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

Abstract

Provided is a method for producing a fluorine-containing polymer with which it is possible to efficiently produce a fluorine-containing polymer without requiring an emulsifier even when an aqueous medium having a small environmental burden is used. This method for producing a fluorine-containing polymer produces a second fluorine-containing polymer by polymerizing a monomer containing a fluorine-containing monomer in an aqueous dispersion containing a first fluorine-containing polymer and an aqueous medium, the first fluorine-containing polymer containing a TFE unit and a PAVE unit, the unit based on perfluoro (alkyl vinyl ether) is 20-60 mol% with respect to the total of the unit based on tetrafluoroethylene and the unit based on perfluoro (alkyl vinyl ether), the average particle diameter of the first fluorine-containing polymer is 1-150 nm, and the content of the first fluorine-containing polymer is 0.01-4.0 mass% with respect to the total mass of the aqueous dispersion before the start of monomer polymerization.

Description

Technical Field

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

[0002] Since fluoropolymers such as ethylene / tetrafluoroethylene copolymers are excellent in heat resistance, chemical resistance, flame retardancy, weather resistance, etc., they are used in various industrial fields. As a method for producing a fluoropolymer, a method of emulsion polymerizing a fluoromonomer in an aqueous medium using a fluorinated emulsifier can be cited (see Patent Document 1). Prior Art Documents Patent Documents

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

[0004] Since the method for producing a fluoropolymer of Patent Document 1 uses an aqueous medium, its environmental load is small. However, when a large amount of the emulsifier as an essential component remains in the obtained aqueous dispersion, the emulsifier must be removed depending on the use.

[0005] The technical problem to be solved by the present invention is to provide a method for producing a fluoropolymer that can efficiently produce a fluoropolymer without an emulsifier even when using an aqueous medium with a small environmental load. In addition, the technical problem to be solved by the present invention is also to provide an aqueous dispersion and particles. Technical Solution for Solving the Technical Problem

[0006] The present inventors have conducted intensive studies and found that the above technical problems can be solved by the following configuration. [1] A method for producing a fluoropolymer, which is a method for producing a second fluoropolymer different from the above first fluoropolymer by polymerizing a monomer containing a fluoromonomer in an aqueous dispersion containing a first fluoropolymer and an aqueous medium, the first fluoropolymer containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), wherein in the above first fluoropolymer, the units based on perfluoro(alkyl vinyl ether) are 20 to 60 mol% relative to the total of the units based on tetrafluoroethylene and the units based on perfluoro(alkyl vinyl ether), the average particle diameter of the above first fluoropolymer is 1 to 150 nm, before the start of the above monomer polymerization, the content of the above first fluoropolymer is 0.01 to 4.0% by mass relative to the total mass of the above aqueous dispersion. [2] The method for producing a fluoropolymer as described in [1], wherein, before the start of monomer polymerization, the concentration of sulfate ions is 5 mass ppm or less relative to the total mass of the aqueous medium in the aqueous dispersion. [3] The method for producing a fluoropolymer as described in [1] or [2], wherein the fluoromonomer contains at least one selected from tetrafluoroethylene, chlorotrifluoroethylene, and vinylidene fluoride. [4] The method for producing a fluoropolymer as described in any one of [1] to [3], wherein the monomer contains ethylene. [5] The method for producing a fluoropolymer as described in any one of [1] to [4], wherein the usage amount of the monomer is 1 to 50 parts by mass relative to 100 parts by mass of the usage amount of the aqueous medium. [6] The method for producing a fluoropolymer as described in any one of [1] to [5], wherein the monomer is polymerized in the presence of a polymerization initiator. [7] An aqueous dispersion, which is an aqueous dispersion containing an aqueous medium and particles containing a fluoropolymer, wherein, the average particle diameter of the particles is 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% relative to all the units of the fluoropolymer, 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 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 NH4, and n each independently represents 8 or 10. [8] The aqueous dispersion as described in [7], wherein the particles contain at least one unit selected from units based on tetrafluoroethylene, units based on chlorotrifluoroethylene, and units based on vinylidene fluoride. [9] The aqueous dispersion as described in [7] or [8], wherein the particles contain units based on ethylene.

[10] A particle, which is a particle containing a fluoropolymer, wherein, the average particle diameter of the particles is 1 μm or less, The above-mentioned particles contain units based on tetrafluoroethylene, units based on ethylene, and units based on perfluoro(alkyl vinyl ether). The above-mentioned units based on ethylene are 20 to 70 mol% relative to the total of the above-mentioned units based on tetrafluoroethylene and the above-mentioned units based on ethylene. The total content of the units based on tetrafluoroethylene and the units based on ethylene is 80 mol% or more relative to all the units of the above-mentioned fluoropolymer. The content of the above-mentioned units based on perfluoro(alkyl vinyl ether) is 0.1 to 1.0 mol% relative to all the units of the above-mentioned fluoropolymer. The contents of the compound represented by formula (S1) and the compound represented by formula (S2) are each 100 mass ppb or less relative to the total mass of the above-mentioned 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 NH4, and n each independently represents 8 or 10. Advantages of the Invention

[0007] According to the present invention, there can be provided a method for producing a fluoropolymer that can efficiently produce a fluoropolymer without an emulsifier even when using an aqueous medium with a small environmental load. In addition, according to the present invention, there can also be provided an aqueous dispersion and particles. Detailed Embodiments

[0008] The meanings of the terms in the present invention are as described below. The numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. In the numerical ranges described hierarchically in this specification, the upper limit value or the lower limit value described in a certain numerical range can be replaced with the upper limit value or the lower limit value of other hierarchically described numerical ranges. In addition, in the numerical ranges described in this specification, the upper limit value or the lower limit value described in a certain numerical range can also be replaced with the value shown in the examples. In this specification, each component can be used alone as one substance corresponding to each component, or two or more can be used in combination. Herein, when two or more substances are used in combination for each component, as long as there is no particular limitation, the content of the component refers to the total content of the substances used in combination. In this specification, the combination of two or more preferred forms is a more preferred form. "Unit" refers to the general term for atomic groups directly formed by polymerization of monomers from one molecule of the above-mentioned monomer 1 and atomic groups obtained by chemically converting a part of the above-mentioned atomic groups. "Unit based on monomer" is also simply referred to as "unit" hereinafter. The content (mass% or mol%) of all units contained in the polymer can be determined by analyzing the polymer by solid nuclear magnetic resonance (NMR) method. However, generally, the content of each unit calculated from the feed amount of each monomer is approximately the same as the actual content of each unit.

[0009] [Manufacturing method of fluoropolymer] The manufacturing method of the fluoropolymer of the present invention (hereinafter also referred to as "this manufacturing method") is a manufacturing method for polymerizing a monomer containing a fluoromonomer (hereinafter also referred to as "specific monomer") in an aqueous dispersion containing a first fluoropolymer and an aqueous medium (hereinafter, the aqueous dispersion used in this manufacturing method is also referred to as "aqueous dispersion (a)") to manufacture a second fluoropolymer different from the above-mentioned first fluoropolymer. The first fluoropolymer contains units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether). In this manufacturing method, in the first fluoropolymer, the units based on perfluoro(alkyl vinyl ether) are 20 to 60 mol% relative to the total of the units based on tetrafluoroethylene and the units based on perfluoro(alkyl vinyl ether). In addition, in this manufacturing method, the average particle diameter of the above-mentioned first fluoropolymer is 1 to 150 nm. In addition, in this manufacturing method, before the start of polymerization of the above-mentioned monomer, the content of the above-mentioned first fluoropolymer is 0.01 to 4.0 mass% relative to the total mass of the above-mentioned aqueous dispersion.

[0010] As the reason for efficiently manufacturing the second fluoropolymer without an emulsifier in this manufacturing method, it is speculated that by using an aqueous dispersion containing a first fluoropolymer with an average particle diameter and the ratio of each unit contained within a specified range in a specified amount, the first fluoropolymer functions as a good polymerization site for the second fluoropolymer.

[0011] <Aqueous dispersion> This manufacturing method uses an aqueous dispersion (a) containing a first fluoropolymer and an aqueous medium.

[0012] (First fluoropolymer) The first fluoropolymer 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 speculated that during the polymerization of a specific monomer, the first fluoropolymer incorporates the specific monomer by adsorbing it in the hydrophobic part to solubilize the specific monomer, and a polymerization initiator is added thereto to polymerize the specific monomer inside the specific particles. In addition, it is speculated that the first fluoropolymer contributes to dispersion stabilization in an aqueous medium and an organic solvent.

[0013] From the viewpoints of excellent polymerization reactivity during the production of the first fluoropolymer and more efficient production of the second fluoropolymer, PAVE is preferably a monomer represented by the formula (1). CF2=CF-O-R f1 (1) In the formula (1), R f1 represents a perfluoroalkyl group having 1 to 10 carbon atoms. From the viewpoint of more excellent polymerization reactivity, the carbon number of R f1 is preferably 1 to 8, more preferably 1 to 6, still more preferably 1 to 5, and particularly preferably 1 to 3. The perfluoroalkyl group may be 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 them, from the viewpoint of more efficient production of the second fluoropolymer, PMVE and PPVE are preferred, and PMVE is more preferred.

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

[0016] The first fluoropolymer may further contain units based on other monomers other than TFE and PAVE, but from the viewpoint of more efficient production of the second fluoropolymer, it preferably substantially does not contain units based on other monomers. Substantially not containing units based on other monomers means that the content of units based on other monomers is 0.01 mol% or less based on all the units of the first fluoropolymer, and more preferably 0 mol%. In the case of containing units based on other monomers, hexafluoropropylene is preferably used as the other monomer.

[0017] Before the monomers for the polymerization of the second fluoropolymer start to polymerize, the content of the first fluoropolymer is 0.01 to 4.0% by mass relative to the total mass of the aqueous medium in the aqueous dispersion (a). From the perspective of more efficiently manufacturing the second fluoropolymer, it is preferably 0.01 to 0.6% by mass, and more preferably 0.01 to 0.5% by mass.

[0018] In this specification, "before the monomers for the polymerization of the second fluoropolymer start to polymerize" refers to the time point before the polymerization is about to start. Here, examples of the "time point when polymerization starts" include the time point when the monomers and the polymerization initiator coexist in the reactor after the temperature in the reactor reaches above the polymerization temperature, and the time point when the temperature in the reactor reaches above the polymerization temperature after the monomers and the polymerization initiator coexist in the reactor.

[0019] From the perspective of suppressing the coloring of the second fluoropolymer, before the monomers for the polymerization of the second fluoropolymer start to polymerize, the concentration of sulfate ions is preferably 10 mass ppm or less, and more preferably 5 mass ppm or less, relative to the total mass of the aqueous medium in the aqueous dispersion (a). As the lower limit, 0 mass ppm can be cited. As an example of the method for bringing the concentration of sulfate ions to the above value, a method of removing sulfate ions using an anion exchange resin during the production of the first fluoropolymer can be cited. Here, sulfate ions are derived, for example, from the polymerization initiator (especially 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 speculated that keeping the content of sulfate ions at 10 mass ppm or less (especially 5 mass ppm or less) can suppress the formation of end groups with low heat resistance on the second fluoropolymer, and thus the coloring of the second fluoropolymer is suppressed.

[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. From the perspective of more efficiently manufacturing the second fluoropolymer, it is preferably 10 to 120 nm, and more preferably 50 to 120 nm. The average particle size of the first fluoropolymer is the particle size calculated by analyzing the autocorrelation function obtained by the dynamic light scattering method using the monodisperse cumulant method.

[0021] The manufacturing method of the first fluoropolymer preferably involves polymerizing monomers containing TFE and PAVE in an aqueous medium in the presence of a polymerization initiator. Thereby, the first fluoropolymer dispersed in the form of particles in the aqueous medium can be obtained. The aqueous medium in which the particles of the first fluoropolymer are dispersed thus obtained can be directly used as the above aqueous dispersion (a), or it can also be used as the above aqueous dispersion (a) after further adding other aqueous media. In addition, the solvent can be replaced to disperse the first fluoropolymer in other aqueous media and then used as the above aqueous dispersion (a). The aqueous dispersion (a) is preferably subjected to sulfate ion removal means such as an anion exchange resin to remove sulfate ions. As the polymerization initiator used in the production of the first fluoropolymer, a water-soluble polymerization initiator is preferred, and ammonium persulfate, sodium persulfate, potassium persulfate and other persulfate compounds, organic polymerization initiator compounds such as dibutyl peroxydisuccinate and azodiisobutyramidine dihydrochloride are more preferred, persulfate compounds are further preferred, and ammonium persulfate is particularly preferred. As the aqueous medium used in the production of the first fluoropolymer, water or a mixed solvent of water and a water-soluble organic solvent can be cited. 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 this production method contains an aqueous medium. As described above, the aqueous medium contained in the aqueous dispersion (a) can be the polymerization solvent used in the production of 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 the production of the first fluoropolymer described above. Before the monomers for the polymerization of the second fluoropolymer start to polymerize, the content of the aqueous medium is preferably 60 to 99.9% by mass, more preferably 96 to 99.9% by mass, and further preferably 98 to 99.9% by mass relative to the total mass of the aqueous dispersion (a).

[0023] (Other components) The aqueous dispersion (a) used in this production method may further contain other components other than the first fluoropolymer and the aqueous medium. Specific examples of other components that the aqueous dispersion (a) may further contain include a chain transfer agent, an emulsifier, and a pH adjuster. Specific examples of the chain transfer agent include ethyl acetate, methanol, ethanol, tert-butyl methyl ether, diethyl ether, n-pentane, cyclohexane, methane, and propane. Specific examples of the emulsifier include fluorinated 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 Emulsifier Co., Ltd. As specific examples of the pH adjuster, inorganic salts can be cited. As specific examples of the inorganic salts, phosphates such as disodium hydrogen phosphate and sodium dihydrogen phosphate, carbonates such as sodium hydrogen carbonate and sodium carbonate, etc. can be cited. As more preferred specific examples of the phosphates, disodium hydrogen phosphate dihydrate, disodium hydrogen phosphate dodecahydrate, etc. can be cited. 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. In addition, 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 further preferably 0.1 to 10 parts by mass relative to 100 parts by mass of the amount of the specific monomer described later. 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] From the viewpoint of polymerization stability, before the polymerization of the monomers for the polymerization of the second fluoropolymer starts, the concentration of fluoride ions is preferably 100 mass ppm or less, more preferably 50 mass ppm or less relative to the total mass of the aqueous dispersion (a). As the lower limit, 0 mass ppm can be cited. As an example of the method for bringing the concentration of fluoride ions to the above value, a method of using an anion exchange resin to remove sulfate ions during the production of the first fluoropolymer can be cited. Here, fluoride ions are generated by the reaction of a polymerization initiator (such as ammonium persulfate) with a fluorine-containing monomer, and they are sometimes contained in the aqueous dispersion.

[0025] <Specific monomer> The specific monomer contains a fluorine-containing monomer. The fluorine-containing monomer preferably contains at least 1 kind selected from TFE, chlorotrifluoroethylene (hereinafter also referred to as "CTFE"), and vinylidene fluoride (hereinafter also referred to as "VdF"), more preferably contains at least 1 kind selected from TFE, CTFE, and VdF (hereinafter also referred to as "specific fluorine-containing monomer"), more preferably contains TFE, and further preferably is TFE. Two or more kinds of fluorine-containing monomers can 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 further preferably 40.0 to 60.0 mol% relative to the amount of the specific monomer used. In addition, it is also preferable to further contain fluorine-containing monomers other than the specific fluorine-containing monomer (hereinafter also referred to as "other fluorine-containing monomers"). Specific examples of the other fluorine-containing monomers include fluoroalkyl vinyl (hereinafter also referred to as "FAE"), PAVE, and hexafluoropropylene. Two or more of the other fluorine-containing monomers can 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. PAVE is the same as PAVE in the above-mentioned first fluorine-containing polymer, and the preferred form is also the same. The usage amount of the other fluorine-containing monomers relative to the usage amount of the specific monomer is preferably 0.1 to 30.0 mol%, more preferably 0.1 to 10.0 mol%, and still more preferably 0.5 to 5.0 mol%.

[0026] The specific monomer preferably further contains monomers other than the fluorine-containing monomer (hereinafter also referred to as "other monomers"). Specific examples of the other monomers include ethylene, propylene, vinyl chloride, and vinylidene chloride. Among them, the other monomers preferably contain ethylene, and more preferably ethylene. Two or more of the other monomers can be used in combination. The usage amount of the other monomers relative to the usage amount of the specific monomer is preferably 10.0 to 70.0 mol%, more preferably 20.0 to 60.0 mol%, and still more preferably 30.0 to 50.0 mol%.

[0027] The usage amount of the specific monomer relative to 100 parts by mass of the aqueous medium contained in the above aqueous dispersion (a) is preferably 1 to 50 parts by mass, more preferably 1 to 40 parts by mass, and still more preferably 1 to 30 parts by mass.

[0028] <Polymerization initiator> In this manufacturing 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, and a water-soluble redox catalyst are preferred. Specific examples of the oil-soluble radical initiator include oil-soluble organic peroxides such as tert-butyl peroxyneopentanoate (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 succinic peroxide, glutaric peroxide, and tert-butyl hydroperoxide (hereinafter also referred to as "TBHP"). As a water-soluble redox catalyst, a combination of an oxidizing agent such as hydrobromic acid or its salt, hydrochloric acid or its salt, persulfuric acid or its salt, permanganic acid or its salt, hydrogen peroxide, etc. and a reducing agent such as sulfurous acid or its salt, bisulfite or its salt, thiosulfuric acid or its salt, organic acid, inorganic salt, etc. is preferred. As the persulfate, potassium persulfate and ammonium persulfate are preferred. As the sulfite, sodium sulfite is preferred. As the inorganic salt, combinations of sulfate anions, sulfite anions and chloride anions with metal ions can be cited. As the metal ion, transition metals are preferred, and ions of manganese, iron, cobalt, nickel, copper, zinc, cerium and silver can be cited, and ferric ion is more preferred among them. 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, and from the viewpoint of more efficiently producing a fluoropolymer, an oil-soluble radical initiator is more preferred, and an oil-soluble organic peroxide is further preferred. Two or more polymerization initiators can 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 further preferably 0.01 to 2 parts by mass with respect to 100 parts by mass of the amount of the specific monomer used.

[0030] <Other components> When polymerizing the specific monomer, components other than the above (hereinafter also referred to as "other components") can be further used. As a specific example of the other components, a reducing agent can be cited. The amount of the other components used is preferably 0.1 to 2 parts by mass with respect to 100 parts by mass of the amount of the specific monomer used.

[0031] <Process> In this production method, the second fluoropolymer is produced by polymerizing the specific monomer in the above aqueous dispersion (a).

[0032] The second fluoropolymer obtained by this production method contains units based on the above fluoromonomer (hereinafter also referred to as "fluorine-containing units"), preferably contains fluorine-containing units and units based on the above other monomers (hereinafter also referred to as "other units"), and preferably contains a copolymer of TFE and ethylene (hereinafter also referred to as "ETFE"). In addition, the first fluoropolymer and the second fluoropolymer can be copolymerized.

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

[0034] The specific monomer is introduced into the reaction system (i.e., the polymerization reaction vessel) by a conventional method. For example, the specific monomer can be introduced continuously or intermittently into the reaction system so that the polymerization pressure reaches a specified pressure. Alternatively, the specific monomer can be dissolved in an aqueous medium, and the resulting solution can be introduced continuously or intermittently into the reaction system. When using a polymerization initiator, the polymerization initiator can be added to the reaction system at once or in batches.

[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. The polymerization time is preferably 90 to 1000 minutes, more preferably 90 to 700 minutes in the case of batch processing.

[0036] The polymerization of the specific monomer is preferably carried out in the substantial absence of an emulsifier. As the emulsifier, known emulsifiers can be cited, and general surfactants can be cited. The substantial absence of an emulsifier means an environment where the content of the emulsifier is 0.03 mass ppm or less, preferably 0.02 mass ppm or less, and more preferably 0 mass ppm, relative to the total mass of the aqueous medium contained in the above aqueous dispersion (a).

[0037] As described above, it is presumed that the specific monomer polymerizes inside the particles of the first fluoropolymer during the polymerization of the specific monomer. Therefore, it is considered that particles containing the first fluoropolymer and the second fluoropolymer are generated in this production method. That is, according to this production method, it is presumed that the second fluoropolymer is obtained in the form of particles containing the first fluoropolymer and the second fluoropolymer. In this case, according to this production method, an aqueous dispersion (a) in which particles containing the first fluoropolymer and the second fluoropolymer are dispersed in the above aqueous medium can be obtained.

[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 diameter of the above specific particles is 1 μm or less. In addition, in this aqueous dispersion, the above-mentioned specific particles contain PAVE units, and the content of the above-mentioned PAVE units is 0.1 to 1.0 mol% based on all the units of the above-mentioned fluoropolymer. In addition, in this aqueous dispersion, the content of the compound represented by the following formula (S1) and the content of the compound represented by the following formula (S2) are each 100 mass ppb or less based on the total mass of the above-mentioned specific particles.

[0039] <Specific Particles> This aqueous dispersion can be easily obtained by the above-mentioned production method. Therefore, the specific particles contained in this aqueous dispersion are preferably particles containing the above-mentioned first fluoropolymer and the above-mentioned second fluoropolymer. In this case, the fluoropolymer contained in the specific particles contains two types of fluoropolymers, namely the first fluoropolymer and the second fluoropolymer.

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

[0041] In this specification, when the fluoropolymer contained in the specific particles is only one type, "all the units of the fluoropolymer" means all the units contained in this one type of fluoropolymer. In addition, when the fluoropolymer contained in the specific particles is two or more types, "all the units of the fluoropolymer" means all the units contained in two or more types of fluoropolymers.

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

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

[0044] A preferred form of the specific particles is a form containing PAVE units, fluorine-containing units other than PAVE units, and other units, and among them, a form containing PAVE units, TFE units, and E units is more preferred. In addition, the PAVE units, fluorine-containing units other than PAVE units, and other units contained in the specific particles are preferably contained in the fluoropolymer in the specific particles.

[0045] From the viewpoint of the dispersion stability of the specific particles, the content of the specific particles is preferably 1 to 50% by mass, more preferably 1 to 40% by mass, and still more preferably 1 to 30% by mass relative to the total mass of the present aqueous dispersion.

[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. From the viewpoint of cohesiveness, the average particle size of the specific particles is preferably 50 nm or more, more preferably 70 nm or more, and still more preferably 100 nm or more. The average particle size of the specific particles is the particle size calculated by analyzing the autocorrelation function obtained by the dynamic light scattering method using the 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 first fluoropolymer. From the viewpoint of the dispersion stability of the specific particles, the content of the aqueous medium is preferably 50 to 99% by mass, more preferably 60 to 99% by mass, and still more preferably 70 to 99% by mass relative to the total mass of the present aqueous dispersion.

[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 may be generated when polymerizing the above-mentioned specific monomers (especially TFE) in the presence of a polymerization initiator, a chain transfer agent, and an emulsifier (especially a hydrocarbon emulsifier). Therefore, since the production amount of the compound represented by formula (S1) and the compound represented by formula (S2) can be suppressed when the fluoropolymer contained in this aqueous dispersion is produced without using an emulsifier, the content of these compounds is likely to be 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 NH4, 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 further preferably 0 mass ppb (i.e., the compound represented by formula (S1) is not contained) relative to the total mass of the specific particles.

[0051] <Others> This aqueous dispersion preferably contains substantially no emulsifier. The emulsifier is as described above. That this aqueous dispersion contains substantially no emulsifier means that the content of the emulsifier is 0.03 mass ppm or less, preferably 0.02 mass ppm or less, and more preferably 0 mass ppm relative to the total mass of this aqueous dispersion.

[0052] <Use> As described above, this aqueous dispersion does not require an emulsifier, and thus it is also easy to obtain a dispersion in an organic solvent such as N-methylpyrrolidone or acetone by solvent substitution. For example, by mixing this aqueous dispersion with an organic solvent and dehydrating by evaporation or using anhydrous sodium sulfate, etc., a dispersion in an organic solvent can be obtained.

[0053] Even though this aqueous dispersion does not contain an emulsifier, the fluoropolymer is still stably dispersed. Therefore, it can be applied to coating uses, adhesives, etc.

[0054] In addition, by coagulating the specific particles from this aqueous dispersion, a powder of the specific particles can be obtained. Further, by making the powder of the specific particles obtained by coagulation more homogeneous by melt-kneading, etc., a shaped material in the form of granules, pellets, etc. can be obtained. In addition, a shaped article can also be obtained by melt-forming the powder of the specific particles obtained by coagulation.

[0055] Examples of the coagulation method include, but are not limited to, freeze coagulation, acid coagulation, alkali coagulation, and coagulation using a coagulant. In the case of freeze coagulation, the coagulation temperature is preferably -20 to 0°C. The coagulation 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 present aqueous dispersion is preferred. Examples of the added acid 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% by mass, more preferably 1 to 30% by mass, and further preferably 1 to 10% by mass. As alkali coagulation, a method of adding an alkali-containing solution to the present aqueous dispersion is preferred. Examples of the added alkali include sodium hydroxide, potassium hydroxide, ammonium carbonate, etc., and sodium hydroxide is preferred. The concentration of the alkali in the alkali-containing solution is preferably 0.1 to 50% by mass, more preferably 1 to 30% by mass, and further preferably 1 to 10% by mass. As coagulation using a coagulant, a known coagulant can be used. Examples of the known coagulant include ammonium salts, calcium salts, and magnesium salts. Specifically, aluminum sulfate, alum represented by the general formula M’Al(SO4)2·12H2O [where M’ is a monovalent cation other than lithium], calcium nitrate, and magnesium sulfate can be mentioned. Alum is preferred, and potassium alum with M being potassium is more preferred. As the coagulation method, alkali coagulation is preferred from the viewpoint of easy 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. In addition, the present particles contain TFE units, E units, and PAVE units. The E units account for 20 to 70 mol% of the total of the TFE units and E units, the total content of the TFE units and E units is 80 mol% or more relative to all the units of the above fluoropolymer, and the content of the above PAVE units is 0.1 to 1.0 mol% relative to all the units of the above fluoropolymer. In addition, 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 mass ppb or less relative to the total mass of the present particles. The present particles are preferably particles obtained by the coagulation method using the above present aqueous dispersion. The preferred form of the present particles is the same as the preferred form of the specific particles contained in the above present aqueous dispersion, and thus the description thereof is omitted. Here, the average particle size of the present particles refers to the arithmetic average obtained by photographing the present particles obtained after coagulation with a scanning electron microscope and determining the particle sizes of 5 different particles from the obtained SEM image. Example

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

[0058] [Measurement and Evaluation Methods] Various measurement methods and evaluation methods are as described below.

[0059] <Average Particle Diameter of Particles in Raw Material Liquid> Using the raw material liquid as a sample, measurement is performed using a laser diffraction / scattering particle size distribution analyzer (ELSZ manufactured by Otsuka Electronics Co., Ltd.). In addition, if the average particle diameter of the particles in the aqueous dispersion corresponding to the raw material liquid is measured by the same method as the raw material liquid, it is the same as the average particle diameter of the particles in the raw material liquid.

[0060] <Average Particle Diameter of Particles after Drying> After drying and aggregating the particles obtained in each example, they are photographed with a scanning electron microscope (for example, JSM-IT700HR InTouchScope manufactured by JEOL Ltd.). The diameters of 5 different particles measured from the obtained SEM images are determined, and the arithmetic mean is taken.

[0061] <Ratio of Each Unit in Polymer> The ratio of each unit in the polymer is obtained by 19 19F-NMR analysis and infrared absorption spectroscopy analysis.

[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 the particles in the aqueous dispersion obtained in each example described below is calculated by the method using the aqueous dispersion in the measurement method using a liquid chromatography mass spectrometer described in paragraphs

[0710] to

[0720] of International Publication No. 2018 / 181904. In addition, an Agilent 1260 series HPLC / 6460S is used as the device, and a cadenza CD-C18 manufactured by Imtakt is used as the column.

[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 the particles in the aqueous dispersion obtained in each of the following examples was calculated by the method using the aqueous dispersion in the measurement method using a liquid chromatography mass spectrometer described in paragraphs

[0721] to

[0732] of International Publication No. 2018 / 181904. In addition, Agilent 1260 series HPLC / 6460S was used as the apparatus, and cadenza CD-C18 manufactured by Imtakt was used as the column.

[0064] <Concentration of sulfate ion> The concentration of sulfate ion relative to the total mass of the aqueous medium in the aqueous dispersion was measured as follows. The aqueous dispersion was frozen and coagulated and then filtered, and the obtained aqueous medium was analyzed by ion chromatography. Among them, the analysis by ion chromatography was performed using ion chromatography ICS-5000 (manufactured by Thermo Fisher Scientific). Dionex IonPac AS-19 was used as the separation column, Dionex IonPac AG-19 was used as the guard column, and KOH was used as the eluent.

[0065] [Production of stock solution A] Ultra-pure water (1162 g), 28% NH3 aqueous solution (1 drop), PMVE (70 g), and TFE (14 g) were added to a 2.1 L stainless steel pressure-resistant reactor, and the temperature was raised to 80 °C while stirring at 600 rpm. Then, an aqueous ammonium persulfate solution (5.9 mass%, 5 cc) was added to start the polymerization. Since the pressure in the reactor decreased with the start of polymerization, TFE was added to maintain the pressure constant. After 24 g of TFE was pressured in, the reactor was cooled to end the polymerization reaction. After the residual gas in the reactor was recovered, the liquid in the reactor was taken out. This liquid was designated as stock solution A. In stock solution A, particles of fluoropolymer 1A (average particle diameter 110 nm) were dispersed in the aqueous medium, and the content of fluoropolymer 1A was 3.0 mass% relative to the total mass of stock solution A. Stock solution A was frozen and coagulated and then filtered, and the obtained fluoropolymer 1A was washed with ultra-pure water. Then, it was dried under vacuum at 100 °C. NMR analysis was performed on the obtained fluoropolymer 1A, and as a result, PMVE unit / TFE unit = 34.4 / 65.6 (molar ratio).

[0066] [Production of stock solution B] To the above raw material liquid A (200 g), ultrapure water (200 g) was added, and it was stirred at 200 rpm. To this, an ion exchange resin (Purolite A300 (manufactured by Purolite Co., Ltd.), an anion exchange resin, 4 g) was added, and it was stirred for 20 minutes. Thereafter, the above ion exchange resin (2 g) was added every 20 minutes. After 60 minutes from 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 diameter 110 nm) were dispersed in an aqueous medium, and the content of fluoropolymer 1A was 1.5 mass% with respect to the total mass of raw material liquid B.

[0067] [Production of Raw Material Liquid C] In a 1.3 L stainless steel pressure-resistant reactor, ultrapure water (717 g), 28% NH3 aqueous solution (1 drop), PMVE (63 g), and TFE (10 g) were added, and it was heated to 80 °C while stirring at 500 rpm. Then, an aqueous ammonium persulfate solution (3.6 mass%, 5 cc) was added to start the polymerization. Since the pressure in the reactor decreased with the start of polymerization, TFE was added to maintain the pressure constant. After injecting 5 g of TFE, the reactor was cooled to end the polymerization reaction. After recovering the residual gas in the reactor, the liquid was withdrawn. This liquid was designated as raw material liquid C. The raw material liquid C was frozen and coagulated and then filtered, and the obtained fluoropolymer 1C was washed with ultrapure water. Thereafter, it was dried under vacuum at 100 °C. NMR analysis was performed on the obtained fluoropolymer 1C, and as a result, PMVE unit / TFE unit = 47.3 / 52.7 (molar ratio).

[0068] [Production of Raw Material Liquid D] To the above raw material liquid C (200 g), (Purolite A300 (manufactured by Purolite Co., Ltd.), an anion exchange resin, 4 g) was added, and it was stirred at 200 rpm for 20 minutes. Thereafter, the above ion exchange resin (2 g) was added every 20 minutes. After 60 minutes from the start of stirring, the raw material liquid and the ion exchange resin were separated by filtration to obtain raw material liquid D. In raw material liquid D, particles of fluoropolymer 1C (average particle diameter 89 nm) were dispersed in an aqueous medium, and the content of fluoropolymer 1C was 1.1 mass% with respect to the total mass of raw material liquid D.

[0069] [Production of Raw Material Liquid E] After degassing a 2100 mL stainless steel pressure reactor containing anchor wings, ultrapure water (1170 g), disodium hydrogen phosphate (10.48 g), and polymethyl methacrylate (37 mg) were added. Subsequently, the solution in the reactor was stirred, and 75 g of PMVE and 14 g of TFE (TFE:PMVE = 25:75 (molar ratio)) were added. The temperature was raised, and when the internal temperature reached 80 °C, the reactor internal pressure was 1.3 MPa [gauge pressure]. 5 mL of an ammonium persulfate solution (20 mass%) dissolved in ultrapure water was added to initiate polymerization. As the polymerization proceeded, TFE was injected at the time point when the reactor internal pressure decreased to 1.29 MPa [gauge pressure] to raise the reactor internal pressure to 1.3 MPa [gauge pressure]. This operation was repeated, and every time 8 g of TFE was injected, 7 g of PMVE was also injected. At the time point when the total added mass of TFE reached 160 g, the addition of the monomers injected after the start of polymerization (hereinafter also referred to as "post-added monomers") was stopped, the reactor internal temperature was cooled to 10 °C to stop the polymerization reaction, the gas remaining in the reactor was recovered, and then the liquid was withdrawn. This liquid was designated as raw material liquid E. The raw material liquid E was frozen and coagulated and then filtered, and the obtained fluoropolymer 1E was washed with ultrapure water. Subsequently, it was dried under vacuum at 100 °C. NMR analysis was performed on the obtained fluoropolymer 1E, and the result was PMVE / TFE = 34 / 66 (molar ratio). In addition, the average particle diameter of the fluoropolymer 1E was 290 - 300 nm.

[0070] [Production of Raw Material Liquid F] To the above raw material liquid E (20 g), ultrapure water (180 g) and an ion exchange resin (Purolite A300 (manufactured by Purolite Co., Ltd.), an anion exchange resin, 4 g) were added, and the mixture was stirred at 200 rpm for 20 minutes. Subsequently, the above ion exchange resin (2 g) was added every 20 minutes. Sixty 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 the fluoropolymer 1E (average particle diameter 326 nm) were dispersed in an aqueous medium, and the content of the fluoropolymer 1E was 2.0 mass% relative to the total mass of raw material liquid F.

[0071] [Example 1] To a 1.2 L stainless steel pressure reactor, ultrapure water (428 g), feed liquid B (185 ml), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) were added 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 pressured in until the reactor pressure 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 start the polymerization. The pressure in the reactor decreased with the start of polymerization, so 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 mass%, 1 cc) and the above reducing agent (1 cc) were added. After 80 g of the mixed CG gas was pressured in, the reactor was cooled to end the polymerization reaction. The polymerization time was 167 minutes. In addition, the content of the fluoropolymer 1A was 0.44 mass% relative to the total mass of aqueous dispersion B. Furthermore, when the usage amount of the aqueous medium in aqueous dispersion B used for polymerization was 100 mass parts, the usage amount of the monomers (TFE and ethylene) for polymerization was 12.3 mass parts. In addition, after aqueous dispersion B was frozen and coagulated, it was filtered and separated, and the resulting aqueous medium was analyzed by ion chromatography. As a result, the concentration of sulfate ions was less than 0.1 mass ppm. After the residual gas in the reactor was recovered, the liquid was withdrawn. This liquid was designated as aqueous dispersion 1. Aqueous dispersion 1 was a dispersion in which particles of fluoropolymer 2A (average particle size 196 nm) were dispersed in an aqueous medium, and the solid content concentration was 10.6 mass%. After the obtained particles were coagulated and dried, the composition was calculated using NMR. As a result, the TFE unit / E unit / PMVE unit = 56.3 / 43.1 / 0.6 (molar ratio). In addition, the average particle size of the dried particles was 156 nm. 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) were both 100 mass ppb or less relative to the total mass of the above particles in aqueous dispersion 1.

[0072] [Example 2] To a 1.2 L stainless steel pressure reactor, ultrapure water (520.5 g), feed liquid A (92.5 ml), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) were added 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 pressured into the reactor. However, the pressure started to drop during the pressurization. Therefore, at the time point when the pressure 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 the polymerization. The pressure inside the reactor dropped with the start of the polymerization. Therefore, a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) was added to maintain the pressure constant. Every 5 minutes from the start of the polymerization, an aqueous TBHP solution (0.2 mass%, 1 cc) and the above reducing agent (1 cc) were added. After 80 g of the mixed CG gas was pressured in, the reactor was cooled to end the polymerization reaction. The polymerization time was 181 minutes. In addition, the content of the fluoropolymer 1A was 0.22 mass% relative to the total mass of the aqueous dispersion B. Furthermore, when the amount of the aqueous medium used in the aqueous dispersion A used for the polymerization was 100 mass parts, the amount of the monomers (TFE and ethylene) used for the polymerization was 12.7 mass parts. In addition, after the aqueous dispersion A was frozen and coagulated, it was filtered and separated, and the resulting aqueous medium was analyzed by ion chromatography. As a result, the concentration of sulfate ions was 8.2 mass ppm. After the residual gas inside the reactor was recovered, the liquid was withdrawn. 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 the solid content concentration was 10.2 mass%. After the obtained particles were coagulated and dried, the composition was calculated using NMR. As a result, the TFE unit / E unit / PMVE unit = 56.9 / 42.4 / 0.7 (molar ratio). In addition, the average particle diameter of the dried particles was 182 nm. In addition, in the aqueous dispersion 2, the contents of the compound represented by the above formula (S1) and the compound represented by the above formula (S2) were both 100 mass ppb or less relative to the total mass of the above particles in the aqueous dispersion 2.

[0073] [Example 3] To a 1.2 L stainless steel pressure-resistant reactor, ultrapure water (428 g), raw material liquid D (185 ml), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) were added to obtain 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 pressured in 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 start the polymerization. The pressure in the reactor decreased with the start of polymerization, so 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 mass%, 1 cc) and the above reducing agent (1 cc) were added. After 92 g of the mixed CG gas was pressured in, the reactor was cooled to end the polymerization reaction. The polymerization time was 97 minutes. In addition, the content of the fluoropolymer 1C was 0.32 mass% relative to the total mass of the aqueous dispersion D. Furthermore, when the amount of the aqueous medium used in the aqueous dispersion D for polymerization was 100 mass parts, the amount of the monomers (TFE and ethylene) used for polymerization was 14.6 mass parts. In addition, after the aqueous dispersion D was frozen and coagulated, it was filtered and separated, and the resulting aqueous medium was analyzed by ion chromatography. As a result, the concentration of sulfate ions was less than 0.1 mass ppm. After the residual gas in the reactor was recovered, the liquid was withdrawn. This liquid was designated as aqueous dispersion 3. Aqueous dispersion 3 was a dispersion in which particles of fluoropolymer 2C (average particle diameter 461 nm) were dispersed in an aqueous medium, and the solid content concentration was 12 mass%. After the obtained particles were coagulated and dried, the composition was calculated using NMR. As a result, the TFE unit / E unit / PMVE unit = 55.9 / 43.7 / 0.4 (molar ratio). In addition, the average particle diameter of the dried particles was 427 nm. In addition, in the aqueous dispersion 3, the contents of the compound represented by the above formula (S1) and the compound represented by the above formula (S2) were both 100 mass ppb or less relative to the total mass of the above particles in the aqueous dispersion 3.

[0074] [Example 4] Add ultrapure water (613 g), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) to a 1.2 L stainless steel pressure-resistant reactor, and heat to 60 °C while stirring at 320 rpm. Press in a mixed gas (TFE / ethylene = 86 / 14 (molar ratio)) until the pressure in the reactor reaches 2.6 MPaG. Add an aqueous TBHP solution (0.2 mass%, 2 ml) and a reducing agent (BRUGGOLITE (registered trademark) FF6M, 0.396 mass%, 2 cc), and start the polymerization. The pressure in the reactor decreases with the start of polymerization, so add a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) to maintain the pressure constant. Every 5 minutes from the start of polymerization, add an aqueous TBHP solution (0.2 mass%, 1 cc) and the above reducing agent (1 cc). After pressing in 92 g of the mixed CG gas, cool the reactor to end the polymerization reaction. The polymerization time is 340 minutes. After recovering the gas remaining in the reactor, the liquid was withdrawn, but it all coagulated, and an aqueous dispersion in which fluoropolymer particles were dispersed in an aqueous medium could not be obtained.

[0075] [Example 5] Add ultrapure water (513 g), raw material liquid F (100 ml), disodium hydrogen phosphate dodecahydrate (0.5 g), and t-BuOH (16.5 g) to a 1.2 L stainless steel pressure-resistant reactor to obtain aqueous dispersion F. Heat aqueous dispersion F to 60 °C while stirring at 320 rpm. Press in a mixed gas (TFE / ethylene = 86 / 14 (molar ratio)) until the pressure in the reactor reaches 2.6 MPaG. Add an aqueous TBHP solution (0.2 mass%, 2 ml) and a reducing agent (BRUGGOLITE (registered trademark) FF6M, 0.396 mass%, 2 cc), and start the polymerization. The pressure in the reactor decreases with the start of polymerization, so add a mixed CG gas (TFE / ethylene = 54 / 46 (molar ratio)) to maintain the pressure constant. Every 5 minutes from the start of polymerization, add an aqueous TBHP solution (0.2 mass%, 1 cc) and the above reducing agent (1 cc). After pressing in 80 g of the mixed CG gas, cool the reactor to end the polymerization reaction. The polymerization time is 370 minutes. In addition, the content of fluoropolymer 1E is 0.32 mass% relative to the total mass of aqueous dispersion F. Furthermore, after freeze-coagulating aqueous dispersion F, it was separated by filtration, and the resulting aqueous medium was analyzed by ion chromatography. As a result, the concentration of sulfate ions was less than 0.1 mass ppm. After recovering the gas remaining in the reactor, the liquid was withdrawn, but it all coagulated, and an aqueous dispersion in which fluoropolymer particles were dispersed in an aqueous medium could not be obtained.

[0076] [Evaluation] <Coloring> The particles contained in the 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 using a colorimeter (SM colorimeter, manufactured by Suga Test Instruments Co., Ltd.), Y.I. and CIE W were obtained by transmission. The values of Y.I. and CIE W are shown in Table 1. Here, "Y.I." is an index indicating yellow, and the higher this value, the yellower the color can be said to be. In addition, "CIE W" is an index indicating white, and the lower this value, the blacker the color can be said to be. In addition, the 1-mm-thick sheet was obtained by heating 2 g of the aggregate at 300 °C for 10 minutes, then press-forming under the conditions of 300 °C for 5 minutes and 10 MPa, and then cooling. In addition, aqueous dispersions were not obtained in Examples 4 and 5, so coloring evaluation could not be carried out.

[0077] [Table 1]

[0078] It can be seen that according to the method for producing a fluoropolymer of the present invention, even when using an aqueous medium with a small environmental load, a fluoropolymer can be efficiently produced without an emulsifier (Examples 1 to 3). In addition, from the comparison of Examples 1 to 3, it can be seen that when an aqueous dispersion in which the concentration of sulfate ions is 5 mass ppm or less relative to the total mass of the aqueous medium contained in the aqueous dispersion is used for polymerization, the coloring of the fluoropolymer can be sufficiently suppressed (Examples 1 and 3). In contrast, fluoropolymers could not be efficiently produced in Examples 4 and 5.

[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 manufacturing a fluoropolymer, which is a method for polymerizing a monomer containing a fluoromonomer in an aqueous dispersion containing a first fluoropolymer and an aqueous medium to manufacture a second fluoropolymer different from the first fluoropolymer, the first fluoropolymer containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), wherein, In the first fluoropolymer, the units based on perfluoro(alkyl vinyl ether) are 20 to 60 mol% relative to the total of the units based on tetrafluoroethylene and the units based on perfluoro(alkyl vinyl ether), The average particle size of the first fluoropolymer is 1 to 150 nm, Before the start of the monomer polymerization, the content of the first fluoropolymer is 0.01 to 4.0% by mass relative to the total mass of the aqueous dispersion.

2. The method for producing a fluoropolymer according to claim 1, wherein, Before the start of the monomer polymerization, the concentration of sulfate ions is 5 mass ppm or less relative to the total mass of the aqueous medium in the aqueous dispersion.

3. The method for manufacturing a fluoropolymer according to claim 1 or 2, wherein, The fluoromonomer contains at least one selected from tetrafluoroethylene, chlorotrifluoroethylene, and vinylidene fluoride.

4. The method for manufacturing a fluoropolymer according to claim 1 or 2, wherein, The monomer contains ethylene.

5. The method for manufacturing a fluoropolymer according to claim 1 or 2, wherein, The usage amount of the monomer is 1 to 50 parts by mass relative to 100 parts by mass of the usage amount of the aqueous medium.

6. The method for producing a fluoropolymer according to claim 1 or 2, wherein, The monomer is polymerized in the presence of a polymerization initiator.

7. An aqueous dispersion, which is an aqueous dispersion containing an aqueous medium and particles containing a fluoropolymer, wherein, The average particle size of the particles is 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% relative to all the units of the fluoropolymer, 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 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 NH4, and n each independently represents 8 or 10.

8. The aqueous dispersion according to claim 7, wherein, The particles contain at least one type of unit selected from units based on tetrafluoroethylene, units based on chlorotrifluoroethylene, and units based on vinylidene fluoride.

9. The aqueous dispersion according to claim 7 or 8, wherein The particles contain units based on ethylene.

10. A particle, which is a particle containing a fluoropolymer, wherein, The average particle size of the particle is 1 μm or less, The particle contains units based on tetrafluoroethylene, units based on ethylene, and units based on perfluoro(alkyl vinyl ether), The units based on ethylene are 20 to 70 mol% relative to the total of the units based on tetrafluoroethylene and the units based on ethylene, The total content of the units based on tetrafluoroethylene and the units based on ethylene is 80 mol% or more relative to all the units of the fluoropolymer, The content of the units based on perfluoro(alkyl vinyl ether) is 0.1 to 1.0 mol% relative to all the units of the fluoropolymer, 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 relative to the total mass of the particle: Formula (S1): H-(CF2) n -COOM Formula (S2): H-(CF2) n -SO3M In formula (S1) and formula (S2), M independently represents a hydrogen atom, Na, K or NH4, and n independently represents 8 or 10.

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