Method for producing fluorine-containing polymer, aqueous dispersion and particles

Abstract

Provided is a method for producing a fluorine-containing polymer, with which a fluorine-containing polymer can be produced with good efficiency using an aqueous medium having a low environmental burden and without the need for an emulsifier.　This method for producing a fluorine-containing polymer produces a second fluorine-containing polymer by polymerizing, in an aqueous dispersion liquid containing an aqueous medium, a monomer, which includes a fluorine-containing monomer, and a first fluorine-containing polymer having a TFE unit and a PAVE unit. In the first fluorine-containing polymer, the ratio of units derived from a perfluoro(alkyl vinyl ether) is 20 to 60 mol% relative to the total amount of units derived from tetrafluoroethylene and units derived from a perfluoro(alkyl vinyl ether). The average particle diameter of the first fluorine-containing polymer is 1 to 150 nm. Prior to initiation of polymerization of the monomer, the content of the first fluorine-containing polymer is 0.01 to 4.0 mass% relative to the total mass of the aqueous dispersion liquid.

Description

[Technical field]

[0001] The present invention relates to a method for producing a fluoropolymer, an aqueous dispersion and particles thereof. [Background technology]

[0002] Fluorine-containing polymers such as ethylene / tetrafluoroethylene copolymers are used in various industrial fields because of their excellent heat resistance, chemical resistance, flame retardancy, weather resistance, and the like. As a method for producing a fluorine-containing polymer, there is mentioned a method in which a fluorine-containing emulsifier is used to emulsion-polymerize a fluorine-containing monomer in an aqueous medium (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2007 / 046377 Summary of the Invention [Problem to be solved by the invention]

[0004] The production method for a fluoropolymer disclosed in Patent Document 1 has a small environmental impact since an aqueous medium is used. However, when a large amount of an emulsifier, which is an essential component, remains in the aqueous dispersion obtained by polymerization, it becomes necessary to remove the emulsifier depending on the application.

[0005] An object of the present invention is to provide a method for producing a fluoropolymer, which can efficiently produce a fluoropolymer using an aqueous medium which has a small environmental load and does not require an emulsifier as an essential component. Another object of the present invention is to provide an aqueous dispersion and particles. [Means for solving the problem]

[0006] As a result of intensive research, the present inventors have found that the above problems can be solved by the following configuration. [1] A method for producing a fluoropolymer, comprising polymerizing a monomer containing a fluorine-containing monomer in an aqueous dispersion containing a first fluoropolymer containing a unit based on tetrafluoroethylene and a unit based on perfluoro(alkyl vinyl ether), and an aqueous medium, to produce a second fluoropolymer different from the first fluoropolymer, the first fluorine-containing polymer contains 20 to 60 mol % of 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); the first fluoropolymer has an average particle size of 1 to 150 nm, The method for producing a fluoropolymer, wherein the content of the first fluoropolymer is 0.01 to 4.0% by mass based on the total mass of the aqueous dispersion before the initiation of polymerization of the monomers. [2] The method for producing a fluorinated polymer according to [1], wherein the concentration of sulfate ions is 5 ppm by mass or less based on the total mass of the aqueous medium in the aqueous dispersion before initiation of polymerization of the monomers. [3] The process for producing a fluorine-containing polymer according to [1] or [2], wherein the fluorine-containing monomer comprises at least one selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene and vinylidene fluoride. [4] The method for producing a fluorine-containing polymer according to any one of [1] to [3], wherein the monomer contains ethylene. [5] The method for producing a fluorinated polymer according to any one of [1] to [4], wherein the amount of the monomer used is 1 to 50 parts by mass per 100 parts by mass of the aqueous medium used. [6] The method for producing a fluorine-containing polymer according to any one of [1] to [5], wherein the monomer is polymerized in the presence of a polymerization initiator. [7] An aqueous dispersion comprising an aqueous medium and particles containing a fluorine-containing polymer, The particles have an average particle size of 1 μm or less, the particles contain units based on perfluoro(alkyl vinyl ether), and the content of the units based on perfluoro(alkyl vinyl ether) is 0.1 to 1.0 mol % based on all units of the fluorine-containing polymer, The aqueous dispersion has a content of the compound represented by formula (S1) and a content of the compound represented by formula (S2) each of which is 100 ppb by mass or less, relative to the total mass of the particles. Formula (S1): H-(CF2) n -COOM Formula (S2): H-(CF2) n -SO3M In formula (S1) and formula (S2), M each independently represents a hydrogen atom, Na, K, or NH 4 , and n each independently represents 8 or 10. [8] The aqueous dispersion according to [7], wherein the particles contain at least one unit selected from the group consisting of units based on tetrafluoroethylene, units based on chlorotrifluoroethylene, and units based on vinylidene fluoride. [9] The aqueous dispersion according to [7] or [8], wherein the particles contain units based on ethylene.

[10] A particle containing a fluorine-containing polymer, The particles have an average particle size of 1 μm or less, the particles contain tetrafluoroethylene-based units, ethylene-based units, and perfluoro(alkyl vinyl ether)-based units; the ethylene-based units constitute 20 to 70 mol % of the total of the tetrafluoroethylene-based units and the ethylene-based units; the total content of units based on tetrafluoroethylene and units based on ethylene is 80 mol % or more based on all units of the fluorine-containing polymer, the content of units based on the perfluoro(alkyl vinyl ether) is 0.1 to 1.0 mol % based on all units of the fluorine-containing polymer, Particles, wherein the content of the compound represented by formula (S1) and the content of the compound represented by formula (S2) are each 100 ppb by mass or less, relative to the total mass of the particles. Formula (S1): H-(CF2) n -COOM Formula (S2): H-(CF2) n -SO3M In formula (S1) and formula (S2), M each independently represents a hydrogen atom, Na, K, or NH 4 , and n each independently represents 8 or 10. Effect of the Invention

[0007] According to the present invention, there can be provided a method for producing a fluoropolymer, which can efficiently produce a fluoropolymer using an aqueous medium which has a small environmental load and does not require an emulsifier as an essential component. The present invention also provides aqueous dispersions and particles. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The terms used in the present invention have the following meanings. A numerical range expressed using "~" means a range including the numerical values ​​before and after "~" as the upper and lower limits. In the numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. In addition, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples. In the present specification, each component may be used alone or in combination of two or more substances corresponding to each component. When two or more substances are used in combination for each component, the content of the component refers to the total content of the substances used in combination, unless otherwise specified. As used herein, a combination of two or more preferred embodiments is a more preferred embodiment. The term "unit" refers collectively to an atomic group derived from one molecule of a monomer that is formed directly by polymerization of the monomer, and an atomic group obtained by chemically converting a part of the 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 the total 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 charged amount of each monomer is approximately equal to the actual content of each unit.

[0009] [Method of producing fluorine-containing polymer] The production method for a fluoropolymer of the present invention (hereinafter also referred to as "the 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 containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), and an aqueous medium (hereinafter the aqueous dispersion used in the production method is also referred to as "aqueous dispersion (a)") In the present production method, the first fluorine-containing polymer contains 20 to 60 mol % of units based on perfluoro(alkyl vinyl ether) relative to the total of units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether). In the present production method, the first fluoropolymer has an average particle size of 1 to 150 nm. In the present production method, before the start of polymerization of the monomers, the content of the first fluorine-containing polymer is 0.01 to 4.0% by mass based on the total mass of the aqueous dispersion (a).

[0010] The reason why the second fluoropolymer was efficiently produced by this production method without requiring an emulsifier is presumed to be that the first fluoropolymer functioned as a good polymerization site for the second fluoropolymer by using an aqueous dispersion containing a predetermined amount of the first fluoropolymer having an average particle size and a ratio of each unit contained within a predetermined range.

[0011] <Aqueous dispersion> In the present production method, an aqueous dispersion (a) containing a first fluorine-containing polymer and an aqueous medium is used.

[0012] (First fluoropolymer) The first fluorine-containing polymer contains units based on tetrafluoroethylene (hereinafter also referred to as "TFE") and units based on perfluoro(alkyl vinyl ether) (hereinafter also referred to as "PAVE"). 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 the specific monomer is polymerized within the specific particles by adding a polymerization initiator thereto. It is also presumed that the first fluorine-containing polymer contributes to dispersion stabilization in aqueous media and organic solvents.

[0013] As the PAVE, a monomer represented by formula (1) is preferred from the viewpoints of excellent polymerization reactivity in producing the first fluoropolymer and of enabling the second fluoropolymer to be produced more efficiently. CF2=CF-OR f1 (1) In formula (1), R f1 R represents a perfluoroalkyl group having 1 to 10 carbon atoms. f1 The number of carbon atoms in the alkyl group is preferably 1 to 8, more preferably 1 to 6, further preferably 1 to 5, and particularly preferably 1 to 3, from the viewpoint of superior polymerization reactivity. The perfluoroalkyl group may be linear or branched.

[0014] 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"). Among these, PMVE and PPVE are preferred, with PMVE being more preferred, from the viewpoint of enabling the second fluorine-containing polymer to be produced more efficiently.

[0015] In the first fluoropolymer, the PAVE units are 20 to 60 mol % based on the total of the TFE units and the PAVE units, and from the viewpoint of more efficient production of the second fluoropolymer, it is preferably 25 to 60 mol %, more preferably 30 to 55 mol %.

[0016] The first fluorine-containing polymer may contain units based on monomers other than TFE and PAVE, but from the viewpoint of more efficient production of 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 derived from other monomers means that the content of units derived from other monomers is 0.01 mol % or less, more preferably 0 mol %, based on the total units of the first fluorine-containing polymer. When units based on another monomer are contained, the other monomer is preferably hexafluoropropylene.

[0017] Before starting polymerization of the monomers to be used in polymerization of the second fluoropolymer, the content of the first fluoropolymer is 0.01 to 4.0 mass% based on the total mass of the aqueous medium in the aqueous dispersion (a), and from the viewpoint of producing the second fluoropolymer more efficiently, it is preferably 0.01 to 0.6 mass%, more preferably 0.01 to 0.5 mass%.

[0018] In this specification, "before initiating polymerization of the monomers used in the polymerization of the second fluoropolymer" means immediately before the start of polymerization. Here, examples of "the start of polymerization" include the time when the monomers and the polymerization initiator are made to coexist in the reactor 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 made to coexist in the reactor.

[0019] Before starting polymerization of the monomers used in the polymerization of the second fluorine-containing polymer, the concentration of sulfate ions is preferably 10 ppm by mass or less, more preferably 5 ppm by mass or less, based on the total mass of the aqueous medium in the aqueous dispersion (a), from the viewpoint of suppressing coloration of the second fluorine-containing polymer.The lower limit is 0 ppm by mass. One example of a method for adjusting the sulfate ion concentration to the above range is to remove sulfate ions using an anion exchange resin during the production of the first fluoropolymer. Here, the sulfate ion originates, for example, from a polymerization initiator (particularly, ammonium persulfate) used in producing the first fluoropolymer, and may be contained in the aqueous dispersion (a) containing the first fluoropolymer. It is presumed that the sulfate ion content of 10 ppm by mass or less (particularly, 5 ppm by mass or less) can suppress the formation of terminal groups with low heat resistance in the second fluoropolymer, thereby suppressing coloration of the second fluoropolymer.

[0020] The first fluoropolymer is dispersed in the aqueous medium in the form of particles. The average particle size of the first fluoropolymer is from 1 to 150 nm, and from the viewpoint of enabling more efficient production of the second fluoropolymer, it is preferably from 10 to 120 nm, more preferably from 50 to 120 nm. The average particle size of the first fluoropolymer is a particle size calculated by analyzing an autocorrelation function obtained by dynamic light scattering using the monodisperse cumulant method.

[0021] The method for producing the first fluoropolymer is preferably a method for polymerizing monomers containing TFE and PAVE in an aqueous medium in the presence of a polymerization initiator, thereby obtaining the first fluoropolymer dispersed in the aqueous medium in the form of particles. The aqueous medium thus obtained in which the particles of the first fluorine-containing polymer are dispersed may be used as the aqueous dispersion (a) as it is, or another aqueous medium may be added and used as the aqueous dispersion (a). Alternatively, the first fluorine-containing polymer may be dispersed in another aqueous medium by solvent replacement and used as the aqueous dispersion (a). It is preferable that sulfate ions are removed from the aqueous dispersion (a) by a sulfate ion removing means such as an anion exchange resin. The polymerization initiator used in the production of 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, further preferably a persulfate, and particularly preferably ammonium persulfate. The aqueous medium used in the production 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 include tert-butanol, propylene glycol, dipropylene glycol, dipropylene glycol monomethyl ether, and tripropylene glycol.

[0022] (aqueous medium) The aqueous dispersion (a) used in the present production method contains an aqueous medium. The aqueous medium contained in the aqueous dispersion (a) may be the polymerization solvent used in the production of the first fluoropolymer, as described above. Specific examples of the aqueous medium contained in the aqueous dispersion (a) are the same as the specific examples of the aqueous medium used in the production of the above-mentioned first fluorine-containing polymer. Before the start of polymerization of the monomers used in the polymerization of the second fluorine-containing polymer, the content of the aqueous medium is preferably from 60 to 99.9 mass%, more preferably from 96 to 99.9 mass%, and even more preferably from 98 to 99.9 mass%, based on the total mass of the aqueous dispersion (a).

[0023] (Other ingredients) The aqueous dispersion (a) used in the present production method may contain other components in addition to the first fluorine-containing polymer and the aqueous medium. Specific examples of other components that may be contained in the aqueous dispersion (a) include a chain transfer agent, an emulsifier, and a pH adjuster. Specific examples of chain transfer agents include ethyl acetate, methanol, ethanol, t-butyl methyl ether, diethyl ether, n-pentane, cyclohexane, methane, and propane. Specific examples of emulsifiers include fluorine-containing emulsifiers such as CF3CF2-O-CF2CF2-O-CF2COONH4 and CF3CF2CF2OCF(CF3)COOH, sodium lauryl sulfate, Pelex SS-H manufactured by Kao Chemical Co., Ltd., and Newcol 1305-SN manufactured by Nippon Nyukazai Co., Ltd. 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 hydrogen carbonate and sodium carbonate. More preferred specific examples of phosphates include disodium hydrogen phosphate dihydrate and disodium hydrogen phosphate dodecahydrate. When the aqueous dispersion (a) contains a chain transfer agent, the content of the chain transfer agent is preferably 0.1 to 5 parts by mass relative to 100 parts by mass of the aqueous medium, and 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, relative to 100 parts by mass of the specific monomer described below. When the aqueous dispersion (a) contains an emulsifier, the content of the emulsifier is preferably from 0.01 to 5 parts by mass with respect to 100 parts by mass of the aqueous medium. When the aqueous dispersion (a) contains a pH adjuster, the content of the pH adjuster is preferably 0.01 to 3.0 parts by mass relative to 100 parts by mass of the aqueous medium.

[0024] Before starting polymerization of the monomers used in the polymerization of the second fluorine-containing polymer, the concentration of fluoride ions is preferably 100 ppm by mass or less, more preferably 50 ppm by mass or less, based on the total mass of the aqueous dispersion (a) from the viewpoint of polymerization stability. The lower limit is 0 ppm by mass. One example of a method for adjusting the fluoride ion concentration to the above range is to remove sulfate ions using an anion exchange resin during the production of the first fluorinated polymer. Here, the fluoride ions may be generated by the reaction between a polymerization initiator (eg, ammonium persulfate) and a fluorine-containing monomer, and may be contained in the aqueous dispersion.

[0025] <Specific monomer> The specific monomer includes a fluorine-containing monomer. The fluorine-containing monomer preferably contains at least one selected from the group consisting of TFE, chlorotrifluoroethylene (hereinafter also referred to as "CTFE"), and vinylidene fluoride (hereinafter also referred to as "VdF"), more preferably contains at least one selected from the group consisting of TFE, CTFE, and VdF (hereinafter also referred to as "specific fluorine-containing monomer"), more preferably contains TFE, and even more preferably is TFE. Two or more kinds of fluorine-containing monomers may be used in combination. The amount of the fluorine-containing monomer used is preferably from 10.0 to 100.0 mol %, more preferably from 30.0 to 70.0 mol %, and even more preferably from 40.0 to 60.0 mol %, based on the amount of the specific monomer used. It is also preferable to contain a fluorine-containing monomer other than the specific fluorine-containing monomer (hereinafter, also referred to as "other fluorine-containing monomer"). Specific examples of the other fluorine-containing monomer include fluoroalkylethylene (hereinafter, also referred to as "FAE"), PAVE, and hexafluoropropylene. Two or more kinds of the other fluorine-containing monomers may be used in combination. Specific examples of FAE include CH2=CH(CF2)2F, CH2=CH(CF2)3F, CH2=CH(CF2)4F (hereinafter also referred to as "C4OLF"), CH2=CF(CF2)3H, and CH2=CF(CF2)4H, with C4OLF being preferred. The PAVE is the same as the PAVE in the above-mentioned first fluorine-containing polymer, and the preferred embodiments are also the same. The amount of the other fluorine-containing monomer used is preferably from 0.1 to 30.0 mol %, more preferably from 0.1 to 10.0 mol %, and even more preferably from 0.5 to 5.0 mol %, based on the amount of the specific monomer used.

[0026] The specific monomer preferably contains a monomer other than the fluorine-containing monomer (hereinafter, also referred to as "other monomer"). Specific examples of the other monomer include ethylene, propylene, vinyl chloride, and vinylidene chloride. Among them, the other monomer preferably contains ethylene, and more preferably is ethylene. Two or more of the 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 %, based on the amount of the specific monomer used.

[0027] 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 (a).

[0028] <Polymerization initiator> In the present production method, the specific monomer is preferably polymerized in the presence of a polymerization initiator. As the polymerization initiator, an oil-soluble radical initiator, a water-soluble radical initiator, or a water-soluble oxidation-reduction catalyst is preferable. Specific examples of oil-soluble radical initiators 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 water-soluble radical initiators include persulfates such as ammonium persulfate and potassium persulfate, water-soluble organic peroxides such as disuccinic acid peroxide, bisglutaric acid peroxide, and tert-butyl hydroperoxide (hereinafter also referred to as "TBHP"). As the water-soluble redox catalyst, a combination of an oxidizing agent such as bromic acid or its salt, chloric acid or its salt, persulfuric acid or its salt, permanganic acid or its salt, or hydrogen peroxide, and a reducing agent such as sulfurous acid or its salt, hydrogen sulfite or its salt, thiosulfuric acid or its salt, organic acid, or inorganic salt, is preferred. As the persulfate, potassium persulfate or ammonium persulfate is preferred. As the sulfite, sodium sulfite is preferred. As the inorganic salt, a combination of sulfate anion, sulfite anion, and chloride anion with a metal ion is preferred. As the metal ion, a transition metal is preferred, and examples thereof include manganese, iron, cobalt, nickel, copper, zinc, cerium, and silver ions, and among these, iron ion is preferred. As the inorganic salt, iron (II) sulfate is preferred. The polymerization initiator is preferably an oil-soluble radical initiator or a water-soluble radical initiator, more preferably an oil-soluble radical initiator from the viewpoint of more efficient production of the fluorine-containing polymer, and further preferably an oil-soluble organic peroxide. Two or more types of polymerization initiators may be used in combination.

[0029] The amount of the polymerization initiator used is preferably 0.01 to 5 parts by mass, more preferably 0.01 to 3 parts by mass, and even more preferably 0.01 to 2 parts by mass, based on 100 parts by mass of the specific monomer used.

[0030] <Other ingredients> 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 components is a reducing agent. The amount of the other components used is preferably 0.1 to 2 parts by mass per 100 parts by mass of the specific monomer used.

[0031] <Process> In the present production method, the above specific monomer is polymerized in the above aqueous dispersion (a) to produce the second fluorine-containing polymer.

[0032] The second fluorine-containing polymer obtained by the present production method contains units based on the above-mentioned fluorine-containing monomer (hereinafter also referred to as "fluorine-containing units"), preferably contains fluorine-containing units and units based on the above-mentioned other monomer (hereinafter also referred to as "other units"), and preferably contains a copolymer of TFE and ethylene (hereinafter also referred to as "ETFE"). The first fluorine-containing polymer and the second fluorine-containing polymer may be copolymerized.

[0033] ETFE contains TFE units and units based on ethylene (hereinafter also referred to as "E units"). In a copolymer having TFE units and E units, the proportion of E units to the total of E units and TFE units is preferably 20 to 70 mol%, more preferably 25 to 60 mol%, and further preferably 35 to 55 mol%. The total proportion of E units and TFE units relative to all units constituting ETFE is preferably 80 mol% or more, more preferably 85 mol% or more, and even more preferably 90 mol% or more, and is preferably 100 mol% or less, more preferably 99.5 mol% or less, and even more preferably 99 mol% or less.

[0034] The specific monomer is 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 becomes a predetermined pressure. Alternatively, the specific monomer may be dissolved in an aqueous medium, and the obtained 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.

[0035] The polymerization temperature is preferably from 10 to 95°C, more preferably from 15 to 90°C. The polymerization pressure is preferably from 0.5 to 4.0 MPaG, more preferably from 0.6 to 3.5 MPaG. In the case of batch processing, the polymerization time is preferably from 90 to 1000 minutes, more preferably from 90 to 700 minutes.

[0036] The polymerization of the specific monomer is preferably carried out in the substantial absence of an emulsifier. The emulsifier may be any known emulsifier, including a common surfactant. "In the substantial absence of an emulsifier" means an environment in which the content of the emulsifier is 0.03 ppm by mass or less relative to the total mass of the aqueous medium contained in the aqueous dispersion (a), preferably 0.02 ppm by mass or less, and more preferably 0 ppm by mass.

[0037] As described above, it is presumed that the specific monomer is polymerized within the particles of the first fluorine-containing polymer during polymerization of the specific monomer, and therefore, in this production method, particles containing the first fluorine-containing polymer and the second fluorine-containing polymer are generated. That is, it is presumed that, in this production method, 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 production method of this invention provides an aqueous dispersion (a) in which particles containing the first fluorine-containing polymer and the second fluorine-containing polymer are dispersed in the aqueous medium.

[0038] [Aqueous dispersion] The aqueous dispersion of the present invention (hereinafter also referred to as "the present aqueous dispersion") is an aqueous dispersion containing an aqueous medium and particles containing a fluorine-containing polymer (hereinafter also referred to as "specific particles"). In the present aqueous dispersion, the specific particles have an average particle size of 1 μm or less. In the present aqueous dispersion, the specific particles contain PAVE units, and the content of the PAVE units is 0.1 to 1.0 mol % based on the total units of the fluoropolymer. In addition, in this aqueous dispersion, the content of the compound represented by formula (S1) described below and the content of the compound represented by formula (S2) described below are each 100 ppb by mass or less relative to the total mass of the specific particles.

[0039] <Specific particles> The present aqueous dispersion is easily obtained by the present production method described above. Therefore, the specific particles contained in the present aqueous dispersion are preferably particles containing the above-mentioned first fluorine-containing polymer and the above-mentioned second fluorine-containing polymer. In this case, the fluorine-containing polymer contained in the specific particles contains two kinds of fluorine-containing polymers, the first fluorine-containing polymer and the second fluorine-containing polymer.

[0040] The specific particles contain PAVE units. Details of the PAVE units are the same as those of the PAVE units contained in the above-mentioned first fluorine-containing polymer, and preferred embodiments are also the same. The content of the PAVE units contained in the specific particles is 0.1 to 1.0 mol %, preferably 0.1 to 0.9 mol %, and more preferably 0.1 to 0.8 mol %, based on all units of the fluoropolymer contained in the specific particles. If the content of the PAVE unit is 0.1 mol % or more, the polymerization stability is more excellent, and if the content of the PAVE unit is 1.0 mol % or less, the mechanical properties are more excellent.

[0041] In this specification, when the specific particle contains only one type of fluoropolymer, "all units of the fluoropolymer" means all units contained in that one type of fluoropolymer. When the specific particle contains two or more types of fluoropolymer, "all units of the fluoropolymer" means all units contained in the two or more types of fluoropolymer.

[0042] The specific particles preferably contain PAVE units and units based on the above-mentioned fluorine-containing monomers other than PAVE units (i.e., the above-mentioned fluorine-containing units), more preferably contain PAVE units and at least one unit selected from the group consisting of TFE units, CTFE units, and VdF units, and even more preferably contain PAVE units and TFE units. When the specific particle contains fluorine-containing units other than PAVE units, the content of the fluorine-containing units other than PAVE units is preferably 30 to 70 mol %, more preferably 40 to 60 mol %, and even more preferably 45 to 60 mol %, based on the total units of the fluorine-containing polymer contained in the specific particle.

[0043] The specific particles preferably contain units based on the other monomers described above (ie, the other units described above), and more preferably contain the E units described above. When the specific particles contain other units, the content of the other units is preferably 30 to 70 mol %, more preferably 40 to 60 mol %, and even more preferably 40 to 55 mol %, based on all fluoropolymer units contained in the specific particles.

[0044] A preferred embodiment of the specific particle is an embodiment containing PAVE units, fluorine-containing units other than PAVE units, and other units, and among these, an embodiment containing PAVE units, TFE units, and E units is more preferred. Moreover, the PAVE units, fluorine-containing units other than the PAVE units, and other units contained in the specific particles are preferably contained in a fluorine-containing polymer in the specific particles.

[0045] The content of the specific particles is preferably 1 to 50 mass %, more preferably 1 to 40 mass %, and even more preferably 1 to 30 mass %, based on the total mass of the aqueous dispersion, from the viewpoint of dispersion stability of the specific particles.

[0046] The average particle size of the specific particles is 1 μm or less, and from the viewpoint of the dispersion stability of the specific particles, it is preferably 500 nm or less, and more preferably 400 nm or less. The average particle size of the specific particles is preferably 50 nm or more, more preferably 70 nm or more, and even more preferably 100 nm or more, from the viewpoint of aggregation. The average particle size of the specific particles is a particle size calculated by analyzing an autocorrelation function obtained by dynamic light scattering using a monodisperse cumulant method.

[0047] <Aqueous medium> Specific examples of the aqueous medium contained in the present aqueous dispersion are the same as the specific examples of the aqueous medium used in the production of the above-mentioned first fluoropolymer. The content of the aqueous medium is preferably 50 to 99 mass %, more preferably 60 to 99 mass %, and even more preferably 70 to 99 mass %, based on the total mass of the aqueous dispersion, from the viewpoint of dispersion stability of the specific particles.

[0048] <Compound represented by formula (S1) and compound represented by formula (S2)> The compound represented by formula (S1) and the compound represented by formula (S2) are components that can be generated when the above-mentioned specific monomer (particularly, TFE) is polymerized in the presence of a polymerization initiator, a chain transfer agent, and an emulsifier (particularly, a hydrocarbon-based emulsifier). Therefore, when no emulsifier is used in the production of the fluoropolymer contained in the present aqueous dispersion, the generation amount of the compound represented by formula (S1) and the compound represented by formula (S2) can be suppressed, and it becomes easy to keep the content of these compounds within the range described below.

[0049] Formula (S1): H-(CF2) n -COOM Formula (S2): H-(CF2) n -SO3M In formula (S1) and formula (S2), M each independently represents a hydrogen atom, Na, K, or NH 4 , and n each independently represents 8 or 10.

[0050] In this aqueous dispersion, the content of the compound represented by formula (S1) and the content of the compound represented by formula (S2) are each 100 mass ppb or less, preferably 50 mass ppb or less, more preferably 25 mass ppb or less, and even more preferably 0 mass ppb (i.e., no compound represented by formula (S1)) relative to the total mass of the specific particles.

[0051] <Other> The aqueous dispersion is preferably substantially free of an emulsifier. The emulsifier is as described above. The aqueous dispersion being substantially free of emulsifier 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 aqueous dispersion.

[0052] <Application> As described above, since the present aqueous dispersion does not require an emulsifier, it is easy to make it into a dispersion in an organic solvent such as N-methylpyrrolidone or acetone by solvent substitution. For example, the aqueous dispersion of the present invention can be mixed with an organic solvent and dehydrated using evaporation or anhydrous sodium sulfate or the like to give a dispersion in the organic solvent.

[0053] In this aqueous dispersion, the fluoropolymer is stably dispersed even without the inclusion of an emulsifier, and therefore it is suitable for use in coating applications, as a binder, etc.

[0054] In addition, the specific particles can be aggregated from the aqueous dispersion to obtain a powder of the specific particles. The powder of the specific particles obtained by aggregation can be homogenized by melt kneading or the like to form a molding material in the form of pellets, granules, or the like, and the powder of the specific particles obtained by aggregation can be melt molded to form a molded product.

[0055] Flocculation methods include, but are not limited to, freeze flocculation, acid flocculation, base flocculation, and flocculation using a coagulant. 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 coagulation, a method of adding an acid-containing solution to the aqueous dispersion is preferred. Examples of the acid to be added include hydrochloric acid, nitric acid, sulfuric acid, oxalic acid, hydrofluoric acid, etc., and hydrochloric acid is preferred. The concentration of the acid in the acid-containing solution is preferably 0.1 to 50 mass%, more preferably 1 to 30 mass%, and even more preferably 1 to 10 mass%. The base coagulation is preferably carried out by adding a solution containing a base to the aqueous dispersion. Examples of the base to be added include sodium hydroxide, potassium hydroxide, and ammonium carbonate, and sodium hydroxide is preferred. The concentration of the base in the solution containing the base 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 the aggregation using a coagulant, a known coagulant can be used. Known coagulants include aluminum salts, calcium salts, and magnesium salts. Specific examples include aluminum sulfate, alum represented by the general formula M'Al(SO4)2·12H2O (wherein M' is a monovalent cation other than lithium), calcium nitrate, and magnesium sulfate. Alum is preferred, and potassium alum, where M is potassium, is more preferred. As the aggregation method, base aggregation is preferred since aggregation is particularly likely to proceed.

[0056] [particle] The particles of the present invention (hereinafter also referred to as "the present particles") are particles containing a fluoropolymer, and the average particle size of the present particles is 1 μm or less. The particles further contain TFE units, E units, and PAVE units, with the E units being 20 to 70 mol% based on the total of the TFE units and the E units, the total content of the TFE units and the E units being 80 mol% or more based on all units of the fluoropolymer, and the content of the PAVE units being 0.1 to 1.0 mol% based on all units of the fluoropolymer. In the present particles, the content of the compound represented by the above formula (S1) and the content of the compound represented by the above formula (S2) are each 100 ppb by mass or less relative to the total mass of the present particles. The present particles are preferably particles obtained by the aggregation method using the present aqueous dispersion described above. The preferred embodiments of the present particles are the same as the preferred embodiments of the specific particles contained in the present aqueous dispersion described above, and therefore the description thereof will be omitted. Here, the average particle size of the present particles means the arithmetic mean value of the particle sizes of five different particles obtained by photographing the present particles obtained after aggregation with a scanning electron microscope and determining the particle sizes from the resulting SEM image. EXAMPLES

[0057] The present invention will be described in detail below with reference to examples. Examples 1 to 3 are working examples, and Examples 4 and 5 are comparative examples. However, the present invention is not limited to these examples.

[0058] [Measurement and evaluation methods] The various measurement and evaluation methods are as follows.

[0059] <Average particle size of particles in the raw material liquid> The raw material liquid was used as a sample and was measured using a laser diffraction / scattering particle size distribution analyzer (Otsuka Electronics Co., Ltd., ELSZ). The average particle size of the particles in the aqueous dispersion corresponding to the raw material liquid was measured in the same manner as for the raw material liquid, and was found to be the same as that of the particles in the raw material liquid.

[0060] <Average particle size after drying> The particles obtained in each example were dried and aggregated, and then photographed using a scanning electron microscope (e.g., JSM-IT700HR InTouchScope, manufactured by JEOL Ltd.). The particle sizes of five different particles measured from the obtained SEM images were calculated and the arithmetic mean was calculated.

[0061] <Proportion of each unit in the polymer> The ratio of each unit in the polymer is 19 The values ​​were determined by F-NMR analysis and infrared absorption spectroscopy.

[0062] <Content of compound represented by formula (S1)> The content of the compound represented by the above formula (S1) relative to the total mass of particles in the aqueous dispersion obtained in each example described below was calculated by a method using an aqueous dispersion among the measurement methods using a liquid chromatograph mass spectrometer described in paragraphs

[0710] to

[0720] of International Publication No. 2018 / 181904. The apparatus used was an Agilent 1260 series HPLC / 6460S, and the column used was an Imtakt Cadenza CD-C18.

[0063] <Content of compound represented by formula (S2)> The content of the compound represented by the above formula (S2) relative to the total mass of particles in the aqueous dispersion obtained in each example described below was calculated by a method using an aqueous dispersion among the measurement methods using a liquid chromatograph mass spectrometer described in paragraphs

[0721] to

[0732] of International Publication No. 2018 / 181904. The apparatus used was an Agilent 1260 series HPLC / 6460S, and the column used was an Imtakt Cadenza CD-C18.

[0064] <Sulfate ion concentration> The concentration of sulfate ions relative to the total mass of the aqueous medium in the aqueous dispersion was measured as follows: The aqueous dispersion was freeze-flocculated, and then filtered, and the resulting aqueous medium was analyzed by ion chromatography. The analysis by ion chromatography was performed using an ion chromatograph ICS-5000 (manufactured by Thermo Fisher Scientific). The separation column was Dionex IonPac AS-19, the guard column was Dionex IonPac AG-19, and the eluent was KOH.

[0065] [Production of raw material solution A] Ultrapure water (1162 g), 28% NH3 aqueous solution (1 drop), PMVE (70 g), and TFE (14 g) were charged into a 2.1 L stainless steel pressure reactor, and the temperature was raised to 80 ° C. while stirring at 600 rpm. Next, an aqueous ammonium persulfate solution (5.9 mass%, 5 cc) was added to start polymerization. Since the pressure in the reactor decreased with the start of polymerization, TFE was added to keep the pressure constant. When 24 g of TFE was injected, the reactor was cooled and the polymerization reaction was terminated. After recovering the gas remaining in the reactor, the liquid in the reactor was extracted. This liquid was used as raw material liquid A. In raw material liquid A, particles of fluoropolymer 1A (average particle size 110 nm) were dispersed in an aqueous medium, and the content of fluoropolymer 1A was 3.0 mass% with respect to the total mass of raw material liquid A. After freeze-aggregation of the raw material liquid A, it was filtered, and the obtained fluoropolymer 1A was washed with ultrapure water and then vacuum-dried at 100° C. The obtained fluoropolymer 1A was analyzed by NMR, and as a result, it was found that the ratio of PMVE units / TFE units was 34.4 / 65.6 (molar ratio).

[0066] [Production of raw material solution B] Ultrapure water (200 g) was added to the above raw material liquid A (200 g) and stirred at 200 rpm. Anion exchange resin (Purolite A300 (manufactured by Purolite), anion exchange resin, 4 g) was added thereto and stirred for 20 minutes. Thereafter, the above ion exchange resin (2 g) was added every 20 minutes. 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 B. In raw material liquid B, particles of fluoropolymer 1A (average particle size 110 nm) were dispersed in the aqueous medium, and the content of fluoropolymer 1A was 1.5 mass% based on the total mass of raw material liquid B.

[0067] [Production of raw material solution C] Ultrapure water (717 g), 28% NH3 aqueous solution (1 drop), PMVE (63 g), and TFE (10 g) were charged into a 1.3 L stainless steel pressure reactor, and the temperature was raised to 80 ° C. while stirring at 500 rpm. Next, an aqueous ammonium persulfate solution (3.6 mass%, 5 cc) was added to start polymerization. As the polymerization started, the pressure in the reactor decreased, so TFE was added to keep the pressure constant. When 5 g of TFE was injected, the reactor was cooled to terminate the polymerization reaction. After the gas remaining in the reactor was collected, the liquid was drawn off. This liquid was used as raw material liquid C. After freeze-aggregation of the raw material liquid C, it was filtered, and the obtained fluoropolymer 1C was washed with ultrapure water. It was then vacuum-dried at 100° C. The obtained fluoropolymer 1C was analyzed by NMR, and as a result, it was found that the molar ratio of PMVE unit / TFE unit was 47.3 / 52.7.

[0068] [Production of raw material solution D] To the above raw material liquid C (200 g) was added (Purolite A300 (manufactured by Purolite Corporation), anion exchange resin, 4 g), and the mixture was stirred at 200 rpm for 20 minutes. Thereafter, the above ion exchange resin (2 g) was added every 20 minutes. 60 minutes after the start of stirring, the raw material liquid was filtered to separate the ion exchange resin from the raw material liquid, thereby obtaining raw material liquid D. In raw material liquid D, particles of fluoropolymer 1C (average particle size 89 nm) were dispersed in the aqueous medium, and the content of fluoropolymer 1C was 1.1 mass % based on the total mass of raw material liquid D.

[0069] [Production of raw material solution E] After degassing a stainless steel pressure reactor with an internal volume of 2100 mL equipped with an anchor blade, ultrapure water (1170 g), disodium hydrogen phosphate (10.48 g), and polymethyl methacrylate (37 mg) were charged. Next, 75 g of PMVE and 14 g of TFE were charged while stirring the solution in the reactor (TFE:PMVE = 25:75 (molar ratio)). The temperature was raised, and when the internal temperature reached 80 ° C, the pressure inside the reactor was 1.3 MPa [gauge]. 5 mL of a solution (20 mass%) in which ammonium persulfate was dissolved in ultrapure water was added to start polymerization. As the polymerization progressed, TFE was injected when the pressure inside the reactor dropped to 1.29 MPa [gauge], and the pressure inside the reactor was increased to 1.3 MPa [gauge]. This was repeated, and 7 g of PMVE was also injected every time 8 g of TFE was injected. When the total amount of TFE added reached 160 g, the addition of the monomer to be injected after the start of polymerization (hereinafter also referred to as "post-added monomer") was stopped, the temperature inside the reactor was cooled to 10°C, the polymerization reaction was stopped, the gas remaining in the reactor was recovered, and the liquid was withdrawn. This liquid was designated as raw material liquid E. After freeze-aggregation of the raw material liquid E, it was filtered, and the obtained fluoropolymer 1E was washed with ultrapure water. It was then vacuum-dried at 100° C. The obtained fluoropolymer 1E was analyzed by NMR, and found to have a PMVE / TFE molar ratio of 34 / 66. The average particle size of the fluoropolymer 1E was 290 to 300 nm.

[0070] [Production of raw material solution F] Ultrapure water (180 g) was added to the above raw material liquid E (20 g), and anion exchange resin (Purolite A300 (manufactured by Purolite Corp.), anion exchange resin, 4 g) was added, followed by stirring at 200 rpm for 20 minutes. Thereafter, the above ion exchange resin (2 g) was added every 20 minutes. 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 F. In raw material liquid F, particles of fluoropolymer 1E (average particle size 326 nm) were dispersed in the aqueous medium, and the content of fluoropolymer 1E was 2.0 mass% based on the total mass of raw material liquid F.

[0071] [Example 1] Ultrapure water (428 g), raw material liquid B (185 ml), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) were charged into a 1.2 L stainless steel pressure-resistant reactor to obtain an aqueous dispersion B. The aqueous dispersion B was heated to 60° C. while stirring at 320 rpm. A mixed gas (TFE / ethylene = 86 / 14 (molar ratio)) was injected until the pressure in the reactor reached 2.6 MPaG, and a TBHP aqueous solution (0.2 mass%, 2 ml) and a reducing agent (BRUGGOLITE (registered trademark) FF6M, 0.396 mass%, 2 cc) were added to start polymerization. Since the pressure in the reactor decreased with the start of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (mol%)) was added to keep the pressure constant. A TBHP aqueous solution (0.2 mass%, 1 cc) and the above reducing agent (1 cc) were added every 5 minutes from the start of polymerization. When 80 g of the mixed CG gas was injected, the reactor was cooled and the polymerization reaction was terminated after 167 minutes. The content of the fluoropolymer 1A in the aqueous dispersion B was 0.44% by mass. In addition, when the amount of the aqueous medium in the aqueous dispersion B used in the polymerization was taken as 100 parts by mass, the amount of the monomers (TFE and ethylene) used in the polymerization was 12.3 parts by mass. In addition, after the aqueous dispersion B was freeze-aggregated and filtered, the resulting aqueous medium was analyzed by ion chromatography, and the concentration of sulfate ions was found to be less than 0.1 ppm by mass. After collecting the gas remaining in the reactor, the liquid was drawn off. This liquid was designated as Aqueous Dispersion 1. Aqueous Dispersion 1 was a dispersion in which particles (average particle diameter 196 nm) containing the fluoropolymer 2A were dispersed in an aqueous medium, and had a solid content concentration of 10.6% by mass. The particles were aggregated and dried, and the composition was calculated by NMR to be TFE unit / E unit / PMVE unit=56.3 / 43.1 / 0.6 (molar ratio). The average particle size of the dried particles was 156 nm. In addition, in aqueous dispersion 1, the contents of the compound represented by formula (S1) and the compound represented by formula (S2) relative to the total mass of the particles in aqueous dispersion 1 were both 100 ppb by mass or less.

[0072] [Example 2] Ultrapure water (520.5 g), raw material liquid A (92.5 ml), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) were charged into a 1.2 L stainless steel pressure-resistant reactor to obtain an aqueous dispersion A. The aqueous dispersion A was heated to 60 ° C. while stirring at 320 rpm. A mixed gas (TFE / ethylene = 86 / 14 (molar ratio)) was injected into the reactor, but the pressure started to drop at the same time as the injection, so when the injection reached 2.6 MPaG, an aqueous TBHP solution (0.2 mass%, 2 ml) and a reducing agent (BRUGGOLITE (registered trademark) FF6M, 0.396 mass%, 2 cc) were added to start polymerization. As the pressure in the reactor decreased with the start of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to keep the pressure constant. Every 5 minutes from the start of polymerization, a TBHP aqueous solution (0.2 mass%, 1 cc) and the above reducing agent (1 cc) were added. When 80 g of mixed CG gas was injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 181 minutes. The content of the fluoropolymer 1A was 0.22% by mass based on the total mass of the aqueous dispersion A. In addition, when the amount of the aqueous medium in the aqueous dispersion A used in the polymerization was taken as 100 parts by mass, the amount of the monomers (TFE and ethylene) used in the polymerization was 12.7 parts by mass. Furthermore, after the aqueous dispersion A was freeze-aggregated and filtered, the resulting aqueous medium was analyzed by ion chromatography, and the sulfate ion concentration was found to be 8.2 ppm by mass. After the gas remaining in the reactor was collected, the liquid was drawn off. This liquid was designated as aqueous dispersion 2. Aqueous dispersion 2 was a dispersion in which particles (average particle diameter 212 nm) containing fluoropolymer 2B were dispersed in an aqueous medium, and had a solid content concentration of 10.2% by mass. The particles were aggregated and dried, and the composition was calculated by NMR to be TFE unit / E unit / PMVE unit=56.9 / 42.4 / 0.7 (molar ratio). The average particle size of the dried particles was 182 nm. In addition, in aqueous dispersion 2, the contents of the compound represented by formula (S1) and the compound represented by formula (S2) relative to the total mass of the particles in aqueous dispersion 2 were both 100 ppb by mass or less.

[0073] [Example 3] Ultrapure water (428 g), raw material liquid D (185 ml), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) were charged into a 1.2 L stainless steel pressure-resistant reactor to obtain an aqueous dispersion D. The aqueous dispersion D was heated to 60° C. while stirring at 320 rpm. A mixed gas (TFE / ethylene = 86 / 14 (molar ratio)) was injected until the pressure in the reactor reached 2.6 MPaG, and a TBHP aqueous solution (0.2 mass%, 2 ml) and a reducing agent (BRUGGOLITE (registered trademark) FF6M, 0.396 mass%, 2 cc) were added to start polymerization. Since the pressure in the reactor decreased with the start of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to keep the pressure constant. A TBHP aqueous solution (0.2 mass%, 1 cc) and the above reducing agent (1 cc) were added every 5 minutes from the start of polymerization. When 92 g of the mixed CG gas was injected, the reactor was cooled and the polymerization reaction was terminated after 97 minutes. The content of the fluoropolymer 1C in the aqueous dispersion D was 0.32% by mass based on the total mass of the aqueous dispersion D. In addition, when the amount of the aqueous medium used in the aqueous dispersion D used in the polymerization was taken as 100 parts by mass, the amount of the monomers (TFE and ethylene) used in the polymerization was 14.6 parts by mass. Furthermore, after Aqueous Dispersion D was freeze-aggregated and then filtered, the resulting aqueous medium was analyzed by ion chromatography, and the concentration of sulfate ions was found to be less than 0.1 ppm by mass. After the gas remaining in the reactor was collected, the liquid was drawn off. This liquid was designated as Aqueous Dispersion 3. Aqueous Dispersion 3 was a dispersion in which particles (average particle diameter 461 nm) containing the fluoropolymer 2C were dispersed in an aqueous medium, and had a solid content concentration of 12 mass%. The particles were aggregated and dried, and the composition was calculated by NMR to be TFE unit / E unit / PMVE unit=55.9 / 43.7 / 0.4 (molar ratio). The average particle size of the dried particles was 427 nm. In addition, in aqueous dispersion 3, the contents of the compound represented by formula (S1) and the compound represented by formula (S2) were both 100 ppb by mass or less relative to the total mass of the particles in aqueous dispersion 3.

[0074] [Example 4] Ultrapure water (613 g), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) were charged into a 1.2 L stainless steel pressure reactor, and the temperature was raised to 60 ° C. while stirring at 320 rpm. A mixed gas (TFE / ethylene = 86 / 14 (molar ratio)) was injected until the pressure in the reactor reached 2.6 MPaG, and a TBHP aqueous solution (0.2 mass%, 2 ml) and a reducing agent (BRUGGOLITE (registered trademark) FF6M, 0.396 mass%, 2 cc) were added to start polymerization. Since the pressure in the reactor decreased with the start of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to keep the pressure constant. A TBHP aqueous solution (0.2 mass%, 1 cc) and the above reducing agent (1 cc) were added every 5 minutes from the start of polymerization. When 92 g of the mixed CG gas was injected, the reactor was cooled to terminate the polymerization reaction. The polymerization time was 340 minutes. After the gas remaining in the reactor was collected, the liquid was extracted, but all of it had aggregated, and it was not possible to obtain an aqueous dispersion in which fluoropolymer particles were dispersed in an aqueous medium.

[0075] [Example 5] Ultrapure water (513 g), raw material liquid F (100 ml), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) were charged into a 1.2 L stainless steel pressure-resistant reactor to obtain an aqueous dispersion F. The aqueous dispersion F was heated to 60° C. while stirring at 320 rpm. A mixed gas (TFE / ethylene = 86 / 14 (molar ratio)) was injected until the pressure in the reactor reached 2.6 MPaG, and a TBHP aqueous solution (0.2 mass%, 2 ml) and a reducing agent (BRUGGOLITE (registered trademark) FF6M, 0.396 mass%, 2 cc) were added to start polymerization. Since the pressure in the reactor decreased with the start of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to keep the pressure constant. A TBHP aqueous solution (0.2 mass%, 1 cc) and the above reducing agent (1 cc) were added every 5 minutes from the start of polymerization. When 80 g of the mixed CG gas was injected, the reactor was cooled and the polymerization reaction was terminated after 370 minutes. The content of the fluoropolymer 1E was 0.32% by mass based on the total mass of the aqueous dispersion F. In addition, after the aqueous dispersion F was freeze-aggregated and filtered, the resulting aqueous medium was analyzed by ion chromatography, and the concentration of sulfate ions was found to be less than 0.1 ppm by mass. After the gas remaining in the reactor was collected, the liquid was extracted, but all of it had aggregated, and it was not possible to obtain an aqueous dispersion in which fluoropolymer particles were dispersed in an aqueous medium.

[0076] [evaluation] <Coloring> The particles contained in the aqueous dispersions 1 to 3 of each example were aggregated and dried to obtain an aggregate corresponding to each example. A sheet having a thickness of 1 mm was prepared from the obtained aggregate, and YI and CIE W were measured in transmission using a color meter (SM color meter, manufactured by Suga Test Instruments Co., Ltd.). The values ​​of YI and CIE W are shown in Table 1. Here, "YI" is an index representing yellowness, and the higher this value, the more yellowish it is. Also, "CIE W" is an index representing whiteness, and the lower this value, the more blackish it is. A sheet having a thickness of 1 mm was obtained by heating 2 g of the aggregate at 300° C. for 10 minutes, press-molding the aggregate at 300° C. for 5 minutes under 10 MPa, and then cooling the aggregate. In addition, for Examples 4 and 5, an aqueous dispersion was not obtained, and therefore coloring could not be evaluated.

[0077] [Table 1]

[0078] According to the process for producing a fluoropolymer of the present invention, it was shown that a fluoropolymer can be efficiently produced using an aqueous medium with a small environmental load and without requiring an emulsifier (Examples 1 to 3). Furthermore, comparison of Examples 1 to 3 shows that when an aqueous dispersion having a sulfate ion concentration of 5 ppm by mass or less based on the total mass of the aqueous medium contained in the aqueous dispersion is used for polymerization, coloration of the fluoropolymer can be sufficiently suppressed (Examples 1 and 3). In contrast, in Examples 4 and 5, the fluoropolymer could not be produced efficiently.

[0079] In addition, the entire contents of the specification, claims and abstract of Japanese Patent Application No. 2022-201048 filed on December 16, 2022 are hereby incorporated by reference as the disclosure of the present invention.

Claims

1. A method for producing a fluorine-containing polymer, comprising polymerizing a monomer containing a fluorine-containing monomer in an aqueous dispersion containing a first fluorine-containing polymer containing a unit based on tetrafluoroethylene and a unit based on perfluoro(alkyl vinyl ether), and an aqueous medium, to produce a second fluorine-containing polymer different from the first fluorine-containing polymer, comprising: in the first fluorinated polymer, the unit based on perfluoro(alkyl vinyl ether) is 20 to 60 mol % relative to the total of the unit based on tetrafluoroethylene and the unit based on perfluoro(alkyl vinyl ether), the first fluorine-containing polymer has an average particle size of 1 to 150 nm, The method for producing a fluorinated polymer, wherein the content of the first fluorinated polymer is 0.01 to 4.0% by mass based on the total mass of the aqueous dispersion before initiation of polymerization of the monomers.

2. The process for producing a fluoropolymer according to claim 1, wherein the concentration of sulfate ions is 5 ppm by mass or less based on the total mass of the aqueous medium in the aqueous dispersion before the initiation of polymerization of the monomers.

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

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

5. The process 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.

6. 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.

7. A particle containing a fluorine-containing polymer, The average particle size of the particles is 1 μm or less, the particles comprise tetrafluoroethylene-based units, ethylene-based units, and perfluoro(alkyl vinyl ether)-based units; the ethylene-based units constitute 20 to 70 mol % of the total of the tetrafluoroethylene-based units and the ethylene-based units; the total content of the units based on tetrafluoroethylene and the units based on ethylene is 80 mol % or more based on all units of the fluorine-containing polymer, the content of units based on the perfluoro(alkyl vinyl ether) is 0.1 to 1.0 mol % based on all units of the fluorine-containing polymer, A particle, wherein the content of the compound represented by formula (S1) and the content of the compound represented by formula (S2) are each 100 ppb by mass or less relative to the total mass of the particle. Formula (S1): H-(CF ２ ) ｎ -COOM Formula (S2): H-(CF ２ ) ｎ -SO ３ M In formula (S1) and formula (S2), M is independently a hydrogen atom, Na, K, or NH ４ and each n independently represents 8 or 10.

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