Method for producing fluorine-containing polymer, aqueous dispersion liquid, and particle

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

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

[0001] The present invention relates to a method for producing a fluorine-containing polymer, an aqueous dispersion, and particles thereof.

[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, etc. Examples of methods for producing fluorine-containing polymers include a method of emulsion polymerization of fluorine-containing monomers in an aqueous medium using a fluorine-containing emulsifier (see Patent Document 1).

[0003] International Publication No. 2007 / 046377

[0004] The process for producing a fluoropolymer disclosed in Patent Document 1 uses an aqueous medium and therefore has a small environmental impact. However, when a large amount of an emulsifier, which is an essential component, remains in the aqueous dispersion obtained by polymerization, it may be 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 that has a small environmental impact and does not require an emulsifier. Another object of the present invention is to provide an aqueous dispersion and particles.

[0006] As a result of intensive investigations, 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 a monomer containing a fluorine-containing 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, to produce a second fluoropolymer different from the first fluoropolymer, wherein the units based on perfluoro(alkyl vinyl ether) account for 20 to 60 mol % of the total of the units based on tetrafluoroethylene and the units based on perfluoro(alkyl vinyl ether) in the first fluoropolymer, the average particle size of the first fluoropolymer is 1 to 150 nm, and the content of the first fluoropolymer before the start of polymerization of the monomers is 0.01 to 4.0 mass % of the total mass of the aqueous dispersion. [2] The method for producing a fluoropolymer according to [1], wherein the concentration of sulfate ions is 5 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion before initiation of polymerization of the monomer. [3] The method for producing a fluoropolymer 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 fluoropolymer according to any of [1] to [3], wherein the monomer comprises ethylene. [5] The method for producing a fluoropolymer according to any 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 fluoropolymer according to any 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, wherein the particles have an average particle size of 1 μm or less, the particles contain units based on perfluoro(alkyl vinyl ether), the content of the units based on perfluoro(alkyl vinyl ether) being 0.1 to 1.0 mol % based on all units of the fluorine-containing polymer, and 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 based on the total mass of the particles. Formula (S1): H-(CF. ２ ) ｎ -COOM Formula (S2): H-(CF ２ ) ｎ -SO ３ M 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] Particles comprising a fluoropolymer, wherein the average particle size of the particles is 1 μm or less, the particles comprise units based on tetrafluoroethylene, units based on ethylene, and units based on perfluoro(alkyl vinyl ether), the ethylene-based units account for 20 to 70 mol% of the total of the tetrafluoroethylene-based units and the ethylene-based units, the total content of the tetrafluoroethylene-based units and the ethylene-based units is 80 mol% or more of all units of the fluoropolymer, the content of the perfluoro(alkyl vinyl ether)-based units is 0.1 to 1.0 mol% of all units of the fluoropolymer, and 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 of the total mass of the particles. Formula (S1): H-(CF ２ )ｎ -COOM Formula (S2): H-(CF ２ ) ｎ -SO ３ M In formula (S1) and formula (S2), M each independently represents a hydrogen atom, Na, K, or NH ４ and n's each independently represent 8 or 10.

[0007] According to the present invention, there is 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. Furthermore, according to the present invention, there are also provided an aqueous dispersion and particles.

[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 containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), and an aqueous medium (hereinafter the aqueous dispersion used in the present production method is also referred to as "aqueous dispersion (a)"). In this production method, the first fluoropolymer 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 addition, in this production method, the average particle size of the first fluoropolymer is 1 to 150 nm. In addition, in the present production method, the content of the first fluoropolymer is 0.01 to 4.0 mass % based on the total mass of the aqueous dispersion (a) before the start of polymerization of the monomers.

[0010] The reason why the second fluorine-containing polymer was efficiently produced by the present production method without the essential use of an emulsifier is presumed to be that the first fluorine-containing polymer functioned as a good polymerization site for the second fluorine-containing polymer by using an aqueous dispersion containing a predetermined amount of a first fluorine-containing polymer 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 fluorinated polymer and an aqueous medium is used.

[0012] (First Fluorine-Containing Polymer) 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] The PAVE is preferably a monomer represented by formula (1) from the viewpoint of excellent polymerization reactivity in producing the first fluoropolymer and enabling more efficient production of the second fluoropolymer. ２ =CF-O-R ｆ１ (1) In formula (1), R ｆ１ represents a perfluoroalkyl group having 1 to 10 carbon atoms. ｆ１ From the viewpoint of better polymerization reactivity, the number of carbon atoms in the perfluoroalkyl group is preferably 1 to 8, more preferably 1 to 6, even more preferably 1 to 5, and particularly preferably 1 to 3. 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"), and among these, PMVE and PPVE are preferred, with PMVE being more preferred, from the viewpoint of enabling more efficient production of the second fluorinated polymer.

[0015] In the first fluorine-containing polymer, the content of PAVE units relative to the total of TFE units and PAVE units is 20 to 60 mol %, and from the viewpoint of more efficient production of the second fluorine-containing polymer, it is preferably 25 to 60 mol %, more preferably 30 to 55 mol %.

[0016] The first fluorine-containing polymer may contain units based on other monomers other than TFE and PAVE, but preferably does not substantially contain units based on other monomers, from the viewpoint of more efficient production of the second fluorine-containing polymer. "Substantially does not contain units based on other monomers" means that the content of units based on 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 other monomers are contained, the other monomer is preferably hexafluoropropylene.

[0017] Before the start of polymerization of the monomers to be used in polymerization of the second fluorine-containing polymer, the content of the first fluorine-containing polymer is 0.01 to 4.0 mass% relative to the total mass of the aqueous medium in the aqueous dispersion (a), and from the viewpoint of enabling more efficient production of the second fluorine-containing polymer, 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 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.

[0019] Before starting polymerization of the monomers used in the polymerization of the second fluoropolymer, the sulfate ion concentration is preferably 10 ppm by mass or less, more preferably 5 ppm by mass or less, relative to the total mass of the aqueous medium in the aqueous dispersion (a), from the viewpoint of suppressing discoloration of the second fluoropolymer. An example of the lower limit is 0 ppm by mass. One example of a method for adjusting the sulfate ion concentration to the above value is a method in which sulfate ions are removed using an anion exchange resin during the production of the first fluoropolymer. Here, the sulfate ions are derived, for example, from the polymerization initiator (particularly ammonium persulfate) used during the production of the first fluoropolymer, and may be contained in the aqueous dispersion (a) containing the first fluoropolymer. It is presumed that by keeping the sulfate ion content at 10 ppm by mass or less (particularly 5 ppm by mass or less), it is possible to suppress the formation of terminal groups with low heat resistance in the second fluoropolymer, thereby suppressing discoloration 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 1 to 150 nm, and from the viewpoint of enabling more efficient production of the second fluoropolymer, is preferably 10 to 120 nm, more preferably 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 in which monomers containing TFE and PAVE are polymerized in an aqueous medium in the presence of a polymerization initiator. This results in the first fluoropolymer dispersed in the aqueous medium in the form of particles. The aqueous medium thus obtained, in which the first fluoropolymer particles are dispersed, may be used as the aqueous dispersion (a) as is, or another aqueous medium may be added and used as the aqueous dispersion (a). Alternatively, the first fluoropolymer may be dispersed in another aqueous medium after solvent substitution and used as the aqueous dispersion (a). It is preferable that sulfate ions have been removed from the aqueous dispersion (a) by a sulfate ion removal means such as an anion exchange resin. The polymerization initiator used in the production of the first fluoropolymer 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, more preferably a persulfate, and particularly preferably ammonium persulfate. The aqueous medium used in producing 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. As described above, the aqueous medium contained in the aqueous dispersion (a) may be the polymerization solvent used in producing the first fluoropolymer. 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 producing the first fluoropolymer described above. Before starting polymerization of the monomers used in polymerizing the second fluoropolymer, the content of the aqueous medium is preferably 60 to 99.9 mass%, more preferably 96 to 99.9 mass%, and even more preferably 98 to 99.9 mass%, based on the total mass of the aqueous dispersion (a).

[0023] (Other Components) The aqueous dispersion (a) 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 (a) may contain include chain transfer agents, emulsifiers, and pH adjusters. 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 CF ３ CF ２ -O-CF ２ CF ２ -O-CF ２ COONH ４ , C.F. ３ CF ２ CF ２ OCF (CF ３ Fluorine-containing emulsifiers such as methyl acrylate, methyl meth ... When the aqueous dispersion (a) contains an emulsifier, the content of the emulsifier is preferably 0.01 to 5 parts by mass relative 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 fluoride ion concentration is preferably 100 mass ppm or less, more preferably 50 mass ppm or less, based on the total mass of the aqueous dispersion (a), from the viewpoint of polymerization stability.The lower limit can be 0 mass ppm.One example of a method for adjusting the fluoride ion concentration to the above value is a method in which sulfate ions are removed using an anion exchange resin during the production of the first fluorine-containing polymer.Here, fluoride ions may be generated by the reaction between a polymerization initiator (e.g., 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 includes 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 includes at least one selected from the group consisting of TFE, CTFE, and VdF (hereinafter also referred to as "specific fluorine-containing monomer"), more preferably includes TFE, and even more preferably is TFE. Two or more types of fluorine-containing monomers may be used in combination. The amount of the fluorine-containing monomer used is preferably 10.0 to 100.0 mol%, more preferably 30.0 to 70.0 mol%, and even more preferably 40.0 to 60.0 mol%, relative to the amount of the specific monomer used. It is also preferable to include 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 of the other 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(CF２ ) ３ H and CH ２ =CF(CF ２ ) ４ H is exemplified, with C4OLF being preferred. PAVE is the same as the PAVE in the first fluorinated polymer described above, and preferred embodiments are also the same. The amount of the other fluorinated monomer used is preferably 0.1 to 30.0 mol %, more preferably 0.1 to 10.0 mol %, and even more preferably 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 these, the other monomer preferably contains ethylene, and more preferably is ethylene. Two or more 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. 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 ions. Transition metal ions are preferred, including manganese, iron, cobalt, nickel, copper, zinc, cerium, and silver ions, with iron ions 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. From the viewpoint of more efficient production of fluoropolymers, 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.

[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, per 100 parts by mass of the specific monomer used.

[0030] <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 0.1 to 2 parts by mass per 100 parts by mass of the specific monomer used.

[0031] <Step> In the present production method, the specific monomer is polymerized in the 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 even more preferably 35 to 55 mol%. The proportion of the total of E units and TFE units 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 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 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.

[0035] 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.

[0036] The polymerization of the specific monomer is preferably carried out in the substantial absence of an emulsifier. Examples of the emulsifier include known emulsifiers, such as common surfactants. The substantial absence of an emulsifier means an environment in which the content of the emulsifier is 0.03 mass ppm or less, preferably 0.02 mass ppm or less, more preferably 0 mass ppm, relative to the total mass of the aqueous medium contained in the aqueous dispersion (a).

[0037] 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 provides an aqueous dispersion (a) in which particles containing the first fluorine-containing polymer and the second fluorine-containing polymer are dispersed in the above 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 fluoropolymer (hereinafter also referred to as "specific particles"). In the present aqueous dispersion, the average particle size of the specific particles is 1 μm or less. Furthermore, 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. Furthermore, in the present 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 based on the total mass of the specific particles.

[0039] <Specific particles> The present aqueous dispersion can be 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 types 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 first fluoropolymer described above, 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 the total units of the fluoropolymer contained in the specific particles. If the content of PAVE units is 0.1 mol% or more, the polymerization stability is more excellent. Furthermore, if the content of PAVE units is 1.0 mol% or less, the mechanical properties are more excellent.

[0041] In this specification, when the specific particles contain only one type of fluorine-containing polymer, "all units of the fluorine-containing polymer" means all units contained in that one type of fluorine-containing polymer. When the specific particles contain two or more types of fluorine-containing polymer, "all units of the fluorine-containing polymer" means all units contained in the two or more types of fluorine-containing polymer.

[0042] The specific particles preferably contain PAVE units and units based on the above-mentioned fluorine-containing monomer 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 particles contain 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 still more preferably 45 to 60 mol%, based on the total units of the fluorine-containing polymer contained in the specific particles.

[0043] The specific particles preferably contain units based on the other monomers described above (i.e., the other units described above), and more preferably contain the units E. When the specific particles contain the 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 the total units of the fluorine-containing polymer contained in the specific particles.

[0044] A preferred embodiment of the specific particles 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. Furthermore, the PAVE units, fluorine-containing units other than PAVE units, and other units contained in the specific particles are preferably contained in the fluorine-containing polymer in the specific particles.

[0045] The content of the specific particles is preferably 1 to 50% by mass, more preferably 1 to 40% by mass, and even more preferably 1 to 30% by 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 dispersion stability of the specific particles, it is preferably 500 nm or less, more preferably 400 nm or less. From the viewpoint of aggregation, 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. 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 producing the first fluoropolymer 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 present aqueous dispersion, from the viewpoint of dispersion stability of the specific particles.

[0048] <The compound represented by formula (S1) and the compound represented by formula (S2)> The compound represented by formula (S1) and the compound represented by formula (S2) are the components that can be generated when polymerizing above-mentioned specific monomer (particularly TFE) in the presence of polymerization initiator, chain transfer agent and emulsifier (particularly hydrocarbon-based emulsifier).Therefore, when no emulsifier is used during the production of the fluorine-containing polymer contained in this aqueous dispersion, the generated amount of the compound represented by formula (S1) and the compound represented by formula (S2) can be suppressed, so that the content of these compounds can be easily set within the range described below.

[0049] Formula (S1): H-(CF ２ ) ｎ -COOM Formula (S2): H-(CF ２ ) ｎ -SO ３ M In formula (S1) and formula (S2), M each independently represents a hydrogen atom, Na, K, or NH ４ and n's each independently represent 8 or 10.

[0050] In the present 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] <Others> The present aqueous dispersion preferably does not substantially contain an emulsifier. The emulsifier is as described above. "The present aqueous dispersion is substantially free of an emulsifier" means that 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 present aqueous dispersion.

[0052] <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.

[0053] The aqueous dispersion of the present invention stably disperses the fluoropolymer even without the addition of an emulsifier, and is therefore suitable for use in coating applications, binders, etc.

[0054] Furthermore, by aggregating the specific particles from the aqueous dispersion, a powder of the specific particles can be obtained. Furthermore, 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 also be melt molded or the like to form a molded product.

[0055] Aggregation methods include, but are not limited to, freeze aggregation, acid aggregation, base aggregation, and aggregation using a coagulation agent. 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 present aqueous dispersion is preferred. Examples of acids to be added include hydrochloric acid, nitric acid, sulfuric acid, oxalic acid, and hydrofluoric acid, with hydrochloric acid being preferred. The acid concentration 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. In the case of base aggregation, a method in which a base-containing solution is added to the present aqueous dispersion is preferred. Examples of bases to be added include sodium hydroxide, potassium hydroxide, and ammonium carbonate, with sodium hydroxide being preferred. The base concentration 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 aggregation using a coagulation agent, known coagulation agents 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. Base coagulation is preferred as a coagulation method, as it is particularly easy to proceed with coagulation.

[0056] [Particles] 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 present particles contain TFE units, E units, and PAVE units, with the E units accounting for 20 to 70 mol% of 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 of all units of the fluoropolymer, and the PAVE unit content being 0.1 to 1.0 mol% of all units of the fluoropolymer. In the present particles, the content of the compound represented by formula (S1) above and the content of the compound represented by formula (S2) above 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 an aggregation method using the present aqueous dispersion described above. Preferred embodiments of the present particles are the same as those of the specific particles contained in the present aqueous dispersion described above, and therefore, 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.

[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] Various measurement and evaluation methods are as follows.

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

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

[0061] <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.

[0062] <Content of the compound represented by formula (S1)> The content of the compound represented by 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 WO 2018 / 181904. The apparatus used was an Agilent 1260 series HPLC / 6460S, and the column used was a Cadenza CD-C18 manufactured by Imtakt.

[0063] <Content of the compound represented by formula (S2)> The content of the compound represented by 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 WO 2018 / 181904. The apparatus used was an Agilent 1260 series HPLC / 6460S, and the column used was a Cadenza CD-C18 manufactured by Imtakt.

[0064] <Sulfate Ion Concentration> The sulfate ion concentration relative to the total mass of the aqueous medium in the aqueous dispersion was measured as follows. The aqueous dispersion 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 ICS-5000 (manufactured by Thermo Fisher Scientific). A Dionex IonPac AS-19 separation column, a Dionex IonPac AG-19 guard column, and KOH were used as the eluent.

[0065] [Production of Raw Material Solution A] Ultrapure water (1162 g), 28% NH ３An aqueous solution (1 drop), PMVE (70 g), and TFE (14 g) were charged, and the temperature was raised to 80°C while stirring at 600 rpm. Next, an aqueous ammonium persulfate solution (5.9% by mass, 5 cc) was added to initiate polymerization. Since the pressure in the reactor decreased with the start of polymerization, TFE was added to maintain the pressure constant. When 24 g of TFE had been injected, the reactor was cooled to terminate the polymerization reaction. 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% by mass relative to the total mass of raw material liquid A. Raw material liquid A was freeze-aggregated and then filtered, and the obtained fluoropolymer 1A was washed with ultrapure water. Thereafter, it was vacuum-dried at 100°C. The resulting fluoropolymer 1A was analyzed by NMR, and as a result, it was found that the ratio of PMVE units to TFE units was 34.4 / 65.6 (molar ratio).

[0066] [Production of Raw Material Solution B] Ultrapure water (200 g) was added to the raw material solution A (200 g) and stirred at 200 rpm. To this was added an ion exchange resin (Purolite A300 (manufactured by Purolite Corporation), an anion exchange resin, 4 g) and stirred for 20 minutes. Thereafter, the ion exchange resin (2 g) was added every 20 minutes. 60 minutes after the start of stirring, the raw material solution and the ion exchange resin were separated by filtration to obtain raw material solution B. In raw material solution B, particles of fluoropolymer 1A (average particle size 110 nm) were dispersed in an aqueous medium, and the content of fluoropolymer 1A was 1.5 mass% based on the total mass of raw material solution B.

[0067] [Production of Raw Material Solution C] Ultrapure water (717 g), 28% NH ３An aqueous solution (1 drop), PMVE (63 g), and TFE (10 g) were charged, 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 initiate polymerization. Since the pressure in the reactor decreased with the start of polymerization, TFE was added to maintain the pressure constant. When 5 g of TFE had been injected, the reactor was cooled to terminate the polymerization reaction. The gas remaining in the reactor was recovered, and the liquid was then withdrawn. This liquid was used as raw material liquid C. Raw material liquid C was freeze-coagulated and then filtered, and the resulting fluoropolymer 1C was washed with ultrapure water. Thereafter, it was vacuum-dried at 100°C. The resulting fluoropolymer 1C was analyzed by NMR, and the ratio of PMVE units to TFE units was 47.3 / 52.7 (molar ratio).

[0068] [Production of Raw Material Liquid D] To the above raw material liquid C (200 g) was added 4 g of Purolite A300 (manufactured by Purolite Corporation), 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 separated from the ion exchange resin by filtration to obtain raw material liquid D. Raw material liquid D contained particles of fluoropolymer 1C (average particle size 89 nm) dispersed in an 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 2100 mL stainless steel pressure reactor equipped with an anchor blade, ultrapure water (1170 g), disodium hydrogen phosphate (10.48 g), and polymethyl methacrylate (37 mg) were charged. Next, while stirring the solution in the reactor, 75 g of PMVE and 14 g of TFE were charged (TFE: PMVE = 25:75 (molar ratio)). The temperature was raised, and when the internal temperature reached 80 ° C., the internal pressure of the reactor was 1.3 MPa [gauge]. 5 mL of a solution (20 mass%) of ammonium persulfate dissolved in ultrapure water was added to initiate polymerization. As the polymerization progressed, when the internal pressure of the reactor dropped to 1.29 MPa [gauge], TFE was injected, and the internal pressure of 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 added mass of TFE reached 160 g, the addition of the monomer 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 then withdrawn. This liquid was used as raw material liquid E. Raw material liquid E was freeze-coagulated and then filtered, and the obtained fluoropolymer 1E was washed with ultrapure water. Thereafter, it was vacuum-dried at 100°C. The obtained fluoropolymer 1E was analyzed by NMR, and as a result, it was found that the ratio of PMVE / TFE was 34 / 66 (molar ratio). The average particle size of the fluoropolymer 1E was 290 to 300 nm.

[0070] [Production of Raw Material Liquid F] Ultrapure water (180 g) was added to the above raw material liquid E (20 g), and an ion exchange resin (Purolite A300 (manufactured by Purolite Corporation), 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 an 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 solution 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 reactor to obtain aqueous dispersion B. 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 inside the reactor reached 2.6 MPaG, and 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 initiate polymerization. As the pressure inside the reactor decreased with the initiation of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (mol%)) was added to maintain the pressure constant. Every 5 minutes from the start of polymerization, an aqueous TBHP solution (0.2% by mass, 1 cc) and the reducing agent (1 cc) were added. When 80 g of the mixed CG gas was injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 167 minutes. The content of fluoropolymer 1A was 0.44% by mass relative to the total mass of aqueous dispersion B. When the amount of the aqueous medium used in 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. After freeze-coagulation of aqueous dispersion B, the aqueous medium obtained was filtered and analyzed by ion chromatography. The sulfate ion concentration was found to be less than 0.1 ppm by mass. The gas remaining in the reactor was recovered, and the liquid was then withdrawn. This liquid was designated aqueous dispersion 1. Aqueous dispersion 1 is a dispersion in which particles (average particle diameter 196nm) containing fluorine-containing polymer 2A are dispersed in an aqueous medium, and its solid content concentration is 10.6% by mass.The particles obtained are aggregated and dried, and then the composition is calculated using NMR, and the result is that TFE unit / E unit / PMVE unit=56.3 / 43.1 / 0.6 (molar ratio).In addition, the average particle diameter of the particles after drying is 156nm.In addition, in aqueous dispersion 1, the contents of the compound represented by the above formula (S1) and the compound represented by the above formula (S2) relative to the total mass of the particles in aqueous dispersion 1 are all 100 mass ppb or less.

[0072] [Example 2] Ultrapure water (520.5 g), raw material solution 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 reactor to obtain aqueous dispersion A. 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 a pressure drop began immediately after the injection. Therefore, 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 initiate polymerization. As the pressure in the reactor decreased with the initiation of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to maintain a constant pressure. Every 5 minutes from the start of polymerization, an aqueous TBHP solution (0.2% by mass, 1 cc) and the reducing agent (1 cc) were added. When 80 g of the mixed CG gas had been injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 181 minutes. The content of fluoropolymer 1A was 0.22% by mass relative to the total mass of aqueous dispersion A. When the amount of the aqueous medium used in aqueous dispersion A used in polymerization was taken as 100 parts by mass, the amount of the monomers (TFE and ethylene) used in polymerization was 12.7 parts by mass. After freeze-coagulation of aqueous dispersion A and filtration, the resulting aqueous medium was analyzed by ion chromatography. The sulfate ion concentration was 8.2 ppm by mass. The gas remaining in the reactor was recovered, and the liquid was then withdrawn. This liquid was designated aqueous dispersion 2. Aqueous dispersion 2 is a dispersion in which particles (average particle diameter 212 nm) containing fluorine-containing polymer 2B are dispersed in an aqueous medium, and the solid content concentration is 10.2 mass%. The obtained particles are aggregated and dried, and then the composition is calculated using NMR, and the result is that TFE unit / E unit / PMVE unit=56.9 / 42.4 / 0.7 (molar ratio). The average particle diameter of the particles after drying is 182 nm. In addition, in aqueous dispersion 2, the contents of the compound represented by the above formula (S1) and the compound represented by the above formula (S2) relative to the total mass of the particles in aqueous dispersion 2 are both 100 mass ppb or less.

[0073] [Example 3] Ultrapure water (428 g), raw material solution 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 reactor to obtain aqueous dispersion D. 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 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 initiate polymerization. As the pressure in the reactor decreased with the initiation of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to maintain the pressure constant. Every 5 minutes from the start of polymerization, an aqueous TBHP solution (0.2% by mass, 1 cc) and the reducing agent (1 cc) were added. When 92 g of the mixed CG gas had been injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 97 minutes. The content of fluoropolymer 1C was 0.32% by mass relative to the total mass of aqueous dispersion D. When the amount of aqueous medium used in aqueous dispersion D used for polymerization was taken as 100 parts by mass, the amount of monomers (TFE and ethylene) used in polymerization was 14.6 parts by mass. After freeze-coagulation of aqueous dispersion D, the aqueous medium obtained was filtered and analyzed by ion chromatography. The sulfate ion concentration was found to be less than 0.1 ppm by mass. The gas remaining in the reactor was recovered, and the liquid was then withdrawn. This liquid was designated aqueous dispersion 3. Aqueous dispersion 3 was a dispersion in which particles containing fluoropolymer 2C (average particle size 461 nm) were dispersed in an aqueous medium, and had a solids concentration of 12% by mass. After obtaining particle is aggregated and dried, calculate composition by NMR, and as a result, TFE unit / E unit / PMVE unit=55.9 / 43.7 / 0.4 (molar ratio).In addition, the average particle diameter of particle after drying is 427nm.In addition, in aqueous dispersion 3, the content of the compound represented by above-mentioned formula (S1) and the compound represented by above-mentioned formula (S2) relative to the total mass of the above-mentioned particle in aqueous dispersion 3 is all 100 mass ppb or less.

[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 inside the reactor reached 2.6 MPaG, and 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 initiate polymerization. As the pressure inside the reactor decreased with the start of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to maintain the pressure constant. An aqueous TBHP solution (0.2 mass%, 1 cc) and the 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 recovering the gas remaining in the reactor, the liquid was withdrawn, 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 solution 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 reactor to obtain aqueous dispersion F. 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 inside the reactor reached 2.6 MPaG, and 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 initiate polymerization. As the pressure inside the reactor decreased with the initiation of polymerization, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to maintain the pressure constant. Every 5 minutes from the start of polymerization, an aqueous TBHP solution (0.2% by mass, 1 cc) and the reducing agent (1 cc) were added. When 80 g of the mixed CG gas was injected, the reactor was cooled and the polymerization reaction was terminated. The polymerization time was 370 minutes. The content of fluoropolymer 1E was 0.32% by mass relative to the total mass of aqueous dispersion F. Furthermore, aqueous dispersion F was freeze-aggregated and then filtered, and the resulting aqueous medium was analyzed by ion chromatography. As a result, the concentration of sulfate ions was less than 0.1 ppm by mass. After recovering the gas remaining in the reactor, the liquid was withdrawn, 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 aqueous dispersions 1 to 3 of each example were aggregated and dried to obtain aggregates corresponding to each example. A 1 mm thick sheet was prepared from the obtained aggregates, and the Y.I. and CIE W were measured by transmittance using a color meter (SM Color Meter, manufactured by Suga Test Instruments Co., Ltd.). The Y.I. and CIE W values ​​are shown in Table 1. Here, "Y.I." is an index representing yellowness, with a higher value indicating a more yellowish color. "CIE W" is an index representing whiteness, with a lower value indicating a more blackish color. A 1 mm thick sheet was obtained by heating 2 g of the aggregate at 300°C for 10 minutes, press-molding at 300°C for 5 minutes under 10 MPa, and then cooling. Note that for Examples 4 and 5, an aqueous dispersion was not obtained, so color evaluation could not be performed.

[0077]

[0078] It was shown that the process for producing a fluoropolymer of the present invention allows for efficient production of a fluoropolymer using an aqueous medium with a small environmental load and without the essential use of an emulsifier (Examples 1 to 3). Furthermore, a comparison of Examples 1 to 3 showed that when an aqueous dispersion having a sulfate ion concentration of 5 ppm by mass or less relative to the total mass of the aqueous medium contained in the aqueous dispersion was used for polymerization, discoloration of the fluoropolymer could be sufficiently suppressed (Examples 1 and 3). In contrast, in Examples 4 and 5, a fluoropolymer could not be produced efficiently.

[0079] The entire contents of the specification, claims and abstract of Japanese Patent Application No. 2022-201048, filed on December 16, 2022, are incorporated herein by reference as part of 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.