene (TFE), chlorotrifluoroethylene, vinylidene fluoride, vinyl fluoride, and hexafluoropropylene. Among them, 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.
The fluorine-containing olefins may be used alone or in combination of two or more kinds.

In the production of the fluorine-containing polymer (C'), in addition to the fluorine-containing monomer (C') and the fluorine-containing olefin, other monomers may be used. Specific examples of the 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), and 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.
The content of units based on other monomers is preferably at most 30 mass % based on all units in the fluoropolymer (C') from the viewpoint of maintaining the ion exchange performance.

(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')), since the effects of the present disclosure can be more effectively exhibited.
The copolymerization method may be a known method such as solution polymerization, suspension polymerization, or emulsion polymerization.

The fluorine-containing olefin may be any of those exemplified above, with TFE being preferred from the viewpoints of monomer production cost, reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer (S').
The fluorine-containing olefins may be used alone or in combination of two or more kinds.
The content of units based on fluorine-containing olefin based on all units contained in the fluorine-containing polymer (S') is preferably from 65 to 95 mol %.

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 type functional group.
As the fluorine-containing monomer (S'), a compound represented by formula (2) is preferred from the viewpoints of the production cost of the monomer, the reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer (S').
Formula (2) CF ₂ =CF-L-(A) _n

L is an (n+1) valent perfluorohydrocarbon group which may contain an oxygen atom.
The oxygen atoms may be located at the terminal ends or between the carbon atoms in the perfluorohydrocarbon group.
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.

As L, a perfluoroaliphatic hydrocarbon group having a valence of n+1 which may contain an oxygen atom is preferred, and a divalent perfluoroalkylene group having an aspect of n=1 which may contain an oxygen atom, or a trivalent perfluoroaliphatic hydrocarbon group having an aspect of n=2 which may contain an oxygen atom is more preferred. The divalent perfluoroalkylene group may be either linear or branched.

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 the group that can be converted into a sulfonic acid type functional group include -SO ₂ F, -SO ₂ Cl, and -SO ₂ Br. When n is 2, the two As may be the same or different.

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

R ^f1 is a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.
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.
r is 0 or 1.
In the formula, R ^f2 is a single bond or a perfluoroalkylene group which may contain an oxygen atom between the carbon atoms.
The definition of A in the formula is as described above.

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.

r is 0 or 1.
m is 0 or 1.

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 F
CF ₂ =CF-O-CF ₂ CF(CF ₃ )-O-(CF ₂ ) _w -SO ₂ F
CF ₂ =CF-[O-CF ₂ CF(CF ₃ )] _x -SO ₂ F

Specific examples of the compound represented by formula (2-2) include the following compounds:
CF ₂ =CF-(CF ₂ ) _w -SO ₂ F
CF ₂ =CF-CF ₂ -O-(CF ₂ ) _w -SO ₂ F

The compound represented by formula (2-3) is preferably a compound represented by formula (2-3-1). In the formula, R ^f4 is a linear perfluoroalkylene group having 1 to 6 carbon atoms, and R ^f5 is a single bond or a linear perfluoroalkylene group having 1 to 6 carbon atoms which may contain an oxygen atom between the carbon atoms. The definitions of r and A in the formula are as described above.

Specific examples of compounds represented by formula (2-3-1) include the following:

As the compound represented by formula (2-4), the compound represented by formula (2-4-1) is preferred.

In the formula, R ^f1 , R ^f2 and A are defined as above.

Specific examples of compounds represented by formula (2-4-1) include the following:

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

[pellet]
The pellets obtained by the production method of the present disclosure (hereinafter also referred to as the present pellets) contain a fluoropolymer (fluoropolymer (I')) having a group that can be converted into an ion-exchange group.
When the 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.

The light transmittance of the pellets is preferably 30 to 60%, more preferably 30 to 50%, and even more preferably 30 to 40%. The pellets having a light transmittance within the above range are presumed to have a roughened surface. This reduces the contact area between the pellets, which is believed to suppress adhesion between the pellets. This suppresses pressure fluctuations during film molding, and provides an ion exchange membrane with excellent uniformity in film thickness.
The light transmittance of the pellets means the visible light transmittance (measured at a wavelength of 400 to 700 nm) measured using a visual transmittance meter (manufactured by Asahi Spectroscopy, MODEL 304 or an equivalent device), and the specific measurement method is as follows.

First, the luminous transmittance meter is adjusted so that the visible light transmittance is 100% when no sample holder described below is placed on the sample stage of the luminous transmittance meter. Next, a sample holder with a hole of a predetermined size for fitting a pellet (e.g., a rectangular hole 2 to 3 mm long and 2 to 3 mm wide) is placed on the sample stage, and the light intensity is adjusted so that the visible light transmittance is 25% before the pellet is fitted into the hole of the sample holder.
Next, a pellet of the same size as the hole of the sample holder is fitted into the hole of the sample holder, and the visible light transmittance is measured. The visible light transmittance of the pellet is measured at multiple points for one pellet, and the arithmetic average value is calculated. For example, if the pellet is cylindrical, the pellet is fitted into the hole of 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, and the visible light transmittance is measured at three points for one pellet, and the arithmetic average value is calculated.
Then, the visible light transmittance of the pellet is calculated when the visible light transmittance (25%) before the pellet is fitted into the hole in the sample holder is converted to 100% (i.e., the measured visible light transmittance of the pellet is multiplied by 4), and this is regarded as the light transmittance (%) of the pellet.

The hydrolysis rate 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%.

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

[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 described.
An example of a suitable method for producing the present ion exchange membrane includes a method in which a precursor membrane containing a fluoropolymer having a group that can be converted into an ion exchange group (fluoropolymer (I')) is formed using the present pellets, and then the group that can be converted into an ion exchange group, which is contained in the precursor membrane, is converted into an ion exchange group to obtain the present ion exchange membrane containing a fluoropolymer having an ion exchange group (fluoropolymer (I)).

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

The precursor membrane may have a reinforcing material embedded therein. The reinforcing material can be embedded in the precursor membrane by known methods. For example, when forming a multi-layered ion exchange membrane, the reinforcing material can be sandwiched between the precursor membranes. The reinforcing material can also be embedded in the precursor membrane by coating both sides of the reinforcing material with a melt of the pellets.

Specific examples of reinforcing materials include reinforcing cloth (preferably woven cloth), fibrils, and porous bodies, with reinforcing cloth being preferred among these.

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

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

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, and potassium hydroxide is preferred. The alkali metal hydroxide may be used alone or in combination of two or more kinds.
In this specification, the water-soluble organic solvent is an organic solvent that is easily dissolved in water, and specifically, the solubility in 1000 ml of water (20° C.) is preferably 0.1 g or more, and more preferably 0.5 g or more. 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 solvent may be used alone or in combination of two or more.

Specific examples of the aprotic organic solvent 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 amino alcohols 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.

The content of the alkali metal hydroxide in the alkaline aqueous solution is preferably 1 to 60% by mass.
The content of the water-soluble organic solvent in the alkaline aqueous solution is preferably from 1 to 60% by 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, and the productivity of the present ion exchange membrane is improved.
The content of water in the alkaline aqueous solution is preferably 39 to 80% by mass.

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, the alkaline aqueous solution may be removed by 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 subjected to a drying treatment. The drying treatment is preferably a heat treatment, and the heating temperature is preferably 50 to 160° C. The heating time is preferably 0.1 to 24 hours.

After the groups in the precursor membrane that can be converted to ion exchange groups are converted to ion exchange groups, the ion exchange membrane may be contacted 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 membrane can be subjected to alkaline water electrolysis in an environment in which the replaced cations are present, improving the dimensional stability of the ion exchange membrane.

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 of the surfaces of the precursor membrane or the present ion exchange membrane.
A specific example of the hydrophilic layer is an inorganic particle layer containing inorganic particles. The inorganic particles are preferably excellent in corrosion resistance against acid or alkali and have hydrophilicity. Specifically, at least one selected from the group consisting of oxides, nitrides and carbides of Group 4 elements or Group 14 elements is preferable, at least one selected from the group consisting of SiO ₂ , SiC, ZrO ₂ and ZrC is more preferable, and ZrO ₂ is particularly preferable.
The hydrophilic layer may contain a binder. As the binder, any known binder used in known hydrophilic layers (gas releasing 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.

The ion exchange membrane may be a single layer or a multilayer structure. A multilayer ion exchange membrane can be produced, for example, by using a precursor membrane obtained by laminating multiple layers of a fluorine-containing polymer having groups that can be converted into ion exchange groups by a co-extrusion method.

The thickness of the ion exchange membrane is preferably 30 μm or more, more preferably 40 μm or more, in order to maintain a certain strength, and is preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 180 μm or less, in order to increase current efficiency and voltage efficiency.

[Fluoropolymer (I)]
The fluoropolymer (I) is a fluoropolymer obtained by converting groups that can be converted into ion-exchange groups of the fluoropolymer (I') contained in the precursor membrane into ion-exchange groups.
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)) from the viewpoint of enabling the effects of the present disclosure to be more effectively exhibited.
Each of the fluoropolymers will be described in detail below.

(Fluorine-containing polymer (C))
The fluoropolymer (C) is preferably obtained by converting a group of the above-mentioned fluoropolymer (C') which can be converted into a carboxylic acid type functional group, into a carboxylic acid group.
The fluoropolymer (C) preferably contains units based on a fluorine-containing olefin and units based on a monomer having a carboxylic acid type functional group and a fluorine atom.
The fluorine-containing olefin may be any of those exemplified above.
The fluorine-containing olefin-based unit may be contained in one type alone or in two or more types.

As the unit based on a monomer having a carboxylic acid 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 is 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.

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=0-1, t=0-3, and u=0-1 is preferred.
-[CF2 _- CF(O- _{CF2CF2 -} ^COOMC )]-,
-[CF2 _- CF( _O - _{CF2CF2CF2 -} ^COOMC )]-,
- _[ CF2 _- CF(O _{- CF2CF2CF2CF2CF2 -} ^COOMC )]-,
- _[ CF2 _- CF(O- _CF2CF2 - _{O- CF2CF2-} ^COOMC )]-,
-[CF2 _- CF ₍ O- _CF2CF2 - _{O -} CF2CF2CF2 _- ^COOMC )]-,
-[CF2 _- CF(O- _CF2CF2 - _{O -} CF2CF2CF2CF2CF2 _- ^COOMC ) _] - _,
- [CF2 _- CF ₍ O- _CF2CF2CF2 -O _- CF2CF2 _- ^COOMC )]-,
-[CF2 _- CF(O- _CF2CF ( _CF3 )-O- _{CF2CF2 -} ^COOMC )]-,
-[CF2 _- CF(O- _CF2CF ( _CF3 )-O _- CF2CF2CF2 _- ^COOMC ) _] -.
The unit based on a monomer having a carboxylic acid type functional group and a fluorine atom may be contained alone or in combination with two or more kinds.

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 type functional group and a fluorine atom.
Specific examples of the other monomers include those exemplified above. The content of units based on other monomers is preferably 30 mass% or less based on all units in the fluoropolymer (C) from the viewpoint of maintaining the ion exchange performance.

(Fluorine-containing polymer (S))
The fluoropolymer (S) is preferably obtained by converting a group of the above-mentioned fluoropolymer (S') which can be converted into a sulfonic acid type functional group into a sulfonic acid group.
The fluoropolymer (S) preferably contains units based on a fluorine-containing olefin and units based on a monomer having a sulfonic acid type functional group and a fluorine atom.
The fluorine-containing olefin may be any of those exemplified above.
The fluorine-containing olefin-based unit may be contained in one type alone or in two or more types.

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

In formula (2S), the definitions of L and n are the same as those in formula (2) above.
^MS is a hydrogen atom, an alkali metal or a quaternary ammonium cation. When n is 2, the two ^MS may be the same or different.

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).
Formula (2S-1) -[CF ₂ -CF(-O-R ^f1 -SO ₃ M ^S )]-
Formula (2S-2) -[CF ₂ -CF(-R ^f1 -SO ₃ M ^S )]-

In formulae (2S-1) to (2S-4), the definitions of R ^f1 , R ^f2 , R ^f3 , r and m are the same as those in formulae (2-1) to (2-4) above.
M 2 ^S is a hydrogen atom, an alkali metal or a quaternary ammonium cation.

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. The definitions of M and ^S in the formula are as described above.
-[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 )]-

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. The definitions of M and ^S in the formula are as described above.
-[CF ₂ -CF(-(CF ₂ ) _w -SO ₃ M ^S )]-
-[CF ₂ -CF(-CF ₂ -O-(CF ₂ ) _w -SO ₃ M ^S )]-

The unit represented by formula (2S-3) is preferably a unit represented by formula (2S-3-1), in which M and ^S are defined as above.

In formula (2S-3-1), the definitions of R ^f4 , R ^f5 and r are the same as those in formula (2-3-1) above. The definition of ^{M 2 S} is as described above.

Specific examples of units represented by formula (2S-3) include the following:

The unit represented by formula (2S-4) is preferably a unit represented by formula (2S-4-1), in which R ^f1 , R ^f2 and M are as defined above.

Specific examples of units represented by formula (2S-4-1) include the following:

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

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 type functional group and a fluorine atom.
Specific examples of the other monomers include those exemplified above. The content of units based on other monomers is preferably 30 mass% or less based on all units in the fluoropolymer (S) from the viewpoint of maintaining the ion exchange performance.

[Uses of ion exchange membranes]
Specific examples of the use of the present ion exchange membrane include various battery applications such as solid polymer fuel cells, direct methanol fuel cells, redox flow batteries, and air batteries, as well as various electrolysis devices such as solid polymer water electrolysis, alkaline water electrolysis, ozone water electrolysis, salt electrolysis, organic electrolysis, chloride or oxide electrolysis, etc. In addition to the above applications, the membrane can be used as a separator or solid electrode in various types of electrochemical cells for selective cation transport at the binding portion of the cell. In addition to electrochemical applications, the membrane can be used for sensor applications such as various gas sensors, biosensors, light-emitting devices, optical devices, organic sensors, and carbon nanotube solubilization, actuators, and catalyst applications.

The present invention will be specifically described below with reference to 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.

[Ion exchange capacity of fluoropolymer]
A fluoropolymer having an ion exchange group was stored for 24 hours in a glove box in which dry nitrogen was flowed, and the dry mass of the fluoropolymer was measured.Then, 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.The liquid 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.

[Production of Fluorine-Containing Polymer (S'-1)]
CF ₂ ═CF ₂ was copolymerized with a monomer (X1) represented by the following formula (X1) 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) was the above value.
CF ₂ =CF-O-CF ₂ CF (CF ₃ )-O-CF ₂ CF ₂ -SO ₂ F (X1)

[Production of Fluoropolymer (S'-2)]
CF ₂ ═CF ₂ was copolymerized with the monomer (X1) represented by the above formula (X1) 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) was the above value.

[Production of Fluorine-Containing Polymer (S'-3)]
CF ₂ ═CF ₂ was copolymerized with the monomer (X1) represented by the above formula (X1) 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) was the above value.

[Production of Fluoropolymer (S'-4)]
CF ₂ ═CF ₂ was copolymerized with a monomer (X2) represented by the following formula (X2) to obtain a fluoropolymer (S'-4) (ion exchange capacity: 1.90 meq/g resin). The blending ratio of each monomer was adjusted so that the ion exchange capacity of the fluoropolymer (S'-4) was the above value.

[Production of Fluoropolymer (C'-1)]
CF ₂ ═CF ₂ was copolymerized with a monomer (Y1) represented by the following formula (Y1) 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) was the above-mentioned value.
CF ₂ =CF-O-CF ₂ CF ₂ CF ₂ -COOCH ₃ (Y1)

The ion exchange capacity described in the above [Production of fluoropolymer (S'-1)] to [Production of fluoropolymer (S'-3)] and [Production of fluoropolymer (C'-1)] represents the ion exchange capacity of the fluoropolymer having ion exchange groups obtained by treating the fluoropolymers (S'-1) to (S'-3) and (C'-1) according to the following procedure. First, a fluoropolymer having groups that can be converted 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 in the fluoropolymer that can be converted to ion exchange groups and convert them to K-type ion exchange groups, and then washed with water. Then, the polymer is immersed in an aqueous sodium hydroxide solution to convert the terminal groups from K-type to Na-type to obtain a fluoropolymer having ion exchange groups for measuring the ion exchange capacity.

[Example 1]
The fluoropolymer (C'-1) was fed to a melt extruder for producing pellets to obtain a melt of the fluoropolymer (C'-1). The melt was extruded through a die heated to 190°C and cooled by blowing air to obtain a strand (diameter 3.0 mm). The strand was then cut to a length of 3.0 mm to obtain pellets of the fluoropolymer (C'-1).
The cooling by blowing air was performed under the condition that the linear velocity of the air was 2 m/sec. The air was blown in a range of 5 m from the point where the strand was 10 cm from the extrusion from the die toward the opposite side 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 type anemometer, Testo 416) at the point where the strand was 10 cm from the extrusion from the die.
The strand transport speed was 2.5 m/min.
The air to be blown in the above procedure was previously measured to have a temperature of 25° C. and a relative humidity of 61%. In other words, the moisture content of the air was 14 g/m ^3. In the tables below, the moisture content of the air is given in units of g/m ³ .

[Measurement of hydrolysis rate]
The obtained pellets were measured using a Fourier transform infrared spectrometer (IRTracer-100, manufactured by Shimadzu Corporation) to calculate the integrated intensity of the peak ( _-CH , 2883 to 3180 _cm ⁾ derived from a group that can be converted to an ion exchange group (-COOCH) and the peak (-OH, 3180 to 3450 ^cm ).
The surface intensity of the ion exchange group relative to the total surface intensity of the ion exchange group and the surface intensity of the groups that can be converted to ion exchange groups was taken as the hydrolysis rate (unit: %). The hydrolysis rate is shown in Table 1 below.

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

The precursor membrane was then 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 groups in the precursor membrane that could be converted to sulfonic acid functional groups, converting 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 carried out on the obtained ion exchange membrane.

[Unevenness in thickness of ion exchange membrane in the in-plane direction]
The thickness unevenness in the in-plane direction of the ion exchange membrane obtained in the latter step was measured. The thickness unevenness in the in-plane direction of the ion exchange membrane was measured at 100 arbitrary points in a 20 cm square area of the obtained ion exchange membrane using a contact type thickness meter, and the thickness unevenness in the in-plane direction of the ion exchange membrane was evaluated according to the following criteria. Note that, in practical terms, an A rating or a B rating is preferable, and an A rating is more preferable.
A: The thickness measured at any 100 points is within the range of 30 μm ± 2 μm at all points. B: The thickness measured at any 100 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 100 points is not within the range of 30 μm ± 3 μm at any point.

[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 into an ion exchange group was _--SO.sub.2F , 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 1M potassium chloride aqueous solution to obtain an extract. By the above-mentioned extraction treatment, a polymer in which a group (—SO ₂ F) that can be converted to an ion exchange group is hydrolyzed and converted to an ion exchange group (—SO ₃ H) in the pellets is extracted into the extract. The above-mentioned extract was titrated with an alkaline reagent to calculate the number of ion exchange groups (—SO ₃ H).
The ratio of the number of ion exchange groups ( _--SO.sub.3H ) to the number of ion exchange groups determined by the above procedure was taken as the hydrolysis rate (unit: %).

[Examples 10 and 12]
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 with water at 40° C. or less, and the above measurements and evaluations were carried out. The results are shown in Table 1.

[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 blowing air thereon, and the above measurements and evaluations were carried out. The results are shown in Table 1.

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 resulting ion exchange membranes had large in-plane thickness variations.
In addition, in Example 11 in which cooling was performed without blowing air onto the strands, the hydrolysis rate was 0.03%, but the resulting ion exchange membrane had large in-plane thickness unevenness.
On the other hand, in Examples 1 to 9 in which the strands were cooled by blowing air against them, it was confirmed that the unevenness in the thickness of the resulting ion exchange membrane in the in-plane direction was reduced.
From a comparison between Examples 1, 2 and 3, it was confirmed that when the moisture content of the blown air was 22 g/ ^m3 or less, an ion exchange membrane with more suppressed unevenness in membrane thickness was obtained.
From a comparison between Example 2 and Example 4, it was confirmed that when the linear velocity of the blown air is 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), an ion exchange membrane with more suppressed unevenness in membrane thickness can be obtained.

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 (9)

A method for producing pellets, comprising extruding a melt containing a fluoropolymer having a group convertible to an ion-exchange group through a die of a melt extruder to obtain a strand containing the fluoropolymer, and then cutting the strand to obtain pellets containing the fluoropolymer, comprising the steps of:
The method for producing pellets includes an air-cooling treatment in which air is blown onto the strand extruded from the die to cool it. 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. The method for producing pellets according to claim 1 or 2, wherein the water content of the air blown in the air cooling treatment is 22 g/ ^m3 or less. The method for producing pellets according to claim 1 or 2, wherein the linear velocity of the air being blown is 5.0 to 50.0 m/sec. The method for producing pellets according to claim 1 or 2, wherein the temperature of the air blown is 40°C or less. The method for producing pellets according to claim 1 or 2, wherein the ion exchange capacity of the fluoropolymer is 0.9 meq/g resin or more and 2.00 meq/g resin or less. The method for producing pellets according to claim 1 or 2, wherein the pellets are used to produce an ion exchange membrane. Pellets are produced by the method for producing pellets according to claim 1 or 2,
A precursor membrane is obtained by a melt extrusion method using the pellets,
A method for producing an ion exchange membrane, comprising obtaining an ion exchange membrane using the precursor membrane. 　Pellets having a hydrolysis rate of less than 1.00%, produced by the method according to claim 1 or 2.