Aqueous dispersion

Abstract

Provided is an aqueous dispersion which has excellent storage stability and is capable of forming a coating film having excellent water resistance. This aqueous dispersion contains a fluorine-containing polymer and an aqueous medium, the fluorine-containing polymer containing TFE units and PAVE units, the PAVE units being 20-60 mol% relative to the total of the TFE units and the PAVE units, the content of the fluorine-containing polymer being 0.1-40 mass% relative to the total mass of the aqueous dispersion, the average particle diameter of the fluorine-containing polymer being 1-150 nm, and the average particle diameter of the fluorine-containing polymer being 1-150 nm. The concentration of the fluoride ions and the sulfate ions is 50 ppm by mass or less with respect to the total mass of the aqueous medium in the aqueous dispersion, and the concentration of the hydrocarbon emulsifier is 100 ppm by mass or less with respect to the total mass of the fluorine-containing polymer.

Description

Technical Field

[0001] The present invention relates to an aqueous dispersion. Background Art

[0002] Fluoropolymers are used in various industrial fields due to their excellent heat resistance, chemical resistance, flame retardancy, weather resistance, etc. Such fluoropolymers are sometimes used in the form of an aqueous dispersion. As a method for producing such an aqueous dispersion containing a fluoropolymer, Patent Document 1 discloses a method of adding a nonionic surfactant to a mixture containing a perfluoroelastomer obtained by using monomers such as tetrafluoroethylene and perfluoro(methyl vinyl ether) and deionized water and mixing them. Prior Art Documents Patent Documents

[0003] Patent Document 1: Japanese Patent Publication No. 2003-522232 Summary of the Invention Technical Problem to be Solved by the Invention

[0004] In recent years, in the field of coatings, from the viewpoint of environmental protection, aqueous coatings containing coating resins and using only water or a mixture of water and water-soluble organic solvents as a medium are being developed. Such aqueous coatings are required to have excellent storage stability and excellent water resistance of the coating film formed therefrom. The present inventors evaluated using the aqueous dispersion containing a fluoropolymer described in Patent Document 1 as an aqueous coating, and found that there is still room for improvement in the water resistance of the coating film obtained therefrom.

[0005] An object of the present invention is to provide an aqueous dispersion having excellent storage stability and capable of forming a coating film having excellent water resistance. Technical Solution for Solving the Technical Problem

[0006] The present inventors have conducted in-depth research and found that the above technical problems can be solved by the following constitution. [1] An aqueous dispersion which is an aqueous dispersion containing a fluoropolymer and an aqueous medium, the fluoropolymer containing units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), wherein 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 content of the fluoropolymer is 0.1 to 40 mass% relative to the total mass of the aqueous dispersion, the average particle size of the fluoropolymer is 1 to 150 nm, The concentration of fluoride ions is 50 mass ppm or less relative to the total mass of the aqueous medium in the above aqueous dispersion, The concentration of sulfate ions is 50 mass ppm or less relative to the total mass of the aqueous medium in the above aqueous dispersion, The concentration of the hydrocarbon emulsifier is 100 mass ppm or less relative to the total mass of the fluoropolymer above. [2] The aqueous dispersion according to [1], wherein the concentration of the fluorinated emulsifier is 100 mass ppm or less relative to the total mass of the fluoropolymer above. [3] The aqueous dispersion according to [1] or [2], wherein the concentration of the emulsifier is 100 mass ppm or less relative to the total mass of the fluoropolymer above. [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-O-R f1 (1) (In formula (1), R f1 represents a perfluoroalkyl group having 1 to 10 carbon atoms). [5] A coated article, which has a substrate and a coating film formed of the aqueous dispersion according to any one of [1] to [4] disposed on the substrate, and the material of the substrate is an inorganic substance, an organic substance, or an organic-inorganic composite material. Advantages of the Invention

[0007] According to the present invention, an aqueous dispersion can be provided which has excellent storage stability and can form a coating film having excellent water resistance. Detailed Description

[0008] The meanings of the terms in the present invention are as described below. A 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 kinds can be used in combination. Here, 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 means the total content of the substances used in combination. In this specification, a 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 atomic groups. "Unit based on monomer" is also simply referred to as "unit" hereinafter. The content (mass% or mol%) of each unit relative to all the units contained in the polymer can be determined by analyzing the polymer by solid nuclear magnetic resonance (NMR) method, or can be deduced from the feeding amounts of the respective monomers. Usually, the content of each unit calculated from the feeding amounts of the respective monomers is substantially the same as the actual content of each unit. The average particle size of the particles is the particle size calculated by analyzing the autocorrelation function obtained by the dynamic light scattering method using the monodisperse cumulant method.

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

[0010] The storage stability of this aqueous dispersion is excellent. For this reason, it is speculated that by using a fluoropolymer having an average particle size within the above range, the dispersion stability of the fluoropolymer particles in the aqueous medium can be improved. The water resistance of the coating film formed from this aqueous dispersion is excellent. For this reason, it is presumed that by making the content of fluoride ions and sulfate ions in the aqueous dispersion relative to the aqueous medium and the content of the hydrocarbon emulsifier relative to the aqueous dispersion be below specified values, respectively, the attachment and intrusion of water onto the coating film can be suppressed. In addition, it is presumed that by using a fluorine-containing polymer having an average particle diameter within the above range, the particles of the fluorine-containing polymer can be easily closely packed with each other during the formation of the coating film, suppressing the generation of perforations in the coating film and improving the water resistance of the coating film. It is considered that these effects act synergistically to obtain a coating film with excellent water resistance.

[0011] <Specific fluorine-containing polymer> 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] From the viewpoint of excellent polymerization reactivity during the production of the specific fluorine-containing polymer, PAVE is preferably a monomer represented by formula (1). CF2=CF-O-R f1 (1) In 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 can be linear or branched.

[0013] As specific examples of PAVE, 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") can be cited. Among them, from the viewpoint of reactivity with tetrafluoroethylene in an aqueous medium, PMVE and PPVE are preferred, and PMVE is more preferred.

[0014] In the specific fluorine-containing polymer, the PAVE units are 20 to 60 mol% relative to the total of the TFE units and the PAVE units. From the viewpoint of being able to more efficiently produce the second fluorine-containing polymer, it is preferably 25 to 60 mol%, and more preferably 30 to 55 mol%.

[0015] The specific fluorine-containing polymer may also contain units based on other monomers other than TFE and PAVE, but from the viewpoint of more excellent effects of the present invention, it is preferably substantially free of units based on other monomers. Substantially free of units based on other monomers means that the content of units based on other monomers is 0.01 mol% or less, preferably 0 mol%, relative to all the units of the specific fluorine-containing polymer. In the case where the specific fluoropolymer contains units based on other monomers, hexafluoropropylene is preferred as the other monomer.

[0016] The content of the specific fluoropolymer is 0.1 to 40% by mass relative to the total mass of the aqueous dispersion. From the perspective of being able to coat a thick film, it is preferably 5% by mass or more, more preferably 10% by mass or more. From the perspective of storage stability, it is preferably 40% by mass or less, more preferably 30% by mass or less.

[0017] The specific fluoropolymer does not have a melting point.

[0018] The specific fluoropolymer is dispersed in the aqueous medium in the form of particles. The average particle size of the specific fluoropolymer is 1 to 150 nm. From the perspective of more excellent effects of the present invention, it is preferably 50 to 140 nm, more preferably 70 to 130 nm.

[0019] <Aqueous medium> As the aqueous medium, 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. The content of the aqueous medium is preferably 60 to 99% by mass, more preferably 65 to 99% by mass, and further preferably 70 to 99% by mass relative to the total mass of the aqueous dispersion.

[0020] <Fluoride ions and sulfate ions> Fluoride ions are generated by the reaction of a polymerization initiator and tetrafluoroethylene and are sometimes contained in the aqueous dispersion. Sulfate ions are generated, for example, by the thermal decomposition of a polymerization initiator (especially ammonium persulfate) used in the manufacture of the specific fluoropolymer and are sometimes contained in the aqueous dispersion. Here, from the perspective of improving the water resistance of the coating film formed from the aqueous dispersion, fluoride ions and sulfate ions are preferably not contained in the aqueous dispersion at all, or only in trace amounts when contained. Specifically, the concentrations of fluoride ions and sulfate ions in the aqueous dispersion are each 50 mass ppm or less relative to the total mass of the aqueous medium in the aqueous dispersion. From the perspective of more excellent water resistance of the coating film formed from the aqueous dispersion, it is more preferably 30 mass ppm or less, and further preferably 20 mass ppm or less. As the lower limit, 0 mass ppm can be cited. As an example of a method for making the concentrations of fluoride ions and sulfate ions in the aqueous dispersion within the above range relative to the aqueous medium, a method of removing fluoride ions and sulfate ions with an ion exchange resin or the like as described later can be cited.

[0021] <Emulsifier> A hydrocarbon emulsifier refers to an emulsifier in which the hydrophobic part of the hydrophilic part and the hydrophobic part of the emulsifier is mainly composed of a hydrocarbon group. Specific examples of hydrocarbon emulsifiers include: anionic hydrocarbon emulsifiers such as alkylbenzene sulfonates, higher fatty acid salts, alkyl sulfates, alkyl sulfonates, and alkyl ether sulfates; cationic hydrocarbon emulsifiers such as alkylamine salts, alkyl quaternary ammonium salts, and benzalkonium chloride salts; nonionic hydrocarbon emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl esters, sorbitan alkyl esters, polyoxyethylene sorbitan alkyl esters, and glycerol esters. In this aqueous dispersion, the concentration of the hydrocarbon emulsifier is 100 mass ppm or less relative to the total mass of the specific fluoropolymer. From the viewpoint of more excellent water resistance of the coating film formed from this aqueous dispersion, it is more preferably 80 mass ppm or less, and further preferably 50 mass ppm or less. As the lower limit, 0 mass ppm can be cited.

[0022] A fluorine emulsifier refers to an emulsifier in which the hydrophobic part of the hydrophilic part and the hydrophobic part of the emulsifier contains a fluorine atom. Specific examples of fluorine emulsifiers include fluorinated alkanoates and fluorinated ether carboxylic acid compounds. As a suitable form of this aqueous dispersion, a form in which the concentration of the fluorine emulsifier is 100 mass ppm or less relative to the total mass of the specific fluoropolymer can be cited. In this aqueous dispersion, the concentration of the fluorine emulsifier is preferably 100 mass ppm or less, more preferably 75 mass ppm or less, and further preferably 50 mass ppm or less relative to the total mass of the specific fluoropolymer. As the lower limit, 0 mass ppm can be cited. If the concentration of the fluorine emulsifier relative to the total mass of the specific fluoropolymer is 100 mass ppm or less, the water resistance of the coating film formed from this aqueous dispersion is more excellent.

[0023] The emulsifier refers to all emulsifiers, including not only the above-mentioned hydrocarbon emulsifiers and fluorine emulsifiers, but also other emulsifiers (such as silicone emulsifiers in which the hydrophobic part contains a silicon atom, etc.). In this aqueous dispersion, the concentration of the emulsifier is preferably 100 mass ppm or less, more preferably 75 mass ppm or less, and further preferably 50 mass ppm or less relative to the total mass of the specific fluoropolymer. As the lower limit, 0 mass ppm can be cited. If the concentration of the emulsifier relative to the total mass of the specific fluoropolymer is 100 mass ppm or less, the water resistance of the coating film formed from this aqueous dispersion is more excellent.

[0024] As an example of a method for making the concentration of the above various emulsifiers within the above range, a method of manufacturing this aqueous dispersion without using various emulsifiers can be cited.

[0025] <Other components> This aqueous dispersion may further contain other components other than the above within the range where the effects of the present invention can be fully exhibited. Specific examples of other components that this aqueous dispersion may further contain include curing agents, curing catalysts, resins other than the above-mentioned specific fluoropolymers ((meth)acrylic resins, polyurethane resins, epoxy resins, etc.), colorants (dyes, organic pigments, inorganic pigments, bright pigments using metals or mica, etc.), ultraviolet absorbers, matting agents, leveling agents, surface modifiers, degassing agents, fillers, tackifiers, antistatic agents, rust inhibitors, silane coupling agents, antifouling agents, low-pollution treatment agents, plasticizers, adhesives, and other additives that can be contained in ordinary coatings. When this aqueous dispersion contains other components, the content of the other components is preferably 1 to 90% by mass, more preferably 10 to 80% by mass, based on the total mass of this aqueous dispersion.

[0026] <Manufacturing method of the aqueous dispersion> As an example of the manufacturing method of the above-mentioned aqueous dispersion, a form having a polymerization step of polymerizing monomers containing TFE and PAVE in an aqueous medium in the presence of a polymerization initiator to obtain a dispersion containing the above-mentioned specific fluoropolymer and an aqueous medium, and a removal step of removing fluoride ions and sulfate ions from the above-mentioned dispersion can be cited. The aqueous medium in which the particles of the specific fluoropolymer are dispersed thus obtained can be directly used as this aqueous dispersion, or it can also be used as this aqueous dispersion after further adding other aqueous media. In addition, the solvent can be replaced and the specific fluoropolymer can be dispersed in other aqueous media and then used as this aqueous dispersion.

[0027] Specific examples of the aqueous medium used in the polymerization step are the same as those of the aqueous medium contained in the above-mentioned aqueous dispersion. The monomers used in the polymerization step contain TFE and PAVE, and other monomers can be further contained, but it is preferably free of other monomers. The suitable form of PAVE is as described above. The amount of monomers used only needs to be appropriately adjusted so that the content of each unit contained in the obtained specific fluoropolymer is within the above range. The amount of monomers used is preferably 3 to 20 parts by mass, more preferably 5 to 10 parts by mass, based on 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 an organic polymerization initiator such as ammonium persulfate, sodium persulfate, potassium persulfate and other persulfate-based, diperoxysuccinic acid, azodiisobutylamidine dihydrochloride, etc., further preferably persulfate-based, and particularly preferably ammonium persulfate. Two or more polymerization initiators can be used in combination. The usage amount of the polymerization initiator is preferably 0.1 to 3 parts by mass, more preferably 0.2 to 2 parts by mass, relative to 100 parts by mass of the monomer usage amount.

[0029] The component of the chain transfer agent can be used in the polymerization process.

[0030] The monomer is introduced into the reaction system (i.e., the polymerization reaction vessel) by a conventional method. For example, the monomer can be continuously or intermittently introduced into the reaction system to make the polymerization pressure reach a specified pressure. Or the monomer can be dispersed or dissolved in an aqueous medium, and the obtained mixed solution can be continuously or intermittently introduced into the reaction system. The polymerization initiator can be added to the reaction system together or separately.

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

[0032] The polymerization process is preferably carried out in the substantial absence of hydrocarbon emulsifiers, more preferably in the substantial absence of hydrocarbon emulsifiers and fluorine emulsifiers, and further preferably in the substantial absence of emulsifiers. Specific examples of these emulsifiers are as described above. The substantial absence of hydrocarbon emulsifiers means an environment where the content of hydrocarbon emulsifiers is 100 mass ppm or less, preferably 50 mass ppm or less, more preferably 0 mass%, relative to the total mass of the aqueous medium used in the polymerization process. The substantial absence of hydrocarbon emulsifiers and fluorine emulsifiers means an environment where the total content of hydrocarbon emulsifiers and fluorine emulsifiers is 100 mass ppm or less, preferably 50 mass ppm or less, more preferably 0 mass%, relative to the total mass of the aqueous medium used in the polymerization process. The substantial absence of emulsifiers means an environment where the content of emulsifiers is 100 mass ppm or less, preferably 50 mass ppm or less, more preferably 0 mass%, relative to the total mass of the aqueous medium used in the polymerization process.

[0033] In the removal step, fluoride ions and sulfate ions that may be contained in the dispersion are removed. As a means for removing fluoride ions and sulfate ions, an ion exchange resin (preferably an anion exchange resin) can be exemplified. The removal step can be carried out multiple times until the concentrations of fluoride ions and sulfate ions in the obtained aqueous dispersion reach the above ranges.

[0034] <Usage> This aqueous dispersion can be suitably used as a coating material itself or as a raw material for a coating material.

[0035] [Coated article] The coated article of the present invention has a substrate and a coating film formed of the above aqueous dispersion disposed on the substrate.

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

[0037] From the viewpoint of more excellent weather resistance of the coated article, the film thickness of the coating film is preferably 1 to 200 μm, more preferably 10 to 100 μm.

[0038] The manufacturing method of the coated article is a method of coating the aqueous dispersion on the substrate to form a coating film. The coating film can be formed by coating the aqueous dispersion on the substrate and drying and then heating and curing as needed. The aqueous dispersion can be directly coated on the surface of the substrate, or can be coated after subjecting the surface of the substrate to a known surface treatment (such as substrate treatment). Further, a primer coat can be formed on the substrate and then the aqueous dispersion can be coated on the primer coat. In addition, the aqueous dispersion can be coated on an article having the above substrate. As the coating method, a spraying method, a doctor blade method, a flow coating method, a rod coating method, a spin coating method, a dip coating method, a screen printing method, an intaglio printing method, a die coating method, an inkjet method, a curtain coating method, a method using a brush or a spatula, etc. can be exemplified. Examples

[0039] Hereinafter, the present invention will be described in detail with 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 the aqueous dispersion> Using the aqueous dispersion as a sample, measurement was carried out using a laser diffraction / scattering particle size distribution analyzer (ELSZ manufactured by Otsuka Electronics Co., Ltd.).

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

[0042] <Concentrations of fluoride ions and sulfate ions> 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 frozen and coagulated and then filtered, and the resulting aqueous medium was analyzed by ion chromatography. Among them, ion chromatography analysis was performed using an ion chromatograph ICS-5000 (manufactured by Thermo Fisher Scientific). The separation column used was Dionex IonPac AS-19, the guard column used was Dionex IonPac AG-19, and the eluent used was KOH.

[0043] <Concentration of emulsifier> The concentrations of hydrocarbon emulsifier, fluorine emulsifier, and emulsifier relative to the total mass of the fluoropolymer in the aqueous dispersion were calculated based on the feeding amounts.

[0044] [Manufacture of aqueous dispersion 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 mixture was stirred at 600 rpm and heated to 80°C. Then, an aqueous ammonium persulfate solution (5.9% by 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 injected, the reactor was cooled to end the polymerization reaction. After recovering the residual gas in the reactor, it was replaced with nitrogen. Then, the pressure in the reactor was reduced to -0.1 MPaG and heated to 60°C. After removing 980 g of water from the reactor, the liquid in the reactor was cooled and withdrawn. This liquid was designated as aqueous dispersion A. Aqueous dispersion A is a dispersion in which particles of fluoropolymer 1A (average particle size 110 nm) are dispersed in an aqueous medium, and the content of fluoropolymer 1A is 20% by mass relative to the total mass of aqueous dispersion A. The aqueous dispersion A was frozen and aggregated, then filtered, and the obtained fluoropolymer 1A was washed with ultrapure water. Thereafter, it was dried under vacuum at 100 °C. NMR analysis of the obtained fluoropolymer 1A showed that the PMVE unit / TFE unit = 34.4 / 65.6 (molar ratio).

[0045] [Preparation of aqueous dispersion B] To the above aqueous dispersion A (100 g), an ion exchange resin (Purolite A300 (manufactured by Purolite Co.), an anion exchange resin, 15 g) was added, and the mixture was stirred for 60 minutes. Thereafter, the aqueous dispersion and the ion exchange resin were filtered to obtain an aqueous dispersion B. The aqueous dispersion B is a dispersion in which particles of fluoropolymer 1A (average particle size 110 nm) are dispersed in an aqueous medium, and the content of fluoropolymer 1A is 20% by mass based on the total mass of the aqueous dispersion B.

[0046] [Preparation of aqueous dispersion C] To a 2.1 L stainless steel pressure-resistant reactor, ultrapure water (1082 g), disodium hydrogen phosphate dodecahydrate (10.5 g), 30% by mass aqueous EEA solution (aqueous solution of CF3CF2-O-CF2CF2-O-CF2COONH4) (80.1 g), PMVE (70 g), and TFE (14 g) were added, and the mixture was stirred at 600 rpm and heated to 80 °C. An aqueous ammonium persulfate solution (1.0% by mass, 20 cc) was added to start the polymerization. The pressure inside the reactor decreased with the start of polymerization, so TFE was added to maintain the pressure constant. For every 8 g of TFE injected, 7 g of PMVE was injected. After injecting 160 g of TFE, the reactor was cooled to end the polymerization reaction. After recovering the residual gas inside the reactor, the liquid was withdrawn. This liquid was designated as aqueous dispersion C. The aqueous dispersion C is a dispersion in which particles of fluoropolymer 1C are dispersed in an aqueous medium. The aqueous dispersion C was frozen and aggregated, then filtered, and the obtained fluoropolymer 1C was washed with ultrapure water. Thereafter, it was dried under vacuum at 100 °C. NMR analysis of the obtained fluoropolymer 1C showed that the PMVE unit / TFE unit = 34 / 66 (molar ratio).

[0047] [Preparation of aqueous dispersion D] To 100 g of the hydrotropic dispersion C, 1 g of TERGITOL TMN-100X (manufactured by The Dow Chemical Company, nonionic hydrocarbon emulsifier) and 15 g of an ion exchange resin (Purolite A300, manufactured by Purolite Company, anionic exchange resin) were added, and the mixture was stirred for 60 minutes. Thereafter, the aqueous dispersion and the ion exchange resin were filtered to obtain an aqueous dispersion D. The aqueous dispersion D is a dispersion in which particles of fluoropolymer 1C (average particle diameter: 84 nm) are dispersed in an aqueous medium, and the content of fluoropolymer 1C is 21% by mass relative to the total mass of the aqueous dispersion D.

[0048] [Production of Aqueous Dispersion E] To a 2.1 L stainless steel pressure-resistant reactor, 1170 g of ultrapure water, 10.5 g of disodium hydrogen phosphate dodecahydrate, 75 g of PMVE, and 14 g of TFE were added, and the mixture was stirred at 600 rpm and heated to 80°C. Subsequently, an aqueous ammonium persulfate solution (20% by mass, 5 cc) was added to initiate polymerization. Since the pressure inside the reactor decreased with the start of polymerization, TFE was added to maintain the pressure constant. For every 8 g of TFE injected, 7 g of PMVE was injected. After injecting 160 g of TFE, the reactor was cooled to terminate the polymerization reaction. After recovering the residual gas inside the reactor, the liquid was withdrawn. This liquid was designated as the fluoroelastomer dispersion E. The aqueous dispersion E is a dispersion in which particles of fluoropolymer 1E (average particle diameter: 290 nm) are dispersed in an aqueous medium. The aqueous dispersion E was frozen and aggregated and then filtered, and the obtained fluoropolymer 1E was washed with ultrapure water. Thereafter, it was dried under vacuum at 100°C. NMR analysis of the obtained fluoropolymer 1E revealed that the PMVE unit / TFE unit = 33.3 / 66.7 (molar ratio). In addition, the concentration of sulfate ions relative to the total mass of the aqueous medium in the aqueous dispersion E was 2000 ppm by mass.

[0049] [Production of Aqueous Dispersion F] To 100 g of the above aqueous dispersion E, 15 g of an ion exchange resin (Purolite A300, manufactured by Purolite Company, anionic exchange resin) was added, and the mixture was stirred for 60 minutes. Thereafter, the aqueous dispersion and the ion exchange resin were filtered to obtain an aqueous dispersion F. The aqueous dispersion F is a dispersion in which particles of fluoropolymer 1E (average particle diameter: 290 nm) are dispersed in an aqueous medium, and the content of fluoropolymer 1E is 20% by mass relative to the total mass of the aqueous dispersion F.

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

[0051] [Example 2] Use the above aqueous dispersion A as the aqueous dispersion in Example 2.

[0052] [Example 3] Use the above aqueous dispersion D as the aqueous dispersion in Example 3.

[0053] [Example 4] Use the above aqueous dispersion F 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. Add 50 cc of the aqueous dispersion of each example to a centrifuge tube, leave it at room temperature (23 °C) for 2 weeks, read the amount of the precipitate (fluoropolymer particles) deposited at the bottom of the centrifuge tube by scale, and evaluate according to the following criteria. A: 0 - 0.1 cc B: More than 0.1 cc

[0055] <Water Resistance 1> Spray-coat V SELAN (registered trademark) #700 manufactured by Dainippon Paint Co., Ltd. on the surface of a stone plate with a length of 120 mm, a width of 60 mm, and a thickness of 15 mm by air spraying so that the dry film thickness reaches 20 μm, and dry it at 100 °C for 210 seconds to form a bottom coating film. Then, spray-coat the aqueous dispersion of each example on the bottom coating film by air spraying so that the dry film thickness reaches 40 μm, and dry it at 120 °C for 210 seconds to form a coating film, and obtain test plates corresponding to each example. Conduct a test. After immersing the test plate in warm water at 60 °C for 18 hours, immerse it in cold water at 5 °C for 15 hours, and then dry it at 5 °C. After drying, evaluate the appearance of the coating film according to the following criteria. A: No whitening or swelling is confirmed in more than 80% of the area of the coating film surface. B: Whitening or swelling is confirmed in more than 20% of the area of the coating film surface.

[0056] <Water Resistance 2> Conduct a test. Immerse the test plate obtained in the above "Water Resistance 1" in warm water at 60 °C for 2 weeks, and then dry it at 5 °C. After drying, evaluate the appearance of the coating film according to the following criteria. A: No whitening or swelling is confirmed in more than 80% of the area of the coating film surface. B: No whitening or swelling is confirmed in more than 60% and less than 80% of the area of the coating film surface. C: Whitening or swelling is confirmed in more than 40% of the area of the coating film surface.

[0057] [Table 1]

[0058] The aqueous dispersion of the present invention has excellent storage stability and can form a film with excellent water resistance (Example 1). In addition, 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 the disclosure of the present invention.

Claims

1. An aqueous dispersion, which is an aqueous dispersion containing a fluoropolymer and an aqueous medium, and the fluoropolymer contains units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), wherein, 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 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, the concentration of fluoride ions is 50 mass ppm or less relative to the total mass of the aqueous medium in the aqueous dispersion, the concentration of sulfate ions is 50 mass ppm or less relative to the total mass of the aqueous medium in the aqueous dispersion, the concentration of hydrocarbon emulsifier is 100 mass ppm or less relative to the total mass of the fluoropolymer.

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

3. The aqueous dispersion according to claim 1 or 2, wherein the concentration of emulsifier is 100 mass ppm or less relative to the total mass of the fluoropolymer.

4. The aqueous dispersion according to claim 1 or 2, wherein, The perfluoro(alkyl vinyl ether) is a monomer represented by the following formula (1): CF2=CF-O-R f1 (1) In formula (1), R f1 represents a perfluoroalkyl group having 1 to 10 carbon atoms.

5. A coated article, which has a substrate and a coating film formed of the aqueous dispersion according to claim 1 or 2 disposed on the substrate, wherein the material of the substrate is an inorganic substance, an organic substance, or an organic-inorganic composite material.

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