Aqueous dispersion

Abstract

To provide an aqueous dispersion which is excellent in storage stability and is capable of forming a coating film excellent in water resistance.SOLUTION: The aqueous dispersion of the present invention is an aqueous dispersion containing a fluorinated polymer having TFE units and PAVE units, and an aqueous medium, wherein the proportion of the PAVE units is 20 to 60 mol% to the total amount of the TFE units and the PAVE units, the content of the fluorinated polymer is 0.1 to 40 mass% to the total mass of the aqueous dispersion, the average particle size of the fluorinated polymer is 1 to 150 nm, the concentration of fluoride ions and the concentration of sulfate ions are each 50 mass ppm or less to the total mass of the aqueous medium in the aqueous dispersion, and the concentration of a hydrocarbon-based emulsifier is 100 mass ppm or less to the total mass of the fluorinated polymer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to aqueous dispersions. [Background technology]

[0002] Fluorine-containing polymers are used in various industrial fields because of their excellent heat resistance, chemical resistance, flame retardancy, weather resistance, etc. Such fluorine-containing polymers are sometimes used in the form of an aqueous dispersion. As a method for producing an aqueous dispersion containing such a fluorine-containing polymer, Patent Document 1 discloses a method in which a nonionic surfactant is added to a mixture containing a perfluoroelastomer obtained using monomers such as tetrafluoroethylene and perfluoro(methyl vinyl ether) and deionized water, and the mixture is mixed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2003-522232 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, in the field of paints, from the viewpoint of environmental protection, water-based paints containing paint resins and using water alone or a mixture of water and a water-soluble organic solvent as a medium have been developed. Such water-based paints are required to have excellent storage stability and also excellent water resistance of the coating films formed using them. The present inventors have evaluated the use of an aqueous dispersion containing a fluorine-containing polymer as described in Patent Document 1 as an aqueous coating material, and have found that there is room for improvement in the water resistance of a coating film obtained using the same.

[0005] An object of the present invention is to provide an aqueous dispersion that has excellent storage stability and is capable of forming a coating film that is excellent in water resistance. [Means for solving the problem]

[0006] As a result of extensive research, the present inventors have found that the above problems can be solved by the following configuration. [1] An aqueous dispersion comprising a fluorine-containing polymer containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), and an aqueous medium, the content of the perfluoro(alkyl vinyl ether)-based units is 20 to 60 mol % based on the total of the tetrafluoroethylene-based units and the perfluoro(alkyl vinyl ether)-based units, the content of the fluoropolymer is 0.1 to 40% by mass relative to the total mass of the aqueous dispersion, the average particle size of the fluoropolymer is 1 to 150 nm, a fluoride ion concentration of 50 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion; a sulfate ion concentration of 50 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion; An aqueous dispersion characterized in that the concentration of the hydrocarbon emulsifier is 100 ppm by mass or less relative to the total mass of the fluoropolymer. [2] The aqueous dispersion according to [1], wherein the concentration of the fluorine-containing emulsifier is 100 ppm by mass or less based on the total mass of the fluorine-containing polymer. [3] The aqueous dispersion according to [1] or [2], wherein the concentration of the emulsifier is 100 ppm by mass or less relative to the total mass of the fluoropolymer. [4] The aqueous dispersion according to any one of [1] to [3], wherein the perfluoro(alkyl vinyl ether) is a monomer represented by the following formula (1): CF2=CF-OR f1 (1) (In formula (1), R f1 represents a perfluoroalkyl group having 1 to 10 carbon atoms. [5] A coated article comprising a substrate and a coating film disposed on the substrate and formed using the aqueous dispersion according to any one of [1] to [4], wherein the material of the substrate is an inorganic material, an organic material, or an organic-inorganic composite material. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an aqueous dispersion that has excellent storage stability and is capable of forming a coating film that is excellent in water resistance. DETAILED DESCRIPTION OF THE INVENTION

[0008] The terms used in the present invention have the following meanings. A numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. In the numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples. In this specification, each component may be a single substance corresponding to the component, or two or more substances may be used in combination. When two or more substances are used in combination for each component, the content of the component refers to the total content of the substances used in combination, unless otherwise specified. As used herein, a combination of two or more preferred embodiments is a more preferred embodiment. The term "unit" refers collectively to an atomic group derived from one molecule of the monomer that is formed directly by polymerizing the monomer, and an atomic group obtained by chemically converting a part of the atomic group. Hereinafter, a "unit based on a monomer" will also be simply referred to as a "unit." The content (mass % or mol %) of each unit relative to the total units contained in the polymer can be determined by analyzing the polymer by solid-state nuclear magnetic resonance spectroscopy (NMR), but can also be estimated from the amount of each monomer charged. Usually, the content of each unit calculated from the amount of each monomer charged is approximately the same as the actual content of each unit. The average particle size of the particles is a particle size calculated by analyzing the autocorrelation function obtained by dynamic light scattering using the monodisperse cumulant method.

[0009] [Aqueous dispersion] The aqueous dispersion of the present invention (hereinafter also referred to as "the present aqueous dispersion") is an aqueous dispersion containing a fluorine-containing polymer containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether) (hereinafter also referred to as "specific fluorine-containing polymer"), and an aqueous medium. In the specific fluorine-containing polymer, the unit based on the perfluoro(alkyl vinyl ether) is 20 to 60 mol % relative to the total of the unit based on the tetrafluoroethylene and the unit based on the perfluoro(alkyl vinyl ether). In addition, the content of the specific fluorine-containing polymer in the present aqueous dispersion is 0.1 to 40% by mass relative to the total mass of the present aqueous dispersion. In the present aqueous dispersion, the specific fluorine-containing polymer has an average particle size of 1 to 150 nm. In addition, in the present aqueous dispersion, the concentration of fluoride ions is 50 mass ppm or less relative to the total mass of the aqueous medium in the present aqueous dispersion, and the concentration of sulfate ions is 50 mass ppm or less relative to the total mass of the aqueous medium in the present aqueous dispersion. In addition, in the present aqueous dispersion, the concentration of the hydrocarbon-based emulsifier is 100 ppm by mass or less based on the total mass of the specific fluorine-containing polymer.

[0010] The aqueous dispersion has excellent storage stability, which is presumably because the use of a fluoropolymer having an average particle size within the above range improves the dispersion stability of the fluoropolymer particles in the aqueous medium. The coating film formed using this aqueous dispersion is excellent in water resistance. The reason for this is presumably that the adhesion and penetration of water into the coating film can be suppressed by setting the contents of fluoride ions and sulfate ions relative to the aqueous medium in the aqueous dispersion and the content of hydrocarbon emulsifier relative to the aqueous dispersion to below predetermined values. Furthermore, it is presumed that the use of a fluorine-containing polymer having an average particle size within the above range facilitates dense packing of the fluorine-containing polymer particles during coating film formation, thereby suppressing the occurrence of pinholes in the coating film and improving the water resistance of the coating film. It is believed that these effects are exerted synergistically to obtain a coating film with excellent water resistance.

[0011] <Specific fluoropolymer> The specific 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").

[0012] The PAVE is preferably a monomer represented by formula (1) because it has excellent polymerization reactivity when producing the specific fluorine-containing polymer. CF2=CF-OR f1 (1) In formula (1), R f1 represents a perfluoroalkyl group having 1 to 10 carbon atoms. f1 The number of carbon atoms is preferably 1 to 8, more preferably 1 to 6, further preferably 1 to 5, and particularly preferably 1 to 3, in terms of better polymerization reactivity. The perfluoroalkyl group may be linear or branched.

[0013] Specific examples of PAVE include perfluoro(methyl vinyl ether) (hereinafter also referred to as "PMVE"), perfluoro(ethyl vinyl ether) (hereinafter also referred to as "PEVE"), and perfluoro(propyl vinyl ether) (hereinafter also referred to as "PPVE"). Among these, PMVE and PPVE are preferred, with PMVE being more preferred, in terms of reactivity with tetrafluoroethylene in an aqueous medium.

[0014] In the specific fluorine-containing polymer, the PAVE units are 20 to 60 mol % based on the total of TFE units and PAVE units, 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 %.

[0015] The specific fluorine-containing polymer may contain units based on monomers other than TFE and PAVE, but it is preferable that it does not substantially contain units based on other monomers in order to achieve better effects of the present invention. "Substantially free of units derived from other monomers" means that the content of units derived from other monomers is 0.01 mol % or less, preferably 0 mol %, based on the total units of the specific fluorine-containing polymer. When the specific fluorine-containing polymer contains units based on another monomer, the other monomer is preferably hexafluoropropylene.

[0016] The content of the specific fluorine-containing polymer is 0.1 to 40% by mass, based on the total mass of the aqueous dispersion, and is preferably 5% by mass or more, more preferably 10% by mass or more, from the viewpoint of enabling thick film coating, and is preferably 40% by mass or less, more preferably 30% by mass or less, from the viewpoint of storage stability.

[0017] The specific fluorine-containing polymer does not have a melting point.

[0018] The specific fluorine-containing polymer is dispersed in the aqueous medium in the form of particles. The average particle size of the specific fluorine-containing polymer is from 1 to 150 nm, and from the viewpoint of achieving better effects of the present invention, it is preferably from 50 to 140 nm, more preferably from 70 to 130 nm.

[0019] <Aqueous medium> The aqueous medium 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. The content of the aqueous medium is preferably from 60 to 99 mass %, more preferably from 65 to 99 mass %, and even more preferably from 70 to 99 mass %, based on the total mass of the aqueous dispersion.

[0020] <Fluoride ions and sulfate ions> Fluoride ions may be generated by the reaction between a polymerization initiator and tetrafluoroethylene and contained in the aqueous dispersion. Sulfate ions may be generated by, for example, thermal decomposition of a polymerization initiator (particularly ammonium persulfate) used in the production of a specific fluorine-containing polymer and contained in the aqueous dispersion. From the viewpoint of improving the water resistance of a coating film formed using the aqueous dispersion, it is preferable that the aqueous dispersion does not contain any fluoride ions or sulfate ions, or that even if they are contained, they are contained in trace amounts. Specifically, the concentrations of fluoride ions and sulfate ions in the present aqueous dispersion are each 50 ppm by mass or less, based on the total mass of the aqueous medium in the present aqueous dispersion, and are more preferably 30 ppm by mass or less, and even more preferably 20 ppm by mass or less, in order to improve the water resistance of a coating film formed using the present aqueous dispersion.The lower limit is 0 ppm by mass. One example of a method for adjusting the concentrations of fluoride ions and sulfate ions relative to the aqueous medium in the present aqueous dispersion to fall within the above range is to remove the fluoride ions and sulfate ions using an ion exchange resin or the like, as will be described later.

[0021] <Emulsifier> The hydrocarbon-based emulsifier means an emulsifier in which the hydrophobic portion is mainly composed of a hydrocarbon group, among the hydrophilic portion and the hydrophobic portion of the emulsifier. Specific examples of hydrocarbon-based emulsifiers include anionic hydrocarbon-based emulsifiers such as alkylbenzene sulfonates, higher fatty acid salts, alkyl sulfate ester salts, alkyl sulfonates, and alkyl ether sulfates; cationic hydrocarbon-based emulsifiers such as alkylamine salts, quaternary alkylammonium salts, and benzalkonium salts; and nonionic hydrocarbon-based emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl esters, sorbitan alkyl esters, polyoxyethylene sorbitan alkyl esters, and glycerol esters. In the present aqueous dispersion, the concentration of the hydrocarbon-based emulsifier is 100 ppm by mass or less, based on the total mass of the specific fluorine-containing polymer, and from the viewpoint of better water resistance of a coating film formed using the present aqueous dispersion, is more preferably 80 ppm by mass or less, and even more preferably 50 ppm by mass or less. The lower limit is 0 ppm by mass.

[0022] The fluorine-containing emulsifier refers to an emulsifier in which the hydrophobic moiety contains a fluorine atom, among the hydrophilic moiety and the hydrophobic moiety of the emulsifier. Specific examples of the fluorine-containing emulsifier include fluorine-containing alkanoates and fluorine-containing ether carboxylic acid compounds. One of the preferred embodiments of the present aqueous dispersion is one in which the concentration of the fluorine-containing emulsifier is 100 ppm by mass or less relative to the total mass of the specific fluorine-containing polymer. In the present aqueous dispersion, the concentration of the fluorine-containing emulsifier is preferably 100 ppm by mass or less, more preferably 75 ppm by mass or less, and even more preferably 50 ppm by mass or less, relative to the total mass of the specific fluorine-containing polymer. The lower limit is 0 ppm by mass. When the concentration of the fluorine-containing emulsifier relative to the total mass of the specific fluorine-containing polymer is 100 ppm by mass or less, the water resistance of the coating film formed using the present aqueous dispersion is better.

[0023] The term "emulsifier" refers to all emulsifiers, including not only the hydrocarbon-based emulsifiers and fluorine-based emulsifiers mentioned above, but also other emulsifiers (for example, silicon-based emulsifiers whose hydrophobic moieties contain silicon atoms). In the present aqueous dispersion, the concentration of the emulsifier is preferably 100 ppm by mass or less, more preferably 75 ppm by mass or less, and even more preferably 50 ppm by mass or less, relative to the total mass of the specific fluorine-containing polymer. The lower limit is 0 ppm by mass. If the concentration of the emulsifier relative to the total mass of the specific fluorine-containing polymer is 100 ppm by mass or less, the water resistance of the coating film formed using the present aqueous dispersion will be better.

[0024] As an example of a method for adjusting the concentrations of the various emulsifiers described above to fall within the above ranges, there is a method for producing the present aqueous dispersion without using any of the various emulsifiers.

[0025] <Other ingredients> The present aqueous dispersion may contain other components in addition to those described above, as long as the effects of the present invention can be fully exhibited. Specific examples of other components that may be contained in the present aqueous dispersion include additives that may be contained in ordinary paints, such as curing agents, curing catalysts, resins other than the above-mentioned specific fluorine-containing polymers (e.g., (meth)acrylic resins, urethane resins, epoxy resins), colorants (e.g., dyes, organic pigments, inorganic pigments, luster pigments using metals or mica), ultraviolet absorbers, matting agents, leveling agents, surface conditioners, degassing agents, fillers, thickeners, antistatic agents, rust inhibitors, silane coupling agents, antifouling agents, stain-reduction treatment agents, plasticizers, and adhesives. When the present aqueous dispersion contains other components, the content of the other components is preferably from 1 to 90% by mass, more preferably from 10 to 80% by mass, based on the total mass of the present aqueous dispersion.

[0026] <Method of manufacturing aqueous dispersion> One example of the method for producing the present aqueous dispersion described above includes an embodiment comprising: a polymerization step of polymerizing a monomer containing TFE and PAVE in an aqueous medium in the presence of a polymerization initiator to obtain a dispersion containing the specific fluorine-containing polymer and an aqueous medium; and a removal step of removing fluoride ions and sulfate ions from the dispersion. The aqueous medium thus obtained, in which the particles of the specific fluorine-containing polymer are dispersed, may be used as the present aqueous dispersion as it is, or another aqueous medium may be added thereto and the resulting mixture may be used as the present aqueous dispersion. Alternatively, the specific fluorine-containing polymer may be dispersed in another aqueous medium by solvent substitution and the resulting mixture may be used as the present aqueous dispersion.

[0027] Specific examples of the aqueous medium used in the polymerization step are the same as the specific examples of the aqueous medium contained in the present aqueous dispersion described above. The monomers used in the polymerization step include TFE and PAVE, and may further include other monomers, but preferably do not include other monomers. Preferred embodiments of PAVE are as described above. The amount of the monomers used may be appropriately adjusted so that the content of each unit contained in the resulting specific fluorine-containing polymer falls within the above-mentioned range. The amount of the monomer used is preferably 3 to 20 parts by mass, more preferably 5 to 10 parts by mass, per 100 parts by mass of the aqueous medium used in the polymerization step.

[0028] The polymerization initiator is preferably a water-soluble polymerization initiator, more preferably a persulfate such as ammonium persulfate, sodium persulfate, or potassium persulfate, or an organic polymerization initiator such as disuccinic acid peroxide or azobisisobutylamidine dihydrochloride, still more preferably a persulfate, and particularly preferably ammonium persulfate. Two or more polymerization initiators may be used in combination. The amount of the polymerization initiator used is preferably 0.1 to 3 parts by mass, more preferably 0.2 to 2 parts by mass, based on 100 parts by mass of the amount of the monomer used.

[0029] In the polymerization step, a chain transfer agent component may be used.

[0030] The monomer is introduced into the reaction system (i.e., polymerization reaction vessel) by a conventional method. For example, the monomer may be introduced into the reaction system continuously or intermittently so that the polymerization pressure reaches a predetermined pressure. Alternatively, the monomer may be dispersed or dissolved in an aqueous medium, and the resulting mixture may be introduced into the reaction system continuously or intermittently. The polymerization initiator may be added to the reaction system all at once or in portions.

[0031] The polymerization temperature is preferably from 10 to 95°C, more preferably from 15 to 90°C. The polymerization pressure is preferably from 0.5 to 4.0 MPaG, more preferably from 0.6 to 3.5 MPaG. In the case of batch processing, the polymerization time is preferably from 30 to 1000 minutes, more preferably from 50 to 700 minutes.

[0032] The polymerization step is preferably carried out in the substantial absence of a hydrocarbon-based emulsifier, more preferably in the substantial absence of a hydrocarbon-based emulsifier and a fluorine-based emulsifier, and even more preferably in the substantial absence of an emulsifier. Specific examples of these emulsifiers are as described above. "In the substantial absence of hydrocarbon-based emulsifiers" means an environment in which the content of hydrocarbon emulsifiers is 100 ppm by mass or less, preferably 50 ppm by mass or less, and more preferably 0 ppm by mass, relative to the total mass of the aqueous medium used in the polymerization step. The condition substantially free of hydrocarbon-based emulsifiers and fluorine-based emulsifiers means an environment in which the total content of hydrocarbon emulsifiers and fluorine-based emulsifiers is 100 ppm by mass or less, preferably 50 ppm by mass or less, and more preferably 0 ppm by mass, relative to the total mass of the aqueous medium used in the polymerization step. "In the substantial absence of an emulsifier" means an environment in which the content of the emulsifier is 100 ppm by mass or less, preferably 50 ppm by mass or less, and more preferably 0 ppm by mass, relative to the total mass of the aqueous medium used in the polymerization step.

[0033] In the removal step, fluoride ions and sulfate ions that may be contained in the dispersion are removed. Examples of a means for removing fluoride ions and sulfate ions include an ion exchange resin (preferably an anion exchange resin). The removal step may be carried out multiple times until the concentrations of fluoride ions and sulfate ions in the resulting aqueous dispersion fall within the above-mentioned ranges.

[0034] <Application> The aqueous dispersion is suitable for use as a paint itself or as a raw material for paint.

[0035] [Painted items] The coated article of the present invention has a substrate and a coating film disposed on the substrate and formed using the above-described aqueous dispersion.

[0036] Specific examples of the material of the substrate include inorganic materials, organic materials, and organic-inorganic composite materials. Specific examples of inorganic materials include concrete, natural stone, glass, and metals (iron, stainless steel, aluminum, aluminum alloys, copper, brass, titanium, etc.). Specific examples of organic materials include plastics, rubber, adhesives, and wood. Specific examples of organic-inorganic composite materials include fiber-reinforced plastics, resin-reinforced concrete, and fiber-reinforced concrete. The substrate may be subjected to a known surface treatment (such as a chemical conversion treatment), or may have a resin layer (such as a polyester resin layer, an acrylic resin layer, or a silicone resin layer) formed by applying a primer or the like to the surface of the substrate.

[0037] The thickness of the coating film is preferably 1 to 200 μm, more preferably 10 to 100 μm, in order to provide a coated article with better weather resistance.

[0038] The method for producing a coated article is a method for forming a coating film by applying the present aqueous dispersion to a substrate, and then drying and curing the coating film by heating, as necessary. The aqueous dispersion may be applied directly to the surface of a substrate, or may be applied after a known surface treatment (priming treatment, etc.) has been performed on the surface of the substrate. Furthermore, the aqueous dispersion may be applied onto a primer layer formed on the substrate. The aqueous dispersion may also be applied to an article having the above-mentioned substrate. Examples of the coating method include spray coating, squeegee coating, flow coating, bar coating, spin coating, dip coating, screen printing, gravure printing, die coating, inkjet coating, curtain coating, and methods using a brush or spatula. [Example]

[0039] The present invention will be described in detail below with reference to examples. Example 1 is an example, and Examples 2 to 4 are comparative examples. However, the present invention is not limited to these examples.

[0040] [measurement] <Average particle size of particles in aqueous dispersion> The aqueous dispersion was used as a sample and measured using a laser diffraction / scattering particle size distribution analyzer (Otsuka Electronics Co., Ltd., ELSZ).

[0041] <Proportion of each unit in the polymer> The ratio of each unit in the polymer is 19 It was determined by F-NMR analysis.

[0042] <Fluoride ion and sulfate ion concentrations> The concentrations of fluoride ions and sulfate ions relative to the total mass of the aqueous medium in the aqueous dispersion were 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 (Thermo Fisher Scientific). The separation column was a Dionex IonPac AS-19, the guard column was a Dionex IonPac AG-19, and the eluent was KOH.

[0043] <Emulsifier concentration> The concentrations of the hydrocarbon-based emulsifier, the fluorine-based emulsifier and the emulsifier relative to the total mass of the fluoropolymer in the aqueous dispersion were calculated from the amounts charged.

[0044] [Production of aqueous dispersion A] A 2.1 L stainless steel pressure reactor was charged with ultrapure water (1162 g), 28% NH3 aqueous solution (1 drop), PMVE (70 g), and TFE (14 g). The mixture was heated to 80 °C while stirring at 600 rpm. Next, an aqueous ammonium persulfate solution (5.9 mass%, 5 cc) was added to initiate polymerization. As the polymerization began, the pressure inside the reactor decreased, so 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. The remaining gas in the reactor was recovered and then replaced with nitrogen. Next, the pressure inside the reactor was reduced to -0.1 MPaG and the reactor was heated to 60 °C. After 980 g of water was removed from the reactor, it was cooled and the liquid inside the reactor was removed. This liquid was designated as aqueous dispersion A. Aqueous dispersion A was a dispersion in which particles of fluoropolymer 1A (average particle size 110 nm) were dispersed in an aqueous medium, and the content of fluoropolymer 1A was 20 mass % based on the total mass of aqueous dispersion A. Aqueous dispersion A was freeze-aggregated and then filtered, and the resulting fluoropolymer 1A was washed with ultrapure water and then vacuum-dried at 100° C. The resulting fluoropolymer 1A was analyzed by NMR, and the ratio of PMVE units to TFE units was 34.4 / 65.6 (molar ratio).

[0045] [Production of aqueous dispersion B] An ion exchange resin (Purolite A300 (manufactured by Purolite), an anion exchange resin, 15 g) was added to the above aqueous dispersion A (100 g) and stirred for 60 minutes. Thereafter, the aqueous dispersion and the ion exchange resin were separated by filtration to obtain aqueous dispersion B. Aqueous dispersion B was a dispersion in which particles of fluoropolymer 1A (average particle size 110 nm) were dispersed in an aqueous medium, and the content of fluoropolymer 1A was 20 mass % relative to the total mass of aqueous dispersion B.

[0046] [Production of aqueous dispersion C] A 2.1 L stainless steel pressure reactor was charged with ultrapure water (1082 g), aqueous disodium hydrogen phosphate dodecahydrate (10.5 g), 30 wt% EEA solution (aqueous solution of CF3CF2-O-CF2CF2-O-CF2COONH4) (80.1 g), PMVE (70 g), and TFE (14 g). The mixture was stirred at 600 rpm and heated to 80 °C. An aqueous ammonium persulfate solution (1.0 wt%, 20 cc) was added to initiate polymerization. As the polymerization began, the pressure in the reactor decreased, so TFE was added to maintain a constant pressure. For every 8 g of TFE, 7 g of PMVE was injected. After 160 g of TFE had been injected, the reactor was cooled to terminate the polymerization reaction. The remaining gas in the reactor was recovered, and the liquid was withdrawn. This liquid was designated as aqueous dispersion C. Aqueous dispersion C is a dispersion in which particles of fluoropolymer 1C are dispersed in an aqueous medium. Aqueous dispersion C was freeze-aggregated and then filtered to obtain fluoropolymer 1C, which was washed with ultrapure water and then vacuum-dried at 100° C. The obtained fluoropolymer 1C was analyzed by NMR, and the ratio of PMVE units to TFE units was found to be 34 / 66 (molar ratio).

[0047] [Production of aqueous dispersion D] To aqueous dispersion C (100 g) were added 1 g of TERGITOL TMN-100X (a nonionic hydrocarbon-based emulsifier manufactured by Dow) and 15 g of an ion exchange resin (Purolite A300 (manufactured by Purolite), an anion exchange resin), and the mixture was stirred for 60 minutes. Thereafter, the aqueous dispersion and the ion exchange resin were separated by filtration to obtain aqueous dispersion D. Aqueous dispersion D was a dispersion in which particles of fluoropolymer 1C (average particle size 84 nm) were dispersed in an aqueous medium, and the content of fluoropolymer 1C was 21 mass % relative to the total mass of aqueous dispersion D.

[0048] [Production of aqueous dispersion E] A 2.1 L stainless steel pressure reactor was charged with ultrapure water (1170 g), aqueous disodium hydrogen phosphate dodecahydrate (10.5 g), PMVE (75 g), and TFE (14 g). The mixture was heated to 80 °C while stirring at 600 rpm. Next, ammonium persulfate solution (20% by mass, 5 cc) was added to initiate polymerization. As the polymerization began, the pressure in the reactor decreased, and TFE was added to maintain a constant pressure. Every time 8 g of TFE was injected, 7 g of PMVE was injected. When 160 g of TFE had been injected, the reactor was cooled to terminate the polymerization reaction. The remaining gas in the reactor was recovered, and the liquid was withdrawn. This liquid was designated fluoroelastomer dispersion E. Aqueous dispersion E is a dispersion of fluoropolymer 1E particles (average particle size 290 nm) dispersed in an aqueous medium. Aqueous dispersion E was freeze-aggregated and then filtered, and the resulting fluoropolymer 1E was washed with ultrapure water and then vacuum-dried at 100° C. The resulting fluoropolymer 1E was analyzed by NMR, and the ratio of PMVE units to TFE units was found to be 33.3 / 66.7 (molar ratio). The concentration of sulfate ions relative to the total mass of the aqueous medium in aqueous dispersion E was 2000 ppm by mass.

[0049] [Production of aqueous dispersion F] An ion exchange resin (Purolite A300 (manufactured by Purolite), an anion exchange resin, 15 g) was added to aqueous dispersion E (100 g) and stirred for 60 minutes. Thereafter, the aqueous dispersion and the ion exchange resin were separated by filtration to obtain aqueous dispersion F. Aqueous dispersion F was a dispersion in which particles of fluoropolymer 1E (average particle size 290 nm) were dispersed in an aqueous medium, and the content of fluoropolymer 1E was 20 mass % relative to the total mass of aqueous dispersion F.

[0050] [Example 1] The above aqueous dispersion B was used as the aqueous dispersion in Example 1.

[0051] [Example 2] The above aqueous dispersion A was used as the aqueous dispersion in Example 2.

[0052] [Example 3] The above aqueous dispersion D was used as the aqueous dispersion in Example 3.

[0053] [Example 4] The above aqueous dispersion F was used as the aqueous dispersion in Example 4.

[0054] [Evaluation test] <Storage stability> The storage stability of the aqueous dispersion was evaluated by the following test. 50 cc of the aqueous dispersion of each example was placed in a centrifuge tube and left to stand at room temperature (23°C) for 2 weeks. The amount of precipitate (fluorine-containing polymer particles) that had sunk to the bottom of the centrifuge tube was read from the scale and evaluated according to the following criteria. A: 0~0.1cc B: More than 0.1cc

[0055] <Water resistance 1> V-Celan (registered trademark) #700 manufactured by Dai Nippon Toryo Co., Ltd. was applied to the surface of a slate board measuring 120 mm in length, 60 mm in width, and 15 mm in thickness using an air spray to a dry film thickness of 20 μm, and then dried at 100°C for 210 seconds to form a primer film. Next, the aqueous dispersion of each example was applied to the primer film using an air spray to a dry film thickness of 40 μm, and then dried at 120°C for 210 seconds to form a coating film, and a test board corresponding to each example was obtained. The test was carried out by immersing the test plate in hot water at 60°C for 18 hours, then in cold water at 5°C for 15 hours, and then drying at 5°C. After drying, the appearance of the coating film was evaluated according to the following criteria. A: No whitening or blistering was observed on 80% or more of the coating surface. B: Whitening or blistering was observed over 20% of the coating surface.

[0056] <Water resistance 2> The test panels obtained in "Water Resistance 1" above were immersed in warm water at 60°C for 2 weeks, and then dried at 5°C. After drying, the appearance of the coating film was evaluated according to the following criteria. A: No whitening or blistering was observed on 80% or more of the coating surface. B: No whitening or blistering was observed over 60% or more but less than 80% of the coating surface. C: Whitening or blistering was observed over 40% of the coating surface.

[0057] [Table 1] It was shown that the aqueous dispersion of the present invention has excellent storage stability and can form a coating film with excellent water resistance (Example 1). The entire contents of the specification, claims and abstract of Japanese Patent Application No. 2022-201101, filed on December 16, 2022, are hereby incorporated by reference as part of the disclosure of the present invention.

Claims

1. An aqueous dispersion comprising a fluorine-containing polymer containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), and an aqueous medium, the content of the perfluoro(alkyl vinyl ether)-based units is 20 to 60 mol % based on the total of the tetrafluoroethylene-based units and the perfluoro(alkyl vinyl ether)-based units; the content of the fluorine-containing polymer is 0.1 to 40% by mass relative to the total mass of the aqueous dispersion, the average particle size of the fluoropolymer is 1 to 150 nm, a fluoride ion concentration of 50 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion; a sulfate ion concentration of 50 ppm by mass or less relative to the total mass of the aqueous medium in the aqueous dispersion; An aqueous dispersion characterized in that the concentration of the hydrocarbon emulsifier is 100 ppm by mass or less relative to the total mass of the fluoropolymer.

2. 2. The aqueous dispersion according to claim 1, wherein the concentration of the fluorine-containing emulsifier is 100 ppm by mass or less based on the total mass of the fluorine-containing polymer.

3. 3. The aqueous dispersion according to claim 1, wherein the concentration of the emulsifier is 100 ppm by mass or less relative to the total mass of the fluoropolymer.

4. 3. The aqueous dispersion according to claim 1, wherein the perfluoro(alkyl vinyl ether) is a monomer represented by the following formula (1): CF ２ =CF-O-R ｆ１ (1) (In formula (1), R ｆ１ represents a perfluoroalkyl group having 1 to 10 carbon atoms.

5. A coating film comprising a substrate and a coating film formed on the substrate using the aqueous dispersion according to claim 1 or 2, A coated article, characterized in that the material of the substrate is an inorganic material, an organic material, or an organic-inorganic composite material.

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