Method for producing fluorine-containing polymer, and solid composition

Abstract

To provide a method for producing a fluorine-containing polymer capable of producing a fluorine-containing polymer having a high molecular weight.　A method for producing a fluorine-containing polymer according to the present invention is a method in which a monomer containing a fluorine-containing monomer is polymerized in an aqueous dispersion containing a first fluorine-containing polymer and an aqueous medium to produce a second fluorine-containing polymer different from the first fluorine-containing polymer, wherein the total concentration of cations contained in the aqueous dispersion is 2 mass ppm or less before starting polymerization of the monomer and the total concentration of anions other than fluoride ions, sulfate ions, and chloride ions contained in the aqueous dispersion is 9 mass ppm or less before starting polymerization of the monomer.

Description

Method for producing fluorine-containing polymer, solid composition

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

[0002] Fluorine-containing polymers such as tetrafluoroethylene copolymers are used in various industrial fields due to their excellent heat resistance, chemical resistance, flame retardancy, weather resistance, etc. Patent Document 1 discloses a method for producing a fluorine-containing polymer, in which a fluorine-containing monomer is emulsion-polymerized in an aqueous medium in the presence of a radical initiator and a polyfunctional dispersant. Here, the polyfunctional dispersant is -SO ３ It is disclosed that the polyfunctional dispersant has a repeating unit derived from an ethylenically unsaturated functional group monomer containing Xa (Xa is H, an ammonium group, or a monovalent metal), and that anions and cations are removed using an ion exchange resin during production.

[0003] Special Publication No. 2020-510737

[0004] In recent years, there has been a demand for high molecular weight fluorine-containing polymers. When the present inventors attempted to produce a fluorine-containing polymer by referring to the method for producing a fluorine-containing polymer described in Patent Document 1, they found that in some cases it was not possible to obtain a high molecular weight fluorine-containing polymer, and that there is room for improvement in the method for producing a fluorine-containing polymer.

[0005] An object of the present invention is to provide a method for producing a fluoropolymer, which can produce a high molecular weight fluoropolymer. Moreover, from the viewpoint of reducing the environmental load, it is desirable to use substantially no emulsifier, and in particular to use substantially no emulsifier containing a fluorine atom. Another object of the present invention is to provide a fluoropolymer having a high molecular weight equivalent to or greater than that produced when a conventional emulsifier is used, without using substantially any emulsifier.

[0006] As a result of intensive studies, the present inventors have found that the above-mentioned problems can be solved by the following configuration. [1] A method for producing a fluoropolymer, comprising polymerizing monomers containing a fluorinated monomer in an aqueous dispersion containing a first fluoropolymer and an aqueous medium to produce a second fluoropolymer different from the first fluoropolymer, wherein, before polymerization of the monomers is initiated, the total concentration of cations contained in the aqueous dispersion is 2 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion, and before polymerization of the monomers is initiated, the total concentration of anions excluding fluoride ions, sulfate ions and chloride ions contained in the aqueous dispersion is 9 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion. [2] The method for producing a fluoropolymer according to [1], wherein, before polymerization of the monomers is initiated, the total concentration of fluoride ions, sulfate ions and chloride ions contained in the aqueous dispersion is 50 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion. [3] The method for producing a fluoropolymer according to [1] or [2], wherein the first fluoropolymer is a polymer having a hydrophilic group. [4] The method for producing a fluoropolymer according to any of [1] to [3], wherein the first fluoropolymer has a glass transition temperature of 10°C or lower. [5] The method for producing a fluoropolymer according to any of [1] to [4], wherein, before polymerization of the monomers is initiated, the content of the first fluoropolymer is 0.01 to 4.0% by mass relative to the total mass of the aqueous dispersion, and before polymerization of the monomers is initiated, the concentration of the fluorine-containing emulsifier contained in the aqueous dispersion is 100 ppm by mass or less relative to the total mass of the first fluoropolymer in the aqueous dispersion. [6] The method for producing a fluoropolymer according to any of [1] to [5], wherein the first fluoropolymer contains units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether).[7] The method for producing a fluorine-containing polymer according to [6], wherein the first fluorine-containing polymer contains 20 to 60 mol% of the units based on perfluoro(alkyl vinyl ether) relative to the total of the units based on tetrafluoroethylene and the units based on perfluoro(alkyl vinyl ether). [8] The method for producing a fluorine-containing polymer according to any of [1] to [7], wherein the amount of the monomer used is 1 to 50 parts by mass per 100 parts by mass of the aqueous medium used. [9] The method for producing a fluorine-containing polymer according to any of [1] to [8], wherein the monomer is polymerized in the presence of a polymerization initiator.

[10] The method for producing a fluorine-containing polymer according to any of [1] to [9], wherein the fluorine-containing monomer comprises at least one selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene and vinylidene fluoride.

[11] The method for producing a fluorine-containing polymer according to any of [1] to

[10] , wherein the fluorine-containing monomer comprises tetrafluoroethylene.

[12] The method for producing a fluoropolymer according to any of [1] to

[11] , wherein the aqueous dispersion is obtained by a step of contacting a raw material liquid containing the first fluoropolymer and the aqueous medium with an anion exchange resin.

[13] The method for producing a fluoropolymer according to any of [1] to

[12] , wherein the aqueous dispersion is obtained by a step of contacting a raw material liquid containing the first fluoropolymer and the aqueous medium with a cation exchange resin.

[14] A solid composition comprising a first fluoropolymer and a second fluoropolymer different from the first fluoropolymer, wherein the total concentration of cations is 2 ppm by mass or less relative to the total mass of the solid composition.

[15] The solid composition according to

[14] , wherein the total content of metal elements in the solid composition is 2 ppm by mass or less relative to the total mass of the solid composition.

[0007] According to the present invention, there can be provided a method for producing a fluoropolymer, which can produce a high-molecular-weight fluoropolymer, and a solid composition containing a fluoropolymer.

[0008] The meanings of terms used in the present invention are as follows. A numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the upper and lower limits. In the numerical ranges described in this specification in stages, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another staged numerical range. 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. In this specification, each component may be used alone or in combination with two or more substances corresponding to the component. Herein, when two or more substances are used in combination for each component, the content of that component refers to the total content of the substances used in combination, unless otherwise specified. In this specification, a combination of two or more preferred embodiments is a more preferred embodiment. A "unit" is a collective term for an atomic group derived from one molecule of the above-mentioned monomer, formed directly by polymerization of the monomer, and an atomic group obtained by chemically converting a portion of the above-mentioned atomic group. Hereinafter, a "unit based on a monomer" will also be simply referred to as a "unit." The content (mass % or mol %) of each unit relative to all units contained in the polymer is determined by analyzing the polymer by solid-state nuclear magnetic resonance spectroscopy (NMR), and usually, the content of each unit calculated from the amount of each monomer charged substantially coincides with the actual content of each unit.

[0009] [Method for producing a fluoropolymer] The method for producing a fluoropolymer of the present invention (hereinafter also referred to as "the present production method") is a method for producing a second fluoropolymer different from the first fluoropolymer by polymerizing a monomer containing a fluorine-containing monomer (hereinafter also referred to as "specific monomer") in an aqueous dispersion containing a first fluoropolymer and an aqueous medium (hereinafter also referred to as "aqueous dispersion X"). In this production method, before the start of polymerization of the specific monomer, the total concentration of cations contained in the aqueous dispersion X is 2 ppm by mass or less, relative to the total mass of the aqueous medium in the aqueous dispersion X. Furthermore, in the present production method, before the start of polymerization of the monomer, the total concentration of anions excluding fluoride ions, sulfate ions and chloride ions (hereinafter also referred to as "other anions") contained in the aqueous dispersion X is 9 ppm by mass or less, relative to the total mass of the aqueous medium in the aqueous dispersion X.

[0010] According to this production method, a high-molecular-weight second fluorine-containing polymer is obtained. The details of the reason for this have not yet been clarified, but it is presumed to be due to the following reasons. If the total concentration of cations in aqueous dispersion X is high, the dispersibility of the first fluorine-containing polymer in aqueous dispersion X becomes unstable, and polymerization of the specific monomer in aqueous dispersion X may be inhibited. Furthermore, other anions may function as chain transfer agents depending on their types, so if the total concentration of other anions is high, polymerization of the specific monomer may be inhibited. To address these problems, the aqueous dispersion X used in this production method has a sufficiently low total concentration of cations and other anions, so it is presumed that polymerization of the specific monomer proceeds well in aqueous dispersion X, resulting in the production of a high-molecular-weight second fluorine-containing polymer.

[0011] <Aqueous Dispersion X> In the present production method, an aqueous dispersion X containing a first fluorinated polymer and an aqueous medium is used.

[0012] (First Fluorine-Containing Polymer) The aqueous dispersion X may contain a first fluorine-containing polymer. It is presumed that the first fluorine-containing polymer solubilizes the specific monomer by adsorbing and incorporating the specific monomer at the hydrophobic portion during polymerization of the specific monomer, and that by adding a polymerization initiator thereto, the specific monomer is polymerized within the particles of the first fluorine-containing polymer. It is also presumed that the first fluorine-containing polymer contributes to the dispersion stabilization of various components in the aqueous medium. It is preferable that the first fluorine-containing polymer is a polymer different from the second fluorine-containing polymer.

[0013] The first fluorine-containing polymer is a polymer having fluorine atoms. The first fluorine-containing polymer is preferably a fluorine-containing polymer having a hydrophilic group. Specific examples of the hydrophilic group include a hydroxyl group and an ionic functional group. The ionic functional group may be either a cationic functional group or an anionic functional group, with an anionic functional group being preferred. Specific examples of the anionic functional group include a carboxylic acid group (-COO － ), sulfonic acid group (—SO ３ － ), sulfate group (-SO ４ ２－ ), a phosphonic acid group (—PO ３ ２－ ) and a phosphate group (-PO ４ ３－ ) and other anionic functional groups. The hydrophilic group is preferably a monovalent group. The fluorine-containing polymer having a hydrophilic group may have one or more hydrophilic groups. The fluorine-containing polymer having a hydrophilic group may have a hydrophilic group in a side chain or at an end. The hydrophilic group is a group derived from a polymerization initiator, or a hydrophilic group contained in a unit based on a monomer having a hydrophilic group.

[0014] The glass transition temperature (hereinafter also referred to as "Tg") of the first fluoropolymer is preferably 10°C or lower, more preferably 5°C or lower, even more preferably 3°C or lower, and particularly preferably 0°C or lower, from the viewpoint of efficient adsorption of the specific monomer. The Tg of the first fluoropolymer is preferably -50°C or higher, more preferably -45°C or higher, and even more preferably -40°C or higher, from the viewpoint of thermal stability after molding. The Tg of the first fluoropolymer is measured by differential scanning calorimetry (DSC), and detailed measurement conditions are as described in the Examples section below. Examples of a method for adjusting the Tg of the first fluoropolymer within the above range include, for example, adjusting the type and amount of monomer used in producing the first fluoropolymer.

[0015] The first fluorine-containing polymer preferably contains units based on tetrafluoroethylene (hereinafter also referred to as "TFE") (hereinafter also referred to as "TFE units") and units based on perfluoro(alkyl vinyl ether) (hereinafter also referred to as "PAVE") (hereinafter also referred to as "PAVE units"), since it is easy to adjust the Tg within the above range and the effects of the present invention are more excellent.

[0016] The PAVE is preferably a monomer represented by formula (1) in that it has excellent polymerization reactivity when producing the first fluoropolymer and that it can produce the second fluoropolymer more efficiently.

[0017] CF ２ =CF-O-R ｆ１ (1) In formula (1), R ｆ１ is a perfluoroalkyl group having 1 to 10 carbon atoms. ｆ１ In terms of better polymerization reactivity, the number of carbon atoms is preferably 1 to 8, more preferably 1 to 6, still more preferably 1 to 5, and particularly preferably 1 to 3. The perfluoroalkyl group may be either linear or branched.

[0018] Specific examples of PAVE include perfluoro(methyl vinyl ether) (hereinafter also referred to as "PMVE"), perfluoro(ethyl vinyl ether) (hereinafter also referred to as "PEVE"), and perfluoro(propyl vinyl ether) (hereinafter also referred to as "PPVE"), and PMVE or PPVE are preferred, with PMVE being more preferred, in that the second fluorinated polymer can be produced more efficiently.

[0019] When the first fluorine-containing polymer contains TFE units and PAVE units, the content of PAVE units in the first fluorine-containing polymer is preferably 20 to 60 mol%, more preferably 25 to 60 mol%, and even more preferably 30 to 55 mol%, based on the total amount of TFE units and PAVE units. The preferred amount used is also similar when PMVE or PPVE is used as PAVE. When the first fluorine-containing polymer contains TFE units and PAVE units, the total content of TFE units and PAVE units in the first fluorine-containing polymer is preferably 90 to 100 mol%, more preferably 95 to 100 mol%, and even more preferably 99 to 100 mol%, based on all units in the first fluorine-containing polymer.

[0020] The first fluorine-containing polymer may contain units based on other monomers other than TFE and PAVE. The other monomer is preferably hexafluoropropylene. In order to more efficiently produce the second fluorine-containing polymer, it is preferable that the first fluorine-containing polymer is substantially free of units based on other monomers. "Substantially free of units based on other monomers" means that the content of units based on other monomers is 0.01 mol% or less, preferably 0 mol%, based on the total units of the first fluorine-containing polymer.

[0021] Before the start of polymerization of the monomer (specific monomer) to be used in polymerization of the second fluorine-containing polymer, the content of the first fluorine-containing polymer is preferably from 0.01 to 10.0 mass% relative to the total mass of the aqueous medium in aqueous dispersion X, and from the viewpoint of enabling the second fluorine-containing polymer to be produced more efficiently, it is more preferably from 0.01 to 5.0 mass%, and even more preferably from 0.01 to 1.0 mass%.

[0022] In this specification, "before initiating polymerization of the monomers used in the polymerization of the second fluoropolymer" means immediately before the initiation of polymerization. Here, "the initiation of polymerization" includes the time when the monomers and the polymerization initiator are brought into the reactor together after the temperature inside the reactor is raised to the polymerization temperature or higher, and the time when the temperature inside the reactor is raised to the polymerization temperature or higher after the monomers and the polymerization initiator are brought into the reactor together.

[0023] The first fluoropolymer is preferably dispersed in the aqueous medium in the form of particles. The average particle size of the first fluoropolymer is preferably 1 to 150 nm, more preferably 10 to 120 nm, and even more preferably 50 to 120 nm, from the viewpoint of enabling more efficient production of the second fluoropolymer. The average particle size of the first fluoropolymer is determined by measuring the particle size distribution by a laser diffraction / scattering method, determining a cumulative curve with the total volume of the particle population set to 100%, and measuring the particle size (D50) at the point on the cumulative curve where the cumulative volume is 50%.

[0024] <Cations> The cations are derived, for example, from the polymerization initiator used in producing the first fluoropolymer, and may be contained in the first fluoropolymer-containing aqueous dispersion X. Specific examples of the cations include sodium ions, potassium ions, ammonium ions, quaternary ammonium cations, manganese ions, iron ions, cobalt ions, nickel ions, copper ions, zinc ions, cerium ions, silver ions and chromium ions.

[0025] Before starting polymerization of the monomers used in the polymerization of the second fluorine-containing polymer, the total concentration of cations contained in aqueous dispersion X is 2 ppm by mass or less relative to the total mass of the aqueous medium in aqueous dispersion X. From the viewpoint of better effects of the present invention, it is preferably 1 ppm by mass or less, more preferably 0.1 ppm by mass or less, and even more preferably less than the lower limit of quantitation. An example of the lower limit is 0 ppm by mass. One example of a method for adjusting the total concentration of cations contained in aqueous dispersion X to the above value is a method (described later) in which a cation exchange resin is used during the production of aqueous dispersion X.

[0026] The total concentration of cations in aqueous dispersion X relative to the total mass of the aqueous medium is calculated from the total area of ​​the detected cation peaks when aqueous dispersion X or a raw material liquid (or purified raw material liquid) described below used in producing aqueous dispersion X is measured by ion chromatography, using a calibration curve showing the relationship between the peak area and amount of ammonium ions. Details of the measurement method will be described in the Examples section below. Here, the total concentration of cations contained in aqueous dispersion X may be measured using aqueous dispersion X, or, as described in the Examples section below, the raw material liquid (or purified raw material liquid) used in producing aqueous dispersion X may be used. When measuring using the raw material liquid (or purified raw material liquid), the total concentration of cations in the raw material liquid (or purified raw material liquid) is determined in the same manner as described above, and then the value of the total concentration of cations in the raw material liquid (or purified raw material liquid) is converted to the value of the total concentration of cations in aqueous dispersion X based on the mass ratio of aqueous dispersion X to the raw material liquid (or purified raw material liquid), thereby obtaining the value of the total concentration of cations in aqueous dispersion X. When the aqueous dispersion X is produced using a purified raw material liquid, the above measurement is carried out using the aqueous dispersion X or the purified raw material liquid.

[0027] <Other anions> "Other anions" refers to anions other than fluoride ions, sulfate ions, and chloride ions. Here, during the production of the first fluorine-containing polymer, fluoride ions, sulfate ions, and chloride ions, which are decomposition products of the polymerization initiator, polymerization by-products, etc., may be generated. When a specific anion exchange resin is used during the production of aqueous dispersion X to remove such fluoride ions, sulfate ions, and chloride ions, other anions may be mixed into aqueous dispersion X. The details of the reason for this have not yet been clarified, but it is presumed that when an anion exchange resin that is easily decomposed by peroxide is used, other anions generated from the anion exchange resin are mixed into aqueous dispersion X. Specific examples of other anions include anions having a hydrocarbon group, such as formate ions and acetate ions.

[0028] Before starting polymerization of the monomers used in the polymerization of the second fluorine-containing polymer, the total concentration of other anions contained in aqueous dispersion X is 9 ppm by mass or less relative to the total mass of the aqueous medium in aqueous dispersion X. From the viewpoint of better effects of the present invention, it is preferably 1 ppm by mass or less, more preferably 0.5 ppm by mass or less, and even more preferably less than the lower limit of quantitation. The lower limit can be 0 ppm by mass. One example of a method for adjusting the total concentration of other anions contained in aqueous dispersion X to the above value is a method (described below) in which an anion exchange resin is used during the production of aqueous dispersion X, and it is particularly preferable to use an anion exchange resin that is less decomposable to peroxides.

[0029] The total concentration of other anions contained in aqueous dispersion X is calculated using a calibration curve showing the relationship between the peak area and amount of trifluoroacetate ion based on the area obtained by excluding the peak area derived from fluoride ion, the peak area derived from sulfate ion, and the peak area derived from chloride ion from the total area of ​​the peaks of the detected anions when aqueous dispersion X or a raw material solution (or purified raw material solution) described below used in the production of aqueous dispersion X is measured by ion chromatography. Details of the measurement method will be described in the Examples section below. Here, the total concentration of other anions contained in aqueous dispersion X may be measured using aqueous dispersion X, or, as described in the Examples section below, the raw material solution (or purified raw material solution) used in the production of aqueous dispersion X may be used. When the measurement is carried out using the raw material liquid (or purified raw material liquid), the total concentration of other anions in the raw material liquid (or purified raw material liquid) is determined by the above-mentioned method, and then the value of the total concentration of other anions in the raw material liquid (or purified raw material liquid) is converted to the value of the total concentration of other anions in aqueous dispersion X based on the mass ratio of aqueous dispersion X to the raw material liquid (or purified raw material liquid), thereby obtaining the value of the total concentration of other anions in aqueous dispersion X. Note that when aqueous dispersion X is produced using a purified raw material liquid, the above measurement is carried out using aqueous dispersion X or the purified raw material liquid.

[0030] <Fluoride ions, sulfate ions and chloride ions> Fluoride ions, sulfate ions and chloride ions (hereinafter also referred to as "specific anions") are derived, for example, from the polymerization initiator used in producing the first fluoropolymer, and may be contained in the aqueous dispersion X containing the first fluoropolymer.

[0031] Before starting polymerization of the monomers used in the polymerization of the second fluorine-containing polymer, the total concentration of the specific anions contained in the aqueous dispersion X is preferably 50 ppm by mass or less, more preferably 10 ppm by mass or less, and even more preferably 1 ppm by mass or less, relative to the total mass of the aqueous medium in the aqueous dispersion X, in order to achieve better effects of the present invention. The lower limit can be 0 ppm by mass. One example of a method for adjusting the total concentration of the specific anions contained in the aqueous dispersion X to the above value is a method (described later) in which an anion exchange resin is used during the production of the aqueous dispersion X.

[0032] <Metal Elements> Metal elements may be contained in the aqueous dispersion X containing the first fluoropolymer, for example, as derived from the polymerization initiator used in the production of the first fluoropolymer. Specific examples of metal elements include manganese, iron, cobalt, nickel, copper, zinc, cerium, silver, and chromium. Before the start of polymerization of the monomers used in the polymerization of the second fluoropolymer, the total concentration of metal elements contained in the aqueous dispersion X is 2 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion X, and more preferably 1 ppm by mass or less from the viewpoint of obtaining a fluoropolymer with a higher molecular weight. The lower limit can be 0 ppm by mass. One example of a method for adjusting the total concentration of metal elements contained in the aqueous dispersion X to the above value is a method in which a cation exchange resin is used in the production of the aqueous dispersion X. The total concentration of metal elements contained in the aqueous dispersion X is measured by ICP atomic emission spectroscopy of the aqueous dispersion X or the raw material liquid (or purified raw material liquid) described below used in the production of the aqueous dispersion X. Details of the measurement method will be described in detail in the Examples section below.

[0033] (Aqueous Medium) The aqueous dispersion X used in the present production method contains an aqueous medium. The aqueous medium contained in the aqueous dispersion X may be the polymerization solvent used in the production method of the aqueous dispersion X described below. Examples of the aqueous medium contained in the aqueous dispersion X include water and a mixed solvent of water and a water-soluble organic solvent. Specific examples of the water-soluble organic solvent include tert-butanol, propylene glycol, dipropylene glycol, dipropylene glycol monomethyl ether, and tripropylene glycol. Before starting polymerization of the monomers used in polymerization of the second fluorine-containing polymer, the content of the aqueous medium is preferably 60 to 99.9 mass%, more preferably 80 to 99.9 mass%, and even more preferably 90 to 99.9 mass%, based on the total mass of the aqueous dispersion X.

[0034] (Other Components) The aqueous dispersion X used in the present production method may contain other components in addition to the first fluorinated polymer and the aqueous medium. Specific examples of other components that the aqueous dispersion X may contain include a chain transfer agent, an emulsifier other than a fluorinated emulsifier, a pH adjuster, and a wax.

[0035] Specific examples of chain transfer agents include ethyl acetate, methanol, ethanol, t-butyl methyl ether, diethyl ether, n-pentane, cyclohexane, methane, and propane.

[0036] A fluorine-based emulsifier refers to an emulsifier in which the hydrophobic moiety contains a fluorine atom in the hydrophilic and hydrophobic moieties of the emulsifier. Specific examples of fluorine-based emulsifiers include fluorine-containing alkanoates and fluorine-containing ether carboxylic acid compounds. Specific examples of emulsifiers other than fluorine-based emulsifiers include sodium lauryl sulfate, Perex SS-H manufactured by Kao Chemical Corporation, and Newcol 1305-SN manufactured by Nippon Nyukazai Co., Ltd.

[0037] Specific examples of pH adjusters include inorganic salts. Specific examples of inorganic salts include phosphates such as disodium hydrogen phosphate and sodium dihydrogen phosphate, and carbonates such as sodium bicarbonate and sodium carbonate. More preferred specific examples of phosphates include disodium hydrogen phosphate dihydrate and disodium hydrogen phosphate dodecahydrate.

[0038] The wax is more preferably paraffin wax. Paraffin wax may be liquid, semi-solid, or solid at room temperature. Among them, saturated hydrocarbons having 12 or more carbon atoms are preferred. The melting point of the paraffin wax is preferably 40 to 65°C, more preferably 50 to 65°C.

[0039] When aqueous dispersion X contains a chain transfer agent, the content of the chain transfer agent is preferably 0.1 to 5 parts by mass per 100 parts by mass of the aqueous medium. Furthermore, the amount of the chain transfer agent used is preferably 0.1 to 20 parts by mass, more preferably 0.1 to 15 parts by mass, and even more preferably 0.1 to 10 parts by mass per 100 parts by mass of the specific monomer described below. When aqueous dispersion X contains an emulsifier other than a fluorine-based emulsifier, the content of the emulsifier other than a fluorine-based emulsifier is preferably 0.01 to 5 parts by mass per 100 parts by mass of the aqueous medium. When aqueous dispersion X contains a pH adjuster, the content of the pH adjuster is preferably 0.01 to 3.0 parts by mass per 100 parts by mass of the aqueous medium. When aqueous dispersion X contains a wax, the content of the wax is preferably 1 to 10 parts by mass per 100 parts by mass of the aqueous medium.

[0040] Before starting polymerization of the specific monomer used in polymerization of the second fluorine-containing polymer, the concentration of the fluorine-containing emulsifier is preferably 100 ppm by mass or less, more preferably 50 ppm by mass or less, even more preferably 25 ppm by mass or less, and particularly preferably 5 ppm by mass or less, relative to the total mass of the first fluorine-containing polymer in aqueous dispersion X, in order to obtain better effects of the present invention. The lower limit can be 0 ppm by mass. An example of a method for adjusting the concentration of the fluorine-containing emulsifier to the above-mentioned range is a method for producing aqueous dispersion X without using a fluorine-containing emulsifier. Specifically, a method for producing the first fluorine-containing polymer without using a fluorine-containing emulsifier can be mentioned.

[0041] (Method for producing aqueous dispersion X) One example of a method for producing the aqueous dispersion X used in the present production method is a method in which a monomer (preferably a monomer mixture containing TFE and PAVE) is polymerized in an aqueous medium in the presence of a polymerization initiator and in the absence of an emulsifier to obtain a raw material solution containing an aqueous medium and a first fluorine-containing polymer. The first fluorine-containing polymer contained in the raw material solution is preferably dispersed in the aqueous medium in the form of particles. The raw material solution thus obtained may be used as the aqueous dispersion X as it is, or another aqueous medium may be added thereto and the resulting solution may be used as the aqueous dispersion X. Alternatively, the first fluorine-containing polymer may be dispersed in another aqueous medium by solvent substitution and the resulting solution may be used as the aqueous dispersion X. Alternatively, the above-mentioned other components may be added to the raw material solution and the resulting solution may be used as the aqueous dispersion X.

[0042] The polymerization initiator used for polymerizing the first fluorine-containing polymer is preferably a water-soluble polymerization initiator, more preferably a persulfate such as ammonium persulfate, sodium persulfate or potassium persulfate, or an organic polymerization initiator such as disuccinic acid peroxide or azobisisobutylamidine dihydrochloride, still more preferably a persulfate, and particularly preferably ammonium persulfate. Also preferred is a water-soluble oxidation-reduction catalyst described below.

[0043] The aqueous medium used for the polymerization of the first fluorine-containing polymer may be water or a mixed solvent of water and a water-soluble organic solvent. Specific examples of the water-soluble organic solvent are as described above.

[0044] The content of the first fluorinated polymer contained in the raw material liquid is preferably 0.01 to 30% by mass, more preferably 0.01 to 10% by mass, and even more preferably 0.01 to 1% by mass, relative to the total mass of the aqueous medium in the raw material liquid. The content of the aqueous medium contained in the raw material liquid is preferably 60 to 99.9% by mass, more preferably 80 to 99.9% by mass, and even more preferably 98 to 99.9% by mass, relative to the total mass of the raw material liquid.

[0045] The method for producing aqueous dispersion X used in the present production method preferably includes a step of contacting the obtained raw material liquid with an anion exchange resin after obtaining the raw material liquid. By carrying out this step, the raw material liquid is purified to obtain a purified raw material liquid. By going through this step, it becomes easy to adjust the total concentration of fluoride ions, sulfate ions, and chloride ions, as well as the total concentration of other anions, in the aqueous dispersion X used in the present production method to the above-mentioned ranges.

[0046] It is considered preferable that the anion exchange resin is one that is low in decomposition property against peroxides (for example, the polymerization initiator used in the production of the first fluoropolymer). It is considered that the use of an anion exchange resin that is low in decomposition property against peroxides can further prevent other anions derived from the anion exchange resin from being mixed into the aqueous dispersion X. As the anion exchange resin, commercially available products can be used, and examples of anion exchange resins that are low in decomposition property against peroxides include Amberlite (registered trademark) HPR4200 Cl (manufactured by DuPont) and Diaion (registered trademark) SA10AOH (manufactured by Mitsubishi Chemical Corporation).

[0047] The ion exchange group of the anion exchange resin is preferably a trialkylammonium group, since the effects of the present invention are more excellent.

[0048] The amount of the anion exchange resin used is preferably 0.1 to 20 parts by mass, more preferably 0.2 to 10 parts by mass, per 100 parts by mass of the raw material solution used.

[0049] Specific examples of methods for contacting the raw material liquid with the anion exchange resin include a method of mixing the raw material liquid with the anion exchange resin and a method of passing the raw material liquid through a column packed with the anion exchange resin. When contacting the raw material liquid with the anion exchange resin, the temperature of the raw material liquid is preferably 0 to 80°C, more preferably 10 to 50°C. When mixing the raw material liquid with the anion exchange resin, the contact time between the anion exchange resin and the raw material liquid is preferably 10 to 120 minutes, more preferably 30 to 90 minutes. When passing the raw material liquid through a column packed with the anion exchange resin, the passage rate is preferably 10 to 60 BV / h, more preferably 15 to 40 BV / h.

[0050] The method for producing the aqueous dispersion X used in the present production method preferably includes a step of contacting the obtained raw material liquid with a cation exchange resin after obtaining the raw material liquid, which makes it easy to adjust the total cation concentration in the aqueous dispersion X used in the present production method to the above-mentioned range.

[0051] The ion exchange group of the cation exchange resin is preferably a sulfonic acid group, since this makes it easier to adjust the total cation concentration to the above range. Commercially available cation exchange resins can be used, such as Dowex Monosphere 650C manufactured by DuPont.

[0052] The amount of the cation exchange resin used is preferably 0.05 to 20 parts by mass, more preferably 0.1 to 10 parts by mass, per 100 parts by mass of the raw material solution used.

[0053] Specific examples of the method for contacting the raw material liquid with the cation exchange resin are the same as the method for contacting the raw material liquid with the anion exchange resin described above. When contacting the raw material liquid with the cation exchange resin, the temperature of the raw material liquid is preferably 0 to 80°C, more preferably 10 to 50°C. The contact time between the cation exchange resin and the raw material liquid is preferably 10 to 120 minutes, more preferably 30 to 90 minutes. When the raw material liquid is passed through a column packed with the cation exchange resin, the passage rate is preferably 10 to 60 BV / h, more preferably 15 to 40 BV / h.

[0054] The method for producing aqueous dispersion X used in the present production method preferably includes a heating step of heating the obtained raw material liquid after obtaining the raw material liquid. This deactivates the polymerization initiator present in the raw material liquid, so that the polymerization of the second fluoropolymer is less affected by the polymerization initiator used in the polymerization of the first fluoropolymer. As a result, a second fluoropolymer having a high molecular weight is more likely to be obtained. The heating temperature in the heating step is preferably 70 to 100°C, more preferably 80 to 98°C, and even more preferably 85 to 95°C, from the viewpoint of further accelerating the deactivation of the polymerization initiator in the aqueous medium.

[0055] The method for producing the aqueous dispersion X used in the present production method may include a step of diluting the raw material liquid or the purified raw material liquid with an aqueous medium (e.g., water). In this step, other components such as wax may be added together with the aqueous medium.

[0056] <Specific Monomer> The present production method uses a specific monomer. The specific monomer includes a fluorine-containing monomer. Specific examples of the fluorine-containing monomer include TFE, chlorotrifluoroethylene (hereinafter also referred to as "CTFE"), vinylidene fluoride (hereinafter also referred to as "VdF"), fluoroalkylethylene (hereinafter also referred to as "FAE"), PAVE, and hexafluoropropylene. Two or more types of fluorine-containing monomers may be used in combination. Specific examples of FAE include CH ２ =CH(CF ２ ) ２ F, CH ２ =CH(CF ２ ) ３ F, CH ２ =CH(CF ２ ) ４ F (hereinafter also referred to as "C4OLF"), CH ２ =CF(CF ２ ) ３ H and CH ２ =CF(CF ２ ) ４H is exemplified, with C4OLF being preferred. PAVE is the same as the PAVE in the first fluoropolymer described above, and preferred embodiments are also the same. Among these, the fluorine-containing monomer preferably contains at least one selected from the group consisting of TFE, CTFE, and VdF, preferably contains TFE, and more preferably is TFE. The amount of the fluorine-containing monomer used is preferably 97 to 100 mass%, more preferably 98 to 100 mass%, and even more preferably 99 to 100 mass%, based on the amount of the specific monomer used. The amount of the fluorine-containing monomer used may also be 10.0 to 100.0 mol%, based on the amount of the specific monomer used, and in this case, is more preferably 30.0 to 70.0 mol%, and even more preferably 40.0 to 60.0 mol%. The amount of the fluorine-containing monomer used may be 90.0 to 99.9 mol % relative to the amount of the specific monomer used, and when emphasis is placed on melt moldability, it is preferably 95.0 to 99.0 mol %.

[0057] The specific monomer may contain a monomer other than the above-mentioned monomers (hereinafter also referred to as "other monomer"). Specific examples of the other monomer include ethylene, propylene, vinyl chloride, and vinylidene chloride, with ethylene being preferred. Two or more types of other monomers may be used in combination. The amount of the other monomer used is preferably 10.0 to 70.0 mol%, more preferably 20.0 to 60.0 mol%, and even more preferably 30.0 to 50.0 mol%, relative to the amount of the specific monomer used. On the other hand, it is also preferred that the specific monomer does not contain any other monomer.

[0058] The amount of the specific monomer used is preferably 1 to 50 parts by mass, more preferably 1 to 40 parts by mass, and even more preferably 1 to 30 parts by mass, per 100 parts by mass of the aqueous medium contained in the aqueous dispersion X.

[0059] <Polymerization Initiator> In the present production method, the specific monomer is preferably polymerized in the presence of a polymerization initiator. Examples of the polymerization initiator include an oil-soluble radical initiator, a water-soluble radical initiator, and a water-soluble redox catalyst. Specific examples of the oil-soluble radical initiator include oil-soluble organic peroxides such as tert-butyl peroxypivalate (hereinafter also referred to as "PBPV") and diisopropyl peroxydicarbonate (hereinafter also referred to as "IPP"). Specific examples of the water-soluble radical initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble organic peroxides such as disuccinic acid peroxide, bisglutaric acid peroxide, and tert-butyl hydroperoxide (hereinafter also referred to as "TBHP"). The water-soluble redox catalyst is preferably a combination of an oxidizing agent such as bromic acid or a salt thereof, chloric acid or a salt thereof, persulfuric acid or a salt thereof, permanganic acid or a salt thereof, or hydrogen peroxide, and a reducing agent such as sulfurous acid or a salt thereof, hydrogen sulfite or a salt thereof, thiosulfuric acid or a salt thereof, organic acid, or inorganic salt. Potassium persulfate and ammonium persulfate are preferred as persulfates. Sodium sulfite is preferred as sulfites. Inorganic salts include combinations of sulfate anions, sulfite anions, and chloride anions with metal elements. Transition metals are preferred as metal elements, including manganese, iron, cobalt, nickel, copper, zinc, cerium, and silver, with iron being preferred. Iron (II) sulfate is preferred as an inorganic salt. The polymerization initiator is preferably an oil-soluble radical initiator or a water-soluble radical initiator, and from the viewpoint of more efficient production of a fluoropolymer, oil-soluble radical initiators are more preferred, and oil-soluble organic peroxides are even more preferred. Two or more polymerization initiators may be used in combination.

[0060] The amount of the polymerization initiator used is preferably 1 to 1,000 ppm by mass, more preferably 5 to 750 ppm by mass, and even more preferably 10 to 500 ppm by mass, relative to 100 parts by mass of the specific monomer used.

[0061] <Other Components> When polymerizing the specific monomer, components other than those described above (hereinafter also referred to as "other components") may be further used. A specific example of the other component is a reducing agent. The amount of the other component used is preferably 1 to 2000 ppm by mass relative to 100 parts by mass of the specific monomer used.

[0062] <Step> In the present production method, the specific monomer is polymerized in the aqueous dispersion X to produce the second fluorine-containing polymer.

[0063] The second fluorine-containing polymer obtained by the present production method contains units based on a fluorine-containing monomer, preferably units based on TFE (hereinafter also referred to as "TFE units"), and is more preferably a homopolymer of TFE (polytetrafluoroethylene, hereinafter also referred to as "PTFE"). The first fluorine-containing polymer and the second fluorine-containing polymer may be copolymerized.

[0064] The content of units based on a fluorine-containing monomer (particularly TFE units) is preferably from 99.0 to 100.0 mass%, more preferably from 99.5 to 100.0 mass%, and even more preferably from 99.9 to 100.0 mass%, based on all units of the second fluorine-containing polymer.

[0065] In this production method, each component may be added all at once or in portions. The order of addition of each component is not particularly limited. The specific monomer can be added to the reaction system (i.e., polymerization reaction vessel) by a conventional method. For example, the specific monomer may be added to the reaction system continuously or intermittently so that the polymerization pressure reaches a predetermined pressure. Alternatively, the specific monomer may be dissolved in an aqueous medium, and the resulting solution may be added to the reaction system continuously or intermittently. When a polymerization initiator is used, the polymerization initiator may be added to the reaction system all at once or in portions.

[0066] The polymerization temperature is preferably 10 to 95° C., more preferably 15 to 90° C. The polymerization pressure is preferably 0.5 to 4.0 MPaG, more preferably 0.6 to 3.5 MPaG. In the case of batch processing, the polymerization time is preferably 90 to 1,000 minutes, more preferably 90 to 700 minutes.

[0067] According to this production method, the second fluorine-containing polymer can be produced using an aqueous medium with a small environmental load, without requiring an emulsifier. Therefore, it is preferable to carry out the polymerization of the specific monomer under conditions in which an emulsifier is substantially absent. Examples of the emulsifier include the above-mentioned fluorine-based emulsifiers and emulsifiers other than fluorine-based emulsifiers. Under conditions in which an emulsifier is substantially absent means an environment in which the content of the emulsifier is 0.03 ppm by mass or less, preferably 0.02 ppm by mass or less, and more preferably 0 ppm by mass, relative to the total mass of the aqueous medium contained in the aqueous dispersion X.

[0068] As mentioned above, it is presumed that the specific monomer polymerizes within the particles of the first fluorine-containing polymer during polymerization of the specific monomer, and therefore it is thought that particles containing the first fluorine-containing polymer and the second fluorine-containing polymer are produced in the present production method. That is, it is presumed that the second fluorine-containing polymer is obtained in the form of particles containing the first fluorine-containing polymer and the second fluorine-containing polymer. In this case, the present production method gives a second aqueous dispersion in which particles containing the first fluorine-containing polymer and the second fluorine-containing polymer are dispersed in the above aqueous medium.

[0069] [Second Aqueous Dispersion] The second aqueous dispersion contains an aqueous medium and the second fluoropolymer obtained by the present production method, and may further contain the first fluoropolymer separately from the second fluoropolymer.

[0070] <Aqueous medium> The aqueous medium is the same as the specific examples of the aqueous medium used in producing the second fluorine-containing polymer described above. The content of the aqueous medium is preferably from 50 to 99 mass%, more preferably from 60 to 99 mass%, and even more preferably from 70 to 99 mass%, based on the total mass of the aqueous dispersion, from the viewpoint of dispersion stability of the particles containing the second fluorine-containing polymer.

[0071] <First Fluorine-Containing Polymer and Second Fluorine-Containing Polymer> The present aqueous dispersion may contain a first fluorine-containing polymer. The first fluorine-containing polymer is the same as the first fluorine-containing polymer in the present production method described above, and preferred embodiments are also the same. When the present aqueous dispersion contains the first fluorine-containing polymer, the content of the first fluorine-containing polymer is preferably 0.01 to 10.0 mass%, more preferably 0.01 to 5.0 mass%, and even more preferably 0.01 to 1.0 mass%, relative to the total mass of the present aqueous dispersion.

[0072] The second fluorine-containing polymer is the same as the second fluorine-containing polymer in the above-mentioned production method of the present invention, and preferred embodiments are also the same. The content of the second fluorine-containing polymer is preferably from 5 to 50 mass%, more preferably from 8 to 40 mass%, and even more preferably from 10 to 35 mass%, based on the total mass of the present aqueous dispersion.

[0073] When the second aqueous dispersion contains the first fluoropolymer, the total content of the first fluoropolymer and the second fluoropolymer is preferably from 5 to 50 mass%, more preferably from 8 to 40 mass%, and even more preferably from 10 to 35 mass%, relative to the total mass of the aqueous dispersion.

[0074] When the second aqueous dispersion contains a first fluorine-containing polymer, the first fluorine-containing polymer and the second fluorine-containing polymer may exist separately in this aqueous dispersion, but are preferably present in the form of particles containing the first fluorine-containing polymer and the second fluorine-containing polymer.In this case, from the viewpoint of dispersion stability, the average particle size of the particles is preferably 500 μm or less, more preferably 450 μm or less, and even more preferably 400 μm or less.Furthermore, from the viewpoint of aggregation, the average particle size of the particles is preferably 50 nm or more, more preferably 80 nm or more, and even more preferably 100 nm or more.The average particle size of the particles is determined by measuring particle size distribution by laser diffraction / scattering method, determining a cumulative curve with the total volume of the particle group as 100%, and the particle size on the cumulative curve is the particle size at the point where the cumulative volume is 50%.

[0075] <Others> The second aqueous dispersion preferably contains substantially no emulsifier. Examples of emulsifiers include the above-mentioned fluorine-based emulsifiers and emulsifiers other than fluorine-based emulsifiers. "Substantially no emulsifier" in the present aqueous dispersion means that the content of emulsifier is 0.03 ppm by mass or less, preferably 0.02 ppm by mass or less, and more preferably 0 ppm by mass, relative to the total mass of the present aqueous dispersion. 0 ppm by mass means that no emulsifier is used. <Cations in the Second Aqueous Dispersion> The total concentration of cations contained in the second aqueous dispersion is 2 ppm by mass or less, relative to the total mass of the aqueous medium in the aqueous dispersion. From the viewpoint of obtaining a fluoropolymer with a higher molecular weight, it is preferably 1 ppm by mass or less, and more preferably 0.1 ppm by mass or less. The lower limit is 0 ppm by mass. One example of a method for adjusting the total concentration of cations contained in the aqueous dispersion to the above value is production using aqueous dispersion X having a cation concentration in a suitable range. The total concentration of cations contained in the aqueous dispersion is calculated from the total area of ​​the peaks of cations detected when the fluoropolymer in the aqueous dispersion is precipitated and the supernatant is measured by ion chromatography, using a calibration curve showing the relationship between the peak area and the amount of ammonium ions. Details of the measurement method will be described in the Examples section below.

[0076] <Uses> As described above, the present aqueous dispersion does not require an emulsifier, and therefore can be easily converted into a dispersion in an organic solvent such as N-methylpyrrolidone, acetone, etc. by solvent substitution. For example, the present aqueous dispersion can be converted into a dispersion in an organic solvent by mixing the present aqueous dispersion with an organic solvent and dehydrating it by evaporation or using anhydrous sodium sulfate, etc.

[0077] The second aqueous dispersion stably disperses the fluoropolymer even without containing an emulsifier, and is therefore suitable for use in coating applications, binders, etc.

[0078] Furthermore, by agglomerating the first fluorine-containing polymer and the second fluorine-containing polymer (preferably particles containing the first fluorine-containing polymer and the second fluorine-containing polymer) from the second aqueous dispersion, powders of the first fluorine-containing polymer and the second fluorine-containing polymer can be obtained.

[0079] Examples of aggregation methods include, but are not limited to, mechanical aggregation, freeze aggregation, acid aggregation, base aggregation, and aggregation using a coagulant. A specific example of mechanical aggregation is a method in which the aqueous dispersion is diluted with water to a concentration of the first fluoropolymer and the second fluoropolymer of 8 to 20% by mass, and then the aqueous dispersion is vigorously stirred to apply shear force to aggregate the primary particles of the first fluoropolymer and the second fluoropolymer. If necessary, the pH of the second aqueous dispersion may be adjusted, and an aggregation aid such as an electrolyte or a water-soluble organic solvent may be added. Examples of pH adjusters include sodium carbonate and sodium bicarbonate. The aggregation may also be carried out in the presence of one or more compounds selected from the group consisting of ammonia, ammonium salts, and urea. Examples of electrolytes include inorganic salts such as potassium nitrate, sodium nitrate, sodium carbonate, and sodium bicarbonate. Examples of organic solvents include alcohols and acetone. In the case of freeze aggregation, the aggregation temperature is preferably −20 to 0°C. The aggregation time is preferably 1 hour or more, more preferably 2 hours or more. In the case of acid aggregation, a method in which an acid-containing solution is added to the aqueous dispersion is preferred. Examples of the acid to be added include hydrochloric acid, nitric acid, sulfuric acid, oxalic acid, and hydrofluoric acid, with hydrochloric acid being preferred. The concentration of the acid in the acid-containing solution is preferably 0.1 to 50% by mass, more preferably 1 to 30% by mass, and even more preferably 1 to 10% by mass. A preferred method of base coagulation is to add a solution containing a base to the aqueous dispersion. Examples of bases to be added include sodium hydroxide, potassium hydroxide, and ammonium carbonate, with sodium hydroxide being preferred. The concentration of the base in the base-containing solution is preferably 0.1 to 50% by mass, more preferably 1 to 30% by mass, and even more preferably 1 to 10% by mass. For coagulation using a coagulant, known coagulants can be used. Known coagulants include aluminum salts, calcium salts, and magnesium salts. Specific examples include aluminum sulfate, coagulants represented by the general formula M'Al(SO4)2.12H2O (where M' is a monovalent cation other than lithium). Among these, alum is preferred, and potassium alum, where M is potassium, is more preferred. As the coagulation method, mechanical coagulation or base coagulation is preferred because coagulation proceeds particularly easily.

[0080] [Solid Composition] The solid composition of the present invention (hereinafter also referred to as "the present solid composition") contains a second fluorine-containing polymer, and may further contain a first fluorine-containing polymer.

[0081] In this specification, a solid composition means a composition having a solid content of 99% by mass or more. Here, the solid content is calculated by the following method based on the mass before and after heating. After heating 2.0 g of the solid composition at 170°C for 20 minutes, the mass of the residue is weighed and the solid content is calculated by the following formula: Solid content (mass%) = 100 x (mass of residue) / (mass of solid composition)

[0082] The present solid composition can be obtained from the second aqueous dispersion. Specifically, it is preferably obtained by an aggregation method using the present aqueous dispersion described above. Preferred embodiments of the present solid composition are the same as the preferred embodiments of the first fluorine-containing polymer and the second fluorine-containing polymer contained in the present aqueous dispersion described above, and therefore description thereof will be omitted. When the present solid composition contains the first fluorine-containing polymer, the first fluorine-containing polymer and the second fluorine-containing polymer may be present separately in the present solid composition, but it is preferable that they are present in the form of particles containing the first fluorine-containing polymer and the above-mentioned second fluorine-containing polymer.

[0083] When the solid composition contains a first fluoropolymer, the content of the first fluoropolymer is preferably 0.01 to 50% by mass, more preferably 0.1 to 30% by mass, and even more preferably 0.3 to 25% by mass, relative to the total mass of the solid composition. The content of the second fluoropolymer is preferably 50 to 100% by mass, more preferably 70 to 100% by mass, and even more preferably 75 to 100% by mass, relative to the total mass of the solid composition. When the solid composition contains a first fluoropolymer, the total content of the first and second fluoropolymers is preferably 98 to 100% by mass, more preferably 99 to 100% by mass, relative to the total mass of the solid composition.

[0084] <Cation Content in Solid Composition> The cation content in the present solid composition is 2 ppm by mass or less, based on the total mass of the solid composition. From the viewpoint of obtaining a fluoropolymer with a higher molecular weight, 1 ppm by mass or less is more preferred. The lower limit can be 0 ppm by mass. One example of a method for adjusting the total cation concentration in the solid composition to the above value is production using an aqueous dispersion X having a cation concentration within a suitable range. The total cation concentration in the solid composition is calculated from the total area of ​​the cation peaks detected when an extract of the solid composition is measured by ion chromatography, using a calibration curve showing the relationship between the peak area and amount of ammonium ions. The extract can be obtained by adding water to the solid composition, subjecting the mixture to ultrasonic treatment, centrifuging the mixture to precipitate the fluoropolymer, and extracting the supernatant. Details of the measurement method will be described in the Examples section below. <Total Content of Metal Elements in Solid Composition> The total content of metal elements in the present solid composition is 2 ppm by mass or less, based on the total mass of the solid composition. From the viewpoint of obtaining a fluoropolymer with a higher molecular weight, 1 ppm by mass or less is more preferred. The lower limit can be 0 ppm by mass. One example of a method for adjusting the total concentration of metal elements contained in the solid composition to the above value is to produce the solid composition using an aqueous dispersion X in which the total content of metal elements is within a suitable range. The total content of metal elements contained in the solid composition can be obtained by ashing the solid composition, extracting it with an aqueous sulfuric acid solution, and measuring the extract using ICP-MS. Details of the measurement method will be described in the Examples section below.

[0085] <DSC Endothermic Peak Ratio> The present solid composition obtained by aggregating the present aqueous dispersion is heated to 380°C at 10°C / min in an air atmosphere using a differential scanning calorimeter in a state that has not had a history of heating to a temperature of 300°C or higher. When the length measured from the largest endothermic peak present at 340°C or higher on the differential thermal analysis curve toward the baseline is taken as the endothermic peak height A (hereinafter also referred to as "A"), and the length measured from a point on the baseline 10°C lower than the intersection point measured from the maximum point toward the baseline to the differential thermal analysis curve is taken as the endothermic peak height B (hereinafter also referred to as "B"), the DSC endothermic peak height ratio B / A is preferably 0.740 or less, and since this results in a fluoropolymer of higher molecular weight, it is more preferably 0.700 or less, and even more preferably 0.500 or less. The DSC endothermic peak ratio is an index corresponding to the molecular weight of the fluoropolymer; the smaller this value, the higher the proportion of highly crystalline polymers having high melting points, i.e., the higher the molecular weight of the fluoropolymer. Furthermore, if there is no peak at 340° C. or higher, it can be said that the molecular weight of the fluorine-containing polymer is small.

[0086] The present invention will be described in detail below with reference to examples. Examples 1-1, 2-1, and 2-2 are working examples, and Examples 1-2, 2-3, and 2-4 are comparative examples. However, the present invention is not limited to these examples.

[0087] [Measurement and Evaluation Methods] Various measurement and evaluation methods are as follows.

[0088] <Glass transition temperature (Tg)> Tg was measured using a NEXTA DSC600 manufactured by Hitachi High-Technologies Corporation. Specifically, 5 mg of a sample for measurement was weighed into an aluminum sample pan, and the sample was heated to 100°C at a heating rate of 10°C / min under a nitrogen atmosphere. Thereafter, the sample was cooled to -60°C at a rate of 10°C / min. Once the predetermined temperature was reached, the temperature was again raised to 100°C at 10°C / min. Tg was estimated from the inflection point observed in this second heating operation.

[0089] <Average particle size of particles in aqueous dispersion> Measurement was carried out using a laser diffraction / scattering particle size distribution measuring device (ELSZneo, manufactured by Otsuka Electronics Co., Ltd.).

[0090] <Proportion of each unit in the polymer> The proportion of each unit in the polymer is １９ It was determined by F-NMR analysis and infrared absorption spectrum analysis.

[0091] <Anion Concentration> The anion concentration relative to the total mass of the aqueous medium in the aqueous dispersion was measured as follows. The raw material solution was freeze-aggregated and then filtered, and the resulting aqueous medium was analyzed by ion chromatography. The ion chromatography analysis was performed using an ion chromatograph (Thermo Fisher Scientific, ICS-5000). AS-19 was used as the separation column, and an aqueous potassium hydroxide solution was used as the eluent. The anion concentration relative to the total mass of the aqueous medium in the aqueous dispersion was calculated from the anion concentration in the raw material solution and the mass ratio of the raw material solution to the aqueous dispersion.

[0092] The concentrations of sulfate ions, chloride ions, and fluoride ions were determined by creating a calibration curve for each substance and integrating the peaks. The concentrations of each ion were summed to calculate the total concentration of sulfate ions, chloride ions, and fluoride ions (specific anions) relative to the total mass of the aqueous medium in the aqueous dispersion.

[0093] (Calibration curves for sulfate ions, chloride ions, and fluoride ions) Four levels of aqueous solutions containing known concentrations of sulfate ions, chloride ions, and fluoride ions ranging from 0.01 to 1 ppm by mass were prepared and measured using an ion chromatograph. Using a linear approximation from the concentration of each sample and the integral value of the peak, a was calculated using the following relational expression (1). The lower limit of quantitation was 0.01 ppm by mass. Y1 = a × X1 (1) Y1: peak area of ​​each anion X1: concentration of each anion (ppm by mass)

[0094] The total concentration of anions other than sulfate ions, chloride ions, and fluoride ions (other anions) was determined by preparing a calibration curve for trifluoroacetate ions and converting the peak integral value into trifluoroacetate ions.

[0095] (Calibration curve for trifluoroacetate ions) Four levels of aqueous solutions containing known concentrations of trifluoroacetate ions ranging from 0.1 to 5 ppm by mass were prepared and measured by ion chromatography. Using a linear approximation from the concentration of each sample and the integral value of the peak, b was calculated using the following relational expression (2). The lower limit of quantitation was 0.1 ppm by mass. Y2 = b × X2 (2) Y2: peak area of ​​trifluoroacetate ions X2: concentration of trifluoroacetate ions (ppm by mass)

[0096] <Total Concentration of Cations> The total concentration of cations relative to the total mass of the aqueous medium in the aqueous dispersion was measured as follows. The starting solution was freeze-aggregated and then filtered, and the resulting aqueous medium was analyzed by ion chromatography. The total concentration of cations relative to the total mass of the aqueous medium in this aqueous dispersion was measured as follows. 6 mL of the starting solution was centrifuged (8,000 rpm, 10 minutes) to precipitate the fluoropolymer, and the supernatant was used as an extract and analyzed by ion chromatography. The total concentration of cations relative to the total mass of the solid composition was measured as follows. 5 mL of ultrapure water was added to 2.5 g of the solid composition, and the mixture was subjected to ultrasonic treatment at 50°C for 2 hours, followed by centrifugation (5,000 rpm, 5 minutes) to precipitate the fluoropolymer. The supernatant was used as an extract and analyzed by ion chromatography. Note that an ion chromatograph (Shimadzu Corporation, HIC-SP) was used for the ion chromatography analysis. A Shim-pack IC-C4 was used as the separation column, and an aqueous oxalic acid solution was used as the eluent. The total concentration of cations relative to the total mass of the aqueous medium in the aqueous dispersion was calculated from the concentration of cations in the raw material liquid and the mass ratio of the raw material liquid to the aqueous dispersion.

[0097] The total concentration of cations was determined by preparing a calibration curve for ammonium ions and converting the peak integral value into ammonium ions.

[0098] (Calibration curve for ammonium ions) Four levels of aqueous solutions containing known concentrations of ammonium ions ranging from 0.1 to 10 ppm by mass were prepared and measured by ion chromatography. Using a linear approximation from the concentration of each sample and the integral value of the peak, c was calculated using the following relational expression (3). The lower limit of quantitation was 0.1 ppm by mass. Y3 = c × X3 (3) Y3: ammonium ion peak area X3: ammonium ion concentration (ppm by mass)

[0099] <Metal Elements in Aqueous Dispersion> The total content of metal elements relative to the total mass of the aqueous medium in Aqueous Dispersion X was measured as follows. The raw material liquid was freeze-aggregated and then filtered. The resulting aqueous medium was diluted with hydrochloric acid and analyzed by ICP-AES (Agilent Technologies, 720-ES). <Metal Element Content in Solid Composition> The total content of metal elements in the solid composition was measured as follows. 0.1 g of the solid composition was placed in a platinum boat and incinerated at 600°C for 10 minutes in an oxygen atmosphere. The resulting ash was dissolved in dilute hydrochloric acid and the volume was adjusted to 20 mL. The metal elements in this solution were measured using ICP-MS (Agilent Technologies, Agilent 7700x) and quantified using the absolute calibration curve method. The metal element species to be measured are 29 types of metal elements (Li, Be, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Zr, Mo, Ag, Cd, In, Sn, Cs, Ba, Pb, and Bi).

[0100] <Melting point> Using a differential scanning calorimeter (DSC7200, manufactured by SII), the melting point was determined from the endothermic peak when heated to 380°C at a rate of 10°C / min in an air atmosphere. When there were multiple endothermic peaks, the peak temperature of the largest endothermic peak was used.

[0101] <DSC Endothermic Peak Ratio> Using a differential scanning calorimeter (DSC7200, manufactured by SII), heating was performed in an air atmosphere to 380°C at 10°C / min. The length measured from the largest endothermic peak present at 340°C or higher on the differential thermal analysis curve toward the baseline was defined as A. The length measured from a point on the baseline 10°C lower than the intersection point measured from the maximum point toward the baseline to the differential thermal analysis curve was defined as B. The value B / A was defined as the DSC endothermic peak ratio.

[0102] [Production of Raw Material Liquid A] Ultrapure water (33 kg) and PMVE (2.1 kg) were charged into a 60.5 L stainless steel pressure reactor, and the temperature was raised to 90°C while stirring at 170 rpm. Next, TFE (210 g) and an aqueous ammonium persulfate solution (5.6 mass%, 150 g) were added to initiate polymerization. As the polymerization started, the pressure inside the reactor decreased, so TFE was added to maintain the pressure constant. When 120 g of TFE had been injected, the reactor was cooled and the polymerization reaction was terminated. This liquid was used as raw material liquid A. Raw material liquid A contained a fluoropolymer 1A. Raw material liquid A was freeze-coagulated and then filtered. The obtained fluoropolymer 1A was analyzed by NMR, and as a result, the TFE unit / PMVE unit ratio was 52 / 48 (molar ratio) and the Tg was -5°C.

[0103] [Production of Raw Material Solution B] Ultrapure water (33 kg) and PMVE (2.1 kg) were charged into a 60.5 L stainless steel pressure reactor, and the temperature was raised to 90°C while stirring at 170 rpm. Next, TFE (210 g) and an aqueous ammonium persulfate solution (5.6 mass%, 150 g) were added to initiate polymerization. As the polymerization started, the pressure in the reactor decreased, so TFE was added to maintain the pressure constant. When 120 g of TFE had been injected, the reactor was cooled to terminate the polymerization reaction. After recovering the gas remaining in the reactor, the temperature was raised to 90°C and the mixture was stirred at 20 rpm for 3 hours. Next, the reactor was cooled, and the liquid was withdrawn. This liquid was used as raw material solution B. Raw material solution B contains fluoropolymer 1B. After freeze-coagulating the raw material liquid B, it was filtered off and the resulting fluoropolymer 1B was analyzed by NMR, and as a result, it was found that the TFE unit / PMVE unit ratio was 52 / 48 (molar ratio) and the Tg was -5°C.

[0104] [Production of Anion Exchange Resin 1] Anion exchange resin 1 was obtained by adding an aqueous sodium hydroxide solution (8% by mass, 100 g) to anion exchange resin AmberLite HPR4200 Cl (manufactured by DuPont, 50 g) and stirring the mixture for 60 minutes.

[0105] Example 1-1 To the above-mentioned raw material liquid A (490 g) was added a cation exchange resin Dowex Monosphere 650C (manufactured by DuPont). 60 minutes after the start of stirring, the raw material liquid and the ion exchange resin were separated by filtration. To the filtered raw material liquid was added anion exchange resin 1 (20 g). 60 minutes after the start of stirring, the raw material liquid and the ion exchange resin were separated by filtration to obtain raw material liquid A-1 (corresponding to the above-mentioned purified raw material liquid). In raw material liquid A-1, particles of fluoropolymer 1A were dispersed in an aqueous medium, and the content of fluoropolymer 1A was 0.5% by mass relative to the total mass of raw material liquid A-1. A 1.0 L stainless steel pressure-resistant reactor was charged with paraffin wax (28 g), ultrapure water (121 g), and raw material liquid A-1 (475 g), to obtain aqueous dispersion A-11 (corresponding to the above-mentioned aqueous dispersion X). According to the methods described above in "<Concentration of Anions>", "<Concentration of Cations>" and "Metal Elements in Aqueous Dispersion>", the total concentration of specific anions, the total concentration of other anions, the total concentration of cations and the metal element content were determined using raw material liquid A-1. Based on the obtained values, the total concentration of specific anions, the total concentration of other anions, the total concentration of cations and the metal element content relative to the aqueous medium in aqueous dispersion A-11 were calculated. The results are shown in the table below. The content of fluoropolymer 1A was 0.4% by mass relative to the total mass of aqueous dispersion A-11. The concentration of the fluorine-containing emulsifier was 0 ppm by mass relative to the total mass of fluoropolymer 1A in aqueous dispersion A-11.

[0106] The aqueous dispersion A-11 was heated to 70°C and stirred at 260 rpm. TFE was injected until the pressure in the reactor reached 1.4 MPaG, and disuccinic acid peroxide (0.17 mmol) was added to initiate polymerization. As the polymerization started, the pressure in the reactor decreased, so TFE was added to maintain the pressure constant. When 90 g of TFE had been injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 194 minutes, and the polymerization rate calculated from the TFE consumption was 47 g / L / h. The gas remaining in the reactor was recovered, and the liquid was withdrawn. This liquid was designated aqueous dispersion 1-1 (corresponding to the second aqueous dispersion described above). Aqueous dispersion 1-1 was a dispersion in which particles (average particle size 237 nm) containing fluoropolymer 1A and fluoropolymer 2A were dispersed in an aqueous medium, and had a solids concentration of 13% by mass. The total cation concentration was determined using the obtained aqueous dispersion 1-1 according to the method described above in "<Concentration of cations in aqueous dispersion>". The total cation concentration relative to the aqueous medium in aqueous dispersion 1-1 was calculated based on the obtained value. The results are shown in the table below. The same applies hereinafter. Note that when the amount of aqueous medium used in aqueous dispersion A-11 used in polymerization was taken as 100 parts by mass, the amount of monomer (TFE) used in polymerization was 14 parts by mass. A solid composition was obtained by agglomerating particles in the obtained aqueous dispersion 1-1 and filtering them. The solid composition dried at 120°C had a melting point of 343.9°C and a DSC endothermic peak ratio B / A of 0.465. The total cation concentration and metal element content were determined using the obtained solid composition according to the methods described above in "<Cation content in solid composition>" and "Metal elements in solid composition". The results are shown in the table below. The same applies hereinafter.

[0107] Example 1-2 Raw material solution A-2 was obtained in the same manner as in Example 1, except that the anion exchange resin was changed to A300OH manufactured by Purolite Corporation. Raw material solution A-2 contained particles of fluoropolymer 1A dispersed in an aqueous medium, and the content of fluoropolymer 1A was 0.5% by mass based on the total mass of raw material solution A-2. Paraffin wax (28 g), ultrapure water (121 g), and raw material solution A-2 (475 g) were charged into a 1.0 L stainless steel pressure reactor to obtain aqueous dispersion A-12. The total concentration of specific anions, the total concentration of other anions, and the total concentration of cations relative to the aqueous medium in aqueous dispersion A-12 were calculated in the same manner as in Example 1-1, except that raw material solution A-2 was used. The results are shown in the table below.

[0108] The aqueous dispersion A-12 was heated to 70°C and stirred at 260 rpm. TFE was injected until the pressure in the reactor reached 1.4 MPaG, and disuccinic acid peroxide (0.17 mmol) was added to initiate polymerization. As the polymerization began, the pressure in the reactor decreased, so TFE was added to maintain the pressure constant. When 90 g of TFE had been injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 227 minutes, and the polymerization rate calculated from the TFE consumption was 40 g / L / h. The gas remaining in the reactor was recovered, and the liquid was then withdrawn. This liquid was designated aqueous dispersion 1-2. Aqueous dispersion 1-2 was a dispersion in which particles (average particle size 227 nm) containing fluoropolymer 1A and fluoropolymer 2B were dispersed in an aqueous medium, and had a solids concentration of 11% by mass. The particles in the resulting aqueous dispersion 1-2 were agglomerated and filtered to obtain a solid composition. The solid composition dried at 120°C had a melting point of 338.0°C, and the DSC endothermic peak ratio B / A was unmeasurable.

[0109]

[0110] As shown in Table 1, when an aqueous dispersion containing fluoropolymer 2 (second fluoropolymer) was produced under the same conditions of initiator concentration and TFE injection amount, the DSC endothermic peak ratio showed that the fluoropolymer (Example 1-1) obtained by the method for producing a fluoropolymer of the present invention had a higher molecular weight.

[0111] Example 2-1 Raw material solution B-1 (corresponding to the above-mentioned purified raw material solution) was obtained in the same manner as in Example 1-1, except that raw material solution A was changed to the above-mentioned raw material solution B. In raw material solution B-1, particles of fluoropolymer 1B were dispersed in an aqueous medium, and the content of fluoropolymer 1B was 0.5% by mass based on the total mass of raw material solution B-1. Paraffin wax (28 g), ultrapure water (121 g), and raw material solution B-1 (475 g) were charged into a 1.0 L stainless steel pressure reactor to obtain aqueous dispersion B-11 (corresponding to the above-mentioned aqueous dispersion X). The total concentration of specific anions, the total concentration of other anions, and the total concentration of cations relative to the aqueous medium in aqueous dispersion B-11 were calculated in the same manner as in Example 1-1, except that raw material solution B-1 was used. The results are shown in the table below. The content of fluoropolymer 1B was 0.4% by mass based on the total mass of aqueous dispersion B-11. The concentration of the fluorine-containing emulsifier was 0 ppm by mass relative to the total mass of the fluorine-containing polymer 1B in the aqueous dispersion B-11.

[0112] The aqueous dispersion B-11 was heated to 70°C and stirred at 260 rpm. TFE was injected until the pressure in the reactor reached 1.4 MPaG, and disuccinic acid peroxide (0.10 mmol) was added to initiate polymerization. As the polymerization started, the pressure in the reactor decreased, so TFE was added to maintain the pressure constant. When 120 g of TFE had been injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 429 minutes, and the polymerization rate calculated from the TFE consumption was 28 g / L / h. The gas remaining in the reactor was recovered, and the liquid was then withdrawn. This liquid was designated aqueous dispersion 2-1 (corresponding to the second aqueous dispersion described above). Aqueous dispersion 2-1 was a dispersion in which particles (average particle size 292 nm) containing fluoropolymer 1B and fluoropolymer 2C were dispersed in an aqueous medium, and had a solids concentration of 19% by mass. When the amount of the aqueous medium used in the aqueous dispersion B-11 used in the polymerization was taken as 100 parts by mass, the amount of the monomer (TFE) used in the polymerization was 19 parts by mass. The particles in the obtained aqueous dispersion 2-1 were agglomerated and filtered to obtain a solid composition. The solid composition dried at 120°C had a melting point of 343.9°C and a DSC endothermic peak ratio B / A of 0.462.

[0113] Example 2-2 Raw material solution B-2 (corresponding to the purified raw material solution described above) was obtained in the same manner as in Example 2-1, except that the anion exchange resin was changed to DIAION SA10AOH manufactured by Mitsubishi Chemical Corporation. Raw material solution B-2 contained particles of fluoropolymer 1B dispersed in an aqueous medium, and the content of fluoropolymer 1B was 0.5% by mass based on the total mass of raw material solution B-2. Paraffin wax (28 g), ultrapure water (121 g), and raw material solution B-2 (475 g) were charged into a 1.0 L stainless steel pressure reactor to obtain aqueous dispersion B-12 (corresponding to aqueous dispersion X described above). The total concentration of specific anions, the total concentration of other anions, and the total concentration of cations relative to the aqueous medium in aqueous dispersion B-12 were calculated in the same manner as in Example 2-1, except that raw material solution B-2 was used. The results are shown in the table below. The content of fluoropolymer 1B was 0.4% by mass based on the total mass of aqueous dispersion B-12. The concentration of the fluorine-containing emulsifier was 0 ppm by mass relative to the total mass of the fluorine-containing polymer 1B in the aqueous dispersion B-12.

[0114] The aqueous dispersion B-12 was heated to 70°C and stirred at 260 rpm. TFE was injected until the pressure in the reactor reached 1.4 MPaG, and disuccinic acid peroxide (0.10 mmol) was added to initiate polymerization. As the polymerization started, the pressure in the reactor decreased, so TFE was added to maintain the pressure constant. When 120 g of TFE had been injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 321 minutes, and the polymerization rate calculated from the amount of TFE consumed was 38 g / L / h. The gas remaining in the reactor was recovered, and the liquid was then withdrawn. This liquid was designated aqueous dispersion 2-2. Aqueous dispersion 2-2 was a dispersion in which particles (average particle size 294 nm) containing fluoropolymer 1B and fluoropolymer 2D were dispersed in an aqueous medium, and had a solids concentration of 18% by mass. When the amount of the aqueous medium used in the aqueous dispersion B-12 used in the polymerization was taken as 100 parts by mass, the amount of the monomer (TFE) used in the polymerization was 19 parts by mass. The particles in the obtained aqueous dispersion 2-2 were agglomerated and filtered to obtain a solid composition. The solid composition dried at 120°C had a melting point of 344.2°C and a DSC endothermic peak ratio B / A of 0.383.

[0115] Example 2-3 Raw material solution B-3 was obtained in the same manner as in Example 1-2, except that raw material solution A was changed to raw material solution B. In raw material solution B-3, particles of fluoropolymer 1B were dispersed in an aqueous medium, and the content of fluoropolymer 1B was 0.5% by mass based on the total mass of raw material solution B-3. Paraffin wax (28 g), ultrapure water (121 g), and raw material solution B-3 (475 g) were charged into a 1.0 L stainless steel pressure reactor to obtain aqueous dispersion B-13. The total concentration of specific anions, the total concentration of other anions, and the total concentration of cations relative to the aqueous medium in aqueous dispersion B-13 were calculated in the same manner as in Example 2-1, except that raw material solution B-3 was used. The results are shown in the table below. Aqueous dispersion B-13 was heated to 70°C and stirred at 260 rpm. TFE was injected until the pressure inside the reactor reached 1.4 MPaG, and disuccinic acid peroxide (0.10 mmol) was added to initiate polymerization. As the polymerization started, the pressure inside the reactor decreased, so TFE was added to maintain the pressure constant. When 120 g of TFE had been injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 370 minutes, and the polymerization rate calculated from the amount of TFE consumed was 34 g / L / h. The gas remaining in the reactor was recovered, and the liquid was then withdrawn. This liquid was designated as aqueous dispersion 2-3. Aqueous dispersion 2-3 was a dispersion in which particles (average particle size 302 nm) containing fluoropolymer 1B and fluoropolymer 2E were dispersed in an aqueous medium, and had a solids concentration of 19% by mass. The particles in the resulting aqueous dispersion 2-3 were agglomerated and filtered to obtain a solid composition. The solid composition dried at 120°C had a melting point of 338.2°C, and the DSC endothermic peak ratio B / A was unmeasurable.

[0116] Example 2-4 To the above-mentioned raw material solution B (490 g), Diaion SA10AOH (20 g) manufactured by Mitsubishi Chemical Corporation was added. 60 minutes after the start of stirring, the raw material solution was filtered to separate the ion exchange resin from the raw material solution, thereby obtaining raw material solution B-4 (corresponding to the above-mentioned purified raw material solution). In raw material solution B-4, particles of fluoropolymer 1B were dispersed in an aqueous medium, and the content of fluoropolymer 1B was 0.5 mass% based on the total mass of raw material solution B-4. Paraffin wax (28 g), ultrapure water (121 g), and raw material solution B-4 (475 g) were charged into a 1.0 L stainless steel pressure reactor to obtain aqueous dispersion B-14. The total concentration of specific anions, the total concentration of other anions, and the total concentration of cations in aqueous dispersion B-14 were calculated in the same manner as in Example 2-1, except that raw material solution B-4 was used. The results are shown in the table below. The aqueous dispersion B-14 was heated to 70°C and stirred at 260 rpm. TFE was injected until the pressure in the reactor reached 1.4 MPaG, and disuccinic acid peroxide (0.10 mmol) was added to initiate polymerization. As the pressure in the reactor decreased with the initiation of polymerization, TFE was added to maintain the pressure constant. Since the polymerization reaction became significantly more difficult to proceed when 60 g of TFE had been injected, the reactor was cooled when 73 g of TFE had been injected, and the polymerization reaction was terminated. The polymerization time was 360 minutes, and the polymerization rate calculated from the amount of TFE consumed was 21 g / L / h. After recovering the gas remaining in the reactor, the liquid was withdrawn. This liquid was designated aqueous dispersion 2-4. Aqueous dispersion 2-4 was a dispersion in which particles (average particle size 309 nm) containing fluoropolymer 1B and fluoropolymer 2F were dispersed in an aqueous medium, and had a solids concentration of 11% by mass. The particles in the resulting aqueous dispersion 2-4 were aggregated and filtered to obtain a solid composition. The solid composition dried at 120°C had a melting point of 344.2°C and a DSC endothermic peak ratio B / A of 0.750.

[0117]

[0118] As shown in Table 2, when aqueous dispersions containing fluoropolymer 2 (second fluoropolymer) were produced under the same conditions of initiator concentration and TFE injection amount, the DSC endothermic peak ratio showed that the fluoropolymers obtained by the method for producing a fluoropolymer of the present invention (Examples 2-1 to 2-2) had higher molecular weights. The entire contents of the specification, claims, and abstract of Japanese Patent Application No. 2023-213841 filed on December 19, 2023 are hereby incorporated by reference as if fully set forth herein.

Claims

1. A method for producing a fluoropolymer, comprising polymerizing a monomer containing a fluorine-containing monomer in an aqueous dispersion containing a first fluoropolymer and an aqueous medium to produce a second fluoropolymer different from the first fluoropolymer, wherein before polymerization of the monomer is initiated, the total concentration of cations contained in the aqueous dispersion is 2 ppm by mass or less, relative to the total mass of the aqueous medium in the aqueous dispersion, and before polymerization of the monomer is initiated, the total concentration of anions, excluding fluoride ions, sulfate ions and chloride ions, contained in the aqueous dispersion is 9 ppm by mass or less, relative to the total mass of the aqueous medium in the aqueous dispersion.

2. A method for producing a fluorine-containing polymer according to claim 1, wherein the total concentration of fluoride ions, sulfate ions and chloride ions contained in the aqueous dispersion before the start of polymerization of the monomers is 50 ppm by mass or less, based on the total mass of the aqueous medium in the aqueous dispersion.

3. The method for producing a fluorine-containing polymer according to claim 1 or 2, wherein the first fluorine-containing polymer is a polymer having a hydrophilic group.

4. The method for producing a fluorine-containing polymer according to claim 1 or 2, wherein the glass transition temperature of the first fluorine-containing polymer is 10°C or lower.

5. The method for producing a fluorine-containing polymer according to claim 1 or 2, wherein the content of the first fluorine-containing polymer is 0.01 to 4.0 mass% based on the total mass of the aqueous dispersion before polymerization of the monomer is initiated, and the concentration of the fluorine-containing emulsifier contained in the aqueous dispersion is 100 ppm by mass or less based on the total mass of the first fluorine-containing polymer in the aqueous dispersion before polymerization of the monomer is initiated.

6. A method for producing a fluorine-containing polymer according to claim 1 or 2, wherein the first fluorine-containing polymer contains units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether).

7. The method for producing a fluorine-containing polymer according to claim 6, wherein in said first fluorine-containing polymer, the units based on perfluoro(alkyl vinyl ether) account for 20 to 60 mol % of the units based on perfluoro(alkyl vinyl ether) relative to the total of the units based on tetrafluoroethylene and the units based on perfluoro(alkyl vinyl ether).

8. The method for producing a fluorine-containing polymer according to claim 1 or 2, wherein the amount of the monomer used is 1 to 50 parts by mass per 100 parts by mass of the aqueous medium used.

9. The process for producing a fluorine-containing polymer according to claim 1 or 2, wherein the monomers are polymerized in the presence of a polymerization initiator.

10. A process for producing a fluorine-containing polymer according to claim 1 or 2, wherein the fluorine-containing monomer comprises at least one member selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene and vinylidene fluoride.

11. The process for producing a fluorine-containing polymer according to claim 1 or 2, wherein the fluorine-containing monomer comprises tetrafluoroethylene.

12. The method for producing a fluoropolymer according to claim 1 or 2, wherein the aqueous dispersion is obtained via a step of contacting a raw material liquid containing the first fluoropolymer and the aqueous medium with an anion exchange resin.

13. The method for producing a fluoropolymer according to claim 1 or 2, wherein the aqueous dispersion is obtained via a step of contacting a raw material liquid containing the first fluoropolymer and the aqueous medium with a cation exchange resin.

14. A solid composition comprising a first fluoropolymer and a second fluoropolymer different from the first fluoropolymer, the solid composition having a total concentration of cations of 2 ppm by mass or less based on the total mass of the solid composition.

15. The solid composition according to claim 14, wherein the total content of metal elements in the solid composition is 2 ppm by mass or less, based on the total mass of the solid composition.

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