Method for producing fluorine-containing polymer

Abstract

Provided is a method for producing a fluorine-containing polymer capable of producing a fluorine-containing polymer having excellent melt elongation and having low ion exchange capacity when converted into acid form.　A method for producing a fluorine-containing polymer according to the present invention comprises copolymerizing TFE and a monomer m1 in a reaction vessel to obtain a fluorine-containing polymer F1, and then heating the fluorine-containing polymer F1 at 250°C or higher to obtain a fluorine-containing polymer F2. The fluorine-containing polymer F1 has a TFE unit / m1 unit (molar ratio) of 6.5-12.5.　Formula (m1): CF2=CF-O-Rf1-A. In formula (m1), Rf1 is a perfluoroalkylene group which may contain an oxygen atom between carbon atoms, and A is a group which can be converted into a sulfonic acid type functional group.

Description

Method for producing fluorine-containing polymer

[0001] The present invention relates to a method for producing a fluorine-containing polymer.

[0002] The ion exchange membrane (electrolyte membrane) of a polymer electrolyte fuel cell or a water electrolysis device is obtained by forming a fluoropolymer having an ion exchange group such as a sulfonic acid group into a membrane. As a method for producing such a fluoropolymer having an ion exchange group, Example 1 of Patent Document 1 describes a method for producing a fluoropolymer containing tetrafluoroethylene and CF ２ = CFOCF ２ CF (CF ３ ) OCF ２ CF ２ SO ２ A monomer represented by F is copolymerized in the presence of a radical polymerization initiator at a temperature of 100 to 200°C, and then -SO ２ A method is disclosed in which the F group is hydrolyzed to convert it into an acid form and into a sulfonic acid group.

[0003] International Publication No. 2016 / 104379

[0004] The sulfonic acid group-containing fluoropolymer obtained by the method described in Example 1 of Patent Document 1 has a high ion exchange capacity, but in recent years, depending on the application, there has been a demand for a fluoropolymer having a low ion exchange capacity when converted to an acid form. The present inventors produced a fluoropolymer that has a low ion exchange capacity when converted to an acid form by referring to the method described in Example 1 of Patent Document 1, and found that there is room for improvement in the melt elongation of the obtained fluoropolymer.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing a fluoropolymer which can produce a fluoropolymer which has a low ion exchange capacity when converted to an acid form and is excellent in melt elongation.

[0006] As a result of intensive investigations into the above-mentioned problems, the present inventors have found that the desired effects can be obtained by subjecting a fluoropolymer in which the content of units based on tetrafluoroethylene and the content of units based on a monomer represented by formula (m1) described below are in a predetermined molar ratio to a heat treatment at a predetermined temperature or higher, and have arrived at the present invention.

[0007] That is, the inventors have found that the above problems can be solved by the following constitution: [1] A method for producing a fluoropolymer, comprising copolymerizing tetrafluoroethylene with a monomer m1 represented by formula (m1) in a reaction vessel to obtain a fluoropolymer F1 having units based on the tetrafluoroethylene and units based on the monomer m1, and then heating the fluoropolymer F1 at 250°C or higher to obtain a fluoropolymer F2, characterized in that the molar ratio of the content of units based on tetrafluoroethylene to the content of units based on monomer m1 in the fluoropolymer F1 is 6.5 to 12.5. ２ =CF-O-R ｆ１ -A In formula (m1), R ｆ１ is a perfluoroalkylene group which may contain an oxygen atom between the carbon atoms, and A is a group which can be converted into a sulfonic acid type functional group. [2] A method for producing a fluoropolymer according to [1], wherein the fluoropolymer F1 is cooled after being heated. [3] A method for producing a fluoropolymer according to [2], wherein the cooling rate in the cooling is 100°C / min or more. [4] A method for producing a fluoropolymer according to any of [1] to [3], wherein the copolymerization is carried out under conditions substantially in the absence of a chain transfer agent. [5] A method for producing a fluoropolymer according to any of [1] to [4], wherein the heating temperature of the fluoropolymer F1 is 300°C or more. [6] A method for producing a fluoropolymer according to any of [1] to [5], wherein the fluoropolymer is used for producing an ion exchange membrane of an electrolytic hydrogenation apparatus used in the electrolytic hydrogenation of an aromatic compound. [7] A method for producing a fluoropolymer according to any of [1] to [5], wherein the fluoropolymer is 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 or CF ２ =CF-[O-CF２ CF (CF ３ )] ｖ -SO ２

[10] The method for producing a fluoropolymer according to any of [1] to [6], wherein the molar ratio is F. (In the formula, w is an integer of 1 to 8, and v is an integer of 1 to 5.) [8] The method for producing a fluoropolymer according to any of [1] to [7], wherein the molar ratio is 7.5 or more and 11.5 or less. [9] The method for producing a fluoropolymer according to any of [2] to [8], wherein the cooling is carried out until the temperature of the fluoropolymer F2 reaches 100°C or less.

[0008] According to the present invention, there can be provided a method for producing a fluoropolymer which can produce a fluoropolymer which has a low ion exchange capacity when converted to an acid form and is excellent in melt elongation.

[0009] The following definitions of terms apply throughout the present specification and claims unless otherwise specified. A "sulfonic acid functional group" refers to 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. 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 known treatments such as hydrolysis treatment and acidification treatment.

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

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

[0012] "Total amount of A added" means the total amount of A added into the reactor to be used in polymerizing the polymer, and for example, when A is added into the reactor before and during polymerization, it means the total amount of A added into the reactor before polymerization and A added into the reactor during polymerization. Note that "A" is a component used in polymerization, and examples thereof include TFE, monomer m1, or a radical polymerization initiator, which will be described later.

[0013] [Method for producing a fluoropolymer] The method for producing a fluoropolymer of the present invention comprises copolymerizing tetrafluoroethylene (hereinafter also referred to as "TFE") with a monomer represented by formula (m1) described below (hereinafter also referred to as "monomer m1") in a reactor to obtain a fluoropolymer F1 (hereinafter also referred to as "polymer F1") having units based on TFE (hereinafter also referred to as "TFE units") and units based on monomer m1 (hereinafter also referred to as "m1 units"), and then heating the fluoropolymer F1 at 250°C or higher to obtain a fluoropolymer F2 (hereinafter also referred to as "polymer F2"). In addition, the molar ratio of the content of TFE units to the content of m1 units in polymer F1 is 6.5 to 12.5. In this specification, a fluoropolymer obtained by converting a group convertible into a sulfonic acid type functional group in the fluoropolymer obtained by the present production method (i.e., polymer F2) into a sulfonic acid type functional group by a known treatment such as hydrolysis treatment or treatment for conversion to an acid form is also referred to as "polymer H".

[0014] According to this production method, obtain a fluoropolymer that has low ion exchange capacity when converted to acid form and has excellent melt elongation.The details of this reason have not yet been clarified, but it is presumed to be due to the following reasons.When the structure of continuous TFE unit increases in fluoropolymer, crystallization occurs in fluoropolymer, which causes the decrease of melt elongation.When attempting to form a membrane using such a fluoropolymer with low melt elongation, there are problems such as difficulty in thinning.In particular, the fluoropolymer that has low ion exchange capacity when converted to acid form contains many structures of continuous TFE unit, so this problem occurs significantly.To address this problem, it is considered that by subjecting fluoropolymer to heat treatment at 250 ℃ or more, the crystallization caused by the structure of continuous TFE unit present in fluoropolymer melts, and obtains a fluoropolymer that has excellent melt elongation.

[0015] <Monomer m1> Monomer m1 is a monomer represented by the following formula m1: Formula (m1): CF ２ =CF-O-R ｆ１ -A In formula (m1), Rｆ１ is a perfluoroalkylene group which may contain an oxygen atom between the 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. In formula (m1), A is a group which can be converted into a sulfonic acid type functional group. The group which can be converted into a sulfonic acid type functional group is preferably a functional group which can be converted into a sulfonic acid type functional group by hydrolysis. Specific examples of groups which can be converted into a sulfonic acid type functional group include -SO ２ F, -SO ２ Cl, —SO ２ Br is an example.

[0016] The monomer m1 is preferably a compound represented by formula (m1-1): Formula (m1-1) CF ２ =CF-(OCF ２ CFY) ｘ -O-(CF ２ ) ｙ -SO ３ In formula (m1-1), x is an integer of 0 to 2, y is an integer of 1 to 4, and Y is F or CF ３ is.

[0017] Specific examples of the compound represented by formula (m1) include the following compounds: In the formula, w is an integer of 1 to 8, and v 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 )] ｖ -SO ２ F

[0018] <Organic Solvent> The copolymerization in the present production method may be carried out in the presence of an organic solvent that serves as a polymerization solvent. Specific examples of the organic solvent include chlorofluorocarbons, hydrochlorofluorocarbons (e.g., 1-chloro-2,3,3-trifluoro-1-propene (HCFO-1233yd)), hydrofluorocarbons, and hydrofluoroethers, with hydrofluorocarbons and hydrofluoroethers being preferred. The organic solvents may be used alone or in combination of two or more.

[0019] The number of carbon atoms in the hydrofluorocarbon is preferably 4 to 10, more preferably 4 to 8. The ratio of the number of hydrogen atoms to the number of fluorine atoms on a molar basis in the hydrofluorocarbon (hereinafter referred to as H / F) is preferably 0.05 to 20, more preferably 0.06 to 1. When the H / F ratio is 0.05 or more, the solubility of the radical polymerization initiator is good. When the H / F ratio is 20 or less, the chain transfer constant of the polymerization reaction becomes appropriate, making it easy to obtain a fluoropolymer with a desired molecular weight.

[0020] The molecular structure of the hydrofluorocarbon may be linear or branched. Specific examples of hydrofluorocarbons include the following compounds: CF ３ CF ２ CH ２ CH ３ CF ３ CH ２ CF ２ CH ３ CHF ２ CF ２ CF ２ CHF ２ CH ３ CF ２ CHFCF ３ CF ３ CF ２ CHFCF ３ CF ３ CF ２ CF ２ CF ２ H (CF ３ ) ２ CFCH ２ CH ３ CH ３CHFCF ２ CF ２ CH ２ CH ３ CH ３ CF ２ CF ２ CF ２ CHF ２ CF ３ CHFCHFCF ２ CF ３ CF ３ CF ２ CF ２ CF ２ CH ２ CH ３ CF ３ CF ２ CH ２ CH ２ CF ２ CF ３ CF ３ CF ２ CF ２ CF ２ CF ２ CF ２ H (CF ３ ) ２ CFCHFCHFCF ３ CH ３ CF ２ CF ２ CF ２ CF ２ CF ２ CF ２ H CF ３ CF ２ CF ２ CF ２ CF ２ CF ２ CF ２ H CF ３ CF ２ CF ２ CF ２ CF ２ CF ２ CH ２ CH ３ CF ３ CF ２ CF ２ CF ２ CF ２ CF ２ CF ２ CHF ２

[0021] Hydrofluorocarbons are preferred because they have an appropriate boiling point and can produce high molecular weight fluorine-containing polymers. ｎ＋ｍ F ２ｎ＋１ H ２ｍ＋１ (where n is an integer of 2 to 8, and m is an integer of 0 to 3), and CF ３ CF ２ CF ２ CF ２ CF ２ CF ２ H (1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorohexane, H / F ratio = 0.077), CF ３ CF ２ CF ２ CF ２ CH ２ CH ３ (1,1,1,2,2,3,3,4,4-nonafluorohexane, H / F ratio = 0.56), CF ３ CF ２ CF ２ CF ２ CF ２ CF ２ CH ２ CH ３ (1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorooctane, H / F ratio=0.38) is more preferred.

[0022] The hydrofluoroether is preferably a compound represented by formula (X) (hydrofluoroalkyl ether, hereinafter referred to as HFE). ｘ１ -O-R ｘ２ (X)

[0023] R ｘ１ and R ｘ２ is a polyfluoroalkyl group, and R ｘ１ and R ｘ２ At least one of R ｘ１ and R ｘ２ The total number of carbon atoms in R is 3 to 8. The polyfluoroalkyl group is preferably a linear or branched hydrofluoroalkyl group or a perfluoroalkyl group. ｘ１ and R ｘ２When one of R is a perfluoroalkyl group, the other is a hydrofluoroalkyl group. ｘ１ and R ｘ２ may be the same or different polyfluoroalkyl groups. ｘ１ and R ｘ２ The total number of fluorine atoms contained in R is preferably greater than the total number of hydrogen atoms. ｘ１ and R ｘ２ The total number of fluorine atoms contained in R is preferably 60% or more, more preferably 65% ​​or more, based on the total number of hydrogen atoms and fluorine atoms. ｘ１ and R ｘ２ The total number of carbon atoms is preferably 3 to 8, and more preferably 4 to 6.

[0024] As HFE, CF ３ CH ２ OCF ２ CF ２ H, CHF ２ CF ２ CH ２ OCF ２ CF ２ H, and CF ３ CF ２ CH ２ OCF ２ CF ２ At least one selected from the group consisting of H is preferred, and CF ３ CH ２ OCF ２ CF ２ H is more preferred.

[0025] <Radical Polymerization Initiator> The copolymerization in the present production method is preferably carried out in the presence of a radical polymerization initiator. Specific examples of the radical polymerization initiator include diacyl peroxides (e.g., disuccinic acid peroxide, benzoyl peroxide, perfluorobenzoyl peroxide, lauroyl peroxide, bis(pentafluoropropionyl)peroxide), azo compounds (e.g., 2,2'-azobis(2-amidinopropane) hydrochlorides, 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(dimethyl isobutyrate), azobisisobutyronitrile), and peroxyesters. (for example, t-butyl peroxyisobutyrate, t-butyl peroxypivalate), peroxydicarbonates (for example, diisopropyl peroxydicarbonate, bis(2-ethylhexyl)peroxydicarbonate), hydroperoxides (for example, diisopropylbenzene hydroperoxide, t-butyl hydroperoxide), dialkyl peroxides (for example, di-t-butyl peroxide, perfluoro-di-t-butyl peroxide). The radical polymerization initiator may be used alone or in combination of two or more.

[0026] <Other Components> In the present production method, components such as a polymerization inhibitor and a chain transfer agent (for example, methanol, chloroform, cyclohexane, n-pentane, n-hexane) may be used.

[0027] <Steps> In this specification, the step of copolymerizing TFE and monomer m1 in a reactor to obtain polymer F1 having TFE units and m1 units is also referred to as "step 1." Furthermore, the step of heating polymer F1 at 250°C or higher to obtain polymer F2 is also referred to as "step 2." Each step will be described in detail below.

[0028] (Step 1) Specific examples of the copolymerization method in Step 1 include bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization, and solution polymerization is preferred. Specific examples of the organic solvent used in the solution polymerization method are as described above.

[0029] The copolymerization temperature of monomer m1 and TFE is preferably 100°C or less, more preferably 95°C or less, even more preferably 90°C or less, particularly preferably 80°C or less, from the viewpoint of controlling side reactions; and from the viewpoint of further improving the reaction rate of monomer, it is preferably 20°C or more, more preferably 40°C or more, even more preferably 60°C or more.

[0030] In step 1, it is preferable to charge monomer m1 into the reactor in advance and then initiate copolymerization. TFE may be added to the reactor together with monomer m1 before the start of copolymerization, or it may be added to the reactor already charged with monomer m1. When a radical polymerization initiator is used, the radical polymerization initiator may be added to the reactor together with monomer m1 before the start of copolymerization, or it may be added to the reactor already charged with monomer m1. TFE, monomer m1, and the initiator may each be added continuously, sequentially, or all at once to the reactor. Among these, TFE and monomer m1 are preferably added continuously or sequentially. The initiator is preferably added all at once. Here, in the present invention, "sequential addition" refers to a method in which the additive to be used in polymerization (e.g., monomer, radical polymerization initiator) is added intermittently in portions, in which a period in which the additive to be added and a period in which the additive to be added are not added are alternately repeated, with the period in which the additive to be added occurring twice or more times. In the present invention, "continuous addition" refers to a method of adding a substance to be added (e.g., a monomer, a radical polymerization initiator) to be used in polymerization without interruption within a predetermined period, and does not add the substance outside the predetermined period. In the present invention, "lump addition" refers to a method of adding the entire amount of a substance to be added (e.g., a monomer, a radical polymerization initiator) to be used in polymerization at once.

[0031] The amounts of TFE and monomer m1 added to the reactor may be appropriately set so that the molar ratio of the content of TFE units to the content of m1 units in the resulting polymer F1 falls within the ranges described below.

[0032] The copolymerization in step 1 may be carried out in the presence of a chain transfer agent, or may be carried out under conditions in which a chain transfer agent is substantially absent. Conditions in which a chain transfer agent is substantially absent mean that the content of the chain transfer agent present in the reaction vessel is 1000 mass ppm or less, preferably 500 mass ppm or less, more preferably 100 mass ppm or less, based on the total amount of TFE and monomer m1 added in the reaction vessel. When copolymerization is carried out under conditions in which a chain transfer agent is substantially absent, hydrocarbon groups derived from the chain transfer agent are less likely to be introduced into the end of polymer F1, and therefore the chemical durability of polymer F2 is improved.

[0033] Polymer F1 obtained by step 1 has TFE units and m1 units. In polymer F1, the molar ratio of the TFE unit content to the m1 unit content (TFE unit content / m1 unit content) is 6.5 to 12.5. The molar ratio (TFE unit content / m1 unit content) is preferably 7.0 or more, more preferably 7.5 or more, from the viewpoint of further improving the melt elongation of polymer F2, and is preferably 12.0 or less, more preferably 11.5 or less, from the viewpoint of further suppressing the generation of the above-mentioned crystals. The content (unit: mole) of each unit in polymer F1 is determined by the method described in the Examples section below.

[0034] (Step 2) The heating temperature of polymer F1 is 250°C or higher, preferably 280°C or higher, more preferably 300°C or higher, and even more preferably 320°C or higher, from the viewpoint that the above-mentioned crystals are more easily melted and the melt elongation of the resulting polymer F2 is better. The heating temperature of polymer F1 is preferably 400°C or lower, more preferably 370°C or lower, and even more preferably 350°C or lower, from the viewpoint that thermal degradation of the polymer can be further suppressed. The heating time of polymer F1 is preferably 1 minute or longer, more preferably 5 minutes or longer, and even more preferably 10 minutes or longer, from the viewpoint that the effect of heating is more effectively exhibited. Furthermore, from the viewpoint that thermal degradation of the polymer can be further suppressed, it is preferably 600 minutes or shorter, more preferably 300 minutes or shorter, and even more preferably 100 minutes or shorter. The heating means is not particularly limited, and examples thereof include an electric furnace, a gas furnace, and an infrared heating furnace.

[0035] Preferably, step 2 further includes a cooling treatment after subjecting polymer F1 to the heating. By cooling polymer F1 after heating, the formation of crystals in polymer F2 can be further suppressed. This results in better melt elongation of the resulting polymer F2. The cooling rate is preferably 100°C / min or more, more preferably 200°C / min or more, even more preferably 300°C / min or more, and particularly preferably 500°C / min or more, in order to further demonstrate the effects of cooling. The higher the cooling rate, the more preferable, and the upper limit is not particularly limited. In order to suppress thermal degradation of polymer F2, cooling is preferably carried out until the temperature of the resulting polymer F2 drops to 200°C or less, more preferably 150°C or less, and even more preferably 100°C or less. The cooling means is not particularly limited, and examples include air cooling using a blower or high-pressure device, wind cooling, liquid cooling, mist cooling, and other contact-type cooling devices.

[0036] <Physical Properties> The TQ value of polymer F2 is preferably 180° C. or higher, more preferably 190° C. or higher, and even more preferably 200° C. or higher, and is preferably 300° C. or lower, more preferably 280° C. or lower, and even more preferably 260° C. or lower. The TQ value is a value related to the molecular weight of the polymer, and ３ The TQ value of Polymer F2 is determined by the method described in the Examples section below.

[0037] The melt elongation of polymer F2 is preferably 5 m / min or more, more preferably 15 m / min or more, and even more preferably 25 m / min or more, from the viewpoint of being able to form a thin film, and is preferably 300 m / min or less, more preferably 250 m / min or less, and even more preferably 200 m / min or less, from the viewpoint of preventing the polymer from being stretched by its own weight and becoming oriented. The melt elongation of polymer F2 is determined by the method described in the Examples section below.

[0038] As described above, polymer H is obtained by converting the groups of polymer F2 that can be converted to sulfonic acid functional groups into sulfonic acid functional groups by known treatments such as hydrolysis and acidification. The ion exchange capacity of polymer H is preferably 1.0 meq / g dry resin or less, more preferably 0.9 meq / g dry resin or less, and even more preferably 0.8 meq / g dry resin or less, from the viewpoint of suitability for the production of ion exchange membranes in the electrolytic hydrogenation apparatus described below. The ion exchange capacity of polymer H is preferably 0.4 meq / g dry resin or more, more preferably 0.5 meq / g dry resin or more, and even more preferably 0.6 meq / g dry resin or more, from the viewpoint of better performance of the electrolytic hydrogenation apparatus described below. The ion exchange capacity of polymer H is determined by the method described in the Examples section below.

[0039] <Applications> The application of the polymer F2 obtained by this production method is not particularly limited, but it is preferably used for the production of ion exchange membranes of electrolytic hydrogenation apparatuses used for the electrolytic hydrogenation of aromatic compounds (e.g., benzene, toluene, naphthalene). Details of the electrolytic hydrogenation apparatus used for the electrolytic hydrogenation of aromatic compounds are as described in WO 2021 / 157639.

[0040] The present invention will be described in detail below with reference to examples. Examples 1 to 7 are working examples, and Examples 8 to 11 are comparative examples. However, the present invention is not limited to these examples. Furthermore, the blending amount of each component in the tables below is based on mass unless otherwise specified.

[0041] [Ion Exchange Capacity] Using the fluoropolymer having fluorosulfonyl groups obtained in each example, pressure press molding was carried out at a temperature 10°C higher than the TQ value or 260°C, whichever was lower, and at 4 MPa (gauge pressure) to obtain a membrane of the fluoropolymer having fluorosulfonyl groups. The membrane of the fluoropolymer having fluorosulfonyl groups was immersed in an alkaline aqueous solution (aqueous solution A: potassium hydroxide / water = 20 / 80 (mass ratio)) at 80°C for 16 hours to remove -SO of the fluoropolymer having fluorosulfonyl groups. ２ F is hydrolyzed to form -SO ３The polymer film was then immersed in a 3 mol / L aqueous hydrochloric acid solution at 50°C for 30 minutes, and then in ultrapure water at 80°C for 30 minutes. The cycle of immersion in an aqueous hydrochloric acid solution and immersion in ultrapure water was repeated five times in total, and the -SO ３ K to -SO ３ H. Washing with ultrapure water was repeated until the pH of the water in which the polymer membrane was immersed reached 7. The polymer membrane was sandwiched between filter papers and air-dried to obtain a membrane of a fluoropolymer containing sulfonic acid groups. The membrane of the fluoropolymer containing sulfonic acid groups was vacuum-dried at 120°C for 12 hours, and then immersed in a 0.85 mol / g sodium hydroxide solution (solvent: water / methanol = 10 / 90 (mass ratio)) to neutralize the ion exchange groups. The sodium hydroxide solution after neutralizing the ion exchange groups was back-titrated with 0.1 mol / L hydrochloric acid to determine the ion exchange capacity of the fluoropolymer containing sulfonic acid groups.

[0042] [TQ value] A flow tester (Shimadzu Corporation, Capillary Tube Rheometer Flow Tester, CFT-500D) equipped with a nozzle having an inner diameter of 1 mm and a length of 1 mm was used to measure the TQ value. ２ The fluoropolymer having fluorosulfonyl groups obtained in each example and filled in a cylinder of 1.0 mm was extruded from a nozzle at 260° C. under a load of 30 kg and a pressure of 2.94 MPa. At that time, when the extrusion rate of the polymer became stable, the volume flow rate (mm ３ / sec) was taken as the Q value at 260°C. Next, the volumetric flow rate Q value measured at different temperatures was ３ The temperature at which the temperature reached 1 / second was taken as the TQ value.

[0043] [Melt elongation] Using a capillary rheometer (Capillograph (registered trademark) Model F-1, manufactured by Toyo Seiki Seisakusho) equipped with a capillary having a diameter of 1 mm and a length of 10 mm, the fluoropolymer having fluorosulfonyl groups obtained in each example and filled into a cylinder having a diameter of 9.55 mm was extruded from a nozzle at 270°C under a load of 25 kN. The extruded fluoropolymer (in the form of a strand) was extruded using a pulley at an acceleration of 5 m / s ２The speed (m / min) at which the fluoropolymer strand broke was measured, and this speed was taken as the melt elongation. ◎: 25 m / min or more ◯: 15 m / min or more but less than 25 m / min △: 5 m / min or more but less than 15 m / min ×: less than 5 m / min

[0044] [Ratio of Each Unit] The ratio of each unit in the polymer derived from the monomer unit was calculated by converting the measured ion exchange capacity into a molar ratio.

[0045] Example 1 A 2.7 L stainless steel reactor (autoclave) equipped with a stirrer was degassed under vacuum and then charged with CF as the polymerization solvent. ３ CF ２ CF ２ CF ２ CF ２ CF ２ 1,652 g of 1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorohexane (hereinafter also referred to as "CH"), CF ２ = CFOCF ２ CF (CF ３ ) O(CF ２ ) ２ SO ２184 g of PSVE (hereinafter also referred to as "PSVE") was charged, and the internal temperature of the reactor was raised to 75 ° C. Next, 74 g of tetrafluoroethylene (hereinafter also referred to as "TFE") and 17 g of an initiator solution prepared by dissolving 2,2'-azobis(dimethyl isobutyrate) (hereinafter also referred to as "V-601") as an initiator in a polymerization solvent at a concentration of 10 mass% were charged, and polymerization was initiated. During the polymerization reaction, TFE was continuously added so as to maintain the same pressure as at the start of polymerization. When the amount of TFE introduced from the start of the reaction reached 134 g, the reactor was cooled to 10 ° C., and then unreacted TFE was released outside the system to terminate the polymerization. Subsequently, methanol was added to the slurry contents in the reactor and stirred, causing the polymer to flocculate and settle. The polymer was recovered from the coagulated crude liquid by suction filtration and dried at 80°C for 10 hours to obtain 177 g of fluoropolymer F1-1, a TFE / PSVE copolymer, as a white powder. The TQ value of fluoropolymer F1-1 and the ion exchange capacity (hereinafter also referred to as "IEC") of the fluoropolymer obtained by converting the fluorosulfonyl groups of fluoropolymer F1-1 to sulfonic acid groups are shown in Table 1 (physical properties (before heat treatment)). In Table 1, "meq / g" means "milliequivalents / g dry resin", which is a unit of ion exchange capacity.

[0046] Subsequently, the obtained fluoropolymer F1-1, while placed on a stainless steel container, was placed in a thermostatic bath at 350°C and heated for 40 minutes, and the fluoropolymer was then taken out of the thermostatic bath and cooled to 40°C by air cooling at a cooling rate of 1000°C / min while measuring the temperature with a thermocouple thermometer, to give fluoropolymer F2-1. The TQ value of fluoropolymer F2-1 and the IEC of fluoropolymer H-1 obtained by converting the fluorosulfonyl groups of fluoropolymer F2-1 to sulfonic acid groups are shown in Table 1 (physical properties (after heat treatment)). The evaluation results of the melt elongation of fluoropolymer F2-1 are also shown in Table 1.

[0047] Examples 2 to 5 Fluoropolymers F2-2 to F2-5 in Examples 2 to 5 were obtained in the same manner as in Example 1, except that the heat treatment temperature and cooling rate were set under the conditions shown in Table 1. The TQ values ​​of fluoropolymers F2-2 to F2-5 and the IEC of fluoropolymers H-2 to H-5 obtained by converting the fluorosulfonyl groups of fluoropolymers F2-2 to F2-5 to sulfonic acid groups are shown in Table 1 (physical properties (after heat treatment)). The evaluation results of the melt elongation of fluoropolymers F2-2 to F2-5 are also shown in Table 1.

[0048] Examples 6 to 7 Fluoropolymers F1-6 to F1-7 (before heat treatment) were obtained in the same manner as in Example 1, except that the amounts of each component were changed as shown in Table 1, and these were used to obtain fluoropolymers F2-6 to F2-7 (after heat treatment) in Examples 6 to 7. The TQ values ​​of fluoropolymers F2-6 to F2-7 and the IEC of fluoropolymers H-6 to H-7 obtained by converting the fluorosulfonyl groups of fluoropolymers F2-6 to F2-7 to sulfonic acid groups are shown in Table 1 (physical properties (after heat treatment)). The evaluation results of the melt elongation of fluoropolymers F2-6 to F2-7 are also shown in Table 1.

[0049] Example 8 The melt elongation was measured using the fluoropolymer F1-1 obtained in Example 1 (that is, the fluoropolymer which had not been subjected to the heating treatment or cooling treatment). The results are shown in Table 1.

[0050] Example 9 Fluoropolymer F2-8 in Example 9 was obtained in the same manner as in Example 1, except that the heat treatment temperature and cooling rate were set under the conditions shown in Table 1. The TQ value of fluoropolymer F2-8 and the IEC of fluoropolymer H-8 obtained by converting the fluorosulfonyl groups of fluoropolymer F2-8 to sulfonic acid groups are shown in Table 1 (physical properties (after heat treatment)). The evaluation results of the melt elongation of fluoropolymer F2-8 are also shown in Table 1.

[0051] Example 10 Fluoropolymer F2-9 in Example 10 was obtained in the same manner as in Example 1, except that the amounts of each component were changed as shown in Table 1. The TQ value of fluoropolymer F2-9 and the IEC of fluoropolymer H-9 obtained by converting the fluorosulfonyl groups of fluoropolymer F2-9 to sulfonic acid groups are shown in Table 1 (physical properties (after heat treatment)). The evaluation results of the melt elongation of fluoropolymer F2-9 are also shown in Table 1.

[0052] Example 11 The melt elongation of the fluoropolymer F1-6 obtained in Example 6 (that is, the fluoropolymer which had not been subjected to the heating treatment or cooling treatment) was measured. The results are shown in Table 1.

[0053] In Table 1, "TFE / PSVE" represents the molar ratio of TFE units to PSVE units in the fluoropolymer F1-1 before heat treatment.

[0054]

[0055] As shown in Table 1, it was confirmed that when a fluoropolymer obtained by copolymerizing TFE with a monomer m1 described later, in which the molar ratio of TFE units to m1 units is within a predetermined range, is subjected to a heat treatment at 250°C or higher, a fluoropolymer which has a low ion exchange capacity when converted to an acid form and is excellent in melt elongation can be produced (Examples 1 to 7).

[0056] The entire contents of the specification, claims, and abstract of Japanese Patent Application No. 2023-147520, filed on September 12, 2023, are incorporated herein by reference as part of the disclosure of the present invention.

Claims

1. A method for producing a fluoropolymer, comprising copolymerizing tetrafluoroethylene with a monomer m1 represented by the following formula (m1) in a reaction vessel to obtain a fluoropolymer F1 having units based on the tetrafluoroethylene and units based on the monomer m1, and then heating the fluoropolymer F1 at 250°C or higher to obtain a fluoropolymer F2, characterized in that the molar ratio of the content of units based on the tetrafluoroethylene to the content of units based on the monomer m1 in the fluoropolymer F1 is 6.5 to 12.

5. ２ =CF-O-R ｆ１ -A In formula (m1), R ｆ１ is a perfluoroalkylene group which may contain an oxygen atom between the carbon atoms, and A is a group which can be converted into a sulfonic acid type functional group.

2. A process for producing a fluoropolymer according to claim 1, wherein said fluoropolymer F1 is cooled after being heated.

3. The method for producing a fluoropolymer according to claim 2, wherein the cooling rate in the cooling step is 100° C. / min or more.

4. The method for producing a fluoropolymer according to claim 1 or 2, wherein the copolymerization is carried out in the substantial absence of a chain transfer agent.

5. The process for producing a fluoropolymer according to claim 1 or 2, wherein the heating temperature of said fluoropolymer F1 is 300° C. or higher.

6. The method for producing a fluoropolymer according to claim 1 or 2, wherein the fluoropolymer is used for producing an ion exchange membrane of an electrolytic hydrogenation apparatus used for the electrolytic hydrogenation of an aromatic compound.

7. The monomer m1 is CF ２ =CF-O-(CF ２ ) ｗ -SO ２ F, C.F. ２ =CF-O-CF ２ CF (CF ３ )-O-(CF ２ ) ｗ -SO ２ F or CF ２ =CF-[O-CF ２ CF (CF ３ ) ] ｖ -SO ２ The method for producing a fluoropolymer according to claim 1 or 2, wherein w is an integer of 1 to 8, and v is an integer of 1 to 5.

8. The process for producing a fluoropolymer according to claim 1 or 2, wherein the molar ratio is 7.5 or more and 11.5 or less.

9. The process for producing a fluoropolymer according to claim 2 or 3, wherein the cooling is carried out until the temperature of the fluoropolymer F2 becomes 100° C. or lower.

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